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Cathodoluminescence spectroscopy of Lithium in-diffusion in ZnO
|Matthew R. Phillips 1, Katie E. McBean 1, Geoffrey M. McCredie , Cuong Ton-That
1. University of Technology (UTS), PO Box 123, Broadway, NSW 2007, Sydney, Australia
Li doped ZnO has been investigated using temperature, depth and power-density resolved cathodoluminescence (CL) spectroscopy. The incorporation of Li was achieved by evaporating 50 nm of Li metal at ~ 1 mPa onto the surface of 1 mm thick, 5 x 5 mm a-plane ZnO sections (hydrothermally grown by the MTI Corporation) and then heating to 950ºC for 30 min without breaking vacuum. CL spectra of the as-received ZnO specimens consisted of the near band edge at 3.25 eV as well as three broad defect luminescence bands centred at 1.95 eV (orange, OL), 2.15 eV (yellow, YL) and 2.42 eV (green, GL). As the temperature was decreased from 300 K to 80 K the dominant GL disappeared giving rise to an OL around 170 K followed by an increase of the YL and reappearance of the GL with further cooling. This behaviour is in agreement with the re-population of thermally ionised shallow donors in donor-acceptor pair (DAP) transitions and also implies that the GL can be attributed to a number of different defect centres. Following heat treatment, an intense broad OL peak centred at 1.96 eV was observed on the Li metal coated side but not the opposite face, which exhibited a weak GL peak. Depth-resolved CL demonstrated that Li in-diffusion had occurred to at least 3 μm (the electron range in ZnO at 30 kV). When cooling to 80K, the Li-related OL peak increased in intensity and no other CL peaks emerged. These results confirm that Li in-diffusion incorporates Li on Zn cation sites (LiZnO) and that the OL originates from a radiative recombination of a free electron with a bound hole at a neutral Li acceptor. This assignment is consistent with the results of power-density resolved CL experiments where OL saturation and a slight red shift were observed with increasing beam current. The Li in-diffusion data also indicates that the OL in the as-received specimen arises from a DAP involving a Lithium acceptor and a shallow donor.
Presentation: Invited oral at E-MRS Fall Meeting 2009, Symposium C, by Matthew R. Phillips
See On-line Journal of E-MRS Fall Meeting 2009
Submitted: 2009-05-11 09:26 Revised: 2009-06-07 00:48