Tunnel Junction Based AlGaN Ultraviolet Lasers

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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.