Interface property and band offset investigation of GaN based MOS heterostructures with diffusion-controlled interface oxidation technique

This paper presents the interface analysis and band offset of Al2O3/AlGaN/GaN metal-oxide-semiconductor (MOS) heterostructures with diffusion-controlled interface oxidation (DCIO) treatment. After conventional surface pre-treatment with wet cleaning and nitridation plasma, 1 nm Al2O3 was prepared with atomic layer deposition, followed by in situ plasma-assisted interface oxidation for 30 min. The interface oxidation process is limited by oxidant diffusion through the pre-deposited Al2O3 layer, contributing to the formation of high quality crystalline interfacial oxide layer. For MOS heterostructures with 24.1 nm Al2O3, a positive threshold voltage shift by 1.8 V was obtained by using DCIO technique. The energy band structures and band offset at Al2O3/AlGaN interface was investigated with X-ray photoelectron spectroscopy (XPS). XPS results show that DCIO treatment causes an increase in conduction band offset from 2.29±0.37 eV to 2.92±0.36 eV. There is also a decrease in Al2O3 band slope, indicating a decrease in internal electrical field strength and interface charges. The Al2O3 energy band for these two cases may also cross with each other at a certain point, defining the critical thickness of gate oxide. Generally, the decrease in interface charges by DCIO causes a positive voltage shift, while the voltage shit will change sign with Al2O3 thickness smaller than the critical value. The MOS heterostructures with and without DCIO exhibit very nice relations between threshold voltage and Al2O3 thickness, giving a critical oxide thickness about 1 nm. DCIO results in a decrease in the slope of linear function by about 0.08 V/nm, indicating the reduced interface charges by 3.96×1012 cm-2.