Temperature profiles induced by a cw laser beam in a semiconductor are calculated. The calculation is done for an elliptical scanning beam and covers a wide range of experimental conditions. The limiting case of a circular beam is also studied. This calculation is developed in the particular cases of silicon and gallium arsenide, where the temperature dependence of the thermal conductivity has been taken into consideration. Using a cylindrical lens to produce an elliptical beam with an aspect ratio of 20, a 1-mm-wide area of an ion-implanted silicon wafer was annealed in a single scan. The experimental data are consistent with the extrapolation of solid-phase epitaxial regrowth rates to the calculated laser-induced temperatures.
We demonstrate that selective intermixing of GaAs/AlGaAs quantum well heterostructures induced by SiO2 capping and subsequent annealing can be spatially localized on a length scale compatible with the lateral confinement of carriers into quantum wires. Low temperature optical spectroscopy measurements including linear polarization anisotropy analysis show evidence of the formation of one-dimensional subbands. A mechanism involving the ability of the thermal stress field generated in the heterostructure by the patterned SiO2 film to pilot the diffusion of the excess Ga vacancies, which are responsible for the enhanced interdiffusion under SiO2 is suggested to account for the high lateral selectivity achievable with this novel process.
The study of solid-phase epitaxy in ion-implanted amorphized silicon is presented via optical reflectivity measurements. The effect of impurities on recrystallization is studied in detail through accurate measurements of growth rate. Enhancement of the growth rate and decrease of the activation energy are shown. These parameters are studied as a function of the impurity concentration. This phenomenon is understood in terms of an original model which introduces band bending and an electric field at the disordered layer-crystalline substrate interface. The electric field acts on recrystallization through the enhancement of defect migration at the interface. The agreement of the model with experimental results is shown to be excellent, and a parallel is drawn with the phenomenon of enhanced dislocation mobility with doping, which behaves in a strikingly similar way.
The phase transformation induced by a picosecond laser pulse in implanted amorphized silicon has been studied. A single 30-ps pulse at 1.06- and 0.532-μm wavelengths from a mode-locked neodymium:yttrium aluminum garnet laser was used to generate a multiannular (up to five rings) recrystallization pattern on an implanted silicon substrate. A Raman microprobe with a 1-μm spatial resolution was utilized to investigate the annealed spots. These measurements combined with polarized light scattering experiments resulted in a detailed spatial analysis (parallel and perpendicular to the surface) of the recrystallization pattern, that was related to the picosecond laser energy and wavelength. At high incident energies single crystal silicon is observed in the central spot and in the first recrystallized ring of the annealed area.
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