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As greenhouses and other forms of protected agriculture modernize their systems and adopt data-driven techniques in reaction to climate change, wireless agricultural sensing is rapidly becoming an emergent area of research. Due to high energy and resource consumption in greenhouses, low-power systems capable of autonomous greenhouse control will be critical for next-gen greenhouses in order to help address food security and climate resilience. To fill this gap, I develop an ultra-low power wireless communication technique that can be utilized for precision agriculture in greenhouses. The proposed solution uses a chirp spread spectrum (CSS) based visible light communication (VLC) downlink that is capable communicating 2 meters using agricultural grow light light emitting diodes (LEDs). VLC coupled with the high noise resilience of CSS permits the transmission of data even under high irradiance conditions such as greenhouses, thereby enabling locationally-relevant sensor placement. In this thesis, in addition to describing the CSS VLC downlink, I also present performance results obtained through simulation experiments as well as experiments with the current CSS VLC prototype.

This dissertation presents measurements of the photocurrent in forward biased III-Nitride light emitting diodes (LEDs). This photocurrent is observed to be in the forward direction, and is therefore attributed to the hot carriers being generated by Auger recombination in the active region of the device. There is strong evidence that Auger recombination is the cause of the well-known phenomenon of efficiency droop in III-Nitride LEDs, which significantly limits the efficiency of high brightness LED lighting. Therefore, the photocurrent measurements presented in this dissertation are a step towards a benchtop technique to more easily study the physical origins of efficiency droop in commercial LEDs.First, single quantum well (SQW) devices are studied, as these are the simplest platform to interpret the results of the measurements. Next, devices with multiple quantum wells (MQW) are studied, as all commercial LEDs are MQW devices. Additionally, measurements of LEDs grown by ammonia molecular beam epitaxy (NH3 MBE) are presented. These LEDs show no droop, even out to high drive current densities. Finally, measurements of the photocurrent in LEDs processed from commercial epi are presented.

The impact of solid state lighting on the world has been remarkable. The improvement in efficiency and lifetime, the reduced environmental impact and the new design space of solid state lighting when compared to traditional incandescent or compact fluorescent lighting is dramatic. Still, improvements can be made to improve and accelerate the adoption of this technology. Light Emitting Diodes (LEDs), the basis of solid state lighting, are impacted by the problem of efficiency droop, whereby the efficiency of LEDs peaks at low current density and declines dramatically with increasing current. This limits the power out per chip and requires many chips to be used to create an incandescent equivalent replacement fixture. In this dissertation, several ways of addressing this problem and other efficiency limits of solid state lighting will be addressed.Utilizing novel properties of alternative planes of the gallium nitride (GaN) crystal, LEDs can be fabricated with reduced droop. These semipolar planes differ from the c-plane (the plane all commercial devices are grown on) in that they have reduced polarization-related effects, which increases the quantum efficiency. Semipolar LEDs with wider active regions to reduce carrier density and limit Auger recombination are demonstrated.Laser Diodes (LDs) can also be used to improve the efficiency of solid state lighting. LDs have higher brightness than LEDs and are not impacted by efficiency droop above threshold, so it is possible to run LD chips at high currents and obtain orders of magnitude higher power per chip, with the potential to reduce size and thus cost of solid state lighting fixtures. Semipolar planes can again be used to overcome efficiency limits of c-plane LDs as well.The design of LEDs and LDs are presented in this dissertation. Beginning with metalorganic chemical vapor deposition (MOCVD) growth, continuing through nanofabrication, packaging, and testing, all facets of semipolar nitride devices will be discussed. Simulations are used throughout to gain additional insight into experimental results and predict future behavior.Semipolar LEDs with low droop and power of 1 W from a small 0.1 mm2 chip and continuous wave (CW) semipolar LDs with 11 % wall plug efficiency are demonstrated. The internal loss and injection efficiency of semipolar LDs is reported for the first time. An accurate absorption model is used to predict gain and transparency current density. Reduced threshold current and improved differential efficiency LDs with fewer quantum wells in the active region are demonstrated. Many considerations to improve the wall-plug efficiency of LDs are discussed, and a path to world record wall plug efficiency is described at the conclusion.

The effect of efficiency droop in light emitting diodes (LEDs) is a huge roadblock for consumer lighting adoption. To prevent loss of efficiency from droop, LEDs must be operated at lower current density, requiring a larger epitaxial area and increasing the overall cost. Proposed mechanisms responsible for droop include, among others, carrier delocalization from indium rich regions, overshoot and leakage current, and Auger recombination. Current methods used to understand droop mechanisms are indirect, and often rely on models that have no unique solution. It is clear that a direct measurement method and a deeper understanding of the fate of injected carriers not contributing to radiative recombination is needed to focus improvement efforts on materials and structures to help identify and mitigate the relevant droop mechanism(s). By analyzing the energy of vacuum emitted electrons from a forward biased LED, we can gain direct information of their origin internally.The study of vacuum emitted electrons has existed for almost 130 years with the discovery of the photoelectric effect. Advancements in electron energy analysis techniques have led to the direct measurement of conduction band structures and transport properties in many commonly used semiconductors such as: InP, GaAs, Si, and recently GaN. The kinetic energy of the vacuum emitted electrons from an InGaN/GaN LED was analyzed and three peaks were identified: First, a low energy peak, resulting from photoexcited electrons generated by diode light. Second, a mid-energy peak, generated by the accumulation of thermalized electrons at the bottom of the Γ conduction band valley. Third, a high-energy peak is generated by an accumulation of electron at the bottom of a low lying side-valley “L”. Auger recombination is uniquely identifiable as it is the only proposed droop mechanism capable of generating hot carriers and solely responsible for the population of electrons found in the L-valley. Two control experiments were carried out to strengthen our interpretation. First a simple GaN pn junction was measured and generated only a single Γ-valley peak. Second, selective detection of photoemission under modulated light from an LED in forward bias confirms that only the low energy peak is photogenerated and that LED light incapable of generating the higher energy Γ or L-valley peaks. Lastly, we discuss the new UCSB electron energy analyzer as well as some proposed future experiments to advance the electron emission technique.

ZnO based homojunction light emitting diode, double heterojunction light emitting diode, embedded heterojunction random laser diode and Fabry-Perot nanowire laser devices were fabricated and characterized. A variety of measurements such as X-ray diffraction, photoluminescence, scanning electron microscope and secondary ion mass spectrometry were used to characterize material properties. Current-voltage, capacitance-voltage and electroluminescence measurements were used to characterize device performance.ZnO p-n junction diode was successfully grown on Si substrate by using Sb and Ga for p-type and n-type dopants respectively. The diode device was fabricated by using standard photolithography process. Due to the improved material quality, dominant ultraviolet light emission from ZnO homojunction diode was firstly observed and the emission properties were studied with increased injection currents. To improve the efficiency, A MgZnO/ZnO double heterojunction structure was realized by molecular beam epitaxy growth on Si substrate. Compared to homojunction structure, the carrier confinement effect in double heterojunction structure gave rise to larger output power at same injection current. With smaller heterojunction well size, ZnO based random laser diode was developed. A thin MgZnO/ZnO/MgZnO well embedded in a ZnO p-n junction was realized by molecular beam epitaxy growth. Random lasing emissions were observed at different injection currents at room temperature. Lasing mechanism was discussed and proved by deep investigation of its nano-structure. Fabry-perot type laser was also developed in our lab, to lower lasing threshold, distributed Bragg reflector was proposed. Vertically aligned ZnO nanowires were firstly successfully grown on distributed Bragg reflectors made of SiO2/SiNx alternative layers. By means of optical excitation, Fabry-Perot type lasing was observed and characterized. Compared to the same length nanowires without distributed Bragg reflectors, better lasing performance was achieved. The discussion of lasing mechanism revealed the function of distributed Bragg reflectors in improving lasing performance.

Background Skin fibrosis is a significant global health problem that affects over 100 million people annually and has a profoundly negative impact on quality of life. Characterized by excessive fibroblast proliferation and collagen deposition, skin fibrosis underlies a wide spectrum of dermatologic conditions ranging from pathologic scars secondary to injury (e.g., burns, surgery, trauma) to immune-mediated diseases. Effective anti-scarring therapeutics remain an unmet need, underscoring the importance of developing novel approaches to treat and prevent skin fibrosis. Our in vitro data show that light emitting diode-red light (LED-RL) can modulate key cellular and molecular processes involved in skin fibrosis. In two phase I clinical trials (STARS 1 and STARS 2), we demonstrated the safety and tolerability of LED-RL at fluences of 160 J/cm2 up to 480 J/cm2 on normal human skin. Methods/design CURES (Cutaneous Understanding of Red-light Efficacy on Scarring) is a dose-ranging, randomized, parallel group, split-face, single-blind, mock-controlled phase II study to evaluate the efficacy of LED-RL to limit post-surgical skin fibrosis in subjects undergoing elective mini-facelift surgery. Thirty subjects will be randomly allocated to three treatment groups to receive LED-RL phototherapy or temperature-matched mock irradiation (control) to either periauricular incision site at fluences of 160 J/cm2, 320 J/cm2, or 480 J/cm2. Starting one week post-surgery (postoperative days 4–8), treatments will be administered three times weekly for three consecutive weeks, followed by efficacy assessments at 30 days, 3 months, and 6 months. The primary endpoint is the difference in scar pliability between LED-RL-treated and control sites as determined by skin elasticity and induration measurements. Secondary outcomes include clinical and photographic evaluations of scars, 3D skin imaging analysis, histological and molecular analyses, and adverse events. Discussion LED-RL is a therapeutic modality of increasing importance in dermatology, and has the potential to limit skin fibrosis clinically by decreasing dermal fibroblast activity and collagen production. The administration of LED-RL phototherapy in the early postoperative period may optimize wound healing and prevent excessive scarring. The results from this study may change the current treatment paradigm for fibrotic skin diseases and help to pioneer LED-RL as a safe, non-invasive, cost-effective, portable, at-home therapy for scars. Trial registration ClinicalTrials.gov, NCT03795116. Registered on 20 December 2018. Electronic supplementary material The online version of this article (10.1186/s13063-019-3546-6) contains supplementary material, which is available to authorized users.