Kihyun Kim a, Aleksey E. Bolotnikov b, Ralph B. James c
a School of Health and Environmental Science, Korea University, Seoul, 02841, Republic of Korea
b Brookhaven National Laboratory, Upton, 11973, NY, USA
c Savannah River National Laboratory, Aiken, 29808, SC, USA
## Abstract
Te secondary phase defects are known as major obstacle for improving charge carriers transport properties of CdTe compounds material. The enhanced detector performance was reported for Te inclusions free CZT detector implemented through two-step annealing (i.e., first in Cd and second in Te). We applied the time-of-flight (TOF) technique to measure the electron lifetime in as-grown and two-step annealed CZT material. TOF has several advantages over Hecht equation fitting such as independence of the electric field distribution, direct measurement of carrier lifetime, and high accuracy for high mobility-lifetime product CZT material. The detector performance improved drastically for the two-step annealed CZT detector except which containing inordinate number and size of prismatic punching defects. The average electron lifetime increased more than 10 times after successful two-step annealing.
## Introduction
CdZnTe (CZT) is the leading material for large-volume room temperature X- and gamma-ray detectors. High performance radiation detectors require good electrical properties such as high carrier mobility and long carrier lifetime. Te secondary phase defects in CdTe compounds refer to the presence of Te-rich secondary phase within the crystal structure, like Te inclusions and Te precipitates. Te inclusions which bigger than 5 μm are most critical effect on the charge trapping [1] and internal electric field distribution [2]. Te inclusions are generated during the CZT growth process at the unstable liquid–solid interface by capturing of the Te-rich melts. Te inclusions can be removed easily by Cd overpressure annealing between 600 and 800 °C but the resistivity of CZT samples drop down from 10 -10 Ω cm to 10 -3 Ω cm. There is several research which report successful Te inclusions removal while keeping its resistivity through two-step annealing (i.e. first annealing under Cd vapor and second annealing in Te ambient environment) [3], [4] or Te vapor annealing [5], [6].
The detrimental effect of Te secondary phase defect can be overcome applying high bias voltage for 2–3 mm thick planar CZT detectors. However, cumulativeness of small-sized Te inclusions in large volume yield critical effect on the non-uniform internal electric fields and severe electron clouds trapping.
The principle of time-of-flight (TOF) technique is measuring the dependence of the collected charge versus the drift time across a long sample. TOF has several advantages over Hecht equation fitting. It is independent on the electric field distribution in sample and offer a direct measurement of carrier lifetime. Also, it is very sensitive technique for measuring carrier lifetime in high carrier lifetime and mobility products (>10−2 cm2/V) materials [7].
Te inclusions free CZT exhibited improved carrier lifetime-mobility products and enhanced detector performance in the gamma-ray spectroscopy. It might be caused by the minimized carrier trapping and scattering due to the Te secondary phase defect [8]. In this paper, the effects of two-step annealing on the electron lifetime was quantitatively analyzed by TOF technique.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/evaluation-of-electron-lifetime-for-te-inclusions-free-cdznte.html