Dominant conduction mechanism and the effects of device temperature on electrical characteristics of Al/ZnPc/n-Si structures

Aluminum/Zinc Phthalocyanine/n-Si metal semiconductor contact with organic interfacial layer has been developed and characterized by Current-Voltage-Temperature (I-V-T) measurements for the study of its junction and charge transport properties. The junction parameters, such as diode ideality factor (n), barrier height (φ_b) and series resistance (R_S), of the device were found to shift with device temperature. The diode ideality factor was found to increase with the device temperature up to 323 K. However, a decreasing trend in the value of n was observed beyond this temperature. The barrier height and series resistance were found to increase and decrease, respectively with increasing device temperature. The peak of interface state energy distribution curves was shifted, in terms of Ess-Ev value, from 0.622 eV to 0.827 eV with 52 meV activation energy of the charge carriers. The data analysis implies that the Fermi level of the organic interfacial layer shifts as function of device temperature. In terms of dominant conduction mechanism, the I-V-T data analysis confirms the relationship log (IV⁴) ∝V^1/2 with the device temperature in the range of 313-343 K and the Poole-Frenkel type is found to be the dominant conduction mechanism for the hybrid device.     

Evidence of current stabilization after long-time decay in high-TC superconductors

In this work, we have studied flux creep phenomenon over a long time period viz. about 1.5 × 10⁷ s (i.e. for six months) at liquid nitrogen temperature. For this purpose, a high current was induced in four high-T_C superconducting rings by means of a field-cooling (one sample) or a ferromagnetic field-cooling procedure (three samples). The resulting current decay was measured using a Hall probe system and the results obtained revealed lower relaxation rates in the ferromagnetic field-cooled samples. Also, the experimental data were found to leave the prediction of the classical Anderson-Kim model after a time long enough. The slope of the logarithmic current decay plot exhibited an oscillatory phenomenon at approximately 2 × 10⁵ s (about 55 h). Oscillations vanished at approximately 4-5 × 10⁶ s (46-58 days), after which the induced current remained stable throughout the remainder of the experiencing period.     

Investigation of the electrical parameters of Ag/p-TlGaSeS/C Schottky contacts

p-type TlGaSeS single crystal was used to fabricate a Schottky device. Silver and carbon metals were used as the Ohmic and Schottky contacts, respectively. The device which displayed wide RF band at 13.200 and narrow band at 62.517 kHz with Q value of 1.4 and of 6.3 × 10⁴, respectively, is characterized by means of current (I)–voltage (V), capacitance (C)–voltage characteristics as well as capacitance–frequency (f) characteristics. The device series resistance, ideality factor and barrier height are determined from the I–V curve as 35.8 MΩ, 1.2 and 0.74 eV, respectively. The apparent acceptor density and the build in voltage of the device increased with increasing ac signal frequency. The high Q value, observed at 62.517 kHz, indicated a much lower rate of energy loss relative to the stored energy of the device. The energy loss (Q⁻¹) is much less than 0.001% of the stored value. The device was tested and found to remain at the same mode of resonance for several hours. It never switched or ceased unless it was tuned off.     

Optimization of L10-FePt/MgO/CrRu thin films for next-generation magnetic recording media

L1₀-FePt thin films were deposited on silicon substrates with the structure of Si/CrRu/MgO/FePt. The magnetic and microstructural properties were optimized by varying the FePt sputter pressure and temperature, as well as the thicknesses of all three layers. High coercivity films greater than 1.8 T were grown when the FePt sputter pressure was at 1.33 Pa with a thickness of only 4 nm, on CrRu and MgO underlayers as thin as 10 nm and 2 nm, respectively.     

H2 absorption at ambient conditions by anodized aluminum oxide (AAO) pattern-transferred Pd nanotubes occluded by Mg nanoparticles

The anodized alumina oxide (AAO) pattern-transferred Pd nanotubes occluded by Mg nanoparticles were easily manufactured by the use of vacuum evaporation technique at room temperature. The nanostructures were subjected to the H₂ absorption at ambient conditions. The material can absorb H₂ very fast (the equilibration between the hydrogen gas and within the material at each target pressure occurs in a time shorter than 110 s) reaching approximately 1 wt.% of H₂ at only 0.4 kPa of H₂ pressure. At 100 kPa the system absorbs up to 2 wt.% of H₂ at room temperature. After hydrogenation a drastic change in the structure and morphology of the nanostructures, as an effect of the metal lattice expansion in all three dimensions, was observed which was characterized by the appearance of a regular array of columns with a substantial increase in volume as compared to the nanostructures before hydrogenation. The application of the system in a potential hydrogen sensing device is envisaged.     

Characterization and humidity sensing of ZnO/42° YX LiTaO3 Love wave devices with ZnO nanorods

Love mode surface acoustic wave devices based on ZnO/42° YX LiTaO₃ were characterized with the thickness of the sputtered ZnO guiding layer varied from 250 nm to 1.18 μm. Phase velocity, temperature coefficient of resonant frequency, sensitivity, electromechanical coupling coefficient and humidity sensing of the Love mode SAW devices were studied as a function of the ZnO layer thickness. With increasing ZnO thickness over the range of thickness values we have examined, the sensitivity of 42° YX LiTaO₃ to liquid loading increased and the values of electromechanical coupling coefficient decreased. The device with a thickness of 250 nm showed the best humidity response. ZnO nanorods were grown on this device and its humidity sensing performance has been further improved due to their large surface-to-volume ratio of the ZnO nanorods.     

Effects of annealing on the polymer solar cells based on CdSe-PVK electron acceptor

CdSe-poly(N-vinylcarbazole) (CdSe-PVK) nanocomposite was synthesized and utilized as the electron acceptor in the active layer of polymer solar cells. The photovoltaic properties of the polymer solar cells based on poly(3-hexylthiophene) (P3HT):CdSe-PVK as the active layer were investigated in detail. The effects of annealing temperature (100-200 °C) and time (5-60 min) on the device performance were studied. At annealing temperature of 150 °C for 30 min, the device demonstrated an optimal efficiency of 0.235% under AM 1.5 (100 mW cm⁻²) solar simulated light irradiation. The improved efficiency under the optimal conditions was confirmed by the highest light harvest in UV-vis spectra due to the increased crystallinity of P3HT after thermal annealing. Photoluminescence of these devices also exhibited that the quench effect increases with the increasing of annealing temperature, indicating that the charge separation between electron-donating (P3HT) and electron-accepting (CdSe-PVK) molecules was increased after heat treatment. Atomic force microscopy (AFM) images showed that the phase segregation and 3D interpenetrating networks of P3HT:CdSe-PVK were responsible for the enhancement of the device efficiency.     

Evolution of the optical transitions in AlxGa1 − xAs/GaAs quantum well structures grown on GaAs buffers with different surface treatments by molecular beam epitaxy

Al_0.3Ga_0.7As/GaAs Quantum Well structures were grown by molecular beam epitaxy (MBE) on a 500 nm thick GaAs buffer layer subjected to the following surface processes: a) in-situ Cl₂ etching at 70 °C and 200 °C, b) air-exposure for 30 min. The characteristics of these samples were compared to those of a continuously grown sample with no processing (control sample). We obtained the quantum wells energy transitions using photoreflectance spectroscopy as a function of the temperature (8-300 K), in the range of 1.2 to 2.1 eV. The sample etched at 200 °C shows a larger intensity of the quantum well peaks in comparison to the others samples. We studied the temperature dependence of the excitonic energies in the quantum wells (QWs) as well as in GaAs using three different models; the first one proposed by Varshni [4], the second one by Viña et al. [5], and the third one by Pässler and Oelgart [6]. The Pässler model presents the best fitting to the experimental data.     

In situ immuno-magnetic concentration-based biosensor systems for the rapid detection of Listeria monocytogenes

A plastic module for in situ immuno-magnetic concentration (IMC) was devised by engraving acrylics and physically combining the system with a rapid test device to detect foodborne pathogen. The IMC module-installed analytical system consisted of three compartments for magnetic separation, sample medium absorption, and analysis of the target microorganism. To experimentally simulate a practical situation, immuno-magnetic beads were prepared by coupling monoclonal antibodies specific to Listeria monocytogenes, which was used as a model analyte, to chemically functionalized beads. Under the optimal conditions, the IMC module condensed the medium by a factor of 100 (e.g., from 10 mL to 100 μL) within 5 min and enriched the microorganism by about 60-fold. This concentrated sample was then analyzed using two different analytical systems based on lateral flow, i.e., ELISA-on-a-chip and immuno-chromatographic assay, which had high detection capabilities, 3.6 × 10² and 6.6 × 10³ cells mL^− 1, respectively. Thus, the IMC module-installed biosensor system was able to sequentially condense a large sample volume and detect the presence of contaminants within, for example, 30 min. Therefore, this approach could be suitable for early screening of food products that may be contaminated with microorganisms.     

Fabrication of Al/MgO/C and C/MgO/InSe/C tunneling barriers for tunable negative resistance and negative capacitance applications

In this work, the design and characterization of magnesium oxide based tunneling diodes which are produced on Al and InSe films as rectifying substrates are investigated. It was found that when Al thin films are used, the device exhibit tunneling diode behavior of sharp valley at 0.15 V and peak to valley current ratio (PVCR) of 11.4. In addition, the capacitance spectra of the Al/MgO/C device show a resonance peak of negative capacitance (NC) values at 44.7 MHz. The capacitance and resistance–voltage characteristics handled at an ac signal frequency of 100 MHz reflected a build in voltage (V_bi) of 1.29 V and a negative resistance (NR) effect above 2.05 V. This device quality factor (Q)–voltage response is ~10⁴. When the Al substrate is replaced by InSe thin film, the tunneling diode valley appeared at 1.1 V. In addition, the PVCR, NR range, NC resonance peak, Q and V_bi are found to be 135, 0.94–2.24 and 39.0 MHz, ~10⁵ and 1.34 V, respectively. Due to the wide differential negative resistance and capacitance voltage ranges and due to the response of the C/MgO/InSe/C device at 1.0 GHz, these devices appear to be suitable for applications as frequency mixers, amplifiers, and monostable–bistable circuit elements (MOBILE).     

Magneto-electrical transport in V-patterned La0.7Sr0.3MnO3 nanostructures

We investigate the electrical transport and magnetic field dependence of nano-patterned La_0.7Sr_0.3MnO₃ devices. We find that the resistivity versus temperature dependence is the same as that observed in thin films, indicating that our nano-patterning preserves the fundamental properties of the material. At temperatures below 20 K there is resistivity upturn of ~ 5 % in the smallest and thinnest device. Structures in a “V” pattern were fabricated in order to investigate domain wall resistance. We find a much smaller resistance area product as compared to previous reports observed in nanoconstrictions and also that the switching field matches that in micromagnetic simulations.     

Structure, properties and gas sensing behavior of Cr2 − xTixO3 films fabricated by electrostatic spray assisted vapour deposition

Single-phase eskolaite crystalline Cr_2 − xTi_xO₃ films (CTO) with a uniform porous microstructure were fabricated via an electrostatic spray assisted vapour deposition (ESAVD) method. The sensing behavior upon exposure to ammonia and ethanol was characterized in a CTO film-based sensor device in terms of response, reproducibility, humidity constraints and sensor stability. The ESAVD process has been shown to be capable of producing CTO films at low temperature (650 °C) and more importantly, it results in a more uniform titanium distribution and better microstructural control than processes based on solid-state chemical reactions. The material with a nominal composition of Cr_1.7Ti_0.3O₃ exhibited the highest sensitivity among the different Cr_2 − xTi_xO₃ compositions examined towards ammonia over the temperature range of 200-500 °C with a peak sensitivity of 2.90 at 200 °C. The CTO materials, when used as sensors, also exhibit excellent responses to ethanol concentration in air. The sensitivity was 0.64 for 10 ppm ethanol, 0.85 for 25 ppm, and 0.92 for 50 ppm, respectively.     

Antimony assisted low-temperature processing of CuIn1 − xGaxSe2 − ySy solar cells

Application of the Sb-doping method to low-temperature (≤ 400 °C) processing of CuIn_1 − xGa_xSe_2 − yS_y (CIGS) solar cells is explored, using a hydrazine-based approach to deposit the absorber films. Power conversion efficiencies of 10.5% and 8.4% have been achieved for CIGS devices (0.45 cm² device area) processed at 400 °C and 360 °C, respectively, with an Sb-incorporation level at 1.2 mol % (relative to the moles of CIGS). Significant Sb-induced grain size enhancement was confirmed for these low processing temperatures using cross-sectional scanning electron microscopy, and an average 2-3% absolute efficiency improvement was achieved in Sb-doped samples compared to their Sb-free sister samples. With Sb inclusion, the CIGS film grain growth temperature is lowered to well below 450 °C, a range compatible with flexible polymer substrate materials such as polyimide. This method opens up access to opportunities in low-temperature processing of CIGS solar cells, an area that is being actively pursued using both traditional vacuum-based as well as other solution-based deposition techniques.     

Temperature effects on the optoelectronic properties of AgIn5S8 thin films

Polycrystalline AgIn₅S₈ thin films are obtained by the thermal evaporation of AgIn₅S₈ crystals onto ultrasonically cleaned glass substrates under a pressure of ~ 1.3 × 10⁻³ Pa. The temperature dependence of the optical band gap and photoconductivity of these films was studied in the temperature regions of 300-450 K and 40-300 K, respectively. The heat treatment effect at annealing temperatures of 350, 450 and 550 K on the temperature dependent photoconductivity is also investigated. The absorption coefficient, which was studied in the incidence photon energy range of 1.65-2.55 eV, increased with increasing temperature. Consistently, the absorption edge shifts to lower energy values as temperature increases. The fundamental absorption edge which corresponds to a direct allowed transition energy band gap of 1.78 eV exhibited a temperature coefficient of −3.56 × 10⁻⁴ eV/K. The 0 K energy band gap is estimated as 1.89 eV. AgIn₅S₈ films are observed to be photoconductive. The highest and most stable temperature invariant photocurrent was obtained at an annealing temperature of 550 K. The photoconductivity kinetics was attributed to the structural modifications caused by annealing and due to the trapping-recombination centers' exchange.     

The role of transparent conducting oxides in metal organic chemical vapour deposition of CdTe/CdS Photovoltaic solar cells

A systematic study is made between the relationship of Cd_0.9Zn_0.1S/CdTe photovoltaic (PV) device properties for three different commercial transparent conducting oxide (TCO) materials and some experimental CdO to determine the role of the TCO in device performance. The resistance contribution from the TCO was measured after depositing the gold contact architectures directly onto the TCOs. These were compared with the Cd_0.9Zn_0.1S/CdTe device properties using the same contact arrangements. Series resistance for the commercial TCOs correlated with their sheet resistance and gave good agreement with the PV device series resistance for the indium tin oxide (ITO) and fluorine doped tin oxide (FTO) 15 Ω/Sq. superstrates. The devices on the thicker FTO 7 Ω/sq superstrates were dominated by a low shunt resistance, which was attributed to the rough surface morphology causing micro-shorts. The device layers on the CdO substrate delaminated but devices were successfully made for ultra-thin CdTe (0.8 μm thick) and compared favourably with the comparable device on ITO. From the measurements on these TCOs it was possible to deduce the back contact resistance and gave an average value of 2 Ω.cm². The correlation of fill factor with series resistance has been compared with the predictions of a 1-D device model and shows excellent agreement. For high efficiency devices the combined series resistance from the TCO and back contact need to be less than 1 Ω.cm².     

Preparation and crystal structure of H-BaTa2O6-type K1.83Ba4.17Nb12.18O36 and dielectric properties of the related compounds

Single crystal of a novel compound, K_1.83Ba_4.17Nb_12.18O₃₆, has been synthesized in the course of investigation on the K₂O-BaO-Nb₂O₅ system. The crystal structure was determined by single crystal X-ray diffraction data. The space group of this compound was found to be P6/mmm (#191) with the lattice parameters of a = 21.109(4) and c = 3.967(1) Å. The final R-factors were R = 0.039 and Rw=0.042 for unique 508 reflections. The crystal structure had the same tunnel structure as that of hexagonal BaTa₂O₆ (H-BaTa₂O₆), which is the high temperature form in three modifications of BaTa₂O₆. The chemical composition of K_1.83Ba_4.17Nb_12.18O₃₆ was close to that of the tetragonal tungsten bronze (TTB) type KBa₂Nb₅O₁₅ and the powder samples within this composition were prepared so as to clarify the boundary between H-BaTa₂O₆ and TTB-type structures. The H-BaTa₂O₆-type structure appears in (K + Ba)/Nb ≤ 0.500 and the TTB-type structure is in (K + Ba)/Nb ≥ 0.575. The dielectric constants of these samples were measured from room temperature to 500 °C for sintered body. The TTB-type compounds exhibited ferroelectric temperature dependence with the Curie points of 284-372 °C and the H-BaTa₂O₆-type compounds were not ferroelectrics as predicted from the crystal structure analysis.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Deposition of Al-doped ZnO thin-films with radio frequency magnetron sputtering for a source/drain electrode for pentacene thin-film transistor

Al-doped ZnO thin-films were deposited with the radio frequency magnetron sputtering technique at various temperatures and sputtering powers for a source/drain electrode in the pentacene thin-film transistor. With the increase in the deposition temperature and the decrease in the radio frequency sputtering power, the crystallinity was increased and the surface roughness was decreased, which lead to the decrease in the electrical resistivity of the film. Al-doped ZnO film deposited at 200 °C and sputtering power of 50 W showed a low resistivity (9.73 × 10⁴ μΩcm), high crystallinity, low roughness and uniform surface morphology. The pentacene thin-film transistor fabricated with Al-doped ZnO film as a source/drain electrode showed a device performance, (mobility: 7.89 × 10^− 3 cm²/Vs and on/off ratio: ~ 5 × 10⁴) which is comparable with an indium tin oxide electrode grown at room temperature.     

Ag and O ion irradiation induced improvement in dielectric properties of the Ba(Co1/3Nb2/3)O3 thin films

We present the preliminary results of temperature and frequency dependent dielectric measurements on Ba(Co_1/3Nb_2/3)O₃ (BCN) thin films. These films were prepared on indium tin oxide (ITO) coated glass substrates by the pulse laser deposition (PLD) technique. It exhibits single-phase hexagonal symmetry. These films were irradiated with Ag¹⁵⁺ (200 MeV) and O⁷⁺ (100 MeV) beams at the fluence 1 × 10¹¹, 1 × 10¹², and 1 × 10¹³ ions/cm². On irradiating these films, its dielectric constant (?′) and dielectric loss (tan δ) parameters improve compared to un-irradiated film. Compared to O⁷⁺ irradiation induced point/cluster defects Ag¹⁵⁺ induced columnar defects are more effective in reducing/pinning trapped charges within grains. The present paper highlights the role of swift heavy ion irradiation in engineering the dielectric properties of conductive samples to enable them to be useful for microwave device applications.     

Polyvinyl chloride seal layer for improving the durability of electrochromic switchable mirrors based on Mg-Ni thin film

Electrochromic switchable mirrors consisting of a multilayer of Mg₄Ni/Pd/Al/Ta₂O₅/H_XWO₃/indium tin oxide on a glass substrate degrade when subjected to changes in humidity and temperature. Polyvinyl chloride was applied as a seal layer to such a device in an attempt to improve its durability. The optical switching properties of the device were investigated through an accelerated degradation test conducted at a constant temperature of 30 °C and a relative humidity of 80%. Although the device with a seal layer was thicker, it showed superior optical switching properties with a transmittance as high as 54% in the transparent state due to interference at the multilayer. Furthermore, the device with a seal layer exhibited higher durability after 35 days in an accelerated degradation environment, as well as higher switching durability.