Ultraviolet-Photoluminescence and Raman Properties of Mg$_{x}$Zn$_{1-x}$O Nanopowders$^{\ast }$

The Mg$_{x}$Zn$_{1-x}$O alloy system may provide a new UV optically tunable family of wide bandgap materials. ZnO has the hexagonal wurtzite structure of bandgap $\sim $ 3.3 eV while MgO has the NaCl cubic structure of bandgap $\sim $ 7.5 eV. Bandgap engineered alloys at the range $\sim $ 3.3 -- 7.0 were achieved. In this communication we present studies on the UV photoluminescence (PL) and Raman properties of wurtzite Mg$_{x}$Zn$_{1-x}$O nanopowders of average size $\sim $ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range of $0\le x\le 0.26$, the room temperature UV-PL was found to be tuned by $\sim $ 0.25 eV towards the UV-spectral range, and the PL emission was established to be due to an excitonic-type recombination mechanism. The first-order LO Raman mode was found to exhibit a blueshift of $\sim$~33 cm$^{-1}$ and the second-order LO a shift of $\sim$~60 cm$^{-1}$. The LO-mode of the nanopowders is discussed in terms of a mixed A$_{1}$-E$_{1}$ symmetry phonon known as a quasi-LO mode. \newline $^{\ast }$L. Bergman, et. al, Appl. Phys. Lett. \textbf{88}, 023103 (2006)