Surface acoustic wave (SAW) interactions with semiconductor surfaces represent a critical area of study in modern acoustoelectronics and integrated photonics. The coupling between piezoelectric materials and semiconductors enables precise control over wave propagation, leading to applications in signal processing, sensing, and lab-on-chip systems. Key materials such as lithium niobate (LiNbO₃) and zinc oxide (ZnO) serve as foundational substrates due to their strong piezoelectric coefficients and compatibility with semiconductor fabrication techniques.

Piezoelectric coupling is the mechanism by which mechanical strain induces an electric field and vice versa. In SAW devices, interdigital transducers (IDTs) patterned on piezoelectric substrates generate surface waves through alternating voltage application. The resulting deformation propagates along the surface, with energy confined within a depth of approximately one wavelength. The coupling efficiency depends on the electromechanical coupling coefficient (K²), which quantifies the conversion between electrical and mechanical energy. For LiNbO₃, K² can reach up to 5.3% for certain crystallographic orientations, while ZnO exhibits values around 1-2%, depending on deposition quality and crystal alignment.

Wave propagation in layered structures such as LiNbO₃/ZnO heterostructures introduces additional design flexibility. When ZnO thin films are deposited on LiNbO₃ substrates, the acoustic wave velocity and dispersion characteristics are modified due to differences in elastic and piezoelectric properties. The phase velocity in LiNbO₃ typically ranges between 3400-4000 m/s, whereas ZnO films exhibit velocities around 2700-3200 m/s. The boundary conditions at the interface influence wave confinement, with higher ZnO film thicknesses leading to stronger energy localization within the semiconductor layer. This effect is exploited in guided SAW devices, where waveguiding improves device performance by reducing losses and enhancing sensitivity to surface perturbations.

The interaction between SAWs and semiconductor surfaces also involves acoustoelectric effects, where charge carriers in the semiconductor modulate wave attenuation and velocity. In piezoelectric-semiconductor systems, the electric field accompanying the SAW interacts with free carriers, leading to phenomena such as acoustoelectric current and nonlinear wave mixing. For instance, in a ZnO-coated LiNbO₃ device, the conductivity of the semiconductor layer can be tuned to control SAW attenuation, enabling applications in tunable filters and delay lines.

SAW-based filters are among the most established applications of these interactions. The frequency response of a SAW filter is determined by the IDT geometry and substrate material properties. For example, a LiNbO₃-based filter operating at 1 GHz with a wavelength of 4 µm requires IDT finger widths of 1 µm. The high electromechanical coupling of LiNbO₃ allows for wide bandwidths, while ZnO integration can improve temperature stability due to its lower temperature coefficient of delay. Modern communication systems utilize such filters for frequency selection in RF front-end modules, where low insertion loss and sharp roll-off are critical.

Lab-on-chip systems leverage SAW-semiconductor interactions for microfluidic actuation and biosensing. When SAWs propagate through a fluid-loaded surface, they generate acoustic streaming and radiation forces, enabling precise manipulation of droplets and particles. In a typical setup, a LiNbO₃ substrate with patterned IDTs generates SAWs that couple into a microfluidic channel bonded to the surface. The resulting acoustic energy can drive mixing, sorting, and pumping of fluids at scales below 100 µm. Additionally, the perturbation of SAW propagation by mass loading or viscosity changes in the fluid provides a mechanism for label-free sensing. For instance, a ZnO-functionalized SAW sensor can detect biomolecular binding events through shifts in resonant frequency, with sensitivities reaching sub-nanogram per square millimeter levels.

Emerging trends in SAW-semiconductor research include the integration of 2D materials and hybrid piezoelectric systems. Transition metal dichalcogenides like MoS₂ can be combined with traditional piezoelectric substrates to enhance acoustoelectric coupling or introduce new functionalities such as strain-tunable electronic properties. Furthermore, advances in nanofabrication have enabled the development of high-frequency SAW devices operating in the 5-10 GHz range, which are relevant for next-generation wireless technologies.

The study of SAW interactions with semiconductor surfaces continues to evolve, driven by demands for miniaturization, energy efficiency, and multifunctional integration. From RF filters to biomedical diagnostics, the synergy between piezoelectric materials and semiconductors enables innovative solutions across diverse fields. Future developments will likely focus on optimizing material interfaces, exploring novel heterostructures, and expanding the operational limits of SAW-based devices.