Etch pit density

Under the dislocation density ρ is defined as the total length of dislocation lines per unit volume in a crystalline solid. It has the unit

An increased dislocation density in a metal increases the strength (see hardening ):

As well as the formability and the electrical conductivity decreases.

For example, the dislocation density in specially bred Kupfereinkristallen is about 108 m / m³ and can increase on heavy deformation up to about 1015 m / m³.

Measurement

Direct methods

Dislocation sets / EPD determination

The oldest known method of visualization of dislocations and determining their density is to etch the crystals concerned. In this case, atoms from the tension of near-surface dislocations are more easily removed. There arise so-called etch pits whose density may be counted in a light microscope. The resulting thereby etch pit density, Eng. Etch pit density, short EPD is a measure of the quality of semiconductor wafers, particularly in the semiconductor industry.

Silicon wafers for microelectronics or epitaxy have are usually a relatively low etch pit density of < 103 cm - 2 for example, GaAs wafer is of the order 105 cm -2.

The determination of the etch pit density is regulated in DIN 50454-1 and ASTM F 1404.

TEM

The stress field around dislocations (TEM ) can be made directly visible in the transmission electron microscope. This also a determination of the dislocation density on image analysis is possible. However, since the observable in an image volume is very small, resulting in a high cost.

Infrared light microscopy

In semiconductors, the dislocation density can also be a special light microscopes that use IR light is determined. Many semiconductors are transparent in this spectral range - the dislocation lines are visible and can be counted.

Indirect methods

The dislocation density can also be determined by diffraction methods. The most commonly used method is the X-ray profile analysis, which measures the profile broadening of X-ray peaks due to the ( integral ) lattice distortion in the vicinity of dislocations.