Schottky-type Barrier Research Articles

Resistance to Pressure in Laser-doped p–n and Schottky-type Barrier Junctions

Resistance to Pressure in Laser-doped p–n and Schottky-type Barrier Junctions

Effects of Schottky barrier height fluctuations on conductivity: Consequences on power-law response in tin oxide gas sensors

Potential barriers formed at the intergrain of polycrystalline metal-oxide semiconductors, such as tin oxide, present fluctuations that are usually ignored in the study of gas sensors responses. Fluctuations in the Schottky-type barriers arise from the punctual character and distribution of the charges at the depletion region. Accordingly, we present a numerical model that allows to determine the distribution of the barrier heights. We then analyze the influence of fluctuations on the electrical intergrain conductivity as a function of oxygen pressure, since the signal detection in polycrystalline metal-oxide semiconductors gas sensors comes from the change of material conductivity after interaction with target gas molecules. To do so, we incorporate the fluctuation distribution found in our numerical model in the calculations of conductance. Based on these results, we could also relate the oxygen pressure with the adsorbate coverage and the fluctuating intergranular barrier. Finally, consequences on the known power-law response for gas sensors are discussed.

Role of deep levels and barrier height lowering in current-flow mechanism in 150 μm thick epitaxial n-type 4H–SiC Schottky barrier radiation detectors

Schottky barrier detectors (SBDs) require larger surface areas than conventional electronics to increase the detection efficiency although such SBDs manifest large diode ideality factors due to inhomogeneous areal distribution of surface barrier height (SBH). Inhomogeneous SBH distributions lead to various current flow mechanisms in SBDs, which need to be identified to optimize detector performance. In this Letter, we identify the current flow mechanism in large area Schottky barrier diodes for radiation detection fabricated on 150 μm thick n-4H–SiC epitaxial layers. The analysis of temperature-dependent forward current–voltage (I–V–T) characteristics of SBDs revealed two linear regions in current–voltage curves up to 450 K, one corresponding to the current flow through a low barrier patch, while the other corresponds to that of average barrier distribution. Applying a SBH distribution model to the reverse I–V–T characteristics, an activation energy of 0.76 eV for the current flow over the Schottky barrier was calculated. The activation energy did not directly correspond to any of the defect levels observed from the deep level transient spectroscopy (DLTS). Above 450 K, a Schottky type barrier lowering suggested a current flow through a low barrier patch of ≈ 0.8 eV. The absence of any SBH lowering below 450 K indicated that the current corresponded to a neutrally charged trap level at ≈ 0.6 eV below the conduction band edge, which was consistent with DLTS measurements revealing the presence of an electron trap level Z1/2 at 0.59 eV below the conduction band edge.

Evaluation of Ni0, NiO, and NiS as a Cocatalyst Modifier on TiO2 Nanotubes Matrix for the Enhancement of Photoelectrocatalytic Oxidation of Penicillin G

The development of an effective cocatalyst for the modification of TiO2 arrays has become highly desirable for the improvement of their photoelectrocatalytic properties. We investigated the modification of TiO2 with nickel-based cocatalysts (nickel (Ni0), nickel oxide (NiO), and nickel sulphide (NiS)) by electrochemical deposition. The formation of n-p heterojunctions for the Ti/TiO2/NiO and Ti/TiO2/NiS and the generation of Schottky-type barrier for the Ti/TiO2/Ni0 led to the enhancement of visible light absorption and the reduction of the band gap energy, contributing significantly to improving the electrogeneration of the electron−hole and lifetime of the holes. The Ti/TiO2/NiS photocurrent was found to be almost two times higher than that of the Ti/TiO2 (5.39 mA.cm−2 at +1.50 V); this result was attributed to the excellent distribution of the cocatalyst around the TiO2 nanotube walls, which helped enhance the chemical interaction on the TiO2/NiS interface. Under optimized conditions (+0.50 V and UV–vis irradiation), the application of the Ti/TiO2/NiS electrode for the photoelectrocatalytic treatment of penicillin-G—(PG) (10 mg l−1) resulted in complete removal and mineralization after 60 min at pH 7. The results showed that one can potentially increase the photoelectrocatalytic efficiency of Ti/TiO2 through a simple modification of the material.

Resistance Switching Effect in Octahedral framework oxide

Resistive Random-Access Memories (ReRAM) are an alternative way to create new memory devices. This is physically possible due to the existence in the material, of two resistive states clearly discreditable, as a function of voltage value and polarity first parameter under control to pass from one state to another one. However, the mechanism of the resistance switching is not simple and is under debate. We present in the present chapter all the factors entering in the switching process in tetragonal tungsten bronze (TTB) type structure oxide thin films deposited by PLD technique onto MgO or STO substrates. Results show that GdK2Nb5O15 (GKN) thin films deposited on MgO and STO substrates are resistively switchable. It was found that the nature of the substrate strongly affects the resistance ratio: GKN on SRO/LSCO/MgO showed a large hysteresis compared to GKN on SRO/STO. Substrate effect and oxygen vacancy on resistance switching in GKN thin film were studied in the same experimental conditions. The study of resistance switching in the GKN/MgO and GKN/STO thin films has confirmed that for low voltages, below the threshold value of 1.3 V, the electric transport is dominated by the formation of a Schottky type barrier, which allows a minimum leakage current. Resistance switching in GKN is attributed to the oxygen vacancies migration which can be controlled by the substrate or the frequency sweep.

Impedance spectroscopy of Al/AlN/n-Si metal-insulator-semiconductor (MIS) structures

In this work, a comprehensive characterization of metal-insulator-semiconductor structures by impedance spectroscopy is demonstrated for the case of electrically insulating, highly c-axis oriented, 600 nm sputter-deposited AlN films on n-Si substrates with Al top electrodes. Direct visual analysis and equivalent circuit fitting of the dielectric data were performed. For the latter procedure, the circuit model consisted of three series resistor-capacitor connection elements for the three dielectric contributions detected. The three contributions were identified as the AlN film, n-Si substrate, and an interface barrier effect. Several essential device parameters were determined separately, by visual or equivalent circuit fitting analysis, such as the dielectric permittivity of the AlN layer, the temperature dependence of the AlN permittivity, and the resistances of the AlN layer, the n-Si substrate, and the interface contribution. Furthermore, DC bias dependent impedance measurements allowed the identification of a Schottky-type interface barrier.

Effect of thermal history on structural, microstructural properties and J – E characteristics of CaCu3Ti4O12 polycrystalline ceramic

We report, the effect of heat treatments slow cooling, rapid thermal cooling (quenching) and microwave-assisted heating, on structural, microstructural and J – E characteristics of CaCu3Ti4O12 polycrystalline ceramic. Rietveld refinement of X-ray data confirms the emergence of the mono-phasic cubic perovskite structure, diffraction line broadening and change in the relative intensity of diffraction peaks while scanning electron microscopy revealed completely different grain morphology. The observed modifications have been discussed in detail. The strong nonlinear J – E characteristics over the wide temperature range (300–823 K) indicated the existence of a Schottky-type electrostatic potential barrier at the grain boundaries. The parameters, nonlinear coefficient, threshold electric field, potential barrier height suggest suitability of the materials for low voltage varistor applications.

Non-ohmic properties of CaCu3Ti4O12 thin films deposited By RF-sputtering

Calcium copper titanate (CaCu3Ti4O12, CCTO), thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO2/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The CCTO thin film present a cubic structure with lattice parameter a = 7.379 ± 0.001 A free of secondary phases. Dielectric spectroscopy was employed to examine the polycrystalline behaviour of CCTO material and the mechanisms responsible for the barrier-layer capacitances associated with Schottky-type barriers and the non-Ohmic properties. The film presents an electric breakdown field (Eb = 203 V cm−1) and then nonlinear coefficient (α = 6), which is even lower than that of the ZnO and SnO2 based varistors The observed electrical features of CCTO thin films are highly dependent on the [CaO12], [CaO4], [CuO11], [CuO11Vo x] and [TiO5· $$V_{O}^{ \bullet }$$ ] clusters.

Enhancement of non-linear properties in SnO2 varistors by ZnO doping

Abstract The effect of Zn doping content in SnO 2 -based varistors was investigated. The general formula was ((Sn 0.985 Co 0.01 Nb 0.005 O β ) 1−x (ZnO) x ) ; x = 0, 0.005, 0.01, 0.02 and 0.04. The powders synthesized by chemical precipitation were 4.4 – 6.1 nm in crystallite size along both [110] and [101]. Only cassiterite (SnO 2 ) phase was observed within the limit of detection of X-ray diffraction (XRD). Zn doping yielded great enhancement in sinterability. High densities were achieved in the x = 0.04 sample at 1150 °C whereas it took 100 °C higher to achieve the similar density for the undoped sample. Substitution of Zn cations into the Sn lattice was believed to generate oxygen vacancies, facilitating diffusion and mass transport. As a result, the average grain sizes initially increased at x = 0.005 and gradually decreased at higher Zn content possibly due to existence of secondary phases. The optimal composition with the greatest breakdown voltage (6384 V cm −1 ) and non-linear coefficient (17.2) occurred at x = 0.02. This improvement in varistor properties has been attributed by formation of more effective Schottky-type potential barrier. The correlations between varistor properties and other microstructural aspects were also discussed.

Preparation and characterization of mixed caesium-tin mixed salt of Keggin-type phosphovanadomolybdate. Application to photocatalytic chromate reduction

In this study, we are interested by the caesium-tin mixed salt of 1-vanado-11-molybdophosphoric acid (CsSnPMo11VO40) with Cs/Sn ~ 1, micro and mesoporous material with a large specific surface area and a good thermal stability. Caesium atoms in Cs3HPMo11VO40 are partially substituted by tin in order to facilitate the electron transfer in the redox processes. The synthesis and characterization by BET measurement, inductively coupled plasma (ICP), X-ray diffraction (XRD), 31P NMR, thermal analysis (TGA and DTA), UV–Vis and Raman spectroscopies, infrared technique of the Keggin-type compound CsSnPMo11VO40 are reported. The forbidden band (1.72eV) is due to the crystal field splitting of Mo6+: 4d orbital in octahedral site and the compound exhibits a chemical stability at neutral pH and semiconducting properties due to molybdenum mixed valences. The intensity-potential (J-E) characteristic shows a small hysteresis while the capacitance measurement (C−2 - E) of the junction CsSnPMo11VO40/electrolyte in Na2SO4 electrolyte exhibits a rectifying behavior with a Schottky type barrier; the positive slope is characteristic of n type conduction with a flat band potential of −0.30 VSCE. The electrochemical impedance spectroscopy (EIS) shows two overlapped semicircles attributed to the bulk contribution (Rb=168Ωcm2) and grains boundaries (Rgb=806Ωcm2). The depletion angle (−16°) is ascribed to a constant phase element (CPE, 67µFcm−2). The photosensitivity in the visible light involves d–d transition and the energy diagram shows the feasibility of the material for the HCrO4- reduction upon visible light (109mWcm−2). The disappearance rate is found to be 4mmolmn−1 (mg catalyst)−1 and the chromate reduction obeys to a first order kinetic with a half-life of 83mn.

The lowered dielectric loss tangent and grain boundary effects in fluorine-doped calcium copper titanate ceramics

The effects of F− concentration on the dielectric properties and electric modulus of CaCu3Ti4O12−x F x ceramics (0 ≦ x ≦ 0.8) were investigated in detail. Proper amount of F− substitution in CaCu3Ti4O12−x F x ceramics reduced the dielectric loss tangent. When x = 0.8, the dielectric loss tangent of CaCu3Ti4O12−x F x ceramic was about 0.05 (at 5 kHz), which was the lowest value and exhibited the highest grain boundary resistance (1.18 MΩ); meanwhile, the samples retained a relatively high dielectric constant above 7000 over a wide frequency range of 100 Hz to 1 MHz. The grain size was reduced as the F− doping concentration increased, which enhanced grain boundary resistance, and the decreased dielectric loss tangent was attributed to the increased grain boundary resistance. With the increase of F− concentration, the dielectric relaxation behaviors correlated with the grain boundary effects were significantly enhanced. The thermal-activated Schottky-type barrier was formed in the grain boundaries, and CaCu3Ti4O12−x F x ceramics had a nonlinear-Ohmic property.

Effect of semiconductive grain and microstructure on the dielectric properties of CaCu3Ti4O12 ceramics with Sr2+ doping

Abstract Compositions based on Ca1−xSrxCu3Ti4O12(x = 0, 0.05, 0.1, 0.2) were prepared via the conventional solid state synthesis methods, influence of semiconductive grain and microstructure on the dielectric properties of CaCu3Ti4O12 ceramics was studied. UV–vis absorption spectra and photoluminescence measurement indicated that the semiconductive of grains changed with Sr2+ doping, and the band gap of materials increased from 2.25 eV to 2.30 eV. According to the model Schottky-type barrier formed between the grain boundaries (GBs), we analyzed that increased band gap would lead to increase of the potential barrier of height, thus grain boundary resistance increased and the dielectric constants decreased. However, microstructure of Ca1−xSrxCu3Ti4O12 contributed to the dielectric properties were researched, the grain size became larger with Sr2+ doping, thus the dielectric constants increased and grain boundary resistance decreased with Sr2+ doping. The results showed that Schottky-type barriers between GBs were the factors to form the special dielectric properties of CCTO, and microstructure would influence the dielectric properties notably.

First-Principles Study of the Band Diagrams and Schottky-Type Barrier Heights of Aqueous Ta3N5 Interfaces.

The photo(electro)chemical production of hydrogen by water splitting is an efficient and sustainable method for the utilization of solar energy. To improve photo(electro)catalytic activity, a Schottky-type barrier is typically useful to separate excited charge carriers in semiconductor electrodes. Here, we focused on studying the band diagrams and the Schottky-type barrier heights of Ta3N5, which is one of the most promising materials as a photoanode for water splitting. The band alignments of the undoped and n-type Ta3N5 with adsorbents in a vacuum were examined to determine how impurities and adsorbents affect the band positions and Fermi energies. The band edge positions as well as the density of surface states clearly depended on the density of ON impurities in the bulk and surface regions. Finally, the band diagrams of the n-type Ta3N5/water interfaces were calculated with an improved interfacial model to include the effect of electrode potential with explicit water molecules. We observed partial Fermi level pinning in our calculations at the Ta3N5/water interface, which affects the driving force for charge separation.

Sensitivity of metal oxide gas sensors to non-parabolic intergranular barriers

We analyze the electrical conductivity of polycrystalline semiconductor solids due to the presence of Schottky-type potential barriers formed at intergrains. The density of charged dopants along the grains is usually considered constant leading to parabolic intergranular potential barriers. If temperature is high enough to allow sufficiently mobility to the dopants, their resulting equilibrium distribution is far from constant leading to potential barriers that show a strong non-parabolic character. Implications for the electrical conductivity and then on the sensitivity to barrier height and sample doping in metal oxide gas sensors are discussed.

A comparison of the effect of CdCl2 and MgCl2 processing on the transport properties of n-CdS/p-CdTe solar cells and a simple approach to determine their back contact barrier height

A simple approach, which can estimate the barrier height of non-Ohmic back contacts for CdS/CdTe solar cell by using its temperature dependent forward biased current-voltage data, is explained. The method involves modelling the forward J–V characteristics using a double exponential expression for the main junction and by a reverse biased Schottky barrier for the back contact. Cells processed with both CdCl2 and MgCl2 are compared, with the current transport phenomena in both kinds of cells also being analysed. Performance loss due to limitation of the forward bias hole current, and its dependence on the post-deposition chloride processing, is discussed. The forward current transport is mainly dominated by recombination at CdS/CdTe interfacial region with pronounced tunnelling effects. Classical Schottky-type conduction, as described by the Richardson-Schottky formula, is a good fit to the reverse biased current-voltage behaviour of an Au/CdTe junction above ∼240K. Below this temperature, the current limiting effect due to the increasing contribution from interfacial defect states can be satisfactorily explained by Bardeen’s model for a modified Schottky type barrier at back contact interface.

Evidence of the pressure-induced conductivity switching of yttrium-doped SrTiO3

The electrical transport properties of undoped and yttrium-doped strontium titanate (Sr(Ti1 – xYx)O3 – δ, x = 0, 0.02) under high pressure were investigated with in situ impedance spectroscopy measurements. A pressure-induced conductivity switching for undoped and 2 mole% Y-doped strontium titanate is observed at around ~10.0 and 7.0 GPa respectively, which are caused by a cubic to tetragonal I4/mcm phase transition. The decrease of the phase transition point of 2 mole% Y-doped strontium titanate can be attributed to larger Y3+ atoms occupying the B-site and the creation of more oxygen vacancies, which lead to octahedra tilting and symmetry breaking. The results of the voltage-bias dependence of grain-boundary impedance of undoped and 2 mole% Y-doped strontium titanate at different pressures revealed that Schottky-type potential barriers formed at grain boundaries are the key factor for the accumulation of oxygen vacancy at the interface under pressure.

Effects of Postsynthesis Thermal Conditions on Methylammonium Lead Halide Perovskite: Band Bending at Grain Boundaries and Its Impacts on Solar Cell Performance

We studied the effects of postsynthesis thermal treatments on material properties of CH3NH3PbI3 perovskite films and their photovoltaic performance. Kelvin probe force microscopy revealed the existence of a positive potential barrier at grain boundaries, which is known to be beneficial by suppressing carrier recombination. The height of the barrier increased with the annealing duration, which directly correlated with the device performance. The origin of the potential barrier appeared to be chemical inhomogeneity as revealed by Nano-Auger electron spectroscopy. Qualitatively similar temperature dependence of current-density vs bias characteristics was observed regardless of the annealing duration, where efficiency collapsed at low temperatures due to a diverging series resistance. The freeze-out of free carriers and/or a Schottky-type barrier(s) at the interface(s) are likely responsible for the diverging series resistance.

Photo and electroluminescence of ZnSe: Sn and ZnSe:(Sn, Pr) phosphors

We have prepared ZnSe (luminescent grade) phosphor doped with Sn and (Sn,Pr) with varying concentration in an inert atmosphere in a silica tubular furnace at temperature of (780 ± 20) °C for 1 hr to obtain ZnSe:Sn and ZnSe: (Sn,Pr) phosphors. The photo luminescence (PL) and electroluminescence (EL) spectra of these phosphors have been studied at room temperature and results were discussed in the light of existing models. Dependence of EL emission on the voltage frequency has also been carried out. It is found that the plot between the integrated light intensity versus 1/√Vrms is a straight line suggesting the existence of Mott–Schottky type barrier on the metal semiconductor interface.

Dielectric constant versus voltage and non-Ohmic characteristics of Bi2/3Cu3Ti4O12 ceramics prepared by different methods

Bi2/3Cu3Ti4O12 (BCTO) ceramics were successfully prepared by traditional solid-state reaction method (BCTO-SS) and sol–gel method (BCTO-SG). Pure perovskite phase and dense structure were obtained in BCTO ceramics prepared by both methods. BCTO-SG ceramics showed a large dielectric constant of ~1.1×104 while BCTO-SS ceramics exhibited a low dielectric constant of ~3200. At 100kHz, the dielectric constant of BCTO-SS ceramics decreased with applied voltage increasing, while the dielectric constant of BCTO-SG ceramics increased with applied voltage increasing. Further study of the relationship between dielectric constant and voltage suggested BCTO-SG ceramics had larger defect concentration than BCTO-SS ceramics. The investigation of complex impedance indicated that the electrical properties of grain boundaries for all BCTO ceramics were evidently affected by applied voltages and the electrical properties of grains were independent of applied voltages. In addition, the non-Ohmic properties of BCTO ceramics were studied in detail. The non-linear coefficients of BCTO-SS and BCTO-SG ceramics were 1.65 and 1.01, respectively. The breakdown electric fields of BCTO-SS and BCTO-SG ceramics were found to be 1.21 and 0.48kV/cm, respectively. The potential barrier heights of BCTO-SS and BCTO-SG ceramics were calculated to be 0.549 and 0.485eV, indicating that the potential barriers at the grain boundaries for BCTO-SS and BCTO-SG ceramics are the Schottky-type barrier.

Junction Effect on Transport Properties of a Single Si Nanowire Metal–Semiconductor–Metal Device

We report measurements of electrical transport properties and impedance spectroscopy studies on a single Si nanowire (diameter ~ 50 nm) metal-semiconductor-metal device, fabricated using electron beam deposited Pt. The temperature-dependent four-probe resistivity of the nanowire exhibits freezing of carriers below 30 K, while at higher temperature, it resembles the temperature variation seen in bulk doped crystals. The device shows reproducible nonlinear and asymmetric current-voltage ( I-V) characteristics which were quantitatively analyzed and were found to arise from unequal Schottky-type barriers at the two ends which also showed temperature dependence. The measured contact resistance is bias as well as temperature dependent and reduces as the bias is increased.