Temperature dependence of self-trapped exciton luminescence in nanostructured hafnia powder

TL;DR

This study investigates how temperature affects self-trapped exciton luminescence in nanostructured hafnia powder, revealing mechanisms behind emission properties relevant for optoelectronic applications.

Contribution

It provides new insights into the temperature-dependent luminescence mechanisms of nanostructured hafnia, including exciton behavior and thermal quenching effects.

Findings

Emission peak at 4.2 eV below 200 K

Activation energy for thermal quenching is 140 meV

Radiative decay of self-trapped excitons dominates the emission

Abstract

The intrinsic optical properties and peculiarities of the energy structure of hafnium dioxide largely determine the prospects for applying the latter in new generation devices of optoelectronics and nanoelectronics. In this work, we have studied the diffuse reflectance spectra at room temperature for a nominally pure nanostructured $HfO_2$ powder with a monoclinic crystal structure and, as well its photoluminescence in the temperature range of 40 - 300 K. We have also estimated the bandgap $E_g$ under the assumption made for indirect (5.31 eV) and direct (5.61 eV) allowed transitions. We have detected emission with a 4.2 eV maximum at T < 200 K and conducted an analysis of the experimental dependencies to evaluate the activation energies of thermal quenching (140 meV) and enhancement (3 meV) processes. Accounting for both the temperature behavior of the spectral characteristics and the estimation of the Huang-Rhys factor S >> 1 has shown that radiative decay of self-trapped excitons forms the mechanism of the indicated emission. In this case, the localization is mainly due to the interaction of holes with active vibrational modes of oxygen atoms in non-equivalent ($O_{3f}$ and $O_{4f}$) crystal positions. Thorough study of the discussed excitonic effects can advance development of hafnia-based structures with a controlled optical response.