The active site for dissociative adsorption of H2: Was Langmuir right?

The kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide (0 0 1) has been determined using reflectance difference spectroscopy to monitor the phosphorus coverage in real time. Assuming a Langmuir adsorption mechanism, phosphine exhibited an initial reactive sticking coefficient at 500 °C of 8.7 ± 1.0 × 10−2, 3.5 ± 1.0 × 10−2 and 1.0 ± 0.2 × 10−3 on the GaP (2 × 4), GaP (1 × 1) and InP (2 × 4) reconstructions, respectively. The sticking coefficient increased with temperature on the gallium phosphide surfaces, exhibiting an activation energy of 0.5 ± 0.2 eV, while on indium phosphide, no temperature dependence was observed. The desorption of phosphorus from the GaP (2 × 1) surfaces was first-order in coverage with rate constants of 5.0 × 1015 (s−1) exp(−2.6 ± 0.2 (eV)/kT). These results may be used to estimate the feed rate of phosphine relative to the group III precursors during the metalorganic vapor-phase epitaxy of gallium and indium phosphide.