Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well

III-nitride nanowire light-emitting diodes: design and characterization

III-nitride semiconductors have been intensively studied for optoelectronic devices, due to the superb advantages offered by this materials system. The direct energy bandgap III-nitride semiconductors can absorb or emit light efficiently over a broad spectrum, ranging from 0.65 eV (InN) to 6.4 eV (AlN), which encompasses from deep ultraviolet to near infrared spectrum. However, due to the lack of native substrates, conventional III-nitride planar heterostructures generally exhibit very high dislocation densities that severely limit the device performance and reliability. On the other hand, nanowire heterostructures can be grown on lattice mismatched substrates with drastically reduced dislocation densities, due to highly effective lateral stress relaxation. Nanowire light-emitting diodes (LEDs) with emission in the ultraviolet to visible wavelength range have recently been studied for applications in solid-state lighting, flat-panel displays, and solar-blind detectors. In this thesis, investigation of the systematic process flow of design and epitaxial growth of group III-nitride nanoscale heterostructures was done. Moreover, demonstration of phosphor-free nanowire white LEDs using InGaN/AlGaN nanowire heterostructures grown directly on Si(111) substrates by molecular beam epitaxy was made. Full-color emission across nearly the entire visible wavelength range was realized by controlling the In composition in the InGaN active region. Strong white-light emission was recorded for the unpackaged nanowire LEDs with an unprecedentedly high color rendering index of 98. Moreover, LEDs with the operating wavelengths in the ultraviolet (UV) spectra, with emission wavelength in the range of 280-320 nm (UV-B) or shorter wavelength hold tremendous promise for applications in phototherapy, skin treatments, high speed dissociation and high density optical recording. Current planar AlGaN based UV-B LEDs have relatively low quantum efficiency due to their high dislocation density resulted from the large lattice mismatch between the AlGaN and suitable substrates. In this study, associated with the achievement of visible LEDs, the development of high brightness AlGaN/GaN nanowire UV-LEDs via careful design and device fabrication was shown. Strong photoluminescence spectra were recorded from these UV-B LEDs. The emission peak can be tunable from 290 nm to 320 nm by varying the Al content in AlGaN active region which can be done by optimizing the growth condition including Al/Ga flux ratio and also the growth temperature. Such visible to UV-B nanowire LEDs are ideally suited for future smart lighting, full-color display, phototherapy and skin treatments applications

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications

EFFICIENCY AND RADIATIVE RECOMBINATION RATE ENHANCEMENT IN GAN/ALGAN MULTI-QUANTUM WELL-BASED ELECTRON BLOCKING LAYER FREE UV-LED FOR IMPROVED LUMINESCENCE

In this paper, an electron blocking layer (EBL) free GaN/AlGaN light emitting diode (LED) is designed using Atlas TCAD with graded composition in the quantum barriers of the active region. The device has a GaN buffer layer incorporated in a c-plane for better carrier transportation and low efficiency droop. The proposed LED has quantum barriers with aluminium composition graded from 20% to ~2% per triangular, whereas the conventional has square barriers. The resulted structures exhibit significantly reduced electron leakage and improved hole injection into the active region, thus generating higher radiative recombination. The simulation outcomes exhibit the highest internal quantum efficiency (IQE) (48.4%) indicating a significant rise compared to the conventional LED. The designed EBL free LED with graded quantum barrier structure acquires substantially minimized efficiency droop of ~7.72% at 60 mA. Our study shows that the proposed structure has improved radiative recombination by ~136.7%, reduced electron leakage, and enhanced optical power by ~8.084% at 60 mA injected current as compared to conventional GaN/AlGaN EBL LED structure

III-Nitride Nanocrystal Based Green and Ultraviolet Optoelectronics

Extensive research efforts have been devoted to III-nitride based solid-state lighting since the first demonstration of high-brightness GaN-based blue light emitting diodes (LEDs). Over the past decade, the performance of GaN-based LEDs including external quantum efficiency (EQE), wall-plug efficiency, output power and lifetime has been improved significantly while the cost of GaN substrate has been reduced drastically. Although the development of blue and near ultraviolet (UV) LED is mature, achieving equally excellent performance in other wavelengths based on III-nitrides is still challenging. Especially, the significant efficiency droop in the green wavelength, known as “green gap” and the extremely low EQE in the UV regime, known as “UV threshold”, have become two most urgent issues. Green LEDs emit light that is most sensitive to human eye, implicating its importance in a variety of applications such as screen- and projection-based displays. UV light sources have a variety of applications including water and air purification, sterilization/disinfection of medical tools, medical diagnostics, phototherapy, sensing, which make solid-state deep UV (DUV) light sources with compactness, low operating power and long lifetime highly desirable. The deterioration of performance with green LEDs originates from increased indium content of the active region, which could degrade material quality and increase quantum confined Stark effect due to the high polarization fields in c-plane InGaN/GaN quantum wells (QWs). Meanwhile, limiting factors in III-nitride UV LEDs include low internal quantum efficiency due to large densities of dislocations, poor carrier injection efficiency and low light extraction efficiency. In this dissertation, we have investigated the molecular beam epitaxial growth, structural characterization, and electrical and optical properties of low-dimensional III-nitride nanocrystals as potential solutions to above-mentioned issues. Through a combination of theoretical calculation and experimental investigation, we show that defects formation in AlN could be precisely controlled under N-rich epitaxy condition. With further optimized p-type doping, AlN nanowire-based LEDs emitting at 210 nm were fabricated. We report DUV excitonic LEDs with the incorporation of monolayer GaN with emission wavelengths of ~238 nm, and exhibit suppressed Auger recombination, negligible efficiency droop and a small turn on voltage ~5 V. To enhance the light extraction efficiency of AlGaN nanowires grown on Si substrate, we demonstrated epitaxy of AlGaN nanowires on Al coated Si(001) substrate wherein Al film functions as a UV light reflective layer to enhance the light extraction efficiency. AlGaN nanowire-based DUV LEDs on Al film were successfully grown and fabricated and measured with a turn-on voltage of 7 V and an electroluminescence emission at 288 nm. Green-emitting InGaN/GaN nanowire LEDs on Si(001) substrate were demonstrated, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers. The incorporation of planar p-GaN layer significantly reduces the fabrication complexity of nanowire-based devices and improves the robustness of electrical connection, leading to a more stable device operation. We also demonstrated micrometer scale InGaN photonic nanocrystal green LEDs with ultra-stable operation. The emission features a wavelength of ~548 nm and a spectral linewidth of ~4 nm, which is nearly five to ten times narrower than that of conventional InGaN QW LEDs in this wavelength range. Significantly, the device performance, in terms of the emission peak and spectral linewidth, is nearly invariant with injection current. Work presented in this thesis provides a new approach for achieving high-performance green and DUV LEDs by using III-nitride nanostructures.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163122/1/ypwu_1.pd

Beyond solid-state lighting: Miniaturization, hybrid integration, and applications og GaN nano- and micro-LEDs

Gallium Nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting and ubiquitous white light sources has developed around the inception of the Nobel-price-winning blue GaN LEDs. Today GaN optoelectronics is developing beyond lighting, leading to new and innovative devices, e.g. for micro-displays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (microLEDs and nanoLEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN microLEDs with silicon microelectronics in a fully hybrid approach. GaN LED technology today is even spreading into the realm of display technology, which has been occupied by organic LED (OLED) and liquid crystal display (LCD) for decades. In this review, the technological transition towards GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art

Design, Growth, and Characterization of III-Sb and III-N Materials for Photovoltaic Applications

abstract: Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps. III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells. III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of ii InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation. In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

μLEDs for optogenetics

Optogenetics is unfolding new ways for us to study the nervous system and could one day be a standard approach to treat neurological diseases like epilepsy. To selectively study the effects on a subcellular level, microscopic light sources are needed. Nanostructure, light-emitting diodes (LEDs) can realize this criteria but processing to connect and protect them is necessary before any fruitful optogenetic tests can be conducted. In this work, micron sized, III-nitride, LED light sources were created using microfabrication techniques such as lithography, etching and thin film deposition. Experimental biointegration and passivation schemes were then used to build a prototype optogenetic device for stimulation of primary neurons grown [i]in vitro[/i] onto the device, in close proximity to the light emitters. Favorable electrical and optical characteristics were obtained for the individual nanostructure LEDs, lighting up brightly at a wavelength around 470 nm. However, larger devices revealed process related and uniformity challenges to overcome. Additionally, the biointegration design would prove too complex and in need of further improvement. This effort, while not outputting a fully functioning device, has contributed to development of the utilized nanostructure LED technology so that we may see more of it in the future.Imagine if I said there was a way to control brain cells with light. You might first think of the scary mind control applications but would you also consider the potential to one day eradicate neurological diseases like epilepsy? Optogenetics is a fairly new technique in medical science and it is still a long way away from fulfilling either of these scenarios but that makes it no less interesting.[/b] Today, optogenetics allow researchers to control nerve impulses by simply shining a light on cells that have been genetically modified with light sensitive properties of fluorescent algae. A common practice in optogenetics is to make cells sensitive to blue light and as luck would have it, blue light-emitting diodes, or LEDs for short, are relatively mature and straight forward to make with high quality. However, to study optogenetic effects subcellulary, for example how stimulation affects individual synapses, light sources would have to be microscopically tiny and this is where we come in. By using tapered hexagonal platelet, gallium nitride μLEDs, less than 1 μm in diameter, situated on a small sapphire chip, we set out to make a prototype device for high resolution optogenetics. LEDs were processed in Lund Nano Lab using microfabrication equipment for lithography, etching and thin film deposition before being characterized in a probe station rig. As we also wanted to be able to test actual nerve cell stimulation, we attempted to package the LEDs and passivate them for a biological environment with conducting fluids and sensitive nerve cells, which would have been grown directly onto the device, in close proximity too the LEDs. Initial testing of the single platelet LEDs showed very promising electrical properties such as the clearly rectifying diode behavior in addition to a rather extraordinary visible light output for such small light source. Continued testing though, revealed short circuiting issues for larger LEDs with several platelets being coupled together in parallel. These issues could be explained by minute variations in original platelet height and be amended with future processing tweaks. Furthermore, actual optogenetic testing had to be abandoned as the complex packaging scheme, featuring thin film oxide passivated, wire bonds, would end up malfunctioning, suggesting a redesign is needed to remove unnecessary points of failure. While we did not fully actualize the very ambitious goals we set out to achieve, our findings have undoubtedly aided in the understanding and fixing of issues with the platelet μLED technique so that development of it can progress. In a broader perspective, the technologies we explored are still highly interesting, combined and individually. Development of smaller LEDs and their use in more and more impressive optogenetic studies are published on a regular basis and inorganic μLED products are even starting to find their way onto the consumer electronics market in direct emitting, high resolution displays. To conclude, I am certain that even if this short text would have been the first time you heard about these topics, it will definitely not be the last