Boron and phosphorus diffusion gettering: Efficiency, mechanisms and applicability to silicon solar cells

Ville Vähänissi

Boron and phosphorus diffusion gettering: Efficiency, mechanisms and applicability to silicon solar cells

Julkaisun otsikon käännös: Boori- ja fosforidiffuusiogetterointi: Tehokkuus, mekanismit ja soveltuvuus piiaurinkokennoihin

Ville Vähänissi

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Erilaiset getterointitekniikat ovat tärkeitä puolijohdekomponenttien valmistuksessa sillä epäpuhtauksien läsnäololta prosesseissa on mahdotonta kokonaan välttyä. Tämän työn tarkoituksena on syventää nykyisille piiaurinkokennoille tärkeimpien getterointitekniikoiden ymmärtämystä teoreettisia ja kokeellisia tutkimusmenetelmiä käyttäen. Yksikiteiselle piimateriaalille tehdyt tutkimukset tarjoavat uutta tietoa sekä booridiffuusio-getteroinnista (BDG) että fosforidiffuusiogetteroinnista (PDG). Fosforin tapauksessa esitettävät tasapainotilan rautatulokset lämpötila-alueelta 650-800°C mahdollistavat raudan segregaatiokertoimen tarkan arvon määrittämisen. Boorilla saavutetaan fosforiin verrattavissa oleva getterointitehokkuus. Tämän korkean tehokkuuden on todettu aiheutuvan B-Si erkaumista, joiden ydintymisen käynnistyminen ja kasvu kuumennuksen aikana vaativat liukoisuusarvon ylittävää boorikonsentraatiota sekä kuumennuksen riittävän pitkää kestoa. Monikiteiselle piimateriaalille tehdyt kokeet osoittavat sekä BDG:n että PDG:n pidentävän huomattavasti vähemmistövarauksenkuljettajien elinaikaa ja merkittävästi pienentävän kiekkojen erityisen huonoa reuna-aluetta. Suuresta metallierkaumien määrästä johtuen, boorin vaatiman korkeamman sisäändiffuusiolämpötilan havaitaan olevan eduksi. BDG:n seurauksena elinajan keskiarvo kiekon reuna-alueilla nousee arvosta 5 us jopa arvoon 270 us. Fosforia ioni-istuttamalla suoritetut getterointitutkimukset paljastavat, että kyseisellä tekniikalla saavutettava tehokkuus on huomattavasti matalampi kuin tavanomaisella diffuusiolla. Toisaalta, jos erkautuminen seostetussa kerroksessa saadaan aktivoitua, voi saavutettava tehokkuus parantua merkittävästi. Erkautumisen käynnistyminen vaatii matalia prosessilämpötiloja ja/tai korkeaa alkuperäistä kontaminaatiotasoa. Lopuksi saavutettujen tulosten avulla optimoituja getterointiprosesseja sovelletaan komponenttitasolla aurinkokennojen valmistuksessa. Hyvinkin korkeasta alkuperäisestä rautakontaminaatiosta (2 × 1014 cm-3) huolimatta kyetään saavuttamaan yhtä hyviä hyötysuhteita kuin puhtailla aurinkokennoilla. Tulokset osoittavat selvästi prosessiparametrien suunnittelun merkityksen: parhaan tuloksen saavuttamiseksi tulisi korkean puhtauden piin getterointiparametrien poiketa huomattavasti korkean epäpuhtaustason materiaalin vastaavista.

Julkaisun otsikon käännös	Boori- ja fosforidiffuusiogetterointi: Tehokkuus, mekanismit ja soveltuvuus piiaurinkokennoihin
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Savin, Hele, Vastuuprofessori
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-6916-6
Sähköinen ISBN	978-952-60-6917-3
Tila	Julkaistu - 2016
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

pii
getterointi
boori
fosfori
rauta
aurinkokenno

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-6917-3Lisenssi: Määrittelemätön

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@phdthesis{84ed694d0f10435a86527c28ca2fd9ce,

title = "Boron and phosphorus diffusion gettering: Efficiency, mechanisms and applicability to silicon solar cells",

abstract = "As it is impossible to completely prevent the presence of impurities in semiconductor device processing, various defect engineering strategies are needed. The purpose of this thesis is to increase the level of understanding of different impurity gettering techniques in silicon, especially from solar cell point-of-view. Experiments made with single crystalline silicon provide new data for both boron diffusion gettering (BDG) and phosphorus diffusion gettering (PDG). In the case of PDG, the presented steady state data at the temperature range of 650-800°C enables the determination of an accurate value for segregation coefficient of iron. For BDG, gettering efficiencies even comparable to PDG are achieved. In this case, the prevailing mechanism is found to be B-Si precipitate gettering, which requires that the boron concentration at the emitter exceeds its solubility at the annealing temperature for a sufficiently long time for nucleation and growth of B-Si precipitates. Studies made on multicrystalline silicon (mc-Si) show that both BDG and PDG lead to a significant increase in minority carrier lifetime and a substantial reduction of the red zone area. Due to the high amount of precipitated metals, the higher in-diffusion temperature of boron is found to be beneficial in mc-Si. After BDG, the average lifetime in the red zone is increased from 5 us up to 270 us. The phosphorus implantation gettering experiments reveal that the obtained gettering efficiency for implanted emitters is significantly lower in comparison to conventional PDG. However, if precipitation at the emitter is activated, considerable improvement in the gettering efficiency is achievable. This requires low gettering temperatures and/or high initial contamination level. Finally, gettering processes optimized based on the material parameter results are applied on device level in solar cell processing. With initial bulk iron concentration as high as 2 × 1014 cm-3, conversion efficiencies comparable with non-contaminated cells are obtained. The results clearly demonstrate the importance of careful process parameter design: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as for a material with high impurity content.",

keywords = "silicon, gettering, boron, phosphorus, iron, solar cell, pii, getterointi, boori, fosfori, rauta, aurinkokenno, silicon, gettering, boron, phosphorus, iron, solar cell",

author = "Ville V{\"a}h{\"a}nissi",

year = "2016",

language = "English",

isbn = "978-952-60-6916-6",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "138",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Boron and phosphorus diffusion gettering: Efficiency, mechanisms and applicability to silicon solar cells

AU - Vähänissi, Ville

PY - 2016

Y1 - 2016

N2 - As it is impossible to completely prevent the presence of impurities in semiconductor device processing, various defect engineering strategies are needed. The purpose of this thesis is to increase the level of understanding of different impurity gettering techniques in silicon, especially from solar cell point-of-view. Experiments made with single crystalline silicon provide new data for both boron diffusion gettering (BDG) and phosphorus diffusion gettering (PDG). In the case of PDG, the presented steady state data at the temperature range of 650-800°C enables the determination of an accurate value for segregation coefficient of iron. For BDG, gettering efficiencies even comparable to PDG are achieved. In this case, the prevailing mechanism is found to be B-Si precipitate gettering, which requires that the boron concentration at the emitter exceeds its solubility at the annealing temperature for a sufficiently long time for nucleation and growth of B-Si precipitates. Studies made on multicrystalline silicon (mc-Si) show that both BDG and PDG lead to a significant increase in minority carrier lifetime and a substantial reduction of the red zone area. Due to the high amount of precipitated metals, the higher in-diffusion temperature of boron is found to be beneficial in mc-Si. After BDG, the average lifetime in the red zone is increased from 5 us up to 270 us. The phosphorus implantation gettering experiments reveal that the obtained gettering efficiency for implanted emitters is significantly lower in comparison to conventional PDG. However, if precipitation at the emitter is activated, considerable improvement in the gettering efficiency is achievable. This requires low gettering temperatures and/or high initial contamination level. Finally, gettering processes optimized based on the material parameter results are applied on device level in solar cell processing. With initial bulk iron concentration as high as 2 × 1014 cm-3, conversion efficiencies comparable with non-contaminated cells are obtained. The results clearly demonstrate the importance of careful process parameter design: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as for a material with high impurity content.

AB - As it is impossible to completely prevent the presence of impurities in semiconductor device processing, various defect engineering strategies are needed. The purpose of this thesis is to increase the level of understanding of different impurity gettering techniques in silicon, especially from solar cell point-of-view. Experiments made with single crystalline silicon provide new data for both boron diffusion gettering (BDG) and phosphorus diffusion gettering (PDG). In the case of PDG, the presented steady state data at the temperature range of 650-800°C enables the determination of an accurate value for segregation coefficient of iron. For BDG, gettering efficiencies even comparable to PDG are achieved. In this case, the prevailing mechanism is found to be B-Si precipitate gettering, which requires that the boron concentration at the emitter exceeds its solubility at the annealing temperature for a sufficiently long time for nucleation and growth of B-Si precipitates. Studies made on multicrystalline silicon (mc-Si) show that both BDG and PDG lead to a significant increase in minority carrier lifetime and a substantial reduction of the red zone area. Due to the high amount of precipitated metals, the higher in-diffusion temperature of boron is found to be beneficial in mc-Si. After BDG, the average lifetime in the red zone is increased from 5 us up to 270 us. The phosphorus implantation gettering experiments reveal that the obtained gettering efficiency for implanted emitters is significantly lower in comparison to conventional PDG. However, if precipitation at the emitter is activated, considerable improvement in the gettering efficiency is achievable. This requires low gettering temperatures and/or high initial contamination level. Finally, gettering processes optimized based on the material parameter results are applied on device level in solar cell processing. With initial bulk iron concentration as high as 2 × 1014 cm-3, conversion efficiencies comparable with non-contaminated cells are obtained. The results clearly demonstrate the importance of careful process parameter design: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as for a material with high impurity content.

KW - silicon

KW - gettering

KW - boron

KW - phosphorus

KW - iron

KW - solar cell

KW - pii

KW - getterointi

KW - boori

KW - fosfori

KW - rauta

KW - aurinkokenno

KW - silicon

KW - gettering

KW - boron

KW - phosphorus

KW - iron

KW - solar cell

M3 - Doctoral Thesis

SN - 978-952-60-6916-6

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Boron and phosphorus diffusion gettering: Efficiency, mechanisms and applicability to silicon solar cells

Abstrakti

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