Effective Potential Barrier Height Research Articles

Efficient and droop-free AlGaN-based UV-C LED using the inverted linearly graded active region and engineered hole source layer

The poor efficiency of the aluminum gallium nitride (AlGaN)-based ultraviolet (UV)-C light emitting diode (LED) remains as the major hindrance towards its commercialization. Thus, to enhance the internal quantum efficiency (IQE) and light output power (LOP) and to mitigate the efficiency droop of AlGaN-based UV-C LED, this simulation work proposes and examines a new engineering strategy, i.e., inverted linearly graded quantum wells along with quantum barriers over a series of samples A, B and C. To further improve the characteristics of AlGaN-based UV-C LED, an engineered linearly graded p-AlGaN layer has been proposed in the device structure. This increases the electron and hole concentrations by ∼2-fold and ∼8-fold, respectively. The 1.2-fold reduction in hole effective potential barrier height improves the hole injection. Also, the 1.5-fold increase in electron effective potential barrier height limits electron spill-off from the active region. The proposed structural-engineering reduces quantum confined stark effect (QCSE) confirmed by the reduction of quantum well slopes. Reducing carrier spill-off and QCSE increases average radiative recombination rate by ∼8-fold. The maximum IQE is improved by ∼44 % and the efficiency droop is also reduced to a promising level of ∼2 % in case of the sample under consideration. Finally, the LOP increases 10-fold, promisingly. Thus, the proposed structure is supposed to be highly potential for improving the UV-C LED performance. The electroluminescence spectra show that despite the gradual differences introduced in the structure, all the samples emit at ∼276 nm with similar full width at half maximum providing a reference for the comparison.

Voltage-margin limiting mechanisms of AlScN-based HEMTs

In this work, the off-state characteristics of AlScN/GaN high electron mobility transistors (HEMTs) grown by metalorganic chemical vapor deposition (MOCVD) were studied and directly compared to an AlGaN- and an AlN-HEMT grown in the same MOCVD. Pinch-off instability and leaky capacitive measurements were observed for AlScN-based HEMTs, which was correlated with a higher ideality factor and lower effective potential barrier height than the AlGaN and AlN-HEMTs. However, the reverse bias characteristics exhibited a sudden drain-current increase without a significant increase in gate-leakage current. The drain-leakage current is assumed to be related to a parasitic channel across the AlScN-barrier as a result of trap-assisted carrier transport with a Poole–Frenkel characteristic. The demonstrated pinch-off instability led to significant gain expansion in load-pull measurements and early soft-breakdown, which, in turn, limits the achievable voltage-margin. The results demonstrate a key issue to reveal the full potential of AlScN-based HEMTs for mm-wave applications.

The marvelous optical performance of AlGaN-based deep ultraviolet light-emitting diodes with AlInGaN-based last quantum barrier and step electron blocking layer.

The optoelectronic characteristics of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with quaternary last quantum barrier (QLQB) and step-graded electron blocking layer (EBL) are investigated numerically. The results show that the internal quantum efficiency (IQE) and radiative recombination rate are remarkably improved with AlInGaN step-graded EBL and QLQB as compared to conventional or ternary AlGaN EBL and last quantum barrier (LQB). This significant improvement is assigned to the optimal recombination of electron–hole pairs in the multiple quantum wells (MQWs). It is due to the decrease in strain and lattice mismatch between the epi-layers which alleviates the effective potential barrier height of the conduction band and suppressed the electron leakage without affecting the holes transportation to the active region. Moreover, to figure out quantitatively, the electron and hole quantity increased by ~ 25% and ~ 15%, respectively. Additionally, the IQE and radiative recombination rate are enhanced by 48% and 55%, respectively, as compared to conventional LED. So, we believe that our proposed structure is not only a feasible approach for achieving highly efficient DUV LEDs, but the device physics presented in this study establishes a fruitful understanding of III nitride-based optoelectronic devices.

Effectively modulating thermal activated charge transport in organic semiconductors by precise potential barrier engineering

The temperature dependence of charge transport dramatically affects and even determines the properties and applications of organic semiconductors, but is challenging to effectively modulate. Here, we develop a strategy to circumvent this challenge through precisely tuning the effective height of the potential barrier of the grain boundary (i.e., potential barrier engineering). This strategy shows that the charge transport exhibits strong temperature dependence when effective potential barrier height reaches maximum at a grain size near to twice the Debye length, and that larger or smaller grain sizes both reduce effective potential barrier height, rendering devices relatively thermostable. Significantly, through this strategy a traditional thermo-stable organic semiconductor (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, DNTT) achieves a high thermo-sensitivity (relative current change) of 155, which is far larger than what is expected from a standard thermally-activated carrier transport. As demonstrations, we show that thermo-sensitive OFETs perform as highly sensitive temperature sensors.

Advances in Novel Low-Macroscopic Field Emission Electrode Design Based on Fullerene-Doped Porous Silicon

Perspective low-macroscopic field (LMF) emission prototype cathodes based on fullerene C60—doped porous silicon were realized via a two-stage technique comprising the electrochemical etching process of a monocrystalline silicon wafer and functionalization of the acquired porous silicon (PS) matrix with silver-doped fullerene-based carbon structures. The resulting LMF cathode prototypes were studied with SEM and EDS techniques. The formation of an amorphous silver-doped C60-based layer consisting of nanosized aggregates on the matrix surface was established. The emission characteristics of the prototypes were analyzed, crucial parameters including threshold field strength values, emission current density, and effective potential barrier height for electrons were considered. A novel LMF emission model is suggested. It was established that the emitter prototypes realized during this study are on par with or superior to modern and promising field cathodes.

Temperature-dependent heterojunction device characteristics of n-ZnO nanorods/p-Si assembly

Heterojunction diode based on n-ZnO nanorods/p-Silicon (Si) assembly was fabricated, examined and reported here. Horizontal quartz tube thermal evaporation technique was used for the growth of ZnO nanorods on Si substrate. The nanorods were characterized by several techniques to examine the structural, morphological, scattering and electrical properties. Wurtzite hexagonal phase of the grown aligned nanorods was observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The appearance of a sharp Raman peak at 438 cm–1 was observed and it is related to the E2(high) mode of the wurtzite hexagonal phase of ZnO. The electrical properties of the fabricated heterojunction assembly were examined at different temperatures (298∼398 K) in both reverse and forward biased conditions, and a good stability was observed over the entire temperature range. A reduction in the turn-on and breakdown voltage was observed with increasing temperature. By increasing the temperature, the effective potential barrier height was increased, while quality factor was decreased. The observed activation energy was found to be ∼93.4 meV, higher than the exciton binding energy of ZnO.

Improved performance of AlGaN-based deep ultraviolet light-emitting diode using modulated-taper design for p-AlGaN layer

AlGaN-based deep ultraviolet light-emitting diodes adopting modulated-taper design for p-AlGaN layer have been investigated. It is found that remarkable enhancements both in light output power (LOP) and internal quantum efficiency (IQE) can be realized using the modulated-taper design compared to the conventional single-Al-component AlGaN ones. It is verified that the upward energy band of the last quantum barrier, especially symmetric ones, can increase effective potential barrier height for electrons and at the same time, can decrease the barrier height for holes, which is beneficial for high hole concentration distribution within the quantum wells, therefore accounting for the improved IQE as well as LOP.

Interfacial thermal and electrical transport properties of pristine and nanometer-scale ZnS modified grain boundary in ZnO polycrystals

Grain boundary plays an important role in energy carrier transport. By choosing ZnO as a model system of technological importance, and by measuring the thermal and electrical transport properties of ZnO polycrystals with a wide range of grain boundary spacing, we determine the interfacial thermal (Kapitza) resistance of the ZnO grain boundary Rk=4.0±0.7×10−9 m2 K W−1, which is relatively independent of grain size. The effective electron potential barrier height and the depletion width of the grain boundary generally increase with spacing, but they collapse below ∼100 nm and become almost invariant above ∼1 μm. When the grain boundary is locally modified by nanometer-thick ZnS thin film, the Kapitza resistance increases by more than three times, up to about 12.9×10−9 m2 K W−1, and the depletion region expands more than twice. The charge carrier concentration is influenced by the effective potential barrier height due to the grain boundary energy filtering effect whereas the electron mobility is related to the depletion width. Our investigations demonstrate the significance of grain boundary characteristics for interfacial and effective bulk transport properties. The findings and the approach are broadly important for polycrystalline materials of which the functional performance can be adjusted via grain boundary engineering.

Evaluation of the effective potential barrier height in nonlinear magnetization dynamics excited by ac magnetic field

An alternating current (ac) magnetic field or spin current can reduce the switching field of a ferromagnet through resonance excitation of a large-angle precession of magnetization. The nonlinear magnetization dynamics of this switching scheme completely differ from the general ferromagnetic resonance phenomenon, which is linearly excited by a small ac magnetic field. To understand these dynamics, it is necessary to evaluate the effective potential barrier height for switching, $\mathrm{\ensuremath{\Delta}}{U}^{\mathrm{eff}}$. However, most previous studies have measured the consequent precession angle in the nonlinear dynamics by magneto-optical methods and/or by applying a magneto-resistive effect. Here, we applied the cooperative switching method, which evaluates the $\mathrm{\ensuremath{\Delta}}{U}^{\mathrm{eff}}$ of the nonlinear dynamics under a sub-ns-wide magnetic field impulse, and observed a nontrivial reduction of $\mathrm{\ensuremath{\Delta}}{U}^{\mathrm{eff}}$ in a submicron-wide NiFe strip. The strong reduction of $\mathrm{\ensuremath{\Delta}}{U}^{\mathrm{eff}}$ under a negative magnetic field was caused by a saddle-node bifurcation in the nonlinear dynamics. In a micromagnetics simulation, we also confirmed that the magnetization is nonuniformly excited at the shallowest $\mathrm{\ensuremath{\Delta}}{U}^{\mathrm{eff}}$.

Optimizing interfacial transport properties of InO2 single atomic layers in In2O3(ZnO)4 natural superlattices for enhanced high temperature thermoelectrics.

In2O3(ZnO)k natural superlattices (where k is an integer), consisting of relatively earth abundant and non-toxic elements with coarsening-resistant nanostructures, are environmentally friendly materials with the potential for high temperature thermoelectric applications. Herein, we report our investigation of the high temperature thermoelectric properties of the In2O3(ZnO)4 superlattice bulk polycrystals that were singly and dually doped with Al and Ce. Transport property measurements revealed that Al and Ce did not only enter the ZnO blocks but also modified the InO2 single atomic layers. The effective electron potential barrier height of the superlattice interfaces can be adjusted by doping, and the optimal value that maximizes the power factor is of the order of kBT above the Fermi level. The interfacial thermal (Kapitza) resistance of the InO2 atomic sheets dramatically increased with doping, primarily accounting for the bulk thermal conductivity reduction. At the optimal crossing of the interfacial thermal resistance and the effective potential barrier height, a maximum ZT of ≈0.22 was achieved at 800 °C in the 1.6 mol% Al-doped superlattice, which was an enhancement of ∼200% over the pristine In2O3(ZnO)4. This work provides a new perspective on enhancing the high temperature thermoelectric performance of nanostructured oxides by synergistically optimizing the interfacial phonon and electron transport properties.

Comparative study on the InGaN multiple-quantum-well solar cells assisted by capacitance-voltage measurement with additional laser illumination

The investigation on the photovoltaic characteristics of two InGaN/GaN multiple-quantum-well (MQW) solar cells with different QW parameters, but having nearly the same spectral response range, is assisted by the capacitance-voltage (C-V) measurements. Based on the laser-assisted C-V technique as well as the X-ray reciprocal space mapping measurements, it is found the enhanced external quantum efficiency of solar cell consisting of the InGaN QWs having thicker well layer and lower In content can be ascribed to the lower effective potential barrier height, the larger active volume and the better material quality. In addition, it suggests that for InGaN MQW solar cells, a proper increase of the InGaN well layer thickness may be a good choice to extend the spectral response into longer wavelength region.

Design of Hole-Blocking and Electron-Blocking Layers in AlxGa1-xN-Based UV Light-Emitting Diodes

The band-engineered structure design for electron-blocking layer (EBL) and hole-blocking layer (HBL) in Al x Ga1– x N-based ultraviolet light-emitting diodes (UV LEDs) is performed and analyzed theoretically. Simulation results show that the severe polarization effect is efficiently mitigated and the downward-bended band profile of the EBL is improved when the EBL is designed with a graded-composition and multiquantum barrier (GMQB) structure. As a result, the capabilities of both electron confinement and hole injection, and also the light output power are promoted. On the contrary, for the HBL, the design of composition graded and/or multiquantum barrier structures reduces the effective potential barrier height for holes in the valence band and, consequently, causes a considerable hole overflow. The UV LED, thus, exhibits superior optical performance when the LED structure is simultaneously designed with a GMQB EBL and a bulk HBL.

Additional electric field in real trench MOS barrier Schottky diode

In real trench MOS barrier Schottky diode (TMBS diode) additional electric field (AEF) the whole is formed in the near contact region of the semiconductor and its propagation space is limited with the barrier metal and the metallic electrodes of MOS structures. Effective potential barrier height TMBS diode is formed via resulting electric field of superposition AEF and electric field of space charge region (SCR) semiconductor. The dependence of the resulting electric field intensity of the distance towards the inside the semiconductor is nonlinear and characterized by a peak at a certain distance from the interface. The thickness of the SCR in TMBS diode becomes equal to the trench depth. Force and energy parameters of the AEF, and thus resulting electric field in the SCR region, become dependent on the geometric design parameters TMBS diode. The forward I-V characteristic TMBS diode is described by the thermionic emission theory as in conventional flat Scottky diode, and in the reverse bias, current is virtually absent at initial voltage, appears abruptly at a certain critical voltage.

Electrophysical properties of Schottky diodes with inhomogeneous contact surface

In Cu–nSi Schottky diode (SD) with inhomogeneous contact surface at the active participation of the additional electric field (AEF) is formed a effective potential barrier height. Forward and initial reverse I–V characteristics SD are determined by the current flowing through almost the entire contact surface and is well described by the thermionic emission theory, as in the idealized homogeneous SD. In a forward and primary reverse bias the effective potential barrier height, the ideality factor (dimensionless coefficient), the contact resistance, the peripheral area and width of the contact surface, the contribution of peripheral current to the total current DS have appropriate similar values. The contribution of peripheral current to the total current SD increases with increasing reverse voltage. The second portion of the reverse I–V characteristics SD, which consists only of peripheral current is represented by a straight line in a semi-logarithmic scale. Its electrophysycal parameters are differed from that for of the first initial portion I–V characteristics SD.

Investigation of carrier transport behavior in amorphous indium–gallium–zinc oxide thin film transistors

Behaviors of carrier transport in amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistors are investigated. It is found that the electron mobility is higher at elevated temperatures, which is contrary to that in crystalline Si devices. Drain current enhancement with regard to temperature at corresponding gate voltage follows the Arrhenius equation. This implies that carrier transport is limited by the potential barrier heights induced by trap states within IGZO, and therefore current conduction is heat-activated to overcome those barriers. In addition, the extracted activation energy decreases with increasing gate voltage, indicating the effective potential barrier height is lowered when abundant electrons are injected into the channel. Furthermore, the relationship between carrier mobility and carrier concentration is also investigated, with the carrier mobility monotonically increasing with carrier concentration. Such behavior can be ascribed to a lowered effective barrier above the conduction band when the Fermi-level rises.

Electrical Polarization Effects on the Optical Polarization Properties of AlGaN Ultraviolet Light-Emitting Diodes

In this paper, the electrical polarization effects on the optical polarization properties of Al 0.35 Ga 0.65 N multiple quantum well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) by varying the Al content in AlGaN quantum well (QW) barriers are qualitatively analyzed. Numerical simulation results show that the varied potential barrier height and different polarization-induced electric field resulting from varying the Al content in AlGaN QW barriers have great influence on the transverse electric (TE) and transverse magnetic (TM) emission intensities. Both the TE- and TM-polarized spontaneous emissions from the Al 0.35 Ga 0.65 N QWs decrease with the increase of the Al content in AlGaN QW barriers, which is attributed to the poor carrier confinement resulting from the more serious band bending effect and the reduced effective potential barrier height in the conduction band, especially, the degree of optical polarization increases with the increase of the Al content in AlGaN QW barriers due to the rearrangement of the valence subbands near the Γ-point of the Brillouin zone. It is consequently concluded that the TE and TM spontaneous emissions and the degree of optical polarization are closely related to the Al content in AlGaN QW barriers of the Al 0.35 Ga 0.65 N MQW UV LEDs.

Enhanced magnetic and electrical properties of Y and Mn co-doped BiFeO3 nanoparticles

Multiferroic (Y, Mn) substituted BiFeO3 had been synthesized by a facial sol–gel method. The single phase polycrystalline nature of samples was confirmed from X-ray diffraction pattern. The average particle size was estimated to be around 30–32nm from transmission electron microscopy. The magnetic properties of codoped nanoparticles had been studied using Bloch equation and the estimated value of the Bloch constant was found to be much larger than usual ferromagnetic materials. Coercivity values for different temperature is used to calculate the blocking temperature and found to lie above room temperature. The dc electrical transport properties were studied in the temperature range 298– 523K and explained using a Motts 3D variable range hopping model and the density of states was estimated near the Fermi level. The ac electrical data were found to follow the correlated barrier hopping model. Well-developed PE hysteresis loops were observed in codoped nanoparticles, which were attributed to a decrease in oxygen vacancies, bismuth volatisation due to doping and an increase of the effective potential barrier height for charge carriers.

Manipulating transport through a single-molecule junction

Molecular Electronics deals with the realization of elementary electronic devices that rely on a single molecule. For electronic applications, the most important property of a single molecule is its conductance. Here we show how the conductance of a single octanethiol molecule can be measured and manipulated by varying the contact's interspace. This mechanical gating of the single molecule junction leads to a variation of the conductance that can be understood in terms of a tunable image charge effect. The image charge effect increases with a decrease of the contact's interspace due to a reduction of the effective potential barrier height of 1.5 meV/pm.

Nano inhomogeneity effects on small Ag/n-Si Schottky diode parameters at high temperature

Schottky diodes with an Ag/n-Si/W/Cu structure and 100 μm in diameter were studied. Analyzing the silver metal surface coating on the n-Si substrate using a scanning probe microscopy (SPM) device showed a large number of nano patches in the surface with dimensions of 0 to 100 nm. The potential distribution of the patches revealed that the potential of each patch with the neighboring patches was different. The electrical characteristics of the devices were studied between temperature ranges of 300 and 380 K. When the temperature ideality factor approximately increases, the potential barrier height decreases. The potential barrier height was calculated separately from the I—V and C—V characteristics. The main reasons for the significant difference between room temperature and higher temperatures were the differences in patch distribution, the different potentials of each patch, and the interactions between them. The effective potential barrier height depended on the degree of inhomogeneity, and thus the operating potential barrier height in the contact surface was smaller than the average value, and the ideality factor was more than unitary. With the increase in the potential value, the ideality factor becomes close to unitary, and with increasing temperatures, the ideality factor is increased. In this case, the maximum potential barrier height accrues at a greater distance from the metal contact. For this reason, at high temperatures the average value of the potential barrier height is smaller. Moreover, with increasing temperature, the ideality factor is increased.

A comparison of attack angle dependence of exchange channel of reaction H + HS (v = 0, 1; j = 0) on 3A″ and 3A′ surfaces

In this theoretical work, we report quasiclassical dynamics predictions for the attack angle-dependence exchange processes for the H + HS (v = 0, 1; j = 0) reaction by using the new triplet 3A″ and 3A′ potential energy surfaces, respectively. The calculated quasiclassical reaction probabilities of exchange reaction channel of reaction H(D)′ + H(D)S for J = 0, 10, 20, 30, 40 are in good agreement with quantum wave packet results over the collision energy range from 0.1 to 2.0 eV on 3A″ surfaces. The attack angle dependence reaction probability of the title reactions at J = 0 are calculated, respectively, on the two surfaces. The reaction probability was found to be strongly dependent on the attack angle. It may be ascribe to the significant difference of the effective potential barrier height in the two reactions. Besides, the reaction probabilities of exchange reaction channel of reaction H(D)′ + H(D)S for J = 0, 10, 20, 30, 40 are also predicted on 3A′ surfaces. © 2013 Wiley Periodicals, Inc.