Improvement of depth resolution of VEPAS by a sputtering...

Improvement of depth resolution of the positron beam spectroscopy by a sputtering technique

MARTIN‐LUTHER‐UNIVERSITY HALLE‐WITTENBERGINSTITUTE OF PHYSICS

Improvement of depth resolution of VEPAS by a sputtering technique R. Krause‐Rehberg, M. John, R. Böttger, W. Anwand and A. Wagner

Martin‐Luther‐University Halle & HZDR Dresden

Martin‐Luther‐University Halle


MARTIN‐LUTHER‐UNIVERSITY HALLE‐WITTENBERGINSTITUTE OF PHYSICS 2

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• implantation profile often approximated as Makhov profile

• extreme broad distribution of implanted positrons at E > 3 keV

Problem: Implantation Profile becomes very broad at higher Ee+



Test sample a‐Si/SiO2/Si Profile

a-Si SiO2 Si(100)485nm 600 nm bulk

• depth resolution mostly not restricted by positron diffusion

• mainly due to positron implantation profile of monoenergetic positrons

• in practice: defect distribution often approximated as block profile

• not possible to extract the actual defect profile

• problem: defects at interfaces or in layers can easily be overlooked

480 nm a-Si

600 nm SiO2bulk Si(100)

layer structure



Depth resolution improvement: etching of samples



• after self‐implantation of Si in Si @3.5 MeV – two defect‐rich layers are found

• at Rp: Si self‐interstitials form dislocation loops

• at Rp/2 (1.7 µm): ???

• the defect layers are in a depth of 1,7 µm and 2,8 µm corresponding to E+ = 18 keV and 25 keV

• implantation profile too broad to discriminate between the two zones

• simulation of S(E) curve gives the same result for assumed defect profile

R. Krause‐Rehberg, F. Börner, and F. Redmann; Applied Physics Letters 77, 3932 (2000)

Example for limitation of depth profiling: Rp/2 effect in Si self‐implantation



• After stepwise removal of the surface the real defect profile becomes visible

• First peak has different defect character (S‐W‐plot)

• Defect peaks correspond with gettered Cu SIMS profile

R. Krause‐Rehberg, F. Börner, and F. Redmann; Applied Physics Letters 77, 3932 (2000)

Example for limitation of depth profiling: Rp/2 effect in Si self‐implantation



Ar+• Ar+ Ions penetrate into surface

• create displacement cascades

• some cascades reach surface

• surface atoms are released

•Defect layer in nm range

• Diffusing defects??

Surface removal by sputtering



Simulated defect profiles created during sputtering

• in the moment: 1 keV @ 42° defect depth ≈ 25 nm in Si

• only change of surface S parameter

Ar+ beam:• 1 keV• α = 42°



0 10 20 300.96

0.98

1.00

1.02

1.04

1.06 n-Si as-received n-Si sputter step 1 n-Si sputter step 2 simulation

positron energy (keV)

S/S bu

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• sputter conditions: 2 keV @ 60°

• no change of positron diffusion length due to sputtering

• depth resolution is limited by e+diffusion, not by implantation profile or diffusing defects

L+=221 13 nm

L+=213 18 nm

Sputter process changes surface parameter only



Sputter station at POSSY

sample chamber

sputter gun (≈ $25,000)

differential pumping station

e+

Ge detector

Ar gasinlet

plate valve

two apertures inside

10‐3 mbar

10‐6 mbar



Sputtern during positron experiment?

• sputter thinning of near‐surface layer during sputtering ‐> advantage of automated measurement

• possible problems: charging of non‐conductive sample? Magnetic field? Vacuum?

e+ beam

Ar+ ions up to 10 mA

H field = 100 G

electron beam for charge compensation

• Charging: at 5 mA and sample capacitance ≈1 pF

• in 1h: U = Q/C = 2x1013 V ‐> spark discharge

• sample discharge with electron beam will not work

• no chance to measure non‐conducting samples

• sputtering and measurement must be done in sequence

• effect of magnetic field of 100Go ion Ar+ radius: about 2.8 m

o electron radius: about 1 cm

• Vacuum in beam system must be < 10‐5 mbar

• Ar pressure in sputter gun must be ≈ 10‐3 mbar Ar

• Differential pumping station and apertures necessary

sample



Test sample: a‐Si/SiO2/Si

a-Si SiO2 Si(100)485nm 600 nm bulk

• depth resolution mostly not restricted by positron diffusion

• mainly due to positron implantation profile of monoenergetic positrons

• in practice: defect distribution often approximated as block profile

• not possible to extract the actual defect profile

• problem: defects at interfaces or d layers can easily be overlooked

480 nm a-Si

600 nm SiO2bulk Si(100)

layer structure



SiO2 (600 nm)a-Si (485nm) Si bulk

surface oxid

0,0 0,2 0,4 0,6 0,8 1,0 1,20,44

0,45

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sputter depth (µm)

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0,020

0,025

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0,035

0,040

0,045

sample 1: closed symbolessample 2: open symbols

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Test sample



Another layered test sample: Au/Cr/Au/Cr/Si

• with S(E) no defects measurable

• better depth resolution then S(E)‐scan

• Interfaces sharply visible

Prove of principle with layer system Au/Cr/Au/Cr/Si

Sputter parameters: IB = 4 mA; UB = 400 V



Cu(In,Ga)Se2 ‐ (CIGSe)

• direct semiconductor

• Eg =1,04 eV – 1,67 eV

• used in thin film solar cells

Experiments at CIGS layers



• CIGS = Cu(In,Ga)Se2 photovoltaic layer

• no vacancies beyond of 10 nm

• e+ energy so that:‐ no influence of surface‐ still sharp e+ implantation profile

• eventually several positrons energies at one sputter depth …

Simulation of vacancy profile by Ar bombardment in CIGS

SRIM‐Simulated defect profile for CIGS

Sputtering conditions:EAr+ = 1keVIncident angle: 42°



• Investigation of the whole solar cell, especially near the back contact

• In‐situ sputtering‐> no air contact between the measurements‐> no degradation on the surface‐> shorter overall measurement time

Results

Sputter parameters: IB = 6 mA; UB = 400 V



Photovoltaic CIGS layers – Se variation series

• a CIGS series

• variation of Se content

• surprising: although three‐stage co‐evaporation process –no defect profile visible

very sharp decrease

• positron diffusion length is very short

• must have a large number of trapping centers: defects



Photovoltaic CIGS layers – Se variation series

• samples of a CIGS series of different thickness


• surprising: although three‐stage co‐evaporation process – no defect profile visible



Photovoltaic CIGS layers – Ga variation series

• samples of a CIGS series of different thickness


• surprising: although three‐stage co‐evaporation process – no defect profile visible



Idea: Measurement of absolute Vacancy concentration in an implantation profile

• He implantation in Si

• SRIM simulations give absolute vacancy concentrations

• however model rather simple:

amorphous solid

no defect annealing or diffusion

no defect interaction

• Plan: first direct measurement of defect profile

• implantation of He+ in dose range 1010 … 1016 cm‐2

SRIM



SRIM: Stopping and Range of Ions in Matter (initial release 1983)

J. F. Ziegler (2004). "SRIM-2003". Nucl. Instr. Meth. B. 219-220: 1027www.srim.org



Simulated vacancy concentration from SRIM results

• SRIM simulation for our implantation dose range

• Simulated vacancy concentration is too large

• Cvac > 1020 not possible



Conventional S(E) Plot

200‐ and 500 keV He+ implantation dose: 1010 … 1016 cm‐2

1010

1012

1013

1015 cm‐2

1014



Simulated vacancy concentration from SRIM results

1015 cm‐3

5x1018 cm‐3

• vacancy concentration is too large

• large discrepancy to positron results

• by sputter profiles: concentration should be measurable

According to positron sensitivity range for mono‐ or di‐vacancies



Sputter profiles not yet measured

• This slide should have shown the sputter profiles of the implanted Si samples

• However, the sputter gun needs repair

• We need spare parts from US which will arrive end of May

• I will add the profiles ‐> look later to http://positron.physic.uni‐halle.de

• We will measure also the lifetime at MePS to see the number of vacancies/defect



• distinctly better depth resolution is possible by sputtering

‐> real defect profiling

• interfaces become sharply visible

• depth resolution is no more limited by positron implantation profile but only by effective positron diffusion length (fundamental barrier)

• disadvantages:

o not anymore non‐destructive

o depth scan over 4 µm last about 40 h (100nm/h)

Conclusions

We acknowledge the support of the BMBF under project number 05K13NHA.

Improvement of depth resolution of VEPAS by a sputtering...

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