Schottky-barrier and interface formation of Cs/GaSb(110) and Rb/GaSb(110) at room temperature

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Abstract
The Schottky-barrier and interface formation of Cs/GaSb(110) and Rb/GaSb(110) at room temperature are investigated by soft-x-ray core level photoemission spectroscopy using synchrotron radiation. A large splitting (0.9 eV) is found at the Cs 4d core level and indicates two different adsorption sites (having a large difference in ionicity) for the alkali-metal atoms on the GaSb(110) surface with bonding to both cation and anion. The large Fermi-level overshootings observed at very low alkali-metal coverages [0.03 monolayer (ML) Cs or Rb] for the p-type GaSb(110) samples are directly related to the specific donor character of alkali-metal atoms with charge transfer into the empty Ga dangling bond. The unique room-temperature growth properties of alkali metals are also likely to be of relevance. Further Cs or Rb deposition is found to be highly disruptive with large reactive components at Sb 4d but also Ga 3d core levels and a higher reactivity for the smaller alkali metal (Rb). Interestingly, this interface reactivity, which results from surface defects, is also significantly higher for p-type than for n-type samples, suggesting that the nature of the doping might play some role in the interface chemistry. The final Fermi-level pinning position for both p- and n-type GaSb(110) is already achieved at an alkali-metal coverage of about 0.3 ML, corresponding to significant changes in the mode of growth and to the threshold of surface disruption. This pinning position is located at about 0.1 eV above the valence-band maximum, in agreement with a defect acceptor state model.