Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Abstract

Cd1−xZnxTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)h of the CZT detector measured by DC photoconductivity is 7.0 × 10−4 cm2/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D0, X) or neutral acceptors (A0, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D0, X) depends linearly on the incident laser intensity, while (A0, X) has a nonlinear dependence.