Excitation Intensity and Temperature Dependent Photoluminescence Behavior of Silicon Nanoparticles
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E. WERWA, A.A. SERAPHIN, AND K.D. KOLENBRANDER Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, MA 02139, [email protected], [email protected], [email protected] ABSTRACT The luminescence properties of silicon nanoparticles have been studied as a function of the excitation light intensity, the temporal nature of the excitation source, and of sample temperature. The excitation intensity dependence of the luminescence was found to depend strongly on the temporal nature of the excitation source. Under high intensity excitation from a pulsed 355 nm source, the photoluminescence (PL) intensity saturates and the peak PL wavelength shifts to the blue at room temperature. This behavior persists at reduced temperature. In contrast, under high intensity excitation using a cw 488 nm source at room temperature, the PL intensity saturates but does not shift in wavelength. At reduced temperatures, there is no saturation of luminescence intensity with high intensity cw excitation. These differences indicate that photogenerated carrier recombination occurs via different pathways depending on the temporal profile of the excitation, with cw excited samples following the expected Auger pathway while pulsed samples exhibit a state filling mechanism. Auger models for the pulsed behavior are found to be inconsistent with the experimental data. The temperature dependence of the PL from a pulsed excited sample for a constant excitation intensity was also monitored. The variation of the peak emission wavelength was found to be similar inmagnitude to that observed for amorphous silicon, suggesting that structural disorder may play a role in the luminescence. The change in emission intensity was fairly weak, indicating enhanced carrier confinement, as would be expected ina quantum confined system. INTRODUCTION There has been much interest in the photoluminescence behavior of silicon nanostructures. Several potential mechanisms for the light emission have been put forth, with the two most popular being excitonic recombination in quantum confined semiconductor nanoparticles' and recombination insemiconductor particles through some undefined surface states.2'3 Much of this work has been performed on porous silicon, synthesized by etching of bulk silicon wafers to produce a pore network within which there have been shown to be nanometer sized particles.4 Unfortunately, the intricate morphology of porous silicon has made it difficult to identify the luminescence mechanism, leading to the various theories of emission. We have chosen to study silicon nanoparticles synthesized by pulsed laser ablation supersonic expansion in order to circumvent the problems of the porous silicon morphology. This will allow us to shed light on the behavior of the actual silicon particles, rather than particles whose behavior may be masked by the porous silicon structure as a whole. In order to examine the luminescence mechanism active in the particles, we have chosen to study the .excitation intensity dependence of the luminescen
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