Reduced Carrier Mobility Research Articles

Exponentially reduced carrier mobility of natural ester via blocking effect of 2D hexagonal boron nitride nanosheets

In this work, the boron nitride (BN) nanosheets were dispersed in natural esters to fabricate the dielectric nanofluid. Microstructures and chemical compositions of the nanosized BN are determined. The stability, viscosity, and thermal conductivity of the BN nanofluid, were obtained. And the dissipation factor, electrical conductivity, and relative permittivity of the BN nanofluid, were measured. The ion mobilities and migrating times of the nanofluid were measured under different volumetric fractions of BN nanosheets and temperatures. Results show that the electrical conductivity and dissipation factor of the nanofluid decreased by 54% and 48% with the addition of only 0.1% of BN nanosheets under 110°C. Both the half-reduced carrier mobility of the nanofluid and the declined ion concentration by blocking of BN nanosheet contributed to the exponentially reduced electrical conductivity and enhanced dielectric performances of the BN nanofluid. The suppression coefficient k is proposed to quantitatively describe the hinder effect of ion migration in nanofluids by 2D BN nanosheet. Results provide a strategy to design and develop advanced nanofluids with low-dielectric loss by aids of surface adsorption of ion and steric hindrance of 2D nanosheets.

Influence of affinity, band gap and ambient temperature on the efficiency of CIGS solar cells

The performance of CI1-xGxS solar cells was studied by investigating the effects of composition (x) on affinity and band gap according to an experimental equation using a 2D-ATLAS simulator. Based on the simulation results, the highest efficiency of 22.69% was obtained at an optimum composition of x = 0.3. The influence of x on other parameters of solar cells including short-circuit current (JSC), open-circuit voltage (VOC), and fill factor (%FF) was also examined. Investigating the effect of ambient temperature on the above-mentioned parameters at the optimum composition revealed a reduction in reliability when increasing the ambient temperature. This phenomenon was found to originate from lattice scattering and a reduction in carrier mobility.

Analysis of Temperature Effect in Quadruple Gate Nano-scale FinFET

Quadruple gate FinFET is a promising candidate among other multi-gate MOS devices due to it’s better scalability and higher short channel effect suppression capability in advanced technology node. In this work, we report the effect of temperature on various performance metrics of quadruple gate FinFET like electrostatic integrity, carrier mobility, switching ratio, analog/RF, and linearity parameters with the help of well-calibrated numerical device simulation-based study (TCAD). While increasing the temperature from 250 °K to 450 °K, due to the enhanced bulk phonon scattering, degradation in carrier effective mobility is observed. Sub-threshold swing and drain induced barrier lowering coefficient vary linearly with temperature. At high temperature, due to the reduced carrier mobility, degradation in the analog figure of merits is observed. RF performance parameters also degrade primarily due to the enhanced gate capacitance and reduced trans-conductance. However, some linearity parameter improves at high temperature due to the reduced magnitude of the higher-order derivative of trans-conductance. Another major contribution of this work is, we demonstrate the inflection point or temperature compensation point in some analog/RF performance metrics where the device characteristics almost invariant with temperature.

Homo-epitaxial growth of n-GaN layers free from carbon-induced mobility collapse and off-angle-dependent doping variation by quartz-free hydride vapor phase epitaxy

Certain undesired phenomena are observed in n-GaN layers grown by metal–organic chemical vapor deposition (MOCVD) due to the unavoidable C-induced carrier compensation. They are a drastic reduction in carrier mobility, called mobility collapse, and significant non-uniformity in the carrier concentration due to the off-angle dependence of the C-incorporation efficiency of the process. These phenomena are particularly severe for low doping levels between 1015 and 1016/cm3, which are suitable for fabricating drift layers used in vertical-type GaN power devices that operate in the range of a few kilovolts to tens of kilovolts. However, the C-related undesired characteristics are absent in homo-epitaxial n-GaN layers grown by quartz-free hydride vapor phase epitaxy (QF-HVPE), recently developed by us. The utilization of C-free raw materials alongside quartz-free parts enables the growth of highly pure GaN crystals with negligible Si, C, and O incorporations. These crystals exhibited an electron concentration in the low-1015/cm3 range with the highest reported room temperature electron mobility, μ, of 1470 cm2/V s among GaN crystals, whereas n-GaN layers with similar carrier concentrations but containing C-compensation, as in the case of those grown by MOCVD, exhibited a severe mobility collapse (μ = 288 cm2/V s). High uniformity in the carrier concentration with a small standard deviation of 4.0% was observed in a 2-in. n-GaN wafer grown by QF-HVPE on a GaN substrate with an off-angle variation of 0.3°. On the other hand, the standard deviation of the carrier concentration in wafers grown by MOCVD was approximately 17% because of the off-angle-dependent C-incorporation.

Ultrahigh energy storage performance and fast charge-discharge capability in Dy- modified SrTiO3 linear ceramics with high optical transmissivity by defect and interface engineering

The excellent energy storage and pulse charge-discharge performance ceramics with high temperature stability and optical transmissivity are competitive for the development of electronic devices. In this work, comprehensive improved performances are simultaneously realized in DyxSr1-xTiO3 (DST) ceramics through defect and interface engineering. Meanwhile, increased high insulation grain boundary and reduced carrier mobility promote the increase of grain boundary resistance, and the increase of grain resistance primarily originates from the fact that DyTi' can easily attract oxygen vacancy, forming defect dipoles and restraining the movement of oxygen vacancies. Additionally, the relative density of ceramics is increased by sintering in O2 and doping Dy ions. Hence, x = 0.008 ceramics exhibit high coverable energy storage density of 4.00 J/cm3, high energy storage efficiency of 89.49%, excellent frequency (1Hz-1 kHz) and temperature stability (20-120 °C) and transmittance of >60% are achieved at 510 kV/cm at 0.2 mm. Furthermore, x = 0.008 ceramics possess the prominent current density of 420.1 A/cm2 and the prominent power density of 19.2 MW/cm3. Meanwhile, the short discharge rate of 20 ns and excellent stability of temperature (20-120 °C) are also achieved, suggesting that it is one candidate with strong potential for the application of lead-free high power capacitors.

Influence of boron doping amount on properties of ZnO:B films grown by LPCVD technique and its correlation to a-Si:H/μc-Si:H tandem solar cells

Boron-doped ZnO:B (BZO) films with various doping levels have been prepared on large-area substrates by low pressured chemical vapor deposition technique. The influence of doping amount on electrical and optical properties of BZO films has been investigated. It is found that ZnO phase synthesis is hardly affected when the doping gas flow varies from 25 to 100 sccm, but the preferential orientation of grain growth is influenced progressively. It is interesting that there should be a threshold value of doping gas flow of 75 sccm that will cause an abrupt reduction in grain size of BZO and therefore dramatically weakens the light-scattering capacity of the film. It is also noted that the boron atoms doped in BZO films are partly electrically active, and moreover, the heavier doping level, the more inactive B atoms, which not only reduces carrier mobility, but also boosts a stronger light absorption due to enhanced impurity scattering. When the doping gas flow is 75 sccm, the BZO film can achieve a proper comprehensive property with a Rsq of 15.2 Ω/□, an average haze of 21.3% and an average TT of 80.2%. Using this film as the front electrode of a-Si:H/μc-Si:H solar cell, the optimum performance of the solar cell with a Jsc of 12.68 mA/cm2, a Voc of 1.385 mV, and an initial efficiency (η) of 11.83% was obtained.

Water-Dispersed High-Quality Graphene: A Green Solution for Efficient Energy Storage Applications.

Graphene has been the subject of widespread research during the past decade because of its outstanding physical properties which make it an ideal nanoscale material to investigate fundamental properties. Such characteristics promote graphene as a functional material for the emergence of disruptive technologies. However, to impact daily life products and devices, high-quality graphene needs to be produced in large quantities using an environmentally friendly protocol. In this context, the production of graphene which preserves its outstanding electronic properties using a green chemistry approach remains a key challenge. Herein, we report the efficient production of electrode material for micro-supercapacitors obtained by functionalization of water-dispersed high-quality graphene nanosheets with polydopamine. High-frequency (terahertz) conductivity measurements of the graphene nanosheets reveal high charge carrier mobility up to 1000 cm-2 V-1 s-1. The fine water dispersibility enables versatile functionalization of graphene, as demonstrated by the pseudocapacitive polydopamine coating of graphene nanosheets. The polydopamine functionalization causes a modest, i.e., 20%, reduction of charge carrier mobility. Thin film electrodes based on such hybrid materials for micro-supercapacitors exhibit excellent electrochemical performance, namely a volumetric capacitance of 340 F cm-3 and a power density of 1000 W cm-3, thus outperforming most of the reported graphene-based micro-supercapacitors. These results highlight the potential for water-dispersed, high-quality graphene nanosheets as a platform material for energy-storage applications.

Nitrogen cluster doping for high-mobility/conductivity graphene films with millimeter-sized domains.

Directly incorporating heteroatoms into the hexagonal lattice of graphene during growth has been widely used to tune its electrical properties with superior doping stability, uniformity, and scalability. However the introduction of scattering centers limits this technique because of reduced carrier mobilities and conductivities of the resulting material. Here, we demonstrate a rapid growth of graphitic nitrogen cluster-doped monolayer graphene single crystals on Cu foil with remarkable carrier mobility of 13,000 cm2 V-1 s-1 and a greatly reduced sheet resistance of only 130 ohms square-1. The exceedingly large carrier mobility with high n-doping level was realized by (i) incorporation of nitrogen-terminated carbon clusters to suppress the carrier scattering and (ii) elimination of all defective pyridinic nitrogen centers by oxygen etching. Our study opens up an avenue for the growth of high-mobility/conductivity doped graphene with tunable work functions for scalable graphene-based electronic and device applications.

Elastic behavior and intrinsic carrier mobility for monolayer SnS and SnSe: First-principles calculations

Abstract Two-dimensional (2D) SnS and SnSe like black phosphorus have recently attracted great attention for their potential applications in next-generation electronics. Here, first-principles calculations are employed to investigate uniaxial strain behaviors and carrier mobilities of these monolayer structures. They are more structurally stable in armchair strain ( y axis strain) than in zigzag strain ( x axis strain), because zigzag deformation of the latter can be spontaneously transformed into former. Band gaps exhibit abnormal behavior with armchair strain changing from e y = −5 % to e y = 15 % , which is elucidated through the orbital hybridization between p states of S or Se atoms and the p states of Sn atoms. Surprisingly, it found that hole mobility is mainly determined by longitudinal acoustic phonon scattering, while electron mobility is mainly dominated by optical phonon scattering. The mobility can be tuned by uniaxial strain. Tensile strain along armchair direction will reduce carrier mobility. At compressive strain of e y = −5 % , the mobility of SnSe is increased to μ x =1.40× 10 3 and μ y =2.35× 10 3 cm 2 V −1 s −1 for electron, and μ x = 1.31 × 10 3 and μ y = 1.46 × 10 3 cm 2 V −1 s −1 for hole. Excellent elastic property, moderate band gap and tunable carrier mobility make SnSe promising for application in nanoelectronics and optoelectronics.

Modeling and Simulation Study of Reduced Self-Heating in Bottom-Gate β-Ga2O3 MISFETs with a h-BN Gate Insulator

Beta-gallium oxide (β-Ga2O3) is an emerging ultra-wide bandgap semiconductor material for high-power devices. However, one of the major drawbacks is the low thermal conductivity resulting in poor heat dissipation, and the so-called self-heating effect reduces carrier mobility and drain current degradation, and even causes a device reliability issue. Here, we propose a bottom-gate β-Ga2O3 field-effect transistor with a hexagonal boron-nitride (h-BN) gate-insulator and investigate the self-heating effect in comparison with an aluminum oxide (Al2O3) insulator using physics-based TCAD simulations. The h-BN with high thermal conductivity reduces the lattice temperature of the β-Ga2O3 channel and decreases drain current degradation. Furthermore, as the thickness of the insulator decreases below 50 nm and the channel length is scaled down to 5 μm, the reduced self-heating effect becomes more prominent. The results imply that the highly thermal-conductive h-BN insulator is promising for achieving high performance β-Ga2O3 metal insulator semiconductor field-effect transistor (MISFET) with the bottom-gate configuration.

Energy‐Efficient Metal–Insulator–Metal‐Semiconductor Field‐Effect Transistors Based on 2D Carrier Gases

Abstract2D electron and hole gases (2DEG or 2DHG) based on semiconductor surface/interface or 2D materials are promising for next‐generation integrated circuits and high‐frequency and ‐power electronics. To reduce power consumption, normally off operation and low‐switching‐loss metal‐oxide field‐effect transistors (MOSFETs) based on 2DEG or 2DHG are highly in demand. The present methods to achieve normally off MOSFETs have various shortcomings such as reduction in carrier mobility, sacrifice of drain current, high uncertainty in threshold voltage, and poor reliability. Here a universal device concept called a metal–insulator–metal‐semiconductor field‐effect transistor (MIMS‐FET) to achieve normally off operation is shown by combining the advantages of MOSFETs and metal‐semiconductor FETs. High‐performance MIMS‐FETs based on 2DHG of diamond with a low subthreshold swing of ≈76 mV dec−1, on–off current ratio exceeding 109, and high thermal stability up to 623 K are realized. The current work provides a strategy to develop next‐generation energy‐efficient electronic circuits as well as fail‐safety power devices. The proposed concept also adds diversity to the semiconductor devices family.

Computational Modelling of Thermoelectricity, Carrier Mobility and Electro-Catalytic Oxygen Reduction Reaction: a Few Examples

Computational Modelling of Thermoelectricity, Carrier Mobility and Electro-Catalytic Oxygen Reduction Reaction: a Few Examples

Phase Pure 2D Perovskite for High-Performance 2D-3D Heterostructured Perovskite Solar Cells.

Three-dimensional (3D) metal-halide perovskite solar cells (PSCs) have demonstrated exceptional high efficiency. However, instability of the 3D perovskite is the main challenge for industrialization. Incorporation of some long organic cations into perovskite crystal to terminate the lattice, and function as moisture and oxygen passivation layer and ion migration blocking layer, is proven to be an effective method to enhance the perovskite stability. Unfortunately, this method typically sacrifices charge-carrier extraction efficiency of the perovskites. Even in 2D-3D vertically aligned heterostructures, a spread of bandgaps in the 2D due to varying degrees of quantum confinement also results in charge-carrier localization and carrier mobility reduction. A trade-off between the power conversion efficiency and stability is made. Here, by introducing 2D C6 H18 N2 O2 PbI4 (EDBEPbI4 ) microcrystals into the precursor solution, the grain boundaries of the deposited 3D perovskite film are vertically passivated with phase pure 2D perovskite. The phases pure (inorganic layer number n = 1) 2D perovskite can minimize photogenerated charge-carrier localization in the low-dimensional perovskite. The dominant vertical alignment does not affect charge-carrier extraction. Therefore, high-efficiency (21.06%) and ultrastable (retain 90% of the initial efficiency after 3000 h in air) planar PSCs are demonstrated with these 2D-3D mixtures.

Variations in the Composition of the Phases Lead to the Differences in the Optoelectronic Properties of MAPbBr3 Thin Films and Crystals

Photoluminescence (PL) spectra from thin films (TFs) and bulk crystals (BCs) of hybrid organo-halide perovskites are significantly different, the origin of which and their impact on the efficiency of the perovskite-based photoactive devices have been debated. We have used two-photon PL to study the temperature-dependent changes in the spectra of the TFs and the BCs of methylammonium lead bromide (MAPbBr3) perovskites in order to clarify the origin of the differences. Our results show that the differences in the spectra are due to the variation in the phase composition. At room temperature, the tetragonal (TE) phase is dominant in the BCs, while the orthorhombic (OR) phase is dominant in the TFs. The PL spectra of the TFs also show discernible contributions from the TE and the cubic phases. At lower temperatures, the increase in excitonic recombination causes a red shift of the PL spectra from the TFs, while a phase transition from the TE to the OR phase results in a blue shift of the PL from the BCs. The temperature-dependent narrowing of the PL linewidths shows a stronger coupling between the longitudinal optical phonons and the free carriers in the OR phase as compared to the TE phase, implying a reduced carrier mobility. However, as the OR phase is metastable at the room temperature, the slow phase transition to the TE phase should improve the photocurrent yield in the TFs, provided that the sample is shielded from other types of degradation.

Enhanced perovskite phototransistor by multi-step slow annealing strategy

Methylammonium lead halide perovskites have received substantial attention in the research photovoltaic communities, because of excellent optoelectronic properties, including long electron-hole diffusion distance, large absorption coefficients in the UV–Vis spectral region, low-cost, solution-based processing and low binding energy of exciton. However, the crystal defects of perovskite films such as pinholes, defect and grain boundaries lead to carrier capture, trapping and scattering, so that reduce carrier mobility in the channel, which has become the bottleneck of high device performance and limited the application in photo detection. Here we report a simple and straightforward strategy based on a multi-step annealing process, which can effectively improves the coverage, smoothness, uniformity and crystallinity by restraining defects of pinhole formation. The field-effect mobilities of perovskite photodetector treated by one-step (OS) direct annealing method at room temperature shows mobility as 0.256 (0.129) cm2V−1s−1 for holes (electrons), which increase to 2.32 (1.18) cm2V−1s−1 for that treated by muti-step (MS) annealing method. The latter exhibits high figure of merit as detectivity of 8.94 × 1011 Jones, a responsivity of 32 A/W and a short response time of 42 μs . Moreover, it shows better stability with polymethylmethacrylate film covered, which can maintain 90% of initial performance for 10 days in ambient air. These findings demonstrate that MS annealing methods is simple yet effective and feasible to prepared high-efficient perovskite based photo detectors.

Effect of ambient air flow on resistivity uniformity of transparent Ga-doped ZnO film deposited by atmospheric pressure plasma jet

Transparent gallium-doped zinc oxide films were deposited on large area (117 mm × 185 mm) glass substrates by atmospheric pressure plasma jet method, using two different enclosure conditions—with and without side opening boundary. The opening allows gas exchange between the growth ambient and outside atmosphere. The spatial resistivity distribution was apparently influenced by the air flow field. The case with side opening has higher overall non-uniformity, and the film near the opening has the highest resistivity, caused by considerable reduction in carrier concentration (−32%) and mobility (−15%). Computer fluid dynamics simulations show that air was drawn in, through the side opening, from outside atmosphere, resulting in a much higher oxygen mass fraction (+50%) near the opening. As such, the film near the opening was deposited in O-rich conditions. The degradation of electrical conductivity in O-rich conditions cannot be explained by oxygen vacancies. We suggest that it is due to the formation of dopant-defect complex GaZn−VZn (the lowest formation energy) and reduction of hydrogen impurity (reduced hydrogen partial pressure) in O-rich conditions. Therefore, the tool enclosure, gas flow field, and scanning trajectory must be carefully designed to achieve minimum resistivity non-uniformity on large-area substrates.

Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn.

TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation between the amount of interstitial Ni and lattice parameter is determined from neutron powder diffraction. High-resolution synchrotron X-ray powder diffraction reveals the occurrence of strain broadening upon hot pressing, which is attributed to the metastable arrangement of interstitial Ni. Hall measurements confirm that interstitial Ni causes weak n-type doping and a reduction in carrier mobility, which limits the power factor to 2.5–3 mW m−1 K−2 for these samples. The thermal conductivity was modelled within the Callaway approximation and is quantitively linked to the amount of interstitial Ni, resulting in a predicted value of 12.7 W m−1 K−1 at 323 K for stoichiometric TiNiSn. Interstitial Ni leads to a reduction of the thermal band gap and moves the peak ZT = 0.4 to lower temperatures, thus offering the possibility to engineer a broad ZT plateau. This work adds further insight into the impact of small amounts of interstitial Ni on the thermal and electrical transport of TiNiSn.

CaP3: A New Two-Dimensional Functional Material with Desirable Band Gap and Ultrahigh Carrier Mobility.

Two-dimensional (2D) semiconductors with direct and modest band gap and ultrahigh carrier mobility are highly desired functional materials for nanoelectronic applications. Herein, we predict that monolayer CaP3 is a new 2D functional material that possesses not only a direct band gap of 1.15 eV (based on HSE06 computation) but also a very high electron mobility up to 19 930 cm2 V-1 s-1, comparable to that of monolayer phosphorene. More remarkably, contrary to bilayer phosphorene which possesses dramatically reduced carrier mobility compared to its monolayer counterpart, CaP3 bilayer possesses even higher electron mobility (22 380 cm2 V-1 s-1) than its monolayer counterpart. The band gap of 2D CaP3 can be tuned over a wide range from 1.15 to 0.37 eV (HSE06 values) through controlling the number of stacked CaP3 layers. Besides novel electronic properties, 2D CaP3 also exhibits optical absorption over the entire visible-light range. The combined novel electronic, charge mobility, and optical properties render 2D CaP3 an exciting functional material for future nanoelectronic and optoelectronic applications.

Enhanced thermoelectric performance of SnTe film with optimized carrier transport induced by facile post-annealing process

Annealing treatment plays a vital role in tuning the microstructure and improving the performance of the thermoelectric film. In this work, SnTe thin films were fabricated by thermal evaporation technique and then annealed at a series of temperatures. The effects of annealing temperature on the element composite, surface morphology, crystal structure and thermoelectric properties were investigated. The electrical conductivity of the annealed films was increased while the Seebeck coefficient was declined slightly, due to the largely enhanced carrier concentrations and slightly reduced carrier mobility. In specific, the maximum Seebeck coefficient of 155 μV K−1 and power factor of 29 μW cm−1 K−2 were achieved at the annealing temperature of 400 °C. Our results may provide an effective strategy to improve the properties of thermoelectric films.

The competition between mechanical stability and charge carrier mobility in MA-based hybrid perovskites: insight from DFT

The combined effects of steric and hydrogen-bonding interactions enhance the elastic properties of MA-based hybrid perovskites relative to their inorganic counterparts. However, MA-induced octahedral tilting increases the effective carrier masses which results in a reduction in carrier mobility.