Adatom diffusion at GaN (0001) and (0001̄) surfaces

Abstract

The diffusion of Ga and N adatoms has been studied for the technologically relevant wurtzite (0001) and (0001) surfaces employing density-functional theory. Our calculations reveal a very different diffusivity for Ga and N adatoms on the equilibrium surfaces: While Ga is very mobile at typical growth temperatures, the diffusion of N is by orders of magnitudes slower. These results give a very detailed insight of how and under which growth conditions N adatoms can be stabilized and efficiently incorporated at the surface. We further find that the presence of excess N strongly increases the Ga diffusion barrier and discuss the consequences for the growth of GaN.Recently, great progress in fabricating highly efficient GaN-based devices has been achieved [1,2]. Nevertheless, there are still substantial problems concerning growth optimization and insight into the fundamental mechanisms is rather shallow. Investigations of molecular beam epitaxy (MBE) growth have shown that the film structure and morphology are very sensitive to the III/V ratio [3,4]. GaN films grown under more N-rich conditions are rough and faceted while going towards more Ga-rich conditions a smoother surface morphology and better film quality are obtained [3].In order to improve growth in a systematic way it is essential to understand the underlying kinetic processes such as adsorption, desorption, and surface diffusion. In particular, adatom diffusion on surfaces is considered to be a key parameter controlling the growth rate, the material quality, and the surface morphology. Experimentally, an analysis of surface diffusion is difficult: So far only effective diffusion barriers have been obtained for GaN [4,5]. It is also not clear whether the cation or anion surface diffusion is the rate limiting process [4]. A problem in growing GaN is also the high N vapor pressure and considerable efforts have been made to enhance the N incorporation. However, the mechanism by which N is incorporated at the surface could not be identified. Theoretically, only few studies to describe GaN growth have been reported, which were either based on thermodynamic models [6] or Monte Carlo simulations [7]. These approaches are useful to model growth on a mesoscopic scale. However, the microscopic meaning of the effective parameters and the underlying microscopic processes remain unclear. In order to identify the fundamental growth mechanisms, but also to improve the above mentioned methods, the correct microscopic parameters are needed [8].We have therefore performed a comprehensive and detailed study of the migration and energetics of Ga and N adatoms on GaN surfaces, employing totalenergy density-functional theory calculations with ab initio pseudopotentials. We will focus on the two technologically relevant orientations for wurtzite GaN, the (0001) and the (0001) surfaces. The adatom substrate system is modeled by supercell geometries with at least 9 layers of GaN, a 14 bohr vacuum region and (2 × 2) periodicity. One side of the slab is passivated by fractional pseud...

Adatom Kinetics On and Below the Surface: The Existence of a New Diffusion Channel

“…In order to test this hypothesis, we have calculated diffusion pathways and migration energy barriers for Ga and N adatoms on the In-adlayer surface and compare these with prior calculations for bare GaN (0001) (i.e., in the absence of the In adlayer) [18]. For Ga adatoms on top of the In adlayer, we find qualitatively the same adsorption sites and diffusion pathway as on bare GaN (0001).…”

“…We note that at high temperatures concerted interaction may modify this value. In contrast, diffusion of N adatoms on the bare surface occurs with a barrier of 1.3 eV [18]. An analysis of the PES for on-surface diffusion showed that diffusion barriers are significantly larger ( 1:5 eV) and that this PES is connected to the PES for AELD: For both PES the energetically most stable adatom configuration is the subsurface (H3) hollow site.…”

Ga Adlayer Coverage and Surface Morphology Evolution during GaN Growth by Plasma-assisted Molecular Beam Epitaxy

“…The smoothing of the surface and the decrease of pit density are attributed to the Ga adatom surface diffusion. Since the surface adatom mobility is higher under Ga-rich conditions, faster adatom transport facilitates the filling of the surface pit defects and planarizes the morphology [13]. Once the coverage exceeds 2ML [ Fig.…”

Adatom density kinetic Monte Carlo: A hybrid approach to perform epitaxial growth simulations

“…This approximation is expected to be well fulfilled, since previous theoretical and experimental studies showed that diffusion barriers on semiconductor surfaces are of the order of a few tenths of an eV up to a few eV. 8 Under these conditions transition state theory applies and the transition rate for an adatom jump from site i to iϩ1 is given by 9,10 …”