Browsing by Author "Öztürk, M. K."
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Item Open Access Analysis of the mosaic defects in graded and non graded InxGa1‐xN solar cell structures(Süleyman Demirel Üniversitesi, 2017) Kars Durukan, İ.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelIn 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.Item Open Access Analyzing the AlGaN/AlN/GaN heterostructures for HEMT applications(American Scientific Publishers, 2018-03) K. Durukan, İ.; Akpınar, Ö.; Avar, C.; Gultekin, A.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelIn the present work, we have reported two high-electron-mobility transistor structures (HEMTs) with different thick AlN buffer deposited on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). AlN buffer layer has a critical important for the device performance therefore its effect of hererostructures analyzed using mosaic model. AlGaN/AlN/GaN heterostructures were examined by X-ray diffraction (XRD) and Atomic-force microscope (AFM). In order to evaluate the mosaicity of GaN, AlGaN, AlN layers in the structures, mosaic model was also used. These layers of lateral and vertical crystal size, dislocations, tilt and stain are investigated with HR-XRD device by Vegard and William Hall (WH) semi-experimental methods. According to the XRD results; As the buffer layer thickness decreases, the FWHM values of symmetric (002) and asymmetric peaks (105) of epitaxial layers increase. The thicker buffer layer makes the structure more crystalized. According to the AFM results the lower thickness AlN buffer has more frequent pits and hillocks and has more rough. As a result, the 520 nm thick AlN buffer layer showed a better structural performance.Item Open Access Analyzing the InGaN LED structures for white LED applications(Gazi Üniversitesi, 2017) Durukan, İ. K.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelIn this paper, blue-light InGaN/GaN light-emitting diodes were deposited on sapphire substrate by the Metal Organic Chemical Vapor Deposition (MOCVD) to investigate the properties of blue LEDs with various well thickness having different indium composition. Structural properties of LEDs was studied by high-resolution X-ray diffraction (HRXRD), Photoluminescence (PL) and ultraviole (UV). Our aim is to increase the quality of the LED structure by taking advantage of the mosaic structure calculations. The use of LED in commercial areas has increased. But, there are great difficulties in preventing defects. Lateral and vertical crystal size, dislocations, tilt and twist properties are investigated with HR-XRD device by Vegard and William hall semi-experimental methods. While dislocation value of the first sample is lower than first sample with less indium content ration, stress value of first sample is higher than second sample. In addition, The twist angle of first sample is lower. This shows that while the structure is crystallized, the tension is much greater, which is an interesting result. This is due to the mismatch when the diode is cooled to lower temperatures than the growth temperature.Item Open Access Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD(Springer New York LLC, 2017) Arslan, E.; Öztürk, M. K.; Tıraş, E.; Tıraş, T.; Özçelik, S.; Özbay, EkmelHigh-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.Item Open Access Characterization of AlInN/AlN/GaN heterostructures with different AlN buffer thickness(Springer New York LLC, 2016) Çörekçi, S.; Dugan, S.; Öztürk, M. K.; Çetin, S. Ş.; Çakmak, M.; Özçelik, S.; Özbay, EkmelTwo AlInN/AlN/GaN heterostructures with 280-nm- and 400-nm-thick AlN buffer grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and Hall-effect measurements. The symmetric (0002) plane with respect to the asymmetric (101 ¯ 2) plane in the 280-nm-thick AlN buffer has a higher crystal quality, as opposed to the 400-nm-thick buffer. The thinner buffer improves the crystallinity of both (0002) and (101 ¯ 2) planes in the GaN layers, it also provides a sizeable reduction in dislocation density of GaN. Furthermore, the lower buffer thickness leads to a good quality surface with an rms roughness of 0.30 nm and a dark spot density of 4.0 × 108 cm−2. The optical and transport properties of the AlInN/AlN/GaN structure with the relatively thin buffer are compatible with the enhancement in its structural quality, as verified by XRD and AFM results.Item Open Access Characterization of an AlN buffer layer and a thick-GaN layer grown on sapphire substrate by MOCVD(Springer, 2010-10-23) Çörekçi, S.; Öztürk, M. K.; Bengi, A.; Çakmak, M.; Özçelik, S.; Özbay, EkmelAn AlN buffer layer and a thick-GaN layer for high-electron-mobility transistors (HEMTs) were grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The structural and morphological properties of the layers were investigated by high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) techniques. The optical quality of the thick-GaN layer was also evaluated in detail by a photoluminescence (PL) measurement. It was found that the AlN buffer layer possesses high crystal quality and an atomically flat surface with a root-mean-square (rms) roughness of 0.16 nm. The screw-and edge-type dislocation densities of the thick-GaN layer were determined as 5.4 9 107 and 5.0 9 109 cm-2 by means of the mosaic crystal model, respectively. It was observed that the GaN layer has a smooth surface with an rms of 0.84 nm. Furthermore, the dark spot density of the GaN surface was estimated as 6.5 9 108 cm-2 over a scan area of 4 μm2. © Springer Science+Business Media, LLC 2010.Item Open Access Comparison of the transport properties of high quality AlGaN/AlN/GaN and AllnN/AlN/GaN two-dimensional electron gas heterostructures(AIP Publishing LLC, 2009-01-07) Tülek, R.; Ilgaz, A.; Gökden, S.; Teke, A.; Öztürk, M. K.; Kasap, M.; Özçelik, S.; Arslan, E.; Özbay, EkmelThe transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) 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 8.44 X 10(12) cm(-2) electron density, which resulted in a two-dimensional sheet resistance of 435 Omega/square 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 4.23 X 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 Omega/square 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 Al0.88In0.12N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs. c 2009 American Institute of Physics.Item Open Access A detailed study on optical properties of InGaN/GaN/Al2O3 multi quantum wells(Springer, 2019) Bilgili, A. K.; Akpınar, Ö.; Öztürk, M. K.; Özçelik, S.; Suludere, Z.; Özbay, EkmelIn this study optical properties of InGaN/GaN/Al2O3 multi-quantum well (MQW) structures are investigated in detail. Three samples containing InGaN/GaN/Al2O3 MQWs are grown by using metal organic chemical vapor deposition technique. Sapphire (6H–Al2O3) is used as the substrate. Forbidden energy band gaps (Eg) 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 Eg 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.Item Open Access Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs(Pleiades Publishing, 2013) Çörekçi, S.; Öztürk, M. K.; Yu, H.; Çakmak, M.; Özçelik, S.; Özbay, EkmelEffects 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 microscopy 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 108 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.Item Open Access Effects of the AlN nucleation layer thickness on the crystal structures of an AlN epilayer grown on the 6H-SiC substrate(Taylor & Francis, 2019-04-08) Arslan, Engin; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelThe influence of the LT-AlN(NL) growth times on the mosaic structure parameters of the AlN layer grown on the LT-AlN(NL)/6H-SiC structures as well as the dislocation densities and the strain behaviours in the AlN epilayers has been investigated using XRD measurements. The growth times of the LT-AlN(NL) were changed to 0, 60, 120, 180, and 240 s. We observed that the mosaic structure parameters of the AlN epilayers were slightly affected by the LT-AlN(NL) growth times. However, the dislocation densities in the AlN layer are affected by the growth times of the LT-AlN(NL) layer. The highest edge dislocation density (5.48 × 1010 ± 2.3 × 109 cm−2) was measured for the sample in which 120 s grown LT-AlN(NL) was used. On the other hand, highest screw type dislocation density (1.21 × 1010 ± 1.7 × 109 cm−2) measured in the sample E that contains 240 s growth LT-AlN(NL). The strain calculation results show that the samples without LT-AlN(NL) suffered maximum compressive in-plane strain (−10.9 × 10−3 ± 1.8 × 10−4), which can be suppressed by increasing the LT-AlN(NL) growth times. The out-of-plane strain also has a compressive character and its values increase with LT-AlN(NL) growth times between 60 and 180 s. Same out-of-plane strain values were measured for the LT-AlN(NL) growth times of 180 and 240 s. Furthermore, the form of the biaxial stress in the AlN epilayer changed from compressive to tensile when the LT-AlN(NL) growth times were greater than 120 s.Item Open Access Examination of the temperature related structural defects of InGaN/GaN solar cells(Academic Press, 2015) Durukan, İ. K.; Bayal, Ö.; Kurtuluş, G.; Baş, Y.; Gültekin, A.; Öztürk, M. K.; Çörekçi, S.; Tamer, M.; Özçelik, S.; Özbay, EkmelIn 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.Item Open Access InGaN/GaN LED yapıların mozaik yapı analizi(Osman Sağdıç, 2018) Durukan, İ. K.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelBu 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.Item Open Access Microstructural analysis with graded and non-graded indium in InGaN solar cell(American Scientific Publishers, 2017) Durukan, I. K.; Öztürk, M. K.; Çörekçi, S.; Tamer, M.; Baş, Y.; Özçelik, S.; Özbay, EkmelIn this study are graded and non graded InGaN/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 InGaN/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 InGaN 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.Item Open Access Microstructural defect properties of InGaN/GaN blue light emitting diode structures(Springer US, 2014-06-28) Bas, Y.; Demirel, P.; Akin, N.; Başköse, C.; Özen, Y.; Kınacı, B.; Öztürk, M. K.; Özcelik, S.; Özbay, EkmelIn this paper, we study structural and morphological properties of metal-organic chemical vapour deposition-grown InGaN/GaN light emitting diode (LED) structures with different indium (In) content by means of high-resolution X-ray diffraction, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and current-voltage characteristic (I-V). We have found out that the tilt and twist angles, lateral and vertical coherence lengths of mosaic blocks, grain size, screw and edge dislocation densities of GaN and InGaN layers, and surface roughness monotonically vary with In content. Mosaic defects obtained due to temperature using reciprocal lattice space map has revealed optimized growth temperature for active InGaN layer of MQW LED. It has been observed in this growth temperature that according to AFM result, LED structure has high crystal dimension, and is rough whereas according to PL and FTIR results, bandgap energy shifted to blue, and energy peak half-width decreased at high values. According to I-V measurements, it was observed that LED reacted against light at optimized temperature. In conclusion, we have seen that InGaN MQW structure's structural, optical and electrical results supported one another.Item Open Access Structural analysis of an InGaN/GaN based light emitting diode by X-ray diffraction(Springer, 2009-04-18) Öztürk, M. K.; Hongbo, Y.; SarIkavak, B.; Korçak, S.; Özçelik, S.; Özbay, EkmelThe important structural characteristics of hexagonal GaN in an InGaN/GaN multi quantum well, which was aimed to make a light emitted diode and was grown by metalorganic chemical vapor deposition on c-plain sapphire, are determined by using nondestructive high-resolution X-ray diffraction in detail. The distorted GaN layers were described as mosaic crystals characterized by vertical and lateral coherence lengths, a mean tilt, twist, screw and edge type threading dislocation densities. The rocking curves of symmetric (00.l) reflections were used to determine the tilt angle, while the twist angle was an extrapolated grown ω-scan for an asymmetric (hk.l) Bragg reflection with an h or k nonzero. Moreover, it is an important result that the mosaic structure was analyzed from a different (10.l) crystal direction that was the angular inclined plane to the z-axis. The mosaic structure parameters were obtained in an approximately defined ratio depending on the inclination or polar angle of the sample.Item Open Access Structural and optical properties of an InxGa1-xN/GaN nanostructure(Elsevier BV * North-Holland, 2007) Korçak, S.; Öztürk, M. K.; Çörekçi, S.; Akaoğlu, B.; Yu, H.; Çakmak, M.; Sağlam, S.; Özçelik, S.; Özbay, EkmelThe structural and optical properties of an InxGa1-xN/GaN multi-quantum well (MQW) were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and photoluminescence (PL). The MQW structure was grown on c-plane (0 0 0 1)-faced sapphire substrates in a low pressure metalorganic chemical vapor deposition (MOCVD) reactor. The room temperature photoluminescence spectrum exhibited a blue emission at 2.84 eV and a much weaker and broader yellow emission band with a maximum at about 2.30 eV. In addition, the optical gaps and the In concentration of the structure were estimated by direct interpretation of the pseudo-dielectric function spectrum. It was found that the crystal quality of the InGaN epilayer is strongly related with the Si doped GaN layer grown at a high temperature of 1090 °C. The experimental results show that the growth MQW on the high-temperature (HT) GaN buffer layer on the GaN nucleation layer (NL) can be designated as a method that provides a high performance InGaN blue light-emitting diode (LED) structure.Item Open Access Structural properties of InGaN/GaN/Al2O3 structure from reciprocal space mapping(Springer, 2018) Bilgili, A. K.; Akpınar, Ö.; Kurtuluş, G.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelBy using metal organic chemical vapor deposition technique, InGaN/GaN solar cell (SC) structure is deposited over sapphire (Al2O3) wafer as GaN buffer and GaN epitaxial layers. Structural properties of InGaN/GaN/Al2O3 SC structure is investigated by using high resolution X-ray diffraction technique dependent on In content. By using reciprocal space mapping, reciprocal space data are converted to w-θ data with a software. These w-θ data and full width at half maximum data are used for calculating lattice parameters. When compared with w-θ measurements in literature it is seen that especially a- lattice parameter is found very near to universal value from RSM. It is calculated as 3.2650 nm for sample A (S.A) GaN layer and 3.2570 nm for sample B (S.B) GaN layer on (105) asymmetric plane. Strain and stress calculations are made by using these lattice parameters. Strain and stress are calculated as 0.02363 and 8.6051 GPa for S.A GaN layer respectively. Other results are given in tables in the results and discussion section of this article. Edge, screw and mixed type dislocations are calculated as mosaic defects. All these calculations are made for two samples on (002) symmetric and (105) asymmetric planes. As a result it is seen that measurements by using RSM give more sensitive results. a- lattice parameter calculated with this technique is the best indicator of this result.Item Open Access Structural, morphological, and optical properties of AlGaN/GaN heterostructures with AlN buffer and interlayer(AIP Publishing LLC, 2007) Çörekçi, S.; Öztürk, M. K.; Akaoğlu, B.; Çakmak, M.; Özçelik, S.; Özbay, EkmelAlxGa1-xN/GaN (x similar to 0.3) heterostructures with and without a high-temperature (HT) AlN interlayer (IL) have been grown on sapphire (Al2O3) substrates and AlN buffer/Al2O3 templates by metal organic chemical vapor deposition. The effects of an AlN buffer layer (BL) grown on an Al2O3 substrate and an AlN IL grown under the AlGaN ternary layer (TL) on structural, morphological, and optical properties of the heterostructures have been investigated by high-resolution x-ray diffraction, spectroscopic ellipsometry, atomic force microscopy, and photoluminescence measurements. The AlN BL improves the crystal quality of the AlGaN TL. Further improvement is achieved by inserting an AlN IL between GaN BL and AlGaN TL. However, experimental results also show that a HT AlN IL leads to relatively rough surfaces on AlGaN TLs, and an AlN IL changes the strain in the AlGaN TL from tensile to compressive type. In addition, an AlN BL improves the top surface quality of heterostructures.Item Open Access Swanepoel method for AlInN/AlN HEMTs(Springer, 2020) Akpınar, Ö.; Bilgili, A. K.; Başköşe, Ü. C.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelIn this study, AlInN/AlN high electron mobility transistor (HEMT) structure is grown on c-oriented sapphire substrate using metal organic chemical vapor deposition method. Optical properties of the structure are investigated by photoluminescence (PL) and ultraviolet (UV–Vis.) spectras. According to PL results, direct bandgap of AlN is determined around 2.80 eV. In UV–Vis. spectra it is seen that conduction of AlInN layer starts at 360 nm. Swanepoel envelope method is applied on transmission spectra and some optical properties such as refractive index (n), film thickness (t), absorption coefficient (α), and extinction coefficient (k) are determined. Forbidden energy bandgap is determined again from Tau method and it is compared with the value gained from PL spectra. This study is a rare one that presents optical properties of HEMTs using Swanepoel and Tau methods. In addition to this, it helps estimating how optical properties of HEMTs effect electrical properties.