Bipolar Conduction Research Articles

(Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge2Sb2Te5

We calculate critical electronic conduction parameters of the amorphous phase of Ge2Sb2Te5 (GST), a common material used in phase change memory. We estimate the room temperature bandgap of metastable amorphous GST to be Eg (300K) = 1.84 eV based on a temperature dependent energy band model. We estimate the free carrier concentration at the melting temperature utilizing the latent heat of fusion to be 1.47 x 1022 cm-3. Using the thin film melt resistivity, we calculate the carrier mobility at melting point as 0.187 cm2/V-s. Assuming that metastable amorphous GST is a supercooled liquid with bipolar conduction, we compute the total carrier concentration as a function of temperature and estimate the room temperature free carrier concentration as p(300K) ≈ n(300K) = 1.69×1017 cm-3. Free electrons and holes are expected to recombine over time and the stable (drifted) amorphous GST is estimated to have p-type conduction with p(300K) ≈ 6×1016 cm-3.

Controlling Tunneling Characteristics via Bias Voltage in Bilayer Graphene/WS2/Metal Heterojunctions.

Van der Waals heterojunctions, formed by stacking two-dimensional materials with various structural and electronic properties, opens a new way to design new functional devices for future applications and provides an ideal research platform for exploring novel physical phenomena. In this work, bilayer graphene/WS2/metal heterojunctions (GWMHs) with vertical architecture were designed and fabricated. The tunneling current–bias voltage (It − Vb) properties of GWMHs can be tuned by 5 × 106 times in magnitude for current increasing from 0.2 nA to 1 mA with applied bias voltage increasing from 10 mV to 2 V. Moreover, the transfer properties of GWMHs exhibit n-type conduction at Vb = 0.1 V and bipolar conduction at Vb = 2 V; these findings are explained well by direct tunneling (DT) and Fowler–Nordheim tunneling (FNT), respectively. The results show the great potential of GWMHs for high-power field-effect transistors (FETs) and next-generation logic electronic devices.

Study of Phase Formation Behavior and Electronic Transport Properties in the FeSe2 -FeTe2 System

In this study, we investigated the phase formation behavior of the FeSe2-FeTe2 system and the evolution of their thermoelectric transport properties. A series of Fe(Se1-xTex)2(<i>x</i> = 0, 0.2, 0.6, 0.8 and 1) compounds were synthesized by conventional solid-state reaction. Single phase orthorhombic structures were formed, except for FeSe0.4Te1.6(<i>x</i> = 0.2). For FeSe0.4Te1.6(<i>x</i> = 0.2), in which a mixture of FeSe2 and FeTe2-based phases was observed. The electrical conductivity of FeSe2 was as low as 31 S/cm at room temperature, and greatly increased to 1295 S/cm in FeSe0.8Te1.2. Then it decreased to 335 S/cm for FeTe2. It was shown that the bipolar conduction of electrons and holes pronouncedly exists in the sample. For FeSe2 and FeTe2, the Seebeck coefficient was positive at room temperature, becoming negative at high temperatures. For the other samples, the Seebeck coefficient was negative at room temperature and the magnitude of Seebeck coefficient increased with temperature. As results, the maximum power factor of 0.19 mW/mK<sup>2</sup> was observed for FeSe0.8Te1.2(<i>x</i> = 0.6) at room temperature mostly due to its high electrical conductivity. The maximum power factor observed at 600 K was 0.34 mW/mK<sup>2</sup> for FeSe2 (<i>x</i> = 0) with the largest Seebeck coefficient of -101 µV/K. The Hall carrier concentration was measured and compared, and effective masses were calculated to further investigate electrical transport in the samples.

Enhancing the thermoelectric properties of powder metallurgically produced BiSbTe alloy through feasible electroless surface coating method

The powder metallurgical techniques capable of powder mass production but lacks in achieving high zT. Therefore, to improve zT, electroless surface coating was performed on water atomized Bi0.5Sb1.5Te3 (BST) powder and stimulated their conducting nature. Surface chemical composition studies displays fine elemental coating on the BST powders and remarkably, the Seebeck coefficient increased till 450 K. The surface coated bulk sample shows greatly improved power factor about ~45% higher than uncoated sample. Indeed, at high temperature we obtained reduced lattice thermal conductivity which confirms the remarkable suppression of bipolar thermal conductivity. Consequently, a maximum zT of 1.16 at 400 K achieved by coated samples which is ~102% enhancement compared to original BST. Also, we attained high average zT of ~0.99 which helps in employing thermoelectric devices across wide temperature. Therefore, electroless surface coating could be a feasible method for improving the properties of mass-produced BST powder.

Synergistic Optimization of the Thermoelectric and Mechanical Properties of Large-Size Homogeneous Bi0.5Sb1.5Te3 Bulk Samples via Carrier Engineering for Efficient Energy Harvesting.

Manufacturing an economically viable, efficient commercial thermoelectric (TE) module is essential for power generation and refrigeration. However, mediocre TE properties, lack of good mechanical stability of the material, and significant difficulties involved in the manufacturing of large-scale powder as well as bulk samples hinder the potential applications of the modules. Herein, an economically feasible single-step water atomization (WA) is employed to synthesize BST powder (2 kg) by Cu doping within a short time and consolidated into large-scale bulk samples (500 g) for the first time with a diameter of 50 mm and a thickness of about 40 mm using spark plasma sintering (SPS). The incorporation of Cu into BST greatly boosts the carrier concentration, leading to a significant increase in electrical conductivity, and inhibits the bipolar thermal conductivity by 73%. Synchronously, the lattice contribution (κL) is greatly reduced by the effective scattering of phonons by comprising fine-grain boundaries and point defects. Therefore, the peak ZT is shifted to the mid-temperature range and obtained a maximum of ∼1.31 at 425 K and a ZTave of 1.24 from 300 to 500 K for the BSTCu0.05 sample, which are considerably greater than those of the bare BST sample. Moreover, the maximum compressive mechanical strength of large-size samples manufactured by the WA-SPS process is measured as 102 MPa, which is significantly higher than commercial zone melting samples. The thermoelectric module assembled with WA-SPS-synthesized BSTCu0.05 and commercial n-type BTS material manifests an outstanding cooling performance (-19.4 °C), and a maximum output power of 6.91 W is generated at ΔT ∼ 200 K. These results prove that the BSTCux samples are eminently suitable for the fabrication of industrial thermoelectric modules.

Thermoelectric Properties of p‐ and n‐type Eu5Sn2As6

AbstractEu5Sn2As6 is a Zintl phase crystalizing in the orthorhombic space group Pbam with one‐dimensional chains of corner‐shared SnAs4 tetrahedra running in the c‐direction. Eu5Sn2As6 has an impressive room temperature Seebeck of >100 μV/K and < – 100 μV/K at 600 K crossing from p‐ to n‐type at 650 K. The maximum thermoelectric figure of merit, zT, for Eu5Sn2As6 is small (0.075), comparable to that of the Zintl phase Ca5Al2Sb6 whose thermoelectric performance was improved by doping Na onto the Ca sites. In this study, we show that the thermoelectric properties of Eu5Sn2As6 can be improved by substituting with K or La. The series Eu5‐xKxSn2As6 provides an increase in maximum zT of 0.22 for x=0.15 due to a decrease resistivity while the onset of bipolar conduction systematically increases in temperature. Upon La substitution, Eu5‐xLaxSn2As6 results in a new n‐type Zintl phase across the temperature range of 300–800 K.

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Low bandgap thermoelectric materials suffer from bipolar effects at high temperatures, with increased electronic thermal conductivity and reduced Seebeck coefficient, leading to a reduced power factor and a low ZT figure of merit. In this work we show that the presence of strong transport asymmetries between the conduction and valence bands can allow high phonon-limited electronic conductivity at finite Seebeck coefficient values, leading to largely enhanced power factors. The power factors that can be achieved can be significantly larger compared to their maximum unipolar counterparts, allowing for doubling of the ZT figure of merit. We identify this behavior in low-bandgap cases from the half-Heusler material family. Using both advanced electronic Boltzmann transport calculations for realistic material band structures and model parabolic electronic bands, we elaborate on the parameters that determine this effect. We then develop a series of descriptors that can guide machine learning studies in identifying such classes of materials with extraordinary power factors at nearly undoped conditions. For this we test more than 3000 analytical band structures and their features, and more than 120 possible descriptors, to identify the most promising ones that contain: (i) only band structure features for easy identification from material databases and (ii) band structure and transport parameters that provide much higher correlations, but for which parameter availability can be somewhat more scarce.

Resonance states and hyperconvergence induced by tungsten doping in SnTe: Multiband transport leading to a propitious thermoelectric material

Discovery of dopants which can engineer the electronic structure of the thermoelectric materials beneficially to improve the figure of merit has been receiving a lot of attention. In this work, we study one such unique dopant, tungsten in SnTe by implementing first principles density functional theory approach. We predict that tungsten is a n-type resonant dopant which not only increases the band gap but causes convergence of valence sub-bands leading to increased Seebeck co-efficient due to increase in the effective mass and decrease in the bipolar conduction. We show for the first time, the introduction of hyperconvergence in the conduction sub-bands, a feature which was observed only in valence bands of SnTe and GeTe. In addition to the above features, it also introduces multiple electronic valleys near the Fermi level excluding the use of a co-dopant to exploit the benefits of the electronic structure engineering. A maximum ZT of ~1.61 theoretically achieved by tuning the chemical potential at 800 K makes this material worth being explored experimentally.

Prominent electron-hole asymmetry in thermoelectric transport of LaCoO3

We have measured the electrical resistivity and thermopower of electron-doped perovskite cobaltites ${\mathrm{LaCo}}_{1\ensuremath{-}y}{\mathrm{Te}}_{y}{\mathrm{O}}_{3}$. In contrast to hole-doped systems such as metallic ferromagnets ${\mathrm{La}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{CoO}}_{3}$ $(M=\text{Ca}, \mathrm{Sr}, \mathrm{Ba})$, the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from a high-spin ${\mathrm{Co}}^{2+}$ to low-spin ${\mathrm{Co}}^{3+}$ site is energetically suppressed. We find that, despite electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly distinct from the hole-doped case where it comes close to zero with doping. This prominent electron-hole asymmetry seen in the thermopower originates from a bipolar conduction which consists of a slight number of mobile holes and the main immobile electrons, demonstrating the impact of a spin-state blockade on thermoelectric transport.

Multiple relaxation, reversible electrical breakdown and bipolar conductivity of pyrochlore–type Bi2Cu0.5Zn0.5Ta2O9 ceramics

Cu-doped bismuth zinc tantalate pyrochlore was synthesized for the first time using standard ceramic technology. Electrical properties were investigated by impedance spectroscopy in a wide frequency range of 10 Hz – 1 MHz and a temperature range of 25–450 °C. As a result of modeling the Nyquist curves, the structure of the equivalent circuit of the ceramic is determined. Two mechanisms of ion-migration polarization caused by ionic transfer of copper and oxygen have been discovered. The activation energy of conduction at temperatures above 200 °C was 0.796 ± 0.009 eV. Current-voltage characteristics were investigated in the potential range 0.1–2 V and temperature 150–450 °C. The calculation of the potential barrier on the electrodes showed that at temperatures above ~250 °C, the potential barrier changes sign and magnitude by an order of magnitude from - 150 mV to +10 mV, which is associated with a change in the nature of charge carriers and activation of oxygen conductivity. In ceramics at 400–450 °C, a reversible effect is observed, which is characteristic of electrochromic materials. When measuring the current in the sample at a fixed potential, a transition from a direct current to an increasing current was detected, and then its sharp drop almost to zero. The instant of the onset of the growing current and electrical breakdown, as well as the recovery time of the electrical characteristics of the sample, depend on the applied potential and the temperature of the sample.

High-Throughput Computational Screening for Bipolar Magnetic Semiconductors.

Searching ferromagnetic semiconductor materials with electrically controllable spin polarization is a long-term challenge for spintronics. Bipolar magnetic semiconductors (BMS), with valence and conduction band edges fully spin polarized in different spin directions, show great promise in this aspect because the carrier spin polarization direction can be easily tuned by voltage gate. Here, we propose a standard high-throughput computational screening scheme for searching BMS materials. The application of this scheme to the Materials Project database gives 11 intrinsic BMS materials (1 experimental and 10 theoretical) from nearly ~40000 structures. Among them, a room-temperature BMS Li2V3TeO8 (mp-771246) is discovered with a Curie temperature of 478 K. Moreover, the BMS feature can be maintained well when cutting the bulk Li2V3TeO8 into (001) nanofilms for realistic applications. This work provides a feasible solution for discovering novel intrinsic BMS materials from various crystal structure databases, paving the way for realizing electric-field controlled spintronics devices.

Defect calculations using a combined SCAN and hybrid functional in γ-CsPbI3

Defect calculations of γ-CsPbI3 using a combination of SCAN Meta-GGA functional and HSE hybrid functional to demonstrate a suppressed bipolar conductivity and defects with multiple charge states.

Over 1.8 GW/cm2 beveled-mesa NiO/β-Ga2O3 heterojunction diode with 800 V/10 A nanosecond switching capability

In this Letter, we demonstrate a large-area (1-mm2) beveled-mesa p-NiO/β-Ga2O3 bipolar heterojunction diode (HJD) with a high Baliga's figure of merit of 1.84 (2.87) GW/cm2 from DC (pulsed) measurements. Benefiting from the suppression of electric field crowing at the designed mesa edge and bipolar current conductivity modulation, the resultant device exhibits advantageous characteristics, including a low subthreshold slope of 65 mV/decade, a low DC (pulsed) differential specific on-resistance of 2.26 (1.45) mΩ cm2, a high current density of 2 kA/cm2, and a high breakdown voltage of 2.04 kV. In particular, the Ga2O3 HJD exhibits an 800 V/10 A extreme switching capability with 16.4-ns reverse recovery characteristics, as well as high operation stability at a high temperature of 200 °C. This work, thus, makes a significant step toward reaching the promise of a high figure-of-merit in β-Ga2O3 power devices.

Analysis of Transient Surge Current Mechanism in SiC MPS Diode With the Transition Region

In this work, Sentaurus TCAD was used to study the transient surge current mechanism of the 4H-SiC merged pin and schottky (MPS) diode. For the first time, a structure combining multicells with transition region was used to analyze the transient surge current mechanism of the MPS diode. The electrothermal simulation analysis of the surge characteristics of the MPS diode is carried out. Internal carrier distribution and lattice temperature in the surge current transient process of the device are mainly studied. The simulation results show that the hole current distribution and the electron current distribution in the MPS diode are not uniform. In the transient process of surge current, the transition region first enters the bipolar conduction mode, and then, the cells in the active region enter the bipolar conduction mode in the order from outside (close to transition region) to inside. The voltage-time (V-t), maximum temperature-time ( $T_{max}$ -t), and I-V curves of MPS diode in surge current events are obtained by simulation. Combined with the current density distribution diagram, the device behavior of multiple critical time nodes is analyzed in detail, and a reasonable explanation is given. In addition, the influence of different transition region contact types on the internal current distribution of the MPS diode is studied, and the simulation results are reasonably explained.

Interfacial engineering effect and bipolar conduction of Ni- doped MoS2 nanostructures for thermoelectric application

Transition Metal Chalcogenides play an important role in the thermoelectric (TE) application due to their unique properties such as stability, easily tunable carrier transport and charge carrier mobility which enhance the power factor. Molybdenum disulphide (MoS2) is a hopeful TE material with a layered structure, but it possesses low electrical conductivity. Herein, our present work focuses to improve the electrical conductivity of MoS2 by Ni-doping and the effect of Ni-concentration was studied extensively. Undoped and Ni-doped MoS2 samples were successfully prepared by hydrothermal route. The hexagonal structured 2H-MoS2 and the formation of nickel sulphide (NiS) secondary phase at 10 at% of Ni-concentration was confirmed by XRD analysis. HRTEM confirmed the influence of Ni content in MoS2 layers which create the bonding between the layers and reduces the interlayer distance of MoS2. The reduced thermal conductivity was observed around 0.48 Wm−1K−1 for 7.5 at% of Ni-doped MoS2 which is two times lower than that of undoped MoS2. Meanwhile, the enhancement of electrical conductivity of Ni-doped samples from 0.012 S/cm to 0.049 S/cm observed for 7.5 at% of Ni-doped MoS2 sample. Because of NiS, the conduction behavior was changed from p-type to n-type Seebeck coefficient. The maximum zT was obtained as 0.28 × 10−3 for undoped sample. However, Ni-doping was experimentally proved to the simultaneous optimization of enhanced electrical conductivity and the suppressed thermal conductivity, the change in carrier type due to nickel sulphide (NiS) phase reduced the figure of merit.

Studies on phase formation behavior and thermoelectric transport properties of Cu-doped Bi2Te3–Bi2S3 system

Herein, we investigated the influence of sulfur (S) addition on the phase formation behavior and thermoelectric transport properties of n-type Cu0.008Bi2Te3. A complete single phase of rhombohedral Bi2Te3 was formed in Cu0.008Bi2Te2.8S0.2, whereas a mixture of Bi2Te3 and Bi2Te2S1 phases was generated at higher S contents. We elucidated the complex effects of S substitution at Te-site on transport properties of Cu0.008Bi2Te3; (1) increase in density-of-states effective mass, (2) decrease in weighted mobility ratio, (3) reduction in lattice thermal conductivity, and (4) reduction in bipolar thermal conductivity. Despite the slight deterioration in electronic transport properties, a maximum thermoelectric figure of merit (zT) of 0.82 was achieved at 400 K in Cu0.008Bi2Te2.8S0.2 due to the synergetic effect of intensified point defect phonon scattering in a wide temperature range and suppressed bipolar thermal conduction at elevated temperatures.

Electrical Conductivity Modeling of Polymer Composite Bipolar Plates

Polymer composites offer the potential for manufacturing lightweight, low-cost, fuel cell bipolar plates [1]. In this work, we develop a micromechanical model to predict the electrical conductivity of these composites. Using the Fiber Contact Model, conductivity was predicted based upon direction in the material, fiber alignment, fiber length and diameter, fiber concentration, and fiber conductivity [2]. Fiber alignment was determined experimentally by imaging cross sections of injection-molded nylon/carbon fiber composites. In addition to modeling electrical properties, the Halpin-Tsai equations are used to model the elastic modulus and ultimate strain of the composite. Conductivity and elastic modulus are predicted to increase in the injection direction of the composite with higher fiber loadings as well as with improved alignment of the fibers [3]. Development of an objective function for optimizing cost, ultimate strain, and conductivity, leads to optimal fiber volume fractions in the longitudinal and transverse directions of the composite.In addition to modeling efforts, electrical conductivity measurements were also performed on injection-molded nylon/carbon fiber composites. Modeling predictions are compared to experimental conductivity data for nylon composites with carbon fiber loadings of 10, 20, 30, and 40 wt%. The analytical predictions show good correlation within 10% of the experimental results. For 20, 30, and 40 wt% samples, conductivities were above the United States DOE technical target for bipolar plate conductivity (> 100 S/cm), reaching 250 S/cm. While higher carbon fiber loadings would further increase conductivity, beyond 50 wt%, the increased viscosity of the polymer blends can inhibit proper injection molding. Mighri, F.; Huneault, M. A.; Champagne, M. F., Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application. Polymer Engineering and Science 2004, 44 (9), 1755-1765.Weber, M.; Kamal, M. R., Estimation of the volume resistivity of electrically conductive composites. Polymer Composites 1997, 18 (6), 711-725.Zameroski, R.; Kypta, C. J.; Young, B. A.; Sanei, S. H. R.; Hollinger, A. S., Mechanical and Electrical Properties of Injection-Molded MWCNT-Reinforced Polyamide 66 Hybrid Composites. Journal of Composites Science 2020, 4 (4), 14. Figure 1

Modeling and Injection Molding of Electrically Conductive Polymer Composite Bipolar Plates

In this work, we are exploring electrically-conductive polymer composites as an alternative to the traditional metal alloys commonly used for fuel cell bipolar plates. Polymer composites can reduce both the weight and cost of the bipolar plate, while still providing sufficient electrical properties. The main objective of this research is to model and injection mold polymer composites that will meet the United States Department of Energy technical target for bipolar plate electrical conductivity (> 100 S/cm). A micromechanical model was developed to predict conductivity based upon direction in the material, fiber alignment, fiber length and diameter, fiber concentration, and fiber conductivity. The conductivity predictions of this model were then compared to injection molded samples. For these samples, conductive fillers (carbon fiber and carbon nanotubes) were added to nylon at weight percentages ranging from 0 to 50%, and conductivity was measured experimentally using a four-point probe method. The analytical and experimental results show good correlation over this range of filler weight percentages. Conductivities greater than 250 S/cm were achieved, exceeding the DOE technical target. Ultimately, the combined modeling and experimental results presented here demonstrate the potential for using polymer composite bipolar plates in fuel cells.

Ultrafast bipolar conductivity driven by intense single-cycle terahertz pulses in a topological insulator Bi2Se3

We present the measurement of ultrafast terahertz (THz) conductivity of a 50 nm thick topological insulator Bi2Se3 at low temperature 5 K by using a THz pump and THz probe spectroscopy scheme. The THz conductivity driven by a single-cycle intense THz pump exhibits a bipolar lineshape and frequency-dependent relaxation dynamics due to the separated surface and bulk charge carriers with distinctly different scattering rates and relaxation times. We also demonstrate THz pump field-dependent excitation of the surface and bulk spectral weights, which suggests one of the optimal THz excitation conditions for generating the most charge carriers from the topology-protected surface state relative to the bulk. This also allows THz control of surface and bulk transport channels in a selective way.

Understanding bipolar thermal conductivity in terms of concentration ratio of minority to majority carriers

Bi2Te3 is a good candidate to be used in thermoelectric generators. For a higher efficiency of the generators, shifting the temperature at which Bi2Te3 performs best to higher temperatures is required. Bipolar thermal conductivity suppression is the most effective approach to improve high-temperature thermoelectric performance. However, characterization of the bipolar thermal conductivity is challenging because it is related to individual contribution to Seebeck coefficient and electrical conductivity from the majority and minority carriers. Two-band model calculations are performed using reported band parameters of n-type Bi2Te3 to estimate the effects of band gap and minority to majority carrier concentration ratio in the bipolar thermal conductivity suppression. Individual Seebeck coefficient, electrical conductivity, and carrier concentrations from the majority and minority carriers are evaluated while varying chemical potential with a different band gap. It was demonstrated that increasing the band gap and chemical potential increased the individual Seebeck coefficient from minority carriers while decreasing the individual electrical conductivity and concentration from minority carriers. As a result, it was shown that the band gap increase and especially the magnitude of the minority to majority carrier concentration ratio decrease was effective in the bipolar thermal conductivity suppression.