Electron Transport In Devices Research Articles

Ultrafast Bragg coherent diffraction imaging of epitaxial thin films using deep complex-valued neural networks

Domain wall structures form spontaneously due to epitaxial misfit during thin film growth. Imaging the dynamics of domains and domain walls at ultrafast timescales can provide fundamental clues to features that impact electrical transport in electronic devices. Recently, deep learning based methods showed promising phase retrieval (PR) performance, allowing intensity-only measurements to be transformed into snapshot real space images. While the Fourier imaging model involves complex-valued quantities, most existing deep learning based methods solve the PR problem with real-valued based models, where the connection between amplitude and phase is ignored. To this end, we involve complex numbers operation in the neural network to preserve the amplitude and phase connection. Therefore, we employ the complex-valued neural network for solving the PR problem and evaluate it on Bragg coherent diffraction data streams collected from an epitaxial La2-xSrxCuO4 (LSCO) thin film using an X-ray Free Electron Laser (XFEL). Our proposed complex-valued neural network based approach outperforms the traditional real-valued neural network methods in both supervised and unsupervised learning manner. Phase domains are also observed from the LSCO thin film at an ultrafast timescale using the complex-valued neural network.

Direct π-extension of a conjugated carbon nanohoop using a zipper method.

The π-extension of carbon nanorings towards ultrashort carbon nanotubes (CNTs) is a great challenge for synthetic chemists. Herein, we report the synthesis, characterization, and properties of a nanographene-embedded carbon nanoring (NECR) by a direct zipper method. In this approach, a long linear phenyl chain is fused onto the CPP backbone by a simple Scholl reaction, similar to zipping two pieces of fabric together. Its photophysical properties were studied by UV-vis and photoluminescence spectroscopy. The potential application of NECR in electron-transport devices were further investigated.

Band-to-band tunneling switches based on two-dimensional van der Waals heterojunctions

Quantum mechanical band-to-band tunneling is a type of carrier injection mechanism that is responsible for the electronic transport in devices like tunnel field effect transistors (TFETs), which hold great promise in reducing the subthreshold swing below the Boltzmann limit. This allows scaling down the operating voltage and the off-state leakage current at the same time, and thus reducing the power consumption of metal oxide semiconductor transistors. Conventional group IV or compound semiconductor materials suffer from interface and bulk traps, which hinder the device performance because of the increased trap-induced parasitics. Alternatives like two-dimensional materials (2DMs) are beneficial for realizing such devices due to their ultra-thin body and atomically sharp interfaces with van der Waals interactions, which significantly reduce the trap density, compared to their bulk counterparts, and hold the promise to finally achieve the desired low-voltage operation. In this review, we summarize the recent progress on such devices, with a major focus on heterojunctions made of different 2DMs. We review different types of emerging device concepts, architectures, and the tunneling mechanisms involved by analytically studying various simulations and experimental devices. We present our detailed perspective on the current developments, major roadblocks, and key strategies for further improvements of the TFET technology based on 2D heterojunctions to match industry requirements. The main goal of this paper is to introduce the reader to the concept of tunneling especially in van der Waals devices and provide an overview of the recent progress and challenges in the field.

Crystal phase engineering of silicene by Sn-modified Ag(111).

The synthesis of silicene by direct growth on silver is characterized by the formation of multiple phases and domains, posing severe constraints on the spatial charge conduction towards a technological transfer of silicene to electronic transport devices. Here we engineer the silicene/silver interface by two schemes, namely, either through decoration by Sn atoms, forming an Ag2Sn surface alloy, or by buffering the interface with a stanene layer. Whereas in both cases Raman spectra confirm the typical features as expected from silicene, by electron diffraction we observe that a very well-ordered single-phase 4 × 4 monolayer silicene is stabilized by the decorated surface, while the buffered interface exhibits a sharp phase at all silicon coverages. Both interfaces also stabilize the ordered growth of a phase in the multilayer range, featuring a single rotational domain. Theoretical ab initio models are used to investigate low-buckled silicene phases (4 × 4 and a competing one) and various structures, supporting the experimental findings. This study provides new and promising technology routes to manipulate the silicene structure by controlled phase selection and single-crystal silicene growth on a wafer-scale.

Electronic Transport in 2D‐Based Printed FETs from a Multiscale Perspective

AbstractAs 2D materials (2DMs) gain the research limelight as a technological option for obtaining on‐demand printed low‐cost integrated circuits with reduced environmental impact, theoretical methods able to provide the necessary fabrication guidelines acquire fundamental importance. Here, a multiscale modeling technique is exploited to study electronic transport in devices consisting of a printed 2DM network of flakes. The approach implements a Monte Carlo scheme to generate the flake distribution. By means of ab initio density functional theory calculations together with non equilibrium Green's functions formalism, detailed physical insights on flake‐to‐flake transport mechanisms are provided. This later feeds a 3D drift‐diffusion and Poisson solution to compute self‐consistently transport and electrostatics in the device. The method is applied to MoS2 and graphene‐based dielectrically gated FETs, highlighting the impact of the structure density and variability on the mobility and sheet resistance. The prediction capability of the proposed approach is validated against electrical measurements of in‐house printed graphene conductive lines as a function of film thickness, demonstrating its strong potential as a guide for future experimental activity in the field.

CuP: A new type of anisotropic and very stable Dirac cone material

Abstract The anisotropic Dirac cone has the advantage of anisotropic carrier mobility, which is different from other Dirac materials and can be applied to direction-dependent quantum devices. By first-principle calculation, we systematically research the Dirac cone and quantum properties of CuP. The calculation results show that CuP monolayer is a new type of anisotropic Dirac material. In addition, there is no virtual frequency in the phonon spectrum of this structure. At 300 K, the CuP monolayer can be remained well and there is no loose fracture of the structure. Based on grounds of Born criteria, CuP monolayer has good mechanical stability. Interestingly, considering the effect of spin-orbit coupling (SOC), the Dirac cone does not open the band gap. Moreover, the addition of stress does not destroy the Dirac cone. The material is mechanically and thermally stable. The results of Poisson's ratio and Young's modulus show that the structure has obvious anisotropy. It is worth noting that, the Fermi velocity to the right is 2.3 × 105 m/s and the left is 4.5 × 105 m/s, which is in the same order of magnitude as the Fermi velocity of graphene (8.0 × 105 m/s). The calculation results provide an ideal basis for the design of directional electron transport devices in spintronics.

The structure, and electronic and magnetic properties of MX (M=GA, IN; X=S, SE, TE) nanoribbons

We used the first principle of density functional theory to perform detailed calculations regarding the structure, and the electronic and magnetic properties of MX (M[Formula: see text]=[Formula: see text]Ga, In; X[Formula: see text]=[Formula: see text]S, Se, Te) nanoribbons. The armchair nanoribbons (ARNs) are nonmagnetic semiconductors, which have even or odd oscillations of bandgaps. All small-sized zigzag nanoribbons (ZRNs) were found to break the six-membered ring structure and move to the center, thereby exhibiting nonmagnetic semiconductor behavior owing to the quantum confinement effect. However, among the large ZRNs, which are all metals, MTe ZRNs are nonmagnetic; this differs from the case of graphene, MoS2 and Ti2CO2 nanoribbons. MX (M[Formula: see text]=[Formula: see text]Ga, In; X[Formula: see text]=[Formula: see text]S, Se) ZRNs exhibited ferromagnetism owing to the presence of the unpaired electrons on the metal-edge side and the magnetic moment of each pair of molecules, which was controlled by the size of the nanoribbons. The results provided a theoretical reference that can be used in the future to produce MX materials for application in low-dimensional semiconductor devices, spin electron transport devices and new magnetoresistance devices.

Monolayer C7N6 : Room-temperature excitons with large binding energies and high thermal conductivities

Two-dimensional (2D) carbon nitrides compounds have attracted wide attention in recent years due to their diverse structures and excellent electronic, thermal, and optical properties. Here, by using first-principles approach, we investigate in details the stability, many-body effect, electronic/thermal transport properties, and thermoelectric performance of monolayer ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$, as a new kind of 2D carbon nitride compounds composed of $s{p}^{2}\text{\ensuremath{-}}\mathrm{hybridized}$ carbon atoms forming hexagonal lattice. Our results show that ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ monolayer is a direct band-gap semiconductor with a band-gap value of 3.56 eV under the accurate ${G}_{0}{W}_{0}$ method. Ab initio molecular dynamics simulations demonstrate that ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ maintains stable up to $1500\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Two exciton absorption peaks can be observed within the band gap with the respective large binding energies of $0.84$ and $0.09\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, which means both excitons can exist at room temperature. Monolayer ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ possesses high carrier mobility with the order of ${10}^{2}--{10}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Moreover, we find that the lattice thermal conductivity for ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ is as high as $134.55\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{mK}$ at room temperature, thus the thermoelectric figure of merit for ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ is relatively low. Our work suggests that ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ is a promising candidate for nanoscale (opto-)electronic and heat transport devices.

Prediction of tetragonal monolayer CuN with a quantum spin Hall state

Abstract Two-dimensional (2D) quantum spin Hall (QSH) insulator are promising candidates for miniaturized electronic and dissipationless transport devices. The bottleneck preventing applications from QSH phase, however, is lack of feasible 2D materials in experiments so far. By using first-principles calculations, here we propose a realistic 2D QSH phase in tetragonal CuN, which is dynamically and mechanically stable. Band structure analysis reveals that it is a nontrivial QSH phase robust against external strains. The origin of QSH effect can be identified by a px-pz band inversion, topological invariant Z2, and helical edge states within the bulk gap. Also, a heterostructure composed of CuN deposited on semiconducting AlAs substrate remains topologically nontrivial with a sizable gap. These findings provide a promising platform to design new QSH insulators in tetragonal lattices, which show potential applications in multifunctional spintronics devices.

On-demand tuning of charge accumulation and carrier mobility in quantum dot solids for electron transport and energy storage devices

Assemblies of colloidal quantum dots (CQDs) are attractive for a broad range of applications because of the ability to exploit the quantum confinement effect and the large surface-to-volume ratio due to their small dimensions. Each application requires different types of assemblies based on which properties are intended to be utilized. Greater control of assembly formation and optimization of the related carrier transport characteristics are vital to advance the utilization of these materials. Here, we demonstrate on-demand control of the assembly morphology and electrical properties of highly crosslinked CQD solids through the augmentation of various assembly methods. Employment of electric-double-layer (EDL) gating on these assembly structures (i.e., an amorphous assembly, a hierarchical porous assembly, and a compact superlattice assembly) reveals their intrinsic carrier transport and accumulation characteristics. Demonstrations of high electron mobility with a high current modulation ratio reaching 105 in compact QD films and of a record-high areal capacitance of 400 μF/cm2 in an electric-double-layer supercapacitor with very thin (<100 nm) QD hierarchical porous assemblies signify the versatility of CQDs as building blocks for various modern electronic devices.

Sharply Increased Current in Asymmetrically Aligned Polycrystalline Polymer Transistors With Sub-Domain-Size Channels

Shortening channels to the feature length of polymer domains is important to understand the charge transport in semiconducting polymers from macroscopic to microscopic scales. In this work, we fabricated polymer transistors with sub-domain-size channels by electron-beam lithography and asymmetrically aligned domains by off-center spin coating, in which a dry self-patterning method with hexagonal boron nitride was developed to eliminate parasitic leakage. The current sharply increased by 60 times when the channel length was shortened from 200 to 80 nm. The device with an 80 nm-long channel in the direction parallel to the aligned polymer domains delivered a saturation current 160 times higher than that of the orthogonal counterpart. These results and a velocity calculation suggested that the bottleneck of charge transport in electronic devices is domain boundaries. These results highlight the importance of high-efficiency intra-domain charge transport and also the inter-domain limitation. Our findings will contribute to further polymer optimization to improve the performance of polymer devices.

Cytomorphic Electronics With Memristors for Modeling Fundamental Genetic Circuits.

Cytomorphic engineering attempts to study the cellular behavior of biological systems using electronics. As such, it can be considered analogous to the study of neurobiological concepts for neuromorphic engineering applications. To date, digital and analog translinear electronics have commonly been used in the design of cytomorphic circuits; Such circuits could greatly benefit from lowering the area of the digital memory via memristive circuits. In this article, we propose a novel approach that utilizes the Boltzmann-exponential stochastic transport of ionic species through insulators to naturally model the nonlinear and stochastic behavior of biochemical reactions. We first show that two-terminal memristive devices can capture the non-linear and stochastic behavior of biochemical reactions. Then, we present the design of several building blocks based on analog memristive circuits that inherently model the biophysical mechanisms of gene expression. The circuits model induction by small molecules, activation and repression by transcription factors, biological promoters, cooperative binding, and transcriptional and translational regulation of gene expression. Finally, we utilize the building blocks to form complex mixed-signal networks that can simulate the delay-induced oscillator and the p53-mdm2 interaction in the cancer signaling pathway. Our approach can provide a fast and simple emulative framework for studying genetic circuits and arbitrary large-scale biological networks in systems and synthetic biology. Some challenges may be that memristive devices with frequent learning and programming do not have the same longevity as traditional transistor-based electron-transport devices, and operate with significantly slower time constants, which can limit emulation speed.

Conductive surface oxide on CrN(001) layers

Epitaxial CrN(001) layers that are exposed to an O2-containing atmosphere exhibit a conductive 2D surface oxide with a sheet conductance Gs,oxide = 5.9 × 10−5 [Ω/◻]−1. This is demonstrated using in situ transport measurements in a 90% Ar–10% O2 mixture with continuously increasing pressure from &amp;lt;10−6 to 240 Pa, showing a conductance increase that is independent of the CrN thickness d = 10 and 300 nm but is absent for control samples that are capped with insulating AlN prior to oxygen exposure. This suggests n-type doping of semiconducting CrN through substitutional replacement of N surface atoms with O. Cooling to 77 K leads to a decrease in Gs,oxide to 3.9 × 10−5 [Ω/◻]−1, indicating that the conduction electrons are not fully delocalized. The overall results indicate a path towards 2D electron transport devices in refractory transition metal nitrides and may explain the large variation in previously reported transport properties of CrN.

Disorder Driven Weak Localization and Phase Coherent Electron Transport in Ga Doped (Zn:V)O Thin Films

We have observed disorder driven weak localization with phase coherent electron transport in Ga doped (Zn:V)O thin films grown by pulsed laser deposition. All the Ga doped (Zn:V)O thin films with fixed 1 at% V and varying (0–5 at%) Ga concentration exhibit metal like behavior by Mott's definition while temperature coefficient of resistivity (TCR) systematically changes its sign with variation of temperature. This anomaly has been understood by considering quantum corrections to classical Boltz mann conductivity where significant contribution of weak localization has been confirmed from the observation of negative magneto resistance (MR) in temperature dependent magneto-transport measurements at varying magnetic fields. Phase coherent length of Ga doped (Zn:V)O thin films varies with both Ga concentration and measurement temperature and attains a maximum value ∼320 nm at 5K for 2 at% of Ga concentration. The nature of temperature dependence on phase coherent length demonstrates electron-electron scattering as the dominant phase breaking mechanism in the Ga doped (Zn:V)O thin films. Phase coherent electron transport and its tunability with Ga concentration in (Zn:V)O thin films is expected to have potential in futuristic phase coherent electron transport devices.

Piezotronic materials and large-scale piezotronics array devices

Abstract

Study of Coulomb attractive random telegraph signal in surface acoustic wave based single electron transport (SAW/SET) devices

We investigated the random telegraph signal (RTS) in surface acoustic wave based single electron transport (SAW/SET) devices. In our experiment, the ratio of the average time at the high acoustoelectric (AE) current state to the average time at the low AE current state, τhigh/τlow, increases when the gate voltage is progressively positive, as well as the radio frequency (rf) power is increased and the offset voltage is gradually decreased. The experiment result indicates that the trap center is positively charged before trapping an electron. In this paper, we describe one kind of RTS attributed to the positively charged trap center. And we can get information on the spin properties of a single trap by investigating the magnetic field dependence of the average time ratio τhigh/τlow of the RTS.

Ultrafast terahertz snapshots of excitonic Rydberg states and electronic coherence in an organometal halide perovskite

How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Here we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes. The nearly ∼1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ∼13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.

Molecular Origin of Donor- and Acceptor-Rich Domain Formation in Bulk-Heterojunction Solar Cells with an Enhanced Charge Transport Efficiency

Understanding the origin of different morphologies in bulk-heterojunction solar cells can provide effective guidelines to rational control of the morphologies in the active layer. Here, we have uncovered the importance of molecular interactions on the morphologies for not only donor materials but also for fullerene acceptors in organic solar cells through the multiscale coarse-graining molecular dynamic simulations at the real device level (∼83 nm × 83 nm × 83 nm). It is found that oligothiophene donors with polar end groups could not only facilitate the formation of continuous donor network but also promote the aggregation and connection of fullerenes toward efficient hole and electron transport. On the contrary, fullerenes are well dispersed at the molecule levels in the less polar oligothiophene matrix and thus contribute to the poor electron transport mobility and device performance, which is consistent with the observed differences in both morphology and charge transport properties of these two systems. These results would provide effective guidelines for the rational molecule design and morphology control to further enhance the device performance of organic solar cells.

Boosting Hole Mobility in Coherently Strained [110]-Oriented Ge-Si Core-Shell Nanowires.

The ability of core–shell nanowires to overcome existing limitations of heterostructures is one of the key ingredients for the design of next generation devices. This requires a detailed understanding of the mechanism for strain relaxation in these systems in order to eliminate strain-induced defect formation and thus to boost important electronic properties such as carrier mobility. Here we demonstrate how the hole mobility of [110]-oriented Ge–Si core–shell nanowires can be substantially enhanced thanks to the realization of large band offset and coherent strain in the system, reaching values as high as 4200 cm2/(Vs) at 4 K and 1600 cm2/(Vs) at room temperature for high hole densities of 1019 cm–3. We present a direct correlation of (i) mobility, (ii) crystal direction, (iii) diameter, and (iv) coherent strain, all of which are extracted in our work for individual nanowires. Our results imply [110]-oriented Ge–Si core–shell nanowires as a promising candidate for future electronic and quantum transport devices.

Predicted Quantum Topological Hall Effect and Noncoplanar Antiferromagnetism in K_{0.5}RhO_{2}.

The quantum anomalous Hall (QAH) phase is a two-dimensional bulk ferromagnetic insulator with a nonzero Chern number in the presence of spin-orbit coupling (SOC) but in the absence of applied magnetic fields. Associated metallic chiral edge states host dissipationless current transport in electronic devices. This intriguing QAH phase has recently been observed in magnetic impurity-doped topological insulators, albeit, at extremely low temperatures. Based on first-principles density functional calculations, here we predict that layered rhodium oxide K_{0.5}RhO_{2} in the noncoplanar chiral antiferromagnetic state is an unconventional three-dimensional QAH insulator with a large band gap and a Néel temperature of a few tens of Kelvins. Furthermore, this unconventional QAH phase is revealed to be the exotic quantum topological Hall effect caused by nonzero scalar spin chirality due to the topological spin structure in the system and without the need of net magnetization and SOC.