Adsorbate interactions on the GaN(0001) surface and their effect on diffusion barriers and growth morphology

Abstract

Studying the adsorbate interactions on a surface helps in understanding the growing surface morphologies and calculating the effective surface diffusion barriers. We study the interaction between Ga-Ga, N-N and Ga-N adatom pairs on the polar GaN(0001) surface using ab initio calculations based on density functional theory. The interaction energy between two adatoms on the surface does not seem to follow definite trends with increasing distance between the adatoms. The presence of a number of possible reconstructions on clean GaN(0001) and periodic effects due to the finite size complicate the analysis of the interactions. Various components of the total interaction energy are separated. We find that there is a large substrate lattice distortion caused due to Ga and N adatoms. The resulting elastic interaction is a major component of the interactions between the adatoms on the GaN(0001) surface. The dipolar interaction is much smaller in magnitude. We also evaluate the component of the interaction energy due to the substrate-mediated electronic interactions. The barriers for surface hopping of adatoms are significantly modified in the presence of other adatoms. We identify several possible surface hopping processes for Ga and N adatoms and calculate their barriers. In particular, we find that the N adatom has a lower barrier to move to an adjoining site on the other side of a neighboring Ga adatom. Kinetic Monte Carlo simulations are performed to see the effect of adatom interactions on the growing surface morphologies of GaN(0001). At the submonolayer growth stage, the fast diffusion of N adatoms located near Ga adatoms leads to more regular island features. In this way, we illustrate the role of adatom interactions in the initial surface nucleation and the morphologies of the growing GaN(0001) film.

Dynamics of Atomic Steps on GaN (0001) during Vapor Phase Epitaxy

“…To interpret the combined parameters obtained from the fits, it is useful to estimate the adatom diffusivity D and equilibrium adatom density ρ 0 eq . Ab initio calculations of the activation energy for Ga diffusion on the Ga-terminated (0001) surface have given values of ∆H m = 0.4 eV 57 and ∆H m = 0.5 eV 58 , and similar values have been obtained for 3d transition metal adatoms 59 . An estimate based on spatial correlations in the surface morphology of GaN films grown at two temperatures gave ∆H m = 1.6 ± 0.5 eV 60 .…”

Why and how In composition fluctuations appear in InGaN?

“…Indeed, In is a bigger atom than Ga and bonds are distorted both in the volume and at the surface. Surface reconstruction also occurs in order to minimize the related elastic energy [37,38]. The effect of strain on the formation enthalpy of an isolated Ga or Al atom on a surface as a function of strain has been estimated, showing that the strain modifies this energy by about 0.4 eV [39].…”

Progress in Modeling Compound Semiconductor Epitaxy: Unintentional Doping in GaN MOVPE

“…Qian et al investigated the relationship between stoichiometry and surface reconstruction in GaAs(001), using chemical potentials. Northrup classified stable structures for Si(001)H using the chemical potential of H. In their study of III-nitride semiconductors, Chugh and Ranganathan calculated the migration potential of Ga and N adatoms and their interactions on the surface of GaN(0001). However, all of these approaches incorporated the ground-state geometry of the crystal surface at absolute zero (Figure a, states (5) and (6); surface and solid state at absolute zero, respectively).…”