The developments of high performance 1-10 micron size InGaN based RGB MicroLEDs are discussed. Through novel epitaxial growth and processing, and transparent packaging we have achieved external quantum efficiencies as high as 58% EQE at 450nm for microLEDs. The critical challenges of µLEDs, namely full-color scheme, decreasing pixel size and mass transfer technique, and their potential solutions are explored. Recently, we have demonstrated efficient microLEDs emitting in the blue to red at dimensions as small of 1 micron. Using strain relaxation methods we have also extending the wavelength range of the InGaN alloys as into the red with emission as long as 640nm. Red InGaN based red MicroLEDs with efficiencies of 6% has been fabricated, and they display superior temperature performance in comparison to AlGaInP based devices. This work was supported by the Solid State Lighting and Energy Electronics Center(SSLEEC) at UC Santa Barbara.
GaN based edge-emitting lasers traditionally rely on index contrast from InGaN or AlGaN cladding for modal confinement. However, defects introduced by lattice mismatch limit the feasible composition and layer thickness of alloyed cladding layers. Alternatively, porous GaN offers high refractive index contrast while remaining lattice-matched, making it a suitable candidate for cladding in green lasers. Edge-emitting laser diodes with nano-porous cladding were fabricated and reached electrical injection at 524 nm. In this work, a deep ridge waveguide structure was used to improve electrochemical etch selectivity of the porous cladding, resulting in higher efficiency and lower loss compared to shallow ridge devices.
Achieving high quantum efficiency in long-wavelength nitride LEDs has proved challenging due to stress induced from high In%, piezoelectric polarization, and defect generation. A leading technique for increasing the efficiency in long-wavelength InGaN is V-defect engineering which uses naturally occurring pyramidal (10-11) defects to achieve deeper hole injection through lateral pathways on the semi-polar sidewalls of the V-defects. In this work we demonstrate V-defect engineered red LEDs achieving a peak EQE of 6.5% at 600 nm on patterned sapphire. We discuss formation and distribution of V-defects as well as luminescence properties and efficiency of InGaN red LEDs.
In this work, the device performances of three laser architectures are examined. All the laser epitaxial structures are grown on freestanding m-plane GaN substrates by Metalorganic Chemical Vapor Deposition (MOCVD), with a peak emission wavelength of about 405 nm. The three laser device architectures are shallow-etch ridge design, which the ridges are defined by etching into the p-GaN layer and not through the active region, and the deep-etch ridge structure that etches through the active region with or without Atomic Layer Deposition (ALD) sidewall passivation. By utilizing ALD sidewall passivation, the optical and electrical characteristics show significant improvements than the other two device designs.
The developments of high performance InGaN based micro-light-emitting diodes (µLEDs) are discussed. Through novel epitaxial growth and processing, and transparent packaging we have achieved external quantum efficiencies as high as 58% EQE at 450nm for MmicroLEDs. The critical challenges of µLEDs, namely full-color scheme, decreasing pixel size and mass transfer technique, and their potential solutions are explored. Recently, we have demonstrated efficient microLEDs emitting in the blue to green at dimensions as small of 1 micron. Red InGaN based red MicroLEDs with efficiencies of 2.5% has also been fabricated.
A fluidic assembly of III-nitride micro-light-emitting diodes (μLEDs) is demonstrated by applying external forces. This work introduces a solution-based mass transfer approach for inorganic semiconductor devices with the advantages in being material- and dimensional-agnostic. This method offers precise control in device alignment and orientation. The alignment of μLEDs in solution is controlled by acoustic focusing and the induced pressure forces, and the device orientation is modulated using magnetic field. The detailed mechanisms of the use of acoustic focusing and magnetic field are discussed. This fluidic assembly technique allows high throughput rate and roll-to-roll manufacturing, which is critical to the mass transfer of μLEDs for display and functional fabric applications.
InGaN based blue and green LEDs are quite attractive owing to their high efficiency, low power consumption, and high performance even at scaled dimensions. However, for red emission higher indium content in the QWs leads to poor performance due to increased strain in the structure. We will discuss a novel elastic strain relaxation technique using porous GaN pseudo-substrates which will allow enhanced indium uptake in the quantum wells for high-efficiency red InGaN micro-LEDs outperforming AlInGaP based micro-LEDs. Results including green and orange luminescence from micro-LEDs (<5 µm) fabricated on these pseudo-substrates will be discussed.
The reduction in efficiency by shrinking device dimensions of micro-light-emitting diodes (μLEDs) has been identified as one of the main drawbacks in the literature. The decrease in efficiency is attributed to surface recombination and sidewall damage due to dry etching, where the efficiency drop is more severe in the AlGaInP material system because of the higher surface recombination velocity and the greater minority carrier diffusion length, compared to the III -nitride system. In this work, the device performance with and without dielectric sidewall passivation using plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD) are first compared, since PECVD is the common method for dielectric sidewall passivation. It is shown that ALD is more effective in terms of suppressing leakage current and enhancing light output power. Moreover, the efficiency of devices with ALD sidewall passivation is partially recovered, indicating that the efficiency drop in μLEDs can be lessened or mitigated by post-etch fabrication techniques. To further improve the efficiency characteristic, the combination of chemical treatments and ALD sidewall passivation are employed to demonstrate size-independent efficiency performance of III-nitride and AlGaInP μLEDs.
We examine the MOCVD growth conditions on c-plane semi-relaxed InGaN substrates necessary for morphological improvement, defect reduction, and elimination of V-pits during epitaxy prior to the active region growth. V-pit defects can propagate through the crystal as epitaxy continues, causing serious morphological degradation. These defects may also be a source of leakage current if they form a low-resistance path through p-n junction. By employing an InGaN/GaN periodic structure, thick base layers can be grown with the morphology improving as the epitaxy proceeds, allowing for high quality layers to be achieved. High temperature (HT) GaN interlayers in the InGaN/GaN base layer structure then continue to reduce defects significantly, notably eliminating the V-pit type defect, and significantly improving growth morphology. Resulting microLEDs on these improved base layers exhibit a nearly three order of magnitude reduction in leakage current density at 1V, far below the μLED turn-on threshold, and significantly lower dynamic resistance. This result indicates the reduction in base layer defects and layer morphology improvement results in significant improvement in electrical performance and enables production of viable LEDs.
A novel approach to realize DFB gratings on GaN based laser diodes is presented and single longitudinal mode operation is achieved. For lasers with plasma-etched surface gratings, single mode operation was maintained until 900 mA and the spectral width FWHM was less than 5 pm with a SMSR of more than 29 dB. Moreover, several issues limiting the performance of semipolar III-Nitride DFB laser diodes with the etched grating are also addressed in this work. Besides these first order gratings that were formed by electron beam lithography and shallow plasma etching, an improved grating design based on dielectric teeth imbedded into ITO is described, along with the design’s impact on power and spectral performance. Particularly, by utilizing the HSQ resist, we focus on reducing the high operating voltage by imbedding the grating inside the transparent conductive oxide layer without dry etching. This new design with a non-etched imbedded grating successfully reduces the threshold voltage and achieves an output power of more than 200 mW under pulsed operation from an anti-reflection coated facet.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.