Charge Carrier Dynamics of Polycrystalline ZnO: Comparing Ground-State Bleaching to Ground-State Absorption

Charge carrier dynamics of ZnO receive considerable attention, in particular because its stable excitons at room temperature make it one of the few semiconductor oxides suitable for many optical applications. In this work, we have studied by pump-probe transient absorption spectroscopy (TAS) the ground-state bleaching (GSB) region of polycrystalline ZnO with different crystallite sizes and compared it to ground-state absorption (GSA) measured by UV–vis spectrometry. It was found that the shift in energy of the GSB with delay time during TAS is related to the density of excitons, with higher density (above the Mott transition) resulting in lower GSB energy due to screening of the exciton electrostatic interaction. Moreover, it was found that the first derivative of the GSA with respect to the wavelength (dA/dλ) well tracks the shape and energy shifts of the GSB region. The kinetic trace of the GSB signal fitted by a biexponential function is traced to the shift of the band gap (band gap renormalization) with the first small time constant (ca. 20 ps) attributed to the formation of high density of excitons while the second (ca. 200 ps) is linked to the slow decrease of the GSB signal reverting back to its high energy position when the exciton density decreases. In addition, it appears that the width of dA/dλ tracks the density of state, with the highly crystalline sample having the narrowest (10 nm) full width at half-maximum (fwhm), while those of the less crystalline samples were larger (ca. 15 nm) and had higher energies. The similarity between the shape of dA/dλ as obtained from steady-state measurements and the GSB as obtained from pump–probe TAS measurements might be more general for oxide semiconductors because a similar trend is seen for TiO₂.