Arsenic in ZnO and GaN: Substitutional Cation or Anion Sites?
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0994-F01-03
Arsenic in ZnO and GaN: Substitutional Cation or Anion Sites? Ulrich Wahl1,2, Joao Guilherme Correia1,2, Elisabete Rita2, Ana Claudia Marques2,3, Eduardo Alves1,2, Jose Carvalho Soares2, and The ISOLDE Collaboration3 1 Dep. Fisica, Instituto Tecnologico e Nuclear, Estrada Nacional 10, Sacavem, 2686-953, Portugal 2 Centro de Fisica Nuclear da Universidade de Lisboa, Av. Prof. Gama Pinto 2, Lisbon, 1649003, Portugal 3 CERN, Geneva 23, 1211, Switzerland
ABSTRACT We have determined the lattice location of ion 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. 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. In contrast, in GaN the preference of As for substitutional cation sites is less pronounced and about half of the implanted As atoms occupy Ga and the other half N sites. Apparently, the smaller size-mismatch between As and N and the chemical similarity of both elements make it feasible that As partly substitutes for N atoms. INTRODUCTION Modifying the properties of ZnO and GaN by means of incorporating group V impurities is of interest in both of these wide band gap semiconductors, although for different reasons. In the case of the technologically promising II-VI compound ZnO, besides N [1,2] the heavy group-V elements P [1,2], As [2-9], and Sb [10-12] have been reported in the literature as possible p-type dopants. However, there is an ongoing debate whether for P, As, and Sb the p-type character results from these impuritities simply replacing O atoms, thus acting as simple “chemical” dopants [3,4,8], or is due to the formation of more complicated defect complexes [9,12-14]. The relative sizes of Zn, O and P, As, or Sb atoms favour incorporation of the heavy group-V elements on cation sites. Theoretical calculations have suggested that the acceptor action of arsenic in ZnO results from its incorporation on Zn sites in the form of electrically active AsZn−2VZn complexes [13-14]. If this hypothesis were correct, the intentional introduction of such a particular defect would constitute a novel approach to semiconductor doping and present a challenging task for defect engineering. In the case of GaN, the doping with As has been studied with respect to its luminescence behaviour [15-22] and, at higher concentrations, with regards to the formation of GaAsxN1−x alloys and the related modification of the GaN band gap [17,21,23-26].
Unfortunately the growth of GaAsxN1−x compounds encounters significant difficulties, one of the reasons being that the most stable polytype of GaN is hexagonal wurtzite while GaAs crystallizes in the cubic zinc blende structure. However, while, on the As-rich side of the phase
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