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Illumination & Displays

New crystalline structure yields reliable thin-film transistors

Oxide semiconductor thin-film transistors with an active layer of highly crystalline indium gallium zinc oxide are very dependable and have extremely low off-state current.
13 September 2012, SPIE Newsroom. DOI: 10.1117/2.1201209.004452

Oxide semiconductor thin-film transistors (TFTs) have attracted attention as leading devices for next-generation displays because of their transparency, flexibility, high-speed operations, and larger sizes. These features are not available simultaneously in existing silicon-based TFTs. The current drivability of amorphous silicon TFTs is insufficient for use in organic LED (OLED) displays, and the poor uniformity of polycrystalline silicon TFTs is a critical issue for large-size displays. Oxide semiconductor TFTs are expected to overcome such shortcomings. Regarding indium gallium zinc oxide (IGZO)-based semiconductor materials, the crystalline structure of InGaZnO4 was found by Kimizuka and colleagues in 1985, and IGZO-related materials were then studied in detail for more than a decade. 1, 2

Figure 1(a) shows that the unit cell of single-crystal InGaZnO4 is hexagonal in the a-b plane, and has a layered structure, including three InO2 (In:O=1:2) layers with a compositional ratio of In:O=1:2 and six (Ga,Zn)O layers with a compositional ratio of Ga:Zn:O=0.5:0.5:1 along the c-axis. So far, amorphous IGZO has been mainly investigated as a TFT channel material. However, reliability issues such as degradation under bias temperature stress have not been solved sufficiently. And from an industrial point of view, device reliability is one of the most important factors.

We have successfully fabricated a highly crystalline IGZO thin film with c-axis alignment, which we call a c-axis aligned crystal (CAAC) film.3 We found that CAAC IGZO-TFTs have high reliability4 and low off-state leakage current at the yA/μm (10−24A/μm) level.5 We have applied them to low-power-consumption displays.6–9 We also have applied CAAC IGZO-TFTs to non-displays using extremely low off-state current such as in non-volatile oxide semiconductor RAM (NOSRAM),10 a normally-off processor,11 and an image sensor12 as novel applications of an oxide semiconductor.

A CAAC is closer to a single crystal than an amorphous material. It has c-axis alignment like a single crystal, but its a-b plane has a mosaic pattern and no grain boundaries. The CAAC differs from a polycrystal, which generally has a, b, and c axes oriented at random and with clear grain boundaries. Figure 1(b) and (c) shows plane and cross-sectional transmission electron microscopy (TEM) images of our CAAC IGZO thin film, respectively. The plane TEM image indicates that the crystal structure in the film is hexagonal when seen from the c-axis. In addition, the film has a layered structure when seen from the direction perpendicular to the c-axis. These features closely resemble the single crystal shown in Figure 1(a).

Figure 1. (a) Single-crystal InGaZnO4, (b) plane, and (c) cross-section transmission electron microscopy (TEM) images of a c-axis-aligned indium gallium zinc oxide (CAAC IGZO) thin film (In:Ga:Zn=1:1:1). The InO2 layer has a compositional ratio of In:O=1:2 and the (Ga,Zn)O layer has a compositional ratio of Ga:Zn:O=0.5:0.5:1. In: Indium. Ga: Gallium. Zn: Zinc. O: Oxygen.

Figure 2(a) shows out-of-plane x-ray diffraction (XRD) of CAAC and amorphous IGZO films. A peak (009) appears in the CAAC IGZO spectrum, which means that the c-axis is oriented perpendicular to the film. Figure 2(b) shows in-plane XRD of CAAC IGZO. In this case, the spectrum is irregular, meaning that CAAC IGZO does not necessarily have a- and b-axes alignment across the entire in-plain region. We fabricated TFTs using the CAAC IGZO film, and measured the Id-Vg (drain current–gate voltage) characteristics: see Figure 3(a). We found that variations among the TFTs rarely occur even on the substrate of 600×720mm, and that the film has good uniformity.

Figure 2. (a) Out-of-plane x-ray diffraction (XRD) of CAAC and amorphous IGZO films. (b) In-plane x-ray diffraction of a CAAC IGZO film. AU: Arbitrary units.

Figure 3. (a) Variation of Id-Vg(drain current–gate voltage) characteristics at drain currents Vd=1Vand 10V among 20 CAAC IGZO-TFTs with channel length L=3μm and channel width W=50μm on a substrate of 600×720mm. (b) Gate temperature tests under light (80°C, Vg=30V, 2000sec, white LED 10,000lx) for a CAAC IGZO-TFT with L=6μm and W=50μm. The threshold voltage shift ▵Vthis +0.27V and –0.23V for positive and negative gate bias stresses, respectively.

Next, we measured the density of the CAAC IGZO film by x-ray reflection. The density of the CAAC film was 6.22g/cm3, while the theoretical value of the single crystal was 6.375g/cm3. This result shows that the CAAC film is similar to a single-crystal film. We thought that the CAAC film could improve reliability because the film was dense. We thus conducted gate bias temperature tests under light: see Figure 3(b). And in fact, the fluctuation of the Id-Vg characteristics was very small.

Figure 3(a) shows that the off-state current of our CAAC IGZO-TFTs is below the measurement limit of typical semiconductor parameter analyzers. We devised a method for calculating the off-state current from the potential change of a capacitor in relation to time.5 The change is obtained with the capacitor charged and then discharged by off-state current. Using this calculation method, we found that the off-state current per micrometer of channel width was 100yA/μm (1022 A/μm) or lower at 85°C. The Arrhenius plot of the off-state current had good linearity at high temperature, suggesting that a thermal activation barrier of the off-state current exists. The extrapolated value to room temperature can be below the level of yA/μm (10−24A/μm), which means that a switching device such as the IGZO-TFT has extremely high charge retention.

In one application, we used CAAC IGZO-TFTs as the backplane of a flexible OLED display.3 There are two methods for manufacturing flexible displays, one by which TFTs are directly formed on a flexible substrate and another by which they are formed on glass and then transferred to a flexible substrate. We fabricated the display using the latter method. Figure 4 shows the 13.5-inch flexible OLED display and its specifications.

Figure 4. A 13.5-inch flexible organic LED using CAAC IGZO-TFTs as the backplane and the specifications. RGB: Red, green, blue. qHD: One-quarter high definition.

In summary, we found a new crystalline structure of IGZO with c-axis alignment and named it CAAC. The CAAC IGZO-TFTs are highly reliable and have an extremely low off-state current. In the future, we will link these features to the mass production of low-power-consumption displays and other applications such as new memory devices and normally-off processors.

Shunpei Yamazaki
Semiconductor Energy Laboratory Co., Ltd.
Atsugi-shi, Japan

Shunpei Yamazaki, president of Semiconductor Energy Laboratory, is also a foreign member of the Royal Swedish Academy of Engineering Sciences and an IEEE life fellow. He received Japan's Medal with Purple Ribbon for innovation of MOS (metal oxide semiconductor) LSI element technology in 1997 and the Okouchi Memorial Technology Prize in 2010. He holds more than 6314 patents (the 2011 Guinness World Record).

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2. N. Kimizuka, M. Isobe, M. Nakamura, Syntheses and single-crystal data of homologous compounds, In2O3(ZnO)m (m = 3, 4, and 5), InGaO3(ZnO)3, and Ga2O3(ZnO)m (m = 7, 8, 9, and 16) in the In2O3-ZnGa2O4-ZnO system, J. Solid State Chem. 116, p. 170-178, 1995. doi:10.1006/jssc.1995.1198
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