Growth and morphology of group-III nitrids
J. Neugebauer1, T. Zywietz1, M. Scheffler1, J. E. Northrup2,
and R. M. Feenstra3
1Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, D-14195 Berlin, Germany
2Xerox Palo Alto Research Center
3333 Coyote Hill Road, Palo Alto, CA 94304, USA
3Department of Physics, Carnegie Mellon University,
Pittsburgh, Pennsylvania 15213, USA
A crucial step towards controlling and improving MBE growth is a better understanding of the fundamental mechanisms during growth. First principles total energy calculations employing density-functional theory have emerged as a powerful and accurate tool to study various static and dynamic properties of surfaces such as atomic geometry, stability, electronic structure, and adatom kinetics. For the example of group-III nitrides we will show, how these methods can be used to obtain a very detailed insight into the structure, stability and formation of surfaces. We will focus on two topics. First, we will present results concerning the structure and adatom kinetics of clean GaN surfaces . It will be shown that these surfaces exhibit properties not observed for "traditional" III-V semiconductors: surfaces may become metallic, novel surface structures are formed and surfaces tend to be cation rich. These properties have interesting implications for the growth  and the formation of extended defects .
Second, the effect of indium on the surfaces will be discussed. Experimentally In is employed to form InGaN quantum wells which are used as active regions in optoelectronic devices. Recent experiments indicate that small amounts of In result in smoother surface morphologies. We have therefore studied the atomic structure, energetics and adatom kinetics of In-covered GaN surfaces. For the technologically relevant wurtzite (0001) surfaces our results show that indium adatoms reduce the surface energy and form a liquid-like metallic film consisting of two and more layers. This film strongly influences the migration paths and the diffusion barriers: The presence of the metallic film opens an unexpected but very efficient subsurface diffusion channel which strongly enhances the mobility for N adatoms. Based on these results we discuss optimum growth conditions, the role of In as a surfactant, as source of extended defects  and its tendency to form spatial fluctuations in InGaN alloys .
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