Mist chemical vapor deposition of Al_1−xTi_xO_y thin films and their application to a high dielectric material

We investigated the synthesis of amorphous aluminum titanium oxide Al_1−xTi_xO_y thin films from a Al(acac)₃ and Ti(acac)₄ mixture using CH₃OH/H₂O as a solvent through mist chemical vapor deposition (mist-CVD) for application as a high dielectric material. The Ti composition ratio x in the Al_1−xTi_xO_y thin films depends on the Al(acac)₃ and Ti(acac)₄ mixing ratios and CH₃OH/H₂O volume ratio. A bandgap energy of Al_1−xTi_xO_y films was decreased from 6.38 to 4.25 eV and the surface roughness also decreased when the Ti composition ratio was increased from 0 to 0.54. The capacitance–voltage plot revealed that the dielectric constant of Al_1−xTi_xO_y thin films increased from 6.23 to 25.12. Consequently, Al_1−xTi_xO_y thin films with a bandgap energy of 5.12 eV and a dielectric constant of 13.8 were obtained by adjusting the ratio x of 0.26. This Al_0.74Ti_0.26O_y layer was applied as a gate dielectric layer for metal-oxide-semiconductor field-effect transistors (MOSFETs) using a mechanically exfoliated two-dimensional (2D) transition metal dichalcogenide (TMDC), MoSe₂, and As-doped WSe₂ flakes as a channel layer. The MoSe₂-based MOSFETs with source/drain gold electrodes exhibit n-channel behavior with a field-effect mobility of 85 cm²/(V s), a threshold voltage of 0.92 V. On the other hand, an on/off ratio of ∼10⁶. As-doped WSe₂-based MOSFETs with source/drain platinum electrodes also showed an ambipolar behavior, which was applied for use in logic applications. These findings suggest that Al_0.74Ti_0.26O_y by mist-CVD is promising as a high-k material for TMDC-based MOSFETs.