A High Mobility of p-Type Sno Thin Films Grown By Atomic Layer Deposition for Thin Film Transistors

Abstract

Oxide semiconductors have attracted great interest as channel materials for use in various emerging electronic applications including transparent and flexible displays, sensors, logic devices, and so on. Great progress has been made in n-type oxide semiconductors for thin film transistors (TFTs), which show superior properties such as high electron mobility and Ion/off ratio. Despite rapid development of n-type oxides, progress in the development of p-type oxides has been delayed. Only a few oxides including SnO, CuO, and NiO have been reported for p-type TFTs, and not yet sufficient for applications. Furthermore, most of the p-type oxides have been fabricated by a reactive sputtering technique, which is not applicable in industry. In this study, we propose atomic layer deposition (ALD) of stable and high quality p-type SnO thin films for p-type TFTs. The SnO films were deposited with dimethylamino-2-methyl-2-propoxy-tin(II) and H2O as Sn and O sources, respectively. The growth of the SnO films was performed in a growth temperature range from 150 to 210 oC. From the XPS analysis, the ALD-grown SnO films are mainly composed of Sn2+ ions and O2- ions, and a peak from Sn4+ ions in the XPS spectra is not observed. As increasing the growth temperature, the Hall mobility increased from 1.26 to 3.8 cm2/Vs and Sn/O atomic ratio of the films approached the stoichiometric value from less unity (O-rich phase). We also fabricated TFTs with the ALD-grown SnO layer to characterize the properties of the p-channel layer in the TFTs. To exclude the influence of environment, we deposited a 15 nm-thick Al2O3 passivation layer on the TFTs by ALD. The field effect mobility increased with increasing the growth temperature, consistent with the change in the Hall mobility. A high field effect mobility of 1.5 cm2/Vs was achieved at a growth temperature of 210 oC. A high Ion/off ratio of ~ 106 was also achieved in the SnO film grown at 210 oC. This is attributed to the formation of dense and stoichiometric SnO films at 210 oC.