Effect of a ZTO buffer layer on the structural, optical, and electrical properties of IGZO thin films

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Abstract

Transparent and conducting InGaZnO4 (IGZO) thin films were deposited by RF magnetron sputtering on zinc tin oxide (ZTO) coated glass substrates to investigate the effect of the ZTO buffer layer on the structural, optical, and electrical properties of the IGZO/ZTO films.

X-ray diffraction (XRD) analysis revealed that although all the films were in the amorphous phase, IGZO 90 nm/ ZTO 10 nm bi-layered films had a lower resistivity (1.1×10−2 Ωcm) than the other films prepared in this study. The enhanced optical transmittance of IGZO 80 nm/ZTO 20 nm films (85.5%) was attributed to the two-fold lower RMS roughness of these films than that of IGZO single films, because a flat surface reduces optical scattering and absorption at the film surface.

These results indicate that a ZTO thin film is an effective buffer layer that can enhance the optical transmittance and electrical conductivity of the IGZO upper film.

Introduction

There is considerable interest in the use of metal doped ZnO films such as AZO [1], GZO [2] and IGZO [3] films as transparent and conducting oxide (TCO) layers for transparent electrodes because of their high optical transmittance in the visible wavelength range, chemical stability, and lower cost of production than conventional Sn doped In2O3 (ITO) films. IGZO films have been used as an active layer in transparent thin film transistors (TTFTs) because the films can be deposited as an amorphous phase, while retaining high carrier mobility [4]. However, relatively high substrate temperatures are needed to produce IGZO films with the required optical and electrical properties [5].

One way to improve the optical and electrical properties of IGZO films without substrate heating is to use an IGZO/metal/IGZO (OMO) structure, which has lower resistivity than IGZO single layer films of the same thickness [6].

Another attempt that has been used to enhance the electrical properties of IGZO films is the insertion of a TCO buffer layer under the IGZO film. Kim et al. demonstrated that adding an optimized 5 nm ITO buffer layer between the IGZO active layer and SiO2 dielectric layer increase the carrier mobility, threshold voltage, and off-current of the TFT [7].

In this study, IGZO/ZTO bi-layered films were deposited by radio frequency (RF) magnetron sputtering on glass substrates. Since the ZTO film is an inexpensive and non-toxic oxide material that grows in an amorphous phase at low temperatures, it has been used as a buffer layer for IGZO film in TTFT applications.

For these reasons, we investigated the effects of IGZO/ZTO thickness on the structural, optical, and electrical properties of films using X-ray diffractometer (XRD), atomic force microscopy (AFM), UV–vis spectrometer, Hall effect measurements, and UV photoelectron spectroscopy (UPS), respectively.

Section snippets

Experimental details

IGZO single layer and IGZO/ZTO bi-layered films were deposited by radiofrequency (RF) magnetron sputtering on glass substrate (Corning 1797, 2×2 cm2) without intentional substrate heating. Sintered IGZO (In2O3: Ga2O3: ZnO=1:1:1 At%) and ZTO (ZnO:SnO2=1:1 At%), targets both had 3-inch diameters. Glass substrates were degreased in a dilute detergent solution, rinsed in de-ionized water, and blown dry in nitrogen gas before they were set on the substrate holder. The distance between the target and

Results and discussion

Fig. 1 shows the XRD patterns of IGZO single layer and IGZO/ZTO double-layered films of different thicknesses. The observed broad peak in the range of 30° to 35° is considered as an amorphous halo peak that comes from the IGZO film [8]. It means that the IGZO films with the composition of In:Ga:Zn=1:1:1 At.% have an amorphous crystal structure [9].

Kim et al. [10] reported that ZTO film maintained a stable amorphous structure even when heated to 600 °C and Cho et al. reported that when the

Conclusions

Both IGZO single layer and IGZO/ZTO bi-layered films were deposited on glass substrates without intentional substrate heating by RF magnetron sputtering. The structural, optical, and electrical properties of the IGZO films were dependent on the ZTO buffer layer.

By examining AFM images, it is concluded that the ZTO buffer layer enhanced the flatness of IGZO films. Thus the high optical transmittance of 85.5% of IGZO 80 nm/ZTO 20 nm films is attributed to their low RMS roughness. Electrical

Acknowledgement

This work was supported by 2013 Research Fund of University of Ulsan.

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