Selective area epitaxy of degenerate n-GaN for HEMT ohmic contact by MOCVD

This Letter reports low-temperature (700 °C) growth of heavily Si-doped GaN (n++GaN) by metal-organic chemical vapor deposition with a resistivity as low as 1.9 × 10−4 Ω·cm and an atomically smooth surface. Indium adatoms added during the growth of n++GaN play an important role in improving both the surface morphology and free electron concentration. On the one hand, acting as surfactant, they greatly boost the adatoms surface mobility at low growth temperature and mitigate Si-induced anti-surfactant effect. On the other hand, they can effectively suppress the formation of compensating defects, thus contributing to an extremely high electron concentration of 2.8 × 1020 cm−3. This high-quality n++GaN was further applied to the realization of Ohmic contacts with an ultra-low contact resistance for AlGaN/GaN high electron mobility transistors. The carrier gas was carefully modulated for the selective area epitaxy (SAE) of n++GaN to facilitate the nucleation of GaN on the dielectric mask, which effectively suppressed the undesired mass transport and resulted in a uniform SAE of n++GaN in the recessed source/drain regions. A nearly defect-free interface between the n++GaN and two-dimensional electron gas channel has been also realized, and the resistance induced by the interface was only 0.03 Ω·mm. As a result, an ultra-low contact resistance of 0.07 Ω·mm has been realized. This work lays a solid foundation for further improving the performance of GaN-based RF and power devices.