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Atomic force microscopy images of (a) a c-plane NA-sapphire substrate, (b) a 38 nm V2O3 film grown on NA-sapphire, (c) a c-plane AN-sapphire substrate annealed at 1050 °C for 90 min to produce an ultra-smooth surface, and (d) a 43 nm V2O3 film grown on AN-sapphire. The root-mean square roughness of these four surfaces are 1.2 Å, 2.9 Å, 0.9 Å, and 2.4 Å, respectively.
(a)–(c) RHEED patterns taken along the azimuth before, during, and after deposition of 38 nm of V2O3 onto NA-sapphire. (d)–(f) Equivalent images taken before, during, and after deposition of 43 nm of V2O3 onto AN-sapphire. (g) Examples of specular spot intensity oscillations and streak spacing as a function of time for two different V2O3 films deposited on AN-sapphire under similar conditions. The discontinuous jumps in the oscillation data between 2 and 3 min are due to manual readjustment of the electron beam’s grid voltage. No oscillations occurred for films grown on NA-sapphire. Note that while c-plane V2O3 has a 4.1% larger in-plane lattice parameter than sapphire at room temperature, the mismatch at a growth temperature of 700 °C increases to about 5% due to the difference in the thermal expansion coefficients of V2O3 and Al2O3. Also note that the period of oscillation is approximately 48 s, corresponding to a deposition rate of 0.05 Å/s, or 0.3 nm/min.
(a) Typical θ-2θ x-ray diffraction scans showing the out-of-plane peaks for V2O3 films on both NA- and AN-sapphire. The expected peak position for bulk V2O3 was taken from JCPDS reference 00-034-0187. (b) A plot versus thickness of the extracted FWHM of V2O3’s peak for films deposited on both NA- and AN-sapphire. Also plotted is the FWHM that would exist purely due to thickness broadening as predicted by the Scherrer equation.22 The inset cartoon depicts how good epitaxy might lead to island-like growth and various types of peak broadening.
Four-point probe resistivity measurements versus temperature for a representative selection of V2O3 film thicknesses deposited onto (a) NA-sapphire and (b) AN-sapphire. The data were collected at a sweep rate of 5 °C/min, and the arrows in (a) denote the direction of hysteresis for the 96 nm film on NA-sapphire.
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