Semiconductor Layers Research Articles

Synthesis and Light-Matter Interaction of Low-Dimension Ordered-Disordered Layered Semiconductors.

Well-structured and ordered 2D layered semiconducting materials have excellent optical properties but limited advanced optoelectronic applications in their natural state. Altering their natural arrangement, through artificial heterostructures, strain and pressure engineering, chemical doping, intercalation, and alloying, can impart them with unusual optical properties and potentially enhance their performance in various applications. Among these approaches, alloying is generally difficult to control and disrupts the well-ordered homophilic crystal phase of these 2D crystals, albeit with the capability to control materials as thin as a single atomic layer. In this work, the synthesis of a low-dimension ordered-disordered layered 2D alloy of Mo(1-x)WxSe2 with clearly ordered in-plane segregations of nano-sized islands of individual MoSe2 and WSe2 across the material surface is reported. The optical analysis of this ordered-disordered layered Mo(1-x)WxSe2 reveals unique interfacial and interlayer coupling physics, such as the co-existence of intralayer (interfacial) and interlayer excitons and enhanced valley polarization of up to 50%, which is traditionally absent in ordered MoSe2, WSe2, or their heterostructures. Furthermore, the structure exhibits implies open circuit voltage (up to 1130mV), signifying its excellent open-circuit voltage potential if employed in photovoltaic devices. Overall, the reported low-dimension ordered-disordered semiconductor alloys can be useful in various optoelectronic applications.

Analytical Model for Investigating Linear and Nonlinear Behavior of Thin Semi-Conductive Layers on Insulator Surface

Analytical Model for Investigating Linear and Nonlinear Behavior of Thin Semi-Conductive Layers on Insulator Surface

Positive Oscillating Magnetoresistance in a van der Waals Antiferromagnetic Semiconductor

In all van der Waals layered antiferromagnetic semiconductors investigated so far, a negative magnetoresistance has been observed in vertical transport measurements, with characteristic trends that do not depend on applied bias. Here, we report vertical transport measurements on a layered antiferromagnetic semiconductor CrPS4 that exhibit a drastically different behavior, namely, a strongly bias-dependent, positive magnetoresistance that is accompanied by pronounced oscillations for devices whose thickness is smaller than 10 nm. We establish that this unexpected behavior originates from transport being space-charge limited and not injection limited as for the layered antiferromagnetic semiconductors studied earlier. Our analysis indicates that the positive magnetoresistance and the oscillations only occur when electrons are injected into in-gap defect states, whereas when electrons are injected into the conduction band, the magnetoresistance vanishes. We propose a microscopic explanation for the observed phenomena that combines concepts typical of transport through disordered semiconductors with known properties of the CrPS4 magnetic state, thus capturing all basic experimental observations. Our results illustrate the need to understand, in detail, the nature of transport through vdW magnets in order to extract information about the nature of the order magnetic states and its microscopic properties. Published by the American Physical Society 2025

Surface-sensitive magneto-optical detection of nonequilibrium spins and orbitals

Abstract The magneto-optical effect provides a direct probing tool for angular momentum in materials and has been used to detect ferromagnetic orders. Notably, owing to its surface sensitivity, the magneto-optical Kerr effect has been extended to detect nonequilibrium spin and orbital accumulations in initially nonmagnetic materials. The spatial distribution and temporal dynamics of spins and orbitals provide key aspects of the generation, transport, and relaxation mechanisms. With an accurate determination of the magneto-optic constant for spin and orbital, a distinctive evaluation of the magnitude of the spin and orbital accumulation is possible. The magneto-optical Kerr effect also allows for the investigation of spin-orbit torque in bilayers of nonmagnet and ferromagnet. The spin-orbit torque enables a quantitative analysis of the generation and transport of spins and orbitals, and the orbital-to-spin conversion process. In this article, we review recent studies on magneto-optical detection of nonequilibrium spin and orbital accumulations in single layers of semiconductors and metals and spin-orbit torque in bilayers including heavy metals, where spin generation is dominant, and light metals, where orbital generation is dominant.

Epitaxial growth of transition metal nitrides by reactive sputtering

Implementing transition metal nitride (TM-nitride) layers by epitaxy into group-III nitride semiconductor layer structures may solve substantial persisting problems for electronic and optoelectronic device configurations and subsequently enable new device classes in the favorable nitride semiconductor family. As a prominent example, the integration of the group-III-transition metal nitride AlScN enabled an improved performance of GaN based transistor structures due to stronger polarization fields as has been recently demonstrated. For other transition metal nitrides (TMNs) and their alloys with group-III nitrides a range of other interesting properties is expected to enable novel devices and applications. We investigated the compatibility of TM-nitride layers with the growth of GaN-based structures on silicon substrates. As we show TiN layers are compatible and particularly suited as highly conducting, metallic-like buffer layer enabling true vertical conduction without elaborate backside processing. Also, we demonstrate epitaxial growth of alloys based on ScN and AlN as well as of HfN layers on Si(111) substrates by reactive sputtering using high purity gases and targets. Particularly, we analyzed the crystal structure and the quality of Sc-rich AlxSc1-xN. For HfN layers, we find a unique impact on the growth polarity of MOVPE-grown GaN layers on Si(111) which changes to N-polar growth. This represents a simple and technologically scalable approach for N-polar GaN-based layers on Si substrates.

Volatile MoS2 Memristors with Lateral Silver Ion Migration for Artificial Neuron Applications

Layered 2D semiconductors have shown enhanced ion migration capabilities along their van der Waals (vdW) gaps and on their surfaces. This effect can be employed for resistive switching (RS) in devices for emerging memories, selectors, and neuromorphic computing. To date, all lateral molybdenum disulfide (MoS2)‐based volatile RS devices with silver (Ag) ion migration have been demonstrated using exfoliated, single‐crystal MoS2 flakes requiring a forming step to enable RS. Herein, present volatile RS with multilayer MoS2 grown by metal‐organic chemical vapor deposition (MOCVD) with repeatable forming‐free operation is presented. The devices show highly reproducible volatile RS with low operating voltages of ≈2 V and fast‐switching times down to 130 ns considering their micrometer‐scale dimensions. The switching mechanism is investigated based on Ag ion surface migration through transmission electron microscopy, electronic transport modeling, and density functional theory. Finally, a physics‐based compact model is developed and the implementation of the volatile memristors as artificial neurons in neuromorphic systems is exploredd.

Study of High Performance Nanoscale Channel Length Vertical Transistors with a Self-Aligned Blocking Layer.

A transistor design employing all vertically stacked components has attracted considerable attention due to the simplicity of the fabrication process and the high conductivity easily realized by achieving nanolevel short channel lengths with two-dimensional current paths. However, fundamental issues, specifically the blocking of the gate electrical field to the semiconductive channel layer and high leakage current at the "off" state, have impeded this configuration in becoming a major transistor design. To address these issues, it has been proposed to introduce a blocking layer (BL) with embedded hole structures and source electrode with embedded hole structures, enhancing gate field penetration and carrier modulation. The hole structure embedded in the source and the BL on the drain induced a desirable combined effect of gate field penetration and carrier pathway modulation. The align accuracy and the hole size difference between BL and source electrode were confirmed as the most important design parameters for high performance of a transistor. We therefore proposed a self-aligning lithography method using a built-in mask that allows high alignment accuracy between the source hole structure and the BL hole structure on the drain over a large area without a high-resolution process system. This method also enables easy and fast fabrication of nanoscale channels with high performance. This design resulted in a transistor with an output of 28 mA/cm2 and an on-off ratio exceeding 106 at 1 mV of VDS. However, at 3 V of VDS, the off-current increased significantly due to short-channel effects in the all metal electrode design. To solve this issue, Fermi level-tunable graphene replaced metal electrodes, maintaining an off-current below 10 pA and an on-off ratio around 107 at 3 V. In addition, the device demonstrates robust electrical properties to light without any special treatment and is stable with a threshold voltage shift of less than 1 V under bias stress. This study demonstrates that the proposed vertical transistor design is a viable candidate as a new major transistor design for various applications.

Quasi-phase-matched up- and down-conversion in periodically poled layered semiconductors

Quasi-phase-matched up- and down-conversion in periodically poled layered semiconductors

Improving the thermoelectric properties of septuple atomic-layer SnBi2Se4 by regulating the carrier concentration through Nb doping

Layered semiconductor materials have garnered significant attention in the thermoelectric field due to their excellent electrical property and intrinsically low lattice thermal conductivity. The septuple atomic-layered ternary compound SnBi2Se4 is reported as a promising thermoelectric material in both bulk and single-layer structures based on theoretical calculations, though experimental investigation remains unexplored. In this work, the melting and hot-press sintering methods were adopted to synthesize the septuple atomic-layered SnBi2Se4. Its unique layered crystal structure contributed to significant anisotropic transport properties and reduced thermal conductivity. However, its thermoelectric performance is constrained by a low carrier concentration that limits electrical conductivity. To solve this issue, the high-valent transition metal Nb was doped at Bi site to provide additional electrons. This doping resulted in a noticeable improvement in the performance of septuple atomic-layered SnBi2Se4 due to increased electrical conductivity and decreased thermal conductivity. Finally, a peak ZT ∼ 0.17 was obtained for SnBi1.97Nb0.03Se4 at 723 K, suggesting the effectiveness of Nb doping in enhancing the performance. These results indicate that septuple atomic-layered SnBi2Se4 is a highly promising thermoelectric material, though further performance improvements are needed.

Room-temperature spin-conserved electron transport to semiconductor quantum dots using a superlattice barrier.

To realize the optical transfer of electron spin information, developing a semiconductor layer for efficient transport of spin-polarized electrons to the active layers is necessary. In this study, electron spin transport from a GaAs/Al0.3Ga0.7As superlattice (SL) barrier to In0.5Ga0.5As quantum dots (QDs) is investigated at room temperature through a combination of time-resolved photoluminescence and rate equation analysis, separating the two transport processes from the GaAs layer around the QDs and SL barrier. The electron transport time in the SL increases for a thicker quantum well (QW) of SL due to the weaker wavefunction overlap between adjacent QWs. Additionally, the degree of conservation of spin polarization during transport varies with QW thickness. Rate equation analysis demonstrates an electron transport from SL to QDs while maintaining a high spin polarization for thick QWs. The achieved spin-conserved electron transport can be attributed to the combination of electron transport being sufficiently faster than the spin relaxation in SL and the suppressed spin relaxation in the p-doped GaAs layer capping the QDs. The findings indicate that SL is a promising candidate as an electron spin transport layer for optical spin devices.

Molecular precursors for the electrodeposition of 2D-layered metal chalcogenides.

Two-dimensional transition metal dichalcogenides (TMDCs) are highly anisotropic, layered semiconductors, with the general formula ME2 (M = metal, E = sulfur, selenium or tellurium). Much current research in this fieldfocusses on TMDCs for catalysis and energy applications; they are also attracting great interest for next-generation transistor and optoelectronic devices. Thelatter high-tech applications place stringent requirements on the stoichiometry, crystallinity, morphology and electronic properties of monolayer and few-layer materials. As a solution-based process, wherein the material grows specifically on the electrode surface, electrodeposition offers great promise as a readily scalable, area-selective growth process. This Review explores the state-of-the-art for TMDC electrodeposition, highlighting how the choice of precursor(or precursors), solvent and electrode designs, with novel 'device-ready' electrode geometries, influence their morphologies and properties, thus enabling the direct growth of ultrathin, highly anisotropic 2D TMDCs and much scope for future advances.

Surface Engineering of 2D Metal-Porphyrin Metal-Organic Frameworks Z-Scheme Heterostructure for Boosting and Stable Photocatalytic Hydrogen Evolution.

How to improve the stability and activity of metal-organic frameworks is an attractive but challenging task in energy conversion and pollutant degradation of metal-organic framework materials. In this paper, a facile method is developed by fabricating titanium dioxide nanoparticles (TiO2 NPs) layer on 2D copper tetracarboxylphenyl-metalloporphyrin metal-organic frameworks with zinc ions as the linkers (ZnTCuMT-X, "Zn" represented zinc ions as the linkers, the first "T" represented tetracarboxylphenyl-metalloporphyrin (TCPP), "Cu" represented the Cu2+ coordinated into the porphyrin macrocycle, "M" represented metal-organic frameworks, the second "T" represented TiO2 NPs layer, and "X" represented the added volume of n-tetrabutyl titanate (X = 100, 200, 300 or 400)). It is found that the optimized ZnTCuMT-200 showed greatly and stably enhanced H2 generation, which is ≈28.2 times and 47.0 times as high as those of the original ZnTCuM and TiO2, respectively. Combined with the results of free radical capture, X-ray photoelectron spectra (XPS), electron spin resonance (ESR), and theoretical calculation, a direct Z-scheme electron transfer mechanism is achieved to fully explain the enhanced photocatalytic performance. It demonstrates that facilely designing Z-scheme heterostructures based on porphyrin MOFs modified with an inorganic semiconductor layer can be an advantageous strategy for enhancing the stability and activity of photocatalytic hydrogen evolution.

Performance enhancement of InSnZnO thin-film transistors by modifying the dielectric-semiconductor interface with colloidal quantum dots.

Thin film transistors (TFTs) with InSnZnO (ITZO) and Al2O3 as the semiconductor and dielectric layers, respectively, were investigated, aiming to elevate the device performance. Chemically synthesized CuInS2/ZnS core/shell colloidal quantum dots (QDs) were used to passivate the semiconductor/dielectric interface. Compared with the pristine device, the device with the integrated QDs demonstrates remarkably improved electrical performance, including a higher electron mobility and a lower leakage current. Moreover, the integration of QDs largely mitigates hysteresis in the bidirectional transfer characteristics of the device. Improved negative bias stress stability is also observed in the device with QDs. The performance enhancement is ascribed to the reduction of the trap states induced by the defects in Al2O3, and the screening of electrical dipoles at the Al2O3/ITZO interface. This work proposes a new strategy to passivate the semiconductor/dielectric interface, which not only improves TFT performance, but also holds potential for optoelectronic applications.

The enhanced lifetime of printed GaS-based photodetectors with polymer encapsulation

Exhibiting excellent absorption in the UV–visible wavelength range makes layered gallium sulfide (GaS) semiconductor material a promising candidate for use in electronics and optoelectronics applications. Recently, a fully printed GaS-based photodetector has been proposed and fabricated, rendering a low-cost fabrication process in flexible electronics. However, the degradation of the semiconductor layer due to environmental conditions causes reliability issues and shortens their lifetime. Thus, in this study, an attempt has been made to encapsulate printed GaS-based photodetector using different polymers to hinder the degradation. It is demonstrated that encapsulating the printed GaS-based photodetector by utilizing the polymer-capping method with styrene co-polymers, Polystyrene-block-polyisoprene-block-polystyrene, highly hydrogenated poly(styrene)-block-poly(butadiene), partially hydrogenated poly(styrene)-block-poly(butadiene), increases the performance of the photodetector. The efficiency of the GaS-based photodetector printed on flexible polyethylene terephthalate (PET) substrate has reached up to 123 % in responsivity in 6 weeks after the polymer coating. Also, the device figure of merit, the detectivity value of the printed photodetector, has increased more than three times after the polymer coating compared to its as-deposited state. Meanwhile, it is observed that the fall and rise times of the printed GaS photodetector have remained constant. Based on these results attained in this study, it can be claimed that the polymer coating provides high performance and long stability in the printed GaS-based photodetectors on flexible substrates, which will pave the way for the further implementations of III-VI group layered semiconductor materials in electronics and optoelectronics applications.

Forming and compliance-free operation of low-energy, fast-switching HfOxSy/HfS2 memristors.

We demonstrate low energy, forming and compliance-free operation of a resistive memory obtained by the partial oxidation of a two-dimensional layered van-der-Waals semiconductor: hafnium disulfide (HfS2). Semiconductor-oxide heterostructures are achieved by low temperature (<300 °C) thermal oxidation of HfS2 under dry conditions, carefully controlling process parameters. The resulting HfOxSy/HfS2 heterostructures are integrated between metal contacts, forming vertical crossbar devices. Forming-free, compliance-free resistive switching between non-volatile states is demonstrated by applying voltage pulses and measuring the current response in time. We show non-volatile memory operation with an RON/ROFF of 102, programmable by 80 ns WRITE and ERASE operations. Multiple stable resistance states are achieved by modulating pulse width and amplitude, down to 60 ns, < 20 pJ operation. This demonstrates the capability of these devices for low-energy, fast-switching and multi-state programming. Resistance states were retained without fail at 150 °C over 104 s, showcasing the potential of these devices for long retention times and resilience to ageing. Low-energy resistive switching measurements were repeated under vacuum (8.6 mbar) showing unchanged characteristics and no dependence of the device on surrounding oxygen or water vapour. Using a technology computer-aided design (TCAD) tool, we explore the role of the semiconductor layer in tuning the device conductance and driving gradual resistive switching in 2D HfOx-based devices.

ANALYSIS OF THE STRUCTURE OF INTERFERENCE COATINGS FOR THE OPTIMIZATION OF THE PARAMETERS OF NARROWBAND OPTICAL FILTERS

To monitor the parameters of semiconductor layers deposited from solid solutions of A3B5 compounds during the epitaxy, optical control methods are used. The choice of optical monitoring methods is determined by the reactor design, which requires non-contact, fast, and precise measuring of wafer surface parameters. During the growth of epitaxial layers, the deposition of semiconductor compounds causes changes in the optical parameters of the wafer surface. To ensure precise measurements, it is essential to monitor these parameters within a narrow spectral range. In electro-optical monitoring systems, narrowband interference filters are used to select the desired spectrum. However, this type of filters is sensitive to several factors, such as environmental conditions, aging of the coating, and the angle of incidence. As a result, the manufacturing of narrowband optical filters with stable and precisely controlled optical characteristics presents a complex challenge. This article analyzes the impact of various factors on the optical characteristics of interference coatings. Quantitative analysis of these shows that shift of the selected spectral band with central wavelength λmax can approach or even exceed the full width at half maximum (FWHM). These changes in the optical characteristics of narrowband filters lead to a decrease in the accuracy required for optical monitoring during epitaxial processes, which leads to inaccuracies in measurements. The results of this analysis will guide the optimization of thin-film structures used in narrowband optical filters. The study of the shift in the selected band also enables, within certain limits, controlled changes to λmax or the compensation of its shift its shift during regular factor variations. Ultimately, it enables to take these changes into account when designing electro-optical systems for monitoring the epitaxial growth of semiconductor heterostructures, which increases the accuracy and stability of optical measurements in the required accuracy range.

TiO2/ZnO activated carbon composite for the removal of benzotriazole from water by a combination of sorption and photocatalytic processes

Organic xenobiotic substances are emitted into the environment from a number of preparations used in industry and households. Some are relatively persistent and difficult to biodegrade, so they pass relatively easily through wastewater treatment plants into the surface waters and contaminate drinking water sources. The goal of this study is to develop an innovative technological material that will effectively eliminate organic xenobiotics in water. The principle of our method is to combine carbon-based sorbent (biochar and Hydraffin) with a semiconductor layer (TiO2 or ZnO) to synthesize a photoreactive nanocomposite material which, in conjunction with UV/VIS exposure, can efficiently and safely degrade captured organic xenobiotics (benzotriazole, BTR in this study) in water through the processes of sorption and photocatalytic degradation. This nanocomposite should act as more effective alternative to the widely discussed composite biochar-TiO2. Specifically, the composite coated with ZnO provided the highest degradation efficiency of the photochemical process and it also had the highest sorption capacity for BTR because of the interactions with Zn.In this study, nevertheless, both types of composites are tested and compared their efficiency during removal of selected micropollutant representatives from waters.Graphical

Introducing Noncovalent Interactions in Conjugated Polymers to Enhance Backbone Coplanarity and Aggregation at the Interface to Improve Carrier Mobility.

In organic field-effect transistors (OFETs), the high carrier mobility of conjugated polymers (CPs) is significantly influenced by the maintenance of excellent coplanarity and aggregation, especially at the interface between the organic semiconductor and dielectric layer. Unfortunately, CPs typically exhibit poor coplanarity due to the single bond rotations between donor and acceptor units. Furthermore, there is relatively little research on the coplanarity of CPs at the interface. Herein, we propose a strategy of introducing noncovalent interactions to enhance the coplanarity of the backbone and promote the aggregation of the polymer at the interface, which should lead to significant enhancements in carrier mobility. The idea is proved by incorporating different volume fractions of oleic acid (OA) into poly(indacenodithiophene-co-benzothiadiazole) (IDTBT). OA can form hydrogen bonds, which has been verified by Fourier transform infrared spectroscopy (FT-IR). OA promotes the migration of IDTBT toward the interface, thereby enhancing aggregation, as verified by film-depth-dependent light absorption spectroscopy (FLAS) and contact angle (CA) experiments. The results from film-depth-dependent Raman spectroscopy (FRS), two-dimensional grazing incidence wide-angle X-ray scattering (2D GIWAXS), atomic force microscopy (AFM), and density functional theory (DFT) calculations suggest that films treated with OA exhibit enhanced backbone coplanarity and aggregation at the interface, resulting in an increase in carrier mobility to 4.24 ± 0.11 cm2 V-1 s-1 with the addition of OA.

High-temperature electrical superconductivity of layered nanoscale “heterostructures”

The paper discusses the possibility of realizing a superconducting state at temperatures of ≈102 K in layered nano-sized heterostructures – thin-film objects consisting of alternating layers of metals and semimetals (or semiconductors) with a thickness of (1–102) nm. The numerical estimates show that in such structures, an increase in the electron density in semimetals (semiconductors) leads to a change in the electrical conductivity up to the appearance of superconductivity. If we assume that superconductivity is realized as a result of electron-phonon interaction, causing the formation of Cooper electron pairs (Bardeen–Cooper–Schrieffer mechanism), then an increase in the electron density in the semiconductor layer of a nano-sized heterostructure should cause an increase in the pairing constant and so-called high-temperature superconductivity can be realized.

Scanning probe spectroscopy of sulfur vacancies and MoS2 monolayers in side-contacted van der Waals heterostructures

Abstract We investigate the interplay between vertical tunneling and lateral transport phenomena in electrically contacted van der Waals heterostructures made from monolayer MoS2, hBN, and graphene. We compare data taken by low-temperature scanning tunneling spectroscopy to results from room-temperature conductive atomic force spectroscopy on monolayer MoS2 with sulfur vacancies and with varying hBN layers. We show that for thick hBN barrier layers, where tunneling currents into the conductive substrate are suppressed, a side-contact still enables addressing the defect states in the STM via the lateral current flow. Few-layer hBN realizes an intermediate regime in which the competition between vertical tunneling and lateral transport needs to be considered. The latter is relevant for device structures with both a thin tunneling barrier and a side contact to the semiconducting layers.