Mustafa Kemal ÖZTÜRK | Gazi University (original) (raw)

Papers by Mustafa Kemal ÖZTÜRK

IEEE Transactions on Electron Devices

Low-resistance ohmic contacts in AlGaN/GaN high-electron-mobility transistor (HEMT) devices requi... more Low-resistance ohmic contacts in AlGaN/GaN high-electron-mobility transistor (HEMT) devices require high-temperature (HT) annealing (>800 • C) which can deteriorate material quality, surface morphology, and edge acuity of the metal stacks. This article demonstrates the highfrequency and high-power performance of the AlGaN/GaN HEMT devices with low-temperature metal-organic chemical vapor deposition (MOCVD) regrown degenerately doped InGaN ohmic contacts compared with GaN-based regrown contacts. Ohmic contacts fabricated by regrowth methods could be a valuable alternative for both metal-based alloyed ohmic contacts and implantation-based ohmic contacts. Using a T-gate and MOCVD regrown InGaN ohmic contacts, the AlGaN/GaN HEMT with an L g of 150 nm and S-D spacing of 2.5 μm demonstrated a maximum drain current of 0.94 A/mm and a peak transconductance of 337 mS/mm. The same device exhibited a forward current gain frequency f t of 36.8 GHz and a maximum frequency of oscillation f max of 75.0 GHz. A power density of 3.07 W/mm with a 60% drain efficiency was measured at 35 GHz with a V ds of 20 V and a quiescent current of 100 mA/mm. Index Terms-AlGaN/GaN high-electron-mobility transistor (HEMT), metal-organic chemical vapor deposition (MOCVD), regrown InGaN ohmic contacts. I. INTRODUCTION G ALLIUM-NITRIDE (GaN)-based high-electronmobility transistors (HEMTs) have had a great interest due to their unique material properties which make them promising candidates for microwave power devices compared

Silicon, 2021

In this study, structural and electrical properties of Ag/TiO2/n-InP/Au Schottky barrier diodes a... more In this study, structural and electrical properties of Ag/TiO2/n-InP/Au Schottky barrier diodes are investigated. Particle size, d-spacing, micro-strain, ideality factor and barrier heights of two samples are determined for two different interfacial TiO2 layer thickness. X-ray diffraction (XRD) and current-voltage (I-V) measurements are employed for mentioned parameters. It is seen that sample with 60 Å TiO2 interfacial layer is a more ideal diode. The reason for this is argued in main text and conclusion section.

In this study, X-ray diffraction peaks of Si, Ti, Au and ZnO grown on Ge substrate with thickness... more In this study, X-ray diffraction peaks of Si, Ti, Au and ZnO grown on Ge substrate with thickness of 500 nm by using sputtering method are analyzed to determine correlation length and dislocation density. It is seen that in most dense region of peaks, peak behaviour is in accordance with Gauss function. Right and left tails of peaks are in good accordance with q3 law. For randomized dislocations, obeying q3 law is typical and they can be monitored with w-scans by using open detectors. Whole profile is fitted with a limited dislocation dispersion. Edge dislocation density and correlation length are determined in the degree of 1010cm-2 and 103 nm, respectively. In order to gain these values, semi-experimental equations in Kragner method are used. For making a good fit, fit iteration step is taken as 9x106.

Journal of Polytechnic, 2021

Highlights  Absorption coefficient is calculated with a different method/Soğurma katsayısı farkl... more Highlights  Absorption coefficient is calculated with a different method/Soğurma katsayısı farklı bir yöntemle hesaplandı  XRD and Raman analysis is made/XRD ve Raman analizleri yapıldı  SEM and AFM techniques are employed for thickness/Kalınlık için SEM ve AFM teknikleri kullanıldı

Journal of Materials Science: Materials in Electronics, 2019

In this study optical properties of InGaN/GaN/Al 2 O 3 multi-quantum well (MQW) structures are in... more In this study optical properties of InGaN/GaN/Al 2 O 3 multi-quantum well (MQW) structures are investigated in detail. Three samples containing InGaN/GaN/Al 2 O 3 MQWs are grown by using metal organic chemical vapor deposition technique. Sapphire (6H-Al 2 O 3) is used as the substrate. Forbidden energy band gaps (E g) of these three samples are determined from photoluminescence and absorption spectra. Results gained from these two spectra are compared with each other. It is found that E g values are between 2 and 3 eV. For determining refraction index, absorption coefficients, extinction coefficients and thickness of the films a rare method called Swanepoel envelope method is used. It is seen that results gained from this method are consistent with those in literature.

Applied Physics A, 2019

In this study, two-dimensional electron gas (2DEG) mobility analysis of AlN/GaN and InAlN/GaN str... more In this study, two-dimensional electron gas (2DEG) mobility analysis of AlN/GaN and InAlN/GaN structures with ultrathin barrier layers by metal organic chemical vapor deposition (MOCVD) has been performed with Hall effect measurements which is carried out under temperature from 15 to 350 K and a single magnetic field of 0.5 T. As a result of the scattering analysis made with Matthiessen's rule, it is shown that while the interface roughness scattering mechanism is dominated on the 2DEG mobility at low temperatures, the 2DEG mobility has been dominated by the polar optical phonon-scattering mechanism at high temperatures. Also, the acoustic phonon-scattering mechanism is effective on the 2DEG mobility at middle temperature. Furthermore, the interface and the quantum well parameters such as deformation potential, quantum well width, and correlation length of the interface are determined for each. As well as experimental measurements, the conduction band energy diagrams of the studied samples have been calculated using one-dimensional (1D) self-consistent Schrödinger-Poisson equations. A 2D quasitriangular quantum well formation has been shown for each studied samples. 2DEG probability density of samples has been investigated.

European Journal of Science and Technology, 2018

Bu çalışmada, MOCVD yöntemi ile üretilen InGaN/GaN ışık yayan diyot (LED) yapılarının mozaik yapı... more Bu çalışmada, MOCVD yöntemi ile üretilen InGaN/GaN ışık yayan diyot (LED) yapılarının mozaik yapıları analiz edildi. Safir alttaş üzerine biriktirilen, farklı In kompozisyonuna sahip InGaN/GaN bariyer tabakasının mozaik yapı üzerine etkisi Yüksek Çözünürlü X Işını Kırınımı (HR-XRD) yöntemi ile karakterize edildi. Amacımız mozaik yapı hesaplarından yararlanarak LED yapısının kalitesini arttırmaktır. Günümüzde LED' lerin geniş bir kullanım alanına sahip olmasından dolayı sektörde büyük bir pazar payı bulunmaktadır. Özellikle InGaN gibi, GaN bazlı LED' ler arsenik bazlı LED' lerden daha yüksek güç, sıcaklık ve frekans aralıklarında çalışabilirler. Ancak GaN tabanlı LED' ler hala yüksek kusur yoğunluklarına sahiptir. Bu nedenle çalışmalarımızda mozaik yapı analizi yapıldı. Hesaplamalarımızda Vegard ve William Hall yarı deneysel metotları kullanıldı. Kompozisyonun, kenar ve vida türü kusurların azalmasına yol açan önemli bir faktör olduğunu söyleyebiliriz.

International Journal of Computational Methods and Experimental Measurements, 2015

Structural, optical and electronic properties and elastic constants of Be 1-x Zn x S alloys have ... more Structural, optical and electronic properties and elastic constants of Be 1-x Zn x S alloys have been studied by employing the commercial code Castep based on density functional theory. The generalized gradient approximation and local density approximation were utilized as exchange correlation. Using elastic constants for compounds, bulk modulus, band gap, Fermi energy and Kramers-Kronig relations, dielectric constants and the refractive index have been found through calculations. Apart from these, X-ray measurements revealed elastic constants and Vegard's law. It is seen that results obtained from theory and experiments are all in agreement.

Süleyman Demirel Üniversitesi Fen Bilimleri Enstitüsü Dergisi, 2017

In this study, graded (A) InxGa1-xN (10.5 ≤ x ≤ 18.4) and non graded (B) InxGa1-xN (13.6 ≤ x ≤ 24... more In this study, graded (A) InxGa1-xN (10.5 ≤ x ≤ 18.4) and non graded (B) InxGa1-xN (13.6 ≤ x ≤ 24.9) samples are grown on c-oriented sapphire substrate using the Metal Organic Chemical Vapour Deposition (MOCVD) technique. The structural, optical and electrical features of the grown InGaN/GaN solar cell structures are analyzed using High Resolution X-Ray Diffraction (HRXRD), Photoluminescense (PL), Ultraviolet (UV), current density and potential (JV) measurements. According to the HRXRD results; it is determined that the InGaN layer of the graded structure has a lower FWHM (Full width at half maximum) value. From the PL measurements, it is observed that the GaN half-width peak value of the graded sample is narrower and the InGaN peak width value of the graded sample is larger. From UV measurements, that the graded sample has a greater band range. JV measurements determine that the performance of the graded structure is higher.

Journal of Nanoelectronics and Optoelectronics, 2017

In this study are graded and non graded lnGaN/GaN samples grown on c-oriented sapphire substrate ... more In this study are graded and non graded lnGaN/GaN samples grown on c-oriented sapphire substrate using the Metal Organic Chemical Vapour Deposition (MOCVD) technique. The structural and morphological properties of the grown lnGaN/GaN solar cell structures are analyzed using High Resolution X-ray Diffraction (HRXRD), atomic force microscopy (AFM). Each structures c and a lattice parameters strain, biaxial strain , hydrostatic strain, stress , lattice relax, tilt angle, mosaic crystal size, dislocation densities of GaN and lnGaN layers are determined by XRD measurements. In accordance with these calculations , the effect of graded structure on the defects, are discussed. As a dramatic result; although values of full width at half maximum (FWHM) are broad, a considerable decrease at dislocations is noticed. The AFM observations have revealed that the two dimensional growth of the graded sample is more significant and its roughness value is lower. JV measurements shown that the performance of the graded structure is higher. It is determined that all test results are consistent with each other.

Materials Science in Semiconductor Processing, 2015

The structural, electronic, optical properties and elastic constants of Be1-xZnxSe alloys were st... more The structural, electronic, optical properties and elastic constants of Be1-xZnxSe alloys were studied thoroughly using a program called CASTEP that grounds on the density function theory (DFT). The exchange correlation, GGA (Generalized Gradients Approximation) was also used. The elastic constants, mechanical properties, forbidden band gap, Kleinman parameter, dielectric constants, refractive index, absorption coefficient, and loss coefficient calculations have been calculated for these compounds. In addition to this, elastic constants, and bulk modulus were obtained via an X-Ray values depending on Vegard's law. As an important result, the bulk module and forbidden band gap decreases with Zn doping to the structure. The obtained results are consistent with the available experimental and theoretical values.

Journal of Crystal Growth, 2010

Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of... more Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of high temperature AlN/AlGaN buffer, which is directly deposited on r-plane sapphire by pulse flows. Compared to the a-plane GaN grown on AlN buffer, X-ray rocking curve analysis reveals a remarkable reduction in the full width at half maximum, both on-axis and off-axis. Atomic force microscopy image exhibits a fully coalesced pit-free surface morphology with low root-mean-square roughness ($ 1.5 nm). Photoluminescence is carried out on the a-plane GaN grown on r-plane sapphire. It is found that, at low temperature, the dominant emission at $ 3.42 eV is composed of two separate peaks with different characteristics, which provide explanations for the controversial attributions of this peak in previous studies.

Journal of Applied Physics, 2009

The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/ Al... more The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/ AlN/GaN two-dimensional electron gas ͑2DEG͒ heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm 2 / V s along with 8.44ϫ 10 12 cm −2 electron density, which resulted in a two-dimensional sheet resistance of 435 ⍀ / ᮀ for the Al 0.2 Ga 0.8 N / AlN/ GaN heterostructure. The sample designed with an Al 0.88 In 0.12 N barrier exhibited very high sheet electron density of 4.23ϫ 10 13 cm −2 with a corresponding electron mobility of 812 cm 2 / V s at room temperature. A record two-dimensional sheet resistance of 182 ⍀ / ᮀ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al 0.88 In 0.12 N / AlN/ GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al 0.88 In 0.12 N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors ͑HEMTs͒, after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.

In this study, InGaN/GaN structures are grown by using Metal Organic Chemical Vapor Deposition (M... more In this study, InGaN/GaN structures are grown by using Metal Organic Chemical Vapor Deposition (MOCVD) technique. Some structural, optical and morphological properties of InGaN/GaN structures are investigated in detail. For structural analysis, X-ray diffraction (XRD), for optical, Raman and morphological, Atomic Force Microscopy (AFM) measurement techniques are used. In XRD analysis both samples presented hexagonal crystal structure. XRD peaks for these two samples showed small differences dependent on growth conditions. Strain and stress values are determined from XRD and they are compared with Raman results. In Raman analysis, five different chemicals are determined in both samples. Raman analysis results are in good accordance with XRD, growth conditions and AFM images. In AFM images, there can be seen hills and holes and they are partly homogeneous. There are also some white regions in AFM images. According to Raman peak center data, these white regions are detected as white rust.

gazi university journal of science, 2015

Between n- GaN and p- AlGaN+GaN contacts, the blue light emitting diode (LED) structure with InGa... more Between n- GaN and p- AlGaN+GaN contacts, the blue light emitting diode (LED) structure with InGaN/GaN multiple quantum well has been grown using metalorganic vapor phase epitax (MOCVD) on c-oriented sapphire substrate. In order to research the strain and the stress of the lattice in crystal form, a reciprocal lattice space was mapped using a High Resolution X-Ray Diffractometer. In this study, by taking the qualities of the GaN epitaxial structure as reference point; relaxation, strain, hydrostatical strain and biaxial strain parameters are researched for the point defects taking the increasing temperatures into consideration. All parameters except the growth temperature of the InGaN layer were kept fixed for all samples. Depending on the growth temperature values, the results indicated a monotonous increasing-decreasing or decreasing-increasing manner. Additionally, the point defects of the blue LED structure on AlGaN layer have been compared to those of InGaN layer. While the lat...

Superlattices and Microstructures, 2015

In this study the effects of the annealing temperature on the InGaN/GaN solar cells with differen... more In this study the effects of the annealing temperature on the InGaN/GaN solar cells with different In-contents grown on sapphire substrate by the Metal Organic Chemical Vapor Deposition (MOCVD) are analyzed by High Resolution X-ray Diffraction (HRXRD) and an Atomic Force Microscope (AFM). The plane angles, mosaic crystal sizes, mixed stress, dislocation intensities of the structure of the GaN and InGaN layers are determined. According to the test results, there are no general characteristic trends observed due to temperature at both structures. There are fluctuating failures determined at both structures as of 350°C. The defect density increased on the GaN layer starting from 350°C and reaching above 400°C. A similar trend is observed on the InGaN layer, too.

Journal of Polytechnic

In this study, five periodic InGaN / GaN LED (light emitting diode) structures grown by the Metal... more In this study, five periodic InGaN / GaN LED (light emitting diode) structures grown by the Metal Organic Vapor Deposition (MOCVD) at different active layer growth temperatures were studied. These structures were grown as InGaN / GaN multiple quantum wells (MQW) between c-oriented sapphire substrate and n-GaN and p-AlGaN + GaN contacts. These constructions were characterized by the high-resolution X-ray diffraction (HR-XRD) system. HRXRD patterns obtained by X-ray diffraction and Reciprocal space maps were performed from the same data. One of the most effective ways of studying the crystal lattice is reciprocal space mapping with HR-XRD technique. This technique does not damage the sample. Information can be obtained from the internal system of the sample or from the intermediate layer including substrate. Using the FWHM (βhkl) values and the elastic coefficients of the structures obtained for each of the three samples separately with the inverse mesh technique, D (nm) particle size, σ (GPa) uniform stress, ε strain, u (kJm-3) anisotropic energy density parameters were calculated. These calculations were done in a Scherrer method and Uniform Deformation Model (UDM) which is the Williamson Hall method, modified uniform Williamson stresses model (USDM) and Uniform Deformation Energy Density Model (UDEDM). The results show that the stretching in the crystal size is very little. Line expansion in HR-XRD is due to small crystal size and lattice strain. UDEDM, one of the W-H methods, has emerged as the most suitable model for stretching..

Effects on AlGaN/GaN high electron mobility transistor structure of a high temperature AlN buffer... more Effects on AlGaN/GaN high electron mobility transistor structure of a high temperature AlN buffer on sapphire substrate have been studied by high resolution x ray diffraction and atomic force micros copy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root mean square of 0.25 nm and a step termination density in the range of 10 8 cm -2 . Due to the high temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.

IEEE Transactions on Electron Devices

Low-resistance ohmic contacts in AlGaN/GaN high-electron-mobility transistor (HEMT) devices requi... more Low-resistance ohmic contacts in AlGaN/GaN high-electron-mobility transistor (HEMT) devices require high-temperature (HT) annealing (>800 • C) which can deteriorate material quality, surface morphology, and edge acuity of the metal stacks. This article demonstrates the highfrequency and high-power performance of the AlGaN/GaN HEMT devices with low-temperature metal-organic chemical vapor deposition (MOCVD) regrown degenerately doped InGaN ohmic contacts compared with GaN-based regrown contacts. Ohmic contacts fabricated by regrowth methods could be a valuable alternative for both metal-based alloyed ohmic contacts and implantation-based ohmic contacts. Using a T-gate and MOCVD regrown InGaN ohmic contacts, the AlGaN/GaN HEMT with an L g of 150 nm and S-D spacing of 2.5 μm demonstrated a maximum drain current of 0.94 A/mm and a peak transconductance of 337 mS/mm. The same device exhibited a forward current gain frequency f t of 36.8 GHz and a maximum frequency of oscillation f max of 75.0 GHz. A power density of 3.07 W/mm with a 60% drain efficiency was measured at 35 GHz with a V ds of 20 V and a quiescent current of 100 mA/mm. Index Terms-AlGaN/GaN high-electron-mobility transistor (HEMT), metal-organic chemical vapor deposition (MOCVD), regrown InGaN ohmic contacts. I. INTRODUCTION G ALLIUM-NITRIDE (GaN)-based high-electronmobility transistors (HEMTs) have had a great interest due to their unique material properties which make them promising candidates for microwave power devices compared

Silicon, 2021

In this study, structural and electrical properties of Ag/TiO2/n-InP/Au Schottky barrier diodes a... more In this study, structural and electrical properties of Ag/TiO2/n-InP/Au Schottky barrier diodes are investigated. Particle size, d-spacing, micro-strain, ideality factor and barrier heights of two samples are determined for two different interfacial TiO2 layer thickness. X-ray diffraction (XRD) and current-voltage (I-V) measurements are employed for mentioned parameters. It is seen that sample with 60 Å TiO2 interfacial layer is a more ideal diode. The reason for this is argued in main text and conclusion section.

In this study, X-ray diffraction peaks of Si, Ti, Au and ZnO grown on Ge substrate with thickness... more In this study, X-ray diffraction peaks of Si, Ti, Au and ZnO grown on Ge substrate with thickness of 500 nm by using sputtering method are analyzed to determine correlation length and dislocation density. It is seen that in most dense region of peaks, peak behaviour is in accordance with Gauss function. Right and left tails of peaks are in good accordance with q3 law. For randomized dislocations, obeying q3 law is typical and they can be monitored with w-scans by using open detectors. Whole profile is fitted with a limited dislocation dispersion. Edge dislocation density and correlation length are determined in the degree of 1010cm-2 and 103 nm, respectively. In order to gain these values, semi-experimental equations in Kragner method are used. For making a good fit, fit iteration step is taken as 9x106.

Journal of Polytechnic, 2021

Highlights  Absorption coefficient is calculated with a different method/Soğurma katsayısı farkl... more Highlights  Absorption coefficient is calculated with a different method/Soğurma katsayısı farklı bir yöntemle hesaplandı  XRD and Raman analysis is made/XRD ve Raman analizleri yapıldı  SEM and AFM techniques are employed for thickness/Kalınlık için SEM ve AFM teknikleri kullanıldı

Journal of Materials Science: Materials in Electronics, 2019

In this study optical properties of InGaN/GaN/Al 2 O 3 multi-quantum well (MQW) structures are in... more In this study optical properties of InGaN/GaN/Al 2 O 3 multi-quantum well (MQW) structures are investigated in detail. Three samples containing InGaN/GaN/Al 2 O 3 MQWs are grown by using metal organic chemical vapor deposition technique. Sapphire (6H-Al 2 O 3) is used as the substrate. Forbidden energy band gaps (E g) of these three samples are determined from photoluminescence and absorption spectra. Results gained from these two spectra are compared with each other. It is found that E g values are between 2 and 3 eV. For determining refraction index, absorption coefficients, extinction coefficients and thickness of the films a rare method called Swanepoel envelope method is used. It is seen that results gained from this method are consistent with those in literature.

Applied Physics A, 2019

In this study, two-dimensional electron gas (2DEG) mobility analysis of AlN/GaN and InAlN/GaN str... more In this study, two-dimensional electron gas (2DEG) mobility analysis of AlN/GaN and InAlN/GaN structures with ultrathin barrier layers by metal organic chemical vapor deposition (MOCVD) has been performed with Hall effect measurements which is carried out under temperature from 15 to 350 K and a single magnetic field of 0.5 T. As a result of the scattering analysis made with Matthiessen's rule, it is shown that while the interface roughness scattering mechanism is dominated on the 2DEG mobility at low temperatures, the 2DEG mobility has been dominated by the polar optical phonon-scattering mechanism at high temperatures. Also, the acoustic phonon-scattering mechanism is effective on the 2DEG mobility at middle temperature. Furthermore, the interface and the quantum well parameters such as deformation potential, quantum well width, and correlation length of the interface are determined for each. As well as experimental measurements, the conduction band energy diagrams of the studied samples have been calculated using one-dimensional (1D) self-consistent Schrödinger-Poisson equations. A 2D quasitriangular quantum well formation has been shown for each studied samples. 2DEG probability density of samples has been investigated.

European Journal of Science and Technology, 2018

Bu çalışmada, MOCVD yöntemi ile üretilen InGaN/GaN ışık yayan diyot (LED) yapılarının mozaik yapı... more Bu çalışmada, MOCVD yöntemi ile üretilen InGaN/GaN ışık yayan diyot (LED) yapılarının mozaik yapıları analiz edildi. Safir alttaş üzerine biriktirilen, farklı In kompozisyonuna sahip InGaN/GaN bariyer tabakasının mozaik yapı üzerine etkisi Yüksek Çözünürlü X Işını Kırınımı (HR-XRD) yöntemi ile karakterize edildi. Amacımız mozaik yapı hesaplarından yararlanarak LED yapısının kalitesini arttırmaktır. Günümüzde LED' lerin geniş bir kullanım alanına sahip olmasından dolayı sektörde büyük bir pazar payı bulunmaktadır. Özellikle InGaN gibi, GaN bazlı LED' ler arsenik bazlı LED' lerden daha yüksek güç, sıcaklık ve frekans aralıklarında çalışabilirler. Ancak GaN tabanlı LED' ler hala yüksek kusur yoğunluklarına sahiptir. Bu nedenle çalışmalarımızda mozaik yapı analizi yapıldı. Hesaplamalarımızda Vegard ve William Hall yarı deneysel metotları kullanıldı. Kompozisyonun, kenar ve vida türü kusurların azalmasına yol açan önemli bir faktör olduğunu söyleyebiliriz.

International Journal of Computational Methods and Experimental Measurements, 2015

Structural, optical and electronic properties and elastic constants of Be 1-x Zn x S alloys have ... more Structural, optical and electronic properties and elastic constants of Be 1-x Zn x S alloys have been studied by employing the commercial code Castep based on density functional theory. The generalized gradient approximation and local density approximation were utilized as exchange correlation. Using elastic constants for compounds, bulk modulus, band gap, Fermi energy and Kramers-Kronig relations, dielectric constants and the refractive index have been found through calculations. Apart from these, X-ray measurements revealed elastic constants and Vegard's law. It is seen that results obtained from theory and experiments are all in agreement.

Süleyman Demirel Üniversitesi Fen Bilimleri Enstitüsü Dergisi, 2017

In this study, graded (A) InxGa1-xN (10.5 ≤ x ≤ 18.4) and non graded (B) InxGa1-xN (13.6 ≤ x ≤ 24... more In this study, graded (A) InxGa1-xN (10.5 ≤ x ≤ 18.4) and non graded (B) InxGa1-xN (13.6 ≤ x ≤ 24.9) samples are grown on c-oriented sapphire substrate using the Metal Organic Chemical Vapour Deposition (MOCVD) technique. The structural, optical and electrical features of the grown InGaN/GaN solar cell structures are analyzed using High Resolution X-Ray Diffraction (HRXRD), Photoluminescense (PL), Ultraviolet (UV), current density and potential (JV) measurements. According to the HRXRD results; it is determined that the InGaN layer of the graded structure has a lower FWHM (Full width at half maximum) value. From the PL measurements, it is observed that the GaN half-width peak value of the graded sample is narrower and the InGaN peak width value of the graded sample is larger. From UV measurements, that the graded sample has a greater band range. JV measurements determine that the performance of the graded structure is higher.

Journal of Nanoelectronics and Optoelectronics, 2017

In this study are graded and non graded lnGaN/GaN samples grown on c-oriented sapphire substrate ... more In this study are graded and non graded lnGaN/GaN samples grown on c-oriented sapphire substrate using the Metal Organic Chemical Vapour Deposition (MOCVD) technique. The structural and morphological properties of the grown lnGaN/GaN solar cell structures are analyzed using High Resolution X-ray Diffraction (HRXRD), atomic force microscopy (AFM). Each structures c and a lattice parameters strain, biaxial strain , hydrostatic strain, stress , lattice relax, tilt angle, mosaic crystal size, dislocation densities of GaN and lnGaN layers are determined by XRD measurements. In accordance with these calculations , the effect of graded structure on the defects, are discussed. As a dramatic result; although values of full width at half maximum (FWHM) are broad, a considerable decrease at dislocations is noticed. The AFM observations have revealed that the two dimensional growth of the graded sample is more significant and its roughness value is lower. JV measurements shown that the performance of the graded structure is higher. It is determined that all test results are consistent with each other.

Materials Science in Semiconductor Processing, 2015

The structural, electronic, optical properties and elastic constants of Be1-xZnxSe alloys were st... more The structural, electronic, optical properties and elastic constants of Be1-xZnxSe alloys were studied thoroughly using a program called CASTEP that grounds on the density function theory (DFT). The exchange correlation, GGA (Generalized Gradients Approximation) was also used. The elastic constants, mechanical properties, forbidden band gap, Kleinman parameter, dielectric constants, refractive index, absorption coefficient, and loss coefficient calculations have been calculated for these compounds. In addition to this, elastic constants, and bulk modulus were obtained via an X-Ray values depending on Vegard's law. As an important result, the bulk module and forbidden band gap decreases with Zn doping to the structure. The obtained results are consistent with the available experimental and theoretical values.

Journal of Crystal Growth, 2010

Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of... more Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of high temperature AlN/AlGaN buffer, which is directly deposited on r-plane sapphire by pulse flows. Compared to the a-plane GaN grown on AlN buffer, X-ray rocking curve analysis reveals a remarkable reduction in the full width at half maximum, both on-axis and off-axis. Atomic force microscopy image exhibits a fully coalesced pit-free surface morphology with low root-mean-square roughness ($ 1.5 nm). Photoluminescence is carried out on the a-plane GaN grown on r-plane sapphire. It is found that, at low temperature, the dominant emission at $ 3.42 eV is composed of two separate peaks with different characteristics, which provide explanations for the controversial attributions of this peak in previous studies.

Journal of Applied Physics, 2009

The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/ Al... more The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/ AlN/GaN two-dimensional electron gas ͑2DEG͒ heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm 2 / V s along with 8.44ϫ 10 12 cm −2 electron density, which resulted in a two-dimensional sheet resistance of 435 ⍀ / ᮀ for the Al 0.2 Ga 0.8 N / AlN/ GaN heterostructure. The sample designed with an Al 0.88 In 0.12 N barrier exhibited very high sheet electron density of 4.23ϫ 10 13 cm −2 with a corresponding electron mobility of 812 cm 2 / V s at room temperature. A record two-dimensional sheet resistance of 182 ⍀ / ᮀ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al 0.88 In 0.12 N / AlN/ GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al 0.88 In 0.12 N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors ͑HEMTs͒, after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.

In this study, InGaN/GaN structures are grown by using Metal Organic Chemical Vapor Deposition (M... more In this study, InGaN/GaN structures are grown by using Metal Organic Chemical Vapor Deposition (MOCVD) technique. Some structural, optical and morphological properties of InGaN/GaN structures are investigated in detail. For structural analysis, X-ray diffraction (XRD), for optical, Raman and morphological, Atomic Force Microscopy (AFM) measurement techniques are used. In XRD analysis both samples presented hexagonal crystal structure. XRD peaks for these two samples showed small differences dependent on growth conditions. Strain and stress values are determined from XRD and they are compared with Raman results. In Raman analysis, five different chemicals are determined in both samples. Raman analysis results are in good accordance with XRD, growth conditions and AFM images. In AFM images, there can be seen hills and holes and they are partly homogeneous. There are also some white regions in AFM images. According to Raman peak center data, these white regions are detected as white rust.

gazi university journal of science, 2015

Between n- GaN and p- AlGaN+GaN contacts, the blue light emitting diode (LED) structure with InGa... more Between n- GaN and p- AlGaN+GaN contacts, the blue light emitting diode (LED) structure with InGaN/GaN multiple quantum well has been grown using metalorganic vapor phase epitax (MOCVD) on c-oriented sapphire substrate. In order to research the strain and the stress of the lattice in crystal form, a reciprocal lattice space was mapped using a High Resolution X-Ray Diffractometer. In this study, by taking the qualities of the GaN epitaxial structure as reference point; relaxation, strain, hydrostatical strain and biaxial strain parameters are researched for the point defects taking the increasing temperatures into consideration. All parameters except the growth temperature of the InGaN layer were kept fixed for all samples. Depending on the growth temperature values, the results indicated a monotonous increasing-decreasing or decreasing-increasing manner. Additionally, the point defects of the blue LED structure on AlGaN layer have been compared to those of InGaN layer. While the lat...

Superlattices and Microstructures, 2015

In this study the effects of the annealing temperature on the InGaN/GaN solar cells with differen... more In this study the effects of the annealing temperature on the InGaN/GaN solar cells with different In-contents grown on sapphire substrate by the Metal Organic Chemical Vapor Deposition (MOCVD) are analyzed by High Resolution X-ray Diffraction (HRXRD) and an Atomic Force Microscope (AFM). The plane angles, mosaic crystal sizes, mixed stress, dislocation intensities of the structure of the GaN and InGaN layers are determined. According to the test results, there are no general characteristic trends observed due to temperature at both structures. There are fluctuating failures determined at both structures as of 350°C. The defect density increased on the GaN layer starting from 350°C and reaching above 400°C. A similar trend is observed on the InGaN layer, too.

Journal of Polytechnic

In this study, five periodic InGaN / GaN LED (light emitting diode) structures grown by the Metal... more In this study, five periodic InGaN / GaN LED (light emitting diode) structures grown by the Metal Organic Vapor Deposition (MOCVD) at different active layer growth temperatures were studied. These structures were grown as InGaN / GaN multiple quantum wells (MQW) between c-oriented sapphire substrate and n-GaN and p-AlGaN + GaN contacts. These constructions were characterized by the high-resolution X-ray diffraction (HR-XRD) system. HRXRD patterns obtained by X-ray diffraction and Reciprocal space maps were performed from the same data. One of the most effective ways of studying the crystal lattice is reciprocal space mapping with HR-XRD technique. This technique does not damage the sample. Information can be obtained from the internal system of the sample or from the intermediate layer including substrate. Using the FWHM (βhkl) values and the elastic coefficients of the structures obtained for each of the three samples separately with the inverse mesh technique, D (nm) particle size, σ (GPa) uniform stress, ε strain, u (kJm-3) anisotropic energy density parameters were calculated. These calculations were done in a Scherrer method and Uniform Deformation Model (UDM) which is the Williamson Hall method, modified uniform Williamson stresses model (USDM) and Uniform Deformation Energy Density Model (UDEDM). The results show that the stretching in the crystal size is very little. Line expansion in HR-XRD is due to small crystal size and lattice strain. UDEDM, one of the W-H methods, has emerged as the most suitable model for stretching..

Effects on AlGaN/GaN high electron mobility transistor structure of a high temperature AlN buffer... more Effects on AlGaN/GaN high electron mobility transistor structure of a high temperature AlN buffer on sapphire substrate have been studied by high resolution x ray diffraction and atomic force micros copy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root mean square of 0.25 nm and a step termination density in the range of 10 8 cm -2 . Due to the high temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.