Improved Techniques for Making P+-I-N+ Diode Detectors

Techniques for achieving a thin dead layer exhibiting low sheet resistance on the n+ side of a p+-i-n+ diode made by the lithium drift process have been developed. A controlled quantity of lithium was diffused through a 1-to 2-micron phosphorus doped layer on the silicon diode. The phosphorus doped layer provided low sheet resistance. Because most of the lithium diffused layer was drifted into the bulk material, dead layers of less than 7 microns thickness were achieved. Detectors made by this technique have given 23 kev (fwhm) resolution for gamma rays and monoenergetic electrons at room temperature limited by diode noise. Detectors cooled to 78°-195°K gave 6.5 kev resolution for Cs137 conversion electrons (625, 655 kev) and Pb207 x-rays (74, 90 kev). Detectors stored without bias voltage at room temperature did not change performance over a four-month period. An analysis of the drift parameters has shown that the lithium drift rate depended upon the power dissipated in the diode during drift. An automatic control system was developed which allows the lithium drift operation to proceed at power dissipations in excess of 50 watts.