Martin Luther University Halle 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 INSTITUTE OF PHYSICS
Problem: Implantation Profile becomes very broad at higher E e+ implantation profile often approximated as Makhov profile extreme broad distribution of implanted positrons at E > 3 kev P( z, E ) mz z m 1 m 0 z z 0 m INSTITUTE OF PHYSICS 2
Test sample a Si/SiO 2 /Si Profile a-si SiO 2 Si(100) 485nm 600 nm bulk layer structure 480 nm a-si 600 nm SiO 2 bulk Si(100) 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 INSTITUTE OF PHYSICS 3
Depth resolution improvement: etching of samples INSTITUTE OF PHYSICS 4
Example for limitation of depth profiling: Rp/2 effect in Si self implantation after self implantation of Si in Si @3.5 MeV two defectrich 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) INSTITUTE OF PHYSICS 5
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) INSTITUTE OF PHYSICS 6
Surface removal by sputtering Ar + Ar + Ions penetrate into surface create displacement cascades some cascades reach surface surface atoms are released Defect layer in nm range Diffusing defects?? INSTITUTE OF PHYSICS 7
Simulated defect profiles created during sputtering Ar+ beam: 1 kev α = 42 in the moment: 1 kev @ 42 defect depth 25 nm in Si only change of surface S parameter INSTITUTE OF PHYSICS 8
Sputter process changes surface parameter only S/S bulk 1.06 n-si as-received L + =221 13 nm n-si sputter step 1 1.04 n-si sputter step 2 simulation 1.02 1.00 0.98 0.96 L + =213 18 nm 0 10 20 30 positron energy (kev) 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 INSTITUTE OF PHYSICS 9
Sputter station at POSSY Ge detector two apertures inside sputter gun ( $25,000) 10 3 mbar e+ 10 6 mbar Ar gas inlet sample chamber plate valve differential pumping station INSTITUTE OF PHYSICS 10
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? Charging: at 5 ma and sample capacitance 1 pf H field = 100 G e + beam in 1h: U = Q/C = 2x10 13 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 sample Ar + ions up to 10 ma electron beam for charge compensation effect of magnetic field of 100G o 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 INSTITUTE OF PHYSICS 11
Test sample: a Si/SiO 2 /Si a-si SiO 2 Si(100) 485nm 600 nm bulk layer structure 480 nm a-si 600 nm SiO 2 bulk Si(100) 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 INSTITUTE OF PHYSICS 12
Test sample 0,55 0,54 surface oxid a-si (485nm) SiO 2 (600 nm) Si bulk 0,045 0,53 0,52 0,040 surface S parameter 0,51 0,50 0,49 0,48 0,47 0,46 sample 1: closed symboles sample 2: open symbols 0,035 0,030 0,025 surface W parameter 0,45 0,44 0,0 0,2 0,4 0,6 0,8 1,0 1,2 sputter depth (µm) 0,020 INSTITUTE OF PHYSICS 13
Another layered test sample: Au/Cr/Au/Cr/Si Prove of principle with layer system Au/Cr/Au/Cr/Si with S(E) no defects measurable better depth resolution then S(E) scan Interfaces sharply visible Sputter parameters: I B = 4 ma; U B = 400 V INSTITUTE OF PHYSICS 14
Experiments at CIGS layers Cu(In,Ga)Se 2 (CIGSe) direct semiconductor E g =1,04 ev 1,67 ev used in thin film solar cells INSTITUTE OF PHYSICS 15
Simulation of vacancy profile by Ar bombardment in CIGS SRIM Simulated defect profile for CIGS Sputtering conditions: E Ar+ = 1keV Incident angle: 42 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 INSTITUTE OF PHYSICS 16
Results 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 Sputter parameters: I B = 6 ma; U B = 400 V INSTITUTE OF PHYSICS 17
Photovoltaic CIGS layers Se variation series very sharp decrease a CIGS series variation of Se content surprising: although threestage co evaporation process no defect profile visible positron diffusion length is very short must have a large number of trapping centers: defects INSTITUTE OF PHYSICS 18
Photovoltaic CIGS layers Se variation series samples of a CIGS series of different thickness variation of Se content surprising: although three stage co evaporation process no defect profile visible INSTITUTE OF PHYSICS 19
Photovoltaic CIGS layers Ga variation series samples of a CIGS series of different thickness variation of Se content surprising: although three stage co evaporation process no defect profile visible INSTITUTE OF PHYSICS 20
Idea: Measurement of absolute Vacancy concentration in an implantation profile SRIM 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 10 10 10 16 cm 2 INSTITUTE OF PHYSICS 21
SRIM: Stopping and Range of Ions in Matter (initial release 1983) J. F. Ziegler (2004). "SRIM-2003". Nucl. Instr. Meth. B. 219-220: 1027 www.srim.org INSTITUTE OF PHYSICS 22
Simulated vacancy concentration from SRIM results SRIM simulation for our implantation dose range Simulated vacancy concentration is too large C vac > 10 20 not possible INSTITUTE OF PHYSICS 23
Conventional S(E) Plot 200 and 500 kev He + implantation dose: 10 10 10 16 cm 2 10 15 cm 2 10 14 10 13 10 12 10 10 INSTITUTE OF PHYSICS 24
Simulated vacancy concentration from SRIM results 5x10 18 cm 3 10 15 cm 3 According to positron sensitivity range for mono or divacancies vacancy concentration is too large large discrepancy to positron results by sputter profiles: concentration should be measurable INSTITUTE OF PHYSICS 25
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 INSTITUTE OF PHYSICS 26
Conclusions 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) We acknowledge the support of the BMBF under project number 05K13NHA. INSTITUTE OF PHYSICS 27