Arsenic has been reported in the literature as one of the few p-type dopants in the technologically promising II-VI semiconductor ZnO. 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. We have determined the lattice location of implanted As in ZnO by means of conversion-electron emission channeling from radioactive 73 As. 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 but in its large majority substitutional Zn sites. Arsenic in ZnO (and probably also in GaN) 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.

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The lattice location of rare earth 167m Er in single-crystalline hexagonal ZnO was studied by means of the emission channeling technique. Following 60-keV room-temperature implantation of the precursor isotope 167 Tm at doses of 1.3−2.8×10 13 cm −2 and annealing up to 900°C, the angular distribution of conversion electrons emitted by the radioactive isotope 167m Er was measured by a position-sensitive electron detector . Interest in zinc oxide has recently increased due to the fact that its structural and semiconducting properties are similar to hexagonal GaN, but that, in comparison to GaN, highquality single crystals of ZnO are easier to grow [13].In analogy with GaN, where the optically active sites of rare earths are considered to be substitutional Ga sites [14,15], optical activity of Er in ZnO is likely to be associated with REs occupying substitutional Zn sites. Several methods for producing RE-doped ZnO have been described in the literature, including sintering [3][4][5], wet-chemical synthesis [6,11], laser ablation [7-10] and co-deposition following evaporation [12]. However, all these methods result in polycrystalline samples, and there exists evidence that the rare earths accumulate at the grain boundaries of polycrystalline ZnO [4]. Ion implantation, on the other hand, which is a widely used method in semiconductor technology for doping single crystals, has so far not been investigated for rare earth doping of ZnO.In this letter we report on the lattice location of ion implanted rare earth atoms in single-crystalline hexagonal ZnO using the emission channeling technique [16]. For that purpose we have measured the angular distribution of conversion electrons emitted by the radioactive isotope 167m Er (t 1/2 = 2.27 s) following implantation of the precursor isotope 167 Tm (t 1/2 = 9.25 d). The recoil energy of the 167 Tm → 167m Er decay is 0.7-0.9 eV only, indicating that the 167m Er lattice site is inherited from 167 Tm. We have already applied this method to study the lattices sites of In order to identify the lattice sites of Er, we have compared the experimental channeling yields to simulated patterns for 167m Er emitter atoms on substitutional Zn sites (S Zn ) and substitutional O sites (S O ) with varying root mean square (rms) displacements. Besides, we also considered a large variety of interstitial sites the location of which has been described in Ref. The electron channeling simulations were carried out with the "many beam" formalism using a number of 20 beams, i.e. representing the electron wave functions by 21×21 Fourier components. In order to describe the crystal structure of ZnO, we adopted two different structural models. While in both models lattice constants of a = 3.2495 Å and c = 5.2069 Å [22] were used, the first approach assumed the Zn-O c-axis bond length to be z = 0.375 c, as in an ideal wurtzite structure. In addition, this approach used isotropic root mean square (rms) displacements of u 1 (Zn) = 0.082 Å and u 1 (O) = 0.085 Å [23], which would correspond to Debye temperatur...