Reaction Pathway of Surface-Catalyzed Ammonia Decomposition and Nitrogen Incorporation in Epitaxial Growth of Gallium Nitride

Abstract

We report density functional calculations that clarify atom-scale mechanisms of vapor-phase epitaxial growth of GaN with gas sources of trimethylgallium and ammonia. We identify various stable adsorption structures of a Ga atom and NH x molecules (x = 0–3) on GaN(0001) surfaces and find that NH2 and NH units spontaneously intervene in Ga–Ga surface bonds on Ga-rich GaN(0001) surface. We then explore the reaction in which NH3 on the surface is decomposed and becomes an N-incorporated structure, −Ga–(NH)–Ga–, on the Ga-rich GaN(0001) and find that the reaction occurs with the energy barrier of 0.63 eV. Further exploration reveals that a reaction of H2 desorption occurs with the energy barrier of 2 eV, leaving the N atom incorporated in the Ga–N network. This barrier can be overcome when we consider the chemical potential of an H2 molecule in the gas phase at the growth temperature. This N incorporation on GaN is a new growth mechanism catalyzed by the growing surface.

Computics Approach toward Clarification of Atomic Reactions during Epitaxial Growth of GaN

“…In this article, we present our efforts to clarify microscopic mechanisms of Metal-Organic Vapor-Phase Epitaxy (MOVPE) of gallium nitride (GaN) [9] based on the DFT calculations. These efforts just get started and preliminary in computics viewpoints but the obtained results are already interesting [10][11][12]. We find that the existence of Ga-Ga weak bonds on the growing surface is essential for the epitaxial growth.…”

“…We have found two possibilities (Fig. 1): One is on the top Ga atom (T1 site) and the other is on the Ga adtom (Tad site hereafter) [10]. As shown in Fig.…”

“…ON GAN(0001) Thermodynamical analyses combined with DFT calculations [15] have shown that the growing surface in GaN MOVPE at the typical range of temperature and pressure of gas sources is Ga rich. We have calculated structures and binding energies of a Ga adatom on Ga-face GaN (0001) surface and found that the Ga adatom is adsorbed on top of N atoms (T4 site [10]). Relying on the electron counting rule (ECR) [18,19], the coverage of Ga adatoms is expected to be 0.25 where all the Ga dangling bond become unoccupied.…”

A two-dimensional liquid-like phase on Ga-rich GaN (0001) surfaces evidenced by first principles molecular dynamics

“…On the theoretical front, recent first principle simulations in our group could provide a detailed atomic-level picture of a Ga rich surface, showing that Ga-Ga weak bond are realized on the exposed surface and these can become active sites for subsequent N incorporation [8,9]. Yet, a fundamental knowledge about both the actual microscopic structure of the Ga-rich growing surface and the NH3 decomposition and incorporation in the growing material at growth temperature is demanded.…”

“…Template for JJAP Regular Papers (Jan. 2014)8 both the surface atomic distribution, in terms of PCFs, and the motion of the atoms at the various increasing temperatures, in terms of diffusion coefficients, has revealed that at the experimental growth temperature a two-dimensional liquid state is realized. When this regime is reached, the Ga adatoms incorporated at the surface of the growing structure are rapidly destabilized and their weak bonds with the surface underneath are easily broken.…”