【International Papers】Mist-chemical vapor deposition grown Ge-doped α-Ga₂O₃ thin films with high electron mobility

      Researchers from the Kyoto University have published a dissertation titled "Mist-chemical vapor deposition grown Ge-doped α-Ga₂O₃ thin films with high electron mobility" in Applied Physics Letters.

Background

      Ga₂O₃ is a promising material for power electronics, high-frequency devices, and ultraviolet detectors because of its large bandgap (~5 eV). Thin-film growth, fundamental electrical properties, and applications of β-Ga₂O₃—the most thermodynamically stable Ga₂O₃ phase—have been actively investigated. Another polymorph of Ga₂O₃, α-Ga₂O₃, has attracted considerable attention because of its wide bandgap (5.3–5.6 eV) and high estimated breakdown field (9.5 MV/cm), which exceed those of β-Ga₂O₃ (4.5–4.8 eV and 6.5 MV/cm, respectively). Although the metastable α-Ga₂O₃ phase generally tends to transform into thermodynamically stable β-Ga₂O₃ at 600–700 °C, the use of an AlO_x capping layer or selective area growth can raise the temperature of this transition to 800–900 °C.

Abstract

      Unintentionally doped and Ge-doped α-Ga₂O₃ films were grown on m-plane sapphire substrates via mist-chemical vapor deposition at a substrate temperature of 600 °C and a growth rate of 3.3 μm/h. Hall-effect measurements at room temperature indicated that the unintentionally doped film exhibited a low carrier concentration of 3 × 10¹⁶ cm⁻³, and the highest electron mobility reached 99 cm² V⁻¹ s⁻¹ for the Ge-doped α-Ga₂O₃ film with a carrier concentration of 5.8 × 10¹⁷ cm⁻³. Temperature-dependent Hall-effect measurements clarified the transport mechanism: scattering caused by dislocations and ionized impurities dominate at low temperatures in films with carrier concentrations of 1.7 × 10¹⁷ and 5.8 × 10¹⁷ cm⁻³, respectively, whereas polar optical phonons are the main origin of electron scattering around room temperature. The average densities of threading screw and edge dislocations in Ge-doped films prepared herein are 6.0 × 10⁸ and 1.1 × 10⁹ cm⁻², respectively, which are lower than those for α-Ga₂O₃ films grown on sapphire substrates to date. The low carrier concentration and high electron mobility achieved by reducing the dislocation density suggest the promising application potential of α-Ga₂O₃ in power electronics and high-frequency devices.

Conclusion

      In conclusion, UID and Ge-doped α-Ga₂O₃ films were grown on m-plane sapphire substrates using mist-CVD at the growth temperature of 600 °C and growth rate of 3.3 lm/h. A low carrier concentration (3 × 10¹⁶ cm^-3) was achieved for the UID α-Ga₂O₃ film. The highest electron mobility reached 99 cm² V^-1 s^-1 at a carrier concentration of 5.8 × 10¹⁷ cm^-3. The room-temperature mobilities of the present α-Ga₂O₃ films with carrier concentrations higher than 10¹⁷ cm^-3 are comparable to the mobilities of β-Ga₂O₃ with similar carrier concentrations. The results of temperature-dependent Hall-effect measurements showed that the dominant scattering factor changes from dislocation scattering to ionized impurity scattering with increasing carrier concentration. The densities of threading edge and screw dislocations in the present α-Ga₂O₃ films were estimated from XRD rocking curves as 6.0 × 10⁸ and 1.1 × 10⁹ cm^-2, respectively. These values are comparable to those for α-Ga₂O₃ films with buffer layers. The lower dislocation density leads to higher mobility at low carrier concentrations, as described above. These results prove the promising potential of α-Ga₂O₃ for applications in high-voltage and high-frequency power electronics devices.