Silicon Photodiode for Visible Spectral Region

Abstract

Photodiodes with high sensitivity in the short-wavelength portion of the visible region of the spectrum, fabricated based on high-ohmic silicon, are widely used in scintillation devices for visualization of gamma radiation and x-radiation in nuclear physics and medicine [1]. The following have been used to improve collection of minority charge carriers generated by short-wavelength photons in the immediate vicinity of the silicon surface: a shallow p-n junction with a vertical [2] or a horizontal [3, 4] structure, a graded p-n junction formed by diffusion of boron through the lattice of a thin layer of silicon oxide on the substrate surface [5], and an inversion layer photodiode [6]. The use of an inversion layer photodiode is limited by the narrow range of linearity typical of the spectral responsivity. A general disadvantage of photodiodes formed by boron diffusion into n-type silicon is the presence in the near-surface layer of silicon of a barrier layer under the oxide, preventing electrons generated by light in the p-region of the silicon from crossing into the p-n junction [7]. Photodiodes based on n-p junctions formed by diffusion of phosphorus or arsenic are more stable in the short-wavelength portion of the visible region of the spectrum [8, 9]. Their sensitivity at O 1 [12]. This allows us to ensure the minimum depth for the non-photoactive zone on the silicon surface. Arsenic is selected because of the following considerations. Its low diffusion coeffi cient makes it possible to form shallow junctions. The depth of the arsenic-doped n-region in our case is 0.2 om. Due to the relatively low atomic weight and the similar size of the ionic radii for silicon and arsenic, mechanical strains arising when doping with arsenic are relatively small, which lets us decrease the number of defects in the n-region and reduce the effect of recombination processes compared with the phosphorus-doped n + -regions [13]. The ohmic contact to the n-region is formed on the periphery of the arsenic-doped n-region. A guard ring reduces the surface component of the dark current. It is designed to be combined with the ohmic contact to the substrate. The dimensions of the photosensitive region of the photodiode are 5 × 5 mm. A two-layer SiO2/Si3N4 antirefl ective coating is formed on the photosensitive surface of the n-i-p junction.