Study of radiation damage in silicon detectors for high ... - F9

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24 2. Operation and Radiation Damage of Silicon Detectorsments in the laboratory were taken at 5 CandFDV as obtained from 10 kHz measurementwere used for comparison. Measurements at the reactor (i.e. during and early after irradiation)were however taken at the irradiation temperature and were aected by thisphenomenon. A more detailed description of measuring methods used in this work canbe found in section 3.5.2.2 Radiation Induced DefectsDamage caused by radiation can be divided into two groups, surface damage and bulkdamage. Due to the interest of electronic industry, the surface damage is better understoodand can be controlled to a certain extent by proper design and manufacturingprocess. Surface damage mainly manifests as charge accumulation in the oxide andsubsequent breakdown. Charge accumulation at the silicon-oxide interface signicantlydecreases the inter-strip resistance of microstrip detectors. Therefore the signal has to becollected very quickly to minimise the loss due to leakage to neighbouring strips.The bulk damage in high resistivity silicon is however much less understood. Anextensive study of the bulk damage for the purpose of experiments at high luminositycolliders is therefore ongoing.In tracking detectors full depletion of the bulk is required. According to eq. 2.10,2.13 the full depletion voltage is proportional to the eective dopant concentration. Sincelow operation voltages are sought to avoid breakdown and extensive heating, high resistivitysilicon is used in tracking devices. There the concentration of irradiation induceddefects can be comparable or even much larger than the initial dopant concentration.Thus construction of trackers for new large experiments, where they will operate in highirradiation environments, demands an intensive study of the irradiation induced bulkdefects and to those the further discussion is focused on.2.2.1 Defect GenerationThe energy loss by interaction of an incoming particle with matter can be divided intotwo parts: ionization and non-ionising energy loss (NIEL). Due to fast recombination ofcharge carriers the ionising energy loss does not lead to bulk damage. NIEL containsdisplacements of lattice atoms and nuclear reactions. They both can result in long termbulk damage and will be described in the following sections.
2. Operation and Radiation Damage of Silicon Detectors 25Nuclear Reactions The most important nuclear reaction resulting from neutron irradiationis transmutation to phosphorus30 Si + n ! 31 Si + 31 Si ! 30 P + e ; +: (2.46)Transmutations to Al and Mg are also possible, but have a few orders of magnitude lowercross-sections, so we shall rst estimate the rate of phosphorus production.The integral radiative capture cross-section for the reaction 2.46 is 96 mb for athermal spectrum and 670 b for a typical ssion (Watt) spectrum. At the irradiationsites used in this work the ux in both parts (thermal and fast) of the spectrum iscomparable 12 . Due to the larger cross-section thus only the thermal part needs to beconsidered. Change of phosphorus concentration N P after neutron uence is given asa product N P = R P where R P is the phosphorus introduction rate. The introductionrate can be estimated byR P = A( 30 Si)N Si : (2.47)With atomic percent abundance of 30 Si isotope A( 30 Si)=3.1% and N Si = N Av SiA=510 22cm ;3 we obtain 30 P introduction rate of R P =1:5 10 ;4 cm ;1 . As will be shown later it ismore than two orders of magnitude lower compared to introduction rates of active latticedefects and thus negligible. Since cross-sections for transmutation to Al and Mg are evensmaller they can also be neglected.Lattice Atoms Displacements The most important process for radiation damage insilicon is the displacement of lattice atoms. An incoming particle can transfer a signicantpart 13 of its energy to a Si atom. Bulk damage occurs when the transfer of kinetic energyis sucient to displace a silicon atom from its lattice site 14 . Displaced atoms may cometo rest in interstitial positions (I), leaving vacancies (V) at their original locations. If thekinetic energy of the recoiling atom is sucient it can displace further Si atoms, creatinga cluster of displacements. Most of the resulting vacancies and interstitials recombinewhile others diuse away and eventually create stable defects.2.2.2 Comparing Radiation Damage MeasurementsOnly non-ionising energy loss (NIEL) is to be taken into account when damages causedby dierent particles or particles of dierent energies are compared. According to this12Measurements of the spectra are presented in chapter 3.13As much as 13% of the incoming neutron energy can be transferred to the Si atom as can be seenfrom the equation E max =E n =4m n m Si =(m n + m Si ) 2 , describing the collision kinematics.14Around 15 eV of recoil energy is required [24].
Page 1: UNIVERSITY OF LJUBLJANAFACULTY OF M
Page 5: Najprej gre moja zahvala delovnima
Page 8 and 9: PovzetekSevalne poskodbe v siliciju
Page 10 and 11: 3.5 Measurement Setup, Methods and
Page 13 and 14: 1IntroductionSilicon detectors play
Page 15 and 16: 1. Introduction 7Figure 1.2: Schema
Page 17 and 18: 1. Introduction 9R(cm)Z(cm)Figure 1
Page 19 and 20: 2Operation and Radiation Damage of
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Page 43 and 44: 3Irradiation Facility3.1 The Reacto
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Page 47 and 48: 3. Irradiation Facility 39Fission p
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Page 51 and 52: 3. Irradiation Facility 43Figure 3.
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Page 57 and 58: 3. Irradiation Facility 49width of
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Page 63 and 64: 4Time Development of DefectsIn this
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4. Time Development ofDefects 75Fig
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4. Time Development ofDefects 77Fig
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4. Time Development ofDefects 79An
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5Dose Rate Dependence5.1 Motivation
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5. Dose Rate Dependence 83Figure 5.
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5. Dose Rate Dependence 85Figure 5.
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5. Dose Rate Dependence 87could be
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6Inuence of Bias VoltageIt was a ge
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6. Inuence of Bias Voltage 91energy
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6. Inuence of Bias Voltage 93Figure
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6. Inuence of Bias Voltage 99that t
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6. Inuence of Bias Voltage 101(abou
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6. Inuence of Bias Voltage 103Figur
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7Comparison with Other GroupsWhile
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7. Comparison with Other Groups 107
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8ConclusionsA systematic study of r
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8. Conclusions 111Implications for
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9Povzetek doktorskega dela9.1 UvodS
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9. Povzetek doktorskega dela 115Ski
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9. Povzetek doktorskega dela 117sre
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9. Povzetek doktorskega dela 1199.2
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9. Povzetek doktorskega dela 121Sli
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9. Povzetek doktorskega dela 123Ce
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9. Povzetek doktorskega dela 127Cep
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9. Povzetek doktorskega dela 129Mer
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9. Povzetek doktorskega dela 131Sam
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9. Povzetek doktorskega dela 1379.6
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10Appendix: List of Symbols and Abb
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10. Appendix: List of Symbols and A
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References[1] J. Straver et al., Nu
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[27] A.M. Ougang et al.: Dierential
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[57] M. Huhtinen et al, HU-SEFT-R-1
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