Polarization and band offsets of stacking faults in AlN and GaN
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Internet Journal Nitride Semiconductor Research
Polarization and band offsets of stacking faults in AlN and GaN J. A. Majewski1 and P. Vogl1 1Walter
Schottky Institut, Technische Universität München,
(Received Saturday, July 25, 1998; accepted Tuesday, September 15, 1998)
We have performed systematic first-principles pseudopotential local density functional calculations of stacking faults in GaN and AlN. Their band offsets and the charge accumulation at stacking fault interfaces has been investigated, taking fully into account the effects of lattice relaxation and electric polarization. We find the stacking fault junctions to be of type I in both materials. However, the intrinsic valence band offsets are close to zero, so that the conduction band offsets result mostly from the differences in the energy gaps between the cubic and wurtzite phases. The charge accumulated at the interface between the cubic and wurtzite phase is found to be 0.009 and 0.003 C/m2 for the AlN and GaN stacking fault, respectively.
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Introduction
Owing to their unique material properties, III-V nitrides have a great potential for optoelectronic and high temperature, high-power microelectronic applications. A key property of the nitrides is their large spontaneous and piezoelectric internal fields [1] that allow a novel type of tailoring of the carrier dynamics and optical properties of nitride devices. One of the characteristic features of the nitrides is the large number of extended defects in these materials, which are mainly caused by the growth on lattice mismatched substrates. The influence of such defects on optical and electric properties of nitrides is not well understood up to now. However, the recent theoretical studies [2] of threading and screw dislocations in wurtzite GaN revealed that these defects are electrically inactive (i.e. without levels in the gap). Stacking faults or zinc-blende inclusions in the wurtzite matrix of AlN and GaN constitute a common type of extended defects [3] [4] [5] in these materials. In addition, it has been suggested that Mg doping can enhance the formation of zinc-blende regions. [6] Indeed, recent optical experiments suggest that the high concentration of stacking faults correlates with the presence of a deep bound exciton. [7] [8] [9] The strong pyro- and piezoelectric character of these materials leads to a pileup of charge at the interface between two different crystallographic phases. This affects the performance of the nitride devices significantly. [10] In the present paper, we have per-
formed ab-initio calculations of zinc-blende inclusions embedded within hexagonal bulk AlN and GaN, and studied their band offsets, and the electric field resulting from the charge accumulation at the wurtzite/zincblende interfaces. In our calculations, the effects of lattice relaxation, and electric polarization have been fully taken into account. Additionally, we have calculated the band structure, and examined the appearance of electrically active interface gap states. Model calculations [11] as w
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