Correlation between Defects and Transparency in Crystalline AlN for Optoelectronic Devices: Insights from EPR Spectroscopy

Progress in AlN growth has paved the way for the production of deep ultraviolet (UV) LEDs and high-power electronics. In our research, we employed X-band photo-induced electron paramagnetic resonance (EPR) spectroscopy between 4 and 80 K to explore AlN crystals with UV transmittance of 65% and 35%. In the 65% sample, our investigation revealed the presence of shallow donors (SDs) with isotropic g value 1.99 and substitutional oxygen (O_N) - aluminum vacancy (V_Al-O_N) complexes with g value 2.0039. The SDs exhibited a negative correlation energy and may arise from Si or O substitution at N sites. The threshold for observing the V_Al-O_N signal was between 3.5 eV and 4.8 eV, confirming previous theoretical results and optical absorption measurements. In the darker samples, we identified another signal with a g-value of 2.0039 and unresolved Al hyperfine which is likely substitutional oxygen or carbon (O_N or C_N). Notably, this signal is seen only in the darker samples, and the 1.99 signal is exclusive to the clear samples suggesting that these defects control the transmittance. At the conference, we will discuss the relationship between the defects in AlN and the UV transmittance. This work was supported by ULTRA, an EFRC funded by DOE-BES, Award No. DE-SC0021230.