Reflection high energy electron diffraction (RHEED) images measured along the substrate <11‾20> direc­tion during the growth of a 40-nm-thick La2O3 film. Images represent the following stages of growth: (a) bare (0001)-oriented GaN, (b) 2 nm of La2O3, (c) 3 nm La2O3, (d) 5 nm La2O3, (e) 10 nm La2O3, & (f) 40 nm La2O3. Arrows indicate secondary diffrac­tion spots that are attributed to a cubic polymorph.


  • Jon Ihlefeld (in the Electronic, Optical, & Nanostructured Materials Dept.),
  • Mike Brumbach (in the Materials Characterization and Performance Dept.), and
  • Stan Atcitty (in the Energy Storage Technology and Systems Dept.)

recently published the article “Band offsets of La2O3 on (0001) GaN grown by reactive molecular-beam epitaxy” in Applied Physics Letters outlining research to prepare high-dielectric-constant gate oxides on gallium nitride (GaN).

Preparation of high-quality gate oxides on wide bandgap semiconductors remains an important barrier/challenge toward realizing efficient high-performance power elec­tronic devices such as metal-oxide-semiconductor (MOS) field effect transistors (MOSFETs).

Several factors make oxide integration difficult. For GaN electronics, in particular, the lack of a low-defect-density native oxide interface and the limited number of compati­ble oxides that have a sufficiently large bandgap to mini­mize electrical leakage are significant hurdles for oxide integration. Lanthanide series oxides are a potential group of materials that may be promising for use in gate insulator applications. These oxides possess relatively large band­gaps and permittivity values as high as 30.

The team prepared La2O3 films on (0001)-oriented GaN substrates via reactive molecular-beam epitaxy and assessed film orientation and phase using reflection high-energy electron and X-ray diffraction (RHEED). Films were observed to grow as predominantly hexagonal La2O3 for thicknesses less than 10 nm while film thickness greater than 10 nm favored mixed cubic and hexagonal symmetries. The team characterized band offsets by X-ray photoelectron spectroscopy on hexagonally symmetric films and measured valence band offsets of 0.636±0.04 eV at the La2O3/GaN interface. A conduction band offset of approximately 1.5 eV could be inferred from the measured valence band offset.

The measured valence band offset at the interface between hexagonal La2O3 and (0001)-oriented GaN is slightly larger than the 0.4 eV valence band offset measured for the high-permittivity Sc2O3/GaN interface, which suggests that this larger valence band offset may be promising for device applications. However, growth challenges associated with the interplay between dielectric layer thickness, film roughness, and cubic polymorph formation may make La2O3 a poor choice as a gate dielectric for a GaN metal-oxide-semiconductor device.

The research was funded through the U.S. Department of Energy’s Office of Electricity Delivery and Energy Reliability (OE) Energy Storage Program.

Read the abstract at Applied Physics Letters.