Structural, electrical, and terahertz transmission properties of VO_2 thin films grown on c-, r-, and m-plane sapphire substrates

摘要

The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO_2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO_2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO_2β angle of 122.2° and the two angles of V~(4+)-V~(4+) chain deviating from the a_m axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO_2 was determined to be epitaxially deposited with VO_2 (011)_T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (211) and (200) orientations forming a twinned structure. VO_2 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 9×10~4 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 VO_2 films grown on r-plane sapphire substrates revealed intensity modulation depth as large as 98% over a broadband THz region, suggesting that VO_2 films are ideal material candidates for THz modulation applications.