Germanium-Silicon Separate Absorption and Multiplication Avalanche Photodetectors Fabricated with Low Temperature High Density Plasma Chemical Vapor Deposited Germanium

Abstract

A desire to monolithically integrate near infrared (NIR) detectors with silicon complementary metal oxide semiconductor (CMOS) technology has motivated many investigations of single crystal germanium on silicon (Ge/Si) diodes [1-3]. Reduction of the epitaxy thermal budget below the typical chemical vapor deposition (CVD) in-situ clean temperature (Tin-situ clean > 780°C) is also increasingly desired to reduce integration complexity. Reduced temperature growth approaches have included p⁺-Ge/n-Si detectors formed with low temperature poly-Ge (e-beam evaporation) or heavily dislocated single crystal germanium (molecular beam epitaxy, T ~ 450°C), which have had dark currents of ~5 mA/cm² and responsivities of ~15 mA/W at 13¹⁰ nm, despite the large number of defects in and at the Ge/Si interface. Responsivities in these materials are however low and believed to be limited by a small diffusion length (i.e., 5-30 nm [2, 4]) due to fast electron recombination in the defect rich germanium. In this paper, we evaluate a commercially available high density plasma chemical vapor deposition (HDP-CVD) process to grow low temperature (i.e., Tin-situ & Tepitaxy < ~450°C) germanium epitaxy for a p⁺-Ge/p-Si/n⁺-Si NIR separate absorption and multiplication avalanche photodetectors (SAM-APD). This device structure is of interest both to examine ways to enhance the responsivity with internal gain as well as to examine alternatives to InGaAs-InP structures for NIR Geiger mode (GM) detection. A silicon avalanche region is highly desirable for GM to reduce after-pulsing effects, which are related to defect density that are smaller in Si than in InP [5]. Despite the high defect densities in the Ge and at the interface, the Ge-Si APDs in this work are found to have relatively low dark count rates in Geiger mode.