Double Schottky Barrier Model Research Articles

A back-to-back diode model applied to van der Waals Schottky diodes

The use of metal and semimetal van der Waals contacts for 2D semiconducting devices has led to remarkable device optimizations. In comparison with conventional thin-film metal deposition, a reduction in Fermi level pinning at the contact interface for van der Waals contacts results in, generally, lower contact resistances and higher mobilities. Van der Waals contacts also lead to Schottky barriers that follow the Schottky-Mott rule, allowing barrier estimates on material properties alone. In this study, we present a double Schottky barrier model and apply it to a barrier tunable all van der Waals transistor. In a molybdenum disulfide (MoS2) transistor with graphene and few-layer graphene contacts, we find that the model can be applied to extract Schottky barrier heights that agree with the Schottky-Mott rule from simple two-terminal current-voltage measurements at room temperature. Furthermore, we show tunability of the Schottky barrierin-situusing a regional contact gate. Our results highlight the utility of a basic back-to-back diode model in extracting device characteristics in all van der Waals transistors.

Understanding the transport properties of perovskite compounds as a function of temperature, frequency and DC-bias voltage using experimental measurements and appropriate theoretical models.

The present paper provides interesting measurements and methodologies to investigate the key factors controlling the electrical response of perovskite systems. The studied system was successfully prepared using a solid-state route. The chemical analysis and the X-ray diffraction results confirm the formation of the desired perovskite phase. As a result, the DC-resistivity analysis shows that the transport properties are governed by hopping mechanisms above the transition temperature. In this case, thermal agitation allows the charge-carriers to hop across the insulating barrier in the form of grain boundaries. Then, the dominance of the grain boundary contribution is proved. AC-resistivity spectra are investigated in terms of numerous power laws (UDR, SPL and NCL). Accordingly, the decrease in the resistivity values at high frequencies is explained through hopping and tunneling processes. Indeed, it is proved that the electrical transport phenomena are governed by QMT and CBH models. The coexistence of direct (C-C) and indirect (C-A-C) interactions explains the multi-behavior of the AC-resistivity. The scaling representation displays a single muster curve describing the universal dynamic response (UDR). Thus, it reveals the universality of the electrical resistivity of the system. However, the double Schottky barrier model is used to explain the grain boundary effects. A critical frequency "νc" is detected under temperature and DC-bias voltage effects. These observations disclose, in a different way, the separate contributions of the electro-active regions of the sample in the conduction phenomenon.

Study of Temperature Rise Characteristics and Voltammetry Characteristics of ZnO Varistor under 10/350μs Pulse Current

To study the effect of 10/350μs pulse current on the electrical performance of ZnO varistor, a high-current impulse experimental study was conducted in this paper. Firstly, the varistor was subjected to different amplitude impulse discharge experiments, and the temperature change of the varistor after the impulse was recorded. Then, the temperature of the varistor at the moment of 1 second after the pulse impulse was suffered by the Asymptotic function, and the Levenberg-Marquardt optimization algorithm was calculated to compare the temperature rise of the varistor under different current amplitudes. Finally, the upward shift of voltammetry characteristics in the high-current region after the impulse was explained according to the double Schottky barrier model and the linear chain aging theory. The results show that the higher the pulse current amplitude is, the higher the temperature rise of the varistor is at the 1s moment. The highest voltammetry characteristic upshift is caused by the 6.4kA pulse current. The temperature rise and the voltammetry characteristic drift in the high-current region are positively correlated. The results of the study can provide a reference for manufacturers to produce ZnO varistors for direct lightning protection of buildings.

Position–sensitive electric property of flash–sintered 3Y–TZP ceramics based on DC bias assisted impedance analysis

In this study, impedance spectroscopy with direct current (DC) bias was used to analyze the electric property and oxygen vacancies of flash–sintered 3 mol% Y 2 O 3 doped tetragonal zirconia polycrystalline (3Y–TZP) ceramics at anode, middle, cathode and no field vicinity, respectively. The electric property was obtained using modified–Debye circuit model and the oxygen vacancies concentration was calculated using double Schottky barrier model. Interestingly, a phenomenon of position–sensitive electric property and oxygen vacancies concentration was observed. It means after flashing the tested samples become ununiform electrically at anode, middle and cathode vicinity. The neutral oxygen vacancies generated at cathode vicinity are responsible for this unique phenomenon fundamentally. The finding here is useful to explore the mechanism of flash sintering and tune the electric property of ZrO 2 ceramics electrolyte.

Electrical properties of n-conducting barium titanate ceramics over a wide temperature range under voltage load

Bulk and grain boundary resistivities as well as grain boundary capacitances of PTCR (positive temperature coefficient of resistivity) thermistors have been investigated as a function of voltage load and temperature ranging from 30 to 820 °C by application of impedance spectroscopy. In addition, current – voltage curves have been measured and the resistivities extracted from these dc measurements are in close agreement with those obtained from impedance spectroscopy. The resistance – temperature characteristics are typical for n-type barium titanate – based PTCR ceramics, viz. a steep increase of the grain boundary resistance above the Curie – temperature (PTCR effect) and decreasing resistance with increasing temperature in the NTC (negative temperature coefficient) regime above approximately 200 °C. The grain boundary capacitance shows a sharp peak at the Curie – temperature (around 120 °C) and obeys the Curie – Weiss law in the paraelectric state. Basically, the grain boundary resistivities decrease significantly under voltage load. However, at elevated temperatures (above 600–700 °C) this non-linear effect vanishes and linear ohmic (or even sub-ohmic) behavior can be observed. The electrical properties can be interpreted in terms of a modified double Schottky barrier model. Reasonable coincidence between simulated and measured current - voltage curves as well as grain boundary conductivities has been found in a wide temperature range (up to 800 °C) under high field conditions (up to an external field strength of 1000 V cm−1).

Effect of temperature on internal potential barriers in polymer-derived amorphous ceramics

Abstract The behavior of the internal potential barrier in a polymer-derived amorphous ceramic was studied by measuring its complex impedance spectra at various dc bias and temperatures. The results showed that the bias-dependent interfacial capacitance can be described by the double Schottky barrier model. Increasing temperature caused the increase in the charge-carrier concentration and decrease in the barrier height, following Arrhenius dependence. The phenomena are correlated to the electronic structure of the free-carbon phase and the amorphous matrix phase in the material.

Investigation of Improved Reliability in BaTiO3 -Based Ceramics via Two-Step Sintering by Impedance Spectroscopy and Schottky Barrier Model

The ceramics prepared by conventional sintering (CS) method and two-step sintering (TSS) method are investigated in the temperature range of 490–570 °C through impedance spectroscopy, which is correlated with the improved reliability of TSS specimen. There is an improvement in the resistances (Rg and Rgb) of TSS specimen mainly due to the reduced grain size. And according to the grain boundary double Schottky barrier model, not merely the smaller potential barrier height (Фgb) but also the larger depletion layer thickness (δgb) for TSS specimen indicate that the effective concentration of charged acceptor dopants and hence oxygen vacancy concentration in TSS specimen are much lower than these in CS specimen. These facts can account for the improved reliability in BaTiO3-based ceramics via two-step sintering.

Non-Ohmic properties of MgTiO3 doped CaCu3Ti4O12 thin films deposited by magnetron sputtering method

CaCu3Ti4O12-xMgTiO3 (x = 0, 0.05, 0.1 and 0.2) thin films were prepared through magnetron sputtering method and sintered at 850 °C for 1 h. Second phase of CuO and TiO2 were detected in MgTiO3 doped ceramic targets and CaTiO3 phase was observed when x = 0.2. SEM results showed that grain size increased with MgTiO3 doping, which is corresponding to the decrease of threshold voltage. Remarkable non-Ohmic behaviors of all samples were observed at different temperatures. Nonlinear coefficient increased from 2.803 (x = 0) to 3.799 (x = 0.2) and Schottky barrier height increased firstly then decreased as MgTiO3 doping. Double Schottky barrier model was used to explain the non-Ohmic characteristic of CCTO thin films.

Interface characteristics of polymer-derived amorphous SiCN ceramics investigated by impedance spectroscopy under direct current bias

The interface characteristics of polymer derived ceramics determine their dielectric properties, useful for sensing applications. In this contribution, the interface characteristics of polymer-derived amorphous SiCN ceramics were studied using direct current (dc) bias superimposed on alternating current (ac) impedance spectroscopy. Double Schottky barrier model was applied to determine the interface characteristics. Potential height and donor concentration at the interface were estimated to 0.99eV and 2.91 × 1018cm−3, respectively. Interface density was recorded as 2.69 × 1013cm−2. Interface resistance reduced as bias rose, resulting in increased dielectric loss. Interface capacitance also declined as bias increased, thus reducing dielectric constant. These changes were related to potential barrier height and width of barrier region. Overall, these findings look promising and should benefit further understanding on interfacial polarization processes of polymer-derived ceramics.

Nonlinear current–voltage behavior in PZT thin films

In this paper, Pb(Zr0.52Ti0.48)O3 (PZT) thin films were prepared by sol–gel synthesis and characterized by X-ray diffraction, field emission scanning electron microscopy and current–voltage measurements. Here, we demonstrate that in addition to the outstanding ferroelectric and dielectric properties, the PZT films also have remarkably nonlinear current–voltage characteristics. Considering the contact of semi-conductive grains in the PZT films, a double Schottky barrier (DSB) model may be responsible for such phenomena. The test results show that with the decrease of annealing temperature and the increase of the film thickness, the threshold voltages (V th) increase obviously. The maximum V th value of 60.95 V and the minimum value of 6.9 V in our experiments were obtained from the five-layered samples annealed at 600 °C and the two-layered samples annealed at 700 °C, respectively. As a result, PZT thin film may lead to efficient switching and sensing devices.

Strain engineering of Schottky barriers in single- and few-layer MoS2 vertical devices

We study the effect of local strain in the electronic transport properties of vertical metal-atomically thin MoS2-metal structures. We use a conductive atomic force microscope tip to apply different load forces to monolayer and few-layer MoS2 crystals deposited onto a conductive indium tin oxide (ITO) substrate while measuring simultaneously the I–V characteristics of the vertical tip/MoS2/ITO structures. The structures show rectifying I–V characteristics, with rectification ratios strongly dependent on the applied load. To understand these results, we compare the experimental I–Vs with a double Schottky barrier model, which is in good agreement with our experimental results and allows us to extract quantitative information about the electronic properties of the tip/MoS2/ITO structures and their dependence on the applied load. Finally, we test the stability of the studied structures using them as mechanically tunable current rectifiers.

Enhanced formaldehyde photoelectric response on ZnO film illuminated with visible light

Photoelectric responses of ZnO to dry air and formaldehyde under visible light illumination were studied in this work. ZnO film was prepared on Al2O3 substrate by screen printing. Fundamental characteristics of ZnO film were examined through XRD, FE-SEM and PL spectra measurements. Time-dependent photocurrent properties of ZnO were tested in dry air and formaldehyde under the illumination of white light, respectively. The results revealed that, in comparison with dry air, ZnO in formaldehyde exhibited an enhanced photocurrent response. To illustrate it, explanations based on the double Schottky barrier model were proposed. In addition, to demonstrate the authenticity of the proposed mechanism, experiments on the time-dependent photocurrent of ZnO under white light illumination in the light-on and light-off conditions were also carried out.

Influence of thermal treatment on the nonlinear current–voltage behavior and restoration hysteresis effect of CaCu3Ti4O12 thin films

CaCu3Ti4O12 (CCTO) thin films were prepared through sol–gel method. The influences of sintering temperature and rapid thermal process (RTP) in oxygen-rich atmospheres on its nonlinear current–voltage (I–V) behavior and restoration hysteresis effect were investigated. The phase identification of the films was characterized by using X-ray diffractometer, the results show that in the as-sintered films which were treated through RTP in oxygen atmospheres, a weak CuO and TiO2 diffraction peaks could be detected. Field emission scanning electron microscope was performed to observe the microstructure of the films. A semiconductor parameter analyzer was used to determine the I–V behaviors. The results show that with sintering temperature increase, the threshold voltages of CCTO thin films decreased significantly. After the films were treated through RTP in oxygen-rich atmosphere, the height of Schottky barriers would increase obviously, resulting in the increase of threshold voltages of CCTO films, and the longer the time of RTP, the higher the Schottky barrier. The influence of RTP on restoration hysteresis effect of CCTO films was reported for the first time, and RTP could decrease the initial current and increase the resistivity. The double Schottky barrier model and the tunnel junction model were used to explain these phenomena, and the conduction mechanism in CCTO film was discussed to describe the restoration hysteresis effect.

Nonlinear current–voltage behavior in La-doped CaCu3Ti4O12 thin films derived from sol–gel method

The effect of La3+ doping at Ca2+ sites in CaCu3Ti4O12 (CCTO) thin film was investigated. Samples with compositions of Ca(1−3x/2)LaxCu3Ti4O12 (x = 0, 0.05, 0.1, 0.15 and 0.2) were synthesized by sol–gel method. Remarkable non-Ohmic behaviors in all the samples were presented through a semiconductor parameter analyzer. Results showed that with the increase of dopant’s amount, the threshold voltage of CCTO thin films firstly increased and then decreased and the nonlinear coefficient decreases, good thermal and electric stability was found in all samples. XRD results confirmed the single phase formation of all the samples with similar cubic structure of CCTO and characterization with SEM showed a completely opposite trends between the grain size and the threshold voltage. The double Schottky barrier model is used to explain the nonlinearity of CCTO thin films and the relationship between grain size and threshold voltage in the thin films. The presence of Schottky barrier at the grain boundary is confirmed by a linear relationship between ln(J) and E1/2. A possible way is put forward to control the non-Ohmic properties of CCTO thin films based on this research.

Fabrication and different photoelectric responses of nanocrystalline ZnO film irradiated with UV and white light in dry air

In this paper, nanocrystalline ZnO film with porous structure was successfully prepared on alumina substrate by the technology of screen printing and the subsequent heat treatment. The fundamental characteristics of the as-prepared sample were examined through XRD, FE-SEM, EDX and PL spectra measurements. Meanwhile, photoelectric responses of it were tested under UV and white light irradiation, respectively. Different photocurrent curves were obtained. Under UV light, the photocurrent with comparatively high amplitude of each cycle could mostly recover upon the light off. While for white light, the photocurrent with low amplitude just partially recovered when the light was turned off. This phenomenon indicated that, after the white light off, a large number of free electrons still remained within the materials. To calculate the amount of the remained free electrons, three photocurrent parameters, which are related to the density of free electrons in ZnO, were defined for the first time. Furthermore, the explanations for the different photoelectric responses of ZnO based on the double Schottky barrier model were also proposed.

High pressure study of B12As2: Electrical transport behavior and the role of grain boundaries

Using a diamond anvil cell, the evolutions of alternate-current impedance spectra and direct- current resistivity in B12As2 have been investigated up to 51.9 GPa. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The grain boundary resistance shows a relatively smaller contribution to the total resistance above 16.8 GPa. By using the double-Schottky barrier model, the space charge potential was obtained. A pressure-induced inversion of charge defect concentration in the space charge layer was found at 20.7 GPa. The high-temperature resistivity measurements indicate that the transport activation energy is determined by defect energy levels in the band gap.

Nonlinear current–voltage behavior of CaCu3Ti4O12 thin films derived from sol–gel method

CaCu3Ti4O12 (CCTO) thin films were synthesized by a sol–gel method followed with spin-coating and sintering process in order to investigate the effects of sintering temperature and thickness of CCTO thin films on the electrical properties as a varistor. The phase identification and morphology of the films have been characterized using X-ray diffractometer and field emission scanning electron microscope. A semiconductor parameter analyzer was used to determine the non-Ohmic (I–V) behaviors. The results showed that the breakdown voltage firstly decreased and then increased with the rising of sintering temperature and was directly proportional to the thickness of the film. Furthermore, the threshold voltage of each sample exhibited nearly twofold relationship as functions of two different measuring mode, surface-to-surface and surface-to-substrate. The lowest leakage current (I L) and the highest nonlinear coefficient (α) of CCTO thin films were found in the sample sintered at 700 °C. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was employed to explained the non-Ohmic behaviors. A linear relationship between ln(J) and E 1/2 indicated that a Schottky barrier should exist at the grain boundary.

Modeling of electrical properties of grain boundaries in n-conducting barium titanate ceramics as a function of temperature and dc-bias

Abstract A double Schottky barrier model suitable for the description of the grain boundary resistivity of n-conducting BaTiO 3 ceramics has been modified by taking account of frozen-in diffusion profiles of cation vacancies at the grain boundaries which are formed during the cooling process after sintering. The space charge model has been extended in order to predict the electrical properties of PTC ceramics (positive temperature coefficient of resistivity) under voltage load for maximum dc-bias values around 0.3 – 0.5 V/grain boundary. The effect of the voltage drop across the Schottky barrier on the concentration profiles of electrons in the depletion zone as well as the space charge potential has been elaborated in detail.

Analysis of Grain-Boundary in SrCoO<sub>3–</sub>Doped ZnO Varistors and its Electrical Characteristics

The grain-boundary and its electrical characteristics in SrCoO3doped ZnO varistors were studied. The grain-boundary around ZnO-grain is probably composed of SrCoO3, and its electrical behavior is clearly different from two conventional types of Bi-and Pr-based ZnO varitors. The non-linearity and characteristic behavior could be explained by considering the n-p-n hetero-structure at the grain boundary. SrCoO3 in the grain-boundary region should play crucial roles of not only the appearance of non-liner property but also the formation of different hetero-structure from double Schottky-barrier model on conventional varistors.

Dielectric spectra of ZnO varistor ceramics

In this paper, the dielectric spectra of ZnO varistor ceramics are measured by Novocontrol wide band dielectric spectrometer in a temperature range of -160℃-200℃ and frequency range of 0.1 Hz-0.1 MHz. It is found that electron transportation can be characterized by the flat region on a low frequency side of σ'-f curve. The Schottky barrier height is 0.77 eV obtained from σ'-f curve, which is consistent very well with the data from I-V curves given in other literature. On the basis of back-to-back double Schottky barrier model, Schottky barrier height corresponding to electron transportation across grainboundary is explained to be the energy difference between interface state and barrier top. According to this explanation, Schottky barrier height will increase linearly with the increase of DC voltage applied. The linear variation of barrier height with the increase of DC voltage applied is confirmed experimentally. Finally, the theoretical value of averaged grain size is obtained to be 6.8 μm, which is almost identical to 6.5 μm measured from SEM images. Therefore, the macroscopic electrical properties and the microstructure can be expressed at the same time by dielectric spectra.