Characterization of in-gap states in epitaxial CoFe2O4(111) layers grown on Al2O3(111)/Si(111) by resonant inelastic x-ray scattering

TL;DR

This study investigates in-gap states in epitaxial CoFe2O4(111) layers on Al2O3(111)/Si(111) using RIXS, revealing the role of Fe2+ cations at octahedral sites and their dependence on film thickness.

Contribution

It demonstrates the effectiveness of combining RIXS, ellipsometry, and XAS to analyze in-gap states and their relation to Fe2+ cations in thin-film oxide heterostructures.

Findings

In-gap states are linked to Fe2+ (Oh) cations at the heterointerface.

The intensity of below-band-gap excitation increases with decreasing film thickness.

In-gap states are more prominent near the heterointerface, affecting the material's electronic properties.

Abstract

We have studied in-gap states in epitaxial CoFe2O4(111), which potentially acts as a perfect spin filter, grown on a Al2O3(111)/Si(111) structure by using ellipsometry, Fe L2,3-edge x-ray absorption spectroscopy (XAS), and Fe L2,3-edge resonant inelastic x-ray scattering (RIXS), and revealed the relation between the in-gap states and chemical defects due to the Fe2+ cations at the octahedral sites (Fe2+ (Oh) cations). The ellipsometry measurements showed the indirect band gap of 1.24 eV for the CoFe2O4 layer and the Fe L2,3-edge XAS confirmed the characteristic photon energy for the preferential excitation of the Fe2+ (Oh) cations. In the Fe L3-edge RIXS spectra, a band-gap excitation and an excitation whose energy is smaller than the band-gap energy (Eg = 1.24 eV) of CoF2O4, which we refer to as "below-band-gap excitation (BBGE)" hereafter, were observed. The intensity of the BBGE was strengthened at the preferential excitation energy of the Fe2+ (Oh) cations. In addition, the intensity of the BBGE was significantly increased when the thickness of the CoFe2O4 layer was decreased from 11 to 1.4 nm, which coincides with the increase in the site occupancy of the Fe2+ (Oh) cations with decreasing the thickness. These results indicate that the BBGE comes from the in-gap states of the Fe2+ (Oh) cations whose density increases near the heterointerface on the bottom Al2O3 layer. We have demonstrated that RIXS measurements and analyses in combination with ellipsometry and XAS are effective to provide an insight into in-gap states in thin-film oxide heterostructures.