Quantum Chemistry Study on the Adduct Reaction Paths as Functions of Temperature in GaN/AlN MOVPE Growth

Abstract

By quantum chemistry calculations with density functional theory method, the adduct reaction paths in GaN/AlN MOVPE growth as functions of temperature were investigated. By comparing the changes of enthalpy ΔH and Gibbs free energy ΔG for reactions between TMX (X = Ga or Al) and NH3 at different temperatures, the most probable reaction paths were predicted. For the formation of adduct TMX:NH3, there exists a critical temperature Teq (Teq ≈ 500 K). When T < Teq, the formation of adduct is highly favored because ΔG < 0 and there is no energy barrier; when T > Teq, ΔG > 0, and the adduct tends to dissociate back into TMX and NH3 with a small energy barrier equivalent to the heat release in the adduct formation; when T = Teq, ΔG = 0 and the adduct formation and dissociation are in equilibrium. For the adduct decomposition into amide, DMXNH2, ΔG < 0 for all calculated temperature range (T = 293.15 K–1473.15 K), and ΔG decreases (more negative) with T increasing. Thus the formation of amide occurs spontaneously provided an energy barrier for the transition state is surpassed. While higher temperature is favored for the amide formation, the reaction requires the preformation of adduct TMX:NH3 which occurs near room temperatures (T < Teq ≈ 500 K). Therefore, the formation of amide DMXNH2 requires a proper temperature gradient. When considering the two-NH3 involved adduct reaction, either the formation of amide-adduct, NH3:DMXNH2, or the formation of 2:1 adduct, NH3:TMX:NH3, ΔH drops down after the second NH3 is combined in, while ΔG > 0 for almost all T's, and ΔG increases with T due to the entropy decrease in the association reactions. Thus the two-NH3 involved adduct formation and amide-adduct formation are disfavored compared to the one-NH3 involved adduct reactions, and this result strengthens the similar hypothesis from both theoretical calculation by Simka et al. and experimental observation by Wang and Creighton.