Picosecond Decay Dynamics in Porous Silicon
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PICOSECOND DECAY DYNAMICS IN POROUS SILICON T. Matsumoto', T. Futagi*, H. Mimura * and Y. Kanemitsu•* SElectronics Research Laboratories, Nippon Steel Corporation, 5 - 10 - 1 Fuchinobe, Sagamihara, Kanagawa 229, Japan Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
ABSTRACT Picosecond decay dynamics of luminescent porous silicon has been studied using the second harmonics (SH) of a cw modelocked YLF laser and a synchroscan streak camera. Picosecond luminescence decay shows nonexponential behavior that becomes large with decreasing emission energy. When increasing hydrogen termination on the surface of a Si microcrystal occurs, this picosecond luminescence decay becomes faster. Our experimental results indicate that there are two luminescent states in porous Si: a weak luminescent quantum confinement state and a strong luminescent surface localized state.
INTRODUCTION Porous silicon (Si) fabricated by electrochemical anodization has attracted much interest because of the strong luminescence at room temperature[1]. The structure of this material is similar to a condensed state of nanometer (nm) order Si microcrystals[2,3]. The luminescence peak [1] and the absorption edge [4] shift to the higher energy side with increasing chemical dissolution. For these reasons, the quantum confinement effect [1 4] is suggested as the origin of the strong luminescence. However, other mechanisms such as an amorphous effect or a siloxene effect[5] have also been proposed, and the luminescent mechanism still remains unknown. To clarify the luminescent mechanism in porous Si, it is necessary to understand the transient decay dynamics such as scattering thermalization, or recombination through ultrafast spectroscopy. In this paper, we have studied the picosecond decay dynamics. The picosecond luminescence decay shows nonexponential behavior that becomes fast with increasing emission energy [6]. This picosecond decay is largely affected by the hydrogen termination on the surface of porous Si : the picosecond luminescence decay becomes faster with an increase in it. Due to these optical , properties and the structural understandings for porous Si, we consider the origin of luminescnce concerning two 5 nm states : a weak luminescent quantum confinement state and a strong luminescent surface Figure 1. TEM micrograph showing Si localized state. microcrystallites.
Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society
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EXPERIMENTAL The picosecond luminescence decay measurements were performed with the SH of cw modelocked YLF laser (A = 527 nm) and a synchroscan the streak camera through a spectrometer. The pulse duration was 40 ps, repetition rate 76 MHz and the excitation power density 100 mW/cm2 . The time resolution of the synchroscan streak camera was 10 ps. The substrates we used were (100) -oriented 3.5---4.5 Q .cm resisitivity p-type Si. The anodizations were carried out in HF-ethanol solution (HF: H20 :2 C2H5OH = 1: 1: 2) at constant current density in the range of 10 mA for 5 minutes. D
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