Chia-Hui Fang

Anomalous luminescence phenomena of indium-doped ZnO nanostructures grown on Si substrates by the hydrothermal method

In recent years, zinc oxide (ZnO) has become one of the most popular research materials due to its unique properties and various applications. ZnO is an intrinsic semiconductor, with a wide bandgap (3.37 eV) and large exciton binding energy (60 meV) making it suitable for many optical applications. In this experiment, the simple hydrothermal method is used to grow indium-doped ZnO nanostructures on a silicon wafer, which are then annealed at different temperatures (400°C to 1,000°C) in an abundant oxygen atmosphere. This study discusses the surface structure and optical characteristic of ZnO nanomaterials. The structure of the ZnO nanostructures is analyzed by X-ray diffraction, the superficial state by scanning electron microscopy, and the optical measurements which are carried out using the temperature-dependent photoluminescence (PL) spectra. In this study, we discuss the broad peak energy of the yellow-orange emission which shows tendency towards a blueshift with the temperature increase in the PL spectra. This differs from other common semiconductors which have an increase in their peak energy of deep-level emission along with measurement temperature.

Exciton wavefunction coupled surface plasmon resonance for In-rich InGaN film with perforated aluminum cylindrical micropillar arrays

The optical characterization of excitons coupled with surface plasmon resonance (SPR) for InGaN/GaN heterostructures with perforated cylindrical micropillar arrays is investigated. We analyze the optical characteristics of excitons coupled with SPR for InGaN/GaN heterostructures with perforated cylindrical micropillars, as shown in measurements of the photoluminescence (PL) spectra over a broad range of temperatures between 20 and 300 K. From the temperature-dependent PL spectra, we observe the better SPR coupling effects, resulting in less carrier confinement in the InGaN energy band. The magnitude of the redshift of the emission peak shown by the sample with the coated aluminum (Al) pattern is larger than that shown by the sample with no metal film. This was due to the presence of more exciton coupling surface plasmons within the Al/InGaN interface. The enhancement of the PL intensity of the sample with the deposited Al pattern film can be attributed to a stronger SPR coupling interaction with the excitons. The experimental results indicate that a perforated Al cylindrical micropillar array can significantly affect carrier confinement, enhancing the quantum efficiency of Al/In-rich InGaN heterostructures due to the interaction of the SPR coupling effect between the InGaN quantum dot-like region and the Al film.

Lie group study of Raman spectra of the Gurken gradient in Drosophila oogenesis.

We carried out a Lie group study of the micro-Raman tissue spectra of the Gurken gradients of Drosophila oogenesis. Matrix representations (2 × 2) resulting from the polarized Raman scattering were employed to assess the roles of the ligand-receptor complexes in follicle cell. It was found that the Gurken expansion caused by overexpressing Dally-like protein (Dlp) revealed an X1 Lie point symmetry, while the Gurken distribution in the wild-type egg showed an X23 Lie point symmetry. The correlation between the corresponding continuous symmetry operations and the observed Gurken localization were a corroboration of the significance of the Lie group analysis by means of the reaction–diffusion equation in a prolate spheroidal coordinate system. These investigations suggested that the group-theoretical approach can be applied to characterize the fluctuating asymmetry and the developmental stability in a wide variety of organisms.

Performance enhancements in optoelectrical properties of InGaN/GaN light-emitting diodes with micro-hole arrayed indium-tin-oxide layer

We demonstrated that the InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) with micro-hole arrays exhibit better performance in optoelectrical properties than do the conventional LEDs. With injection current of 20 mA, by employing the micro-hole array on top surface of the LED structure, the room-temperature output power conversion efficiency and external quantum efficiency were increased 28.7% and 14.3%, respectively, compared with the conventional broad area devices. From room temperature I-V characteristics of the LEDs, it has found that series resistance and leakage current were related with hole dimensions and etching depths, respectively. Interestingly, the leakage current of the transparent conductive layer (TCL) was dominated by the contribution of the micro-hole side-wall, the number of the etching micro-holes, and the wet-etched depths. We conclude that a well-designed micro-hole array structure can indeed significantly not only inhibit the leakage current of ITO transparent conductive layer, but also enhance external quantum efficiency and extraction efficiency by using wet etching process, over a broad temperature ranges.

Bias dependence of phonon behaviors in the InGaN/GaN multiple quantum well with multiquantum barriers

To elucidate the unique correlations between the electrical and optical characteristics of InGaN/GaN MQW LEDs, it is necessary to examine the radiative recombinations of carriers confined from aspects of the phonon trainsitions.

Effect of Excitonic Interactions on Abnormal Luminescence Behaviour of InGaN/GaN Light-Emitting Diodes with Electron Tunneling Layer

We studied the unique correlations between the electrical and optical characteristics of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with n-AlGaN layer and n-InGaN/GaN superlattice electron tunneling layer (ETL). It was found that the ideality factor of the two devices increased with decreasing temperature from 300 to 20 K. Levines model was used to characterize the anomaly, and the ideality factor could be shown as a function of temperature by n = 1 + To/T. The device with ETL inherently exhibited a large pseudo-temperature To with a large characteristic energy and charge population in the multilayer interface, over wide temperature and voltage ranges. Owing to the more interface state distribution and the less effective density of state (DOS) in the quantum wells, the excitons formed in the ETL sample augment the spectral radiations at temperatures higher than 150 K. The carrier tunneling processes via the extent of charge population consequently caused anomalous To and resulted in the abnormal deterioration of the EL intensities with a small density of state.