held-jap-85-7697

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Structure and composition of GaN (0001) A and B surfaces

R. Held, G. Nowak, B.E. Ishaug, S.M. Seutter, A. Parkhomovsky, A.M. Dabiran, P.I. Cohen, I. Grzegory, S. Porowski

Journal of Applied Physics 85(11), 7697 (1999).

The growth of GaNon GaN.000 N 1/ could be modeled by a set of rate equations with the following main assumptions: (1) Ga could adsorb into either of two weakly adsorbed states, (2) Ga from these weakly adsorbed states could, in turn, either chemisorb or desorb, (3) NH3 only incorporated N at step edges, (4) Ga was more mobile on a Ga covered surface than on a nitrided surface. With these assumptions the main features of the measured desorption data could be described under conditions where there was either an incident Ga flux and no NH3 flux or where there were both Ga and NH3 fluxes incident on the sample. In the case of Ga adsorption without an NH3 flux incident on the sample, the DMSmeasurements depended on sample history. For GaN.000 N 1/ annealed in NH3 there was a knee in the Ga DMS data that was understood as a competition between desorption from a weak precursor state and chemisorption into a strongly bound state. The surface before and after Ga adsorption was examined with AFM, and distinctly different morphologies were found. The surface annealed in NH3 had an island morphology; after Ga adsorption the surface was featureless. We found that annealing a surface in NH3 could never completely nitride the surface, with between 0.1 and 0.25 ML of strongly adsorbed, non-nitrided Ga remaining. Further, during adsorption of Ga onto a nitrided surface, fitting the rate equations showed that only about half of the incident flux could be used to fill strongly bound, chemisorbed states. Fitting the measured DMS data during growth was more difficult since it had to account for the abrupt change in growth mode, from island nucleation to step flow, while still using the parameters determined from the adsorption experiments without NH3. Further, we observed that on gallided surfaces nitridation began at step edges. Hence we needed to build in that NH3 only reacted at step edges, either those at islands or step edges due to the local miscut. We found terms that could be added to the rate equations, corresponding to growth at step edges, that gave good agreement with the abrupt crossover between excess Ga and excess N growth conditions. Also, the growth rates as a function of substrate temperature and of fluxes gave qualitative agreement with the model. Two small issues in the comparison with model and experiment were noted. First, the exact temperature of the crossover between step flow and island nucleation predicted by the rate equation model, in which terms first order in island perimeter were included, decreased slightly more quickly with increasing temperature than that measured. Second, the actual growth rate would depend on the defect density present, which could change vs time. Nonetheless, the important result is that the rate equation model presented here shows what processes need to be emphasized in a more complete simulation and that the step edges play a key role.

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Contributed by R. Held from pub56k-21-153.dialup.umn.edu. on Tuesday, June 1, 1999 11:55:38 PM
Modified by R. Held from pub56k-20-111.dialup.umn.edu. on Friday, June 4, 1999 8:28:23 AM

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