Slide 1 DERF June 2008 APCVD TiO 2 vs PECVD SiN x : The Battle for Antireflection Supremacy (Findings from a 3-yr ARC Linkage Project between ANU, SierraTherm & SunPower) 06-June-2008

Slide 2 Project details Contributions over three years (AUD): ContributorCashIn kind AgreedActualAgreedActual ARC$340,000$385,000 – – SierraTherm$120,000$120,000$492,000$270,000 SunPower – –$339,000$138,000 Direct Personnel:  Research Fellow: Keith/Bryce ( )  MPhil student: Luke ( )  Part-time contributions from - Mihail Ionescu (ANSTO) - Jeff Simpson, Bob Spekkart (SierraTherm) - Dick Swanson (SunPower) Indirect Personnel:  2 x PhD students: Andy ( ), Bijaya ( )  3 x honours students: Amarjit (2006), Simeon (2008), Lachlan (2008).  3 x foreign students: Efthimios (2005), Joan (2007), Maria (2008)  2 x summer students: Yunki (2006), Teng ( )

Slide 3 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x

Slide 4 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine

Slide 5 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher

Slide 6 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher 3. Passivation of bare Si TerribleGood

Slide 7 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher 3. Passivation of bare Si TerribleGood 4. Passivation of SiO 2 -Si GoodExcellent

Slide 8 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher 3. Passivation of bare Si TerribleGood 4. Passivation of SiO 2 -Si GoodExcellent 5. Static chargeUnknownModerate Positive

Slide 9 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher 3. Passivation of bare Si TerribleGood 4. Passivation of SiO 2 -Si GoodExcellent 5. Static chargeUnknownModerate Positive 6. Hydrogen content Little, if anyHigh

Slide 10 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher 3. Passivation of bare Si TerribleGood 4. Passivation of SiO 2 -Si GoodExcellent 5. Static chargeUnknownModerate Positive 6. Hydrogen content Little, if anyHigh 7. Barrier to water vapour NoYes

Slide 11 Comparison at beginning PropertyAPCVD TiO 2 PECVD SiN x 1. Anti-reflectionGoodFine 2. CostLow~3 x higher 3. Passivation of bare Si TerribleGood 4. Passivation of SiO 2 -Si GoodExcellent 5. Static chargeUnknownModerate Positive 6. Hydrogen content Little, if anyHigh 7. Barrier to water vapour NoYes 8. Reaction with EVA PossiblyNo

Slide 12 Hope vs time

Slide 13 Project goal Address properties 4, 5, 6 & 7 to: Attain equivalent surface recombination (J 0E ) with TiO 2 /SiO 2 /Si compared to SiN x /SiO 2 /Si for textured silicon after accelerated testing (damp-heat, UV). TiO 2 or SiN x SiO 2 n+ diffused silicon n-type silicon

Slide 14 Property 4: Passivation of SiO 2 -Si Efficiency of rear-contact cells strongly affected by J 0E SiN x /SiO 2 TiO 2 /SiO 2

Slide 15 Causes for lower J 0E (Andy) Not contamination  Phew! Not moisture in reactants  Good. Loss of hydrogen during deposition at 400 oC  Reversible with FGA  Does not occur at lower 250 oC Stress on texture (?)  Hmmm No atomic H with APCVD, unlike PECVD  Probably not a big deal – attain similar D it Static charge  Need to add some!

Slide 16 Property 5: Static Charge TiO 2 /SiO 2 SiN x /SiO 2 5 ohm-cm n-type All donor defects n = 10 15

Slide 17 Add charge by corona? Adding charge by corona discharge has problems: our corona damages Si-SiO 2 interface - will add gate (Tom/Klaus) charge is slowly removed by water vapour in atmosphere - add moisture barrier (Simeon/Maria), - anneal TiO 2 (Simeon/Maria)

Slide 18 Property 6: Hydrogen Measurement  ERDA (Andy) - Almost underway...  SIMS  RTA & PC (Teng) - increase/decrease in recomb as H evolves As deposited TiO 2 is permeable to H 2 and H +  H 2 lost during deposition at 400 oC  post-deposition FGA replaces H 2  FGA within N2 curtains replaces H 2. Annealed TiO 2 appears impermeable to H 2 (Maria) H not bonded in significant concentration to TiO 2 when FGA in N 2 curtains.  Will test FGA in separater N 2  H known to alter crystal state but not necessarily bonded to Ti and O.

Slide 19 Property 7: Moisture barrier Occurs on * planar and texture (Thimios) * annealed, alnealed, RTA (Thimios) * n-type & p-type (Luke) * P & B diffusions (Joan) Does it matter? Yes! Damp heat test degrade Si-SiO 2.

Slide 20 Property 7: Moisture barrier Does APCVD TiO2 prevent it degradation No! (Maria, Luke)

Slide 21 Passivation of SiO 2 -Si after damp heat Efficiency of rear-contact cells strongly affected by J 0E SiN x /SiO 2 TiO 2 /SiO 2 after damp-heat

Slide 22 DH degradation: cause / solution Cause is not  removal or deposition of surface charge (Joan)  removal of H at interface Increase in J 0E corresponds to increase in D it (Luke). Possibly due to  wet oxidation at 85 oC  silicic acid in SiO 2 creating stress at Si-SiO2 interface Solution requires moisture barrier  HMDS: no success so far  Annealed TiO 2 : anneal removes H  Module encapsulation  (PECVD Si x N y )

Slide 23 Conclusion – the best hope... Si x N y /SiO 2 TiO 2 /SiO 2 2) Embed high Q without damage (gated corona?) 3) Engineer moisture barrier to prevent Q loss & DH deg. 1) Deposit hydrogenated TiO 2 3a) Anneal TiO 2 3b) Add barrier 3c) Vapour-tight module

Slide 24 What affects J 0E Charge in dielectrics, Q. Density of interface states, D it (E), of donors and acceptors. Capture cross sections,  p (E) &  p (E). Surface doping N s or bulk doping N b Illumination n (at moderate and high injection) Q TiO2 Q SiO2 CB VB FL nn pp