硅在氮化硅涂层上的形核SiO2

Nucleation of silicon on Si3N4coated SiO2I.BrynjulfsenÃ,L.ArnbergDepartment of Materials Science and Engineering,Norwegian University of Science and Technology,7491Trondheim,Norwaya r t i c l e i n f oArticle history:Received30May2011Received in revised form6July2011Accepted8July2011Communicated by P.RudolphAvailable online19July2011Keywords:A1.NucleationA1.SolidificationA2.UndercoolingB1.Silicona b s t r a c tControl of the nucleation during directional solidification of solar cell silicon is important in order to beable to control the growth and number of grains formed.A certain amount of undercooling is requiredto obtain dendritic growth with faceted twins(which has shown promising results for structurecontrol),but a too high undercooling will lead to extensive nucleation which will oppose the positiveeffect of a small number of large grains with controlled growth directions.In the present experiments,the nucleation undercooling of silicon on Si3N4coated SiO2with variation in coating parameters hasbeen investigated.Experiments were performed with the sessile drop method,and with differentialthermal analysis,with a cooling rate of20K/min.There were no significant differences in nucleationundercooling between the different variations in coating.The undercooling does not seem to bedependent on coating thickness,oxygen concentration,wetting angle or roughness at the givencooling rate.&2011Elsevier B.V.All rights reserved.1.IntroductionThe solar cell industry is developing fast in several directions,andin order for the multicrystalline solar cell to be able to compete withmonocrystalline cells and other new alternatives,the efficiency hasto be improved.The solidification process of multicrystalline siliconis important for thefinal efficiency of the solar cell.Grain size,grainorientation,and impurity distribution/concentration are all proper-ties dependent on solidification parameters.Some of these char-acteristics like the number of,the size,and orientation of grains areagain dependent on the nucleation of silicon,and it is thereforeimportant to be able to control this mechanism.Recently several solidification experiments have been per-formed by Fujiwara et al.[1–4].They studied grain growth,andwere able to increase the crystals size by an initial faceteddendritic growth followed by traditional planar front directionalsolidification.The dendritic growth results in fewer and largergrains,which again lead to less grain boundaries were recombi-nation can take place.Fujiwara et al.[1]investigated howdifferent cooling rates influenced the size of undercooling neededto obtain faceted dendritic growth.The present work has beenperformed in order to study how/if the substrate on which thesilicon grows will influence the undercooling and nucleation ofsilicon.Si3N4coated SiO2has been chosen as a substrate sincemulticrystalline silicon ingots are normally cast in Si3N4coatedSiO2crucibles.Nucleation is the dominant process in the beginning of solidifica-tion and leads to the establishment of thefinal grain number.Heterogeneous nucleation undercooling depends strongly on thewetting angle between the nucleus and the nucleating substrate.This implies that the nucleation is dependent on the substrateroughness,composition,thickness,etc.[5].Another aspect is impu-rities.Impurities in the bulk have been studied by several authors,and it has been shown that silicon often nucleate from Si3N4-orSiC-particles[6].This nucleation can cause the formation of anequiaxed zone instead of the desired columnar zone[7,8].It hasbeen documented that particles like this are present in the bottomof the ingot[9].The substrate’s influence on undercooling for solidification ofsilicon has not been studied thoroughly,but some investigationsin the area has been done.Appapillai et al.[10]investigatednucleation undercooling for silicon samples coated with differentmaterials among others Si3N4.They foundfinely spaced nuclea-tion sites near the edge of the samples coated with dry oxides(high undercooling),which indicated that the nucleation startedin this region.For the silicon nitride coated samples the nuclea-tion sites were further apart.This resulted in the conclusion that alower undercooling gave fewer grains,which is consistent withclassical nucleation theory.This shows the importance of theability to control the nucleation more precisely.They also showedthat the chemical composition played an important role innucleation.The oxides had a higher interfacial stability resultingin a higher undercooling than the Si3N4.The present work has been performed to investigate whichcoating parameters influence the nucleation undercooling ofsilicon on Si3N4coated SiO2.This is done in order to be able toContents lists available at ScienceDirectjournal homepage:/locate/jcrysgroJournal of Crystal Growth0022-0248/$-see front matter&2011Elsevier B.V.All rights reserved.doi:10.1016/j.jcrysgro.2011.07.003ÃCorresponding author.Tel.:þ4773594903;fax:þ4773550203.E-mail address:ingvild.brynjulfsen@material.ntnu.no(I.Brynjulfsen).Journal of Crystal Growth331(2011)64–67control the nucleation and undercooling needed for a dendritic growth more accurately,since this growth has shown promising results for structure control of silicon ingots.2.Experimental procedureThree different parameters have been studied in this work;the coating thickness;the amount of oxygen in the coating;and the roughness of the coating.Two types of experiments have been used to study the nucleation undercooling.The sessile drop method performed in a wettability furnace and differential thermal analysis (DTA).The samples for the wettability furnace,SiO2pieces with the dimensions0.9Â0.9Â0.3cm3,were coated both manually with a spray gun and with a coating robot.Before coating the samples were preheated on a heater to a temperature of1501C,but this dropped rapidly when the samples were coated.The samples were coated with a variation in number of layers.The samples were dried between each layer of coating to ensure a better adherence to the substrate and a smooth coating.The amount of coating was weighed and the coating thickness was both estimated and measured after the nucleation experiments,with correlating numbers.The samples were thenfired at11001C for4h as a standardfiring routine.Firing temperatures of900and12001C,and holding time of6h were used to obtain different oxygen levels in the coating.The oxygen concentration was measured by LECO.Similar samples were also produced by a commercial crucible producer,Vesuvius.For the nucleation study in the wettability furnace a small piece of silicon(approximately20mg)was placed on the Si3N4 coated substrate and mounted on the sample holder.The proce-dure and specifications for using the wetting furnace are explained elsewhere[11].The sample was heated at a rate of 5K/min when it was close to melting and to the preset holding temperature.When the sample reached the holding temperature, which was always less than50K above the melting point,it was cooled at a rate of20K/min.The temperature was measured with a thermocouple placed directly below the sample holder,and the melting point of silicon was used as a reference point for the amount of undercooling.When the droplet started solidifying it expanded in most cases,as can be seen in Fig.1.The solidification temperature was set to be the temperature when thefirst visible changes in size and/or reflection in the middle of the droplet took place.The high cooling rate was chosen in order to be able to visually see the point of solidification.A high cooling rate will give higher values of undercooling[12],but since the cooling rate was the same for all experiments it should not have influenced the parameters under investigation in this study.Some experiments were performed in a differential thermal analysis(DTA)equipment to confirm the undercooling measured by the sessile drop method.The same heating and cooling routine as for the wetting experiments were used in these experiments. The crucibles were Al2O3coated with Si3N4,and therefore an Al2O3substrate was also tested in the wetting furnace.Because of the small diameter of the DTA crucibles the coating was not easy to spray uniformly.Some crucibles were spray coated and some painted with coating to investigate if this led to differences.3.Results and discussion3.1.Wetting experimentsAs explained in Introduction a good wetting between solid and substrate leads to a lower energy barrier for nucleation,thus a lower nucleation undercooling.The surface energy balance for a solid droplet in a liquid is shown in Fig.2(a).For good wetting between the solid and the substrate, gLiquidÀSubstratemust be higher than the sum of the other interface energies:gLiquidÀSubstrateZ g SolidÀSubstrateþg LiquidÀSolid cos yð1ÞA large g LiquidÀSubstrate will give wetting of the solid nucleus in Fig.2(a).On the other hand,if g LiquidÀSubstrate is large it can beseenFig.1.Melted droplet of silicon(a)before cooling and(b)solidified.Fig.2.Thefigure illustrates the difference between contributions affecting the wetting angle for the two cases:(a)Solid nucleus in liquid silicon.(b)Liquid drop on substrate in vapour atmosphere,sessile drop experiment.I.Brynjulfsen,L.Arnberg/Journal of Crystal Growth331(2011)64–6765from Fig.2(b)that this can lead to non-wetting in the liquid on substrate case.Wetting conditions in this second case is also dependent on the liquid–vapour tension and vapour–substrate tension.Both of these interface tensions are dependent on the coating,because a high oxygen concentration can lead to the formation of an oxide film and/or a high production of SiO gas.This will affect the wetting and nucleation conditions on the edges of the droplet,but not the nucleation conditions inside the droplet.Therefore it is difficult to predict whether or not non-wetting/wetting in sessile drop experiments indicate wetting/non-wetting of the solid nucleus forming inside the droplet.Another factor from theory that will contribute to a good wetting is a rough coating which will lead to more nucleation points [13].In addition,in the sessile drop experiments a low oxygen concentration will lead to a higher degree of wetting [11].Since the dendritic growth requires a certain amount of under-cooling to take place,non-wetting in Fig.2(a)is wanted.The aim of this study is therefore not only to investigate what factors influence the nucleation undercooling,but also see which gives the highest undercooling at the given cooling rate.Since several factors are affecting the wetting conditions,the only conclusion that can be drawn is that wetting in Fig.2(b)gives a higher probability for non-wetting in Fig.2(a).A consideration that must be taken into account,and cannot be left out of experiments with solar cell silicon,is that non-wetting in Fig.2(b)is important to prevent the silicon from sticking to the crucible.Large variations in the undercooling were measured,from the lowest of 12K to the highest of 37K.The undercooling did not seem to be dependent on the oxygen concentration in the coating,see Fig.3(a),or the measured wetting angle,see Fig.3(b).A thicker coating will limit the diffusion of oxygen from the silica substrate and in this way affect the reactions taking place between silicon and substrate.It was therefore believed that the thickness would influence the undercooling,but variations in coating thicknessdid neither show a trend;see Fig.4(a).These samples were produced with the same firing routine and hence had the same oxygen concentrations.Samples produced by Vesuvius with an up to 20times thicker coating also gave undercooling in the same range.The variations in coating thickness were applied due to earlier investiga-tions of wetting and oxygen concentration [11].The comparison of samples with different roughness from Vesuvius did neither show any trend;see Fig.4(b).The average roughness given in the figure is smaller than 5and 10m m Ra.The last value marked as 20m m Ra is for all the experiments with variation in coating thickness.The two finest coatings are some-what thinner than the rough coatings.The coating fired at 12001C for 6h displayed a different behaviour then the rest.It looked like the droplet reacted with the substrate after melting and formed an oxide layer around the droplet.It lost its round shape and imploded.This implies an effect of the oxygen concentration on the surface tension and hence the nucleation.Because of this behaviour the data for the coating fired at 12001C were only plotted in Fig.4(a).The present results are not directly comparable with the experiments in the study by Appapillai et al.[10],since in their case they coated the silicon samples completely with the sub-strate.But,as for Appapillai,the coated Al 2O 3substrates in the present project gave a somewhat lower undercooling than the coated silica substrates.Silicon wet these substrates most,and the spreading was faster and more significant.This implies that the chemical composition of the silicon nitride coating alone does not determine the undercooling.All the samples in the present experiments were covered by a black layer,indicating a low interfacial stability,which can contribute to decrease the undercooling.This behaviour together with the clear reaction for the samples fired at 12001C showed,in accordance with Appapillai et al.[10],Koh et al.[14]and Vallat-Sauvain et al.[15],the importance of chemical composition.0510152025303540Undercooling as a function of oxygen concentration in coatingOxygen concentration (wt.%)U n d e r c o o l i n g (K )1100°C, 4h 1100°C, 6h 900°C, 2h 1200°C, 4h0510152025303540Undercooling as a functionof wetting angleWetting angle (°)U n d e r c o o l i n g (K )Author VesuviusCoated alumina Pure silica Pure aluminaFig.3.Variation in undercooling with:(a)oxygen concentration.The tempera-tures and time displayed are the firing parameters for the coating.(b)Liquid-substrate wettingangle.Undercooling as a function ofcoating thickness0510152025303540Coating thickness (µm)U n d e r c o o l i n gAuthorVesuviusUndercooling as a function of roughness- samples from Vesuvius0510152025303540Roughness (µm)U n d e r c o o l i n g (K )Fig. 4.Display of variation in undercooling with coating thickness (a)and roughness (b).I.Brynjulfsen,L.Arnberg /Journal of Crystal Growth 331(2011)64–6766As mentioned above there is also the possibility for the nucleation to take place at inclusions in the melt.Si 3N 4particles can be formed due to dissolution and re-precipitation of the coating,and carbon is also available in the furnace atmosphere giving the possibility for formation of SiC.The substrates without coating were not exposed to nitrogen.(The coating has been identified as the main source of nitrogen in silicon.)If Si 3N 4particles were the main cause of nucleation of silicon,the substrates without coating should show a higher undercooling.This is not the case in this work,but SiC can not be ruled out as a compound influencing nucleation.3.2.DTA-analysisTwo series of differential thermal analysis experiments were performed on Si 3N 4coated Al 2O 3crucibles,with the same heating and cooling cycle as in the wetting furnace.Because of the small diameter of the crucible an even coating was very difficult to obtain.The first series of coating were therefore still on the experimental level and not very complete or uniform.Two crucibles were coated in this way and are marked M in Fig.1.In the second series,two crucibles were spray coated,S,and three painted with coating,P.The average undercooling was ca.21K.The shapes of two of the DTA curves were varied from the others.These were the spray coated and most evenly coated crucibles.They have a somewhat higher undercooling than the others,but not significantly enough to conclude with a real difference,see Table 1.The undercooling of the samples in the DTA were in the same range as the sessile drop experiments.This means that even if the DTA is a more accurate measuring method,since the sessile drop experiments rely on a visual observation of start of nucleation,sessile drop experiments can be used to measure undercooling.Two of the DTA-values were somewhat lower,but as commented above these were not coated as good as they should have.This leads to an uncertainty in their accuracy.All the experiments are summarized in Fig.5.4.ConclusionThe results from the wetting experiments do not indicate that the coating alone plays an important role in the nucleation under-cooling.Different oxygen concentration,thickness,and roughness gave undercooling in the same range.There where on the other hand some variations in undercooling with different substrates,as the coated alumina substrates showed a lower undercooling,see Fig.5.A very high oxygen concentration did affect the undercooling in the way that a clear reaction and deformation affected the silicon leading to a lower degree of undercooling.Even if the DTA results and the results from the sessile drop experiments were in the same range,the DTA-results showed variations,and a further study of the coating roughness should be done.Nucleation in the melt caused by inclusion should not be ruled out either.Further work is required to study if the nucleation undercooling experiments in the present investigation can be used to predict nucleation conditions during solidification of large silicon ingots.AcknowledgementThis work was performed within The Norwegian Research Centre for Solar Cell Technology project number 193829,a Centre for Environment-friendly Energy Research co-sponsored by the Norwegian Research Council and research and industry partners in Norway.References[1]K.Fujiwara,W.Pan,K.Sawada,M.Tokairin,ami,Y.Nose,A.Nomura,T.Shishido,K.Nakajima,Directional growth method to obtain high quality polycrystalline silicon from its melt,Journal of Crystal Growth 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substrate are placed next to each other.I.Brynjulfsen,L.Arnberg /Journal of Crystal Growth 331(2011)64–6767。