Browsing by Subject "Optical characteristics"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item Open Access Enhanced optical characteristics of light emitting diodes by surface plasmon of Ag nanostructures(SPIE, 2011) Jang L.-W.; Ju J.-W.; Jeon J.-W.; Jeon, D.-W.; Choi J.-H.; Lee, S.-J.; Jeon, S.-R.; Baek J.-H.; Sarı, Emre; Demir, Hilmi Volkan; Yoon H.-D.; Hwang, S.-M.; Lee I.-H.We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface of the roughened p-GaN. © 2011 SPIE.Item Open Access Low thermal-mass LEDs: Size effect and limits(Optical Society of American (OSA), 2014) Lu, S.; Liu W.; Zhang, Z.-H.; Tan, S.T.; Ju, Z.; Ji, Y.; Zhang X.; Zhang, Y.; Zhu, B.; Kyaw, Z.; Hasanov, N.; Sun X.W.; Demir, Hilmi VolkanIn this work, low thermal-mass LEDs (LTM-LEDs) were developed and demonstrated in flip-chip configuration, studying both experimentally and theoretically the enhanced electrical and optical characteristics and the limits. LTM-LED chips in 25 × 25 μm2, 50 × 50 μm2, 100 × 100 μm2 and 200 × 200 μm2 mesa sizes were fabricated and comparatively investigated. Here it was revealed that both the electrical and optical properties are improved by the decreasing chip size due to the reduced thermal mass. With a smaller chip size (from 200 μm to 50 μm), the device generally presents higher current density against the bias and higher power density against the current density. However, the 25 × 25 μm2 device behaves differently, limited by the fabrication margin limit of 10 μm. The underneath mechanisms of these observations are uncovered, and furthermore, based on the device model, it is proven that for a specific flip-chip fabrication process, the ideal size for LTM-LEDs with optimal power density performance can be identified. ©2014 Optical Society of AmericaItem Open Access Metal-semiconductor-metal ultraviolet photodetectors based on gallium nitride grown by atomic layer deposition at low temperatures(SPIE, 2014) Tekcan, B.; Ozgit Akgun, C.; Bolat, S.; Bıyıklı, Necmi; Okyay, Ali KemalProof-of-concept, first metal-semiconductor-metal ultraviolet photodetectors based on nanocrystalline gallium nitride (GaN) layers grown by low-temperature hollow-cathode plasma-assisted atomic layer deposition are demonstrated. Electrical and optical characteristics of the fabricated devices are investigated. Dark current values as low as 14 pA at a 30 V reverse bias are obtained. Fabricated devices exhibit a 15× UV/VIS rejection ratio based on photoresponsivity values at 200 nm (UV) and 390 nm (VIS) wavelengths. These devices can offer a promising alternative for flexible optoelectronics and the complementary metal oxide semiconductor integration of such devices. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE).Item Open Access Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Butun, B.; Özbay, EkmelIn this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 μm to 7.3 μm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design. © 2017 Optical Society of America.Item Open Access Optical characteristics of nanocrystalline AlxGa1-xN thin films deposited by hollow cathode plasma-assisted atomic layer deposition(AVS Science and Technology Society, 2014) Goldenberg, E.; Ozgit-Akgun, C.; Bıyıklı, Necmi; Kemal Okyay, A.Gallium nitride (GaN), aluminum nitride (AlN), and AlxGa 1-xN films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 °C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and Al xGa1-xN films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 °C). For all films, the average optical transmission was ∼ 85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and AlxGa1-xN were lower compared to GaN thin films. The refractive index of as-deposited films decreased from 2.33 to 2.02 (λ = 550 nm) with the increased Al content x (0 ≤ x ≤ 1), while the extinction coefficients (k) were approximately zero in the VIS spectrum (>400 nm). Postdeposition annealing at 900 °C for 2 h considerably lowered the refractive index value of GaN films (2.33-1.92), indicating a significant phase change. The optical bandgap of as-deposited GaN film was found to be 3.95 eV, and it decreased to 3.90 eV for films annealed at 800 °C for 30 min and 2 h. On the other hand, this value increased to 4.1 eV for GaN films annealed at 900 °C for 2 h. This might be caused by Ga 2O3 formation and following phase change. The optical bandgap value of as-deposited AlxGa1-xN films decreased from 5.75 to 5.25 eV when the x values decreased from 1 to 0.68. Furthermore, postdeposition annealing did not affect the bandgap of Al-rich films. © 2014 American Vacuum Society.