How perfect is a perfect crystal Part-per-billion level mosaicity measurements in silicon

A novel method for measuring mosaicity in nearly perfect crystals is presented. The method employs a monolithic Laue case two-crystal diffractometer cut entirely from the crystal under investigation for measuring rocking curve broadening. The advantages of this arrangement as compared to conventional separate crystal diffractometers are greatly enhanced stability and complete elimination of alignment problems. High-order or symmetry-forbidden reflections having a very narrow intrinsic half-width of ∼8 ms of arc are used to obtain high resolution. Measurements on a commercial dislocation-free silicon crystal show line broadening of (54±16)×10^-9 rad at different points on the crystal. The broadening is discussed in terms of mosaic tilt and block size. The method is shown to have an ultimate resolution of a few parts in 10¹⁰ for these quantities.