Extraction of Trap Parameters in HgCdTe P/N MWIR Focal Plane Arrays Using Photocurrent Transients Induced by Optical Excitation

Abstract Since their initial synthesis in 1958, HgCdTe compounds have been extensively developed as a highly effective material for infrared detection. The performance of these materials has improved continuously in terms of the first-order figures of merit, including dark current, quantum efficiency, and signal-to-noise ratio. Nowadays, the optimization of second-order performances is a key objective, with a particular focus on low-amplitude transients of photocurrents induced by scene illumination variation. In this study, we conducted measurements of photocurrent transients induced by optical excitation on HgCdTe mid-wave infrared (MWIR) focal plane arrays (FPAs). The HgCdTe detection circuit is flip-chipped onto a silicon direct injection readout integrated circuit (DI ROIC). The transients were measured at various temperatures, wavelengths, and levels of illumination. Furthermore, the bias of the photodiodes was varied in order to examine the impact of the space charge region extension. The FPA exhibits remarkably brief transients at operational temperatures (2.6 ms at 130 K) and low amplitudes relative to typical operational requirements. Two exponential regimes with two distinct time constants were identified in each transient. By plotting the aforementioned time constants with regard to temperature in Arrhenius plots, the activation energies and cross sections of two families of traps responsible for the transients were extracted. Both trap families exhibit energies that are in close proximity to the mid-gap, situated at 40% and 60% above the valence band edge. The energies and cross sections are in accordance with the established literature on deep-level transient spectroscopy (DLTS). The measurements conducted at various wavelengths and photodiode polarizations, in conjunction with the diverse photodiode designs available on the array, led to the conclusion that the traps responsible for the transients are situated within the space charge region (SCR) of the HgCdTe photodiodes. It is our contention that this methodology could be employed as a standard diagnostic tool for the characterization of trap parameters within the SCR of photodiodes, as well as for the assessment of material quality at the ultimate stage of fabrication for an FPA using a DI ROIC.