High quantum efficiency, back-illuminated, crystallographically etched, silicon-on-sapphire avalanche photodiode with very wide dynamic range, for manufacturable high resolution imaging arrays

Abstract

There is a growing need in industrial and scientific research applications for very wide dynamic range, high sensitivity imaging methods. To fill this need, an advanced back-illuminated avalanche photodiode (APD) design is presented based on crystallographically etched (100) epitaxial silicon on R-plane sapphire (SOS), enabling large single photon sensitive, solid-state focal plane arrays with very wide dynamic range. The design will enable reliable and low cost fabrication. When (100) silicon is properly etched with KOH:IPA:H2O solution through a thermally grown oxide mask, square based pyramidal frustum or mesa arrays result with the four mesa sidewalls of the APD formed by (111) silicon planes that intersect the (100) planes at a crystallographic angle of 54.7 degrees. The APD is fabricated in the silicon mesa. Very wide dynamic range operation at high quantum efficiencies exceeding 50% for 250 nm<lambda<400 nm and exceeding 80% for 400 nm<lambda<700 nm, is enabled by a carefully optimized p-type boron spatial impurity profile, fabricated by high temperature diffusion. Monte Carlo modeling confirms the majority of electron-hole pairs are photogenerated outside of the narrow device multiplication region, resulting in a very low APD excess noise factor.