Incorporation of Al or Hf in atomic layer deposition TiO₂ for ternary dielectric gate insulation of InAlN/GaN and AlGaN/GaN metal-insulator-semiconductor-heterojunction structure

This article investigates high dielectric constant gate insulators for GaN-based devices. Exploiting TiO₂ as a high-κ insulator typically compromises leakage current and temperature stability of the film. In this work, the authors compare TiO₂ mixed with either Al₂O₃ or HfO₂ to form composite films Ti-Al-O and Ti-Hf-O, respectively, deposited by atomic layer deposition on both AlGaN/GaN and InAlN/GaN substrates. The authors investigated the compositional effects of the ternary compounds by varying the Al or Hf concentration, and the authors find that leakage current is reduced with increasing Al or Hf content in the film; with a maximum Al-content of 45%, leakage current is suppressed by about 2 orders of magnitude while for a maximum Hf-content of 31%, the leakage current is suppressed by more than 2 orders of magnitude compared to the reference TiO₂ sample. Although the dielectric constant is reduced with increasing Al or Hf content, it is maintaining a high value down to 49, within the investigated compositional range. The crystallization temperature of the insulators was also studied and the authors found that the crystallization temperature depends on both composition and the content. For a Ti-Al-O film with Al concentration of 45%, the crystallization temperature was increased upward of 600 °C, much larger compared to that of the reference TiO₂ film. The interface trap densities of the various insulators were also studied on both AlGaN/GaN and InAlN substrates. The authors found a minimal trap density of $2.2×1012 eV−1 cm−2$ for the Ti-Hf-O compound with 35% Hf. In conclusion, our study reveals that the desired high-κ properties of TiO₂ can be adequately maintained while improving other insulator performance factors. Moreover, Ti-Hf-O compounds displayed overall better performance than the Ti-Al-O composites.