Geçiş metali karbürlerin yapısal, elektronik ve titreşim özelliklerinin yoğunluk fonksiyon teorisi ile incelenmesi

06.03.2018 tarihli ve 30352 sayılı Resmi Gazetede yayımlanan “Yükseköğretim Kanunu İle Bazı Kanun Ve Kanun Hükmünde Kararnamelerde Değişiklik Yapılması Hakkında Kanun” ile 18.06.2018 tarihli “Lisansüstü Tezlerin Elektronik Ortamda Toplanması, Düzenlenmesi ve Erişime Açılmasına İlişkin Yönerge” gereğince tam metin erişime açılmıştır

Bizmut ve azot içeren III-V grubu yarıiletken alaşımların fiziksel özelliklerinin incelenmesi

06.03.2018 tarihli ve 30352 sayılı Resmi Gazetede yayımlanan “Yükseköğretim Kanunu İle Bazı Kanun Ve Kanun Hükmünde Kararnamelerde Değişiklik Yapılması Hakkında Kanun” ile 18.06.2018 tarihli “Lisansüstü Tezlerin Elektronik Ortamda Toplanması, Düzenlenmesi ve Erişime Açılmasına İlişkin Yönerge” gereğince tam metin erişime açılmıştır.Bu çalışmanın amacı teknolojide büyük öneme sahip 1x1x1 süper-hücre kullanılarak III-V grubu GaAs, InAs, GaN, InN ve GaBi ikili bileşiklerinin, GaInAs, GaAsN ve GaAsBi üçlü ve GaInAsN dörtlü alaşımlarının fiziksel özelliklerinin çinko-sülfür (ZnS) yapıda araştırılmasıdır. Fiziksel özellikler kapsamında bu malzemelerin yapısal, elektronik ve optiksel özellikleri incelendi. Elektronların değiş-tokuş korelasyon enerjileri için yerel yoğunluk ve genelleştirilmiş gradyent yaklaşımları kullanıldı. Yapılan hesaplamalar yoğunluk fonksiyonel teorisini esas alan WIEN2k koduyla elde edildi. Öncelikle çalışılan malzemelerin taban durumu örgü parametresi hesaplandı. Elde edilen örgü parametreleri deneysel ve teorik çalışmalar uyumlu bulunması yapılacak olan elektronik ve optiksel çalışmaların doğruluğunu arttıracağı tespit edildi. Üçlü alaşımlar için elde edilen örgü parametrelerinin Vegard kanunuyla uyuştuğu görüldü. Daha sonra elektronik özellikler kapsamında incelenen ikili bileşiklerden GaBi metalik özellik gösterirken, diğer ikili bileşikler ise açık şekilde doğrudan bant aralığına sahip yarıiletken davranış gösterdiği tespit edildi. Üçlü ve dörtlü alaşımlar ise, GaAsBi üçlü alaşımı hariç, tüm konsantrasyonlarda doğrudan bant aralığına sahip yarıiletken oldukları görüldü. GaAsBi ise %44.2 Bi konsantrasyonuna kadar yarıiletken bu orandan büyük (>%44.2) konsantrasyonlarda ise metalik özellik gösterdiği elde edildi. GaAsBi alaşımının sahip olduğu geniş bant aralığı yakın kızıl ötesi ve kızıl ötesinde yer alması bu malzemenin özellikle Tera-Hertz uygulamaları ve fiber-optik alanlarda kullanılabileceği tespit edildi. Elektronik özellikler kapsamında GaAsN ve GaAsBi üçlü alaşımlarının elektronik bant yapılarında meydana gelen bükülme parametreleri incelenerek bizmutun valans bandını azotun ise iletkenlik bandını etkilediği tespit edildi. Son olarak tez kapsamında incelenen tüm malzemeler için kompleks dielektrik fonksiyonu hesaplandı. İmajiner dielektrik fonksiyonunda meydana gelen temel soğurma sınırı malzemelerin yasak bant enerjilerine karşılık geldiği tespit edildi. Reel ve imajiner dielektrik fonksiyonlar kullanılarak diğer optiksel sabitler olan soğurma katsayısı, yansıma spektrumu, kırılma indisi, sönüm katsayısı ve enerji kayıp fonksiyon spektrumları elde edildi.The aim of this work is investigation of physical properties of GaAs, InAs, GaN, InN and GaBi binary compounds, GaInAs, GaAsN and GaAsBi ternary and GaInAsN quaternary alloys of III-V group using 1x1x1 supercell at zinc-blende (ZnS) structure. As part of physical properties, the structural, electronic and optical properties have been investigated. The local density and generalized gradient approximation are used for exchange and correlation energy. The calculations are obtained by WIEN2k code based on density functional theory. Firstly, ground state lattice parameter is calculated for studied materials. The obtained lattice parameters are in good agreement with experimental and theoretical studies. That result increases reliability of electronic and optical results. The calculated lattice parameters of ternary alloys are coincidence with Vegard's law. Within the scope of electronic properties, GaBi compound behaves metallic, but other binary compounds behaves semiconductor with direct band gap. With the exception of GaAsBi alloy, all of ternary and quaternary alloys have direct band gap for all concentration. Reach up to % 44.2 Bi concentration, GaAsBi ternary alloy behaves semiconductor, but bigger than that concentration it becomes metal material. So, GaAsBi alloy has a large infra-red spectrum which is important for tera-hertz and fiber-optic applications. And, the bowing parameter of GaAsN and GaAsBi is obtained. Also, it is found that bismuth effects valance bands, while nitrogen effects conduction band. Finally, complex dielectric function is calculated for all studied materials. It is determined that, fundamental absorption limit is correspond to forbidden band energy of materials. Using the real and imaginer dielectric function, the other optical constants, such as absorption coefficient, reflectivity, refractive index, extinct coefficient and energy loss function, are obtained

Ground state properties and thermoelectric behavior of Ru(2)VZ (Z=Si, ge, sn) half-metallic ferromagnetic full-Heusler compounds

The ground state properties namely structural, mechanical, electronic and magnetic properties and thermoelectric behavior of Ru(2)VZ (Z =Si, Ge and Sn) half-metallic ferromagnetic full-Heusler compounds are systematically investigated. These compounds are ferromagnetic and crystallize in the Heusler type L2(1) structure (prototype: Cu2MnAl, Fm-3m 225). This result is confirmed for Ru2VSi and Ru2VSn by experimental work reported by Yin and Nash using high temperature direct reaction calorimetry. The studied materials are half-metallic ferromagnets with a narrow direct band gap in the minority spin channel that amounts to 31 meV, 66 meV and 14 meV for Ru2VSi, Ru2VGe, and Ru2VSn, respectively. The total spin magnetic moment (Mtot) of the considered compounds satisfies a Slater-Pauling type rule for localized magnetic moment systems (M-tot=(N-V-24)mu(B)), where N-V=25 is the number of valence electrons in the primitive cell. The Curie temperature within the random phase approximation (RPA) is found to be 23 K, 126 K and 447 K for Ru2VSi, Ru2VGe and Ru2VSn, respectively. Semi-classical Boltzmann transport theories have been used to obtain thermoelectric constants, such as Seebeck coefficient (S), electrical (sigma/tau) and thermal conductivity (kappa/tau), power factor (PF) and the Pauli magnetic susceptibility (chi). ZT(MAX) values of 0.016 (350 K), 0.033 (380 K) and 0.063 (315 K) are achieved for Ru2VSi, Ru2VGe and Ru2VSn, respectively. It is expected that the obtained results might be a trigger in future experimentally interest in this type of full-Heusler compounds. (C) 2016 Elsevier B.V. All rights reserved

Effects of IIIB transition metals on optoelectronic and magnetic properties of HoMnO3: A first principles study

The optoelectronic and magnetic properties of pure HoMnO3 and Ho0.67T0.33MnO3 (T = La, Y) alloys in hexagonal phase are theoretically investigated by using the first-principles calculations. The investigations are performed by means of the density functional theory through using the spin polarized generalized gradient approximation plus the Hubbard potential (SPGGA + U, Ueff = 3 eV). The studied material HoMnO3 exhibits two indirect band gaps: 1.58 eV for the spinup state and 0.72 eV for the spin-down state along the S-G direction within the SPGGA + U approximation. It is found that the band gap of pure HoMnO3 for the spin-up state increases with increasing La and Y dopants. The results show that all of the studied materials have semi-metallic behaviors for the spin-up state and semiconducting character for the spin-down state. The substitutions of La and Y for Ho in HoMnO3 cause the static dielectric constant (epsilon(0)) to increase in the x direction but to decrease in the z direction. The calculated optical conductivity spectrum of HoMnO3 in a low energy range is in good agreement with the recent experimental data

Thermoelectric properties of stannite-phase CuZn(2)AS(4) (CZAS; A=Al, Ga and In) nanocrystals for solar energy conversion applications

The current study aimed to comprehensively investigate structural, electronic, optical and transport properties of quaternary semiconductor CuZn(2)AS(4) (CZAS; A= Al, Ga and In) nanocrystals (NCs). Based on energy considerations, the stannite structure (I-42m; No. 121) is found to be more stable than the kesterite (I-4; No. 82) and wurtzite (P6(3)mc; No. 186) type structures. By means of hybrid functional calculations, these nanocrystals have direct band gap of 0.81-1.71 eV with a high absorption coefficient of >10(4) cm(-1), which are well-suited for use in solar energy-conversion applications. Some of the latest advances in applications of these nanocrystals in thermoelectric applications are also highlighted in the current study. It is observed that transport coefficients of these materials are found to be nearly direction independent and isotropic. All three samples are p-type conductors at room temperature. Especially, the Seebeck coefficient of CuZn2AlS4 is even larger than that of CuZn2GaS4 and CuZn2InS4 under the studied carrier concentration and temperature region. The maximum figure of merit (ZT) reaches 0.982 (0.977), 0.984 (0.974) and 0.53 (0.955) for p-type (n-type) CuZn2AlS4, CuZn2GaS4, and CuZn2InS4, respectively, at 300 K. The high Seebeck coefficients, high figure of merit and low thermal conductivities make these systems good candidates for high-efficiency thermoelectric conversion applications

Structural, mechanical and thermodynamic properties of N-dope BBi compound under pressure

.The structural, mechanical and thermodynamic properties of N-dope BBi compound have been reported in the current study. The structural and mechanical results of the studied binary compounds (BN and BBi) and their ternary alloys BBi1−x N x structures are presented by means of density functional theory. The exchange and correlation effects are taken into account by using the generalized gradient approximation functional of Wu and Cohen which is an improved form of the most popular Perdew–Burke–Ernzerhof. The quasi-harmonic Debye model is used for the thermodynamic properties of studied materials. The basic physical properties of considered structures such as the equilibrium lattice parameter (a 0), bulk modulus (B 0), its pressure derivative (B′), elastic constants (C 11, C 12 and C 44), Kleinman’s internal-strain parameter (ƺ), shear modulus anisotropy (A), the average shear modulus (G), Young’s modulus (Y) and Poisson’s ratio (v), B 0/G ratio, microhardness parameter (H), Cauchy pressure (C″), and 1st and 2nd Lame constants (λ, μ), debye temperature (θ D), wave velocities (ν l, ν t and ν m), melting temperature (T m) and minimum thermal conductivity (κ min) have been calculated at zero pressure. In order to obtain more information, thermodynamic properties, such as internal energy (U), Helmoltz free energy (F), entropy (S), Debye temperature (θ D), thermal expansion (α), constant volume and pressure heat capacities (C V and C P ), are analyzed under the whole range from 0 to 20 GPa and temperature range from 0 to 1500 K. The obtained results of the studied binary compounds are in coincidence with experimental works

The first-principles study on physical properties and phase stability of Boron-V (BN, BP, BAs, BSb and BBi) compounds

The physical properties of Boron - group V compounds have been investigated systematically by using density functional theory, based on the full-potential linearized augmented plane-wave method. Generalized gradient approximation is performed to calculate the structural and elastic properties of all studied Boron compounds. We have investigated the phase stability of Boron-V compounds in zinc-blend, rock-salt and wurtzite crystallographic phases. The structural properties such as the equilibrium lattice parameter, bulk modulus and its pressure derivative of Boron-V compounds have been calculated in three phases and compared with other theoretical and experimental works. The elastic properties of the studied compounds are only investigated in the most stable calculated phase. We have obtained Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, Kleinmann parameter, wave velocities and melting temperature by the aid of the calculated elastic constants, which are of great technological importance to Boron related compounds. And, we have also found the Debye temperature of the compounds from the average wave velocity. Our results are in reasonable agreement with the available theoretical and experimental data. Detailed comparisons are made with the results obtained in previous theoretical studies and available measured values. (C) 2013 Elsevier B.V. All rights reserved

Structural, mechanical, electronic and optical properties of BBi, BP and their ternary alloys BBi1-xPx

The ground state properties of BBi, BP and their ternary alloys BBi1-xPx are reported using first-principles calculations based on density functional theory (DFT). The modified Becke-Johnson (mBJ) potential together with the generalized gradient approximation (GGA) for the correlation potential has been used here as it is a superior method for estimating band inversion strength and band order. The zincblende phase is found to be more stable than the other phases for all studied materials. The calculated lattice constants exhibit a small deviation from the linear Vegard's law with a downward bowing value of 0.11 angstrom. The calculated ground state parameters for the studied binary compounds agree with available theoretical and experimental results. The bandgap value of the studied materials calculated with the mBJ potential is considerably enhanced with respect to values from the GGA functional. Optical properties have been calculated and analysed with photon incident energy up to 21.0 eV. The spin-orbit interaction (SOI) has also been considered for structural and electronic calculations and the results are compared with those of non-SOI calculations. The real and imaginary parts of the dielectric function have also been calculated and discussed

Investigation of structural, mechanical, electronic, optical, and dynamical properties of cubic BaLiF3, BaLiH3, and SrLiH3

The structural, mechanical, electronic, optical, and dynamical properties of BaLiF3, BaLiH3, and SrLiH3 cubic perovskite materials are theoretically investigated by using first principles calculations. Obtained results are in reasonable agreement with other available theoretical and experimental studies. The considered materials are found to be mechanically stable in the cubic structure. We found that all materials are brittle. The modified Becke-Johnson (mBJ) exchange potential has been used here to obtain an accurate band order. The calculated band-gap energy value of BaLiF3 (8.26 eV) within the mBJ potential agrees very well with the experimentally reported value of 8.41 eV. In order to have a deeper understanding of the bonding mechanism and the effect of atomic relaxation on the electronic band structure, the total and partial density of states have also been calculated. We have investigated the fundamental optical properties, such as the real epsilon(1) (omega) and imaginary epsilon(2) (omega) parts of the dielectric function, absorption coefficient alpha(omega), reflectivity R(omega), and refractive index n(omega) in the energy range from 0 to 40 eV within the mBJ potential. The band-gap energy obtained from the absorption spectrum is around 8.76, 3.99, and 3.31 eV for BaLiF3, BaLiH3, and SrLiH3 crystals, respectively. It should be noted that BaLiF3 could be a strong potential candidate as a laser material for the development of a vacuum-ultraviolet light emitting diode once direct transition is confirmed by experimental studies. Finally, we have calculated the lattice dynamical properties of BaLiF3, BaLiH3, SrLiH3, and SrLiF3 crystals. The full phonon dispersion curves of these materials are reported for the first time. Our results clearly indicate that the materials are dynamically stable, except for SrLiF3, in the cubic structure. The obtained zone-center phonon frequencies of BaLiF3, BaLiH3, and SrLiH3 accord very well with previous experimental measurements

Structural and electronic properties of Ga1-xInxAs1-yNy quaternary semiconductor alloy on GaAs substrate

We have presented structural and electronic properties of binary (GaAs, GaN and InAs), ternary (Ga1-xInxAs and GaAs1-yNy) and quaternary (Ga1-xInxAs1-yNy) semiconductor alloys by using a first-principles pseudopotential technique. The structural and electronic properties of Zinc-Blende phase of these materials have been calculated by using the local density approximation (LDA) of the density-functional theory (DFT). To obtain the lattice parameter and band gap energy of the (GaInAsN) quaternary semiconductor alloy we separately calculated the lattice constant and band gap energies of ternary semiconductor alloys, namely GaAsN and GaInAs. The calculated lattice constant, bulk modulus and the direct band gaps for studied semiconductors showed great parallelism with the previous available theoretical and experimental studies. (C) 2011 Elsevier B. V. All rights reserved