ELECTRIC FIELD AND CHARGE MULTIPLICATION IN RADIATION-DAMAGED SILICON DETECTORS

Od zagona trenutno največjega in najmočnejšega pospeševalnika električno nabitih osnovnih delcev, Velikega trkalnika hadronov (Large Hadron Collider, okrajšano LHC), ki deluje v okviru Evropske organizacije za jedrske raziskave (CERN) v Ženevi, je ta projekt veliko prispeval k znanosti na področju fizike in tudi tehnike. LHC je v osnovi namenjen trkanju gruč protonov (žarkov) z najvišjo kinetično energijo trkov do sedaj (do 14 TeV) Glavni cilj je raziskovanje veljavnosti in morebitnih pojavov onkraj Standardnega modela in s tem boljšemu razumevanju obnašanja osnovnih gradnikov snovi. Odkritje Higgsovega bozona leta 2012, ki je odgovoren za maso gradnikov, je nedvomno eden izmed največjih znanstvenih dosežkov zadnjih petdesetih let, kljub temu pa pomeni šele začetek raziskav. Največji in eden izmed dveh najpomembnejših LHC eksperimentov je ATLAS (A large Toroidal LHC ApparatuS). Na tem orjaškem detektorju velikosti 5‐nadstropne stolpnice, sodeluje tudi Odsek za eksperimentalno fiziko osnovnih delcev Instituta Jožef Stefan (IJS) v Ljubljani. Detektor ATLAS je zaradi zapletenosti procesov, ki jih opazuje, sestavljen iz številnih pod‐sistemov, prikazanih na sliki 10.1. Vsak od sestavnih delov detektorja ima posebno vlogo pri sledenju in zaznavanju delcev. Točki interakcije najbližji del je notranji detektor (Inner detector ID), tudi sestavljen iz več plasti detektorjev (slika 10.2): točkovnega oziroma tako imenovanega piksel detektorja, polprevodniškega sledilca (SemiConductor Tracker ‐ SCT) in sledilca prehodnega sevanja (Transition Radiation Tracker ‐ TRT). Naloga notranjega sledilca je čimbolj natančna meritev sledi nabitih delcev po trku protonov. Točkovni detektor je najbližje točki interakcije. Sestavljen je iz točkovnih (pixel) detektorjev, ki so razporejeni v tri koncentrične valjaste lupine, zaprte s tremi diski na obeh straneh. Podatke zajemamo z v ta namen izdelanim integriranim vezjem (ASIC), ki je s tehnologijo krogličnih povezav (»bump bond«) spojen z detektorjem. Naslednji sloj notranjega sledilca je SCT, zgrajen iz silicijevih mikropasovnih detektorjev. Štirje detektorji veliki 6.4x6.4 cm2 (po dva na vsaki strani) skupaj z bralno elektroniko in hladilnim sistemov tvorijo modul. Moduli so porazdeljeni v štiri valjaste lupine v centralnem valjastem (Barrel) delu. Takšno valjasto strukturo zapirata dva pokrova imenovana End‐cap. Vsakega od njiju sestavlja devet diskov. Zunanji podsistem notranjega detektorja predstavlja TRT, ki je zgrajen iz drobnih cilindričnih celic (cevk) napolnjenih s plinom (Xe), ki se ionizira ob prehodu nabitega delca. Med cevkami je snov, ki omogoča prehodno sevanje žarkov‐X ob prehodu relativističnih nabitih delcev. Z zaznavo teh dobimo poleg sledi tudi možnost identifikacije delcev. Kljub temu, da notranji sledilec služi zaznavanju preleta nabitih delcev pa morajo biti njegovi sestavni imeti majhno maso (radiacijsko dolžino), da je njihov vpliv na energijo in smer primarnega delca zanemarljiv. V nasprotju z notranjim detektorjem pa delci v kalorimetrih izgubijo vso energijo. Tako služi elektromagnetni kalorimeter zaznavanju elektronov in fotonov, hadronski kalorimeter zaznavanju hadronov (protoni, nevtroni in pozitroni). Mioni in nevtrini zapustijo detektor. Še nevtrinov neposredno ne moremo zaznati, pa mione, ki so nabiti, pred tem zaznamo v mionskih komorah. Pomemben del spektrometra je tudi magnetni sistem, ki s svojim magnetnim poljem ukrivlja pot delcev in tako omogoča določitev gibalne količine in s tem identifikacijo delcev. Na sliki 10.3 je prikazan princip interakcije in zaznavanja delcev s posameznimi deli ATLAS detektorja.The radiation damage is the main limitation for the operation of position sensitive silicon detectors at future high energy physics experiments, which aim for ever larger energies of colliding particles with larger energies. Silicon detectors have been widely used in all experiments over the last decades. It was however expected, that the signal from planar silicon detectors would degrade with irradiation to a level where the efficient operation of the innermost tracking detectors at the upgraded LHC experiments would become impossible. However, recent measurements with planar detectors where n+ side is segmented for readout (n+‐p or n+‐n detectors) showed a charge collection efficiency sufficient for the efficient operation even at the highest fluences expected at the HL‐LHC, in excess of 1.6∙1016 hadrons/cm2. The key condition was the operation at very high bias voltages of around 1000 V. Several groups reported charge collection efficiencies much larger than expected, in some operating conditions even exceeding the one before irradiation. This is a clear evidence for charge multiplication in silicon detector, due to impact ionization. As charge multiplication may well be the reason for successful operation of heavily irradiated silicon detector, the main focus of this thesis is on this phenomenon, as well as understanding the device model and operation of heavily irradiated silicon detectors. Both planar and so called 3D devices of different thicknesses (75, 150, 300 μm), coming from different manufacturers (HPK, Micron, MPI‐HLL, Soitec/MPI‐HLL, CNM) and irradiated with different reactor neutrons and 200 MeV pions (and a combination of both) up to fluences of 1016 cm‐2, were investigated using different detector characterization techniques: Edge‐Transient Current Technique (Edge‐TCT) and multichannel readout of induced charge by custom made ASICs. Edge‐TCT is a novel technique utilizing an short pulses (~ 100 ps) of infra red light (1060 nm) directed at a polished edge of the detector. Electron hole pairs generated along the narrow beam (spot size FWHM < 10 μm) are separated by electric field in the detector and consequently induce currents in the electrodes. As the position of the beam is externally controlled by moving stages the profiling of the electric field at different depths is possible in accurate way. The analysis doesn’t depend on time evolution of the induced current pulse hence the precise knowledge of effective trapping times is not required for determination the drift velocity, charge collection and electric field profiles in heavily irradiated silicon detectors. The Edge‐TCT measurements of the induced current gave first direct observations of charge multiplication in heavily irradiated silicon strip detectors, taking place in high electric fields near the main junction (strips). The amplification was found to increase with detector post‐irradiation annealing, which in this work was studied up to 40960 min at 60 ⁰C. Long term annealing causes build up of negative space charge at the n+‐p junction, consequently resulting in very high electric fields, sufficient for initiating impact ionization. A strong correlation between the increase of charge collection and the increase of the leakage current was also found. These findings were also confirmed by charge collection measurements with 90Sr electrons. TCT measurements where detector surface was illuminated were also performed on special types of miniature detectors, with junction implants not fully covered by metal, allowing proper analysis of charge multiplication at implant edges, where it was confirmed to be the highest. Charge sharing between electrodes due to trapping (incomplete carrier drift) was also studied. According to the obtained results, an appropriate modeling of the electric field in irradiated detectors was proposed. A simple model, assuming two space charge regions at each side of the detector and neutral bulk in‐between was found to describe the field profile in neutron irradiated detectors. Pion‐irradiated detectors were found to have strikingly different profiles and attributed to large oxygen concentration in the detector bulk. The model parameters were also studied during long term annealing and it was found that the space charge near the main junction shrinks, which leads to these high electric fields and consequently impact ionization. The model parameters extracted from the measurements were also fed to the device simulation program, which showed reasonable agreement between the simulated and measured data at lower fluences. Effects of long term applied bias were also studied using both multi‐electrode charge readout system (ALiBaVa) and Edge‐TCT. A significant drop in both collected charge and the leakage current was observed after keeping the detectors under bias for longer periods of time (> 1000 min). The time evolution of the charge decrease was found to be fully reproducible under any bias or temperature, influencing long term annealing induced charge multiplication only. Applying sufficient bias voltage however (≥ 800 V), results in obtaining a more stable and high enough SNR, providing optimum detector performance. Both charge collection efficiency and the leakage current were found to fully recover in late annealing stages after keeping the detectors at room temperature and no bias for more than 24h