Publications
Materials Challenges of AlGaN-Based UV Optoelectronic Devices
Over the past 15 years, tremendous progress has been made in AlGaN-based optoelectronic devices, including light-emitting diodes (LEDs) and laser diodes (LDs) in the deep UV (DUV) region of the spectrum. However, performance levels are still lagging those of InGaN light emitters in the visible region due to outstanding materials challenges of the wider band gap AlGaN alloys. In this review, we focus on two of the most significant materials roadblocks to higher-performing AlGaN devices: doping and substrates. For each topic, we present the state of the art as well as exploratory concepts for enabling future device advances. On the topic of p-type doping, we describe the concomitant challenges of large acceptor activation energy, dopant solubility, and compensating defects and describe growth optimization approaches to mitigate those issues. We further present polarization engineering approaches to enhance p-type doping, including Mg-doped superlattices, distributed polarization doping, and tunnel-junction-enabled LEDs. Limitations to n-type doping for high-Al-composition AlGaN alloys are also reviewed along with insights into the origins these doping challenges. On the topic of substrates, we report the challenges of heteroepitaxy on lattice-mismatched substrates, describe the impact of high dislocation densities on AlGaN emitters, and overview strategies for dislocation reduction. State-of-the-art UV LD performance, enabled by these defect reduction strategies, is also presented. Limitations due to electrically insulating substrates are described as well as promising approaches to achieving vertical-injection UV light emitters. Overall, common themes of employing material and device structures at the micro/nanoscale and leveraging the polarization properties of nitride heterostructures reveal approaches for realizing next-generation UV light emitters.