Grain Boundary Types and Relative Grain-boundary Energy in Undoped Silicon Films with Equiaxed and Fibrous Structure

Abstract

The structure and relative grain-boundary energy of undoped silicon films prepared by low-pressure chemical vapor deposition, were investigation in a wide range of film thicknesses by the methods of transmission electron microscope and atomic force microscope, respectively. We used the method of grain-boundary grooves formed at the intersection of grain boundaries with the free surface. It was shown that > 85 nm films are characterized by equiaxial structure with an average grain size of 20 nm. There are no defects inside the grain. In these films, the grain boundaries are high-angle and the relative grain-boundary energy is the lowest. At film thicknesses $\gt85$ nm, the equiaxed structure of the films is transformed into a fibrous structure. Grains in the fibrous structure are twin complexes, which were formed by multiple twinning. Their sizes vary from $\sim100$ nm with a film thickness of 500 nm to 750 nm with a thickness of 2200 nm. Increase in relative grain-boundary energy with increasing film thickness is due to the presence of a large number of low-angle boundaries in the fibrous films and the dislocation structure of grain boundaries. Dislocations have a strong field of elastic stresses, which makes a significant contribution to the increase in relative grain-boundary energy. With an increase in the film thickness up to 2200 nm, a tendency toward a decrease in the relative grain-boundary energy is observed. This is due to relaxation processes in the structure of the films due to the replacement of high-energy low-angle grain boundaries by high-angle grain boundaries with lower energy. We can assume that the mechanism of relaxation processes in films, by analogy with metals, is the slip of dislocations. Sliding dislocations can interact with dual dislocations, and often seat dislocations are formed. The presence of a large number of moving dislocations in the fibrous structure leads to the formation of metatable configurations and an increase in the stability of the film structure.