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MICROSTRUCTURE AND ELECTRICALPROPERTIES OF PLASMA SPRAYED

POLYCRYSTALLINE SILICONR. Suryanarayanan, G. Zribi

To cite this version:R. Suryanarayanan, G. Zribi. MICROSTRUCTURE AND ELECTRICAL PROPERTIES OFPLASMA SPRAYED POLYCRYSTALLINE SILICON. Journal de Physique Colloques, 1982, 43 (C1),pp.C1-375-C1-380. �10.1051/jphyscol:1982150�. �jpa-00221807�

JOURNAL DE PHYSIQUE

Colloque Cl, supplément au n°10, Tome 43, octobre 1982 page Cl-375

MICROSTRUCTURE AND ELECTRICAL PROPERTIES OF PLASMA SPRAYED

POLYCRYSTALLINE SILICON

R. Suryanarayanan and G. Zribi

Laboratoire de Physique des Solides, C.N.R.S., 92190 Meudon Bellevue, France

Résigné. Une des méthodes pour réduire le coût de la fabrication des piles solaires au Silicium est d'utiliser le matériau polycristallin.Parmi les différentes techniques disponibles pour l'obtenir, la technique de TORCHE A PLASMA, est intéressante pour des raisons suivantes: (a) elle est rapide-on obtient 200yUm en moins d'une minute sur une surface quelconque de 10 cm2

(b) la dimension de grain d pourrait être aussi grande que 100/1 m. Nous avons obtenu le Silicium polycristallin de type n et p par cette technique et étudié l'influence de la taille de poudre, la pureté et la microstructure sur les propriétés électriques. La microstructure est hétérogène avec 10 < d^100ytlm. Les analyses qualitatives et quantitatives des impuretés de surface et en volume ont été obtenues. L'analyse à la sonde ionique montre une distribution homogène de P sur la surface et en volume de notre échantillon. La mobilité , à la suite d'un traitement thermique est 10 cm2/V/sec.

Abstract. One of the ways to reduce the cost of Silicon solar cells is to fabricate them from polycrystalline material. Among the various techniques available to obtain this, PLASMA SPRAY seems to be attractive for the foll­owing reasons: (a) it is a rapid method capable of giving 200/tm in less than a minute over any surface of 10 cm2 (b) the grain size d could be as large as 100>im. We have obtained n and p type poly Si using this technique and studied the influence of powder particle size, purity and micro structure on the electrical properties. The microstructure is heterogeneous with 10 < d ̂ 100/Am. Qualitative and quantitative surface and bulk analyses were performed. SIMS indicate a homogeneous distribution of P on the surface and inside our sample. The mobility following a heat treatment is 10 cm2/v/sec.

1. Introduction. Silicon has been and will continue to be, no doubt, the most imp­ortant feed material for the electronic industry and especially for the solar cells. However, to-day single crystal Si and solar cells made out of them are generally considered to be expensive. Thus , a great amount of effort is being directed in many laboratories to reduce the cost (1). One approach would be to reduce the cost of preparation of Si, say, by obtaining polycrystalline Si (poly Si) of 50 - 200/im thick. We have attempted to achieve this goal by using the plasma spray technique (PLAST). A preliminary report on PLAST was recently published though details were not given either on the impurity content or on the electrical properties of poly Si (2). We present here the preparation, bulk and surface characteisation, microstru­cture and electrical properties of poly Si deposits obtained by PLAST.

2. Principle of the technique. The PLAST is a well known technique in the aerona­utic industry and elsewhere to obtain reliable polycrystalline coatings/deposits of metals, alloys and ceramics (3). The apparatus consits mainly of a hand held plasma torch (fig.1) in which an electrical discharge is created between W and Cu electrodes (50V, 600 A) in the presence of a gas like argon. A "plasma flame" is thus obtained where temperatures can reach of the order of 10000 K. The material to be deposited is taken in a powder form and is injected into this flame by a neutral

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982150

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Fig.1. ( l e f t ) Cross section of the LETCO plasma spray gun. (right) Actual deposi- tion. A i r cooling of the substrate through 3 tubes can be seen.

gas l ike argon. The par t ic les pf the powder.at-e rapidly melted and accelerated t o produce a stream of molten par t ic les that are projected onto a substrate placed a t 8 t o 15 cms i n front of the torch and cooled by an a i r stream from behind. Large t-erature and velocity gradients exis t i n the flame with peak values rang- ing upto 10000 K and 1000 m/sec respectively (2). The velocity of the par t ic les impinging on the substrate could range from 100 t o 400 m/sec. The deposit or coat- ing is fonned by the build-up of successive layers of l iquid droplets which f l a t t e n and sol id i fy on impact to give the lenticular microstructure usually observed.

3. Advantages of PLAST. This technique offers several advantages some of which are l i s t ed below': - powder, often a by product i n the raw material preparation, can be used as the

s tar t ing material - ease and rapidity of p re~ara t ion resulting i n a high growth ra te i n a continuous

manner extendable to large surfaces e.g., 300pm i n 30 sec on 200 rnmz - any substrate can be used - no serious "crucible" ef fects - good adhesion to the substrate - though the deposit has a rough surface, it can be eas i ly polished - maxim~nn grain s ize could be 100,Um - lends i t s e l f fo r automation with re la t ively low level of technical s k i l l

4. Preparation conditions. ??e have used a commercially available plasma torch of type METCO 7 M B with a coFDer nozzle of type GH for our experiments. In PLAST, there are more than 100 variables which can be varied and which are interrelated(2). We have mainly concentrated on arc voltage, current, powder flow,particle s ize and carr ier gas flow ( which carries the s i l icon powder into the plasma flame) i n order t o obtain f a i r l y reproducible S i de-posits of 200 to 300pm thick on 100 mm2 Al2O3 substrates i n about 20 sec. The deposition conditions were normally as follows: arc voltage 65 V, arc current 500 A, argon 90 cf t /h , hydrogen 10 t o 1 5 cf t /h , powder flow 1 2 gm/ min with argon as the carr ier gas. In addition to A120 , a

variety of substrates l i k e Fe,Cu,Al,Si and graphite were a lso tr ied. The sugstrates except Si , were usually sand blasted with 177,um corindon powder and well rinsed with alcohol or trichloroethylene. A s for Si , they were well polished single cryst- a l s o r Wacker polycrystals.The substrates were a i r cooled from behind and kept a t 8 to 10 cm i n front of the torch which was hand held. They were generally pre-

(a) (b) Fig. 2. SEM photos of polished and etched samples. (a) scale 2n) powder used 80 < +< 125pm ; (b) scale powder used 80 ( < 250,um.

heated for about 5 to 10 secs when the maxim temperature was w 800 C. A successful deposit on polished Si substrate was obtained only by preheating for a longer time. In this paper we confine ourselves to deposits obtained on A1203. Sin e we could procure high purity Si 6N only in the form of chuncks, size reductiozand sieving were carried out to obtain particles ranging in size from 10 to 40, 40 to 80, 80 to 125 and 125 to 290pm. Silicon of 3N5 purity available in powder form was also used. In situ doping was tried by adding suitable quantities of P or B2O3 directly to the powder. By properly cooling the substrates, we were also able to detatch the substrate to obtain Si ribbons of 20X 5 X 0.3 nun.

5. Results and discussion . 5.1. Mechanical properties. The samples thus obtained including the ribbons were easy to handle and could stand the mechanical treatnent given while polishing. About 20 to 80pm of the deposit was usually lost on polishing. The porosity was estimated to be less than 8 % from Cambridge stero scan images.

5.2, Microstructure. The microstructure of plasma sprayed deposits of metals and oxides have been reported in detail (2). The grain size d is generally found to depend strongly on the powder particle size $ . However, for +7/ 125pm , d was always less than 125,um. The particles on impinging the substrate seem to break into smaller droplets which result in a decrease of d. For 80 < 9 < 125 m, some samples (polished and etched with acids) showed regions containing grains of 7 00p m In general, a heterogeneous microstructure was observed with 1 0 4 d ,< 1 00p m for 80< +<125~m (fig.2). Though the histogram of grainsize distribution was not performed, it is estimated that at lest 50 % had d close to 50)lm. Perhaps a search for better control in plasna parameters and avoiding irregular shapes of the starting powder would be necessary to improve the uniformity in the micro- structure. Some amount of cracking on the polished surface was observed either due to increased porosity or due to abrasion caused by foreign particles like Fe and Ni in the deposit while polishing.

5.3. X- ra analysis. The samples on A1203 and the ribbons were found to be well crysta-tensities of different (hkl) coinciding fairly well those reported in the ASP4 cards. It is possible that samples deposited on Si could show a texture which is currently under investigation.

5.4. Surface and bulk anqlyses. The samples were qualitatively and quantitatively analysed for impurities using EPMA ,AUGER, S IMS and emission spectroscopy. Table I., lists some of the impurities by emission spectroscopy in the starting ingot Si 6N before crushing and in the sample which was scraped off the A1203

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Table . 1. Quantitative inpurity analysis by emission spectroscopy of the start ing material S i 6N and the plasma sprayed sample ( i n atppm ) . Element A1 B Be B i Ca Cd Co C r Cu Fe Mo Na N i Pb Ti V Zn

Starting m a t e r i a l s i < 5 <2<0 .4 < 2 < 2 0 < 5 (5 <2 <5<5 (2<40 < 3 < 3 (2<2<20

Plasma sprayed sample 800 3C0.4 (2 2 0 ~ 5 <5 10 9 440 C2<40 103 3 8<2<20

substrate. We note a dramatic increase i n Fe,A1 and N i . The increase i n Fe contentis perhaps due t o the crushing procedure adopted wheras the increase i n A1 and N i can be explained due to contamination from the industrial environment i n which the plasma spray was done. It should. be pointed out that the plasma spray equipment has been used for years to make Ni-A1 coatings. I t is also possible that A 1 conta- mination could come from (a) some moving par ts of the powder distributor due t o erosion by S i powder for+780,um and (b) f r m the substrate due t o scraping. The increase i n Cu content is mostly due to electro erosion of Cu electrode. Eowever, images obtained by SIMS and EPMA reveal irregular presence and distribution of O,Fe,Al,Ni and Cu.

The vola t i le impurities l ike cer ta in carbohydrides originally pre- sent i n the s tar t ing material are to ta l ly absent as shown by SIMS.Further, K,Na and Ca were present mostly on the surface. The SIMS prof i le of a P doped sample revealed a more o r less hmogeneous distribution of P on the surface and inside the

bulk of the sample ( f i g .3) .

Fig. 3. SIMS prof i le of P i n the sample as a function of sputtering time. Intensity i n arbi t rary units.

Preliminary Auger spectra indicate an oxidised surface and one sample contained as much as 4 a t % of oxygen. I t seems the f i r s t layer an& the top layer are heavily oxidised. The s tar t ing material, both the ingot and the powder revealed the presen- ce of oxygen. Eowever, a major contribution is from the a i r i n which the sample is prepared. This could be greatly reduced by flushing with a j e t of argon during spr- aying or depositing i n a chamber f i l l e d with cer ta in amount of argon.

5.5. Resist ivity and mobility. Van der Pauw technique was used t o measure and Hall effect . For a given purity depends strongly on # . We have obtained a l inear relation between log f and log (f ig.4) . The lowest value i n g obtained for 125< 9 <20OPm is very close t o the measured on the s tar t ing material.

Fig.4. Relation between res i s t iv i ty f and the par t ic le s i z 4 o f the s t a h powder. Fig.5. M o b i l i t y p a s function of carr ier concentration for P doped samples.

Doping by T or B203 reduced f from 6 . 1 0 3 n an t o 0.1 cm. Pbbi l i t of P dop~f samples were usually less than 5 anz /~ / sec with n ranging from 4 4 l o 1 $ t o 2 -10 / an3. Fig.5 showsp as a function of n. Though more data are required, one could appreciate a decrease i n J & a s n s t a r t s t o increase similar t o that observed i n the case of CVD films (4). Sowever, low values ofpin our samples could be due t o the impurities (see Table 1 ) and perhaps due t o an oxidised grain boundary as shown by preliminary Auger spectra.

A short study was undertaken to see the effect of heat treatment under argon on and,&. Table 2 sumarises the f indings.For example, i n the case of E3 heat treated a t 800 C fo r 7 hr . , Q decreases by a factor of 30. On the other hand

for P doged sarple E l l , fdecreses by a f a tor of 3 w i t h p incresing from 0.5 to 5.6 5 cm2/v/sec. The highest value o f p =I0 cm /V/sec was obtained a f t e r heat treating a t 8OOCfor l h r . ( $ i g s . ( )

Table. 2. Effect of heat treatment on 9 andp . S w l e Purity doping par t ic le

s ize pfim 97 3N5 - 125-290

treatnent

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Different models have been proposed to pders tand the e lect r ica l properties of poly S i with grain sizes f r m less than 1000 A upto 50pm (5).Though there are disagreements i n the deta i ls among them, there is a general agreement that trapping a t grain boundaries creates barriers and affects both the f ree carr ier den- s i t y and mobility. The heat treatment somehow seem t o decrease or anneal out the defects thereby reducing the number of traps. This in turn reduces the potential energy barrier a t the grain boundary resulting i n an increse i n p a s observed.

6.Conclusions. We have used the plasma spray 5echnio;ue t o obtain n and p type poly Si of 300Am on different substrates of 100 mm i n less than 30 sec. Silicon ribbons of 20X5x0.3 mm were also obtained. The only additional impurity i n the deposit is Cu and is < 10 atppn. Spraying i n a i r introduces upto 4 a t % of 0xygen.A wide range o f f was obtained depe~ding on the par t ic le s ize and the d,oping. The microstructure is heterogeneous with 10 < d ,< 100,Um. SIMS indicate a honogeneous distribution of 2 P i n the sample. Tnough the maximum nobili ty is 10 an /V/sec, we hope t o improve the

properties by treatments l ike laser, graphite heater or lamp annealing.

This work was pa r t i a l ly supported by a contract from Elect r ic i t6 de France. We would l i k e tthankMr S.Leguay of Soci6t6 de Construction e t de REparation de Mat6riel ABronautique for the expert tec'nnical assistance i n the plasma spray, MrJ.Clement for carefully polishing the sm-ples,~elld4.Rmmeluere7:~e M.Miloche and M r . R.Moreau res9ectively for EPMA, SEM and SIMS data. We a9preciate very much Prof.V.Brabers fo r having kindly provided data on SIMS prof i le and Auger analysis. Helpful and encouraging advice from Dr.Y.Marfaing and Dr.M.Rodot is greatly appreciated.

References. 1. For a c r i t i c a l review of the methods for preparing poly Si , see Helmreich th i s conference; Dietel J., Helmreich D and S i r t l E i n " Crystals- Growth,Properties and Applications 5 " (Springer Verlag,Berlin 1981) p43; M.Rodot i n Ad. Solar Energy (1982) to be published. 2. Alaee M.S. and hashem T 3rd E C Thotovoltaic Solar Energy Conference(Reide1, Dordrecht, 1380) p 584. 3. For review a r t i c l e s on plasma spray see Mash D.R. Weare N.E. and !qalker D.C. J.Metals 13 (1961) 473; Fisher I.A. Int . Metallurg. Rev. 17 (1972) 177; Mc~hersonx. Thin Solid Films 83 (1 981) 297; Vardelle ? ~ . ~ T V a r d e l l e A., Besson J . L . and Fauchais P. Rev.Phys.Ap~1.-5 (1 981) 425 4. Seto J.Y.W. J.Appl.Phys. 46 (1975) 5247 5. For a recent review see Saraswat K.C. i n " Grain Boundaries i n Semiconductors " Ed.Leamy H.J., Pike G.E. and Seager C.H. ( North Holland, Amsterdam 1982) p 261 and references therein.