Metamorphic Substrate and Fabrication Process for Solar-Blind UV Imaging Arrays

Imaging in the ultraviolet (UV) is important for heliophysics, astrophysics, exoplanet survey and solar system exploration missions. Many of the currently used UV detectors are sensitive to longer wavelength (i.e. visible) light pollution, and so instruments employ filters to block wavelengths longer than those of interest; however, these filters also attenuate the UV radiation, reducing the external quantum efficiency (EQE) of the instrument. Visible wavelengths of light have energies less than the bandgaps of InGaN and AlGaN, and so are not absorbed by them, thus UV detectors based on (In,Al)GaN are insensitive to visible light (i.e. solar blind) and do not require any filters, which increases their EQE. A significant problem preventing AlGaN and InGaN-based UV imaging devices from realizing their true potential is the presence of dislocation defects that result from growing active layers of (In,Al)GaN on lattice-mismatched substrates. The availability of a low-dislocation, lattice-matched virtual substrate would present significant benefits to the availability and performance of (In,Al)GaN UV imaging devices. Other semiconductor technologies make use of lattice-matched virtual substrates that are created by depositing thick buffer layers on a suitable, although lattice-mismatched substrate. This approach fails for (In,Al)GaN because the predominant dislocation types are sessile under biaxial stress and do not glide to interact and reduce their concentration. We propose that nanometer-scale patterning of the substrate (e.g. AlN, GaN, SiC, Si or sapphire) can induce dislocations to glide and reduce in concentration within the buffer layer. The resulting virtual substrate would have a dislocation density low enough to support the fabrication of high-performance (In,Al)GaN transistors and photonic devices.

Solar-blind UV imaging is important for heliophysics and astrophysics, as the Lyman series is used to observe hydrogen in stellar coronas and around dark phenomena, as planned to be used in LUVOIR. Exoplanet surveys, such as HabEx, search for atomic and molecular oxygen emission lines in the UV. (In,Al)GaN can be fabricated as linear-mode or Geiger-mode avalanche photodiodes (APD), enabling solar-blind single-photon detection of UV radiation from atmospheric fluorescence caused by cosmic rays, which is important to Physics of the Cosmos.

Solar-blind UV detectors aid in the early detection of ballistic missiles. A low-dislocation virtual substrate will improve the performance of AlGaN/GaN high-electron mobility transistors, providing advantages for a applications in RF amplification and power control and conversion. The virtual substrate will also provide benefits for ultraviolet light emitting diodes and diode lasers.

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