TIGHT-BINDING MOLECULAR DYNAMICS STUDIES OF CLUSTERS AND DEFECTS IN GAAS AND ALAS

Abstract

In this work, we investigate the structural properties of (1) GaAs and AlAs clusters and (2) point defects in bulks GaAs and AlAs using the recently developed tight-binding molecular dynamics (TBMD) simulation method. Various possible structures for small GamAsn, and AlmAsn, clusters (m + n = 2 - 6) were fully relaxed using the simulated annealing technique. Relative stabilities were determined based on the cohesive energies per atom of the relaxed structures. The obtained ground state geometries and the relative stabilities of various metastable structures are in good agreement with those obtained in previous studies, which include experimental results as well as more accurate results from ab initio calculations, indicating that the tight-binding scheme, parametrized based on the bulk material properties, is reliable in predicting structures and relative stabilities of clusters. Small AlAs clusters are investigated for the first time. Their structures and properties were found to be very similar to those of GaAs clusters. Being interested in composition dependence of cluster stability, we carried out TBMD simulation of 13-atom clusters of both GaAs and AIAs as well as 55-atom clusters of AIAs. The icosahedral structure was used as the initial configuration for each cluster and it was then fully relaxed using the simulated annealing technique. For both 13-atom and 55-atom clusters, we found that the stability increases monatomically with As/Ga or As/Al ratio which suggests a typical molecular behavior, rather than crystalline. Our investigation on point defects were concentrated in the atomic relaxations near the defects, which were determined by molecular dynamics simulation. Compared to more rigorous ab initio calculation, TBMD may not provide as accurate information concerning electronic structures, but it does allow longer simulation and therefore thorough atomic relaxation. This is clearly shown by the large magnitudes of relaxation obtained in our study. We found that three of the four nearest neighbouring Ga atoms of the As vacancy in GaAs relax outwards while the other Ga atom relaxes inwards. This is in agreement with the results of an earlier TBMD study but contradicts the results of an ab initio calculation. Based on the large atomic relaxation and the dynamic behaviour of the system, we conclude that the structure obtained in the TBMD studies is favoured energetically. In contrast, all the four nearest neighbouring As atoms of the Ga vacancy in GaAs were found to relax inwards in our study. This. However, is in excellent agreement with both results of first principle and the earlier TBMD study. Different atomic relaxations around vacancies were found in AlAs. All the four nearest neighbouring Al atoms were found to relax towards the As vacancy while three of the four nearest neighbouring As atoms relax towards the Al vacancy and the other one relaxes away from it. Similar atomic relaxations were observed around the antisite defects with typically large breathing mode components and relatively smaller pairing mode components. All the four nearest neighbouring of the antisite defects relax towards the defective atoms for both cation and anion types of defects and in both GaAs and AlAs. However, it is found that the perturbation induced by a cation antisite is much larger than that due to an anion antisite. For substitutional defects, all the four nearest neighbouring As atoms of the impurity Al atom at the site for Ga in GaAs relax away from the impurity while all the four nearest As atoms or the impurity Ga atom at the site of Al in AlAs relax towards the impurity. The breathing mode in each case relaxes by an amount which is virtually a constant whereas the pairing modes arc negligible. Thus, the local tetrahedral symmetry is preserved in each case.