Stacking faults and planar defects are critical imperfections in semiconductor crystals that influence mechanical, electronic, and optical properties. These defects arise from deviations in the ideal stacking sequence of atomic planes, leading to localized disruptions in crystallographic order. Understanding their atomic-scale structure, formation mechanisms, and impact on device performance is essential for semiconductor engineering.

In face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices, stacking faults occur due to errors in the layer-by-layer arrangement of atoms. The FCC structure follows an ABCABC stacking sequence, where each layer is offset from the previous one. A stacking fault disrupts this periodicity, introducing a local HCP-like sequence such as ABCBCABC. This disruption creates a thin region where the coordination of atoms differs from the bulk crystal. In HCP materials, which have an ABAB stacking sequence, faults may introduce an FCC-like segment, such as ABABCAB. The energy cost of forming these faults depends on the material's stacking fault energy (SFE), with lower SFE favoring fault formation.

Partial dislocations are closely associated with stacking faults. In FCC crystals, a perfect dislocation with Burgers vector (a/2)<110> may dissociate into two Shockley partial dislocations, (a/6)<112>, separated by a stacking fault. This dissociation reduces elastic strain energy but creates a planar defect between the partials. The width of the faulted region is determined by the balance between the repulsive forces of the partials and the SFE. In HCP materials, partial dislocations can similarly bound stacking faults, though the Burgers vectors differ due to the hexagonal symmetry.

Twin boundaries are another type of planar defect where the crystal lattice is mirrored across a specific plane. In FCC materials, twins form through shear deformation or growth accidents, producing a sequence like ABCBACBA. Twin boundaries exhibit low energy due to the coherent alignment of atoms across the boundary, making them less disruptive than stacking faults. However, they still alter local electronic properties by modifying the periodicity of the crystal.

The presence of stacking faults and planar defects significantly impacts semiconductor performance. Electrically, these defects introduce localized states within the bandgap, acting as traps or recombination centers for charge carriers. In silicon, for example, stacking faults can degrade minority carrier lifetime, reducing the efficiency of solar cells and bipolar transistors. In compound semiconductors like GaAs, faults may scatter electrons, increasing resistivity and impairing high-frequency device operation. Optical properties are also affected, as defects can quench photoluminescence or introduce additional emission peaks due to localized exciton states.

Mechanical properties are similarly influenced. Stacking faults impede dislocation motion, leading to work hardening in deformed crystals. However, excessive faulting can embrittle materials by facilitating crack propagation along fault planes. In wide-bandgap semiconductors like SiC, planar defects reduce thermal conductivity by scattering phonons, which is detrimental for power electronics applications requiring efficient heat dissipation.

Characterization of stacking faults and planar defects relies on advanced microscopy and diffraction techniques. Transmission electron microscopy (TEM) is the most direct method, enabling atomic-scale imaging of defect structures. High-resolution TEM (HRTEM) reveals the stacking sequence and partial dislocation cores, while diffraction contrast TEM highlights faulted regions through fringe contrast. Weak-beam dark-field imaging enhances dislocation visibility by suppressing background intensity.

X-ray diffraction (XRD) provides complementary information by detecting peak broadening or shifts due to fault-induced strain. For example, FCC stacking faults cause asymmetric peak broadening, with the extent of broadening proportional to the fault density. Raman spectroscopy can also probe defects in some materials, as faults modify vibrational modes, leading to peak shifts or additional scattering features.

In summary, stacking faults and planar defects are intrinsic to semiconductor crystals, with their formation governed by crystallographic stacking sequences and material-specific energetics. These defects influence electronic, optical, and mechanical behavior, often degrading device performance but sometimes enabling novel functionalities. Advanced characterization techniques like TEM and XRD are indispensable for studying their atomic-scale structure and distribution. Mitigating or engineering these defects is crucial for optimizing semiconductor materials for applications ranging from microelectronics to optoelectronics.