N-Type Conductivity in Single-Phase r-GeO₂ Thin Films

Rutile-structured germanium dioxide (r-GeO₂) is attracting attention as a next-generation power device material with a large bandgap (4.68 eV) ^[1], and both p-type and n-type conductivity are theoretically predicted ^[2]. The reason for this is that when it is possible to create p-type and n-type materials using doping techniques, there will be no need to create expensive Fin-FET structures, and it will also be possible to apply it to normally-off MOSFETs, which account for over 90% of the power semiconductor market. However, due to high saturation vapors pressure and similar formation energies of crystal polymorphs, it is a hard work to produce single phase r-GeO₂ films. N-type conductivity has been confirmed in alloy thin films mixed with tin dioxide (r-SnO₂),which is easy to crystallize. ^[3]. However, alloy thin films has smaller band gap against r-GeO₂ because band gap of r-SnO₂ is 3.7eV. From the view point of future power device applications and social implementation, fabrication of conductive single-phase r-GeO₂ is a milestone technology. In this study, we fabricated n-type conductive r-GeO₂ thin films and structural and electrical property evaluation are conducted. In this study, we fabricated n-type conducting thin films because controlling the conductivity of single-phase r-GeO₂ films is more important than that of mixed phase films from the perspective of device applications.
Atmospheric pressure CVD systems with ultrasonic transducer were employed to create r-GeO₂ thin films on rutile-structured titanium oxide (r-TiO₂) (001). We chose Sb ion as an n-type dopant and evaluated the structure and electrical properties using X-ray diffraction (XRD) measurement devices and Hall effect measurements system. From XRD 2Θ/ω scanning measurement, 2Θ peaks derived from 002 of r-TiO2 substrate and r-GeO₂ thin film were confirmed the others peaks from other crystal phases were not detected. These results indicate obtained thin films were oriented grown on the substrates and these are single-phase films. N-type doped thin films were also grown on r-TiO₂ substrates using these growth condutions. The molar cncentarations (mol%) of Sb dopant in the precursor source solutions were varied as 3, 5 or 7 mol%. Hall effect measurements were performed on each sample using van der Pauw method. Electron carrier densities were confirmed as around 10²⁰ / cm³ for all samples. The electron liabilities were recorded as 6 or 12 cm²/Vs for the sample with 3 mol% or 5mol% Sb dopants in the source solutions, respectively.
On the day of the presentation, we plan to discuss the results of detailed electrical and structural characterization.

[1] K. A. Mengle, et al. Appl. Phys. Lett. 126, 085703 (2019).
[2] S. Chae, et al. Appl. Phys. Lett. 114, 102104 (2019).
[3] H. Takane, et al. Physical Review Materials 6, 084604 (2022).