D.C. I–V characteristics and steady-state photoconductivity of Au/Pb2CrO5/SnO2 sandwich-structure films under illumination in the visible region

The room temperature d.c. current–voltage (I–V) characteristics of an Au/Pb2CrO5/SnO2 sandwich-structure 1.39 μm thick film have been measured for d.c. voltages, Vd.c., in the range 0.25 V≤Vd.c.≤5.0 V. These measurements were carried out under both dark and visible-light illumination conditions. For Vd.c.<2.5 V, the I–V curves of the sample in both dark and light environments were found to be non-linear and conform to space-charge-limited (SCL) current governed by traps uniformly distributed in energy. At higher d.c. voltages, a nearly Mott–Gurney V2 behaviour of the dark current has been observed, whereas the I–V behaviour of the illuminated specimen was a combination of an ohmic conduction and a V2 dependence at low illumination levels and became highly ohmic at large light intensities. This behaviour can be understood in terms of a reduction in the SCL dark current in favour of a larger ohmic d.c. photocurrent as a result of neutralization of the majority-carrier space charge by the photogenerated minority carriers of the electron–hole pairs produced under the illumination with visible light of energy ℏω≅EG(∼2.1–2.3 eV for the Pb₂CrO₅ material). The d.c. photocurrent, I_phot, at a fixed d.c. voltage, was found to follow a power-law dependence on light intensity, F, of the form I_phot∝F^γ, with the exponent γ being dependent on the applied d.c. voltage. At the low-voltage side (V_d.c.<1.5 V), γ∼0.5, a value usually obtained when the photoconductivity behaviour is governed by bimolecular recombination mechanisms. As the d.c. voltage is increased further, γ increases monotonically until it saturates at a value of about 0.9 for d.c. voltages beyond 3.5 V, where monomolecular recombination processes seem to be more operative with increasing d.c. voltage. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of Cr barrier on interfacial reaction of Au/Zn/Au/Cr/Au contacts to p-type InGaAs/InP

Alloy-type metal is widely used to reduce contact resistance in optoelectronic devices. Among the alloy-types, Au/Zn is one of the most common metallization systems. In this paper, we studied the alloy morphology of p-InP/p-InGaAs/Au/Zn/Au/Cr/Au systems. We found that the amount of Au-Zn alloy depended upon the thickness of the Cr layer. When Cr thickness was reduced to 135 Å, both Au-rich and GaAs-rich excessive compound formation started to occur. The Au diffusion punched through the InGaAs layer and penetrated into the InP. Comparison of Au/Zn/Au and Au/Zn/Au/Cr/Au suggested that the top Au layer maybe very influential during the alloy reaction. The Au-Zn alloy was significantly less in the Au/Zn/Au than that in the Au/Zn/Au/Cr/Au.     

Optical absorption and EPR studies of borate glasses with PbCrO4 and Pb2CrO5 microcrystals

The optical absorption (in the 400–1000 nm region) and electron paramagnetic resonance (EPR) spectra of 50PbO-20Cr₂O₃-(30-y)B₂O₃-yAl₂O₃ glasses and those crystallized by suitable thermal treatments have been studied. The results are applied to account for the crystallization process, which revealed the crystalline products due to PbCrO₄ and Pb₂CrO₅, and crystallization yield was found to be drastically enhanced (by as much as 85 vol%) with Al₂O₃ additives. The addition of Al₂O₃ in the present glasses and/or the thermal treatments induce the optical absorption intensity in the visible region at the expense of the near infrared region and reflect a gradual decrease in the EPR linewidth, H, at g2. The chromium in these cases most likely exists in a thermodynamic equilibrium between Cr³⁺ and Cr⁶⁺ oxidation states and the Al₂O₃ favours the Cr³⁺Cr⁶⁺ nucleation transformation to accord with the above observations. In addition, the crystalline PbCrO₄ and Pb₂CrO₅ comprise CrO ₄ ^2– groups with the Cr⁶⁺ oxidation state of chromium. This presents a phenomenological correlation for a local symmetry similarity of the CrO ₄ ^2– groups in glass and the microcrystalline products and observation of an enhanced crystallization yield due to Pb₂CrO₅ crystallites in the glasses using Al₂O₃ additives. The chromium occupying the tetragonal CrO ₄ ^2– anion sites migrates rather easily from the glass (subject to the thermal treatment) to the growing nucleation sites of PbCrO₄/Pb₂CrO₅, whereas those Cr³⁺ usually occupying the octahedral sites, which are associated with characteristically large stabilization energy, hardly play a direct role in the crystallization.     

Current-voltage and capacitance-voltage characteristics of polypyrrole/p-InP structure

The polypyrrole/p-InP structure has been made by the electrochemical polymerization of the organic polypyrrole onto the p-InP substrate. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of diode have been determined at room temperature and different frequencies. At each frequency, the measured capacitance decreases with increasing frequency due to a continuous distribution of the interface states in the frequency range 50 kHz-1 MHz. From the I-V characteristics of the polypyrrole/p-InP structure, ideality factor and barrier height (BH) values of 1.68 and 0.59 eV, respectively, were obtained from a forward-bias I-V plot. The diode shows non-ideal I-V behavior with ideality greater than unity. This is attributed to the interfacial layer, the interface states and barrier inhomogeneity of the device. As expected, the C-V curves gave a BH value higher than those derived from I-V measurements. This discrepancy can be explained from the fact that due to the different nature of the C-V and I-V measurement techniques, BHs deduced from them are not always the same.     

Current-voltage characteristics from an asymptotic analysis of the MOSFET equations

Phenomenological formulae for the current-voltage characteristics of a MOSFET are derived by extending the asymptotic method of Ward. Practical methods for combining the perturbation approximations and numerically implementing the Ward equations are developed. A detailed comparison with real MOSFET data is presented and the model is shown to be effective over a range of device geometries.     

Bilayer Cr/Au contacts on n-GaN

Cr/Au (40/65 nm) metal layers have been deposited by thermal evaporation onto n-GaN epitaxial layers grown by metal-organic chemical-vapour deposition (MOCVD) on a sapphire substrate. The samples have been annealed at 400, 700 and 900 °C for 10 min in vacuum. Techniques of TEM, EDS, HRTEM, FESEM, XRD and I-V characteristics have been used to characterize the micro-, and nanostructure, morphology, composition and electrical properties of the contacts before and after annealing. A binary phase of Cr₃Ga₄ and Au₇Ga₂ were identified at the interface of the n-GaN/Cr/Au contacts after annealing in vacuum at 700 and 900 °C. Current-voltage characterizations showed that the as-deposited and annealed Cr/Au contacts are rectifying up to 600 °C. After heat treatment in vacuum at 700 °C and 900 °C the Cr/Au contacts were linear.     

A.c. electrical properties of Au/CIAIPc/Au device

The a.c. electrical properties of chloro aluminum phthalocyanine (CIAIPc) thin film sandwiched between two gold (Au) electrodes have been studied in the frequency range 10 Hz to 100 kHz and the temperature range 293 K to 400 K. The a.c. conductivity Σ(Ω) was found to vary as Ω ^s , with the index s < 1, indicating that a hopping process of conduction is dominant at low temperatures and higher frequencies. At higher temperatures and lower frequencies a free carrier conduction mechanism is likely to dominate. At low temperatures the capacitance of the Au-CIAIPc-Au device is temperature-insensitive and increases rapidly above 300 K. The capacitance and loss tangent decreased with frequency and increased with temperature. Such characteristics were found to be in good agreement with the devices Au/CuPc/Au and Au/PbPc/Au.     

Effect of annealing on the characteristics of Au/Cr bilayer films grown on glass

Au layers with thickness of about 110 nm were sputter-deposited on unheated glass substrates coated with a Cr layer about 20 nm thick. The chamber was evacuated to a pressure of 2 Pa and then sputtering was carried out at Ar pressure of 4 Pa. The Au/Cr bilayer films were annealed in a vacuum of 5×10⁻⁴ Pa at 170°C, 180°C, 200°C and 250°C for from 5 to 120 min, respectively. Atomic force microscopy was used to observe the structural characteristic of the bilayer films. Auger electron spectroscopy was used to analyze the composition inside the Au layers. The sheet resistance of the films was measured using the four-point probe technique. The grain size of the bilayer film gradually increases with an increase in annealing temperature while its average surface roughness ranging from 4.5 to 6.8 nm does not show any systematic change with annealing temperature and time. No impurities such as carbon, nitrogen and oxygen are detected inside all of the Au layers. When the annealing temperature reaches 200°C and the annealing time exceeds 30 min, chromium atoms markedly diffuse into the Au layer. Furthermore, for the bilayer films annealed at 250°C, chromium atoms have markedly diffused into the Au layer even for annealing time of 5 min. Regardless of the increase in grain size of the Au layer, the diffusion of chromium atoms into the Au layer causes an increase in the resistivity of the bilayer film.     

Current-voltage characteristics of metal-titanium dioxide-silicon structures

Metal-titanium dioxide-silicon capacitors were fabricated using electron- beam-evaporated titanium dioxide as the insulating layer. The effects of the titanium dioxide deposition conditions and post-deposition oxidation on the current-voltage characteristics were investigated. A time-dependent current was observed and modeled. Diffusion of the titanium from the titanium dioxide layer into the p-type silicon substrates during oxidation of the titanium dioxide film resulted in an n-type diffused layer. Leakage current densities as low as 10^-9 A in^-2 at 10 V applied bias were obtained for a 1000 Å film of titanium dioxide.     

Current-voltage characteristics and grain growth of Li2CO3-doped tungsten trioxide ceramics

Ceramics samples of tungsten trioxide doped with lithium carbonate from 0.5 to 5 mol% were prepared by conventional electroceramic technique. The current-voltage characteristics of these ceramics were measured under various ambient temperatures. All of the I-V curves showed non-ohmic electrical properties with obvious negative-resistance characteristic at room temperature. It is found that there exists a direct correlation between the negative-resistance phenomenon in the I-V curves and the electrical history of these samples. The suitability of some models regarding the negative-resistance characteristics is discussed. X-ray diffraction (XRD) revealed coexistence of two phases of tungsten trioxide, which depends on the amount of lithium. Scanning electron microscope (SEM) showed great differences for both grain shape and size between the Li-doped and undoped WO₃ ceramics, and this indicates that Li₂CO₃ doped into WO₃ influences strongly the growing of WO₃ during sintering process.     

Current-voltage characteristics and emf of an electric-arc heater operating with various gases

The stationary and dynamic current-voltage characteristics of an electric-arc heater with an argon, nitrogen, air, and oxygen stabilized arc are determined. An empirical expression for calculating the currentvoltage characteristics for various working gases is proposed.     

Current-voltage studies on β-FeSi2/Si heterojunction

I-V characteristics of both β-FeSi₂/n-Si and β-FeSi₂/p-Si were studied at room temperature. The junctions were formed by depositing Fe on Si selectively followed by thermal annealing and some samples were later treated by pulsed laser. Temperature of thermal annealing and diode area were also varied.I-V studies on all these samples were done and ideality factors were computed. Results obtained were interpreted. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

Calculations of impurity doping effects in CrO/sub 2/

The effects of impurity doping in CrO/sub 2/ have been calculated based on the Cr/sub 9/O/sub 40/ cluster using the method of DV-X /spl alpha/ molecular orbital calculation. Elements (Mo, W) belonging to the same family as Cr and the ones (Zr, Ru, Sn) whose dioxides have the same crystalline structure as the rutile were selected for impurity doping. The calculated results show the large enhancements of the magnetic moment and the Curie temperature by the doping of Zr and Ru, respectively.     

Current-voltage characteristics of a four-terminal superconducting weak link

The current-voltage characteristics of a new type of Josephson-like junction consisting of four superconductors in connection with a common region via separate weak links are investigated. This arrangement permits strong interaction and proximity effects to occur inside the composite junction, and the observed variations in theI–V curves can be explained mainly in terms of an exchange of dc and ac quasiparticle currents between phase-slip centers. In particular, direct frequency locking among three Josephson oscillators and induced dc voltage in an unbiased junction have been found for the first time. These effects are of interest for considerations of developing large-scale integrated superconducting circuits.