Role of hydrogen species in promoting photoluminescence from Eu3+-doped ZnO thin films via bandgap excitation

We studied the role of hydrogen-containing species (OH and/or H) in promoting photoluminescence (PL) from 1 at. % Eu3+ ions doped in ZnO thin films. The hydrogen concentration in the films was systematically changed by varying the substrate temperature and the vapor pressure of H2O gas supplied during sputter deposition. The correlation between the PL spectra via bandgap excitation and the degree of oxidization/hydrogenation was investigated by x-ray diffraction and Fourier-transform infrared spectroscopy. Films deposited at room temperature under H2O partial pressures higher than 10−2 Pa were sufficiently hydroxylated, as confirmed by the appearance of diffractions peaks from Zn(OH)2 coexisting with ZnO(002). Eu3+ emissions were observed after post-annealing in a vacuum or O2 atmosphere. When the H2O pressure was lower than 10−2 Pa, the ZnO:Eu films were so oxygen-deficient as to exhibit a metallic character, which deactivated the Eu3+ emission. Deposition at temperatures above 200 °C reduced the OH and/or H species incorporated in the ZnO films and only a faint Eu3+ emission was observed. The H2O pressure under which a sharp Eu3+ emission could be obtained was between 1.0 and 2.5 × 10−2 Pa if subsequent post-annealing was done in a vacuum. For more oxidized films deposited at 3.5 × 10−2 Pa, reduction by post-annealing in an H2 atmosphere was effective to generate a sharp and intense Eu3+ emission signal through reduction and hydrogenation, confirming that a moderate oxidization/hydrogenation state is a necessary condition. Codoped hydrogen species will facilitate substituting Zn2+ sites with emission-active Eu3+ ions and stabilize them.