Optical and Photodetector Properties of Si/ZnO/HfO2 Heterostructures Formed by Internal Oxidation under Controlled Environment

Abstract

This study presents a comparative analysis of Si/ZnO/HfO2 heterostructures formed by annealing metallic Zn deposited beneath HfO2 in oxygen and nitrogen atmospheres. It investigates oxidation behavior, structural transformations, defect dynamics, and the fabrication of a photodetector based on the grown material. We systematically examined the thermal evolution of the heterostructure using in situ X-ray diffraction (XRD), with particular emphasis on the Zn-to-ZnO conversion and associated changes in lattice parameters and thermal expansion characteristics. Zn transforms into wurtzite ZnO at approximately 450 °C under an N2 atmosphere through diffusion of lattice O2 from HfO2, whereas the same occurs at around 400 °C in an O2 atmosphere due to accelerated oxidation. X-ray photoelectron spectroscopy (XPS) confirms oxygen migration under N2 conditions, resulting in sub-stoichiometric HfO1.17, while O2 annealing maintains HfO1.72 stoichiometry and facilitates more pronounced oxidation. Photoluminescence analysis indicates enhanced excitonic emission and diminished defect states in samples annealed under N2, contrasted by increased defect-related luminescence and stronger exciton–phonon coupling following O2 annealing. These variations significantly impact device performance: the device annealed in N2 demonstrates superior response at a wavelength of 488 nm, attributed to efficient excitonic recombination, whereas the O2-annealed device exhibits improved detection at 514 nm, driven by defect-assisted photogating mechanisms. This study highlights an annealing-atmosphere-dependent trade-off between excitonic purity and defect-mediated response and demonstrates that internal oxidation from HfO2 enables high-quality ZnO without external oxygen for tunable optoelectronic applications.