Electron velocity enhancement in silicon drift detectors by means of deep n-implants

Abstract

We present a technique to enhance the average electron drift velocity in Silicon Drift Detectors. Regions of deep n-implants have been introduced underneath the p+ field strips in close proximity to one detector surface. The presence of the deep n-implants reduces the residual ripple in the bottom of the channel caused by the segmentation of the p+ field strips. In this way higher average drift velocities at a given drift field and proper work even at very low drift fields with respect to the conventional case (no deep implantation is present) can be obtained. Moreover deep n-implants guarantee a better stability in the electron transport. A Silicon Drift Detector has been designed to experimentally verify the effect of deep n-implants on the electrons' transport. Drift fields from values as low as 25 V/cm up to 500 V/cm have been tested. At 300 V/cm with the electrons drifting at 11 /spl mu/m from the detector front surface, the measured average drift velocity is 0.38 cm//spl mu/s, showing a reduction of 9% with respect to the ideal value (bulk electron mobility /spl times/ applied drift field). Without deep n-implants the reduction in the average electrons velocity is 17%. The presence of deep n-implants allows proper electron drift also at drift fields as low as 25 V/cm. The achieved average drift velocity is 0.027 cm//spl mu/s, showing a reduction of less than 23% with respect to the nominal drift velocity.