Concentration Of Free Charge Carriers Research Articles

THE COMPOSITION OF IMPURITIES AND DEFECTS IN Cd1-XMgXTe:In, NECESSARY TO ENSURE STABLE DETECTOR PROPERTIES

A model study of the promising new material Cd0.92Mg0.08Te:In, intended for X-ray and gamma radiation detectors operating at room temperature, was carried out. 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 Cd0.92Mg0.08Te:In. An analysis of the calculated values of resistivity ρ and concentrations of free charge carriers, life time of non-equilibrium electrons τn, and holes τp, charge collection efficiency η with different composition of impurities and defects in this material at temperature T = 298 K was carried out. The optimal ranges of energy and concentration of alloying deep donor, which ensure a stable high-resistive state and acceptable values of η, are established. Compensation of cadmium vacancies with indium admixture was studied. Assumption was made regarding possibility of increasing the operating time of the detector having semi-insulating properties and great charge collection efficiency. A direction for further research has been formulated in order to clarify the nature of a suitable doping deep donor that ensures stable properties of the detector.

Enhancement of gasochromic response to hydrogen of WO3 thin films by post-process modification and catalyst selection

The gasochromic properties of WO3 thin films deposited by electron beam evaporation were investigated, focusing on their structural, morphology and optical changes caused by heat treatment. Moreover, the influence of the thickness and type of catalyst on the gasochromic properties of WO3 was examined. X-ray diffraction measurements revealed that WO3 thin films annealed up to 473 K remained amorphous, whereas annealing at 673 K and above, initiated crystallization into a monoclinic structure. Gasochromic behaviour was observed in response to hydrogen exposure, with the most significant changes occurring in films annealed at 673 K with a thin adlayer of palladium catalyst. A gasochromic response of 1.84 was achieved for H2 concentration as low as 25 ppm. The process of gasochromic colouration was correlated with the changes of free charge carrier concentration calculated based on Drude free electron theory. XPS analysis confirms WO3 reduction under hydrogen exposure, validating proposed colouration mechanisms. These comprehensive findings offer insights into optimizing gasochromic thin films for various sensing applications.

Investigations on structural and opto-electrical characterization of Ag-TCNQ (metal-organic frameworks) as complex thin films for potential device applications

The silver- (7, 7, 8, 8-tetracyanoquinodimethane (Ag-TCNQ) organometallic complex was synthesized chemically as a nano-powder and thereafter deposited as a thin film by thermal evaporation. The monoclinic, needle-like, polycrystalline structure of the Ag-TCNQ complex was analyzed employing x-ray diffraction (XRD) and a scanning electron microscope (SEM). Whereas their chemical structure and stability were explored using Fourier transformation infrared (FTIR) techniques. The findings imply a charge transfer of degree −0.5 between TCNQ° and TCNQ−, as revealed by the shift of the bands of the C≡N stretching and the (C═C-H) bond positioned at 2198 and 824 cm−1, respectively. As well as the optical properties of Ag-TCNQ thin film were studied using spectrophotometric measuring in the wavelength ranging 200 to 2500 nm. The measurement of transmittance and reflectance spectra were employed to calculate the refractive index (n), dielectric constant, absorption index (k), surface and volume energy loss functions, and optical conductivity. In the normal dispersion zone, optical characteristics such free charge carrier concentration, infinity dielectric constant (ε ∞), lattice dielectric constant (ε L), oscillator energy (Eo), and dispersion energy (Ed) were estimated. Furthermore, the optoelectronic nonlinear optical features and electronic transitions for the Ag-TCNQ thin film were assessed. Finally, these findings are promising milestones on the path to providing convincing evidence for the synthesis of stoichiometric thermally stable Ag-TCNQ thin film by conventional thermal evaporation technique that is potential to be utilized in optoelectronic devices applications.

Variation of the metal-insulator phase transition temperature in VO2: An overview of some possible implementation methods

The great interest in VO2, which has stimulated a large number of studies and publications in recent decades, is caused by the reversible metal-insulator phase transition (MIT) that occurs at T = 68 °C and is accompanied by the transformation of a low-temperature dielectric (semiconductor) monoclinic phase into a high-temperature metallic phase with a rutile structure. Despite the ongoing discussion about the physical mechanism of this transition, the concomitant rapid change in the electrical and optical characteristics of the material by several orders of magnitude already finds numerous applications in optics, optoelectronics and sensors. At the same time, it became obvious that both the number and performance of the applications of VO2 would greatly increase, if it were possible to decrease the temperature of the phase transition without deterioration of other properties. This issue has become the subject of numerous studies. Mechanical stress and oxygen vacancies in the VO2 lattice, the concentration of free charge carriers, tuned by impurity doping or implantation, have been investigated and discussed as the main factors affecting the transition temperature. In this review, we intend to summarize and analyze the literature data on these ways, primarily those which are most efficient in influencing the transition temperature while maintaining a significant change in the modulation characteristics.

Plasmonic Near-Infrared-Response S-Scheme ZnO/CuInS2 Photocatalyst for H2O2 Production Coupled with Glycerin Oxidation.

Solar fuel synthesis is intriguing because solar energy is abundant and this method compensates for its intermittency. However, most photocatalysts can only absorb UV-to-visible light, while near-infrared (NIR) light remains unexploited. Surprisingly, the charge transfer between ZnO and CuInS2 quantum dots (QDs) can transform a NIR-inactive ZnO into a NIR-active composite. This strong response is attributed to the increased concentration of free charge carriers in the p-type semiconductor at the interface after the charge migration between ZnO and CuInS2, enhancing the localized surface plasmon resonance (LSPR) effect and the NIR response of CuInS2. As a paradigm, this ZnO/CuInS2 heterojunction is used for H2O2 production coupled with glycerin oxidation and demonstrates supreme performance, corroborating the importance of NIR response and efficient charge transfer. Mechanistic studies through contact potential difference (CPD), Hall effect test, and finite element method (FEM) calculation allow for the direct correlation between the NIR response and charge transfer. This approach bypasses the general light response issues, thereby stepping forward to the ambitious goal of harnessing the entire solar spectrum.

P‐108: High conductivity transparent electrode with In2O3 – ZnO periodic structure and gradient oxygen concentration

Multilayer structures based on the In‐Zn‐O (IZO) system with modulated oxygen doping have been developed as a TCF (transparent conductive film) alternative to ITO or IGZO layers in the LC and OLED displays. The minimum resistivity (3.37 Ohm*cm) and the maximum concentration of free charge carriers correspond to the layers synthesized in an atmosphere of pure argon, while the maximum mobility (39.18 cm2/V*s) corresponds to the layers synthesized in the atmosphere of both argon and oxygen 99.6%/0.4% mixture. Thin‐film periodic structures with gradient modulation of oxygen content in thickness have been fabricated by periodically changing the oxygen concentration in the composition of the working gas during spraying at a substrate temperature of 100 °C. The structures consist of alternating layers with a high concentration of free charge carriers and layers with high mobility. As a result of the single layers' thicknesses optimization the thin‐film periodic structures with a minimum resistivity of 2.89 Ohm*cm have been made. The resistivity is lower than in homogeneous layers of the same composition. The alternation of layers of optimal thickness provides high carrier mobility, it is inherent to layers synthesized in the atmosphere of 99.6/0.4 argon and oxygen mixture at high concentration. The mobility value is close to the value achieved in the layers synthesized in the pure argon atmosphere.This material provides fabrication of high quality, stable and low cost transparent electrodes or TFT layers with performances corresponding to parameters of traditional materials.

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.

Self-assembled hollow CuS@AuNRs/PDA nanohybrids with synergistically enhanced photothermal efficiency.

The design of multifunctional nanocarriers with enhanced photothermal efficiency is of great significance for the photothermal therapy of cancer. In this study, hollow CuS@gold nanorods/polydopamine (HCuS@AuNRs/PDA) nanohybrids with synergistically enhanced photothermal efficiency were prepared by electrostatic self-assembly method. The high photothermal conversion efficiency of HCuS@AuNRs (55.88%) is attributed to the interfacial electron transfer between CuS and AuNRs, as well as the increase in free charge carrier concentration. The excellent adhesion performance and strong negative charge of PDA ensure a high doxorubicin hydrochloride (DOX) loading efficiency of 96.08% for HCuS@AuNRs/PDA. In addition, HCuS@AuNRs/PDA reveals outstanding NIR/pH dual-responsive drug release properties owing to the weakened interaction between PDA and DOX in acidic media and the distinct NIR responsiveness of HCuS@AuNRs. In vitro cell viability results confirm that HCuS@AuNRs/PDA could efficiently kill tumor cells under the dual effect of acidic media and NIR laser. This study presents a novel nanocarrier with synergistically enhanced NIR photothermal responsiveness and high drug loading capacity, which provides a versatile platform in intelligent drug release and photothermal therapy.

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.

Measurement of Photoelectrophysical Characteristics of Conductive Layers of CsPbBr3 Colloidal Quantum Dots

Colloidal quantum dots of perovskites having the chemical composition CsPbBr3 have been synthesized. For these particles, the average size of an ensemble of particles and sample polydispersity have been determined by steady-state spectrofluorometry. Conducting layers were made from the obtained particles, and the electrophysical characteristics of these layers were measured. The hole nature of the conductivity wasestablished, and the layer conductivity (0.04 S/m), mobility (0.8 cm2/(V s)), and number concentration of free charge carriers (3.01 × 1021 m−3) were measured. In accordance with published data, the measured mobility value is higher by one to two orders of magnitude than published typical values. It is shown that high polydispersity has a weak effect on the electrophysical and transport characteristics in the resulting layers.

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.

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.

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.