Surface Structures, Surfactants and Diffusion at Cubic and Wurtzite GaN
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Internet Journal Nitride Semiconductor Research
Surface Structures, Surfactants and Diffusion at Cubic and Wurtzite GaN T. Zywietz1, Jörg Neugebauer1, M. Scheffler1, J. Northrup2 and Chris G. Van de Walle 2 1Fritz-Haber-Institut 2Xerox
der MPG, Palo Alto Research Center,
(Received Friday, June 26, 1998; accepted Wednesday, September 23, 1998)
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.
1
Introduction
Despite progress in growing high quality material fo rdevice fabrication, an understanding of the fundamental growth mechanisms of GaN is still in its infancy. Knowledge of the structure of clean and adsorbate covered surfaces and the mechanisms of incorporation and diffusion of adatoms is still incomplete. However, an understanding of these mechanisms is crucial to gain a deeper insight into the relevant growth mechanisms on an atomic scale and to improve growth in a controlled fashion. First-principles density functional theory (DFT) calculations have been applied successfully to understand defects, impurities, doping, and surface reconstructions of non-polar and polar III-nitride surfaces [1] [2] [3] [4]. In this work we review the application of DFT calculations to model the behavior of surfactants (e.g. As) and to determine the energetics of adatom migration on GaN surfaces. We summarize the results obtained for the (001) surface of cubic GaN and the (0001) and (0001) surfaces of wurtzite GaN. The outline of the paper is as follows: In the first part the computational method is described. We then discuss the properties and structure of cubic and wurtzite equilibrium GaN surfaces and the effect of surfactants on the reconstructions and energies of these surfaces. Finally,
the migration paths and diffusion barriers of N and Gaadatoms on selected surfaces are described. Based on these results we discuss consequences for the growth of GaN in a thermodynamic and kinetic context. 2
Formalism
The energy necessary to create a surface is called the surface energy. This energy depends on the specific thermodynamic conditio
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