Phase Transition Study Of VO 2 Functional Films

Vanadium dioxide is a binary compound that has a variety of crystal structures.The crystal structures of VO₂ mainly are: M phase,D and B phase of monoclinic system;T phase in triclinic crystal system;R phase of Rutile structure;and A phase of tetragonal crystal system.VO₂ can present a metal-insulator phase transition?MIT?at 68 ?,which transforms from M phase of monoclinic crystal structure at low temperature to R phase of rutile structure at high temperature.And the phase change properties of vanadium dioxide are stongly affected by different preparation conditions on the process.At present papers based on vanadium dioxide application in various fields have sprung up.In this paper,the phase transition process of functional vanadium dioxide thin films are studied,which has certain referential significance in the practical application of vanadium dioxide.In this paper,phase transition,materials and researching methods on phase transition are briefly introduced.Raman scattering spectroscopy and Spectroscopic ellipsometry are introduced and described systematically on basic principle and analyzing methods.The phase transformation process of vanadium dioxide thin films prepared by magnetron sputtering are investigated using Raman scattering spectroscopy and Spectroscopic ellipsometry.Both Raman scattering spectroscopy and Spectroscopic ellipsometry are distinctive and complementary in the study of phase transition.Firstly,the effect of substrate temperature on vanadium dioxide thin films phase transition is studied.vanadium dioxide film deposited at 450 ? shows no phase transition,otherwise vanadium dioxide film deposited at 550 ? presents the best Raman signal intenstity.The fitting results of ellipsometry consistent with Raman analysis.It is found that vanadium dioxide is an indirect band gap semiconductor of M1 phase from T-L band gap of harmonic oscillator analysis.A small amount of T phase arises in vanadium dioxide thin film deposited at high substrate temperature,and the phase transition process is mainly transformed from M phase to the R phase.Secondly,the different substrate voltage has a great influence on the phase transition properties of VO₂ thin films.The new T phase appeared at room temperature for-125 V samples.With the increase of the voltage,the T phase changes to M2 phase,rather VO₂ phase transition temperature decreased.The thickness of both VO₂ and buffer layers tend to increase during the phase transition from spectroscopic ellipsometry analysis.Finally,phase transitions show up as transforming from M1 phase to R phase in VO₂ thin films with different thickness of ZnO buffer layer.There is no new phase appeared during the phase transition.Therefore,it is concluded that there is no stable M2 phase structure in VO₂ thin films.The phase transition process of undoped VO₂ thin films is mainly from M1 phase to R phase.