Recent Developments in High Resistivity Detector-Grade CdTe
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RECENT DEVELOPMENTS IN HIGH RESISTIVITY DETECTOR-GRADE CdTe M. HAGE-ALI and P. SIFFERT Centre de Recherches Nuclaires (IN2P3), Laboratoire PHASE (UPR 292 du CNRS), BP 20, 67037 Strasbourg Cedex 2 (France) ABSTRACT We present a brief review of results concerning the doping or compensation of CdTe by different elements like halogen, group IV, hydrogen and 3d transition elements for high resistivity materials. Comparison with theoretical models and discussion of results under this light is provided. INTRODUCTION Detector-grade CdTe materials must follow the general requirements of nuclear detector materials, namely high Z, band gap of more than 1.3 eV for room temperature operation, large g.tcproduct for electrons and holes for good transport characteristics and high resistivity for low dark current and large sensitive volume in the mm to cm thickness range. These latter two parameters are the most important ones and the most difficult to reach. Theoretical modeling shows that the maximum of resistivity for high purity CdTe must be around 108 Ql.cm. However, experimentally obtained values reach a maximum of only 106 a.cm for the best travelling heater method (THM) material, and the more common value is around 104 Q.cm. Concerning the gt'r product, which is intimately related to impurities and defect levels in the band gap, the situation in CdTe is even more complicated. When only a few defect levels are seen in Si or GaAs, more than 20-30 levels are identified in CdTe [1-31 with different ionization states and concentrations. This leads to a large spreading in electrical characteristics mainly for p and gt which decrease as the defect concentration increases. Enhancement of material and devices quality needs an the inversion of this situation. While ultimate purification removes the greatest part of the chemical impurities, structural defects are harder to suppress, and until now, apart from one case, improvement of resistivity up to semiinsulating CdTe has been done by chemical doping or compensation by a group III element as In [4] or a group VII element (halogen) [5-71. The last group (mainly Cl) leads to p > 109 flcm with ([t), > 10-3 cm 2/V, but with some spread of results, indicating the presence of active defects at different concentrations. Recently, many high resistivity materials have been achieved with doping by elements like Cu, Fe, V, etc... and by H. Semi-insulating CdTe and CdZnxTel.x (0.04 < x < 0.20) using the high pressure Bridgman method without compensation have also been reported. These facts make a general review of some dopants or methods applied on CdTe desirable to improve our knowledge of the material and the fabricated devices. EXPERIMENTAL RESULTS
Chlorine Chlorine, in particular, and the halogens, in general, are widely used with success in CdTe compensation [5-7], the general idea being that Cl acts as a shallow donor to allow the compensation of the acceptor, generally believed to3 be a Cd vacancy VCd. However, the situation is not completely clear today. Up to 1018 cm- added3 CI is nec
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