Lattice structure predetermines physical properties of crystals and crystalline perfection plays a major role in many semiconductor and optical applications. That explains the special attention given to this stage of quality control and the variety of methods used to check crystals for internal defects, namely: - visual inspection under focused white light; - visual inspection with a polariscope; - visual inspection through a microscope; - X-Ray diffraction.

Crystalline defects classification

Name of defect	Description	Method of control	Notes
Inclusion	a particle of alien material in the bulk of a crystal	1. Visual inspection under focused white light 2. Visual inspection through a microscope	Method 2 is used when extra high quality material is required
Block	a crystal part whose orientation in different from the orientation of the whole crystal	1. Visual inspection under focused white  light 2. Visual inspection with a polariscope
Block mark	a boundary between parts of the crystal with different orientations	1. Visual inspection under focused white light. 2. Visual inspection with a polariscope
Twin	a crystal part whose lattice is a direct reflection of the lattice of the whole crystal	1. Visual inspection under focused white light. 2. Visual inspection with a polariscope
Slip line	a trace of plastic deformation of the crystal when atomic planes slip over each other. It looks like a straight line (stria) or several lines that can be revealed after polishing or annealing.	1. Visual inspection under focused white  light. 2. Visual inspection with a polariscope
Bubble	a pore of vacuum space or gas in the bulk of a crystal which appears during crystallization	1. Visual inspection under focused white light. 2. Visual inspection with a polariscope. 3. Visual inspection through a microscope	Bubbles are always present is some II-VI crystals, e.g. ZnTe Method 3 is used when extra high quality material is required
Microbubble	a bubble under 50 µm in diameter	1. Visual inspection under focused white  light 2. Visual inspection with a polariscope 3. Visual inspection through a microscope	Bubbles are always present is some II-VI crystals, e.g. ZnTe Method 3 is used when extra-high quality material is required
Macrobubble	a bubble over 50 µm in diameter	1. Visual inspection under focused white  light; 2. Visual inspection with a polariscope	Bubbles are always present is some II-VI crystals, e.g. ZnTe
Bubble line	a gathering of bubbles extending along the crystallization front, the distance between the bubbles in less than 2 mm	1. Visual inspection under focused white light. 2. Visual inspection with a polariscope 3. Visual inspection through a microscope	Bubbles are always present is some II-VI crystals, e.g. ZnTe Method 3 is used when extra-high quality material is required
Low grain boundary (lineage)	a boundary between parts of the crystal with slight orientation differences (up to 2 degrees)	1. Visual inspection with a polariscope 2. X-Ray diffraction	The defect is acceptable for LED and some optical applications Method 2 is used for specific purposes
Soft low grain boundaries (soft lineage)	a boundary between parts of the crystal with slight orientation differences (a few minutes)	1. Visual inspection with a polariscope 2. Comparison of an interference picture of the tested crystal with an interference picture of an etalon crystal of the same thickness +/-10 mm and dislocation density of 5000 / cm2 3. X-Ray diffraction	The defect is acceptable for LED, RF and most optical applications. Method 3 is used for specific purposes

 

Visual inspection under focused light is carried out with the help of a laboratory lamp (power 75-100 Watt) with a slit-shaped aperture, or a laser, with a fiber-optic light guide. The inspected sample is wetted with an index-matching fluid (glycerin) to make the sample translucent and improve visibility. A trained inspector scans the total bulk of the crystal moving the lamp or the laser along the total surface of the sample. Bubbles, inclusions and other defects in the crystal absorb, reflect, or disperse the light beam. The inspector can detect bubbles and inclusions of up to 50 microns with an unaided eye. To see micro-bubbles the sample is scanned through a microscope with a magnification of ten-fold and over (left image). 



Visual inspection with a polariscope is used predominantly to detect lattice distortions (blocks, twins, lineage). It requires specific training and solid experience. The sample is wetted with an index-matching fluid and inspected with the help of a polarized light beam (Polariscope PCS-250M). The interference pattern reflects any discontinuity and asymmetry in the crystal lattice (right image).

Soft lineage in sapphire

Etalon sapphire crystal

X-Ray diffraction analysis is a popular method of studying the crystallographic structure and physical properties of solid crystals. It is based on observing the scattered intensity of an X-Ray beam hitting a sample as a function of incident and scattered angle, polarization, and wavelength or energy. The sample mounted onto a goniometer is gradually rotated and at the same time is being bombarded with X-Rays. The crystal atoms scatter the X-Rays which land onto an X-Ray detector forming a diffraction pattern of spots.
The analysis is performed with the help of a high resolution X-Ray diffractometer and supplemental software. The results are reflected in rocking curves.

Rocking curves of a sapphire sample. The average width is 12.66’ (the nominal width of the rocking curve for KY sapphire is <=30’)