Electron Carrier Concentration Research Articles

Ion implantation effects on the characteristics of β-Ga2O3 epilayers grown on sapphire by MOCVD

In this study, the Si-ions implantation technique with different doses from 1 × 1014 to 1 × 1015 cm−2 and dose energy 30, 40 and 50 keV was used to tune the electrical properties in unintentionally doped (UID) β-Ga2O3 epilayers grown on the sapphire substrates by metalorganic chemical vapor deposition (MOCVD). A high quality UID β-Ga2O3 epilayers were fabricated using the optimized growth parameters of MOCVD. The UID and Si-ions implanted β-Ga2O3 epilayers were examined and results were compared with the help of X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Si-ions implantation parameters were also simulated by stopping and range of ions in matter software (SRIM) and actual Si-ions concentration was measured by secondary ions mass spectroscopy. The electrical properties of the implanted β-Ga2O3 epilayers were measured by transmission length method and Hall measurements. The sheet resistivity for the β-Ga2O3epilayers with Si-ion dose of 1 × 1014, 6 × 1014 and 1 × 1015 cm−2 were found as 2.047, 0.158 and 0.144 Ω cm, respectively measured by Hall measurements and the electron carrier concentrations for the above doses were 4.39 × 1018, 6.86 × 1018 and 7.98 × 1019 cm−3. From the above results, the ion implantation was demonstrated to effectively reduce the resistivity with the high carrier concentrations.

Diffusion behaviors of Sn dopant in ITO films upon supercritical CO2 treatment and annealing

Indium Tin Oxide (ITO) thin films have been extensively studied for their excellent photoelectric performance. Nevertheless, the diffusion mechanism of doped-tin in ITO thin film is not sufficiently illuminated to date. Herein, the diffusion behaviors of Sn dopant in ITO films fabricated by RF-sputtering were investigated. As-deposited ITO thin films were treated by supercritical CO2 (SCCO2) fluid with aqua ammonia, then they were annealed at different temperatures. X-ray diffraction patterns (XRD) showed that the crystallization of ITO films occurred upon annealing above 300 °C, which resulted in the higher mobility of electron carriers. Relatively lower electron carrier concentrations were found for the ITO films after SCCO2 treatment and successive annealing at temperatures of 500 and 600 °C. X-ray photoelectron spectroscopy (XPS) analysis revealed that the content of tin element near the surface of ITO thin films can be reduced by SCCO2 fluid treatment. Subsequently, the annealing process results in the diffusion of the tin element to the sub-surface region of the films at different levels depending on the annealing temperature. The present work provides direct evidence for bleaching of tin dopant in the sub-surface region of ITO films by SCCO2 treatments and diffusion of tin from deeper regions to the sub-surface region of the ITO films upon annealing.

Bi Doped Sb2S3 Thin Film Synthesized by a Two-Step Approach with Enhanced Photoelectrochemical Water Splitting Performance

Photoelectrochemical water splitting (PEC) is believed as a promising approach to address the energy and environmental crisis. Here, we adopted a simple two-step method to synthesize bare Sb2S3 film, and then employed a Bi element doping strategy to improve the PEC performance of obtained bare Sb2S3. The obtained Bi-Sb2S3 exhibited an increased photocurrent density, 2.8 mA cm−2 at 1.23 VRHE and a negative shift of onset potential, compared to bare Sb2S3. The enhancement in PEC performance of Bi-Sb2S3 could be attributed to the expanded light absorption range, increased electron carrier concentration and improved charge transfer ability.

Graphite Nanosheets as Multifunctional Nanoinclusions to Boost the Thermoelectric Performance of the Shear‐Exfoliated Bi2O2Se

AbstractAs an eco‐friendly oxide‐based thermoelectric material, Bi2O2Se exhibits considerable potential for practical device application, but its low electrical conductivity needs to be further improved to achieve higher thermoelectric performance. Here, a record‐high figure of merit, ZT of >0.7 at 773 K in the shear‐exfoliated nanostructured Bi2O2Se with graphite nanosheets as multifunctional secondary nanoinclusions, is achieved. The introduced graphite nanosheets regularize the arrangement of Bi2O2Se nanograins, strengthen the anisotropy, and act as the “expressway” to improve the electrical conductivity by simultaneously enhancing the electron carrier concentration and mobility of the hybrid materials, leading to a high power factor of ≈6.0 µW cm–1 K–2 at 773 K. Also, the liquid‐phase shear exfoliation refines both graphite and Bi2O2Se into nanosheets. Moreover, the as‐sintered hybrid bulk materials composed of these nanosheets possess dense grain and phase boundaries, as well as various lattice imperfections, such as lattice distortions and stacking faults formed by physical shearing, which can significantly scatter the phonons with different wavelengths and in turn contribute to a low thermal conductivity of only 0.63 W m–1 K–1 at 773 K, both contributing to a competitive ZT of ≈0.73 at this temperature, indicating the great potential for practical applications.

The magnetic ordering transition of Li(ZnFe)As regulated via electron carriers

First-principles calculations based on the density functional theory (DFT) were performed to study the electronic structure and magnetic coupling mechanism of Li(ZnFe)As with different concentrations of electron carriers. The electron carrier was introduced by the treatment of Se nonequivalent substitution for As. The results manifested that Fe atoms were apt to short-range antiferromagnetic coupling for the Fe-3d electrons superexchange interactions. When an extra electron carrier was introduced, the antiferromagnetic stability of the co-doped system was weakened. In the case of two additional electron carriers, the magnetic ordering of the co-doped system changed from antiferromagnetic state to ferromagnetic state, which benefited from the electron carrier-mediated p-d exchange mechanism. The values of energy difference were slightly larger than zero and exhibited plateau behavior despite the variation of Fe doping distance, indicating that electron carriers promoted the co-doped system weak and long-range ferromagnetic stability.

Valence Disproportionation of GeS in the PbS Matrix Forms Pb5Ge5S12 Inclusions with Conduction Band Alignment Leading to High n-Type Thermoelectric Performance.

Converting waste heat into useful electricity using solid-state thermoelectrics has a potential for enormous global energy savings. Lead chalcogenides are among the most prominent thermoelectric materials, whose performance decreases with an increase in chalcogen amounts (e.g., PbTe > PbSe > PbS). Herein, we demonstrate the simultaneous optimization of the electrical and thermal transport properties of PbS-based compounds by alloying with GeS. The addition of GeS triggers a complex cascade of beneficial events as follows: Ge2+ substitution in Pb2+ and discordant off-center behavior; formation of Pb5Ge5S12 as stable second-phase inclusions through valence disproportionation of Ge2+ to Ge0 and Ge4+. PbS and Pb5Ge5S12 exhibit good conduction band energy alignment that preserves the high electron mobility; the formation of Pb5Ge5S12 increases the electron carrier concentration by introducing S vacancies. Sb doping as the electron donor produces a large power factor and low lattice thermal conductivity (κlat) of ∼0.61 W m-1 K-1. The highest performance was obtained for the 14% GeS-alloyed samples, which exhibited an increased room-temperature electron mobility of ∼121 cm2 V-1 s-1 for 3 × 1019 cm-3 carrier density and a ZT of 1.32 at 923 K. This is ∼55% greater than the corresponding Sb-doped PbS sample and is one of the highest reported for the n-type PbS system. Moreover, the average ZT (ZTavg) of ∼0.76 from 400 to 923 K is the highest for PbS-based systems.

Computational prediction of high thermoelectric performance in MPtSn (M = Ti, Zr, and Hf) half-Heusler compounds by first-principle study

The excellent thermoelectric efficiencies of half-Heusler compounds mainly originate from their distinctive crystal structure, high-thermal stability, and outstanding electrical transport performance. In this work, we investigate the elastic constants, electronic structure, and thermoelectric properties of MPtSn (M = Ti, Zr, and Hf) compounds using semi-classical Boltzmann transport theory and deformation potential model. The predicted maximum ZT value of n-type TiPtSn reaches up to 2.19 at the optimal electron carrier concentration of 5.98 × 1020 cm−3 at 1000 K. This ZT value is much higher than the previously experimental reported value. The electronic structure and phonon dispersion of the p-type HfPtSn compound suggest that doping at the Pt position can decrease the lattice thermal conductivities and modify the carrier concentrations. It is expected that these investigations could provide valuable guidance for exploring high-performance thermoelectric materials.

Г-L intervalley separation and electron mobility in GaAsSb grown on InP: Transport comparison with the GaInAs and GaInAsSb alloys

GaAs0.51Sb0.49 is lattice-matched to InP and finds electron transport applications in base or absorber layers in high-speed heterostructure bipolar transistors or photodiodes, because its staggered (“type-II”) band alignment with InP favors electron injection across abrupt heterojunctions. Little remains known about electron transport properties and band structure details of GaAsxSb1−x near x = 0.5. Particularly, based on the Γ-L intervalley separation in binary constituents (ΔΓL = 84 meV in GaSb and 290 meV in GaAs at 300 K), interpolation suggests a low Γ-L separation in GaAs0.51Sb0.49 before considering energy gap bowing effects. To gain insight into electron transport in GaAs0.51Sb0.49, we characterized experimental Hall electron mobilities vs carrier concentration at 300 and 77 K in n-type GaAs0.51Sb0.49, Ga0.47In0.53As, and Ga0.76In0.24As0.67Sb0.33 alloys nearly matched to InP. In marked contrast to the other two alloys, GaAs0.51Sb0.49 exhibits a sharp rise in 77 K electron mobility, which evidences L-valley de-population for lower electron concentrations. A two-band transport analysis reveals a Γ-L valley separation ΔΓL = 91 meV at 77 K, significantly lower than values recommended in the literature. Based on the reported temperature variations of ΔΓL in GaAs and GaSb, 84 < ΔΓL < 95 meV is expected at 300 K. The corresponding GaAsxSb1−x L-valley bowing parameter is cL = 1.63 eV, significantly higher than the 1.1–1.2 eV recommended in the literature. In contrast to GaAsxSb1−x, GaInAsSb grown on InP displays a strong alloy scattering, which limits its low-temperature electron mobility.

Effects of the addition of graphene on the defect-related photoluminescent and electrical properties of n-type ZnO thin films

This study determines the effect of the addition of graphene on the photoluminescent and electrical properties of ZnO films that are prepared using the sol-gel method. ZnO and graphene-doped ZnO thin films that are annealed at 500 °C for 30 min in nitrogen exhibit n-type conductivity. The correlation between photoluminescence (PL) spectral features and defects of ZnO means that there are oxygen vacancies (VO), zinc vacancies (VZn) and oxygen interstitials (Oi) in a ZnO thin film and there are VO, VZn, Oi in a graphene-doped ZnO thin film. The PL result shows that the incorporation of graphene into ZnO leads to a decrease in the number of VO, VZn and Oi and a transition from VZn to the substitution of carbon for zinc (CZn). The decreased number of VO and Oi is attributed to the substitution of VO by Oi, so equal numbers of acceptors and donors are annihilated and there is no change in the net electron carrier density. The transition from VZn to CZn increases the electron carrier concentration, which is opposite to the electrical observation. The results from PL, X-ray diffraction and X-ray photoelectron spectroscopy measurements demonstrate that a CZn+2Oi complex is formed in a graphene-doped ZnO thin film. The decrease in the electron concentration in a graphene-doped ZnO thin film is associated with the formation of the acceptor-like CZn+2Oi complex.

The effective mass, fitness function and enhanced thermoelectric properties in CuSbS2 for p-type thermoelectric material applications: Density functional theory approach

The thermoelectric properties, electronic fitness function (EFF) and transport inverse effective mass (m*) of CuSbS2 were predicted using Density Functional Theory along with Boltzmann transport theory. The Generalised Gradient Approximation as embedded in projector – augmented wave was used in this work. From our results, CuSbS2 exhibits an indirect narrow bandgap of 0.78 eV. The Power factor (PF) as well as the Seebeck coefficient (S) increase with temperature at every hole carrier concentration, and while PF is temperature dependent, S is almost independent of temperature for electron carrier concentrations making the electron minority carrier in the material. It was also revealed that this material has light m* and high EFF at 300, 500, 700 and 800 K. The obtained EFF implies that the thermoelectric performance of this material increases with temperature and can also be enhanced via doping. We obtained an electronic Figure of merit of 0.2 at room temperature which increase to 0.49 at 800 K. These results revealed that CuSbS2 is a promising P-type thermoelectric material.

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Microstructure control and thermoelectric figure of merit of Bi2O3-added WO3 ceramics

Microstructure control for Bi2O3-added WO3 ceramics is explored by changing the atomic ratio of Bi/W and the firing temperature. Densified WO3 ceramics with two distinct microstructures are obtained for Bi/W = 0.005; the ceramic fired at 1123 K consists of WO3 fine grains measuring several micrometers in size, whereas those fired at 1173 K and 1223 K consist of large WO3 grains measuring several 10 μm in size. The grain growth is enhanced by a Bi2W2O9 liquid phase formed among the WO3 particles during the firing. Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ are measured at high temperatures for the three ceramics to elucidate the dimensionless figure of merit ZT, where Z is defined as S2σκ−1 and T is the absolute temperature. The ceramics consisting of larger WO3 grains display higher power factors S2σ due to the higher electron carrier concentrations. The ZT values are consequently higher for these ceramics because the κ values are lower, due most likely to the larger phonon scattering by the grain boundaries developed among the larger WO3 grains. The highest ZT is 2.4 × 10−3 at 873 K for the ceramic fired at 1223 K.

Enhancing thermoelectric performance of n-type Bi6Cu2Se4O6 through introducing transition metal elements

Compared with p-type oxyselenide thermoelectric material, the corresponding n-type material research progress is slow. In this work, combining low thermal conductivity of BiCuSeO and high electron carrier concentration of n-type Bi2O2Se, a new-type two-dimensional layered oxyselenide Bi6Cu2Se4O6 was synthesized. Through increasing electron carrier concentration via introducing transition metal elements (Ti, Zr, Ce), the electrical transport properties were enhanced and the peak ZT value ~ 0.16 at 873 K was obtained, which is 60 % higher than that in Bi6Cu2Se3.6Cl0.4O6. This work indicates that transition metal element is an effective n-type dopant to enhance the thermoelectric performance of Bi6Cu2Se4O6.

Revealing the inner rules of PREN from electronic aspect by first-principles calculations

This study provides insight into the empirical PREN formula by studying the effects of alloying elements (Cr, Mo, N, Ti) on the surface acidity and electronic structures of α-Fe2O3. Incorporation of Cr, Mo and N into α-Fe2O3 (0001) surface can weaken the surface acidity and reduce the susceptibility to Cl−, while the influence of Ti is opposite. The doped N into α-Fe2O3 behaves as an acceptor and lowers the concentration of electron carriers due to compensation. A negatively-charged center has formed to repel Cl− after the compensation and ionization of acceptor. The mechanism of pitting resistance improving by Cr, Mo or N is proposed.

Disorder-Induced Anderson-Like Localization for Bidimensional Thermoelectrics Optimization

Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, couldplay an important role in global sustainable energy. However, improving thermoelectric efficiency has proven difficult, largelydue to the complex interdependence of electronic properties of solids. Early work by Ioffe has developed into the standardthermoelectric optimization paradigm of tuning the electronic carrier concentration in semiconductors. Although the localizationtheory of electrons by Anderson and Mott has developed in parallel, its potential for thermoelectrics optimization has notbeen explored. Here, we show that electron localization induced by structural disorder also provides an effective optimizationstrategy for thermoelectric materials. By using a transport model that includes the relevant physics of localization it is shownthat the maximum thermoelectric figure-of-merit can be increased ~20% by tuning both carrier concentration and disorder.This approach is demonstrated in two model Ge-Sb-Te material systems and confirms that the peak figure-of-merit occursin slightly disordered materials. Particularly for highly degenerate semiconductors this bidimensional optimization strategyprovides a new methodology to attain high thermoelectric performance.

Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Thermoelectric properties of tetragonal HfH2 under doping effect: First principles study

The effect of doping on the transport properties of HfH2 is explored using first-principles method and semi-classical Boltzmann approach. Thermoelectric properties such as electrical conductivity, Seebeck coefficient, electron and lattice thermal conductivity of doped and pure samples are calculated under various temperatures. The calculated results show that electrical and thermal conductivities increased with electron carrier concentrations similar to results reported by other studies in the low doping regime. While in the hole carrier concentration regime, irregular behavior is observed, especially in density of carriers equal to n = 4.6 × 1023 cm−3. Also, the lattice thermal conductivity and absolute value of Seebeck coefficient have been decreased with electron carrier concentration but these properties have irregular trend in the hole doping regime. Our results suggest that for achieving lower electron thermal conductivity with respect to pure HfH2, hole doping is much effective. Furthermore, the dispersive and large contribution of 5d orbital of Hf near the Fermi state and very low variation of L (Wiedemann-Franz factor) compared to the value of the free electron model L0, for pure and doped samples indicate that electrons in HfH2 have itinerant behavior and free-electron model is suitable to describe the transport properties of pure and doped samples of HfH2.

Effect of GaN Substrate Properties on Vertical GaN PiN Diode Electrical Performance

Present GaN technology consists primarily of heteroepitaxial, lateral high electron mobility transistors; however, high-power devices would be much more efficiently manufactured with a vertical geometry due to better blocking voltage scaling. This technology has yet to be realized due to the inconsistency of GaN wafer properties. These inconsistencies can be detected with several long-range, non-destructive techniques including Raman spectroscopy, optical profilometry, and photoluminescence mapping. In particular, Raman spectroscopy is an effective tool for determining the carrier concentration of GaN substrates and whether the wafers are uniform or inhomogeneous. In this work, vertical p-i-n GaN diodes are fabricated using both uniform and inhomogeneous wafers determined by using the A1 Raman peak position to monitor carrier concentration. The inhomogeneous samples have regular patterns of varying conductivity as a result of electron-donating defects brought about by changes in crystal stress. By avoiding these defects, we have found that diode performance improves by increasing the +/− 10 V rectification ratio by up to a factor of 5X and reducing the 200 V reverse bias leakage current by several orders of magnitude. Using the higher quality substrates with uniform electron carrier concentrations can improve the rectification ratio by another order of magnitude.

A comparative study of thermoelectric Cu2TrTi3S8 (Tr = Co and Sc) thiospinels: Enhanced Seebeck coefficient via electronic structure modification

We studied the electronic structures of Cu2TryTi4–yS8 (Tr = Sc, Co) thiospinels by combining thermoelectric measurements and first-principles calculations. The thiospinels showed n-type and metallic properties governed by a conductive network composed of edge-shared (Ti/Tr)S6 octahedra. The substitutions of Sc and Co for Ti decreased the electron carrier concentration in a similar manner, leading to increases in both the electrical resistivity and Seebeck coefficient. Although the electrical resistivity was equivalent for Tr = Sc and Tr = Co, the Seebeck coefficient was remarkably enhanced for Tr = Co as compared to that for Tr = Sc. The enhanced Seebeck coefficient in the Co-substituted system was ascribed to the increased density of states of conduction band near the Fermi level due to the partial replacement of Ti-3d orbitals by Co-3d orbitals. Owing to the electronic structure modification, a higher dimensionless thermoelectric figure of merit (ZT = 0.2 at y = 1–1.5) was achieved for the Co-substituted samples at 673 K.

Inorganic Lead‐Free Cs2AuBiCl6 Perovskite Absorber and Cu2O Hole Transport Material Based Single‐Junction Solar Cells with 22.18% Power Conversion Efficiency

AbstractCs2AuBiCl6 is considered to be a potential lead‐free double perovskite alternative for perovskite solar cells. Its electronic and optical properties are investigated using density functional theory. The electronic properties of Cs2AuBiCl6 material ensure a bandgap of 1.40 eV (without considering SOC) and 1.12 eV (with SOC) using mBJ exchange‐correlation functional, close to the optimal bandgap for solar cell application as per the Shockley–Queisser limit. Optical properties suggest a high absorption coefficient ≈105 cm−1 with low reflectance, making it the optimal absorber material. Furthermore, the photovoltaic performance of Cs2AuBiCl6 based single‐junction transparent conducting oxide (TCO)/IDL1/Cs2AuBiCl6/IDL2/Cu2O solar cell is investigated using SCAPS‐1D device simulation program. The impact of electron affinity, thickness, carrier concentration, defect density, and interface defect density is examined using interface defect layer (IDL) on the photovoltaic performance. The maximum photoconversion efficiency (PCE) of ≈22.18% is noticed for optimized material's parameters. These studies on TCO/IDL1/Cs2AuBiCl6/IDL2/Cu2O solar cell will provide guidelines for designing and developing an efficient lead‐free perovskite‐based solar cell as an alternative to conventional halide perovskite materials based solar cell.