Fermi Level Position Research Articles

First-Principles Disordered Local-Moment Study on Temperature Dependence of Spin Polarization in Co <sub>2</sub>Fe(Ga <sub>0.5</sub>Ge <sub>0.5</sub>) Heusler Alloy

Magnetoresistance (MR) devices fabricated with half-metallic Co-based Heusler alloys (Co2YZ) have a large MR ratio but are subject to substantial temperature degradation. Reduction of the spin polarization in the bulk electrode at finite temperatures is one possible reason for the reduction in the MR ratio. In this study, we investigated the temperature dependence of the spin polarization of Co2Fe(Ga0.5Ge0.5) (CFGG) using density-functional theory and the disordered local-moment method. We found that the reduction in the spin polarization at finite temperatures is smaller in CFGG compared to the well known Co2MnSi (CMS) half-metal, which is attributed to the higher Curie temperature of CFGG than CMS. On the other hand, it was also found that modulation of the Fermi level position in the half-metallic gap of CFGG within the rigid band model by electron and hole doping can improve the temperature dependence of the spin polarization. However, off-stoichiometric Co-rich and Fe-rich CFGG corresponding to electron and hole doping show a reduction in spin polarization compared to that of stoichiometric CFGG at 0 K. Therefore, we propose that modulation of the Fermi level position through varying the Y and Z site compositions of Co2YZ will be necessary to improve the temperature dependence of the spin polarization.

Validity of the effective injection barrier in organic field-effect transistors

The injection barrier is a key parameter that governs the charge and current density in organic field-effect transistors (OFETs). However, the energetic disorder of organic disordered semiconductors (ODSs) hinders a clear definition of the injection barrier. We study the validity of effective injection barrier at the metal/organic disordered semiconductor by means of systematic numerical simulation of OFETs in full consideration of the Gaussian density-of-states. The contact resistance from the numerical simulation and analytical model are compared to verify this procedure. By varying the Gaussian width, the effective injection barrier is validated for low degree of disorder at which the semiconductor is under non-degenerate condition. The position of Fermi level with respect to the Gaussian width distinguishes the non-degenerate and degenerate condition of ODSs, indicating clearly the range of validity of the effective injection barrier.

Engineering of Fermi Level by Construction of Hybrid Systems

Hybrid heterostrures provide a versatile and largely enhanced systems in comparison to their single components. However, construction of effective visible-light responding and stable hybrid heterojunction based photoelectrodes remains a challenge due to stricte requirements regarding band alignment in such constructions. Therefore implementation of a conductive mid-layer of reduced graphene oxide (RGO) might be a crucial factor in band structure engineering. Herein we demonstrate the formation of an outperforming hybrid heterojunction photocatalyst containing diversified synthetic approaches RGO and thus allowing to engineer the Fermi level position within the working architecture.Shifting the position of the Fermi level makes possible to increase the size of the depletion layer and PEC efficiency of the entire system increases.

Terahertz photoresistivity of a high-mobility 3D topological insulator based on a strained HgTe film

We report on a detailed study of terahertz (THz) photoresistivity in a strained HgTe three-dimensional topological insulator (3D TI) for all Fermi level positions: inside the conduction and valence bands and in the bulk gap. In the presence of a magnetic field, we detect a resonance corresponding to the cyclotron resonance (CR) in the top surface Dirac fermions (DFs) and examine the nontrivial dependence of the surface state cyclotron mass on the Fermi level position. We also detect additional resonant features at moderate electron densities and demonstrate that they are caused by the mixing of surface DFs and bulk electrons. At high electron densities, we observe THz radiation-induced 1/B-periodic low-field magneto-oscillations coupled to harmonics of the CR and demonstrate that they have a common origin with microwave-induced resistance oscillations previously observed in high mobility GaAs-based heterostructures. This observation attests the superior quality of a 2D electron system formed by helical surface states in strained HgTe films.

Tunable quantum Shubnikov-de Hass oscillations in antiferromagnetic topological semimetal Mn-doped Cd3As2

Three-dimensional Dirac semimetal Cd3As2 has been considered as an excellent candidate for applications of electronic devices owing to its ultrahigh mobility and air-stability. However, current researches are focused mainly on the use of gate-voltage to control its carrier transport tunability, while the manipulation of transport properties by element-doping is quite limited. Here we report the tunable magneto-transport properties by adjusting Mn-doping in the Cd3As2 compound. We find that Mn-element doping has a strong influence on the Fermi level positions, and the Fermi energy approaches to Dirac point with higher Mn-doping. More importantly, the introduction of Mn atoms transforms diamagnetic Cd3As2 to antiferromagnetic (Cd, Mn)3As2, which provides an approach to control topological protected Dirac materials by manipulating antiferromagnetic order parameters. The Shubnikov-de Hass oscillation originates from the surface states, and the Landau fan diagram yields a nontrivial Berry phase, indicating the existence of massless Dirac fermions in the (Cd1-xMnx)3As2 compounds. Our present results may pave a way for further investigating antiferromagnetic topological Dirac semimetal and expand the potential applications in optoelectronics and spintronics.

Highly Tunable Spin-Orbit Torque and Anisotropic Magnetoresistance in a Topological Insulator Thin Film Attached to Ferromagnetic Layer.

We investigate spin-charge conversion phenomena in hybrid structures of topological insulator thin films and magnetic insulators. We find an anisotropic inverse spin-galvanic effect that yields a highly tunable spin-orbit torque. Concentrating on the quasiballistic limit, we also predict a giant anisotropic magnetoresistance at low dopings. These effects, which have no counterparts in thick topological insulators, depend on the simultaneous presence of the hybridization between the surface states and the in-plane magnetization. Both the inverse spin-galvanic effect and anisotropic magnetoresistance exhibit a strong dependence on the magnetization and the Fermi level position and can be used for spintronics and spin-orbit-torque-based applications at the nanoscale.

First-principles theory of the Dirac semimetalCd3As2under Zeeman magnetic field

Time-reversal broken Weyl semimetals have attracted much attention recently, but certain aspects of their behavior, including the evolution of their Fermi surface topology and anomalous Hall conductivity with Fermi-level position, have remained underexplored. A promising route to obtain such materials may be to start with a nonmagnetic Dirac semimetal and break time-reversal symmetry via magnetic doping or magnetic proximity. Here we explore this scenario in the case of the Dirac semimetal Cd$_{3}$As$_{2}$, based on first-principles density-functional calculations and subsequent low-energy modeling of Cd$_{3}$As$_{2}$ in the presence of a Zeeman field applied along the symmetry axis. We clarify how each four$-$fold degenerate Dirac node splits into four Weyl nodes, two with chirality $\pm 1$ and two higher-order nodes with chirality $\pm 2$. Using a minimal \kdotp model Hamiltonian whose parameters are fit to the first-principles calculations, we detail the evolution of the Fermi surfaces and their Chern numbers as the Fermi energy is scanned across the region of the Weyl nodes at fixed Zeeman field. We also compute the intrinsic anomalous Hall conductivity as a function of Fermi-level position, finding a characteristic inverted-dome structure. Cd$_{3}$As$_{2}$ is especially well suited to such a study because of its high mobility, but the qualitative behavior revealed here should be applicable to other Dirac semimetals as well.

Signature of gate-controlled magnetism and localization effects at Bi2Se3/EuS interface

Proximity of a topological insulator (TI) surface with a magnetic insulator (MI) can open an exchange gap at the Dirac point leading to exploration of surface quantum anomalous Hall effect. An important requirement to observe the above effect is to prevent the topological breakdown of the surface states (SSs) due to various interface coupling effects and to tune the Fermi level at the interface near the Dirac point. In this work, we demonstrate the growth of high-quality c-axis oriented strain-free layered films of TI, Bi2Se3, on amorphous SiO2 substrate in proximity to an MI, europium sulfide (EuS), that show stronger weak anti-localization response from the surface than previous studies with epitaxially interfaced heterostructures. Importantly, we find gate and magnetic field cooling modulated localization effects in the SSs, attributed to the position of interface Fermi level within the band gap that is also corroborated from our positron annihilation spectroscopy measurements. Furthermore, our experiments provide a direct evidence of gate-controlled enhanced interface magnetism in EuS arising from the carrier mediated Ruderman–Kittel–Kasuya–Yosida interactions across the Bi2Se3/EuS interface. These findings demonstrate the existence of complex interfacial phenomena affecting the localization response of the SSs that might be important in proximity engineering of the TI surface to observe surface quantum Hall effects.

Hybridisation of ZnO with Ti 3 C 2 as a co‐catalyst for enhanced photocatalytic activity

In this study, a typical MXene material, Ti3C2, was selected as a co-catalyst and then integrated with ZnO via a facile hydrothermal strategy. The phase composition, morphology, and photophysical properties of as-prepared samples were investigated by XRD, field emission SEM, ultraviolet–visible spectrophotometer, and fluorescence spectrophotometer, respectively. In addition, the results of the photocatalysis experiment showed that the photocatalytic activity of ZnO can be improved significantly through the hybridisation with Ti3C2, which arises from the inhibition of the photogenerated carriers recombination. Furthermore, the theoretical analysis indicated that the high-quantum efficiency arises from the appropriate Fermi level position of Ti3C2. This work demonstrated that Ti3C2 will show great potential for constructing novel and efficient photocatalysts.

Formation and control of the center in implanted β-GaO by reverse-bias and zero-bias annealing

Deep-level transient spectroscopy measurements are conducted on β-GaO thin-films implanted with helium and hydrogen (H) to study the formation of the defect level ( = 0.71 eV) during heat treatments under an applied reverse-bias voltage (reverse-bias annealing). The formation of during reverse-bias annealing is a thermally-activated process exhibiting an activation energy of around 1.0 eV to 1.3 eV, and applying larger reverse-bias voltages during the heat treatment results in a larger concentration of . In contrast, heat treatments without an applied reverse-bias voltage (zero-bias annealing) can be used to decrease the concentration. The removal of is more pronounced if zero-bias anneals are performed in the presence of H. A scenario for the formation of is proposed, where the main effect of reverse-bias annealing is an effective change in the Fermi-level position within the space-charge region, and where is related to a defect complex involving intrinsic defects that exhibits several different configurations whose relative formation energies depend on the Fermi-level position. One of these configurations gives rise to , and is more likely to form if the Fermi-level position is further away from the conduction band edge. The defect complex related to can become hydrogenated, and the corresponding hydrogenated complex is likely to form when the Fermi level is close to the conduction band edge. Di-vacancy defects formed by oxygen and gallium vacancies (V−V) fulfill several of these requirements, and are proposed as potential candidates for .

How Photoinduced Gate Screening and Leakage Currents Dynamically Change the Fermi Level in 2D Materials

An in‐depth analysis of physics in 2D materials like transition metal dichalcogenides requires the measurement of many material properties as a function of Fermi level position within the electronic band structure. This is normally done by changing the charge carrier density of the 2D material via the gate electric field effect. Herein, a comparison of gate‐dependent measurements, which are acquired under different measurement conditions, is shown to encounter significant problems due to the temporal evolution of the charging of trap states inside the dielectric layer or at its interfaces. The impact of, e.g., the gate sweep direction and the sweep rate on the overall gate dependence gets especially prominent in optical measurements due to photoexcitation of donor and acceptor states. Under such conditions, the same nominal gate voltage may lead to different gate‐induced charge carrier densities and, hence, Fermi level positions. It is demonstrated that a current flow from or even through the dielectric layer via leakage currents can significantly diminish the gate tunability in optical measurements of 2D materials.

Unintended Deviation of Fermi Level from Band Edge in Fractal Silicon Nanostructures: Consequence of Dopants’ Zonal Depletion

The location of Fermi level in a semiconductor is of utmost importance, thus understanding its relative drift from the expected location contains a lot of technologically significant information. A fabrication process induced spatial deviation in the Fermi level position has been reported here in silicon quantum structures prepared using chemical etching. Experimental (direct) and theoretical Raman spectroscopy analyses (indirect) reveal low dopant concentration in the nanosilicon, which explains several reports on its low conductivity. An experimental dopant mass migration based hypothesis (similar to the zone melting process), duly validated using Raman spectroscopy, has been used to explain the observed phenomenon.

Fermi level pinning and hydrostatic pressure effect in electron irradiated GaSb

The effect of 2 MeV electron bombardment up to total electron dose of on the electrical and piezo resistive properties of n- and p-type GaSb crystals was studied. The electron irradiation leads to saturation of the hole concentration at about as a result of Fermi level pinning near the valence band top in all irradiated samples. With this in mind, the calculated data on the energy position of the charge neutrality level in GaSb were analyzed. The pressure coefficient of resistivity was measured as a function of Fermi level position in the electron irradiated samples. Isochronal annealing of radiation-induced defects was investigated in the temperature range of 20–400 °C.

Suppression of Persistent Photoconductivity of Rubrene Crystals using Gate-Tunable Rubrene/Bi2 Se3 Diodes with Photoinduced Negative Differential Resistance.

Organic single-crystalline semiconductors show great potential in high-performance photodetectors. However, they suffer from persistent photoconductivity (PPC) due to the charge trapping, which has severely hindered high-speed imaging applications. Here, a universal strategy of solving the PPC by integrating with topological insulator Bi2 Se3 is provided. The rubrene/Bi2 Se3 heterojunctions are selected as an example for general demonstration due to the reproducibly high mobility and broad optoelectronic applications of rubrene crystals. By virtue of high carrier concentration on Bi2 Se3 surface and the strong built-in electrical field, the photoresponse of the heterotransistor is significantly reduced for more than two orders (from over 10 s to 54ms), meanwhile the photoresponsivity can reach 124 A W-1 . To the best of knowledge, this operating speed is among the fastest responses in organic-inorganic heterojunctions. The heterotransistor also shows unique negative differential resistance under positive gate bias, which can be explained by photoinduced de-trapping of electron trap states in the bulk rubrene crystals. Besides, the rubrene/Bi2 Se3 heterojunction behaves as a gate-tunable backward-like diode due to the inhomogenous carrier distribution in the thick rubrene crystal and inversion of relative Fermi level positions. The findings demonstrate versatile functionalities of the rubrene/Bi2 Se3 heterojunctions for various emerging optoelectronic applications.

Enhancement of Stability by Applying HAT-CN for Hole Modification With Good Water Resistance and Hole Extraction

A creative application of using HAT-CN as a hole modification layer (HML) in positive perovskite solar cells (PSCs) is demonstrated herein. This measure successfully improves the performance of devices in many aspects. Although HAT-CN owns a similar position of Fermi level with MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , there have been proved to be a much smoother interface. The devices using HAT-CN had a higher V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> at 1.10 V over 1.06 V that was shown by devices using MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Optimized devices remained PCE over 18% for 188 h resulting from compressing recombination better for owning higher hydrophobicity. The measure that using HAT-CN for modification proposed in this work was proved that HAT-CN provides a better ability of extracting holes and owns better water-resistance to be more stable.

Double-atomic-layer Tl-Mg compound on a Si(111) surface with advanced electronic properties

We report on the formation and comprehensive study of atomic and electronic structures of a new Tl-Mg two-dimensional compound reconstruction with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ periodicity on a Si(111) substrate. Owing to properties of its constituent elements and crystal structure with a breathing kagome lattice of Mg and a dense Tl layer (4/3 monolayer), this double-layer system displays promising features, including spin-split metallic bands, nearly flat bands, and a saddle point with corresponding divergences in the local density of states. These features indicate the possibility that strongly correlated electronic states exist in this structure after tuning the Fermi level position. In accordance with the ${C}_{3}$ symmetry, we observe spin-orbit-coupling-driven Rashba-type spin splitting with spin degeneracy at the $\overline{M}$ point and Zeeman-like splitting at the $\overline{K}$ point. In contrast to the previously reported cases, splitting at the $\overline{K}$ point is significant to produce fully out-of-plane spin-polarized cones. Due to opposite Berry curvatures at the $\overline{K}$ and ${\overline{K}}^{\ensuremath{'}}$ points of the hexagonal surface Brillouin zone, spins at these valleys have opposite signs, which ensure effective spin-pseudospin coupling.

Success in both p-type and n-type of a novel transparent AgCuI alloy semiconductor system for homojunction devices

Although optically transparent semiconductor diodes and electronics can have a wide range of important potential applications, the development bottleneck is encountered due to the lack of a transparent semiconductor system that can be either p- or n-type. In this work, we report our design and success in the development of a novel AgCuI alloy semiconductor system that can be either p-type or n-type depending on Ag concentration. We have studied the electronic and optical properties of AgCuI alloyed thin films (AgxCu1-xI, 0 ≤ x ≤ 1). The crystal structure is of γ-CuI for Cu rich, while it is of β-AgI for Ag rich. By tuning Ag/Cu composition in thin films, we could engineer the energy bandgap from 2.78 to 3.02 eV with excellent transparency. Significantly, all the AgxCu1-xI alloy films are direct-gap semiconductors showing PL emission from exciton band-band transition and the wavelength is found tunable with respect to composition. The electronic valence band, conduction band and Fermi level positions determined from ultraviolet photoelectron spectroscopy show continuous shifts with respect to composition, marking a p-type (Cu rich) to n-type (Ag rich) conduction conversion. The composition analysis presented a full picture of energy levels for AgxCu1-xI alloys, which can serve as reference in band alignments. The achievement in both p-type and n-type for a transparent AgCuI semiconductor system, the light emitting property and tunable electronic bands with desired electrical conductivity in AgxCu1-xI alloys may offer new opportunities in invisible electronics and optoelectronic applications.

Ultrafast demagnetization mechanism in half-metallic Heusler alloy thin films controlled by the Fermi level

The electronic band structure-controlled ultrafast demagnetization mechanism in Co2FexMn1-xSi Heusler alloy is underpinned by systematic variation of composition. We find the spin-flip scattering rate controlled by spin density of states at Fermi level is responsible for non-monotonic variation of ultrafast demagnetization time ({\tau}M) with x with a maximum at x = 0.4. Furthermore, Gilbert damping constant exhibits an inverse relationship with {\tau}M due to the dominance of inter-band scattering mechanism. This establishes a unified mechanism of ultrafast spin dynamics based on Fermi level position.

RETRACTED: Efficient band gap opening in single-layer stanene via patterned Ga-As codoping: Towards semiconducting nanoelectronic devices

Abstract Stanene is considered as an auspicious candidate for the design and exploitation of future prosperous electronic devices. The equilibrium structures and electronic behaviors of the Ga, As and Ga-As codoped stanene nanosheets were scrutinized to fully expedite the advances in the field of semiconducting materials technology. The formation energy calculations indicated that Ga/As doped/codoped stanene monolayers are thermodynamically stable. The variability of the band gap of the intrinsic stanene through codoping of Ga and As atoms was probed in detail. The emergence of band gap around the Fermi level of stanene was also accomplished by patterned Ga and As codoping. Thus, Ga-As codoped stanene behaves as a productive semiconductor material. Both Ga-doped and As-doped stanene nanosheets display metallicity because of the remarkable shifts in the position of Fermi level to the valence and conductance bands. The total charge density schemes indicate the formation of covalent bond between the Ga and As, as well as Ga/As and adjacent Sn atoms. As a result, Ga-As codoped stanene exhibits superior semiconducting behavior compared to the perfect system, and can be applied as a promising material for nanosensors and nanoelectronic devices.

Design, engineering, and performance of nanorod-Fe2O3@rGO@LaSrFe2-nConO6 (n = 0, 1) composite architectures: The role of double oxide perovskites in reaching high solar to hydrogen efficiency

A visible light responsive heterojunction system composed of Fe2O3 nanorods (NF), reduced graphene oxide (rGO) and double oxide perovskites (LaSrFe2–nConO6, n = 1 (LSFC) and 0 (LSFF)) was rationally designed for an effective photoelectrochemical water splitting. The photoelectrodes were characterized in terms of structure, surface, physicochemical and functional properties with a focus on the electronic structure (e.g., the density of electronic states, band-edge, Fermi level positions), which was gauged from UPS, Kelvin probe (KP) and spectroelectrochemical (SE-DRS) measurements. The KP measurements not only revealed the successful and efficient contact of the materials with different work functions but also showed the drift direction of photogenerated charges. In the studied systems, the transparent, thin layer of rGO facilitates the hole transfer from NF (photoabsorber) to perovskites (electrocatalyst). The hydrogen conversion efficiency under visible and solar simulated irradiation for NF@rGO@LSFF reached remarkable values of 3.61 % and 1.13 % (STH), respectively.