Preparation,Characterization And Device Study Of Anatase TiO₂ Single Crystal Films

Compared with the first and second generation semiconductors,wide band gap semiconductor materials possess higher breakdown field strength as well as excellent high temperature and radiation resistances,which satisfy the requirements for the performance and stability of high frequency and high power devices in extreme conditions.Wide band gap semiconductors have broad application prospects in the fields of national defense military,information technology and space exploration,and have become a commanding height of global strategic competition.Titanium dioxide?TiO₂?is a typical wide band gap oxide semiconductor with many advantages such as excellent visible light transparency,physicochemical stability,biocompatibility as well as non-toxic and low cost,which makes it widely focused in the fields of dye-sensitized solar cells,light catalysts and biomedicine,etc.TiO₂ has three main phases in natural state:rutile?r-TiO₂?,anatase?a-TiO₂?and brookite?b-TiO₂?.Among them,a-TiO₂ has higher photocatalytic activity and has been deeply studied in the fields of photocatalytic water splitting,decomposition of organic pollutants as well as self-cleaning coatings.A-TiO₂ possesses a wide band gap close to those of GaN,ZnO and SiC,a high refractive index matching with GaN,as well as a relatively high carrier mobility.These advantages make a-TiO,a promising transparent photoelectric film material.At present,there are few researches on a-TiO₂ epitaxial films and their applications in semiconductor devices,mainly because a-TiO₂ single crystal films are difficult to prepare,and intrinsic a-TiO₂ films possess quite high resistivity,which limit their applications in the field of semiconductor devices.Therefore,depositing a-TiO₂ single crystal thin films and controlling their electrical properties by doping are the basic work and necessary steps for the practical application of a-TiO₂ in semiconductor devices.Metal organic chemical vapor deposition?MOCVD?is a typical semiconductor thin film epitaxial process with advantages of good step coverage,high uniformity and fast deposition rate.It is suitable for the epitaxial growth of a-TiO₂ film.In consideration of the mechanical strength,physicochemical stability and lattice matching with a-TiO₂,epitaxial GaN,SrTiO3?STO?and MgAl₆O₁₀ single crystal wafers were selected as the substrate materials to deposite a-TiO₂ films.The effects of substrate temperature on crystal structures,morphologies and optical properties of the films were investigated.On this basis,a-TiO₂ epitaxial films doped with In,Nb and Ta elements were deposited,the electrical properties of which were well controlled and the doping mechanism along with the conduction mechanism were analyzed.Finally,metal-semiconductor-metal?MSM?structured ultraviolet photodetectors and thin film transistors?TFT?based on intrinsic and Ta-doped a-TiO₂ epitaxial films were fabricated,and the device performances were investigated.The main research contents and results of this dissertation are as follows:1.Intrinsic a-TiO₂ single crystal epitaxial films were deposited on epi-GaN?0001?/?-Al₂O₃,STO?100?and MgAl₆O₁₀?100?substrates by MOCVD.?1?.TiO₂ films were deposited on epi-GaN?0001?/?-Al₂O₃ at different temperatures?550,600,650 and 700 ??.Chemical composition measurements showed the deposited films approximated stoichiometric TiO₂.The film deposited at 650 ? was a-TiO₂ epitaxial film with high crystallization quality,the out-of-plane and in-plane epitaxial growth relationships between a-TiO₂ film and GaN epilayer were a-TiO₂?001?? GaN?0001?and a-TiO₂[110]? GaN<1100>,respectively.The average transmittance in visible range of the 650?-deposited film was about 90%,and its optical band gap was about 3.33 eV.?2?.TiO₂ films were prepared on STO?100?substrates at different substrate temperatures?500.550,600,and 650 °C?.X-ray diffraction?XRD?and transmission electron microscopy?TEM?results indicated the film prepared at 550 ? was a-TiO₂ single crystal film with high quality.The out-of-plane and in-plane epitaxial relationships between a-TiO₂ film and STO substrate were a-TiO₂?001??STO?100?and a-TiO₂[110]? STO[001],respectively,and the lattice mismatch between a-TiO₂[100]and the STO[001]direction was about 3.1%.XPS measurements showed the deposited film approximated stoichiometric TiO₂.The average transmittances in visible range of the films deposited at 500,550,600 and 650? were 93.0%,85.1%,88.6%and 93.6%,respectively.?3?.TiO₂ films were synthesized on MgAl₆O₁₀?100?substrates at different temperatures?500,550,600,and 650 ??.XRD and Raman tests results showed that the film prepared at 550 0C was a-TiO₂ epitaxial film with high crystallization quality while the films prepared at 600 and 650 ? presented anatase-brookite mixed phase.The out-of-plane and in-plane epitaxial relationship between a-TiO₂ and MgAl₆O₁₀ substrate were a-TiO₂?001?? MgAl₆O₁₀?100?and a-TiO₂[100]? MgAl₆O₁₀[001],respectively.The a-TiO₂ film prepared at 550 ? possessed an average transmittance of 95.1%in visible range,an optical band gap of about 3.46 eV and a refractive index of about 2.41 at the wavelength of 600 nm.2.Based on the preparations and researches of intrinsic a-TiO₂ single crystal films,In,Nb and Ta doped a-TiO₂ films were deposited on STO substrates and Ta doped a-TiO₂ films were also deposited on MgAl₆O₁₀ substrates via MOCVD.And the electrical properties of a-TiO₂ films were successfully modified.?1?.TiO₂ films with various In doping concentrations were deposited on STO?100?substrates at 550 ?.Structural measurements indicated the TiO₂ films with In doping concentration less than or equal to 1.80%to be[001]single oriented anatase phase while the films with doping concentration more than 1.80%to be polycrystalline.Room temperature Hall effect measurement showed that the prepared films were n-type semiconductors.And the doping mechanism analyses manifested that In mainly existed in the form of interstitial atoms in TiO₂ lattice.As In doping concentration increased,the resistivities of the films first decreased and then increased slowly.The minimum resistivity value was 8.68×10-2 ?·cm,which was obtained at In concentration of 1.80%and was about 6 orders of magnitude lower than that of intrinsic a-TiO₂,and the carrier concentration and Hall mobility of this film were 6.5× 1018 cm^-3 and 10.9 cm²V^-1s^-1,respectively.The average transmittances of the prepared films were about 85%in visible range.?2?.TiO₂ films with Nb doping concentrations of 0,0.15%,0.30%,0.60%,1.20%,1.50%,and 3.00%were prepared on STO?100?substrates at 550 ?.Structural tests manifested the films with different Nb concentrations were all a-TiO₂ epitaxial films grown along[001]direction.The resistivities of the films decreased first and then increased slightly with the increase of Nb doping concentration,and the minimum resistivity 5.75 × 10-2 ? cm was obtained at Nb content of 1.20%,which was around 6 orders of magnitude lower than that of undoped a-TiO₂ film.As Nb doping concentration increased from 0.30%to 3.00%,the Hall mobility decreased from 16.5 cm²V^-1s^-1 to 3.07 cm²V^-1s^-1,and the carrier concentration increasedfrom1.79 × 1018 cm^-3 to 1.99 × 1019 cm^-3.The average transmittances of the prepared films exceeded 88.0%in visible light range.?3?.TiO₂ films with Ta doping concentrations of 0,1.0%,2.0%,4.0%,6.0%and 8.0%were deposited on STO?100?substrates at 550 ? Crystal structure measurements indicated that the prepared films were all a-TiO₂ epitaxial films,and the in-plane and out-of-plane epitaxial growth relationships were a-TiO₂[100]? STO[001]and a-TiO₂?001?? STO?100?,respectively.Chemical composition measurements manifested that the actual Ta concentration values of the prepared films approached their designed values,and Ta distributed uniformly in a-TiO₂ films only in the form of+5 valence.Room temperature Hall measurement showed that the prepared films presented n-type conductivity.1.0%Ta-doped a-TiO₂ film had the highest Hall mobility of 15.4 cm²·V^-1·s^-1,and the film with Ta content of-4.0%has the minimum resistivity of 8.2×10-2 ? cm,which was about 6 orders of magnitude lower compared with the intrinsic a-TiO₂ film.The average visible light transmittances of the prepared films exceeded 92.0%.As Ta doping concentration increased from 0 to 8.0%.the optical band gaps of the films increased from 3.48 to 3.57 eV.?4?.Ta-doped TiO₂ films were deposited on MgAl₆O₁₀?100?substrates at 550?.Chemical composition measurements of the films showed that Ta only presented +5 valence in the films,and the actual doping concentration values were close to the designed values.Structural tests indicated the prepared films with low Ta doping concentrations to be[001]single oriented a-TiO₂ epitaxial films while the films with Ta contents more than 2.0%to be polycrystalline.The root mean square?RMS?surface roughnesses of TiO₂ films with Ta doping concentrations of 0,1.0%,2.0%,and 4.0%were 1.96,2.79,3.03,and 3.69 nm,respectively.As Ta doping concentration increased from 1.0%to 6.0%,the resistivities of the films first decreased and then increased slightly,and the minimum resistivity value was around 1.2×10-1 ? cm,which was obtained at Ta content of 4.0%;the carrier concentrations of the films increased monotonically from 1.1×1017 cm^-3 to 1.0×1019 cm^-3,and the carrier mobility decreases from 13.3 cm²·V^-1·s^-1 to 4.4 cm²·V^-1·s^-1.The average transmittances of the films in visible range were more than 87.0%.As Ta doping concentration increased from 0 to 4.0%,the optical band gaps of the films increased from 3.38 to 3.52 eV.3.Based on the aforementioned researches,ultraviolet photodetectors and TFTs based on intrinsic and Ta-doped a-TiO₂ films were fabricated,and the devices performances were studied.?1?.MSM UV photodetectors based on Ta-doped a-TiO₂ films with different Ta doping concentrations?0,1.0%,2.0%and 4.0%?on MgAl₆O₁₀?100?substrates were fabricated.Among them,the detectors based on 2.0%and 4.0%Ta-doped a-TiO₂ films had large dark currents,which did not change significantly when irradiated by ultraviolet light,indicating that a-TiO₂ films with high Ta doping levels were not suitable for fabricating ultraviolet photodetectors.The detectors based on undoped and 1.0%Ta-doped a-TiO₂ films presented good visible-blind characteristics and their peak photoresponsivities were 19.3 and 32.3 A/W.respectively.In addition,the ultraviolet-visible suppression ratios of the devices reached 104,showing good visible light-blind characteristic.When ultraviolet light was turned on/off,the photocurrent rise/fall times of the device based on undoped and 1.0%Ta-doped films were 4.7/2.0 and 5.0/3.0 s,repeatability.Besides,the devices presented good detection repeatability,indicating a stable ultraviolet light detection capability.?2?.TFTs based on Ta-doped a-TiO₂ films with various Ta contents?0,0.5%,2.0%and 4.0%?on MgAl₆O₁₀?100?substrates were prepared.The device based on 0.5%Ta-doped a-TiO₂ film exhibited typical n-channel depletion mode device characteristics with a switching current ratio of 4.0×108 and a subthreshold swing of only 0.60 V/dec.In addition,the saturation mobility of the device was about 4.4 cm²·V^-1·s^-1 which was better than that of amorphous Si TFT and close to the saturation mobility of IGZO TFT,indicating Ta-doped a-TiO₂ film to be a TFT channel material with great research value.