Defect annealing in ultra-thin polycrystalline silicon films on glass: Rapid thermal versus laser processing

Abstract

Conventional thermal and line-focus diode laser annealing was applied to ultra-thin, 160nm, solid-phase crystallized phosphorous-doped Si film on glass to improve their crystal and electronic quality. Electronic quality improvement is reflected in higher dopant activation and lower sheet resistance. The active dopant concentration increases from 5.6×1019cm−3 in as-crystallized film to 7.6×1019cm−3 and 1.0×1020cm−3 after thermal and laser annealing respectively. Sheet resistance decreases from 75Ω/□, to 54Ω/□ and 45.1Ω/□ after thermal and laser annealing respectively. However, electron mobility after laser annealing (86.6cm2/Vs) is lower than mobility in as-crystallized (92.5cm2/Vs) films and in the films after thermal annealing (95.3cm2/Vs). The lower mobility is likely due to scattering effects by higher concentration of ionized dopant. Crystal quality improvement is evidenced by TEM imaging. The observed intragrain defect (microtwins and dislocations) density is reduced after either thermal or laser annealing, the latter resulting in the lowest defect density. Thus, laser annealing allows achieving better crystal and electronic poly-Si film quality compared to conventional thermal annealing. Such a laser annealed poly-Si film can be used as a high quality seed layer for epitaxial thickening for solar cell and TFT applications.