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.