Flat GaAs Research Articles

High-density quantum wires naturally formed on (7 7 5)B-oriented GaAs substrates by molecular beam epitaxy

GaAs ( GaAs ) 2 ( AlAs ) 2 quantum wires (QWRs) were naturally formed in a thin GaAs ( GaAs ) 2 ( AlAs ) 2 quantum well (QW) with a regularly corrugated AlAs GaAs upper interface (a period of 12 nm) and a flat GaAs AlAs lower interface grown on (7 7 5)B-oriented GaAs substrates by molecular beam epitaxy. The QWRs were formed side by side with an extremely high density of 8 × 10 5 QWRs/cm, which is the same to that of the previous GaAs AlAs QWRs grown on the (7 7 5)B substrate (the highest density of QWRs ever reported [8]). A photoluminescence from the QWRs formed in the QW with an average well width ( L w ) of 2.1 nm, which have a cross section of about 12 × 2 nm 2 , showed a strong polarization dependence (the polarization degree P ≡ [ (I ∥ − I ⊥ ) (I ∥ + I ⊥ ) ] = 0.21 ). The polarization degree is about twice as large as that of the previous GaAs AlAs QWRs with an average L w of 3.3 nm grown on the (7 7 5)B substrate. This improvement of the polarization degree is mainly due to the further reduced QW width, which results in an improved one-dimensional confinement of carriers in the present QWRs.

Initial Growth of Microcrystalline Silicon on Atomically Flat Hetero-Substrate

ABSTRACTInitial growth of microcrystalline silicon μ-Si:H) deposited on an atomically flat GaAs (001) wafer using a RF glow-discharge decomposition of hydrogen diluted monosilane gas mixture has been studied by means of atomic force microscope (AFM), Auger electron spectroscopy (AES), and cross-sectional transmitssion electron microscopy (XTEM).It is shown that the initial growth of μc-Si:H deposited at a substrate temperature of 50∼250°C consists of four successive stages, i.e., (1) a layer-by-layer growth of a-Si:H up to d ∼5 Å, (2) island formation of a-Si:H, (3) the coalescence of the islands and the nucleation of microcrystallite at d∼ 10–40 Å depending on the growth temperature, and (4) a rapid roughening with microcrystalline growth.

Effect Of Atomic-Hydrogen Treatment Of (001) Gaas Substrate At “High Temperatures” On Rf Plasma-Assisted Molecular Beam Epitaxy Of Cubic Gan

AbstractsGrowth of high quality c-GaN on atomic-H treated (001) GaAs was examined by rf plasma-assisted MBE. First the initial growth stages (atomic-H treatment of GaAs, nitridation/deposition of a thin GaN buffer layer, the post deposition annealing of the buffer layer, and epitaxy of c-GaN) were studied by RHEED and AFM observations. It was found that atomically flat GaAs surface with one monolayer-height steps and ragged step edges could be obtained by the atomic-H treatment at high temperatures. It was found that the atomic-H treated GaAs surface was preferable to grow high quality c-GaN; the FWHM of the X-ray rocking curve for the (002) c-GaN was as small as 70 – 90 arcsec.

Reduced nonradiative recombination in etched/regrown AlGaAs/GaAs structures fabricated by in situ processing

Effectiveness of in situ processing, which combines Cl2 gas etching with subsequent molecular beam epitaxy regrowth in an ultrahigh vacuum environment, is quantitatively evaluated with nonradiative carrier recombination velocity, S, of etched/regrown AlGaAs/GaAs heterointerfaces. When AlGaAs is in situ regrown on Cl2 gas-etched flat GaAs surface, the S value is as low as 1.3×103 cm/s. This value is lower than that of air-exposed/regrown (or ex situ processed) interfaces, thus representing the essential superiority of in situ processing over ex situ. Buried GaAs/AlGaAs multiquantum-well (MQW) mesa-stripes are also fabricated by in situ Cl2 gas-etching/regrowth. The S value at the MQW mesa-sidewall regrown with AlGaAs is 6.0×103 cm/s, which is reduced by more than one order of magnitude from that of a nonburied case.

Ab-initio calculations on the dissociative reaction of an As 2-molecule approaching a Ga-terminated GaAs(001)-surface

We calculate by using a Ga 5As 2H 8 cluster the reaction path of an As 2-molecule approaching a Ga-terminated flat GaAs(001) surface along the symmetry axis C 2. We calculate the reaction path by minimization the total energy of the system (cluster, molecule) for twenty different heights of the As 2-molecule above the surface. In these positions we account for the possibility of the As 2-molecule to rotate around the C 2 symmetry axis and for a change in the distance between the As-atoms. The reaction path is thus given by the angle and the corresponding distance between the As-atoms for each selected height.

Method of linear combination of structural motifs for surface and step energy calculations: Application to GaAs(001).

First-principles calculations of the atomic structure and formation energies of semiconductor surfaces and surface steps are often complicated by the existence of complex structural patterns. We suggest here a simpler, algebraic (not differential) approach that is based on two observations distilled from previous first-principles calculations. First, a relatively large collection of equilibrium structures of surfaces and bulk point defects can be built from a limited number of recurring local ``structural motifs,'' including for GaAs tetrahedrally bonded Ga and As and miscoordinated atoms such as threefold-coordinated pyramidal As. Second, the structure is such that band-gap levels are emptied, resulting in charged miscoordinated atoms. These charges compensate each other. We thus express the total energy of a given surface as a sum of the energies of the motifs, and an electrostatic term representing the Madelung energy of point charges. The motif energies are derived by fitting them to a set of pseudopotential total-energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters is shown to suffice to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential approach. Application of the ``linear combination of structural motif'' (LCSM) method to flat GaAs(001) surfaces reveals the following: (i) The observed h(2\ifmmode\times\else\texttimes\fi{}3) surface may be a disordered c(8\ifmmode\times\else\texttimes\fi{}6) surface. (ii) The observed (2\ifmmode\times\else\texttimes\fi{}6) surface is a metastable surface, only 0.03 eV/(1\ifmmode\times\else\texttimes\fi{}1) higher than the \ensuremath{\alpha}(2\ifmmode\times\else\texttimes\fi{}4) surface having the same surface coverage. (iii) We confirm the recent suggestion by Hashizume et al. that the observed \ensuremath{\gamma}(2\ifmmode\times\else\texttimes\fi{}4) phase of the (2\ifmmode\times\else\texttimes\fi{}4) surface is a mixture of the \ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4) and c(4\ifmmode\times\else\texttimes\fi{}4) surfaces. In particular, we examined an 8\ifmmode\times\else\texttimes\fi{}7 surface structure which has a lower energy than the earlier proposed \ensuremath{\gamma}(2\ifmmode\times\else\texttimes\fi{}4) structure. Application of the LCSM method to prototype steps on the GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) surface is illustrated, comparing the LCSM results directly to pseudopotential results. \textcopyright{} 1996 The American Physical Society.

Formula omitted] resonant tunneling diodes with atomically flat interfaces grown on (411)A GaAs substrates by MBE

Al 0.3Ga 0.7As (10 nm)/GaAs (5 nm)/Al 0.3Ga 0.7As (10 nm) double barrier resonant tunneling diodes (RTDs) have been fabricated on a (411)A GaAs substrate by molecular beam epitaxy (MBE), which has atomically flat GaAs Al 0.3Ga 0.7As interfaces over a device area. The (411)A RTDs showed a larger peak to valley current ratio by about 40% at 77K comparing to RTDs grown on a conventional (100) GaAs substrate simultaneously. Reduction of the valley current (0.61 times smaller) is the main cause for this larger peak to valley current ratio, which is probably due to improved interface roughness and defects of the (411)A RTDs.

Surface diffusion of AlAs on GaAs in metalorganic vapor phase epitaxy studied by high-vacuum scanning tunneling microscopy

After depositing a 1/6 monolayer of AlAs on a very flat GaAs (001) surface by metalorganic vapor-phase epitaxy, we have studied AlAs two-dimensional (2D) nuclei by high-vacuum scanning tunneling microscopy. AlAs 2D nuclei elongate in the [110] direction, like GaAs. The density of AlAs 2D nuclei in the saturation region was 5×1010 cm−2 at 580 °C. The saturated AlAs 2D nucleus density decreased as the temperature increased. From the saturated AlAs 2D nucleus densities the surface diffusion coefficient of AlAs on GaAs was calculated to be 1.5×10−7 cm2/s at 530 °C. This is one order of magnitude smaller than that of GaAs on GaAs.

Characterization of InP islands on [formula omitted]: effect of deposition temperature

Atomic force microscopy has been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs(001) substrates as a function of deposition temperature. In particular, the deposition time required for island formation as a function of deposition temperature is determined at a constant reactant flux, and the effect of InP deposition temperature on the island shape and size distributions is explored. The qualitative behavior of the growth changes very little with temperature, whereas the size and spatial distributions are significantly affected. In addition, the effect of InGaP surface morphology on the size and density of InP islands is compared at constant deposition times. From a detailed analysis of the islands on the various surfaces, the island density and base area are observed to be strong functions of surface morphology, while the height of the islands remains unchanged. These observations indicate that the size, uniformity, and density of the coherent islands are not dominated by strain energy, and, thus, they can be manipulated by changes in substrate morphology and growth conditions.

Surface-diffusion and step-bunching mechanisms of metalorganic vapor-phase epitaxy studied by high-vacuum scanning tunneling microscopy

On a very flat GaAs surface we grow a 1/6 monolayer amount of GaAs by metalorganic vapor-phase epitaxy and observe two-dimensional (2D) nuclei by high-vacuum scanning tunneling microscopy. From the 2D nucleus densities we calculate the surface diffusion coefficient of 2×10−6 cm2/s at 530 °C. During growth, the bunched step (multistep) separation saturates and is independent of the substrate misorientation angle. These results can be explained by a mechanism that takes into account both 2D nuclei formation on a wider terrace and their coalescence on ascending steps. A step-bunching simulation based on our model reveals that the saturated multistep separation is proportional to the 2D nucleus separation, i.e., the inverse of the square root of the density.

Scanning tunneling microscopy study of GaAs(001) surface prepared by deoxygenated and de-ionized water treatment

This letter presents the scanning tunneling microscopy (STM) images of GaAs(001) surfaces prepared by deoxygenated and de-ionized water (DODIW) treatment. The STM images reveal that the missing-dimer rows and unit cells of the 2×4 structure and atomic-layer steps of Ga–As clearly appear after heating the samples above 500 °C in an ultrahigh vacuum. These experimental results suggest the DODIW treatment can attain atomically flat GaAs(001) surface.

Selectively regrown carbon-doped (Al)GaAs by chemical beam epitaxy with novel gas sources

Selective-area epitaxy of carbon-doped (Al)GaAs by chemical beam epitaxy (CBE) on (100) GaAs substrates patterned with SiO 2 has been studied. Triethylgallium, trimethylaluminum, tris-dimethylaminoarsenic (TDMAAs) and diiodomethane were used as precursors. True selectivity of GaAs and AlGaAs with a growth rate of 0.5 μm/h is achieved at a growth temperature as low as 470°C. A flat GaAs regrown surface can be obtained, however, only by thermally cracking the TDMAAs source at 350°C. The growth rate of GaAs:C layers is independent of the stripe openings down to about 5 μm. A growth rate enhancement is observed when the feature size is less than 5 μm. The “rabbit ear” thickness can be significantly reduced if the stripe opening is less than 5 μm or if the V III incorporation ratio is low. A heterojunction bipolar transistor (HBT) with selectively regrown GaAs/AlGaAs:C external base layers has been successfully fabricated. It exhibits at least a 60% improvement in the base sheet resistance compared to the original HBT structure, demonstrating the potential of selective-area growth by CBE.

Characterization of MOCVD-grown InP on [formula omitted

Atomic force microscopy and transmission electron microscopy have been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs(001) substrates. From a detailed structural analysis of the deposited InP, we observe coherent Stranski-Krastanov growth, i.e., after the initial formation of a two-dimensional layer, two types of coherently strained islands appear above a defined thickness. At higher coverages a third type of larger, incoherent and defected island is observed. The two types of coherently strained islands can be characterized by their distinct sizes. For low coverages the coherently strained islands possess irregular dimensions in the plane of the substrate and have a maximum height of 6 nm. At higher coverages the second type of coherently strained islands appears. These are cap-shaped and have a rather narrow distribution of base diameter (120 ± 10 nm) and height (22.5 ± 2.5 nm). An appreciable density of incoherent and defected relaxed islands also becomes evident at increased coverages. These have a hexagonal shape exposing clear facets and are elongated along the [110] crystallographic direction of the substrate.

Structure and formation energy of steps on the GaAs(001)-2 × 4 surface

The energies of various steps on the As-terminated GaAs(001)-2 × 4 surface are evaluated using a novel, approximate method of “linear combination of structural motifs”. It is based on the observation that previous total energy minimizations of semiconductor surfaces produced invariably equilibrium structures made of the same recurring local structural motifs, e.g. tetrahedral fourfold Ga, pyramidal threefold As, etc. Furthermore, such surface structures were found to obey consistently the octet rules as applied to the local motifs. We thus express the total energy of a given semiconductor surface as a sum of (i) the energies ε M of the local structural motifs appearing in the surface under consideration and (ii) an electrostatic term representing the Madelung energy of point charges resulting from application of the octet rule. The motif energies are derived from a set of pseudopotential total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters suffices to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential total energy approach. Application to GaAs(001)-2 × 4 surfaces with steps reveals the following. (i) “Primitive steps”, defined solely according to their geometries (i.e. step heights, widths and orientations) are often unstable. (ii) Additional, non-geometric factors beyond step geometries such as addition of surface adatoms, creation of vacancies and atomic rebonding at step edges are important to lower step energies. So is step-step interaction. (iii) The formation of steps is generally endothermic. (iv) The formation of steps with edges parallel to the direction of surface As dimers (A steps) is energetically favored over the formation of steps whose edges are perpendicular to the As dimers (B steps).

Highly Uniform GaAs/AlAs Quantum Wires Grown on [001] Ridges of GaAs(100) Patterned Substrates by Molecular Beam Epitaxy

Highly uniform GaAs/AlAs ridge quantum well (QW) structures have been grown on ridges with various widths of about 0.1–1.3 µ m in the [001] direction on GaAs (100) patterned substrates by molecular beam epitaxy (MBE). A GaAs/AlAs quantum wire (QWR) with the cross-section of about 40± 7 nm (width)× 10–15 nm (thickness) grown on a [001] ridge with width of 0.45 µ m showed a very uniform cathodoluminescence (CL) at 78 K along the QWR. Linewidths of the CL peaks of the QWRs up to 75 µ m in wire length were as small as 20–26 meV. The CL peak intensity from only one GaAs/AlAs QWR was much larger than that from a GaAs/AlAs quantum well simultaneously grown on a flat GaAs (100) substrate when the excitation electron beam was 1 µ m in diameter, indicating the high optical quality of the QWRs.

Anisotropic structural, electronic, and optical properties of InGaAs grown by molecular beam epitaxy on misoriented substrates

We have investigated the structural, electronic, and optical properties of partially strain-relaxed InxGa1−xAs layers, grown by molecular beam epitaxy on both misoriented and nominally flat (001) GaAs substrates. We find large anisotropies in bulk strain relaxation, interfacial misfit dislocation density, dark-line defect density, and electron mobility, as well as a polarization anisotropy in cathodoluminescence for epilayers grown on misoriented substrates, in comparison with those grown on flat substrates.

Reflection high energy electron diffraction and reflectance difference studies of surface anisotropy in InGaAs chemical beam epitaxy on flat and vicinal (001) GaAs

InGaAs quantum wells (QWs) were grown in a chemical beam epitaxy (CBE) machine with trimethylindium (TMI), triethylgallium (TEG) and tertiarybutylarsine (TBA) as precursors. Growth was monitored in-situ by reflectance difference (RD) and reflection high energy electron diffraction (RHEED), on both flat and vicinal (2° off in the 〈111〉 A direction) (001)GaAs substrates. The RD was monitored at 632.8 nm. At this wavelength the RD signal from a GaAs surface is primarily related to the absorption by Ga dimers. When InGaAs had been grown, both the average RD signal and the amplitude of the RD oscillations for the subsequent growth of GaAs increased significantly, compared to GaAs growth on GaAs. This In influence was found to persist even after the growth of 20–30 ML of pure GaAs. As a result we were able to monitor growth oscillations with RD and RHEED simultaneously during growth of quantum wells of InGaAs in GaAs. As a conclusion to these observations we suggest that the group III dimer bond concentration, detected in the RD signal, increases.

Multiorientational growth of Al on GaAs(001) studied with scanning tunneling microscopy.

Scanning tunneling microscopy has been used to study the initial stages of Al growth on decapped (2\ifmmode\times\else\texttimes\fi{}4)/c(2\ifmmode\times\else\texttimes\fi{}8) GaAs(001). Three orientations have been observed for the clusters of Al that form. On nominally flat GaAs(001) surfaces, Al(001)- and Al(011)-oriented clusters are dominant. These two types of clusters nucleate with the same probabilities for Al depositions at 60, 200, and 300 \ifmmode^\circ\else\textdegree\fi{}C, indicating the importance of the intrinsic properties of the growth system rather than temperature in determining the Al crystal orientation. A model that considers energy arguments is used to rationalize the observed population of Al orientations. In contrast, Al(011)- and Al(011)R-oriented clusters dominate for growth on stepped GaAs(001), an effect related to an increase in the interface free energy for Al(001)-oriented clusters at steps. Previous results of this growth system are discussed in the context of this model.

Anisotropic Structural and Electronic Properties of InGaAs/GaAs Heterojunctions

ABSTRACTWe have investigated the structural and electronic properties of partially strain-relaxed InxGal-xAs/GaAs heterojunctions, grown by molecular beam epitaxy (MBE) on both misoriented and nominally flat (001) GaAs substrates. Mobility measurements using Hall bars aligned along the [110] and [110] in-plane directions reveal an asymmetry in bulk InGaAs electron mobility. This asymmetry is correlated with an anisotropic bulk strain relaxation and interfacial misfit dislocation density, determined from high-resolution x-ray rocking curves (XRC), as well as a polarization anisotropy in cathodoluminescence (CL).

Cluster model study on GaAs epitaxial crystal growth by arsenic molecular beam: I. As 2 adsorption on a GaAs surface

We study the chemisorption of an As 2 cluster on a flat GaAs(100) surface and at the step site of this surface with the Hartree-Fock geometry optimization method followed by the energy calculation with the second-order Møller-Plesset perturbation method. On the flat surface, the activation energies for both molecular and dissociative adsorption are high and the molecular adsorption is more favorable than the dissociative one, so that the As 2 cluster is hardly adsorbed and dissociated. At the step site, on the other hand, the dissociative adsorption occurs smoothly: the dissociative adsorption becomes more favorable than the molecular one and the activation energy for the molecular adsorption is only 9.6 kcal/mol. Therefore, the As 2 cluster arriving at the step site is easily adsorbed and dissociated and thus one As layer is added on a Ga surface. We explain the mechanism of this reaction and the difference in the reactivity between the flat surface and the step site.