Optical constants of As2Se3–Ag4SSe–SnTe amorphous thin films

Journal of Non-Crystalline Solids 353 (2007) 1618–1623 www.elsevier.com/locate/jnoncrysol

Optical constants of As2Se3–Ag4SSe–SnTe amorphous thin films S. Boycheva a

a,b,*

, A. Krasilnikova Sytchkova c, J. Bulir W. Kulisch a, A. Piegari c

c,d

, C. Popov a,

Institute of Nanostructure Technologies and Analytics (INA), University of Kassel, Heinrich-Plett-Street 40, 34132 Kassel, Germany b Technical University of Sofia, Department of Thermal and Nuclear Engineering, 8 Kl. Ohridsky Blvd., 1000 Sofia, Bulgaria c ENEA Research Center, Optical Coatings Group, Via Anguillarese 301, 00060 Rome, Italy d Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18121 Prague 8, Czech Republic Received 3 August 2006; received in revised form 15 January 2007 Available online 19 March 2007 This manuscript is dedicated to the memory of our colleague Dr Enrico Masetti

Abstract Amorphous thin films from the As2Se3–Ag4SSe–SnTe system were prepared by thermal vacuum evaporation from the corresponding bulk glasses. Their transmission, reflection, scattering and ellipsometric spectra were measured in the UV, VIS and NIR regions, and the results were mathematically simulated using the Tauc–Lorentz dispersion model in order to obtain the refractive index and extinction coefficient. The compositional dependence of the derived optical properties was found and discussed.  2007 Elsevier B.V. All rights reserved. PACS: 78.66.J; 78.66.D Keywords: Chalcogenides; Optical properties

1. Introduction The optical characteristics of As- and Ag-containing chalcogenide glasses (ChGs) belong to the most important properties of these materials closely related with their potential advanced applications. ChGs are low-phonon energy materials with a large optical window extended to the mid-infrared spectral region [1]. This wide range of high transparency combined with an appropriate viscosity–temperature dependence make these materials suitable for IR fibers with ultra-low optical losses [1,2], waveguides [2] and optical fiber sensors [3–5]. The non-linear optical properties observed for these materials are a consequence of the presence of external lone-pair electrons and large polariz* Corresponding author. Address: Technical University of Sofia, Department of Thermal and Nuclear Engineering, 8 Kl. Ohridsky Blvd., 1000 Sofia, Bulgaria. Tel./fax: +359 29652537. E-mail address: [email protected] (S. Boycheva).

0022-3093/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.026

able anions, and they are utilized in all-optical switching devices [2]. A number of reversible and irreversible photoinduced phenomena, such as photodarkening, photocrystallization, photoconductivity, photoluminescence, photodoping, etc., are registered under light-irradiation of the ChG media due to their flexible and defective structure with a high density of localized levels in the band gap. The photoinduced effects are accompanied by structural transformations and considerable changes in the optical constants, and are successfully applied for holographic information recording [6] and creation of high-efficiency holographic diffraction gratings [7,8]. ChGs are proven as reliable membrane materials for chemical sensors, microsensors and multisensor systems due to a number of advantages with respect to their polycrystalline analogous [9]. Knowledge concerning their optical properties could elucidate the network structure, electron state distribution and dielectrical properties of

S. Boycheva et al. / Journal of Non-Crystalline Solids 353 (2007) 1618–1623

these materials and help to clarify the potential-generating mechanisms and charge transport phenomena in sensor membranes. The multicomponent chalcogenide glasses from the As2Se3–Ag4SSe–SnTe system attracted our scientific interest because of the unique combination of the constituents: As2Se3 is an excellent glass-former; Ag4SSe is a networkmodificator, which cross-links the two-dimensional As2Se3 network and in combination with SnTe enhances the ionic conductivity of the vitreous material. These glassy alloys are expected to fulfil the requirements for some advanced applications of amorphous chalcogenide materials, such as chemical sensor membranes, optical windows and IR optical fibers, because they combine the properties of the excellent optical material As2Se3, the narrow-gap semiconductor Ag4SSe and the ionic conductivity contributing component SnTe. In the pseudo-ternary As2Se3–Ag4SSe– SnTe system a wide region of glass-formation was observed and the synthesized glasses are characterized with a high glass-transition temperature (102–173 C) and good stability against crystallization (the difference between glass-crystallization and glass-transition temperatures is higher than 100 C) [10]. The present paper is a continuation of previous investigations on the optical properties of amorphous thin films from the As2Se3–Ag4SSe–SnTe system and reports new data for the film optical constants computed by mathematical simulation models of the experimental spectra measured [11].

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SnTe films were prepared by thermal vacuum evaporation of the glassy materials in a standard high-vacuum set-up. Evaporation conditions allowing the preparation of uniform and homogeneous films with compositions close to those of the bulk multicomponent materials were established [11,12]. The evaporation process was carried out at a residual pressure of about 1.33 · 10 3 Pa, evaporation temperature range of 900–1000 C depending on the glass composition and a source–substrate distance of 0.12 m. Optical ‘corning glass 3003’ substrates were used for the purposes of our investigation; they were rotated during the deposition to prevent non-uniformities of the thickness of the thin films. Samples enriched in As2Se3, which provides a high glassformation ability and stability against crystallization, were chosen for the optical investigations. Optical transmission, reflection and scattering measurements of the As2Se3–Ag4SSe–SnTe thin films were performed in the spectral range 500–2500 nm with an UV/VIS/NIR double channel spectrophotometer (Perkin Elmer Lambda 900). The spectral ellipsometric measurements were carried out using a home-made null-type ellipsometer, in the spectral region between 250 and 860 nm and at the incidence angle of 70. The measured spectra were mathematically simulated using the Tauc–Lorentz dispersion model to determine the refractive index, the extinction coefficient and the optical band gap of the films. 3. Results

2. Experimental The As2Se3–Ag4SSe–SnTe bulk glasses were synthesized according to the conventional melt-quenching technique from preliminary prepared starting compounds. The results for the glass-formation in the As2Se3–Ag4SSe–SnTe system and the regimes of the direct mono-temperature synthesis of the vitreous alloys are described in details elsewhere [10]. For the proposes of our investigation, different compositions consecutively rich in As2Se3 glass-former, Ag4SSe network-modificator and SnTe ionic conductive additive were selected from the As2Se3–Ag4SSe–SnTe glass-formation region to draw information about the influence of each of the constituents on the optical properties behavior of the multicomponent chalcogenide alloys. The investigated compositions are given in Table 1. Thin As2Se3–Ag4SSe– Table 1 Optical band gap (DEg,opt) derived from the simulation of the optical and ellipsometric spectra, thickness and rms roughness of As2Se3–Ag4SSe– SnTe films Composition (mol%) As2Se3

Ag4SSe

SnTe

81 68 72

9 22 8

10 10 20

DEg,opt (eV)

Film thickness (nm)

rms roughness (nm)

1.56 1.44 1.41

653 453 468

0.54 0.58 0.34

3.1. Basic properties of As2Se3–Ag4SSe–SnTe thin films Composition, morphology, structure and some mechanical properties of As2Se3–Ag4SSe–SnTe thin films prepared by thermal evaporation technique have been thoroughly studied and discussed in details in previous publications [11,12]. The major results of this characterization concerning the basic film properties can be briefly summarized as following: The investigated films possess uniform, homogeneous, featureless and smooth surfaces typical for an amorphous phase, as revealed by scanning electron microscopy (SEM). Surface topology studies by atomic force microscopy (AFM) confirmed that the As2Se3–Ag4SSe–SnTe thin films are characterized by low rms roughness values (Table 1). Wavelength depressive X-ray (WDX) analysis carried out at five different points of each sample showed that the As2Se3–Ag4SSe–SnTe films are homogeneous with compositions close to those of the corresponding bulk glasses, with exception of differences exceeding 7 at.% observed for the silver concentration as a result of losses during the deposition (Table 2) [11]. The film stress was evaluated by the bending method using silicon micromachined cantilevers, on which the investigated films were deposited. The films possess tensile stresses,

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Table 2 Composition of the As2Se3–Ag4SSe–SnTe bulk glasses and the corresponding thin films (As2Se3)81(Ag4SSe)9(SnTe)10 (mol%)

(As2Se3)72(Ag4SSe)8(Sn Te)20 (mol%)

Composition (at.%)

Bulk glass Thin film Dispersion

Composition (at.%)

As

Se

Ag

S

Sn

Te

As

Se

Ag

S

Sn

Te

33.8 36.7 ±0.41

52.6 57.6 ±0.30

7.5 0.1 ±0.04

1.9 1.5 ±0.04

2.1 1.7 ±0.05

2.1 2.4 ±0.16

32.1 31.7 ±0.55

50.0 56.6 ±0.29

7.1 0.1 ±0.05

1.8 1.8 ±0.06

4.5 6.4 ±0.11

4.5 3.5 ±0.26

60 50 40 30 20 10 0

which increases with the concentration of Ag4SSe in the glassy alloy. The addition of up to 10–20 mol% Ag4SSe leads to a densification of the AsSe3/2 two-dimensional layered structure of the As2Se3 glass-former [12].

3.2.1. Transmission, reflectivity and scattering The transmission and reflectivity spectra of the asdeposited amorphous As2Se3–Ag4SSe–SnTe films are plotted in Figs. 1 and 2, respectively. Regions of high absorption caused by interband electron transitions are found below 700 nm. A wide transmission window with wellexpressed interference oscillations was observed covering the entire NIR region for all investigated compositions. The results from the measurements of the total scattering are shown in Fig. 3. They can be related to the results for the rms roughness of the films obtained by AFM given in Table 1. The film with the composition (As2Se3)72(Ag4S-

Reflectivity, %

3.2. Optical properties of As2Se3–Ag4SSe–SnTe thin films

60 50 40 30 20 10 0

c

b

50 a 40 30 20 10 0 500

1000

1500

2000

2500

Wavelength,nm 100 80

Fig. 2. Optical reflection spectra of amorphous (As2Se3)x(Ag4SSe)y(SnTe)z films, x + y + z = 100 mol%: (a) (As2Se3)81(Ag4SSe)9(SnTe)10; (b) (As2Se3)68(Ag4SSe)22(SnTe)10; and (c) (As2Se3)72(Ag4SSe)8(SnTe)20.

c

60

Se)8(SnTe)20, which possesses the smoothest surface is characterized with scattering below 0.5% in the whole measured spectral range from 380–900 nm. In correspondence with the higher surface roughness of the (As2Se3)68(Ag4SSe)22(SnTe)10 thin film, the total scattering exceeds 1% at k < 400 nm; at higher wavelengths the total scattering drastically drops down. Interference oscillations are seen in the long-wavelength end of the spectra.

40 20

Transmission, %

0 100 80

b

60 40 20 0 100 80

a

60 40 20 0 500

1000

1500

2000

2500

3.2.2. Spectral ellipsometry In addition to the spectrophotometric investigations shown above, spectral ellipsometric measurements have been performed. Typical spectral distributions of the ellipsometric angles of the As2Se3–Ag4SSe–SnTe films are presented in Fig. 4. Oscillations can be observed in all measured ellipsometic spectra, most probably as a result of interference effects.

Wavelength, nm Fig. 1. Optical transmission spectra of amorphous (As2Se3)x(Ag4SSe)y(SnTe)z films, x + y + z = 100 mol%: (a) (As2Se3)81(Ag4SSe)9(SnTe)10; (b) (As2Se3)68(Ag4SSe)22(SnTe)10; and (c) (As2Se3)72(Ag4SSe)8(SnTe)20.

3.2.3. Tauc–Lorentz dispersion model The measured transmission, reflection, scattering and ellisometric spectra were used as an input for the fitting

0.5 0.4 0.3 0.2 0.1 0.0

c

2.0

b

1.0

x=68; y=22; z=10 x=72; y=8; z=20 x=81; y=9; z=10

3.6 3.4 3.2 3.0 2.8

0.5

2.6

0.0

0.8

(As2Se3)x(Ag4SSe)y(SnTe)z

3.8

1.5

1.0

1000

a

2500

(As2Se3)x(Ag4SSe)y(SnTe)z 400

500

600

700

800

900

Fig. 3. Total scattering of amorphous (As2Se3)x(Ag4SSe)y(SnTe)z films, x + y + z = 100 mol%: (a) (As2Se3)81(Ag4SSe)9(SnTe)10; (b) (As2Se3)68 (Ag4SSe)22(SnTe)10; and (c) (As2Se3)72(Ag4SSe)8(SnTe)20.

0.5 0.4

Extinction coefficient

300

Wavelength (nm)

tan (psi)

2000

0.30

0.4

cos (delta)

1500

Wavelength (nm)

0.6

0.2 200

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4.0

Refractive index

Scattering. %

S. Boycheva et al. / Journal of Non-Crystalline Solids 353 (2007) 1618–1623

0.25

x=68; y=22; z=10 x=72; y=8; z=20 x=81; y=9; z=10

0.20 0.15 0.10 0.05 0.00

0.3

1000

0.2

1500

2000

2500

Wavelength (nm)

0.1 0.0 1.0

Fig. 5. Dispersion of the refractive index (a) and the extinction coefficient (b) of amorphous As2Se3–Ag4SSe–SnTe thin films obtained from the Tauc–Lorentz model.

0.5 0.0 -0.5 -1.0 200

300

400

500

600

700

800

900

Wavelength (nm) Fig. 4. Experimental ellipsometric spectra of (As2Se3)81(Ag4SSe)9(SnTe)10 thin film.

procedure applied for calculation of the optical constants refractive index (n), extinction coefficient (k) and absorption coefficient (a) in wide spectral range 350–2500 nm. The experimental and simulated spectra were fitted with the well-known Tauc–Lorentz dispersion model, which is based on multiplying the imaginary part of the complex dielectric function (e2) determined by the Lorentz oscillator model with the Tauc density of the states [13,14]. This approach has been developed especially for the parameterization of the optical function of amorphous semiconductors and insulators [15] and has been reliably applied for the modeling of the optical constants of a number of amorphous chalcogenide films [16].

The computed spectral distributions of n and k for the investigated films are plotted in Fig. 5. The refractive index of the As2Se3–Ag4SSe–SnTe films varies in the range 2.7 6 n 6 3.7 in the VIS and NIR spectral regions; these values are typical for amorphous chalcogenide semiconductors. The functions of n(k) and k(k) drop down rapidly with increasing the wavelength up to ca. 1000 nm, while at higher wavelengths n(k) is almost constant and k is zero for all investigated compositions (Fig. 5). The obtained values for the DEg,opt and the film thickness as a result of the spectra simulation procedure are reported in Table 1. Interference-free Urbach regions due to the structural disorder of the amorphous materials are situated at wavelengths between 550 and 650 nm. The absorption coefficient a was derived by detailed analysis of the absorption edge using the data for the refractive index and the thickness from the simulation with Tauc–Lorentz model; the obtained a(k) distribution is shown in Fig. 6. The position of the absorption edge changes slightly with the film composition (Fig. 6); an infrared shift is registered with increasing the SnTe content at a constant

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5

Absorption coefficient (cm-1)

1.2x10

(As2Se3)x(Ag4SSe)y(SnTe)z

5

1.0x10

x=81; y=9; z=10 x=72; y=8; z=20 x=68; y=22; z=10

4

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

0.0 550

600

650

700

Wavelength (nm) Fig. 6. Absorption edge of amorphous As2Se3–Ag4SSe–SnTe thin films.

Ag4SSe/(As2Se3 + Ag4SSe) relation, and when the Ag4SSe/ (As2Se3 + Ag4SSe) relation increases at constant SnTe content in the parent glasses. 4. Discussion The amorphous As2Se3–Ag4SSe–SnTe thin films are characterized by high values of the refractive index (Fig. 5(a)). For example, for the (As2Se3)72(Ag4SSe)8 (SnTe)20 film, it varies between 2.9 and 3.7 in the spectral region 600 6 k 6 2500 nm. In the region of high absorption caused by interband electron transitions, n(k) drastically increases when decreasing k, while in the high transmission region above 1000 nm n hardly changes with k. The refractive index depends significantly on the film composition and increases with the SnTe concentration, most probably related to the increased polarizability of the Te atoms, which are larger than the S and Se atoms. A similar trend in the compositional dependence of n has been observed for amorphous As–Se–S containing chalcogenide films [17]. The investigated films are characterized with low extinction coefficients for k > 550 nm, which become zero for k > 1000 nm. DEg,opt varies between 1.41 and 1.56 eV for the investigated compositions (Table 1). Some discrepancies in the DEg,opt with previous published values were observed [11], due to the differences in the approaches applied. Our suggestion is that the DEg,opt obtained using Tauc–Lorentz fitting is more accurate due to the simultaneous simulation of the experimental transmission, reflection and ellipsometric spectra, and the scattering effect correction. Tauc–Lorentz fitting model has been evaluated to give DEg,opt values with an accuracy of about ±0.01 eV [13]. Despite of the discrepancies in the value, the observation that the band gap narrows for the composition with 20 mol% SnTe which is connected with the creation of charged defect centers,

corresponding to the appearance of localized energy states close to the band edges, coincides with that made in the previous manuscript [11]. 5. Conclusions Thin amorphous film from the As2Se3–Ag4SSe–SnTe system were prepared by vacuum thermal evaporation and characterized with respect to their optical properties in the UV–VIS–NIR regions using spectrophotometric and ellipsometric measurements. The films are transparent at wavelengths above 700 nm. The spectra were mathematically simulated using the Tauc–Lorentz fitting model, and the spectral distributions of the refractive index and the extinction coefficient, as well as the optical band gap were derived. The refractive index varies in a wide range in the investigated spectral region (2.7 6 n 6 3.7) and depends on the film composition, taking values typical for chalcogenide glass semiconductors. A strong dispersion of n(k) and k(k) is observed for k < 1000 nm, while at higher wavelengths n(k) is almost constant and k is 0. The optical band gap, obtained as a fitting parameter in the computational model used, is in the interval 1.41–1.56 eV depending on the film composition. Acknowledgements The authors would like to thank V. Vassilev and P. Petkov (University of Chemical Technology and Metallurgy, Sofia, Bulgaria) for the glass syntheses and film deposition. S.B gratefully acknowledges the financial support of NATO under the Reintegration Grant PDD(CP)(CBP.EAP.RIG 981850) and Alexander von Humboldt foundation. References [1] A.B. Seddon, J. Non-Cryst. Solids 184 (1995) 44. [2] A. Zakery, S.R. Elliot, J. Non-Cryst. Solids 330 (2003) 1. [3] J. Keirsse, C. Boussard-Pledel, O. Loreal, O. Sire, B. Bureau, B. Turlin, P. Leroyer, J. Lucas, J. Non-Cryst. Solids 226–327 (2003) 430. [4] K. Michel, B. Bureau, C. Boussard-Pledel, T. Jouan, J.L. Adam, K. Staubmann, T. Baumann, Sens. Actuator. B 101 (2004) 252. [5] B. Bureau, X.H. Zhang, F. Smektala, J.L. Adam, J. Troles, H. Ma, C. Boussard-Pledel, J. Lucas, P. Lucas, D. Cog, M.R. Riley, J.H. Simmons, J. Non-Cryst. Solids 345&346 (2004) 276. [6] A.M. Andriesh, M.S. Iovu, S.D. Shutov, J. Optoelectron. Adv. Mater. 4 (2002) 631. [7] A.V. Stronski, M. Vlcek, A. Sklenar, P.E. Shepeljavi, S.A. Kostyukevich, T. Wagner, J. Non-Cryst. Solids 266–269 (2000) 973. [8] J. Teteris, M. Reinfelde, J. Non-Cryst. Solids 326&327 (2003) 494. [9] V. Vassilev, S. Boycheva, Talanta 67 (1) (2005) 20. [10] L. Aljihmani, V. Vassilev, P. Petkov, J. Optoelectron. Adv. Mater. 5 (2003) 1187. [11] V. Vassilev, S. Boycheva, C. Popov, L. Aljihmani, P. Petkov, K. Kolev, B. Monchev, J. Non-Cryst. Solids 351 (2005) 299. [12] V. Vassilev, C. Popov, S. Boycheva, L. Aljihmani, P. Petkov, K. Kolev, B. Monchev, Mater. Lett. 58 (2004) 3802. [13] G.E. Jellison Jr., F.A. Modine, Appl. Phys. Lett. 69 (1996) 371; G.E. Jellison Jr., F.A. Modine, Appl. Phys. Lett. 69 (1996) 2137.

S. Boycheva et al. / Journal of Non-Crystalline Solids 353 (2007) 1618–1623 [14] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley, 1998, ISBN 0-471-05772. [15] S. Bosch, J. Ferre-Borrull, J. Sancho-Parramon, Solid State Electron. 45 (2001) 703.

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[16] D. Fanta, I. Ohlidal, M. Frumar, J. Jedelsky, Appl. Surf. Sci. 175&176 (2001) 555. [17] J.M. Gonzalez-Leal, R. Prieto-Alcon, J.A. Angel, E. Marquez, J. Non-Cryst. Solids 315 (2003) 134.