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Point defects in zinc oxide nanowires: growth and optical properties

Simon Watkins, Simon Fraser University
Thursday, May 30, 2019 - 12:30pm to 1:30pm
PAT C-520

Zinc oxide has been under intense investigation as a low cost earth abundant replacement for gallium nitride in optoelectronics applications. The control of impurities, particularly p-dopants is one of the key limitations hindering the roll-out of ZnO based light emitting diodes. ZnO nanowires (NWs) have also received much attention as candidates for nano LEDs, nano UV detectors, and gas and biological sensors. The low substrate contact area of nanowires makes it feasible to grow nanocrystals of ZnO on foreign substrates like sapphire which are free of structural defects, and as a result can exhibit very narrow spectral linewidths, making them potential candidates for qubits. Unlike conventional III-V materials like GaAs, ZnO NWs exhibit extremely high emission intensity due to a lack of surface recombination. The MOCVD growth process with its extremely high control of dopant incorporation makes it possible to explore various dopants that have been proposed as potential p-dopants for optoelectronics, as well as potential n-dopants for qubit candidates.  We show that large atom group V dopants such as Sb, which have been proposes as p-dopants, actually  behave as shallow donors in ZnO.  DFT calculations indicate that Sb has the lowest formation energy when residing on a group II lattice site, in which case it donates a single electron to the conduction band and behaves like a shallow donor. Our PL studies show that the Sb defect is a shallow donor with the lowest binding energy yet reported in  ZnO. Impurity complexes are also of interest as potential paths to achieving p-type conduction. We discuss our recent work to identify one such example of a defect complex, the I10 center, which we show to contain a Sn-Li pair defect consisting of a Sn double donor bound to a Li single acceptor. Finally we discuss a ubiquitous emission associated with OH radicals at the surface of air-exposed ZnO, the so called surface exciton emission.  Photoluminescence excitation spectra show that this highly efficient recombination band directly captures free excitons which then relax into radiative emission states with a range of binding energies.