Physical Properties of an Ultrathin Al2O3/HfO2 Composite Film by Atomic Layer Deposition and the Application in Thin-Film Transistors

A high-quality ultrathin dielectric film is important in the field of microelectronics. We designed a composite structure composed of Al₂O₃/HfO₂ with different Al₂O₃/HfO₂ cycles prepared by atomic layer deposition (ALD) to obtain high-quality ultrathin (1-12 nm) dielectric films. Al₂O₃ protected HfO₂ from interacting with the Si substrate and inhibited the crystallization of the HfO₂ film. High permittivity material of HfO₂ was adopted to guarantee the good insulating property of the composite film. We investigated the physical properties as well as the growth mode of the composite film and found that the film exhibited a layer growth mode. The water contact angle and grazing-incidence small-angle X-ray scattering analyses revealed that the film was formed physically at 3 nm, while the thickness of the electrically stable film was 10 nm from grazing-incidence wide-angle X-ray scattering and dielectric constant analyses. The composite film was applied as a dielectric layer in thin-film transistors (TFTs). The threshold voltage was decreased to 0.27 V compared to the organic field-effect transistor with the single HfO₂ dielectric, and the subthreshold swing was as small as 0.05 V/dec with a carrier mobility of 49.2 cm²/V s. The off-current was as low as 10^-11 A, and the on/off ratio was as high as 5.5 × 10⁶. This ALD-prepared composite strategy provides a simple and practical way to obtain the high-quality dielectric film, which shows the potential application in the field of microelectronics.