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Polarization charge properties of low-temperature atomic layer deposition of AlN on GaN

Low-temperature (<250 °C) plasma-enhanced atomic layer deposition (PEALD) is used to fabricate aluminum nitride (AlN)/gallium nitride (GaN) heterojunctions for tunnelling contacts in devices where selective contacts are required, such as GaN metal–oxide semiconductor field-effect transistors. AlN is grown on GaN templates with via low-temperature plasma-enhanced ALD, and compared in order to extract their surface two-dimensional electron gas concentration. A peak electron density of >2 × 1013 cm−2 was observed for an approximately 4.5 nm AlN thickness on GaN wafers with a higher initial doping concentration while for the lightly doped GaN samples, a peak carrier concentration of 1.9 × 1013 cm−2 was observed for a thickness of 5.7 nm AlN. Polarization is strongest near the AlN critical thickness of strain relaxation for low-temperature deposition methods. This suggests that greater initial dopant concentrations are more conducive to enhanced polarization characteristics under these low growth temperatures because the critical thickness is realized at lower thicknesses. This approach also yields a low contact resistance of 0.45 Ω mm with Al contacts.