Heteroepitaxial Growth Of GaAs Research Articles

From Salt to Electronics: Heteroepitaxy and GaAs Solar Cells

AbstractThis paper presents work on the heteroepitaxy of salts, specifically fluorides, on semiconductors and heteroepitaxy of semiconductors on salts. Fluorides layers are deposited on commercial Gallium Arsenide (GaAs) wafers followed by the heteroepitaxial growth of GaAs using metal‐organic chemical vapor deposition (MOCVD). The fluoride layers consist of 2 lattice‐engineered layers of alkaline‐earth compounds to match with GaAs, and are used to sandwich another alkaline‐earth compound with higher water‐solubility as a sacrificial layer. The triple fluoride layers enable liftoff of free‐standing semiconductor films which can be further transferred to desirable substrates. 2D‐X‐ray Diffraction (2D‐XRD) measurements confirm epitaxial growth of both the fluorides and the subsequently grown GaAs films. Single junction (SJ) solar cell devices based on thus prepared films show a power conversion efficiency (PCE) of 10.3% under 1 sun illumination. After the completion of device fabrications, the GaAs film is lifted off from the substrate by a novel water‐assisted epitaxial liftoff (H2O‐ELO) technique and transferred to a cheaper substrate. The original GaAs wafer is recycled and reused twice. Devices based on reused substrates show no significant degradation in performance. The semiconductor‐salt‐semiconductor scheme has great implications in high‐performance, flexible, and large‐area electronics.

Heteroepitaxial growth of GaAs on (100) Ge/Si using migration enhanced epitaxy

In this paper, heteroepitaxial growth of GaAs on nominal (100) Ge/Si substrate was investigated. The root-mean square surface roughness of the sample where the first few monolayers of the GaAs were nucleated by migration enhanced epitaxy (MEE) is four times smaller compared to the sample without such a process, indicating better surface planarity. From the (004) x-ray diffraction rocking curve measurement, the full width at half maximum of the GaAs layer nucleated by MEE is 40% lower compared to that of the GaAs layer without such a process, indicating better crystal quality. Furthermore, it was found that the sample where the GaAs layer was nucleated by MEE experienced early relaxation. As the MEE process promotes two-dimensional growth, the GaAs layer where nucleation was initiated by such a process has fewer islandlike formations. This leads to a pseudomorphically grown GaAs layer, which experiences higher strain compared to the GaAs layer with more islandlike formations, where most relaxation occurs on the free surface of the islands. Therefore, for the same layer thickness, the GaAs layer on (100) Ge/Si substrate where nucleation was initiated by MEE relaxed first.

Heteroepitaxial growth of GaAs on Si by MOVPE using a-GaAs/a-Si double-buffer layers

In this work, a buffer structure of a-GaAs/a-Si double amorphous layers was used for the epitaxial growth of GaAs on Si substrate by metal-organic vapor phase epitaxy. The atomic force microscopy (AFM) images exhibited that the root-mean-square (RMS) value of the surface morphology was only 1.331 nm. The full-width at half-maximum (FWHM) of the double crystal X-ray rocking curve in the (4 0 0) reflection was about 102 arcsec. The top-surface etch-pit density (EPD) revealed by molten KOH etching was lower than 10 6 cm −2. From the material characterizations described above, the use of a-GaAs/a-Si buffer structure was found to be a very simple and effective approach for obtaining high-quality GaAs on Si for solar cell applications.

Growth of high-quality GaAs on Ge∕Si1−xGex on nanostructured silicon substrates

Heteroepitaxial growth of GaAs∕Ge∕SiGe films on submicrostructured Si substrates is reported. One-dimensional, nanometer-linewidth, submicrometer period features were fabricated in Si substrates using interferometric lithography, reactive ion etching, and wet-chemical etching techniques. The quality of the GaAs layers grown on these structures was investigated using high-resolution x-ray diffraction, transmission electron microscopy, scanning electron microscopy, photoluminescence, and etch pit density measurements. The defect density of GaAs epilayers grown on submicrostructured Si at ∼6×105cm−2 was two orders of magnitude lower compared with that grown on planar silicon. The optical quality of the GaAs∕Ge∕SiGe on submicrostructured Si was comparable to that of single crystal GaAs.

GaAs Growth on Micro and Nano Patterned Ge/Si1-XGeX and Si Surfaces

AbstractWe show heteroepitaxial growth of GaAs on Ge/SiGe grown on nanometer-scale grating structures. Conventional lithography techniques were combined with reactive ion and wet-chemical etching to fabricate 1-D patterns of silicon posts. The quality of the GaAs layers was investigated using high-resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), photoluminescence (PL) and etch pit density (EPD) measurements. Our results show significant improvement in the quality of heteroepitaxial layers grown on nano patterned structures compared to those on the unpatterned silicon. The optical quality of the GaAs/Ge/SiGe on nano-scale patterned silicon was comparable to that of single crystal GaAs.

Beam Induced Lateral Epitaxy of GaAs on a GaAs/Si Template

ABSTRACTBeam induced lateral epitaxy (BILE) on truncated ridges was applied to the heteroepitaxial growth of GaAs on a Si substrate. A GaAs buffer layer was formed on the Si substrate, and then this GaAs/Si template was used as a substrate for the BILE process. As a result, overgrown regions of GaAs of widths as large as 6.5 μm were grown laterally from the sides of the truncated ridges. The growth regions had a flat, smooth top surface consisting of a (111) facet. Although stacking faults from the GaAs/Si template remained in the growth region, which are unfavorable for device applications, the lateral grown region has no dislocations. Thus, the BILE method is useful for reducing dislocations in heteroepitaxy.

GaAs MESFETs fabricated on Si substrates using a SrTiO3 buffer layer

Heteroepitaxial growth of GaAs on an Si substrate has been achieved through the use of crystalline SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (STO) and amorphous SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffer layers. The buffer layers serve to accommodate some of the lattice mismatch between the substrate and the GaAs epilayers. Field-effect transistors fabricated in the GaAs epilayers show performance comparable to similar devices fabricated on GaAs substrates. The mobility in the GaAs/STO/Si sample is 2524 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs compared to a GaAs/GaAs sample with mobility of 2682 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs. A 0.7 μm gate length device has I/sub d max/ of 367 mA/mm and G/sub m max/ of 223 mS/mm. These devices also have good RF performance with f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 14.5 GHz and class AB power density of 90 mW/mm with an associated power-added efficiency of 38% at 1.9 GHz. This RF performance is within experimental error of similar devices fabricated on GaAs substrates. Preliminary reliability results show that after 800 h at 200/spl deg/C, the GaAs/STO/Si sample showed 1.2% degradation in drain current.

Heteroepitaxial growth of GaAs on Si substrates using low energy Ga and As ion beams

GaAs thin films were heteroepitaxially grown on (1 0 0) and (1 1 1) Si substrates at a temperature of 500°C using Ga and As ion beams at energies lower than about 50 eV. The growth rates increased with decreasing ion energy. However, the GaAs films were heteroepitaxially grown on the (1 0 0) and (1 1 1) Si substrates by mixed beams containing larger amounts of the neutral Ga and As molecular beams added to the ion beams even when the ion beams were accelerated to energies higher than 60 eV.

Suppression of twins in GaAs layers grown on a GaP(111)B substrate by liquid phase epitaxy

Heteroepitaxial growth of GaAs on GaP(111)B was demonstrated by liquid phase epitaxy. In order to suppress the generation of rotational twins, initial saturation conditions of GaAs solution contacting with a GaP substrate were investigated at 800°C. Supersaturated and undersaturated solutions caused a number of twins in the grown GaAs layer, while saturated solution led only a few percents in volume. It was concluded that near equilibrium contact is effective to reduce the twins.

Growth mode transition processes in a GaAs/lnP system studied by scanning tunneling microscopy

The growth mode transition processes of GaAs heteroepitaxial growth on an InP substrate were investigated using a scanning tunneling microscopy (STM) multi-chamber system equipped with a molecular beam epitaxy facility. During the initial stage, the growth mode transition from two-dimensional (2D)-to three-dimensional (3D)-island growth occurred with more than 2.0 ML of GaAs deposition onto the InP surface. This 3D-island structure gradually became flat upon increasing the amount of GaAs deposition, and finally recovered to a GaAs 2D structure. Furthermore, an extended dislocation line can be observed in the STM image of the recovered GaAs surface.

Growth Process in Initial Stage of GaAs/GaP(001) by Molecular Beam Epitaxy

The growth-mode transition of strained GaAs heteroepitaxial growth on GaP(001) is studied in detail using reflection high-energy electron diffraction (RHEED). The lattice parameter oscillation during layer-by-layer growth and the increase of the critical thickness of the growth-mode transition with increase of the growth rate reflect the elastic strain relaxation around the edge of two-dimensional nuclei and the existence of the kinetically controlled evolution of islands, respectively. The transformation from a strained metastable layer structure to a relaxed stable island structure is revealed through analyses of the RHEED pattern. We propose a model to explain the increase of the critical thickness, which assumes two processes at the surface, the formation of a metastable layer and its transformation to islands.

GaAs heteroepitaxial growth on vicinal Si(110) substrates by molecular beam epitaxy

We report the growth mechanism of GaAs on vicinal Si(110) substrates by molecular beam epitaxy, and investigate the effects of off-angle and -direction on the crystalline quality of grown films. The quality was improved with the increase of the off-angle and strongly influenced by the off-direction. Off-direction toward the [001] was better than that toward the [11̄0] for obtaining high-quality films. The cause is related to the structure and density of the steps. We infer from reflection high-energy electron diffraction results that GaAs films on vicinal Si(110) with off-angles above 2° toward the [001] direction grew up to a thickness of 3 nm in step-flow-like mode. The spotty reflection high-energy electron diffraction patterns characteristic of three-dimensional growth in GaAs/Si(100) were not observed at a thickness above 3 nm.

Heteroepitaxial Growth of GaAs on (100) GaAs and InP by Selective Liquid Phase Epitaxy

Experimental conditions to obtain heteroepitaxial (100) GaAs/InP structure were studied using the selective liquid phase epitaxy (SLPE) technique. It was observed that maximum epitaxial layer overgrowth (ELO) can be obtained by aligning the growth seed along the 75° off [011] direction. The ELO width decreased as the mask width decreased, probably due to As depletion above the oxide mask, which could be partly compensated by convecting the melt. The ELO width was substantially increased by adding Si, P or Se to the Ga growth melt. Se addition, in particular, prevented meltback of substrates during the growth of GaAs on InP, resulting in a continuous specular GaAs film. Cross-sectional transmission electron microscopy (TEM) study of the layers revealed that propagation of the misfit dislocations at the GaAs/InP interface is effectively suppressed by the presence of the SiO2 mask, except for some dislocations that propagate toward the top of the mask by bending at the mask edge.

Scanning Tunneling Microscopy of Initial Stages of GaAs Heteroepitaxy on Lattice-Mismatched Substrates

The island-formation processes of GaAs heteroepitaxial growth on InP (lattice constant ( a) is 3.7% larger than that of GaAs) and InAs ( a is 7.2% larger than that of GaAs) surfaces were investigated using a scanning tunneling microscope (STM) multichamber system equipped with a molecular beam epitaxy facility. In the case of GaAs/InP heteroepitaxial growth, two-dimensional (2D) growth was observed when less than 2.0 monolayers (MLs) of GaAs were deposited on the InP surfaces. On the other hand, three-dimensional (3D) growth was confirmed for more than 2.0 ML GaAs deposition. In the case of GaAs/InAs heteroepitaxial growth, the same transition from 2D to 3D island growth occurred when more than 0.75 ML of GaAs was deposited.

Island Formation and Misfit Strain Relaxation in GaAs Heteroepitaxial Growth on GaP(001).

This paper describes a mechanism of the growth mode transition from layer-by-layer to three dimensional island formation due to the large lattice mismatch. The island formation and strain relaxation of GaAs heteroepitaxial growth onto GaP (001) by molecular beam epitaxy is studied by means of an analysis of the reflection high-energy electron diffraction (RHEED) pattern. The anisotropic strain relaxation of formed islands is in agreement with the prediction of the model which considers an elastic strain relaxation within coherent islands. The model is also supported by the oscillation of the lattice parameter of the growing film during the layer-by-layer growth. The change of the RHEED intensity profile reveals the transformation of the film structure from layer to islands and the relaxation of the misfit strain even after the growth has stopped. This finding shows the kinetically limited pseudomorphic structure of the GaAs film and suggests the presence of kinetically controlled critical thickness of the growth mode transition.

Improvements in the heteroepitaxial growth of GaAs on Si by MOVPE

We describe methods of improving the metalorganic vapour phase epitaxy (MOVPE) growth of heteroepitaxial GaAs on Si. The low temperature growth step at the start is optimized by using the new alternative group III precursor dimethyl-ethyl-amine-gallane (GaH 3·N(CH 3) 2C 2H 5 (DMEAGa)) as a substitute for the conventional group III compound TMGa. Further improvements in surface morphology and X-ray diffraction linewidth are obtained by applying the modulated atomic layer epitaxy (ALE) for this growth step. Thermal cycle annealing which is much more effective in dislocation reduction than constant temperature annealing has been optimized with respect to the high as well as the low temperature steps. We present a model of low temperature behaviour of dislocations to explain the role of cooling the samples during thermal cycle annealing. An additional improvement of layer quality is achieved by introducing a thin (AlGa)As-layer after the low-temperature-GaAs-layer leading to a confinement of dislocation after performing thermal cycle annealing as established by TEM-investigations.

Charge-Balanced Heteroepitaxial Growth of GaAs on Si

We propose charge-balanced heteroepitaxy, where the GaAs/Si interface is neutralized by inserting one monolayer of column II elements in place of the Ga atoms at the GaAs/Si interface. We have constructed this structure using Be atoms, and have investigated the initial stages of GaAs molecular beam epitaxial growth on this charge-balanced surface by high-energy electron diffraction, and ultraviolet and X-ray photoemission spectroscopy. These measurements reveal that the charge-balanced structure neutralizes the GaAs/Si interface, and the GaAs growth mode proceeds in a Stranski-Krastanow mode.

Co-integration of optoelectronics and submicrometer CMOS

The rapid emergence of multichip modules (MCMs) and the continuing interest in wafer scale integration (WSI) provide important opportunities for successful insertion of high performance optical interconnections into real systems. The large area substrates and the distances between packaged wafer-level modules introduce distances of sufficient length that propagation of very high-speed digital signals along electrical lines will be difficult. At the same time, the substrates allow use of thin film technologies for fabrication of optoelectronic devices, optical waveguides, and other optical elements, drawing on the natural alignment accuracy of photolithographic definition of optical components to avoid several practical problems arising when optical elements are surface mounted. For such reasons, large area silicon wafers provide an important potential application for more aggressive use of optical interconnections. An important issue is growth of GaAs semiconductor regions within a silicon WSI or MCM substrate containing high performance silicon CMOS circuitry, seeking to co-integrate optical and silicon VLSI devices. Experimental studies of submicrometer CMOS device characteristics following thermal simulation of GaAs heteroepitaxial growth are summarized. >

Desorption of Indium during the Growth of GaAs/InGaAs/GaAs Heterostructures by Molecular Beam Epitaxy

The desorption of indium during molecular beam epitaxy (MBE) growth of InGaAs and GaAs/InGaAs/GaAs quantum wells has been investigated by reflection high energy electron diffraction (RHEED). The indium desorption activation energy calculated from the temperature dependence of the InAs growth rate is shown to be almost same as the enthalpy of InAs decomposition. It was found that the RHEED pattern transition time, t1, from (4×2) of InGaAs to (2×4) of GaAs after GaAs heteroepitaxial growth on InGaAs began, was the desorption time of indium, which segregated to the growth front from the topmost layer of InGaAs. The activation energy determined from this process is close to the desorption enthalpy of indium from indium liquid.

Growth of GaAs on hydrogen-fluoride treated Si (100) surfaces using migration-enhanced epitaxy

Heteroepitaxial growth of GaAs has been successfully achieved for the first time using migration-enhanced epitaxy (300 °C) on HF treated Si substrates which have been subjected to an initial in situ pre-heat treatment in the range of 350–620 °C and without the customary high-temperature Si oxide desorption step. High quality GaAs on Si exhibiting double-crystal x-ray rocking curve full-width at half-maximum value of 280 arcsec was obtained by using two-step growth technique MEE(300 °C)/MBE (580 °C). Secondary-ion mass spectrometry studies indicated 0.05 (monolayer) ML of oxygen at the interface of GaAs and HF treated Si substrate. The photoluminescence spectra at 10 K for GaAs grown on HF treated Si substrates was dominated by sharp excitonic related peaks and no carbon acceptor related emission could be detected.