Effects of accelerated long term annealing in highly irradiated n⁺-p strip detector examined by Edge-TCT

Abstract

Effects of long term annealing on the charge collection properties of a miniature p-type micro-strip detector, irradiated to a fluence of 10 16 n eq /cm 2 with reactor neutrons, were examined with the Transient Current Technique with edge-on laser injection (Edge-TCT). The detector was annealed at 60 • C in steps to a cumulated time of 10240 min. A large increase of the measured charge was observed even at low bias voltages at late annealing stages. Long term annealing causes build up of negative space charge at the n + -p junction (strip implants), consequently resulting in very high electric fields, sufficient for initiating the process of impact ionization, leading to charge multiplication. The increase of charge collection is correlated with the increase of leakage current.

Annealing effects on operation of thin Low Gain Avalanche Detectors

“…We had two parallel detectors and a detector irradiated to Φ eq = 3 • 10 15 cm −2 (shown in figure 10(b)) and all showed the same behavior. Such a behavior was also observed with ATLAS strip detectors where substantial increase of charge occurred only after > 3000 min annealing [17,18]. This improvement can be attributed to a larger impact of the bulk on electric field in the gain layer as described in the introduction.…”

First annealing studies of irradiated silicon sensors with modified ATLAS pixel implantations

“…Thus, if one of these effects depend on the pixel design, different hit detection efficiencies might be observed. Also charge multiplication could explain the higher efficiencies, which was also observed in [17,18], where neutron irradiated n-in-p micro-strip sensors showed higher electrical fields and therefore charge multiplication close to the strips.…”

“…The higher hit detection efficiencies might be explained with charge multiplication which is observed in n-in-p micro-strip detectors after neutron irradiation and long term annealing [17,18]. To understand if the higher efficiency is caused by charge amplification, which was the intention of the REINER pixel designs, TCT measurements at different voltages and annealing steps are planned for irradiated modules.…”

Edge-TCT measurements with the laser beam directed parallel to the strips

“…The experimental setup for the E-TCT measurements at the Jožef Stefan Institute in Ljubljana has been described in detail in several publications [3][4][5][6]. A very brief description to facilitate further reading will be given here.…”

“…In this way the charge carriers are released at a known depth in the detector, so enabling the direct probing of the electric field, which is especially interesting in the case of irradiated detectors. Usually, in E-TCT [3][4][5][6], an IR laser beam with λ = 1064 nm and an absorption depth of over 1 mm (at -20 • C) in silicon, is directed at the edge of the detector, which runs parallel with the strips. Light with such a long penetration depth is used because it must traverse the area of the detector hosting guard and bias rings, and the signals are measured several (∼ 5) strips away from the edge so that the effects of the edge can be neglected.…”