Effect of electron irradiation on characteristics of Mo Schottky barriers on epitaxial silicon

The results of studies on influence of 6 MeV electron irradiation on avalanche breakdown voltage (U_b) and on forward voltage (U_F) at different values of direct current (I_F) for the Mo Schottky diodes on epitaxial silicon of n-type conductivity are presented. It was found out that the avalanche breakdown voltage of the diodes is very sensitive to electron irradiation. A decrease in U_b was observed after electron irradiation with a fluence as low as 1 × 10¹¹ cm^?2. An increase in electron irradiation fluence from 1 × 10¹¹ cm^?2 to 5 × 10¹⁴ cm^?2 resulted in 30% decrease in U_b, however, further increase in electron irradiation fluence from 5 × 10¹⁴ cm^?2 to 3 × 10¹⁶ cm^?2 led to some increase in the avalanche breakdown voltage. Monotonic increases in U_F values at different I_F with the increase in electron irradiation fluence were observed starting from a fluence of 5 × 10¹⁴ cm^?2. Radiation-induced changes in U_b were unstable at room temperature and a significant recovery of U_b occurred after maintaining the irradiated diodes at room temperature for 30 days. Annealing at 120 °C for 20 min resulted in the almost complete recovery of U_b. Radiation-induced changes in U_F values were stable up to 300 °C. Mechanisms of the observed radiation-induced changes in the U_b and U_F values and defects responsible for the changes are discussed.     

Electrical transport characteristics of Au/n-GaN Schottky diodes

The current–voltage measurements were performed in the temperature range (80–300 K) on Au/ n-GaN Schottky barrier type diodes. The Schottky diode shows non-ideal I(V_G) behaviour with ideality factors n equals to 1.18 and 1.81 at 300 K and 80 K, respectively, and are thought to have a metal-interface layer-semiconductor configuration. Under forward bias and for T ≥ 200 K, the electrical current transport was controlled by the thermionic emission (TE) process. However, for T ≤ 200 K, The current was controlled by the thermionic field emission (TFE). The characteristic energy E₀₀ = 3.48 meV was obtained from the I(V_G, T) measurements and agreed very well with the value of E₀₀ = 3.62 meV calculated theoretically. The zero-bias barrier height ϕ_B0 determined from the I(V_G) measurements was 0.84 eV at 300 K and decreases to 0.49 eV at 80 K.     

Electrical and photocurrent characteristics of Au/PVA (Co-doped)/n-Si photoconductive diodes

In this work, we investigate the some main electrical and photocurrent properties of the Au/PVA(Co-doped)/n-Si diodes by using current–voltage (I–V) measurements at dark and various illumination intensity. Two types of diodes with and without polyvinyl alcohol (PVA) (Co-doped) polymeric interfacial layer were fabricated and measured at room temperature. Results show that the polymeric interfacial layers and series resistance (R_s) strongly affect the main electrical parameters of these structures. Also, metal/polymer/semiconductor (MPS) diode with PVA (Co-doped) interfacial organic layer is very sensitive to the light such that the current in reverse bias region increase by 10³–10⁴ times with the increasing illumination intensity. The open circuit voltage V_oc and short-circuit current I_sc values of this MPS diode under 100 mW/cm² illumination intensity were found as 0.28 V and 19.3 μA, respectively.     

Analysis of the device characteristics of AlGaN/GaN HEMTs over a wide temperature range

In this study, we investigate the behavior of the current–voltage (I–V) characteristics of AlGaN/GaN HEMT in the temperature range of 223–398 K. Temperature dependent device characteristics and the current transport mechanism are reported. It is observed that the Schottky barrier height Φ increases and the ideality factor n decreases with temperature. There is a linear relationship between the barrier height and the ideality factor, which is attributed to barrier height inhomogeneities of AlGaN/GaN HEMT. The estimated values of the series resistances (R_s) are in the range of 144.2 Ω at 223 K to 74.3 Ω at 398 K. The Φ, n, R_s, G_m and Schottky leakage current values are seen to be strongly temperature dependent.     

Effects of the intercalation time on the dielectric properties of Na2xMn1−xPS3

Manganese thiophosphate powders have been intercalated with sodium ions at two different intercalation times (150 min and 180 min) in order to evaluate the influence of this parameter on the dielectric properties of the obtained compounds labeled like Na_2xMn_1?xPS₃. In particular, dielectric permittivity measures have been carried out as a function of temperature (80 K–350 K) and frequency (20 Hz to 1 MHz) and compared with each other and with those of the pure matrix and of the Na_2xMn_1?xPS₃ compound corresponding to a 120 min intercalation time. Both investigated compounds show a dielectric response characterized by a loss peak at low temperatures, by a strong dispersion at low frequency and at not very high temperatures and by a crossover frequency showing an Arrhenius temperature dependence in agreement with what already observed in Na_2xMn_1?xPS₃ (120 min). The results have been also discussed in terms of ac conductivity, Maxwell–Wagner–Sillar (MWS) polarization, electrode polarization and dc conductivity and all the above cited features have been attributed to the Na⁺ ions which are, by their nature, hopping charge carriers that behave like jumping dipoles in their alternate motions and simultaneously show conducting characteristics resulting from their extended hopping over many sites. This attribution allows us to classify the analyzed sodium compounds like hopping charge carrier systems in which a greater Na intercalation time translates into a decrease in the loss peak intensity and an increase in the activation energy associated to the crossover frequency.     

Enhanced field emission from the ZnO nanowires by hydrogen gas exposure

ZnO nanowires (ZNWs) were synthesized on Co-coated Si wafer via a carbon thermal reduction vapor transport method. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy investigations show that these ZNWs present a high-quality single-crystalline hexagonal structure. Field emission (FE) characteristics of the ZNWs film were measured. A low turn-on voltage for driving a current density of 0.1 μA/cm² is about 3.9 V/μm. The field enhancement factor was determined to be ∼ 1180 for ZNWs film. Exposure of H₂ during FE causes a permanent increase in the FE current and a decrease in the turn-on field. Also, the field enhancement factor γ was finally increased from 1180 ± 20 to 1510 ± 20 after FE saturation.     

Influence of copper substitution on magnetic and electrical properties of MgCuZn ferrite prepared by microwave sintering method

Polycrystalline MgCuZn ferrites with chemical formula Mg_0.50-xCu_xZn_0.50Fe₂O₄ (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared by microwave sintering method. These powders were calcined, compacted and sintered at 950 °C for 30 min. Structural, microstructural and elemental analyses were carried out using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS), respectively. The lattice parameter is found to increase with increasing copper content. A remarkable densification is observed with the addition of Cu ions in the ferrites. The sintered ferrite was characterized for initial permeability, dielectric constant and dielectric loss tangent and ac conductivity measurements. The temperature variation of the initial permeability of these samples was carried out from 30 °C to 200 °C. The dielectric constant, dielectric loss tangent and ac conductivity have been measured in the frequency range of 100 Hz to 1 MHz. Initial permeability and dielectric constant were found to increase and dielectric loss decreased with Cu substitution for Mg, up to x = 0.20. The ferrite powder prepared is suitable for the application in multilayer chip inductor due to its low-temperature sinterability, good magnetic properties and low loss at high frequency.     

Formation and investigation of p–n diode structures based on lanthanum manganites and Nb-doped SrTiO3

High quality La_2 / 3Ba_1 / 3MnO₃ (LBMO), La_2 / 3Ca_1 / 3MnO₃ (LCaMO) and La_2 / 3Ce_1 / 3MnO₃ (LCeMO) thin films were grown on Nb 0.1 wt.% doped conducting SrTiO₃(100) (STON) substrates. Asymmetric current–voltage relations measured for the LBMO/STON, LCaMO/STON and LCeMO/STON heterostructures at T = 78/300 K certified hole-doping of the manganite films. The diffusion voltage, corresponding to a steep current increase at forward bias has been estimated. The LCaMO/STON heterojunction showed possible impact of interfacial strain on the rectifying behavior, meanwhile, the LaCeMO/STON heterostructures demonstrated evidence of phase separation of the manganite film at the interface.     

AC loss in YBCO coated conductors at high dB/dt measured using a spinning magnet calorimeter (stator testbed environment)

A new facility for the measurement of AC loss in superconductors at high dB/dt has been developed. The test device has a spinning rotor consisting of permanent magnets arranged in a Halbach array; the sample, positioned outside of this, is exposed to a time varying AC field with a peak radial field of 0.566 T. At a rotor speed of 3600 RPM the frequency of the AC field is 240 Hz, the radial dB/dt is 543 T/s and the tangential dB/dt is 249 T/s. Loss is measured using nitrogen boiloff from a double wall calorimeter feeding a gas flow meter. The system is calibrated using power from a known resistor. YBCO tape losses were measured in the new device and compared to the results from a solenoidal magnet AC loss system measurement of the same samples (in this latter case measurements were limited to a field of amplitude 0.1 T and a dB/dt of 100 T/s). Solenoidal magnet system AC loss measurements taken on a YBCO sample agreed with the Brandt loss expression associated with a 0–0.1 T I_c of 128 A. Subsequently, losses for two more YBCO tapes nominally identical to the first were individually measured in this spinning magnet calorimeter (SMC) machine with a B_max of 0.566 T and dB/dt of up to 272 T/s. The losses, compared to a simplified version of the Brandt expression, were consistent with the average I_c expected for the tape in the 0–0.5 T range at 77 K. The eddy current contribution was consistent with a 77 K residual resistance ratio, RR, of 4.0. The SMC results for these samples agreed to within 5%. Good agreement was also obtained between the results of the SMC AC loss measurement and the solenoidal magnet AC loss measurement on the same samples.     

The simulation of the electrochemical cathodic Ca–P deposition process

Electrochemical cathodic deposition has been widely employed for fabrication of Ca–P films onto surface of titanium. However, systematical and theoretical analyses for this process are rarely reported. In this study, a model of Ca–P nucleation for electrochemical deposition process was proposed based on classical crystallization theories of thermodynamics and kinetics. The obtained results from this model referred to that: the following results are obtained: the Ca–P phase precipitated on titanium is DCPD under lower loading current (I₀ < 0.5 mA) and shorter loading time (t < 50 s); the preferential precipitated phase on titanium is OCP under higher loading current (I₀ > 0.5 mA) and longer loading time (t > 50 s). Inspiringly, this theoretical model was successfully validated with our experimental results analyzed through high-resolution TEM micrographs.     

Temperature dependent transport properties in molybdenum oxide doped α-NPD

The temperature dependent transport properties of molybdenum oxide (MoO₃) doped N,N′-di(1-naphthyl)-N,N′-diphenylbenzidin (α-NPD) were studied over a frequency range of 100 Hz to 1 MHz. The value of trap density and mobility calculated by detailed analysis of current–voltage (I–V) characteristics are 9.43 × 10²⁶ m^?3 and 1.23 × 10^?6 cm² V^?1 s^?1, respectively. The relaxation time for the carriers in the bulk and in the interface region decreases with temperature. The Cole–Cole plot indicates the device can be modeled as the combination of two parallel resistor–capacitor (R–C) circuits with a series resistance of around 70 Ω. The dc conductivity shows two different regions in the studied temperature range with activation energy of E_a ～ 0.107 eV (region I) and E_a ～ 52 meV (region II), respectively. The ac conductivity follows the universal power law and the onset frequency increases with increase of temperature. The temperature dependent conduction mechanism can be explained by correlated hopping barrier (CBH) model.     

Effects of LiF/Al back electrode on the amorphous/crystalline silicon heterojunction solar cells

To improve the quantum efficiency (QE) and hence the efficiency of the amorphous/crystalline silicon heterojunction solar cell, we have employed a LiF dielectric layer on the rear side. The high dipole moment of the LiF reduces the aluminum electrode's work–function and then lowers the energy barrier at back contact. This lower energy barrier height helps to enhance both the operating voltage and the QE at longer wavelength region, in turn improves the open-circuit voltage (V_oc), short-circuit current density (J_sc), and then overall cell efficiency. With optimized LiF layer thickness of 20 nm, 1 cm² heterojunction with intrinsic thin layer (HIT) solar cells were produced with industry-compatible process, yielding V_oc of 690 mV, J_sc of 33.62 mA/cm², and cell efficiencies of 17.13%. Therefore LiF/Al electrode on rear side is proposed as an alternate back electrode for high efficiency HIT solar cells.     

Study on the ITO work function and hole injection barrier at the interface of ITO/a-Si:H(p) in amorphous/crystalline silicon heterojunction solar cells

For this study we focused on the front contact barrier height of HIT (ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)) solar cell. The ITO films with low resistivity of 1.425 × 10^?4 Ω cm were deposited by pulsed DC magnetron sputtering as a function of substrate temperature (T_s). There were improvement in Φ_ITO from 4.15 to 4.30 eV and delta hole injection barrier from 0 to 0.129 eV for the HIT solar cell. The results show that the high values of Φ_ITO and the delta hole injection barrier at the front interface of ITO/p-layer lead to an increase of open circuit voltage (V_oc), fill factor (FF) and efficiency (η). The performance of HIT device was improved with the increase of T_s and the best photo voltage parameters of the device were found to be V_oc = 635 mV, FF = 0.737 and η = 14.33% for T_s = 200 °C.     

Synergetic effects of pulse constraints and additives in electrodeposition of nanocrystalline zinc: Corrosion,structural and textural characterization

Pulse electrodeposition was to produce nanocrystalline (nc) zinc from alkaline non-cyanide electrolyte with primary and secondary additives. The combined effect of pulse parameters (ON-time (T_ON), OFF-time (T_OFF), pulse peak current density (I_P)) and additives on the corrosion properties (evaluated using electrochemical techniques) of zinc electrodeposits are elucidated in terms of surface morphology (using scanning electron microscope), topography and root mean square (RMS) roughness (using atomic force microscope), crystallite size, its orientations and relative texture co-efficient (RTC, %) were evaluated using X-ray diffraction. The corrosion resistance of zinc electrodeposits obtained at constant T_ON and I_P enhanced (i.e., low I_corr and high R_ct values) with increased T_OFF. At constant T_OFF and I_P, the I_corr values increased and R_ct values decreased with T_ON while the former decreases and latter increases with I_P at constant T_ON and T_OFF. The inclusion of primary and secondary additives into the electrolyte produced nc zinc electrodeposits at 5 Adm^?2, showed enhanced protective properties (I_corr—16 μA cm^?2 and R_ct—481.8 Ω cm^?2). Fine grained due to high negative overpotential, reduced roughness and higher percentage of basal plane [0 0. 2] orientation have major impact for the enhanced corrosion resistances.     

Synthesis of As-grown superconducting MgB2 thin films by sputtering method

As-grown superconducting MgB₂ thin films were prepared by very simple RF sputtering method using a single target without post-annealing process. A two-component target comprising of many small chunks of B on a Mg disk was used. The films were deposited onto polished single crystal sapphire (R-plane) or SrTiO₃ (1 1 1) substrates at rather low temperatures ranging from 100 to 110 °C. Five samples on SrTiO₃ substrate and one on sapphire showed zero resistivity at about 15 K and onset at about 20 K with measuring the resistance versus temperature (R–T curves) and the current versus voltage curves (I–V curve).     

Effect of magnetic fields on the abnormal electroresistance behavior in epitaxial thin films of La0.8Ca0.2MnO3

We report an unusual electroresistance (ER) behavior induced by a current and its response to magnetic fields in La_0.8Ca_0.2MnO₃ epitaxial thin films. These thin films were fabricated on SrTiO₃ (1 0 0) substrate using pulsed laser deposition (PLD) technique. It is found that the electric resistivity in these films is significantly enhanced by applying a dc current over a threshold value. Simultaneously, an abnormal electroresistance behavior appears in the temperature range from 10 to 300 K. The enhanced resistance turns out to be very sensitive to a weak current. Even a very small dc current can remarkably depress the high resistance, showing an unusual colossal ER effect. The ER reaches ∼1175% at temperatures lower than ∼50 K, and ∼705% at 300 K for a current changing from 0.72 to 10.5 μA. The influence of magnetic fields on the transport was also studied. The I–V curves can be strongly influenced by a low magnetic field even at room temperature. The deduced magnetoresistance (MR) reaches 120% at 300 K upon applying a magnetic field of 0.25 T. An interesting phenomenon is that the observed MR is current dependent.     

Variants of the Bi6TiP2O16 structure: The preparation and crystal structure of the isomorph Bi6(Mn0.6Bi0.4)P2O15

Compounds with the composition Bi₆(Bi_1−yM_y)X₂O_16−z, M = transition metal or Pb, X = P, V, As, display pseudo-tetragonal crystal systems. They are, however, monoclinic with space group I2 and the heavy atom positions mimic the δ-Bi₂O₃ structure. The title compound is monoclinic, a = 11.284(2) Å, b = 5.4259(11) Å, c = 11.112(2) Å, β = 96.25(3)°, I2, Z = 2. Least-squares refinement of single-crystal X-ray diffraction data on F² converged to R1 = 0.050, wR2 = 0.130. The crystal is twinned by two-fold rotation about [0 1 0] and each twin consists of its inverted component forming a racemate. The structure consists of chains of edge sharing (OBi₄) tetrahedra parallel to [1 0 −1]. The chains are bridged parallel to [1 0 1] by linked PO₄ tetrahedra and (Mn/Bi)O₆ octahedra parallel to [1 0 −1], into a three-dimensional structure. The lone-pair electrons of adjacent Bi atoms along the chain point in opposite directions along the b-axis. The Bi atoms are in distorted trigonal prismatic coordination that has one or two faces capped. The BiO bond lengths vary from 2.08(5) to 3.05(2) Å. The Mn/Bi atoms are disordered around the two-fold axis. Three oxygen atom sites contain vacancies.     

Preparation and thermomechanical properties of Ag-PVA nanocomposite films

Metal–polymer hybrid nanocomposites have been prepared from an aqueous solution of polyvinyl alcohol (PVA) and silver nitrate (AgNO₃). The silver nanoparticles were generated in PVA matrix by the reduction of silver ions with PVA molecule at 60–70 °C over magnetic stirrer. UV–vis analysis, X-ray diffraction studies, transmission electron microscopy, scanning electron microscopy and current–voltage analysis were used to characterize the nanocomposite films prepared. The X-ray diffraction analysis reveals that silver metal is present in face centered cubic (fcc) crystal structure. Average crystallite size of silver nanocrystal is 19 nm, which increases to 22 nm on annealing the film at 150 °C in air. This result is in good agreement with the result obtained from TEM. The UV–vis spectrum shows a single peak at 433 nm, arising from the surface plasmon absorption of silver nanocolloids. This result clearly indicates that silver nanoparticles are embedded in PVA. An improvement of mechanical properties (storage modulus) was also noticed due to a modification of PVA up to 0.5 wt% of silver content. The current–voltage (I–V) characteristic of nanocomposite films shows increase in current drawn with increasing Ag-content in the films.     

Microstructure and transport properties of [0 0 1]-tilt bicrystal grain boundaries in iron pnictide superconductor,cobalt-doped BaFe2As2

Relationships between microstructure and transport properties of bicrystal grain boundary (BGB) junctions were studied in cobalt-doped BaFe₂As₂ (BaFe₂As₂:Co) epitaxial films grown on [0 0 1]-tilt bicrystal substrates of MgO and (La, Sr)(Al, Ta)O₃ with misorientation angles θ_GB = 3–45°. The θ_GB of BaFe₂As₂:Co BGBs were exactly transferred from those of the bicrystal substrates. No segregation of impurities was detected at the BGB junction interfaces, and the chemical compositions of the BGBs were uniform and the same as those in the bulk film regions. A transition from a strongly-coupled GB behavior to a weak-link behavior was observed in current density–voltage characteristics under self-field around θ_GB ～ 9°. The critical current density decreased from (1.2–1.6) × 10⁶ A/cm² of the intragrain transport to (0.7–1.1) × 10⁵ A/cm² of θ_GB = 45° because supercurrent becomes more governed by Josephson current with increasing θ_GB.     

Investigation on structural,optical and dielectric properties of Co doped ZnO nanoparticles synthesized by gel-combustion route

We report the synthesis of Co doped ZnO nanoparticles by combustion method using citric acid as a fuel for 0%, 1%, 3%, 5% and 10% of Co doping. The structural, optical and dielectric properties of the samples were studied. Crystallite sizes were obtained from the X-ray diffraction (XRD) patterns whose values are decreasing with increase in Co content up to 5%. The XRD analysis also ensures that ZnO has a hexagonal (wurtzite) crystal structure and Co²⁺ ions were successfully incorporated into the lattice positions of Zn²⁺ ions. The TEM image shows the average particle size in the range of 10–20 nm for 3% Co doped ZnO nanoparticles. The energy band gap as obtained from the UV–visible spectrophotometer was found gradually increasing up to 5% of Co doping. The dielectric constants (?′, ?″), dielectric loss (tan δ) and ac conductivity (σ_ac) were studied as the function of frequency and composition, which have been explained by ‘Maxwell Wagner Model’.