Generation-recombination effects on dark currents in CdTe-passivated midwave infrared HgCdTe photodiodes

Abstract

The effect of an abrupt CdTe∕HgCdTe passivation heterointerface on generation recombination and dark currents in n-on-p midwave infrared photodiodes with 5.2-μm cut-off wavelength has been investigated. Experimentally, it was observed that the zero-bias-dynamic resistance, R0, at low temperatures scales with the perimeter of the n-on-p junction, rather than with the junction area, suggesting that surface effects are dominant. The diode current–voltage characteristics at low temperatures indicate significant contributions from tunneling effects, which is the dominant leakage current mechanism for reverse bias greater than approximately 30mV. These two observations suggest that the region where the junction terminates at the CdTe∕HgCdTe abrupt interface is responsible for the above effects. A two-dimensional model has been developed to investigate the dark current mechanisms in the vicinity of the junction termination at CdTe∕HgCdTe interface, which also takes into account the effect of dislocations on generation-recombination processes. Calculated profiles of the energy bands and electric field along different cross sections of the photodiode indicate that the electric field achieves a maximum value of the order of mid −105V∕cm in the region where the junction terminates at the CdTe∕HgCdTe interface. The presence of such a high localized electric field in this area decreases the ionization energy of trap levels in the band gap and, hence, increases efficiency of the Shockley–Read–Hall generation-recombination processes. In addition to diffusion, generation recombination, and trap-assisted tunneling mechanisms, the calculations of dynamic resistance include Poole–Frenkel and phonon-trap-assisted tunneling effects. The best fit to the zero bias dynamic resistance versus temperature results has been obtained using an aerial dislocation density in the bulk of the HgCdTe layer equal to 106cm−2. Although the direct band-to-band tunneling and impact ionization processes were also considered, their contributions are shown to be insignificant.