Optical and electrical performance of titanium-gallium-zinc oxide transparent semiconductor thin films

GU Jinhua，WAN Xin，LONG Hao2,，CHEN Shoubu，ZHONG Zhiyou2,*

（1 Experimental Teaching and Engineering Training Center，South-Central Minzu University，Wuhan 430074，China；2 Hubei Key Laboratory of Intelligent Wireless Communications，South-Central Minzu University，Wuhan 430074，China；3 College of Electronic Information Engineering，South-Central Minzu University，Wuhan 430074，China）

Abstract The transparent semiconductor thin films of titanium-gallium-zinc oxide（TiGaZnO）were prepared on glass substrates by magnetron sputtering.The effects of deposition pressure on optical and electrical characteristics were studied by UV-Vis spectrophotometer and Hall-effect measurement system.The results demonstrate that all the prepared samples possess high transparency in the visible light region.The average transmittance in the visible range exceeds 81.0% for all the prepared samples regardless of the deposition pressure.The deposition pressure has a significant impact on the optical and electrical characteristics of the prepared samples.The thin film fabricated at deposition pressure of 0.4 Pa has relatively better optoelectronic performance，with a high figure of merit of 4.034×103 Ω-1·cm-1 and low resistivity of 1.685×10-3 Ω·cm.Keywords magnetron-sputtering technique；transparent semiconductor；optical and electrical performance

Gallium-zinc oxide（GaZnO）is a promising semiconductor material which has numerous applications in modern technologies such as organic electroluminescent devices［1-4］，liquid-crystal displays［5-7］，thin film transistors［8-9］，solar cells［10-13］and chemical sensors［14-16］.Apart from high optical transparency and electrical conductivity，the GaZnO thin films have several advantages such as nontoxicity，low manufacture cost，high exciton binding energy（60 meV），wide direct optical bandgap（3.30 eV）and high stability under hydrogen plasma in comparison with the transparent conducting indium-tin oxide thin films［17-20］.In recent years，the dopants such as aluminum（Al），fluorine（F），titanium（Ti）and magnesium（Mg）are added to further improve the physical properties of GaZnO thin films［21-28］.Although several experimental studies have been reported on the microstructural，morphological and electrical properties of co-doped thin films，there are few reports on the figure of merit of them in the available literature.In this study，the transparent semiconductor thin films of titanium-gallium-zinc oxide（TiGaZnO）were prepared on glass substrates by using radiofrequency（RF）magnetron-sputtering method under various deposition pressures.The optical and electrical characteristics were studied by Hall-effect measurement system and UV-Vis double beam spectrophotometer，respectively.

1 Experimental procedures

A ceramic target（TiO2：1.5 wt.%，Ga2O3：1.5 wt.%，ZnO：97 wt.%，4 N in purity）was used as the sputtering source material，and a common glass was employed as the substrate.The TiGaZnO thin film samples were grown on the cleaned glass substrates by RF magnetron-sputtering method at various deposition pressures in an argon gas atmosphere，using MS-560C sputtering system.Prior to the TiGaZnO samples deposition，the pre-sputtering for about 15 min was performed in order to attain stability and to remove the ceramic target's surface contamination.The processing parameters for preparing the TiGaZnO samples were tabulated in Tab.1.

Tab.1 The processing parameters for preparing thin film samples表1制備薄膜樣品的工藝參數(shù)

The optical reflectance and transmittance of the prepared samples were measured over the wavelength range 300-850 nm by using a TU-1901 UV-Vis double beam spectrophotometer，and the reflection and transmission spectra were recorded at room temperature.The thickness of the prepared samples was determined by an Alpha-step 500 surface profiler.The electrical characteristics of the investigated samples were characterized at room temperature using a RH-2035 Hall-effect measurement system.All measurements were carried out in ambient air.

2 Results and discussion

Fig.1 displays the dependence of the optical transmission spectra（T-λ）of the investigated samples in the wavelength region 300-850 nm as a function of deposition pressure.As can be seen from the figure，all theT-λcurves exhibit an interference pattern where the transmittance drops rapidly at the edge of the bands，which indicates excellent crystallinity and low surface roughness.For the samples prepared under the deposition pressure of 0.3，0.4，0.5 and 0.6 Pa，the maximum optical transmittance are found to be 97.5%，97.8%，96.4%and 94.6%，respectively.The results indicate that all the prepared samples possess high transparency in the visible light region.

Fig.1 Transmission spectra of all the prepared samples圖1所有薄膜樣品的透射光譜

Fig.2 depicts the dependence of the average transmittance（Tvis）in the visible range of the prepared samples on deposition pressure.As the figure illustrating，theTvisvalue exceeds 81.0% for all the samples regardless of the deposition pressure.TheTvisincreases with the increase of deposition pressure from 0.3 to 0.4 Pa，and then decreases when the deposition pressure exceeds 0.4 Pa.The maximumTvisvalue of 86.3% of the prepared sample can be obtained when the deposition pressure is 0.4 Pa.

Fig.2 The Tvis values for all the prepared samples圖2所有薄膜樣品的可見光平均透過率

Fig.3 gives the dependence of the optical reflection spectra（R-λ）of the investigated samples in the wavelength region 300-850 nm as a function of deposition pressure.As can be seen in the figure，all theR-λcurves exhibit an interference pattern in the visible light region.The average reflectance values of the prepared samples in visible range were found to be ranging from 13.1%to 14.4%.

Fig.3 Reflection spectra of all the prepared samples圖3所有薄膜樣品的反射光譜

The optical bandgap（Eg）of the prepared samples can be evaluated by applying the Tauc model and the Davis and Mott model in the high absorbance region［29-30］：

whereαis the absorption coefficient，λis the wavelength of incident light，Bis a constant of proportionality，cis the light speed，Eis the photon energy，his the Planck constant，andnis an exponent which can assume different values depending on the nature of the electronic transitions［31］.Theαvalues of the prepared samples were calculated from the optical transmission and reflection spectra using the following formula［32］：

wheredis the thickness of the thin film，Tis the transmittance of the thin film，andRis the reflectance of the thin film.Fig.4 presents the Tauc's plots of（αE）2as a function of photon energyEfor the samples prepared at different deposition pressures.A good straight line can be obtained in the band edge region for all the samples，suggesting the transition is direct in nature［31］.The straight-line portion of the curve，when extrapolated to zero，gives the direct optical bandgapEg.The dependence of optical bandgapEgon deposition pressure are shown in Fig.5.From the data of the figure，the values ofEgfor the prepared samples with various deposition pressures are 3.456，3.481，3.463 and 3.476 eV，respectively，and theEgvalue for the undoped ZnO thin film is about 3.30 eV.The results indicate that the direct optical bandgaps of all the prepared samples are larger than that of undoped ZnO thin film.The increase in direct optical bandgap may be due to the Burstein-Moss effect［33-34］.Similar results have been reported by many researchers［35-38］.

Fig.4 Tauc’s plots of（aE）2-E for all the prepared samples圖4所有薄膜樣品的（aE）2-E曲線圖

Fig.5 The Eg values for all the prepared samples圖5所有薄膜樣品的光學(xué)能隙

Fig.6 shows the dependence of the electrical characteristics of the prepared samples on deposition pressure.As the deposition pressure rises from 0.3 to 0.4 Pa，the Hall mobility（μ）and carrier concentration（ne）increase，which brings about the drop ofρfor the prepared samples.When the deposition pressure is 0.4 Pa，the TiGaZnO thin film exhibits the minimumρvalue of 1.685×10-3Ω·cm.The minimum resistivity at 0.4 Pa may be the reason of the improvement of crystal quality.Additionally，the maximumμvalue of 14.105 cm2·V-1·s-1and the highestnevalue of 2.629×1020cm-3are also found when the deposition pressure is at 0.4 Pa.

The figure of merit（FTC）is introduced in order to quantify the optoelectronic performance of the prepared thin films［39］.It is defined as：

whereTvisis the average visible transmittance of thin film，andρis the resistivity of thin film.

The dependence of figure of meritFTCof the prepared samples on deposition pressure is shown in Fig.7.As the deposition pressure rises，theFTCincreases first and subsequently drops and reaches the peak value of 4.034×103Ω-1·cm-1when the deposition pressure is at 0.4 Pa.The increase ofFTCvalue with deposition pressure was due to the decrease of resistivity and the increase of average visible transmittance.As we all know，the higher the figure of merit，the better the optoelectronic performance of the transparent semiconductor thin film.Thus，it can be concluded that the optimal deposition pressure is 0.4 Pa in this work.

Fig.7 The FTC values for all the prepared samples圖7所有薄膜樣品的品質(zhì)因數(shù)

3 Conclusion

In summary，the transparent semiconductor thin films of TiGaZnO were prepared onto common glass substrates under various deposition pressures.The influence of deposition pressure on the optical，electrical and optoelectronic performance of the prepared thin films was investigated.The results show that the average transmittance of the prepared samples is above 81.0%in the wavelength range of the visible spectrum，and the optical，electrical and optoelectronic characteristics of the samples are subjected to the deposition pressure.When the deposition pressure is at 0.4 Pa，the prepared sample exhibits relatively better optoelectronic performance with the low resistivity（1.685×10-3Ω·cm）and the high figure of merit（4.034×103Ω-1·cm-1）.Additionally，the optical bandgaps of the investigated samples were evaluated according to the extrapolation method，and observed to be in the range of 3.456 to 3.481 eV.