Important point defects after γ and proton irradiation investigated by TSC technique

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1 Important point defects after γ and proton irradiation investigated by TSC technique I. Pintilie a),b), E. Fretwurst b), G. Kramberger c) G. Lindström b) and J. Stahl b) a) National Institute of Materials Physics, Bucharest-Magurele, P.O.Box MG-7, Romania b) Institute for Experimental Physics, Hamburg University, D-22761, Germany c) DESY Hamburg, Germany

2 Outlines What means important defects for RD5 community? Thermally Stimulated Current (TSC) as powerfull technique for detection of electrically active defects after high levels of irradiation. Results after irradiation with Co 6 gammas and protons - point defects of crucial importance for the radiation hardness of silicon detectors

3 What means important defects for RD5 community? Limiting factors for Si detectors in LHC experiments: the changes in the N eff and CCE V dep [V] CA: FZ standard CB: DOFZ 24 h/115 C CC: DOFZ 48 h/115 C CD: DOFZ 72 h/115 C N eff [1 11 cm -3 ] N eff [1 12 cm -3 ] standard FZ neutrons pions protons oxygen rich FZ neutrons pions protons V dep [V] (3µm) dose [Mrad] Φ eq [1 14 cm -2 ] Fig. 1. Examples of change in the Neff measured at RT after irradiation with diferent particles GeV/c Protons CCE Fig. 2. Change of the CCE with the 24 GeV/c proton fluence.8.75 EPI/CZ CZ (similar with STFZ and DOFZ) Φ eq [1 14 cm -2 ]

4 Electrical properties of Point Defects in the SCR 1) Contribution to Neff - given by the steady state occupancy of the defect/impurity levels in SCR n acceptor T ( T ) = N TE e n ep ( T ) ; n ( T ) + e ( T ) p donor T ( T ) = N 2) Contribution to the Leakage current density TE e n en ( T ) ( T ) + e p ( T ) α E acceptor q en ( T ) nt ( T ) + q e p ( T ) donor ( T ) = nt ( T ) What we need to know for evaluating the effect of defects on detector properties are: - type of defect (donor or acceptor) - the exact concentration N TE - emission rates (level position in the bandgap and both capture cross sections!)

5 * I. Pintilie, L. Pintilie, M. Moll, E. Fretwurst and G. Lindström, Appl. Phys. Lett. 78 (4) 55 (21) Thermally Stimulated Currents Method 1) filling process the injection of carriers at low temperature (electrons, holes or both) 2) recording the reverse biased diode is heated with constant rate and the current due to charge emission from the traps is recorded as function of temperature TSC signal from only one defect (if diode full depleted): I TSC ( T ) = A * 2 1 * q * e n ( T ) * N t exp( e n ( T `) dt β Evaluation of activation enthalpy - the thermal cleanning procedure d T T ` TSC (pa) electron traps 1e 3e 5 7e 8e 9e 15e Log(TSC) e 8e 9e 7e 2 min. at 3 C /T(K) 3e E a3 =.95 ev E a5 =.166 ev E a7 =.22 ev E a8 =.3 ev E a9 =.32 ev E a12 =.37 ev E a15 =.45 ev Correct values* - only when the guard ring is grounded and the diode is fully depleted - one type of carriers should be injected ( especially for close to midgap levels)!!

6 Concentration of defects by integrating the TSC peak Problems when the diode cannot be fully depleted over the TSC temperature range The SCR width can vary with T as well and introduce false TSC peaks! Examples of TSC spectra for diodes not depleted over entire temperature range TSC (pa) 35 neutron irradiated E&H(147) E&H(165) RB= 2 V RB= 15 V RB= 1 V fit 2 V fit 15 V fit 1 V TSC (pa) 6 RB = 1 V E&H(147K) protons neutrons 5 1 SCR width (µm) neutron irradiated RB= 2 V RB= 15 V RB= 1 V SCR width (µm) proton irradiated RB= 1 V

7 Co 6 -gamma irradiation Acceptors which can influence the diode performance at RT The I center *,**: detected by both DLTS (4-42Mrad) and TSC (up to 5 Mrad) The single acceptor state: Ea = Ec.545 ev σ n = (1.7±.2)x1-15 cm 2 - direct measurement TSCurrent (pa) 4 2 cooling from RT under V and then forward injection 284 Mrad dose STFZ DOFZ BD tail E(5K) VO i + C i C s I +/ (97K) C i O i RB = 3 V I /- (2K) Γ H σ p = (9±1)x1-14 cm 2 -fromn T DLTS (T) Its steady state occupancy at room temperature in SCR is 85%! 85% from the concentration of I center is negatively charged at RT For [I]=1 12 cm -3 (~26 Mrad) Neff= 8.48x1 11 cm -3 LC/vol=.43 µa/cm 3 * I. Pintilie, E. Fretwurst, G. Lindstroem and J. Stahl, Appl. Phys. Lett. 81 (1) 165 (22) ** I. Pintilie, E. Fretwurst, G. Lindström and J. Stahl, NIM A, 514 (1-3),18-24, (23)

8 Detection of single acceptor state of the I center by TSC method Electrons in the conduction band N P + N A (-) N D (+) / Steady state occupancy in the SCR - n T (T) ~ 85%N T Holes in the valence band Itscp n Itscn. 1 n ItscF n N + what can be detected after filling this center with nonequilibrium carriers is: 15% N T emission of electrons - if only electron injection is performed 85% N T emission of holes - if only holes injection is performed 8% N T emission of holes after forward injection T 22 n

9 Dose dependence of I center I center - detected through 2 levels (one corresponding to the single acceptor state and second to the donor state) cooling from RT under V and then forward injection Concentration of defects (cm -3 ) STFZ 1 1 Dose (Mrad) I(2K): I /- H(97K): I +/ slope 1.96 DOFZ I(2K): I /- slope 1.98 TSCurrent (pa) Mrad dose STFZ DOFZ BD tail E(5K) VO i + C i C s I +/ (97K) C i O i RB = 3 V I /- (2K) Γ H I defect - almost quadratic dose dependence second order defect - experiments indicate that I defect is formed via VO center - thermal stable up to 32 C * * I. Pintilie, E. Fretwurst, G. Kramberger, G. Lindström, J. Stahl and Z. Li, Phys. B, in press

10 I center& device performance after Co 6 -gammas At RT only the acceptor state can influence the diode characteristics N eff (cm -3 ) 1.x x x x x x1 11 n type "p" type T = 293 K determined from C-V calculated - only I level LC (µa/cm 3 ) K experimental (from I-V) calculated - only I level -4.x Co 6 - gamma irradiation dose (Mrad) Co 6 - gamma irradiation dose I center *,** - responsable for type inversion in STFZ material and increase of the leakage current in both STFZ&DOFZ silicon * I. Pintilie, E. Fretwurst, G. Lindstroem and J. Stahl, Appl. Phys. Lett. 81 (1) 165 (22) ** I. Pintilie, E. Fretwurst, G. Lindström and J. Stahl, NIM A, 514 (1-3),18-24, (23)

11 What the I defect might be? induced by irradiation, supressed in oygen rich material, high thermal stability Quadratic dose dependence up to 4 Mrad, formed via VO center Possible defects formed via a second order process 1. VO+V V 2 O most probably since is the only one reaction which does not require high temperature annealing (EPR 1 results and theory 2,3 ). The reaction V 2 + O V 2 O may happen only after annealing at T > 22 C 2. V+O 2 VO 2 ; VO 2 + V V 2 O 2 - less probable since cannot explain the suppresion of this defect in oxygen rich material but contrary (should appear in Si riched in oxygen dimers). In adition V 2 O 2 was detected only after annealing at high temperatures (EPR 1, FTIR 4 ). 3. VV+V V 3 even less probably because : a) [VV] is about 5 times smaller than of [VO]; b) V 3 was detected in heavy irradiated Si only after annealing at temperatures higher then 34 C (EPR 5 ) 4. Does anybody knows other possibility? 1 Y. H. Lee and J. Corbett, Phys. Rev. B 13 (6), 2653 (1976); Y. H. Lee, T. D. Bilash and J. Corbett, Radiat. Eff. 29, 7 (1976) 2 B. MacEvoy and G. Hall, Mater. Sci. in Semicon. Process (2) 3 M. Pesola, J. von Boehm, T. Matila and R. M. Niemen, Phys. Rev. B 6 (1999) J. L. Lindström et al, NIM B, 186, , (22) 5 Y. H. Lee, T. D. Bilash and J. Corbett, Radiat. Eff. 29, 7, (1976)

12 Check list for assignment to V 2 O not closed yet! Features of V 2 O(EPR and theory) Induced by irradiation via VO+V reaction High thermal stability Two acceptor states and one donor state Formed via VV+O during treatment at T > 22 C Responsable for type inversion in STFZ (the V 2 O model)?? I center /3 + X center + +2/3? (because [X] can be > [VV])

13 Shallow Donors which can influence the Neff at RT Generation of shallow donors is enhanced in oxygen rich material * TSCurrent (pa) 1 1 Forward injection at 2 K VO -/ DOFZ Mrad 1 V 3 V 5 V BD(98K) TSCurrent (pa) 1.1 RB = 5 V; Ea TSC =.128 ev RB = 3 V; Ea TSC =.139 ev RB = 1 V; Ea TSC =.16 ev /T (K -1 ) H=.225 ev -.26xRB 1/4 (ev) * I. Pintilie, E. Fretwurst, G. Lindström and J. Stahl, NIM A, 514 (1-3),18-24, (23)

14 BD(98K) is a bistable donor detected in DOFZ the generation of BDs is very much enhanced in EPI/CZ TSCurrent (pa) DOFZ Mrad: Cooling from RT under V after: exposure to day light keeping in dark at RT for 2 days BD H(42K) E(5K) VO -/ BD(98K) CiOi +/ I H (V 2 O /- ) Γ H (24K) TSC signal (na/cm 3 ) Co- gamma irradiation E(5K) VO -/ X 1 BD I +/ C i O i +/ STFZ - 5 Mrad DOFZ - 5 Mrad EPI/CZ Mrad VV -/ I / beneficial effect - This bistable donor can overcompensate the negative space charge introduced by deep acceptors at RT

15 Dose dependence of BD defects N eff introduced by BD (cm -3 ) DOFZ 4.x x x x x x1 11 BD (98K) Dose (Mrad) BD defect non-linear dose dependence - bistability similar to the earlier TDDs - largely produced in samples with high concentration of oxygen dimers oneof theearliertdds?

16 Proton irradiation (24GeV) * TSC signal (na/cm 3 ) x1 14 cm GeV - proton irradiation H(42K) IO 2 -/ VO -/ x 5 L(9K) BD VOH? C i O i +/ L(17K) STFZ CZ EPI I center more pronounced in oxygen lean material (appears also in DOFZ but in smaller concentration) can account only for 1-15% to the changes in Neff and LC (the main damage due to clusters) BDs strongly generated in material rich in oxygen dimers * J. Stahl, E. Fretwurst, G. Lindström and I. Pintilie, Phys. B, in press I /- TSC signal (pa) BD - Poole Frenkel effect Reverse bias 1 V 2 V 5 V 1 V BD Φ = 6x1 14 p + /cm

17 Defect engineering Epitaxial silicon / CZ substrate V D [V] CERN Szenario (24 GeV p+) STFZ DOFZ Epi/Cz Epi/CZ * no type inversion, - almost constant N eff / V D - but no difference in LC * G. Kramberger, D. Contarato, E. Fretwurst, F. Hönniger, G. Lindström, I. Pintilie, R. Röder, A. Schramm and J. Stahl, NIM A- in press Proton Fluence [cm -2 ] oxygen dimers are the precursors for the thermal donors High concentration of oxygen dimers are in Cz material These oxygen dimers can diffuse into the high purity epitaxial layers (during the growth process) and after irradiation can form the ealier thermal donors The formation of BDs is the reason for such an improvement! (because they can compensate the effect of deep acceptors on Neff)

18 Conclusions Co 6 gammas: Two defects are mainly responsible for the macroscopic behavior of the diodes - The I center (acceptor like at RT) is the main source for both introduction of negative space charge and the increase of the LC - The BD defects are the source for the increase of the positive space charge with the dose in DOFZ diodes and can compensate the deep acceptors effect!! 24 GeV proton irradiation: Both I and BD centers can be detected after proton irradiation. Very promising results with respect to radiation hardness in the case of EPI/CZ silicon most likely due to a large concentration of oxygen dimers migrating from the CZ substrate which after irradiation lead to the formation of bistable donors! Defect engineering possibility - the beneficial oxygen effect results not only in suppression of deep acceptors (responsible for the type inversion in oxygen lean material) but also in the creation of donors. Maintaining the desired balance between the two defect formation mechanisms depends largely on how silicon is engineered prior to irradiation. But, we should have an idea about the structure of these defects. Any help for the identification of I and BD centers is gratefully acknowledged!

19 TSC program used in Hamburg (done by G. Kramberger ) We can offer it to whom is interested