Fermi Level Position Research Articles

Thermoelectric transport properties of single-crystalline ZrCoBi half-Heusler

Half-Heusler compounds are one of most promising thermoelectric materials for power generation at high temperatures. Recent studies focus on fine-grained polycrystalline samples because of their lower thermal conductivity, and the induced defects are found to play an important role in the thermoelectric transport properties. Here, we report the thermoelectric transport properties of single-crystalline ZrCoBi. Two samples from different batches clarify the same charge carrier concentration of ~1020 cm-3, denoting the robust Fermi level position in the ZrCoBi single crystals. The high electron density is attributed to the Zr interstitial point defects. Moreover, a high power factor of over 3.3 mWm-1K-2 is achieved in the single-crystalline ZrCoBi. By comparing the thermoelectric properties of single-crystalline and fine-grained polycrystalline samples, we reveal the role of grain boundary scattering in reducing the thermal conductivity from ~11.5 Wm-1K-1 to ~9 W/mK at 300 K. The present work declares the significance of defects in tuning the transport properties of ZrCoBi half-Heusler compound.

The influence of Fermi level position at the GaN surface on carrier transfer across the MAPbI3/GaN interface.

Both gallium nitride (GaN) and hybrid organic-inorganic perovskites such as methylammonium lead iodide (MAPbI3) have significantly influenced modern optoelectronics. Both marked a new beginning in the development of important branches in the semiconductor industry. For GaN, it is solid-state lighting and high-power electronics, and for MAPbI3, it is photovoltaics. Today, both are widely incorporated as building blocks in solar cells, LEDs and photodetectors. Regarding multilayers, and thus multi-interfacial construction of such devices, an understanding of the physical phenomena governing electronic transport at the interfaces is relevant. In this study, we present the spectroscopic investigation of carrier transfer across the MAPbI3/GaN interface by contactless electroreflectance (CER) for n-type and p-type GaN. The effect of MAPbI3 on the Fermi level position at the GaN surface was determined which allowed us to draw conclusions about the electronic phenomena at the interface. Our results show that MAPbI3 shifts the surface Fermi level deeper inside the GaN bandgap. Regarding different surface Fermi level positions for n-type and p-type GaN, we explain this as carrier transfer from GaN to MAPbI3 for n-type GaN and in the opposite direction for p-type GaN. We extend our outcomes with a demonstration of a broadband and self-powered MAPbI3/GaN photodetector.

Enhancement-mode β-Ga2O3 U-shaped gate trench vertical MOSFET realized by oxygen annealing

Vertical metal–oxide–semiconductor field effect transistor (MOSFET) is essential to the future application of ultrawide bandgap β-Ga2O3. In this work, we demonstrated an enhancement-mode β-Ga2O3 U-shaped gate trench vertical metal–oxide–semiconductor field effect transistor (UMOSFET) featuring a current blocking layer (CBL). The CBL was realized by high-temperature annealing under oxygen ambient, which provided electrical isolation between the source and drain electrodes. The CBL thicknesses of different annealing temperatures were derived from C–V measurements and the Fermi level position of the sample surfaces of different annealing temperature was characterized by x-ray photoelectron spectroscopy measurements, indicating good process controllability. Furthermore, photoluminescence spectra were measured to study the effect of oxygen annealing. The fabricated UMOSFET showed normally off with a Vth of 11.5 V, an on-state resistance of 1.48 Ω cm2, a maximum on-state current of 11 A/cm2, an on–off ratio of 6 × 104, and a three-terminal breakdown voltage over 100 V. This work paves a way to form a CBL and broadens the design space for high-power β-Ga2O3 vertical transistors.

Phase stability and half-metallic character of off-stoichiometric Co2FeGa0.5Ge0.5 Heusler alloys

We investigate the effects of off-stoichiometric compositional variations from the Co2Fe(Ga0.5Ge0.5) (CFGG) full-Heusler alloy on its half-metallic electronic structure. First-principles calculations predict that the Co antisite defects that occupy Fe-sites (CoFe) lead to a finite DOS in the half-metallic gap of CFGG. Fe antisites defects in Co-sites (FeCo) introduced by excessing Fe composition, which could suppress the formation of CoFe, preserves the half-metallic gap but reduces spin polarization because the Fermi level shifts to the lower energy. We found that, in Fe-excess CFGG, Ge-excess has an important role to enhance the spin polarization by lifting up the Fermi level position and suppressing the formation of CoFe. To confirm the effect of the Fe and Ge-excess off-stoichiometric composition on spin polarization and phase-purity experimentally, we fabricated CFGG epitaxial thin films with various composition ratios (Co2−αFe1+α) (Ga1−βGeβ)1+γ with small positive γ (=0.09–0.29). It turns out that Co1.75Ge or Fe1.7Ge secondary phase often forms in the films for β≥0.69 in Fe-deficient (α≤0.21) and excess (α≥0.49) compositions. This secondary phase can be suppressed by tuning the Ge and Fe compositions, and the L21-phase pure film was found in Co39.4Fe29.3Ga13.4Ge17.9 (α=0.28,β=0.57,γ=0.25). The measurements of conventional magnetoresistance effects qualitatively indicate higher spin polarization in the Co39.4Fe29.3Ga13.4Ge17.9 film compared to other Co-excess and Ge-deficient films, which evidences the benefit to make Fe- and Ge-excess off-stoichiometric CFGG for obtaining the half-metallic nature of CFGG.

Atomic and electronic structures of nitrogen vacancies in silicon nitride: Emergence of floating gap states

We report the density functional calculations with the hybrid approximation to the exchange-correlation energy that clarify the atomic and electronic structures of the N vacancy (${\mathrm{V}}_{\mathrm{N}}$) in ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$. We find that the gap states induced by the Si dangling bonds around ${\mathrm{V}}_{\mathrm{N}}$ show interesting interplay between the spin splitting and the Jahn-Teller splitting depending on their charges. More surprisingly, we find that a peculiar electron state distributed in the internal space of the host SiN is hidden in the conduction bands and converted to a localized floating state in the fundamental energy gap due to the presence of ${\mathrm{V}}_{\mathrm{N}}$. This theoretical finding indicates ${\mathrm{V}}_{\mathrm{N}}$ being multiply charged from $+1$ to $\ensuremath{-}5$: The charge states from $+1$ to $\ensuremath{-}3$ are stable at respective Fermi-level positions in the gap, whereas the states $\ensuremath{-}4$ and $\ensuremath{-}5$ are metastable for the Fermi level near the conduction band bottom. A possibility of multilevel memory function of ${\mathrm{V}}_{\mathrm{N}}$ in SiN is suggested.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications.

Photocatalytic water splitting, CO2 reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H2 generation potential, and an excellent ability to capture and activate CO2 molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H2 evolution, CO2 reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

Self-Enhancing Photoelectrochemical Properties in van der Waals Ferroelectric CuInP2S6 by Photoassisted Acid Hydrolysis.

Transition metal thiophosphate, CuInP2S6 (CIPS), has recently emerged as a potentially promising material for photoelectrochemical (PEC) water splitting due to its intrinsic ferroelectric polarization for spontaneous photocarrier separation. However, the poor kinetics of the hydrogen evolution reaction (HER) greatly limits its practical applications. Herein, we report self-enhancing photocatalytic behavior of a CIPS photocathode due to chemically driven oxygen incorporation by photoassisted acid oxidation. The optimal oxygen-doped CIPS demonstrates a >1 order of magnitude enhancement in the photocurrent density compared to that of pristine CIPS. Through comprehensive spectroscopic and microscopic investigations combined with theoretical calculations, we disclose that oxygen doping will lower the Fermi level position and decrease the HER barrier, which further accelerates charge separation and improves the HER activity. This work may deliver a universal and facile strategy for improving the PEC performance of other van der Waals metal thiophosphates.

Doping of Graphene Films: Open the way to Applications in Electronics and Optoelectronics

AbstractGraphene films have been regarded as potential materials for future applications in electronics and optoelectronics. Among various synthetic approaches, chemical vapor deposition (CVD) approach can produce large‐area graphene films with excellent scalability, controllability, and quality. Graphene doping, that is capable of improving carrier concentration and shifting the Fermi level position of graphene, has become an important pathway for realizing its desired applications. Physical adsorption of dopants on graphene surface can dope the graphene system through surface charge transfer and preserve the graphene lattice; however, the low doping stability strongly hinders its applications. The substitutional doping can achieve fine doping stability by incorporating heteroatoms, such as nitrogen and boron, into graphene lattice, but suffers from low carrier mobility owing to the presence of defects and disorders. In this review, the aim is to provide a comprehensive understanding of application‐related doping performance including doping stability, doping uniformity, carrier concentration, carrier mobility, electrical conductivity, and optical transparency. The aim of the review is also to provide an outlook for future doping techniques of CVD graphene films toward various applications.

Hidden symmetry in 1D localisation

ABSTRACT Resistance ρ of an one-dimensional disordered system of length L has the log-normal distribution in the limit of large L. Parameters of this distribution depend on the Fermi level position, but are independent on the boundary conditions. However, the boundary conditions essentially affect the distribution of phases entering the transfer matrix and generally change the parameters of the evolution equation for the distribution . This visible contradiction is resolved by the existence of the hidden symmetry, whose nature is revealed by the derivation of the equation for the stationary phase distribution and by analysis of its transformation properties.

Modeling of thickness-dependent energy level alignment at organic and inorganic semiconductor interfaces

We identify a universality in the Fermi level change of Van der Waals interacting semiconductor interfaces. We show that the disappearing of quasi-Fermi level pinning at a certain thickness of semiconductor films for both intrinsic (undoped) and extrinsic (doped) semiconductors over a wide range of bulk systems including inorganic, organic, and even organic–inorganic hybridized semiconductors. The Fermi level (EF) position located in the energy bandgap was dominated by not only the substrate work function (Φsub) but also the thickness of semiconductor films, in which the final EF shall be located at the position reflecting the thermal equilibrium of semiconductors themselves. Such universalities originate from the charge transfer between the substrate and semiconductor films after solving one-dimensional Poisson's equation. Our calculation resolves some of the conflicting results from experimental results determined by using ultraviolet photoelectron spectroscopy (UPS) and unifies the general rule on extracting EF positions in energy bandgaps from (i) inorganic semiconductors to organic semiconductors and (ii) intrinsic (undoped) to extrinsic (doped) semiconductors. Our findings shall provide a simple analytical scaling for obtaining the “quantitative energy diagram” in the real devices, thus paving the way for a fundamental understanding of interface physics and designing functional devices.

Surface charge-transfer doping a quantum-confined silver monolayer beneath epitaxial graphene

Recently the graphene/SiC interface has emerged as a versatile platform for the epitaxy of otherwise unstable, monoelemental, two-dimensional (2D) layers via intercalation. Intrinsically capped into a van der Waals heterostructure with overhead graphene, they compose a new class of quantum materials with striking properties contrasting their parent bulk crystals. Intercalated silver presents a prototypical example where 2D quantum confinement and inversion symmetry breaking entail a metal-to-semiconductor transition. However, little is known about the associated unoccupied states, and control of the Fermi level position across the bandgap would be desirable. Here, we n-type dope a graphene/2D-Ag/SiC heterostack via in situ potassium deposition and probe its band structure by means of synchrotron-based angle-resolved photoelectron spectroscopy. While the induced carrier densities on the order of $10^{14}$ cm$^{-2}$ are not yet sufficient to reach the onset of the silver conduction band, the band alignment of graphene changes relative to the rigidly shifting Ag valence band and substrate core levels. We further demonstrate an ordered potassium adlayer ($2\times 2$ relative to graphene) with free-electron-like dispersion, suppressing plasmaron quasiparticles in graphene via enhanced metalization of the heterostack. Our results establish surface charge-transfer doping as an efficient handle to modify band alignment and electronic properties of a van der Waals heterostructure assembled from graphene and a novel type of monolayered quantum material.

Prediction of half-metallic gap formation and Fermi level position in Co-based Heusler alloy epitaxial thin films through anisotropic magnetoresistance effect

We have investigated the temperature $(T)$ dependence of the anisotropic magnetoresistance (AMR) effect of ${\mathrm{Co}}_{2}{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) epitaxial thin films having different compositions and atomic orders to examine the relation between AMR and the half-metallic electronic structure based on a developed theoretical model. The $T$ dependence of the resistance change (\ensuremath{\Delta}\ensuremath{\rho}) of the AMR normalized at 10 K is minimal in the CFGG films having a standard composition and high atomic order. In contrast, the films having a large atomic disorder due to the Co-rich composition exhibit a large reduction of \ensuremath{\Delta}\ensuremath{\rho} with $T$. Our theoretical model of AMR well explains this behavior; namely, the half-metallic ferromagnets having the Fermi level $({E}_{\mathrm{F}})$ around the center of the half-metallic gap are predicted to show a small $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} because of no/few localized $d\ensuremath{\downarrow}$ states at ${E}_{\mathrm{F}}$. On the contrary, a large change of \ensuremath{\Delta}\ensuremath{\rho} with $T$ is expected for the half-metallic materials having ${E}_{\mathrm{F}}$ close to gap edges or large in-gap states because of the contribution of thermally excited s-d scattering involving the $d\ensuremath{\downarrow}$ states. Moreover, we have also investigated the variation of \ensuremath{\Delta}\ensuremath{\rho} with $T$ for the thin films of various half-metallic Co-based Heusler alloys and found the behavior that agrees with our theoretical prediction. The present study proves that the formation of a half-metallic gap and the position of ${E}_{\mathrm{F}}$ in a half-metallic material are readily predictable from the $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} of the AMR effect, which can be a facile way for efficient screening of half-metallic materials.

Effect of semiconducting nature of ZnO interfacial layer on inverted organic solar cell performance

The light-soaking effect is one of the major drawbacks for inverted organic solar cells (OSCs) if metal oxides are used as the electron transport layer (ETL). The oxide ETL primarily originates the above effect from the energy barrier, deep level defects, and excess carriers tunneling. Here, electron-beam evaporated high-quality pristine and post-treated e-ZnO thin films are utilized to fabricate inverted OSC as the ETL between the transparent cathode and active bulk-heterojunction PBDB-T-2Cl:PC61BM layer to study the influence on device performance. Various experimental techniques, including AFM, XRD, XPS, and UPS, are utilized to identify the surface and semiconducting properties of differently treated interfacial e-ZnO films precisely. XPS results reveal the variation of oxygen vacancies and adsorbed oxygen species on the surface of e-ZnO layers. The semiconducting nature of various e-ZnO thin films for the use of ETL are also probed with the help of UPS results, which accurately locate the valence band maximum and Fermi level position. After correlating the property of e-ZnO systematically with the respective OSC device performance, it is found that the deeper valence band top and higher n-type nature of e-ZnO is desirable to depict the light soaking free highest solar cell efficiency and large open-circuit voltage of about 0.97 V in a single junction. The presence of lesser chemisorbed oxygen species over the e-ZnO surface might be an added advantage to demonstrate the light soaking free operation in inverted OSC devices.

Modification of β-gallium oxide electronic properties by irradiation with high-energy electrons

We present a study of the modifications of the electronic properties of β-gallium oxide crystals by 2.5-MeV electron irradiation. This type of irradiation produces exclusively local point defects in Ga2O3, predominantly gallium vacancies, which act as acceptor centers. Starting with a highly n-doped sample, we establish a quantitative linear relation between the irradiation dose and the concentration of generated acceptor centers. This gives the possibility to tune the Fermi level position within the bandgap by choosing an appropriate irradiation dose. At high doses, with a very deep position of the Fermi level, the n-type sample becomes compensated, reaching a semi-insulating state. The downward shift of the Fermi level with irradiation allows us to reveal the presence of latent impurities of transition metals (like Cr and Fe), which are inactive in electron paramagnetic resonance and luminescence spectra of pristine samples. This study confirms the potential of electron irradiation as a tool for tailoring the electronic properties of gallium oxide.

Efficient degradation of organic pollutants by enhanced interfacial internal electric field induced via various crystallinity carbon nitride homojunction

Herein, a homojunction carbon nitride photocatalyst integrated with three crystallization levels (Tri-crystallinity) is developed to enhance the degradation activity of organic pollutants. The increase in crystallinity induces the Fermi level and band position to decrease. This difference constructs an internal electric field (IEF) at the interface with a lower to higher crystallinity direction. The multiple contact interfaces in Tri-crystallinity reinforce the interfacial IEF twice compared to a conventional single-interface. The interfacial IEF improves charge separation and transfer, and the degradation of antibiotics by Tri-crystallinity carbon nitride is at least 20 times greater than primary carbon nitride. Further, the Tri-crystallinity carbon nitride loading on a nonwoven fabric in a continuous-flow reactor achieves 91.0% purification of the organic wastewater at a flow rate of 14.2 L h−1 m−2 from 11:00 a.m. to 3:00 p.m. This work provides a strategy for engineering interfacial IEF of future efficient photocatalysts.

Alternative electronic density of states model for metastable crystalline phase of Ge2Sb2Te5

Motivated by future data storage requirements, Ge2Sb2Te5 is studied for application in phase-change random access memory. The currently accepted density of states (DOS) models for the cubic crystalline phase, based on first-principles calculations, are reviewed. An alternative DOS model, which incorporates band tails and an antimony vacancy multivalent defect, is proposed. Solar cell capacitance simulation results reveal that the alternative model is successful in predicting a free hole concentration and Fermi level position consistent with previous Hall effect and thermopower measurements respectively. The conduction band tail, which has not previously been incorporated within the DOS model of the crystalline phase, is shown to contribute to this success.

Quantum point defects in 2D materials - the QPOD database

Atomically thin two-dimensional (2D) materials are ideal host systems for quantum defects as they offer easier characterisation, manipulation and read-out of defect states as compared to bulk defects. Here we introduce the Quantum Point Defect (QPOD) database with more than 1900 defect systems comprising various charge states of 503 intrinsic point defects (vacancies and antisites) in 82 different 2D semiconductors and insulators. The Atomic Simulation Recipes (ASR) workflow framework was used to perform density functional theory (DFT) calculations of defect formation energies, charge transition levels, Fermi level positions, equilibrium defect and carrier concentrations, transition dipole moments, hyperfine coupling, and zero-field splitting. Excited states and photoluminescence spectra were calculated for selected high-spin defects. In this paper we describe the calculations and workflow behind the QPOD database, present an overview of its content, and discuss some general trends and correlations in the data. We analyse the degree of defect tolerance as well as intrinsic dopability of the host materials and identify promising defects for quantum technological applications. The database is freely available and can be browsed via a web-app interlinked with the Computational 2D Materials Database (C2DB).

Fe(001) angle-resolved photoemission and intrinsic anomalous Hall conductivity in Fe seen by different ab initio approaches: LDA and GGA versus GW

Many material properties such as the electronic transport characteristics depend on the details of the electronic band structure in the vicinity of the Fermi level. For an accurate ab initio description of the material properties, the electronic band structure must be known and theoretically reproduced with high fidelity. Here, we ask a question which of the ab initio methods compare the best to the experimental photoemission intensities from bcc Fe. We confront the photoemission data from Fe(001) thin film grown on Au(001) to the photoemission simulations based on different ab initio initial band structures: density functional theory (DFT) in the local density approximation (LDA) and the generalized gradient approximation (GGA) and GGA corrected with many-body perturbation theory in the $GW$ approximation. We find the best comparison for the $GW$ results. As a second step, we discuss how the calculated intrinsic anomalous Hall conductivity (AHC) in bcc Fe depends on the choice of the method that describes the electronic band structure and Fermi level position. We find very large differences in AHC between the three theoretical approaches and show that the AHC found for the experimental Fermi level location within the $GW$ band structure is the closest to the literature results of transport experiments. This finding improves our understanding of not only the anomalous Hall effect itself, but also other related phenomena, such as the anomalous Nernst effect.

Nonlinear intensity dependence of photogalvanics and photoconductance induced by terahertz laser radiation in twisted bilayer graphene close to magic angle

We report on the observation of the nonlinear intensity dependence of the bulk photogalvanic current and photoconductivity in the twisted graphene with small twist angles close to the second magic angle. We show that terahertz radiation results in photoresponses, which are caused by indirect optical transitions (free carrier absorption), direct interband transitions, and optical transitions between moir\'e subbands. The relative contribution of these absorption channels depends on the Fermi-level position with respect to the multiple moir\'e subbands of the twisted graphene. The interplay of these absorption channels results in oscillations of the photoresponses with variation of the gate voltage. We show that the photoresponse saturates at high intensities. For different absorption channels it has different intensity dependencies and saturation intensities. The latter depends nonmonotonically on the Fermi-level position, which is controlled by the gate voltage.

Simulation and experimental studies of the dissolution corrosion of 4H-SiC in liquid Pb/Bi

The dissolution corrosion of liquid Pb/Bi on 4H-SiC (0001) surfaces are studied based on first-principles calculations and experimental studies. The calculation models of 4H-SiC (0001) surfaces are proposed according to the experimental results, and the rationality of the models is verified by our repeated experiments. Clean surface stability is mainly determined by the strength of surface states due to the dangling bonds. Pb/Bi adatoms accelerate the dissolution of C atoms more severely than that of Si atoms and liquid Bi is more corrosive than liquid Pb, which are consistent with our experimental results. Energetics, microstructural and electronic properties are analyzed to reveal the corrosion mechanism of liquid Pb/Bi. Moreover, we provide a theoretical scheme to improve the corrosion resistance of 4H-SiC by adjusting the charges of vacancies and changing the relative position of Fermi level.