High Conductivity Tunnel Junctions for Next-Generation UV Emitters

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Ultra violet light sources can greatly impact health and well-being by enabling rapid water purification, biological sensing. However, the light sources available today are too costly, bulky, and inefficient for widespread adoption. Semiconductor based ultra-violet sources can in theory overcome these challenges and enable compact and efficient ultra-violet sources, but they face some fundamental challenges. Ultra-violet light sources are based on wide band gap semiconductors, and these semiconductors have very poor conductivity of positive charge carriers (holes). The poor conductivity of these holes has been a challenge for the last several decades, leading to poor efficiency and low power in solid state emitters. The proposed project will circumvent this problem through the use of quantum mechanical tunneling that allows positively charged carriers to be injected. The research proposed is expected to enable a new class of compact, efficient, and bright ultra-violet light sources that would find direct applications in water treatment plants, disease prevention, and many other critical applications. The investigator will develop touch sensitive applications (apps) for learning and teaching semiconductor device physics, with an emphasis on intuitive and visual learning of concepts. In addition, the investigator will continue outreach activities by engaging high school students and undergraduates in research projects, as well as by promoting international exchange and experiences. In the proposed work, a transformative new design is proposed for ultra violet light emitting diodes where non-equilibrium tunneling transport is used to inject holes into the active region of the light emitting diode. Short wavelength ultra-violet emitters (350 nm) have low efficiency due to the unique challenges presented by wide band gap materials such as AlGaN. Hole doping and transport remain one of the main challenges for AlGaN-based ultra-violet emitters and account for a large portion of the losses. High acceptor ionization energy leads to low hole concentration and high resistivity, as well as imbalance between electron and hole injection, leading to electron overflow. The challenges of making p-contact necessitate absorbing p-type GaN contact layers that lead to further losses. Tunneling injection of holes removes constraints due to thermal ionization, enhances hole injection, thereby increasing the injection efficiency. Advanced modeling and growth techniques will be used to design and demonstrate highly efficient tunneling for the first time in ultra-wide band gap AlGaN materials. Tunnel junctions will be integrated with ultra-violet emitters to show the efficacy of tunneling injection, and to enable devices without any p-contacts. The proposed experimental work will be supported by detailed theory and simulation to verify and predict device operation, and will lead to detailed understanding of tunneling transport in the III-nitride system. The ideas proposed on nanometer scale engineering of tunneling transport and non-equilibrium carrier injection will lead to higher efficiency and new design paradigms for III-nitride ultra-violet emitters, and could also be relevant for bipolar devices based on several other wide band gap semiconductors.