Mesostructured MTES-Derived Silica Thin Film with Spherical Voids Investigated by TEM: 2. Dislocations and Strain Relaxation

A mesostructured silica thin film with nanosized voids arranged in a body-centered cubic (bcc) array with a slight distortion was studied by transmission electron microscopy (TEM) for line defects. This film was engineered by a preferential solvent evaporation-induced sol-gel and self-assembly process and subsequent pyrolysis to remove its structure-directing agent. Methyl triethoxysilane [MTES, Si(OCH2CH3)3CH3] was the silica precursor. Two types of dislocations were observed from a cross-sectional TEM sample of this non-free-standing film on a Si(001) substrate. One is an edge dislocation; the other is a dislocation dipole. The edge dislocation, with its Burgers vector b ) a[010] and dislocation line direction ê ) [100], was formed by the reaction of two regular dislocations: b ) b1 + b2, while b1 ) (a/2)[1,1,-1] and b2 ) (a/2)[-1,1,1]. The origin of this edge dislocation is related to the tensile strain developed in the film because of film shrinkage during the fabrication; its development is argued to arise from the partial relief of developed strain. A new concept, namely, critical mesostructure thickness for the occurrence of the stress relaxation, is proposed and computed using an elastic strain energy argument. The possible factors for the termination site of the edge dislocation are discussed briefly. The dislocation dipole has the Burgers vectors b ) ((a/2)[-1,1,1] on a (0,1,-1) plane.