Excitonic stimulated emission from MgxZn1−xO films due to enhanced exciton binding energy

We report synthesis and excitonic emission properties of high-quality micrometer-thick $\mathrm{M}{\mathrm{g}}_x\mathrm{Z}{\mathrm{n}}_{1\ensuremath{-}x}\mathrm{O}$ films ($x=0.04$ and 0.10) grown on $a$-sapphire substrates by pulsed laser deposition. The band gap ${E}_g$ values of the $x=0.04$ and 0.10 samples have been shown to be larger than that of pure ZnO by \ensuremath{\sim}0.1 and \ensuremath{\sim}0.3 eV, respectively. From the investigations of the stimulated emission due to the exciton-exciton (ex-ex) scattering process at 3 K, the exciton binding energies ${E}_b$ of the $x=0.04$ and 0.10 samples are determined to be $80\ifmmode\pm\else\textpm\fi{}5$ and $120\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, respectively, which are substantially larger than that of pure ZnO (${E}_b\phantom{\rule{0.28em}{0ex}}=60\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$). The present observation demonstrates that ${E}_b$ of ZnO-based materials can be increased to 100% by controlling the synthetic and alloying conditions. Purely excitonic stimulated emissions are observed in the temperature range from 3 to 300 K without showing any symptoms of electron-hole plasma emission. The mechanism of the optical gain changes from the ex-ex to ex-electron (ex-el) scattering process at 200 and 250 K for the $x=0.04$ and 0.10 samples, respectively, with increasing temperature. We argue that the enhancement of ${E}_b$ occurs as a consequence of a complex, combined effect of Mg addition on the effective electron/hole masses and the dielectric constant of the $\mathrm{M}{\mathrm{g}}_x\mathrm{Z}{\mathrm{n}}_{1\ensuremath{-}x}\mathrm{O}$ films.