ZrO₂ gate dielectrics produced by ultraviolet ozone oxidation for GaN and AlGaN/GaN transistors

We investigated the suitability of ZrO₂ as a high-k dielectric for GaN material systems. Thin Zr films (4 nm) were deposited by electron-beam evaporation at room temperature on n-type GaN and Al_0.22Ga_0.78N (29 nm)/GaN high electron mobility transistor (HEMT) structures. The Zr-coated samples were subsequently oxidized at temperatures in the range of 200–400 °C in an ozone environment. Atomic force microscopy studies after oxidation show that the ZrO₂ forms a conformal layer on the underlying GaN template. Cross-section transmission electron microscopy studies showed little intermixing of the ZrO₂ with the AlGaN/GaN. The relative dielectric constant of the ZrO₂ was determined to be 23. In comparison with HEMTs with bare gates (no dielectric between the gate metal and AlGaN), the HEMTs with ZrO₂ showed two to three order of magnitude reduction in gate leakage current. Optimization of the HEMT process on sapphire substrates with ZrO₂ under the gates yielded devices with powers of 3.8 W/mm and 58% power-added efficiency at 4 GHz.