Gongbin Tang

The growing demand for higher speed, wider bandwidth, and enhanced power handling in electronic, photonic, and radio‐frequency systems drives the rapid advancement of monolithic heterogeneous integration. The piezoelectric‐on‐insulator (POI) architecture isolates high‐quality single‐crystal piezoelectric thin films from supporting substrates through engineered heterointerfaces, enabling simultaneous confinement of optical modes, acoustic waves, and electric fields within the active layer. This configuration provides a versatile platform for multifunctional devices targeting 5G/6G communications, high‐power electronics, and quantum photonics. This review systematically examines the physical mechanisms governing POI heterointerfaces, with a focus on interface engineering strategies across key material systems, including LiNbO ₃ /Si, LiNbO ₃ /SiC, LiNbO ₃ /diamond and AlN/diamond heterostructures. The strategies discussed encompass atomic‐scale bonding optimization, interfacial nanostructuring, and the design of ultrathin phonon‐bridge interlayers. A notable emerging capability is voltage‐tunable thermal conductivity enabled by ferroelectric domain wall engineering. Furthermore, this review addresses current challenges in scalable synthesis, defect control, and thermomechanical stress management, while outlining future directions, such as monolithic multifunctional system integration, dynamic interface tuning, and quantum photonic integration. By repositioning the heterointerface as an actively tunable functional element, this review offers a systematic framework for the design of POI interfaces in next‐generation electronic–photonic integrated systems.

Lithium niobate, as an important piezoelectric substrate material, is widely used in various acoustic and microwave devices. When lithium niobate crystals are grown from a melt at high temperatures, they typically contain a high concentration of intrinsic point defects. These point defects (either intrinsic or related to impurities or external fields) often alter the material's physical constants and increase acoustic losses. In this study, in order to study the effect of defects on the performance of SAW devices, we prepared black lithium niobate crystals with different concentrations of bipolarons, lithium vacancies, niobium anti-sites and other defects in an argon–hydrogen atmosphere. The types and concentrations of defects in the blackened lithium niobate wafers at different reduction temperatures were analyzed using UV-vis and IR spectroscopy. The chemical state of oxygen on the surface of the sample was analyzed by XPS. Dual-port electrodes were fabricated on the samples using micro-nano processing technology, and the relationship between the substrate dielectric loss and defects in lithium niobate was established through S-parameter testing. The results indicate that the chemical reduction treatment increases the defects in lithium niobate, leading to lattice distortion and the formation of defect dipoles, thereby increasing the dielectric loss of the devices. These findings provide profound insights into defect engineering for the application of lithium niobate in acoustic devices.

Lithium niobate (LiNbO₃) has remarkable ferroelectric properties, and its unique crystal structure allows it to undergo significant spontaneous polarization. Lithium niobate plays an important role in the fields of electro-optic modulation, sensing and acoustics due to its excellent electro-optic and piezoelectric properties. Thin-film LiNbO₃ (TFLN) has attracted much attention due to its unique physical properties, stable properties and easy processing. This review introduces several main preparation methods for TFLN, including chemical vapor deposition (CVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), magnetron sputtering and Smartcut technology. The development of TFLN devices, especially the recent research on sensors, memories, optical waveguides and EO modulators, is introduced. With the continuous advancement of manufacturing technology and integration technology, TFLN devices are expected to occupy a more important position in future photonic integrated circuits.

In this article, we studied the effect of radio frequency (RF) loss caused by parasitic surface conductivity (PSC) to surface acoustic wave (SAW) devices through a physical-model-based and universal method. First, through technology computer-aided design (TCAD) tools, the phenomenon of PSC attracting minority carriers to top part of Si to form an inversion layer was clearly displayed. Then, this influence was transformed into the reduction in the effective resistivity ( ${R}_{\text {eff}}{)}$ of homogeneous Si substrate using equivalent circuit model. Finally, ${R}_{\text {eff}}$ was converted into the dielectric loss tangent, which can be conveniently introduced to ANSYS electromagnetics (EM) models. This method is first used for coplanar waveguide (CPW), which is an excellent device for characterizing substrate RF loss. Using a similar method, the specific impact of PSC on SAW resonators based on 42°YX-LiTaO3 (LT)/SiO2/PSi/Si and 42°YX-LT/SiO2/Si substrate was also studied. It has been verified that PSC causes a decrease in Bode-Q of SAW resonators and deterioration in insertion loss (IL). And by calculating the changes in Bode-Q, we successfully separated substrate RF loss from other losses, such as acoustic loss. Simulations for both CPW and SAW devices have achieved good agreement with measured results. This method provides a convenient approach for analyzing the impact of PSC that exist in all similar structures on the performance of RF devices.

This paper discusses SAW-BAW-based band reject filters. They are composed of the impedance converters, where capacitive elements are replaced with SAW/BAW resonators. First, basic properties of the unit cell are studied. It is shown how basic properties of a unit cell change with the design. It is also shown that when two notches caused by the resonators are placed in proximity, two synergy effects occur: (i) an extra matching point appears on one side of the transition band. This make the insertion loss at the point smaller and the transition band steeper, and (ii) the dip level becomes deeper, and the total rejection level becomes better. Then, the wide rejection band filter is designed by cascading multi-stages, and effectiveness of the device configurations is demonstrated.