The work aims to improve the electrical characteristics and negative gate bias stress stability of indium doped zinc oxide thin film transistors (IZO TFT). In this paper, firstly, the ITO glass substrate is ultrasonically cleaned with deionized water and isopropanol in sequence, and then dried in a constant temperature oven. Subsequently, the Al2O3 thin film is prepared on the ITO glass substrate with atomic layer deposition (ALD) as the gate dielectric layer for the IZO TFT. Next, the indium-doped zinc oxide (IZO) thin film is deposited on the Al2O3 thin film using radio frequency magnetron sputtering through a patterned metal mask, serving as the active layer of the IZO TFT. Then, the source and drain electrodes of the IZO TFT are prepared by sputtering an Al target using direct current sputtering. Finally, an indium-doped zinc oxide thin-film transistor with a bottom-gate top-contact structure is obtained. After the prepared IZO TFT is removed from the radio frequency magnetron sputtering equipment, it is directly transferred to the reaction chamber of the plasma equipment. CF4 gas is then introduced into the reaction chamber, and under the effect of electric field-induced coupling, the CF4 gas is ionized into CF4 plasma composed of F atoms, CF2-, and CF3- radicals. The CF4 plasma bombards the surface of the IZO active layer of the IZO TFT under the action of the electric field, thereby completing the treatment of the IZO TFT back-channel surface. Lastly, the IZO TFT after CF4 plasma surface treatment is placed on a heating platform for post-annealing treatment. The effects of CF4 plasma RF power and treatment time on the electrical properties and bias stress stability of IZO active layer and IZO TFT devices are studied in detail.
The results indicate that when the radio frequency power for CF4 plasma treatment on the back-channel surface of the IZO TFT is appropriate (15 W), F atoms replace the oxygen in weak M—O bonds to form more stable In—F and Zn—F bonds, effectively reducing the oxygen vacancies in the IZO thin film and thus enhancing the negative gate bias stress stability of the IZO TFT device. However, when the radio frequency power for the CF4 plasma back-channel surface treatment is too high (25 W), the weak M—O bonds in the IZO thin film are destroyed under high-energy ion bombardment, leading to the generation of new defect states and subsequently deteriorating the negative gate bias stress stability of the IZO TFT. When the duration of the CF4 plasma treatment on the back-channel surface of the IZO TFT is appropriately increased, there is a certain degree of improvement in the negative gate bias stress stability of the IZO TFT, as the F atoms continuously passivate the oxygen vacancy defects in the IZO thin film with the increase in treatment time. Nevertheless, when the treatment time is excessively long (150 s), the F atoms diffuse to the interface between the channel and the gate dielectric, and even enter the interior of the gate dielectric layer of the IZO TFT to create new defect states, resulting in a slight degradation of the negative gate bias stress stability of the IZO TFT. Therefore, selecting appropriate radio frequency power and time for the CF4 plasma treatment on the back-channel surface of the IZO TFT can significantly improve the electrical performance and negative gate bias stress stability of the IZO TFT.
The results show that after treating the back channel surface of IZO TFT with CF4 plasma, stronger In—F and Zn—F bonding bonds are formed due to the replacement of oxygen in weak M—O bonding bonds by F atoms, effectively reducing oxygen vacancies in the IZO film. Therefore, appropriate CF4 plasma RF power and treatment time can significantly improve the electrical properties and negative gate bias stress stability of IZO TFT. When the RF power is 15 W and the treating time is 100 s, the electrical properties and negative gate bias stability of IZO TFT are relatively good. The mobility, threshold voltage, subthreshold swing, and switching current ratio of IZO TFT are 35 cm2/(V·s), 0.4 V, 64 mV/dec, and 4 × 109, respectively. When the gate bias stress of -10 V is applied for 1 h, the threshold voltage drift is -1.8 V.
Key words
indium doped zinc oxide (IZO) /
CF4 plasma treatment /
thin-film transistor /
negative gate bias stress
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Funding
The Key Fields of the Higher Education Institutions of Guangdong Province (2020ZDZX3125); Basic and Applied Basic Research Foundation of Guangdong Province (2024A1515011719); The National Natural Science Foundation of China (61871195)
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