Non-invasive digital etching of van der Waals semiconductors

Performance limit of all-wrapped monolayer MoS2 transistors

All-wrapped transistors consisting of two-dimensional transition-metal dichalcogenide channels are appealing candidates for post-silicon electronics. Based on the Boltzmann transport theory, here we report a comprehensive theoretical survey on the performance limits for monolayer MoS2 transistors with three prototypical gate dielectrics (Al2O3, HfO2 and BN), by including primary extrinsic charge scattering mechanisms present in practical devices. A concept of "dead space" between the dielectrics and channels is proposed and used in calculation to ameliorate the general overestimation in scattering intensity of surface optical phonons, which enables an accurate description of electronic transport behavior. Crucial device indices, including charge mobility and current density, are thoroughly analyzed for transistors at post-silicon technological nodes beyond 1 nm. The on-state current is estimated to be generally greater than 2 mA μm-1 at channel lengths below 10 nm. The results clarify the potential benefits in performance from extremely miniaturized monolayer-channel transistors for More-Moore electronics.

2D materials readiness for the transistor performance breakthrough

As the size of the transistor scales down, this strategy has confronted challenges because of the fundamental limits of silicon materials. Besides, more and more energy and time are consumed by the data transmission out of transistor computing because of the speed mismatching between the computing and memory. To meet the energy efficiency demands of big data computing, the transistor should have a smaller feature size and store data faster to overcome the energy burden of computing and data transfer. Electron transport in two-dimensional (2D) materials is constrained within a 2D plane and different materials are assembled by the van der Waals force. Owning to the atomic thickness and dangling-bond-free surface, 2D materials have demonstrated advantages in transistor scaling-down and heterogeneous structure innovation. In this review, from the performance breakthrough of 2D transistors, we discuss the opportunities, progress and challenges of 2D materials in transistor applications.

Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling

Layered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime and hold promise for integrated spintronic devices. While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies by optical excitation and detection, achieving spin wave control with microwaves is highly desirable, as modern integrated information technologies predominantly are operated with these. The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a hybrid approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultra-thin flakes of Cr₂Ge₂Te₆ (CGT) as thin as 11 nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets.

Noninvasive Photodelamination of van der Waals Semiconductors for High-Performance Electronics

Atomically thin 2D van der Waals semiconductors are promising candidate materials for post-silicon electronics. However, it remains challenging to attain completely uniform monolayer semiconductor wafers free of over-grown islands. Here, the observation of the energy-funneling effect and ambient photodelamination phenomenon in inhomogeneous few-layer WS₂ flakes under low-illumination fluencies down to several nW µm^-2 and its potential as a noninvasive atomic-layer etching strategy for selectively stripping the local excessive overlying islands are reported. Photoluminescent tracking on the photoetching traces reveals relatively fast etching rates of around 0.3-0.8 µm min^-1 at varied temperatures and an activation energy of 1.7 eV. By using crystallographic and electronic characterization, the noninvasive nature of the low-power photodelamination and the highly preserved lattice quality are also confirmed in the as-etched monolayer products, featuring a comparable density of atomic defects (≈4.2 × 10¹³ cm^-2 ) to pristine flakes and a high electron mobility of up to 80 cm² V^-1 s^-1 at room temperature. This approach opens a noninvasive postetching route for thickness uniformity management in 2D van der Waals semiconductor wafers for electronic applications.

Intrinsic ferromagnetism and the quantum anomalous Hall effect in two-dimensional MnOCl2 monolayers

Due to their potential application in spintronic devices, two-dimensional (2D) ferromagnetic materials are highly desired. We used first-principles calculations and Monte Carlo simulations to investigate the electronic structure and magnetic characteristics of the MnOCl₂ monolayers. We discovered two stable monolayer structures, Pmna-MnOCl₂ and Pmmn-MnOCl₂. Our findings show that the Pmna-MnOCl₂ monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.152 eV and a Curie temperature (T_C) of 202 K, while the Pmmn-MnOCl₂ monolayer is an intrinsic ferromagnetic Dirac semimetal with a high T_C (910 K) and triaxial magnetic anisotropy. We also show that a Pmmn-MnOCl₂ monolayer with a nontrivial band gap of 6.2 meV can achieve the quantum anomalous Hall effect (QAHE) with Chern number C = 1. Additionally, the existence of a gapless edge state can be flexibly regulated by choosing the terminal edges. Our studies reveal that the Pmmn-MnOCl₂ monolayer can serve as a candidate material to achieve high-temperature QAHE.