Electronic Structure and Optical Properties of ZnGeN 2
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G6.11 (1999) ABSTRACT The electronic band structure, structural and bonding, and some linear and nonlinear optical properties are calculated for a new ternary nitride compound ZnGeN2 using firstprinciples methods. Good agreement is obtained with crystallographic data and with absorption data on the band gap. The prospects for use as nonlinear optical material are discussed. INTRODUCTION The most common way to achieve band-structure engineering of semiconductor compounds is to make alloys using cations or anions from the same column of the parent compound. For example, GaN can be modified by making InxGa1-xN alloys, i.e. replacing Ga by another group III element In. On the other hand, one may also replace every other group-III element by a group II and a group IV element. This will lead to a new kind of ternary compounds II-IV-N2, such as, e.g. ZnGeN2. This kind of chemical substitution is well known in more traditional III-V compounds: e.g. GaP →ZnGeP2. Typically because the replacement of a group III by a group II and IV element leads to stringent conditions on local charge neutrality, this leads to a well defined cation-ordered crystal structure rather than a disordered alloy. This may avoid the disadvantages of alloy disorder scattering in the transport. In addition, most semiconductor alloys are in principle only metastable and some (like InxGa1-xN alloys) suffer from rather severe phase segregation problems. In most III-V semiconductors the III→(II,IV) replacement leads to a superstructure of the zincblende structure, known as the chalcopyrite structure. The ordering is also typically accompanied by a local structural distortion of the tetrahedral first nearest neighbor environment. In other words, the anion makes different bond lengths with each of the two cations and thus is displaced off-center from the cation tetrahedron surrounding it. The latter may in addition slightly distort from the regular tetrahedron shape. This leads to an overall c/a distortion of the tetragonal lattice. These anisotropies in the structure translate in anisotropies in the electronic and optical properties. Among others, this makes the chalcopyrites suitable for second order nonlinear optics: the anisotropy in index of refraction leads to the possibility of phase matching light beams of different frequencies that can be generated by second-order nonlinear optical susceptibilities because the structure is noncentrosymmetric. It also splits certain degeneracies in the band structure and thus plays a similar role as strain in strained superlattices which is well known to be an efficient way to tailor the band structure of semiconductors. The difference is that in II-IV-V2 compounds, this can be done in the bulk semiconductor instead of only in thin films. In the case of nitrides, a similar family of compounds may be conceived but because the natural stacking favored by nitrides is hexagonal (because of the higher ionicity), we now
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