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Optical investigation of electron-beamdeposited tungsten-tellurite (TeO2)100- x (WO3) x amorphous films M. Emam-Ismail Shaltout

a e

, E.R. Shaaban

b e

, M. El-Hagary

c e

& I.

d

a

Physics Department, Faculty of Science , Ain Shams University , 11566 Cairo, Egypt b

Physics Department, Faculty of Science , Al-Azhar University , 71452 Assuit, Egypt c

Physics Department, Faculty of Science , Helwan University , 11792 Cairo, Egypt d

Physics Department, Faculty of Science , Al-Azhar University , Nasser City 18841, Cairo, Egypt e

Physics Department, College of Science , Qassim University , P.O. 6644, 5145, Buryadh, Kingdom of Saudi Arabia Published online: 28 Jun 2010.

To cite this article: M. Emam-Ismail , E.R. Shaaban , M. El-Hagary & I. Shaltout (2010) Optical investigation of electron-beam-deposited tungsten-tellurite (TeO2)100- x (WO3) x amorphous films, Philosophical Magazine, 90:25, 3499-3509, DOI: 10.1080/14786435.2010.489890 To link to this article: http://dx.doi.org/10.1080/14786435.2010.489890

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Philosophical Magazine Vol. 90, No. 25, 7 September 2010, 3499–3509

Optical investigation of electron-beam-deposited tungsten-tellurite (TeO2)100Zx(WO3)x amorphous films M. Emam-Ismailae*, E.R. Shaabanbe, M. El-Hagaryce and I. Shaltoutd


a

Physics Department, Faculty of Science, Ain Shams University, 11566, Cairo, Egypt; b Physics Department, Faculty of Science, Al-Azhar University, 71452 Assuit, Egypt; c Physics Department, Faculty of Science, Helwan University, 11792, Cairo, Egypt; d Physics Department, Faculty of Science, Al-Azhar University, Nasser City 18841, Cairo, Egypt; ePhysics Department, College of Science, Qassim University, P.O. 6644, 5145, Buryadh, Kingdom of Saudi Arabia (Received 31 January 2010; final version received 26 April 2010) Amorphous films of (100 x)TeO2–xWO3 with compositions 7.5 x 40 mol. % were prepared by electron-beam evaporation. The compositional dependence of the optical properties of the prepared films was analyzed by the Swanepoel envelope method, which revealed that the refractive index increases with increasing tungsten oxide content. The Wemple–DiDomenico dispersion model was used to explain the refractive index increase in terms of the formation of WOTe bonds, which have a higher energy than that of TeOTe bonds. A fitting of the spectral dependence of the absorption coefficient to the Tauc relation allowed a determination of the optical band gap, E opt g , which is found to decrease linearly with increasing tungsten oxide percentage. Keywords: tungsten-tellurite glass; thin film; refractive index; optical properties

1. Introduction Investigation of the physical properties of bulk tungsten-tellurite (TeO2–WO3) glasses is an attractive and promising field of research since they are useful for optoelectronic devices, integrated optics, nonlinear optical effects and many other optical applications [1–6]. TeO2–WO3 glasses are non-hygroscopic and can be easily prepared in large sizes because they have a low melting point (800 C) and a low glass transformation temperature (310–360 C) [7–9]. In addition, the glasses have a wide range of spectral transparency throughout the visible to the near infrared (0.35–5.5 mm), a high refractive index (2.1–2.3) and a high dielectric constant (30–32). The structure, thermal and optical properties of bulk tungsten-tellurite [(100 x)TeO2–xWO3 (5 x 50 mol. %)] have been investigated using Raman spectroscopy, differential calorimetry and Fourier transform infrared spectroscopy [10,11]. Generation of second harmonic signals has been reported from poled tungsten-tellurite glass disks of fixed composition (85TeO2–15WO3) [12,13]. *Corresponding author. Email: [email protected] ISSN 1478–6435 print/ISSN 1478–6443 online ß 2010 Taylor & Francis DOI: 10.1080/14786435.2010.489890 http://www.informaworld.com

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M. Emam-Ismail et al.


The structure and vibrational properties over a full composition range of bulk tungsten-tellurite glasses have been theoretically investigated using a continuous network model based on quantum chemical calculation and Raman spectra analysis [14]. Recently, tungsten-tellurite [(100 x)TeO2–xWO3 (5 x 30 mol. %)] thin films doped with Er3þ prepared by radio-frequency magnetron sputtering have been studied using Raman scattering and photoluminescence techniques [15,16]. In this paper, a detailed investigation is reported of the optical properties of tungsten-tellurite amorphous thin films [(100 x)TeO2–xWO3 (7.5 x 40 mol. %)] prepared by electron-beam deposition.

2. Sample preparation and experimental techniques Bulk glass samples of composition (100 x)TeO2–xWO3 (7.5 x 40 mol. %) were prepared from reagent-grade TeO2 (Alfa Johnson Matthey Electronics, purity 99.995%) and WO3 (Alfa Inorganic Ventron, purity 99.996%) powders as starting materials. These were weighed according to the atomic weight percentages to obtain the intended composition. They were then milled together in an agate mortar for about 30 min. The milled mixtures were then placed in a platinum crucible and melted in air in a preheated furnace at 800 C for about 30 min. For other compositions (33 x 40 mol. %), the temperature of the preheated furnace was raised slowly from 800 C to 1000 C for the same period of time. These conditions were found to produce good homogeneity of the glasses. Once complete fusion was achieved, the molten glass was removed from the furnace and poured onto a stainless-steel plate and cooled rapidly to room temperature in air. The color of the glass pieces changes from yellow to light green to dark green as the percentage of WO3 increases from 7.5% to 40%. The amorphous nature of the prepared glass samples was confirmed by X-ray diffraction (for further details, see [10,11,17]). Amorphous thin films of (100 x)TeO2–xWO3 (7.5 x 40 mol. %) with different compositions were obtained by electron-beam evaporation in an Edward 306 Auto high-vacuum coating unit. To ensure good quality deposited films, the substrates of either glass or fused silica were pre-cleaned using an ultrasonic hot bath, distilled water and pure acetone. Before depositing the films, the graphite boat containing the fragments of bulk glass was heated up slowly to release most of the oxygen imbedded within the glass. After oxygen release, the vacuum chamber was evacuated to a base pressure of about 9 107 Pa. During the deposition process, the substrates were kept at room temperature (300 K). The deposition rate was adjusted to be 5 nm/s. To obtain homogenous and smooth films, the substrates were rotated at 3 revolutions/min. The thickness of the produced films was monitored during the deposition process using an FTM6 thickness monitor to an accuracy of 5 nm (for further details, see [18]). The films thicknesses were checked independently after removal from the chamber using a F20 profile meter. The optical transmittance, T, and reflectance, R, of the deposited films were measured in the wavelength range 400–2500 nm using a computer-controlled UV– visible–NIR JASCO-670 double-beam spectrophotometer. The transmittance spectra were collected at normal incidence without a substrate in the reference arm, whereas the reflectance spectra were measured close to normal incidence ( 6 ) using

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a reflection attachment. In our measurements of R and T, the effect of slit correction was eliminated by adjusting the spectrophotometer slit width to 6 nm, which is much less than the width of the interference peaks observed in the transparent region of the investigated samples. All the optical measurements were performed at room temperature (300 K). The values of refractive index, n, and dispersion energy, Ed, were calculated by analyzing the room-temperature spectral transmittance, T(), and reflection, R(), as a function of tungsten content using the Swanepoel envelope method and the Wemple–DiDomenico dispersion model. The correlation between the variation of the refractive index of the prepared films and the composition has been explained using a single effective oscillator model that directly relates the strength of interband transitions and coordination numbers to the excess tungsten atom content. In addition, the linear variation of the absorption coefficients versus incident photon energy was used to determine the optical band gap as a function of the tungsten oxide content.

3. Results and discussion The amorphous character of the deposited films was examined and confirmed using X-ray powder diffraction. No prominent peaks were observed in the X-ray spectra. The transmittance and reflectance spectra of the (100 x)TeO2–xWO3 (7.5 x 40 mol. %) films as a function of incident photon wavelength are shown in Figure 1. As can be seen, the transmission spectra show ‘non-shrinking’ interference fringes at long wavelength (1000–2500 nm), which indicates good homogeneity and smoothness of the deposited films. Furthermore, the absorption

0.8

0.6 T&R


Philosophical Magazine

92.5TeO2-7.5WO3

T

85TeO2-15WO3 75TeO3-25WO3

0.4

R

70TeO3-30WO3 60TeO3-40WO3

0.2

0.0 400

800

1200 1600 Wavelength (nm)

2000

2400

Figure 1. Optical reflection, R(), and transmission, T(), spectra as a function of wavelength for amorphous films of composition (100 x)TeO2–xWO3 (7.5 x 40) evaporated on glass and fused silica substrates using electron-beam deposition.

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edge shifts toward longer wavelength as the WO3 content increases from 7.5 up to 40 mol. %.

Values of the film thickness, d, the refractive index dispersion, n() and absorption coefficient of the films were calculated by applying the envelope method first suggested by Manifacier et al. [19], applied by Swanepoel [20] and widely used by many researchers [21,22]. As shown in Figure 2, the refractive index can be directly calculated from the transmission spectrum. Following Swanepoel, the upper and lower envelopes of the transmission spectrum are calculated using a simulation program generated by applying more than one calculation procedure [23]. The refractive index of the deposited films is calculated in the transparency region, where the absorption within the film is almost negligible ( 0), using the following equations: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n ¼ N þ N2 s2 , ð1Þ where N ¼ 2s

TM Tm s2 þ 1 : þ 2 TM Tm

ð2Þ

Here s is the refractive index of the bare substrate and is given by [20]: 12 1 1 sðÞ ¼ þ 1 , Ts Ts

ð3Þ

92.5TeO2-7.5WO3

1.0

Range of calculating refractive index

TM

0.8 T&R


3.1. Calculation and analysis of refractive index and film thickness

Tm

0.6 T R

0.4

0.2

0.0 400

800

1200

1600

2000

2400

Wavelength (nm)

Figure 2. A typical optical transmission–reflection spectrum of 92.5TeO2–7.5WO3 thin film. The top and bottom envelopes, TM and Tm, of the transmittance spectrum are plotted as dashed lines.


3503

where Ts is the transmission of the uncoated substrate and TM and Tm are the values at the wavelengths in which the upper and lower envelopes show extrema (see Figure 2). Note that either TM or Tm is an experimentally defined interference maximum, whereas the interference minimum is extracted from the envelope method calculation. If ne1 and ne2 are the refractive indices at two adjacent tangent points (maximum or minimum positions) with wavelengths 1 and 2, then the film thickness d is given by d¼

1 2 : 2ð1 ne2 2 ne1 Þ

ð4Þ


Using different values of the film thicknesses, d, along with the value of n calculated from Equation (1) at a specific wavelength, the order of interference m can be obtained using the following equation: 2nd ¼ m:

ð5Þ

In Equation (5), the orders of interference, m, take integer values for maxima and half-integer values for minima. For more details of Swanepoel’s method, see [20–22]. The spectral variations of refractive index corresponding to films of (100 x)TeO2–xWO3 (7.5 x 40) are presented in Figure 3. The experimentally extracted dispersion curves for different compositions are shown in Figure 3 with different symbols, along with fitting to a Cauchy dispersion function drawn as a continuous line. The refractive index, n, is fitted to a second-order dispersion function of the form, n() ¼ a þ b/2, which can be extrapolated over the whole of wavelength range, as can be seen in Figure 3. A least-squares fit of the n values

Figure 3. Variation of refractive index, n, as a function of wavelength of the incident photons for different thin films of composition (100 x)TeO2–xWO3 (7.5 x 40). The experimental data points are well described by a Cauchy dispersion function (continuous line).

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Table 1. Values of Cauchy dispersion parameters, WDD dispersion parameters, Eo and Ed, static refractive index, n(0), energy gap, Eg, total number of valance electrons, Ne, the chemical valence of the anion, Za, and the coordination number (CN) calculated for (100 x)TeO2– xWO3 (7.5 x 40) films prepared by electron-beam deposition. Cauchy’s parameters Composition x


7.5 15 25 30 40

a

b

Eo (eV)

Ed (eV)

NC

Ne

Za

Eg (eV)

n(0) (1%)

1.98 2.02 2.06 2.11 2.15

2.54 105 2.60 105 2.83 105 2.92 105 3.02 105

3.11 3.18 3.19 3.23 3.24

16.73 17.78 18.79 19.86 20.24

4.15 4.30 4.50 4.60 4.80

8 8 8 8 8

2 2 2 2 2

1.26 1.17 1.01 0.97 0.84

2.53 2.57 2.62 2.67 2.69

corresponding to five different compositions give the Cauchy parameters that are listed in the second column of Table 1. Figure 3 shows clearly that the refractive index of the deposited films increases with increasing tungsten oxide content. This is consistent with reported results for bulk tungsten-tellurite glass [10]. The spectral dependence of the refractive index, n(), and its dependence on doping with tungsten oxide (WO3) of different percentage has been analyzed in terms of the Wemple–DiDomenico (WDD) dispersion model. This model describes the response of dielectric materials for electronic transitions below the optical gap by reducing the complicated valence–conduction band system to a two-level system with a simple single-oscillator formula having the following form [24,25]: n2 ¼ 1 þ

Eo Ed , E2o E2

ð6Þ

where E is the incident photon energy, Eo is the single-oscillator energy (which is associated with the average energy gap) and Ed is a dispersion energy, which measures the oscillator strength of interband optical transitions. It is worth mentioning that the WDD model has been tested for many materials with either covalent or ionic bonding, and for crystalline and amorphous structures. According to Equation (6), a plot of (n2 1)1 against photon energy squared (E2) should be a straight line with slope equal to (EoEd)1 and an intercept on the vertical axis of (Eo/ Ed) from which the values of Eo and Ed can be deduced. Figure 4 shows the linear fit to (n2 1)1 versus E2 for all deposited films. The values of the WDD dispersion parameters, Eo and Ed, for all film samples obtained from the linear fitting shown in Figure 4 are summarized in Table 1. As can be seen, a slight increase in the value of Eo is observed as the tungsten oxide percentage increases. The term Eo is considered as a measure of the average band gap (WDD gap) and corresponds to the distance between the centers of gravity of the valence and conduction bands. Therefore, Eo is directly related to the average molar bond energy of the different bonds existing in the studied films. Accordingly, we conclude that in the investigated system, (100 x)TeO2–xWO3 (7.5 x 40), the increase of Eo with increasing tungsten oxide content arises mainly from the larger bond energy of the WOTe linkages compared to the TeOTe linkages [14,26,27].



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Figure 4. A two-dimensional plot showing the variation of the refractive index dispersion factor, (n2 1)1, against photon energy squared, E2, for [(100 x)TeO2–xWO3 (7.5 x 40)] amorphous films.

More insight into the behavior of the dispersion in the refractive index in terms of the WWD model can be obtained by correlating the values of the Ed, which is related to oscillator strength of interband transitions, to the structure of the system under investigation through the following empirical relation [28]: Ed ¼ Nc Za Ne ,

ð7Þ

where is a constant that can take two values, either 0.26 0.03 eV for ionic materials or 0.37 0.04 eV for covalent materials, Nc is the coordination number of the cation nearest neighbor to the anion, Za is the formal chemical valence of the anion and Ne is the total number of valence electrons (core electrons are excluded) per anion. In Table 1, it can be seen that the values of Ne and Za have fixed values for oxide glasses and consequently the values of Ed depend only on the value of Nc [18]. Therefore, the variation of the refractive index of oxide glass can be understood as arising mainly from changes occurring in the cation coordination number with composition. Thus, the changes of refractive index behavior as a function of composition are principally attributed to the variation of the band gap with composition. An equation describing the static refractive index, n(0), for the system under investigation is obtained by letting the value of the incident photon energy, E, approach zero in Equation (6). This has the following form: Ed 1=2 : ð7Þ n¼ 1þ Eo The variation of the static refractive index n(0) as a function of composition is depicted in Figure 5 for all thin film samples studied. As can be clearly seen in Figure 5, a slight increase in the value of n(0) as a function of WO3

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M. Emam-Ismail et al. 2.75 2.70

n(0)

2.65 2.60 2.55

5

10

15

20

25

30

35

40

45


WO3 content (mol%)

Figure 5. Compositional dependence of the static refractive index, n(0).

Figure 6. Variation of (h)1/2 as a function of photon energy for (100 x)TeO2–xWO3 (7.5 x 40) thin films using the Tauc relation. The inset shows the variation of the energy gap as a function of photon energy.

content is observed. This increase of n(0) with increasing WO3 content is attributed to the large atomic polarizability of the W atom (with atomic radius 274 pm) in comparison with the atomic polarizabilities of both Te and O atoms of smaller atomic radii, namely 137 pm and 120 pm, respectively [29].

3.2. Calculation of absorption coefficient The variation of the optical absorption coefficient, , as a function of the incident photon energy for the investigated films can be directly calculated from the



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Figure 7. Variation of the optical band gap, E opt g , with different concentrations of WO3 (mol. %) in the films.

experimental transmittance, T() and reflectance, R(), spectra and film thickness, d, according to the following relation [30]: " 1=2 # ð1 RÞ2 þ ð1 RÞ4 þ 4R2 T2 1 ¼ ln : d 2T

ð8Þ

The calculated absorption spectra for all the films corresponding to different WO3 contents are displayed in Figure 6. A clear shift of the optical absorption edge toward lower energy (red shift) with increasing WO3 content from 7.5 mol. % up to 40 mol. % is observed. This red shift of the absorption edge should be associated with an increase in the refractive index values, as experimentally realized in Figure 3, which is consistent with what is expected from Kramers–Kronig dispersion relations [31]. The variation of the optical band gap as a function of composition can be obtained using the relation proposed by Tauc and applied successfully to explain the photon energy dependence of the absorption coefficient of amorphous semiconductors and oxide glasses [32]: ðhÞ ¼

2 Kðh E opt g Þ , h

ð9Þ

where K is constant dependent on the extent of the band tailing and E opt is the g optical band gap. Figure 6 shows the linear relationship between (h)1/2 and photon for the energy for (100 x)TeO2–xWO3 (7.5 x 40) films. The values of E opt g different compositions were determined by linear extrapolation of the straight line to the abscissa, shown magnified in the inset of Figure 6. These values are listed in Table 1 and represented graphically in Figure 7. It can be seen from Figure 7 that

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E opt g decreases linearly with increasing WO3 percentage. Such a decrease is attributed to the decrease in the concentration of Te–O–Te bonds when W–O–Te linkages replace Te–O–Te bonds.

4. Conclusions


Films of (100 x)TeO2–xWO3 (7.5 x 40 mol. %) have been successfully prepared using electron-beam deposition. The refractive index of the investigated films increases with increasing tungsten oxide content, a behavior that is attributed to WOTe bonds having a higher energy than those of TeOTe. The optical band gap, E opt g , of the films decreases linearly with increasing tungsten oxide concentration.

Acknowledgements The authors would like to thank the Deanship of scientific research at Qassim University, Kingdom of Saudi Arabia for support. In addition, the authors would like to thank Professor E.A. Davis and the referees for their valuable comments and suggestions.

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