Carrier Mobility Research Articles

Impact of high-k insulators on electrical properties of junctionless double gate strained transistor

High-k dielectric insulators are required to reduce leakage and increase transistor performance. They are able to impact the mobility of carriers in transistors positively, leading to better device performance in advanced transistor architecture. Nevertheless, an in-depth analysis of how high-k dielectric insulators influence transistor characteristics must be carried out to determine their suitability. The objective of this study is to investigate the impact of high-k insulators towards electrical properties of junctionless double gate strained transistor. The simulation works is done using process/device simulator Silvaco Athena/Atlas. Based on the retrieved results, the magnitude of I<sub>ON</sub>, on-off ratio, g<sub>m</sub>, and C<sub>int</sub> for TiO<sub>2</sub>-based device are approximately 63%, 99%, 62%, and 89% respectively higher than the lowest permittivity material-based device. The TiO<sub>2</sub>-based device also exhibits the lowest magnitude in I<sub>OFF</sub> and SS compared to others. However, a significant degradation in f<sub>T</sub> magnitude have been observed for TiO<sub>2</sub>-based device significantly due to its large capacitances

Blue-emission crystalline OLED doped with DMAC-DPS TADF material.

Doping thermally activated delayed fluorescence (TADF) materials with high exciton utilization into crystalline hosts with high carrier mobility is an effective approach for developing novel OLEDs. This approach harnesses the strengths of both materials to realize high-performance blue light-emitting crystalline organic light-emitting diodes (C-OLEDs). Nevertheless, the high triplet energy levels of blue emitting TADF materials may facilitate the outflow of triplet excitons through Dexter energy transfer to the lower energy levels within the crystalline host, thus leading to efficiency losses in the device. In this study, we present a pioneering strategy designed to improve the exciton utilization efficiency of TADF materials in C-OLED by leveraging the up-conversion capability of TTA materials to reclaim triplet excitons. With a well-designed energy level structure, this device achieves a maximum EQE of 5.6% and a low turn-on voltage of 2.7 V. The benefits of the crystalline host allowed for fast turn-on, and a rapid increase in brightness and current density, leading to significantly improved blue photon output and a lower series resistance Joule heat loss ratio. This work introduces a novel approach to employ TADF materials in crystalline hosts and manage excitons within the emissive layer of devices, aiming to develop high-performance C-OLEDs.

Exploring Ferroelectricity and Photocatalytic Performance in Wurtzite Zn2VN3: A Novel Approach for Hydrogen Evolution

Photocatalytic hydrogen production represents a sustainable and eco-friendly approach to energy generation. Employing Density Functional Theory (DFT) as the analytical framework, the study demonstrates that wurtzite-structured Zn2VN3 exhibits exceptional ferroelectric polarization, significantly surpassing that of conventional ferroelectric materials. Additionally, Zn2VN3 is characterized by enhanced carrier mobility, a critical attribute for augmenting photocatalytic efficiency. Of particular note is the advantageous alignment of Zn2VN3's band edge positions for visible light photocatalysis, coupled with a high solar-to-hydrogen (STH) efficiency, thereby positioning it as a formidable candidate for hydrogen production. The synthesis of a Zn2VN3@MoS2 heterojunction structure is shown to substantially enhance its photocatalytic properties, as evidenced by significant improvements in the hydrogen evolution reaction (HER) performance. Overall, this meticulous analysis positions Zn2VN3 as a material of considerable potential, poised to advance optoelectronic technology and contribute significantly to efficient and sustainable photocatalytic hydrogen production endeavors.

Two-Dimensional MXene-Based Functional Composites for Photocatalysts: Current Status and Perspectives

Abstract: MXenes, as novel two-dimensional (2D) transition metal carbides, nitrides or carbonitrides, have excellent metal conductivity, high carrier mobility, and surface-terminated groups regulated band structure. It can be thus used as a cocatalyst in photocatalytic systems to improve the photocatalytic properties. This review represented recent research progress on the controllable construction of MXene-based functional composites with zero-dimensional (0D), one-dimensional (1D), 2D, and three-dimensional (3D) semiconductor photocatalysts and their applications for photocatalysts. Extensive information related to 2D MXene-Based composites for photocatalysts and their associated patents were collected. The construction methods and photocatalytic enhancement mechanisms of 2D MXene-based composite photocatalysts were given. Due to their excellent physical and chemical properties, 2D MXene composites have been widely used in pollutant removal, hydrogen production, CO2 reduction, and nitrogen fixation. Through the construction of 2D MXene-based functional composite photocatalysts with novel structures and excellent performance, it provides a new perspective for the design and construction of high-efficiency photocatalysts. The future research directions of MXene-based composite photocatalysts was proposed.

Optoelectronic and Transport Properties of Layer-dependent Two-Dimensional Perovskite Cs3Bi2I9

The emergence of two-dimensional (2D) perovskite provides an ideal platform for designing and fabricating microelectronic and optoelectronic devices. Here, we present detailed ab initio calculations to comprehensively explore the layer-dependent optoelectronic and transport properties of the newly synthesized 2D perovskite Cs3Bi2I9. The calculations reveal that the indirect band gap of monolayer, bilayer, and trilayer Cs3Bi2I9 decreases from 2.41 to 2.35 eV. The charge carriers of few-layered perovskite Cs3Bi2I9 are dominated by electrons, and the highest electron and hole carrier mobilities are 140 and 62 cm²V⁻¹s⁻¹ along the b axis for bilayer Cs3Bi2I9. Exciton binding energies decrease from 2.48 to 1.19 eV with an increment of layer, and the calculated exciton level drops down into the valence band to generate potential exciton insulator. 2D Cs3Bi2I9 exhibits potential in the field of ultraviolet detection and photoluminescent devices due to large exciton energy and ultraviolet absorption.

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.

Extreme electron-hole drag and negative mobility in the Dirac plasma of graphene.

Coulomb drag between adjacent electron and hole gases has attracted considerable attention, being studied in various two-dimensional systems, including semiconductor and graphene heterostructures. Here we report measurements of electron-hole drag in the Planckian plasma that develops in monolayer graphene in the vicinity of its Dirac point above liquid-nitrogen temperatures. The frequent electron-hole scattering forces minority carriers to move against the applied electric field due to the drag induced by majority carriers. This unidirectional transport of electrons and holes results in nominally negative mobility for the minority carriers. The electron-hole drag is found to be strongest near room temperature, despite being notably affected by phonon scattering. Our findings provide better understanding of the transport properties of charge-neutral graphene, reveal limits on its hydrodynamic description, and also offer insight into quantum-critical systems in general.

Enhanced Efficiency and Intrinsic Stability of Wide-Bandgap Perovskite Solar Cells through Dimethylamine-Based Cation Engineering.

High efficiency and stable wide-bandgap perovskite solar cells (PSCs) are important for perovskite-based tandem solar cells. However, the efficiency and stability of wide-bandgap PSCs suffers from severe phase segregation and surface defects. In this study, we propose a cation engineering strategy for wide-bandgap perovskite deposited by a two-stepsequential methodthrough the incorporation ofdimethylamine hydroiodide (DMAI)into the lead halide complex in the first step. The DMAI additive modifies the crystal structure and grain growth of perovskite film, resulting in enhancing crystal quality, suppressing photo-induced halide segregation, reducing defect density, and improving charge carrier mobility. As a result, we achieved a champion photoelectric conversion efficiency (PCE) of 21.9% for 1.68 eV wide-bandgap PSCs. Additionally, the stability of PSCs based on DMA doped perovskite was strongly enhanced, after being exposed to ambient air for 1500 hours, the unencapsulated device maintained an impressive 80.6% of their initial efficiency, demonstrating great potential for stable and efficient wide-bandgap PSCs.

Graphene Oxide-Doped Indium Oxide Buffer Film Sandwiched between Titanium Oxide Layers for the Development of Photosensitive Resistive Memory Devices.

Over the past decade, metal oxide semiconductors have attracted considerable attention because of their transparency, high intrinsic charge carrier mobility, and charge carrier density. Metal oxide semiconductors also provide a promising route to develop resistive memory devices because of the tunability of their conductivity via the removal of oxygen ions, forming oxygen vacancies that can act as electron donors. Here, this paper reports the fabrication of a resistive random-access memory (ReRAM) device with TiO2 and TiO2-x layers and introduces a solution-processed In2O3-graphene oxide (GO) buffer layer from a water-based precursor solution to tune the switching characteristics. The devices were compared with a ReRAM device with no buffer layer, and the device composition was optimized by tuning the GO concentration in the layers. The devices showed hysteresis under cyclically scanned bias between positive and negative voltages with clear switching between the low resistance state (LRS) and the high resistance state (HRS). The optimized device also showed a more than 3 orders of magnitude difference in resistance between the LRS and HRS and a stable current retention. The devices also showed photoresponsive behaviors. In the LRS, the current increased significantly under illumination, while in the HRS, the current deteriorated slowly when constantly illuminated. These results highlight the dual role of the GO flakes in the buffer layer by increasing the resistance in the HRS and providing a photoresponsive switching capability. The ReRAM device was connected to a TFT device, and its feasibility as a memory component in a digital circuit was successfully demonstrated. These results provide a new avenue for developing metal-oxide-based ReRAM devices with GO-containing active layers.

Analyzing Carrier Density and Hall Mobility in Impurity‐Free Silicon Virtually Doped by External Defect Placement

AbstractImpurity doping at the nanoscale for silicon is becoming less efficient with conventional techniques. Here, an alternative virtual doping method is presented for silicon that can achieve an equivalent carrier density while addressing the primary limitations of traditional doping methods. The doping for silicon is carried out by placing aluminum‐induced acceptor states externally in a silicon dioxide dielectric shell. This technique can be referred to as direct modulation doping. The resistivity, carrier density, and mobility are investigated by Hall effect measurements to characterize the carrier transport using the new doping method. The results thereof are compared with carrier transport analysis of conventionally doped silicon at room‐temperature, demonstrating a 100% increase in carrier mobility at equal carrier density. The sheet density of hole carriers in silicon due to modulation doping remains nearly constant, ≈4.7 × 1012 cm−2 over a wide temperature range from 300 down to 2 K, proving that modulation‐doped devices do not undergo carrier freeze‐out at cryogenic temperatures. In addition, a mobility enhancement is demonstrated with an increase from 89 cm2 Vs−1 at 300 K to 227 cm2 Vs−1 at 10 K, highlighting the benefits of the new method for creating emerging nanoscale electronic devices or peripheral cryo‐electronics to quantum computing.

Two-Dimensional Pentagonal Materials with Parabolic Dispersion and High Carrier Mobility

Materials with high carrier mobility, represented by graphene, have garnered significant interest. However, the zero band gap arising from linear dispersion cannot achieve an ideal on–off ratio in field-effect transistors (FETs), limiting practical applications in certain fields. In contrast, parabolic dispersion usually exhibits extremely high carrier mobility and an appropriate band gap. In this work, we predicted a planar pentagonal lattice composed entirely of pentagons (namely penta-MX2 monolayer), where M = Ni, Pd and Pt, X = group V elements. Using first-principles calculations, we demonstrated a parabolic dispersion within this framework, which results in intriguing phenomena, such as a direct band gap (0.551–1.105 eV) and extraordinary high carrier mobility. For penta-MX2 monolayer, the carrier mobility can attain ~1 × 108 cm2 V−1 s−1 (PBE), surpassing those of black phosphorene, graphene and 2D hexagonal materials. This monolayer also displays anisotropic mechanical properties and significant absorption peaks in the ultraviolet spectrum. Remarkably, 2D penta-MX2 monolayers are promising for successful experimental exfoliation, particularly when X is a nitrogen element, opening up new possibilities for designing two-dimensional semiconductor materials characterized by high carrier mobility.

Synthesis, Structures and Air-stable N-type Organic Field-effect Transistor (OFET) Properties ofFunctionalized-phenanthrene Conjugated Asymmetric N-heteroacenes.

The development of stable high-performance n-type organic semiconductors for applications in organic field-effect transistors (OFETs) under ambient conditions is desirable but challenging. To address this issue, we here synthesized a series of functionalized-phenanthrene conjugated asymmetric N-heteroacenes, where the phenanthrene moiety was modified by N substitution or Br functionalization at different positions to induce various degrees of asymmetry in their structures. The photophysical and electrochemical properties of these molecules were studied, and their packing patterns were analysed. The OFETs based on these materials were fabricated through simple spin-coating method, and the as-resulted thin films were treated with different conditions. The devices exhibit typical n-type performances under ambient conditions with charge carrier mobilities up to 4.27 × 10-3 cm2V-1s-1. The crystallinities and morphologies of these thin films were studied to investigate the correlations between the device performances and thin-film characteristics. Our study suggests that phenanthrene conjugated N-heteroacenes can be developed as promising air-stable solution-processable n-type semiconducting materials, and Br modification at certain positions of phenanthrene is beneficial in adjusting the thin-film properties for the improvement of OFET performances.

A-Site Engineering with Thiophene-Based Ammonium for High-Efficiency 2D/3D Tin Halide Perovskite Solar Cells.

Tin halide perovskites are the most promising candidate materials for lead-free perovskite solar cells (PSCs) thanks to their low toxicity and ideal band gap energies. The introduction of 2D/3D mixed perovskite phases in tin-based PSCs (TPSCs) has proven to be the most effective approach to improving device efficiency and stability. However, a 2D perovskite phase normally shows relatively low carrier mobility, which will be unfavorable for carrier transfer in the devices. In this work, we used a thiophene-based cation 2-(thiophen-3-yl)ethan-1-aminium (3-TEA) as a spacer to form a novel 2D perovskite phase in TPSCs, which shows the most promising effect on the performance enhancement in comparison with other cations like 2-(thiophen-2-yl)ethan-1-aminium (2-TEA) and benzene-based 2-phenylethan-1-aminium (PEA). Theoretical calculations reveal that 3-TEA enables the most compact crystal packing of [SnI6]4- octahedral layers, resulting in the lowest hole effective mass and formation energy in the 2D phase. This effect significantly enhances device efficiency and stability by facilitating more efficient carrier transfer within the 2D phase. These findings indicate that thiophene-based 2D perovskites are well-suited for high-performance TPSCs.

A‐Site Engineering with Thiophene‐Based Ammonium for High‐Efficiency 2D/3D Tin Halide Perovskite Solar Cells

AbstractTin halide perovskites are the most promising candidate materials for lead‐free perovskite solar cells (PSCs) thanks to their low toxicity and ideal band gap energies. The introduction of 2D/3D mixed perovskite phases in tin‐based PSCs (TPSCs) has proven to be the most effective approach to improving device efficiency and stability. However, a 2D perovskite phase normally shows relatively low carrier mobility, which will be unfavorable for carrier transfer in the devices. In this work, we used a thiophene‐based cation 2‐(thiophen‐3‐yl)ethan‐1‐aminium (3‐TEA) as a spacer to form a novel 2D perovskite phase in TPSCs, which shows the most promising effect on the performance enhancement in comparison with other cations like 2‐(thiophen‐2‐yl)ethan‐1‐aminium (2‐TEA) and benzene‐based 2‐phenylethan‐1‐aminium (PEA). Theoretical calculations reveal that 3‐TEA enables the most compact crystal packing of [SnI6]4− octahedral layers, resulting in the lowest hole effective mass and formation energy in the 2D phase. This effect significantly enhances device efficiency and stability by facilitating more efficient carrier transfer within the 2D phase. These findings indicate that thiophene‐based 2D perovskites are well‐suited for high‐performance TPSCs.

Comparing the Efficacies of Electrospun ZnO and TiO2 Nanofibrous Interlayers for Electron Transport in Perovskite Solar Cells

ZnO and TiO2 are both well-known electron transport materials. Their comparison of performance is considered advantageous and novel. Therefore, a viable electrospinning route was considered for the development of highly polycrystalline TiO2 and ZnO nanofibers as an electron transport material (ETM) for perovskite solar cells. The materials were well-characterized in terms of different analytical techniques. The X-ray diffraction detected polycrystalline structural properties corresponding to TiO2 and ZnO. Morphological analysis by scanning electron microscopy revealed that the nanofibers are long, uniform, and polycrystalline, having a diameter in the nanometer range. Optoelectronic properties showed that TiO2 and ZnO exhibit absorption values in the ultraviolet and visible ranges, and band gap values for TiO2 and ZnO were 3.3 and 3.2 eV, respectively. TiO2 bandgap and semiconductor nature were more compatible with Electron Transport Layer (ETL) compared to ZnO. Electrical studies revealed that TiO2 nanofibers have enhanced values of conductivity and sheet carrier mobility compared to ZnO nanofibers. Therefore, higher photovoltaic conversion efficiency was achieved for TiO2 nanofibers (10.4%) compared to ZnO (8.5%).

First-principles study of the electron and phonon transport properties in monolayer boron monosulfide

Abstract Motivated by the excellent electronic properties of theoretical proposed monolayer boron monosulfides (BS) binary materials, which belong to a light-element member of the group IIIA chalcogenides family, we have investigated the electron and phonon transport properties of three monolayer phases of BS (α, β and γ) using first-principles and Boltzmann transport theory methods. The calculated electronic structures show that α- and β-BS are semiconductors with indirect bandgaps of 4.01 eV and 3.87 eV, respectively, whereas the γ-BS is a semiconductor with a direct bandgap of 2.97 eV. Additionally, due to their light carrier effective masses, superior high carrier mobilities are observed, thus bringing high Seebeck coefficients. We also find that due to the absence of imaginary phonon frequencies, the three monolayer phases of BS show dynamical stability. The phonon group velocity and relaxation time are also calculated to analyze the phonon thermal conductivity. We find that strong phonon scattering makes γ-BS monolayer possess extremely low phonon thermal conductivity (1.2 and 2.7 Wm−1K−1 along the x and y directions at 300 K, respectively), whereas α- and β-BS monolayers show a relatively high phonon thermal conductivity. Therefore, the highest predicted ZT value of 1.7 ( n -type) at 700 K is obtained for monolayer γ-BS along the x direction. However, α- and β-BS monolayers have extremely low ZT values. Our results reveal that the γ-BS monolayer has wider promising applications in high performance thermoelectric material than α- and β-BS monolayers.

Decreasing Carrier Mobility under Concentrated Illumination and Its Effect on the Series Resistance and Conversion Efficiency of Silicon Concentrator Solar Cells

The carrier mobility of solar cell materials is a critical physical parameter that determines the series resistance. Furthermore, series resistance is an essential factor affecting the conversion efficiency of concentrator solar cells under high sunlight illumination. However, there has been slight discussion regarding the relationship between these factors. In the previous study, it was reported that concentrated illumination decreased carrier mobility; however, the scattering mechanism has not yet been clarified. Moreover, carrier–carrier scattering was discussed; however, it did not cause a mobility reduction. Thus effective mass changes are discussed as a possible cause of the decrease in carrier mobility under concentrated illumination. The cause of the decrease in mobility under concentrated sunlight illumination cannot be fully elucidated; however, it is a change in lattice scattering. Furthermore, the effect of the decrease in mobility due to concentrated illumination on the series resistance and conversion efficiency of Si solar cells using numerical calculations is clarified. The increase in the series resistance caused by the mobility reduction is ≈10% under 30 suns.

Production of Ultrathin and High-Quality Nanosheet Networks via Layer-by-Layer Assembly at Liquid-Liquid Interfaces.

Solution-processable 2D materials are promising candidates for a range of printed electronics applications. Yet maximizing their potential requires solution-phase processing of nanosheets into high-quality networks with carrier mobility (μNet) as close as possible to that of individual nanosheets (μNS). In practice, the presence of internanosheet junctions generally limits electronic conduction, such that the ratio of junction resistance (RJ) to nanosheet resistance (RNS), determines the network mobility via μNS/μNet ≈ RJ/RNS + 1. Hence, achieving RJ/RNS < 1 is a crucial step for implementation of 2D materials in printed electronics applications. In this work, we utilize an advanced liquid-interface deposition process to maximize nanosheet alignment and network uniformity, thus reducing RJ. We demonstrate the approach using graphene and MoS2 as model materials, achieving low RJ/RNS values of 0.5 and 0.2, respectively. The resultant graphene networks show a high conductivity of σNet = 5 × 104 S/m while our semiconducting MoS2 networks demonstrate record mobility of μNet = 30 cm2/(V s), both at extremely low network thickness (tNet < 10 nm). Finally, we show that the deposition process is compatible with nonlayered quasi-2D materials such as silver nanosheets (AgNS), achieving network conductivity close to bulk silver for networks <100 nm-thick.

High Charge Carrier Mobility in Porphyrin Nanoribbons

Polydisperse edge‐fused nickel(II) porphyrin nanoribbons have been synthesized by Yamamoto coupling followed by gold(III)‐mediated fusion, with average degrees of polymerization of up to 37 repeat units (length 31 nm). Time‐resolved optical‐pump terahertz spectroscopy measurements indicate that photo‐generated charge carriers have dc mobilities of up to 205 cm2 V–1 s–1 in these nanoribbons, exceeding the values previously reported for most other types of nanoribbon or π‐conjugated polymer.

Ultrafast terahertz conductivity in epitaxial graphene nanoribbons: an interplay between photoexcited and secondary hot carriers

Abstract Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 µm wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier–carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.