Papers by Author: D.H. Kang

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Authors: Jae Chul Song, D.H. Kang, Byung Young Shim, Eun A Ko, Dong Wook Kim, Kannappan Santhakumar, Cheul Ro Lee
Abstract: GaN epilayers were grown on lens shaped patterned sapphire substrate (PSS) (0001) and unpatterned sapphire substrate (UPSS) (0001) by metal-organic chemical vapor deposition (MOCVD). The quality of the grown GaN epilayers on the PSS and UPSS were compared. Structural characteristics, surface morphology and optical properties of the GaN epilayers were investigated using double crystal X-ray diffraction (DCXRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and photoluminescence (PL). A lens shaped pattern was formed on the sapphire substrate to reduce threading dislocation (TD) density and also to improve the optical emission efficiency by internal reflection on the lens. Scanning electron microscopy images show the growth of GaN epilayers at various times. Full coalescence is observed at a growth time of 80 min. It is seen from the DCXRD rocking spectrum that full width at half maximum (FWHM) of the GaN grown on PSS was 438.7 arcsec which is less than UPSS value. The lower value of FWHM indicates that the crystalline quality of the GaN epilayers grown on PSS is improved compared to GaN grown on UPSS. It is clearly seen from the AFM images that the dislocation density is less for the GaN grown on PSS. A strong and sharp photoluminescence (PL) band edge emission was observed for the GaN grown on PSS compared to UPSS. Defect related yellow luminescence was observed for GaN grown on UPSS which did not appear for PSS. The FWHM at the 364.3 nm peak position was evaluated to be 50.7 meV from the PL spectra for GaN grown on PSS. The above result indicates GaN epilayers can be grown on PSS with low TD density and will be useful for optical emission.
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Authors: D.H. Kang, Jae Chul Song, Dong Wook Kim, J.S. Kim, Kannappan Santhakumar, Cheul Ro Lee
Abstract: We report the microstructure and optical properties of gallium nitride (GaN) epilayers grown on lens shape patterned sapphire substrate (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth times. A lens shaped pattern was used to reduce the threading dislocation density and to improve optical emission efficiency. A scanning electron microscope (SEM) image shows flat and smooth surface of GaN grown on PSS at 80 min which could be achieved by lateral growth from the trench region. From the DCXRD spectra, full width at half maximun (FWHM) value was decreased with increasing growth time. FWHM of the sample grown at 80 min was 473.5 arc sec. This indicates there is an improvement in crystalline quality of the GaN grown on PSS as the growth time increases. From photoluminescence (PL) spectra, an increase in band edge emission intensity and a decrease in defect related yellow luminescence was observed for GaN on PSS as the growth time increased. From the PL spectra, FWHM was 82.2 meV at peak position 363.9 nm for the sample grown for 80 min. It is clearly seen that the threading dislocations can be reduced by lateral growth improving the light emission efficiency by internal light reflection on the lens surface for GaN grown on PSS.
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Authors: Jae Chul Song, D.H. Kang, Seon Ho Lee, Eun Su Jang, Dong Wook Kim, Kannappan Santhakumar, Cheul Ro Lee
Abstract: Vertical GaN nano-columns arrays were grown on Au-coated silicon (111) substrate by Au+Ga alloy seeding method and pulsed flow of Gallium and ammonia using metalorganic chemical vapor deposition (MOCVD). A gold thin film was deposited on Si using an ion coating system. The Au coated Si substrate was annealed at 800 oC under hydrogen ambient for 5 min. The pre-deposition of gallium and nitrogen was performed for 60 sec to form Au+Ga and nitrogen solid solution, which acts as the initial nucleation islands. Then Gallium and ammonia were let in pulse method. Scanning electron microscopy (SEM) image reveals a vertical growth and cylindrical in shape GaN nano-column. From the sharp PL peak intensity it is clearly seen that the dislocation density is reduced considerably and the optical quality of the nano-column is improved.
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Authors: Kannappan Santhakumar, D.H. Kang, Jae Chul Song, Dong Wook Kim, J.S. Kim, M.D. Kim, Cheul Ro Lee
Abstract: Vertically aligned GaN nanocolumn arrays were grown by molecular beam epitaxy on Gallium coated silicon substrate. The dense packing of the NCs gives them the appearance of a continuous film in surface view, but cross-sectional analysis shows them to be isolated nanostructures. The GaN nanocolumns have uniform diameters of 85 nm, lengths up to 720 nm and possess a pyramid like tip. Photoluminescence measurements of NCs show excitonic emission with a dominant, narrow peak centered at 363 nm and FWHM of 68 meV. From the Raman spectrum, peaks at 566.9 and 730 cm-1 are assigned to the E2 and A1(LO) GaN phonons modes which clearly indicates that the grown nanocolumns are highly crystalline. The grown nanocolumns are highly oriented and perpendicular to the growth surface.
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Authors: D.H. Kang, Jae Chul Song, H. Song, Dong Wook Kim, I.H. Lee, Kannappan Santhakumar, Cheul Ro Lee
Abstract: Structural and optical properties of gallium nitride (GaN) epilayers grown on lens shape patterned sapphire substrate (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth times were evaluated. From Raman spectra, a blue shift and reduction in the FWHM of Raman modes of GaN grown on PSS were observed when compared to GaN grown on unpatterend sapphire substrate (UPSS). From the DCXRD spectra, full width at half maximum (FWHM) value was decreased with increasing growth time. FWHM of the sample grown at 80 min was 473.5 arc sec. This indicates that there is an improvement in crystalline quality of the GaN grown on PSS as the growth time increases. From photoluminescence (PL) spectra, an increase in band edge emission intensity and a decrease in defect related yellow luminescence were observed for GaN on PSS as the growth time increased.
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