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Revolutionising displays with indium, gallium and zinc oxide technology

A new material should enable even higher resolutions, lower power consumption, and higher performance touch screens, as well as narrower bezel widths for LCD display panels used in mobile devices such as smartphones

Sharp Corporation and Semiconductor Energy Laboratory (SEL) have jointly developed a new oxide semiconductor (IGZO) technology with high crystallinity.

This jointly developed new IGZO technology imparts crystallinity in an oxide semiconductor composed of indium, gallium and zinc. Compared to current amorphous IGZO semiconductors, Sharp says this new material enables even smaller thin-film transistors to be achieved and provides higher performance.

Sharp believes that IGZO will be adopted for use in LCD displays for mobile devices such as smartphones where the trend toward higher screen resolutions is growing increasingly strong. What's more, it can also be adapted for use in organic EL displays which hold out high expectations for the future.

Although challenges to commercialisation remain in terms of both service life and production, the two companies will continue to push ahead with R&D in anticipation of future market needs.

Aiming towards early commercialisation in LCD displays using this new IGZO technology, the two companies will also be pursuing R&D to expand the use of this material in non-display devices and to develop applications other than displays in the future.

The LCD display will have a screen size of 4.9 inch by 6.1 inch and a pixel resolution of 720 x 1280 and 2560 x 1600. The pixel density will be 302 ppi and 498 ppi. Envisioned applications include mobile devices, in particular smartphones.

During the development of the oxide semiconductor, Sharp and SEL found a new crystalline structure and have named it CAAC (C-Axis Aligned Crystal) structure.

Single crystal IGZO is characterised by having a hexagonal structure when seen from the c-axis direction and a layered form when seen from the direction perpendicular to thec-axis (see Figs. 1 (a) and (b)).

The researchers found the IGZO layer is a crystalline hexagonal structure, as shown in a plane TEM image and cross-sectional TEM image, depicted in Figs. 2 (a) and (b)

Another cross-sectional TEM image shows the relationship between a surface of the layer and the c-axis: the c-axis of the IGZO crystal is aligned in a perpendicular direction to the surface of the layer see Fig. 3(a), (b))The name of the structure, theC-Axis Aligned Crystal(CAAC)structure, comes from this relation.

A conventional amorphous IGZO TFT has had a problem of shift in electrical characteristics in the gate bias temperature (BT) stress tests, especially in the gate BT stress tests with light irradiation. Meanwhile, a CAAC-IGZO TFT is less influenced by (is resistant to) the gate BT stress tests with light irradiation and has high reliability (see Fig. 4(a), (b)), which means that the TFT can be stable with CAAC-IGZO.

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