Materials Science

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    Thgorgticali Vestigatlon of Supgrconouctivity in (Sr, Na)-Grapheng Shget
    (Addis Ababa University, 2013-06) Haile, Mesfin; Mazher, Javed(Prof)
    Graphcne hfl,s many remFlrkablc properti es) but superconductivity is notably absent. Hccent observation of proxi mity effeeL has ignited in superconductivity of graphel'" sheel,. In this work wc explore. using density fu nctiona l theory (OFT) iI.nd density functioned pert.urbation t heory (DFPT) ca.lcu lation the possibility of inducing !';upcrconductivity in strontium and sodium doped graphenc. 'vVe have g iven <-\, clear evidence that slIperconductivity cri t ica l temperature J~ is easily affected by the contrihlltion of inplrtlle phonon vi hration of intercR.IRt.e and outof- p lane phonon (ZO) of the graphene sheet. P arti cularly Sr and Na graphene sheet leads t.o superconductivit.y, with), = 0.667 with T, of 7.5 K and 0.41 with T, of 2 K for sodium and strontium interca.lated grap henc res pectively.
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    Effects of Aromatic Ring (C6) Fictionalization on Electronic- Transport Properties of Armchair Graphene Nanribbons
    (Addis Ababa University, 2013-03) Sinu, Melese; Mazher, Javed(Prof)
    Two dimensional structures like graphene nanostructures exhibit vanous phys ical properties on in-plane anchoring of fun ctional groups. We have investi gated the electronic band structure, the density of states (DOS), and the current-voltage (I-V) characteri stics of pri stine and armchair nanoribbons (AGNRs) functionali zed with aromatic ring (C6) by first- princi ples calculations with the help of ab-initio DFT code. A C6 ring structure is in-plane anchored to the nano-ribbon to maximize the effects of functionali zation on the properties of the nanostructures of various widths. Samples are theoretically modeled, both in the periodic and open system nano-device configurations, by using arm-chair nanoribbons of different widths with and without ring attachments. Subsequently, the as-prepared nano-devices are simulated and their I-V characteri stics are studied. The resul ts of these simulations are analysed by using the charge transfer, which takes place during the final self consistent calculation routines amongst the C-ring and its ribbons. We found a significant amount of change in the electro nic density of states of the semiconducting ribbons due to the presence of C6-rings. Also, we have demonstrated that the C-ring attachment can be effective ly used to tailor the energy bandgaps of the semiconducting ribbons. In addition to this we have also showed the nano-device feasibility of such ring functionalized ribbons. The property of bandgap dependence on aromatic ring is also studied. Finally the prospects of functionalized nanostruclllres of graphene are also discussed in this thesis, especially, the electronic and transport properties of armchair ribbons are di scussed with respect to ring functionalizations.
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    Gloally Convergent Modification of Newto's Method
    (Addis Ababa University, 2003) Kedir, Girma; Deumlich, R. (Prof.)
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    Invest Ligations of Electronic and Phonon Mediated Superconductivity of (Rb, K) Doped Graphene
    (Addis Ababa University, 2013-06) Gebreamlak, Gebrekirstos; Mazher, Javed(Prof)
    We performed ab-initio calculations on a novel class of superconductors, which comprise of alkali doped graphene. The graphene is doped wit h the potassium and rubidium atoms. We theoretically studied lattice dynamics by performing density functional perturbation t heory based simulations for studying doping effects on graphene's phonon modes. Spectra of electronic and phonon density of states are compared in between undoped and K and Rb doped graphene. Besides, the study of electron-phonon int eractions is also performed and the signifi cant amount of electron-phonon coupling is found to be present among the K and Rb doped graphene. That is attributed as one of the reasons for as-found induced superconductivity at certain crystal symmetry points of interest in Brillouin zone of this material, specifically, at the (r and K) points. The higher values of electronphonon coupli ngs of 0.3679 and 0.1907 correspondingly for the potassium and rubidium doped graphene are found to be responsible for the superconductivity. Nevertheless, during our calculations the el-ph coupling strengths for pristine graphene is always found to be zero at various special points. In addi tion , the prospects of the phenomena of su perconducti vi ty amongst the alkali doped graphene are discussed in detail.
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    Ab-Initio Calculations of High Pressure Phase Transition, Structural and Electronic Properties of Bexznl _Xs (X = 9.00, 0,41, 0.66 and 1.00) Ternary Alloys
    (Addis Ababa University, 2013-06) Fufa, Fatana; Mazher, Javed(Prof)
    In this piece of research, we have performed self-consistent ab-initio calculations to study the structural and e lectronic properties of BexZnl -xS mixed crystals in the various low and high pressure crystallographic phases of thc alloy. The zinc blend phase and rock salt phase of compositionally variant zinc-bery llium-sulfide have been used for the calc ul ations of' structura l and electronic properties within the local density approximation of density fu nctional theory. The concent rat ion of Be (X) changes f'rom 0.00, 0.41 , and 0.66 to 1.00 in the al loy. Super cells (54 atoms) of the all oys are constructed and fully relaxed. Al l the values of ternary and binary alloy lattice constants, bulk modulus and cnergy band gap are calculated on these relaxed structures. Our results are comparable to the contemporary theoreti cal and experimental observations. Our values for the structural parameters obtained in the LOA approximation are also in good agreement with the experimental values. We have also observed direct band to indirect band gap (r ..... r) ..... (r ..... X) by increasi ng beryl lium concentrations along with some LOA related under estimation s of gap widt hs. For al l the samples of the alloy, high pressure phase tran sitions are found to present and reported in the range of' giga pascal pressures. At the same time, increase of hardening of the al loy is reported due to Be content and the bulk modulus correspondingly increases in the alloy. We have also analyzed the concentration related effects on electronic density of states and bowing parameters.
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    Simulation of Five Layers Solid-State Memory Capacitor
    (Addis Ababa University, 2010-07) Kebede, Eyob; Bekele, Mulugeta (PhD)
    Tn t his work we provided th e simulation of a five layer charge-cont rolled memory capacitor in which periodic and non-period ic metamaterials are embeddcd between the plates of a rcgul ar paralic! plate capacitor. The simulation result shows hysteretic, negative and diverging in capacitance-voltage behavior and pinchcd hysterctic charge-voltage propcrt ies in both periodic and non-periodic memcapacit ive systems. Even though the chargevo ltage hysteresis loops of both perio dic and non periodic metamaterial memcapacit ive systems are pinched, the hysteresis loop of t he per iodic system is relatively wider t han that of the non-periodic and on the average the rem nant polarized charge on the periodic memcapacitve system is higher than that of the non-periodic. This property of the periodic metamater ial memcapacitive system makes it a good candidate for non-volatile memory application over t he non-periodic one.
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    Investigation of Calcite Fineness on the Physico-Mechanical Properties of Ordinary Portland Cement
    (2013-03) Belew, Eyob; Redi, Mesfin (PhD)
    The use of limestone as a partial rep lacement of Ordinary Portland Cement (OPC) has several advantages like Technical, Economical and Env ironmenta l. The present study aimed at investigating the phys ico-mechanical properties of fresh and hardened cement pastes of Portland limestone cement (PLC) made by blend ing clinker, gypsum and fined ca lc ite. Where the percentages of limestone are 0%, 5%, 10%, 15%, 20% and 25% by mass and the particle s izes are 6 11m, 12 11m, 18 11m and 24 11m used to replace a part of Ordinary Portland cement. The resuiting specimens were compared for, standard cons istency, sett ing time, so undness, and compress ive strength. Generally, the results show that as the Ca lcite fineness increase, the compress ive strengths of PLC also increases for some amount whereas, when the amount of fined calcite increases in PLC the compressive Strength of the mortars decreases. However, the specimens have competitive strength for the rep lacement value of Calc ite powder up to 20% by mass of clinker with particle s ize 6 11m, These represents a significant reduction o f energy, raw material consumption, costs and environmental advantages by reducing CO, emiss ion over ord inary Portland cements. Setting time, both init ial and final setting times, result were decreased with an increase Calcite in PLC. Furthermore, at the same level replacement, the cement pastes of 611m and 1211m of limesto ne show lower setting time than those us ing, I 811m and 24!lm, respective ly.
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    Effect of Solvent Additives and P3HT on PDTSTTz/PCBM Based Bulk Heterojunction Solar Cells
    (Addis Ababa University, 2013-05) Ergete, Asegid; Yohannes, Tcketel (Prof.)
    The dcpcndcncy o fthc performance of organic bulk heteroj uct ion (B HJ) so lar cell s on the nanomorphology of the photoactive blend can be co ntro lled by so lvent additi ves. In this work BI-IJ so lar cells ba sed on PDTS'ITz:PCBM (1:1) which processcd by us ing a host solvcnt di-chl orobc nzcnc and 2.5% (v/v) different solvent add iti ves such as iodobutane, iodoethane, di-iodometha ne. and iodomcthane were constructed. The effects of the aforementioned addit ives on the photovolta ic parametcrs of the so lar cells wcre in vcstigated. Deviccs processed in 2.5% (v/v) of iodobutanc cx hibited the hi ghest power convers ion e fTI cicncy (PCE) up to 2.02% as compared to the pri st ine PDTST f z:PCBM deviccs (without add itive) (0.39%). The UY-Yis absorption spectra of films showed the existence of red shift in th c prcsence of thcse addi tivcs res ulting from thc growth of enhanced local structure with d istinct, opt imized phase scpa ratcd morph ology. Bcsides, the introduction of P3HT donor into PDTSTTzlPCBM system forming a ternary bl end a lso showed improvcd cfficicncy (2.42%) following thc incrcascd sho rt ci rcu it current up to 11.08 mi\/cm' through improvcd photon harvesting. KEYWORDS - Organic Solar Cell, Additi ve, Morphology, Bulk Il eterojunction
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    Spin Dependent Transport Properties of Graphene Nanoantidots: A First-principles Study
    (Addis Ababa University, 2013-03) Dekama, Allah; Mazher, Javed(Prof)
    Inherently, Graphene - a perfect 20 hexagonal crystal of C-atoms, is a non-spintronics material due to spin moment cancellation of C-atoms, especiall y, at the edges. Spintro nics, or spin electronics, involves the study of acti ve control and manipul at ion of spin degrees of freedom in the solid-state systems with non- equili brium spin populations. The spins transport can be introduced either by doping or creating exotic nanostructures of graphene such as antidots. We have performed a systematic simulational study of bare and doped graphene nanostructures for their spin transport properti es. We studi ed the transport properties of eight different O-shaped graphene nanoantidots (GNAOs) with di ffe rent electrode contact configurations by using non-equili brium Green' s functions (NEGF) in combination with the density-functional theory (OFT). Our calculations indicate the presence of spi ntro ni city in the graphene nanoanti dot 's GNAOs' geometrical conformations. Among the two important antidots (0 [, and O2,) , it is found that the former is more spintronic in comparison to the latter structure. Surprisingly, Oh becomes more spintronic when the device is connected asymmetrically to its electrodes. This spin flip behavior is suitably ex plained in the thesis on the basis of zigzag edge spin-charge contributions to the devices. We have also studied effect of the presence of Co atom at center of GNAO on its spintronicity -- by comparing spin up and spin down conduction channel s. The prospects of spin control among graphene nanostructures for spintronics applications are also discussed in the thesis.
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    Effects of Carbon Content in Ferrite and Martensite on Lattice Parameter and it’s Diffusion Process (Computer Simulation)
    (Addis Ababa Universty, 2011-01) Mengistu, Yilma; Bekele, Mulugeta (PhD)
    Molecular dynamics and molecular static simulation has been performed to investigate the structural transition of martensite and the diffusion properties of single carbon atom in the bulk of ferrite. For both types of simulations embedded atom potential was applied to describe atomic interaction. This potential considers short-range interaction of iron and non-interacting defects which can be valid only for small concentration. After carrying out molecular static simulation we found that octahedral sites are the preferential position of carbon that have lowest formation energy -6.277 eV with minimum energy configuration than tetrahedral sites which have -5.349 eV formation energy. The migration energy of carbon calculated using this method when carbon moves from one preferential position to the next preferential position is 0.89 eV. This result is in good agreement with Ab initio calculation of migration energy 0.90 eV and experimental results 0.83 eV. From direct molecular dynamics simulation result of equilibrium lattice constant at room temperature we found that ferrite that contains carbon less than 0.081 wt % have cubical structure (that have equal values of lattice constant in the three direction (100), (010) and (001)). When the amount of carbon contained reaches 0.081 wt % the equilibrium lattice constant of martensite started splitting in to two values a and c. This splitting shows ferrite-martensite structural transition. Thus this content in our case is the minimum concentration (critical point) at which structural transition of martensite from body centered cubic to body centered tetragonal occur at room temperature. The diffusion coefficient calculated using molecular dynamic method is 9.736 × 10−8m2/s.
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    Role of Surface Chemistry in Modifying the Optical Properties of Silicon Nanostructure
    (Addis Ababa Universty, 2010-06) Belege Tsegaye; Ghoshal S.K. (PhD)
    We combine two model used earlier. We vary the fitting parameters C and α (which depend on surface, geometry, confinement, size, porosity and materials properties). We use surface energy (Es = 0.05eV), binding energy (Eb = 0.07eV), C, and α using these as input we generate Eg and PL intensity spectra. The Eg and PL spectrum are compared with TB and PP data. The position of PL peak is used to find the band gap and compared with the experiment. The main objective is to use the already existing model, combining them and use fitting parameters to calculate optical properties like PL intensity and band gap for varying size. The results show qualitative agreement with the experiment. This is due to the simplicity of the model. However, we found it is interesting to use the fitting parameters C and α to show that our model is in good agreement with the experimental observation.
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    Investigations of Electronic Structure of Bare and Passivated Nanocrystalline Silicon Clusters and their Transport Properties
    (Addis Ababa Universty, 2009-06) Teklahymanot, Solomun; Mazher, Javed (Professor)
    Silicon low dimensional structures are found to be extremely stable in comparison to bulk silicon along with variety of interesting electronic properties. They are established as potential candidates for various promising applications in advanced nano-electronics devices especially in a nano-switch to be specific. Here we are presenting our most recent and most detailed results for electronic and transport properties of ground-state structures of Sin, SinH and SinO clusters using DFT- LDA-NEGF simulations. From our present set of studies, a high degree of structural stability is observed in very small Sin nanoclusters of various sizes; n = 4, 6, 7 and 8. Amongst several diverse and stable clusters, the Si6 is found to possess a maximum stability. The energy gaps in between LUMO-HOMO of Si clusters are also found to be decreasing with the “n” in accordance with general principles of a quantum confinement with an exception of the Si6 cluster. Density of states spectra (DOS), transmission spectrum (TS) and currentvoltage (I-V) calculations are also preformed. Various TS spectra results are mostly discussed in contest to charge transfer phenomena, states injection in between various components of the nanodevices and their ensuing effects on the nanoswitching. We have also ascertained increased cluster stability with the passivation and the presence of a significant contributing states at fermi energies in nano devices that are made up of these clusters. A novel phenomenon of a negative differential resistance becomes pragmatic in I-V curves of these samples and it is discussed in relation to the Q-confinement induced singularities among the concerned nanostructures. I-V characteristics curve of a device ZGNR/Si8H/ZGNR shows strong nonlinearity as a result it can be fruitfully deployed for the nano-switch. Various lurid prospects of nano-swtcing using Sin nanoclusters are also conferred in the present work.
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    Simulation of Three-Layer Solid State Memcapacitor
    (Addis Ababa Universty, 2010-06) Wami, Shiferaw; Bekele, Mulugeta (PhD)
    Memcapacitors are capacitors whose capacitance depends on the past states through which the system has evolved. The focus of this thesis is on the possible realization of a three-layer solid-state memory capacitive (memcapacitive) system. The functioning of this device is based on the slow polarization rate of a medium between plates of a regular capacitor. To achieve this goal, this work considers a three-layer structure embedded in a capacitor. The three layer structure is formed by metallic layers separated by an insulator so that non-linear electronic transport between the layers can occur. Unlike conventional capacitor, the capacitance of this device depends on the history of the system. Our calculation shows non-pinched hysteretic charge-voltage and capacitance-voltage curves, and both negative and diverging capacitance within certain ranges of the field. This property of memcapacitive system makes it a good candidate for non-volatile memory application.
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    Study the Effect of Ultraviolet Radiation on Polycrystalline Silicon Solar Cell
    (Addis Ababa Universty, 2009-03) Teklu Nigatu; Asfaw Araya (PhD)
    The effect of ultraviolet(UV)radiation on polycrystalline silicon solar cell was studied. The reflectance spectrum, performance parameter and external quantum efficiency was analyzed by varying exposure time for different fluence of UV radiation. Reflectance spectrum showed increase for 10 minutes of exposure and starts to decrease and come back to nearly original value after 190 minutes exposure. Maximum potential power (Pmax) showed no change for all fluence of UV radiation. Short circuit current density (JSC) increase from 0.037 A cm2 to 0.041 A cm2 for 10 minutes of UV radiation exposure and external quantum efficiency (EQE) of polycrystalline silicon solar cell showed reduction for 10 minutes exposure and for the rest of fluences of UV radiation very small change of EQE was observed.
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    Investigation of Damping Effects on Power Loss in an Optical Fiber Due to Higher Order Nonlinear Term
    (Addis Ababa Universty, 2009-07) Abebe Mulualem; Ghoshal S.K. (PhD)
    Many properties of optical fibers namely power loss, noise, distortion, attenuation, absorption, damping etc are guided by nonlinearities. They must be checked and calculated their effects before put into applications. We used a developed an-harmonic oscillator model (by Sunita Sharma, S. K. Ghoshal and Devendra Mohan) in which two oxygen atoms are connected to the silicon atom by springs which undergoes into an-harmonic vibration. This formulation is targated for nanosecod pulses in long haul optical communication. For such laser signal a nonlinearity is quite prominent. It is this an-harmonic motion that leads to nonlinear effects. The equations for secondorder, third-order, and fifth-order linear and nonlinear susceptibilities are derived from this an-harmonic model. The equations for the dielectric constants and for the index of refraction are also derived. The power loss due to imaginary part of the higher-order nonlinear refractive index for Pure Silica Core Fiber, Dispersion Shifted Fiber and Dispersion Compensating Fiber is explicitly calculated. The variation of power loss with damping constant is also calculated at 2mW. The power loss by the Kerr and the electrostrictive nonlinear refractive index and also the total power radiated is theoretically calculated from the model. Our results demonstrate that the electrostriction, the Kerr the damping effects are significant in optical fibers. These results confirm some of the recent theoretical and experimental observations. The model is quite general and is suitable for calculating many other nonlinear properties of fiber materials.
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    First Principle Determination of Electronic Structures of Bimno3 Using Hubbard U Correction
    (Addis Ababa Universty, 2009-06) Atlaw, Mesfin; Marzari, Nicola (Professor)
    In this research work the electronic structure properties and lattice instability of BiMnO3 are examined by employing first principle DFT based techniques. First we perform a pseudopotential calculation using general gradient approximation without Hubbard U. The calculations are further extended adding U term to observe near effects to the Fermi. We also examine phonon dispersion for simple cubic ferromagnetic phase. Results for phonon show clear lattice instability to the off centered displacement driven by strong covalent bonding between Bi (6p) and O (2p) states. The entire work is carried out using Quantum-ESPRESSO. Some of our results are quite new and some matches with other recent findings.
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    Investigation of Calcite and Volcanic Ash for their Utilizations as Cement Filling and Additive Materials
    (Addis Ababa Universty, 2010-06) Aregaw, Mengistu; Redi, Mesfin (PhD)
    The purpose of this study is to use calcite and volcanic soil as cement filling and additive materials in order to maximize cement productions in Ethiopia. In this study the characteristics of calcite and volcanic soil, suitable mix design of OPC raw materials, sources of calcite, chemical properties of calcite, volcanic soil, basic cement raw materials and clinker, and the effect of calcite and volcanic soil on mechanical performance of concrete were investigated. The additions of calcite up to 15% by weight improves early strength and additions above 15% results little decreasing early strength but develop high lateral strength. Use of volcanic soil from 28% to 35% by weight results a decrease of early strength and develops lateral strength. The additions of these materials did not affect quality of cement and mechanical performance of concrete. The investigation shows that up to 15% Calcite filled cements have superior properties to the current Portland cement and Portland Pozzolanic Cements such as high early strength, excellent volume stability, minimum cost of productions and easy manufacture process.
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    First Principle Simulation for Electronic Structures and Transport Properties of Edge Doped Graphene Nanostructures for Applications in Glucose Sensors
    (Addis Ababa Universty, 2012-06) Semu, Gishu; Mazher, Javed (Professor)
    Glucose as adsorbed species on edges of the graphene nanostructures are found to create some significant variations among the nanostructure’s electronic properties and the changes are fruitfully tapped for futuristic nano-sensing applications. The present study is focused on ab-initio transport measurement for a nano-scale sensor which comprises of either H or O-edge doped zigzag graphene nanoribbons (zGNRs). A range of diverse transport phenomenon is observed by either variation of gating, edge doping, and biomolecular (glucose) attachments or by a combination of all these factors. Our results of these measurements point towards the suitability of ribbon’s zigzag edges as glucose attachment sites for the sensing purpose. Furthermore, transformations of conductivity, density of states, and current-voltage characteristics are studied in the nanostructural two-dimensional forms of carbon with foreign atoms doped in or in vicinity of the honeycomb lattice. In addition, we proposed some capable sensing device architectures for exploiting the newly explored unique functionalities. It is important to precisely control and study the density and character of the charge carriers, nature of chemical doping, and biomolecular adsorbates for tailoring the proposed in-vivo glucose sensor’s properties. First principle two probe methods are used in this work for the nanodevice simulation in conjunction with density functional theory (DFT) and nonequilibrium Green function (NEGF) methods in a standard electron-correlation approximation scheme of local density (LDA).
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    Experimental Technique to characterize Macrobending Loss in Single Mode Fiber
    (Addis Ababa Universty, 2010-06) Getu, Girum; Gholap, A.V. (PhD)
    An experimental technique of macrobending losses for a single mode fiber (SMF) with different radius of curvature is presented. Macrobending losses for SMF are investigated experimentally, showing that a tight bent of fiber has a significant impact on the power loss. This experimental measurement suggests how this method is crucial to characterize macrobending at wavelength of interest even to calculate the power loss due to bending, to determine the spectral window at which the fiber can effectively operates, to predict the bend loss for a higher wavelength from the trend of shorter wavelengths. This thesis work also investigates the variation of the constants, CR and C , which depends on the waveguide dimension, with respect to shorter wavelengths using He-Ne laser and Mercury Vapor Lamp sources of light that emits different wavelengths in the visible and ultraviolet region of an optical spectrum using an optic beam launch method. Corresponding experimental tests are presented, which agree with the theoretical results that show, the bent fiber at different bend radii has a significant influence on the total loss of the fiber. Key words: macrobending ; optic beam launch; optical spectrum; radius of curvature: Single mode fiber: spectral window; power loss
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    Ab–initio Calculations of Structural and Electronic Properties of a BexZn1-xSe Ternary Alloy
    (Addis Ababa Universty, 2012-06) Mulualem, Getasew; Mazher, Javed (Professor)
    We have used the ab-initio SIESTA code within the framework of DFT, LDA method to calculate the structural and electronic properties of BexZn1-xSe alloy for different compositions x = 0.0, 0.33, 0.66, and 1.0. The system is modelled in various possible configurations using a large 54-atom supercell. It is noteworthy to mention that the determination of structural and electronic properties of a BexZn1-xSe ternary alloy at x = 0.33 and 0.66 have not been reported earlier to the best of our knowledge. We analyze composition effect on lattice constants, bulk modulus, pressure derivative, bandgap, and density of states. Deviations of the lattice constant from Vegard’s law and the bulk modulus from linear concentration dependence are observed. It was deduced that increasing the Be composition in the alloy increases the hardness of the materials. In addition, the calculated band structures showed that the bandgap undergoes a direct-to-indirect transition at the composition of 0.84. The bandgap is found to vary non-linearly with Be composition. Using the approach of Bernard and Zunger, the microscopic origins of bandgap bowing is also explained. It is concluded that the energy bandgap bowing is primarily due to volume deformation effect. Furthermore, the structural phase transformations of ZnSe under high pressure are also studied by similar method. It is found that ZnSe undergoes a first-order phase transition from the zinc blende structure to the rock salt structure at approximately 13.75 GPa. The ground state properties of the phases of ZnSe are also calculated. Our results are in good agreements with experimental observations.