Advanced electrical characterization of defects in silicon photodiodes

Abstract

Photodiodes made of silicon, the predominant material in the semiconductor-based sensor industry, are so pure, that already spurious amounts of contamination or a tiny fractional amount of crystallographic mismatch may cause detrimental effects on the device performance. The fundamental reason for this device degradation is, that electronic trap states within the band-gap are created, and charge carriers may interact with these states disturbing the nominal operation. Therefore, the characterization of such defects in photodiodes is essential to understand, identify and eradicate trap-related performance degradation, as well as to design trap-resistant devices. An important parameter for opto-electronic devices, that is directly related to trap states, is the minority carrier lifetime. However, the extraction of lifetimes in photodetectors poses a formidable challenge, since state-of-the-art devices often consist of pn-junctions embedded in epitaxial layers grown on a substrate. In this work, the extraction of lifetimes on such structures has been reviewed and compared among different selected methods. It was found that the ratio of diffusion length to the thickness of the epitaxial layer is a critical parameter resulting in effective lifetimes possibly much lower than the trap related minority carrier lifetimes. The reverse recovery and open circuit voltage decay methods are excellent candidates to extract these effective carrier lifetimes. With the combined current-voltage and capacitance voltage method, however, lifetimes corresponding to diffusion lengths much larger than the layer thickness can be extracted, because the main signal investigated by this method originates from the junction depletion zone. Another state-of-the-art concept in nowadays photodiodes is the utilization of deep trenches filled with an insulating material as terminator or separator between individual devices. However, by means of measurements and simulations it is shown that trap-related injection-dependent interface recombination at the sidewalls of the trenches may lead to non-linear responsivities with respect to the radiant flux. Different concepts to mitigate this issue are proposed and discussed, which is supported by simulation results. To draw conclusions about the nature of the trap states located at the sidewalls, a deep level transient spectroscopy study has been performed on dedicated test samples. Based on these results, it is shown that silicon dangling bonds at the silicon to silicon-dioxide interface may be the root cause of the observed injection-dependent recombination rates.