Reflectivity and transmission of amorphous thin films at the melting temperature

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Abstract
A method to evaluate the evolution of the optical properties (reflectivity and transmission) of thin solid films as a function of temperature has been developed. A pulsed Ar+ laser focused on the sample surface to a 1/e‐beam radius of 4 μm is used as a local heat source while the optical properties of the irradiated surface are monitored in real time by means of another laser focused to 1.7 μm. The use of laser pulses combined with real time optical measurements provides a suitable means to determine accurately the optical properties of thin‐film materials for temperatures up to the melting point. In addition, the irradiation pulse can be sufficiently shortened to avoid crystallization prior to melting in the case of amorphous films and therefore the optical properties of amorphous films up to the melting point can be also determined. The analysis of the optical transients with a simple thermal model allows either the determination of the thermal conductivity of a transparent substrate if the film melting temperature is known or the film melting temperature if the thermal properties of the substrate are well characterized. The results obtained in Ge and Si amorphous thin films prove the accuracy of the method and the melting temperature of amorphous Ge is determined to be 987 K.