  Strain-engineering is a powerful means to tune the polar, structural, and
electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar
instability and shows anisotropic superconductivity in electron-doped samples.
Here, we demonstrate growth of high quality KTaO3 thin films by molecular-beam
epitaxy. Tantalum was provided by both a suboxide source emanating a TaO2 flux
from Ta2O5 contained in a conventional effusion cell as well as an
electron-beam-heated tantalum source. Excess potassium and a combination of
ozone and oxygen (10 \% O3 + 90 \% O2) were simultaneously supplied with the
TaO$_2$ (or tantalum) molecular beams to grow the KTaO$_3$ films. Laue fringes
suggest that the films are smooth with an abrupt film/substrate interface.
Cross-sectional scanning transmission electron microscopy does not show any
extended defects and confirms that the films have an atomically abrupt
interface with the substrate. Atomic force microscopy reveals atomic steps at
the surface of the grown films. Reciprocal space mapping demonstrates that the
films, when sufficiently thin, are coherently strained to the SrTiO$_3$ (001)
and GdScO$_3$ (110) substrates.
