Use of the drift-time method to measure the electron lifetime in long-drift-length CdZnTe detectors

Abstract

The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias to evaluate the μτ product, which in turn can be converted into the carrier lifetime. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the distance. This condition is not fulfilled in practice at low applied biases, where the Hecht equation is most sensitive to the μτ product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low μτ-product material, <10−3 cm2/V, but give significantly underestimated values for the case of high μτ-product crystals. In this work, we applied the drift-time method to measure the electron lifetimes in long-drift-length (4 cm) standard-grade CZT detectors produced by the Redlen Technologies. We found that the electron μτ product of tested crystals is in the range 0.1–0.2 cm2/V, which is an order of the magnitude higher than any value previously reported for a CZT material. In comparison, using the Hecht equation fitting, we obtained μτ = 2.3 × 10−2 cm2/V for a 2-mm thin planar detector fabricated from the same CZT material.

Carrier Lifetime and Mobility Characterization Using the DTU 3-D CZT Drift Strip Detector

“…and knowing the electron drift length y d , we can estimate the µ e τ e -product. For an estimation of the error in the calculated µ e τ e -product error propagation is applied to (2). The electric field strengths are assumed to be known accurately, which results in the uncertainty in N 1 , N 2 , and y d contributing to the error.…”

“…Estimation of the electron lifetime τ e τ e is estimated utilizing electron drift times together with photopeak positions measured at different voltage bias between anode and cathode. Given the decay of the freecarrier concentration as a function of time, and assuming that the collected charge is proportional to the amplitude of the photopeak position, we have for two measurements at different bias voltage settings [2]…”

“…The electron drift distance, y d , is used to determine the µ e τ e -product as stated in (2). For a planar detector, the electron drift distance can be determined as the interaction depth (yposition) in the detector, since the drift path is perpendicular to the anode.…”

“…However, events within the boundary volumes vary close to 0.4mm in electron drift distance. Since we use the drift distance to determine the µ e τ eproduct in (2), this does impose an uncertainty introduced from the drift distance, especially for the boundary volumes.…”

“…To build a reliable model for simulation of the pulse shape formation within a semiconductor detector, the detector material properties must be well understood. A method for determining the electron mobility (µ e ) and lifetime (τ e ) has previously been described [1] [2], estimating uniform material properties of the detector. In this paper, we investigate how to use these methods to create a 3D map of µ e and τ e within the 3D CdZnTe (CZT) drift strip detector utilizing its 3D position and spectroscopic capabilities.…”