Spontaneous and stimulated emission in the mid-ultraviolet range of quantum-well heterostructures based on AlGaN compounds grown by molecular beam epitaxy on $c$-sapphire substrates

This paper reports on the results of investigations of the spontaneous and stimulated luminescence in AlGaN heterostructures with a single quantum well and a high Al content (up to $\sim$80 mol% in barrier layers), which were grown by plasma assisted molecular beam epitaxy (PAMBE) on $c$-sapphire substrates. It has been demonstrated that the stimulated emission occurs in the mid-ultraviolet range of the spectrum at wavelengths of 259, 270, and 289 nm with threshold excitation power densities of 1500, 900, and 700 kW/cm$^2$, respectively. It has been shown that there exists a possibility of TE polarization $(\mathbf{E}\perp\mathbf{c})$ of both stimulated and spontaneous luminescence down to wavelengths of 259 nm.