In this study, we demonstrate the ability to exclude the thermal effect and detect the generation of non-thermal hot carriers by surface plasmon using an AlGaN/GaN high-electron-mobility transistor. We will also provide a theoretical model to explain the detecting mechanism. This proposed platform is very sensitive, which is at least two orders of magnitude more sensitive compared to the previous reports, can detect the hot carriers generated from discrete nanostructures illuminated by a continuous wave light. The quantitative measurements of hot carrier generation also open a new way to optimize the plasmonic nanoantenna design in many applications.
With the rapid development of GaN light-emitting diodes (LEDs), LEDs have been utilized in various ways. However, the quality of the GaN epi-structure has been a popular topic. In order to achieve higher internal quantum efficiency (IQE), LEDs have to be made with few defects during the epitaxy growth. Here we propose an AlN nanorod template grown on the sapphire substrate by vapor-liquid-solid (VLS) method. The voids near the AlN nanorods indicate a modification of dislocation with a lateral overgrowth. A strain relaxation and a better IQE in the epi-layer are observed in the Raman spectroscopy and temperature-dependent photoluminescence (PL). As a result, the IQE of the device with the proposed AlN nanorod template is increased 12.2% as compared with the reference sample without AlN nanorods.
GaN-based flip-chip light emitting diodes (FC-LEDs) with embedded air voids grown on a selective-area Arimplanted AlN/sapphire (AIAS) substrate was demonstrated in this study. The proposed FC LED with an embedded light scattering layer can destroy the light interference and thereby increase the LEE of GaN-based flip-chip LEDs. The epitaxial layers grown on Ar-implanted regions exhibited lower growth rates compared with those grown on implantation-free regions. Accordingly, air voids formed over the implanted regions after merging laterally grown GaN facet fronts. The light-output power of LEDs grown on AIAS was greater than that of LEDs grown on implantation free sapphire substrates. At an injection current of 700 mA, the output power of LEDs grown on AIAS was enhanced by 20% compared with those of LEDs without embedded air voids. The increase in output power was mainly attributed to the scattering of light around the air voids, which increased the probability of photons escaping from the LEDs. This study on FC LEDs with embedded light-scattering layer highlights the potential application of these LEDs as an alternative to conventional patterned sapphire substrates for improving the LEE of GaN/sapphire-based LEDs. Based on ray tracing simulation, if the height and the width of bottom of gaps were increased to 3 μm, the Lop could be enhanced over 60%.
In recent year, InGaN-based alloy was also considered for photovoltaic devices owing to the distinctive material properties which are benefit photovoltaic performance. However, the Indium tin oxide (ITO) layer on top, which plays a role of transparent conductive oxide (TCO), can absorb UV photons without generating photocurrent. Also, the thin absorber layer in the device, which is consequent result after compromising with limited crystal quality, has caused insufficient light absorption. In this report, we propose an approach for solving these problems. A hybrid design of InGaN/GaN multiple quantum wells (MQWs) solar cells combined with colloidal CdS quantum dots (QDs) and back side distributed Bragg reflectors (DBRs) has been demonstrated. CdS QDs provide down-conversion effect at UV regime to avoid absorption of ITO. Moreover, CdS QDs also exhibit anti-reflective feature. DBRs at the back side have effectively reflected the light back into the absorber layer. CdS QDs enhance the external quantum efficiency (EQE) for light with wavelength shorter than 400 nm, while DBRs provide a broad band enhancement in EQE, especially within the region of 400 nm ~ 430 nm in wavelength. CdS QDs effectively achieved a power conversion efficiency enhancement as high as 7.2% compared to the device without assistance of CdS QDs. With the participation of DBRs, the power conversion efficiency enhancement has been further boosted to 14%. We believe that the hybrid design of InGaN/GaN MQWs solar cells with QDs and DBRs can be a method for high efficiency InGaN/GaN MQWs solar cells.
In this study, hetero-structure p-i-n type epitaxy wafers were deposited on the GaN/sapphire templates with different
buffer layers by the MOVPE system. The absorption layers sandwiched in top p-GaN and bottom n+-GaN layers were
designed into different short-period InGaN/GaN superlattice structures with specific pair numbers to maintain a total
absorption thickness of 200 nm. As the buffer layer was properly adjusted, the VOC and JSC were enhanced by 35% and
95%, respectively. In addition to material qualities, the thickness of GaN buffer layers and piezoelectric-induced stain in
the InGaN film itself also influenced the PV device performance.
To enhance the efficiency of photogenerated electron collection in the n-type working electrode, Indium Tin oxide (ITO)
finger-type ohmic contacts were immersed in NaCl electrolyte because ITO is a well-known transparent and conductive
optical film and the ITO/n-GaN contact exhibited ohmic property when the carrier concentration of n-GaN were close to
1×1019/cm3. We found that the performances of the n-GaN photoelectrochemical cells with finger-type ITO ohmic
contacts in photocurrent densities and hydrogen gas generation rates were both better than the n-GaN without finger-type
ITO ohmic contacts. Related analyses have been performed and will be presented in this paper to explain the possible
mechanism from the point of view of electrochemical analysis. Besides, after the photoelectrochemical measurements
we observed that the adhesion of ITO/n-GaN contacts was pretty good. Finally, we did the surface analysis by scanning
electron microscope (SEM) before and after the photoelectrochemical measurements to conform the surface morphology
of ITO almost did not change in the NaCl electrolyte. This indicates that ITO is a good candidate material for the
immersed ohmic contact in water splitting system.
Optical and crystal properties of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical
vapor deposition (MOCVD) were characterized using room-temperature photoluminescence (PL) and high-resolution Xray
diffraction (HRXRD), respectively. The near bandgap excitonic peak decreased from 2.77 eV to 2.68 eV while there
was a 10 Å increase in the well thickness, probably caused by variations of quantized energy levels. In addition, higher
growth temperature of MQW structures had a small influence on the pair thickness, but the emission wavelength showed
a blueshift attributed to the decrease in average of indium mole fraction. However, the near bandgap excitonic peak
remained constant for the thicker quantum barriers. For the PL emission intensity of InGaN/GaN MQW structures, it was
enhanced with a thinner quantum well width and a thicker quantum barrier, which could be resulted from the
improvement of optical confinement in the quantum well. Moreover, by using the higher growth temperature, enhanced
PL intensity was achieved due to the improvement of structure quality for the InGaN/GaN heterostructure. Therefore,
these results suggest that the emission wavelength and intensity of the InGaN/GaN MQW-based optical device could be
modulated by designing thicknesses of quantum wells as well as growth temperatures of MQW structures.
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