  Lithium niobate is a multi-functional material with wide reaching
applications in acoustics, optics, and electronics. Commercial applications for
lithium niobate require high crystalline quality currently limited to bulk and
ion sliced material. Thin film lithium niobate is an attractive option for a
variety of integrated devices, but the research effort has been stagnant due to
poor material quality. Both lattice matched and mismatched lithium niobate are
grown by molecular beam epitaxy (MBE) and studied to understand the role of
substrate and temperature on nucleation conditions and material quality. Growth
on sapphire produces partially coalesced columnar grains with atomically flat
plateaus and no twin planes. A symmetric rocking curve shows a narrow linewidth
with a full width at half-maximum (FWHM) of 8.6 arcsec (0.0024{\deg}) which is
comparable to the 5.8 arcsec rocking curve FWHM of the substrate, while the
film asymmetric rocking curve is 510 arcsec FWHM. These values indicate that
the individual grains are relatively free of long-range disorder detectable by
x-ray diffraction (XRD) with minimal measurable tilt and twist and represents
the highest structural quality epitaxial material grown on lattice mismatched
sapphire without twin planes. Lithium niobate is also grown on lithium
tantalate producing high quality coalesced material without twin planes and
with a symmetric rocking curve of 193 arcsec, which is nearly equal to the
substrate rocking curve of 194 arcsec. The surface morphology of lithium
niobate on lithium tantalate is shown to be atomically flat by atomic force
microscopy (AFM).
