Preparation And Characteristic Regulation Of Epitaxial Tin Oxide And Zinc Stannate Single Crystal Films

The third-generation semiconductor materials possess large band gap,high thermal conductivity and high strength of breakdown electric field,which are very suitable for high-temperature,high-frequency and high-power applications.The third-generation semiconductor materials have become one of the crucial points for the strategic competition between great powers in the military and civilian technologies such as military radar,5G communications,high-speed train and electric vehicles.Due to the excellent photoelectrical properties,non-toxic and rich resource reserves,wide band gap oxide semiconductors have become a research hotspot of the third generation of semiconductors after SiC and GaN,being considered to have a broad application prospect in the fields of high-voltage power devices and ultraviolet optoelectronics.Single crystal semiconductor materials own the characteristics of complete lattice structure,few defects,high carrier mobility and excellent optical properties,thus enjoying the obvious advantages in the manufacture of semiconductor optoelectronic devices.The regulation of the electrical properties of single crystal materials by doping is one of the effective methods to improve device performance.Rutile phase SnO2,as a typical wide band gap oxide semiconductor,has a band gap of 3.6 eV with good transparency in the near ultraviolet and visible light regions,and has been received extensive attention and research in solar cells,lithium ion batteries,and gas sensors.Because there is no SnO2 bulk single crystal material,it is necessary to prepare SnO2 single crystal film by hetero-epitaxial method.The SnO2 films prepared by traditional methods such as magnetron sputtering and sol-gel have a polycrystalline structure.Even using the epitaxial method,the prepared SnO2 films are inclined to appear domain structures because of the different lattice structure between the substrate and SnO2 as well as the constraints of lattice mismatching,which will decrease the crystalline quality of the films.Antimony(Sb)and fluorine(F)are normally used in the doping of SnO2 film,which can effectively regulate the electrical properties of the films.However,due to the large differences of ionic radii between the dopant elements and SnO2 host elements,the crystal quality of single crystal film is significantly reduced after doping.Therefore,systematic research on the preparation of domain-free SnO2 single crystal films and the selection of doping elements which have less effect on the crystalline quality of SnO2 to regulate the electrical properties of the films is a very necessary part for the current SnO2 research.The inverse-spinel phase Zn2SnO4 is a ternary wide band gap oxide semiconductor with excellent electrical and optical properties,which is considered to be highly potential in the fields of dye-sensitized solar cells,photocatalysis,gas sensors,humidity sensors and resistive random access memory.Zn2SnO4 films prepared by traditional methods such as solution,sol-gel and sputtering are mostly mixed-phase,polycrystalline and nanostructures.The crystal quality of these film is poor,and there are a large number of defects in the material.At present,there are no Zn2SnO4 bulk single crystal material,and no report about Zn2SnO4 single crystal films published at home and abroad,which limits the research and application of Zn2SnO4 materials in the field of semiconductor photoelectric devices.Therefore,the preparation of Zn2SnCO4 single crystal film with high crystal quality and research on its application in semiconductor devices will contribute to expand the application of Zn2SnO4 material.The thesis mainly contains three parts:In the first part,the domain-free SnO2 single crystal films were successfully prepared on MgF2 single crystal wafers using MOCVD method.The structure,optical and electrical properties of the obtained films were studied in detail.In the second part,based on the investigation of first part,Nb and Ta doped SnO2 epitaxial films were deposited by MOCVD.The effect of doping concentration on the structure of SnO2 and the regulation rules on the electrical properties of the films were systemically studied.In the third part,epitaxial Zn2SnO4 single crystal films were obtained on MgO single crystal wafers using the PLD and post-annealing methods.The epitaxial relationships,microstructure and optical properties of the obtained samples were investigated in detail.On the basis of the study above,the preparation of the Ta doped Zn2SnO4 films was further performed.The effect of Ta concentration on the structure,morphology and optical properties of the Zn2SnO4 films were systemically studied.The UV-detectors based on the Ta doped Zn2SnO4 films were fabricated,and the performances of these devices were studiedThe main research contents of the thesis:1.Epitaxial SnO2 single crystal films were deposited on MgF2(001)and MgF2(110)substrates by MOCVD.(1)SnO2 films were deposited on MgF2(001)substrates at different temperatures(540-660?).The chemical compositions of the prepared films were close to the stoichiometric SnO2.The obtained films were all rutile phase SnO2 grown along the[110]orientation.Among them,the SnO2 film prepared at 620? was an epitaxial single crystal film with the best crystal quality.The out-of-plane epitaxial relationship between the film and substrate was SnO2(110)//MgF2(001).Double domain structures existed in the SnO2 films,so there were two equal probability kinds of in-plane epitaxial relationships of SnO2[001]//MgF2[110]and SnO2[001]//MgF2[110].The Sn02 film prepared at 620? had the highest carrier Hall mobility of 10.4 cm2·V-1·s-1 with the corresponding resistivity of 0.75 ?·cm and the carrier concentration of 8.1×1017 cm-3.The average transmittances in the visible range of SnO2 films prepared at different substrate temperatures exceeded 86%,with the optical band gaps ranging from 3.93 to 4.05 eV.(2)SnO2 films were prepared on MgF2(110)substrates at different substrate temperatures(580-700 ?).XRD results showed that the obtained films were rutile phase SnO02 and grew along a single orientation of[110].When the substrate temperature was 660?,the obtained SnO2 film was a single crystal film with the best crystal quality.The out-of-plane and in-plane epitaxial relationships between the film and substrate were SnO2(110)//MgF2(110)and SnO2[001]//MgF2[001],respectively.The XPS measurements indicated that the elemental composition of the film prepared at 660? accorded with the stoichiometric SnO2.The SnO2 film prepared at 660?had the highest Hall mobility of 21.0 cm2·V-1·s-1,,and the corresponding resistivity was 0.76 ?·cm with the carrier concentration of 3.9 × 1017 cm-3.The average transmittance of the 660? deposited SnO2 film in the visible light region exceeded 93%,and the optical band gap was 3.89 eV.2.Using MOCVD method,Nb and Ta doped SnO2 epitaxial films were prepared on MgF2(110)substrates,and Ta doped SnO2 epitaxial films were prepared on a-Al2O3(012)substrate.(1)SnO2 films with different Nb concentrations(0-8.4 at.%)were deposited on MgF2(110)substrates.The obtained films were all rutile phase SnO2 and grew along a single orientation of[110].The out-of-plane and in-plane epitaxial relationships between the film and substrate were SnO2(110)//MgF2(110)and SnO2[001]//MgF2[001],respectively.The XPS results showed that Nb presented in the SnO2 film was mainly pentavalent state.When Nb concentration increased from 0 to 8.4 at.%,the carrier concentration of the film monotonically increased,which ranged from 3.9×1017 to 3.3×1019 cm-3.The Hall mobility increased first and then decreased.The resistivity of the prepared films rapidly decreased at the beginning and then slightly increased,with the control range exceeding two orders of magnitude.The 4.3 at.%Nb-doped SnO2 film had the lowest resistivity of 2.9×10-3?·cm and the highest Hall mobility of 84.0 cm2·V-1·s-1,with an optical band gap of 4.04 eV.The average transmittances of SnO2 films with different Nb concentrations in the visible region were above 80%.(2)SnO2 films with different Ta concentrations(0-8.0 at.%)were prepared on MgF2(110)substrates.The chemical composition results showed that Ta mainly existed in the SnO2 film with a pentavalent state,and the actual Ta concentrations of the prepared films approximated the experimental set values.XRD and cross-section TEM analyses showed that the prepared Ta doped SnO2 films were epitaxial films,of which the out-of-plane and in-plane epitaxial relationships between the film and the substrate were SnO2(110)//MgF2(110)and SnO2[001]//MgF2[001],respectively.As the Ta doping concentration increased from 0 to 8.0 at.%,the carrier concentration of the film monotonically increased from 3.9×1017 cm-3 to 3.9×1020 cm-3,and then remained unchanged.The Hall mobility increased first and then decreased.The 5.0 at.%Ta doped SnO2 film owned the highest Hall mobility of 74.2 cm2·V-1·s-·1.The film resistivity decreased first and then slightly increased.The lowest resistivity presented in the 6.0 at.%Ta doped SnO2 film,which was about 2.5×10-4 ?·cm with three orders of magnitude lower than that of the undoped SnO2 film.The prepared films had average transmittances in the visible region of more than 87%and optical band gaps ranging from 3.89 to 4.32 eV.The PL spectra of the obtained films manifested a photoluminescence region from blue to yellow.(3)SnO2 films with different Ta concentrations(0-8.0 at.%)were prepared on a-Al2O3(012)substrates.The obtained films were rutile phase SnO2 epitaxial films grown along a single orientation of[101].The out-of-plane and in-plane epitaxial relationships between the film and substrate were SnO2(101)//Al2O3(012)and SnO2[010]//Al2O3[1210],respectively.The resistivity of SnO2 films decreased first and then had a slightly rise with the increase of Ta concentration.When the Ta concentration was 6.0 at.%,the resistivity of SnO2 film had a minimum value of 4.0×10-4 ?·cm,which had been reduced by more than three orders of magnitude compared with the undoped SnO2 film.The Hall mobility of SnO2 film had a maximum value of 58.1 cm2·V-1·s-1 at 4.0 at.%Ta concentration.The average transmittances of the obtaind films in the visible region were above 88%,and the optical band gaps ranged from 3.96 to 4.30 eV.3.Using PLD method and post-annealing process,inverse-spinel phase Zn2SnO4 films were prepared on MgO(110)and MgO(100)substrates,among which hetero-epitaxial Zn2SnO4 single crystal films were successfully prepared on MgO(100)substrates.On the basis above,Ta-doped Zn2SnO4 films on MgO(100)substrates and ultraviolet light detectors based on these films were prepared and fabricated,besides the properties of Ta-doped Zn2SnO4 films and the related properties of devices were studied.(1)Zn2SnO4 films were prepared on MgO(110)substrates and annealed at different temperatures(700-900?)in air atmosphere.The as-deposited Zn2SnO4 film was amorphous.The films annealed at 700 and 800? were polycrystalline Zn2SnO4 films with multiple growth orientations.The film annealed at 900? was inverse-spinel phase Zn2SnO4 polycrystalline film with a cubic structure grown along a single orientation of[110].The optical band gaps of the as-deposited film and films annealed at 700,800,and 900? were 3.32,3.80,3.98,and 4.09 eV,respectively,with the average transmittances in the visible region exceeding 93%.(2)Zn2SnO4 films were deposited on the MgO(100)substrates and annealed at different temperatures(700-900?)in air atmosphere.The as-deposited and 700?annealed Zn2SnO4 films were amorphous films.The films annealed at 800 and 900?were inverse-spinel phase Zn2SnO4 single crystal films with a cubic structure grown alone a single orientation of[100].Among them,the 800? annealed Zn2SnO4 film had the best crystal quality,of which the out-of-plane and in-plane epitaxial relationships between the film and the substrate were Zn2SnO4(100)//MgO(100)and Zn2SnO4[001]//MgO[001],respectively.The root-mean-square roughness of the film surface had a range of 0.70-1.49 nm and increased with the annealing temperature rising.The atomic ratio of Zn and Sn for the 800? annealed film was close to the stoichiometric Zn2SnO4.The average transmittances of the as-deposited,700,800,and 900? annealed samples in the visible region were 81.8%,81.9%,82.8%,and 82.5%,while the optical band gaps were 3.29,3.48,3.97,and 4.11 eV,respectively.The refractive index at a wavelength of 600 nm for the single crystal Zn2SnO4 film annealed at 800? was about 1.80.(3)Zn2SnO4 films with different Ta concentrations(0-5.0 at.%)were deposited on MgO(100)substrates and annealed at 800? in air.The metal-semiconductor-metal(MSM)UV detectors based on the obtained films were also fabricated.After Ta doping,Zn2SnO4 film maintained the original[100]single growth orientation,the crystal quality decreased gradually as the Ta concentration increased.Ta doping reduced the surface roughness of Zn2SnO4 films,with the RMS range of 1.51-1.86 nm.The average transmittances of the prepared Zn2SnO4 films with different Ta concentrations in the visible region were all above 95%,and the optical band gaps were between 3.98 and 4.10 eV.The UV detector based on the 3.0 at.%Ta doped Zn2SnO4 film had the highest responsivity with the detection wavelength range concentrated at 200-280 nm.When the 3.0 at.%Ta doped Zn2SnO4 UV-detector worked at 5 V bias,the responsivity for 254 nm UV light was 23.3 A/W,while the ratio of light-dark currents was above 104.Besides,the detection rise and fall times were 0.73 and 0.58 s,respectively,which were both shorter than the detection times of UV detectors based on the undoped Zn2SnO4 film.The detectors also showed good detection repeatability.