Influence of nanostructure size on the luminescence behavior of silicon nanoparticle thin films
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Influence of nanostructure size on the luminescence behavior of silicon nanoparticle thin films A. A. Seraphin,a) E. Werwa,b) and K. D. Kolenbranderc) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 30 January 1997; accepted 20 March 1997)
We demonstrate the effect of particle size and quantum confinement on the luminescence properties of nanoscale silicon thin films. Thin films of agglomerated silicon nanoparticles are synthesized using pulsed laser ablation supersonic expansion. Following deposition, standard semiconductor processing techniques are employed to reduce the nanoparticle size. Films are oxidized both in air and chemically to reduce the silicon core dimensions, resulting in a shift of the luminescence emission peak to shorter wavelengths. Removal of the oxide using hydrofluoric acid (HF) results in further blueshifting of the luminescence, as does subsequent reoxidation in air and using nitric acid. The luminescence properties of samples are also studied as a function of excitation intensity. For room temperature excitation with a pulsed 355 nm source, a saturation of the photoluminescence intensity at high excitation intensity is observed, along with a blueshift of the peak PL wavelength. This behavior is found to persist at reduced temperature. A saturation of PL intensity, but no blueshift, is observed for high excitation intensity using a cw 488 nm source at room temperature. At reduced temperatures, no saturation of emission intensity occurs for high intensity 488 nm cw excitation. Both the irreversible shifting of the peak PL wavelength with size reducing treatments and the PL behavior at high excitation intensities indicate that quantum confinement determines the luminescence wavelength.
I. INTRODUCTION
Semiconductor devices based upon quantum confinement effects are of great technological importance. Visible light emission different from that seen in bulk systems has been observed from various nanoscaled semiconductor systems such as quantum wells1 and quantum dots.2,3 In these materials, the explanation for this difference is the expansion of the energy gap due to the quantum confinement of carriers. When visible light emission was first observed in nanostructured silicon systems such as nanocrystalline silicon,4 porous silicon,5 and silicon nanoclusters,6 a similar mechanism was proposed. Over the intervening years, there has been spirited debate over the validity of this proposal. Many researchers support the quantum confinement model,7–9 while others propose a mechanism in which the luminescence pathway involves “surface states” arising from various possible sources.10–12 We present evidence here that indicates that in thin films of pulsed laser ablation supersonic expansion synthesized silicon nanoparticles, size does control the peak photoluminescence emission wavelength. We also show that the luminescence patha)
Present address: Institute for Defense Analyses
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