Photoluminescence due to Group IV impurities in ZnO
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Photoluminescence due to Group IV impurities in ZnO J. Cullen1, K. Johnston2, 3, M. O. Henry1,2 and E. McGlynn1 School of Physical Sciences, Dublin City University, Collins Avenue, Dublin 9, Ireland 2 ISOLDE Collaboration, CERN, CH-1211 Geneva 23, Switzerland 3 Technische Physik, Universitat des Saarlandes, D66041 Saarbrucken, Germany 1
ABSTRACT We report the results of photoluminescence measurements on ZnO bulk crystals implanted with both stable and radioactive species involving the group IV impurities Ge, Si and Sn. We previously confirmed the identity of a line emerging at 3.3225 eV as being related to Ge and present here uniaxial stress data which show that the defect responsible has trigonal symmetry. Experiments with Si provide circumstantial evidence of a connection with the well-known line at 3.333 eV. Our measurements indicate that for the case of Sn on the Zn site luminescence is not observed. We also confirm that the I9 and I2 lines are due to substitutional In impurities. INTRODUCTION Steady progress has been made in recent years in unambiguously identifying the principal band edge bound exciton (In) lines in the photoluminescence (PL) spectra of ZnO, and several of these lines have now been assigned to bound exciton recombination at either neutral or ionised group III donor impurities on the Zn site. Nevertheless, several of the dominant In lines remain to be positively identified, in addition to some new lines recently reported [1, 2] in the PL spectra below the usual bound exciton region. These new lines are sharp, with full width at half maximum values similar to those of the In lines. In contrast to the In lines, however, their thermal binding energy is much lower than their spectral binding energy [2]. Ion implantation allows for controlled doping of impurities and for unparalleled control over the depth of these impurities after doping. However, the implantation process can induce defects not native to the semiconductor which may survive annealing procedures, making spectral features that appear after ion implantation difficult to assign unambiguously to the implanted impurity. Radioisotopes have been used in conjunction with traditional spectroscopic techniques for decades [3] and their use removes some of the ambiguity by correlating the decaying (increasing) intensity of a feature to the mother (daughter) isotope in the sample. Radioisotopes have already been combined with PL to accurately study the behaviour of specific dopants in a variety of semiconductors, including ZnO [3,4]. In this paper we expand on earlier work [2] that used radioisotopes to identify a sharp PL line at 3.3225 eV with Ge impurities. We provide more detail on the properties of this Ge-related line, and we also examine samples implanted with Si and with radioactive Ag that decays through In to stable Sn. We also note results obtained by other workers for the case of the group IV impurity Pb [5].
EXPERIMENT For radiotracer ion implantation high quality single crystal ZnO obtained from Tokyo Denpa Ltd. (Tokyo, Japan) an
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