Basic Limitations in the Fabrication of Radiation Detectors by Gamma Compensation of Germanium

The process of ¿-compensation as a method for fabricating Ge radiation detectors has been studied in some detail. Particular attention has been focused on the energy distribution and concentration of the levels created by the ¿-irradiation, on the differences between crystals before and after irradiation, on the field configurations in junctions with deep traps, and on obtaining the carrier trapping cross-sections due to irradiation. It is found that crystals with impurity concentrations of 5 × 1010 to 5 × 1012 (cm)-3 can be quite different from each other before and after irradiation. Compensation of n-type material to a very low n or p-type residual conductance is always feasible, although the compensation mechanism for devices at near thermal equilibrium is different from that of diodes with bias applied. From measurements with several compensated detectors, it is found that effective hole trapping cross-sections of 10-12 to 10-13 cm2 are commonly obtained, resulting in poor resolution even with initial materials of the highest purity. One exceptional crystal gave excellent resolution after compensation but suffered catastrophic failure with a small amount of over-irradiation. This particular result has helped to propose a two threshold mechanism, which, in combination with the knowledge gained from the calculation of field distributions in junctions with deep levels, may explain the results observed. Emphasis is then placed on the apparent need for extreme crystal uniformity, which, if confirmed, will result in declaring the ¿-compensation process impractical for the fabrication of radiation detectors.