Quantum confinement in nanostructured semiconductors, such as nanowires and superlattices, significantly enhances thermoelectric performance by modifying both phonon transport and electronic density of states. Unlike bulk materials, where phonon and electron transport are less restricted, low-dimensional systems introduce discrete energy levels and boundary effects that drastically alter thermal and electronic properties. This article explores the mechanisms through which quantum confinement improves the thermoelectric figure of merit (ZT) by suppressing phonon conduction and optimizing electronic transport.

In bulk semiconductors, the thermoelectric figure of merit is defined as ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity. High ZT requires a large power factor (S²σ) and low κ. However, in bulk materials, these parameters are interdependent, making optimization challenging. Quantum confinement decouples these relationships by independently tuning electronic and thermal transport at the nanoscale.

Phonon scattering is a critical mechanism for reducing thermal conductivity in confined systems. In nanowires and superlattices, the mean free path of phonons is significantly shortened due to boundary scattering. Phonons, which are quantized lattice vibrations, encounter increased scattering at interfaces, surfaces, and defects. For nanowires with diameters comparable to or smaller than the phonon mean free path, boundary scattering dominates, leading to a reduction in lattice thermal conductivity (κ_l). Superlattices introduce additional phonon scattering through interfacial roughness and periodicity mismatches between layers. Studies have shown that silicon nanowires with diameters below 50 nm exhibit κ_l reductions of up to two orders of magnitude compared to bulk silicon. Similarly, GaAs/AlAs superlattices demonstrate thermal conductivity reductions due to coherent phonon interference and interface scattering.

Beyond phonon scattering, quantum confinement modifies the electronic density of states (DOS) near the Fermi level. In bulk materials, the DOS is continuous, but in nanowires and superlattices, it becomes quantized due to spatial confinement. This quantization leads to sharp peaks in the DOS, which can enhance the Seebeck coefficient (S) without significantly degrading electrical conductivity (σ). The Seebeck coefficient is proportional to the energy derivative of the DOS at the Fermi level. By engineering the confinement dimensions, the DOS can be tuned to maximize S while maintaining high σ through selective doping or band alignment. For example, Bi₂Te₃ superlattices exhibit enhanced power factors due to the sharp DOS peaks introduced by quantum well structures.

The interplay between confinement-induced DOS modification and energy filtering further improves thermoelectric performance. Energy filtering occurs when carriers with low energy are selectively scattered, while high-energy carriers contribute to conduction. In nanowires, the presence of discrete subbands allows for selective carrier transport, increasing the average energy per carrier and thus the Seebeck coefficient. Experimental studies on PbTe nanowires have demonstrated enhanced S values due to energy filtering effects arising from quantum confinement.

Another advantage of quantum-confined systems is the ability to exploit resonant states and miniband formation in superlattices. When quantum wells are closely spaced, wavefunctions overlap, forming minibands that facilitate carrier transport while maintaining sharp DOS features. This miniband transport can enhance electrical conductivity without sacrificing the Seebeck coefficient. In InAs/GaSb superlattices, miniband engineering has been shown to simultaneously improve σ and S, leading to higher ZT values compared to bulk counterparts.

Material choice also plays a crucial role in leveraging quantum confinement for thermoelectrics. Narrow-bandgap semiconductors, such as Bi₂Te₃, PbTe, and SiGe, are commonly used due to their favorable band structures for confinement effects. For instance, Bi₂Te₃ nanowires exhibit strong quantum confinement effects at diameters below 10 nm, leading to significant improvements in ZT. Similarly, PbSe/PbTe core-shell nanowires demonstrate reduced κ_l and enhanced power factors due to the combined effects of phonon scattering and DOS engineering.

The impact of dimensionality on thermoelectric performance is further illustrated by comparing 1D nanowires and 2D superlattices. Nanowires offer stronger phonon boundary scattering due to their high surface-to-volume ratio, while superlattices provide more precise control over electronic states through layer thickness and composition. For example, Si/Ge nanowires exhibit extremely low κ_l values due to enhanced surface scattering, whereas InGaAs/InAlAs superlattices achieve high power factors through miniband conduction.

Despite these advantages, challenges remain in realizing practical quantum-confined thermoelectric devices. Fabrication uniformity, interfacial defects, and scalability are critical issues that affect performance reproducibility. Advanced growth techniques, such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), have enabled precise control over nanowire and superlattice structures, but further optimization is needed for large-scale applications.

In summary, quantum confinement in nanowires and superlattices enhances thermoelectric performance through two primary mechanisms: phonon scattering and electronic density of states engineering. By reducing lattice thermal conductivity and optimizing the power factor, low-dimensional systems achieve higher ZT values than bulk materials. Continued advancements in nanofabrication and material design will further unlock the potential of quantum-confined thermoelectrics for energy harvesting and cooling applications.