Amplified luminescence of heavily doped Al_xGa_1-xN structures under optical pumping

Spectral, temporal, and polarisation characteristics of luminescence of heavily doped Al_xGa_1-xN films on a sapphire substrate are studied under pulsed pumping at the wavelength λ = 266 m. Spectra of spontaneous emission related to donor-acceptor transitions are inhomogeneously broadened with the FWHM of above 0.5 eV and cover the entire visible range. Spectra of radiation emitted from an edge of investigated structure comprise several narrow-band equidistant components, each of them being split to TE and TM modes with mutually orthogonal polarisations. This is related to plane waves propagating inside a plane waveguide along a zigzag path in the conditions of total internal reflection from waveguide surfaces. The optical gains measured for Al_0.5Ga_0.5N/AlN at λ ≈ 510 nm, Al_0.74Ga_0.26N/AlN at λ ≈ 468 nm, and AlN/Al_0.6Ga_0.4N/AlN/Al₂O₃ at λ ≈ 480 nm were, respectively, ∼70, 20, and 44 cm^-1. The luminescence quantum efficiencies measured for Al_0.74Ga_0.26N, Al_0.65Ga_0.35N, and Al_0.5Ga_0.5N films are, respectively, 0.79, 0.49, and 0.14; the transition cross sections calculated at emission band centres are ∼10^-18 cm².