Strain-Correlated Piezoelectricity in Quasi-Two-Dimensional Zinc Oxide Nanosheets

Characteristics of 3D-Integrated GaN Power Module Under Multi Heat Source Coupling

3D-integrated GaN power modules can effectively reduce parasitic parameters and enhance the power system's performance. However, the heat from each power chip during operation can lead to a mutual thermal coupling effect, potentially causing performance drift of the GaN power chips. This work investigates the impact of the thermal coupling effect in a 3D-integrated GaN power module on the characteristics of its GaN power chips. The GaN power chips' characteristics are measured before and after the other power chips in the 3D-integrated GaN power module and after applying VGS/VDS = 3 V/1 V for 60 s. The results indicate that the thermal coupling effect in 3D-integrated GaN power modules can cause a rightward shift in the threshold voltage, reduce the response speed and on-state current, and also increase the leakage current of GaN power chips. In severe cases, the threshold voltage drift can reach up to 0.26 V, the device's response time can increase by as much as 217 μs, the on-state current can decrease by 1.7 A, and the off-state leakage current can increase by more than 80 times. The impact of the thermal coupling effect is related to the direction of heat flow and the distance between chips. The closer the chips are to each other, the stronger the thermal coupling. It has a greater impact on the performance of chips near the bottom substrate and a lesser impact on the performance of chips at the top of the module. Typically, the influence of the thermal field generated by two chips working simultaneously is more significant than that of the thermal field generated by a single chip working alone.

Temperature Dependence on Microstructure, Crystallization Orientation, and Piezoelectric Properties of ZnO Films

This study has investigated the effects of different annealing temperatures on the microstructure, chemical composition, phase structure, and piezoelectric properties of ZnO films. The analysis focuses on how annealing temperature influences the oxygen content and the preferred c-axis (002) orientation of the films. It was found that annealing significantly increases the grain size and optimizes the columnar crystal structure, though excessive high-temperature annealing leads to structural degradation. This behavior is likely related to changes in oxygen content at different annealing temperatures. High resolution transmission electron microscopy (HR-TEM) reveals that the films exhibit high-resolution lattice stripes, confirming their high crystallinity. Although the films exhibit growth in multiple orientations, the c-axis (002) orientation remains the predominant crystallographic growth. Further piezoelectric property analysis demonstrates that the ZnO films annealed at 400 °C exhibit enhanced piezoelectric performance and stable linear piezoelectric behavior. These findings offer valuable support for optimizing the piezoelectric properties of ZnO films and their applications in piezoelectric sensors.

Excellent Hole Mobility and Out-of-Plane Piezoelectricity in X-Penta-Graphene (X = Si or Ge) with Poisson's Ratio Inversion

Recently, the application of two-dimensional (2D) piezoelectric materials has been seriously hindered because most of them possess only in-plane piezoelectricity but lack out-of-plane piezoelectricity. In this work, using first-principles calculation, by atomic substitution of penta-graphene (PG) with tiny out-of-plane piezoelectricity, we design and predict stable 2D X-PG (X = Si or Ge) semiconductors with excellent in-plane and out-of-plane piezoelectricity and extremely high in-plane hole mobility. Among them, Ge-PG exhibits better performance in all aspects with an in-plane strain piezoelectric coefficient d11 = 8.43 pm/V, an out-of-plane strain piezoelectric coefficient d33 = -3.63 pm/V, and in-plane hole mobility μh = 57.33 × 103 cm2 V-1 s-1. By doping Si and Ge atoms, the negative Poisson's ratio of PG approaches zero and reaches a positive value, which is due to the gradual weakening of the structure's mechanical strength. The bandgaps of Si-PG (0.78 eV) and Ge-PG (0.89 eV) are much smaller than that of PG (2.20 eV), by 2.82 and 2.47 times, respectively. This indicates that the substitution of X atoms can regulate the bandgap of PG. Importantly, the physical mechanism of the out-of-plane piezoelectricity of these monolayers is revealed. The super-dipole-moment effect proposed in the previous work is proved to exist in PG and X-PG, i.e., it is proved that their out-of-plane piezoelectric stress coefficient e33 increases with the super-dipole-moment. The e33-induced polarization direction is also consistent with the super-dipole-moment direction. X-PG is predicted to have prominent potential for nanodevices applied as electromechanical coupling systems: wearable, ultra-thin devices; high-speed electronic transmission devices; and so on.

Giant Electromechanical Effect in Piezoelectric Nanomembranes Based on Hf0.5Zr0.5O2

Since ultrathin ferroelectric HfO2 films can be conformally grown by atomic layer deposition even on complex three-dimensional structures, new horizons in the development of next-generation piezoelectric devices are opened. However, hafnium oxide has a significant drawback for piezoelectric applications: its piezoelectric coefficients are much smaller than those of classical materials currently used in piezoelectric devices. Therefore, new approaches to the development of high-performance piezoelectric devices based on exploiting the unique properties of HfO2 are of paramount importance. In this work, a giant electromechanical effect in miniature piezoelectric membrane devices based on a 10 nm-thick ferroelectric Hf0.5Zr0.5O2 (HZO) film is experimentally demonstrated. Compared to the pure piezoelectric effect in the HZO film, the gain of the electromechanical response in membrane devices reaches 25 times. Numerical simulations confirm that this effect stems from the asymmetric shape of the membranes and can be further improved by designing the device geometry. Furthermore, according to first-principles calculations, an additional opportunity to improve the piezoelectric coefficient, and hence, the device efficiency is provided by the engineering of the mechanical stress in the HZO film. The proposed approach enables the development of new promising piezoelectric devices including miniature reflectors, nanoactuators, and nanoswitches.

Energy Harvesting Using ZnO Nanosheet-Decorated 3D-Printed Fabrics

In this work, we decorated piezoresponsive atomically thin ZnO nanosheets on a polymer surface using additive manufacturing (three-dimensional (3D) printing) technology to demonstrate electrical-mechanical coupling phenomena. The output voltage response of the 3D-printed architecture was regulated by varying the external mechanical pressures. Additionally, we have shown energy generation by placing the 3D-printed fabric on the padded shoulder strap of a bag with a load ranging from ∼5 to ∼75 N, taking advantage of the excellent mechanical strength and flexibility of the coated 3D-printed architecture. The ZnO coating layer forms a stable interface between ZnO nanosheets and the fabric, as confirmed by combining density functional theory (DFT) and electrical measurements. This effectively improves the output performance of the 3D-printed fabric by enhancing the charge transfer at the interface. Therefore, the present work can be used to build a new infrastructure for next-generation energy harvesters capable of carrying out several structural and functional responsibilities.