Luminescence properties of heavily doped Al_xGa_1-xN/AlN films grown on sapphire substrate

Luminescent properties of the Al_xGa_1-xN films with Si dopant concentration more than 10²⁰ cm⁻³ grown by molecular beam epitaxy on sapphire substrates with AlN buffer film were investigate at room temperature. Time-resolved photoluminescence spectroscopy was employed to study the donor-acceptor pair transitions in the Al_xGa_1-xN/AlN/Al₂O₃ structures with Al mole fraction x from 0.47 to 1 under action of Nd:YAG (λ = 266 nm) laser radiation. The radiation inside planar waveguide consists of spontaneous emission and amplified spontaneous emission. The spontaneous emission spectra demonstrated inhomogeneous broadening with FWHM of 0.58 eV covers full visible range and propagate randomly in all directions. The amplified spontaneous emission propagate at near the critical angle of incidence along zigzag path under total internal reflection conditions at the interfaces of the waveguide. Its spectrum consists of several TE_m and TM_m modes, which have mutually orthogonal polarizations. The optical measured gains are equals to g ≈ 58 cm⁻¹ for Al_0.65Ga_0.35N at 510 nm and g ≈ 20 cm⁻¹ for Al_0.74Ga_0.26N films at λ = 528 nm. The measured values of quantum yields of spontaneous emission for Al_xGa_1-xN films grow monotonically from 0.11 to 0.79 with increasing x from 0.47 to 0.74. Estimated emission cross-sections for the donor-acceptor pair transitions are more than 10⁻¹⁸ cm².