Orientational and long range effects in epitaxy

The mechanism of orientation in epitaxy has been studied on the electron microscope scale using as a model process the crystallization of silver chloride on ferroelectric triglycine sulphate (TGS) crystals both directly on the surface and via interfacial amorphous selenium layers. At the initial deposition stage, the AgCl crystals are distributed at random with respect to the TGS crystal surface. Orientation begins only at the coalescence stage, after the AgCl crystals have reached certain critical sizes. The AgCl crystals are at first positioned at an angle of 30° (±3°) to one another on the domains of both charge signs. Then, on the positive domains, reorientation of crystals by rotation solely in one direction takes place, i.e. for the first time a transition from a random distribution of initially generated crystals at first to one, and later to the other, azimuthal orientation is detected. Crystallization of silver chloride via amorphous Se layers does not cause any change in the mechanism of orientational effects. As a result, on the domains of both signs, including those coated with amorphous layers, network structures of AgCl crystals in an identical azimuthal orientation occur. The orientational and long range effects are explained in terms of an electrically active network structure existing in real crystal substrates. Nucleation takes place on the point defects of the substrate surface and on their “copies” on the surface of interfacial layers. The orientational effects are accounted for by linear elements of the network structure of the crystal substrates which interact with the particles of the crystallizing material after the latter reach certain critical sizes. Epitaxy as a whole is treated as a matrix-replicating process programmed in the electrically active real structure of the substrate.