Full-three dimensional quantum approach to evaluate the surface-roughness-limited magnetoresistance mobility in SNWT

We present a theoretical method to simulate magnetotransport in silicon nanowire (Si-NW) MOSFET including the effect of Surface Roughness (SR). We use a full three dimensional (3D) real-space self-consistent Poisson-Schrödinger solver based on Non Equilibrium Green’s function Formalism (NEGF) which can treat the influence of an external magnetic field on the device. By comparing magnetoconductance curves with the classical Drude formula we extract magnetoresistance (MR) mobility for nanowires with and without roughness. From the preliminary results it seems that the MR mobility is not dramatically reduced for the SR parameters considered in this work.     

Transport calculationof Semiconductor Nanowires Coupled to Quantum Well Reservoirs

Semiconductor nanowires are possible candidates to replace the metal-oxide-semiconductor field-effect transistors (MOSFET) since they can act both as active devices or as device connectors. In this article, the transmission coefficients of Si and GaAs nanowires with arbitrary transport directions and cross sections are simulated in the nearest-neighbor sp³d⁵s* semi-empirical tight-binding method. The open boundary conditions (OBC) are calculated with a new scattering boundary method where a normal eigenvalue problem of reduced size is solved. Two different types of contacts are studied. In the ideal case, semi-infinite reservoirs (the source and the drain) that are the prolongation of the device are assumed. In a more realistic configuration, the active nanowire is embedded between two quantum well (QW) reservoirs. The electrical properties of the device are obtained by a non-equilibrium Green’s function (NEGF) calculation.     

Transport in silicon nanowires: role of radial dopant profile

We consider the electronic transport properties of phosphorus (P) doped silicon nanowires (SiNWs). By combining ab initio density functional theory (DFT) calculations with a recursive Green’s function method, we calculate the conductance distribution of up to 200 nm long SiNWs with different distributions of P dopant impurities. We find that the radial distribution of the dopants influences the conductance properties significantly: surface doped wires have longer mean-free paths and smaller sample-to-sample fluctuations in the cross-over from ballistic to diffusive transport. These findings can be quantitatively predicted in terms of the scattering properties of the single dopant atoms, implying that relatively simple calculations are sufficient in practical device modeling.     

Controlled growth of silicon nanowires on silicon surfaces

This paper reports on silicon nanowire growth on oxidized silicon substrates using different approaches for gold catalyst deposition. The gold coated surfaces and the resulting nanowires were characterized using scanning electron microscopy. The gold catalysts were made up of gold nanoparticles (50 nm diameter), which were either dispersed or spotted at different concentrations using a robot, or were formed on a patterned Si/SiO₂ substrate by metal evaporation (63 nm diameter). The subsequent silicon nanowire growth was accomplished by CVD decomposition of silane gas (SiH₄) at high temperature (400–500^°C) in a vapor-liquid-solid (VLS) process. Under these conditions, a high density of silicon nanowires (SiNWs) was achieved on the oxidized silicon surfaces, but the distribution of the nanowires was found to be inhomogeneous in the case of the gold nanoparticles. Such result is attributed to the aggregation of the nanoparticles during the growth process. Alternatively, when gold nanodot catalysts were lithographically patterned on the surface, the nanowires were obtained in the patterned regions.     

Nanowire transistor modeling: influence of ionized impurity and correlation effects

This study presents two relevant effects influencing the electronic transport of nanowire transistors. We first focus on the ionized impurity impacts and calculate the current characteristics with a self-consistent three-dimensional (3D) Green’s function approach. The results show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. In a second part, we report a numerical study of the self-energy correction due to correlation effects from dynamic screening of the moving electron in silicon nanowire transistors. This many-body effect, which is not included in the usual Hartree approximation, is then incorporated self-consistently into a non-equilibrium Green’s function (NEGF) code. The results pinpoint the importance of dielectric confinement whose magnitude can not be neglected compared to its quantum counterpart in ultimate nanowire transistors.     

NEGF simulation of the RTD bistability

The simulation of I-V characteristics of Al_0.3Ga_0.7As-GaAs and AlAs-GaAs resonant tunneling diodes (RTD) is presented. The nonequilibrium Green function (NEGF) based 1D quantum transport simulator Wingreen is used in our case. The plateau region on the IV characteristics usually present only by the Wigner function equation (WFE) based simulation appeared now by the NEGF simulation of our AlAs-GaAs RTD and its shape is comparable with our experimental measurements. Analysis of our results from point of view of the scattering and geometrical parameters of the RTD structure is presented.     

Growth of flower-like patterns of TiO2 nanorods over FTO substrate

ABSTRACT

Nanostructures such as nanoparticles, nanowires, nanorods, nanotubes have attracted great interest with the understanding of shape and size dependence in the electronic and sensing properties. This work demonstrates growth and characterization of one-dimensional nanorods of titanium-dioxide (TiO₂) on the fluorine doped tin oxide (FTO) substrate by using hydrothermal method. Field effect scanning electron microscopy shows flower-like morphology, and energy-dispersive X-ray spectroscopy confirms the presence of Ti and O. The X-ray diffraction peaks agree well with the JCPDS data. Initially, the 1D nanorods are formed and high concentration of precursor assists evolution of the short branches to form flower-like structures.     

Modeling Challenges in Molecular Electronics on Silicon

Molecular electronics on silicon substrates hold promise for new functional possibilities on silicon. One such possibility is a hybrid silicon-molecule device that shows negative differential resistance (NDR) when conduction through the molecular levels are cut-off as the levels are driven into the silicon band-gap region. We demonstrate through a self-consistent solution based on Non-equilibrium Green’s Function (NEGF) formalism that the NDR peaks show a clear polarity dependence, appearing for only positive bias on a p-doped substrate. We further discuss what determines the position of the NDR peaks and how the locations are affected by self-consistency.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Microfabrication of Piezoelectric MEMS

In this paper we present an overview of processes for fabrication of piezoelectric thin film devices using PZT (Pb(Zr _x Ti_{1 ? x} )O₃) in planar structures. These structures are used in cantilever-like and membrane configurations for sensing and actuation. Elaboration of a compatible wet and dry etching sequence for patterning of PZT, electrodes, SiO₂ and silicon substrate is the key issues. The method for compensation of mechanical stresses to obtain flat, multilayer structures is demonstrated. Definition of membrane thickness and release of the structures are obtained by Deep Reactive Ion Etching of silicon (SOI—silicon on insulator substrates) or by surface micromachining. The complete process has been used for fabrication of cantilever arrays, ultrasonic transducers and pressure sensors. Excellent permittivity and transverse piezoelectric coefficient of PZT have been obtained with the final devices. Other examples of applications like: ferroelectric memories, nanopatterning and local growth of PZT are presented as well. The microfabrication of piezoelectric MEMS was found to be a complex task where all aspects from device design, material properties and microfabrication to assessment of performance are closely interconnected.     

First-principles versus semi-empirical modeling of global and local electronic transport properties of graphene nanopore-based sensors for DNA sequencing

Using first-principles quantum transport simulations, based on the nonequilibrium Green function formalism combined with density functional theory (NEGF+DFT), we examine changes in the total and local electronic currents within the plane of graphene nanoribbon with zigzag edges (ZGNR) hosting a nanopore which are induced by inserting a DNA nucleobase into the pore. We find a sizable change of the zero-bias conductance of two-terminal ZGNR + nanopore device after the nucleobase is placed into the most probable position (according to molecular dynamics trajectories) inside the nanopore of a small diameter \(D=1.2\) nm. Although such effect decreases as the nanopore size is increased to \(D=1.7\) nm, the contrast between currents in ZGNR + nanopore and ZGNR + nanopore + nucleobase systems can be enhanced by applying a small bias voltage \(V_b \lesssim 0.1\) V. This is explained microscopically as being due to DNA nucleobase-induced modification of spatial profile of local current density around the edges of ZGNR. We repeat the same analysis using NEGF combined with self-consistent charge density functional tight-binding (NEGF+SCC-DFTB) or self-consistent extended Hückel (NEGF+SC-EH) semi-empirical methodologies. The large discrepancy we find between the results obtained from NEGF+DFT vs. those obtained from NEGF+SCC-DFTB or NEGF+SC-EH approaches could be of great importance when selecting proper computational algorithms for in silico design of optimal nanoelectronic sensors for rapid DNA sequencing.     

An extended Hückel theory based atomistic model for graphene nanoelectronics

An atomistic model based on the spin-restricted extended Hückel theory (EHT) is presented for simulating electronic structure and I–V characteristics of graphene devices. The model is applied to zigzag and armchair graphene nano-ribbons (GNR) with and without hydrogen passivation, as well as for bilayer graphene. Further calculations are presented for electric fields in the nano-ribbon width direction and in the bilayer direction to show electronic structure modification. Finally, the EHT Hamiltonian and NEGF (Nonequilibrium Green’s function) formalism are used for a paramagnetic zigzag GNR to show 2e ²/h quantum conductance.     

First principles modeling of disorder scattering in graphene

It is important to investigate impurity scattering phenomena when modeling graphene nanoscale devices, as impurities are invariably present in any realistic system and can significantly influence graphene carrier transport. We present a short review of quantum transport where density functional theory (DFT) is carried out within the nonequilibrium Green’s function formalism (NEGF), focusing on a recent extension of this framework in the form of nonequilibrium vertex correction (NVC) that captures random graphene impurity scattering in a systematic fashion. Our results show that disorder effects significantly alters the electronic and transport properties of graphene devices. We argue that disorder effects should not be ignored if one were to model graphene nanoscale devices in realistic situations, including arriving at fundamental electronic properties such as Ohm’s law.     

Study on Assembly Method for GaAs Nanowires Device

The assembly and fabrication method for Gallium arsenide (GaAs) nanowires nano devices was implemented. Assembly of GaAs nanowires field effect transistor (FET) was realized by dielectrophoresis approach. Before deposition of the contacts, GaAs nanowires were treated wet etching in an ammonium polysulfide ((NH₄)₂S) solution to remove a surface oxide layer. The assembled devices were characterized by atomic force microscopy. The experimental results showed that the efficient assembly of GaAs nanowires was obtained when the applied alternating current voltage has a frequency of 1.5MHz and an amplitude of 10 V, and the success rate of ideal assembly for GaAs nanowires FET had been assessed. Meanwhile, it also provided an effective assembly method for other one-dimensional nanomaterials assembly of nano devices.     

Emission and absorption of optical phonons in Multigate Silicon Nanowire MOSFETs

In this paper we study the influence of emission/absorption processes due to optical phonons on the electrical properties of multigate silicon nanowire transistors. We show that low-energy phonons reduce drain current through backscattering of carriers by emission/absorption processes while high-energy phonons redistribute the current energy spectrum along the nanowire channel through phonon emission without significantly reducing the drain current drive. The influence of emission/absorption is investigated in different multigate silicon FET structures with uniform channel, single impurity, random doping atom distribution and oxide tunnel barriers. A three-dimensional quantum mechanical device simulator based on the NEGF formalism in coupled mode-space approach is used to model electron transport in the presence of optical phonon scattering mechanism. Electron-phonon scattering is accounted for by adopting the self-consistent Born approximation and using the deformation potential theory.     

Nanowires: a new pathway to nanotechnology-based applications

The synthesis and the characterisation of silicon nanowires (SiNWs) have recently attracted great attention due to their potential applications in electronics and photonics. As yet, there are no practical uses of nanowires, except for research purposes, but certain properties and characteristics of nanowires look very promising for the future.

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Graphical abstract Semiconductor nanowires are attracting more and more interest for their applications in nanoscience and nanotechnology. The characteristic of the nanowires is their geometry with a diameter in the range of a few nanometers and a length far greater than their diameter. The structural defects often lead to mechanical defects. By reducing the number of defects per unit length, decreasing the lateral dimensions, crystalline nanowires are expected to be more resistant than the solid. Recently nanowires are attracting intense interest for solar energy conversion. In this review, we summarize the different methods of nanowires production and their applications. Special focus will be kept on silicon nanowires.

     

Scanning Probe Parallel Nanolithography with Multiprobe Cantilever Array Fabricated by Bulk Silicon Micromachining

This work describes a scanning probe parallel nanolithography (SPNL) technique for high throughput in nanometric patterning on single‐crystal silicon (SCS) substrates. Two types of multiprobe cantilever arrays used for SPNL were fabricated by conventional micromachining. All the probes mounted on the free end of each cantilever were made of quasitrihedral pyramidal shape composed of (311) and (411) planes using the originally designed mask. Negative and positive types of nanolithography were performed on the basis of field‐enhanced anodization and self‐assembled monolayer mask techniques, respectively, and they succeeded in drawing a number of nanometric patterns of silicon dioxide (SiO₂) on SCS substrates. After anisotropic wet etching of the SCS substrates using the SiO₂ films as the mask material, we were also able to fabricate nanowires and nanogrooves. The effects of the applied voltage and scan time of cantilever arrays on wire and groove dimensions were systematically examined by atomic force microscopy (AFM) observations. An optimum condition for the parallel SPNL is proposed on the basis of this research. © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.