Date of Award

Summer 2022

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Asif Khan

Abstract

III-Nitride materials-based visible emission LEDs have emerged as a disruptive technology in the fields of lighting,i) communications,ii,iii,iv) and displays.v,vi) Shorter wavelength LEDs in the DUV spectral region (210nm – 360nm) with ultra-wide bandgap (UWBG) AlxGa1-xN active layers are now poised to displace toxic Mercury-based light sources.vii) Over the past decade AlGaN LEDs operating in the deep ultra-violet (DUV) spectral region (200 nm < λemission < 300 nm) have been deployed in novel applications including autonomous drone-based sterilization and sanitization systems, viii) point-of-use water purification systems, ix) photo-therapeutics,x) gas sensors,xi) and non-line-of-sight (NLOS) communications.xii) Similarly, DUV light detectors using ultra-wide bandgap (UWBG) AlxGa1-xN hetero-junctions have also been reported by several research groups.xiii,xiv,xv) Currently, several DOD early threat warning systems employ such solarblind DUV photodiodes.xvi) These detectors have also garnered attention for environmental safety applications in flame and radiation monitoring systems.xvii) Furthermore, ultra-wide Bandgap (UWBG) AlGaN materials-based devices are robust to the harsh conditions of outer space.xviii) Hence, the integration of UWBG (> 4 eV) AlGaN-based DUV optoelectronics and electronic devices is of great interest for future miniaturized systemon-chip (SOC) applications.xix) Unlike visible emission materials platforms, at the onset of this work, there were no reports of high brightness DUV emitters, DUV Photonic Integrated Circuits (PIC), nor deeply-scaled DUV micro-LEDs. In this work, we designed and characterized the world’s first monolithically integrated DUV PICs comprised of AlGaN MQW-based light emitters and detectors and an AlGaN waveguide, demonstrated sub-20 µm sized DUV micro-LEDs (also referred to as micropixel LEDs) with record kW/cm2 class brightness, and integrated these ultra-bright emitters into a modularly interconnected array architecture with excellent power and area scalability. This work is divided into two sections with the first focused on providing proof-of-concept and experimental characterizations of DUV PICs. The second revolves around the development and characterization of deeply-scaled sub-20 µm diameter DUV emitters with aim to improve the electrical, optical, and thermal performance in addition to opening the door to future applications where traditional large-area DUV LEDs are unsuited such as high speed data transfer, direct write lithography, and high-resolution UV displays.

In our pioneering work on DUV PICs, we first qualified MQW AlGaN LED epitaxial layers for use as both an emitter and detector. We next established the suitability of the n-AlGaN contact layer for waveguiding DUV radiation. Using a symmetric array of micro-LEDs with a pixel size of 30 µm, we determined the directional dependence of DUV radiation within the AlGaN waveguide layer. Then, using a neighboring emitter and detector, we etched a trench between the devices down to the sapphire substrate and measured the photocurrent after each etching iteration to determine the distribution of guided light among the epilayers. We next fabricated DUV PICs with different emitterdetector spacings and extracted the optical losses for both planar and ridge waveguides.xx,xxi) In those studies, successful detection of DUV emission at waveguide channel lengths up to 3 mm was realized for the first time. With continued development, such next-generation AlGaN materials-based PICs will have profound impacts in the fields of DUV-based gas and bio-chemical sensing as well as covert and quantum communications.

To realize high brightness DUV emitters with theorized high modulation bandwidths, AlGaN-based micro-LEDs (micropixel LEDs) were designed and fabricated with sub-20 µm mesa diameters, slanted sidewalls, and a monolithically integrated heatspreader. Despite the reduced emission area for these AlGaN-based micro-LEDs, the brightness (W/cm2 ) is remarkably enhanced due to their efficient light generation at kA/cm2 -level current densities enabled by a superior uniformity of current injection and removal of the self-generated heat from the device active region.xxii) At these levels of injection current density, the dynamic carrier lifetimes, which chiefly dictate the maximum modulation bandwidth in the case of visible emission micro-LEDs, is significantly reduced.xxiii,xxiv) Further tailoring of our micro-sized devices for applications requiring a high dose of UVC radiation, we developed a novel device layout architecture for large arrays via a hierarchically interconnected micropixel geometry which we showed to decrease the series resistance and thermal impedance of the devices while increasing the EQE, maximum LOP, and peak brightness compared to a single macro-LED with an equal emission area.xxv,xxvi,xxvii) The first-generation 5 µm micropixel of this work,xxii) with vertical sidewalls, had a record-setting brightness of 291 W/cm2 at a current density of 10.2 kA/cm2 under continuous wave (CW) operation. Second generation micro-LEDs with slanted sidewalls and an optimized fabrication procedure yielded a CW brightness of up to 600 W/cm2 at 15 kA/cm2 and a pulsed-mode brightness as high as 10.2 kW/cm2 at a current density of 50 kA/cm2 without flip-chip packaging nor encapsulation.xxviii) This kW/cm2 - class performance is an order of magnitude brighter than some of the most luminescent blue LEDs found in the literature, the current benchmark.xxix,xxx,xxxi) To date, these are the smallest and brightest DUV LEDs globally.

This technology, when further matured, will be particularly useful in the areas of DUV direct-write lithography, time-resolved fluorescence, optically pumped polymerbased lasers, charge control systems like envisaged in the evolved Laser Interferometer Space Antenna (eLISA) mission, and optical communications. Future development of selective area growth (SAG) techniques to marry electronic control and read-out devices with DUV micro-LED-based PIC technology is expected to lead to demonstrations of highbandwidth multi-functional UWBG AlGaN-based SOCs.

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