Exciton-phonon stimulated emission in ZnO crystalline film at room temperature

The near-edge luminescence of zinc oxide epitaxial film grown by molecular-beam epitaxy on a sapphire substrate was studied. Under an optical excitation increase at room temperature, the luminescence spectrum changes radically and a new band appears with a maximum of $\sim$3.17 eV. It has features of stimulated emission i.e. a threshold of nonlinear growth and narrowing of the half-width. Using the model of a one-dimensional amplifier and experimental data, the gain spectrum was calculated, with maximal value being 170 cm$^{-1}$. The analysis of theoretical approaches for calculating the Mott concentration is carried out. For the first time it is shown that nearby the threshold intensity the observed stimulated emission is originated from the second phonon replica of the exciton.