High Vacancy Concentration Research Articles

Improved Thermoelectric Performance of P-type SnTe through Synergistic Engineering of Electronic and Phonon Transports

SnTe has been regarded as a potential alternative to PbTe in thermoelectrics because of its environmentally friendly features. However, it is a challenge to optimize its thermoelectric (TE) performance as it has an inherent high hole concentration (nH∼2 × 1020 cm-3) and low mobility (μH∼18 cm2 V-1 s-1) at room temperature (RT), arising from a high intrinsic Sn vacancy concentration and large energy separation between its light and heavy valence bands. Therefore, its TE figure of merit is only 0.38 at ∼900 K. Herein, both the electronic and phonon transports of SnTe were engineered by alloying species Ag0.5Bi0.5Se and ZnO in succession, thus increasing the Seebeck coefficient and, at the same time, reducing the thermal conductivity. As a result, the TE performance improves significantly with the peak ZT value of ∼1.2 at ∼870 K for the sample (SnGe0.03Te)0.9(Ag0.5Bi0.5Se)0.1 + 1.0 wt % ZnO. This result proves that synergistic engineering of the electronic and phonon transports in SnTe is a good approach to improve its TE performance.

Binder-free SnS2 sheet array with high sulfur vacancy concentration for enhanced lithium storage performance

Vacancy could not only offer extra active sites for electrochemical reactions, but also induce the change of band gap of electrode materials to enhance the electronic conductivity. In this work, binder-free SnS2 sheet array with high S vacancy concentration was fabricated on carbon paper (CP) by hydrothermal synthesis. When employed as the anode for lithium-ion battery, the SnS2/CP delivers greatly improved capacity, coulombic efficiency, high-rate discharge-ability as well as cycling performance from that of SnS2 powders. The results were analyzed and discussed with density functional theory based theoretical calculation, electrochemical impedance spectra combining diverse physicochemical techniques.

Vacancy dependent mechanical behaviors of high-entropy alloy

An abundance of defects would be inevitably generated during manufacturing and service in high-entropy alloys (HEAs). However, the mechanical properties of the damaged HEAs with nanoscale defects have been rarely considered. Meanwhile, an additional challenge is a dearth of the effective reliable method for an in-depth study of damaged HEAs, due to the restriction of the complicated experimental measurement. Here, the effect of zero-dimensional defect, such as vacancy, on mechanical properties in FeNiCoCrCu HEA is studied through the method combining molecular dynamics simulation with machine learning. Atomic simulation results show that the existence of vacancy clusters breaks the continuity of stacking faults and dislocations, and the high vacancy concentrations reduce the strength due to the increase of stacking fault spacing at room temperature. Compared with traditional alloys, HEAs can reduce the property reduction when vacancy exits there. In addition, the anisotropy of vacancy effect is found. The effect of vacancy on the yield strength is more obvious under [100] loading direction than that under the [110] and [111] loading direction. The discrepancy of the stair-rod partial dislocation proportion would be regarded as the primary reason for different yield strengths. The change of vacancy concentration has a significant effect on the deformation mechanism in [100] direction, which is different from that in the [110] and [111] direction. Meanwhile, the effects of vacancy concentration in combination with temperature on the yield strength and dislocation density at yield point of the FeNiCoCrCu HEA are predicted by machine learning, which is on the basis of a set of data obtained from molecular dynamics simulations. The current work provides valuable guidance for HEA design based on the viewpoint of defect regulation.

Improvement of thermoelectric performance of SnTe-based solid solution by entropy engineering

SnTe is a good alternative to PbTe in the thermoelectric (TE) applications, in that it is a compound with no toxic element Pb. Besides, the compound SnTe has a relatively narrow bandgap (0.3–0.4 eV) and high Sn vacancy concentration (Sn<sub>v</sub>) as well. Accordingly, it gives a high carrier concentration (10<sup>21</sup> cm<sup>–3</sup>) at room temperature (RT), which is not favorable for thermoelectrics, therefore the regulation of both the electronic and phonon scattering mechanisms is strongly required. Up to date, there have been many approaches to improving its TE performance. The typical examples are those involving the valence band convergence, nanostructuring, substitutional and interstitial defects, and lattice softening, which are all practical and effective to improve the TE performance of SnTe. However, in this work the entropy is taken as an indicator to design the SnTe-based TE material with multicomponents and then optimize its TE performance. The detailed scheme involves the chemical composition design step by step. At first, SnTe alloys with 5% GaTe to form a solid solution Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te, aiming to increase the solubility of the foreign species. The second step is to form another solid solution (Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> via the alloying Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te with 5% Ag<sub>2</sub>Se. The purpose of this step is to reduce the p-type carrier concentration of the system, for the species Ag<sub>2</sub>Se is a typical n-type semiconductor. The last step is to form a series of solid solutions (Sn<sub>0.95–<i>x</i></sub>Ge<sub>0.05</sub>Bi<sub><i>x</i></sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> by substituting different amounts of Bi on Sn in (Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub>, to further enhance the configurational entropy (Δ<i>S</i>). Because of the above approaches, both the carrier concentration and thermal conductivity decrease while the highest TE figure of merit (<i>ZT</i>) increases from 0.22 for the pristine SnTe to ~0.8 for the alloy (Sn<sub>0.95–<i>x</i></sub>Ge<sub>0.05</sub>Bi<sub><i>x</i></sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> (<i>x</i> = 0.075). This result proves that the entropy engineering is a practical way to improve the TE performance of SnTe, and at the same time it illustrates that it is very important to harmonize the entropy engineering with other electronic and phonon scattering mechanisms, in order to improve the TE performance of SnTe effectively.

Surface-Engineered Homostructure for Enhancing Proton Transport.

Ultra-wide bandgap semiconductor samarium oxide attracts great interest because of its high stability and electronic properties. However, the ionic transport properties of Sm2 O3 have rarely been studied. In this work, Ni doping is proposed to be used for electronic structure engineering of Sm2 O3 . The formation of Ni-doping defects lowers the Fermi level to induce a local electric field, which greatly enhances the proton transport at the surface. Furthermore, ascribed to surface modification, the high concentration of vacancies and lattice disorder on the surface layer promote proton transport. A high-performance of 1438mW cm-2 and ionic conductivity of 0.34 S cm-1 at 550 °C have been achieved using 3%mol Ni doped Sm2 O3 as electrolyte for fuel cells. The well-dispersed Ni doped surface in Sm2 O3 builds up continuous surfaces as proton channels for high-speed transport. In this work, a new methodology is presented to develop high-performance, low-temperature ceramic fuel cells.

Intrinsic color centers in 4H-silicon carbide formed by heavy ion implantation and annealing

We study the generation and transformation of intrinsic luminescent centers in 4H-polytype of silicon carbide via heavy ion implantation and subsequent annealing. Defects induced by the implantation of germanium (Ge) or tin (Sn) have been characterized by photoluminescence (PL) spectra recorded at cryogenic temperatures. We find three predominant but as-yet-unidentified PL signatures (labeled as DI1–3) at the wavelength of 1002.8 nm (DI1), 1004.7 nm (DI2), and 1006.1 nm (DI3) after high dose implantation (> 4 × 1013 cm−2) and high temperature annealing (> 1700°C). The fact that the DI lines co-occur and are energetically close together suggest that they originate from the same defect. Regardless of the implanted ion (Ge or Sn), a sharp increase in their PL intensity is observed when the implantation damage becomes high (vacancy concentration > 1022 cm−3), indicating that the lines stem from an intrinsic defect caused by the damage. By tracking the PL signals after stepwise annealing, we examine how the overall intrinsic defects behave in the temperature range of 500 – 1800°C; the silicon vacancies formed by the implantation transform into either divacancies or antisite-vacancy pairs with annealing at about 1000°C. These spectral signatures are strongly reduced at 1200°C where the so-called TS defects are maximized in luminescence. As a final stage, the DI defects, which are most likely formed of antisites and vacancies, emerge at 1700°C. Our results provide a knowledge on how to incorporate and manipulate the intrinsic luminescent centers in SiC with ion implantation and annealing, paving the way for fully integrated quantum technology employing SiC.

High seebeck coefficient and low thermal conductivity in Bi and In co-doped GeTe thermoelectric material

Pure GeTe shows inferior thermoelectric performance due to the large carrier concentration caused by the intrinsic high concentration of Ge vacancies. In this study, we report improved thermoelectric performance of Bi and In co-doped GeTe based thermoelectric material where a figure of merit, ZT ∼ 1.7 at 623 K was realized through synergetic effect of tuning the carrier concentration, resonant states, and suppression of thermal conduction. In doping induces resonant states in the density of states near the Fermi energy level. Bi sharply reduces lattice thermal conductivity by formation of extensive solid solution point defects. Moreover, Bi and In co-doping decreases the phase transformation temperature to widen the better thermoelectric performance of cubic GeTe at low temperatures. In addition, microstructural characterization showed herringbone structures, high-density of domain boundaries, and twinning. These, together with the point defects, lead to a significantly reduced thermal conductivity.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2 surfaces

To study the dependence of the relative stability of surface (V A ) and subsurface (VB ) oxygen vacancies with the crystal facet of CeO2, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce3+ polarons, are facet- and concentration-dependent. At the higher vacancy concentration, the V A is more stable than the V B at the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both the V A and the VB have similar stability. The stability of the V A vacancies, compared to that of the V B , is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce3+−Ce3+ interactions.

Green emission in Fe- and Mn-doped ZnO nanowires studied by magneto-photoluminescence

The green emission in ZnO, which was reported to result from deep-level transitions and proportional to the concentration of singly ionized oxygen vacancy, is critical for the visible-band optoelectronic application. However, the effective modulation of the green emission intensity is challenging. In this study, the green emission in ZnO nanowires is greatly suppressed by Fe doping and enhanced by Mn-doping during the growth process. Temperature dependent PL spectra demonstrate strong electron-phonon coupling, while magneto-PL show shrinkage of electron wavefunction for ZnO:Fe, these indicate both the electron-dopant energy transfer and decrease of oxygen vacancies in ZnO:Fe suppress the green emission; whereas for ZnO:Mn, its PL spectra show weak temperature and magnetic field dependence, which signify very high concentration of the oxygen vacancies that enhance the green emission.

Annealing effects on the microstructure, stoichiometry, and ferromagnetic/antiferromagnetic fraction in NiO nanoparticles

The effect of the annealing atmosphere (air and oxygen) and temperature on the microstructural and magnetic properties of sol–gel synthesized NiO nanoparticles is investigated. A structural refinement of the X-ray diffraction pattern show an increase in particle size, with a decrease in lattice constant and bond length with increasing annealing temperature. Air annealing leads to a larger variation in sizes as compared to oxygen annealing. For moderate oxygen annealing upto 600 °C, a Ni phase is observed which is almost undetectable for air annealing. Air annealing at 400 °C shows a high concentration of Ni vacancies which reduced with increasing temperature. The colour variation with annealing show the stoichiometric improvement. Magnetic properties are extracted from the law of approach to saturation fit of the magnetic hysteresis curves. The two-magnon mode observed in the Raman spectrum correlates with the antiferromagnetic/ferromagnetic transition which can be tailored with the microstructure.

Key role of interaction between dislocations and hydrogen-vacancy complexes in hydrogen embrittlement of aluminum: discrete dislocation plasticity analysis

With the development of experimental techniques and characterization methods at the microscopic scale, a high concentration of hydrogen-induced vacancies and their clusters have been detected in a large variety of metals and alloys charged with hydrogen, which is supposed to play an important role in hydrogen-induced fracture process. In hydrogen environment, vacancies and their clusters can trap a certain number of H atoms to form vacancy-H complexes or clusters. To investigate the role of hydrogen-induced vacancies and their clusters in brittle-like failure in hydrogen environment, multi-scale modelling was performed in this work, with a particular attention on the interaction between dislocations and vacancy-H complexes. Firstly, the H-enhanced vacancy formation due to H-induced reduction in vacancy formation energy in Al was modelled by first principles calculations, and a hydrogen-influenced vacancy concentration model was proposed. Then, to capture the interaction between gliding edge dislocations and vacancy-H complexes and clusters, atomistic simulations were performed, and a mesoscale model quantitatively describing the pinning effect of vacancy-H complexes and clusters on the gliding edge dislocations was obtained. Finally, these quantitative models were introduced into a newly developed XFEM-based discrete dislocation plasticity scheme. With this scheme, mode-I crack propagation was simulated, where dislocation emission from crack-tip, dislocation evolution and cohesive crack propagation were considered, with a particular attention to the key role of the hydrogen-influenced dislocation evolution in the brittle-like failure of the H-charged metals. Based on these multi-scale simulations, a pinning effect of H-enhanced vacancies and vacancy-H complexes on edge dislocations was identified, which was proposed to be responsible for the brittle-like failure of Al in hydrogen environment. This pinning effect presents a good agreement with the hydrogen-restricted dislocation mobility observed in in situ fracture experiments.

Synthesis of nonstoichiometric NiO nanocatalysts and growth of bamboo‐like carbon nanotubes

AbstractThe growth behavior of carbon nanotubes from NiO nanoparticles was investigated in terms of vacancy concentration and nanostructure. The NiO nanoparticles were prepared using anhydrous ethanol as a solvent to lower hydroxyl groups, compared to distilled water. The Ni vacancies in NiO via less hydrolysis were confirmed by lattice expansion and the increased mean oxidation state of Ni3+ through X‐ray absorption spectroscopy. The increased Ni3+ ratio determined from the photoelectron spectrum and ferromagnetism implied a high concentration of Ni vacancies on the surface. Nonstoichiometric NiO exhibited a small particle size of 10.13 nm and a narrow size distribution. For the growth of CNTs, the NiO nanocatalysts with vacancies resulted in thinner CNTs. The internal structures of the CNTs obtained from the defective particles were grown into bamboo‐like CNTs, identifying the variation in the carbon diffusion path.

Origins of size effects in initially dislocation-free single-crystal silver micro- and nanocubes

We report phenomenal yield strengths—up to one-fourth of the theoretical strength of silver—recorded in microcompression testing of initially dislocation-free silver micro- and nanocubes synthesized from a multistep seed-growth process. These high strengths and the massive strain bursts that occur upon yield are results of the initially dislocation-free single-crystal structure of the pristine samples that yield through spontaneous nucleation of dislocations. When the pristine samples are exposed to a focused ion-beam to fabricate pillars and then compressed, the dramatic strain burst does not occur, and they yield at a quarter of the strength compared to the pristine counterparts. Regardless of the defect-state of the samples prior to testing, a size effect is apparent—where the yield strength increases as the sample size decreases. Since dislocation starvation and the single-arm-source mechanisms cannot explain a size effect on yield strength in dislocation-free samples, we investigate the dislocation nucleation mechanisms controlling the size effect through careful experimental observations and molecular statics simulations. We find that intrinsic or extrinsic symmetry breakers such as surface defects, edge roundness, external sample shape, or a high vacancy concentration can influence dislocation nucleation, and thus contribute to the size effect on yield strength in initially dislocation-free samples.

Molecular dynamic study for concentration-dependent volume relaxation of vacancy

In this paper, large-scale molecular dynamic (MD) simulation is performed to investigate the concentration-dependent vacancy volume relaxation in Al, Cu, and Au, respectively. The vacancy volume relaxation factor is calculated and correlated to the microstructure change based on MD results. It is found that the vacancy volume relaxation factor is nearly a constant at low to mid-vacancy concentration level, i.e., from 10−6 to 10−3 of the lattice concentrations. However, the volume of vacancies will completely collapse at the high vacancy concentration, and the coalescence of vacancies will form massive dislocations. The simulation results are in good agreement with the existing data from both experiments and simulations in the literature. A uniform empirical equation is developed to obtain the vacancy volume relaxation factor as a function of vacancy concentration. Furthermore, the hydrostatic stress is also calculated based on MD simulations. This paper also discusses the relationship between the vacancy volume relaxation and the diffusion-induced strain, such as in the application of electromigration. For a constrained condition, the hydrostatic stresses obtained from MD-based vacancy volume relaxation and a commonly used stress-vacancy equation are compared. An insight on the critical vacancy concentration at which electromigration failure would occur is discussed in detail.

Modeling of vibrational and configurational degrees of freedom in hexagonal and cubic tungsten carbide at high temperatures

Transition metal carbide is a class of materials characterized by high hardness, high melting points, and low chemical reactivity. It is widely used in industrial applications involving exposure to elevated temperatures, aggressive media, and heavy loads, and is thus of technological and industrial importance. In this paper the high-temperature thermodynamic properties of tungsten carbide, WC, is studied. At most temperatures below melting, WC assumes a hexagonal structure with essentially no vacancies ($\ensuremath{\delta}$-WC). Only at very high temperatures (around 3000 K), WC crystallizes in the cubic rocksalt structure ($\ensuremath{\gamma}$-WC), which is more common for the transition metal carbides and in the case for WC can contain up to 40% carbon vacancies. At lower temperatures, $\ensuremath{\gamma}$-WC can, however, form as thin interfacial structures or nanoparticles. Hence, the thermodynamic properties of both $\ensuremath{\gamma}$-WC and $\ensuremath{\delta}$-WC are of relevance. Here, we conduct a first-principles density-functional theory based computational investigation of the $\ensuremath{\gamma}$-WC and $\ensuremath{\delta}$-WC phases, which requires modeling of high carbon vacancy concentrations at high temperatures. The configurational degrees of freedom are modeled with an alloy cluster expansion and sampled through Monte Carlo simulations. To account for the dynamic instability of the cubic $\ensuremath{\gamma}$-WC phase at low temperatures, the vibrational degrees of freedom are treated using effective harmonic models constructed from $ab$ initio molecular dynamics simulations. Finally, we obtain a part of the W-C phase diagram in reasonably quantitative agreement with experimental data.

On the ionic conductivity of some zirconia-derived high-entropy oxides

The reported High Entropy Oxides (HEOs) up to now exhibit lower ionic conductivity values than those of classical SOFC electrolytes. Multi-cations oxides, stabilized with the fluorite-type structure are investigated here in order to examine whether the high entropy is relevant to enhance the anionic conductivity of such HEOs, or not. The two synthesis routes that are used do not show significant impact on material properties. Based on configurational entropy (≥ 1.5 kB/f.u. for HEOs) and ionic radius difference calculations, new compositions are designed and prepared: i) the (Hf1/3Ce1/3Zr1/3)1-x(Gd1/2Y1/2)xO2-x/2 series in which the x ratio is increased so as to promote a high vacancy concentration, ii) the (HfxCeyZr1-x-y)0.85Yb0.15O1.93 series based on the critical radius concept. The ionic conductivity of these HEOs is slightly improved compared to previously reported data but does not exceed 4 × 10−4 S.cm−1 at 600 °C. Possible causes of such a low ionic conductivity value are discussed.

Phase transition dynamics in one-dimensional halide perovskite crystals

Triiodide perovskites $${\rm CsPbI}_{3}$$ , $${\rm CsSnI}_{3}$$ , and $${\rm FAPbI}_{3}$$ (where FA is formamidinium) are highly promising materials for a range of optoelectronic applications in energy conversion. However, they are thermodynamically unstable at room temperature, preferring to form low-temperature (low-T) non-perovskite phases with one-dimensional anisotropic crystal structures. While such thermodynamic behavior represents a major obstacle toward realizing high-performance devices based on their high-temperature (high-T) perovskite phases, the underlying phase transition dynamics are still not well understood. Here we use in situ optical micro-spectroscopy to quantitatively study the transition from the low-T to high-T phases in individual $${\rm CsSnI}_{3}$$ and $${\rm FAPbI}_{3}$$ nanowires. We reveal a large blueshift in the photoluminescence (PL) peak (~38 meV) at the low-T/high-T two-phase interface of partially transitioned $${\rm FAPbI}_{3}$$ wire, which may result from the lattice distortion at the phase boundary. Compared to the experimentally derived activation energy of CsSnI3 (~1.93 eV), the activation energy of $${\rm FAPbI}_{3}$$ is relatively small (~0.84 eV), indicating a lower kinetic energy barrier when transitioning from a face-sharing octahedral configuration to a corner-sharing one. Further, the phase propagation rate in CsSnI3 is observed to be relatively high, which may be attributed to a high concentration of Sn vacancies. Our results could not only facilitate a deeper understanding of phase transition dynamics in halide perovskites with anisotropic crystal structures, but also enable controllable manipulation of optoelectronic properties via local phase engineering. Metal halide perovskites are a new class of semiconductors with great promise for a variety of optoelectronic applications. Owing to their soft ionic lattice, halide perovskites often exhibit rich phase transitions between different crystal structures, frequently from the “active” perovskite phases to undesirable “inactive” non-perovskite phases. Understanding and controlling this transition is vital for developing stable, high-performance devices. However, there is limited understanding on how different symmetry, crystal structure, and defect would impact such a phase transition process. In this report, in situ optical micro-spectroscopy is used to systematically investigate the phase transitions from non-perovskite to perovskite phases in individual CsSnI3 and FAPbI3 wires. Compared to the transition from an edge-sharing octahedral non-perovskite structure to a corner-sharing perovskite structure in CsSnI3, the activation energy for the FAPbI3 phase transition is relatively small, indicating a lower energy barrier when starting from a face-sharing octahedral structure in FAPbI3. The high concentration of Sn vacancies is probably responsible for the much higher phase propagation rate in CsSnI3 when compared to a CsPbBrxI3-x system with the same crystal structure but different halide vacancies. Our experimental results expand the knowledge of phase transition in halide perovskites and offer important guidance toward rationally designing more stable and efficient perovskite devices.

As-doped SnSe single crystals: Ambivalent doping and interaction with intrinsic defects

We performed ambivalent doping study on single crystals of two sets, $\mathrm{Sn}{\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{As}}_{x}$ and ${\mathrm{Sn}}_{1\ensuremath{-}x}{\mathrm{As}}_{x}\mathrm{Se}$, with the aim to explore the interaction of doping species with intrinsic defects. We found that As atoms substitute preferentially for Se atoms in both sets forming the extrinsic substitutional point defect ${\mathrm{As}}_{\mathrm{Se}}$. In the first set, As lowers the concentration of Sn vacancies, ${V}_{\mathrm{Sn}}$, by an order of magnitude compared to undoped stoichiometric SnSe crystal. The remaining Sn vacancies are preferentially coordinated with As atoms. Importantly, a very low concentration of As led to healing process of hosting structure in terms of intrinsic point defects and eventual $\mathrm{Sn}{\mathrm{Se}}_{2}$ inclusions. This is reflected in an increase of the Hall mobility and drop of the Hall concentration. In the second set, the concentration of Sn vacancies markedly increases upon doping in contrast to the first set. Additionally, the coordination of Sn vacancies by As atoms is less evident due to the high concentration of vacancies. The substitutional defect ${\mathrm{As}}_{\mathrm{Se}}$ is a deep-level defect that produces no free carriers at room temperature. Moreover, the coupling of ${V}_{\mathrm{Sn}}$ to ${\mathrm{As}}_{\mathrm{Se}}$ defects increases their activation energy. This results in an unprecedentedly low Hall concentration in SnSe which stays below ${10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--3}$ for $x=0.0075$. The present study indicates that doping of SnSe is a rather complex process that generally includes a strong interaction of doping atoms with the hosting structure. On the other hand, such doping allows adjustment of the type and concentration of defects. The present study reveals a general tendency of point defects to clustering, which modifies the properties of point defects markedly.

Electronic properties of co-doped nonstoichiometric germanium telluride

GeTe-based alloys have been considered as p-type thermoelectric materials for decades. Yet the fact that they possess intrinsically high concentration of cation germanium vacancies have triggered the thermoelectric community in finding alternatives and strategies to optimize their efficiency. While most of the published studies mainly discuss the stoichiometric alloy, here we demonstrate the introduction of excess germanium/tellurium to the system. The nonstoichiometric alloy is doped with bismuth, acting as a donor simultaneously with an increased number of vacancies, enabling higher solubility of other impurities (iodine/indium). This strategy and the mechanisms involved, has been investigated and correlated to the microstructural characteristics. ZT of ~1.6 was obtained but more importantly, it opens the opportunity for a potential route for further thermoelectric enhancement.

Investigation of switching uniformity in resistive memory via finite element simulation of conductive-filament formation

Herein, we present simulations of conductive filament formation in resistive random-access memory using a finite element solver. We consider the switching material, which is typically an oxide, as a two-phase material comprising low- and high-resistance phases. The low-resistance phase corresponds to a defective and conducting region with a high anion vacancy concentration, whereas the high-resistance phase corresponds to a non-defective and insulating region with a low anion-vacancy concentration. We adopt a phase variable corresponding to 0 and 1 in the insulating and conducting phases, respectively, and we change the phase variable suitably when new defects are introduced during voltage ramp-up for forming. Initially, some defects are embedded in the switching material. When the applied voltage is ramped up, the phase variable changes from 0 to 1 at locations wherein the electric field exceeds a critical value, which corresponds to the introduction of new defects via vacancy generation. The applied voltage at which the defects percolate to form a filament is considered as the forming voltage. Here, we study the forming-voltage uniformity using simulations, and we find that for typical planar-electrode devices, the forming voltage varies significantly owing to the stochastic location of the initial defects at which the electric field is “crowded.” On the other hand, a protruding electrode can improve the switching uniformity drastically via facilitating the deterministic location of electric-field crowding, which also supported by the reported experimental results.