We demonstrate that dopant inhomogeneity strongly suppresses thermal conductivity in Cd/Eu co-doped, non-polar a-plane-oriented ZnO films grown on r-plane sapphire (Al₂O₃) by plasma-assisted molecular beam epitaxy. X-ray diffraction confirms the a-plane-oriented ZnO without detectable secondary phases. Cross-sectional scanning electron microscopy shows continuous films with well-defined interfaces, and secondary-ion mass spectrometry depth profiling identifies Cd/Eu incorporation through the film thickness and a sharp Zn/O drop at the substrate interface. Cross-plane thermal transport was measured at room temperature using frequency-domain photothermal infrared radiometry (PTR) and analyzed by fitting the complex PTR amplitude and phase with a multilayer heat-diffusion model. The extracted thermal conductivity ($\kappa$) spans ~3.7–6.3 W·m⁻¹·K⁻¹. The lowest $\kappa$ values correlate with increased Eu-distribution inhomogeneity, consistent with enhanced phonon scattering and reduced effective cross-plane heat transport.

We demonstrate that dopant inhomogeneity strongly suppresses thermal conductivity in Cd/Eu co-doped, non-polar a-oriented ZnO films grown on r-plane sapphire (Al₂O₃) by plasma-assisted molecular beam epitaxy. Structural characterization by θ-2θ XRD confirms a-oriented ZnO without detectable secondary phases. Cross-sectional SEM shows continuous films with well-defined interfaces, and SIMS depth profiling verifies Cd/Eu incorporation through the film thickness and a sharp Zn/O drop at the substrate interface. Optical transmittance and Tauc analysis reveal composition-dependent shifts of the absorption edge and band gap. Cross-plane thermal transport was measured at room temperature using frequency-domain photothermal infrared radiometry (PTR) and analyzed by fitting the complex PTR amplitude and phase with a multilayer heat-diffusion model. The extracted thermal conductivity spans ~3.7 - 6.3 Wm-1K-1. The lowest k values correlate with increased defect non-uniformity, consistent with enhanced phonon scattering and reduced effective cross-plane heat transport.