ALBERT

Description: In this work, we present the terrace etching technique to obtain excessive thickness series of atomic layer deposition (ALD) grown Al 2 O 3 and HfO 2 on GaN-cap/AlGaN/GaN heterostructures allowing for the detailed study of oxide charge distribution and its impact of the metal-insulator-semiconductor high electron mobility transistor (MISHEMT) threshold voltage. By modeling the experimental plot of threshold voltage versus oxide thickness on the basis of experimentally determined two-dimensional electron gas charge density in AlGaN/GaN MISHEMTs, we separated the interface and bulk charge components and determined the oxide-metal barrier height for the investigated gate dielectrics. In both Al 2 O 3 and HfO 2 gate dielectrics, the oxide charges are mainly located at the oxide/GaN interface. Determining the interface trap charges from comparison of the pulsed capacitance-voltage (CV) technique with very fast voltage sweep to the modulation type CV method with slow DC voltage ramp, we extracted positive fixed charges of N Ox = 2.7 × 10 12  cm −2 for Al 2 O 3 and N Ox = 7.8 × 10 12  cm −2 for HfO 2 . We found a strong V th shift of opposite direction for both high-k materials, corresponding to negatively charged up trap states at the HfO 2 /GaN interface and positively charged up trap states at the Al 2 O 3 /GaN interface. The evaluation of the metal-oxide barrier height in dependence of the metal work function followed the trend of the Schottky model, whereas HfO 2 showed less Fermi level pinning compared to Al 2 O 3 indicating the presence of an increased number of interface states in Al 2 O 3 on GaN.