关键词: 光抽运-太赫兹探测/
光密度/
光生载流子/
瞬态电导率
English Abstract
Photocarrier dynamics in zinc selenide studied with optical-pump terahertz-probe spectroscopy
Li Gao-Fang1,2,Ma Guo-Hong3,
Ma Hong4,
Chu Feng-Hong1,
Cui Hao-Yang1,
Liu Wei-Jing1,
Song Xiao-Jun1,
Jiang You-Hua1,
Huang Zhi-Ming2,
Chu Jun-Hao2
1.College of Electronic and Information Engineering, Shanghai University of Electric Power, Shanghai 200090, China;
2.Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China;
3.Department of Physics, Shanghai University, Shanghai 200444, China;
4.College of Physics and Electronics, Shandong Normal University, Jinan 250014, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11404207, 11674213), the Shanghai Natural Science Foundation, China (Grant No. 14ZR1417500), the Local Colleges and Universities Capacity Building Program of the Shanghai Science and Technology Committee, China (Grant Nos. 15110500900, 14110500900), the Innovation Program of Shanghai Municipal Education Commission, China (Grant No. 15ZZ086), the Colleges and Universities Young Teachers Training Funding Program of Shanghai Municipal Education Commission, China (Grant No. ZZsdl15106), and the Shanghai University of Electric Power Scientific Research Fund, China (Grant No. K2014-028).Received Date:13 October 2016
Accepted Date:08 November 2016
Published Online:05 December 2016
Abstract: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 J/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 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.
Keywords: optical pump-terahertz probe/
pump flux/
photocarriers/
transient conductivity
