xMgxO Alloy Research Articles

Magnesium Diffusion from MgO Substrates in Sol–Gel‐Derived NiO Epitaxial Films: Effects of Heat Treatment Temperature and Li Doping

The effects of heat treatment temperature and Li doping on the structural properties of sol–gel‐derived NiO epitaxial films on MgO (100) substrates are investigated. X‐ray diffraction measurements indicate that the out‐of‐plane lattice parameters of undoped NiO films increase as the heat treatment temperature increases. This increase is particularly significant in films treated above 900 °C, in which out‐of‐plane and in‐plane lattice parameters are larger than the expected values for bulk NiO. This result cannot be explained by the strain originating in the lattice mismatch between NiO and MgO. In addition, the bandgap drastically increases in undoped films treated above 900 °C. Secondary ion mass spectrometry depth profiling demonstrates that undoped films treated above 900 °C are Ni1−xMgxO alloys generated by the diffusion of Mg from MgO substrates into NiO films, resulting in the anomalous characteristics of the prepared films. Furthermore, it indicates that the diffusion of Mg is reduced in Li‐doped films, which can be explained on the basis of defect chemistry, in which Ni vacancies decrease with the increase in Li concentration. Consequently, the increases in the lattice parameter and bandgap at high heat treatment temperatures are reduced in Li‐doped films.

Structural, electronic, and optical properties of inhomogeneous Ca1−xMgxO alloys

The structural, electronic, and optical properties of homogeneous and inhomogeneous Ca1−xMgxO alloys are studied with the density functional theory (DFT) under the assumption of polymorphism. Large supercells (2×2×2 and above) with different lattice symmetries and varying Mg molar fraction x were constructed, representing distinct solid phases that may coexist in micro/nanodomains of inhomogeneous alloys. We demonstrate that these polymorphs exhibit rich phenomenology like similar formation enthalpies for a given concentration x, but different electronic and optical properties. For example, Ca0.5Mg0.5O may have crystallites with four possible lattice symmetries using a 2×2×2 supercell for its description, with bandgaps varying between 3.26 eV (direct) and 4.46 eV (indirect). The DFT-simulated X-ray diffraction shows that polymorphism causes broadening and shift of the diffraction peaks. We also performed a detailed calculation of the bandgaps, optical absorption, and dielectric constants as a function of x for each polymorph, and they exhibit a structured bandgap behavior with maxima and minima in the 0.0<x<1.0 range, which is in marked contrast to Vegard’s law rule of mixtures. We also report a direct-to-indirect bandgap transition occurring between 0.072≤x≤0.1094 for the FM3¯M symmetry.

Influence of size and ternary mixed crystals on optical absorption of excitons in Zn[formula omitted]Mg[formula omitted]O/ZnO/Zn[formula omitted]Mg[formula omitted]O quantum wells

Optical absorption of exciton transitions in asymmetry Zn1−xMgxO/ZnO/Zn1−yMgyO quantum wells (QWs) is investigated and the effects of size and ternary mixed crystals (TMCs) are discussed in a wider TMC region. The different exciton states and interstate transition energies are obtained by the combination of a variational method and numerical calculations. Based on the density matrix approach and our weight model, the mixed phase of wurtzite and zinc blende in Zn1−xMgxO alloys within 0.37<x<0.62 is considered to discuss the optical absorption coefficients (OACs) in the presence of built-in electric fields and the Hartree potential. The results show that the peaks of OACs appear a merged phenomenon under size modulation. The change of OACs in wurtzite–wurtzite–wurtzite QWs is more obvious than that in zinc blende–wurtzite–zinc blende ones with increase of Mg component to demonstrate the TMCs effect. Our results are better agreement with the photoluminescence spectra of exciton transitions given by experiments.

Bandgap bowing in Ni1−xMgxO alloy

It is crucial to understand the compositional dependence of the bandgap of the Ni1–xMgxO alloy as it is a promising bandgap-tunable material for ultraviolet (UV) and deep-UV photodetectors. However, the dependence of the bandgap of this material on the Mg content, x, is still a subject of debate, as the experimentally measured optical bandgap of Ni1–xMgxO is 2–3 eV lower than that of pure MgO (7.77 eV), in contrast to that predicted by conventional bandgap bowing theories. Here, we demonstrate that the Ni1−xMgxO alloy has two bandgaps: (i) the Ni-3d bandgap Eg3d, i.e., the bandgap between the valence band (O 2p) and Ni-3d(eg) bands, where the charge-transfer (CT) transition energy weakly depends on the Mg content and (ii) the alloy bandgap EgNi1−xMgxO, i.e., the bandgap between the valence band and the conduction band of the Ni1−xMgxO alloy, which is related to the Ni 4s/Mg 3s states; the alloy bandgap obeys the conventional bandgap bowing model. The Ni1−xMgxO absorption spectra at low Mg contents are difficult to deconvolute because the electronic bands originating from the Ni-3d bands and the alloy conduction band overlap. The band structure described above elucidates the anomalous characteristics of the bandgap of Ni1–xMgxO, i.e., most of the optical bandgap values reported so far are smaller than the expected value corresponding to the transition from the valence band to the alloy conduction band because they were evaluated based on the absorption due to the O 2p-Ni 3d(eg) CT transition.

First-principles calculations of electronic and optical properties of Fe1−xZnxS2 and Zn1−xMgxO alloys

Electronic properties of Fe1−xZnxS2 and Zn1−xMgxO alloys are performed by using full potential linearized augmented plane wave method (FP-LAPW). Band gap energies have been calculated by using concomitantly LDA with Tran Blaha modified Becke-Johnson (TB-mBJ) potentials. The corrected positions related to valence band maximum (VBM) and conduction band minimum (CBM) have been evaluated by many-body perturbation theory in GW approximation. The electron affinities of these alloys are determined by knowledge the exact position of CBM from the fit of total density of states (TDOS). In the case of 0 at % of (Zn, Mg) alloyed elements, pyrite FeS2 and wurtzite ZnO, respectively, exhibit positive electron affinities of 3.34 eV and 4.34 eV, which are in close with experimental measurements. As for optical properties, the absorption coefficient spectra and refraction index variations are performed from momentum matrix elements and interpreted via the projected density of states. The studied alloys show significant responses in visible range and a blue shift in connection with increasing the alloyed elements.

Luminescent properties of ZnO and ZnMgO epitaxial layers under high hydrostatic pressure

Studies of ambient-pressure and high-pressure behaviour of photoluminescence (PL) as well as the phase transition pressure from wurtzite to cubic structure for a series of Zn1−xMgxO alloys as a function of the Mg-content x (0 ≤ x ≤ 0.34) are presented. The measured PL peak energy is shown to increase approximately linearly with increasing magnesium content from 3.36 eV to 4.11 eV, and the phase transition pressure from wurtzite to cubic structure decreases with increasing x from 12.6 GPa to 5.9 GPa. The phase transition is accompanied by a strong decrease of the near band-gap PL intensity. The value of the PL peak pressure coefficient dEPL/dp changes non-monotonically. It varies between 18.6 meV/GPa and 25.2 meV/GPa, and seems to depend on the layer thickness rather than on the layer composition, exhibiting strong scattering for several alloys with similar magnesium content. This behaviour can be due to the stabilizing influence of sapphire substrate, as is indicated both by the pressure coefficients and by the values of pressure of phase transition from wurtzite to rock salt structure in Zn1−xMgxO layers with the same or similar composition and different layer thicknesses.

Mechanical and electronic properties of Ca1−xMgxO alloys

The structural, mechanical, elastics anisotropy and electronic properties of Ca1−xMgxO in the cubic structure are investigated using density functional theory calculations. The lattice parameters, elastic constants and elastic modulus are in excellent agreement with the experimental and others theoretical data. The sound velocities and the Debye temperatures are calculated for all the Ca1−xMgxO alloys using the calculated elastic constants and elastic modulus. The elastic anisotropy are characterized by calculating several different anisotropic indexes and describing the three dimensional surface constructions. Finally, electronic structure studies show that Ca1−xMgxO alloys are direct band gap semiconductors.

Surface compositions of atomic layer deposited Zn1−xMgxO thin films studied using Auger electron spectroscopy

In this paper, the authors present Auger electron spectroscopy (AES) studies of Zn1−xMgxO (ZMO) films grown via interrupted atomic-layer deposition (ALD) techniques. The ZMO films were fabricated by alternating ALD deposition of ZnO and MgO layers up to 1000 cycles. Zn1−xMgxO films with progressively decreasing Mg/Zn ratios (Mg/Zn = 1/1, 1/2, 1/3, 1/4, 1/5, 1/6, 1/9, and 2/8, 3/12, 4/16, and 5/20) were fabricated for this study. The AES results exhibit an abrupt drop of Mg composition on the ZMO surface when the Mg/Zn &amp;lt; 1/3. Additionally, the surface composition ratios of O to Mg, O to Zn, and Mg to Zn were estimated with known Auger sensitivity factors. The results indicate that Mg ions diffuse into the bulk, forming Zn1−xMgxO alloys.

纤锌矿Zn1-xMgxO极化特性的第一性原理 GGA+U方法研究

Two-dimensional electron gas( 2DEG) with high-mobility was found in wurtzite Zn O /Zn1- xMgxO heterostructures which probably arises from the polarization discontinuity at the Zn O /Zn1- xMgxO interface. In this paper,we studied the polarization properties of Zn1- xMgxO alloy at different Mg composition using first-principles calculations with GGA + U method,and the polarization properties were calculated according to Berry-phase method. In addition,the polarization was divided into three parts: electronic polarization,iron polarization and piezoelectric polarization. The results indicate that the piezoelectric polarization is the most important part in these contributions.

Temperature-dependent optical transitions of sol–gel derived Zn1−xMgxO alloy films

The effect of alloy-composition on optical properties of sol–gel derived Zn1−xMgxO films (0⩽x⩽0.12), was investigated by absorption and photoluminescence spectroscopy. Alloy-clustering induced broadening of emission-linewidth is negligible in the sol–gel derived Zn1−xMgxO alloys. Analyzing the temperature-dependent band-gap shrinkage in the Zn1−xMgxO alloys based on Pässler’s two-oscillator model enables us to precisely determine the strength of electron–phonon interaction as a function of Mg-concentration. The results reveal that the coupling-strength slightly decreases as Mg-composition increases in the Zn1−xMgxO alloys. Moreover, the phonon-dispersion coefficient, associated with an average effective phonon-energy (∼38meV), is approximately 0.5 and hardly changes with Mg-composition in the Zn1−xMgxO alloy films.

Etch-Resistant Zn1–xMgxO Alloys: An Alternative to ZnO for Carboxylic Acid Surface Modification

Zinc oxide (ZnO) is a high bandgap transparent metal oxide used extensively in organic electronic devices and dye-sensitized solar cells (DSSCs) as an electron accepting and transporting material. Carboxylic and other organic acid attachment schemes are often employed to sensitize or functionalize the surface of ZnO in these applications. Unfortunately, one weakness of ZnO is its high susceptibility to etching by even weak acids. This has a substantial negative impact on ZnO DSSCs which use carboxylic acid groups to attach dyes to ZnO and also influences attachment techniques for functionalizing ZnO in organic electronic devices. By substituting Mg for Zn atoms to form Zn1–xMgxO (ZnMgO) alloys, a material with similar electronic properties but higher etch resistance is achieved. Here, we show that the etch rate of Zn0.8Mg0.2O, when exposed to the prototypical modifier benzoic acid (BA), is an order of magnitude lower than that of ZnO. Infrared spectroscopic characterization of BA-modified ZnMgO indicates ...

First-principles study of structural, electronic and optical properties of Zn1−xMgxO ternary alloys using modified Becke–Johnson potential

The structural, electronic and optical properties of binary ZnO, MgO and their ternary Zn1−xMgxO alloys (0≤x≤1) in zinc blende phase have been studied using the modified Becke–Johnson (mBJ) potential within the framework of density functional theory (DFT). Wu–Cohen (WC), PBE for solids (PBEsol) and Perdew–Burke–Ernzerhof (PBE) generalized gradient approximations (GGA) were also used to calculate the exchange–correlation and electronic band gap energies. The composition effect on lattice constants is analyzed. In addition, we found that the calculated band structures show that the crystal is a semiconductor with a direct band gap, and the contribution of Zn-3d and O-2p is prominent in the density of states (DOS). The dielectric function, refractive index, optical reflectivity, absorption and optical conductivity are predicted. Furthermore, the binary compound results are in agreement with existing experimental and theoretical values.

Material properties of Cd1− x Mg x O alloys synthesized by radio frequency sputtering

We have studied structural, electrical, and optical properties of sputter deposited ternary CdMgO alloy thin films with total Mg concentration as high as 44%. We found that only a fraction (50%–60%) of Mg is incorporated as substitutional Mg contributing to the modification of the electronic structures of the alloys. The electrical and optical results of the Cd1−xMgxO alloys are analyzed in terms of a large upward shift of the conduction band edge with increasing Mg concentration. With the increase of the intrinsic bandgap, appropriately doped Cd-rich CdMgO alloys can be potentially useful as transparent conductors for photovoltaics.

Band engineering of Ni1−xMgxO alloys for photocathodes of high efficiency dye-sensitized solar cells

Density functional theory calculations were carried out for Ni1−xMgxO alloys using both GGA+U method and hybrid exchange-correlation functional HSE06. We find that the band gap of Ni1−xMgxO is a nonlinear function of MgO concentration with a strong bowing behavior at high Mg content. Band edge alignment is determined using heterojunction superlattice models. The valence-band-maximum of Ni1−xMgxO is shown to be tunable within a range of 0.90 eV. By comparing with the highest-occupied-molecular-orbital levels of some of the most widely used dye molecules, we propose that Ni1−xMgxO is a promising alternate to replace NiO photocathode in dye-sensitized solar cells with an enhanced open-circuit voltage and transparency of cathode films.

Optical and Electronic Properties of Wurtzite Structure Zn1−xMgxO Alloys

Optical and electronic properties of Zn1−xMgxO ternary alloys of wurtzite structure are calculated by using first-principles based on the framework of generalized gradient approximation to density functional theory with the introduction of the on-site Coulomb interaction. The use of the U parameter on Zn-3d and O-2p orbits is obviously crucial, which can improve the GGA to predict the electronic properties and bandgap of the Zn1−xMgxO (0 ≤ x ≤ 0.25) system reasonably. It is further demonstrated that the bandgap widens with an increasing Mg concentration from 3.217 eV of ZnO to 3.877 eV of Zn0.75Mg0.25O. Therefore, the theoretical results show that Zn1−xMgxO ternary alloys are potential candidates for optoelectronic materials, especially for UV photon emitters and detectors.

Carrier dynamics in (ZnMg)O alloy materials

AbstractCarrier recombination dynamics in Zn1–xMgxO alloys were studied by time‐resolved photoluminescence as function of Mg concentration and lattice temperature for different emission and excitation energies. Disorder and carrier localization effects are found to play a significant role, becoming increasingly important for lower temperatures and higher Mg concentrations.Emission energy dependent dynamics were analyzed by the application of a theoretical model, yielding a characteristic localization energy of 60 ± 15 meV for the sample with the highest Mg concentration of x = 0.21. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth characteristics of single‐crystalline ZnMgO layers by ultrasonic spray assisted mist CVD technique

AbstractA simple, safe, and cost‐effective ultrasonic spray assisted mist chemical vapor deposition (CVD) method has been applied for the growth of Zn1−xMgxO alloy thin films on a‐plane sapphire substrates with ZnO buffer layers. Single‐crystalline (0001)‐oriented wurtzite Zn1−xMgxO thin films have been prepared for x of between x = 0 and 0.25. The optical bandgap can be tuned from 3.28 to 3.96 eV with a pure wurtzite phase (x = 0–0.25). At x = 0.31 slight segregation of rocksalt ZnMgO was seen in wurtzite ZnMgO, but the bandgap was as large as 4.10 eV. Zn1−xMgxO alloys exhibited room temperature ultraviolet cathode luminescence (CL) at energies between 3.28 (x = 0) and 4.04 eV (x = 0.31) without no observable deep level emissions. This novel growth technique can contribute to evolution of various oxide thin films of unique functions.

Conduction-Band Structures of Wurtzite ZnO Solid Solutions by First Principles Calculations

Solid solutions of Zn1� xCdxO and Zn1� xMgx O( 0 � x � 1) with wurtzite structures are systematically investigated by first principles calculations with special interests on the dependence of energetics and electronic structures on the alloy structure. Alloying ZnO with CdO shows a linear increase in the lattice parameters when we choose energetically favorable alloy structures. On the other hand, they do not show such a linear trend when alloyed with MgO. Formation energy with reference to the end-member oxides of wurtzite structures is positive for Zn1� xCdxO alloys, whereas it is slightly negative for Zn1� xMgxO alloy. They are consistent with the experimental fact that highly concentrated solid solutions are easier to be formed in Zn1� xMgxO alloys. The band gap shows a monotonous decrease in Zn1� xCdxO and increase in Zn1� xMgxO with the solute concentration. In Zn1� xCdxO, Cd has an orbital component as large as that of Zn at the bottom of the conduction band. On the other hand, the contribution of Mg is much smaller in Zn1� xMgxO. They are consistent with the changes in the band gap with the composition. [doi:10.2320/matertrans.MC200816]

Calculations of electronic structure and density of states in the wurtzite structure of Zn1−x Mg x O alloys using sp3 semi-empirical tight-binding model

Calculating the electronic structure and the den- sity of states in the wurtzite structure of Zn1 − xMgxO (ZMO) alloys using sp 3 semi-empirical tight-binding model, we observed increases of both band gap and electron effec- tive mass that agree with the experimental results as increas- ing Mg composition up to x = 0.3. From the calculated total density of states, the increasing electron effective mass is a result of less orbital overlap of cation sites due to extra density of modes coming from Mg3s and Mg3p orbitals as introducing more Mg composition. Additionally, reducing electronegative characteristic of oxygen was caused by that the O2p was less localized around the oxygen atom.

Hard x-ray photoemission spectroscopy in wurtzite-type zinc magnesium oxide solid-solution films grown by pulsed-laser deposition

The electronic structure of Zn1−xMgxO alloy thin films was determined by hard x-ray photoemission spectroscopy (HX-PES) and optical transmittance measurements. HX-PES measurements revealed that the binding energies of valence band, Zn 2p, and O 1s levels increased with increasing MgO fraction. The energy shift correlated with a widening of the band gap and change in ionicity of chemical bonds by alloying. The Fermi level in the alloy compounds was found to be close to the bottom of conduction band regardless of the MgO fraction.