Electronic Structure and Optical Properties of ZnGeN 2
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Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
have to think of structures derived from wurtzite by cation ordering. It turns out that a well-defined ordering also exists and has been found in a number of II-IV-N 2 compounds.[1-4] The same cation ordering is also found in LiGaO 2 [5], which can be thought of as a I-III-VI 2 analog of ZnO. As a starting point for studying the properties of this family of materials we here present computational results for ZnGeN 2. ZnGeN2 can be synthesized in bulk form by reacting Zn or Zn 3N2 with Ge 3N4 powders [1,2] or in polycrystalline thin film form by a HCI +NH 3 based process similar to the one used for GaN synthesis [6] and more recently has been grown epitaxially by MOCVD [7]. Its crystal structure was determined by neutron diffraction [8]. Here we present the first electronic structure study of this material to the best of our knowledge. METHOD OF COMPUTATION The density functional method is used in the local density approximation and solved using the full-potential linearized muffin-tin orbital method [9]. Care must be taken to treat the Zn 3d bands which are overlapping with the bottom of the N2p valence band. In order to also treat the higher lying 4d like atomic wave function character mixed in with the valence and conduction band states a so-called coupled panel calculations [10,11] is used. Because the interatomic spacings in the nitrides is unusually small, and the structure is open because of the tetrahedral bonding, fairly small Ge and Zn spheres have to be used. This means that it becomes advisable to treat even rather deep semicore levels such as Ge3d as bands, which was thus done here.
Figure 1: Crystal structure of ZnGeN 2 : small spheres N, dark medium spheres Zn, light large spheres Ge.
Figure 2: Brillouin zone of ZnGeN2 . X, Y, Z correspond to a, b, c with a horizontal, b normal to the page, and c vertical in Fig. 1.
As far as structures are concerned, we consider an idealized GaN wurtzite derived structure with the proper cation ordering but without bond relaxation, a structure in which the anion position is relaxed inside its nearest neighbor tetrahedron without cation sublattice distortions and finally, the actual experimental crystal structure. The latter is orthorombic with space group Pna21 . Its relation to the wurtzite can be described as follows. The a-axis of the orthorhombic structure is along the [01-10] axis (or a2-a3) of wurtzite and the unit cell is double the lattice spacing of wurtzite in that direction, i.e.
approximately a,,0x3 with a,, the wurtzite a-lattice constant, the b-axis is along the wurtzite [10001 direction (or a , ) and also takes a doubled lattice spacing, and the c-axis is
along c of wurtzite and takes the same lattice spacing as wurtzite. The actual lattice parameters are a=0.545 nm, b=0.6441 nm and c=0.5194 nm, which is close to a=0.5524 nm, b=0.6378 nm and c=0.5185 nm which one would obtain from GaN lattice constants in the above described way. The crystal structure is shown in Fig. I and th
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