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In this study, the total and partial magnetism and phase transform temperature properties of Co2NbGa Full Heusler half-metallic ferromagnet alloy were investigated with Effective Field Theory (EFT) developed by Kaneyoshi [1,2]. The research involved temperature dependence of total and partial magnetizations and M-H examinations. Shell and core interactions of each component were considered in the data results. First, the atomic positions of atoms Co, Nb, and Ga have located a, b, c =0.5 a0 Wyckoff positions, and M – T and M – H examinations were performed with a total of 1000, and 2000 steps, respectively. The program performed 50 sub-calculations for each step and chose the best result. Second, Coshell, Cocore, Nb, Ga, bMtCo, bMTCo2NbGa and bMTICo2NbGa parameters gained with M-T actions 4.74309E-6, 3.17012E-6, 1.90929E-7, 6.12596E-8, 7.9132E-6, 8.16539E-6 and 1.20752E-05, respectively. Then the total magnetic moment was obtained as 2.00 μB/f.u. Partial magnetic contributions of Cos, Coc, Nb, and Ga were calculated, and most contributions originated from Cos. Finally, the magnetic phase transforms of Co2NbGa alloy were observed at Tc= 351 K Curie temperature using the EFT method. This result is in full agreement with the experimental value obtained [3]. Additionally, the Curie temperature of Co2NbGa alloy was theoretically obtained as 451 K [4,5]. The magnetic properties obtained in EFT studies of Co2NbGa full Heusler compound obtained in the literature studies were fully compatible.

Electronic, magnetic and elastic calculations of a new Ti2RuTl full Heusler compound have been made using the Vienna Ab-initio Simulation Package (VASP). The computations were carried out for the ferromagnetic (FM) state in the F- 43 m (No.216) space group. Ground-state values of the lattice parameter, bulk modulus, volume and energy were obtained. The lattice parameter of the cubic structure was 6.478 Å. According to the electronic calculations, the spinup electrons were found to have metallic properties while the spin-down electrons were found to have a semiconductor nature. Therefore, these metallic and semiconductor spin states showed that the Ti2RuTl full Heusler compound is a half-metallic (HM) ferromagnet with 1.00 μB/f.u. magnetic moment. Finally, the elastic calculations showed that the compound is an elastically stable, ductile material and could be a new candidate material for using in spintronics.

Bu tez çalışmasında farklı püskürtme güçleri altında üretilen n-ZnO/AlN/p-GaN yakın uv mavi ışık yayan diyotların (led) elektriksel, optik, yapısal ve morfolojik özellikleri incelendi. Bu çalışmada p-n yapıları elde etmek için farklı ince film büyütme yöntemleri kullanıldı. İlk olarak safir (Al2O3, (0 0 0 1) yönelimli) alttaş üzerine Metal-Organik Kimyasal Buhar Biriktirme (MOCVD) tekniği kullanılarak nitrit tabakaları (AlN, GaN) büyütüldü. Daha sonra n-tipi tabakası için rf magnetron püskürtme yöntemiyle 25 W, 50 W, 75 W, 90 W ve 100W püskürtme gücü değişkenleri altında ZnO büyütmesi yapıldı. p-GaN ve n-ZnO katmanları üzerine kontaklar (Ni (200 Å)/Au (2000 Å) ve Ti (300 Å)/Au (2000 Å) yapılarak Led yapısı tamamlandı. Numunelerin elektriksel, yapısal, optik ve morfolojik özellikleri; Akım-Voltaj (I-V) ölçümleri, X-ışını kırınımı (XRD), Fotolüminesans (PL), Elektrolüminesans (EL) dataları ve Atomik Kuvvet Mikroskobu (AFM) sonuçları değerlendirilerek belirlendi. AFM sonuçlarının rms (root-mean-square) değerlerine göre püskürtme gücü arttıkça, tane boyutu arttı. Ayrıca PL spektrumundan 422nm, 465nm, 425nm, 420nm ve 421nm’de ZnO pikleri, 423 nm’de GaN piki görüldü. Led'in oda sıcaklığı elektrolüminesansı (EL) sırasıyla 25 W, 50 W, 75 W, 90 W ve 100 W püskürtme güçlerinde biriktirilen ZnO ince filmler için 455, 442, 430, 420 ve 395 nm'de güçlü mor-mavi emisyon spektrumu gösterdi. Led'lerin I-V eğrileri, 25 W, 50 W, 75 W, 90 W ve 100 W değerleri için 6,8 V, 6,4 V, 5,5 V, 3,3 V ve 5,2 V eşik gerilimleriyle iyi düzelme eğilimi gösterdi.Anahtar KelimelerLED, ZnO, MOCVD, Püskürtme yöntemi In this thesis, the electrical, optical, structural and morphological properties of n-ZnO/AlN/p-GaN near uv-blue light Emiiting Diodes (Led) fabricated under different sputtering powers were examined. In this study, we have used different thin film deposition techniques to obtain p-n junctions. Firstly, nitrite layers were deposited on Sapphire (Al2O3, with (0 0 0 1) orientation) substrates using Metal-Organic Chemical Vapor Deposition (MOCVD) technique. Then ZnO depositions were performed for the n-type layers under 25 W, 50 W, 75 W, 90 W and 100W sputtering powers with rf Magnetron sputtering method. The Led structure is completed by making contacts on p-GaN and n-ZnO layers. The electrical, structural, optical and morphological properties of the samples were determined by evaluating current-voltage (I-V) measurements, X-ray diffractions (XRD), Photoluminescence (PL), Electroluminescence (EL) datas and Atomic Force Microscope (AFM) results. According to the rms values of AFM results. As the sputtering power increased, grain size increased. In addition, PL peaks were observed at 422nm, 465nm, 425nm, 420 nm, 421 nm for ZnO and at 423nm for GaN layers at 25 W, 50 W, 75 W, 90 W and 100 W sputtering powers, respectively. The room temperature Electroluminescence (EL) of the Leds showed strong violet-blue emission spectra at 455, 442, 430, 420 and 395 nm for ZnO thin films deposited at 25 W, 50 W, 75 W, 90 W and 100 W sputtering powers, respectively. The I-V curves of the Leds showed good rectifying behavior with threshold voltages of 6,8 V, 6,4 V, 5,5 V, 3,3 V and 5,2 V for 25 W, 50 W, 75 W, 90 W and 100 W valuesKey WordsLED, ZnO, MOCVD, Sputtering method

In this study, the changes in the half-metallic and magnetic characters of the compounds obtained by doping Cr to the nearly half-metallic CoTcMnSb quaternary compound, which has a lattice parameter of 6.16 Å, a direct bandgap of 0.489 eV, and a total magnetic moment of 4.00 μB/f.u., were investigated. It was observed that the band gaps at the L point of the spin-down band structures increased as the amount of Cr doping increased. While x= 0.25 valence band electrons cut the Fermi energy level, half-metallic characters appeared at 0.50 and 0.75 ratios. The total magnetic moment values were obtained as 3.75, 3.50, and 3.25 μB/f.u for the 0.25, 0.50, and 0.75 ratios, respectively.

High quality single crystalline n-type ZnO films, with a thickness of 200 nm, were grown on top of AlN/GaN layers under 25 W, 50 W and 100 W sputtering powers. The AlN and p-type GaN layers were deposited by metalorganic chemical vapor deposition (MOCVD). From X-ray diffraction measurements, the samples exhibited the (000l) peaks corresponding to both ZnO, GaN and AlN monocrystalline layers. As the sputtering power was increased, the ZnO grain size increased and the dislocation density decreased. This result is supported by a reduction of the rms values obtained on the ZnO layers from Atomic Force Microscopy (AFM) results. In addition, photoluminescence peaks of ZnO at 372 nm, 375 nm and 380 nm were seen as dependent on the sputtering power. We have used an AlN electron blocking layer between ZnO and GaN films to improve the electroluminescence from the n-ZnO side. Room temperature electroluminescence (EL) of the LEDs demonstrated near UVblue emission consisting of predominating peaks centred at 405 nm, 390 nm and 380 nm for the device with ZnO deposited at 25 W, 50 W and 100 W sputtering powers, respectively. Moreover, the I-V curves of the LEDs showed a rectifying behavior with 6.8 V, 6.4 V, 5.2 V threshold voltages for 25 W, 50 W and 100 W values.

Electronic, structural and elastic properties of Pd2CrPb compound were examined with Density Functional Theory (DFT). The lattice constant in state of balance and the total magnetic moment value were calculated and 6.454 A0 and 3.678 µB f.u. values were obtained respectively. The lattice constant and total magnetic moment value obtained for Pd2CrPb compound were compared to existing values in the literature, and the results obtained were found to be consistent with the literature results. The calculation of bulk modulus in this study is non-existent in the literature. The bulk modulus of this molecule is presented in the literature in this sense. The aim of the study is to publish the results of studies on non-existent electronic, structural and elastic properties for Pd2CrPb compound conducted using density functional theory. The electronic band structure, total and partial density graphs for Pd2CrPb compound were drawn. Bulk modulus (B), Shear modulus (G), B/G ratio, Young modulus (E), Poisson ratio, C11, C12 and C44 values were also calculated for this compound. Pd2CrPb compound meets Born stability principles and calculated elastic stabilities show that this compound is mechanically stable., Yoğunluk Fonksiyonel Teorisi (DFT)ile Pd2CrPb bileşiğinin elektronik, yapısal ve elastik özellikleri incelenmiştir. Denge durumundaki örgü sabiti ve toplam manyetik moment değeri hesaplanmıştır ve sırasıyla değerleri 6.454 A0 ve 3.678 µB f.u. bulunmuştur. Pd2CrPb bileşiği için elde edilen örgü sabiti ve toplam magnetik moment değerleri literatürde mevcut olan değerler ile karşılaştırılmış ve elde edilen sonuçların literatür sonuçlar ile uyumlu olduğu gözlemlenmiştir. Bu çalışmada yapılan bulk modülü hesaplaması literatürde mevcut değildir. Bu anlamda literature bu molekülün bulk modülü sunulmuştur. Pd2CrPb bileşiği için literatürde mevcut olmayan elektronik, yapısal ve elastic özellikleri ile ilgili çalışmalar yoğunluk fonksiyon teorisi yardımı ile gerçekleştirilerek elde edilen sonuçların literature kazandırılması amaçlanmaktadır. Pd2CrPb için elektronik bant yapıları, toplam ve kısmi durum yoğunluğu grafikleri çizilmiştir. Ayrıca bu bileşik için Bulk modülü (B), Shear modülü (G), B/G oranı, Young modülü, Poisson oranı, C11, C12 ve C44 hesaplanmıştır. Pd2CrPb bileşiği Born kararlık ilkelerini sağlamaktadır ve hesaplanan elastik sabitleri bu bileşiğin mekaniksel olarak kararlı olduğunu göstermiştir.

Doğrusal boyutu L= 4, 6, 8, olan yedi boyutlu ising modelin en yakın komşu etkileşmeliperiyodik sınır şartlı örgülerde ve sonsuz örgü kritik sıcaklığı yakınında beş “bit”li demonlarkullanılarak Creutz “cellular automaton”ında simülasyonları yapılmıştır. Yedi boyutlu isingmodelinde düzen parametresi olasılık dağılımları kritik sıcaklıkta hesaplanmıştır. Düzenparametresi olasılık dağılımı için sonlu örgü ölçekleme ilişkisi Creutz Cellular Automatonsimülasyonları ile test edilmiş ve literatür ile uyumluluğu doğrulanmıştır. Analitik olasılıkfonksiyonlarının sabitleri kritik noktada sayısal olarak oluşturulan olasılık fonksiyonuna fitedilerek elde edilmiştir. Büyük örgü değerlerinde analitik olasılık fonksiyonu evrensel birbiçime sahiptir. The seven dimensional ising model with nearest-neighbor pair interactions has beensimulated on the Creutz cellular automaton by using five bit demons near the infinite-latticecritical temperature with the linear dimensions L=4,6,8. Moreover the order parameterprobability distribution for seven dimensional ising model have been calculated at the criticaltemperature. The finite size scaling relation for the order parameter probability distiributionhas been tested and verified numerically by the Creutz Cellular Automaton simulation. Theconstants of the analytical function have been estimated by fitting to probability functionobtained numerically at the finite size critical point. For the large finite size, the analyticalfunction has been described the universal shape of order parameter probability distributionfunction.