Physical-Chemical Considerations for Semiconductor Room-Temperature Radiation Detectors
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PHYSICAL-CHEMICAL CONSIDERATIONS FOR SEMICONDUCTOR ROOM-TEMPERATURE RADIATION DETECTORS M. SCHIEBER, H. HERMON AND M. ROTH School of Applied Science, The Hebrew University of Jerusalem, 91904, Israel. ABSTRACT Physical properties of large band gap semiconductors such as: HgI 2 , CdTe, Cd 0 .8Zn0 .2 Te, CdSe, Cd 0 .7 Zn 0 .3 Se, GaAs, Pbi 2 and TlBr are briefly reviewed and discussed in terms of their use as room temperature operating x-ray and gamma ray radiation detectors. It is shown that
HgI2 which has the largest drift length for holes, AL~prE, i.e., the product of the mobility Ph,
life time m and the electrical field E, is at present the leading material, being followed by the newly developed Cd 0 .8Zn0 .2 Te. Chemical defects in HgI2 were enhanced by doping the material with aliphatic, aromatic and oxyhydrocarbons as well as with excess Hg and 12 and the increase in unit cell parameter was studied as a function of the amount of dopant. The value of r was measured as a function of dopant concentration and it was found that Hg doping causes the most severe trapping defects. Low temperature studies of mh down to 170K allowed the identification of the trapping energy levels and concentration of electrically active defects. Shallow traps of 0. 13-0.18 eV stemming from deviation from stoichiometry of H-gI 2 were found to be in the ppm level whereas deeper traps of 0.4-0.5 eV stemming from hydrocarbons were found to be in the ppb level. It is concluded that only extensive research on the physical, chemical and structural defects correlated with improved crystal growth and device fabrication methods, would lead, in the future, to improvements in 2 h also of the other large Eg semiconductor detector materials. INTRODUCTION Room temperature operating x-ray and yf-ray semiconductor radiation detectors which are efficient in their charge collection and which are also available commercially, have long been needed by the scientific community. Only lately (1992) instruments based on HgI 2 x-ray spectrometers entered the market. The other alternative material, CdTe, has generally produced only radiation counters and it entered the market a few years earlier. Both materials have been studied in depth and some early reviews have been published by one of the authors [ 1,2]. Since then the literature has been flooded by high quality papers on these two materials, but both of them still have problems and limitations [3]. Many attempts were also made to study other room temperature materials. Several reviews are available, including [4,5] which describe the status of semiconductor radiation detectors [6,7]. The materials are listed below in Table I of this paper. What are the criteria required to select room temperature operating semiconductor radiation detectors? Some of these criteria are mentioned in various review articles cited in the references. Most are based on energy gap, Eg, average atomic number, Z, or charge transport properties such as mobilities, p, or lifetime, r, or their products, pet e or /-hrh, for electrons a
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