Layered Semiconductor Structures Research Articles

Impact of Solar Cell Infrastructures on Energy Efficiency in Power Grid Integration

Photovoltaic technology harvest electrical energy by stimulating liberated electrons within the semiconductor layers using solar radiation. Photovoltaic technology produces electrical energy by collecting electrons that are liberated in a semiconductor pn-junction by solar radiation. Photovoltaic solar cells have layered semiconductor structures and this study utilised for this objective. Current researches on energy storage with solar cells, focused to optimise the utilisation of the generated energy with cell efficiency. This study offers a thorough analysis of the energy efficiency of solar cells based on their infrastructures. The study involved obtaining computational visuals and doing efficiency verification. This was done by comparing the impact of different chemical structures on energy production. The MATLAB software was used with fixed parameters and varying efficiency. The results show that the Monocrystalline N-Type IBC model exhibits the maximum efficiency in terms of PV cell structure. The MIBC structure is more efficient than polycrystalline cells and also standard monotypes with high temperatures. This allows the cell to reflect itself and passivise the cell base, resulting in a 5% or more increase in energy production. Standard monotype cell has %16.2 efficiency and Monotype IBC has %20.1 efficiency results achieved with PVsyst and Matlab softwares. The results of the calculations were applied in real time and confirmed by testing the impact of structural differences on efficiency with real climate data

Rotation effects on propagation of shear horizontal surface waves in piezomagnetic-piezoelectric semiconductor layered structures

In this work, the propagation characteristics of shear horizontal waves in a piezomagnetic and piezoelectric semiconductor layered structure with rotation are studied. The dispersion equation of shear horizontal waves in the rotating multiferroic composite semiconductor is obtained in the framework of coupled field theory including Coriolis and centrifugal forces. By solving the dispersion equation in complex domain, the influence of rotation on the shear horizontal waves is deeply analyzed, and the effects of steady-state carrier concentration and biasing electric field in the presence and absence of rotation are obtained, respectively. Similar to the semiconductor properties, the results show that the rotation will also cause dispersion and attenuation of shear horizontal waves. Besides, the wave speed decreases with the increase of the angular rate and the frequency shift occurs. The rotation sensitivity at long wavelengths near the surface is greater than those at short wavelengths deep into the half-space. Importantly, the increase in wave speed and amplitude gain can be achieved by applying an appropriate biasing electric field regardless of rotation or non-rotation. Specially, several critical points such as attenuation from large to small with rotation, wave speed from decrease to increase and wave amplitude from attenuation to gain in the biasing electric field are obtained. The results could be helpful for the design of surface acoustic wave gyroscopes and piezoelectric semiconductor devices related to magnetic fields.

The optimized electrochemical deposition of bismuth-bismuth telluride layered crystal structures

Underpotential deposition, i.e. the cathodic deposition above reversible potential E(Men+/Me), produces an atomic layer of a metal on a semiconductor electrode, such as e.g. bismuth telluride. This phenomenon allows electrodeposition of superlattices formed of building blocks of a layered semiconductor structure joined by biatomic metal interlayer. This work outlines the optimized pulse potential controlled electrodeposition of (Bi2)m(Bi2Te3)n films produced under mentioned above technique. The influence on the morphology of the electrodeposited films of key-parameters as applied pulse frequency, duty cycle, a routine of sodium dodecyl sulfate introduction in the electrolyte is discussed. The optimized procedure comprises a short (about 10 s) cathodic pre-treatment at high overpotential of the cathodic reaction, the subsequent periodic switching for 120 min between potentials of electrodeposition and refinement at 0.1 Hz and 5% duty cycle with addition of surfactant 60 min after the start of the electrodeposition.

Picosecond‐Range Avalanche Switching Initiated by a Steep High‐Voltage Pulse: Si Bulk Samples Versus Layered pn Junction Structures

The study is devoted to picosecond‐range avalanche switching initiated by application of a steep high‐voltage ramp. It is focused on comparing layered semiconductor structures with bulk semiconductors. Experimental measurements and numerical simulations of ultrafast transients in layered Si structures with pn junctions and bulk Si samples are presented. The experimental setup allows measuring both device current and voltage during ultrafast high voltage switching transient with time resolution better than 50 ps. The focus is on the inner spatio‐temporal dynamics of electron–hole plasma generation, plasma concentration, and triggering mechanism. It is shown that in contrast to layered pn junction structures bulk samples with Ohmic contacts exhibit uniform switching in the whole structure volume. This comes at the price of steeper triggering pulse and absence of pulse sharpening capability. Our findings establish pulse‐triggered avalanche switching as a general method to create large (fractions of cubic mm) volumes of electron–hole plasma in semiconductors.

A Study on the Electronic Properties of SiOxNy/p-Si Interface

In this study, we investigated the electrical properties of Sn/SiOxNy/p-Si metal-insulator layer-semiconductor (MIS) structure. Silicon oxynitride (SiOxNy) thin film was grown on chemically cleaned p-Si substrate by the plasma nitridation process. The chemical composition and surface morphology of the thin film were analyzed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Electrical measurements of the devices (e.g. current-voltage (I-V ), capacitance-voltage (C-V ), capacitance and conductance-frequency characteristics (C-f and G-f )) were performed at room temperature. The characteristic parameters of the SiOxNy/p-Si interface such as energy position, interface state density and relaxation time constant were obtained from admittance measurements over a wide range of frequencies (from 1 to 500 kHz) for the values of the forward bias between 0.0 V ≤ V ≤ 1.1 V. The values of the interface state density and their relaxation time constant changed from 3.684 × 1013 cm− 2 eV− 1 to 3.216 × 1012 cm− 2eV− 1 and from 1.770 × 10− 5 s to 6.277 × 10− 7 s, respectively. The obtained values of the interface state density were compared to those of the oxides grown by the other techniques. The experimental results clearly show that the density and location of interface states has a significant effect on electrical characteristics of the MIS structure.

Near-field microwave tomography of planar semiconductor microstructures

A method for determining the parameters of a layered semiconductor structure, using the data obtained by near-field microwave probing with a micron-size lateral resolution, was developed and tested experimentally. We have measured a frequency spectrum of the impedance of a coaxial antenna formed on a test structure surface. The corresponding inverse problem has been solved based on the quasistatic theory for the impedance of a monopole antenna interacting with a layered medium, which was proposed earlier [A. N. Reznik and S. A. Korolyov, J. Appl. Phys. 119, 094504 (2016)]. This method was applied to a low-barrier Mott diode structure with a nearly 100 nm thick undoped layer grown on a conducting substrate GaAs. Computer simulation allowed us to establish the optimal frequency intervals and estimate the accuracy of determining the structure parameters. Measurements were taken in the frequency range of 0.1–67 GHz on commercially available equipment. Three antennas with a radius of the central conductor of 5.5, 11, and 25 μm, respectively, were used. The accuracy of the experimental evaluation of the layer thickness d and conductivity σ was ∼1–3%, and for the substrate conductivity, it came to about 15%. As an example, we also present the parameters σ and d in four points of the sample surface image. These data show strong lateral inhomogeneity of the structure under study.

Polarization Pyrometry of Layered Semiconductor Structures under Conditions of Low-Temperature Technological Processes

Principal issues of using pyrometry for temperature monitoring in low-temperature processes in the technology of production of semiconductor structures are considered by an example of growing mercury–cadmium–telluride (MCT) layers on the GaAs substrate by the method of molecular beam epitaxy. Optical and thermophysical models are proposed to describe the processes of radiant heat transfer in a vacuum chamber. Based on these models, it is demonstrated that radiation from the heater and the signal reflected from the chamber walls, which are comparable in magnitude with the measured radiation emitted by the sample, should be taken into account in interpreting data measured by a pyrometer. Methods of useful signal identification are found. Experiments on temperature measurement by a pyrometer mounted on the MCT growth chamber are performed. Results of these experiments are in good agreement with theoretical predictions.

Three-dimensional interface roughness in layered semiconductor structures and its effect on intersubband transitions

A general model for treating the effects of three dimensional interface roughness (IFR) in layered semiconductor structures has been derived and experimentally verified. Configurational averaging of the IFR potential produces an effective grading potential in the out-of-plane direction, greatly altering the energy spectrum of the structures. IFR scattering self-energy is also derived for the general case; when IFR is strong, its scattering effect is shown to dominate over phonon interaction and impurity scattering. When applied to intersubband transitions, the theoretical predictions explain the experimental observation of the anomalous energy shift and unusual broadening of the ISB transitions in III-Nitride thin-layered superlattices.

Thermal diffusivity and heat capacity of SiGe/Si superlattice from 374 K to 674 K

In this work, we examine the thermal diffusivity of Si/SiGe thin-film superlattice (SL) structures and use these results with pervious thermal conductivity results to calculate the heat capacity from 374 K to 674 K. The thermal properties of semiconductor layered structures can be altered through the use of SL structures. This alteration occurs through two possible mechanisms: increased phonon scattering due to rough interfaces and phonon zone folding due to boundary conditions of the propagating waves. Examining the heat capacity allows for the observation of phonon zone folding effects while limiting effects due to scattering. Structures studied here consist of SiGe SLs grown at different temperatures and with varying SL spacing allowing the examination of period and crystallinity effects on thermal properties. Previously reported results show that for SL structures both crystalline and polycrystalline have a thermal conductivity of approximately 1 W/mK measured over temperatures ranging from 374 K to 674 K. In this work, thermal diffusivity was measured through laser flash analysis, with crystalline SL structures showing values <1 mm2/s, while the thermal diffusivity of the polycrystalline structure was found to be twice that of the crystalline structure over the temperature range. In all instances, the heat capacities for the SL structures are found to be lower than that for a uniform thin film alloy, indicating a significant contribution of phonon dispersion modification to the heat capacity.

Ballistic Energy Transport in Oligomers.

The development of nanocomposite materials with desired heat management properties, including nanowires, layered semiconductor structures, and self-assembled monolayer (SAM) junctions, attracts broad interest. Such materials often involve polymeric/oligomeric components and can feature high or low thermal conductivity, depending on their design. For example, in SAM junctions made of alkane chains sandwiched between metal layers, the thermal conductivity can be very low, whereas the fibers of ordered polyethylene chains feature high thermal conductivity, exceeding that of many pure metals. The thermal conductivity of nanostructured materials is determined by the energy transport between and within each component of the material, which all need to be understood for optimizing the properties. For example, in the SAM junctions, the energy transport across the metal-chain interface as well as the transport through the chains both determine the overall heat conductivity, however, to separate these contributions is difficult. Recently developed relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy is capable of studying energy transport in individual molecules in the time domain. The transport in a molecule is initiated by exciting an IR-active group (a tag); the method records the influence of the excess energy on another mode in the molecule (a reporter). The energy transport time can be measured for different reporters, and the transport speed through the molecule is evaluated. Various molecules were interrogated by RA 2DIR: in molecules without repeating units (disordered), the transport mechanism was expected and found to be diffusive. The transport via an oligomer backbone can potentially be ballistic, as the chain offers delocalized vibrational states. Indeed, the transport regime via three tested types of oligomers, alkanes, polyethyleneglycols, and perfluoroalkanes was found to be ballistic, whereas the transport within the end groups was diffusive. Interestingly, the transport speeds via these chains were different. Moreover, the transport speed was found to be dependent on the vibrational mode initiating the transport. For the difference in the transport speeds to be explained, the chain bands involved in the wavepacket formation were analyzed, and specific optical bands of the chain were identified as the energy transporters. For example, the transport initiated in alkanes by the stretching mode of the azido end group (2100 cm(-1)) occurs predominantly via the CH2 twisting and wagging chain bands, but the transport initiated by the C=O stretching modes of the carboxylic acid or succinimide ester end groups occurs via C-C stretching and CH2 rocking bands of the alkane chain. Direct formation of the wavepacket within the CH2 twisting and wagging chain bands occurs when the transport is initiated by the N═N stretching mode (1270 cm-1) of the azido end-group. The transport via optical chain bands in oligomers involves rather large vibrational quanta (700-1400 cm(-1)), resulting in efficient energy delivery to substantial distances. Achieved quantitative description of various energy transport steps in oligomers, including the specific contributions of different chain bands, can result in a better understanding of the transport steps in nanocomposite materials, including SAM junctions, and lead towards designing systems for molecular electronics with a controllable energy transport speed.

Defect Formation and Strain Relaxation in graded GaPAs/GaAs, GaNAs/GaAs and GaInNAs/Ge Buffer Systems for high-efficiency Solar Cells

Transmission electron microscopy of cross-section specimens and high-resolution X-ray diffraction analyses have been applied to investigate the formation of defects and the relaxation of layer strain in step-graded GaPxAs1−x and GaNyAs1−y buffer layer systems grown by metal-organic vapour phase epitaxy on GaAs (001) substrates with 6° miscut towards (111)A. The investigations have been complemented by characterization of the layer surfaces employing optical microscopy. The comparison of the different buffer concepts reveals characteristic differences in the formation of defects and in the relaxation of tensile layer strain. For GaPAs layers dislocations and microtwins form, releasing the major part of the tensile misfit strain. In contrast, for GaNAs dislocations and microtwins are largely absent, at least in the upper part of the buffer structure, and microcracks are generated. Consequently, during subsequent growth of layers with tensile strain, strain relaxation and defect formation can be effectively hindered by introducing intermediate GaNyAs1−y layers with concentrations y > 2 % into a GaAs1−xPx buffer structure [1]. A similar concept can be used for layer systems with compressive strain, however, modified by using layers of differing alloy composition. The use of dilute nitride layers appears to offer a new concept for engineering defect distributions and layer strain in lattice-mismatched compound semiconductor layer structures. Such concepts are of particular interest not only but especially also for applications in high-efficiency III-V solar cells.

Characterization of vertical Mo/diamond Schottky barrier diode from non-ideal I–V and C–V measurements based on MIS model

In this study, we investigated non-ideal characteristics of a diamond Schottky barrier diode with Molybdenum (Mo) Schottky metal fabricated by Microwave Plasma Chemical Vapour Deposition (MPCVD) technique. Extraction from forward bias I–V and reverse bias C−2–V measurements yields ideality factor of 1.3, Schottky barrier height of 1.872eV, and on-resistance of 32.63mΩ·cm2. The deviation of extracted Schottky barrier height from an ideal value of 2.24eV (considering Mo workfunction of 4.53eV) indicates Fermi level pinning at the interface. We attributed such non-ideal behavior to the existence of thin interfacial layer and interface states between metal and diamond which forms Metal-Interfacial layer-Semiconductor (MIS) structure. Oxygen surface treatment during fabrication process might have induced them. From forward bias C–V characteristics, the minimum thickness of the interfacial layer is approximately 0.248nm. Energy distribution profile of the interface state density is then evaluated from the forward bias I–V characteristics based on the MIS model. The interface state density is found to be uniformly distributed with values around 1013eV−1·cm−2.

Determination of heteroepitaxial layer relaxation at growth temperature from room temperature X-ray reciprocal space maps

Lattice-mismatched heteroepitaxial growth in compound semiconductor layer structures, e.g. metamorphic buffers, is frequently used to combine materials with favorable properties. A detailed understanding of the strain relaxation during growth is important to optimize the buffer layers and to achieve high material quality. We present a method to determine the epilayer relaxation at growth temperature from room temperature X-ray diffraction measurements. For this the lattice parameters are measured with (004) and (224) reciprocal space maps along two orthogonal <110> directions to account for anisotropic misfit strain relaxation. Dissimilar thermal expansion coefficients of substrate and epilayer are taken into consideration when calculating the lattice parameters and epilayer relaxation parameters at growth temperature. The application of this method is discussed for GaAsxP1−x metamorphic buffer structure on Si with eight lattice-mismatched layers showing a relaxation parameter of over 130% at room temperature. All layers were found to be nearly 100% relaxed at growth temperature. The use of this method for other compound semiconductors is discussed.

Holographic arrays based on semiconductor microstructures

A concept of complex reflectivity modulation is proposed based on the electrical control of quantum well exciton resonances that influence the propagation of light in a layered semiconductor structure. By variation in exciton energies, both the intensity and the phase of reflected light can be fully controlled. Unlike previous devices, for full complex light modulation, the design is based on a single device in a single structure. The device allows complete 100$%$ intensity contrast and allows for the construction of small pixel sizes with fast response times.

Numerical Study on Metal Cavity Couplers for Terahertz Quantum-Well Photodetectors

Metal cavities composed of metal gratings, layered semiconductor structures, and bottom metal layers are proposed as light couplers for terahertz quantum-well photodetectors (QWPs). Due to a strong waveguide effect, the coupling properties of the metal cavities are significantly changed in comparison with the case of metal-grating-coupled terahertz QWPs. Numerical calculations show that there are two resonant coupling peaks for the sub-wavelength metal cavity couplers, with one located at the lower side of the first grating diffraction mode and the other at the upper side. A simple ray propagation picture explains the behaviors of the two resonant coupling peaks on a qualitative level. Our numerical data show that the coupling efficiencies of metal cavity couplers are several tens of times higher than that of the 45-degree facet coupling scheme.

Nature and spatial localization of electroluminescence sources in the metal-composite layer-semiconductor structures

Variations in the electroluminescence spectra and intensity of the metal-composite layer-semi� conductor (Au-SiOxNy(Si)-cSi) structures as functions of the characteristics of the luminescentactive tran� sition region at the interface of the cSi substrate and the SiOxNy(Si) composite layer are studied. New infor� mation on localization of the electroluminescence sources in the transition region is obtained. It is found that the transition region contains various luminescenceacti ve silicon inclusions, which contribute to both the shortwavelength ( λ ≈ 500-1000 nm) and longwavelength ( λ ≈ 1000-1600 nm) branches of electrolumines� cence. The effect of technological factors on the electroluminescence spectra, intensity, and quantum effi� ciency is analyzed.

General R-matrix approach for integrating the multiband [formula omitted] equation in layered semiconductor structures

A Fortran 90 code is provided for calculating the electron reflection and transmission coefficients in semiconductor heterostructures within the 14-band k ⋅ p approximation. The code may easily be adapted for use with any k ⋅ p model, including magnetic field and/or strain effects, for example. Numerical instability, which is problematic in type-II systems due to the simultaneous presence of propagating and evanescent states, is reduced by developing a novel log-derivative R-matrix approach based on the Jost solution to the k ⋅ p equation. Program summary Program title: multiband-kp Catalogue identifier: AEKG_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKG_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 7088 No. of bytes in distributed program, including test data, etc.: 90 237 Distribution format: tar.gz Programming language: Fortran 90 Computer: HP 128-node cluster (8 Intel 3.0 GHz Xeon processors per node) Operating system: RedHat Enterprise Linux 5.1 RAM: 11 MB Classification: 7.3, 7.9 External routines: LAPACK [1], ODE [2] Nature of problem: Calculating the electron transmission (or reflection) coefficient for single, double or multiple semiconductor quantum wells. Solution method: Makes use of a log-derivative reflection matrix approach which is based on obtaining the Jost solution to the multiband envelope function k ⋅ p equation. Restrictions: Accuracy depends on the limitations of the k ⋅ p model. In this implementation a “bare” 14-band model is used. Unusual features: By default Intelʼs math-kernel-library (MKL) [3] runs in serial mode. MKL also has built in parallel matrix algorithms which can be invoked without explicit parallelization in the source code. In this case all of the 8 CPUs in one node are used by the LAPACK subroutines. Running time: The given sample output, for the transmission coefficient at 750 different energies, required 376.51 CPU seconds (less than 7 minutes on a single CPU).

Quantized space-charge waves in quasi-2D

Hot-electron transport theory of standard semiconductor layered structures predicts the occurrence of negative differential resistance (NDR) associated with inter-valley electron transfer, negative effective mass, and real-space transfer. An analysis of the growth of quantized space-charge waves in single and double heterostructures in which no real-space transfer occurs is presented. It is shown that, in contrast to the situation in bulk material, growth is a complicated function of wave-vector because of quantization effects. As a consequence, growth is limited by quantization effects and diffusion to small wave-vectors. To illustrate the effect of quantization clearly, the analysis is limited to growth within the lowest sub-band. In spite of quantization differences, a quantitative comparison can be made by a simple rescaling. Growth is shown to be severely limited to small wave-vectors and its rate turns out to be independent of the details of quantization, being that for extreme confinement. The rate maximizes for a wave-vector of about 4 × 105 cm−1 corresponding to a frequency of 700 GHz. The criterion for domain formation in GaN systems becomes n02L≥1020cm-3, where n0 is the areal density of electrons and L is the sample length. As far as growth rate is concerned, the restriction to the lowest sub-band represents a worst-case scenario.

Effect of interfacial scattering on spin-dependent thermoelectric properties in diluted magnetic semiconductor layered structures

We propose a theoretical model to analyze spin-dependent surface and interface roughness effects on the magnetoelectric and thermomagnetoelectric properties of heterostructures based on diluted magnetic semiconductors (DMSs). Our calculations are based on spin-dependent Boltzmann semiclassical equation (BSE) in the relaxation-time approximation. We consider a typical GaMnAs/GaAs/GaMnAs trilayer in current in plane (CIP) geometry, and calculate the currents for parallel and anti-parallel alignments to deduce the magnetoresistance and thermopower. In the boundary conditions, we include the parameter P which denotes the surface roughness probability, while interfacial roughness is characterized by the ratio S, where both are spin-dependent. These kinds of roughness can either increase or decreases the electric and thermoelectric quantities. The magnitude of the effects depends on the surface and interface scattering parameters and the width of the layers. The magnetoresistance (MR) and spin polarization (SP) reach to about 100%, when the interface layer scatters only one type of majority or minority carriers and the influence of outer surfaces on the scattering carriers is infinitesimal. In particular, it is seen that the MR increases when the interface scattering decreases. The presented numerical results are specially useful for designing spin electronics devices.

Electromagnetic wave transmission and reflection by a quasi-periodic layered semiconductor structure

The specific features of electromagnetic wave reflection and transmission through the periodic and quasi-periodic layered semiconductor structures placed into a parallel plate waveguide are studied. The effect of the structure parameters and dissipation in semiconductor layers on the reflectance (transmittance) of electromagnetic wave is examined. The comparative analysis of the electromagnetic wave reflectance characteristics in the structures with different types of symmetry is carried out.