Study On Optical,Electrical Properties And Applications Of VO₂ Thin Films Fabricated By Chemical Vapor Deposition

VO2(M)is of special interest for application to smart windows and switching devices.However,it is required to conquer some drawbacks such as high processing temperature of current synthesis techniques and low thermochromic properties.VO2(B)thin films are assumed to be the preferred phase structure for the application in uncooled infrared detectors as the thermal sensitive coating.While high-vacuum techniques are extensively used nowadays which together with high temperature post-annealing are complicated in operation and expensive.This study was on the preparation and performance improvement of VO2-based thermochromic smart windows and thermal sensitive coating.A low-temperature,cost-effective chemical vapor deposition(CVD)process for the preparation of VO2 thin films was employed.To improve the thermochromic and thermal sensitive properties of the VO2 thin films,doping films were fabricated.The main results are summarized as follows:1.Properly tuning of the CVD process parameters is found to be critical in fabricating thickness-controllable highly-crystallinine VO2 films.For a-62 nm thick VO2 film,the visible transmittance of 52.3%(annealed at 400 ?)and 52.7%(annealed at 350 ?)were obtained,respectively.The corresponding solar energy modification ability(?Tsol)were 9.7%and 7.1%,and the transition temperatures were 45.1 0C and 50.9 ?.The output results are comparable to the reported values of the pure VO2(M)thin films and reveal lower transition temperatures.The underlying microscopic mechanism was studied by first-principles calculations and results indicated that upswing performances including low transition temperatures that were achieved by properly controlling the annealing temperatures could be ascribed to the combined effect of strain and oxygen vacancy.Moreover,the initial use of a pre-grown seed layer induced a fast grain growth,which is in favor of further decreasing the deposition and annealing temperature to 325 ?.2.The improvement of the thermochromic properties of VO2(M)thin films by doping techniques,including single-doping and co-doping was investigated.Mo or Al single-doped and Mo-Al co-doped VO2(M)thin films were prepared by CVD method.The results shown that doping did not significantly change the visible light transmittance of VO2(M)films.The phase transition temperatures of the Mo single-doped and Mo-Al co-doped VO2(M)thin films were lower than that of the undoped VO2(M)thin film,and the phase transition temperature decreased with the increase of the concentration of Mo.In addition,compared with undoped VO2(M)thin films,the thermal hysteresis loop widths of the doped films decreased obviously,especially for A1 single-doped VO2(M)films.The microscopic mechanism for the performance improvement of the phase transition temperature and the hysteresis loop width of doping films was discussed.3.The CVD method successively achieved the facile and scalable synthesis of thermal sensitive VO2(B)thin films with high TCRs(from-2.4%/K to-2.89%/K)and favorable square resistances(10 k?/??40 k?/?)at a growth temperature range of 375 ?-450 ?.The output results are comparable to the reported values of the high-vacuum synthesized VO2(B)thin films,proved our method suitable for uncooled infrared detector applications.4.Futhermore,the co-doping technology was used to improve the thermal sensitive properties of VO2(B)thin films.Mo-Al co-doped VO2(B)thin films were synthesized with high temperature-coefficient-of-resistance(TCR,-3.6%/K)and favorable square resistances(16.2 k?/?).Both the TCR and resistance are superior to that of undoped and A1 single-doped VO2(B)thin film,and the TCR is larger than that of Mo single-doped VO2(B)thin film.The underlying microscopic mechanism for the performance improvement might be that Mo-Al co-doping creates less lattice deformation,strain and inhomogeneity of carrier concentration due to dimension and charge compensation than that of single-doping and the partly release of 3d electrons around V4+-V4? pairs in co-doping films.Besides,the special two-dimensional octahedral structure of monoclinic(C2/m)VO2(B)favors the strain control with doping.The films have been subjected to synchrotron radiation based X-ray photoelectron spectroscopy(SRPES)and X-ray absorption near edge structure(XANES)techniques to study the local atomic and electronic structure around V ions.The results indicated that the dopants of Mo and Al were incorporated into the VO2 lattice and occupied some of the V lattice sites,resulting in the formation of the V1-x-yMozAlyO2 substitution solid solution.V L-edge and 0 K-edge XAS results identically verified that Mo-Al co-doping might create the least lattice deformation in films.