We present a novel ultra-wide bandgap alloy system based on rutile-structured oxides (SnO2-GeO2-SiO2), and clearly show their superior points from a point of view of both experiments and first-principles calculations. Experimentally, we demonstrate that mist-CVD method is effective for the fabrication of r-GexSn1−xO2 alloy films. The r-GexSn1−xO2 alloy films (x≤0.96) are composed of single-crystalline compounds, while phase separation is found in the r-GeO2 films. The lattice parameters of r-GexSn1−xO2 alloy films decrease with an increase in fraction of Ge. Additionally, the bandgaps increase as the fraction of Ge increases. The compositional dependence of lattice parameters and bandgaps determined experimentally for the r-GexSn1−xO2 alloy films is in good agreement with those derived by the calculations. Our measurements of electrical properties indicate that the r-GexSn1−xO2 (x≤0.57) films are n-type semiconductors. Finally, we suggest band alignments calculated for r-GexSn1−xO2 and r-GexSi1−xO2 alloys. The results suggest a possibility of p-type doping in r-GeO2 and highly Ge-rich r-GexSn1−xO2. It is also suggested that it is preferable to use r-SiO2 and highly Si-rich r-GexSi1−xO2 as a blocking layer for other rutile-structured devices.
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