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 Appliquee, 1987, 22 (7), pp.663-669.
<10.1051/rphysap:01987002207066300>. <jpa-00245593>
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Submitted on 1 Jan 1987
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Revue
Phys. Appl.
Classification
Physics Abstracts
61.70W - 66.30
22
-
(1987)
663-669
Amarray and J.
segregation of metal impurities
P. Deville
in silicon
poly-
(*)
Equipe d’Etude des Surfaces, UA 795 du C.N.R.S.,
Université Louis-Pasteur, 4, rue Blaise-Pascal, 67000
(Reçu
663
1987
68.60J
Role of oxygen in surface
and bicrystals
E.
JUILLET
le 6 octobre 1986, révisé le 25
mars
Strasbourg, France
1987, accepté le 9 avril 1987)
Nous avons caractérisé, au moyen des méthodes d’analyse des surfaces, les impuretés métalliques
Résumé.
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.
2014
Metal impurities at surfaces of polycrystalline silicon ribbons have been characterized by surface
Abstract.
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.
2014
out methods for
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
REVUE DE PHYSIQUE
APPLIQUÉE. - T. 22,
should be sent.
passivation of the electrically
active
ones.
For example, it has been demonstrated that implantation 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 improvements 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
N° 7, JUILLET 1987
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01987002207066300
45
664
out of the active
region ? Is it relevant to
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.
impurities
use
2.
some
Experimental.
RAD silicon ribbons were obtained
2.1 SAMPLES.
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 atmosphere at temperatures ranging from 1000 °C to
1200 °C during 1 hour, resulting in two self-supporting 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 concentrations are in the 1017 at.cm-3 range and can locally
exceed the limit of solubility at equilibrium [9]. Barlike 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
long axis of the bar,
ii) the easily cleavable (111) plane, perpendicular
to the
The samples were etched in dilute HF and no
thermal treatment applied prior their introduction in
the UHV chamber
3.
methods.
concentration profiles of RAD samples
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 boundaries, it allows to study surface domains about
30 03BCm large.
Since the low-energy Auger peaks of metal impurities 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.
Impurity
were
4. Results
on
RAD
polycrystalline
ribbons.
4.1 GENERAL OUTLINE OF THE IMPURITY DISTRIBUTION IN RAD RIBBONS.
We previously described 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 :
-
to this axis.
Grooves were made either right along the intersection 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.
Analytical
i)
carbon and oxygen, which
were
always present
at concentrations equal or higher than their solubility
limit at the melting point of silicon, respectively
x 1017 and 1018 at. cm-3 [9]. Their concentration
profiles were identical in all samples. Oxygen is a byproduct of the reaction between the silicon melt and
the quartz crucible ; carbon comes mainly from the
support but also from the decomposition of the
5
residual carbon monoxide present in the furnace.
665
ii)
metal
impurities, which were not always presame places of the ribbon but which,
when they were observed, were not randomly distributed. 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
sent at the
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
POCI3
OF IMPURITIES IN BURNT-OFF AND
RAD. - When the burn-off proof 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
DIFFUSED
cess
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 contamination. 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
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 (extrinsic 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].
junction
5. Model studies
It
on
silicon
bicrystals.
clear from the results obtained on RAD
that oxygen plays an important
role on the surface segregation of impurities since a
clear relation between their presence and concentrations 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
seems
polysilicon ribbons
[18].
To test the role of oxygen in gettering (or in surface
segregation) we studied the model materials described 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
For this purpose, we measured by Auger Electron
Spectroscopy (AES) surface concentration profiles
of impurities after isochronous annealing processes
at. cm-3).
450 °C (1 hour), 750 °C
and 1 250 °C (5 min) on :
at
(1 hour), 950 °C (1 hour)
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
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].
they
are
5.1 SEGREGATION AT CLEAN SURFACES.
In
figure 1 are shown surface concentrations of impurities on cleaned surfaces. In this case, the surface
is cleaned by argon ion-bombardment after
every
heat treatments.
Oxygen and carbon concentrations reach a maximum 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 steadily 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.
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
1.
AES surface concentrations on cleaned silicon
surfaces at the initial state i and after heat treatments.
a) Silicon concentrations, b) impurity concentrations.
Fig.
-
gives
the eroded thickness.
interface. There are still noticeable concentrations in
iron as far as 60 Â deep.
In figure 3 are shown the surface concentrations of
impurities, respectively before and after a room-
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 ultrathin oxide layer and they reach the surface when the
temperature is large enough to allow the outdiffusion
of oxygen atoms.
impurities
5.2 SEGREGATION
AT
ROOM-TEMPERATURE
OX-
Two kinds of room temperature oxidation have been investigated : one in ambient 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 identical results.
A typical in-depth concentration profile of impurities taken on a room-temperature oxidized sample 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 stoichiometry 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
IDIZED SURFACES.
-
Fig. 3. AES surface concentration before (i), after (ox)
a room-temperature oxidation in UHV and after heat
treatments at given temperatures. a) Silicon concentrations (left scale) and thickness of the oxide (right scale),
b) impurity concentrations.
-
667
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 oxidationat
450 °C |p(O2) : 2 x 10-5 Torr, 3 houris 1and 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 Â thick, is annealed there is an important
decrease of the impurity content. Its minimum is
reached at 750 °C.
-
It is clear from the results that metal impurities
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 oxidation showing also large concentrations. It should be
pointed out that the spatial distribution of these
impurities is the same whatever the oxidation temperature is and whatever the oxide thickness is.
i)
are
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 correspond 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 diffusion 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-interstitials 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 mechanisms of this enhanced diffusivity.
We have shown that if
room-temperature
ox-
SiO,, layer, were able to draw
impurities out of the bulk, giving rise to a kind of
intrinsic getter effect, subsequent heat treatments
idations, leading
did not
modify
to a
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 selfinterstitials which would be possible only because of
the strain induced by the misfit between silica and
AES surface concentrations before (i), aftei
made in UHV at 450 °C and after heal
oxidation
(ox)
treatments at given temperatures. a) Silicon and oxyger
concentrations (left scale) and thickness of the oxide (righ1
scale), b) impurity concentrations.
Fig.
4.
-
an
6. Discussion.
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.
Two
silicon.
How the final step (i.e. the diffusion of siliconmetal 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
if the surface is oxidized at low
possible through intrinsic and extrinsic getter protemperature). cesses induced by the precipitation of the dissolved
Which kind of mechanism is possible ? If it is the oxygen atoms during the burn-off process of the
affinity of a given metal impurity to oxygen as it was carbon support and during the POCl3 diffusion cycle
postulated in [10], why is these impurity not bound for RAD ribbons. These effects have been demonto oxygen as it is evidenced by AES ?
strated for silicon bicrystals when the surface is
A possible explanation could be that metal im- oxidized at temperatures higher than 450 °C and
purities, such as iron and chromium are only catalysts then annealed at 750 °C. However, it is thought that
for silicon oxidation : they would favour an oxygen oxygen does not act only as a nucleus for giving
dissociative adsorption process on silicon surfaces.
straining Si04 units, leading to the injection of fastViefhaus and Rossow [25] have found that in a Fe- diffusing silicon self-interstitials but also as a chemical driving force able to induce the segregation of
Si 6 at. % alloy, surface segregated silicon is more
metal impurities in the area rich in oxygen through a
easily oxidized than pure silicon. The same observation has been made by Mosser, Srivastava and catalytic mechanism in which oxygen, silicon and
Carrière [26] who noticed that Fe-Si oxidation at metal impurities are strongly cooperative.
We have shown that intrinsic and extrinsic getter
temperatures higher than 500 °C would lead to a
effects
or surface segregation may be effective in
silicon dioxide segregated overlayer topping unoxidized iron. The formation of silicides has been also polysilicon technology. It is thus possible to take
evidenced in similar experiments as in the POCl3 advantage of the thermal treatments occurring either
the growth or during the diffusion process to
getter process [27]. In our case, we could not show during
improve the electrical properties of polysilicon solar
by AES or XPS that these silicides really exist.
cells.
occurs even
temperature (450 °C and
even room
7. Conclusion.
Acknowledgments.
Surface analytical methods have been used to investigate the diffusion and segregation of metal impurities
at the surface of RAD polysilicon ribbons and silicon
bicrystals. High surface concentrations of chromium, copper, iron, potassium, iron and nickel have
been evidenced. We believe that this segregation is
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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