Researcher: Kemal Berk Sonmez, Supervisor: Sefer Bora Lisesivdin
MMIC chips, which are used in almost all of the microwave components that constitute the basic building blocks of monolithic microwave integrated circuits (MMIC), radar and electronic warfare applications, are among today’s critical technologies. Developed with technically equipped infrastructure support, GaN-based MMIC chips have become the preferred choice for different working subjects due to the technology developed in recent years due to their high voltage and high temperature performance due to their high voltage and high frequency frequencies. MMIC technology is a technology used in the manufacture of specially designed structures for active and passive circuits on a single crystal bottomstone. The production and characterization of resistive structures, which serve as passive circuit elements in MMIC structures, with RF magnetron sputter system, were obtained according to the design with a resistivity of 30 ohm / sq.
Researcher:Mehmet Taha Haliloglu, Supervisor: Sefer Bora Lisesivdin
Transistors have a significant role in semiconductor technology. Transistors are mainly composed of three components as source, drain and gate metals. These structures provide electrical conduction by placing the source, drain and gate metals on a semiconductor. High power, high frequency and fast switching operations can be made by these structures. Recently, transistors or monolithic microwave integrated circuits (MMIC) are designed and manufactured according to different needs. Various methods have been developed to improve the performance and optimization of these devices produced for different purposes. One of these methods is the via hole application. In the part of so-called back side processes of semiconductor device production, a well opening process for contacting the material with the device metals on the front side is referred to as the via hole. With this method substrates are thinned to 100 µm from back-side, contact is provided to weld metal on the front side. The weld metal is grounded from the bottom of the substrate by this contact. This method has been developed by aiming to reduce the inductance of source metal electrically. The via hole process was performed on the silicon carbide (SiC) material known as the hardest material after diamond. The optimization and dry etching mechanism of the via hole process with fluorine-based dry etching method of SiC material by using reactive ion etching system (ICP RIE) with inductive coupled plasma has been reported on this study. With describing whole via hole dry etching process back-side operations are discussed. The SEM system was used to analyze the results of the etching process. As a result of DC measurement, it has been show that the resistance of SiC via hole structures were analyzed.
Researcher: Gokhan Atmaca, Supervisor: Sefer Bora Lisesivdin
AlGaN/GaN heterostructures have a widely usage area in high power, high frequency and high temperature applications for modern electronics. Despite of this widely usage area, AlGaN/GaN based electronics devices have some problems during the operation. One of these problems is current collapse in current-voltage characteristics. For long years, Si3N4 surface passivation is used in surface of heterostructure to prevent current collapse. Because electrons trapped by surface states in surface leads to current collapse and this passivation layer reduces surface states in surface. Therefore, number of trapped electrons is decreased and current collapse is suppressed at certain amount. Low-field transport and hot-electron transport investigations can play an important role in better understanding the behind mechanisms of this method widely used in high power and high frequency electronics devices. In this thesis study, the effect of Si3N4 passivation layer on two dimensional electron gas in AlGaN/GaN heterostructures is investigated with temperature dependent Hall effect and hot-electron dynamics measurements of different sample groups. In low-field transport investigations, since carrier density is increased with surface passivation, it was found that effect of interface roughness scattering mechanism on mobility is increased. In analyses with hot-electron dynamics measurements, the experimental observation of the negative differential resistance phenomena in drift velocity-electric field characteristics of AlGaN/GaN heterostructures was firstly achieved.
Researcher: Canan Bektas, Supervisor: Beyza Lisesivdin
GaAs/AlGaAs structures’electrical characteristics have been investigated in the frequency range of 7 kHz-700 kHz by using admittance measurements technique which is including various capacitance-voltage (C-V) and conductance-voltage (G/w-V) measurements. The C and G/w values are found a strong function of frequency. The C-V plots show two peaks which are corresponding to the inversion and accumulation regions. The first peak was attributed to the particular density distribution of surface states/interface states (Nss) localized at metal/semiconductor interface. Second peak which is called anomalous peak and its magnitude decreases with increasing frequencies and it can be attributed to the series resistance (Rs), Nss, and interfacial layer native or deposited. Therefore, the voltage dependent profile of Nss was obtained using the low-high frequency capacitance (CLF-CHF) technique. Nss-V plot shows two distinctive peaks at about -10 V and 0 V, respectively. The Nss value was also obtained using Hill-Coleman technique for each frequency and the value of Nss decreases with increasing frequency. The experimental results confirmed that both the values of Nss and Rs considerably affect a the admittence considerably effect on the admittance measurements in the GaAs/AlGaAs structures.
Researcher: Rokaia Ibrahem, Supervisor: Sefer Bora Lisesivdin
In this study, the electronic and the optical properties of wz-ZnO and wz- MgxZn1-xO have been calculated for different Mg mole fractions using the Atomistic Toolkit-Virtual Nano Lab (ATK-VNL) software based on density functional theory (DFT). Our calculations are performed using the hybrid-generalized gradient approximation (GGA+U) formalism, where the Hubbard parameters are applied to Zn-3d electrons and O-2p electrons. The electronic properties calculations include the electronic band structures, the density of states and electron effective masses, whereas the optical properties calculations include the static dielectric functions, refractive indexes, extinction coefficients and absorption spectra of all the studied structures. The results show that the band gap energies increase as the Mg mole fractions increase, which corresponds with the previous experimental results. The electron effective masses calculation shows a linear dependence of the Mg mole fraction, but their values show overestimation. The absorption edges of studied structures move toward the higher energies region (the lower wavelengths region) as Mg mole fraction of MgxZn1-xO increases. The static dielectric constant of MgxZn1-xO decreases as Mg mole fraction increases. The dielectric constants of the high frequency of MgxZn1-xO are found to be very similar to the experimental results. In addition, the refractive indexes and the extinction coefficients move toward the higher energy region (the lower wavelength region).
Researcher: Oznur Omeroglu, Supervisor: Sefer Bora Lisesivdin
Within the scope of this thesis, the electronic properties of graphene sheets doped with group IIB elements Zn, Cd and Hg are investigated by using Local Density Approach (LDA) with Density Functional Theory (DFT). Adatom doping of the impurity atoms, Zn, Cd and Hg have been calculated at three different positions of the two dimensional graphene sheet as hollow (H), top (T) and bridge (B) locations. Also, substitutional doping of the same impurity atoms instead of carbon atoms have been investigated within the graphene sheet. Calculations of the basic electronic properties such as density of states (DOS) and the electronic band structures achieved by adatom and substitutional doping of the graphene sheet have been done. The binding energies have been calculated with the help of the total energies of the structures which are formed by adatom doping. In addition, the positions which structural stabilities of the impurity atoms are established within the lattice and has the lowest binding energies, have been investigated. Doped graphene sheets differ greatly by changing the impurity atoms or by changing the location of the impurity atoms. The possible changes in the electronic properties of graphene structure due to this phenomenia have been investigated.
Researcher: Mehmet Ozturk, Supervisor: Sefer Bora Lisesivdin
In this study, Hall effect and sheet carrier density measurements of ultrathin barrier AlN/GaN and In0.17Al0.83N/GaN high electron mobility transistor (HEMT) structures by MOCVD technique were performed at a temperature of 15-300K and 0.5T magnetic field. As a result of measurements, 2-dimensional electron gas (2DEG) sheet carrier densities and mobility were calculated. Analysis of scattering mechanisms affecting electron mobility for each sample was made using Hall effect measurement results based on temperature. As a result of the scattering analysis, the parameters related to the samples as deformation potential, correlation length and quantum well width were determined.
Researcher: Ece Kutlu, Supervisor: Sefer Bora Lisesivdin
In this study, the electronic and optical properties of Si3N4 crystal with and without Al, Ga, In and As impurities are explained with the help of the density fuctional theory (DFT) calculations which use local density approximation (LDA). Hexagonal beta-Si3N4 crystal structure was used in the calculations. Stable positions for the impurity atoms were calculated using the formation energy and binding energies of the related structures and atoms. Basic electronic properties like electronic band structures and density of states (DOS) were determined for pure structures and structures with impurities. In addition to electronic property calculations, optical properties like static dielectric coefficient, photon energy dependent refractive index, extinction coefficient, absorption coefficient and reflection coefficient were calculated with photon energy dependent real and imaginary dielectric functions. The electronic and optical properties of the beta-Si3N4 crystal were found to change drastically with the introducing impurities to the system, and the possible related electronic and optical applications were discussed.
Researcher:Jangeez Mostafa M Jameel Al Abbas
In this study, 2-Dimensional Electron Gas (2DEG) properties of GaN-based high electron mobility transistor (HEMT) heterostructures with ultrathin AlN, AlGaN and AlInN barriers were investigated by using the solutions of one-dimensional self-consistent SchrödingerPoisson equations. For each heterostructures with ultrathin AlN, AlGaN and AlInN barriers, the effects of different GaN cap layer and barrier layer thicknesses and doping concentrations on the conduction band profile, electron probability densities and sheet carrier densities were examined. In addition, some of these heterostructures were evaluated according to different InGaN channel thicknesses and doping concentrations. Using the obtained results, optimized GaN-based HEMT heterostructures with ultrathin barrier, which have the highest sheet carrier density and 2DEG carriers less affected from scattering mechanisms, were suggested for future device investigations
Researcher: Cem Gunes, Supervisor: Sefer Bora Lisesivdin
In this study, electron and magneto transport properties of 4 different ultrathin barrier AlInN/GaN and AlInN/AlN/GaN/AlN structures were investigated. Resistivities were measured at a temperature of 30-300 K, Hall mobilities and Hall carrier densities were measured at the same temperature range with magnetic field intensities 0<B<1.4 T, 2 dimensional electron gas (2DEG) carrier densities and mobilities are calculated from these measurements. Results of Hall measurements were used to investigate scattering mechanisms of each structure. Also, material related parameters named well width of the quantum well, correlation length and deformation potential were determined with the help of scattering analyses.
Researcher: Gokhan Atmaca, Supervisor: Sefer Bora Lisesivdin
In this study, we investigated 2-Dimensional Electron Gas (2DEG) properties of AlQN/AlN/GaN/InGaN/GaN/AlGaN/AlN/GaN (Q=Ga,In) heterostructures based InGaN/GaN multiple quantum well with AlGaN and InAlN barrier by solving 1-dimensional self-consistent Schrödinger-Poisson equations. As a result, electron mobilities are analyzed dependent temperature and barrier thickness for heterostructures based multiple quantum well with InAlN barrier. Current-voltage characteristics are examined using this electron mobility values via Simba mobility model for different barrier thicknesses, channel lenghts and gate lenghts. 2DEG properties and current-voltage characteristics, which obtained via numerical and analytical calculations, help to produce sample and devices which have better electronic properties for further investigations.
Researcher: Abdullah Fatih Kuloglu, Supervisor: Sefer Bora Lisesivdin
In this study, electronic properties variation of graphene nanoribbons with armchair edges depending on ribbon width and saturation of free bond at the edges were investigated with DFT. For exchange-correlation calculation, LDA was chosen. Calculations were performed for six different widths (Na = 5, 7, 9, 11, 13, 15) which are symmetrical. First of all, calculations performed for bare conditions. After, electronic properties variation of the structure depending on both-side H-terminated, single-side H terminated and another-side Pd terminated and both-side Pd terminated were investigated. Especially, in the Pd-terminated conditions, we observed that electronic properties of the structure dramatically changed. Obtained results important because of investigation of Pd and graphene relationship and it may be a guide for experimental studies.
Researcher: Kenan Elibol, Supervisor: Sefer Bora Lisesivdin
In this study, the properties of the surface and magnetoconductivity of exfoliated and epitaxially grown graphene samples were investigated. The properties of the surface of graphene samples were investigated by using optical microscope, AFM, and SEM. The Raman spectrum was measured to determine the number of layers which the epitaxial graphene samples have. The graphene devices were fabricated by using optical and e-beam lithography methods for Hall effect and I-V measurements. I-V characteristics of the exfoliated and epitaxially grown graphene samples were measured. To investigate the effect of light on the properties of magnetoconductivity of graphene, under of a fixed magnetic field, Hall effect measurements were carried out under dark and then illumination conditions at the 25-300 K temperature range. To investigate the effects of helium gas and water on Hall measurements of epitaxial graphene, under of a fixed magnetic field, Hall effect measurements were carried out at the 30-300 K temperature range. To better understand the interaction of helium with graphene, DFT calculations were performed.
Researcher: Gulser Karakoc, Supervisor: Sefer Bora Lisesivdin
In this study, Hall measurement and conductivity analyse have been investigated for 3 samples of AlInN/AlN/(InGaN)/GaN multi-structures with InxGa1-xN- quantum well grown by MOVPE crystal growth method. Resistivities, Hall mobilities and carrier concentrations were measured in a temperature range of 15-300K and 0.4T magnetic field. Electron transport characteristics of the samples with the values of x =0.00, 0.12 and 0.18 were explained. Also, upon of scattering mechanisms that effect the mobility of electrons were discussed. Resistivity analyses were performed which are dependent to the resistivity measurements.