Pressure-dependent thermal conductivity in semiconductors is a critical area of study due to its implications for thermal management in high-performance electronics, optoelectronics, and energy conversion systems. The thermal conductivity of semiconductors is primarily governed by phonon transport, which is highly sensitive to external pressure. Understanding how pressure alters phonon scattering mechanisms and thermal transport properties provides insights into material behavior under extreme conditions and guides the design of next-generation devices.

At ambient conditions, thermal conductivity in semiconductors is dominated by phonon-phonon scattering, defect scattering, and boundary scattering. However, under high pressure, these mechanisms undergo significant modifications due to changes in interatomic distances, bond stiffness, and crystal symmetry. The application of hydrostatic pressure compresses the lattice, increasing the force constants between atoms and altering the phonon dispersion relations. This typically leads to an increase in phonon group velocities and a reduction in anharmonic phonon-phonon scattering, resulting in higher thermal conductivity. For instance, in silicon, thermal conductivity increases monotonically with pressure up to several gigapascals due to enhanced phonon lifetimes and suppressed Umklapp scattering.

However, not all semiconductors exhibit this straightforward behavior. In materials with strong anharmonicity or complex crystal structures, pressure can induce anomalous thermal conductivity trends. For example, in boron arsenide (BAs), which already exhibits ultrahigh thermal conductivity at ambient pressure, further pressure application leads to a non-monotonic response due to competing effects of increased phonon velocities and enhanced three-phonon scattering processes. Similarly, in skutterudites and clathrates, pressure-induced structural transitions or rattling atom dynamics can lead to unexpected reductions in thermal conductivity despite lattice stiffening.

Graphene presents a particularly intriguing case where pressure-dependent thermal conductivity deviates from conventional expectations. At low pressures, graphene’s thermal conductivity initially increases due to reduced out-of-plane phonon scattering and enhanced in-plane bonding. However, beyond a critical pressure, the formation of wrinkles and strain-induced defects introduces additional phonon scattering centers, leading to a decrease in thermal conductivity. This behavior contrasts sharply with three-dimensional semiconductors and highlights the unique role of dimensionality in thermal transport under pressure.

Pressure also influences the relative contributions of different phonon modes to thermal conductivity. In many semiconductors, optical phonons have negligible contributions at ambient pressure due to their low group velocities and high scattering rates. However, under high pressure, optical phonon branches can stiffen and couple more effectively with acoustic phonons, leading to increased contributions to heat transport. This effect has been observed in materials like gallium nitride (GaN), where high pressure enhances the thermal conductivity by activating optical phonon participation.

The impact of pressure on defect-mediated phonon scattering is another critical consideration. Point defects, dislocations, and grain boundaries typically reduce thermal conductivity by scattering phonons. Under pressure, the scattering cross-sections of these defects can change due to altered strain fields and defect configurations. In some cases, pressure can even anneal defects or induce defect ordering, leading to improved thermal transport. For example, in diamond, high-pressure treatment has been shown to reduce vacancy clusters and improve thermal conductivity.

High-pressure phases and phase transitions further complicate the thermal conductivity landscape. Many semiconductors undergo pressure-induced phase transitions, such as the zinc-blende to rocksalt transition in III-V materials or the wurtzite to rocksalt transition in II-VI compounds. These transitions often involve drastic changes in bonding and symmetry, leading to abrupt changes in thermal conductivity. For instance, the thermal conductivity of gallium arsenide (GaAs) drops significantly upon transitioning from the zinc-blende to the rocksalt phase due to increased phonon scattering at the phase boundary and altered phonon dispersion.

The implications of pressure-dependent thermal conductivity are profound for thermal management in extreme environments. In high-power electronic devices, operating under mechanical stress or in high-pressure environments, understanding these effects ensures reliable performance. For example, silicon carbide (SiC) power devices used in aerospace applications must account for pressure-induced thermal conductivity changes to prevent overheating. Similarly, in thermoelectric materials, pressure can be leveraged to optimize the trade-off between electrical and thermal transport properties.

Emerging research directions include the study of hybrid and nanostructured materials under pressure. Heterostructures, superlattices, and nanocomposites exhibit unique pressure-dependent thermal transport properties due to interfacial phonon scattering and confinement effects. For instance, in silicon-germanium superlattices, pressure can modulate the phonon coherence length and interfacial scattering rates, offering tunability in thermal conductivity. These materials hold promise for tailored thermal management solutions in advanced electronics.

In summary, pressure-dependent thermal conductivity in semiconductors is governed by complex interplay between phonon dynamics, defect scattering, and structural transformations. While most materials show increased thermal conductivity under pressure, anomalies arise due to anharmonicity, dimensionality, and phase transitions. Graphene and other two-dimensional materials exhibit unique behaviors that challenge traditional models. These insights are crucial for designing materials and devices operating under extreme conditions, enabling advances in electronics, energy conversion, and thermal management technologies. Future work will likely explore the role of pressure in emerging quantum materials and heterostructures, further expanding the frontiers of thermal transport science.