L. Ion

Multisegment CdTe nanowire homojunction photodiode

Electrochemical deposition in nanoporous ion track membranes is used for the preparation of multisegment CdTe--homojunction diode nanowires. Our study is based on the fact that the deposition overpotential strongly influences the composition of the compound semiconductor nanowires. Therefore, the transport behavior of the nanowire devices can be tailored by appropriately choosing a certain sequence of electrodeposition potentials. The wires were characterized using scanning electron microscopy, energy dispersive x-ray analysis, optical spectroscopy and x-ray diffraction. The current-voltage characteristics measured prove that, by appropriately choosing the voltage pulse pattern, one can fabricate nanowires with ohmic or rectifying behavior. The semiconducting nanowires are sensitive to light, their spectral sensitivity being characteristic of CdTe. The preparation of functional nanostructures in such a simple approach provides, as a major advantage, an increase in the process reproducibility and opens a wide field of potential optoelectronic applications.

Electrical properties of electron irradiated thin polycrystalline CdSe layers

Electrical properties of nonirradiated and electron-irradiated thin layers of CdSe, sandwiched between two gold electrodes, were investigated. Thin films of CdSe, prepared by thermal-vacuum evaporation on glass substrate at a temperature of 220 °C, were subjected to two sessions of irradiation with 7 MeV electrons to the fluences of 2×1015 and 4×1015 e/cm2, respectively. The current–voltage characteristics, recorded at temperatures in the range 150–350 K, showed that the Ohm’s law is followed at low-applied voltages, in both nonirradiated and irradiated CdSe layers. In the range of high-applied voltages, the space-charge-limited current (SCLC), controlled by a Gaussian trap distribution, placed in the vicinity of the Fermi level, has been identified as the dominant conduction mechanism. An analysis in the frame of SCLC theory allowed us to obtain the parameters characterizing the trap distribution and their changes induced by electron irradiation.

Self-consistent potentials and linear regime conductance of cylindrical nanowire transistors in the R-matrix formalism

One of the major difficulties in solving the coupled Schrodinger–Poisson equations for open quantum systems is providing the wave functions for a large energy set. In this context, the R-matrix formalism provides an alternative method to obtain efficiently the wave functions. In a first step, which is energy independent, the eigenvalue problem associated with the quantum system is solved only once using fixed boundary conditions. Then, in a second step, the wave functions and transmission coefficients are obtained with a much lower computational effort for each energy. As an application, self-consistent potential and charge distribution, as well as the ballistic source-drain conductance, are calculated for a cylindrical nanowire transistor. The numerical accuracy with respect to basis cardinality is also discussed.

Electrical properties of single CuO nanowires for device fabrication: Diodes and field effect transistors

High aspect ratio CuO nanowires are synthesized by a simple and scalable method, thermal oxidation in air. The structural, morphological, optical, and electrical properties of the semiconducting nanowires were studied. Au-Ti/CuO nanowire and Pt/CuO nanowire electrical contacts were investigated. A dominant Schottky mechanism was evidenced in the Au-Ti/CuO nanowire junction and an ohmic behavior was observed for the Pt/CuO nanowire junction. The Pt/CuO nanowire/Pt structure allows the measurements of the intrinsic transport properties of the single CuO nanowires. It was found that an activation mechanism describes the behavior at higher temperatures, while a nearest neighbor hopping transport mechanism is characteristic at low temperatures. This was also confirmed by four-probe resistivity measurements on the single CuO nanowires. By changing the metal/semiconductor interface, devices such as Schottky diodes and field effect transistors based on single CuO p-type nanowire semiconductor channel are obtained. These devices are suitable for being used in various electronic circuits where their size related properties can be exploited.

Electron-irradiation effects on CdSe thin films investigated by thermally stimulated current method

Defects determining the electrical properties of CdSe thin films, before and after irradiation with high-energy electrons, have been investigated by thermally stimulated current technique. Thin films of CdSe, 30 μm thick, prepared by thermal-vacuum evaporation on glass substrate at a temperature of 220 °C were subjected to irradiation with 6-MeV electrons to a fluency of 5×1013e∕cm2. The main defect (D1), controlling the electrical properties of the films both before and after irradiation, is located at 0.38 eV below the conduction-band edge. Some other defects existing in lower densities and having lower ionization energies (0.24 eV, D2; 0.17 eV, D3; and 0.14 eV, D4) were also identified. Electron irradiation induces in significant increase in the peaks associated with the defects D1, D2, and D3, especially in the first one. The parameters characterizing all the detected traps were determined.

Magnetic behavior and clustering effects in Mn-doped boron nitride sheets

Ab initio calculations are performed in the framework of density functional theory on Mn-doped boron nitride sheets, which are candidates for two-dimensional diluted magnetic semiconductors (DMSs). Each type of substitution reveals a qualitatively different magnetic behavior encompassing ferromagnetic, anti-ferromagnetic and spin glass ordering. The ability of formation of these defects is also discussed. We analyze the dependence of the exchange couplings on the distance between impurities and the typical range and distribution are extracted. Multiple-impurity configurations are considered and the results are mapped on an Ising-type Hamiltonian with higher order exchange interactions, revealing deviations from the standard two-spin models. The percolation of interacting magnetic moments is discussed and the critical concentration is determined for the underlying transition from a ferromagnetic to a super-paramagnetic state. We conclude our study by providing the optimal conditions for doping in order to obtain a ferromagnetic DMS.

Optical phonon spectrum and the Fröhlich Hamiltonian in würtzite-type nanotubes.

Dispersion laws of the full optical phonon spectrum of a nanotube made of würtzite-type materials as well as the corresponding Fröhlich electron-phonon interaction terms are derived and studied within the framework of the dielectric continuum model for a uniaxial crystal. The coupling coefficients describing electron-phonon interaction are obtained in an analytical closed form, depending on the dispersion law of the involved phonon branch. We present and discuss results of numerical calculations of optical phonon spectrum and Fröhlich coupling coefficients for some chosen würtzite AlN nanotubes. Observed features, induced by the anisotropy of würtzite-type materials, are discussed.

Effects of graded distribution of scattering centers on ballistic transport

The transmission coefficient of a two dimensional scattering region connected to ideal leads was calculated for the case of electrons interacting with an inhomogeneous distribution of repulsive or attractive scattering centers. The scattering centers with Gaussian profiles were positioned at regular intervals perpendicular to the transport direction, but were spaced according to a power law along this direction. The transmission function was obtained using a scattering formalism based on the R-matrix method. The simulations revealed that although, overall, the transmission coefficient decreases and becomes almost monotonously dependent on energy as the inhomogeneity of both attractive and repulsive scattering centers increases, the redistribution of transmission between open channels depends on the type of scattering centers.

Quantum limits to the electron field emission from tapered conductive sheets

A model has been constructed in order to study the effects of quantum confinement on the electron population on tapered conductive sheets. A two-dimensional rounded nanocone was considered for this study and the Schrodinger equation was solved analytically on the entire system. The average number of electrons on the tip region was shown to be strongly influenced by the geometrical parameters of the system, such as the tip and base radius and the overall length. Field emission from such structures was also investigated using a simple one-dimensional Wentzel–Kramers–Brillouin approximation. The limitations on the electron population imposed by the strong quantum confinement at the tip region will have important consequences on the field emission current obtained from these structures.

Ballistic electron transport in wrinkled superlattices

Abstract Inspired by the problem of elastic wave scattering on wrinkled interfaces, we studied the scattering of ballistic electrons on a wrinkled potential energy region. The electron transmission coefficient depends on both wrinkle amplitude and periodicity, having different behaviors for positive and negative scattering potential energies. For scattering on potential barriers, minibands appear in the electron transmission, as in superlattices, whereas for scattering on periodic potential wells the transmission coefficient has a more complex form. Besides suggesting that tuning of electron transmission is possible by modifying the scattering potential via voltages on wrinkled gate electrodes, our results emphasize the analogies between ballistic electrons and elastic waves even in scattering problems on non-typical configurations.