GaAs/InGaAsN heterostructures for multi-junction solar cells

Solar-cell heterostructures based on GaAs/InGaAsN materials with an InAs/GaAsN superlattice, grown by molecular beam epitaxy, are studied. A p-GaAs/i-(InAs/GaAsN)/n-GaAs p–i–n test solar cell with a 0.9-μm-thick InGaAsN layer has an open-circuit voltage of 0.4 V (1 sun, AM1.5G) and a quantum efficiency of >0.75 at a wavelength of 940 nm (at zero reflection loss), which corresponds to a short-circuit current of 26.58 mA/cm² (AM1.5G, 100 mW/cm²). The high open-circuit voltage demonstrates that InGaAsN can be used as a material with a band gap of 1 eV in four-cascade solar cells.     

Raman and infrared spectroscopy of GaN nanocrystals grown by chloride-hydride vapor-phase epitaxy on oxidized silicon

Raman and infrared spectroscopy were applied to study nanocrystalline GaN films grown by chloride-hydride vapor-phase epitaxy on SiO₂/Si(111) substrates at T=520°C. It was ascertained that GaN nanocrystals are formed on the oxidized silicon surface at a rate of 10^?2 nm/s. It was shown that the peaks in the Raman spectra E₂(high)=566 cm^?1 and A₁(LO)=730 cm^?1 correspond to the elastically strained GaN wurtzite structure. It was detected that a peak related to E₁(TO)=558 cm^?1 arises in the infrared spectra, which shows that elastic stresses in the nanocrystals are insignificant.     

The <Emphasis Type="Italic">U</Emphasis> peak in the DLTS spectra of <Emphasis Type="Italic">n</Emphasis>-GaAs irradiated with fast neutrons and 65-MeV protons

The origin of a broad U band in spectra obtained using deep-level transient spectroscopy (DLTS) of n-GaAs irradiated with fast neutrons and 65-MeV protons was investigated. It is believed that this band is presumably a superposition of two peaks related to two defects P2 and P3 which have been well documented in GaAs and reside within defect clusters. The DLTS spectra were calculated taking into account the nonuniform distribution of these defects in a sample and the built-in electric fields induced by corresponding inhomogeneities.     

Electrical characteristics of Au/<Emphasis Type="Italic">n</Emphasis>-GaAs structures with thin and thick SiO<Subscript>2</Subscript> dielectric layer

The aim of this study, to explain effects of the SiO₂ insulator layer thickness on the electrical properties of Au/n-GaAs Shottky barrier diodes (SBDs). Thin (60 Å) and thick (250 Å) SiO₂ insulator layers were deposited on n-type GaAs substrates using the plasma enganced chemical vapour deposition technique. The current-voltage (I–V) and capacitance-voltage (C-V) characteristics have been carried out at room temperature. The main electrical parameters, such as ideality factor (n), zero-bias barrier height (? _Bo ), series resistance (R _s ), leakage current, and interface states (N _ss ) for Au/SiO₂/n-GaAs SBDs have been investigated. Surface morphologies of the SiO₂ dielectric layer was analyzed using atomic force microscopy. The results show that SiO₂ insulator layer thickness very affects the main electrical parameters. Au/n-GaAs SBDs with thick SiO₂ insulator layer have low leakage current level, small ideality factor, and low interface states. Thus, Au/n-GaAs SBDs with thick SiO₂ insulator layer shows better diode characteristics than other.     

Determination of the fractal dimension for the epitaxial <Emphasis Type="Italic">n</Emphasis>-GaAs surface in the local limit

Atomic-force microscopy studies of epitaxial n-GaAs surfaces prepared to deposit barrier contacts showed that major relief for such surfaces is characterized by a roughness within 3–15 nm, although “surges” up to 30–70 nm are observed. Using three independent methods for determining the spatial dimension of the surface, based on the fractal analysis for the surface (triangulation method), its section contours in the horizontal plane, and the vertical section (surface profile), it was shown that the active surface for epitaxial n-GaAs obeys all main features of behavior for fractal Brownian surfaces and, in the local approximation, can be characterized by the fractal dimension D _f slightly differing for various measuring scales. The most accurate triangulation method showed that the fractal dimensions for the studied surface of epitaxial n-GaAs for measurement scales from 0.692 to 0.0186 μm are in the range D _f = 2.490?2.664. The real surface area S _real for n-GaAs epitaxial layers was estimated using a graphical method in the approximation δ → 0 δ is the measurement scale parameter). It was shown that the real surface area for epitaxial n-GaAs can significantly (ten times and more) exceed the area of the visible contact window.     

Modification of Au-Ti(W,Cr, TiB<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)-GaAs contacts properties caused by external influences

In this paper it is considered an influence of gamma-radiation ⁶⁰Co, microwave and ultrasonic processing on electro-physical properties and relaxation of internal stresses in Au-Ti(W, Cr, TiB _x )-GaAs contacts, based on GaAs plate, containing n-n ⁺ structures of GaAs. Correlation between radius of curvature of GaAs plates and contact parameters is detected. It is shown experimentally, that modification of electro-physical parameters of diodes with Schottky barrier, based on GaAs is specified by internal stresses relaxation in gallium arsenide structures with contacts.     

Optical properties of <Emphasis Type="Italic">p–i–n</Emphasis> structures based on amorphous hydrogenated silicon with silicon nanocrystals formed via nanosecond laser annealing

Silicon nanocrystals are formed in the i layers of p–i–n structures based on a-Si:H using pulsed laser annealing. An excimer XeCl laser with a wavelength of 308 nm and a pulse duration of 15 ns is used. The laser fluence is varied from 100 (below the melting threshold) to 250 mJ/cm² (above the threshold). The nanocrystal sizes are estimated by analyzing Raman spectra using the phonon confinement model. The average is from 2.5 to 3.5 nm, depending on the laser-annealing parameters. Current–voltage measurements show that the fabricated p–i–n structures possess diode characteristics. An electroluminescence signal in the infrared (IR) range is detected for the p–i–n structures with Si nanocrystals; the peak position (0.9–1 eV) varies with the laser-annealing parameters. Radiative transitions are presumably related to the nanocrystal–amorphous-matrix interface states. The proposed approach can be used to produce light-emitting diodes on non-refractory substrates.     

Phonon spectra of (GaAs)<Subscript>n</Subscript>(Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript>x</Subscript>As)<Subscript>m</Subscript> superlattices according to the Keating model

Phonon spectra of (GaAs)_n(Ga_1?xAl_xAs)_m superlattices were calculated using the Keating model with allowance made for long-range forces. The calculated summed longitudinal acoustical branches are consistent with observed experimental Raman doublets. The tendency towards a shift to higher frequencies of longitudinal optical modes as the number of GaAs monolayers increases, which is observed both in calculations and in experiments, is caused by localization of these modes within the GaAs layer.     

Experimental studies of the effects of atomic ordering in epitaxial Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>P alloys on their optical properties

The properties of epitaxial Ga _x In_{1 – x} P alloys with an ordered arrangement of atoms in the crystal lattice are studied by a number of spectroscopic methods. The alloys are grown by metal-organic chemical vapor deposition onto single-crystal GaAs(100) substrates. It is shown that, under conditions of the coherent growth of an ordered Ga _x In_{1 – x} P alloy on a GaAs(100) substrate, atomic ordering results in radical modifications of the optical properties of the semiconductor compared to the properties of disordered alloys. Among these modifications are a decrease in the band gap and an increase in the luminescence intensity. From the data of dispersion analysis of the infrared dispersion spectra and from ultraviolet spectroscopy data obtained in the transmittance–reflection mode of measurements, the basic optical characteristics, specifically, the dispersion of the refractive index and the high-frequency permittivity of Ga _x In_{1 – x} P alloys with ordering are determined. All of the experimental data are in good agreement with the developed theoretical concepts.     

Properties of Ge nanocrystals formed by implantation of Ge<Superscript>+</Superscript> ions into SiO<Subscript>2</Subscript> films with subsequent annealing under hydrostatic pressure

The influence of hydrostatic compression on the implantation-induced synthesis of Ge nanocrystals in SiO₂ host was studied. It is found that high-temperature annealing under pressure leads to retardation of Ge diffusion in SiO₂. It is shown that unstressed Ge nanocrystals are formed as a result of conventional annealing (under atmospheric pressure). Annealing under pressure is accompanied by formation of hydrostatically stressed Ge nanocrystals. The stress in Ge nanocrystals was determined from optical-phonon frequencies in the Raman spectra. The shift of Raman resonance energy (E₁, E₁ + Δ₁) corresponds to the quantization of the ground-state energy for a two-dimensional exciton at the critical point M₁ of germanium. It is ascertained that a photoluminescence band peaked at 520 nm is observed only in the spectra of the films which contain stressed Ge nanocrystals.     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Amorphization of the surface region in epitaxial <Emphasis Type="Italic">n</Emphasis>-GaAs treated with atomic hydrogen

It was found that the atomic-hydrogen treatment of n-GaAs epitaxial samples having initially high-quality surfaces both with a SiO₂ protective film on the n-layer surface and without it can lead to the amorphization of these surfaces and a thin (≈7 nm) surface layer, which is accompanied by the formation of a hydride phase. The lack of a hydrogen sublattice in the near-surface layer can imply that the main driving forces in the amorphization of epitaxial n-GaAs are short-range chemical interactions between hydrogen atoms and also between hydrogen atoms and atoms of the basic matrix of the crystal.     

Oscillations of induced photopleochroism in ZnO/GaAs heterojunctions

Anisotype and isotype ZnO/GaAs heterojunctions were formed by magnetron sputtering of thin n-ZnO:Al films on epitaxial layers of n-and p-GaAs. It is shown that the heterostructures obtained have a high photosensitivity (～5×10³ V/W at 300 K) in a wide spectral range (1.5–3.2 eV), which oscillates due to the radiation interference in thin ZnO films. Under oblique incidence of linearly polarized radiation on a ZnO film, photopleochroism is induced in a heterojunction, whose value oscillates within ～1–55% at θ=85°. The photopleochroism oscillations are also due to the radiation interference in the ZnO film. It is concluded that the heterojunctions obtained are promising candidates for selective photodetectors of linearly polarized radiation.     

Photoluminescence Studies of Si-Doped Epitaxial GaAs Films Grown on (100)- and (111)A-Oriented GaAs Substrates at Lowered Temperatures

The electrical properties and photoluminescence features of uniformly Si-doped GaAs layers grown on GaAs substrates with the (100) and (111)A crystallographic orientations of the surface are studied. The samples are grown at the same As₄ pressure in the growth temperature range from 350 to 510°C. The samples grown on GaAs(100) substrates possess n-type conductivity in the entire growth temperature range, and the samples grown on GaAs(111)A substrates possess p-type conductivity in the growth temperature range from 430 to 510°C. The photoluminescence spectra of the samples exhibit an edge band and an impurity band. The edge photoluminescence band corresponds to the photoluminescence of degenerate GaAs with n- and p-type conductivity. The impurity photoluminescence band for samples on GaAs(100) substrates in the range 1.30–1.45 eV is attributed to V_As defects and Si_As–V_As defect complexes, whose concentration varies with sample growth temperature. Transformation of the impurity photoluminescence spectra of the samples on GaAs(111)A substrates is interpreted as being a result of changes in the V_As and V_Ga defect concentrations under variations in the growth temperature of the samples.     

Chromium diffusion in gallium arsenide

Chromium diffusion in GaAs was studied by measuring the thickness of high-resistivity layers formed during diffusion of chromium (a deep acceptor) in n-GaAs. The dependence of the chromium diffusivity in GaAs on the temperature, arsenic-vapor pressure, conductivity type, and carrier density was determined. The temperature dependence of the diffusivity is described by the Arrhenius equation with the parameters D₀=8×10⁹ cm²/s and E=4.9 eV. The dependence of the diffusivity on the arsenic-vapor pressure is described by the expression \(D \propto P_{As_4 }^{ - m} \), where m≈0.4. The experimental data obtained are interpreted in terms of the concept of the dissociative mechanism of migration of Cr atoms in GaAs.     

Study of deep levels in GaAs <Emphasis Type="Italic">p–i–n</Emphasis> structures

An experimental study of the capacitance–voltage (C–V) characteristics and deep-level transient spectroscopy (DLTS) of p⁺–p⁰–i–n⁰ structures based on undoped GaAs, grown by liquid-phase epitaxy at two crystallization-onset temperatures T_o (950 and 850°C), with optical illumination switched off and on, are performed. It is shown that the p⁰, i, and n⁰ layers of epitaxial structures are characterized by the presence of defects with deep donor- and acceptor-type levels in concentrations comparable with those of shallow donors and acceptors. Interface states are found, which manifest themselves in the C–V characteristics at different measurement temperatures and optical illumination; these states form an additive constant. A distinct temperature dependence of the steady-state capacitance of the structures is revealed. It is found that the injection of minority carriers under an applied positive filling pulse and optical recharging lead to modification of the structure and, correspondingly, the DLTS spectra of the p⁺–p⁰–i–n⁰ structures. It is revealed that the p⁺–p⁰–i–n⁰ GaAs structures grown at T_o = 850°C are characterized by a lack of interface states and that the recharging of acceptor-type deep traps under illumination does not change the C–V characteristics. The conventionally measured DLTS spectra reveal the presence of two hole traps: HL5 and HL2, which are typical of GaAs layers.     

Ab initio calculations of phonon dispersion in CdGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>

The density of phonon states and the phonon dispersion in the Brillouin zone are calculated by the density functional method of. The displacements of atoms in the elementary cell are constructed for vibrations of the A, B, and E symmetries. The calculated frequencies of optical phonons are consistent with the experimentally determined frequencies from the IR-absorption (infrared) and Raman-scattering spectra. In the xy plane, the intersection of low-frequency optical phonons with acoustic phonons is observed.     

Photosensitivity of the Ni-<Emphasis Type="Italic">n</Emphasis>-GaAs Schottky barriers

The method of chemical deposition is used to form the structures with the Ni-n-GaAs Schottky barrier. The thickness of the Ni layers with a specular outer surface was varied within the range of 150–220 Å. It was experimentally observed for the first time that photosensitivity of the obtained barriers with the semitransparent Ni layers illuminated is practically absent in the Fowler region of the spectrum at hv = 0.9?1.5 eV. This circumstance is related mainly to the fact that, in this case, the Ni layer side of the structure was illuminated, and radiation with the photon energy hv < 1.3 eV was effectively reflected from the nickel surface. It is established that the developed Ni-n-GaAs structures can be used as high-efficiency wide-band photoconverters of both visible and ultraviolet radiation.     

Spin effects in magnetoresistance induced in an <Emphasis Type="Italic">n</Emphasis>-In<Subscript>x</Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs double quantum well by a parallel magnetic field

Magnetoresistance in n-In_xGa_1?xAs/GaAs (x ≈ 0.18) heterostructures with double quantum wells (DQWs) was studied in the magnetic field parallel to the DQW layer. Specific features of the magnetoresistance, related to the passing of the tunnel gap edges across the Fermi level, are revealed and studied. Agreement between the calculated and experimental positions of the observed features is obtained when the spin splitting of the energy spectrum is taken into account. Earlier, similar features were observed in the magnetoresistance of n-GaAs/Al_xGa_1?xAs DQW heterostructures, but the spin effects did not manifest themselves.     

Composition and Band Structure of the Native Oxide Nanolayer on the Ion Beam Treated Surface of the GaAs Wafer

Detailed information on GaAs oxide properties is important for solving the problem of passivating and dielectric layers in the GaAs-based electronics. The elemental and chemical compositions of the native oxide layer grown on the atomically clean surface of an n-GaAs (100) wafer etched by Ar⁺ ions have been studied by synchrotron-based photoelectron spectroscopy. It has been revealed that the oxide layer is essentially enriched in the Ga₂O₃ phase which is known to be a quite good dielectric as compared to As₂O₃. The gallium to arsenic ratio reaches the value as high as [Ga]/[As] = 1.5 in the course of oxidation. The Ga-enrichment occurs supposedly due to diffusion away of As released in preferential oxidation of Ga atoms. A band diagram was constructed for the native oxide nanolayer on the n-GaAs wafer. It has been shown that this natural nanostructure has features of a p–n heterojunction.