Chung-Hui Chen

Strong and stable visible luminescence from Au-passivated porous silicon

We report on porous silicon (PS) samples with strong and stable red photoluminescence (PL) prepared by chemical anodization of gold-plated substrate. We demonstrate that the structural stability of Au-passivated porous silicon is much better than that of normal PS. It is also found that the PL intensity of Au-passivated PS can be enhanced by a factor of 3 when prepared under the same condition as that for normal PS. The infrared absorption spectra reveal that the photoluminescence stability can be attributed to the formation of stable Au–Si bonds on the surface of porous silicon. In addition, our study provides further evidence to support the quantum confinement model of the red emission of porous silicon.

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.

Optical properties of n-type porous silicon obtained by photoelectrochemical etching

Abstract The optical studies of n-type porous silicon prepared by the photo-assisted chemical etching are reported here. The optical properties of samples obtained under different conditions have been investigated by photoluminescence and Fourier transform infrared absorption measurements, and they are compared with that of p-type porous silicon. Our results clearly demonstrate that the blue emission in porous silicon originates from surface compounds. From the infrared absorption measurement, we point out that the surface compounds are Si–OH complexes. This conclusion is further supported by a recent calculation which shows that Si–OH complexes can emit the photon energy in the range observed here. We show that the optical properties of the n-type porous silicon are more stable than that of the p-type porous silicon. The result provides the evidence to support the fact that the n-type porous silicon is a better candidate for the application in optoelectronics.

Observation of persistent photoluminescence in porous silicon: Evidence of surface emission

We report on the observation of persistent photoluminescence (PPL) in oxidized porous silicon. The PPL decay can be well described by a stretched-exponential function, and its decay rate is not sensitive to the change of temperature. We point out that the PPL behavior can be interpreted in terms of the picture that the emission arises from the excited surface complexes, which is produced by capture of photocarriers tunneling from the nearest shallow trap in the nanocrystalline silicon. To explore the microscopic origin of the surface compounds, we performed infrared absorption, and found that the PPL intensity correlates well with Si–OH vibration mode. Further evidence is provided by the recent theoretical calculation showing that the Si–OH complex can emit the photon energy in the range observed here. We thus provide concrete evidence to support the fact that the PL signal of porous silicon does contain surface emission.

Persistent photoconductivity in InxAlyGa1−x−yN quaternary alloys

The optical properties of InxAlyGa1−x−yN quaternary alloys were investigated by photoconductivity (PC), persistent photoconductivity (PPC), photoluminescence (PL), and photoluminescence excitation (PLE) measurements. Quite interestingly, persistent photoconductivity was observed. Through the combination of our optical studies, we show that the PPC effect arises from composition fluctuations in InxAlyGa1−x−yN quaternary alloys. From the analysis of the decay kinetics, the localization depth caused by composition fluctuations was determined. A comparison between the PL, PLE, and PC measurements gives a direct access to the Stokes’ shift. The Stokes’ shift can be explained in terms of localization due to the existence of nanoscale clusters, and it is consistent with the PPC result. The results shown here provide concrete evidence to support our previously proposed model that the existence of InGaN-like clusters is responsible for the strong luminescence in InxAlyGa1−x−yN quaternary alloys.

Behaviors of Surface Plasmon Coupled Light-Emitting Diodes Induced by Surface Ag Nanoparticles on Dielectric Interlayers

The enhanced surface plasmon (SP) coupling effects in a blue light-emitting diode (LED) with regularly patterned (REG) surface Ag nanoparticles (NPs) on a dielectric interlayer (DI) of a lower refractive index overgrown on p-GaN are demonstrated. Without a DI, the surface Ag NPs-induced SP coupling with the quantum wells (QWs) in the LED can lead to the increases of internal quantum efficiency and LED output intensity, the reduction of the external quantum efficiency droop effect, and the enhancement of modulation response. By adding a DI, the SP coupling effect is enhanced, resulting in the further improvements of all the aforementioned factors. We compare the SP coupling effects in the LEDs with REG Ag NPs on DIs to those of randomly distributed (RAN) Ag NPs previously reported. Although the variation trends of the localized surface plasmon (LSP) resonance peaks and hence the SP coupling behaviors of REG and RAN Ag NPs are similar, their LSP resonance strengths at the QW emission wavelength are different due to their different spectral patterns of LSP resonance. In other words, although the REG Ag NPs can produce stronger collective LSP resonance with a narrower spectral width, the SP coupling effect depends mainly on the LSP resonance strength at the QW emission wavelength.

Surface plasmon coupled light-emitting diode: Experimental and numerical studies

First, the experimental implementations and theoretical/numerical investigations of surface plasmon (SP) coupled InGaN/GaN quantum-well light-emitting diodes (LEDs) are reviewed. If the p-GaN layer in an LED can be thin, surface metal nanoparticle (NP) is an inexpensive structure for inducing effective SP coupling. When the p-GaN layer is thick, a few metal structures, including metal protrusion, buried metal NP, and embedded metal NP, can be used for effective SP coupling. In the numerical study, an algorithm, including the feedback effect of the induced SP resonance on the radiating behavior of the source dipole, has been proposed for studying the SP coupling effects with an embedded metal NP, a surface metal NP, and a metal protrusion. Then, the theoretical formulations and numerical algorithms for evaluating the radiated power enhancement in the coupling process between two radiating dipoles and the localized surface plasmon (LSP) induced on a nearby Ag NP are built. Three mechanisms are considered in the coupling process for radiated power enhancement, including the interference of the two phase-retarded radiation contributions from the two dipoles, the interaction between the two dipoles, and the LSP resonant coupling.

Localized Surface Plasmon Coupled Light-Emitting Diodes With Buried and Surface Ag Nanoparticles

Two sets of light-emitting diodes (LEDs) based on two epitaxial structures of different p-GaN layer thicknesses for demonstrating the effects of localized surface plasmon (LSP) coupling with the quantum wells (QWs) in the LEDs are compared. In the first set based on the epitaxial structure of thick p-GaN, to reduce the distance between the Ag nanoparticles (NPs) and QWs for increasing the LSP coupling strength, Ag NPs are filled into a hole array fabricated on the p-GaN layer. In the second set based on the epitaxial structure of thin p-GaN, Ag NPs are fabricated on the top surface of the p-GaN layer. The LSP-coupled LEDs show the significant enhancements of internal quantum efficiency and LED output intensity even though the coverage of the transparent conductor, GaZnO, red-shifts the LSP resonance peak such that the LSP resonance at the QW emission wavelength becomes weaker.

Light-emitting diodes on Si (110) substrate

We study the morphologies, material properties, and optical characteristics of an InGaN/GaN QW light-emitting diode (LED) structure grown on a <;1-10>-oriented one-dimensional trench-patterned Si (110) substrate with other samples of different trench orientations on Si (110) substrate, flat Si (110) substrate, and Si (111) substrate are demonstrated. By comparing the performances of the fabricated LEDs based on the three samples of continuous top surfaces, it is found that the sample of <;1-10>-oriented trench has the strongest output power, lowest device resistance, and smallest spectral shift range in increasing injection current.

The Optimization of SiGe Hetero-Structure Thin-Film Solar Cell by the Theoretical Calculation and Quantitative Analysis

The characteristics of SiGe hetero-structure solar cell such as short circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and efficiency with optimal Ge concentration are investigated in this work. The average Ge concentration was systematically changed in the range from 0% to 30%. The appropriate addition of Ge in crystal Si solar cell is an effective way to increase the short circuit current density, without affecting on the open-circuit voltage, due to the modulation of the material band-gap and hetero-structure. Therefore, the solar cell efficiency can be further improved. With the optimization of Ge concentration and clean process condition, the overall efficiency of a Si0.9Ge0.1 solar cell is found to be improved about ~4% than it in the control Si solar cell. The band-gap of the SiGe material, the key parameter for the solar cell design, can be extracted by an Electron-Hole Plasma (EHP) model at different temperatures. Finally, the SiGe-based solar cell has also been observed that it has less operated temperature sensitivity than Si solar cell for the real application. The theoretical calculation and simulation in this work can help us to understand and engineer the high-efficiency SiGe solar cell qualitatively.