Extrinsic Dopants as Growth Modifiers in Cu-Cr-O delafossites: A Study of Incorporation Limits and Film Properties

TL;DR

This study investigates how extrinsic dopants affect the structure and properties of Cu-Cr-O delafossite thin films grown by chemical vapor deposition, revealing limited dopant incorporation but notable impacts on strain and optical properties.

Contribution

It provides new insights into dopant incorporation limits and their subtle effects on film structure and optical behavior in Cu-Cr-O delafossites.

Findings

Dopant incorporation is below 5%, with Al and Sc detected above the threshold.

Films are epitaxial on sapphire but polycrystalline on silicon.

Doped films show a slight red-shift in optical absorption edge.

Abstract

Cu-Cr-O delafossite thin films were grown by metal-organic chemical vapor deposition with various extrinsic dopants (Al, Mg, Mn, Sc, Y, and Zn) targeted at 5 at % to investigate how such doping influences their structure and properties. X-ray photoelectron spectroscopy revealed that the actual dopant incorporation is well below the nominal 5 %, with only Al and Sc present above detection. An off-stoichiometric Cu-Cr-O composition is determined, with no secondary phases detected. Transmission electron microscopy indicates that films grown on c-plane sapphire are epitaxial near the substrate interface but relax into a polycrystalline structure beyond 20 nm, while films on silicon are polycrystalline throughout. All films show high p-type conductivity (on the order of 10-10^2 Scm-1) attributable to the excess oxygen, with no significant variation among different dopants. Optical transmission measurements indicate a slight red-shift (~20 nm) of the absorption edge for all doped films, likely arising from strain effects and subtle structural disorder introduced during growth. We discuss the influence of lattice strain (investigated by X-ray diffraction sin {\psi} squared measurements showing residual strain) and small polaron absorption behavior in these films. Despite limited incorporation of dopants, subtle structural and optical shifts suggest that dopant precursor chemistry and growth conditions play a significant role in influencing film stoichiometry and properties.