DC sputtered amorphous In–Sn–Zn–O thin-film transistors: Electrical properties and stability

Today, flat-panel displays (FPDs) and imagers (FPIs) based on thin-film electronic devices are widely used in our daily lives, e.g. TVs and imagers. Current FPD and FPI industries are dominated by the more than 30 years old amorphous silicon (a-Si:H) thin-film transistor (TFT) active-matrix arrays. However, the low field-effect mobility (~0.5 cm²/Vs) of the a-Si:H TFTs is not adequate for next generation super high definition, e.g. 8k × 4k, fast frame rate (> 240 Hz) displays. As an alternative to a-Si:H TFTs, low-temperature polysilicon (LTPS) TFTs have been widely used for small area, active-matrix organic light emitting displays (AM-OLEDs). LTPS technology achieves high field-effect mobility ranging from 50 to 100 cm²/Vs to drive organic-light emitting devices (OLEDs) used for small area FPDs, but it is very difficult to realize LTPS active-matrix arrays over large area substrates with an acceptable manufacturing yield. Also current medical imagers based on passive pixel sensor (PPS) technology suffer from the limited resolution, a low frame rate, and a high electronic noise owing to the low-mobility of the a-Si:H TFTs.  Hence, there is a critical need to develop TFT active-matrix array technology that can achieve a high field-effect mobility and has a low noise that is suitable for a large area fabrication process with the acceptable yield using existing a-Si:H TFT manufacturing infrastructure. All fabricated devices must be electrically stable during operation and over time.

Amorphous oxide TFTs that can be fabricated by DC sputtering method, such as amorphous In-Ga-Zn-O (a-IGZO) TFTs with a field-effect mobility higher than 10 cm²/Vs, have been extensively studied since 2004 and demonstrated its potential use for ultra-high definition (UHD), e.g. 4k × 2k, FPDs. Both electrical stability and an acceptable manufacturing yield still need to be demonstrated for these devices. Another interesting metal oxide is DC sputtered amorphous In-Sn-Zn-O (a-ITZO) TFTs that boost even higher field-effect mobility (> 30 cm²/Vs), a low threshold voltage and low noise; carrier mobility > 30 cm²/Vs is required for super high vision FPDs operating at high refresh rates. To investigate the device properties of a-ITZO TFTs, an analytical field-effect mobility model has been proposed and key parameters have been extracted using different approaches. The device SPICE model was also developed to help out with the circuits design. During this study, it was demonstrated that good device performance can be realized through the a-ITZO channel thickness and oxygen gas flow ratio optimization. The bias-temperature stress induced electrical instability was evaluated to demonstrate stable operation of the a-ITZO TFTs. The obtained results demonstrated that, in comparison to a-IGZO TFTs, higher electrical performance and better electrical stability can be achieved using a-ITZO TFTs.

The new a-ITZO TFT technology combines the advantages of both the a-Si:H TFTs for large area mass production and the LTPS TFTs for high mobility. The finding of this work is significant in understanding the a-ITZO TFT device physics and optimization of the fabrication process to realize high production yield. The high field-effect mobility a-ITZO TFTs are very promising for large area, fast frame rate active-matrix liquid crystal displays (AM-LCDs) and AM-OLEDs. It also provides possible avenue for development of active pixel sensor (APS) a-ITZO TFT active-matrix arrays for next generation medical high-resolution imaging applications such as digital tomosynthesis and computed tomography. The a-ITZO TFT APS can achieve a large charge gain on the pixel circuit level to amplify the input signal and minimize the electronic noise. In summary, we would expect breakthroughs in both the flat-panel display and imager industries using the new a-ITZO TFT technology in the near future.