Electric Field Stiffening Effect in c-Oriented Aluminum Nitride Piezoelectric Thin Films

Abstract

Aluminum nitride offers unique material advantages for the realization of ultrahigh-frequency acoustic devices attributed to its high ratio of stiffness to density, compatibility with harsh environments, and superior thermal properties. Although, to date, aluminum nitride thin films have been widely investigated regarding their electrical and mechanical characteristics under alternating small signal excitation, their ultrathin nature under large bias may also provide novel and useful properties. Here, we present a comprehensive investigation of electric field stiffening effect in c-oriented aluminum nitride piezoelectric thin films. By analyzing resonance characteristics in a 2.5 GHz aluminum nitride-based film bulk acoustic resonator, we demonstrate an up to 10% linear variation in the equivalent stiffness of aluminum nitride piezoelectric thin films when an electric field was applied from -150 to 150 MV/m along the c-axis. Moreover, for the first time, an atomic interaction mechanism is proposed to reveal the nature of electric field stiffening effect, suggesting that the nonlinear variation of the interatomic force induced by electric field modulation is the intrinsic reason for this phenomenon in aluminum nitride piezoelectric thin films. Our work provides vital experimental data and effective theoretical foundation for electric field stiffening effect in aluminum nitride piezoelectric thin films, indicating the huge potential in tunable ultrahigh-frequency microwave devices.

Control and Theoretical Modeling of the Growth Process of AlN Six-fold and Multifold Armed Dendritic Crystals

“…Its wurtzite phase (w-AlN) is a member of the hexagonal crystal system and consists of tetrahedrally coordinated silicon and nitrogen atoms. w-AlN is a wide band gap (6.0–6.2 eV) semiconductor material with excellent thermal conductivity, good electrical resistance, low dielectric loss, high piezoelectric response, and an ideal thermal expansion coefficient. , Several recent pioneering studies report that w-AlN nanocrystals can be synthesized in the forms of nanoparticles, nanowhiskers, nanorods, nanobelts, nanoprisms, or complex hierarchical structures with six-fold corners. − The morphological diversity has triggered the application of AlN nanocrystals in a variety of high-frequency devices such as surface acoustic wave devices (SAW), resonators, high-frequency filters, and pressure sensors. − In particular, AlN dendritic crystals are distinguished by their high symmetry and parallel signal processing capabilities, which may provide many opportunities for applications in the next generation optical, electronic, and energy devices, such as photovoltaic cells, passive heat exchanger in electronic circuits, energy harvesting, and acoustic devices. − In order to facilitate the widespread application of dendritic-AlN nanocrystals, the experimental growth process, which enables the fine-tuning of the morphology of AlN crystals, should be optimized. In this context, combined, in-depth theoretical understanding of the growth mechanism would accelerate the rational synthesis of AlN crystals.…”

Interface Engineering of Metal Oxynitride Lateral Heterojunctions for Photocatalytic and Optoelectronic Applications

“…Electronic structure modulation via these lateral heterojunctions is evident from Figure 2B. We observe a trend of decreasing band gap with an increasing ZnO c o n c e n t r a This resulting reduction in the band gap from that of the pure phase ZnO, AlN, and GaN is often attributed to the Zn 3d, O 2p, and N 2p repulsion 18,22 and allows for these heterojunctions to perform in visible light applications. These heterojunctions are intrinsically strained as they are evident from the lattice constant values.…”

“…These results are promising for material property tuning simply by composition variation. This resulting reduction in the band gap from that of the pure phase ZnO, AlN, and GaN is often attributed to the Zn 3d, O 2p, and N 2p repulsion , and allows for these heterojunctions to perform in visible light applications.…”

“…Moreover, it is also revealed that biaxial strain is more effective than vertical strain in tuning the band gap of these lateral heterojunctions. This may be because of the fact that the VBE is comprised mostly of the N 2p orbital, followed by Zn 3d and O 2p, , and therefore the p–d repulsion between Zn 3d and N 2p, which is accentuated by strain normal to the interface (i.e., biaxial), will have the greatest effect on the band gap. Figure E–H depicts the shift of the DOS of the Zn 3d orbital across the VBE and CBE of (ZnO) 0.5 (AlN) 0.5 .…”

“…Zinc oxide, aluminum nitride, and gallium nitride are well-known II–VI and III–V materials mostly stable in the hexagonal wurtzite phase, being used for a vast range of applications such as gas sensors, LEDs, transistors, and piezoelectric and optoelectronic systems. ,− Their solid solutions are well-known for catalysis and have even demonstrated the highest visible light activity for water-splitting. − Platinum-doped zinc oxide–gallium nitride solid solutions have demonstrated CO 2 photoreduction as well . ZnO, AlN and GaN have been used in electronics for a long time and their interfaces are a matter of interesting study for efficient charge transport. , These metal oxynitrides, both at bulk and interface sites, allow for a vast range of composition variation, exhibiting a wide range of electronic density of states (DOS).…”