A theory for image persistence in HgCdTe photodiodes

Abstract

Image persistence, an elevated dark current caused by prior illumination, has been a chronic problem in near infrared focal plane arrays. It has the potential to interfere with astronomical surveys requiring precision photometry, astrometry and/or shape measurement. We posit a mechanism whereby charge is captured by traps exposed to carriers as the depletion width shrinks when photo-generated charge accumulates on the electrically isolated photodiode. After the depletion width is reestablished by resetting the diode, trapped charge is slowly released, appearing to be a signal during the subsequent exposure(s). We present data illustrating the complex behavior of persistence for several 1.7 µm cutoff HgCdTe arrays on Teledyne H2RG multiplexers, and show that our simple model predicts this behavior at least qualitatively. We identify one inconsistency that has yet to be resolved and discuss mitigation options.

A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope’s Wide Field Camera 3 Near-IR Detector and Its Applications to Transiting Exoplanets and Brown Dwarfs

“…These studies aimed to describe the effects of charge trapping on the data by fitting empirical functions to the observed trends, an approach that only works imperfectly and only applicable within sets of self-similar observations. The model we introduce here invokes the same physically-motivated conceptual picture as previous studies(e.g., Smith et al 2008a); however, our approach differs from the past qualitative and quantitative models in two important aspects: First, our model assumes that charge trapping occurs immediately after the photons are absorbed and not later, i.e., charge trapping occurs only when the detector is illuminated. This difference results in a different behavior of the detector, which we demonstrate is more consistent with the existing observations than the predictions of the traditional model that assumes continuing trapping even in the non-illuminated state of the detector.…”

“…The width of the undepleted regions (see Figure 1 of Smith et al 2008a) increases with fluence level, which potentially enlarges the number of available traps. Indeed, Long et al (2012Long et al ( , 2015a showed that the persistence of the WFC3 IR detector surged when fluence reached near saturation, which could suggest significant trap density increase at saturation.…”

“…This can change the response of the detector during a series of exposures (Rieke 2012), and is suggested to be the primary source of image persistence (Smith et al 2008a tion region of the P-N junction are responsible for the image persistence, and based on this assumption established a qualitative theoretical framework to explain the image persistence phenomenon. Ibid, charge carriers can be trapped as they diffuse across the depletion region.…”

“…Our model is based on the charge carrier trapping theory of Smith et al (2008a). However, instead of fitting empirically derived exponential functions, our model enables us to quantitatively model the charge carrier trapping processes, so that we can precisely calibrate ramp-effect-impacted timeresolved observations made with WFC3.…”

“…The theories of charge carrier trapping (e.g., Smith et al 2008a) suggest that the total number of charge carrier traps can increase if the illumination fluence levels are very high. The width of the undepleted regions (see Figure 1 of Smith et al 2008a) increases with fluence level, which potentially enlarges the number of available traps.…”