H. W. Lo

Raman measurements of temperature during cw laser heating of silicon

We present the first Raman data from laser‐heated silicon obtained in situ during the laser irradiation. Analysis of the Stokes to anti‐Stokes ratios yields temperatures at the center of the laser spot which range from room temperature to the melting point as laser power is increased. The data agree well with the nonlinear heat diffusion calculations of Lax for Gaussian beam profiles when the temperature dependence of the silicon absorption coefficient is included. In addition, we show that analysis of the spectral profile of the Raman line can yield information on strain effects produced in the laser‐heated spot.

Pulsed Raman measurement of the onset of recrystallization in laser annealing

We have made the first time‐resolved measurement of the onset of recrystallization following pulsed laser irradiation of ion‐implanted silicon. We find recrystallization to be nearly complete 25 nsec after a 7‐nsec annealing pulse with focused energy density of 0.6 J/cm2.

Time‐resolved optical transmission of pulsed laser‐irradiated silicon

The time‐resolved optical transmission of silicon has been observed at l = 1.15 mm during irradiation by an 8‐nsec pulsed laser at 485 nm with several energy densities in the range of 0.25 to 1.2 J/cm2. The transmission exhibits a sudden brief drop consistent with the rise and fall of the reflectivity enhancement. However, the transmission does not exhibit the strong absorption expected of molten silicon with a skin depth of ∼100 A.

Pulsed Raman measurements of lattice temperature: Validity tests

We measure the temperature dependence of the Raman correction factors and present data on the spot size and transverse beam quality of lasers used in the pulsed Raman measurements of lattice temperature in Si. Recent criticisms are also evaluated and shown to be inappropriate or in error. Finally we measure the shift of the 520‐cm−1 Raman line and find it also to be consistent with the observed Stokes/anti‐Stokes ratios indicating optic phonon populations characteristic of ∼450 °C.

Time-resolved transmission of GaAs under intense laser excitation

Abstract Time-resolved transmission and reflectivity of bulk GaAs and GaAs on sapphire have been studied at λ = 1152 and 633 nm following excitation by an ≈8 ns pulse at 485 nm. As in the case of silicon, the spectral dependence of the transient absorption coefficient, from 1.1 eV to 2.5 eV, is inconsistent with the existence of a metallic molten state during pulsed laser annealing.

Validity of Raman and transmission data reaffirmed

The simple experimental test of blocking the 1.15‐μm cw probe beam shows that Si luminescence is not a problem in our time‐resolved transmission measurements. The phonon wave vector probed in our Raman experiments is q = 1.5×106 cm−1, so that the Raman experiment is, in fact, sensitive to direct phonon heating by carrier relaxation.

Nanosecond optical transmission studies of laser annealing in ion-implanted silicon-on-sapphire

Abstract Time-resolved optical transmission has been studied using 633 and 514 nm CW probes on ion-implantation-amorphized silicon-on-sapphire during annealing by a 10 nsec, ⊥ J/cm2 pulse at either 532 nm or 485 nm. As recrystallization sets in the transmitted signal at 514 nm rises by ⊥ 103 in ⊥ 60 nsec and provides a measure of regrowth velocity. Beyond 200 nsec the much slower transmission rise is used to provide an estimate of the Si cooling rate. The difference in transmission observed between initially crystalline and initially amorphous Si provide an estimate of the latent heat of recrystallization of the amorphous phase.

Time-Resolved Raman Scattering and Transmission Measurements during Pulsed Laser Annealing,

Abstract : Raman scattering from a 7 nsec pulsed dye laser has been used to determine the onset of recrystalization following an 8 nsec dye laser excitation pulse in ion-implanted silicon. We find essentially complete recrystallization 59 nsec after the first excitation pulse and from Stokes-anti-Stokes ratios we find at 59 nsec a crystalline lattice temperature of 600 + or - 200 C. Time-resolved transmission measurements at lambda = 1.15 microns also demonstrate that no molten phase has occurred even though the usual reflectivity enhancement is observed. (Author)

RAMAN TEMPERATURE MEASUREMENTS DURING LASER HEATING OF SILICON

We describe the use of anti-Stokes-to-Stokes Raman ratios for measuring lattice temperatures during laser heating of crystalline silicon. The observed peak temperatures, which range from room temperature to the melting point, are compared with the nonlinear heat diffusion calculation of Lax for Gaussian beam profiles. The results show the importance of including in the theory the temperature dependence of the silicon absorption coefficient when small spot diameters are used. Analysis of the asymmetric Raman line shape provides evidence for thermal expansion and strain effects in the heated spot. Finally we discuss the use two pulsed dye lasers for temporally monitoring the temperature following a 10 nsec heating pulse.