Direct observation of potential phase at joining interface between p-MgO and n-MgFe₂O₄

Visualization of the depletion layer is a significant a guideline for the material design of gas sensors. We attempted to measure the potential barrier at the interface of core-shell microspheres composed of p-MgO/n-MgFe₂O₄/Fe₂O₃ from the inside out by means of Kelvin probe force microscopy (KPFM) as a first step to visualizing enlargement of the depletion layer. As determined by high-angle annular dark-field scanning transmission electron microscopy, ca. 70% of the microspheres were hollow with a wall thickness of ca. 200 nm. Elemental mapping revealed that the hollow particles were composed of ca. 20 nm of MgO, ca. 80 nm of MgFe₂O₄, and ca. 100 nm of Fe₂O₃. A difference of 0.2 V at the p-MgO/n-MgFe₂O₄ interface was clarified by KPFM measurements of the hollow particles, suggesting that this difference depends on the formation of a p-n junction. The potential barrier enlarged by the formation of a p-n junction was considered to increase the resistance in air (R_a), since the R_a of the core-shell hollow microspheres was higher than that of MgO, Fe₂O₃, MgO-Fe₂O₃, and MgO/MgFe₂O₄/Fe₂O₃ particles with irregular shapes. Measurement of the potential barrier height by KPFM is a promising potential approach to tuning the gas sensitivity of oxide semiconductors.