The decomposition of nitrous oxide over germanium surfaces

Abstract

The decomposition of nitrous oxide over six different samples of intrinsic, p-type and n-type doped crushed germanium single crystals was studied. Experiments were performed between 150 and 650°C at 1 to 20 Torr in a recirculated batch system, and at 10 −6 to 10 −4 Torr in a continuous flow system. Changes in electrical resistance of clean germanium wafers caused by adsorption and decomposition of nitrous oxide were observed in an ultrahigh vacuum system at 10 −9 Torr. For the measurements at high pressures (1 to 20 Torr), the decomposition rates at the critical temperature (the temperature at which the concentrations of intrinsic and extrinsic majority carriers are equal) were highest for samples of highest p-type and n-type doping (log k = − 6.0 and − 7.9 respectively), and were lowest for the least doped sample (log k = − 14.9). Energies of activation for decomposition on the most highly doped p-type and n-type samples were respectively 23.7 and 22.8 kcal/mole, and for decomposition on the least doped sample was 42 kcal/mole. For low pressure (10 −6 to 10 −4 Torr) experiments, activation energies were also low for samples of highest doping and increased to a maximum in the intrinsic range. Thus, at both low and high pressures with bulk p-type or n-type doping, the rate of nitrous oxide decomposition on the surface of germanium was increased. Arrhenius plots obtained in high pressure experiments indicate a compensation effect, with an inversion temperature of 437 °C. In addition, the increase in resistance of an n-type germanium wafer and decrease in resistance of a p-type wafer with nitrous oxide adsorption in UHV indicated that the initial step in the decomposition reaction on a clean surface is electron transfer to the adsorbate.