Abstract
Edge-TCT method was used to extract velocity profiles in heavily irradiated silicon micro-strip detectors. Detectors were irradiated up to 1016 c -2 with reactor neutrons, 200 MeV pions and a combination of both. A simple electric field model assuming two space charge regions at each side of the detector and neutral bulk in-between was found to describe the field profile. It was observed that after heavy irradiation a sizeable electric field is present in the entire detector volume. For pion-irradiated detectors strikingly different profiles were obtained and attributed to the large oxygen concentration in the detector bulk. The model parameters were also studied during the long term annealing. The space charge region near the strips was found to shrink which in turn leads to larger electric field and impact ionization. The model parameters extracted from the measurements were fed to the device simulation program which showed reasonable agreement between simulated and measured data at lower fluences.
Highlights
The induced current is proportional to the velocity of charge carriers, the electric field profile can be extracted from its time evolution
Up to 2 · 1015 cm−2 the agreement with low fluence data [2, 18], obtained from C-V is satisfactory. This means that voltage drop in electrically neutral bulk (ENB) and the back space charge regions (SCR) is small compared to the drop at the front junction
A silicon micro-strip n-on-p type sensor was irradiated with neutrons to fluences up to Φeq = 1 · 1016 cm−2
Summary
The basic principle of the Edge-TCT technique is shown in figure 1. A carefully polished edge of the detector is illuminated with a narrow laser beam of infrared light (λ = 1064 nm, ≈ 40 ps pulse width, 200 Hz repetition rate). The beam position, and by that the depth at which the carriers are generated, is controlled by moving stages with sub-micron precision. The carriers start to drift in the electric field and induce a signal, read out by the current amplifier (MITEQ AM-1309, 10 kHz–1 GHz) connected to one of the Al strips. An average of 400 pulses is recorded by a 1.5 GHz oscilloscope at each scan position. The measurements were done on three p-type micro-strip detectors processed by HPK1 on float zone silicon. Detectors had 1 cm long AC coupled n+ strips with a pitch of 100 μm and implant width of 20 μm
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