Tunnel Junction Based AlGaN Ultraviolet Lasers

虽然可见光蓝光激光器已被证明具有优异的性能，但实现波长短于300 nm的紫外激光器仍然具有挑战性，尽管各个研究小组已经努力了十多年。电驱动激光器的主要问题涉及在电流将流过的激光材料中实现合理的p电导率。这使得在合理的电功率下操作激光器具有挑战性。该项目提出了一种利用隧道结（TJ）的创新方法，该方法在不牺牲器件光学性能的情况下解决了激光材料的p电导率问题。这些短波长紫外激光器最近被发现可用于对表面或物体进行消毒，作为预防冠状病毒（COVID-19）全球传播的预防措施之一。可以说，这种应用可以通过在这种波长范围内的发光二极管（LED）来实现。然而，迄今为止实现的节能LED体积大、复杂且昂贵，这基本上限制了它们在这些关键领域的应用。除了高影响力的研究进展，该项目还将支持纳米科学和纳米技术的跨学科教育活动。由于拟议的研究项目跨越科学和工程的不同学科，如光学，材料科学，电气工程，物理和化学，它将导致一系列潜在的，动手学习活动，可以吸引不同背景的学生。此外，这项研究所带来的科学见解和技术进步也将通过在这一欠发达的光谱区域实现操作来广泛影响光子学领域。由于包括植物生长照明、水消毒、痕量气体感测、固化聚合物和刺激抗癌物质形成的广泛新兴应用，在紫外（UV）线B（320 - 280 nm）和UV-C（280-200 nm）波长范围内，存在对电泵浦（EP）和连续波（CW）操作的AlGaN基二极管激光器的巨大需求。拟议研究的主要目标是设计和演示发射波长范围为320 nm至280 nm的隧道注入EP和CW操作紫外激光器。p型掺杂和低阻p型欧姆接触的形成是电泵浦紫外激光二极管的关键挑战。这项工作提出了使用新的带间隧道结的超宽带隙AlGaN高达70%的铝成分，以克服这一主要挑战。在该项目中进行的工作将产生关于AlGaN材料系统及其几个重要特性的新的基础知识，包括折射率，通过带尾态和带隙变窄的载流子带间隧穿，以及具有量子限制的超薄层中的吸收。拟议的研究涉及一种新的器件概念，以实现基于超宽带隙材料的高要求光源。从这项研究中获得的设备知识将为演示在整个深紫外光谱范围内发射的激光设备奠定基础。该项目的早期阶段旨在展示在有源区两侧使用高带隙n型AlGaN包层区域的大面积法布里-珀罗激光器。然后将在脉冲模式下对这些器件进行测试，这将有助于确定高Al成分结构的各种未知材料特性。在该项目的第二阶段，该项目旨在通过采用具有优化外延结构的窄脊结构来展示适合应用的CW操作激光器。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

While visible blue lasers have been demonstrated with excellent performance, realizing ultraviolet lasers at wavelengths shorter than 300 nm is still challenging despite efforts over more than a decade by various research groups. The principal problem for electrically-powered lasers is related to achieving a reasonable electrical p-conductivity in laser materials through which electrical current will flow. This makes it challenging to operate lasers at a reasonable electrical power. This project proposes an innovative approach that utilizes tunnel junctions (TJs) which alleviates the p-conductivity problem of laser materials without sacrificing optical performance of the device. These short-wavelength ultraviolet lasers are recently found to be useful for sterilizing surfaces or objects, as one of the precautionary steps to prevent the global spread of the coronavirus (COVID-19). Arguably, this application could be enabled by light emitting diodes (LEDs) in this wavelength regime. However, energy-inefficient LEDs achieved to-date are large, complicated, and expensive, which essentially limits their applicability in these key areas. In addition to high impact research advancement, this project will also support interdisciplinary education activities in nanoscience and nanotechnology. Because the proposed research project crosses different disciplines of science and engineering, such as optics, materials science, electrical engineering, physics, and chemistry, it will lead to a range of potential, hands-on learning activities that can engage students of varying backgrounds. In addition, the scientific insights and technological advances stemming from the research will also broadly impact the field of photonics by enabling operation in this underdeveloped spectral region. There is a tremendous need for electrically-pumped (EP) and continuous-wave (CW) operating AlGaN-based diode lasers in the ultraviolet (UV)­B (320­280 nm) and UV-C (280–200 nm) wavelength regimes due to a wide range of emerging applications including plant growth lighting, water sterilization, trace gas sensing, curing polymers, and stimulating the formation of anti-cancerogenic substances. The primary objective of the proposed research is to design and demonstrate tunnel-injected EP and CW-operating UV lasers with wavelengths of emission ranging from 320 nm to 280 nm. P-type doping and formation of low-resistive p-ohmic contacts are the key challenges for electrically-pumped UV laser diodes. This work proposes to use novel interband tunnel junctions for ultra-wide band gap AlGaN up to 70% aluminum composition in order to overcome this principal challenge. The work performed within this project will generate new fundamental knowledge on the AlGaN-material system and its several important properties including refractive index, carrier interband tunneling through band-tail states and bandgap narrowing, as well as absorption in ultra-thin layers with quantum confinement. The proposed research involves a novel device concept to realize such highly demanding light sources based on the ultra-wide band gap materials. The device knowledge gained from this research will establish a foundation for demonstrating laser devices with emission in the entire deep-UV spectral regime. The early stage of the project aims to demonstrate broad-area Fabry-Pérot lasers using high-bandgap n­AlGaN cladding regions on both sides of the active region. The devices will then be tested in pulsed mode, which will help determine various unknown material properties of high Al composition structures. In the second stage of the project, this project aims to demonstrate application-suited CW-operating lasers by employing narrow­ridge structures with optimized epitaxial structures. The radically new approach proposed here will enable new scientific understanding in the areas of ultra-wide band gap materials and optical devices as well as could establish the platform for a new class of AlGaN-based UV laser technology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.