Optical and Magnetic Resonance Studies of Na-Diffused ZnO Bulk Single Crystals

Photoluminescence (PL) and optically-detected magnetic resonance (ODMR) at 24 GHz were performed on bulk ZnO crystals after diffusion of Na impurities that were explored as an alternate doping source for p-type conductivity. PL at 2K revealed strong bandedge excitonic recombination at 3.361 eV and a broad ``orange'' PL band at 2.17 eV with FWHM of $\sim$0.5 eV. This ``orange'' emission is very similar to that reported previously\footnote{D. Zwingel and F. Gartner, Solid State Commun. 14, 45 (1974).} from thermoluminescence measurements of intentionally Na-doped bulk ZnO and, thus, strongly suggests the incorporation and activation of the Na-diffused impurities. ODMR performed on this ``orange'' PL revealed two signals. The first was a sharp feature with g-value of $\sim$1.96 and is a well-known ``fingerprint'' of shallow donors in ZnO. The second signal consisted of a pair of lines with an intensity ratio of $\sim$3:1 and with g-tensors (g$_{\parallel}$,g$_{\perp}$$\sim$2.008-2.029) very similar to ESR signals attributed previously\footnote{Ibid.} to holes bound to Na impurities located at the axial and non-axial Zn host lattice sites in Na-doped ZnO. Thus, the ``orange'' PL can be tentatively assigned to radiative recombination between residual shallow donors and deep Na-related hole traps.