GaN decomposition in H 2 and N 2 at MOVPE temperatures and pressures

Abstract

GaN decomposition rates were measured in H 2, N 2, and mixed H 2 and N 2 flows for pressures and temperatures typically encountered in metalorganic vapor phase epitaxy. The rates for GaN decomposition, Ga desorption, and Ga droplet accumulation, were obtained from weight measurements before and after annealing the GaN films in a close-spaced showerhead reactor. In H 2 at constant temperature, the GaN decomposition rate is enhanced when the reactor pressure is greater than 100 Torr. Unlike H 2, the decomposition rate in N 2 did not change as a function of pressure. The enhanced GaN decomposition rate in H 2 is not due to an increase in the Ga desorption rate, which is constant vs. pressure, but instead is due to H 2 dissociation on the surface followed by NH 3 formation and desorption. NH 3 formation is suggested by the cubic decrease in the GaN decomposition rate as N 2 is substituted for H 2. The measured activation energies, E A, for the GaN decomposition range from 0.34 to 3.62 eV and depend strongly on the annealing conditions. By comparing measured and literature values of the E A, four distinct groupings of the E A are observed. The four distinct groupings of the E A imply that there are possibly four different reactions which limit the GaN decomposition rate. Connections between the GaN decomposition and improved GaN growth are discussed. This includes a discussion of changes that occur in the nucleation layer during the ramp from low to high temperature, as well as increases in GaN grain size as the growth pressure is increased.