High-density nanodot structures on silicon solar cell surfaces irradiated by ultraviolet laser pulses below the melting threshold fluence

The surface morphology of silicon solar cells irradiated with KrF excimer laser pulses (lambda = 248 nm, tau = 20 ns) was investigated below the experimentally observed melting threshold fluence (F th) of 0.47 J cm-2 (+/- 20%). At laser fluences of 0.23-0.48 J cm-2 (equivalent to 0.49F th to 1.0F th), nanodot structures with a height and width of approximately 60-120 nm were periodically formed with an interdot spacing similar to the laser wavelength. The observed nanodot density (29 dots mu m2) was higher than that previously obtained at longer wavelengths. Furthermore, crystallinity analysis by micro-Raman spectroscopy revealed a Raman shift of 519.56 cm-1 after irradiation (N= 1500 pulses), compared with 518.27 cm-1 prior to irradiation. A laser fluence of 0.41 J cm-2 ( = 0.87F th) was found to induce compressive stress on the silicon solar cell surface.