Reconstructions Of GaAs Research Articles

Kinetic Monte Carlo simulation of GaAs(001) MBE growth considering the V/III flux ratio effect

The kinetic Monte Carlo method is used to study the initial stage of GaAs molecular beam epitaxial growth on the GaAs(001) surface via fluxes of Ga atoms and As tetramers. The developed algorithm allows efficient investigation of the effects of the V/III flux ratio upon the geometrical characteristics of islands in the temperature range at which the (2 × 4) reconstruction of GaAs(001) is exclusively observed. The island density increases with an increasing V/III flux ratio, whereas the average island size decreases. The temperature dependences of the island characteristics are observed to be stronger at a smaller V/III flux ratio because large arsenic fluxes compensate the enhanced desorption at high temperatures. The island size distribution is more uniform at small V/III flux ratios, while a higher concentration of small clusters is observed as the arsenic flux increases. The analysis of the island morphology and size distribution function suggests that an increase in the V/III flux ratio and a decrease in the substrate temperature have similar effects upon island characteristics, which are attributed to suppression of surface diffusion in both cases.

Diffusion of Ga adatoms at the surface of GaAs(001)c(4×4)αandβreconstructions

Diffusion of Ga adatom at the As rich, low temperature c(4$\times$4) reconstructions of GaAs(001) surface is analyzed. We use known energy landscape for the motion of Ga adatom at two different $\alpha$ and $\beta$ surface phases to calculate diffusion tensor by means of the variational approach. Diffusion coefficient describes the character of low density adatom system motion at the surface. The resulting expressions allow to identify main paths of an adatom diffusion and to calculate an activation energy of this process. It is shown that diffusion at the $\alpha$ surface is slower and more anisotropic than this for the $\beta$ surface.

Effect of low As pressure annealing on the morphology and reconstruction of GaAs (001)

In the low As beam equivalent pressure condition, the in-situ annealing treatment is carried out for the previously atomically flat GaAs(001) βup 2(2×4) reconstruction surface. Utilizing scanning tunneling microscopy, the surface is found to change its morphology simultaneously with the surface reconstruction during the increase of low As beam equivalent pressure annealing time. The surface morphology undergos from ordered flat to disordered flat and then gradually returns to the ordered flat state again. The surface reconstruction turns from βup 2(2×4) to (2×6) and then changes to “zig-zag” (2×6) state. And there is a correlation between the evolution of the surface morphology and surface reconstruction.

Surface reconstructions and stabilizing mechanism of the GaAs(311)Asurface

We investigated various surface reconstructions of a GaAs(311)$A$ surface by using first-principles calculations. The relative stability of the reconstructions was compared by estimating the formation energy taking into account the chemical potentials of Ga and As. We found many metastable reconstructions. Moreover, we found that the electron counting model is not so effective for this surface, although it is a strong guiding principle for determining the stable surface reconstruction of GaAs(100). Further investigations suggested that $s{p}^{2}$-like bond formation effectively works on the (311)$A$ surface. Competition between the $s{p}^{2}$-like bond formation and the conventional stabilization mechanism of As-dimer formation would explain why the (311)$A$ surface has many metastable surface reconstructions.

Optical anisotropy induced by cesium adsorption on the As-richc(2×8)reconstruction of GaAs(001)

Upon adsorption of Cs, the As-rich $c(2\ifmmode\times\else\texttimes\fi{}8)/(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of GaAs(001) is found to exhibit an intense negative signal between 3 eV and 5 eV in the reflectance anisotropy spectrum. This signal has a universal character in that similar features also appear on the Ga-rich surface. The mechanism of this signal is interpreted using ab initio calculations of Cs adsorption at As and Ga sites of the As-rich surface. The calculations succeed in explaining the universality of the signal. In the vicinity of the ${E}_{0}^{\ensuremath{'}}$ bulk critical point at 4.5 eV, the signal arises from perturbation of bulk states terminating at the surface. At lower energies, the signal arises from the creation of new surface resonances induced by the Cs adatom.

Early stages of cesium adsorption on the As-richc(2×8)reconstruction of GaAs(001): Adsorption sites and Cs-induced chemical bonds

We investigate the adsorption of Cs on the As-rich $c(2\ifmmode\times\else\texttimes\fi{}8)/(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of GaAs(001) at low coverages using a combination of theoretical and experimental techniques. Density-functional-theory local-density-approximation total-energy calculations and x-ray diffraction experiments find only minimal Cs-induced surface relaxation and identify three preferential adsorption sites within the partially disordered overlayer. These sites are, in order of decreasing occupation probability, the arsenic dimer bridge D site, the gallium dangling bond ${T}_{2}^{\ensuremath{'}}$ site, and the arsenic ${T}_{3}$ trench site. Detailed analysis of the wave functions and electronic charge densities allows us to clarify the bonding mechanisms at the three sites. At the gallium site, the bonding is strongly ionic and involves significant charge transfer to a new Cs-induced state reminiscent of the ${p}_{z}$ orbital of the gallium atom in the ${\mathrm{sp}}^{{}_{2}}$ configuration. In sharp contrast, at the arsenic sites, the charge transfer is minimal and the bonding rather occurs through mixing with a relatively delocalized state of the clean surface. The ionization energy decreases are estimated and compared for the three sites.

Do WE UNDERSTAND THE STRUCTURE OF THE GALLIUM-RICH SURFACE of GaAs(001)? EXPERIMENTAL AND THEORETICAL APPROACHES

While the As-rich 2 × 4 reconstruction of GaAs(001) is well explained by the so-called β2 structure, the atomic structure of the Ga-rich 4 × 2 phase has been discussed for a long time. In this review, the most important structural models for the GaAs(001) (4 × 2)/c(8 × 2) surface are compared from different theoretical and experimental points of view. The selected reconstructions include the recently proposed ζ model, a new mixed dimer model, and the well-known β, β2, Cerdà and Skala models. The different structures are compared on the basis of total energy calculations, simulations of STM experimental images and interpretation of X-ray diffraction data. Only the ζ model satisfies all criteria, and provides therefore a satisfactory explanation of the atomic structure of GaAs(001)-(4 × 2).

Surface phase diagram of (2×4) and (4×2) reconstructions of GaAs(001)

Total-energy calculations for a series of $(2\ifmmode\times\else\texttimes\fi{}4)$ and $(4\ifmmode\times\else\texttimes\fi{}2)$ reconstructed GaAs(001) surfaces not included in previous theoretical studies are presented. A $(2\ifmmode\times\else\texttimes\fi{}4)$ surface model containing single anion dimers in the first and third atomic layers is predicted for a balanced surface stoichiometry. It is more stable than the two-As-dimer $\ensuremath{\alpha}$ structure assumed previously, due to its lower electrostatic energy. Our results for the $(4\ifmmode\times\else\texttimes\fi{}2)$ reconstructed surface confirm the two-Ga-dimer $\ensuremath{\beta}2$ structure suggested by Biegelsen and co-workers. Nearly degenerate in energy, however, are mixed Ga-As dimers adsorbed on a Ga-terminated substrate.

Theory of the (3×2) reconstruction of the GaAs(001) surface

We discuss an ab initio density functional theory investigation of a (3×2) reconstruction of the Ga-rich GaAs(001) surface. Recent experiments identified this new reconstruction as a stable surface of epitaxially grown GaAs doped with indium or carbon (L. Li, et al., Appl. Phys. A 66 (1998) S501; L. Li, et al., Ultramicroscopy 73 (1998) 229). Using our recently developed real-space pseudopotential density functional code, we investigate this (3×2) reconstruction and evaluate stabilization mechanisms for the model proposed by the experimentalists, which does not satisfy the electron counting rule. In order to study the effect of In substitution, we consider the dependence of the surface energies of competing reconstructions on surface strain and relative chemical potentials. The effects of carbon substitution are modeled using a charged (3×2) reconstruction. These studies support and clarify the proposed structure and stabilization mechanisms. We provide a simple physical explanation for the behavior of the surface energy of the (3×2) reconstruction.

Phase transition between charge-induced long-period and (2×4) reconstructions of GaAs(001) surface

A phase diagram of GaAs(001) with Si δ-doped in the subsurface layer has been constructed as a function of temperature and As pressure. The c(4×4), low-temperature (2×4)/c(2×8), long-period and high-temperature (2×4)/c(2×8) reconstructions occur in order of increasing the surface As coverage. The long-period reconstruction induced by surface charges occurs in the middle of the (2×4)/c(2×8) phase region, with a narrow range of the As coverage. The formation of an ideal structure of the long-period reconstruction at a specific As coverage is suggested by the RHEED observation. The phase transition of the long-period reconstruction has been described in terms of the As dimer row kinks and disappearance of kink ordering which is driven by the desorption of As.

Hydrogen adsorption on GaAs (001) reconstructions

Hydrogen adsorption on the c(4×4), (2×4), (2×6), and (4×2) reconstructions of GaAs (001) have been characterized by internal-reflection infrared spectroscopy. The infrared spectra contain up to 15 bands due to the stretching vibrations of arsenic hydrides (2150–1950 cm−1), terminal gallium hydrides (1950–1800 cm−1), and bridging gallium hydrides (1800–950 cm−1). These features arise from hydrogen adsorption on arsenic and gallium dimers, and second-layer arsenic and gallium atoms. The large number of peaks observed indicates that the surface atoms exist in a variety of different chemical environments.

Surface phases of GaAs and InAs (001) found in the metalorganic vapor-phase epitaxy environment

We have characterized the (2×4) and (4×2) reconstructions of GaAs and InAs (001) that are present in a metalorganic vapor-phase epitaxy (MOVPE) reactor. Scanning tunneling micrographs show that these surfaces are terminated with arsenic and gallium (or indium) dimers. The (2×4) dimer row exhibits a mottled appearance, which is ascribed to the adsorption of alkyl groups on some of the sites. On the (4×2), <10% of the surface is covered with small (2×4) islands. These results show that, in the MOVPE environment, the GaAs and InAs surface structures are nearly the same as those found in ultrahigh vacuum molecular beam epitaxy.

The reaction of carbon tetrachloride with gallium arsenide (001)

Carbon tetrachloride dissociatively adsorbs on the Ga-rich (4×2) reconstruction of GaAs (001) at 200 °C. Upon heating to 440 °C, the chlorine desorbs as GaCl, which etches the surface. Scanning tunneling micrographs reveal that this reaction transforms the (4×2) into a Ga-rich (3×2) structure that is interlaced with As-rich (2×4) phases. The (3×2) is well ordered, while the (2×4) phases exhibit a high degree of disorder. This work establishes the surface reaction pathway for carbon doping of GaAs with CCl4.

In-induced (4 × 2) reconstructions of GaAs(001) surfaces

We present first-principles total-energy calculations on In-terminated GaAs(001)(2 × 4) surfaces. The atomic structures and energetics of two structural models suggested from recent experiments are discussed. We favour a surface structure similar to the GaAs(001) β2(4 × 2) surface, where all Ga dimers are replaced by indium. Its stability, however, is limited to In- and As-rich conditions. The surface electronic structure is characterized by filled In sp2-like states energetically close to the bulk valence band edge and empty π-bonding and π*-antibonding combinations of In pz orbitals in the upper half of the GaAs bulk band gap.

(4×2) and (2×4) reconstructions of GaAs and InP(001) surfaces

We report results of first-principles total-energy calculations. The atomic and electronic structure and the formation energy of the Ga-rich GaAs(001)-β2(4 x 2) surface is compared with recent results for the As-rich GaAs(001)-(2 x 4) surface reconstructions. The relaxed β2(4 x 2) structure is characterized by an appreciable decrease of the distance between the first and second atomic layer and gives rise to unoccupied Ga-dimer-derived surface states in the upper half of the GaAs bulk band gap. Its stability is limited to extreme Ga-rich conditions. For the InP(001) surface, we investigate a series of (4 x 2) and (2 x 4) reconstructions that were suggested in order to explain recent experiments. Among these models, a structure containing three- and four-fold coordinated In atoms is energetically favourable, independent of the chemical potentials of the surface constituents.

Reaction of dimethylzinc and diethylzinc on the As-rich GaAs(100)-c(4×4) surface

The reactions of dimethylzinc (DMZ) and diethylzinc (DEZ) on the As-rich c(4×4) reconstruction of GaAs(100) were studied using temperature programed desorption. Both DMZ and DEZ dissociatively adsorbed on the c(4×4) surface to form adsorbed Zn atoms and alkyl groups. The primary reaction pathway for surface methyl groups formed via dissociative adsorption of DMZ was desorption as methyl radicals between 500 and 720 K. A small fraction of the methyl groups also underwent dehydrogenation to form adsorbed hydrogen atoms and deposit carbon on the surface. In contrast, for surface ethyl groups formed from DEZ, the primary reaction pathway was β-hydride elimination to produce ethylene. Comparison of the results obtained in this study with those reported previously for the Ga-rich (4×2) reconstruction of GaAs(100) indicates that surface Ga atoms enhance the dehydrogenation activity of GaAs(100) surfaces.

STM studies of island formation and surface ordering of Si on GaAs (001), (2�4) and c(4�4): Implications for ?-doping

Scanning tunnelling microscopy (STM) and reflection high energy electron diffraction (RHEED) have been used to study the deposition of Si below 400‡C onto GaAs (001) surfaces grown in situ by molecular beam epitaxy (MBE). The emphasis is on the island formation and growth, as well as surface ordering, for submonolayer quantities of Si (up to 0.2 ML) deposited on two different As-rich reconstructions of GaAs (0 0 1) (2 × 4) and c(4 × 4). For deposition on the c(4 × 4) surface, an asymmetric (3 × 1) RHEED pattern is formed, a consequence of anisotropic “needle-like” islands, which grow adjacent to each other along the [1 1 0] direction and produce a three-fold periodic superstructure. Individual islands grow by a site exchange process in which the additional As layer of the c(4 × 4) structure acts as a surfactant and enables the Si atoms to occupy Ga sites in the GaAs lattice. In contrast, Si deposition on the (2 × 4) surface does not lead to any new surface periodicities as monitored by RHEED. The Si atoms form poorly ordered clusters distributed randomly across the surface. The site exchange process cannot occur in this case as the (2 × 4) surface is terminated with only one layer of arsenic. Instead, the Si atoms occupy sites on top of the outer arsenic layer.

The influence of surface structure on the reaction of dimethylzinc on GaAs(100)

The reactions of dimethylzinc (DMZ) on the (4 × 2), (2 × 4) and c(4 × 4) reconstructions of GaAs(100) were studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The results of the TPD experiments demonstrate that the strength of the Zn-surface interaction is a function of the surface As Ga ratio. Zn atoms produced via dissociative absorption of DMZ desorb at 565, 600 and 620 K from the (4 × 2), (2 × 4) and c(4 × 4) reconstructions, respectively. In contrast, methyl groups were found to desorb from all three reconstructions at 610 K. HREELS data for the (4 × 2) and c(4 × 4) reconstructions show that the energy of the surface-C stretching mode for absorbed methyl groups is the same on these surfaces. These results indicate that the absorption sites for methyl groups are similar on the three reconstructions. Based on these experimental results, models of the absorption sites for Zn atoms and methyl groups on the (2 × 4) and c(4 × 4) reconstructions of GaAs(100) are proposed.

Reaction of I2 with the (001) surfaces of GaAs, InAs, and InSb. I. Chemical interaction with the substrate.

InAs(001)-c(8\ifmmode\times\else\texttimes\fi{}2), InSb(001)-c(8\ifmmode\times\else\texttimes\fi{}2), and several reconstructions of GaAs(001) are exposed at room temperature to iodine molecules (${\mathrm{I}}_{2}$). Low-energy electron diffraction (LEED) and synchrotron soft x-ray photoelectron spectroscopy (SXPS) are employed to study the surfaces as a function of ${\mathrm{I}}_{2}$ dose and sample anneal. In the exposure range studied, GaAs and InAs become saturated with ${\mathrm{I}}_{2}$, resulting in removal of the clean surface reconstruction and the formation of a very strong 1\ifmmode\times\else\texttimes\fi{}1 LEED pattern. Iodine bonds primarily to the dominant elemental species present on the clean surface, whether it is a group-III or -V element. The InSb(001)-c(8\ifmmode\times\else\texttimes\fi{}2) reconstruction is also removed by ${\mathrm{I}}_{2}$ adsorption, and a strong 1\ifmmode\times\else\texttimes\fi{}1 LEED pattern is formed. SXPS data, in conjunction with scanning tunneling microscopy images, however, reveal that InSb(001)-c(8\ifmmode\times\else\texttimes\fi{}2) does not saturate at room temperature, but is instead etched with a preferential loss of In. Heating the iodine-covered group-III-rich InAs(001)-c(8\ifmmode\times\else\texttimes\fi{}2) and InSb(001)-c(8\ifmmode\times\else\texttimes\fi{}2) surfaces causes removal of the iodine overlayer and transformation to a group-V-rich reconstruction. When the iodine-covered As-rich GaAs(001)-c(2\ifmmode\times\else\texttimes\fi{}8) surface is heated to remove iodine, however, the c(2\ifmmode\times\else\texttimes\fi{}8) reconstruction is simply regenerated. \textcopyright{} 1996 The American Physical Society.

Growth of GaAs epitaxial layers on porous silicon

We have reviewed the initial stage and the reconstruction of GaAs epitaxial layers grown on Si substrates by various methods, the formation and the properties of porous Si films, the growth of GaAs epitaxial layers on porous Si by various methods, and the electrical, structural and optical properties of GaAs epitaxial layers grown on porous Si. In addition, we observed new interesting results such as the structural, optical and electrical properties of GaAs/porous Si heterostructures, and the advantages of and the possible applications for the GaAs/porous Si heterostructures are suggested.