Effective Density Of States Research Articles

Modeling the Conduction Mechanism in Chemoresistive Gas Sensor Based on Single-Crystalline Sn3O4 Nanobelts: A Phenomenological In Operando Investigation.

Investigating the sensing mechanisms in semiconducting metal oxide (SMOx) gas sensors is essential for optimizing their performance across a wide range of potential applications. Despite significant progress in the field, there are still many gaps in comprehending the phenomenological processes occurring in one-dimensional (1D) nanostructures. This article presents the first insights into the conduction mechanism of chemoresistive gas sensors based on single-crystalline Sn3O4 nanobelts using the operando Kelvin Probe technique. From this approach, direct current (DC) electrical resistance and work function changes were simultaneously measured in different working conditions, and a correlation between the conductance and the surface band bending was established. Appropriate modeling was proposed, and the results revealed that the conduction mechanism in the single-crystalline one-dimensional nanostructures closely aligns with the behavior observed in single-crystalline epitaxial layers rather than in polycrystalline grains. Based on this assumption, relevant parameters were further estimated, including Debye length, concentration of free charge carriers, effective density of states in the conduction band, and position of the Fermi level. Overall, this study provides an effective contribution to understanding the role of surface chemistry in the transduction of the electrical signal generated from gas adsorption in single-crystalline one-dimensional nanostructures.

Experimental determination of intrinsic carrier density in 4H-SiC based on electron diffusion current in an npn bipolar junction transistor

The intrinsic carrier density of 4H-SiC at temperatures ranging from 294 to 595 K was derived by analyzing a collector current in an npn-type SiC bipolar junction transistor, the structure of which was designed based on a device simulation. The obtained intrinsic carrier density was in good agreement with the value calculated from the bandgap and effective densities of states taking multiple and non-parabolic SiC bands into account. The coincidence of the intrinsic carrier density obtained by these two different approaches indicates the usefulness of the proposed method and the validity of the evaluated value of intrinsic carrier density. The temperature dependence of the bandgap was also estimated from the deduced intrinsic carrier density and compared with an empirical formula. The derived bandgap agreed well with the empirical formula showing bandgap shrinkage at high temperatures. The errors in evaluating the intrinsic carrier density and the bandgap caused by the estimation of the hole density and electron mobility in the base layer are also discussed for the proposed method.

Density of states effects on emission and scattering of photons in plasmas

Plasma supports electromagnetic waves propagation for frequencies higher than plasma frequency but features dielectric permittivity less than 1. This property leads to photon density of states (DOS) lower than in vacuum and should result in subnatural spectral linewidths, sub-Planckian spectrum of thermal radiation, and sub-Rayleigh scattering as well as in lower inelastic photon scattering including Raman scattering. Lamb shift will be altered as well though the decisive contribution from high-energy modes in this case makes the photon DOS effect rather small since plasma DOS converges with the vacuum one in the limit of infinite frequencies. We emphasize the basic character of all these phenomena though absolute values of corrections in many real experiments may appear to be small as compared to other factors. We found that dissipative losses make possible DOS effects smaller though not vanishing and additionally bring about indefinite growth of DOS in the low-frequency limit.

Improvement of Absorption and Emission Phenomena of 1.55µm Quantum Dot Laser using Indium Nitride

This paper presents the improvement of emission and absorption phenomena of 1.55µm quantum dot laser by enhancing the stability of oscillation frequency and minimizing absorption loss. Among the key parameters related to these absorption and emission phenomena, the effective density of state, rate of change of carrier mobility, and optical feedback level were extensively investigated to reduce the fluctuation of the oscillation frequency of the emitted light as well as the absorption loss. These phenomena were investigated through mathematical analysis and numerical simulation using AlN, GaN, and InN quantum dots as the active material of the laser structure. The numerical results were compared, confirming that the InN quantum dot in the active layer significantly improved the absorption and emission phenomena of the quantum dot laser. At the same time, InN is capable of emitting light at a wavelength of 1.55µm due to its band gap of 0.7eV. Consequently, InN has a superior potentiality to other existing materials to fabricate the quantum dot laser operating at the 1.55µm wavelength, which is the window of an optical fiber communication system that offers the lowest attenuation. Therefore, the InN quantum dot laser is expected to be a promising candidate not only in the field of semiconductor technology but also in the field of optical communication in the imminent future.

Voltage Deficit in Wide Bandgap Perovskite Solar Cells: The Role of Traps, Band Energies, and Effective Density of States

Wide‐bandgap (1.7 eV) perovskite solar cells (PSCs) are plagued by relatively low open‐circuit voltages. This is problematic as they are key to achieving perovskite silicon tandems, which can boost the potential of silicon solar cells. Performance in PSCs is widely considered to be limited by recombination at the interface between the perovskite and the transport layer (TL). Here, a number of design rules to increase the open‐circuit voltage of wide‐bandgap PSCs are introduced. A numerical device model that includes a detailed description of the interfacial recombination processes is presented. The combined effects of interface traps, ions, band alignment, and transport properties are introduced to identify the critical parameters for improving the open‐circuit voltage. A large number of devices are simulated by picking random combinations of parameters and are looked for trends. It is shown that interface recombination can be suppressed by reducing the minority carrier density close to the interface with the TLs. It is demonstrated that the alignment of energy levels is only part of the story; the effective densities of states are of equal importance. The results pave the way to achieving high open‐circuit voltages, despite a significant density of interface defects.

Towards a DFT-based layered model for TCAD simulations of MoS2

In this work, we employ the results of atomistic DFT calculation to extract useful parameters for the simulation of few-layers MoS2 structures with traditional TCAD tools. In particular, we focus on the charge distribution, which allows us to obtain a layered model for the dielectric constant, and on the effective densities of states in the conduction and valence bands taking into account the full 2D density of states. Using this model, we compute the capacitance of a metal–oxide–semiconductor structure and compare it to the one obtained employing a uniform model with averaged effective parameters.

Density of States Effects on Emission and Scattering of Photons in Plasmas

Plasma supports electromagnetic waves propagation for frequencies higher than plasma frequency but features dielectric permittivity less than 1. This property leads to photon density of states (DOS) lower than in vacuum and should result in subnatural spectral linewidths, sub-Planckian spectrum of thermal radiation, and in sub-Rayleigh photon scattering as well as in lower inelastic photon scattering including Raman scattering in molecular and crystalline structures. We emphasize the basic character of these phenomena though absolute values of corrections from this property of plasmas in many real experiments may appear to be small as compared to other factors. We found that dissipative losses when being taken into account make possible DOS effects smaller though not vanishing and additionally bring about indefinite growth of DOS in the low-frequency limit.

Simulation of the performance of organic solar cells based on D1-BT-EDOT-BT-D2-A/PCBM structures

The analysis of microelectronic and photonic structures-one dimension (AMPS-1D) was used to study and simulate the performance of organic heterojunction solar cells based on D1 –B-Edot-B-D2-A as electron donors, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor. The organic photovoltaic cell devices using T3-B-Edot-B-T3-A/PCBM showed the improved open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, and power conversion efficiency PCE values for the optimum thickness of 120 nm and the effective state density of electrons and holes of 1021cm−3.The P3-B-Edot-B-T3-A/PCBM and P3-B-Edot-B-P3-A/PCBM based devices exhibited a power conversion efficiency (PCE) of 9.295% and 8.735%, respectively, which outperformed the corresponding T3-B-Edot-B-T3-A/PCBM, Cbz-B-Edot-B-T3-A/PCBM, F-B-Edot-B-T3-A/PCBM, and A-B-Edot-B-T3-A/PCBM based devices (7.330, 6.622, 7.226, and 7.327%). More importantly, the P3-B-Edot-B-T3-A/PCBM and P3-B-Edot-B-P3-A/PCBM -based device delivered the highest PCE of 14.432% and 15.031% respectively, when we deposit a layer of PEDOT between the indium tin oxide (ITO) and the active layer, which is a clear improvement over other results in the literature.

Experimental Studies on the Dynamic Memcapacitance Modulation of the ReO3@ReS2 Composite Material-Based Diode.

In this study, both memcapacitive and memristive characteristics in the composite material based on the rhenium disulfide (ReS2) rich in rhenium (VI) oxide (ReO3) surface overlayer (ReO3@ReS2) and in the indium tin oxide (ITO)/ReO3@ReS2/aluminum (Al) device configuration is presented. Comprehensive experimental analysis of the ReO3@ReS2 material properties’ dependence on the memcapacitor electrical characteristics was carried out by standard as well as frequency-dependent current–voltage, capacitance–voltage, and conductance–voltage studies. Furthermore, determination of the charge carrier conduction model, charge carrier mobility, density of the trap states, density of the available charge carrier, free-carrier concentration, effective density of states in the conduction band, activation energy of the carrier transport, as well as ion hopping was successfully conducted for the ReO3@ReS2 based on the experimental data. The ITO/ReO3@ReS2/Al charge carrier conduction was found to rely on the mixed electronic–ionic processes, involving electrochemical metallization and lattice oxygen atoms migration in response to the externally modulated electric field strength. The chemical potential generated by the electronic–ionic ITO/ReO3@ReS2/Al resistive memory cell non-equlibrium processes leads to the occurrence of the nanobattery effect. This finding supports the possibility of a nonvolatile memory cell with a new operation principle based on the potential read function.

Effect of CuIn1−xAlxSe2 (CIAS) thin film thickness and diode annealing temperature on Al/p-CIAS Schottky diode

Al/p-CIAS Schottky diodes were fabricated by depositing aluminium (Al) on different flash evaporated copper–indium–aluminium–diselenide (CIAS) films of varying thickness. Further, all diodes were annealed at 573 K for an hour. The influence of p-CIAS film thickness and the thermal annealing of Al/p-CIAS Schottky diode were investigated by observing current–voltage (I–V) and capacitance–voltage (C–V) characteristics at room temperature. Various diode parameters, such as ideality factor (η), barrier height (ϕbo) and series resistance (Rs) were calculated using Cheung’s and Norde methods. ϕbo found to increase with annealing as well as with increase in the film thickness. However, the value of η and Rs decreases with annealing and CIAS thickness. The effective density of states (Nv), acceptor density of states (NA) and barrier height have been calculated from C–V measurements. Values obtained from CV analysis were well matched with I–V results. The value of Nv decreases and the value of NA increases with the increase in the film thickness. Using I–V and C–V parameters, energy band gap for the prepared Al/p-CIAS diodes has been reconstructed.

Effect of different device parameters on tin-based perovskite solar cell coupled with In2S3 electron transport layer and CuSCN and Spiro-OMeTAD alternative hole transport layers for high-efficiency performance

ABSTRACT SCAPS 1-D was used for the simulation of lead-free environmentally benign methylammonium tin-iodide (CH3NH3SnI3) based solar cell. Indium sulfide (In2S3) was utilized as the electron transport layer (ETL) for its high carrier mobility and optimized band structure, unlike traditional titanium oxide (TiO2) ETL. Traditional expensive spiro-OMeTAD (C81H68N4O8) and cheaper cuprous thiocyanate (CuSCN) were utilized alternatively as hole transport layer (HTL) to observe the effect of different HTL on cell performance. We investigated the trend in electrical measurements by altering parameters such as thickness, defect density, valence band (VB) effective density of state and bandgap of the absorber layer, interfacial trap densities, and defect density of ETL. At optimum condition, the device revealed the highest efficiency of 18.45% for CuSCN (HTL) and 19.32% for spiro-OMeTAD (HTL) configuration. The effect of working temperature, the wavelength of light and band-to-band radiative recombination rate was also observed for both configurations. All these simulation results will help to fabricate eco-friendly high-efficiency perovskite solar cell by replacing the commonly used toxic lead-based perovskite.

Density of states of a coupled-channel system

We demonstrate how an effective density of states can be derived from the S-matrix describing a coupled-channel system. Besides the locations of poles, the phase of the determinant of the S-matrix encodes essential details in characterizing the dynamics of resonant and non-resonant interactions. The density of states is computed for the two channel scattering problem ($\pi\pi, K \bar{K}$, S-wave), and the influences from the various dynamical structures: poles, roots, branch cuts, and Riemann sheets, are examined.

Correlating charge and thermoelectric transport to paracrystallinity in conducting polymers

The conceptual understanding of charge transport in conducting polymers is still ambiguous due to a wide range of paracrystallinity (disorder). Here, we advance this understanding by presenting the relationship between transport, electronic density of states and scattering parameter in conducting polymers. We show that the tail of the density of states possesses a Gaussian form confirmed by two-dimensional tight-binding model supported by Density Functional Theory and Molecular Dynamics simulations. Furthermore, by using the Boltzmann Transport Equation, we find that transport can be understood by the scattering parameter and the effective density of states. Our model aligns well with the experimental transport properties of a variety of conducting polymers; the scattering parameter affects electrical conductivity, carrier mobility, and Seebeck coefficient, while the effective density of states only affects the electrical conductivity. We hope our results advance the fundamental understanding of charge transport in conducting polymers to further enhance their performance in electronic applications.

Experimental studies on the conduction mechanism and electrical properties of the inverted Ba doped ZnO nanoparticles based memristor

In this study, the determination of the charge carrier conduction in a memristor based on barium doped zinc oxide (Ba0.95ZnO1-x) nanoparticles (NPs) in an indium tin oxide (ITO)/Ba0.95ZnO1-xNP/aluminium (Al) configuration, as well as an explanation of the resistance switching mechanism, was successfully carried out. The ITO/Ba0.95ZnO1-xNP/Al memristor charge carrier conduction was found to be dependent on the bulk charge conduction model of the space charge limited current. Furthermore, comprehensive quantitative analysis of the electrical properties of the memristor, such as the density of the trap states (Nt), capture cross section of trap (σt), frequency of escape (f), available charge carrier density (no), effective density of states in the conduction band (NC), maximum of dielectric relaxation time (τd), and charge carrier mobility (μ), was conducted based on the experimental data fitting. The switching behavior was found to be governed by an oxygen vacancy migration, and conducting filaments grow and rupture. The nonideality of the memristance of the ITO/Ba0.95ZnO1-xNP/Al device points to an excellent prospect of its application as an active platform for the development of novel nonlinear memristor based security systems.

Origin of the Below-Bandgap Turn-On Voltage in Light-Emitting Diodes and the High VOC in Solar Cells Comprising Colloidal Quantum Dots with an Engineered Density of States.

The turn-on voltage of a light-emitting diode (LED) is an important parameter as it determines the power consumption of the LED and influences the effective power conversion efficiency. LEDs based on nanoscale engineering of the blended PbS [mixture of quantum dots (QDs) with two different bandgaps] colloidal QDs have recently shown record performance in the infrared region. One of the most intriguing findings for these blended devices is the substantially lower-than-bandgap turn-on voltage and the achievement of an open circuit voltage ( VOC), approaching the radiative limit. In this work, we provide new insight into the origin of these phenomena. We posit that the decrease in the effective density of states in the conduction and valence band of the emitter QDs in the blended structures modifies the chemical potential, which controls the turn-on voltage for LEDs through the modification of the photon flux and VOC for photovoltaic devices with quasi-Fermi level modification.

Simulation of dark current characteristics of type-II InAs/GaSb superlattice mid-wavelength infrared p–i–n photodetector

For photosensitive devices such as infrared (IR) photodetectors, dark current is an important mechanism limiting the performance because it causes a decrease in the signal-to-noise ratio and the responsivity. The main objective of this work is to understand and analyze the InAs/GaSb type-II superlattice (SL) IR photodiode using a device simulator especially focusing on the dark current characteristics. Physical parameters such as the effective density of states and the effective masses are extracted from the k·p band calculation, and included into a two-dimensional device simulator based on the drift-diffusion model. Simulation results demonstrate that the leakage current of the IR photodiode depends on the SL thickness ratio even with the same cut-off wavelength. In the low reverse bias conditions, the “InAs-rich” SL detectors exhibit smaller dark current than the “GaSb-rich” ones, which originates from the difference in the intrinsic carrier density. On the other hand, under the higher voltage and the higher doping level, the larger effective mass of “GaSb-rich” SL can suppress the dark current caused by the tunneling-related leakage mechanisms.

Short Note on the Density of States in 3D Weyl Semimetals.

The average density of states in a disordered three-dimensional Weyl system is discussed in the case of a continuous distribution of random scattering. Our results clearly indicate that the average density of states does not vanish, reflecting the absence of a critical point for a metal-insulator transition. This calculation supports recent suggestions of an avoided quantum critical point in the disordered three-dimensional Weyl semimetal. However, the effective density of states can be very small such that the saddle-approximation with a vanishing density of states might be valid for practical cases.

Charge carrier transport and electroluminescence in atomic layer deposited poly-GaN/c-Si heterojunction diodes

In this work, we study the charge carrier transport and electroluminescence (EL) in thin-film polycrystalline (poly-) GaN/c-Si heterojunction diodes realized using a plasma enhanced atomic layer deposition process. The fabricated poly-GaN/p-Si diode with a native oxide at the interface showed a rectifying behavior (Ion/Ioff ratio ∼ 103 at ±3 V) with current-voltage characteristics reaching an ideality factor n of ∼5.17. The areal (Ja) and peripheral (Jp) components of the current density were extracted, and their temperature dependencies were studied. The space charge limited current (SCLC) in the presence of traps is identified as the dominant carrier transport mechanism for Ja in forward bias. An effective trap density of 4.6 × 1017/cm3 at a trap energy level of 0.13 eV below the GaN conduction band minimum was estimated by analyzing Ja. Other basic electrical properties of the material such as the free carrier concentration, effective density of states in the conduction band, electron mobility, and dielectric relaxation time were also determined from the current-voltage analysis in the SCLC regime. Further, infrared EL corresponding to the Si bandgap was observed from the fabricated diodes. The observed EL intensity from the GaN/p-Si heterojunction diode is ∼3 orders of magnitude higher as compared to the conventional Si only counterpart. The enhanced infrared light emission is attributed to the improved injector efficiency of the GaN/Si diode because of the wide bandgap of the poly-GaN layer and the resulting band discontinuity at the GaN/Si interface.

Instability of insulator state towards nanocrystallinity in [formula omitted] compound: Enhancement of low field magnetoresistance

The present study shows the modification of the insulator state to metallic state with reduction of particle size in (La0.5Y0.5)0.7Ca0.3MnO3 compound by magnetotransport measurements. The decrease in the activation energy as well as increase in the effective density of states near Fermi level is observed with particle size reduction. This modification leads to the decrease in the resistivity in the nanoparticle in the low-temperature regime (T<150K). On the other hand, the temperature dependent dc susceptibility data shows the evolution of the non-Griffiths phase to Griffiths phase with reduction of particle size in the temperature range 100K<T⩽200K. This evolution from non-Griffiths phase to Griffiths phase leads to the formation of large ferromagnetic clusters at the expense of antiferromagnetic interactions which is responsible for the non-Griffiths phase. The presence of the large ferromagnetic clusters helps to create percolation path for electronic transport and is the probable reason for the enhancement of magnetoresistance in this temperature range. On the other hand, enhancement of low field magnetoresistance (LFMR) observed below 100 K has been attributed to the increased spin polarized tunneling (SPT) component due to the increase in the size of the ferromagnetic clusters in the nanoparticle compares with that of bulk.

NUMERICAL SIMULATION OF THERMAL BEHAVIOR AND OPTIMIZATION OF a-Si/a-Si/C-Si/a-Si/A-Si HIT SOLAR CELL AT HIGH TEMPERATURES

Purpose. Silicon heterostructure solar cells, particularly Heterojunction with Intrinsic Thin layer (HIT) cells, are of recommended silicon cells in recent years that are simply fabricated at low processing temperature and have high optical and temperature stability and better efficiency than homojunction solar cells. In this paper, at first a relatively accurate computational model is suggested for more precise calculation of the thermal behavior of such cells. In this model, the thermal dependency of many parameters such as mobility, thermal velocity of carriers, band gap, Urbach energy of band tails, electron affinity, relative permittivity, and effective density of states in the valence and conduction bands are considered for all semiconductor layers. The thermal behavior of HIT solar cells in the range of 25-75 °C is studied by using of this model. The effect of the thickness of different layers of HIT cell on its external parameters has been investigated in this temperature range, and finally the optimal thicknesses of HIT solar cell layers to use in wide temperature range are proposed .