Concentration Of Free Charge Carriers Research Articles

Effect of preparation conditions on the surface morphology and edge absorption of ZnGa2O4:Cr thin films

Thin ZnGa2O4:Cr films were obtained by RF ion-plasma sputtering in argon and argon-oxygen atmospheres mixtures on single-crystal NaCl and amorphous υ-SiO2 substrates. The surface morphology of the films was studied by atomic force microscopy. It was found that during the annealing of ZnGa2O4:Cr films on υ-SiO2 substrates in oxygen, the grains forming the film grow along the film surface, and during annealing in an argon atmosphere, the growth of grains is observed normal to the film surface. The region of the fundamental absorption edge was studied by optical spectroscopy. The variation of the optical band gap of Eg depending on the deposition and annealing atmosphere was determined. The total effective mass of free charge carriers and the concentration of free charge carriers are estimated. It is shown that the shift of the fundamental absorption edge in ZnGa2O4:Cr thin films after annealing in an argon atmosphere and when oxygen is added to the sputtering atmosphere is due to the Burstein-Moss effect.

Electrostatic conductive disc singularity resolved

The conventional electrostatic solutions for two-dimensional (2D) electrodes possess edge singularities for the surface charge density σ and the normal component of the electric field En. These singularities are generally non-physical because they admit infinite gradients of the concentration of free charge carriers. In particular, they are unacceptable in the studies of the local field sensitive effects, such as the electric breakdown and the ferroelectric domain reversal. We claim that account for diffusion of free charge carriers leads to the disappearance of the edge singularities. This generalization occurs consistently within the same basic concept of conduction. Specifically, we consider the case of U-biased circular disc electrode of radius a. Account for diffusion leads here to a strongly nonlinear integral 2D equation for the electrostatic potential φ(r). Numerical solution of this equation shows that the law σ(a)∝U2/a takes place. Outside a close vicinity of the disc edge, we stay close to the conventional electrostatic solution for φ and σ.

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.

Growth, structural, thermal, and optical characteristics of L-asparagine monohydrate doped magnesium sulphate heptahydrate semiorganic crystals

A novel semi-organic crystal has been grown using slow evaporation technique by doping organic compound L-asparagine monohydrate (C4H8N2O3·H2O) with inorganic material Magnesium sulphate heptahydrate (MgSO4·7H2O). The crystallographic parameters like strain, dislocation density and crystallite size were calculated by powder X-ray diffraction method. Functional groups were identified and bond length, force constants were calculated from FT-IR spectroscopy. Energy dispersive X-ray (EDX) analysis was used to identify the constituent elements of the crystal. Kinetic and thermodynamic parameters, such as, activation energy Ea, change in Gibb's free energy (ΔG) and change in enthalpy (ΔH) have been determined by thermogravimetric analysis (TGA) analysis. Ea, ΔH and ΔG show positive values and change in entropy (ΔS) shows negative ones. The thermal degradation behavior of the crystals has been analyzed by differential scanning calorimetry (DSC) analysis. Various optical constants such as optical band gap, lattice dielectric constant, absorbance, extinction coefficient, the ratio of free charge carrier concentration to the effective mass, Urbach energy, optical and electrical conductivities were estimated from UV–vis transmittance data. High optical conductivity (1010 s−1) justifies the good photo response nature of the semi-organic crystal.

Large enhancement of visible light photocatalytic efficiency of ZnO films doped in-situ by copper during atomic layer deposition growth

In the present study, we investigate the photocatalytic activity of ZnO thin films doped by copper atoms in successive steps during the film growth by the atomic layer deposition method. Scanning electron microscopy and x-ray diffraction measurements reveal a polycrystalline structure of samples with degrading crystallinity for films with higher Cu content. X-ray photoelectron spectroscopy analysis is consistent with the presence of copper in the Cu+ state for samples with a relative Cu content lower than 1 at.%. Those samples exhibit p-type conductivity indicating that copper ions occupy substitutional, CuZn, acceptor sites in ZnO. As established from UV–vis measurements, Cu-doped ZnO films also show enhanced optical absorption in the visible region. A reduced electron-hole recombination rate, due to much lower intrinsic free charge carrier concentrations in the doped samples, and the increased light absorption in the visible region, lead to a large enhancement of photocatalytic efficiency observed in doped films under simulated sunlight irradiation.

Correlating elemental compositions and charge carrier profiles in ultra-pure GaN/AlGaN stacks grown by molecular beam epitaxy

Inconsistencies in the concentrations of unintentional donor impurities and free charge carriers in GaN/AlGaN layer stacks hosting a two-dimensional electron gas (2DEG) can be attributed to the measurement procedure and solely depend on the way in which the free charge carrier concentration is extracted. Particularly, when the 2DEG acts as the bottom electrode in capacitance versus voltage measurements, unphysically low concentrations of free charges are calculated. This originates from the depletion of the 2DEG and the accompanying disappearance of the bottom electrode. It is shown that, for the case of a defined (non-vanishing) bottom electrode, the levels of donor impurities and resulting free charges consistently match.

Influence of the content of impurities and structural defects on the properties of the Cd0.9Mn0.1Te:V-based detector

The paper highlights the results of quantitative studies of the influence of the content of impurities and structural defects on the electrophysical and detector properties of Cd0.9Mn0.1Te:V — resistivity and concentrations of free charge carriers, life time of nonequilibrium charge carriers τ, charge collection efficiency η. The optimal ranges of energy change and deep donor concentration, which ensure a high-resistive state and acceptable values of τ and η, are established. The authors study the compensation of cadmium vacancies with vanadium admixture.

Characterization of boron-doped single-crystal diamond by electrophysical methods (review)

A critical analysis of the existing methods of controlling the concentration of impurity and majority charge carriers in wide bandgap semiconductors and the issues of improvement of modern diagnostics of the main electrophysical properties of single-crystal diamond are considered based on the results of our studies and the works of other authors. It was found that independent assessment of impurity concentration and concentration of free charge carriers is of fundamental importance for semiconductor diamond due to very low (less than 1%) degree of ionization of the introduced impurity. The advantages and prospects of admittance spectroscopy as a diagnostic method for ultrawide bandgap semiconductors are shown and solutions aimed at the correct interpretation of the experimental data are proposed. The high ionization energy of boron impurity in diamond (370 meV) results in a strong frequency dispersion of the measured barrier capacitance. It is shown that under disturbance of quasi-static conditions in capacitance-voltage measurements, low frequencies and high temperatures should be used for correct assessment of the charge carrier concentration. The results of electrophysical studies are compared with traditional measurements of impurity concentration in diamond by optical methods. A decrease of hole activation energy from the boron impurity level from 325 to 100 meV was found upon increasing the boron concentration NA from 2·1016 to 4·1019 cm-3. The transition to the hopping mechanism of conductivity within the impurity (acceptor) band with thermal activation energy of 10-20 meV was registered for NA≥5·1018 cm-3 at temperatures of 120-150 K. Keywords: single-crystal diamond, boron impurity, charge carrier concentration, activation energy, admittance spectroscopy, capacitance-voltage measurements.

Influence of the content of impurities and structural defects on the properties of the Cd0.9Mn0.1Te:V-based detector

The paper highlights the results of quantitative studies of the influence of the content of impurities and structural defects on the electrophysical and detector properties of Cd0.9Mn0.1Te:V — resistivity and concentrations of free charge carriers, life time of nonequilibrium charge carriers τ, charge collection efficiency η. The optimal ranges of energy change and deep donor concentration, which ensure a high-resistive state and acceptable values of τ and η, are established. The authors study the compensation of cadmium vacancies with vanadium admixture.

Microwave volt-impedance spectroscopy of semiconductor structure

Microwave voltage-impedance spectroscopy is used to study a semiconductor structure in the form of a doped n-GaAs film grown on a conducting n^+-GaAs substrate with a buffer sublayer. A system of concentric barrier contacts is formed on the structure surface. A technique has been developed for measuring complex impedance spectrum Z(f,U) of the sample as a function of DC bias voltage U. Spectra Z(f,U) were measured using a Cascade Microtech probe station in the frequency range 0.01-40 GHz with a lateral resolution of 15-30 μm at U=0-10 V. The main electrophysical characteristics of a semiconductor film were determined from the spectra: type, concentration and mobility of free charge carriers, electrical conductivity. An excess resistance was found in the range f=0.1-20 GHz. This effect is interpreted as the deep states (traps) recharging for two types of traps --- low-frequency l and high-frequency h with characteristic time tau_l=10-9 s, tau_h=4.2·10-11 s. A model description is proposed that explains the characteristic shape of the trap resistance spectrum, its dependence on the contact area and voltage U. Keywords: microwave band, near field, impedance, semiconductor, barier contact, deep states, electrophysical characteristics.

Микроволновая вольт-импедансная спектроскопия полупроводниковой структуры

Microwave voltage-impedance spectroscopy is used to study a semiconductor structure in the form of a doped n-GaAs film grown on a conducting n+-GaAs substrate with a buffer sublayer. A system of concentric barrier contacts is formed on the structure surface. A technique has been developed for measuring complex impedance spectrum Z(f,U) of the sample as a function of DC bias voltage U. Spectra Z(f,U) were measured using a Cascade Microtech probe station in the frequency range 0.01 – 40 GHz with a lateral resolution of 15 – 30 μm at U = 0 – 10 V. The main electrophysical characteristics of a semiconductor film were determined from the spectra: type, concentration and mobility of free charge carriers, electrical conductivity. An excess resistance was found in the range f = 0.1 – 20 GHz. This effect is interpreted as the deep states (traps) recharging for two types of traps – low-frequency l and high-frequency h with characteristic time τl = 10^(-9) s, τh = 4.2∙10^(-11) s. A model description is proposed that explains the characteristic shape of the trap resistance spectrum, its dependence on the contact area and voltage U.

MOF Structure engineering to synthesize core-shell heterostructures with controllable shell layer thickness: Regulating gas selectivity and sensitivity

Metal-organic framework (MOF)-derived metal oxide semiconductors have received significant attention for gas sensing applications. Herein, we reported core-shell In2O3@ZnO n-n heterostructures by depositing ZIF-8 derivative onto wrinkled In2O3 sphere, realizing the control of ZnO shell thickness (12.6–72.4 nm) through controlling MOF growth time. Due to the formation of n-n heterojunction at the core-shell interface, the tuning of shell thickness can lead to the radial modulation of the electron-accumulation layer in ZnO, and realizing the control of free charge carrier concentration that participated in gas sensing reaction. What’s more, the MOF-derived ZnO shell with rich oxygen vacancies is beneficial for oxygen chemisorption. Accordingly, compared with the In2O3 based sensor, the In2O3@ZnO based sensor exhibits higher sensitivity to trace-level acetone (100 ppb), faster response time (2 s vs. 100 ppm), better selectivity, and stronger anti-humidity capacity at operating temperature 300 °C, while the thickness of ZnO shell is 55.3 nm. In addition, the increase of ZnO shell thickness can lead to the selectivity change from ethanol to acetone of In2O3@ZnO owing to the inherent catalytic oxidation activity. Thus, the remarkable performance of the In2O3@ZnO sensor mainly relies on ZnO shell layer.

Nanoscale ITO Films for Plasmon Resonance-Based Optical Sensors

The developing field of plasmonics has led to the possibility of creating a new type of high-speed, highly sensitive optical sensors for the analysis of chemical and biological media. The functional conducting layers of surface plasmon resonance (SPR) optical sensors are almost always nanoscale thin films of noble metals. To enhance the plasmon resonance, nanostructured films of transparent conductive oxides are introduced into the optical sensors. However, such modified optical sensors operate in the infrared region of the spectrum. In this work, we demonstrate that the use of indium tin oxide (ITO) films with a high concentration of charge carriers makes it possible to shift the surface plasmon resonance into the visible radiation region. The work presents the results of the development of magnetron deposition technology for ITO thin films, with optimal parameters for optical sensors based on surface plasmon resonance operating in the visible range of the spectrum. Their optical and electrical characteristics are investigated. Excitation of the surface and volume plasmon resonance at the dielectric-ITO film interface, using the Kretschman configuration, is studied. It is shown that SPR is excited in the investigated ITO films with a concentration of free charge carriers of the order of 1021–1022 cm−3, when irradiated with a beam of light with TM polarization in the wavelength range of 350–950 nm. At the same time, the addition of various analytes to the surface of an ITO film changes the excitation wavelength of the SPR.

Structural, optical, dielectric, and surface properties of polyimide hybrid nanocomposites films embedded mesoporous silica nanoparticles synthesized from rice husk ash for optoelectronic applications

In this study, mesoporous silica nanoparticles previously prepared from the rice husk ash were utilized as nanofillers to fabricate thin films of polyimide/silica hybrid nanocomposites with different ratios (0, 6, 8, 10, and 12%). Subsequently, all hybrid films were further subjected to comprehensive characterization using XRD, SEM, AFM, and contact angle analyzers. The films exhibited a variety of optoelectronic properties depending on the silica nanoparticles' content. Where the silica nanofillers affected the optical clarity of polyimide films and increasing the silica ratio resulted in decreasing in films transmittance which led to reducing the transparency and enhanced the absorption coefficient of films in the UV range. Besides, the dielectric constant value and free charge carrier concentrations have increased which promoted the optical conductivity of the films. Moreover, increasing silica content resulted in converting the films from hydrophobic to hydrophilic surfaces, and has improved their wettability at all pH values.Graphical abstract

Thermal insulation performance of novel orange fabric coated with Fe3+-doped La2Zr2O7 NIR solar reflectivity pigment

Novel orange thermal insulation coated fabrics were prepared by applying La2Zr2-xFexO7-δ (0.00 ≤ x ≤ 0.25) pigments. The formation of Fe3+-doped La2Zr2O7 pigment was ensured by analyzing XRD and XPS patterns. The pigment samples have near-spherical particles with particle sizes between 30 and 70 nm. The NIR solar reflectance of the pigments is very high, ranging from 75.97 % to 96.18 %, and its change is caused by the variation in free charge carrier concentration. Doping with Fe3+ ions makes the color of the pigment appear orange and reduces the band gap vaule from 5.17 eV to 2.32 eV, mainly attributed to the charge transfer transition of La 5d ↔ Fe 3d. The La2Zr2-xFexO7-δ-coated fabrics exhibit excellent orange color and outstanding NIR solar reflectance (R* = 51.08 % − 64.47 %). Experimental simulations revealed that the designed coated fabrics have lower internal surface temperatures, with a temperature difference of 7.9–14.7 ℃ compared to conventional orange coated fabric, which shows superb thermal insulation performance. The fastness of washing, acids, and alkalis was excellent for fabric. In conclusion, the La2Zr2-xFexO7-δ-coated fabrics can achieve thermal insulation and energy savings by means of NIR solar reflectivity pigments.

Enhancement of Thermoelectric Performance in Bi0.5Sb1.5Te3 Particulate Composites Including Ferroelectric BaTiO3 Nanodots

An increasing number of studies have reported producing composite structures by combining thermoelectric and functional materials. However, combining energy filtering and ferroelectric polarization to enhance the dimensionless figure of merit thermoelectric ZT remains elusive. Here we report a composite that contains nanostructured BaTiO3 embedded in a Bi0.5Sb1.5Te3 matrix. We show that ferroelectric BaTiO3 particles are evenly composited with Bi0.5Sb1.5Te3 grains reducing the concentration of free charge carriers with increasing BaTiO3 content. Additionally, as a result of the energy-filtering effect and ferroelectric polarization, the Seebeck coefficient was improved by ∼10% with a ∼10% improvement in power factors. The BaTiO3 phase can effectively scatters phonons reducing lattice thermal conductivity κl (0.5 W m-1 K-1) and increasing ZT to 1.31 at 363 K in Bi0.5Sb1.5Te3 composites with 2 vol % BaTiO3 content giving an improvement of ∼25% over pure Bi0.5Sb1.5Te3. Our work indicates that the introduction of ferroelectric nanoparticles is an effective method for optimizing the ZT of Bi0.5Sb1.5Te3-based thermoelectric materials.

Organic semiconductor nanostructures: optoelectronic properties, modification strategies, and photocatalytic applications

Organic semiconductors (OSCs) possess diverse chemical structures and tailored optoelectronic properties via simple chemical modifications, so increasing use of them are found in efficient visible-light photocatalysis. However, the weak chemical bonds and the poor charge behavior (e.g., low concentration of free charge carriers, low carrier mobility) intrinsic in them, always incur quite limited stability and efficiency. Therefore, the assembly of them into refined nanostructures or nanocomposites is usually proposed to enhance their optoelectronic properties, as well as the photocatalytic efficiency and reliability. Zero-dimensional (0D) nanoparticles are low in size and hence high specific surface area (SSA); One-dimensional (1D) nanostructures are usually arranged in an orderly long range thus leading to low surface defect density and increased carrier mobility; Two-dimensional (2D) nanostructures are particularly capable of enhancing the photogenerated charge utilization because of their large reaction sites and shortened charge transport length. Furthermore, the building of heterogeneous interfaces in the nanocomposites can effectively facilitate the special charge separation. All these highlight the importance of organic nanostructures in improving the photocatalytic activity and stability. Therefore, organic semiconductor nanostructures (OSNs) have been increasingly used in the photocatalytic water splitting into H2 and O2, CO2 reduction, pollutant decomposition, disinfection, etc. In this review, we first examine the important optoelectronic properties of OSNs that govern the photocatalytic processes; we then analyze different classes of OSNs and their mechanistic pathways, with an emphasis on the structure-property relationships; we also introduced various photocatalytic applications of OSNs; we lastly propose the challenges and future outlook in real use.

Investigation of low-confinement surface phonon polariton launching on SiC and SrTiO3 using scanning near-field optical microscopy

Surface phonon polaritons (SPhPs) are important building blocks of nanophotonics, as they enable strong light–matter interaction on the nanoscale, are well-suited for applications in the mid- to far-infrared regime, and can show low losses. SrTiO3 is an interesting material for SPhPs, because it allows for reversible, nonvolatile doping with free charge carriers via oxygen vacancies and for local switching with conductive AFM tips. As a result, SrTiO3 could enable programmable nanophotonics with tunable SPhPs and direct writing of metasurfaces. Surface polariton properties can be determined by mapping their real-space propagation using scattering-type scanning near-field optical microscopy (s-SNOM), which is sensitive to the high local electric fields with nanoscale lateral resolution. Low-confinement (LC) SPhPs with wavevectors close to that of free-space radiation, such as in SrTiO3 and the model polar dielectric SiC, can be difficult to investigate in s-SNOM due to interference effects with the incident illumination and fringe spacings exceeding the scan range or the size of the focus spot. Here, we present s-SNOM measurements of LC-SPhPs on SiC and SrTiO3 launched at gold stripes, retrieve physical quantities such as launching amplitude and phase, and show that they are influenced strongly by gold stripe geometry as well as illumination angle. Using two complementary measurements, we show a convenient way to determine the out-of-plane angle of the s-SNOM setup. Finally, we predict how control over the free charge carrier concentration in SrTiO3 could enable tunable LC-SPhPs, showing the potential of SrTiO3 for programmable nanophotonics.

Nickel Nanoparticles Encapsulated in Nitrogen‐Doped Carbon Nanofibers as Excellent Bifunctional Catalyst for Hydrogen and Oxygen Evolution Processes

AbstractThe potential of nitrogen‐doped carbon nanofibers with encapsulated Ni nanoparticles as a bifunctional catalyst in H2 and O2 evolution reactions (HER and OER, respectively) was studied. The proposed heterostructure is fabricated via a facile chemical vapor deposition on Ni supported on graphitic carbon nitride (GCN) as a precursor. HER was an effect of photocatalytic water splitting and catalytic dehydrogenation of lactic acid used as sacrificial electron donor. However, photocatalysis played the dominant role in the acceleration of the reaction rate which was 17 times and 330 times higher in respect to the sample prior the CVD and to pristine GCN, respectively. The catalyst exhibited boosted electrocatalytic activity in OER and outstanding stability in respect to RuO2 in alkaline medium. The overpotentials calculated from linear sweep voltammetry at 10 mA cm−2 is ∼383 mV for the fabricated electrocatalyst in respect to 490 mV for the RuO2 and Tafel slope is calculated to be 106 mV dec−1 in comparison to 136 mV dec−1 for the reference which is above the state‐of the‐art bifunctional catalysts reported to date. The results reveal that the catalytic processes were boosted due to several key effects such as enhanced visible light absorption, better charge carriers transfer and separation, increased free charge carrier concentration and higher reducing ability of electrons.

Density of states and interband light absorption in Y2O3 and Sc2O3 thin films

The long-wavelength edge of the fundamental absorption band of thin Y2O3 and Sc2O3 films obtained by the method of discrete evaporation in vacuum is investigated. On the basis of its temperature dependence, the excitons - phonon interaction is investigated, which made it possible to interpret the absorption edge as the absorption of self-trapped excitons. To analyze the experimental results, we used a model of a heavily doped or defective semiconductor in the quasi-classical approximation. The use of this model made it possible to estimate the radius of the ground electronic state a and the screening radius rS and the concentration of free charge carriers N in the films under study.