Molecular Beam Epitaxial GaAs Research Articles

Influence of physisorbed arsenic on RHEED intensity oscillations during low-temperature GaAs molecular beam epitaxy

In a recent work, RHEED specular spot intensity oscillations were obtained during low-temperature molecular beam epitaxy (MBE) growth of GaAs when near stoichiometric V III flux ratios (≤2:1) were used. The cause of these oscillations has not been fully explained. In this work, we have developed a stochastic model of growth which correctly describes the RHEED intensity dynamics over a wide range of growth conditions. A critical and novel improvement to the stochastic model is the inclusion of a physisorbed state. The experimental RHEED responses for various growth conditions were matched by taking into account the build-up of a physisorbed As layer and its effect on the specular spot RHEED intensity. The dynamics of the physisorbed As layer were introduced into the stochastic model by including the thermally activated processes of chemisorption and evaporation of the As physisorbed state. Model results indicate that for typical low- T GaAs growth temperatures (200°C), the steady-state coverage of physisorbed As ranged from 0.72 at a V III flux ratio of 2:1 to 0.24 at a flux ratio of 1:1. Increased scattering of the RHEED beam due to the higher physisorbed As coverage at 2:1 leads to a factor of 5 decrease in the steady-state amplitude of the RHEED oscillations compared to the 1:1 case. This is in excellent agreement with the experimental results of Ibbetson et al. The RHEED intensity oscillates under these conditions due to layer-by-layer growth and a periodic variation in the coverage of physisorbed As, even though the surface migration rates are small. A factor in maintaining this growth mode is that arsenic stays in the physisorbed state with lifetimes in the range of 10 −3 to 10 −5 s and incorporates only when an appropriate configuration of Ga atoms forms on the surface. The temperature dependence of the evaporation and chemisorption time constants of physisorbed As yield activation energies of 0.24 eV and 0.39 eV, respectively, which are in excellent agreement with the experimental data.

On Hall scattering factors for holes in GaAs

Hall scattering factors for electrons and holes in molecular beam epitaxial GaAs layers have been determined by comparing carrier concentrations measured by the Hall effect with those measured by the electrochemical capacitance–voltage technique. The conclusion is that both the electron and hole scattering factors are near unity for n ranging from 2×1016 to 7×1017 cm−3, and p ranging from 5×1016 to 4×1019 cm−3. This conclusion is consistent with the present theory for electrons, but not with that for holes.

Andreev reflections at interfaces between δ-doped GaAs and superconducting Al films

By placing several Si δ-doped layers close to the surface of a GaAs molecular beam epitaxy–grown crystal, we achieve a compensation of the Schottky barrier and obtain a good Ohmic contact between an in situ deposited (without breaking the vacuum) Al metallization layer and a highly modulation doped (n++) conduction layer embedded below the δ-doped layers in the GaAs crystal. When cooled to below the critical temperature (≊1.2 K) of Al, superconductivity is induced in the conductive layer of the semiconductor. We have studied the current voltage (I–V) characteristics in a planar geometry where the Al has been removed in a thin stripe. We find a manifestation of the superconducting energy gap and a rich fine structure at injection energies both below and above the gap.

Interaction of dopants with a host GaAs lattice: The case of low-temperature grown molecular beam epitaxial GaAs(Si)

Superlattice structures of alternately undoped and [Si]=1×1019 cm−3 doped GaAs have been grown by molecular beam epitaxy at a substrate temperature of 250 °C. X-ray diffraction profiles give an average value of lattice constant in the structures in between the values measured in single, nominally undoped and [Si]=1×1019 cm−3 doped epilayers grown under identical conditions. Transmission electron microscopy dark-field (004) beam images of the as-grown structures contain bands of strain contrast and hybrid diffraction/imaging reveals periodic variations in the position of the higher-order Laue zone lines in the superlattice structures. We believe the results signify a modulation of the lattice constant along the growth direction: the lattice constant being smaller in the doped regions than in the undoped regions due to a reduction of excess As concentration at this high doping level. On annealing, dense bands of large As precipitates appear in the undoped regions with bands of fewer, smaller precipitates in the doped regions. Hall-effect measurements on the as-grown structures yield electron concentrations comparable with those measured in heavily Si-doped, low-temperature grown single epilayers, and the mobilities are indicative of band conduction in the doped regions of the superlattices.

Basic analysis of atomic-scale growth mechanisms for molecular beam epitaxy of GaAs using atomic hydrogen as a surfactant

A basic analysis is given to understand how a continual irradiation of atomic hydrogen (H) during molecular beam epitaxy (MBE) of GaAs could result in production of atomically smooth surfaces, abrupt heterointerfaces, and high-quality epitaxial layers as required for many applications. Some interesting results related to the atomic-scale growth mechanisms and atomic interactions are presented, and a growth model is proposed for atomic H-assisted homoepitaxial MBE of GaAs. It is thought that atomic H is an effective surfactant reducing the surface and total energy of GaAs (001), and acts to provide favorable kinetic and energetic pathways to promote an ideal layer-by-layer two-dimensional growth mode.

Femtosecond time-resolved photoemission study of hot electron relaxation at the GaAs(100) surface

Abstract Time-resolved two-photon photoemission spectroscopy is used to study the relaxation of hot electrons as a function of excitation energy for a variety of device grade and molecular beam epitaxial grown GaAs(100) surfaces. Relaxation times ranging from tens of femtoseconds to greater than 400 fs are observed depending on the energy of the intermediate state, the surface quality, and the bulk material quality. This study provides detailed information on electron relaxation dynamics in the corresponding energy range and spatial distribution along the surface normal relevant to hot electron transfer processes at interfaces.

Characterization of Annealed Low-Temperature GaAs Layers Grown by Molecular Beam Epitaxy with n-i-n Structure

Deep levels and resistivity in annealed low-temperature molecular beam epitaxial (LT-MBE) GaAs layers were characterized using n- i-n structure. We found that annealing at 620°C resulted in the highest attainable resistivity of LT GaAs layers. However, deep-level transient spectroscopy (DLTS) and photocapacitance quenching measurements showed no signal of the midgap (EL2) level. Only a “shallower” EL3-like level was observed in the layer of high resistivity. The fact that no midgap level was detected in the highest-resistivity LT GaAs layer means that the “compensation" mechanism does not play an important role in controlling the semi-insulating properties of LT GaAs layers. On the other hand, a midgap level appeared upon annealing at higher than 620°C, although the resistivity of LT GaAs layers decreased more than five orders of magnitude. The photoquenching measurements suggest that this level is identical to the EL2 level. These results support the “buried Schottky barrier" model.

Native defects in gallium arsenide grown by molecular beam epitaxy and metallorganic chemical vapour deposition: effects of irradiation

Gallium arsenide (GaAs), doped n with silicon nominally to 10 15 and 10 16 cm −3, grown by molecular beam epitaxy (MBE) and by metallorganic chemical vapour deposition (MOCVD), was characterized by photoluminescence (PL) spectroscopy. In MOCVD GaAs, we found the signature of the arsenic antisite (As Ga) at 0.702 eV, but in MBE GaAs we found the signature of the gallium antisite (Ga As) at 1.441 eV as well as silicon at the arsenic site (Si As) at 1.483 eV. We used proton (0.6–10 MeV) irradiation to increase the number of intrinsic defects. The fluence range was 10 10 to 10 13 cm −2 for MOCVD GaAs, and 10 10 to 10 14 cm −2 for MBE GaAs. At low fluences, the effect of irradiation was to reduce the PL intensity, which became 10% at 10 11 cm −2 in MOCVD GaAs, and 10% at 10 12 cm −2 in MBE GaAs. Annealing the samples to 550 °C for 30 min resulted in the total recovery of the PL intensity for MBE GaAs, but only a 70% recovery in MOCVD GaAs even at these low fluences. The signature of gallium vacancies (V Ga) appeared only in the irradiated MOCVD samples, at a fluence of 10 11 cm −2 and higher, but never in the MBE samples. We conclude that MOCVD GaAs was grown under arsenic rich conditions but MBE GaAs under gallium rich conditions. MBE material is about ten times more resistant to radiation than MOCVD GaAs. The presence of V Ga in this material may limit the optical output of devices depending on electron-hole recombination. In the case of as-grown MBE GaAs, the presence of Si As and Ga As lowers the electron-hole recombination efficiency.

Heavily carbon-doped GaAs layers prepared by low-energy ion-beam impingement during molecular beam epitaxy

Low-energy (170 eV) C + ion-beam doping during molecular beam epitaxy of GaAs was carried out using a combined ion beam and molecular beam epitaxy (CIBMBE) system. Incorporated C atoms were both electrically and optically well-activated as acceptors with doping levels up to 4 × 10 19 cm −3 in the as-grown condition. In low temperature (2 K) photoluminescence (PL) spectra, C-related specific emissions were clearly observed, indicating good quality of the films. To understand the advantages of low-energy process by CIBMBE, film quality and the rate of C activation were compared with those obtained from 400 keV hot implantation of C at substrate temperatures up to 600°C. Through Raman scattering, 2 K PL and Hall effect measurements, it was found that the use of a low-energy C + ion beam offers advantages such as low damage doping and good activation rate of incorporated C atoms.

Effect of hydrogen on the surface-diffusion length of Ga adatoms during molecular-beam epitaxy

Systematic measurements were carried out on the surface-diffusion length of Ga adatoms during the molecular-beam epitaxy of GaAs in the presence of hydrogen atoms (H⋅) or hydrogen molecules (H2). The spatial variation of the growth rate on the (100) surface adjacent to the (111)A surface was measured from the period of the reflection high-energy electron diffraction (RHEED) intensity oscillations using in situ scanning microprobe RHEED. The surface-diffusion length of Ga adatoms, which was derived from the spatial variation of the growth rate, becomes larger along with an increase in the H⋅ or H2 pressure. It also increases as the substrate temperature is raised under H⋅ or H2 pressure. The diffusion length in the case of H⋅ introduction is larger than that in the case of H2 introduction.

Effects of Al doping on deep levels in molecular-beam-epitaxy GaAs

The effects of Al as an isoelectronic dopant have been investigated on the deep level defects in GaAs grown by molecular-beam epitaxy (MBE), using deep level transient spectroscopy. Two different compositions containing 0.1% Al (Al0.001Ga0.999As) and 1% Al (Al0.01Ga0.99As) have been studied. At least nine different deep levels have been detected. Their detailed characteristics consisting of emission rate signatures, capture cross sections, concentrations, and junction depth profiles have been determined. The deep levels observed have been compared with the M levels normally found in MBE n-GaAs. The emission rates of deep levels have been found to shift to higher values with decrease in Al concentration. This fact has been attributed to lattice strain and random alloy effects. The relative concentrations of deep levels are seen to undergo large changes as the Al concentration is increased from 0.1% to 1%. Al doping upto 0.1% does not seem to reduce the deep level concentration, unlike the case of other isovalent dopants such as In and Sb in MBE GaAs. Further increase in the Al doping to 1% is found to lead to a pronounced increase in the overall deep level concentration. These data along with the other measured characteristics are interpreted in terms of the possible models for the various defects.

Study of the mechanism of GaAs(001) molecular-beam epitaxy

In this paper the nucleation and growth processes of GaAs(001) molecular-beam epitaxy were studied by Monte Carlo simulation. The density of islands and the density of isolated Ga adatoms were obtained for different growth temperatures, and the island size distribution at low coverage, as well as the correlation function between atoms and its relation with the temperature were studied in detail.

Current transport in as-grown and annealed intermediate temperature molecular beam epitaxy grown GaAs

Molecular beam epitaxy (MBE) GaAs grown in the intermediate temperature range of about 400 °C may provide combination of low lifetime, high resistivity, and high mobility. We compare current conduction in unannealed and annealed material grown at 400 °C by fabricating photodetectors on substrates grown between temperature ranges of 250–500 °C. The unannealed version of the device grown at 400 °C shows substantial difference of conduction properties in dark and under light. It is shown that while at low biases the unannealed material may be semi-insulating, at high biases more current is conducted than in annealed material. We attribute this to the effect of intergap states on current conduction and suggest that defect state assisted tunneling is the dominant current transport mechanism in these ranges. Quenching of response by light suggests that occupancy of traps can eliminate their role in current conduction.

A monolithic nonlinear transmission line system for the experimental study of lattice solitons

We describe a new method, applying molecular beam epitaxial (MBE) growth and microfabrication techniques, to experimentally model nonlinear phenomena. Specifically, we have built a periodically loaded monolithic nonlinear transmission line which has been fabricated on a specially designed MBE GaAs wafer to duplicate a theoretical physics model, the one-dimensional exponential lattice, which was proposed by Toda. An advantage of the current approach over previous lumped element circuits is that it precisely reproduces the physics of the model without depending upon mathematical manipulations and corrections. To accomplish this, the line capacitance and parasitic capacitance were canceled by proper design of the MBE wafer profile. The conditions required to apply this approach to other nonlinear systems have also been studied. The experimentally measured pulse shapes for the Toda soliton system were compared against calculated results, with good agreement.

Semi-insulating GaAs layers grown by molecular-beam epitaxy

Temperature dependent conductivity and Hall-effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 420 °C and separated from the substrate. The layers exhibit semi-insulating properties with a room temperature resistivity of (6–7)×106 Ω cm. The electron Hall mobility is proportional to temperature as ∼T−1.1 and the room temperature value is 5900 cm2 V−1 s−1. From the (nHT−3/2) vs T−1 plot, a donor activation energy of 0.68 eV, different than in bulk semi-insulating GaAs, has been evaluated. A similar activation energy is observed in as-grown and annealed GaAs layers grown at 200–350 °C. This indicates that the high resistivity of GaAs grown at low temperatures might be explained by deep donor defects rather than As precipitates.

Defect density measurements of low temperature grown molecular beam epitaxial GaAs by photothermal deflection spectroscopy

The subgap optical absorption of GaAs layers grown by low temperature molecular beam epitaxy is measured by photothermal deflection spectroscopy (PDS). The absorption increases as the growth temperature decreases at a fixed wavelength. Defect densities evaluated from the absorption spectra and the known absorption cross sections are between 1018 and 1019 cm−3. It is shown that complementary PDS phase spectra can be used to separate the absorption of the epitaxial layers from the bulk.

Modeling the diffusion of grown-in Be in molecular beam epitaxy GaAs

The diffusion of Be in GaAs is studied in samples which are molecular beam epitaxy GaAs with grown-in Be. The Be diffusion profiles of samples annealed under various conditions are obtained using secondary ion mass spectrometry. SUPREM-IV.GS, a simulator for GaAs and Si processing technology, is used to compare the experimental results with our models and to extract parameters. The Be diffusion profiles show a kink feature and a time-dependent Be diffusivity which are successfully simulated. The intrinsic Be diffusivity, the Ga interstitial diffusivity, and the equilibrium concentration of Ga interstitials, all as a function of temperature, are obtained from this study: D+1Be=0.17 exp(−3.39 eV/kBT) cm2 s−1, DI=6.4×10−5 exp(−1.28 eV/kBT) cm2 s−1, and CI*int=4.7×1028 exp(−3.25 eV/kBT) cm−3. The role of nonequilibrium Ga point defects in the anomalous Be diffusion behavior is addressed.

Growth modes in atomic hydrogen-assisted molecular beam epitaxy of GaAs

It has been shown that irradiation with atomic hydrogen during the growth of GaAs in molecular beam epitaxy (MBE) promotes an ideal layer-by-layer two-dimensional nucleation and step-flow growth mode on GaAs(001) substrates, thereby resulting in atomically flat surfaces. Fundamentally important observations related to elementary processes have been presented based on the reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements. A growth model for the atomic hydrogen-assisted GaAs MBE has been postulated.

Infrared measurements in annealed molecular beam epitaxy GaAs grown at low temperature

We have utilized an infrared absorption technique to study deep level defects in molecular beam epitaxy GaAs grown at 250 °C. From an observed broad absorption band below the conduction edge, the concentration of defects is estimated to be ≊5×1019 cm−3 in an as-grown sample. The concentration of defects decreases exponentially by one order of magnitude due to annealing of the sample at temperatures of 400–500 °C. From the temperature dependence of the defect concentration, the migration energy of defects is calculated to be 0.52±0.02 eV. The measured migration energy shows that part of the defects are arsenic interstitials.

Effect of 20–95 eV Ar ion bombardment of GaAs(001): In pursuit of damage-free ion-assisted growth and etching

Defect generation due to very-low-energy ion bombardment of GaAs(001) has been studied. Ar+ bombardment was performed at energies E=20–95 eV and current densities J=0.1–0.75 mA/cm2 on surfaces maintained at 615 °C in an As4 flux, both without growth and during GaAs molecular beam epitaxy. Reflection high-energy electron diffraction patterns exhibited 2×4 reconstruction and strong Kikuchi lines, indicative of flat ordered surfaces, independent of E and J. Cross-sectional transmission electron microscopy showed that subsurface dislocation loops were generated when E≥55 eV. The defect density increased with increasing E and J. Films grown with E≤50 eV were free of extended defects, showing that the beneficial effects of ion-assisted molecular beam epitaxy can be achieved without causing ion damage. Measurable sputtering rates were observed for E≥50 eV at J=0.5 mA/cm2.