Measurement of transient photoabsorption and photocurrent of BiFeO${}_3$ thin films: Evidence for long-lived trapped photocarriers

We have studied the optical response and dynamical behavior of photocarriers in BiFeO${}_3$ thin films by means of transient absorption (TA) and photocurrent (PC) measurements. PC and absorption spectroscopy indicate that BiFeO${}_3$ thin films have an indirect band gap energy of ∼2.4 eV. The TA and PC decay dynamics have fast (∼1 ns) and slow (∼100 ns) components that are attributed to the localization of free carriers to shallow trap states and the recombination of trapped carriers, respectively. The long decay time of the PC is caused by the thermal activation of trapped carriers into the conduction band. Long-lived trapped photocarriers can be linked to the ferroelectricity and give rise to unique photoinduced phenomena in BiFeO${}_3$.

Figure 1

Optical absorption and PC excitation spectra of (a) BiFeO${}_3$/SrTiO${}_3$ and (b) BiFeO${}_3$/DyScO${}_3$. The insets show Tauc plots of the optical absorption and PC. The dashed lines are a guide for the eye.

Figure 2

TA spectra of (a) BiFeO${}_3$/SrTiO${}_3$ and (b) BiFeO${}_3$/DyScO${}_3$ for different delay times. Dashed curves represent the two Gaussian functions that compose the 2.3-eV TA band. The inset illustrates the energy levels of BiFeO${}_3$.

Figure 3

Normalized TA decay profiles of (a) BiFeO${}_3$/SrTiO${}_3$ and (b) BiFeO${}_3$/DyScO${}_3$ for different excitation densities. The inset shows the TA decay profiles in the submicrosecond time region and the calculated double-exponential function (blue curve).

Figure 4

(a) Schematic of the photocarrier relaxation and recombination processes in BiFeO${}_3$. (b) PC dynamics of BiFeO${}_3$/SrTiO${}_3$ at different bias voltages (2, 4, 6, 8, and 10 V). The inset shows the time-integrated PC as a function of inverse temperature.