Using GaN-based light-emitting diodes (LEDs) as a radio source for visible light communication (VLC) is one of alternative choice in a high-speed data system. However, the spontaneous radiative recombination lifetime in the multiple quantum wells (MQWs) usually restrict the modulation bandwidth of LEDs. For LEDs accompanied photonic crystal (PhC) structure, the guided photonic modes can be extracted with a shorter radiative recombination lifetime; therefore, improve the performance of the devices for VLC. In this paper, we compare various PhC structures with corresponding dynamic behaviors in both small- and large-signal modulation. Faster transient responses and higher efficiency of the out-coupled modes were obtained in the room-temperature time-resolved photoluminescence (TRPL) and Raman scattering measurement. Here, sub-GHz modulation of GaN-based PhCLED is demonstrated, and the PhCLED exhibits a higher bandwidth than the conventional LED structure. Our study also indicates that we can not just keep scaling down the masa size of LEDs to increase the operation frequency owing to the light output power may become dull and reduce the performance of VLC system
As internet of things (IOT) has become a popular topic in current consumer electronics, there is a demand for cost-effective
sensors to monitor bio-signals. Traditional optical sensors employ low-dimensional gratings and high-resolution
spectrometers to detect the refractive index changes of the solutions. In this work, we develop an alternative
approach to correlate the concentration of molecules to the band diagrams of the photonic crystals. A relatively low-resolution
spectrum analyzer can be employed, yet achieves higher sensitivity than traditional approaches.
Light emitting diodes (LEDs) for visible light communication (VLC) as radio sources is a solution to channel crowding
of radio frequency (RF) signal. However, for the application on high-speed communication, getting higher bandwidth of
LEDs is always the problem which is limited by the spontaneous carrier lifetime in the multiple quantum wells. In this
paper, we proposed GaN-based LEDs accompanied with photonic crystal (PhC) nanostructure for high speed
communication. Using the characteristic of photonic band selection in photonic crystal structure, the guided modes are
modulated by RF signal. The PhC can also provide faster mode extraction. From time resolved photoluminescence (TRPL)
at room temperature, carrier lifetime of both lower- and higher-order modes is shortened. By observing f-3dB -J curve, it
reveals that the bandwidth of PhC LEDs is higher than that of typical LED. The optical - 3-dB bandwidth (f-3dB) can be
achieved up to 240 MHz in the PhC LED (PhCLED). We conclude that the higher operation speed can be obtained due to
faster radiative carrier recombination of extracted guided modes from the PhC nanostructure.
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.
In this work, the angular light output enhancements of LEDs were investigated from the spontaneous emission and light scattering of devices with different photonic crystal (PhC) geometries. The emitted photon coupled into a leaky mode is differentiated by the manipulation of the quality factor in various spatial frequencies. Therefore, light extraction in this light-emitting device is determined by the modal extraction lengths and the quality factor obtained from the measured photonic bands. Furthermore, the higher- and lower-order mode spontaneous emissions are affected by the nonradiative process in the PhC structures with different periods. In our cases, the photonic crystal device with the largest period of 500 nm exhibits the highest lower-order mode extraction and quality factor. As a result, a self-collimation behavior toward the surface-normal is demonstrated in the 3D far-field pattern of such a device. We conclude that, with the coherent light scattering from the PhC region, the spontaneous emission of the material and spatial behavior of the extracted mode can be both managed by the proper design of the device.
Tumor detection is a significant health issue, but it is still a limit to identify cancer cells during tumor resection by using traditional methods such as fluorescence. In this study, zinc oxide (ZnO) nanorods bonded to antibodies was
investigated as nanoprobes for sensing cancer cells. The result shows that antibodies toward epidermal growth factor
receptor (EGFR) can be connected to ZnO nanorods and EGFR receptors of squamous cell carcinoma (SCC). The cancer
cells can be recognized via the observation of purple light emission from these probes by using naked eye or an optical microscope. By contrast, the HS68 cells with less EGFR expression had no purple light emission as the probes were washed off. Besides, from the photoluminescent spectra, the intensity ratio between the purple light (from ZnO nanorods) and green band (from the autofluorescence of cells) is much higher in SCC than in HS68 cells, which suggest that the cancer cells can be detected by comparing the peak intensity ratio. The probes have the potential clinical application for real-time tumor detection, and the cancer cells can be excised more precisely with the help of purple light emission.
Two comparative blue emitting InGaN/GaN multiple quantum well (MQW) structures, for lighting and laser diode
applications, with and without pre-strained layer, were grown by MOCVD. Temperature dependent photoluminescence
(TDPL) and time-resolved (TR) PL were used to study their optical and transient properties. PL signals from InGaN
MQWs were divided into two parts: one is the band to band transition of InGaN; the other is the broad defect band. It is
indicated that the InGaN/GaN MQW structure with prestrained layer has larger activation energy. TRPL measurements
were performed in 10-300 K and with the detection wavelength cross over the emission peak. It is found that the MQW
sample with prestrained layer has deeper localization depth. Temperature dependence of PL decay time shows an
interesting behavior of an increase from 10K to 30K and then a decrease till 300K.
Photonic crystals (PhCs) were typically fabricated on the light emitting surface of light emitting
diodes (LEDs) to improve light extraction, which is regarded as the weak coupling between the
laterally propagated light in the epi-layers and the surface nanostructure. This work demonstrates
GaN-based LEDs with the PhC structure on the mesa surface and nanohole arrays surrounding the light
emitting mesa. Our new device (SHLED) shows a 56% higher optical output power than the planar
structure (PLED), as compared with the 40% improvement of the surface PhC device (SLED) over
PLED. The output power of SHLED is higher than that of SLED due to the enhanced diffraction of low
order modes propagated in the lateral direction, in addition to the higher order mode light diffraction
from the surface PhCs. From the relative angular spectra, the interaction of in-plane optical wave with
the nanoholes (which are etched through MQWs) is much stronger than that with surface PhCs,
suggesting an efficient light diffraction to the surface normal by nanoholes.
For InGaN/GaN based nanorod devices using top-down etching process, the optical output power is affected by
non-radiative recombination due to sidewall defects (which decrease light output efficiency) and mitigated quantum
confined Stark effect (QCSE) due to strain relaxation (which increases internal quantum efficiency). Therefore, the
exploration of low-temperature optical behaviors of nanorod light emitting diodes (LEDs) will help identify the
correlation between those two factors. In this work, low-temperature EL spectra of InGaN/GaN nanorod arrays was
explored and compared with those of planar LEDs. The nanorod LED exhibits a much higher optical output percentage
increase when the temperature decreases. The increase is mainly attributed to the increased carriers and a better spatial
overlap of electrons and holes in the quantum wells for radiative recombination. Next, while the nanorod array shows
nearly constant peak energy with increasing injection currents at the temperature of 300K, the blue shift has been
observed at 190K. The results suggest that with more carriers in the quantum wells, carrier screening and band filling
still prevail in the partially strain relaxed nanorods. Moreover, when the temperature drops to 77K, the blue shift of both
nanorod and planar devices disappears and the optical output power decreases since there are few carriers in the quantum
wells for radiative recombination.
Natural lithography is realized by spin-coating of nanoparticles. The surface textured LED s and
nanorod structured LEDs have been made successfully. Concentrated radiation profiles of nanorod structured and surface
textured LED are both observed. We fabricate these nano-structure by nature lithography method which use silica
nanosphere as etching mask. The SiO2 grown by PECVD is used as a space layer to realize the nanorod LED.
Furthermore, we suggest a light guided concept of the highly concentrated phenomena. The highest output power density
of nanorod LEDs is achieved. The reversed current of nanorod LEDs is also reduced down to nA level.
We propose an on-wafer heat relaxation technology by selectively ion-implanted in part of the p-type GaN to decrease
the junction temperature in the LED structure. The Si dopant implantation energy and concentration are characterized to
exhibit peak carrier density 1×1018 cm-3 at the depth of 137.6 nm after activation in nitrogen ambient at 750 °C for 30
minutes. The implantation schedule is designed to neutralize the selected region or to create a reverse p-n diode in the p-GaN layer, which acts as the cold zone for heat dissipation. The cold zone with lower effective carrier concentration and
thus higher resistance is able to divert the current path. Therefore, the electrical power consumption through the cold
zone was reduced, resulting in less optical power emission from the quantum well under the cold zone. Using the diode
forward voltage method to extract junction temperature, when the injection current increases from 10 to 60 mA, the
junction temperature of the ion-implanted LED increases from 34.3 °C to 42.3 °C, while that of the conventional one
rises from 30.3 °C to 63.6 °C. At 100 mA, the output power of the ion-implanted device is 6.09 % higher than that of the
conventional device. The slight increase of optical power is due to the increase of current density outside the cold zone
region of the implanted device and reduced junction temperature. The result indicates that our approach improves
thermal dissipation and meanwhile maintains the linearity of L-I curves.
Many researches report that the mobility in organic material is dependent on not only the gate field but also the grain size.
There is also some evidence to prove that the gate length is strongly related to the carrier mobility. We construct both the
analytical model of organic thin film transistor and the large signal circuit model designed by T-CAD to fit the measured
I DS - V DS curves. We first apply basic I DS - V DS equations in both triode and saturation regions with mobility μ best
fitted to measured I-V curves. The "best-fitted" μ increases with the gate length, and is related to the increase of total
channel resistance due to the presence of small grains size of pentacene next to source/drain electrodes. We then use the
Advanced Design System software to design the large signal circuit model. Similar to the MOSFET, we add the
additional parameters to fit the I DS - V DS curves, ex: Rgd, Rgs, and Rp. Here, Rgd. With the circuit simulation, we find
that Rgd presents the leakage current from gate to source, and it affects the slope of curves in the saturation region in the
I DS - V DS curves. The equivalent circuit can fit the I DS - V DS curves very well with the proper parameter set.
In this paper, we have demonstrated the current increase with repeated measurements of Id-Vds curves with different Vg values which results from the non-uniform carrier accumulation in the channel region of a pentacene-based thin film transistor (TFT). The mobility of our device reaches 0.07 cm2/Vs even the substrate was not heated during pentacene deposition. Besides, the devices show good air-stable properties. The magnitude of Id decreased less than 30% after exposure in air for 2 weeks. By repeating the Id-Vds measurements from 0 to -50 V with the Vg values of 0, -10, -20, -30, -40, and -50 V for 10 minutes, we observed a four times current increase from -0.75 to -2.8 μA at Vg = -50V and Vds = -50V. The current increase comes from the holes accumulation near the drain. When the source and drain were exchanged, the current decreases to the 0.08 μA. After another 10 minutes operation, the current will recover back to the original values. Such a process is reversible and shows the potential of the memory device base on this pentacene transistor.
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