Phosphorus-doped Silicon Nanowires Research Articles

Time-efficient simulations of tight-binding electronic structures with Intel Xeon PhiTM many-core processors

Modelling of multi-million atomic semiconductor structures is important as it not only predicts properties of physically realizable novel materials, but can accelerate advanced device designs. This work elaborates a new Technology-Computer-Aided-Design (TCAD) tool for nanoelectronics modelling, which uses a sp3d5s∗ tight-binding approach to describe multi-million atomic structures, and simulate electronic structures with high performance computing (HPC), including atomic effects such as alloy and dopant disorders. Being named as Quantum simulation tool for Advanced Nanoscale Devices (Q-AND), the tool shows nice scalability on traditional multi-core HPC clusters implying the strong capability of large-scale electronic structure simulations, particularly with remarkable performance enhancement on latest clusters of Intel Xeon PhiTM coprocessors. A review of the recent modelling study conducted to understand an experimental work of highly phosphorus-doped silicon nanowires, is presented to demonstrate the utility of Q-AND. Having been developed via Intel Parallel Computing Center project, Q-AND will be open to public to establish a sound framework of nanoelectronics modelling with advanced HPC clusters of a many-core base. With details of the development methodology and exemplary study of dopant electronics, this work will present a practical guideline for TCAD development to researchers in the field of computational nanoelectronics.

Atomistic study on dopant-distributions in realistically sized, highly P-doped Si nanowires.

The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger-Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼ 2 × 10(19) cm(-3) and have 12 nm-28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm. Insufficient room to screen donor ions and shallower impurity bands are the primary reasons of the nonuniform dopant-distributions in smaller nanowires. Being firmly connected to the recent experimental study (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15254-15258), this work establishes the first theoretical framework for understanding dopant-distributions in over-10 nm highly doped silicon nanowires.

Electronic Level Scheme in Boron- and Phosphorus-Doped Silicon Nanowires

We report the first observation of the electronic level scheme in boron (B)- and phosphorus (P)-doped nanowires (NWs). The NWs' morphology dramatically depends on the doping impurity while a few deep electronic levels appear in both kinds of nanowires, independently of the doping type. We demonstrate that the doping impurities induce the same shallow levels as in bulk silicon. The presence of two donor levels in the lower half-bandgap is also revealed. In both kinds of NWs, B- and P-doped, the donor level (0/+) at E(v) + 0.36 eV of the gold-hydrogen complex is observed. This means that the gold diffusion from the NW tip introduces an electronically active level, which might negatively affects the electrical characteristics of the NWs. In P-doped NWs, we observed a further donor level at 0.26 eV above the valence band due to the phosphorus-vacancy pairs, the E-center, well-known in bulk silicon. These findings seriously question both diffusion modeling of impurities in NWs and the technological aspects arising from this.

Exchange-Induced Electron Transport in Heavily Phosphorus-Doped Si Nanowires

Heavily phosphorus-doped silicon nanowires (Si NWs) show intriguing transport phenomena at low temperature. As we decrease the temperature, the resistivity of the Si NWs initially decreases, like metals, and starts to increase logarithmically below a resistivity minimum temperature (T(min)), which is accompanied by (i) a zero-bias dip in the differential conductance and (ii) anisotropic negative magnetoresistance (MR), depending on the angle between the applied magnetic field and current flow. These results are associated with the impurity band conduction and electron scattering by the localized spins at phosphorus donor states. The analysis on the MR reveals that the localized spins are coupled antiferromagnetically at low temperature via the exchange interaction.

In Situ Electron Microscopy Mechanical Testing of Silicon Nanowires Using Electrostatically Actuated Tensile Stages

Two types of electrostatically actuated tensile stages for in situ electron microscopy mechanical testing of 1-D nanostructures were designed, microfabricated, and tested. Testing was carried out for mechanical characterization of silicon nanowires (SiNWs). The bulk micromachined stages consist of a comb-drive actuator and either a differential capacitive sensor or a clamped-clamped beam force sensor. High-aspect-ratio structures (height/gap = 20) were designed to increase the driving force of the geometrically optimized actuator and the sensitivity of the capacitive sensor. The actuator stiffness is kept low to enable high tensile force to be exerted in the specimen rather than in the suspensions of the comb drive. Individual SiNWs were mounted on the devices by in situ scanning electron microscopy nanomanipulation, and their tensile properties were determined to demonstrate the device capability. The phosphorus-doped SiNWs, which were grown in a bottom-up manner by the vapor-liquid-solid process, show an average Young's modulus of (170.0 ± 2.4) GPa and a tensile strength of at least 4.2 GPa. Top-down electroless chemically etched SiNWs, with their long axis along the [100] direction, show a fracture strength of 5.4 GPa.

Quantized electron transfer through random multiple tunnel junctions in phosphorus-doped silicon nanowires

We have demonstrated numerically and experimentally that quantized electron transfer can be achieved in single-gated random multiple tunnel junctions. Extensive Monte Carlo simulations based on Coulomb blockade orthodox theory show that nonhomogeneous distributions of capacitances energetically favor one-by-one electron shuttling between the electrodes during each cycle of a gate voltage. This numerical prediction is supported by our experimental results on Si nanowire field-effect transistors with the channel moderately doped with phosphorus. Ionized dopants within the device channel locally modulate the potential, creating a naturally random one-dimensional multiple-tunnel-junction array. Under ac-gate operation, small current plateaus or inflections aligned at $\ifmmode\pm\else\textpm\fi{}nef$ appear in the ${I}_{d}\text{\ensuremath{-}}{V}_{d}$ characteristics, suggesting that quantized electron transfer is achievable in such naturally disordered systems.

Surface Segregation and Backscattering in Doped Silicon Nanowires

By means of ab initio simulations, we investigate the structural, electronic, and transport properties of boron and phosphorus doped silicon nanowires. We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Important differences between p- and n-type doping are observed. Our results rationalize several experimental observations.

Use of Phosphine as an n-Type Dopant Source for Vapor−Liquid−Solid Growth of Silicon Nanowires

Phosphine (PH3) was investigated as an n-type dopant source for Au-catalyzed vapor-liquid-solid (VLS) growth of phosphorus-doped silicon nanowires (SiNWs). Transmission electron microscopy characterization revealed that the as-grown SiNWs were predominately single crystal even at high phosphorus concentrations. Four-point resistance and gate-dependent conductance measurements confirmed that electrically active phosphorus was incorporated into the SiNWs during VLS growth. A transition was observed from p-type conduction for nominally undoped SiNWs to n-type conduction upon the introduction of PH3 to the inlet gas. The resistivity of the n-type SiNWs decreased by approximately 3 orders of magnitude as the inlet PH3 to silane (SiH4) gas ratio was increased from 2 x 10(-5) to 2 x 10(-3). These results demonstrate that PH3 can be used to produce n-type SiNWs with properties that are suitable for electronic and optoelectronic device applications.

Phosphorus-doped silicon nanowires studied by near edge x-ray absorption fine structure spectroscopy

The electronic structure and chemical bonding of heavily phosphorus-doped silicon nanowires have been investigated by near edge x-ray absorption fine structure spectroscopy using total electron yield and x-ray fluorescence yield. The results show that the wires are encapsulated within a silicon oxide layer and that the core of the nanowires was crystalline silicon doped with phosphorus. Phosphorus is found to be inside the core of the Si wire as well as at the silicon oxide–core silicon wire interface but not on the surface of the as-prepared nanowire.

The effect of cross-coupling in a bidirectional electron pump

Abstract Bidirectional electron pumps consisting of two multiple tunnel junctions formed along 2×10 19 cm −3 phosphorus-doped silicon nanowires were realised in silicon-on-insulator material. Both devices are separated by one macroscopic main island to which an rf-signal is applied, providing the energy needed for pump operation. The electrical characteristics of the circuit are discussed with reference to cross-coupling effects of the rf-signal towards the multiple tunnel junctions. We find that cross-coupling has a major impact on the circuit characteristic and needs to be considered if further circuit miniaturisation is desired.

Silicon nanowires and their application in bi-directional electron pumps

Multiple tunnel junctions formed along 2×10 19 cm −3 phosphorus-doped silicon nanowires of different length were realised in silicon-on-insulator material. The electrical characteristics of these Coulomb blockade devices indicate that the gate voltages provide better control of the transport through the tunnel barriers if their wire length is reduced. This results in more distinct Coulomb blockade oscillations which are needed for the successful operation of bi-directional electron pumps consisting of two short nanowires separated by one macroscopic main island to which an rf signal is applied.