硅在氮化硅涂层上的形核SiO2

Nucleation of silicon on Si3N4coated SiO2

I.BrynjulfsenÃ,L.Arnberg

Department of Materials Science and Engineering,Norwegian University of Science and Technology,7491Trondheim,Norway

a r t i c l e i n f o

Article history:

Received30May2011

Received in revised form

6July2011

Accepted8July2011

Communicated by P.Rudolph

Available online19July2011

Keywords:

A1.Nucleation

A1.Solidification

A2.Undercooling

B1.Silicon

a b s t r a c t

Control of the nucleation during directional solidification of solar cell silicon is important in order to be

able to control the growth and number of grains formed.A certain amount of undercooling is required

to obtain dendritic growth with faceted twins(which has shown promising results for structure

control),but a too high undercooling will lead to extensive nucleation which will oppose the positive

effect 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 has

been investigated.Experiments were performed with the sessile drop method,and with differential

thermal analysis,with a cooling rate of20K/min.There were no significant differences in nucleation

undercooling between the different variations in coating.The undercooling does not seem to be

dependent on coating thickness,oxygen concentration,wetting angle or roughness at the given

cooling rate.

&2011Elsevier B.V.All rights reserved.

1.Introduction

The solar cell industry is developing fast in several directions,and

in order for the multicrystalline solar cell to be able to compete with

monocrystalline cells and other new alternatives,the efficiency has

to be improved.The solidification process of multicrystalline silicon

is important for thefinal efficiency of the solar cell.Grain size,grain

orientation,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 are

again dependent on the nucleation of silicon,and it is therefore

important 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,and

were able to increase the crystals size by an initial faceted

dendritic growth followed by traditional planar front directional

solidification.The dendritic growth results in fewer and larger

grains,which again lead to less grain boundaries were recombi-

nation can take place.Fujiwara et al.[1]investigated how

different cooling rates influenced the size of undercooling needed

to obtain faceted dendritic growth.The present work has been

performed in order to study how/if the substrate on which the

silicon grows will influence the undercooling and nucleation of

silicon.Si3N4coated SiO2has been chosen as a substrate since

multicrystalline silicon ingots are normally cast in Si3N4coated

SiO2crucibles.

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 the

wetting angle between the nucleus and the nucleating substrate.

This implies that the nucleation is dependent on the substrate

roughness,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-or

SiC-particles[6].This nucleation can cause the formation of an

equiaxed zone instead of the desired columnar zone[7,8].It has

been documented that particles like this are present in the bottom

of the ingot[9].

The substrate’s influence on undercooling for solidification of

silicon has not been studied thoroughly,but some investigations

in the area has been done.Appapillai et al.[10]investigated

nucleation undercooling for silicon samples coated with different

materials 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 started

in this region.For the silicon nitride coated samples the nuclea-

tion sites were further apart.This resulted in the conclusion that a

lower undercooling gave fewer grains,which is consistent with

classical nucleation theory.This shows the importance of the

ability to control the nucleation more precisely.They also showed

that the chemical composition played an important role in

nucleation.The oxides had a higher interfacial stability resulting

in a higher undercooling than the Si3N4.

The present work has been performed to investigate which

coating parameters influence the nucleation undercooling of

silicon on Si3N4coated SiO2.This is done in order to be able to

Contents lists available at ScienceDirect

journal homepage:/locate/jcrysgro

Journal of Crystal Growth

0022-0248/$-see front matter&2011Elsevier B.V.All rights reserved.

doi:10.1016/j.jcrysgro.2011.07.003

ÃCorresponding author.Tel.:þ4773594903;fax:þ4773550203.

E-mail address:ingvild.brynjulfsen@material.ntnu.no(I.Brynjulfsen).

Journal of Crystal Growth331(2011)64–67