Improvement in Instability of Transparent ALD ZnO TFTs Under Negative Bias Illumination Stress With SiO2/Al2O3 Bilayer Dielectric

The theory of oxygen vacancy related deep energy defects and valence band offset (VBO) between gate insulator and channel codetermining the threshold voltage shift (Delta V-TH) of ZnO thin film transistor under negative gate bias and illumination stress (NBIS) is proposed and investigated systematically. Two kinds of ZnO thin film transistors are fabricated by atomic layer deposition with different gate oxide structures, a control sample with Al2O3 gate oxide and an improved sample with SiO2/Al2O3 gate oxide structures. Among two kinds of devices, the device with SiO2/Al2O3 gate oxide achieves a smaller Delta V-TH under the NBIS with SiO2 thin film acting as a holes-blocking layer, despite the existence of more defects than control device. The improvement in stability is attributed to large VBO up to 3.08 eV. Moreover, the device with SiO2/Al2O3 gate oxide is evaluated on a 500-nit LCD back light to simulate the practical working environment in displays, which exhibits good stability of Delta V-TH = -0.3 V for 3600 seconds.

Automated summary

This study proposes and investigates a theory on the effects of oxygen vacancy related deep energy defects and valence band offset on the threshold voltage shift of ZnO thin film transistors under negative gate bias and illumination stress. The results show that ZnO thin film transistors with SiO2/Al2O3 gate oxide structures achieve a smaller threshold voltage shift, despite having more defects. This improvement in stability is attributed to a large valence band offset.