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Defects in Silicon Crystals - Some Very Basic Remarks

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CERN, TA1/SSD coffee-meeting 11.6.2004 Defects in silicon crystals - Some very basic remarks - Michael Moll CERN - Geneva - Switzerland People are like crystals. It is the defects in them that make them interesting. Sir F.Charles Frank Perfection has one grave defect : it is apt to be dull William Somerset Maugham · Defects give silicon crystals new properties · These properties can be useful (e.g. doping, defect engineering) or not (e.g. radiation damage, metal contaminations) CERN, TA1/SSD coffee meeting 11.6.2004 Outline • Silicon and Silicon crystal structure Easy • Defect types in silicon crystals level • Silicon doping (Dopants = Defects) • Radiation induced defects More • What are defects doing to detectors? difficult level • How to measure defects? • Coffee } Relax level CERN TA1/SSD Silicon Atomic number = 14 Atomic mass 28.0855 amu · Most abundant solid element on earth 50% O, 26% Si, 8% Al, 5% Fe, 3% Ca, ... · 90% of earth’s crust is composed of silica (SiO2) and silicate ! ? Very pure sand or quartz Silicon (e.g. from Australia or Nova Scotia in Canada) » 99% SiO2 Another coffee meeting ? » 1% Al2O3, Fe2O3, TiO2 (CaO,MgO) Michael Moll – TA1/SD meeting – June 2004 -3- CERN TA1/SSD Unit Cell in 3-D Structure Unit cell Michael Moll – TA1/SD meeting – June 2004 -4- CERN TA1/SSD Faced-centered Cubic (FCC) Unit Cell Michael Moll – TA1/SD meeting – June 2004 -5- CERN TA1/SSD Silicon Crystal Structure Silicon has the basic diamond crystal Basic FCC Cell Merged FCC Cells structure: two merged FCC cells offset by a/4 in x, y and z Omitting atoms Bonding of Atoms outside Cell Michael Moll – TA1/SD meeting – June 2004 -6- CERN TA1/SSD Silicon Crystal Structure Link: BC8 Structure of Silicon Link: Silicon lattice (wrl) Link: Silicon lattice with bonds (wrl) Silicon crystallizes in the same pattern as Diamond, in a structure called "two interpenetrating face-centered cubic" primitive lattices. The lines between silicon atoms in the lattice illustration indicate nearest-neighbor bonds. The cube side for silicon is 0.543 nm. Michael Moll – TA1/SD meeting – June 2004 -7- CERN, TA1/SSD coffee meeting 11.6.2004 Outline • Silicon and Silicon crystal structure • Defect types in silicon crystals •Lattice defects (Dislocations) •Point defects (e.g. Impurities) •Cluster defects and Precipitates • Silicon doping (Dopants = Defects) • Radiation induced defects • What are defects doing to detectors? • How to measure defects? • Coffee CERN TA1/SSD Dislocations Burger’s Vector = b Shear stress The caterpillar or rug-moving analogy Michael Moll – TA1/SD meeting – June 2004 -9- CERN TA1/SSD Point Defects Vacancy defect Interstitial defect Link: Split-interstitital (wrl) Frenkel defect Michael Moll – TA1/SD meeting – June 2004 -10- CERN TA1/SSD Point Defects · Intrinsic defects: · The Vacancy (denoted V): an atom is removed. · The Self-interstitial (denoted I ): a host atom sits in a normally unoccupied site or interstice (various sites: bond centres, tetrahedral sites, interstitial + displaced regular atom). · Extrinsic defects: due to an impurity. These can be: · Substitutional, such as carbon substitutional (denoted Cs) · Interstitial (such as the carbon interstitial (denoted Ci). Michael Moll – TA1/SD meeting – June 2004 -11- CERN TA1/SSD Point Defects - Lattice strain Ge, Sn (a) A vacancy in the crystal. (b) A substitutional impurity in the crystal. The impurity atom is larger than the host atom. Link: Vacancy - Hydrogen Defect C s Ci, Oi (c) A substitutional impurity in (d) An interstitial impurity in the crystal. It the crystal. The impurity atom occupies an empty space between host atoms. is smaller than the host atom. Michael Moll – TA1/SD meeting – June 2004 -12- CERN, TA1/SSD coffee meeting 11.6.2004 Outline • Silicon and Silicon crystal structure • Defect types in silicon crystals • Silicon doping (Dopants = Defects) • Intrinsic silicon • n-type silicon • p-type silicon • Radiation induced defects • What are defects doing to detectors? • How to measure defects? • Coffee CERN TA1/SSD Covalent Bonding of Pure Silicon Energy Si Si Si Si Si Conduction Band (CB) Si Si Si Si Si Si Si Si Si Si Eg =1.1eV Si Si Si Si Si Valence Band (VB) Si Si Si Si Si Silicon atoms share valence electrons to form insulator-like bonds. Michael Moll – TA1/SD meeting – June 2004 -14- CERN Electrons in N-Type Silicon TA1/SSD with Phosphorus Dopant Donor atoms provide excess electrons to form n-type silicon. Si Si Si Si Si Conduction Band (CB) Si Si Si P Si Si P Si Si Si Si Si Si P Si Valence Band (VB) Si Si Si Si Si Excess electron (-) Phosphorus atom serves as n-type dopant Michael Moll – TA1/SD meeting – June 2004 -15- CERN TA1/SSD Conduction in n-Type Silicon Positive terminal from power supply Negative terminal from power supply Electron Flow Free electrons flow toward positive terminal. Figure 2.24 Michael Moll – TA1/SD meeting – June 2004 -16- CERN TA1/SSD Holes in p-Type Silicon with Boron Dopant Acceptor atoms provide a deficiency of electrons to form p-type silicon. Si Si Si Si Si Conduction Band (CB) Si Si Si B Si Si B Si Si Si Si Si Si B Si Valence Band (VB) Si Si Si Si Si + Hole Boron atom serves as p-type dopant Michael Moll – TA1/SD meeting – June 2004 -17- CERN “Hole Movement in Silicon” TA1/SSD Boron is neutral, but Boron is now Only thermal energy to nearby electron may a negative ion. kick electrons jump to fill bond site. from atom to atom. The empty silicon bond sites (holes) are thought of as being positive, since their presence makes that Hole moved from region positive. 2 to 3 to 4, and will move to 5. Michael Moll – TA1/SD meeting – June 2004 -18- CERN TA1/SSD Conduction in p-Type Silicon Positive terminal from voltage supply Negative terminal from voltage supply Electron Flow Hole Flow -Electrons flow toward positive terminal +Holes flow toward negative terminal Figure 2.26 Michael Moll – TA1/SD meeting – June 2004 -19- CERN, TA1/SSD coffee meeting 11.6.2004 Outline • Silicon and Silicon crystal structure • Defect types in silicon crystals • Silicon doping (Dopants = Defects) • Radiation induced defects •Point defects and clusters •Particle dependence •Defect kinetics •Example of DLTS measurement • What are defects doing to detectors? • How to measure defects? • Coffee CERN Radiation Damage – Microscopic Effects TA1/SSD ¨ Spatial distribution of vacancies created by a 50 keV Si-ion in silicon. (typical recoil energy for 1 MeV neutrons) M.Huhtinen 2001 van Lint 1980 V I I V V Vacancy point defects + particle Si EK>25 eV S I Interstitial (V-O, C-O, .. ) EK > 5 keV point defects and clusters of defects Michael Moll – TA1/SD meeting – June 2004 -21- CERN Radiation Damage – Microscopic Effects TA1/SSD V Vacancy point defects + particle Si EK>25 eV S I Interstitial (V-O, C-O, .. ) EK > 5 keV point defects and clusters of defects ·60Co-gammas ·Electrons ·Neutrons (elastic scattering) ·Compton Electrons ·Ee > 255 keV for displacement · En > 185 eV for displacement with max. Eg »1 MeV ·E > 8 MeV for cluster · E > 35 keV for cluster (no cluster production) e n Only point defects point defects & clusters Mainly clusters 10 MeV protons 24 GeV/c protons 1 MeV neutrons Simulation: Initial distribution of vacancies in (1mm)3 after 1014 particles/cm2 [Mika Huhtinen NIMA 491(2002) 194] Michael Moll – TA1/SD meeting – June 2004 -22- CERN Primary Damage and secondary defect formation TA1/SSD · Two basic defects I - Silicon Interstitial V - Vacancy · Primary defect generation V I I, I2 higher order I (?) Þ I -CLUSTER (?) V, V2, higher order V (?) Damage?! Þ V -CLUSTER (?) I · Secondary defect generation V Main impurities in silicon: Carbon (Cs) Oxygen (Oi) I+Cs ® Ci Þ Ci+Cs ® CiCS Ci+Oi ® CiOi Ci+Ps ® CiPS V+V ® V2 V+V2 ® V3 V+O ® VO Þ V+VO ® V O i i i 2 i Damage?! (“V2O-model”) V+Ps ® VPs I+V2 ® V I+VOi ® Oi ....................... Michael Moll – TA1/SD meeting – June 2004 -23- CERN Example of defect spectroscopy TA1/SSD - neutron irradiated - Deep Level Transient Spectroscopy 0.8 (-/0) C (-/0) ? + VV + ? Introduction Rates 0.6 i (-/0) N /F : ] CiCs t eq F p [ 0.4 ) E(35K) (-/0) Ci : 1 E(40K) VOi -1 b (--/-) 1.55 cm ( E(45K) 0.2 VV l a n g i 0 s - S C C : C O : T i s i i -0.2 -1 -1 L H(220K) 0.40 cm 1.10 cm D 60 min -0.4 170 days (+/0) (+/0) Ci CiOi -0.6 50 100 150 200 250 Temperature [ K ] 14 -2 example : Feq = 1´10 cm § Introduction rates of main defects 1 cm-1 » defects » 1´1014 cm-3 12 -3 § Introduction rate of negative space charge » 0.05 cm-1 space charge » 5´10 cm Michael Moll – TA1/SD meeting – June 2004 -24- CERN, TA1/SSD coffee meeting 11.6.2004 Outline • Silicon and Silicon crystal structure • Defect types in silicon crystals • Silicon doping (Dopants = Defects) • Radiation induced defects • What are defects doing to detectors? • How to measure defects? • Coffee CERN Impact of Defects on Detector properties TA1/SSD Inter-center charge Shockley-Read-Hall statistics transfer model (standard theory) (inside clusters only) charged defects Trapping (e and h) generation enhanced generation Þ Neff , Vdep Þ CCE Þ leakage current Þ leakage current e.g. donors in upper shallow defects do not Levels close to Þ space charge and acceptors in contribute at room midgap lower half of band temperature due to fast most effective gap detrapping Impact on detector properties can be calculated if all defect parameters are known: sn,p : cross sections DE : ionization energy Nt : concentration Michael Moll – TA1/SD meeting – June 2004 -26- CERN, TA1/SSD coffee meeting 11.6.2004 Outline • Silicon and Silicon crystal structure • Defect types in silicon crystals
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