III-N semiconductors and their alloys are important materials for optoelectronic device applications such as light-emitting diodes and lasers. Recently, InN has attracted significant attentions because experimentally measured band gaps of InN show a wide range of variation from 0.6 to 2.0 eV. The origin of this variation is currently still under debate. Using a band-structure method that includes the correction to the band gap error in the local density approximation [S.-H. Wei et al., Phys. Rev. B 67, 165209 (2003)], we find that the band gap for stoichimetric InN is 0.8 ± 0.1 eV, in good agreement with recent experimental data, but is much smaller than previous experimental value of ~1.9 eV. The unusually small band gap for InN is explained in terms of the high electronegativity of nitrogen and consequently the small band gap deformation potential of InN. To understand the origin of some of the experiments, which show large band gap of InN, we have performed detailed analysis of the band structure of InN. The possible origins of the measured large band gaps are discussed in terms of the non-parabolicity of the bands, the Moss-Burstein shift, and the effect of oxygen. We find that the Moss-Burstein shift plays the dominant role in determining the measured InN band gap. The formation of InN_xO_1-x alloys reduces the band gap, whereas the formation of (InN)/(In₂O₃) superlattices increases the band gap with respect to InN. The effect of non-stoichimetric InN will also be discussed.

Work was done with P. Carrier and supported by the U.S. DOE, under contract No. DE-AC36-98-GO10337

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