Semiconductor Layers Research Articles

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.

Lead Sulfide Quantum Dots as the Semiconductor Layer in Thin-Film Phototransistors for Infrared Thermal Image Sensing

Abstract This paper utilizes lead sulfide quantum dots, known for their high absorption coefficients, ease of solution processing, and high stability in air, as the semiconductor layer in thin-film phototransistors. These are combined with the most mature glass panel processing technology to create an infrared sensing device characterized by simple manufacturing processes, low production costs, small sensing areas, and large-area sensing capabilities. The novel lead sulfide quantum dot infrared sensing device proposed here exhibits remarkable performance at room temperature and under uncooled operation. It demonstrates a temperature sensitivity above 300oC for radiation temperature measurement, a fast total response time of 69 µs, and performance metrics such as noise of 1.9×10-12 A/Hz1/2, a maximum responsivity of 103 A/W, and a maximum detectivity of 2.04×1013 Jones. Therefore, the experimental results obtained here are significant for the research and design of future advanced quantum dot-based thermal imaging sensing applications.

Inkjet printing of sustainable ZTO/AlOx thin film transistors

Abstract Even though printed metal oxide thin film transistors (TFTs) have been a central topic of research in the past decade, the most notable results still require scarce elements such as gallium and indium, or high annealing temperatures (≥ 400 °C) when used non-critical raw materials such as zinc and tin.
In this work, safe, abundant and inexpensive materials such as zinc, tin and aluminum are explored to reach low-cost thin films and devices with both the semiconductor and dielectric layers deposited by inkjet printing and annealed at lower temperatures (300 °C). Alumina (AlOx) and zinc tin oxide (ZTO) inks containing a theoretical optimal V% of ethylene glycol (EG) were optimized for production of uniform and reproducible AlOx/ZTO thin film layers. Common ink parameters (such as the reverse Ohnesorge, capillary, Webber and Reynolds numbers) were evaluated and compared with relevant literature on inkjet drop formation mechanisms. Inks within theoretical optimal parameter values were printing optimized in terms of drops per inch, number of layers, UV substrate surface activation, print speed, and post annealing. A high-quality dielectric of two alumina layers was printed, having a breakdown field above 2.93 ± 0.33 MV.cm-1, and a dielectric constant of 7.74 ± 0.73 at 1 kHz. Thin film transistors (TFTs) of inkjet printed (IJP) ZTO/AlOx layers were produced with a maximum IOn/IOff ratio of 103 and a saturation mobility of 2.2 cm2.V-1.s‑1. This approach not only advances the field of printed electronics but also addresses concerns related to material scarcity, thermal budget, and production costs.

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.

Chiral Phonon, Valley Polarization, and Inter/Intravalley Scattering in a van der Waals ReSe2 Semiconductor.

Exploring valley manipulatable layered semiconductors is highly significant for valleytronic devices. Here, we report the phonon chirality and resulting inter/intravalley scattering in valley polarized van der Waals (vdW) layered ReSe2 by linearly/circularly polarized Raman (L/CPR), transmission (L/CPT), and photoluminescence (L/CPL) spectroscopic techniques. L/CPR combined with scanning transmission electron microscopy determines the Re chains' direction and displays the existence of chiral phonons. The LPT discloses the energetic valley polarization between the Re chains and its perpendicular crystal axis directions. Intriguingly, the valley polarization strength depends on the layer thickness even in a micrometer scale and abnormaly increases with temperature increase. Further, CPT manifests the optical rotation in ReSe2 due to strong chiral phonon-photon coupling. More essentially, L/CPL unveils a strong exciton-like effect (Stokes shift) that can be interpreted from the inter/intravalley scattering related to the (chiral) phonon-carrier coupling. This investigation suggests a promising platform based on a low-symmetry vdW ReSe2 semiconductor for exploring valley physics and fabricating valley(opto)tronic nanodevices.

Significantly improved energy storage performance of polyetherimide-based dielectric composites via employing core-shell organic-semiconductor@BaTiO3 nanoparticles

With fast development of modern industries and electrical systems, polymer dielectrics are urgently demanded to have high discharged energy density (Ud) in elevated temperature. In this paper, novel core-shell poly [2-((3,6,7,10,11-pentakis (hexyloxy) triphenylene-2-yl) oxy) ethyl methacrylate] (PHT) coated barium titanate nanoparticles (BT) (denoted as PHT@BT) are prepared, and then incorporate into polyetherimide (PEI) matrix via solution blending method. Semi-conductive organic shell layer can not only promote the dispersion and compatibility of BT nanoparticles but also construct deep trap. As a result, 0.3 wt% PHT@BT/PEI composites achieve maximal Ud of 7.62 J cm−3 at 641 MV m−1 and room temperature, which is 1.93 times that of pure PEI film (3.93 J cm−3 at 461 MV m−1). Importantly, the Ud of 4.86 J cm−3 is obtained at 150 °C. This work provides superior interfacial modifier for inorganic nanofiller, which is of great significance for the fabrication of polymer-based nanocomposites with superior Ud.

Electronic vs optical bandgap of stoichiometric V2O5: Effects of surface transfer doping and electron accumulation layer

The experimental and theoretical bandgap reported for stoichiometric V2O5, a layered semiconductor of great technological importance, spans a wide range of 1.7–4.8 eV. Using combined photoemission, absorption, and photoluminescence measurements, we show that the fundamental electronic gap of V2O5 is 1.85 eV, which is in close agreement with the value of 1.8 eV predicted by the first density functional theory studies, but is lower than the value of 2.2–2.8 eV obtained from optical absorption and photoemission studies. It is shown that this difference between the fundamental and optical gaps is due to the presence of a surface electron accumulation layer, which results in a Burstein–Moss shift of the Fermi level well into the conduction band. The underlying cause of degenerate electron doping is due to “surface transfer doping” by ambient water molecules as a result of the high electron affinity of the semiconductor.

High Mobility, High Carrier Density SnSe2 Field‐Effect Transistors with Ultralow Subthreshold Swing and Gate‐Controlled Photoconductance Switching

Abstract2D and layered semiconductors are considered as promising electronic materials, particularly for applications that require high carrier mobility and efficient field‐effect switching combined with mechanical flexibility. To date, however, the highest mobility has been realized primarily at low carrier concentration. Here, it is shown that few‐layer/multilayer SnSe2 gated by a solution top gate combines very high room‐temperature electron mobility (up to 800 cm2 V−1s−1), along with large on‐off current ratios (>105) and a subthreshold swing below the thermodynamic limit (50 mV per decade) in field‐effect devices, at exceptionally large sheet carrier concentrations of ≈1013 cm−2. Observed mobility enhancements upon partial depletion of the channel point to near‐surface defects or impurities as the mobility‐limiting scattering centers. Under illumination, the resulting gap states give rise to gate‐controlled switching between positive and negative photoconductance. The results qualify SnSe2 as a promising layered semiconductor for flexible and wearable electronics, as well as for the realization of advanced approaches to photodetection.

Nanoscale Mapping of Carrier Distribution Regulated by Polarization in 2D FeFETs.

The emergence of 2D ferroelectrics, sliding ferroelectrics, and 2D ferroelectric semiconductors has greatly expanded the potential applications of two-dimensional ferroelectric field-effect transistors (2D FeFETs) in nonvolatile memory, neuromorphic synapses, and negative capacitance. However, the interaction between ferroelectric and semiconductor layers remains not well understood, and characterization methods to correlate carriers and polarization dynamics at the nanoscale are still lacking. Utilizing in situ scanning microwave impedance microscopy and piezoresponse force microscopy measurements, we employed a Pb(Zr0.2Ti0.8)O3/MoS2-based 2D FeFET as an example to reveal, with high spatial resolution, the microscopic redistribution of carriers. This study uncovers the microscopic behavior of ferroelectric-semiconductor heterojunctions, paving the way for a deeper understanding of ferroelectric-gating effects and retention issues at the nanoscale in 2D FeFETs.

Research on the Carrier Characteristics of Power Cables Considering the Aging Status of Insulation and Semiconducting Layers

The 10 kV XPLE cable is widely used in highly cabled transmission and distribution networks. It is necessary to closely monitor the transient current, harmonic content, and electric field distribution of each layer of the insulation and semiconductive layers of the cable when they age and deteriorate, so as to promptly carry out circuit breaking treatment and prevent safety accidents. Considering the frequency sensitivity and dielectric sensitivity of the distributed Runit, Lunit, Gunit, and Cunit parameters of long cables, this paper quantitatively analyzes the frequency variation of 10 kV cable parameters under different aging states. Reconstructing the frequency variation process of typical electrical quantities through MATLAB PSCAD joint simulation, constructing fault circuits for cable insulation and semiconducting layers, obtaining transient currents in each layer of the cable under aging conditions, and conducting total harmonic distortion (THD) analysis to provide theoretical guidance for the subsequent monitoring and fault diagnosis of distribution cable status.

Enhancement of Synaptic Behavior in Organic Electrochemical Transistors via the Introduction of Layer‐by‐Layer Grown Metal‐Organic Framework

AbstractOrganic electrochemical transistors (OECTs) are promising for neuromorphic architectures as they can generate multiple electrical states through the control of ion transport. However, conventional OECTs face limitations in mimicking a fully functional biological synapse due to their inability to achieve long‐term plasticity. In this study, a metal‐organic framework (MOF)‐enhanced OECT (MOECT) is fabricated by introducing MOF into the ion‐organic semiconductor (OSC) layer. MOFs are synthesized using the layer‐by‐layer (LBL) method, and additional cross‐linked OSC is introduced to prevent damage to the semiconductor layers during synthesis. The synthesized MOF layers hinder the rediffusion of ions in OECT, allowing ions to remain in the OSC for an extended period. In this study, the MOECT showed a change in current depending on the doping level, recording a current state 4.4 × 107 times higher than that of pristine OECT. Ultimately, the developed MOECTs are applied as synaptic transistors. MOECTs show 14% higher excitatory postsynaptic current (EPSC) after 130 s compared to pristine OECT, thereby strengthening the long‐term plasticity characteristics of neuromorphic devices. This method enhances the performance of synaptic transistors by introducing MOF, offering various possibilities through the selection of different MOF structures and ions, indicating it is a methodological approach with high potential.

The Impact of Non‐Monolithic Semiconductor Capacitance on Organic Electrochemical Transistors Performance and Design

AbstractThe existing device models for organic electrochemical transistors (OECTs) fail to provide any device design guidelines for optimized performance parameters such as transconductance that are pivotal for the applications OECTs in sensing. Moreover, the current models are based on the questionable assumption of a homogenous organic semiconductor layer, and all predict a linear behavior of the resistance with the OECT channel length. Consequently, the experimentally observed nonlinear resistance behavior in OECTs has been overlooked thus far. Here, an OECT device model is developed that accurately describes the nonlinear behavior of the OECT channel resistance and offers the first guidelines for maximizing transconductance. The model is inherently nonlinear and the nonlinearity stem from the non‐monolithic capacitance of the organic semiconductor layer. Moreover, the model provides a consistent and reliable estimations for the contact resistance in OECTs. The success of the model in accurately describing and providing predictions of the OECT operation by relating the device's geometrical parameters with electrochemical parameters of the semiconductor layer paves the way toward unlocking OECT potentials in diverse applications, from biosensing to neuromorphic computing and flexible electronics.

Quasi-particle Propagation Across Semiconductor–Mott Insulator Interfaces

As a prototypical example for a heterostructure combining a weakly and a strongly interacting quantum many-body system, we study the interface between a semiconductor and a Mott insulator. Via the hierarchy of correlations, we derive and match the propagating or evanescent (quasi) particle solutions on both sides and assume that the interactions among the electrons in the semiconducting regions can be absorbed by an effective potential. While the propagation is described by a band-like dispersion in both the weakly and the strongly interacting case, the inverse decay length across the interface follows a different dependence on the band gap in the Mott insulator and the semiconductor. As one consequence, tunnelling through a Mott insulating layer behaves quite differently from a semiconducting (or band insulating) layer. For example, we find a strong suppression of tunnelling for energies in the middle between the upper and lower Hubbard band of the Mott insulator.

Atom-Vacancy-Defect-Derived Electric Hysteresis Loops and Stochastic Low-Frequency Noises in Few-Atom Layer MoS2.

Atom-vacancy-defects present in various materials yield numerous interesting physical phenomena, even obstructing high performance in some cases. On the other hand, their valuable applications to novel devices, such as nitrogen vacancy centers in diamond for quantum bits, have gathered significant attention. In particular, these tendencies become more substantial in two-dimensional (2D) (atomically) thin van der Waals layers. However, correlations with various kinds of atom defects are still under exploration. Herein, we find the stochastic behaviors of large hysteresis loops with strong photoresponse in the static electrical properties in few-atom layer semiconductors, molybdenum disulfide (MoS2). The temperature dependence and transmission electron microscopy reveal that they arise from pairs of two neighboring in-plane S-vacancy defects, which predominantly present only around the interface at the MoS2 flake/substrate, with activation energies ∼0.35 eV. The low-frequency (f) (LF) noise measurements clarify a high f shift in the two 1/f2-dependent regimes, implying stochastic behaviors of electric charges through the S-vacancy pairs with high-speed charge(spin) transitions across low kinetic energy barriers between narrow discrete states. The shallow energy sates are formed from the highly uniform S-vacancy pairs interacting with Mo atoms, which act like quantum dots. The observed stochastic operation holds promise for various application, particularly for probabilistic neuromorphic computation in artificial intelligence.

Heterogeneous Integration of Complementary Field-Effect Transistors for High-Performance Micro LED Displays.

This study investigates a micro light-emitting diode (µLED) pixel circuit using the heterogeneous integration of complementary field-effect transistors (CFETs). The CFETs are fabricated using a semiconductor layer composed of tellurium (Te) and indium-gallium-zinc oxide (IGZO) layers. Te and IGZO layers in the heterostructure IGZO/Te film exhibit hexagonal and amorphous phases, respectively, indicating that each layer maintains independent material characteristics. The fabricated IGZO/Te CFETs exhibit ambipolar behavior with a turn-on voltage of 2.0V and field-effect mobility of 0.74 and 1.42 cm2 V-1 s-1 for p-type and n-type channels, respectively. Inverters comprising IGZO/Te CFET and IGZO TFT exhibit inverting behavior. A µLED pixel circuit is designed using IGZO/Te CFETs based on pulse width modulation (PWM). The proposed circuit uses an inverter structure with IGZO/Te and IGZO to control the emission time, suppressing the wavelength shift of µLEDs depending on the µLED current levels. The operation of the proposed pixel circuit is investigated through simulation and measurement of the fabricated circuit. The fabricated µLED pixel circuit successfully exhibits PWM operation, controlling the emission time and luminance. Consequently, IGZO/Te CFETs show promise as devices for high-quality µLED displays.

Emergence of mixing-induced polymorphic phase with higher mobility in alkylated layered organic semiconductors

Emergence of mixing-induced polymorphic phase with higher mobility in alkylated layered organic semiconductors

Analysis of the mobility behavior of MOS2 2D FETs

In this work we analyze the behavior of 2D FETs, with channel length greater than the mean free path, and using MOCVD or CVD deposition method for the deposition of the 2D semiconductor layer, with different dielectric materials and EOTs. We show that transfer, output and conductance characteristics can be modeled with precision, considering the hopping transport mechanism as the predominant one, similarly to amorphous or polycrystalline TFTs. It was also observed that for the devices with channel length above one micrometer, mobility increased with the gate voltage as a power law. For channel lengths of 1 µm and 100 nm, mobility decreased with voltage, which in this case, can be attributed to other extrinsic effects, as the presence of high series resistance at the drain and source, which becomes more important as the channel length reduces, modifying its behavior with gate voltage.

Self-activated epitaxial growth of ScN films from molecular nitrogen at low temperatures

Unlike naturally occurring oxide crystals such as ruby and gemstones, there are no naturally occurring nitride crystals because the triple bond of the nitrogen molecule is one of the strongest bonds in nature. Here, we report that when the transition metal scandium is subjected to molecular nitrogen, it self-catalyzes to break the nitrogen triple bond to form highly crystalline layers of ScN, a semiconductor. This reaction proceeds even at room temperature. Self-activated ScN films have a twin cubic crystal structure, atomic layering, and electronic and optical properties comparable to plasma-based methods. We extend our research to showcase Sc’s scavenging effect and demonstrate self-activated ScN growth under various growth conditions and on technologically significant substrates, such as 6H–SiC, AlN, and GaN. Ab initio calculations elucidate an energetically efficient pathway for the self-activated growth of crystalline ScN films from molecular N2. The findings open a new pathway to ultralow-energy synthesis of crystalline nitride semiconductor layers and beyond.

Design and simulation of highly efficient CZTS/CZTSSe based thin-film solar cell

Thin-film solar cells are a substitute for more common crystalline silicon solar cells, which consist of thin semiconductor layers. Thin-film materials comprise direct bandgap and can absorb sunlight more efficiently than silicon. In this article, a double-absorber-based thin-film solar cell comprising CZTS/CZTSSe is designed and optimized through numerical simulation. The proposed solar cell structure consists of a transparent window layer made of aluminum-doped zinc oxide, followed by an intrinsic zinc oxide layer, an n-type cadmium sulfide layer, and a p-type combined absorber layer of copper zinc tin sulfide (Cu2ZnSnS4) (CZTS) and copper zinc tin sulfur-selenium alloy (Cu2ZnSn(S,Se4)) (CZTSSe). The structure is further optimized by introducing two interfacial layers between CZTSSe/CZTS and CZTS/CdS. The highest conversion efficiency is achieved by adjusting the thicknesses of the layers, the doping densities in different layers, and the defect densities in the two absorber layers. The optimized model, with a total thickness of 2.01 μm, demonstrates an open-circuit voltage (Voc) of 0.7669 V, a short-circuit current (Jsc) of 48.57740 mA/cm2, a fill factor (FF) of 70.61%, and efficiency (η) of 26.31%. These results suggest that CZTS is a promising candidate for replacing other thin-film photovoltaic materials, such as CdTe. The proposed double-absorber thin-film solar cell optimizes doping concentration, thickness, and defect density to enhance performance metrics and efficiency while utilizing non-toxic materials to promote cost-effective, environmentally friendly energy solutions.

Device simulation study of multilayer MoS2 Schottky barrier field-effect transistors

Molybdenum disulfide (MoS2) is a representative two-dimensional layered transition-metal dichalcogenide semiconductor. Layer-number-dependent electronic properties are attractive in the development of nanomaterial-based electronics for a wide range of applications including sensors, switches, and amplifiers. MoS2field-effect transistors (FETs) have been studied as promising future nanoelectronic devices with desirable features of atomic-level thickness and high electrical properties. When a naturallyn-doped MoS2is contacted with metals, a strong Fermi-level pinning effect adjusts a Schottky barrier and influences its electronic characteristics significantly. In this study, we investigate multilayer MoS2Schottky barrier FETs (SBFETs), emphasizing the metal-contact impact on device performance via computational device modeling. We find thatp-type MoS2SBFETs may be built with appropriate metals and gate voltage control. Furthermore, we propose ambipolar multilayer MoS2SBFETs with asymmetric metal electrodes, which exhibit gate-voltage dependent ambipolar transport behavior through optimizing metal contacts in MoS2device. Introducing a dual-split gate geometry, the MoS2SBFETs can further operate in four distinct configurations:p - p,n - n,p - n, andn - p. Electrical characteristics are calculated, and improved performance of a high rectification ratio can be feasible as an attractive feature for efficient electrical and photonic devices.