Nano-Structured Silicon-Based Films with Visible Light Emission Synthesized by Laser Ablation
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ABSTRACT Applying laser ablation technique, we have synthesized two types of Si0 2 films that include nanometer-sized Si particles. One is synthesized by alternative deposition of Si nanoparticles layers and Si0 2 layers. The synthesized film exhibits red photoluminescence (PL) with a peak energy below 1.5 eV. The other is synthesized by annealing at 10000C of SiO•, films, which are formed by laser ablation in diluted 02 gas. We find that there is a narrow range of composition for efficient red PL. Based on the experimental results, we tentatively discuss a possible model for the origin of the red PL.
INTRODUCTION Nanostructured Si-based materials can exhibit bright light emission in the visible wavelength range [1-8], although crystalline Si has a 1.1-eV indirect band gap. The structure responsible for the light emission is still controversial partly because most of the materials contain undesirable chemical impurities such as hydrogen and fluorine. For further investigation, it is important to develop a clean synthesis method utilizing laser ablation [2, 3] and annealing of SiO,, films [4]. It is shown that nanometer-sized Si particles can be synthesized by ablating a Si target by laser light in He gas and collecting the final products on a substrate [2]. Shimizu-Iwayama et al. showed that Si nanocrystallites with visible photoluminescence (PL) precipitate in a Si0 2 matrix when Si+-implanted Si0 2 films are annealed [4]. In the present work, applying the laser ablation technique, we have synthesized two types of Si0 2 films including Si nanoparticles. One was synthesized by alternative deposition of Si nanoparticle layers and Si0 2 layers. The other was synthesized by annealing of SiO,, films deposited in diluted 02 gas. These films exhibit red PL with a peak energy below 1.5 eV. SiO,, films with particular compositions are found to exhibit efficient PL. Based on the experimental results, we propose that the red PL is due to excitons with large lattice distortion, localized at the interfaces of the precipitated Si core and the Si0 2 matrix. Si NANOPARTICLE LAYERS ISOLATED BY S102 LAYERS The laser ablation was performed by focusing a Q-switched Nd:YAG laser beam (A=532 nm. FWHM=10 ns, 10 J/cm2) on Si targets in He gas filled in a vacuum chamber. The chamber was pumped down to 2 x 106 Torr before filling with the gas. The synthesized Si particles deposited on Si substrates were observed using an atomic force microscope (AFM). The AFM has space resolution of 0.05 nm in height and 20 nm in the direction parallel to the sample surface. For the AFM measurement, the Si target was ablated 50 times 135 Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
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Figure 1: (a) An image of final products observed by an atomic force microscope. The products were synthesized by laser ablation of a Si target in He gas. (b) A section of the image along a straight line indicated in Fig. 1 (a). (c) Maximum of the size distrib
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