Effects of bulk and surface conductivity on the performance of CdZnTe pixel detectors

Abstract

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metalsemiconductor-metal system with two back-to-back Schottkybarrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material.This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip.

Synchrotron radiation response characterization of coplanar grid CZT detectors

“…It should be mentioned, that similar variations in the device response were also observed with pixel [11], drift-field [12], coplanar-grid [13], and other devices that employ steering electrodes.…”

Characterization of CdTe and (CdZn)Te detectors with different metal contacts

“…That way produced detector has a lower leakage current than a detector with two ohmic contacts; however, detector is polarizing under the Schottky contact, thus worsening the detection characteristics 8,9 . Another option is to create guarding electrode -guard ring (GR) which is a useful tool for the separation of leakage current from current flowing through the sample volume 10,11 . Basically it is a metal contact fully surrounding the cathode.…”

Non‐uniformity effects in CdZnTe coplanar‐grid detectors

“…According to the results in Refs. [28,29], at low differential biases, <10 V, the surface between the contacts behaves like a field-effect transistor. At intermediate biases, when the surface channel is fully depleted, the current becomes diffusion limited (CZT is a slow semiconductor).…”