Radiative and nonradiative excitonic transitions in nonpolar (112̄ 0) and polar (0001̄) and (0001) ZnO epilayers Appl. Phys. Lett. 84, 1079 (2004); 10.1063/1.1646749 Defects in ZnO thin films grown on ScAlMgO 4 substrates probed by a monoenergetic positron beam The internal quantum efficiency ͑ int ͒ of the near-band-edge ͑NBE͒ excitonic photoluminescence ͑PL͒ in ZnO epilayers was significantly improved by eliminating point defects, as well as by the use of ZnO high-temperature-annealed self-buffer layer ͑HITAB͒ on a ScAlMgO 4 substrate as epitaxial templates. Negatively charged Zn vacancy ͑V Zn ͒ concentration was greatly reduced by high-temperature growth, and slower postgrowth cooling ͑annealing͒ under minimum oxygen pressure further reduced the gross concentration of positively and negatively charged and neutral point defects, according to the suppression of nonequilibrium defect quenching. The nonradiative PL lifetime ͑ nr ͒ at room temperature was increased by decreasing the gross concentration of point defects, as well as by decreasing the concentration of V Zn . Accordingly, certain point defect complexes incorporated with V Zn ͑V Zn -X complexes͒ are assigned to the dominant nonradiative recombination centers. As a result of the elimination of point defects, a record long nr ͑3.8 ns͒ at 300 K was demonstrated. Because the radiative lifetime ͑ r ͒ is in principle constant in bulk and epitaxial ZnO, the increase in nr gave rise to the increase in int . Rich structures originating from exciton-polaritons and excited states of excitons were eventually observed in the low-temperature PL spectrum of the improved ZnO epilayer on HITAB, of which int of the NBE emission was 6.3% at 300 K.
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