  Precise control of the chemical valence or oxidation state of vanadium in
vanadium oxide thin films is highly desirable for not only fundamental
research, but also technological applications that utilize the subtle change in
the physical properties originating from the metal- insulator transition (MIT)
near room temperature. However, due to the multivalent nature of vanadium and
the lack of a good understanding on growth control of the oxidation state,
stabilization of phase pure vanadium oxides with a single oxidation state is
extremely challenging. Here, we systematically varied the growth conditions to
clearly map out the growth window for preparing phase pure epitaxial vanadium
oxides by pulsed laser deposition for providing a guideline to grow high
quality thin films with well-defined oxidation states of V2(+3)O3, V(+4)O2, and
V2(+5)O5. A well pronounced MIT was only observed in VO2 films grown in a very
narrow range of oxygen partial pressure P(O2). The films grown either in lower
(< 10 mTorr) or higher P(O2) (> 25 mTorr) result in V2O3 and V2O5 phases,
respectively, thereby suppressing the MIT for both cases. We have also found
that the resistivity ratio before and after the MIT of VO2 thin films can be
further enhanced by one order of magnitude when the films are further oxidized
by post-annealing at a well-controlled oxidizing ambient. This result indicates
that stabilizing vanadium into a single valence state has to compromise with
insufficient oxidation of an as grown thin film and, thereby, a subsequent
oxidation is required for an improved MIT behavior.
