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Role of oxygen in surface segregation of metal impuritiesin 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 impuritiesin silicon poly-and bicrystals. Revue de Physique Appliquee, 1987, 22 (7), pp.663-669.�10.1051/rphysap:01987002207066300�. �jpa-00245593�

663

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 métalliquessituées sur des rubans de silicium polycristallin. L’oxygène et les traitements thermiques semblent une forcemotrice 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 typeCzochralski. Nous avons étudié deux facteurs principaux de ségrégation superficielle : le rôle d’une couched’oxyde très mince et celui de traitements thermiques. Nous avons remarqué que le maximum de purificationdes surfaces était obtenu après le recuit à 750 °C d’une surface préalablement oxydée à 450 °C. Nous avonsrelié cela à la formation d’amas de SiO, suivie d’une coalescence donnant des unités de type SiO4 entraînantl’injection d’auto-interstitiels de silicium dans le réseau.

Abstract. 2014 Metal impurities at surfaces of polycrystalline silicon ribbons have been characterized by surfacesensitive methods. Oxygen and heat treatments were found to be a driving force for surface segregation ofthese impurities. To better analyse their influence and their possible incidence in gettering, model studies wereundertaken on Czochralski grown silicon bicrystals. Two main factors of surface segregation have beenstudied : the role of a ultra-thin oxide layer and the effect of heat treatments. The best surface purification wasobtained after an annealing process at 750 °C of a previously oxidized surface at 450 °C. This was related to theformation of SiO clusters, followed by a coalescence of SiO4 units leading to the subsequent injection of siliconself-interstitials in the lattice.

Revue Phys. Appl. 22 (1987) 663-669 JUILLET 1987

Classification

Physics Abstracts61.70W - 66.30 - 68.60J

1. Introduction.

Polycrystalline silicon, often referred to as

« polysilicon », is obtained either by casting ingotsvia a Bridgman-like growth process or by setting upribbon technologies based on shaped crystal growth.These technologies are developed to reduce the

cost of terrestrial solar cells by minimizing siliconconsumption and/or by using cheaper, degradedsilicon as starting material. Possible applications tomicroelectronics should be also born in mind for thefuture.

Final materials obtained by such methods have alarge amount of structural, chemical and electricaldefects. Thus, the main objective of research in thelast decade has been to identify and classify thesedefects, to investigate which ones are the mostdetrimental 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 activeones.

For example, it has been demonstrated that im-

plantation of molecular or atomic hydrogen improvesthe electrical properties such as the diffusion lengthin both ingots and ribbons [1-5]. Diffusion at lowtemperature of selected impurities such as Cu and Alinto polycrystalline Si was also found to improve theminority carrier diffusion length [6].

In the case of ribbon technologies, it has been alsoshown that thermal treatments could, in certain

cases, improve the diffusion length [4, 7]. The im-provements have been related to intrinsic getteringeffects in which fast-diffusing species are offeredenergetically favorable sites outside the electricallyactive region of the material.

Surface physics methods have been thought to beuseful in this perspective since the active area ofphotovoltaïc devices are located in the top fewmicrometers. Is it possible to draw detrimental

45

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

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impurities out of the active region ? Is it relevant touse some of the classical gettering processes in

polysilicon technology ? To answer these questionswe applied surface science techniques to understandparticularly the role of oxygen and heat treatmentsin the segregation of metal impurities towards thesurface of silicon samples.

In this paper, after having briefly recalled theRAD growth process and the experimental set-up,we shall sum up the early surface analytical resultsfound on RAD ribbons, then we shall present recentinvestigations on model systems (silicon bicrystals).No attempt has been made, however, to measuredifferences in electrical properties during thesemodel studies since ultra-high vacuum is needed forsurface analysis and prevent one to realize easilyreliable electrical measurements.

2. Experimental.

2.1 SAMPLES. - RAD silicon ribbons were obtained

by a shaped crystal growth in which a carbon supportis continuously pulled through a p-doped silicon meltvia a slot located at the bottom of a RF inductionheated quartz crucible. Details about the varioustechnical requirements and achievements of the

process can be found in [8]. After growth, thecarbon 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 outsidefaces are oxidized and the inner faces are coveredwith a discontinuous SiC layer. The thicknesses ofthe oxide layers range from 0.2 to 1 03BCm. These twooverlayers are chemically etched off before makingN+ /p homojunctions by a classical POCl3 diffusionprocess at 850 °C. In most cases, we studied samplesas 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 theLETI (Grenoble). Oxygen and carbon concen-

trations are in the 1017 at.cm-3 range and can locallyexceed 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 perpendicularto the long axis of the bar,

ii) the easily cleavable (111) plane, perpendicularto this axis.

Grooves were made either right along the intersec-tion of the grain boundary plane with the bar oralong a (111) plane. It was then possible to cleaveour samples in UHV and to get clean virgin siliconsurfaces and, sometimes, the boundary plane itself.In this paper, we shall discuss only incidentally therole of the grain boundary which is not the majorparameter of interest.

The samples were etched in dilute HF and nothermal treatment applied prior their introduction inthe UHV chamber

3. Analytical methods.

Impurity concentration profiles of RAD sampleswere determined by means of X-Ray PhotoelectronSpectroscopy (XPS) in a VG ESCA III apparatus.Depth profiles down to 1 )JLm were obtained byargon-ion sputter etching the sample step by step(about 100 Â per ion bombardment). The sputteredarea had a larger diameter than the analyzed one.

Silicon bicrystals were analysed by Auger ElectronSpectroscopy (AES) using a RIBER CMA in aUHV chamber fitted with several accessories (argonion bombardment, cleavage system, Knudsen cellsfor metal evaporation, ...). The diameter of the spotis 10 03BCm ; too large to investigate the grain bound-aries, it allows to study surface domains about30 03BCm large.

Since the low-energy Auger peaks of metal im-purities are superimposed to the silicon Auger LVVfine structure, we used the LMM peaks of Cr, Feand Ni at respectively 529, 703, and 848 eV to deriveconcentrations. Of course, the differences in meanfree paths between the various Auger transitionshave been taken into account in these calculations.Concentrations are given in atom per cent and areaveraged over thicknesses of about 8 Â.The ratioes between the high- and low-energy

peak intensities have been also studied to precise thelocation of given impurities with respect to thesurface.

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 ribbonsand we summarize here the main results [10]. At thattime burning off the carbon support was not yet inuse. On the samples received as grown, surface

analysis showed oxide layers thicknesses of whichwere ranging from 200 to 1 000 A, depending on thegrowth conditions. The oxide was generally not

three-dimensional silica as observed by AES andXPS [11,12].

Impurities could be classified in two catégories :i) carbon and oxygen, which were always present

at concentrations equal or higher than their solubilitylimit at the melting point of silicon, respectively5 x 1017 and 1018 at. cm-3 [9]. Their concentrationprofiles were identical in all samples. Oxygen is a by-product of the reaction between the silicon melt andthe quartz crucible ; carbon comes mainly from thesupport but also from the decomposition of theresidual carbon monoxide present in the furnace.

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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 wasalways the same, i.e. Cu, Na, Bi, Zn, Ni and Sn arein the top 20 A, Ca and Mg are just above theSi/Si02 interface, Fe and Cr are just under thisinterface and in the bulk. Bulk concentrations havebeen measured by neutron activation analysis(NAA) and found to be in the range of 1012 to1014 at. cm-3 showing effective partition coefficientsbetween the melt and the ribbon in the range10-1 to 10-3 [13]. In our experiments, surfaceconcentrations can be estimated to 1012 to

1014 at. cm-2 which, averaged in the volume probedby the method, would correspond to 1015 to morethan 1017 at. cm- 3. This gives enrichment factor ofabout 1 000 for RAD ribbons. These impurities wereincorporated from the silicon melt and came initiallyfrom 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 lessthan 1 jim thick covers the bulk silicon film [14].This layer is always three-dimensional stoichiometricsilica as evidenced by AES and EELS [12,15]. Wedid not find metal impurities in this layer, within thelimits of detection of XPS which are somewhat poorif the impurity is equally distributed in a largevolume (10 ppm).

If the silica layer is dissolved in diluted HF and thesample 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 sameprofiles as described in § 1. Control samples, madeof FZ single-crystalline silicon, do not show theseimpurities after heat treatments equivalent to theburn-off process. It is thus clear that during thisprocess, surface segregation of metal impuritiesoccurs in RAD ribbons.

We also studied some samples where a N+ /pjunction had been formed (details on the procedurecan be found in Ref. [27]). The depth of the junctionis 0.6 03BCm. Impurities (Cu, Fe, Cr) were found with aprofile looking much alike the one described above,viz. copper near the surface, chromium and iron inthe vicinity of the junction. In this case, the getteringeffect is probably due to the POCl3 diffusion (extrin-sic gettering) since impurities are located within thejunction. This type of gettering, like the intrinsic oneleads to lattice strain and to the injection of siliconinterstitials, believed to be responsible for the gettereffect [16, 17].

5. Model studies on silicon bicrystals.

It seems clear from the results obtained on RADpolysilicon ribbons that oxygen plays an importantrole on the surface segregation of impurities since aclear relation between their presence and concen-trations in oxygen has been evidenced [10]. It hasalso been shown that heat treatments could improvesome of the electrical properties [7]. So there aresome analogies with the so-called getter effect usedin microelectronics and reviewed recently by Richter[18].To test the role of oxygen in gettering (or in surfacesegregation) we studied the model materials de-scribed in the experimental section. These crystals,grown especially to offer an alternative material topolysilicon in fundamental investigations are simplerto study since there is only one grain boundary.Metalloïdic impurities (carbon, oxygen) have aboutthe same bulk concentrations as in polysilicon, i.e.between 1017 and 1018 at. cm-3 as measured by IRspectrometry [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 limitsfor Cr, Cu, Fe,... (below 1012-1013 at. cm-3).For this purpose, we measured by Auger Electron

Spectroscopy (AES) surface concentration profilesof impurities after isochronous annealing processesat 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 becausethey are characteristic respectively of the formationof the first thermal donor, of the second (new)donor, of the precipitation of silica and, finally, of itsdissolution [20].

5.1 SEGREGATION AT CLEAN SURFACES. - Infigure 1 are shown surface concentrations of im-purities on cleaned surfaces. In this case, the surfaceis cleaned by argon ion-bombardment after everyheat treatments.

Oxygen and carbon concentrations reach a maxi-mum at 750 °C and then decrease. At 750 °C thesilicon Auger fine structure is characteristic of silica.The thickness of the oxide layer is evaluated at about5 Á. Potassium, not shown in the figure, presentafter the first annealing process is dissorbed before750 °C.Chromium and iron concentrations increase stead-

ily with temperature above 750 °C and the Augerspectra of these impurities show that they are notoxidized. Since their local concentration is higherthan 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 Â 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 donot 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 thesegregated layer present after the heat treatment at450 °C ; chromium and iron are just below the ultra-thin oxide layer and they reach the surface when thetemperature is large enough to allow the outdiffusionof 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 about20 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 oxidizedsilicon are not shown. Auger spectroscopy showsthat this oxide is not silica but SiOx ; this is evidencedboth by the Auger fine structure and the stoichiomet-ry calculated from the height of the oxidized siliconpeak and the oxygen peak. Nickel is present on thetop of the layer ; chromium and iron reach theirmaximum concentration at the silicon/silicon oxide

Fig. 3. - AES surface concentration before (i), after (ox)a room-temperature oxidation in UHV and after heattreatments at given temperatures. a) Silicon concen-

trations (left scale) and thickness of the oxide (right scale),b) impurity concentrations.

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temperature oxidation in UHV, and after each heattreatment. One can see that RT oxidation indeed

brings impurities near the surface. In this case,

subsequent heat treatments do not modify drasticallythe impurity concentrations and the final state, afterthe whole annealing cycle, is nearly the same as theinitial one, before oxidation.

5.3 SEGREGATION AT SURFACES OXIDIZED AT

450 °C. - Figure 4 shows the concentration of

impurities respectively before and after oxidation at450 °C |p(O2) : 2 x 10-5 Torr, 3 houris 1 and aftereach heat treatment. The starting surface was, in thiscase, 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 importantdecrease 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 healtreatments at given temperatures. a) Silicon and oxygerconcentrations (left scale) and thickness of the oxide (righ1scale), b) impurity concentrations.

6. Discussion.

Two points appear worth to be discussed, the

segregation of metal impurities toward the surface inpolycrystalline RAD ribbons or bicrystals and therole of oxygen as a driving force for gettering.

i) It is clear from the results that metal impuritiesare present at much higher concentrations in the toplayers of RAD polycrystalline ribbons than in thebulk. 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 bepointed out that the spatial distribution of theseimpurities is the same whatever the oxidation tem-

perature is and whatever the oxide thickness is.

In our experiments, surface concentrations can beestimated to 1012 to 1014 at. cm-2 which, averaged inthe volume probed by AES or XPS, would corre-spond to 1015 to more than 1017 at. cm- 3. This givesenrichment factors of about 1 000 for RAD ribbonsand about 105 for bicrystals. Even if classical diffu-sion or segregation coefficients could explain such alarge segregation for RAD ribbons, it is not the casefor oxidation of bicrystals, at room-temperature or450 °C. Intrinsic gettering is thus believed to occur inboth cases.

It is indeed well known that oxygen plays animportant 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 oxygenand 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 drawimpurities out of the bulk, giving rise to a kind ofintrinsic getter effect, subsequent heat treatmentsdid not modify the surface concentration of metalimpurities. On the contrary, if there is a ultra-thinlayer 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 ofthe strain induced by the misfit between silica andsilicon.

How the final step (i.e. the diffusion of silicon-metal complexes) occurs is not yet fully understoodeven if metal impurities have been characterized inthe vicinity of Si02 precipitates [24].

ii) On another hand, one can argue that a 12 ASi02 layer is probably not able to inject enough Siself-interstitials in the bulk and it would seem thatthe role of oxygen is not only to induce intrinsicgettering after its precipitation, it is also a chemicaldriving force. We have indeed the evidence that, insilicon 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 waspostulated in [10], why is these impurity not boundto oxygen as it is evidenced by AES ?A possible explanation could be that metal im-

purities, such as iron and chromium are only catalystsfor silicon oxidation : they would favour an oxygendissociative adsorption process on silicon surfaces.

Viefhaus and Rossow [25] have found that in a Fe-Si 6 at. % alloy, surface segregated silicon is moreeasily oxidized than pure silicon. The same obser-vation has been made by Mosser, Srivastava andCarrière [26] who noticed that Fe-Si oxidation at

temperatures higher than 500 °C would lead to asilicon dioxide segregated overlayer topping unox-idized iron. The formation of silicides has been alsoevidenced in similar experiments as in the POCl3getter process [27]. In our case, we could not showby 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 impuritiesat the surface of RAD polysilicon ribbons and siliconbicrystals. High surface concentrations of chro-mium, copper, iron, potassium, iron and nickel havebeen evidenced. We believe that this segregation is

possible through intrinsic and extrinsic getter pro-cesses induced by the precipitation of the dissolvedoxygen atoms during the burn-off process of thecarbon support and during the POCl3 diffusion cyclefor RAD ribbons. These effects have been demon-strated for silicon bicrystals when the surface isoxidized at temperatures higher than 450 °C andthen annealed at 750 °C. However, it is thought thatoxygen does not act only as a nucleus for givingstraining 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 ofmetal impurities in the area rich in oxygen through acatalytic mechanism in which oxygen, silicon andmetal impurities are strongly cooperative.We have shown that intrinsic and extrinsic getter

effects or surface segregation may be effective inpolysilicon technology. It is thus possible to takeadvantage of the thermal treatments occurring eitherduring the growth or during the diffusion process toimprove the electrical properties of polysilicon solarcells.

Acknowledgments.

The authors wish to thank Dr. Belouet from C.G.E.for the supply of the polysilicon RAD samples andthe « Groupe Silicium Polycristallin » for the manyenlighting discussions that it has initiated.The work was made under the financial support of

PIRSEM and COMES.

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