1. Radiation hardness of Si-sensors N.Zamjatin, JINR, Dubna The 3rd Work Meeting of the CBM-MPD STS Consortium “Technical challenges of the CBM and MPD Silicon Tracking Systems 2009” 1 – 4 June 2009, Doctor Winter’s Lodge Resort, Sortavala, Karelia, Russia

2. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 2 Main effects in Silicon at Non Ionization Energy Loss (NIEL) Radiation damage of Si: -Removing of the atom Si from crystal lattice with vacancy on stital (V) and interstital Si (I ); -V and I is electrical activity deep levels: V V; VО; VР; IС; IVР; et al. Deep level effects: - generation/recombination - «traping/detraping e-h» - removing donors Ес Ev е h Dark current Charge collection Compensation donors DL

3. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 3 A. The detector leakage current increases linearly with hadrons fluence:  =  I  I    V   fluency, V  the detector bulk,  I = (5  1)  10 -17 А  см -1 for fast neutrons at T = + 20º C (no self-annealing) Possible solution at high hadrons fluence: work at low temperature -(30-50)°C, detector leakage current doubles it’s value every 8 degrees; Detectors module design required very good thermo contact between Si-sensor and cooling plate for the excluding of the heating and thermo break down /3/

4. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 4 I-V and C-V measurements for detector RWS-01/01-69-7 before and after neutrons /3/ irradiation 4.9×10 14 n/cm 2 Blue color curves – measurements at +20°C before irradiation Red color curves – measurements at -28°C after irradiation Topology of detector: 32-p + strips; 63×63×0.3 mm 3 ; 5 kOhm×cm n-type Si- Wacker,

5. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 5 7.5. Рост мощности рассеяния на ППД (P=I  V). До облучения: Р дет =10 -6 А×150В=1.5×10 - 4 Вт (Для V det =(63×63×0.3)мм 3 ). После 5 лет (CMS/SE Ф n =10 14 см -2 ): Р дет =5×10 -4 А×500В=0.25Вт (при Т = -5º). ! Проблема теплового пробоя (саморазогрев облученного детектора)

6. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 6 B. Decreasing of the charge collection efficiency (CCE) with increasing hadrons fluence The CCE decreases 8% /10 14 n/cm 2 and does not depend on initial silicon type, extrapolation of this curve to 10 15 n/cm 2 give the value for signal equal 20% from initial amplitude /3/ What to do??? to change of Si-planes after each 5×10 14 n/cm 2 ???; to work with (20-10)% of CCE; to work at low temperatures and at forward bias (experience of RD39, LAZARUS effect /1/ )

7. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 7 C. The detector full depletion voltage U fd increases with hadrons fluence (after point inversion of type conductivity for n-type Si). This effect could be diminished /2, 4/ applying the oxygenated Si and low resistivity Si as a starting detectors material: CZ (MCZ) has higher oxygen concentration in crystal ingot, low resistivity material (100 – 800 Ohm  cm) FZ-Si-p – no conductivity type inversion FZ-Si-(n- or p-type) + O 2 during the detectors technology fabrication Remark: in the real experiments with irradiated detectors Ud>Ufd for the attainment of plato CCE=f(Ud) and max S/N

8. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 8 Inversion of bulk type conductivity (n-type to p-type) at irradiation Si-sensors Sensor (20×20×0.4) mm 3 at bias Ud=15V irradiated by fast neutrons Capacity of detector measured (F=1 кГц) directly at irradiation on the neutrons chanel Initial parameters of Si: - n-type,, (FZ-Wacker); - ρ=6 кОm×сm (Ufd=80 В) -Point inversion for this detector equal (2÷3)×10 12 n/сm 2 Direct method for the measuring of the point inversion bulk type conductivity /5/

9. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 9 Is it an alternative to SILICON to construct a detector with large area in radiation hard environment??? Possible candidates: CVD-C, CdTe, GaAs and??? CVD-C: (-) commercial wafers with big area???, big cost, small (50-150mkm) CCLength; (+) radiation hardness!!! CdTe(CdZnTe): (-) small area of wafers (1-5 cm 2 ); high cost (5-10 $/mm 3 ) for mono crystal; (+) high radiation condition of policrystaline (CERN-R&D for luminosity monitor) GaAs: (-) more expensive that Si-detectors, mechanical conditions lower that Si; (+) bigger radiation hardness that Si! (need to study of rad.hard. for real detectors)

10. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 10 DS-strip detector: n+side with p+stop ring/n+strip n+ strips p+ stop rings

11. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 11 DS-strip sensors need to study (surface) radiation effects on the both sides What difference between n+ and p+ sides for DS-strip sensors? each n+ strip isolated by p+ stop ring inter n+ strip region need bigger value of full depletion voltage in compare with bulk depletion with over depletion increase electric field in inter n+strip region and possible break down n+ strip to p+ stop ring detectors from difference manufactures have individual parameters (thickness of dielectric layers, density of charge in SiO 2, profile of implantation layers, et al)

12. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 12 (I-V) at +20° for Si PAD-detector (8×8×0.3 mm 3 ) after irradiation 2×10 12 alpha/cm 2, Eα=3.5 MeV, α-particles illuminated n+ side (Ohmic contact) U full depletion

13. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 13 (I-V) for Si PAD-detector (8×8×0.3 mm 3 ) after irradiation 2×10 12 alpha/cm 2, Eα=3.5 MeV, α-particles illuminated p+ side (junction contact)

14. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 14 References: 1.V.G.Palmieri et al., “Evidence for charge collection efficiency recovery in heavily irradiated silicon detectors operated at cryogenic temperatures”, NIM A 413 (1998) 475-478. 2.B.Dezillie, Z.Li et al., “Improved Neutron Radiation Hardness for Si- detectors: Application of Low Resistivity Starting Material and/or Manipulation of Neff by Selective Filling of Radiation-induced Traps at Low Temperatures”, IEEE Transactions on Nuclear Science, Vol.46, No.3, June 1999. 3.Ph. Bloch, N.Zamiatin et al. “Performance of Si sensors irradiated to 5 ×10 14 n/cm 2 ”, NIM A517 (2004) 121-127. 4.RD-48, ROSE Collaboration Reports. 5.Н.И. Замятин и др., Экспериментальный метод определения инверсии n–типа проводимости кремния при облучении быстрыми нейтронами. Сообщение ОИЯИ, Р13-2001-281.

15. N.Zamjatin, JINR, Dubna 3-rd meeting CBM-MPD, 1-3 June'09 15 Conclusion: Bulk damage effects in Si crystal are very good study and understood (big experimental data of ROSE collaboration for LHC experiments, RD48) Surface damage effects dependent from topology and technology sensors (inter strip distance, p+ stop ring on n+ side, thickness of SiO 2, et all) DS-strip sensor need to study radiation hardness for both sides n+ and p+ before- and after point inversion (inter strip resistance, inter strip capacitance, cross talk, S/N, break down on p+ or n+ side?) Sensors after high hadron fluence need very good thermo contact with cooling system (special design modules/ladders)