Preparation And Characterization Of High Quality SnO₂ Films

In recent years,there has been increasing interest in the research of wide band gap semiconductors due to the huge commercial desire for short-wavelength luminescent devices.ZnO and GaN have been the research focus all along,but now it seems that there is more attention being paid to SnO₂.As a wide band gap semiconductor with direct bandgap,SnO₂ has very stable physical and chemical properties.Compared with ZnO and GaN,SnO₂ has some advantages,such as a wider band gap of 3.6 eV at room temperature,a higher exciton binding energy of 130 meV, a lower growth temperature,and higher chemical stability.All the virtues mentioned above make SnO₂ a promising UV and blue luminescence material.As a key functional material,SnO₂ has been widely used in the fields of transparent conducting oxides(TCO)and gas sensors.In the past,the investigation of SnO₂ films was mainly focused on these two fields.For the optical properties,much attention was paid on the transmission,reflection and refraction,seldom on the photoluminescence(PL).Until now,no study was reported on the available room temperature PL for SnO₂.If we can obtain SnO₂ films with stable PL properties through the study of preparation and PL mechanisms,we can get a new semiconductor material with UV-violet luminescent properties.However,the SnO₂ films prepared by the common methods such as sputtering,CVD,and spray pyrolysis are defective with bad PL properties and difficult to manufacture luminescent devices.Under such background,the preparation and characterization of high quality SnO₂ films are investigated in this article.The content of this article consists of two parts.In the first part,Sb-doped SnO₂ films were deposited by RF magnetron sputtering,the structural and optical properties of the films were investigated,and some new PL phenomena were observed.In the second part,high quality SnO₂ films,Sb-doped SnO₂ films,and In-doped SnO₂ films were prepared by MOCVD.The structural,optical and electrical properties of the films were investigated in detail. The major research work and results of the first part are as follows:1.Sb-doped SnO₂ films were prepared by RF magnetron sputtering,and the experimental procedure was described.The structural,optical and PL properties of the films as a function of substrate,sputtering power and annealing were investigated in detail,and the corresponding physical mechanisms were discussed.The XRD results indicated that the prepared samples were polycrystalline films having rutile structure of pure SnO₂ with preferred orientation of(110)direction;the lattice constants of the films were larger than the standard value of SnO₂ and films prepared on sapphire (0001)had the most appropriate lattice constants;both increased sputtering power and annealing could enhance the preferred orientation,enlarge the grain size and improve the crystallization of the films.The transmittance spectrum for the 150 W deposited SnO₂:Sb film exhibited high transmittance,with absolute average transmittance over 90%in the visible region and optical band gap of 3.80 eV obtained.2.The PL spectra of the 150 W deposited SnO₂:Sb film were measured at room temperature.A ultra-violet(UV)PL peak near 334 nm was observed for the first time. In the meanwhile,two other PL peaks located near 393 and 430 nm were also observed.After annealing at 600℃,the PL peaks near 334 and 393 nm became more intense,while the intensity of the PL peak near 430 nm decreased.The PL peaks near 334,393,and 430 nm were ascribed to the electron transition between the donor level formed by Sb⁵⁺and the valence band,the electron transition between the donor level and the acceptor level formed by Sb ions,and the electron transition related to the structural defects or impurities formed during the growth,respectively.The major research work and results of the second part are as follows:1.High-quality SnO₂ films were successfully prepared by MOCVD.Ultra high purity Sn(C₂H₅)₄,N₂ and O₂ were used as the organometallic source,carrier gas,and oxidant,respectively.XRD and SEM results indicated that SnO₂ films deposited on sapphire(0001)had the best crystalline quality.The structural,optical and electrical properties of the SnO₂ films deposited on sapphire(0001)at different substrate temperatures were investigated in detail.XRD and AFM results showed that as substrate temperature increased,the films changed from single crystalline to polycrystalline,and SnO₂ film prepared at 600℃had the best crystalline quality.The HRXRD and HRTEM results indicated that the 600℃prepared sample was single crystalline film having the rutile structure of pure SnO₂ with a clear epitaxial relationship of SnO₂(100)//Al₂O₃(0001).The full width at half maximum of theω-rocking curve of(200)reflection is only 0.11°,indicating a high quality crystalline structure obtained.To our knowledge,this is the smallest value obtained until now. The optical transmittance spectra showed that the average transmittance in the visible range for all films was over 90%.The optical band gap changed with the substrate temperature,with a minimum value of 3.58 eV obtained at 600℃.The Hall measurements indicated that resistivity,Hall mobility and carrier concentration for the films also changed with the substrate temperature.Film deposited at 600℃exhibited the highest Hall mobility of 27.67 cm²Â·v^-1·s^-1.The PL spectra of the SnO₂ film deposited at 600℃on sapphire(0001)were measured.An intense and sharp UV PL peak near 331 nm was observed at room temperature for the first time.While measured at low temperature,the intensity of this UV PL peak increased sharply and the peak position shifted towards higher energy.In the meanwhile,three other PL peaks near 369,375,and 500 nm were also observed. The origin of the UV PL peak near 331 nm was ascribed to the electron transition from the conduction band to the valence band.The PL peaks near 369,375,and 500 nm were ascribed to the band-acceptor transition,the donor-acceptor pair transition, and the electron transition related to the structural defects or impurities formed during the growth,respectively.2.Sb-doped SnO₂ transparent conducting films were prepared on sapphire(0001) substrates by MOCVD for the first time.High purity Sn(C₂H₅)₄ and Sb(CH₃)₃ were used as the organometallic sources.The structural,optical and electrical properties of the films dependent on doping level were investigated in detail.The XRD results showed that all the films had the tetragonal rutile structure and a single orientation along a-axis.The optical transmittance spectra and Hall measurements indicated that the optical band gap,resistivity,carrier concentration and Hall mobility for the films changed with the doping level.The 5%Sb-doped SnO₂ film exhibited the best optical and electrical properties with average transmittances over 90%in the visible range and optical band gap of 3.75 eV.The resistivity,carrier concentration and Hall mobility for the 5%Sb-doped SnO₂ film were 9.1×10^-4Ω·cm,5.33×10²⁰cm^-3and 12.73 cm²Â·v^-1·s^-1,respectively.The low temperature Hall measurements indicated that ionized impurity scattering was dominant in the low temperature range and lattice vibration scattering was dominant in the high temperature range.The high temperature annealing showed that all the SnO₂:Sb films had good thermal stability under 900℃,and the 7%Sb-doped SnO₂ film still exhibited good electrical and optical properties even at 1100℃.3.In-doped SnO₂ films were prepared on sapphire(0001)substrates by MOCVD for the first time.High purity Sn(C₂H₅)₄ and In(CH₃)₃ were used as the organometallic sources.All the films had the tetragonal rutile structure of SnO₂ and a single orientation alongα-axis with the best single crystalline structure obtained at 4%of In concentration.HRXRD and HRTEM results indicated that the 4%In-doped sample was single crystalline film of rutile structure with a clear epitaxial relationship of SnO₂(100)//Al₂O₃(0001)and full width at half maximum of 0.055°for theω-rocking curve of(200)reflection.The transmittance spectrum for the 4%In-doped SnO₂ film exhibited high transmittance,with absolute average transmittance over 90%in the visible region and optical band gap of 3.64 eV obtained.The PL spectra of the 4%In-doped SnO₂ film were measured.Only a UV PL peak near 339 nm was observed at room temperature.While measured at low temperature,the intensity of this UV peak increased sharply and the peak position shifted toward higher energy.In the meantime,two other PL peaks near 369 and 493 nm were also observed.The PL peaks near 339,369,and 493 nm were ascribed to the electron transition from the conduction band to valence band,electron transition from the conduction band to the acceptor level formed by In³⁺,and electron transition related to the structural defects or impurities formed during the growth,respectively.