Next Generation SiC APD Deep-UV Single-Photon Detectors

We design and fabricate avalanche-breakdown photodetectors (APD) that operate in the deep ultraviolet (UV). The APDs will be able to operate in Geiger mode, where an electrical pulse is generated when a deep UV photon strikes it. CoolCAD uses the wide bandgap semiconductor SiC to fabricate these APDs. SiC has a number of advantages, especially in high-sensitivity visible-blind UV detection, and the advantage of an extremely small intrinsic carrier concentration, which allows SiC photodiodes to display very low noise without the need for cooling. CoolCAD has already fabricated SiC APDs for different applications and different specifications. For the present work CoolCAD plans to revise its design and processing to build SiC deep UV APDs to meet the requirement of the program call. These specifications include: (1) Single Photon Detection Efficiency (SPDE) greater than 15 percent, over a 50 nm range within the 200-375 nm range.Ā (2) Dark Count Rate Density (DCRD) less than 1.0 MHz square mm, with no, or minimal, cooling. (3) Maximum Count Rate that is greater than 1.0 MHz. (4) UV/Visible Rejection Ratio that is greater 1000. (5) Develop High Resolution Readout Electronics that, in addition to counting Geiger pulses, is able to optimize bias and threshold detection levels to maximally differentiate between dark and real photons initiated counts. SiC APDs depart from ideal performance due to bulk defects as well as defects and dangling bonds at the surface of the material. To mitigate these defects, we will utilize high temperature annealing in nitric oxide, as well other gaseous anneals in hydrogen and argon. Ā Also, we will introduce carbon to passivate carbon lattice vacancies that cause traps that reduce carrier lifetime. To identify and passivate traps quantitatively, we will apply advanced Density Functional Theory and chemical theoretical modeling, which will guide the fabrication, especially for defect passivation. In addition, reduced photon detection efficiency also comes from some photon absorption that is too close to the APD surface to generate electron-hole pairs that can initiate avalanches, and thereby not produce Geiger mode pulses. We will fabricate SiC APDs using a three-level epitaxial layer structure grown on a SiC N-type handle wafer. From the top down, we will fabricate both, N+/N/P+ and P+/P/N+ layered APD structures, with a 1.1um thick avalanche region which is designed using TCAD to achieve minimum volume for dark electron-hole pair generation, as well as top built-in field for e-h separation near the surface. The characteristic of N vs. P structures will be analyzed and down selected according to ability to meet specs. The APDs will be etched mesa structures with beveled sidewall with optimal pitch so that the voltage drops over an extended length to reduce the sidewall electric field to avoid surface currents and sidewall breakdown.