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

Abstract

First principles molecular dynamics simulations have been used as a tool to investigate at an atomistic level the finite temperature behavior of a typical GaN growing surface. At 1300 K, Ga adatoms show a high degree of destabilization and a departure of the diffusivity from an ordinary Arrhenius trend. Two signatures, a drastic change in the radial distribution function and a sudden increase of the diffusion constant, allow to quantify such a diffusion process. Such a high diffusivity results in the arising of a pronounced peak at about 2.6 Å, typical of Ga-Ga bonds, indicating a tendency to cluster together of the Ga adatoms. Along with the remarkable diffusivity observed at 1300 K, this result indicates rather clearly that at the typical experimental temperature, the epitaxial growth occurs not on a rigid surface in which Ga adatoms are tightly bound to specific sites, but rather on a two-dimensional liquid state.

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.…”

“…The epitaxial growth is usually conducted on GaN(0001) surface inclined by less than a few degrees toward either [11][12][13][14][15][16][17][18][19][20] or direction. Such vicinal surfaces consist of terraces and steps and the reactions of the decomposition and the incorporation discussed above occur at the step edges especially in the step-flow growth.…”

“…The initial distance of 6.4 Å corresponds to the relaxed system from which dynamical simulations were started. (after Bui et al[12])…”

“…The dashed red line shows the Arrhenius plot obtained by excluding the point at 1300 K, where the system deviates from the standard trend. (after Bui et al[12])…”

An atomistic insight into reactions and free-energy profiles of NH3 and Ga on GaN surfaces during the epitaxial growth

“…Along this line, total-energy electronic-structure calculations based on DFT have been attempted to unveil atomic reactions during GaN epitaxial growth [17][18][19]. The proposed reaction from those DFT calculations is that NH3 is adsorbed on the Ga-rich surface, the subsequent NH unit after NH3 decomposition diffuses easily to the surface step edges, and then N is incorporated into GaN films Former studies assuming H2 desorption in the final product, although insightful, were lacking all dynamical aspects of the reaction and a thorough investigation of other reaction channels.…”