Modeling of surface-state induced inter-electrode isolation of n-on-p devices in mixed-field and γ-irradiation environments

Position sensitive $n$-on-$p$ silicon sensors will be utilized in the tracker and in the High Granularity Calorimeter (HGCAL) of the Compact Muon Solenoid (CMS) experiment at High Luminosity Large Hadron Collider (HL-LHC). The detrimental effect of the radiation-induced accumulation of positive net oxide charge on position resolution in $n$-on-$p$ sensors has typically been countered by the application of isolation implants like $p$-stop or $p$-spray between $n^{+}$-electrodes. In addition to the positively charged layer inside the oxide and close to the Si/SiO₂-interface, surface damage introduced by ionizing radiation in SiO₂-passivated silicon particle detectors includes the accumulation of trapped-oxide-charge and interface traps. A previous study of either n/$\gamma$ (mixed field)- or $\gamma$-irradiated Metal-Oxide-Semiconductor (MOS) capacitors showed evidence of substantially higher introduction rates of acceptor- and donor-type deep interface traps ($N_{\text{it,acc/don}}$) in mixed-field environment. In this work, an inter-pad and -strip resistance (or resistivity (${\rho }_{\text{int}}$)) simulation study of $n$-on-$p$ sensors with and without $p$-stop isolation implants was conducted for both irradiation types. Higher levels of ${\rho }_{\text{int}}$ showed correlation to higher densities of deep $N_{\text{it,acc/don}}$, with the inter-pad isolation performance of the mixed-field irradiated sensors becoming independent of the presence of $p$-stop implant between the $n^{+}$-electrodes up to about 100 kGy. The low introduction rates of deep $N_{\text{it,acc/don}}$ in $\gamma$-irradiated sensors resulted in high sensitivity of ${\rho }_{\text{int}}$ to the presence and peak doping of $p$-stop above the lowest dose of about 7 kGy in the study. As a consequence of the advantageous influence of radiation-induced accumulation of deep $N_{\text{it}}$ on the inter-electrode isolation, position sensitive $n$-on-$p$ sensors without isolation implants may be considered for future HEP-experiments where the radiation is largely due to hadrons.