Negative differential capacitance in n-GaN/p-Si heterojunctions

Negative differential capacitance (NDC) has been observed in n-GaN/p-Si heterojunctions grown by plasma assisted molecular beam epitaxy (PAMBE). The NDC is observed at low frequencies 1 and 10 kilohertz (kHz) and disappeared at a higher testing frequency of 100 kHz. The NDC is also studied with temperature and found that it has disappeared above 323 ^°C. Current-Voltage (I-V) characteristics of n-GaN /p-Si heterojunction were measured at different temperatures and are attributed to the space-charge-limited current (SCLC). A simple model involving two quantum states is proposed to explain the observed NDC behavior.     

Leakage current and capacitance characteristics of Si/SiO2/Si single-barrier varactor

We investigate, both experimentally and theoretically, current and capacitance (I–V/C–V) characteristics and the device performance of Si/SiO₂/Si single-barrier varactor diodes (SBVs). Two diodes were fabricated with different SiO₂ layer thicknesses using the state-of-the-art wafer bonding technique. The devices have very low leakage currents (about 5×10^-2 and 1.8×10^-2 mA/mm²) and intrinsic capacitance levels of typically 1.5 and 50 nF/mm² for diodes with 5-nm and 20-nm oxide layers, respectively. With the present device physical parameters (25-mm² device area, 760-μm modulation layer thickness and ≈10¹⁵-cm^-3 doping level), the estimated cut-off frequency is about 5×10⁷ Hz. With the physical parameters of the present existing III–V triplers, the cut-off frequency of our Si-based SBV can be as high as 0.5 THz. Received: 9 February 2001 / Accepted: 9 February 2001 / Published online: 23 March 2001     

Small-signal ac response of Ba1−xLaxF2+x crystals

The small-signal ac response of single crystalline Ba_1?xLa_xF_2+x solid solutions has been studied in the temperature region 300–820 K, and in the frequency range 4×10^?2–5×10⁴ Hz. At low temperatures the frequency dispersion is dominated by processes in the bulk. Apart from the bulk conductance these processes include a dielectric response determined by broad distributions of dipole-like relaxations. The increase of the static dielectric constant with increasing solute content is explained. At higher temperatures the frequency dispersion reflects interfacial phenomena. In addition to an almost ideal interface capacitance, phenomena are observed which indicate the growth of a surface layer due to a reaction between the electrolyte, and residual oxygen and/or water vapour from the ambient.     

Wheat starch + NaI: a high conducting environment friendly electrolyte system for energy devices

Energy storage devices are the lifeline of today’s society, and hence, researchers are trying to achieve 3Es (economical, environment friendly, and easy to prepare) along with other required technical data. Here, a new wheat starch + NaI system is synthesized which not only have good technical data such as high ionic conductivity (_˜3.61 × 10⁻² S/cm), very low dielectric relaxation time (_˜13.2 μs), and wide electrochemical stability window (_˜2.6 V) but it also satisfies 3Es conditions because it uses starch which is very cheap and abundant biopolymer available in large variety. The approximate cost of preparing 1 cm² of the sample is approximately INR 15. The prepared material is flexible, free standing film which can be easily stretched to 1.9 times to its length, and can be easily twisted up to 90°. The phase angle value approaches to −73° and maintains its value from 10 Hz to 9 kHz frequency range. The ESR value (of sample having thickness 0.48 mm and area 1.21 cm²) is very low (1.10 Ω). Parallel capacitance (C_p) to series capacitance (C_s) ratio is >0.9, and the value is maintained up to a wide frequency range (10 Hz to 10 kHz). Self-resonance frequency (the frequency limit below which energy can be stored efficiently) is also quite high (_˜0.1 MHz) for the present system. Hence, the studied system is a potential candidate for future electrochemical applications.

     

Frequency dispersion of dielectric coefficients of layered TlGaS<Subscript>2</Subscript> single crystals

Frequency dependence of the dissipation factor tanδ, the permittivity ɛ, and the ac conductivity σ_ac across the layers in the frequency range f=5×10⁴−3×10⁷ Hz was studied in layered TlGaS₂ single crystals. A significant dispersion in tanδ was observed in the frequency range 10⁶−3×10⁷ Hz. In the range of frequencies studied, the permittivity of TlGaS₂ samples varied from 26 to 30. In the frequency range 5×10⁴−10⁶ Hz, the ac conductivity obeyed the f ^0.8 law, whereas for f>10⁶ Hz σ_ac was proportional to f ². It was established that the mechanism of the ac charge transport across the layers in TlGaS₂ single crystals in the frequency range 5×10⁴−10⁶ Hz is hopping over localized states near the Fermi level. Estimations yielded the following values of the parameters: the density of states at the Fermi level N _F=2.1×10¹⁸ eV⁻¹ cm⁻³, the average time of charge carrier hopping between localized states τ=2 μs, and the average hopping distance R=103 ?. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 6, 2004, pp. 979–981. Original Russian Text Copyright ? 2004 by Mustafaeva.     

Swift heavy ion induced capacitance and dielectric properties of Ni/n-GaAs Schottky diode

The in-situ capacitance and dielectric properties of 25 MeV C⁴⁺ ion irradiated Ni/n-GaAs Schottky barrier diode (SBD) were studied at 100 kHz in the fluence range 5 × 10¹⁰ – 5 × 10¹³ ions/cm². The investigation shows reduction in capacitance and charge density with increase in ion fluence. Consequent changes were observed in other related parameters like conductance, dielectric constant, dielectric loss, loss tangent and electrical modulus. The results were interpreted in terms of generation of swift heavy ion induced acceptor trap states by electronic energy loss mechanism. Besides, the switch over characteristics of depletion to inversion regions in the CV plot reveals minority carrier recombination centers also. The dispersion and relaxation peaks observed in bias dependent dielectric plots were ascribed to the polarization and relaxation mechanism due to the interfacial trap states. The traps and recombination centers were found to alter the barrier characteristics of the fabricated SBD depending upon the ion fluence.     

A study of hand vibration on chipping and grinding operators,part III: Power levels into the hands of operators of pneumatic tools used in chipping and grinding operations

This paper presents a method for calculating power transmitted to the hands of operators who use vibrating hand tools. Results that relate to a comprehensive multidisciplined NIOSH field study of several hundred chipper and grinder workers who used pneumatic hand tools are presented. The results of this study indicated that the power in the frequency range of 6·3 Hz to 1000 Hz transmitted to the hand ranged from 1·08 × 10³ to 7·23 × 10³ J/s for the chisel and from 8·52 × 10^?1 to 1·57 × 10² J/s for the handle of chipping hammers. For pneumatic grinders the power transmitted to the hands of the tool operators was in the range of 6·58 × 10^?3 to 2·35 × 10^?3 J/s over the same frequency range.     

Equivalent circuit of silicon diodes subjected to high-fluence electron irradiation

Silicon diodes with a p ⁺-n junction made in a 48-μm-thick phosphorus-doped silicon epilayer (resistivity ρ = 30 Ω cm) grown on antimony-doped Si(111) wafers (ρ = 0.01 Ω cm) are studied. The diodes are irradiated by high-energy (3.5 MeV) electrons with fluences from 5 × 10¹⁵ to 2 × 10¹⁶ cm⁻². It is shown that the conventional equivalent circuit of the diode that consists of a parallel RC network and a series-connected resistor inadequately describes the dependence of the dielectric loss tanδ on variable current frequency f in the range 1 × 10²–3 × 10⁷ Hz. Another equivalent circuit is suggested that includes not only the capacitance and resistance of the n-base (the latter increases because radiation-induced defects are compensated for by shallow donors) but also the f dependence of the capacitance of the space-charge region, which is due to retarded charge exchange between deep-level radiation-induced defects.     

AC electrical properties of FeIn2S4

The frequency and temperature dependences of the ac capacitance and resistivity of FeIn₂S₄ semiconductors are studied. Resonances are observed at certain temperatures in the frequency range (2.5–5.0) × 10⁵ Hz. The permittivity of the crystals and the activation energy of charge carriers are determined. It is found that electrical conduction in the given temperature interval is governed by an activation mechanism. The activation energy is frequency-dependent, because the relaxation time of barrier layers decreases with rising frequency.     

Determination of l-proline based on anodic electrochemiluminescence of CdTe quantum dots

A novel flow injection method for detection of l-proline was proposed in the presence of CdTe quantum dots (QDs). This method is based on the enhanced anodic electrochemiluminescence (ECL) emission of CdTe QDs l-proline in aqueous system. CdTe QDs were modified with thioglycolic acid to obtain stable water-soluble QDs and intensive anodic ECL emission in Na₂CO₃–NaHCO₃ buffer solution at an indium tin oxide (ITO) electrode, which was used for the sensitive detection of ECL enhancement using our homemade flow cell. Under the optimal conditions, the ECL intensity was correlated linearly with the concentration of l-proline over the range of 1.0×10^?8?1.0×10^?4 g mL^?1 (r=0.9996) and the detection limit was 5.0×10^?9 g mL^?1. The relative standard deviation was 1.12% for 6.0×10^?5 g mL^?1 l-proline (n=11). The possible mechanism was discussed. This method put forward a new efficient ECL methodology for enhancement-related determination of l-proline successfully.     

Proton-conducting hybrid membranes for medium temperature (>100 °C) fuel cells

Hybrid membranes doped with silicotungstic acid (STA) were prepared by sol–gel process with 3-glycidoxypropyltrimethoxysilane, 3-aminopropyltriethoxysilane, phosphoric acid, and tetraethoxysilane as chemical precursors. The thermogravimetry and differential thermal analysis measurements confirmed that the hybrid membranes were thermally stable up to 350 °C. Relatively, a high proton conductivity of 2.85 × 10⁻² S/cm was obtained for 10 mol% STA-doped hybrid membrane at 120 °C under 90% RH. The hydrogen permeability was found to decrease in the temperature range 20–120 °C from 1.64 × 10⁻¹⁰ to 1.36 × 10⁻¹⁰ mol/cm.s.Pa.     

Impedance characteristics of ITO/Alq3/Al organic light-emitting diodes depending on temperature

Temperature-dependent impedance characteristics of ITO/Alq₃/Al organic light-emitting diodes were studied in the frequency range from 40 to 10⁸ Hz, and the temperature was varied from 10 to 300 K. At each temperature, the frequency-dependent complex impedance was measured under discrete bias voltages from −6 to +20 V, and the voltage-dependent impedance was measured at 10² Hz, 10³ Hz, 10⁴ Hz, and 10⁵ Hz. A Cole–Cole plot shows that there is one relaxation, and a parallel capacitor–resistor network in series with a contact resistance could be considered as an equivalent electrical circuit to this device. As the temperature decreases, a radius in the Cole–Cole plot increases, which indicates an increase of resistance of the device.     

Al/CdSe/GaSe/C resonant tunneling thin film transistors

An Al/CdSe/GaSe/C thin film transistor device was prepared by the physical vapor deposition technique at a vacuum pressure of 10⁻⁵ mbar. The x-ray diffraction measurements demonstrated the polycrystalline nature of the surface of the device. The dc current-voltage characteristics recorded for the Al/CdSe/C and Al/CdSe/GaSe/C channels displayed a resonant tunneling diode features during the forward and reverse voltage biasing, respectively. In addition, the switching current ratio of the Al/CdSe/C increased from 18.6 to 9.62×10³ as a result of the GaSe deposition on the CdSe surface. Moreover, the alternating electrical signal analyses in the frequency range of 1.0 MHz to 1.8 GHz, showed some remarkable properties of negative resistance and negative capacitance spectra of the Al/CdSe/GaSe/C thin film transistors. Two distinct resonance-antiresonance phenomena in the resistance spectra and one in the capacitance spectra were observed at 0.53, 1.04 and 1.40 GHz for the Al/CdSe/C channel, respectively. The respective resonating peak positions of the resistance spectra shift to 0.38 and 0.95 GHz when GaSe is interfaced with CdSe. These features of the thin film transistors are promising for use in high quality microwave filtering circuits and also for use as ultrafast switches.     

Interaction of Folic Acid with Mn2+ Doped CdTe/ZnS Quantum Dots: In Situ Detection of Folic Acid

To utilize the nanomaterials as an effective carrier for the drug delivery applications, it is important to study the interaction between nanomaterials and drug or biomolecules. In this study GSH functionalized Mn²⁺-doped CdTe/ZnS QDs has been utilized as a model nanomaterial due to its high luminescence property. Folic acid (FA) gradually quenches the FL of GSH functionalized Mn²⁺???doped CdTe/ZnS QDs. The Stern-Volmer quenching constant (K_sv), binding constant (K_s) and effective quenching constant (Ka) for the FA-QDs system is calculated to be 1.32?×?10⁵ M^?1, 1.92?×?10⁵ and 0.27?×?10⁵ M^?1, respectively under optimized condition (Temp. 300 K, pH 8.0, incubation time 40 min.). The effects of temperature, pH, and incubation time on FA-QDs system have also been studied. Statistical analysis of the quenched FL intensity versus FA concentration revealed a linear range from 1?×?10^?7 to 5.0?×?10^?5 for FA detection. The LOD of the current nano-sensor for FA was calculated to be 0.2 μM. The effect of common interfering metal ions and other relevant biomolecules on the detection of FA (12.0 μM) have also been investigated. L-cysteine and glutathione displayed moderate effect on FA detection. Similarly, the common metal ions (Na⁺, K⁺, Ca²⁺ and Mg²⁺) produced minute interference while Zn²⁺ Cu²⁺ and Fe³⁺ exert moderate interference. Toxic metal ions (Hg²⁺ and Pb²⁺) produced severe interferences in FA detection.

Graphical abstract

GSH-Mn²⁺ CdTe/ZnS QDs based Fluorescence Nanosensor for Folic acid

     

Narrow bandgap mid-infrared photodetectors based on InAsSbP quantum dots

The results of investigations of mid-infrared photodetectors based on InAsSbP quantum dot (QD) grown on InAs(100) substrate by modified liquid phase epitaxy are presented. The atomic force microscope measurements have shown that the surface density of grown QDs is (4–8) × 10⁹ cm^?2. Also, the morphology and crystalline quality of grown QDs are investigated by a scanning tunneling microscope. Photodetectors based on n-InAs(100) substrate with InAsSbP QDs on its surface were fabricated in the form of a photoconductor cell. The photoresponse spectrum extended up to 4 μm was observed. The optical properties of fabricated structures were investigated under He–Ne laser irradiation with wavelength of 1.15 μm. It was found that the relative surface conductance increases by 16% at power density of 0.15 W/cm². Capacitance hysteresis with maximal remnant capacitance of 2.17 nF at 10³ Hz was observed as well.     

Growth of InAs/GaAs quantum dots on germanium-on-insulator-on-silicon substrate for silicon photonics

We report the growth of self-assembled InAs/GaAs quantum dots (QDs) on germanium-on-insulator-on-silicon (GeOI/Si) substrate by antimony-mediated metal organic chemical vapor deposition. The influence of various growth procedures for the GaAs buffer layer on the QD formation and optical quality was investigated. We obtained QDs with density above 10¹⁰ cm⁻², and ground state emission in the 1.3 μm band at room temperature. These results demonstrate the promising suitability of germanium-on-insulator for the monolithic integration of QD-based and other GaAs-based photonic devices on silicon.     

Improvement of the optical property and uniformity of self-assembled InAs/InGaAs quantum dots by layer-by-layer temperature and substrate rotation

The influence of layer-by-layer temperature and substrate rotation on the optical property and uniformity of self-assembled InAs/In_0.2Ga_0.8As/GaAs quantum dots (QDs) gown with an As₂ source was investigated. An improvement in the optical property of QDs was obtained by the precise control and optimization of growth temperature utilized for each layer, i.e., InAs QDs, InGaAs quantum wells, GaAs barriers and AlGaAs layers, respectively. By using a substrate rotation, the QD density increased from ∼1.4×10¹⁰ to ∼3.2×10¹⁰ cm⁻² and its size also slightly increased, indicating a good quality of QDs. It is found that the use of an appropriate substrate rotation during growth improves the room-temperature (RT) optical property and uniformity of QDs across the wafer. For the QD sample with a substrate rotation of 6 rpm, the RT photoluminescence (PL) intensity is much higher and the standard deviation of RT-PL full-width at half-maximum is decreased by 35% compared to that grown without substrate rotation.     

Surface-initiated ring-opening metathesis polymerization (SI-ROMP) to attach a tethered organic corona onto CdSe/ZnS core/shell quantum dots

Core–shell CdSe/ZnS quantum dots (QDs) are useful as tunable photostable fluorophores for multiple applications in industry, biology, and medicine. However, to achieve the optimum optical properties, the surface of the QDs must be passivated to remove charged sites that might bind extraneous substances and allow aggregation. Here we describe a method of growing an organic polymer corona onto the QD surface using the bottom-up approach of surface-initiated ring-opening metathesis polymerization (SI-ROMP) with Grubbs catalyst. CdSe/ZnS QDs were first coated with mercaptopropionic acid by displacing the original tri-octylphosphine oxide layer, and then reacted with 7-octenyl dimethyl chlorosilane. The resulting octenyl double bonds allowed the attachment of ruthenium alkylidene groups as a catalyst. A subsequent metathesis reaction with strained bicyclic monomers (norbornene-dicarbonyl chloride (NDC), and a mixture of NDC and norbornenylethylisobutyl-polyhedral oligomeric silsesquioxane (norbornoPOSS)) allowed the construction of tethered organic homo-polymer or co-polymer layers onto the QD. Compounds were characterized by FT-IR, 1H-NMR, X-ray photoelectron spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Atomic force microscopy showed that the coated QDs were separate and non-aggregated with a range of diameter of 48–53 nm.     

Infralow-frequency dielectric response of Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> polycrystalline layers

This paper reports on the results of the investigation into the frequency dispersion of the capacitance and dielectric loss in capacitor structures based on red lead Pb₃O₄. It is established that, in the range of frequencies f = 1.50 × 10^?3?0.25 Hz, the capacitance decreases and the dielectric loss tangent increases with increasing frequency. The frequency dependence of the electrical conductivity in an alternating-current electric field indicates the applicability of the hopping model of charge transfer under normal conditions. The role of a lone electron pair of Pb²⁺ cations in dielectric polarization is discussed.     

Dielectric response of doped Bi<Subscript>12</Subscript>TiO<Subscript>20</Subscript>: Ru crystals in an alternating electric field

The results of examination of AC dependences of capacitance and dielectric loss tangent of sillenite Bi₁₂TiO₂₀ crystals doped with ruthenium on frequency are presented. Non-Debye dispersion of dielectric coefficients is found in the frequency interval of 5 × 10²–10⁵ Hz, and a resonance phenomenon is observed. Polarization processes in the studied samples are attributed to relaxators associated with metal–oxygen vacancies and structural elements incorporating 6s² lone-pair electrons.