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We have performed a generalization of earlier experimental and theoretical studies of the steady-state solid–melt patterns that form on silicon thin films irradiated by a beam from a λ = 10.6-μm CO2 laser. Here we report on the morphologies that form for s-, p-, and circularly polarized light as a function of angle of incidence. As with the earlier normal-incidence studies we observe two morphological types: irregular structures consisting of isolated molten or solid lamellae and grating structures composed of alternating strips of solid and molten silicon with a period equal to a multiple of the incident wavelength. Irregular structures could be observed for sufficiently large spot size at all angles of incidence θ and polarizations. With s- and circularly polarized light for sufficiently small spot size, gratings were observed whose wave numbers vary as cos θ for angles of incidence as large as 60°. For p-polarized light, gratings were observed only at normal incidence and for 30° ≤ θ ≤ 60°, with the patterns at the large angles consisting of orthogonally oriented gratings with wave numbers that vary as (1 − sin θ) and cos θ. The observed grating spacings and orientations are a much smaller subset of those seen when nanosecond pulsed lasers induce periodic structures on semiconductors. It is clear that various feedback processes play an important role in selecting the specific steady-state patterns. An extension of the theoretical model that accounts for the formation of grating structures at normal incidence successfully accounts for the nonnormal-incidence gratings.
© 1990 Optical Society of America
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