Photoemission spectroscopy and microscopy of n-, p-GaAs(110) homostructures

Zinc-blend III–V semiconductors being cleaved along nonpolar (110) planes yield atomically clean surfaces due to their charge neutrality, which can be employed as mirror planes for optical resonators. As a result of the Ga3d core level photoemission measurements after the n-, p-GaAs (110) plane cleavage in situ of doped semiconductors with 14 nominally identical p-n homojunctions having optically flat cleaved surface under the optical microscope inspection, the estimate of the Fermilevel positions on energy scale were acquired for p- and n-epilayers. The two-dimentional photoemission image for p-n homojunction was studied as well, using the focused synchrotron radiation beam (diameter 0.7 μm, 95 eV, ～10¹⁰ phot./s) at the scanning photoemission microscope. The average value of surface band bending for p-epilayer measured from the energy position of ideal flat-band conditions, was equal to 0.12 ± 0.05 eV, and that for n-epilayer—0.16 ± 0.08 eV, the spectrometer energy resolution being <0.15 eV. The detailed analysis of the experimental data was carried out in order to clarify the deviation fsrom ideal flat band conditions characteristic for absence of surface cleavage defects. Comparison was done with the well-known literature data on theoretical and experimental investigations of cleavage defects. The experimental results leading to different deviations from ideal flat-band conditions for each cleavage are explained by extrinsic local microscopic defects of cleavage which can appear accidentally and unpredictably, since the system dynamics during cleavage is a highly nonlinear many-body process with many degrees of freedom.     

The electronic structure of Si/GaP(110) interface and superlattice

It is argued that the (110) interface between group IV and III–V semiconductors are more likely to lend themselves to fabrication via MBE techniques. Results for the electronic structure of Si-GaP(110) superlattice are reported for the first time. Both, interface states and two dimensionally confined states are found. Sensitivity of the energy and charge distribution of these states to the interfacial geometry and the band edge discontinuity is investigated. The results are contrasted with the situation found for lattice matched III–V compound semiconductor systems such as GaAs/AlAs and GaSb/InAs.     

Reconstruction of the (110) surface of III–V semiconductor compounds

We have used ab initio quantum chemical methods to determine the structure of the clean (110) surface of the nine III–V compounds with III = B, Al or Ga, and V = N, P or As. The theoretical results for small clusters give geometries in good agreement with the experimental geometry for GaAs (the only case for which an experimental structure is available), supporting the use of such clusters in studying the reconstruction of the other systems. We find in all cases that the reconstruction of these (110) surfaces is determined by local valence hybridization forces on the surface atoms. To show the effects of these valence forces we have also carried out extensive calculations on the trihydrides of the above-mentioned elements, for which we have calculated bond distances, bond angles, and the variation of the total energy with bond angle, leading to inversion barriers for the trihydrides of column V elements.     

Quasiparticle corrections to the electronic properties of anion vacancies at GaAs(110) and InP(110)

We propose a new method for calculating optical defect levels and thermodynamic charge-transition levels of point defects in semiconductors, which includes quasiparticle corrections to the Kohn-Sham eigenvalues of density-functional theory. Its applicability is demonstrated for anion vacancies at the (110) surfaces of III-V semiconductors. We find the (+/0) charge-transition level to be 0.49 eV above the surface valence-band maximum for GaAs(110) and 0.82 eV for InP(110). The results show a clear improvement over the local-density approximation and agree closely with an experimental analysis.     

Low-energy electron energy-loss spectroscopy with Ge:GaAs (110) heterostructures

Electronic properties and chemical composition of Ge films grown by molecular beam epitaxy on GaAs (110) surfaces were studied in situ by electron energy-loss spectroscopy. The loss peaks involving core-level excitations proved that As atoms segregate at the surface of the growing film. The well known 20 eV loss peak of the clean GaAs (110) surface, being attributed to transitions from Ga(3d) to Frenkel-type excitons of dangling-bond surface states, was found to persist, slightly shifted to 19.7 eV, with the growing Ge (110) film. Since the Ga coverage amounts to below approximately 0.05 of a monolayer this transition seems to contain a strong intraatom contribution.     

Photoemission studies of the electronic structure of III–V semiconductor surfaces

The photoemission technique using synchroton radiation in the photon energy range 5–450 eV has been applied to the study of the electronic structure of some III–V semiconductor surfaces, prepared by cleavage in situ under ultrahigh vacuum conditions, ? 10^?11 Torr. For p-type GaAs(110), the Fermi level is pinned at the top of the valence band and thus no filled surface states extend into the band-gap. The situation is more complicated for n-type GaAs(110), where band bending easily can be introduced by extrinsic effects (impurities, cleavage quality, etc.) and push the Fermi level down to about midgap. Chemical shifts of inner core levels (3d for Ga and As) are used to obtain information on the bonding site of oxygen on the (110) surface. GaAs(110) can be exposed to atmospheric pressure of molecular oxygen without breaking the bonds between the surface atoms and the bulk. Oxygen is predominantly bonded to the As atoms on the surface. The oxidation behavior is strikingly different for GaSb(110) with formation of gallium and antimony oxides on the surface directly upon oxygen exposure. Heavier oxidation of GaAs(110) and breaking of the surface bonds will also be reported.     

Surface states calculation for the (100), (110) and (111) faces of Si

Using essentially the Heine and Jones model of the band structure for diamond-type semiconductors and a 3-dimensional surface state secular equation, we have calculated the bands of surface states for the (100), (110) and (111) orientations of the surface plane of ideal Si. Our bands for the (110) surface are very similar to those obtained by Jones. Based on these bands of surface states, we can explain reasonably both optical and electrical measurements on the (111) surface.     

Charge transfer from chemical shifts at (110) surfaces of III–V compound semiconductors

Surface charge-transfer are calculated from surface core-level shifts determined by Eastman et al. and Taniguchi et al. by using soft X-ray photoemission spectroscopy. A simple electrostatic model which was first applied by Shevchik et al. to the analysis of bulk data is adapted for surfaces of compound semiconductors. The charge transferred from cations to anions is found to be the same in the bulk and at the (110) surfaces of GaP, GaAs, GaSb, and InSb.     

Calcul des vibrations des atomes de volume et de la surface (110) des composés III–V application à l'arsénieure de gallium

Analytic expressions are given for the mean-square displacements of volume and (110) surface atoms of III–V compounds. An application is made in the case of GaAs for which experimental data are available.     

Oxidation properties of InSb(110) Surfaces

The structural and electronic properties of InSb(110) surfaces, which were cleaned by argon bombardment annealing technique, have been investigated by LEED and surface conductivity measurements during oxygen adsorption. The diffraction patterns before and during exposure exhibit only diffraction spots which are compatible with the bulk periodicity. The exposures resulted in a gradual decrease of all beams. The surface conductivity increases during exposures. The magnitude of the adsorption induced changes is determined by the coverage and by the density of surface defects. In view of these results the oxidation process on the InSb(110) sur face is discussed.     

A LEED-AES study of the oxidation of Fe (110) and Fe (100)

Simultaneous LEED and AES observations have been used to study the initial stages of oxidation of the Fe(110) and Fe(100) single crystal surfaces at 300 K and 400 K and of a clean Fe polycrystal at 300 K. Accurate surface lattice spacings of the precursory oxide structures have been measured and attempts have been made to quantitatively evaluate the corresponding surface oxygen density.On the (110) single crystal surface the final structure is FeO-like with a lattice spacing 4% larger than that of bulk FeO. The transition to the FeO-like structure starts with a surface lattice expansion in the [11&#x0304;0] direction followed by an expansion in the [001] direction in order to accommodate the (111) face of the FeO-like structure. On the (100) single crystal face the oxygen and iron form an fcc (100) face which initially contracts and then expands with increasing oxygen doses. The structure formed at 300 K is spinel-like but heat treatment causes a transition to FeO(100).The changes of the surface unit cell dimensions are interpreted as the result of an interaction between adsorbate and substrate. This interaction is strongest in a direction parallel to the close packed rows of the substrate, making the corresponding periodicity of the adsorbate more resistant to lattice changes.In the case of the polycrystal a hexagonal structure was observed after oxygen adsorption with no simple relation to the oxide structures observed on the single crystals. The initial sticking coefficients in the interval 0–10^?5 torr sec ranged from 0.07 to 0.36 depending on temperature and crystal face observed. The latter dependence is explained in terms of the surface structure.     

Two-dimensional unoccupied electronic band structure of clean Cu(110) and (1 × 2) Na/Cu(110)

Using the technique of angle-resolved inverse photoemission, we have measured the dispersion of an unoccupied Cu(110) surface state for the clean Cu(110) surface and for the (1 × 2) reconstructed Na/Cu(110) surface along the symmetry lines. The dispersion of the crystal-induced surface state of clean Cu(110) at 2.05 eV above the Fermi energy at the point of the SBZ is free-electron-like with an effective mass of (1.0 ± 0.2)m_e at the point, which is in good agreement with other experimental results as well as a theoretical calculation. This surface state shifts to 2.5 eV above the Fermi energy for the (1 × 2) phase of Na/Cu(110) with a coverage of 0.25 ML, and the dispersion along the direction is considerably reduced compared to the clean surface. On the other hand, the dispersion of this state for (1 × 2) Na/Cu(110) (0.25 ML) along the direction is close to that of clean Cu(110). We account for these results within a missing-row picture of the Na-induced reconstruction.     

Accurate He-Ag(110) interaction potential determination by selective adsorption

The first accurate energy level measurements of a He-metal interaction potential are reported for the Ag(110) surface as obtained from selective adsorption data. The fitting of the experimental data with a suitable phenomenological potential leads to a well depth of 6.0 ± 0.1 meV. The potential curve is considerably softer than those obtained for He interaction with either insulators or semiconductors and is in good agreement with theoretical predictions.     

Determination of atomic positions in the GaSb(110) and InAs(110) surfaces by medium-energy ion blocking

In medium energy ion scattering measurements on cleaved GaSb(110) and InAs(110) backscattering contributions from both elements in the compound are resolved. Measurements of blocking directions for each signal provide an independent, real-space determination of the surface relaxation. Bonds connecting first-layer atoms are rotated through an angle ω = 28.5° ± 2.6° out of the (110) plane for GaSb and through ω = 30.0° ± 2.4° for InAs.Bond lenghts are essentially conserved. The root-mean-square thermal vibration amplitudes of the surface atoms are enhanced by a factor 1.6–1.7.     

Molecular dynamics simulation of surface melting behaviours of the V（110） plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V（110） plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V（110） surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V（110） surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.     

Electronic structure of graphitic carbon on Ni(110)

Photoemission experiments show that graphitic overlayers obtained by cracking ethylene on Ni(110) have a significantly different structure from graphite carbon on Ni(111) or Ni(100). Analyses of our data suggest that a complete overlayer of graphite in register with the substrate cannot be formed because of the rough structure of the Ni(110) face. Nevertheless a graphitic-like structure with much of the p_z orbitals saturated can grow along the channels of the (110) surface. These findings are consistent with a previous model deduced by surface extended energy loss experiments performed on the same system.     

关于RHEED振荡技术优化GaAs(110)量子阱生长的研究

在GaAs(110)衬底上生长的半导体材料有诸多优良性能，使得在非极性GaAs(110)衬底上获得高质量各类异质结材料，成为近年来分子束外延生长关注的课题.考虑GaAs(110)表面是Ga和As共面，最佳生长温度窗口很小；反射式高能电子衍射的(1×1)再构图案对生长温度和V/Ⅲ束流比不敏感，难于通过观察再构图案的变化，准确地找到最佳生长条件.作者在制备GaAs(110)量子阱过程中，观察到反射式高能电子衍射强度振荡呈现出的单双周期变化.这意味着不同工艺条件下，在 GaAs(110)衬底上量子阱有单层和双层两种生长模式.透射电子显微镜和室温光致荧光光谱测量结果表明：在双层生长模式下量子阱样品光学性能较差，而在单层生长模式下量子阱光学性能较好，但是界面会变粗糙.利用这一特点，我们采用反射式高能电子衍射强度振荡技术，找到了一种在GaAs(110)衬底上生长高质量量子阱的可行方法. 关键词： 反射高能电子衍射 量子阱 分子束外延     

Photoelectron spectroscopy of sulfur heterocycles adsorbed onto a Cu(110) surface

Photoelectron spectra of Cu(110) after exposure to thiacyclobutane, thiacyclopentane, and thiacyclohexane, as well as of the clean Cu(110) surface, have been obtained using He(I) radiation. Comparison of the spectra of the absorbates with gas-phase spectra suggests a strong sulfur-to-copper dative bond. Sulfur lone-pair orbital stabilization energies were obtained for thiacyclobutane (1.4 eV), thiacyclopentane (1.1 eV), and thiacyclohexane (0.6 eV), and these stabilization energies are correlated with the dative sulfur-to-surface bond energies. Ring-strain changes are suggested as the reason for additional small shifts of absorbate-bond energies in the thiacyclobutane spectrum. Relaxation energies were found to decrease in the order 4- > 5- > 6-membered ring. An empirical linear relation was found between relaxation shifts and lone-pair orbital stabilization energies for monoheteroatom heterocycles bonded to Cu(110) by a lone-pair dative bond. All adsorbates studied showed marked angular dependence, suggesting nonrandom orientation on the surface. Al Kα X-ray data were used to calculate percentage surface coverages. Other sulfur-containing adsorbates are discussed, including thiophene, allyl mercaptan, and thiacyclopropane. Thiacyclopropane was found to decompose upon exposure to copper.     

Optical detection of surface states in GaAs(110) and GaP(110)

We present results on the optical detection of surface states in GaAs(110)and GaP(110)by the method of the fractional change of external reflectivity. Optical transitions are observed at ～3.1 eV m GaAs(110) and ～3.4 eV in GaP. A comparison with existing theories suggests a rotational relaxation model for the surface, with partial relaxation for GaAs(110) and full relaxation for GaP(110).