Effects of Annealing Temperature and Gas on Pentacene OTFTs With HfLaO as Gate Dielectric

Pentacene organic thin-film transistors (OTFTs) with high-κ HfLaO as gate insulator were fabricated. HfLaO film was prepared by sputtering method. To improve the film quality, the dielectric was annealed in N₂, NH₃, or O₂ at two temperatures, i.e., 200°C and 400°C, respectively. The I-V characteristics of the OTFTs and C-V characteristics of corresponding organic capacitors were measured. The OTFTs could operate at a low operating voltage of below 5 V, and the dielectric constant of the HfLaO film could be above ten. For all the annealing gases, the OTFTs annealed at 400°C achieved higher carrier mobility than their counterparts annealed at 200°C (with the one annealed in NH₃ at 400°C showing the highest carrier mobility of 0.45 cm²/V · s), which could be supported by SEM images which indicate that pentacene tended to form larger grains on HfLaO annealed at 400°C than on that annealed at 200°C. The C-V measurement of the organic capacitors indicated that the localized charge density in the organic semiconductor/oxide was lower for the 400°C annealing than for the 200°C annealing. Furthermore, through the characterization of gate current leakage, HfLaO film annealed at 400°C achieved much smaller leakage than that annealed at 200°C. Since the maximum processing temperature of ITO glass substrates is around 400°C, this study shows that 400°C is suitable for the annealing of HfLaO film in high- performance OTFTs on glass substrate.