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The transient grating method has been applied to study both heating and crystallization dynamics of amorphous Ge and GaAs films in the nanosecond time scale. The observed time behavior of the diffracted signals allows measurements of the onset of melting and of the full solidification time. From the model Calculations, we show that the free-carrier diffusion is a predominant process in laser heating under high excitation power and leads to the saturation of carrier density before melting. The heating of amorphous Ge in a limited volume gives evidence of a decrease in melting temperature.