Electrical,structural and morphological characteristics of rapidly annealed Ni/Pd Schottky rectifiers on n‐type GaN

Schottky rectifiers are fabricated on n‐type GaN using Ni/Pd metallization scheme and its characteristics have been investigated by current‐voltage (I‐V), Capacitance‐Voltage (C‐V), X‐Ray Diffraction (XRD) and SIMS measurements as a function of annealing temperature. The calculated Schottky barrier height of the as‐deposited contact was found to be 0.60 eV (I‐V), 0.71 eV (C‐V) with an ideality factor of 1.44. However, the barrier height slightly increases after annealing at 300, 400 and 500 °C. On the basis of the experimental results, a high‐quality Schottky contact with barrier height and ideality factor of 0.81 eV (I‐V), 0.88 eV (C‐V) and 1.13 respectively, can be obtained after annealing at 600 °C for 1 min in a nitrogen atmosphere. Further, after annealing at 700 °C, it is found that the barrier height slightly decreased to 0.74 eV (I‐V) and 0.85 eV (C‐V). From the above observations, one can note that Ni/Pd Schottky contact exhibits excellent electrical properties after a rapid thermal annealing at 600 °C. According to the SIMS and XRD analysis, the formation of gallide phases at the Ni/Pd/n‐GaN interface could be the reason of the barrier height increase at elevated annealing temperatures. The Atomic Force Microscopy (AFM) results show that the overall surface morphology of Ni/Pd Schottky contacts on n‐GaN is fairly smooth. The above observations reveal that Ni/Pd Schottky metallization scheme was a good choice for the fabrication of high‐temperature and high‐power device applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrical properties of polypyrrole/p‐InP structure

The polypyrrole/p‐InP structure has been fabricated by the electrochemical polymerization of the organic polypyrrole onto the p‐InP substrate. The current–voltage (I–V), capacitance–voltage (C–V), and capacitance–frequency (C–f) characteristics of the PPy/p‐InP structure have been determined at room temperature. The structure showed nonideal I–V behavior with the ideality factor and the barrier height 1.48 and 0.69 eV respectively. C–f measurements of the structure have been carried out using the Schottky capacitance spectroscopy technique and it has been seen that there is a good agreement between the experimental and theoretical values. Also, it has been seen that capacitance almost show a plateau up to a certain value of frequency, after which, the capacitance decreases. The higher values of capacitance at low frequencies were attributed to the excess capacitance resulting from the interface states in equilibrium with the p‐InP that can follow the a.c. signal. The interface state density N_ss and relaxation time τ of the structure were determined from C–f characteristics. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1572–1579, 2006     

Characteristics of metal/p-SnS Schottky barrier with and without post-deposition annealing

With growing interest in SnS for solar photovoltaic device fabrication, the barrier characteristics to this semiconductor with respect to different metal contacts have become increasingly important. In this work we have studied barrier characteristics of polycrystalline SnS thin films metallized with indium, aluminium, copper and silver under different annealing conditions. Indium has been observed to form ohmic contact to p-SnS under all annealing conditions. With the other three metals, Schottky diodes were fabricated and subsequently the contact parameters were extracted under forward bias using indium top contact under different annealing conditions. Although aluminium formed Schottky contact to polycrystalline SnS, annealing at 350 °C rendered it ohmic. EDX analysis confirmed desulfurization from SnS thin films due to annealing. Breakdown voltages of the Al/SnS Schottky barrier diode were determined and were in the decreasing trend with higher annealing temperature, supporting the increase in the doping profile with annealing temperature. Photoluminescence spectra of SnS films were studied and correlated to surface trap centers generated due to annealing.     

Fabrication and electrical characterization of nickel/p-Si Schottky diode at low temperature

In this study the current–voltage and capacitance–voltage characteristics of metal semiconductor Ni/p-Si(100) based Schottky diode on p- type silicon measured over a wide temperature range (60–300 K) have been studied on the basis of thermionic emission diffusion mechanism and the assumption of a Gaussian distribution of barrier heights. The parameters ideality factor, barrier height and series resistance are determined from the forward bias current–voltage characteristics. The barrier height for Ni/p-Si(100) Schottky diode found to vary between 0.513 eV and 0.205 eV, and the ideality factor between 2.34 and 8.88 on decreasing temperature 300–60 K. A plot involving the use of ϕ_b versus 1/T data is used to gather evidence for the occurrence of a Gaussian distribution of barrier height and obtain the value of standard deviation. The observed temperature dependences of barrier height and ideality factor and non-linear activation energy plot are attributed to the Gaussian distribution of barrier heights at the metal-semiconductor contact. The barrier height of Ni/p-Si(100) Schottky diode was also measured over wide temperature from the capacitance-voltage study.     

Schottky barrier and catalytic effect induced high gas sensing of one-step synthesized Pd–SnO2 nanorods

Uniformly loaded Pd–SnO₂ nanorods are synthesized via a simple one-step hydrothermal route. The gas sensors fabricated from Pd–SnO₂ nanorods exhibit high sensitivity and fast response. The sensor response at 300 °C is up to 9.9, 36.8, 55.6, 89.1 and 168.2 upon exposure to 100, 200, 300, 500 and 1000 ppm ethanol, respectively. And the work temperature can be lowered down to 200 °C. Such behaviors can be attributed to Schottky barrier at Pd/SnO₂ interface and catalytic effect of Pd nanoparticles. Our results open a way for uniform modification of SnO₂ nanorods with Pd nanoparticles and enhancing their gas sensing performance.     

The density of interface states and their relaxation times in Au/Bi4Ti3O12/SiO2/n‐Si(MFIS) structures

Bismuth titanate (Bi₄Ti₃O₁₂) (BTO) thin films were fabricated on an n‐type Si substrate and annealed by rapid thermal annealing methods. The I‐V measurement shows that the device has properties of Schottky diode with the Φ_B0 of 0.76 eV, n of 2.42, and leakage current of about 10^?7 A at ? 8 V. The experimental C‐V‐f and G/w‐V‐f characteristics of metal‐ferroelectric‐insulator‐semiconductor (MFIS) structures show fairly large frequency dispersion especially at low frequencies due to interface states N_ss. The energy distribution of (N_ss) has been determined by using the high‐low frequency capacitance (C_HF? C_LF) and conductance method. The relaxation time (τ) of interface states was calculated from the conductance method. It has been shown that both the N_ss and relaxation time increase almost exponentially with bias, which activates traps located at deeper gap energies. Copyright © 2011 John Wiley & Sons, Ltd.     

Conductance and capacitance–frequency characteristics of polypyrrole/p‐type silicon structures

We formed a polypyrrole/p‐type silicon device by an anodization process. An aluminum electrode was used as an ohmic contact. From the current–voltage characteristics of the device, barrier height and ideality factor values of 0.662 eV and 1.734, respectively, were obtained from a forward‐bias current–voltage plot. Low capacitance–frequency and conductance–frequency measurements from 0.00 to 0.30 V with steps of 0.02 V were made. At each frequency, the measured capacitance decreased with increasing frequency because of a continuous distribution of the interface states in the frequency range of 5.0 Hz to 2.0 MHz. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1334–1338, 2003     

Controlling the electrical characteristics of Au/n‐Si structure with and without (biphenyl‐CoPc) and (OHSubs‐ZnPc) interfacial layers at room temperature

In order to interpret well whether or not the organic or polymer interfacial layer is effective on performance of the conventional Au/n‐Si (metal semiconductor [MS]) type Schottky barrier diodes (SBDs), in respect to ideality factor (n ), leakage current, rectifying rate (RR ), series and shunt resistances (R_s , R_sh ) and surface states (N_ss ) at room temperature, both Au/biphenyl‐CoPc/n‐Si (MPS₁) and Au/OHSubs‐ZnPc/n‐Si (MPS₂) type SBDs were fabricated. The electrical characteristics of these devices have been investigated and compared by using forward and reverse bias current–voltage (I–V ) characteristics in the voltage range of (?4 V)–(4 V) for with and without (biphenyl‐CoPc) and (OHSubs‐ZnPc) interfacial layers at room temperature. The main electrical parameters of these diodes such as reverse saturation current (I₀ ), ideality factor (n ), zero‐bias barrier height (Φ_B0 ), RR , R_s and R_sh were found as 1.14 × 10^?5 A, 5.8, 0.6 eV, 362, 44 Ω and 15.9 kΩ for reference sample (MS), 7.05 × 10^?10 A, 3.8, 0.84 eV, 2360, 115 Ω and 270 kΩ for MPS1 and 2.16 × 10^?7 A, 4.8, 0.7 eV, 3903, 62 Ω and 242 kΩ for MPS₂, respectively. It is clear that all of these parameters considerably change by using an organic interfacial layer. The energy density distribution profile of N_ss was found for each sample by taking into account the voltage dependence of effective barrier height (Φ_e ) and ideality factor, and they were compared. Experimental results confirmed that the use of biphenyl‐CoPc and OHSubs‐ZnPc interfacial layer has led to an important increase in the performance of the conventional of MS type SBD. Copyright © 2015 John Wiley & Sons, Ltd.     

Phosphorylated PEG (PPEG) as a new support for generation of nano‐Pd(0): application to the Heck–Mizoroki and Suzuki–Miyaura coupling reactions

Polyethylene glycols (PEGs) with different molecular weights (M_w = 200, 400, 1000) were phosphorylated to their bis‐diphenyl phosphinite derivatives as stable solids which are melted in the range 140–160°C. These phosphorylated PEGs were used as ligands and reducing agents to generate nano‐Pd(0) catalysts in 2.5–8.3 nm. The nano‐Pd(0) particles supported on phosphorylated PEG₂₀₀ were applied for the efficient Heck–Mizoroki carbon–carbon coupling reactions of ArX (X = Cl, Br, I) at 80–100°C under solvent‐free conditions and for the Suzuki–Miyaura coupling reaction in ethanol at 70°C. The catalyst was recycled easily and reused for several runs. Copyright © 2013 John Wiley & Sons, Ltd.     

Photoemission studies of Mn/ZnO(000‐1) interface

The O‐terminated ZnO(000‐1) surface and Mn/ZnO(000‐1) interface have been investigated by synchrotron radiation photoemission spectroscopy (SRPES), low energy electron diffraction (LEED) and X‐ray photoelectron spectroscopy (XPS) systematically. Our results show that ordered O‐polar ZnO(000‐1) surface can be prepared by annealing in an oxygen ambience and this polar surface expresses good chemical stability. At room temperature, metallic Mn film is deposited onto the cleaned ZnO(000‐1)surface and grows in a layer‐by‐layer mode. During the process of Mn film deposition a downward Fermi level movement is observed, and the final resultant Schottky barrier height is 1.07 ± 0.05 eV. High temperature annealing is performed and the interfacial reaction happens evidently. The interfacial chemical reaction and the effect of interfacial dipole layer have been briefly discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Electrical and photoconducting properties of nanorod in based spinel compound/p-Si photodiode by sol–gel spin coating technique

A new Schottky diode (InFe₂O₄/p-Si/Al) was fabricated using the sol–gel spin coating technique. The current–voltage (I–V) characteristics of the Schottky diode were investigated under various illumination intensities. The value of ideality factor (n) and zero-bias barrier height (Φ_B0) for all illuminations was determined by using the forward-bias I–V measurements, and were found to be about 4.20 and 0.72 eV, respectively. The reverse current of the diode in the reverse bias increases with the increasing illumination intensities. Also, the photocurrent under illumination is higher than the dark current. In addition, the capacitance–voltage (C–V) and conductance–voltage (G–V) measurements of the diode were studied in the frequency range of 10 kHz–1 MHz. The measured values of the C decrease with the increasing frequency. The decrease in capacitance was explained on the basis of interface states. To obtain the real C and G of the diode, the measured values of C and G were corrected to eliminate the effect of series resistance. The obtained results suggest that the diode can be used as a photodiode in optoelectronic applications.     

Hydrogen sensing properties of a Pd/SiO2/AlGaN-based MOS diode

Hydrogen sensing properties of a Pd/AlGaN-based Schottky diode are improved by the deposition of SiO₂ at the metal/semiconductor (MS) interface. The wide Schottky barrier height variation of the MOS diode could be attributed to the large electric field across the SiO₂ layer. This leads to the presence of more hydrogen dipoles caused by the polarization effect. The sensing response of the MOS diode at room temperature (1.3 × 10⁵) is comparable to that of the MS one at 150 °C (2.04 × 10⁵). Thus, the MOS-type sensing device shows the benefit of low-temperature operation. Kinetic analyses confirm that the short response times of the MOS diode are attributed to high reaction rate at the Pd/SiO₂ interface.     

Neutron bombardment of single wall carbon nanohorn (SWCNH): DSC determination of the stored Wigner-Szilard energy

Single wall carbon nanohorn (SWCNH) were neutron-bombarded to a dose of 3.28 × 10¹⁶ n/cm². The Wigner or stored energy was determined by a differential scanning calorimeter and was found 5.49 J/g, 50 times higher than the Wigner energy measured on graphite flakes treated at the same neutron dose. The activation energy for the thermal annealing of the accumulated radiation damage in SWCNH was determined in the range 6.3–6.6 eV against a typical activation energy for the annealing of the radiation-damaged graphite which is in the range of 1.4–1.5 eV. Furthermore the stored energy in neutron-damaged SWCNH is released at 400–430 °C while the main peak in the neutron-damaged graphite occurs at 200–220 °C. The radiation damaged SWCNH were examined with FT-IR spectroscopy showing the formation of acetylenic and aliphatic moieties suggesting the aromatic C=C breakdown caused by the neutron bombardment.     

Thermal and spectral stability of electroluminescent hyperbranched copolymers containing tetraphenylthiophene‐quinoline‐triphenylamine moieties

Fluorescent hyperbranched copolymers (HB‐x, x = 1–4) with inherent tetraphenylthiophene, triphenylamine (TPA) and quinoline (Qu) moieties were prepared to study the influence of the TPA branching point on the thermal and the spectral stability. All the HB‐x copolymers exhibited high glass transition temperatures (T_gs = 245–315 °C) with the detected values increasing with the increasing branching TPA content in the HB‐x. The solid HB‐x films possess high emission efficiency with the resulting quantum yields (?_Fs) in the ranges of 0.72–0.74. More importantly, the HB‐x copolymers and the derived light‐emitting devices exhibit high photoluminescence (PL) and electroluminescence (EL) stability towards thermal annealing at temperatures higher than 200 °C. After annealing at 200 °C (or 300 °C), no change was observed in the respective PL and EL spectra of HB‐1 (or HB‐4) copolymers. The spectral stability was found to correlate with T_g and with the highest branching density, HB‐4 copolymer possesses the highest thermal stability among all HB‐xs and show no EL spectral change after annealing at 300 °C for 4 h. The results indicate that all the branched HB‐x copolymers are promising candidates for the polymer light‐emitting diodes due to their high quantum yield and spectral stability. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Surface Dynamics of A Vanadyl Pyrophosphate Catalyst for n‐Butane Oxidation to Maleic Anhydride: An In Situ Raman and Reactivity Study of the Effect of the P/V Atomic Ratio

This work focused on investigating the effect of the P/V atomic ratio in vanadyl pyrophosphate, catalyst for n‐butane oxidation to maleic anhydride, on the nature of the catalytically active phase. Structural transformations occurring on the catalyst surface were investigated by means of in situ Raman spectroscopy in a non‐reactive atmosphere, as well as by means of steady‐state and non‐steady‐state reactivity tests, in response to changes in the reaction temperature. It was found that the nature of the catalyst surface is affected by the P/V atomic ratio even in the case of small changes in this parameter. With the catalyst having P/V equal to the stoichiometric value, a surface layer made of α_I‐VOPO₄ developed in the temperature interval 340–400 °C in the presence of air; this catalyst gave a very low selectivity to maleic anhydride in the intermediate T range (340–400 °C). However, at 400–440 °C δ‐VOPO₄ overlayers formed; at these conditions, the catalyst was moderately active but selective to maleic anhydride. With the catalyst containing a slight excess of P, the ratio offering the optimal catalytic performance, δ‐VOPO₄ was the prevailing species over the entire temperature range investigated (340–440 °C). Analogies and differences between the two samples were also confirmed by reactivity tests carried out after in situ removal and reintegration of P. These facts explain why the industrial catalyst for n‐butane oxidation holds a slight excess of P; they also explain discrepancies registered in the literature about the nature of the active layer in vanadyl pyrophosphate.     

Study of barrier height inhomogeneity at zinc oxide/polymer:fullerene interface in polymer solar cells

Dark current density‐voltage (J‐V) characteristics of polymer solar cell with inverted structure have been measured in the temperature range 190 to 350 K. Ideality factor (n) and dark saturation current (J₀) have been extracted from forward bias J‐V characteristics at different temperatures. The ideality factor is found to decrease, and J₀ increases with the increase in temperature. Estimated zero‐bias barrier height (Φ_B) with the temperature also shows similar trend like J₀. This observed behavior is attributed to the presence of inhomogeneous Schottky barrier at the ZnO/polymer:fullerene interface. Further, this has been confirmed that the barrier height inhomogeneities at the interface have Gaussian distribution with mean zero‐bias barrier height  = 1.15 eV and zero‐bias standard deviation σ_S0 = 0.155 V. We calculated the Richardson constant (A*) using modified versus 1/T plot and obtained value as approximately 48.11 A cm⁻² K⁻², which is close to the value taken from the literature.     

Effect of residual water and free volume on the glass‐transition temperature and heat capacity in polystyrene/poly(vinyl acetate‐co‐butyl acrylate) structured latex films

The dynamic heat capacity and glass‐transition temperature of polystyrene (PS)/poly(vinyl acetate‐co‐butyl acrylate) (VAc–BA) (50:50 w/w) structured latex films as a function of annealing time at 70, 77, and 85 °C were examined with modulated‐temperature differential scanning calorimetry. The PS and poly(vinyl acetate‐co‐n‐butyl acrylate) components were considered to be the cores and shells, respectively, in the structured latex. The dynamic heat capacity decreased with time. The glass‐transition temperatures of the PS and VAc–BA phases shifted to higher values after annealing. The results of thermogravimetry showed that there existed about 1.8% residual water in the films. The mean free volume and relative concentration of holes at room temperature (before and after annealing) and 85 °C, as a function of time, were obtained with positron annihilation lifetime spectroscopy (PALS). The PALS results indicated no significant change in free volume during annealing. It is believed that the loss, by diffusion, of residual water mainly caused a decrease in heat capacity and an increase in the glass‐transition temperatures. As little as 1.8% residual water in the structured latex films had a significant influence on the thermal properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1659–1664, 2001     

Interface formation of metals and poly(p-phenylene vinylene): surface species and band bending

We used X-ray photoemission spectroscopy (XPS) to investigate the surface species of poly(p-phenylene vinylene) (PPV) and its interface formation with Ca and Al. PPV surfaces compositions varied with sample preparation. For relatively "clean'' surfaces with 4–5% O, analysis of the O 1s peak revealed four types of oxygen species, namely carbonyl (C=O), hydroxyl (C–OH), ether (C–O–C) and the carboxylic groups (HO–C=O). The oxygen groups, excluding ether, reacted with Al or Ca to form the corresponding metal oxides. Chemical interactions between the metals and the phenylene and vinylene units to yield new species were not detected. For sulfur-free surfaces, a C 1s peak shift of +0.5 eV followed the deposition of 15–30 Å of Ca on PPV. For sulfur-containing surfaces, the C 1s peak shift was −0.5 eV. We attribute this difference to the interaction of metal atoms with the sulfur impurities. For Al/PPV, a C 1s peak shift occurred at <2 Å of Al deposition and reached a constant value of about +0.4 eV after ⪅8 Å of Al. Again, the direction of the peak shift depended on the presence of sulfur impurities. We attribute the C 1s peak shifts to surface band bending and to Schottky barrier formation. Since surface oxidation of PPV can inhibit band-bending, our overall results suggest that the barrier height at the metal/PPV interface is highly sensitive to the surface preparation and relatively insensitive to the work function of the metals. The shift seen by XPS in the C 1s core level spectra of PPV points clearly to charge transfer and Schottky barrier formation at the interface as a result of metal deposition. These results imply that the metal/polymer interface is not rigid and that triangular barrier tunneling fails to take into account the effect of barrier formation. We propose a band-bending modified tunneling (BBMT) model to explain charge transfer at the Ca/polymer interface. © 1997 John Wiley & Sons, Ltd.     

Synthesis and characterization of new carbazole/fluorene‐based derivatives for blue‐light‐emitting devices

2,7‐Bis(9‐ethylcarbazol‐3‐yl)‐9,9‐di(2‐ethylhexyl)fluorene and a segmented copolymer composed of the same chromophores alternated with hexamethylene fragments were synthesized. The obtained materials possess good solubility in common organic solvents, high thermal stability with 1% weight loss temperature of 350–370 °C, and suitable glass transition temperatures. Both derivatives show blue fluorescence in dilute solutions as well as in solid state, demonstrating that excimers are not formed in the thin films. The fluorescence spectra of the materials do not show any peaks in the long‐wavelength region even after annealing at 200 °C in air. An organic LED with the configuration of ITO/copolymer/Al generates blue electroluminescence with the maximum peak at 416 nm, rather low turn‐on voltage (4.0 V), and brightness of about 400 cd/m². The heterostructure device based on model derivative emitted stable blue light with low operation voltage (100 cd/m² at ～11 V) and demonstrated luminescence efficiency of 0.8 cd/A. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5987–5994, 2006     

Current–voltage and capacitance–voltage characteristics of the ITO/polyaniline doped boron trifloride/Al Schottky diode

The electrical characteristics of the ITO/polyaniline (PANI) doped boron trifloride (BF₃)/Al Schottky diode have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) methods. The diode indicates a rectification behavior with the ideality factor of 4.78. An ideality factor higher than unity can result from the interface state and electronic properties of the PANI doped BF₃ organic semiconductor. The barrier height of the diode was determined from both I–V and C–V characteristics. The barrier height obtained from the C–V measurements is higher than that obtained from the I–V measurements. At higher forward bias voltages, the space charge‐limited current is the dominant transport mechanism, whereas at reverse bias voltages, the current flow in the ITO/PANIBF₃/Al diode is controlled by Schottky emission mechanism. Copyright © 2008 John Wiley & Sons, Ltd.