Texture and Preferred Orientation

Abstract

The distribution of the crystallite orientations in a polycrystalline thin film is only rarely isotropic. On the contrary, in a large number of studies a certain crystallographic direction [hkl] was found to be preferentially oriented with respect to the sample reference frame. This anisotropy of crystallite orientation is named texture or preferred orientation and both terminologies will be used interchangeably for denoting the phenomenon. The effect may easily be recognized in a symmetric θ/2θ diffraction pattern by a pronounced enhancement of certain Bragg ref lections I hkl -and reduction of others -when compared with a powder pattern of randomly oriented grains. Since many physical properties of polycrystals sensitively depend on the distribution of grain orientation, this structural anisotropy is directly mapped into a property anisotropy and the issue thus is a highly relevant one for technological applications.Techniques for the investigation of textures were initially developed for bulk metallic materials like steel plates and wires that were subjected to forming processes like rolling, cold-working or deep-drawing and subsequent annealing that all have pronounced effects on the crystallite orientation distribution and the mechanical properties of macroscopic work pieces. Accordingly, many notations and methods in the field are related to their metallographic roots. The evaluation of texture in thin polycrystalline films started with retardation, but may now routinely be applied because of the development of measuring and analysis tools over the last few decades.The full solution to the texture problem is given by the orientation distribution function (ODF). This distribution function describes the density of orientations that transform the sample reference frame {s i } into the reference frames of individual crystallites {c i } j as shown schematically in Fig. 5.1. Most texture investigations of thin films, however, are not concerned with the determination of the ODF, but make use of simplified methods like the determination of texture factors, the measurement of pole figures of selected Bragg peaks, ω scans (rocking curves) or φ scans. By application of these techniques the thin-film grower has initial quantitative measures that allow for the optimization of orientation-dependent properties and these will be introduced in the following sections. In addition, the more elaborate techniques of determining the ODF are presented, since the full solution of texture is required if orientational averages of physical properties are to be calcu- Texture and Preferred OrientationThin Film Analysis by X-Ray Scattering. M. Birkholz