1,721,269 research outputs found
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Development of Efficient and Spectrally Stable c-plane True Red III-Nitride Light Emitting Diodes
Get PDFIn 2020, lighting accounted for 14% of total U.S. electricity usage, and of that light emitting diodes (LEDs) accounted for 31%. It goes without saying how important improving LED efficiency is, but many technologies, such as data communications, AR/VR displays, and high dynamic range screens, depend on LEDs with stable color emission across a broad range of brightnesses. Much of the ubiquitous integration of LEDs into many aspects of modern society is attributed to the development of GaN and its III-N alloys. Color stability and efficiency are major challenges in red III-N LEDs. As current density increases, a monotonic decrease in peak wavelength is observed and the LED blueshifts, meaning at low brightnesses the LED can appear red and at higher brightnesses it may appear yellow or even green. As peak emission wavelength extends from blue to red, the efficiency drops from over 90% for blue LEDs to 30% for red LEDs. In this work, we demonstrate relatively high external quantum efficiency (EQE) red LEDs with negligible blueshift and improved spectral, and therefore color, stability from 0.01 A/cm2 to 150 A/cm2. Additionally, I will outline the nuances between peak wavelength, which is the maximum intensity of the LEDs emitted radiometric flux, dominant wavelength, which is the physiological color perceived by the human eye, and why it is relevant specifically for true red III-N LEDs.The 1st generation of these efficient and improved color stability LEDs had a total cumulative peak wavelength blueshift of only 7 nm, substantially lower than the 50+ nm reported elsewhere in red III-N LED literature, while achieving a peak EQE and wall-plug efficiency (WPE) of 7.5% and 4.8%, respectively. The 2nd and 3rd generations of these LEDs are 50% and 90% more efficient than the 1st generation, respectively, and more optically red while still demonstrating improved color stability. Furthermore, the 2nd and 3rd generation LEDs are their most efficient when they are at their longest dominant and peak wavelengths, which has never been observed before in III-N LEDs. Lastly, I will discuss the benefits of incorporating V-Defects into our LED structures to improve their WPE and its current development status. This work opens up the possibility to design explicitly for LEDs to be their most efficient at the desired color and/or brightness of their application.Future work should focus on incorporating V-Defects into the LEDs to further improve WPE while maintaining this color stability, processing these LED wafers into µLEDs less than 25 µm2, which have demonstrated >2x efficiency improvements, and experiments to investigate delaying the onset of blueshifting to higher current densities
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III-Nitride Vertical-Cavity Surface-Emitting Lasers: Growth, Fabrication, and Design of Dual Dielectric DBR Nonpolar VCSELs
Get PDFVertical-cavity surface-emitting lasers (VCSELs) have a long history of development in GaAs-based and InP-based systems, however III-nitride VCSELs research is still in its infancy. Yet, over the past several years we have made dramatic improvements in the lasing characteristics of these highly complex devices. Specifically, we have reduced the threshold current density from ~100 kA/cm2 to ~3 kA/cm2, while simultaneously increasing the output power from ~10 µW to ~550 µW. These developments have primarily come about by focusing on the aperture design and intracavity contact design for flip-chip dual dielectric DBR III-nitride VCSELs. We have carried out a number of studies developing an Al ion implanted aperture (IIA) and photoelectrochemically etched aperture (PECA), while simultaneously improving the quality of tin-doped indium oxide (ITO) intracavity contacts, and demonstrating the first III-nitride VCSEL with an n-GaN tunnel junction intracavity contact. Beyond these most notable research fronts, we have analyzed numerous other parameters, including epitaxial growth, flip-chip bonding, substrate removal, and more, bringing further improvement to III-nitride VCSEL performance and yield. This thesis aims to give a comprehensive discussion of the relevant underlying concepts for nonpolar VCSELs, while detailing our specific experimental advances. In Section 1, we give an overview of the applications of VCSELs generally, before describing some of the potential applications for III-nitride VCSELs. This is followed by a summary of the different material systems used to fabricate VCSELs, before going into detail on the basic design principles for developing III-nitride VCSELs. In Section 2, we outline the basic process and geometry for fabricating flip-chip nonpolar VCSELs with different aperture and intracavity contact designs. Finally, in Section 3 and 4, we delve into the experimental results achieved in the last several years, beginning with a discussion on the epitaxial growth developments. In Section 4, we discuss the most noteworthy accomplishments related to the nonpolar VCSELs structural design, such as different aperture and intracavity contact developments. Overall, this thesis is focused on the nonpolar VCSEL, however our hope is that many of the underlying insights will be of great use for the III-nitride VCSELs community as a whole. Throughout this report, we have taken great effort to highlight the future research fronts that would advance the field of III-nitride VCSELs generally, with the goal of illuminating the path forward for achieving efficient CW operating III-nitride VCSELs
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Developing High Efficiency AlGaN-based UV LEDs on Sapphire: Overcoming the Light Extraction Barrier
No full textGermicidal ultraviolet (GUV) radiation is well known to have an efficient deactivation effect on many pathogens. According to most reports, GUV disinfection efficacy peaks with 265 nm illumination, but significant efficacy has been reported up to 290 nm. More widespread GUV disinfection has been implemented due to the benefits of solid-state UV light sources. However, current commercial GUV LEDs are 3-5% in WPE (wall-plug efficiency) at best, compared to the 25% deemed necessary to fully replace conventional UV mercury vapor lamps. Technological challenges in the MOCVD growth, active region design, and fabrication of UV LEDs must be overcome in order for GUV LEDs to reach that 25% WPE.In this dissertation, I will give a comprehensive overview of our progress in UV LED technology at UCSB. I will highlight the methods of achieving highly conductive n-AlGaN through the use of an ultralow V/III ratio growth condition, and the difficulties in implementing this condition in UV LEDs. I will report on how we achieved reproducible UV LED epi with quicktest power >4 mW at 20 mA, and the many experiments we performed to reach this milestone. I will report on the discovery of a low areal density p-GaN hole injection layer that significantly increased light extraction efficiency and resulted in 18-20% peak external quantum efficiency (EQE) at 300-310 nm. I will report on the successful demonstration of a fully transparent tunnel junction UV LED, the discoveries made to get there, and the operating behavior of the device. Finally, I will discuss the remaining unsolved problems in UV LEDs, their potential solutions, and our next steps in UV LED research
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MOCVD Growth and Loss Optimization for Tunnel Junction Contacts for III-Nitride Laser Diodes
Get PDFThe III-Nitride edge-emitting laser diode (LD) shows promise in diverse applications such as directed illumination e.g. automotive and spot lighting, communication e.g. Li-Fi communication, fundamental science, and others. While significant recent progress has been made in improving the efficiency of the III-Nitride edge-emitting LD, continuing progress must be made for it to become and remain competitive in these applications. The tunnel junction (TJ) presents unique design opportunities for many III-Nitride devices, including in edge-emitting LDs where it is most often used as a substitute p-side contact that in principle allows p-type material on one side of the TJ to be replaced with less absorbing and more conductive n-type material on the other side. However, the TJ presents challenges in p-type GaN activation, where typical metal-organic chemical vapor deposition (MOCVD) growth conditions are known to introduce hydrogen and re-passivate p-type material. The TJ presents additional challenges in absorption, where the highly-doped layers of the TJ can contribute significant optical absorption loss to LDs, reducing device efficiencies. Both of these challenges must be addressed for the tunnel junction to be a part of viable III-Nitride edge-emitting LD designs.In this work, we demonstrate high-power LDs using TJ contacts grown by molecular beam epitaxy (MBE), which preserves p-GaN activation, and we also demonstrate LDs using TJ contacts grown by MOCVD that employ a p-GaN activation scheme utilizing lateral diffusion of hydrogen through the LD ridge sidewalls. Next, we model the lasing mode and internal optical absorption loss profiles of III-Nitride edge-emitting LD designs using the transfer matrix method and identify new designs showing reduced modeled internal optical absorption loss in III-Nitride edge-emitting LDs using TJ contacts. Last, LDs using TJ contacts and distributed feedback (DFB) gratings in tandem are designed and evaluated in a joint film mode matching-analytical model, showing the tunnel junction facilitates DFB LD designs with higher-order gratings, benefiting fabrication tolerances and complexity
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Bulk Group-III Nitride Crystal Growth in Supercritical Ammonia-Sodium Solutions
Get PDFGallium nitride (GaN) and its alloys with indium nitride (InGaN) and aluminum nitride (AlGaN), collectively referred to as Group-III Nitride semiconductors, have enabled white solid-state lighting (SSL) sources and power electronic devices. While these technologies have already made a lasting, positive impact on society, improvements in design and efficiency are anticipated by shifting from heteroepitaxial growth on foreign substrates (such as sapphire, Si, SiC, etc.) to homoepitaxial growth on native, bulk GaN substrates. Bulk GaN has not supplanted foreign substrate materials due to the extreme conditions required to achieve a stoichiometric GaN melt (temperatures and pressures in excess of 2200°C and 6 GPa, respectively). The only method used to produce bulk GaN on an industrial scale is hydride vapor phase epitaxy (HVPE), but the high cost of gaseous precursors and relatively poor crystal quality have limited the adoption of this technology. A solution growth technique known as the ammonothermal method has attracted interest from academia and industry alike for its ability to produce bulk GaN boules of exceedingly high crystal quality. The ammonothermal method employs supercritical ammonia (NH3) solutions to dissolve, transport, and crystallize GaN. However, ammonothermal growth pressures are still relatively high (~200 MPa), which has thus far prevented the acquisition of fundamental crystal growth knowledge needed to efficiently (i.e. through data-driven approaches) advance the field. This dissertation focused on addressing the gaps in the literature through two studies employing in situ fluid temperature analysis. The first study focused on identifying the solubility of GaN in supercritical NH3-Na solutions. The design and utilization of in situ and ex situ monitoring equipment enabled the first reports of the two-phase nature of supercritical NH3-Na solutions, and of Ga-alloying of Ni-containing autoclave components. The effects of these error sources on the gravimetric determination of GaN solubility were explored in detail. The second study was aimed at correlating autoclave dissolution and growth zone fluid temperatures with bulk GaN crystal growth kinetics, crystal quality, and impurity incorporation. The insights resulting from this analysis include the identification of the barrier between mass transport and surface integration-limited GaN growth regimes, GaN crystal shape evolution with fluid temperature, the sensitivity of (0001)-orientation crystal quality with fluid temperature, and impurity-specific incorporation activated from the dissolution and growth zones of the autoclave. The results of the aforementioned studies motivated a paradigm-shift in ammonothermal growth. To address this need, a fundamentally different crystal growth approach involving isothermal solutions and tailor-made Group-III alloy source materials was developed/demonstrated. This growth method enabled impurity incorporation reduction compared to traditional ammonothermal GaN growth, and the realization of bulk, ternary Group-III Nitride crystals
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Development of Semipolar III-Nitride Vertical-Cavity Surface-Emitting Lasers
Get PDFIII-N vertical-cavity surface-emitting lasers (VCSELs) show promise for numerous communications, lighting, display, and sensor applications due to their low threshold current, high beam quality, and arraying capabilities. Primarily, research has been focused on using c-plane based devices, but non-basal growth planes provide an interesting alternative due to a reduced quantum confined Stark effect; higher material gain; lower transparency current density; and inherent polarized emission. The anisotropic gain leads to VCSELs and VCSEL arrays where each laser is polarization locked along the a-direction. At UCSB, an m-plane VCSEL was first demonstrated in 2012 under pulsed injection and in 2018 under CW operation. Through that time, the device performance has improved and the polarization properties of the VCSELs has been experimentally verified. However, the wavelength of m-plane lasers is severely limited due to poor indium incorporation and high defect formation, inhibiting their adoption in many applications. This led to the question of how the benefits of using m-plane can be retained, such as the inherent polarization, while expanding the available wavelengths. The answer that was developed in this thesis is the use of a semipolar growth plane with higher indium uptake. After developing an epitaxial growth recipe and optimizing processing parameters for semipolar planes, we achieved the first demonstration of semipolar (20(21) ̅) VCSELs which were experimentally shown to be polarization locked along the a-direction and emit in the blue region. The devices had a 5λ cavity length, an ion implanted aperture, and a dual dielectric DBR design and showed an improvement in the differential efficiency, threshold current density and total output power relative to m-plane VCSELs with the same design. However, there were issues. The devices were only able to lase under pulsed operation, up to a 70% duty cycle. Focused ion beam images in conjunction with COMSOL modeling was used to identify the key structural features that contributed to the high measured thermal impedance. Nearfield images suggest that the LP01 mode was lasing near the edge of the aperture. This commonly observed spatial misalignment introduced additional sources of loss beyond the expected material absorption loss, including mode overlap with the implanted region and the metal contacts. The effect of these absorbing layers on the device performance relative to simulation models was estimated and highlighted the need for proper mode control.To improve the optical confinement, devices using a buried tunnel junction (BTJ) scheme to confine the current were fabricated and were found to lack the excess losses due to absorption seen on the initial semipolar samples. Significant filamentation was observed on these samples and several characterization methods, including optical and thermal nearfield images, were used to identify the source of the filamentation. Further comparison of multiple BTJ samples with different index guiding showed that the mode behavior was driven by the interplay of inhomogeneous current injection and index guiding. The cause of the inhomogeneous current injection is projected to be due to doping variations in the p-GaN but still requires further investigation for verification
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Development of Long-Cavity III-Nitride Vertical-Cavity Surface-Emitting Lasers
Get PDFGaN vertical-cavity surface-emitting lasers (VCSELs) show promise for numerous lighting, display, communications, and sensor applications due to their visible wavelength emission, low threshold current, high beam quality, and arraying capabilities. Primarily, research has been focused on short to medium cavity (L<5λ) VCSEL designs, prioritizing single longitudinal mode operation. However, GaN VCSELs struggle with thermal management due to self-heating from higher input power requirements, high optical losses from p-type GaN and current spreaders, and poor heatsinking from the typically low thermal conductivities of the bottomside distributed Bragg reflectors (DBRs). These issues result in a high thermal impedance, generally >1000 K/W, quick thermal rollover, and low device lifetimes. Recently, long cavity (L>>5λ) GaN VCSEL designs have shown significant promise towards addressing the issues of thermal stability and cavity length control but require substrate polishing and complex fabrication, limiting scalability for mass production. To address these issues, a topside lens fabrication method is developed. Then, a 65λ GaN VCSEL with a topside lens, a buried tunnel junction current aperture, and bottomside epitaxial nanoporous GaN DBR was fabricated using standard microfabrication techniques. First, a topside GaN lens was demonstrated, with CW lasing achieved at lower current densities than comparable planar cavity VCSELs. However, the output power was limited by the high temperature regrowth required to fabricate the GaN lens as well as the high turn-on voltage. Next, a topside dielectric lens was developed which enabled CW lasing performance above 2mW for a GaN VCSEL with a partially etched porous DBR, and single transverse mode operation for other VCSELs with fully etched porous DBRs. The devices show high thermal stability due to the long cavity, with an estimated thermal impedance of 600K/W measured on-chip
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Strain Relaxation Engineering towards Efficient III-Nitride based Laser Diodes
No full textIII-Nitride based laser diodes (LDs) have brought far-reaching impacts on multiple fields of industry and research. The high lighting efficiency together with the wide spectrum spanning ultra-violet and the entire visible wavelengths by III-Nitride materials have enabled numerous applications, including solid state lighting, microdisplays, material processing and communication, and even quantum computing and photonic integrated circuits. Despite the promising properties, the high misfit strain between III-Nitride alloys has posed great challenges in both material epitaxy and improving the efficiency. For visible LDs, the high misfit compressive strain of InGaN active region to GaN substrate not only causes the generation of defects but also hinders the incorporation of indium towards longer wavelengths due to composition pulling effects. For polar substrate orientation, the strain further exacerbates quantum-confined Stark effect (QCSE), which significantly degrades the optical gain of LDs. On the other hand, AlGaN thin film on GaN substrates suffers from cracking due to the tensile misfit strain, thereby limiting its application of cladding layers and for UV-A LDs. Therefore, being able to relax the misfit strain and engineer the relaxation becomes important and can open new epitaxial design pathways to improve efficiency of LDs.
In this dissertation, strain engineering for InGaN based LDs and AlGaN layers for cladding and UV-A LDs on c-plane GaN are presented. For InGaN based LDs, based on the previously demonstrated strain relaxed template (SRT), we successfully demonstrated blue LDs with 61.9% strain relaxed InGaN buffer. Furthermore, a patterned SRT method was successfully developed, which greatly reduces the threading dislocation density (TDD) on SRT while maintaining partially relaxed InGaN layers. LDs with a 15% relaxed InGaN buffer showed a threshold current density (Jth) as low as 7.4 kA/cm2, which is much lower than the Jth over 40 kA/cm2 from previous SRT. Moreover, an enhanced optical gain as well as differential gain than conventional c-plane LDs was demonstrated. Polarization dependent micro-photoluminescence (μ-PL) showed a degree of linear polarization within local areas for patterned SRT LDs, indicating the presence of local anisotropic strain relaxation. We attribute such local anisotropic strain to be the main mechanism of enhanced optical gain. By improving the strain relaxation of InGaN buffer to 52%, blue LDs with a Jth of 4.7 kA/cm2 were demonstrated, and a higher wall-plug efficiency than conventional c-plane was exhibited. Moreover, the threshold is comparable to the best reported values in the literature for strain relaxed InGaN LDs.
In addition, we demonstrated the capability of anisotropic strain engineering in a macroscopic scale for both InGaN and AlGaN on GaN. Through the line patterning by holographic lithography, anisotropic strain relaxed InGaN buffer was demonstrated on SRT. Moreover, for AlGaN, an uniaxially tensile strain thin film was achieved, with a TDD of 1 – 2E9/cm2, exhibiting great reduction compared to the non-patterned sample. With further optimization, it is promising that it can further improve the efficiency of LDs
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