K. Y. Lai

National Central University | NCU · Department of Optics and Photonics

Cell cultivation is a multi-billion dollar industry. The industry is currently in great need of a glucose-monitoring tool to maximize the yield of biological products. However, detecting glucose in a cell culture medium is no easy task. This is because the medium contains complex cell nutrients, from which the interfering noises make it extremely d...

The range of applications of the common III-nitride semiconductors (Al, Ga, In)N can be extended through bandgap engineering with the inclusion of boron and forming their heterojunctions. In this study, the band alignments of B(Al, Ga)N alloys with common III-nitrides are investigated using x-ray photoemission spectroscopy. A type-I straddling-gap...

Surface-enhanced Raman scattering (SERS) is a technique that can deliver label-free, real-time, and multiplex detection of target molecules. However, the development of this potential tool has been impeded by an obstacle: reliability. Because SERS detection relies on the very localized (< 10 nm) hot spot, severe intensity fluctuation occurs as the...

The large bandgap and high p-type conductivity of sp²-bonded boron nitride (BN) make the compound very attractive for deep ultraviolet light-emitting diodes (DUV LEDs). However, integrating the promising sp² material in the DUV LED structure is challenging. This is because the reported growth conditions for scalable high-quality BN, including the h...

A 1.5 μm AlN epilayer with a root-mean-square surface roughness of 0.25 nm was grown by metal-organic chemical vapor deposition at the single substrate temperature below 1200 °C. The ultra-flat surface is achieved with 30 min annealing performed in the initial growth, during which abrupt AlN hillocks are removed by pulsed H2 etching controlled via...

The desire to improve human lives has led to striking development in biosensing technologies. While the ongoing research efforts are mostly dedicated to enhance speed and sensitivity of the sensor, a third consideration has become increasingly important: compactness, which is strongly desired in emergency situations and personal health management....

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

A novel method to stabilize the correlated color temperature in pcW-LEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. Under the normal operation condition, it can stabilize the CCT of a pcW-LED by the positive matching of the blue LED peak wavelength to the phosphor excitation spectrum. When the operating te...

1.5-μm AlN grown by metal-organic chemical vapor deposition (MOCVD), with a single substrate temperature of 1180 °C, exhibits atomically flat surface and the XRD (102) peak width of 427 arcsec. The results are achieved with a pulsed NH3-flow condition, serving as an alternative for the commonly used temperature-varied buffer structure, which is oft...

We have synthesized a metal-free composite ink that contains graphene dots (GDs) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) that can be used on paper to serve as the counter electrode in a flexible dye-sensitized solar cell (DSSC). This paper-based GD/PEDOT:PSS electrode is low-cost, light-weight, flexible, environmental...

GaInP/GaAs/Ge triple-junction concentrator solar cells with significant efficiency enhancement were demonstrated with antireflective ZnO nanoneedles. The novel nanostructure was attained with a Zn(NO3)2-based solution containing vitamin C. Under one sun AM 1.5G solar spectrum, conversion efficiency of the triple-junction device was improved by 23.7...

Photon management is essential in improving the performances of optoelectronic devices including light emitting diodes, solar cells and photo detectors. Beyond the advances in material growth and device structure design, photon management via nanoscaled phenomena have also been demonstrated as a promising way for further modifying/improving the dev...

An effective light-harvesting scheme for InGaN-based multiple quantum well solar cells is demonstrated using stacking layers of polystyrene nanospheres. Light-harvesting efficiencies on the solar cells covered with varied stacks of nanospheres are evaluated through numerical and experimental methods. The numerical simulation reveals that nanosphere...

Fused-silica packaging glass fabricated with a hierarchical structure by integrating small (ultrathin nanorods) and large (honeycomb nanowalls) structures was demonstrated with exceptional light-harvesting solar performance, which is attributed to the subwavelength feature of the nanorods and an efficiently scattering ability of honeycomb nanowalls...

Alx Ga1-x N thin-film-based piezotronic strain sensors with ultrahigh strain sensitivity are fabricated through alloying of AlN with GaN. The strain sensitivity of the ternary compound Alx Ga1-x N is higher than those of the individual binary compounds GaN and AlN. Such a high performance can be attributed to the piezoelectric constant enhancement...

Semipolar {101¯1} InGaN quantum wells are grown on (001) Si substrates with an Al-free buffer and wafer-scale uniformity. The novel structure is achieved by a bottom-up nano-heteroepitaxy employing self-organized ZnO nanorods as the strain-relieving layer. This ZnO nanostructure unlocks the problems encountered by the conventional AlN-based buffer,...

The dip of external quantum efficiency (EQE) is observed on In0.15Ga0.85N/GaN multiple quantum well (MQW) solar cells upon the increase of incident optical power density. With indium composition increased to 25%, the EQE dip becomes much less noticeable. The composition dependence of EQE dip is ascribed to the competition between radiative recombin...

In this paper, we start from the study on the packaging efficiency of the phosphor-converted white LED via a new way to measure and calculate the blue light from the blue die to the encapsulation lens. Then we try to estimate the limit of luminous efficacy of a pcW-LED with Type VII structure. In the calculation, with the EQE of 81% of the blue die...

The photovoltaic performance of a GaN p-n-n homojunction is characterized using the air-mass (AM) 1.5 G solar spectrum and a 532-nm green laser. The n-type active region is doped with Si in order to create an intermediate band for sub-bandgap photon absorption. When the doping level in the active region increases from 0 (unintentionally doped) to n...

The unique geometry and intriguing physical properties of nanostructure-based solar cells gives them great potential to achieve the goals of cost-effectiveness and high-efficiency. With nanostructured solar cells it is expected to be possible to break the Shockley–Queisser limit. This potential has driven widespread research and development in phot...

III–nitride blue (λ = 450 nm) LEDs were fabricated on ceramic substrates using thin-film and high-voltage processes. The device, comprising sixteen cascaded sub-LEDs, exhibits excellent current spreading and thermal resilience. Under the driving power of 1800 W/cm 2 (J = 450 A/cm 2), the high-voltage thin-film LED delivers unsaturated output power...

High-voltage thin-film GaN LEDs with the emission wavelength of 455 nm were fabricated on ceramic substrates (230 W/m·K). The high-voltage operation was achieved by three cascaded sub-LEDs with dielectric passivation and metal bridges conformally deposited on the side walls. Under the driving power of 670 W/cm², the high-voltage LEDs exhibit much a...

We report an efficient light-extraction scheme employing the hierarchical structure, p-GaN microdomes/SiO2 nanorods (NRs), on GaN light-emitting diodes (LEDs). Compared with the flat LED, the LEDs with hierarchical surfaces exhibits a light-output improvement of 36.8%. The considerable enhancement in light-extraction efficiency is attributed to the...

Creating a pyramidal structure on an n-GaN surface is considered the most effective approach for maximizing light extraction of n-side-up GaN-based light-emitting diodes (LEDs). This letter shows that the light extraction efficiency of a pyramidal n-GaN surface can be further enhanced by growing ZnO nano-rods (NRs) specifically on the tips of the p...

A hybrid structure (HS) made of one-dimensional ZnO nanorods (NRs) and a two-dimensional synthesized graphene sheet was successfully constructed in this study. The uniform ZnO NRs were obtained by hydrothermal method and grown on a graphene surface that had been transferred to a polyethylene terephthalate substrate. The HS exhibited high transmitta...

InGaN/GaN multiple-quantum-well solar cells were grown on (111) Si substrates. AlN/AlGaN superlattice and self-assembly Si x N y masking islands were employed to alleviate the material mismatches between Si and GaN. The devices were characterized under the illumination of AM 1.5 G with different solar concentrations. As the concentration ratios inc...

We experimentally and theoretically demonstrated the hierarchical structure of SiO(2) nanorod arrays/p-GaN microdomes as a light harvesting scheme for InGaN-based multiple quantum well solar cells. The combination of nano- and micro-structures leads to increased internal multiple reflection and provides an intermediate refractive index between air...

We employ a ZnO nanorod/Si(3)N(4)-coated Si microgroove-based hierarchical structure (HS) for a light-harvesting scheme in 5 inch single crystalline Si solar cells. ZnO nanorods and Si microgrooves were fabricated by a simple and scalable aqueous process. The excellent light-harvesting characteristics of the HS, such as broadband working ranges and...

SiO2 nanorod arrays (NRAs) are fabricated on InGaN-based multiple quantum well (MQW) solar cells using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching. The SiO2 NRAs effectively suppress the undesired surface reflections over the wavelengths from 330 to 570 nm, which is attributed to the light-trapping effect...

InGaN-based multiple quantum well (MQW) solar cells (SCs) employing the p-GaN microdome were demonstrated to significantly boost the conversion efficiency by 102%. The improvements in short-circuit current density (Jsc, from 0.43 to 0.54 mA/cm2) and fill factor (from 44% to 72%) using the p-GaN microdome are attributed to enhanced light absorption...

Antireflective Si/oxide core-shell nanowire arrays (NWAs) were fabricated by galvanic etching and subsequent annealing process. The excellent light-harvesting characteristics of the core-shell NWAs, such as broadband working ranges, omnidirectionality, and polarization-insensitivity, ascribed to the smooth index transition from air to the substrate...

InGaN/GaN quantum wells (QWs) grown at identical conditions on m-plane GaN and c-plane sapphire substrates were characterized by several techniques, aiming to clarify the reason for different emission wavelengths often observed in similar LED structures with m- and c-plane surface orientations. Cathodoluminescence (CL) spectra of m-plane QWs reveal...

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°–85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of obliq...

Periodic sub-wavelength SiO2 nano-honeycombs are fabricated on GaN-based multiple quantum well solar cells by self-assembly polystyrene nanosphere lithography and reactive ion etching. The nano-honeycombs are found to be effective in suppressing the undesired surface reflections over a wide range of wavelengths. Under the illumination of air mass 1...

Large-area, periodic Si nanopillar arrays (NPAs) with the periodicity of 100 nm and the diameter of 60 nm were fabricated by metal-assisted chemical etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NPAs serve an antireflection function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to...

InGaN/GaN quantum wells (QWs) grown at identical conditions on m-plane GaN and c-plane sapphire substrates were characterized by several techniques, aiming to clarify the reason for different emission wavelengths often observed in similar LED structures with m-and c-plane surface orientations. Cathodoluminescence (CL) spectra of m-plane QWs reveale...

Syringe-like ZnO nanorod arrays (NRAs) synthesized by a hydrothermal method were applied as the light-harvesting layer on InGaN-based multiple quantum well (MQW) solar cells. Theoretical calculations show that the NRAs with an abrupt shrinkage of tip diameter can further suppress surface reflectance in comparison with the flat NRAs. InGaN-based MQW...

Electronic structures of well-aligned Er-doped ZnO (ZnO:Er) nanorod arrays (NRAs) synthesized by a solution-based hydrothermal process were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray absorption fine structure (XAFS). HRTEM and angular dependent X-ray absorption near-edge structure analysis at O K and Zn L3 e...

The nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (<0.1%) in the wavelengths of 200–850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidi...

An effective antireflection coating of syringelike ZnO NRAs is demonstrated for GaAs solar cells. Energy conversion efficiency is significantly enhanced by up to 32%. GaAs is focussed on as a model system, but the hydrothermal growth of syringelike ZnO nanostructures is readily compatible with the existing processes in the photovoltaic industry, an...

InxGa1-xN/GaN multiple quantum-well (QW) (MQW) solar cells with x = 0.30 and 0.15 were characterized. The MQWs with x = 0.30 show deteriorated performances due to the inferior crystal qualities. At the temperatures above 200 K, the conversion efficiency (η) for x = 0.30 exhibits an abrupt increase led by the thermally activated carriers. Two potent...

Sinanorod arrays (NRAs) mimicking moth-eye structures were fabricated with colloidal lithography and reactive ion etching. Compared with that on polished surface, the reflectance on NRA structures is significantly reduced by more than 10 times. The reflectance is decreased with the height of the NRAs. The anti-reflection (AR) ability of the NRAs is...

Galvanic wet etching was adopted to fabricate Sinanowire arrays (NWAs) as a near-perfect subwavelength structure (SWS), which is an optically effective gradient-index antireflection (AR) surface and also exhibits super-hydrophobicity with an extremely high water contact angle (159°). Fresnel reflection and diffuse reflection over the broad spectrum...

Slope-tunable Si nanorod arrays (NRAs) were fabricated with colloidal lithography and reactive ion etching (RIE). Sharpened NRAs fabricated by increasing the SF6/O2 flow ratio during RIE exhibit enhanced antireflection (AR) and hydrophobic properties, which are attributed to the smooth gradient in the effective refractive index of NRAs, and the enl...

Periodic Si nanopillar arrays (NPAs) were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wa...

The effect of m-plane GaN substrate miscut on the growth of InGaN/GaN quantum wells (QWs) was investigated. It was found that the miscut toward [0001] c+-axis resulted in an increase of In incorporation efficiency and in a green-shift of the QW emission, while the miscut toward [1120] a-axis resulted in even higher In compositions but it also led t...

Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature...

InGaN/GaN quantum wells (QWs) grown with identical conditions on m-plane and c-plane GaN substrates were studied by cathodoluminescence spectroscopy. At a low current of 10 nA, the emission intensity and wavelength of the m-plane aligned QWs were found to be about two times stronger and 19.5 nm blueshifted with respect to that of the c-plane aligne...

The surface of HVPE grown GaN substrates were treated with two different polishing procedures. Both procedures were successful in producing highly smooth and featureless surfaces. However, subsurface damage was observed in the sample treated by one of the procedures. The subsurface damage was revealed by cathodoluminescence (CL) spectroscopy imagin...

M-plane non-polar bulk GaN substrates were regrown by hydride vapour phase epitaxy (HVPE), aiming to enlarge their lateral size. The non-polar GaN substrates were synthesized by growing thick GaN boules by HVPE along the c-axis, and wafering transversely to expose non-polar (m- or a-plane) surfaces. Non-polar GaN substrates obtained in this manner...

Non-polar surfaces of HVPE grown GaN were characterized by cathodoluminescence (CL), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). Both of a- and m-plane GaN were prepared by growing thick GaN along the c-axis, and cutting in transverse orientations. The exposed non-polar surfaces were prepared by mechanical polish...

Large-area, periodic Si nanopillar arrays (NPAs) with the periodicity of 100 nm and the diameter of 60 nm were fabricated by metal-assisted chemical etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NPAs serve an antireflection function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to...

Nonpolar (m-plane or a-plane) gallium nitride (GaN) is predicted to be a potential substrate material to improve luminous efficiencies of nitride-based quantum wells (QWs). Numerical calculations indicated that the spontaneous emission rate in a single In0.15Ga0.85N/GaN QW could be improved by ˜2.2 times if the polarization-induced internal field w...