Ultra-High Mobility Transistors Via Metal Induced Conductive Region
Authors: Park Ji-Min , Kim Hyun-Suk 
Published: December 2020

The amorphous oxide semiconductors (AOSs) such as In-Ga-Zn-O (IGZO) and Zn-Sn-O (ZTO) have been extensively studied as the active layers for thin film transistors (TFTs) in display backplanes due to its high mobility (>10cm2/Vs), transparency, low process temperature and large area uniformity. However, many studies on AOSs have reported that there is trade-off relationship between electrical performance and device stability. Generally, the mobility increases with increasing indium or tin content, while the stability is degraded simultaneously. Furthermore, because the indium is a rare metal and expensive, incorporating indium in active materials is one of the critical reasons for increasing overall process costs. For this reason, the development of an indium-free oxide semiconductor materials with high mobility and stability is continuously required.

Recently, (Zn1-xBax)SnO3 (ZBTO) has been reported to be a very stable oxide semiconductor with high electron mobility results from its small effective mass and less defective structure. Based on these desirable material properties, it has been adopted as an active material for cost-effective and high-performance TFTs. Also, in order to improve electrical performance dramatically, an additional metal layers are introduced.

The metal capping layers have been reported to improve the performance of TFTs by various mechanisms, including the elimination of the scattering/trapping centers, low-temperature crystallization, and electron injection. However, although many research have been performed to demonstrate the origin of the mobility enhancement by the metal capping layer, the exact mechanism has not been confirmed. Also, former reports of this method showed several drawback such as the induced crystallization and need for stabilization time, which may limit its application on large-area displays.

To investigate the effect of metal capping layer, the device was fabricated using In-free active material. The ZBTO active layer were deposited via RF magnetron sputtering on the p++ Si/SiOx substrate. All sample was annealed at 350℃ for 1 hour in air ambient, and patterned using shadow masks. The channel width (W) and length (L) were 800 and 200 μm, respectively. In case of metal capped device, 20 nm-thick Al or Ti metal layer was sputtered selectively between the source/drain electrodes with dimensions W/L = 1300/100 μm.

In this work, the formation of conductive region in active layer induced by metal capping layer was suggested as new mechanism of the mobility improvement. Due to the strong oxidation power of Al metal, oxygen deficiency is easily induced in active layer near the interface between Al and ZBTO during Al oxidation. This conductive region in active layer generates free electrons, which can be transported without serious scattering owing to the low defect density of ZBTO. These phenomena were confirmed by TEM and XPS results. As a result, the ZBTO TFTs with an Al metal layer exhibited significantly improved field-effect mobility from 20.8 cm2/Vs to 153.4 cm2/Vs without degradation of other transfer parameters. Furthermore, the ZBTO TFTs showed far superior mobility improvement compared to other conventional oxide TFTs. The carrier transport mechanisms was investigated by 1/f noise measurements, which indicated that carrier transport of ZBTO TFTs with the Al capping layer is enhanced by the reduced bulk trap sites. Moreover, reliability tests including NBS, PBS indicated that the front channel of ZBTO active layer was relatively less affected. Our results indicate that adopt of stable active material with small defect density, in this case ZBTO, is a key factor in the fabrication of metal-assisted TFTs capable of ultra-high performance.