3-5 June 2020
Europe/Zurich timezone

Measurement of the charge collection for the irradiated $\text{n}^{+}\text{p}\text{p}^+$ pad diode in the region of the $\text{n}^{+}\text{p}$ interface


Mohammadtaghi Hajheidari (Hamburg University (DE))


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.

Primary authors

Mohammadtaghi Hajheidari (Hamburg University (DE)) Erika Garutti (Hamburg University (DE)) Dr Joern Schwandt (Hamburg University (DE)) Robert Klanner (Hamburg University (DE))

Presentation materials