Types Of Charge Carriers Research Articles

Влияние электронного и дырочного допирования на транспортные характеристики халькогенидных систем

The electrical properties and the Hall effect in semiconductor compounds Ag0.01Mn0.99S and Tm0.01Mn0.99S have been studied in the temperature range 80–400 K in a magnetic field of 12 kOe. The mechanism of conduction is established, which depends on the type of doping and concentration from the current - voltage characteristics. At the replacement of manganese by silver, the Mott type was found, and the replacement by thulium causes ohmic conductivity. The mobility and type of charge carriers are found from the Hall constant.

Substantial and stable magnetoresistance and spin conductance in phosphorene-based spintronic devices with Co electrodes.

Designing devices with excellent spin-polarized properties has been a challenge in physics and materials science. In this work, we report a theoretical investigation of the spin injection and spin-polarized transport properties of monolayer and bilayer phosphorene devices with Co electrodes. Based on the analysis of transmission coefficients, spin-polarized current, magnetoresistance (MR) (or tunnel MR) ratio and spin injection efficiency (SIE), both devices show superior spin-polarized transport properties. As phosphorene in the device is changed from monolayer to bilayer, the charge carrier type can be tuned from n-type to p-type. For the monolayer phosphorene device, the tunnel MR ratio reaches about 210% and the SIE is about 80.7% at zero bias. Notably, the SIE and tunnel MR ratio maintain almost constant values against bias voltage and gate voltage, which makes it suitable for magnetic sensors. As for the bilayer phosphorene device, it not only exhibits a considerable tunnel MR ratio, but also shows significantly enhanced conductance, beneficial to the sensitivity of spintronic devices. Further analysis shows that the improvement of conductance is attributed to the low barrier height between the bilayer phosphorene channel and Co electrodes. According to our results, the studied phosphorene devices with Co electrodes demonstrate superior spin injection and transport properties. We believe that these theoretical findings will be a strong asset for future experimental works in spintronics.

Локализация носителей заряда в самоорганизованных квантовых точках InAs

We considered the problem of localization of electrons and holes taking for instance the pyramidal InAs quantum dots in GaAs. The problem of quantum mechanics was solved for the localizing potential taking into account the geometry, chemical composition and built-in fields of the mechanical stress and strain. We found that the strongest localization of both types of charge carriers can be achieved if the ratio of the pyramid height to its base is about 0.2.

Interplay between ferroelectricity and metallicity in BaTiO3.

We explore the interplay between ferroelectricity and metallicity, which are generally considered to be contra-indicated properties, in the prototypical ferroelectric barium titanate, BaTiO3. Using first-principles density functional theory, we calculate the effects of electron and hole doping, first by introducing a hypothetical background charge, and second through the introduction of explicit impurities (La, Nb and V for electron doping, and K, Al and Sc for hole doping). We find that, apart from a surprising increase in polarization at small hole concentrations, both charge-carrier types decrease the tendency towards ferroelectricity, with the strength of the polarization suppression, which is different for electrons and holes, determined by the detailed structure of the conduction and valence bands. Doping with impurity atoms increases the complexity and allows us to identify three factors that influence the ferroelectricity: structural effects arising largely from the size of the impurity ion, electronic effects from the introduction of charge carriers, and changes in unit-cell volume and shape. A competing balance between these contributions can result in an increase or decrease in ferroelectricity with doping.

Electrical impedance spectroscopy study of unsubstituted palladium (II) phthalocyanine

A pressed pellet of unsubstituted palladium (II) phthalocyanine (PdPc) was synthesized and characterized by UV-Vis transmittance and impedance spectroscopy. Optical absorbance analysis revealed that upon increasing the concentration, the absorption at 630 nm became more intense, demonstrating strong aggregation at high concentrations. For the synthesized PdPc pellet, the absorbance decreased beyond 800 nm, and the direct band gap energy was 1.95 eV. The electric properties of the sample were studied for a range of frequency (40–107 Hz) and temperature (300–660 K) using complex impedance analysis. The activation energy and the relaxation time values for both time and frequency ranges were close to each other, suggesting that the electrical conduction and relaxation phenomena were controlled by the same type of charge carrier in the sample. Notably, the sample exhibited a semiconductor–metal transition at temperature TSM = 560 K. The AC-conductance was satisfactorily explained using the Jonscher universal power law. The electrical transport mechanism was dominated by the overlapping large polaron tunneling (OLPT) model. Furthermore, impedance spectroscopy analysis revealed a non-Debye-type relaxation. The Nyquist plot indicates the presence of grains and grain boundary effects. The results demonstrate that this material may be a promising candidate for photovoltaic applications.

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers.

Molecular dopants are often added to semiconducting polymers to improve electrical conductivity. However, the use of such dopants does not always produce mobile charge carriers. In this work, ultrafast spectroscopy is used to explore the nature of the carriers created following doping of conjugated push-pull polymers with both F4 TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and FeCl3 . It is shown that for one particular push-pull material, the charge carriers created by doping are entirely non-conductive bipolarons and not single polarons, and that transient absorption spectroscopy following excitation in the infrared can readily distinguish the two types of charge carriers. Based on density functional theory calculations and experiments on multiple push-pull conjugated polymers, it is argued that the size of the donor push units determines the relative stabilities of polarons and bipolarons, with larger donor units stabilizing the bipolarons by providing more area for two charges toco-reside.

Bipolar conductivity in amorphous Cu–Al–O thin films prepared by r.f. magnetron sputtering

Bipolar electronic behavior has been demonstrated in amorphous Cu–Al–O thin films prepared on soda-lime glass substrate via radio frequency (r.f.) magnetron sputtering deposition technique. The tunable majority charge carrier type is as a result of the intentional adjustment of the percentage chemical composition of the material. Signs of Hall coefficients from Hall effect measurement and slopes of Mott-Schottky plots obtained for the films indicate the majority charge carrier type of each films. Negative Hall coefficient and a positive Mott-Schottky slope were obtained for the Al-rich film showing its n-type nature. Positive Hall coefficient and negative Mott-Schottky slope were obtained for the Cu-rich film showing its p-type nature. X-Ray Photoelectron Spectroscopy (XPS) was used to explain the variation in charge carrier type as a result of the adjustments made to the percentage chemical composition of the film. The I–V curves obtaine from simple devices fabricated show that the homojunction formed from the two films exhibit rectifying and photovoltaic properties. The rectifying behavior was studied between a bias of −1 V to +1 V. In addition, a VOC of 0.33 V, a JSC of 8.00 mA/cm2, an FF of 0.30 and a PCE of 0.79% were obtained from the photovoltaic device fabricated.

WSe2 Homogeneous in-Plane Pin Junction Diode with Electric Double Layers

Transition metal dichalcogenide (TMDC) materials are in the spotlight as next-generation semiconductor materials because of their advantages over other semiconductor materials. Research on WSe2 among the materials is underway, and the feature of WSe2 has an ambipolar characteristic, and the fermi level change and the doping effect are different according to the electrode type.In the present study, we have fabricated a homogeneous WSe2 PIN junction diode. Our in-plane multilayer WSe2 p-i-n homogeneous diode exhibited photovoltaic properties by asymmetric physical contact doping effect with electric double layer(EDL). Owing to the EDL placed between the cathode and anode, local charge carrier type and density profile are modulated in the same layers.Compared to general diode mode, PIN mode enhance the photo response of the device in our visible measurement range. The diode exhibits excellent rectifying behavior with a current rectification ratio of 104 or higher. Photovoltaic performance of short circuit current and open circuit voltage achieved 3.2 nA, 0.55 V, respectively under the 550 nm green light. And spectrum responsivity is about ~100 mA/W (max point) at 3.2 eV to 1.6 eV. Figure 1

Impact of nanorod morphology on microstructure, magnetic and electrical properties of NixCo1-xFe2O4

A series of NixCo1-xFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1) was successfully synthesized via hydrothermal method. From XRD analysis, the pure phase formation of Ni–CoFe2O4 (NCFO) was confirmed and the crystallite size ranged from 33 to 44 nm. Besides, the variation of theoretical lattice parameter with x-content was in a good agreement with experimental ones. Field Emission Scanning Electron Microscope (FESEM) revealed the formation of nanorod shapes with gradual decrease in length and diameter with addition of Ni2+ ions. Energy Dispersive X-ray analysis (EDX) confirmed the purity of prepared samples. Magnetic parameters (Ms, Mr and Hc) showed a gradual decrease with addition of Ni2+ ions. Meanwhile, both Coercivity and Curie temperature were enhanced in our studied nanorod compositions compared to their corresponding nanosphere compositions. Variation of DC resistivity and thermoelectric power coefficient were studied in the temperature range 385–548 K. The sign of the thermoelectric power coefficient could determine the type of charge carriers. Moreover, the conduction mechanism was deduced from activation energy values. Finally, the cation distribution was proposed such that it could verify the variation of lattice parameter and magnetization of each composition as well as the behavior of resistivity and thermoelectric power.

Electric gating of the multichannel conduction in LaAlO3/SrTiO3 superlattices* *Project supported by the National Basic Research Program of China (Grant Nos. 2016YFA0300701, 2017YFA0206300, 2017YFA0303601, and 2018YFA0305704), the National Natural Science Foundation of China (Grant Nos. 11520101002, 51590880, 11674378, 11934016, and 51972335), and the Key Program of the Chinese Academy of Sciences.

The electric gating on the transport properties of two-dimensional electron gas (2DEG) at the interface of LaAlO3/SrTiO3 (LAO/STO) heterostructure has attracted great research interest due to its potential application in field-effect devices. Most of previous works of gate effect were focused on the LAO/STO heterostructure containing only one conductive interface. Here, we systematically investigated the gate effect on high-quality LAO/STO superlattices (SLs) fabricated on the TiO2-terminated (001) STO substrates. In addition to the good metallicity of all SLs, we found that there are two types of charge carriers, the majority carriers and the minority carriers, coexisting in the SLs. The sheet resistance of the SLs with a fixed thickness of the LAO layer increases monotonically as the thickness of the STO layer increases. This is derived from the dependence of the minority carrier density on the thickness of STO. Unlike the LAO/STO heterostructure in which minority and majority carriers are simultaneously modulated by the gate effect, the minority carriers in the SLs can be tuned more significantly by the electric gating while the density of majority carriers is almost invariable. Thus, we consider that the minority carriers may mainly exist in the first interface near the STO substrate that is more sensitive to the back-gate voltage, and the majority carriers exist in the post-deposited STO layers. The SL structure provides the space separation for the multichannel conduction in the 2DEG, which opens an avenue for the design of field-effect devices based on LAO/STO heterostructure.

A multiscale framework to predict electrochemical characteristics of yttrium doped Barium Zirconate based solid oxide cells

A multiscale model is developed combining physics at atomistic, meso and continuum scales to predict the electrochemical characteristics of Current-Voltage curves in Yttrium doped Barium Zirconate based solid oxide cells. The most probable reaction pathway involving proton transfer from the surface of electrodes to the electrode/air/electrolyte Triple Phase Boundary from where it moves to the electrolyte is proposed, and their reaction barriers have been predicted from Density Functional Theory calculations. The model is validated against experimental observations. The environment, such as the amount of H2 (reducing) and O2 (oxidizing) gases the electrolyte is equilibrated in, affects the type of charge carriers while the temperature affects their rate of transport. Both these factors affect the rate of reactions in the proposed pathway. These effects are well manifested in the predicted electrochemical characteristics. The reducing environments are suitable for fuel cell mode operation and, the oxidizing environments are suitable for electrolyzer cell mode operations. The activation energy for conductivity in oxidizing environments is higher making it less amenable for low-temperature operation than in reducing environments. A data-driven sensitivity analysis is performed from which a non-intuitive parameter, permittivity of the electrolyte, is predicted to be important for Solid Oxide Cell performance.

Near-infrared holographic photorefractive recording under applied electric field in undoped Bi12TiO20 sillenite crystal

Abstract Direct holographic recording in undoped Bi12TiO20 crystal at 1064 nm is investigated aiming the characterization of diffraction efficiency under action of applied dc electric field (E0). An enhancement of 12-fold in the diffraction efficiency was revealed when E0 increased from 0.0 to 4.2 kV/cm. The theoretical dependence of the diffraction efficiency and the holographic erasure time upon E0 was investigated using the standard model of Kukhtarev for photorefractivity and the results showed a good experimental data fitting in both cases, allowing the computation of the effective trap concentration (ND)eff ≈ 5.62 × 1015 cm−3 which is responsible by the recording mechanism into the crystal sample. In particular, the theoretical-experimental investigation using the holographic erasure time also allowed to determine others relevant parameters for Bi12TiO20 such as the diffusion transport length LD, the Maxwell relaxation time τ M , and the quantum efficiency Φ that can explain the relatively slow dynamics of photorefractive recording and erasure in near-infrared region. The type of charge carrier involved in the recording mechanism in the near-infrared region is also discussed.

Dual-Plasmonic Gold@Copper Sulfide Core-Shell Nanoparticles: Phase-Selective Synthesis and Multimodal Photothermal and Photocatalytic Behaviors.

Although the intriguing plasmonic properties of noble metal nanoparticles originate from the collective oscillations of free electrons in the conduction band, nanoparticles of doped semiconductors may also exhibit metal-like, plasmonic features that are dictated by the resonantly excited free hole oscillations in the valence band. Here, we combine Au, a representative free electron metal, with copper sulfides, a class of plasmonic p-type semiconductors, in a core-shell nanoparticle geometry to construct dual-plasmonic hetero-nanostructures displaying unique multiplex optical characteristics dominated by plasmonic hole oscillations in the semiconductor shells, plasmonic electron oscillations in the metallic cores, and interband electronic transitions from the valence to conduction bands. Through deliberately designed colloidal synthesis, we are able to selectively grow nanoshells comprising copper sulfides of specifically targeted crystalline phases and Cu/S stoichiometries, such as covellite (CuS), digenite (Cu1.8S), and nonstoichiometric Cu2-xS, on the surfaces of Au nanoparticle cores. Our synthetic approach enables us not only to finely control the core and shell dimensions but also to systematically adjust the free hole concentrations in the semiconductor shells, which forms the keystone for the fine tuning of multiple optical resonance modes supported by these hybrid hetero-nanostructures. The dual-plasmonic Au@copper sulfide core-shell nanoparticles exhibit unique multimodal photothermal and photocatalytic behaviors upon selective photoexcitations of different optical transitions at their characteristic resonant frequencies, allowing us to quantitatively evaluate and rigorously compare the intrinsic photothermal and photocatalytic efficacies of multiple types of hot charge carriers, all photoexcited in the same hybrid nanoparticles but with distinct photophysical origins, excited-state lifetimes, energy distributions, and transfer pathways.

High temperature Seebeck coefficient, small-polaron conduction and work function of LiMn1.5Ni0.5O4 thin films

We report on the high temperature Seebeck coefficient, conduction mechanism and room temperature work function of LiMn1.5Ni0.5O4 thin film prepared using the radio frequency sputtering technique. Cubic spinel LiMn1.5Ni0.5O4 thin film exhibits a room temperature average work function value of 4.94 eV. DC electrical conductivity exhibits a small-polaron hopping mechanism with the activation energy of 0.47 eV. The polaron radius, inverse localization length and the Debye temperature values of LiMn1.5Ni0.5O4 thin films are predicted using the variable range hopping model and Raman studies. The Seebeck coefficient and the power factor values for the LiMn1.5Ni0.5O4 thin film is seen to vary from −1000 to −600 μV K−1 and 0.09 to 0.5 μW mK−2, respectively, in the temperature range of 420 to 560 K. Electrical transport studies reveal the fact that the LiMn1.5Ni0.5O4 thin film posseses n type charge carriers with a high Seebeck coefficient, high power factor and small-polaron hopping conduction mechanism in the temperature range of 420–560 K.

K+ doping effect on grain boundary passivation and photoelectronics properties of NiOx/perovskite films

As a new type of solar cells, perovskite solar cells (PSCs) have received widespread attention. The grain growth and surface morphology of perovskite thin films are considered to have an important influence on the performance of PSCs. In this paper, the K+ was introduced into the CH3NH3PbI3 (MAPbI3) and the effect of different doping concentration on the morphology, photophysical properties, and semiconductor characteristics of the perovskite films was investigated. From experiments, we found that the incorporation of low-concentration K+ has a passivation effect on the grain boundaries, which reduces the defect density at interface. The phase transition and obvious red shift of MAPbI3 could be seen under high doping concentration in XRD pattern and absorption spectrum, respectively. The modulation of the perovskite layer major charge carrier type was observed when K+ was introduced, which laid a basis for future investigation on inverted homo-junction solar cells.

Structural characterization, dielectric, and magnetic properties of Ti-doped YFeO3 multiferroic compound

Crystal structure, and magnetic and dielectric properties were studied in Ti-doped YFeO3 polycrystalline samples. The crystal structure and chemical state of the Fe/Ti cations were characterized by a combination of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It is found that a narrow solid-solubility limit is achieved (~ 6%) and the d0–ness of Ti4+ is not favored in the YFeO3 matrix. A change in the valence states of Fe and Ti cations explains not only the low solubility limit but also the anomalous increase of the volume cell. Soft magnetic hysteresis curves in the M vs H curves indicate that the weak ferromagnetic contribution prevails for all the studied samples. Furthermore, by means of differential calorimetry (DSC) it was possible to identify the AFM transition which decreases with increasing Ti content. On the other hand, the change of the step-like anomaly to a broad peak in the permittivity (e′) and loss tangent (tan δ) as Ti content increases resemble to a relaxor ferroelectric transition. The endothermic peak (DSC) around the magnetic transition and the broad peaks in the e′(T) data seem to indicate the existence of a local polarization coupled with AFM transition at ~ 650 K. Two types of charge carriers were found in the doped systems, oxygen vacancies at high temperatures, and small polarons at low temperatures, respectively. Besides, it is found that the introduction of those charge carriers in the YFeO3 matrix occurs by the change of oxidation state from Fe3+ to Fe2+ through Ti3+.

Effects of sintering temperature on structural, infrared, magnetic and electrical properties of Cd0.5Zn0.5FeCrO4 ferrites prepared by sol–gel route

In this work, an attempt is made to study the effects of sintering temperature on the structural, infrared, magnetic and electrical properties of Cd0.5Zn0.5FeCrO4 ferrites. Samples were prepared by sol–gel route and sintered at 900 and 1100 °C. The structural characterization of the synthesized powders by X-ray powder diffraction showed that only single phase with cubic spinel structure existed. Estimated values of unit cell parameter and average grains size show an increase with sintering temperature. The intensities of absorption bands, spontaneous magnetization, electrical conductivity, and dielectric constants were found to increase with increasing sintering temperature. The non-overlapping small polaron tunneling (NSPT) model dominates the conduction process for the studied samples which present a relaxation phenomenon with non-Debye nature. Close activation energies values were found from analyzes of relaxation time and dc-conductivity indicating that the relaxation and the conduction processes may be attributed to the same type of charge carriers. The equivalent electrical circuit (Rg+ Rgb//CPEgb) was used for modeling the Nyquist data.

Detecting Chirality in Two-Terminal Electronic Nanodevices.

Central to spintronics is the interconversion between electronic charge and spin currents, and this can arise from the chirality-induced spin selectivity (CISS) effect. CISS is often studied as magnetoresistance (MR) in two-terminal (2T) electronic nanodevices containing a chiral (molecular) component and a ferromagnet. However, fundamental understanding of when and how this MR can occur is lacking. Here, we uncover an elementary mechanism that generates such an MR for nonlinear response. It requires energy-dependent transport and energy relaxation within the device. The sign of the MR depends on chirality, charge carrier type, and bias direction. Additionally, we reveal how CISS can be detected in the linear response regime in magnet-free 2T nanodevices, either by forming a chirality-based spin-valve using two or more chiral components or by Hanle spin precession in devices with a single chiral component. Our results provide operation principles and design guidelines for chirality-based spintronic nanodevices and technologies.

Surface charge transfer doping for two-dimensional semiconductor-based electronic and optoelectronic devices

Doping of semiconductors, i.e., accurately modulating the charge carrier type and concentration in a controllable manner, is a key technology foundation for modern electronics and optoelectronics. However, the conventional doping technologies widely utilized in silicon industry, such as ion implantation and thermal diffusion, always fail when applied to two-dimensional (2D) materials with atomically-thin nature. Surface charge transfer doping (SCTD) is emerging as an effective and non-destructive doping technique to provide reliable doping capability for 2D materials, in particular 2D semiconductors. Herein, we summarize the recent advances and developments on the SCTD of 2D semiconductors and its application in electronic and optoelectronic devices. The underlying mechanism of STCD processes on 2D semiconductors is briefly introduced. Its impact on tuning the fundamental properties of various 2D systems is highlighted. We particularly emphasize on the SCTD-enabled high-performance 2D functional devices. Finally, the challenges and opportunities for the future development of SCTD are discussed.

Tuning the type of charge carriers in N-heterocyclic carbene-based molecular junctions through electrodes* *Project supported by the National Natural Science Foundation of China (Grants Nos. 11874242 and 21933002) and the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2019PA022).

Designing tunable molecular devices with different charge carriers in single-molecule junctions is crucial to the next-generation electronic technology. Recently, it has been demonstrated that the type of charge carriers depends on and can be tuned by controlling the molecular length and the number of interfacial covalent bonds. In this study, we show that the type of charge carriers can also be tuned by controlling the material and shape of electrodes. N-heterocyclic carbenes (NHCs) have attracted attention because of their ability to form strong, substitutional inert bonds in a variety of metals. Also, NHCs are more stable than the widely used thiol group. Therefore, we use electrodes to tune the type of charge carriers in a series of NHCs with different side groups. The ab initio calculations based on non-equilibrium Green’s formalism combined with density functional theory show that the dominant charge carrier switches from electrons to holes when gold electrodes are changed into platinum ones. The nature of the charge carriers can be identified by variations in the transport spectra at the Fermi level (E F), which are caused by the side groups. The projections of transport spectra onto the central molecules further validate our inferences. In addition, the transmission coefficient at E F is found to be dependent on the atomic interface structure. In particular, for the NHC without methyl or ethyl side groups, connecting a protruding atom on the electrode surface significantly enhances the transportability of both electrode materials. Overall, this study presents an effective approach to modifying transport properties, which has potential applications in designing functional molecular devices based on NHCs.