Physical-chemical considerations for semiconductor room-temperature radiation detectors

Physical properties of large band gap semiconductors such as: HgI₂, CdTe, Cd_0.8Zn_0.2Te, CdSe, Cd_0.7Zn_0.3Se, GaAs, PbI₂ 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, λ = μτE, i.e., the product of the mobility μ_h, lifetime τ_h and the electrical field E, is at present the leading material, being followed by the newly developed Cd_0.8Zn_0.2Te. Chemical defects in HgI₂ were enhanced by doping the material with aliphatic, aromatic and oxyhydrocarbons as well as with excess Hg and I₂ and the increase in unit cell parameter was studied as a function of the amount of dopant. The value of τ 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 τ_h down to 170 K 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 HgI₂ 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 λ_h also of the other large E_g semiconductor detector materials.