[show abstract][hide abstract] ABSTRACT: Graphene oxide coated on GaN was simultaneously reduced and doped with nitrogen via excimer laser irradiation. Nitrogen dopant was originated from the GaN during the laser-induced dissociation at high energies. This phenomenon was confirmed by the absence of C–N bond formation in laser irradiated graphene oxide on SiO2. A top-gated field-effect transistor based on laser reduction of graphene oxide channel on GaN showed n-type behavior via the gate voltage modulation. The present findings indicate a paradigm for the formation of graphene-nitride semiconductor interfaces.
[show abstract][hide abstract] ABSTRACT: In this Letter, a light-emitting diode (LED) with prism-shaped-air-ring microstructures (PSAMs) formed on flat sapphire substrate is demonstrated as an alternative design to face-up LEDs on patterned sapphire substrate (PSS) for enhanced light extraction efficiency. In this LED design, the emitted photons can be deflected to the top of the chip for its effective extraction, contrary to the PSS-LED wherein photons are guided to sapphire and get absorbed by packaging materials. The PSAM-LED showed an enhancement in the radiometric power as high as 10% with a low far-field angle of 129° over that of a PSS-LED under an injection current of 20 mA.
[show abstract][hide abstract] ABSTRACT: The effect of the combination of triangular prisms and cones as air-void structures arrays on the enhancement of light extraction efficiency of InGaN light-emitting diodes (LEDs) is investigated. The arrays embedded at the sapphire/GaN interface act as light reflectors and refractors, and thereby improve the light output power due to the redirection of light into escape cones on both the front and back sides of the LED. Enhancement in radiometric power as high as 74% and far-field angle as low as 125° is realized with a compact arrangement of arrays compared with that of a conventional LED.
[show abstract][hide abstract] ABSTRACT: The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.
[show abstract][hide abstract] ABSTRACT: This paper reports on the evaluation of the impact of introducing interlayers and post-metallization annealing on the graphene/p-GaN ohmic contact formation and performance of associated devices. Current-voltage characteristics of the graphene/p-GaN contacts with ultrathin Au, Ni and NiOx interlayers were studied using transmission line model with circular contact geometry. Direct graphene/p-GaN interface was identified to be highly rectifying and post-metallization annealing improved the contact characteristics as a result of improved adhesion between the graphene and the p-GaN. Ohmic contact formation was realized when interlayer is introduced between the graphene and p-GaN followed by post-metallization annealing. Temperature dependent I-V measurements reveled that the current transport was modified from thermionic emission for the direct graphene/p-GaN contact to tunneling for the graphene/metal/p-GaN contacts. The tunneling mechanism results due to the interfacial reactions that occur between the metal and p-GaN during the post-metallization annealing. InGaN/GaN light-emitting diodes with NiOx/graphene current spreading electrode offered a forward voltage of 3.16 V comparable to that of its Ni/Au counterpart, but ended up with relatively low light output power. X-ray photoelectron spectroscopy provided evidence for the occurrence of phase transformation in the graphene-encased NiOx during the post-metallization annealing. The observed low light output is therefore correlated to the phase change induced transmittance loss in the NiOx/graphene electrode. These findings provide new insights into the behavior of different interlayers under processing conditions that will be useful for the future development of opto-electronic devices with graphene-based electrodes.
[show abstract][hide abstract] ABSTRACT: The effect of air-gap/GaN DBR structure, fabricated by selective lateral wet-etching, on InGaN light-emitting diodes (LEDs) is investigated. The air-gap/GaN DBR structures in LED acts as a light reflector, and thereby improve the light output power due to the redirection of light into escape cones on both front and back sides of the LED. At an injection current of 20 mA, the enhancement in the radiometric power as high as 1.91 times as compared to a conventional LED having no DBR structure and a far-field angle as low as 128.2° are realized with air-gap/GaN DBR structures.
[show abstract][hide abstract] ABSTRACT: This paper describes a detailed systematic study based on the fabrication and performance of InGaN/GaN blue light-emitting diodes (LEDs) with multilayer graphene film as a current spreading electrode. Two facile approaches to improve the electrical coupling between graphene and p-GaN layer are demonstrated. Using chemical charge transfer doping, the work function (Φ) of graphene is tuned over a wide range from 4.21 to 4.93 eV with substantial improvements in sheet resistance (Rs). Compared with pristine graphene, the chemically modified graphene on p-GaN yields several appealing characteristics such as low specific contact resistance (ρc) and minimized barrier height. In addition, insertion of a thin gold interlayer between graphene and p-GaN profoundly enhances the contact properties at the interface. Combining these two approaches in a single LED, the current spreading and thus the device forward voltage (Vf) are considerably improved comparable to that of an LED fabricated with an indium tin oxide electrode. The importance of pre-metal deposition oxygen plasma treatment and rapid thermal annealing in improving the contact characteristics is also addressed.
Journal of Physics D Applied Physics 03/2012; 45(14):145101. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper reports on the deflection effect of ellipsoidal air tunnel (EAT) array as an embedded deflector with low refractive index in InGaN/GaN light-emitting diodes (LEDs). Light extraction efficiency (LEE) for different height-to-width (h/w) ratios of air structure is analyzed by using an optical simulation program. Maximum LEE was obtained at a h/w ratio of 0.3, which corresponds to an ellipsoidal shape of the air structure. An LED structure with EAT array embedded between the sapphire substrate and the GaN epitaxial layer has been fabricated. The reflectance spectra measured over the entire visible spectral range showed reflectance modulations by the EAT structures. Moreover, the light-output power of the EAT-embedded LED was enhanced by 2.3 times compared to a conventional LED without EAT arrays. This could be attributed to the enhanced light deflection from EAT structures.
[show abstract][hide abstract] ABSTRACT: This letter reports on the implementation of multilayer graphene (MLG) as a current spreading electrode in GaN-based blue light-emitting diodes. We demonstrate two facile strategies to maneuver the electrical coupling between p-GaN layer and MLG. Using a work-function-tuned MLG and a thin gold (Au) metal interlayer, the current spreading and thus the device forward voltage are considerably improved. We attribute these improvements to the diminution in work function difference between p-GaN and MLG, the decrease of specific contact resistance, and the enhancement in the conductivity of MLG film as a result of doping. In addition, rapid thermal annealing at elevated temperature is found to provide additional pathway for enhanced carrier injection.
[show abstract][hide abstract] ABSTRACT: The effect of triangular air prism (TAP) arrays with different distance-to-width (d/w) ratios on the enhancement of light extraction efficiency (LEE) of InGaN light-emitting diodes (LEDs) is investigated. The TAP arrays embedded at the sapphire/GaN interface act as light reflectors and refractors, and thereby improve the light output power due to the redirection of light into escape cones on both the front and back sides of the LED. Enhancement in radiometric power as high as 117% and far-field angle as low as 129° are realized with a compact arrangement of TAP arrays compared with that of a conventional LED made without TAP arrays under an injection current of 20 mA.
[show abstract][hide abstract] ABSTRACT: This study attempts to identify in what aspects China and India are different and similar in forming consumer attitudes and purchase intentions toward US apparel brand goods. To this end, this study proposed a composite model incorporating the theory of planned behavior and a modified Fishbein model, and empirically compared the model with data collected in China and India. Structural equation modeling results yielded both similarities and differences in patterns of significance and the direction of effects for specific hypothesized paths. Differences were found in the paths of face saving → attitude, face saving → purchase intention (paths were positive and significant in the Chinese sample but non-significant in the Indian sample), group conformity → attitude and group conformity → purchase intention (paths were positive and significant in the Indian sample but non-significant in the Chinese sample). Similarities between two countries were found in the rest of the proposed paths. Based on the findings, discussions and implications were provided.
Journal of The Textile Institute - J TEXT INST. 01/2012;
[show abstract][hide abstract] ABSTRACT: We demonstrate enhanced light emission in blue light-emitting diodes (LEDs) by multiple Mie scattering from embedded silica nanosphere stacking layers (SNSL). A honeycomb cone structure is introduced in the GaN epilayer to confine a maximum number of silica nanospheres (SNs). We found that the light is predominantly directed vertically by scattering and geometrical effect in SNSL embedded LEDs. Consequently, the light output power is enhanced by 2.7 times, which we attribute to the improvement in light extraction efficiency due to the multiple Mie scattering of light from the embedded SNSL. The experimental results are verified by simulation using finite difference time domain method (FDTD).
[show abstract][hide abstract] ABSTRACT: We demonstrate the performance improvement of GaN-based light-emitting diodes (LEDs) using zinc oxide (ZnO) nanoparticles inserted between the p-GaN and the indium tin oxide (ITO) layers. Upon deposition of an ITO film over the dispersed ZnO nanoparticles, the ITO surface tends to attain a nano-rough morphology due to the presence of ZnO nanoparticles. The light output power of the fabricated LEDs with ZnO nanoparticles is 39% higher than that of conventional LEDs at an injection current of 20 mA. This is attributed to the improved light extraction favored by the light scattering tendency of ZnO nanoparticles and the nano-roughened ITO film. In addition, the intermediate refractive index (n ∼2) of ZnO materials between those of the p-GaN (n ∼2.5) and the ITO (n ∼1.9) results in a broader critical angle and a reduction of total internal reflection.
Journal of Applied Physics 05/2011; 109(9):093116. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Purpose – The purpose of this article is to assist US apparel firms to successfully market in China. It aims to accurately predict Chinese consumers' purchase intentions toward a US brand. For this purpose, it proposes a composite model incorporating two behavioral intention models, Lee's model and decomposed Ajzen's model, and empirically tests the composite model. Design/methodology/approach – A total of 747 consumer data were collected in three cities of China, Shanghai, Beijing, and Guangzhou, utilizing a series of mall intercept method at two levels of shopping malls in each city. Findings – Among the proposed direct paths to purchase intention (PI), attitude toward the US brand apparel was the most important in explaining contemporary Chinese consumers' purchase intentions, followed by external perceived behavioral control (PBC) and subjective norm (SN). Two Confucian values, face saving and group conformity, did not directly affect PI at all, and face saving only influenced PI indirectly through the increase of attitude toward US brand apparel. Among external PBC (one's controllability toward a purchase) and internal PBC (one's self-efficacy), only external PBC led to PI. Practical implications – By examining the two aspects of PBC, the study validated the importance of consumers' resource controllability (i.e. external PBC) in Chinese purchase behaviors in the midst of rapid socioeconomic changes. This approach advances previous studies that primarily viewed PBC as one dimension. Originality/value – The paper is one of earliest endeavors to consider the decomposed PBC in a composite model. Also, the paper provides early proof that external PBC is more important than internal PBC in purchase contexts.
Journal of Consumer Marketing 05/2011; 28(3):187-199.
[show abstract][hide abstract] ABSTRACT: The US apparel and textiles industry operates within an interdependent global system, necessitating workforces competent for day‐to‐day operations. The US workforce lacks preparedness in working globally; this study tests an infusion method of curriculum internationalisation to enhance students' global understanding. Four cognitive and experiential instructional modules focused on China were developed and infused into four existing courses at a US university. Pre‐ and post‐test assessment of students' understanding in each module revealed that the number of correct answers was increased nearly fifteen percent across modules after implementation. The developed modules and implementation were assessed very favourably by the participants.
Innovations in Education and Teaching International 05/2011; 48(2):171-181. · 0.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the spectral tunability of white light by localized surface plasmon (LSP) effect in a colour converting hybrid device made of CdSe/ZnS quantum dots (QDs) integrated on InGaN/GaN blue light-emitting diodes (LEDs). Silver (Ag) nanoparticles (NPs) are mixed with QDs for generating LSP effect. When the plasmon absorption of Ag NPs is synchronized to the QW emission at 448 nm, the NPs selectively absorb the blue light and subsequently enhance the QD emission. Using this energy transfer scheme, the (x, y) chromaticity coordinates of the hybrid white LED was tuned from (0.32, 0.17) to (0.43, 0.26), and thereby generated warm white light emission with correlated colour temperature (CCT) around 1800 K. Moreover, a 47% enhancement in the external quantum efficiency (EQE) was realized.
[show abstract][hide abstract] ABSTRACT: The various surface texturing effects of InGaN light emitting diodes (LEDs) have been investigated by comparison of experimented data and simulated data. The single-layer and double-layer texturing were performed with the help of ITO nanospheres using wet etching, where the ITO ohmic contact layer and the p-GaN layer are textured using ITO nanospheres as an etch mask. In case of single-layer texturing, p-type GaN layer texturing was more effective than ITO ohmic contact layer texturing. The maximum enhancement of wall-plug efficiency of double-layered textured LEDs is 40% more than conventional LEDs, after packaging at an injected current of 20 mA. The increase of light scattering at the textured GaN surfaces is a major reason for increasing the light extraction efficiency of LEDs.