Molecular Beam Epitaxial GaAs Research Articles

Multiclass classification of reflection high-energy electron diffraction patterns using deep learning

Reflection high-energy electron diffraction (RHEED) is widely used because it allows in-situ observations of the surface behavior of a sample during molecular beam epitaxy (MBE) growth. In particular, RHEED patterns have been used for the calibration of growth conditions because they change considerably depending on the sample temperature, amount of material supplied, and supply ratio. However, RHEED pattern analysis depends on the expertise of the operator and there is also a time limit; therefore, it cannot be readily applied to real-time feedback control. In our previous study, we reported a model that acquires RHEED images of MBE GaAs grown on a GaAs substrate via machine learning using a convolutional neural network. However, the model that classifies the two patterns was a binary classification model and extensibility via data addition was limited. In this study, we report a classification model that can classify three or more classes. This suggests the possibility of a generalized RHEED pattern classifier.

Ultra-high-quality two-dimensional electron systems.

Two-dimensional electrons confined to GaAs quantum wells are hallmark platforms for probing electron-electron interactions. Many key observations have been made in these systems as sample quality has improved over the years. Here, we present a breakthrough in sample quality via source-material purification and innovation in GaAs molecular beam epitaxy vacuum chamber design. Our samples display an ultra-high mobility of 44 × 106 cm2 V-1 s-1 at an electron density of 2.0 × 1011 cm-2. These results imply only 1 residual impurity for every 1010 Ga/As atoms. The impact of such low impurity concentration is manifold. Robust stripe and bubble phases are observed, and several new fractional quantum Hall states emerge. Furthermore, the activation gap (Δ) of the fractional quantum Hall state at the Landau-level filling (ν) = 5/2, which is widely believed to be non-Abelian and of potential use for topological quantum computing, reaches Δ ≈ 820 mK. We expect that our results will stimulate further research on interaction-driven physics in a two-dimensional setting and substantially advance the field.

Epitaxy of High-Quality Single-Crystal Hexagonal Perovskite YAlO3 on GaAs(111)A Using Laminated Atomic Layer Deposition

Single-crystal metastable hexagonal perovskite YAlO3 (H-YAP) films were epitaxially grown on GaAs(111)A by utilizing sub-nanolaminated (snl) and nanolaminated (nl) atomic layer deposited (ALD) Y2O3/Al2O3 multilayers in situ on freshly prepared pristine molecular beam epitaxy (MBE) GaAs. We have studied and compared the structures of H-YAP between the snl-ALD and nl-ALD samples using synchrotron radiation X-ray diffraction (SR-XRD) and scanning transmission electron microscopy (STEM). The evolution of SR-XRD data under different annealing conditions suggested the lowest temperature of 900 °C for forming H-YAP. The observation of pronounced Pendellosung fringes indicated an excellent crystallinity of H-YAP and sharp H-YAP/GaAs interface despite an ∼8.4% large lattice mismatch between H-YAP and GaAs. Narrow full width at half-maximum (fwhm) ≈ 0.019° of H-YAP(0004) θ-rocking scan of the snl-ALD sample is close to that of substrate GaAs(111) ≈ 0.013° and 1 order of magnitude smaller than that of H-YAP(0004) of...

Atomic Nature of the Growth Mechanism of Atomic Layer Deposited High-κ Y2O3 on GaAs(001)-4 × 6 Based on in Situ Synchrotron Radiation Photoelectron Spectroscopy.

Y2O3 was in situ deposited on a freshly grown molecular beam epitaxy GaAs(001)-4 × 6 surface by atomic layer deposition (ALD). In situ synchrotron radiation photoemission was used to study the mechanism of the tris(ethylcyclopentadienyl)yttrium [Y(CpEt)3] and H2O process. The exponential attenuation of Ga 3d photoelectrons confirmed the laminar growth of ALD-Y2O3. In the embryo stage of the first ALD half-cycle with only Y(CpEt)3, the precursors reside on the faulted As atoms and undergo a charge transfer to the bonded As atoms. The subsequent ALD half-cycle of H2O molecules removes the bonded As atoms, and the oxygen atoms bond with the underneath Ga atoms. The product of a line of Ga–O–Y bonds stabilizes the Y2O3 films on the GaAs substrate. The resulting coordinatively unsaturated Y–O pairs of Y2O3 open the next ALD series. The absence of Ga2O3, As2O3, and As2O5 states may play an important role in the attainment of low interfacial trap densities (Dit) of <1012 cm–2 eV–1 in our established reports.

On the origin of reflectance-anisotropy oscillations during GaAs (0 0 1) homoepitaxy

We report on the first spectroscopic study of reflectance-anisotropy (RA) oscillations during molecular beam epitaxy (MBE) GaAs homoepitaxy. Real-time RA spectra measured during epitaxial growth were carried out with a recently developed rapid RA multichannel spectrometer with 100 ms per spectrum acquisition time. An analysis of the time-resolved RA spectra shows that RA oscillations are mostly due to the periodic modulation of the surface orthorhombic strain associated to surface reconstruction. Results reported here demonstrate the power of real-time RA spectroscopy as a probe for the study of epitaxial growth processes. In particular, given its sub monolayer surface-strain sensitivity, RA spectroscopy results a very convenient tool to study epitaxial growth mechanisms in real-time with sub monolayer resolution. This capability allows for real-time RA spectroscopy to be used as a probe for the in situ, real-time control of epitaxial growth, with the additional advantage of operating in higher pressure systems such as CVD, where RHEED monitoring cannot be implemented.

MBE growth of GaAs and InAs nanowires using colloidal Ag nanoparticles

Ag colloidal nanoparticles were used as a catalyst for molecular beam epitaxy of GaAs and InAs nanowires on the Si(111) substrates. The scanning electron microscopy measurements revealed that nanowires obtained are uniform and have small size distribution.

Influence of surface reconstruction on dopant incorporation and transport properties of GaAs(Bi) alloys

We report on the influence of surface reconstruction on silicon dopant incorporation and transport properties during molecular-beam epitaxy of GaAs(Bi) alloys. GaAs(Bi) growth with an (n × 3) reconstruction leads to n-type conductivity, while growth with a (2 × 1) reconstruction leads to p-type conductivity. We hypothesize that the presence or absence of surface arsenic dimers prevents or enables dopant incorporation into arsenic lattice sites. We consider the influence of bismuth anions on arsenic-dimer mediated dopant incorporation and the resulting electronic transport properties, demonstrating the applicability of this mechanism to mixed anion semiconductor alloys.

Effect of interaction in the Ga–As–O system on the morphology of a GaAs surface during molecular-beam epitaxy

A thermodynamic analysis of processes of interphase interaction in the Ga–As–O system has been performed and their theoretical laws have been determined, taking into account nonlinear thermal physical properties of the compounds, the oxide film compositions, and modes of molecular-beam epitaxy of GaAs. The processes of interaction of the native oxide of GaAs with the substrate material and also with Ga and As4 from a vapor gaseous phase have been studied experimentally. The experimental results correlate with the results of the thermodynamic analysis. The laws of influence of the removal of the proper oxide on the evolution of the GaAs surface morphology under conditions of the molecular-beam epitaxy have been proposed.

Fast atom diffraction inside a molecular beam epitaxy chamber, a rich combination

Two aspects of the contribution of grazing incidence fast atom diffraction (GIFAD) to molecular beam epitaxy (MBE) are reviewed here: the ability of GIFAD to provide in-situ a precise description of the atomic-scale surface topology, and its ability to follow larger-scale changes in surface roughness during layer-by-layer growth. Recent experimental and theoretical results obtained for the He atom beam incident along the highly corrugated [11¯0] direction of the β2(2×4) reconstructed GaAs(001) surface are summarized. We also discuss the measurements and calculations for the beam incidence along the weakly corrugated [010] direction where a periodicity twice smaller than expected is observed. The combination of the experiment, quantum scattering matrix calculations, and semiclassical analysis allows structural characteristics of the surface to be revealed. For the in situ measurements of GIFAD during molecular beam epitaxy of GaAs on GaAs surface we analyze the change in elastic and inelastic contributions in the scattered beam, and the variation of the diffraction pattern in polar angle scattering. This analysis outlines the robustness, the simplicity and the richness of the GIFAD as a technique to monitor the layer-by-layer epitaxial growth.

Single-Crystal Y2O3 Epitaxially on GaAs(001) and (111) Using Atomic Layer Deposition

Single-crystal atomic-layer-deposited (ALD) Y2O3 films 2 nm thick were epitaxially grown on molecular beam epitaxy (MBE) GaAs(001)-4 × 6 and GaAs(111)A-2 × 2 reconstructed surfaces. The in-plane epitaxy between the ALD-oxide films and GaAs was observed using in-situ reflection high-energy electron diffraction in our uniquely designed MBE/ALD multi-chamber system. More detailed studies on the crystallography of the hetero-structures were carried out using high-resolution synchrotron radiation X-ray diffraction. When deposited on GaAs(001), the Y2O3 films are of a cubic phase and have (110) as the film normal, with the orientation relationship being determined: Y2O3//GaAs. On GaAs(111)A, the Y2O3 films are also of a cubic phase with (111) as the film normal, having the orientation relationship of Y2O3//GaAs. The relevant orientation for the present/future integrated circuit platform is (001). The ALD-Y2O3/GaAs(001)-4 × 6 has shown excellent electrical properties. These include small frequency dispersion in the capacitance-voltage (CV) curves at accumulation of ~7% and ~14% for the respective p- and n-type samples with the measured frequencies of 1 MHz to 100 Hz. The interfacial trap density (Dit) is low of cm−2eV−1 as extracted from measured quasi-static CVs. The frequency dispersion at accumulation and the Dit are the lowest ever achieved among all the ALD-oxides on GaAs(001).

ChemInform Abstract: Molecular Beam Epitaxial Growth and Magnetotransport Properties of GaMnAs

AbstractReview: 65 refs.

Conduction mechanisms on annealed semi-insulating GaAs samples

We have measured electrical resistivity in the range of 100–500 K on six semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples that were exposed to annealing treatment. Samples 2, 3, 4, 5 and 6 were annealed with temperatures of 350 °C, 400 °C, 450 °C, 500 °C and 550 °C, while sample 1 was not exposed to the annealing process. We used the differential activation energy method to precisely identify the temperature region where the different conduction regimes dominate, i.e. band conduction, nearest neighbor hopping and variable range hopping. We will also show that the change on the density of states in the impurity band caused by annealing significantly alters the conduction mechanisms present in those samples.

Gold-free growth of GaAs nanowires on silicon: arrays and polytypism

We report growth by molecular beam epitaxy and structural characterization ofgallium-nucleated GaAs nanowires on silicon. The influences of growth temperature andV/III ratio are investigated and compared in the case of oxide-covered and oxide-free substrates.We demonstrate a precise positioning process for Ga-nucleated GaAs nanowires using ahole array in a dielectric layer thermally grown on silicon. Crystal quality is analyzed byhigh resolution transmission electron microscopy. Crystal structure evolves from purezinc blende to pure wurtzite along a single nanowire, with a transition region.

Partial arsenic pressure and crystal orientation during the molecular beam epitaxy of GaAs on SrTiO3(001)

A study of epitaxial growth of GaAs islands on (2×1) reconstructed SrTiO3(001) surface is presented. Under low arsenic partial pressures (PAs), GaAs islands are (001)-oriented, and increasing PAs leads to the progressive formation of (111)-oriented GaAs islands. This results from the competition between the formation of Ga–O and Ga–As bonds at the early stages of the growth, as supported by the analysis of the evolution of the island density with respect to PAs.

Laplace DLTS of molecular beam epitaxy GaAs grown on (100) and (211)B substrates

AbstractDeep Level Transient Spectroscopy (DLTS) and Laplace DLTS (LDLTS) techniques have been employed to study defects in n‐type GaAs grown by MBE on (100) and (211)B GaAs planes. The DLTS spectra were different for the two GaAs substrate orientations. Five and four defect states are found in samples grown on (100) and (211)B GaAs planes, respectively with activation energies ranging from 0.054 eV to 0.570 eV. For all of the traps observed in our samples we obtained small activation energies as compared to the previous data published in literature on n‐GaAs samples grown by MBE. This can be explained by the fact that the emission of the carriers depends on the applied electric field and temperature dependence of the carrier concentration. These two phenomena seem to explain the small trap energies seen in our samples. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The hypothesis of charge components of the conductivity of a GaAs molecular beam epitaxy epi-layer

This paper presents our hypothesis on how to calculate the fractions of total conductivity based in part on the conductivities of a GaAs molecular beam epitaxy (MBE) epi-layer. The method allows the calculation of the carrier concentration and mobility of each component of a supposed multi-carrier system. The partial concentrations extracted from the solution of the neutrality equation are used to characterize the particular charge transport portions in the active layer in the form of conductance, G (S), values of these portions. It is important to note that it was assumed that the neutrality equation could be solved for the entire layer; however, it is likely that more layers (according to the physical sense) are present. Furthermore, scattering events for the investigated samples are presented. The analysis of the experimental results demonstrates the utility of our method in comparing the conductance of each part of the multi-layer system as a function of temperature. We present a different and alternative look at the conductivity of a GaAs MBE epi-layer with a mobility spectrum analysis. We introduce the physical element, X (or fitting parameter), to explain and allow for a fit of the experimental data with the calculated values.

Formation Mechanism of AlGaAs/GaAs Stripe Structure Made of (11n)A Facets in Selective Molecular-Beam Epitaxy

The evolution of the shape of a stripe mesa during molecular-beam epitaxy of GaAs is studied under different growth conditions (e.g., growth temperature, As pressure). If the stripe mesa is along the <110> axis on a (001) GaAs substrate, {11n}A facets (n=1, 3, 4, 5) are formed. We found that {111}A and {113}A predominate if the stripe width is less than 1.5 µm. If the stripe width is wider than 2.0 µm, however, we have a shallow groove made of {115}A facets on the top of the mesa. Thus, the cross-sectional shape (appearance/disappearance of each facet) depends on the growth time. The overall evolution is well explained numerically assuming diffusion of adatoms on the facets. We found that the mass transport is the dominant mechanism to determine the cross-sectional shape of the stripe. Slight discrepancy between the experimental and numerical results is attributed to the presence of a potential barrier for the diffusion across the boundary between different facets.

Impact of the Reconstruction Condition on Gallium Arsenide Surface Morphology during Molecular-Beam Epitaxy and Vacuum Annealing

The peculiarities of morphological evolution during GaAs(001) molecular beam epitaxy are studied by RHEED and AFM methods. A clear correlation is established between surface superstructure and morphological evolution. Thermodynamic conditions are experimentally determined which provide the most perfect GaAs(001) epilayer surface. For the (2 × 4) surface reconstruction growth conditions are established which give rise to surface roughening. A new technique is proposed and tested to significantly increase the efficacy of surface smoothening procedure under As flux.

Controlling chaos with magnetic field in semi-insulating GaAs

Chaos control has stimulated a large amount of work. We have studied the effect of an external parallel magnetic field on the low-frequency current oscillations observed on a molecular beam epitaxy GaAs sample grown at 265 deg. C, and we have shown that it can be efficiently used for chaos control. The study of the magnetoresistance indicates that the effect of the magnetic field on the charges of the hopping conduction mechanism induces changes in the low-frequency oscillations. Due to this, we have used the magnetic field to control chaos assessed through direct observation low-frequency oscillations, their attractors, and bifurcation diagrams. We also found that the magnetic field interferes indirectly with the Coulombian interaction between the free charges in the conduction band and the hopping carriers, as well as with the recombination mechanism of field enhanced trapping. Controlling the low-frequency oscillations in semi-insulating GaAs by means of an external magnetic field permits probing the interaction of the slow hopping carriers and the fast free carriers in the electric-field domains.

Effect of rapid thermal annealing on the noise properties of InAs∕GaAs quantum dot structures

Self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on n+-GaAs substrates, capped between 0.4μm thick n-type GaAs layers with electron concentration of 1×1016cm−3. The effect of rapid thermal annealing at 700°C for 60s on the noise properties of the structure has been investigated using Au∕n-GaAs Schottky diodes as test devices. In the reference sample without containing QDs, the noise spectra show a generation-recombination (g-r) noise behavior due to a discrete energy level located about 0.51eV below the conduction band edge. This trap is ascribed to the M4 (or EL3) trap in GaAs MBE layers, related to a chemical impurity-native defect complex. In the structure with embedded QDs, the observed g-r noise spectra are due to a midgap trap level ascribed to the EL2 trap in GaAs, which is related to the InAs QDs dissolution due to the thermal treatment.