Sites for hydrogen adsorption on GaAs(001)

Abstract

The adsorption of hydrogen atoms on the c(2 × 8) and (1 × 6) reconstructions of GaAs(001) has been studied by multiple internal-reflection infrared spectroscopy. A series of vibrational bands are observed which are due to arsenic hydrides, bridging gallium hydrides and terminal gallium hydrides. These species result from hydrogen adsorption on arsenic dimers, second-layer As atoms, gallium dimers, and second-layer Ga atoms, respectively. The hydrides on these sites form at about the same rate during dosing and disappear at about the same rate during heating. The increase in coverage with dosage follows a Langmuir adsorption isotherm with an initial sticking probability of 0.1–1.0 at 303 to 433 K. Assuming a second-order reaction, the average frequency factor and activation energy for H 2 desorption are 1.0 ± 0.5 × 10 8 s −1 and 14 ± 2 kcal/mol (60 ± 8 kJ/mol). However, these rate constants may be shifted below their true values as a result of hydrogen desorbing off multiple surface sites.