We propose a mechanism for the nucleation of highly aligned biaxially textured MgO on amorphous Si 3 N 4 during ion beam-assisted deposition. Using transmission electron microscopy, reflection high-energy electron diffraction, energy dispersive x-ray analysis, and ellipsometery, we have observed that highly aligned biaxially textured grains emerge from a "diffraction-amorphous" film when the film thickens from 3.5 to 4.5 nm. Transmission electron microscopy dark-field images also show the onset

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... of rapid grain growth during this same film thickness interval. These results suggest biaxial texturing through aligned solid phase crystallization. Biaxially textured MgO is technologically interesting since it provides a suitable path for silicon integration of single-crystal-like films on amorphous substrates for many important perovskite oxide thin-film materials. Singlecrystalline MgO ͑001͒ has already been used as a substrate for BaTiO 3 and Pb͑Zr,Ti͒O 3 heteroepitaxy. 1,2 Ion beamassisted deposition ͑IBAD͒ creates biaxially textured films ͑polycrystalline films with a preferred in-plane and out-ofplane grain orientation͒ on amorphous substrates. 3 Incorporation of biaxially textured ferroelectric films with silicon integrated circuits would enable new types of actuators for microelectrical mechanical systems. Previous work has demonstrated high-quality heteroepitaxy of perovskites on Si, 4 but typical integrated circuit fabrication processes do not leave single-crystal Si available for oxide heteroepitaxy. By eliminating the requirement for a pre-existing heteroepitaxial template, IBAD MgO may provide an opportunity to incorporate ferroelectric materials on top of amorphous dielectric films in silicon integrated circuits following interconnect fabrication. In contrast to materials like yttria-stabilized zirconia ͑YSZ͒ where biaxial texture evolves slowly during one micron of IBAD growth, 5 the biaxial texture of IBAD MgO develops rapidly during the nucleation phase. Biaxial texturing mechanisms such as anisotropic sputtering, ion channeling, and anisotropic grain damage 6,7 have been proposed to explain biaxial texture evolution during growth of YSZ, but do not specifically address the nucleation-mediated biaxial texturing seen for MgO. It has been suggested that IBAD MgO grains nucleate with biaxial texture because surface energy is minimized with a ͑001͒ fiber texture, leaving inplane alignment to be achieved by ion channeling along the ͓011͔ zone axis. 3 High-temperature physical vapor deposition of MgO on amorphous SiO 2 favors nucleation with a ͑001͒ fiber texture, 8 but kinetic limitations result in nucleation with random orientation at room temperature. 9 We have used transmission electron microscopy ͑TEM͒, electron dispersive x-ray analysis ͑EDAX͒, ellipsometery, and in situ reflection high-energy electron diffraction ͑RHEED͒ to investigate IBAD MgO biaxial texture during the first few nanometers of film growth. Using electron-beam evaporation, films of MgO were deposited by roomtemperature IBAD onto 30 nm thick Si 3 N 4 TEM windows at the rate of 0.17 nm/s with simultaneous ion bombardment of 750 eV Ar ϩ ions from a Kaufman ion gun. The ions impinged on the surface at a 45°incidence angle with an ion/ MgO molecule flux ratio of 0.43. The growth of each sample was stopped when the RHEED image exhibited the desired relative contributions from diffraction rings and spots. RHEED was performed with 25 keV electrons at a 2.6°incidence angle and images were taken with a 16-bit dynamic range, 1024ϫ1024 pixels, charge coupled device camera. In order to increase the sensitivity to weak diffraction intensities, the diffuse RHEED background was removed by subtracting a RHEED image of the amorphous Si 3 N 4 substrate from all subsequent RHEED images. RHEED pattern development for IBAD MgO grown on amorphous Si 3 N 4 is shown in Fig. 1 . Film thicknesses were determined by measuring the final MgO film thickness by ellipsometery and then assuming a constant growth rate. The evolution from diffraction rings ͓Fig. 1͑a͔͒ to diffraction a͒ Electronic mail: rhett@its.caltech.edu FIG. 1. In situ RHEED images from a continuous IBAD MgO growth experiment where the film thickness is equal to: 2.5 nm ͑a͒, 3.1 nm ͑b͒, 3.6 nm ͑c͒, and 4.2 nm ͑d͒. The field of view contains diffraction spots from ͑024͒, in the upper left-hand side corner, to ͑046͒ in the lower right-hand side corner.