Indian J(lurnal o f Pure & Applied Physics VIII. ," i , Seplemher 2()OI. pp. 574-5R I

,.. ,

( I!' ;...>.

(Bh(Sl-X, Sexh thin fi!!D composites: synthesis and properties _~ /Bhosale, ~shmukh * &~ o/Patil -

/ ' <. Deparlment of Chemi stry . Shivaji University. Kolhnpur 41 () 004

':D epartmenl of Ph ysics (Applie ectronics). Shivaji Un iversity Centre for PG Studies, Solapur 413 00:;

R ec~ bnuary 2001: rev ised 12 Apri l 200 1: nccepted 16 Apri l 2001

l Chemical synlhes i ~ of the hismulh sui hoselenide (Bi , (S ,., .SeJ.1 ) composile thin Illms is presented i~)afl() r for lhe l il"\l lime. The composi lcS werCohlained o'nlo the piane microscopic nmorphous g l asse~ by a simple electroless SO IUl ioll )! ro \\' tll proces,. The different preparation conditions and pnrameters (tempemlUre. lime. So lulion cOmPOSilion and liH elc ) WelT I'in ;di/.cd in the ini t ial still!eS of lhe work nndthe reacti on ki netics and growth mechanism h<lve been di scussed i~c( Thl" c() 'lI e lll ~ of l3i and Se ill ' lhe halh and in the film were determined b; the speclrophotometri c and atomic ; I b~orpli(ln spectrnph()[Omelri c lechniques, The X-rny diffraction anal ysis showed that the composi tes arc microcryslalline in n;ilu rc ;11ld are mixed lernary ehalcogens of the general formul a Bi, (S, ., . Sexh. The optical slUdies revealed that the films ilre higill y ;Ihsorplive with a hand to band type tr<lnsitions and the energy gap decreased. lypic<lll y. from 1.9 eY for Bi ,S , to ()()7eY for Bi, Se " The eleclri ca l conducli vily and thermoeleelrie power measurements showed increase in the conducli vil y wi lh incrcased Se-conlelll (x) in the film and n-l ype conduction of the samples. respecti ve ly. The thermo power is of lhe

ordCrlJfPY:J

I ntroductioll

Mi x<.:d metal cha l cogenide~ are currently proml stn l! malerial s because of their proven pot <.: nti;tI ahilit y in a variety of applications and can lk' :--y nthes ized at ease with a re lati ve ly non ex pensive procedure". The procedure allows for large area depos ition and a va riety of substrate material s could be coated since it is a slow and low teillperature process that avoids mainly the ox idat inn or corrosion of meta II ic substrates(,,7 . Recentl y alloys of metal - Se/Te based materials (S hTe and BiSe) are emerging as the best media for 11lL' e r<l sah le opti cal and phase change recording ,; y\letn,, ' ", It has further been shown that Bi-Se fi 1111 :-- ;Ire besl su ited for phase change opt ica l r(' c ()n lin ~ media ancl that the ir amorphous phase is the Illost stabl e with the optical properti es invariant hy the normal environmental conditions" ".

A so lid solution of Bi-Se with Bi-Te is reported ;1:-- an excellent thermoelectric cool ing mat enal "'11 ,. 1(" Additionally these materials have their energy gaps suitable for applications in the hi gh frequency power sensors, thermopiles, wide halld radiation detectors. temperature controllers and "train guages '" '; ,,, "' ,

BI'S , i:-- a third candidate of thi s group and has ;lclequ,lIe handgap useful for conversi on of the so lar

radiations into an equivalent electrical counterpart. However, the electrical reS istiVities of these materials are very hi gh that put limit on their use as solar energy convertors. These materials ha ve individuall y been obtained by a vari ety of techniques and their crystallographic. microscopic. optical and e lectrical transport properties are reported. The studies pertaining to the mixed/ alloyed forms of these material s are very rare. It is therefore aimed, through these investiga ti ons, to sy nthesise mixed/alloyed Bi 2S,/Bi 2Se, thin composite films to investigate thei r fe w of the properties .

2 Preparation Techniques

Methods

2.1 Preparation methods

and Measurement

The Bi2S,Bi2Se, and Bi 2(SI.x'se, )) composite thin films were obtained onto the amorphous glas ~

microslides under the same ex perimental conditions except the solution composition for the mixed films . The AR type bismuth nitrate pentahydrate (0. 1M), triethanolamine (20 ml) , anhydrous sodi um sulphite. thioacetamide (0.1 M) and sodium selenosulphate (0. 1 M) were used as the basic starting m3leriah . The stoichiometric proportion of the above ingredients were fix ed in the solution state to obtain Bi 2S" Bi 2Se, and Bi 2(SI .x,Sexh (0 ::; x ::; I) thin films.

BHOSALE el al.:B i2 (S I., ' SeJ 1 THIN FILM COMPOSITES 575

respecti ve ly. All the chemica ls were prepared in a doubly di stilled water. In actual preparation of the Bi2(SI _x,Se,h thin films was followed up as fo llows' '' :

The depos ition was carried out using the glass "uh :-- trat L:S c leaned chemo- mec hanica ll y and IIlrr;1."onic;d ly. For Bi 2S·\. the reaction mixture was prcparlx l in ~I 100 ml beaker by addition of 10 ml ((l J M) 11IS murh nitrate and 10 ml (0.1 M ) thioacctami de so luti ons. The above cleaned suhstrates were mounted on a speciall y des igned sllhst rate ho lder and attached to a shaft of a constant spL: L: ci gear motor and finall y who le assemb ly was r()t~lted at a 50 rpm speed in a 55 ± 0.2 °C oil bath . T hL' p H of the reaction so lution was measured by a digita l p H meter and was maintained 8.5 ± 0.2 t hr(l ll ~ h out the deposition time . The sa mpl es were t,l~ c n of from the bath and washed well with the di "lilleci w;l ler and dried. The sa me procedure was repeated for Bi1Se1 films with change of thioacetami de by sodium se lenosulphate .

For preparati on of the Bi 2(SI -x,Sexh composites, concentrati on of hi smuth nitrate was kept fix ed and Ihose of thi oacetamide and sod ium selenosulphite we re chan ged so as to va ry x from () to I.

2.2 Measurement techniques

The hath and the film co mpositions (i.e content s oj Hi ,\\lei Se ) were determined by the spectropholollletri c '.IIld atomic absorpti on SPL:ctI'OSCOP Il' techniques. For thi s. a known weight ()f the sa mpl e was dissolved In a medium concentrated hydrochl oric ac id and was diluted to a ppm leve l. The resultin g so luti on was ana lysed by a spectrop hotometer usin g a standard blank and an ilt omi c absorption spectrophotometer. The layer thi ckness was meas ured by a weight difference de ll sit\ meth od.

T hL: XRD traces were obtained for 28 = IO-SO" lI"'Il ~; 1 Philips PW- 17 10 X-ray diffractometer with Cul( " ( 1.'i4()6 A) rad iation.The opti ca l absorption "' pec tr;1 were obtained for a ll the films in the range of wave lengths between 3500- 13000 A. The clt:ct ri ca l conducti vity and thermoe lectric power Ill C; 1.~ l1re me nt s were performed on these sa mpl es in the ,,()()-5(){) K and 30()-450 K temperature ranges, res pec ti ve ly. Two point probe methods were empl oyed for these purposes. Si lver emul sion was ,Ip pli ecl to hoth ends of the sampl es for getting

ohmic contacts. The working temperature was sensed by a Cr-AI thermocouple whereas thermovoltage was measured by a sensitive 61/2 di git HP microvoltmeter.

3 Results and Discussion

The chalcogenides of y th group e lements are most stabl e and can be obtained by a simple, cheap and extremely convenient e lectro less so lution growth process developed in their laboratori ·H ,.211. In the process, the chalcogen ions are made free in an alkaline medium and are a ll owed to react with the metal ions in a bound complex state. The metal ions get free in the same alkaline medium and as a result of their strong affinity towards chalcogen, get depos ited on the substrate surface. The growth or the thin films is therefore a functi on of several parameters such as concentration of the basi c ingredi ents. deposition temperature, time, pH . speed of substrate rotation, etc.

3.1 Kinetic studies

The molar concentration of the bas ic constituents were therefore varied from a.oos to 0.5 M for obtaining both Bi 2S ~ and Bi 2Sel films . The growth rate was measured in te rms of layer thickness. It was observed that growth rate is initiall y linear and saturates at hi gh molar concentrati ons (beyond 0.1 M). The temperature and time dependent growth rates were also examined for the pure Bi 2S1 and Bi 2(SI -x ,Sexh films . It is apparent that the growth rate is influenced by both depositi on temperature and time211

. The temperature dependence of growth rate revealed that the growth rate is initiall y small indicat in g very thin fi lm layers. As the deposition temperature was increased (from 20-70°C) , the growth rate increased almost linearl y and decreased at hi gher temperature ow ing to the precipitation rather than the film formation . Thi s i:-­shown in Fig. ( I a). The optimum te mperature se lected for the deposition is 55 (Ie. The growth rate was also studied for variollS depositi on durations (90-180 min ). Fig. 1 b shows the film layer thickness for different depositi on times . It is seen that the variation is initiall y quasi-linear (up to 2 hI') and saturates thereafte r.

The effects due to the pH of the reac ti on mixture and speed of the substrate rotati on (rate of mechanica l churnin g) were also studi ed. At low pH va lues « 7), the films were difficu lt to obtain

INDIAN J PURE & APPL PHYS. VOL 39. SEPTEMBER 2001

because of the unavailability of the HS- and HSe­iO ll s whereas hi gher pH values(> II) yie lded very thin films . Therefore. a pH value of 8.5 ± 0.2 was :-,e lec ted for the quality deposition . Similarly the speed of the substrate rotation (mechanical c hll rnill ~) was va ried from lO-80 rpm. At low speed 1<3n rpm ) thi ck. porous. less st icky films were for illed. whil e very thin, adherent and reflecting dep()sih were obtained at re lative ly higher speeds (>6) rpm). In thi s case, the speed of the substrate rotation was kept around 50 rpm. The film thickness was also measured for various x va lues from 0 to I . Fi~ I c is 01 sketch of the thickness versus fi lm c()mposition. x . It is seen that the film thickness increased with increase in the Se-content in the bath ; the vari ation bein g linear.

:'-2 Reaction mechanism

Deposition of the binary and or mixed/alloyed chal eof!cnidc thin fi lms hy a solution growth pmces~ is now thoroughl y understood and in many of the cases it is reported. The primary aim of this \,\fork is to deposit the Bi2S1 and Bi2Se1 thin films individuall y to understand the film formation process. deposition kinetics and reaction mechanism of the Bi 2(S, _" Sexh mixed type films and then to examine their few of the properties . The formation t11" thc,c film '. and reaction mechanism could be lI ll de r~II)(ld in three different steps.

J .2.1 Mechanism of'the BhS" formation

T IllS in vo lves hydrolysis of thioacetamide in an aqueous alkaline medium to give S2- ions :

s II

SH I

SH

I . .. ( I )

H,c ' - C = NH + O H- ---) H,C - C- NH2 + HS- . . . (2) Acteamide

1 )

! I H.c -C- NH ::,+20H-+2 HS

---)CH3COOH+NH40H+2S 2-

acetic acid ... (3)

The ahove reaction sequence shows that the S2-~pL'c i c:-, I S a predominant species in the solution and

Bi 3+ is made available from dissoc iat ion of the

bi smuth triethanolamine complex givi ng ri se to:

rBiN(CH2CH20H h f+ ---) Bi 3+ + N ( CH2CH20H )1

E ~

0:35

.; 015 11\

" C x u E 0-15 I-

(0)

0

40 (b)

30

E c .; 11\ 20 III c x

/ 0

TEA . . . (4)

--- . -------

20 40 60 Temperature ,DC _ .

.....--. . . / /' Bif.le3 /0

° / I

I u /

J .c: I-

10

40 80 120 160 Deposition time,min

35 (c) /

/" / .

E 30 /0 c

.; /0 11\ III

/ • c .>< u

/ .c: 25 ° I- / .

/ °

20/ 0 0·2 0·1. 0-6 08

Se-content in bath (x )

Fi g. I - (a) Temperature dependent growth rate lor: Bi 2Se, (. ) and Bi2(S o. ~. Se 0.5) (0): (b) Growth rate as a function of the deposition time for Bi 2Se, layer: (c) Variation in 111m I<lyer thickness with the composition parameter. x

BHOSALE ef al .: Bi1 (SI_X' SeJ .1 THIN FILM COMPOSITES 577

The ove ra ll reac ti on is the refore g i ven as:

2 1 BiN ( CH "CH ~OH )l 1:1+

S

II pH=8.5±O.2

Bi ,S,+2N ( CH 1C H20H )1 + 3CH,CONH2 + 3 H20 TEA Acetamide

.. .(5)

The acetamide furthe r di ssoc iates to form acetic acid and ammonium hydroxide as react ion solution a:--:

C I-I ,COO H + NH~OH - H CH1COONH4 + 3H20 ., .(6)

He re ace ti c ac id and ammonium acetate act as a hllller that contro ls the IJH of the reac ti on solution thro ughout .

3.2.2 Mechanism of the BhSe.1 formation

III thi :-; case. Bi-triethanolamine compl ex IS

a ll owed to reac t with the Se-ions re leased by hydro lys is of sodium se lenosulphate as "":

(\ >SeSO, + O H- H Na2S04 + HSe­

HSt.: - + O H-H Se 2- + H~O

... (7)

. . . (8)

Thl' reacti o n:-; g iven in Eqs (4) and (8) revea l thaI the Bi '+ and Se2

- ions condense o n the ion-by­inn hilsi s on the substrate support. Thus:

Aqueous

+ 3 Na2SeSO, ---------------~

medium

. .. (9)

3.2.3 Mechanism of the Bh(S I_"Se,h formation

Here the bismuth conte nt in the bath was ke pt constant and the sulphur and se lenium contents were va ri ed to obey I-x and x, res pec tive ly. Obvious ly fi I illS were formed when the ionic product of the Hi \+ . S2. and Se 2

. exceeds the so lubility product of Bi ,(S ,."Se,) 1 T he reacti on could be formulated as"":

2fBiN( CH2CH20H), 13+ Bi-TEA complex

S

II + nH,C - C- NH2 + n .Na2SeSO, ---------~

Thioacetamide N a-selenosu I phate

Bi 2(SI _" Se,h +2fN(CH2CH20Hh l TEA

o II

+ n.NaZS04 +n(H1C-C-NH2) acetamide

. .. ( 10)

It is reminded that the depositi on conditions namely , temperature, time, pH , substrate rotati on speed, concentration of the bas ic in gredients e tc were ke pt identical as earli e r, whereas onl y the vo lume of Sand Se were changed so as to obtain ( I­x) and x. For the above set of ex pe rime nta l conditi ons. it has been found that the terminal layer thickness for BizS, is always less than the Bi1Se , and can obviously be related to lower so lubility product of Bi zS, than that of the Bi 2Se, 1 Ksp

(Bi zSe3 ) = I x 1O-97 10r highe r reac tivity of Se towards Bi than S .

3.3 Properties of Bi2(SI .x,Sexh composites

3.3.1 Chemical analysis

The compositio nal analysis was carried out by the ato mi c absorpti on spectroscopy and spectrophotometric techniques for both bath and the film contents. The bath and the film contents are shown in Table I . It appears from Table I that approximately one third of the total content taken in bath ente red in the film (unfortunate ly sulphur content in both cases could not be ana lysed du E' to ex perimenta l limits). The bismuth content observed in the film re mained the same throughout the composition range, whereas the selenium cont ent varied (as it was varied in bath) to approximat e ly one th i rd of that taken in the bath. It is a I so notab Ie that these results match within the error limit with that determined from the AAS technique (Table I). It can therefore be sa id that, the ana lys is method developed by us is sensitive enough compared to the AAS technique for the elemental analysis.

INDl AN] PURE & APPL PHYS. VOL 3LJ. SEPTEMBER 2001

T;lhlc I - Compos it ional an~ l ys i s ancl rew or the m~ t eri a l s propert ies or Bi2(S I_\ ' Sex), thin rilm structures

~I ( '0 1Il - Chelllic-, 11 anal ys i, /\AS analysis Crys- Power Ac ti vation ;\ I, plls i- ta ll ite ractor energies (c V )

t ll> ll size B;ll h contcnl Fi I III w nlenl B:11h COlllenl Fi 1111 cOllient A 11/ I~ "" C,'"

(pplll ) (ppm ) (ppm) (ppm ) (i-I T ) (LT)

Bi Se l3i Se Bi Se Bi Se

() 15(1 (I 5() 0 157 0 SO 0 62.4 0.45 062 1 0. 124 ~ 01 150 6.75 50 2.3R 152.3 6.5 49.9 1 2.25 0.50 O.6!):; 0. 124 -~ 0:: 150 1:1.6 50 4.H 153 13.2 49.R7 4.SR 76 0.55 0.695 n.ms

-1 0 .. ' 1')0 20.4 50 7.14 154. 1 20 49.90 7.75 n.53 0.570 (). ()91)

') 11.-1 1')0 27.26 50 R.OS 154.2 26.9R 49.63 R.2S 90 n.s:I 0.720 O. ( 1)9 h II .. ~ ISO .1-t 05 50 10.12 154.2 34.26 49.76 10.65 90 0.53 O. ()<)) 0. 124

11.(, I ')tl -10 .X 5(1 12 .24 ISS -11. 2 1 49 .5 12.25 90 055 o Ill») 0. 124 ;-.; 11 7 1.')0 -1 7') 50 14.62 I S3. X 4(,.4H 49. 32 14. 12 O.S I 0.670 o 14') ' ) tl .X I,)() ')4 .4 50 16.56 154.61 54.5 49.47 17 .0 I I 10 0.49 0.546 n. 17-1 I I I tl. ') 15(1 6:1.4 SO 17.62 154 115 49.3 I 17.H5 0.54 05·'1(, O. 14X I I 10 15(1 70.2 :')(1 20 152 72.2 49.26 19.09 130 OS; OA<J7 014:1

Tlhlc 2 - Compari son o r the standard ancl ohserved cla ta o r the Bi2S,. Bi2Se.1 and Bi 2( SI_x.SeJ ri l ills with the .l CPD clal;}

S;lllljli<' ()hserved /\STM 11/",:" 11/",:" IJ kl L all iee parameters

tlt: \ I d( A I (ohs) ASTM Observed A STM A A

~ . h-1 56.') 20.67 :20 020 S.Il')2 5 04 ~ 6. ~ II) 120 {/ = 11 .25 (/ = I I I ) _,t)(,t> l, t n -15.5 JX 220 .1.5(,0 35(, 100 100 J30 h = I J.29 /, = I I. l0 ~_2)2 :1 .256 HO I H 021

I ~ I,.'> ; 2J,I 2 2.64 1 54.5 24 3 1 I (. = 3.R57 (. = l .<)X

2.49:1 2.499 II 1:1 420 ::()<)2 2.0l)(, 13.6 II 250 1 -l·' ) 1 1.4l)O 5.5 6 242 .',()l5 3. 0:10 56 100 015 ~.)3 2.54 40 SO OIH 0=4. 144 {/ = -1.I TI ~.2 1 i 2.2l (,-I 60 j() I 0 I t)61 \';107 HO lO 0015 (' = 2R. 6 1 i ' = 2H.62

11 1,\ (' , IS'}.7 1.5 1 ') 10 20 02 10

1.-171 1.-175 6H 10 20 I I

1.352 1.35') 10 10 0021

-' .05X .' .022 100 100 01 7

11,,1 \. SC I, 2.55X 2.527 12 10 10 1 I a=4. 144 (/ = 4.15

I.Hm I .H 65 32 20 002 1

\ = II 7 1.511') 1 51 3 20 20 0216 (' = :19. I H (' = ]9. 1 t)

I .:i 7() UX I 1(, 10 11 2 1

BHOSALE et al.:Bi2 (Sr . ., Sex) 3 THIN FILM COMPOSITES '579

3.3.2 Structural studies

The X-ray diffractogram traces of the as­deposited Bi 2S \, Bi2Sel and Bi 2(S'.x,Sexh thin film sa mples were obtained in the 28 range from 20-80" with CuK" radiation (1.5406 A). The diffractograms showed microcystalline nature of the as-deposited samples with the peak intensities improved si<>nificantlv with the addition of selenium in the B~S .\. The 'observed d values of Bi2S), Bi2Se) and Bi ;(S ,.x,Sexh are in close consonance with the standard d-values. The comparison of the d-values for few of the reflections is given in Table 2. Films of lower thi ckness show little crystallinity however, crvstallinitv of the film improved with the increase in ' thickne~s . The average crystallite size was then determined for all the compositions (using slow scan) hy a FWHM method The size of a crystallite (d) is related to the wavelength of X-rays (A), half

angular width (B) and B~agg s angle (8 ) as:

O.94A rf = --­

HcosA .. . ( I I )

O LJ~SO~--------~5~50~--------~7t50'-----~ Wavele ngth ,," ( nrn)

Fi ~ :: - Ofllica l absorption spectra for various film structures: Ii) x = (J .n (0): (ii) x = 0. 1(.); (i ii ) x = 0.3 (0):

( iv ) x = 0.5 (~ ): (v) x = 0.7 (x) ; (v i) x = 1.0 (M

The crysta llite sizes for various x values are listed in Table I. The detailed invest igations on the

crystallographic aspects coupled with the microscopic features are under press2ll

.

3.3.3 Optical studies

The optical absorption spectra in the wavelength range 350-1300 nm were recorded for these fi I m structures (Fig. 2). The spectra revealed that the

absorption coefficient, (a) is larger for Bi 2Se" while comparatively smaller for Bi2S, and

Bi2(So5,SeoshThe a's for other compositions have (ntermediate values, however it has no systemattc dependence with the composition . It is also to be noted that the a values of Bi 2Se, and Bi2(S()7,Seo:1 h are comparable. An important indication from these studies is that the absorption edge is shifted towards hiaher wavelenath side as x was increased from () to b b

I . The energy gaps of all the fi I m structures were therefore computed from this data and its variation with x is shown in Fig. 3. It seems that Eg decreased, typically from 1.9 eV for Bi lS, to 0 .97 eV for Bi2Se3. The plots of (ahv)2 versus hv are non-linear and this is in good agreement with results of others. It is also to be noted that these plots are linear in the high energy region . This is an indication of the band to band direct type transitions involved in these films22.25 . The nature of transitions22.25.2f> was also tested by plotting In ahv

versus In (ahv - Eg)22.252f>. The plots are straight lines whose slopes are nearly 0 .5 (listed in Table I) confirming that the film structures are of the direct type22.25.2f> .

1.92~--------------, •

>~ ~ 1. 28 • 0' -------. W --.-e-·. 0-o 0' -0 0.64 c o III

o 0.2 0,4 0.6 o,a 1.0

80th composition

Fig. 3 - Variation in optical gap with Se content in ~ BiiS r." Sexl.1 tllm structu res

3.3.4 Electrical transport studies

The electrical transport studies were conducted on these films via the electrical conductivity and

5XO INDI AN J PURE & APPL PHYS. VOL 39. SEPTEMB ER 200 I

the rmopower measuremenr techniques. A two point prohe press contac t meth od was used f or these purpOSL:s. T he vari ati on of an elec tri ca l conducti v ity ~ I ~ ; 1 functi on or hath temperature and Se content in the fi lm was examined in the 300-550 K

temperature ran ge (Fig. 4). It is seen that the

b cr. - 2 n

-3

2.0 2.4 2.8 3.2 lOoo/r , K- 1

Fi ~ 4 - V;lrialion or lo!! () versus temper~ l ure and setenium conl enl inlhc Bi ~ (S , " Se,l, Ih in li lms: (i).\' = 0.0 ( ): I i i ) \ = (I I ( I): (iii ).\ = 0 :1 10 ): (iv).\' = 0.) ( -'. ): (v).\' = 0.6

IDi: (vi)r = O.X (. ): Ivi i ).\'= 1.0 (0)

temperature vari ati on o f an elec tri ca l conducti v ity showed A rrhenius behaviour showing two

conducti on regions; a high temperature region

w herein log () versus ilT variati ons is almost linear

and va ri ati on in log (j are sensiti ve to the vari at ions

in te mperature and a low temperature region in \V hich the electrical conducti vit y is less sensiti ve to ;Ippli ecl temperatu re. T he acti va ti on energies of an elec tri ca l conducti on in both regions have been c ilcul atcd and are g i ven in T ab le I . As f ar as the

variat ions o f an electri ca l conducti v ity with Se content is considered. it is seen that. the electrica l co nducti v ity is enhanced as the Se-content is increased . A t thi s stage, these vari ati ons are att ri buted to the decreased bandgap of the structures and the increased mobility for low gap materi als would ;il so contri bute to the increase in the electri cal conci l ict iv it y. T he temperature dependent thermo­e llli was also measured In the 300-450 K temper;l ture range ror all the samples . The samples are or 11-1 ype conduct ion and the generated thermo-

vo ltage is found to be direct ly proporti onal to thc

temperature di fference across the two ends of the samples. It appears that the thermo-emf increased w ith the increase in Se content in the films. In F ig. S, temperature variation of thermo-emf is shown for

f i ve typica l samples.

120

~ 90

---> ~

~ ..... E 60' CIJ

o E '­CIJ

.I:. f- 30

350 400

llT, K 450

Fig. ) - Vari~ li o l1 of thermo-e lectri c power with lemper;ilulT difference for various films: (i) x = 0.0 (e ): ( i i) .r = O.l( n): (iii ) x = 0.5 (L\); (iv) X = O.R ( A ) and (v) x = 1.0 (x )

4 Conclusions

The method of preparat ion deve loped to depos it Bi2(S I_x, Sexh is the most convenien t. ex tremely simple and cheap compared to other cos t intensive techniques. The quality of the deposits is found to be dependent on several preparati ve conditi ons and parameters. The deposits obtained by thi s technique are mixed ternary crystalline chalcogens of the

general composi t ion Bi2(S l -x ,Se,h. C rystal I ites o r the nanometer in size can eas il y be obtained and size of the crystal went on increasing w ith increase in se lenium content. The compos ites are highl y

absorpti ve (0.= 104_105 cm- I) and the opti ca l gap

decreased continuously w ith increased Se- content in the film . Transiti ons are of the direct type. T ransport studi es showed increase in an elec tri ca l conducti v ity w ith increase in x. The compos ites are of the n-type conduction.

BHOSALE el a/.:Bi2 (SI .,' SeJ .1 THIN FILM COMPOSITES SRI

Rcfcrences

Kaur I. Pilildya 0 K & Chopra K L. .I £Ieetrochem Soc. 127 ( I <)XO) !)43 .

Dcshillukil L P & I-Iolikalli S G . .I PhI's D .' AIJp l Pin's. 27 ( 1!)94) In(,.

:; Estral];] C A. N;lir P K . Nair M T S. Zi ngaro R A & Meyers [A . .l Gf' l' trt)('heIl/Soc. 141 ( 1994)802.

-1 Dcshllluk h I . P & Shahanc G S. IlItem atirJ//al .I Of'f· tmllif·s. x:; 11(97) 26l .

'i Nouri R N . Kohl P A & Bard A .I . .I Electrll,ltem Soc. 125 ( l!nX ) 1, 7::' .

II Sli ryanar;lyana C V. Lakshmanan A S. Subraillanian V & Kumar R K. /JIIII Eleetmchell/ . 2 ( 1986) 57.

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