Iteratively derived optical constants of MoO3 polycrystalline thin films prepared by CVD

~7"70

7"7

Thin Solid Films 304 (1997) 39-44

ELSEVIER

Iteratively derived optical constants of MoO 3 polycrystalline thin films prepared by CVD A. Abdellaoui a, G. Lrv~,que b, A. Donnadieu c, A. Bath d, B. Bouchikhi a a D@artement de Physique FacuItd des Sciences, Unicersit~ MouIay Ismail, Bdni M'hamed B.P, 4010 Mdkn~s, Morocco b Laboratoire d'Analyse des Interfaces et Nanophysique, Universitg Montpellier II Sciences et Techniques de Languedoc Place Eugene BatailIon 34095, Montpellier Cedex 5, France c Laboratoire d' Electrooptique des Couches minces Universitg Montpellier II PLace Eugene BataiIIon 34095, Montpellier Cedex 5, France d Laboratoire Interfaces Composants et Microdlectroniqne, CLOES-Supdlec, Universitd de Metz 2 Rue E. Belin 57078, Metz Cedex 3, France

Received 5 June 1996; accepted 6 February 1997

Abstract

Polycrystalline MoO 3 thin films have been prepared by oxidation at high temperature of molybdenum compound layers deposited by chemical vapour deposition (CVD) from molybdenum hexacarbonyl Mo(CO) 6. In this paper is presented the experimental procedure to prepare polycrystalline MoO 3 coatings by annealing at various temperatures and in various annealing environments of "black molybdenum" or "reflective molybdenum" thin films. From X-ray diffraction measurements the structure of the films has been determined. The optical constants n and k of films were calculated by numerical iteration, using the Fresnel laws and dispersion relations of n and k. From these results the optical gap Eg has been deduced. All the samples exhibited the electrochromic effect. © 1997 Elsevier Science S.A. Keywords: Chemical vapour deposition; Molybdenum oxide; Optical properties; Oxides

1. Introduction

During the past decade a great interest has been shown in the study of transition metal oxide (WO 3 and MOO3) thin solid films. The reason is that these transition metal oxides present a number of interesting optical and electrical properties [1-10]. Among these properties is the electrochromic effect. Electrochromic devices are of particular interest for information displays, variable transmittance, variable-reflectance mirrors and variable-emittance surfaces [11-17]. Until now, various methods (thermal or electron beam evaporation, r.f. sputtering, spray, anodic oxidation, solgel, hydrothermal treatment) have been used to prepare amorphous and polycrystalline WO 3 or M o O 3 thin films [1-17]. In this paper is presented the experimental procedure to prepare polycrystalline M o O 3 thin film coatings by annealing, at various temperatures and in various annealing environments, "black molybdenum" or "reflective molybdenum" thin films deposited by chemical vapor deposition (CVD) from molybdenum hexacarbonyl Mo(CO) 6. The structure of different types of thin films obtained have been determined. The optical constants: refractive index n and extinction coefficient k of films 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PH S 0 0 4 0 - 6 0 9 0 ( 9 7 ) 0 0 0 9 2 - 8

were calculated by numerical iteration, using the Fresnel laws. From these results the optical gap ~ was deduced. Electrochromism is observed in these compounds.

2. Experimental procedure The black molybdenum (BMo) films were chemical vapor deposited by pyrolysis of molybdenum hexacarbonyl Mo(CO) 6 in the presence of an oxygen bleed in a radiatively heated quartz reaction chamber. The reflective molybdenum (RMo) films were prepared by the same method but in the absence of an oxygen. The Mo(CO) 6 is sublimed in a source vessel which is heated with an oil bath to 70 °C. The temperature of the plumbing between the source and reaction vessel must be held between 70 and 150 °C to prevent either condensation or premature decomposition of the carbonyl. About 1700 cm 3 min-i of argon carrier gas (25% through the source) is combined or not with less than 2 cm 3 min-1 of oxygen, and directed into the reaction chamber. The pressure inside the reactor is maintained at 1 atm, and the graphite susceptor is heated up to 300 °C. Fused quartz (for optical measurement) or

40

A. Abdetlaoui et at. / Thin Solid Fihns 304 (1997) 39-44

fluorine-doped SnOa-coated pyrex (for electrochromic measurement) substrates were used. Substrate cleaning included the following steps: acetone rinse, ultrasonic agitation (in distilled water), distilled water rinses and a dry nitrogen blow-off. After deposition, the BMo or RMo films were annealed, until the moment they became transparent, in three different ways: at 600 or 500 °C in air, and at 500 °C in air enriched with oxygen flowing at a rate of 1.5 cm 3 min- ~ (with 10% of oxygen and 90% of argon). In the remainder of this paper, these procedures are denoted as the first, second, and third mode, respectively. After oxidation, the samples were kept in the chamber until room temperature was reached. More details of the preparation method are given in Refs. [18-20]. X-ray diffraction (XRD) measurements were made with a Philips X-ray diffractometer using monochromatized the Cu Koe incident wavelength (3. = 0.15405 rim). The current across the tube was 20 mA, and the power 800 W. The measurements were made by the accumulation method [211. The transmittance T~p(3.) and reflectance Rexp(3.) spectra were recorded between 0.25 Ixm and 2.5 tzm on a Beckman UV5240 spectrophotometer equipped with an integrating sphere.

3.

Optical

properties

determination

The experimental data concerning M o O 3 were obtained in the form of reflectance, Rexp(3.), and transmittance, Te×p(3.), curves. They present interference fringes, particularly in the visible region. As a first approximation, the refractive index n values can be estimated from the wavelengths of reflectance maxima and minima, using the classical expressions: n = L3.mi~/2d or n = ( L + 1 / 2 ) A m , x / 2 d , when the substrate index is lower than the film index, with 3.m~ and 3.m~xthe wavelengths at the interference extrema, d the film thickness and L being an integer. Unfortunately this simple method has two drawbacks. Firstly there are very few extrema in the R(A) curves and subsequently few n(3.) values. Secondly the method neglects the absorption index k(3.) which induces a phase shift in boundary reflections, and may be important in the high energy part of the spectrum (above the optical gap). To avoid these difficulties, we may use the complete Fresnel formalism [22], using the following equations depending on the complex index ~ = n + ik: R = ] ~ ] 2 /5

T = n,l?l 2 ~ =

71 + ?2 ~2 1 ÷ ?172 y2 ~1t'2 ~'2 1 + rlrze-

where: 1-~ ?!

2

i +fi

1 +fi ~,2= - 2~

fi--ns

with F = e x p ( i ~ ) , d is the film thickness, 3. is the wavelength, n~ is the substrate index, F1, 7I, and ?2, ?'2 are the respective Fresnel reflection and transmission coefficients for the vacuum-layer and the layer-substrate boundaries. The determination of the optical constants n and k consists of solving the following system [23-25]: R ( n , k , A ) = R,xp(3. ) r(n,k,3.)

=

The system is easily solved numerically by successive approximations of n and k, which minimize the quality function:

o= (r al -

+

2

The iterative solution is stopped when a defined low Q value is achieved. The main difficulty lies in the precise determination of the film thickness d, which is not straightforward in any method. We consider here the thickness as an adjustable parameter, determined in order to obtain consistent n(3.) values as detailed below: - For any d value, the Fresnel equations give multiple n(3.) and k(3.) solutions, due to the periodic nature of the ~ expression. If we plot the n(A) values, we obtain several continuous curves in the 02,3.) plane [26]. - Among all these curves, one is the true n(A) solution, and it can be identified by using the dispersion relation binding the n(3.) and k(3.) curves, 1 - f0 n(e)) = 1 + -~-e)

~ d[ w'k(e)')] de)'

[of+e)[ , In --W-Z--~_ ~ dw

This relation implies that in the transparent region (k = 0) the index curve n(~o) is monotonic and slowly increasing versus o)(with w--2";rC/A). - For a given d value, we do not generally obtain any n(3.) curve satisfying the above criterion. This is due to the fact that for some A values, small errors in the experimental data give large deviations of the resulting n(A) values [27]. For other d values this phenomenon does not occur; then a convenient n(A) curve can appear as several segments suggesting, a continuous increasing curve. - If experimental R(3.) and T(3.) values are exact, only one d value will give a consistent continuous curve comprising the above segments. Practically, several n(A) solutions are computed for different d values, and the best result is selected as the most continuous n(3.) curve.

41

A. Abdellaouf et aL / T h i n Solid Films 304 (1997) 3 9 - 4 4

To resume, we obtain the correct d value by a self-consistent process, based on the assumption that the true n(3.) curve is continuous and slowly increasing versus energy. This method uses the dispersion equation only qualitatively, which seems satisfying in our case where k(3.) is always lower than I. In more complex cases, an iterative method, using the dispersion relation quantitatively has been used [26].

Table 1 Position of X R D peaks O for M o O 3 Prepared f r o m reflective m o l y b d e n u m at: 600 °C in air

Ref. [28] 6.388 11.663 12.843 I3.663 16.878 17.744 19.480

500 °C in air

6.44 11.71 12.90 13.70 16.91 19.56

500 °C in o x y g e n

6.40 11.69 12.87 I3.69 16.90 19.48

6.41 11.71 12.88 13.70 16.91 19.51

hkl

020 I10 040 021 111 121 060

4. Results and discussion

4.1. Structure and composition X-ray diffraction spectra of three samples prepared from RMo by the three annealing modes are shown in Fig. 1. The spectra for samples obtained from RMo reveal a marked texture with the (110) plane, whereas the samples obtained from BMo showed a marked texture with the (010) plane [28]. These planes are predominantly oriented parallel to the substrate. It is seen that the nature of the initial material (RMo or BMo) and the annealing mode, influence the magnitude of the peaks but not their position. The values of the angle of incidence O measured in our spectra for the strongest X-ray lines are given in Table 1. These values are in agreement with those calculated from

the parameters of the orthorhombic modification of MoO 3 (a = 3.9628, b = 13.8550, c = 3,6964 A) [29]. The slight differences observed in the O values reported in Table 1 can be due to the polycrystalline state of the films and to the imperfect orientation of the crystallites. RBS measurements were also performed [30]. For all the samples, the stoichiometry was M o / O - 0 . 3 3 , and thus the formula of the various studied samples is M o O 3. We can conclude that all our films deposited by CVD are polycrystalline, and that the MoO 3 obtained is crystallized in the orthorhombic form.

4.2. Optical results Fig. 2(a) and 2(b) shows typical spectra of hemispherical transmittance and reflectance of three films, prepared

RMo annealed at : 600°C in a i r 500°C in a i r - - - - - 500°C in oxygen - - - - -

(a) 1.0 0,8

,=_

(11o)

t

b 0.2

0.3

'

.Z,

~

.5

.

6~

~

,7

~

.8

~

12

~ t[6

'

t

2,0

I

J

24

% (am

(b) 1.0 RMo annealed at : 600°C in air 500°C in s.ir - - -- - 500°C in oxygen,

0.8

Illo)

c

0.6 0.4

,

,

0.2

I[(o o)l

(o2o)

....11 I1

03

,Z,'

.; .

;.

.

'7

8

,S

.

.

. . 24 2,0

k(9- Ill) 1

~

~

IC rl

12 I~ 14 F5 IS IG 18 ~ 20 2r

o(°)

Fig. 1. X R D spectra for M o O s films prepared f r o m R M o annealed at: (a) 600 °C in air, (b) 500 °C in air, and (c) 500 °C in oxygen.

Fig. 2. (a) Transmittance spectra versus w a v e l e n g t h for M o O 3 films annealed f r o m R M o by the three annealing modes. (b) Reflectance spectra versus w a v e l e n g t h for M o O 3 films annealed f r o m R M o by the three different modes.

42

A. Abdellaoui et a L / Thin Solid Films 304 (1997) 39-44

0,4

3,0 IGMo - A n n e a l e d at:

0,3

600°C ta a t r - X

.g ..=

500~C m an- . . . .

2,5

500°C in o x y g e n - 600~C m m r •. =

2,0

0,2

5 o o ~ c ~. =

.....

/

u~

0,I

1,5

I

•

1

:

q

I

q

r

2 3 Photon energy (eV)

I

,

0,0

i

4

J ,.2 '

I

0

I

1

I

'

l

l

I

2 3 Photonenergy (eV)

4

i

5

Fig. 3. Variation of refractive index, n, as a function of photon energy for M o O 3 films annealed from RMo by the three annealing modes.

Fig. 5. Variation of extinction coefficient, k, as a function of the photon energy for M o O 3 films annealed from RMo.

by annealing of RMo with the three different modes. Interferences are evident in all three spectra. The transmittance edge is about the same for all the samples. In the visible region the films are transparent. For MoO 3 samples prepared from BMo layers the results are similar [28]. In the following we present the optical results, calculated by the iterative method, concerning the samples obtained from RMo as well as from BMo. Fig. 3 shows typical spectra of the variation of refractive index n, obtained by the use of the numerical method, as a function of photon energy, for three films of M o O 3 film prepared by the three different annealing modes from RMo. We observe that the values of n are almost constant in the visible region and increase slowly in the high energy part of the spectrum (UV range). This variation of n with photon energy is similar for all the samples prepared from either reflective (Fig. 3) or black molybdenum (Fig. 4). For a given energy, n(A) varies with the annealing mode. As an example at A = 0 . 5 btm (2.5 eV) the value of n is included between t.96 and 2.08 for oxidized RMo and between 2.00 and 2.10 for oxidized BMo. At this wave-

length the values of the refractive index of the oxidized RMo and BMo samples with the second mode are slightly higher than those of the oxidized samples with the first and third mode. Our values are in overall agreement with those given in the literature for MoO 3 thin films prepared by other methods: 2.00-2.30 [2] and 2.38-2.70 [1,10,31]. The observed variations of n with preparation conditions (black or reflective deposited films, annealing modes) can be attributed to the influence of several parameters: crystallization, roughness, crystaltite size, point defects as confirmed from the SEM micrographs [28] and the XRD spectra, which affect differently the optical properties of the films [14]. The variation of the extinction coefficient k, as a function of photon energy, for the same samples are presented in Figs. 5 and 6. The values of k are nearly zero between 1 and 3 eV, which explains the transparency of these kind of films in the visible range. For the higher energies, the increase of k is similar for all samples and the values do not exceed 0.4 in the region studied.

3,0

0,4 BMo - ~ne~ed

at:

600#C ~ alr - 500~C m m r . . . .

x 2,5 .,=

0,3

/

~C~C m oxygea - -

/"

B M o - a n n e a l e d at.

"

*d

"-~ 2,0

600oC m ~ r

j

~u--

0,2

ua

0,I 1,5 i

0

I

1

r

I

2

t

P

i

3

I

4

Photon energy (eV) Fig. 4. Variation of refractive index, n, as a function of photon energy for MoO3 films annealed from BMo by the three annealing modes.

0,0

,

I

t

~

1

2

,

I

,

3

I

4

5

Photon energy (eV) Fig. 6. Variation of the extinction coefficient, k, as a function of photon energy for MoO 3 films annealed from BMo.

A. Abdellaoui et aL / T h i n Solid Films 304 (1997) 39-44

800

/

RMo - amica]ed at.

600

600°C in air

,

500°C in air

•

5®~C in oxygea

"7

i

43

values which give a correct self-consistency in the above described method. This leads to a " d " uncertainty in the order of 50 A. This uncertainty induces a dispersion in the n and k values which can be estimated as An = _+0.02 and A k = +_0.002.

•

400

5. C o n c l u s i o n

,

200

Z

,, :,:

~:::,,::;

~ .,,':'":' '~ ~" ~--'~; ~ I

2

,

,

3 4 Photon energy (eV)

Fig. 7. (o~E) W2 versus photon energy for three MoO 3 films prepared by the three annealing modes from RMo.

The optical gap Eg, was determined from Tauc's formula [32]: (c~e) 1/2 = A ( E - E g ) where e~ is the absorption coefficient, E is the photon energy and A is a constant. The variation of (oeE) t / 2 v e r s u s photon energy is shown in Figs. 7 and 8 for three samples prepared by oxidation of RMo and three samples by oxidation of BMo with the three annealing modes. Plots of (oeE) ~/2 versus photon energy are linear above Eg. The optical gap was deduced from the intersection of the extrapolated linear part with the energy axis.The values of the optical gap for all the films of MoO 3 are about 2.79-3.1 eV with a slight decrease in Eg value for the samples oxidized with the third mode (500 °C in oxygen).The values are in agreement with the literature results. It was found that 2.70 < Eg < 3.20 eV pertained to films made by evaporation and sputtering methods [2,33,34]. An estimation of the accuracy in the determination of n(A) and k(X) is obtained by observing the interval of d

800

---'- 600

' .7/ t

/;/

BMo - a.nne aJcd at 6 0 0 ' C in air

•

$00'C in ah"

*

500~Cin ox3,gem

•

/

'o~

400

//

2O0 Tlli~iP I

1

II

itnlll

I

2

l

/ [

1

l

3 4 Photon energy (eV)

5

Fig. 8. (c~E) W2 versus photon energy for three MoO 3 films as in Fig. 4 prepared by the three annealing modes from BMo.

The present values of the optical constants, obtained from the numerical iterative procedure, are more satisfactory than those reported earlier [28] obtained by the classical method based on the expressions n = L,'kmin/2d or n = (L + 1/2)Xmax/2d, which are limited to the zone of interferences (essentially in the visible range), and thus the number of points for the calculations is limited by the number of fringes. For thinner films, for which the number of fringes is reduced, the precise localization of extrema becomes somewhat difficult. Moreover, the classical method works poorly in the wavelength range where the film index may approach the substrate index n s. The optical constants of polycrystalline MoO 3 thin films prepared by CVD have been calculated using a numerical method, based on the use of Fresnel's formalism. The optical gap, Eg has been deduced from these results. These optical properties vary with the preparation conditions, which influence the crystallinity and the morphology of the films. It could be possible that these kind of materials has an interest in electrochromic devices, for display and windows applications.

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A. Abdetlaoui et al. /Thin Solid Films 304 (1997) 39-44

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