Unoccupied electronic states on Ni(100) induced by A c(2 × 2) oxygen overlayer

Bremsstrahlung isochromat spectra at hω₀ = 9.7 eV for electrons normally incident on a clean Ni(100) surface are compared to emission from Ni(100) with an ordered c(2 × 2) oxygen overlayer. We observe strong emission due to adsorbate induced antibonding electronic states near E_F and a simultaneous attenuation of previously identified direct bulk interband transitions. The results are in accord with theoretical predictions.     

Unoccupied electronic states in adsorbate systems

Experimental work on unoccupied electronic states in adsorbate systems on metallic substrates is reviewed with emphasis on recent developments. The first part is devoted to molecular adsorbates. Weakly chemisorbed hydrocarbons are briefly discussed. An exhaustive inverse photoemission (IPE) study of the CO bond to the transition metals Ni, Pb, and Pt is presented. Adsorbed NO is taken as an example to demonstrate the persisting discrepancies in the interpretation of IPE spectra. Atomic adsorbates are discussed in the second part. The quantum well state model is applied to interpret the surface states in reconstructing and non-reconstructing adsorption systems of alkali metals and hydrogen. A recent controversy on the unoccupied electronic states of the Cu(110)/O p(2×1) surface is critically reviewed. The quantum well state model is then compared to tight binding and local-density-functional calculations of the unoccupied bands and the deficiencies of the various approaches are pointed out. Finally, the relation between the surface state model and more chemically oriented models of surface bonding is briefly discussed.     

Surface and interface electronic properties of AlGaN(0001) epitaxial layers

AlGaN layers with Al content varying over the whole range of compositions were grown by molecular beam epitaxy (MBE) on n-6H-SiC substrates. The band gap energy is obtained from the vanishing of Fabry–Pérot oscillations in a fit to optical reflection spectra near the band gap absorption edge. The surface potential was determined by in-situ X-ray photoemission spectroscopy (XPS) and is found to increase as a function of the Al content from (0.5±0.1) eV to (1.3±0.1) eV, from GaN to AlN. A Si₃N₄ thin passivation layer was formed in-situ onto a 2DEG AlGaN/GaN structure. The mechanism underlying the passivation of high electron mobility transistor (HEMT) structures is suggested to be based on the formation of interface states, which keep the Fermi level fixed at a position close to that of the free AlGaN surface. PACS 73.20.-r; 73.40.-c; 73.40.Kp     

Time-resolved two-color interferometric photoemission of image-potential states on Cu(100)

We describe an interferometric time-resolved photoemission technique that makes it possible to simultaneously observe the decay of optical induced polarizations and populations at surfaces in a two-color excitation scheme. In this scheme initially unoccupied electronic surface states are coherently excited by the interaction of laser pulses with frequency ω_a and the two-photon polarization which is induced by laser pulses with frequency ω_a/2. Interference is observed by changing the delay between both laser pulses using an actively stabilized two-color Mach–Zehnder interferometer. We demonstrate this technique for excitation of the n=1 image-potential state on a Cu(100) surface. PACS 78.47.+p; 79.60.Bm; 73.20.-r; 82.53.Kp; 42.50.Md     

A photoelectron spectroscopy study of ultra-thin epitaxial alumina layers grown on Cu(111) surface

Thin epitaxial alumina layers were grown on the Cu(111) surface using simultaneous aluminum deposition and oxygen exposure. Different substrate temperatures during the deposition resulted in layers with different thicknesses, growth rates, crystallinity and epitaxy. Low energy electron diffraction measurements confirmed the epitaxial growth for substrate temperatures above 870 K. The Al 2p doublet was studied by means of photoelectron spectroscopy in order to determine the alumina termination at the metal-oxide interface. A strong dependence on the preparation temperature was found and both aluminum and oxygen terminated interfaces were created.     

Photoelectron spectroscopy of Ar/Cu(100) interface states

Buried interface states in Ar/Cu(100) were studied by means of one- and two-photon photoemission experiments. With increasing Ar overlayer thickness, a transition from broad electron scattering resonances in the Ar conduction band into a hydrogen-like series of quasi-bound states at the Ar/Cu interface was observed. The thickness dependence of energies and lifetimes is compared to theoretical resonance positions and linewidths derived from a parameterized one-dimensional potential. PACS 73.20.-r; 73.40.Ns; 79.60.-i; 78.47.+p     

Influence of Ar,Kr, and Xe layers on the energies and lifetimes of image-potential states on Cu(100)

The influence of well-ordered adlayers of Ar, Kr, and Xe on the energetic and dynamical properties of image-potential states on Cu(100) has been investigated in a comprehensive study using time-resolved two-photon photoemission (2PPE). The effect of these insulating films varies systematically with the electron affinity EA of the condensed rare gases and with the film thickness. For the electron-repulsive Ar layers (EA=-0.25 eV), a strong lifetime increase of the n=1 state from 40 fs on clean Cu(100) to as much as 10 ps at a coverage of 5 monolayers is observed. For Kr and Xe layers (EA=+0.3 and +0.5 eV, respectively), decoupling from the metal is less efficient. These layers exhibit quantum-well-like resonances of the n=2 state as a function of layer thickness. The energies of the series of states depend characteristically on the affinity level and the dielectric constant of the films. A microscopic model is developed that includes the discrete atomic structure of the adsorbate layers. It is capable of describing the experimental results to a high degree of quantitative agreement. PACS 78.47.+p; 73.20.At; 77.55.+f     

Unoccupied electronic states and the interface formation between oligo(phenylene-vinylene) films and a Ge(111) surface

Thin films of tri-oligo(phenylene-vinylene) end-terminated by di-butyl-thiole (tOPV) were thermally deposited in UHV on Ge(111) substrates. The surface potential and the structure of unoccupied electron states (DOUS) located 5–20 eV above the Fermi level (E _F) were monitored during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The electronic work function of the surface changed during the film deposition until it reached a stable value of 4.3±0.1 eV at a tOPV film thickness of 8–10 nm. Deposition of the tOPV under 3 nm led to the formation of intermediate DOUS structures that were replaced by another DOUS structure along with an increase in the tOPV deposit thickness up to 8–10 nm. The occurrence of the intermediate DOUS structure is indicative of a substantial reconfiguration of the electronic structure of the tOPV molecules due to the interaction with the Ge(111) surface. Analysis of the TCS data allowed us to assign the unoccupied electronic bands in tOPV located at 5.5–6.5 and 7.5–9.5 eV above the E _F as π* bands and at 11–14 and 16–19 eV above E _F as σ* bands.