Change In Carrier Concentration Research Articles

Impact of Hierarchical Dopant‐Induced Microstructure on Thermoelectric Properties of p‐Type Si‐Ge Alloys Revealed by Comprehensive Multi‐Scale Characterization

AbstractDopant‐induced microstructure in thermoelectric materials significantly affects thermoelectric properties and offers a potential to break the interdependence between electron and phonon transport properties. However, identifying all‐scale dopant‐induced microstructures and correlating them with thermoelectric properties remain a huge challenge owing to a lack of detailed microstructural characterization encompassing all length scales. Here, the hierarchical boron (B)‐induced microstructures in B‐doped Si80Ge20 alloys with different B concentrations are investigated to determine their precise effects on thermoelectric properties. By adopting a multi‐scale characterization approach, including X‐ray diffraction, scanning and transmission electron microscopy, and atom probe tomography, five distinctive B‐induced phases within Si80Ge20 alloys are identified. These phases exhibit different sizes, compositions, and crystal structures. Furthermore, their configuration is comprehensively determined according to B doping concentrations to elucidate their consequential impact on the unusual changes in carrier concentration, density‐of‐states effective mass, and lattice thermal conductivity. The study provides insights into the intricate relationship between hierarchical dopant‐induced microstructures and thermoelectric properties and highlights the importance of investigating all‐scale microstructures in excessively‐doped systems for determining the precise structure‐property relationships.

Graphene quality evaluation by Van der Pauw-Hall Method

Evaluating graphene quality is crucial for both industrial applications and academic research. Traditional methods like Optical Microscope, Raman Spectroscope, Scanning and Transmission Electron Microscope, while common, face challenges in convience and providing electrical performance. To address these limitations, a cost-effective and rapid method was introduced for assessing macroscopic quality of graphene. This technique involves fine-tuning carrier concentration over 1013 cm-2 by controlling hydrochloric acid (HCl) doping time. Utilizing the Van der Pauw-Hall system, it tracks carrier concentration changes of graphene, allowing for mobility measurement under consistent carrier conditions. This facilitates a direct comparison of different graphene samples. The method shows reproducibility in evaluating HCl-doped graphene, which is consistent with the Raman characterizations. This advancement in graphene evaluation, focusing on mobility comparison at identical carrier concentrations, sets the stage for standardized assessments in the graphene industry.

Effects of Pb and carbonate substitutions on the superconducting properties of TlSr4Cu2OzCrO4

We have investigated the effects of Pb and carbonate substitutions on the phase formation and the superconducting properties of the intergrowth superconductor TlSr4Cu2OzCrO4. X-ray diffraction data showed that nearly single-phase samples with tetragonal structure were obtained in the compositions (Tl1-xPbx)Sr4Cu2OzCrO4 (0 ≤ x ≤ 0.5) and (Tl0.5Pb0.5)Sr4Cu2Oz(CrO4)1-y(CO3)y (0 ≤ y ≤ 1.0). Resistivity measurements indicated that the transition temperature decreased significantly as the Pb doping content x increased to x = 0.5. In contrast to the Pb doping effects, the carbonate substitution in the (Tl0.5Pb0.5)Sr4Cu2Oz(CrO4)1-y(CO3)y system resulted in an improvement of the superconducting transition temperature up to y = 0.9. The y = 1.0 sample showed a reduced transition temperature, compared to the x = 0.9 sample. The experimental results are discussed in conjunction with the change in hole carrier concentration due to the Pb and carbonate substitutions, based on the data from room temperature thermoelectric power measurements.

Piezo-optoelectronic coupling in heterojunctions for self-powered and ultrasensitive pressure sensing

It is highly desirable to discover sensing mechanisms for developing self-powered and ultrasensitive pressure sensing. Here, we report that the piezo-optoelectronic coupling in a n+-3C-SiC/p-Si heterojunction not only enables energy harvesting but also features massive strain sensitivity. To demonstrate the proof of the concept, a pressure sensing device is developed with active and reference electrodes arranged on SiC along strain-sensitive [100] and strain-insensitive [110] orientations, respectively. Under the illumination of 637-nm laser at 2.0 mW, experimental results show the great response of the photogenerated voltage to the change in both static and dynamic pressures. Gauge factor is found to be −670.2 which is much larger than the highest gauge factor of n-type 3C-SiC. The underlying physics behind is the strain modulation on the effective mass and energy level of photogenerated charge carriers. This results in the change in carrier concentration detected by the electrodes along the strain-sensitive and strain-insensitive orientations. These findings could contribute significantly to the development of self-powered mechanical sensors using solid-state electronics.

Tuning magnetic properties of two-dimensional antiferromagnetic <i>M</i>P<i>X</i><sub>3</sub> by organic cations intercalation

Electrical control of magnetism of two-dimensional (2D) antiferromagnetic (AFM) materials combines the advantages of controlling magnetism by purely electrical means, compatibility with semiconductor process, low energy consumption, heterogeneous integration of 2D materials with van der Waals (vdW) interface, and AFM materials with no stray field, resistance to external magnetic field interference, and high intrinsic frequency, and thus becomes a research focus in the field. The carrier concentration control is the main mechanism of electrical control of magnetism, and has been proved to be an effective way to control the magnetic properties of materials. The intralayer-antiferromagnetic materials have net-zero magnetic moments, and it is a challenging task to measure their regulated magnetic properties. Therefore, there is limited research on the electrical control of magnetism of intralayer-antiferromagnetic materials, and their potential mechanisms are not yet clear. Based on the diversity of organic cations, the present work systematically modulates the carrier concentrations of 2D intralayer-antiferromagnetic materials <i>M</i>P<i>X</i><sub>3</sub> (<i>M</i> = Mn, Fe, Ni; <i>X</i> = S, Se) by utilizing organic cations intercalation, and investigates the influence of electron doping on their magnetic properties. Phase transitions between AFM-ferrimagnetic (FIM)/ferromagnetic (FM) depending on carrier concentration changes are observed in <i>M</i>P<i>X</i><sub>3</sub> materials, and the corresponding regulation mechanism is revealed through theoretical calculations. This research provides new insights into the carrier-controlled magnetic phase transition of 2D magnetic materials, and opens up a pathway for studying the correlation between the electronic structure and magnetic properties of 2D magnets, and designing novel spintronic devices as well.

Synthesis and enhanced room-temperature thermoelectric properties of CuO-MWCNT hybrid nanostructured composites.

This work presents the synthesis of novel copper oxide-multiwalled carbon nanotube (CuO-MWCNT) hybrid nanostructured composites and a systematic study of their thermoelectric performance at near-room temperatures as a function of MWCNT wt% in the composite. The CuO-MWCNT hybrid nanostructured composites were synthesized by thermal oxidation of a thin metallic Cu layer pre-deposited on the MWCNT network. This resulted in the complete incorporation of MWCNTs in the nanostructured CuO matrix. The thermoelectric properties of the fabricated CuO-MWCNT composites were compared with the properties of CuO-MWCNT networks prepared by mechanical mixing and with the properties of previously reported thermoelectric [CuO]99.9[SWCNT]0.1 composites. CuO-MWCNT hybrid composites containing MWCNTs below 5 wt% showed an increase in the room-temperature thermoelectric power factor (PF) by ∼2 times compared with a bare CuO nanostructured reference thin film, by 5-50 times compared to mixed CuO-MWCNT networks, and by ∼10 times the PF of [CuO]99.9[SWCNT]0.1. The improvement of the PF was attributed to the changes in charge carrier concentration and mobility due to the processes occurring at the large-area CuO-MWCNT interfaces. The Seebeck coefficient and PF reached by the CuO-MWCNT hybrid nanostructured composites were 688 μV K-1 and ∼4 μW m-1 K-2, which exceeded the recently reported values for similar composites based on MWCNTs and the best near-room temperature inorganic thermoelectric materials such as bismuth and antimony chalcogenides and highlighted the potential of CuO-MWCNT hybrid nanostructured composites for applications related to low-grade waste heat harvesting and conversion to useable electricity.

Enhanced thermoelectric figure-of-merit in ‘defective’ half-Heusler Nb0.8CoSb

The half Heusler alloys are potential mid-to-high temperature range thermoelectrics due to their high power factor, solid structural stability and high mechanical strength. However, the high lattice thermal conductivity inherent to these materials reduces the zT, rendering them less useful for practical applications. The ‘defective’ half-Heuslers provide an attractive alternative to mitigate these issues. In these materials, the intrinsic lattice defects present in the form of vacancies lower the lattice thermal conductivity substantially. Here, we study the effect of excess Nb and Sn doping in the defective half Heuslers Nb0.8+δCoSb1−xSnx. We show that Sn doping allows for: (i) optimizing the carrier concentration with a much finer control than if only δ is increased, and (ii) incorporation of a higher concentration of Nb in the structure. A combination of optimized carrier concentration and synergistic changes in the band structure, including the appearance of a new flat band near 50 meV above the Fermi energy due to excess Nb, suppresses the bipolarity and enhances the thermopower further at high temperatures as the average band effective mass of the carriers increases. We, therefore, obtain a high zT exceeding 1 at 1100 K for Nb0.85CoSb0.95Sn0.05. This value is ≈ 15% higher than the highest zT previously reported.

Towards energy filtering in Mg2X-based composites: Investigating local carrier concentration and band alignment via SEM/EDX and transient Seebeck microprobe analysis

A comprehensive understanding of multi-phase materials requires the characterization of transport properties at the micro/nano-scale. Optimized alignments of the conduction bands and valence bands at the interfaces of multi-phase materials can enhance the properties of thermoelectric materials by filtering out undesired charge carriers and is a promising path towards high performance thermoelectric devices. Here, a micro-scale characterization technique using transient Seebeck microprobe analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and electronic transport modelling based on the Boltzmann transport equation modeling is introduced to assess the band diagram and estimate the band offset in multi-phase materials. This characterization technique is applied to a composite of magnesium silicide-based materials. This material class is prone to form self-assembling nano-structured composites driven by a miscibility gap in the solid solutions series. Our analysis reveals changes in carrier concentration upon composite formation and considerable band offsets for both conduction and valence band when synthesizing nominally undoped composites. Employing Bi as dopant we show that Bi exhibits a preference for the Sn-rich phase, changing the carrier concentration differently in the Sn-rich matrix phase compared to the Si-rich secondary phase, effectively altering the band alignment of the composite. This demonstrates that our approach can be utilized to measure and manipulate the band offset within the composite to achieve optimal thermoelectric performance.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 μW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

Manufacturing-Enabled Tunability of Linear and Nonlinear Epsilon-Near-Zero Properties in Indium Tin Oxide Nanofilms.

Epsilon-near-zero (ENZ) materials have attracted great interest due to their exotic linear and nonlinear responses, which makes it significant to tune ENZ wavelengths for wavelength-dependent applications. However, studies to achieve tunability in a wide spectral range and link the fabrication parameters with linear and nonlinear ENZ properties have been uncovered. ENZ indium tin oxide (ITO) nanofilms are fabricated by magnetron sputtering, through which the control of ENZ properties is demonstrated. Factors in the sputtering process, such as the gas ratio and annealing, have a great impact on the ITO samples. Tunable ENZ parameters are listed to provide a beneficial database for ENZ ITO, mainly attributed to the change of carrier concentration. The influence of ENZ parameters on optical characteristics via annealing treatment is further explored. The ENZ wavelength is blue-shifted by 609 nm, and the intrinsic loss is reduced by 63.2%, while the ITO samples exhibit better linear scattering properties and stronger field intensity enhancement. Additionally, the laser testing system illustrates the change from reverse saturable absorption to saturable absorption with an absolute modulation depth of 21.9%, improved by 222.1%, and the nonlinear refractive index n2 and nonlinear absorption coefficient β are 2.07 × 10-16 m2 W-1 and -3.16 × 10-10 m W-1 for post-annealed ITO samples, respectively. The proposed sputtering protocol offers a feasible technique to control the linear and nonlinear ENZ performance, which has great potential in laser technology and nanophotonics.

Decoding the physiological response of plants to stress using deep learning for forecasting crop loss due to abiotic, biotic, and climatic variables

This paper presents a simple method for detecting both biotic and abiotic stress in plants. Stress levels are measured based on the increase in nutrient uptake by plants as a mechanism of self-defense when under stress. A continuous electrical resistance measurement was used to estimate the rate of change of nutrients in agarose as the growth medium for Cicer arietinum (Chickpea) seeds. To determine the concentration of charge carriers in the growth medium, Drude’s model was used. For identifying anomalies and forecasting plant stress, two experiments were conducted and outliers were found in electrical resistance and relative changes in carrier concentration. Anomaly in the first iteration was detected by applying k-Nearest Neighbour, One Class Support Vector Machine and Local Outlier Factor in unsupervised mode on electrical resistance data. In the second iteration, the neural network-based Long Short Term Memory method was used on the relative change in the carrier concentration data. As a result of the change in resistance of growth media during stress, nutrient concentrations shifted by 35%, as previously reported. Farmers who cater to small communities around them and are most affected by local and global stress factors can use this method of forecasting.

Induced nonstoichiometric effects of Sb in p-type skutterudite thermoelectrics

Sublimation of antimony (Sb) during synthetic processes can be fatal to the thermoelectric (TE) properties of MT4Sb12 (M: filler atoms; T = Fe, Co, Ni)-based skutterudite (SKD). In this study, the nonstoichiometric effects of Sb on the phase structure formation and, thereby, on the TE properties of p-type SKD with Nd0.9Fe3.5Co0.5Sb12+x compounds were systematically investigated. Research is carried out with x values varying from −0.8 to 1.0, and samples are prepared by combining induction melting and melt-spinning, followed by a spark plasma sintering (SPS) process. The analysis showed that the Sb-deficiency formed NdSb and FeSb2 secondary phases along with the SKD main phase, while the Sb-excess effectively suppressed the FeSb2 phase formation and a small amount of the Sb phase appeared at a high Sb-excess level. The appearance of the secondary phases induced by Sb nonstoichiometry directly affected the change in the filling fraction of Nd in the SKD crystal structure, which led to the carrier concentration change. Our results demonstrate that the TE performance of MT4Sb12-based compounds can be significantly changed by the Sb nonstoichiometry, which should be profoundly considered along with other strategies to improve the TE efficiency of Sb-based SKD materials.

Conductivity versus structure dependence investigation of Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) through composition optimization by adjusting Si/P ratio

Presently, the low ionic conductivity of Na super ionic conductor (NASICON) electrolytes is one of the main obstacles to the development of solid-state sodium batteries. Most researchers focus on the modified NASICON by doping with foreign elements and have not paid much attention to the effect of the composition changing by adjusting the ratio of elements in Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3) on the ionic conductivity. In order to clarify the relationship between the ionic conductivity and the structure/composition of Na1+xZr2SixP3-xO12, the ionic conductivity dependence on the Si/P ratio is systematically investigated in this study. The obtained results demonstrate that the ionic conductivity variation of NASICON is close to an order of magnitude i.e. the ionic conductivity of Na1+xZr2SixP3-xO12 is much more sensitive to the Si/P ratio in the structure. This means that it is crucial to control the composition during the preparation process to obtain the electrolyte with high ionic conductivity. The strong composition-dependent property of Na1+xZr2SixP3-xO12 can be attributed to the change of current carrier concentration, crystal structure, and sintering property resulting from the Si/P ratio. The results are insightful for further understanding the conducting characteristics of Na1+xZr2SixP3-xO12 material and its derivatives.

Regulation of the Electronic Properties of Graphene via Organic Molecular Intercalation

Stable, efficient, and flexible regulation of the carrier types and concentrations of graphene is important to its multifunctional application. Here, we have synthesized n- and p-type graphene superlattices which are named as Gr/TBA and Gr/Ac via the electrochemical intercalation of organic tetrabutyl ammonium and acetate, respectively. The crystal structure characterization demonstrated that the interlayer spacing of the high-quality Gr/TBA and Gr/Ac was larger than that of pristine graphene. Hall effect measurements indicated that Gr/TBA and Gr/Ac showed n-type and p-type conductive behaviors, respectively. The carrier concentrations and mobility can be effectively tuned by the intercalated organic molecules. The regulation of the electronic properties of graphene via intercalation should result from the electron transfer between graphene and the corresponding organic molecules. Moreover, we further show that the resulting Gr/TBA and Gr/Ac feature excellent stability (chemicals and air), with no apparent change in carrier concentration. Our work provides a new way to regulate the electronic properties of graphene and broaden the application of graphene in multifunctional electrical devices.

Research on Infrared Emissivity and Laser Reflectivity of Sn1−xErxO2 Micro/Nanofibers Based on First-Principles

Sn1−xErxO2 (x = 0%, 8%, 16%, 24%) micro/nanofibers were prepared by electrospinning combined with heat treatment using erbium nitrate, stannous chloride and polyvinylpyrrolidone (PVP) as raw materials. The target products were characterized by thermogravimetric analyzer, X-ray diffrotometer, fourier transform infrared spectrometer, scanning electron microscope, spectrophotometer and infrared emissivity tester, and the effects of Er3+ doping on its infrared and laser emissivity were studied. At the same time, the Sn1−xErxO2 (x = 0%, 16%) doping models were constructed based on the first principles of density functional theory, and the related optoelectronic properties such as their energy band structure, density of states, reflectivity and dielectric constant were analyzed, and further explained the mechanism of Er3+ doping on SnO2 infrared emissivity and laser absorption from the point of electronic structure. The results showed that after calcination at 600°C, single rutile type SnO2 was formed, and the crystal structure was not changed by doping Er3+. The calcined products showed good fiber morphology, and the average fiber diameter was 402 nm. The infrared emissivity and resistivity of the samples both decreased first and then increased with the increase of Er3+ doping amount. When x = 16%, the infrared emissivity of the sample was at least 0.71; and Er3+ doping can effectively reduce the reflectivity of SnO2 at 1.06 μm and 1.55 μm, when x = 16%, its reflectivity at 1.06 μm and 1.55 μm are 50.5% and 40%, respectively, when x = 24%, the reflectivity at 1.06 μm and 1.55 μm wavelengths are 47.3% and 42.1%, respectively. At the same time, the change of carrier concentration and electron transition before and after Er3+ doping were described by firstprinciple calculation, and the regulation mechanism of infrared emissivity and laser reflectivity was explained. This study provides a certain experimental and theoretical basis for the development of a single-type, light-weight and easily prepared infrared and laser compatible-stealth material.

Quasi-3-dimensional simulations and experimental validation of surface leakage currents in high operating temperature type-II superlattice infrared detectors

The surface leakage in InAs/GaSb type-II superlattice (T2SL) is studied experimentally and theoretically for photodiodes with small sizes down to 10 × 10 μm2. The dependence of dark current density on mesa size is studied at 110 and 200 K, and surface leakage is shown to impact both generation–recombination (GR) and diffusion dark current mechanisms. A quasi-3-dimensional model to simulate the fabrication process using surface traps on the pixel's sidewall is presented and is used to accurately represent the dark current of large and small pixels with surface leakage in the different temperature regimes. The simulations confirmed that the surface leakage current has a GR and diffusion component at low and high temperature, respectively. Finally, the surface leakage current has been correlated with the change in minority carrier concentration at the surface due to the presence of donor traps.

Significant Lifetime Enhancement in QLEDs by Reducing Interfacial Charge Accumulation via Fluorine Incorporation in the ZnO Electron Transport Layer

HighlightsQuantum dots light emitting devices (QLEDs) with extremely long half-lifetimes at 100 cd m−2, 2,370,000 h were successfully fabricated using CF4 plasma-treated ZnO nanoparticle electron transport layers.A new experimental approach that probes changes in exciton lifetime under current flow was used to investigate the changes in carrier concentration in QLEDs.Evidence of the dependence of QLED stability on electron and hole concentrations at the QD/HTL interface was revealed.

Selective High-Resistance Zones Formed by Oxygen Annealing for -GaO Schottky Diode Applications

Selective area doping technique is essential for diversifying semiconductor device structures. In this letter, selective high-resistance zones on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -Ga2O3 wafers were successfully achieved by a high-temperature oxygen annealing process. Polysilicon, which proved to have an ideal blocking capability against oxygen annealing ambient, was employed as an annealing cap layer to prevent local carrier concentration changes during annealing. Based on this unique process approach, we further demonstrate a high-resistance anode edge termination of Schottky barrier diodes, which can considerably reduce the leakage current and increase the breakdown voltage of the devices. This research broadens the device manufacturing method and promotes the development of Ga2O3 devices.

Review of Radiation-Induced Effects on β-Ga2O3 Materials and Devices

β-Ga2O3 has become an ultimate choice of emerging new-generation material for its wide range of compelling applications in power electronics. In this review, we have explored the available radiations in the atmosphere and the effects of radiation on the β-Ga2O3 material and devices. The focus in this review summarizes various studies covering different radiation such as swift heavy ions, protons, neutrons, electrons, Gamma, and X-rays to understand the radiation-induced effects on the structure and their reliable performance in harsh environments. In addition, we focused on the various pre-existing defects in β-Ga2O3 and the emergence of radiation-induced defects that provoke a severe concern, especially from the device performance point of view. This review presents the irradiation-induced effects on the devices such as high-power devices such as Schottky barrier diodes (SBDs), field-effect transistors (FETs), metal-oxide-semiconductor (MOS) devices, and photodetectors. Some key studies including the changes in carrier concentration with a removal rate, Schottky barrier height (SBH), ideality factor, defect dynamics dielectric damage, interface charge trapping, a thermally activated recovery mechanism for charge carriers at elevated temperature, and diffusion length of minority charge carriers. These reports show that β-Ga2O3-based devices could be deployable for space or high-radiation terrestrial applications. These results provide/suggest a better device design based on the radiation degradation studies in the state-of-the-art β-Ga2O3 devices.

Enhanced acidic gas adsorption performance of arsenene by Pt mediation

Based on first-principle calculations, we have theoretically investigated the effect of Pt mediation (doping and embedding) on the adsorption behavior of the arsenene monolayer (As-ML) for several acidic gas molecules (H2S, SO2, and NO2). We find that the shape and orientation of each gas molecule in the most stable adsorption configuration are changeable depending upon the type of substrate used; however, the changing trend for adsorption energy is well correlated with the adsorption distance. The results reveal that Pt-embedded As-ML is more sensitive to all gas molecules than Pt-doped As-ML due to its larger adsorption energy. The gas molecules interact with Pt-embedded As-ML via weak chemical bonds, which induces a significant change in the bandgap of the adsorption system. Charge transfer occurs from the substrate to gas molecules, and the gas molecule acts as an electron acceptor, which will affect the change in the charge carrier concentration, thereby reflecting a change in the electrical conductance of the arsenene system. However, taking into account the feasibility of the desorption process, Pt-doped As-ML is more suitable as an H2S sensor compared to Pt-embedded As-ML. Overall, this work will help to guide scientists to develop fabricating arsenene-based gas sensors in the future.