Doping, compensation, and photosensitivity of detector grade CdTe
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J. Franca) Charles University, Faculty of Mathematics and Physics, Institute of Physics, Prague CZ 121 16, Czech Republic
A. Fauler and M. Fiederle Materialforschungszentrum, Freiburg D-79104, Germany
R.B. James Nonproliferation and National Security Directorate, Brookhaven National Laboratory, Upton, New York 11973 (Received 12 February 2008; accepted 4 March 2008)
We studied the resistivity, photosensitivity, photoluminescence, and surface photovoltage of CdTe crystals doped with Ge or Sn to extend our knowledge of the influence of the deep-donor level on compensation and afterglow effects. We demonstrated a strong correlation between photosensitivity caused by photoelectrons with Fermi-level variations near the GeCd0/2+ or SnCd0/2+ energy levels. Surface photovoltage measurements confirmed that when the concentration of residual acceptors varied along the direction of growth, then trapping conditions dramatically changed as a defect was converted from a neutral state to doubly charged positive one.
I. INTRODUCTION
Electronic deep levels in CdTe and CdZnTe (CZT) materials used in x-ray and gamma-ray detectors are of considerable scientific and practical importance because they greatly affect resistivity, charge collection, and the afterglow effect.1,2 High resistivity is vitally important for detector applications of semiconductors because of the need for having a low-leakage current. Nonetheless, while semi-intrinsic resistivity is a necessary condition, it alone does not suffice for CdTe and CZT to be used as detectors; other important conditions are low trapping and afterglow effects. In growing semi-intrinsic CdTe and CZT semiconductors, we must compensate for the surplus of shallow-level acceptors because their concentration, ranging from 1015 to 1016 cm−3, far exceeds the intrinsic concentration of the free charges.3,4 Deep-level donors help in tuning of compensation conditions by transferring their electrons to acceptors and, importantly, stabilize the compensation condition against spatial variations in the concentrations of shallow-level donors. For example, when the concentration of shallow donors decreases, the neutral deep donors give up their electrons
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0198 J. Mater. Res., Vol. 23, No. 6, Jun 2008
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to the acceptors. In other words, deep donors act as a buffer, and, in so doing, they are transformed from a neutral state to a positively charged one, thereby increasing the concentration of electron traps.2,5 However, their concentration in spectroscopic grade CZT must not exceed 1011 to 1012 cm−3.2,5,6 When the concentration of deep-level impurity exceeds this value, then the ionized part of this impurity must not be greater than 1011 to 1012 cm−3. Under such conditions, thermally ionized deep levels supply free electrons in the C-band, and, to minimize the conductivity value, their contribution must be comparable with the intrinsic concentra
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