Oxygen and hydrogen adsorption on GaAs(110)

Abstract

The interaction of molecular, ‘‘unexcited’’ oxygen and of atomic hydrogen with clean, cleaved GaAs surfaces was studied by Auger-electron and low-energy electron energy-loss spectroscopy as well as by a Kelvin probe. Up to ≊3×105 L of O2 the coverage slightly increases only up to θ=0.05–0.1, depending on the ‘‘quality’’ of the cleave. This chemisorption, which is attributed to cleavage-induced imperfections, is further correlated with the annihilation of the 20 eV energy-loss, usually characterized as an excitonic Ga(3d) core level-to-dangling bond states transition. The main chemisorption is observed after exposures above 3×105 L and yields θ=0.8 after 1010 L of O2. Two successive stages can be distinguished. The first one saturates at θ=0.4–0.5 after 3×106 L of O2. Larger exposures further increase the coverage and cause two new losses to grow, an O(2p) related one at 7 eV and an As(3d) related one shifted by 2.5 eV to larger binding energies. Illumination of the surface with the light of a Xe-arc lamp dramatically stimulated the oxygen uptake. Under the present experimental conditions, the initial sticking coefficient at RT is increased by a factor of 100, and the coverage reaches θ=1.5 after 1010 L of O2. The chemisorption of hydrogen causes the work function to increase on n-type and to decrease on p-type substrates, by this indicating the formation of depletion layers on both substrates. After exposures of up to 5 L the chemisorption-induced surface states of acceptor- and donor-type, respectively, pin the Fermi level at approximately Evs+0.85 eV and Evs+0.63 eV for n- and p-doped samples, respectively. Above exposures of 3 L the 20 eV energy-loss peak is observed to decay, and it has vanished after 3×104 L of hydrogen. This result clearly demonstrates that hydrogen atoms do not initially bond to the surface Ga atoms. In the exposure range from 50 to 3×104 L the intensity ratio of the surface sensitive As(31 eV) and Ga(55 eV) AES peaks decreases by approximately 20% while the Ga(55 eV) signal itself remains unchanged with respect to the primary peak. This result corresponds to a chemisorption-induced loss of approximately 75% of the As atoms in the topmost layer.