Amorphized Si has been irradiated using a 7.5-ns frequency-doubled neodymium:yttrium aluminum garnet laser. For low energy density pulses, time-resolved reflectivity measurements and Rutherford backscattering spectrometry of Cu implantation profiles show that the melted layer solidifies from the surface as well as from the liquid-solid interface. From interferences in the reflectivity, growth from the surface is found to occur at a velocity of 1.5 m/s. At higher energy densities sufficient to obtain epitaxial regrowth of the amorphous layer, solidification from the surface does not occur.
We have investigated the temperature dependence of the photoluminescence (PL) decay kinetics of a series of GaAs/AlAs quantum-well structures where the GaAs thickness was kept constant at 25 Å and the AlAs was varied between 41 and 19 Å. In these structures the band alignment is type II and the dominant photoluminescence process at 4 K is due to recombination of excitons involving electrons confined at the AlAs X point and holes in the GaAs. At 4 K on the low-energy side of the zero-phonon type II transition the PL decay is a single exponential over at least two decades. The time constant of this decay is a strong function of the AlAs layer thickness. The variation of this decay time is described by a change in the oscillator strength of the type II process due to the change in the mixing between the Xz (AlAs) electron states and the Γ (GaAs) electron states. At higher temperatures (T>15 K) the photoluminescence intensity and the decay time decrease very rapidly with increasing temperature. This is due to the increased influence of nonradiative processes as the type II excitons become delocalized.
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