Impurity Energy Research Articles

Reduction in Absorption Edge of the Digital-Alloyed ZnMnO Thin Films Grown Using Atomic Layer Deposition

Transition metal doping has been used as a common approach for tuning the bandgap energy of zinc oxide (ZnO) based materials. In this work, manganese-doped ZnO (ZnMnO) thin films were grown using the atomic layer deposition (ALD) and used to study the effect of manganese incorporation in ZnO films. The effect of digital alloying on reducing the absorption edge of the ZnMnO thin films is studied by changing the ZnO to MnO cycle ratio during each ALD super-cycle. Room-temperature optical spectroscopic characterization showed a significant reduction in bandgap energy from 3.30 eV for pure ZnO to around 2.25 eV for the ZnMnO thin films with the ZnO:MnO cycle ratio of 5:3. This is likely due to the effect of band bending, which can be related to the formation of Mn-related impurity energy levels in the band structure of the ZnMnO. A notable change in the deduced optical bandgap energy was observed by further increasing the growth sequence of ZnO and MnO layers in each super-cycle. This was attributed to less Mn incorporation in these films caused by a highly suppressed ALD growth rate of MnO on ZnO. The observed changes in the optical bandgap of ZnMnO thin films demonstrates the potential for using them as the active light-absorbing or the window layer in high-efficiency multi-junction solar cells or photodiodes.

Calculation of band structure of the strained germanium nanofilm, doped with a donor impurity

Abstract The band structure of the doped germanium nanofilm grown on the G e ( x ) S i ( 1 − x ) (001) substrate was calculated on the basis of the theory of deformation potential. It was shown that internal mechanical strains exceeding 2% arise in the germanium nanofilm with increasing S i content in the substrate. The presence of such strains leads to the ( L 1 − Δ 1 ) type inversion of the absolute conduction band minimum of the nanofilm. As a result of such radical reconstruction of germanium band structure under deformation, the ionization energy of the donor dopant in the nanofilm is increasing which is explained by the increase of the effective mass of electron and, accordingly, the decrease of the characteristic size of the wave function of electron that is localized on the donor impurity. The dependence of the ionization energy of impurity on the magnitude of the internal strains is due to different values of the baric coefficients of the shift of the energy level of impurity and of the lowest conduction band minimum of the nanofilm. Obtained results can be used in design and modeling of electronic devices and sensors for modern nanoelectronics with pre-set operating characteristics on the basis of strained germanium nanofilm doped with various donors.

Peculiarities of admittance spectroscopy study of wide bandgap semiconductors on the example of diamond

To improve the performance characteristics of power and high-frequency electronics, wide bandgap semiconductors are now widely used. This paper presents consideration of features arising during exploration of electronic characteristics of wide bandgap materials. We use the admittance spectroscopy method for analyzing free carrier concentration and boron-impurity activation energy in semiconductor diamond. The special aspect that should be taken into account while studying wide bandgap materials is incomplete ionization of impurity. In this work we adjust the experimental conditions, basing on the previous experience, in particular reduce signal/noise ratio and reckon with heat capacity of the samples and substrate. As a result we obtained high quality conductance spectra and activation energy of boron impurity in low-doped diamond.

Impact of QW coupling on the binding energy in InGaN/GaN under the effects of the size, the impurity and the internal composition

In the present paper, the binding energy of hydrogenic shallow-donor impurity in simple and double coupled quantum wells based on unstrained wurtzite (In,Ga)N/GaN is investigated. Considering the effective-mass and dielectric mismatches between the well and its surrounding matrix, the numerical calculations are performed within the framework of the parabolic band and the single band effective-mass approximations under the finite potential barrier using finite element method (FEM). According to our results, it appears that the main effect of the wells coupling is to enhance the binding energy. It is also obtained that the binding energy is strongly sensitive to the internal and external parameters and can be adjusted by the quantum well/barrier width, the impurity position and the internal Indium composition. Our results are in good agreement with the finding especially for those obtained by the variational approach.

Theoretical study on Schottky regulation of WSe<sub>2</sub>/graphene heterostructure doped with nonmetallic elements

采用平面波超软赝势方法研究了非金属元素掺杂对二硒化钨/石墨烯肖特基电子特性的影响. 研究表明二硒化钨与石墨烯层间以范德瓦耳斯力结合形成稳定的结构. 能带结果表明二硒化钨与石墨烯在稳定层间距下形成n型肖特基势垒. 三维电子密度差分图表明石墨烯中的电子向二硒化钨移动, 使二硒化钨表面带负电, 石墨烯表面带正电, 界面形成内建电场. 分析表明, 将非金属原子掺杂二硒化钨可以有效地调控二硒化钨/石墨烯肖特基势垒的类型和高度. C, O原子掺杂二硒化钨时, 肖特基类型由p型转化为n型, 并有效降低了肖特基势垒的高度; N, B原子掺杂二硒化钨时, 掺杂二硒化钨体系表现出金属性, 与石墨烯接触表现为欧姆接触. 本文结果可为二维场效应晶体管的设计与制作提供相关指导.

Donor impurity energy and optical absorption in spherical sector quantum dots

The properties of the conduction band energy states of an electron interacting with a donor impurity center in spherical sector-shaped GaAs-Al0.3Ga0.7As quantum dots are theoretically investigated. The study is performed within the framework of the effective mass approximation through the numerical solution of the 3D Schrödinger equation for the envelope function via the finite element method. The modifications undergone by the spectrum due to the changes in the conical structure geometry (radius and apical angle) as well as in the position of the donor atom are discussed. With the information regarding electron states the linear optical absorption coefficient associated with transition between confined energy levels is evaluated and its features are discussed. The comparison of results obtained within the considered model with available experimental data in GaAs truncated-whisker-like quantum dots shows very good agreement. Besides, our simulation leads to identify the lowest energy photoluminescence peak as donor-related, instead of being associated to acceptor atoms, as claimed after experimental measurement (Hiruma et al. (1995) [14]). Also, a checking of our numerical approach is performed by comparing with analytical solutions to the problem of a spherical cone-shaped GaN with infinite confinement and donor impurity located at the cone apex. Coincidence is found to be remarkable.

Adsorption performance of SO2 gases over the transition metal/P‒codoped graphitic carbon nitride: A DFT investigation

The adsorption performance of SO2 on the P‒doped and transition metal (TM)/P‒codoped graphitic carbon nitride (gCN) systems (TM = Co, Rh, and Ir elements) were studied by DFT methods. The adsorption energy (Eads) results demonstrated that the interaction energy between SO2 and TM/P‒codoped gCN systems are stronger than that of the pristine gCN. Furthermore, it was found that SO2 molecule prefers to adsorbed on the Ir/P‒codoped gCN with superior Eads of −3.52 eV. Hence, the Ir/P‒codoped gCN is a suitable candidate for removing and detecting SO2 molecules from the atmosphere, in comparison with that of the other adsorbents. Besides, the results revealed that the presence of SO2 molecules over surface of doped and codoped gCN systems led to considerable changes in the magnetic, and structural characteristics of these systems. Furthermore, the Lowdin charges analysis demonstrated that with adsorption of SO2 over the TM/P‒codoped gCN systems, electrons remarkable transfer from the TM/P‒codoped systems to SO2 gas molecules owing to the strong adsorption of this gas. The results of electronic band structure displayed that with adsorption of SO2 gas and also codoping of TM/P elements, the electrical conductivity of gCN systems significantly decreased owing to the induced new impurity energy states near the Fermi energy level (Ef). The results of partial density of states (PDOS) plots for the P‒doped, TM/P‒codoped, and SO2‒adsorbed gCN systems revealed that the sharp peak are localized near the Ef in SO2‒adsorbed Ir/P‒codoped system, which confirmed the strong interaction between SO2 gas molecules and this system. Thus, it can be deduced that the Ir/P‒codoped system with the highest Eads can be efficiently used for the removing and sensing of SO2 gas from the environment.

First-excited-state energy of hydrogenic impurity bound polaron in an anisotropic quantum dot in an electric field

The first-excited-state (ES) binding energy of hydrogenic impurity bound polaron in an anisotropic quantum dot (QD) is obtained by constructing a variational wavefunction under the action of a uniform external electric field. As for a comparison, the ground-state (GS) binding energy of the system is also included. We apply numerical calculations to KBr QD with stronger electron–phonon (E–P) interaction in which the new variational wavefunction is adopted. We analyzed specifically the effects of electric field and the effects of both the position of the impurity and confinement lengths in the xy-plane and the [Formula: see text] direction on the ground and the first-ES binding energies (BEs). The results show that the selected trial wavefunction in the ES is appropriate and effective for the current research system.

In situ growth of g-C3N4 on TiO2 nanotube arrays: Construction of heterostructures for improved photocatalysis properties

In this work, we successfully constructed g-C3N4/TNTs heterostructures via in situ growth of g-C3N4 on the surface of TiO2 nanotube arrays (TNTs). Varying concentrations of urea precursor were adopted to prepare the binary composites for the photodegradation of methylene blue (MB). Advanced microscopic and spectroscopic approaches such as FESEM, PL-Raman, UV–vis DRS, XRD and etc examined the topography, structural and optical properties attributed to the presence of g-C3N4 in the heterostructures. The morphological analysis showed that the in-situ growth of g-C3N4 onto the surface of TNTs significantly increased the wall thickness of the nanotubes. The least band energy of 1.8 eV was obtained by g-C3N4/TNTs (1.5 g) due to the formation of an impurity energy level induced by the presence of g-C3N4. The electron transfer between the heterojunction of g-C3N4 and TNTs was revealed by the quenching of PL emission intensity. When the urea content was optimized at 1.0 g, the build-in electric field at the interface of g-C3N4/TNTs stimulated the electrons transfer and prolonged lifetime of carriers, thus enhancing the degradation efficiency by 1.25 times higher than that of pure TNTs. However, the aggregation of g-C3N4 as a result of increasing urea content (1.5–2.0 g) reduced the interfacial adhesion at the heterojunction between the g-C3N4 and TNTs, thus dominating its excellent optical and charge separation properties and diminishing the degradation efficiency of MB.

On the Characteristic Features of the Impurity Energy Spectrum in Arsenides

The impurity energy spectrum of undoped n-type GaAs, InAs, CdSnAs2, CdGeAs2, and CdTe, ZnO bulk crystals is studied based on a quantitative analysis of the baric and temperature dependences of the kinetic coefficients. It is found that the deep-level donor center corresponds to the intrinsic vacancy defect in the anion sublattice in the above-listed semiconductors. The conclusion on the nature of the donor center, i.e., the vacancy in the anion sublattice, is based on the fact that, in contrast to shallow impurity centers which trace the intrinsic band under uniform pressure, to which they are genetically related, the energy of deep impurity centers with respect to absolute vacuum remains constant under isotropic compression of the lattice. Therefore, it seems favorable to study the evolution of the carrier energy spectrum in semiconductors under uniform-pressure conditions. The energy-level positions with respect to the conduction-band edge and pressure coefficients of the energy gaps between them and the corresponding conduction-band bottom are determined. The shift of the energy level of the deep donor center to the conduction-band depth with decreasing band gap is observed.

First-principles study on electronic and optical properties of S, N single-doped and S-N co-doped ZnO

The electronic and optical properties of undoped, N single-doped, S single-doped, and S-N co-doped ZnO were systematically investigated by first-principles calculations. The lattice parameters of S single-doped and S-N co-doped ZnO clearly increased. After N-doping, a strongly localized impurity energy level of N was formed near the Fermi level at the top of valence band (VB). In S-N co-doped ZnO, the localization of N weakened, and the Fermi level went deeper into the VB, indicating that the acceptor energy level of N formed in S-N co-doped system became shallower due to the effect of 3p state of S. Therefore, S-N co-doping is beneficial to obtain p-type ZnO with a higher hole concentration than N single-doping. Compared with undoped ZnO, the static dielectric constant, absorption coefficient, refractive index, energy loss function, and reflectivity of N single-doped, S single-doped, and S-N co-doped ZnO exhibited an increase in low-energy area.

TiO x N y /TiO2 Photocatalyst for Hydrogen Evolution under Visible Light Irradiation II Degradation of Photocatalytic Activity of TiO x N y /TiO2 with Mild Oxidation.

We have studied degradation of photocatalytic activity of TiOxNy for water splitting under visible light irradiation with heat treatment in O2/N2 mixed gas. The reduction of the N content by oxidation through the formation of O–N–O species (NOx) was confirmed as the result of the reduction of the catalytic activity. The catalytic activity is not simply related to the amount of N remained but that of N taking the chemical state of O–Ti–N in TiOxNy, which is the active species for visible light responsiveness on hydrogen evolution. N in TiOxNy is first oxidized to NO2 species during the oxidation, which reduces the activity. Then, O–N–O species (NOx) is removed as NOx gas from the surface. Because the formation of O–N–O in TiOxNy could induce an impurity energy level to enhance charge recombination, the loss of catalytic activity might be influenced by the formation of O–N–O species (NOx) rather than the loss of the N content from TiOxNy.

Vibrational spectra and optical properties ofFe1−xCrxVO4solid solutions: With a group theory analysis

The present manuscript reports vibrational spectra and optical studies of polycrystalline Fe1-xCrxVO4 solid solutions through FT-IR spectroscopy augmented with a group theory (G.T.) analysis and UV-Visible DRS spectroscopy. Full set of IR and Raman modes are determined by G. T. for various crystal symmetries in FeVO4-CrVO4 solid solutions where Triclinic, Monoclinic and Orthorhombic structures evolve with increasing Cr concentration. Experimentally obtained vibrational modes support the structural phase transitions and confirm formation of continuous solid solutions in Fe1-xCrxVO4. The Diffuse Reflectance Spectra (DRS) of Fe1-xCrxVO4 depicts the electronic structure and different optical transitions due to absorption of photon energy. The d-d transitions are manifested for all compounds in terms of crystal field stabilization energy (CFSE) caused by distorted lattice sites. The band gap energy of Fe1-xCrxVO4 is calculated using Tauc formula. It shows a red shift initially within triclinic structure then blue shift with the increase of Cr concentration. Urbach energy (Eu) tails in the spectra show the electronic structural disorder in Fe1-xCrxVO4 due to impurity energy levels of Cr ions within band gap region. It is observed that Eu decreases with the doping concentration due to the increase in crystal symmetry corresponding to the structural phase transitions in Fe1-xCrxVO4.

Evaluation of structural, electronic, thermoelectric, and optical results of C‐doped HfO 2 by first‐principle's investigation

In this work, we report the ab initio numerical simulation investigation on the crystal lattice, electronic structure, optical, and transport properties of pure and C-doped crystalline hafnium dioxide (c-HfO2) using FP-LAPW method. Different exchange correlation functionals like generalized gradient approximation (GGA) of PBE-sol and Tran and Blaha's modified Becke-Johnson exchange potential (mBJ) within density functional theory have been used. Two kinds of defects in cubic pure HfO2 have been investigated: one is substitution of Hf atom by C impurity and other substitution of O atom by C impurity in crystalline HfO2. The computed results indicate that impurity energy bands as a result of 2p states of C are found to present in the band gap of c-HfO2. Few of these bands are present at the conduction band minimum, which results to a noteworthy band gap contraction, and hence electrons close to Fermi level get transferred in doped c-HfO2. We have also analysed the dielectric function, absorption coefficient, optical conductivity, optical function, electron energy loss, and reflectivity for both pure HfO2 and doped with C. Furthermore, the temperature-dependent transport properties of C-doped HfO2 are also discussed in terms of Seeback coefficient, thermal conductivities, electronic conductivities, power factor, and figure of merit in the temperature range 0 to 1200 K. The calculated value of PF for pure HfO2 was found to increase from 0.01 × 1012 WK−2m−1s−1 at 50 K to 1.79 × 1012 WK−2m−1s−1 at 1200 K and for HfO2(1 − x)Cx it was found to increase from 0.06 × 1012 WK−2m−1s−1 at 50 K to 0.25 × 1012 WK−2m−1s−1 at 1200 K.

Transparent ohmic contact for boron doped diamond using p-type NiO film synthesized through oxidation

Transparent electrode is promising for diamond optoelectronic device application. Herein, a natural p-type conductive NiO synthesized by thermal oxidation is adopted as the ohmic electrode for boron-doped diamond. The NiO/diamond contact shows an ohmic-like current-voltage curve with a weak rectification effect. The interfacial band alignment between NiO and diamond is measured by X-ray photoelectron spectroscopy. The corresponding band offsets are deduced to approximately 1.07 eV (valence band) and 0.74 eV (conduction band), respectively. Based on the results above, the ohmic contact is supposed to be the tunneling mechanism with the assistance of the impurity energy bands.

The preparation and photocatalytic properties of Na doped ZnO porous film composited with Ag nano-sheets

Abstract The precursors with porous skeleton structures were prepared by a sol-gel process with the different mass ratios of Zn source to glucose as the raw materials. Then Na doped ZnO films were deposited on the precursors by a chemical vapor deposition (CVD) method. Finally, the sample prepared with optimized conditions was composited with Ag nano-sheets via a chemical method, and a sample of Na doped ZnO porous film composited with Ag nano-sheets was obtained. The photocatalytic characteristics of the samples were investigated with the photocatalytic degradation of methyl orange (MO) solution at room temperature. With the modifications of the precursor structures, the following Na doping, and the final composite with Ag nano-sheets, the photocatalytic properties of the samples showed progressive improvement. The optimum photocatalytic performance was achieved in the sample composited with Ag sheets, with the mass ratio of Zn source to glucose as 8:1 precursor and the carrier flow rate of 75 sccm Na doping. The excellent photocatalytic performance attributed to the porous structure, the additional impurity energy levels, as well as the prolongation of exciton life.

Effect of Cu, N co-doping on conductive properties of AgSnO2 contact

The structure, impurity formation energy, energy band and density of states of SnO2-N,SnO2-Cu and SnO2-Cu-N were studied and analyzed theoretically by using the first- principles method based on the density functional theory in this study. AgSnO2 contacts with different additives were prepared by using sol-gel method and powder metallurgy method, and a series of experiments were carried out on the samples. The simulation results show that the impurity formation energy of Cu,N co-doped SnO2 is the smallest, and its structure is the most stable compared with other doping systems. In the vicinity of Fermi energy level, there is a strong hybridization between Cu atoms and N atoms, and more impurity energy levels are introduced into the forbidden band, which increases the carrier concentration, decreases the band gap and enhances the conductivity. The experimental results show that the doping does not change the structure of SnO2, so the doped SnO2 still belongs to the tetragonal crystal system. And the Cu,N co-doping can greatly improve the wettability between SnO2 and Ag, so that the aggregation of the SnO2 on the contact surface can be effectively prevented, and the contact resistance is reduced. Among the four kinds of AgSnO2, AgSnO2-N, AgSnO2-Cu, AgSnO2-Cu-N contacts, the conductivity of AgSnO2-Cu-N contact is maximum, which is 56.18%IACS, and the contact resistance, arc duration, arc energy and welding force are the smallest, which are 0.946 mΩ, 1.90 ms, 174.31mj and 28.05cN, respectively. Therefore, Cu,N co-doping can fully combine the advantages of Cu single doping and the advantages of N single doping to make up for the shortcomings of AgSnO2 contact, which further improves its conductivity. Moreover, Cu,N co-doping also improves the arc corrosion resistance and anti-welding behaviour of the contact.

Effective Correlation of Two Holes in a Semimagnetic Quantum Well Wire: Influence of Shape of Confining Potential on Coulomb Interaction**Supported by University Grants Commission, New Delhi, India under Major Research Project F. No. 42-816/2013(SR)

The hole-hole interaction has been considered in a Semimagnetic Quantum Well Wire (SQWW). The influence of the shape of the confining potential like square well and parabolic well type on the binding energy of an acceptor impurity with two holes and their Coulomb interaction between them has been studied for various impurity locations. Magnetic field has been used as a probe to understand the carrier-carrier correlation in such Quasi 1-Dimensional QWW since it alters the strength of the confining potential tremendously. In order to show the significance of the correlation between the two holes, the calculations have been done with and without including the correlation effect in the ground state wavefunction of the hyderogenic acceptor impurity and the results have been compared. The expectation value of the Hamiltonian, , is minimized variationaly in the effective mass approximation through which has been obtained.

Density functional study on electric structure and optical properties in Na-doped 3C-SiC

Structural stability along with the electronic and the optical properties of intrinsic 3C-SiC, [Formula: see text] and [Formula: see text] are studied by the first-principles calculations. [Formula: see text] system possesses the most considerate stability with lowest binding energy [Formula: see text] and formation energy [Formula: see text] compared to [Formula: see text]. It is observed that the non-filled impurity energy levels in the vicinity of the Fermi level can subsequently give rise to an enhancement of electrical conductivity of 3C-SiC. Through the analysis of charge difference density maps, it is found that covalence of bonds between the Na atom and nearby C atom reduces in varying degrees. In different concentrations of Na doping systems, especially for the supercell of [Formula: see text], the real and imaginary parts of the dielectric constant are visibly added in the energy range of 0–0.5 eV, demonstrating that the dielectric loss property of the 3C-SiC is improved evidently. These features confirm that the Na-doped 3C-SiC semiconductor is propitious to the wide application of 3C-SiC in the field of absorbing materials.

Tunable magnetic order in transition metal doped, layered, and anisotropic Bi2O2Se : Competition between exchange interaction mechanisms

$\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ is a novel layer-structured material with high electron mobility, while its efficiency could be greatly improved by doping different elements to introduce a magnetic spin order. We investigated the electronic and magnetic properties of various transition metal (TM) ($\mathrm{TM}=\mathrm{Mn}$, Cr, Fe, Co, and Ni) doped $\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ within a framework of density functional theory (DFT), and discovered that $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}n}{\mathrm{X}}_{n}{\mathrm{O}}_{2}\mathrm{Se}$ exhibits long-range magnetic ordered structure via competition among double-exchange, p-d exchange, and superexchange interaction. The magnetic order of the bulk phase in which the magnetic atoms form interlayer coupling would vary with the type and concentration of doped atoms, but all the layered phases in which the magnetic atoms are in-plane coupled show ferromagnetic order. By combing DFT calculations with the Monte Carlo scheme, we solve the exchange interaction constants for the Heisenberg model and further evaluate the Curie temperatures of $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}n}{\mathrm{X}}_{n}{\mathrm{O}}_{2}\mathrm{Se}$. Ferromagnetic order for most doped systems exhibit to be robust with high Curie temperature, some of which overcomes room temperature (for 12.5% Co-doped layer $\mathrm{B}{\mathrm{i}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$). It is also worth mentioning that the appearance of impurity energy levels narrows the band gap and enhances the spin-orbit coupling of $d$ orbitals and therefore increase large magnetic anisotropy energy. Our study demonstrates a potential pathway to design new dilute magnetic semiconductors through doping of $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}n}{\mathrm{X}}_{n}{\mathrm{O}}_{2}\mathrm{Se}$ by magnetic transitional elements.