Micro-size light-emitting diodes (μLEDs) have attracted huge attentions as the next-generation display technology for the wide applications, such as wearable watches, virtual/augmented reality, micro-projectors, and ultra-large televisions. InGaN LEDs or laser diodes with epitaxial tunnel junctions (TJs) are attractive due to several advantages, such as simple fabrication process, higher efficiency, and lower efficiency droop by improved current spreading, less loss. In the first part, high performance InGaN µLEDs with MOCVD-grown TJs were successfully demonstrated using selective area growth (SAG) and p+GaN/InGaN/n+GaN structure. SAG apertures provide paths for the out diffusion of H+ atoms, which passivated Mg atoms in the p-type GaN during the overgrowth of TJs. The forward voltage (Vf) in the SAG TJ µLEDs is independent on the device sizes, suggesting that the H+ atoms are effectively removed through the holes on top of the p-GaN surface. The Vf @ 20 A/cm2 in the TJ µLEDs utilizing SAG is significantly reduced to 3.24 V. Moreover, the output power of TJ µLEDs with SAG is ~10% higher than the common µLEDs with indium tin oxide (ITO) contact. The InGaN TJs µLEDs show a significant reduction of Vf by ~0.6 V compared to the common n-GaN TJs µLEDs. The external quantum efficiency (EQE) of the packaged TJ µLEDs was improved by 6% compared to the common µLEDs with common ITO contact. These demonstrations solve the key challenges of MOCVD-grown TJs. Then, we present fully MOCVD-grown InGaN cascaded µLEDs with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a TJ, and a green emitting diode. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently. Finally, we present the state-of-the-art InGaN-based red μLEDs, which have become research focus now in the whole nitride community. The common AlInGaP red μLEDs show a dramatical reduction of the EQE with decreasing the devices area and poor thermal property. We demonstrated several remarkable InGaN device performances with emission wavelength greater than 600 nm. First, we reported that the EQE of InGaN red µLEDs has less influence from the size effect due to the lower surface recombination velocity, compared to conventional AlInGaP red µLEDs. Moreover, we investigated the temperature-dependent electrical luminesce characteristics of InGaN red µLEDs, and we found that InGaN red µLEDs have superior device performances even operating at high temperature up to 400 K. We realized 5×5 µm2 InGaN devices with a peak EQE value of 2.6%. We also present red InGaN 60×60 µm2 µLED with SAG TJ contact, which show a peak EQE of 4.5% at 623 nm. These values about InGaN red µLEDs are one of the best reported among the literatures. A 568 nm stimulated optically lasing from our InGaN red MQWs was also achieved.

Solid-state lighting based on III-nitride by light emitting diodes (LEDs) has been developed as a high efficient light source and has been successfully commercialized in the market. In addition to the function of lighting, visible light communication (VLC) has gained momentum to solve the data traffic of the radio frequency (RF) wireless communication. Many progresses in LED based VLC have been reported previously. However, since LEDs are limited by the carrier lifetime of its spontaneous emission and RC parasitics, the modulation bandwidth is typically on the order of MHz.

For white lighting communication, the bandwidth and data rate are significantly affected by a slow phosphor response due to its relaxation decay time on the order of 100 ns ~ 1 μs.

As an alternative to LED technology, the use of laser diodes (LDs) as a transmitter presents a viable approach to overcome the limitations of modulation bandwidth associated with transmitters and phosphors. Despite of these merits, only a few studies have been reported for LD based VLC without a significant improvement in modulation bandwidth and data rate. In this dissertation, I discuss the materials, fabrication, and device characterization of III-nitride visible laser diodes on semipolar (202 ̅1 ̅) orientation for high-speed performance. Furthermore, I demonstrated laser based white lighting communication system containing different phosphors in supporting the merits in high-speed data transmission resuling in world-record of high speed performance.

In the first part of this thesis, the initial VLC system by using commercial high-power laser diodes is discussed. It is important to study the capability of data transmission by utilizing the high-power designed laser diode because not only it suggests the guideline beyond the LED based VLC, but also high-power operation is necessary for LiFi application. Moreover, the bandwidth limiting effect by the phosphor commonly observed in LED based system is investigated for laser based system and an understanding of these effects is used to achieve high data rate of white lighting communication without filtering phosphor converted photons. In the second part of this thesis, the optimization of epitaxial structure, optical mode, and fabrication process of semipolar laser diodes are discussed. These include the relatively conventional structure with well-optimized topside ohmic n-type contact for high-speed microwave probing rather than backside n-type contact. Dynamic characteristics and a high-speed performance of LDs emitting at 410 nm are investigated resulting in a 3-dB bandwidth of 5 GHz and 5 Gbit/s direct modulation with on-off keying (OOK), which were limited by the bandwidth of the photodetector used for the measurements. The differential gain of the LDs was determined to be 2.5 ± 0.5 × 10-16 cm2 by comparing the slope efficiency for different cavity lengths. Analysis of the frequency response showed that the K-factor, the gain compression factor, and the intrinsic maximum bandwidth were 0.33 ns, 7.4 × 10-17 cm3, and 27 GHz, respectively.

Finally, I will conclude with a demonstration of white lighting gigabit data transmission by utilizing red-, green-, and blue-emitting phosphors based on processed semipolar 410 nm LD. The advantage of near UV or violet LD in white lighting data communication involves lowest ambient light that can be significant noise level in other spectrum range.

The impact of III-Nitride material and solid-state lighting on the world has been far-reaching. The last decade has seen the III-Nitride LEDs and laser didoes (LDs) steadily becoming essential components in a broad range of applications. Compared with traditional lighting systems, their great improvement in efficiency and portability for lighting systems, the reduced environmental impact as well as the potentials for the future visible light communication systems is profound. Properly engineered distributed feedback (DFB) laser diodes (LDs) are well known for their excellent portability, easy integration, and ability to offer robust single longitudinal mode stability. Among them, III-nitride material based visible DFB-LDs are gaining special attention due to many emerging applications such as atomic clocks, medical sensing, laser cooling and visible light communication. Previous GaN DFB-LDs have been conventionally fabricated with epitaxially buried gratings, which is often difficult and costly. In this dissertation, the design of DFB-LDs with a novel surface grating structure and their enhanced performance will be presented. Starting with the design and optical simulation of the devices, continuing through nanofabrication and testing, major parts and analysis of these novel III-nitride devices with surface gratings will be discussed. We show that under continuous-wave operation, the 445 nm laser diode with a uniform first-order etched surface grating emits with a FWHM (full width half maximum) of 5 pm, SMSR (side mode suppression ratio) of more than 25 dB and output power of more than 80 mW from a single facet. Moreover, an enhanced performance has been also achieved in laser diodes with phase-shifted third-order non-etched hydrogen silsesquioxane (HSQ) gratings. These improved single-mode blue lasers achieve a high output power, SMSR of more than 35 dB and FWHM of less than 2 pm even under high current injection. In addition to the DFB-LDs, surface gratings have also been historically employed in many other optical and photonic devices as an approach to realize novel functions. Efficient polarized light source, as a key component in mainstream LCD displays, plays an important role in energy saving. Specifically, in this dissertation, we also demonstrate that, by combining an ITO surface grating and Al surface coating, a highly polarized dual-wavelength semipolar InGaN micro-LED can be achieved. Such devices exhibit a 1.7-fold enhancement of the polarization ratio as compared to the conventional semipolar micro-LEDs.

High efficiency III-nitride light-emitting diodes (LEDs) have drastically improved solid-state lighting. They are sold in stores and are gradually replacing compact fluorescent lightbulbs because they use less power and last longer. III-nitrides are on the cusp of entering another market. As the size of mobile electronics shrink over time, display technologies must also move to smaller form factors while maintaining high efficiencies. To achieve these goals, III-nitride LEDs are once again a candidate to overtake the state of the art.

Incumbent technologies, such as liquid crystal displays (LCDs) and organic LED (OLED) displays, have major issues with power efficiency. A new technology, termed the micro-LED (uLED) display, is poised to enter the market in the next few years. A uLED display is made of inorganic LEDs (such as InGaN or AlGaInP) with dimensions typically below 40 um. uLED displays are promising due to higher luminance (brightness) than OLEDs, wider viewing angles, and significantly higher energy efficiencies.

In this thesis, advances in InGaN uLED epitaxial design, nanofabrication, and mass transfer are discussed. Chapter 1 introduces the III-V families (arsenides/phosphides/nitrides) and provides insight into uLED design. While the majority of IngaN uLEDs are grown on sapphire or silicon, there are many reasons to explore homoepitaxial growth on freestanding GaN (particularly on semipolar planes). Chapter 2 presents a comparison of external quantum efficiency (EQE) amongst various sized uLEDs and shows that high EQEs (40-45%) are sustained as the size of the uLED drops. Reasons for efficiency loss are presented and designs aimed at improving uLED efficiencies are highlighted. Chapter 3 discusses the incorporation of tunnel junction contacts to uLEDs to enable new design space (n-type mirrors and multiple active region growths). Chapter 4 examines ideas to eliminate the drop of efficiency with decreasing uLED size that incorporates a current aperture and reduction of dry etch damage at the active region sidewall. Chapter 5 highlights a new mass transfer method that is applicable to uLEDs grown on freestanding GaN, sapphire, or other substrates. Most of the commercial mass transfer techniques today use laser lift-off (LLO), which is incompatible with uLEDs grown on freestanding GaN. The technique in Chapter 5 combines photoelectrochemical (PEC) etching and transfer printing. With this method, red, green, and blue InGaN uLEDs have been transferred from their growth substrates (sapphire and freestanding GaN) to the same transparent and/or flexible substrates such as glass or acrylic without damage to the uLED. This thesis reports the first demonstration of red, green, and blue uLEDs all with an InGaN active region that have been transferred to the same substrate.

Gallium nitride (GaN) has become an important semiconductor for the optoelectronics and power electronics fields in the pursuit of high efficiency devices. However, the lack of a natural native GaN substrate has forced growth of GaN devices on foreign substrates such as sapphire, silicon carbide, and silicon. To further enhance efficiency and develop devices with longer lifetimes, the number of defects present in devices must be reduced. The development of a native GaN substrate of high crystalline quality would directly enable defect reduction.

The ammonothermal method of GaN growth has shown significant promise as a technique for the production of high quality GaN crystals of industrially significant size (crystals on the order of centimeters in the largest dimension). The ammonothermal method is a solvothermal method that uses a mineralizer (here ammonium chloride) with supercritical ammonia to transport GaN from a source material from one temperature zone to grow a seed crystal in another temperature zone. High pressures, high temperatures, and the presence of a highly corrosive chemistry make development of an economical growth reactor challenging. This body of work outlines the development of a growth reactor capable of high temperature ammonothermal growth of GaN using ammonium chloride mineralizer.

Initial development of the ammonothermal reactor required identification of suitable reactor materials. A materials stability study was conducted by exposing samples of materials to the ammonothermal environment and measuring mass loss as well as any chemical or mechanical changes that occurred. An Inconel 625 alloy reactor was employed, although the reactor itself was somewhat susceptible to corrosion from the ammonothermal environment. The study yielded a subset of materials that may be suitable for use as gaskets and other single use items which include niobium, molybdenum, titanium, vanadium, tungsten, gold, and platinum. Alloys of molybdenum and cobalt may also be useful. High strength titanium-zirconium-molybdenum (TZM) was also identified as a corrosion resistant material and was selected for reactor design.

A TZM reactor was then designed and fabricated. Subsequent high pressure, high temperature tests indicated that TZM was essentially inert and growth of GaN crystals followed. All GaN growth was accomplished at or above 650°C using seed crystals grown by hydride vapor phase epitaxy. Seeds were characterized by micrometer measurements for growth thickness, x-ray diffraction (XRD) for crystalline quality, and secondary ion mass spectrometry (SIMS) for impurity concentrations. The growth quality appeared to match the seed quality as measured by XRD. Growth coloration ranged from slightly gray to green or yellow with growth rates up to 191 µm/day. Most seeds exhibited significant faceting at the edges of the sample, forming semipolar planes. SIMS was performed on a couple of samples which indicated oxygen concentrations of ~1018 cm¬-3.

As the quest for high-efficiency light-emitting diodes (LEDs) intensifies, researchers are increasingly focusing on advanced materials such as III-Nitride semiconductors, including AlN, GaN, and InN. Despite their potential, the performance of these LEDs is often impeded by factors such as defect density, quantum-confined Stark effect (QCSE), and suboptimal light extraction efficiency. In this investigation, we explore the impact of incorporating an aluminum gallium nitride (AlGaN) interlayer on the efficiency of III-Nitride LEDs, employing a combination of experimental and computational methodologies. The synergy of experimental results and computational simulations reveals that the introduction of an AlGaN interlayer significantly enhances the overall efficiency of LEDs. This improvement is achieved by ameliorating threading dislocations, attenuating QCSE, and elevating light extraction efficiency. Moreover, the interlayer facilitates superior carrier confinement and diminished electron leakage, resulting in an increased internal and external quantum efficiency [1]. By conducting meticulous lab experiments and employing sophisticated software simulations, we optimized the growth parameters of the AlGaN interlayer, encompassing aspects such as thickness, and alloy composition. The optimized AlGaN interlayer-based III-Nitride LED demonstrated a substantial increase in efficiency compared to its conventional counterparts lacking the interlayer. Additionally, the AlGaN interlayer imparts enhanced thermal stability and robustness to the LEDs [2]. This research elucidates the significance of integrating AlGaN interlayers into III-Nitride LED structures, resulting in markedly improved device performance. The insights garnered from the experimental and computational approaches contribute to the advancement of next-generation high-efficiency LEDs, promoting sustainable and energy-efficient lighting solutions across a diverse range of applications.

Due to rapid developments of wearable and portable devices in recent years, displays with better efficiency and higher resolution performances are greatly desired. Inorganic micro-light-emitting diodes (μLEDs) have been considered as one of the promising candidates for next-generation display applications, and demonstrations of μLED displays have shown outstanding performances as compared to liquid crystal displays (LCDs) and organic-light-emitting-diode (OLED) displays. However, it has been shown that the maximum efficiency drops as the device dimensions shrink. This dissertation describes post-etch fabrication techniques that suppress or mitigate the size effect in InGaN and AlGaInP μLEDs.

In this dissertation, the concepts of current display technologies, namely LCDs and OLED displays, and μLEDs will be first introduced. Moreover, the mechanism of the reduction in efficiency will also be addressed. Followed by the first chapter, a series of fabrication methods and their improvements to the device performances will be discussed in the chapters 2-4. In chapter 2, the optoelectrical improvements in the InGaN μLEDs with dielectric sidewall passivation using atomic layer deposition (ALD) will be illustrated. This is the first demonstration that the InGaN μLED efficiency can be partially recovered using post-etch technique. Due to the greater surface recombination velocity and minority carrier diffusion length, the effects of ALD sidewall passivation on AlGaInP μLEDs are investigated in chapter 3. This chapter also reveals the excellent optical properties of AlGaInP red emitters, and the potential elimination of the size effect in AlGaInP devices is also shown. In chapter 4, a detailed report on the performances of InGaN devices with the combination of chemical treatment and ALD sidewall passivation is presented, where this is the first demonstration of size-independent efficiency from 100×100 to 10×10 μm2 devices. Since the size effect is mitigated in InGaN and AlGaInP devices, chapter 5 shows a possible mass transfer method that employs fluidic assembly with external forces. Lastly, other critical challenges in μLED displays, brief μLED device process follower, and outlook are discussed concisely in chapter 6.

Basal plane III-nitride laser diodes have been commercialized for wide ranging technologies, including pico projectors for solid state RGB displays, optical data storage and automotive headlights. Active research is focused in solid state lighting and visible light communications. Despite widespread commercialization, c-plane devices are affected by the inherent spontaneous and piezoelectric polarizations of the basal plane. These polarization effects are significantly reduced in semipolar planes of GaN, creating a vast design space for III-nitride optoelectronic devices.

III-nitride semipolar laser design and performance are considerably affected by the material composition and growth conditions of the cladding layers. The bottom cladding design is limited by stress relaxation of ternary alloys while the top cladding is limited by the growth time and temperature of the p-type (Al,In,Ga)N layers. These design limitations have been independently addressed by using limited area epitaxy (LAE) to enable n-AlGaN bottom cladding layers and by using thin p-GaN and transparent conducting oxide (TCO) top cladding layers.

In this work, we investigate a new laser design that simultaneously incorporates LAE-enabled n-AlGaN bottom cladding and thin p-GaN and TCO top cladding layers in ( III-nitride laser structures. We evaluate the performance of two different TCOs as the top cladding layer: indium-tin-oxide (ITO) and zinc oxide (ZnO). Thorough optical modeling of the LAE-TCO laser design will be discussed for various Al compositions in the LAE-enabled n-AlGaN bottom cladding and varying p-GaN thicknesses in the top cladding. The LAE-TCO laser design was first demonstrated using ITO as the top cladding layer, with pulsed lasing achieved at 446 nm with a threshold current density of 8.5 kA/cm2 and a threshold voltage of 8.4 V. Insights from the optical modeling in conjunction with improvements in the LAE-TCO laser fabrication process led to the demonstration of the LAE-TCO design using ZnO as the top cladding layer, with pulsed lasing achieved at 445 nm with a threshold current density of 5.6 kA/cm2 and a threshold voltage of 6.7 V. This notable improvement in threshold current density and voltage led to the first continuous wave (CW) operation of blue ( ) LAE-ZnO laser structures.

This work presents the first Nitride non polar Current Aperture Edge Emitting Blue Laser Diode (CA-LD) fabricated using the Photo-Electro-Chemical Etching (PECE) technique.

The main features of this design are represented by the deep etching of the laser diode ridge through the active region, the controlled etching of the active region by PECE and the increase of the p-contact area with respect to the active region area.

Preliminary experiments manifest that CA-LD has similar threshold current density, slope efficiency and peak output power of the more commonly used shallow etch ridge design and it has also shown a reduction in the series resistance down to ~40% with respect to the shallow-etch LDs indicating the potential of the CA-LD design in high-efficient, high-power, high-frequency LD applications.

The highly efficient blue InGaN-based light emitting diode (LED) has given rise to the solid-state lighting revolution, replacing its inefficient predecessors in home lighting, flashlights, displays, projectors, automotive lighting and much more. However, long-wavelength (green & red) InGaN-based LEDs have lagged in efficiency, often referred to as the ‘green gap’ in the LED industry. Addressing the green gap has become an increasingly important area of research for color-mixed home lighting and a wide range of display applications including AR and VR displays. Long-wavelength III-N LEDs have more non-radiative recombination and higher internal voltage barriers which hinder LED performance, especially the forward voltage and wall-plug efficiency (WPE).In the last decade, LED researchers have discovered that strategic use of V-defects (or V-pits) can help address these issues by creating energetically favorable pathways for carrier (electron and hole) transport. This novel approach to LEDs, makes use of the anisotropic electrical properties in III-N crystals by using c-plane for epitaxial growth and light emission and the {101 ̅1} plane (V-defect sidewall) for carrier transport. The lateral injection through the semipolar V-defect sidewalls reduces the polarization barriers and leads to reduced forward voltage and better WPE. In a few short years, V-defect approaches to green and red LEDs have been adopted by most major LED companies. In this work, we study the structure and formation of V-defects in LEDs grown by metal-organic chemical vapor deposition (MOCVD). Through advanced microscopy we investigate how V-defect structure, formation and density evolve in III-N epitaxial growth on (111) Si and sapphire. We demonstrate novel ways of forming V-defects and generating the threading dislocations that lead to V-defect nucleation. We also studied extended defects in LED active regions, their impact on LED efficiency, and provide experimental evidence of lateral injection through V-defects. We achieved high external quantum efficiencies of 6.5% in red LEDs and 30% in green LEDs grown on PSS with V-defect engineering. Furthermore, we show low voltage operation (2.8 V at 20 A/cm2) in green LEDs with V-defects. These results help provide a pathway to solving the green gap through V-defect engineering and advance the scientific understanding of V-defect formation and structure in III-N epitaxy.