Simulation of charge accumulation in silicon photomultipliers under the influence of soft X-rays

Abstract

A model of the cell structure of silicon photomultipliers (SiPM) is herein created in the software complex “Silvaco”. The cells are n+–p–p+-structures optically isolated from each other. The optical isolation of the cells is realized by trenches filled with metal after passivation of the walls with a SiO2 layer. Simulations are carried out for two variants of SiPM structures when the trench metal is electrically connected to the n+- (the first design) or p+ (the second design) region of the cell. The cells are irradiated by X-ray quanta with 10 keV energy up to a dose of 105 rads at the reverse bias values of Ub = –30 V (active electrical mode) and Ub = 0 V (passive electric mode). We obtain the distribution of the volume density of the accumulated charge Q in the oxide layer of the separation trench. It is established that the maximal Q value depends upon the irradiation mode. In the passive mode, the Q value is minimal and similar for both variants of the structures. In the active mode, Q increases in comparison to the passive mode by 2.5 times for SiPM with the structure of the second variant and by 5.9 times for the structure of the first variant. The obtained result can be explained by an increase of the hole charge yield under the influence of the appropriately distributed electric fields in the oxide layers of the separating trenches of the investigated SiPM’s cells.