The Enabling Role of Surface Passivation in Visible Photoluminescence from Si Nanoparticles
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PHOTOLUMINESCENCE FROM Si NANOPARTICLES A.A. SERAPHIN, E. WERWA, L.A. CHIJ*, AND K.D. KOLENBRANDER Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. *Intel Corporation, Santa Clara, CA 95052. ABSTRACT Silicon nanocrystallites have been studied in a variety of passivating environments to study the role of surface passivation in visible light emission from the particles. Thin films of Si
nanocrystallites have been deposited by a laser ablation supersonic expansion technique. The films show significant room temperature photoluminescence (PL) behavior only after processing to achieve surface passivation. Passivation effects on light emission are studied through PL emission spectroscopy on clusters in the gas phase, as well as films in a variety of passivating media. The intensity of PL emission seems to scale with the extent of surface passivation, but the specific nature of the passivating species is not critical in defining the wavelength of emitted light.
INTRODUCTION The light emission characteristics of nanostructured silicon have been studied for a
variety of materials [1-3]. The efficient light emission seen by many researchers is unexpected, because these effects are not seen in indirect gap bulk Si. The quantity and diversity of work in
the field has shown that the wavelength and intensity of the emission behavior seems to be strongly dependent on the synthesis and processing of the materials. After much debate, it is now widely acknowledged that the visible light emission from porous silicon and similar materials is due to quantum confinement effects. However, the degree of surface passivation of the nanostructured materials seems to control the intensity of the emission, through its control of the relaxation pathways of excited states. Dangling bonds at semiconductor surfaces act as non-radiative recombination centers. These bonds are manifested by a continuous distribution of energy states. When excited carriers are within a diffusion length of the surface, they will recombine, and the transition through the continuum of states is readily non-radiative [4]. This effect is enhanced in nanocrystalline materials, whose surface area-to-volume ratios are very large. By limiting the number of dangling bonds in a system, it would seem to be possible to change the kinetics of the excited state relaxation process such that radiative pathways are favored and light emission is more probable. Therefore, a perfectly capped nanocrystallite should exhibit highly efficient emission, while an unpassivated particle should show no emission behavior. We have studied nanocrystallites with unpassivated surfaces, as well as Si nanocrystallite thin films in a variety of passivating environments, in order to examine the role of passivation in controlling PL emission intensity. EXPERIMENTAL Silicon nanocrystallites were produced using a pulsed laser ablation supersonic expansion source. A doubled Nd:YAG laser (X= 532 nm, 5 mJ/pulse, 7 nsec pulse width, 20 Hz repetition 4
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