J. S. Preston

Laser‐induced periodic surface damage and radiation remnants

We report a detailed experimental study of the periodic surface damage induced by laser irradiation at 1.06 μm on nominally smooth Ge samples; we include novel observations of the damage obtained by studying its optical diffraction pattern, which reveals a previously unappreciated richness in the damage structure. The usual ’’surface scattered wave’’ explanation of the damage is criticized; we argue that the damage results from the electromagnetic fields generated by surface inhomogeneities but that, due to the presence of the interface between vacuum and bulk Ge, these fields are not radiative; these ’’radiation remnants’’ are discussed.

Confinement of laser‐generated carriers in semiconductors by induced lattice temperature gradients

The generation of large carrier densities in semiconductors by pulsed laser excitation can be accompanied by a large lattice temperature gradient near the surface. We formulate the coupled transport equations which describe the evolution in space and time of the carrier density and the carrier/lattice temperature below the melting point. A large temperature gradient is seen to influence the diffusion of carriers through the thermoelectric effect (which enhances diffusion) and energy band‐gap gradients (which in most materials counteract diffusion). For high laser intensities we conclude that the latter effect dominates, leading to a region of carrier confinement and enhanced lattice heating near the surface.

Nonequilibrium Phases and Phase Transitions in the Surface Melt Morphology of Laser Irradiated Silicon

The increased use of lasers in material processing is directly related to the coherence of the laser beam. It is the spatial coherence property which allows one to direct and/or focus the modest intensity of laser beams onto small areas of semiconductors, metals or insulators, in such high temperature1 processing applications as annealing and melting, or low temperature2 processing applications, such as catalysis, etching or chemical vapor deposition. The temporal coherence or quasi-monochromaticity of the beam, permits one to initiate state specific effects. The coherence aspect of laser-material processing is often overlooked by many researchers who would wish to regard the laser as little more than a directed energy source or an ultrafast heat-gun. It has become apparent3–5, however, that the coherence of the laser beam is responsible for development of surface patterns in the processed material. These patterns range from random morphologies to highly periodic structures whose spacing is related to the wavelength of light. It is clear that the development of these structures is related to the interference of the incident beam with surface scattered fields associated with surface polaritons or radiation remnants (lateral waves)5. The generation of surface periodic structures or “ripples” can also be viewed4,6 as a stimulated surface scattering process or a stimulated Wood’s anomaly. The specific types of patterns are determined4 by material properties such as surface roughness and dielectric constant and beam properties such as wavelength, coherence, polarization and angle of incidence. Because of the interference effects there is often a significant variation in the surface fields and beam intensities leading to inhomogeneous melting, vapor deposition, etc. In some cases, such as in the production of grating structures, this effect is seen as desirable, but more often, it is regarded as deleterious and efforts are taken to suppress it, usually with a sacrifice of some other aspect of the process. It is clearly desirable to have a fundamental understanding of the influence of beam coherence and polarization on surface processing and to a large extent we have developed such an understanding over the past five years.

Observation of a soret-dufour effect for laser-generated carriers in germanium

Abstract The absorption of high-intensity laser radiation in semiconductors can generate high carrier densities and temperatures. If the absorption coefficient is high, large lattice temperature gradients and, therefore, band-gap gradients are produced. Under certain conditions it is possible for temperature gradients and not density gradients to dominate the carrier diffusion process, with carrier confinement near the surface being possible. We present preliminary evidence of this effect based on 10.6 μm reflectivity probing of carriers generated by 0.53 and 1.06 μm excitation pulses in germanium.