Buffer Layer Technique Research Articles

Significantly improved energy storage stabilities in nanograined ferroelectric film capacitors with a reduced dielectric nonlinearity

For the development of high energy density ferroelectric film/multilayer capacitors, which have applications in electric power systems and advanced pulsed-discharge devices, a stable energy storage performance in a wide range of temperature, frequency and cycling times has become extremely important. Here, we propose an effective method to boost the energy storage stabilities of ferroelectric film capacitors through the use of a nanoengineered film structure with a reduced dielectric nonlinearity. We show in (Ba0.95,Sr0.05)(Zr0.2,Ti0.8)O3 (BSZT) ferroelectric films that such an approach is facilitated by reducing the size of sputtered columnar nanograins via a synergetic effect between a low temperature deposition process and a buffer-layer technique. In comparison with the BSZT films sputtered at 500 °C which are better ferroelectrics, the 400 °C-sputtered BSZT films not only showed an improved energy storage density and efficiency, but also displayed a reduced dielectric nonlinearity in its polarization response. The latter has led to a significantly improved energy storage stability under a changing temperature (−175 oC ∼ 200 °C) or frequency (1 Hz ∼ 20 kHz), or after a large amount of charge–discharge cycles (up to 2 × 109).

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaOx buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm−2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaOx semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Low bandgap GaInAsSb thermophotovoltaic cells on GaAs substrate with advanced metamorphic buffer layer

Thermophotovoltaic (TPV) devices based on GaInAsSb lattice matched to GaSb (100) substrates have demonstrated high external quantum efficiencies (EQEs) in the mid-infrared spectral range, making them promising candidates for waste heat recovery from high temperature “blackbody” sources. In this work, the GaInAsSb alloy has been integrated onto more cost-effective GaAs (100) substrates by using advanced metamorphic buffer layer techniques in molecular beam epitaxy (MBE), which included an interfacial misfit (IMF) array at the GaSb/GaAs interface followed by GaInSb/GaSb dislocation filtering layers. The threading dislocations in the GaInAsSb region can be efficiently supressed, resulting in high quality materials for TPV applications. To determine the performance of the GaInAsSb TPV on GaAs it was compared with a cell grown lattice matched onto GaSb substrate having the same structure. The resulting TPV on GaAs exhibited similar dark current-voltage characteristics with that on GaSb. Under illumination from an 800 °C silicon nitride emitter, the short circuit current density (Jsc) from the GaInAsSb TPVs on GaAs reached more than 90% of the control cell on GaSb, and the open circuit voltage (Voc) exceeded 80% of the cell on GaSb. The EQE from the TPV on GaAs reached around 62%, the highest value reported from this type of TPV on GaAs. With improved TPV structure design, large area GaInAsSb TPV panels on GaAs substrates can be realized in the future for waste heat energy recovery applications.

Structural and Electrical Characterization of Sputter-Deposited Gd0.1Ce0.9O2−δ Thin Buffer Layers at the Y-Stabilized Zirconia Electrolyte Interface for IT-Solid Oxide Cells

The use of a doped Ceria buffer layer and Physical Vapour Deposition (PVD) techniques for Solid Oxide Fuel Cells (SOFC) fabrication can limit the former, the formation of electrical insulating lanthanum, and strontium zirconates at the cathode/electrolyte interface, whereas the latter allows a better control of the materials interfaces. These effects allow for operation at intermediate temperature ranges. In this work, we study the structural and electrical properties of Gadolinium Doped Ceria (GDC) barrier layer deposited via the room temperature RF Sputtering technique on anode supported electrolytes and then annealed at high temperature. The crystal structure and the surface morphology of the GDC barrier layers have been analyzed and optimized varying the temperature ramp of the post-growth annealing procedure. The electrical behavior of the obtained samples has been investigated by Electrochemical Impedance Spectroscopy and compared to that of standard SOFC with screen-printed GDC barrier layers, the former showing a maximum high frequency and low frequency resistances reduction of about 50% and 46%, respectively, with respect to the latter at an operating temperature of 650 °C. The results clearly show an important improvement of SOFC performances when using sputter deposited GDC layers, linking the electrical properties to the structural and stoichiometric ones.

Fabrication and characteristics of high performance SOI-based Ge PIN waveguide photodetector

Silicon-based photonics has aroused an increasing interest in the recent year, mainly for optical telecommunications or optical interconnects in microelectronic circuits. The waveguide photodetector is one of the building blocks needed for the implementation of fast silicon photonics integrated circuits. The main considerations for designing such a device are the bandwidth, the power consumption and the responsivity. Germanium is now considered as an ideal candidate for fully integrated receivers based on silicon-on-insulator (SOI) substrates and complementary metal oxide semiconductor (CMOS)-like process because of its large optical absorption coefficient at the wavelength for optical communication. Therefore, the study of high speed and high responsivity Ge waveguide photodetectors is necessary. In this paper, high concentration phosphor doped SOI substrate is achieved by using solid-state source diffusion at first. Secondly, the high quality epitaxial germanium (Ge) is grown on phosphor doped SOI substrate by using low temperature Ge buffer layer technique based on the UHV/CVD system. The surface profile, crystal quality and strain of epitaxial Ge film are characterized by using atomic force microscopy, X-ray diffraction (XRD), and Raman scattering spectrum. The results show that the Ge film has a smooth surface of 1.12 nm roughness and about 0.2% tensile strain, which is verified by XRD characterization result. Thirdly, ptype Ge region is formed by BF2+ implantation, and rapid thermal annealing to repair the implantation damages and activate impurity. Finally, the highperformance Ge PIN waveguide photodetectors with different sizes are fabricated by standard COMS technology. Moreover, the device performances, in terms of dark current versus voltage characteristics, photocurrent responsivity and 3 dB bandwidth, are well studied. The results show that the detector with a size of 4 m20 m demonstrates a dark current density of 75 mA/cm2 at -1 V and a photocurrent responsivity of 0.58 A/W for 1.55 m optical wavelength. In addition, an optical band width of 5.3 GHz at -2 V for 1.55 m is also demonstrated, which is far below theortical value of about 40 GHz. This can mainly be attributed to two aspects. On the one hand, Ge PIN structure contains low temperature Ge buffer layer, which has highdensity dislocation because of large lattice mismatch between Si and Ge. Those dislocations or defects can trap and release the photo-generated carrier, which increases the transit time. On the other hand, the contact characteristics of Al with n+-Si and p+-Ge are not very good, leading to a large contact resistance and RC delay. Through improving the above two aspects, the performance of Ge PIN waveguide photodetector will be further enhanced.

Room Temperature 3.5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math> </inline-formula> Mid-Infrared InAs Photovoltaic Detector on a Si Substrate

We report a mid-infrared InAs p-i-n photovoltaic detector on a Si substrate operating at room temperature with a cutoff wavelength at 3.5 $\mu \text{m}$ . The large 11.6% lattice mismatch between the device layer (InAs) and the Si substrate is accommodated by employing a compositionally graded buffer layer technique using solid-source molecular beam epitaxy and a commercially available GeSi substrate. An Al x In1– x As graded buffer layer is used to linearly scale the lattice constant from AlAs to InAs with an increasing $x$ value. The photovoltaic detector has a responsivity of 0.57 A/W, measured using a blackbody source maintained at 700 °C.

Reflectance analysis on the MOCVD growth of AlN on Si(111) by the virtual interface model

AbstractThe reflectance‐time profile of AlN‐on‐Si(111) growth by MOCVD as acquired through an in‐situ optical reflectance monitor is analyzed. It is found that, similar to the case of GaN‐on‐sapphire nucleation and growth through the low temperature GaN or AlN buffer layer techniques, the reflectance profile for AlN‐on‐Si also contains information that relate to the growth mechanism, which can then be correlated to the as‐grown material quality. Based on the equations for the Virtual Interface model, a set of 6 parameters can be used to curve‐fit the reflectance traces of AlN growth as acquired by the in‐situ optical monitor. By analyzing a range of 405 nm AlN reflectance‐time data from single layer AlN growth to multilayer AlN/AlGaN/GaN stack grown on Si(111), the “goodness of fit” statistics are found to correlate with material quality metrics such as the XRD (002) rocking curve FWHM for AlN, microscopic morphological roughness of the AlN surface, as well as the resultant wafer bow and curvature for GaN/AlGaN grown on the AlN/Si(111) substrate. The correlations provide useful insight into the desirable mechanism and growth mode for the high temperature MOCVD growth of AlN and GaN on Si(111). When fully established, the technique has the potential of being used as an in‐situ pass/fail screen for the MOCVD growth of GaN‐on‐Si. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and mechanical properties of diamond films on cemented carbide with buffer layers

The main hindrance to applications of diamond coatings on cemented carbide cutting tools is poor adhesion between diamond films and the substrates, caused mainly by Co acting as a catalyst for the formation of graphite during the growth of the diamond films. The buffer-layer technique is commonly used to reduce the elemental diffusion or residual stress in the interface between the film and substrate. In this paper, four kinks of buffer layers, namely TiN, CrN, TiC, and SiN, were applied to (111)-textured diamond films grown on mirror-polished cemented carbide substrates using the hot-filament chemical vapor deposition method. The adhesion strengths of different samples are measured by Vickers hardness tests and compared to show the different effects of the buffer layers. The sample with an 800-nm-thick CrN layer and processed by 10-min chemical etching has the best adhesion among the four buffer layers.

Characterisation of Ga<sub>1-x</sub>In<sub>x</sub>Sb quantum wells (x~0.3) grown on GaAs using AlGaSb interface misfit buffer

GaInSb multiple quantum wells (MQW) grown on GaAs using an AlGaSb interface misfit (IMF) metamorphic buffer layer technique exhibit superior infrared photoluminescence (PL) at room temperature compared with MQW grown directly on GaSb. PL emission was obtained in the range from 1.7 μm (4 K) to 1.9 μm (300 K) from Ga1-xInxSb samples containing five compressively strained QW with In content x~0.3. Structural and optical characterisation confirms that the AlGaSb IMF growth technique is promising for the development of photonic devices operating at extended wavelengths based on GaAs substrates.

GaAs buffer layer technique for vertical nanowire growth on Si substrate

Gold catalyzed vapor-liquid-solid method is widely applied to III–V nanowire (NW) growth on Si substrate. However, the easy oxidation of Si, possible Si contamination in the NWs, high defect density in the NWs, and high sensitivity of the NW morphology to growth conditions largely limit its controllability. In this work, we developed a buffer layer technique by introducing a GaAs thin film with predefined polarity as a template. It is found that samples grown on these buffer layers all have high vertical NW yields in general, due to the single-orientation of the buffer layers. Low temperature buffer with smoother surface leads to highest yield of vertical NWs, while high temperature (HT) buffer with better crystallinity results in perfect NW quality. The defect-free property we observed here is very promising for optoelectronic device applications based on GaAs NW. Moreover, the buffer layers can eliminate Si contamination by preventing Si-Au alloy formation and by increasing the thickness of the Si diffusion barrier, thus providing more flexibility to vertical NW growth. The buffer layer technique we demonstrated here could be easily extended to other III-V on Si system for electronic and photonic applications.

Long-Range Ordered Single-Crystal Graphene on High-Quality Heteroepitaxial Ni Thin Films Grown on MgO(111)

Large-area single crystal monolayer graphene is synthesized on Ni(111) thin films, which have flat terraces and no grain boundaries. The flat single-crystal Ni films are heteroepitaxially grown on MgO(111) substrates using a buffer layer technique. Low-energy electron diffraction and various spectroscopic methods reveal the long-range single crystallinity and uniform monolayer thickness of the graphene. When transferred onto an insulating wafer, continuous millimeter-scale single domain graphene is obtained.

Buffer Layers for Highly Ordered L1$_{0}$ FePt-Oxide Thin Film Granular Media at Reduced Processing Temperature

In this work, we present an experimental technique for obtaining highly ordered L10 FePt-oxide thin film media at moderate deposition temperatures. In most previous studies, a FePt-Oxide layer is directly deposited on a (001) textured MgO layer. By introducing a buffer layer in between the FePt-oxide layer and the MgO underlayer, we are able to substantially enhance the L10 ordering of the FePt-oxide layer while lowering the deposition temperature to 400 . The buffer layer also yields a significantly enhanced (001) texture of the formed L10 FePt structure. With the order parameter near unity, the coercivity of the resulting granular L10 FePt-oxide film exceeds 20 kOe with an average grain size about 8 nm. With the buffer layer technique, 18kOe coercivity has also been achieved for L10 FePt-oxide film at a grain size of about 4.5 nm. In this work, the detailed material composition choice of the buffer layers and the corresponding results are presented.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Grown on Epitaxial AlN/Sapphire Templates

AlGaN films and deep ultraviolet light-emitting diode (UV-LED) structures with AlGaN multi-quantum wells (MQWs) were grown directly on 2-in.-diameter epitaxial AlN/sapphire template (AlN template) by metalorganic chemical vapor deposition (MOCVD). It was confirmed that crack-free and good-crystal-quality AlGaN films and UV-LED structures can be grown on AlN template without the use of any buffer layer technique such as superlattice structures or pulsed atomic layer deposition. This seemed to be mainly due to the in-plane compressive stress caused by using AlN template as underlying substrates. Deep UV-LED devices were successfully fabricated using MOCVD-grown wafers. Electroluminescence spectra for those LEDs exhibited sharp near-band emissions ranging from 265 to 277 nm, depending on their Al compositions in MQWs. It was also confirmed that the insertion of a thin AlN layer between the active region and the p-type cladding layer can drastically suppress a parasitic sub-band emission around 320 nm.

Optical studies of InN epilayers on Si substrates with different buffer layers

We have investigated the photoluminescence (PL) and time-resolved PL from the InN epilayers grown on Si substrates with different buffer layers. The narrowest value of the full width at half maximum of the PL peak is 52 meV with the AlN/AlGaN/GaN triple buffer layer, which is better than previous reports on similar InN epilayers on Si substrates. Based on the emission-energy dependence of the PL decays, the localization energy of carriers is also the least for the InN with a triple buffer layer. According to the x-ray diffraction measurements, we suggest that the reduced lattice mismatch between the InN epilayer and the top buffer layer is responsible for improvement of sample quality using the buffer-layer technique.

Atomic Layer Deposition for Nano-Fabrication of 3D Optoelectronic Devices

We present recent investigations of the atomic layer deposition (ALD) of high transparency, high index, luminescent, and optoelectronic materials into self-assembled opal, lithographic, and biologically derived templates. Investigations on inverse opal based structures, which have the potential to sustain a complete photonic band gap, are presented and have been infiltrated with depositions of TiO2, Al2O3, ZnS:Mn and GaP. It is demonstrated that derivatives of the opal structure can be obtained by the use of a sacrificial buffer layer technique. Consequently, structures can be inverted, precisely replicated, and formed from composite or multilayered materials that allow a high degree of functionality. Additionally, this process enables temperature-sensitive polymer structures to be inverted by low temperature ALD to a high temperature compatible material that then serves as a high temperature template. Recent work is presented on the application of this technique to tune the properties of 2D photonic crystal slab waveguides and for the coating of biological scaffolds.

High-efficiency blue multilayer polymer light-emitting diode based on poly(9,9-dioctylfluorene)

A highly efficient blue polymer light-emitting diode based exclusively on commercial poly(9,9-dioctylfluorene) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl)) diphenylamine)] is demonstrated. High electroluminescent efficiency is achieved by enhancing electron currents and making devices in multilayered structures. CsF∕Al is used as the efficient electron injection cathode, and the fabrication process is in the glove box to enhance electron mobility by reducing oxygen adsorption. The multilayer structure is prepared by the liquid buffer layer technique. The maximum efficiency is 2.5 cd∕A at deep blue with the corresponding external quantum efficiency of 2%.

ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.

Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers grown on Si substrate

Epitaxially grown Ge layers on Si substrate are shown to reveal an enhanced absorption of near-infrared light, which is effective for the photodiode application in Si-based photonics. Ge layers as thick as 1μm were grown on Si substrate by ultrahigh-vacuum chemical-vapor deposition with a low-temperature buffer layer technique. X-ray-diffraction measurements showed that the Ge layer possesses a tensile strain as large as 0.2%, which is generated during the cooling from the high growth temperature due to the thermal-expansion mismatch between Ge and Si. Photoreflectance measurements showed that the tensile strain reduces the direct band-gap energy to 0.77 eV (c.f. 0.80 eV for unstrained Ge), as expected from the theory. Reflecting the band-gap narrowing, photodiodes fabricated using the Ge layer revealed an enhanced absorption of near-infrared light with the photon energy below 0.80 eV, i.e., with the wavelength above 1.55μm. This property is effective to apply the photodiodes to the L band (1.56–1.62μm) in the optical communications as well as the C band (1.53–1.56μm). It is shown that the experimental absorption spectrum agrees with the theoretical one taking into account the splitting of light-hole and heavy-hole valence bands accompanied by the band-gap narrowing. Based on the calculation, the performance of the photodiode using the tensile-strained Ge is discussed.

Modelling fluid flow: the state of the art

Application of CFD-Techniqus in Fluid Machinery.- Modelling Separation from Curved Surfaces with Anisotropy-Resolving Turbulance Closures and Related Supplementary Observations on LES.- Development of Fluid Mechanics Methods in the 20th Century and the Application to Laminar and Turbulent Flow Investigations.- Theodor Von Karman: A Global Life.- Workshop on Complex Flows with Chemical Reactions.- Numerical Modelling of Turbomachinery fluid dynamics.- Challenges in Optimisation of Axial Flow Turbomachinery Blades for 3D Flow, Including Sweep and Dihedral Effects.- Flame Stabilization of Highly Diffusive Gas Mixtures in Porous Inert Media.- A Hybrid Approach to Model Combustion Instabilities.- Modelling of Reaction and Transport in Chemical Vapor Deposition: Predictions of Growth and Uniformity of Tin Oxide Films.- Electromagnetically Forced Convection during Solidification of a Binary Metal Alloy.- Sound Generated by an Unsteady Flow Field, Using a Hybrid Method.- Influence of Cross-Sectional Configuration on Karman Vortex Excitation.- A New Wall-Function Strategy Applied to Mixed Convection in Vertical Annular Passages.- RANS simulations for film-cooling analysis and design.- Very Large Eddy Simulation for the Prediction of Unsteady Vortex Motion.- A New Stabilized Finite Element Method for Advection-Diffusion-Reaction Equations Using Quadratic Elements.- Non-Uniform Flow in a High Speed Compressor due to Asymmetric Tip Clearance.- Deterministic Unsteady Vorticity Field in a Driven Axisymmetric Cavity Flow.- An Application of the Buffer Layer Technique to Computations of Flow in Turbomachinery.- CFD-Analysis of Secondary Flows and Pressure Losses in a NASA Transonic Turbine Cascade.- Numerical Optimisation of High Pressure Steam Turbine Stage.- An Experimental Investigation of the Unsteady Flow in a Two-Stage Low-Pressure Research Turbine.- About the Onset of Partload Instability of Swept and Unswept Axial Pump Blades.- Mixing Interface Algorithm for 3D Turbulent Flow Analysis of the GAMM Francis Turbine.- Study of Blade to Blade Flows and Circumferential Stall Propogation in Radial Diffusers and Radial Fans by Vortex Cloud Analysis.- New Approach to Radial Compressor Return Channel Design.- The Flow and Head Distribution within the Volute of a Centrifugal Pump in Comparison with the Characteristics of the Impeller without Casing.- Investigation of Unsteady Flow Phenomena in a Side Channel Vacuum Pump by Particle- image Velocimetry.

In 0.25Ga 0.75As growth on low-temperature thin buffer layers formed on GaAs (0 0 1) substrate

High-quality In 0.25Ga 0.75As films were grown on low-temperature (LT) ultra-thin GaAs buffer layers formed on GaAs (0 0 1) substrate by molecular beam epitaxy. The epilayers were studied by atomic force microscopy (AFM), photoluminescence (PL) and double crystal X-ray diffraction (DCXRD). All the measurements indicated that LT thin buffer layer technique is a simple but powerful growth technique for heteroepitaxy.