Chi-Feng Huang

Surface plasmon coupling effect in an InGaN∕GaN single-quantum-well light-emitting diode

The authors demonstrate the coupling effects between the quantum well (QW) and surface plasmon (SP) generated nearby on the p-type side in an InGaN∕GaN single-QW light-emitting diode (LED). The QW-SP coupling leads to the enhancement of the electroluminescence (EL) intensity in the LED sample designed for QW-SP coupling and reduced SP energy leakage, when compared to a LED sample of weak QW-SP coupling or significant SP energy loss. In the LED samples of significant QW-SP coupling, the blueshifts of the photoluminescence and EL emission spectra are observed, indicating one of the important features of such a coupling process. The device performance can be improved by using the n-type side for SP generation such that the device resistance can be reduced and the QW-SP coupling effect can be enhanced (by further decreasing the distance between the QW and metal) because of the higher carrier concentration in the n-type layer.

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

Phosphor-free white-light light-emitting diode of weakly carrier-density-dependent spectrum with prestrained growth of InGaN∕GaN quantum wells

The authors grew a white-light InGaN∕GaN quantum-well (QW) light-emitting diode epitaxial structure with its electroluminescence spectrum close to the ideal condition in the Commission International de l’Eclairage chromaticity based on the prestrained metal-organic chemical vapor deposition technique. The prestrained growth leads to the efficient yellow emission from three InGaN∕GaN QWs of increased indium incorporation. The color mixing for white light is implemented by adding a blue-emitting QW at the top of the yellow-emitting QWs. The blueshifts of the blue and yellow spectral peaks of the generated electroluminescence spectra are only 1.67 and 8nm, respectively, when the injection current increases from 10to70mA. Such small blueshifts imply that the piezoelectric fields in their QWs are significantly weaker than those previously reported.

Enhanced and partially polarized output of a light-emitting diode with its InGaN/GaN quantum well coupled with surface plasmons on a metal grating

The enhanced and partially polarized output of a green light-emitting diode (LED), in which its InGaN/GaN quantum well (QW) couples with surface plasmons (SPs) on a surface Ag grating structure, is demonstrated. Compared with a LED sample without (flat) Ag coating, the total output intensity of an LED of SP-QW coupling can be enhanced by ∼59% (∼200)% when the grating period and groove depth are 500 and 30 nm, respectively. Also, a bottom-emission polarization ratio of 1.7 can be obtained under the condition of 15 nm in groove depth.

Prestrained effect on the emission properties of InGaN∕GaN quantum-well structures

The authors demonstrate the spectral redshift of the quantum wells (QWs) designated for green emission into the orange range in a light-emitting diode by adding a violet-emitting QW at the bottom in metal-organic chemical vapor deposition. An electroluminescence redshift of 53nm was obtained. The cathodoluminescence spectra indicated that the long-wavelength QWs close to the violet one were strongly influenced by this added QW and mainly emitted the orange photons. Those near the top were less affected. This influence is supposed to originate from the prestrained effect in the barrier layer right above the violet QW. Such a prestrained effect is expected to be more effective when the underlying QW is well shaped and the heterojunction strain is strong, like the case of the violet QW. This effect is weak between the high-indium QWs, in which the formation of indium-rich clusters releases the strain.

Temperature dependence of the surface plasmon coupling with an InGaN∕GaN quantum well

The authors demonstrate the temperature-dependent behavior of the surface plasmon (SP) coupling with two InGaN∕GaN quantum-well (QW) structures of different internal quantum efficiencies. The SP modes are generated at the interface between the QW structures and Ag thin films coated on their tops. It is observed that the SP-QW coupling rate increases with temperature. Such a trend may rely on several factors, including the availability of carriers with sufficient momenta for transferring the energy and momentum into the SP modes and possibly the variation of the SP density of state with temperature. Although the required momentum matching condition only needs the thermal energy corresponding to a few tens of Kelvins, the carrier delocalization process results in a significantly higher probability of SP-carrier momentum matching and hence SP-QW coupling.

Reduced injection current induced blueshift in an InGaN∕GaN quantum-well light-emitting diode of prestrained growth

The authors demonstrate the smaller blueshift in increasing injection current level of an InGaN∕GaN quantum-well (QW) light-emitting diode (LED) of a longer electroluminescence (EL) peak wavelength based on the prestrained growth technique when compared with the result of a LED of a shorter EL peak wavelength based on the conventional growth technique. The smaller blueshift can be attributed to more contribution to light emission from the deeper QWs of higher indium contents when the injection current level is increased in the prestrain sample. It can also be attributed to the stronger carrier localization because of the stronger composition clustering in the prestrain sample of higher indium contents. Carrier localization can reduce the influences of the quantum-confined Stark effect and its screening process.

Formation of various metal nanostructures with thermal annealing to control the effective coupling energy between a surface plasmon and an InGaN/GaN quantum well

We demonstrate the variations of the photoluminescence (PL) spectral peak position and intensity through the surface plasmon (SP) coupling with an InGaN/GaN quantum well (QW) by forming Ag nanostructures of different scale sizes on the QW structure with thermal annealing. By transferring an Ag thin film into a nanoisland structure, we can not only enhance the PL intensity, but also adjust the SP dispersion relation and hence red-shift the effective QW emission wavelength. Such an emission spectrum control can be realized by initially coating Ag films of different thicknesses. Although the screening process of the quantum-confined Stark effect, which can result in PL spectrum blue-shift and intensity enhancement, also contributes to the variations of the emission behaviour, it is found that the SP-QW coupling process dominates in the observed phenomena.

Polarization dependent coupling of surface plasmon on a one-dimensional Ag grating with an InGaN∕GaN dual-quantum-well structure

The authors report the observation of a polarization-dependent surface plasmon (SP) feature on a one-dimensional Ag-grating structure through the SP coupling with an InGaN∕GaN dual-quantum-well structure closely below the metal grating. Polarized photon output is observed because only the momentum matching condition of the SP mode propagating in the direction perpendicular to the grating grooves can be reached through the diffraction of the fabricated grating and, thus, the SP radiation efficiency is significantly enhanced only in this polarization. The dispersion curve of the observed SP mode shows a group velocity of 2.4×108m∕s, which manifests the SP characteristics in the air/Ag∕GaN grating structure.