Intrinsic unoccupied surface states at GaAs(110)

Abstract

The exact energetic positions of the empty electronic surface states at GaAs(110) have long been in question. Excitonic effects prevent any spectroscopic technique involving the excitation of core levels from yielding more than an estimate of the binding energy. We have therefore employed Bremsstrahlungs isochromat spectroscopy to measure the density of unoccupied electronic states at GaAs(110). An Al-absorption foil in front of a photocathode was used together with the potential modulation technique to form an energy selective detector with a pass energy of 72.7 eV. Thus, a maximum of surface sensitivity could be obtained. The experimental data were analysed in terms of surface and bulk densities of empty states using the r-factor concept of Zanazzi and Jona since the emphasis was on structural peculiarities rather than on best fit in the least squares sense. A surface contribution of almost 50% to the total signal could be determined. It was also possible to identify the theoretically predicted empty surface states b′ 1, S′ 1, B′ 2 and S′ 2. Their energetic positions as well as the relative strengths are in excellent agreement with calculations by Chadi based on the bond relaxation model. Furthermore, while PYS places the B′ 1 state at 0.3 eV below CBM, our analysis rather indicates 0.7 eV above CBM yielding an excitonic binding energy of 1.0 eV.