Radiative and nonradiative processes in strain-free AlxGa1−xN films studied by time-resolved photoluminescence and positron annihilation techniques

Radiative and nonradiative processes in nearly strain-free ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately −0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for $\text{x⩽0.7.}$ Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for $\text{x=0.5.}$ These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies $({\mathrm{V}}_{\mathrm{III}})$ and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to $\text{x=0.7.}$ Consequently, certain trapping mechanisms associated with ${\mathrm{V}}_{\mathrm{III}}$ where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency ${\mathrm{(\eta }}_{\mathrm{int}})$ with increasing x for $\text{x⩽0.7.}$ Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the ${\mathrm{V}}_{\mathrm{III}}$ -related trap density.