Approach to In-Ga-Zn-O Thin-Film Transistor via Atomic Layer Deposition: Properties, Processes and Precursors Characteristics

Abstract

Amorphous InGaZnOx (a-IGZO) thin film transistors (TFTs) are currently used in flat-panel displays due to their beneficial properties. However, the mobility of ~10 cm2/Vs for the a-IGZO TFTs used in practical displays such as OLED TVs is not satisfactory for high-resolution display applications such as virtual and augmented reality (VR and AR, respectively) applications. In this work, an approach to quaternary oxide IGZO was designed from binary oxide (InOx) to ternary oxide (IZO and IGO). Both of thermal and plasma enhanced ALD methods were investigated utilizing different indium precursors. The results of ternary oxide (IZO and IGO) applied TFT indicated multi-component oxide semiconductors deposited by ALD are promising for high performance oxide TFTs. However, the dependence of precursor adsorption on surface terminate species influences the composition and growth rate of thermal ALD deposited thin films. Moreover, In2O3 thin films deposited by thermal ALD using InCA-1 precursor were observed containing carbon impurity at low temperature and Si impurity at high temperature, while no nitrogen and carbon impurities existed in PEALD grown In2O3 thin films using DADI precursor. Thus, PEALD process using DADI precursor is suggested as a solution for IGZO TFT applications.

The electrical properties of amorphous oxide semiconductors, in general, change a lot according to their chemical composition; the Indium (In)-rich IGZO achieves a rather high mobility of 50 cm2/Vs. However, the In-rich IGZO TFTs possess another issue of negative threshold voltage owing to intrinsically high carrier density. Therefore, the development of effective way to carrier density suppression in In-rich IGZO will be a key strategy to the realization of practical high-mobility a-IGZO TFTs.

In this work, PEALD method is used to grow hetero-structure IGZO thin film, where In2O3 inter-layers are inserted between random Ga+ and Zn+ions, similar structure to sc-IGZO reported by Hosono’s group. The thicknesses of the sub-nanometer In2O3 layer are accurately controlled by PEALD super-cycle process and the deposited thin films are designed as heterogeneous IGZO thin films and homogeneous IGZO thin films. For the heterogeneous IGZO thin films, device performance is found highly dependent on crystallinity of films. A 5nm thick GaZnO sublayer is able to act a barrier to prevent indium and electron vertical diffusion. Although heterogeneous IGZO thin films with crystalline phase perform relative high mobility, the (a) large negative shift of Vth originate from high carrier concentration; (b) weak In-O bond induced carrier generation under bias limit their application on switching device. For the homogeneous IGZO thin films, a high mobility of 74.3 cm2 /(V s), with threshold voltage of −1.3 V, on-off ratio of 8.9 × 108, and subthreshold swing of 0.26 V/decades, are realized in an IGZO1.8nm TFT fabricated by sub-nano level vertical stacking InOx , ZnOx and GaOx atomic layers. It is found that the ratio of InOx to Ga-Zn-O is critical to TFT performance parameters such as mobility, threshold voltage and stability under positive bias stability stress. This study illustrates the potential advantages of ALD processes for fabricating ultra-high mobility oxide TFTs as well as flexible device products.