Characterization of dark noise in CdZnTe spectrometers

Abstract

Systematic measurements of dark noise spectra of CdZnTe x-and γ-ray spectrometers, correlated with the dc I-V characteristics and detector technology, are reported. The dark noise of two innovative CdZnTe spectrometer configurations are studied: metal-semiconductor-metal (MSM) resistive detectors with three terminals as well as heterostructure PIN detectors with thermally evaporated n + CdS and p + ZnTe contacts, which are fabricated on high pressure Bridgman CdZnTe (Zn = 10%) crystals. The two innovative CdZnTe spectrometer configurations presented here exhibit very low dark (leakage) current. By reducing the dc value of the dark (leakage) current below 1 nA, shot noise becomes the dominant noise mechanism and the contribution of 1/f noise becomes negligible. The use of non-injecting contacts (evaporated gold) for the MSM detectors and the operation of the PIN detector in the reverse bias mode prevent generationrecombination noise which becomes dominant with injecting contacts (for example MSM detectors with evaporated indium and titanium contacts) or when operating the PIN detector in the forward bias mode. Surface leakage is reduced by applying surface passivation but is eliminated only by using the three terminal MSM configuration which exhibits simple shot noise instead of the suppressed shot noise observed in the two terminal MSM spectrometers. The noise measurements are useful for optimizing detector technology.

Noise in CdZnTe detectors

“…The surface current, although much larger than the bulk current, apparently did not contribute any significant noise. The suppression of shot noise in surface leakage current has also been reported in earlier publications [6], [7].…”

“…This indicates that the observed 1 noise at the preamplifier output orig- inates as a -type noise source in parallel to the amplifier input, i.e., between the two strips. This preamplifier-output referred 1 noise must be distinguished from the 1 noise discussed in some of the publications, which refers to the current noise generated at the device [6], [7]. If integrated on a charge-sensitive amplifier, such 1 current noise source would produce a 1 -type noise at the output.…”

“…It is therefore important to understand the origin of noise in these types of devices so that methods to minimize their adverse effects on detector performance can be developed. There are only a small number of publications concerning the detailed investigation of noise behavior in CdZnTe [6] and CdTe [7], [8]. In these publications, the noise measurements are presented in terms of spectral distributions, which makes interpretation in terms of energy resolution less direct.…”

Spectroscopic evaluation of n-type CdZnTe gamma-ray spectrometers

“…The model considers the effect of the variance of the collected charge and ignores the effect of additional noise sources such as detector dark noise 22,23 and the noise contributed by the readout electronics. It is worthwhile noting that 0 eN ∫ S(Q)dQ = 1 and this can be used to check the calculation of the spectrum S(Q), which is given by Eq.…”

Effective Surface Passivation of CdMnTe Materials

“…1,2 Dark currents, including those in the surface and bulk, are the leading source of electronic noise and are responsible for degrading the detector's energy resolution. 3 The detector material itself determines the bulk leakage current, but the surface leakage current can be controlled by appropriate passivation layers. The main mode of passivation is via the formation of oxides or sulfides on the semiconductor surface to eliminate any dangling bonds.…”