In any theoretical treatise on the interaction of photons with a crystal field, the field is assumed to be regular and periodic. Practical researchers are only too aware, however, that the optical output can frequently vary significantly in amplitude and spatial quality from point to point within a crystal. Finding the "sweet spot" is an acceptable part of a day's work for an academic researcher but the kiss of death in a production environment. Inspecting crystals for quality, therefore, is critical.
If growth is not properly controlled, the solid/liquid interface first forms striae (shown in this YLiF2 crystal), then breaks down to give solute trails and polycrystals.
Crystal growth involves directional solidification at a rate controlled by an applied, dynamic temperature gradient. The rate of cooling or translation of the temperature gradient must be sufficiently slow to allow impurities or excess native components (non-stoichiometry) to be removed from the region of the advancing solid/liquid interface. If the rate of cooling or translation is too fast, striations can form in the crystal (see figure). Eventually, as the level of impurity or non-stoichiometry in the residual melt increases, the planar growth interface can no longer be maintained; solute trails presage the complete breakdown of single-crystal growth.
Techniques for identifying defects in crystals range from holding them up to the light to laser-induced mass analysis in which volumes of material as low as 10-12 m3 can be laser ionized and chemically analyzed.1 Other intermediate-scale techniques include Twyman-Green interferometry2, scanning electron microscopy, and energy-dispersive analysis by x-rays.
Probably the greatest pitfall in crystal testing arises from complete reliance on any one technique rather than the use of several complementary analysis techniques with extensive cross-checking between the results of each. A classic case is the use of chemical analysis to derive quantitative or semi-quantitative information about alloy composition without experimental data from compositional standards analyzed under similar conditions. Frequently, such standards must be made up separately, which involves time and expense. Without them, however, results can be rendered meaningless due to the fact that the same element in different matrices can produce widely differing ion yields.
Other analytical errors arise from failure to consider the uptake of impurities from the crucible material, particularly elements such as silicon, sodium, lithium, and potassium from natural fused quartz or chlorine from synthetic quartz. In mass spectrometry, cross contamination from recent analyses made in the same instrument is a frequent hazard, especially in the case of techniques dependent on vaporization of a small quantity of the material.
The measurement of non-stoichiometry is a source of concern in materials with volatile components. For example, although we conventionally write the equation
the loss of either cadmium (Cd) or tellurium (Te) from the crystal leads to the creation of vacant crystal lattice sites, so that the formula of the practical crystals grown can range from approximately Cd0.98Te1.02 to Cd1.01Te0.99. Knowledge of the extent of this position in the phase range of the actual composition of a particular crystal is important in determining the growth conditions required to minimize point defects in the crystal lattice.
The most common method to determine deviations from stoichiometry is the use of x-ray diffraction to measure the parameters of the lattice unit cell. In practice, however, the changes in lattice parameter from one extreme of the phase range to the other may only be around one part in 104, so great care must be taken to ensure accurate measurement of diffraction peak maxima to eliminate instrumentation errors and distortions due to neighboring peaks or internal strain in the crystal. oe
1. Aktos Vewrtes, Renaat Gijbels, et al. (Eds.), Laser Ionisation Mass Spectroscopy, John Wiley & Sons Inc., New York (1993).
2. Daniel Malacara (Ed.), Optical Shop Testing 2nd edition, John Wiley & Sons Inc., New York (1992).
3. A. Vere, Crystal GrowthPrinciples and Progress, Plenum Press, New York (1987).
Tony Vere is CEO of The Crystal Consortium Ltd., Glasgow, UK.