Dong Su Shin

Enhanced light extraction from GaN based light-emitting diodes using a hemispherical NiCoO lens

We report on the improvement of light extraction efficiency from GaN-based light-emitting diodes (LEDs) using Ni(1-x)Co(x)O nanoparticles (NPs) formed on the p-GaN layer. After formation of Ni(1-x)Co(x)O hemispherical lens arrays on the blue LEDs by drop-casting colloidal NPs, electroluminescence (EL) and photoluminescence (PL) measurements are conducted to investigate the electrical and optical properties. The PL and EL intensities from the blue LEDs with the Ni(1-x)Co(x)O NPs are 1.74 and 1.61 times greater, respectively, than a conventional LED. Finally, a hybrid approach using ZnO nanorod arrays grown on the NiCoO hemispherical lens shows further increase of light extraction by 3.8 and 6.2 times compared to LEDs with bare NiCoO NPs and without any NPs, respectively. Finite-difference time-domain (FDTD) simulation results also agree well with the experimental results. The enhancement of light extraction from LEDs with ZnO nanorods and NiCoO NPs can be attributed to an enlarged escape angle cone and increased probability of light scattering.

A simple sonochemical approach of Mn2+ doped ZnO nanopowder: structural, optical and magnetic studies

Mn2+ doped ZnO nanopowder was prepared by simple sonochemical method. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, optical absorption (UV–Vis), photoluminescence (PL), Electron Paramagnetic Resonance (EPR), Fourier transform infrared (FT-IR) spectroscopy and magnetization have been used to characterize the sample. XRD study revealed that Mn2+ doped ZnO had wurtzite hexagonal structure. Composition analysis by EDX indicated the presence of small amounts of manganese. Crystal field (Dq) and interelectronic repulsion (B, C) parameters are evaluated from optical absorption. Spin-Hamiltonian and bonding parameters are evaluated from the EPR study. Optical and EPR confirms that the Mn2+ entered into ZnO lattice as distorted octahedral site symmetry. FT-IR spectrum exhibited vibrational bands of Zn–O bonds. PL spectrum shows the emission bands in UV and visible region. Magnetization loop was measured and clearly shows typical ferromagnetic saturation behavior. The results suggest oxygen vacancies, especially singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the Mn doped ZnO system.

Ultraviolet photodetector using pn junction formed by transferrable hollow n-TiO 2 nano-spheres monolayer

We report an ultraviolet (UV) photodetector with a universally transferable monolayer film with ordered hollow TiO2 spheres on p-GaN. After forming a TiO2 monolayer film by unidirectional rubbing of hollow TiO2 spheres on a polydimethylsiloxane (PDMS) supporting plate, we used a 5% polyvinyl alcohol (PVA) aqueous solution to transfer the film onto the target substrate. The PVA/TiO2 monolayer film was detached from the PDMS film and transferred to the p-GaN/Al2O3 substrate. To investigate the effects of crystallized phases of the TiO2 hollow spheres, anatase and rutile TiO2 sphere monolayers prepared by combining template synthesis and thermal treatment. The responsiveness of the UV photodetectors using anatase and rutile hollow n-TiO2 monolayer/p-GaN was 0.203 A/W at 312 nm and 0.093 A/W at 327 nm, respectively.