p-i-n High-Speed Photodiodes for X-Ray and Infrared Imagers Fabricated by In Situ-Doped APCVD Germanium Homoepitaxy

Design, simulation, fabrication, and measurement have been performed using a new method for obtaining high-speed p-i-n germanium photodiodes for use in backside imaging arrays. These devices are proof-of-principal in situ -doped vertical mesa structures fabricated using single-wafer, atmospheric-pressure chemical vapor deposition (APCVD) homoepitaxy on Czochralski-grown Ge substrates. Excellent impurity profiles resulting from in situ doping, as measured by spreading-resistance profilometry, were ${5} \times {10}^{{16}}$ , ${2} \times {10}^{{15}}$ , and ${2} \times {10}^{{18}}$ (cm−3) for the p-type anode, i-region absorber, and n+ cathode, respectively. Scanning electron microscopy analysis revealed very-low defect densities in the epitaxial layers. The diodes, with diameters ranging 100– $500~\mu \text{m}$ , exhibited excellent ideality ( ${n} = {1.15}$ ) and less than 1- $\mu \text{A}$ dark current, corresponding to up to 80 kV-cm−1 electric field in the smallest devices. Avalanche breakdown bias was consistently at a magnitude of 50 V or lower. External quantum efficiencies in the near-infrared spectrum peaked at 70%. Pulsed illumination experiments, both in the infrared and X-ray regions, showed pulse rise times in the 100–140 ps range, indicating excellent applicability of this method to high-speed X-ray or infrared applications.