Early stages of melting in Si under nanosecond laser pulse irradiation: A time-resolved study

Abstract

Time-resolved reflectivity (TRR) measurements are performed in crystalline Si under UV and visible wavelength irradiation. The former are carried out with ArF excimer laser pulses whereas the latter are performed in micron-sized areas irradiated with Ar+ laser pulses by means of a novel experimental setup. It is the first time that TRR measurements in the nanosecond regime are performed in micron-sized irradiated areas although they are very suitable to characterize processes in phase change optical storage and microelectronics applications. The energy density melting thresholds at both Ar+ and ArF laser wavelengths are determined. The reflectivity values obtained for pulse fluences just above the melting threshold show that melting proceeds inhomogeneously being the near-surface region formed by a mixture of solid and liquid phases without a well-defined interface. The comparison of the results obtained with uv and visible irradiation indicates that inhomogeneous melting is a general phenomenon which does not depend on the irradiation wavelength. It is present in the early stages of the melting process and its origin is related to the phase nucleation process itself. As the laser fluence is increased, the evolution of the melt duration exhibits a ‘‘different’’ behavior which is related to the formation of a homogeneous molten layer on top of the surface.