Phase transformation of molecular beam epitaxy-grown nanometer-thick Gd2O3 and Y2O3 on GaN.

High quality nanometer-thick Gd2O3 and Y2O3 (rare-earth oxide, R2O3) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R2O3 epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R2O3 layers have a hexagonal phase with the epitaxial relationship of R2O3 (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R2O3 film thickness, the structure of the R2O3 films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R2O3 (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd2O3 films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

Nanometer-Thick Single-Crystal Hexagonal Gd2 O3 on GaN for Advanced Complementary Metal-Oxide-Semiconductor Technology.

Hexagonal-phase single-crystal Gd2 O3 is deposited on GaN in a molecular beam epitaxy system. The dielectric constant is about twice that of its cubic counterpart when deposited on InGaAs or Si. The capacitive effective thickness of 0.5 nm in hexagonal Gd2 O3 is perhaps the lowest on GaN-metal-oxide-semiconductor devices. The heterostructure is thermo dynamically stable at high temperatures and exhibits low interfacial densities of states after high-temperature annealing.