Time-resolved optical studies of silicon during nanosecond pulsed-laser irradiation

The time-resolved optical transmission (at 1152-nm wavelength) and reflectivity (at both 633 and 1152 nm) of crystalline silicon have been measured with ∼nsec resolution during and immediately after pulsed-ruby-laser irradiation (694 nm, full width at half maximum pulse duration 14 nsec), over a range of pulsed-laser energy densities $E_l$. For $E_l\ge 0.8$ J/${\mathrm{cm}}^2$ the transmission is found to go to zero and to remain at zero for a time proportional to $E_l$ (during which time the reflectivity is also at a maximum value), and then to recover (in ∼500 nsec) to its initial value. The zero-transmission result during the high-reflectivity phase contradicts reports of other similar experiments. Measured reflectivities during the high-reflectivity phase agree with reflectivities calculated from the known optical constants of molten silicon, at both the 633- and 1152-nm probe wavelengths. Intense near-band-gap photoluminescence is also observed from our silicon samples, for $E_l$ both above and below the threshold for the high-reflectivity phase. The results of detailed calculations using the thermal-melting model are presented. Good quantitative agreement is found between the results of these calculations and the measured melting threshold of 0.8 J/${\mathrm{cm}}^2$, and with the reflectivity and transmission of crystalline silicon, as functions of time and $E_l$. The small (≲10%) absorption due to long-lived laser-induced free carriers is also calculated and found to be in satisfactory agreement with the measured transmission for long (≳100 nsec) times after pulsed-laser irradiation. The results are discussed in relation to other recent time-resolved measurements during pulsed-laser irradiation of silicon.