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In this work, AlN polarization-enhancement interlayer (AlN-PEL) is adopted to enhance two-dimensional hole gas (2DHG) density in a p-GaN/AlN-PEL(∼2 nm)/AlGaN(<6 nm)/GaN heterostructure, aiming at monolithic integration of p/n-channel field effect transistors (p-FETs) on GaN-on-Si substrate. Owing to the strong built-in polarization of the AlN-PEL, high density 2DHG over 2.3 × 1013 cm−2 with good immunity to thermal freeze out effect is realized. Assisted by a two-step gate trench etching process, enhancement-mode (E-mode) buried-channel GaN p-FETs with temperature independent ON-resistance RON, and ON/OFF current ratio ION/IOFF (>108), have been fabricated. The fabricated p-FETs also deliver thermally stable subthreshold swing as well as threshold voltage Vth, and smaller Vth shift than that of p-FETs without the AlN-PEL, which is primarily due to enhanced 2DHG confinement by the AlN-PEL. The proposed structure is an attractive platform for monolithic integration of GaN-based logic and power devices for cryogenic applications as low as 10 K. [ABSTRACT FROM AUTHOR]

The effects of interface oxidation on the transport behavior of the 2-D electron gas (2DEG) in AlGaN/GaN heterostructures by plasma-enhanced-atomic-layer-deposited AlN (PEALD-AlN) passivation were investigated using temperature-dependent Hall-effect and X-ray photoelectron spectroscopy (XPS) characterizations. AlGaN/GaN heterostructure with a 4-nm-thick PEALD-AlN passivation exhibits good 2DEG transport behavior and stability at moderately high temperature (e.g., 275 °C). However, serious oxidation of the AlN/GaN (cap layer) interface occurs as the sample is heated up to 400 °C in low-pressure atmosphere, as verified by an increased Ga-O bond in Ga 3d core-level spectra. The oxidation leads to a significant reduction of 2.47 × 1012 cm-2 in the 2DEG density in the channel. A modified AlN passivation structure with Al2O3/AlN (10/4 nm) stack is shown to be able to effectively suppress the oxidation of the AlN/GaN interface, demonstrating an enhanced 2DEG density and high-temperature stability even when the sample is heated up to 500 °C. Based on XPS and 2DEG recovery experiments, it is suggested that acceptor-like deep levels have been generated in the near-surface region of AlGaN/GaN heterostructure because of the oxidation, and trapping of these deep levels results in significant depletion of the 2DEG in the channel. The effects of PEALD-AlN passivation on the strain in the AlGaN barrier of AlGaN/GaN heterostructures are also evaluated with high-resolution X-ray diffraction technique. [ABSTRACT FROM AUTHOR]

The variations in surface potential and the Schottky barrier height [lowercase_phi_synonym]B in fluorine-plasma-treated AlxGa1-xN/GaN heterostructures are systematically studied by x-ray photoelectron spectroscopy, providing insights to the mechanisms underlying the strong threshold voltage (Vth) modulation achieved by the F plasma treatment technology. It is found that a large amount of AlF3 appeared on the surface after the treatment, indicating a fluorinated surface. In addition, the surface potential of Al0.25Ga0.75N/GaN heterostructure was increased by ∼0.38 eV during the first 60 s of the treatment and then rises slowly with additional treatment. Annealing at 400 °C in N2 ambient for 10 min does not affect the surface potential, but results in quick reduction in AlF3, consistent with the relative poor thermal stability of AlF3 reported in literature. [lowercase_phi_synonym]B between Ni and F-plasma-treated Al0.25Ga0.75N surface was extrapolated from the shift in Ga 2p3/2 core-level and exhibits a small increase of 0.20 eV. The enhanced [lowercase_phi_synonym]B is much smaller than the positive shift in Vth observed from high electron mobility transistors fabricated with the same treatment conditions, suggesting that the primary factor responsible for the conversion from depletion-mode to enhancement-mode AlxGa1-xN/GaN by F plasma treatment is not the surface modifications but rather the negative fixed charges carried by F ions in AlxGa1-xN/GaN heterostructures. [ABSTRACT FROM AUTHOR]