REVISTA MEXICANA DE FíSICA 44 SUPLEMENTO 3. 254-259 DICIEMBRE 199K

CdTe polycrystalline films grown by double proccss closc-spacc vapor transporttechnique

A. Martel, R. Castro-Rodríguel., J.L. Peña. F Cahallero-Briones, and W. CauiehCentro de Im'estigación y Estudios Avanzados de/l1Istituto Politecnico Nacional, Unidad Mérida

97310 Mérida, Yucatán, Mexic(}

E. PurónInstituto de Materiales y Reacti\'os para la Electrónica, Unil'ersit/ai/ de La Habana

Zapa/a y G, Vedado, 10-100 La Haballa. ellba

Recihido el 9 de fehrero de 199H: aceptado el J de julio de 1998

The characterislics 01' Ihe CdTe polycrystalline films grown by Ihe new technique 01' double process~close-space vapor transport techniqucare presented. This process can be divided in two steps: al the ¡¡rst step, a graphile source hoal packed with CdTe powdcr acts as the CJTesource and other graphile block with or without Corning glass cover is used as auxiliary substrale. At the second stcp, the tilm depositeJin the tirsl step over graphilc block or Corning glass is used as CdTc source. Different substrates were useJ lo obtain polycrystalline CliTelilms. We describe Ihe oxygcn and lempcralure influences on the slruclural and the l1lorphological properties of differenl 1Ilrns: CdTc/glass,CdTclSnO,/glass and CdTclCdSISnO,lglass.

Kt!y",ords: CdTe; polycrystalline films: CSVT

Se presentan las caraclensticas de las películas dclgad¡j~ policrisl¡jlina~ de CdTe crccidas por la nueva lécnica de transporte de vapor cnespacio reducido con doble proceso. Este proceso puede ser dividido en dos pasos: en el primero, un bole de gralito cargado con polvode Cáfe actúa como fuenle de CdTe. lIs;:índose un bloque de grafito cubierto o no con un vidrio Corning corno substralo auxiliar. En elsegundo paso, la película depositada en el primer paso sobre el bloque de grafilo o sobre el substrato auxiliar. se usa corno fuente de CdTe.Se utilizan diferentcs subslratos para obtener películas policrislalinas dc CdTe. Se describe la innuencia del oxígeno y la ternrx:ratura sobrelas propiedades morfológicas y estruclurales en pclículas edTe/vidrio y CdTeJSn02/vidrio. CdTcfCdS/Sn02/vidrio.

D('scriptores: Cáfe; películas policrislalinas; evaporación

PACS, 81.05.Dz: 81.15.Ef; 68.55.Nq: 68.55.Jk

1. Introduction

and is Iransporled lO, and condenses upon a slightly coolersubSlrale which is placed in close proximity lo Ihe sourcc,

CdTc is one of lhe leading absorber materials Cm fahricat-ing polyeryslalline thin film solar eells [1]. With a handgapof 1.5 eV. it possesses optimum characleristics for produc-ing high voltage and good eurrent colleclion when malchedto the incident solar spectrum. At presento the best solar eon-version efficicncies are achicved by using this p-type materi-als as lhe ahsorher layer in a superslrale wilh SnOz and CdSas Ihe Il-type window laycrs, Several fabricalion lechniquesha ve beco uscd lo achievc conversioo cffieicncics exceeding10%. These inelude evaporalion. eleelrodeposilion, spray py-rolisis, and elose-spaeed suhlimalion (CSS). CdTe thin lilmsgrown by CSS have produeed Ihe highest effieieney polyerys-talline CdTe solar eelllo dale 11, 2J. In lhis prueess, a soureeof CdTe is healed to a lemperature of ahoul 600-660°C in alow pressure amhient (1 to 30 Torr) consisting of He and O2.The CdTe deeomposcs according lo the rcaetion:

CdTe(s) =Cd(g) + 0.5Te,(g). (I)

thus the designation close-spaced sublimation. For solar celldcvice fahrication, lhe substrates eoosist 01' glass pre-coaledwilh SnO, and CdS layers in a CdS/SnOz/glass eonligura-tion,

Several aUlhors 11-3J have used and invesligated Ihe rolenI' oxygen during the growth 01' lhe CdTe absorber layer inlhe CdTe/CdS solar eell hy e10se spaee suhlimalion. Usinglhis leehnique Brin 11 J has made lhe moSl effieient eell ofthis kind.

Many authors [4--61 have reponed lhe Icndcllcy of oxygento incrcase acceplor densily or compensatc donors in CdTefilms, and Ihen lo increase lhe p-typc conductivily, Rose el

al. [2J have poinled out the contradiction in using oxygcn:the oxygcn inereases lhe nucleation and the intcrdiffusion alIhe hctcrojunction and decreascs Ihe pin hole formation, bUIdecreascs the grain size and the CdTe bulk erystallinily andworscn the surface morphology as welL In all these works,Ihe oxygen innuence is rclatcd to nucleation of CdTe onloCdS suhstrates.

In our work wc made a distinction betwcen oxygen andsubstrate inllucnce. With that scope wc have grown CdTcfilms undcr diffcrent oxygen content and temperature. us-

CdTe POLYCRYSTALLlNE FILMS GROWN BY DOUIlLE PROCESS CLOSE-SI'ACE VAPOR TRANSPORT TECHNIQUE 255

ing lhree types of sobslrate: Corning 7059 glass, Sn02and CdS. Films are comparcd using scanning electron mi-croscopy (SEM) ami X-ray diffraelion (XRD).

2. Experimental details

For the preparation of the CdSISn02: F/glass substrate. theCdS layer was dcpositcd on the substratc by chcrnical bathdcposition (eSD) using a modification o" a proccss dcscribcdin lhe literature [71. Specifically, afler thorough c1eaning wilh1% Liquinox soap and dcionizcd (DI) water. ¡he subSLrateswcrc placcd in a stirrcd and covcrcd jackcted bcakcr al 89°Cwith 550 mi DI waler, 8 mi of 0.033 M Cadmium acetate,5.3 mi of I M ammonium acelale, 15 mi of 15 M ammonia,and 8 mi of 0.667 M thiourea.

The thiourea was added in 4 aliquots of 2 mi, 10 minutesapart lOminimizc homogcncous grov•.'th in the solution. Priorlo CdTe depositioll 011(he CdS suhstratc. an in-silll hydrogcnanneal was pcrforrncd. Anncaling the substratc al 400°C for15 minules in 20 mbar H2. This step has been found helpfulin aehieving large fill faetors in CdSTe/CdS solar cells [1].The DCSS process can be divided in lwo steps: First, us-ing a grapbite source boal packed with 99.999%-pure CdTepowder as lhe CdTe source and Corning 7059 glass as sub-strate lo obtain a CdTe film in a H2 ambient, and second usinglbe Corning glass wilh the CdTe film deposiled in lbe firststep as source (secondary source) of CdTe anu a CdS filmdeposiled from CBO method as substratc to obtain polyerys-talline CdSICdTe films. In both cases the CSS technique havebeen used. Tbe substrale is loadeu such that the CdS film is[aec up anu in close proximity lo the CdTc sourcc. Thc CdTcsourcc are typically used for I lo 2 dcpositions. dcpcnding onthe thickness ol' the CdTc sccondary sourcc.

Al the beginning only the hclium is introduccd in ¡heehamhce, and the Icmperaturc is f31llP ur. During this stagcthe sccondary CdTc sourcc and the suhstratc are scparntcdanu covcrcd by shulters. Thc oxygcn conlrollcr ncedle valveis 0pcllcd 35 scconus (the O2 prcssurc set up time) hefore thetempcraturcs reach thcir slationary valucs. \Vhcn these valucsare rcacheu (he shulIcrs are quickly opcncd ami the sourccand substratc are hrought togethcr (3 mm scparation), al Ihismomen! the CdTc dcposition proccss propcrly hcgins. Dur-ing all the proccss the temperatures are monitorcd and finefixed at stationary values hy a high performance temperatureconlrol systel11 [8J.

CSS of CdTe is baseu on the principie of reversible disso-cialion of CuTe al high lemperalure [91. Tbe elemental gasesdiffuse lo the suhstrate, which is in c10se proximity to theCdTe source (3 mm in our case). The gases thcn recomhincon the substrate. which is kept at a lowcr temperature thanthe source. According 10. this, our processes are resumed inTable 1.

Using such a kind DI"source 8.5 mm diamcter CdTc filmswcrc depositcd using different composition O2+ He mixtureand tcmpcratures. The deposition cnvironmcnt is shown inTable !l.

TABLF: 1.Douhle c10se space suhlimalion processes.

SECONDARYSOURCE FORMATIONGas environment: H2 al pressure oC 20 mbar

CdTe souree CdTe(s)= Cd(g)+0.5Te,(g) 700°CCorniog sub,trato Cd(g)+O.5Te,(g)=CdTe(s) 600°C

FILM DEPOSITIONGas environmcnt : Ol + He mixlure

Secondary source CJTe(s) ==> Cd(g)+O.5Tc2(g) 6S0-660°C

Substralo Cd(g)+0.5Te,(g)=CdTe(s) 580-ó00°C

TABLE 11.Deposition cnvironmenl of the CdTe films.

Film Dcposilion Environmenl

T T P l' SubstralcNo. source suhstralc O, lIe Kind

oC oC mbar mbarlOl 660 580 2 I~102 660 580 2 27 OlO3 660 580 2 38201 650 585 2 14202 650 585 2 27 o2m 650 585 2 38301 650 590 2 14 L.302 650 600 2 11Sub'lralc Kiud: O 7059. o SoO,17059. L. CdSISIl0,17059

A Siemens D5000 X-ray diffractometer with a CuKa radia-tion source (), = O.1540G nm) was used to identify the crys-talline phases of the lilms. The scanning electron rnicroscopy(SEM) wa, earried out in a leol lSM-35C ,canning electronmicroscopc operating al 25 KV.

The Auger elcc(ron speclroscopy (AES) depth profile,was perforlllou in a Perkin-Elmer PHI 560mSCA-SAM sys-tem with a douhle cylindrical rnirror analyzer and a base pres-sure 01"approximatcly 2 x 10-7 Torr. For lhe analysis a 3 kcVelec(ron bcalll al 200 nA current was used,

3. Rcsults and discussion

3.1. Thickncss hOl11oRcncit}'

Al (he beginning of our expcrimcnls we used lWicc the sec-ondary CdTe sourcc. The rcsults of such handle are shown inFig. 1.

Frolll Fig. 1 and Table 1II, it is obvious thatthe filllllbiek-ncss homogcneity from the secend deposit is worsc. The filmlhickness hOlllogeneity is an important featurc in solar cel!teehnology. Albin el al. 19] have l/sed lhe secondary CdTesource up lo eighl (imes. but they do nol repon thc lhickness

Re". Mex. Fí.<.-l4S3 (1998) 254-259

256 A. MARTEL t'f al.

TABLE 11l. Statistical ••..a1ues ol' (he ¡¡1mlhickness distribution uponsuhslratc.

Table IY.- Film Lhickness and grown rate as a funclion of the oxy-gen conten!.

Using He Pressure Mean Thickness SD Error Substrale Time O¿ Content Thickness Grown RateNo.Times mbar /,m ¡ml % Kind mino % ¡ml!min.pm

14 10.9 0.38 1.9 101 5 12.5 10.8 2.162 27 13.3 1.26 9.5 102 O 5 6.9 13.7 2.74

38 25.9 0.54 2.1 103 5 5.0 23." 4.72T (:-;pcoll(iary Cd Tf' ~oHrce) = 6600 C. 201 4 12.5 9.S 245T(slIh:-;trate) = 580°C. 202 o 4 6.9 12.5 3.15P(O,) = 2 mbar, 203 4 5.0Tirnc= 5 mino 301 4 12.5 16 4Lo.

302 3 154 7.1 2.36

FIGURE l. Thickness homogencity along the diametcr fm theCdTe/Corning 7059 f1lms J.cpositcd al T(sOHrcc) = GGO°c.T(suhstrate) = 580°C. deposition time= 5 mino In (A) thesource is only one time lIscd. in (B) the source is twice used

FIGURE 2. SEM morphology 01' (he CdTe/Corning 7059 films dt>posileJ at T(sourcc) = 660°C. T(suhstrate) = ;J80°C. depositiontime = 5 min .. and P(02) = 2 mbar. (101) P(lIe) = 14 mbar.(102) P(He) = 27 mbar and (103) P(He) = .38 mbar.

1064 6Position (a.u.)

_ p(He)=14 rrbaf

~ p(He)=27 rrbaf

- p(He)=38 rrbaf

2

30

25

20

E.>~ 15~~uF "

5

oo

3,2, SEM morphology of lhe films

homogeneily of the IiIms. In Fig. 1, we can scc lhaL whcnthc source is used twice, the worsL hOl11ogcneity is prescntal the pcriphery of the film, where the ceramic ring scparalOris plaeed. \Ve have ohserved preferred yellow powder (CdO)formal ion in this rcgion. To avoid this effecl, the secondaryCdTe so urce was only one time uscd for all the olher IiIms 01'Lhis \,l,'ork.

Table 11I shows (he S1alis(ical values 01' the film (hieknessdislrihution upon substrale.

Table IV shows the film Ihickncss and Ihe growth rate asa l'Ullction 01' Lhe oxygcn contcnt (parlial prcssurc). From Ta-ble IV. it is important to rcmark Ihal (he oxygen conlenl is thekey paramctcr 01' the process: the growth rale gocs up \vhcnthe oxygen content goes Jown, in spiLC than the He pressurcis rinsing (sec also Taole 11). It was useo 10 adjusL the filmthickness (o aboul 7 1"" (film 30 1) whieh is the usual valuein CdTe solar eells.

Figure 2 shows the SEM morphology of the CdTe/Corning7059 films. In these photographs we ean see (hat (he bestmorphology has the 101 film, whieh is done with (he largeroxygen eonlen( (12.5%) and Ihe wors( morphology belongsto 103 film, which is done wilh the lowcr oxygcn conlent(Ylr,). Al the firsL (ime it looks cOlllradictory, out il can heunderstood oy laking into accounl ¡hat the 10 I film has lhelarger grain sizc (mean value 01'7 11m) anJ thc smallcr thick-ness (mean values of 10.8 /,m), while (he 103 film has juslthc inverse siluation: 5.2 11m grain sil.e mean value and 23.6¡lIn lhickncss mean valucs. That Illeans that lhe 101 filmmust have colurnnar grain struelUre [lO), bu( (he 103 doesnol havc iI.

Figure 3 shows lhe SEM morphology 01' theCdTe/Sn02/Corning 7059 tilms. Thc morphological hehav-ior is the S31lle Lhan in the CJTe/Corning 7059 lilms, but thegrain sizc is srnaller.

Re\'. M('x. }-ú. +; SJ (199X) 254-259

CdTe POLYCRYSTALLlNE FIL~lS GROIVN BY DOUIlLE PROCESS CLOSE-SPACE VAPOR TRANSPORT TECHNIQUE 257

FIGURE 3. SEM morphology of lhe CdTc/SnO.dCorning 7059Jilms deposilcd al T(sonrce) = 650°C, T(substrate) = 585°c'depo~itioH time = 4 min .. and P(Oz) = 2 mbar. (201) P(He) =H mbar. (202) P(lIe) = 27 mbarand (203) P(He) = 38 mbar.

FIGURE 5. Compar:l1ive SEM morphology of the CdTc Illms de-positcd upon different subslrates under similar valucs of the growncnvironmem and thickness.

(220)

7060

(400) (331)

40 5020 (degree)

3020

(111)

fIGURE 6. X-ray diffraction pauern uf the as grown 101CdTe/Cnrning 7059 111m.The intcnsity of the pcaks was normal-ized to (he predominam one.

J"

FIGURE 4. SEM morphology ofthc CdTc/CdS/SnOl/Corning 705911lmsdcpositcd ¡-¡t T(sOluTe) = G50°C. P(02) = 2 mbar. (301)P(lIe) = 14 mbar. T(substrate) = 590°c' deposit.ioll time =4 mino (302) P(lle) = 11 mbar, T(,ubs!ra'e) = 600'C,deposition time = 3 mino

Figure 4 shows the SEM morphology 01' theCdTe/CdS/SnO,/Corning 7059 films. The morphology 01'the both fHms is practically lhe same, in accordancc lo thesimilar oxygcn content, bUI the grain size is smallcr than inthe CdTe/SnO,/Corning 7059 films.

Figure 5 shows a comparativc SEM morphology uf theCdTe films deposiled upon differenl suhstrales. The filmshave similar values of the grown environment and thickness(see Tahles II and 11I)but Ihe grain shupe and size is quite dif-ferent: the CdTe17059 film has the largesl grain (mean value01'7 I"n), the majority 01' Ihe grains are huge and they havewell defined hexagonal fonn. The grains in Ihe CdTe/SnO,film are smaller (mean value of 4.4 pm), the densily 01' higgrains is less lhan in the CdTe17059 flITn and lhe grain formis more diffuscd. The CdTe/CdS t1lm has the smallest grainsize (mean value of 3 /1In) and lhe most diffused formo

3.3. X-ray diffraction patterns

Figure 6 shows lhe X-ray diffracLion paltern of Ihe as grown101 CdTe/Corning 7059 film. The pattern is a Iypical CdTecubic strucLure reponed by other authors [11 j where the(1,1, 1) peak is the predominant one and withuut superficialoxides.

Figure 7 shows Ihe X.ray difTraction pauern uf the asgrown 20 I CdTe/SnO, /Corning 7059 film. Frol1l this fig-ure. using 1 degrec X-ra)' incidence, we can sce the same(1,1,1) CdTe predominant peak and the presence of severalCdTe oxides, prohahly Cd,Te207 and CdTe,05' Howeverthe 20 degrcc incidence paltern demonstrates that those ox-ides are superficial. Prom the aboye paltern, we can point oullhat here the (3,1,1) peak is now the predominant one. Thiscan be understand kccping in mind that for deepcr layers thcinlluence oi" the Sn02 sutJstrale may modify the growing filmoricntation.

Figure 8 shows lhe X-ray diffraction pattern 01" the asgrown 302 CdTe/Cd5/SnO, ICorning 7059 film. In this lig-ure, for the case of the 3 dcgrec incidence, \Ve can alsu notea pattcrn Iikc Lhe typical onc shown in Fig. 6 going with thesamc CdTe oxidcs as in Ihe ahoye figurc. The liule peak al

ReJ'. Mex. FIs. 44 53 (1998) 254-259

676345Spultering time (h)

2o10"

A. MARTEL el lIl.

~ El S.

Cd,!1\~ Too.'"¡;<•:.

(.00) (331)

258

<¡;

~:(220) o:

" .-!1\

v (311)

~~~.<•¡¡~•~X (111)

20'

20 30 " "2e (degfee)

FIGURE 7. X-ray diffraction panem of the as grown 201CdTelSn02/Corning 7059 film. The intcnsity {lfthe peaks was nor-malized to the predominant onc.

FIGURE 9. AES depth profilc of the as grown 302CdTc/CdS/Sn02/Corning 7059 film. Before the analysis the CdTefilm was sputtered by argon ion beam during six hours.

FIGURE 8. X-ray diffraction panero (lf the as grown 302CdTeJCdS/Sn02/Corning 7059 film. Thc intensity of the peaks wasnormalizcd 10 the predominant cne.

Acknowledgments

We have investigaled the morphologieal and struelural prop-erties of lhe CdTe/Coroing 7059, CdTe/SnO, ICoroing 7059and CdTe/CdS/SnO, ICoroing 7059 films deposited in 0,-helium mixture by double proccss close-space vapor trans-port tcchnique. We have found a big difference betwecn thegrain shape and size of lhe films deposiled onlo differentsuhstrates wilh similar values of the grown environmcnt amIlhiekness: lhe CdTel7059 film has lhe largesl grain, lhe ma-jorily oflhe grain are huge and lhey have good defined hexag-onal formo The grain in lhe CdTelSnO, film are smaller, lhedensily of the big grains is less lhan in the CdTe17059 filmand lhe grain form is more diffused. The CdTe/CdS film hasthe smallcst grain sizc, and lhe most diffused formo

The CdTe/Coroing 7059 film has a lypieal CdTe eubiestrueture X-ray diffraelion pattero, where lhe (1,1, 1) peakis the predominant ane and without superficial oxides. TheCdTe/SnO:¿ film prcsents some superficial oxides and thesame (1, 1,1) CdTe predominant peak at the surfaee, but in lhedeep layers lhe (3, 1,1) peak beco mes lhe predominanl one.

The pattero of lhe CdTe/CdS al lhe surfaee is like lheabove eited for CdTel Coroing 7059. However in lhe deeplayers lhe pallero is quile differenl, the (1, 1,1) peak is sl¡lIlhepredominanl one, but the (3,1, 1), (4,0,0) and (3,3,1) peaks be-carne very strang, speeially lhe lasllwo. A CdTe oxide peak ispresent too, perhaps duc to the presence of a notable amountof oxygen.

4. Conclusions

The aulhors gratefully aeknowledge lhal fael lhat lhis workwas supported in part by CONACyT (Mexieo) under projeelnumber 211085-5-23676P-E9509. The aulhors are also grate-fui 10 eolleagues of the CINVESTAV-Merida: for lhe use-

.0 502(-) (degree)

30"

(111) @illo:

~ (220)

(31r ~~- o"

,o I 5= .(.00)

<¡;• (331),o"

1.-.- !lO' ~

~.

26.5 degree may be eaused by relleetions eoming from lheCdS/SnO, subslrale zane non eovered by CdTe film. Forthe 20 degree ineidenee lhe pattero is quile differen!: the(111) peak is still lhe predominanl one, bul the (311), (400)and (331) peak s beeame very slrong, specially lhe lasl two.An olher differenee belween lhe patleros is lhe presenee oflhe CdTe oxide peak in deeper layers. To understand bel-ler lhis effeet, the AES deplh profile of the as grown 302CdTe/CdS/SnO, ICoroing 7059 film was earried out.

Figure 9 shows lhe profile. From lhe O, peak we can seethar a notable amouot of oxygcn is present at interface re-gion. From the figure we can also note the strong intermixingof lhe elemenls in lhis zane, although any signifieanl CdSpeak is present in 20 degree incidence pattern. It may meanthal all lhe CdS was eonsumed during the proeess by lhe in-lerfaee ehemieal reaetion, or be eaused by diffieully lo deleellhe absorption in the liek CdTe layer (7.1 ¡.¡m) [12).

Rev. Mex. Fis. 44 53 (1998) 254-259

CdTe POLYCRYSTALLlNE FILM S GROWN BY DOUBLE PROCESS CLOSE.SPACE VAPOR TRANSPORT TECHNIQUE 259

fuI discussions to Dr. Pascual Bartolo; for the assistancc 10

Ing. Daniel Perez and for the CdS films preparalion; to Ing. F.Gamboa for eleetronie works; 10 Me. M. Herrera and lng. O.

1. J. Briu and C. Ferekides. Apl'/ Phys. Lea. 62 (1993) 2851.

2. D.H. Rose. el. al.• in Proceedings o/lhe Maferiais Researcl!Socief)' Symposiw'l. Vol. 426, cdited by D. Ginlcy el (l/., (l\la-tcrials Rcscarch Society. Pittsburgh. PA. 1(96) p. 337.

3. 0.5. Albin. el al., Proc. 26/£EI:: Photovoltaic Sl'ecialis/J COIJ-

faena. Anaheim. CA.. Vol. 67 (1990) p. 4165.

4. S. Ikekami and A. Nakano./nt. 1. Solar E"ergy 12 (1992) 25.

5. J. Windheim. 1. Reneaud, and M. Cocivcra, 1. ,.11'1'1. 1'11)'5; S.Ikekami and A. Nakano./nt. 1. Solar E"ergy 12 (1992) 53.

6. B.M. Baso!' !rll. 1. Solar F.llergy 12 (1992) 25.

i. O. De !\1e1o. E. Vasco. and E. Puron. Mar. Leu. 2S (1995) 205.

8. V. Rejón. R. Castro-Rodriguez. 1\1. Bonilla. and A. Ramircz.

Gómcl. for mechanical works; lo Lic. Fabio Chale andMr. Roberto Sanchcl for tcchnical suppon and to Msc. CesarR. Acosta 1(" editing help.

Proc. of the 35th Conference on Decision and Control, Kobe,Japa/l. /996. p. 2690.

9. 0.5. Albin et al.. in Proceedings o/the MateriaIJ Research So-ciet)' Symp0!iium. Vol. 410. (Matcrials Rescarch Sociely. Pítts.burgh. PA. 1996). p. 45.

10. R.G.Dhcrcel al.. Proc. 26/£££ Pholovoltaic SfJecialües Con.faence. Anaheim, CA., 1997.

11. YA. Chao et a/ .• in Proceedi"gs o/the Marerials Research 50-ciety 5ymposium. Vol. 426. (!\1aterials Research Society. Pitts.burgb. PA. 1996). p. 379.

12. D.G. Jcnsen. B.E. Mac. Candless. and R.W. Birkmire .. in Pro-ceedings of tite Materia/s Research SocielY Sympo.üwn. Vol.426. (Matcrials Rcscarch Society. Píttsburgh. PA. 1996). p. 325.

Re" Mex. f'ís. 44 53 (1998) 254-259