Theoretical investigation of Er-O co-doping in hexagonal GaN
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Theoretical investigation of Er-O co-doping in hexagonal GaN Simone Sanna, Uwe Gerstmann and Wolf Gero Schmidt Lehrstuhl für Theoretische Physik, Universität Paderborn, Warburger Straße 100, 33098 Germany. ABSTRACT The co-doping of hexagonal GaN with Er and O is investigated by means of density functional calculations. Predominantly Er-O defect-pairs characterized by a binding energy around 0.5 eV are formed. Different geometric configurations with various orientations (i.e. axial and basal pairs with C3v or C1h symmetry) are expected with similar formation energies. Independent of the particular configuration, the presence of oxygen does not deeply affect the atomic structure and the electronic charge distribution around the Er centers. The relatively high binding energy suggests that Er-O pairs should survive thermal treatment. An investigation of the binding energy per bond indicates that on the other hand Er-Ox complexes (x=2,3,4) are not likely to be formed (differently from Er-O co-doped Si). Rather, as long as the oxygen fluence does not overtake the Er fluence, different oxygen ions will be bound to different Er-centers. INTRODUCTION Rare earth (RE) doped semiconductors are attracting an increasing interest due to their application in light emitting devices and lasers. The optical emission from RE ions consists of very sharp lines (ranging from the UV to IR) which are not affected by the host material and whose wavelength is determined by the energy of the corresponding transition between the 4fstates [1]. Different factors, such as the thermal quenching of the luminescence, the energy transfer from the host to the rare-earth 4f-shells and the efficiency of the luminescent transitions (i.e. the radiative/non-radiative decay ratio) have represented up to date important limits to the realization of customer-ready commercial devices [2]. However, several steps towards the understanding of the excitation and emission mechanisms in different hosts as well as towards the identification of the luminescent centers have been done recently [2,3]. Concerning the optimization of the luminescent devices, both the use of high quality RE-doped samples and codoping with light elements such as C and O have lead to important improvements [5]. It has been demonstrated that in semiconductors such as Si and GaAs the co-implantation of rare earths with oxygen is beneficial for the luminescence [4-7]. In Si and GaAs the co-doping has the dual purpose to increase the luminescence intensity and to reduce the temperature quenching of the emission. This is achieved by the formation of particular RE-O complexes, e.g. Er-O2 [17,18] in GaAs and Er2O3 in Si [19,20]. The effect of co-doping in other hosts with higher electronic band gap is still under debate, though. The knowledge of the effect of oxygen co-doping in RE doped GaN and AlN would be highly desirable, as these semiconductors are particularly suitable hosts for the RE. Indeed, it has been shown that the quenching of the emission at room temperature decreases with increasing ban
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