The paradox of characteristics of silicon detectors operated at temperature close to liquid helium

The aim of this study is to give characterization of silicon p$^+$/n/n$^+$ detectors for the monitoring systems of the Large Hadron Collider machine at CERN with the focus on justifying the choice of silicon resistivity for the detector operation at the temperature of 1.9–10 K. The detectors from n-type silicon with the resistivity of 10, 4.5, and 0.5 k$\Omega$ cm were investigated at the temperature from 293 up to 7 K by the Transient Current Technique with a 660 nm pulse laser and alpha-particles. The shapes of the detector current pulse response allowed revealing a paradox in the properties of shallow donors of phosphorus, i.e., native dopants in the n-type Si. There was no carrier freeze-out on the phosphorus energy levels in the space charge region (SCR), and they remained positively charged irrespective of temperature, thus limiting the depleted region depth. As for the base region of a partially depleted detector, the levels became neutral at $T < \lt 28 $K, which transformed silicon to an insulator. The reduction of the activation energy for carrier emission in the detector SCR estimated in the scope of the Poole-Frenkel effect failed to account for the impact of the electric field on the properties of phosphorus levels. The absence of carrier freeze-out in the SCR justifies the choice of high resistivity silicon as the only proper material for detector operation in a fully depleted mode at extremely low temperature.