Y. Nomura

Surface diffusion length of Ga adatoms on (1̄1̄1̄)B surfaces during molecular beam epitaxy

The spatial variation of the growth rate on mesa‐etched GaAs (111)B substrates during molecular beam epitaxy of GaAs is measured from the period of the reflection high‐energy electron diffraction (RHEED) intensity oscillation using in situ scanning microprobe RHEED. The surface diffusion length of Ga adatoms on the (111)B surface is determined from the spatial variation of the growth rate. The surface diffusion length on the (111)B surface increases as the substrate temperature is raised or the arsenic pressure is decreased. The typical value of the diffusion length is about 10 μm at a substrate temperature of 580 °C and an arsenic pressure of 5.7×10−4 Pa, which is an order of magnitude larger than that on the (100) surface along the [011] direction. The activation energy of the surface diffusion length changes with the surface reconstruction. Anisotropic diffusion, as reported for the (100) surface, is not observed on the (111)B surface.

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.

Molecular Beam Epitaxy of GaAs/AlAs on Mesa Stripes along the [001] Direction for Quantum-Wire Fabrication

We report on the growth of GaAs/AlAs layers by molecular beam epitaxy on GaAs (100) substrates patterned with mesa stripes oriented along the [001] direction. The GaAs growth led to the formation of {110} facets which were smoother than the facets formed on [01]- and [0]-oriented mesa stripes. It was also found that the GaAs growth rate on the {110} facets is extremely low. We fabricated quantum wire-like structures by narrowing the width of the mesa-top (100) facet, which is limited by the (10) and (110) facets, to the nanometer scale, and then growing an AlAs/GaAs quantum well. The resulting structure was as narrow as ~30 nm with a thickness of ~30 nm at its center.

Surface diffusion lenght during MOMBE and CBE growth measured by μ-RHEED

The growth rates of layers grown on a mesa-etched (001) GaAs surface were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion lenght of the surface adatoms of column III elements was determined from the gradient of the variation of the growth rates in the cases of MBE, MOMBE using trimethylgallium (TMGa) and CBE using TMGa or triethylgallium (TEGa) and arsine (AsH3). The obtained values of the diffusion lengths were of the order of a micrometer in every case of the source-material combination. In the case of metalorganic materials as Ga source, it was found that the diffusion length was larger than that of Ga atom from metal Ga source. Since the substrate temperature of the present experiment is high enough to decompose TMGa and TEGa on the surface, Ga adatoms are considered to be responsible to the surface diffusion. Therefore, it is considered that the derivatives of the metalorganic molecules such as methyl radicals affect the diffusion of Ga adatoms.

Lateral Metalorganic Molecular Beam Epitaxy of GaAs on Patterned (1̄1̄1̄)B Substrates

GaAs layers were grown on patterned ()B substrates having ()A sidewalls with various arsenic fluxes at a fixed temperature of 480°C by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMGa) and metal arsenic. Vertical growth rate on the top and bottom ()B surfaces decreased rapidly as the arsenic flux was increased. For arsenic fluxes of 2.0×10-3 Pa and more, only lateral epitaxy on the ()A sidewall was achieved. Real-time scanning microprobe reflection high-energy electron diffraction (µ-RHEED) observations showed that the surface smoothness of the epitaxial layer was maintained throughout the growth time under the optimized condition.

Real-time scanning microprobe reflection high-energy electron diffraction observations of the cleaning process of GaAs substrates

Abstract GaAs surface cleaning using atomic hydrogen (H·) prior to molecular beam epitaxy has been compared to the conventional thermal treatment of GaAs surfaces. The surface morphology was observed in real time using in situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). GaAs surfaces have been found to be uniformly cleaned at temperatures of about 400°C using the H· treatment. On the other hand, a local initiation of the desorption of the oxide layer has been observed during the conventional thermal treatment at about 580°C.

Effect of hydrogen radicals on the reduction of carbon incorporation into GaAs grown by using trimethylgallium

Carbon incorporation into GaAs grown under an ultra-high vacuum (UHV) environment was investigated in a trimethylgallium (TMGa)-As4 or AsH3 (cracked at 850° C) system with hydrogen molecules ( H2) or hydrogen radicals ( H•). The residual carbon concentrations in GaAs layers grown at a substrate temperature of 490° C were measured by secondary ion mass spectrometry (SIMS). In the case of GaAs growth by simultaneous source supply, the introduction of H• reduced the residual carbon concentrations in epitaxial layers from 8×1019 to 7×1018 cm-3 for TMGa-As4, and from 4×1019 to 5×1018 cm-3 for TMGa-AsH3. In the case of an alternating source sup- ply of TMGa and AsH3 without hydrogen, a higher level of carbon concentrations ( 1–2×1020 cm-3) than that of the simultaneous source supply case was observed irrespective of the purge duration (10–250 s) of TMGa in UHV. The residual carbon concentrations were reduced to 6×1018 cm-3 by the injection of H• after TMGa-exposure in an alternating supply cycle, although H2 did not affect the carbon incorporation. This result indicates that the adsorbed carbon-containing species derived from TMGa, which has a residence time of more than 600 s on GaAs surfaces, can be desorbed by a reaction with H• .

Real‐time scanning microprobe reflection high‐energy electron diffraction observations of InGaAs surfaces during molecular‐beam epitaxy on InP substrates

The microscopic surface features were observed during the molecular‐beam epitaxy of InxGa1−xAs on InP (100) substrates by scanning microprobe reflection high‐energy diffraction in real time as a function of x from 0.41 to 0.96. In the case of In0.41Ga0.59As (the lattice mismatch of the strained layer was f=−0.77%, where the minus sign represents a smaller lattice constant of an epitaxial layer than that of a substrate), three‐dimensional island growth was observed from the start of growth. On the other hand, two‐dimensional layer‐by‐layer growth was maintained during growth of InxGa1−xAs with x=0.45–0.75 (f=−0.47 to +1.48%, where the plus sign represents a larger lattice constant of an epilayer than that of a substrate). While the growth of compressive epilayers with f between +0.36% and +1.48% proceeded, a surface crosshatched morphology was observed after the growth of certain film thicknesses, which were dependent on the lattice mismatch. A rough textured morphology was observed instead of a crosshatch...

Influence of hydrogen radicals on the reduction of carbon incorporation into chemical beam epitaxial GaAs

Abstract In order to investigate the influence of hydrogen radicals (H·) on carbon incorporation into chemical-beam epitaxial GaAs, H· generated by flowing H 2 through a hot tungsten filament was intentionally introduced into a trimethylgallium (TMGa)-AsH 3 (cracked at 850°C) and a TMGa (or metal Ga)-trisdimethylaminoarsine (TDMAAs) system. In the case of the TMGa-AsH 3 -H· system, the residual carbon concentrations, measured by secondary ion mass spectrometry, in epitaxial layers grown at 490°C rapidly decreased along with an increase in the H 2 flow rate in a low-flow region, and saturated at around 1 × 10 18 cm -3 in a higher flow region. On the other hand, carbon incorporation (6 × 10 17 cm -3 at 490°C) in TMGa-TDMAAs was less than that in TMGa-AsH 3 . Since no residual carbon over the detection limit ((1−2) × 10 17 cm -3 ) was detected in metal Ga-TDMAAs, the carbon in TMGa-TDMAAs was clarified as having been derived from TMGa. However, the introduction of H· did not reduce the carbon incorporation in TMGa-TDMAAs. Influence of injected H· on the carbon reduction is discussed in relation to the adsorption of uncracked H 2 and surface species derived from TDMAAs on a growing surface.

Real-Time Observations on the Cleaning Process of Patterned GaAs Substrates.

We report on real-time scanning microprobe reflection high-energy electron diffraction (µ-RHEED) observations on the cleaning process of mesa-etched GaAs (100) surfaces for the first time. Both the initial (100) surfaces and the (111)A sidewall have been found to be uniformly cleaned at temperatures of about 400° C using atomic hydrogen ( H•); further, RHEED intensity oscillations more than 50 periods have been observed during direct molecular-beam epitaxy (without a GaAs buffer layer) of GaAs on the cleaned (100) surface. On the other hand, an inhomogeneous desorption of the oxide layer has been observed during conventional thermal cleaning under an As4 pressure at about 600° C. The results indicate that the H• treatment of patterned substrates is useful for obtaining smooth and clean surfaces without the growth of a GaAs buffer layer.