DENSITY CHANGES AND VISCOUS-FLOW DURING STRUCTURAL RELAXATION OF AMORPHOUS-SILICON

Structural relaxation of amorphous silicon (a-Si) surface layers made by ion irradiation has been studied during heating using wafer curvatu re measurements. These measurements, which determine the stress in the amorphous layer, are sensitive to both plastic deformation and densit y changes. The amorphous layer first expands (0.1%) on heating from ro om temperature to 250-degrees-C and then densifies (0.1%) on heating f urther to 500-degrees-C. A larger expansion (greater-than-or-equal-to 0.3%) is observed on heating liquid-nitrogen-temperature irradiated a- Si to room temperature. This behavior reveals the existence of two dis tinct relaxation regimes, and is explained in terms of the annihilatio n of complementary features of the amorphous covalent network. In addi tion to density changes, shear deformation was observed during heating the a-Si layers. This deformation was characterized by a Newtonian sh ear viscosity of roughly 3 X 10(12) N s/m2. The thermal-expansion coef ficient of the a-Si was determined to be roughly 6.5% smaller than tha t of crystalline Si. Stress changes due to crystallization by epitaxia l regrowth were observed between 600 and 700-degrees-C and revealed ev idence for the existence of large compressive stresses at the amorphou s-crystalline interface.