Materials Science and Engineering A 374 (2004) 362–368

Plasma assisted metal-organic chemical vapor deposition of hardchromium nitride thin film coatings using chromium(III)

acetylacetonate as the precursor

Arup Dasguptaa,∗, P. Kuppusamia, Falix Lawrencea, V.S. Raghunathana,P. Antony Premkumarb, K.S. Nagarajab

a Physical Metallurgy Section, Materials Characterization Group, Indira Gandhi Centre for Atomic Research,Kalpakkam 603102, India

b Department of Chemistry, Loyola College, Chennai 600034, India

Received 15 October 2003; received in revised form 4 March 2004

Abstract

A new technique has been developed for depositing hard nanocrystalline chromium nitride (CrN) thin films on metallic and ceramic substratesusing plasma assisted metal-organic chemical vapor deposition (PAMOCVD) technique. In this low temperature and environment-friendlyprocess, a volatile mixture of chromium(III) acetylacetonate and either ammonium iodide or ammonium bifluoride were used as precursors.Nitrogen and hydrogen have been used as the gas precursors. By optimizing the processing conditions, a maximum deposition rate of∼0.9�m/h was obtained. A comprehensive characterization of the CrN films was carried out using X-ray diffraction (XRD), microhardness,and microscopy. The microstructure of the CrN films deposited on well-polished stainless steel (SS) showed globular particles, while arelatively smooth surface morphology was observed for coatings deposited on polished yittria-stabilized zirconia (YSZ).© 2004 Elsevier B.V. All rights reserved.

Keywords: Metal-organic chemical vapor deposition (MOCVD); Plasma processing; Hard coatings; Nitrides; Surface morphology; X-ray diffraction

1. Introduction

Hardfacing or deposition of a hard coating is commonlypracticed to improve the chemical and mechanical propertieslike surface hardness, wear resistance, corrosion resistanceand fatigue resistance of materials. Metallic oxides, nitrides,carbides, and carbonitrides are some of the choices as hard-facing materials. Among the nitrides, chromium nitride (CrNor Cr2N) can attain hardness up to 2400 HV[1]. They alsohave excellent thermal stability up to∼723◦C [2]. The su-perior properties of chromium nitride coatings as comparedto TiN coatings, especially with respect to lower coefficientof friction and better oxidation resistance can help to ex-tend the life of cutting tools, forming tools and automotiveparts, as well as specialized machinery[3]. Hard chromiumnitride coatings may also be used in nuclear power plants in

∗ Corresponding author. Tel.:+91-4114-280202;fax: +91-4114-280381.

E-mail address: arup@igcar.ernet.in (A. Dasgupta).

preference to Co based alloys (eg. Stellite 6 having hardness∼400 HV) and Ni-based alloys (eg. Colmonoy 6 havinghardness∼660 HV) because of significantly lower inducedradioactivity and superior mechanical properties. Also, CrNcoating on certain ceramic materials like yittria-stabilizedzirconia (YSZ) is extremely useful to shield the ceramicoxide from reducing environments. Interestingly, chromiumnitride thin films deposited on rough copper substrateshave been reported to exhibit moderately high solar absorp-tion [4].

In industry, chromium nitride coatings are popularly syn-thesized either by a physical vapor deposition (PVD) tech-nique or gas nitriding or plasma nitriding. The range ofPVD processes include cathodic arc vapor deposition[5,6],magnetron sputtering[7,8], and combined magnetron andarc processes[9]. Unbalanced magnetron sputtering, whichis a modification of conventional sputtering is adopted forcoating inside large holes at small depths[9]. However,PVD processes are line of site processes and are not suit-able for coating on work pieces with complex geometries.

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2004.03.021

A. Dasgupta et al. / Materials Science and Engineering A 374 (2004) 362–368 363

A two-step process comprising for CrN coating has alsobeen used[10]. It involves the formation of a chromiumlayer by electroplating or chromising process followed bynitriding. The chromium layer may be nitrided using ei-ther the tuftriding method, which uses a toxic cyanide fusedsalt, or conventional gas nitriding using hazardous ammoniagas at temperatures of the order of 800◦C. Plasma nitrid-ing of the chrome-plated layer using a mixture of nitrogenand hydrogen has been recently developed as an alternativeenvironment-friendly process[11,12].

Chemical vapor deposition (CVD) of amorphous andpolycrystalline chromium oxide[13] and amorphouschromium carbonitrides[14] have been reported usingmetal-organic sources such as chromium acetylacetonate,chromium hexacarbonyl and bis(benzene) chromium. Inthese studies, air[13], and hydrazine[14] have been used asthe gas precursors. In general, metal-organic chemical vapordeposition techniques (MOCVD) involve thermal crackingof the metal-organic complex on a heated substrate. Such atechnique is not yet an industrially viable one. On the otherhand, plasma assisted chemical vapor deposition (PACVD)of titanium nitrides and carbides are industrially acclaimedprocesses[15]. The introduction of plasma brings in nu-merous advantages, such as: (i) low processing temperature(200–600◦C) as compared to a conventional thermal CVD(1000◦C and above); (ii) ability to form uniform depositionover large area on metallic as well as ceramic substrates;(iii) deposition on any geometry that is exposed to theplasma; and (iv) low substrate distortion. Attempts havebeen made to combine the PACVD and MOCVD processesfor depositing thin films of various nitrides and oxides suchas YBa2Cu3O7−x [16], NbN [17], TiN/BON [18], carboni-trides of Ti and Zr[19], and metal containing amorphoushydrogenated carbon (Me–C:H)[20,21]. The process ispopularly referred as plasma assisted metal-organic chemi-cal vapor deposition (PAMOCVD) technique. In this paper,we report a new PAMOCVD technique for deposition ofCrN, using dc plasma for cracking the metal-organic pre-cursors[22].

Fig. 1. Schematic diagram of the system used for deposition of CrN coating by PAMOCVD technique, where A: water-cooled CVD vacuum chamber;B: anode plate; C: cathode plate, D: substrate; E: substrate heater, F: vaporizer chamber (heated); G: carrier gas line; H: heated gas line; I & J: N2 &H2 gas cylinders; K: gas line; L: high pressure pirani gauge; M: insulating stand for anode; N: insulating stand for cathode; O: pumping line; P: rotarypump; Q: exhaust pine and (⊗) valve.

2. Experimental

In PAMOCVD technique vapors of metal-organic com-pound such as chromium(III) acetylacetonate (Cr(acac)3)have been used as the precursor for Cr. Crystals ofCr(acac)3 complexes were prepared from high purity(99.4%) CrCl3·6H2O [23]. In addition, commercially avail-able ammonium halide of purity 99.9% (NH4I or NH4FHF)was mixed with Cr(acac)3. The PAMOCVD equipment wasindigenously designed and custom built by M/s VacuumTechniques (India). The schematic diagram of the system isshown inFig. 1. A rotary vacuum pump was used to evacu-ate the system. The substrates were placed on the cathode.A laboratory built dc power supply (0–1000 V) was usedto generate the plasma. The cathode plate was heated usingan external heater. The substrate temperature was measuredusing a K-type (Chromel–Alumel) thermocouple that hasbeen previously calibrated with respect to the actual sub-strate temperature. Temperature was controlled using a PIDtemperature controller (Indotherm MPC-500) with an ac-curacy of± 2◦C. Care was taken to electrically isolate thethermocouple from the cathode power supply. Powders ofmetal-organic precursors, mixed with required concentra-tions of the ammonium halide were pelletized and placed inthe vaporizer chamber, F. Vapors of these precursors werecarried into the deposition chamber, A, through a heatedgas line H, using nitrogen (99.9% pure) as the carrier gas.Additional N2 and H2 (99.9% pure) gases were also fedto the deposition chamber. All the depositions have beencarried out at sub-atmospheric pressures, typically 0.2 kPaand a substrate temperature of 550◦C. During the heatingcycle of the substrates, the chamber was maintained at apartial pressure of∼0.5 kPa of hydrogen, in order to pre-vent formation of a stable passive oxide (Cr2O3) film onthe hot stainless steel (SS). The deposition time for all theexperiments was fixed at 8 h. The hydrogen to nitrogen gasflow ratio (r) and the type and amount of the ammoniumhalide additive were varied during the experiments. Poly-crystalline CrN thin films were deposited on well-polished

364 A. Dasgupta et al. / Materials Science and Engineering A 374 (2004) 362–368

316LN SS and yittria-stabilized zirconia. The SS substrateswere polished using fine (∼1�m size) diamond suspension.The roughness of this substrate is thus expected to be ofthis order.

X-ray diffraction (XRD) of the coatings were carried outusing a Philips PW-1730 X-ray diffractometer with Cu K�radiation (wavelength,λ = 1.5418 Å) in θ–2θ geometry. Itmay be pointed out here that substrate contributes signif-icantly to XRD peaks due to small thickness of the films(2–7�m). For example, (1 1 1) and (2 0 0) peaks from SS(�-Fe) almost coincide with (2 0 0) and (3 1 1) peaks of CrN,respectively[24]. There is no such coincidence for XRDpeaks from YSZ and CrN. Therefore, YSZ substrates wereused for XRD analysis of the coating.

The thermo gravimetric analyses of the precursors werecarried out in helium environment using a Seiko 320�m hor-izontal balance analyzer[25]. To measure the hardness of thecoatings, a Vicker’s micro hardness tester (Leitz Miniload-2)was used.

The scanning electron microscope (SEM) image of thecross-section of the coating was taken using a PhilipsXL30 ESEM (Environmental SEM) at an operating volt-age of 20 kV. For this purpose, two identically coated YSZsubstrates were glued face to face using Gatan G1 glue,mounted in araldite and polished. The viewing surface wasgold coated to avoid charging effects and to improve con-trast. The coatings were also characterized using a LeicaMEF4A optical microscope.

3. Results and discussion

The XRD patterns of CrN coatings deposited on SS andYSZ substrates are shown inFig. 2. The hydrogen to nitro-gen flow ratio (r) during the deposition of these thin filmshas been maintained at 1.05. Cr(acac)3 with 10 wt.% ofNH4FHF has been used as the precursor. Diffraction from

30 40 50 60 70 80

SS316 substrate

YSZ substrate

CrN

+

-Fe

(311

)

(22

0)

CrN

(220

)

CrN

+

-F

e(2

00)

(

111)

CrN

(111

)

2 θ (degrees)

Inte

nsity

(arb

itrar

y un

its)

γ γ

Fig. 2. X-ray diffraction pattern of CrN coatings on stainless steel (SS316)and YSZ substrates under identical deposition conditions ofr = 1.05 and10 wt.% of NH4FHF.

(1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face centered cu-bic (fcc) CrN are clearly identified in the case of CrN coatedon YSZ substrate. Thus, the polycrystalline nature of theCrN coating is established. As mentioned before, some ofthe X-ray reflections from SS substrate overlap with thoseof CrN coating. However, the CrN peaks corresponding tothe planes (1 1 1) and (2 2 0) confirm the deposition of poly-crystalline CrN films on stainless steel substrates as well.Peaks corresponding to oxides and carbides of Cr or its othernitride such as Cr2N have not been observed in the XRDspectra. Therefore, the feasibility of the PAMOCVD tech-nique in depositing a mono phase polycrystalline CrN coat-ing is demonstrated. Formation of pure CrN has also beenreported using PVD techniques[5–9]. This may be con-trasted with the formation of a mixture of CrN and Cr2N ina diffusion controlled processes, such as, (a) gas nitrided Crin a NH3–H2 environment[10], and (b) plasma nitriding ofelectroplated Cr[11].

Fig. 3shows the effect of the ammonium halides (10% byweight of the metal-organic compound) on the XRD patternsof the CrN coatings.r was maintained at 1.05 during the de-position of these thin films. The figure shows that additionof NH4I and NH4FHF improves the crystallinity of the coat-ing. It is proposed that the halogen may have been helpful inpreferentially etching out any amorphous nuclei of CrN dur-ing the deposition process. Analogy has been drawn fromSi:H thin films deposited on corning glass or quartz, wherehydrogen helps in preferentially etching out the amorphousphase of Si and thereby promoting crystallinity[26,27]. Inthis regard, between F, and I the former is expected to bemost helpful in crystallizing CrN due to higher electroneg-ativity. In fact, Fig. 3 reveals that addition of the fluorideresults in stronger and sharper XRD peaks indicating supe-rior crystallinity of the CrN. Also, halogens are known to begood etchants for chromium oxides[1]. Therefore, they notonly help to improve the crystallinity of films, but also etchaway the native chromium oxide film on SS surface making

Fig. 3. X-ray diffraction pattern of CrN coatings on YSZ substrates usingr = 1.05 and different precursors, such as, A: Cr(acac)3, B: Cr(acac)3+ NH4I and C: Cr(acac)3 + NH4FHF.

A. Dasgupta et al. / Materials Science and Engineering A 374 (2004) 362–368 365

30 40 50 60 70 80

H2/ N

2=2.45(noCrNpeaksobserved

andhencenotshown)

H2/ N

2=1.05

H2/ N

2=0.01

H2/ N

2=0

(311)

(220)

(200)

(111)

2 θ

Inte

nsity

Fig. 4. X-ray diffraction pattern of CrN coatings deposited on YSZsubstrates under various hydrogen dilution ratios. 10 wt.% of NH4FHFhas been used for these experiments.

available a clean and rough metallic surface for nucleationof CrN and thereby promoting adhesion.

Fig. 4 shows the effect ofr on the crystallinity of CrNcoatings deposited on YSZ substrates, while using Cr(acac)3and 10 wt.% NH4FHF as the precursor. It is seen from thefigure that whenr is 0, the film is amorphous as exhibitedby the broad and diffuse peaks of the corresponding XRDpattern. From the XRD patterns corresponding tor = 0.01and 1.05, it is noted that hydrogen addition increases thecrystallinity of CrN films significantly. Therefore, at least1% hydrogen dilution is necessary for the growth of CrNcrystallites. It has been suggested[28] that hydrogen is re-quired to promote surface diffusion of the precursors andprevent random nucleation on the substrate leading to amor-phisation of the deposited film. In general, the film nucleatesand grows by island formation according to Volmer–Webermode[29,30]. Optimum hydrogen coverage of the substratecan increase surface diffusion length of the incoming Crand N species. This would prevent instantaneous adsorptionof the growth species, and instead promote surface diffu-sion till a suitable growth site for CrN nuclei is encoun-tered. Such a growth mechanism has been established inSi thin films [26–28]. But at a high hydrogen dilution ra-tio of 2.45, the deposited film did not contain CrN. Thisis believed to be a result of loss of nitrogen precursor atthe growth site. Presence of excess hydrogen may lead tothe formation of ammonia gas by the following reactions inthe plasma:

N2 → N + NH2 → H + HN + 3H → NH3 ↑

(1)

This in essence reduces the availability of N for the forma-tion of CrN and retards the deposition process. Therefore,concentration of NH4FHF in the metal-organic precursorand hydrogen dilution of the precursors are two importantparameters governing the formation of polycrystalline CrNthin films.

50 100 150 200 250 3000

20

40

60

80

100

0%FHF 1%FHF 5%FHF 10%FHF 100%FHF

Wei

ght l

oss

(%)

Temperature (oC)

Fig. 5. Thermo-gravimetric analysis of Cr(acac)3 precursor in He envi-ronment using various concentrations of NH4FHF.

The lattice parameters and crystallite sizes of the CrNcoatings deposited on YSZ substrates using different halideprecursors and hydrogen dilution ratios have been evaluatedfrom the corresponding X-ray diffraction patterns (Table 1).The results have been compared with the standard data avail-able in JCPDS[24]. Clearly, the measured lattice parameteris in good agreement with the standard value of 4.14 Å forCrN. The crystallite sizes of the CrN films have been cal-culated using the Scherrer formula. For the same hydrogendilution (r = 1.05) CrN crystallite size reduces from∼10 to∼5 nm when the halide precursor is changed from NH4FHFto NH4I. But for the same NH4FHF precursor, crystallite sizeremains almost same whenr is reduced from 1.05 to 0.01.Thus, nanocrystalline CrN coatings were obtained througha right choice of precursor and hydrogen dilution.

Fig. 5 shows the thermo gravimetric analysis of theCr(acac)3 precursor mixed with various concentrations ofthe NH4FHF compound, in helium environment. It is seenthat the mixture does not decompose in the temperaturerange tested. The pure Cr(acac)3 sublimates in the tempera-ture range 215–250◦C. When 10 wt.% of NH4FHF is used,the temperature over which sublimation occurs is extendedto 50–270◦C. Thus, the precursor partial pressure can beeasily regulated over a wide range just by controlling theprecursor temperature. It may be pointed out here that,in general, precursors that sublimate within a small rangeof temperature require precise and rather expensive hightemperature flow controllers.

The etching effect of the bifluoride on the overall growthrate of the CrN films deposited on stainless steel substratesand the microhardness of the films were studied and theresults are summarized inTable 2. For each of these tests,rwas fixed at 1.05. Interestingly, it is seen that the depositionrate of the film increases from 0.67�m/h to 0.88�m/h whenNH4FHF is increased from 0 to 5 wt.%. The deposition ratedoes not change significantly till 10 wt.%. Beyond 10 wt.%,the deposition rate of CrN film is reduced to 0.35�m/h.The increase in deposition rate is possibly due to formationfavorable nucleation sites for CrN by preferential removal

366 A. Dasgupta et al. / Materials Science and Engineering A 374 (2004) 362–368

Tabl

e1

X-r

aydi

ffrac

tion

para

met

ers,

such

asin

terp

lane

rsp

acin

g(

d),

latti

cepa

ram

eter

(a)an

dpa

rtic

lesi

ze(D)

ofC

rNco

atin

gsde

posi

ted

with

diffe

rent

halid

esan

dhy

drog

endi

lutio

n(

r)

Sr.

No.

Pre

curs

orH 2/

N2flo

wra

tio(r

)2θ

(◦)

d-va

lues

(Å)

Cry

stal

plan

eLa

ttice

para

met

er,a

(Å)

Ave

rage

,a(Å

)P

artic

lesi

ze(D

)

Cal

cula

ted

JCP

DS

@19

99C

alcu

late

dJC

PD

S@

1999

FW

HM

(11

1)(

◦ )D

(Å)

1C

r(ac

ac) 3

+10

wt.%

NH 4

I1.

0537

.59

2.39

412.

3902

111

4.14

674.

1347±

0.01

584.

140

∼1.9

5∼5

043

.86

2.06

402.

0700

200

4.12

8063

.73

1.46

001.

4637

220

4.12

95

2C

r(ac

ac) 3

+10

wt.%

NH 4

FH

F1.

0537

.46

2.39

892.

3902

111

4.15

504.

1470±

0.00

820.

7611

3±15

43.7

12.

0693

2.07

0020

04.

1386

63.4

21.

4655

1.46

3722

04.

1451

76.0

11.

2510

1.24

8231

14.

1491

3C

r(ac

ac) 3

+10

wt.%

NH 4

FH

F0.

0137

.38

2.40

382.

3902

111

4.16

354.

1511±

0.00

820.

8510

1±15

43.6

22.

0733

2.07

0020

04.

1466

63.4

01.

4659

1.46

3722

04.

1462

76.0

31.

2508

1.24

8231

14.

1484

The

‘d’

and

‘a’

valu

esha

vebe

enco

mpa

red

with

JCP

DS

(@19

99).

of unstable and weak bonds. On the other hand, the decreasein deposition rate can be ascribed to excessive use of halidewhich etches away some of the deposited material. In fact,excess NH4FHF was found to corrode even the chamberwall. Thus, 10 wt.% of NH4FHF was found to be an optimumconcentration.

Table 2also shows that for a fixed load of 50 g, the hard-ness of the coating deposited on SS substrates increases from772 HV to values in excess of 1000 HV with increase ofconcentration of NH4FHF. And, for a fixed concentrationof NH4FHF (10 wt.%), the hardness values decrease as theload is increased from 50–200 g. Since the thickness of thecoating (2.8–7.1�m) is small, this behavior is expected be-cause of the influence from the underlying soft substrate. Itis known that if the ratio of the indentation depth to coatingthickness exceeds a certain critical value (normally∼0.07)[31,32], the measured hardness is no longer characteristicof the coating alone but also includes a contribution fromthe substrate. In addition, poor contrast of the CrN film alsoresults in underestimation of hardness. These have been themajor difficulties in hardness measurement of the thin CrNcoating. Therefore, it can be definitely said that the actualhardness of the CrN coating is higher than those measured.In literature, the hardness values of CrN coatings have beenreported to vary depending on the deposition technique. Forexample, it is 1560–2270 HV for Cathodic arc ion plating[33], 1900 HV for organometallic CVD[14], 1400–1600 HVfor gas nitriding of Cr[1], and 1500 HV for plasma nitrid-ing of Cr [34]. Therefore, taking into account all the factorsmentioned above, hardness of the CrN coating developedusing PAMOCVD (>1000 HV) compares well with reportedvalues and is far superior to other popular hard coatings suchas Co-based stellite (400 HV) or Ni-based colmonoy (660HV).

The optical micrograph of the CrN coatings deposited si-multaneously on well-polished SS and YSZ substrates, us-ing optimum deposition conditions of 10 wt.% NH4FHF andr = 1.05, are shown inFig. 6. A distinct difference betweenthe two is visible; while the particles appear to be globular incase of SS substrate, a smooth surface is observed for YSZsubstrate. A rough surface morphology is often attributed tomacro droplet formation in a cathodic arc process wherein,

A. Dasgupta et al. / Materials Science and Engineering A 374 (2004) 362–368 367

Fig. 6. Optical micrograph of CrN coatings deposited under optimum conditions ofr = 1.05 and 10 wt.% of NH4FHF, on, (a) SS, and (b) YSZ substrates.

Fig. 7. SEM of the cross-section of CrN coating shown inFig. 6b.

the fast evaporation leads to coalescence of excess neutralatoms[35,36]. This kind of morphology has also been re-ported for TiN coatings deposited on Cu and s45c (JIS des-ignation) type steel substrates using PAMOCVD technique[37], and has been attributed to surface damage. Althoughcoalescence of excess neutral atoms cannot be ruled out ina plasma CVD process, absence of globular particles on theco-deposited YSZ substrate rules out formation of macrodroplets in this case. Since a dc source is used to power theelectrodes, the electrical conductivity of the substrate mayhave a major role to play in governing the morphology ofthe coating. Stainless steel is an electrically conducting sub-strate while YSZ is a ceramic insulator. Hence, YSZ will notattract the charged species in the plasma and growth shalloccur only from the neutrals. On the other hand, SS will at-tract the charged species, which may lead to surface damage[37] resulting in the alteration of surface morphology.

The scanning electron micrograph of the cross-section ofCrN coating deposited on two YSZ substrates, which have

been glued face to face, is shown inFig. 7. The coating thick-ness is found to be about 2.5�m and it is free of micro-pores.Absence of any pit at the substrate-coating interface indi-cates full coverage of the substrate surface during deposi-tion, which is important for good adhesion.

4. Conclusion

By a plasma assisted metal-organic chemical vapor de-position technique, using chromium acetylacetonate and anoptimum concentration of ammonium bifluoride precursors,a method for the deposition of CrN thin films has been devel-oped successfully. Nanocrystalline (crystallite size∼10 nm)adherent CrN films could be deposited both on metallic(SS) and ceramic (YSZ) substrates. The crystallinity andcrystallite grain sizes were analyzed using X-ray diffractiontechnique. The deposited films contained only CrN; no othernitrides such as Cr2N were found. A minimum hydrogen

368 A. Dasgupta et al. / Materials Science and Engineering A 374 (2004) 362–368

dilution (r = 0.01) of the precursors was found to be nec-essary to obtain crystalline CrN films, while excess dilution(r = 2.45) is detrimental to the film formation. Concentra-tion of ammonium bifluoride in chromium acetylacetonatewas found to affect the deposition rate of CrN thin films.A maximum deposition rate of∼0.9�m/h was obtained ata concentration of 10 wt.% bifluoride. Thermogravimetricanalysis of the precursor (Cr(acac)3 + NH4FHF) was usefulin identifying the parameters for good control of the depo-sition process. Microstructure of the films deposited on SSsubstrates suggests that the coating is composed of globularparticles while it is relatively smooth when deposited onYSZ substrate. This dissimilarity is attributed to the dif-ference in their electrical conductivity. The cross-sectionalSEM of the CrN coating deposited on YSZ substrate indi-cated an adherent coating, which is free of micro-pores.

Acknowledgements

The authors would like to thank Dr. M.P. Janwadkar forsurface profile analysis of the thin films, Dr. N. Pankajavalli,Dr. C. Mallika and Dr. O.M. Sreedharan for thermo gravi-metric data on precursors. The support and encouragementfrom Shri S.B. Bhoje, Director, IGCAR and Dr. Baldev Raj,Director, MCRG, IGCAR are gratefully acknowledged. Theauthors are also grateful to Dr. S.K. Ray for his valuablesuggestions.

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