Carrier Doping Effect Research Articles

Ca substitution effects on structure transformation and physical properties of (Eu,Ca)FeAs2 co-doped with La and Co

In this study, Ca substitution and carrier doping effect on phase formation temperature and physical/superconducting properties of (Eu0.9-xCaxLa0.1)(Fe0.97Co0.03)As2 ((Eu,Ca)112, x = 0–0.4) compounds were investigated. The best synthesis condition for obtaining pure phases vary (850–900 °C) depending on the Ca substitution amount. Ca substitution in the EuFeAs2 structure induced a structural transformation from orthorhombic I2mm to monoclinic P21/m symmetry. Single crystals of EuFeAs2 and (Eu0.9La0.1)FeAs2 were grown and their crystal structures were determined. Density functional theory calculations confirmed the transformation and stability of the La-doped Eu112 crystal structure with Ca substitution. Compared with the parent compound EuFeAs2, the effective substitution of Ca reduced the lattice constants. The magnetic and transport properties of (Eu0.9-xCaxLa0.1)(Fe0.97Co0.03)As2 compounds were studied using the temperature and field dependence of the magnetization and electrical resistivity measurements. Ca substitution suppressed the magnetic ordering of Eu and gradually reduced the Néel temperature TNEu. As determined by the magnetization and electrical resistivity data, the onset of critical temperature (Tc) values increased with Ca substitution increment from 26 K to 32.2 K and 31 K–34.8 K, respectively. Ca substitution slightly increased the upper critical field values, while the coherence length ξ values were slightly decreased. The Ca substitution and sintering temperature dependence of superconductivity, upper critical fields, lower critical fields, and Eu-related magnetic transitions were extracted. The new series of Eu-containing iron-based superconductors with high Tc provides an interesting playground in this field.

The magnetic and electric transport properties in erbium-doped layered perovskite iridate Sr2–xErxIrO4

The 5d oxide Sr2IrO4 with a layered perovskite structure has been investigated due to its peculiar Jeff = 1/2 insulating ground state. In this research, we investigate the evolution of magnetic and electric transport properties of Er-doped polycrystalline perovskite iridates Sr2–xErxIrO4 (0 ≤ x ≤ 0.06). Er3+ is magnetic and can thus provide magnetic interactions with Ir. Also, Er3+ doping introduces a carrier-doping effect. We find that substituting Sr2+ with Er3+ still preserves the I41/acd crystal structure, while it causes the rotation of IrO6 octahedra φ to decrease. In this case, the Ir–O1–Ir bond angle increases and the tetragonal distortion c/a decreases. The magnetic properties present a suppressed transition temperature, a decreased Curie–Weiss temperature, and an increased effective magnetic moment. The electrical resistivity of the doped samples decreases with doping, which originates from the crystal distortion and electron doping. Theoretical calculations represent an increase in band filling and an improvement in the conduction property with doping.

Comparison of carrier doping in ZnSnO3 and ZnTiO3 from first principles.

Ferroelectric materials have attracted increasing attention due to their rich properties. Unlike perovskite ferroelectric oxides, in the LiNbO3-type ferroelectric oxides of ABO3, ferroelectrically active cations are not necessary. While the effects of carrier doping on perovskite ferroelectric oxides have been extensively studied, the studies on LiNbO3-type ferroelectric oxides are rare. We consider two LiNbO3-type ferroelectric oxides ZnSnO3 and ZnTiO3, where the former has no ferroelectrically active cation and the latter has ferroelectrically active cation Ti4+, and study the effect of carrier doping by performing first-principles calculations. Comparison results indicate that the B-site cation has significant effects on the polar distortion in LN-type ferroelectrics. Our studies show that LN-type materials can maintain the coexistence of ferroelectricity and conductance over a very wide range of concentrations. The polar displacement is even enhanced under hole doping. More importantly, ZnSnO3 can be doped by electrons up to a high level to realize the conducting ferroelectrics of high mobility due to its isolated s conduction band.

Control of interlayer friction in two-dimensional ferromagnetic CrBr3.

Two-dimensional (2D) magnetic materials may offer new opportunities in the field of lubrication at the nanoscale. It is essential to investigate the interfacial properties, particularly magnetic coupling, at the interfaces of 2D magnetic materials from the point of view of friction. In the present study, we investigated the tribological and interfacial properties at the interface of bilayer CrBr3 by performing first-principles calculations. The effects of normal load, biaxial strain and carrier doping on interlayer magnetic coupling were also studied. Our calculations identify the ferromagnetic (FM)-antiferromagnetic (AFM) conversion of the interlayer magnetic couplings, which leads to the reduction of the sliding energy barriers. Importantly, our calculations demonstrate the lower sliding energy barrier at the interface of 2D FM CrBr3, implying lower friction and better lubricating properties. Additionally, we found that a normal load of 0.5-1.0 eV Å-1, a biaxial compressive strain of 0% to -5%, and a carrier doping of -0.2 to 0.2 e f.u.-1 are effective in reducing the sliding energy barrier and the friction. It is also found that the biaxial strain tunes the interlayer electron redistribution and thus alters the interlayer interaction and friction. The differences between the lubricating properties of 2D magnetic CrX3 (X = Cl, Br and I) have also been studied. The present findings are inspiring for the application of 2D magnetic materials as solid lubricants in the fields of lubrication at the nanoscale.

Controllable Carrier Doping in Two-Dimensional Materials Using Electron-Beam Irradiation and Scalable Oxide Dielectrics.

Two-dimensional (2D) materials, characterized by their atomically thin nature and exceptional properties, hold significant promise for future nano-electronic applications. The precise control of carrier density in these 2D materials is essential for enhancing performance and enabling complex device functionalities. In this study, we present an electron-beam (e-beam) doping approach to achieve controllable carrier doping effects in graphene and MoS2 field-effect transistors (FETs) by leveraging charge-trapping oxide dielectrics. By adding an atomic layer deposition (ALD)-grown Al2O3 dielectric layer on top of the SiO2/Si substrate, we demonstrate that controllable and reversible carrier doping effects can be effectively induced in graphene and MoS2 FETs through e-beam doping. This new device configuration establishes an oxide interface that enhances charge-trapping capabilities, enabling the effective induction of electron and hole doping beyond the SiO2 breakdown limit using high-energy e-beam irradiation. Importantly, these high doping effects exhibit non-volatility and robust stability in both vacuum and air environments for graphene FET devices. This methodology enhances carrier modulation capabilities in 2D materials and holds great potential for advancing the development of scalable 2D nano-devices.

Electronic phase transition, perpendicular magnetic anisotropy and high Curie temperature in Janus FeClF

How to enhance the spin polarization, the Curie temperature and the perpendicular magnetic anisotropy (PMA) is crucial for the applications of 2D magnets in spintronic devices. In this work, based on the experimental FeCl2 flakes and the predicted in-plane magnetic anisotropy (IMA) and lower Curie temperature of FeCl2 monolayer, we use first-principles and Monte Carlo simulation to explore the strain and carrier-doping effects on the electronic and magnetic properties of Janus FeClF monolayer. The structure is stable within −10% to 2% biaxial strain. Janus FeClF monolayer can experience transitions from a half-semiconductor to a spin gapless semiconductor (SGS) around the −6% compressive strain, and from the IMA to the PMA at the −7% compressive strain. The super-exchange Fe–F/Cl–Fe interaction induces the ferromagnetic coupling, and the Curie temperature can be considerably enhanced from 56 K to 281 K at the −10% compressive strain. The half-metallicity can be achieved whether under electron doping or hole doping. The Fe-d orbitals and the spin–orbit coupling interaction between occupied and unoccupied intraorbital states are responsible for the electronic phase transition and the magnetic anisotropy, respectively. Remarkably, the compressive −10% strain and the 0.02 e doping collectively increase the Curie temperature to near room temperature (286 K). The high spin polarization (exhibiting SGS and half-metal), the PMA and the near-room-temperature ferromagnetism induced by strain and doping make Janus FeClF a promising candidate for 2D spintronic applications, which will stimulate experimental and theoretical broad studies on this class of Janus monolayers FeXY (X,Y = F, Cl, Br, and X ≠ Y).

Effect of carrier doping on the electronic states of earth-abundant Fe–Al–Si thermoelectric materials

Fe–Al–Si-based thermoelectric (FAST) materials are non-toxic and low-cost materials that can be used for autonomous power supplies to drive internet-of-things wireless sensor devices. The conduction type can be controlled by changing the Al/Si ratio, which is suitable for fabricating reliable thermoelectric power-generation modules consisting of materials with similar thermal expansion coefficients. In this work, we evaluated the electronic structures of p- and n-type FAST materials with relatively large absolute values of the Seebeck coefficient by photoemission spectroscopy to obtain deeper insight into controlling the p-n characteristics of FAST materials. The core-level spectra suggested that the FAST materials have a covalent bonding nature. The chemical-potential shift should be the dominant factor of the core-level shift, which is consistent with the expected behavior of carrier doping of thermoelectric semiconductors, that is, rigid-band-like behavior. The size of the core-level shift of ∼0.15 eV is close to the band gap of ∼0.18 eV obtained from transport measurements. The observed electronic structure can qualitatively explain the experimental results.

Anomalous enhancement of charge density wave in kagome superconductor CsV3Sb5 approaching the 2D limit

The recently discovered kagome metals AV3Sb5 (A = Cs, Rb, K) exhibit a variety of intriguing phenomena, such as a charge density wave (CDW) with time-reversal symmetry breaking and possible unconventional superconductivity. Here, we report a rare non-monotonic evolution of the CDW temperature (TCDW) with the reduction of flake thickness approaching the atomic limit, and the superconducting transition temperature (Tc) features an inverse variation with TCDW. TCDW initially decreases to a minimum value of 72 K at 27 layers and then increases abruptly, reaching a record-high value of 120 K at 5 layers. Raman scattering measurements reveal a weakened electron-phonon coupling with the reduction of sample thickness, suggesting that a crossover from electron-phonon coupling to dominantly electronic interactions could account for the non-monotonic thickness dependence of TCDW. Our work demonstrates the novel effects of dimension reduction and carrier doping on quantum states in thin flakes and provides crucial insights into the complex mechanism of the CDW order in the family of AV3Sb5 kagome metals.

Effective carrier doping and quantum capacitance manipulation of graphene through two-dimensional solid electrolytes of ScI3 and YBr3

Effective carrier doping would be expected to enhance the electrical conductivity and further improve the quantum capacitance (Cq) of graphene. In this work, two-dimensional solid electrolytes of ScI3 and YBr3 as buffer layers have been proposed to modulate the carrier density and improve the Cq. By using first-principles calculations, the effective carrier doping has been examined by building heterostructures of graphene/ScI3 and graphene/YBr3 combined with Li, Na, K, Ca, F dopants. The n-type doping with electron density and p-type doping with hole density of graphene reach up to 1.14 × 1014 cm−2 and 6.66 × 1013 cm−2, respectively. Remarkably, the linear band dispersions of graphene are maintained near the Dirac point. Moreover, the Cq is significantly increased around the zero bias due to the increased density of states near the Fermi level. For Li-, Na-, K-, and Ca-doped systems, the Cq in the negative bias region is effectively enhanced while the Cq is improved in the positive bias region for F-doped system. These results provide an ideal route to implement the effective carrier doping and the Cq manipulation of graphene, facilitating the applications of graphene as electrodes in high-performance optoelectronics and energy storage devices, such as light-emitting diodes, solar cells and supercapacitors.

Electron-doping induced tunable magnetisms in 2D Janus TiXO (X = S, Se)

The effects of carrier doping on magnetic and electronic properties of two-dimensional (2D) Janus TiXO (X = S, Se) systems are investigated by first-principles calculations. It is found the electronic instability of systems is related with the flat conduction-band edge and high density of states near Fermi energy. At a concentration of electron doping n ranging from ∼10 13 to ∼10 14 cm −2 , the system maintains in a fully spin-polarized state and the Curie temperature ( T C ) is estimated to be above room temperature; while the nonmagnetic semiconductor TiXO monolayer could become a magnetic half-metal when n is varied. The doping-induced magnetisms and T C could be modulated substantially by applying strains and alternating the number of monolayers in Janus TiXO multilayers. The results indicate that the Janus TiXO possesses tunable magnetic properties, providing promising 2D materials for electronics and spintronics applications. • The Janus TiXO (X = S, Se) monolayers are explored by first-principles calculations. • The 2D monolayers and bilayers exhibit ferromagnetic properties. • The magnetic properties can be modulated by electron doping and applied strains.

Enhanced Spin–Orbit Coupling in a Correlated Metal

We investigate the correlation effects on spin-orbit coupling (SOC) in a two-orbital Hubbard model on a square lattice by applying the variational Monte Carlo method. We consider an effective SOC constant $\lambda_{\text{eff}}$ in the one-body part of the variational wave function and mainly discuss the cases of the electron number per site $n=1$, that is, quarter filling. We find that $\lambda_{\text{eff}}$ is proportional to the bare value $\lambda$ and depends on the electron-electron interactions through $(U'-J')$ in a relatively wide parameter range in the paramagnetic (PM) phase, where $U'$ is the interorbital Coulomb interaction and $J'$ is the pair hopping interaction. Increasing the electron-electron interactions in the PM phase leads to a transition to an effective one-band state, in which the upper band becomes empty due to the enhanced $\lambda_{\text{eff}}$. We also construct phase diagrams considering magnetic order. The carrier doping effect on $\lambda_{\text{eff}}$ is also investigated. We find that $\lambda_{\text{eff}}$ enhances in a strongly correlated region around the Mott transition point and it is necessary to include the correlation effects beyond the Hartree-Fock approximation to describe the enhanced SOC properly.

Enhancement of n-type conductivity of hexagonal boron nitride films by in-situ co-doping of silicon and oxygen

Effective doping of ultra-wide band gap semiconductors is of crucial importance, yet, remains challenging. Here, we report the enhancement of n-type conductivity of nanocrystalline hexagonal boron nitride (h-BN) films with simultaneous incorporation of Si and O while deposition by radio frequency magnetron sputtering method. The resultant h-BN films are of ∼50 nm in thickness, containing nitrogen vacancy (VN) defects. Incorporation of O together with Si results in effective healing of VN defects and significantly reduces electric resistivity in h-BN thin films. X-ray photoelectron spectroscopy results reveal that under B-rich condition, the substitutional O in VN bonding with B leads to the formation of Si–N, which thus plays an important role to the n-type conductivity in h-BN films. The temperature dependent electrical resistivity measurements of the Si/O co-doped h-BN films reveal two donor levels of 130 and 520 meV at room temperature and higher temperatures, respectively. The n-h-BN/p-Si heterojunctions demonstrate apparent rectification characteristics at room temperature, where the tunneling behavior dominates throughout the injection regimes due to the effective carrier doping. This work proposes an effective approach to enhance the n-type conductivity of h-BN thin films for future applications in electronics, optoelectronics and photovoltaics.

Doped Mott insulator on a Penrose tiling

We study the effect of carrier doping to the Mott insulator on the Penrose tiling, aiming at clarifying the interplay between quasiperiodicity and strong electron correlations. We numerically solve the Hubbard model on the Penrose-tiling structure within a real-space dynamical mean-field theory, which can deal with a singular self-energy necessary to describe the Mott insulator and spatial inhomogeneity. We find that the strong correlation effect produces a charge distribution unreachable by a static mean-field approximation. In a small doping region, the spectrum shows a site-dependent gap just above the Fermi energy, which is generated by a singularly large self-energy emergent from the Mott physics and regarded as a real-space counterpart of the momentum-dependent pseudogap observed in a square-lattice Hubbard model.

Variation between Antiferromagnetism and Ferrimagnetism in NiPS3 by Electron Doping

AbstractTo electrically control magnetic properties of material is promising toward spintronic applications, where the investigation of carrier doping effects on antiferromagnetic (AFM) materials remains challenging due to their zero net magnetization. In this work, the authors find electron doping dependent variation of magnetic orders of a 2D AFM insulator NiPS3, where doping concentration is tuned by intercalating various organic cations into the van der Waals gaps of NiPS3 without introduction of defects and impurity phases. The doped NiPS3 shows an AFM‐ferrimagnetic (FIM) transition at a doping level of 0.2–0.5 electrons/cell and a FIM‐AFM transition at a doping level of ≥0.6 electrons/cell. The authors propose that the found phenomenon is due to competition between Stoner exchange dominated inter‐chain ferromagnetic order and super‐exchange dominated AFM order at different doping level. The studies provide a viable way to exploit correlation between electronic structures and magnetic properties of 2D magnetic materials for realization of magnetoelectric effect.

Atomic Intercalation Induced Spin-Flip Transition in Bilayer CrI3.

The recent discovery of 2D magnets has induced various intriguing phenomena due to the modulated spin polarization by other degrees of freedoms such as phonons, interlayer stacking, and doping. The mechanism of the modulated spin-polarization, however, is not clear. In this work, we demonstrate theoretically and computationally that interlayer magnetic coupling of the CrI3 bilayer can be well controlled by intercalation and carrier doping. Interlayer atomic intercalation and carrier doping have been proven to induce an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in the spin-polarization of the CrI3 bilayer. Our results revealed that the AFM to FM transition induced by atom intercalation was a result of enhanced superexchange interaction between Cr atoms of neighboring layers. FM coupling induced by O intercalation mainly originates from the improved superexchange interaction mediated by Cr 3d-O 2p coupling. FM coupling induced by Li intercalation was found to be much stronger than that by O intercalation, which was attributed to the much stronger superexchange by electron doping than by hole doping. This comprehensive spin exchange mechanism was further confirmed by our results of the carrier doping effect on the interlayer magnetic coupling. Our work provides a deep understanding of the underlying spin exchange mechanism in 2D magnetic materials.

Self-consistent analysis of doping effect for magnetic ordering in stacked-kagome Weyl system

We theoretically study the carrier doping effect for magnetism in a stacked-kagome system $\rm{{Co}_3{Sn}_2{S}_2}$ based on an effective model and the Hartree-Fock method. We show the electron filling and temperature dependences of the magnetic order parameter. The perpendicular ferromagnetic ordering is suppressed by hole doping, wheres undoped $\rm{{Co}_3{Sn}_2{S}_2}$ shows magnetic Weyl semimetal state. Additionally, in the electron-doped regime, we find a non-collinear antiferromagnetic ordering. Especially, in the non-collinear antiferromagnetic state, by considering a certain spin-orbit coupling, the finite orbital magnetization and the anomalous Hall conductivity are obtained.

Dimensionality-Inhibited Chemical Doping in Two-Dimensional Semiconductors: The Phosphorene and MoS2 from Charge-Correction Method.

Despite the great appeal of two-dimensional semiconductors for electronics and optoelectronics, to achieve the required charge carrier concentrations by means of chemical doping remains a challenge due to large defect ionization energies (IEs). Here, by decomposing the defect IEs into three parts based on ionization process, we propose a conceptual picture that the large defect IEs are caused by two effects of reduced dimensionality. While the quantum confinement effect makes the neutral single-electron point defect levels deep, the reduced screening effect leads to high energy cost for the electronic relaxation. The first-principles calculations for black phosphorus and MoS2 do demonstrate the general trend. Using BP monolayer either embedded into dielectric continuum or encapsulated between two hBN layers, we demonstrate the feasibility of increasing the screening to reduce the defect IEs. Our analysis is expected to help achieve effective carrier doping and open ways toward more extensive applications of 2D semiconductors.

Effects of Si doping on the ferromagnetic properties of delta doped GaMnN nanorods

Delta doping (δ-doping) of group-III nitride-based nanostructures such as nanorods (NRs) with transition metals such as manganese (Mn) can lead to one-dimensional (1D) diluted magnetic semiconductors (DMSs). In order to investigate the effects of free carrier doping on the structural, electrical, and magnetic properties of such delta-doped 1D structures, we have used nanosphere lithography to grow uniform arrays of vertically aligned NRs with fixed aspect ratios on single crystal Al2O3 substrates using plasma-assisted molecular beam epitaxy (PAMBE). The precise control of the elemental flux intensity and duration, facilitated by PAMBE, enables the growth of phase-pure nanostructures, resulting in spatial separation on the order of few nanometers, between the δ-Mn layer and the free carriers in the Si:GaN layer. Chemical quantification verifies the presence of Mn and Si, while Raman spectroscopy shows that Si doping enhances the local vibration mode associated with Mn bonded to N as well as the disorder-activated mode. The free carriers do not diminish the inherent magnetic ordering in these 1D structures, while magnetic measurements show a stability in the signal.

Enhancement of figure of merit for Nernst effect in Bi77Sb23 alloy by Te-doping

The effect of carrier doping on the figure of merit for the Nernst effect zNernst is investigated using Bi77Sb23 alloys, aiming at the enhancement of the dimensionless figure of merit zNernstT at room temperature. Four Bi77Sb23 alloys—undoped, 0.1-, 0.2-, and 0.3-at.% Te-doped—are produced by spark plasma sintering and annealing. The Nernst thermopower, electrical resistivity, and thermal conductivity of undoped and Te-doped Bi77Sb23 alloys are measured in magnetic fields of up to 6 T at temperatures from 10 K to 300 K to determine zNernstT. The magnitude of the Nernst thermopower increases by 102% at 300 K because of the modification of the electron and hole carrier mobility by 0.1-at.% Te-doping. In addition, the magnetoresistance effect is suppressed over the entire temperature range owing to the fact that charge neutrality is destroyed by Te-doping, and this contributes to the enhancement of zNernstT. The thermal conductivity in the magnetic field is increased by Te-doping due to the increased electron thermal conductivity. Thus, zNernstT for the Bi77Sb23 alloy at 300 K is increased by 329% as a result of 0.1-at.% Te-doping.

Carrier and Transition Metals (M = Nb, Cr and Fe) Doping Effects on Structure and Electronic Structure in Spinel Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> Compounds

Spinel Li4Ti5O12 (LTO) is one of the most promising candidate anode material for Li-ion battery (LIB) known, as zero strain material, it has poor intrinsic electronic properties. In order to enhance it, we have investigated effect of doping on electronic conductivity of spinel LTO phase structure. We consider the carrier and transition metal doping effect on structure and electronic structure of spinel LTO. It is shown that the doping can improve the electronic conduction of spinel LTO. Our calculations were based on the projector augmented wave (PAW) method with the generalized gradient approximation (GGA+U+J0) including the Hubbard U parameter for exchange correlation functional within the framework of density functional theory (DFT).