A novel regime of crystal growth and segregation has been observed. Amorphous Si layers were uniformly doped with Au and epitaxial crystallization was induced in the temperature range 250–420°C using 2.5 MeV Ar ion irradiation. The Au segregation at the amorphous/crystal interface is analogous to behavior at liquid/solid interfaces except that the interfacial segregation coefficient of 0.007 at 320°C is independent of velocity between 0.6 and 6A/sec. This process results in the trapping of Au in crystalline Si at concentrations some ten orders of magnitude in excess of equilibrium concentration.

The amorphous to polycrystalline transformation of silicon implanted with high doses of In, Bi, Ga, and Sn is investigated. Each of these elements forms a low temperature eutectic with crystalline silicon and the details of the phase transformations in these systems are found to be very similar. A general model for the transformation based on the nature of the binary solutions is presented.

Z-contrast scanning transmission electron microscopy has been used to study the connection between dopant precipitation and phase transformation in high dose In+ and Sb+ implanted Si. In the case of In, the observations confirm a heterogeneous nucleation model. Images of the precursor precipitates give the first measurement of the diffusion coefficient in amorphous Si, with an enhancement of 107 over tracer crystalline values. With Sb+ implants enhanced homogeneous nucleation is observed. The connection between these results and the transient enhanced diffusion observed in crystallized Si is discussed.

Microcrystalline Si, as produced by explosive crystallization of an amorphous Si layer on (100) Si, shows a two-stage annealing behaviour. Initially, solid phase epitaxial regrowth occurs very rapidly at temperatures at, or above 800°C. After a few seconds, the regrowth rate slows down to the value typical for alignment of poly-Si. Solid phase epitaxial regrowth of microcrystalline Si is suggested to be strongly dependent on grain size and structure.

Ion implanted, amorphous layers on (100) GaAs have been recrystallised epitaxially by 1.5 MeV Ne+ irradiation at temperatures between 75°C and 135°C. The extent of regrowth essentially increases linearly with ion fluence in the early stages of crystallization and the process is characterized by an activation energy of 0.16eV, about an order of magnitude lower than that for thermal - only epitaxy. Beam annealing produces dislocations in the underlying GaAs crystal, the density of which increases with depth up to the Ne+ ion range.

A method of producing microcrystalline material from thin films of hydrogenated amorphous silicon was investigated. Exposure to gamma and neutron radiation, and silicon self ion implantation were used to induce nucleation in the amorphous material. According to the preliminary results, neutron irradiation represents a most promising method for promoting crystallization.

We have measured the pressure dependence of the solid phase epitaxial growth (SPEG) rate of self-implanted Si (100) by using the in-situ time-resolved reflectivity technique [1] in a hightemperature and high-pressure diamond anvil cell (DAC). With fluid argon as the pressure transmission medium, a clean and perfectly hydrostatic pressure environment is achieved around the sample. The external heating geometry employed in the DAC provides a uniform temperature across the sample. At temperatures in the range of 530 – 550 °C and pressure up to 50 kbars (5 GPa), the growth rate is enhanced by up to a factor of ten over that at 1 atmosphere pressure. The results are characterized by a negative activation volume of approximately −3.0 cm3/mole (−27% of the atomic volume). These preliminary results show a significantly weaker pressure dependence than does the previous work of Nygren et al. [2], who found an activation volume of −8.7 cm3/mole. The implications of these results for the nature of the defect responsible for thermal SPEG and irradiation enhanced SPEG is discussed.

The microstructure of high-energy (0.5–6.0 MEV) As-ion implanted Si and rapid thermal annnealed (RTA'd) Si has been studied by transmission electron microscopy (TEM). The implantations formed buried amorphous layers that recrystallized during RTA at different temperatures and became either single crystal or polycrystalline depending on their implation energy and fluence. At energies > 2.5 MeV and fluences < 1015 cm−2, recrystallization occurred below 400°C and regowth was single crystal. At an energy of 6 MeV and fluence of 5 × 1015 cm−2 recrystallization occurred above 600°C and regrowth was polycrystalline. When the implantation energy and fluence were reduced to 0.5 MeV and 2 × 1014 cm−2, respectively, recrystallization occurred above 600°C and regrowth was polycrystalline. The above results are explained by both the formation mechanisms of amorphous Si resulting from ion implantation and the structural order of a-Si.

We have studied the solid phase epitaxial regrowth (SPER) of implantation (31P+11B+ (73Ge+ preamorphized)) amorphized silicon in the temperature range 500–600°C induced by Rapid Thermal Annealing (RTA) using Rutherford Backscattering and channeling measurements (RBS). Our results show rate enhancements (≃ 3.5–6.5) of the velocities of regrowth in all studied cases with respect to literature-reported values for furnace-induced SPER. Also, the ratio VB/VP (velocity of regrowth in the presence of boron with respect to phosphorus) gives a value of approximately 3 in both RTA and furnace-induced kinetics. These results are explained by a model which takes into account the role of electrically-active interfacial defect sites during SPER.

The atmospheric effects of oxygen and water vapor on the epitaxial regrowth behavior of ion-implanted CaTiO3 have been studied. Cross-sectional transmission electry microsgopy (TEM) samples prepared from crystals implanted with 1(1015) Pb/cm2 at 540 KeV, were annealed at 450–650°C for 40 – 80 minutes, in environments containing different percentages of oxygen and water vapor. Regrowth was studied for implantation directions along both the <010> and <100> crystallographic directions of this orthorhombic Perovskite structure, space group Pcmn. TEM on unannealed crystals indicated the presence of an amorphous region extending approximately 190 nm in from the edge of the sample as a result of ion-implantation. Annealing produced an epitaxial regrowth of the amorphous region. The regrowth process began at the original amorphous/crystalline interface and proceeded outward to the surface of the sample. It was found that the epitaxial regrowth rate increased as the amount of water vapor in the annealing atmosphere was increased. In addition, it was observed that the amount of oxygen present during annealing did not alter the rate of recrystallization in this system.

We describe investigations of carrier dynamics in GaAs and AlGaAs semiconductors and thin gold films using femtosecond optical techniques. Transient absorption saturation measurements in semiconductors permit an investigation of the fundamental scattering processes of optically excited carriers. Measurements of energy relaxation provide evidence for transient nonthermal carrier distributions. Studies in metals allow an investigation of nonequilibrium electron and phonon effects since the electrons are decoupled from the lattice on a femtosecond time scale. Nonequilbrium temperatures and energy transport in the electron gas have been observed.

Melt-in velocities of picosecond laser-heated Si and GaAs were obtained from optical reflectivity measurements. We observed a velocity increase from about 100 m/s near the melting threshold to many hundred m/s at higher fIuences. With a new technique for picosecond time-resolved observation of atomic desorption we have been able to resolve the evaporation of Ga.

The complex dielectric function of molten silicon produced after picosecond illumination is found to be well described by a Drude model from 1.1 eV to 3.5 eV. Close to the melting temperature, we obtain ωp = 2.50 1016rad/s and τ = 212 10−18s. Transiently, the liquid temperature can exceed the melting temperature or even the boiling temperature Tb. We observe this transient heating and model it with a relatively simple computer code which includes superheating of the liquid above Tb. These measurements are possible thanks to a novel pump and probe technique.

We have employed pump-and-probe techniques coupled to ellipsometry to measure the transient complex index of refraction at 1.064 μm of higly oriented pyrolitic graphite (HOPG) interacting with 20 ps, .532 μm laser pulses. When the laser pump fluence exceeds the threshold value for melting, measurements indicate a substantial decrease of both real and imaginary parts of the index of refraction, thus confirming that molten graphite becomes less metallic. Measurements provide also direct evidence of the insensitivity of our picosecond results to evaporation from the irradiated surface.

Nanosecond resolution visible (633 nm) and near-infrared (1152 nm) reflectivity measurements have been used, together with transmission electronmicroscopy (TEM), to study pulsed KrF (248 nm) laser melting and subsequent solidification of thick (190–410 nm) amorphous (a) silicon layers. The measurements cover the entire laser energy density (El) range between the onset of melting (∼ 0.12 J/cm2) and the completion of epitaxial crystallization (∼1.1 J/cm2). Four distinct El-regimes of melting and solidification are found for the 410-nm thick a-Si layers. For El > 0.25 J/cm2, the time of formation, velocity and final depth of “explosively” propagating undercooled liquid layers were measured in specimens that had been uniformly implanted with Si, Ge, or Cu. TEM shows that the “fine-grained polycrystalline Si” produced by explosive crystallization (XC) actually contains large numbers of disk-shaped Si flakes that have largely amorphous centers and are visible only in plan view. The optical and TEM measurements suggest (1) that flakes are the crystallization events that initiate XC, and (2) that lateral heat flow (parallel to the sample surface) must be taken into account in order to understand flake formation. Results of new two-dimensional (2-D) model calculations of heat flow and solidification are presented. These calculations confirm the importance of 2-D heat flow and crystallite growth early in the solidification process. For 0.3 4 < El > 1.0 J/cm2, pronounced changes in both the shape and the duration of the reflectivity signals provide information about the growth of polycrystalline grains; this information can be correlated with post-irradiation plan and cross-section view TEM microstructural measurements.

Following pulsed laser melting, supercoolings of 505 K in Si and 430 K in Ge were observed prior to bulk nucleation. These large supercoolings are obtained because of the extremely high thermal quench which follows laser irradiation. Nucleation rates were estimated to be ∼1029 events/m3/s. Assuming that homogeneous nucleation was achieved, surface energies are estimated to be 0.34 J/m2 for Si and 0.24 J/m2 for Ge. These results are in reasonable agreement with traditional homogeneous nucleation experiments sensitive to rates of only ∼1010 events/m3/s. This laser melting technique is applicable to nucleation studies in a wide variety of materials.

Nanosecond-resolution x-ray diffraction has been used to measure the interface and lattice temperatures of silicon during rapid, pulsed-laser induced melting and regrowth in silicon. Measurements have been carried out on <100> and <111> oriented silicon using the (100) and (111) reflections to measure the thermal strain during 30 ns, 1.1 J/cm2 KrF laser pulses. The results indicate overheating to be low (< 2 K/m/s) for both orientations with undercooling rates of 5.6 K/m/s and 11.4 K/m/s for the <100> and <111> orientations, respectively. Observations of higher than expected temperature gradients below the liquidsolid interface have been discussed in terms of restricted heat flow under high gradients.

We report transient conductance measurements of liquid/solid interface velocities during pulsed laser melting of amorphous Si (a-Si) films on crystalline Si (c-Si), and a more accurate, systematic procedure for analyzing these measurements than described in previous work [1]. From these analyses are extracted relations between the melting velocities of a-Si and c-Si at a given interface temperature, and between the temperatures during steady-state melting of a-Si and c-Si at a given interface velocity.

A transient resistance technique has been developed which allows monitoring the temperature of a thin film sample at a fixed depth from the surface following pulsed laser irradiation. The technique utilizes the temperature dependence of a thin, electrically insulated, semiconducting or metallic layer. Temperature determinations with nanosecond resolution, an absolute accuracy of ±50 K, and a relative accuracy of ±5 K are demonstrated. Combined with simultaneous interface position and velocity measurements, the undercooling at the interface during rapid solidification may be obtained. Preliminary results using this technique during the solidification of thin Ge films are presented.

Consequences of the need to transport density to or from a fast-moving liquid-solid interface are examined. There can be a severe limitation of the freezing velocity and a change in the amount of heat which must be conducted. The latter effect produces a net latent heat which can be significantly different from the equilibrium value and even change sign.