Evaluation of performance and reliability of TFT devices with ultra-thin HfTiO dielectric layer deposited by plasma enhanced atomic layer deposition

Abstract

Titanium (Ti)-doped hafnium oxide (HfO₂) films have garnered dramatical interest by virtue of their high dielectric constant (k). Ti-doped HfO₂ (HfTiO) films were prepared using plasma-enhanced atomic layer deposition at a substrate temperature of 300 °C by varying cycle ratio between HfO₂ and titanium oxide (TiO₂) deposition. The influence of Ti doping, with cycle ratios ranging from 0 to 40 %, on the properties of HfTiO films was identified by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity, field-emission scanning electron microscopy, and UV–vis spectroscopy. For the electric measurements, aluminum (Al) /HfO₂ or HfTiO films/Si capacitors were prepared through evaporating Al dots. SE measurements revealed that the deposition rate of HfTiO films decreased as the TiO₂ cycle ratio increased. GIXRD analysis showed that HfTiO films are polycrystalline and the intensity of the (−221) and (221) crystallite orientations in the HfTiO film with a 10 % TiO₂ cycle ratio was reduced compared to that of HfO₂ film. The band gap of HfTiO film was lower than that of the HfO₂ film. The electrical characterization of the HfTiO films showed a high k value of 30.0 for the 10 % TiO₂ cycle ratio sample, which is higher than the k value of 19.7 for the undoped HfO₂ film. The leakage current density of HfTiO film was minimized at the 10 % TiO₂ cycle ratio. To assess the potential application of HfTiO film as gate dielectrics in thin film transistor (TFT) devices, indium‑gallium‑zinc oxide (IGZO) was applied as a channel layer. The optimized IGZO/HfTiO (10 nm) TFTs exhibited an on/off current ratio of 8.9 × 10⁸, a subthreshold swing of 74 mV/decade, and a saturation mobility of approximately 7.6 cm²/V·s at a low threshold voltage of −0.08 V.