多孔硅在太阳电池中应用有着很大的 潜力。
478R.Bilyalovetal./SolarEnergyMaterials&SolarCells65(2001)477}485
lowporositycouldserveasaseedinglayerforepitaxialgrowthwhileontheotherhandahigh-porositylayeredisneededforasu$cientlightre#ection[3].Amulti-layeredPSstructurehasbeenproposedasapossiblesolutionin[4].Indeed,high-porosityPScanactasanexcellentlightre#ector,whichisanecessaryrequire-mentforlighttrappinginultra-thinSi"lms.Moreover,atthesametimethisPSlayercouldbeagetteringbarrier[5]preventingimpuritydi!usionfromthelow-costsubstrateintotheactiveSilayer.Inthisworkwepresentforthe"rsttimetheresultsoftheuseofPSwithdoubleporosity(%)asanopticalandimpuritybarrierlayerforthinSi"lmsdepositedbyconventionalCVDatmediumtemperature(725}8003C).TheCVDperformedonPSatthesetemperaturesistobeseenasanalternativeapproachbesidestheassistedCVDtechniquesatlowtemperatures(400}6003C)[6]andthehigh-temperature(1000}11003C)thermalCVDonstabilizedPS[7].Further-more,ourstudyfocusesonadualporositystructurebeingacompromisebetweensingle-andmulti-layeredPSstructure.
2.Experimental
TwokindsofSimaterialareusedinthiswork:highlyborondopedCz-andMG-Si.ThesingleanddoubleporosityPSarepreparedbyconventionalelectrochemicalanodizationinaHF-basedelectrolyte[8].ThethicknessofPSisintherangeof400}700nm.Si"lmsaregrowninacommerciallyavailableepitaxialreactorusingSiHCl(DCS)asasourcegas.The"rst300nmofSiisgrownat7253Ctoobtainan appropriateepi-layerqualityandtoavoidreorganizationofPS[3].Thengrowthiscontinuedat8003Ctoreachasu$cientgrowthrate.The"nishedSi/PS/SubstratestructuresareusedforamesasolarcellprocessdevelopedatIMECforthin"lmsonnon-conductivesubstrates[9].
3.Resultsanddiscussions
3.1.Morphologicalanalysis
Morphologicalanalysisiscarriedoutbymeansofhigh-resolutioncross-sectionaltransmissionelectronmicroscopy(XTEM)analysisonCz-SisamplesinordertostudythestructuralqualityoftheepitaxialSilayer.Furthermore,spectroscopicellipsometry(SE)andRutherfordbackscatteringspectroscopy(RBS)wereusedtoinvestigatechangesinporosityafterthedepositionoftheepitaxialSilayers.
Fig.1showstheXTEM-imageof150nmSi"lmgrownona400nmPSlayerwith%ofporosity.Thedefectdensity(cm\ )hasbeenderivedfromplanviewTEM-analysisforthinSi"lmsepitaxiallygrownonbothsingleanddualporouslayersusingDCSat7253C(Table1).Adefectdensitybelow10 cm\ isobservedinthetopregionoftheinvestigatedlayers(i.e.'100nmabovetheSi/porousSiinterface),iftheSilayerisgrownontopofasingle20%porouslayerorontopofadualporouslayerwithatoplayerof20%porosity.Thiscon"rmsthatalow-porositytoplayeris