The visible-blind detection of UV light has important applications in planetary imaging and spectroscopy, astronomy, communications, and defense-related imaging. Future instruments for imaging in the ultraviolet will require improvements in detector capabilities. An all-solid-state ultraviolet detector will enable substantial improvements in mass, volume, complexity, power, and robustness compared with conventional image-tube-based technologies. One new class of solid-state UV detectors includes those based on the gallium nitride (GaN) family of materials. The electronic passivation methods described here are one promising way to produce detectors with the required low dark current characteristics, and show a significant improvement over current state-of-the-art passivation methods. These methods will contribute to a next-generation solar-blind, solid-state UV detector for a wide range of space-based UV instruments.
Wide bandgap semiconductors such as those in the GaN family are inherently insensitive to lower-energy photons and can be fabricated to achieve high responsivity and high gain in the UV in a structure such as an avalanche photodiode (APD). This work focuses on APDs fabricated using GaN and AlGaN layers epitaxially grown in the vertical direction. Individual diodes or pixels are formed by geometrical isolation, which requires the formation of mesa structures. The discontinuity of the crystal structure at the vertical sidewall of these mesa APDs can result in the formation of electrically active defects that degrade device performance through increased dark current or premature breakdown.
In order to reduce the impact of these sidewall surfaces on device performance, mesa structures are typically coated with a passivation material to satisfy dangling bonds and reduce the overall density of active surface states. In this work, dielectrics formed by atomic layer deposition (ALD) are explored as side-wall passivation layers for mesa-isolated gallium nitride APDs. ALD is a technique that is capable of depositing continuous thin films via a series of alternating, self-limiting chemical reactions with the substrate surface, in contrast to more traditional methods like chemical vapor deposition (CVD) in which precursors react in the vapor phase and are physisorbed onto the intended substrate.