A comparison of gallium gadolinium oxide and gadolinium oxide for use as dielectrics in GaN MOSFETs

Amorphous gadolinium gallium oxide (GGG) and crystalline gadolinium oxide (Gd₂O₃) have been investigated as dielectrics for fabrication of GaN metal oxide semiconductor field effect transistors. GGG, deposited by e-beam evaporation in a molecular beam epitaxy (MBE) chamber was found to produce a breakdown field of 12 MV/cm indicating a low concentration of defects in the dielectric. MOSFETs fabricated with this dielectric showed modulation at forward voltages up to 3 V. Gd₂O₃ was deposited by molecular beam epitaxy using elemental Gd and an electron cyclotron resonance (ECR) oxygen plasma in an MBE chamber. Both reflection high energy electron diffraction (RHEED) and X-ray diffraction confirm the single crystal nature of the deposited Gd₂O₃. However, cross-sectional transmission electron microscopy indicates the presence of a large concentration of dislocations and other structural defects. These defects cause significant leakage in the Gd203 and necessitated the addition of an SiO₂ overlayer in order to reduce the gate leakage current in depletion mode devices. Using this double layer structure, devices with modulation at forward voltages up to 7 V were obtained, as compared to 3 V. For devices fabricated with GGG. Both dielectrics appear to be stable at temperatures up to 950-1000°C as determined by X-ray diffraction and Auger electron spectroscopy