| Wide ban gap materials have the advantages of high breakdown field strength,high chemical stability and strong radioresistance,which is suitable to fabricate the electronic devices with the radioresistance,high frequency,high power and high-density integration.They are widely applied in the fields of new-generation lighting,ultrahigh voltage power transmission,aerospace,5G communication and so on.At present,GaN and SiC materials have received the very wide application.Oxide semiconductor is one of the important research fields for the wide band gap semiconductor and has received the wide attention and research,which has been applied in newgeneration display technology.Tantalum pentoxide(Ta2O5)is a highly potential wide band gap semiconductor material and its band gap is 4.0-4.5 eV,dielectric constant is above 20,breakdown field strength is up to 4.5 MV/cm,and leakage current is as low as 10-8 A/cm2,which has received wide attention and research in the fields of new optical waveguide,photo catalysis,biomedical and semiconductor devices.The intrinsic Ta2O5 exhibits the high resistance and the research about Ta2O5 doping is usually in order to improve its photocatalytic activity.Nowadays,Ta2O5 material is usually used as the insulator dielectric layer in the semiconductor devices and its lattice structure usually is amorphous or polycrystalline.There are few reports on the research of Ta2O5 used as active layer in semiconductor devices,which is due to the lack of the preparation for its single-crystal materials and research for its electronic-properties improvement.In order to expand the application of Ta2O5 in the semiconductor devices,it is necessary to research the epitaxial growth,doping and optoelectronic properties of Ta2O5 single-crystal films.It is necessary to look for the suitable doping elements to improve the electronic properties of Ta2O5 films.The electron valence state of tungsten(W)element is +6,and the ionic radius of W6+is close to that of Ta5+.Thus,we pick W as the doping element.The preparation and properties of the orthorhombic(β phase)and hexagonal(δ phase)Ta2O5 epitaxial films are mainly researched in this dissertation.In this dissertation,β-Ta2O5 epitaxial films were prepared on α-Al2O3-based epitaxial GaN(epi-GaN)(0001)substrates grown along(001)plane using metal-organic chemical vapor deposition(MOCVD)method.δ-Ta2O5 epitaxial films were deposited on α-Al2O3(0001)grown along the(1011)plane.δ-Ta2O5 epitaxial films were prepared on YVO4(100)and YSZ(111)substrates grown along(0001)plane,respectively.The properties of the prepared films were studied.Then,our study found that the highest crystal quality of the prepared δ-Ta2O5 films prepared on YSZ(111)substrates.On the basis of the above work,W doped δ-Ta2O5 single-crystal films were prepared on YSZ(111)substrates using pulsed laser deposition(PLD)method and the properties of the films were researched.The ultraviolet(UV)detectors were fabricated based on W doped δ-Ta2O5 films,and the performance of the detectors were researched.Main content of this dissertation.1.Ta2O5 films were prepared on epi-GaN(0001)substrates using MOCVD method.The growth temperature of the films was 800 and 850℃,respectively.And then,the 850℃-prepared samples were annealed in air for 30 mins.The annealed temperature was 900 and 950℃,respectively.The lattice structure,epitaxial mechanism,chemical composition and band gap of the films were analyzed.The prepared films are β-Ta2O5 structure and grown along the single orientation of[001].The thermal annealing has the low effect on the crystal quality of the films.The film prepared at 850℃ exhibits the best crystal quality,which contains the triple domain structure rotated by 120°.The epitaxial relationship between the β-Ta2O5 film and GaN substrate is β-Ta2O5(001)‖GaN(0001)with-Ta2O5<100>‖GaN<1100>.The band gap of the 850℃prepared film is about 4.34 eV.2.Ta2O5 films were deposited on α-Al2O3(0001)substrates using MOCVD method.The growth temperature of the films was set to be 650,700,750 and 800℃,respectively.The surface topography,lattice structure,epitaxial mechanism,chemical composition and optical properties of the films were researched in detail.The 700 and 750℃ prepared films are all exhibited δ-Ta2O5 structure and grown along the δ-Ta2O5(1011)plane.800℃-prepared film exhibits the mix phase structure containing δ-Ta2O5 and tetragonal Ta2O5.The 750 ℃-prepared δ-Ta2O5 film shows the best crystal quality,which contains the triple domain structure rotated by 120°,and the corresponding epitaxial relationship between the film and substrate is δ-Ta2O5(1011)‖α-Al2O3 (0001)with δ-Ta2O5<1210>‖α-Al2O3(1010).The optical band gap of the films is 4.20-4.22 eV.The refractive index of the films deposited at 650 and 750℃ is in the range of 2.1-2.5.3.Ta2O5 films were prepared on YVO4(100)substrates using MOCVD method.The growth temperature of the films was set to be 800 and 850℃,respectively.The 850 ℃-prepared films were annealed in air at 900 and 950℃,respectively.The surface topography,structure properties,epitaxial mechanism,chemical composition,band gap and optical properties of the films were researched.The prepared films all exhibit δ-Ta2O5 structure.The growth plane of the 800℃prepared film is δ-Ta2O5(1010).Whereas,the growth plane of the films prepared at 800℃,annealed at 900 and 950℃ is δ-Ta2O5(0001).The 900℃-annealed film exhibits the best crystal quality,and the corresponding epitaxial relationship between the film and substrate isδ-Ta2O5(0001)‖YVO4(100)with δ-Ta2O5[1210]‖YVO4[010].The optical band gap of the 900℃annealed film is about 4.29 eV.4.Ta2O5 films were deposited on YSZ(111)substrates by MOCVD.The growth temperature of the films was set to be 700,750,800 and 850℃,respectively.The structure properties,epitaxial mechanism,chemical composition and optical properties of the films were researched.The prepared films all exhibit the δ-Ta2O5 structure grown along the single growth direction of[0001].800℃-prepared film shows the best crystal quality and the full width of half maximum(FWHM)of the XRD co-rocking curve is only 0.066°.There is no domain structure in the 800℃-prepared film.The epitaxial relationship between the film and substrate is δ-Ta2O5(0001)‖ YSZ(111)with δ-Ta2O5[2110]‖ YSZ[101].The valence state of the Ta element in the films is+5,and the atomic ratio of O/Ta is about 2.47,which is consistent with the stoichiometric ratio of Ta2O5.The optical band gap of the prepared films is 4.37-4.50 eV.5.The Ta2O5 films were grown with the different growth rate by regulating the metal-organic flow using MOCVD method.The substrates are YSZ(111)and the growth temperature is 800℃.The effect of growth rate on surface topography,structure,crystal quality and composition of the films were studied.The films all show the δ-Ta2O5 structure and the growth direction of the films is[0001].The prepared films all show the high-quality single crystal without domain structure.The crystal quality of the prepared films is slightly decreased with the increase of growth rate and the corresponding FWHM of the XRD ω-rocking curve is in the range of 0.067°-0.085°.The epitaxial relationship between the film and substrate is δ-Ta2O5(0001)‖YSZ(111)with δ-Ta2O5[2110]‖YSZ[101].6.The W doped Ta2O5 films were deposited on YSZ(111)substrates using PLD method.The growth temperature was 750℃.The doping concentration were 0.5%,1%and 2%,respectively.The effect of the W doping concentration on the surface topography,crystal quality and electronic properties of the films were systematically studied.The surface root mean square roughness of the films is increased from 0.38 to 0.72 nm with the increase of the doping concentration,whereas,the crystal quality of the films exhibits the slight decrease trend.All the films exhibit δ-Ta2O5 structure.The FWHM of the XRD rocking curve for 2%W doped δ-Ta2O5 film is only 0.075° and exhibit the high-quality single crystal without domain structure.The 1%and 2%W doped δTa2O5 films all exhibit n-type semiconductor electronic properties.The MSM UV detectors were fabricated based on the W doped δ-Ta2O5 single-crystal films.The performance of the UV detectors was researched.The UV detector based on 2%W doped δ-Ta2O5 single-crystal film exhibits the good detection performance and the corresponding light-dark current ratio,responsivity,fast rising and descending time under the 222 nm UV light with the 3 V voltage is about 1.3×104,10.32 A/W,0.26 and 0.16 s,respectively. |
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