Local Carrier Concentration Research Articles

Thermal boundary conductance of artificially and systematically designed grain boundaries of Silicon measured by laser heterodyne photothermal displacement method

The overall physical properties of polycrystalline materials vary depending on the microscopic individual grain boundary (GB) properties and their structures. Unlike previous studies that only examined the structure and properties of a specific GB, this study focuses on understanding the thermal boundary conductance (TBC) through artificial and systematic changes in the GB structures. This is achieved by combining an advanced technique to map local thermal expansion displacement using the laser heterodyne photothermal displacement method and a unique crystal growth method that induces spontaneous changes in the GB structures. As a result, we could quantify the TBC of the GB in silicon, considering the changes in three structural parameters of GB: azimuthal misorientation (α), asymmetry angle (β), and deviation angle (θ) from the growth direction. Our findings reveal that the TBC increases with increasing θ, whereas parameters α and β have negligible effects. The underlying physics of this relationship is discussed in terms of local carrier concentration and impurity segregation. These results demonstrate the crucial role of the GB structures in influencing the local TBC, shedding light on potential avenues for enhancing the macroscopic properties of polycrystalline materials by engineering GBs.

Effect of repeating hydrothermal growth processes and rapid thermal annealing on CuO thin film properties.

This paper presents an investigation into the influence of repeating cycles of hydrothermal growth processes and rapid thermal annealing (HT+RTA) on the properties of CuO thin films. An innovative hydrothermal method ensures homogeneous single-phase films initially. However, their electrical instability and susceptibility to cracking under the influence of temperature have posed a challenge to their utilization in electronic devices. To address this limitation, the HT+RTA procedure has been developed, which effectively eliminated the issue. Comprehensive surface analysis confirmed the procedure's ability to yield continuous films in which the content of organic compounds responsible for the formation of cracks significantly decreases. Structural analysis underscored the achieved improvements in the crystalline quality of the films. The implementation of the HT+RTA procedure significantly enhances the potential of CuO films for electronic applications. Key findings from Kelvin probe force microscopy analysis demonstrate the possibility of modulating the work function of the material. In addition, scanning capacitance microscopy measurements provided information on the changes in the local carrier concentration with each repetition. These studies indicate the increased usefulness of CuO thin films obtained from the HT+RTA procedure, which expands the possibilities of their applications in electronic devices.

An Accurate Electro-Thermal Coupling Model of a GaAs HBT Device under Floating Heat Source Disturbances.

Taking into consideration the inaccurate temperature predictions in traditional thermal models of power devices, we undertook a study on the temperature rise characteristics of heterojunction bipolar transistors (HBTs) with a two-dimensional cross-sectional structure including a sub-collector region. We developed a current-adjusted polynomial electro-thermal coupling model based on investigating floating heat sources. This model was developed using precise simulation data acquired from SILVACO (Santa Clara, CA, USA). Additionally, we utilized COMSOL software (version 5.6) to simulate the temperature distribution within parallel power cells, examining further impacts resulting from thermal coupling. The research findings indicate that the rise in current induces modifications in the local carrier concentration, thereby prompting variations in the local electric field, including changes in the heat source's peak location and intensity. The device's peak temperature exhibits a non-linear trend regulated by the current, revealing an error margin of less than 1.5% in the proposed current-corrected model. At higher current levels, the drift of the heat source leads to an increase in the heat dissipation path and reduces the coupling strength between parallel devices. Experiments were performed on 64 GaAs (gallium arsenide) HBT-based power cells using a QFI infrared imaging system. Compared to the traditional temperature calculation model, the proposed model increased the accuracy by 6.84%, allowing for more precise predictions of transistor peak temperatures in high-power applications.

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.

Near-field thermophotovoltaic energy conversion analysis based on enhanced radiative absorption distribution

Near-field radiation has been widely shown to greatly boost the electrical power of thermophotovoltaic (TPV) cells. However, there is a lack of theoretical analysis exploring the important influences of near-field effects on radiative absorption distributions as well as TPV energy conversion performances. This work investigates the electrical performances of near-field TPV cells made of InGaSb coupled with different practical emitters such as plain tungsten (W), indium tin oxide (ITO) film, and alternate W and alumina multilayer in detail. A comprehensive analysis is conducted to systematically compare the impacts of evanescent wave tunneling, surface plasmon resonance, and hyperbolic modes on spatial distributions of radiative absorption and the profiles of local carrier concentrations. The detailed and accurate analysis reveals the crucial role of near-field radiation emitted by various emitters in charge collection efficiency, thermal photon flux penetration depth, and photocurrent generation. Thus, the results certify that the electric power could be enhanced by utilizing ITO and multilayer emitters instead of a plain W emitter. The efficiency for an ITO emitter increases with decreasing vacuum gap owing to the suppressed bulk recombination but decreases when the vacuum gap falls below 18 nm due to increased surface recombination. While the efficiency for a multilayer emitter is comparatively lower due to the larger sub-bandgap photons and inefficient n-region. Furthermore, we verify the strategies for performance improvement via decreasing the surface recombination and optimizing the p-region thickness. The underlying mechanism is interpreted based on the spatial distribution and the collection efficiency of minority carriers.

Construction of PdSe2/ZnIn2S4 heterojunctions with covalent interface for highly efficient photocatalytic hydrogen evolution

Constructing semiconductor heterojunctions can enable novel schemes for highly efficient photocatalytic activity. However, introducing strong covalent bonding at the interface remains an open challenge. Herein, ZnIn2S4 (ZIS) with abundant sulfur vacancies (Sv) is synthesized with the presence of PdSe2 as an additional precursor. The sulfur vacancies of Sv-ZIS are filled by Se atoms of PdSe2, leading to the Zn–In–Se–Pd compound interface. Our density functional theory (DFT) calculations reveal the increased density of states at the interface, which will increase the local carrier concentration. Moreover, the length of the Se-H bond is longer than that of the SH bond, which is good for the evolution of H2 from the interface. In addition, the charge redistribution at the interface results in a built-in field, providing the driving force for efficient separation of photogenerated electron-hole. Therefore, the PdSe2/Sv-ZIS heterojunction with strong covalent interface exhibits an excellent photocatalytic hydrogen evolution performance (4423 μmol g−1h−1) with an apparent quantum efficiency (λ > 420 nm) of 9.1 %. This work will provide new inspirations to improve photocatalytic activity by engineering the interfaces of semiconductor heterojunctions.

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.

Phosphonate/Phosphine Oxide Dyad Additive for Efficient Perovskite Light‐Emitting Diodes

AbstractAdditives play a critical role for efficient perovskite light‐emitting diodes (PeLEDs). Here, we report a novel phosphonate/phosphine oxide dyad molecular additive (PE‐TPPO), with unique dual roles of passivating defects and enhancing carrier radiative recombination, to boost the device efficiency of metal–halide perovskites. In addition to the defect passivation effect of the phosphine oxide group to enhance the photoluminescence intensity and homogeneity of perovskite film, the phosphonate group with strong electron affinity can capture the injected electrons to increase local carrier concentration and accelerate the carrier radiative recombination in the electroluminescence process. Owing to their synergistic enhancement on device efficiency, quasi‐two‐dimensional green PeLEDs modified by this dyad additive exhibit a maximum external quantum efficiency, current efficiency, and power efficiency of 25.1 %, 100.5 cd A−1, and 98.7 lm W−1, respectively, which are among the reported state‐of‐the‐art efficiencies.

Phosphonate/Phosphine Oxide Dyad Additive for Efficient Perovskite Light-Emitting Diodes.

Additives play a critical role for efficient perovskite light-emitting diodes (PeLEDs). Here, we report a novel phosphonate/phosphine oxide dyad molecular additive (PE-TPPO), with unique dual roles of passivating defects and enhancing carrier radiative recombination, to boost the device efficiency of metal-halide perovskites. In addition to the defect passivation effect of the phosphine oxide group to enhance the photoluminescence intensity and homogeneity of perovskite film, the phosphonate group with strong electron affinity can capture the injected electrons to increase local carrier concentration and accelerate the carrier radiative recombination in the electroluminescence process. Owing to their synergistic enhancement on device efficiency, quasi-two-dimensional green PeLEDs modified by this dyad additive exhibit a maximum external quantum efficiency, current efficiency, and power efficiency of 25.1 %, 100.5 cd A-1 , and 98.7 lm W-1 , respectively, which are among the reported state-of-the-art efficiencies.

Controlling exciton many-body states by the electric-field effect in monolayer MoS2

We report magneto-optical spectroscopy of gated monolayer MoS$_2$ in high magnetic fields up to 28T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels in both $K/K^{\prime}$ valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavour that is defined by the Landau level quantized spin and valley texture.

Contactless probing of graphene charge density variation in a controlled humidity environment

The electronic properties of graphene are highly sensitive to atoms and molecules adsorbed on its surface and to changes in its environment, such as temperature and humidity. In this paper, we examine the effect of humidity on the local carrier concentration of different types of graphene prepared by mechanical exfoliation, chemical vapour deposition and epitaxial growth on SiC, using in-situ Raman spectroscopy. We present a systematic and comparative study of the changes in Raman response using a vector analysis method to produce spatial maps of doping variation as a function of humidity. We also quantify the humidity induced carrier concentration change for different types of graphene. This study illustrates the effects of humidity on the electronic properties of graphene and provides a simple, contactless and quantitative method to directly evaluate water-induced doping effects in graphene, that are crucial for tailored device performance.

Piezoelectricity in Monolayer Hexagonal Boron Nitride.

2D hexagonal boron nitride (hBN) is a wide-bandgap van der Waals crystal with a unique combination of properties, including exceptional strength, large oxidation resistance at high temperatures, and optical functionalities. Furthermore, in recent years hBN crystals have become the material of choice for encapsulating other 2D crystals in a variety of technological applications, from optoelectronic and tunneling devices to composites. Monolayer hBN, which has no center of symmetry, is predicted to exhibit piezoelectric properties, yet experimental evidence is lacking. Here, by using electrostatic force microscopy, this effect is observed as a strain-induced change in the local electric field around bubbles and creases, in agreement with theoretical calculations. No piezoelectricity is found in bilayer and bulk hBN, where the center of symmetry is restored. These results add piezoelectricity to the known properties of monolayer hBN, which makes it a desirable candidate for novel electromechanical and stretchable optoelectronic devices, and pave a way to control the local electric field and carrier concentration in van der Waals heterostructures via strain. The experimental approach used here also shows a way to investigate the piezoelectric properties of other materials on the nanoscale by using electrostatic scanning probe techniques.

Microscopic Determination of Carrier Density and Mobility in Working Organic Electrochemical Transistors

A comprehensive understanding of electrochemical and physical phenomena originating the response of electrolyte-gated transistors is crucial for improved handling and design of these devices. However, the lack of suitable tools for direct investigation of microscale effects has hindered the possibility to bridge the gap between experiments and theoretical models. In this contribution, a scanning probe setup is used to explore the operation mechanisms of organic electrochemical transistors by probing the local electrochemical potential of the organic film composing the device channel. Moreover, an interpretative model is developed in order to highlight the meaning of electrochemical doping and to show how the experimental data can give direct access to fundamental device parameters, such as local charge carrier concentration and mobility. This approach is versatile and provides insight into the organic semiconductor/electrolyte interface and useful information for materials characterization, device scaling, and sensing optimization.

Hyperspectral time-domain terahertz nano-imaging.

Terahertz (THz) near-field microscopy has wide and unprecedented application potential for nanoscale materials and photonic-device characterization. Here, we introduce hyperspectral THz nano-imaging by combining scattering-type scanning near-field optical microscopy (s-SNOM) with THz time-domain spectroscopy (TDS). We describe the technical implementations that enabled this achievement and demonstrate its performance with a heterogeneously doped Si semiconductor sample. Specifically, we recorded a hyperspectral image of 40 by 20 pixels in 180 minutes and with a spatial resolution of about ~170 nm by measuring at each pixel with a time domain spectrum covering the range from 0.4 to 1.8 THz. Fitting the spectra with a Drude model allows for measuring-noninvasively and without the need for Ohmic contacts-the local mobile carrier concentration of the differently doped Si areas. We envision wide application potential for THz hyperspectral nano-imaging, including nanoscale carrier profiling of industrial semiconductor structures or characterizing complex and correlated electron matter, as well as low dimensional (1D or 2D) materials.

Mg Deficiency in Grain Boundaries of n‐Type Mg3Sb2 Identified by Atom Probe Tomography

AbstractHighly resistive grain boundaries significantly reduce the electrical conductivity that compromises the thermoelectric figure‐of‐merit zT in n‐type polycrystalline Mg3Sb2. In this work, discovered is a Mg deficiency near grain boundaries using atom‐probe tomography. Approximately 5 at% of Mg deficiency is observed uniformly in a 10 nm region along the grain boundary without any evidence of a stable secondary or impurity phase. The off‐stoichiometry can prevent n‐type dopants from providing electrons, lowering the local carrier concentration near the grain boundary and thus the local conductivity. This observation explains how nanometer scale compositional variations can dramatically determine thermoelectric zT, and provides concrete strategies to reduce grain‐boundary resistance and increase zT in Mg3Sb2‐based materials.

Surface Defect Engineering in 2D Nanomaterials for Photocatalysis

Abstract2D Nanomaterials, with unique structural and electronic features, have shown enormous potential toward photocatalysis fields. However, the photocatalytic behavior of pristine 2D photocatalysts are still unsatisfactory, and far below the requirements of practical applications. In this regard, surface defect engineering can serve as an effective means to tune photoelectric parameters of 2D photocatalysts through tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, recent progress in the design of surface defects with the classified anion vacancy, cation vacancy, vacancy associates, pits, distortions, and disorder on 2D photocatalysts to boost the photocatalytic performance is summarized. The strategies for controlling defects formation and technique to distinguish various surface defects are presented. The crucial roles of surface defects for photocatalysis performance optimization are proposed and advancement of defective 2D photocatalysts toward versatile applications such as water oxidation, hydrogen production, CO2 reduction, nitrogen fixation, organic synthesis, and pollutants removal are discussed. Surface defect modulated 2D photocatalysts thus represent a powerful configuration for further development toward photocatalysis.

Another approach to the problem of room temperature superconductivity

The superconductivity at room temperature $$T_{\mathrm{m}}$$ can be accomplished by pursuing two distinct strategies. One approach is to create a new superconductor, whose critical temperature $$T_{\mathrm{c}} \sim T_{\mathrm{m}}$$ .Here we discuss viability of another approach involving integration of a conventional superconductor (S) with the Peltier cooler (P) made of an atomic monolayer (AM). Owing to remarkable properties of AM, the hybrid SP thermoelectric device promises a tremendously high efficiency, sufficient to diminish the local temperature of S below its $$T_{\mathrm{c}}$$ amid the ambient temperature $$\sim T_{\mathrm{m}}>>T_{\mathrm{c}}$$ . In particular, the chirality and Klein tunneling of electric charge carriers in graphene allow creating topologically protected quantized state, immune to electron scattering on lattice imperfections. Efficient doping with the gate electrodes allows changing the local carrier concentration in wide limits, leading to a high thermopower. The nanostructuring and alloying diminish the phonon part of the thermal flux by orders of magnitude. By combining the above methods, one accomplishes the nanoscale cooling of S locally down to $$T_{\mathrm{c}}<<T_{\mathrm{m}}$$ .

Carrier properties of B atomic-layer-doped Si films grown by ECR Ar plasma-enhanced CVD without substrate heating

The atomic-layer (AL) doping technique in epitaxy has attracted attention as a low-resistive ultrathin semiconductor film as well as a two-dimensional (2-D) carrier transport system. In this paper, we report carrier properties for B AL-doped Si films with suppressed thermal diffusion. B AL-doped Si films were formed on Si(100) by B AL formation followed by Si cap layer deposition in low-energy Ar plasma-enhanced chemical-vapor deposition without substrate heating. After fabrication of Hall-effect devices with the B AL-doped Si films on unstrained and 0.8%-tensile-strained Si(100)-on-insulator substrates (maximum process temperature 350°C), carrier properties were electrically measured at room temperature. Typically for the initial B amount of 2 × 1014 cm−2 and 7 × 1014 cm−2, B concentration depth profiles showed a clear decay slope as steep as 1.3 nm/decade. Dominant carrier was a hole and the maximum sheet carrier densities as high as 4 × 1013 cm−2 and 2 × 1013 cm−2 (electrical activity ratio of about 7% and 3.5%) were measured respectively for the unstrained and 0.8%-tensile-strained Si with Hall mobility around 10–13 cm2 V−1 s−1. Moreover, mobility degradation was not observed even when sheet carrier density was increased by heat treatment at 500–700 °C. There is a possibility that the local carrier (ionized B atom) concentration around the B AL in Si reaches around 1021 cm−3 and 2-D impurity-band formation with strong Coulomb interaction is expected. The behavior of carrier properties for heat treatment at 500–700 °C implies that thermal diffusion causes broadening of the B AL in Si and decrease of local B concentration.

Self-Assembled Lead Halide Perovskite Nanocrystals in a Perovskite Matrix

Hybrid metal halide perovskite materials are produced with facile routes, but their morphology is sensitive to water, oxygen, temperature, and exposure to light. While phase separation and self-assembly of perovskite nanostructures have been demonstrated, the realization of controlled perovskite–perovskite heterostructures has been limited up to now. We demonstrate here the growth of stable CH3NH3PbI3–xBrx nanocrystals in a CH3NH3PbBr3 matrix. Optical emission from the nanocrystals can be reversibly activated upon illumination through a photobrightening process. Optical microscopy images show that nanocrystals are stable in time, through several illumination cycles. Ultrafast photoluminescence measurements imply that optical excitations are funneled from the matrix into the lower bandgap nanocrystals. Because the nanocrystals represent <2% of the materials volume, the local carrier concentration is higher in the nanocrystals than in the matrix, leading to an increase in the photoluminescence quantum yield...

Sensitive, Selective, and Fast Detection of ppb-Level H2S Gas Boosted by ZnO-CuO Mesocrystal.

ZnO-CuO mesocrystal was prepared via topotactic transformation using one-step direct annealing of aqueous precursor solution and assembled into a H2S sensor. The ZnO-CuO mesocrystal-based sensor possesses good linearity and high sensitivity in the low-concentration range (10–200 ppb). Compared to pure CuO, the as-prepared ZnO-CuO mesocrystal sensor exhibited superior H2S sensing performance with a response ranging from 8.6 to 152 % towards H2S concentrations from 10 ppb to 10 ppm when applied at the optimized working temperature of 125 °C. The sensor showed excellent repeatability and good selectivity towards H2S gas even at a concentration four orders of magnitude lower than the interfering gases, such as H2, CO2, CO, NO2, acetone, and NH3. The improved sensitivity could be attributed partially to the effective diffusion of analyte gas through the mesocrystal surface and the abundant accessible active sites. Moreover, the nanoscale p-n junctions within the mesocrystal, which could effectively manipulate the local charge carrier concentration, are also beneficial to boost the sensing performance.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1688-y) contains supplementary material, which is available to authorized users.