Crystal orientation
Under crystal orientation is defined as the indication of how a particular crystal or even just one of its surfaces with respect to its internal structure is oriented, ie whether its outer surface parallel to one of (for example) cube faces of its unit cell is located, or even the whole crystal as such is oriented so on. The specification is usually expressed using Miller indices , for example (111) or (110) planes (and orientations parallel or perpendicular to them).
Specifying an orientation is only useful for a single crystal . So it doesn't just have to be a solid, but also a consistently uniformly oriented body. In the case of multicrystalline solids , the totality of the orientations is called texture . B. can specify as orientation density distribution function.
origin
There are two main sources of single crystal material with a defined orientation: natural crystals with an obvious orientation (example: NaCl crystals in the cubic crystal system, which also crystallize as macroscopic cubes) or single crystals artificially produced with the help of seed crystals, especially with the Czochralski process . Another manufacturing process is the zone melting process .
Measurement
If there is a single crystal of unknown orientation, there are various ways of determining its exact orientation.
In the case of crystals produced using the Czochralski process, the orientation is recognizable by the vertical seams on the outside, which clearly stand out from the actually round crystal body. Due to the constant rotation in this manufacturing process, the crystal becomes predominantly round, cylinder-symmetrical, but new atoms are particularly well deposited in the excellent directions of symmetry, so that an additional application takes place here. Depending on the orientation of the vertical axis of rotation, i.e. the [100], [110], [111] direction in the crystal, three or four seams are formed on the circumference.
Quantitative measurements for orientation are carried out using X-ray methods, especially the Laue method . For the smallest crystals, electron microscopic diffraction methods are used, such as electron backscattering , Kossel or Kikuchi diagrams .
A more qualitative measuring method is to etch the polished crystal surface . So-called etch pits form . These have the shape of upside-down hollow pyramids, depending on the orientation of the surface, three or four-fold, i.e. with a triangular or square (rectangular) base. If you don't want to etch away too much, you have to do this examination with the microscope.
application
Crystals with a defined orientation are widely used in quartz crystals . Here it is decisive for the temperature constancy of the oscillation frequency and thus for the long-term accuracy of clocks built with it, for example. The " AT cut " is one of the preferred configurations.
In science, single crystals of different orientations are required when taking measurements to determine the details of the ribbon model of the material used. If spectroscopy is carried out with polarized light in a known orientation for the orientation of the crystal material, band properties can be determined in the individual directions of symmetry of the material.
Knowledge of the orientation of crystals plays a major role in technology. The base material ( substrate ) in semiconductor technology are high-purity silicon monocrystals that are processed in the form of thin disks ( wafers ). For different processing technologies it is important to know the crystal orientation. For example, when doping by ion implantation, a preferred direction can develop when the ions traverse the substrate, the so-called grid guide effect . This prevents an exact process control, since the profile of the penetration depth can no longer be precisely calculated. To remedy this, the substrate can be tilted slightly, which means that only the calculable scattering of the ions in the crystal is effective - standard (100) silicon wafers are tilted by approx. 7 °, for example, or thin layers of silicon dioxide are applied as an alternative. Another example is the crystal orientation-dependent wet etching of silicon with potassium hydroxide solution in microsystem technology . Different mechanical elements can be produced here depending on the crystal orientation of the substrate or its surface and masking.
literature
- Charles Kittel: Introduction to Solid State Physics . 14th edition. Oldenbourg, 2005, ISBN 3-486-57723-9 .
- Werner Schatt, Hartmut Worch: Materials Science . 9th edition. Wiley-VCH, 2003, ISBN 3-527-30535-1 .