Some of the fundamental aspects of pulsed laser interactions with semiconductors are reviewed within the frame of recent experimental work with picosecond laser pulses. Main emphasis is placed on data obtained in space-time resolved measurements of transmission and reflectivity of picosecond pulse irradiated silicon at different probing wavelengths. Four distinct interaction regimes can be defined, depending on the fluence of the laser pulse. Special attention is devoted to the phase transition from solid to liquid state at the surface. The optical heating model is found to be valid even on a time scale of picoseconds. Firm data of the electron-hole plasma density and lattice temperature are derived. The thermal nature of the phase transition is confirmed.

Advances in the study of high speed crystal growth from the melt are reviewed, with special emphasis on the fast melting and solidification of silicon achieved by use of Q-switched laser radiation pulses. Rapid melting of amorphous Si is confirmed to yield a liquid undercooled by several hundred Kelvins and, under suitable conditions, explosive crystal growth processes can occur. The latter involve the self-sustaining propagation of melt bands buried within the initially amorphous material. When the highest quench-rate conditions are established melting of even crystalline Si can yield a final amorphous solid phase. This breakdown in crystal growth is orientation dependent and can give regimes of crystal defect formation when amor-phization does not take place. The processes which characterize this limiting growth behaviour are discussed.

Recent progress in studies of temperature dependent kinetic competition during solid phase crystallization of silicon is reviewed. Specific areas which are emphasized include: the enhancement of solid phase epitaxial growth rates by impurity-induced changes in electronic properties at the crystal/amorphous interface, the influence of impurity diffusion and precipitation in amorphous silicon on the kinetics of epitaxial growth, the effects of impurities on the kinetic competition between solid phase epitaxy and random crystallization, and the kinetics of solid phase crystallization at very high temperatures in silicon.

Pulsed or scanned energy beams have been shown to be very useful for many semiconductor applications, ranging from annealing of ion-implanted damage to preparation of materials. An overview of this important field is given and its prospects assessed.

Simultaneous measurements of the transient conductance and time-dependent surface reflectance of the melt and solidification dynamics produced by pulsed laser irradiation of Si are reviewed. These measurements demonstrate that the melting temperature of amorphous Si is reduced 200 ± 50 K from that of crystalline Si and that explosive crystallization in amorphous Si is mediated by a thin (≤ 20 nm) molten layer that propagates at ~ 15 m/sec. Studies with 3.5 nsec pulses permit an estimate of the dependence of the solidification velocity on undercooling. Measurements of the effect of As impurities on the solidification velocity demonstrate that high As concentrations decrease the melting temperature of Si (~ 150 K for 7 at.%), which can result in surface nucleation to produce buried melts. Finally, the silicon-germanium alloy system is shown to be an ideal model system for the study of superheating and undercooling. The Si50Ge50 alloy closely models amorphous Si, and measurements of layered Si-Ge alloy structures indicate superheating up to 120 K without nucleation of internal melts. The change in melt velocity with superheating yields a velocity versus superheating of 17 ± 3 k/m/sec.

Since the first time-resolved Raman studies of pulsed laser annealing (PLA) effects in Si, a number of cw Raman studies have been performed which provide a much improved basis for understanding the consequences on Raman spectra of temperature-dependent resonance effects, high carrier density effects, phonon anharmonicity, and strain effects. Here we briefly review these effects and then analyze the latest pulsed Raman studies of PLA including Stokes/anti-Stokes ratios, the shift and shape of the first order line, and time-resolved second-order spectra. The Raman data indicate the existence of a Raman-silent phase followed by a rapidly cooling solid which begins within 300 K of the normal melting temperature of Si. The Raman data also give evidence of carrier densities in the recrystallizing solid of ~1−2×l019/cm3 .

Four different regimes of photoelectric emission are observed over a vide fluence range of UV-laser pulses irradiating single-crystal silicon samples. The role of the electron-hole plasma in the nonlinear photoemission is demonstrated by temporal correlation measurements. A regime where ion thermal evaporation processes take place is observed above the critical fluence for melting. At higher laser fluences nonlinear ion acceleration is demonstrated by direct time-of-flight measurements.

Electron energy distributions, extending from equilibrium surface states through transiently populated conduction band states, have been observed in laser excited silicon by means of transient photoelectron spectroscopy. Atomically clean (111) wafers in UHV were excited by 90 nsec, 2.33 eV or 65 nsec, 4.66 eV pulses. Photoelectron spectra were obtained with the 4.66 eV pulses. Photovoltage shifts have been measured and found to saturate. We have identified a feature in the excited state portion of the spectra due to electrons which have equilibrated in the conduction band minima (CBM). Electron temperatures as a function of 2.33 eV pump fluence have been extracted from the spectra and are shown to be in equilibrium with the lattice above 1100 K. Very hot non-equilibrium electrons are also observed to ~ 4 eV above the CBM. A method of determining the diffusion depth of CBM electrons has been developed and applied to our data.

The technique of picosecond electron diffraction is used to time resolve the laser-induced melting of thin aluminum films. It is observed that under rapid heating conditions, the long range order of the lattice subsists for lattice temperatures well above the equilibrium point, indicative of superheating. This superheating can be verified by directly measuring the lattice temperature. The collapse time of the long range order is measured and found to vary from 20 ps to several nanoseconds according to the degree of superheating. Two interpretations of the delayed melting are offered, based on the conventional nucleation and point defect theories. While the nucleation theory provides an initial nucleus size and concentration for melting to occur, the point defect theory offers a possible explanation for how the nuclei are originally formed.

The free carrier absorption in InSb was found to depend on the pulse duration of the picosecond CO2 laser pulse. This is interpreted as the effect of incomplete carrier relaxation within the laser pulse duration. An electron energy relaxation time of 5.6 ps was derived from the observation. The present experiment presents a new method of measuring relaxation times in semiconductors and metals.

A KrF (248 nm) pulsed laser was used to melt 90-, 190-, and 440-nm thick amorphous silicon layers produced by Si ion implantation into (100) crystalline Si substrates. Time-resolved reflectivity measurements at two different probe wavelengths (633 nm and 1.15 μm) and post-irradiation TEM measurements were used to study the formation of an undercooled liquid Si phase and the subsequent solidification processes. The time-resolved measurements provide new experimental information about the nucleation of fine-grained Si crystallites in undercooled liquid Si, at low laser energy densities (Eℓ), and about the growth of large-grained Si in the near-surface region at higher Eℓ. Measurements with the infrared probe beam reveal the presence of a buried, propagating liquid layer at low ??. Model calculations indicate that this liquid layer is generated in part by the release of latent heat associated with the nucleation and growth process.

Time-resolved reflectivity measurements have been performed on single-crystal silicon irradiated by intense 48 ps pulses of 1 micron radiation. The temporal evolution of the spatial profile of the laser-induced reflectivity changes has been monitored using spatial correlation and surface imaging techniques. These measurements resolve apparent discrepancies between previous spatially and temporally averaged measurements at this wavelength and similar measurements in the visible.

Several advances in time-resolved optical measurement techniques have been made, which allow a more detailed determination of the optical properties of silicon immediately before, during, and after pulsed laser irradiation. It is now possible to follow in detail the time-resolved reflectivity signal near the melting threshold; measurements indicate that melting occurs in a spatially inhomogeneous way. The use of time-resolved ellipsometry allowed us to accurately measure the optical properties of the high reflectivity (molten) phase, and of the hot, solid silicon before and after the laser pulse. We obtain n = 3.8, k = 5.2 (±10.1) at λ = 632.8 nm for the high reflectivity phase, in minor disagreement with the published values of Shvarev et al. for liquid silicon. Before and after the high reflectivity phase, the time-resolved ellipsometry measurements are entirely consistent with the known optical properties of crystalline silicon at temperatures up to its melting point.

Synchrotron x-ray pulses from the Cornell High Energy Synchrotron Source (CHESS) have been used to carry out nanosecond resolution measurements of the temperature distrubutions in Ge during UV pulsed-laser irradiation. KrF (249 nm) laser pulses of 25 ns FWHM with an energy density of 0.6 J/cm2 were used. The temperatures were determined from x-ray Bragg profile measurements of thermal expansion induced strain on <111> oriented Ge. The data indicate the presence of a liquid-solid interface near the melting point, and large (1500-4500°C/pm) temperature gradients in the solid; these Ge results are analagous to previous ones for Si. The measured temperature distributions are compared with those obtained from heat flow calculations, and the overheating and undercooling of the interface relative to the equilibrium melting point are discussed.

We have performed experimental and theoretical studies of the origins of anisotropic second harmonic generation observed in reflection from (111) and (100) faces of crystal Ge and Si. Results, using 25 psec and 20 nsec 1.06 and 0.53 μm pump beams allow us to conclude that induced plasmas have no role in the effect and that the mechanism for the nonlinearity is associated with quiescent surface dipolar or bulk quadrupolar effects. Dispersion in the anisotropic behaviour is presented and the application of anisotropic harmonic generation to structural determinations of Si and Ge during laser annealing will be discussed.

The total energy reflected from a silicon single crystal irradiated by a 100 femtosecond laser pulse is measured. We observe a plasma resonance at wavelengths of 620 nm and 310 nm indicating electron-hole densities higher than 1022 cm. The result are interpreted using a highly non linear theory. Very short relaxation times are observed and attributed to electron-hole collisions. The study of the light scattered by the silicon surface shows a sharp decrease at high fluences that we interprete by a possible screening of irregularities by emitted electrons .A pump-test experiment is also reported showing the emission of Si particles. A possible mechanism for the extraction of these particles is proposed.

Laser pulses, of a duration of the order of 100 femtosecondsare a very unique tool to study the physical mechanisms of energy transfer from the electron-hole (e-h) plasma to the lattice in semiconductors. The incident photons are absorbed by the electrons, creating a hot and dense electron-hole plasma and breaking covalent bonds thus softening the lattice. After the pulse, the electron-hole pairs recombine, the plasma expands, and through electron-phonon interaction the energy is transferred to the lattice. Several experiments have recently been reported using femtosecond pulses to create a high density e-h plasma in silicon and study its time evolution [1,2,3]. The use of such intense and short pulses raises the possibility of breaking so many covalent bonds that the melting temperature of the crystal can be lowered [4,5] significantly. In a first period, a new phase is obtained, with atoms almost immobile (having a low kinetic energy) but imbedded in a dense hot plasma. In a time of the order of several electron-phonon relaxation times (τe-p) the energy is transferred to the atoms and the normal liquid phase is obtained. The understanding of the exact nature of the melting induced by very short pulses relies on a good knowledge of the energy transfer from the laser pulse to the sample. In this paper, we report measurements of the total amount of energy of a 100 fs, 620 nm and 310 nm of wavelengths, light pulse reflected by a silicon single crystal and its variation with pulse intensity (self reflectivity with no test beam). We also give measurement of the light scattered from the surface to see changes of the surface roughness. Finally, we give the result of a pump-test experiment showing the formation of a "blackhole" in the center of the incident spot, as already reported by other authors [6].

Detailed analysis of the second harmonic signals in reflection from crystalline GaAs surfaces reveals that the structural transition associated with surface melting occurs in less than 2 ps. Preliminary results of picosecond time-resolved reflectivity measurements are also presented.

Impurity segregation at the liquid-solid interface, interfacial instabilities due to constitutional supercooling and precipitation are compared for the growth of a crystalline or of an amorphous phase from rapid solidification. Segregation at low concentration and interfacial instabilities at high concentrations are shown to give the same information. The impurity behaviour shows a distinct similarity between the crystalline and amorphous phases.

Two solute trapping models are compared. They are shown to predict identical behavior at any given end of a phase diagram but different behavior as the phase diagram is traversed. The segregation behavior of dilute solutions of Ge in Si (ke < 1) and of Si in Ge (ke > 1) during regrowth from pulsed-laser melting is being studied using transient conductance, high resolution RBS, and SIMS. Our results to date suggest a significant amount" of solute trapping (k —> 1) of Ge in Si and of Si in Ge. Such a result would be inconsistent with the predictions of one of the models.

Recently available experimental data indicate that the solidification of undercooled molten silicon prepared by pulsed laser melting of amorphous silicon is a complex process. Time-resolved reflectivity and electrical conductivity measurements provide information about near-surface melting and suggest the presence of buried molten layers. Transmission electron micrographs show the formation of both fine- and large-grained polycrystalline regions if the melt front does not penetrate through the amorphous layer. We have carried out extensive calculations using a newly developed computer program based on an enthalpy formulation of the heat conduction problem. The program provides the framework for a consistent treatment of the simultaneous formation of multiple states and phase-front propagation by allowing material in each finite-difference cell to melt, undercool, nucleate, and solidify under prescribed conditions. Calculations indicate possibilities for a wide variety of solidification behavior. The new model and selected results of calculations are discussed here and comparisons with recent experimental data are made.