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

The advancement in the growth of aluminum nitride (AlN) crystals has opened exciting possibilities for the development of deep ultraviolet (UV) light-emitting diodes (LEDs) and high-power electronic devices. During our research, we utilized 10 GHz photo-induced electron paramagnetic resonance (EPR) spectroscopy, from 4 to 80 K, to investigate two distinct sets of AlN crystals with varying UV transmittance characteristics: one with a transmittance of 65% and another with 35%. Our analysis of the AlN crystal with 65% UV transmittance revealed the presence of shallow donors (SDs) exhibiting an isotropic g value of 1.99. Shallow donors are a type of defect in the crystal lattice that can introduce charge carriers. In this case, the SDs exhibited a negative correlation energy, suggesting that they may be attributed to Si_Al or O_N. In contrast, the darker AlN samples presented a different signal with a g-value of 2.0039, which is likely associated with O_N or C_N defects. Notably, this signal was exclusively observed in the darker AlN samples, while the 1.99 signal was unique to the clear samples. This observation implies that these defects, specifically the 2.0039 signal associated with O_N or C_N defects, play a role in controlling the UV transmittance characteristics of the AlN crystals. Our presentation will focus on elucidating the relationship between these defects within AlN crystals and their impact on the UV transmittance properties. This work was supported by ULTRA, an EFRC funded by DOE-BES, Award No. DE-SC0021230.