Horng-Shyang Chen

Strain relaxation and quantum confinement in InGaN/GaN nanoposts

Nanoposts of 10?40?nm top diameter on an InGaN/GaN quantum well structure were fabricated using electron-beam lithography and inductively coupled plasma reactive ion etching. Significant blue shifts up to 130?meV in the photoluminescence (PL) spectrum were observed. The blue-shift range increases with decreasing post diameter. For nanoposts with significant strain relaxation, the PL spectral peak position becomes less sensitive to carrier screening. On the basis of the temperature-dependent PL and time-resolved PL measurements and a numerical calculation of the effect of quantum confinement, we conclude that the optical behaviours of the nanoposts are mainly controlled by the combined effect of 3D quantum confinement and strain relaxation.

White light generation with CdSe-ZnS nanocrystals coated on an InGaN-GaN quantum-well blue/Green two-wavelength light-emitting diode

We grew and processed a blue/green two-wavelength light-emitting diode (LED) based on the mixture of two kinds of quantum wells (QW) in epitaxial growth. The X-ray diffraction and photoluminescence measurements indicated that the crystalline structure and the basic optical property of individual kinds of QW are not significantly changed in the mixed growth. The relative electroluminescence (EL) intensity of the two colors depends on the injection current level, which controls the hole concentration distribution among the QWs. At low injection levels, the top green-emitting QW dominates in EL. As the injection current increases, the blue-emitting QWs beneath become dominating. We also coated CdSe-ZnS nanocrystals on the top of the two-wavelength LED for converting blue photons into red light. With the coating of such nanocrystals, the device emits blue, green, and red lights for white light generation

The structure and crystallization characteristics of phase change optical disk material Ge1Sb2Te4

The crystallization characteristics of amorphous Ge1Sb2Te4 thin films were studied by means of time‐resolved transition measurements. It was found that a metastable phase appeared at the first stage of the crystallization process and then the metastable phase was transformed into a stable crystalline phase at higher annealing temperatures. The x‐ray diffraction and transmission electron microscopy results indicated the metastable phase was identified as a face‐centered‐cubic structure and the stable crystalline phase corresponded to a hexagonal structure. Our experimental results show that the Ge1Sb2Te4 materials are applicable for phase change erasable optical storage.

Surface plasmon coupled light-emitting diode with metal protrusions into p-GaN

An Ag protrusion array is fabricated on the p-GaN layer of an InGaN/GaN quantum-well (QW) light-emitting diode (LED) for generating surface plasmon coupling with the radiating dipoles in the QWs and hence LED emission enhancement. The tips of the Ag protrusions penetrating into the p-GaN layer are close to the QWs such that the induced near field around the tips can strongly interact with the dipoles in the QWs. With the Ag protrusions, the fabricated flip-chip LED shows a 74.6% enhancement in output intensity at 100 mA in injection current, when compared with a control sample of no Ag protrusion. The simulation results of Ag protrusion absorption agree reasonably well with the experimental data of protrusion reflectance. The simulation also shows a strong near field distribution around the tip of an Ag protrusion for coupling with the radiating dipoles in the QWs.

Efficiency improvement of a vertical light-emitting diode through surface plasmon coupling and grating scattering

The enhancement of output intensity, the generation of polarized output, and the reduction of the efficiency droop effect in a surface plasmon (SP) coupled vertical light-emitting diode (LED) with an Ag nano-grating structure located between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN quantum wells (QWs) are demonstrated. In fabricating the vertical LED, the patterned sapphire substrate is removed with a photoelectrochemical liftoff technique. Based on the reflection measurement from the metal grating structure and the numerical simulation result, it is found that the localized surface plasmon (LSP) resonance induced around the metal grating crest plays the major role in the SP-QW coupling process although a hybrid mode of LSP and surface plasmon polariton can be generated in the coupling process. By adding a surface grating structure to the SP-coupled vertical LED on the n-GaN side, the output intensity is further enhanced, the output polarization ratio is further increased, and the efficiency droop effect is further suppressed.

Geometry and composition comparisons between c-plane disc-like and m-plane core-shell InGaN/GaN quantum wells in a nitride nanorod

With the nano-imprint lithography and the pulsed growth mode of metalorganic chemical vapor deposition, a regularly-patterned, c-axis nitride nanorod (NR) array of quite uniform geometry with simultaneous depositions of top-face, c-plane disc-like and sidewall, m-plane core-shell InGaN/GaN quantum well (QW) structures is formed. The differences of geometry and composition between these two groups of QW are studied with scanning electron microscopy, cathodoluminescence, and transmission electron microscopy (TEM). In particular, the strain state analysis results in TEM observations provide us with the information about the QW width and composition. It is found that the QW widths are narrower and the indium contents are higher in the sidewall m-plane QWs, when compared with the top-face c-plane QWs. Also, in the sidewall m-plane QWs, the QW width (indium content) decreases (increases) with the height on the sidewall. The observed results can be interpreted with the migration behaviors of the constituent atoms along the NR sidewall from the bottom.

Cross-sectional sizes and emission wavelengths of regularly patterned GaN and core-shell InGaN/GaN quantum-well nanorod arrays

The cross-sectional sizes of the regularly patterned GaN nanorods (NRs) and InGaN/GaN quantum-well (QW) NRs of different heights and different hexagon orientations, which are grown on the patterned templates of different hole diameters, pitches, and crystal orientations, are compared. It is found that the cross-sectional size of the GaN NR, which is formed with the pulsed growth mode, is mainly controlled by the patterned hole diameter, and the thickness of the sidewall QW structure is mainly determined by the NR height. The cross-sectional size variation of GaN NR is interpreted by the quasi-three-dimensional nature of atom supply amount for precipitating a two-dimensional disk-shaped NR segment. The variation of the sidewall QW structure is explained by the condition of constituent atom supply in the gap volume between the neighboring NRs. Also, we compare the cathodoluminescence emission wavelengths among those samples of different growth conditions. Generally speaking, the QW NR with a smaller height,...

Surface plasmon coupling with a radiating dipole near a Ag nanoparticle embedded in GaN

The localized surface plasmon (LSP) coupling behaviors of a radiating dipole with a Ag nanosphere (NS) embedded in GaN are evaluated based on a numerical algorithm, which takes account of the induction of the LSP resonance on the Ag NS by the radiating dipole and the feedback of the LSP resonance to the radiating behavior of the dipole. In particular, we compute the enhancements of dipole strength and radiated power through such a coupling process for the radiating dipoles oriented along the radial and orbital directions with respect to the Ag NS. The enhancements of radiated power of a radial and an orbital dipole are induced through the coupling with the LSP dipole and higher-order resonance, respectively. These results are interpreted with the interference behaviors between the radiations of the source dipole and the LSP mode of the Ag NS.

Fabrication of surface metal nanoparticles and their induced surface plasmon coupling with subsurface InGaN/GaN quantum wells.

Based on the fabrication of Ag nanoparticles (NPs) with controlled geometry and surface density on an InGaN/GaN quantum well (QW) epitaxial structure, which contains indium-rich nano-clusters for producing localized states and free-carrier (delocalized) states in the QWs, and the characterization of their localized surface plasmon (LSP) coupling behavior with the carriers in the QWs, the interplay behavior of LSP coupling with carrier delocalization in the QWs is demonstrated. By using the polystyrene nanosphere lithography technique with an appropriate nanosphere size and adjusting the post-fabrication thermal annealing condition, the induced LSP resonance wavelength of the fabricated Ag NPs on the QW sample can match the QW emission wavelength for generating the coherent coupling between the carriers in the QWs and the induced LSP. The coupling leads to the enhancement of radiative recombination rate in the QWs and results in increased photoluminescence (PL) intensity, red-shifted PL spectrum, reduced PL decay time, and enhanced internal quantum efficiency. It is found that the observed effects are mainly due to the LSP coupling with the delocalized carriers in the QWs.

Further reduction of efficiency droop effect by adding a lower-index dielectric interlayer in a surface plasmon coupled blue light-emitting diode with surface metal nanoparticles

Further reduction of the efficiency droop effect and further enhancements of internal quantum efficiency (IQE) and output intensity of a surface plasmon coupled, blue-emitting light-emitting diode (LED) by inserting a dielectric interlayer (DI) of a lower refractive index between p-GaN and surface Ag nanoparticles are demonstrated. The insertion of a DI leads to a blue shift of the localized surface plasmon (LSP) resonance spectrum and increases the LSP coupling strength at the quantum well emitting wavelength in the blue range. With SiO2 as the DI, a thinner DI leads to a stronger LSP coupling effect, when compared with the case of a thicker DI. By using GaZnO, which is a dielectric in the optical range and a good conductor under direct-current operation, as the DI, the LSP coupling results in the highest IQE, highest LED output intensity, and weakest droop effect.