Microstructures of Luminescent nc-Si by Excimer Laser Annealing of a-Si:H
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ABSTRACT We have reported for the first time on visible photoluminescence (PL) in crystallized aSi:H/a-SiN×:H multilayer structures by CW Ar ion laser annealing treatments. In this paper we present new results on visible PL from crystallized a-Si:H by using KrF excimer pulse laser (wavelength 248 rn) irradiating treatments. The transmission electron microscopy and Raman scattering studies reveal the microstructures of crystallized Si films, which depend on the pulse number and the pulse energy density of KrF laser. When the laser pulse energy density is higher than 520 mJ/cm 2 , the nanosized Si crystallites (nc-Si) can be formed from a-Si:H layers with a thickness of 100 nm and strong PL with a peak wavelength of 610 nm has been observed at room temperature.
1. INTRODUCTION To prepare and study luminescent silicon based materials is one of the active subjects in the field of semiconductor optoelectronics, especially after the discovery of strong room temperature photoluminescence (PL) from porous silicon [1]. Recently many improvements of the electrochemical dissolution method for fabrication of porous silicon have been reported [2] but all these approaches were based on chemical solution (wet) synthesis techniques. A dry process for fabricating light emitting silicon films would be ideal for integrating this material into the current silicon based microelectronics and very large scale integrated circuits technology and consequently would provide new possibilities in the realization for monolithic integration of silicon with optical signal processing [3-5]. We have reported on room temperature visible PL in crystallized Si:H/SiNx:H multiquantum well structures by a CW Ar ion laser annealing technique [6]. This new approach for fabricating luminescent silicon is a fully dry process which is different from the electrochemical dissolution method to obtain luminescent porous silicon. This is also a good way to avoid the surface contamination which plays a significant role in strong visible luminescence in porous silicon layers [7,8]. In this paper, we present new results on room temperature visible PL from crystallized a-Si:H films by using KrF excimer pulse laser irradiating technique. The main purpose of this work is to investigate the influence of the pulse 803
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
number and pulse energy density on the microstructures of luminescent nc-Si films. The distinct microstructural changes were observed for energy densities from 160 to 560 mJ/cm2 , and the changing tendency is similar to the observations reported in reference 9. 2.
EXPERIMENTAL 2.1
Fabrication of a-Si:H films
The a-Si:H thin films were prepared in a capacitively coupled single reaction chamber by a rf plasma enhanced chemical vapor deposition (PECVD) method with a reactant gas of pure silane (SiH 4). The films were deposited on fused quartz and Si0 2 /Si substrates which were heated to 250 'C. The SiO2 /Si substrates were prepared by means of thermal oxidation of c-Si wafers and the t
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