Thermal boundary resistance (TBR) is a critical parameter governing heat dissipation across interfaces in semiconductor heterostructures. It quantifies the impediment to heat flow when phonons encounter a boundary between dissimilar materials. The magnitude of TBR depends on the mismatch in vibrational spectra, interface quality, and scattering mechanisms. Understanding and controlling TBR is essential for optimizing thermal performance in applications such as high-power electronics, optoelectronics, and thermoelectric devices.

Phonon scattering mechanisms dominate TBR at semiconductor interfaces. When phonons propagate from one material to another, their transmission depends on the overlap of phonon density of states and the conservation of energy and momentum. Acoustic mismatch model (AMM) and diffuse mismatch model (DMM) are two foundational theories describing phonon transport across boundaries. AMM assumes purely elastic scattering and specular reflection, valid for atomically smooth interfaces at low temperatures. DMM accounts for inelastic scattering and diffuse reflection, better suited for rough or disordered interfaces at higher temperatures. Real interfaces often exhibit behavior between these extremes, with additional contributions from interfacial defects, dislocations, and chemical intermixing.

Experimental techniques for measuring TBR include time-domain thermoreflectance (TDTR) and Raman thermometry. TDTR employs ultrafast laser pulses to generate and probe thermal transients. A pump laser heats the sample surface, while a delayed probe laser monitors the temperature-dependent reflectivity decay. By fitting the thermal decay profile with a thermal model, TBR can be extracted with high precision. TDTR offers sub-picosecond temporal resolution and is widely used for studying interfaces like Si/Ge and GaN/diamond. Raman thermometry leverages the temperature-dependent shift of Raman-active phonon modes. A focused laser locally heats the sample, and the resulting temperature gradient near the interface is mapped via Raman spectroscopy. This technique provides spatial resolution on the order of micrometers and is non-destructive, making it suitable for layered materials and nanostructures.

Engineering approaches to reduce TBR focus on enhancing phonon transmission through interfacial modifications. Alloying is one strategy, where graded composition layers bridge the vibrational mismatch. For example, a SiGe alloy interlayer between Si and Ge reduces TBR by gradually transitioning the acoustic impedance. Nanostructuring introduces periodic features or phononic crystals to alter phonon dispersion relations. Textured interfaces with controlled roughness can increase phonon scattering angles, promoting inelastic processes that improve transmission. Another approach involves inserting ultra-thin adhesion layers or bonding materials with intermediate Debye temperatures. In GaN/diamond interfaces, a few nanometers of silicon nitride or aluminum nitride can significantly lower TBR by improving lattice matching and reducing interfacial strain.

Reference systems like Si/Ge and GaN/diamond highlight the impact of material properties on TBR. The Si/Ge interface exhibits high TBR due to large differences in acoustic impedance and phonon velocities. Measured values range from 10 to 30 m²K/GW depending on interface quality and measurement technique. Alloying with SiGe interlayers can reduce TBR by up to 50%. GaN/diamond interfaces are of interest for high-power electronics, where diamond’s exceptional thermal conductivity is leveraged for heat spreading. However, TBR values between 20 and 60 m²K/GW limit heat extraction. Nanoscale adhesion layers and surface treatments have shown promise in reducing this resistance by up to 40%.

Advanced characterization and modeling continue to refine understanding of TBR. Molecular dynamics simulations reveal atomic-scale details of phonon transport, including the role of interfacial dislocations and localized modes. First-principles calculations predict phonon transmission coefficients based on bonding characteristics and vibrational spectra. Experimental advances, such as frequency-domain thermoreflectance, provide additional insights into spectral phonon contributions. Together, these tools enable targeted design of interfaces with minimized TBR.

Practical applications demand balancing TBR reduction with other performance metrics. In semiconductor lasers, excessive interfacial thermal resistance can lead to overheating and efficiency droop. In power devices, poor heat dissipation accelerates degradation and limits operational lifetimes. Optimized interfaces must maintain electrical properties while enhancing thermal conduction. For instance, in GaN-on-diamond substrates, minimizing TBR without introducing parasitic capacitance or carrier trapping is crucial.

Future directions include exploring hybrid interfaces with tailored phonon spectra. Superlattices and metamaterials offer opportunities to engineer phonon bandgaps and resonant tunneling effects. Two-dimensional materials like graphene and hexagonal boron nitride present unique interfacial thermal properties due to their anisotropic phonon dispersion. Integrating these materials into heterostructures could enable ultra-low TBR through van der Waals bonding and reduced lattice mismatch.

In summary, thermal boundary resistance at semiconductor interfaces is governed by phonon scattering mechanisms, measurable via advanced techniques like TDTR and Raman thermometry. Engineering solutions such as alloying, nanostructuring, and interfacial layers provide pathways to mitigate TBR, as demonstrated in systems like Si/Ge and GaN/diamond. Continued progress in characterization and modeling will further enable precise control of interfacial thermal transport for next-generation semiconductor technologies.