M. Prietsch

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 θ≊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 (θ>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.

Core-level binding energies and Auger electron energies in epitaxial rare-gas layers on graphite (001)

Abstract Layer-dependent shifts of core-level binding energies and Auger electron kinetic energies are reported for epitaxial multilayers for Xe, Kr, and Ar adsorbed on graphite (001). Despite negligible changes of the substrate work function, clearly resolved shifts are observed, which are described by layer-dependent hole screening effects. As with metallic substrates, the Auger electron energies shift three times more than the core-level binding energies, but the magnitudes of the shifts are reduced due to the small electron density in graphite.

Distinction of Band Bending Mechanisms at the Na/GaAs(110) Interface

Detailed As-3d and Ga-3d photoemission studies of the Na/GaAs(110) interface reveal substantial differences in band bending behaviour for n- and p-type substrates and for different substrate temperatures. For nonreactive interfaces grown at 85 K, a strong initial band bending of 1.0 eV is observed for p-GaAs at very low Na coverages ( 0.01 ML) due to electron transfer from Na to p-GaAs holes, while for n-GaAs the opposite mechanism is inhibited. For reactive interfaces formed at 300 K, on the other hand, defect states contribute strongly to band bending at low coverages. At higher coverages, band bending is dominated in both cases by virtual gap states.

Layer resolved photoemission study of the Cs/Si(111)2×1 interface

For Cs multilayers grown on Si(111)2×1 at 130 K, Cs 5p and Cs 4d core levels exhibit shifts to higher binding energies for second-layer Cs atoms as compared to first-layer atoms by 1.0 and 1.1 eV, respectively. For thick Cs layers, Cs 5p emission from bulk and surface atoms is resolved, resulting in a surface core level shift of 0.23 eV. These results are quantitatively described on the basis of the (Z+1) approximation using a Born-Haber cycle. In addition, layer dependent differences of the photoemission line widths and the Cs 5p spin-orbit splitting are reported as well as an anomalous energy dependence of the electron mean free path.

Ballistic‐electron emission microscopy on the Au/n‐Si(111)7×7 interface

Ballistic‐electron emission microscopy (BEEM), performed under ultrahigh vacuum conditions at the room‐temperature‐grown Au/n‐Si(111)7×7 interface, allows a measurement of the BEEM current for tip biases up to ≊ 8 V without a noticeable change in ballistic transmissivity. The differences of the present results to previous reports, where either no BEEM current was observed or the transmissivity was modified when applying high tip voltages, can be explained by the absence of intermixing at the Au/Si interface. Scanning tunneling microscope images of ≊40‐A‐thick Au films reveal a characteristic topography of the metal surface with ≊2.5 A high circular terraces stacked in up to four stages.

Probing the CaF2 density of states at Au/CaF2/n‐Si(111) interfaces with photoelectron spectroscopy and ballistic‐electron emission microscopy

The electronic properties, chemistry, and spatial structure of Au/CaF2/n‐Si(111) metal–insulator–semiconductor (MIS) structures, with epitaxially grown CaF2 layers of a few monolayers (ML) thickness, have been studied by photoelectron spectroscopy, scanning‐tunneling microscopy, and ballistic‐electron emission microscopy. CaF2 films on Si are characterized by flat surfaces with defect lines about 500 A apart, and band bending in Si reduces gradually with increasing CaF2 layer thickness. Au grows on top of the CaF2 layer in the form of hexagonal terraces. A Si segregation to the surface, as observed in case of the bare Au/Si interface, is strongly reduced by the CaF2 intralayer. Ballistic‐electron emission microscopy shows a strong influence of the CaF2 density of states for electron transport through the intralayer. For a 4 ML thick CaF2 intralayer, the position of the CaF2 conduction‐band minimum is derived from the onset of the collector current at 3.3 V. The valence‐band offset at the CaF2/Si interface...

Ballistic‐electron emission microscopy study of the Au/Si(111)7×7 and Au/CaF2/Si(111)7×7 interfaces

Ballistic‐electron emission microscopy (BEEM) has been performed on Au/n‐Si(111)7×7 and Au/CaF2/n‐Si(111)7×7 in UHV. In both cases, the topography of the Au surface is characterized by ≊2.5 A height terraces, stacked in several stages, with rounded shapes for Au/Si, and hexagonal shapes for Au/CaF2/Si. BEEM up to tip voltages of 8 V on Au/Si is not altering the ballistic transmissivity, in contrast to previous work on Au/Si interfaces which involved chemical preparations of the Si surfaces. The shape of the BEEM spectra on Au/CaF2/Si depends on spectral features of the density of states of the CaF2 thin film.

Enhanced oxidation of GaAs(110) by adsorbed K atoms

Abstract Data taken with a variety of experimental techniques ranging from Auger electron spectroscopy and work function changes to synchrotron-based photoelectron spectroscopy prove that K adatoms can efficiently enhance the oxidation of GaAs in close analogy to their role in Si oxidation. Alkali-covered GaAs substrates display an enhancement in the kinetics of oxidation much larger than those previously observed in electron or laser irradiated surfaces. Complete removal of the alkali adlayer by thermal treatments has not been achieved without changing the stoichiometry of the substrate oxides, in contrast to the case of Si.

Ballistic‐electron emission microscopy at metal/GaP(110) interfaces: Electron transport and Schottky‐barrier heights

Ballistic‐electron emission microscopy (BEEM) was performed for Au and Mg films on n‐type GaP(110). BEEM spectra taken with tip voltages up to 6 eV reveal a strong dependence of the spectral shape on metal‐film thickness and on the type of the metal. Monte Carlo simulations of the electron transport allow a quantitative description of the BEEM current, and provide information on the tunneling process, on hot‐electron transport in the metal film, on transmission across the interface, and on impact ionization within the semiconductor. In addition, the simulations quantitatively account for the observed dependence of the BEEM current on topographic features. The Schottky‐barrier heights were also found to increase with the thickness of the metal film, demonstrating that Schottky‐barrier formation is not yet completed when the overlayer metallizes.

Surface photovoltage at Cs/GaAs(110): Photoemission experiments and theoretical modeling

The effects of a surface photovoltage on band bending were investigated with photoemission for interfaces of thin Cs films with p‐type GaAs(110) as a function of substrate dopant concentration n and temperature T. Theoretical calculations, which consider recombination by both thermionic emission and tunneling, were found to be in excellent agreement with experimental data. As a result, surface photovoltages are found to contribute only for low dopant concentrations and at low temperatures, with limits depending on the Schottky‐barrier height, ΦB : n 2.5 ML), the surface photovoltage is strongly reduced due to edge‐leakage currents. Finally, general predictions for the occurrence and magnitude of a surface photovoltage will be given, and recent data will be critically reviewed.