Photocarrier dynamics in zinc selenide studied with optical-pump terahertz-probe spectroscopy

Optical pump-terahertz (THz) probe spectroscopy is employed to investigate the photo-excited carrier relaxation process and the evolution of terahertz conductivity in ZnSe. With the pump pulse at a wavelength of 400 nm, the carrier relaxation process can be well fitted to a biexponential function. We find that the recombination process in ZnSe occurs through two components, one is the fast carrier recombination process, and the other is the slow recombination process. The fast carrier relaxation time constant is in a range from a few tens of picoseconds to hundreds of picoseconds, and slow carrier relaxation time constant ranges from one to several nanoseconds. We find that both the fast and the slow carrier relaxation time constant increase with the power density of pump beam increasing, which is related to the density of defects in the sample. Upon increasing the excitation power density, the defects are filled by the increased photo-excited carriers, which leads to an increase in the fast carrier relaxation time. While, the slow carrier relaxation time increasing with pump flux can be attributed to the filling of surface state. We also present the THz complex conductivity spectra of ZnSe at different delay times with a pump flux of 240 μJ/cm². It is shown that the real part of the conductivity decreases with increasing the pump-probe delay time. The real part of the conductivity is positive and increases with frequency in each of the selective three delay times (2, 20, and 100 ps), while the imaginary part is negative and decreases with frequency. The transient conductivity spectra at terahertz frequency in different delay times are fitted with Drude-Smith model. According to the fitting results from Drude-Smith model, with the pump-probe delay time increasing, the average collision time τ and the value of c₁ decrease. Generally, a higher carrier density leads to a more frequent carrier-carrier collision, which means that the collision time should decrease with carrier density increasing. The abnormal carrier density dependence of collision time implies a predominance of backscattering in our ZnSe. The predominance of backscattering is also observed for the negative value of c₁. The negative value of c1 indicates that some photocarriers are backscattered in ZnSe. With a delay time of 2 ps, the value of c1 approaches to -1, which indicates that the direct current (DC) conductivity is suppressed, and the maximum conductivity shifts toward higher frequency. With increasing the delay time, the value of c1 decreases: in this case DC conductivity dominates the spectrum. The study of the dynamics of photoinduced carriers in ZnSe provides an important experimental basis for designing and manufacturing the high speed optoelectronic devices.