Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n- and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions.     

Controlling heterojunction abruptness in VLS-grown semiconductor nanowires via in situ catalyst alloying

For advanced device applications, increasing the compositional abruptness of axial heterostructured and modulation doped nanowires is critical for optimizing performance. For nanowires grown from metal catalysts, the transition region width is dictated by the solute solubility within the catalyst. For example, as a result of the relatively high solubility of Si and Ge in liquid Au for vapor-liquid-solid (VLS) grown nanowires, the transition region width between an axial Si-Ge heterojunction is typically on the order of the nanowire diameter. When the solute solubility in the catalyst is lowered, the heterojunction width can be made sharper. Here we show for the first time the systematic increase in interface sharpness between axial Ge-Si heterojunction nanowires grown by the VLS growth method using a Au-Ga alloy catalyst. Through in situ tailoring of the catalyst composition using trimethylgallium, the Ge-Si heterojunction width is systematically controlled by tuning the semiconductor solubility within a metal Au-Ga alloy catalyst. The present approach of alloying to control solute solubilities in the liquid catalyst may be extended to increasing the sharpness of axial dopant profiles, for example, in Si-Ge pn-heterojunction nanowires which is important for such applications as nanowire tunnel field effect transistors or in Si pn-junction nanowires.     

Photoconductive characteristic in individual WO(3-x) nanowire

Tungsten Oxide nanowires, with their natural excellent structure, possess unique physical and chemical properties. In this paper, photoconductivity of single tungsten trioxide nanowire (WO3 NW) and single tungsten suboxide nanowire (WO(3-x) NW) have been studied respectively on a photoconductivity testing system. Under 514 nm wavelength laser illumination, an unsaturated photocurrent and a slow photoconductive responsivity could be expressed in single WO3 NW device. In WO(3-x) NW device, photoconductive responsivity was determined by the illuminating position. About 100 nA photocurrent could be generated and a fast optoelectric responsivity with a recover time about 90 ms from the device was observed. Not only photoconductive effect but also the photovoltaic effect was obtained from individual WO(3-x) NW device. According to the results of X-ray photoelectron spectroscopy, the mechanisms of photoconductive and photovoltaic effects in these two kinds of devices have been discussed and the oxygen vacancy played an important role in the photoconductive phenomenon. All these effects could be of practical use in the design and fabrication of photodetectors based on single tungsten oxide nanowire.     

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm², which is important for potential applications in concentrator solar cells.     

Direct observation of field emission in a single TaSi2 nanowire

The electric potential change in a single TaSi2 nanowire during field emission was visualized by means of electron holography. During the field emission, the interference fringes of the electron hologram were blurred locally between the TaSi2 nanowire and anode. This phenomenon was interpreted as being due to a change in the electric potential of approximately 1 V in the TaSi2 nanowire after each ballistic emission. The experiments on the single TaSi2 nanowire field emission behavior provide the useful information for understanding the field emission in the nano-field-emitting device.     

Photovoltaic device on a single ZnO nanowire p-n homojunction

A photovoltaic device was successfully grown solely based on the single ZnO p-n homojunction nanowire. The ZnO nanowire p-n diode consists of an as-grown n-type segment and an in situ arsenic-doped p-type segment. This p-n homojunction acts as a good photovoltaic cell, producing a photocurrent almost 45 times larger than the dark current under reverse-biased conditions. Our results demonstrate that the present ZnO p-n homojunction nanowire can be used as a self-powered ultraviolet photodetector as well as a photovoltaic cell, which can also be used as an ultralow electrical power source for nanoscale electronic, optoelectronic and medical devices.     

Degradation pattern of SnO(2) nanowire field effect transistors

The degradation pattern of SnO(2) nanowire field effect transistors (FETs) was investigated by using an individual SnO(2) nanowire that was passivated in sections by either a PMMA (polymethylmethacrylate) or an Al(2)O(3) layer. The PMMA passivated section showed the best mobility performance with a significant positive shift in the threshold voltage. The distinctive two-dimensional R(s)-μ diagram based on a serial resistor connected FET model suggested that this would be a useful tool for evaluating the efficiency for post-treatments that would improve the device performance of a single nanowire transistor.     

Electrical Characteristics of Hybrid Nanoparticle–Nanowire Devices

Gold nanoparticles synthesized by a colloidal method were deposited in an Al₂O₃ dielectric layer of an omega-gated single ZnO nanowire FET. These gold nanoparticles were utilized as localized trap sites. The adsorption of the gold nanoparticles on an Al₂O₃-coated ZnO nanowire was confirmed by high-resolution transmission electron microscopy. In this study, a hybrid nanoparticle-nanowire device was fabricated by conventional Si processing. Its electrical characteristics indicated that electrons in the conduction band of the ZnO nanowire can be transported to the localized trap sites by gold nanoparticles for gate voltages greater than 1 V, through the 10-nm-thick Al ₂O₃ tunneling oxide layer.     

Process Optimization and Downscaling of a Single-Electron Single Dot Memory

This paper presents the process optimization of a single-electron nanoflash electron memory. Self-aligned single dot memory structures have been fabricated using a wet anisotropic oxidation of a silicon nanowire. One of the main issue was to clarify the process conditions for the dot formation. Based on the process modeling, the influence of various parameters (oxidation temperature, nanowire shape) has been investigated. The necessity of a sharp compromise between these different parameters to ensure the presence of the memory dot has been established. In order to propose an aggressive memory cell, the downscaling of the device has been carefully studied. Scaling rules show that the size of the original device could be reduced by a factor of 2. This point has been previously confirmed by the realization of single-electron memory devices.      

Broadband ZnO single-nanowire light-emitting diode

We present a novel technique for reliable electrical injection into single semiconductor nanowires for light-emitting diodes and lasers. The method makes use of a high-resolution negative electron-beam resist and direct electron-beam patterning for the precise fabrication of a metallic top contact along the length of the nanowire, while a planar substrate is used as a bottom contact. It can be applied to any nanowire structure with an arbitrary cross section. We demonstrate this technique by constructing the first zinc oxide single-nanowire light-emitting diode. The device exhibits broad sub-bandgap emission at room temperature.     

Carrier depletion and exciton diffusion in a single ZnO nanowire

Carrier depletion and transport in a single ZnO nanowire Schottky device have been investigated at 5?K, using cathodoluminescence measurements. An exciton diffusion length of 200?nm has been determined along the nanowire axis. The depletion width is found to increase linearly with the reverse bias. The origin of this unusual dependence in semiconductor material is discussed in terms of charge location and dimensional effects on the screening of the junction electric field.     

Fast switching electrochromic display using a viologen-modified ZnO nanowire array electrode

We report an electrochromic (EC) display using a viologen-modified ZnO nanowire array as the EC electrode. The ZnO nanowire array was grown directly on an indium tin oxide (ITO) glass by a low temperature aqueous thermal decomposition method and then modified with viologen molecules. The ZnO nanowire electrochromic device shows fast switching time (170 and 142 ms for coloration and bleaching respectively for a 1 cm (2) cell), high coloration efficiency (196 C (-1) cm (2)) and good stability. The improved performance of the ZnO nanowires EC device can be attributed to the large surface area and high crystalline and good electron transport properties of the ZnO nanowire array.     

The impact of nanocontact on nanowire based nanoelectronics

Nanowire-based nanoelectronic devices will be innovative electronic building blocks from bottom up. The reduced nanocontact area of nanowire devices magnifies the contribution of contact electrical properties. Although a lot of two-contact-based ZnO nanoelectronics have been demonstrated, the electrical properties bringing either from the nanocontacts or from the nanowires have not been considered yet. High quality ZnO nanowires with a small deviation and an average diameter of 38 nm were synthesized to fabricate more than thirty nanowire devices. According to temperature behaviors of current-voltage curves and resistances, the devices could be grouped into three types. Type I devices expose thermally activated transport in ZnO nanowires and they could be considered as two Ohmic nanocontacts of the Ti electrode contacting directly on the nanowire. For those nanowire devices having a high resistance at room temperatures, they can be fitted accurately with the thermionic-emission theory and classified into type II and III devices according to their rectifying and symmetrical current-voltage behaviors. The type II device has only one deteriorated nanocontact and the other one Ohmic contact on single ZnO nanowire. An insulating oxide layer with thickness less than 20 nm should be introduced to describe electron hopping in the nanocontacts, so as to signalize one- and high-dimensional hopping conduction in type II and III devices.     

Performance analysis of a Ge/Si core/shell nanowire field-effect transistor

We ana/lyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp3d5s* tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 and 85% of the ballistic limit. For this approximately 15 nm diameter Ge nanowire, we also find that 14-18 modes are occupied at room temperature under ON-current conditions with ION/IOFF = 100. To observe true one-dimensional transport in a 110 Ge nanowire transistor, the nanowire diameter would have to be less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on a common basis nanowire transistors of various materials and structures.     

High-Performance UV-Visible-NIR Broad Spectral Photodetectors Based on One-Dimensional In(2)Te(3) Nanostructures

For the first time, high quality In(2)Te(3) nanowires were synthesized via a chemical vapor deposition (CVD) method. The synthesized In(2)Te(3) nanowires are single crystals grown along the [132] direction with a uniform diameter of around 150 nm and an average length of tens of micrometers. Further, two kinds of photodetectors made by 1D In(2)Te(3) nanostructures synthesized by CVD and solvothermal (ST) methods respectively were fabricated. To our best knowledge, this is the first time photoresponse properties of In(2)Te(3) nanowire have been studied. The CVD grown nanowire device shows better performance than the ST device, which demonstrates a fast, reversible, and stable photoresponse and also a broad light detection range from 350 nm to 1090 nm, covering the UV-visible-NIR region. The excellent performance of the In(2)Te(3) nanowire photodetectors will enable significant advancements of the next-generation photodetection and photosensing applications.     

Investigation of electrical characteristics on surrounding-gate and omega-shaped-gate nanowire FinFETs

In this paper, electrical characteristics of small nanowire fin field-effect transistor (FinFET) are investigated by using a three-dimensional quantum correction simulation. Taking several important electrical characteristics as evaluation criteria, two different nanowire FinFETs, the surrounding-gate and omega-shaped-gate devices, are examined and compared with respect to different ratios of the gate coverage. By calculating the ratio of the on/off current, the turn-on resistance, subthreshold swing, drain-induced channel barrier height lowering, and gate capacitance, it is found that the difference of the electrical characteristics between the surrounding-gate (i.e., the omega-shaped-gate device with 100% coverage) and the omega-shaped-gate nanowire FinFET with 70% coverage is insignificant. The examination presented here is useful in the fabrication of small omega-shaped-gate nanowire FinFETs. It clarifies the main difference between the surrounding-gate and omega-shaped-gate nanowire FinFETs and exhibits a valuable result that the omega-shaped-gate device with 70% coverage plays an optimal candidate of the nanodevice structure when we consider both the device performance and manufacturability.     

Possibility of Transport Through a Single Acceptor in a Gate-All-Around Silicon Nanowire PMOSFET

Temperature-dependent electrical transport measurements of cylindrical shaped gate-all-around silicon nanowire p-channel MOSFET were performed. At 4.2 K, they show current oscillations, which can be analyzed by single hole tunneling originated from nanowire quantum dots. In addition to this single hole tunneling, one device exhibited strong current peaks, surviving even at room temperature. The separations between these current peaks corresponded to the energy of 25 and 26 meV. These values were consistent with the sum of the bound-state energy spacing and the charging energy of a single boron atom. The radius calculated from the obtained single-atom charging energy was also comparable to the light-hole Bohr radius.      

Controlling electron overflow in phosphor-free InGaN/GaN nanowire white light-emitting diodes

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.     

Cover Picture: A Single Polymer Nanowire Photodetector (Adv. Mater. 18/2006)

Redmond and co‐workers report on p. 2379 the successful application of solution‐assisted template wetting to the high‐yield controlled synthesis of poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(bithiophene)] (F8T2) nanowires (main picture and lower inset). Photoconductivity measurements (schematic in center) yield single‐nanowire device responsivities of ca. 0.4 mA W^–1 and external quantum efficiencies of ca. 0.1 % under monochromatic illumination, comparable with data reported for single inorganic nanowire devices.     

A two-colour heterojunction unipolar nanowire light-emitting diode by tunnel injection

We present a systematic study of the current-voltage characteristics and electroluminescence of gallium nitride (GaN) nanowire on silicon (Si) substrate heterostructures where both semiconductors are n-type. A novel feature of this device is that by reversing the polarity of the applied voltage the luminescence can be selectively obtained from either the nanowire or the substrate. For one polarity of the applied voltage, ultraviolet (and visible) light is generated in the GaN nanowire, while for the opposite polarity infrared light is emitted from the Si substrate. We propose a model, which explains the key features of the data, based on electron tunnelling from the valence band of one semiconductor into the conduction band of the other semiconductor. For example, for one polarity of the applied voltage, given a sufficient potential energy difference between the two semiconductors, electrons can tunnel from the valence band of GaN into the Si conduction band. This process results in the creation of holes in GaN, which can recombine with conduction band electrons generating GaN band-to-band luminescence. A similar process applies under the opposite polarity for Si light emission. This device structure affords an additional experimental handle to the study of electroluminescence in single nanowires and, furthermore, could be used as a novel approach to two-colour light-emitting devices.