Improvement in leakage characteristics of Ge-on-Si photodetectors

Germanium (Ge) offers a distinct advantage over silicon (Si) in terms of its superior electron and hole mobility. In addition, its small direct bandgap of 0.8 eV at room temperature makes it possible to design efficient photodetectors operating in the low-loss optical fiber range of 1.3-1.6 μm
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. Silicon, on the other hand, is a well established material for integrated circuits and has superior mechanical and thermal characteristics. The origin of dark currents in Ge-on-Si photodetectors is primarily due to the high density of threading dislocations (TDD) associated with the 4.2% lattice mismatch between Si and Ge. Most of the solutions to date for reducing dark currents have focused on the substrate/Ge film interface. Annealing and the use of SiGe buffer layers has been shown to be effective in the reduction of dark current in Ge-on-Si photodiodes. Dark currents between 296°K and 400°K have been shown to be associated with defect-related generation centers close to half the band gap energy of Ge, indicating prevalence of the Shockley-Read-Hall process. However, the measured dark current is the superposition of current contributions from three regions: bulk, depletion and surface. In this paper, we study the impact of bulk and the surface contributions on the total dark current. We demonstrate that the Ge-film/doped contact interface can be modified to reduce dark currents, which provides an additional option in the design of photodetectors with low leakage current. The modification at this interface not only leads to the reduction in the bulk leakage, but surface leakage component as well.