Bandgap Variation at the Isostructural Phase Transformation of Wurtzite InN
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 14
Bandgap Variation at the Isostructural Phase Transformation of Wurtzite InN L. Bellaiche National Renewable Energy Laboratory K. Kunc, M. Besson CNRS and Université P. and M. Curie This article was received on June 3, 1996 and accepted on October 7, 1996.
Abstract The pressure variation of the bandgap, at the isostructural phase transition of wurtzite InN, is determined theoretically, using the first-principles total-energy pseudopotential method. It is found that the bandgap (as well as the structural parameters) exhibit three different types of behavior, in three regions of pressure. Optical experiments at low temperatures could then be employed to directly identify the two different wurtzite phases of InN.
By using the first-principles total-energy pseudopotential method, we recently studied [1] the behavior of wurtzite InN under hydrostatic pressure. All the structural parameters were relaxed in [1], at all volumes considered, and the relative variations of the volume, of the lattice constant, and of the c/a axial ratio were found to be in remarkable agreement with experiment [2], over the whole range of the measured pressures (see Figure 1). These calculations point to the existence of a second order isostructural phase transition which occurs in the same pressure range as the wurtzite–rocksalt first-order phase transformation, i.e. between 12 and 15.5 GPa. This isostructural phase transition completely changes the behavior of the structural parameters, as can be seen from Figure 1. For instance, it induces a sharp decrease of the axial ratio c/a and an increase of the internal parameter u, which indicates that the In and N sublattices slide relative to each other until a new stable position is reached around 16 GPa. In this respect, InN behaves quite differently from GaN, for which c/a is nearly independent of pressure [2]. Since it was shown in Ref. [1] that a specific second-neighbor interaction between indium atoms was instrumental in the isostructural phase transition process, the difference of behavior between InN and GaN can be related to the difference of the cation atomic sizes between the two nitrides. We also proposed in [1] that this new phase transition, which still awaits experimental observation, can be considered as a pretransitional effect announcing the reconstructive first order transition. The existence of pretransitional phenomena is not unknown to solid state physics, but these effects have been so far established in rather few semiconductors. For instance, in the mercury chalcogenides HgSe and HgTe [3] (which are tetrahedrally coordinated, too), the first order transition from the zincblende to the cinnabar structure is preceded by a strongly non-linear behavior of the elastic constants, leading to their fast decrease with pressure, just prior to the transition. Pretransitional effects have also been proposed in Ref. [4], as an explanation of the Si and InSb diffraction patterns recorded, as well, just prior t
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