Impact of Aluminum Content on Neutral Donor States in Undoped Bulk AlN and Si-doped AlGaN Investigated by EPR

Aluminum nitride (AlN) and Aluminum Gallium nitride (AlGaN) are potential materials for high-power and high-frequency applications due to their ultra-wide bandgaps. Si donors can introduce compensation or self-compensation which limit the n-type conductivity. We investigate the neutral donors in Si-doped AlGaN films with 65%, 85%, or 97.5% Al and undoped bulk AlN using electron paramagnetic resonance (EPR) spectroscopy from 4 to 80 K. The AlGaN films doped with Si ~10^19 cm⁻³, were grown using molecular beam epitaxy on AlN substrates, while the undoped bulk AlN sample was grown by physical vapor transport. In the 65% and 85% AlGaN, the neutral donor is observed in the dark as expected. However, in AlGaN with 97.5% aluminum and undoped bulk AlN, no signal is found in the dark, but a strong signal appears when illuminated by an LED, typical of a compensated donor. Interestingly, when heated in the dark, the donor is quenched at 70 K in AlGaN and 60 K in AlN. The lower quenching temperature for the AlN sample compared to the 97.5% AlGaN implies that thermal ionization does not occur by excitation to the conduction band. We suggest that the temperature dependence along with optical response indicates that the compensation mechanism is formation of a DX center, a donor with a negative correlation energy.