Photoelectron diffraction effects in XPS angular distributions from GaAs(110) and Ge(110) single crystals

Angular distribution measurements of XPS intensities have been made for various spectral lines from GaAs(110) and Ge(110) single-crystal surfaces. Observed angular distribution curves (ADC's) showed steep intensity variations and sharp peaks due to X-ray photoelectron diffraction (XPED) phenomena. The effects of the type of transition process (photoelectron or Auger), electron kinetic energy and crystal structure on the XPED patterns were examined. Considerably different ADC patterns were observed for high-energy photoelectrons and Auger electrons and for low-energy photoelectrons. ADC's for Ga 3d, As 3d and Ge 3d showed almost the same patterns for scans of the type [110] → [100] → [1$\text{1}$0], but they showed substantially different patterns for [110] → [11$\text{1}$] → [00$\text{1}$] scans. These features correspond well with the structural characteristics of GaAs and Ge crystals. A discussion of the applicability of XPS angular distribution measurements to the geometric analysis of crystal surfaces is presented.     

Photon-induced reactions of No adsorbed on GaAs(110)

The interaction of cw laser radiation with NO adsorbed on GaAs(110) at 90 K has been studied over a wide range of wavelengths from 457 to 900 nm by high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and laser induced desorption spectroscopy (LIDS). Adsorption of molecular NO is observed. By varying the incident laser power, it is found that desorption and dissociation of molecular NO are induced by a nonthermal process. By measuring the time profile and the power and wavelength dependence of the desorption signal, the observed desorption and dissociation of NO are attributed to interactions of the adsorbed NO with the photogenerated carriers which migrate to the surface.     

Surface states on GaAs(110)

The electronic structure of the (110) surface of GaAs is recalculated using the relaxation geometry recently obtained from analyzing elastic low-energy electron diffraction intensity data and a self-consistent ab initio pseudopotential approach. Better agreement is found for the occupied surface states compared with photoemission data, giving support for the new structural model. The influence of convergence of the plane-wave expansion and relativistic effects on the surface states is also examined.     

Bonding of column 3 and 5 atoms on GaAs (110)

Experimental data, in strong disagreement with current theoretical work, shows that the bonding of column 3 elements to GaAs (110) is mainly nondirectional (metallic), with no significant change in GaAs surface lattice reconstruction. Adsorbed Sb, however, is characterized by highly directional bonding. Based on these conclusions, a model of the mechanism of molecular beam epitaxy is presented.     

Photoemission study of alkali/GaAs(110) interfaces

A detailed core-level photoemission study of interfaces between thin alkali films andn-orp-type GaAs (110) formed at different substrate temperatures 85 K and 300 K) is reported. All the interfaces grown at 85 K (with Na, K, Rb, and Cs) were found to be non-reactive, while at 300 K, the interface with Na is reactive and that with Cs remains non-reactive. In case of the non-reactive interfaces, a strong band bending of 1.0 eV is observed forp-GaAs at alkali coverages as low as 0.01 monolayers, but practically none forn-GaAs. This striking asymmetry in band bending is interpreted as a consequence of the donor character of the alkali atoms. On the other hand, an approximately symmetric band bending at low coverages is observed for the reactive interfaces of Na withn- andp-GaAs and assigned to defect states. For high alkali coverages (>2 monolayers), the final band bending is characterizeds by the same Fermilevel position forn- andp-GaAs, independent of the reactivity of the interface, and assigned to metal-induced gap states. Furthermore, systematic trends along the alkali series in Fermi-level position ionization energy, plasmon-loss features, and layer-dependent binding-energy shifts of alkali core levels are discussed.     

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 ± 1.6 Å-thick mixed oxide consisting primarily of As₂O₃ with a small Ga₂O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al₂O₃ layer, but TiCl₄ exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO₂ coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 °C to 135 °C, but no TiO₂ was present from 170 °C to 230 °C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO₂ deposition even after 50 full ALD cycles of TiCl₄ and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297 °C. This allowed TiO₂ deposition at 300 °C in single TiCl₄ pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl₄ exposure deposited TiO₂.     

Growth of Sb overlayers on GaAs(110)

The GaAs(110)-Sb system is studied with AES, EELS, LEED, ellipsometric spectroscopy and SEM. As indicated by EELS Sb atoms are adsorbed first on Ga sites. The AES spectra can be explained by assuming a simultaneous growth of multiple layers on top of a well ordered homogeneous first monolayer (MSM growth mode). The results of ellipsometric spectroscopy confirm the inhomogeneity of the Sb-film as proposed by the MSM mode. Desorption experiments and EELS demonstrate a strong chemical bonding between the first Sb monolayer and the substrate.     

Angular-resolved photoemission from GaAs(110) surfaces with adsorbed Al

GaAs(110) surfaces with adsorbed Al were studied by a combination of angular-resolved valence band photoemission, Ga 3d core level photoemission, low energy electron loss spectroscopy and Auger electron spectroscopy. At room temperature Al is adsorbed on top of GaAs. After heat treatment the compound AlAs is formed at the surface, which is used as a substrate for Ga adsorption to form the inverted structure of the system GaAs + Al.     

The surface geometry of GaAs(110)

Results of a fully dynamical low-energy electron diffraction calculation show that the GaAs(110) surface reconstructs with a top-layer tilt-angle of 27° ≤ ω ≤ 31°. The smaller tilt angle 7° ≤ ω ≤ 10° reported earlier is outside the error limits of the analysis and can be clearly ruled out. The results are independent of which R-factor or which set of existing experimental data is used. Lateral shifts larger than 0.1 Å for the surface atoms are necessary to obtain acceptable agreement with the measured intensity spectra.     

LEED-AES-TDS characterization of Sb overlayers on GaAs(110)

The GaAs(110)-Sb system is studied with Auger Electron Spectroscopy, Low Energy Electron Diffraction, Soft X-Ray Photoemission Spectroscopy and Thermal Desorption. Sb evaporated at room temperature forms a continuous film and a sharp interface with the substrate. For a coverage of one monolayer, LEED indicates a well ordered (1 × 1) structure which produces diffracted intensities very different from those measured from the clean substrate. Thermal desorption experiments show the particularly strong bonding between the first Sb monolayer and the substrate, confirming the large chemical stability observed during photoemission experiments. Two types of structures were considered for GaAs(110)-Sb(1 ML). Sb chains extending along the (110) direction, parallel and anti-parallel to the top layer Ga-As chains, and Sb dimers placed above the surface with one Sb bond to the surface Ga. The preliminary LEED analysis favors the later model.