Han Zhang

This paper investigates a GaN planar Gunn diode with regions of high–low two-dimensional electron gas (2DEG) sheet carrier density. By altering the length of the high 2DEG sheet density region (H-region), this diode achieves a large increase in the operating frequency compared to conventional planar Gunn diodes of the same size, while retaining the power output. Under certain conditions, it can potentially double the frequency of conventional planar Gunn diodes. This research studies the length and sheet density characteristics of the H-region and the mechanisms by which they affect device performance. For the device with a low-density region length of 0.6 μm, a minimum H-region length of 0.2 μm is required to increase the output frequency from 518.16 to 926.28 GHz. For a low-density region length of 0.5 μm, an H-region length of 0.3 μm or more is required to reach a device frequency of 1.149 THz, all while maintaining the DC-to-AC conversion efficiency at about 1.63%. Instead of simply reducing device sizes, this study employs a step-like structural design to significantly increase the device's upper frequency limit and operating frequency.

Twisted bilayer transition metal dichalcogenides, governed by a single twisted interface, offer a versatile platform for manipulating interlayer coupling and emergent optical behavior. However, despite extensive studies on single twisted interface systems, how multi‐interface twisted stacking influences nonlinear optical behavior remains unclear. Here, we address this gap by systematically investigating the nonlinear optical performance of chemical vapor deposition (CVD) grown twisted trilayer MoS ₂ (TT‐MoS ₂ ) with multi‐interface twisted stacking. We fabricate all nine distinct trilayer MoS ₂ configurations by CVD. Raman and photoluminescence show that twisted interfaces modulate interlayer coupling, giving rise to stacking‐dependent phonon and excitonic responses. Atomic‐resolution scanning, transmission electron microscopy, and selected area electron diffraction confirm dual twisted interfaces and a supermoiré lattice. Guided by these structural insights, we examine the second harmonic generation (SHG) response across all nine trilayer configurations. Extending the coherent SH field superposition model to a trilayer system with two independent twisted interfaces, we develop a framework that describes how the dual interfaces govern the SHG intensity and polarization orientation across stacking configurations. Our findings establish TT‐MoS ₂ as a versatile nonlinear platform and open new avenues for understanding and exploiting multi‐interface twisted coupling to engineer nonlinear optical responses in van der Waals materials.

Altermagnets, featuring momentum-dependent spin splitting without net magnetization, provide a promising platform for spintronic functionalities beyond conventional ferromagnets and antiferromagnets. Here, we propose an altermagnetic spin photovoltaic battery consisting of a nonmagnetic centrosymmetric semiconductor sandwiched between two altermagnetic electrodes. Using first-principles quantum-transport simulations, we show that a V2Te2O/ZnSe/V2Te2O junction supports a pure spin photocurrent for opposite Néel vectors in the two altermagnetic electrodes, with spin-up and spin-down photocurrents equal in magnitude and opposite in sign. The effect persists under both linearly and circularly polarized light and remains tunable with photon energy and polarization angle. Our results establish a realistic route toward light-driven pure spin-current generation in altermagnetic junctions.

Underwater images in turbid environments suffer from scattering-induced contrast loss and structural degradation. We propose a frequency-decoupled underwater polarimetric image restoration network guided by a polarization-difference prior. Two-level wavelet decomposition separates low-frequency structures from high-frequency details. Wave-LFRM collaboratively restores multi-polarization low-frequency components, while HFGain uses restored structures to modulate high-frequency sub-bands. A polarization-consistency loss further constrains restoration. On a self-collected dataset, the method achieved PSNR/SSIM values of 33.606/0.960, 30.722/0.935, and 29.714/0.892 under low, medium, and high turbidity, respectively. For 1024 × 1024 inputs, the model required 1.74 million parameters and 46.87 GFLOPs and achieved a 17.38 ms inference time, demonstrating accurate and computationally efficient restoration suitable for video-rate processing.

Hydrostatic deformation is an effective approach for tuning the quantum properties of color centers in diamond, with significant implications for quantum sensing, computing, and communication. Compared to the widely studied nitrogen-vacancy (NV) centers, silicon-vacancy (SiV) centers exhibit more than a tenfold increase in coherent photon emission. In this work, we investigate the effects of hydrostatic pressure and tension on the SiV center in diamond using first-principles calculations with the r2SCAN meta-GGA (Generalized Gradient Approximation) functional. We demonstrate that under hydrostatic tension corresponding to an isotropic expansion exceeding 4%, the SiV center undergoes spontaneous symmetry breaking from the inversion-symmetric D3d structure to the asymmetric C3v configuration, similar to that of the NV center. Within the hydrostatic compression and tension range corresponding to isotropic deformations of −8%–4%, the optical properties and hyperfine parameters of the SiV center change monotonically, indicating promising potential for pressure- or deformation-sensing applications. A microscopic explanation of these trends is provided from an electronic structure perspective. The r2SCAN meta-GGA functional shows high accuracy in calculating hyperfine parameters, in agreement with experimental results. This study enhances our understanding of the optical properties and hyperfine interactions of SiV defects in diamond, laying the groundwork for their potential use in hydrostatic pressure or strain sensing applications.

Conventional gate-all-around (GAA) Si nanowire field-effect transistors (NWFETs) are reaching their fundamental scaling limits as channel length reduction exacerbates short-channel effects (SCEs). In this work, the performance of sub-5 nm gate-length (L_g) GAA SiGe NWFETs is systematically investigated using first-principles quantum transport simulations, revealing superior performance over conventional Si FETs. The results reveal that at L_g = 3 and 5 nm, key performance metrics such as on-state current (I_on), subthreshold swing (SS), delay time(τ), and power dissipation (PDP) meet the International Technology Roadmap for Semiconductors (ITRS) high-performance (HP) standards. At L_g = 5 nm, the n- and p-type devices demonstrate ultra-low SS values of 63 mV dec⁻¹ and 89 mV dec⁻¹, representing 42.2% and 14.4% reductions compared to conventional Si FETs, respectively. Furthermore, applying a −1% compressive strain significantly improves the performance of FETs, increasing the I_on by 41% and reducing the subthreshold swing, delay time, and power dissipation by 10%, 39%, and 14%, respectively. These findings underscore the significant potential of SiGe GAA NWFETs for future high-performance nanoelectronics.

Two-dimensional germanium is considered a promising new channel material to replace silicon owing to its lower effective mass and larger electron–hole mobility. To investigate the transport characteristics of single-layer germanane transistors with gate lengths (Lg) below 5 nm, we utilize an ab initio quantum transport methodology. It was found that the n-type germanane transistors having Lg of 3 and 5 nm satisfy the International Technology Roadmap for Semiconductors (ITRS) requirements for the on-state current (Ion), effective delay time, and power-delay products of high-performance (HP) devices. Notably, by introducing a negative capacitive (NC) dielectric layer, the p-type germanane transistor having an Lg of 5 nm is almost able to meet the ITRS demands for HP devices. Despite reducing the gate length to 2 nm through the incorporation of the NC dielectric layer, the on-state currents for both n-type and p-type still satisfy approximately 80% of the ITRS standard. Therefore, monolayer germanane presents promising potential as a channel material in a sub-5 nm scale for HP applications.