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The primary interaction is the absorption of photons by electrons. In metals free-free transitions increase the energy of the electron gas. In semiconductors and insulators electron-hole pairs are created, if the photon energy exceeds the band gap. If it is less, only multiphoton processes can initiate energy transfer from the light beam. In nearly all solid materials Auger processes and electron-phonon interactions occur on a picosecond time scale for the high density and energy of the carrier gas created by intense short laser pulses. Thus melting and evaporation of the material can occur on this time scale. These processes may be considered as the initial phases in the creation of laser produced plasmas. They have been studied by time-resolved measurements of the complex index of refraction, by electron and ion emission, by second harmonic generation, by electrical conductivity and other techniques. Fast time resolution is essential. The dynamic behavior of atoms and phase transitions in the picosecond and femtosecond regime has been opened up for experimental investigation.
The advent of femtosecond optical pulse techniques has provided new opportunities for the investigation of the dynamical properties of highly excited semiconductors. In this paper we describe recent investigations of delayed Auger heating in Si.
Molecular beam scattering from solid surfaces has long been recognized as a powerful means for investigation of gas-surface reaction dynamics. With the help of the recently developed laser-induced fluorescence and ionization techniques for state-selective detection, one can now measure the angular and velocity distributions of the scattered molecules together with their internal energy distributions. Such measurements fully describe the average energy and momentum exchanges between molecules and surfaces and give thus full information on the dynamics of the interaction. Recently, also the scattering of vibrationally excited NO molecules was investigated. The paper gives a review of new experiments with emphasis on the investigation of the scattering of NO molecules from a pyrographite surface. A simple model using transport properties of the solid is presented which accounts surprisingly well for the observed features.
The recent status of applications of nonlinear optics to surface science is reviewed. The basic theory of wave mixing on a surface layer, and the possibility of using various nonlinear optical processes for surface probing are briefly discussed. Emphasis is on surface second harmonic generation, which is shown with many illustrations to be a rather unique and versatile tool for surface studies.
The collisions of an individual energetic ion in a solid set atoms of the solid in motion and excite electronic states. The moving atoms collide with others to form a collision cascade and the electronic excitation also forms a branched trail around the track of the ion. If the densities of the energy deposition in either of these modes is high the result is called a “spike”. The low density regimes of collision and ionization cascades and evidence for the transition to non-linear high density spikes are summarized. The understanding of the latter is still quite incomplete as is also understanding of the transfer of energy between the kinetic and the electronic modes, particularly at high energy densities.
The thermodynamic interrelation between the amorphous semiconducting (a-sc), the diamond cubic (c-sc) and the liquid metal (lm) states of Ge and Si is reviewed with especial emphasis on the question of the thermodynamic uniqueness of the a-sc state following its structural relaxation. The experience on the occurrence of the direct lm→a-sc transition and its reverse is surveyed and interpreted. This experience, in conjunction with the lm undercooling studies of Devaud and the author, indicates that the formation, in the metastable regime, of a-sc from lm results from preferential growth rather than preferential nucleation.
This review examines recently observed phenomena associated with amorphisation and crystallisation of silicon under ion bombardment and furnace annealing. Ideally, heavy ion damage should completely amorphise the silicon surface layers so that the underlying crystal can provide a perfect template for subsequent epitaxial growth. However, in practise the ion bombardment and annealing behaviour can be decidedly more complex. During ion bombardment of silicon, several correlated processes can take place depending on the target temperature and the precise bombardment conditions. These processes include: defect production; amorphisation; diffusion and segregation of defects and impurities; and ion-beam-induced (epitaxial) crystallisation. During subsequent heat treatment, amorphous layers can exhibit anomalous impurity diffusion and precipitation effects, nucleation of random crystallites, and solid phase epitaxial growth. In addition, the kinetics of the epitaxial growth process are sensitive to the type and state of implanted impurities present in the silicon. The competition between random nucleation and epitaxy is also dominated by impurity effects. Finally, correlations between all of these phenomena provide i) considerable insight into impurity and defect behaviour in amorphous and crystalline silicon, and ii) a better understanding of the amorphous to crystalline phase transition, including mechanisms of solid phase epitaxial growth.
Nanosecond pulsed laser irradiation of silicon results in the melting of the surface with subsequent solidification from a crystalline substrate. In the presence of impurities or alloys, the solidification dynamics are greatly affected by the nature of the impurities. Five classes of materials have been investigated: amorphous Si, mutually soluble alloys, high solubility impurities, low solubility impurities, and compound forming materials. Solidification of each of these classes of materials is discussed. Several anomalous kinetic regimes are observed, including explosive crystallization, surface nucleation of solid, and internal nucleation of melt. These results are interpreted in terms of thermodynamic modifications of the melting temperature in the alloy regions.
Nanosecond resolution time-resolved x-ray diffraction measurements of thermal strain have been used to measure the interface temperatures in silicon during pulsed-laser irradiation. The pulsed-time-structure of the Cornell High Energy Synchrotron Source (CHESS) was used to measure the temperature of the liquid-solid interface of <111> silicon during melting with an interface velocity of 11 m/s, at a time of near zero velocity, and at a regrowth velocity of 6 m/s. The results of these measurements indicate 110 K difference between the temperature of the interface during melting and regrowth, and the measurement at zero velocity shows that most of the difference is associated with undercooling during the regrowth phase.
We report time-resolved X-ray absorption and extended X-ray absorption fine structure (EXAFS) measurements on amorphous silicon under nanosecond pulsed-laser irradiation. Each measurement was performed with one laser shot in the X-ray energy range from 90 to 300 eV. An X-ray absorption spectrum for induced liquid Si (liq*Si) was first observed above an energy density of 0.17 J/cm2. It differs significantly from the spectrum for amorphous Si and characteristically shows the disappearance of the Si-L(II,III) edge structure at around 100 eV. This phenomenon is interpreted in terms of a significant reduction in the 3s-like character of the unfilled part of the conduction band of liq*Si compared to that of amorphous Si. This is the first direct evidence that liq*Si has a metallic-like electronic structure. Timeresolved EXAFS results are also discussed briefly.
Real-time measurements of the molten layer thickness and simultaneous measurements of the melt duration at the surface reveal that melt nucleates internally when Si implanted with low solid-solubility impurities such as In is melted with a 30 nsec laser pulse at 694 nm. Internal nucleation of melt was observed for all energy densities examined. Furthermore, at energy densities insufficient to melt the entire amorphous layer, internal nucleation of melt is followed by an explosive-like process in which a buried molten layer propagates toward the irradiated surface.
The behavior of pulsed laser-induced “explosively” propagating buried molten layers (BL) in ion implantation-amorphized silicon has been studied in a time- and spatially-resolved way, using nanosecond time-resolved reflectivity measurements, “Z-contrast” scanning transmission electron microscope (STEM) imaging of implanted Cu ions transported by the BL, and helium ion backscattering measurements. Infrared (1152 nm) reflectivity measurements allow the initial formation and subsequent motion of the BL to be followed continuously in time. The BL velocity is found to be a function of both its depth below the surface and of the incident KrF laser energy density (El); a maximum velocity of about 14 m/s is observed, implying an undercoolingvelocity relationship of about 14 K/(m/s). Z-contrast STEM measurements show that the final BL thickness is less than 15 nm. Time-resolved optical, TEM and ion backscattering measurements of the final BL depth, as a function of E1, are also found to be in excellent agreement with one another.
During cw laser induced melting, over a large range of intensities, silicon films phase separate into patterns of coexisting solid and molten regions. We have identified several distinct and reproduceable morphologies of this inhomogeneous coexistence region ranging from random lamellae (“amorphous”) structures to periodically ordered (“crystalline”) strips. The type of morphology formed is a function of laser intensity and spot size and these parameters can be viewed as constituting the two axes for a steady state “phase diagram” of the structures. “Phase transitions” between these structures occur for small changes of the experimental parameters with the fraction of liquid being an order parameter.
Time-resolved reflectivity measurements of silicon and germanium have been made during pulsed KrF excimer laser irradiation. The reflectivity was measured simultaneously at both 1152 and 632.8 nm wavelengths, and the energy density of each laser pulse was monitored. The melt duration and the time of the onset of melting were measured and compared with the results of melting model calculations. For energy densities just above the melting threshold, it was found that the melt duration was never less than 20 ns for Si and 25 ns for Ge, while the maximum reflectivity increased from the value of the hot solid to that of the liquid over a finite energy range. These results, along with a reinterpretation of earlier time-resolved ellipsometry measurements, indicate that, during the melt-in process, the near-surface region does not melt homogeneously, but rather consists of a mixture of solid and liquid phases. The reflectivity at the onset of melting and in the liquid phase have been measured at both 632.8 and 1152 nm, and are compared with the results found in the literature.
We study the composition, stress and structure variations across periodic surface undulations produced by pulsed laser illumination of semiconductors, by explosive crystallization of amorphous films, and by laser-assisted CVD. These variations are mapped out with a one micron spatial resolution using a Raman microprobe. Similarities and differences between the three cases are pointed out. These results are also compared to those obtained by deliberately exposing the sample to interfering beams.
We describe what appears to be a first attempt to melt and recrystallize macroscopic (10-20 μm deep) silicon mechanical damage that is induced from wafer modification such as slicing and lapping. Recrystallized surfaces appear mirror shiny, with significantly improved surface smoothness, as compared to the coarse texture of damaged surfaces. The crystallinity also appears good in general. Through the depth of melt were observed indications of impurity migration, probably caused by accumulated segregation at the advancing solid-liquid interface. Recrystallized surfaces, despite their smoothness, remain topologically uneven as a result of lateral mass transport. In addition, the extensive heat required to melt thick layers of silicon causes slip dislocations.
We report new optical and structural properties of p-type GaAs that result from the absorption of high-intensity 10.6 μm radiation. Prior to the onset of surface melting, we find that the absorption coefficient decreases with increasing intensity in a manner predicted by an inhomogeneously broadened two-level model. As the energy density of the CO2 laser radiation is increased further, the surface topography shows signs of melting, formation of ripple patterns, and vaporization. Auger spectroscopy and electron-induced x-ray emission show that there is loss of As, compared to Ga, caused by the melting of the surface. Using plain-view TEM we find that Ga-rich islands are formed near the surface during the rapid solidification of the molten layer. Auger and SIMS measurements are used to study the incorporation of oxygen in the near-surface region, and the results show that oxygen incorporation can occur for GaAs samples that have been irradiated in air.
The time resolved reflectivity technique is shown to give informations on the amorphous-crystalline interface evolution during solid phase epitaxial (SPE) regrowth in semiconductors. Two specific cases have been treated here. The first case is encountered in laser annealing when the growth front exhibits a curvature due to the combination of an inhomogeneous temperature distribution and a steep dependence of SPE growth rates with temperature. A computer simulation is carried out from an analytical determination of the laser induced temperature profiles to shape up the resultinq reflectivity signal. The second case is obtained when there is an evolution of interface roughness during regrowth. In order to simulate this effect a simple model is developed to treat the influence of diffusion of the reflected light at the interface, on the reflectivity modulation during SPE regrowth.
We have investigated the incorporation of oxygen into heavily doped silicon during UV excimer laser irradiation. For the case of repetitive laser irradiations in air, we observe that the amount of oxygen incorporated into Si depends markedly on the dopant. For As and Sb implanted silicon, there is no anomalous oxygen incorporation. By contrast, increasing amounts of 0 are incorporated into In implanted silicon as a function of number of laser shots. The incorporation of 0 is associated with degradation of the optical and structural properties of the surface, and a deep diffusion of the dopant. This behavior is believed to be partly related to specific chemical reactions between oxygen and indium present in the surface at high concentrations as the result of dopant segregation during solidification.
Sputter deposited TiO2 and ZrO2 films on silica have been characterized with regard to thickness, phase purity, and residual film stress using nondestructive Raman spectroscopic techniques. Irradiated anatase coatings exhibit vibrational features that irreversibly shift to higher frequencies, suggesting an increase in localized stress. Catastrophic damage is manifested by the appearance of a higher density rutile phase. Rutile coatings examined after irradiation show only intensity changes, while amorphous coatings were found to rapidly crystallize under high-pulse energy irradiation. Results of these studies are compared with equilibrium Raman measurements of thermally-induced transformations in these materials.