The mechanical properties of semiconductors are profoundly influenced by the presence of point and line defects, which include vacancies, interstitials, and dislocations. These defects disrupt the ideal crystalline lattice, leading to changes in hardness, ductility, and fracture resistance. The interactions between defects and their response to thermal treatments such as annealing further modulate these mechanical behaviors. Understanding these effects is critical for optimizing semiconductor performance in applications requiring mechanical reliability, such as flexible electronics, MEMS devices, and high-power systems.

Point defects, such as vacancies and interstitials, introduce localized strain fields into the crystal lattice. A vacancy, the absence of an atom in the lattice, creates a tensile strain, while an interstitial atom induces compressive strain. These strain fields interact with dislocations, which are line defects representing misalignments in the lattice structure. The presence of dislocations allows plastic deformation to occur through their motion under stress. However, when point defects are present, they can pin dislocations, impeding their movement and leading to material hardening. This phenomenon is well-documented in silicon, where vacancies and oxygen interstitials increase yield strength by acting as obstacles to dislocation glide.

Dislocations themselves are primary contributors to mechanical behavior. Edge dislocations, characterized by an extra half-plane of atoms, and screw dislocations, with a helical lattice distortion, both facilitate shear deformation. In high-purity single crystals, dislocations can move relatively freely, resulting in lower hardness and higher ductility. However, in heavily defected or polycrystalline materials, dislocation density increases, leading to entanglement and the formation of dislocation networks. These networks hinder further dislocation motion, increasing the material's strength through work hardening. For instance, deformed germanium exhibits a significant rise in hardness due to dislocation multiplication and interaction.

The interaction between point defects and dislocations is a key factor in mechanical property modification. Point defects can segregate around dislocations, forming Cottrell atmospheres that anchor the dislocations in place. This segregation reduces dislocation mobility, increasing the critical resolved shear stress required for plastic deformation. In compound semiconductors like gallium arsenide, arsenic vacancies have been observed to cluster near dislocations, enhancing mechanical stability at elevated temperatures. Conversely, excessive defect concentrations can lead to softening if the defects promote dislocation climb or cross-slip, mechanisms that allow dislocations to bypass obstacles.

Dislocation-dislocation interactions also play a major role in mechanical behavior. When dislocations intersect, they form jogs or kinks that impede further motion. In materials with high dislocation densities, such as plastically deformed silicon carbide, these interactions create a tangled microstructure that resists deformation. However, if the dislocation density becomes too high, the stress fields of neighboring dislocations may cancel each other out, leading to partial softening. This balance between hardening and softening is highly dependent on the semiconductor's crystallographic orientation, temperature, and defect distribution.

Thermal annealing is a common method for modifying defect populations and restoring mechanical integrity. During annealing, point defects diffuse and recombine, reducing their concentration. Dislocations can also reorganize into lower-energy configurations, such as subgrain boundaries, through recovery and recrystallization processes. For example, annealing deformed zinc oxide at moderate temperatures reduces dislocation density and relieves internal stresses, resulting in a softer but more ductile material. However, excessive annealing can lead to grain growth in polycrystalline materials, potentially weakening grain boundaries and reducing fracture toughness.

The temperature dependence of defect behavior further complicates mechanical property evolution. At low temperatures, point defects and dislocations are relatively immobile, leading to brittle fracture and high hardness. As temperature increases, defects become more mobile, enabling dislocation glide and climb. This transition is evident in indium phosphide, which exhibits a marked decrease in yield strength above 600°C due to thermally activated dislocation motion. High-temperature applications must therefore account for defect-mediated creep and stress relaxation.

In polycrystalline semiconductors, grain boundaries interact with dislocations and point defects to influence mechanical properties. Grain boundaries act as barriers to dislocation motion, contributing to the Hall-Petch strengthening effect. However, they can also serve as sinks for point defects, reducing their concentration in the grain interiors. In materials like polycrystalline silicon, grain boundary sliding becomes significant at high temperatures, leading to reduced hardness and increased susceptibility to fracture.

Defect engineering is a powerful tool for tailoring mechanical properties. Intentional introduction of controlled defect populations can enhance hardness without compromising other characteristics. For instance, nitrogen implantation in silicon creates dislocation loops that strengthen the material while maintaining reasonable ductility. Similarly, in diamond semiconductors, controlled vacancy generation via ion irradiation increases fracture toughness by promoting localized plastic deformation.

The mechanical effects of defects are not uniform across all semiconductor classes. Covalent semiconductors like silicon and germanium exhibit strong directional bonding, making them more susceptible to dislocation-induced hardening. In contrast, ionic semiconductors such as zinc oxide display more complex behavior due to the interplay between electrostatic interactions and defect mobility. The anisotropy of certain crystal structures, such as the hexagonal lattice of gallium nitride, further influences defect-mediated mechanical responses.

Understanding these defect-related mechanisms is essential for semiconductor processing and device fabrication. Mechanical stresses during wafer handling, dicing, or packaging can induce defect generation, leading to unexpected failures. Techniques like X-ray topography and electron microscopy are indispensable for characterizing defect distributions and their impact on mechanical integrity.

In summary, point and line defects significantly alter the mechanical properties of semiconductors through hardening, softening, and complex interaction mechanisms. The balance between these effects depends on defect type, density, and thermal history. Annealing processes can mitigate some detrimental effects but must be carefully controlled to avoid unintended property changes. Defect engineering offers opportunities to optimize mechanical performance for specific applications, ensuring reliability in demanding environments. Continued research into defect dynamics will further refine the design and processing of advanced semiconductor materials.