Abstract
Al2O3 films, electron beam evaporated from a high-purity single crystal sapphire source in UHV (a molecular beam epitaxy (MBE) approach), have been found to be epitaxially grown on Si (111) substrate. The structural studies were carried out by single crystal x-ray diffraction and transmission electron microscopy (TEM) with the initial epitaxial growth observed using in-situ reflection high energy electron diffraction (RHEED). The film morphology has also been studied by atomic force microscopy (AFM). The Al2O3 films grown in our MBE system have a cubic γ-phase with a very uniform thickness and a highly structural perfection, even for films as thin as 3.8 nm. The film thickness has been studied using low-angle x-ray diffraction (x-ray reflectivity) and cross-sectional TEM. The film normal was found to be <111>, which is well aligned with the Si substrate normal. All the in-plane unit cell vectors of the film are parallel to those of the substrate. A mosaic scan of the Al2O3 (222) peak (with no in-plane component) shows a 0.3 degree (or 18’) spread for a film 11 nm thick, while a 0.6 degree spread for a film 3.8 nm thick. A degenerate component (of ~ 10%) and 3o splitting were observed in x-ray phi-cone scans. A capping Si layer grown on top of the oxide film at a substrate temperature of 800 □C is in a form of islands, not a smooth layer, as observed using x-ray reflectivity, TEM, and AFM. Nevertheless, the capping Si has the same orientation as that of the underlying Si (111) substrate. The interface between the γ-Al2O3 film and Si (111) was atomically sharp as observed using x-ray reflectivity and cross-section TEM. The electrical measurements show a low leakage current density of 10-7 A/cm2 and a breakdown field of 7 MV/cm for the oxide film 7.5 nm and a leakage current density of 10-1 A/cm2 and a breakdown field of ~10 MV/cm for the oxide film 3.8 nm thick.