Single Crystal GaAs Substrates Research Articles

Conversion efficiency improvement of ELO GaAs solar cell, deposited on water soluble sacrificial buffer

We demonstrate the improvement of power conversion efficiency of an epitaxially-lifted-off single junction GaAs solar cell deposited on a water-soluble sacrificial buffer architecture by introducing an additional germanium (Ge) interlayer between GaAs and the fluoride buffer. The epitaxial lift-off (ELO) technique has been extensively used to separate III-V device layers from their single crystal GaAs substrates. However, conventional ELO requires the use of concentrated hydrofluoric acid (HF) for extended times to etch out the sacrificial layer, subsequently degrading the surface roughness of the parent wafer. As a result, the wafer has to undergo expensive and intensive chemical mechanical polishing (CMP) processes, costing about 25 % of a pristine 6-inch GaAs substrate. In our previous work, we demonstrated a method to eliminating the need for CMP post-processing by using water-assisted ELO (H2O-ELO). A water-soluble, 3-layer buffer architecture was developed using alkaline earth compounds. However, devices suffered low performance due to a high defect density in the GaAs active layers. A Ge interlayer was introduced to provide a more favorable surface energy for GaAs growth, which leads to an improvement of GaAs crystal quality. The Ge deposition conditions were optimized to achieve a high-quality Ge layer on triple-layer fluoride buffer. Single junction GaAs devices fabricated on Ge/(Ca,Sr)F2/BaF2/(Ca,Sr)F2 showed improvement of solar cell performance parameters featuring increases in Voc by 23.7 %, and fill factor (F.F.) by 4.9 %, which results in an overall improvement of power conversion efficiency from 10.3 % to 12.69 %.

Структура, морфология и фотоэлектрические свойства гетероструктуры n-GaAs--p-(GaAs)1--x(Ge2)х

The paper studies the structural properties of the thin-film (GaAs)1--х(Ge2)x solid solutions grown by liquid-phase epitaxy on single crystal GaAs substrates. The obtained epitaxial layers were 10 μm thick, had a 0.17 Ω · cm resistivity of p-type conductivity. X-ray diffraction analysis showed that the obtained films are monocrystalline with nanoinclusions with block sizes up to 49 nm, with a crystallographic orientation (100), and have a sphalerite structure of the ZnS type. Morphological studies using AFM showed that the nanocones on the surface of the epitaxial (GaAs)1--х(Ge2)x layers are due to Ge atoms in them. The analysis revealed that the diameter of the nanocones' bases is in the range from 70 to 90 nm, and the height is from 3 to 12 nm. The sizes of such nanocones, determined by the X-ray diffraction method, were close to these values. The photosensitivity spectra of these solid solutions exhibit peculiar fluctuations due to various complexes of charged atoms in them. Analysis of the photosensitivity spectrum of n-GaAs--p-(GaAs)1--х(Ge2)x heterostructures using the Wolfram Mathematics 7 program showed that the spectrum consists of three peaks due to As--Ge, Ge--Ge and Ga--Ge

Investigation into Microwave Absorption in Semiconductors for Frequency-Multiplication Devices and Radiation-Output Control of Continuous and Pulsed Gyrotrons

The results of experimental investigation into the dielectric losses in GaAs, InP:Fe, and Si semiconductor crystals in the millimeter wavelength range (80–260 GHz) using the original precise method of measuring the reflectance and dielectric-loss tangent tanδ based on open high-quality Fabry–Perot cavities are presented. It is shown that the losses in the frequency range from 100 to 260 GHz in ultrapure semiconductor single-crystal GaAs substrates are mainly determined by lattice absorption, while the main loss mechanism in single-crystal silicon is absorption by free carriers; herewith, tan δ ≈ (1–2) × 10–4 even for a noticeable, at a level of 1012 cm–3, free carrier concentration. In contrast with GaAs and Si, tanδ in compensated InP:Fe crystals is almost independent of frequency in the range from 100 to 260 GHz, which is associated with the material conductivity and optimization of microwave semiconductor devices, in particular, frequency-multiplication devices and devices of the controlled emission output of continuous and pulsed gyrotrons.

GaAs solar cells grown on intentionally contaminated GaAs substrates

III-V materials such as GaAs and GaInP have some of the best electronic and optical properties of any semiconductor materials, but deposition of these materials relies on high-quality single crystal GaAs substrates for their superior performance. Unfortunately, the cost of these substrates makes these high-efficiency devices only accessible in high-value or niche markets. Here, we explore the effect of growing bulk GaAs crystals with lower purity input materials in order to reduce their cost. We observe that intentional impurities added to the melt before the crystal growth occurs segregate to the top of the boule. Single junction GaAs solar cells grown on substrates made from contaminated boules showed no performance degradation compared to a high-purity control substrate. These results suggest that lower purity Ga and As source materials can be used during crystal growth to reduce the cost of substrates.

Исследование микроволнового поглощения в полупроводниках для устройств умножения частоты и управления выводом излучения непрерывных и импульсных гиротронов

Abstract The results of experimental investigation into the dielectric losses in GaAs, InP:Fe, and Si semiconductor crystals in the millimeter wavelength range (80–260 GHz) using the original precise method of measuring the reflectance and dielectric-loss tangent tanδ based on open high-quality Fabry–Perot cavities are presented. It is shown that the losses in the frequency range from 100 to 260 GHz in ultrapure semiconductor single-crystal GaAs substrates are mainly determined by lattice absorption, while the main loss mechanism in single-crystal silicon is absorption by free carriers; herewith, tan δ ≈ (1–2) × 10^–4 even for a noticeable, at a level of 10^12 cm^–3, free carrier concentration. In contrast with GaAs and Si, tanδ in compensated InP:Fe crystals is almost independent of frequency in the range from 100 to 260 GHz, which is associated with the material conductivity and optimization of microwave semiconductor devices, in particular, frequency-multiplication devices and devices of the controlled emission output of continuous and pulsed gyrotrons.

Synthesizing InxGa1−xAs using molten In and GaAs by the sessile drop method at 400–600 °C

We studied the surface reaction between metallic In and a single-crystal GaAs substrate at 400–600 °C induced by the improved sessile drop method. Studying the surface morphology and microstructure using scanning electron microscopy, we found that many large particles had formed on the GaAs substrate, indicating a violent reaction between the In and GaAs. This reaction became more violent with increasing temperature. Characterizing the generated phases by X-ray diffraction and transmission electron microscopy, we identified the particles as InxGa1−xAs compounds. Our results show that indium (In) reacted with single-crystal GaAs through this method to form InxGa1−xAs compounds. Moreover, during the synthesis of InxGa1−xAs by the sessile drop method, the molten In and GaAs exhibited wettability at 400–600 °C. This experiment lays the foundation for synthesizing InxGa1−xAs compounds only using the metal In and GaAs substrate by a simple method.

Ellipsometric and reflectometric characterization of thin films exhibiting thickness non-uniformity and boundary roughness

In this paper epitaxial ZnSe thin films prepared by molecular beam epitaxy onto GaAs single crystal substrates exhibiting two defects, i.e. boundary roughness and thickness non-uniformity, are optically characterized using a combination of spectroscopic ellipsometry and near-normal spectroscopic reflectometry. The influence of boundary roughness is included into optical quantity formulae by the combination of the scalar diffraction theory and Rayleigh–Rice theory. Very thin overalyers modelled by rough thin films with identically rough boundaries are taken into account on the upper boundaries of the ZnSe thin films. Two approximations are used to express the local reflection coefficient of the rough ZnSe thin films covered with the overlayers within combination of both the theories. Thickness non-uniformity is incorporated by means of averaging the elements of the unnormalized Mueller matrices. The universal dispersion model of the optical constants of the ZnSe thin films based on parametrization of the joint density of electronic states is used. The spectral dependencies of the optical constants of the ZnSe thin films are determined within the wide spectral range (0.12–8.7eV). Moreover, the mean thickness of the ZnSe thin films and thickness of overlayers are determined together with the other structural parameters characterizing the defects. The values of roughness parameters, determined by the optical method, are verified by a comparison with results achieved by atomic force microscopy. It is also shown that the approximations of the local reflection coefficient presented are usable for processing the experimental data.

Regularities of ion-beam-induced crystallization and properties of InAs-QD/GaAs(001) semiconductor nanoheterostructures

A novel method of the ion-beam-induced crystallization of quantum-size semiconductor eterostructures has been proposed. Using atomic force (AFM) and transmission electron microscopy (TEM), the capacitance–voltage (C–V) method, and photoluminescence (PL) measurements, we have studied the main regularities of ion-beam-induced crystallization and the properties of InAs quantum dots (QDs) on GaAs single crystal substrates with (001) crystallographic orientation as functions of temperature, ion current, and ion energies. It is shown that, in order to grow InAs hut structures, the optimal temperature range is from 500 to 550°C. An intense development of dome clusters is observed at higher temperatures. It is found that an increase in the ion current in an interval from 60 to 120 μA at a temperature of 500°C and an ion energy of 150 eV inconsiderably affects the average sizes of nanoislands. It is shown that, in an ion energy range from 150 to 200 eV and at a constant temperature of the process of 500°C and bam current of 120 μA, bands of stability of medium sizes (∼15 nm) and surface density (∼1011 cm–2) are observed.

Investigation of γ′-Fe4N thin films deposited on Si(1 0 0) and GaAs(1 0 0) substrates by facing target magnetron sputtering

γ′-Fe4N thin films with good soft magnetic properties were successfully prepared on single crystal Si(100) and GaAs(100) substrates with Fe buffer layer by facing target magnetron sputtering. The microstructure and magnetic properties of the samples at low temperature were investigated. Both structural and magnetic properties of the films were shown to be influenced by the substrate. Specifically, the film deposited on the GaAs exhibited a strong preferred orientation along the (100) plane, while the film deposited on Si exhibited higher saturation magnetization (Ms) and lower coercivity (Hc). The results suggest that the coercivity is probably related to the intrinsic stress and that Ms increases whereas Hc decreases with decreasing lattice mismatch. The Mr/Ms ratio decreased with decreasing temperature due to the effect of the thermal disturbance on the magnetization. Both films had a single easy magnetized direction parallel to the substrate plane.

Single-Crystal CdTe Homojunction Structures for Solar Cell Applications

We report two different CdTe homojunction solar cell structures. Single-crystal CdTe homojunction solar cells were grown on GaAs single-crystal substrates by metalorganic chemical vapor deposition. Arsenic and iodine were used as dopants for p-type and n-type CdTe, respectively. Another homojunction solar cell structure was fabricated by growing n-type CdTe directly on bulk p-type CdTe single-crystal substrates. The electrical properties of the different layers were characterized by Hall measurements. When arsine was used as arsenic source, the highest hole concentration was ~6 × 1016 cm–3 and the activation efficiency was ~3%. Very abrupt arsenic doping profiles were observed by secondary ion mass spectrometry. For n-type CdTe with a growth temperature of 250°C and a high Cd/Te ratio the electron concentration was ~4.5 × 1016 cm–3. Because of the 300 nm thick n-type CdTe layer, the short circuit current of the solar cell grown on the bulk CdTe substrate was less than 10 mA/cm2. The open circuit voltage of the device was 0.86 V. According to a prediction based on measurement of short circuit current density (J sc) as a function of open circuit voltage (V oc), an open circuit voltage of 0.92 V could be achieved by growing CdTe solar cells on bulk CdTe substrates.

Temperature dependences of surface magnetoelastic constants of ultrathin Fe/GaAs (001) films

The magnetoelastic constants of epitaxial iron films prepared by dc magnetron sputtering on single crystal GaAs (001) substrate in argon atmosphere and covered with a protective Si layer have been investigated in the temperature range 10–300 K by means of the strain modulated ferromagnetic resonance. It has been shown that the magnetoelastic constants strongly depend on the thickness of the film. The surface components of the magnetoelastic constants have been determined and analyzed within the Néel and dipolar models. The proposed analysis of experimental data gives chance for deeper insight into mechanisms responsible for magnetostriction of iron thin films.

Metastable bcc-Ni and bcc-NiFe Single-Crystal Films Prepared on GaAs Single-Crystal Substrates With Different Orientations

Ni and Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> (at. %) epitaxial thin films are prepared on GaAs single-crystal substrates of (100), (111), and (110) orientations by ultra-high vacuum RF magnetron sputtering. The film growth behavior and the crystallographic properties are studied by reflection high energy electron diffraction and X-ray diffraction. Metastable bcc-Ni and metastable bcc-NiFe single-crystals of (100), (111), and (110) orientations respectively nucleate on GaAs substrates of (100), (111), and (110) orientations, where the bcc structure is stabilized through hetero-epitaxial growth. With increasing the film thickness, the bcc structure in Ni and NiFe films starts to transform into more stable fee structure. The bcc-fcc phase transformation orientation relationships observed for Ni and NiFe films on GaAs(100) and (111) substrates are fcc{100}{011} ||bcc{100}〈 001〉, whereas those for Ni and NiFe films on GaAs(110) substrates are fcc(111)[112̅] ||bcc(110)[11̅0] and fcc(111)[1̅1̅2] ||bcc(110)[11̅0]. The resulting films thus consist of mixtures of bcc and fee crystals. Metastable bcc structure is more stable in Ni film than in NiFe film on all the GaAs substrates. The lattice constants of bcc-Ni and bcc-NiFe crystals are determined to be α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bcc-Ni</sub> = 0.292 nm and α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bcc-NiFe</sub> = 0.291 nm, respectively. The in-plane magnetization properties of Ni and NiFe films grown on GaAs(110) substrates are reflecting the magnetocrystalline anisotropies of bcc-Ni and bcc-NiFe crystals, respectively.

Structural Analysis of MgO/Fe Bi-Layer Films Epitaxially Grown on GaAs Single-Crystal Substrates with Different Orientations

MgO/Fe bi-layer films were prepared on GaAs single-crystal substrates of (100), (110), and (111) orientations by ultra-high vacuum radio-frequency magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth process and the crystallographic properties were investigated by reflection high energy electron diffraction and X-ray diffraction. Fe single-crystal layers of (100), (110), and (111) orientations epitaxially grow on GaAs substrates of (100), (110), and (111) orientations, respectively. MgO(100) single-crystal layers are obtained on the Fe(100) layers, whereas MgO layers epitaxially grow on the Fe(110) layers with two (111) variants whose orientations are rotated around the film normal by 180° each other. On the contrary, MgO polycrystalline layers are formed on the Fe(111) layers. The out-of-plane lattice spacing of MgO(100) layer is expanded, while the in-plane spacing is shrunk with respect to the lattice constant of bulk MgO crystal due to an accommodation of lattice mismatch with the Fe(100) layer. The strain in the MgO(100) layer decreases by employing a higher substrate temperature for thin film deposition.

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100) B3 , (110) B3 , and (111) B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100) B3 substrates, Fe(100) bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110) bcc and Fe(111) bcc single-crystal films are respectively obtained on GaAs(110) B3 and GaAs(111) B3 substrates at RT–300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0–9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc–Fe crystal.

Properties of single crystal Fe1−xGax thin films

Molecular beam epitaxy was used to deposit single crystal thin film Fe1−xGax samples on ZnSe buffer layers grown on (001) and (110) single crystal GaAs substrates. The crystal quality of the GaAs surface and each deposited layer was monitored in situ by reflection high energy electron diffraction. The magnetic properties of the samples were characterized by vibrating sample magnetometry and ferromagnetic resonance (FMR). The FMR linewidth increases dramatically with Ga concentration while the cubic anisotropy term K1 switches sign.

X-ray photoelectron and auger spectroscopy analysis of Ge:Mn-based magnetic semiconductor layers

Thin (∼60 nm) germanium layers supersaturated with a manganese impurity of 10–16 at % have been studied by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The layers have been fabricated by pulsed laser deposition onto a semi-insulating single-crystal GaAs substrate. The results of XPS analysis of the Ge:Mn layers reveal a change in the line shape of germanium and manganese (2p) in the surface region compared to deeper layers, which indicates a transition from the oxidized form of the base material (Ge2+ and Ge1+) and impurity (Mn2+) near the surface to the unoxidized state of germanium (Ge 0) and manganese (Mn0) in the interior of the layer. The XPS spectra of the valence electrons of the Ge:Mn structure indicate that the density of states in the valence band of the ferromagnetic Ge:Mn structures is caused not only by mechanical mixing of germanium and manganese. The composition of heterogeneous inclusions in Ge:Mn films has been studied using scanning Auger microscopy.

Caracterización eléctrica de películas delgadas de Al2O3 depositadas sobre GaAs por la técnica de rocío pirolítico

Se estudiaron las características eléctricas de películas delgadas de óxido de aluminio preparadas por la técnica de rocío pirolítico ultrasónico. Las películas delgadas se depositaron a partir de una solución de acetilacetonato de aluminio en N,N-dimetilformamida sobre substratos monocristalinos de GaAs (100) tipo-p. La temperatura de depósito fue de 300 a 600 ºC. Las propiedades eléctricas de las películas en función de la temperatura de substrato se determinaron por medidas de capacitancia y corriente contra voltaje mediante la incorporación de las películas en estructuras tipo MOS (metal-óxido-semiconductor). La densidad de estados de interfaz resultó del orden de 10(12) 1/eV-cm² y el dispositivo MOS soportó campos eléctricos mayores a 5MV/cm, sin mostrar rompimiento dieléctrico. El índice de refracción se determinó por elipsometría a 633nm, con un valor máximo del orden de 1.64.

Interfacial and mechanical characterization of wafer-bonded GaSb/amorphous α-(Ga,As)/GaAs structure for GaSb-on-insulator applications

In this study, the feasibility of using wafer-bonding technology to fabricate a GaSb semiconductor on GaAs substrates for potentially creating a GaSb-on-insulator structure has been demonstrated. A GaSb wafer has been bonded on two types of GaAs substrates: (1) a regular single crystal semi-insulating GaAs substrate and (2) the GaAs wafers with pre-deposited low-temperature amorphous α-(Ga,As) layers. The microstructures and interface adhesion studies have been carried out on these wafer-bonded semiconductors. It has been found that the GaSb-on-α-(Ga,As) wafers have shown enhanced interface adhesion and lower temperature bonding capability.

Microstructure and morphology evolution in chemically deposited semiconductor films: 4. From isolated nanoparticles to monocrystalline PbS thin films on GaAs(100) substrates

Thin lead sulfide films were grown on single crystal GaAs(100) substrates by chemical deposition using Pb(NO3)2 and CS(NH2)2 with excess of NaOH in aqueous solution at a range of deposition temperatures 0–50 °C. The microstructure and morphology evolution were studied as a function of the deposition conditions, resulting in a wide range of microstructures. Ultrahigh resolution scanning electron microscopy and atomic force microscopy indicated a systematic change in particle shape and surface morphology as a function of deposition temperature and deposition time. X-ray diffraction of 200–500 nm thick films indicated a dominant texture throughout the deposition temperature range. At deposition temperatures above 40 °C, single crystal films were obtained. Cross-sectional transmission electron microscopy analyses showed a unique (011)PbS||(100)GaAs and [100]PbS||[011]GaAs orientation relationship.

Kinetics of GaN radical-beam gettering epitaxy on GaAs substrates

The growth of thin GaN layers on single-crystal GaAs substrates during annealing in an atmosphere containing nitrogen atoms (radicals) has been analyzed in terms of a kinetic model. The nitridation of the surface of GaAs substrates has been studied by x-ray photoelectron spectroscopy. The results demonstrate that, at annealing temperatures below 500°C, both Ga-N and As-N bonds are formed on the substrate surface. Above 500°C, only Ga-N bonds are formed. The growth of GaN layers is shown to follow a quasi-epitaxial mechanism, in accordance with the proposed model.