CO 2 laser annealing synthesis of silicon nanocrystals buried in Si-rich SiO 2
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A19.2.1
CO2 laser annealing synthesis of silicon nanocrystals buried in Si-rich SiO2 Chun-Jung Lin1, Yu-Lun Chueh2, Li-Jen Chou2, Hao-Chung Kuo1 and Gong-Ru Lin1* Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan R. O. C. 2 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan R. O. C.
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ABSTRACT Localized synthesis of 3-8 nm Si nanocrystals (nc-Si) in PECVD-grown Si-rich SiO2 (SRSO) film is demonstrated using CO2 laser annealing at an intensity below the ablation-threshold (6.0 kW/cm2). At an optimized surface temperature of 1285oC, the precipitated nc-Si in CO2-laser-annealed SRSO film results in near-infrared photoluminescence (PL) at 806 nm, whereas the ablation damage induced at higher laser intensities as well as temperatures results in blue PL at 410 nm related to structural defects. The refractive index of the laser-annealed SRSO at 633 nm increases from 1.57 to 2.31 as the laser intensity increases from 1.5 to 6.0 kW/cm2. Transmission electron microscopy analysis reveals that the average size and volume density of Si nanocrystals embedded in the SRSO film are about 6 nm and 4.5×1016 cm-3, respectively. The CO2 laser annealing with controlled intensity and spot size can potentially accomplish in-situ, localized annealing of the SRSO film without causing irreversible damage to nearby electronics. INTRODUCTION Silicon-rich silicon oxide (SRSO) films are known as a new-class of materials for light-emitting devices in next-generation optoelectronic integrated circuits (ICs). Typically, a high-temperature annealing (over 1000oC) is required to precipitate Si nanocrystals (nc-Si) buried within the SRSO matrix, which leads to near-infrared photoluminescence (PL) at 750-900 nm [1]. The conventional furnace annealing method is usually not a localized treatment, however, which could seriously damage the other electronic ICs on the same wafer. Laser irradiation has been employed recently for precipitating localized nc-Si in the SRSO layer, but such a process also induces power-dependent structural changes and phase segregation in alloys or samples with different constituents. Rossi et al. [2] demonstrated the formation of nc-Si with diameters of 2.5-12 nm in hydrogenated sub-stoichiometric amorphous silicon oxide (a-SiOx:H) using continuous-wave Ar+ laser (λ=514.5 nm) treatment at a intensity of 105 W/cm2. In another study, it was demonstrated that using a pulsed, frequency-tripled Nd:YAG laser (τ=8 ns, λ=355 nm) with laser intensity increasing from 30 to 360 mJ/cm2, the size of the nc-Si in a-SiOx:H (with oxygen concentrations between 10 and 30 at. %) can be enlarged from 20 to 200 nm [3]. Later on, Gallas et al. [4] reported that the threshold laser intensity is 85 mJ/cm2 for annealing SiOx without any ablation using a 248 nm-KrF pulsed excimer laser. Nonetheless, it is relatively difficult to precipitate nc-Si in SRSO at such a low laser intensity due to the negligibly small absorption coefficient of SiO2
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