Investigation of a Charge Pumping Effect in Silicon P-I Diodes at Cryogenic Temperatures.

An attempt has been made to design and test a P-nu-N diode suitable for use as a cryogenic charge storage particle detector. In order to reduce noise caused by self injection after the ejection of stored charge, the devices were designed to enhance a barrier to injection that exists at the N-nu interface. Some of the devices commercially fabricated for this investigation were found to have barriers to injection sufficient to prohibit such self injection. When these diodes were cooled to temperatures near 4.2^ circK and a short reverse bias pulse of 100 -200V was applied they exhibited unusual charge pumping behavior. The dependence of this effect on temperature, pulse amplitude and pulse width strongly suggests that cryogenic charge storage is involved in this phenomenon. An explanation of the charge pumping effect has been proposed that is consistent with all the available data. Charge pumping is believed to result from charge entering the nu region of the diode by tunneling through deep levels from the P-side. The fields that make this tunneling possible result from the strong space charge fields present after ejection of stored charge and the unusual geometry of the {rm P} ^{+} region of these devices. The charge is then trapped in shallow impurities and stored until the next ejection pulse. This explanation is supported by experimental evidence from six diodes produced in three fabrication runs and the results of a three dimensional computer simulation.