The biaxial modulus and coefficient of thermal expansion of ion-beam-sputtered amorphous Si and Ge thin films were determined from curvature changes induced by differential thermal expansion. Viscous flow was measured by stress relaxation and was found to be Newtonian. The viscosity increased linearly with time as a result of structural relaxation, and its isoconfigurational activation enthalpy was 1.8 rtO.3 and 2.6* 1.3 eV for amorphous Si and Ge, respectively. An atomistic model, based on a chain reaction of broken bond rearrangements, is proposed to describe the observation.
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