J Shanghai Univ (Engl Ed), 2011, 15(4): 242–244 Digital Object Identifier(DOI): 10.1007/s11741-011-0729-1

Dependence of electrical and optical properties of IGZO films on oxygen flow

ì  CHEN Long-long (í  )

SHI Ji-feng ( ), ZHANG Jian-hua (

),



LI Qian (

Í),

), LI Xi-feng (

Key Laboratory of Advanced Display and System Application (Shanghai University), Ministry of Education, Shanghai 200072, P. R. China ©Shanghai University and Springer-Verlag Berlin Heidelberg 2011

Abstract Amorphous InGaZnO (a-IGZO) films were deposited on the corning eagle XG (EXG) glass substrates using magnetron sputtering method. The structure, surface morphology, electrical and optical properties of these films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), semiconductor parameter analyzer and spectrophotometry, respectively. The influence of oxygen flow on the electrical properties of IGZO thin films was studied, showing that increasing oxygen flow changes the resistivity with six orders of magnitude. The contact resistance of ITO/IGZO is 7.35×10−2 Ω·cm2 , which suggests that a good ohmic contact exists between In2 O3 : Sn (ITO) and IGZO film. Keywords InGaZnO (IGZO), sputtering, contact resistance

Introduction In the past few years, organic light-emitting diode (OLED) display has attracted considerable attention because of several fundamental advantages compared with LCD TVs, such as lower power consumption, very fast response time, higher contrast ratio, and wider viewing angle. However, there are many challenges to bring these properties together into a high quality display. One of the most challenging problems for activematrix OLED (AMOLED) is the need for a stable and cost-effective thin-film transistor (TFT) backplane. Recently, transparent oxides are viewed as novel candidates for channel materials of TFTs[1−2] . Amorphous InGaZnO (a-IGZO) TFTs have several advantages over other transparent conductive oxide (TCO) TFTs, such as high field-effect mobility, small subthreshold voltage swing, and high electrical reliability[3−6] . Furthermore, a-IGZO TFTs can be fabricated at low temperatures (< 300 ◦ C) and most parts of the fabrication process are compatible with the existing technology employed for mass production of a-Si:H TFTs. These features make the a-IGZO TFT a promising candidate for AMOLED. The contact interfaces between amorphous oxide semiconductor channels, and source/drain contact materials have strong influences on the performance of devices. Thus, the influence of S/D electrodes, contact area,

ohmic contact quality, and parasitic series resistance should be considered. However, so far, less attention is paid to the systematic studies about the low-resistance ohmic contacts to a-IGZO. In this paper, the contact property of IGZO/ITO films is focused. a-IGZO film is deposited on the glass substrate by radio-frequency sputtering, and the effect of oxygen flow on the property of IGZO thin film is studied.

1 Experimental a-IGZO films were deposited on unheated glass substrates (Corning EXG 200 mm×200 mm) using ULVAC SME-200E rf-magnetron sputtering equipment with a polycrystalline InGaZnO4 ceramic sputtering target. The films were grown in Ar and O2 ambient, oxygen flow was adjusted from 0 to 1.5 mL/min, and the RF power of sputtering war set at 300 W. The films with the thickness of 100 nm were obtained. The chemical compositions of as-deposited IGZO film were estimated by ICP which were In : Ga : Zn=1 : 0.9 : 0.8 in atomic ratio. The optical transmission of the IGZO thin films was analyzed by UV/VIS spectrophotometer (U-3000 spectrophotometer). The characteristic of the a-IGZO thin films’ structure was analyzed by thinfilm X-ray diffraction (XRD) (DLMAX-2200 type), and the scanning electron microscopy (SEM Hitachi S4800)

Received Apr.23, 2011; Revised May 16, 2011 Project supported by the Science and Technology Commission of Shanghai Municipality (Grant Nos.09111100702, 10DZ1100102) Corresponding author LI Xi-feng, Ph D, Assoc Prof, E-mail: [email protected]

J Shanghai Univ (Engl Ed), 2011, 15(4): 242–244

was used to analyse the surface morphology, operating at the electron acceleration voltage of 10 kV. Effective contact resistance (RC-eff) were extracted by the wellknown transmission line method (TLM) using a different lengths resistance structure. All electrical characterizations were conducted with a semiconductor parameter analyzer (Agilent HP 4155C) at the room temperature in the dark.

2 Results and discussion

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ranges from −1 V to 1 V. Figure 5 shows the resistivity of as-deposited IGZO films as a function of oxygen flow. As is shown, the resistivity decreases from 2×10−1 Ω·cm to 8.5×105 Ω·cm with the increase of oxygen flow from 0 to 1.5 sccm. The conductivity monotonically increases in logarithmic scale with increasing the oxygen flow. The result indicates that the resistivity of the sputtered a-IGZO films can be controlled by the sputtering condition.

Figure 1 shows the XRD patterns for the asdeposited IGZO. It can be seen that the diffraction patterns only consist of halo peaks, indicating amorphous structure regardless of the prepared conditions in this study. Figure 2 presents the typical surface SEM images of IGZO films. Featureless contrast typically for an amorphous film is seen over the whole morphology, further indicating that the films show the amorphous structures, which have been verified by the XRD pattern. As is shown in Fig.2, the films display a very smooth surface and a uniform grain size.

Fig.1

Fig.3

I-V curves of IGZO films deposited in different oxygen flow

Fig.4

I-V curves of IGZO films deposited when oxygen flows are 0.5, 1.0, 1.5

Fig.5

Resistivity of as-deposited IGZO films as a function of oxygen flow

XRD pattern of typical as-deposited IGZO film

Fig.2

SEM micrographs for a typical IGZO film

Figure 3 shows current-voltage (I-V ) characteristics of the ITO contact to a-IGZO with different oxygen flow. They were obtained from TLM contact pads with a spacing of 50 μm. The as-deposited IGZO films showed a nearly linear I-V characteristic regardless of oxygen flow, indicating a good ohmic behaviour in the voltage

The TLM was adopted to evaluate the contact resistance of different electrodes. By defining the total

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J Shanghai Univ (Engl Ed), 2011, 15(4): 242–244

resistance (RT ) as RT = V /L, RT can be expressed as L + RC , Wd where ρIGZO is resistivity of the IGZO films, RC the series resistance of ITO/IGZO contacts, W the width of transmission line, L the length of transmission line, and d the thickness of the IGZO films. As is illustrated in Fig.6, the total resistance is plotted as a function of transmission line length, and then fit the experimental values with linear curves. The series resistance can be obtained from the y-axis intercept. The resistance of IGZO contact to ITO is 7.35×10−2 Ω·cm2 . Figure 7 shows the optical transmission spectra of the IGZO films ranging from 200 nm to 800 nm with different oxygen flows. The results indicate that at the low oxygen flow, the average visible transmission is relatively low, and with the increase of the oxygen flow, the transmittance increases and the average visible transmission is more than 80% at oxygen flow of above RT = RIGZO + RC = ρIGZO

0 mL/min, which meets the industrial requirement of transmittance exceeding 80%. Low oxygen flow will result in the drop of average transmittance, which may be ascribed to the nonstoichiometry structure of the films due to the oxygen deficiency.

3 Conclusions This study has demonstrated that rf sputtering is a feasible method to prepare high quality IGZO thin fims and the electrical resistivity is varied in the wide range from 10−1 Ω·cm to 10−5 Ω·cm. ITO/IGZO contact shows a good ohmic behavior, with a contact resistance of 7.35×10−2 Ω·cm. The average visible optical transmission of IGZO exceeds 80% including glass substrate.

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Fig.6

Relation of resistance of IGZO films and transmission line length

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Fig.7

Optical transmission of IGZO films deposited with different oxygen flow

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