Limited Electron Mobility Research Articles

Efficient planar mixed-cation perovskite photovoltaics with low-temperature-processed indium sulfide as electron transport material

Electron transport materials(ETMs) are crucial for efficient transport of photocarriers from the perovskites to the cathode in perovskite solar cells (PSCs). Although titanium dioxide (TiO2) is frequently employed as an ETM, PSC performance and applicability are hindered by its high-temperature manufacturing and limited electron mobility. In this work, a low-temperature solution-processable indium sulfide (In2S3) was incorporated into triple-cation perovskite solar cells to be an efficient ETM for the first time. After solvent-thermal synthesis of In2S3, followed by spin-coatingpreparation of In2S3 ETM, aPSC is constructed. The photoelectrical measurements show that the In2S3-based PSC has lower density of charge traps, higher conductivity, better electron transportation at the In2S3/perovskite interfaces, and a lower transfer resistance and carrier recombination. The PSC-based In2S3 yields a champion efficiency of 18.182%, and the PSC-based TiO2 gets an efficiency of 15.288%. Without encapsulation, the In2S3-based devices demonstrate increased stability in ambient environments. Our ETM layer and perovskite were made at low temperatures, which may open up new possibilities for solution-processed flexible electronics.

Nontrivial role of polar optical phonons in limiting electron mobility of two-dimensional Ga2O3 from first-principles.

The exfoliated two-dimensional (2D) Ga2O3 opens new avenues to fine-tune the carrier and thermal transport properties for improving the electro-thermal performance of gallium oxide-based power electronics with their enhanced surface-to-volume ratios and quantum confinement. Yet, the carrier transport in 2D Ga2O3 has not been fully explored, especially considering their large Fröhlich coupling constants. Herein, we mainly investigate the electron mobility of monolayer (ML) and bilayer (BL) Ga2O3 from first-principles by adding polar optical phonon (POP) scattering. The results show that POP scattering is the dominant factor limiting the electron mobility for 2D Ga2O3, accompanied by a large 'ion-clamped' dielectric constant Δε. The value of Δε is 3.77 and 4.60 for ML and BL Ga2O3, respectively, indicating a large change in polarization in the external field. The electron mobility of 2D Ga2O3 enhances with increasing thickness despite the enhanced electron-phonon coupling strength and Fröhlich coupling constant. The predicted electron mobility for BL and ML Ga2O3 at a carrier concentration of 1.0 × 1012 cm-2 is 125.77 cm2 V-1 s-1 and 68.30 cm2 V-1 s-1 at room temperature, respectively. This work aims to unravel the scattering mechanisms beneath engineering electron mobility of 2D Ga2O3 for promising applications in high-power devices.

Contact Conductance Governs Metallicity in Conducting Metal Oxide Nanocrystal Films

Although colloidal nanoparticles hold promise for fabricating electronic components, the properties of nanoparticle-derived materials can be unpredictable. Materials made from metallic nanocrystals exhibit a variety of transport behavior ranging from insulators, with internanocrystal contacts acting as electron transport bottlenecks, to conventional metals, where phonon scattering limits electron mobility. The insulator-metal transition (IMT) in nanocrystal films is thought to be determined by contact conductance. Meanwhile, criteria are lacking to predict the characteristic transport behavior of metallic nanocrystal films beyond this threshold. Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied electron concentration, size, and contact area, we assess the IMT as it depends on contact conductance and show how contact conductance is also key to predicting the temperature-dependence of conductivity in metallic films. The results establish a phase diagram for electron transport behavior that can guide the creation of metallic conducting materials from nanocrystal building blocks.

Enhancement of optical phonons limited electron mobility in an AlGaN step-shaped quantum well

Achieving high conductivity in n-type AlGaN of deep ultraviolet light-emitting diodes is still a challenge nowadays. In the Lei–Ting equilibrium equation method framework, a four-layer quantum well with n-Al0.7Ga0.3N barriers and an Al0.55Ga0.45N/AlxGa1−xN bi-component well could obtain a much higher electron mobility μT than that in a single-component quantum well over a relatively broad Al content range 0.56 < x < 0.68. While the built-in electric field induces strong confinement, pushing the electron to the interface, the optical phonon mode at this interface could be component-modulated to disappear due to the ternary mixed crystal effect. A significant enhancement of electron mobility can be achieved by avoiding the scattering from optical phonons at the interface where the electron gathers. The optical phonon limited μT could reach 7966 cm2/V s at x = 0.58, which is almost five times of 1518 cm2/V s in the case of x = 0.55 and ten times of 822 cm2/V s for x = 0.70. Such a step-shaped quantum well with high electron mobility could be an alternate or insert layer to the high Al content n-layer to alleviate the current crowding.

Optical phonon limited electron mobility in ZnO nanowires wrapped by MgZnO shells

MgxZn1−xO shells are commonly used as a passivation barrier for improving electron mobility in ZnO nanowires by preventing electrons from charged surfaces. However, a high Mg mole fraction x instead makes lower electron mobility, which is usually attributed to the appearance of mixed-phase MgxZn1−xO as x increases. This work aims to find the optimal x for optical phonon limited electron mobility by considering the phase transformation in the MgZnO shell from wurtzite to rock salt, leading to a mixed-phase range of x. Our calculations show that the electron mobility μT can be effectively enhanced by keeping x below 0.057 when confined (CO1) optical phonons are only permitted for small wave vectors, and there is no interface (IF) optical phonon. Once x gets over 0.057, the propagating optical phonons are transformed into IF ones while CO1 phonons become permitted for all wave vectors resulting in a largely strengthened scattering effect and thus a drastic drop in the total electron mobility μT from 1215 to 310 cm2/V s. From then, μT begins to fall slowly as x increases even when the rock salt component in the shell appears to take the place of the wurtzite part, while the scattering from CO1 optical phonons remains primary. Furthermore, the enlarging core radius can weaken the electron–CO1 phonon interaction to enhance mobility.

Structure-property relationships and mobility optimization in sputtered La-doped BaSnO3 films: Toward 100cm2V−1s−1 mobility

The wide band gap semiconducting perovskite $\mathrm{BaSn}{\mathrm{O}}_{3}$ is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin film research has focused on pulsed laser deposition or molecular beam epitaxy. Here, we present an exhaustive optimization of the mobility of ${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$ films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space mapping, rocking curve analysis, scanning transmission electron microscopy, atomic force microscopy, and temperature-dependent electronic transport measurements, detailed understanding of synthesis-structure-property relationships is attained. Optimized room temperature mobility of $96\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ is achieved in vacuum-annealed $\mathrm{GdSc}{\mathrm{O}}_{3}$(110)/$\mathrm{BaSn}{\mathrm{O}}_{3}$(120 nm)/${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$(200 nm) heterostructures, as well as $92\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ on unbuffered substrates and $87\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ without postdeposition annealing. These results, including important trends in defect densities and a surprising dependence of mobility on lattice mismatch, substantially expand the understanding of the interplay between deposition conditions, microstructure, and transport in doped $\mathrm{BaSn}{\mathrm{O}}_{3}$ films, establishing competitive mobilities in films fabricated via a scalable, high-throughput, industry-standard technique.

Morphology control of SnO2 layer by solvent engineering for efficient perovskite solar cells

Perovskite solar cells (PSCs) are considered among the most promising alternatives to Si-based solar cells, owing to the ongoing rise in their power conversion efficiencies (PCEs). While TiO2 has been widely used as an electron transport layer (ETL) in the n-i-p (regular) type PSCs, series of its disadvantages, including limited bulk electron mobility and photocatalytic character, restrict further improvement on the photovoltaic performance and environmental stability of devices. Therefore, SnO2 has attracted much attention as an alternative, because of its relatively low fabrication temperature, suitable energy alignment and high electron mobility. In this study, we effectively controlled the morphology of the SnO2 ETL by adding an organic solvent (ethanol) into the commercialized colloidal SnO2 precursor. This approach improved not only the wettability of the precursor on the glass substrate, but also the stability of the SnO2 colloidal dispersion. Thus, an aggregation-free SnO2 film with homogenous morphology was obtained with the assistance of ethanol, which also contributed to the improved crystal quality of the perovskite. The PSCs fabricated using ethanol-processed SnO2 exhibited a high PCE of 18.84% and improved environmental stability. Thus, our investigation provides a simple and effective approach toward achieving high-performance SnO2-based PSCs.

Phonon limited mobility in 3D Dirac semimetal Cd3As2

We report the numerical calculations of phonon limited electron mobility, μ in Cd3As2 three dimensional Dirac semimetal using Boltzmann transport theory over a temperature range 20<T<300K. By employing the Ritz iteration technique, we have directly solved the 3D linearized Boltzmann transport equation to obtain the first order perturbation distribution function Φ−1(Ek) as a function of electron energy Ek and hence the mobility, μ. we have considered electrons scattering by acoustic phonons via deformation potential scattering and polar optical phonons. The μ due to the latter is dominating the former for T>80K, for electron concentration ne = 1x1018cm−3. Thiscrossover shifts to lower T, with decreasing ne. The calculations are qualitatively agreeing with the experimental results.

Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response.

Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2'-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3- d:2',3'- d']-s-indaceno[1,2- b:5,6- b']dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1 H-indene-2,1(3 H)-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ photovoltaic performance of a series of ITIC variants, ITIC- nF ( n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor polymer. Single-crystal diffraction reveals that the ITIC- nF molecules cofacially interleave with ITIC-6F having the shortest π-π distance of 3.28 Å. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV, and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC-0F, and ITIC-4F, and track a positive correlation between the ITIC- nF space-charge limited electron mobility and n. Finally, a heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived end-groups (EGs) of IDTT-based NFAs, i.e., EG1-IDTT-EG1 + EG2-IDTT-EG2 ⇌ 2 EG1-IDTT-EG2, with implications for the entire ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.

Phonon Limited Electron Mobility in Germanium FinFETs: Fin Direction Dependence**Supported by the National Natural Science Foundation of China under Grant Nos 61534004, 61604112 and 61622405.

We investigate the phonon limited electron mobility in germanium (Ge) fin field-effect transistors (FinFETs) with fin rotating within (001), (110), and (111)-oriented wafers. The coupled Schrödinger–Poisson equations are solved self-consistently to calculate the electronic structures for the two-dimensional electron gas, and Fermi’s golden rule is used to calculate the phonon scattering rate. It is concluded that the intra-valley acoustic phonon scattering is the dominant mechanism limiting the electron mobility in Ge FinFETs. The phonon limited electron motilities are influenced by wafer orientation, channel direction, fin thickness Wfin, and inversion charge density Ninv. With the fixed Wfin, fin directions of , and within (001), (110), and (111)-oriented wafers provide the maximum values of electron mobility. The optimized Wfin for mobility is also dependent on wafer orientation and channel direction. As Ninv increases, phonon limited mobility degrades, which is attributed to electron repopulation from a higher mobility valley to a lower mobility valley as Ninv increases.

Oxygen vacancy-assisted recovery process for increasing electron mobility in n-type BaSnO3 epitaxial thin films

The scattering of charge carriers by line defects, i.e., threading dislocations (TDs), severely limits electron mobility in epitaxial semiconductor films grown on dissimilar substrates. The density of TDs needs to be decreased to further enhance electron mobility in lattice-mismatched epitaxial films and heterostructures for application in high-performance electronic devices. Here, we report a strategy for the post-treatment of epitaxial La-doped BaSnO3 (LBSO) films by delicately controlling the oxygen partial pressure p(O2), which achieved a significant increase in the room temperature (RT) electron mobility (μe) to μe = 122 cm2 V−1 s−1 at a carrier concentration of 1.1 × 1020 cm−3. This mobility enhancement is mostly attributed to an oxygen vacancy-assisted recovery process that reduces the density of TDs by accelerating the movement of dislocations in ionic crystals under a p(O2)-controlled treatment despite an increase in the density of charged point defects. Our finding suggests that accurate control of the interactions between point defects and line defects can reduce dominant carrier scattering by charged dislocations in epitaxial oxide semiconductors that have dissimilar substrates. This method provides alternative approaches to achieving perovskite oxide heterostructures that have high RT μe values.

Comparison of alloy disorder scatterings in Ga- and N-polar AlGaN/GaN heterostructures

The two-dimensional electron gas (2DEG) mobilities limited by alloy disorder (AD) scattering in both Ga- and N-polar AlGaN/GaN heterostructures are investigated. It was found that the AD scattering limited electron mobility in N-polar heterostructures is on the order of 103–104 cm2/Vs, which is comparable to the optical phonon scattering at room-temperature. In comparison, the AD scattering in Ga-polar samples is much less important. Moreover, the electron mobility decreases with the 2DEG density in the Ga-polar device but shows a reverse trend in the N-polar counterpart. This is found to be caused by the rather different electric field distributions in Ga- and N-polar AlGaN/GaN heterostructures. In addition, we find that an AlN interlayer can effectively reduce the alloy scattering, mainly due to the large band offset between AlN and GaN. The calculated mobilities have been compared with the experiment results and good agreements are found. We believe that our results are important for the design of AlGaN/GaN heterostructure-based devices, especially the N-polar ones.

Influence of Temperature in Scattered SiNW MOSFET

The impact of temperature in carrier transport of scattered SiNW MOSFET has been developed. Scattered SiNW MOSFET model includes the effects of elastic scattering, optical phonon emission, surface roughness scattering and random discrete dopants. The temperature effect of above mentioned scatterings in the device limits electron mobility, decreasing device current and transconductance. This work discusses the detailed behavior of analog parameters like transconductance (gm) and early voltage (VA). The validity of the proposed model has been confirmed by comparing the analytical results with the technology computer aided design simulation results.

Investigation of electron mobility and saturation velocity limits in gallium nitride using uniaxial dielectric continuum model

Here we introduce a uniaxial dielectric continuum model with temperature-dependent phonon mode frequencies to study temperature- and orientation-dependent polar-optical-phonon limited electron mobility and saturation velocity in uniaxial semiconductors. The formalism for calculating electron scattering rates, momentum relaxation rates, and rate of energy change as a function of the electron kinetic energy and incident electron angle with respect to the c-axis are presented and evaluated numerically. Electron–longitudinal-optical-phonon interactions are shown to depend weakly on the electron incident angle, whereas the electron–transverse-optical-phonon interactions around the emission threshold energy are observed to depend strongest on the electron incident angle when varied from π/4 to π/2 (with respect to the c-axis). We provide electron mobility and saturation velocity limits in different GaN crystal orientations as a function of temperature and electron concentration. At room temperature and for an electron density of 5 × 1018 cm−3, electron mobility limit of ∼3200 cm2/V s and electron saturation velocity limit of 3.15 × 107 cm/s are calculated. Both GaN electron mobility and saturation velocity are observed to be governed by the longitudinal-optical-phonon interaction, and their directional anisotropy is shown to vary less than 5% as the electron incident angle with respect to the c-axis is varied from 0 to π/2. Overall, we develop a theoretical formalism for calculating anisotropic properties of uniaxial wurtzite semiconductors.

Electron mobility in polarization-doped Al0-0.2GaN with a low concentration near 1017 cm−3

In this letter, carrier transport in graded AlxGa1-xN with a polarization-induced n-type doping as low as ∼1017 cm−3 is reported. The graded AlxGa1-xN is grown by metal organic chemical vapor deposition on a sapphire substrate, and a uniform n-type doping without any intentional doping is realized by linearly varying the Al composition from 0% to 20% over a thickness of 600 nm. A compensating center concentration of ∼1017 cm−3 was also estimated. A peak mobility of 900 cm2/V·s at room temperature is extracted at an Al composition of ∼7%, which represents the highest mobility achieved in n-Al0.07GaN with a carrier concentration of ∼1017 cm−3. A comparison between experimental data and theoretical models shows that, at this low doping concentration, both dislocation scattering and alloy scattering are significant in limiting electron mobility and that a dislocation density of &amp;lt;107 cm−2 is necessary to optimize mobility near 1016 cm−3. The findings in this study provide insights into key elements for achieving high mobility at low doping levels in GaN, a critical parameter in the design of novel power electronics taking advantage of polarization doping.

A Model for Neutral Defect Limited Electron Mobility in Strained-Silicon Inversion Layers

On the strained silicon metal–oxide–semiconductor field-effect transistors (MOSFETs), we show how to derive a formalism dealing with the scattering of a 2-D electron by a neutral defect. The corresponding neutral defect limited inversion-layer electron mobility, $\mu _{n}$ , is calculated in the momentum relaxation time approximation. The calculated results lead to a new analytical model: $\mu _{n} =cN_{n} ^{-1}$ , where $N_{n}$ is the neutral defect density per unit area and ${c}$ is the coefficient independent of the inversion-layer density, the strain, and the temperature. The validity and applicability of the model are confirmed by citing three independent experiments on strained silicon MOSFETs undergoing different implantation sources and different annealing budgets. Importantly, this paper clarifies for the first time that strain will not change neutral defect limited mobility unless changing the neutral defect density. This reasonably explains the two experimental observations during implantation and annealing: 1) the implantation-induced strain relaxation in strained sample does not occur and 2) the neutral defect density is much higher in strained sample than in unstrained sample.

Intrinsic electron mobility limits in β-Ga2O3

By systematically comparing experimental and theoretical transport properties, we identify the polar optical phonon scattering as the dominant mechanism limiting electron mobility in β–Ga2O3 to &amp;lt;200 cm2/V s at 300 K for donor doping densities lower than ∼1018 cm–3. Despite similar electron effective mass of β–Ga2O3 to GaN, the electron mobility is ∼10× lower because of a massive Fröhlich interaction, due to the low phonon energies stemming from the crystal structure and strong bond ionicity. Based on the theoretical and experimental analysis, we provide an empirical expression for electron mobility in β–Ga2O3 that should help calibrate its potential in high performance device design and applications.

Limitation of electron mobility from hyperfine interaction in ultraclean quantum wells and topological insulators

The study of electron transport and scattering processes limiting electron mobility in high-quality semiconductor structures is central to solid-state electronics. Here, we uncover an unavoidable source of electron scattering which is caused by fluctuations of nuclear spins. We calculate the momentum relaxation time of electrons in quantum wells governed by the hyperfine interaction between electrons and nuclei and show that this time depends greatly on the spatial correlation of nuclear spins. Moreover, the scattering processes accompanied by a spin flip are a source of the backscattering of Dirac fermions at conducting surfaces of topological insulators.

Influence of Scattering in Near Ballistic Silicon NanoWire Metal-Oxide-Semiconductor Field Effect Transistor.

The present work explores the impact of scattering in carrier transport of near-ballistic SiNW MOSFET. For the first time, a scattered SiNW MOSFET model is revealed which includes the effects of optical phonon emission, elastic scattering, surface roughness scattering and random discrete dopants. Impact of above mentioned combined scatterings in the device limits electron mobility which makes a remarkable decrease in device current and transconductance compared with our previous model and Natori's Ballistic transport model. This work discusses the detailed behavior of analog parameters like Transconductance (g(m)), Transconductance generation factor (g(m)/I(d)) and Early Voltage (VA). The proposed model has been validated by comparing the analytical results with the TCAD simulation results and it shows the good agreement.

Electron mobility limited by scattering from threading dislocation lines within gallium nitride

Theoretical as well as experimental studies in the literature suggest that defect sites associated with the threading dislocation lines within [Formula: see text]-type gallium nitride (GaN) act to trap free electrons from the bulk of this semiconductor material. As a result, the core of the threading dislocation lines become negatively charged. The charge accumulated along the core of a threading dislocation line should be screened by a charge of opposite polarity and equal in absolute value per unit length along the dislocation line. In the present work, we model this screened charge buildup along the threading dislocation lines by two concentric space-charge cylinders. Quantum mechanical theory of scattering in cylindrical coordinates is then employed in order to numerically compute the electron mobility limited by scattering from the charged threading dislocation lines. The dependence of the computed electron mobility on the dislocation line density and on the amount of charge accumulated per unit length along the core of the dislocation lines is also investigated in this work. Our computed electron mobility results are compared with results from existing calculations of the GaN dislocation scattering limited electron mobility in the literature.