Density changes and viscous flow during structural relaxation of amorphous silicon

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Abstract
Structural relaxation of amorphous silicon (a‐Si) surface layers made by ion irradiation has been studied during heating using wafer curvature measurements. These measurements, which determine the stress in the amorphous layer, are sensitive to both plastic deformation and density changes. The amorphous layer first expands (0.1%) on heating from room temperature to 250 °C and then densifies (0.1%) on heating further to 500 °C. A larger expansion (≥0.3%) is observed on heating liquid‐nitrogen‐temperature irradiated a‐Si to room temperature. This behavior reveals the existence of two distinct relaxation regimes, and is explained in terms of the annihilation of complementary features of the amorphous covalent network. In addition to density changes, shear deformation was observed during heating the a‐Si layers. This deformation was characterized by a Newtonian shear viscosity of roughly 3×1012 N s/m2. The thermal‐expansion coefficient of the a‐Si was determined to be roughly 6.5% smaller than that of crystalline Si. Stress changes due to crystallization by epitaxial regrowth were observed between 600 and 700 °C and revealed evidence for the existence of large compressive stresses at the amorphous‐crystalline interface.