Reduction In Hole Concentration Research Articles

Intercalation-Induced Monolayer Behavior in Bulk NbSe2.

Superconductivity at the 2D limit is significant for advancing fundamental physics, leading to extensive research on monolayer two-dimensional materials. In particular, monolayer transition metal dichalcogenides such as NbSe2 exhibit Ising superconductivity due to broken in-plane inversion symmetry and strong spin-orbit coupling, which has garnered significant attention. In this letter, we adopted an organic cation intercalation technique to modulate the interlayer interaction of NbSe2 by expanding the interlayer distance, thereby making intercalated NbSe2 behave similarly to monolayer NbSe2. The interlayer distances of NbSe2 intercalated with THA+, CTA+, and TDA+ cations are almost double or triple that of pristine NbSe2. The superconducting transition temperature (Tc) of THA-NbSe2 is comparable to that of pristine NbSe2, while the charge density wave (CDW) transition temperature is higher. The Tc of intercalated NbSe2 decreases with a reduced hole concentration, and the enhanced CDW is ascribed to the dimensional reduction. Notably, the in-plane upper critical field of intercalated NbSe2 significantly exceeds the Pauli paramagnetic limit, which is similar to the Ising superconductivity observed in monolayer NbSe2. Our work demonstrates that the organic cations intercalated two-dimensional materials exhibit behavior similar to their monolayer counterparts, providing a convenient platform for exploring and modulating physical phenomena at the two-dimensional limit.

Efficient rhombohedral GeTe thermoelectrics for low-grade heat recovery

The existence of Ge vacancy induces an inherently hole concentration up to 1021 cm−3 in pristine thermoelectric GeTe, which results in a need of ∼10 % aliovalent dopants for optimization. However, the extra carrier scattering by the dopants would notably deteriorate the carrier mobility. In this work, CuSbTe2-alloying is revealed to increase the formation energy of Ge-vacancy, leading to a rapid reduction in hole concentration down to 1.3 × 1020 cm−3 but with a mobility above 100 cm2/V-s. Along with a further Pb-substitution at Ge sites for lattice thermal conductivity reduction and band alignment, a peak figure of merit zT of ∼2.4 at 625 K is eventually realized in the rhombohedral structure, leading to a specific power density of as high as 70 W/m and a conversion efficiency of ∼10 % under a temperature difference of 302 K. This illustrates r-GeTe as truly efficient thermoelectrics particularly for low-grade heat recovery applications.

The Remarkable Role of Indium in Synergistically Optimizing Carrier Concentration and Phase Distribution of AgCuTe-Based Materials.

Carrier regulation has proven to be an effective approach for optimizing the thermoelectric performance of materials. One common method to adjust the carrier concentration is through element doping. In the case of AgCuTe-based materials, it tends to form with cation vacancies, resulting in a high hole concentration and complex phase composition at low temperatures, which also hinders material stability. However, this also offers additional opportunities to manipulate the carrier concentration. In this study, the improved performance of AgCuTe through indium doping is reported, which leads to a reduction in hole concentration. In combination with a significant increase in the effective mass of the carriers, the enhanced Seebeck coefficient is also realized. Particularly, a notable improvement in power factor is observed in the hexagonal phase near room temperature. Furthermore, a lower electron thermal conductivity is achieved, contributing to an average figure of merit value of ≈1.21 (between 523 and 723K). Additionally, the presence of indium inhibits the formation of the second phase and ensures a homogeneous phase distribution, which reduces the instability arising from phase transition. This work significantly enhances the potential of AgCuTe-based materials for low to medium-temperature applications.

Enhancement of Thermoelectric Performance by Doping to Reduce Degeneracy

Increasing valley degeneracy via doping is one approach to enhance thermoelectric performance. Herein, it is shown that reducing degeneracy can also have a positive impact on performance, particularly for materials with high degeneracy around their undoped band edges. Specifically, doping NbFeSb with Zr or Hf atoms split the valence band edge, resulting in a 1.33 reduction in hole concentration, compared with the experimental carrier concentrations. The reduction increases Seebeck coefficients and overall thermoelectric performance to better agreement with the experimental ones. Notably, for potentially effective thermoelectric materials with high degeneracy, if the doped atom has significantly lower (higher) electronegativity, it is expected to produce a high‐performance p‐type (n‐type) thermoelectric material.

Strongly overdoped La2−xSrxCuO4 : Evidence for Josephson-coupled grains of strongly correlated superconductor

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) with $x\ensuremath{\ge}0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at ${T}_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at ${T}_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at ${T}_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to ${T}_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

P-GaN Gate HEMTs With 10.6 V Maximum Gate Drive Voltages by Mg Doping Engineering

We present a novel p-gallium nitride (GaN) gate HEMT structure with reduced hole concentration near the Schottky interface by doping engineering in metal-organic chemical vapor deposition (MOCVD), which aims at lowering the electric field across the gate. By employing the additional unintentionally doped GaN (u-GaN) layer, the gate leakage current is suppressed and the gate breakdown voltage is boosted from 10.6 to 14.6 V with negligible influence on the threshold voltage and ON-resistance. A reduced Mg concentration in the u-GaN layer was confirmed by secondary-ion mass spectrometry. Time-dependent gate breakdown measurements reveal that the maximum gate drive voltage increases from 6.2 to 10.6 V for a ten-year lifetime with a 1% gate failure rate, which effectively expands the operating voltage margin of the p-GaN gate HEMTs without any other additional process step.

A 1.86-kV double-layered NiO/ β -Ga2O3 vertical p–n heterojunction diode

In this Letter, high-performance vertical NiO/β-Ga2O3 p–n heterojunction diodes without any electric field managements were reported. The devices show a low leakage current density and a high rectification ratio over 1010 (at ±3 V) even operated at temperature of 400 K, indicating their excellent thermal stability and operation capability at high temperature. Given a type-II band alignment of NiO/β-Ga2O3, carrier transport is dominated by the interface recombination at forward bias, while the defect-mediated variable range hopping conduction is identified upon strong reverse electric field. By using the double-layer design of NiO with a reduced hole concentration of 5.1 × 1017 cm−3, the diode demonstrates an improved breakdown voltage (Vb) of 1.86 kV and a specific on-resistance (Ron,sp) of 10.6 mΩ cm2, whose power figure of merit (Vb2/Ron,sp) has reached 0.33 GW/cm2. The high breakdown voltage and low leakage current are outperforming other reported Ga2O3 based p–n heterojunctions and Schottky barrier diodes without field plate and edge termination structures. TCAD simulation indicates that the improved Vb is mainly attributed to the suppression of electric field crowding due to the decreased hole concentration in NiO. Such bipolar heterojunction is expected to be an alternative to increase the breakdown characteristics of β-Ga2O3 power devices.

Influence of air atmosphere on electrical characteristics of p-type MoTe2 FETs under DC and pulsed mode operation

Herein, ambient effects on electrical characteristics of p-type MoTe2 FETs under DC and pulse mode measurements were systematically investigated. As a result of reduced hole concentration in a high vacuum condition, threshold voltage increased as compared to air condition. Moreover, reduction of charge trapping and scattering in vacuum condition leads to electrical performance improvement such as contact resistance, mobility, and interface trap density. Pulsed I-V measurement and CYTOP passivation was additionally introduced to investigate into intrinsic ambient effects by excluding external issues near interface and top channel region. Lastly, temperature dependent mobility variation was examined to validate scattering mechanism in the channel of MoTe2 FETs.

Manipulating the Ge Vacancies and Ge Precipitates through Cr Doping for Realizing the High-Performance GeTe Thermoelectric Material.

GeTe alloy is a promising medium-temperature thermoelectric material but with highly intrinsic hole carrier concentration by thermodynamics, making this system to be intrinsically off-stoichiometric with Ge vacancies and Ge precipitations. Generally, an intentional increase of formation energy of Ge vacancy by element substitution will lead to an effective dissolution of Ge precipitates for reduction in hole concentration. Here, an opposite direction of decreasing the formation energy of Ge vacancies is demonstrated by substituting Cr at Ge site. This strategy produces more but nearly homogenously distributed Ge precipitations and Ge vacancies, which provides enhanced phonon scattering and effectively reduces the lattice thermal conductivity. Furthermore, Cr atom carries one more electron than Ge and serves as an electron donor for decreasing the hole carrier concentrations. Further optimization incorporates the effect of Bi substitution for facilitating band convergence. A maximum figure of merit (ZT) of 2.0 at 600 K with average ZT of over 1.2 is achieved in the sample of Ge0.92 Cr0.03 Bi0.05 Te, making it one of the best thermoelectric materials for medium-temperature application.

Vacancy Manipulation for Thermoelectric Enhancements in GeTe Alloys.

Optimization of carrier concentration plays an important role on maximizing thermoelectric performance. Existing efforts mainly focus on aliovalent doping, while intrinsic defects (e.g., vacancies) provide extra possibilities. Thermoelectric GeTe intrinsically forms in off-stoichiometric with Ge-vacancies and Ge-precipitates, leading to a hole concentration significantly higher than required. In this work, Sb2Te3 having a smaller cation-to-anion ratio, is used as a solvend to form solid solutions with GeTe for manipulating the vacancies. This is enabled by the fact that each substitution of 3 Ge2+ by only 2 Sb3+ creates 1 Ge vacancy, because of the overall 1:1 cation-to-anion ratio of crystallographic sites in the structure and by the charge neutrality. The increase in the overall Ge-vacancy concentration facilitates Ge-precipitates to be dissolved into the matrix for reducing the hole concentration. In a combination with known reduction in hole concentration by Pb/Ge-substitution, a full optimization on hole concentration is realized. In addition, the resultant high-concentration point defects including both vacancies and substitutions strongly scatter phonons and reduce the lattice thermal conductivity to the amorphous limit. These enable a significantly improved thermoelectric figure of merit at working temperatures of thermoelectric GeTe.

Superconductivity of Y2O3 and BaZrO3 nanoparticles co-added YBa2Cu3O7−δ bulks prepared using co-precipitation method

In this work, polycrystalline samples of nominal composition YBa2Cu3O7−δ with co-addition of 5.0 mol% of Y2O3 and x mol% of BaZrO3 (BZO) nanoparticles (x = 0.0, 1.0, 2.0, 3.0, 5.0 and 7.0) were prepared using co-precipitation (COP) method. Data of X-ray diffraction (XRD) showed that all the samples were composed of Y-123 as the major phase and Y-211 as the minor phase. XRD peak of BZO was also observed in the samples co-added with BZO nanoparticles. Refinement of lattice parameters of a, b, and c-axis showed that the orthorhombic structure of the samples was retained without occurance of orthorhombic-tetragonal phase transition. The average grain size was increased from 0.30 ± 0.02 µm for the pure sample to 0.47 ± 0.03 µm for the sample with 7.0 mol% BZO as revealed by the scanning electron microscope images. Plots of normalized resistance versus temperature showed metallic behavior in the normal state and a single step transition in the samples. Tc-onset was decreased with co-addition of Y2O3 and BZO probably because of reduced hole concentration. The higher Josephson’s current, Io of the samples with co-addition of 0.0–2.0 mol% BZO compared with that of the pure one is likely to be due to improved grain coupling as shown by the AC susceptibility measurement. The calculated intergranular critical current density, Jcm based on the Bean critical state model is 1.88 A/cm2 at Tp = 84.8 K for the pure sample. The highest Jcm obtained is 2.10 A/cm2 at Tp = 85.4 K for 2.0 mol% BZO co-added sample.

Demonstration of high-performance p-type tin oxide thin-film transistors using argon-plasma surface treatments

In this work, we investigated the effects of low-temperature argon (Ar)-plasma surface treatments on the physical and chemical structures of p-type tin oxide thin-films and the electrical performance of p-type tin oxide thin-film transistors (TFTs). From the x-ray photoelectron spectroscopy measurement, we found that SnO was the dominant phase in the deposited tin oxide thin-film, and the Ar-plasma treatment partially transformed the tin oxide phase from SnO to SnO2 by oxidation. The resistivity of the tin oxide thin-film increased with the plasma-treatment time because of the reduced hole concentration. In addition, the root-mean-square roughness of the tin oxide thin-film decreased as the plasma-treatment time increased. The p-type oxide TFT with an Ar-plasma-treated tin oxide thin-film exhibited excellent electrical performance with a high current on–off ratio (5.2 × 106) and a low off-current (1.2 × 10−12 A), which demonstrates that the low-temperature Ar-plasma treatment is a simple and effective method for improving the electrical performance of p-type tin oxide TFTs.

Simultaneous optimization of electrical and thermal transport properties of Bi0.5Sb1.5Te3 thermoelectric alloy by twin boundary engineering

The strong interdependence between the Seebeck coefficient, the electrical and thermal conductivity makes it difficult to obtain a high thermoelectric figure of merit, ZT. It is of critical significance to design a novel structure that manages to decouple these parameters. Here, we combine a liquid state manipulation method for solidified Bi0.5Sb1.5Te3 alloy with subsequent melt spinning, ball milling, and spark plasma sintering processes, to construct dedicated microstructures containing plenty of 60° twin boundaries. These twin boundaries firstly scatter the very low-energy carriers and lead to an enhancement of the Seebeck coefficient. Secondly, they provide a considerable high carrier mobility, compensating the negative effect of the reduced hole concentration on the electrical conductivity. Thirdly, both experimental and calculated results demonstrate that the twin-boundary scattering dominates the conspicuous decrease of the lattice thermal conductivity. Consequently, the highest ZT value of 1.42 is achieved at 348K, which is 27% higher than that of the sample with less twin boundary treated without liquid state manipulation. The average ZT value from 300K to 400K reaches 1.34. Our particular sample processing methods enabling the twin-dominant microstructure is an efficient avenue to simultaneously optimize the thermoelectric parameters.

Efficiency enhancement of ultra-thin Cu(In,Ga)Se2 solar cells: optimizing the absorber bandgap profile by numerical device simulations

Thin-film chalcopyrite Cu(In1−x,Gax)Se (CIGS) solar cells have recently achieved an efficiency of ∼22% at the lab scale, making the technology more promising for commercial applications than most other thin-film solar cells. Using numerical device simulations, this study provides approaches to enhance the efficiency of ultra-thin CIGS solar cells. Effects of various Ga grading profiles in the CIGS absorber and of surface bandgap modifications are simulated. Our simulation results reveal that, in ultra-thin CIGS solar cells, back grading is an effective and practical approach to increase the cell efficiency, while front grading is unfeasible due to unacceptable current and fill factor losses. The quality of the back surface is of particular importance in moderately graded cells, while interface and bulk defect properties dominate in extremely graded cells. By introducing an ordered vacancy compound (OVC) layer with a downward-shifted valence band at the CIGS surface, the interface recombination losses can be significantly suppressed due to the reduced hole concentration. The thickness of the OVC layer and the valence band offset (VBO) between the OVC and CIGS materials are critical parameters for the cell efficiency. The simulations reveal that an optimized CIGS cell with a 300 nm thick CIGS absorber, a back-graded absorber profile and a 70 nm thick OVC layer at the CIGS surface can reach a 1-Sun efficiency of over 12%.

Restoration of long range order of Na ions in NaxCoO2 at high temperatures by sodium site doping

We have systematically investigated the NaxCoO2 system doped with Cu, Y, Sn, W, Au and Bi for x=0.5, 0.75 and 1.00 using density functional theory. Sn, W and Bi always substitute a Co while Au always substitute a Na regardless of Na concentration. However, for, Cu and Y, the substitution site depends on Na concentration. When compared to the available experimental data, we find that thermoelectric performance is enhanced when the dopants substitute a Na site. In this case, surprisingly, resistivity decreases despite the reduced hole concentration caused by carrier recombination. We propose improved carrier mobility to be the cause of observed reduced resistivity.

Effects of air-annealing on the electrical properties of p-type tin monoxide thin-film transistors

We have investigated the effects of air-annealing on the electrical performance of the p-type tin oxide thin-film transistors (TFTs). The air-annealing of the tin oxide thin-film was made using a mini furnace at various temperatures. From the x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) data, it is demonstrated that the phase of tin oxide partially transforms from SnO to SnO2 with an air-annealing process, and it accelerates as the annealing temperature increases. The electrical performance of the p-type tin oxide TFT with a channel thickness of 25 nm exhibits much improved electrical performance when air-annealed at 230 °C for 1 h, but a decrease of the on-current is observed with an ambipolar operation in 260 and 290 °C air-annealed devices. Based on the XPS, XRD, and Hall measurement data, the reduced hole concentration inside the channel due to the recombination with electrons from SnO2 is believed to be the reason for the electrical performance improvement in 230 °C air-annealed p-type tin oxide TFTs, and a partial formation of n-type SnO2 channel is considered as the plausible reason for the ambipolar operation in tin oxide TFTs with high annealing temperatures. Our experimental results show that there is an optimum air-annealing temperature which can improve the electrical performance in p-type tin oxide TFTs.

Crystallographic & Electrical Properties of Pr Substituted Gd-123 Nanometre Sized High Temperature Superconductors

The samples of Gd1-xPrxBa2Cu3O7-δ with compositions x = 0.0, 0.1 & 0.3 were prepared by ceramic route. The XRD study of all samples is carried out. The XRD studies of the samples prepared by ceramic route show GdPr-123 system have an orthorhombic pervoskite structure. The lattice parameters, oxygen content, volume of unit cell, orthorhombic distortion, hole concentration in CuO2 plane and charge on Cu-O plane were evaluated using XRD data. It is found that the lattice parameters, volume of unit cell increase while oxygen content orthorhombic distortion, hole concentration (Psh) and average charge (p) decrease as doping percentage of Pr increases. The resistivity measurement shows the transition temperature decreases while transition width increases with increasing Pr concentration. The suppression of superconductivity may be due to reduction in hole concentration or magnetic pair breaking. We report such qualitative results of Pr substituted Gd-123 in this paper quantitatively.

Role of Pr in Eu-123 high Tc nanometer-sized superconductors

Samples of Eu1−xPrxBa2Cu3O7−δ with compositions x=0.0, 0.1 and 0.3 were prepared by the solid state reaction route. The structural, electrical and microstructural properties were investigated by X-ray diffraction, Physical Property Measurement System (PPMS) and scanning electron microscopy (SEM) techniques respectively. The lattice parameters, oxygen deficiency, hole concentration in the CuO2 plane, average charge on Cu–O plane were evaluated from X-ray diffraction data. From XRD data, it is observed that the lattice parameters and oxygen deficiency increase with increasing Pr concentration. It was also observed that the hole concentration in CuO2 plane and average charge on Cu–O plane decrease with increasing Pr concentration. The electrical resistivity of all samples was measured in the temperature range 300 K to 4 K. It was found that the normalized transition temperature decreases with increasing Pr concentration. This may be due to reduction in hole concentration or average charge on Cu–O plane. From scanning electron microscopy images, it was observed that grain size increases with increasing Pr concentration. This increase may be the possible cause for suppression of Tc.

Phase behavior and mixed ionic–electronic conductivity of Ba4Sb2O9

The 6H-type perovskite phase Ba4Sb2O9, which decomposes in air below 600K, is found to survive to room temperature in a CO2-free atmosphere. It shows substantial mixed protonic, oxide ionic and electronic conductivity. Compared to Ba4Nb2O9 and Ba4Ta2O9, Ba4Sb2O9 shows higher ionic conductivity due to the relatively easy reducibility of Sb5+, but lower electronic conductivity due to the predominantly n-type conductivity provided by the Sb5+/Sb3+ redox couple which leads to reduced hole concentration under oxidizing conditions. Variable temperature synchrotron X-ray diffraction studies carried out in situ under controlled atmospheres reveal a strong monoclinic distortion below 1150K. The hexagonal to monoclinic transition is slow, does not show second-order behavior, is strongly dependent on atmosphere, and coincides with the loss of ~0.4 molecules of H2O per formula unit of Ba4Sb2O9. All of this suggests an important structural role for protons or hydroxide ions in the monoclinic phase.

Effects of MgO on N Dissolution of p-Type Al-N Codoped MgxZn1-xO Films

The effects of MgO on nitrogen dissolution and p-type conduction of sol-gel Al-N codoped MgxZn1-xO films were investigated. Substitution of MgO for ZnO significantly increased the solubility of nitrogen in the MgxZn1-xO films. The formation energies of NO and (N2)O were found to decrease with increasing MgO content, resulting in higher concentrations of NO and (N2)O. The acceptor levels of NO were estimated to be about 114 and 163 meV above the valence band maximum, respectively, for x = 0 and x = 0.05 films. The bandgap and the acceptor ionization energy increase with the MgO content, leading to a reduction in hole concentration and an increase in resistivity. The homojunction formed between a Al-N codoped MgxZn1-xO and a n-type Ga0.01Zn0.99O film displayed a typical rectifying behavior with a turn-on voltage of 2V, confirming the p-type conduction of the Al-N codoped MgxZn1-xO films.