Theory of Defects, Doping, Surfaces and Interfaces in Wide Gap Nitrides
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light-emitting diodes and lasers, and in high-temperature electronics [1, 2]. The recent demonstration of stimulated emission in the blue region has served to further increase awareness of the potential of nitride-based devices [3] and elicited the interest of a number of theoretical groups [4]. In this paper we describe several of our recent results [5, 6, 7, 8, 9, 10] concerning substitutional group-IV impurities in wurtzite GaN and AIN, electron affinity of AIN surfaces in the wurtzite structure, and the zinc-blende (001) interfaces of AIN/GaN/InN. The group IV impurities are potentially important dopants (e.g., Si is frequently used as a n-type dopant of epitaxial GaN). Both C and Si may also be unintentionally incorporated as contaminants during growth. In general, a group-IV atom is likely to become a donor when incorporated on the cation site, and an acceptor on the anion site. This possibility of the amphoteric behavior critically depends on the conditions of growth. We have recently shown [7] that C is preferentially incorporated on the N site under Ga-rich conditions of growth, while both Si and Ge occupy cation sites under N-rich conditions. However, at high concentrations self-compensation, i.e., a simultaneous incorporation of the dopant on both cation and anion sublattice, is expected, which would limit the doping efficiency. We therefore also discuss the electronic structure of Xcation, XN, and Xcation-XN nearestneighbor pairs. We further consider a possible transition of Xatio, from the substitutional 1
also at Institute of Physics, PAN, 02-668 Warsaw, Poland. 465 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
to the DX-like configuration. Such a transition is commonly accompanied by a capture of electron on a stable (or metastable) state, which quenches the doping efficiency. Some of the results for C were published in [6]. Motivated by the experimental discovery of negative electron affinity at AIN surfaces [11], we have investigated the electron affinity [8]. We find that the 2x2 aluminium-vacancy reconstruction on the aluminium-terminated face has a negative electron affinity and that the 1 x 1 hydrogen-passivated nitrogen-terminated surface has a very small electron affinity. The electronegativity difference between the adatom and the atomic species comprising the surface is found to be a guide for anticipating the presence of NEA. In particular, NEA may occur when the adatom is much more electropositive than the surface atoms. We have also investigated the zinc-blende (001) interfaces of A1N/GaN/InN [9]. The elastic energy of GaN grown on AIN is lower than AIN grown on GaN, indicating that the quality of pseudomorphic growth should be higher when AIN is used as the substrate. Further, the effects of strain on the valence-band offset are significant, as is illustrated by
the calculated values of the valence-band offset for AIN/GaN (-0.58 eV) and GaN/AlN (-0.70 eV). The band offsets of A1N/GaN/InN were computed using the AIN in-plane lattice constant and including strai
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