First-principles study of defects and carrier compensation in semiconductor radiation detector materials
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1164-L08-02
First-principles study of defects and carrier compensation in semiconductor radiation detector materials Mao-Hua Du, Hiroyuki Takenaka, and David J. Singh Materials Science & Technology Division and Center for Radiation Detection Materials and Systems, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ABSTRACT We discuss defect engineering strategies in radiation detector materials. The goal is to increase resistivity by defect-induced Fermi level pinning without causing defect-induced reductions in the carrier drifting length. We show calculated properties of various intrinsic defects and impurities in CdTe. We suggest that the defect complex of a hydrogen atom and an isovalent impurity on an anion site may be an excellent candidate in many semiconductors for Fermi level pinning without carrier trapping. INTRODUCTION Semiconductor radiation detectors need to have high resistivity in order to suppress free carrier noise.1 The compound semiconductors that give large enough band gaps for room temperature operation cannot be sufficiently purified to obtain intrinsic semiconductor behavior in contrast to small-gap elemental semiconductors like germanium. High resistivity in large-gap compound semiconductors approaching the theoretical intrinsic semiconductor limit can be made possible by the presence of midgap impurity or defect levels that pin the Fermi level, as schematically shown in Fig. 1(a). A typical example is semi-insulating GaAs, whose high resistivity is made possible by As antisite (AsGa) defects that induce a midgap energy level.2 However, the deep levels are also efficient nonradiative recombination centers that greatly reduce the carrier drifting length, the figure of merit for semiconductor radiation detectors. This problem severely limits the applicability of GaAs as a room-temperature radiation detector. The same carrier compensation mechanism, i.e., Fermi level pinning by anion antisites, has been suggested for CdZnTe (CZT),3, 4 the best room temperature radiation detector available now. What is puzzling is that if the anion antisite is the common Fermi level pinning mechanism for both GaAs and CZT, why does the same limitation on carrier drifting length not affect CZT as severely as GaAs. We note that, in contrast to GaAs, in which the midgap donor related to AsGa has been confirmed by numerous experiments,2 the midgap donor in CdTe or CZT, supposedly TeCd by conventional wisdom, has never been unambiguously observed experimentally. A midgap level at Ec – 0.75 eV has been found in n-type CdTe by deep level transient spectroscopy5, and was tentatively assigned to TdCd4. However, this level was observed only after the CdTe was annealed under saturated Cd vapor. As we will show later, this level is very likely a Cd interstitial level. Moreover, the recent thermoelectric effect spectroscopy measurements did not find any midgap donor levels in CZT.6 The carrier compensation mechanism shown in Fig. 1(a) is intrinsically problematic, as a large number of deep donors leads to effective ele
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