1. Antimatter in materials research: defect spectroscopy and study of porosity using positrons
Laszlo Liszkay
CEA Saclay
DSM/IRFU/SACM/LEDA

2. Defect spectroscopy with positrons
introduction – why positrons?
positron interactions in solids
Doppler broadening spectroscopy
positron lifetime spectroscopy
study of porosity in nanoporous films

3. Introduction
positron: selective and sensitive non-destructive probe of lattice defects with open volume
vacancies in semiconductors
vacancies and voids in irradiated solids
vacancies in metals
non-destructive test of porosity in nanoporous oxides (silica, alumina)
low-k dielectrics in semiconductor industry
silica films with well-controlled porosity (sensors, templates for nanostructures...)

4. Positron-electron annihilation
25 % para-Ps (singlet)
125 ps lifetime 2 g photons (511 keV) e+
positronium (Ps)
e+-e- atom
from b+ decay or pair production
75 %
ortho-Ps (triplet)
142 ns lifetime 3 g photons ( keV)
(slowing down)
0-540 keV (b+) or 0-25 keV (“slow positron beam”)
annihilation 2 g photons (511 keV)
from a condensed matter e-

5. Conventional positron annihilation spectroscopy
100 mm
0-540 keV e+ (22Na)
diffusion (E~kT)
diff length L~100 nm
slowing down ~ ps
g 1.28 MeV
“bulk” annihilation from Bloch state
tb~ ps
higher momentum
“trapping” in a vacancy tv>tb lower momentum
positronium (Ps) formation in voids 1-2 ns
in some pores (Ø 1-50 nm): ns

6. much higher efficiency than velocity selection!
Positron moderator
annihilation
fast e+
~200 keV e+
efficiency: 10-4 (W) 10-2 (Ne)
thermalization, diffusion
slow (eV) e+ Ps
thin (~ mm) W (Ni, Pt) foil (negative e+ work function), solid Ne (Kr)
much higher efficiency than velocity selection!

7. Positron spectroscopy with “slow” (keV) positrons
Makhovian profile
Mean impl. range
e+ ~1-30 keV
surface state (450 ps)
diffusion to surface
e+ emission (~ eV, negative work function)
positronium (Ps) emission (o-Ps 142 ns, p-Ps 125 ps) thermal (3/2kT) or fast (few eV)
annih. in crystal (see before)

8. Detectables: gamma energy distribution (Doppler spectroscopy)
vacuum
the 511 keV annihilation peak e+
Sample
High purity Ge detector
measurement of the Doppler broadening of the annihilation radiation due to the Doppler shift
where pL is the longitudinal momentum component of the electron-positron pair
proportional with electron momentum (e+ thermalized)
two lineshape parameters: S (low momentum) : valence electrons W (high momentum): core electrons  chemical information
S-W plot: identification of the defect

9. Energy-dependent Doppler broadening spectrum
implantation induced defects
bulk
surface
defect-free crystal
defect profile

10. Vacancy-fluor complex in implantation-induced defect structure
as-implanted Si
the same sample after annealing at 600 °C
implantation  divacancies
annealing  F decorated vacancies

11. Identification of the local chemical environment by coincidence Doppler measurements
annihilation with core electrons
(Doppler coincidence measurement: HPGe and NaI detectors in coincidence)

12. Detectables: lifetime
sample e+
511 keV annihilation g photon
pulse or “start” g-photon
(BaF2 scintillator)
Schema of the pulsed positron beam in Munich
Ii intensities – proportional with vacancy concentration
ti lifetimes – characteristic value for each vacancy type typically ps (bulk solid, vacancies) 1-2 ns (large voids, positronium)
more information, more sensitivity than in the case of Doppler sp.

13. Defect spectroscopy using slow positrons: implantation-induced defects
250 MeV Kr and 710 MeV Bi in sapphire (Al2O3)
homogeneous defect concentration in the positron range
vacancies and larger defects can be identified
trapping in larger defects (500 ps)
trapping in vacancies
saturated trapping in vacancies

14. Schema: pulsed positron beam (Garching, near Munich)
~ eV
100 keV...MeV
1-30 keV
main buncher
moderator
prebuncher
chopper
target
source
good time resolution (<250 ps FWHM) and low, flat background (peak/bgd ~ 10-4) are essential
brightness of the possible sorce/moderator structures is low  difficult to form short pulse
target region: backscattering etc  background problems

15. The intense source based positron spectrometer in Garching (FRM-II)
113Cd(n,γ)114Cd
Pt moderator
~ 3 x 108 slow e+/s
B~70 Gauss
C. Hugenschmidt et al, NIM B 198, 220 (2002)
magnetic transport
remoderation stage
(brightness enhancement)

16. Lifetime spectroscopy with positrons: identification of defects in semiconductors
e+ Doppler
thin Mg doped GaN layers (2 mm) (slow positron beam only)
problem: electrical compensation of dopant (Mg) that limits p type doping
shallow positron traps + vacancy defects (S parameter measurements)
vacancies + vacancy clusters (lifetime measurements)
identification of VN-MgGa complex with 180 ps characteristic lifetime(lifetime + Doppler coincidence measurement)
15 keV
e+ lifetime
Doppler coincidence
S. Hautakangas, J. Oila, M. Alatalo, K. Saarinen, L. Liszkay, D. Seghier and H. P. Gislason, Phys. Rev. Letters 90, (2003)

17. defect spectroscopy with positrons
sensitive to defects with free volume (vacancy, vacancy complex, voids)
sensitivity up to (lifetime changes below 1 ps are reliably observed)
open volume defects: important role in mechanical failure (metals), dopant compensation (compound semiconductors), radiation damage (reactor pressure vessel steels, implantation)
non-destructive probe, in most cases does not require special sample treatment

18. Positrons in semiconductors: charged defects

19. sensitivity range
depth range: surface, ~10 nm – ~5 mm
sensitivity: defect dependent
e.g. in silicon:

20. Study of porosity using positrons
nanoporous silica films:
SiO2, porosity in nm range
low k dielectrics in semiconductor device technology
possibility to grow films with well controlled pore structure (sensors, filters, template for nanostructures)
using o-Ps annihilation to study
pore size
pore interconnectivity

21. Porous silica films prepared by the sol-gel method
silicon: TEOS, (tetraethyl orthosilicate)
porogen: polymer or surfactant (removed by solvent or heating)
silanol (Si-OH) groups
deposition by spin coating ( nm thickness)
removal of porogen by heating in air at 400 °C
pure SiO2 structure (amorphous walls)
micropores
mesopores

22. Orthopositronium in nanoporous layers
2 nm
SiO2
ortho-positronium (o-Ps)
o-Ps e+ e+
o-Ps annihilation is a closed pore  size-dependent lifetime
o-Ps reemission from the surface  142 ns vacuum lifetime, intensity proportional with the degree of interconnectivity of the pore system (percolation)

23. Positron study of thin nanoporous silica films: open/closed porosity
142 ns (vacuum o-Ps) component (o-Ps reemission from the film)
lifetime spectrometer at CERN (with P. Crivelli, U. Gendotti, A. Rubbia ETHZ)
porogen content
closed porosity
open porosity
(with J-P Boilot, L. Raboin, Ecole Polytechnique PMC)

24. Positron sources
classical source: 22Na
limit of activity ~ 100 mCi  ~ 106 slow e+/s max
nuclear reactor-based sources
Delft ( ~8x107 slow e+/s Doppler broadening and angular correlation)
Munich (~ 3x108 slow e+/s; lifetime, scanning positron lifetime microscope)
accelerator-based sources
Rossendorf (EPOS at ELBE: 40 MeV 40 kW Linac; under construction)
Tsukuba (lifetime, TOF)
need of intense source
shorter measuring time (with 22Na typicaly 1-2 spectra/day)
e+ microscope, angular correlation, fast processes ...