Surface Structures, Surfactants and Diffusion at Cubic and Wurtzite GaN

Abstract

Clean and As covered zinc-blende and wurtzite GaN surfaces have been investigated employing density-functional theory calculations. For clean GaN surfaces our calculations indicate the stability of several novel surface structures that are very different from those found on traditional III-V semiconductors. Adding impurities commonly present in significant concentrations during growth strongly modifies surface reconstructions and energies. In particular, we find that arsenic has a low solubility and significantly stabilizes the cubic GaN (001) surface making it interesting as a potential surfactant. Finally, we have studied the diffusion of Ga and N adatoms on both the equilibrium and non-equilibrium surfaces. Our calculations reveal a very different diffusivity for Ga and N adatoms: While Ga adatoms are very mobile at typical growth temperatures, the diffusion of N adatoms is slower by several orders of magnitude. These results give insight into the fundamental growth mechanisms and allow conclusions concerning optimum growth conditions.

Structure of clean and arsenic-covered GaN(0001) surfaces

“…However, as the Ga flux at the surface decreases and with sufficient arsenic coverage, it becomes energetically favorable for the surface to convert to an arsenic covered 2×2 structure. A simple model for this structure would consist of one As-adatom per 2×2 unit cell, [20] although as described below an As-trimer model is also possible. We have also observed that depositing small amounts of Ga on the 2×2 surface at temperatures near 250°C leads to the formation of 4×4 and 5×5 reconstructions, in agreement with the observations of Xue et al [11] and thus providing additional evidence that the arsenic-induced 2×2 structure observed here is the same as that seen of Xue et al A simple model for the 4×4 structure would consist of 3 arsenic adatoms and 1 Ga adatom per 4×4 cell, yielding STM contrast consistent with that reported by Xue et al (dangling bonds are filled on As adatoms and empty on Ga adatoms).…”

“…These techniques have been employed previously to study clean GaN surfaces [22] as well as adsorption of foreign species such as As [20,23] and H [24] on these surfaces. Previous calculations for As-adatoms on the GaN(0001) surface [20] indicate that the As-terminated surface is more stable than the Ga-terminated surface when the As is equilibrated with GaAs. We will show here that the 2×2 Asadatom structure is more stable than the Ga-terminated surfaces even when the As pressure is as low as 10 -9 Torr.…”

GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

“…59 Furthermore, the surfactant segregates at the growth front, without being incorporated. Concerning GaN MBE growth, the addition of As, 60,61 H, 62 or In ͑Refs. 63-65͒ has been reported to favor 2D growth under slightly N-rich conditions.…”

Growth of AlN and GaN Bulk Crystals from the Vapor Phase

“…Since the diffusion barrier for N is high (~1.4eV for (0001) and 0.9eV for (000-1) [29,30], the migration is slow and consequently the residence time of N should be reasonably large. On the other hand, Ga adatoms are very mobile (~0.4eV for (0001) and 0.2 eV for (000-1) [29,30], the probability that Ga atoms capture N atoms is much higher than the other process where N atoms form molecules and desorb from the surface. Thus, if there are enough Ga atoms present, which is the case under Ga-rich growth conditions, the incorporation probability of N atoms is enhanced.…”