Measurement of carrier mobility in silicon nanowires

We report the first direct capacitance measurements of silicon nanowires (SiNWs) and the consequent determination of field carrier mobilities in undoped-channel SiNW field-effect transistors (FETs) at room temperature. We employ a two-FET method for accurate extraction of the intrinsic channel resistance and intrinsic channel capacitance of the SiNWs. The devices used in this study were fabricated using a top-down method to create SiNW FETs with up to 1000 wires in parallel for increasing the raw capacitance while maintaining excellent control on device dimensions and series resistance. We found that, compared with the universal mobility curves for bulk silicon, the electron and hole mobilities in nanowires are comparable to those of the surface orientation that offers a lower mobility.     

Effects of strain on the carrier mobility in silicon nanowires

We investigate electron and hole mobilities in strained silicon nanowires (Si NWs) within an atomistic tight-binding framework. We show that the carrier mobilities in Si NWs are very responsive to strain and can be enhanced or reduced by a factor >2 (up to 5×) for moderate strains in the ± 2% range. The effects of strain on the transport properties are, however, very dependent on the orientation of the nanowires. Stretched 100 Si NWs are found to be the best compromise for the transport of both electrons and holes in ≈10 nm diameter Si NWs. Our results demonstrate that strain engineering can be used as a very efficient booster for NW technologies and that due care must be given to process-induced strains in NW devices to achieve reproducible performances.     

Electron emission from individual indium arsenide semiconductor nanowires

A procedure was developed to mount individual semiconductor indium arsenide nanowires onto tungsten support tips to serve as electron field-emission sources. The electron emission properties of the single nanowires were precisely determined by measuring the emission pattern, current-voltage curve, and the energy spectrum of the emitted electron beam. The two investigated nanowires showed stable, Fowler-Nordheim-like emission behavior and a small energy spread. Their morphology was characterized afterward using transmission electron microscopy. The experimentally derived field enhancement factor corresponded to the one calculated using the basic structural information. The observed emission behavior contrasts the often unstable emission and large energy spread found for semiconductor emitters and supports the concept of Fermi-level pinning in indium arsenide nanowires. Indium arsenide nanowires may thus present a new type of semiconductor electron sources.     

Electrothermal dynamics of semiconductor nanowires under local carrier modulation

Charge transfer, surface/interface, defect states, and internal fields strongly influence carrier statics and dynamics in semiconductor nanowires. These effects are usually probed using spatially resolved scanning current techniques, where charge carriers are driven to move by diffusion force due to a density gradient, drift force due to internal fields, and thermoelectric force due to a temperature gradient. However, in the analysis of experimental data, analytical formulas are usually used which are based on the assumption that a single component of these forces dominates the carrier dynamics. In this work we show that this simplification is generally not justified even in the simplest configurations, and the scanning microscopy data need to be analyzed with caution. We performed a comprehensive numerical modeling of the electrothermal dynamics of free charge carriers in the scanning photocurrent microscopy configuration. The simulation allows us to reveal and predict important, surprising effects that are previously not recognized, and assess the limitation as well as potential of these scanning current techniques in nanowire characterization.     

Bending-induced conductance increase in individual semiconductor nanowires and nanobelts

Reliable ohmic contacts were established in order to study the strain sensitivity of nanowires and nanobelts. Significant conductance increases of up to 113% were observed on bending individual ZnO nanowires or CdS nanobelts. This bending strain-induced conductance enhancement was confirmed by a variety of bending measurements, such as using different manipulating tips (silicon, glass or tungsten) to bend the nanowires or nanobelts, and is explained by bending-induced effective tensile strain based on the principle of the piezoresistance effect. This article is published with open access at Springerlink.com     

Giant enhancement of the carrier mobility in silicon nanowires with diamond coating

We show theoretically that the low-field carrier mobility in silicon nanowires can be greatly enhanced by embedding the nanowires within a hard material such as diamond. The electron mobility in the cylindrical silicon nanowires with 4-nm diameter, which are coated with diamond, is 2 orders of magnitude higher at 10 K and a factor of 2 higher at room temperature than the mobility in a free-standing silicon nanowire. The importance of this result for the downscaled architectures and possible silicon-carbon nanoelectronic devices is augmented by an extra benefit of diamond, a superior heat conductor, for thermal management.     

Correlated micro-photoluminescence and electron microscopy studies of the same individual heterostructured semiconductor nanowires

To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (μ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to μ-PL or in NW device processing. The correlation between the three main μ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.     

Plasmon-assisted local temperature control to pattern individual semiconductor nanowires and carbon nanotubes

We demonstrate a new versatile strategy to rapidly heat and cool subdiffraction-limited volumes of material with a focused light beam. The local temperature rise is obtained by exploiting the unique optical properties of metallic nanostructures that facilitate efficient light-to-heat conversion through the excitation of surface plasmons (collective electron oscillations). By locally heating nanoscale metallic catalysts, growth of semiconductor nanowires and carbon nanotubes can be initiated and controlled at arbitrarily prespecified locations and down to the single nanostructure level in a room-temperature chamber. This local heating strategy can be orders of magnitude (>10(5)) more energy efficient than conventional chemical vapor deposition (CVD) tools in which an entire chamber/substrate is heated. For these reasons, it has great potential for use in process- and energy-efficient assembly of nanowires into complementary metal-oxide-semiconductor (CMOS) compatible device architectures. In general, the high degree of spatial and temporal control over nanoscale thermal environments afforded by this method inspires new pathways for manipulating a range of important thermally stimulated processes and the development of novel photothermal devices.     

Electronic properties of semiconductor nanowires

This paper provides a review of the state-of-the-art electronic-structure calculations of semiconductor nanowires. Results obtained using empirical k.p, empirical tight-binding, semi-empirical pseudopotential, and with ab initio methods are compared. For conciseness, we will restrict our detailed discussions to free-standing plain and modulated nanowires. Connections to relevant experimental data, particularly band gaps and polarization anisotropy, will be made since these results depend crucially on the electronic properties. For completeness, a brief review on the synthesis of nanowires is included.     

Optical coupling of deep-subwavelength semiconductor nanowires

Systems of coupled resonators manifest a myriad of exciting fundamental physical phenomena. Analogous to the synthesis of molecules from single atoms, the construction of photonic molecules from stand-alone optical resonators represents a powerful strategy to realize novel functionalities. The coupling of high quality factor (Q) dielectric and semiconductor microresonators is by now well-understood and chipscale applications are abound. The coupling behavior of low-Q nanometallic structures has also been exploited to realize high-performance plasmonic devices and metamaterials. Although dense arrays of semiconductor nanoparticles and nanowires (NWs) find increasing use in optoelectronic devices, their photonic coupling has remained largely unexplored. These high refractive index nano-objects can serve as low-Q optical antennas that can effectively receive and broadcast light. We demonstrate that the broad band antenna response of a pair of NWs can be tuned significantly by engineering their optical coupling and develop an intuitive coupled-mode theory to explain our observations.     

半导体材料少子寿命测试仪的研制开发

少数载流子寿命(简称少子寿命)是半导体材料的一项重要参数，它对半导体器件的性能、太阳能电池的效率都有重要的影响。我们采用微波反射光电导衰减法研制了一台半导体材料少子寿命测试仪，本文将对测试仪的实验装置、测试原理及程序计算进行了较详细的介绍，并与国外同类产品的测试进行比较，结果表明本测试仪测试结果准确、重复性高，适合少子寿命的实验室研究和工业在线测试。     

Measuring the capacitance of capacitors with large losses

Conclusions The application range of the beating method and similar methods for measuring capacitances with large losses are limited by errors which are inherent in these methods and are due to the loss resistances affecting the measurement results (12).The effect of losses on the capacitance measurements is completely eliminated in measuring circuits with modulated parameters. The utilization of circuits with an external modulation serves to raise the sensitivity of capacitance measurements. The automation of these circuits makes them very promising for designing commercial instruments.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 58–61, August, 1968.     

Electrical failure behaviors of semiconductor oxide nanowires

Electrical failure studies on semiconductor oxide nanowires (NWs) were performed in situ inside a transmission electron microscope (TEM). A high driven current leads to a sudden fracture of the SnO(2) NW and creates ultra-sharp and high aspect ratio tips at the broken ends, which provides a simple and reliable way for in situ nanoprobe fabrication. As a comparison, the TiO(2) NW fails due to Joule-heating-induced melting and retracts back into a nanosphere. The distinct behaviors are rooted in the different bonding nature. The strong ionic bonding between titanium and oxygen ions preserves the stoichiometry, while the covalently bonded SnO(2) NW decomposes before melting. The decomposition process is observed by resistively heating an SnO(2)/TiO(2) core-shell structure. It has been demonstrated that the needle-like geometry greatly enhanced field emission properties of SnO(2) NWs.     

Three-dimensional nanoscale composition mapping of semiconductor nanowires

We demonstrate the three-dimensional composition mapping of a semiconductor nanowire with single-atom sensitivity and subnanometer spatial resolution using atom probe tomography. A new class of atom probe, the local electrode atom probe (LEAP) microscope, was used to map the position of single Au atoms in an InAs nanowire and to image the interface between a Au catalyst and InAs in three dimensions with 0.3-nm resolution. These results establish atom probe tomography as a uniquely powerful tool for analyzing the chemical composition of semiconductor nanostructures.     

Characterization of semiconductor nanowires using optical tweezers

We report on the optical trapping characteristics of InP nanowires with dimensions of 30 (±6) nm in diameter and 2-15 μm in length. We describe a method for calibrating the absolute position of individual nanowires relative to the trapping center using synchronous high-speed position sensing and acousto-optic beam switching. Through brownian dynamics we investigate effects of the laser power and polarization on trap stability, as well as length dependence and the effect of simultaneous trapping multiple nanowires.     

Confinement-guided shaping of semiconductor nanowires and nanoribbons: "writing with nanowires"

To fully exploit their full potential, new semiconductor nanowire building blocks with ab initio controlled shapes are desired. However, and despite the great synthetic advances achieved, the ability to control nanowire's geometry has been significantly limited. Here, we demonstrate a simple confinement-guided nanowire growth method that enables to predesign not only the chemical and physical attributes of the synthesized nanowires but also allows a perfect and unlimited control over their geometry. Our method allows the synthesis of semiconductor nanowires in a wide variety of two-dimensional shapes such as any kinked (different turning angles), sinusoidal, linear, and spiral shapes, so that practically any desired geometry can be defined. The shape-controlled nanowires can be grown on almost any substrate such as silicon wafer, quartz and glass slides, and even on plastic substrates (e.g., Kapton HN).     

Structure-mechanical property of individual cobalt oxide nanowires

We present a comprehensive approach to address the correlation between mechanical properties of nanowires (NWs) with their characteristic size, microstructure, and chemical composition. Using this technique, the Young's modulus of Co3O4 NWs with different sizes was evaluated. Thermal annealing in inert atmosphere was found to induce chemical reduction of as-grown Co3O4 NWs into CoO NWs without modifying their geometrical shape. Both Co3O4 and CoO NWs exhibited a size-dependent variation in Young's modulus.     

Growth of semiconductor nanowires at large diffusion lengths

A universal asymptotic expression for the law of nanowire (NW) growth in cases where the diffusion lengths for adatoms on the substrate surface are much greater than the NW radius and the diffusion lengths for adatoms on the side surface of the growing crystal are much greater than the NW length. The main stages of growth, which are characterized by different relations between the NW length and its radius and the growth time, are determined. Possible asymptotic regimes of NW epitaxy are considered, including the cases of exponential growth and limited growth to a certain critical thickness, which depend on the direction of the diffusion flux of adatoms on the side surface of the growing crystal.     

Ultrafast dynamics of exciton formation in semiconductor nanowires

The dynamics of free electron-hole pairs and excitons in GaAs-AlGaAs-GaAs core-shell-skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron-hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron-hole charge pair densities of 2-4 × 10(16) cm(-3) . The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.