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

Abstract

The time-resolved optical transmission (at 1152-nm wavelength) and reflectivity (at both 633 and 1152 nm) of crystalline silicon have been measured with \ensuremath{\sim}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 ${E}_{l}\ensuremath{\ge}0.8$ 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 \ensuremath{\sim}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 (\ensuremath{\lesssim}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 (\ensuremath{\gtrsim}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.