Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

Transcription

1 Revue des Eeries Reouvelables Vol. 8 (05) 8 Adequae mehod o sudy he surface assivaio effeciveess i HEM mulicrisalli silico wafers D. Bouhafs,. Khelifai, A. Boucheham ad B. Palahouae Divisio Déveloeme des Disosiifs de Coversio à Semi-coduceurs Cere de Recherche e Techoloie des Semi-coduceurs our l Eeréique, CRTSE, B d Fraz Fao, 7-Merveilles, B.P. 40, 6038, Aliers, Aleria (reçu le 7 Février 04 acceé le 0 Javier 05) Absrac - I his work we have examied he effeciveess of surface assivaio o ascu mulicrysallie silico (mc-si) wafers usi differe echiques. The sudy is based o mioriy carrier lifeime measuremes wih quasi seady sae hoo-coducace, QSSPC. Effecive mioriy carrier lifeime (τeff) measured values of.4, 8.9, 4.9 ad 3. µsec are obaied resecively wih four silico surface assivaio echiques: - Shallow hoshorous diffusio emier ( + ), - Iodie-Ehaol (I-E), 3-Hydrofluoric acid (HF) emersio ad 4- Six layer deosiio. These resuls sues ha he shallow + emier ives he eff close o he bulk lifeime (τb) due o he beer surface assivaio qualiy. Simulaios made wih Horbeck-Hayes model idicae ha he τeff imroveme ca be correlaed wih he decrease of he surface recombiaio velociy (SRV) ad he icreme of bulk lifeime. Résumé Das ce ravail, ous avos examié l efficacié de la assivaio des surfaces des laquees de silicium oly crisalli coué (mc-si) avec différees echiques. Cee éude es basée sur des mesures de la durée de vie des oreurs mioriaires das u éa quasi saique de hoo-coducace, QSSPC. La mesure de la durée de vie effecive des oreurs mioriaires (τeff) a doé lieu aux résulas suivas:.4, 8.9, 4.9 ad 3. µsec qui o éé reseciveme obeus avec quare echiques de assivaio des surfaces de silicium: - Emeeur eu rofod de diffusio de hosore ( + ), - Iode -Ehaol (I-E), 3- Emersio de l acide fluorhydrique (HF) e 4- Six Disosiio des couches. Ces résulas suère que l émeeur eu rofod + doe le τeff roche de l alloeme de la durée de vie e rofodeur. Keywords: Lifeime measureme - Mulicrysallie silico - Surface assivaio - Mioriy carrier.. ITRODUCTIO P-ye mulicrysallie silico (mc-si) wafers used i he hoovolaic (PV) idusry rereses 60 % of modules roduced aually []. Duri he rocess fabricaio of solar cells, he isecio of elecrical chare carrier lifeime i he bulk of he mc-si wafer is bes ool for moiori duri wafer rocessi o redic ad evaluae he imac of each echoloical se o his imora hysical arameer duri he fabricaio of he solar cell. Differe echiques are used o measure effecive lifeime by meas of a averaed value over a area or a mai of he wafer surface i he rasie [, 3] or he seady sae SS ad quasi seady sae QSS modes [4, 5]. I is based o he decay hoocoducace via he eeraio/recombiaio mechaisms of chare carriers. Techiques wih he rasie mode is more aroriae for hih lifeime >00 µsec ad are less sesiive o he surface recombiaio velociy SRV of mioriy carrier. Techiques which use he SS ad QSS modes like he QSSPC, he SRV facor affec heavily he measured values of eff ad for his reaso he surface mus be effecively assivaed. bouhafsd@ yahoo.fr

2 D. Bouhafs e al. Acually, wo echiques are commoly used by he hoovolaic commuiy o isec he lifeime i silico wafers; he Microwave hoocoducace decay (MW- PCD) i he rasie mode [6] ad he QSSPC wih he QSS mode [7). This las are very fas ad simle o use i a mc-si bare wafers characerized by a low carrier lifeime (<00 µsec). Before each measure se, he surface are chemically cleaed ad assivaed usi a well-kow echique such as hermally rowh silico dioxide (SiO ) layer, floai + jucio, PECVD silico iride (Si x) layer ad Iodie- Ehaol or Mehaol (I-E soluio) [7]. The I-E soluio is o used sysemaically comared o hermally SiO, PECVD Si x layer ad + floai jucio. I some cases, measuremes are reeaed a few imes o reach he sabiliy sae wih a accuracy of 0% [9]. I he rese work we have iseced he effecive lifeime of P-ye mc-si wafers usi QSSPC wih a differe surface assivaio ways: Hydrofluoridric acid (HF), I-E soluio, PECVD Si x layer ad floai + jucio. To deermie he SRV values from QSSPC lifeime curves ad o ideify he orii of eff variaio for each assivaio way as a fucio of he chare carrier desiy ijecio level ( ) we have used he Horbeck-Hayes model.. SURFACE PASSIVATIO AD BULK LIFETIME MEASUREMET Geerally he mioriy carrier lifeime secroscoy i semicoducors is based o he eeraio-recombiaio mechaisms. Theoreically, here exis four mechaisms which moior his hysical arameer: Shockley-Read-Hall (SRH) recombiaio, Auer recombiaio, Radiaive recombiaio ad Surface recombiaio. Taki io accou all his mechaisms, we ca wrie he effecive lifeime as follow [0]: eff SRH auer rad s b I he -ye silico moderaely doed by boro aoms ( a.5x0 6 cm 3 ), he Auer comoe ca be eleced. Also, he silico is a semicoducor maerial characerized by a idirec bad-a which reduces he robabiliy of he radiaive oical rasiio ad he recombiaio mechaism is eliible comared o SRH comoe. Cosideri hese asecs ad associai he SRH o he bulk comoe, he equaio () ca be simlified o: eff SRH s b s The surface mioriy carrier lifeime s ca be relaed o he SRV ad he wafer hickess W. If we assume ha boh fro ad rear surface recombiaio velociies have he same values ( Sfro Srear S), he formulaios below ive s deedi o he surface recombiaio sreh: For low values of SRV < 50 cm.s - [0] W s (3.a) S For hih values of SRV > 0 5 cm.s - [] () ()

3 Adequae mehod o sudy he surface assivaio effeciveess i HEM 3 W s (3.b) D Where D is he diffusiviy cosa of mioriy carrier i he bulk. Geerally D varies bewee 0 cm.s - o 30 cm.s - deedi o he defec desiy i he silico wafer. I he rae 50 cm.s - < SRV < 0 5 cm.s - s are exressed as follow, W W s (4) S D We ijec he erm of {eq. (4)} i {eq. ()}, he effecive lifeime ca be wrie, eff b W S D W Comui he {eq. (5)}, fiure illusraes he behavior of eff as a fucio of he surface recombiaio velociy for differe bulk lifeime b values (-50) µsec. The evoluio of eff curves suess ha he effec of SRV o effecive lifeime are more roouced i he iermediae rae (0 cm.s - < S < cm.s - ) for bulk lifeime values b < 5 µsec. We ca see also ha eff b for S < 0 cm.s - ad for S > cm.s - as show i Fi.. For low SRV values, carrier lifeime is domiaed by he erm ive i {eq. (3.a)} ad he eff is racically equal o he bulk life ime b. The domiace of his erm decrease i hih SRV values ad he he surface recombiaio is overed by he secod erm described i {eq. (3.b)}. I he oher had whe bulk lifeime is relaively hiher b > 5 µsec he eff values are sroly deede o he SRV values which corresod o S < cm.s -. I coclusio, his sudy allow us o verify if eff imroveme is due oly o he assivaio qualiy or o, as discussed below. (5) Fi. : Effecive lifeime vs. mioriy excess carrier desiy deedi o surface recombiaio velociy sreh

4 4 D. Bouhafs e al. 3. EXPERIMET Samles used i his sudy are.5 Ω.cm -ye mc-si as-cu wafers wih 300 µm of hickess. All he wafers are adjace ad hey were seleced from he cere reio of he same io. Firsly, hey are chemically eched i aoh: H O a 80 C o remove he saw damae iduced by slici se followed by he Piraha ech clea rocess: H SO 4:H O a 80 C + HF ad DI waer rise. Before lifeime measuremes surface wafers were assivaed by several ways: HF-di ad Iodie-Ehaol immersio assivaio usi a olyehylee ba, elaborai a + floai jucio hrouh hoshorous diffusio characerized by a shee resisiviy of 50 Ω/ ad rowi o he fro ad he rear surface sides of a 80 m Si x layer i a Plasma Ehaced Chemical Vaor Deosiio reacor. Lifeime measuremes are erformed usi he quasi-seady-sae hoocoducace echique (Sio Cosuli, WCT-0). For he we assivaio measuremes were imlemeed i he firs 0 miues. 4. RESULTS AD DISCUSSIO Measured aare lifeime vs. mioriy excess carrier desiy curves obaied afer each assivaio mehod, are illusraed i he fiure. We observe ha he maximum excess carrier desiy ( cm -3 ) has bee reached wih + floai jucio + 0 m of hermally SiO layer ad he eff a = 0 5 cm -3 reach.4 µsec. For I-E soluio, HF Di ad Si x assivaio, τ eff measured values are resecively 8., 4.9 ad 3. µsec. The maximum excess carrier desiy obaied wih Si x is cm -3 ad he ive lifeime corresod o his value (Fi. ). O he oher had, he Si x layer does o ive he bes mc-si surface assivaio ad reardi he shae of eff ( ) curves idicae ha he ras aciviy is dimiished for < 0 4 cm -3 leadi o a low aare lifeime i low ijecio level reio. To exlai his behavior we susec he formaio of a ew dee recombiaio ceers a he Si/Si x ierface duri Si x deosiio. The same behavior of eff vs. wih Si x assivai layer was observed by Peres e al. [], where he assivaio qualiy of Si x layer was foud lower ha ha of he hermally SiO. The lifeime curve relaed o he Si x assivaio (Fi. ) cao be exlaied by he recombiaio model described by Horbeck-Hayes based o SRH recombiaio ceers [3] which lead o a hih aare lifeime values i he reio of low carrier ijecio as show wih samles assivaed wih + jucio (Blue curves). The lifeimes decrease i he carrier low ijecio level ca be oly aribued o aoher ye of recombiaio ceers. This heomeo was observed ad aalyzed by Harder e al. ad hey roose a secod ye of ra saes recombiaio acive ra [4]. I heir sudy he lifeime measuremes were erformed usi Phoolumiescece (PL) echique comared by QSSPC oe. I ay case hey o observed his behavior wih QSSPC curves. Usi Horbeck-Hayes model, he fi of measured lifeime curves allows us o esimae he SRV ad he ra desiy of maerial afer each ye of assivaio. I his model, he o recombiaive ras desiy is cosidered. The sreh of he ras desiy alers he value of mioriy carrier ijecio level ( ). I his case we obai aare carrier desiies ( a ) which deed o several hysical arameers:, ad which corresod o he ras desiy, rai ad emissio ime cosas resecively. The carrier desiy relaed o he rai heomea is ive by:

5 Adequae mehod o sudy he surface assivaio effeciveess i HEM 5 Fi. : Effecive lifeime vs. mioriy excess carrier desiy corresodi o each assivaio mehod. (6) Ad he aare value off he excess carrier desiy is ive by: av a (7) Duri he measureme, he free elecros desiy ad he coduciviy are overesimaed ad he correc value of he effecive lifeime vs. ijecio carrier desiy ca be relaced by a aare lifeime which is ive by he followi formulaio aki io accou he o-recombiaio ras: / a eff a eff a eff a 4 (8) We ca say ha he erm of he {eq. (6)}, lay a imora role i he deermiaio of he measured lifeime ad i is resosible o he hih hoocoduciviy i he low carrier ijecio level as showed i he QSSPC measured curves. The elecros emissio by he ras i P-ye silico wafer ive a hih hoocoduciviy i a low ijecio level reios ( < cm -3 ). This behavior is o observed i he moo crysallie wafers which are ras free. I he case of he mulicrisallies subsraes, his orecombiaive ras effec ca be aeuaed by usi a bias lih wih a haloe lam. Also someoe use he hoolumiescece PL lih source o elimiae his heomeo. I our exerime we have used he msec. Xeo flash lam ad he

6 6 D. Bouhafs e al. obaied resuls are show i he ise of fiure. We oe ha he value of SRV decreases sysemaically wih he assivaio mehods: Si x, HF, I-E ad +, while he ra desiy icreases from o bewee he use HF ad + mehod. QSSPC measuremes wih Si x layer show a excelle bulk euralizaio of ras desiy ad he aare lifeime is o overesimaed (Black curve) i he low ijecio reio ( <0 4 cm -3 ) reardi he oher assivaio ways ( Colored curves) which ive lifeimes several orders reaer ha ha measured a =0 5 cm -3 o avoid he ras effec o he hoocoduciviy. A correlaio bewee he measured lifeime ad he calculaed SRV idicaes ha he imroveme lifeime is due o he ood surface assivaio. However, he measured lifeime is also a fucio of he bulk lifeime. So, o check if here is a chae i he bulk lifeime wih he erformed assivaio rocess, he {eq. (5)} was used o fi he four ois of he aare lifeime vs. SRV obaied for each assivaio mehod; cm.s -, wih + floai jucio, cm.s - wih I-E soluio, cm.s - wih HF immersio ad cm.s - wih Six layer (ois i blue i fiure 3). Fi. 3: Bulk lifeime values obaied from he fi of he four ois of he effecive lifeime vs. SRV for each assivaio echique The corresodi b values of he rocessed mc-si wafers are: 3 µsec, µsec, 0.8 µsec ad 5.9 µsec resecively. We foud ha is o ossible o fi he four ois wih oly oe curve. Therefore, we have loed a curve for each oi associaed o a fixed value of bulk lifeime, idicai ha he bulk lifeime also chaes wih he SRV accordi o he assivaio mehod. As a resul, surface assivaio has a direc imac o he SRV ad he bulk lifeime as well as he ra desiy. From he erformed QSSVoc measuremes o he mc-si wafers wih he differe assivaio ways, we have observed ha he imlied oe circui volae values follow he effeciveess of he surface assivaio duri he lifeime measuremes. For he Si x samle, he corresodi oical cosa of 0.9 was cosidered. As illusraed i fiure 4 ad by exraolaio, he values of he imlied volae are resecively: 55, 58, 59 ad 6 mv obaied wih Si x, HF di, I-E soluio ad + floai jucio assivaio mehods.

7 Adequae mehod o sudy he surface assivaio effeciveess i HEM 7 Fi. 4: Measured Imlied volae values correlaed wih assivaio echique These resuls rove he effeciveess of he used rocesses as show i fiure ad correlae bewee he icreases of τ eff from 3. µs o.4 µs. The hysical meai of he imlied volae is he elecrical oeial which ca roduce each wafer i he fiished solar cell. 5. COCLUSIO I his work we have examied he behavior of he carrier mioriy lifeime vs. excess carrier o he HEM muli crysallie wafers usi he QSSPC echique uder differe surface assivaio codiios. The chemical surface re-clea of he silico wafer is a criical arameer ad affecs he a measured values via he assivaio qualiy. We have observed ha he we chemical surface assivaio like he Iodie Ehaol soluio ad he HF Di are cosiderably alered by he surface clea wih ad wihou iraha ech se. I such case he I-E soluio ives he correc a value for he bare Mc-Si wafers. Also bulk lifeime of he rocessed wafers is deermied usi QSSPC a measureme ad he b values corresodi o 3 µsec ( + floai jucio), µsec (I-E), 0.8 µsec (HF di) ad 5.9 µsec (Six) are exraced usi fii curves of he effecive lifeime vs. surface recombiaio velociy. Ackowledeme- This work is realized a Phoovolaic Divisio/CRTSE ad fiaced by he aioal Fud for Research -DGRSDT. REFERECES [] Ch. Breyer, Ch. Birker, F. Kerse, A. Gerlach, G. Sryi-Hi, J.Ch. Goldschmid, D.F. Mooro, M. Riede, Research ad Develome Ivesmes i PV A limii Facor for a fas PV Diffusio?, 5 h Euroea Phoovolaic Solar Eery Coferece ad Exhibiio,6-0 Seember 00, Valecia, Sai.

8 8 D. Bouhafs e al. [] A. Zuschla, J. Jue, S. Sere ad G. Hah, Evaluaio of rocessi ses reardi lifeime of iro/coer coamiaed mc Si wafers, 4 h EU PVSEC, Seember -5, 009 Hambur, Germay. [3] B. Herzo, G. Hah, M. Hofma, I. Romij ad A.W. Weeber, Bulk hydroeaio i mc-si by PECVD SiOx Deosiio usi direc ad remoe lasma, 3 rd Euroea Phoovolaic Solar Eery Coferece, , Valecia, Sai, Seember -5, 008. [4] R.A. Sio ad A. Cuevas, Coacless Deermiaio of Curre-Volae Characerisics ad Mioriy-Carrier Lifeimes i Semicoducors from Quasi- Seady-Sae Phoocoducace Daa, Alied Physics Leers, Vol. 69, 7,. 50 5, 996. [5] M.J. Kerr ad A. Cuevas, Geeralizaio of he Illumiaio Iesiy Vs. Oe- Circui Volae Characerisics of Solar Cells, 7 h Euroea Phoovolaic Solar Eery Coferece, , Muich, Germay, -6 Ocober 00. [6] K Lauer, A Laades, H Ubesee, H Mezer ad A Lawerez, Deailed Aalysis of he Microwave-Deeced Phoo coducace Decay i Crysallie Silico, Joural Alied Physics, Vol. 04, 0, 04503, 008. [7] A. Cuevas, The Effec of Emier Recombiaiso o he Effecive Lifeime of Silico Wafers, Solar Eery Maerials ad Solar Cells, Vol. 57, 3, , 999. [8] B. Soori, P. Ruowski, J. Ael, V. Meha, C. Li ad S. Johso, Wafer Prearaio ad Iodie-Ehaol Passivaio Procedure for Reroducible Mioriy- Carrier Lifeime Measureme, 33 rd IEEE Phoovolaic Secialiss Coferece,. 4, May 6, 008, Sa Dieo, Califoria, USA. [9] A. Hauser, M. Sieel, P. Fah ad E. Bucher, Ivesiaios o Hydroe i Silico by meas of Lifeime Measuremes, 8 h IEEE Phoovolaic Secialiss Coferece, , Seember 5 -, 000, Achorae, Alaska. [0] S. Rei, Lifeime secroscoy, A Mehod of Defec Characerizaio i Silico for Phoovolaic Alicaios, Srier Series i Maerial Sciece, Vol. 85, 005. [] A.B. Sroul, Dimesioless Soluio of he Equaio Describi he Effec of Surface Recombiaio o Carrier Decay i Semicoducors, Joural of Alied Physics, Vol. 76, 5, , 994. [] R. Peres, J. Libal, T. Buck, R. Koecek, M. Veer, R. Ferre, I. Marí, D. Borcher ad P. Fah, Imrovemes i he Passivaio of P+-Si Surfaces by PECVD Silico Carbide Films, IEEE Elecro Devices Sociey: IEEE 4h World Coferece o Phoovolaic Eery Coversio, Vol.,. 0 05, 7- May 006, Waikoloa, Hawaii. [3] J.A. Horbeck ad J.R. Hayes, Trai of mioriy carriers i silico, I. P-Tye silico, Physics Review, Vol. 97,. 3, 955 [4].P. Harder, R. Gooli ad R. Bredel, Trai-relaed recombiaiso of chare carriers i silico, Alied Physics Leers, Vol. 97,, 00.