Atomic and electronic structures of GaAs (001) surface (original) (raw)

2006, Russian Physics Journal

https://doi.org/10.1007/S11182-006-0227-1

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Abstract

The atomic and electronic structures of α, β, β2, and ζ reconstructions for the Ga-terminated GaAs (001)-(4 × 2) surface are investigated in the framework of the pseudopotential approach. Total, surface, and local densities of electron states, electron-energy spectra, and relative surface energies of the structures under consideration are calculated.

A UNION OF THE REAL-SPACE AND RECIPROCAL-SPACE VIEW OF THE GaAs(001) SURFACE

International Journal of Modern Physics B, 2001

A union of the real-space and reciprocal space view of the GaAs(001) surface is presented. An optical transmission temperature measurement system allowed fast and accurate temperature determinations of the GaAs(001) substrate. The atomic features of the Ga A s (001)-(2×4) reconstructed surface are resolved with scanning tunneling microscopy and first principles density functional theory. In addition, the 2D lattice-gas Ising model within the grand canonical ensemble can be applied to this surface to understand the thermodynamics. An algorithm for using electron diffraction on the GaAs(001) surface to determine the substrate temperature and tune the nanoscale surface roughness is presented.

GaAs(001): Surface Structure and Optical Properties

physica status solidi (a), 2001

The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 Â 4) and b2(2 Â 4) as well as for the stoichiometric a2(2Â4) and the Ga-rich z(4 Â 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E 1 critical point energy, characteristic of the b2(2 Â 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 Â 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the z(4 Â 2) model for the surface geometry.

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

Applied Surface Science, 1998

We present first-principles total-energy calculations on In-terminated GaAs(001)(2 X 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)/32(4 X 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 sp2-1ike states energetically close to the bulk valence band edge and empty 7r-bonding and 7r *-antibonding combinations of In p~ orbitals in the upper half of the GaAs bulk band gap.

The GaAs(001)-(2 × 4) Surface: Structure, Chemistry, and Adsorbates

The Journal of Physical Chemistry B, 1997

A series of ab initio simulations, based on density functional theory, of the structure of the clean GaAs-(001)-(2 × 4) surface and of C 2 H 2 , C 2 H 4 , and trimethylgallium (TMGa) adsorbates are described. This surface was selected because of its importance in the growth of GaAs by molecular beam epitaxy. After summarizing briefly the theoretical basis of the computational methods used in the paper, we review critically what is known from experiment and theory about the structure of the clean surface. We argue that there is now strong evidence in favor of the "trench dimer" model for the -phase of the clean surface, while the structures of the R and γ phases are less settled. We then present ab initio simulations of the trench dimer, the three dimer, and the gallium rebonded models of the clean GaAs(001)-(2 × 4) surface and discuss their common structural and bonding features. Ab initio simulations of C 2 H 2 and C 2 H 4 adsorbates at arsenic dimers of the GaAs(001)-(2 × 4) surface are then presented. The changes in the bonding configurations of both the adsorbates and the surface arsenic dimers are explained in terms of changes in the bond orders and local hybridization states. The As dimer bond is broken in the stable chemisorbed states of the molecules. However, an intermediate state, in which the As dimer is still intact, provides a significant barrier to chemisorption in both cases. This barrier, and its absence at the Si(001) surface, stems from the two extra electrons in the As dimer compared with the Si dimer. We then go on to describe the results of 14 ab initio simulations of structures connected with the chemisorption and decomposition of TMGa on the GaAs(001)-(2 × 4) surface. TMGa is commonly used in the growth of GaAs crystals from the vapor phase. The results of these simulations are used to explain a number of experimental observations concerning the surface coverage and the decomposition of TMGa to dimethylgallium and monomethylgallium. Significant technical aspects of the calculations, notably the number of relaxed layers in the slab calculations and the necessity to use gradient-corrected adsorption energies, are stressed. The paper also contains critical comments about ab initio simulations of the GaAs-(001)-(2 × 4) clean surface and about the model based on a "linear combination of structural motifs". Discussion of related experimental results appears throughout the paper.

New Ga-enriched reconstructions on the GaAs(001) surface

JETP Letters, 2009

To prepare structure-ordered GaAs(001) surfaces at low temperatures, GaAs(001) surfaces coated with native oxides were exposed in an atomic hydrogen flow in the temperature range 280-450 ° C. The new Ga-enriched GaAs(001) surfaces with the (4 × 4) and (2 × 4)/c(2 × 8) reconstructions were prepared and studied by the methods of X-ray photoelectron spectroscopy, low-energy electron diffraction, and high-resolution characteristic electron energy loss spectroscopy. For the GaAs(001)-(2 × 4) surface, the structure of the Ga-stabilized surface has been proposed and ab initio computed within the (2 × 4) Ga-trimer unit cell model.

GaAs(001) Surface under Conditions of Low As Pressure: Evidence for a Novel Surface Geometry

Physical Review Letters, 2000

Using density-functional theory we identify a new low-energy structure for GaAs(001) in an As-poor environment. The discovered geometry is qualitatively different from the usual surface-dimer based reconstructions of III-V semiconductor (001) surfaces. The stability of the new structure, which has a c͑8 3 2͒ periodicity, is explained in terms of bond saturation and favorable electrostatic interactions between surface atoms. Simulated scanning tunneling microscopy images are in good agreement with experimental data, and a low-energy electron diffraction analysis supports the theoretical prediction. PACS numbers: 68.35.Bs, 61.14.Hg, 68.35.Md, 73.20.At III-V semiconductors play an increasing role in microelectronics, such as light-emitting diodes and high frequency, low noise devices for mobile phones, and are important candidates for the development of devices in the emerging field of spin electronics. The knowledge of the surface atomic structure is a prerequisite to achieve understanding and controlling of the surface or interface electronic properties. As we will show below, however, up to date analyses of surface structures of III-V semiconductors are hindered by some prejudice on the type of structures considered. For an example of the GaAs(001) surface, we show the existence of a new type of surface reconstruction.

Theoretical study of the atomic and electronic structure of the c-4 × 4 reconstructed GaAs(100) surface

Surface Science, 1982

Electronic structures of several atomic wires on an H-terminated Si͑100͒2ϫ1 surface have been examined by using first-principles calculations within the local-density-functional approach. Several dangling-bond ͑DB͒ wires, which are constructed by extracting H atoms from the surface, have been examined and found to have different characteristics depending on their structures. Electronic states near the Fermi energy are localized around the wire on the atomic scale in DB wires along the dimer rows on the surface, while they are much more delocalized around a DB wire in the direction across the dimer rows. Ga adsorbate atomic wires, which are formed by Ga adsorbates around the above wires, have also been examined. Several metastable geometries of Ga adsorbates were found. It was found that formation of Ga dimers was stable on this surface.

Surface structure of GaAs(2 5 11)

Physical Review B, 2002

GaAs samples with orientations vicinal to ͑2 5 11͒ within 1°were prepared by molecular beam epitaxy and analyzed in situ by scanning tunneling microscopy, low-energy electron diffraction, and reflection high-energy electron diffraction. In addition, first-principles electronic structure calculations were carried out. GaAs͑2 5 11͒ is a stable surface whose orientation is located within the stereographic triangle. For a wide range of As-rich conditions a (1ϫ1) reconstruction forms that is characterized by an inclined series of three As dimers and that fulfills the electron counting rule. The terrace size is limited only by the macroscopic off-orientation of the samples. The surface is perturbed by thin stripes of the nearby orientation ͑3 7 15͒. While the dangling bond densities of GaAs͑2 5 11͒ and GaAs͑3 7 15͒ are almost equal, GaAs͑3 7 15͒ violates the electron counting rule. The analysis of this perturbation suggests that, in general, on semiconductor surfaces the gain in stability arising from the minimization of the number of dangling bonds is significantly greater than the gain arising from reaching a semiconducting ground state. Upon annealing of the samples in ultrahigh vacuum, a fairly rough surface structure develops whose mean orientation is different from ͑2 5 11͒.

Surface electronic structure of GaAs(311)A studied by angle-resolved photoelectron spectroscopy

Surface Science, 1996

The valence and core electronic structure of the sputtered and annealed 1 × 1 periodic GaAs(311)A surface has been studied by angle-resolved photoelectron spectroscopy. Five surface bands are identified and their dispersions along high symmetry lines in the surface Brillouin zone are mapped out. While electron-counting indicates a metallic surface, the experiment show no evidence for partly occupied surface bands. Analysis of the core level spectra reveals one As and two Ga surface shifted components. The results are discussed in terms of surface geometry models.

Structure and energetics of Ga-rich GaAs() surfaces

Surface Science, 2002

The atomic structures and energies of Ga-rich GaAs(0 0 1) surface reconstructions are examined by means of firstprinciples total-energy calculations based on a real-space multigrid method. Our calculations confirm the existence of the novel f(4 Â 2) structure suggested by Lee et al. [Phys. Rev. Lett. 85 (2000) 3890]. (4 Â 6) surface reconstructions suggested to explain STM experiments are found to be unstable. The calculations indicate that the adsorption of Ga adatoms in the trenches of the f(4 Â 2) surface could possibly explain the observed structures. The diffusion of Ga/As adatoms on the Ga-rich GaAs surface is predicted to be anisotropic and should preferably take place parallel to the [1 1 0]/[1 1 1 0] direction, respectively. Ó

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

Materials Science and Engineering: B, 1999

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.

GaAs(001) surface reconstructions: geometries, chemical bonding and optical properties

Applied Surface Science, 2002

We re-examine the GaAs(0 0 1) surface by means of first-principles calculations based on a real-space multigrid method. The cð4 Â 4Þ; ð2 Â 4Þ and ð4 Â 2Þ surface reconstructions minimize the surface energy for anion-rich, stoichiometric and cation-rich surfaces, respectively. Structural models proposed in the literature to explain the Ga-rich GaAs(0 0 1) ð4 Â 6Þ surface are dismissed on energetic grounds. The electronic properties of the novel zð4 Â 2Þ structure are discussed in detail. We calculate the reflectance anisotropy of the energetically most favoured surfaces. A strong influence of the surface geometry on the optical anisotropy is found. #

Geometry and electronic structure of GaAs(001)(2 x 4) reconstructions

Physical review. B, Condensed matter, 1996

Structural and electronic properties of the As-rich GaAs͑001͒͑2ϫ4͒ reconstructions are investigated by means of converged first-principles total-energy calculations. For an As coverage of ⌰ϭ3/4, we find the two-dimer ␤2 phase to be energetically preferred over the three-dimer ␤ phase. As the As chemical potential decreases, the ␣ phase of GaAs͑001͒ represents the ground state of the surface. All geometries are characterized by similar structural elements as As dimers with a length of about 2.5 Å, dimer vacancies, and a nearly planar configuration of the threefold-coordinated second-layer Ga atoms leading to a steepening of the dimer block. Consequently, the resulting electronic properties also have similar features. The surface band structures are dominated by filled As-dimer states and empty Ga dangling bonds close to the bulk valence-and conduction-band edge, respectively. The measured Fermi-level pinning cannot be related to intrinsic surface states. The calculated surface states and ionization energies support the ␤2 structure as the surface geometry for an As coverage of ⌰ϭ3/4. ͓S0163-1829͑96͒04248-8͔

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

Physical Review B, 2000

Total-energy calculations for a series of (2ϫ4) and (4ϫ2) reconstructed GaAs͑001͒ surfaces not included in previous theoretical studies are presented. A (2ϫ4) 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-Asdimer ␣ structure assumed previously, due to its lower electrostatic energy. Our results for the (4ϫ2) reconstructed surface confirm the two-Ga-dimer ␤2 structure suggested by Biegelsen and co-workers. Nearly degenerate in energy, however, are mixed Ga-As dimers adsorbed on a Ga-terminated substrate.

Antimony-stabilized GaAs(001)(2×4) reconstructions

Physical Review B

Structural and electronic properties of the Sb-stabilized GaAs͑001͒(2ϫ4) reconstructions are studied by means of first-principles pseudopotential calculations. Six structural models with Sb coverages of ⌰ϭ 3 4 , 1 2 , and 1 4 are considered. The atomic geometries are optimized by means of total-energy calculations. Models with one or two Sb dimers in the first atomic layer describe the stable surface depending on the chemical potentials of the surface constituents. The model widely accepted so far, containing three dimers in the outermost layer, is unfavorable from the energetical point of view. The lengths of the Sb dimers are about 2.9 Å, in close agreement with recent x-ray standing-wave measurements. The surface electronic structures are similar to those of As-rich GaAs͑001͒ surfaces and dominated by filled Sb-dimer states and empty Ga dangling bonds close to the GaAs valence-and conduction-band edges, respectively. ͓S0163-1829͑97͒04919-9͔

Analytic many-body potential for GaAs(001) homoepitaxy: Bulk and surface properties

Physical Review B, 2011

We employ atomic-scale simulation methods to investigate bulk and surface properties of an analytic Tersoff-Abell type potential for describing interatomic interactions in GaAs. The potential is a modified form of that proposed by Albe and colleagues [Phys. Rev. B 66, 035205 (2002)] in which the cut-off parameters for the As-As interaction have been shortened. With this modification, many bulk properties predicted by the potential for solid GaAs are the same as those in the original potential, but properties of the GaAs(001) surface better match results from first-principles calculations with density-functional theory (DFT). We tested the ability of the potential to reproduce the phonon dispersion and heat capacity of bulk solid GaAs by comparing it to experiment and the overall agreement is good. In the modified potential, the GaAs(001) β2(2 × 4) reconstruction is favored under As-rich growth conditions in agreement with DFT calculations. Additionally, the binding energies and diffusion barriers for a Ga adatom on the β2(2 × 4) reconstruction generally match results from DFT calculations. These studies indicate that the potential is suitable for investigating aspects of GaAs(001) homoepitaxy.

Observation of the atomic surface structure of GaAs(001) films grown by metalorganic vapor-phase epitaxy

Surface Science, 1998

We present atomically resolved scanning tunneling micrographs of the surfaces of GaAs(001) films grown by metalorganic vaporphase epitaxy (MOVPE). Thin films deposited in an MOVPE reactor were transferred to an (ultra high) vacuum system without air exposure. After heating the samples from 480 to 580°C high-quality images of the (2 x 4)/c(2 x 8) (1 x 6)/(2 x 6) and (4 x 2)/c(8 x 2) reconstructions were obtained. In addition, a new Ga-rich (3 x 2)/(3 x n) phase was observed that forms during annealing at 540°C. This structure consists of single dimer rows running along the [ 1 lo] direction with a spacing of 12 A. The rows vary in length, and are separated by line defects which occur on average every 20 A (n = 5). A model is proposed for the (3 x 2) which consists of rows of Ga dimers alternating between the first and third layers. Since this structure exhibits a deficit of one electron, line defects are required to expose As dimers in the second layer and neutralize the surface charge.

Photoemission from α and β phases of the GaAs(001)-c(4×4) surface

Surface Science, 2009

We prepared aand b surface phases of GaAs(0 0 1)-c(4 Â 4) reconstruction by molecular beam epitaxy (MBE) using As 4 and As 2 molecular beams, respectively, and examined them by angle-resolved ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation as an excitation source. The UPS valence band spectra and the XPS 3d core level data show pronounced differences corresponding to the surface composition and the atomic structure of the two phases, as proposed in the literature. In UPS, the b phase is characterized by an intensive surface state 0.5 eV below the top of the valence band at low photon energy, while an analogous peak in the a phase spectra is missing. The surface state is interpreted in terms of dangling bonds on As dimers. The As3d and Ga3d core level photoelectron lines exhibit phase-specific shapes as well as differences in the number, position and intensity of their deconvoluted components. The location of various atoms in the surface and subsurface layers is discussed; our photoemission results support models of the b phase and the a phase with As-As dimers and Ga-As heterodimers, respectively.

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 GaAs001 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 GaAs001 surfaces reveals the following: i The observed h23 surface may be a disordered c86 surface. ii The observed 26 surface is a metastable surface, only 0.03 eV/11 higher than the 24 surface having the same surface coverage. iii We confirm the recent suggestion by Hashizume et al. that the observed 24 phase of the 24 surface is a mixture of the 224 and c44 surfaces. In particular, we examined an 87 surface structure which has a lower energy than the earlier proposed 24 structure. Application of the LCSM method to prototype steps on the GaAs001-24 surface is illustrated, comparing the LCSM results directly to pseudopotential results.

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First-principles study of structural, electronic, and optical properties of surface defects in GaAs(001) - β2(2x4)

AIP Advances, 2018

We performed first-principles calculations based on density functional theory (DFT) to investigate the role of point defects in the structural, electronic, and optical properties of the GaAs(001)-β2(2x4). In terms of structural properties, As Ga is the most stable defect structure, consistent with experiments. With respect to the electronic structure, band structures revealed the existence of sub-band and midgap states for all defects. The induced sub-bands and midgap states originated from the redistributions of charges towards these defects and neighboring atoms. The presence of these point defects introduced deep energy levels characteristic of EB3 (0.97 eV), EL4 (0.52 eV), and EL2 (0.82 eV) for As Ga , Ga As , Ga V, respectively. The optical properties are found to be strongly related to these induced gap states. The calculated onset values in the absorption spectra, corresponding to the energy gaps, confirmed the absorption below the known bulk band gap of 1.43 eV. These support the possible two-step photoabsorption mediated by midgap states as observed in experiments.