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Ab-initio DFT-FP-LAPW/TB_mBJ/LDA-GGA investigation of structural and electronic properties of Mg x Zn 1−x O alloys in Würtzite, Rocksalt and Zinc-Blende phases
Abstract
We report on ab-initio DFT/FP-LAPW/TB-mBJ/GGA and LDA investigation of structural and electronic properties of MgxZn1−xO alloys in Würtzite (WZ), Rocksalt (RS) and Zinc-Blende (ZB) phases. TB-mBJ corrections of electronic exchange and correlation interactions make it possible to improve considerably the computational results which we have found very close to the experimental data. As expected, our results show that at 0.375 < x < 0.5, MgxZn1−xO undergoes a phase transition from the tetrahedrally coordinated non-centrosymmetric covalent WZ phase to the octahedrally coordinated ionic RS phase, and the ZB phase remains metastable in the whole 0 ≤ x≤1 range. Our structural properties results show that the lattice parameters vary nonlinearly with x in all three phases. In ZB phase, aZB increases with x very slightly (ΔaZB/aZB≤+0.6%) while in RS phase, aRS decreases with x more strongly (ΔaRS/aRS ≈ −1.7%). The strongest lattice parameters variations with x are found in WZ phase where increasing x results in increasing aWZ (ΔaWZ/aWZ≈+1.4%), decreasing cWZ (ΔcWZ/cWZ ≈ −2.9%), decreasing cWZ/aWZ ratio (Δ(cWZ/aWZ)/(cWZ/aWZ)≈-3.31%), and increasing u internal parameter (Δu/u≈+2.8%). Our electronic properties results show that the fundamental energy bandgap EG is Γ-direct in WZ and ZB MgxZn1−xO in the whole 0 ≤ x≤1 range. In RS MgxZn1−xO, EG is indirect throughout 0 ≤ x < 1 range including RS ZnO and excluding RS MgO where EG is Γ-direct. In all three phases, EG increases with x first linearly in the low x range (x < 0.5), then nonlinearly at higher x (x≥0.5). At x < 0.5, EGZB(x) = 2.609 + 2.861×x eV, EGWZ(x) = 2.863 + 1.990x eV, and EGRS(x) = 3.190 + 1.508x eV. At x≥0.5, nonlinear EG(x) variations show strong bowing parameters depending on the crystal phase: bZB = 2.709eV, bWZ = 4.079eV, and bRS = 16.83eV. These strong EG(x) variations which are confined to a narrow high x range, are tightly correlated with the addition of Zn to MgO and are the strongest in RS MgxZn1−xO. Densities of states and energy band structure data analysis show dominant influence of strong exchange and correlation interactions between hybridized O:2p-Zn:3d orbitals which take place as soon as Zn is introduced into MgO. Our results confirm early findings that O:2p-Zn:3d interactions do play a leading role in overall MgxZn1−xO physical properties whatever the phase, with strongest effects in RS phase, especially in huge EG bowings and in RS-WZ phase transition.
Supplementary resource (1)
... Calculations have suggested nonlinear behaviour (e.g. Shimada et al., 2013;Djelal et al., 2017). Our results show a small decrease in the a 0 lattice parameter with decreasing X wz ZnO and a small increase in c 0 , resulting in almost constant molar volume. ...
At ambient pressure, MgO crystallizes in the rock salt (B1) structure, whereas ZnO crystallizes in the wurtzite structure (B4). The asymmetric miscibility gap between these two structures in the MgO–ZnO binary system narrows with increasing pressure, terminating at the wurtzite-to-rock-salt phase transition in pure ZnO, which occurs at approximately 5 GPa at 1000 ∘C. Despite their essential simplicity, the pressure–temperature–composition (P–T–X) relations in the MgO–ZnO binary system have been sparsely studied experimentally, with disparate results that are inconsistent with available thermodynamic data. Here we report the experimental determination of the P–T–X relations of the miscibility gap from 940 to 1500 ∘C and 0 to 3.5 GPa, which we combine with calorimetric and equation-of-state data from the literature and on the transition in endmember ZnO, to build a thermodynamic model that resolves many of the inconsistencies. The model treats the rock salt phase as an ideal solution (no excess Gibbs free energy of mixing), while in the wurtzite phase the MgO component follows Henry's law and the ZnO component Raoult's law in the range of compositions accessed experimentally. However, there is an inconsistency between the partial molar volume of wurtzite-structured MgO deduced from this model and that inferred from lattice parameter measurements by X-ray diffraction in the quenched samples. This discrepancy may be caused by unquenchable disordering of some significant fraction of the substituting Mg2+ into normally vacant octahedral interstices of the wurtzite structure.
... d [34]. e [35]. f [26]. ...
The inter-band optical absorption of electron transitions in asymmetric Mg x Zn 1- x O/ZnO/Mg y Zn 1- y O quantum wells (QWs), which are the prime functional units of multi-color ultraviolet photodetectors (UVPDs), is investigated with the consideration of mixed phases (0.37< x(y) <0.62) by using the density matrix method combined with a weight model. Our results show that the intensity and number of absorption peaks can be modulated by controlling the Mg composition x in single or mixed phases with a given y in mixed phases. The single peak or dual absorption peaks will appear for x being in a single phase. While x is in the mixed phases, at most three observable peaks can be observed to instruct the designation of multi-color UVPDs. Meanwhile, the size dependence of the left barrier and well in the QWs is also discussed. It is shown that relative wider left barrier and narrower well is more beneficial to obtain multi-detectable absorption peaks for photodetectors.
... La caracterización de las nanopartículas sintetizadas mediante el método sol-gel se realizó mediante el análisis de difracción de rayos X, en función del contenido de cobalto (Fig. 2a). Los resultados obtenidos para las muestras dopadas pueden ser correlacionadas con el ZnO tipo Würtzita, según la carta JCPDS 98-002-6170, correspondiente a la fase hexagonal de grupo cristalino P 63 m c., la cual es la más estable de las estructuras de ZnO (Djelal et al. 2017 hasta el 20 % de Co (Kaphle et al. 2019). Para el caso de las muestras analizadas, es posible afirmar que el Co forma una solución sólida sustitucional con el ZnO a partir del hecho de que se observa un desplazamiento en los máximos de difracción hacia mayores valores de ángulo. ...
El incremento del uso de compuestos farmacéuticos es uno de los factores que ha ocasionado un impacto sobre la salud de los ambientes acuáticos. Las tecnologías convencionales de las plantas de tratamiento de aguas residuales han mostrado tener una baja eficiencia para la remoción de estos contaminantes. Para contribuir a resolver este problema, en la presente investigación se evaluó la degradación de los fármacos bezafibrato, gemfibrozil, sulfametoxazol, diclofenaco e indometacina mediante fotocatálisis heterogénea utilizando nanopartículas de óxido de zinc (ZnO) con dopaje de cobalto de 0, 1, 3 y 5 % en peso de cobalto, con diferentes concentraciones (1, 2, 5, 6 y 10 g/L) y tres temperaturas diferentes (25, 40 y 55 ºC) durante la degradación de los fármacos, bajo una radiación UV λ = 375 nm durante cuatro horas de exposición. Se encontró que el dopaje con cobalto disminuyó la velocidad de remoción de los fármacos. Además, la mayor eficiencia de remoción para todos los compuestos evaluados (98.8-100 %), se obtuvo con una concentración de 5 g/L de nanopartículas de ZnO a una temperatura de 25 ºC. Estos hallazgos resultan promisorios debido a las ventajas del ZnO en la remoción de productos farmacéuticos en aguas residuales o en hospitales en donde se realizan las formulaciones para pacientes hospitalizados. No obstante, se requiere continuar con estudios sobre condiciones similares a las reales para determinar la viabilidad de su aplicación.
... We were able to compute the primary cell constants using the Density Function Theory (DFT) and the Siesta software, and we subsequently discovered the values of 3.327Å for constant a and 5.025Å for constant c. It should be noticed that these findings substantially agree with the conclusions of applied theory included in Table 1 results of previous theoretical and experimental work [13,[21][22][23][24][25][26]. Additionally, the percentage of error in the obtained values has been calculated. ...
In this study, the Density Function Theory (DFT), Generalized Gradient Approximation (GGA) and Local Density Approximation (LDA) were used, based on the Siesta code, in order to study the magnesium oxide compound (MgO) and focus on the (B4) Wurtzite phase. This is to find the primary cell constants and energy gap at 0 pressure, which are consistent with previous results. The effect of pressure on the energy gap and the size of the primary cell as well as the dielectric constant were studied. The study also revealed that the (B4) phase can move to the (B2) phase at the pressure of 45.86 GPa and from the h-MgO phase to the (B2) phase at 70 GPa.
... A theoretical investigation is required to put together the characteristics (structural, optical, and electrical) of (KNbO 3 ), and first-principles calculations are the most useful instrument for this [14][15][16][17][18][19]. In the case of first principle calculations, however, there are just a few computations for (KNbO 3 ) [20,21]. The ferromagnetic and electromechanical characteristics of strontium niobate crystals are valuable for optical properties [22]. ...
A First-principles study based on density functional theory was conducted to examine the different properties of pure and Sr-doped-KNbO3-perovskites materials such as structural, optical, and electronic properties. The current research was conducted by using CASTEP (Cambridge Serial Total Energy Package) code with USP (Ultra-soft Pseudo-potential) and PBE (Perdew Burke-Ernzerhof) exchange co-relational functional of GGA (Generalized Gradients Approximation). By using a generalized gradient approximation (GGA) and an ultra-soft pseudo potential. The Sr effect on structural, electronic, and optical properties of Sr-doped-KNbO3 was investigated using a generalized gradient approximation (GGA) and an ultra-soft pseudo-potential. The band gap of Sr-doped-KNbO3 was found to remarkably increased from 1.50 eV to 1.83 eV after the doping of Strontium (Sr) with various percentages (0%, 12.5%, 25%, 37.5%). Under the DFT study, strontium(Sr) is appropriate material for increasing the band gap of KNbO3. Owing to the difference in ionic radii of Mo and Sr volume of crystal cells also decreased from 82.68 to 42.89A3 after the doping of Strontium (Sr). The nature of the band gap was found indirect moreover band gap indicated that the material was a prominent semiconductor. A significant increment was found in the band gap and optical conductivity. After the substitution of strontium (Sr) impurities, the energy absorption peaks increased. It was also observed that after doping with Sr optical conductivity shifted toward larger energies because of the band gap. Sr-doped-KNbO3 has high optical conductivity, refractive index, and energy absorption making it an appropriate material for Perovskite solar cell applications.
... In particular, it is important to investigate phase stability of both wz-and rs-ZMO compounds in order to evaluate potential of low lattice mismatch substrate applicability. Theoretical computer modelling of ZMO alloys have been carried out within the past decade to investigate morphology of their bulk phase transitions and the fundamental electronic structure [25], [26]. It should also be noted that simulations of these materials are considerably less numerous than those of more common materials, such as TiO 2 . ...
In this research, density functional theory accompanied with linear combination of atomic orbitals (LCAO) method is applied to study the atomic and electronic structure of wurtzite and rocksalt ZnxMg1−xO pseudobinary compounds in their bulk phases. Calculated band gaps of ZnxMg1−xO solid solutions under study are further validated by means of spectroscopic ellipsometry and optical absorption. In agreement with an experiment, it is predicted that increase of Zn content in ZnxMg1−xO leads to narrowing of its band gap for both wurtzite and rocksalt phases. The calculated infra-red (IR) spectra show that the IR peaks are shifted towards larger frequencies along with decrease of Zn content. Presence of imaginary phonon frequencies in rocksalt ZnxMg1−xO of x > 0.625 allows us to suggest that it is necessary to use properly oriented substrates for epitaxial growth to overcome polycrystallinity inZnxMg1−xO thin films at concentration x = 0.4 – 0.6.
... c [27]. d [28]. e [29]. ...
We have explored the structural, electronic, optical, and elastic properties of ZnO1−xSex (x = 0, 0.25, 0.50, 0.75 and 1) ternary alloys with wurtzite structure (WZ). We have used the full potential linearized augmented plane wave method in the framework of density functional theory. The lattice parameters, bulk modulus, and first pressure derivative are determined in the WZ phase using the equation of state of Birch–Murnaghan. The variations of structural parameters versus the concentration x deviate slightly from Vegard's law. The electronic properties including bandgap energy, are studied for ternary alloys with their binary constituents, and the studied materials fall in the domain of direct bandgap semiconductors. Moreover, the optical properties like dielectric function, refractive index, reflectivity and absorption coefficient are examined. The findings of this works could be worthful for the development and preparation of these materials for wide-range of optoelectronic applications.Graphical abstract
Imaginary parts ε2(ω) versus photon energy of the ZnO1−xSex ternay alloys
... Taking into account that the structure is hexagonal, we will find that in all cases a = b. Accordingly, it is noted that these results are in great agreement with the theoretical and applied results mentioned in the previous references table [24][25][26][27][28][29][30][31]. In addition, the error ratio was calculated for the obtained values, by making a comparison with the experimental values taken as a reference, to find that the error value was estimated at 1.34 % for the constant a and the value of 1.37 % for the constant c in the case if we use the approximation GGA, And to find that the error value is 0.15 % and that for the constant a and the value of 1.79 % for the constant c if we use the LDA approximation, then on the whole the calculated error values to us seem small, which suggests that the method of calculation is logical and reasonable. ...
The pseudo ab initio ability is based on density function theory (DFT), use of generalized gradient approximation (GGA), local density approximation (LDA).We use of the Siesta symbol for the first time in studying this particular compound and the wurtzite phase that enabled us to find the structural and optical properties of MgO in its crystal structure (B4) wurtzite. Where the structural results indicated that the wurtzite phase has lattice constants very close to what was found previously in applied studies, and all the calculated properties such as absorption coefficient, reflectivity, extinction, refractive index, imaginary and real part of the constant show that the dielectric has an energy gap greater than 3.27 eV, meaning that it can be used in applications in the ultraviolet (UV) region, and all properties calculated by approximation (GGA) give slightly better results than the use case approximation (LDA).The results obtained when we study the compound MgO wurtzite are a reference for further theoretical and experimental studies.
... This relatively large discrepancy between GGA-PBE and TB-mBJ for TlSrF 3 suggests that the use of other functionals for this compound is necessary to have more comprehensive information on its electronic properties. Improved band gap energies obtained from TB-mBJ results compared to GGA for various compounds have also been reported elsewhere [57][58][59][60][61][62][63]. ...
This study aims to use Density Functional Theory (DFT) to investigate structural and electronic properties of cubic fluoroperovskites TlBF3 (B = Ge, Sn, Pb, Zn, Cd, Hg, Mg, Ca, Sr, and Ba). The calculation was performed using the Quantum Espresso package using GGA-PBE functional. The optimized lattice parameters for the perovskites ranged from 4.01 Å to 5.08 Å while direct and indirect band gaps were observed, with band gaps between 0.75 eV and 4.48 eV. Results were in good agreement with other theoretical studies using different functionals and packages. It was also found that for all compounds analyzed separately based on three different columns of the periodic table, the larger the atomic radii of the B cations, the larger their equilibrium lattice constants. In addition, an increase in the electronegativity of cations leads to a decrease in covalent character of B–F bonds while Tl–F bond is mainly ionic for all compounds.
... Other than the wurtzite hexagonal structure, few reports were available on the rock saltstructured ZnMgS having an indirect bandgap. The indirect bandgap materials are having a phonon addition between electron and photon which results from the optical inactiveness [28,29]. Consequently, such direct bandgap semiconductor materials state their dominance in optoelectronics technology. ...
Here, we report the chemical synthesis of Zn0.7Mg0.3S columnar nanorods onto a commercial glass slide. The synthesized films were used for further studies. Structural, morphological, optical, and electrical properties were analyzed via X-ray diffraction, FE-SEM, TEM, UV–Vis spectroscopy, and two-probe I–V characteristics, respectively. A stable hexagonal structure with the nanocrystalline size of the ZnMgS material has been confirmed from X-ray and TEM analysis and also structural parameters were determined. FE-SEM micrographs show the uniformity of the grown nanorods on the surface of the film; combined EDX spectra display the presence of Zn, Mg, and S. UV–Vis absorption spectra show a prominent peak at ~ 310 nm which confirms the blue emission spectra. The semiconducting nature of synthesized ZnMgS films was confirmed by the electrical study and the photosensing ability was examined. Finally, ZnMgS is a direct bandgap semiconductor which is confirmed from first-principle calculations.
... Deep UV photon sensors based on ZnO-related compounds can be used as biological and chemical sensors for ozone detection and as detectors for water purification, as well as sensors for detection of pollution levels in any biological agent [31]. Up to now, the most prospective ZnO combinations were reported with the use of MgO (direct band gap with Eg = 7.8 eV [32][33][34]), producing alloy thin films and heterostructures with different techniques (PLD, MOCVD, magnetron sputtering, plasma-assisted MBE, etc.), using different substrates (ZnO, sapphire or quartz wafers, SiO2, and others) [35, 36 and references therein] and different buffer layers. Thus, a-plane MgZnO films were grown on a-GaN/r-sapphire by PLD [37], ZnMgO thin films were produced by plasma-assisted MBE on c-plane sapphire substrates using MgO/ZnMgO buffer layer [38]. ...
Wurtzite Zn1-xMgxO epilayers (x=0, 0.26, 0.44, 0.49, 0.66) grown by the plasma-assisted molecular beam epitaxy on ScAlMgO4 substrate were characterized using the methods of optical spectroscopy: spectroscopic ellipsometry (SE), optical absorption (OA), and photoluminescence (PL). The complex dielectric function in the spectral range of 210 to 1690 nm, band gap width, exciton absorption and emission parameters, and film quality were studied and discussed. Individual characterization of samples was provided by combining SE and OA measurement results. The observed increase of the band gap up to 4.35 eV with the rise of the MgO content allowed the recommendation of the wurtzite Zn1-xMgxO epilayers as material for UV sensors. The origin of defects hampering the practical application of the materials was discussed.
... It is noted that these results largely correspond to the applied theoretical results [10][11][12][13][14] shown in Table 1. In addition, the percentage of error in the obtained values was calculated. ...
The ab initio pseudopotential method is based on Density Functional Theory (DFT), in which the Generalized Gradient Approximation (GGA) according to the scheme described by Perdew-Burke-Ernzerhof (PBE) and the Local Density Approximation (LDA) according to the scheme described by Ceperly-Alder (CA) are used. The method is implemented using the Siesta program to investigate the structural and electronic properties of the wurtzite (B4) phase of magnesium oxide (MgO) compound. Indeed, it is a useful method to predict the crystal structure of MgO. Actually, the calculated structural parameters of this compound are consistent with the available experimental data, so these results can be considered as a good prediction. Both the lattice constants and band gap at zero pressure are found to be in agreement with previous theoretical and experimental results. Besides, the binding energy is verified and compared with previous work. Electronic properties, especially the Total Density of States (TDOS) and Partial Density of States (PDOS), also show that the contribution of the oxygen atom to DOS in the region close to the Fermi level is significant. Comparison of the calculated lattice parameters and all electronic properties with the available experimental values reveals the compatibility between them. These results are in broad agreement with experimental results.
... By using PBE-GGA approximation, our calculations revealed that the pristine CaS is a semiconductor with a broad indirect band gap of 3.1522 eV, located between Γ and X highsymmetry points, as displayed in Fig. 6 and Table 3. To overcome the problem posed by PBE-GGA approximation which tends to underestimate the value of the electronic band gap [38][39], modified Becke-Johnson exchange potential approach is implemented. It is found that the electronic band gap is considerably enhanced and reached the value around 4.04545 eV. ...
In this research paper, we investigated the structural, electronic, and magnetic features of titanium atom substituting calciumatom in rock salt structure of CaS to explore the new dilute magnetic semiconductor compounds. The calculations are carried out using the full potential linearized augmented plane wave (FP-LAPW) method based on spin-polarized density functional theory, implemented in WIEN2k code. The generalized gradient approximation and Tran-Balaha modified Becke-Johnson exchange potential. The stability of Ti doped CaS in ferromagnetic state is provided by the total energy released from the optimized structures and defect formation energies. The classical model of Heisenberg is employed to estimate Curie temperature of these compounds. It is found that the room temperature ferromagnetism is achieved at low concentrations. The studied materials exhibit half metallic ferromagnetic demeanor. The half metallic gaps (GHM) are the extremely significant factors to consider for spintronic applications. The insertion of impurity significantly decreased the value of GHM due the broadening of 3d Ti states in the gap of the minority spin. Furthermore, to evaluate the effects of the exchange splitting process, the pd exchange splitting and the exchange constants are predicted.
... Transition metal Nickel and Iron doped KNbO 3 is a single-phase, ferroelectric of high-temperature Curie (Tc=830) with anti-ferromagnetism below Nel. (TN=370) temperature [6]. In the perovskite structures (ABX 3 ), 'A' site elements deal with ferroelectricity, while 'B' site elements deliberate ferromagnetism. ...
The First principle study based on Density Function Theory (DFT) was accomplished to explore the electronic bandgap configurations of KNbO3 by transition metal doping such as Iron [Fe], and Nickel [Ni] using PBE-GGA (Perdew–Burke–Ernzerhof- Generalized Gradient Approximation) for the exchange-correlation potentials. In the current study K1−xYxNbO3 doped with various percentages (12.5%, 25%, 50%, and 75%) of [Ni] and [Fe] metal ions. Different unique properties such as electronic, optical conductivity, and magnetic properties of cubic K1−xYxNbO3(Y = Fe, Ni) compounds have been calculated through the FP-LAW WEIN2k software within DFT. The bandgap of KNbO3 can be reduced by doping various metal ions such as [Ni] and [Fe]. The spin up band structures was observed semiconductor but spin down metallic behaviour. The bandgap structure of overall K1−xYxNbO3(Y = Fe, Ni) compound after doping [Fe] and [Ni] with various concentrations become half metallic compound. Under the DFT scheme, iron [Fe] and nickel [Ni] are reliable as dopants for reducing the bandgap of KNbO3. After substituting various impurity concentration (12.5%, 25%, 50%, and 75%) of [Fe] and [Ni] the energy absorption peaks are 8.2 to 8.43eV for K1−xYxNbO3(Y = Fe, Ni). It is also observed that optical conductivity starting points shift towards the large energy because of bandgap enhancement when the doping concentration (12.5%, 25%, 50%, and 75%) of [Fe] and [Ni] increased in K1−xYxNbO3(Y = Fe, Ni) compound. Magnetic moment was increasing 1.00153 to 3.02210 μB by the increment of doping [Fe] and [Ni] concentrations. K1−xYxNbO3(Y = Fe, Ni) compounds are appropriate perovskite oxides materials for promising optical, magnetic and photovoltaic device applications.
... 而且, ZnO和ZnMgO之间的晶格 失配小于GaN和AlGaN, 这也使得ZnO/ZnMgO异质结 量子阱中的压电极化导致的内建电场比GaN/AlGaN 异质结量子阱中的更低 [57] . 但是类似地, 未来研制 [62][63][64] . 但是, Zn 1−x Mg x O结构中载流子迁移 率随Mg成分单调增加, 其原因主要是由AlGaN/GaN异 质结构的导带偏移增加所引起的屏蔽效应的增强和合 金散射的抑制 [65] . ...
... a-In the case of the pure material By using PBE-GGA approximation, our calculations revealed that the pristine CaS is a semiconductor with a broad indirect band gap of 3.1522 eV, located between Γ and X high-symmetry points, as displayed in Fig. 6 and Table 3. To overcome the problem posed by PBE-GGA approximation which tends to underestimate the value of the electronic band gap [36,37], the modified Becke-Johnson exchange potential approach is implemented. It is found that the electronic band gap is considerably Table 2 Calculated band gap E g (eV), half-metallic gap G HM (eV) of minority spin states, half-metallic ferromagnetic gap G Γ−Γ HMF (eV), and energy difference between ferromagnetic and anti-ferromagnetic states ΔE for Ca 1 − x Ti x S compounds Fig. 6 that the majority spin and the minority spin states are well overlapped suggesting the nonmagnetic performance of the pristine CaS. ...
In this research paper, we investigated the structural, electronic, and magnetic features of titanium atom substituting calcium atom in rock-salt structure of CaS to explore the new dilute magnetic semiconductor compounds Ca1 − xTixS. The calculations are carried out using the full potential-linearized augmented plane wave (FP-LAPW) method based on spin-polarized density functional theory (SP-DFT), implemented in WIEN2k code. The exchange and correlation potential are described by the generalized gradient approximation (PBE-GGA) and Tran-Balaha modified Becke-Johnson exchange potential (TB − mBJ). The stability of Ca1 − xTixS ternary alloys in ferromagnetic state is provided by the total energy released from the optimized structures and defect formation energies. The classical model of Heisenberg is employed to estimate Curie temperature of these compounds. It is found that the room temperature ferromagnetism is achieved at low concentrations. The studied materials exhibit half-metallic ferromagnetic demeanor. The half-metallic gaps (GHM) are the extremely significant factors to consider for spintronic applications. The insertion of impurity significantly decreased the value of GHM due the broadening of Ti − 3d states in the gap of the minority spin. Furthermore, to evaluate the effects of the exchange splitting process, the p − d exchange splitting ΔEC, ΔEv and the exchange constants N0⍺, N0β are predicted.
... Due to the very wide band gap of MgO, it is used as the substrate and catalyst [5,6], and it is also used in superconductor products [7], thermal shock transducers, protecting layers for AC plasma display panels [8] and spintronics devices [9]. Meskine et al. [10] used MgO for doping Mn using two different approximations (LSDA and LSDA + U), and Djelal et al. [11] studied Mg x Zn 1−x O in deferent phases such as rocksalt, wurtzite, and zinc blend structure, using TB-mBJ approach. Gueddin et al. [9] investigated the elastic properties of Mg 1−x Fe x O ternary compounds. ...
We report on the investigation of the structural, electronic, and optical properties of binary compounds (MgO and MgSe) and their ternary () alloys within the density functional theory based on the full-potential linearized augmented plane wave method as implemented in the WIEN2k code. We have used the revised Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBEsol) to calculate the structural properties and analyze the effect of the Se composition on the lattice constant and the bulk modulus of . The calculated electronic properties by employing the GGA-PBEsol and TB-mBJ approaches show that alloys have a direct band gap – for and 0.75, suggesting the possibility of their use in the long wavelength optoelectronic applications. The optical properties such as the real and imaginary parts of the dielectric function, the refractive index, and the reflectivity of are computed by using the accurate TB-mBJ potential. The wide band gaps larger than 3.1 eV mean that alloys can be used in the applications of the ultraviolet region of the spectrum. Our data for all studied bowing parameters of may serve as references for future experimental studies.
The purpose of this study is to conduct theoretical investigations on the electronic, structural and magnetic
properties of lithium niobate (LiNbO3) doped with different concentrations of the transition metal chromium (Cr). All
properties of LiNb1-xCrxO3 compounds (x = 0.25, 0.50 and 0.75) are achieved with spin-polarized density functional
theory by the GGA-PBE and the TB-mBJ approximations. To ensure the stability of the LiNb1-xCrxO3 quaternary alloy in
the ferromagnetic state, the total energy released from the optimized structure and the defect formation energy were
calculated. The classical Heisenberg model estimated the Curie temperature of the compounds. The material studied
exhibits a 100% spin-polarized semi-metallic ferromagnetism behaviour. In all compounds, the data of the magnetic
moment demonstrated that the chromium impurity atom is the most important source by comparison with the Nb and Li,
which have minor contributions. In addition, the magnetic moment of the Cr atoms decreases with doping; this comes
from the p–d hybridization between the Cr-3d and O-2p states. The half-metal gap (GHM), the most important factor in
spintronics applications, was also calculated. In addition, to assess the effects of the exchange separation process, the
crystal field separation energy, the exchange separation parameters, and the exchange constants N0a, N0b, and DEC, DEV
are predicted.
MgZnO barriers are commonly applied to passivate wurtzite ZnO films to enhance electron mobility, while the Mg mole fraction x is usually controlled below 0.4 to avoid phase separation. Few theoretical analyses have focused on electron mobility at large x since the phase separation leads to a complex scattering mechanism. This work investigates the effects of asymmetric MgZnO barriers on electron mobility, which is one source of complexity. Four asymmetric quantum wells simultaneously contribute to the electron mobility in proportions when the wurtzite and rock salt coexist in the mixed-phase MgZnO barriers with large Mg mole fractions. Besides, built-in electric fields also contribute to the asymmetry by tilting the bands. The polar optical phonon-limited electron mobility in asymmetric Mg x Zn 1− x O/ZnO/Mg 0.45 Zn 0.55 O quantum wells is simulated between 176 and 333 cm ² /V s as x ranges from 0.1 to 1. Our calculations show that confined optical phonons play a leading role in the quantum well with wurtzite barriers. Interface optical phonons are primary in the wells with rock salt barriers since most electrons are pushed close to the interface by the strong built-in electric field. The results indicate that wurtzite barriers are more favorable to achieving stable high mobility above 238 cm ² /V s as the Mg mole fraction ranges from 0.14 to 0.33, which is commonly applied in practice.
The structure, electronic and magnetic properties of the MgO bulk of (1x2x2) and (1x1x1) atoms for the B4 wurtzite phase, doped by Manganese Mn have been studied. Accordingly, the Mn atom location in the far and near spots was taken into account, as well as recognizing the magnetic interaction between both spots. Such initiative was provided thanks to the use of the density function theorem (DFT). As for the energy gap of the semiconductor MgO, it was calculated by the linearly increasing planar method, and by the local density approximation (LDA), not to mention the generalized gradient approximation (CGA).It is found that the calculated results agree well with other theoretical and experimental findings. Whereas, the energy gap and the total magnetic torque have been recorded for the Mn doped MgO in the (1x2x2) super Celle. Therefore, our given results have shown that the use of the classification-generalized approximation could enable us to provide more precise results of the d orbital composites, and they also added new properties to the new compound.
In this work, the modulation effects of external magnetic and electric fields on the inter-subband optical absorption coefficients and refractive index changes in asymmetric wurtzite MgZnO/ZnO double quantum wells are investigated considerably. The linear and nonlinear characteristics among the three lowest confined states are calculated here. The obtained results show that the optical properties of the structure are very sensitive to the external field. The optical characteristics related to higher energy levels which have been ignored in past works are as important as the transition related to the ground state. The transition from the ground state to the second excited state is the most sensitive to the external fields. The optical parameters can be risen by two orders of magnitude through the manipulation of the applied electric and magnetic fields.
The structural and electronic properties of ternary Zn1-xMgxO alloys were investigated at the Mg compositions from x = 0 to 0.375in the wurtzite phase. The latter were carried out through an experimental method and first principle calculations based on density functional theory (DFT).ZnO and ZnMgO thin films were synthesized by a sol-gel process using dip coating technique. The XRD analysis has confirmed that Zn1-xMgxO thin films conserve the hexagonal wurtzite structure throughout the whole studied concentrations. The lattice parameter “a" increases and “c" decreases with Mg content, which are in agreement with the theoretical results. The optical properties studied by a dual-beam UV–vis spectrophotometer revealed a band gap ranging from 3.30 eV for x = 0–3.29eV for x = 0.375.The surface characteristics of the ZnO and Zn1-xMgxO thin films were then evaluated with Atomic Force Microscopy (AFM). The results revealed that the Zn1-xMgxO film with higher contents of Mg has higher surface roughness. In addition to the experimental work, DFT calculations were performed. The structural properties were computed by LDA and PBEsol, while the electronic properties were calculated with the modified Becke-Johnson (mBJLDA) potential for a better band gap accuracy with the results compared to our experimental data.
MgxZn1−xO shells are commonly used as a passivation barrier for improving electron mobility in ZnO nanowires by preventing electrons from charged surfaces. However, a high Mg mole fraction x instead makes lower electron mobility, which is usually attributed to the appearance of mixed-phase MgxZn1−xO as x increases. This work aims to find the optimal x for optical phonon limited electron mobility by considering the phase transformation in the MgZnO shell from wurtzite to rock salt, leading to a mixed-phase range of x. Our calculations show that the electron mobility μT can be effectively enhanced by keeping x below 0.057 when confined (CO1) optical phonons are only permitted for small wave vectors, and there is no interface (IF) optical phonon. Once x gets over 0.057, the propagating optical phonons are transformed into IF ones while CO1 phonons become permitted for all wave vectors resulting in a largely strengthened scattering effect and thus a drastic drop in the total electron mobility μT from 1215 to 310 cm²/V s. From then, μT begins to fall slowly as x increases even when the rock salt component in the shell appears to take the place of the wurtzite part, while the scattering from CO1 optical phonons remains primary. Furthermore, the enlarging core radius can weaken the electron–CO1 phonon interaction to enhance mobility.
We report on a theoretical investigation of energy band gaps of III-nitrides, InN, GaN, AlN and their alloys, InxGa1-xN, InxAl1-xN and AlxGa1-xN. Our theoretical framework is based on a DFT full-potential linearized augmented plane wave method within both GGA and LDA exchange-correlation functional. Tran Blaha modified Becke-Johnson exchange potential was also invoked to accurately provide band gaps. We find strong nonlinear compositional dependence of In-containing alloys band gaps, in contrast to AlxGa1-xN which is nonlinear compositional dependence of Al-containing alloys band gaps is smaller. The reported bowing parameters for both functionals are; InxAl1-xN ̴ 4.5 eV, InxGa1-xN ̴ 1.8 eV while; AlxGa1-xN ̴ 0.8 eV is weaker. Our findings, in particular for the LDA, are in close agreement to the major experimental and theoretical data reported so far.
When MgZnO serves as the shell to passivate a ZnO nanowire, the proportion of Mg is usually small to avoid the unexpected spectrum from the wurtzite to rock salt transformation. Using the effective mass approximation, we investigate the impact of a mixed-phase MgxZn1−xO shell on the optical absorption spectra in ZnO nanowires. The results show that the dual absorption peaks from the coexisting two sets of band offset tend to appear as an intrinsic line broadening. This is because the spacing of dual peaks is small and even less than the full width at half maximum to be distinguished. The dual peaks get closer by increasing x or core size since the energy levels become less insensitive to the difference of the potentials. Enhanced confinement of an electron from higher x and smaller core size induces not only a blueshift and a slower saturation but also a sharper peak. The above two aspects make the dual peaks appear always as a broadening in inter-band V1–C1 transitions, while only appear as a broadening in inter-subband C1–C2 transitions when the core radius gets larger than the critical value for a certain x. The broadening from the mixed-phase MgZnO-coated ZnO nanowire could be restricted by increasing the core size or the proportion of Mg in the shell.
Rocksalt ZnxMg1−xO alloys are theoretically and experimentally investigated for near- and deep-UV optoelectronics with a tunable band gap of 4.2–7.8 eV. Regarding the key question about the composition x, at which there is a transition between the direct and indirect gaps, we performed ab initio calculations for various Zn concentrations and all possible atomic arrangements in eight- to 64-atom supercells. We show that, depending on the detailed Zn distribution (clustered, random, or uniform distribution), the alloy band gap can vary by as much as 1.27 eV. The band gap is indirect for clustered and random Zn arrangements in the supercell. For uniform Zn arrangements, the gap is also indirect, except for x<0.5 and atom uniform arrangements excluding Zn-O-Zn nearest neighbor bridges, for which the direct gap can be lowered below the indirect gap by about 0.1 eV. The mechanisms of band-gap fluctuation, Zn clustering, and direct-indirect band-gap transitions are analyzed and explained in terms of atomic contributions to band structures by projecting Bloch functions onto localized Wannier functions. Simultaneously, cathodoluminescence measurements were performed on a set of ZnxMg1−xO multiquantum wells grown by molecular beam epitaxy on MgO substrates. We observed strong and broad emission bands, redshifting with increasing Zn concentration but featuring no clear-cut evidence for any direct to indirect band-gap crossover. We argue that these alloys are well suited for deep-UV optoelectronics, thanks to the rare combination of strong exciton binding energy, coupling to phonons, and carrier localization, which is favored by the marked flattening of the top valence bands by both short-range and long-range Zn-Zn interactions.
The optical absorption of exciton interstate transition in Zn 1 − xl Mg xl O/ZnO/Zn 1 − xc Mg xc O/ZnO/Zn 1 − xr Mg xr O asymmetric double quantum wells (ADQWs) with mixed phases of zinc-blende and wurtzite in Zn 1 − x Mg x O for 0.37 < x < 0.62 is discussed. The mixed phases are taken into account by our weight model of fitting. The states of excitons are obtained by a finite difference method and a variational procedure in consideration of built-in electric fields (BEFs) and the Hartree potential. The optical absorption coefficients (OACs) of exciton interstate transition are obtained by the density matrix method. The results show that Hartree potential bends the conduction and valence bands, whereas a BEF tilts the bands and the combined effect enforces electrons and holes to approach the opposite interfaces to decrease the Coulomb interaction effects between electrons and holes. Furthermore, the OACs indicate a transformation between direct and indirect excitons in zinc-blende ADQWs due to the quantum confinement effects. There are two kinds of peaks corresponding to wurtzite and zinc-blende structures respectively, and the OACs merge together under some special conditions. The computed result of exciton interband emission energy agrees well with a previous experiment. Our conclusions are helpful for further relative theoretical studies, experiments, and design of devices consisting of these quantum well structures.
The present work deals with electronic band structure and derived optical spectra of MgxZn1−xO in the hypothetical rocksalt structure. The computations are performed using full-potential linearised augmented plane wave method. The exchange–correlation potential is described using the Wu-Cohen and Tran-Blaha modified Becke–Johnson generalised gradient approximation (TB-mBJ-GGA). The calculated lattice parameter deviates by less than 1% from experiment showing a net improvement when compared with previous calculations. Moreover, its variation with respect to x does not violate Vegard's law. The TB-mBJ-GGA approach improves the magnitude of the fundamental band gap with respect to experiment. The rocksalt MgxZn1−xO is found to be an indirect gap semiconductor for x = 0, 0.25, 0.50 and 0.75 and a direct gap semiconductor for x = 1. The nature of the gap for rocksalt MgxZn1−xO is still in controversy and further investigations are required in this respect. The optical spectra of MgxZn1−xO are analysed and discussed. Our findings yield values of 1.55 and 1.25 for the static refractive index and 2.4 and 1.55 for the static dielectric constant for rocksalt ZnO and rocksalt MgO, respectively.
In this study, the electronic and optical properties of wurtzite MgxZn1−xO structures for different Mg mole fractions (x) are studied using Density Functional Theory (DFT). In calculations, the generalised gradient approximation (GGA + U) formalism is used with the Hubbard parameters (U) are applied to Zn-3d and O-2p electrons of ZnO. The calculated electronic band structures show that the band gap energies of the investigated structures increase linearly with increasing Mg mole fraction from 0 to 31.25% which is also quantitatively consistent with the previous experimental results. In addition, the electron effective masses of investigated MgxZn1−xO structures are calculated. The electron effective masses of investigated structures show an increment linearly with increasing Mg mole fractions. The optical results show that the absorption edges of the structures move toward the higher energies region as the Mg mole fractions increase.
The current paper reports on the alloying effect on the structural and electronic properties of the hexagonal wurtzite MgxZn1-xO. The calculations are carried out using full-potential linearized augmented plane wave method. The Wu-Cohen and Tran-Blaha modified Becke-Johnson generalized gradient approximations are used to describe the exchange-correlation potential. Our results show generally reasonable accord with experiment. By alloying ZnO with different concentrations of Mg, the variation of the lattice parameters versus Mg content violates slightly the Vegard's law. The analyses of the band structure and density of states show that MgxZn1-xO (0 ≤ x ≤ 1) exhibits a semiconducting character with a direct band-gap (Г-Г). The variation of the latter versus alloy composition x is found to be non-linear showing a band-gap bowing parameter of −1.33 eV. Keywords: Band structure, Crystal structure, Electronic properties, Wurtzite MgxZn1-xO, Ab initio
We present a theoretical simulation study of spontaneous (Psp) and piezoelectric (Ppz) polarizations in ZnO/MgxZn1-xO heterostructures (HS) and their influence on electronic properties through Current-Voltage (I–V) characteristics investigation. By combining ab-initio and Schrödinger-Poisson simulations, we determine Psp and Ppz respective contributions in the formation of the huge internally built-in electric field (Eip≥1 MV/cm) and highly degenerate 2DEG (ns ≥ 10¹³cm⁻²) in 2D ZnO/MgxZn1-xO Quantum Well HS at x ≤ 0.4. 3D heterojunction (HJ) case was then studied by investigating the 2D/3D transition. Fundamental changes are found by tracking the effects of increasing ZnO interface active layer width (LW) above a critical value (hc). At LW ≥ hc, we find that in addition to interface 2DEG confined electrons, there is a 2DHG hole gas (ps ≈ nS) accumulating at the opposite side of the totally depleted i-ZnO interface layer, at a distance d ≈ hc away from HJ interface. The resulting 2DHG/i-ZnO/2DEG structure forms a polarization built-in p⁺-i-n⁺ Esaki-type tunnel junction at ZnO/MgxZn1-xO HJ interface. In agreement with some experimental data, our I–V simulations show typical behavior of dual junction consisting of low bias Eip(x)-controlled Esaki-like ZnO p⁺-i-n⁺ tunneling junction, embedded within interface region of a high forward bias potential barrier controlled 3D ZnO/MgxZn1-xO HJ.
Optical absorption of exciton transitions in asymmetry Zn 1−x Mg x O/ZnO/Zn 1−y Mg y O 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 Zn 1−x Mg x O 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.
Observation of deep ultraviolet (UV) cathodoluminescence peaks around 4.88–5.86 eV and optical transmittance measurements in the far UV spectral range enabled us to find a relatively large Stokes-like shift of 0.7–0.8 eV in rocksalt-structured (RS) MgxZn1−xO films with x = 0.61–0.92 grown on (001) MgO substrates by using the mist chemical vapor deposition method. Electronic structure calculations suggested the existence of bandgap energy (Eg) fluctuations induced by differences in the local arrangement of Mg and Zn atoms in the RS-MgxZn1−xO alloy. The Eg fluctuations and resultant exciton localization were determined to be possible origins of the large Stokes-like shift.
In this work, we have successfully fabricated bottom gate fully transparent tin-doped zinc oxide thin film transistors (TZO TFTs) fabricated on flexible plastic substrate at low temperature by RF magnetron sputtering. The effect of O2/Ar gas flow ratio during channel deposition on the electrical properties of TZO TFTs was investigated, and we found that the O2/Ar gas flow ratio have a great influence on the electrical properties. TZO TFTs on flexible substrate has very nice electrical characteristics with a low off-state current (Ioff) of 3 pA, a high on/off current ratio of 2 × 107, a high saturation mobility (μsat) of 66.7 cm2/V•s, a steep subthreshold slope (SS) of 333 mV/decade and a threshold voltage (Vth) of 1.2 V. Root-Mean-Square (RMS) roughness of TZO thin film is about 0.52 nm. The transmittance of TZO thin film is about 98%. These results highlight that the excellent device performance can be realized in TZO film and TZO TFT can be a promising candidate for flexible displays.
Structural and electronic properties of MgZnO and BeMgZnO alloys are studied by the ab-initio Density Functional Theory method. Large band gap bowings are found for both kinds of alloys. The total energies as functions of the lattice constants are calculated and used to determine the ranges of composition in which the alloys are stable in the wurtzite structure. It is shown that the addition of 6% of Be can already help in stabilization of the MgZnO alloy in the wurtzite structure. The band gap can reach 7 eV for the wurtzite Bex Mg 0.5 Zn 0.5-xO alloys with x approaching 0.5 and about 5.0 eV for Be0.125 Mg x Zn 0.875-xO type alloys for x approaching 0.6. Varying the alloy composition according to the presented stabilization diagram showing ranges of the x, y, for which Bex Mg y Zn 1- x-yO is stable in the wurtzite phase, one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering.
This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.45+/-0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed (4.6 eV at 10 GPa), with a positive pressure coefficient (around 40+/-3 meV/GPa between 5 and 19 GPa). These results are interpreted on the basis of first-principles electronic band structure calculations.
A homogeneous wurtzite-to-rocksalt phase transformation in ZnO is studied using a first-principles pseudopotential method. The calculated transformation enthalpy barrier at the phase equilibrium transition pressure is much lower than the barriers previously studied in SiC and GaN. SiC, GaN, and AlN are found experimentally to transform at a pressure significantly higher than their respective phase equilibrium transition pressures. Interestingly we note that the experimentally observed transition pressure in SiC, GaN, and AlN occur for roughly the same value of the enthalpy barrier. In contrast, ZnO readily transforms at the phase equilibrium transition pressure consistent with the fact that its enthalpy barrier at that pressure is well below this critical barrier value.
The spontaneous polarization and the piezoelectric constants of ZnO and BeO are calculated at an ab initio quantum-mechanical level by using two alternative strategies, namely, through the Berry phase scheme applied to delocalized crystalline orbitals, and through the definition of well-localized Wannier functions. The two sets of results, obtained in the same computational conditions (both schemes are implemented in the CRYSTAL code) compare extremely well, and are in good agreement with available experimental data.
The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by
Bunn [Proc. Phys. Soc. London 47, 836 (1935)
], studies of its vibrational properties with Raman scattering in 1966 by
Damen et al. [Phys. Rev. 142, 570 (1966)
], detailed optical studies in 1954 by
Mollwo [Z. Angew. Phys. 6, 257 (1954)
], and its growth by chemical-vapor transport in 1970 by
Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)
]. In terms of devices, Au Schottky barriers in 1965 by
Mead [Phys. Lett. 18, 218 (1965)
], demonstration of light-emitting diodes (1967) by
Drapak [Semiconductors 2, 624 (1968)
], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by
Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)
], ZnO/ZnSe n-p junctions (1975) by
Tsurkan et al. [Semiconductors 6, 1183 (1975)
], and Al/Au Ohmic contacts by
Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)
] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by
Look and Claflin [Phys. Status Solidi B 241, 624 (2004)
]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
The ground- and metastable-state properties of II-VI oxides in
wurtzite (h-MgO), zincblende and rocksalt structures are
systematically investigated using first-principles. We study the
phase stability of these three structures energetically, and find
that CaO, SrO and BaO prefer h-MgO instead of wurtzite. This is
consistent with the fact that ionic compounds prefer a high
coordination. We also examine the influence of the crystallographic
structure and cations on elastic constants, bulk moduli, spontaneous
polarisation, piezoelectricity, band structures and optical
properties. The band offsets for the common semiconductors (BeO,
MgO, ZnO and CdO) in the zincblende structure are calculated. Our
calculated results are in good agreement with other theoretical and
experimental data.
Ultraviolet (UV) photodetection has drawn a great deal of attention in recent years due to a wide range of civil and military applications. Because of its wide band gap, low cost, strong radiation hardness and high chemical stability, ZnO are regarded as one of the most promising candidates for UV photodetectors. Additionally, doping in ZnO with Mg elements can adjust the bandgap largely and make it feasible to prepare UV photodetectors with different cut-off wavelengths. ZnO-based photoconductors, Schottky photodiodes, metal-semiconductor-metal photodiodes and p-n junction photodetectors have been developed. In this work, it mainly focuses on the ZnO and ZnMgO films photodetectors. We analyze the performance of ZnO-based photodetectors, discussing recent achievements, and comparing the characteristics of the various photodetector structures developed to date.
The influence of the built-in electric field on the binding energy of a bound polaron and the polaron effect in a wurtzite ZnO/MgxZn1−xO quantum well are studied using the improved Lee-Low-Pines intermediate coupling method. The ground-state binding energy, the contributions from different branches of optical phonons to the energy and the binding energy are presented as the functions of well width, impurity position and composition. In the numerical calculations, the anisotropic properties of the frequencies of the different branches of optical phonons, electron effective mass, dielectric constant, the electron-optical phonon interaction and the impurity center-optical phonon interaction are considered. The results show that the built-in electric field has obvious influence on the energy, the binding energy and the polaron effect, and it affects the contributions of different phonon modes to the energy and the binding energy with different degrees. The built-in electric field significantly increases the total phonon contribution to the energy, but it reduces the total phonon contribution to the binding energy. The binding energy of the bound polaron with the built-in electric field is less than that without the built-in electric field, and it declines rapidly with increasing well width. Because of the built-in electric field effects, the contributions from different branches of phonons to the energy and the binding energy and the functions of binding energy with well width and impurity center position are different from the cases without the built-in electric field. The built-in electric field in the wurtzite ZnO/MgxZn1−xO quantum wells has a great impact on the binding energy and polaron effect, and the polaron effect in the wurtzite ZnO/MgxZn1−xO quantum wells is significantly greater than that in the zinc blende GaAs/AlxGa1−xAs QWs, hence, it is necessary to discuss the built-in electric field and polaron effect when considering the problem of electronic state in such systems.
Zn1–xMgxO epitaxial films with Mg concentrations 0 ≤ x ≤ 0.3 were grown by plasma-assisted molecular beam epitaxy on a-plane sapphire substrates. Precise determination of the Mg concentration x was performed by elastic recoil detection analysis. The bandgap energy was extracted from absorption measurements with high accuracy taking electron-hole interaction and exciton-phonon complexes into account. From these results a linear relationship between bandgap energy and Mg concentration is established for x ≤ 0.3. Due to alloy disorder, the increase of the photoluminescence emission energy with Mg concentration is less pronounced. An analysis of the lattice parameters reveals that the epitaxial films grow biaxially strained on a-plane sapphire.
A detailed simulation on the intersubband absorption for 1–2, 2–3, and 1–3 optical transitions in ZnO/MgxZn1−xO quantum wells is presented. The quantum-confined Stark effect induced by the internal polarization field on the absorption process is effectively controlled through an external electric field. It is easy to obtain the structural optimization of light absorption in different terahertz ranges in our numerical analysis. The absorption wavelengths corresponding to almost all of the transitions increase as the applied field varies from −500 kV/cm to 500 kV/cm. The small absorption coefficient corresponding to the 1–3 optical transition is increased by enhancing the structural asymmetry of quantum-well potential. The present work can be extended to the structural design and simulation of new optoelectronic devices based on other low-dimensional structures such as quantum wires and quantum dots.
We report the results of a combined experimental and theoretical investigation on the stability and the volume behavior under hydrostatic pressure of the rocksalt (B1) phase of ZnO. Synchrotron-radiation x-ray powder-diffraction data are obtained from 0 to 30 GPa. Static simulations of the ZnO B1 phase are performed using the ab initio perturbed ion method and the local and nonlocal approximations to the density-functional theory. After the pressure induced transition from the wurtzite phase, we have found that a large fraction of the B1 high-pressure phase is retained when pressure is released. The metastability of this ZnO polymorph is confirmed through the theoretical evaluation of the Hessian eigenvalues of a nine-parameter potential energy surface. This allows us to treat the experimental and theoretical pressure-volume data on an equal basis. In both cases, we have obtained values of the bulk modulus in the range of 160–194 GPa. For its zero-pressure first derivative, the experimental and theoretical data yield a value of 4.4±1.0. Overall, our results show that the ZnO B1 phase is slightly more compressible than previously reported.
Band structures of both wurtzite and rock-salt ZnO were investigated using the ab initio pseudopotential method with both local density approximation (LDA) and GW approximation. The error in approximating 3d electrons as core electrons was investigated for both LDA and quasiparticle calculations. The differences between the band structures obtained by the GW approximation and LDA were explained. The quasiparticle band structures were compared with experimental results. The spin–orbit splitting was calculated for both wurtzite and rock-salt ZnO with the LDA. The density of states was investigated with the GW approximation. © 2002 American Institute of Physics.
We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition. We found that the composition, structure, and band gaps of the MgxZn1−xO thin-film alloys depend critically on the growth temperature. The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at. %) (1 ≥ x ≥ 0.5) which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature. Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys. © 2002 American Institute of Physics.
We report on the growth of single cubic-phase MgZnO thin films by reactive
electron beam evaporation on sapphire substrates. A detailed theoretical
procedure has been employed to analyse the transmission profile for information
on composition non-uniformity, in addition to the exact determination of the band
gap energy. The study of composition non-uniformity has been further extended
to both the typically reported hexagonal and cubic MgZnO thin films. It is found
that the composition non-uniformity strongly depends on the Mg content,
which can be well explained by the ZnO?MgO phase diagram.
The band structures and effective masses of III-V semiconductors InP, InAs, InSb, GaAs, and GaSb are calculated using the GW method, the Heyd, Scuseria, and Ernzerhof hybrid functional, and modified Becke-Johnson combined with the local-density approximation MBJLDA—a local potential optimized for the de-scription of the fundamental band gaps F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 2009. We find that MBJLDA yields an excellent description of the band gaps at high-symmetry points, on par with the hybrid functional and GW. However, the effective masses are generally overestimated by 20– 30 % using the MB-JLDA local multiplicative potential. We believe this to be related to incorrect nearest-neighbor hopping ele-ments, which are little affected by the choice of the local potential. Despite these shortcomings, the MBJLDA method might be a suitable approach for predicting or interpolating the full band dispersion, if only limited experimental data are available. Furthermore, the method is applicable to systems containing several thousand atoms where accurate quasiparticle methods are not applicable.
Several hundred thousands of tons of ZnO are used by per year, e.g. as an additive to concrete or to rubber. In the field of optoelectronics, ZnO holds promises as a material for a blue/UV optoelectronics, alternatively to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits, which are transparent in the visible or for semiconductor spintronics. The main problem is presently, however, a high, reproducible and stable p-doping. We review in this contribution partly critically the material growth, fundamental properties of ZnO and of ZnO-based nanostructures, doping as well as present and future applications, with emphasis on the electronic and optical properties including stimulated emission. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
We propose a widegap II–VI semiconductor alloy, Mg x Zn 1-x O , for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO. The c -axis oriented Mg x Zn 1-x O films were epitaxially grown by pulsed laser deposition on ZnO epitaxial films and sapphire (0001) substrates using ceramic targets. Solid solution films were prepared with Mg content up to x=0.33, achieving a band gap of 3.99 eV at room temperature. MgO impurity phase segregated at x≥0.36. Lattice constants of Mg x Zn 1-x O films changed slightly (∼1%), increasing in a axis and decreasing in c -axis direction with increasing x. These films showed ultraviolet photoluminescence at energies from 3.36 (x=0) to 3.87 eV (x=0.33) at 4.2 K. © 1998 American Institute of Physics.
The complete piezoelectric tensors of both the wurtzite and zinc blende polymorphs of ZnO and ZnS have been computed by ab initio periodic linear combination of atomic orbitals (LCAO) methods, based mainly on the Hartree–Fock Hamiltonian, with an all-electron Gaussian-type basis set. The computational scheme was based on the Berry phases theory, yielding directly the proper piezoelectric stress coefficients eik=(∂Pi/∂εk)E; also the strain coefficients dik=(∂εk/∂Ei)τ were obtained, by intermediate calculation of the full elasticity tensors of all four crystals. In particular, the e15 wurtzite shear constants were included for the first time in such calculations. A careful study of the clamped-ion and internal-strain piezoelectric components shows that the latter ones are well simulated by classical point-charge calculations including quantum-mechanical structural relaxation. The much larger piezoelectric response of ZnO with respect to ZnS is explained by analysing signs and ratios of the respective clamped-ion and internal-strain components.
A modified version of the exchange potential proposed by Becke and Johnson [J. Chem. Phys. 124, 221101 (2006)10.1063/1.2213970] is tested on solids for the calculation of band gaps. The agreement with experiment is very good for all types of solids we considered (e.g., wide band gap insulators, sp semiconductors, and strongly correlated 3d transition-metal oxides) and is of the same order as the agreement obtained with the hybrid functionals or the GW methods. This semilocal exchange potential, which recovers the local-density approximation (LDA) for a constant electron density, mimics very well the behavior of orbital-dependent potentials and leads to calculations which are barely more expensive than LDA calculations. Therefore, it can be applied to very large systems in an efficient way.
The II-VI semiconductors ZnO and ZnSe have been investigated by x-ray and 67ssbauer spectroscopy at high external pressures. In ZnSe, the recoilfree fraction f increases from f=0.50% at ambient pressure to 1.19% at 6.1 GPa. It then decreases to f=0.92% as the pressure is further raised to 8.2 GPa. This decrease of f is caused by softening of phonon modes which occurs far below the crystallographic phase transition (13.5 GPa). In the high-pressure phase of ZnO (NaCl structure), low-frequency acoustic-phonon modes become harder and high-frequency optic modes become softer as compared to ZnO (wurtzite structure). Modern theoretical Hartree-Fock cluster and full potential scalar-relativistic linearized-augmented plane-wave calculations have been performed. These calculations reveal that in both systems covalent contributions to the chemical bond determine the change of the s electron density rho(0) at the Zn nucleus between the different crystallographic phases as well as the electric-field-gradient tensor in ZnO (wurtzite). In particular, rho(0) in ZnO (NaCl phase) is reduced compared to rho(0) in ZnO (wurtzite phase) by -1.15e/a30. Thus, contrary to observation for ZnSe, the electrical conductivity in ZnO (NaCl phase) is not expected to increase in comparison with the low-pressure wurtzite structure.
We propose a simple analytic representation of the correlation energy εc for a uniform electron gas, as a function of density parameter rs and relative spin polarization zeta. Within the random-phase approximation (RPA), this representation allows for the r-3/4s behavior as rs-->∞. Close agreement with numerical RPA values for εc(rs,0), εc(rs,1), and the spin stiffness alphac(rs)=∂2εc(rs, zeta=0)/deltazeta2, and recovery of the correct rslnrs term for rs-->0, indicate the appropriateness of the chosen analytic form. Beyond RPA, different parameters for the same analytic form are found by fitting to the Green's-function Monte Carlo data of Ceperley and Alder [Phys. Rev. Lett. 45, 566 (1980)], taking into account data uncertainties that have been ignored in earlier fits by Vosko, Wilk, and Nusair (VWN) [Can. J. Phys. 58, 1200 (1980)] or by Perdew and Zunger (PZ) [Phys. Rev. B 23, 5048 (1981)]. While we confirm the practical accuracy of the VWN and PZ representations, we eliminate some minor problems with these forms. We study the zeta-dependent coefficients in the high- and low-density expansions, and the rs-dependent spin susceptibility. We also present a conjecture for the exact low-density limit. The correlation potential musigmac(rs,zeta) is evaluated for use in self-consistent density-functional calculations.
The optimized effective potential (OEP) for exchange was introduced some time ago by Sharp and Horton and by Talman and Shadwick. The integral equation for the OEP is difficult to solve, however, and a variety of approximations have therefore been proposed. These are explicitly orbital dependent and require the same two-electron integrals as Hartree-Fock theory. We have found a remarkably simple approximate effective potential that closely resembles the Talman-Shadwick potential in atoms. It depends only on total densities and requires no two-electron integrals.
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