The current understanding of the interaction between ultrashort laser pulses and condensed matter is demonstrated on a few selected samples. Hot electronic carrier relaxation in GaAs, plasmon aided recombination in highly excited silicon and ultrafast energy transport in metals are discussed.

Real-time techniques were used to study rapid melt and solidification dynamics in silicon. In crystalline Si, the interface response function was characterized and found to be asymmetric for large deviations from the melting temperature, which will require reevaluation of conventional transition state treatments of melt and solidification. In amorphous Si, the mechanism of explosive crystallization was studied. The explosive transformation is mediated by a buried liquid layer, and detailed measurements have led to the suggestion that polycrystalline Si nucleates at the moving liquid-amorphous interface. For certain conditions, this process could yield fine-grained polycrystalline Si; for other conditions it permits epitaxial regrowth from the underlying crystalline Si for maximum melt thickness much less than the original amorphous layer thickness.

The deviations from local equilibrium at a rapidly moving solid-liquid interface are well documented. The fraction of solute atoms in the liquid at the interface that joins the crystal during rapid solidification approaches unity and the interface temperature drops. Experimental and theoretical work on impurity incorporation and interfacial undercooling is reviewed. Past and future experiments to test the theories are discussed.

The technique of ion beam deposition (IBD) is utilized to investigate low-energy, ion-induced damage on Si and Ge; to study reactive ion cleaning of Si and Ge; to fabricate amorphous isotopic heterostructures; and to fabricate and study the low-temperature epitaxial deposition of 74Ge on Ge(100), 30Si on Si(100), and 74Ge on Si(100). The techniques of ion scattering/channeling and cross-sectional TEM are combined to characterize the deposits.

Low-energy (often < 100 eV) ion bombardment during thin film deposition is commonly used in such diverse application areas as microelectronics, optical coatings. magnetic recording layers. and hard wear resistant coatings to modify the microstructure and microchemistry of films deposited by a variety of techniques (e.g. sputtering, primary ion deposition, plasma-assisted CVD, and accelerated-beam MBE). Ion irradiation has been shown to affect every phase of deposition including nucleation and growth kinetics, crystal structure and phase stability, the average grain size and degree of preferred orientation of polycrystalline films, the epitaxial temperature of single-crystal films, defect concentrations, elemental incorporation probabilities, surface segregation, and, hence, film properties. As discussed in this brief review, a detailed understanding of many of these processes is beginning to emerge. Effects such as trapping, preferential sputtering, enhanced diffusion, and collisional mixing have been used to interpret and, in some cases, model experimental results. Nevertheless, there are still large gaps in our knowledge of the role of ion bombardment on fundamental processes such as nucleation kinetics.

A overview of the recent developments in laser chemical processing is provided. The review covers advances in understanding fundamental process physics, the development of new techniques, and the implementation of applications.

The transition between amorphous regrowth and explosive crystallization of a 220nm thick amorphous Si layer on a crystalline Si substrate has been studied using time-resolved reflectivity, transmission electron microscopy, and Rutherford backscattering spectroscopy. Upon irradiation by 7.5ns FWHM pulses from a frequency-doubled Nd:YAG laser, interferences in the reflectivity indicate growth of amorphous Si from the surface. The observation of a narrow Cu peak, buried below the surface, points towards solidification from both the rear and the front. Transmission electron microscopy studies revealed the occurrence of small patches of polycrystalline Si. The relative amount of this polycrystalline Si is increased by longer laser pulses, higher substrate temperatures, and thicker amorphous Si layers. The results are discussed in terms of the temperature distribution and the time available for the nucleation of polycrystalline Si at the liquid-solid interface.

The velocity distributions of atoms evaporated from the surface of GaAs during laser heating with nanosecond and picosecond pulses are measured. The atomic velocities provide information about the surface temperature. For picosecond heating we observe a continuous transition of the temperature across the melting point, whereas for nanosecond heating the melting point is marked by a distinct plateau of the temperature curve. From these observations we conclude that the solid is strongly superheated during picosecond irradiation. A detailed analysis suggests superheating of typically a few hundred degrees above the melting point.

Measurements of the competition beween solid phase epitaxy, solid phase random nucleation, and melting in amorphous Si on a microsecond time scale are reported. We find that the behavior of amorphous Si under microsecond pulsed dye laser irradiation depends strongly on film thickness and temperature. In “thin” (≲1000 Å) films solid phase epitaxy is observed at temperatures up to and exceeding 1300°C with random nucleation dominating at T>1330° C; however, melting of amorphous Si does not occur. In contrast, in “thick” (2600 Å) amorphous films melting is observed at T˜1190°C. These results are discussed with respect to measurements obtained previously in the nanosecond time regime using Q-switched laser heating and in the 0.1–1 millisecond regime using “chopped beam” cw laser heating.

A brief review is given of transition-state theory, both for the case of unimolecular reactions in the gas phase, and for reactions in condensed phases. An argument is made that, within the context of this theory, heterogeneous freezing in Si is limited to rates much lower than collision rates by the difference between the entropies of the solid and the liquid.

Statistical thermodynamics is used to derive the reaction rate for melting and freezing by considering an atomically sharp interface with or without an activated intermediate state. The resulting predictions differ substantially from those of the classical kinetic rate theory at large deviations from equilibrium. The model may be appropriate for the analysis of pulsed laser melting experiments where large deviations are expected.

A novel method of pulsed laser processing of ion-implanted silicon is presented, in which samples are irradiated in water ambient. The water layer in contact with the silicon during irradiationh as a considerable influence on melting and solidificationd ynamics. Still, perfect epitaxy of a thin amorphous layer can be obtained using this method.

For epitaxy to occur on a sample irradiated under water, 40 % more absorbed energy is necessary than for a sample irradiated in air. This indicates the occurrence of a considerable heat-flow from the silicon into the water layer during the laser pulse. From impurity redistribution after irradiation it is found that by processing a sample under water liquid-phase diffusion is reduced. Diffusion theory arguments indicate that this can be due to a reduction in total melt duration by about afactor 2–3. This can be due to faster cooling of the liquid silicon layer after the laser pulse whereas the melt-in time might be influenced as well. As a consequence, shallower impurity profiles can be obtained in crystalline silicon. No oxygen incorporation is detected and the surface morphology is not disturbed using this new process.

PLZT, a ferroelectric material with a high quadratic electro-optic coefficient, is combined with silicon technology via chemical vapor deposition to enable the realization of Si/PLZT spatial light modulators. Laser crystallization is required to produce device quality silicon deposited on PLZT. The critical issues in the laser crystallization are (i) achieving crystallization in the absence of a good seed, and (ii) preventing damage to the PLZT. To prevent PLZT damage during the laser heating, a 3.5 μm silicon dioxide layer is used as a thermal buffer between the silicon and the PLZT and the effective scanning time is shortened to 100 μs. A double humped cw argon ion laser with a shaped beam is used to achieve successful crystallization with good surface smoothness and large grain size. This technology and devices fabricated on these samples will be discussed.

New results on the ultrafast laser-induced formation of high-quality patterned silicon dioxide (SiO2) layers on silicon substrates at room temperature are presented. Using a pulsed ArF excimer laser (λ = 193 nm) operating at pulse energies near the melting threshold of silicon, SiO2 layers can be grown at a rate of 3–4 Å/pulse with an oxygen pressure of 500 torr. This growth rate exceeds by a factor of 5 the rate found using a XeCI excimer laser (λ = 308 nm) indicating that oxygen atoms produced by the direct photodissociation of oxygen by the ArF excimer laser play a role in achieving this extremely fast oxidation rate. Careful analysis of the laser exposed regions following each laser shot was performed using angle-resolved XPS with 150 micron spatial resolution. After one laser shot, a nonstoichiometric SiOx interface layer (<5Å) is clearly visible. Subsequent laser shots show the oxide peak shifting to higher energy as the oxide thickness increases until the characteristic ∼4.5 eV chemical shift for SiO2 (relative to the bulk Si 2p peak) is realized at a thickness of ∼20Å. The nonstoichiometric SiOx layer is still present at this oxide thickness but is confined to the interface region (<10Å) as observed in thermally grown oxides. Oxide thicknesses obtained by angle-resolved XPS were confirmed using sputter depth-profiling. XPS measurements on samples irradiated at low oxygen pressure (5 torr) show evidence for oxidation of the silicon substrate outside the focused laser beam (2X). At this pressure the recombination time for oxygen atoms is long enough to allow diffusion and apparent reaction with the silicon substrate at reduced temperatures.

Carrier concentrations exceeding 1019/cm3 in GaAs implanted with Si (2 × 1014/cm2 @ 140 keV) have been obtained by pulsed laser annealing with either a dye laser (λ = 728 nm) or a XeCl excimer laser (λ = 308 nm). Carrier concentrations were measured by plasmon Raman scattering over a wide range of anneal energy densities. Compared with capless laser annealing, much higher carrier activations were achieved when the annealing laser pulse was incident through a Si3N4 cap.

We have measured the microwave-induced damage to the near-surface region of silicon for 1.9-μs pulses at a frequency of 2.856 GHz and a pulse power of up to 7.2 MW. Rectangular samples were irradiated in a test section of WR-284 waveguide that was filled with freon to a pressure of 30 psig. Incident, transmitted and reflected powers were monitored with directional couplers and fast diodes. The results of the time-resolved optical measurements show that the onset of surface damage is accompanied by a large increase in the reflected power. Examination of the irradiated surfaces shows that the degree of damage is greatest near the edges of the samples. Using secondary ion mass spectrometry to profile the implanted As, we find that the microwave pulses can melt the near-surface region of the material for pulse powers exceeding a threshold value.

Pulsed laser melting of the low-temperature σ (tetragonal, D8b) phase has been used to generate a liquid undercooled with respect to the melting point of the higher-temperature, equilibrium α (bcc) solid solution in equiatomic Fe-V alloys. From calculations based on reported thermodynamic data and equilibrium transformation temperatures, the metastable melting point of the σ phase is about 1720 K for an Fe-50 at.% V alloy, which is 54 K below the melting temperature of the α phase. During rapid heating of well-annealed σ-phase material with a 30 ns laser pulse to above melt threshold, the σ → α reaction is suppressed, so that the melt zone is undercooled by ∼ 54 K with respect to the equilibrium α phase. The α phase nucleates from the undercooled molten surface layer and is retained during the subsequent rapid cooling (∼ 1010 K/s) because of the relatively sluggish α → σ transformation. X-ray diffraction (Read camera) and TEM identified the α phase in the near-surface after melting σ with incident laser energies (1.0–1.41 J/cm2) which are well above the melt threshold as determined by changes in reflectivity (∼ 0.7 J/cm2). The α phase nucleated from the undercooled liquid within ∼ 20 ns.

Melting transitions of metastable quasicrystalline phases of Al-Mn have been observed using rapid electron-beam heating of fine-grained icosahedral surface layers. The congruent melting-point for icosahedral Al80 Mn20 was directly measured to be 910±20°C. Heating to higher temperatures shows another transition which is inferred to correspond to the liquidus of the decagonal phase at 965±20°C for 20 at.% Mn. The microstructure and formation kinetics of the decagonal phase are discussed, and its electron diffraction is described.

Irradiation of metals in gaseous and liquid nitrogen by KrF excimer laser pulses shows the way in which vapor or plasma pressure fields determine the nature of the rapidly solidified surface topography. Wave patterns formed on aluminum by irradiation in N2 gas arise from recoil pressure differences produced by highly localised differential surface evaporation. The solidification time, calculated from the observed ripple wavelength, is in good agreement with that deduced from a heat flow calculation. For irradiation in liquid N2, the homogeneous pressure within the center of the dense plasma zone results in a smooth surface. Beyond this region the progressively diminishing pressure induces surface ripples, fragmentation and droplet formation. Intriguingly, immersion of Ni, Ti, Zn, and Al in liquid N2 (to a depth of 2 to 4 mm) considerably increases the area and depth of irradiation-modified regions.

A KrF laser was used to produce ions from an aluminum surface. The threshold energy fluence was found to be ∼30 mJ/cm2 which was lower than similar studies on the ablation of metals. The measured threshold however, was of the same order of magnitude for atomic desorption from semiconductor surfaces. This process can be used to perform surface analysis without ablation damage.