Preparation And Optoelectrical Properties Of TGZO Transparent Conductive Thin Films

Transparent conductive oxide （TCO） thin film is one kind of important materialsof optoelectronic information with high transparency and low resistivity. The largeamount of market requirements on transparent thin film are mainly in the fields of solarcells, flat-panel displays （FPDs） and touch panels. At present, indium-tin oxide （ITO）thin film is commonly used as the mainstream products in industry. However, theindium resources in the earth is very limited, and the demand for transparentconductive film grows unbated, seeking the alternatives is a task which brooks nodelay. Znic oxide （ZnO） film is a cheap and non-toxic semiconductor material ofexcellent optoelectrical performance comparable with ITO, and it is considered as oneof the potential alternatives for ITO. Though, there are many reports about singleelements doped ZnO thin film, the optical and electrical properties should be improvedstill further.In this paper, titanium-gallium co-doped zinc oxide （TGZO） thin films weredeposited on the glass substrates by radio-frequency magnetron sputtering techniqueusing a sintered ceramic of ZnO mixed with TiO2（1.5%） and Ga2O3（1.5%） as a target.The microstructural, optical and electrical properties of the TGZO films were studied.The main works and conclusions are shown as follows:（1） According to the spectral fitting theory, the spectral gradient optimizationalgorithm was used in the solving process, a computer simulation program forprocessing transmission spectral evaluation of the refractive index （n）, extinctioncoefficient （k）, and thickness （d） was developed. Meanwhile, the optical bandgap （Eg）values of the deposited films were obtained by extrapolation method. Furthermore, theoptical constants and thickness were determined by the envelope method and scanningelectron microscope （SEM）, respectively. The results were consonant with thosedetermined from the spectrum fitting program, which indicates the fitting program iseffective and creditable. The spectrum fitting method is simple and feasible without theneed of complex and expensive hardware equipments, therefore, it has generalapplicability.（2） The microstructure, composition and morphology of the films wereinvestigated by X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS） andSEM, respectively. The grain sizes estimated by Scherrer formula are consistent withthe SEM results. The prepared films are polycrystalline in nature having a hexagonal wurtzite type crystal structure with a preferred grain orientation in the （002） direction.The doping process does not change the crystal structure of ZnO films, however, thesputtering parameters obviously affect the grain size and crystalline quality of the films.The TGZO films fabricated at the substrate temperature of350°C exhibit the bestcrystalline quality, which have the maximum intensity of （002） diffraction peak（1.42×10⁶cps）, the minimum full width at half maximum （0.092°） and the largest grainsize （82.33nm）.（3） The electrical properties of the fims were studied by four-point probe. Theresults show that the electrical properties of TGZO films are in the marked degreesubjected to the sputtering parameters such as working pressure, substrate temperature,sputtering power and sputtering time. With increasing the values of sputteringparameters, the electrical resistivity is found to decrease firstly and then increase, andthe lowest electrical resistivity is close to10-4Ω cm. The electrical properties of TGZOfilms are closely related to the microstructure. The larger the grain size of the films, thebetter the conductivity of the films.（4） The transmittance, optical constant （n, k） and optical bandgap （Eg） of the filmswere studied by UV-visible spectrophotometer and the method of optical spectrumfitting, respectively. The results calculated by the inverse fitting program show that theoptical dispersion properties of TGZO films are highly dependent on the depositionconditions. All the prepared films obey the normal dispersion relation in the visibleregion, and the gradient of n and k is very small. The values of n range from2.0to2.5,and k value is close to0. However, the gradient of n and k increase rapidly in the UVregion, and some samples exhibit the anomalous dispersion characteristics. The opticalbandgap Egof TGZO films are greater than that of the pure ZnO due to theBurstein-Moss effect. The Egvalues are obviously broadened for the doping process,ranging from3.40eV to3.55eV.（5） The optoelectronic properties of TGZO films were quantitatively evaluatedusing figure of merit （ФTC） defined by Haccke. The ФTCvalues are observed to rangefrom0.29×10^-2Ω^-1to1.38×10^-2Ω^-1for the TGZO films deposited with the differentsputtering parameters, which indicates that the optoelectronic properties of the filmswere obviously affected by the deposition conditions and the selection of sputteringparameters for preparing the transparent conductive TGZO films is very important.（6） Based on the investigation of the microstructure and optoelectrical propertiesof the TGZO thin films, the optimum deposition conditions were obtained and given asfollows: working pressure,0.4Pa; substrate temperature,350°C; sputtering power, 200W; target-substrate distance,70mm; argon rate of flow,15sccm; and sputteringtime,30min. The TGZO films grown at the optimum deposition conditions exhibit theexcellent optical and electrical performance. The average transmission in the visiblerange is close to86%, the electrical resistivity is as low as1.78×10^-3Ω·cm, and thefigure of merit reaches up to1.38×10^-2Ω^-1. The dispersion behavior of the refractiveindex was analyzed in terms of the single-oscillator Wemple-DiDomenico （W-D）model, and the oscillator parameters of the films were achieved. The resultsdemonstrate that the refractive index dispersion of TGZO films obey the single-oscillator W-D model.