Structural, electrical, and terahertz transmission properties of VO2 thin films grown on c-, r-, and m-plane sapphire substrates

Abstract

The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT property superior to the other two samples, with a resistivity change as large as 9 × 104 times and a transition window as narrow as 3.9 K, and it has the highest resistivity with the lowest free carrier density in the insulating phase. THz transmission measurements on VO2 films grown on r-plane sapphire substrates revealed intensity modulation depth as large as 98% over a broadband THz region, suggesting that VO2 films are ideal material candidates for THz modulation applications.

Temperature dependence of the optical properties of VO2deposited on sapphire with different orientations

“…These T MIT values agree with our prior resistivity (ρ) measurements 22 and are all noticeably different from the accepted T MIT for bulk VO 2 . Our results thus corroborate the presence of tensile C R strain in sample c, and compressive strain in both m and r. 6 Upon cooling sample c, differences in the thermal expansion coefficients of sapphire and VO 2 will produce tensile strain in C R , thereby raising T MIT .…”

“…The temperature at which this jump takes place correlates with T MIT based on the ω P dependence and ρ measurements. 22 These changes in the optical properties near T onset and T MIT are also observed in the other bands obtained from our SE measurements, as seen in Fig. 3.…”

“…22 For sample m we observe a temperature range below T MIT over which E g ~ 0 eV. This dependence shows that the collapse of the energy gap is gradual and that E g is close to zero before the MIT.…”

“…22 SE measurements were carried out between photon energies of 0.6 and 6.5 eV at 70° angle of incidence using the approach detailed by Majumdar, et al 25 Results were modeled using commercial software. Raman measurements were done in the direct backscattering configuration with 514.5 nm laser excitation kept at low power (< 10 mW).…”

“…In this paper, we report SE studies of VO 2 deposited on sapphire having different crystal orientations, denoted c, m, and r plane (herein samples c, m, and r). Under identical deposition conditions, described previously along with extensive characterization, 22 the sapphire influences the VO 2 orientation. For deposition on c-plane sapphire, the VO 2 is predominantly oriented with the V-V chains of the R phase in the plane of the sapphire.…”