Tai-Yuan Lin

Mechanism of luminescence in InGaN/GaN multiple quantum wells

We report a firm evidence of luminescence from InN clusters in InGaN/GaN multiple quantum wells. Photoluminescence, photoluminescence excitation, and Raman scattering measurements have been employed to study the optical properties of InGaN/GaN multiple quantum wells. A careful examination of the low energy shoulders of the main peak luminescence reveals the fact that their separation is in good agreement with the longitudinal optical phonon energy of pure InN film measured by Raman scattering. A large Stokes-like shift between the emission peak energy and the absorption edge is found; it increases with increasing indium content. All these observations can be explained in a consistent way by the effect of localization due to self-organized InN clusters within InGaN layers. Our results thus strongly suggest that the emission mechanism of InGaN/GaN quantum wells originates from radiation recombination within the localized states of self-organized InN clusters.

Strong luminescence from strain relaxed InGaN/GaN nanotips for highly efficient light emitters

Semiconductor heterostructures represent the most important building block for current optoelectronic devices. One of the common features of semiconductor heterostructures is the existence of internal strain due to lattice mismatch. The internal strain can tilt the band alignment and significantly alter the physical properties of semiconductor heterostructures, such as reducing the internal quantum efficiency of a light emitter. Here, we provide a convenient route to release the internal strain by patterning semiconductor heterostructures into nanotip arrays. The fabrication of the nanotip arrays was achieved by self-masked dry etching technique, which is simple, low cost and compatible with current semiconductor technologies. By implementing our approach to InGaN/GaN multiple quantum wells, we demonstrate that the light emission can be enhanced by up to 10 times. Our approach renders an excellent opportunity to manipulate the internal strain, and is very useful to create highly efficient solid state emitters.

Photoassisted Synthesis of Luminescent Mannose–Au Nanodots for the Detection of Thyroglobulin in Serum

We have employed mannose-modified gold nanodots (Man-Au NDs) as a luminescence sensor for the detection of the thyroid-cancer marker thyroglobulin (Tg) in homogeneous solutions. The luminescent Man-Au NDs are prepared through the reaction of 2.9 nm-diameter gold nanoparticles (Au NPs) with 11-mercapto-3,6,9-trioxaundecyl-alpha-D-mannopyranoside (Man-RSH) under the irradiation of a light-emitting diode (LED). We have found that the irradiation enhances the quantum yield (approximately 11%), alters the emission wavelength and lifetimes, and shortens the preparation time. A luminescence assay has been developed for Tg based on the competition between Tg and Man-Au NDs for the interaction with the concanavalin A (Con A). Because luminescence quenching of the Man-Au NDs by Con A is inhibited by Tg selectivity, we have obtained a highly sensitive and selective assay for Tg.

Effects of alloy potential fluctuations in InGaN epitaxial films

Results of photoluminescence and photoconductivity measurements in epitaxial films are presented. The photoluminescence peak energy and intensity show several anomalous behaviours. The peak energy changes with temperature exhibiting an inverted S-shape dependence, where it decreases, then increases with increasing temperature in the range 40-100 K and finally decreases with increasing temperature. The intensity shows a temperature dependence similar to that of amorphous semiconductors and disordered superlattices. A blue shift of the photoluminescence energy with increasing excitation intensity is observed. A large Stokes shift between the photoluminescence peak position and the band edge transition energy is found; it decreases with decreasing indium content. A persistent photoconductivity effect has been detected up to room temperature with a stretched-exponential function for its decay rate. All these observations can be explained in a consistent way by alloy potential fluctuations, and these clearly indicate the existence of compositional fluctuations. These two related effects thus appear to constitute the mechanism for the widely observed localized excitons in InGaN-based devices.

Terahertz response of GaN thin films

The indices of refraction, extinction constants and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz are obtained using THz time-domain spectroscopy. The results correspond well with the Kohlrausch stretched exponential model. Using the Kohlrausch model fit not only provides the mobility of the free carriers in the GaN film, but also estimates the relaxation time distribution function and average relaxation time.

Transferable and Flexible Label‐Like Macromolecular Memory on Arbitrary Substrates with High Performance and a Facile Methodology

A newly designed transferable and flexible label-like organic memory based on a graphene electrode behaves like a sticker, and can be readily placed on desired substrates or devices for diversified purposes. The memory label reveals excellent performance despite its physical presentation. This may greatly extend the memory applications in various advanced electronics and provide a simple scheme to integrate with other electronics.

In-rich In1−xGaxN films by metalorganic vapor phase epitaxy

Single crystalline In1−xGaxN films containing high In content (70%–100%) were grown by metalorganic vapor phase epitaxy. A linear relation was observed between the lattice constants and gas phase Ga∕In ratios. The surface morphology changed from pyramid for InN to more planar ones for the InGaN alloys with increasing Ga content. The electron mobility decreased rapidly from 1200cm2∕Vs for InN to less than 100cm2∕Vs for In0.7Ga0.3N, with a carrier concentration of low- 1019cm−3 for all the as-grown films. Using photoluminescence a single emission peak was observed at 1.4–1.6μm for the In-rich InGaN with decreasing wavelengths up to below 20% of Ga. Two peaks were observed for the In0.80Ga0.20N, however, indicating possible phase separation. The x-ray photoelectron spectroscopic measurement showed shifts to higher binding energies for both In and Ga with increasing Ga content. The estimated alloy composition, however, depended sensitively on the sputtering conditions of the samples.

Biologically inspired flexible quasi-single-mode random laser: An integration of Pieris canidia butterfly wing and semiconductors

Quasi-periodic structures of natural biomaterial membranes have great potentials to serve as resonance cavities to generate ecological friendly optoelectronic devices with low cost. To achieve the first attempt for the illustration of the underlying principle, the Pieris canidia butterfly wing was embedded with ZnO nanoparticles. Quite interestingly, it is found that the bio-inspired quasi-single-mode random laser can be achieved by the assistance of the skeleton of the membrane, in which ZnO nanoparticles act as emitting gain media. Such unique characteristics can be interpreted well by the Fabry-Perot resonance existing in the window-like quasi-periodic structure of butterfly wing. Due to the inherently promising flexibility of butterfly wing membrane, the laser action can still be maintained during the bending process. Our demonstrated approach not only indicates that the natural biological structures can provide effective scattering feedbacks but also pave a new avenue towards designing bio-controlled photonic devices.

Optical quenching of the photoconductivity in n-type GaN

Results of optical quenching of photoconductivity measurements in undoped n-type and Se-doped GaN epitaxial thin films are presented. The spectral distribution of quenching phenomena shows a broadband centered around 1.26 eV. Transient changes in photoconductivity on application or removal of the quenching radiation are shown to exhibit a metastable behavior. The results reveal that the origin of the optical quenching phenomena is closely related to the defects corresponding to the persistent photoconductivity effects and the yellow luminescence band observed in most n-type GaN. In addition, this result indicates that these defects can have multiple charge states. It is found that the quenching ratio increases with increasing Se-doping concentration. We point out that the origin of the defects responsible for the optical quenching can be attributed to nitrogen antisite and/or Ga vacancy.

Concentration dependence of carrier localization in InN epilayers

The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.