Speakers
Description
The Low Gain Avalanche Detector (LGAD) is developed with the aim to serve as a timing detector for two leading HEP experiments in CERN, CMS and ATLAS. This means that it is adjusted for work with Minimum Ionizing Particles (MIP). However, its excellent timing resolution and good spatial resolution, have made LGAD an attractive solution in experiments where much higher charge is generated than those generated by MIP as well. When Highly ionized Particles (HIP) cross the sensor, the plasma formation can not be avoided. Moreover, if HIP particles cross the LGAD at an angle that differs from the perpendicular one, it also means that charge is at the initial step, mainly induced in no-gain region. In the past a lot of attention has been given to the investigation of the pad region when HIP particles cross this region, while the no-gain region has been less investigated. All factors considered above indicate that it is also important to investigate the no-gain region under high intensity injections.
In this contribution, we report the results from an investigation on dynamics of charge transport when the charge density is such that conditions for plasma formation are met and plasma is created in bulk.
Two effects of plasma creation in LGAD on dynamics of charge transport and charge collection have been investigated: 1) the level of doping and 2) the density of plasma in both, the low and the highly doped bulk region. For this purpose the two selected regions of standard segmented LGAD are investigated: the inter-pad region with no gain layer beneath the n++ electrode, and the pad region with gain layer beneath the n++ electrode.
By increasing the laser power we varied the plasma density.
Contrary to the expectations, it was observed that the transient signal in LGAD becomes faster and shorter if plasma is denser. Moreover, the ratio between the amplitudes of signals from the pad and the inter-pad region has been increasing with the decreased plasma density.
The observed behavior can only be explained by additional underlying mechanism which is more dominant then it is plasma induced screening of local electric field; the charge screening of local field would slow down the charge velocity, and thus the probability for impact ionization would be reduced. Instead of observing the slower signal with the increased plasma density as it would be if only charge screening of local el field dominates the dynamics of charge collection, we observed the opposite effect: the faster signal. The wavelength of fs-laser is 800 nm, this corresponds to 20 microns of absorption depth in silicon that is far away from the bottom electrodes in LGAD. Also, only Single Photon Absorption of TCT technique was used. Therefore, the reflection of the laser beam from the bottom electrode can not be the reason.
We think that the thermal effect is responsible for faster plasma expansion in denser plasma, broadening the plasma charge cloud; as result the induced signal becomes faster in denser plasma. The repulsion effect of the same sign charge clouds may also contribute.
However, more data and larger pool of LGAD prototypes are needed for more conclusive statement. This will be researched during next LGAD campaign at ELI ERIC.
