Luminescent properties of the AlxGa1-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 AlxGa1-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 TEm and TMm modes, which have mutually orthogonal polarizations. The optical measured gains are equals to g ≈ 58 cm−1 for Al0.65Ga0.35N at 510 nm and g ≈ 20 cm−1 for Al0.74Ga0.26N films at λ = 528 nm. The measured values of quantum yields of spontaneous emission for AlxGa1-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.