88
4 Structural Defects
Fig. 4.25 Dislocation
density (as revealed by etch
pits) for GaAs and InP as a
function of the carrier
concentration for various
concentrations of
impurities (S, Te, and Zn).
Adapted from [343]
4.3.3 Impurity Hardening
It has been found that the addition of impurities can lead to a substantial reduction of the dislocation
density. This effect is known as impurity hardening and is caused by a hardening of the lattice due
to an increase of the so-called critical resolved shear stress [342]. In Fig. 4.25 the dependence of the
dislocation density in GaAs and InP is shown as a function of the carrier density that is induced by the
incorporation of (electrically active) group-II or group-VI atoms (acceptors or donors, cf. Sect. 7.5).
The high carrier concentration is unwanted when semi-insulating substrates (cf. Sect. 7.7.8) or low
optical absorption (cf. Sect. 9.9.1) are needed. Thus the incorporation of isovalent impurities, such as
In, Ga or Sb in GaAs and Sb, Ga or As in InP, has been investigated and found to be remarkably
effective. Material containing such impurities in high concentration (>10
19 cm
−3 ) must be considered
a low-concentration alloy. The lattice constant is thus slightly changed, which can cause problems in
the subsequent (lattice-mismatched) epitaxy of pure layers.
4.4 Extended Defects
4.4.1 Micro-cracks
If the stress in a material becomes too big to be accommodated by dislocations, cracks may form
to release strain energy.
8 In Fig. 4.26 an example is shown. In this case, micro-cracks have formed
in a bulk mercury indium telluride crystal upon incorporation of residual stress and thermal stress
during cooling of the material from growth temperature (about 1000 K) to room temperature. See also
Fig. 12.19 for micro-cracks in an epitaxial layer.
8 We note that in elasticity theory a continuous deformation is assumed. Obviously the separation (fracture) into two
unstrained blocks is the lowest strain energy state of a stressed piece of material.
4 Structural Defects
Fig. 4.25 Dislocation
density (as revealed by etch
pits) for GaAs and InP as a
function of the carrier
concentration for various
concentrations of
impurities (S, Te, and Zn).
Adapted from [343]
4.3.3 Impurity Hardening
It has been found that the addition of impurities can lead to a substantial reduction of the dislocation
density. This effect is known as impurity hardening and is caused by a hardening of the lattice due
to an increase of the so-called critical resolved shear stress [342]. In Fig. 4.25 the dependence of the
dislocation density in GaAs and InP is shown as a function of the carrier density that is induced by the
incorporation of (electrically active) group-II or group-VI atoms (acceptors or donors, cf. Sect. 7.5).
The high carrier concentration is unwanted when semi-insulating substrates (cf. Sect. 7.7.8) or low
optical absorption (cf. Sect. 9.9.1) are needed. Thus the incorporation of isovalent impurities, such as
In, Ga or Sb in GaAs and Sb, Ga or As in InP, has been investigated and found to be remarkably
effective. Material containing such impurities in high concentration (>10
19 cm
−3 ) must be considered
a low-concentration alloy. The lattice constant is thus slightly changed, which can cause problems in
the subsequent (lattice-mismatched) epitaxy of pure layers.
4.4 Extended Defects
4.4.1 Micro-cracks
If the stress in a material becomes too big to be accommodated by dislocations, cracks may form
to release strain energy.
8 In Fig. 4.26 an example is shown. In this case, micro-cracks have formed
in a bulk mercury indium telluride crystal upon incorporation of residual stress and thermal stress
during cooling of the material from growth temperature (about 1000 K) to room temperature. See also
Fig. 12.19 for micro-cracks in an epitaxial layer.
8 We note that in elasticity theory a continuous deformation is assumed. Obviously the separation (fracture) into two
unstrained blocks is the lowest strain energy state of a stressed piece of material.