Interfacial Potential Barrier Research Articles

Temperature dependent electron effective mass and barrier height in HfO2 based metal/oxide/metal devices

Electrical measurements are realized on Cu/HfO2/Pt capacitors to extract the electron effective mass in HfO2 and the barrier height at the Cu/HfO2 interface. The dominant conduction mechanisms are found to be the Schottky emission at medium voltages and the Fowler–Nordheim tunneling at high voltages. Previous research has usually been carried out by assuming a constant value for either the electron effective mass in oxide or the interfacial potential barrier height to determine the other parameter. However, in contrast to that general practice, an iterative method was proposed in the present study to determine, at the same time, the electron effective mass in HfO2 and the barrier height at the Cu/HfO2 interface without making any prior assumption about their values. The temperature dependence of these two parameters was also studied in the 298–423 K range. It is found that they strongly vary with temperature. The effective mass decreases quadratically with temperature, while the barrier height increases linearly with temperature.

Interfacial Electron Transfer Barrier at Compact TiO2 /CH3 NH3 PbI3 Heterojunction.

Low-temperature solution-processed CH3 NH3 PbI3 interfaced with TiO2 has recently been demonstrated as a highly successful type-II light harvesting heterojunction with ≈20% efficiency. Therefore, an efficient ultrafast photoexcited electron transfer from CH3 NH3 PbI3 to TiO2 is expected. However, by probing the photoexcited charge carrier dynamics in CH3 NH3 PbI3 /quartz, CH3 NH3 PbI3 /TiO2 (compact), and CH3 NH3 PbI3 /PCBM in a comparative study, an electron transfer potential barrier between CH3 NH3 PbI3 and the compact TiO2 (prepared with the spray pyrolysis method) formed by surface states is uncovered. Consequently, the CH3 NH3 PbI3 photoluminescence intensity and lifetime is enhanced when interfaced to compact TiO2 . The electron accumulation within CH3 NH3 PbI3 needed to overcome this interfacial potential barrier results in the undesirable large current-voltage hysteresis observed for CH3 NH3 PbI3 /TiO2 planar heterojunctions. The findings in this study indicate that careful surface engineering to reduce this potential barrier is key to pushing perovskite solar cell efficiencies toward the theoretical limit.

MWNTs/PANI/PVDF复合热电材料的制备及特性研究

为开发新型有机聚合物热电材料，本文通过将不同重量百分比的多壁碳纳米管(MWNTs)、聚苯胺(PANI)导电聚合物与聚偏二氟乙烯(PVDF)有机聚合物三种材料进行超声分散并混合后，将其喷涂于聚对苯二甲酸乙二酯(PET)塑料片上，制备出了MWNTs/PANI/PVDF涂层型复合热电材料。实验表明，这种新型热电材料充分利用了均匀分散于复合体中的大量MWNTs及PANI导电聚合物颗粒，提高了复合体的导电特性。同时，利用MWNTs因交叉、链接及三种材料相互复合嵌套产生的大量界面势垒对声子及低能电子的散射效应及弥散于MWNTs及PANI之间的PVDF聚合物颗粒具有的低导热特性，使复合热电材料的塞贝克(Seebeck)系数及热电优值比单纯的碳管/聚合物热电材料有一定提高。研究结果对探索开发新型复合热电材料具有重要的参考价值。 For the development of new organic polymer thermoelectric materials, multi-walled carbon na-notubes (MWNTs), conducting polyaniline (PANI) and polyvinylidene fluoride (PVDF) polymer materials were ultrasonic mixed with different weight ratio and sprayed them to the surface of polyethylene terephthalate (PET) plastic sheets. A new-type MWNTs/PANI/PVDF polymeric ther- moelectric composite coatings was prepared. And the test showed that the synthesized composite thermoelectric coating materials were of interest for several reasons. 1) For the introduction of one-dimensional conducting pathways of the MWNTs and the PANI conductive particles, the elec-trical conductivity of composite is dramatically increased. 2) By adding the MWNTs and PANI particles into the PVDF polymer, a lot of interfacial potential barriers such as tube-tube junctions and scattering point for phonon and lower energy electrons among the MWNTs, PANI particles, and the PVDF polymer were introduced into composite matrix. As a result, the Seebeck coefficient and ZT value of the composite thermoelectric materials were enhanced obviously than the corresponding data of simplest carbon nanotube based organic thermoelectric composites. The research results could provide the important basis for the applications of new-type composite thermoelectric materials.

Space-charge dominated epitaxial BaTiO3 heterostructures

Epitaxial BaTiO3 thin films (∼300nm thick) with various conductive oxide bottom electrodes including (La0.5Sr0.5)CoO3, SrRuO3 and LaNiO3 were prepared on single-crystalline SrTiO3 substrates using a radiofrequency magnetron sputtering technique in an oxygen-rich atmosphere and at a deposition temperature of 700°C. The charge transport phenomena of these films were systematically investigated. Comparison of leakage current levels with respect to different BaTiO3–bottom electrode interfaces gave a direct proof of the p-type semiconducting behavior in the BaTiO3 films, whose I–V characteristics were well described by a modified Schottky contact model up to an applied field of ∼60MVm−1. By adapting this model, we obtained interface potential barriers, space-charge and free charge carrier densities, as well as depletion layer width and interface layer thickness for the epitaxial BaTiO3 films. The dominance of space-charges (∼1020cm−3) over intrinsic polarizations in these films well explained the apparent reduction in remnant polarization and tilting of the P–V hysteresis loops, as well as an enhanced dielectric strength. Further increase of the applied electric field will result in complete depletion of the BTO films. Consequently, charge transport in these films will become bulk-controlled. This prediction was validated by experimental J–V curves acquired for a broad range of applied electric field, where ionic conduction and Pool–Frenkel emission currents consecutively dominated in the high field (60–220MVm−1) and ultrahigh field (220–300MVm−1) regions. Finally, the potential of these lead-free heterostructures as high-energy density dielectric capacitors was demonstrated.

Thickness scaling effect on interfacial barrier and electrical contact to two-dimensional MoS2 layers.

Understanding the interfacial electrical properties between metallic electrodes and low-dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS2 interfaces. For MoS2 thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS2 thickness. By contrast, the contact resistivity sharply increases with reducing MoS2 thickness below 5 layers, mainly governed by the quantum confinement effect. We find that the interfacial potential barrier can be finely tailored from 0.3 to 0.6 eV by merely varying MoS2 thickness. A full evolution diagram of energy level alignment is also drawn to elucidate the thickness scaling effect. The finding of tailoring interfacial properties with channel thickness represents a useful approach controlling the metal/semiconductor interfaces which may result in conceptually innovative functionalities.

Hopping-tunneling model to describe electric charge injection at metal/organic semiconductor heterojunctions

This paper describes a theoretical model which takes into account charge injection by thermally stimulated hopping (HP), as proposed by Arkhipov, and Fowler–Nordheim tunneling (FNT), for steady-state response of a metal/organic semiconductor hetero-junction in a polymeric light-emitting diode (PLED). The reason for combining these two models is that both injection processes are usually observed in metal/organic semiconductor hetero-junctions. The present model takes into account the material disorder and the escape probability introduced by Arkhipov et al. for both injection processes. The proposed model was tested with experimental results obtained from a PLED ITO/MEH-PPV/Al. Analysis of the experimental results took into account the well-known characteristic of the MEH-PPV in which the charge transport is dominated by holes. Because of this, the electronic current component is assumed to be negligible compared to that for holes. Considering that the device is composed of two different metal/semiconductor junctions, that is, ITO/MEH-PPV and MEH-PPV/Al, the heights of the different potential barriers could be determined. The injection potential barrier, for holes, was determined as 0.4 eV for direct bias (ITO as anode), and as 0.8 eV for reverse bias (aluminum as anode). It was verified that the FNT process is preferable to the HP process in the following situations: (i) high interface potential barrier, (ii) high electric field, and (iii) low temperatures. On the other hand, that is, for a low interface potential barrier, low electric field and high temperatures, the HP process is dominant. With the application of the proposed model, which combines HP and FNT, it was possible to fit the current versus voltage curves for the studied PLED in a wide range of temperatures and electric fields, for direct and reverse bias. Additionally, it was shown that the condition where the HP injection probability is neglected, the non-linearity in FNT plots, versus , obtained experimentally by several authors, may be a consequence of the material disorder. When the HP process is associated with the FNT a more significant non-linearity in the FNT plot was verified.

Zero-energy bound state at the interface between ans-wave superconductor and a disordered normal metal with repulsive electron-electron interactions

In recent years, there has been a renewed interest in the proximity effect due to its role in the realization of topological superconductivity. Here, we study a superconductor--normal metal proximity system with repulsive electron-electron interactions in the normal layer. It is known that in the absence of disorder or normal reflection at the superconductor--normal metal interface, a zero-energy bound state forms and is localized to the interface [Fauch\`ere et al., Phys. Rev. Lett. 82, 3336 (1999).]. Using the quasiclassical theory of superconductivity, we investigate the low-energy behavior of the density of states in the presence of finite disorder and an interfacial barrier. We find that as the mean free path is decreased, the zero-energy peak in the density of states is broadened and reduced. In the quasiballistic limit, the bound state eliminates the minigap pertinent to a noninteracting normal layer and a distinct peak is observed. When the mean free path becomes comparable to the normal layer width, the zero-energy peak is strongly suppressed and the minigap begins to develop. In the diffusive limit, the minigap is fully restored and all signatures of the bound state are eliminated. We find that an interfacial potential barrier does not change the functional form of the density of states peak but does shift this peak away from zero energy.

Resistive switching effects in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3/Pt heterostructures prepared by pulse laser deposition method

Abstract The heterostructural junctions of CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3/Pt (CeO2/LSCMO/Pt) were prepared using pulse laser deposition technique. Their resistive switching (RS) behavior was investigated. As compared to the metal/manganite/Pt junction, the CeO2/LSCMO/Pt device displayed an improved switching characteristic. The RS effects with characteristics of bipolar, threshold, and complementary were realized by adjusting the thicknesses of the CeO2 layer in the CeO2/LSCMO/Pt junctions. Under a higher external bias voltage, the threshold and complementary switching modes of the junctions could turn into bipolar switching mode. The switching behavior shows strong dependence on the O2 partial pressure during the fabrication, indicating that the amount and behavior of the oxygen at the interface play an important role in the determination of the RS behavior. The observed switching behavior is related to the modification of the accumulation/depletion layers as well as the interfacial potential barrier due to the migration of the oxygen vacancies.

Role of chemical termination in edge contact to graphene

Edge contacts to graphene can offer excellent contact properties. Role of different chemical terminations is examined by using ab initio density functional theory and quantum transport simulations. It is found that edge termination by group VI elements O and S offers considerably lower contact resistance compared to H and group VII element F. The results can be understood by significantly larger binding energy and shorter binding distance between the metal contact and these group VI elements, which results in considerably lower interface potential barrier and larger transmission. The qualitative conclusion applies to a variety of contact metal materials.

Motion and energy dissipation of single-walled carbon nanotube on graphite by molecular dynamics simulation

The motion and energy dissipation of carbon nanotubes (CNTs) have important influence on their application. This paper investigated the sliding of single-walled carbon nanotubes (SWCNTs) on a graphite substrate using molecular dynamics simulation. An interesting phenomenon was observed in the commensurate interface. The carbon nanotube slid in not only the initial velocity direction but also perpendicular to the initial velocity with only a slight rolling in the later stage of motion. The direction and distance of motion were related to the initial interfacial configuration when the CNT started to slide and presented a certain degree of randomness. This phenomenon could be attributed to the SWCNT not having a platform on the bottom, which caused a low interfacial potential barrier. Therefore, no obvious ‘stick-slip’ occurred. In the incommensurate state, the SWCNT slid on the graphite substrate in the initial direction for the process, and the velocity slowly decreased. The temperature slightly influenced the carbon nanotube’s motion mainly because of random thermal fluctuations.

GaAs heterojunction devices with MDMO-PPV thin films

Au/MDMO-PPV/n-GaAs heterojunction devices were fabricated. The electronic properties of these devices were investigated at room temperature. The I–V characteristics of Au/MDMO-PPV/n-GaAs heterojunction devices show rectification behavior. The important diode parameters such as series resistance (Rs), ideality factor (n) and barrier height (BH) were determined by performing different plots from the forward bias ln I–V, Cheung and Norde plots. It is seen that there is not an agreement between the values of the junction parameters obtained from these methods. The thin film of MDMO-PPV was introduced as an efficient interlayer for achieving modification of the interfacial potential barrier of metal/n-GaAs diode. Also, the MDMO-PPV thin films have been prepared by spin coating on glass substrate. The obtained films have been characterized by ultraviolet–vis (UV–vis) spectroscopy. UV–vis absorption measurements indicate that the band gap of MDMO-PPV film is 2.20 eV.

ZnO/aSi interface charge carriers transport in Li-ion secondary cell anodes

Electron and Li-ion transport at the n-ZnO/p-aSi (amorphous Si) heterojunction interface is analyzed for the initial charging conditions of a secondary battery anode. The ohmic and diode-type current–voltage characteristics of the junction are investigated for varying doping levels of aSi and ZnO layers. The interface potential barrier impacts the electrons supply to control the Li + ZnO → Li2O + LixZn reaction. The interface electric field could exceed ∼105 V cm−1 and draws in Li ions from zinc oxide into the silicon layer. Relatively low-level doping (∼1018 cm−3) of the semiconductors is preferred for the optimum draw-in effect. During the initial charging, when the Li content in ZnO (as substitution LiZn acceptors) does not exceed the solubility level (∼1019 cm−3), the overall doping maintains the n-type, and the interface electric field continues to draw in Li ions towards silicon. Under further increase of Li content at interstitials, the layer conductivity is converted, and the heterojunction becomes n−–n–p (or even p–n–p) type. During the subsequent transport of Li ions, the interface potential barrier diminishes and vanishes, and the current–voltage characteristics become ohmic. The importance of doping level control for both the materials is emphasized. The results are applicable for interface engineering in LIB anodes.

Schottky barrier height tuning by Hybrid organic-inorganic multilayers

ABSTRACTSemiconducting and insulating polymers and copolymers/Au nanograins based hybrid multilayers (HyMLs) were fabricated on p-Si single-crystal substrate by an iterative method that involves, respectively, Langmuir-Blodgett and spin-coating techniques (for the deposition of organic film) and sputtering technique (for the deposition of metal nanograins) to prepare Au/HyMLs/p-Si Schottky device. The electrical properties of the Au/HyMLs/p-Si Schottky device were investigated by current-voltage (I–V) measurements in the thickness range of 1-5 bilayers (BL).At different number of layers, current-voltage (I–V) measurements were performed. Results showed a rectifying behavior. Junction parameters, such as barrier height (BH), from the I–V measurements for example for the PMMA-b-PS based Au/HyMLs/p-Si structure were obtained as 0.72±0.02 eV at 1BL and 0.64±0.02eV at 5BL. It was observed that the BH value of 0.61 eV obtained for the 5 BL PS based Au/HyMLs/p-Si structure was lower than the value of 0.68 eV of conventional Au/p-Si Schottky diodes. Thus, modification of the interfacial potential barrier for Au/p-Si diodes has been achieved using a thin MLs of different polymers based HyMls semiconductor.

The effect of interface hopping on inelastic scattering of oppositely charged polarons in polymers

The inelastic scattering of oppositely charge polarons in polymer heterojunctions is believed to be of fundamental importance for the light-emitting and transport properties of conjugated polymers. Based on the tight-binding SSH model, and by using a nonadiabatic molecular dynamic method, we investigate the effects of interface hopping on inelastic scattering of oppositely charged polarons in a polymer heterojunction. It is found that the scattering processes of the charge and lattice defect depend sensitively on the hopping integrals at the polymer/polymer interface when the interface potential barrier and applied electric field strength are constant. In particular, at an intermediate electric field, when the interface hopping integral of the polymer/polymer heterojunction material is increased beyond a critical value, two polarons can combine to become a lattice deformation in one of the two polymer chains, with the electron and the hole bound together, i.e., a self-trapped polaron—exciton. The yield of excitons then increases to a peak value. These results show that interface hopping is of fundamental importance and facilitates the formation of polaron—excitons.

Barrier height enhancement of metal/semiconductor contact by an enzyme biofilm interlayer

A metal/interlayer/semiconductor (Al/enzyme/p-Si) MIS device was fabricated using α-amylase enzyme as a thin biofilm interlayer. It was observed that the device showed an excellent rectifying behavior and the barrier height value of 0.78 eV for Al/α-amylase/p-Si was meaningfully larger than the one of 0.58 eV for conventional Al/p-Si metal/semiconductor (MS) contact. Enhancement of the interfacial potential barrier of Al/p-Si MS diode was realized using enzyme interlayer by influencing the space charge region of Si semiconductor. The electrical properties of the structure were executed by the help of current–voltage and capacitance–voltage measurements. The photovoltaic properties of the structure were executed under a solar simulator with AM1.5 global filter between 40 and 100 mW/cm2 illumination conditions. It was also reported that the α-amylase enzyme produced from Bacillus licheniformis had a 3.65 eV band gap value obtained from optical method.

Metal and annealing atmospheres dependence of resistive switching in metal/Nb0.7wt%-SrTiO3 interfaces

We investigated the resistance switching (RS) effect of metal/Nb-doped SrTiO3 interfaces under different treating conditions. Two types of I–V characteristics appeared due to the modification of Schottky-like barrier and the formation of insulating layer. According to X-ray photoelectron spectroscopy analysis, the change in interface potential barrier was contributed to the migration of oxygen vacancies and electrons trapping/detrapping of carriers in the vicinity of interface. Nonlinear fitting is applied to the curves to study the conduction mechanism of metal/NSTO. For “barrier height” style, Schottky emission and Poole–Frenkel (P–F) emission are dominating; for “insulating layer” style, space-charge-limited current, controls the conduction.

The modification of Schottky barrier height of Au/p-Si Schottky devices by perylene-diimide

Perylene-diimide (PDI) thin film was fabricated by spin coating method on p-Si single-crystal substrate to prepare Au/PDI/p-Si Schottky device. The electrical properties of the Au/PDI/p-Si Schottky device were investigated by current-voltage (I–V) measurements in the temperature range 80–300 K and room temperature capacitance-voltage (C–V) measurement. Results showed a rectification behavior. Junction parameters such as ideality factor (n), barrier height (ϕB0), series resistance (Rs) interface state density (Nss), built-in potential (Vbi), carrier concentration (NA), and the width of the depletion layer (WD) were obtained from the I–V and C–V measurements. The values of ideality factor (n) and barrier height (BH) for the Au/PDI/p-Si structure from the I–V measurements were obtained as 1.77 and 0.584 eV at 300 K, 7.78 and 0.176 eV at 80 K, respectively. It was seen that the BH value of 0.584 eV calculated for the Au/PDI/p-Si structure was significantly larger than the value of 0.34 eV of conventional Au/p-Si Schottky diodes at room temperature. Thus, modification of the interfacial potential barrier for Au/p-Si diodes has been achieved using a thin interlayer of the peryleen-diimide organic semiconductor; this has been ascribed to the fact that the peryleen-diimide interlayer increases the effective barrier height because of the interface dipole induced by passivation of the organic layer. Furthermore, the energy distribution of the interface state density determined from I–V characteristics increases exponentially with bias from 1.11 × 1012 eV−1 cm−2 at (0.556−Ev) eV to 11.01 × 1013 eV−1 cm−2 at (0.449−Ev) eV.

Local potential profiling of operating carbon nanotube transistor using frequency-modulation high-frequency electrostatic force microscopy

The local potential measurement of an operating carbon nanotube transistor provides information on the defects in the nanotube and the interfacial potential barriers. While Kelvin-probe force microscopy is a powerful technique to measure the local surface potential, its accuracy is often degraded by the charges trapped on the surrounding insulator surface. Here, we introduce an alternative method to measure the local potential profile along the nanotube being less affected by those charges. We identified the location of a defect and detected the variation in the potential profile for different gate bias voltages, which were not detected by the conventional method.

Enhancing the stability of polymer solar cells by improving the conductivity of the nanostructured MoO3 hole-transport layer

This article demonstrates improvements in the operational stability of organic solar cells (OSCs) by taking advantage of the relationship between oxygen stoichiometry and conductivity in nanostructured metal oxide semiconductors (n-MOS). OSCs in the inverted device configuration of ITO/Ca/P3HT:PCBM/MoO3/Ag were employed in the present study. A high degree of oxygen defects were introduced in the hole-conducting MoO3 layer by annealing the devices under vacuum (≥10(-5) mbar) for nominal temperature (120 °C) and time (10 min). The above devices had much higher operational stability, when tested following the ISOS-D-1 (shelf) protocol, than control devices annealed conventionally, i.e., in nitrogen atmosphere. Employing current-voltage measurement as functions of temperature and photon flux, we show that the devices annealed under vacuum have a lesser density of traps than those annealed in nitrogen. The lesser trap density is shown to be beneficial in reducing the rate of electron recombination thereby increasing the operational stability of the corresponding device. A number of experiments were undertaken to show that the difference in the operation stability of the device results from the difference in conductivity of the nanostructured MoO3 hole transporting layer. The charge extraction by linear increasing voltage spectroscopy shows that charges are relaxed at the trap states in the device annealed in nitrogen whereas they are efficiently transported in the other device. We identify that building up of an interfacial potential barrier as a result of the charge relaxation at the trap states and the corresponding chemical changes in the devices annealed conventionally is the source of degradation of the device performance over time. To our knowledge, this is the first report that successfully overcomes hole-conductivity induced degradation in organic solar cells.

Enhanced thermoelectric performance through energy-filtering effects in nanocomposites dispersed with metallic particles

The thermoelectric properties of semiconductor materials with metallic nanoinclusions were investigated by using the Boltzmann transport equation under the relaxation time approximation. The results showed that the Seebeck coefficient can be significantly enhanced due to interface potential barrier induced by metallic nanoinclusions, leading to the optimized power factor at T < 700 K as barrier height is around kBT larger than the Fermi energy (here, kB is the Boltzmann constant). Additionally, it was found that high-concentration nanoinclusions with radius 1–2 nm can effectively enhance the thermoelectric performance of nanocomposites through energy-selective carrier scattering.