AlxGal-xAs alloy was obtained on semi-insulating GaAs by laser beam interaction. For this purpose, thin layers (~150 A) of AlAs and GaAs were deposited by MICVD on the semi-insulating undoped GaAs substrate and irradiated with a high power laser beam. For a certain critical value of incident power (7.1 watts/mm2), a layer of AlxGal-xAs alloy was formed. The nature of the alloy was examined by Auger and Far infrared reflectance spectroscopy. The later technique reveals the characteristic mode of AlxGal-xAs confirming its formation. Disorder induced LA mode was also observed.

Recent results by Coriell and Sekerka [J. Crystal Growth, 61, 499(1983)] on the oscillatory instability of a planar rapidly solidifying binary melt are extended to include diffusion in the solid phase. Under assumptions equivalent to those made by Coriell and Sekerka, it is shown that no matter how small the diffusion coefficient is in the solid, the system is stable to all oscillatory and non-oscillatory disturbance modes if the modified constitutional supercooling criterion is satisfied and if the nonriequilibrium segregation coefficient is zero. Thus, a range of the non-equilibrium segregation parameter exists where these results allow the possibility of instability, whereas Coriell and Sekerka predict that the system will be stable.

System stability is increased for both oscillatory and non-oscillatory modes. It is necessary for the diffusivity ratio Ds/D1 to be nearly 0.1 before oscillatory modes are affected. Both the critical wavelength of the disturbance as well as the oscillation frequency are reduced slightly from the case where diffusion in the solid is ignored.

The dynamics of the electron-hole plasma in silicon and germanium samples irradiated by 20 ps, 532 nm laser pulses has been investigated in the near infrared by time-resolved picosecond optical spectroscopy. The experimental reflectivities and transmissions are compared with the redictions of the thermal model for degenerate carrier distributions through the Drue iformalism. Above a certain fluence, a significant deviation between measured and calculated values indicates a strong increase of the recombination rate as soon as the plasma resonances become comparable with the band gaps. These new plasmon-aided recombination channels are particularly pronounced in germanium.

Plan and cross-sectional transmission electron microscopy (TEM) has been used to examine the various bulk and surface structural changes observed in crystalline silicon following melting caused by the absorption of 1-μm pulses that are 4 ps in duration. We show for the first time that for picosecond excitation polycrystalline Si (p-Si), rather than singlecrystal Si, is always formed when the melt resolidifies. Specifically, cross-sectional TEM analyses indicate that a thin layer of fine-grained p-Si is formed at the interface of the melted region with the bulk. When the incident fluence is more than 10% above the melting threshold, the region between the surface and the fine-grained p-Si regrows as largergrain p-Si. This is the first reported observation of a large-grain p-Si layer on a fine-grain p-Si base in crystalline material. If the incident fluence is less than 10% above threshold, the region near the surface resolidifies as amorphous Si, but the narrow layer of fine-grained p-Si remains at the interface with the single crystal material. Present models for resolidification must be modified to account for these features.

The surface temperature of GaAs during pulsed laser heating is obtained from the measured velocity distributions of evaporated atoms. For nanosecond pulses the observed solid-liquid transition temperature agrees with the equilibrium melting point TM. With picosecond pulses the temperature rises above TM by several hundred degrees before melting can be detected. This result is interpreted as evidence of superheating.

The interactions of picosecond laser pulses at 532 nm wavelength with GaAs surfaces have been studied with time-resolved reflectivity and transmission measurements at three probe wavelengths. At fluences below the melting threshold, the laser-generated electron-hole plasma is limited to a density below ≈1020cm−3. The high reflectivities of molten GaAs observed at fluences above the tlhreshold have a wavelength dependence inconsistent with a simple Drude model for a metallic GaAs molten layer. At high fluences, the evolution of a laser-induced vapor cloud is observed.

Experimental evidence for laser melting of graphite, by irradiation with 30ns pulses from a ruby laser, is presented. RBS-channeling analysis, Raman scattering and TEM measurements reveal that the surface of graphite melts at a threshold energy density of about 0.6 J/cm2. For laser pulse energy densities above 0.6 J/cm2, the melt front penetration depth increases nearly linearly with increasing energy density. An intense emission of carbon particles during and after irradiation is observed. The thickness of the carbon layer removed in this process also increases nearly linearly with increasing pulse fluence. A dramatic redistribution of ion implanted impurities is also observed. Furthermore, the crystalline structure of the resolidified material is shown to depend on the energy density of the laser pulse. In order to explain these phenomena, a model for laser melting of graphite at high temperatures to form liquid carbon has been developed in which a free electron gas approximation is used to describe the properties of liquid carbon. The model is solved numerically to give the time and depth dependences of the temperature as a function of the laser pulse energy density. Very good agreement is found between the observed melt depth dependence on laser pulse energy density, as determined by RBS-channeling, and the model calculations. The redistribution of ion implanted impurities and the modification of the crystalline structure, caused by the pulsed laser irradiation, are also consistent with the model and permit the determination, for the first time, of interfacial segregation coefficients for impurities in liquid carbon. The model also predicts that liquid carbon at low pressure (p < 1 kbar) has metallic properties.

The pump-and-probe technique is employed to perform picosecond timeresolved measurements of the reflectivity changes in highly oriented pyrolitic graphite excited by 0.532-μm pump pulses. At low pump fluences, the presence of a short-lived plasma and a high-temperature gradient gives rise to an increase in the reflectivity probed at 1.9 μm but causes a decrease at 1.064 μm. At the threshold fluence, 140 mJ/cm2, the reflectivity drops abruptly, marking a phase transformation. Above the threshold, the reflectivity drops to -0.2 from its original value of 0.42 at 1.064 μm and to -0.4 from its ambient value of 0.50 at 1.9 μm. This new phase persists only for a few nanoseconds.

Time resolved reflectivity measurements at the surface of pulsed laser irradiated graphite are found to be complicated by the evolution of carbon atoms from the surface which attenuates the probe beam. The evolution of the species occurs concomitant with the observation of the melt on a time scale of <2 nsec. Further, the emitted species have velocities of:≤ 2 x 105 cm/s. The experimental results suggest the formation of large clusters at the molten surface though more experiments are required to clearly distinguish the effect of clusters from those due to the formation of a randomly reflecting plasma.

Emission of ionic clusters from surfaces of group IV elements has been analyzed by means of time-of-flight measurements under nitrogep laser irradiation in its peak intensity ranging from 50 to 200 MW/cm with pulse duration of 10 ns. The measurements on Si, Ge and β-Sn with a straight flight tube show a continuous distribution of spectra in the flight time corresponding to the mass-to-charge ratio between 1 and 2 in units of the singly charged atom of respective elements. The line shape analysis indicates that emitted ions immediately after laser excitation are coagulated in the form of large clusters having nearly an equal mass-to-charge ratio between 5/4 and 3/2. They eventually decompose into monoatoms and their ions in a time scale of 10 ps as confirmed by a quadrupole mass analysis. These results are in remarkable contrast with the case of graphite which exhibits a series of sharp spectral peaks. The clear distinction between graphite and other group IV elements is attributed to the intrinsic difference in their chemical bonds under highly excited conditions.

The surface structures of laser-irradiated samples of highly oriented pyrolytic graphite (HOPG) have been investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The samples were irradiated with a 30 ns ruby laser (λ = 6943 !A) pulse with energy fluences ranging from 0.1 to 3.0 J cm−2. Optically, the specimens show a damage region approximately 5 mm in diameter. The surface structure displays three characteristic regions: an outer boundary characterized by submicron carbon spheroids resting on the surface; an inner boundary characterized by both submicron spheroids and 1 to 5 [m ‘torn’ carbon layers which appear to have broken away from the graphite surface after irradiation; a central region characterized by a uniform density of spheroids and a pattern of surface upheavals which trace out a grain pattern similar to that of the pristine substrate. Electron diffraction patterns taken on the irradiated region indicate an ultra-fine grain 2-dimensionally ordered carbon. Qualitative trends in the areal density of different microstructural features are presented. In addition, a simple model explaining the observed features is given. All observations are consistent with the rapid solidification of liquid carbon.

Pure crystalline Ga films (α-Ga, β-Ga) have been irradiated at low temperatures (≤ 20 K) with an Excimer laser. By measuring the superconducting transition temperature Tc and the residual resistivity ≤o, the resulting Ga phases (α-Ga, β-Ga, a-Ga) can be identified.

Both crystalline Ga phases can be transformed into the amorphous phase.

The threshold energy density for the β→ a transition depends on the film thickness, whereas the α →. a transition occurs always at about 225 mJ/cm2 This behavior is in agreement with earlier observations that a-Ga can grow on top of the in-phase but not on the β-phase.

The results of laser quenching are compared with other non-equilibrium techniques for the production of a-Ga, such as vapor quenching and low temperature ion iradiation.

The kinetics of rapid crystallization in metals is investigated by measuring transient optical property changes in laser irradiated gold and copper films in the picosecond and nanosecond regimes. Melt thresholds are determined by examining concentration profiles of multi-layered films before and after irradiation.

We report time-resolved electrical-resistance measurements during pulsed-laser melting of a metal, aluminum. The resistances are correlated with the thresholds for partial and full melting. We describe a semianalytic solution, based on an impulse-response function, for the response, to a heating pulse, of a thin film of good thermal conductor supported by an infinite substrate. The results agree well with the resistance measurements, and confirm our interpretation of the data. In addition, time-resolved reflectance measurements establish that, in this geometry, melting and solidification proceed via the motion of a well-defined, planar liquid/solid interface, whose position can be deduced from the resistance measurements. These measurements permit the first real-time determinations of melt-depths and quenching histories during rapid-solidification processing of metals.

We report on time-resolved measurements of laser-induced transformations in Te thin films. Rapid thermal transformations are effected utilizing a KrF excimer laser (12 ns duration, 249 nm). The kinetics of the transformation is investigated utilizing transient conductance and infrared absorption measurements as well as reflectivity and transmissivity changes at 633 nm. The threshold for melting is accurately determined as well as melt velocities pertinent to the recrystallization kinetics of the molten films. Criteria for glass formation of Te are also discussed.

Manganese has four allotropes with an equilibrium melting point of the high temperature δ-phase at 1517 K and calculated metastable melting points for the ϒ, β and α phases at 1501 K, 1481 K and 1395 K, respectively. Our observations for Mn irradiated with a pulsed laser and supporting estimates of maximum allotropic transition rates indicate that transformations between allotropes are suppressed during heating with ~ 25 ns laser pulses, as well as during subsequent cooling. Upon pulsed heating of β-Mn to the melt threshold, the melt is undercooled 122 K below the δ-Mn melting point. For incident laser pulse energy densities near the melting threshold, resolidification involves regrowth of β-Mn from the substrate. At energy densities well above threshold, the ϒ-Mn phase forms by separate nucleation and growth from the undercooled melt, and is retained upon rapid solidification. From these results and analyses, we conclude that significant melt undercooling, which may exceed 100 K, can occur during pulsed laser melting of metallic crystals and that the resulting crystalline structure is determined by both thermodynamics and nucleation kinetics.

We have studied laser induced crystallization and amorphization reactions in tellurium and tellurium alloy thin films, which are of interest for use in optical data storage. The writing characteristics of these films are determined by plotting changes in film reflectivity against the parameters of laser power and pulse width. The laser pulse conditions necessary for film melting, crystallization, and amorphization are identified and then checked against the results of a numerical heat diffusion model. In this way we determine the critical quenching rate for formation of the amorphous phase in pure tellurium (2 × 109 C/sec). Minimum crystallization times are determined and found to increase by orders of magnitude with addition of alloying elements such as germanium or selenium. TEM analysis reveals significant differences in the crystallization mechanisms of different alloy systems.

We report on the growth and redistribution of Au clusters caused by nanosecond laser interaction of Aux(TeO2 )1−x thin films with intense excimer laser radiation. This laser-induced phenomenon is studied in a time-resolved manner using transient reflectivity and transmissivity techniques. Structural and compositional changes are investigated using Rutherford Backscattering, XPS depth profiling, x-ray diffraction and conductivity measurements. Our studies indicate that melting of the binary structure initializes segregation, growth and coalescence of Au crystallites in the amorphous TeO2 matrix.

Ion-beam induced epitaxy is shown to be essentially athermal over the temperature range 200-400°C, and to exhibit no dependence on substrate orientation and little dependence on doping in this regime. On the other hand, the formation and propagation of defects during growth and the interaction of the advancing crystal-amorphous interface with implanted impurities is essentially identical for both thermally induced and ion-beam induced epitaxy. These observations lead to a simple model for ion-beam induced epitaxial crystallization in which epitaxial growth is nucleated by defects generated at, or near, the crystal-amorphous interface by the ion beam. Comparisons of ion-beam induced epitaxy and thermally induced epitaxy suggest that the 2.7 eV activation energy associated with the latter process is dominated by a 2.0 eV nucleation step.

The damage produced by high current density ∿l0µA/cm2 implants of 120 keV P+ into <111> and <100> silicon wafers, 500 °m thick, has been investigated in the fluence range 1×l01 5/cm2-l×l016 /cm2 by ion channeling and by transmission electron microscopy. For both orientations the thickness of the damage layers increases with the fluence up to 2×1015 /cm2 and then decreases. The rate of regrowth is a factor two faster for the <100> with respect to the <111> oriented Si crystals. Similar ratios have been found in pre-amorphized samples and irradiated with Kr+ ions in the temperature range 350°C-430°C. The TEM analysis reveals the presence of hexagonal silicon and of twins in small amounts for both orientations. The beam induced epitaxial growth depends also on the species present in the amorphous layer. A comparison between self-annealing and beam annealing in Si <100> preamorphized with Ar+ or P+ shows a noticeable retardation of the growth rate in the presence of Ar+.