Radiation Damage and Leakage Current Measurement in Silicon Detector - Ken Chow 7/21/2003

1. Radiation Damage and Leakage Current Measurement in Silicon Detector-Ken Chow 7/21/2003 Rainer Wallny UCLA - Silicon Particle Detectors

2. Overview • A little bit on semiconductor physics • Overview of P-N junction • Radiation Damage in the Silicon detector • Leakage Current • Some results so far Rainer Wallny UCLA - Silicon Particle Detectors

3. Contemporary Silicon Detector Modules CDF SVX IIa half-ladder: two silicon sensors with readout electronics (SVX3b analog readout chip) mounted on first sensor ATLAS SCT barrel module: four silicon sensors with center-tapped readout electronics (ABCD binary readout chip) Rainer Wallny UCLA - Silicon Particle Detectors

4. Cross section view of the detector Rainer Wallny UCLA - Silicon Particle Detectors

5. Semiconductor Basics – Band Gap • In a gas, electron energy levels are discrete. In a solid, energy levels split and form a nearly-continuous band. • If the gap is large, the solid is an insulator. If there is no gap, it is a conductor. A semiconductor results when the gap is small. • For silicon, the band gap is 1.1 eV, but it takes 3.6 eV to ionize an atom. The rest of the energy goes to phonon exitations (heat). Rainer Wallny UCLA - Silicon Particle Detectors

6. _ + _ + _ + _ + Semiconductor– Principle of Operation • Basic motivation: charged particle position measurement • Use ionization signal (dE/dx) left behind by charged particle passage • In a semiconductor, ionization produces electron hole pairs • Use an electric field to drift the electrons and holes to oppositely charged electrodes BUT: • in pure intrinsic (undoped) silicon, there are many morefree charge carriers than those produced by a particle.Ex.: in this volume have 4.5x108 free charge carriers and only 3.2x104 produced by MIP particle • Also, electron –hole pairs quickly re-combine … Need to deplete number of free charge carriers and separate e-holes ‘quickly’! Rainer Wallny UCLA - Silicon Particle Detectors

7. pn-Junction • p-type and n-type doped silicon forms a region that is depleted of free charge carriers • The depleted region contains a non-zero fixed charge and an electric field. In the depletionzone, electron – hole pairs won’t recombine but rather drift along field lines • Artificially increasing this depleted region by applying a bias voltage allow charge collection from a larger volume p n    – –  + – – – + + + + + +   – +  + – – + + + + + – Dopant concentration Space charge density Carrier density Electric field Electric potential Rainer Wallny UCLA - Silicon Particle Detectors

8. Current –vs- Bias Voltage Rainer Wallny UCLA - Silicon Particle Detectors

9. We get a larger depletion volume by applying a revered biased voltage

10. Leakage Current Two main sources of (unwanted) current flow in reversed-biased diode: • Diffusion current, charge generated in the undepleted zone adjacent to the depletion zone which diffuse into the depletion zone (otherwise they would quickly recombine) negligible in a fully depleted device • Generation current Jg, charge generated in the depletion zone by defects or contaminants Rainer Wallny UCLA - Silicon Particle Detectors

11. Where does the leakage current comes from????

12. Radiation Damage in Silicon Close proximity to the interaction region means the sensors are subject to high doses of radiation • Two general types of radiation damage • “Bulk” damage due to physical impact within the crystal • “Surface” damage in the oxide or Si/SiO2 interface • Cumulative effects • Increased leakage current (increased Shot noise) • Silicon bulk type inversion (n-type to p-type) • Increased depletion voltage • Increased capacitance • Sensors can fail from radiation damage by virtue of… • Noise too high to effectively operate • Depletion voltage too high to deplete • Loss of inter-strip isolation (charge spreading) Rainer Wallny UCLA - Silicon Particle Detectors

13. Bulk Damage • Bulk damage is mainly from hadrons displacing primary lattice atoms (for E > 25 eV) • Results in silicon interstitial, vacancy, and typically a large disordered region • 1 MeV neutron transfers 60-70 keV to recoiling silicon atom, which in turn displaces ~1000 additional atoms • Defects can recombine or migrate through the lattice to form more complex and stable defects • Annealing can be beneficial, but… • Defects can be stable, unstable, or bi-stable • Displacement damage is directly related to the non-ionizing energy loss (NIEL) of the interaction • Varies by incident particle type and energy Vacancy/Oxygen Center O Vacancy Disordered region Interstitial C Carbon Interstitial Carbon-Carbon Pair C C Di-vacancy Phosphorous dopant Carbon-Oxygen pair P O C Rainer Wallny UCLA - Silicon Particle Detectors

14. Bulk Damage – Leakage Current • Defects created by bulk damage provide intermediate states within the band gap • intermediate states act as ‘stepping stones’ of thermal generation of electron/hole pairs • Some of these states anneal away; the bulk current reduces with time (and temperature) after irradiation Rainer Wallny UCLA - Silicon Particle Detectors

15. Leakage Current: • DI = a(t)FV • Current depends on a(t) (annealing function), V (volume), and F (fluence). • Annealing reduces the current • Independent of particle type • Depletion Voltage: Vdep = q|Neff|d2/2ee0 • Depends on effective dopant concentration (Neff = Ndonors – Nacceptors), sensor thickness (d), permitivity (ee0). • Depletion voltage is often parameterized in three parts: • Short term annealing (Na) • A stable component (Nc) • Long term reverse annealing (NY) Rainer Wallny UCLA - Silicon Particle Detectors

16. Some sample results so far….(without temperature correction)

17. Look at stores 1665-2155 100.97 pb-1 Temperature was flat during that time Rainer Wallny UCLA - Silicon Particle Detectors

18. Biased voltage –vs- time Rainer Wallny UCLA - Silicon Particle Detectors

19. The φ and z dependence of current in LOO Rainer Wallny UCLA - Silicon Particle Detectors