PASSIVATION OF A METAL CONTACT WITH A TUNNEL LAYER

Transcription

1 PASSIVATION OF A METAL CONTACT WITH A TUNNEL LAYER X. LOOZEN, J. B. LARSEN, F. DROSS, M. ALEMAN, T. BEARDA, B. J. O SULLIVAN, I. GORDON AND J. POORTMANS

2 Literature overview Highest efficiency cell on Si in 1984 with front contacts passivated by thin SiO 2 (full Al-BSF cell, 18.4%) M.A. Green et al, IEEE TED (1984) Tunnel SiO 2 under front Al contacts on a n-type emitter (PERC cell) ISFH K. Jaeger-Hezel et al, EU PVSEC (1995) J. Schmidt et al, Prog. Photovolt.: Res. Appl. (2008) V oc gain of ~35mV by use of passivated contacts on IBC cells (best cell 721mV, 24.2%) - High potential for contact passivation P.J. Cousins et al, IEEE PVSC (2010) - Need for characterization V oc gain of 12mV by using an ALD Al 2 O 3 tunnel layer (best cell 673mV, 21.7%) D. Zielke et al, Phys. Status Solidi RRL (2011) CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 2

3 Outline Literature overview AlO x candidate for contact passivation? AlO x contact passivation seen by photoluminescence Contact resistance Conclusion CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 3

4 Outline Literature overview AlO x candidate for contact passivation? AlO x contact passivation seen by photoluminescence Contact resistance Conclusion CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 4

5 Use of AlO x for surface passivation Lifetime and S eff on bulk p- and n-si passivated by 10nm AlO x + FGA 350ºC 2 Ωcm p-si 2.4 Ωcm n-si 2 Ωcm p-si 2.4 Ωcm n-si Effective passivation of n- & p-si by thick AlO x CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 5

6 Can we use AlO x for contact passivation? Target: Reduce recombination underneath metal contacts via a tunneling layer Question: Does an ultra-thin (~nm) AlO x layer provide good surface passivation? 1) On base material? 2) On highly doped n- and p-type regions? - Emitter - FSF/BSF CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 6

7 Can we use AlO x for contact passivation? Target: Reduce recombination underneath metal contacts via a tunneling layer Question: Does an ultra-thin (~nm) AlO x layer provide good surface passivation? 1) On base material? 2) On highly doped n- and p-type regions? - Emitter - FSF/BSF CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 7

8 Passivation of base material by thin AlO x Lifetime versus AlO x thickness after FGA 350ºC 2nm Effective Very low passivation S eff already with 2nm of thermal n- & ALD p-si AlOwith x - 8 cm/s on n-si (polished FZ) # ALD cycles ~ # Å of AlO x very thin AlO x - 20 cm/s on p-si (polished FZ) CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 8

9 Can we use AlO x for contact passivation? Target: Reduce recombination underneath metal contacts via a tunneling layer Question: Does an ultra-thin (~nm) AlO x layer provide good surface passivation? 1) On base material? 2) On highly doped n- and p-type regions? - Emitter - FSF/BSF CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 9

10 AlO x passivation on n + diffused surfaces Lifetime of AlO x passivated n + surfaces versus R sheet after FGA 350ºC Symmetrical deposition of thick 30nm AlO x on POCl 3 diffused surfaces eff limited to ~100µs for 100 Ω/ doped n + surfaces As diffused with AlO x passivation. Why? After oxidation at 1000ºC CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 10

11 Why poorer passivation with higher doping? AlO x with negative charges n n Base Si Base Si Base Si Base n-si: strong induced inversion Medium n + doping: weak induced inversion High n ++ doping: very weak or no induced inversion Floating junction passivation works well for base n-si, but not for highly doped n+ surfaces CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 11

12 Outline Literature overview AlO x candidate for contact passivation? AlO x contact passivation seen by photoluminescence Contact resistance Conclusion CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 12

13 AlOx passivation seen by photoluminescence Target: passivation of front contact in i-perc cells For PL measurement - Suppress rear metallization - Use tunnel candidate on front - Compare passivated/non-passivated areas SiO x /SiN x passivation stack Al BSF PL illumination SiO x /SiN x passivation stack CZ p-si 100 Ω/ n + emitter CZ p-si n + SiN x ARC SiN x ARC CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 13

14 AlOx passivation seen by photoluminescence Target: passivation of front contact in i-perc cells For PL measurement - Suppress rear metallization - Use tunnel candidate on front - Compare passivated/non-passivated areas SiO x /SiN x passivation stack Al BSF PL illumination SiO x /SiN x passivation stack CZ p-si 100 Ω/ n + emitter CZ p-si n + SiN x ARC Tunnel AlO x CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 14

15 AlOx passivation seen by photoluminescence Target: passivation of front contact in i-perc cells For PL measurement - Suppress rear metallization - Use tunnel candidate on front - Compare passivated/non-passivated areas SiO x /SiN x passivation stack Al BSF PL illumination SiO x /SiN x passivation stack CZ p-si 100 Ω/ n + emitter CZ p-si n + SiN x ARC Tunnel AlO x CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 15

16 AlOx passivation seen by photoluminescence Comparison passivated/nonpassivated areas Area for QSSPC calibration Reflection on metal no direct comparison SiO x /SiN x passivation stack Al BSF Ti/Pd/Ag Tunnel AlO x + Ti/Pd/Ag No coating (native oxide) Tunnel AlO x SiO x /SiN x passivation stack CZ p-si 100 Ω/ n + emitter CZ p-si n + SiN x ARC Tunnel AlO x CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 16

17 Lifetime (µs) AlO x passivation seen by photoluminescence Ti/Pd/Ag No coating (native oxide) 6.4µs 23µs nm Al 2 O 3 + Ti/Pd/Ag 1.5nm Al 2 O 3 CZ 180µm p-si, with 100Ω / POCl 3 emitter Low AlO x reference lifetime - Limited AlO x passivation on n + Si 17.4µs 25µs 10 Dn= cm -3 on non-metalized area and lower on metalized areas Significant increase of lifetime under metal with 15 AlO x ALD cycles (~1.5nm) 5 0 CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 17

18 Lifetime versus AlO x thickness Average lifetime on the Si areas covered by AlO x and/or metal 1.5-2nm Ultra-thin AlO x under contact improves passivation CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 18

19 Outline Literature overview AlO x candidate for contact passivation? AlO x contact passivation seen by photoluminescence Contact resistance Conclusion CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 19

20 Influence of thickness on contact resistance 500 µm Ti/Pd/Ag dots AlO x V n+, 120 Ω/ p-si Flat emitter Departure from ohmic contact from 20 AlO x ALD cycles (~2 nm) Textured emitter Large current and ohmic contact through all AlO x layers, even through 20nm AlO x!!! CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 20

21 Conclusion Reduced recombination under Ti/Pd/Ag contacts with AlO x tunnel layers is observed with PL Ohmic behavior observed on flat emitters with up to 1.5 nm AlO x (= optimal thickness for passivation) Contact resistance on textured emitters not clear, but no show-stopper indication CONTACT PASSIVATION 3 RD METALLIZATION WORKSHOP, CHARLEROI 2011 X. LOOZEN 21

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