Multifunctional binary and complex oxides films and nanostructures for nanoelectronics and energy applications - II

Resume : The origin of the so-called Bulk Photovoltaic Effect (BPE) in non-centrosymmetric crystals is fundamentally different from that of classical PE. Irrespective of particular microscopic mechanism, as the photo-excited “hot” carriers thermalize in the conduction band, they get a shift in space lo. Although the voltage generated in BPE can be much higher than the band gap of a crystal, the energy conversion efficiency (e.c.e.) is extremely low. However, the situation can radically change at the nanoscale, when the thickness of the crystal is comparable to lo, typically in a 10 nm range. To measure BPE in thin non-centrosymmetric films, we used Pt/BaTiO3/Pt heterostructures comprising 20 or 50 nm thick epitaxial ferroelectric BaTiO3 layer, grown on a single crystalline MgO(001) substrate. The set of light-emitting diodes with the emission band around ~360 nm (Eg ~ 3.4 eV) and the controlled light power density on the sample surface up to 0.5 W/cm2 was used for illumination. By analyzing the set of the obtained I-V curves, we get e.c.e. ~ 0.6%, which is 4.5 orders of magnitude larger as compared to the bulk crystals. This value is still much lower than the commercial solar cells. However, the calculations indicate that for alternative thin film materials with non-centrosymmetric crystal structure (e.g. LiNbO3:Fe) the BPE efficiency can be further enhanced, and this might provide a viable alternative for the effective use of BPE in photovoltaic applications.

Resume : Transition metal oxides can present a plethora of interesting coupled functionalities including the coupling between transport properties and light, traditionally exploited making use of semiconductor materials. A particular subclass of transition metal oxides are ferroelectric (FE) materials, which are insulating in nature and in most cases with large bandgaps that make them rather transparent to visible light. However, in thin films, due the presence of defects, interface and electrode effects, visible light can generate photocarriers. We will report on the photoresponse of BaTiO3 thin films, where remarkable photoinduced effects have been observed at appropriate wavelengths. More specifically, we have explored the dependence of the induced photocurrents on the direction of the polarization and the strength of the imprint field. We show that the measured short-circuit photocurrent resembles the FE retention properties of the studied material. We argue that this fact results from the dependence of both parameters on the depolarizing electric field, therefore it is stated that depolarizing electric field is the main driving force for the generated photocurrent. Interestingly, it is observed that the FE imprint fielddoes not importantly collaborate in the generation of photocurrent. The present study suggest that: photocurrent is univocally determined by the depolarizing field, which is promising in terms of electro-optic read-out of information on FE based memories.

Resume : Ferroelectrics are attractive to design agile devices thanks to the large electric field dependence of their dielectric permittivity. Device performance depends on the ferroelectric characteristics of the integrated thin films. During the last decade, intensive studies were focused on BaxSr1-xTiO3, which is a quite low loss perovskite compound, for which TC can be monitored by choosing an adequate composition. Alternative multifunctional materials were also investigated, such as KTa1-xNbxO3 which also offers to tune TC. This solid solution shows high tunability at a moderate electric field, but its use is still limited because of significant dielectric losses. For the different materials, various approaches are investigated in order to obtain low loss agile materials, such as the screening of composition, doping, the control of high quality epitaxial films, the association with dielectrics in composites. The challenge is to find the best trade-off in order to reduce the losses without drastically damaging the tunability. It was also shown that the deposition conditions strongly influence the final performance of the materials. In this way, it has been evidenced that strain effects can give access to the monitoring of ferroelectric properties in nanostructured composite films. Deposition conditions can also give access to new materials, with specific nanostructuration, obtained by a phase diagram approach. New piezoelectric epitaxial niobates films were obtained by this way.

Resume : The evaluation of the dielectric response in thick films of dielectric materials and in mixtures of dielectric and metals is not as straightforward as in bulk materials, due to the presence of porosity. To obtain the effective dielectric function of these films, porosity can be taken into account using the Lichtenecker model, but a more generalized effective medium model with percolation threshold and critical percolation exponents was proposed by McLachlan. The case of a mixture of a ferroelectric with a metal, is very interesting because the dielectric response can be greatly enhanced, and near the percolation threshold the permittivity diverges. This effect is discussed for a set of thick Pb(Zr,Ti)O3 films and composite films Pb(Zr,Ti)O3 −xPb2Ru2O6.5 [1]. Films were prepared by screen printing on sapphire substrates, and their homogeneity and phonon response were studied by Raman spectroscopy and Fourier-transform infrared (FTIR) reflectivity. The dielectric response below phonons was measured using time-domain terahertz transmission spectroscopy. The compositions, up to 25 vol% Pb2Ru2O6.5, are below and above the electrical percolation threshold, known to be about 17% in the bulk composites. Using the dielectric functions of bulk PZT ceramics and Pb2Ru2O6.5 single crystals, the effective dielectric functions can be compared with those obtained using the theoretical models. [1] D Nuzhnyy, E Buixaderas et al, J. Physics D: Appl. Phys 47 (2014) 495301 (2014).

Resume : Lead zirconate titanate (Pb(Zr1-xTix)O3, PZT) is well known for its excellent piezoelectric and ferroelectric properties and its subsequent applications. Tunable composition makes it versatile and morphotropic phase boundary (MPB, x=0.48) is especially attractive for the large piezoelectric constant, considerable remnant polarization and great dielectric constant. Despite the fact that lead in not environment friendly, the study based on PZT continues intensively along with other complex oxides due to the lack of equivalent substitutes. PZT(52/48) films with thickness from 33 nm to 200 nm have been deposited on SrTiO3 substrate, with an SrRuO3 interlayer as bottom electrode, by two methods: a) sol-gel, which is low cost and has short fabrication cycle; b) sputtering, producing high homogeneous, large scale, precise thickness controlled thin film. X-Ray Diffraction indicates single crystalline PZT is epitaxially grown on the substrate. Surface roughness is less than 1 nm (root mean square), characterized by Atomic Force Microscopy. At nanoscale, piezoresponse force microscopy shows the inverse piezoelectric effect. At macroscale, electrical properties of the PZT thin films were measured through current-voltage (I-V), capacitance-voltage (C-V) and PUND (Positive Up Negative Down). These characterizations allowed to discriminate between the differents conduction models and also between the ferroelectric domaisn switching behaviour.

Resume : Sustainable removal of organic pollutants from wastewater represents an alternative to the traditional methods and to the mono-component fotocatalytic materials which show sever limitations in terms of production and operation costs and durability. This paper presents the development and properties of high efficient photocatalytic multi-tandem structures based on metallic oxides (SnO2, TiO2) and sulfides (CuxS). The thin films are obtained by spray pyrolysis deposition, using inorganic precursors and alcoholic solvents. Post-deposition annealing is used to improve the crystallinity of the tandem components. The design of the multi-layered structures is done considering the band energy levels to obtain complex multi-tandems activated mostly by Vis radiation, sustainable and stable in the working environment. The compatibility of the crystallization systems (tetragonal for SnO2, TiO2 and body-centered tetragonal for CuxS), the morphology (dense granular for metal oxides and porous granular for CuxS) and the surface energy (dominated by polar component) support the adjustment of the energy levels in bi- and tri-component tandem structures. The photocatalytic tests were doe at two methylene blue (MB) and phenol concentrations (4ppm and 10ppm) and at different pH (3, 5, 7, 8, 9). The results shows significant differences in the photocatalytic efficiency of multi-tandem structures which are correlated with the influence pH influence on the energy levels values. The photocatalytic experiments under UV-Vis irradiation shows that MB degradation using CuxS/TiO2/SnO2 tandem structure reach 60% efficiency in 4h and 80% efficiency after 6h of irradiation.

Resume : One of the emerging fields in nano-electronics is the adaptive electronics based on resistive memories integrating functional oxides. In this context, the most promising memories and devices make use of the Mott Metal-Insulator Transition (MIT) in which electric charge injection into a transition metal compound induces a transition from a strongly correlated insulator into a weakly electron correlated metal (Mott or Mott-Hubbard transition). The transition is accompanied by large resistance changes of the material, on very short timescales. Among the possible materials exhibiting a MIT, vanadium dioxide (VO2) is of particular interest since it exhibits a huge resistivity change between the two stable phases (5 orders of magnitude). In this work, pure and W-doped VO2 epitaxial films have been grown on Al2O3 and TiO2 substrates of various orientations using both pulsed laser deposition and electron beam evaporation. The films have been characterized using high-resolution X-ray diffraction, ellipsometry and 4-probe electrical resistivity measurements. We show that there is clear dependence between the magnitude of the MIT and the deposited film thickness. This behavior can be explained by the fact that upon decreasing film thickness the level of strain in the films increases, which results in a lowering of the electrical gap in the insulating state by ~0.1 eV and hence a degraded resistivity and resistivity ratios reduced by 2 orders of magnitude for the thinnest films.

Resume : ZnO nanoparticles were successfully synthesized via simple precipitation method by controlling different parameters of the precipitation process such as solution concentration and calcination temperature. The structural and morphological properties of these samples were investigated by SEM and XRD analysis. SEM images showed uniformity in the particles size and shape for ZnO nanoparticles calcined at different temperatures compared to the other synthesized samples. The average crystallite size increases with increasing the calcination temperature. We used UV-VIS and FTIR spectrophotometers to get the reflectivity data of the selected samples. By using Kramers-Kronig (K-K) method, we then determined the optical constants, n and k with the real and imaginary parts of the dielectric constant in the IR region. The K-K method was then combined with a newly developed technique to compute the optical parameters in the UV-VIS region. A shift in the absorption peak in the extinction coefficients of the samples was observed due to defects by varying the calcination temperature. Green emission was observed from 36 nm grain size ZnO samples and yellow emission was observed from 470 nm grain size ZnO samples. Finally, to verify the validity of our approach, we back calculated the UV-VIS reflectivity spectra from the deduced optical parameters, and good agreement was found between the measured and calculated spectra. This confirms the importance of our approach in developing a new numerical technique for accurate measurement of the optical parameters at the UV-VIS wavelengths. The reflectivity based technique described in this work could be applied to study ZnO nanoparticles of smaller sizes reaching the quantum confinement region.

Resume : We investigated the electrical, optical, structural, and morphological properties of Ti-doped In2O3 (TIO) films grown by DC magnetron sputtering using 1-, 3-, and 5-wt% Ti-doped TIO targets as a function of film thickness. Compared with the TIO films prepared from 3 and 5 wt % Ti doped TIO targets, the 300 nm thick TIO films fabricated using a 1 wt % TIO target showed a lower resistivity of 2.4 × 10-4 Ohm-cm, an mobility of 49 cm2/Vs, and higher average transmittance of 80% in the near infrared wavelength (800-2000nm) region due to an effective Ti dopant composition. In addition, a different structural evolution of the TIO columnar with increasing thickness significantly influenced on the resistivity and optical transmittance of the TIO films. Based on Hall measurements, UV/visible spectrometer, X-rays diffraction and field emission scanning electron microscopy analysis results, we correlated the properties of TIO films with thickness and TIO target composition

Resume : The variation of optical and magnetoresistive properties of La-Sr-Mn-O films at different regimes of annealing procedure is investigated. The films with the thicknesses 40 -100 nm were deposited onto single crystal Gd3Ga5O12 (111) substrates by dc magnetron sputtering of ceramic La0.7Sr0.3MnO3 target in argon-oxygen atmosphere [1]. The optical conductivity investigated in energy spectrum range of 1-5 eV. Magnetoresistance was measured in the temperature range 80–310 K. We observed no significant dc-conductivity and no 1 eV polaron feature in the optical conductivity spectra for the unannealed films. The annealing carried out in air at temperature 870 K has small influence on the films properties. When the annealing temperature increased to 1170 K the refractive index and the optical conductivity spectra of the annealed films become similar to the ones of epitaxial La0.7Sr0.3MnO3 films (the 1 eV polaron feature appears). However these spectra are shifted by ~ 0.5 eV to higher energy. The annealed films with thicknesses of more than 60 nm have the temperature maximum of resistance at ~ 180 K and widened temperature range of negative magnetoresistance, which is typical for polycrystalline films [2]. The films with thicknesses of less than 60 nm have no maximum of resistance. 1. Yu.M. Nikolaenko, A.B. Mukhin, V.A. Chaika and V.V. Burkhovetskii, Tech. Phys., 55, 8, 1189 (2010); 2. Abdullah Goktas, Ferhat Aslan, Ibrahim Halil Mutlu, J Mater Sci: Mater Electron, 23, 605 (2012).

Resume : With the current trend of decreasing device sizes whilst improving performance, silica-based resistive switches offer the potential for simple fabrication, using existing infrastructures, of high-density, high-efficiency data storage media. Here we present recent work on memory functionality through reversible switching between discrete states of resistance in 37 nm silicon suboxide layers. However, in order for such devices to be optimised and integrated into consumer technologies it is necessary to fully understand the switching mechanism. Current research in oxide-based resistive RAM focuses on the migration of oxygen atoms through an insulating layer under electrical stress. While it has become increasingly clear that oxygen movement plays a major role in device behaviour, there has remained a lack of direct evidence correlating switching behaviour and oxygen motion. Along with pre- and post-switching characterisation of the active material using atomic force microscopy and x-ray photoelectron spectroscopy, we present recent work in which we have successfully observed, using primary and secondary ion mass spectroscopy, the ejection of oxygen-based species from our devices during operation. The novel methods used and subsequent data obtained highlight the dynamic behaviour of our silica memory.

Resume : Many methods such as magnetron sputtering, atomic layer deposition, spin coating, sol-gel and chemical vapor deposition (CVD) have been used for the deposition of thermochromic vanadium dioxide layers on glass substrates. In this work, vanadium dioxide coatings were fabricated using vanadyl (IV) acetylacetonate as vanadium precursor in an atmospheric pressure CVD (APCVD) system on SnO2-precoated glass substrates. The samples were characterized by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, UV-Vis-NIR spectroscopy at temperatures below and above transition temperature as well as transmittance measurements as a function with temperature at an incident radiation of 1500 nm. The effect of deposition temperature and N2 flow rate through the vanadium precursor on the thermochromic characteristics of the coatings is discussed.

Resume : A’A’’B’B’’O3-delta-type complex perovskites with a large oxygen deficiency deltaexhibit a perceptible ionic conductivity, leading to their use as electrolytes ((La,Sr)(Ga,Mg)O3-delta) or materials for oxygen permeation membranes and solid oxide fuel cell (SOFC) cathodes ((La,Sr,Ba)(Mn,Fe,Co)O3-delta) [1,2]. The oxygen migration in these materials occurs by the vacancy mechanism with the migration through a ”critical triangle” formed by one B- and two A– site cations, as the bottleneck [3]. Oxygen ionic conductivity is important not only for its transport though the cathode body, but also for the surface chemistry. Oxygen vacancy formation and migration energies were found to be the major factors determining the surface oxygen reduction rate [1]. These two quantities were analyzed for a series of complex (Ba,La,Sr)(Co,Fe)O3-delta (BSCF and LSCF) perovskites by means of first principles DFT calculations [3]. The atomic relaxation, electron charge redistribution, and transition states energies for oxygen ion migration are obtained and discussed in detail with special emphasis on the vacancy formation and migration at the surface. [1]. M. Kuklja, E.Kotomin, R. Merkle, Yu, Mastrikov, J.Maier, Phys. Chem. Chem. Phys. 15, 5443 (2013). [2] R. Merkle, Yu.A. Mastrikov, E.A. Kotomin, M.M. Kuklja, J. Maier. J Electrochem.Soc. 159, B 219 (2012). [3] Yu.A. Mastrikov, R. Merkle, E.A. Kotomin, M.M. Kuklja, J. Maier, Phys. Chem. Chem. Phys. 15, 911 (2013)

Resume : TiOx layers are implemented in various devices ranging from metal/insulator tunnel transistors to Memristors. Nevertheless the Ti/TiOx interfaces lack a thorough characterization towards a better understanding of occurring phenomena. We report here on the behavior of lateral MIM structures fabricated by Local Anodic Oxidation (LAO) of Ti thin films. The samples are composed of a thin Ti channel, a few µm wide and 5-10 nm thick fabricated by means of optical lithography. A transverse oxide line is then patterned by LAO lithography to define the Ti/TiOx /Ti planar structure. Using PtSi tips in contact mode, 85nm wide and 8nm thick junctions are obtained. Under air, the TiOx junctions exhibit evolving I(V) characteristics with a continuous increase in resistance at each cycle. After several cycles the devices invariably gets in a high resistance mode, consistent with a continuing oxidation process of the Ti electrode induced by the electrical stress, the Ti/TiOx interface being open to the environment. This self-limiting process leads to a junction’s width of around 1 µm. Structural and chemical analysis performed by EELS and EDX via HRTEM before and after the electrical stress highlight the growth of a rutile TiOx phase initiated at the amorphous oxide line. This transformation mechanism is totally suppressed if the electrical stress is applied in a vacuum environment, thus indicating the role of ionic motion in the process. Further issues will be discussed in the final paper.

Resume : Ultrathin ferroelectric (FE) films have recently got much attention in the context of the non-volatile memory devices. Over the last decade, the rapidly improving expertise in the fabrication of the structurally perfect complex oxide FE layers have led to the fabrication of heterostructures with intriguing physical and functional properties. In particular, ultrathin heteroepitaxial BaTiO3 (BTO) films have been utilized as a storage medium in the FE tunnel junctions [1], and exhibited giant bulk photovoltaic effect [2]. Pulsed Laser Deposition (PLD) has recently emerged as an excellent tool to grow epitaxial oxide heterostructures, particularly comprising ultrathin FE BTO and SrTiO3 (STO) layers. In this work, we report on the effect of PLD growth conditions of BTO and STO on MgO and Si substrates on their structural and FE properties. We show that the structural properties of the oxide layer strongly depends on a precise control over the partial O2 pressure in the growth chamber, substrate temperature and laser ablation parameters. The structure of 3-100 nm PLD grown layers was monitored in situ by RHEED, and further analyzed ex situ by XRD, XRR, AFM and TEM. The ferroelectric properties were investigated by piezoresponse force microscopy. BTO and STO thin films grown at the optimized conditions are monodomain ferroelectrics with the polarization direction perpendicular to the substrate. 1. A. Zenkevich et al. APL 102 062907 (2013). 2. A. Zenkevich et al. PRB 90 161409 (2014)

Resume : Tb0.3Dy0.7Fe2/Pt/PbZr0.56Ti0.44O3 (Terfenol-D/Pt/PZT) artificial multiferroic structure was sputtered on Pt/TiO2/SiO2/Si substrate. An intermediate platinum layer of 80 nm was used to minimize atomic diffusion between magnetostrictive Terfenol-D layer and piezoelectric PZT films. Ferroelectric and magnetic properties were characterized at room temperature. Then, direct and converse magnetoelectric coupling was evidenced in the synthesized heterostructure. A direct magnetoelectric voltage coefficient of 1.27 V/cm Oe was measured, which is a large value for thin films considering the clamping effect of the substrate. The magnetic domain patterns were locally imaged by magnetic force microscopy, and strong changes were observed when a dc electric field was applied to the ferroelectric layer. A reversible control of the ferromagnetic domains in the multiferroic heterostructure was obtained by removing the external electrical field. This significant finding was explained by the residual tensile stress imposed by the clamping effect of the silicon substrate. These results emphasize that the combination of Terfenol-D and PZT materials in thin films form is very promising to design integrated magnetoelectric devices operating at room temperature.

Resume : Several TCO thin films (ITO, AZO, SnO2), with thickness in the range 200–400 nm, have prepared onto unheated glass substrates, using rf magnetron sputtering and vacuum thermal evaporation. Using the two sputtering targets during the rf magnetron sputtering deposition, the surface of the substrate was heated at 350 - 375 K, which resulted in a good adherence of the thin film on the respective substrate. After deposition, the samples were annealed in air at temperatures up to 750 K. The structural and optical properties of both as-deposited and annealed samples were investigated. The morphological and structural characteristics of the obtained structures were investigated by AFM, SEM and XRD respectively. The transmittance spectra, in double beam were recorded in the 190–3000 nm wavelength range and the electrical conductivity was measured with four points method. The samples surface morphology appeared as granular and polycrystalline with high optical transparency and have a good electrical conductivity. Depending on the preparation conditions and the annealing temperature, value of the optical bandgap, Eg, of the corresponding thin films ranged between 3.36 eV and 3.45 eV. The obtained results are discussed in correlation with the structure of the thin films and the role of doping materials in oxide thin films.

Resume : High-κ gate dielectric implementation is one of several strategies to allow further miniaturization of amorphous oxide-based thin film transistors (TFTs) [1]. Gate dielectrics play a crucial role in decreasing subthreshold slope and operating voltage due to their large capacitive coupling between the active layer and gate. However, achieving an equivalent thickness reduction with a reasonably small leakage current and obtaining the intended threshold voltage coexisting with good electrical stability under stress are still big challenges. Therefore, all the reliability issues must be understood and minimized for producing High-κ/metal gate stacks. Throughout this investigation, we employ the IGZO-TFTs using different gate dielectric configurations of a high-κ, amorphous Ta2O5 mixed with SiO2 in multicomponent films and/or multilayer structures. The dynamic of gate positive bias stress (PBS) and recovery time is studied for each TFT. At the initial stages of PBS experiment, all TFTs exhibit anomalous negative threshold voltage shift (ΔVth). However, the negative ΔVth is smaller for cosputtered Ta2O5/SiO2-IGZO devices compared to Ta2O5-IGZO TFTs. Interestingly, inserting silicon layers into the cosputtered Ta2O5/SiO2 dielectric suppresses the negative ΔVth and with increasing stress time, the shift direction switches to the positive side. Note that SiO2-gated IGZO-TFTs show normal positive ΔVth, so this unusual shift cannot be caused by the channel layer exclusively. Two possible reasons can be the origin of this negative ΔVth: (i) ion migration within the gate dielectric or (ii) charge trapping/detrapping in dielectric. The ion drift is an irreversible process while all the TFTs show a reasonable fast recovery time after PBS. Therefore, this hypothesis is discarded. However, it is hard to believe that holes can be generated in the n-channel device without light illumination. In addition, it is already reported that hole current cannot be injected into the dielectric by the carrier separation technique on leakage current [2]. Therefore, it can be concluded that the negative shift is not due to hole trapping, but due to electron detrapping from Ta2O5. In order to clarify an instability model, positive bias stress temperature instability (PBTI) process is applied for devices showing only negative ΔVth. The negative shift increases with increasing temperature. The time evolution of negative ΔVth is given by a power law with almost the same exponent 0.7 for all TFTs. We suggest that negatively charged defect that releases electrons is a negatively charged interstitial oxygen defect. Such interstitial oxygen defects are easily generated as already reported for the HfO2 gate dielectric, a close relative of Ta2O5 [3]. References: [1] L. Zhang, J. Li, X. W. Zhang, X. Y. Jiang, and Z. L. Zhang, “High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric,” Thin Solid Films, vol. 518, no. 21, pp. 6130–6133, Aug. 2010. [2] M. Sato, K. Yamabe, T. Aoyama, Y. Nara, and Y. Ohji, “Origin of the Hole Current in n-type High- k /Metal Gate Stacks Field Effect Transistor in an Inversion State,” Jpn. J. Appl. Phys., vol. 46, no. No. 44, pp. L1058–L1060, Nov. 2007. [3] C. Kaneta and T. Yamasaki, “Oxygen-related defects in amorphous HfO2 gate dielectrics,” Microelectron. Eng., vol. 84, no. 9–10, pp. 2370–2373, Sep. 2007.

Resume : High-k dielectrics are extensively studied as alternatives to SiO2 as gate dielectrics for the next generation of field-effect transistors employing metal-oxide semiconducting channels. Amongst the wide range of high-k dielectrics LaAlO3 combines the advantages of the high permittivity of La2O3 with the chemical and thermal stability of Al2O3 and exhibits desirable chemical and electrical properties without the shortcomings of each individual material retaining at the same time a relatively high permittivity and large band gap. LaAlO3 films have been deposited by a variety of techniques including MBE, PLD, magnetron sputtering, sol-gel, ALD and MOCVD. Here, we report the deposition of a-LaAlO3 thin films as a function of the aluminium to lanthanum atomic ratio using spray pyrolysis, a simple and large-area-compatible deposition technique. The films were studied by means of atomic force microscopy, X-ray diffraction, impedance spectroscopy, UV–Visible absorption spectroscopy, spectroscopic ellipsometry, and field-effect measurements. Analyses reveal amorphous LaAlO3 that exhibit wide band gap (6.2 eV), low roughness (1.3 nm), high dielectric constant (17), and high breakdown voltage in excess of 3 MV/cm. Thin film transistors based on a-LaAlO3 gate dielectrics employing spray coated ZnO channels exhibit excellent electron transport characteristics with negligible hysteresis, high on/off current modulation ratio (>10^7) and electron mobility in excess of 12 cm^2 V−1 s−1.

Resume : The fundamental material requirements for alternative gate dielectrics to provide performance comparable to SiO2 to be achieved, constitute a very challenging topic. Rare earth oxides (REO) are among the potential candidates for the next generation of gate oxides, because of their high relative permittivity, large bandgap, high crystallisation temperature and thermodynamic stability in contact with a number of semiconducting materials. A wide range of techniques has been used for the deposition of rare earth oxide films including, ion-beam and magnetron sputtering, MBE, PLD, sol–gel, MOCVD and ALD. Here, we report on the deposition and characterisation of high-k neodymium oxide (Nd2O3) dielectrics grown by spray pyrolysis in air at moderate temperatures. Nd2O3 films on ITO were investigated by means of X-ray diffraction, AFM, admittance spectroscopy, UV–Vis absorption spectroscopy, spectroscopic ellipsometry, and field-effect measurements. Depending on the deposition temperature (between 400 oC and 600 oC), analyses reveal smooth films (RRMS<2 nm) of amorphous, cubic or hexagonal phase with high dielectric constant in the range between 12 and 19 and high breakdown voltage (>2.9 MV/cm). Thin film transistors employing Nd2O3 gate dielectrics employing spray coated ZnO semiconducting channels exhibit excellent electron transport characteristics with electron mobility in excess of 30 cm^2 V−1 s−1, high on/off current ratio (10^6) and hysteresis-free operation.

Resume : The architecture of a polymer solar cell (PSC) plays a crucial role on its stability because some of the materials used to build the device can suffer from degradation when exposed to air. In the inverted device configuration, the indium tin oxide (ITO) electrode, covered by a n-type metal oxide is used as the cathode and an air-stable high-work-function metal (e.g. Ag) is used as the anode. Such structure doesn’t suffer from the top metal contact oxidation and ITO/PEDOT:PSS interface stability of the conventional structure. In this work, we report the application of low-temperature solution processed zinc oxide or titanium oxide as ETLs in inverted PSCs. The metal oxide thin films were deposited on ITO/glass substrates by spin coating. Photovoltaic devices with the configuration ITO/ETL/blend/MoOx/Ag were then realized in order to investigate the performance of the metal oxides. The photoactive layer is a blend of Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) and [6,6]-phenyl C71 butyric acid methyl ester ([70]PCBM) (1:1.5 w/w). We made a comparative study of the photovoltaic behavior of devices realized using different ETLs. All the devices were characterized by UV-VIS spectroscopy, IV light, IV dark and quantum efficiency measurements. The best polymer solar cells reached a power conversion efficiency up to 9.5%.

Resume : Complex functional oxides exhibit a fascinating range of properties: from colossal magnetoresistance and high-temperature superconductivity to ferroelectricity and multiferroicity. Oxides can therefore potentially provide solutions to many of the technological challenges facing human society, including lower energy consumption devices and transition to renewable sources of energy. In addition, silicon is nowadays the most fundamental technological material in electronic industry. Therefore, it is highly desirable to integrate monolithically oxide films and nanostructures on silicon, since it could open the way to the fabrication of efficient devices applications. In this regard, the presented work takes advantage of all the benefits of soft chemistry to overcome the main challenges for the monolithic integration on silicon of a high temperature ferromagnetic-metallic material [1]. In this work we describe the polymer assisted deposition (PAD) [2] of epitaxial La0.7Sr0.3MnO3 (LSMO) thin films on SrTiO3/Si. A buffer layer of crystalline STO was previously deposited on top of Si (001) with Sr-passivated surface by molecular beam epitaxy (MBE). The transport and magnetic properties of ultrathin films of LSMO show that it is possible to integrate functional thin films of complex oxides on silicon substrates by exploiting the possibilities of this soft chemical technique. [1] A. Carretero-Genevrier et al. Nanoscale, 6, 14025 (2014) [2] Q. X. Jia et al. Nature Mat. 3, 529 (2004)

Resume : KTaO3, a I-V ABO3 perovskite, is attracting substantial interest following the discovery of a two-dimensional electron gas (2DEG) on its (001) surface [1]. Little is, however, known of its surface structure unlike the widely studied II-IV, SrTiO3 counterpart [2]. The polar (001) KTaO3 surface, with alternating +1 and -1 charged terminating planes must undergo a reconstruction to become stable. Here, we reveal a new stabilisation mechanism which involves K enrichment of the surface in agreement with experiment [3]. Reconstructions of the TaO2 terminated surface with inter-planar cation exchange result in stable configurations, in which highly charged Ta is fully co-ordinated and only K is exposed on the surface. A global optimisation yields a number of low-energy minimum (2 x 2) reconstructions mediated by this mechanism. We propose that this mechanism is general to polar I-V perovskite surfaces and that it will give rise to novel surface electronic properties. [1] King, P. D. C. et al. Subband structure of a two-dimensional electron gas formed at the polar surface of the strong spin-orbit perovskite KTaO3. Physical review letters 108 (2012). [2] Enterkin, J. A. et al. A homologous series of structures on the surface of SrTiO3 (110). Nature Materials 9, 245–248 (2010). [3] Szot, K. et al. Chemical inhomogeneity in the near-surface region of KTaO3 evolving at elevated temperatures. Journal of Physics: Condensed Matter 12, 4687–4697 (2000).

Resume : Transparent oxide materials have drawn considerable attention due to their unique electrical and optical properties, such as high electron mobility, and optical transparency. In particular, thin film transistors (TFTs) employing amorphous oxide semiconducting channels seem to constitute promising candidates for large-area electronics such as AMOLED displays because of the superior device characteristics over their polycrystalline counterparts or a-Si. Several amorphous oxide semiconductors (AOS), such as gallium indium zinc oxide (GIZO) and Indium zinc oxide (IZO), have been reported as alternative channel materials and fundamental electronic circuits have already been realised. Despite their attractive properties, however, AOS TFTs are usually realised using stringent and potentially costly manufacturing techniques. Here we demonstrate how spray pyrolysis, a simple and large-area-compatible deposition technique, can be used for the processing of high quality GIZO semiconducting channels. The GIZO semiconductors were spray coated onto thermal SiO2 substrates at temperatures in the range between 300 and 600 oC from soluble precursor solutions with varying the Ga, In, and Zn composition. At the optimum deposition temperature and composition, TFTs employing spray coated GIZO semiconducting channels exhibit excellent electron transport characteristics with negligible hysteresis operation, electron mobility in excess of 40 cm^2 V−1 s−1, and high on/off current ratio (10^6).

Resume : Thermal property of gate dielectric in MOSFETs is crucial and fundamental information for managing the heat to avoid performance drop or device failure come from the released heat of the devices. The electric devices are getting smaller until few nano meters to maximize the degree of integration, therefore the nano scale thermal properties should be also provided to design the heat dissipation system properly. However, the bulk properties of material cannot be applied because most properties of nano scale films are not same as that of well-known bulk’s value owing to the phonon scattering at the boundary or phonon confinement effects. Furthermore, the measuring the thermal conductivity of thin film is very hard due to the fact that the measuring the difference of temperature between film’s cross section is too small and difficult to detect. Here, we introduce the 3ω method that can measure the thermal conductivity stably with small errors less than 10% and report the thermal conductivity of thin film Al2O3 grown by atomic layer deposition (ALD) which is the representative high-k material generally used as a gate dielectric material. The film is also characterized by XPS, RBS to investigate the film quality, such as density and composition of the Al2O3. Our study provides more accurate value of thermal properties which was changed in nano scale so we expect it can help designing the heat managing in integrated circuit more efficient.

Resume : We study the tunneling resistance of superconductor/ferroelectric/normal-metal vertical junctions based on ultrathin (4 to 8 nm thick) BiFeO3-Mn grown on YBa2Cu3O7 by pulsed laser deposition. The junctions are defined via a series of nanofabrication steps which include e-beam lithography, normal-metal deposition (NbN or Co capped with Pt) and lift-off. We use conductive-tip atomic force microscopy and Piezoresponse Force Microscopy to characterize the transport across the barrier as a function of its ferroelectric state. The observed electro-resistance is discussed in terms of different microscopic mechanisms and variable electric-field screening in the normal and superconducting states. Work supported by DIM Oxymore

Resume : ZnO is an important wide-gap n-type semiconductor. Accurate characterisation of surface structure and properties is therefore of vital importance. Recently, ZnO wurtzite films with non-polar surfaces have attracted attention as new materials which show higher emission efficiency for blue or ultra-violet LEDs. Two of the most important issues to address in the study of the non-polar surface structural properties of ZnO are the character of the atomic relaxation at clean surfaces compared to bulk and the electronic structure of such surfaces. Structurally, both: the (10-10) and the (11-20) ZnO surfaces have been investigated extensively using both theoretical and experimental techniques; however, the nature of surface relaxations remains controversial. We will discuss the structure of these surfaces using interatomic potential and ab-initio methods and the stability of features such as steps, dimer vacancies and grooves. The electronic structure of the non-polar ZnO surfaces was predicted using ab-initio methods. A correct positioning of the band edges of ZnO is essential to calculate a great variety of physicochemical properties, which are crucial in the design of electronic devices. We employed a method recently developed by Logsdail et al., which gives a better approximation to the experimental bulk ionization potential values. The last point we will discuss will be the band bending of the VBM and CBM across the surface on the ZnO non-polar surfaces.

Resume : Charge-trapping memories such as SONOS and MONOS have attracted considerable attention as promising alternatives for next-generation flash memories due to dielectric layer’s scalability, process simplicity, power economy, operation versatility. Nevertheless, the continued miniaturization of the devices forces an application of high-k dielectrics. In this work high-k stacked dielectric structures based on the combination of Hf-based and SiNx materials were fabricated. Their structural and electrical properties versus deposition conditions are studied by means of FTIR-ATR and high-resolution TEM techniques. Front- and back-side Al contacts were deposited by thermal evaporation and patterned by photolithography in order to fabricate MIS memory capacitors for electrical characterization. All samples demonstrated smoothed surface (with a roughness below 1 nm) and abrupt interfaces between the different stacked layers. No crystallization of Hf-based layers was observed after annealing at 800°C for 30 min, demonstrating their amorphous nature and phase stability upon annealing. Uniform C-V characteristics were measured along the wafers for all stacks. Besides, after round-voltage sweep they demonstrate significant Vfb hysteresis due to charging of the stack caused by carrier injection from the substrate. These phenomena were found to be more pronounced for the structures with pure HfO2 layers. The associated mechanisms and their tuning parameters will be discussed in details.

Resume : Due to the waste heat in ever more compact microelectronic devices, the harvesting of thermal energy has become interesting for self-powering small devices. Pyroelectric materials which couple a change in temperature to a change in electrical polarization can be used for the conversion of the thermal energy to an electric energy with temperature temporal variation [1]. According to recent reports, single-crystalline pyroelectric films should provide an enhanced conversion energy efficiency with respect to bulk or polycrystalline materials [2]. Here, we show the integration of pyroelectric PbZr0.52Ti0.48O3 thin films by sol-gel process on Si(001) via a thin SrTiO3 buffer layer grown by molecular beam epitaxy (MBE) and a sputtered SrRuO3 bottom electrode. These heterostructures are compared with the same ones grown on SrTiO3(001) substrate. The structural characterizations by RHEED, XRD and AFM show that the heterostructures are epitaxial with flat surfaces and with mixed c/a domains. Ferroelectric hysteresis loops at different temperatures give pyroelectric coefficients close to the bulk value (~250 µC.m-2.K-1), and provide an estimation of the harvested pyroelectric energy of 50 mJ.cm-3.cycle-1. Direct pyroelectric measurements are done and compared to the indirect ones, and the pyroelectric efficiency of single-crystalline films are also compared to that of poly-crystalline ones. [1]: S.B. Lang, Phys. Today 58, 31 (2005) [2]: G. Sebald et al., Smart Mater. Struct. 18, (2009)

Resume : Recent discovery in ZnO nanogenarator has spurred tremendous interest in micro-sensor power application. The fundamental principle of ZnO nanogenarator is to utilize the environmental mechanical energy, which is available everywhere from irregular vibrations, light airflow, noise and human activity with a wide spectrum of frequencies and time-dependent amplitudes. The first prototyping of a nanogenarator by means of ZnO piezoelectric nanowire (NW) arrays have demonstrated to able to drive sensor network nods in micro-power range. ZnO piezoelectric thin films have also been successfully transferred onto flexible substrates for stretchable energy harvesting, which suggests new possible applications of piezoelectric nanomaterials. However so far the fundamental mechanism from thin film piezoelectric generator is still unclear. To further clarify the effect of stress-induced and nonsymetry charge center promoted electrical potential is critical for large power output from ZnO nanogenerator. Power output from NW nanogenarator is still in the power range micro watt, which is still far way from milli-power output source requested by most applications of individual sensor and sensor network system. The lacking of high power output in current ZnO NW generator is partially attribute to non-well c-axis oriented crystalline of ZnO NW material and low yield in NW device[5], both are synthesized by chemical method. Therefore to explore new approach for ZnO NW material synthesis to enhance piezoelectric effect is desirable. In this paper, the effects of the stress on piezoelectric response for ZnO film grown by pulsed-laser deposited (PLD) are investigated. To promote differerent film stress, the ZnO films are deposited at different thcknessa and different temperature. Structurally, special emphasis is placed on XRD characterizations of the films. Additional characterizations using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) will be detailed at a later date. The piezo-electric properties of these films are characterized by Piezoelectric Force Microscopy (PFM). The observed corresponds of piezoelectric (PE) property to stress is also theoretically investigated.

Resume : Ferroelectric oxides integrated on semiconductor substrates are of particular interest for various silicon-based electronic and photonic devices. Among them, the perovskite BaTiO3 is an attractive candidate for integrated photonics and low power logic devices. The control of the crystalline orientation of the ferroelectric tetragonal cell (c- versus a-axis orientation) as a function of the processing parameters is a key issue. In order to support the MBE growth strategy on SrTiO3-buffered Si of epitaxial BaTiO3 thin films with the desired orientation, high-resolution transmission electron microscopy (HRTEM) and high angle annular dark-field (HAADF) atomic structure images of various films were acquired and treated quantitatively using geometric phase analysis. Different deposition conditions were investigated (temperature, oxygen pressure, annealing). In this work, maps of the strain in the BaTiO3 films with respect to the Si substrate are determined at the nanometric scale (1-2nm) to evidence the local tetragonality (orientation and c/a ratio) of the BaTiO3 films. Correlations with the local cation composition are also proposed on the basis of electron energy loss spectroscopy (EELS) performed at the atomic scale. HRTEM work is performed on an image corrected Hitachi HF3300S microscope (I2TEM-Toulouse) and HAADF-EELS on a FEI Titan Low-Base 60-300 (Zaragoza).

Resume : Selective synthesis for integrated nanomaterials with controllable morphology and composition represents an emerging research area in nanoscience and nanotechnology because the intrinsic properties behind nanostructures are generally phase-, shape-, and size- dependent. In this direction, the present work shows the capabilities of nanoporous organic and inorganic template systems directly supported on different substrates for the confined growth of epitaxial ferromagnetic complex oxides nanostructures. In particular, we describe the versatility and potentiality of sol-gel precursor solutions combined with nanoporous templates [1] to synthesize i) polycrystalline La0.7Sr0.3MnO3 nanorods on top of single crystal perovskites, ii) single crystalline manganese based octahedral molecular sieves (OMS) nanowires on silicon [2,3], and iii) epitaxial directional grown single crystal OMS nanowires on fluorite-type substrates [4]. The influence of the distinct growth parameters on the nanostructural evolution of the resulting nanostructures and their magnetic properties are studied in detail. We demonstrate that the combination of soft-chemistry and epitaxial growth opens new opportunities for the effective integration of novel functional complex oxides nanomaterials. . 1 A Carretero-Genevrier et al Chem.Soc.Rev 43 2042 (2014) 2 A Carretero-Genevrier et al JACS 133 4053 (2011) 3 A Carretero-Genevrier et al Chem.Mater 26 1019 (2014) 4 A Carretero-Genevrier et al Chem.Comm 48 6223 (2012)

Resume : Barium strontium titanate (BaxSr1-xTiO3, BST) is a well known ferroelectric material. Moreover this environmentally friendly material - thanks to its lead-free composition - is attractive for microwaves applications, energy harvesting, and electro-optics. In this work, 200nm-thick BST (70/30) has been deposited using two methods: a) sol-gel, which is low-cost and has short fabrication cycle; b) sputtering, producing high homogeneous, large scale, thickness precisely controlled thin film. Two types of bottom electrode were used and their influence on ferroelectric properties and domains switching was investigated: SrTiO3 (STO) substrate, with a SrRuO3 (SRO) interlayer as bottom electrode to get epitaxial BST film; platinum buffered silicon, which is the classical industrial template. Diffraction indicates crystalline BST is c-axis oriented on both substrates. Moreover on SRO-buffered STO the film is epitaxially grown. At nanoscale, the samples were characterized by piezoresponse force microscopy. At macroscale, BST thin films show different behaviors with respect to the different bottom electrodes, measured through current-voltage, capacitance-voltage and PUND (Positive Up Negative Down). These characterizations will be compared with those performed with 200nm-thick PZT films deposited by sputtering and sol-gel on SRO-STO and Pt buffered silicon templates.

Resume : Graphene is a semiconductor with zero band gap, linear energy dispersion, and linear density of electronic states. One of its most important properties is a strong electric field effect which leads to an electrostatically tunable carrier density in the range of n < 1e14 cm-2. Together with high carrier mobilities for both electrons and holes (as high as 10000 cm2/V.s at room temperature), this attracts a lot of attention to graphene as a possible material for a future high-speed field effect transistor (FET). Graphene layer was synthetized from silicon carbide wafers (SiC) by evaporating Si atoms from 4H-SiC surface at high temperature thanks to a high energy electron beam in UHV. The formation of single layer graphene has been confirmed by Raman spectroscopy: peaks at 1580 cm-1 (G-peak) and 2690 cm-1 (2D peak), originated from graphene in-plane vibrational modes. After deposition of SiO2 functional oxide, the whole layers were patterned to realize a FET device. The graphene channel has width about 40µm and length between 2µm and 16µm. The application of voltage up to 9V at room temperature on the binary oxide gate leads to on-off working operation with a variation about 600% of the drain current.

Resume : ZnO is both a piezo-electric and electro-optic material, and a semiconductor which possesses a wide band gap (~3.3 eV) [1] with as its most unique property an exciton binding energy of 60 meV [2]. However, the properties of ZnO thin film depend greatly on the crystallographic texture, and the control of the crystallization and growth direction is therefore essential. For example, (002)-oriented ZnO have practical applications such as light emitting diodes and sensor arrays [3]. Similarly, other specific orientations may exhibit different interesting properties and the precise control of the final texture of the films is therefore a key parameter for their wider development. With significantly lower investment costs and a highly-controlled way for ZnO thin films elaboration, the sol-gel process has emerged as being very attractive compared to other deposition methods. Zinc acetate dihydrate, ethanol and monoethanolamine were used as starting materials, solvent and stabilizer, respectively. The multi thin layers are prepared by spin-coating onto glass substrates, and are transformed into ZnO upon annealing at 500°C. The structural, morphological, and optical properties of the thin films as a function of precursor concentration have been investigated using X-Ray Diffraction (XRD, Rietveld and texture analysis), Scanning Electronic Microscopy (SEM), Atomic Force Microscopy (AFM) and Cathodo-Luminescence (CL). The results obtained from the different characterization techniques will be presented and discussed. ω – 2 θ scans and texture measurements were performed by XRD. On all samples, the different analyses show two main texture components (fibers) <00.2> and <05.2>. From the ODF (Orientation Distribution Function), volume fractions of these two components were calculated. Keywords: Sol-gel, ZnO thin films, Spin-coating, Texture, ODF, Volume fractions [1] A.B.M.A. Ashrafi, Y. Segawa, K. Shin, J. Yoo, T. Yao, Appl. Surf. Sci. 249 (2005) 139–144. [2] K.K. Kim, J.H. Song, H.J. Jung, S.J. Park, J.H. Song, J.Y. Lee, J. Vac. Sci. Technol. A 18 (6) (2000) 2864–2868. [3] C.S. Lao, Q. Kuang, Z.L. Wang, M.C. Park, Y. Deng, Appl. Phys. Lett. 90 (2007) 262107.