S. Antohe

The effect of the electron irradiation on the electrical properties of thin polycrystalline CdS layers

The electrical properties of nonirradiated and electron irradiated structures, containing a polycrystalline thin layer of CdS, sandwiched between two gold electrodes, were investigated. The thin films of CdS, obtained through thermal-vacuum evaporation on the glass substrate at a temperature of 220 °C, were subjected to two sessions of irradiation with 7 MeV electrons to the fluences of 4×1015 and 6×1015 e/cm2, respectively. In the case of nonirradiated structures, under low voltages, the Ohm’s law is followed with a thermally activated electron concentration of n0≅3×1016 cm−3, an electron mobility μ0≅0.1 cm2/V s and a room temperature electrical conductivity σ0≅4×10−4 Ω−1 cm−1. In this range of voltage the electron irradiation induces a small increase in the activation energy of mobility, determining, of course, a small decreasing of the mobility. At high-applied voltage, there is a space-charge-limited conductivity controlled by a single trap level having the depth of Ec−Et≅0.086 eV and the total trap c...

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.

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.

Direct laser deposition of nanostructured tungsten oxide for sensing applications

Nanostructured tungsten trioxide (WO 3 ) thin films are deposited by pulsed laser deposition (PLD) and radio-frequency (RF) assisted PLD onto interdigitated sensor structures. Structural characterization by x-ray diffraction and Raman spectroscopy shows the WO 3 films are polycrystalline, with a pure monoclinic phase for the PLD grown films. The as-fabricated WO 3 sensors are tested for ammonia (NH 3 ) detection, by measuring the electrical response to NH 3 at different temperatures. Sensors based on WO 3 deposited by RF-PLD do not show any response to NH 3 . In contrast, sensors fabricated by PLD operating at 100 °C and 200 °C show a slow recovery time whilst at 300 °C, these sensors are highly sensitive in the low ppm range with a recovery time in the range of a few seconds. The microstructure of the films is suggested to explain their excellent electrical response. Columnar WO 3 thin films are obtained by both deposition methods. However, the WO 3 films grown by PLD are porous, (which may allow NH 3 molecules to diffuse through the film) whereas RF-PLD films are dense. Our results highlight that WO 3 thin films deposited by PLD can be applied for the fabrication of gas sensors with a performance level required for industrial applications.

Current-voltage characteristics of electron irradiated polycrystalline Au/CdSe/Au thin layers

Polycrystalline thin CdSe layers, obtained by thermal-vacuum deposition on glass substrate at temperature of 220/spl deg/C, were subjected to two consecutive sessions of irradiation with 7 MeV electrons to fluences of 2 /spl times/ 10/sup 15/ and 4 /spl times/ 10/sup 15/ electrons/cm/sup 2/, respectively. After irradiation, the electrical properties of polycrystalline thin CdSe films, sandwiched between two gold electrodes, were investigated by current voltage measurements. Under low-voltages, Ohms law is followed with an electron mobility /spl mu/ of 52 cm/sup 2/V/sup -1/ s/sup -1/ and a room temperature electrical conductivity /spl sigma//sub 0/ of 4,627 /spl times/ 10/sup -5/ /spl Omega//sup -1/ cm/sup -1/. At high-applied voltages, there is a space-charge limited conductivity (SCLC) controlled by an uniform trap distribution with density per unit energy range /spl rho/(E) of 2.39 /spl times/ 10/sup 14/ cm/sup -3/ eV/sup -1/.

Structural and electrical properties of N doped SiC nanostructures obtained by TVA method

Ionized nitrogen doped Si-C thin films at 200°C substrate temperature were obtained by Thermionic Vacuum Arc (TVA) method. To increase the energy of N, C and Si ions, -400V, -600V and -1000V negative bias voltages was applied on the substrate. The 400nm, 600nm and 1000nm N-SiC coatings on glass was deposed. To characterize the structure of as-prepared N-SiC coatings, Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM), X-Ray and Photoelectron Spectroscopy (XPS) techniques was performed. Electrical conductivity was measured comparing the potential drop on the structure with the potential drop on a series standard resistance in a constant current mode. To justify the dependence of measured electrical conductivity by the temperature, we assume a thermally activated electrical transport mechanism.

The effect of the substrate temperature and the acceleration potential drop on the structural and physical properties of SiC thin filmsdeposed by TVA method

Crystalline Si-C thin films were prepared at substrate temperature between 200°C and 1000°C using Thermionic Vacuum Arc (TVA) method. To increase the acceleration potential drop a negative bias voltage up to -1000V was applied on the substrate. The 200nm thickness carbon thin films was deposed on glass and Si substrate and then 200-500 nm thickness Si-C layer on carbon thin films was deposed. Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM), X-Ray Photoelectron Spectroscopy (XPS), and electrical conductivity measurement technique characterized the structure and physical characteristics of as-prepared SiC coating. At a constant acceleration potential drop, the electrical conductivity of the Si-C films deposed on C, increase with increasing of substrate temperature. On the other part, significant increases in the acceleration potential drop at constant substrate temperature lead to a variation of the crystallinity and electrical conductivity of the SiC coatings XPS analysis was performed using a Quantera SXM equipment, with monochromatic AlKα radiation at 1486.6eV. Electrical conductivity of the Si-C coating on carbon at different temperatures was measured comparing the potential drop on the sample with the potential drop on a series standard resistance in constant mode.