Impact of point defects on the luminescence properties of (Al,Ga)N

Threading dislocations (TDs) in (Al,In,Ga)N semiconductors are known to affect the luminescence efficiency of near-band-edge (NBE) emissions in bulk films and quantum structures. However, the principal role of point defects such as vacancies on the luminescent properties has not been fully understood. In this article, impacts of point defects on the luminescence quantum efficiency of NBE emissions and on the intensity of deep emission bands will be described, based on the results of steady-state and time-resolved photoluminescence (TRPL) and positron annihilation measurements. The room temperature nonradiative lifetime (TNR) of the NBE excitonic photoluminescence (PL) peak in polar (0001) and (000-1) , nonpolar (11-20) and (10-10), and zincblende (001) GaN layers prepared by various growth techniques was shown to increase with the decrease in concentration or size of Ga vacancies (V_Ga) and with the decrease in gross concentration of point defects including complexes, leading to an increase in the NBE PL intensity. As the edge TD density decreased, the concentration or size of V_Ga, tended to decrease and τ_NR tended to increase. However, there existed remarkable exceptions. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of V_Ga, such as V_Ga-defect complexes. Similar relations were found in Al_xGa_1-xN alloy films grown by metalorganic vapor phase epitaxy: i. e. τ_NR at room temperature increased with the decrease in the concentration of cation vacancies (V_III) and with the decrease in gross concentration of point defects. In addition to nonradiative processes, the V_III concentration was found to correlate with the intensity ratio of characteristic deep emission band to the NBE emission (I_deep/I_NBE). For example, I_deep/I_NBE at low temperature for the deep emission bands at 4.6, 3.8, and 3.1 eV of AlN epilayers grown by NH ₃-source molecular beam epitaxy had a linear correlation with the concentration or size of Al vacancies (V_Al). Since the relative intensities of 3.1 eV and 3.8 eV bands increased remarkably with lowering the supply ratio of NH₃ to Al (V/III ratio) and growth temperature (T_g), they were assigned to originate from V_Al-O as well as V_Al-shallow donor complexes. The V_Al concentration could be decreased by adjusting the V/III ratio and T_g. In the case of Al_xGa_1-xN alloys, the concentration or size of V _III and I_deep/I_NBE at 300 K increased simultaneously with the increase in x up to approximately 0.7. Similar to the case for GaN and AlN, the deep emission band was assigned as being due to the emission involving V_III-O complexes.