Photoluminescence Studies of Si Quantum Dot Thin Film Materials
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ABSTRACT A laser ablation supersonic expansion source is used to deposit self-supporting thin films of agglomerated Si nanocrystallites. Average particle size is controlled through variation of deposition process parameters, including relative ablation pulse/gas valve timing and interaction channel length, as well as post-deposition processing steps, including repeated HF etch/oxide regrowth cycles, and oxidation furnace treatments. Films are characterized using photoluminescence (PL) emission spectroscopy. Clear spectral trends in the PL emission data point to a correlation between process parameter control, mean particle size, and peak PL wavelength. We conclude that quantum size effects play a critical role in defining the PL emission wavelength of Si nanocrystallites.
INTRODUCTION Recent work in light emitting semiconductors has focused on the behavior of light emitting silicon nanostructures, especially porous silicon [1]. This work has lead to considerable debate on the source of the efficient visible light emission that in seen in these materials. The 400-800 nmi light emission and 1-10% quantum efficiencies observed are not expected from bulk silicon, whose indirect bandgap lies in the infrared at 1.12 eV. Three models for the visible light emission have gained the most support. One holds that light comes from a surface layer of siloxenes on the silicon [2]. Another is based on emissions which originate from surface states in the bulk silicon energy gap [3]. Finally, the light emission can also be explained as the result of quantum confinement effects in the nanostructured material [4]. By controlling Si particle size and comparing this to trends in PL spectral behavior, we have provided evidence supporting the quantum confinement model. The quantum confinement model postulates that the visible light emission results from an expansion of the band gap of the material due to its size. The emitted light is then thought to be due to a band-to-band or excitonic recombination process, so the energy of the radiative transitions should scale with particle size. The increased energy gap then results in the emission of visible light, whose wavelength will scale with particle size as well. According to theoretical calculations, emission wavelengths of 500-800 nm should correlate with crystalline particles of approximately 2-4 nm in diameter [5].
EXPERIMENTAL Nanocrystallites were produced using a pulsed laser ablation supersonic expansion source [6]. A doubled Nd:YAG laser (532 nm, 3 mJ/pulse, 7 nsec pulse width, 20 Hz repetition 281 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
rate) was focused to the surface of a rotating polycrystalline Si rod (99.9999% pure) in a vacuum chamber at a base pressure of 10-8 torr. A skimmer cone separated the source chamber form a differentially pumped secondary chambei A schematic diagram of the apparatus is shown in Figure 1. A pulsed valve behind the target rod Laser Ablation Semiconductor controlled the flow of high pressure He which Source Nanocluster
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