Heteroepitaxial Cubic (111) Zn2GeO4 Ultrawide Bandgap Semiconductor Thin Films Grown on Cubic (111) MgAl2O4 Substrates by Pulsed Laser Deposition

This study presents a comprehensive analysis of the microstructural, optical, and electrical transport properties of heteroepitaxial cubic spinel (111)-oriented Zn2GeO4 thin films grown on cubic spinel (111) MgAl2O4 substrates by pulsed laser deposition. The in-plane epitaxial relationships are [1-10] Zn2GeO4//[1-10] MgAl2O4 and [11-2] Zn2GeO4//[11-2] MgAl2O4, indicating a cube-on-cube epitaxy. A 316 nm thick (111) Zn2GeO4 epitaxial film has a surface root-mean-square (RMS) roughness of about 0.9 nm and a narrow rocking curve of the (444) reflex with a full width at half maximum of about 0.36 degrees. Temperature-dependent Hall effect measurements indicate that the nominally undoped films exhibit n-type semiconductor behavior. The high-quality 316 nm thick epitaxial film, with a direct optical bandgap of about 4.9 eV at room temperature, shows a notable decline in resistivity from about 60 to about 4 Omega cm, as temperature increases from 100 to 300 K. Concurrently, the Hall electron carrier mobility rises gradually from approximate to 0.5 to 5.5 cm(2) V-1 s(-1) as temperature increases from 100 to 300 K. In contrast, the Hall electron carrier concentration demonstrates minimal temperature dependence, with a value of approximate to 10(17) cm(-3). The native n-type conductivity is likely the result of unintentional dopants introduced during thin-film fabrication.