Van Der Waals Semiconductors Research Articles

Probing and manipulating the Mexican hat-shaped valence band of In2Se3

Ferroelectrics based on van der Waals semiconductors represent an emergent class of materials for disruptive technologies ranging from neuromorphic computing to low-power electronics. However, many theoretical predictions of their electronic properties have yet to be confirmed experimentally and exploited. Here, we use nanoscale angle-resolved photoemission electron spectroscopy and optical transmission in high magnetic fields to reveal the electronic band structure of the van der Waals ferroelectric indium selenide (α-In2Se3). This indirect bandgap semiconductor features a weakly dispersed valence band, which is shaped like an inverted Mexican hat. Its form changes following an irreversible structural phase transition of α-In2Se3 into β-In2Se3 via a thermal annealing in ultra-high vacuum. Density functional theory supports the experiments and reveals the critical contribution of spin orbit coupling to the form of the valence band. The measured band structure and its in situ manipulation offer opportunities for precise engineering of ferroelectrics and their functional properties beyond traditional semiconducting systems.

Spatiotemporal imaging of nonlinear optics in van der Waals waveguides.

Van der Waals (vdW) semiconductors have emerged as promising platforms for efficient nonlinear optical conversion, including harmonic and entangled photon generation. Although major efforts are devoted to integrating vdW materials in nanoscale waveguides for miniaturization, the realization of efficient, phase-matched conversion in these platforms remains challenging. Here, to address this challenge, we report a far-field ultrafast imaging method to track the propagation of both fundamental and harmonic waves within vdW waveguides with femtosecond and sub-50 nanometre spatiotemporal precision. We focus on light propagation in slab waveguides of rhombohedral-stacked MoS2, a vdW semiconductor with large nonlinear susceptibility. Our method allows systematic optimization of nonlinear conversion by determining the phase-matching angles, mode profiles and losses in waveguides without prior knowledge of material optical constants. Using this approach, we show that both multimode and single-mode rhombohedral-stacked MoS2 waveguides support birefringent phase matching, demonstrating the material's potential for efficient on-chip nonlinear optics.

High rectification and gate-tunable photoresponse in 1D-2D lateral van der waals heterojunctions

<p>The self-passivating surfaces and reduced tunneling leakage current enable the creation of ideal Schottky contacts in van der Waals (vdW) semiconductor heterojunctions. However, simultaneously achieving high rectification ratios, low reverse leakage currents, and rapid photoresponse remains challenging. Here, we present a one-dimensional (1D)/two-dimensional (2D) mixed-dimensional heterostructure photodiode to address these challenges. The significant valence band offset and minimal electron affinity difference in this structure ensure high rectification ratios and efficient charge collection. Additionally, the dimensional disparity between the 1D and 2D materials, characterized by a smaller contact area and significant thickness difference, results in low reverse leakage current and a high current on-off ratio. Moreover, it enables gate-tunable band structure transitions. Our device exhibits an exceptional rectifying ratio of 4.7 × 10<sup>7</sup> and a high on-off ratio of 5 × 10<sup>7</sup> (<i>V</i><sub>ds</sub> = 2 V and, <i>V</i><sub>g</sub> = 30 V) at room temperature. Under a gate voltage of 20 V, the photodiode achieves a specific detectivity (<i>D</i><sup><i>*</i></sup>) of 4.9 × 10<sup>14</sup> Jones, a rapid response time of 14 μs, and an extended operational wavelength approaching to <styled-content style-type="number">1550</styled-content> nm. The strategic combination of mixed-dimensional design and band engineering yields a 1D-2D p-n heterojunction photodiode with remarkable sensitivity, repeatability, and fast response, underscoring the potential of vdW semiconductors for advanced optoelectronic applications.</p>

Contact engineering for two-dimensional van der Waals semiconductors

Contact engineering for two-dimensional van der Waals semiconductors

Low-Temperature Evaporation Combined with Pulsed Laser Annealing for the Preparation of High-Quality p-Type Single-Crystal Tellurium-Layered Material Thin Films

Two-dimensional (2D) materials are currently recognized as key materials for device miniaturization due to their advantages such as perfect surface, high carrier mobility, and the ability to reduce short-channel effect. Despite significant progress in n-type two-dimensional semiconductor materials, van der Waals semiconductors with high hole mobility as the main carriers still face challenges. Tellurium layered material thin films have attracted widespread attention due to their low processing temperature and high hole mobility characteristics. Compared to other p-type 2D materials, tellurium layered material thin films offer superior uniformity and environmental durability. In this work, high-quality single-crystal p-type two-dimensional semiconductor tellurium layered material thin films were grown using low-temperature physical vapor deposition and pulsed laser annealing. This technique offers precise control over material thickness, minimal surface roughness, high uniformity, reduced defects, and improved hole carrier mobility. Electron backscatter diffraction (EBSD) analysis revealed that untreated tellurium layered material thin films had grain sizes of up to 3 micrometers. Following pulsed laser treatment, grain sizes increased to over 4 micrometers, with the main grain direction changing from (01-10) to (10-10). X-ray diffraction (XRD) results post-pulsed laser annealing showed higher peak intensities and smaller full width at half maximum (FWHM) compared to untreated and conventionally thermal annealed conditions, confirming significant enhancements in crystallinity and grain size. Additionally, minimal color change was observed in the tellurium layered material thin films after pulsed laser annealing from KAM spectra. Furthermore, significant crystallinity enhancement was also observed from high-resolution transmission electron microscopy (HRTEM) and Raman spectra. Figure 1

Electrically switchable chiral nonlinear optics in an achiral ferroelectric 2D van der Waals halide perovskite.

Two-dimensional (2D) van der Waals (vdW) semiconductors play a key role in developing nanoscale nonlinear optical devices. 2D Ruddlesden-Popper lead halide perovskites (RPPs) expand the potential of using 2D vdW semiconductors in nonlinear optical applications because they exhibit electrically switchable and chiral second-order optical nonlinearity originating from the emergence of ferroelectricity and chirality. However, electrically switchable chiral nonlinear optics has not yet been realized because of the difficulty in electrically manipulating chiral structures. Here, we demonstrate that chiral second-harmonic generation (SHG) can be electrically induced and switched in an achiral biaxial ferroelectric 2D RPP. We observe reversible and continuous electrical switching of SHG circular dichroism and large nonlinear chiroptical activity. Polarization-resolved SHG imaging reveals that electrical poling induces the ferroelectric multidomain structure arising from the biaxial nature of the material, and the planar chirality appears. Our findings show a simple electrical control of the nonlinear chiroptical responses and establish chiral nonlinear optics based on ferroelectric 2D RPPs.

Directing Charge Carriers and Ferroelectric Domains at Lateral Interfaces in van der Waals Heterostructures.

Emergent phenomena in traditional ferroelectrics are frequently observed at heterointerfaces. Accessing such functionalities in van der Waals ferroelectrics requires the formation of layered heterostructures, either vertically stacked (similar to oxide ferroelectrics) or laterally stitched (without equivalent in 3D-crystals). Here, we investigate lateral heterostructures of the ferroelectric van der Waals semiconductors SnSe and SnS. A two-step process produces ultrathin crystals comprising an SnSe core laterally joined to an SnS edge-band, as confirmed by Raman spectroscopy, transmission electron microscopy (TEM) imaging, and electron diffraction. TEM shows a moiré pattern across the SnSe core due to coverage by an ultrathin SnS layer. The ability of the lateral interface (IF) to direct excited carriers, probed by cathodoluminescence, shows electron transfer over 560 nm diffusion length from the SnS edge-band. Large, thin flakes supporting ferroelectricity allow investigating domains and domain wall interactions in uniform crystals and lateral heterostructures. Polarized optical microscopy of sub-20 nm flakes consistently shows ⟨110⟩ oriented stripe domains with mirror-twin domain walls. Heterostructures adopt two domain configurations, with domains either constrained to the SnSe core or propagating across the entire SnSe-SnS flakes. The combined results demonstrate multifunctional van der Waals heterostructures with high-quality IFs presenting extraordinary opportunities for manipulating carrier flows and ferroelectric domain patterns.

Electrical Control of Valley Polarized Charged Exciton Species in Monolayer WS2.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the charged exciton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible nonmonotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a resonant interplay between neutral and charged exciton states.

A high-performance selenium nanoflake-based avalanche photodetector

Photodetectors are now indispensable in our daily lives, and there is a pressing need to explore new materials and mechanisms that can push the boundaries of device performance. Two-dimensional (2D) van der Waals (vdW) semiconductors have emerged recently as a promising material platform with exceptional optoelectronic properties, making them particularly suitable for high-performance photodetectors. However, photoinduced carrier generation in conventional 2D vdW photodetectors are usually limited, and new mechanisms need to be introduced to enhance device performance. Herein, we report a high-performance avalanche photodetector based on selenium (Se) nanoflakes. Our device achieves a high photoresponsivity (R) and specific detectivity (D*) of 361 A·W−1 and 2.4 × 1012 Jones, respectively. These figures of merit are two orders of magnitude higher than that in conventional Se photoconductive photodetectors. As a large bandgap vdW semiconductor, the Se channel allows the application of an extremely large bias voltage across it, and the resulting high electric field leads to the avalanche multiplication of carriers, which lays the groundwork for the improved device performance.

Van der Waals semiconductor InSe plastifies by martensitic transformation.

Inorganic semiconductor materials are crucial for modern technologies, but their brittleness and limited processability hinder the development of flexible, wearable, and miniaturized electronics. The recent discovery of room-temperature plasticity in some inorganic semiconductors offers a promising solution, but the deformation mechanisms remain controversial. Here, we investigate the deformation of indium selenide, a two-dimensional van der Waals semiconductor with substantial plasticity. By developing a machine-learned deep potential, we perform atomistic simulations that capture the deformation features of hexagonal indium selenide upon out-of-plane compression. Unexpectedly, we find that indium selenide plastifies through a martensitic transformation; that is, the layered hexagonal structure is converted to a tetragonal lattice with specific orientation relationship. This observation is corroborated by high-resolution experimental observations and theory. It suggests a change of paradigm, where the design of new plastically deformable inorganic semiconductors can focus on compositions and structures that facilitate phase transformations, going beyond the conventional dislocation slip.

Unveiling anharmonic scattering in van der Waals semiconductor GaPS4 through Raman spectroscopy and theoretical calculation

Unveiling anharmonic scattering in van der Waals semiconductor GaPS4 through Raman spectroscopy and theoretical calculation

Liquid Metal Oxide-Assisted Integration of High-k Dielectrics and Metal Contacts for Two-Dimensional Electronics.

Two-dimensional van der Waals semiconductors are promising for future nanoelectronics. However, integrating high-k gate dielectrics for device applications is challenging as the inert van der Waals material surfaces hinder uniform dielectric growth. Here, we report a liquid metal oxide-assisted approach to integrate ultrathin, high-k HfO2 dielectric on 2D semiconductors with atomically smooth interfaces. Using this approach, we fabricated 2D WS2 top-gated transistors with subthreshold swings down to 74.5 mV/dec, gate leakage current density below 10-6 A/cm2, and negligible hysteresis. We further demonstrate a one-step van der Waals integration of contacts and dielectrics on graphene. This can offer a scalable approach toward integrating entire prefabricated device stack arrays with 2D materials. Our work provides a scalable solution to address the crucial dielectric engineering challenge for 2D semiconductor-based electronics.

Controlling Gold-Assisted Exfoliation of Large-Area MoS2 Monolayers with External Pressure.

Gold-assisted exfoliation can fabricate centimeter- or larger-sized monolayers of van der Waals (vdW) semiconductors, which is desirable for their applications in electronic and optoelectronic devices. However, there is still a lack of control over the exfoliation processes and a limited understanding of the atomic-scale mechanisms. Here, we tune the MoS2-Au interface using controlled external pressure and reveal two atomic-scale prerequisites for successfully producing large-area monolayers of MoS2. The first is the formation of strong MoS2-Au interactions to anchor the top MoS2 monolayer to the Au surface. The second is the integrity of the covalent network of the monolayer, as the majority of the monolayer is non-anchored and relies on the covalent network to be exfoliated from the bulk MoS2. Applying pressure or using smoother Au films increases the MoS2-Au interaction, but may cause the covalent network of the MoS2 monolayer to break due to excessive lateral strain, resulting in nearly zero exfoliation yield. Scanning tunneling microscopy measurements of the MoS2 monolayer-covered Au show that even the smallest atomic-scale imperfections can disrupt the MoS2-Au interaction. These findings can be used to develop new strategies for fabricating vdW monolayers through metal-assisted exfoliation, such as in cases involving patterned or non-uniform surfaces.

A Perspective on tellurium-based optoelectronics

Tellurium (Te) has been rediscovered as an appealing p-type van der Waals semiconductor for constructing various advanced devices. Its unique crystal structure of stacking of one-dimensional molecular chains endows it with many intriguing properties including high hole mobilities at room temperature, thickness-dependent bandgap covering short-wave infrared and mid-wave infrared region, thermoelectric properties, and considerable air stability. These attractive features encourage it to be exploited in designing a wide variety of optoelectronics, especially infrared photodetectors. In this Perspective, we highlight the important recent progress of optoelectronics enabled by Te nanostructures, which constitutes the scope of photoconductive, photovoltaic, photothermoelectric photodetectors, large-scale photodetector array, and optoelectronic memory devices. Prior to that, we give a brief overview of basic optoelectronic-related properties of Te to provide readers with the knowledge foundation and imaginative space for subsequent device design. Finally, we provide our personal insight on the challenges and future directions of this field, with the intention to inspire more revolutionary developments in Te-based optoelectronics.

High In-Plane Thermoelectric Performance of Layered Bi4O4SeCl2.

The van der Waals semiconductor Bi4O4SeCl2 has recently attracted great interest due to its extremely small lattice thermal conductivity, which may find possible application in the field of energy conversion. Herein, we accurately predict the thermoelectric transport properties of Bi4O4SeCl2 using first-principles calculations and Boltzmann transport theory, where the carrier relaxation time is obtained by fully considering the electron-phonon coupling. It is found that a maximum p-type ZT value of 3.1 can be reached at 1100 K along the in-plane direction, which originates from increased Seebeck coefficient induced by multivalley band structure, as well as enhanced electrical conductivity caused by relatively stronger intralayer bonding. Besides, it is interesting to note that comparable p- and n-type ZT values can be realized in certain temperature regions, which is very desirable in the fabrication of thermoelectric modules.

Raman spectrum and phonon thermal transport in van der Waals semiconductor GaPS4

The emergent van der Waals semiconductor GaPS4 is heralding frontiers for gallium-based semiconductors. Despite its potential, the intricacies of its Raman spectrum and phonon heat transport remain elusive. In this research, experimental and theoretical methods are employed to give a comprehensive portrayal. The Raman spectra and phonon calculations obtained were cross-validated, affirming the study's credibility. A total of 28 Raman peaks were identified, with all phonon irreducible representations delineated. Advanced calculations unveiled notable shifts in the transition of GaPS4 from bulk to monolayer. During this process, phonons undergo a red shift, and the vibration contributions of different atoms change. The lifetime and group velocity of low wavenumber phonons are markedly reduced, suppressing the thermal conductivity in the monolayer. The thermal conductivity of GaPS4 bulk at 300 K is 0.5 W/m K, and 0.13 W/m K for monolayer, while the thermal conductivity in the cleavage direction is lower. These findings offer a detailed account of the complex Raman spectra and phonon thermal transport properties of GaPS4, setting the stage for its subsequent exploration and prospective applications in electronic and thermal devices, and contributing to enriching condensed matter theory of phonon thermal transport in van der Waals materials.

Strain engineering of the transition metal dichalcogenide chalcogen-alloy WSSe

Alloying has been a powerful and practical strategy to widen the palette of physical properties available to semiconductor materials. Thanks to recent advances in the synthesis of van der Waals semiconductors, this strategy can be extended to monolayers (MLs) of transition metal dichalcogenides (TMDs). Due to their extraordinary flexibility and robustness, strain is another powerful means to engineer the electronic properties of two-dimensional (2D) TMDs. In this article, we combine these two approaches in an exemplary metal dichalcogenide chalcogen-alloy, WSSe. Highly strained WSSe MLs are obtained through the formation of micro-domes filled with high-pressure hydrogen. Such structures are achieved by hydrogen-ion irradiation of the bulk material, a technique successfully employed in TMDs and h-BN. Atomic force microscopy studies of the WSSe ML domes show that the dome morphology can be reproduced in terms of the average of the elastic parameters and adhesion energy of the end compounds WSe2 and WS2. Micro-photoluminescence measurements of the WSSe domes demonstrate that the exceedingly high strains (ε∼4%) achieved in the domes trigger a direct-to-indirect exciton transition, similarly to WSe2 and WS2. Our findings heighten the prospects of 2D alloys as strain- and composition-engineerable materials for flexible optoelectronics.

Enhanced interlayer electron transfer by surface treatments in mixed-dimensional van der Waals semiconductor heterostructures

We investigate the excitonic species in WS2 monolayers transferred onto III–V semiconductor substrates with different surface treatments. When the III–V substrates were covered with amorphous native oxides, negatively charged excitons dominated the spectral weight in low-temperature near-resonance photoluminescence (PL) measurements. However, when the native oxides of the III–V substrates were reduced, neutral excitons began to dominate the spectral weight, indicating a reduction in the electron density in the WS2 monolayers. The removal of the native oxides enhanced the electron transfer from the WS2 monolayer to the III–V substrate. In addition, an additional shoulder-like PL feature appeared ∼50 meV below the emission of neutral excitons, which can be attributed to the emission of localized excitons. When the III–V substrate surface was passivated by sulfur after the reduction of the native oxides, neutral excitons still dominated the spectral weight. However, the low-energy PL shoulder disappeared again, suggesting the effective delocalization of excitons through substrate surface passivation. Surface engineering of the semiconductor substrates for two-dimensional (2D) materials can provide a novel approach to control the carrier density of the 2D materials, implement deterministic carrier localization or delocalization for the 2D materials, and facilitate the interlayer transfer of charge, spin, and valley currents. These findings open the avenue for novel device concepts and phenomena in mixed-dimensional semiconductor heterostructures.

Hole-Doped Nonvolatile and Electrically Controllable Magnetism in van der Waals Ferroelectric Heterostructures

Electrical control of magnetism in van der Waals semiconductors is a promising step towards development of two-dimensional spintronic devices with ultralow power consumption for processing and storing information. Here, we propose a design for two-dimensional van der Waals heterostructures (vdWHs) that can host ferroelectricity and ferromagnetism simultaneously under hole doping. By contacting an InSe monolayer and forming an InSe/In2Se3 vdWH, the switchable built-in electric field from the reversible out-of-plane polarization enables robust control of the band alignment. Furthermore, switching between the two ferroelectric states (P ↑ and P ↓) of hole-doped In2Se3 with an external electric field can interchange the ON and OFF states of the nonvolatile magnetism. More interestingly, doping concentration and strain can effectively tune the magnetic moment and polarization energy. Therefore, this provides a platform for realizing multiferroics in ferroelectric heterostructures, showing great potential for use in nonvolatile memories and ferroelectric field-effect transistors.

Acoustic and optical phonons in quasi-two-dimensional MPS3 antiferromagnetic semiconductors

We report the results of the investigation of the acoustic and optical phonons in quasi-two-dimensional antiferromagnetic semiconductors of the transition metal phosphorus trisulfide family with Mn, Fe, Co, Ni, and Cd as metal atoms. The Brillouin–Mandelstam and Raman light scattering spectroscopies were conducted at room temperature to measure the acoustic and optical phonon frequencies close to the Brillouin zone center and the Γ−A high symmetry direction. The absorption and index of refraction were measured in the visible and infrared ranges using the reflectometry technique. We found an intriguing large variation, over ∼28%, in the acoustic phonon group velocities in this group of materials with similar crystal structures. Our data indicate that the full-width-at-half-maximum of the acoustic phonon peaks is strongly affected by the optical properties and the electronic bandgap. The acoustic phonon lifetime extracted for some of the materials was correlated with their thermal properties. The results are important for understanding the layered van der Waals semiconductors and assessing their potential for optoelectronic and spintronic device applications.