Structures and composition-dependent polarizabilities of open- and closed-shell Gan Asm semiconductor clusters
Résumé
A systematic investigation of the structures and the dependence of the dipole polarizabilities on the composition of closed and open shell gallium arsenic clusters is presented. Our investigation focuses on nine gallium arsenide (GaAs) clusters with five and six total number of atoms of systematically varying composition. These clusters are the smallest species of small GaAs clusters which have attracted substantial attention due to their strongly oscillating experimental polarizability values. The ground states of those clusters have been determined using a global approach which combines molecular dynamics and an automatic procedure of comparing and selecting cluster structures based on pattern recognition techniques. The polarizabilities have been studied by means of coupled cluster techniques complemented by a semi-empirical hybrid functional which includes corrections from perturbation theory. Our global structural investigation found two different structures for Ga4 As2 and Ga1 As5 which are lower in energy than the previously reported ones. The performed polarizability investigation suggests that open shell GaAs systems composed of five atoms are not more polarizable than closed-shell clusters built by six atoms as is indirectly implied by the reported experimental data. Also, the polarizabilities of those species increase as a function of the number of Ga in a monotonic but not systematic manner. The observed increase type is explained by the large atomic polarizabilities of Ga and in terms of the particular structural and bonding features of a given cluster. Furthermore, the comparison between our theoretical values and earlier experimental polarizability estimations clearly shows that for the five-atomic clusters, the reported experimental polarizability is not largely overestimated as was previously believed. Our results for the six-atomic cluster demonstrate that the polarizability per atom of a six atomic GaAs cluster of any composition is larger than the polarizability of the bulk material, contrary to what has been demonstrated by the experiment. © 2009 The American Physical Society.