Characterization of ternary MgxZn1−xO thin films deposited by electron beam evaporation

Mg_xZn_1−xO (0≤x≤1) thin films were deposited on glass and quartz substrates by electron beam evaporation and effect of the Mg content of the film on its structural, optical and electrical properties were investigated. The structure, surface morphology, optical transmittance, band gap, refractive index and electrical resistivity were found to depend on the Mg content of the film. XRD data revealed that films were polycrystalline in nature. The structure of the films having Mg content in the range of 1–0.74 was cubic, mixed cubic-hexagonal phases for x=0.47 and hexagonal phase for x=0. The composition analysis showed that Mg content in Mg_xZn_1−xO film is high as compared to the corresponding target alloy. It was observed that the optical band gap increases from 3.3 to 6.09 eV, refractive index at 550 nm decreases from 1.99 to 1.75, transmittance increases from about 70% to 90% and electrical resistivity increases from 0.5 to 1.48×10⁶ Ω cm with the increase of Mg concentration in the film from 0 to 1. The results reported in this work are useful for window layer of solar cells and other optoelectronic devices.     

Mechanical and electronic properties of Ca1−xMgxO alloys

The structural, mechanical, elastics anisotropy and electronic properties of Ca_1−xMg_xO 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 Ca_1−xMg_xO 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 Ca_1−xMg_xO alloys are direct band gap semiconductors.     

First-principles calculations of the structural,electronic and optical properties of In1−xBxAsyP1−y quaternary alloys lattice matched to InP and BeS

The structural, electronic, and optical properties of the cubic In_1−xB_xAs_yP_1−y quaternary alloys lattice matched to InP and BeS have been investigated by using the full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT). The generalized gradient approximation (GGA) of Wu and Cohen was used as the exchange correlation potential to calculate the structural and electronic properties. In addition, the alternative GGA proposed by Engel and Vosko and the modified Becke–Johnson potential are utilized to calculate the electronic properties. The computed structural and electronic properties of the binary compounds are in good agreement with the available experimental and theoretical data. For the alloys, non-linear variations of composition x and y with the lattice constant, bulk modulus, direct, indirect band gap, dielectric constant and refractive index are found. All the compounds are direct band gap excluding BP and BAs. The energy band gap of In_1−xB_xAs_yP_1−y quaternary alloys lattice matched to InP and BeS substrates is computed. Finally, the band gap of our materials is less than 3.1 eV. Thus the In_1−xB_xAs_yP_1−y quaternary alloys may possibly be used in visible light devices.     

The study of structural,electronic, elastic and optical properties in Be1−xZnxTe alloys

Structural, optical and electronic properties, elastic constants of Be_1−xZn_xTe alloys have been studied by employing Castep program based on density functional theory (DFT). The Generalized Gradient Approximation (GGA) and Local Density Approximation (LDA) were utilized as exchange correlation. Using elastic constants for compounds, bulk modulus, forbidden band gap, Fermi energy and Kramers–Kronig relations; dielectric constants, the refractive index, absorption coefficient, energy loss function have been found through calculations. Apart from these results the elastic constants and bulk modulus were obtained experimentally, using X-ray measurement and Vegard׳s law with aim comparison. These properties of ternary alloys were explored using the properties of binary alloys. It is seen that results obtained from both methods are all in agreement. Be_1−xZn_xTe alloy also shows the alloy ionic character for x=0.25. When the Zn is increased, the ionic property of all alloys is increases. Furthermore, as the alloy with x=0.25 shows a flexible characteristic property, other alloys become brittle one.     

Structural,optical and electrical properties of MgxZn1−xO ternary alloy films

The structural, optical and electrical properties of Mg_xZn_1−xO (x=0.05–0.3) ternary alloy thin films deposited by the sol–gel method on the glass substrate were investigated. The presence of Mg in deposited samples was confirmed through EDAX. XRD spectra revealed that the deposited Mg doped ZnO films were polycrystalline in nature. The optical band gap of the films was tailored between 3.2 and 3.45 eV by varying Mg mole concentration between 0.05 and 0.3. I–V characteristics showed decrease in current with increase in the Mg mole concentration. These results explore the applicability of MgZnO to form effective and efficient heterostructures with ZnO as an active layer for efficient carrier confinement in light emitting devices.     

Optical spectra of wide band gap BexZn1−x Se alloys

We report optical measurements (photoluminescence, Raman scattering, and infrared reflectance) of direct band gap and of optical phonon energies of Be_xZn_1−x Se alloys grown by MBE on (001) GaAs substrates for a wide range of Be concentrations. The high band gap of BeSe (5.15 eV) suggests the possibility of using isoternary alloys for ultraviolet optoelectronic applications. Be_xZn_1−x Se has the unique advantage that it can be lattice matched to Si at about x=0.5. We observed a strong linear shift of the Be_xZn_1−x Se direct band gap to higher energies with increasing Be content (to 3.63 eV for x=0.34). Furthermore, optical phonon parameters for the entire range of BeSe content have been obtained. Finally, polarized infrared and Raman spectra revealed local atomic ordering (anti-clustering) effects in the group-II sublattice. Fiz. Tekh. Poluprovodn. 33, 1120–1122 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Atomic layer deposition of Zn1−xMgxO buffer layers for Cu(In,Ga)Se2 solar cells

Fabrication of Zn_1−xMg_xO films by atomic layer deposition (ALD) has been studied for use as buffer layers in Cu(In,Ga)Se₂ (CIGS)‐based solar cell devices. The Zn_1−xMg_xO films were grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in the temperature range from 105 to 180°C. Single‐phase ZnO‐like films were obtained for x < 0·2, followed by a two phase region of ZnO‐ and MgO‐like structures for higher Mg concentrations. Increasing optical band gaps of up to above 3·8 eV were obtained for Zn_1−xMg_xO with increasing x. It was found that the composition of the Zn_1−xMg_xO films varied as an effect of deposition temperature as well as by increasing the relative amount of magnesium precursor pulses during film growth. Completely Cd‐free CIGS‐based solar cells devices with ALD‐Zn_1−xMg_xO buffer layers were fabricated and showed efficiencies of up to 14·1%, which was higher than that of the CdS references. Copyright © 2006 John Wiley & Sons, Ltd.     

Structure,luminescence and photocatalytic activity of Mg-doped ZnO nanoparticles prepared by auto combustion method

A series of Zn_1−xMg_xO nanoparticles with x=0 to 0.15 were prepared by auto combustion method using citric acid as the fuel and chelating agent. Structure, luminescence and photocatalytic properties were systematically investigated by means of X-ray diffraction, scanning electron microscopy, photoluminescence spectra, ultraviolet–visible absorbance measurement and photochemical reactions etc. The samples retained hexagonal wurtzite structure of ZnO and single phase below x=0.13, and the sizes of the nanoparticles were 60–70 nm. The photoluminescence spectroscopy demonstrated blue shift of ultraviolet emission with increasing Mg doping concentration. Both optical measurements of the as grown and Mg doped ZnO nanoparticles showed that the optical band gap could be modified from ~3.28 eV to 3.56 eV as the Mg content x increased from 0 to 0.13. The photocatalytic activities of the samples were evaluated by photocatalytic degradation of methyl orange, and the results showed that the doping of Mg into ZnO nanoparticles could enhance photocatalytic activity compared to the undoped ZnO nanoparticles, which was attributed to increased band gap and superior textural properties. In addition, according to the PL and photocatalytic studies, the critical doping content of effective Mg in ZnO is up to 0.09.     

Electronic structure of Ge1 − x − ySixSny ternary alloys for multijunction solar cells

Ternary group‐IV alloys have a wide potential for applications in infrared devices and optoelectronics. In connection with photovoltaic applications, they are among the most promising materials for inclusion in the next generation of high‐efficiency multijunction solar cells, because they can be lattice matched to substrates as GaAs and Ge, offering the possibility of a range of band gaps complementary to III–V semiconductors. Apart from the full decoupling of lattice and band structures in Ge_{1 − x − y}Si_xSn_y alloys, experimentally confirmed, they allow preparation in a controllable and large range of compositions, thus enabling to tune their band gap. Recently, optical experiments on ternary alloy‐based films, photodetectors measured the direct absorption edges and probed the compositional dependence of the direct gap. The nature of the fundamental gap of Ge_{1 − x − y}Si_xSn_y alloys is still unknown, as neither experimental data on the indirect edges nor electronic structure calculations are available, as yet. Here, we report a first calculation of the electronic structure of Ge_{1 − x − y}Si_xSn_y ternary alloys, employing a combined tight‐binding and virtual crystal approximation method, which proved to be useful to describe group‐IV semiconductor binary alloys. Our results confirm predictions and experimental indications that a 1eV band gap is indeed attainable with these ternary alloys, as required for the fourth layer plan to be added to present‐day record‐efficiency triple‐junction solar cells, to further increase their efficiency, for example, for satellite applications. When lattice matched to Ge, we find that Ge_{1 − x − y}Si_xSn_y ternary alloys have an indirect gap with a compositional dependence reflecting the presence of two competing minima in the conduction band. Copyright © 2013 John Wiley & Sons, Ltd.     

Zn1?xMgxO用于 CIGS 太阳电池的研究进展

Zn1-xMgxO透过率高、带隙可调,且与CIGS太阳电池在晶格和能带结构上匹配良好,可用作CIGS太阳电池缓冲层、窗口层,因此制备高质量的Zn1-xMgxO薄膜是提高太阳电池性能的关键。文章介绍了Zn1-xMgxO薄膜的结构特性、光学特性及制备方法;从Mg含量、Zn1-xMgxO膜厚及Zn1-xMgxO/CIGS界面处缺陷密度等方面概述了Zn1-xMgxO用于CIGS太阳电池的研究进展,并比较了Zn1-xMgxO与In2S3,ZnS,CdS等其他材料作缓冲层的CIGS太阳电池性能的差别。     

Effects of ammonia concentration on property of Cd1−xZnxS films by chemical bath deposition

Cd_1−xZn_xS thin films were grown on soda–lime glass substrates by chemical-bath deposition (CBD) at 80 °C with stirring. All the samples were annealed at 200 °C for 60 min in the air. The crystal structure, surface morphology, thickness and optical properties of the films were studied with transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), step height measurement instrument and spectrophotometer respectively. The results revealed that Cd_1−xZn_xS thin films had cubic crystal structure and the intensity of the diffraction peak increased gradually as ammonia concentration rose and the grain size varied from 5.1 to 8.3 nm. All of Cd_1−xZn_xS thin films had a granular surface with some smaller pores and the average granule sizes increased from 92 to 163 nm with an increase in ammonia concentration. The Cd_1−xZn_xS thin films had the highest transmittance with ammonia concentration of 0.5 M L⁻¹, whose thickness was 50 nm and band gap was 2.62 eV.     

First principles study of structural,electronic and optical properties of indium gallium nitride arsenide lattice matched to gallium arsenide

First principles calculations in the framework of the full-potential linearized augmented plane wave (FP-LAPW) scheme have been carried out. The dilute-nitride zinc blende (In_xGa_1−xN_yAs_1−y) was modeled at selected nitrogen compositions of y=3.125%, 6.25% and 9.375% lattice matched to gallium arsenide (GaAs). We pay attention to the In_xGa_1−xN_yAs_1−y alloy which can be perfectly lattice matched to the GaAs over its entire compositional range. In our study, this is achieved when a condition y~2.7x is maintained. The band structure calculations were performed with and without relaxation by using the generalized gradient approximation of Engel and Vosko (EV-GGA) as well as by the modified Becke–Johnson potential exchange (TB-mBJ). The action of the localized potential of subsisted nitrogen atoms was attributed to effect of relaxation. Increasing both indium and nitrogen compositions leads to decreasing energy band gap. In addition a band anti-crossing model (BAC) was also adopted to study the composition dependence of the direct band gap of quaternary alloys, building a bridge between their electronic and linear optical properties.     

Enhanced Electron Mobility Due to Dopant‐Defect Pairing in Conductive ZnMgO

The increase of the band gap in Zn_1‐xMg_xO alloys with added Mg facilitates tunable control of the conduction band alignment and the Fermi‐level position in oxide‐heterostructures. However, the maximal conductivity achievable by doping decreases considerably at higher Mg compositions, which limits practical application as a wide‐gap transparent conductive oxide. In this work, first‐principles calculations and material synthesis and characterization are combined to show that the leading cause of the conductivity decrease is the increased formation of acceptor‐like compensating intrinsic defects, such as zinc vacancies (V_Zn), which reduce the free electron concentration and decrease the mobility through ionized impurity scattering. Following the expectation that non‐equilibrium deposition techniques should create a more random distribution of oppositely charged dopants and defects compared to the thermodynamic limit, the paring between dopant Ga_Zn and intrinsic defects V_Zn is studied as a means to reduce the ionized impurity scattering. Indeed, the post‐deposition annealing of Ga‐doped Zn_0.7Mg_0.3O films grown by pulsed laser deposition increases the mobility by 50% resulting in a conductivity as high as σ = 475 S cm^‐1.     

Some physical properties of chemically sprayed Zn1−xCdxS semiconductor films

The ZnS is a direct and wide band gap material and it is used in optoelectronics and especially in photovoltaic solar cell applications. We tried to improve some physical characteristics of ZnS films by Cd incorporation. In this work, Zn_1−xCd_xS semiconductor films were produced by ultrasonic spray pyrolysis (USP) technique. Zn_1−xCd_xS films were obtained with incorporation of Cd element into ZnS at different concentrations (0⩽x⩽1). The electrical, optical, structural and morphological properties were investigated. The current–voltage characteristics were taken to see the electrical conduction mechanism and to determine the electrical conductivities of the films. Optical band gaps of the films were determined by optical method and structural properties were analyzed using X-ray diffraction (XRD) patterns. Also, scanning electron microscope (SEM) images were taken to see the distribution on the surface and energy dispersive X-ray spectroscopy (EDS) was used for elemental analysis. It was seen that some Cd incorporated films have higher conductivity values and more homogeneous distributions on the surface than ZnS films. Also, crystallinity level of the films increased. Finally, we think that different experimental parameters and Cd incorporation will improve the properties of ZnS films to be used in optoelectronics and photovoltaic solar cells.     

Characterization of UV photodetectors with MgxZn1−xO thin films

In this study the metal-semiconductor-metal (MSM) structure ultraviolet (UV) photodetectors (PDs) based on Mg_xZn_1−xO thin films were fabricated. The Mg_xZn_1−xO thin films were grown on glass substrates by sol-gel method. The results show that the optical absorption has a blue shift and higher transmittance with increasing Mg dopant. The optical band gap were modified by 3.28-3.52 eV, which corresponded to x = 0 and x = 0.16. For a 10 V applied bias, the dark currents of the Mg_xZn_1−xO MSM-PDs were 637 nA (x = 0) to 0.185 nA (x = 0.16) and showed good Schottky contacts. This UV-visible rejection ratio of the Mg_xZn_1−xO UV PDs at x = 0, 0.16, 0.21 and 0.33 were 18.82, 35.36, 40.91 and 42.92, respectively.     

Optical band gap of Cd1−x MnxTe and Zn1−x MnxTe semiconductors

The dependence of the optical band gap for Zn_1?x Mn_xTe and Cd_1?x Mn_xTe semiconductor compounds was investigated by the methods of cathodoluminescence and optical reflection. It was found that, for Zn_1?x Mn_xTe compounds in the region x?0.2, the band gap is additionally broadened by a magnitude of about 0.08 eV, which is related to the high density of interstitial-type defects in single crystals. For x?0.3, the probability of the existence of these defects decreases substantially, which is related to the distortion of tetrahedra of the crystal lattice of Zn_1?x Mn_xTe by Mn atoms, which are incorporated into each tetrahedron.     

Density functional perturbation theory calculations of vibrational and thermodynamic properties of Zn1−xBexO alloys

The elastic, phonon and thermodynamic properties of Zn_1−xBe_xO alloy are investigated by performing density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. The calculated lattice parameters decreases with the increase of Be content that is in good agreement with the available theoretical and experimental data. The effect of Be composition on elastic constants was investigated for Zn_1−xBe_xO alloys. Phonon dispersion curves show that Zn_1−xBe_xO are dynamically stable. Thermodynamic properties, including Helmholtz free energy, enthalpy, entropy and heat capacity, were evaluated under quasi-harmonic approximation using the calculated phonon density of states. Finally, the results show that Zn_1−xBe_xO alloys with lower Be content are more thermodynamically stable. The agreement between the present results and the known data that are available only for ZnO and BeO is generally satisfactory.     

Selenium alloying of indium sulfide: Ab-initio study of structural,electronic and optical features

In the framework of density functional calculations and using the Linear Augmented Plane Waves with local orbital method (LAPW+lo), we have investigated the structural, electronic and optical properties of indium sulfoselenide (InS_1−xSe_x). The present study confirms that InS_1−xSe_x are indirect band gap materials. The non-linear dependence concentration x of the theoretical forbidden energy band is clearly visible and the microscopic origins of gap bowing are identified and calculated. In order to identify the angular momentum characteristics of the bands structure, the total and partial densities of states (DOS and PDOS) are analyzed. The feature bonding is also investigated from charge density study. Using the projected total densities of states (DOS) and the bands structure, we have calculated the linear optical properties, namely, the complex dielectric function ε(ω), refractive index n(ω), absorption coefficient α(ω) and the electron energy loss L(ω). In particular, we take into account the effect of Se composition on anisotropy, real part of the dielectric function and refractive index. The present alloy is found to be a challenging material in optoelectronic, medical and photovoltaic devices. Good agreements are found with the available experimental and theoretical results.     

Optoelectronic properties of cubic BxInyGa1−x−yN alloys matched to GaN for designing quantum well Lasers: First-principles study within mBJ exchange potential

A Full-Potential Linearized Augmented Plane Wave calculation within density functional theory is performed to investigate the electronic and optical properties of cubic B_xIn_yGa_1−x−yN alloys matched to GaN with low-Boron content (x≤0.187). The exchange-correlation potential is treated by the local density approximation (LDA) to calculate the structural properties. The band structure and density of states of these compounds are well predicted by modified Becke–Johnson (mBJ) exchange potential compared to LDA and generalized gradient approximation (GGA). Also, the optical properties are calculated by the mBJ exchange potential. The computed structural parameters are found to be in good agreement with experimental and theoretical data. The B_xIn_yGa_1−x−yN alloy is expected to be lattice matched to GaN substrate for (x=0.125, y=0.187). The incorporation of B and In into GaN substrate allows the reduction of the band gap energy. The real and imaginary parts of the dielectric function, refractive index, reflectivity and absorption coefficient are discussed on the basis on the energy band structure and the calculated density of states. The optical properties of B_xIn_yGa_1−x−yN depend on the incorporated Boron content (with y=0.187). This means that B_xIn_yGa_1−x−yN could constitute an active layer in single quantum well for the design of high-efficiency solar cells and optoelectronic devices as Laser Diodes operating in the UV spectral region.     

Effect of window layer composition in Cd1−xZnxS/CdTe solar cells

To improve CdS/CdTe cell/module efficiencies, CdS window layer thinning is commonly applied despite the risk of increased pin‐hole defects and shunting. An alternative approach is to widen the band gap of the window layer (2.42 eV for CdS) via alloying, for example, by forming compositions of Cd_1−xZn_xS. In this study, the performance of Cd_1−xZn_xS/CdTe thin‐film solar cells has been studied as a function of x (from x = 0 to 0.9), widening the window layer band gap up to and over 3.4 eV. Optimum Cd_1−xZn_xS compositions were clearly identified to be around x = 0.7, and limitations to the achievable photocurrent and conversion efficiencies have been addressed. Copyright © 2012 John Wiley & Sons, Ltd.