Degenerate Semiconductors Research Articles

Electron Scattering from Disordered Grain Boundaries in Degenerate Polycrystalline Al‐Doped ZnO Thin Films

This paper describes the influence of electron scattering from disordered grain boundaries on the carrier transport in polycrystalline Al‐doped ZnO (AZO) thin films prepared with electron concentrations on the order of 1020 cm−3. It is found that in degenerate AZO thin films prepared on glass substrates by various magnetron sputtering depositions, the mobility–carrier concentration (µHall–nHall) relationship with a positive slope (µHall increases as nHall increases) is always obtained when measured by changing the sputter gas pressure and the substrate temperature, the location on the substrate surface, and the exposure time in a moisture‐resistance test. The main scattering mechanism, which limits the carrier transport in AZO thin films, is attributed to grain boundary scattering caused by the reflection of electrons from the potential barrier at the grain boundary between crystallites. The measured µHall–nHall relationship demonstrates fair agreement with the relationship calculated using the Mayadas and Shatzkes (MS) theory. However, conceptual and other difficulties to apply the semi‐classical MS theory to degenerate semiconductors must be considered. The results of this quantum theory‐based investigation show that the positively sloped µHall–nHall relationship attributes to Anderson localization, induced by electron grain boundary scattering from disordered successive grains.

Recovering the Semiconductor Properties of the Epitaxial Group V 2D Materials Antimonene and Arsenene

Antimonene (2D-Sb) is attracting considerable attention because of its environmental stability and exceptional electronic and optical properties. Recently, 2D-Sb was grown on germanium (Ge) substrates, thus laying the groundwork for the integration of 2D-Sb-based devices in standard semiconductor processing. However, the relatively strong Ge–Sb interactions were found to suppress the semiconducting properties expected for monolayer 2D-Sb. To overcome this limitation, this work demonstrates that Ge passivation prior to epitaxy allows tuning the electronic properties of 2D-Sb. Ab initio calculations indicate that hydrogen and methyl passivation yields semiconducting epitaxial 2D-Sb, whereas halogen passivation yields degenerate semiconductors. A similar behavior is observed for 2D-As and 2D-AsSb on silicon. Finally, using molecular beam epitaxy combined with in situ low-energy electron microscopy, we demonstrate that the growth of 2D-Sb can be achieved on passivated Ge. These results will stimulate the deve...

A new approximation of Fermi-Dirac integrals of order 1/2 for degenerate semiconductor devices

There had been tremendous growth in the field of Integrated circuits (ICs) in the past fifty years. Scaling laws mandated both lateral and vertical dimensions to be reduced and a steady increase in doping densities. Most of the modern semiconductor devices have invariably heavily doped regions where Fermi-Dirac Integrals are required. Several attempts have been devoted to developing analytical approximations for Fermi-Dirac Integrals since numerical computations of Fermi-Dirac Integrals are difficult to use in semiconductor devices, although there are several highly accurate tabulated functions available. Most of these analytical expressions are not sufficiently suitable to be employed in semiconductor device applications due to their poor accuracy, the requirement of complicated calculations, and difficulties in differentiating and integrating. A new approximation has been developed for the Fermi-Dirac integrals of the order 1/2 by using Prony's method and discussed in this paper. The approximation is accurate enough (Mean Absolute Error (MAE) = 0.38%) and easy enough to be used in semiconductor device equations. The new approximation of Fermi-Dirac Integrals is applied to a more generalized Einstein Relation which is an important relation in semiconductor devices.

Investigation of Tellurite Based Fire-Through Dielectric (FTD) Ag Paste Contacts for Si Solar Cell with High Sheet Resistance Emitter

This paper reports the importance of PbO/TeO2 ratio in the formation of Ag2Te, which is responsible for increasing the reformed glass conductivity after sintering. The presence of Ag2Te, a degenerate semiconductor, seen in the Raman Spectra, after HNO3 etching of the gridline and its absence in the HF etched counterpart, supports this assertion. Thus, the increase glass conductivity after sintering has the potential of enabling formation of high quality contacts to emitter with high sheet resistance.

Stability enhancement of Cu2S against Cu vacancy formation by Ag alloying

As a potential solar absorber material, Cu2S has proved its importance in the field of renewable energy. However, almost all the known minerals of Cu2S suffer from spontaneous Cu vacancy formation in the structure. The Cu vacancy formation causes the structure to possess very high p-type doping that leads the material to behave as a degenerate semiconductor. This vacancy formation tendency is a major obstacle for this material in this regard. A relatively new predicted phase of Cu2S which has an acanthite-like structure was found to be preferable than the well-known low chalcocite Cu2S. However, the Cu-vacancy formation tendency in this phase remained similar. We have found that alloying silver with this structure can help to reduce Cu vacancy formation tendency without altering its electronic property. The band gap of silver alloyed structure is higher than pristine acanthite Cu2S. In addition, Cu diffusion in the structure can be reduced with Ag doped in Cu sites. In this study, a systematic approach is presented within the density functional theory framework to study Cu vacancy formation tendency and diffusion in silver alloyed acanthite Cu2S, and proposed a possible route to stabilize Cu2S against Cu vacancy formations by alloying it with Ag.

Impact of Silicon Doping on the Properties of Transparent Conducting CdO Thin Films

CdO thin films doped with different amounts of silicon ions have been deposited on glass substrates by the vacuum evaporation method in order to improve their properties for application as transparent conducting oxide (TCO) films. The structural, electrical, and optical properties of the host CdO films were systematically studied. The realization of doping with silicon ions was confirmed and studied by the X-ray diffraction (XRD) method. It was concluded that Si ions occupied locations in interstitial positions and structural vacancies of the CdO lattice. The bandgap of Si-doped CdO was blue shifted following the Moss-Burstein (B-M) effect. The dc-electrical behaviors were studied by the Van der Pauw method showing that the prepared Si-doped CdO films were degenerate semiconductors of controllable opto-electrical properties by doping level. The utmost improvements in mobility (μ) and conductivity (σ) took place with the host CdO film doped with ∼2% Si, so that the mobility increased by ∼10 times and the conductivity by ∼100 times compared to the pristine CdO film. The results show that silicon is effective for CdO utilization in applications in the TCO field.

Unusually large chemical potential shift in a degenerate semiconductor: Angle-resolved photoemission study of SnSe and Na-doped SnSe

We have studied the electronic structure of SnSe and Na-doped SnSe by means of angle-resolved photoemission spectroscopy. The valence-band top reaches the Fermi level by the Na doping, indicating that Na-doped SnSe can be viewed as a degenerate semiconductor. However, in the Na-doped system, the chemical potential shift with temperature is unexpectedly large and is apparently inconsistent with the degenerate semiconductor picture. The large chemical potential shift and anomalous spectral shape are key ingredients for an understanding of the novel metallic state with the large thermoelectric performance in Na-doped SnSe.

Fabrication of CdS/CdTe Solar Cells by Quasiclosed Space Technology and Research of Their Properties

A quasiclosed space technology has been developed for the deposition of CdS and CdTe layers, while fabricating solar cells (SCs). Technological factors affecting the crystal lattice structure, the optical band gap width, and the conductivity in the CdS and CdTe layers are studied and analyzed. A technology to produce an ohmic contact with p-CdTe, by using the degenerate CuxS semiconductor, is proposed. The characteristics of SCs fabricated on substrates covered with various conducting films (Mo, ZnO, ZnO:Al) are analyzed. The measurement results of light and dark voltage-current characteristics testify to the better characteristics of ZnO and ZnO:Al films obtained by the atomic layer deposition from the viewpoint of their application in SCs. The optimum thicknesses of the CdS (67 nm), CdTe (about 1 /um), and CuxS (30 nm) layers, at which the best SC efficiency (n = 1.75÷1.89%) is obtained, are determined. The application of thin films in SC structures is shown to improve the characteristics of the latter.

Many-body theory of effective mass in degenerate semiconductors

We derive the many-body theory of the effective mass in the effective mass representation (EMR). In the EMR, we need to solve the equation of motion of an electron in the presence of electron–electron interactions, where the wavefunction is expanded over a complete set of Luttinger–Kohn wavefunctions. We use the Luttinger–Ward thermodynamic potential and the Green’s function perturbation to derive an expression for the band effective mass by taking into account the electron–electron interactions. Both quasi-particle and the correlation contributions are considered. We show that had we considered only the quasi-particle contribution, we would have missed important cancellations. Thus the correlated motion of electrons has important effects in the renormalization of the effective mass. Considering the exchange self-energy in the band model, we derive a tractable expression for the band effective mass. We apply the theory to n-type degenerate semiconductors, PbTe and SnTe, and analyze the impact of the theory on the anisotropic effective mass of the conduction bands in these systems.

Synthesis and transport properties of p-type lead-free AgSnmSbSe2Tem thermoelectric systems

We report the synthesis, characterization and thermoelectric properties of lead-free AgSnmSbSe2Tem (m = 2 and 10) systems. Powder X-ray diffraction patterns and Rietveld refinement results were consistent with phases belonging to the Pm3¯m space group. The microstructures and morphologies of these systems were investigated using scanning electron microcopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Parallelepiped bars for transport measurements were prepared using two methods: the classical method (CM) from melted samples and the spark plasma sintering (SPS). The AgSnmSbSe2Tem (m = 2 and 10) systems exhibited typical degenerate semiconductor behavior, with a carrier concentration of approximately +1021 cm−3. We determined that the Seebeck coefficient can be substantially increased from approximately +40 μV K−1 (CM) to +70 μV K−1 (SPS) in AgSn2SbSe2Te2 at room temperature. Consequently, the power factor (S2σ) was ∼22 μW cm−1 K−2. On the basis of the electrical and thermal transport properties, ZT values of ∼0.10 were obtained at room temperature.

Selective synthesis of higher manganese silicides: a new Mn17Si30 phase, its electronic, transport, and optical properties in comparison with Mn4Si7

The electronic structure, transport and optical properties of thin films of Mn4Si7 and Mn17Si30 higher manganese silicides (HMS) with the Nowotny “chimney-ladder” crystal structure are investigated using different experimental techniques and density functional theory calculations. Formation of new Mn17Si30 compound through selective solid-state reaction synthesis proposed and its crystal structure is reported for the first time, the latter belonging to I-42d. Absorption measurements show that both materials demonstrate direct interband transitions around 0.9 eV, while the lowest indirect transitions are observed close to 0.4 eV. According to ab initio calculations, ideally structured Mn17Si30 is a degenerate n-type semiconductor; however, the Hall measurements on the both investigated materials reveal their p-type conductivity and degenerate nature. Such a shift of the Fermi level is attributed to introduction of silicon vacancies in accordance with our DFT calculations and optical characteristics in low photon energy range (0.076–0.4 eV). The Hall mobility for Mn17Si30 thin film was found to be 25 cm2/V s at T = 77 K, being the highest among all HMS known before. X-ray photoelectron spectroscopy discloses a presence of plasmon satellites in the Mn4Si7 and Mn17Si30 valence band spectra. Experimental permittivity spectra for the Mn4Si7 and Mn17Si30 compounds in a wide range (0.076–6.54 eV) also indicate degenerate nature of both materials and put more emphasis upon the intrinsic relationship between lattice defects and optical properties.

Thermoelectric Properties of Cu-doped Bi2−xSbxTe3 Prepared by Encapsulated Melting and Hot Pressing

P-type Bi2−xSbxTe3:Cum (x = 1.5–1.7 and m = 0.002–0.003) solid solutions were synthesized using encapsulated melting and were consolidated using hot pressing. The effects of Sb substitution and Cu doping on the charge transport and thermoelectric properties were examined. The lattice constants decreased with increasing Sb and Cu contents. As the amount of Sb substitution and Cu doping was increased, the electrical conductivity increased, and the Seebeck coefficient decreased owing to the increase in the carrier concentration. All specimens exhibited degenerate semiconductor characteristics and positive Hall and Seebeck coefficients, indicating p-type conduction. The increased Sb substitution caused a shift in the onset temperature of the intrinsic transition and bipolar conduction to higher temperatures. The electronic thermal conductivity increased with increasing Sb and Cu contents owing to the increase in the carrier concentration, while the lattice thermal conductivity slightly decreased due to alloy scattering. A maximum figure of merit, ZTmax = 1.25, was achieved at 373 K for Bi0.4Sb1.6Te3:Cu0.003.

Гетероструктура для обращенного диода на основе электроосажденного в импульсном режиме наномассива оксида цинка и изготовленной методом SILAR пленки иодида меди

AbstractA heterostructure promising for designing a backward diode is formed from a zinc-oxide nanorod array and a nanostructured copper-iodide film. The effect of modes of successive ionic layer adsorption and reaction (SILAR) deposition and the subsequent iodization of CuI films on smooth glass, mica, and fluorine-doped tin oxide (FTO) substrates and on the surface of electrodeposited nanostructured zinc-oxide arrays on the film structure and electrical and optical properties is investigated. A connection between the observed variations in the structure and properties of this material and intrinsic and iodination-induced point defects is established. It is found that the cause and condition for creating a backward-diode heterostructure based on a zinc-oxide nanoarray formed by pulsed electrodeposition and a copper-iodide film grown by the SILAR method is the formation of a p ^+-CuI degenerate semiconductor by the excessive iodination of layers of this nanostructured material through its developed surface. The n -ZnO/ p ^+-CuI barrier heterostructure, which is fabricated for the first time, has the I – V characteristic of a backward diode, the curvature factor of which (γ = 12 V^–1) confirms its high Q factor.

Synergistic optimization of carrier transport and thermal conductivity in Sn-doped Cu2Te

Doping Sn into the Cu2Te lattice can synergistically enhance the power factor and decrease thermal conductivity, leading to remarkably optimized zTs. The lone pair electrons from the 5s orbital of Sn can increase the DOS near the Fermi level of Cu2Te to promote PF and reduce κe by decreasing the carrier concentration. This study explores a scalable strategy to optimize the thermoelectric performance for intrinsically highly degenerate semiconductors.

Conductive Tungsten Oxide Nanosheets for Highly Efficient Hydrogen Evolution.

Exploring efficient and economical electrocatalysts for hydrogen evolution reaction is of great significance for water splitting on an industrial scale. Tungsten oxide, WO3, has been long expected to be a promising non-precious-metal electrocatalyst for hydrogen production. However, the poor intrinsic activity of this material hampers its development. Herein, we design a highly efficient hydrogen evolution electrocatalyst via introducing oxygen vacancies into WO3 nanosheets. Our first-principles calculations demonstrate that the gap states introduced by O vacancies make WO3 act as a degenerate semiconductor with high conductivity and desirable hydrogen adsorption free energy. Experimentally, we prepared WO3 nanosheets rich in oxygen vacancies via a liquid exfoliation, which indeed exhibits the typical character of a degenerate semiconductor. When evaluated by hydrogen evolution, the nanosheets display superior performance with a small overpotential of 38 mV at 10 mA cm-2 and a low Tafel slope of 38 mV dec-1. This work opens an effective route to develop conductive tungsten oxide as a potential alternative to the state-of-the-art platinum for hydrogen evolution.

A first-principles study of impurity effects on monolayer MoS2: bandgap dominated by donor impurities

Based on the first principles calculation, the electrical properties and optical properties of monolayer molybdenum disulfide (MoS2) substitutionally doped by the VB and VIIB transition metal atoms (V, Nb, Ta, Mn, Tc, Re) were investigated. It is found that n-type doping or p-type doping tunes the Fermi level into the conduction band or the valence band respectively, leading to the degenerate semiconductor, while the compensatorily doped systems where the number of valence electrons is not alerted remain direct band gap ranging from 0.958 eV to 1.414 eV. According to the analysis on densities of states, the LUMO orbitals of donor impurities play the crucial role in band gap tuning. Hence, the band gap and optical properties of doped MoS2 are dominated by the species of the donor. Due to the reduction of the band gap, doped MoS2 have a lower threshold energy of photon absorption and an enhanced absorption in near infrared region. These results provide a significant guidance for the design of new 2D optoelectronic materials based on transition metal disulfide.

The Electronic Properties of Double and Single Configurations Aurum Adsorbed Armchair Graphene Nanoribbons

The electronic properties of H-terminated armchair graphene nanoribbons (AGNRs), which can be adsorbed with double and single Aurum atoms, are investigated using the local density approximation based on density functional theory. The results indicate that double Aurum adsorbed AGNRs are a direct band gap semiconductor with an energy gap of 0.888 eV due to involving hybridization and charge transfer among C-2p and Au-6s, 6p, 5d electrons near the Fermi level. The H-terminated AGNRs for single Au-adsorbing is an indirect band gap semiconductor with energy gap of 0.543 eV due to hybridization among C-2p and Au-6s, 6p electrons and electrical conductivity of Aurum atoms. The Fermi level is located on conduction band so that the H-terminated AGNRs of single Au-adsorbing become into degenerate semiconductor. The charge density is located around Au atoms. Their band gap decrease with adsorbing Aurum atoms.

Effects of Lu and Tm Doping on Thermoelectric Properties of Bi2Te3 Compound

The Bi2Te3, Bi1.9Lu0.1Te3 and Bi1.9Tm0.1Te3 thermoelectrics of n-type conductivity have been prepared by the microwave-solvothermal method and spark plasma sintering. These compounds behave as degenerate semiconductors from room temperature up to temperature T d ≈ 470 K. Within this temperature range the temperature behavior of the specific electrical resistivity is due to the temperature changes of electron mobility determined by acoustic and optical phonon scattering. Above T d, an onset of intrinsic conductivity takes place when electrons and holes are present. At the Lu and Tm doping, the Seebeck coefficient increases, while the specific electrical resistivity and total thermal conductivity decrease within the temperature 290–630 K range. The increase of the electrical resistivity is related to the increase of electron concentration since the Tm and Lu atoms are donor centres in the Bi2Te3 lattice. The increase of the density-of-state effective mass for conduction band can be responsible for the increase of the Seebeck coefficient. The decrease of the total thermal conductivity in doped Bi2Te3 is attributed to point defects like the antisite defects and Lu or Tm atoms substituting for the Bi sites. In addition, reducing the electron thermal conductivity due to forming a narrow impurity (Lu or Tm) band having high and sharp density-of-states near the Fermi level can effectively decrease the total thermal conductivity. The thermoelectric figure-of-merit is enhanced from ∼ 0.4 for undoped Bi2Te3 up to ∼ 0.7 for Bi1.9Tm0.1Te3 and ∼ 0.9 for Bi1.9Lu0.1Te3.

Synthesis and Thermoelectric Properties of Partially Double-Filled (Ce1-z Pr z ) y Fe4-x Co x Sb12 Skutterudites

Partially double-filled p-type (Ce1−z Pr z ) y Fe4−x Co x Sb12 (z = 0.25, 0.75; y = 0.8; x = 0, 0.5, 1.0) skutterudites were synthesized by encapsulated melting and consolidated by hot pressing. The microstructure, phase, charge transport characteristics, and thermoelectric properties of the hot-pressed specimens were analyzed. Detailed measurements indicated that the skutterudite phase was successfully synthesized, but a small amount of a secondary phase (FeSb2) was also identified. However, the amount of the FeSb2 phase decreased with an increase in the Co substitution. Unlike for the filled Ce1−z Pr z Fe4−x Co x Sb12 skutterudites with y = 1, the (Ce,Pr)Sb2 phases were not formed by partial filling with Ce/Pr. The electrical conductivity decreased with increasing temperature, similar to the behavior shown by degenerate semiconductors. The Hall coefficient and the Seebeck coefficients were positive, indicating that all specimens exhibited p-type characteristics. The electrical conductivity and the electronic thermal conductivity decreased with increasing Pr filling and Co substitution because of the decreased carrier concentration caused by charge compensation. A maximum dimensionless figure of merit, ZTmax = 0.84, was obtained at 623 K for (Ce0.75Pr0.25)0.8Fe3CoSb12.

Conduction-band effective mass and bandgap of ZnSnN2 earth-abundant solar absorber

Pseudo III-V nitride ZnSnN2 is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN2 degenerately doped with oxygen (ZnSnN2−xOx) in the ultraviolet to infrared region to determine the conduction-band effective mass (mc*) and intrinsic bandgap (Eg). ZnSnN2−xOx epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, mc* was determined to be (0.37 ± 0.05)m0 (m0 denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the Eg value of 0.94 ± 0.02 eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.