1. Silicon Radiation Detectors, Manchester 2010 Laplace Deep Level Transient Spectroscopy … a powerful tool for defect analysis in irradiated silicon A.R. Peaker 1, V. Markevich 1, I.D. Hawkins 1, B. Hamilton 1, L. Dobaczewski 2, J. Coutinho 3, R. Jones 4, K. Bonde-Nielsen 5 1 Photon Science Institute, University of Manchester 2 Institute of Physics, Warsaw, Poland 3 I3N, University of Aveiro, Portugal 4 Department of Physics, University of Exeter, UK 5 Institute of Physics and Astronomy, University of Århus, Denmark

2. Silicon Radiation Detectors, Manchester 2010 Outline

3. Silicon Radiation Detectors, Manchester 2010

8. Laplace or high resolution DLTS  removes the instrumental broadening from DLTS  isothermal measurement  replaces Lang’s double boxcar with an inverse Laplace transform of a digitally averaged transient (~1min of averaging).  dynamic determination of regularisation parameters based on Tikhonov approach  can separate defects with very similar emission rates (e 1 /e 2 )~2 ie a resolution of ~2 meV at 100K J. Appl. Phys., 76, 194, (1994)

9. Silicon Radiation Detectors, Manchester 2010 Separation of closely spaced electron emission rates from Si:Au and Si:AuH Hydrogen complexes with gold to form a series of states The gold acceptor and the AuH acceptor G4 have very similar electron emission rates Conventional DLTS (inset) shows near ‘ideal’ DLTS lineshape and width Laplace DLTS clearly separates the two defects which are 15meV apart Appld Phys Lett, 73, 3126 (1998)

10. Silicon Radiation Detectors, Manchester 2010 Laplace or High Resolution DLTS The order of magnitude improvement in energy resolution makes it possible to study aspects of the physics of defects that is not possible by any other techniques but…  sensitivity about an order less than conventional DLTS  the problem of separating exponentials is fundamentally ill posed so result has uncertainty  more difficult to use than conventional DLTS  at the moment only ~50 LDLTS installations worldwide http://www.laplacedlts.eu

11. Silicon Radiation Detectors, Manchester 2010 0 kbar 1 kbar 1.3 kbar 1.6 kbar 1.9 kbar 2.2 kbar 2.5 kbar 2.8 kbar 3.4 kbar 4 kbar 4.6 kbar 5.2 kbar Z axis: Pressure [GPa] 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.001.002.003.004.00 uni-axial stress and defect symmetry uni-axial stress lifts the degeneracy and causes the defect emission line to split in a predictable pattern dependent on the symmetry for practical stress levels this is almost undetectable in conventional DLTS the diagram shows the LDLTS signal from VO in silicon at zero stress (blue) splitting into two components when stress up to 5kBar is applied at 85K

12. Silicon Radiation Detectors, Manchester 2010 Double acceptor state of the silicon di-vacancy applying stress in the three major directions reveals the apparent symmetry of this diamagnetic state in a region <1µm thick comparison with theory of the absolute magnitude of the piezoelectric tensor helps decide if this is the true symmetry and possibly reveals the defect structure. We conclude V 2 2- has static trigonal symmetry with no measurable Jahn Teller effect (unlike V 2 - ) splitting ratios system trigonal (D 3d )3:13:33:0 Phys Rev B 65 113203 (2002)

13. Silicon Radiation Detectors, Manchester 2010 In situ LDLTS of H implanted into Si x Ge 1-x In situ measurements enable us to explore defect configurations unstable at room temperature … eg hydrogen at the bond centre between two silicon atoms is perturbed by germanium E3’ th = 163meV (the H 0/+ transition) E3’(Ge) th =146meV  E3’ th ƒ(Ge)  -17meV Si Phys. Rev. B, 65, 075205, (2002) SiGe Phys. Rev. B, 68, 045204 (2003) Ge Phys. Rev. B, 69, 245207 (2004) x= 0.8%(solid line) x=1.3%(dotted line

14. Silicon Radiation Detectors, Manchester 2010 V 3 in Si EPR (J.-H. Lee and J.W. Corbett, PRB 9, 4351 (1974)) Si- A4 (S=1) EPR signal was observed in n 0 - irradiated Si after annealing in the temperature range 100-250 0 C DLTS (M. Ahmed et. al, NIM-PR-A 457, 588 (2001)) E(=/-) = E c - 0.35 eV (E4a or E4) E(-/0) = E c - 0.45 eV (E4b or E5) E(0/+) = E v + 0.20 eV The levels anneal out in the temperature range 50-90 0 C (E An = 1.1 eV) (R.M. Fleming et. al, JAP 104, 083702 (2008))  E4/E5 is a bistable divacancylike defect in Si damage cascades E4/E5 in n-Si diodes can be restored by forward bias injection (12 A/cm 2 -20 min-RT) “Invisible” E4/E5 transform to “DLTS-visible” E4/E5 upon annealing at T>200 0 C Si-A4

15. Silicon Radiation Detectors, Manchester 2010 DLTS and LDLTS of E4/E5 DLTS spectra for a p + -n diode subjected to: 1) irradiation with 6 MeV electrons to a dose of 8  10 13 cm -2 ; 2) 30-minute annealing at 125 o C; and 3) forward bias injection with a current density 10 A/cm 2 for 10 minutes at 300 K. Table 1. Electronic parameters of V 3 -related acceptor levels in Si. Trap Level E na (eV) A (s -1 K -2 )  na (cm 2 ) E4(E4a) V 3 (2-/-) 0.359 1.4  10 7 2.2  10 -15 E5(E4b) V 3 (-/0) 0.458 1.6  10 7 2.4  10 -15 E 75 V 3 *(-/0) 0.075 2.4  10 7 3.7  10 -15

16. Silicon Radiation Detectors, Manchester 2010 E4/E5  E 75 transformations 1) E4/E5  E 75 ; Thermally activated process with ΔE = 1.16 eV and the pre- exponential factor value characteristic for atomic re-configuration 2) E 75  E4/E5; Carrier injection induced process with a very small (if any) energy barrier; Forward bias injection with I FB = 10 A/cm 2 at 50 K for 5 s was enough for complete E 75  E4/E5 transformation; 3) The above two process are fully reversible; There is no signal loss either for E4/E5 or for E 75 traps upon transformations.

17. Silicon Radiation Detectors, Manchester 2010 L-DLTS of E 75 under stress 1) Results of L-DLTS measurements under uniaxial stress for the E 75 trap indicate unambiguously that the defect, which gives rise to the signal, has trigonal (D 3 ) symmetry

18. Silicon Radiation Detectors, Manchester 2010 Modeling V 3 in silicon Figure 2. Atomic and electronic structure of a) V 3 (C 2v ) and c) V 3 (D 3 ) configurations. c) One- electron Khon-Sham levels within the valence band (VB) and conduction band (CB) edges (band gap of the cluster is E g = 2.4 eV). Occupied states are represented with solid circles. Levels E v +E(++/+) C 2v (planar) 0.15 (0.11) D 3 (4FC) E v +E(+/0)0.25 (0.20) E c -E(-/0)0.50 (0.46)0.23 (0.08) E c -E(=/-)0.28 (0.36) State ++ C 2v (planar) 0.00 D 3 (4FC) 1.19 +0.000.43 00.230.00 - 0.05 =0.000.50 Table 1. Relative energies of V3 complexes (eV).Table 2. Calculated electrical levels of V3 in silicon. All values in eV. The values derived from DLTS measurements are in brackets.

19. Silicon Radiation Detectors, Manchester 2010 Summary of tri-vacancy results  Trivacancy (V 3 ) in Si is a bistable center in the neutral charge state, with a 4FC configuration being lower in energy than the (110) planar one.  V 3 in the planar configuration gives rise to two acceptor levels at E c -0.36 eV and E c -0.46 eV in the gap, while in the 4FC configuration it has trigonal symmetry and an acceptor level at E c -0.075 eV.  V 3 is mobile in Si at temperatures higher than 200 o C and in Si:O it can be trapped by an oxygen atom, that results in the appearance of a V 3 O defect. Full story including V 3 O in: Phys. Rev. B 80, 235207 (2009)

20. Silicon Radiation Detectors, Manchester 2010 Conclusions

21. Silicon Radiation Detectors, Manchester 2010

22. Probing the local environment of Au in SiGe Au in has 4 nearest neighbours and 12 second nearest neighbours which can be Si or Ge The electron binding energy to the gold is modified by the local environment (ie Si or Ge) LDLTS can be used to quantify the local environment and so determine site preferences

23. Silicon Radiation Detectors, Manchester 2010 Site Preference of Au in SiGe peaks show 0, 1 and 2 Ge as nearest neighbours  E th 0/1   E th 1/2  -34meV there is a clear preference for the gold to site near germanium the relative concentration of the 1Ge configuration is about twice as big as expected for a random alloy this site preference of gold can be translated to the energy difference between the 0Ge and 1Ge configurations:  E conf 0/1  64meV. Phys. Rev. Lett., 83, 4582 (1999)

24. Silicon Radiation Detectors, Manchester 2010 LDLTS of Si 0.95 Ge 0.5 :Pt For SiGe:Pt measured at a lower temperature than SiGe:Au the role of the second nearest neighbour Ge can be seen The figures and subscripts denote configurations with the indicated numbers of germanium in the first and second nearest shell, respectively.  E th 0/1  -34meV  E th 0 0 /0 1  -4meV  E th 1 0 /1 1  -4meV Phys Rev B, 63, 235309 (2001)

25. Silicon Radiation Detectors, Manchester 2010 Si:VO splitting under uni-axial stress energy has been calculated from LDLTS peak shift using: e n =Aexp(-E a /kT) assuming A is constant Ole Andersen PhD thesis (2003)

26. Silicon Radiation Detectors, Manchester 2010 re-orientation spectra Si:VO Many defects re-orientate under stress eg one component of a complex may change lattice site in assessing symmetry. The example shows VO measured at 82K before and after alignment Alignment conditions 0.2GPa 140K (bias on, neutral charge state) Ole Andersen PhD thesis (2002)

27. Silicon Radiation Detectors, Manchester 2010 re-orientation rate of Si:VO after alignment VO was allowed to re-orientate at a range of temperatures The re-orientation rate of the neutral charge state agrees with with the EPR work of Watkins and Corbett (1965). The re-orientation in the negative charge state is found to be about two orders slower. Less total energy is required if the negative state looses an electron prior to re-orientation Laurent Rubaldo PhD thesis (June 2001)

28. Silicon Radiation Detectors, Manchester 2010 Si:VO total energy diagram to be published Phys Rev B (May 2003) The total energy diagram for the VO complex stressed along the direction. Below each minimum the corresponding vectors parallel to the Si-Si bond are given. All values of energies are given referred to the energy of the O 4 point for VO 0. The numbers in a bold font are for the zero stress, in italic are values obtained from the alignment experiments and calculated for 1GPa, the underlined values are obtained from Laplace DLTS peak splitting (extrapolated to 1GPa).