Heteroepitaxial Cubic (111) Zn 2 GeO 4 Ultrawide Bandgap Semiconductor Thin Films Grown on Cubic (111) MgAl 2 O 4 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 Zn 2 GeO 4 thin films grown on cubic spinel (111) MgAl 2 O 4 substrates by pulsed laser deposition. The in‐plane epitaxial relationships are [1–10] Zn 2 GeO 4 //[1‐10] MgAl 2 O 4 and [11–2] Zn 2 GeO 4 //[11–2] MgAl 2 O 4 , indicating a cube‐on‐cube epitaxy. A 316 nm thick (111) Zn 2 GeO 4 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°. 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 Ω cm, as temperature increases from 100 to 300 K. Concurrently, the Hall electron carrier mobility rises gradually from ≈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 ≈10 17 cm −3 . The native n‐type conductivity is likely the result of unintentional dopants introduced during thin‐film fabrication.