Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung,

Institut für Ionenstrahlphysik und Materialforschung

Slow positron implantation spectroscopy – a tool to characterize

vacancy-type damage in solids

G. BrauerInstitut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf

Postfach 510119, D-01314 Dresden, Germany

See also:

# G.Brauer W. Anwand, P.G. Coleman, W. Skorupa Slow positron annihilation spectroscopy – a tool to characterize vacancy-type damage in ion-implanted 6H-SiC Vacuum 78 (2005) 131-136

# R.I. Grynszpan, W. Anwand, G. Brauer, P.G. Coleman Positron depth profiling in solid surface layers Annales de Chimie - Science des Materiaux (2007, in press) (18 pp)

Where do positrons come from ?

- (1) radioactive decay


- (2) Bremsstrahlung / pair production

- pair production: Eγ 2 m0c²

n(E)dE

Emean E

22Na

Emean = 225 keV

Emax = 542 keV

β+-decay: p n + e+ + ν


Distribution of positrons from ²²Na in solids (experiment)

ρLi = 0,535 g cm-3

ρSiC = 3,217 g cm-3

In good approximation holds for e+ from ²²Na

zeff = 100 mg cm-2 ρ zmax = 200 mg cm-2

Example: ρFe = 7,841 g cm-3

zeff = 128µm zmax = 255 µm


G. Dlubek, PhD 1975, U Halle


Positron energy vs. Time

Positron trapping by open volume defects

increasing preference

grain boundary edge dislocation monovacancy vacancyagglomerate


TRAPPING MODEL- rate equation approach

(vacancies, dislocations)

- diffusion-limited approach (vacancy agglomerates, shape of the trapping site!)

= µdefect Nd

= 4 D+ Nd

Method Parameter

DB

AC

LT

S , W

H

, mean

sensitivityrange

defectconcentration Nd

0,1 – 200 ppm1012 – 1015 m-2

monovacanciesdislocationsIn metals:

larger sizes seen: 2 – 50 agglomerated mono-vacancies



Principal methods of Positron Annihilation Spectroscopy (PAS)

Definition of the line shape parameters S and W in DB


E = m0c² + 0.5 c x pparallel

pparallel ... electron momentum component parallel to

emission direction of -quantum

S = A1 / A

W= (B1 + B2) / A

low electron momentum parameter, annihilation with valence electrons

high electron momentum parameter,annihilation withcore electrons


Cartoon of a slow positron beam

13,04101 x

E ~ 3eV

W (110), negative workfunction for positrons, ~ 3eV


Cartoon of a positron moderator

„SPONSOR“Slow POsitroN System Of Rossendorf

direction of-fast e+

-γ-rays

Since 1999:

(1,09 ± 0,01) keV FWHM

Positron energy: 30 eV ... 36 keVBeam diameter: ~ 4 mm at all energies



„SPONSOR“


(a) “natural” positrons from 22Na (b) mono-energetic positrons of

energy E as indicated

N(z) is the distribution function obtained by P(E,z) is the distribution function of positrons

integration of P(E,z) over all energies having the energy E(0-542 keV) of positrons from 22Na.

Depth distribution of thermalized positrons in SiC

Goal of Slow Positron Implantation Spectroscopy (SPIS)

depth (nm)


First step: from S(E) to S(d) plot


First step: from S(E) to S(d) plot (mathematical background)

Numerical solution of the positron diffusion equation (one dimensional)

(Software „VEPFIT“: van Veen u.a. in AIP Conf. Proc. 218 (1990) 171)

0),()()()(²

² EzPznzkzn

dz

dD eff

D+ … Diffusion coefficient

keff ... Part of the positrons annihilating in

defectsn(z) ...Positron density at depth z

Makhovian distribution of the implanted positrons

])(exp[),(00

1m

m

m

z

z

z

mzEzP

z0 = zmean/Γ/(1/m+1)zmean = A/ρ*En ... mean penetration

depth of positronsA, m, n ... experimental parameters

62,136Ezmean

zmean : nm

ρ : g cm-3

E : keV


Mean positron depth

Second step: theoretical calculation of positron lifetimes

positron lifetime - specific for bulk and every defect - independent from defect concentration

see e.g.G. Brauer, W. Anwand, P.G. Coleman, A.P. Knights, F. Plazaola, Y. Pacaud, W. Skorupa, J. Störmer,P. Willutzki, Positron studies of defects in ion implantated SiC, Phys. Rev. B54 (1996) 3084-3092



Problems to identify a defect by positron lifetime in a compound semiconductor

Experiment:- already native (grown in) defects can exist on both sublattices- defects may be charged- mostly impossible to create a certain defect on one sublattice only, e.g. by irradiation

Theory:- calculations performed so far for neutral defects only- different approaches to include electron-positron interaction available- adjustment of calculation to reality somehow necessary- possible lattice relaxation around a defect

Theoretical methods in use:(a) ATSUP (atomic superposition method): rigid lattice positions, large defects, gives positron binding energy(b) LMTO (linear muffin tin orbital method): ab initio calculation, small defect configurations only, gives

positron affinity and positron binding energy

Third step: lifetime measurements (pulsed positron beam, Munich)

Result: scaling curve S(N)

see e.g.

W. Anwand, G. Brauer, P.G. Coleman, W. Skorupa, MRS Proc. Vol. 504 (1998) 135-140


Fourth step: defect size depth distribution N(d)

W. Anwand, G. Brauer, P.G. Coleman, R. Yankov,

W. Skorupa, Appl. Surf. Sci. 149 (1999) 140-143

depth d (nm)


W. Anwand, G. Brauer, W. Skorupa,Appl. Surf. Sci. 184 (2001) 247-251

Evolution of ion implantation-caused vacancy-type defects in 6H-SiC

Motivation:- ion beam synthesis of a buried (SiC)1-x(AlN)x layer- layer between 80 nm and 210 nm with x~0.2 to adjust the band gap between 3.0 eV (6H-SiC) and 6.2 eV (2H-AlN)


Experimental details:- {0001}-oriented, n-type 6H-SiC wafer- Fourfold implantation necessary:

Al+ implantation: 100 keV (5.0x1016 cm-2) and 160 keV (1.3x1017 cm-2)

N+ implantation: 65 keV (5.0x1016 cm-2) and 120 keV (1.3x1017 cm-2)- substrate temperature during implantation: 800°C

depth d (nm)

0 100 200 300 400 500

norm

aliz

ed a

tom

s or

vac

anci

es

/ A

ngst

roe

m /

ion

0.2

0.4

0.6

0.8

1.2

0.0

1.0

vacanciesAl Al+ 100 keV

depth d (nm)

0 100 200 300 400 500

norm

aliz

ed a

tom

s or

vac

anci

es /

Ang

stro

em

/ io

n

0.2

0.4

0.6

0.8

1.2

0.0

1.0

vacanciesAl

Al+ 160 keV


Results of TRIM / SRIM calculations

F. Ziegler, J.P. Biersack, The stopping and range of ions in matter, http://www.SRIM.org

http://www.srim.org/



depth d (nm)

0 100 200 300 400 500

norm

aliz

ed a

tom

s or

vac

anci

es /

Ang

stro

em /

ion

0.2

0.4

0.6

0.8

1.2

0.0

1.0

vacanciesN N+ 65 keV

N+ 120 keV

depth d (nm)

0 100 200 300 400 500

norm

aliz

ed a

tom

s or

vac

anci

es /

Ang

stro

em /

ion

0.2

0.4

0.6

0.8

1.2

0.0

1.0

vacanciesN

Results of TRIM / SRIM calculations

F. Ziegler, J.P. Biersack, The stopping and range of ions in matter, http://www.SRIM.org



6H-SiCion-implanted (800°C) + annealed (1.200°C, 10 min)


6H-SiCion-implanted (800°C) + annealed (1.650°C, 10 min)


? Incomplete defect annealing – a surface problem ?


Detection capabilities for various microprobe techniques

(a) defect concentration (b) defect size

(PAS refers to all positron annihilation techniques)

Solved...- more and more sophisticated data collection possible- ambitious theoretical calculations available

Further improvements needed...- increase of available positron beam intensity- depth dependant positron lifetime measurements routinely- combination of PAS results with those from other methods


State – of – the – art in this field was reviewed at SLOPOS – 10Doha / Qatar, March 19 – 25, 2005.

Results see at: Applied Surface Science, Vol. 252 (Feb 2006)

Next meeting: SLOPOS – 11 at Orleans / France, July 9 -13, 2007


Coincidence Doppler Broadening measurements

Principle

CDB – results




EPOS scheme EPOS scheme For a description of the project, see also:

R. Krause-Rehberg, S. Sachert, G. Brauer,A. Rogov, K. NoackAppl. Surf. Sci. 252 (2006) 3106

The End











Detection capabilities for various microprobe techniques

(a) defect concentration (b) defect size

(PAS refers to all positron annihilation techniques)


EPOS scheme EPOS scheme For a description of the project, see also:

R. Krause-Rehberg, S. Sachert, G. Brauer,A. Rogov, K. NoackAppl. Surf. Sci. 252 (2006) 3106

Fourth step: defect size depth distribution N(d)

see e.g.

W. Anwand, G. Brauer, W. Skorupa, Appl. Surf. Sci. 149 (1999) 140-143



(a) “natural” positrons from 22Na (b) mono-energetic positrons of

energy E as indicated

N(z) is the distribution function obtained by P(E,z) is the distribution function of positrons

integration of P(E,z) over all energies having the energy E(0-542 keV) of positrons from 22Na.

Depth distribution of thermalized positrons in SiC

Documents

Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung,