Group IV semiconductors at the nanoscale - towards applications in photonics, electronics and life sciences

Resume : The development of non-equilibrium group IV nanoscale alloys is key to achieve new functionalities, such as direct bandgap, in conventional indirect bandgap elemental semiconductors. Group IV one-dimensional nanoscale systems are critical for the advancement of silicon compatible device modules. In particular, direct bandgap semiconductor materials are needed for new device architectures such as ?band-to-band tunnelling (BTBT)? tunnel FETs (TFET), optical interconnects and for the development of group IV photonics. Here, we describe for the first time the fabrication of uniform diameter, direct bandgap Ge1-xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, through a conventional catalytic bottom-up growth paradigm employing innovative catalysts and precursors. Sn inclusion in the Ge nanowires far exceeded the equilibrium solubility (~ 1 at.%) of Sn in bulk Ge. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 ºC) during cool-down, facilitated the excessive dissolution of Sn in the nanowires. Sn was uniformly distributed throughout the Ge nanowire lattice, as determined by atomic resolution energy electron loss spectroscopy, with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. A direct bandgap has been identified for Ge1-xSnx nanowires with 9.2 at.% Sn through temperature and power dependent photoluminescence. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth.

Resume : Given that nearly 60% of the world's fossil-fuel energy is wasted as heat, if even a fraction of this heat could be converted to useful electric power, both energy efficiency and sustainability would be dramatically increased. However the low power factor of the thermoelectric materials and in turn device efficiency remains too low. Nanostructuration has demonstrated strong impact on thermal conductivity reduction related to phonon dispersion at surfaces coupled to potential increases in electronic conductivity due to confinement effects. We propose the development of a patterned nanowire-based thermoelectrical generator. We intend to exploit a pioneered nanostructuration of Si and Ge NWs by phase transformation leading to 2H/3C heterostructured nanowires [1]. Due to the formation of periodic boundaries and to the concomitant surface roughness, these NWs should exhibit an enhanced figure of merit ZT. The system will take advantages of the use of Si and Ge abundant, non-toxic and recyclable material and compatible with standard silicon technologies. First we performed structural characterizations combined with Raman and IR absorption spectroscopies to get fundamental physical properties of the 2H phase. The main goal is to give an exhaustive insight into the thermoelectric properties of heterostructures 2H/3C in Si, Ge and Si1-xGex NWs. C-AFM measurements performed on both 3C and nanostructured 3C/2H Ge NWs have confirmed that the electronic conductivity is not degraded by the formation of interfaces and 2H domains in the nanowires in agreement with simulations of electronic transmission at the 2H/3C interface. This is the first encouraging point toward an increase of ZT. Regarding the phonon, thermal conductivity measured on Ge NWs show a reduction as a function of NW diameters from 460 to 220 nm and the thermal conduction is in particular strongly reduced at 220 nm for which the phase transformation has occurred. Both results are encouraging for a thermoelectric application of our transformed nanowires. [1] : L. Vincent, G. Patriarche, G. Hallais, C. Renard, C. Gardès, D. Troadec, and D. Bouchier, Nanoletters 2014, 14, pp4828 cla

Resume : Using a top-down approach applied to Silicon on insulator (SOI) wafers, we fabricate Silicon nanoribbons field effect transistors (ISFETs) and investigate their properties as bio-chemical sensors. ISFETs with high-k gate oxide layers (Al2O3 or HfO2) exhibit a very good sensitivity towards protons due to the high density of hydroxyl groups at their surface. The maximum pH response for an ideal oxide surface reaches approximately 60mV/pH (Nernst limit). Using a dual-gate approach, we demonstrated a pH response at the Nernst limit for both oxide surfaces [1]. To help establishing the detection limit of the sensors, we have characterized their low-frequency noise [2,3]. Going beyond pH sensing, we have demonstrated the specific detection of ions (typically K+, Na+) by functionalizing the nanoribbons surface with e.g. polyvinyl chloride (PVC) membranes with embedded ionophores [4] or via ion-binding linkers covalently anchored to gold-coated nanoribbons [5]. We emphasize the importance of functionalization schemes leading to a dense, compact layer to avoid the influence of competing reactions taking place at unfunctionalized sites and model this competition effect [6]. We further illustrate this effect on a experimental system by investigating the influence of residual pH sensitivity on the response of nanoribbons functionalized for the specific detection of ions. Recently, we have also started to investigate the possibilities of our sensors for the detection of small sugar binding proteins [7]. 1. O. Knopfmacher, A. Tarasov, W. Fu, M. Wipf, B. Niesen, M. Calame and C. Schönenberger, Nernst Limit in Dual-Gated Si-Nanowire FET Sensors, Nano Letters, 10, 2268-2274 (2010). 2. A. Tarasov, W. Fu, O. Knopfmacher, J. Brunner, M. Calame and C. Schoenenberger, Signal-to-noise ratio in dual-gated silicon nanoribbon field-effect sensors, Appl. Phys. Lett., 98, 012114 (2011). 3. K. Bedner, V. A. Guzenko, A. Tarasov, M. Wipf, R. Stoop, S. Rigante, J. Brunner, W. Fu, C. David, M. Calame, J. Gobrecht and C. Schönenberge, Investigation of the dominant 1/f Noise Source in Silicon Nanowire Sensors, Sensors and Actuators B, 191 , 270?275 (2014). 4. M. Wipf, R. L. Stoop, A. Tarasov, K. Bedner, W. Fu, M. Calame, and C. Schönenberger. Potassium sensing with membrane-coated silicon nanowire field-effect transistors. , Proceeding of the 17th IEEE International Conference on Solid-State Sensors, Actuators & Microsystems (Transducers & Eurosensors XXVII), Barcelona, Spain, 16-20 June 2013, pages 1182-1185 (2013). 5. M. Wipf, R. L. Stoop, A. Tarasov, K. Bedner, W. Fu, I. A. Wright, C. J. Martin, E. C. Constable, M. Calame and C. Schönenberger, Selective Sodium Sensing with Gold-Coated Silicon Nanowire Field-Effect Transistors in a Differential Setup, ACS Nano, 7 (7) , 5978?5983 (2013). 6. R. L. Stoop, M. Wipf, S. Mueller, K. Bedner, I. A. Wright, C. J. Martin, E. C. Constable, W. Fu, A. Tarasov, M. Calame, and C. Schönenberger, Competing Surface Reactions Limiting the Performance of Ion-Sensitive Field-Effect Transistors, Sensors and Actuators B, 220, 500-507 (2015). 7. M. Wipf, R. L. Stoop, G. Navarra, S. Rabbani, B. Ernst, K. Bedner, C. Schoenenberger and M. Calame, Label-Free FimH Protein Interaction Analysis Using Silicon Nanoribbon BioFETs, submitted.

Resume : For lab-on-a-chip systems, silicon nanowire (SiNW) sensors allow a straightforward electrical read-out and label-free detection, thus rendering them feasible sensors for biomedical diagnostics even in remote areas. However, a crucial concern for nanostructures, including those made of silicon, are their degradation and long-term stability in a physiological environment or in vivo. Functional SiNW biosensors need stable electronic properties and sensitivity over the entire duration of a measurement, which may be hampered by surface reactions leading to a slow dissolution of the device [1]. To systematically determine the extent of the dissolution reactions and their impact on the electrical properties, we first examined quantitatively the dissolution behavior of SiNWs grown from silane by gold catalyzed vapor-liquid-solid synthesis in PBS and cell culture medium using ICP-AES analysis and SEM. Furthermore, I-V measurements of SiNWs integrated as liquid-gate field-effect-transistor by common microfabrication technologies were used to record the SiNW degeneration electrically with respect to the liquid-gate potential and the flow rate of the liquid medium. Additionally, SiNWs exhibiting a rectifying pn-junction were configured as diodes and enabled us to study the effect of contact degradation and dissolution caused diameter variation independently from each other. [1] Iller 1979 in The Chemistry of Silica, Wiley-Interscience, and Zhou et al. 2014, Nano Lett. 14, 1614-1619

Resume : Lead pollution due to industrial wastes is responsible ofmany diseases even forlow exposure level. A lot of technologies have been developed to enable low lead concentration detection. These main techniques are spectrophotometry, colorimetry, and voltammetry. More recently, electrochemical sensors based on diazonium salts showed high sensitivity. However, these detection methods are not compatible with embedded portable microsystems for outdoor real time measurement. On the contrary, silicon based devices are highly integrable and are already used as chemical sensors. In our work, functionalized silicon nanoribbons (< 50 nm) based resistors to specifically detect Pb2 ions are fabricated. The sensitive layer is formed by parallel nanoribbons processed by classical silicon technology. We chose diazonium salts which can be grafted on multiple substrates like polycrystalline silicon and enable heavy metal ions trapping.Analysis of silicon nanoribbons by three different surface analysis techniques (nanoSIMS, contact angle and ellipsometer) revealed a silicon surface modification and shows the feasibility of this functionalization. The use of these resistors for the detection of lead in aqueous solution is studied. The detection of lead ions is carried out by integration of current versus time. The results show that the detection increases in function both of the time and the concentration of the lead ions.

Resume : The organic/oxide/semiconductor field-effect transistors (FETs) shows that the optoelectronic field-effect of organic layer can effectively modulate the channel conductivity of the devices [1]. Based on the optoelectronic field-effect study, optoelectronic double-gate modulation using organic functional film on the FET devices has been proposed. For these purposes, hybrid Si nanowire FETs covered by metal-ion doped sol-gel film are fabricated in this work. The Si nanowire FETs are produced by conventional top-down CMOS process featured with honeycomb structure having better gate controllability [2,3]. The channel conduction of the devices is modulated by an optically sensitive gate formed with a sol-gel derived silicate matrix including metal cations. The film-coated devices show dramatically reduced subthreshold slope compared to conventional back-gate devices due to the nonlinear dielectric polarization of ions in the film. Also, we have investigated the transfer and transconductance characteristics upon light illumination that demonstrates the channel separation in the nanowire induced by gate coupling between the back gate and the optical front gate. The optical gate bias is originated from the photoreduction of metal cations in the film. Furthermore, the device shows linear photoresponse depending on the light power density and the light spectra. [1] E. Baek, S. Pregl, M. Shaygan, L. Römhildt, W. M. Weber, T. Mikolajick, D. A. Ryndyk, L. Baraban, G. Cuniberti, Optoelectronic switching of nanowire-based hybrid organic/oxide/semiconductor field-effect transistors, Nano Research, 2015, 8(4), 1229-1240. [2] T. Rim, K. Kim, S. Kim, C.-K. Baek, M. Meyyappan, Y.-H. Jeong, J.-S. Lee, Improved Electrical Characteristics of Honeycomb Nanowire ISFETs, Electron Device Letters, IEEE, 2013, 34(8), 1059-1061. [3] K. Kim, T. Rim, C. Park, D. Kim, M. Meyyappan and J.-S. Lee, Suspended honeycomb nanowire ISFETs for improved stiction-free performance, Nanotechnology, 2014, 25(34), 345501.

Resume : Silicon nanocrystals (SiNCs) have attracted attention as active materials in a variety of proto-type devices including solar cells, light-emitting diodes, and photodetectors. These, and other device structures require well-defined materials with predictable properties. Traditionally SiNC surfaces have been rendered processable and stable toward oxidation using variations of the general hydrosilylation approach; this class of reactions involves the addition of silicon-hydride bond on the SiNC surface across carbon-carbon double (or triple) bonds of target ligands and can afford ?monolayers? attached through robust silicon-carbon linkages. Recently it has come to light that typical hydrosilylation conditions can also afford insulating polymer/oligomer layers that could limit the utility of SiNCs. This important finding led us to explore alternative functionalization protocols and in doing so, we discovered that surface chemistry provides yet another degree of freedom with which SiNC properties may be tailored. This presentation will include a discussion of our most recent explorations of SiNC surface chemistry and new reactive platforms that have opened the door to new functional materials.

Resume : Here we present a new method of direct synthesis of silicon nanocrystals with a thin passivation shell. The silicon nanocrystals are formed from hydrogen silsesquioxane molecules which have been modified by various chemical groups. This material is annealed at 1000°C in an Ar atmosphere with a few percent of H. While the nanocrystals are being formed, the residues of modifying organic molecules are pushed out on the surface of the newly created nanocrystals and are partially carbonized. These silicon nanocrystals exhibit various linewidth and varying emission peak position, depending on the used modifying molecule, as observed with single dot spectroscopy. For example, the silicon nanocrystals prepared from hydrogen silsesquioxane, modified with methyl isobutyl ketone, exhibit a significantly narrower emission peak of individual nanocrystals at room temperature (average linewidth ~25 meV, spectral range from 530-720 nm (1.7-2.3 eV)) compared to silicon nanocrystals embedded in a silicon oxide shell (150 meV), according to single dot spectroscopy. Recently, we have measured an even narrower single silicon nanocrystal linewidth for acetone modified hydrogen silsesquioxane (~ 14 meV). These extremely narrow emission peaks are observed for the first time for silicon nanocrystals at room temperature and are even narrower than that of single CdSe quantum dots (>50 meV). These nanocrystals show promising optical properties with strong application potential for solar cells and LEDs.

Resume : Silicon quantum dots (SiQDs) are a promising alternative to toxic and rare material QDs that are researched or used in optoelectronics, photonics and bio-imaging. For competitive emission properties, however, the radiative rate needs to be considerably improved. This is the case in SiQDs capped with organic ligands (C:SiQDs). Nevertheless, despite radiative rates approaching those of direct bandgap QDs [1,2], the external quantum yield (EQY) in the visible range remains comparatively low (<20%) [1,2]. Improved understanding of the processes underlying the ensemble photoluminescence (PL) and EQY of these materials can be achieved from single QD spectroscopy, allowing study of the properties of individual emitters that are otherwise obscured in ensemble measurements. As generally in all quantum dot emitters, the EQY is critically influenced by effects related to surface charge, local electric fields and trap states, leading to lower blinking duty cycle and enhanced non-radiative losses. In order to measure radiative rate directly and to be able to distinguish the separate contribution of non-radiative effects, we perform a Drexhage-type experiment. This confirms the high radiative rate of C:SiQDs and – by comparison with the blinking behavior and ensemble quantum yield measurements - allows us to identify the non-radiative mechanisms that prevent high EQY. [1] K. Dohnalova et al., Light: Science and Applications 2 (2013) e47 [2] K. Kusova et al., ACS Nano 4 (2010) 4495

Resume : Metal-semiconductor hybrid nanoparticles (HNPs), which combine metal and semiconductor nano-domains into a single nanoparticle, are expected to exhibit novel optical properties useful for the biomedical applications, solar-energy conversion and photo-catalysts. Recently, there have been great advancements in the synthesis of HNPs based on Cd and Pb chalcogenide quantum dots (QD). On the other hand, the development of Si QD-based HNPs has scarcely been reported despite their advantage in terms of biocompatibility. In this work, we report the synthesis of Si QD-based HNPs with different shapes and compositions. Si-QDs used in this work have very heavily boron (B) and phosphorus (P) codoped Si shells on the surface.[1] Unlike organic-ligand-capped QDs, the surface of codoped Si-QDs are mainly terminated by hydrogen (H). Chemically active H-terminated sites on the surface of codoped Si-QDs have an ability to reduce metal-salts, leading to the growth of metal NPs by simply mixing the QDs and metal salts. This process results in the formation of very uniform HNPs with precise controllability of the metal NP size. We systematically study the structural and optical properties of different metal NPs, which demonstrate that the Si-QD based HNPs can be a promising material for biomedical and optoelectronic applications. [1] H. Sugimoto, et al., J. Phys. Chem. C, 117, 11850 (2013), Nanoscale, 6, 12354 (2014).

Resume : We present a structural and optical study of Si and Ge nanocrystals in the resonator structures. The multilayer nanoperiodic structures were fabricated by physical vapor deposition of SiO (or GeO) and SiO2. Formation of nanocrystals has been achieved by annealing: the SiO/SiO2 structure at 1000-1100°C (L Vaccaro et al. 2015 Phys. Status Solidi 600) and the GeO/SiO2 at 400-600°C. Nanocrystal sizes have ranged from about 3 to 8nm by varying the annealing temperature and the layer thicknesses. The resonators were prepared on silicon substrates and consisted of two Bragg reflectors with an active medium in between filled with the nanocrystals (A Belarouci et al. 2006 J Lum 282). The stationary photoluminescence has been excited with a Nd:YAG laser (532nm) and time-resolved with a pulse N2 laser (337nm, 7ns, 45Hz). The study has been found that the use of such resonators has led to significant narrowing of luminescence bands to 20 nm and enhancing intensity up to 50 times. The band peak is determined by the resonator design: operating range for the SiO/SiO2 structures was 650-850nm and 1000-750nm for GeO/SiO2. So far as the dynamics is concerned, we observed decrease in luminescence time for the resonators compared with the structures without mirrors. This study in general is a good way to add further functionality to silicon photonics and investigate broader the problem of the nanocrystal luminescence as well. The work was supported by the RFBR project 140200119 & 150205086.

Resume : Two years after the birth of silicene, graphene?s nearest cousin, in 2012 [1], ?The growth and properties of silicene? has become fourth among the ten "hottest research fronts" in physics of 2014, according to a citation-based study recently released by Thomson-Reuters [2]. Germanene, its germanium analogue, was synthesized in 2014 [2,3], and, just very recently, stanene, its tin partner, in December 2015 [4]. The first silicene based FET was fabricated mid 2015 [5], showing the promise of these novel 2D allotropes of Si, Ge and Sn, for ultimate scaling of low energy consumption nanoelectric devices [6,7,8]. In this invited talk I will review the advances in these fascinating synthetic elemental 2D materials and discuss progress and challenges in their potential applications. 1. P. Vogt et al., Phys. Rev. Lett., 108, 155501 (2012). 2. C. Day, Physics Today, 25 September 2015. 3. M.E. Dávila et al., New J. Phys., 16, 095002 (2014). 4. M.E. Dávila and G. Le Lay, Sci. Rep., in press. 5. Feng-feng Zhu et al., Nature Mater., 14, 1020 (2015). 6. Li Tao et al., Nature Nanotechnology, 10, 227 (2015). 7. G. Le Lay, Nature Nanotechnology, 10, 202 (2015). 8. A. Dimoulas, Microelectronic Engineering, 131, 68 (2015). 9. C. Grazianetti, E. Cinquanta and A. Molle, 2D Mater., 3, 012001 (2015).

Resume : Graphene-covered metallic nanostructures provide a unique platform for plasmonic-enhanced graphene devices. Recently, single-layer graphene was transferred and studied onto arrays of metallic nanoparticles, fabricated by lithographic methods. Here, we integrate graphene with plasmonic Si substrates. We prepare arrays of nanopillars on the surface of Si (black Si) by femtosecond-laser irradiation in water. Laser nanostructuring of Si provides a simple, maskless, large-area, cost-effective method for the fabrication of plasmonic substrates. Coating the structured Si surface by a thin metallic layer results in the spontaneous formation of metallic nanoparticles, which cover the structured surface, instead of a smooth metallic film. The whole process is scalable and not inherently size-limited. Single layers of graphene are prepared by chemical vapor deposition on transition metal catalytic substrates (Cu foil) and transferred on the plasmonic Si substrates by a PMMA scaffolding method. The properties of black Si-supported graphene are studied by scanning electron microscopy and optical reflectance spectroscopy. We probe the graphene layer for its plasmonic-enhanced Raman spectral signal via Raman spectroscopy. Optical reflectance spectra demonstrate the coupling between localized surface plasmons and graphene. The Raman signal of graphene on black Si, coated with metallic nanoparticles, is enhanced by orders of magnitude, compared with all other reference substrates employed, i.e., (a) graphene on black Si without metallic nanoparticles and (b) graphene on flat Si with or without metallic coating. This result paves the way for future real-world applications of large-area hybrid nanomaterials.

Resume : Free-standing nanometer-thin silicon membranes with pore sizes from ~ 1 nm up to ~ 100 nm in diameter have been fabricated using a bottom-up process that involves the deposition of amorphous silicon films sandwiched between silicon oxide or nitride films followed by annealing. This process is inexpensive, controllable, and scalable. In this presentation, we start by describing how the pores are formed and their properties controlled. We then discuss a number of applications, ranging from applications in biology and microscopy to fundamental studies of molecular transport.

Resume : Photoelectrical and electrical properties of the composite Si-Au layers on crystalline Si were investigated. An improved modification of the laser electrodispersion was used, with two lasers and two separate targets, to prevent mixing of elements in the laser torch plasma and obtain uniform Si-Au composite layer. Atomic-force microscopy proves that the prepared layers consist of nanoparticles. At a certain Au to Si ratio (~50:50), we obtain large photoresponse gain. The photoresponse of such heterostructures is generally determined by rectifying properties of the interface. However, for the structures with the Si-Au nanolayer (50:50) the rectification factor is much smaller, while the responsivity is much larger than those of the structure with purely Si nanolayer. Thus we assume that there is one more photoresponse mechanism, which, moreover, implies photocurrent gain. We propose that the interface in our structures consists of two area types: rectifying and ohmic, and the structures could be represented by an equivalent circuit consisting of a photodiode and a photoconductor in parallel. Contribution of the photoconductivity in the whole photoresponse may be significant. If the time of flight of the photogenerated carriers is shorter than their lifetime, the so called photoconductivity gain appears. Our estimations show that in the Si-Au composite layers this condition is fulfilled, which gives rise to the enhanced responsivity and large photoresponse gain.

Resume : Silicon oxide has for many years provided engineers with an ideal insulator. Silicon microelectronics still relies on its physical, chemical and, above all, electrical durability; modern devices incorporate few-nanometre thick oxide layers in which the electrical stress can be extreme. Here, we report the highly dynamic structural and electrical behaviour of thin silicon oxide films under voltage stress. We show, using a combination of electrical measurements, structural measurements, in situ ion detection and mass spectroscopy, along with DFT and Monte Carlo models, that realistic device voltages can generate major changes to the oxide that are reflected in high contrast resistance switching. In some cases these changes are reversible; in others they are permanent precursors to dielectric breakdown. Our results have major implications for the use of silicon oxide in electronics and photonics ? rather than a passive, stable insulator prior to breakdown it is instead a highly dynamic electrically manipulated system.

Resume : Conductance tomography is a technique that is rapidly growing in interest, offering a novel approach to probing the conductive structure of electronic devices. To generate profiles of conductivity within a device, an atomic force microscope is used in conductive mode with material gradually removed by the scanning tip. This builds up a set of two-dimensional current maps that may be combined into a three-dimensional profile. To date, variations on this method have been used to study such buried features as carrier profiles, conductive filaments and photo-excitation. We have recently probed the formation of conductive filaments in a number of resistance switching devices, predominantly with silicon-rich silica or perovskite active layers. We have demonstrated for the first time the profile of intrinsic filamentation in such devices, demonstrating that conductive pathways conform to the intrinsic structure of the switching layer. Moreover, we have also found that this technique is extremely versatile and surprisingly accessible. Here, we will show some of our key findings, relating to both experimental data and also some particularly appealing aspects of the procedure itself. Our results demonstrate the effectiveness of conductance tomography in our own devices and also its potential for application to a wide variety of devices and working conditions.

Resume : Ge nanocrystals (NCs) in dielectrics are interesting for applications of nonvolatile memories (NVMs) and optical sensors. Ge NCs in HfO2 is a good approach for NVM devices as Ge NCs play the role of nodes with efficient charge storage, improving charge retention and HfO2 matrix is a good dielectric to ensure the device area decrease and to reduce the leakage currents. Our approach to obtain Ge NCs in HfO2 matrix for NVM applications consists in deposition by magnetron sputtering of HfO2/(Ge or Ge-HfO2)/HfO2 trilayers on Si wafer followed by rapid thermal annealing for Ge NCs formation. HRTEM, HAADF-STEM, Raman and XPS are employed for studying Ge NCs formation (NCs size, separation distance, density and location, metallic Ge state). Charge storage properties are studied on MOS-like capacitors (with top and bottom Al contacts) by measuring C–V characteristics at different frequencies (100 kHz–1 MHz). C–V hysteresis loops with 1–3 V memory window are obtained. We demonstrate that the memory effect is given only by Ge NCs [1]. This result is strongly correlated with Ge NCs morphology, the most important being the controlled positioning of Ge NCs with high density at a precise location. The largest memory windows correspond to the denser Ge NCs with good vertical and lateral separation to each other. The separation between Ge NCs is improved if for fabrication of trilayer, the co-sputtered Ge-HfO2 intermediate layer is used. [1] Slav et al., Scripta Mater. 113, 135 (2016)

Resume : Strain-engineered silicon nanocrystals (SiNCs) passivated with organic molecules have recently been shown to possess direct bandgap [1]. Here we present observation of a rich structure in the single-nanocrystal photoluminescence (PL) spectra of such SiNCs in the temperature range of 9?300 K [2]. The intrinsic spectral shape is shown to be a structure that contains three peaks, approximately 150 meV apart, each of which possesses a Si phonon substructure. Narrow spectral lines, reaching < 1 meV at 20 K, are detected. The observed temperature dependence of the spectral structure can be assigned to the radiative recombination of positively charged trions. Our results serve as additional support for the direct-bandgap nature of the investigated SiNCs. We investigated also the blinking dynamics of these SiNCs over a broad interval of time scales and excitation intensities [3]. Both the autocorrelation function and the distribution of on and off times are observed to follow a broken-power-law function, which signifies a change in the power-law dynamics at short time scales (about 1?20 ms). Under the lowest excitation intensities, both the autocorrelation function and the distribution of on times are flat, implying the existence of a non-blinking regime. In addition to the broken-power-law dynamics, the end of the on-times distribution is also cut off by an exponential tail. The different observed regimes are discussed in detail and a qualitative model including the emission of a trion is proposed. [1] K. Kusova et al. Adv. Mater. Interfaces 1 (2014) 1300042, [2] K. Kusova et al. Light Sci. Appl. 4 (2015) e336, [3] K. Kusova et al., Phys. Rev. B 93 (2016) 035412.

Resume : Semiconductor nanocrystals (NCs) feature a lot of attractive properties but their wide-spread applications in practical devices are yet to appear. This is due to the fact that the NCs with the best optical characteristics contain either scarce (In, Ga) or toxic (Cd, As) elements, while the emissivity of Si NCs – the most obvious sustainable alternative remains low. Here, we investigate the emission efficiency limit of the basic, and so far the only truly up-scalable form of Si NC networks: thin-film solid-state dispersions of Si NCs embedded in SiO2. By making use of a low-temperature hydrogen passivation treatment we demonstrate a maximum emission efficiency of approximately 35%. This is the highest value ever reported for this type of material. By cross-correlating PL lifetime with emission efficiencies, we obtain a comprehensive understanding of the efficiency limiting processes and conclude that energy transfer towards Pb-defected NCs is a major mechanism limiting ensemble emissivity. We establish that the observed record efficiency corresponds to an interface density of Pb-centers of 1.3×1012cm2, which is 2 orders of magnitude higher than for the best Si/SiO2 interface. This result implies that Si NCs with up to 100% emission efficiency are feasible.

Resume : Si-nanoparticles (Si-nps) embedded in dielectric matrix such as SiO2 have attracted much interest during the past years as promising candidate for many applications in photonics and electronics: waveguides amplifiers, memory devices… However, light emission and carrier storage properties of Si-nps are directly related to the particle characteristics (size, distribution, density, the interface state…). Therefore, an accurate control of these parameters is crucial. In this context, we fabricate SiOx thin films and study, by the mean of Atom Probe Tomography (APT), the influence of dopants such as rare earth (RE) elements on the phase separation process for different annealing temperatures, silicon excess and dopant content. It has been observed that in the presence of dopant, the growth of Si-nps is highly related to the dopant content in the layer. Moreover, complex snowman-like nanostructure between Si and RE-silicates has been formed in the film at high annealing temperature. This nanostructure leads to a drastic change in the optical properties of the samples analyzed by photoluminescence (PL). Interestingly, under some conditions, PL from both Si-ncs and RE elements is observed. Indeed, such material can be a promising candidate for the development of multi-color light emitting device.

Resume : The ability to tune the band gap of Si nanocrystals (NC) embedded in a high band gap Si-rich alloy allows us to minimize thermalization losses and thereby increase the efficiency of multi-junction solar cells. Embedded Si-NCs can be made by annealing Si-rich Si alloy films at temperatures in excess of 900 °C. However, H effusion occurs between 400-600 °C, leading to an increased defect density in the annealed film. Reincorporation of H in a post-annealing passivation step is an effective method to reduce the defect density, but this requires an extra processing step, complicating the process and increasing the thermal budget. Another approach is to combine annealing and passivation in a single step, by annealing in a H2 containing atmosphere. Studies have reported discordant results, which suggests a detailed study of the effect of H on the NC growth kinetics has been lacking. In this contribution we report the effect of H on the NC crystallization process. We show that H2 gas during annealing leads to a lower sub-band gap absorption, indicating passivation of defects created during annealing. Samples annealed in pure N2 show expected trends according to crystallization theory. Samples annealed in forming gas (90% N2:10% H2), however, deviate from this trend. Their crystallinity decreases for increased annealing time. Furthermore, we observe a decrease in the mean NC size and the size distribution broadens, indicating that H causes a partial amorphization of the Si-NCs.

Resume : We have elaborated nanocomposites based mesoporous silicon (PS) and rhodamine 6G (Rh 6G) by simple impregnation. New luminescence properties were shown. This luminescence could occur by energy transfer from silicon nanocristallites to dye molecules. FTIR spectroscopy suggests the incorporation of rhodamine molecules into the pores of the matrices. An energy transfer was demonstrated from Si nanocrystallites to Rh6G molecules. We also study the effect of the concentration of Rh6G molecule on optical properties of nanocomposites PS/Rh6G. Also we present the thermal conductivity of samples using photothermal deflection thechnique.

Resume : Nano-gap electrodes separated from the molecular scale to the nanometer scale are fundamental building blocks for the fabrication of devices and circuits. Recently, large-area nano-gap electrodes fabrication for single molecules, including diodes, transistors, switches, memory and sensors has been central technical challenge for the device miniaturization. We report a new nanofabrication method for realizing large-area tunable nano-gap (20nm, 50nm, 100nm) electrode arrays with high efficiency and reproducibility. This method is the combination with chemical etching and an innovative technique called by secondary sputtering lithography. "Secondary sputtering lithography” enables fabrication of ultrahigh-resolution and high aspect ratio patterns of 3D various shapes. And this large-area nano-gap electrodes in a precise and controllable manner are fully compatible with previous nano-gap fabrication technology.

Resume : ¨Photoluminescence excitation measurements were performed on oxide-embedded single silicon nanocrystals at 300 K and at 70 K. The structure of the absorption curve was found to consist of four broad, temperature-independent peaks in the 2.0-3.5 eV energy range, above the optically emitting state (in the 1.7-1.9 eV range for probed nanocrystals). Atomistic calculations of the Si nanocrystal energy level structure exhibited good agreement with the experiment. They allowed to identify some of the observed transitions as the indirect band edge X-dominated states with enhanced Γ mixing. Previously, the continuum-based effective mass theory suggested a direct bandgap redshift, based on assigning a negative effective mass to the Si nanocrystal electrons at the Γ point. Here the direct band gap Γ-Γ transition does not show a redshift, nor does it appear in energy below the Γ-valley direct edge of bulk Si - neither in experiment nor calculations. Instead it is found to slightly blue-shift to higher energy with reduced nanocrystal size. The early theory does not appear to be as accurate in nanocrystal level description as atomistic calculations.

Resume : The Porous Silicon layers were formed on p-type silicon wafers by electrochemical etching in an HF:C2H5OH (1:1 by volume) electrolyte at room temperature at a constant current density 20mA/cm2 and etching duration 5min. This paper investigates the effect of Berberine on the passivation of PS. The immersion of as-etched PS in dilute Berberine solution. The immersion duration of berbrine was variable from 5 to 20min. For the optical and morphological characterization of porous films, Photoluminescence spectroscopy, FTIR spectroscopy and Reflectivity spectroscopy were used. A decrease in the reflectivity to about 6% for Berberine/PS annealed at 20minwas obtained. From Photoluminescence (PL) spectra, a blue-shift of the gap and an intensity were observed when the immersion duration is increased to 20min; we correlate these results to the change in chemical composition of the layers in order to find the optimized conditions for a potential application in silicon solar cells.

Resume : In this work, we investigate the thermal oxidation effect on optical and thermal properties of meso-porous silicon (meso-PS) layers using photoluminescence (PL) and photothermal deflection techniques (PDS, PTD). Samples have been successfully prepared by electrochemical anodization process. After a pre-oxidation for 1h 30 min at 300°C, layers are thermally oxidized in dry oxygen at different temperatures (800, 1000°C) and durations. PL measurements show that the annealed layers have comparable spectra with a peak position focused on 2.1 eV. This behavior indicates the formation of luminescent silicon (Si) nanocrystallites with comparable average sizes. From the effective mass theory, the diameter of those nanocrystallites was estimated to be around 2.2 nm. Another estimation of the mean size of the Si nanocrystallites obtained from the evolution of the thermal conductivity of the meso-PS layers based on photothermal deflection technique (PTD) data was close to the values obtained from the PL results. Photothermal deflection spectroscopy (PDS) measurements show that the thermal oxidation affects the absorption edge of the Si substrate. The optical band gap energy of the started substrates determined from the Tauc’s relation is observed to increase with the thermal temperature and duration.

Resume : Despite continued research efforts, the photoluminescence (PL) quantum yield (QY) of Si nanocrystals (NCs) remains below approximately 30%, strongly inhibiting their application potential. In this work, colloidal dispersions of free-standing B/P co-doped Si NCs are developed in an attempt to engineer the most suitable Si NC ensemble for efficient down-conversion applications. An ideal ensemble has a high PL QY (at least 50%) and allows interactive electronic properties, such as exciton transfer and space-separated quantum cutting. Recently [1,2], the importance of NC size and dispersity was demonstrated in relation to PL QY, validating the necessity of being able to attain monodisperse ensembles of NCs at a desired size. Here we report on the results of our pursuit towards NC size selection using high-performance size exclusion chromatography (HPSEC) on free standing colloidal Si NCs. Subsequently, embedding of the colloidal NCs in a glass solid is explored to obtain solid structures with engineered inter-particle interactions. Our final goal is a stable solid state dispersion of Si NCs with high PL QY, and desired interactions. [1] Sugimoto, Hiroshi, et al. The Journal of Physical Chemistry C 117.22 (2013): 11850-11857. [2] Sun, Wei, et al. Advanced Materials 27.4 (2015): 746-749.

Resume : Doping impurities in Si nanocrystals modifies not only the energy state structures, but also the chemical properties. We have shown that simultaneously B and P doped Si nanocrystals are dispersible in polar solvents almost perfectly without any surface functionalization processes. The codoped Si nanocrystals exhibit bright luminescence in the visible to near infrared ranges in water and are considered to be a promising environmentally friendly light source in biomedical fields. In this work, we determine the B acceptor and P donor levels measured from the vacuum level in a very wide size range by photoemission yield spectroscopy and photoluminescence spectroscopy. In relatively large Si-NCs, both levels are within the bulk Si band gap. The levels exhibit much smaller size dependence compared to the valence band and conduction band edges expected from theoretical calculations. We also estimate the Fermi level of codoped Si-NCs. The Fermi level of relatively large codoped Si-NCs is close to the valence band and thus they are p-type semiconductor. The Fermi level approaches the middle of the band gap with decreasing the size. The results suggest that below a certain size, perfectly compensated Si-NCs, i.e., Si-NCs with exactly the same number of active B and P, are preferentially grown.

Resume : The metal catalyzed vapour-liquid-solid (VLS) process has been widely used to fabricate divers silicon nanowires (SiNW) comprising radial and axial heterostructures, kinked or face-selective core-shell structures.[1] Along with this VLS mechanism, Wagner [2] proposed in 1968 also the reversed mechanism – coined solid-liquid-vapour (SLV) – for silicon among other semiconductors, using HCl as etching gas to drill “negative whiskers”. Despite its potential to extend the repertoire of available bottom-up nanotechnologies, very few publications since then have dealt with the SLV mechanism, e.g. in GaAs and InP using CBr4 as etching gas [3] or for SnO2 nanowires under high vacuum conditions [4]. In the present work, we used hydrogen gas as less corrosive and better-to-handle etching gas in combination with a plasma source to achieve SLV etching of bulk silicon substrates. We show that holes can be selectively etched, controlled by the metal catalyst size and location, and discuss the effect of process temperature and hydrogen flow rate on the etching process. Additionally, we compare the feasibility of using gold, platinum or aluminium as catalyst material. The resulting holes and surface features were characterized by profilometry, atomic force microscopy and scanning electron microscopy. [1] Mankin et al. 2015, Nano Lett 15, 4776-4782 [2] Wagner 1968, J Cryst Growth 3-4, 159-161 [3] Tateno et al. 2005, Jap J Appl Phys 44, L1553-L1555 [4] Hudak et al. 2014, ACS Nano 8, 5441-5448

Resume : With intriguing optical properties, Si nanocrystals (NCs) are promising for photon converting applications. Currently, while the photoluminescence (PL) tunability of these NCs is well-established and compatible with these applications, the emission efficiency is still insufficient. In this study, we investigate the excitation dependence of the PL efficiency of a large variety of Si NCs with different mean sizes and concentrations. The investigated materials, in the form of solid-state dispersions of Si NCs within an SiO2 matrix, have been prepared by co-sputtering using different amounts of excess Si and annealing procedures. Based on comprehensive optical characterization we conclude that both the absolute value of the PL efficiency as well as its excitation dependence are strongly influenced by fabrication conditions. Therefore, one NC size for which the highest PL efficiency is reached does not exist (as claimed in previous results of colloidal Si NCs) since it also depends on the particular excitation energy. This study provides a fundamental insight in the PL efficiency of SiO2 embedded Si NCs and is directly relevant for the future implementation of Si NCs in photon converters, showing that size optimization should be performed with a particular pump wavelength in mind.

Resume : The production of diamond nanoparticles containing colour centres has attracted increasingly interest in the last years for quantum computation applications. Particularly, the SiV colour centre has highlighted as a promising photon source among other colour centres in diamond, due to its interesting optical properties at room temperature [1]. SiV colour centres acting as single photon emitters can be created through different approaches in in-situ CVD growth by introducing silicon as a solid source or in a gas phase. Nevertheless, many optical applications demand nanoparticles with highly controlled sizes of around 100 nm. For this reason, nanoparticles’ production methods and accurate particles’ size characterization ones are needed. Diamond nanoparticles with custom colour centres were produced following a milling strategy. First a diamond film containing SiV centres was grown onto a passive substrate, followed by sacrificial etching of the substrate. The resulting diamond film was milled using a planetary mill [2]. To measure particle sizes, different slurries containing the diamond nanoparticles obtained were prepared and characterized by using Dynamic Light Scattering (DLS). Although this is a powerful method to characterize particle size distributions, it has a fundamental drawback: particle size is determined from intensity fluctuations in the Rayleigh scattering off a volume of the particles. As the intensity of Rayleigh scattering is proportional to d6, large particles, dust or aggregates can mask the measurement over smaller particles producing a non-accurate result. To overcome this problem, Nanoparticle Tracking Analysis (NTA) was used, in which individual particles can be tracked, giving a more precise measure. Both methods will be compared. References: [1] E, Neu et al, Optics Express 20, No. 18, (2012). [2] S. Heyer et al, ACS Nano 8, 5757-5764 (2014).

Resume : Silicon nanocrystals (Si NCs) dispersible in water and exhibiting luminescence in the near-infrared range are promising materials for fluorescent markers in bio-medical fields. Recently, we have developed a new type of all-inorganic Si NCs with high boron (B) and phosphorus (P) concentration shells [1,2]. The high B and P concentration shells induce negative potential on the surface and make Si NCs dispersible in water due to the electrostatic repulsions. The all-inorganic Si NCs exhibit a bright and very stable luminescence in water in the near infrared range. In this work, in order to conjugate the all-inorganic Si NCs with bio-materials, we functionalize the surface with amino or carboxyl group-terminated organic molecules. We use the hydrosilylation process to functionalize the surface with acrylic acid and allylamine. The surface functionalization is confirmed by infrared absorption spectroscopy. The functionalized Si NCs are dispersible in water and very clear yellowish solutions are obtained. The effect of the functionalization on the photoluminescence properties is studied in detail. We demonstrate that the luminescence properties of B and P co-doped Si NCs are very insensitive to the chemical reactions on the surface, and bright near infrared luminescence is obtained in functionalized Si NCs. [1] H. Sugimoto et al., Nanoscale 6, 122 (2014). [2] H. Sugimoto et al., J. Phys. Chem. C 117, 11850 (2013).

Resume : SixGeyO1-x-y has been proved as an initial material for growth of Si and Ge quantum dots. Due to quantum confinement, these dots emit light and the emission is wavelength-tunable by controlling the mean size. Si29.9Ge21O49.1 thin films are deposited onto Si substrate by sputtering and are then thermal annealed at 500°C for 3 hours in order to obtain Si and Ge nano-crystals. The nano-crystal size is in the range of 2 to 10 nm. The annealed film is used as active layer for fabrication of MOSLED. ITO and Al layers are used as top and bottom electrodes, respectively. The emission of this LED is observed by naked eyes when a forward bias of +8 V is applied. The EL band is centered at ~550 nm and extends from 350 to 780 nm. However, the emission of this LED is still weak and needs to be enhanced. In this work, we will study the EL enhancement by top patterned metal electrode. Round, linear, and finger-like electrodes of Al or Au are deposited onto ITO layer. Thickness of metal electrode is controlled to ~100 nm. The EL emission of LED with Al round, liner and finger-like electrode is about 154%, 519% and 692% of that without patterned electrode. The finger-like electrode shows the strongest improvement of EL intensity. In comparison to Al finger-like electrode, Au may increase the emission of ~18% due to the higher conductivity. Our results confirm the patterned metal electrode could improve the EL of SixGeyO1-x-y MOSLED. More details on MOSLED characteristics will be discussed.

Resume : Semiconductor nanocrystals (NCs) are famous for their tunable intrinsic emission owing to the size-dependent bandgap. In oxide capped silicon NCs (SiNCs), however, this emission color tunability is hindered by formation of oxide-related states inside the bandgap. There are two widely observed oxide-related bands in oxide capped SiNCs: (i) A broad red band around 620-650 nm, usually ascribed to Si-O-Si bridging and/or Si=O bonds and (ii) a broad blue band around 450-480 nm, ascribed to a not-well defined oxide center specific for very small SiNCs. In this work we introduce additional color centers into the oxide capping of three different types of SiNCs samples by exposure to a high energy electron beam (> 2 keV) and experimentally investigate possible correlations between these emission bands and the well-documented silica color centers emitting in the red (~650 nm), green (~550 nm) and blue (~460 nm) spectral regions. Measurements show that all oxide-capped SiNCs, independently of their material properties and original emission spectrum, display white-appearing emission from these three red-green-blue color centers, with slight variations between the different samples. For reference we also expose oxide-free alkyl-capped SiNCs to the e-beam, where we do not observe any spectral changes.

Resume : The independent behavior of photons and phonons at the nanoscale has reached a good level of understanding, whereas their mutual interaction is still unexplored. Here, we investigate the simultaneous localization of photons and phonons in silicon nanocrystals (Si NCs), in order to boost photoluminescence (PL) emission for photovoltaic applications. PL saturation is well known to set an upper limit to the emissivity of ensembles of Si NCs. Here, we present dedicated investigations of this effect under continuous wave excitation and reveal, in contrast to the generally accepted picture, a persistent increase of PL intensity above the saturation point; full PL saturation is, however, observed under pulsed excitation. By cross-correlating these PL measurements with results obtained by transient induced absorption, we explain and theoretically model the observed phenomenon as being due to the massive phonon production and their localization in the NCs, which results in an enhancement of the radiative emission rate via phonon-assisted optical transitions. These results offer better insight into the mechanisms of energy conversion and dissipation in ensembles of Si NCs in solid matrices, and open novel avenues for efficiency increase in future photovoltaics, but also to dedicated phonon management on the nanoscale and an enhancement of the optical faculty of Si.

Resume : Recent fabrication of monoatomic layer materials such as graphene and silicene has received much attention due to their unique linear band-structure, novel electronic properties and wealth of possible applications. Extremely high electron intrinsic mobility has been observed in suspended graphene even at room temperature. We present a theoretical study of electron transport and diffusion in 2D materials with linear energy bands, more specifically suspended monolayer graphene and silicene. We used a Monte Carlo simulation accounting for scattering on phonons, Coulomb electron-electron scattering and Pauli exclusion principle. Velocity-Field characteristics (VFC) and low field mobility are calculated as a function of temperature T. The 1/T^3 mobility dependence is observed and explained by the peculiarity of the linear band-structure. This property has also a profound influence on VFC, which are sub-linear and can even exhibit negative differential mobility.. Electron diffusion coefficients are obtained from the spreading of a packet of 'excess' particles, governed by linearized Boltzmann equation. The validity of this approach in presence of carrier-carrier scattering is discussed. By varying electron density we highlight the influence of Pauli exclusion and electron-electron scattering. This last phenomenon acts in reducing significantly mobility and diffusivity, especially in graphene for which phonon emission is infrequent due to high LO phonon energy.

Resume : At present there is an increased interest in semiconductor materials containing nanoscale structural elements, which can significantly alters the traditional properties of conventional materials. In this work was investigated distribution of temperature profiles in periodic structures on the surface of silicon. On the surface of a single crystal c-Si periodic structure formed in a height of 1 micron filaments with a diameter of 0.5 microns and the distance between the filaments 1 micron. The distribution of the temperature field in the structure during its thermal annealing, described transient heat equation, is solved the numerical by finite element method. The temperature profile in this structure showed that burning a structure at 1000 0C, filaments throughout the volume heated to a temperature of 10 ns. Dynamics of distribution of temperature profiles may give an explanation of the processes. that occur during annealing such structures.

Resume : Light emission properties of carbon incorporated silica nanopowders are analysed and discussed. Nanopwders have been synthesised by modification of surface of fumed silica (specific area 295 m2/g) with ethanol, buthanol, tetramethoxysilane and phenyltrimethoxysilane followed by thermal annealing in inert atmosphere (vacuum, pure nitrogen). It was demonstrated that incorporation of pyrolitical carbon results in the development of broad band photoluminescence that covers spectral range from near-UV to near-IR. PL of SiO2:C nanopowders exhibited some general properties: emission spectra are composed by at list three bands, and relative contribution of the bands is strongly dependent on annealing temperature i.e. increase of annealing temperature results in quenching of UV band and enhancement of visible emission. Emission, excitation (including X-ray excitation) and kinetic properties of PL have been thoroughly studied along with structural characterization by FTIR, Raman scattering and electron paramagnetic resonance spectroscopy. It is demonstrated that proper tuning of the composition and annealing temperature of nanopowders allows reaching color rendering index of white light emission as high as 97 under excitation by near-UV radiation. Origin of broad band tunable light emission is discussed in term of native surface defects of nanosilica and carbon based surface precipitates/agglomerates/clusters.

Resume : Similarly to silicon nanocrystals (NCs) germanium (Ge) ones are nontoxic and can exhibit efficient luminescence and other physical properties for applications in both optoelectronics and life sciences. For instance, having stable isotopes with half-integer spins, Ge NCs can be used as labels for nuclear magnetic resonance imaging. Therefore, a search of the effective and inexpensive method to prepare Ge NCs is an actual task. We use a method of spark ablation of crystalline germanium (c-Ge) in air by using a Tesla coil to form powder of Ge NCs with sizes of 2-20 nm. The powder was formed during a discharge between two c-Ge electrodes under high voltage applied from a Tesla coil. The latter generated sets of the damped high voltage pulses with durations of about 100 ns, which followed with a repetition rate of 100 Hz. The discharge power was estimated to be about 10 kW for a single pulse. The prepared powder was investigated by means of the transmission electron microscopy (TEM), Raman measurements and photoluminescence spectroscopy. The room temperature PL measurements revealed a broad band centered at 1000 nm with a lifetime of about 1 microsecond, which can be associated with the radiative recombination of excitons confined in oxidized Ge NCs. The obtained results demonstrate a simple and efficient method to prepare photoluminescent Ge NCs for biophotonic applications.

Resume : Luminescing nanoparticles are promising for intra-cell biological research as luminescent markers or for the toxicity studies. Water-based colloidal solutions of single nanoparticles or tens-to-hundreds nm-sized clusters suit the purpose provided they have strong luminescence, high stability and narrow size distribution. In this work we compare structural and optical prop-erties of silicon nanocrystals in colloidal solutions, prepared by two fundamentally different methods: Porous silicon prepared by electrochemical etching of a silicon wafer (a “top-down” method), and microplasma-based synthesis (“bottom-up” technique). The size of investigated clusters prepared by electrochemical etching, as determined by dynamic light scattering meth-od, was from 80 nm to 500 nm, depending on the preparation conditions. Their highly porous structure of interconnected small individual nanocrystals (~2.5 nm in size) leads to strong visi-ble luminescence in the red spectral region (~700 nm). However, the zeta-potential about -20 mV reveals the nanocrystals´ tendency to agglomerate. On the contrary, by using plasma-based synthesis we achieved smaller nanocrystals/clusters (diameter 10-50 nm) with narrower size distribution and better stability; however, their luminescence appears in the blue region (~450 nm) and is substantially weaker, being thus less convenient for intra-cell working. Our research has two main goals: 1) compare structural and luminescence properties of silicon nanocrystals prepared by electrochemical etching and plasma synthesis and 2) prepare luminescent clusters with different sizes for intra-cell working.

Resume : The drop in photoluminescence (PL) intensity with increasing temperature of oxide-embedded silicon nanocrystals (Si-NCs) is usually explained in terms of a thermally-activated non-radiative process. The origin of this process has been difficult to establish so far, while both single-dot and ensemble measurements indicate its µs time-scale. Here we present results of temperature-dependent single-dot lifetime and blinking measurements. We show that very fast blinking, or ?super-blinking?, could explain the variations in PL intensity of single Si-NCs as a function of temperature. We propose a simple blinking model based on temperature-dependent resonant tunneling of an electron in the Si core to a trap state in the oxide with blinking characteristic times approaching the exciton radiative lifetime. In agreement with the experiment, Monte Carlo simulations reveal a shortening of the PL lifetime and a reduction of the blinking duty cycle at elevated temperatures for a single nanocrystal under the ?superblinking? conditions. These results indicate that oxide-embedded silicon nanocrystals are prone to low quantum efficiency at room temperature due to the presence of defect sites in the oxide matrix.

Resume : Since silicon (Si) and carbon (C) are two of the most abundant and environmentally friendly elements in nature that play a fundamental role in a wide range and very important technologies (e.g. electronics, photovoltaics etc.). At the same time, a wealth of new opportunities has become evident in recent years due to unique physical and chemical characteristics exhibited by nanoscale silicon and carbon materials. Very attractive is also the possibility of electronically and optically coupling Si- and C-nanostructures to form advanced nanoscale devices and/or components. Si- and C-based nanostructures can offer the possibility of exploring new nano-device architectures that may open the way to alternative and improved design approaches. One of the most attractive opportunities is for instance offered by Silicon nanocrystal (Si NC)/Carbon nanotube (CNT) junctions that have so far received very limited attention. With silicon’s well established history within the micro-electronics industry, C/Si nano-interfaces are of great interest for a range of energy and environmental applications. A Si NC/CNT system, in principle, would represent an almost ideal fundamental optoelectronic nano-component (i.e. a nanoscale junction between two nanostructures with unique quantum confinement effects) where the CNT can serve as the acceptor to promote exciton dissociation and as an efficient charge transport channel for flow of carriers to the collecting electrode. In addition to optoelectronics, this nano-component would become useful for a range of other applications. For instance the very large volumetric and gravimetric capacities of Si at room temperature represent attractive properties to be combined with high-conductivity CNTs for energy storage. The Si/C nanocomposite could contribute to faster charge/discharge cycles where the mechanical stability of silicon may be enhanced by the rigid CNT structure attached to the nanocrystals. Nonetheless, CNTs growth over non-metallic Si NC catalysts remains challenging and very little is understood about the mechanisms at play. In this work we demonstrate CNT growth directly from quantum confined Si NCs using a microwave plasma enhanced chemical vapor deposition (MW-PECVD) process. We highlight that nanocrystal morphology coupled with a sufficient oxide shell thickness of ~1 nm is imperative for CNT growth. We demonstrate that Si NCs oxidation can be reliably controlled via solvent and implementation of a common fragmentation process. The small size and level of oxidation clearly assists in the unstable physisorption of carbon atoms or for the non-metal catalytic decomposition of the hydrocarbons. The Si NCs quantum confined properties namely room temperature photoluminescence are diminished after the growth of the CNTs. It is however important to note that, although reduced, synthesized Si NCs/CNTs structures still exhibit PL emission. The PL quenching that was experienced for Si NCs post CNT synthesis can be attributed to a number of concurrent factors. Firstly, the strong absorption properties of the highly dense CNTs can limit the emission from our Si NCs. Secondly, following the plasma process, there is the possibility of increased defect density and therefore increased number of non-radiative paths. In part these may be due to the diffusion and/or introduction of defects states and interface stress between Si NCs and the oxide layer. Nonetheless, the prospect of carrier transfers and recombination at the CNTs is possible and particularly intriguing. Importantly, although a direct synthesis from hydrogenated Si NC surfaces does not seem possible, the required oxide interface is sufficiently thin to allow for carrier tunneling and therefore envision the formation of useful opto-electronic nano-junctions. This work is of substantial importance in understanding the growth process of CNT from non-metallic Si NC catalysts and influential to the future development of Si NCs/carbon nanostructured interfaces.

Resume : There is an increasing need for sensitive, low-cost and miniaturized methods of biomolecules detection. The rapid technological advances in fields such as biochemistry and semiconductors in the 1980s made possible a large-scale development of biochips in the 1990s. For the purpose of developing ultrasensitive electrochemical biosensors, nanomaterials have been a key starting point due to their high surface-to-volume ratio, favorable electronic properties as well as electrocatalytic activity. In this sense, nanowires have been extensively used as substrates in electrochemical biosensors. On the other hand, indium tin oxide (ITO) has been extensively used in optoelectronics for its remarkable properties. In this work, we take advantage of nanostructured surfaces together with the benefits of ITO for biosensing applications. For this purpose, ITO was evaporated onto silicon electrodes as thin films and also as nanostructured films. Half the resulting ITO electrodes were submitted to annealing treatment. The resulting electrodes were characterized by cyclic voltammetry (CV). The principle of measurement was based on the reduction/oxidation properties of redox molecules, which can be used to label the target biomolecules. As-deposited and annealed nanostructured ITO electrodes were proved to have about 20% and 30% higher electroactive surface area than their thin film counterparts, respectively. Then, a surface modification chemistry to adapt the ITO electrodes as potential biosensors was performed. As a proof of concept, the oxidation of a ferrocene molecule attached to the modified ITO surface was detected by CV. The current levels for as-deposited and annealed electrodes were the same, although peaks for annealed electrodes were more outstanding than peaks for as-deposited electrodes. Results showed the current intensity through nanostructured electrodes to be four times the intensity through thin film electrodes. This work can be considered as a proof of the viability to use nanostructured ITO electrodes as biosensors.

Resume : Slow light in one-dimensional Quasi-periodic Cantor photonic structure formed by stacking alternative silicon and silica Si/SiO2 layers is studied in the Photonic Band Gap in the infrared range. These effects are well reproduced by theoretical model based on the transfer matrix method and are compared to effect observed on Bragg photonic crystals. The slowing down factor obtained by the Cantor structure is studied as function of the incidence angle and the wavelength reference of exciting light is greater than those reported for other photonic structures and is then promising for interesting applications in the telecommunication area.

Resume : The effect of Si-nanocrystal embedded in the silicon dioxide layer on admittance characteristics of the nc-MOS device can be theoretically described as behavior of oxide traps located inside the insulator. The traps are charged and discharged by processes of tunneling from/to the gate electrode and processes of thermally activated elastic tunneling from/to the semiconductor conduction band and valence band. The geometric location of the nanocrystals determines the efficiencies of the above mentioned processes. The applied theoretical model of the MIS tunnel diode utilizes the steady-state algorithm and a small signal equivalent circuit of traps located inside the insulator layer with their continuous distribution over energy. The proposal will cover the discussion of Si-nanocrystals geometric location effect on static tunnel currents and small-signal admittance characteristics of the considered MOS structure. Simulation results and their comparison with measured data can give useful information on parameters including geometric location of nanocrystals. Theoretical results will be compared with measured characteristics of nc-MOS test structures.

Resume : Metal-insulator-semiconductor structures containing nanocrystals (nC) embedded in the insulator have attracted scientific interest for their potential charge retention and light emitting properties. The two features constitute possible implementation fields: charging nodes in electron memory devices and integrated photonics. In this work the time-dependent capacitance (C-V-t) and current (I-V-t) characteristics of such structures are analysed with the use of a theoretical model. The considered nC-Si MOS device is represented by a double-barrier MOS structure i.e. a silicon substrate capped by two silicon dioxide layers with a silicon well region between them which represents silicon crystals. In the assumed model, the steady state is satisfied by a preservation of the continuity for the currents in the structure. When a steady-state is disturbed by a bias change, the electrostatics problem is solved with the assumption that charge accumulated in the well remains unchanged just after the bias jump. In general it means that the fluxes balance is not fulfilled and the unbalanced flux factors (electron / hole currents) charge or discharge the well region until a new steady-state is acquired. It enables consideration the bias sweep rate as a parameter of the simulation. The nC-Si physical parameters, in respect to the bulk ones, are open to doubt. Therefore comparision of simulated and measured C-V-t and I-V-t characteristics should give information on the physical structure, parameters and position of the nanocrystals in the insulator. Simulation based analysis can help in understanding the physics behind charging and discharging processes in considered nC-Si MOS structures.

Resume : Composite structures consisting of nano-Si inclusions in Si oxide matrix are discussed for applications in electronic devices such as Si based light emitters and solar cells. Such nanocomposites are often produced by phase separation of nonstoichiometric Si oxide (SiOx, x < 2) films during high temperature annealing. Controllable nano-Si formation requires understanding of both the thermodynamic and kinetic mechanisms associated with the SiOx separation into Si and Si oxide phases. This work is devoted to the study of the kinetic mechanisms of the phase separation of SiOx films and the formation of nano-Si inclusions in Si oxide matrix. Analysis of the obtained earlier expression for the Gibbs free energy of SiOx phase is carried out to derive its binodal and spinodal characteristics. The ranges of the values of x corresponding to the stability, metastability, and instability of SiOx with respect to the phase separation, as functions of annealing temperature are determined. The regions of the phase separation process taking place according to the spinodal decomposition as well as the nucleation and growth mechanism are presented. In contrast to the empirical approaches describing the nano-Si formation by Si nucleation, this study demonstrates the phase separation in SiOx films occurring predominantly by spinodal decomposition. Obtained results open a way for the proposal of the macroscopic kinetic theory of the formation of nano-Si/Si oxide composite structures.

Resume : Doped silicon nanowires (SiNW) and nanotubes (SiNT) embedded in a functional architecture can be used as liquid-gate field-effect-transistors (FET) for the ultra-sensitive detection of analytes in minute sample volumes. In order to support a label-free detection with such sensors, a suitable modification with receptor molecules addressing only the desired analytical target is necessary[1]. We chose a silane based system allowing to link receptor molecules such as proteins or DNA strands[2] to the surface. While silane surface modifications are well-known for SiNWs, a modification of the inner nanoscale cavity of SiNTs was not examined so far. In order to study the silanization reactions, the surface modification was monitored electrically exploiting the field effect. Hence, we developed a gas phase silanization setup that can be flexibly integrated into microfabrication processes. Boron-doped SiNW- and SiNT-FETs were assembled on a SiO2/Si substrate using conventional microfabrication. The integrated SiNWs and SiNTs were modified with 3-mercaptopropyltrimethoxysilane (MPTMS) while recording the electrical characteristics. Supported by chemical analysis, these measurements allow a comprehensive discussion of the surface modification reactions of SiNWs and SiNTs. [1] Patolsky, F. et al., MRS Bulletin, 2007, 32, 142-149. [2] Stern, E. et al., Nature, 2007, 445, 519-522.

Resume : Low temperature (≤ 600°C) dual gate field effect transistors based on polycrystalline silicon nanowires have been fabricated in a fully compatible silicon technology. Nanowires are elaborated using a classical fabrication method commonly used in microelectronic industry: the sidewall spacer formation technique. Assets of this technological process rest on the use of low cost lithographic tools, and the possibility to get by direct patterning numerous parallel nanowires with precise location on the substrate. In this method an amorphous silicon layer is deposited on a SiO2 step and crystallized by thermal annealing. An anisotropic reactive ion etching of the polycrystalline silicon layer allows sidewall spacer formation used as nanowires. The analysis of electrical properties of N-channel dual-gate transistors has been carried out on single top-gate or bottom-gate architectures. Bottom and top silicon nanowire/silicon dioxide interfaces qualities are characterized by determination of the surface state distributions using Suzuki’s method based on field effect. Results show that state distribution is higher at the top surface than at the bottom surface of the silicon nanowire. In addition, determination of surface state distribution by this method is experimentally validated for the first time.

Resume : Deoxyribonucleic acid (DNA) from a fish sperm is widely used for study of physicochemical interactions of DNA with different biochemical agents. It often requires to precisely control a structure of resulting compound at ultra-low concentrations. Surface-enhanced Raman scattering (SERS) spectroscopy is a high-sensitive method, which provides a detailed information on oscillation types of chemical bands in a number of complex molecules revealing peculiarities of their structure. Here we report results of study devoted to a study of DNA from herring sperm deposited on substrates of silvered porous silicon (Ag/PS) by SERS-spectroscopy. PS was fabricated by anodic electrochemical etching of highly-doped silicon wafer. Following silver immersion deposition on PS led to formation of a layer of Ag particles and rods with nanoscale dimensions assembled in a quasi-ordered array. Reflectance spectroscopy revealed that Ag film demonstrates expanded band of surface plasmon resonance between 400–650 nm, which is explained by its specific structural parameters. Ag/PS substrates were incubated in low-concentrated solutions of Na-DNA from herring sperm. SERS-spectra of DNA were recoded with 3D scanning confocal microscope Confotec NR500 using 473 and 633 nm lasers. It was found that detection limit of Na-DNA reaches nanomolar concentration for blue laser excitation while red laser resulted in micromolar concentration. Obtained results are discussed and explained in the range of present work.

Resume : An effective approach for the control of the optical properties of photonic nanostructures is presented. Extinction and photoluminescence spectra are studied for a thin multilayered film with silicon nanocrystals and periodic array of gold nanowires deposited on top. It is shown both experimentally and theoretically that metal grating can strongly influence the optical properties of silicon nanocrystals. Calculations performed by the scattering matrix method are found to be in good agreement with the experimental results. Both extinction and photoluminescence spectra of experimental samples are characterized by sharp peaks which are explained by the excitation of quasiguided modes in the layer with silicon nanocrystals. The appearance of these modes leads to enhancement of silicon nanocrystals photoluminescence intensity at the corresponding photon energies. It is demonstrated that the periodicity provides a powerful tool for achieving a high photoluminescence out-coupling efficiency of photonic nanostructures. The work is supported by the Russian Foundation for Basic Research (Grant No. 15-32-21153).

Resume : The mechanism of the formation of Si and, later, Ge nanocrystals (ncs) in SiO2 host has been deeply investigated. However, this issue was not well addressed for the Si- or Ge-ncs embedded in high-k oxides. The present work concentrates on the thermally stimulated formation of Si- or Ge-ncs in Al2O3 and HfO2 films grown by RF magnetron sputtering. Our films were analyzed by means of spectroscopic ellipsometry, FTIR, Raman scattering, XRD, TEM and photoluminescence methods. We found that for the same excess Si content, the Si-ncs form at lower temperatures in HfO2 (900-1000C), than in Al2O3 and SiO2 (1050-1150C). Moreover, whatever the oxide, the formation of Ge-ncs requires lower temperatures (600-700C) than that of Si-ncs. The mechanisms of ncs’ formation in different oxides will be discussed from the thermodynamic point of view. We also found that the decomposition process stimulates also the appearance of visible and near-infrared photoluminescence (PL) and the shape of PL spectrum depends significantly on Si(Ge) content and on the wavelength of the optical excitation. We further found that the exciton recombination inside the nanocrystals dominates for the longer wavelength excitation, whereas the UV-blue excitation results in the radiative recombination via interface or host defects in the materials. Following the above, the competition between different radiative channels will be also discussed and the technological ways to control it will be proposed.

Resume : Ge nanocrystals (NCs) embedded in high-k materials have attracted a wide interest for microelectronic1 and optical2 applications. To further optimize their properties, it is important to develop a deeper understanding how deposition and annealing conditions effects the optical and structural properties of these materials. In this work, Ge-ZrO2 and Ge-TaZrOx composite layers with 500 nm thickness were deposited by confocal rf magnetron sputtering. The Ge content was varied by using different rf power densities applied to the Ge target while keeping the rf power density applied to the ZrO2 and TaZrOx target constant. All samples were annealed for 30 s in a rapid thermal processing tool at temperatures ranging from 300 to 900 °C in nitrogen and studied by ellipsometry, Raman scattering, FTIR, XRD, Auger spectroscopy and TEM. As-deposited films are amorphous and chemically homogeneous. Annealing stimulates a phase separation and the formation of Ge-NCs. The Ge crystallization sets in at about 600-700 °C and depends on the Ge content and matrix material: the higher the Ge content, the lower is the Ge crystallization temperature. Under similar thermal treatment, Ge-NCs embedded in ZrO2 are larger than Ge-NCs embedded in TaZrOx. The ZrO2 matrix crystallizes simultaneously with the Ge, whereas the TaZrOx matrix stays amorphous up to 770°C. The effect of the high-k host material on the formation of the Ge-NCs and their properties are discussed in details. 1. S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E.F. Crabbé, and K. Chan, Appl. Phys. Lett. 68, 1377 (1996). 2. D. Lehninger, L. Khomenkova, C. Röder, G. Gärtner, B. Abendroth, J. Beyer, F. Schneider, D. C. Meyer and J. Heitmann, ECS Transactions, 66 (4) 203-212 (2015).

Resume : The correlated pairs fade away and spatially distributed structures form in the systems, in which annihilation processes have a place. Efficiency of these processes on a porous surface depends on its structural characteristics (pore size, degree of heterogeneity). Convenient way for gathering of information about modification of reagent distribution and cluster formation is a registration of delayed fluorescence, which is accompanied by triplet excitation annihilation at the different temperatures on ratio of reagent concentrations. Investigation of luminescence decay kinetics of aromatic hydrocarbon anthracene, which is measured by making use of the laser photolysis plant. The analysis of experimental results of kinetic processes on the SiO2 surface based on the multifractal formalism allows establish the correlations between fractal properties of system and efficiency of electron excitation energy transfer. The ratio of the reagents molecules concentrations and the temperature of SiO2 surface are control parameters of processes occurring in the system under study. In the region of low temperatures the system tends to maintain its multifractal parameters independently from the concentration of donors and acceptors. The increase of the matrix temperature leads to loss of the system stability of, to the change of the mechanism of energy transfer. Obtained experimental kinetic dependences are compared to computer modeling results of heteroannihilation processes in the systems, which have the multifractal distribution of reagents on a surface. Degree of order, which is a criterion of structure self-organization and allows to determine degree of structure symmetry breakdown, is used for the description of reagent distribution on a surface. The degree of matrix microscopic heterogeneity relates with topology of space, in which annihilation processes have a place that leads to formation and growth of interstitial molecule clusters and exerts influence on kinetics of processes. Degree of order dependence on the temperature has the same nature as dependence of quenching sphere radius on the matrix temperature. Thus on basis of investigation it is established the fact that one can estimate the heterogeneity degree of the matrix and the influence of reagent molecule distribution on kinetics of annihilation processes on a surface by making use of the computer modeling.

Resume : In recent years, the nanoscale multigate MOSFETs have attracted more attentions in nanoelectronics technology due to their high electrical performance for advanced integration circuits in nanoscale domain. However, the accurate modeling of these devices is still an important challenge due to the complex quantum behavior that describes the transport mechanism. In this context, several approaches have been proposed to investigate the nanoscale Double Gate (DG) MOSFET using numerical and analytical approaches. But from the circuit simulation point of view even numerical modeling is an overkill approach in terms of complexity and computational cost. In addition, it is difficult to obtain closed and compact analytical models for nanotransistors due to the approximations taken during the models development. Therefore, modeling tools which can be applied to design nanoscale devices require new modeling approaches taking into account the model accuracy and computation efficiency. The aim of this paper is to investigate the efficiency of a new approach, built upon Kriging metamodeling and non-dominated sorting genetic algorithm II, for the optimal design in terms of RF and analog performances including the hot carrier effects. Data generated according to computer experiments, based on Atlas 2-D simulator, are used to identify and fit Kriging surrogate models. It is emphasized that the obtained models can be used accurately in a multi-objective context to offer several Pareto optimal configurations. Therefore, a wide range of selection possibilities is available to the designer depending on situations under consideration.

Resume : We report on the bulk synthesis of ultra-thin Silicon Nanowires (SiNWs) by means of an Inductively Coupled Plasma (ICP) process. The nanostructural properties of the ICP synthesized SiNWs were systematically investigated by employing state-of-the-art transmission electron microscopy (TEM) techniques (i.e. energy filtered-TEM, scanning-TEM-energy dispersive X-ray spectroscopy and TEM tomography). We were thus able to reveal the existence of two competitive growth mechanisms for ICP-SiNWs, namely the predominant Oxide Assisted Growth (OAG) and Vapor-Liquid-Solid (VLS). In particular, the structural characterization revealed that OAG-SiNWs present an interesting core/shell structure formed both by a thin Si cylinder (diameter ~2nm) and a sort of necklace-like nanostructure, made of spherical or almond-shaped Si nanocrystals (diameter ~4nm) wrapped into a SiO2 outer nanolayer. We proved that such complex nanostructures are due to the Rayleigh instability phenomenon, which can also be controllably achieved via a post-synthesis thermal treatments. By investigating the photoluminescence properties of both as-grown and annealed SiNWs, we were able to point out a clear blue-shift of the PL peak (in the visible range), which confirms the occurrence of quantum confinement effects in these silicon nanostructures.

Resume : While silicon (Si) nanocrystals of sub-10 nm sizes are well investigated, a low-temperature and up-scalable production of Si nanocrystals with well-defined size remains a major challenge. Here, we present photoluminescence (PL) measurements on ensembles of nearly monodisperse large Si nanoparticles with dimensions in the 10-100 nm range, produced at 50 C. These nanoparticles show intense emission in the visible as well as the (near)-infrared range under continuous wave excitation. The PL increases superlinearly with excitation power, with slopes up to ~10 on double logarithmic representation. By studying particles of different sizes and shapes, we show that the PL intensity is directly linked to the porosity of the deposited layer. We conclude that the PL characteristics of the investigated material can be tuned by doping and the porosity of the grain layers, which controls the heat exchange between individual Si nanoparticles, and the overall heat conductivity of the whole layer, opening interesting application fields of this unique form of Si.

Resume : Rare earths doped Si-based materials have been applied for decades as suitable active hosts for many applications, such as lighting, photovoltaics and microphotonics. In this last field, Er has been widely used as dopant due to its optical emission at 1.54 um that falls in a window of minimum losses for silica optical fibers. Owing to its low excitation cross section and the easy formation of optically inactive Er metallic nanoparticles in Si-based hosts, the use of Er-Y mixed silicate thin films have been adopted in our work as a solution to dissolve high amounts of Er ions in Y substitutional positions. The contemporary introduction of Bi will be proposed to further enhance Er efficiency. The influence of Bi ions on the optical and structural properties will be investigated. In particular, by performing conventional Transmission Electron Microscopy (TEM) and analytical STEM-Energy Dispersive X-Ray and -Electron Energy Loss Spectroscopy, it has been evidenced that Bi ions precipitate in nanoparticles whose nature and oxidation state are strongly dependent on the annealing atmosphere. In addition, the Er optical properties at 1.54 um show an enhancement of Er lifetime attributed to a blocking action of Bi silicate nanoparticles, formed only under oxidizing atmosphere, that well isolate Er ions from OH quenching centers. These results demonstrate that Y-Er silicates containing Bi silicates nanoparticles are suitable materials to improve the Er optical efficiency at 1.54 um.

Resume : Silicon nanowires (SiNWs) are synthetized by LP-CVD (Low Pressure Chemical Vapor Deposition) via VLS process (Vapor-liquid-solid) using gold as catalyst on the mono-Si (100) and Corning glass subtrates. Au layers with different thicknesses (from 2 to 10 nm) are deposited by thermal evaporation. Before SiNWs growth an annealing process at 450°C for 30 minutes is performed to achieve the gold layer dewetting. The dewetted Au thin films are characterized by AFM (Atomic force microscopy) analysis. SEM (Scanning electron microscopy) is used to investigate the SiNWs morphology and the Raman spectroscopy for the structural characteristics. All the Raman spectra show a peak around 520 cm -1 which is relative to the optical mode vibration of Si-Si bonds in a crystalline environment. The intensity of this peak increases with the increasing of the Au thickness. Also the diameter of the nanowires increases from 20 to 120nm when the Au thickness increases from 2 to 10 nm. Key words: SiNWs, dewetting, Raman, SEM.

Resume : Straight silicon nanowires (SiNWs) have been obtained by metal assisted chemical etching (MACE) method from a monocrystalline <100> silicon substrate. The SiNWs were fabricated by etching the whole silicon wafer, obtaining lengths up to one hundred microns and diameters ranging from 50 to 500 nanometers. Once fabricated, SiNWs were dispersed in ethanol. The structure, chemical composition and surface morphology of the resulting SiNWs were characterized by different techniques including TEM, XPS, AFM and FE-SEM. In order to measure the electrical properties, the dispersed SiNWs were deposited over an oxidized silicon substrate pre-patterned with pairs of conductive AZO:Al electrodes fabricated by optical lithography. The spacing between electrodes ranged from 4 to 8 microns. The nanowires were individually aligned in the gap between each pair of electrodes by dielectrophoresis, contacting both electrodes and enabling electrical characterization. To improve the contact between the electrodes and each nanowire, aluminum was deposited atop the tips of the nanowire by lithography and lift-off. The I-V characteristics of p-type and n-type SiNWs were obtained by this method. Despite obtaining a high resistance, a good linear characteristic was observed. The authors propose a model for the conductance based on the tunneling current through the oxide, the contact resistances and the possible change of doping level.

Resume : The silicon nanoparticles (NPs) can be synthesised through a number of different processes. In this study, It is demonstrated that Si NPs can be produced using the arc discharge (gas breakdown) method and distributed over a Si wafer surface simultaneously. In this method, it is not required to collect the NPs and prepare a solution or ink to redistribute them on the substrate. The Si NPs produced by arc discharge in the form of "fume" was deposited on the substrate without going through the wet dispersion. The results show that the NPs are distributed on the surface evenly without signs of significant agglomeration and coagulation. The NPs were evaporated from a powder based sintered anode with tungsten cathode to make the arc. The produced NPs from the anode material could fly on the gas flow and land on the surface of the substrate right above the arc. It was observed that if the substrate surface is smooth, the deposition rate will be rather low since the gas flow will agitate the NPs on the surface and moves them away. The processing gas was H2/Ar mixture, using "safety" composition (H2/Ar; ~4%/96% - below the combustion limit). Three substrates were used for deposition: Textured silicon wafer, polished silicon wafer and thin aluminium sheet. The excessive NPs were collected on a Teflon filters at the exhaust line. The NPs on the mentioned substrates were characterized with the scanning electron microscope and the light absorbance was measured. In all cases, it was observed that the NPs are distributed quite evenly on the surface. This cannot be easily obtained using the wet deposition techniques. The NPs were made from p-type Si and the substrates are n-type. Through this process, the evenly coated wafers will make nanoscale p-n junctions. The average primary particle size in all cases was below 30 nm. It was also observed that the arc discharge method is capable of producing quantum dots (QD) at the range of 1 nm to 5 nm. Possible applications of the developed approach will be discussed.

Resume : Nanocomposite materials containing Germanium (Ge) and Carbon(C) phases embedded in a Silicon Oxycarbide matrix have a significant potential in photonic devices, particularly as thermal detectors. The preparation of such materials involves two steps – synthesis of polymeric precursor and their thermal conversion. A hybrid polymeric precursor has been prepared from sol-gel chemistry route through crosslinking of a mixture of triethylchlorogermane ((C2H5)3-Ge-Cl) with vinyltriethoxysilane (C2H3-Si-(OC2H5)3). Pyrolysis of the precursor in inert atmosphere yields a nanocomposite material with 82.5 wt% at 1400 °C. The thermal behaviour was monitored by thermogravimetry coupled with mass spectrometry; the main volatile species that evolve during thermal conversion include H2, H2O, CH4, HCl, and Oligosilanes. The structural modification as a function of pyrolysis temperature was monitored by Fourier transform infrared spectroscopy. Raman spectroscopy reveals the fingerprints of Ge at 295 cm-1 and turbostratic graphitic carbon at 1330 cm-1(D-band), 1560 cm-1(G-band). Furthermore, heat treatment above 1400 °C leads to formation of SiC crystals by carbothermal reduction. A minimum linear shrinkage during the thermal conversion suggests that our approach offers a flexibility to tailor the polymer into an appropriate geometrical form to obtain net-shape components. Finally, properties such as thermal conductivity, absorption, reflection in UV, visible and Infra-red range have been analyzed.

Resume : In this contribution we discuss the luminescence properties of the disk and ring microresonators realized on the basis of Ge(Si)/SOI structures. Ge(Si) nanoislands containing structures are considered as the building blocks of Si based nano- photonic and electronic devices. The resonators with the outer diameters of 3 - 40 um were fabricated on multilayer Ge(Si)-Si structures that were epitaxially grown on the SOI substrates. The reactive ion etching (RIE) in combination with the optical (UV) lithography were applied for the resonators fabrication. The luminescence properties of disk and ring microresonators were analyzed by means of the micro-PL method in the wavelength range of 1 – 2 um. The studies were carried out in the temperature range of 10 – 300 K with the spectral resolution of up to 0.01 nm. The resonant response has been observed in PL spectra of the disk and ring microresonators, intensity of which strongly depends on the resonator quality. The quality factors of such resonators vary between 100 and 1000, and depend on the resonator diameter. To simulate such specific behavior the finite element method (FEM) has been applied. The simulations of disk and ring microresonators were carried out with the different boundary conditions taking into account the radiation losses. The defects of resonators were simulated by holes on the outer disk boundaries. This work was supported by the Russian Foundation for Basic Research (grant #15-02-05272).

Resume : By co-doping silicon nanocrystals with phosphor and boron simultaneously, both donor and acceptor levels are formed inside the band gap, shifting the photon emission energy to values below the bulk silicon band gap. The absorption edge shifts approximately by the same energy as the emission peak, therefore preserving the Stokes shift. Here, we experimentally investigate the excitation dependence of emission spectra, photoluminescence lifetime and quantum yield of a solid state ensemble of co-doped silicon nanocrystals in a silicon dioxide matrix. Transient absorption data complement the set of measurements. Using the latter, in combination with quantum yield and spectral changes, we attempt an interpretation of carrier multiplication effects in co-doped nanocrystals. The results are compared to those obtained from undoped samples with similar NC size and similar active layer thickness. Finally, possible applications for co-doped nanocrystals are discussed in the light of our findings.

Resume : This presentation will cover the optical properties of colloidal silicon (Si) nanocrystals and nanorods. Si nanocrystals are made by the thermal decomposition of hydrogen silsequioxane (HSQ) and etching, followed by organic ligand surface passivation. A size-selective precipitation step is used to obtain relatively uniform nanocrystals. They are sufficiently uniform to assemble into superlattices. These nanocrystals are used as models to measure the size-dependent photoluminescence (PL) spectra, quantum yield, and optical absorption. The size-dependent optical properties are compared to an effective mass approximation. Colloidal Si nanorods are synthesized by a metal nanoparticle-seeded solution-liquid-solid (SLS) growth process. The nanorod diameter is less than 5 nm and quantum confinement effects are observed, including visible PL. The optical properties of the nanorods are compared to the nanocrystals of similar size. Carrier multiplication is also measured and compared between the nanocrystals and the nanorods.

Resume : Nanocrystalline (NC) silicon is a promising material for developing new generation of highly efficient and low-cost optoelectronic devices. One of the most powerful tools for probing NC dynamics is the time-resolved spectroscopy. Photoluminescence (PL) decay in silicon nanostructures is most often described by the stretch exponential (Kohlrausch) function (even if it is sometimes not adequate). This form of decay is believed to originate from a broad distribution of decay rates in inhomogeneous ensembles of nano-objects. The difficulty arises when one needs to represent such a non-trivial decay form by a single decay time (rate). Here we treat this problem mathematically by various approaches: statistical average via integration, averaging several exponential lifetimes etc. An optimum algorithm is worked out and applied to the treatment of PL kinetics under modulated excitation in both colloidal suspensions and solid layers of Si nanocrystals. The determined decay times form an input for calculation of absorption cross section [1] and internal quantum yield of PL in special samples of Si nanocrystals. [1] J. Valenta, M. Greben, Z. Remes et al. Appl. Phys. Lett. 108 (2016) 023102.

Resume : Silicon nanocrystals covered by hydrogen is the classical subject for various types of modulations, but usually the distance between atoms is considered equal to the distance between nearest neighbors in the bulk silicon. We present the investigation of the effect of the crystal lattice deformation in silicon and germanium nanocrystal covered by hydrogen and carbon atoms. The important role of the crystal lattice deformation for silicon nanocrystals covered by CH3 chains has been demonstrated in the [1]. We have modulated the atomic structure of the nanocrystals by the molecular dynamics method. The change of the distance between neighbor atoms is small in comparison with the corresponding values in bulk materials. However, it causes considerable shift of all levels of the energy spectrum and the splitting of the ground energy levels. The most important effect of the lattice deformation is the significant change of the distribution of the electron density as in the real, as in wavevector space. The probability of the direct optical transition from ground and excited states increases, correspondingly. The calculations of energy levels and wavefunction have been provided in the framework of the tight-binding approximation with the sp3d5s* atomic orbital basis [2]. 1. Prokop Hapala,* Kateˇrina K°usov´a, Ivan Pelant, and Pavel Jel´ınek, Physical Review B 87, 195420 (2013). 2. J.-M. Jancu, R. Scholz, F. Beltram, and F. Bassani, Physical Review B 57, 15 (1998).

Resume : In recent, graphene quantum dots (GQDs) have been paid attention mainly due to their unique opto-electronic properties. The GQDs that consists of nano meter-scaled graphene particles that have sp2-sp2 carbon bonds are expected to show characteristic properties such as only size-dependent general quantum dots (QDs) or chemically modified quantum dots having sp2-sp2 carbon bonds. Since GQDs shows a strong photoluminescence (PL), PL related researches become poplar. Additional excellent properties of GQDs having a high transparency and high surface area have proposed for energy and display applications. In addition, due to their nm size, the GQDs particles are dispersed well in various organic solvents to allow lots of organic reactions and solution process possible. GQDs can be modified with functional molecules to show different PL colors, indicating changes in band gap. Thus, GQDs can apply for various other applications. To utilize GQDs in various fields, a solution processible mass production of GQDs with simple purification is highly required. Until now, although many methods to prepare GQDs with high yield and good properties have introduced, there is no report on the practical preparation of GQDs with simple purifications. Herein, we like to introduce the solution-process and mass production synthesis of GQDs by using top-down method, and also a display application for an electro-chromic device and an energy-related applications for supercapacitor.

Resume : Diamond is the hardest known natural material. It has outstanding thermal conductivity (900–2.320 Wm-1K-1) and a bandgap of 5.5 eV. Those are only few of the properties that make it an excellent candidate for future electronics, at the dawn of Si era. Understanding electronic transport and charge-related effects in diamond are therefore crucial for designing and optimizing diamond-based devices. We deposit nanocrystalline diamond (NCD) thin films in sub-100 nm- and μm-thickness, and characterize them at nanoscale, after applying intentional charging of the material locally. By correlating Kelvin probe force microscopy, conductive atomic force microscopy, micro-Raman spectroscopy and scanning electron microscopy data, we show that sp2 phase dominates over diamond grains in local transport and electrostatic charging of NCD. However, several experiments under the same conditions revealed a variation of up to 12x in charged potential amplitude (0.1-1.2 V) due to tip-surface junction changes when scanning in atomic force microscope. Nevertheless, we show that it is possible to use highly charged NCD (>1V) for the self-assembly of nanoparticles. Charging monocrystalline diamond (MCD) solves the reproducibility issues of NCD, as it always exhibits several V of charged amplitude, allowing for the self-assembly to occur every time. This is attributed to the different charging mechanisms (surface only in MCD vs, in the bulk in NCD) which are clarified by detailed models for both systems.

Resume : Understanding the fluorescence of complex systems such as small nanocrystals with various surface terminations in solution is still a scientific challenge. We developed a method based on time resolved emission spectroscopy (TRES) to find and identify spectrally overlapping emission species by studying surface engineered silicon carbide (SiC) colloids. Fluorescent water-soluble SiC nanocrystals had been previously identified as complex molecular systems of silicon, carbon, oxygen, and hydrogen held together by covalent bonds that made the identification of their luminescence centers unambiguous. The aqueous solutions of molecular-sized SiC nanocrystals are exceedingly promising candidates to realize bioinert nonperturbative fluorescent nanoparticles for in vivo bioimaging, and thus the identification of their luminescent centers is of immediate interest. Two emission centers of this complex system were identified: emission from the SiC nanoclasters that is tailored by surface groups involving carbon−oxygen bonds (instead of making well-known surface states) and a defect related emission from silicon−oxygen bonds was recognized that was shade by the bright luminescence of the claster. Our suggested interpretation of TRES proved its potentiality by helping reconcile previous experimental results on the surface and pH-dependent emission of SiC nanocrystals and it could be a versatile tool for understanding the complex recombination mechanisms of other luminescent nanomaterials.

Resume : Silicon carbide (SiC) was once the pioneering material for yellow and blue light emitting diodes, but was eventually overtaken by the more efficient gallium nitride which has a direct bandgap. However, when structured with dimensions near the excitonic Bohr radius, SiC becomes a competitive light emitter as the optical transition probability is greatly increased via quantum confinement effects. In this work, we generate quantum confinement structures in 4H-SiC using a high voltage anodic electrochemical etch to produce mesoporous SiC (Por-SiC). Blue shifted, above bulk bandgap emission was demonstrated, a rare observation that we attribute to the columnar nature of our material, which has pores of 20-30 nm and interpore spacings of less than 3 nm. The Por-SiC can be powdered and suspended in a solvent and a drop of such a suspension was cast onto a dielectric substrate. The subsequently dried film exhibited a photoluminescence (PL) peak that was shifted from 386 nm to 370 nm and had an intensity that was increased by 2 orders of magnitude. The optically enhanced Por-SiC is a step towards fabricating SiC based quantum dots suitable for non-toxic and oxidation resistant biomarkers. Furthermore, the material is solution processible and has enhanced UV-blue absorption/emission indicating its potential as a thin film wavelength down converter for applications such as UV efficient photo-detection.

Resume : The implementation of Si-based lasers for on-chip optical interconnects holds several promises for next generation digital devices: Faster data transfer with increased circuit clock speeds, accompanied by reduced power consumption. Here we demonstrate lasing from epitaxially grown Ge quantum dots (QDs) in a defect-free Si matrix that show such extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In this talk we will present that - in contrast to conventional epitaxial SiGe nanostructures - these GIB-QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Further, we will elaborate on the microscopic origin of the enhanced PL-emission properties from GIB-QDs. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem, also present at room-temperature.

Resume : Ge quantum dots (QD) embedded in insulating matrix show the appealing features of a confined system with interesting properties in the light absorption processes. In particular, among quantum confinement effects, there are the increase of the optical bandgap and the enhanced oscillator strength. In this respect, since in Ge QDs the exciton Bohr radius is larger than in Si QDs (24 nm vs. 5 nm), the modulation of the light absorption coefficient can be easier and stronger, with large potential benefits in several applications. In this work, we present an experimental investigation of light absorption in small Ge QDs (2-3 nm in diameter) grown by PECVD in a multilayer configuration (3-6 nm thick film with Ge QDs, separated by 20 nm thick SiO2 barrier). An unprecedented high light absorption efficiency, 15 times larger than in the bulk, has been observed for these systems. Structural and optical characterizations have been employed to describe the QCE-induced enhancement of optical bandgap (from 0.8 in bulk up to 2.5 eV in multilayer structure) and of oscillator strength (one order of magnitude). Through a detailed electron energy loss spectroscopy (EELS) analysis we characterized the structural and chemical properties of Ge QD. A comparison with Ge QDs in single thick layer is also performed. These results add new insights into the role of QD packaging on confined systems, and open the route for reliable exploitation of QC effects.

Resume : GeSn binary alloys have emerged over the past years as means to overcome the longstanding limitations of group IV materials, in particular in the field of optoelectronics. Epitaxial GeSn layers grown on Ge-buffered Si substrates offer exciting prospects, as evidenced by the very recent demonstration of an optically pumped laser and by the tunable nature of the direct gap [1]. Nevertheless, the low quantum efficiency of such GeSn heterostructures is an unanswered issue with far-reaching consequences from both fundamental and technological perspectives. Here we addressed the impact of strain relaxation on light emission properties of tens-of-nm thick GeSn epitaxial layers with Sn content up to ~10%. An in-depth analysis of the temperature dependence of the integrated photoluminescence intensity is supported by a model of carrier dynamics, which describes the competitive interplay between the radiative band-edge transitions and the trapping of carriers by defects. By doing so, we clarified recent experimental observations and gathered a deeper understanding of radiative recombination processes. By studying the optical properties of GeSn epilayers as a function of strain relaxation we clarify strategies for yielding extremely efficient photonic devices, thus contributing to current worldwide efforts aimed at combining electrical and optical functionalities on a single computer chip. 1. Nature Photon. 9, 88 (2015); PRL 102, 107403 (2009)

Resume : The Materials Genome Initiative (MGI) was lunched to help businesses discover, develop, and deploy new materials twice as fast. Semiconductors playing a key role in a wide assortment of technologies and industries ranging from economy, human well being, and national security are essential part of the materials genome initiative. Here we present demos of semiconductor materials genome to accelerate the development of Si-based light emission and spintronics. Receipts for optics-friendly silicon. Si is the darling of the microelectronics industry, but it has an Achilles? heel: Si can?t absorb or emit light without the help of phonons. This so-called indirect band gap makes it an inefficient option for light-emitting diodes and solar cells. On the other hand, there is an urgent requirement for an optical emitter that is compatible with standard, silicon-based ultra-large-scale integration technology. The discovered Si/Ge magic sequence superlattices [1] or nanowires [2] or core/multishell nanowries [1] exhibit orders more efficient at absorbing (emission) light than their existing counterpart records and approach more than 10% brightness of real direct gap materials. In principle, such magic sequence superlattice could be prepared with molecular beam epitaxy, and the nanowire can be prepared by selective area metalorganic vapor phase epitaxy. Enhanced valley splitting towards a spin qubit in Si. Electronic spins in Si are raising contenders for qubits -- the logical unit of quantum computation-- owing to its outstanding spin coherence properties and compatibility to standard electronics. A remarkable limitation for spin quantum computing in Si hosts is the orbital degeneracy of this material's conduction band, preventing the spin-1/2 states from being an isolated two-level system. Very recently, we numerically inverse designed Si quantum well to isolating a single electron valley state in Si by a magic-sequence of Ge/Si barrier layers [3]. References: [1] M. d?Avezac , J.W. Luo, and Zunger, Phys. Rev. Lett. 108, 027401 (2012); L. Zhang, M. d?Avezac, J.W. Luo, A. Zunger, Nano Lett. 12, 984 (2012) [2] X.W. Jiang, J.W. Luo, and S.S. Li, manuscript in preparation. [3] L. Zhang, J.W. Luo, A.L. Saraiva, B. Koiller, A. Zunger, Nature Communications 4, 2396 (2013).

Resume : We have performed atomistic simulations of the phonon-limited high field carrier transport in <110> Si nanowires with small diameter. The carrier drift velocities are obtained from a direct solution of the non-linear Boltzmann transport equation. The relation between the drift velocity and the electric field considerably depends on the carrier, temperature, and diameter of the nanowires. In particular, the threshold between the linear and non-linear regimes exhibits important variations. The drift velocity can reach a maximum value then drop. These trends can be related to the effects of quantum confinement on the band structure of the nanowires. We also discuss the impact of the different phonon modes, and show that high-energy phonons can, unexpectedly, increase the drift velocity at high field. Please refer to Appl. Phys. Lett. 107, 063103 (2015) for detail.

Resume : Recent experimental studies have demonstrated that Ge nanowires can be formed as a polytypic mixture of cubic-diamond (CD) and hexagonal-wurtzite (HW) phases [1]. As a consequence, the comprehension of the main physical properties of these new crystal structures is acquiring an increasing importance in view of their enormous potential for technological applications [2-3]. In this work, we present results of hybrid Density Functional Theory (DFT) calculations to investigate the structural and electronic properties of different Ge phases and their interfaces [4]. First, we present a comparison of bulk HW with respect to CD revealing three major differences: i) a reduction of the band gap value, ii) a transition from an indirect to a direct semiconductor and iii) a red-shift of the optical spectra. Second, the study of the polytypes junctions, carried out by taking into account the effect of the length of HW segments in a pure CD system, shows that i) the presence of a structural discontinuity creates only minor atomic rearrangements and ii) the CD/HW interface induces a band offset which strongly modifies holes and electrons spatial localization. Our results suggest that fabricating Ge polytype junctions is an alternative route to engineer electronic properties that could add new functionalities to electronic and optoelectronic devices. Keywords: Ge heterostructure, band offsets, hybrid functional [1] L. Vincent et al., Nano Lett. 14, 4828-4836 (2014) [2] C. Rödl et al., Phys. Rev. B 92, 045207 (2015) [3] S. Wippermann, M. Vorös, D. Rocca, A. Gali, G. Zimanyi, and G. Galli, Phys. Rev. Lett. 110, 046804 (2013) [4] T. Kaewmaraya et al. (to be submitted; 2016)

Resume : New generation of transistors are fabricated using nanostructures such as fins or nanowires. The management of heat at these short scales is an issue. It is known from experiments and modelings that the thermal conductivity decreases in nanostructures. This could generate a dramatic self-heating of the device. We study heat conduction in silicon nanowires by means of molecular dynamics simulations. In particular, we want to achieve an understanding of the length dependence. We use approach-to-equilibrium molecular dynamics (AEMD). AEMD is a transient method where a temperature difference is first initialised before the approach-to-equilibrium is monitored. The temperature difference goes to zero exponentially with a decay time directly related to the thermal conductivity of the nanowire, as can be demonstrated from the solution of the heat equation in 1D. This method is faster that stationnary methods and allows to study larger systems. We have obtained the thermal conductivity of nanowires with a radius from 1 to 7 nm and a length up to 800 nm. We obtain a length dependence, even more pronounced at higher radius. The consequences on the phonon mean free path distribution in the nanowire will be discussed.

Resume : Phononic crystals have recently gained a lot of attention due to many potential technological applications from ultrasound imaging to wireless communications and thermoelectric materials for energy harvesting. Phononic crystals are heterostructures that use Bragg reflection to control the propagation of elastic waves. Hypersonic phononic crystals with structure sizes in the nanometer range can be used to tailor the dispersion of lattice vibrations in the GHz - THz range. This work focuses on the vibrational properties of nanoscale silicon phononic crystals. Molecular-dynamics simulations have been used to calculate the vibrational band structure of a multi-million atom model of a phononic crystal made from silicon nanoparticles and nanowires. The results are in qualitative agreement with the band structure of a simple two-dimensional finite-element model. The finite-element model is then used to gain further insight into the nature of the lower bands of the band structure. The different characters of the lowest non-acoustic bands are illustrated and it is shown how the vibrational modes in the nanoscale phononic crystal differs from the behaviour of bulk material as the wave-length approaches the border of the Brillouin zone.