Smart Technology New Technology Presentation Meetings!
GTO oxide-TFT has more potential compared to that of IGZO-TFT

March 2, 2017“Smart Technology New Technology Presentation Meetings!” was held in JST. In this here, oral presentation Rare-metal free oxide-semiconductor thin film transistor by Prof. Kimura of Ryukoku University is picked up.

As you know, among oxide semiconductors, IGZO (In-Ga-Zn-O) is practical use for TFT-LCD and OLED as a backplane. However, considering future's resource depletion, it's ideal to not use In, a rare metal, in oxide semiconductor. For this reason, the research group succeeded to find out a new oxide semiconductor Ga-Sn-O (GTO) without In and Zn. Earth's crust reserve of Sn is approximate 10 times than that of In. And also, its unit price is not expensive. Furthermore, it can be deposited by the sputtering method in common with IGZO and a-Si.

As figure 1, GTO film is almost transparent and has non absorption in region of wavelength 400 nm and over. Also, it's confirmed to be amorphous structure from diffraction of X-ray result.

The research group pilot-produced GTO-TFT on the silicon wafer with thermal oxidation SiO2 film. This device is composed of substrate, thermal oxidation SiO2 film, GTO semiconductor layer, and Ti:Au source/drain. Figure 2 shows transistor characteristic of the first device. Its carrier mobility was 11.3 cm²/V･s. Furthermore, in the latest device, higher mobility same as 25.1 cm²/V･s was gained. This value is about 2 times than that of general IGZO-TFT. In addition, superior Vth same as −1.49V was gained.

As you know, in oxide-TFT inclusive of IGZO-TFT, there are some problems for practical use, concretely, bias stress durability such as PBS (Positive Bias Stress), PBTS (Positive Bias Temperature Stress), PBTS (Positive Bias Illumination Stress), NBS (Negative Bias Stress), NBTS (Negative Bias Temperature Stress), NBIS (Negative Bias Illumination Stress). NBIS is critical issue for display use. For this reason, bias stress durability was estimated in the test device without encapsulation, as a result, Vth shifted for 0.4 V by NIBS test applying voltage for 3600 sec. On the other hand, in IGZO-TFT without encapsulation, Vth shifted for −10 V by NIBS. In short, bias stress durability of GTO-TFT is higher than that of IGZO-TFT.

By the way, the above is based on characteristic of post annealing device at 350 ℃ after sputtering deposition. To research additional potential, RT deposited device without post annealing and mist deposited device (by mist deposition CVD method) without post annealing were produced and estimated. As a result, carrier mobility of the former and the later were 0.83 cm²/V･s and 0.3 cm²/V･s respectively. Of course, these specifications were lower than that of annealing device, however, if device structure and manufacturing process parameter are optimized; device characteristics are expected to be enhanced.

Reference
1)Kimura：Rare-metal free oxide-semiconductor thin film transistor, Smart Technology New Technology Presentation Meetings!, pp.31-35(2017.3)
※This title is translated arbitrarily by Stella News Site because of without English edition of presentation material.