Panja Luukka Helsinki Institute of Physics on behalf of the CERN RD50 Collaboration

1. Development of Radiation Hard Sensors for Very High Luminosity Colliders- CERN RD50 Collaboration - Panja Luukka Helsinki Institute of Physics on behalf of the CERN RD50 Collaboration http://www.cern.ch/rd50 Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

2. Outline • Background and motivation • Introduction to the CERN RD50 Collaboration • Radiation damage in silicon detectors • Approaches to obtain radiation hard sensors • Latest results of defect engineering • Magnetic Czohralski silicon • Epitaxial silicon • Summary Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

3. Background • LHC at CERN is the first experiment to use silicon detectors in a large scale. • Only few proton-proton collisions produce Higgs boson. • The luminosity of the LHC beam is very high, causing a hostile radiation environment. • The radiaton hardness of silicon devices is currently extensively studied subject. • There are 100 institutes within the CERN RD50 and RD39 Collaborations Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

4. Background The charge transportation in particle detectors is based on drift of charge carriers, i.e. electrons and holes. Thus, the detectors need to be fully depleted, i.e. the electric field extends over the entire bulk (~300µm) Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

5. Motivation LHC upgrade to Super-LHC: Luminosity of LHC: L ~ 1034 cm-2s-1 and fluence of fast hadrons at r=4cm ~3·1015cm-2  Super-LHC:L ~ 1035 cm-2s-1, expected fast hadron fluence at r=4cm ~1.61016 cm-2. The main constraint is the survival of the silicon tracker in the hostile radiation environment. Radiation hardness studies are also beneficial before the luminosity upgrade e.g. for other experiments such as linear colliders. Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

6. CERN RD50 Collaboration • formed in November 2001 • approved as RD50 Collaboration by CERN in June 2002 • Main objective: Development of ultra-radiation hard semiconductor detectors for the luminosity upgrade of the LHC to 1035 cm-2s-1 (“Super-LHC”). Challenges: - Radiation hardness up to 1016 cm-2 required - Fast signal collection (collision rate 80 MHz) - Low mass (reducing multiple scattering) - Cost effectiveness (large area has to be covered) • Presently 260 members from 53 institutes Belarus (Minsk), Belgium (Louvain), Canada (Montreal), Czech Republic (Prague (3x)), Finland (Helsinki, Lappeenranta), Germany (Berlin, Dortmund, Erfurt, Freiburg, Hamburg, Karlsruhe, Munich), Israel (Tel Aviv), Italy (Bari, Bologna, Florence, Padova, Perugia, Pisa, Trento, Turin), Lithuania (Vilnius), Norway (Oslo (2x)), Poland (Warsaw(2x)), Romania (Bucharest (2x)),Russia (Moscow), St.Petersburg), Slovenia (Ljubljana), Spain (Barcelona, Valencia), Switzerland (CERN, PSI), Ukraine (Kiev), United Kingdom(Exeter, Glasgow, Lancaster, Liverpool, Oxford, Sheffield, Surrey), USA (Fermilab, Purdue University, Rochester University, SCIPP Santa Cruz, Syracuse University, BNL, University of New Mexico) Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

7. Approaches Scientific strategies: • Material engineering • Device engineering • Change of detectoroperational conditions • Defect Engineering of Silicon • Understanding radiation damage • Macroscopic effects and microscopic defects • Simulation of defect properties & kinetics • Irradiation with different particles & energies • Oxygen rich Silicon • DOFZ, CZ, MCZ, EPI • Oxygen dimer & hydrogen enriched silicon • Pre-irradiated silicon • Influence of processing technology • New Materials • Silicon Carbide (SiC), Gallium Nitride (GaN) • Diamond: CERN RD42 Collaboration • Device Engineering (New Detector Designs) • p-type silicon detectors (n-in-p) • thin detectors • 3D and Semi 3D detectors • Stripixels • Cost effective detectors • Simulation of highly irradiated detectors • Monolithic devices CERN RD39 Collaboration“Cryogenic Tracking Detectors” Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

8. Radiation induced microscopic damage Frenkelpair V Si particle Vacancy+ Interstitial I EK > 25 eV EK > 5 keV point defects (V-V, V-O .. ) clusters Influence of defects on the material and device properties trapping (e and h) CCEshallow defects do not contribute at room temperature due to fast detrapping charged defects Neff , Vdepe.g. donors in upper and acceptors in lower half of the band gap generation leakage currentlevels close to middle of the bandgap most effective Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

9. Vacancy amount and distribution Ratio of point/cluster defects depends on the particle type and energy M. Huhtinen NIMA 491(2002) 194 Only point defects point defects & clusters Mainly clusters 60Co-g, Eg ~ 1MeV Displacement no clusters Neutrons: En > 185 keV displacement En > 35 keV clusters Electrons: Ee > 255 keV displacement Ee > 8 MeV clusters

10. Radiation damage – macroscopic effects Change of leakage current - can be helped with coolingChange of the full depletion voltage Vdep (effective doping concentration Neff). - every p-n-junction has a finite breakdown voltageDecrease of the charge collection efficiency - limited by partial depletion, trapping, type inversion Evolution of the Neff for n-type initial doping: Change of the leakage current: Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

11. V2 in clusters Ec VO V2O(?) EV Defect engineering of silicon • Influence the defect kinetics by incorporation of impurities or defects: • - good example: oxygen • Initial idea: Incorporate oxygen to getter radiation-induced vacancies •  prevent formation of di-vacancy (V2) related deep acceptor levels • Higher oxygen contentless negative space charge • One possible mechanism: V2O is a deep acceptorO VO (not harmful at RT)VVO V2O(negative space charge) DOFZ (Diffusion Oxygenated Float Zone Silicon) RD48 NIMA 465 (2001) 60 Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

12. Magnetic Czohralski silicon • High resistivity magnetic Czochralski silicon (MCZ-Si): • Oxygen has experimentally been found to improve the radiation hardness of silicon detectors (CERN RD48 ”ROSE” Collaboration). • contains intrinsically high level of oxygen (typically 1017-1018 cm-3). • formation of thermal donors  p-type MCZ-Si can be compensated by intentionally introducing thermal donors (TDs). • Depletion voltage of detectors can be tailored by adjusting: a) oxygen concentration in the bulk. b) thermal history of wafers (Thermal Donor killing). • Possibility for internal gettering. • Higher mechanical strength. • Less prone to slip defect formation. • Cost effectiveness compared to e.g. Float Zone silicon (FZ). SNIC - Int Symp Development of Detectors for Particle, Astro-Particle and Synchrotron Radiation Experiments Mara Bruzzi on behlaf of the CERN RD50 Collaboration – Radiation Tolerant Tracking Detectors - SLAC, April 5, 2006

13. Shallow donors in oxygen rich detector materials • Shallow donors have twofold influence on detector propeties: • - shallow oxygen thermal donors (TD) can be utilized to manipulate the Neff during processing. • - shallow donors interact with radiation defects and influence the radiation hardness. • The TDs are shallow donor levels within 0.01-0.2 eV energy range below the conduction band. • - their formation stronly depends on the temperature and the oxygen concentration of the silicon material. • - heat threatment between 400-600ºC can yield to a TD concentration comparable with the initial doping concentration of the high resistivity material Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

14. Thermal activation of TDs in MCZ-Si • The TD activation was performed through an isochronal annealing treatment at 430ºC up to total time of 120 min. • the shallow levels related to TDs were studied by thermally stimulated currents (TSC) in 10-70 K. • TSC spectra of MCZ-Si diodes before and after annealing treatment of 120 min. at 430ºC. • reverese bias 10 V • heating rate 0.1 K/s • deep levels filled at lower temp. by 1 min. forward current injection • The evolution of the space charge density caused by the annealing was related to the activation of TDs by the means of current deep level transient spectroscopy and TSC. Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

15. TSC spectra of proton irradiated MCZ-Si • TSC spectra of proton irradiated MCZ detector. • fluence 4 x 1014 p/cm2 (24 GeV protons) • annealing 1260 min at 60°C. • The TSC speak visible in 30 K has been related to a shallow charged defect • the occurence of the Poole Frenkel defect (evidenced by the characteristic shift of the peak temperature as the applied reverse voltage is changed) indicates that the radiation induced defect should be charged, possibly donor-like. • the peaks related to the TD emissions (TD0/+) and (TD+/++) are not present • when compared to reference FZ-Si samples the MCZ-Si samples have higher concentrations of VO complex and shallow donor at 30 K. Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

16. Bistable donors In addition to the oxygen rich silicon materials (DOFZ, MCZ-Si), detectors made on epitaxial silicon (EPI-Si) have been proved to be very radiation hard in terms of the effective doping concentration evolution. • The effective doping concentration measured after the end of the beneficial annealing as a function of fluence (24 GeV proton irradiation). • for larger fluences the possible creation of acceptors is over-compensated by the donors causing an almost linear increase of Neff. • the effect of stable donor generation is largely depending on the thickness of the device • the differences in the Neff are also correlated with the oxygen concentration profiles measured from the samples. Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

17. Bistable donors The well known point defects like CiOi, double vacancy and the peak at 115 K are measured with concentrations independent of the diode type and hence of the oxygen concentration. The TSC signal due to the shallow donor (BD) have very similar dependence on the material as the average oxygen concentration and the stable damage generation. The strong similarity of the BD complex to the thermal double donors, especially with respect to their bistability and the well known fact that the oxygen dimers (O2i) are one of the precursors for the formation of the thermal donors leads to the assumption: Dimers are involved in the damage produced BD defects. The TSC spectra for n-type EPI-Si diodes after 24 GeV proton irradiation, with Φeq=1.8 x 1014 cm-2. Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)

18. Conclusions • The formation of thermal donors can be utilized to tailor the full depletion voltage of the detectors. • detectors with high oxygen concentration and low Vfd can be fabricated with relatively easy process. • it is suggested that the shallow defect as a donor, is partially compensating the radiation induced deep acceptors  yields to the improved radiation hardness of the MCZ-Si devices compared to standard FZ-Si devices • EPI-Si devices exhibit a very high radiation tolerance. • this is mainly due to the high concentration of oxygen as interstitial atoms (Oi) and so-called oxygen dimers (O2). • the high Oi concentration leads to a suppression of deep acceptors and high concentration of O2 promotes the formation of shallow donors resulting in the compensation of the radiation induced negative space charge Panja Luukka,The Fifth International Forum on Advanced Material Science and Technology (IFAMST5 2006)