Indian Journal of Chemistry A Vol. 44A, October. 2005, pp. 2034-2038

Notes

Structural, optical and electrical characterizations of spray deposited CdMo04

and CdW04 thin films

P K Pandey *, N S Bhave & R B Kharat

Department of Chemistry, Nagpur University, agpur 440 033, India

Email: pandeykp2003 @yahoo .co.uk

Received 24 lulle 2005; revised 28 August 2005

Preparation of cadmium mol ybdate (CdMo04) and cadmium tungstate (CdW04) thin films for the first time by 'spray pyroly­sis' using their ammon ical solution as precursor, is reported here. Growth of the films takes pl ace by pyrol ytic deco mposition of the spraying precursor so lution onto the preheated glass substrates X-ray diffraction studi es con firm the po lycrystalline, single-phase nature of the sintered films. Scanning electron microscopic im­ages of the thin fi lms clearl y show aggregates of microcrystallites. Optical absorpt ion spectrum in the range 350-850 nm shows di­rect as we ll as indirect opti cal transitions in both the material s. The plot of log( a ) versus I IT, within the temperature range 3 10-600 K for both the films indicates thei r semiconducting na­ture and shows a break in the curves. The thermal activation ener­gies below and above the break temperature have been esti mated to be 0.73 eY and 0 . 10 e for CdMo04 ; and 0.98 eV and 0.14 eY for CdW04 , respectively. The data have been analyzed using va­lence and conduct ion band model.

IPC Code: lnt CJ.7 C23C 16122; CO IG 11100

Molybdate and tungstate materia ls are useful because of their optical, chemical and structural prope rties . Among them, CdMo04 and CdW04 have many inter­esting properties such as high chemical stability, high averaged refractive index, high X-ray absorption coef­ficient, short decay time and low afterg low to lumi­nescence. Due to the interesting optical properties of CdW0 4, it has been used as a sc intill ator for detecting X-rays and y-rays in medical applications1.2. CdMo04 and CdW04 show interesting structural properties under pressure. There seems to be a tendency for the Scheeli te form of CdMo04 to transform to the Wo lframite form3 and for the Wolframite form of CdMo04 to transform to the Scheelite form4 under pressure . At room te mperature and atmospheric pres­sure, CdMo04 belongs to a Scheelite type structure5

with a space group of /4 11a=C411 . This structure has e ight symmetry e lements and a body centered ortho­rhombic primiti ve cel l that includes two formula units of CdMo04. Wolframite type structure of CdW04 is

in the monoclinic class with one axis not orthogonal to the other two6

. This Wolframite structure has a space group of Pvc=C 2h· In this structure5

, each tung­sten is surrounded by six oxygen sites in approxi­mately octahedral coordination.

Studies on CdW04 are focused o single crystal s and powders synthesized by Czochra lsk i method and sintered at high temperature?-9. Some studies a re a lso ava ilable on CdW04 thin films deposited by various techniques like pulsed laser deposition 10, sol-gel proc-

. II d I' 'd . 12 R I L / 11 ess mg an tqut ep ttaxy . ccent y, ou et a . · have reported luminescence properties of ZnW04 and CdW04 thin films deposited by spray pyrolysis.

Among all these techniques of thi n film deposition , spray pyro lysis is most advantageous due to its simple operation, low-cost and large a rea of depositi on 14

without the use of vacuum. Noth ing is reported so fa r on the preparation of thin fi lms of CdMoO~ though some reports 15 are avai lable on the e lectronic and op­tical properties of molybdate and tungstate of Cd(TI). Therefore, it was thought worthwhi le to prepare the thin fi lms of CdMo04 and CdW04 by spray pyro lys is and to investigate the ir structural, optical and e lec tri­ca l properties .

Experimental Thin film preparation

CdMo04 and CdW04 thin fil ms were deposited on g lass substrate obtained from Blue-Star. Ind ia by spray pyrolysis using an appa ratus described else­where16. The precursor solutions for CdMo04 and CdW04 thin films were the ammonica l solutions of their respective powdered material s synthesized by the precipitation method 17 The substrates used were ultrasonically cleaned, acetone-treated g lass slides. A 25 rnL of the precursor solution (0.05-0. I 25 M) was sprayed th roug h a specially designed glass nozzle onto the heated g lass su bstrates held at various tem­peratures rang ing from 300-450°C. Compressed a ir was used as carrier gas. The tlow rate, deposition time, nozzle to substrate di stance and frequency of the to-fro motion of the nozzle were kept constant at 5 mL/min , 5 mi n, 40 em and 0 .29 Hz, respective ly. After deposition, the films at an ambient temperature, were al lowed to cool slowly to room temperature and then taken out for further characte rization.

NOTES 2035

Material characterization The as deposited films were first examined under

an optical microscope (Leitz Orthoplan Microscope, Switzerland) to confirm the film deposition pattern, its uniformity and adherence to the substrate. The thin films were then sintered in air atmosphere at 350-850°C for crystallization, since the as deposited films prepared onto preheated substrates are always amorphous 18

. These sintered thin films were then subjected to XRD studies (Philips PW-1710 X-ray diffractometer) using Cu-Ka (radiation source) anode for structural characterization and phase identifica­tion. Surface morphology of the films was studied with scanning electron microscope (SEM) model JXA-840A, from JEOL, Japan with acceleration volt­age 20 kY. A gold coating was deposited on the sam­ples to avoid charging of the surface. The film thick­ness was determined by the weight difference density consideration method 19

• Optical absorption and transmission studies were carried out using Hitachi Spectrophotometer (UV -vis, NIR model 330, Japan) in the wavelength range 350-85 nm. To study the electrical properties of the thin films, dark resistivity measurements were taken using the two point probe method in the temperature range 310-600 K. Silver paste was applied to provide ohmic contact with the film20

.

Results and discussion Optimization of deposition parameters

The films deposited below 350°C at all the con­centrations were found to be non-uniform and not ad­herent to the substrate whereas no film deposition was observed above 350°C as revealed from the optical microscopic studies taken for the films at a magnifi­cation of 200x . Therefore, the substrate temperature for the film deposition was set to be 350°C. The opti­cal microscopic studies of the deposited films using the precursor solution of various concentrations at 350°C revealed that the films prepared at concentra­tion above 0.10 M were porous, non-uniform and non­adherent to the glass substrate. The film formation was not observed at concentration below 0.025 M . This may be due to unsuitable substrate temperature. At higher concentration, complete thermal decompo­sition of solution did not take place. Transparent films were obtained corresponding to the concentrations 0.075 and 0.1 M of the precursor solution. It was re­vealed from the optical microscopic studies that the films obtained from 0.1 M precursor solution were uniform without any agglomeration. Therefore, based

on the optical microscopic studies, the thin film depo­sition temperature was optimized at 350°C and the precursor solution concentration 0.1 M .

XRD and SEM studies Figure l shows the XRD patterns of the grown

films of CdMo04 and CdW04, deposited on the glass substrate at 350°C and also of the thin films annealed at 450°C. The spectra in Fig. la and 1 c indicate that the as deposited films at 350°C are amorphous in na­ture. After annealing at 450°C, the films exhibit an XRD pattern as shown in Figs lb and ld, consistent with the polycrystalline Scheelite structure of CdMo04 and the Wolframite structure of CdW04, respectively. All the peaks in the diffraction patterns were indexed on the basis of JCPDS data cards2

1.22

.

The observed 'd' values are in good agreement with the standard 'd' values . Therefore, the formation and single phase polycrystalline nature of the films are confirmed.

...-.. :::l

~ !/) -c :::l 0 0

Ill

Ill 400

I 2

012

10}

d

c

b

"'"'"t~~~•~•~r~•~•\:,; .. .,ii1Aj41AJ j a 10 20 30 40 50 60 70 80

Fig. I - XRD patterns of the films (a) CdMo04 lhin films as deposited; (b) CdMo04 thin films sintered at 450°C; (c) CdW0 4

lhin films as deposited; and (d) CdW04 lhin films sintered at 450°C.

2036 INDIAN J CHEM, SEC A. OCTOBER 2005

SEM images of the annealed films are presented in the Fig. 2. The CdMo04 film shows porous morphol­ogy (Fig. 2a) with a uniform crystallite size of 1 !J-Ill whereas that of CdW04 has compact morphology (Fig. 2b) with a non-uniform crystallite size. The SEM images of both the films clearl y show the aggre­ga tes of crysta llites. It can be seen that the crystallites are ordered in a layered like structure. This is the spe­cial feature of the spray pyrolysis deposition method23

Thickness of the film The th ickness of the prepared thin films was de­

termined using the relation 19 given in Eq . (1),

p = 111/(A .t) .. . (1)

where, 'm' is the mass of the thin film d1:posited onto the substrate, A is the area of the dep -sition of the

Fig. 2 - SEM images of the thin films at 5000x: (a) CdMo04,

(b) CdW04.

film, ' t ' is the film thickness and p is the density of deposited material which is assume to be the same as that of the bulk material (p = 5.35 g/cm3 for CdM o0 4 and p = 7.90 g/cm3 for CdW04)

24. Thickness of the films was found to be 0.78 and 0.85 ,urn for CdMo04

and CdW04, respectivel y.

Optical properties

The absorption coeffic ient, a, for the films is ob­tained using relation 25 as hown in Eq. (2),

(' )=~ I [ { 1-R(!.. ) ?] a"- t og,o T(!..) ... (2)

where, tis the th ickness of the fi lm, R(A) and T(A.) are the refl ectance and transmittance at pecific wave­length A.. The absorption coefficient is fo und to be of the order 105 cm-1

. To resolve the nature of the optica l transmission in the films, the absorption spectrum data are further analyzed as per the theory of Bardeen et aF6

. The optical band gaps for the deposited thin films are calculated on the basis of the optical spectral absorption using the fo llowing well -known relation27

,

Eq. (3),

h -1 n/ 0

a= k ( v ) (hv- £ 11) - .. . (3)

where, k is the constant, E~ the energy band gap, hv is the photon energy and n is equal to I fo r direct band gap and 4 for indirect band gap. Figures 3a and 3b show the plot of ( ahv )" and ( ahv) 112 versus (hv) for CdW04 and CdMo0.-1 thin films , respectively. The plots are linear, indicating direct and indirect type of optical transitions. The optical band gaps , thus ob­tained by extrapolating the linear portion to energy axis at zero absorption coefficients, are given in Table I. The direct transition is due to spin orbit va­lence band to conduction band while indirect transi­tion is due to transition from virtual state in valence band to conduction band minimum. lt is interesting to note that Abraham et al. also reported similar results15

on the basis of density functional calculations.

Electrical properties Ohmic contacts to the CdMo04 and CdW04 films

are made with si lver paste. The e lectrical resistance was measured in the temperature range 310-600 K. At room temperature, the film possesses res istiv ity of the order of 1010 O.cm and I 0 12 n .cm for CdMo04 and CdW04, respectively. The conducti vity of both the films increases with increase in temperature that indi­cates semiconducting nature of the thi n film materials.

NOTES 2037

3.0

2.5

N ~ 20 X

~

> .c ~ 1.5

0 5

0.0

2.75

N 2.25 0 ~

X

~ ~

Ius ~

1.25

- ------- 4 .00 a ; ·

3.50 .. • Direct band gap I • Indirect band gap 3 .00

/ 2 .50 co 0

I X

,/ . 2 .00N

I >

/ i .c

cs

I~ 1.50 ~

.f 1.00 / ; ..

+/ / : / :

/ . i 0 .50 ___.,'.'! i / _ ___./ I /

~-.-. I 0 .00

1.38 1.47 1.57 1 68 1.81 1.96 2.14 2.35 2.62 2.94 3 .36

Photon energy, hv (eV) -------~8.0

b

/1 :: I 50~

Ill

/ 4.0 ~ 3.0 ~

,/ 2 0

-+- Direct band gap

~ Indirect band gap

// . ' __ ........... /

/ 1.0

138 1 ~~ 1 57 168 1.81 196 2. 14 2.35 2.62 2.94 3 36

Photon energy, hv (eV)

Fig. 3 - Direct and indirect band gap or the thin films: (a) CdMo04 • (h) CdW04.

The plot of inverse absolute temperature versus log( cr) is shown in Fig. 4 which clearly indicates a break in the curves corresponding to the temperature 420 and 460 K for CdMo04 and CdWO.~ thin fi lms, respectively. The variation of log(cr) with liT is linear in the two regions of temperature, showing app lica­bi lity of the well -known exponential law [Eq. (4 )1,

CJ = CJ0 Exp (-E,/2kT) ... (4)

The act ivation energies have been calculated and li sted in the Table 1. The activation energy for con­duction is found to be low in the low temperature re­gion. Thi s low temperature conductivity can be con­sidered to be extrinsic (impurity dominated), whereas, conduction in the higher temperature region range may be regarded as intrinsic28

'29

. One of the major

-6 ,--------------- - --- -

-7

-8

c: ~ -10 OJ 0 _J

-11

-12

. .

a Cadmium molybdate

• Cadmium tungstate

-13~----~~~~--~--~----~~~-----1.5 2 2.5 3 3.5

Temperature, 1000fT (K-1)

Fig. 4- Plot of log( a ) vs. liT for the thin fil ms.

Table I -Physical data obtained for thin film materials

Properties CdMo0 4 CdW0 4

Bulk density (g.cm-3) 5.35 7.90

Thin film thickness (~m) 0.78 0.85

Optical band gap Direct 2.14 2.05

(eV) Indirect 1.42 1.52

Intrinsic 0.73 0.98 Activation energy Conduction

(eV) Extrinsic 0. 10 0. 14 Conduction

Temperature fo r (log (<J) VS 420 460 change of slope l iT plot] (K)

reasons for ex trinsic conductivity for these com­pounds may be due to the presence of other metal im­purities. Even very small amounts of an impurity can drastically mod ify the electrical properties of a semi­conductor. At higher temperature, these impurity at­oms generally are ionized and do not show their ef­fect30. In the compounds like CdMo0 4 and CdW04 ,

the 'd' shells for Cd2+ are fully occupied and there­

fore, hopping conduction31 is not likely to take place. High temperature conductivity for tbese molybdates and tungstates can also be explained using a band model where, a valence band can be thought of as comprising 2p orbital of 0 2

- whereas conduction band may be derived from empty metal orbital and 5d or­bital of Mo6+/W6

+ with antibonding admixtures of

2038 INDIAN J CHEM, SEC A. OCTOBER 2005

oxygen orbital 15• Intrinsic atomjc defects such as oxy­

gen vacancies and non-stoichiometry are most likely to be present in· these' compounds32

'33 which may give

lower energy g~ps in the intrinsic range.

Conclusions The CdMo04 and CdW04 films have been found to

be polycrystalline and sihgle phase in nature after sintering at 450°C, as revealed from their XRD spec­tra. The SEM images show the uniform crystallite size of l ~ for CdMo04 films whereas that for CdW04 films, between l-5 Jlm.

Acknowledgement Sincere thanks are due to Dr. C H Bhosale, Profes­

sor of Physics, Department of Physics, Shivaji Uni­versity, Kolhapur, India for providing necessary labo­ratory and instrumentation facilities for the thin film deposition. One of the authors (RBK) is thankful to University Grants Commission, India for the award of Emeritus Fellowship and the financial support to carry out this research work.

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