Studies On The Fabrication, Microstructure And Properties Of Titanium Oxide Thin Films

Uncooled infrared detectors have been paid considerable attention during the pastdecades, during the process of fabrication of the uncooled thermal detectors, the choiceand property of heat sensitive material is one of the most important issues. Recently,titanium oxide (TiO_x) thin films have been reported as a new candidate for thisapplication.In this paper, by using a direct current reactive sputtering technique, TiO_xthinfilms of different crystalline structures have been successfully fabricated on glasssubstrates under different sputtering conditions. The microstructures of the depositedfilms were characterized by several equipments and methods such as grazing incidencex-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM),atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), X-rayphotoelectron spectroscopy (XPS) and laser Raman spectroscopy (LRS). The growthmechanism of the sputtered films with different crystalline structures has been mainlyinvestigated. Furthermore, we also have illustrated the effect of microstructure on theelectrical and optical properties of the sputtered TiO_xthin films applied for uncooledinfrared detectors. Based on these researches, the main results are as follows:(1) TiO_xthin films of amorphous (a-TiO_x) and polycrystalline rutile single phase(R-TiO_x) structures have been fabricated on glass and silicon substrates simultaneously.During the deposition process of TiO_xthin films, it is found that the sputteringparameters such as substrate temperature (Ts), oxygen partial flux (pO₂) and filmthickness all play important roles in the crystallization of TiO_xthin films. The a-TiO_xthin films of about100nm have been successfully prepared by controlling the TsandpO₂from30oC to200oC and1.5%to9%, respectively. The R-TiO_xthin films of about180nm can be obtained at the Tsof200oC and pO₂from5%to10%, with the increaseof pO₂, the average size of rutile nanocrystals increases from10.88nm to13.66nm, andthe root mean square roughness increases from2.2nm to3.6nm, correspondingly.(2) The effect of oxygen annealing on the microstructure and properties of a-TiO_xthin film of280nm and R-TiO_xthin film of265nm has been investigated. After annealing in oxygen atmosphere at320oC for10min, the crystal structure of a-TiO_xthin film changes from amorphous to mixed phase of anatase and rutile, the average sizeof rutile and anatase nanocrystals are12.5nm and14.6nm, respectively. R-TiO_xthinfilms still have rutile singe phase after annealing, but the size of the nanocrystals in thefilms changes from11nm to12.6nm.(3) The results of the optical measurements of TiO_xthin films with differentstructures show that the Raman scattering peaks belong to TiO_xin the laser Ramanspectra have asymmetric and broadening features due to the phonon confinement effectsand oxygen vacancies in the films. Compared to single crystalline TiO₂, the positions ofthe main Raman scattering peaks belong to rutile and anatase display blueshift andredshift features, respectively. Because of the quantum size effect, it is found that theoptical bandgaps of TiO_xthin films of three different structures all display blueshiftscompared to the bandgap of single crystalline TiO₂.(4) The effects of crystalline structure and composition of the TiO_xthin films onthe electrical properties show that in the a-TiO_xthin films, as the oxygen to titanium(O/Ti) ratio varies from1.72to1.96, the electrical resistivity at room temperature andthe absolute values of temperature coefficient of resistance (TCR) change from0.21cm to55.2cm and1.2%to2.2%, respectively. Furthermore, it is found that the filmsdeposited at200oC have relatively higher absolute values of TCR and lowerresistivities than the films deposited at lower substrate temperatures. The results of themeasurements of electrical properties of TiO_xthin films with different structuresillustrate that the as-annealed R-TiO_xthin films have the best electrical properties.(5) The electrical properties of TiO_xthin films in the temperature range20oC to100oC have been investigated by temperature-dependent conductivity. It has beenobserved that all the films show a thermally activated conduction behavior in thetemperature range, and the carrier transport mechanism in the a-TiO_xis found to obeythe conventional Meyer-Neldel rule or anti Meyer-Neldel rule, but this rule does notexist in the crystalline TiO_xfilms.