Transport and photonics in group IV-based nanodevices

Resume : In recent years, research interest in group-IV semiconductors has shifted from SiGe and SiC to Si-Ge-Sn epitaxial alloys. Besides the potential applications in photonics and optoelectronics, Sn-containing heterostructures are also highly relevant for high-mobility and low-power electronics, thermoelectrics and high-efficiency multijunction solar cells. From a synthesis perspective, recent advances in epitaxy have enabled the growth of GeSn epilayers with very high Sn content, up to ~12%, by CVD using low-cost precursors, despite the large mismatch. However, device fabrication requires formation of ohmic contacts which involves thermal treatments of device’s structure. Due to this it is essential to research thermal stability of GeSn and GeSnSi epilayers. Here we report, for the first time, the comprehensive study of thermal stability of fully strained and GeSn epilayers grown on standard Si(001) substrates by an industrial type RP-CVD. Ranges of Sn concentrations from 2% to 12 % are now routinely achievable, corresponding to small and huge amount of compressive strain in the GeSn, and have been researched. HR-XRD, TEM, AFM and SIMS characterization techniques were utilized in order to acquire comprehensive understanding of strain relaxation mechanisms and Sn(Ge) diffusion in GeSn epilayers. The obtained results are very intriguing and are indispensable for all researchers involved in materials research and fabrication of devices based on strained GeSn epilayers.

Resume : Due to its direct bandgap, the alpha form of tin (Sn) is an interesting material for optical functionality in group-IV based devices. Here we report on the growth of Sn nanocrystals embedded in silicon and on luminescence studies of these, using time-resolved fluorescence spectroscopy. The samples were fabricated by molecular beam epitaxy, where first an epitaxial layer of Si(1-x-y)Sn(x)C(y) was grown, followed by nanocrystal formation due to heat treatment at various temperatures. We present the dependence of the observed luminescence on the temperature of the heat treatment, and we correlate our observations with structural characterization techniques, transmission electron microscopy and Rutherford backscattering spectrometry. Possible mechanisms behind the luminescence will be discussed.

Resume : Monolithic integration and CMOS compatibility of optical and electronic devices based on tin-containing group IV materials (GeSn or SiGeSn) has attracted a large research interest in recent years. On the optical side, the difficulty of conventional, indirect band gap, Si/Ge system to produce stimulated emission can be circumvented by introducing Sn, leading to direct gap material. Recent successes in good-quality material growth give rise to expectation that GeSn will be the basis of Si-compatible injection lasers. The gain achievable in GeSn is discussed based on calculations using 8-band k.p method, and the second-order perturbation method for indirect interband and various free carrier absorption processes (acoustic and optical phonons, intervalley, ionised impurity and alloy disorder scattering). The regions of the parameter space (alloy composition, doping, strain, carrier injection) are identified where good values of gain can be expected. The ternary SiGeSn alloys are unlikely to be useful as gain media, but may be very useful for cladding layers in waveguides and cavities, and their properties are also discussed. On the electronic side, the GeSn and SiGeSn materials can offer high mobilities of electrons and holes, and together with their well/barrier properties, this makes them good candidates for microelectronic components. Calculations of carrier mobilities, using the similar methodology, and including the same processes as for optical gain, are described.

Resume : Group IV-Nitrides having spinel structures i.e. γ-M3N4 (M= Si,Ge,Sn) exhibit electronic bandgaps which span the visible spectrum making them suitable for optoelectronic devices. [SixSn(1-x)]3N4 is the most challenging ternary compound, with a bandgap tunable over a broad area. The USPEX evolutionary structure prediction code interfaced with the VASP code are used in order to predict the structure of Sn3N4. The energetically preferable is found to be the spinel structure, while the second best is the hexagonal β-Si3N4-like structure. Following these results, an in depth analysis of the [SixSn(1-x)]3N4 ternary alloy is performed, resulting in the preferable atom configurations for both cubic and hexagonal [SixSn(1-x)]3N4 for the full range of x. The cubic structure is found to be preferable for small Si content, but before x=0.33 a switch to the hexagonal structure occurs. Finally, hybrid functional calculations are employed to accurately extract the bandstructures of all the examined configurations the corresponding bandgaps are calculated. A quadratic fit is applied and the bowing parameter of the bandgaps is extracted. Sn3N4 has been grown via halide chemical vapor deposition which exhibits metallic-like conductivity and photoluminescence close to the bandgap of the ground state spinel structure. The incorporation of Si in Sn3N4 leads to a systematic increase in resistivity while preliminary measurements of absorption-transmission show an increase in the energy gap with x.

Resume : Charge transport in nano-scale devices is characterized by specific transport mechanisms, related to the quantum confinement in such structures. At the same time, the temporal fluctuations in the current, better known as noise, in small devices becomes dominated by only a few ? sometimes one ? dominant fluctuators. In the case trapping is at the origin of the low-frequency (LF) noise, so-called Random Telegraph Signals (RTSs) appear in the time domain, whereby in the most simple form, the current switches between a high state (carrier capture) and a low state (carrier emission) [1]. Since the first report on RTS in small-area silicon MOSFETs [2], great progress has been made in the understanding of the physics of RTSs [1],[3]-[5]. Not only the trap parameters (activation energy for emission and capture, capture cross section) can be derived from the study of the emission (e) and capture (c) time constants with temperature but also information on the location of the trap can be derived by analyzing among others the current switching amplitude or step I. As will be shown in this paper, there are different ways to extract the RTS parameters [6]. The most direct one is based on time domain measurements, representing the current amplitude in a histogram. Combined with the corresponding frequency domain Lorentzian spectrum, I, e andc can be determined as a function of the operation biases or temperature. However, in the case of complex, multilevel RTSs, a more refined analysis can be based on so-called Time-Lag Plots, representing I(t t)) versus I(t), i.e., the current measured at moment t t versus the current at t. These different methods will be illustrated for various types of nano-devices, ranging from Ultra-thin Buried Oxide (UTBOX) Silicon-on-Insulator (SOI) MOSFETs over Resistive RAM memory devices, to vertical polysilicon transistors for non-volatile memory applications. References [1] M.J. Kirton and M.J. Uren, Adv. in Phys., 38, 367 (1989). [2] K.S. Ralls, W.J. Skocpol, L.D. Jackel, R.E. Howard, L.A. Fetter, R.W. Epworth and D.M. Tennant, Phys. Rev. Lett., 52, pp. 228-231 (1984). [3] P. Restle and A. Gnudi, IBM J. Res. Develop., 34, pp. 227-242 (1990). [4] H.H. Mueller and M. Schulz, J. Mater. Sci.: Mater in Electron., 6, pp. 65-74 (1995). [5] E. Simoen and C. Claeys, Mat. Sci. Eng. B, 91-92, pp. 136-143 (2002). [6] W. Fang, E. Simoen, M. Aoulaiche, J. Luo, C. Zhao and C. Claeys. Accepted for publication in phys. stat. sol. (c), E-MRS Spring Meeting 2014.

Resume : Two-dimensional (2D) materials have tremendous potential for use in optoelectronic devices due to their unique optical and electronic properties. For instance, graphene is a broadband absorber and its optical absorption can be tuned by shifting the Fermi energy. This property allows graphene to create optical modulators that have significant advantages over conventional Si and SiGe modulators including high speed, low-energy consumption and extreme broadband operation. Graphene optical modulators also offer the potential for greatly simplified integration with CMOS since these devices do not require electrical contact to the silicon waveguide. Despite the utility of graphene for optical modulators, graphene photodetectors suffer from relatively low responsivity and high dark current. Recently, a new 2D material, phosphorene (the 2D analog of black phosphorus) has been shown to have high mobility and a tunable band gap between 0.3 eV and 1.5 eV. I will show that phosphorene has tremendous potential to realize high-performance near-IR photodetectors with high-speed, high responsivity and orders of magnitude lower dark current than comparable graphene detectors. Phosphorene can also produce extremely high performance transistors with high transconductance and drive current. These results suggest that the combination of graphene and 2D semiconductors has the potential to create a complete set of a devices that could form the basis of a new technology platform for high-performance integrated optoelectronic circuits.

Resume : Recent developments in LEDs allowed them to be used in environmental lighting and have revealed many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Besides this general lighting application, LEDs are now used in other specific fields such as automotive headlamps, traffic signals, advertising, and camera flashes. However another emerging field of application is in advanced communications technology due to its high switching rates. Thus, the visible light spectrum is currently being used in the Visible Light Communication (VLC) technology, taking advantage of the lighting infrastructure based on white LEDs. These energy-saving white light sources devices were enabled by the invention of efficient blue LEDs. In this paper we propose the use of a multilayered pinpin device based on a-SiC:H to work as a photodetector operating in the pertinent range of operation for VLC (375 nm ? 780 nm) using as optical sources white and visible wavelength LEDs. The device consists of a p-i'(a-SiC:H)-n/p-i(a-Si:H)-n heterostructure with low conductivity doped layers, sandwiched between two transparent contacts. It works as an optical filter in the visible range with tunable spectral sensitivity dependent on both applied bias and type of steady state optical bias (wavelength, intensity and direction of incidence on the device). Optoelectronic characterization of the device is presented and includes with spectral response, transmittance and I-V characteristics, with and without background illumination. Results show that when the device is biased with front optical steady state light of short visible wavelength (400 nm) superimposed with the pulsed light emitted from the optical transmission sources, it exhibits an increased output current in the long part of the spectrum (550-650 nm), and a reduction of the same photocurrent for the short wavelengths (400-500 nm). An opposite behavior is observed when the wavelength of the background is changed to longer values. A comparison of the performance of white LEDs and visible wavelengths is presented. Results show that, front background enhances the light-to-dark sensitivity of the medium, long and infrared wavelength channels and quench strongly the low wavelength, depending optical amplification on the background intensity. The change of the impinging side of the steady state illumination produces the reverse effect, as the output photocurrent is enhanced under short wavelength signals and range and strongly reduced it under the long wavelength. A decoding algorithm for the detection of different optical signals is presented and discussed with a self-recovery error procedure. A capacitive optoelectronic model supports the experimental results and explains the device operation. A numerical simulation will be presented.

Resume : Research in silicon photonics has experienced rapid growth and a number of notable and important results have been achieved. Compact devices such as microrings and photonic crystals made in the silicon on insulator platform have been used to control optical signals in the near infrared where silicon is transparent. Applications range from biological and chemical sensing to electro-optics and optical interconnects. In this presentation, I will focus on the development of various photonic crystal devices in SOI. These include single or coupled microcavities and photonic crystal/microring hybrid devices. In very small microrings that can be used to achieve low power electro-optic modulation, the free spectral range (FSR) increases, which may be undesirable. By slowing down light trapped inside the ring, one can decrease the FSR. I will discuss a family of photonic crystal/microring hybrid devices, in which a periodic index modulation is created by drilling equally-spaced holes. Light then travels well below the normal speed of light in silicon. The design of such devices is different from that of conventional microrings and these differences will be reviewed. The strong backscattering inside the devices leads to several important effects. For example, different resonant modes can be launched into a drop waveguide depending on its position along the photonic crystal/microring hybrid device diameter. Theory and experimental verifications of these and other effects will be presented. One and two dimensional photonic crystal microcavities can be used to focus light on resonance in a very small modal volume. This leads to the development of very sensitive sensors and very low power modulators, as well as interesting fundamental physical effects. I will review recent work using these devices, with a focus on biosensors capable of detecting the capture of a small amount of biological targets such as viruses. In addition, I will discuss very recent research on the effects of coupling two or more photonic crystal microcavities. Simulations show interesting, sometimes unanticipated effects that appear promising for a variety of applications.

Resume : In this study, selective Si1-xGex growth (0.30≤x≤0.40) with boron concentration of 1×1020 cm-3 was used to elevate the source/drain on bulk Si-fins for 14nm FinFET technology node. The quality of SiGe layers, epitaxial profile and the strain amount of the SiGe layers were investigated.The deposition of SiGe on Si-fins is a very sensitive process and the strain may be relaxed if the initial Si-fin has poor quality due to any native oxide islands or undesired residual species after the etch step. In this case, the ex- and in-situ cleaning are important steps prior to the epitaxy process. In order to solve this problem, a series of prebaking experiments (740-825 °C) were performed for in-situ cleaning. The results showed that the thermal budget needs to be limited to 780-800 °C in order to avoid any damages to the shape of Si-fins but to remove the native oxide which is essential for high epitaxial quality of SiGe. The Ge content in epi-layers was measured directly by using high-resolution x-ray diffraction (HRXRD).The cross-section of the SiGe on the Si-fins were performed by high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) in order to study the layer thickness and epi-quality. In the latter analysis, energy dispersive spectroscopy (EDS) technique was also employed to verify the Ge content profile in these layers.

Resume : Energy requirements of the evolving and emerging mobile applications, such as Internet of Things, Big Data analytics and Augmented Reality, are discussed in view of the state of the art microelectronic devices and their scaling limitations. Mechanisms of power dissipation in microcircuits are reviewed and the recent progress in the field of low-power semiconductor technologies is summarized. This includes the principles for power reduction in VLSI with the focus on device and integration issues, such as advantages and limitations of FinFets, FDSOI transistors and FinFets on SOI, interconnects employing low-k dielectrics and 2.5D/3D ICs. Low-power NVM technologies, integration of low-power sensors and specific features of power consumption in analog and RF circuits are discussed using examples from TowerJazz recent research, including the remaining challenges and opportunities. One of the main conclusions of the talk is that ultimately scaled silicon technologies, though allowing increased performance, are hardly suitable for most of the emerging mobile applications due to cost restrictions and increased power consumption of scaled down devices.

Resume : The reduction of system size opens the possibility for coherent transport in modern nanotransistors. This trend favors high-speed signal transport necessary for optoelectronical applications. In this contribution we analyze coherent carrier transport in Si-based nano-MOSFETs. In our semi-empirical model we calculate the I-V traces of seven experimental transistors finding quantitative agreement. Adjusting our semi-empirical model to the experimental traces three system parameters are found: First, the height of the source-drain barrier, second the device temperature and, third, the overlap parameter. The overlap parameter describes the wave function overlap between the source/drain contact and the electron channel. It turns out to be crucial for the device operation: With the aid of the overlap parameter it is possible to classify the transistors in three groups, namely one with good contact-channel coupling and high drain currents, one group with intermediate values and one group with poor contact-channel coupling and small drain currents.

Resume : Silicon nanocrystals are envisioned for optoelectronic devices such as light emitting diodes and solar cells. Owing to various issues induced by the incorporation of nitrogen in Si nanocrystal / SiO2 multilayers such as matrix defects and hindered Si diffusion, we developed a nitrogen free PECVD process [1]. In this work, we present a detailed study of the electrical properties of nitrogen free Si nanocrystal/SiO2 superlattices. The results are compared to previous samples containing up to 10 at% of nitrogen [2]. It is found that the absence of nitrogen in general leads to a reduced defect density and hence to a higher electron mobility within the multilayers. Furthermore, we discuss the transport mechanism and demonstrate why Poole-Frenkel and Fowler-Nordheim tunneling are not suitable to describe the transport characteristics [2]. In contrast, we establish a model based on space-charge limited current in the presence of defects that explains correctly the observed features. Publications: [1] Laube et al. JAP, 116, 223501 [2] Gutsch et al. JAP, 113 133703

Resume : SiC MOSFETs are expected to have a large verity of applications in power electric systems with improved efficiency for energy saving. However, the device performance has mainly been limited by poor carrier transport at the oxide/SiC interface. In this communication, we report a study on the development of a novel technique involving both the surface treatment and the low temperature process for a dielectric layer on SiC. It is found that interface properties were strongly influenced by the way to chemically treat the SiC surface prior to the deposition of dielectric layers, as well as the post annealing at different temperatures and different ambient. It is believed that during the thermal oxidation of SiC, the incomplete oxidation of carbon resulted in C platelets allocated at the interface, which consequently degrade the electron mobility. We therefore engaged an enhanced oxidation pre-treatment to produce an Si-rich top, followed by PECVD deposition of a SiNy/SiOx double stack, and post-annealing in an oxygen-nitrogen mixed ambient. The flat-band voltage VFB of the C-V experiments from the processed SiC-MOS capacitors can then be controlled to < 1V, while the dielectric strength of the SiNy/SiOx double stack was measured to be > 5x106 V/cm, which is compatible to the thermal SiO2 (107 V/cm). The measured transconductance in the preliminary test device was also improved more than a factor of two, compared to the reference with no surface treatment.

Resume : Responsive photodetectors bring many benefits to various applications. In systems such as range finders and IR touch panels, more sensitive detectors enable longer detection distance and larger panel size, respectively. In addition, integration of photodetectors and corresponding circuitry on the same chip enables better system performance and smaller system form factor. Therefore, realizing responsive photodetectors in advanced Si-based process technologies is desired. The photocurrent responsivity seen in typical semiconductor photodiodes, either p-n junction diodes or p-i-n diodes, is around 0.1 A/W. Two-terminal base-floating phototransistors usually show responsivity between 1 A/W and 10 A/W, already meaning a lot of saving in detector area. In this work, it is demonstrated that the responsivity of a two-terminal bipolar phototransistor can be further enhanced by another order of magnitude, simply by reusing the parasitic substrate carriers. Device simulation and measurement showed that the carrier reuse mechanism also shifts the peak location of spectral response from 650 nm toward 800 nm, which benefits near-IR detection. For the two commonly used near-IR wavelengths, namely 850 nm and 940 nm, the phototransistor sample showed high responsivity values of 50 A/W and 20 A/W, respectively. The demonstration has been conducted in a standard 0.18 um SiGe BiCMOS process, revealing the feasibility of high-performance optoelectronic integrated circuits (OEIC).

Resume : In today’s world, optical communications are possible because of the superior optical electrical properties offered by CMOS technology both in digital as well as in analog applications. Optically controlled field effect transistors (OC-FETs) are promising devices to overcome the undesired high power dissipation and limited bandwidth effects. However, the use of uniformly doped gate presents the well-known problem of the low commutation speed and high subthreshold swing effect, which degrade the electrical performance of the device. Therefore, in order to obtain a global view of OC-FETs performance under optical excitation, new designs and optimization approaches are important for the comprehension of the fundamentals of such device characteristics. Based on numerical investigation of OC-FETs, in the present paper a new Gate-Engineering-based-approach to improve the submicron OC-FET electrical and optical performance is presented. The proposed design provides a good solution to improve the drain current and commutation characteristic for high performance optical communication applications. In this context, I-V, voltage gain and subthreshold characteristics of the proposed design are studied by 2-D numerical investigation and compared with conventional OC-FET characteristics.

Resume : The optical confinement is an important parameter for enhancing the efficiency of solar cells. In this context, transparent conductive oxide coating (TCO) with randomly textured surfaces is widely used to improve optical confinement, where many numerical and experimental studies have been carried out to investigate the impact of the surface texture morphology on the electrical performance of the solar cell. However, till now, any analytical investigation is proposed to optimize and improve the electrical efficiency of SiGe-based solar cells, taking into account the texture morphology effects. In this paper we present an analytical investigation including texture morphology effects, in order to optimize the texture morphology and TCO design parameters. In the present paper, we propose an analytical model allowing the electrical efficiency optimization for Glass/ZnO:Al/SiGe/Si heterojunction solar cell, by taking into account the surface texture morphology, Al concentration and Ge mole fraction effects. A multi-objective genetic computation has been used to optimize triangular grating. Solar cell with optimized triangular grating exhibits an enhancement over planar and randomly triangular grating cells. The purpose of this work is to formulate novel design criteria based on analytical and optimization investigation of surface texture morphology that would help in obtaining a high electrical performance.

Resume : ZnSe crystals are well known as perspective semiconductor for laser matrix, blue diodes, X-ray detectors, VIS-IR transparent window for lasers. The overstoichiometric zinc results to n-type conductivity and not so long ago it has been demonstrated for extra pure ZnSe the possibility to obtain p-type conductivity by Se doping. In the research we have studied Se nonstoichiometry in ZnSe under bivariant and monovariant equilibriums by direct physic-chemical method [1]. ZnSe single crystals grown from the melt and vapor were used as an initial material. It has been found out that at T>720 K overstoichiometric Se generates preliminary electrically neutral defects. The ionized defects of p-type can be observed inside the homogeneity range close to congruent sublimation conditions. ZnSe thin films (50-300 nm) were prepared by vacuum thermal sputtering on glass substrates with ITO layer. To control the nonstoichiometry of produced films a special design of two chamber heater was used. The nonstoichiometry and electrical properties of ZnSe films depending on preparation conditions were investigated. It was demonstrated that we could made p-i-n structures by varying the nonstoichiometry of produced ZnSe layers. 1. I. Avetissov, E. Mozhevitina, A. Khomyakov, Tran Khanh. Universal approach for nonstoichiometry determination in binary chemical compounds // Cryst. Res. Technol., 50, No 1, 93-100 (2015)

Resume : The goal to make Si an optoelectronic material has been pursued since long term. From the different approaches, the growth of heterostructures based on the SiGe/Si system is probably the one where more effort has been put. Concerning photovoltaic devices, the inclusion of Ge-quantum dots (QDs) in the photo-electrically active material will increase the light spectrum absorption especially in the infrared region which leads to an overall enhancement of the photocurrent and quantum efficiency of devices. Photoluminescence (PL) is a very useful optical characterization technique for the evaluation of the electronic levels structure. A sample in which 6.5 monolayers of Ge were deposited in a multilayer structure, with a Si spacer layer of 25 nm, and grown on top of a p-doped Si (001) substrate covered with a Si buffer layer with 150 nm, is investigated. TEM measurements reveal the presence of SiGe QDs in each period. Several sharp and broad bands are observed in the PL spectra, and the influence of excitation wavelength and power as well as temperature are discussed in order to evaluate the nature of the observed radiative transitions. The non-radiative de-excitation channels for the QDs related emission are discussed and activation energies for the less bound charge carrier are estimated. Also, the role of defects in these non-radiative de-excitation mechanisms is investigated.

Resume : In this contribution we will discuss the methods of fabrication and luminescent properties of disk microresonators being developed on the basis of light-emitting Si structures. The properties of disk/waveguide coupling schema on Si chip will be also considered. The disk microresonators with the diameters of 6-40 um were realized on Si structures with the active SiGe:Er and Ge(Si) nanoisland containing layers. The focused ion beam (FIB) and reactive ion etching (RIE) techniques in combination with the optical and electron-beam lithography were applied for the structures realization. The specificity of these methods and the etching conditions those ensure production of the high quality microresonators on Si basis (with the Q factors of up to 10^5) will be discussed. The luminescence properties of disk microresonators in the wavelength range of 1–2 um were analyzed by means of the micro-PL method with the spectral resolution of up to 0.05 cm^-1, the studies were carried out in the temperature range of 10–300K. The microresonators studied were simulated by means of the FDTD technique. This work was supported by the Russian Foundation for Basic Research.

Resume : Development of GeSn alloys has attracted considerable attention due to potential applications in fast photodetectors and modulators for optical interconnection. The using of GeSn buffer layers as uniaxial compressive stressors for Ge channels is a promising approach for realizing high mobility Ge-MOSFETs on a Si platform. This study presents the results of X-ray diffraction (XRD), micro-Raman and SIMS investigations of Ge1-xSnx films with thickness of about 500 nm and mole fraction x of about 0.04 and 0.07 grown by MBE on virtual Ge buffer layers. XRD measurements showed Ge buffer layers, which are fully relaxed at the growth temperature, to be under residual biaxial tensile strains in the growth plane due to difference in thermal expansion coefficients of Si and Ge. The formation of GeSn solid solutions in the investigated films was proven by both XRD and Raman investigations. In particular, Sn-Sn, Sn-Ge and Ge-Ge phonon modes were observed in the Raman spectra. The positions of the observed modes reflect the changes in concentration x and strains of the Ge1-xSnx films. The Ge1-xSnx films of x=0.04 were found to be fully strained, while of x=0.07 to be strain-relaxed. The asymmetrical reciprocal lattice patterns of the Ge1-xSnx films of x=0.07 showed the misorientation between the (001) planes in the GeSn layers and Ge possibly due to plastic relaxation. No evidence of phase segregation or surface roughening was found. The mechanism of the strain relaxation is discussed.

Resume : High quality Ge epitaxial layers were grown on Si (001) substrates by Molecular Beam Epitaxy (MBE) atTG = 100oC - 550oC.Real-time growth monitoring was done by Reflection High-Energy Electron Diffraction (RHEED), which for all samples showed that the germanium films were single-crystalline and two-dimensional. High resolution X-ray diffraction (HRXRD) scans further confirmed that all epitaxial Ge films were fully relaxed and single-crystalline. Full width half maxima (FWHM) measured from ω-scans around (004) peak varied from 0.2282o to 0.5360o. Raman spectra of all samples exhibit a single sharp peak around 301cm-1, confirming that the Ge layer grown at the lowest temperature(100oC) also exhibit good crystal quality, comparable to that of high temperature grown films. As TG plays a very important role in achieving layers with sharp contrast in doping concentration (e.g. p-i-n junctions), the present results are promising for realising various devices such as Ge-based NIR photodetectors and lasersand also for application in CMOS technologies. Further, the epitaxial layers can also be used as virtual substrates for growth of GeSn alloy and GaAs on Si. Metal Semiconductor Metal (MSM) photodetectors were fabricated on these Ge epitaxial layers and exhibited a very low dark current of the order of 1nA/μm2.

Resume : A brief review will be given on different epitaxial approaches of III-V materials on Si before a new approach will be presented, which may combine nearly full Si process compatibility with III-V functionality. Currently, active photonic devices on a silicon platform are obtained by bonding techniques, either on a single device chip or a full III-V wafer level. However, in this case the majority of III-V material is wasted by substrate removal. Direct epitaxy on silicon overcome these problems but suffers in most of the cases on lattice mismatch and differences in expansion coefficients. This can be partially accommodated by thick relaxation layers and using quantum dot active regions to reduce the sensitivity against threading dislocations or using complex material compositions, like Ga(N,As,P), which allow lattice matching. However, the III-V layers have to be processed well separated to silicon. A new approach based on III-V Si nanocomposite is under development, which may overcome most of those problems. This approach is based on the growth of III-V quantum dots (QDs) directly on silicon. Based on a GaAs/InAs core-shell geometry high quality optical emission could be obtained. By overgrowing InAs QDs with silicon a defect free single crystal planar silicon layer can be formed with embedded InAs nano clusters. TEM investigations show that the nanoclusters fully relax by closed loop interface dislocations but without initiating threading dislocations in the silicon matrix.

Resume : Semiconductor nanowires (NWs) are attracting the interest of a large scientific community as building blocks for a wide range of future nanoscaled devices. In this work we show that metal-assisted chemical etching of Si substrates is a powerful technique to obtain nanometer-size, high density and low-cost Si NWs with high and controllable aspect ratio. NWs obtained by this technique maintain the same structure and doping of the starting substrate and exhibit a very bright room temperature PL, which is tunable with NWs size, in agreement with the occurrence of quantum confinement effects. Light emitting devices based on Si NWs, showing an efficient room temperature EL emission at low voltage, have also been realized. We demonstrate that the design of new textures of NWs and the optimization of their size and spatial arrangement may play a key role for the improvement of the optical properties, such as light trapping and multiple scattering phenomena. We also realized a room temperature operating hybrid light source by coupling Si NWs and carbon nanotubes (CNT). This system exhibits an emission in the visible range from Si NWs and in the IR from CNT; a detailed study of the PL properties has been performed, and the conditions leading to the prevalence of the visible or of the IR signal have been identified. Finally, we report the structural and optical properties of Si NWs decorated with metallic clusters, a system which has great potentialities for biosensing applications.

Resume : Silicon nanocrystals (SiNCs) are expected to be used in optoelectronic devices as light sources. However those nanostructures have a large luminescence signal and a low quantum yield, which is particularly detrimental for optoelectronic devices. This problem can be overcome by inserting SiNCs into optical microcavities [1,2]. In this work, we first present a study where SiNCs are inserted into planar microcavities. This system is entirely made by a single evaporation run and a post annealing. The technique gives the possibility to both control the average SiNC diameter and to precisely locate them into the cavity. This last one is made with classical Si/SiO2 Bragg mirrors. Results concerning the coupling of SiNCs emission to microcavity modes will be given. A Purcell effect is clearly observed as room temperature yielding a reduction of the spontaneous emission width and decay time, an increase of emission intensity and a redirection of the emitted light. This last result is in good agreement with modelled emission spectra. Finally, few studies have been dedicated to the evaluation of the SiNC emission width using spherical and toroidal microcavities [3,4]. The same analysis realized here using planar microcavities gives equivalent results. [1] A. Belarouci et al., Appl. Phys. B 88 237 (2007). [2] M. Grün, et al., Optical Materials 33, 1248 (2011). [3] A. Meldrum et al., Phys. Rev. Lett. 104, 103901 (2011). [4] Pitanti et al., Phys. Rev. Lett. 104, 103901 (2010).

Resume : Germanium is a prominent candidate for the development of semiconductor spintronics. It is compatible with Silicon microelectronic processing and it has quasi-direct behavior. These latter properties result in direct-gap transitions wich leads to efficient angular momentum transfer from circularly polarized light to the carriers. Germanium has thus the potential of merging spintronics and photonics in the so-called spin-optoelectronics field. We investigate the state of the polarization of the direct-gap photoluminescence (PL) emission in bulk Ge and we provide also an overview of the processes governing the recombination of carriers in Γ valley. It will then be shown how the polarization of excited carriers can be controlled simply by tuning the excitation power density. In this study we achieve the control over the helicity of emitted photons across the direct-gap without the need of any external magnetic field or optical retarder. Our result provides a step forward the implementation of novel polarized light sources, such as Ge-based spin LEDs and spin lasers monolithically integrated on Silicon.

Resume : An optoelectronic integrated pi´n/pin active filter based on a-SiC:H technology is analyzed experimental and theoretically. Several monochromatic pulsed channels in the UV/VIS/NIR range, each one with a specific bit sequence are transmitted received and decoded. Experimental and simulated results show that the output multiplexed signals has a strong nonlinear dependence on the incident light wavelength, bit rate and intensity under unbalanced light generation. Tailoring the channel wavelength in the UV/NIR/VIS was achieved by using near-ultraviolet backgrounds and by changing the device irradiation side and lighting intensity. Results show that the pi´n/pin multilayered structure, acts as data selector in the UV/VIS/NIR ranges. A capacitive active two connected phototransistors model to support the experimental results is presented and gives physical meaning to the use of near-ultraviolet steady state illumination to increase the wavelength selecting ability of a double a-SiC/Si pi’n/pin integrated optical active filter, beyond the visible wavelengths (350 nm-900 nm). Here, the optoelectronic circuit representation is supported by the complete dynamical large signal Ebers-Moll model with series resistances and capacitors. The time periodic linearized state equation is deduced and the simplified block diagram of the state model presented. A graphics user interface computer program was designed within the MATLAB® environment, to perform the numerical simulation task. A good agreement between experimental and simulated results was achieved which shows the ability of the presented model to simulate the sensitivity behavior of the proposed system in the UV/VIS/NIR spectral ranges.

Resume : Avalanche electroluminescence from silicon pn junctions has been studied for about 60 years. As can be expected from an indirect band gap material, the internal quantum efficiency is quite low. However, we have used several techniques and structures to improve not only the internal quantum efficiency, but also the external light extraction efficiency and the external power efficiency.In this work the properties of hot carrier electroluminescence under avalanching conditions in SOI nanowire pn junctions are presented. Of special interest are the spectral emission properties, the operating voltages and the device efficiencies. Silicon pn junctions were manufactured within silicon nanowires using a custom SOI (silicon on insulator) technology. The nanowires have diameters less than 50 nm and lengths ranging from 200 nm to 600 nm. The pn junctions are operated in the reach-through mode of operation, thus increasing the average electric field within the fully depleted nanowire. Experimental results of the emission spectrum indicate that the most dominant photon generating mechanism is intraband hot carrier relaxation processes. Using output optical power measurements in the spectral range 400 nm to 1 600 nm, the SOI nanowire light sources can be compared to light being generated within a comparable bulk CMOS structure. It is reported that the SOI nanowire light source external power efficiency is at least an order of magnitude better than the comparable bulk CMOS light source.