Injection Barrier Height Research Articles

Drift of Schottky Barrier Height in Phase Change Materials.

Phase-change memory (PCM) devices have great potential as multilevel memory cells and artificial synapses for neuromorphic computing hardware. However, their practical use is hampered by resistance drift, a phenomenon commonly attributed to structural relaxation or electronic mechanisms primarily in the context of bulk effects. In this study, we reevaluate the electrical manifestation of resistance drift in sub-100 nm Ge2Sb2Te5 (GST) PCM devices, focusing on the contributions of bulk vs interface effects. We employ a combination of measurement techniques to elucidate the current transport mechanism and the electrical manifestation of resistance drift. Our steady-state temperature-dependent measurements reveal that resistance in these devices is predominantly influenced by their electrical contacts, with conduction occurring through thermionic emission (Schottky) at the contacts. Additionally, temporal current-voltage characterization allows us to link the resistance drift to a time-dependent increase in the Schottky barrier height. These findings provide valuable insights, pinpointing the primary contributor to resistance drift in PCM devices: the Schottky barrier height for hole injection at the interface. This underscores the significance of contacts (interface) in the electrical manifestation of drift in PCM devices.

Analytical modeling of photocurrent in double perovskite Cs[formula omitted]AgBiBr[formula omitted] X-ray detectors

This work has developed a theoretical model to investigate the mechanisms of nonlinear steady-state photocurrent versus dose rate characteristics in Cs2AgBiBr6-based X-ray detectors. The photocurrent analysis on Cs2AgBiBr6-based X-ray detector shows that there is a photocurrent gain, primarily due to the presence of dark current. Charge injection from metal contacts is the primary source of dark current in Cs2AgBiBr6. The accumulation of X-ray induced ions near the electrodes elevates the metal/perovskite interface electric field, and hence increases the charge injection with the X-ray dose. The Schottky barrier height for charge injection is found to be dependent on the X-ray dose as well. Therefore, the variation of dark current under X-ray exposures are mainly responsible for the non-linear steady-state photocurrent. The comparison of the model with the experimental results reveals the accumulated ion concentrations and effective barrier heights.

Unifying and Suppressing Conduction Losses of Polymer Dielectrics for Superior High-Temperature Capacitive Energy Storage.

Superior high-temperature capacitive performance of polymer dielectrics is critical for the modern film capacitor demanded in the harsh-environment electronic and electrical systems. Unfortunately, the capacitive performance degrades rapidly at elevated temperatures owing to the exponential growth of conduction loss. The conduction loss is mainly composed of electrode and bulk-limited conduction. Herein, the contribution of surface and bulk factors is unified to conduction loss, and the loss is thoroughly suppressed. The experimental results demonstrate that the polar oxygen-containing groups on the surface of polymer dielectrics can act as the charge trap sites to immobilize the injected charges from electrode, which can in turn establish a built-in field to weaken the external electric field and augment the injection barrier height. Wide bandgap aluminum oxide (Al2 O3 ) nanoparticle fillers can serve as deep traps to constrain the transport of injected or thermally activated charges in the bulk phase. From this, at 200 °C, the discharged energy density with a discharge-charge efficiency of 90% increases by 1058.06% from 0.31 J cm-3 for pristine polyetherimide to 3.59 J cm-3 for irradiated composite film. The principle of simultaneously inhibiting the electrode and bulk-limited conduction losses could be easily extended to other polymer dielectrics for high-temperature capacitive performance.

High-temperature polymer dielectric films with excellent energy storage performance utilizing inorganic outerlayers

The optimization of high-temperature polymer capacitors is critical to the development of power electronics in harsh environments. The conduction loss of polymers increases dramatically at high temperatures, leading to a decrease in energy density and charge/discharge efficiency, which is a major impediment for capacitor applications. In this work, polyetherimide (PEI) composite films with trilayer structures are designed, in which the boron nitride nanosheet (BNNS) outer layers are optimized for charge blocking effect at multiple interfaces. The modulation of the inorganic layers not only increases the barrier height for charge injection from the electrodes, but also creates a PEI layer/BNNS layer interface that facilitates the formation of traps, thus effectively suppressing further carrier transport. Experiment and simulation verify that the construction of the trilayer structure promotes electric field redistribution, which significantly enhances high-temperature energy storage performance. At 200 °C, the energy density of the trilayer composite film is 3.81 J cm−3 with a charge/discharge efficiency >90 %, which is 766 % higher than PEI film (0.44 J cm−3 with a charge/discharge efficiency >90 %). Notably, the energy storage performance of trilayer composite film at high temperature is far superior to the reported high-temperature polymer dielectric films. This work demonstrates the promising potential of multilayer structures applied to dielectric polymer composite films at high temperatures.

Tunable charge injection from graphene to halide perovskite induced by reversible ion segregation

Migration of point defects can result in various performance instabilities in organometal halide perovskite devices. In particular, the segregation of charged defects can influence the electron transport, but the correlation between ionic and electronic transports remains elusive. In this work, we use a graphene/methylammonium lead iodide capacitor to probe the effect of ion segregation on the charge injection behavior at the graphene/perovskite junction. In our samples, the ion segregation in the perovskite film is induced by a controllable electric field applied through an electrostatic gate. At the same time, the charge injection from graphene to the perovskite is probed by monitoring changes in the graphene conductivity. It is found that a positive (negative) defect accumulation layer near the graphene/perovskite interface can facilitate electron (hole) injection from the graphene electrode to the perovskite. Because the polarity of the defect accumulation layer is switchable by changing the direction of the applied electric field, the graphene/perovskite interface behaves like a Schottky junction with a tunable charge injection barrier height. This behavior should be generally found in other metal/halide perovskite interfaces as well.

Effect of titanium-dioxide nanoparticle on Richardson constant and barrier height of tartrazine dye based Schottky device

Recently Fruit Dyes have achieved a significant interest as organic devices because of they are widely available, customizable and biodegradable. But low conductivity is one of the major limitations. Low conductivity is due to low charge injection from the electrodes to the organic layer and the charge injection process is strongly dependent on the injection barrier height at the metal-dye interface. Higher barrier height causes low charge injection. A detailed analysis is needed to reduce the interfacial barrier to get better conductivity. In this work, we have estimated the interfacial barrier height of Indium tin oxide coated glass/Tartrazine dye/Copper based device with and without titanium dioxide nanoparticles. The devices have been prepared with dye and dye-nanoparticles blends by using the spin coating technique. We have measured the steady state dark current–voltage–temperature characteristics in the range of 288 K to 333 K to estimate the Richardson constant and interfacial barrier height. The Richardson constant was found to be modified from 44.35 × 10–3 to 07.34 × 10–3 Am−2 K−2. We also have found that interfacial barrier height is reduced from 0.71 eV to 0.64 eV in the presence of nanoparticles at room temperature. As the temperature increased, the interfacial barrier height increased to 0.72 eV from 0.71 eV and to 0.68 eV from 0.62 eV for dye based device with and without nanoparticles, respectively. Reduction of the interfacial barrier height indicates an increase in the charge injection through the interface. This work will be informative to improve the charge flow at the metal–fruit dye interface.

The effect of terminal group of hole injection self-assembled monolayers on the performance of optoelectronic devices on ITO anodes

We demonstrate the effect on the performance of conventional (Indium tin oxide (ITO)/N,Nj-Di (1- naphthyl)-N,Nj-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB)/Tris-(8-hydroxyquinoline)aluminum (Alq3)/LiF/Al) optoelectronic devices when the ITO surface is modified with various types of self-assembled monolayer (SAM)s such as 3-(trimethoxysilyl)propylamine (NH2SAM), Trimethoxy (propyl)silane CH3SAM, and Trimethoxy (3,3,3-trifluoropropyl)silane (F3SAM). The head functional group and chain configuration are trimethoxysilylpropyl (-(CH3O)3Si(CH2)2) and the tail groups distinguished SAMs (-NH2, -CH3, and -CF3). The contact angles of NH2SAM, CH3SAM, and F3SAM deposited on ITO and ITO were 70.7°, 81.42°, 68.23°, and 30.85°, respectively. The SAM-modified ITO's surface morphology Rpv and Rq parameters were reduced from 31.02 nm to 2.0 nm and 1.18 nm–0.26 nm. The work functions values of ITO, ITO/NH2SAM, ITO/CH3SAM and ITO/F3SAM measured by UPS were 4.99eV, 4.39, 5.09 and 5.69eV, respectively. The maximum luminance of the SAM-modified device increased to 166-fold, and the current density 105-fold, compared to the unmodified device when the operating voltage is set to 11 V. Current and power efficiency increased by 1.9 and 2-fold, respectively. The hole injection conditions are enhanced and the hole injection barrier height between ITO/NPB is reduced.

Argon Plasma treated Silver-Nanowire-based Perovskite Light Emitting Diode

During the recent decade, various approaches have been utilized to fabricate a commercial Perovskite Light Emitting Diode (PeLED), and, till now, acceptable progress has been made. Despite this, to increase the efficiency of these devices to reach their maximum, it is necessary to modify and optimize them after each development, at each stage. One of the most important options to enhance is the interface between two different layers. In this work, we first used atmospheric Ar-Plasma treated Silver-Nanowire (Ag-NW) to fabricate PeLEDs as a new generation of solid-state light sources. Results demonstrate that the Ag-NW work function increases after exposing Ar-Plasma, and it is one of the easiest cost-effective ways to reduce injection barrier height. Also, modifying the barriers reduces the contact angle for four different treatment times. Besides, sheet resistance reduces, and Ag-NWs’ transparency increases. Therefore, it can provide a positive change in surface morphology and consequently a better coverage for the next layer; so that, the current density increases, which means that the injected carriers are effectively increased; so that, the luminance of the devices reaches 30,000 cd m−2 from 8000 cd m−2.

Study of the effect of density of states distribution on carrier injection at organic/electrode interface through high-sensitivity photoemission spectroscopy and injection simulation

Carrier injection, which is a key factor in controlling and improving organic device properties, has been predominantly studied using the injection barrier height derived from HOMO and LUMO positions. The weak density of states (DOS) within the HOMO–LUMO energy gap is also important to understand the practical injection properties. In this study, the DOS of the α-NPD/electrode model interfaces are investigated using high-sensitivity UV photoemission spectroscopy. The nature of hole injection is discussed based on the observed DOS and a simple simulation. The results indicate that the weak DOS close to the Fermi level is critical for carrier injection.

Investigation of Charge Accumulation and Decay Processes in Epoxy-Based Composites Using Quantum Chemical Calculations Under the Effect of Electric Field

This work investigates the influence of aluminum nitride (AlN) fillers on the charge dynamics of epoxy resin (EP) and its hybrid composites through space charge measurements and quantum chemical calculations (QCCs). QCCs analyze the energy-level orbitals and trap parameters of the molecules under the electric field. The pure EP exhibits a homo-charge accumulation, whereas fewer charges are accumulated in the EP hybrid composites. The AlN filler addition significantly suppresses the space charge accumulation during polarization and facilitates the charge dissipation during the depolarization due to the lower trap density than the pure EP. It is found that the orbital energy level of the combined reaction of all monomers together is not a simple superposition of individual orbital energy levels of different monomers. The increased charge injection barrier height suppresses space charge injection and accumulation. The electric field significantly influences the hole traps more than the electron traps. The positive charges with low mobility are easily trapped inside hole traps compared with the negative charges with high mobility inside the electron traps. This results in positive charge accumulation during the polarization and slow decay during the depolarization. The increase of the electrostatic potential is more uniform in hybrid composites than in the pure EP, and no visible electron migration is observed. It indicates that hybrid composites remain stable under electric fields. Moreover, it is not easy for the hole to transport due to the deep trap after the charge injection, leading to limited visible space charge accumulation near the anode.

Electron-assisted deposition and interface control of naphthalenediimide derivative thin films

Stable film formation and interface control of electron-transporting polymers are important key issues for developing organic devices. In this report, thin films of N,N’-bis(p-vinylbenzyl)naphthalenediimide (PVB-NDI) were prepared by the vapor deposition and an electron-assisted (e-assist) deposition methods on aluminum substrates. The aluminum surface was modified with self-assembled monolayers (SAMs) of (3-mercaptoprppyl) trimethoxysilane (MPTMS) or vinyltriethoxysilane for controlling the film/substrate interface. It was observed that the e-assist deposition can polymerize PVB-NDI in the process of film deposition. The polymerization was induced by the e-assist process, and was not influenced by the surface modification with the SAMs. It was found that the electron injection barrier height can be substantially decreased when the PVB-NDI film was prepared by the e-assist deposition on the aluminum surface modified with the SAM of MPTMS. This result implies that covalent tethering via the thiol-en reaction at the interface contributes to controlling the metal/organic interface.

Study on the influencing factors of space charge dynamic behavior in XLPE insulation under alternating current electric field

The dynamic behavior of space charges is one of the potential factors that cause electrical aging. In this paper, a bipolar space charge transport model was established under an AC electric field in cross-linked polyethylene. The effects of physical parameters on the dynamic behavior of space charges in an AC electric field were explored by changing the injection barrier heights, mobilities, trapping coefficients, and detrapping barrier heights of positive and negative charges. The effects of voltage conditions on space charge behavior were explored by changing the amplitude and frequency. The effects of temperature were studied by setting a temperature gradient similar to that of real cable operation. The asymmetry of the physical parameters could seriously aggravate the accumulation of space charges in the AC field. A set of physical parameters based on these results was identified. Both the increase in voltage amplitude and decrease in frequency enhanced the amount of and penetration into the insulation of the accumulated space charge. The dynamic behaviors of the space charges were more active with increased voltage amplitude and frequency. The temperature gradient in the cable insulation implies that the degree of accumulation, penetration into material, and activity of dynamic behaviors of the space charges at the warmest electrode are greater than those at the coldest. Compared with the DC field, the dynamic behavior of space charges in the AC electric field is more active and releases more energy. This research provides a theoretical basis for inhibiting the accumulation of space charges in an AC electric field.

Steady-state and transient optoelectronic characteristics of styrene-and quinoline-based derivative

Styrene and quinoline groups are commonly incorporated into the organic fluorescent materials for organic light-emitting diodes (OLEDs). In this work, a type of small molecule derived from styrene and quinoline, with a chemical structure of 2,2'-(2,5-dimethoxy-1,4-phylenedivinylene)bis-8- acetoxyquinoline (MPV-AQ), is employed as the emitter and electron transporting material in the OLEDs, and its optoelectronic characteristics such as charge-carrier injection, transporting and recombination are investigated by the steady-state and transient technologies. It is found that the electron injection from the cathode into the MPV-AQ layer shows the Fowler-Nordheim (FN) tunneling characteristic in the N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB)/MPV-AQ bilayer OLED, which is different from the Richardson-Schottky (RS) thermionic emission in the electron-only device based on the MPV-AQ single-layer. The difference in electron injection is attributed to the bend of energy bands of MPV-AQ in the NPB/MPV-AQ device, which can be caused by the charge accumulation at the NPB/MPV-AQ interface. The accumulated charges should mainly be the holes on the side of NPB layer because the electron mobility of MPV-AQ is much lower than the hole mobility of NPB. Owing to the bending of lowest unoccupied molecular orbital (LUMO) of MPV-AQ, the tunneling distance for electrons is significantly reduced, which is favorable for the FN tunneling. The barrier height for electron injection is calculated to be 0.23 eV by fitting the current-voltage curve of the NPB/MPV-AQ bilayer OLED. And the electron mobility of MPV-AQ is determined by the delay time of transient electroluminescence (EL) and shows field-dependence with the value on the order of 10<sup>–6</sup> cm<sup>2</sup>/(V·s). In addition, the electron-hole recombination coefficient is obtained from the long time component of the temporal decay of the EL intensity, and the coefficient is found to decrease with the applied voltage increasing, which is consistent with the efficiency roll-off in this bilayer OLED. This study may provide a foundation for understanding the electronic processes of carrier injection, transport and recombination in the OLEDs, which is helpful in improving the device performance.

Tuning Surface States of Metal/Polymer Contacts Toward Highly Insulating Polymer-Based Dielectrics.

Metal-polymer interface plays a crucial role in controlling the dielectric performance in all flexible electronics. Ideally, the formation of the Schottky barrier due to the large band offset of the electron affinity of the polymer over the work function of the electrode should sufficiently impede the charge injection. Arguably, however, such an injection barrier has hardly been indisputably verified in polymer-metal junctions due to the ever-existing surface states, which dramatically compromise the barrier thus leading to undesired high electrical conduction. Here, we demonstrate experimentally a clear negative correlation between the breakdown strength and the density of surface states in polymer dielectrics. The existence of surface states reduces the effective barrier height for charge injection, as further revealed by density functional theory calculations and photoinjection current measurements. Based on these findings, we present a surface engineering method to enhance the breakdown strength with the application of nanocoatings on polymer films to eliminate surface states. The density of surface states is reduced by 2 orders of magnitude when the polymer is coated with a layer of two-dimensional hexagonal boron nitride nanosheets, leading to about 100% enhancement of breakdown strength. This work reveals the critical role played by surface states on electrical breakdown and provides a versatile surface engineering strategy to curtail surface states, broadly applicable for all polymer dielectrics.

Wavelength conversion through plasmon-coupled surface states

Surface states generally degrade semiconductor device performance by raising the charge injection barrier height, introducing localized trap states, inducing surface leakage current, and altering the electric potential. We show that the giant built-in electric field created by the surface states can be harnessed to enable passive wavelength conversion without utilizing any nonlinear optical phenomena. Photo-excited surface plasmons are coupled to the surface states to generate an electron gas, which is routed to a nanoantenna array through the giant electric field created by the surface states. The induced current on the nanoantennas, which contains mixing product of different optical frequency components, generates radiation at the beat frequencies of the incident photons. We utilize the functionalities of plasmon-coupled surface states to demonstrate passive wavelength conversion of nanojoule optical pulses at a 1550 nm center wavelength to terahertz regime with efficiencies that exceed nonlinear optical methods by 4-orders of magnitude.

Leakage current characteristics and DC resistance degradation mechanisms in Nb doped PZT films

The correlation between defect chemistry, leakage currents, and time-dependent dielectric breakdown was studied for PbZr0.52Ti0.48O3 (PZT) films doped with 0.5, 1, 2, or 4 mol. % Nb. As the samples are nearly intrinsic (that is, close to n- to p-type transition), signatures for both hole hopping between Pb2+ and Pb3+ and electron trapping by Ti4+ were observed. For all doping levels, the dominant conduction mechanism transitioned from Poole–Frenkel emission at lower electric fields to Schottky emission at higher electric fields. The electric field for this transition diminishes from 172 to 82 kV/cm with decreasing Nb concentration. The concomitant modification of the Schottky barrier height from 1.24 to 0.95 eV with decreasing Nb concentration is attributed to Fermi level pinning via oxygen vacancies. The DC resistance degradation was controlled by Schottky emission from 250 to 400 kV/cm. It was found that the lifetime of the films increases with increasing the Nb level. The effective Schottky barrier height for 2 mol. % Nb-doped PZT films decreased from 1.12 to 0.85 eV during degradation. This is related to the movement of oxygen vacancies toward the cathode and the observation of Ti3+ near the cathode, which are proved via thermally stimulated depolarization current and electron energy loss spectroscopy, respectively. Furthermore, Schottky emission starts to control the conduction at lower electric fields after degradation as a result of oxygen vacancy accumulation near the cathode. This, in turn, decreases the potential barrier height for electron injection from the Pt electrode into the PZT films. The mechanisms for time-dependent dielectric breakdown in PZT films will thus be a strong function of the initial oxygen vacancy concentration and its distribution within the PZT films.

Barrier heights of polymer-electrode interfaces measured via photo injection current method

The metal-polymer contact interface plays a major role in the conduction and breakdown process in flexible dielectrics. Enhancing barrier heights has been investigated for effectively blocking the charge injection and thus impeding the degradation of the dielectrics in the field. However, the electronic band structure at the metal-dielectric interface and its relationship with the electrical insulation properties are still not well understood due to the complex morphological details of the interfacial region. Here, we determine the barrier height profiles of various sets of metal electrodes and dielectric polymers systems by utilizing the photo injection current method. Our results reveal that all aromatic polymers tend to have lower barrier heights than aliphatic polymers, and thus we believe that the chemical structure plays a prominent role in determining the barrier heights. Additionally, an important trend for aromatic polymers/metal configurations has been found that those aromatic polymers with higher glass transition temperatures feature however lower injection barrier heights. The correlations uncovered in this work unveil an effective approach of rational modifying the chemical structure to build polymers that can efficiently block charge injection for reliable insulation performance at elevated temperatures.

Contact‐Barrier Free, High Mobility, Dual‐Gated Junctionless Transistor Using Tellurium Nanowire

AbstractThe gate‐all‐around nanowire transistor, due to its extremely tight electrostatic control and vertical integration capability, is a highly promising candidate for sub‐5 nm technology nodes. In particular, the junctionless nanowire transistors are highly scalable with reduced variability due to avoidance of steep source/drain junction formation by ion implantation. Here a dual‐gated junctionless nanowire p‐type field effect transistor is demonstrated using tellurium nanowire as the channel. The dangling‐bond‐free surface due to the unique helical crystal structure of the nanowire, coupled with an integration of dangling‐bond‐free, high quality hBN gate dielectric, allows for a phonon‐limited field effect hole mobility of 570 cm2 V−1 s−1 at 270 K, which is well above state‐of‐the‐art strained Si hole mobility. By lowering the temperature, the mobility increases to 1390 cm2 V−1 s−1 and becomes primarily limited by Coulomb scattering. The combination of an electron affinity of ≈4 eV and a small bandgap of tellurium provides zero Schottky barrier height for hole injection at the metal‐contact interface, which is remarkable for reduction of contact resistance in a highly scaled transistor. Exploiting these properties, coupled with the dual‐gated operation, we achieve a high drive current of 216 μA μm−1 while maintaining an on‐off ratio in excess of 2 × 104. The findings have intriguing prospects for alternate channel material based next‐generation electronics.

Polarization-engineered AlGaN last quantum barrier for efficient deep-ultraviolet light-emitting diodes

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have been identified as a prospective mercury-free UV source. However, the observation of severe electron overflow and low hole injection efficiency in the conventional DUV LED deteriorates the device performance, attributing to the downward band bending as a result of the strong polarization-induced electric fields between the last quantum barrier (LQB) and the electron blocking layer (EBL). In this study, a composition-graded AlGaN layer with linearly increasing of Al composition from 0.5 to 0.65 is proposed to act as the LQB, replacing the conventional flat LQB to reduce the effective barrier height for hole injection while improving the electron blocking ability. Hence, a considerable enhancement of the output power can be obtained. Moreover, further investigations show that the thickness of graded LQB determine the band bending in the LQB and thus significantly suppress the electron leakage, eventually leading to a boosted output power. The thorough investigation on the LQB could pave the way towards the realization of efficient DUV LEDs of the future.

Charge transport properties of semiconducting natural rubber (Cis 1,4-polyisoprene)

For the first time, an effort has been made to evaluate the charge transport properties of semiconducting natural rubber. Four different ratios of iodine-rubber composites are synthesised and tested for charge transport by current density- voltage characteristics (J-V) and impedance spectroscopy. The optimal doping ratio for best mobility value is identified, and the effect of injection barrier height on mobility is discussed. An attempt has also been made to correlate the density of states (DOS) with mobility and doping ratio. The transport properties of semiconducting natural rubber are compared with one of the most popular p-type material, Poly(3-hexylthiophene-2,5-diyl) (P3HT) under the same ambience and experimental conditions to demonstrate its potential as a cost-effective and green alternative organic semiconductor.