Sodium acceptor doping of ZnO crystals

ZnO bulk single crystals were doped with sodium by thermal diffusion. Positron annihilations spectroscopy was employed to confirm the filling of zinc vacancies, to \textgreater 6 $\mu $m deep in the bulk. SIMS measurement shows the diffusion of sodium up to 8 $\mu $m with concentration (1 --- 3.5) $\times$ 10$^{17}$ cm$^{-3}$. Broad photoluminescence excitation peak at 3.1 eV, with onset appearance at 3.15 eV in Na:ZnO is attributed to an electronic transition from a Na$_{\mathrm{Zn}}$ level at $\sim$ (220 -- 270) meV to the conduction band. For electrical measurements, Ohmic contacts were made using a MoO$_{2}$ solution (ethyl glycol:H$_{2}$O:: 60:40) on a hot plate at 200 $^{\circ}$C for 10 min. MoO$_{2}$ has a work function of 6.5 eV and can be useful for electrical Ohmic contacts for wide band gap semiconductors. Resistivity in Na doped ZnO crystals increases up to (4 --- 5) orders of magnitude at room temperature.\\[4pt] [1] N. S. Parmar and K. G. Lynn, \textbf{\textit{Applied Physics Letters }}\textbf{106, }022101 (2015).