Nanowire Contacts Research Articles

Synthesis and electrical characterization of Zn3P2 nanowires

Single crystalline Zn3P2 nanowires were synthesized on Si substrates via vapour phase deposition catalysed by In–Au seeds. Single nanowire devices were fabricated and the metal–Zn3P2 nanowire contacts were studied using a model based on two Schottky barriers as a function of temperature. As far we know, these are the first reported values of Schottky barriers of Ti/Zn3P2 nanowire contacts. The obtained values showed no significant dependence on the temperature, indicating that the defects at the nanowire's surfaces did not affect the device characteristics. We found evidence of an acceptor level at 49 meV, also indicating that the dominant transport mechanism is the thermal activation of carriers as is found in the bulk Zn3P2. It seems that the p-type behaviour is independent of the dimensionality of the Zn3P2 samples and primarily associated with the phosphorous interstitial atoms.

Focused helium ion beam deposited low resistivity cobalt metal lines with 10 nm resolution: implications for advanced circuit editing

Helium ion beam induced cobalt nanowire deposition using dicobalt octacarbonyl (Co2(CO)8) as a precursor is described. 10 nm wide metal lines were fabricated with good repeatability and extremely high purity. The metal lines were deposited on electrical test structures to determine the nanowire resistivity and contact resistance. Measurements reveal that these metal lines have 50–100 μΩ-cm resistivity. The contact resistance of a Co line on a gold pad is 15 Ω with a 0.14 × 1.4 μm contact area. The resulting metal deposit size was determined by helium ion microscopy (HIM) because of its high imaging resolution, and further characterized by high resolution transmission electron microscopy (HR-TEM) and electron energy-loss spectroscopy (EELS). HR-TEM images reveal that these metal lines are composed of 6 ± 2 nm cobalt crystallite grains; and EELS analysis shows that no measurable carbon signal was observed. Single-line and multiple-line patterns were prepared to examine proximity effects around the metal line depositions. Both HIM imaging and electrical measurements of the patterns verify that the collateral proximity deposition between individual lines can be minimized. Finally, factors determining the ultimate line width achievable with the He ion beam are discussed.

Surface depletion effects in semiconducting nanowires having a non-uniform radial doping profile

Doping profile has a significant effect on nanowire (NW) electrostatics, an effect that is expected to influence NW contact and transport properties. Herein, the electrostatic potential of nanowires (NWs) of non-uniform radial doping is calculated by two means: depletion approximation and a numerical calculation. Two profiles are considered: linear and exponential, corresponding to shallow and abrupt distributions; the results are compared to planar systems with similar doping profiles, and to uniformly doped NW systems. For a given average doping distribution, a non-uniform doping profile results in significantly lower carrier concentrations, an effect which intensifies with doping non-uniformity. Furthermore, in some cases, band diagrams obtained for the exponential doping profile vary greatly from any uniform doping, indicating that unique properties are expected for such NWs.

Current Density-Sensitive Welding of a Semiconductor Nanowire to a Metal Electrode

In situ welding of a single ZnSe nanowire (NW) to an Au electrode has been successfully achieved by means of current-induced Joule heating. The parameter governing the welding of semiconductor NW to the metal electrode is highly dominated by the current density at the Au-ZnSe NW (M-S) contact because current density at the M-S contact can change the temperature profile along the NW. The self-heating behaviors of the Au-ZnSe NW-Au (M-S-M) nanostructure can be changed from the electrical failure of the ZnSe NW to the melting of the Au electrode localized at the M-S contact when the current density at the M-S junction was adjusted to be larger than in the NW. Consequently, the self-welding is the current density-sensitive and controllable since the current density at the M-S contact can be controlled by adjusting the contact area between the NW and metal electrode. This controlled self-welding may have potential applications in the construction of a complex nanostructure and improvement of the thermal stability of the M-S contact as well as the enhanced performance of the nanodevices based on the M-S-M nanostructure.

Direct growth of SnO2 nanowires on WOx thin films

Single-crystalline SnO2 nanowires were directly grown on an amorphous WOx thin film, leading to the formation of nano-scale contacts with a near-Ohmic conductance. The WOx facilitated the diffusion of SnO2 on the surface of the WOx thin film, and SnO2 nanowires could be uniformly grown from the diffused SnO2. The contact properties between the metallic WOx and a semiconducting SnO2 nanowire were examined. The resistivity of the WOx–SnO2 nanowire contact was found to be approximately 2.6 × 10−5 Ω cm2. This was comparable to the resistivity of a contact between an Al electrode and a SnO2 nanowire with a contact area. A fabricated SnO2 nanowire transistor exhibited an on-current of approximately 386 nA, a threshold voltage of approximately 3.8 V, a subthreshold slope of approximately 0.26 V/dec and a field-effect mobility of approximately 43 cm2 V−1 s−1.

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires**Contribution of an agency of the US government, not subject to copyright.

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current–voltage (I–V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N2/O2. This degradation originates from the poor wetting behavior of Ni and Au on SiO2 and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces. The void formation can cause cracking and delamination of the metal film as well as reduce the contact area at the metal/NW interface, which increases the resistance. The morphology and composition of the annealed Ni/Au contacts on SiO2 and the p-GaN films were investigated by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) measurements. Adhesion experiments were performed in order to determine the degree of adhesion of the Ni/Au films to the SiO2 as well as observe and analyze the morphology of the film’s underside by SEM. Device degradation from annealing was prevented through the use of a specific adhesion layer of Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition. I–V measurements of NW devices fabricated using this adhesion layer showed a decrease in resistance after annealing, whereas all others showed an increase in resistance. Transmission electron microscopy (TEM) on a cross-section of a NW with Ni/Au contacts and a Ti/Al/Ni adhesion layer showed a lack of void formation at the contact/NW interface. Results of the XRD and TEM analysis of the NW contact structure using a Ti/Al/Ni adhesion layer suggests Al alloying of the Ni/Au contact increases the adhesion and stability of the metal film as well as prevents excessive void formation at the contact/NW interface.

Design of SnO2/ZnO hierarchical nanostructures for enhanced ethanol gas-sensing performance

Abstract Designing nanostructured materials to enhance gas-sensing performance is of important key for the next-generation sensor platforms. In this paper, a design of hierarchical SnO2/ZnO nanostructures for scalable fabrication of high-performance ethanol sensors is developed based on a combination of two simple synthesis pathways. High-quality single crystalline SnO2 nanowire (NW) backbones were first synthesized using the thermal evaporation method, whereas ZnO nanorod (NR) branches were subsequently grown perpendicularly to the axis of SnO2 NWs via the hydrothermal approach. The successful synthesis of SnO2/ZnO hierarchical nanostructures is confirmed by the results of scanning electron microscope, X-ray diffraction and photoluminescence spectrum. The ethanol-sensing properties of the SnO2/ZnO hierarchical nanostructures sensors were systematically investigated and compared to those of the bare SnO2 NWs sensor. The effect of growth manipulation of the SnO2/ZnO hierarchical nanostructures on the ethanol sensing characteristics was also studied. The results revealed that the design of the hierarchical nanostructures enhanced the ethanol gas response and selectivity for interfering gases such as NH3, CO, H2, CO2, and LPG. These enhancements are attributed to the enhancement of homogenous and heterogeneous NW–NW contacts. In addition, the results of this study may serve as a basis for designing various novel hierarchical nanostructures for other applications, including photocatalysis, battery electrode, solar cell, and nanosensors.

Nanoscale contacts between semiconducting nanowires and metallic graphenes

Here we report on the metal–semiconductor junction characteristics of a semiconducting ZnO nanowire grown directly on a metallic graphene film. The extracted specific contact resistivity of the graphene–ZnO nanowire contact (1.5 × 10−5 Ω·cm2) is comparable to that reported for Al–ZnO contacts. Based on the assumption that thermionic-field emission is the dominant mechanism, we obtained a zero-bias effective barrier height of 0.413 eV for the graphene–ZnO nanowire Schottky contact. We thus demonstrate that as a result of the enhanced tunneling at the contact, the graphene–nanowire contact exhibits near-ohmic current–voltage characteristics with a low contact resistance.

Cathode-Control Alloying at an Au-ZnSe Nanowire Contact via in Situ Joule Heating

Controllable interfacial alloying is achieved at a Au-ZnSe nanowire (M-S) contact via in situ Joule heating inside transmission electron microscopy (TEM). TEM inspection reveals that the Au electrode is locally molten at the M-S contact and the tip of the ZnSe nanowire is covered by the Au melting. Experimental evidences confirm that the alloying at the reversely biased M-S contact is due to the high resistance of the Schottky barrier at this M-S contact, coincident to cathode-control mode. Consequently, in situ Joule heating can be an effective method to improve the performance of nanoelectronics based on a metal-semiconductor-metal nanostructure.

The wavelength dependent photovoltaic effects caused by two different mechanisms in carbon nanotube film/CuO nanowire array heterodimensional contacts

Hetrodimensional contacts were fabricated by coating double-walled carbon nanotube (CNT) films on CuO nanowire arrays. Wavelength dependent photovoltaic effects by irradiating the devices with 405, 532, and 1064 nm lasers were observed. Two possible mechanisms responsible for the observed results were discussed. Photoexcitations within CuO nanowires and Schottky barriers in the heterojunctions dominate the photovoltaics in the 405 and 532 nm cases. For the 1064 nm case, the photovoltaic is the result of the excitation within the CNTs and of the heterodimensionality effect. Control experiments on CNT film/CuO granular film hetrodimensional contacts further show the relationship between these two mechanisms.

Ferromagnetic Germanide in Ge Nanowire Transistors for Spintronics Application

To explore spintronics applications for Ge nanowire heterostructures formed by thermal annealing, it is critical to develop a ferromagnetic germanide with high Curie temperature and take advantage of the high-quality interface between Ge and the formed ferromagnetic germanide. In this work, we report, for the first time, the formation and characterization of Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire transistors, in which the room-temperature ferromagnetic germanide was found through the solid-state reaction between a single-crystalline Ge nanowire and Mn contact pads upon thermal annealing. The atomically clean interface between Mn(5)Ge(3) and Ge with a relatively small lattice mismatch of 10.6% indicates that Mn(5)Ge(3) is a high-quality ferromagnetic contact to Ge. Temperature-dependent I-V measurements on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire heterostructure reveal a Schottky barrier height of 0.25 eV for the Mn(5)Ge(3) contact to p-type Ge. The Ge nanowire field-effect transistors built on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) heterostructure exhibit a high-performance p-type behavior with a current on/off ratio close to 10(5), and a hole mobility of 150-200 cm(2)/(V s). Temperature-dependent resistance of a fully germanided Mn(5)Ge(3) nanowire shows a clear transition behavior near the Curie temperature of Mn(5)Ge(3) at about 300 K. Our findings of the high-quality room-temperature ferromagnetic Mn(5)Ge(3) contact represent a promising step toward electrical spin injection into Ge nanowires and thus the realization of high-efficiency spintronic devices for room-temperature applications.

Automated nanomanipulation for nanodevice construction

Nanowire field-effect transistors (nano-FETs) are nanodevices capable of highly sensitive, label-free sensing of molecules. However, significant variations in sensitivity across devices can result from poor control over device parameters, such as nanowire diameter and the number of electrode-bridging nanowires. This paper presents a fabrication approach that uses wafer-scale nanowire contact printing for throughput and uses automated nanomanipulation for precision control of nanowire number and diameter. The process requires only one photolithography mask. Using nanowire contact printing and post-processing (i.e. nanomanipulation inside a scanning electron microscope), we are able to produce devices all with a single-nanowire and similar diameters at a speed of ∼1 min/device with a success rate of 95% (n = 500). This technology represents a seamless integration of wafer-scale microfabrication and automated nanorobotic manipulation for producing nano-FET sensors with consistent response across devices.

A Mobile Sn Nanowire Inside a β‐Ga2O3 Tube: A Practical Nanoscale Electrically/Thermally Driven Switch

Nanoelectromechanical system switches are seen as key devices for fast switching in communication networks since they can be switched between transmitting and receiving states with an electrostatic command. Herein, the fabrication of practical, nanoscale electrically/thermally driven switches is reported based on a mobile Sn nanowire inside a β-Ga2 O3 tube. The melting point of Sn inside the Ga2 O3 tube is found to be as low as 58 °C-far below the value of bulk Sn (231.89 °C)-and its crystal phase (β-Sn) remains unchanged even at temperatures as low as -170 °C. Thus a miniaturization of the unique wide-temperature-range thermometer based on the linear thermal expansion of liquid Sn fillings in the Ga2 O3 tube is realized. In addition, the electrical properties of the Sn-nanowire-filled β-Ga2 O3 tubes are carefully determined: importantly, the resistance demonstrates a sudden drop (rise) when two Sn nanowires contact (separate), due to the thermally driven motion of the liquid Sn fillings inside the tube. Thus this structure can be switched between its on and off states by controlling the motion, merging or splitting, of the Sn nanowires inside the tube, either electrically, by applying a current, or thermally, at a predetermined temperature.

High-Frequency Characterization of Contact Resistance and Conductivity of Platinum Nanowires

Contact resistance and conductivity of individual platinum (Pt) nanowires (NWs) embedded in coplanar waveguide structures are investigated at high frequencies. Two approaches to extract the NW conductivity and contact resistance from two-port S-parameters are developed. The first approach is based on transmission-line theory, while the second approach is based on a lumped-element physics-based model. Full-wave and circuit simulations are used to aid validation and the systematic analysis of both methods. Simulations are compared to calibrated on-wafer measurements of individual Pt NWs. The studies of the transmission-line-based approach reveal that the contact resistance can be determined accurately, but the obtained conductivity is inaccurate. By contrast, the lumped-element approach produces accurate results for both the contact resistance and conductivity of Pt NWs. The lumped-element method is used to determine the contact resistance of about 50 Ω and conductivity of 0.013 times the bulk conductivity of Pt for fabricated Pt NWs with 300-nm diameter.

Fluctuation-induced tunneling conduction through RuO2 nanowire contacts

A good understanding of the electronic conduction processes through nanocontacts is a crucial step for the implementation of functional nanoelectronic devices. We have studied the current- voltage (I-V) characteristics of nanocontacts between single metallic RuO2 nanowires and contacting Au electrodes, which were pre-patterned by simple photolithography. Both the temperature behavior of contact resistance in the low-bias voltage ohmic regime and the I-V curves in the high-bias voltage non-ohmic regime have been investigated. We found that the electronic conduction processes in the wide temperature interval 1-300 K can be well described by the fluctuation-induced tunneling (FIT) conduction theory. Taken together with our previous work [Lin et al., Nanotechnology 19, 365201 (2008)], where the nanocontacts were fabricated by delicate electron-beam lithography, our study demonstrates the general validity of the FIT model in characterizing electronic nanocontacts.

Oxide-Confined Formation of Germanium Nanowire Heterostructures for High-Performance Transistors

Over the past several years, the formation of nanowire heterostructures via a solid-state reaction between a semiconductor nanowire and metal contact pads has attracted great interest. This is owing to its ready application in nanowire field-effect transistors (FETs) with a well-controlled channel length using a facile rapid thermal annealing process. We report the effect of oxide confinement on the formation of Ge nanowire heterostructures via a controlled reaction between a vapor-liquid-solid-grown, single-crystalline Ge nanowire and Ni pads. In contrast to the previous formation of Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni(2)Ge and Ge with the confinement of Al(2)O(3) during annealing. Significantly, back-gate FETs based on this Ni(2)Ge/NiGe/Ge/NiGe/Ni(2)Ge heterostructure demonstrated a high-performance p-type transistor behavior, showing a large on/off ratio of more than 10(5) and a high normalized transconductance of 2.4 μS/μm. The field-effect hole mobility was extracted to be 210 cm(2)/(V s). Temperature-dependent I-V measurements further confirmed that NiGe has an ideal ohmic contact to p-type Ge with a small Schottky barrier height of 0.11 eV. Moreover, the hysteresis during gate bias sweeping was significantly reduced after Al(2)O(3) passivation, and our Ω-gate Ge nanowire FETs using Al(2)O(3) as the top-gate dielectric showed an enhanced subthreshold swing and transconductance. Therefore, we conclude that the Al(2)O(3) layer can effectively passivate the Ge surface and also serve as a good gate dielectric in Ge top-gate FETs. Our innovative approach provides another freedom to control the growth of nanowire heterostructure and to further achieve high-performance nanowire transistors.

Characterization of Resistivity of Sb<sub>2</sub>S<sub>3</sub> Semiconductor Nanowires by Conductive AFM and <i>In Situ</i> Methods

Conductive AFM and in situ methods were used to determine contact resistance and resistivity of individual Sb2S3 nanowires. Nanowires were deposited on oxidized Si surface for in situ measurements and on Si surface with macroelectrodes for conductive AFM (C-AFM) measurements. Contact resistance was determined by measurement of I(V) characteristics at different distances from the nanowire contact with the macroelectrode and resistivity of nanowires was determined. Sb2S3 is a soft material with low adhesion force to the surface and therefore special precautions were taken during measurements.

In Situ Nanomechanics of GaN Nanowires

The deformation, fracture mechanisms, and the fracture strength of individual GaN nanowires were measured in real time using a transmission electron microscope-scanning probe microscope (TEM-SPM) platform. Surface mediated plasticity, such as dislocation nucleation from a free surface and plastic deformation between the SPM probe (the punch) and the nanowire contact surface were observed in situ. Although local plasticity was observed frequently, global plasticity was not observed, indicating the overall brittle nature of this material. Dislocation nucleation and propagation is a precursor before the fracture event, but the fracture surface shows brittle characteristic. The fracture surface is not straight but kinked at (10-10) or (10-11) planes. Dislocations are generated at a stress near the fracture strength of the nanowire, which ranges from 0.21 to 1.76 GPa. The results assess the mechanical properties of GaN nanowires and may provide important insight into the design of GaN nanowire devices for electronic and optoelectronic applications.

Force Analysis and Modeling of Carbon Nanowires Operation in SEM

Due to influence of size effect, the force properties in nanoscale are greatly different from those in macroscale and the traditional models of operation are becoming difficult to meet the development of nanoscale manipulation. To provide guiding theory for practical nano-manipulation, the nanoscale forces of contact and non-contact operation of nanowires are analyzed for nano-manipulation in SEM. The Vander Waals models among the probe, nanowire and substrate are modeled according to the force properties in nanoscale, and then the simplified models are simulated with MATLAB. The influence degree of various factors and the relationship of them during the operation are obtained. At last, experimental system is established to verify the correctness of the proposed models.

Formation and Device Application of Ge Nanowire Heterostructures via Rapid Thermal Annealing

We reviewed the formation of Ge nanowire heterostructure and its field-effect characteristics by a controlled reaction between a single-crystalline Ge nanowire and Ni contact pads using a facile rapid thermal annealing process. Scanning electron microscopy and transmission electron microscopy demonstrated a wide temperature range of 400~500°C to convert the Ge nanowire to a single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure with atomically sharp interfaces. More importantly, we studied the effect of oxide confinement during the formation of nickel germanides in a Ge nanowire. In contrast to the formation of Ni2Ge/Ge/Ni2Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni2Ge with the confinement of Al2O3during annealing. A twisted epitaxial growth mode was observed in both two Ge nanowire heterostructures to accommodate the large lattice mismatch in the NixGe/Ge interface. Moreover, we have demonstrated field-effect transistors using the nickel germanide regions as source/drain contacts to the Ge nanowire channel. Our Ge nanowire transistors have shown a high-performancep-type behavior with a high on/off ratio of 105and a field-effect hole mobility of 210 cm2/Vs, which showed a significant improvement compared with that from unreacted Ge nanowire transistors.