Femtosecond Pump Probe Study Of Ultrafast Carrier Dynamics In ZnSe

There comes a greater need and requirements on the technological information transfer. However, the processing of information is limited in its speed by the performance of semiconductor devices. In the past two decades, there has been significant development in the field of ultrafast carrier dynamics in semiconductor. The driving force behind it is the endless need for the faster response of semiconductor devices. There must be a basic and detailed understanding of the dynamical processes in the semiconductor for the need in microelectronics devices. The semiconductor after excitation is out of equilibrium, and the subsequent relaxation has various mechanism and rates. This article is based on the above discuss. In order to allow the reader to develop a basic understanding of carrier dynamics, a brief overview of various microscopic relaxation is introduced. Momentum and energy relaxation, carrier-carrier scattering, intervalley and intravalley scattering, optical phonon scattering and carrier diffusion have been investigated extensively. In early days, the primary challenge is the limit of ultrashort pulse. Then we review the history of Ti:sapphire laser system. Several important experiment techniques such as pump-probe, four wave mixing, Z scan and fluorescence up conversion are discussed. These techniques allow to probe the ultrafast processes in semiconductors with high resolution limited only by the laser pulse itself, independent of the response rate of high speed electronic instruments. These optical techniques are used to explore the properties of semiconductors on a time scale much shorter than those previously to be attainable. Femtosecond optical techniques for creating and monitoring nonequilibrium carrier distributions provide powerful means for exploring the dynamics of hot carriers in semiconductors. Historically, the ultrafast spectroscopic techniques investigated the carrier dynamics in GaAs and Ⅲ－Ⅴ semiconductors initially photoexcited with a photon energy above the band gap energy. The study of Ⅱ－Ⅵ semiconductors is far behind the Ⅲ－Ⅴ semiconductors since it is difficult for Ⅱ－Ⅵ semiconductors to adulterate. In highly polar semiconductors like GaAs and ZnSe, the electrons will lose their excess energy and thermalize with the lattice predominantly by emission of LO phonons.We present a ultrafast pump-probe transmission technique which is used to monitor the relaxation dynamics of carriers in n-type bulk ZnSe. The 800nm pump and probe pulses are obtained from a Ti:sapphire laser system. The instantaneous signal is due to two-photon absorption since the 2.7eV band gap of ZnSe at room temperature. Initially, momentum relaxing scattering, carrier-carrier scattering and carrier-phonon scattering dephase the excited state. The quasi equilibrium relaxation occurs via elastic and inelastic scattering and the emission of longitudinal optical phonon. The inter-valley scattering and thermalization time are 1.1ps and 94fs respectively. The processes are closely related to carrier concentration and lattice temperature. In our knowledge, the inter-valley and thermalization relaxation time constant of ZnSe pumped at 800nm is first measured. The electron and hole system are described by a Fermi-Dirac distribution with two parameters: the quasi-Fermi level μ and the carrier temperature Tc which is initially higher than the lattice temperature Tc>TL. The hot carrier exchange energy with lattice by emission and absorption of phonons. Cooling of the hot carriers down to lattice temperature TL represents the final relaxation process.