New crystalline structure yields reliable thin-film transistors

Abstract

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:OD1:2) layers with a compositional ratio of In:OD1:2 and six (Ga,Zn)O layers with a compositional ratio of Ga:Zn:OD0.5:0.5:1 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 IGZOTFTs have high reliability4 and low off-state leakage current at the yA/ m (10 24A/ m) 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. 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:ZnD1:1:1). The InO2 layer has a compositional ratio of In:OD1:2 and the (Ga,Zn)O layer has a compositional ratio of Ga:Zn:OD0.5:0.5:1. In: Indium. Ga: Gallium. Zn: Zinc. O: Oxygen.