Preparation And Properties Of Single Crystal Titanium Dioxide Films With Different Crystalline Forms

Comparing to the first and the second generation semiconductor materials,the third generation semiconductor i.e.wide band gap semiconductor materials have several advantages like high breakdown field strength,high thermal conductivity and wide band gap,which make them strongly adaptable in harsh environments of high temperature,high voltage,high power and strong radiation.Wide band gap semiconductor materials also have wide application prospects in transparent photoelectrical devices such as transparent thin film transistors,flat panel displays,ultraviolet detectors,light emitting diodes and semiconductor lasers.With its superior performance and huge market prospect,wide band gap semiconductor materials have become the focus of the global semiconductor market.Titanium dioxide?TiO₂?is a wide band gap transparent oxide semiconductor with a bang gap larger than 3 eV.Due to its physicochemical stability,nontoxicity,abundant reserves and low cost,TiO₂ has attracted lots of attentions in physics,chemistry,medical and functional materials.In addition,its high refractive index and high transparency in visible and near-infrared region make it very prospected in transparent photoelectrical devices.TiO₂ films prepared by traditional methods like sputtering,electron beam evaporation and sol-gel method were almost polycrystalline or nano-structured films with poor crystalline quality,suffering a poor performance stability.TiO₂ has three main phases:anatase,rutile and brookite.So far some researches have been done on the preparation of epitaxial TiO₂ films and most of the prepared films were anatase TiO₂?a-TiO₂?.There are few studies on the other two phases,especially on brookite phase.In addition,the intrinsic TiO₂ films show poor conductivities,which limit their applications in photoelectrical devices.Thus,it is necessary to take a systematic research on the preparation and properties of intrinsic and doped TiO₂ epitaxial films.This research has important scientific significance,not only in terms of potential applications in photoelectrical devices,but also for the fundamental studies of TiO₂ single crystalline.Comparing to other common thin film epitaxial methods,metal organic chemical vapor deposition?MOCVD?is more suitable for commercial mass production.In this dissertation,intrinsic single crystalline TiO₂ epitaxial films of three different phases have been successfully prepared on different substrates by MOCVD.The epitaxial mechanism,surface morphology,chemical composition,structural and optical properties have been studied systematically.On this basis,TiO₂ epitaxial films doped with different elements of three phases have been prepared.The effects of different doping elements and doping concentration on the properties of TiO₂ films have been investigated,and the effective modulation of electrical properties of the films has been achieved.In this dissertation,commercially available Tetrakis-dimethylamino Titanium?TDMAT?,Trimethylindium?TMIn?,Pentaethoxy Niobium?PEN?and Pentaethoxy Tantalum?PET?were used as the organometallic?OM?sources for Ti,In,Nb and Ta,respectively.High purity oxygen and ultra-high purity nitrogen were used as the oxidant and carrier gas,respectively.The research work of this dissertation can be divided into three parts as follows:1.Intrinsic and In-doped single crystalline brookite TiO₂?b-TiO₂?epitaxial films were deposited on Y-stabilized ZrO₂?YSZ??110?substrates by MOCVD.?1?TiO₂ films were prepared on YSZ?110?substrates by MOCVD.Effects of deposition rate on the properties of the films were investigated.The structural analysis indicated that all the obtained films were brookite phase TiO₂ and the films prepared at high deposition rates were polycrystalline structures.The crystalline quality of the films increased with the deposition rate decreasing and the film prepared at deposition rate of 1.25 A/min had the best crystalline quality with only one orientation of b-TiO₂?120?,which had the lowest surface roughness.?2?TiO₂ films were prepared on YSZ?110?substrates at different substrate temperatures?500?650??.During the deposition,the process conditions were the same as those of the TiO₂ film with the best crystalline quality prepared in the first section.The structural analysis indicated that the obtained films were all b-TiO₂ and the film prepared at 550? had the best single crystalline quality without any domains.The out-of-plane epitaxial relationship was b-TiO₂?120?|| YSZ?110?and the in-plane epitaxial relationship was b-TiO₂[001]|| YSZ[001]and b-TiO₂[210]|| YSZ[110].The average transmittances of the prepared films were all over 92%in the visible region.The optical band gaps of the films prepared at 500,550,600 and 650? were 3.66,3.64,3.57 and 3.60 eV,respectively.?3?In-doped b-TiO₂ films with different In concentrations have been prepared on YSZ?110?substrates at 550?.The obtained films showed the same orientation as undoped b-TiO₂ films and the crystalline quality decreased with the In concentration increasing.All the obtained In-doped TiO₂ films showed n-type conductivity.With the increase of In concentration,the resistivity first decreased and then increased slightly.The minimum resistivity was 7.4×10-2 ?·cm obtained at 2.6%?In/?In+Ti?atomic ratio?In-doped film,which was 8 orders of magnitude lower than that of undoped b-TiO₂ film.The hall mobility of the 2.6%In-doped film was 8.3 cm2V-1s-1 and the carrier concentration was about 1.0×1019 cm-3.The optical band gap of the obtained films was broadened slightly as In concentration increased.B-TiO₂ is relatively seldom investigated because it is difficult to be prepared.In this chapter,single crystalline b-TiO₂ film was successfully prepared by MOCVD and its electrical property has been effectively improved by In doping.2.Single crystalline a-TiO₂ epitaxial films were obtained on YSZ?100?substrates by MOCVD.On the basis,Nb and Ta doped a-TiO₂ films were prepared on YSZ?100?substrates to improve the electrical properties.In order to compare the effects of substrate material on properties of doped TiO₂ films,Nb and Ta doped a-TiO₂ films were also prepared on LSAT?100?substrates.Effects of doping concentration on the properties of the films were studied in detail.?1?TiO₂ films were prepared on YSZ?100?substrates at different substrate temperatures?500-650??.The films deposited at 500 and 550? were a-TiO₂ with polycrystalline structures and the films prepared at 600 and 650? only had one orientation of a-TiO₂?004?.The film prepared at 600? exhibited the best single crystalline quality.The out-of-plane epitaxial relationship was a-TiO₂?001?|| YSZ?100?and the in-plane epitaxial relationship was a-TiO₂[110]|| YSZ[001]and a-TiO₂[110]|| YSZ[010].The optical band gap of the single crystalline a-TiO₂ film prepared at 600? was about 3.16 eV and the average transmittance in the visible region was above 93%.?2?Nb-doped a-TiO₂ films with different Nb concentrations?0?3%,?Nb/Nb+Ti?atomic ratio?were deposited on YSZ?100?substrates at 600?.The obtained films showed the same orientation as undoped a-TiO₂ films prepared at 600 ?.The crystalline quality of the doped films decreased with the Nb concentration increasing.The minimum resistivity was obtained at 0.6%Nb-doped sample,which was 1.42?·cm,5 orders of magnitude lower than that of undoped a-Ti02 film.With the Nb concentration increasing to 3%,the film resistivity increased.?3?Ta-doped a-TiO₂ films with different Ta concentrations?0?8%,?Ta/Ta+Ti?atomic ratio?were deposited on YSZ?100?substrates at 6000?.All the obtained films were anatase phase TiO₂ with only one orientation of?004?.With Ta concentration increasing,the resistivity of the films decreased first and then increased slightly.The film with Ta concentration of 6%showed the lowest resistivity,which was 1.4×10-1?·cm.As the Ta concentration increased from 2%to 8%,the carrier concentration increased from 9.1×1016 to 2×1019 cm-3 and the mobility decreased from 13.2 to 0.1 Cm2V-1s-1.?4?The Nb-doped TiO₂ films with different Nb concentrations?0?5%?were prepared on LSAT?100?substrates with the substrate temperature of 600?.The out-of-plane epitaxial relationship of the obtained films was a-TiO₂?001?|| LSAT?100?and the in-plane epitaxial relationship of the films was a-TiO₂[O10]|| LSAT[010]and a-TiO₂[100]|| LSAT[001].The resistivity decreased first and then increased with the Nb concentration increasing.The 0.6%Nb-doped film had the minimum resistivity of 4.0x10-2 ?·cm,which was 8 orders of magnitude lower than that of undoped a-TiO₂ film.With the increase of Nb concentration from 0.15%to 5%,the carrier concentration increased from 9.4×10-7 to 1.7×1019 cm-3,and the electron mobility decreased from 13.5 to 0.5 cm2 V-1s-1.The average transmittance of the films in the visible region exceeded 93%.The optical band gap of the films broadened slightly as the Nb concentration increased.?5?The Ta-doped TiO₂ films with different doping concentrations?0?8%?were prepared on LSAT?100?substrates at 600?.The obtained films were all anatase phase TiO₂ with one orientation of a-TiO₂?004?.As the Ta concentration increased,the resistivity decreased first and then increased.The minimum resistivity of 2.3×10-2?·cm was achieved at 6%Ta-doped TiO₂ film.As the Ta concentration increased from 2%to 8%,the mobility decreased from 12.6 to 0.1 cm2V-1s-1,meanwhile,the carrier concentration increased from 6.6×1017 to 1.2×1020 cm-3.The above research indicated that for the a-TiO₂ films prepared on YSZ?100?substrates,the effects of Ta doping on the electrical properties of the films were better than that of Nb.Comparing to YSZ?100?substrates,LSAT?100?substrates were more suitable for preparing Nb and Ta doped a-TiO₂ films.3.Homoepitaxial rutile TiO₂?r-TiO₂?films were deposited on r-TiO₂?001?substrates by MOCVD.The effects of deposition temperature on the properties of the films have been studied.On this basis,Nb and Ta doped homoepitaxial r-TiO₂ films were prepared to investigate the effects of doping concentration on the properties of the films.?1?TiO₂ films were deposited on r-TiO₂?001?substrates at different substrate temperatures?500?700??.Films prepared at 500?600? were rutile phase TiO₂ polycrystalline films and those prepared at 650 and 700? had only one orientation of r-TiO₂?001?.The structural analysis indicated that the film prepared at 650? had the best single crystalline quality.The optical band gap of the homoepitaxial single crystalline r-TiO₂ film was about 2.99 eV,which is in accord with that of r-TiO₂ substrates.?2?Nb-doped homoepitaxail r-TiO₂ films with different Nb concentrations?0-3%?were prepared on r-TiO₂?001?substrates at 650?.The obtained films were all rutile phase TiO₂.The crystalline quality decreased with the increase of Nb concentration.The high resolution transmission electron microscopy images indicated that the 1.2%Nb-doped film was single crystalline r-TiO₂,With the increase of Nb concentration,the resistivity decreased first and then increased.As the Nb concentration increased from 0.15%to 3%,the mobility decreased from 14.0 to 3.1 cm2V-1s-1,and the carrier concentration increased from 8.5×1016 to 3.3×1018 cm-3.The r-TiO₂ film with Nb concentration of 1.2%had the lowest resistivity of 1.9×10-1 ?-cm,which was 7 orders of magnitude lower than that of undoped homoepitaxial r-TiO₂ film.?3?Ta-doped homoepitaxial r-TiO₂ films with different Ta concentrations?0?8%?were prepared on r-TiO₂?001?substrates at 650?.The obtained films were all along one orientation of r-TiO₂?001?.The resistivity first decreased and then increased with the increase of Ta concentration.The minimum resistivity was obtained at 6%Ta-doped film,which was 3.3×10-1 ?·cm,7 orders of magnitude lower than that of undoped r-TiO₂ film.With the increase of Ta concentration from 2%to 8%,the carrier concentration increased from 2.0×l016 to 3.3×1018 cm-3,and the mobility decreased monotonously from 13.6 to 3.2 cm2V-1s-1.Based on the above research,it can be indicated that for the doped homoepitaxial r-TiO₂ films,the doping efficiency of Nb is obviously higher than that of Ta.The Nb element is more suitable for the preparation of n-type doped homoepitaxial r-TiO₂ films than the Ta element.