Highly Dense and Stable p‑Type Thin-Film Transistor Based on Atomic Layer Deposition SnO Fabricated by Two-Step Crystallization

Over the past several decades, tin monoxide (SnO) has been studied extensively as a p-type thin film transistor (TFT). However, its TFT performance is still insufficient for practical use. Many studies suggested that the instability of the valence state of Sn (Sn2+/Sn4+) is a critical reason for the poor performance such as limited mobility and low on/off ratio. For SnO, the Sn 5s–O 2p hybridized state is a key component for obtaining p-type conduction. Thus, a strategy for stabilizing the SnO phase is essential. In this study, we employ a variety of analytical methods such as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Hall measurement to identify the main contributors to the physical properties of SnO. It is revealed that precision control of the process temperature is needed to achieve both the crystallinity and thermal stability of SnO. In other words, it would be ideal to obtain high-quality SnO thin films at low temperature. We find that atomic layer deposition (ALD) is a quite advantageous process for obtaining high-quality SnO thin films by the following two-step process: (i) growth of highly c-axis oriented SnO at the initial stage and (ii) further crystallization along the in-plane direction by a postannealing process. Consequently, we obtained a highly dense SnO thin film (film density: 6.4 g/cm3) with a high Hall mobility of ∼5 cm2/(V·s). The fabricated SnO TFTs exhibit a field-effect mobility of ∼6.0 cm2/(V·s), which is a quite high value among the SnO TFTs reported to date, with long-term stability. We believe that this study demonstrates the validity of the ALD process for SnO TFTs.