III-As heterostructure field-effect transistors with recessed ex-situ gate oxide by O2 plasma-oxidized GaAs cap

Abstract

III-As heterostructure field-effect transistors (HFETs) and metal–oxide–semiconductor HFETs with gate electrodes insulated by an amorphous layer of ex-situ-prepared mixture of Ga and As oxides are studied. Gate insulator was prepared by O2 plasma oxidation of undoped GaAs cap layer of epitaxially grown transistor structures in standard plasma unit commonly used for photoresist ashing. GaAs cap is gradually consumed by the oxidation turning it into Ga and As oxides and causing bottom surface of the oxide moving closer to the two-dimensional electron gas. Gate electrode “recessing” is a positive byproduct of the process. Expectedly, impact on HFETs' threshold voltage (Vth) was observed and shift from −2.17 to −1.15 V was achieved. X-ray reflectivity confirmed much higher oxidation tendency for N-type GaAs than for undoped GaAs with this oxidation technique. Strong Vth shift can be most likely attributed to negative oxide charge in plasma-grown oxide or its interface with GaAs. Excluding Vth shift, negligible impact of O2 plasma on electrical characteristics was observed. Negligible Fermi level (EF) pinning inherited from the oxidation process can be concluded. Such O2 plasma-grown oxides might serve as an efficient seeding layer for subsequent high-κ gate dielectric growth. The authors believe this method might help to create a high-quality interface reducing number of Fermi level-pinning traps induced by other ex-situ deposition techniques while providing fine control over Vth as well.