Carrier dynamics in porous silicon studied with a near-field heterodyne transient grating method Masahiro Yamaguchi a , Kenji Katayama b, * , Qing Shen c , Taro Toyoda c , Tsuguo Sawada d a Graduate School of Frontier Sciences, Department of Advanced Material Science, The University of Tokyo, 401, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan b Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA c Department of Applied Physics and Chemistry, and Course of Coherent Optical Science, The University of Electro-communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan d Department of Chemical System Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184-8588, Japan Received 9 February 2006; in final form 2 June 2006 Available online 20 June 2006 Abstract The dynamics of excited carriers in porous silicon were investigated using the near-field heterodyne transient grating method, and the fundamental processes related to light emission were determined. The processes include trapping to surface states and two-body recom- bination of excited carriers, with trapping being the dominant source of light emission. Since nonlinear processes, namely two-body recombination, are included, it is necessary to measure the pump intensity dependence of the transient responses and to analyze them with a nonlinear differential equation in order to obtain accurate decay times. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction Since Canham [1] reported on the light emission of por- ous silicon at room temperature, considerable attention has been given to the emission mechanism; however, the mech- anism remains unclear. Canham explained it on the basis of the effect of the quantum confinement of nano-sized silicon particles, where light is emitted via the quantitized energy state. Koch et al. [2] extended Canham’s explanation and proposed that light is emitted from photo-excited carriers that are transferred to defect states when they are elimi- nated due to recombination, while light absorption itself occurs due to the quantitized energy state. To elucidate the mechanism, the dynamics of the photo- excited carriers has been investigated by time-resolved mea- surements [3–11]. The studies indicated that surface states are involved in the emission mechanism. Matsumoto et al. [6,7] studied the correlation between the fluorescence decay time and the number of hydrogen-terminations on sample surfaces and reported that weak emission with a decay time of several hundreds of picoseconds comes from excited carriers at a quantitized state and that strong emis- sion with a lifetime of nanoseconds occurs due to excited carriers at surface states. Others also reported, using time-resolved fluorescence and transient absorption mea- surements, that fluorescence with a faster decay time is due to the recombination of photo-excited carriers in the core of silicon nano-crystals and that fluorescence with a slower decay time is due to the recombination of carriers localized in surface states [8–11]. However, two difficulties are encountered when time- resolved measurements are made for porous silicon. The first comes from the nonlinear recombination processes of excited carriers. Porous silicon has a low-intensity threshold of pump intensity where nonlinear processes start, and the ratio between nonlinear and linear recombination processes then changes according to the experimental pump intensity used, which causes a change in the apparent decay time. Furthermore, since the threshold value changes depending on the sample preparation conditions, it is difficult to get 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.06.041 * Corresponding author. E-mail address: kkatayama@chem.chuo-u.ac.jp (K. Katayama). www.elsevier.com/locate/cplett Chemical Physics Letters 427 (2006) 192–196