Thermomechanical properties of a-Si:H and a-Ge:H

The coefficient of thermal expansion, biaxial modulus and stress of a-Si:H and a-Ge:H films were investigated as a function of the hydrogen concentration. The thermal expansion and the biaxial modulus were measured by the thermally-induced bending technique, and the stress was determined by a bending beam method. The biaxial modulus decreases as the hydrogen concentration increases, due to the reduction of the coordination number. It was observed that the coefficient of thermal expansion of a-Si:H and a-Ge:H is influenced by the network strain, which is related to the macroscopic stress. For high quality films, which are compressive, the thermal expansion coefficient is higher than that of their crystalline counterparts. The structural changing from the crystalline to the amorphous phase appears to interfere little with the magnitude of the thermal expansion coefficient. (C) 2001 Elsevier Science B.V. All rights reserved.