Surface Reconstruction during Molecular Beam Epitaxial Growth of GaN (0001)
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Internet Journal Nitride Semiconductor Research
Surface Reconstruction during Molecular Beam Epitaxial Growth of GaN (0001) A. R. Smith1, V. Ramachandran1, R. M. Feenstra1, D. W. Greve2, A. Ptak3, T. Myers3, W. Sarney4, L. Salamanca-Riba4, M. Shin5 and M. Skowronski5 1Department
of Physics, Carnegie Mellon University, of Electrical and Computer Engineering, Carnegie Mellon University, 3Department of Physics, West Virginia University, 4Department of Materials and Nuclear Engineering, University of Maryland, 5Department of Materials Science and Engineering, Carnegie Mellon University, 2Department
(Received Monday, July 20, 1998; accepted Sunday, August 23, 1998)
Surface reconstructions during homoepitaxial growth of GaN (0001) are studied using reflection high-energy electron diffraction and scanning tunneling microscopy. In agreement with previous workers, a distinct transition from rough to smooth morphology is seen as a function of Ga to N ratio during growth. However, in contrast to some prior reports, no evidence for a 2×2 reconstruction during GaN growth is observed. Observations have been made using four different nitrogen plasma sources, with similar results in each case. A 2×2 structure of the surface can be obtained, but only during nitridation of the surface in the absence of a Ga flux.
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Introduction
Significant progress has been made in the past several years in understanding the kinetics and the equilibrium surface structures formed during growth of GaN by molecular beam epitaxy (MBE) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]. An important aspect of such studies is the reconstruction of the surface formed during growth. As demonstrated by MBE studies of GaAs and other III-V surfaces over the past several decades, surface reconstructions as monitored by reflection high-energy electron diffraction (RHEED) are valuable real-time indicators of temperature and surface stoichiometry during growth. In addition, since the (0001) and (0001) directions of GaN are inequivalent, observation of the surface reconstructions can be used to distinguish the predominant film polarity [11] [12] [13] [14] [15]. Recent studies by scanning tunneling microscopy (STM) combined with first-principles theory have produced an understanding of most of the major reconstructions for both the GaN(0001) (or Ga-face) and GaN(0001) (or N-face) surfaces [11] [12] [13] [14]. Despite the above mentioned progress in understanding the surface structures of GaN, a significant discrepancy exists concerning one particular reconstruction, namely, the 2×2 structure of Ga-face material. This is an important structure since it has been
cited as an indicator of optimal growth conditions during MBE [3] [4]. Growth under Ga-rich conditions in MBE yields flat, smooth surfaces, whereas growth under N-rich conditions yields rough surfaces [8] [9], and we associate the crossover point between these morphologies with the reported transition between 1×1 and 2×2 reconstructed surfaces [4]. A number of groups observe an intense 2×2 RH
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