Preparation and characterization of semiconductor GNR-CNT nanocomposite and its application in FET

So far, little is known about the experimental potential of graphene nanoribbon-carbon nanotube (GNR-CNT) heterostructure as a semiconductor nanocomposite. The present work examined the structural features, topography and electronic properties of GNR-CNT nanocomposite by using Raman spectroscopy, transmission electron microscopy, scanning tunneling microscopy and spectroscopy (STS). The homogenous semiconductor GNR-CNT nanocomposites were produced under optimized synthesis conditions. The narrow band gap was exhibited by optimization of the reduction step. The STS of the micro-scale surface of the nanocomposite shows local density of state in selected areas that represent the 0.08 eV band gap of a homogenous nanocomposite. The potential of the semiconductor nanocomposite was considered for application in stacked graphene nanoribbon-field effect transistors (SGNR-FETs). A simple method of device fabrication is proposed based on a semiconductor stacked GNR nanocomposite. The high hole mobility and rectifying effect of the p–n junction of the SGNR nanocomposite on TiO₂ are demonstrated. The optimal thickness for the back gate TiO₂ dielectric for the tested devices was 40 nm. This thickness decreased leakage current at the p–n junction of the SGNR/TiO₂ interface, which is promising heterojunction for optoelectronics. The thickness of gate dielectric and quantum capacitance of the gate was investigated at the low 40 nm thickness by calculating the mobility. In the proposed SGNR-FET, holes dominate electrical transport with a high mobility of about 1030 cm²/V s.     

Shrinking limits of silicon MOSFETs: numerical study of 10 nm scale devices

We have performed numerical modeling of dual-gate ballistic n-MOSFETs with channel length of the order of 10 nm, including the effects of quantum tunneling along the channel and through the gate oxide. Our analysis includes a self-consistent solution of the full (two-dimensional) electrostatic problem, with account of electric field penetration into the heavily doped electrodes. The results show that transistors with channel length as small as 8 nm can exhibit either a transconductance up to 4000 mS mm ^− 1or gate modulation of current by more than 8 orders of magnitude, depending on the gate oxide thickness. These characteristics make the devices satisfactory for logic and memory applications, respectively, although their gate threshold voltage is rather sensitive to nanometer-scale variations in the channel length.     

Scanning tunneling microscope study of nanosized metal–semiconductor contacts between ErSi2 nanoislands and Si(0 0 1) substrate

The current–voltage (I–V) characteristics of the nanosized metal–semiconductor contacts formed between the epitaxially grown ErSi₂ islands and p-Si(0 0 1) substrate are measured in situ by the scanning tunneling microscope. Experimental results show that the current densities passing through the nanocontacts are five orders of magnitude larger than that of the macroscopic ones and have an obvious dependence on the contact area. Especially, it is found that I–V characteristics of the contacts are sensitive to the sample surface adsorption. Our investigations indicate that surface conduction plays an important role in the electrical transport process from ErSi₂ islands to the Si(0 0 1) substrate. Furthermore, for the nanocontacts with surface currents suppressed effectively, the ideality factor and the effective Schottky barrier height are estimated by using the standard thermionic emission model. Our analysis suggests that the current through the interface between ErSi₂ nanoislands and the p-Si(0 0 1) substrate is enhanced due to the effects of tunneling and image force lowering.     

Characterization of photonic nanojets in dielectric microdisks

The direct imaging of photonic nanojets in different dielectric microdisks illuminated by a laser source is reported. The SiO₂ and Si₃N₄ microdisks are of height 650 nm with diameters ranging from 3 μm to 8 μm. The finite-difference time-domain calculation is used to execute the numerical simulation for the photonic nanojets in the dielectric microdisks. The photonic nanojet measurements are performed with a scanning optical microscope system. The photonic nanojets with high intensity spots and low divergence are observed in the dielectric microdisks illuminated from the side with laser source of wavelengths 405 nm, 532 nm and 671 nm. The experimental results of key parameters are compared to the simulations and in agreement with theoretical results. Our studies show that photonic nanojets can be efficiently created by a dielectric microdisk and straightforwardly applied to nano-photonics circuit.     

Systematic studies of tunneling magnetoresistance in granular films made from well-defined Co clusters

The tunneling magnetoresistance (TMR) of granular films prepared by the co-deposition of well-defined ferromagnetic Co clusters and insulating inert-gas matrix atoms has been studied as function of matrix atoms (Kr, Xe), cluster volume fraction, temperature (4–40 K) and cluster size (4.2–5.2 nm). Tunneling samples with resistivities that differ by about five orders of magnitude have a TMR that is found to be independent of matrix and cluster volume fraction, i.e. is independent of both tunneling barrier height and width. All samples show a characteristic TMR(T)-dependence that can be explained by a model which takes into account that magnetic moments at the cluster surface are becoming misaligned with increasing temperature. The fraction of misaligned moments at a fixed temperature is increasing with decreasing clusters size.     

AFM study of a SiC film grown on Si(1 0 0) surface using a C2H4 beam

A 3C-silicon carbide (SiC) thin film grown on a Si(1 0 0) surface using an ethylene (C₂H₄) molecular beam has been studied by atomic force microscopy. At the center of the irradiation area of the ethylene beam, the shape of the SiC islands was rectangular, the average length of which was 74.5 nm and the average height was 13.1 nm. Each SiC island consists of the SiC particles with the average diameter of 17 nm. Just inside of the boundary region of the beam irradiation, the average size and height of the islands decreased to 50.1 and 8.2 nm, respectively. Just outside of the boundary region, the average size and height decreased to 17.7 and 5.1 nm, respectively. The average reaction probabilities at the above three points were estimated to be 0.14, 0.27 and 2.7%, respectively. New growth mode of the crystal growth is proposed (particles gathering island mode).     

Persistent current with fractional period in a multichannel Wigner crystal ring

We calculate the persistent current in a multichannel semiconductor ring with strongly correlated electrons when they form a Wigner crystal (WC). Pinning this crystal with a smooth gate potential leads to the suppression of the amplitude of the Aharonov–Bohm oscillations and to the fractionalization of the period. A fractional period Φ₀/w (w = 2,3,…,6) appears because of the strong coupling between channels in the tunneling process of the WC through the pinning barrier.     

STM light emission from Si(1 1 1)-(7 × 7) surface using a silver tip

Scanning tunneling microscope-light emission (STM-LE) from the Si(1 1 1)-(7×7) surface has been measured using silver tips. For silver tips photon emission was enhanced by more than 100 times as compared with that for tungsten or platinum–iridium alloy tips. A broad spectrum with a single peak at ∼2.25 eV was observed. The spectrum obtained can be reproduced by a theory based on the macroscopic dielectric response of the tip-sample system, indicating that the observed emission arises from the localized plasmons on the silver tip excited by tunneling electrons. Spatial variations in the emission intensity at the atomic scale was observed even under low bias voltage (2 V) and low tunneling current (1 nA) conditions.     

Ultrasmall optical logic gates based on silicon periodic dielectric waveguides

An ultrasmall silicon periodic dielectric waveguides-based multimode interference all-optical logic gate has been proposed. The device consists of three 205 nm wide single-mode input waveguides, a 1.1 μm wide and 5.5 μm long multimode interference waveguide, and three 205 nm wide single-mode output waveguides. The total length and width of the device are 13.7 μm and 3.2 μm, respectively. By changing the states of the input optical signals and/or control signals launched into the device, multifunctional logic functions including OR, NAND, NOR, and NOT gates are performed, and each logic function can be realized at a specific output waveguide in accordance with the launched control signals. The ultrasmall multifunctional logic device has potential applications in high density photonic integrated circuits.     

Dependence of mark size on STM pulse voltage in heat-assisted magnetic probe recording on a cobalt–nickel–platinum thin film

This paper presents a method of heat-assisted magnetic probe recording on perpendicular medium. Electrical current from scanning tunneling microscope (STM) is employed as the major heating source. Recording medium is a strongly-coupled CoNi/Pt multilayered film. Pulses with amplitude of 3–7 V and duration of 200 ns were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 184 nm were formed for voltages above 4 V. A model is built to simulate the formation of magnetic domains in the processing of probe-based magnetic recording. Simulation results agree with experiments well.     

Influence of rapid thermal annealing on electrical and structural properties of double metal structure Au/Ni/n-InP (1 1 1) diodes

The effect of rapid thermal annealing on the electrical and structural properties of Ni/Au Schottky contacts on n-InP have been investigated by current–voltage (I–V), capacitance–voltage (C–V), auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The Au/Ni/n-InP Schottky contacts are rapid thermally annealed in the temperature range of 200–500 °C for a duration of 1 min. The Schottky barrier height of as-deposited Ni/Au Schottky contact has been found to be 0.50 eV (I–V) and 0.86 eV (C–V), respectively. It has been found that the Schottky barrier height decreased with increasing annealing temperature as compared to as-deposited sample. The barrier height values obtained are 0.43 eV (I–V), 0.72 eV (C–V) for the samples annealed at 200 °C, 0.45 eV (I–V) and 0.73 eV (C–V) for those at 400 °C. Further increase in annealing temperature to 500 °C the barrier height slightly increased to 0.46 eV (I–V) and 0.78 eV (C–V) compared to the values obtained for the samples annealed at 200 °C and 400 °C. AES and XRD studies showed the formation of indium phases at the Ni/Au and InP interface and may be the reason for the increase in barrier height. The AFM results showed that there is no significant degradation in the surface morphology (rms roughness of 1.56 nm) of the contact even after annealing at 500 °C.     

Investigation of complex channel capacitance in C60 field effect transistor and evaluation of the effect of grain boundaries

Frequency and gate voltage dependences of capacitance in a C₆₀ field effect transistor (FET) showed an intriguing power law (C ∝ f^−p, p ∼ 0.3–0.35) irrespective of the gate voltage. In order to interpret this phenomenon, we formulated a complex impedance of the bottom contact FET based on a distributed constant circuit model in cases of both a single grain channel and a multi-grain channel. The power law could be well explained in terms of the complex impedance formula using only a small number of fitting parameters, the results of which indicate the validity of the model. This kind of analysis could usefully characterize the organic FETs consisting of grain boundaries, providing information on the resistance ratio of the grain interior to the grain boundary.     

Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.     

Photo-curable epoxy functionalized cyclotetrasiloxane as a gate dielectric for organic thin film transistors

We synthesized a new photo-curable organic/inorganic hybrid material, cyclotetrasiloxane (CTS) derivative containing cyclohexene-1,2-epoxide functional groups (CTS-EPOXY), and its characteristics are compared with a prototypical organic gate insulator of poly(4-vinylphenol) (PVP) in the organic thin film transistors (OTFTs) using pentacene as an active p-type organic semiconductor. Compared with PVP, CTS-EPOXY shows better insulating characteristics and surface smoothness. A metal/insulator/metal (MIM) device with the 300-nm-thick CTS-EPOXY film shows more than two orders of magnitude lower current (less than 40 nA/cm² over the voltage range up to 60 V) compared with PVP. In addition, the pentacene TFT with CTS-EPOXY as a gate dielectric layer shows slightly higher field-effect mobility of μ_FET = 0.20 cm²/V s compared to that with PVP.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.     

Highly efficient and stable organic light-emitting diode by balancing drift current of charge

Highly efficient and stable OLED device in which hole-drift current and electron-drift current are balanced was fabricated. Drift current characteristics according to the thickness of organic layer were examined using the device with ITO/m-MTDATA/NPB/Al structure that can only move the hole and the device with Al/LiF/Alq₃/LiF/Al structure that can only move the electron. Using the result of such examination, green device with balanced drift current was produced. Device with the structure of m-MTDATA (80 nm)/NPB (20 nm)/C-545T (3%) doped Alq₃ (5 nm)/Alq₃ (59 nm)/LiF (1 nm)/Al (200 nm) showed color purity of (0.309, 0.643) and high efficiency of 7.0 lm/W (14.4 cd/A). Most of light emission was observed inside the green emitting layer. Through the result of EL spectrum for the device also including red emitting layer, same result could be obtained. The device with balanced drift current also showed half life-time of 175 h for initial luminance of 3000 cd/m², which is more stable in comparison to the device without balanced drift current.     

Cobalt metal nanoparticles embedded in SiO2 dielectric layer for the application of nonvolatile memory

Metal–oxide–semiconductor structures (MOS) with the embedded Co nanoparticles (NPs) were efficiently fabricated by utilizing an external laser irradiation technique for the application of nonvolatile memory. Images of high resolution transmission electron microscopy measurements exhibited that the Co NPs of 5 nm in diameter were clearly embedded in SiO₂ gate oxide. Capacitance–voltage measurements certainly exhibited flat-band voltage shift of 2.2 V from 2 V to −8 V in sweeping range. The retention characteristics of MOS capacitors with the embedded Co NPs were also studied as a function of tunnel oxide thickness to confirm the suitability of nonvolatile memory devices with metal NPs. The experimental results reveal that our unique laser process will give possible promise for experimental efficient formation or insertion of metal NPs inside the gate oxide.     

A novel polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber

A kind of polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber has been proposed. The polarization splitter is based on the phenomenon of resonant tunneling. We use the finite element method and the full-vector beam propagating method to analyze the characteristics of three-core photonic crystal fiber. Compare with the silica glass three-core PCF, the ZnTe tellurite glass three-core PCF have higher extinction ratios and lower coupling loss, the extinction ratios ERA = ? 164.2681 dB and ERC = ? 37.1742 dB at the wavelength λ = 1.55 μm, and the coupling loss is lower than 0.02 dB. The 8.7983-mm-long splitter is proposed to achieve extinction ratio better than ? 20 dB and a bandwidthof 20 nm.     

The investigation of transport properties of mesoscopic graphite in high magnetic field

The electrical transport properties of mesoscopic graphite have been investigated in a gate voltage configuration. Few layer graphene structures made from Kish graphite exhibit Shubnikov-de Haas (SdH) oscillations in magnetic fields up to 33 T, with a strong gate voltage dependence. A two band model can be used to explain the linear dependence of the SdH frequency on the gate voltage. The temperature dependence of the SdH oscillation amplitude allows the determination of the effective masses of the carriers, which remain comparable between mesoscopic and bulk graphite samples. However, mesoscopic graphite thinner than 130 nm does not exhibit the field induced charge density wave transition seen in bulk samples above 25 T at low temperatures.     

Schottky-type polycrystalline CdZnTe X-ray detectors

The polycrystalline CdZnTe:Cl thick films which have high resistivity about 5 × 10⁹ Ω cm are grown by thermal evaporation method. The leakage currents of as-deposited CdZnTe layers are still too high to operate as medical applications. The blocking layer of Schottky type was formed on the stoichiometric surface of polycrystalline CdZnTe layers to suppress the leakage current of polycrystalline CdZnTe X-ray detectors. The polycrystalline CdZnTe Schottky barrier diodes with indium contact exhibit the low leakage current (14 nA/cm²) at 40 V due to its high barrier height (ϕ_b = 0.80 eV). In X-ray image acquisition with Schottky-type linear array polycrystalline CdZnTe X-ray detector, we have obtained the promising results and proved the possibility of polycrystalline CdZnTe for applications as a flat panel X-ray detector.