Picosecond time-resolved reflectivity and transmission changes of bulk silicon and silicon-on-sapphire have been measured to study the electron-hole plasma formation and phase transitions in silicon, induced by picosecond green or ultraviolet pulses. The results provide direct evidence of ultrafast energy transfer to the lattice and ultrafast phase transitions in silicon. Lattice heating up to the melting point and overheating of the melt or boiling have been observed during the picosecond pulse duration.

We report measurements of the time resolved reflectivity of Silicon following excitation with an intense 90 femtosecond optical pulse. These measurements clearly time resolve the process of energy transfer from the optically excited electron-hole plasma to the crystal lattice and subsequent melting within the first few picoseconds.

We present time-resolved measurements of spontaneous anti-Stokes and Stokes Raman scattering during pulsed laser heating of crystalline silicon. The time-evolution of the lattice temperature is determined from the measured anti-Stokes/Stokes intensity ratio. In a separate calibration experiment we measure the temperature dependence of the anti-Stokes/Stokes ratio of an oven-heated silicon crystal from 300 K up to 900 K. The phase transition occuring during laser heating is detected by monitoring the changes of the optical reflectivity during laser irradiation. Our data suggest that the phase transition occurs at a lattice temperature of ∼600 K.

Transient optic phonon populations are measured in crystalline Si as a function of 532 nm laser energy density. The use of a continuously tunable pulsed dye laser as the Raman probe allows us to obtain, under exact experimental conditions, all correction factors necessary to extract the phonon population without the necessity of relying on room temperature or oven-heated conditions. We find the shift of the 520 cm−1 Raman-line to be consistent with the observed Stokes/anti-Stokes ratios indicating a maximum optic phonon temperature of 450 ± 100°C. A discussion is also given of the errors in several recent criticisms of the Raman results.

Raman temperature measurements during pulsed laser annealing of Si by Compaan and co-workers are critically examined. It has been shown previously that the Stokes to anti-Stokes ratio depends critically upon the optical properties of silicon as a function of temperature. These dependences, coupled with the large spatial and temporal temperature gradients normally found immediately after the high reflectivity phase, result in large variations in the calculated temperature depending upon the probe laser pulse width and the pulse-to-pulse and spatial variations in the annealing pulse energy density.

We show that the softening of the T.A. phonon mode produces an instability in the electron-hole plasma of a silicon-like semiconductor and suggest that this instability can be the mechanism for melting of this type of material. This allows us to estimate the characteristics of a laser pulse which are necessary to modify the usual melting, i.e. which would lead to a substantial decrease of the melting temperature.

Near surface temperatures and temperature gradients have been studied in silicon during pulsed laser annealing. The investigation was carried out using nanosecond resolution x-ray diffraction measurements made at the Cornell High Energy Synchrotron Source. Thermal-induced-strain analyses of these real-time, extended Bragg scattering measurements have shown that the lattice temperature reached the melting point during 15 ns, 1.1–1.5 J/cm2 ruby laser pulses and that the temperature of the liquid-solid interface remained at that temperature throughout the high reflectivity phase, after which time the surface temperature subsided rapidly. The temperature gradients below the liquid-solid interface were found to be in the range of 107°C/cm.

Fast regrowth of amorphous silicon from liquid silicon films has been directly observed in a time resolved picosecond laser melting experiment. Liquid films up to 100 nm thick were formed on crystalline substrates with 15 picosecond 248 nm pulses from a KrF* excimer laser. The film thickness as a function of time was probed directly by observing attenuation of 1.64 pm 15 psec light pulses transmitted through the melt. Melting and regrowth velocities were compared to a heat diffusion model, and evidence for melt undercooling was observed. The resolidified silicon was amorphous at all values of incident laser intensity.

The transient conductance technique has been used in a detailed study of the liquid-solid interface dynamics during pulsed laser melting of Si and silicon-on-sapphire. Average melt and regrowth velocities, as well as the maximum melt depth, can be obtained with the technique. The measurements are found to agree well with a computer simulation based on a thermal model of the melt and subsequent solidification. The melt-in velocity has been observed to exceed 200 m/sec. Under 2.5 ns UV irradiation, the critical velocity for amorphization of <100> Si has been measured at 15 m/sec.

A pulsed proton beam, ˜200 ns in duration, has been used to melt and regrow single crystal silicon. The protons had an energy of 300 kev, yielding a measured energy density of 0.8–2.0 J/cm2. The method of transient conductivity has been used to determine the melt depths, melt durations, and regrowth velocities. The measured values for 2.0 J/cm2 were, respectively, 1.7 μm, 2 μsec, and 1.4 m/sec.

Computer generated melt curves were compared to experiment with good agreement. The energy required to initiate melt was determined, and a linear dependence of melt depth with energy has been observed.

The use of Q-switched laser melting techniques to investigate new rapid solidification phenomena is described. It has been found that Si, Ge, GaP and GaAs can give rise to orientation dependent, kinetically-controlled defect generation processes during fast recrystallization from the melt. Indeed, these materials yield amorphous phases at sufficiently high solidification rates. Ultra-fast pulsed melting permits the study of the basic thermodynamic properties of amorphous solids. It is shown that amorphous Si melts to give a normal, low viscosity, undercooled liquid and that novel explosive crystal growth processes can occur in this low temperature regime.

It is concluded that large interface undercoolings of ˜ 300 deg are not likely to occur during pulsed laser annealing and that the observed liquid to amorphous phase transition is not a purely thermodynamic effect. It is then shown that the formation of the amorphous phase can be understood on the basis of a kinetic rate model which makes large undercoolings of the interface unnecessary.

Ion mass spectrometry, charged particle yields, and kinetic energy distributions of electrons and ions are used to characterize silicon wafers and vacuum-cleaved silicon surfaces under conditions related to laser annealing. We find that alkali metals dominate the positive ion emission from chemically-cleaned wafers, a mass-72 peak tentatively identified as Si2O+ comprises the main ion emission from the cleaved surface, and ion and electron temperatures can be derived from the energy distribution curves although the Si2O+ emission implies more than a simple thermal evaporation process.

Pulsed 10.6μm TEA CO2 laser light has been used to melt the semiconductors silicon and InSb. Measurements indicate that generation of free carriers necessary for melting may take place by nonlinear processes such as two-photon absorption or intraband avalanche ionization. If the semiconductor is sufficiently doped, melting may also result from linear free carrier absorption. In all cases, it appears that the molten depth exceeds several μm, which is much greater than obtained with lasers of shorter wavelength.

From a model for the thermodiffusion of high density plasmas an electronic mechanism for plasma self-confinement is derived. Due to the increase of the confinement densities with the density of states masses this effect should be most important in laser annealing experiments in Si but much weaker in GaAs. A fast diffusion of the surface generated plasma is found to enhance the impact ionization rate.

The dynamic characteristics of lattice temperature, T attained in Si under the influence of a high power laser beam irradiation are examined analytically over a wide range of laser wavelengths, pulse intensities and durations. The strongly temperature-dependent material parameters, such as optical absorption coefficient α(T) and thermal diffusivity D(T) are incorporated in heat diffusion equation, and their nonlinear coupling effect on the ensuing temperature in laser-processed semiconductors are examined. Specifically, the threshold pulse energy for surface melting is characterized as a function of both material and annealing laser beam parameters. This analytic description of pulsed laser heating of semiconductors should provide considerable insight into the transient heating phenomena and localized material modifications.

We present a numerical model for the calculation of the temperature rise caused by pulsed laser irradiation of a thin film/substrate structure. This model includes phase changes in both the thin film and the substrate. The inclusion of phase changes results in more complex thermal behavior and significantly affects melt durations. This model was applied to the AuGe/GaAs system. Morphological observation using the scanning electron microscope and SIMS profiles provides experimental verification for the numerical calculations.

It is shown how the systematic use of the method of integral transforms greatly simplifies the calculation of the temperature rises in laser irradiated media. In general, this method leads ultimately either to analytical results or to very simple numerical integrals (e.g. no poles, exponential kernels). We focus here on the analytical results, and discuss some aspects of CW laser heating, for large surface absorption, including radial dependance, depth dependance and transient nonlinearities. The new results derived in this treatment are in good agreement with experimental data from other studies.

The thermodynamic interrelations of the crystalline (c-sc), amorphous semiconducting (a-sc) and liquid metal (lm) states of Si and Ge, based on the existing thermal data, are reviewed. Then the kinetics of the interfacial processes in the growth of c-sc and a-sc into undercooled lm and the conditions for transition from c-sc to a-sc growth are discussed.

Crystallization of amorphous Ge (or Si) has been studied as a function of temperature and the flux of ionizing radiation (or doping). The crystallization growth rate Vg takes on the form Vg = vo exp(−E/kT) where vo is an increasing function of flux (or doping). We propose the following to explain these data: A concentration of mobile dangling bonds (DBs) exists in the bulk and near the amorphous-crystalline (a-c) interface. Ionization and doping induce transitions from the uncharged state Do to the charged states D+ and D−. The process controlling crystallization resulting in the above activation energy is discussed. Only certain sites on the a-side of the a-c interface are available for crystallization, and these sites are those which have captured DBs. The charged D+; and D− states have a larger capture cross section than the uncharged Do state. Increased concentrations of charged DBs results in an enhancement of the prefactor in the above equation.