Heavy metal doping of CdTe crystals

CdTe crystals doped with several heavy metals (Hg, Tl, Pb and Bi) in the concentration range of 10¹⁷ - 10¹⁸ at/cm³ have been grown by the vertical Bridgman method. The structural quality and uniformity of the crystals was verified by chemical determination of etch pits density and by X-ray rocking curves. Inductively coupled plasma-mass spectroscopy (ICP-MS) was employed for evaluating the dopant concentration. Low temperature photoluminescence (PL) measurements of the crystals were performed in the 1.2 - 1.6 eV energy range. The crystal electrical properties were studied by Hall and I-V measurements. The obtained PL data show a domination of (D-X) and (DAP) lines in CdTe crystals, suggesting n-type conductivity and the presence of a compensation mechanism due to the heavy dopant incorporation. Investigating the 1.4 eV band, we obtained the Huang-Rhys parameter, the principal energy of zero phonon line and the energy of longitudinal optical phonon for each dopant. Impurities were measured by ICP-MS. A particular PL line at 1.473 eV, correlated to M_Te (M = heavy metal) defect centers, was found. It was found that the heavy metal doped CdTe crystals have resistivity values in the range 10⁷ - 10¹⁰ Ω·cm, with maximum values for Bi and Hg doped materials. Also these materials show a higher dark current stability than the ones doped with the other heavy metals. Detectors made from Hg doped crystals gave response to X and γ radiation.