Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.     

Optical anisotropy of cyclopentene terminated GaAs(001) surfaces

Up to now most of the experimental work regarding the adsorption of organic molecules has been concerned with silicon. Here we study the interface formation on a III–V-semiconductor, GaAs(001). We show that reflectance anisotropy spectroscopy (RAS) is a sensitive technique for investigating the interface formation between organic molecules and semiconductor surfaces. With RAS it is possible to determine the surface reconstruction and the structural changes at the interface during the deposition of organic molecules. These changes and the underlying adsorption process are discussed here for the adsorption of cyclopentene on GaAs(001)c(4×4), (2×4) and (4×2). PACS 61.66.Hq; 72.80.Le; 34.50.Dy; 68.47.Fg     

Atomic structures of two-dimensional strained InAs epitaxial layers on a GaAs(001) surface: in situ observation of quantum dot growth

Scanning tunneling microscopy and reflection high-energy electron diffraction under ultrahigh vacuum conditions were used to make an in situ study of atomic structures at the surface of an InAs/GaAs heterostructure grown by molecular-beam epitaxy. It was observed that the deposition of approximately 0.3 ML of indium on an arsenic-enriched GaAs(001)-2 × 4 surface leads to the formation of the 4 × 2 phase while the deposition of 0.6 ML indium leads to the appearance of a new 6 × 2 reconstruction. It is shown that layer-by-layer two-dimensional epitaxial growth of InAs on GaAs(001) as far as 13 monolayers can only be achieved if the growth front reproduces the 4 × 2 or 6 × 2 symmetry of the substrate and models of 4 × 2 and 6 × 2 reconstructions are proposed. Atomic-resolution images of faceted planes on the surface of three-dimensional islands in an InAs/GaAs(001) system were obtained for the first time and structural models of these were developed.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Bi- and Au-Induced Reconstructions on GaAs（001）-2 × 4 Surface

Submonolayer Bi and Au adsorptions on the GaAs（001）-2× 4 surface are investigated by scanning tunnelling microscopy, low energy electron diffraction and first-principles calculations. The 1 ×4 and 3 × 4 reconstructed surface induced by Bi and Au, respectively, are revealed and their structural models are proposed based on experiments and first-principles calculations. Moreover, the validity of the recently proposed generalized electron counting （GEC） model [Phys. Rev. Lett. 97 （2006） 126103] is examined in detail by using the two surfaces. The GEC model perfectly explains the structural features, such Bi-1 × 4 surface and the 3x arrangement of four-atom Au as the characteristic short double-line structure in the clusters.     

Atomic configurations during Si incorporation on GaAs(001) in As atmosphere evidenced by reflectance difference spectroscopy

The structural ordering of surface atoms during Si deposition on singular and vicinal GaAs(001) surfaces has been studied by reflectance difference (RD) spectroscopy using the difference function between the Si-covered and the bare surface. In dependence on the Si coverage the difference spectra correspond to RD spectra of the bare Si(001)-(1×2) or of the As-terminated Si(001):As(2×1) surface. This finding and the behaviour of RD transients recorded at 3.8 eV photon energy allows to define a (3×2)α phase with Si dimers in the top layer and Ga dimers in the third layer, and a (3×2)β phase with As-dimer rows on top of Si in the second layer.     

Temperature and reconstruction dependence of the initial Al growth on GaAs(001)

The development of the interface between Al and MBE-grown GaAs(001) surfaces has been analyzed up to a mean Al coverage of 0.5 nm. Interdiffusion, chemical reactivity and nucleation have been studied by Auger electron spectroscopy and reflection high energy electron diffraction for the c(2 × 8) and the (4 × 6) surface reconstructions in the temperature interval 268 to 673 K. Above 550 K no nucleation was observed and the growth process was governed by As outdiffusion followed by formation of AlAs. At lower temperatures three characteristic regions of Al coverage were found. The different growth mechanisms in these regions are discussed. An AlGa exchange reaction was observed only on the Ga-rich (4 × 6) structure where it was also correlated to the nucleation. The particular deposition measurement procedure used was found to be an important parameter, since interdiffusion was observed even at room temperature. The coverage at which 3D nucleation occurred was dependent on both substrate temperature and surface reconstruction but always appeared between 1.5 and 3.5 monolayer coverage. A strong temperature dependence of the nucleation was observed.     

Switching of GaAs(001) termination by action of molecular iodine

This paper presents experimental results of an ultrahigh vacuum study of 4 × 2/c(8 × 2) → 2 × 4/c(2 × 8) structural transition on GaAs(001) caused by the thermal removal of the saturated iodine monolayer formed at GaAs(001)-4 × 2/c(8 × 2). It has been found out that the original c(8 × 2) low energy electron diffraction pattern transforms into 4 × 1 at 0.6 ML of iodine coverage and then keeps up to its saturation at 1.0 ML. We have determined that GaI is the only chemical product of the iodine action, its double peak was observed in the thermal desorption spectra at T = 150–370°C. The explanation of surface processes underlying 4 × 2/c(8 × 2) → 2 × 4/c(2 × 8) phase transition is presented below.     

III-V compound semiconductor (001) surfaces

There has been renewed interest in the structure of III-V compound semiconductor (001) surfaces caused by recent experimental and theoretical findings, which indicate that geometries different from the seemingly well-established dimer models describe the surface ground state for specific preparation conditions. I review briefly the structure information available on the (001) surfaces of GaP, InP, GaAs and InAs. These data are complemented with first-principles total-energy calculations. The calculated surface phase diagrams are used to explain the experimental data and reveal that the stability of specific surface structures depends largely on the relative size of the surface constituents. Several structural models for the Ga-rich GaAs (001)(4×6) surface are discussed, but dismissed on energetic grounds. I discuss in some detail the electronic properties of the recently proposed cation-rich GaAs (001)ζ(4×2) geometry. Received: 18 May 2001 / Revised version: 23 July 2001 / Published online: 3 April 2002     

Combined RHEED-AES study of the thermal treatment of (001) GaAs surface prior to MBE growth

Auger analysis and reflection high energy electron diffraction (RHEED) have been used to study the UHV thermal cleaning procedure of different chemically treated (001) GaAs surfaces when heated in ultra high vacuum. It is shown that the ultimate surface composition of the substrate critically depends on the nature and the thickness of the oxide layer formed during chemical treatment. The oxygen removal mechanism has been studied and a comparative analysis of AES and RHEED observations has been drawn. A low residual carbon coverage cleaning procedure is fully investigated and it results that a carbon coverage as low as ∼6×10⁻² monolayer induces surface faceting by heating the GaAs substrate at temperatures higher than 570°C. A (001) GaAs surface heated in an arsenic flux up to 570°C is As-stabilized and (411) facets appear at a temperature ranged between 575 and 585°C.     

Surfactant properties of cesium in molecular beam epitaxy of GaAs(100)

The experimental and ab initio investigations of the effect of a decrease in the binding energy of surface arsenic atoms under the cesium adsorption on an As-stabilized GaAs(001)-(2 × 4) surface have been performed. The cesium-induced redistribution of the charge on the surface atoms reduces the electron density in the As-Ga bond of the upper layer of the GaAs(001) surface; thus, the As-Ga binding energy decreases and, as a result, the diffusion activation energy, as well as the arsenic atom desorption, decreases. An increase in the diffusion coefficient of surface atoms, along with the property of Cs to segregate on the surface of a growing semiconductor film, makes it possible to use cesium as a surfactant in the low-temperature growth of GaAs by molecular beam epitaxy.     

A kinetic study of the initial oxidation of the Ni(110) surface by RHEED and X-ray emission

Nickel (110) surfaces, prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction, were oxidized in pure O₂. The structural aspects of this interaction were studied by reflection high energy electron diffraction (RHEED) and the corresponding kinetics determined by electron excited X-ray emission spectroscopy. On exposure to oxygen the surface was observed to go through three ordered two-dimensional structures. These were a (2 × 1), a (3 × 1) and finally a (9 × 4) structure containing 0.015 microg/cm² of oxygen. From this surface NiO was produced in a (001) epitaxy which was compressed 4.5% in the plane of the surface. With oxidation beyond 0.060 microg/cm² the oxide strain was relieved and a complex oxide epitaxy developed which has been tentatively identified as a (117) oxide parallel to the nickel (110) surface. The kinetics have been explained on the basis of three distinct processes. (i) An initial chemisorption stage (0 to 0.015 microg/cm²) associated with the two-dimensional structures, (ii) Oxide nucleation and spreading to cover the surface, associated with the (001)-NiO epitaxy (0.015 to 0.06 microg/cm²), (iii) Logarithmic film thickening above 0.060 0.06 microg/cm² associated with the development of (117) epitaxy.     

Stoichiometry effects on surface properties of GaAs{100} grown in situ by MBE

In this paper, we report results dealing with the effects of stoichiometry on surface properties of GaAs(001) layers grown by MBE. Three aspects of surface properties were investigated: crystallography, electronic properties and chemical reactivity. Surface crystallography was studied mainly by LEED. The reconstruction of the surface was found to be drastically dependent on the composition of the uppermost atomic layer, i.e. the surface stoichiometry. According to the arsenic surface coverage, many structures from the c(8 × 2) Ga rich to the (1 × 1) arsenic saturated surface have been observed. The influence of stoichiometry on surface electronic properties has been studied by electron loss spectroscopy (ELS) and contact potential difference (CPD) measurements. In the electron loss spectra, two peaks, at about 10.3 and 20.2 eV are very sensitive to the surface composition: they gradually disappear when the arsenic coverage increases, and consequently are associated with surface states on gallium atoms. On the other hand, the CPD measurements have shown that the variation of the work function with the arsenic surface coverage is not monotonic: in particular, an abrupt change of work function of about 300 meV occurs between the (1 × 6) and c(2 × 8) structures which are very similar as far as the arsenic surface coverage (about 0,5 and 0,6 respectively) is concerned. Therefore, it seems that the work function is strongly dependent on the atomic reconstruction occurring at the surface, and not only on its stoichiometry. The connection between stoichiometry and chemical reactivity of the surface is illustrated by the study of H₂S adsorption: a large difference (factor of 10³) in sticking coefficient has been found between surfaces with different arsenic coverages.     

The initial growth of Au on GaAs(001)-c(4 × 4)

The metal growth when depositing a monolayer (ML) of Au at 200° C on MBE-grown surfaces of GaAs(001)-c(4 × 4) was studied by AES and RHEED. The surface interaction can be characterized to proceed in two stages depending on the surface coverage of Au. At a coverage of less than 0.3 ML the gold atoms are mainly dispersed on the surface with a small in depth diffusion. Above 0.4 ML there is a rapid intermixing and a tendency of arsenic accumulation to the surface.     

Influence of the structural surface state on the formation of a relief and morphology of GaAs(001) layers during molecular-beam epitaxy and vacuum annealing

Methods of reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) are used to investigate the special features of morphological changes on the GaAs(001) surface during the molecular-beam epitaxy (MBE) growth and vacuum annealing. A relationship is revealed between the superstructural state of the surface and the character of these changes. Thermodynamic conditions of epitaxial GaAs(001) growth with the most structurally perfect surface are established. The characteristics of the processes causing evolution of the relief during MBE growth in states with reconstruction (2 × 4) are determined. A new technique that allows the efficiency of smoothing of the GaAs(001) surface to be increased considerably by annealing in an arsenic flow is suggested and tested. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 5–13, September, 2008.     

Room temperature hydrogenation of the Si(001)2 × 1 surface studied by angle-resolved electron energy loss spectroscopy

We observed the hydrogen adsorption on the Si(001)2 × 1 surface achieved at room temperature by angle-resolved electron energy loss spectroscopy (AR-ELS) and elastic low-energy electron diffraction. From measurements of the intensities of elastically diffracted beams, we found a characteristic hydrogen covered surface (called Si(001)2 × 1H(RT) surface in this paper), where all the diffracted beam intensities were enhanced drastically and a sharp 2 × 1 LEED pattern was observed. The angular dependence of the elastically diffracted beams on the 2 × 1H(RT) surface was different from that on the monohydride 2 × 1:H surface. On the 2 × 1H(RT) surface the S₃, transition from the back bond surface state disappeared in contrary to the 2 × 1:H surface and two hydrogen induced transitions were observed at 7.0 and 8.0 eV in AR-ELS spectra. We revealed that the 2 × 1H(RT) surface consisted of the monohydride and the dihydride phases with comparable weights. Additionally, we found the new transition S'₁, ascribed to the newly produced dangling bond surface state due to the rupture of the dimerization bond with hydrogen adsorption.     

Hydrogen adsorption and surface structures of silicon

The adsorption of atomic hydrogen on silicon (111)2 × 1 cleaved, (111) 7 × 7, and (100) 2 × 1 surfaces has been studied by using electron energy loss spectrscopy (ELS) and Photoemission spectroscopy (UPS). On all surfaces the hydrogen removes the “dangling bond” surface state and a new peak in the density of states at lower energies corresponding to the SiH bond is found. The LEED pattern of the equilibrium surfaces (111) 7 × 7 and (100) 2 × 1 is not altered by hydrogen adsorption, while on the cleaved (111) 2 × 1 surface the fractional order spots are extinguished. The Haneman surface-buckling model therefore provides an explanation for the surface reconstruction of the cleaved (111) 2 × 1 surfaces. For the equilibrium surfaces, (111) 7 × 7 and the (100) 2 × 1, the data are consistent with the Lander-Phillips model.     

NO在Ag/Pt(110)-(1×2)双金属表面的吸附：亚硝酸盐/硝酸盐表面物种的生成

以俄歇电子能谱、X射线光电子能谱和热脱附谱研究了室温下NO在Ag/Pt(110)-(1×2)双金属表面的吸附. 在该双金属表面上观察到了可能的亚硝酸盐/硝酸盐表面物种,其在更高温度下分解生成N₂. 然而,室温下NO在清洁Pt(110)表面和Ag-Pt合金表面上并不会生成这种亚硝酸盐/硝酸盐表面物种. 亚硝酸盐/硝酸盐表面物种的形成归因于高度配位不饱和Ag粒子的高活性及其与Pt基底之间的协同作用.     

Tight—binding calculation of the electronic states of bulk—terminated GaAs（311）A and B surfaces

We have carried out theoretical investigations on the electronic structure of GaAs(311)A and GaAs(311)B surfaces. The bulk electronic structure of GaAs has been described by the second-neighbour tight-binding formalism and the surface electronic structure was evaluated via an analytic Green function method. First, we present the surface band structure together with the projected bulk band of both Ga-terminated and As-terminated for GaAs(311)A and GaAs(311)B surfaces, respectively. In each case, the number of surface states is determined, and the localized surface features and orbital properties of these surface states along Γ-Y-S-X-Γ high symmetry lines of the surface Brillouin zone are discussed. For the Ga-terminated GaAs(311)A (1×1) surface, we have tested two possible structure models, i.e. "the bridge site" and "the hollow site" models. In comparison with the angle-resolved photoelectron spectroscopy studied recently, the results have shown that the surface electronic states of the hollow site model are in good agreement with the experiments, whereas those of the bridge site model are not. So we have concluded that the hollow site model is favourable for the Ga-terminated GaAs(311) (1×1) surface and the bridge site model should be excluded.     

On the electronic structure of cleaved GaAs(110) surfaces exposed to formic acid

GaAs(110) surfaces cleaved in UHV and exposed to HCOOH have been studied by work function measurements (Kelvin method), electron energy loss spectroscopy (ELS) and by low energy electron diffraction (LEED). From the different changes of the work function on n- and p-type material information about intrinsic and extrinsic surface states is derived. In the loss spectra the adsorbed formate species causes a loss near 9 eV. The intensity of the loss near 20 eV generally ascribed to an excitonic transition from the Ga 3d core level into surface states is reduced only by a factor of two after saturation with HCOOH. This might be related to the c(2 × 2) superstructure observed in LEED, which suggests a saturation coverage of half a monolayer.