SITES FOR HYDROGEN ADSORPTION ON GAAS(001)

The adsorption of hydrogen atoms on the c(2x8) and (1X6) reconstructio ns of GaAs(001) has been studied by multiple internal-reflection infra red spectroscopy. A series of vibrational bands are observed which are due to arsenic hydrides, bridging gallium hydrides and terminal galli um hydrides. These species result from hydrogen adsorption on arsenic dimers, second-layer As atoms, gallium dimers, and second-layer Ga ato ms, respectively. The hydrides on these sites form at about the same r ate during dosing and disappear at about the same rate during heating. The increase in coverage with dosage follows a Langmuir adsorption is otherm 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 a ctivation energy for H-2 desorption are 1.0 +/- 0.5 X 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.