Jeng-Jie Huang

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.

Absorption enhancement of an amorphous Si solar cell through surface plasmon-induced scattering with metal nanoparticles

The simulation results of absorption enhancement in an amorphous-Si (a-Si) solar cell by depositing metal nanoparticles (NPs) on the device top and embedding metal NPs in a layer above the Al back-reflector are demonstrated. The absorption increase results from the near-field constructive interference of electromagnetic waves in the forward direction such that an increased amount of sunlight energy is distributed in the a-Si absorption layer. Among the three used metals of Al, Ag, and Au, Al NPs show the most efficient absorption enhancement. Between the two used NP geometries, Al nanocylinder (NC) are more effective in absorption enhancement than Al nanosphere (NS). Also, a random distribution of isolated metal NCs can lead to higher absorption enhancement, when compared with the cases of periodical metal NC distributions. Meanwhile, the fabrication of both top and bottom Al NCs in a solar cell results in further absorption enhancement. Misalignments between the top and bottom Al NCs do not significantly reduce the enhancement percentage. With a structure of vertically aligned top and bottom Al NCs, solar cell absorption can be increased by 52%.

Enhancing InGaN-based solar cell efficiency through localized surface plasmon interaction by embedding Ag nanoparticles in the absorbing layer

The use of localized surface plasmon (LSP) interaction for significantly enhancing InGaN absorption near its band edge and the overall efficiency of an InGaN-based solar cell by embedding Ag nanoparticles (NPs) in the InGaN absorbing layer is numerically demonstrated. The generation of LSP resonance on the embedded Ag NPs and the NP scattering can produce a field distribution in the InGaN layer for enhancing absorption. It is shown that the embedded Ag NPs do not significantly affect the transport of the photo-generated carriers. The distortion of static electrical stream lines in the solar cell due to the embedded Ag NP leads to a decrease of photocurrent by only a few percents. Based on the material parameter values we use, unless the surface recombination velocity at the interface between the Ag NP and surrounding InGaN is extremely high, Ag NP embedment in the absorbing layer of an InGaN-based solar cell can enhance its efficiency by up to 27%. Such an increase is significantly larger than that achieved by depositing metal NP on the top surface of a solar cell.

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.

Surface plasmon coupling with radiating dipole for enhancing the emission efficiency of a light-emitting diode.

The experimental demonstrations of light-emitting diode (LED) fabrication with surface plasmon (SP) coupling with the radiating dipoles in its quantum wells are first reviewed. The SP coupling with a radiating dipole can create an alternative emission channel through SP radiation for enhancing the effective internal quantum efficiency when the intrinsic non-radiative recombination rate is high, reducing the external quantum efficiency droop effect at high current injection levels, and producing partially polarized LED output by inducing polarization-sensitive SP for coupling. Then, we report the theoretical and numerical study results of SP-dipole coupling based on a simple coupling model between a radiating dipole and the SP induced on a nearby Ag nanoparticle (NP). To include the dipole strength variation effect caused by the field distribution built in the coupling system (the feedback effect), the radiating dipole is represented by a saturable two-level system. The spectral and dipole-NP distance dependencies of dipole strength variation and total radiated power enhancement of the coupling system are demonstrated and interpreted. The results show that the dipole-SP coupling can enhance the total radiated power. The enhancement is particularly effective when the feedback effect is included and hence the dipole strength is increased.

Improved a-plane GaN quality grown with flow modulation epitaxy and epitaxial lateral overgrowth on r-plane sapphire substrate

The authors demonstrate superior crystal quality of a-plane GaN grown on r-plane sapphire substrate based on the flow modulation epitaxy (FME) technique, in which the Ga atom supply is alternatively switched on and off with continuous nitrogen supply. With the FME technique, a high growth rate of 2.3μm∕h can still be achieved. With or without epitaxial lateral overgrowth (ELOG), either c- or m-mosaic condition is significantly improved in the samples of using FME. With ELOG, the surface roughness can be reduced from 1.58to0.647nm in an area of 10×10μm2 microns by using the FME technique. Based on the results of photoluminescence measurement, one can also conclude the better optical property of the FME-grown a-plane GaN thin films. Besides, it is shown that tensile strain is more relaxed in the FME samples.

Surface plasmon leakage in its coupling with an InGaN∕GaN quantum well through an Ohmic contact

We demonstrate the loss of surface plasmon (SP) energy through oscillating electron leakage via the ohmic contact of either p-type or n-type GaN layer in the coupling process between SP and an InGaN/GaN quantum well (QW). The observation implies that in using the SP-QW coupling for enhancing emission in a light-emitting diode, the metals for ohmic contact and SP generation must be separated. A thin dielectric interlayer is required in the region for SP-QW coupling to avoid the leakage of SP energy.

X-ray diffraction study on an InGaN∕GaN quantum-well structure of prestrained growth

We compare the x-ray diffraction (XRD) results of two InGaN∕GaN quantum-well (QW) structures to observe the effects of prestrained growth by depositing a low-indium QW before the growth of five high-indium QWs. From the results of reciprocal space mapping, we observe the fully strained condition in the QWs of the control sample. However, in the sample of prestrained growth, the average strain is partially relaxed. By using an XRD fitting algorithm for calibrating QW parameters, we obtain reasonable values for the compositions and thicknesses of the QWs in both samples. In particular, by assuming a nonuniform strain relaxation distribution among the five high-indium QWs in the prestrained sample, we obtain reasonable composition variations among the QWs. The high-indium QW closest to the low-indium one is most strain-relaxed and has the highest indium incorporation, leading to the longest-wavelength emission. The observed red shift with increasing electron penetration depth in the cathodo-luminescence spec...

MBE-Grown CdZnO/ZnO Multiple Quantum-Well Light-Emitting Diode on MOCVD-Grown p-Type GaN

A CdZnO/n-ZnO multiple-quantum-well (QW) light-emitting diode (LED), with the QWs and n+-ZnO capping layer grown with molecular beam epitaxy on p-GaN, which is grown with metal-organic chemical vapor deposition, is fabricated and characterized. Because of the weak carrier localization mechanism in the ZnO-based LED, its defect emission is quite strong and dominates the LED output when injection current is low. The blue shift of the LED output spectrum in applying a forward-biased voltage and the large blue-shift range in increasing injection current show the different behaviors of such a ZnO-based LED from those of a nitride LED.