Role of oxygen in surface segregation of metal impurities in silicon poly-and bicrystals

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Role of oxygen in surface segregation of metal impurities in silicon poly-and bicrystals

E. Amarray, J.P. Deville

To cite this version:

E. Amarray, J.P. Deville. Role of oxygen in surface segregation of metal impurities in silicon poly-

and bicrystals. Revue de Physique Appliquée, Société française de physique / EDP, 1987, 22 (7),

pp.663-669. �10.1051/rphysap:01987002207066300�. �jpa-00245593�

Role of oxygen ⁱⁿ surface segregation ^{of metal} impurities in silicon poly-

and bicrystals

E.

Amarray

and J. P. Deville

(*)

Equipe

d’Etude des

Surfaces,

^{UA 795 du}

C.N.R.S.,

Université Louis-Pasteur, 4, ^rue

Blaise-Pascal,

⁶⁷⁰⁰⁰

Strasbourg, France (Reçu

le 6 octobre 1986, révisé le 25 mars

1987, accepté

^{le 9 avril}

1987)

Résumé. ²⁰¹⁴ Nous avons

caractérisé,

^au moyen des méthodes

d’analyse

^des

surfaces,

^les

impuretés métalliques

situées sur des rubans de silicium

polycristallin. L’oxygène

^et les traitements

thermiques

^{semblent une force}

motrice pour la

ségrégation superficielle

^{de ces}

impuretés.

Pour mieux étudier leur influence et leurs

possibilités

^{en terme d’effet} getter, nous avons initié des études de modélisation sur des bicristaux de type Czochralski. Nous avons étudié deux facteurs

principaux

^de

ségrégation superficielle :

le rôle d’une couche

d’oxyde

très mince et celui de traitements

thermiques.

^{Nous avons}

remarqué

que le maximum de

purification

des surfaces était obtenu

après

le recuit à 750 °C d’une surface

préalablement oxydée

^{à 450 °C. Nous avons}

relié cela à la formation d’amas de

SiO,

suivie d’une coalescence donnant des unités de type

SiO4

^entraînant

l’injection

d’auto-interstitiels de silicium dans le réseau.

Abstract. ²⁰¹⁴ Metal

impurities

^at surfaces of

polycrystalline

silicon ribbons have been characterized

by

^surface

sensitive methods.

Oxygen

^{and heat} treatments were found to be a

driving

force for surface

segregation

^of

these

impurities.

^{To better}

analyse

their influence and their

possible

incidence in

gettering,

model studies were

undertaken on Czochralski grown silicon

bicrystals.

Two main factors of surface

segregation

^{have been}

studied : the role of a ultra-thin oxide

layer

^{and the effect of heat} treatments. The best surface

purification

^was

obtained after an

annealing

process ^at 750 °C of a

previously

oxidized surface at 450 °C. This was related to the formation of SiO clusters, ^followed

by

^a coalescence of

SiO4

^units

leading

^{to the}

subsequent injection

^{of silicon}

self-interstitials in the lattice.

Classification

Physics

^Abstracts

61.70W - 66.30 ^- 68.60J

1. Introduction.

Polycrystalline silicon,

^{often referred to as}

«

polysilicon

^», is obtained either

by casting ingots

via a

Bridgman-like growth

process ^or

by setting

up ribbon

technologies

^{based on}

shaped crystal growth.

These

technologies

^are

developed

^{to reduce the}

cost of terrestrial solar cells

by minimizing

^silicon

consumption

^and/or

by using cheaper, degraded

silicon as

starting

material. Possible

applications

^to

microelectronics

should be also born in mind for the future.

Final materials obtained

by

such methods have a

large

^{amount of}

structural,

^chemical and electrical defects.

Thus,

^{the main}

objective

of research in the last decade has been to

identify

^and

classify

^these

defects,

^to

investigate

^{which ones are the most}

detrimental

in terms of

photovoltaic yield

^{and to find}

(*)

^{To whom}

correspondence

^{should be sent.}

REVUE DE PHYSIQUE APPLIQUÉE. - ^T. 22, N° 7, JUILLET 1987

out methods for

passivation

^{of the}

electrically

^active

ones.

For

example,

it has been demonstrated that im-

plantation

of molecular or atomic

hydrogen improves

the electrical

properties

^{such as} the diffusion

length

in both

ingots

^{and ribbons}

[1-5].

^{Diffusion at low}

temperature

of selected

impurities

^{such as Cu and Al}

into

polycrystalline

^{Si was} also found to

improve

^the

minority

^{carrier diffusion}

length [6].

In the case of ribbon

technologies,

^{it has been also}

shown that thermal treatments

could,

in certain cases,

improve

^{the diffusion}

length [4, 7].

^{The im-}

provements

^have been related to intrinsic

gettering

effects in which

fast-diffusing species

^{are offered}

energetically

favorable sites outside the

electrically

active

region

^of the material.

Surface

physics

methods have been

thought

^{to be}

useful in this

perspective

since the active area of

photovoltaïc

^{devices are} located in the

top

^few micrometers. Is it

possible

^to draw detrimental

45

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01987002207066300

664

impurities

^out of the active

region ?

Is it relevant to use some of the classical

gettering

processes in

polysilicon technology ?

^{To answer these}

questions

we

applied

^{surface science}

techniques

^{to understand}

particularly

^{the role} of oxygen and heat ^treatments in the

segregation

^{of metal}

impurities

towards the surface of silicon

samples.

In this paper, after

having briefly

recalled the RAD

growth

process and the

experimental set-up,

we shall sum up the

early

^surface

analytical

^results

found on RAD

ribbons,

^{then we shall}

present

^recent

investigations

^{on model}

systems (silicon bicrystals).

No

attempt

^{has been}

made, however,

to measure differences in electrical

properties during

^these

model studies

since ultra-high

^vacuum is needed for surface

analysis

^and

prevent

^{one to realize}

easily

reliable electrical measurements.

2.

Experimental.

2.1 SAMPLES. ^- RAD silicon ribbons were obtained

by

^a

shaped crystal growth

^{in which a carbon}

support

is

continuously pulled through

^a

p-doped

silicon melt via a slot located at the bottom of a RF induction heated

quartz

crucible. Details about the various technical

requirements

^and achievements of the process ^can be found in

[8].

^After

growth,

^the

carbon

support

is burnt-off in a

dry

oxygen ^atmos-

phere

^at

temperatures ranging

^{from 1000 °C to}

1200 °C

during

¹

hour, resulting

^{in two}

self-support- ing

^{Si sheets} less than 100 _03BCm thick. The outside faces are oxidized and the inner faces are covered with a discontinuous SiC

layer.

^The thicknesses of the oxide

layers

range from 0.2 ^to 1 _03BCm. These two

overlayers

^are

chemically

etched off before

making N+ /p homojunctions by

^a

classical POCl3

^diffusion

process ^at 850 °C. In most cases, ^we studied

samples

as obtained after the burnt-off process.

As model

materials,

^{we used CZ}

bicrystals (n -10 ^03A9.cm-1

^and

p -1 03A9.cm-1),

grown ^at the LETI

(Grenoble). Oxygen

and carbon concen-

trations are in the

1017 at.cm-3 range

^{and can}

locally

exceed the limit of

solubility

^at

equilibrium [9].

^Bar-

like

samples (4

^{x 4 x 10}

mm3),

^{cut from the}

ingots,

were oriented

by back-reflection

^Laue

techniques ;

two

kinds

of orientations were chosen :

i)

^the

grain boundary plane being perpendicular

to the

long

axis of the

bar,

ii)

^the

easily

^cleavable

(111) plane, perpendicular

to this axis.

Grooves were made either

right along

^{the intersec-}

tion of the

grain boundary plane

with the bar or

along

^a

(111) plane.

^{It was then}

possible

^{to cleave}

our

samples

^{in UHV and to}

get

^clean

virgin

^silicon

surfaces

and, sometimes,

^the

boundary plane

^itself.

In this paper, ^we shall discuss

only incidentally

^the

role of the

grain boundary which

^{is not the}

major parameter

of interest.

The

samples

^were etched in dilute HF and no

thermal treatment

applied prior

their introduction in the UHV chamber

3.

Analytical

^methods.

Impurity

concentration

profiles

^{of RAD}

samples

were determined

by

^{means of}

X-Ray

Photoelectron

Spectroscopy (XPS)

^{in a VG ESCA III}

apparatus.

Depth profiles

^{down to 1} )JLm ^were obtained

by argon-ion sputter etching

^the

sample step by step (about

¹⁰⁰

^Â

^{per ion}

bombardment).

^The

sputtered

area had a

larger

diameter than the

analyzed

^one.

Silicon

bicrystals

^were

analysed by Auger

^Electron

Spectroscopy (AES) using

^{a RIBER CMA in a}

UHV chamber fitted with several

accessories (argon

ion

bombardment, cleavage system,

Knudsen cells for

metal evaporation, ...).

^{The diameter of the}

spot

is 10 _{03BCm ;} too

large

^to

investigate

^the

grain

^bound-

aries,

it allows to

study

^{surface domains about}

30 _03BCm

large.

Since

the low-energy Auger peaks

of metal im-

purities

^are

superimposed

^to the silicon

Auger

^LVV

fine structure, ^we used the LMM

peaks

^of

Cr,

^Fe

and Ni at

respectively 529, 703,

^{and 848 eV to derive} concentrations. Of _course, the differences in mean

free

paths

between the various

Auger

transitions have been taken into account in these calculations.

Concentrations are

given

^{in atom} per ^cent and are

averaged

^over thicknesses of about 8

Â.

The ratioes between the

high-

^and

low-energy peak

intensities have been also studied to

precise

^the

location of

given impurities

^with

respect

^{to the} surface.

4. Results on RAD

polycrystalline

^ribbons.

4.1 GENERAL OUTLINE OF THE IMPURITY DISTRI- BUTION IN RAD RIBBONS. ^- We

previously

^de-

scribed

depth profiles

^of

impurities

^{in RAD ribbons}

and we summarize here the main results

[10].

^{At that}

time

burning

off the carbon

support

^{was not}

yet

ⁱⁿ

use. On the

samples

^{received as} grown, surface

analysis

^showed

^oxide layers

thicknesses of which

were

ranging

^{from 200 to 1 000}

A, depending

^{on the}

growth

conditions. The oxide was

generally

^not

three-dimensional silica as observed

by

^{AES and}

XPS

[11,12].

Impurities

^could

be

classified in two

catégories : i)

^carbon and oxygen, which ^were

always present

at concentrations

equal

^or

higher

^{than their}

solubility

limit at the

melting point

^of

silicon, respectively

5 x

1017

and

1018

^at.

cm-3 [9].

Their concentration

profiles

^were identical in all

samples. Oxygen

^{is a}

by- product

^of the reaction between the silicon melt and the

quartz crucible ;

^{carbon comes}

mainly

^{from the}

support

^{but also from the}

decomposition

^{of the}

residual carbon monoxide

present

^{in the furnace.}

ii)

^metal

impurities,

^{which were not}

always

pre- sent at the same

places

^{of the ribbon but}

which,

when

they

^were

observed,

^{were not}

randomly

^dis-

tributed. Their

position by respect

^to the surface was

always

^the same, i.e.

Cu, Na, Bi, Zn,

^{Ni and Sn are}

in the

top

²⁰

A,

^{Ca and}

Mg

^are

just

^{above the}

Si/Si02 interface,

Fe and Cr are

just

^{under this}

interface and in the bulk. Bulk concentrations have been measured

by

^neutron activation

analysis (NAA)

^{and found to be} in the range of

1012

to

1014

at.

cm-3 showing

^effective

partition

coefficients between the melt and the ribbon in the range

10-1

^to

10-3 [13].

^{In our}

experiments,

^surface

concentrations can be estimated to

1012

to

1014

at.

cm-2 _which, averaged

in the volume

probed by

^the

method,

^would

correspond

^to

1015

to more

than

1017

at. cm-

3.

This

gives

enrichment factor of about 1 000 for RAD ribbons. These

impurities

^were

incorporated

from the silicon melt and came

initially

from the carbon ribbon.

3.2 GETTERING OF IMPURITIES IN BURNT-OFF AND

POCI3

^DIFFUSED RAD. - When the burn-off _pro-

cess of the carbon

support

^is

used,

^{an oxide}

layer

^less

than 1 _jim thick covers the bulk silicon film

[14].

This

layer

^is

always

three-dimensional stoichiometric silica as evidenced

by

AES and EELS

[12,15].

^We

did not find metal

impurities

^{in this}

layer,

^{within the}

limits of detection of XPS which are somewhat poor if the

impurity

^is

equally

distributed in a

large volume (10 ppm).

If the silica

layer

^{is dissolved in diluted HF and the}

sample quickly

^{returned to} the XPS UHV

chamber,

a

layer

of native oxide 10 to 20 A thick

forms,

covered with a

monolayer

^of carbonaceous con-

tamination. In and under this

layer,

^metal

impurities (Bi, Cu, Fe, Cr, Ni)

^{have been found with the same}

profiles

^{as described}

in §

1. Control

samples,

^made

of FZ

single-crystalline silicon,

^{do not show} these

impurities

after heat treatments

equivalent

^{to the}

burn-off process. It is thus clear that

during

^this

process, surface

segregation

^{of metal}

impurities

occurs in RAD ribbons.

We also studied some

samples

^{where a}

N+ /p junction

^had been formed

(details

^{on the}

procedure

can be found in Ref.

[27]).

^The

depth

^{of the}

junction

is 0.6 _03BCm.

Impurities (Cu, ^Fe, Cr)

^were found with a

profile looking

^{much alike the one described}

above,

viz. copper ^near the

surface,

^chromium and iron in the

vicinity

^{of the}

junction.

^{In this} case, the

gettering

effect is

probably

^{due to the}

POCl3

^diffusion

(extrin-

sic

gettering)

^since

impurities

^are located within the

junction.

^This

type

^of

gettering,

^{like the intrinsic one}

leads to lattice strain and to the

injection

^{of silicon}

interstitials,

^{believed to be}

responsible

^{for the}

getter

effect

[16, 17].

5. Model studies on silicon

bicrystals.

It seems clear from the results obtained on RAD

polysilicon

^{ribbons that} oxygen

plays

^an

important

role on the surface

segregation

^of

impurities

^{since a}

clear relation between their presence and ^concen- trations in oxygen has been evidenced

[10].

^{It has}

also been shown that heat treatments could

improve

some of the electrical

properties [7].

^{So there are}

some

analogies

^{with the}

so-called getter

^{effect used} in microelectronics and reviewed

recently by

^Richter

[18].

To test the

role

of oxygen in

gettering (or

^{in surface}

segregation)

^we studied the model materials de- scribed in the

experimental

section. These

crystals,

grown

especially

^{to offer an} alternative material to

polysilicon

in fundamental

investigations

^are

simpler

to

study

since there is

only

^one

grain boundary.

Metalloïdic

impurities (carbon, oxygen)

^{have about}

the same bulk concentrations as in

polysilicon,

^i.e.

between

1017

and

1018

at.

cm-3

as measured

by

^IR

spectrometry [9] ; ^metals, analysed by

^NAA

[19],

have lower concentrations than in

ribbons,

_e.g.

1.8 x

1013

at.

cm-3

for Na and below detection limits for

Cr, Cu, Fe,... (below 1012-1013 at. cm-3).

For this _purpose, we measured

by Auger

^Electron

Spectroscopy (AES)

surface concentration

profiles

of

impurities

^after isochronous

annealing

processes at 450 °C

(1 hour),

^{750 °C}

(1 hour), ^{950 °C} (1 hour)

and 1 250 °C

(5 min)

^{on :}

- clean

surfaces,

- surfaces oxidized at room

temperature

ⁱⁿ

UHV or in

air,

- surfaces oxidized at 450 °C in UHV.

The

annealing temperatures

^{were chosen}

because they

^are characteristic

respectively

^{of the formation}

of the first thermal

donor,

of the second

(new) donor,

^{of the}

precipitation

^{of silica}

and, finally,

^{of its}

dissolution

[20].

5.1 SEGREGATION AT CLEAN SURFACES. ^- In

figure

^{1 are shown surface} concentrations of im-

purities

^{on cleaned} surfaces. In this _case, the surface is cleaned

by

argon

ion-bombardment

after _every heat treatments.

Oxygen

and carbon concentrations reach a maxi-

mum at 750 °C and then decrease. At 750 °C the silicon

Auger

^{fine structure is}

characteristic

of silica.

The thickness of the oxide

layer

is evaluated at about 5

Á. Potassium,

^{not shown in the}

figure, present

after the first

annealing

process is dissorbed before 750 °C.

Chromium and iron concentrations increase stead-

ily

^with

temperature

^{above 750 °C and the}

Auger

spectra

^{of these}

impurities

^{show that}

they

^{are not}

oxidized. Since their local concentration is

higher

than the limit of

solubility

^{it is}

possible

^{that these}

666

Fig.

^{1. -} AES surface concentrations on cleaned silicon surfaces at the initial ^state i and after heat treatments.

a)

^Silicon concentrations,

b) impurity

concentrations.

Fig.

^{2. - AES}

in-depth

concentration

profile

^{of a room-} temperature ^{oxidized silicon}

sample.

The time scale is related to the time of

argon-ion

bombardment ; ^the distance scale

gives

the eroded thickness.

interface. There are still noticeable concentrations in iron as far as 60

 deep.

In

figure

^{3 are} shown the surface concentrations of

impurities, respectively

before and after a room-

impurities

^are

present

^as silicides.

Up

^{to now we do}

not have evidence of this silicide formation

by

^AES.

These results look very much alike those obtained

on RAD ribbons. Potassium is on the

top

^{of the}

segregated layer present

after the heat treatment at 450

°C ;

^{chromium and iron are}

just

below the ultra- thin oxide

layer

^and

they

reach the surface when the

temperature

^is

large enough

^{to allow the} outdiffusion of oxygen ^atoms.

5.2 SEGREGATION AT ROOM-TEMPERATURE OX- IDIZED SURFACES. ^- Two kinds of room

tempera-

ture oxidation have been

investigated :

^{one in am-}

bient

atmosphere (native

oxide obtained after about 20 min in air after

cleaning),

the other in UHV

|p(O2) :

^{2 x}

^{10- 5} Torr,

³

houris 1. They

gave identi- cal results.

A

typical in-depth

concentration

profile

^{of im-}

purities

^{taken on a}

room-temperature

^{oxidized sam-}

ple having

^{a native oxide}

layer

is shown in

figure

^2.

For sake of

clarity,

the concentrations of oxidized silicon are not shown.

Auger spectroscopy

^shows that this oxide is not silica but

SiOx ;

^{this is evidenced}

both

by

^the

Auger

^{fine structure} and the stoichiomet- ry calculated from the

height

of the oxidized silicon

peak

and the oxygen

peak.

^{Nickel is}

present

^{on the}

top

^{of the}

layer ;

chromium and iron reach their maximum concentration at the silicon/silicon oxide

Fig.

^{3. -} AES surface concentration before

(i),

^after

(ox)

a

room-temperature

oxidation in UHV and after heat treatments at

given

temperatures.

a)

^{Silicon concen-}

trations

(left scale)

and thickness of the oxide

(right scale),

b) impurity

concentrations.

temperature ^oxidation

ⁱⁿ

UHV,

^{and after each heat}

treatment. One can see that RT oxidation indeed

brings impurities

^near the surface. In this case,

subsequent

^heat treatments do not

modify drastically

the

impurity

concentrations and the final state, ^after the whole

annealing cycle,

^is

nearly

^{the same as the}

initial _one, before oxidation.

5.3 SEGREGATION AT SURFACES OXIDIZED AT

450 °C. ^-

Figure

^{4 shows the} concentration of

impurities respectively

before and after

oxidation at

450 °C

|p(O2) :

^{2 x}

^10-5 Torr,

³

houris 1 ^and

^after

each heat treatment. The

starting

^{surface was, in this}

case, rather rich in

impurities.

It is

clear, however, that

^oxidation favours the

segregation

^{of more}

impurities

towards the surface.

But as soon as this

surface,

^{covered now with a silica}

layer

¹⁴

 thick,

^{is annealed there is an}

important

decrease of the

impurity

^content. Its minimum is reached at 750 °C.

Fig.

^{4. -} AES surface concentrations before

(i),

^aftei

(ox)

^an oxidation made in UHV at 450 °C and after heal treatments at

given

temperatures.

a)

Silicon and oxyger concentrations

(left scale)

and thickness of the oxide

(righ1 scale), b) impurity

concentrations.

6. Discussion.

Two

points

appear ^{worth to be}

discussed,

^the

segregation

^{of metal}

impurities

^{toward the surface in}

polycrystalline

RAD ribbons or

bicrystals

^{and the}

role of oxygen ^{as a}

driving

^{force for}

gettering.

i)

It is clear from the results that metal

impurities

are

present

^{at much}

higher

concentrations in the

top layers

^{of RAD}

polycrystalline

^ribbons than in the bulk. In

bicrystals,

^surface

segregation

^{of metal}

impurities, probably incorporated

from the melt

during

^the

growth

process, is

observed

after oxida- tion

showing

^also

large

concentrations. It should be

pointed

^{out that the}

spatial

distribution of these

impurities

^{is the same} whatever the oxidation ^tem-

perature

^{is and} whatever the oxide thickness is.

In our

experiments,

^surface concentrations can be estimated to

1012

to

1014

at.

cm-2 which, averaged

ⁱⁿ

the volume

probed by

^{AES or}

XPS,

^{would corre-}

spond

^to

¹⁰¹⁵

^{to more than}

¹⁰¹⁷

^{at. cm-}

^3.

^This

gives enrichment

factors of about 1 000 for RAD ribbons and

about 105

for

bicrystals.

Even if classical diffu- sion or

segregation

coefficients could

explain

^{such a}

large segregation

^{for RAD}

ribbons,

^{it is not the case}

for oxidation of

bicrystals,

^at

room-temperature

^or 450 °C. Intrinsic

gettering

is thus believed to occur in both cases.

It is indeed well known that oxygen

plays

^an

important

role in this

type

^of

gettering

^effect

[18]

and several mechanisms have been described to

explain

the enhanced

diffusion

of oxygen in silicon

[21, 23].

Some of these

authors

^{think that the}

precipitation

^{of oxygen}

injects

^silicon self-intersti- tials

which

^{are able to make}

complexes

^with oxygen and which have a

large diffusivity. ^Then,

^these

complexes

^should

trap

^metal

impurities.

^Model

studies show in fact that the surface structure of the oxide

layer

^is

important

^{in the}

segregation

process.

They

^{are not able to describe}

yet

^{the exact}

mechan-

isms of this enhanced

diffusivity.

We have shown that if

room-temperature

^ox-

idations, leading

^{to a}

SiO,, layer,

^{were able to draw}

impurities

^{out of the}

^bulk, giving

^{rise to a kind of}

intrinsic

getter effect, subsequent

^{heat treatments}

did not

modify

^{the surface}

concentration

of metal

impurities.

^{On the}

contrary,

if there is a ultra-thin

layer

of silica

(Si02 )

^{instead of}

SiOx,

^{the heat}

treatments draw back the

impurities

in the bulk.

This could be related to the

injection

of silicon self- interstitials which would be

possible only

^{because of}

the strain induced

by

the misfit between silica and silicon.

How the final

step (i.e.

^{the diffusion} of silicon- metal

complexes)

^{occurs is not}

yet fully

^understood

even if metal

impurities

^{have been} characterized in the

vicinity

^of

Si02 precipitates [24].

ii)

On another

hand,

^{one can} argue that ^a 12 A

Si02 layer

^is

probably

^{not able to}

inject enough

^Si

self-interstitials in the bulk and it would seem that the role of oxygen is ^not

only

^{to induce intrinsic}

gettering

^{after its}

precipitation,

it is also a chemical

driving

force. We have indeed the evidence

that,

ⁱⁿ

silicon

bicrystals, impurity

diffusion toward the sur-

668

face occurs even if the surface is oxidized at low

temperature (450 °C

^{and even room}

temperature).

Which kind of mechanism is

possible ?

If it is the

affinity

^{of a}

given

^metal

impurity

^to oxygen ^as it was

postulated

ⁱⁿ

[10], why

^{is these}

impurity

^{not bound}

to oxygen ^{as it is} evidenced

by

^{AES ?}

A

possible explanation

^{could be}

^that

^{metal im-}

purities,

^{such as iron and chromium are}

only catalysts

for silicon oxidation :

they

^{would favour an}

oxygen

dissociative

adsorption

process ^on silicon surfaces.

Viefhaus and Rossow

[25]

^{have found that in a Fe-}

Si 6 at. %

alloy,

^surface

segregated ^silicon

^{is more}

easily ^oxidized

than pure silicon. The same obser- vation has been made

by Mosser,

Srivastava and Carrière

[26]

^{who noticed that} Fe-Si oxidation at

temperatures higher

^{than 500 °C would lead to a}

silicon dioxide

segregated overlayer topping

^unox-

idized iron. The formation of silicides has been also evidenced in similar

experiments

^{as in the}

POCl3

getter

process

[27].

^{In our} case, ^we could not show

by

^{AES or XPS that} these silicides

really

^exist.

7. Conclusion.

Surface

analytical

methods have been used to investi-

gate

^{the diffusion and}

segregation

^{of metal}

impurities

at the surface of RAD

polysilicon

^{ribbons and silicon}

bicrystals. High

^surface concentrations of chro-

mium,

copper,

iron, potassium,

^iron and nickel have been evidenced. We believe that this

segregation

^is

possible through

^{intrinsic and extrinsic}

getter

pro-

cesses induced

by

^the

precipitation

^{of the dissolved}

oxygen atoms

during

^{the burn-off} process of the carbon

support

^and

during

^the

POCl3

^diffusion

cycle

for RAD ribbons. These effects have been demon- strated for silicon

bicrystals

when the surface is oxidized at

temperatures higher

^{than 450 °C and}

then annealed at 750 °C.

However,

^{it is}

thought

^that

oxygen does not act

only

^{as a nucleus for}

giving straining Si04 ^units, leading

^{to the}

injection

^{of fast-}

diffusing silicon

self-interstitials

but

also as a chemi- cal

driving

^{force able to} induce the

segregation

^of

metal

impurities

^{in the area rich in} oxygen

through

^a

catalytic

^{mechanism in which} oxygen, silicon and metal

impurities

^are

strongly cooperative.

We have shown that intrinsic and extrinsic

getter

effects or surface

segregation

may be effective in

polysilicon technology.

^{It is thus}

possible ^to

^take

advantage

^{of the thermal} treatments

occurring

^either

during

^the

growth

^or

during

^{the diffusion} process ^to

improve

the electrical

properties

^of

polysilicon

^solar

cells.

Acknowledgments.

The authors wish to thank Dr. Belouet from C.G.E.

for the

supply

^{of the}

polysilicon

^RAD

samples

^and

the ^«

Groupe

^Silicium

Polycristallin »

for the many

enlighting

discussions that it has initiated.

The work was made under the financial

support

^of PIRSEM and COMES.

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