Properties of the VO$_2$ films produced by the droplet-free PLD method

Problem: Thin films of some materials which dielectric properties actively changeable by optical, electrical or thermal influence are used to control technologically important terahertz and microwave ranges of electromagnetic waves [1]. One of such materials is VO$_2$ in which the metal-semiconductor transition following by considerable change of optical and electric properties occurs at close to the room temperature of 68$^\circ$C [2]. The VO$_2$ thin films stand considerably the bigger number of switchings cycles than single-crystal samples is considerable [3]. The purpose of the real work was production of the VO$_2$ films with the electric and optical properties suitable for generation of terahertz radiation on sapphire single-crystal substrates and silicon substrates by the droplet-free (PLD) pulse laser deposition method. Methodology: The PLD method has certain advantages in comparison with other methods of receiving thin multicomponent oxidic films. The composition of the films which are grown up by the PLD method thanks to nonequilibrium conditions of deposition, completely reproduces the composition of components of a target. The films received by the PLD method crystallize at lower temperatures of a substrate in comparison with other physical methods of steam deposition, thanks to high kinetic energy (>1 эВ) the ionized injected particles in the created laser plasma. Possibility of direction of a power spectrum of a plume allowed to investigate features of growth and direction of properties of the grown-up films. Besides, the PLD method can work with quite high pressure of buffer gas in the deposition chamber therefore it is effectively used at the deposition of oxides thin films. Results: The VO$_2$ thin films were received by the droplet-free PLD method under various conditions of the deposition. Influence of energy density on targets and oxygen pressure in the chamber on the optical and electric properties of the films was investigated. Change specific characteristics of the electric and optical properties of the films in the vicinity of phase transition in a temperature interval from 20$^\circ$C to 100$^\circ$C were studied. From measurements of the films electric properties it was established that values of the (Ts) transition temperature for the films on (0001) sapphire and (111) silicon substrates were equal 67.5$^\circ$C and 67$^\circ$C respectively and hysteresis widths were equal 3$^\circ$C and 2$^\circ$C respectively. From measurements of the films optical properties it was established that the films transmission on waves lengths from 200 nm to 800 nm shows jump of the transmission and a hysteresis at heating and cooling. Discussion of results: It was established that the electric resistance hysteresis was observed at the samples heating and cooling of the VO$_2$ crystal films. The films showed typical semiconductor dependence of the electric resistance reduction with temperature increase to the transition temperature. The electric resistance slightly grew with the temperature increase higher than the transition temperature showing a metal behavior. The hysteresis existence near the temperature of 67$^\circ$C indicated domination of a VO$_2$ phase in the film. For the first time it was established that the optical transmission hysteresis curves of various shape versus wavelength were observed. Transmission change nature of the films on the different wavelengths differed. We connected various behavior of the hysteresis curves with difficult behavior of the films absorption at the temperature change. Practical importance: Thermochromic properties of the VO$_2$ films find a broad application from microelectronics devices to clever coverings of buildings windows [4]. The VO$_2$ unique properties do it suitable as switches and sensors in microwave and terahertz spectral regions [1]. Besides, the films transmission hysteresis at the temperature change assumes possibility of VO$_2$ films use in optical switches devices in visible and near UV spectral regions.