Persistent photoconductivity in MgZnO alloys

The electrical characteristics and persistent-conductivity effects in MgZnO:P alloys grown by the method of pulsed laser deposition on undoped n-ZnO substrates are systematically investigated. It is shown that the pronounced persistent conductivity and the persistent photocapacitance related to the presence of high-barrier electron traps for electron capture are observed in initial unannealed layers. These traps are located in the lower half of the band gap and have the optical ionization threshold of 2.8 eV and the electron-capture barrier height of ～0.4 eV. Alongside such centers, the hole traps with the ionization energy of 0.14 eV are also observed. The annealing at 850°C transforms the material into that of p-type conductivity with the depth of dominant phosphorus-related acceptors close to 0.2 eV. The conductivity compensation and the formation of hole traps with the activation energy of 0.14 and 0.84 eV in the n-ZnO substrate are also observed, and these traps are associated with the acceptor-defect diffusion into the substrate at annealing.     

Stress Controllability in Thermal and Electrical Conductivity of 3D Elastic Graphene‐Crosslinked Carbon Nanotube Sponge/Polyimide Nanocomposite

Stress controllability in thermal and electrical conductivity is important for flexible piezoresistive devices. Due to the strength‐elasticity trade‐off, comprehensive investigation of stress‐controllable conduction in elastic high‐modulus polymers is challenging. Here presented is a 3D elastic graphene‐crosslinked carbon nanotube sponge/polyimide (G_w‐CNT/PI) nanocomposite. Graphene welding at the junction enables both phonon and electron transfer as well as avoids interfacial slippage during cyclic compression. The uniform G_w‐CNT/PI comprising a high‐modulus PI deposited on a porous templated network combines stress‐controllable thermal/electrical conductivity and cyclic elastic deformation. The uniform composites show different variation trends controlled by the porosity due to different phonon and electron conduction mechanisms. A relatively high k (3.24 W m^?1 K^?1, 1620% higher than PI) and suitable compressibility (16.5% under 1 MPa compression) enables the application of the composite in flexible elastic thermal interface conductors, which is further analyzed by finite element simulations. The interconnected network favors a high stress‐sensitive electrical conductivity (sensitivity, 973% at 9.6% strain). Thus, the G_w‐CNT/PI composite can be an important candidate material for piezoresistive sensors upon porosity optimization based on stress‐controllable thermal or electrical conductivity. The results provide insights toward controlling the stress‐induced thermal/electrical conductivities of 3D interconnected templated composite networks for piezoresistive conductors or sensors.     

Pre-breakdown noise in electrically stressed thin SiO2 layers of MOS devices observed with C-AFM

A conductive atomic force microscope (C-AFM) has been used to analyse the degradation stage (before breakdown, BD) of ultrathin (<6 nm) films of SiO₂ at a nanometer scale. Working on bare gate oxides, the conductive tip of the C-AFM allows the electrical characterization of nanometric areas. Due to the extremely small size of the analysed areas, several features, which can be masked by the current that flows through the overall test structure during standard electrical tests, are observed. In particular, switching between different conduction states and sudden changes of conductivity have been measured during ramped voltage tests, which have been related to the trapping and detrapping of single electronic charges in the defects generated during the electrical stress. This phenomenon, which has been observed during constant voltage stresses in the form of random telegraph signals, has been associated to the pre-breakdown noise measured in poly-gated structures. The C-AFM has also allowed to directly measure the I–V characteristics of the fluctuating spot.     

Superior Thermoelectric Performance of Textured Ca3Co4−xO9+δ Ceramic Nanoribbons

Calcium cobaltite Ca₃Co_4−xO_9+δ (CCO) is a promising p-type thermoelectric (TE) material for high-temperature applications in air. The grains of the material exhibit strong anisotropic properties, making texturing and nanostructuring mostly favored to improve thermoelectric performance. On the one hand multitude of interfaces are needed within the bulk material to create reflecting surfaces that can lower the thermal conductivity. On the other hand, low residual porosity is needed to improve the contact between grains and raise the electrical conductivity. In this study, CCO fibers with 100% flat cross sections in a stacked, compact form are electrospun. Then the grains within the nanoribbons in the plane of the fibers are grown. Finally, the nanoribbons are electrospun into a textured ceramic that features simultaneously a high electrical conductivity of 177 S cm⁻¹ and an immensely enhanced Seebeck coefficient of 200 µV K⁻¹ at 1073 K are assembled. The power factor of 4.68 µW cm⁻¹ K⁻² at 1073 K in air surpasses all previous CCO TE performances of nanofiber ceramics by a factor of two. Given the relatively high power factor combined with low thermal conductivity, a relatively large figure-of-merit of 0.3 at 873 K in the air for the textured nanoribbon ceramic is obtained.     

Direct Ink Writing of Adjustable Electrochemical Energy Storage Device with High Gravimetric Energy Densities

3D printing graphene aerogel with periodic microlattices has great prospects for various practical applications due to their low density, large surface area, high porosity, excellent electrical conductivity, good elasticity, and designed lattice structures. However, the low specific capacitance limits their development in energy storage fields due to the stacking of graphene. Therefore, constructing a graphene‐based 2D materials hybridization aerogel that consists of the pseduocapacitive substance and graphene material is necessary for enhancing electrochemical performance. Herein, 3D printing periodic graphene‐based composite hybrid aerogel microlattices (HAMs) are reported via 3D printing direct ink writing technology. The rich porous structure, high electrical conductivity, and highly interconnected networks of the HAMs aid electron and ion transport, further enabling excellent capacitive performance for supercapacitors. An asymmetric supercapacitor device is assembled by two different 4‐mm‐thick electrodes, which can yield high gravimetric specific capacitance (C_g) of 149.71 F g^?1 at a current density of 0.5 A g^?1 and gravimetric energy density (E_g) of 52.64 Wh kg^?1, and retains a capacitance retention of 95.5% after 10 000 cycles. This work provides a general strategy for designing the graphene‐based mixed‐dimensional hybrid architectures, which can be utilized in energy storage fields.     

Manifestation of the upper Hubbard band in the electrical conductivity of two-dimensional p-GaAs-AlGaAs structures

The Hall effect and electrical conductivity were studied in the temperature range of 1.7–300 K in Be-doped p-GaAs/AlGaAs multilayer structures with 15-nm-wide quantum wells. Doping of the well itself and the adjacent barrier layer was used to create a situation when the upper Hubbard band (the A ⁺ centers) was occupied with holes and electrical conduction proceeded over the states in this band. It is shown experimentally that the binding energy of A ⁺ centers increases significantly in the 15-nm-wide wells compared to this energy in the bulk, which is explained by the fact that the well size and the hole radius at the A ⁺ center are almost identical. The above radius was independently estimated from an analysis of the temperature dependence of the hopping conductivity.     

The nature of “heavy” electrons in the p-HgTe zero-gap semiconductor

To interpret the low-temperature features of the electron transport in the p-HgTe zero-gap semiconductor, the model is suggested, according to which, the “heavy” electrons are the electrons for the conduction band localized in the wells of the fluctuation potential. The experimental data on the temperature, magnetic field, and baric dependences of the Hall coefficient R(T, H, P) and electrical conductivity ρ₀(T, P) in lightly doped, moderately compensated, and heavily doped p-HgTe samples are analyzed.     

Thermopower of transmutation-doped Ge:Ga in the region for hopping conductivity

The low-temperature thermopower of transmutation-doped Ge:Ga is investigated experimentally and theoretically. The large values of the thermopower observed in the region for ɛ ₁ conduction and its sharp drop upon the transition to conduction between impurities are interpreted as manifestations of the phonon drag of free holes and its suppression in the region for hopping transport. The positive sign of the thermopower and its magnitude in the hopping-conduction saturation region can be explained theoretically under the assumption that the classical ɛ ₂ conduction channel, which is not manifested explicitly in the electrical conductivity, makes a contribution to the thermopower in the narrow temperature range associated with the transition from ɛ ₁ conduction to hopping conduction. After the transition to variable-range hopping (T⩽2 K), the thermopower decreases sharply and takes anomalous, vanishingly small values. They can be explained within the standard theory of hopping thermopower only under the condition that the contribution caused by the asymmetry of the density of states of the impurity band in the vicinity of the Fermi level and the correlation contribution are compensated. Fiz. Tekh. Poluprovodn. 31, 1174–1179 (October 1997)     

Tuning the Electronic Properties of Graphene Oxide Nanoribbons Through Different Oxygen Doping Configurations

The electronic properties of armchair graphene oxide nanoribbons (AGONRs) with different doped oxygen configurations are studied based on density functional theory using first principle calculations. The electronic properties of the AGONRs are tuned by different oxygen configurations for top edges, center, bottom edges and fifth width. The AGONRs for top-edge O doping configuration are indirect band gap semiconductors with an energy gap of 1.268 eV involving hybridization among C-2p and O-2s, 2p electrons and electrical conductivity of oxygen atoms. The center and bottom edges are direct band gap semiconductors with 1.317 eV and 1.151 eV, respectively. The valence band is contributed from C-2p, O-2p and H-1s for top-edge O doping. The electronic properties of AGONRs are changed due to localization in ?2.94 eV of O-2p states. The center O-doped AGONRs are n-type semiconductors with Fermi levels near the conduction band bottom. This is due to hybridization among C-2s, 2p and O-2p electrons. However, bottom-edge O-doped AGONRs are p-type semiconductors, due to the electrical conductivity of oxygen atoms. The fifth-width O-doped AGONRs are indirect band gap semiconductors with an energy gap of 0.375 eV. The projected density of states shows that the localization and hybridization between C-2 s, 2p, O-2p and H-1s electronic states are rising in the conduction band and valence band from the projected density of states. The localization is induced by O-2p electronic states at a Fermi level.     

Effect of Nano-Ce-Doped TiO<Subscript>2</Subscript> on AC Conductivity and DC Conductivity Modeling Studies of Poly (<Emphasis Type="Italic">n</Emphasis>-Butyl Methacrylate)

Among the unique properties of polymer nanocomposites, electrical conductivity deserves a prominent place due to their wide applications in conducting adhesive, electromagnetic shielding and sensors. The present work focuses on the effect of cerium-doped titanium dioxide (Ce-TiO₂) nanoparticles on the conductivity studies of poly (n-butyl methacrylate), or PBMA, nanocomposites at different temperatures. The frequency-dependent alternating current (AC) electrical conductivity of PBMA/Ce-TiO₂ nanocomposites has been found to increase with increase in temperature and the concentration of Ce-TiO₂ nanoparticles. The activation energy calculated from the AC electrical conductivity has been found to decrease with frequency and increasing temperatures. The frequency exponent factor also showed a decrease with frequency, indicating the hopping conduction in the nanocomposites. The maximum AC conductivity has been observed for the composites with 7 wt.% sample. The direct current (DC) conductivity of PBMA/Ce-TiO₂ composites was also enhanced with the addition of Ce-TiO₂ nanoparticles. Experimental and theoretical investigations based on Scarisbrick, Bueche, McCullough and Mamunya modeling were undertaken to understand the observed DC conductivity differences induced by the addition of Ce-doped TiO₂ nanoparticles to PBMA matrix. The experimental conductivity showed good agreement with the theoretical conductivity observed using the Mamunya model.     

Optical and electrical characterization of hafnium oxide deposited by MOCVD

The paper reports on electrical and optical investigations performed on HfO₂ high-k films deposited by Metal-organic chemical vapor deposition (MOCVD). Spectroellipsometry investigations show the presence of a transition layer between HfO₂ and the silicon substrate, which can be optically modelled as a mixture of Si and SiO₂; this information is further used in the assessment of the electrical measurements. Hysteresis effects have been observed in the Capacitance–Voltage (C–V) measurements for the as-deposited sample as well as the annealed samples. For the samples with large hysteresis, Poole–Frenkel (PF) conduction is the most likely dominant conduction mechanism. The energy of dominant trap level was found to be 0.7 eV.     

Electronic Conductivity and Defect Chemistry of Heterosite FePO4

Electrochemical investigations on polycrystalline orthorhombic FePO₄ (heterosite), the lithium‐poor part of the LiFePO₄/FePO₄ redox couple, gives insight into its charge‐carrier chemistry. The material obtained by chemical delithiation exhibits a predominant electronic conductivity. A residual lithium content of 0.03 wt% was found and has to be considered as lithium interstitials in the FePO₄ ground structure. Compensation by electrons induces n‐type conduction, confirmed by the pO₂ dependence of the electronic conductivity. The pO₂ dependence is primarily ascribed to the formation of an oxidic surface composition leading to bulk depletion of lithium, rather than to filling of oxygen vacancies.     

One-Dimensional Modeling of Thermogenerator Elements with Linear Material Profiles

Graded and segmented thermoelectric elements have been studied for a long time with the aim of improving the performance of thermogenerators that are exposed to a large temperature difference. However, it has been shown that simply adjusting the maximum figure of merit ZT in each segment of a stacked or graded thermoelectric (TE) element is not a sufficient strategy to maximize thermoelectric device performance. Global optimization of a performance parameter is commonly based on a one-dimensional continua-theoretical model. Following the proposal by Müller and coworkers, the temperature profile T(x) can be calculated within a model-free setup directly from the one-dimensional (1D) thermal energy balance, e.g., based on continuous monotonic gradient functions for all material profiles, and independent and free variability of the material parameters S(x), σ(x), and κ(x) is assumed primarily, where S is the Seebeck coefficient, and σ and κ are the electrical and thermal conductivities, respectively. Thus the optimum current density can be determined from the maximum of the global performance parameter. This has been done up to now by means of numerical procedures using a 1D thermoelectric (TE) finite-element method (FEM) code or the algorithm of multisegmented elements. Herein, an analytical solution of the 1D thermal energy balance has been found for constant gradients, based on Bessel functions. For a constant electrical conductivity but linear profiles S(x) and κ(x), first results for the electrical power output of a thermogenerator are presented.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Study of Synthesis and Temperature Dependence of DC Conductivity in the Low Temperature Range for Poly(<Emphasis Type="Italic">N</Emphasis>-Methylaniline)

Poly(N-methylaniline) (PNMA) doped with p-toluenesulfonic acid (PTSA) was prepared chemically. Polymer synthesis was optimized for different dopant-to-monomer and oxidant-to-monomer ratios. The polymer was characterized by Fourier-transform infrared (FT-IR) and ultraviolet-visible absorption (UV–Vis) spectroscopy. The dependence of direct-current (DC) conductivity on temperature was studied in the range from 20 K to 300 K. The conductivity was found to follow the variable-range hopping (VRH) model of charge transport in the higher temperature range, while showing deviation at low temperature. The temperature dependence of the DC conductivity and activation energy data are indicative of the existence of both VRH and mixed conduction for various doping ratios in poly(N-methylaniline).     

Special features of hopping conduction in p-Hg0.78Cd0.22Te crystals under conditions of dual doping

At T=4.2–125 K, the electrical conductivity and Hall effect were studied in p-Hg_0.78Cd_0.22Te crystals that contained 3×10¹⁶ cm^?3 Cu atoms and 1.83×10¹⁶ cm^?3 of Hg vacancies (either simultaneously or independently of each other). In such crystals, the ?₁ conductivity over the valence band is dominant at temperatures above 10–12 K, whereas the hopping conduction is prevalent at temperatures below 8–10 K. In the samples containing copper atoms and mercury vacancies simultaneously, conductivity with variable-range hopping is observed. It is found that the ?₁ conductivity of the copper-doped crystals is independent of the presence of mercury vacancies, whereas the hopping conductivity increases appreciably if these vacancies are introduced into the undoped crystal. This phenomenon is attributed to attachment of holes to the neutral mercury vacancies. The energy of this attachment is calculated, and it is found that this energy is equal to 3.7 meV for the ground state. The fluctuation-related broadening of the impurity band in the solid solutions gives rise to the overlap of the impurity bands formed by the copper acceptor levels and by the levels of holes attached to vacancies.     

Charge transport mechanisms in Schottky diodes based on low-resistance CdTe:Mn

CdTe:Mn crystals with a resistivity of ～1 Ω cm at 300 K and Schottky diodes based on them are investigated. The electrical conductivity of the material and its temperature variations are explained in terms of the statistics of electrons and holes in semiconductors with allowance for the compensation processes. The ionization energy and the degree of compensation of the donors responsible for the conductivity are determined. It is shown that, in the case of forward connection and low reverse biases, the currents in Au/CdTe:Mn Schottky diode are determined by generation-recombination processes in the space-charge region. At higher reverse biases (above 1.5–2 V) the excess current is caused by electron tunneling from the metal to the semiconductor, and at even higher voltages (>6–7 V) an additional increase in the reverse current due to avalanche processes is observed.     

Tensoresistive effect in porous silicon layers with different morphology

The effect of elastic flexural strain on electrical conductivity of porous silicon with different pore morphology and different properties of depletion regions around the pores is studied. Porous layers are formed using anodic electrochemical etching of the p-and n-Si wafers and have a porosity of 5–68%. It is shown that specific features of variations in electrical conductivity of porous silicon under the effect of deformation depend on the structural characteristics of the porous material. In order to explain the results obtained, various physical models of the charge-carrier transport in porous silicon are used.     

The Effects of Polysilastyrene and Au Additions on the Thermoelectric Properties of <Emphasis Type="Italic">β</Emphasis>-SiC/Si Composites

Recently, Yamaguchi et al. proposed a self-cooling device that does not require additional power circuits for cooling because it is Peltier-cooled using its own current in conjunction with a thermoelectric material. Silicon carbide is a promising thermoelectric material for this technology since its electrical conductivity, thermal conductivity, and Seebeck coefficient are higher than those of conventional thermoelectric materials. This study investigates the effects of polysilastyrene and Au additions on the thermoelectric properties of p-type β-SiC/Si polycrystalline semiconductor composites in order to assess whether their addition improves the performance of self-cooling devices.     

Surface‐Dominated Transport Properties of Silicon Nanowires

Surface effects are widely recognized to significantly influence the properties of nanostructures, although the detailed mechanisms are rarely studied and unclear. Herein we report for the first time a quantitative evaluation of the surface‐related contributions to transport properties in nanostructures by using Si nanowires (NWs) as a paradigm. Critical to this study is the capability of synthesizing SiNWs with predetermined conduction type and carrier concentration from Si wafer of known properties using the recently developed metal‐catalyzed chemical etching method. Strikingly, the conductance of p‐type SiNWs is substantively larger in air than that of the original wafer, is sensitive to humidity and volatile gases, and thinner wires show higher conductivity. Further, SiNW‐based field‐effect transistors (FETs) show NWs to have a hole concentration two orders of magnitude higher than the original wafer. In vacuum, the conductivity of SiNWs dramatically decreases, whereas hole mobility increases. The device performances are further improved by embedding SiNW FETs in 250 nm SiO₂, which insulates the devices from atmosphere and passivates the surface defects of NWs. Owing to the strong surface effects, n‐type SiNWs even change to exhibit p‐type characteristics. The totality of the results provides definitive confirmation that the electrical characteristics of SiNWs are dominated by surface states. A model based on surface band bending and carrier scattering caused by surface states is proposed to interpret experimental results. The phenomenon of surface‐dependent transport properties should be generic to all nanoscale structures, and is significant for nanodevice design for sensor and electronic applications.