Investigation On The Preparation And Properties Of Nanometer TiO₂ Films

Titanium dioxide (TiO2) films are widely used for optical andelectrical applications because it has a number of attractiveproperties, including high refractive index, high dielectricconstant, semiconductor properties and chemical stability. MoreoverTiO2 thin films have many applications in photocatalysis and solarenergy cell. There are three kinds of crystalline phases, anatase,rutile and brookite. The rutile and anatase structures have widerapplications, and this paper is mainly about these two structures.Up to now, TiO2 films have been prepared by a variety of depositiontechnique such as the sol-gel process, chemical vapor deposition,physical vapor deposition ,various reactive sputtering techniques,ion beam assisted process, atomic layer deposition, pulsed laserdeposition and filtered arc deposition. In most of these techniquesadditional heating during deposition or post heating is required tosynthesize crystalline phases of TiO2. Reactive rf magnetronsputtering has simple apparatus, and can be controlled easily. It canbe repeated well and can make large number kinds of films. It coulddeposit rutile phase at room temperature and the quality of the filmwas well. In this work we used the rf magnetron sputtering apparatus. Atitanium disk (99.99% purity) of 50mm in diameter was used as a target.The target-substrate spacing was 50mm. The background pressure in theprocess chamber was lower than 3×10-3Pa. We used a mixture of Ar andO2 gases as the sputtering gas. The deposition was performed on non-heated substrates. The substrates we used were polishing NiMnCoalloy slice and Si slice.Firstly, Raman spectroscopy, X-ray diffraction, UV-visspectroscopy and atomic force microscopy (AFM) were used tocharacterize the samples .The thickness and the adhesion of the filmswas also measured. The Raman spectroscopy indicated :(ⅰ)the filmsprepared at 10% oxygen partial pressure and 2.0 Pa total pressure wereanastase .(ⅱ) the films prepared at 10% oxygen partial pressure and0.5 Pa total pressure were rutile. The peak of anatase becomes weakerand that of rutile turn stronger with the increase of oxygen partialpressure in the X-ray diffraction spectra. The UV-vis spectrum showsabsorption peak at 380nm corresponding with the intrinsic absorptionof TiO2. The roughness and grain size were measured by AFM. Theroughness and grain size increase with increasing sputtering time.The films adhered with the substrate closely.Secondly, the influence of experimental parameters on the growthof TiO2 films was investigated by Raman spectroscopy. The resultsindicates: (ⅰ) With increasing rf power the energy of the particlesimpinging on the substrate increases, providing adequate energy fordiffusion and resulting the increase of mobility ratio. Therefore,the molecules in the films arrange more orderly and transform fromamorphous to polycrystalline. The crystallinity becomes better athigher rf power. (ii) The reduction of the total gas pressurehelped to the formation of rutile phase seems govern by thedensity of ionic and excited species. The density of ionized O2increases with decreasing pressure, so it helped to form rutilephase. At second, the formation of rutile phase depends on theenergy of particle impinging on the substrate. With the decreaseof working pressure, the mean free path increased and theprobability of collision decreased, resulting in the increaseof the energy of the particles arriving at the substrate, so therutile phase formed easily. (ⅲ)The increase of O2 partial pressureis in favor of the transition from anatase to rutile . This may bedue to that Ti3+ resulted from the vacancy of oxygen can makeanatase stable. The increase of O2 can remove Ti3+ , that is to say,the increase of O2 can remove the stability of anatase and facilitatethe formation of rutile. (ⅳ)The crystallinity becomes better withthe increase of sputtering time.(ⅴ)the adhesion of films are the bestwithout bias voltage.Then the electrical properties of TiO2 films on differentsubstrates were investigated. The results indicate that theresistivity of the films increases with increasing thickness.The films on NiMnCo substrates had undergone atransformation from conduction to semiconductor, then toinsulator. The Si-TiO2 films had undergone a transformation fromsemiconductor to insulator. The ambipolar layer thickness ofSi-TiO2 is about 94nm and that of NiMnCo-TiO2 is about 145nm.Thedifference of work function determined the depth of conductivelayer. The work function of NiMnCo is lower than that of Si. Thusthe ambipolar layer thickness is different.Finally, the UV-vis absorption spectroscopies of TiO2 filmsdeposited at different conditions were studied. It is known thatthe intrinsic absorption peak of TiO2 is at about 380nm and onlyultraviolet wavelength can be utilized. This kind of band gapmakes it absorb only ultraviolet in sunlight. Therefore, theutilization ratio of solar energy is very low (only about 4%).As we know the wavelength of absorption and the band gap are ininverse proportion .The band gap should become narrow in orderto obtain absorption in visible region. It has been reported thatthe band gap of TiO2 became narrow by doping to make use of visiblelight. In this thesis, the absorption peak can directly move fromultra-violet to visible by adjusting the sputtering time, rfpower, and magnetic field. The increase of grain size is the mainreason for the red-shift of absorption peak. The band gap of thefilms was also calculated. This characteristic perhaps will playimportant role in the scope of solar energy.