Chemical reactions of ammonia with polar and non-polar nitride semiconductor surfaces

We present results from ab-initio local-orbital calculations performed to study the impact of ammonia on GaN. We focus on chemical reactions on the non-polar (<10(1)over bar 0>) and (<11(2)over bar 0>) surfaces as well as the dissociative adsorption of NH3 on GaN(<000(1)over bar>), for which an energy barrier is obtained by computing the potential energy surface. For the adsorption of NH3 on GaN(<000(1)over bar>), finite-temperature molecular-dynamics simulations show that the molecule splits into NH2 and H. The NH2 group binds to two of the three second-layer Ga atoms neighboring the vacancy, while the dissociating H binds to a first-layer nitrogen atom. This reaction lowers the total energy of the system by 2.05 eV. The energy barrier estimated for dissociative adsorption of NPT, and H on the vacancy p(2 x 2) reconstruction of the GaN(<000(1)over bar>) surface is 0.5 eV. (C) 1999 Elsevier Science B.V. All rights reserved.