Free Carrier Density Research Articles

Theoretical Study of Electronic Transport in Two-Dimensional Transition Metal Dichalcogenides: Effects of the Dielectric Environment

We discuss the effect of the dielectric environment (substrate/bottom oxide, gate insulator, and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations (``remote phonon scattering''). We find that the presence of insulators with a high dielectric constant may improve significantly the carrier mobility. However, scattering with the interface hybrid excitations negates this gain and degrades the mobility significantly below its free-standing value. We find that this process is dominated by long-wavelength interactions that, for the carrier sheet density of interest, are strongly affected by the coupling with the 2D plasmons. Considering 2D layers in a double-gate geometry with ${\mathrm{Si}\mathrm{O}}_{2}$ as bottom oxide and various top-gate insulators, we find that the mobility decreases as the top-insulator dielectric constant increases (from hBN to ${\mathrm{Zr}\mathrm{O}}_{2}$), as expected. However, we observe two main deviations from this trend: high mobility is predicted in the case of the weakly polar hBN, and much lower than expected mobility is calculated in the case of gate-insulator/TMD/bottom-oxide stacks in which two or more polar materials have optical phonons with similar resonating frequencies. We also find that the effect of screening by metal gates is noticeable but not particularly strong. Finally, we discuss the effect of the TMD dielectric constant, of the free-carrier density, and of temperature on the transport properties of TMD monolayers.

Anisotropic photoconduction in ultrathin CuO: A nonreciprocal system?

With the recent global surge in the research on perovskite halides, CuO is one of the binary oxides, which gets attention as a hole transport material. In centrosymmetric CuO, parity-time (PT) violation leads to photoconduction. The PTsymmetry can be preserved if the system were non-reciprocal. Thus, in the current work, we fabricated an ultra-thin film of CuO using pulsed laser deposition and observed anisotropic photoconduction. The semiconductor parameters estimated from the photoresponse suggest that the relative value of free charge carrier density is neither altered significantly with thickness reduction nor with light exposure as it is quite low (∼10−7) suggesting high trap (deep) density. Further, anisotropic photocurrent in the absence of an electric field suggests the alteration in electromagnetic potential due to the existence of self-biasing and structural asymmetry. The application of Gauge field variance on 2D photonic metasurface reveals the non-chiral nature. It is suggesting T-symmetry breaking, and, therefore, the possibility of the photonic Aharonov–Bohm effect is expected in CuO thin films.

A rigorous Fermi-Dirac statistics-based MOSFET channel surface potential equation using polylogarithms

The MOSFET channel Surface Potential Equation (SPE), being the fundamental building block of all Surface Potential (SP)-based MOSFET compact models, must be as physically faithful as possible. To that end, in this article we propose a novel alternative form of the SPE, which is valid under many circumstances and operating conditions (e.g. degenerate doping, incomplete dopant ionization, strong carrier inversion and accumulation, cryogenic operation) where the occupation probabilities of charges cannot be well described by the Maxwell-Boltzmann (MB) statistics approximation. Thus, to more accurately describe free charge carrier densities they are defined here by their corresponding Fermi-Dirac Integrals (FDI). But, unlike previous formulations of the SPE directly expressed in terms of FDIs or some analytic approximation thereof, in our formulation we substitute from the beginning the FDIs by their equivalent polylogarithm representations, before integrating Poisson’s equation to obtain the proposed SPE. Since polylogarithms are functions that may be analytically differentiated and integrated, and because there already exist well established algorithms and software routines for their efficient numerical computation, the proposed polylogarithmic form of the MOSFET channel SPE constitutes a more accurate alternative to existing SPE approximate forms.

An Analytical Drain Current Model of ZnO-Based Amorphous Oxide Semiconductor Thin-Film Transistors

An analytical one-piece drain current model of ZnO-based amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) is presented in this article. This model is able to solve the main obstacle in a lack of analytical and physics-based nonregional device modeling for AOSTFTs and is suitable to be implemented into circuit simulators. Based on the effective-charge-density approach, free carrier density and localized carrier density, including tail and deep states, are unified into total carrier density. Then, one-piece surface potential is derived analytically from equivalent 1-D Poisson’s equation described in an effective-charge-density approach. It is also possible that channel carrier mobility is calculated from the ratio between free carrier and effective total carrier densities. Furthermore, drain current expression is derived analytically and validated by experimental data. Good agreements of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> characteristics prove that the proposed model could serve as a useful simulation tool implemented into circuit simulators.

Machine learning for enhanced semiconductor characterization from time-resolved photoluminescence

<h2>Summary</h2> To enhance the accuracy and effectiveness of optoelectronic characterization, Bayesian inference has been applied to the statistical analysis of time-resolved photoluminescence (TRPL) data with large-scale graphics-processing unit (GPU)-based simulations of electron dynamics. A simulated TRPL dataset, derived from a CH₃NH₃PbI_3-xCl_x perovskite absorber, was used as a case study. From a power scan, Bayesian inference extracts values of the (ambipolar) carrier mobility, free-carrier density, radiative recombination rate, and the overall lifetime of the nonradiative recombination processes. Analysis of the experimental TRPL data yields similar parameter values. The independent contributions of bulk and surface recombination can be distinguished via the introduction of an additional sample thickness. Further experiments separate the front and back surface recombination velocities and can distinguish the electron and hole mobilities. Ultimately, Bayesian inference enables a significant increase in the information yield from TRPL measurements.

Tailoring linear and nonlinear surface plasmon responses in borophene nanostructures

The newly reported 2D material ‘borophene’ provides a novel building block for nanoscale materials and devices. In this work, the linear and nonlinear plasmonic response of the electric dipole (ED) moment in metallic borophene is theoretically investigated. In our proposed model, a borophene nanostructure is deposited on the top of a dielectric layer sandwich with a silver layer acting as a mirror. It was found that the scattering at the scattering peak originates mainly from the exciting total ED. Our calculations demonstrate that scattering in the proposed model can be tuned well with carrier relaxation time, effective electron mass and free carrier density. The strongly localized fundamental field induces the desired increase in the second harmonic wave, which is discussed in detail by introducing the second-order nonlinear source. In addition, the evolution of the lifetime of linear and nonlinear plasmonic modes is also investigated, which helps us to study the underlying mechanism of microprocessing in the borophene plasmonic–photonic interaction. The manipulation of plasmonic behaviour and lifetime evolution makes borophene an excellent platform for tunable plasmonic–photonic devices.

Improved solar water splitting performance of BiVO4 photoanode by the synergistic effect of Zr-Mo co-doping and FeOOH Co-catalyst layer

• Synthesis of Zr-Mo:BiVO 4 electrocatalyst via electrodeposition process. • Synergistic effect of Zr-Mo co-doping and FeOOH co-catalytic layer over BiVO 4 electrodes with enriched PEC performances. • A photocurrent of 3.34 mA cm −2 with 13-fold enhancement is achieved. In this work, Zr-Mo co-doping was successfully employed on electrochemically deposited BiVO 4 photoanodes, and tested for photoelectrochemical (PEC) water oxidation. Zr doping induced the formation of relatively smaller morphological features resulting in improved light absorption and PEC activity. Mo doping resulted in structural damage that enhanced the charge carrier separation and increased free carrier density. The synergetic effect of both dopants resulted in ∼5-fold enhancement of the PEC performance. When a FeOOH layer is hydrothermally grown on the Zr-Mo co-doped BiVO 4 electrode, the photocurrent density was enhanced by ∼13-fold achieving 3.34 mA/cm 2 at 1.23 V RHE . In addition, the onset potential was remarkably reduced from ∼0.45 V to <0.1 V.

Broadening the absorption bandwidth based on heavily doped semiconductor nanostructures.

Broadband light absorption is a basis for the proper functionality of various materials, microstructures, and devices. Despite numerous studies, however, many aspects of broadband absorption remain uncovered. In this paper, we demonstrate an inverse-problem approach to designing nanostructures with a very low optical reflection and high absorption through a frequency band. Particular emphasis is made on a subwavelength transparent film as a top layer and anisotropic substrate. The polarization-dependent metamaterial absorber based on a subwavelenth semiconductor multicomponent multilayer structure is proposed and numerically investigated. For an illustration, we consider a four-component heavily doped silicon lattice with a thin undoped silicon top layer. The dielectric response of the structure is engineered by controlling the free carrier density and filling factor of each layer. A simulation study reveals a power law dependence of the bandwidth on the maximum reflectivity within the band.

Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions

We demonstrate a substantial modulation of the optical properties of multilayer graphene (∼100 layers) using a simple device consisting of a multilayer graphene/polymer electrolyte membrane/gold film stack. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI]− [ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]] resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm−1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 μm (2000–6000 cm−1), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. All of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI− ion and the associated change in the free carrier density (Δn = 1020 cm−3). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of ΔEF = 1.1 eV.

Nanoscale mapping of shifts in dark plasmon modes in sub 10 nm aluminum nanoantennas

In this work, we report the fabrication and spectroscopic characterization of subwavelength aluminum nanocavities—consisting of hexamer or tetramer clusters of sub 10 nm width Al nanorods—with tunable localized surface plasmon resonance (LSPR) energies on suspended SiN x membranes. Here the volume plasmon (VP) and LSPR modes of lithographically-fabricated Al nanocavities are revealed by low-loss electron energy-loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM). We show that the existence of grain boundaries (GBs) in these nanocavities results in shifts in the VP energy and a reduction in the VP lifetime. We map the VP energy and lifetime across GBs and we observe a decrease in VP energy and lifetime at GBs that is consistent with a reduction in free carrier density and increased plasmon scattering at these locations. Dipolar LSPR modes resonant in the UV and blue regions of the electromagnetic spectrum as well as higher-energy optically dark quadrupolar and hexapolar LSPR modes are also observed and mapped by STEM and EELS. All LSPR modes are confirmed via electromagnetic simulations based on the boundary element method. Both tetramer and hexamer structures support the excitation of dipolar bright and dipolar dark modes. Finally, we find that asymmetries in fabricated nanorod hexamer and tetramer nanocavities result in a mode mixing leading to a shift in dipolar dark LSPR modes.

Effects of SeO2 epilayer on the structural, morphological, optical and dielectric properties of nanocrystalline ZnSe thin films

Herein the effects of SeO2 coating on the physical properties of thermally evaporated nanocrystalline (NC) ZnSe thin film substrates are explored. The formed ZnSe films showed nanocrystalline Zincblende phase. They contained excess selenium (ZnSe1.07) with relatively large defect density and displayed band gap of value of 3.48 eV, energy band tails of width of 1.2 eV and dielectric quality factor (Q) less than 100. Coating NC-ZnSe with SeO2 induced the crystallization of tetragonal SeO2 and enhanced the light absorbability of SeO2. It also reduced the defect density and stacking faults, increased the crystallites sizes, narrowed the width of band tails, increased the dielectric constant and increase the Q factor up to ∼400 of NC-ZnSe. In addition, analysis of the optical conductivity spectra for the NC-ZnSe/SeO2 bilayers has shown that the drift mobility, plasmon frequency and free carrier density can reach values of 30.42 cm2/Vs, 5.07 GHz and 8.0 ×1018 cm−3, respectively.

Controllable crystallization engineering on amorphous tungsten oxide nanoparticles for highly efficient photochromic response

Photochromic materials are promising for application in the smart window, but typical WO 3 chromogenic ceramics suffer from slow color-switching speed and limited color saturation. Rational construction of amorphous–crystalline ( a–c ) heterostructure offers infinite expectations for developing WO 3 -based composites with a highly efficient photochromic response. Here, the controllable modulation of the WO 3 crystallization for synthesizing a-WO 3 /c-WO 3 hetero-nanoparticles is reported by modulating the ratio of acetic acid/sodium tungstate precursors via the synchronous dual-phase synthetic strategy. Owing to the hierarchical structure of crystalline WO 3 self-embedded amorphous WO 3 , the ultraviolet sensitivity, color difference, and light comprehensive reflectance of the hetero-nanoparticles are enhanced by 4.25, 1.87, and 1.52 times, respectively, compared with pure amorphous WO 3 . Spectroscopic and photoelectrochemical results reveal that hetero-nanoparticles switched from yellow WO 3 to the blue H x WO 3 accompanied by an increased free carrier density, positively shifted energy levels, and narrower bandgap. For the improved photochromic response, it is ascribed to the concerted advantages of each component and the resultant strong a–c interface interaction, promoting water decomposition in the protons supply, charge transfer in the surface hydration layer, and proton intercalation in the ion-to-electron transducer layer. This work enriches the preparation avenue and mechanism interpretation for WO 3 -based photochromic nanocomposites that exhibit NIR shielding properties. • Amorphous–crystalline WO 3 photochromic hetero-nanoparticles are synthesized by using acetic acid. • The hetero-nanoparticles display faster color-switching speed and higher coloration efficiency. • Photochromic behavior increases the free carrier density, shifts the energy level, and narrows the bandgap. • The concepts of protons supply layer, surface hydration layer, and ion-to-electron transducer layer are introduced. • The synergistic effect between hybrid components triggers efficient charge transfer and protons insertion.

Massively parallel FDTD full-band Monte Carlo simulations of electromagnetic THz pulses in p-doped silicon at cryogenic temperatures

In semiconductor samples with a low density of free charge carriers the current response to an intense electromagnetic pulse is weak and can be treated as a perturbation in the finite-difference-time-domain calculation of the electromagnetic field. Up to first order the electric field that drives the particles does not depend on their density and the particles can be simulated independently. This opens the door for massively parallel Monte Carlo simulations that can be processed efficiently on a computer cluster and a large number of particles can be simulated resulting in a very low noise level. By this full-band Monte Carlo finite-difference-time-domain approach the generation of higher harmonics in p-doped silicon at cryogenic temperatures is investigated. It is found that the higher harmonics are mostly due to the warped valence bands, which are discretized on adaptive unstructured tetrahedral grids in the k-space to avoid numerical artifacts.

Permittivity modulation in Si-based PIN diode by electron irradiation

In this paper, we investigate the influence of defects induced by different doses of electron irradiation on the real part of permittivity of Si-based PIN diodes with a deep-level transient spectrum test. For a more precise result, the relationship between the real part of the permittivity of the sample and the irradiation dose, temperature and annealing is studied. The results show that the real part of the permittivity changes significantly with the irradiation dose and temperature. The absolute value of the real part of the permittivity decreases with the decrease in temperature, and the changing trend decreases after annealing. The deep-level transient spectroscopy results illustrate that different states of defects caused by radiation and annealing have changed the free carrier mobility, free carrier density and charge balance in the material, which means they have a significant impact on the electrical characteristics of the samples. The I–V and C–V curves are obtained through experiments to extract the leakage current and carrier concentration. The I–V characteristic and the permittivity are subjected to temperature-variation analysis.

Direct Determination of the Steady State and Time‐Resolved Quasi‐Fermi Level Separation in Organic Solar Cells from Electroluminescence Measurements

AbstractThe detection of photoluminescence (PL) is an important characterization method for many photovoltaic technologies providing direct information about the separation of the quasi‐Fermi levels (QFL), ΔEF. However, for organic solar cells, the PL is dominated by excitons, which decay radiatively before they form free charge carriers via dissociation at donor/acceptor interfaces. This (major) part of the PL signal does therefore not correlate with ΔEF. In contrast, electroluminescence (EL) stems from injected electrons and holes, which recombine via charge transfer (CT) states. This work evaluates whether ΔEF can be derived directly from EL emission and whether this also holds true for transient measurements. To do so, ΔEF data derived from steady‐state EL measurements of highly efficient organic solar cells are compared with the electrical voltage, both in the dark and under illumination at equal recombination currents. Furthermore, ΔEF is also, to the best of the authors’ knowledge for the first time, derived from transient EL experiments, and in all cases, an excellent agreement is found. This means that the occupation of CT states is in equilibrium with the free charge carrier densities and can, thus, be described by the same QFL, even in the transient case.

Plasmonic interactions at the Pb/SeO2 interfaces designed as terahertz/gigahertz optical receivers

Herein semitransparent lead substrates of thicknesses of 200 nm are employed as plasmonic interfaces to enhance the optoelectronic performance of SeO2 thin films. Pb/SeO2 interfaces are fabricated by the physical vapor deposition technique under a vacuum pressure of 10-5 mbar. The structural analyses showed that Pb substrates can induce the evolution of tetragonal phase of selenium oxide. The phase comprises 1.27% cubic PbSe in its structure. Surface morphology studies show that Pb substrates prefer formation of grains of sizes of 250 nm resulting from the accumulation of lead crystallites of sizes of 37 nm. Optically, Pb substrates increased the light absorbability in the visible range of light without altering the value of the energy band gap, improved the dielectric response by 4% at 1.15 eV and by 40% at 1.87 eV. In addition, Durde-Lorentz modeling of the optical conductivity spectra of SeO2 has shown that Pb plasmonic resonators increase the plasmon frequency and free carrier density of Pb/SeO2 interfaces allowing it reaching 5 G/6 G technology needs. Moreover, evaluation of the terahertz cutoff frequency spectra indicated the applicability of the Pb/SeO2 as optical receivers responsive to infrared, visible and ultraviolet bands of light at terahertz cutoff frequency values of 17–160 THz. Furthermore, the capacitance and conductance spectral studies in the frequency domain of 0.01–1.80 GHz indicated the suitability of Pb/SeO2/C devices as negative conductance sources beneficial for signal amplification in the microwave frequency domain.

Determination of the charge carrier density in organic solar cells: A tutorial

The increase in the performance of organic solar cells observed over the past few years has reinvigorated the search for a deeper understanding of the loss and extraction processes in this class of device. A detailed knowledge of the density of free charge carriers under different operating conditions and illumination intensities is a prerequisite to quantify the recombination and extraction dynamics. Differential charging techniques are a promising approach to experimentally obtain the charge carrier density under the aforementioned conditions. In particular, the combination of transient photovoltage and photocurrent as well as impedance and capacitance spectroscopy have been successfully used in past studies to determine the charge carrier density of organic solar cells. In this Tutorial, these experimental techniques will be discussed in detail, highlighting fundamental principles, practical considerations, necessary corrections, advantages, drawbacks, and ultimately their limitations. Relevant references introducing more advanced concepts will be provided as well. Therefore, the present Tutorial might act as an introduction and guideline aimed at new prospective users of these techniques as well as a point of reference for more experienced researchers.

Nanoscale interplay of native point defects near Sr-deficient SrxTiO3/SrTiO3 interfaces

SrTiO3 has many applications involving interfaces with other materials, but defects that affect the properties of SrTiO3 films can also play a significant role at its heterointerfaces and even at junctions with nonstoichiometric SrTiO3. Depth-resolved cathodoluminescence spectroscopy (DRCLS) combined with systematic cation Sr content reduction in SrxTiO3 ultrathin films grown on SrTiO3 showed an interplay of native point defects and electronic structure within the Sr-deficient film and how interplay extends tens of nanometers into the substrate below. Understanding how defects form and affect interface electronic structure during epitaxial growth is central to improving complex oxide devices. Controlling the balance of oxygen vacancy defects with strontium vacancies and other acceptor-like defects can improve control over free carrier densities. Likewise, control over nanoscale defect charge distributions can advance new device features such as two-dimensional hole gases and the performance of existing devices such as ferroelectric tunnel junctions. This study shows how DRCLS directly measures the relative densities and spatial distributions of multiple native defects within and extending away from nanoscale SrxTiO3/SrTiO3 junctions and how their interplay varies with controlled epitaxial growth. In turn, these growth-dependent defect variations can help guide SrTiO3 epitaxial growth with other complex oxides.

Transition from surface phonon-polariton to surface phonon–plasmon-polariton by thermal injection of free carriers

Surface phonon-polariton, surface plasmon-polariton, and surface phonon–plasmon-polariton are evanescent electromagnetic waves confined to the surfaces of different classes of materials, which gives each of them particular characteristics suitable for diverse applications. Natural or forced injection of free carriers in a dielectric may change the surface phonon-polariton into a surface phonon–plasmon-polariton. Understanding this effect provides an insight into the fundamental physics of surface electromagnetic waves on dielectrics and offers tools that can be used to develop new technologies. In this contribution, we experimentally study the transition from surface phonon-polariton to surface phonon–plasmon-polariton on a yttrium-doped aluminum nitride polycrystalline substrate by thermal injection of free carriers. We perform this study using reflectivity measurements in the far- and mid-infrared spectral range and at a variable temperature, taking the necessary precautions to eliminate any errors that may arise from measurement artifacts and inaccurate analysis of the spectra. We demonstrate that thermal injection of a significant free carrier density can tune the surface phonon-polariton into a much shorter mean free path surface phonon–plasmon-polariton.

Pt/SeO2 optical receivers designed for terahertz and 5G/6G technologies

Selenium oxide thin films are highly transparent optical layers proper for optoelectronic technology. However, SeO2 films are rarely studied and observed suffering from clustery surface morphology, low light absorbability and low dielectric constant. For this reason herein, in an attempt to enhance its properties, platinum nanosheets (10–50 nm) are used as plasmonic substrates to grow transparent selenium oxide thin films. The films are deposited onto glass and Pt substrates using thermal evaporation technique under a vacuum pressure of 10−5 mbar. Both films of the glass/SeO2 and Pt/SeO2 are characterized by the techniques of x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, optical spectrophotometry and impedance spectroscopy. While no significant effect of Pt nanosheets on the amorphous nature of structure of SeO2 is observed, remarkable enhancements in the light absorbability by 50 times and in dielectric constant by three times are achieved. In addition, Pt nanosheets form plasmonic interfaces resulting in improving the plasmon frequency, drift mobility and free carrier density of the films. Pt/SeO2 films showed plasmon frequency larger than 6.0 GHz and free carrier density of 1018 cm−3. Moreover, analysis of the terahertz cutoff frequency and impedance spectra have shown that the Pt/SeO2 interfaces can also be employed as terahertz receivers and as low pass filters suitable for 5G/6G technologies.