High-dielectric-constant Ta₂O₅/n-GaN metal-oxide-semiconductor structure

High-dielectric-constant Ta₂O₅ has been grown on the n-GaN epifilm by rf magnetron sputtering. Photoluminescence measurement has been performed to compare the luminescence intensity with and without the dielectrics. Threefold increase in intensity is obtained, and a surface recombination velocity is estimated to be 3×10⁴ cm/s as an upper limit using a modified dead-layer model. A metal-oxide-semiconductor structure has been fabricated with Al on n-GaN as the ohmic contact and on Ta₂O₅ as the gate metal. Capacitance-versus-voltage characteristics have been measured. The doping concentration obtained from the depletion regime is compared with the result of Hall measurement, which is 7.0×10¹⁶ cm^-3. The flat-band voltage is obtained from the high-frequency data, and the effective oxide charge number density is calculated as 4.1×10¹² cm^-2. Indication of strong inversion appears at low reverse bias due to the high dielectric constant of Ta₂O₅, and matches closely with calculated values. Hysteresis is observed and ascribed to positive mobile charges derived as 2.1×10¹² cm^-2. The capacitance dependence on the frequency and the leakage current are discussed.