Since the report of amorphous In-Ga-Zn-O (a-IGZO) based thin film transistors (TFTs), interest in oxide semiconductors has increased due to their high mobility, low off-current, low process temperature, and wide flexibility of compositions and processes. However, oxide semiconductors deposited by conventional processes like physical vapor deposition (PVD) leads a problematic issue to produce high-resolution displays and highly integrated memory devices due to the limited process flexibility and the poor conformality on the structured surface. Thus, a need to replace conventional deposition processes has emerged. Atomic layer deposition (ALD) is an advanced technique which could provide conformal, thickness-controlled, and high-quality thin film deposition. From these backgrounds, recently, studies on ALD based oxide semiconductors have dramatically increased. Even so, the relations of film properties of ALD-oxide semiconductors with the main variables associated with deposition and issues related to applications are still less understood than conventional one. In this review, we introduce ALD-oxide semiconductors by providing: (Ⅰ) a brief history and importance of ALD-based oxide semiconductors in the industry, (Ⅱ) a discussion of the value of ALD in oxide semiconductors (in-situ composition control in vertical distribution /vertical structure engineering / chemical reaction and film properties / insulator and interface engineering), and (Ⅲ) an explanation of the challenging issues of scaling oxide semiconductors and ALD in industrial applications. This review provides a valuable perspective for researchers who have interest in semiconductor material and electronic devices by suggesting the reasons why ALD is important in the application of oxide semiconductors.
Achieving high mobility and reliability in atomic layer deposition (ALD)-based IGZO thin-film transistors (TFTs) with an amorphous phase is vital for practical applications in relevant fields. Here, we suggest a method to effectively increase stability while maintaining high mobility by employing the selective application of nitrous oxide plasma reactant during plasma-enhanced ALD (PEALD) at 200 °C process temperature. The nitrogen-doping mechanism is highly dependent on the intrinsic carbon impurities or nature of each cation, as demonstrated by a combination of theoretical and experimental research. The Ga2O3 subgap states are especially dependent on plasma reactants. Based on these insights, we can obtain high-performance indium-rich PEALD-IGZO TFTs (threshold voltage: −0.47 V; field-effect mobility: 106.5 cm2/(V s); subthreshold swing: 113.5 mV/decade; hysteresis: 0.05 V). In addition, the device shows minimal threshold voltage shifts of +0.45 and −0.10 V under harsh positive/negative bias temperature stress environments (field stress: ±2 MV/cm; temperature stress: 95 °C) after 10000 s.
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