Search

This study demonstrated InGaN-based flexible RGB micro-light-emitting diodes (μLEDs) with size ranging from 20 to 100 μm through laser liftoff and UV-tape-assisted transfer process. The fabrication process of flexible RGB μLEDs released the stress of GaN films, which reduced the bending of energy band and screened the quantum-confined Stark effect in InGaN quantum wells based on theoretical simulation. Thus, a clear blue shift of peak wavelength was observed especially for red μLEDs. The electrical and electroluminescent performance, such as forward voltage, peak wavelength, and full width half maximum of flexible RGB μLEDs, could remain approximately identical under different bending radii from 3 to 7 mm. We realized the monolithic integration of flexible RGB μLEDs and obtained a wide color gamut that covered around 78% of the Rec. 2020 at bending conditions. [ABSTRACT FROM AUTHOR]

Microscopic lattice vibration images of the E2(high) mode (E2H) and another mode of A1(LO) (A1L) or the higher energy branch of LO‐phonon−plasmon coupling mode (LOPC+) in a Ga0.95In0.05N film on a GaN template are obtained by Raman scattering spectroscopy using a 325 nm laser. The increase in temperature by increasing the laser power is obtained from the decrease in the energy of E2H and the theoretical formula comprising two terms based on the mode energy variation of the bulk material and the thermal strain effect. Using the obtained temperature and the energy shift of the LOPC+, the mapping images of the temperature and electron density in the x–y plane are simultaneously obtained. This image provides the spatial variation of photoluminescence (PL) emission efficiency, given as PL intensity per electron. This method enables the quantitative discussion on photo‐emission efficiency even in the regions of low or high carrier density affected by carrier transport. In the investigated area, a region with a lower PL efficiency is found despite a higher electron density and lower temperature increase than the surrounding region. This imaging analysis is feasible in integrating the carrier and thermal energy transports and recombination processes in carrier dynamics study. [ABSTRACT FROM AUTHOR]

To investigate nonradiative recombination processes in indium gallium nitride (InGaN)‐based red light‐emitting diodes (LEDs), an InGaN‐based red LED with a hybrid quantum well (QW) structure consisting of red and blue single quantum wells (SQWs) is characterized by micro‐photoluminescence (μ‐PL) spectroscopy. The μ‐PL mapping of the red emission reveals numerous dark spots with various sizes and contrasts. Not only the red and blue (from a blue SQW) but green emission bands are observed at some red dark spots, suggesting that indium (In) segregation is one of the causes of nonradiative recombination in the red emission. Comparing the blue and green emission images to the red emission image reveals that the dark spots in the intensity map of the red emission can be classified into four types. Through this correlative analysis, the red dark spots associated with the dark areas in the intensity map of the blue emission are attributed to the major nonradiative recombination centers in the red emission. [ABSTRACT FROM AUTHOR]

Herein, a selective passivation of p‐GaN via hydrogen plasma treatment for InGaN single‐quantum‐well (SQW) red light‐emitting diodes (LEDs) is reported. Insulating regions are formed on the p‐GaN top surface via hydrogen plasma treatment, suppressing current injection beneath the p‐pad and along the mesa perimeter to increase light output and mitigate non‐radiative recombination. The fabricated LEDs demonstrate a high on‐wafer light output power density of >88 mW cm−2, a peak on‐wafer external quantum efficiency of 0.65%, and on‐wafer wall‐plug efficiency of 0.41% with a 645 nm peak emission wavelength at 10 mA (7.2 A cm−2) current injection. Further, the temperature dependence of InGaN SQW red LEDs is compared with their AlGaInP counterparts. InGaN SQW red LEDs exhibit a high characteristic temperature of 208 K and a small redshift coefficient of 0.072 nm K−1 at 72 A cm−2 current injection, which are almost 3 and 2 times better than the characteristics of AlGaInP red LEDs, respectively. [ABSTRACT FROM AUTHOR]

The use of μLEDs in self‐luminous displays is crucial for the development of high‐efficiency displays for virtual space services. For this purpose, a stacked monolithic GaInN‐based micro light‐eimtting diodes (μLED) device that emits red, green, and blue (RGB) light is obtained. The RGB layers are connected via tunnel junction (TJ) layers. The pixel density is 330 ppi (35 × 15 μm2 of μLED mesa area corresponding to the emission). The cross‐sectional transmission electron microscopy analysis indicates that there is not a significant increase in threading dislocations even after stacking the three RGB active layers and two TJ layers. In addition, the X‐ray diffraction reciprocal lattice space mapping shows that all the layers grow almost coherently with respect to the GaN substrate. A detailed evaluation of the resulting device shows that the BT.2020 color gamut coverage reaches a maximum of 71%, and the luminance is sufficiently high for head‐mounted display applications. Redshifting of the emission wavelength of the red μLEDs by increasing the InN mole fraction in the GaInN active layer would likely lead to improvement of the color gamut coverage in future studies. [ABSTRACT FROM AUTHOR]

The pump and probe technique in Raman spectroscopy of the E2 (high) mode is exploited to uncover the enhancing factor of the phonon transport across Ga1−xInxN/GaN interfaces. Two samples are investigated: one with a uniform x of 0.09 and another one with a graded variation in x from 0.17 to 0 along the depth direction. Lateral phonon transport is obtained by scanning the 532-nm probing laser from the irradiation position of the 325-nm heating laser. No difference in the lateral diffusion length is observed between the two samples, while the transport probability across the interface is higher for the sample with the graded variation in x than the sample with the uniform x of 0.09. The microscopic images of the decrease in the mode energy or the increase in temperature of the GaN layer reveal that the local phonon transport across the heterointerface is enhanced in regions with low differences in the phonon mode energy between the GaN and GaInN rather than the difference in crystal quality. [ABSTRACT FROM AUTHOR]

We have investigated the use of nanopillar structures on high indium content InGaN amber multiple quantum well (MQW) samples to enhance the emission efficiency. A significant emission enhancement was observed which can be attributed to the enhancement of internal quantum efficiency and light extraction efficiency. The size-dependent strain relaxation effect was characterized by photoluminescence, Raman spectroscopy and time-resolved photoluminescence measurements. In addition, the light extraction efficiency of different MQW samples was studied by finite-different time-domain simulations. Compared to the as-grown sample, the nanopillar amber MQW sample with a diameter of 300 nm has demonstrated an emission enhancement by a factor of 23.8.

We report the growth of N-polar InGaN layers on misoriented ScAlMgO4 (SAM) substrates with offset of 0.3 to 5.8° toward the m-plane. The surface of N-polar InGaN with small-offset substrates exhibited hexagonal hillocks similar to those commonly observed in N-polar GaN layers. Larger misorientation angles resulted in smoother surfaces of the InGaN layers. In contrast, the crystalline quality of InGaN indicated an opposite trend with significantly improved quality observed at smaller misorientation angles. We obtained an unprecedented crystalline quality of N-polar InGaN using SAM substrates with a 0.5° offset, which exhibited a 000 2 ¯ X-ray rocking curve full width at half maximum value of 223 arcsec. The crystalline quality and surface morphology of InGaN were significantly influenced by the step surface of substrates according to atomic force microscopy observations. [ABSTRACT FROM AUTHOR]