Observation of Ge-induced electronic states at the Ge:GaAs(110) interface by means of polarization-dependent UPS

Abstract

Strong evidence for Ge-induced valence band states for ∠0.5 monolayer (ML) Ge on cleaved GaAs(110) is presented. Normal emission energy distribution curves have been measured for two different polarization [A⊥ mirror plane (MP) and A∥MP] for photon energies from hν = 14–27 eV at the University of Wisconsin Synchrotron Radiation Center. Four different kinds of Ge-induced features are observed: (i) attenuation of clean substrate transitions, (ii) energy shifts of substrate transitions, (iii) new transitions with hν dispersion, and (iv) new transition with no dispersion. The latter dispersionless transition observed for hν between 17 and 23 eV must originate from a two- dimensional structure, and we propose that it is due to a Ge:GaAs(110) interface state. With respect to the valence band maximum, its binding energy (EB) is 6.8 eV which compares to EB = 7.95 eV predicted for a coverage of 3 ML Ge by Mazur, Pollmann, and Schmeits for a Ge–Ga interface state. The observed interface state shows a relatively strong polarization dependence which implies that the initial state symmetry is odd. Strong Ge-induced transitions are observed with binding energies between 1 and 3 eV which show large dispersion. This means that the initial states must have a three-dimensional character. They are identified as GaAs(110) substrate transitions whose matrix elements are strongly enhanced by the chemisorption of Ge, making them observable in UPS.