Lattice Site Location Studies of Rare-Earths Implanted in ZnO Single-Crystals
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Lattice Site Location Studies of Rare-Earths Implanted in ZnO Single-Crystals Elisabete M.C. Rita 1,2, Ulrich Wahl 1,2, Armandina L. Lopes 3, João P. Araújo 4, João G. Correia 1,2,5, Eduardo Alves 1,2, José C. Soares 1,2, and the ISOLDE Collaboration 5 1 Instituto Tecnológico e Nuclear, Estrada Nacional 10, PT-2685 Sacavém, Portugal 2 CFNUL, Av. Prof. Gama Pinto 2, PT-1699 Lisboa Codex, Portugal 3 Departamento de Física da Universidade de Aveiro, PT-3800 Aveiro, Portugal 4 IFIMUP, University of Porto, PT-4150 Porto, Portugal 5 CERN-EP, CH-1211 Geneva 23, Switzerland ABSTRACT In this work we report on the lattice site location of rare earths in single-crystalline ZnO by means of the emission channeling (EC) technique. Following low dose (3×1013at/cm2) 60 keV ion implantation of the precursor isotope 169Yb, a position-sensitive electron detector was used to monitor the angular distribution of the conversion electrons emitted from 169Tm* as a function of the annealing temperature up to 600ºC in vacuum. An additional annealing at 800ºC in flowing O2 was performed. The EC measurements revealed that around 95-100% of the rare earth atoms occupy substitutional Zn sites up to an annealing temperature of 600ºC/vacuum. After the 800ºC/O2 annealing, the emission channeling effects decreased considerably. INTRODUCTION Rare earth doped semiconductors exhibit visible or infrared luminescence and are of interest for optoelectronic applications, for instance, as light emitting diodes [1]. Experimental evidence of high luminescence efficiency from rare earths in wide band gap semiconductors [2], such as GaN, pointed out these systems as possible materials for electroluminescent devices. The II-VI compound zinc oxide (ZnO) is one of the materials investigated in that respect. With a band gap of similar width as GaN, this oxide offers the advantage of growing high quality bulk single crystals more easily. Rare earth doping of ZnO during growth has already been described previously in the literature [3-10]. However, the production of doped layers by ion implantation, the main doping technique used in semiconductor technology, has not been exploited to a great extent. A fundamental step to understand the doping effects from implanted elements is to learn their lattice location. In the particular case of low concentrations of implanted elements, this can be achieved using the emission channeling (EC) technique, which makes use of the fact that charged particles emitted from radioactive isotopes in single-crystals experience channeling or blocking effects along crystallographic axis and planes. In this work we report on the lattice location of the rare earth isotope 169Tm* (t1/2=0.66 µs) in a ZnO single crystal using emission channeling, following ion implantation of its precursor 169Yb (t1/2=32 d). The 169Yb probe nuclei decay by electron capture to the excited state 169Tm*, which decays to the 169Tm ground state. The latter transition is accompanied by conversion electron emission, which can be used for EC [11]. Using position-s
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