| AVS 63rd International Symposium & Exhibition | |
| Electronic Materials and Photonics | Monday Sessions |
| Session EM-MoA |
| Session: | Surface and Interface Challenges in Wide Bandgap Materials |
| Presenter: | Kosala Yapabandara, Auburn University |
| Authors: | K. Yapabandara, Auburn University M. Park, Auburn University M.C. Hamilton, Auburn University D.-J. Kim, Auburn University V. Mirkhani, Auburn University S. Wang, Auburn University M. Sultan, Auburn University B. Ozden, Auburn University M.P. Khanal, Auburn University S. Uprety, Auburn University Y. Chung, Auburn University M.H. Sk, Qatar University, Qatar |
| Correspondent: | Click to Email |
ZnO has been widely studied due to its promising material properties such as wide energy bandgap, optical transparency, and high carrier mobility for thin film transistor (TFT) technology. Solution-based ZnO can easily be deposited on large areas of substrates at low temperatures, which makes this material a good candidate for commercial device manufacturing. In the case of device reliability and performance, device stability under electrical stress is of imminent importance.
In this work, we report on the device instability of solution-based ZnO TFTs by studying the electrical characteristics during electrical stressing and subsequent relaxation. In order to elucidate the major source for device instability under electrical stress, the electrical characteristics of the transistors under the vacuum and ambient conditions were measured and compared. The positive shift of threshold voltage (VT) of the device under gate stressing and negative shift under relaxation for both the vacuum and ambient conditions were observed, which suggest that the charge trapping near or at the semiconductor/dielectric interface and charge injection to dielectric layer may be main mechanisms for device instability. However, the continuous degradation of the field effect mobility with electrical bias-stressing in both environmental conditions and a full recovery of the device with a longer relaxation time provided evidence to disregard the assumption of charge injection to the dielectric layer.
Variation in sub-threshold swing (S) with biasing process indicates a new defect level creation. A negligible change in S during gate stressing and relaxation under the vacuum condition, compared to a significant change in S under ambient conditions confirmed that there is no new defect level creation in the absence of oxygen and moisture. Under ambient conditions, oxygen and moisture were adsorbed on the channel surface with the presence of a positive electric field. Upon adsorption, oxygen molecules can capture electrons from the conduction band and a depletion layer can be formed in the ZnO channel layer. Previously, it has been reported that oxygen molecules cannot diffuse into the channel layer at room temperature. However, we have suggested a plausible mechanism that oxygen can be located closer to semiconductor/dielectric interface in thin films upon acceptor-like reaction of H2O that diffused into the channel via voids in grain boundaries. Further confirmation of charge trapping and new defect level creation was carried out by fitting the VT shift vs. time curve with the power law and stretched exponential functions for the vacuum and ambient conditions, respectively.