Theoretical study of point defects in GaN and AlN; lattice relaxations and pressure effects
- PDF / 80,564 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 98 Downloads / 258 Views
Internet Journal o f
Nitride S emiconductor Research
Volume 2, Article 18
Theoretical study of point defects in GaN and AlN; lattice relaxations and pressure effects I. Gorczyca High Pressure Research Center A. Svane, N. E. Christensen Institute of Physics and Astronomy, University of Aarhus This article was received on June 7, 1997 and accepted on August 18, 1997.
Abstract Native defects and some common dopants (Mg, Zn, and C) in cubic GaN and AlN are examined by means of ab initio theoretical calculations using two methods: i) the Green‘s function technique based on the linear muffin-tin orbital method in the atomic-spheres approximation; ii) a supercell approach in connection with the full-potential linear muffin-tin-orbital method. We apply the first method to look mainly at the energetic positions of the defect and impurity states in different charge states and their dependence on hydrostatic pressure. The second method allows us to study lattice relaxations. Whereas small relaxations are found near vacancies and substitutional Mg and Zn, the calculations predict large atomic displacements around antisite defects and the substitutional carbon impurity on the cation site.
1. Introduction The III-V nitrides are important materials for optoelectronic device applications. Their optical properties are strongly influenced by native and introduced defects. The nature of these defects is by far not yet fully understood, but intense experimental and theoretical research activity during the past few years provide constantly new data on these systems. Our work, described here, attempts to add some information by means of ab initio calculations of electronic structure properties of defects in GaN and AlN. This work is based on the density-functional theory (DFT), using two calculational methods. The first method is the linear muffin-tin orbital (LMTO) [1] Green‘s function (GF) method [2] It only allows us to study ideal substitutional defects without taking structural relaxations into account. The results are compared to those obtained by our second method, calculations using a supercell approach and the full-potential [3] LMTO method with lattice relaxation included. In both methods we are using the local-density approximation (LDA) [4] to the DFT, by which exchange and correlation effects are accounted for by a simple local potential. In the LMTO GF method the valence electronic structure of the impurity atom is obtained from the Green‘s function G, which is found by solving the Dyson equation with G0, the Green‘s function of the pure crystal host, and ∆V , the perturbation due to the impurity. In the LMTO method the host Green‘s function is calculated from the band structure of the pure crystal within the atomic sphere approximation (ASA), i.e., the crystal volume is approximated by slightly overlapping atom centered spheres, inside which the potential is made spherically symmetric. The fundamental gaps derived from the LDA bandstructures are generally 50-100% too small. To overcome this problem we have chosen to
Data Loading...
