Synthesis And Properties Of Sb-doped SnO₂ Thin Film

SnO₂ with tetragonal rutile structure is a kind of typical n-type semiconductor material that has wide energy gap(3.6eV) and high exciton binding energy(130meV). Due to its excellent performance in conductivity,optics,thermal stability and corrosion resisting,it has been widely applied in fields of gas sensor,humidity tester, solar cell,chemical electrode,transparent conductive thin films,optical glass,pigment and luminescent materials.Since pure SnO₂ thin film has high resistivity and its carrier concentration,which is decided by oxygen vacancy,is hard to control,doping is taken into account to reduce the resistivity while keep the high light transmittance. Doped Sb works as a donor to increace the carrier concentration so that the film's conductivity has been enhanced.The recent research on Sb-doped SnO₂(ATO) thin films focuses on the improvement of coating process,the influence of dopant on the optical and electrical properties and the extensions of ATO thin film application fields.However,a detailed analysis of the optical constants,refraction index,n and extinction coefficient,k,of the films through the study of spectroscopic ellipsometry has not yet been reported. Sb-doped SnO₂ thin films have been prepared by several methods such as chemical vapor deposition(CVD),spray pyrolysis,magnetron sputtering,pulsed laser deposition and sol-gel,among which sol-gel attracts much attention,for it is an easy way to fabricate large-area high-purity films with strictly controlled amount of dopant and the cost is really low.The achievements of this thesis are as follows:(1) High-quality antimony-doped tin dioxide(ATO) thin films with various concentration of Sb have been successfully prepared on silicon wafer by sol-gel spin-coating technique,using C₂H₅OH,SnCl₂·2H₂O and SbCl₃ as raw materials.In addition,polyethylene glycol(PEG) with different concentrations and molecular weight has been introduced to form porous structure.(2) Microstructure,surface morphology and composition of the films are characterized by using X-ray diffraction(XRD),scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS).XRD patterns show that the films are SnO₂ with rutile structure and Sb dopant causes the peak positions to shift slightly and their relative intensities to change instead of introducing new phases.The results of estimation of the grain size indicate that the growth of SnO₂ grain is greatly restrained by doping Sb.It's evidenced by XPS measurement that ATO film takes shape in the form of Sb⁵⁺ replacing Sn⁴⁺.SEM images indicate that surface morphology and grain size of the samples are influenced by the factors of film depth,annealing temperature,the concentration of Sb dopant and the amount of PEG.The optical properties of the samples are analyzed by spectroscopic technology and the measurement of reflection spectra.Refraction index n and extinction coeffient k,which are gained by spectroscopic ellipsometry measurement,both changes with the incidence wavelength. With increasing the layers in the films,the refraction index is decreased and the extinction coefficient is increased,resulting from that thicker films have fewer defects,lower resistance and consist of more intact and compact crystalline grain;Since appropriate high temperature can promote the amorphous film to develop into the state of completely crystallization with a characterized energy band structure,the increase of the annealing tempreture can lead to energy gap broadened and extinction coefficient reduced;Due to the band gap will be widened and the refraction index n will be reduced from 1.95 to 1.55 in the visible spectral region by donor state induced by Sb doping,we can control the refraction index of the ATO films by varying Sb content;The introduce of PEG leads to the phenomenon of refraction index first reduced and then increased while extinction coefficient first increased and then reduced.The analysis of the reflection spectra further validates that the energy gap will be broadened by increasing annealing tempreture or doping Sb element. The electrical properties of the samples are gained by the measurement of their sheet resistance and dielectric characteristics of the MOS structure(Au/SnO₂(ATO)/n-Si).It's found by analysis of sheet resistance that the resistivity of the sample will dramatically reduce with the increase of the concentration of Sb dopant while will increase when the concentration reach certain value.The capacitance value of the MOS structure is strongly dependent on the frequency and applied voltage and also influenced by interface states when the frequency is relatively low (f＜500 KHz).