Photo-induced excess low frequency noise in HgCdTe photodiodes

Abstract

We have investigated the properties of excess low frequency noise in illuminated mid wavelength infrared and long wavelength infrared HgCdTe photodiodes at zero bias. The current power spectrum (S_i) dependence is usually close to inverse frequency (f), but substantial variations have been observed. The magnitude of l/f spectra is voltage independent for small bias voltages, but is proportional to the square of the photocurrent (I). Consequently, the l/f knee increases, with photocurrent. Variable area device studies indicate that the noise sources are more closely associated with the device area (A_j) than perimeter, indicating bulk limitations. The power spectrum can be represented by an empirical relationship of the form S_i=α_phI ²/fA_j. This defines a figure of merit, α_ph which takes into the account the relationship between current dependence and device geometry. α_ph is device dependent, suggesting that randomly distributed defects play a role in the difference. This is also supported by noting that devices fabricated in material grown on lattice matched substrates have lower α_ph (10⁻¹⁶ cm²) than those fabricated in material grown on nonlattice matched substrates (10⁻¹⁴ cm²), which usually have two orders of magnitude larger dislocation density. We conclude that photo-induced l/f noise can be reduced via defect reduction and is not fundamental. Data on our best devices indicates that α_ph is somewhat lower for smaller band gap material. The temperature dependence of photo-induced excess low frequency noise is much weaker than that of bias induced excess low frequency noise, indicating unrelated generation mechanisms. In addition, photo-induced l/f adds in quadrature with bias induced l/f noise and is not well correlated in magnitude with either bias induced l/f noise or detector dark currents.