High-k dielectrics on n-Al$_{0.25}$Ga$_{0.75}$N via atomic layer deposition

AlGaN/GaN and AlInN/GaN high-electron-mobility transistors (HEMTs) are promising devices for high-temperature and high-power electronics applications. A key issue with these devices is the high gate leakage current, particularly for enhancement-mode HEMTs. There has been an increased interest in developing high quality gate insulators to reduce gate leakage current. Al$_{2}$O$_{3 }$and HfO$_{2}$ layers (21nm thick)$_{ }$were deposited via atomic layer deposition on n-Al$_{0.25}$Ga$_{0.75}$N pretreated with one of two different surface preparations, H$_{2}$O$_{2}$:H$_{2}$SO$_{4}$ (1:5) (piranha) or HF:H$_{2}$O (1:3). Dielectrics were characterized using spectroscopic ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy (AFM), and capacitance-voltage (C-V) measurements. AFM shows that Al$_{2}$O$_{3 }$and HfO$_{2}$ layers are continuous and uniform in thickness on both HF and piranha pretreated surfaces. However, C-V measurement shows smaller (15{\%}) hysteresis for HF pretreated samples. The estimated dielectric constants ($\varepsilon )$ are 9 and 18 for Al$_{2}$O$_{3}$ and HfO$_{2}$ on HF pretreated surfaces, respectively, in general agreement with theoretical values of 9 and 25. Al$_{2}$O$_{3}$ layers on Al$_{0.25}$Ga$_{0.75}$N exhibited a lower leakage (7x10$^{-8}$ A/cm$^{2}$ at 5 V) current and higher forward breakdown voltage of 7.5 MV/cm compared to that of HfO$_{2}$ layer. The higher breakdown voltage and lower leakage current for Al$_{2}$O$_{3}$ is due to larger conduction band offset with Al$_{0.25}$Ga$_{0.75}$N.