Arsenic in ZnO and GaN: Substitutional Cation or Anion Sites?

Modifying the properties of ZnO and GaN by means of incorporating arsenic impurities is of interest in both of these semiconductors, although for different reasons. In the case of ZnO, the group V element As has been reported in the literature as one of the few p-type dopants in this technologically promising II-VI compound. However, there is an ongoing debate whether the p-type character is due to As simply replacing O atoms or to the formation of more complicated defect complexes, possibly involving As on Zn sites [1]. In the case of GaN, the incorporation of high concentrations of As has been studied with respect to the formation of GaAs(x)N(1-x) alloys and the related modification of the GaN band gap and its luminescence behaviour. It has been suggested that As in GaN is amphoteric, with its lattice site preference depending on the doping character of the material, i.e. mostly substitutional Ga in p-type but also substitutional N in n-type [2].We have determined the lattice location of implanted As in ZnO and GaN by means of conversion electron emission channeling from radioactive 73As. In contrast to what one might expect from its nature as a group V element, we find that As does not occupy substitutional O sites in ZnO but in its large majority substitutional Zn sites [3]. Arsenic in ZnO is thus an interesting example for an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system. The results are different in the case of As implanted into GaN, where we found roughly half of the implanted As atoms occupying Ga and the other half N sites. The amphoteric character of As therefore certainly plays a role in explaining the extreme difficulties in growing high quality GaAs(x)N(1-x) alloys with values of x above a few percent.A preliminary report will also be given on ongoing emission channeling lattice location experiments using radioactive 124Sb in ZnO and GaN.