Impurity Band Research Articles

Role of indium in tuning the band structure of Sr1-xInxTiO3 for enhanced photocatalytic water splitting: A GGA + U study

Electronic structures and optical properties of In-doped SrTiO3 (Sr1-xInxTiO3) were systematically analyzed by using first-principles calculations. The calculated formation energies indicate that the Sr1-xInxTiO3 system is stable. Calculated results show that the band gaps of SrTiO3 are significantly reduced when In dopants replace Sr atoms. It is observed that impurity bands formed by the contribution of In-5 s, Ti-3d, and O-2p orbitals appeared in the band gap of the doped system. The photoabsorption spectra and the calculated positions of the band edges indicate that Sr0.875In0.125TiO3 has better visible light absorption activity than pure SrTiO3 and is more suitable for water separation.

Ab initio prediction of optoelectronic and thermoelectric behavior of Eu‐doped Zn2SnO4 using GGA + U functional: A study for optoelectronic devices

AbstractTo understand the electronic structure and optical properties of Zn2SnO4 and Eu‐doped compounds, we utilized the full potential linearized augmented plane wave method with the generalized gradient approximation (GGA). To analyze the electronic, optical, and thermoelectric properties of Eu‐doped Zn2SnO4, we implemented the GGA‐PBE + U method within density functional theory. We obtained the density of states (total and partial DOS) and optical constants (absorption coefficient, refractive index, reflectivity, and optical conductivity). Our findings revealed that the near‐Fermi valence bands (VB) of the band structure are primarily due to the O and Eu atoms. The electronic properties, including the band structure and TDOS/PDOS, particularly the hybridization among Eu s and f along with Zn s and d states, demonstrated that the pristine material's VB and conduction bands are located at the G point, indicating a direct band gap. However, the Eu‐doped material exhibits intermetallic behavior. We used the electronic structures to analyze the dielectric (real and imaginary parts) function and related properties, such as absorption coefficient, reflectivity, energy loss function, and refractive index. Consequently, in the low‐energy zone, the refractive index and the dielectric constant increased. With the presence of impurity bands, the width of the band gap decreased significantly, and the absorption spectrum shifted toward the red region, indicating a robust response in the infrared band, with high utilization efficiency. Moreover, the Zn2SnO4‐doped samples showed the best improvement in optical properties in the low‐energy region.

Effect of Mn, N co-doped LiFePO4 on electrochemical and mechanical properties: A DFT study

In this study, the thermodynamic stability, embedding voltage, volume change rate, electronic structure properties, mechanical properties and lithium-ion diffusion characteristics of the Mn, N co-doped LiFePO4 material are investigated using a first-principles approach based on density generalization theory. The results show that the doped system has a low formation energy and the material meets the thermodynamic stability criteria. During the de-lithium process, the volume change rate of the doped material decreases and the cycling performance is improved, but the battery energy density decreases slightly. It is also found that the doping of N led to the transformation of the material from a p-type semiconductor to an N-type semiconductor, while the doping of Mn and N lead to the creation of impurity bands, narrowing of the band gap and an increase in conductivity. At the same time, Mn, N co-doping greatly improve the ductility of the material, suppress the generation of microcracks, and reduce the possibility of shear deformation. In addition, it is noteworthy that the lithium-ion diffusion energy barrier of the doped system is reduced, which predicts an increase in the diffusion rate of lithium ions in the doped system.

Prospectives for AlN electronics and optoelectronics and the important role of alternative synthesis

Future applications for emerging AlN semiconductor electronics and optoelectronics are facilitated by emerging doping technologies enabled by low temperature, non-equilibrium epitaxy. Defect and impurity compensation can be reduced by controlling the surface chemistry with reducing compensating vacancy concentrations being a key driver for lower temperature growth. Contrary to common understanding, low temperature, metal-rich vacuum processes are shown to have higher diffusion lengths than high temperature nitrogen-rich methods. This feature can be utilized to inhibit silicon-DX center formation without compromises in crystal quality. First principles calculations identify the valence split-off band as the dominant hole band contributing to impurity band formation (as opposed to the heavy and light hole bands in other nitrides). This anomalous band structure causes an impurity band to form at dopant concentrations similar to GaN even though AlN has a deeper isolated acceptor energy and results in hole mobilities that are substantially higher than possible in GaN. AlN hole concentrations of ∼4.4 × 1018 cm−3 and 0.045 Ω cm resistivity and electron concentrations of ∼6 × 1018 cm−3 and ∼0.02 Ω cm resistivity are shown and offer substantial promise for future generations of AlN bipolar electronic and optical devices.

Effects of Cu, Zn Doping on the Structural, Electronic, and Optical Properties of α-Ga2O3: First-Principles Calculations.

The intrinsic n-type conduction in Gallium oxides (Ga2O3) seriously hinders its potential optoelectronic applications. Pursuing p-type conductivity is of longstanding research interest for Ga2O3, where the Cu- and Zn-dopants serve as promising candidates in monoclinic β-Ga2O3. However, the theoretical band structure calculations of Cu- and Zn-doped in the allotrope α-Ga2O3 phase are rare, which is of focus in the present study based on first-principles density functional theory calculations with the Perdew-Burke-Ernzerhof functional under the generalized gradient approximation. Our results unfold the predominant Cu1+ and Zn2+ oxidation states as well as the type and locations of impurity bands that promote the p-type conductivity therein. Furthermore, the optical calculations of absorption coefficients demonstrate that foreign Cu and Zn dopants induce the migration of ultraviolet light to the visible-infrared region, which can be associated with the induced impurity 3d orbitals of Cu- and Zn-doped α-Ga2O3 near the Fermi level observed from electronic structure. Our work may provide theoretical guidance for designing p-type conductivity and innovative α-Ga2O3-based optoelectronic devices.

Ab Initio Time-Domain Study of Charge Relaxation and Recombination in N-Doped Cu2O.

Cu2O is a good photoelectric material with excellent performance, and its crystal structure, electronic structure, and optical properties have been extensively studied. To further illustrate the charge distribution and the carrier transport in this system, the e-h recombination dynamics was studied. It is found that N doping induced a shallower impurity band above the VBM, leading to significant charge localization around the impurity atom. NAMD simulation reveals that the N doping system possesses a longer e-h nonradiative recombination time scale. Therefore, we demonstrate that the formation of the impurity band and charge localization play an essential role in suppressing e-h recombination in N doping systems. This work is conducive for understanding the carrier transport mechanism in N-doped Cu2O.

Electronic structure of boron and aluminum δ-doped layers in silicon

Recent work on atomic-precision dopant incorporation technologies has led to the creation of both boron and aluminum δ-doped layers in silicon with densities above the solid solubility limit. We use density functional theory to predict the band structure and effective mass values of such δ layers, first modeling them as ordered supercells. Structural relaxation is found to have a significant impact on the impurity band energies and effective masses of the boron layers, but not the aluminum layers. However, disorder in the δ layers is found to lead to a significant flattening of the bands in both cases. We calculate the local density of states and doping potential for these δ-doped layers, demonstrating that their influence is highly localized with spatial extents at most 4 nm. We conclude that acceptor δ-doped layers exhibit different electronic structure features dependent on both the dopant atom and spatial ordering. This suggests prospects for controlling the electronic properties of these layers if the local details of the incorporation chemistry can be fine-tuned.

On-chip lateral Si:Te PIN photodiodes for room-temperature detection in the telecom optical wavelength bands.

Photonic integrated circuits require photodetectors that operate at room temperature with sensitivity at telecom wavelengths and are suitable for integration with planar complementary-metal-oxide-semiconductor (CMOS) technology. Silicon hyperdoped with deep-level impurities is a promising material for silicon infrared detectors because of its strong room-temperature photoresponse in the short-wavelength infrared region caused by the creation of an impurity band within the silicon band gap. In this work, we present the first experimental demonstration of lateral Te-hyperdoped Si PIN photodetectors operating at room temperature in the optical telecom bands. We provide a detailed description of the fabrication process, working principle, and performance of the photodiodes, including their key figure of merits. Our results are promising for the integration of active and passive photonic elements on a single Si chip, leveraging the advantages of planar CMOS technology.

JWST MIRI Flight Performance: Detector Effects and Data Reduction Algorithms

The detectors in the Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST) are arsenic-doped silicon impurity band conduction (Si:As IBC) devices and are direct descendants of the Spitzer IRAC long wavelength arrays (channels 3 and 4). With appropriate data processing, they can provide excellent performance. In this paper we discuss the various non-ideal behaviors of these detectors that need to be addressed to realize their potential. We have developed a set of algorithms toward this goal, building on experience with previous similar detector arrays. The MIRI-specific stage 1 pipeline algorithms, of a three stage JWST calibration pipeline, were developed using pre-flight tests on the flight detectors and flight spares and have been refined using flight data. This paper describes these algorithms, which are included in the first stage of the JWST Calibration Pipeline for the MIRI instrument.

Enhanced electrical conductivity and lithium ion diffusion rate of LiFePO4 by Fe site and P site doping

Lithium iron phosphate (LiFePO4) with an olivine-type structure has garnered great interest due to its electrochemical activity. It is considered one of the most promising cathode materials for lithium-ion batteries because of its high discharge platform, large theoretical capacity, and excellent cycling performance and stability. However, the typical crystal structure of LiFePO4 restricts the formation of efficient electron conduction pathways and hampers the embedding and removal motion of lithium ions, resulting in low electron conductivity and a low ion diffusion rate. The experimental results show that element doping can reduce the activation energy of diffusion, increase the diffusion rate of lithium ions, improve the rate capability of lithium-ion batteries, and contribute to the improvement of its conductivity. In this work, the electronic structure and diffusion mechanism of a S/Co/Mn co-doped LiFePO4 system are systematically studied. The introduction of S, Co, and Mn atoms leads to the formation of impurity bands, resulting in a significant reduction in the bandgap from 3.593 eV to 0.190 eV, which is beneficial for improving the conductivity of LiFePO4. Moreover, the diffusion activation energy of lithium ions decreases from 0.73 eV to 0.34 eV, and the diffusion rate of lithium ions increases from 2.7 × 109 cm/s to 1.0 × 1016 cm/s. These results demonstrate that co-doping of S, Co, and Mn can effectively enhance the conductivity and diffusion rate of lithium ions in LiFePO4, providing a reliable reference for improving the electrochemical performance of lithium iron phosphate.

Circular Photogalvanic Current in Ni-Doped Cd3As2 Films Epitaxied on GaAs(111)B Substrate.

Magnetic element doped Cd3As2 Dirac semimetal has attracted great attention for revealing the novel quantum phenomena and infrared opto-electronic applications. In this work, the circular photogalvanic effect (CPGE) was investigated at various temperatures for the Ni-doped Cd3As2 films which were grown on GaAs(111)B substrate by molecular beam epitaxy. The CPGE current generation was found to originate from the structural symmetry breaking induced by the lattice strain and magnetic doping in the Ni-doped Cd3As2 films, similar to that in the undoped ones. However, the CPGE current generated in the Ni-doped Cd3As2 films was approximately two orders of magnitude smaller than that in the undoped one under the same experimental conditions and exhibited a complex temperature variation. While the CPGE current in the undoped film showed a general increase with rising temperature. The greatly reduced CPGE current generation efficiency and its complex variation with temperature in the Ni-doped Cd3As2 films was discussed to result from the efficient capture of photo-generated carriers by the deep-level magnetic impurity bands and enhanced momentum relaxation caused by additional strong impurity scattering when magnetic dopants were introduced.

Determination of electronic band structure of quaternary ferromagnetic Ga0.97-yMn0.03CryAs epitaxial layers

Introducing transition metals to conventional III-V semiconductors anomalously changes their fundamental characteristics, such as electronic, magnetic, and structural properties. In this study, we show that the valence band anti-crossing (VBAC) model can be exploited to calculate the electronic band structure of the quaternary Ga0.97-xMn0.03CrxAs epitaxial layers. In this model, the localized Mn and Cr defect states interact with the valence band states (VB), reconstructing VBs and splitting each VB state. The splitting top of the VB state forms an impurity band (IB) and fundamental VB edge. Photomodulated reflectance (PR) spectroscopy is exploited to determine optical transition energies at room temperature. PR spectra were analyzed with the third derivative functional form (TDFF) signal's line shape. The experimental optical transition energies, including band-to-band and spin split-off band transitions, match the calculated optical transition energies by the VBAC model. In the calculation, the interaction energy between localized Mn/Cr-energy level and valence band edges is experimentally determined as 0.7 eV.

First-principles study on electronic and optical properties of sn-doped topological insulator Bi2Se3

Electronic structures and optical properties of pure and sn-doped Bi2Se3 were investigated by first-principles calculations based on density functional theory (DFT). For Bi2-xSnxSe3 (x = 0, 0.083, 0.167, 0.25), the introduction of Sn induced impurity bands near the Fermi level. Sn@(0,3) configuration with longer sn-Sn distance was more stable for the smallest formation energy. From the charge density result, sn-doped Bi2Se3 was the material with covalent and ionic characteristic. There was obvious blue shifting in the dielectric function and ultraviolet absorption edges for sn-doped Bi2Se3. The results had implications for the fabrication of sn-doped Bi2Se3 optical devices that can be blue-shifted in ultraviolet (UV) region.

Electrical conductivity and Hall effect in n-type CdS

ABSTRACT The experimental data of the temperature dependence of the electrical conductivity σ(T) and the Hall coefficient RH (T) on single crystal samples of n-type CdS reported in literature are reviewed and have been critically analysed including hopping conduction in an impurity band. It is shown that the experimental data of σ(T) and RH (T) reported in almost previous studies can be simultaneously fitted using a common set of the values of material parameters for calculating free-electron conduction, i.e. the conduction and the density-of-state effective mass of electrons, the acoustic-phonon deformation potential, the Fröhlich coupling constant and the piezoelectric coupling coefficient, whereas wide ranges of values for these material parameters had been adopted in different studies. The correct assignment of impurity conduction mechanisms in n-type CdS single crystals is presented. In addition, reasonable explanations are given for unresolved enigmas regarding impurity pairing, unexpected large Hall mobility at low temperatures, and the effect of electric field on hopping Hall mobility.

Efficient degradation of tetracycline over vacancy-modified Cu-doped Bi2O2S via peroxymonosulfate activation and photocatalysis

Herein, Cu-doped Bi2O2S catalysts with abundant oxygen vacancies (abbreviated Cu/BOS-OV) were prepared by calcining one-step hydrothermally synthesized Cu-doped Bi2O2S in a tube furnace. The photodegradation efficiency of tetracycline (TC, 15 mg L−1) over Cu/BOS-OV reached 79% under visible light owing to the introduction of impurity bands by Cu doping and oxygen vacancies, improving the separation efficiency of photogenerated carriers. However, the Cu/BOS-OV catalyst achieved an efficiency of 90.4% for 30 mg L−1 TC degradation within 30 min due to peroxymonosulfate (PMS) activation by oxygen vacancies and copper ions. The intermediates of TC degradation were analyzed by LC‒MS, and the possible decomposition process of TC was proposed. The free radical trapping and ESR experiments showed that ·O2−, 1O2, and ⋅OH/SO4•− participated in the degradation of TC. Furthermore, the free energy diagram of HSO5− dissociation and ·OH/SO4•− desorption processes indicated that the enhanced PMS activation efficiency toward TC degradation was contributed by the decreased adsorption energy and increased O–O bond length of HSO5− on Cu/BOS-OV. This study provides good application prospects for removing organic contaminants based on photocatalysis and PMS activation technology.

Low-temperature radiation damage studies in single-walled carbon nanotube bundles by hopping conductance

The paper shows the results of studying radiation defects in single-walled carbon nanotube bundles at low temperatures. The data were obtained in a unique experiment that inseparably combines 1 MeV electron irradiation at a temperature of ∼10 K and electrical conductivity measurements over a wide temperature range of 5–300 K. The experiment allowed us to reveal and separate the influence of stable and unstable radiation defects on the electrical properties of SWCNTs. It is shown that the 2D Mott variable range hopping conduction regime exhibits resistance to electron irradiation. The hopping conductivity is the dominant charge transfer mechanism in the temperature range of 5–50 K in both the pristine and irradiated SWCNT samples. It was found that the parameters of hopping transport demonstrate high sensitivity to the introduction and annealing of radiation defects. The low-temperature electron irradiation revealed the possibility of the emergence of an additional conduction channel in the irradiated SWCNTs. This channel demonstrates thermally activated behavior with a low activation energy of ∼0.5 meV at temperatures of 5–50 K. The obtained data indicate that unstable radiation defects create conditions for the possibility of impurity band conduction in SWCNT bundles.

Mo-modified (t-m)-ZrO2 with narrowed band gap and strong ability to activate reactants for photocatalytic CO2 reduction

ZrO2 is a promising candidate for photocatalytic CO2 reduction due to its high conduction potential, but it suffers from narrow sunlight absorption range and weak activation ability to reactants. Herein, we explore Mo-modified (t-m)-ZrO2 photocatalysts by a one-step hydrothermal method. As compared to bared ZrO2, their photoresponse extends to the visible light region, being attributed to the impurity band of Mo 4d states embedded in the ZrO2 band gap. Meanwhile, PL and electrochemical tests illustrate that the impurity band also leads to an increase in the carrier separation efficiency. MoO3 nanoparticles and doping Mo in Mo-modified ZrO2 photocatalysts promotes H2O and CO2 activation, respectively. Moreover, in situ FTIR and theoretical calculation revealed that Mo modification is also favorable for the generation of *COOH intermediate and the desorption of CO. As a result, the optimized Mo-modified ZrO2 exhibited a 27.3 μmol·g−1 of the CO yield with ∼100% selectivity, being 13 times higher activity than bared ZrO2. This work provides a guidance for the exploitation of wide bandgap semiconductors in the field of photocatalytic CO2 reduction.

Structure of the impurity band in heavily doped nonmagnetic semiconductors

We study the properties of the impurity band in heavily doped nonmagnetic semiconductors using the Jacobi-Davidson algorithm and the supervised deep learning method. The disorder averaged inverse participation ratio and Thouless number calculation show us the rich structure inside the impurity band. A convolutional neural network model, which is trained to distinguish the extended/localized phase of the Anderson model with high accuracy, shows us the results in good agreement with the conventional approach. Together, we find that there are three mobility edges in the impurity band for a specific on-site impurity potential, which means the presence of the extended states while filling the impurity band.

Photo-responsive Mn-doped TiO2-based superhydrophobic/ underwater superoleophobicity membrane for efficient oil-water separation and photothermal decontamination

In recent years, oily wastewater has become an important factor in environmental pollution. As a transition metal element, manganese is able to introduce new impurity band gaps using d-orbital hybridization to extend the photoresponse range of TiO2, thus enabling TiO2 to be self-cleaned under UV irradiation more easily. Therefore, we synthesized superhydrophobic/submerged superoleophobic Mn@TiO2-modified membranes which possess a self-cleaning property under UV light using manganese element-doped titanium dioxide and cotton fabric as the substrate by hydrothermal method and then investigated the relevance between the doping concentration of Mn and super-hydrophobicity. The results showed that water contact angles of prepared membranes with the Mn ions doping concentrations of 0.5 mol/L, 1.0 mol/L, and 1.5 mol/L were 147.89°, 150.43°, and 153.37°, respectively. The membrane with 1.5 mol/L was effective for the separation of the dichloromethane-water mixture. Under Xenon lamp irradiation, three membranes with different concentrations showed a significant temperature increase with the maximum rising to 53.1 ℃ within 2 min, reflecting an excellent photothermal conversion capability. In addition, the decontamination ability of the Mn@TiO2 membrane was tested under UV light. The Mn@TiO2 membrane was able to degrade the oil-red solution dripping on the surface, showing that the membrane had a good self-cleaning performance, which was significant for industrial applications.

Effect of Mg deficiency on the thermoelectric properties of Mg2(Si, Sn) solid solutions

In this study, we analyze the change in the thermoelectric properties of a Mgx(Si, Sn) system upon varying nominal Mg content x (x = 1.8, 1.9, 2.1, 2.2, 2.3). We find that Mg deficiency induces multiple point defects, such as Mg vacancies and anion anti-site defects, as well as phase separation of crystal structure, both of which alter the band structure. Systematic investigations are followed to elucidate how thermoelectric properties of a Mgx(Si, Sn) system are influeced by the degree of Mg deficiency. Temperature-dependent measurements of the Seebeck coefficient and the Hall coefficient indicate that Mg deficiency induces p-type behavior; this is confirmed by an analysis of the thermoelectric transport properties by using a single parabolic band model at low temperature. The p-type conduction in the Mg-deficient system suggests that Mg vacancies are the predominant defects. However, the efficiency of p-type doping is two to three orders of magnitude lower (∼1018 cm−3) than expected when considering only Mg vacancies as the origin of charge carriers. Density functional theory calculations suggest that the anion anti-site defect on the Mg site is attributable for the low measured carrier concentration and low doping efficiency. Moreover, the energy band structure of the Mg-deficient Mgx(Si, Sn) system is shown to undergo significant deformation and allow the formation of an impurity band at the X point due to the Mg-deficient conditions, enabling the system to have a relatively anion-rich condition. Furthermore, Mg deficiency minimizes the thermal conductivity to 1–1.5 W/m∙K at room temperature owing to an early bipolar contribution and separated structural phases. This study provides the Mg deficiency as a new control parameter that can be utilized to enhance the p-type thermoelectric performance of Mg silicides.