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Description
The charge collection of two $\text{n}^{+}\text{p}\text{p}^+$ pad diodes for light with a wavelength of $660$ nm from a sub-nanosecond laser and $\alpha$-particles with energies, $E_{\alpha}$, between $1.5$ and $2.8$ MeV injected from the $\text{n}^{+}\text{p}$ side, has been measured. The diodes had an area of $25$ $\text{mm}^{2}$, a thickness of $150$ $\mu$m and a doping concentration of $4.5 \times 10^{12} \text{cm}^{-3}$ in the bulk region. The measurements were performed at $ - 20~^\circ$C for bias voltages up to $V_{bias}= 800$ V. One diode had been irradiated by $23$ MeV protons to a 1 MeV equivalent fluence of $\Phi_{eq} = 2 \times 10^{15}$ $\text{cm}^{-2}$, the other one had not been irradiated. As expected, above the depletion voltage the charge measured for the non-irradiated diode, $Q_0$, is independent of the bias voltage. The Charge Collection Efficiency (CCE) for the irradiated diode is obtained from $\text{CCE}_{\Phi}(V_{bias}) = Q_{\Phi}$($V_{bias}$)/$Q_0$, where $Q_{\Phi}$($V_{bias}$) is the charge measured for the irradiated diode. As expected, $\text{CCE}_{\Phi}(V_{bias})$ increases with bias voltage because the higher electric field increases the drift velocity of the holes, which dominate the signal. In addition, it is observed that $\text{CCE}_{\Phi}(V_{bias})$ for $\alpha$-particles increases with increasing $E_{\alpha}$, and at $E_{\alpha} \approx 1.5$ MeV the $\text{CCE}_{\Phi}(V_{bias})$ for $\alpha$-particles (with $\approx 5 ~\mu$m range in silicon) is the same as for the laser light of $660$ nm (with $4.5 ~\mu$m attenuation length at $ - 20~^\circ$C).
The data can be described assuming a $V_{bias}$-independent layer with zero charge collection of thickness $d_0$, followed by an active region with the $V_{bias}$-dependent mean charge collection $\text{CCE}_{\Phi}(V_{bias})$ for the remaining range of the $\alpha$-particles. It is found that $d_0 = 1.15 \pm 0.10~ \mu$m and $\text{CCE}_{\Phi} = 55 ~\pm$ 1 $\%$ at $V_{bias} = 300$ V increasing to $78 ~\pm$ 1 $\%$ at $V_{bias} = 800$ V. The presence of an inactive layer is relevant for the determination of charge-carrier lifetimes using light with a short attenuation length or low energy $\alpha$-particles: Not taking into account $d_0$ underestimates the values for the lifetime.
