Fluorescence lifetime imaging microscopy analysis of defects in multi-tube physical vapor transport grown Cd1−x Zn x Te

Abstract

Cadmium zinc telluride (CZT) is the material of choice for high-energy room-temperature X-ray and γ-ray detectors. However, the performance of pixelated detectors is greatly influenced by the quality of CZT. Crystal defects and impurities are one source of shallow and deep level traps for charge carriers. Fluorescence lifetime of the recombination of optically excited charges may indicate the presence and type of defects and impurities in CZT. Fluorescence lifetime imaging microscopy (FLIM) is used to examine the excited-state lifetime in CZT fabricated by different growth methods and conditions. The FLIM set-up analyzes luminescence emitted from the sample following photo excitation. Samples were optically excited above band gap with a pulsed laser (590 nm) for raster scanning a 220 × 165 µm2 sample area. In-situ room-temperature photoluminescence (PL) and FLIM were recorded simultaneously. In order to analyze the FLIM data, two dominant charge carrier decay processes (τ1, τ2) were identified. The luminescence signal decays with a rapid lifetime of τ1 ≈ 50–200 ps, and a large variety of long-lifetime components τ2 were found in the range of 225–900 ps. CZT grown by multi-tube physical vapor transport (MTPVT) showed extremely long-lived recombination decay times up to 3.5 ns in the vicinity of the interface at growth start. Further away from this interface, the recombination lifetime was in the typical range of fast transitions similar to those found in detector-grade CZT fabricated by travelling heater method. Crystalline material quality strongly influences FLIM lifetime. Time-resolved transients of MTPVT-grown CZT compared with industry-leading detector grade CZT (dots: measured data; lines: fitted exponential decay curves).