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The paper presents a device simulator for computing transport characteristics from first principles. The developed computer program effectively performs large-scale parallel calculation of quasi-one-dimensional quantum transport in realistic nanoscale devices with thousands of atoms in the cross section area of the device channel. Our simulator is based on the real-space Kohn-Sham Hamiltonian in the density functional theory and improved numerical algorithms for reducing computational burden in non-equilibrium Green's function (NEGF) method. Several computational improvements have been introduced in constructing a reduced quantum transport model from the original Kohn-Sham Hamiltonian and implementing the R-matrix computational scheme in the NEGF simulations. PACS numbers: 72.10.Bg, 73.23.-b, 73.40.Qv

We investigate the effects of hydrogen radical treatment on piezoresistance coefficients of germanium under uniaxial stress up to ±500 μ strain over the temperature range −30 °C to 25 °C. Hydrogen radicals are produced by hydrogen gas (99.99%, 2.7 kPa) on a hot tungsten catalyst (1800 °C). The gauge factor and majority carrier on non-dope germanium dramatically changed from n- to p-type after the hydrogen radical treatment. The results suggest that the piezoresistance coefficients of germanium are seriously affected by vacancy-related levels around the midgap, which behave as donors or acceptors depending on the Fermi level and are passivated by the hydrogen radicals.

The effects of strain on the band structure of GaN are investigated by using an empirical tight-binding method. The impacts on its bandgap, carrier effective mass, and group velocity are discussed. By analyzing the orbital components at the top of the valence band, the cause of the variation of the band structure including effective-mass exchange is discussed. Analysis of the average group velocity indicates that tensile uniaxial or compressive biaxial strain may be beneficial for achieving higher breakdown voltage in vertical GaN devices due to the smaller group velocity of the valence band. For the same reason, we also predict higher breakdown voltages due to tensile biaxial strain for horizontal devices.

The accuracy of the equivalent model (EM) in the band-to-band tunneling simulation of semiconductor nanowires is investigated by constructing EMs for various semiconductor nanowires and calculating the tunneling characteristics with the non-equilibrium Green’s function method. As the target window is widened, the EM accurately reproduces not only the traveling states but also the evanescent states. The relative error of the maximum tunnel current is shown to decrease exponentially with the transport window size. It is also shown that semiconductor nanowires can be classified into two types (D-type and I-type) according to the dispersion of the evanescent mode connecting the valence and the conduction band edges, and that the D-type nanowires can be represented with a smaller EM basis size or high accuracy compared to the I-type nanowires.

We propose the methodology for thermoelectric power factor enhancement using nanoscale thermal management. The thermally managed composite is composed of a high electrical conductivity (σ) material...

The electrical transport properties of a turbostratic multilayer graphene nanoribbon (GNR) with various number of layers (1–8 layers) were investigated using a field effect transistor with a single GNR channel. In the turbostratic multilayer GNR with 5 layers or less, the carrier mobility and Ion/Ioff ratio in the FETs were improved by slightly increasing the conductance with increasing the number of layers, meaning that the excellent semiconducting characteristic. The improvement of the carrier transport properties promotes by the turbostratic stacking structure. In the turbostratic multilayer GNR with 6 layers or more, although the Ion/Ioff ratio degraded, the conductance extremely improved with increasing the number of layers. This indicates that the turbostratic multilayer GNR with thicker number of layers becomes the significantly lower resistivity wire as a metallic characteristic. We revealed that the crossover point of the physical properties between the semiconducting and metallic characteristics is determined by the strength to screen the surrounding environment effects such as charged impurity on the substrate. Our comprehensive investigation provides a design guidance for the various electrical device applications of GNR materials.

We propose an equivalent model of simple form suitable for using in a computer program. The model reduces the order of device Hamiltonian and reproduces a narrow transport window of a target band-structure calculated by an atomistic model. We implemented the model with the adaptive moment estimation and the automatic differentiation technique by using an end-to-end open-source platform for machine learning. We tested the equivalent model on semiconducting nanoribbon and nanowire structures, and confirmed that it correctly reproduces not only the band-structures but also the ballistic transmission functions calculated by the non-equilibrium Green's function method.

Intra-layer band-to-band tunneling transmission function T(E) through monolayer transition metal dichalcogenides is calculated using the nonequilibrium Green function method combined with the tight-binding approximation. We focus on the differences in T(E) according to structures (nanosheet and nanoribbon) or materials (MoS2, WS2, MoSe2, WSe2, MoTe2, and WTe2). We find T(E) of the nanoribbon structure becomes much lower than that of the nanosheet structure due to the indirect transition and the small spatial overlap of the wave functions at the conduction band (CB) and valence band (VB) edges. In the nanosheet structure, the material dependence of T(E) is shown to be understood in terms of the tunneling mass and the bandgap energy. In the nanoribbon structure, MoTe2 and WTe2 show large T(E) due to the large spatial overlap of the wave functions at the CB bottom and VB top.

We perform TCAD simulation for TMDC channel TFETs with the material parameters considering ab initio band structure. By using the WKB-based nonlocal band-to-band tunneling (BTBT) model with the above parameters, we find that the current voltage characteristics of the TFETs are in good agreement with those obtained by microscopic NEGF calculation. Based on this approach, we also investigate the dependence of tunnel leakage current on the gate length. Our simulation method paves the way for reliable macroscopic device simulations for TMDC channel TFET.

We describe our recent progress in developing a non-equilibrium Green's function (NEGF) quantum transport simulator based on the real-space density functional theory (RSDFT). The simulator is implemented with the R-matrix theory and the low-dimensional equivalent model (EM), which substantially reduce the computational burden and make it possible to simulate the transport characteristics of realistic semiconductor devices from the first-principles. The simulator can incorporate non-equilibrium polarization charge effects by using the EM method to construct a piece-wise EM representation for a wide energy range of the RSDFT Hamiltonian. Numerical examples of transfer characteristics have shown for Si and Ge nanosheet field-effect transistors (FETs) and Si nanowire FETs.

The analytical formula of the transmission function of the inter-layer intra-band tunneling is derived for coupled narrow two-dimensional materials. Analytical models of the intra-band tunneling conductance G, the transmission function of the inter-layer band-to-band tunneling, and the maximum band-to-band tunneling current I max, are also obtained. G and I max are shown to exhibit different characteristics depending on the channel length.

Thermal conductance, K, of one-dimensional mass-disordered systems is calculated. Two types of dispersion models are considered: linear and quadratic dispersion model. For the linear dispersion model, the thermal conductance shows the super-diffusive nature, which is consistent with the previous research. For the quadratic dispersion model, the thermal conductance exhibits the normal-diffusion in which K is inversely proportional to the system size.

Using the reducing activity of ballistic hot electrons emitted from a nanocrystalline silicon (nc-Si) diode, it is demonstrated that thin Si and Ge films can be deposited under a situation that output electrons directly impinge upon a SiCl4 or GeCl4 solution-coated target substrate located in close proximity to the nc-Si emitter. After the electron incidence, uniform deposition of thin Si or Ge films was clearly observed in the irradiated area. In accordance with spectroscopic characterizations, no signs of contaminations were detected in deposited thin Si and Ge films, and those films exhibit features as oxidized nano-clusters. It is also shown that from a thermodynamic viewpoint, the output electron energy of the nc-Si emitter is suitable for promoting preferential reduction of target ions within the penetration depth in solutions followed by the nuclei formation.

A method for quantum transport simulations of nanowire (NW) field-effect transistors (FETs) with inelastic electron---phonon scattering processes incorporated is presented in this paper. The microscopic device Hamiltonian with realistic phonon spectrum and electron---phonon interaction is transformed into an equivalent low-dimensional transport model with discrete random phonon modes. The electron---phonon coupling constants are optimized in order to reproduce the inelastic scattering effects. Small size of the model and special form of the inelastic self-energy terms in the NEGF formalism make it a powerful tool to study dissipative transport in realistic NW transistors. The utility of the method is demonstrated by computing inelastic transport characteristics in Si NW FETs.

This study presents the material design of Si1−xGex epitaxial films/Si for thin film thermoelectric generators (TFTEGs) by investigating their thermoelectric properties. The thermoelectric films composed of group-IV elements are advantageous due to their compatibility with the Si process. We fabricated Si1−xGex epitaxial films with various controlled x values and strains using various growth methods. Ge epitaxial films without strains exhibited the highest thermoelectric power factor (∼47 μW cm−1 K−2) among various strain-controlled Si1−xGex (x ≠ 1) epitaxial films, which is higher at room temperature than SiGe alloy-based bulks ever reported. On the other hand, strained Si1−xGex epitaxial films showed an ultralow thermal conductivity of ∼2 W m−1 K−1, which is close to the value for amorphous Si. In addition to strained SiGe films with the ultralow thermal conductivity, unstrained Ge films with a high thermoelectric power factor can also be used for future TFTEGs by applying a nanostructuring technique. A preliminary TFTEG of Ge epitaxial films was realized, which generated a maximum power of ∼0.10 μW cm−2 under a temperature difference of 20 K. This demonstrates that epitaxial films composed of group-IV semiconductors are promising materials for TFTEG applications.

Comparative simulation study on the band-to-band (BTB) tunneling current of van der Waals heterojunctions consisting of transition metal dichalcogenides has been performed using a method based on the nonequilibrium Green function combined with a tight-binding approximation. When the channel length is longer, a dip structure of low tunneling transmission is found to appear on the transmission function from the source electrode to the drain electrode. This dip structure appears in the middle of the transport window and originates in the anti-crossing gap of the band-structure. It is also found that the BTB tunneling current is strongly enhanced when a satellite valley (Q-valley) is located within the transport window. This enhancement is attributed to the Q-valley assisted BTB tunneling.

Atomically thin layers of van der Waals (vdW) crystals offer an ideal material platform to realize tunnel field effect transistors (TFETs) that exploit the tunneling of charge carriers across the forbidden gap of a vdW heterojunction. This type of device requires a precise energy band alignment of the different layers of the junction to optimize the tunnel current. Amongst two-dimensional (2D) vdW materials, black phosphorus (BP) and indium selenide (InSe) have a Brillouin zone-centered conduction and valence bands, and a type II band offset, both ideally suited for band-to-band tunneling. Here, we demonstrate TFETs based on BP/InSe heterojunctions with diverse electrical transport characteristics: forward rectifying, Zener-tunneling and backward rectifying characteristics are realized in BP/InSe junctions with different thickness of the BP layer or by electrostatic gating of the junction. Electrostatic gating yields a large on/off current ratio of up to 108 and negative differential resistance at low applied voltages (V ~ 0.2V). These findings illustrate versatile functionalities of TFETs based on BP and InSe, offering opportunities for applications of these 2D materials beyond the device architectures reported in the current literature.

The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our "embedded-ZnO nanowire structure" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

Electron transport in Si double-gate junctionless transistors (JLTs) is simulated on the basis of the multisubband Monte Carlo method, considering acoustic phonons, intervalley phonons, ionized impurities (IIs), and surface roughness (SR) scattering. It is demonstrated that JLTs can actually minimize the mobility degradation caused by SR scattering and improve the current drivability. On the other hand, II scattering is confirmed to degrade the current drivability compared with conventional MOSFETs with an intrinsic channel. However, the performance degradation in JLTs due to II scattering is shown to be insignificant, owing to the screening effect of free carriers and the forward scattering properties of high-speed carriers. Furthermore, by extracting a backscattering coefficient, the screening effect and the forward scattering properties are shown to more mitigate II scattering as the gate voltage increases.

For photosensitive devices such as infrared (IR) photodetectors, dark current is an important mechanism limiting the performance because it causes a decrease in the signal-to-noise ratio and the responsibility. The main objective of this work is to understand and analyze the electrical performance of InAs/GaSb type-II superlattice (SL) IR detectors using a device simulator especially focusing on the dark current characteristics. Physical parameters such as the effective density of states are extracted from the k-p band calculation, and included into a two-dimensional device simulator based on the drift-diffusion model. Simulation results demonstrate that the leakage current of the IR photodetector depends on the SL thickness ratio even with a similar cut-off wavelength due to the difference in the intrinsic carrier density. The InAs-rich SL detectors exhibit smaller dark current than the GaSb-rich ones, which is qualitatively consistent with the previously reported experimental data.

High-field electron transport characteristics in bulk 4H-SiC are simulated with the full-band Monte Carlo method, and anisotropy of the ionization coefficient α is discussed. The simulation results exhibit larger α along 〈1120〉 direction than 〈0001〉 direction, which is consistent with the experimental observation. Furthermore, the dependence of α on the electric field direction θ is investigated. Compared to the existing analytical model, the present simulation results show a steeper increase when θ is tilted from the c-axis, which is originated from the anisotropic nature of the hot-electron heating.

The paper presents a method for the first principles calculations of polarization charge in non-equilibrium states of sihcon nanowire transistor. The method combines the density-functional perturbation theory and piecewise equivalent model representation which enables us to reduce the numerical burden and eliminate erroneous contributions from the occupied electronic states. In the mean field approximation, the polarization response of the device material is taken into consideration by using appropriate dielectric constants in the Poisson solver. Our results suggest that this approximation generally fails and the electric field in the device area may give rise to a nonzero macroscopic polarization charge which is completely ignored in common device simulations.

The frame rate of the digital high-speed video camera was 2000 frames per second (fps) in 1989, and has been exponentially increasing. A simulation study showed that a silicon image sensor made with a 130 nm process technology can achieve about 1010 fps. The frame rate seems to approach the upper bound. Rayleigh proposed an expression on the theoretical spatial resolution limit when the resolution of lenses approached the limit. In this paper, the temporal resolution limit of silicon image sensors was theoretically analyzed. It is revealed that the limit is mainly governed by mixing of charges with different travel times caused by the distribution of penetration depth of light. The derived expression of the limit is extremely simple, yet accurate. For example, the limit for green light of 550 nm incident to silicon image sensors at 300 K is 11.1 picoseconds. Therefore, the theoretical highest frame rate is 90.1 Gfps (about 1011 fps).

The manuscript presents a strategy for enhancing the carrier mobility of single layer CVD graphene (CVD SLG) based on spatially correlated charges. Our Monte Carlo simulations, numerical modeling and the experimental results confirm that spatial correlation between defects with opposite charges can provide a means to control independently the carrier concentration and mobility of planar field effect transistors in which graphene is decorated with a layer of colloidal quantum dots (QDs). We show that the spatial correlation between electrically charged scattering centres close to the graphene/SiO2 interface and the localised charges in a QD layer can smooth out the electrostatic potential landscape, thus reducing scattering and enhancing the carrier mobility. The QD capping molecules influence the distribution and correlation of electrical charges in the vicinity of SLG and provide a means of tuning the carrier concentration and increasing the carrier mobility in graphene. These results represent a significant conceptual advance and provide a novel strategy for control of the electronic properties of 2D materials that could accelerate their utilization in optoelectronic devices.

We report a simultaneous increase of carrier concentration, mobility and photoresponsivity when SiC-grown graphene is decorated with a surface layer of colloidal PbS quantum dots, which act as electron donors. The charge on the ionised dots is spatially correlated with defect charges on the SiC-graphene interface, thus enhancing both electron carrier density and mobility. This charge-correlation model is supported by Monte Carlo simulations of electron transport and used to explain the unexpected 3-fold increase of mobility with increasing electron density. The enhanced carrier concentration and mobility give rise to Shubnikov-de Haas oscillations in the magnetoresistance, which provide an estimate of the electron cyclotron mass in graphene at high densities and Fermi energies up to 1.2 x 10(13) cm(-2) and 400 meV, respectively. Funding Agencies|Leverhulme Trust [RPG-2013-242]; Engineering and Physical Sciences Council [EP/M012700/1]; EU

Quantum-sized nanocrystalline Si (nc-Si) dots behave as a drift layer which efficiently generates ballistic hot electrons. This is due to little phonon scattering losses in nc-Si at the early stage of injection and subsequent multiple-tunneling cascade through nc-Si dots chain interconnected with thin oxides [1]. At a sufficient electric field, generated ballistic hot electrons are ejected through the surface electrode. In solutions, the nc-Si emitter operates as an active electrode supplying highly reducing electrons. This is applicable to deposition of thin films of metals [2], Si [3], and Ge [4]. Details of this thin film deposition scheme based on the ballistic reduction effects is discussed here including the availability for the deposition of thin Ge and SiGe films. The nc-Si emitter is a kind of MIS diode consisting of a thin film surface electrode, a nanosilicon layer (~1 μm thick), a crystalline silicon wafer substrate, and a back contact. The nc-Si layer includes parallel chains of quantum-sized nanosilicon dots (~3 nm in mean diameter) interconnected with tunnel oxides. When a positive voltage is applied to the surface electrode, electrons injected from the substrate into the nc-Si layer are accelerated toward the outer surface, and then some of them are emitted from the surface electrode as ballistic hot electrons [5]. This planar cold cathode emits energetic and directional electrons. The kinetic energy of electrons can be controlled by the applied voltage. At applied voltages of 15-20 V, for instance, the mean energy of output electrons varies from 5 to 7 eV. Besides in vacuum, the nc-Si emitter operates in atmospheric-pressure gases and even in solutions. Since electrons with an extremely high and electrochemical activity are supplied at relatively low applied voltage, positive ion species are directly reduced. Following the experimental confirmation of the usefulness in aqueous solutions as means for H2 generation and pH control, it has been demonstrated that the ballistic hot electron injection into metal-salt solutions leads to the deposition of thin metal (Cu, Ni, Co, Zn, and so on) films without using any counter electrodes. After the device is mounted in a N2 gas filled glove box, a small amount (5~10 μl) of SiCl4, GeCl4, or their mixture (1:1 in volume) was dripped onto the emission surface, and then the emitter was driven at V b=10-15 V. After the emitter operation for several min without using any counter electrodes, thin films were uniformly deposited on the emitting area in the same way as the previous dipping method. The structure and composition of the deposited films were investigated by transmission electron microscope (TEM), scanning electron microscope (SEM), atomic force microscope (AFM), energy dispersive X-ray (EDX) spectra, X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectroscopy (SIMS). In accordance with TEM analyses, the deposited films are amorphous. The observed EDX spectra indicate that the respective deposited films are composed of Si, Ge, and SiGe. The contamination signals of Cl were below the detection limit of XPS measurements. The result of SIMS of thin SiGe films, the compositional ratio of Si to Ge was about 0.9:0.1. This can be controlled by changing the mixture ratio of SiCl4+GeCl4 solution. In contrast to the electro-plating based on exchange of thermalized electrons, the ballistic electro-reduction proceeds with neither Cl2 evolution nor byproducts generation at the counter electrode. Despite a significantly large electrochemical window between the reduction (Si/Si4+) and oxidation (Cl2/Cl-) potentials, injected electrons have energies sufficient enough for unilateral reduction. Being a low-temperature and clean process, this ballistic electro-reduction mode is attractive as an alternative approach for fabrication of group-IV thin-films, multilayered nanostructures, and devices. This work was partially supported by the JSPS through a Grant-in-Aid for Scientific Research and the FIRST Program and by the NEDO through the R&D Project. References 1. N. Mori, H. Minari, S. Uno, H. Mizuta, and N. Koshida, Appl. Phys. Lett. 98, 062104 (2011). 2. T. Ohta, B. Gelloz, and N. Koshida, Electrochem. Solid-State Letters 13, D73 (2010). 3. T. Ohta, B. Gelloz, and N. Koshida, Appl. Phys. Lett. 102, 022107 (2013). 4. N. Koshida, A. Kojima, T. Ohta, R. Mentek, B. Gelloz, N. Mori, J. Shirakashi, ECS Solid State Lett. 3(5), P57 (2014). 5. N. Koshida, X. Sheng, and T. Komoda, Appl. Surf. Sci. 146, 371 (1999).

Apart from the conventional dry process such as chemical vapor deposition (CVD), some electrochemical approaches have been conducted to fabricate thin Si and Ge films. The liquids used to date for electro-deposition of those films are nonaqueous organic electrolytes, 1‐4 tetrachloride solutions (SiCl4, GeCl4), 5,6 ionic liquids, 7‐13 aqueous solutions, 14 and molten salt solutions. 15‐17 In every case, however, deposited films show non-uniform, porous, or contaminant features in structure and composition. Nanocrystalline silicon (nc-Si) diode acts as a planar cold cathode which uniformly emits ballistic hot electrons. 18 The nc-Si emitter is composed of a thin Au (10 nm), an anodized nc-Si layer (∼1 μm), a

We solve the steady-state Barker–Ferry equation by converting it to a zero-eigenvalue problem and investigate the impact of the intra-collisional field effect (ICFE) on the transport characteristics of one-dimensional GaN nanowires. We find that an electron deficiency appears in the distribution function at around the optical-phonon energy for the case that the electron wavevector is antiparallel to the applied electric field. This causes a significant drift-velocity reduction compared to the drift-velocity calculated without the ICFE.

The InN/GaN digital alloy is a superlattice-like nanostructure formed by periodically stacking ultra-thin InN and GaN layers. In this study, we calculate the electron mobility in InN/GaN digital alloy channel high electron mobility transistors (HEMTs) by performing a single-particle Monte Carlo simulation. The results of the simulation show that alloy-induced scatterings have little impact and the electron mobility significantly improves as the effective indium mole fraction of the channel increases. This contrasts with InGaN alloy channel HEMTs, where alloy disorder and random dipole scatterings have a strong impact and the electron mobility decreases as the indium mole fraction of the channel increases.