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

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

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...)

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 (0...511 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-

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~100..300 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): 10-142 ns

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!

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)

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

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

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

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

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 100-300 ps (bulk solid, vacancies) 1-2 ns (large voids, positronium) more information, more sensitivity than in the case of Doppler sp.

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

Schema: pulsed positron beam (Garching, near Munich) ~ 10-100 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

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)

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, 137402 (2003)

defect spectroscopy with positrons sensitive to defects with free volume (vacancy, vacancy complex, voids) sensitivity up to 10-6-10-7 (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

Positrons in semiconductors: charged defects

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

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

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 (300-500 nm thickness) removal of porogen by heating in air at 400 °C pure SiO2 structure (amorphous walls) micropores mesopores

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)

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)

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 ...