Degenerate Semiconductors Research Articles

Enhanced Thermoelectric Properties in Cu₁.₈Se Thin Films: Achieving Superior Power Factor through Phase Control and Optimal Deposition Temperature

Thermoelectric thin films are vital for efficient energy conversion and thermal management. XRD analysis reveals that Cu₁.₈Se films exhibit a mixed α-phase when deposited at room temperature, 100°C, and 300°C, but transition to a pure β-phase at 200°C. The formation of the pure β-Cu₁.₈Se phase at 200°C significantly improves the crystallinity of the films. Increased annealing temperatures lead to greater surface roughness and grain size as observed by AFM and FESEM. Electrical conductivity decreases with higher measurement temperatures, reflecting degenerate semiconductor behavior due to Cu vacancies. The sample deposited at 200°C, exhibiting the pure β-phase, achieves the highest power factor of 5456 μWm⁻1K⁻2 and improved Seebeck coefficient, underscoring the importance of phase purity and controlled surface roughness for optimal thermoelectric performance. Keywodsannealing temperature; Surface Roughness; β-Cu1.8Se thin films; Thermoelectric

Electronic structure and thermoelectric properties of epitaxial Sc1−xVxNy thin films grown on MgO(001)

The electronic structure of Sc1−xVxNy epitaxial films with different alloying concentrations of V are investigated with respect to effects on thermoelectric properties. Band structure calculations on Sc0.75V0.25N indicate that V 3d states lie in the band gap of the parent ScN compound in the vicinity of the Fermi level. Thus, theoretically, the presence of light (dispersive) bands at the Γ point with band multiplicity is expected to lead to lower electrical resistivity, while flat (heavy) bands at X−W−K symmetry points are associated with higher Seebeck coefficients than that of ScN. Hence, to probe the thermoelectric properties experimentally, epitaxial Sc1−xVxNy thin film samples were deposited on MgO(001) substrates. All the samples showed N substoichiometry and pseudocubic crystal structure. The N-vacancy-induced states were visible in the Sc 2p x-ray absorption spectroscopy spectra. The reference ScN and Sc1−xVxNy samples up to x=0.12 were n type, exhibiting carrier concentration of 1021 cm−3, typical for degenerate semiconductors. For the highest V alloying of x=0.15, holes became the majority charge carriers, as indicated by the positive Seebeck coefficient. The underlying electronic structure and bonding mechanisms in Sc1−xVxNy influence the electrical resistivity, Seebeck coefficient, and Hall effect. Thus, this paper contributes to the fundamental understanding to correlate defects and thermoelectric properties to the electronic structure in the Sc-N system with V alloying. Published by the American Physical Society 2024

Facile synthesis and characterization of Cu2Se thin films and self-powered p-Cu2Se/n-Si heterojunction with high-performance photoresponse

A facile, cost-effective, and scalable chemical vapor deposition technique was used to synthesize p-type Cu2Se thin films on glass and n-type Si substrates. Thorough characterization confirmed the films’ β-phase structure with the correct stoichiometric ratio and exceptional crystalline quality, exhibiting behavior akin to a degenerate semiconductor. Measurements unveiled a work function of 4.83 eV and a bandgap of 2.13 eV for Cu2Se. The fabrication of a p-Cu2Se/n-Si heterojunction was achieved by depositing the p-type Cu2Se thin film onto the n-type Si substrate. The resulting heterostructure displayed rectification behavior, and its energy band diagram resembled a Schottky diode. Further exploration into its photoelectric properties showcased the p-Cu2Se/n-Si heterostructure’s favorable self-powered attribute, characterized by fast, steady, reproducible, sensitive, and robust photoresponsive performance. Consequently, it proves highly suitable for applications in high-frequency photodetectors. Additionally, the p-Cu2Se/n-Si heterojunction’s photovoltaic power conversion efficiency exceeded the reported values of the CuO/Si and Cu2O/Si systems. Here, this study contributes significantly to the pivotal evaluation of p-Cu2Se/n-Si heterostructures for promising optoelectronic applications.

Study on the passivation properties of austenitic stainless steel 316LN based on the point defect model

The Point Defect Model was successfully developed to study the passivation of 316LN. Steel's passive film exhibits n-type semi-conductivity with interstitial cations as the dominant point defect. The transfer coefficients αi and rate constants ki are independent of the applied potential but vary with temperature. The electronic structure of the barrier layer (bl) is discussed as a degenerate defect semiconductor in which the bl comprises essentially the depletion zone with the electronic charge and the electron-hole charge being concentrated at the metal/bl and the bl/solution interfaces, respectively. The closest classical analog is postulated to be a tunnel diode.

ABa6Cu31Te22 (A = K, Rb, Cs) Featuring Polyanionic Copper-Telluride Frameworks with Ultralow Thermal Conductivity.

Three polyanionic tellurides, ABa6Cu31Te22 (A = K, Rb, Cs), were synthesized in salt flux. The isostructural tellurides crystallize in a new structure type, in the cubic Pa3 space group with a Wyckoff sequence of d10c2b1 and large unit cell volumes of over 5500 Å3. The structures feature a framework of [CuTe4] tetrahedra and [CuTe3] trigonal pyramids with disorder in the Cu sites. The polyanionic frameworks have large square antiprism and cuboctahedral voids where Ba and alkali metal cations are situated, forming [BaTe8] and [ATe12], respectively. The overall compositions are close to being charge balanced. The large [ATe12] cuboctahedra allowed for significant anisotropic displacement of the A cations, as observed from both single crystal X-ray diffraction and heat capacity studies. Alkali cations rattling together with Cu atom displacement and disorder leads to the dispersion of phonons, thus softening the lattice and subsequently reducing the thermal conductivity. Evaluations of the electronic band structure revealed the occurrence of a narrow bandgap together with the presence of a flat band near the valence band maximum, giving rise to the high thermopower. The Cs and Rb analogues show a slope change in the temperature dependence of electrical resistivity around room temperature, which is typical for semimetals or degenerate semiconductors. For the as-synthesized and unoptimized materials, high values of the thermoelectric figure-of-merit of ∼0.2 were observed at 623 K.

Pulsed laser deposition and structural analysis of Ba2Ti6O13 epitaxial thin film on the (210) surface of SrTiO3 single crystal

A single crystalline thin film of Ba2Ti6O13 was grown on the (210) surface of a SrTiO3 single-crystal substrate by pulsed laser deposition. Scanning transmission electron microscopy shows that the Ba2Ti6O13 thin film grew epitaxially with the a-plane, and the b-/c-axes of Ba2Ti6O13 parallel to the (210) plane and the [00 1¯ ]/[ 1¯ 20] directions of SrTiO3, respectively. The electrical conductance of the Ba2Ti6O13 film was approximately one order larger than that of the reduced SrTiO3−δ single crystal with oxygen deficiency of roughly δ = 0.05. Electronic structure calculations predict that Ba2Ti6O13 is an n-type degenerate semiconductor with spin-split states primarily of Ti 3d orbital.

Domains with Varying Conductance in Tensile Strained SrMnO3 Thin Films Using Out‐of‐Plane Electric Fields

AbstractDomains and domain wall engineering have been extensively explored in ferroic materials for a wide range of applications in nanoelectronics and spintronics. Complex oxides exhibiting strongly correlated properties are model platforms for such studies where response to strain or external stimuli such as electric field, temperature and light can be probed. Here, domains in strained SrMnO3 films, grown on a degenerate semiconductor, allowing for conduction in an out‐of‐plane geometry, are studied using a combination of microscopy probes. Using conductive atomic force microscopy, electrically isolated domains with varying conductance are found and their temporal evolution is investigated. Further, their formation and microstructure are studied using scanning transmission electron microscopy and secondary electron contrast in scanning electron microscopy. An important contribution is establishing that the observed domains are formed by cracks, driven by inhomogeneous strain relaxation throughout the film, resulting in significantly high strain planes. The potential of secondary electrons to detect domain dependent contrast over a large area, ensuing due to the use of a degenerate semiconductor correlates with the conductive properties of the domains and serves as a new direction to probe domains and domain walls in ferroic materials.

Synergistic role of Sn-doping on the thermal and electrical properties of sinnerite Cu6As4S9

Sinnerite Cu6As4S9 has been identified as a promising material for optoelectronic applications, with potential for other energy-related applications; however, knowledge of the electrical and thermal properties as well as the mechanisms underlying transport in sinnerite is lacking. We present an investigation of the synthesis, structural, thermal and electrical transport properties of stoichiometric and Sn-doped Cu6As4S9. Sinnerite has a triclinic lattice structure, with highly distorted local atomic coordination environments that, in part, results in its complex structure and bonding. Our results and analyses indicate As 4s2 lone pair-induced distortions and strong lattice anharmonicity that leads to a relatively short phonon mean free path, resulting in intrinsically very low thermal conductivity. The electrical resistivity for both compositions, Cu6As4-xSnxS9 (x = 0, 0.2), are relatively high and varies little with temperature, typical of degenerate semiconductors. An increase in mobility and electrical conductivity was obtained by Sn doping in Cu6As3.8Sn0.2S9. This work demonstrates an effective route to synthesize bulk sinnerite as well as advances the knowledge of the properties of sinnerite, as this and other ternary chalcogenides continue to be of interest for potential technologically significant applications.

Copper-Enriched Nanostructured Conductive Thermoelectric Copper(I) Iodide Films Obtained by Chemical Solution Deposition on Flexible Substrates

The objects of our research are flexible thin-film thermoelectric materials with nanostructured CuI layers 0.5–1.0 μm thick, fabricated by the chemical solution method Successive Ionic Layer Adsorption and Reaction (SILAR) on flexible polyethylene terephthalate and polyimide substrates. These cubic γ-CuI films differ from films obtained by other chemical solution methods, such as spin-coating, sputtering, and inject printing, in their low resistivity due to acceptor impurities of sulfur and oxygen introduced into CuI from aqueous precursor solutions during SILAR deposition. Energy barriers at the boundaries of 18–22 nm CuI nanograins and a large number of charge carriers inside the nanograins determine the transport properties in the temperature interval 295–340 K characterized by transitions from semiconductor to metallic behavior with increasing temperature, which are typical of nanostructured degenerate semiconductors. Due to the resistivity of about 0.8 mΩ· m at 310 K and the Seebeck coefficient 101 μV/K, the thermoelectric power factor of the CuI film 1.0 μm thick on the polyimide substrate is 12.3 μW/(m · K2), which corresponds to modern thin-film p-type thermoelectric materials. It confirms the suitability of CuI films obtained by the SILAR method for the fabrication of promising inexpensive non-toxic flexible thermoelectric materials.

Space-Confined Growth of Ultrathin P-Type GeTe Nanosheets for Broadband Photodetectors.

As p-type phase-change degenerate semiconductors, crystalline and amorphous germanium telluride (GeTe) exhibit metallic and semiconducting properties, respectively. However, the massive structural defects and strong interface scattering in amorphous GeTe films significantly reduce their performance. In this work, two-dimensional (2D) p-type GeTe nanosheets are synthesized via a specially designed space-confined chemical vapor deposition (CVD) method, with the thickness of the GeTe nanosheets reduced to 1.9nm. The space-confined CVD method improves the crystallinity of ultrathin GeTe by lowering the partial pressure of the reactant gas, resulting in GeTe nanosheets with excellent p-type semiconductor properties, such as a satisfactory on/off ratio of 105. Temperature-dependent electrical measurements demonstrate that variable-range hopping and optical-phonon-assisted hopping mechanisms dominate transport behavior at low and high temperatures, respectively. GeTe devices exhibit significantly high responsivity (6589 and 2.2 A W-1 at 633 and 980nm, respectively) and detectivity (1.67 × 1011 and 1.3 × 108 Jones at 633 and 980nm, respectively), making them feasible for broadband photodetectors in the visible to near-infrared range. Furthermore, the fabricated GeTe/WS2 diode exhibits a rectification ratio of 103 at zero gate voltage. These satisfactory p-type semiconductor properties demonstrate that ultrathin GeTe exhibits enormous potential for applications in optoelectronic interconnection circuits.

Temperature evolution of transverse magnetoresistance due to forming the topological insulator state in single-crystalline n-type Bi2Te2.7Se0.3

Specific features in magnetotransport properties due to gradual forming the topological insulator state in sample of single-crystalline n-type Bi2Te2.7Se0.3 during its cooling were analyzed. The electrical resistivity of sample, measured from 2 K to 240 K, corresponds to partially degenerate semiconductor and dominantly depends on T-effect on electron mobility. The moblity is governed by electron–phonon scattering above T C = 50 K, whereas below T C electron–electron scattering is dominant scattering mechanism. With increasing temperature, electron content linearly increases above T C , whereas below T C electron content is very weakly T-dependent. Transverse magnetoresistance of sample is positive and strongly T-dependent. Two features, which are characteristic for topological insulators, were found in the magnetoresistance. First feature is a crossover from quadratic to linear magnetoresistance, observed within T C < T < 240 K range. Crossover field B C decreases with decreasing temperature. Linear magnetoresistace is quantum one that can be due to presence of Dirac fermions, which occupy the lowest Landau level under magnetic field. Second feature is another crossover from combined quadratic-linear to dip-shaped magnetoresistrance, observed at T ≤ T C . Dip-shaped magnetoresistrance is related to weak antilocalization (WAL) phenomenon. The WAL phenomenon and the electron–electron scattering process coexist at the same temperature range. Dip-shaped magnetoresistrance was analysed by in frames of the Hikami-Larkin-Nagaoka model, developed for systems with strong spin–orbit coupling. At cooling below ∼ 30 K, the effective dephasing length rapidly increases that is dominantly related to the electron–electron scattering process, too. The parameter α, characterizing the number of conduction channels, contributing to electron transport, is close to 0.5. This value α corresponds to a single topologically non-trivial conduction channel.

Fermi-edge singularity and related emission from degenerate semiconductors: Transition from a spontaneous to a stimulated process

Fermi-edge singularity and related emission from degenerate semiconductors: Transition from a spontaneous to a stimulated process

Synthesis of Nb‐Doped‐WS2–NbS2: A Degenerate‐Semiconductor–Metal Heterostructure

AbstractThe synthesis of semiconductor–metal (S–M) heterostructures comprising semiconducting (S)‐transition metal dichalcogenides (TMDs) and metallic (M)‐TMDs via chemical vapor deposition (CVD) has emerged as a highly promising avenue for achieving low contact resistance in TMD‐based devices. However, CVD‐grown S–M heterostructures are mainly focused on non‐degenerate semiconductors, despite degenerate semiconductors also being essential for semiconductor technology, such as negative differential resistance (NDR) device. In this study, a degenerate‐S–M heterostructure, Nb‐doped‐WS2–NbS2, is synthesized via CVD with a liquid‐metal precursor. Optimizing the growth parameters, such as growth temperature, precursor ratio, and H2 content in the mixture gas, affords the desirable degenerate‐Nb‐doped‐WS2–NbS2 heterostructure. Raman and photoluminescence spectroscopies, transmission electron microscopy, and energy‐dispersive spectroscopy clearly clarify the doping signal and layer structure of the heterostructure. A growth mechanism has been proposed using in‐plane and vertical models based on structural analysis. Electrical transport measurements reveal degenerate p‐type behavior in the Nb‐doped‐WS2. In the Nb‐doped‐WS2–NbS2 degenerate‐S–M heterostructure, the device flows about twice as much on‐state current as that by an Nb‐doped‐WS2/Cr contact.

First principle investigation on electronic and thermoelectric properties of CoTiP half-Heusler with Co, Ti, and P vacancies

This study focused on investigating the effect of vacancy-site defects on the thermoelectric properties of the CoTiP half-Heusler (HH) compound. While analyzing the electronic properties of the compounds, it was observed that the material changed from an intrinsic semiconductor with a direct band gap to p-type non degenerate semiconductor in case of Co vacancy, while a compensate semiconductors is obtained in the case of Ti and P vacancies. The figure of merite ZT values are found to be 0.90, 0.94, and 0.95 at 300 K for Co, Ti, and P vacancy-sites, respectively, with a power factors of 7.14 × 10−3 W/m s K2, 8.00 × 10−3 W/m s K2, and 8.79 × 10−3 W/m s K2 at 900 K. This study concludes that vacancies have the potential to enhance the thermoelectric properties of CoTiP HH.

A deep dive into structural, electronic, optical, and mechanical properties of ATiO3 (A = Ba, Th): DFT insights

Different physical characteristics (structural, optical, electronic, and mechanical) of ThTiO3 were explored using DFT and compared to BaTiO3. ThTiO3 has been determined to be mechanically and thermodynamically stable based on the simulation results, which were validated using the Born stability criteria and formation energy. Furthermore, a significant modification in the traits of ThTiO3 has been revealed compared to BaTiO3. For example, after the complete substitution of Ba by Th, in the case of GGA-PBE, the band gap increases from 1.82 eV to 3.37 eV, while in the case of HSE-06, it increases from 3.254 eV to 4.21 eV, also converting from indirect to direct bandgap. Not only that, but ThTiO3 has become an n-type degenerate semiconductor from a conventional semiconductor, which assures potential applications in tunnel diodes, high-frequency transistors, photocatalysts, etc. ThTiO3 is an appropriate material for capacitors, optoelectronic, and high k nanoelectronics devices based on the high dielectric constant value, which is higher than BaTiO3. Moreover, with Th substitution, BaTiO3 transitioned from brittle to ductile, which ensures its suitability for industrial machining processes. Furthermore, this substitution also improved the material’s anisotropic behavior, as the Zener Anisotropy value for ThTiO3 is also higher than BaTiO3. We believe this investigation will open another door in the field of materials for microelectronics and optoelectronics enthusiasts.

Impact of Nd Doping on Electronic, Optical, and Magnetic Properties of ZnO: A GGA + U Study

The electronic, optical, and magnetic properties of Nd-doped ZnO systems were calculated using the DFT/GGA + U method. According to the results, the Nd dopant causes lattice parameter expansion, negative formation energy, and bandgap narrowing, resulting in the formation of an N-type degenerate semiconductor. Overlapping of the generated impurity and Fermi levels results in a significant trap effect that prevents electron-hole recombination. The absorption spectrum demonstrates a redshift in the visible region, and the intensity increased, leading to enhanced photocatalytic performance. The Nd-doped ZnO system displays ferromagnetic, with FM coupling due to strong spd-f hybridization through magnetic exchange interaction between the Nd-4f state and O-2p, Zn-4s, and Zn-3p states. These findings imply that Nd-doped ZnO may be a promising material for DMS spintronic devices.

N-Type Thermoelectric AgBiPbS3 with Nanoprecipitates and Low Thermal Conductivity.

Thermoelectric sulfide materials are of particular interest due to the earth-abundant and cost-effective nature of sulfur. Here, we report a new n-type degenerate semiconductor sulfide, AgBiPbS3, which adopts a Fm3̅m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag2S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS3 exhibits a low room-temperature lattice thermal conductivity of 0.88 W m-1 K-1, owing to the intrinsic low lattice thermal conductivity of Ag2S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS2, AgBiPbS3 demonstrates a significantly improved weighted mobility of >16 cm2 V-1 s-1 at 300 K, leading to an enhanced PF of 1.6 μW cm-1 K-2 at 300 K. The superior electrical transport in AgBiPbS3 can be attributed to the high valley degeneracy of the L point (the conduction band minimum), which is contributed by the Pb s and Pb p orbitals. Further, Ga doping is found to be effective in modulating the Fermi levels of AgBiPbS3, leading to further enhancement of PF with a PFave of 2.7 μW cm-1 K-2 in the temperature range of 300-823 K. Consequently, a relatively high ZTave of 0.22 and a peak ZT of ∼0.4 at 823 K have been achieved in 3% Ga-doped AgBiPbS3, highlighting the potential of AgBiPbS3 as an n-type thermoelectric sulfide.

Tightly bound and room-temperature-stable excitons in van der Waals degenerate-semiconductor Bi4O4SeCl2 with high charge-carrier density

Excitons, which represent a type of quasi-particles consisting of electron-hole pairs bound by the mutual Coulomb interaction, are often observed in lowly-doped semiconductors or insulators. However, realizing excitons in semiconductors or insulators with high charge-carrier densities is challenging. Here, we perform infrared spectroscopy, electrical transport, ab initio calculations, and angle-resolved-photoemission spectroscopy study of the van der Waals degenerate-semiconductor Bi4O4SeCl2. A peak-like feature (α peak) is present around ~125 meV in the optical conductivity spectra at low temperature T = 8 K and room temperature. After being excluded from the optical excitations of free carriers, interband transitions, localized states and polarons, the α peak is assigned as an exciton absorption. Assuming the existence of weakly-bound Wannier-type excitons in this material violates the Lyddane-Sachs-Teller relation. Moreover, the exciton binding energy of ~375 meV, which is about an order of magnitude larger than those of conventional semiconductors, and the charge-carrier concentration of ~1.25 × 1019 cm−3, which is higher than the Mott density, further indicate that the excitons in this highly-doped system should be tightly bound. Our results pave the way for developing optoelectronic devices based on tightly bound and room-temperature-stable excitons in highly-doped van der Waals degenerate semiconductors.

First-principles study of the electronic and optical properties of GeSe/SnSe vertical heterojunction via P-doping

Recent huge progress in the field of chemical doping for modifying the physical properties of nano-materials has inspired us to perform this investigation on the electronic and optical properties of GeSe/SnSe vertical heterojunction via P-doping. The calculated results indicate that the pristine GeSe/SnSe vertical heterojunction is a semiconductor and shows a type-II band alignment. After P-doping, all doped heterojunctions have good structural stability due to the negative binding energies. Interestingly, the typical p-type and n-type degenerate semiconductors can be found. In addition, the optical properties including the absorption coefficient, reflectivity and loss function of P-doped GeSe/SnSe vertical heterojunction are discussed. Particularly, in the ultraviolet light range, the optical absorption coefficient of P-doped systems can reach up to [Formula: see text][Formula: see text]cm[Formula: see text], which suggests that P-doping GeSe/SnSe vertical heterojunctions can serve as a promising material for ultraviolet light detecting. Meanwhile, the absorption and the loss peaks of doped cases exhibit an apparent trend of blue shift with respect to the pristine heterojunction. All these interesting results show that the P-doping gives GeSe/SnSe heterojunction the possibility to tune its electronic and optical properties.

Impedance Analysis of Semiconductor Electrodes Biased in the Accumulation Regime for Fuels Evolution

This work explores the theory and experimental use of impedance analyses of semiconductor electrodes in the accumulation regime of semiconductor electrodes under bias. It contrasts with the established practice of measuring capacitance of the semiconductor-electrolyte biased in the depletion region to obtain semiconductor doping densities and flatband potentials (Ufb). The goal is to establish Ufb and the solution Helmholtz capacitance for an electrode in the working bias region where research is done on semiconductor electrodes for CO2 reduction and H2 evolution.The technique of measuring the capacitance of semiconductor electrodes when polarized in the depletion region was introduced by DeWald1 and is based upon the work of Garrett and Brattain2 where mathematical expressions were derived for the electric field of a semiconductor space charge region. A significant assumption in that work was that a Boltzmann expression could be used to determine the population of the majority charge carriers in the bands created through ionization of dopant. Research in this area is extensive and has been summarized.3 However, for degenerate semiconductors or for bias potentials of the semiconductor where the fermi level at the electrode surface equals the energy of the band edges, a full fermi function must be employed to describe population statistics in the solid. This is the condition commonly found in systems for fuels production at semiconductor electrodes. The solid state physics theory for this situation is known4 and describes the electric field in a semiconductor space charge region. However, it was put aside during the development of impedance evaluations in semiconductor electrochemistry because its mathematical complexity was too tedious to evaluate at that time. Only limits and simplifications were explored.5 With modern computational software, this theory has become straightforward to calculate and it becomes possible to apply it to experimental systems in a description of behavior over the entire range of doping and polarization. The nature of this full theoretical description will be discussed and illustrated through experimental measurements of Si and InP semiconductor electrodes.