Rutile films grown by molecular beam epitaxy on GaN and $AlGaN∕GaN$

Titanium dioxide (⁠$TiO2$⁠, with the rutile structure) was grown on (0001) oriented GaN and (0001) $Al0.33Ga0.67N∕GaN$ heterostructure field effect transistor (HFET) structures by molecular beam epitaxy. X-ray diffraction showed $(100)TiO2‖(0001)GaN(AlGaN)$ and $[001]TiO2‖⟨112¯0⟩GaN(AlGaN)$ with three rotational variants of the $TiO2$⁠. Transmission electron microscopy of $50nm$ thick $TiO2$ films on GaN and $AlGaN∕GaN$ showed sharp interfaces with no intermixing or reaction between the oxide and semiconductor. The $TiO2$ exhibited a columnar film microstructure with a lateral domain size of a few nanometers parallel to $(101)TiO2$ and a few tens of nanometers parallel to $(101¯)TiO2$⁠. Metal–oxide HFETs with $50nm$ thick $TiO2$ dielectric layers under the gate were processed and compared to HFETs without the $TiO2$ dielectric layer. The transconductance of the HFETs with $TiO2$ was $140mS∕mm$⁠, approximately 20% less than HFETs with no dielecric, and the pinchoff voltages of the two stuctures were comparable. The dielectric constant of the $TiO2$ was $∼70$⁠. The gate leakage current of the HFETs with $TiO2$⁠, $∼4×10−6mA∕mm$ at $50V$⁠, was approximately 4 orders of magnitude lower than that of the HFETs with no dielectric. Band offset measurements were performed using x-ray photoelectron spectroscopy, and the valence band of the rutile $TiO2$ and the GaN nearly line up.