Measurement of charge-carrier concentration in indium phosphide by means of an electrolyte-semiconductor contact

An electrolyte-semiconductor contact is used to study the conductivity of epitaxial layers and single crystals of n-type indium phosphide obtained by gas transport. Some of the specimens were alloyed with tin and sulfur. The voltfarad characteristics are used to find the potentials of planar zones, which amount to 0.8–1.3 V for different electrolytes. Values of concentration of charge carriers calculated from measured values of capacitance of the electrolyte-indium-phosphide contact showed good agreement with measurements of the Hall effect on single crystals in the range 10¹⁶-10¹⁸cm^–3. The use of measurements of the capacitance of the electrolyte-semiconductor contact with simultaneous etching of a local region made it possible to study the electron distribution in epitaxial layers of indium phosphide.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 71–74, May, 1987.     

Properties of Mott contacts with an ultralow metal-semiconductor barrier

The current-voltage and capacitance characteristics of Mott contacts with an ultralow metal-semiconductor barrier are investigated. The analysis is based on the analytical solution of the Poisson equation for the space charge of carriers in the “metal-i-layer-n ⁺-substrate” structure without regard for bulk doping of the i layer. For contacts with ultralow metal-semiconductor barriers (comparable in magnitude to the thermal energy of charge carries), it is demonstrated that the reverse current becomes greater than the forward current, the sign of rectification is reversed, and the capacitance of the contact acquires a strong dependence on the voltage. This means that the mechanism of nonlinearity of the structure changes and the nonlinearity governed by charge carriers injected into the i layer becomes dominant. In a specific range of bias voltages close to zero, the differential resistance and the capacitance of the structure exponentially increase with increasing voltage. The observed behavior is not typical of conventional metal-semiconductor contacts. The obtained dependences of the electric current and capacitance on the voltage determine the characteristics of new advanced instruments, in particular, highly sensitive microwave detectors operating without a bias voltage.     

Chitosan-ZnO/polyaniline ternary nanocomposite for high-performance supercapacitor

We report on the synthesis of chitosan-zinc oxide (ZnO)/polyaniline (CS-ZnO/PANI) ternary nanocomposites via in situ polymerization of aniline in the presence of CS-ZnO nanocomposite prepared by simple precipitation method. The structure, morphology, and physicochemical properties of prepared ternary composites are characterized by Fourier transform infrared, UV–visible, X-ray diffraction, SEM, EDXS, TEM, thermogravimetric/differential thermal analysis, and N₂ adsorption/desorption measurements. Their electrochemical properties are also investigated using cyclic voltammetry, galvanostatic charge–discharge tests, and electrochemical impedance spectroscopy. Electrochemical measurements show that the mesoporous CS_0.12-ZnO_2.5/PANI electrode yields larger specific capacitance (587.15 F g^?1) than the corresponding PANI-ZnO electrode without added chitosan and the capacitance retention is 80 % after 1,000 charge/discharge cycles at 175 mA cm^?2 current density in the voltage range of 0 to 0.8 V vs. SCE, due to the synergistic effect among three components which result in enhanced specific capacitance and cycling stability. The resulting composites are promising electrode materials for high-performance, environmentally friendly, and low-cost electrical energy storage devices.     

Investigation of electrical properties (ac and dc) of organic zinc phthalocyanine,ZnPc, semiconductor thin films

We report measurements on the electrical properties of thermally evaporated zinc phthalocyanine, ZnPc, semiconductor thin films. Aluminum and gold metal electrodes were used and both proved to act as ohmic contacts. A relative permittivity, ε_r, of 1.56 was estimated from the dependence of capacitance on film thickness. The room temperature current density–voltage measurements indicated an ohmic conduction at low voltages, while a space–charge-limited conduction at higher voltages. An average value of a thermally generated hole concentration of the order 10¹³ m⁻³ was estimated at room temperature.The ac conductivity, capacitance and loss tangent were measured over a wide range of temperature (from 170 to 430 K) and frequency (between 0.1 and 20 kHz). The ac conductivity of ZnPc films was observed to be proportional to ω^s, where ω is the angular frequency, and the index s is a temperature and frequency-dependent constant. At low temperatures and for higher frequencies the ac conduction was due to hopping. The capacitance, as well as the loss tangent, was found to be dependent on both temperature and frequency, but was constant for all frequencies at low temperatures. Such dependences were accounted for the equivalent-circuit model consisting of inherent capacitance in parallel with a temperature dependent resistive element.     

Characterization of MOVPE-grown GaAs layers by C-V analysis

A computer simulation study of the capacitance of a surface space charge layer in undoped n-GaAs grown by metalorganic vapour phase epitaxy is presented. The effect of the deep donor level EL2 on the surface capacitance of epilayers with an ideal free surface is estimated. In order to approach the as-grown layer surface the model used is extended considering MIS and Schottky-barrier structures and their voltage-capacitance curves are analysed. The theoretical C-V dependences are compared with experimental C-V curves of a real structure including N⁺-GaAs substrate, undoped n-GaAs epitaxial layer containing EL2 levels and thin native oxide. Conditions are determined at which the EL2 levels as well as the native oxide film may influence the capacitance characteristics.     

Hole injection and surface state effects at gallium arsenide electrodes

We report capacitance and luminescence measurements at gallium arsenide electrodes in 1 M H₂SO₄ electrolyte that indicate the injection of holes on cathodic polarization. The steady-state potential-current relations and photoeffects are reviewed critically. We show that the assumption of hole injection is equally plausible to that of electron extraction provided the surface recombination velocity is fast compared to the bulk diffusion and recombination process. In addition to the space charge there is a substantial surface charge of the type commonly observed at III–V semiconductor surfaces.     

Electric double layer capacitance of multi-walled carbon nanotubes and B-doping effect

The double layer capacitance properties of multi-walled carbon nanotubes (MWCNTs) prepared by CVD were investigated using propylene carbonate electrolytes. The original MWCNTsare typically entangled tubes with 10∼20 nm outer diameter, around 5 nm inner diameter, and around 220 m² g^-1 of BET specific surface area. The galvanostatic measurement (40 mA g^-1, 2∼4 V vs. Li/Li⁺) in a three-electrode system showed 15 F g^-1 of gravimetric capacitance and a good rate of this property. Due to the tips opening by thermal oxidation, the specific surface area and the capacitance increased. In contrast, heat-treatment at 3000 °C decreased the surface area and the capacitance of the MWCNT due to defect recovery. A small amount (<1 at. %) of boron was doped to the MWCNT by heat-treatment at 2200 °C or 2300 °C with a B-contained graphite crucible. The B-doping can improve the specific capacitance per surface area for the MWCNTswhile maintaining the tube morphology. This effect of B-doping on the capacitance can be explained by modification of the space charge layer in carbon. PACS 81.07.De; 81.65.Mq; 82.47.Uv; 61.72.Vv     

Barrier height enhancement of metal/semiconductor contact by an enzyme biofilm interlayer

A metal/interlayer/semiconductor (Al/enzyme/p-Si) MIS device was fabricated using α-amylase enzyme as a thin biofilm interlayer. It was observed that the device showed an excellent rectifying behavior and the barrier height value of 0.78?eV for Al/α-amylase/p-Si was meaningfully larger than the one of 0.58?eV for conventional Al/p-Si metal/semiconductor (MS) contact. Enhancement of the interfacial potential barrier of Al/p-Si MS diode was realized using enzyme interlayer by influencing the space charge region of Si semiconductor. The electrical properties of the structure were executed by the help of current–voltage and capacitance–voltage measurements. The photovoltaic properties of the structure were executed under a solar simulator with AM1.5 global filter between 40 and 100?mW/cm² illumination conditions. It was also reported that the α-amylase enzyme produced from Bacillus licheniformis had a 3.65?eV band gap value obtained from optical method.     

Effect of donor-acceptor complex dissociation of the properties of the surface boundary region in P-InP:Zn during its thermal oxidation

The effect of thermal oxidation on the properties of the surface boundary region in p-InP:Zn epilayer is discussed. Thermal oxidation has been carried out at temperatures ranging from 100°C to 550°C. The free hole concentration in the bulk epilayer was in the range of 2–6 × 10¹⁶cm^?3.The analysis of the Schottky barrier capacitance voltage characteristics, the photoluminescence spectra and the differential spectra obtained from the thermally stimulated capacitance in Ag-p-InP Schottky diodes have shown that thermal oxidation in p-InP:Zn led to significant transformations of the parameters of the indium phosphide boundary region. The transformation covered changing of N_a-N_d profiles, variations of absolute values for concentrations of shallow and deep centres, variations of luminescence intensity and types of photoluminescence spectra.The observed effects were interpreted by a model designed to study the dissociation of neutral donor-acceptor Zn-V_p, complexes in the space charge region surface of p-InP, provided the dissociation occurs under varying rates of the generation of phosphor vacancies in the boundary region.     

Excess space charge in strontium titanate

An analysis is made of mechanisms of the formation and redistribution of space charge which influence the dielectric hysteresis in SrTiO₃. The excess space charge density in SrTiO₃ at 4.2 K is estimated by comparing calculated and experimental dependences of the initial capacitance and the dielectric hysteresis parameter ΔC/C ₀ as a function of the crystal thickness. Fiz. Tverd. Tela (St. Petersburg) 40, 245–247 (February 1998)     

Formation of ac Discharges Between Glass Insulated Electrodes in Ne+1% N2 Mixtures. III. Discharge Development and Space Charge

The investigation of the optical space-time-structure of the discharge development was made for various values of p^D (1,3 kPacm, 33,4 kPacm) by means of the variation of the electrode distance for two gas pressures. The broadening of the bistable region with growing electrode distance can be explained by the effect of the positive space charge as a virtual anode. Thus the space charge memory is dependent on the value of p^D. If the space charge breakdown takes place, a light front moves from the anode to the cathode, its velocity, varying from 3 · 10⁴ cm/s to 8 · 10⁵ cm/s. In the volume an optical structure develops which is analogous to that of a medium pressure glow discharge. If the time between successive discharge pulses is too short, then the decay of the positive space charge is incomplete and there is no Townsend phase. The process depends, via the ion drift velocity, on the electrode distance.     

Off-centre charge model of the planar electric double layer for asymmetric 2:1/1:2 valencies

ABSTRACT

Grand canonical Monte Carlo simulation results are reported for the structure and capacitance of a planar electric double layer containing off-centre charged rigid sphere cations and centrally charged rigid sphere anions. The ion species are assigned asymmetric valencies, +2:?1 and +1:?2, respectively, and set in a continuum dielectric medium (solvent) characterised by a single relative permittivity. An off-centre charged ion is obtained by displacing the ionic charge from the centre of the sphere towards its surface, and the physical double layer model is completed by placing the ionic system next to a uniformly charged, non-penetrable, non-polarizable planar electrode. Structural results such as electrode-ion singlet distribution functions, ionic charge density and orientation profiles are complemented by differential capacitance results at electrolyte concentrations of 0.2?mol/dm³ and 1?mol/dm³, respectively, and for various displacements of the cationic charge centre. The effect of asymmetry due to off-centre cations and valency asymmetry on the double layer properties is maximum for divalent counterions and when the cation charge is closest to the hard sphere surface.     

Frequency domain measurements on Na2B4O7−V2O5 semiconducting glass

Summary Dielectric measurements on Na₂B₄O₇(99.5%)−V₂O₅(0.5%) glass system, in the frequency range 10⁻³ to 10⁴ Hz and temperature range 300 to 500 K, have been carried out. The normalized plots of complex capacitance have shown a single mechanism responsible for conduction for both volume and surface measurements with their close values of activation energies (0.67±0.03) eV and (0.64±0.03) eV, respectively. The low-frequency dispersion (LFD) behaviour has been observed to be perturbed by the presence of more than one competing process. The impedance plots have shown a parallel combination of a capacitor (C) and a resistor (R), with some contribution of a dispersive element due to charge accumulation in the vicinity of the electrodes. The values ofR andC were found to be of the same order of magnitude, for both surface and volume measurements. The observedR has shown a decrease with an increase in temperature due to an increase in mobility of Na⁺ ions, whereasC remains practicaly constant. The complex capacitance surface behaviour is dominated by volume, due to hygroscopy of this glass system.     

Synthesis of graphene/nickel oxide composite with improved electrochemical performance in capacitors

A novel reduced graphene oxide/NiO nanosheet composite (r-GO/NiO) (ca. 75 % NiO in weight) was synthesized by a facile two-step method, where the NiO nanosheets were decorated with some voids. The composite was characterized by using X-ray diffraction, transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy. The electrochemical properties of the composite were investigated by cyclic voltammetry, galvanostatic charge, and discharge measurements. The results show that the r-GO/NiO composite exhibits a stable average specific capacitance of ca. 1,139 F g^?1 (at 0.5 A g^?1) during 1,000 charge–discharge cycles, suggesting that the r-GO/NiO composite is a potential supercapacitor material. The main correlation between the electrochemical performance and the structure of the materials was studied, and the formation process of the composite was also discussed.     

The small signal behavior of SCLC-diodes with deep traps

Theoretical and experimental investigations on the small signal behavior of sclc-diodes containing deep levels were performed. The experimental results were obtained from high resistivity Au-compensated n⁺in⁺-silicon samples. The measurements of the frequency dependent conductance and capacitance are consistent with theory up to frequencies ωT>1. From the bias dependence of the capacitance a method is deduced to determine the capture time constant easily and with high accuracy and, if the trap concentration is already known, the capture cross section. The value obtained for the gold acceptor level agrees well with that determined by means of transient current measurements.     

Application of abrupt cut-off models in the analysis of the capacitance spectra of conjugated polymer devices

In this paper, a detailed study of the capacitance spectra obtained from Au/doped-polyaniline/Al structures in the frequency domain (0.05 Hz–10 MHz), and at different temperatures (150–340 K) is carried out. The capacitance spectra behavior in semiconductors can be appropriately described by using abrupt cut-off models, since they assume that the electronic gap states that can follow the ac modulation have response times varying rapidly with a certain abscissa, which is dependent on both temperature and frequency. Two models based on the abrupt cut-off concept, formerly developed to describe inorganic semiconductor devices, have been used to analyze the capacitance spectra of devices based on doped polyaniline (PANI), which is a well-known polymeric semiconductor with innumerous potential technological applications. The application of these models allowed the determination of significant parameters, such as Debye length (≈20 nm), position of bulk Fermi level (≈320 meV) and associated density of states (≈2×10¹⁸ eV⁻¹ cm⁻³), width of the space charge region (≈70 nm), built-in potential (≈780 meV), and the gap states’ distribution.     

Pulsed electrodeposition of mesoporous cobalt-doped manganese dioxide as supercapacitor electrode material

Cobalt-doped MnO₂, as electrode material for supercapacitor, was synthesized by pulse electrodeposition method. The morphology and structure of the products were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM). The results show that the crystal structure of the products is γ-type, and the samples reveals a porous texture composed of manganese oxide nanosheets. Cyclic voltammetry (CV), electrochemical impedance spectrometry (EIS), and galvanostatic charge–discharge tests indicate that doping cobalt has great effect on the electrochemical performance of manganese dioxide material. A specific capacitance of 354 F g^?1 is obtained when the molar ratio of Mn to Co is 200:10. After 100 charge–discharge cycles in 6 M KOH solution, the specific capacitance stabilized at 333.6 F g^?1, exhibiting excellent capacitance retention ability.     

Size dependent electron momentum density distribution in ZnS

In this paper, we present size dependent electron momentum density distribution in ZnS. ZnS nanoparticles of size 3.8 nm and 2.4 nm are synthesized using the chemical route and characterized by X-ray diffraction (XRD). The Compton profile measurements are performed on both the nano-sized as well as bulk ZnS samples employing 59.54 keV gamma-rays from ²⁴¹Am source. The results reveal that the valence electron density in momentum space becomes narrower with reduction of particle size. To evaluate the charge transfer on compound formation, the ionic model based calculations for a number of configurations of Zn^+xS^−x (0.0≤x≤2) are also performed utilizing free atom Compton profiles. These results suggest different amounts of charge transfer in these materials varying from 1.2 to 2.0 electron from Zn to S atom.     

SIMS on ZnO surfaces: The influence of space charge accumulation layers on secondary ion yields and measurement of true hydrogen concentration

On single crystal surfaces of n-conducting ZnO rather strong space charge accumulation layers can be produced by adsorption of atomic hydrogen. It is shown that secondary ion yields depend strongly on this charge carrier concentration at and near the surface. By comparison to electric conductivity measurements it is demonstrated that surface concentrations of hydrogen are proportional to secondary ion yield ratios of type XH^±/X^±. In the experiments it was further noticed that Na, K, Ca represent common impurities on ZnO surfaces.     

Synthesis and performance of nickel hydroxide nanodiscs for redox supercapacitors

Herein, we report the facile synthesis of β-Ni(OH)₂ nanodiscs by chemical precipitation method and their use in supercapacitors. β-Ni(OH)₂ nanodiscs are characterized by FTIR, XRD, FESEM, XPS and TGA analysis. Morphological analysis revealed the uniform nanodisc morphology of β-Ni(OH)₂. The supercapacitor behavior is evaluated by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy measurements in 1-M aqueous KOH solution with 0- to 0.6-V potential window. The specific capacitance of β-Ni(OH)₂ nanodiscs is found to be 400 F g⁻¹. The energy and power densities of the β-Ni(OH)₂ nanodiscs are found to be 7.15 W h kg⁻¹ and 1716 W kg⁻¹, respectively, at the current density of 1 A g⁻¹. The cycle life test shows the good stability of the electrode with 83 % retention capacitance even after 1500 cycles.