1. On behalf of the CERN-RD50 collaboration
Experimental techniques for defect characterization of highly irradiated materials and structures
Ioana Pintilie
National Institute of Materials Physics, Bucharest, Romania
[2]
[3]
[1]
[2]
On behalf of the CERN-RD50 collaboration
VERTEX 2016
NSS, 21 october 2008, Dresden

2. Outline Defect generation, impact on device performance
- Bulk radiation damage (in Si detectors)
- Surface and interface related damage
Experimental techniques for defect characterization
- electrical properties and concentration
- structure and chemical composition
Summary
Acknowledgements
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NSS, 21 october 2008, Dresden

3. Defect generation – bulk radiation damage
Low fluences
– point defects, 1st order process
V+O→VO
I+Cs→Ci
Ci+Cs→CiCs V2
…………..
Medium fluences
- point defects, complexes V3 I3
and 2nd order processes
V+VO→V2O
V + V2 → V3
I+I2→I3
4)….n) ?………..
High fluences
–clusters of point defects
(directly or via higher order processes)
V 4 , V5, V6 ……?..
I 4 , I5, I6 …….. ?
V +V2 O → V3O
…………
Electrically
active defects
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4. Defect generation – bulk radiation damage
Electrical properties of Point Defects in the Space Charge Region (SCR)
1) Contribution to Neff - given by the steady state ocupancy of the defect levels in SCR
2) Contribution to the leakage current
3) Carriers lifetime
- in steady state, instantaneous, related to carrier capture, trapping, emission, recombination etc....
- very sensitive to defect densities and their electrical properties
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NSS, 21 october 2008, Dresden

5. Carrier trapping lifetime
Defect generation – bulk radiation damage
Carrier lifetimes
-from transient MicroWave probed Photoconductivity technique (MW-PC)1-4
Recombination lifetime
- fluence dependence
Carrier trapping lifetime
Recombination lifetime
1) A.Tekorius, et al, Conference Proceedings “Radiation Interaction with Materials: Fundamentals and Applications 2014” (2014)
2) V. Rumbauskas, et al, J. Instrum. 11 (2016) P09004.
3) E. Gaubas, E. Simoen, and J. Vanhellemont, ECS J. Solid State Sci. Technol. 5 (2016) P3108-P3137
4) E. Gaubas, et al, Appl. Phys. Lett. 101 (2012)
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6. Defect generation - Surface and interface related damage
Heterostructures (e.g. MOS)
Interface states (electronic defect states)
Oxide states (mobile, fix, trapped charges)
Surface of any bulk material
Interface states – influence the Capacitance/Conductance shape and frequency dependence
Oxide states – direct influence on flat-band voltage
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NSS, 21 october 2008, Dresden

7. What we need to know about the electrically active defects ?
Electrical characteristics  impact on the device performance
Energy level in the bandgap (Et)
Capture cross sections for electrons and holes (n and p)
Introduction rates (concentration versus irradiation fluence)
Configurations and thermal stability
Structure  defect engineering
Chemical composition
Configurations and thermal stability
- There are no exp. techniques to give all the information about defects
- Correlations between different techniques via the results regarding the thermal stability
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8. Experimental techniques for defect characterization
Based on
Defect parameters
Limitations
C- Deep Level Transient Spectroscopy
Charge capture/emission
- Capacitance transients
Et, n,p, Nt
low density of bulk and interface defects (<Nd/3)
Chemical nature (indirect)
Thermally Stimulated Current (TSC)
– Current –free charged carriers
- high density of bulk defects (up to 1000 Nd)
Thermally Dielectric Relaxation Current (TDRC)
– Displacement Current
- high density of interface states
Photoluminescence
Photon Absorption followed by Photon Emission
PL bands
(Et, )
Only for radiative bulk recombination centers
Infrared Spectroscopy
Absorption of IR energy on molecules vibrational modes
Nt (acc %),
- Defect structure
Large density of defects
(> 1015 cm-3)
Electron Paramagnetic resonance
Zeeman effect and Spins resonance
Chemical nature and vecinity Nt
(> 1016 cm-3)
- Only paramagnetic centers
High Resolution Transmission Electron Microscopy
Electron microscopy
- structure and chemical composition
Large density of defects - clusters
electron beam damage of the sample during the observations
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NSS, 21 october 2008, Dresden

9. Electrical characteristics: Et, n,p, Nt
1) Bulk defects - Thermally Stimulated Current (aplicable for large defect concentration in ohmic and diode-like structures)
injection (uni- or bi- polar) of free carriers into traps at low temperatures
measuring the emission current from the traps during the heating with constant rate by applying a bias
- reverse for p-n diodes (V≥Vdep needed for accurate Nt )5-7
- low, or even 0V for ohmic structures (in this case a non-uniform filling of the traps is required for recording a current)7
Leakage
current
5) Appl.Phys.Lett. 78, (2001), ,
6) Nuclear Instruments and Methods in Physics Research A 556 (2006) 197–208; 3)
7) Nuclear Instruments and Methods in Physics Research A 439 (2000) 221}22
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10. After high irradiation levels only few defects prove to have significant impact on the device properties*
* Nucl. Instr. and Meth. in Phys. Res. A 611 (2009) 52; J. Appl.Phys. 117 (2015) ; Nucl. Instr. and Meth. in Phys. Res. A 514 (2003) 18; Nucl. Instr. and Meth. in Phys. Res. A 556 (2006) 197; Nucl. Instr. and Meth. in Phys. Res. A 583 (2007) 58; Appl. Phys. Lett. 81 (2002) 165; Appl. Phys. Lett. 82, 2169 (2003); Appl. Phys. Lett. 92 (2008)
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11. Ip center in STFZ (O lean material) - deep acceptor (-/0)
Electrical characteristics
Point defects (after gamma and low energy electron irradiation)*
Ip center in STFZ (O lean material) - deep acceptor (-/0)
Ea = Ec – eV
n = (1.70.2)x10-15 cm2 ; p = (91)x10-14 cm2
~ 90% occupied with (-) at RT
Generated via a second order process
BD center –generated in DOFZ (O rich material)
shallow donor (+ at RT)
Ei BD(98K) = Ec eV (0/++ )
Ei BD(50K) = Ec eV (+/++ )
Overcompensates the effect of Ip acceptors!
Gamma irradiation
(Ip and BD point defects determine Neff and LC!)
* APPL. PHYS. LETT.  81 , , 2002; APPL. PHYS. LETT 82, 2169 (2003);, Nucl. Inst. Meth. A 514, 18-24, (2003); Nucl. Instr. and Meth. in Phys. Res. A 611 (2009) 52-68
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12. - independent of impurity content in the material**
Electrical characteristics
Extended defects (E(30K) and H type), predominantly after neutron irrad.
- independent of impurity content in the material**
1 MeV neutron irradiated STFZ samples,  =5x 1013 cm-2
Extended defects - almost entirely responsible for hadron damage and annealing seen in Neff !
** APPL. PHYS. LETT. 92, (2008); Nucl. Inst. Meth. A 611 (2009) 52–68;
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13. Point & Extended defects, after irradiation with charged particles###
*** R. Radu et al. , J. Appl.Phys. 117 (2015)
Changes in Neff - well understood based on defect investigations in all the cases!
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14. Thermally dielectric relaxation current (TDRC)
Electrical characteristics
2) MOS structures - interface and oxide states
Thermally dielectric relaxation current (TDRC)
- TDRC procedure – similar to TSC (filling the traps during cooling or at low T by bringing the MOS in accumulation), measuring TDRC - displacement current, in depletion conditions, during heating when trapped carriers at the interface and in oxide are emitted in the Ec of the semiconductor
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15. Detection/characterization of Interface states in MOS capacitors
1) Si based MOS capacitors - X rays -12 keV photons
10 MGy dose
interface (Dit) position, shape and frequency dependence of C/G-V
near-interface-states in oxide (NITox) shift the VFB in C-V/G-V characteristics towards more negative bias and causes hysterezis effects
Nox influence the VFB ; it can be accurately determined only from experiments where no charge of NITox take place (avoiding biasing in inversion)
J. Synchrotron Rad. (2012). 19, 340–346; J. INSTRUM., 6, C11013  (2011);; J. INSTRUM. 7, C01006 (2012)
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16. Detection and characterization of Interface states#
Electrical characteristics
Detection and characterization of Interface states#
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17. Electrical characteristics
Calculation of C/G-V curves and determination of Nox – inputs from TDRC investigations
Nox determined from the voltage shift between the measured and the calculated curves
rare case - to fit so well both C and G!
<100>
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18. Electrical characteristics
2) N implantation at the interface SiO2 /n-4H-SiC in MOS capacitors, for reducing the Dit ##
Sample #1 – dual implantation with N (2.5 and 5 keV, dose 1.0x1015 cm-3)
Sample #2 – non-implanted
Sample #2 - sharp (0.05V) minima in the C-V curves at T< 150 K.  electron tunnelling from interface states
## Journal of Applied Physics 108, (2010); Thin Solid Films 545 (2013) 22–28
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19. Electrical characteristics
Resonant Tunnelling of electrons from interface states into oxide NIT states
Electron injection into NITfast states (E1 and E2 levels)
Thin Solid Films 545 (2013) 22–28
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20. Electrical characteristics
C/V – G/V simulated characteristics based on the TDRC investigations
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21. Transmission Electron Spectroscopy
Structure and chemical composition
Transmission Electron Spectroscopy
-allow to study, down to atomic scale, the structure and chemical composition of almost any material with two conditions:
- to prepare a thin sample (200 nm / 10 nm) transparent to the electron beam;
- to prevent the electron beam damage of the sample during the observations.
Methods used in modern TEMs for the study of:
extended defects, interfaces and heterostructures: conventional (TEM); high resolution (HRTEM); scanning (STEM); electron diffraction:
local chemical composition and chemical bonds:
- energy dispersive X-ray spectroscopy (EDS)
- electron energy loss spectroscopy (EELS)
Cross section specimen preparation for TEM observations:
- sawing strips (2x1) mm; gluing them face to face; mechanical thinning to ~20 μm followed by ion milling to <100 nm for HRTEM, EDS, and <50 nm for STEM, EELS
Instrument used:
High resolution probe-corrected analytical JEOL JEM-ARM 200F operated at 200 kV with a resolution of 0.19 nm in HRTEM mode and of 0.08 nm in STEM mode. The microscope is equipped with a JEOL JED-2300T EDS spectrometer and a Gatan GIF QuantumSETM Imaging Filter/EELS spectrometer.
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22. Structure and chemical composition
HRTEM on Si irradiated with high energy electrons### or neutrons:
- DOFZ Si irradiated at RT with electrons: energy 27 MeV, fluence 2x1016 cm-2
- 75 µm EPI Si layer deposited on CZ substrate irradiated with 1 MeV neutrons, fluence 1x1016 cm-2
irradiation with high energy electrons or neutrons introduces in Si a high density of clusters of point defects resulting in large distorted regions with dimensions of nm (marked red arrows), formed by the accumulation of excess vacancies {111} and self interstitials {110}, the end products of the collision cascade.
the structure of the damaged zones is highly disturbed, but not completely amorphous.
### R. Radu et al. , J. Appl.Phys. 117 (2015)
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23. Structure and chemical composition
Some aggregates in the {111} or {110} planes, as revealed at higher magnification.
- The contrast at the {111} defect in the HRTEM image results from a Frank vacancy loop formed by accumulating vacancies in the {111} plane. The defect is further stabilized by a partial filling with interstitials [L. Fedina et al Phil. Mag. A 77, 423 (1998)].
- The contrast at the {110} defect can be interpreted as agglomeration of self-interstitials [S.Takeda, T. Kamino PRB 51, 2148 (1995)].
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24. Structure and chemical composition
Ex. Analytical HRTEM/STEM studies on N-implanted SiO2/4H-SiC interface
Electron tunneling can occur
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25. Structure and chemical composition
Composition at the interface by EELS-SI (EELS-Spectrum Imaging)
Low magnification STEM image
EELS-maps at the interface in the N-implanted specimen
EELS-maps at the interface in the N-implanted specimen from
the region marked green in the left image: (a) STEM image showing a variable contrast at the interface, placed approximately in the middle, in which a composition variation is expected, (b)
EELS Spectrum Image, (c) Si-map (yellow), (d) O-map (red), (e) C-map (blue), (f) N-map (green). The maps, extracted from the data-cube, give the relative composition of the elements of interest. The intensity of colors is proportional to the atom concentration in each pixel. The maps show the local distribution of each element in the analyzed region of (3x41) nm2. From the N-map it results that N is present in the whole analyzed region ( ~20 nm on each side of the interface), with a relative larger content on the SiO2 side near the interface.
VERTEX 2016
NSS, 21 october 2008, Dresden

26. Electron Paramagnetic Resonance
Structure and chemical composition
Electron Paramagnetic Resonance
EPR is the best experimental technique for determining the structure of irradiation induced paramagnetic point defects (IPPD) in semiconductors.
EPR = Zeeman spectroscopy (the effect of splitting a spectral line into several components in the presence of a static magnetic field B ≠ 0) of defects with unpaired electron states (S = 1/2, 1, …).
The condition of resonance: h = E = gB, ge = for free electron
  9.5 – 34 GHz, 95 GHz, …
Sensitive: 2 x 1010 spins/Gauss (~ 1 ppb)
B ≠ 0
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NSS, 21 october 2008, Dresden

27. Structure and chemical composition
The EPR method
An unpaired electron can gain or lose angular momentum
change the g-factor value through spin-orbit coupling
 information about the nature of the atomic/molecular orbital containing the unpaired electron - defect ‘s electronic structure
The magnetic moment of a nucleus with I≠0 affects any unpaired electrons associated with that atom.  hyperfine coupling splitting the EPR resonance signal into doublets, triplets …..
M equiv. nuclei, each with I 2MI+1 EPR lines
The g-factor and hf coupling in an atom/ molecule may not be the same for all orientations of the unpaired electron in external magnetic field  spectra anisotropy depending on the electronic structure of the atom/molecule
 reflects the local structure
EPR spectrum of CH3 radical
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28. Ex. EPR results after irrad. with 3.5 MeV electrons, =1017 cm-2
Structure and chemical composition
Ex. EPR results after irrad. with 3.5 MeV electrons, =1017 cm-2
The fluence used is still too small to identify more defects!
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NSS, 21 october 2008, Dresden

29. Summary
The radiation damage get more complex with increasing the fluences.
The defect characterization becomes a more difficult and costly task, requiring several complementary techniques to understand/prevent the consequences for the device performance.
Direct correlation between defect investigations and device properties can be achieved. Moreover, when electrical characteristics of the defects are known then models predicting the device performance in different operational scenarious can be developed
Still missing identification of the chemical nature of the defects that deteriorates the device characteristics – more efforts foreseen within RD50 Collaboration
NSS, 21 october 2008, Dresden

30. Acknowledgements Members of CERN-RD50 Collaboration
Alexander von Humboldt Foundation
UEFISCDI - romanian national funded project PNII-ID-PCE Nr. 72/
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