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Deposition, morphology and stress investigation of amorphous As2Se3–Ag4SSe–SnTe thin films
Materials Letters 58 (2004) 3802 – 3806 www.elsevier.com/locate/matlet
Deposition, morphology and stress investigation of amorphous As2Se3–Ag4SSe–SnTe thin films V. Vassileva, C. Popovb,*, S. Boychevac, L. Aljihmania, P. Petkova,d, K. Koleva, B. Moncheva,d a
University of Chemical Technology and Metallurgy, Department of Non-Ferrous Metals and Semiconductor Technologies, 8 Kl. Ohridsky Blvd., 1756 Sofia, Bulgaria b Institute of Microstructure Technologies and Analytics (IMA), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany c Technical University of Sofia, Dept. of Thermal and Nuclear Engineering, 8 Kl. Ohridsky Blvd., 1000 Sofia, Bulgaria d Department of Physics, Laboratory of Thin Film Technology, 8 Kl. Ohridsky Blvd., 1756 Sofia, Bulgaria Received 23 April 2004; accepted 5 July 2004 Available online 8 September 2004
Abstract Thin films from the As2Se3–Ag4SSe–SnTe system have been prepared by vacuum thermal evaporation from the corresponding bulk glasses. The film structure and surface morphology have been investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The investigated chalcogenide films are amorphous, featureless and with smooth surfaces. The stress measurements have been carried out by a cantilever bending technique, and tensile stress has been observed. The obtained results have been comprehensively discussed with respect to the film composition and structure. The relationship between the stress and other mechanical parameters, like compactness and density, of the amorphous condensates has been established. The obtained dependencies confirmed the existence of a structural threshold in the glassy network at mean coordination numbers Z=2.27 and Z=2.56. D 2004 Elsevier B.V. All rights reserved. Keywords: Glasses; Thin films; Mechanical properties; Multicomponent chalcogenide glasses; Stress
1. Introduction The wide practical application of thin amorphous chalcogenide films, especially of the vitreous As-containing condensates, is closely connected with their transparency in the VIS and near IR spectral regions and with the possibility to create optical media with defined values of the refractive index, dispersion and extinction coefficients [1]. The relatively low energy of the chemical bonds in the Asbased chalcogenide glasses stimulates photostructural transformations and a number of light-induced effects, which are accompanied by considerable changes in the optical constants [2]. These reversible and irreversible photoeffects are successfully applied for optical information storage [3–5].
* Corresponding author. Fax: +49 561 804 4136. E-mail address: [email protected] (C. Popov). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.07.032
Another advanced application of the vitreous chalcogenide films is as membrane materials for ion-selective chemical microsensors and multisensor systems due to their excellent analytical characteristics [6–9]. The requirements for excellent adhesive strength and minimal residual stress are of a great importance for the technological performance of thin films in the field of optical and sensor devices. The film stress appears during the deposition process as a result of different thermal expansion coefficients of the film and the substrate, intrinsic residual thermal strains and structural rigidity. High stress values cause a number of negative consequences, e.g., bending of the film–substrate structure, peeling, cracking, etc. The glass-forming region in the pseudoternary As2Se3– Ag4SSe–SnTe system was determined by electron microscopy and by X-ray and electron diffractions revealing the glassy nature and the uniformity of the synthesized samples [10]. The glasses are located up to 45 mol% SnTe and
V. Vassilev et al. / Materials Letters 58 (2004) 3802–3806
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immersion fluid; their compactness (d) was calculated by the equation #1 ( )" X X M i xi X M i xi Mi xi ð1Þ d¼q qi q i i i
25 mol% Ag4SSe in the binary As2Se3–SnTe and As2Se3– Ag4SSe systems, respectively (Fig. 1). The maximum solubility of the Ag4SSe in the glassy alloys reaches about 30 mol%. The obtained chalcogenide glasses were investigated with respect to their thermal (temperatures of the glass-transition, crystallization and melting) and mechanical (density, microhardness and compactness) properties. These multicomponent chalcogenide glasses are characterized by good glass-forming ability, thermal stability (glass-transition temperature above 370 K), hardness of 0.76–0.89 GPa and density between 4.48 and 5.40 g/cm3, which are satisfactory for many practical applications. The aim of the present work is to deposit amorphous thin films from the bulk As2Se3–Ag4SSe–SnTe glasses by thermal vacuum evaporation and to investigate their structure, morphology, topography and stress with respect to the composition.
where M i , x i and q i are the molar weight, the fraction and the density of each component. Thin As2Se3–Ag4SSe–SnTe films were deposited from the synthesized bulk glasses by thermal vacuum evaporation in a standard high-vacuum installation bHochvacuumQ B 30.2. The process conditions were as following: 1.33103 Pa residual pressure in the chamber; 0.12 m source– substrate distance; the maximum evaporation temperature was varied in the region 1200–1300 K depending on the composition of the glasses. The evaporation time was determined from the desired thickness of the films. An inductively heated covered ribbon evaporator from Ta was used, which allows the preparation of uniform and homogeneous films from sublimating complex materials. The chalcogenide amorphous films were deposited on silicon crystalline substrates for scanning electron microscopy (SEM) and atomic force microscopy (AFM) investigations. The surface roughness of the thin films was examined by an atomic force microscope bNanoScope IIQ in a tapping mode. Their morphology, structure and thickness were investigated by a scanning electron microscope bHitachiQ S-4000. The film stress was evaluated by the bending method using silicon micromachined cantilevers, on which the chalcogenide films were deposited. These cantilever substrates consist of seven beams with a thickness of 45 Am, a width between 0.7 and 2 mm and a length between 2 and 8 mm, and this configuration allows accurate measurement of stress in a wide range. The deflection of the cantilever beams was measured by the depth of the focus of an optical microscope. The cantilever fabrication was described in details in Ref. [11].
2. Experimental
3. Results and discussions
Bulk (As2Se3)x (Ag4SSe)y (SnTe)z glasses, where x+y+ z=100 mol%, situated in the glass-forming region on two tie-lines with concentrations z=10 mol% and z=20 mol% (Fig. 1) were prepared. The compositions of the glassy alloys are listed in Table 1. As2Se3, Ag4SSe and SnTe preliminary synthesized from As, Se, Ag, S, Te and Sn (5N purity) were used as starting materials to guarantee a definite composition of the multicomponent alloys. Conventional direct monotemperature synthesis was carried out in evacuated (~103 Pa) and sealed quartz ampoules using a rotary furnace. Stepwise heating regimes were employed taking into account the physicochemical properties of the starting components [10]. The density of the samples (q) was measured by a hydrostatic method using toluene as
The preparation of homogeneous amorphous thin films from complex chalcogenide systems with a composition
Fig. 1. Glass-forming region of the As2Se3–Ag4SSe–SnTe system (.) glass, (o) crystal, (e) glass+crystal).
Table 1 Composition, thickness, rms roughness and height difference of (As2Se3)x (Ag4SSe)y (SnTe)z thin films, where m=x/(x+y) Composition (mol%) As2Se3
Ag4SSe
SnTe
81 68 58 72 64 52
9 22 32 8 16 28
10 10 10 20 20 20
m
Thickness (nm)
Max. height (nm)
rms roughness (nm)
0.9 0.8 0.6 0.9 0.8 0.7
288 386 324 426 378 318
5.92 5.22 2.79 3.65 14.48 11.56
0.54 0.58 0.31 0.34 1.29 0.74
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Fig. 2. SEM images of the (As2Se3)58(Ag4SSe)32(SnTe)10 thin film (a) top-view image and (b) cross-section.
close to that of the bulk material was carried out under appropriate conditions maintained during the vacuum thermal deposition process: evaporation temperature range 700–800 K, low substrate temperature (b300 K) and low evaporation rate. It can be supposed that the relatively low apparent evaporation rate of 0.12 kg/m2 s guarantees simultaneous evaporation of the structural units building up the As2Se3–Ag4SSe–SnTe glassy network, owing to their close values of the evaporation enthalpy. This leads to low deposition rates (around 0.7 nm/s in our case) which ensure simultaneous condensation of the structural units onto the substrate and consequently the close composition with the bulk materials. On the other hand, the evaporation under high vacuum (1.33103 Pa) ensures the creation of a molecular flow with a steady distribution of the flux on the entire substrate surface, resulting in smooth coatings with uniform thickness. This approach has been successfully used in other cases for deposition of multicomponent amorphous thin films with a definite composition [12]. Typical SEM images of As2Se3–Ag4SSe–SnTe thin films are shown in Fig. 2. The top-view SEM picture (Fig. 2a) reveals uniform, homogeneous, featureless and smooth surface of an amorphous phase. Neither defects nor traces of initial nucleation nor any liquation separation are visible on the film surfaces. However, after longer exposure with high energy electron beam during the SEM observations local districts of initial and progressive nucleation are observed, known as mechanism of bisland crystallizationQ
(Fig. 3). The cross-section view in Fig. 2b shows that the films possess also an internal amorphous compact structure. The lack of features and voids in depth of the films and also at the interface with the substrate surface is indicative for coatings with good quality and adhesion. The values of the film thickness measured from the cross-section SEM images are given in Table 1. The surface images of the investigated thin films taken by AFM were very similar and those of (As2Se3)58(Ag4 SSe)32(SnTe)10 samples are given as an example in Fig. 4 in two different presentations; the calculated data for the maximum height and rms roughness of all films under study are listed in Table 1. The results from topographic analyses are in good agreement with the information obtained by the SEM investigation and confirm that vacuum thermal evaporated thin films possess relatively uniform surfaces with a high degree of smoothness. A slight tendency of increasing the rms roughness with increasing of the SnTe content in the glassy alloy is observed. The film stress (r) was measured by bending cantilever method and was calculated by the help of the Stoney’s equation: r¼
E D2 ; 6ð1 mÞ Rd
ð2Þ
where d is the film thickness, R the radius of the curvature of the substrate, E and m the Young’s modulus and Poisson’s ratio of the substrate, respectively, and D the substrate
Fig. 3. SEM images of a sample showing initial nucleation after longer electron irradiation [in this case (As2Se3)81(Ag4SSe)9(SnTe)10]: (a) top-view image and (b) cross-section.
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Fig. 5. Relationship between the stress and the composition of the (As2Se3)x (Ag4SSe)y (SnTe)z thin films [m=x/(x+y)].
sional structure as a result of the placement of Ag atoms in the microvoids of the glassy network. It is known from the literature that the glassy As2Se3 structure is composed from strongly bonded layers consisting of two-dimensional bonded AsSe3/2 structural units [1]. At higher concentrations of Ag4SSe additional bonds in the glassy network appear resulting in more rigidity structure, and consequently increasing the stress. This topology transition from layered (two-dimensional) to cross-bonded (three-dimensional) structure corresponds to mean coordination numbers Z=2.27 and Z=2.56 and has also been shown up as a peculiarity in the compositional dependences of the density and the compactness of the As2Se3–Ag4SSe–SnTe glasses (Fig. 6). Fig. 4. AFM images of the (As2Se3)58(Ag4SSe)32(SnTe)10 thin film (a) topview image and (b) three-D image.
thickness. In our case d was on the order of 290–430 nm, while D was 45 Am; i.e., the thickness of the films was much smaller than that of the substrate, which allowed us to apply the approximated Stoney’s equation for the stress determination. The magnitude and the sign of the stress are functions of the film composition and structure, as well as of the mechanical and thermomechanical properties. The variation of the stress with the film composition is presented in Fig. 5. It is obvious that the film tensile stress significantly decreases with increasing the concentration of the glassformer As2Se3 in the region 0.6VmV0.8 at constant content of SnTe, and decreases monotonically or remains almost constant when mz0.8. The increase of SnTe content in the As2Se3-rich condensates (mz0.8) leads to reduction of the stress values. Taking into consideration that the structural units building up the network of the bulk glasses are preserved in the corresponding thin films, we could suppose that the addition of Ag4SSe up to 10–20 mol% leads to densification and stabilization of the AsSe3/2 two-dimen-
Fig. 6. Relationships of the density and the compactness of the As2Se3– Ag4SSe–SnTe glasses versus the composition, according to the results published in Ref. [10].
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V. Vassilev et al. / Materials Letters 58 (2004) 3802–3806
The incorporation of atoms with larger atomic radius, like Sn and Te, in the vitreous rigid network (0.6Vm) considerably increases the tensile stress in the films.
University of Chemical Technology and Metallurgy under project No. 10053. The authors would like to thank G. Georgiev and K. Ludolph (University of Kassel) for the SEM and AFM measurements.
4. Conclusions Thin amorphous films from the As2Se3–Ag4SSe–SnTe system were prepared by thermal vacuum evaporation from the synthesized bulk glasses. The coatings are characterized with smooth, featureless and uniform surfaces as shown by morphological and topological studies. These chalcogenide films possess tensile stress revealed by cantilever bending measurements. It was established that the addition of more than 20 mol% Ag4SSe in the glassy alloy leads to higher stress values and changes drastically the structural network of the As2Se3 glass former. The introduction of small amounts of SnTe up to its solubility in the glassy mixtures reduces the rigidity and consequently the stress in the films. A topological threshold is observed in the relationships between the stress and the composition of the condensates corresponding to change in the film structure.
Acknowledgements V. V. and L. A. gratefully acknowledge the financial support of the Ministry of Education and Science, Scientific Investigations Fund under contract TN-1102 and the
References [1] A. Feltz, Amorphous Inorganic Materials and Glasses, VCH Publishers, New York, 1993. [2] V. Lyubin, M. Klebanov, M. Mitkova, T. Petkova, Applied Physics Letters 71 (1997) 2118. [3] E. Marquez, J.M. Gonzalez-Leal, R. Jimenez-Garay, M. Vlcek, Thin Solid Films 396 (2001) 183. [4] Z. Polak, M. Frumari, B. Frumarova, Thin Solid Films 343–344 (1999) 484. [5] M. Vlcek, K. Nejezchleb, T. Wagner, M. Frumar, Mil. Vlcek, A. Vidourek, P.J.S. Ewen, Thin Solid Films 317 (1998) 228. [6] M. Miloshova, E. Buchkov, V. Tsegelnik, V. Strykanov, H. KleweNebenius, M. Bruns, W. Hoffmann, P. Papet, J. Sarradin, A. Pardel, M. Ribes, Sensors and Actuators B 57 (1–3) (1999) 171. [7] J. Schubert, M.J. Schoning, C. Schmidt, M. Siegert, St. Mesters, W. Zander, P. Kordos, H. Luth, A. Legin, Yu.G. Mouzina, B. Seleznev, Yu. G. Vlasov, Applied Physics. A 69 (7) (1999) 803. [8] A.V. Legin, E.A. Bychkov, Yu.G. Vlasov, Sensors and Actuators B 15–16 (1993) 184. [9] A. Guessous, P. Papet, J. Sarradin, M. Ribes, Sensors and Actuators B 24 (1995) 296. [10] L. Aljihmani, V. Vassilev, P. Petkov, Journal of Optoelectronics and Advanced Materials 5 (5) (2003) 1187. [11] A. Klett, A. Malave, R. Freudenstein, M.F. Plass, W. Kulisch, Applied Physics. A 69 (6) (1999) 653. [12] S. Boycheva, V. Vassilev, Journal of Optoelectronics and Advanced Materials 4 (1) (2002) 33.
Deposition, morphology and stress investigation of amorphous As2Se3–Ag4SSe–SnTe thin films V. Vassileva, C. Popovb,*, S. Boychevac, L. Aljihmania, P. Petkova,d, K. Koleva, B. Moncheva,d a
University of Chemical Technology and Metallurgy, Department of Non-Ferrous Metals and Semiconductor Technologies, 8 Kl. Ohridsky Blvd., 1756 Sofia, Bulgaria b Institute of Microstructure Technologies and Analytics (IMA), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany c Technical University of Sofia, Dept. of Thermal and Nuclear Engineering, 8 Kl. Ohridsky Blvd., 1000 Sofia, Bulgaria d Department of Physics, Laboratory of Thin Film Technology, 8 Kl. Ohridsky Blvd., 1756 Sofia, Bulgaria Received 23 April 2004; accepted 5 July 2004 Available online 8 September 2004
Abstract Thin films from the As2Se3–Ag4SSe–SnTe system have been prepared by vacuum thermal evaporation from the corresponding bulk glasses. The film structure and surface morphology have been investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The investigated chalcogenide films are amorphous, featureless and with smooth surfaces. The stress measurements have been carried out by a cantilever bending technique, and tensile stress has been observed. The obtained results have been comprehensively discussed with respect to the film composition and structure. The relationship between the stress and other mechanical parameters, like compactness and density, of the amorphous condensates has been established. The obtained dependencies confirmed the existence of a structural threshold in the glassy network at mean coordination numbers Z=2.27 and Z=2.56. D 2004 Elsevier B.V. All rights reserved. Keywords: Glasses; Thin films; Mechanical properties; Multicomponent chalcogenide glasses; Stress
1. Introduction The wide practical application of thin amorphous chalcogenide films, especially of the vitreous As-containing condensates, is closely connected with their transparency in the VIS and near IR spectral regions and with the possibility to create optical media with defined values of the refractive index, dispersion and extinction coefficients [1]. The relatively low energy of the chemical bonds in the Asbased chalcogenide glasses stimulates photostructural transformations and a number of light-induced effects, which are accompanied by considerable changes in the optical constants [2]. These reversible and irreversible photoeffects are successfully applied for optical information storage [3–5].
* Corresponding author. Fax: +49 561 804 4136. E-mail address: [email protected] (C. Popov). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.07.032
Another advanced application of the vitreous chalcogenide films is as membrane materials for ion-selective chemical microsensors and multisensor systems due to their excellent analytical characteristics [6–9]. The requirements for excellent adhesive strength and minimal residual stress are of a great importance for the technological performance of thin films in the field of optical and sensor devices. The film stress appears during the deposition process as a result of different thermal expansion coefficients of the film and the substrate, intrinsic residual thermal strains and structural rigidity. High stress values cause a number of negative consequences, e.g., bending of the film–substrate structure, peeling, cracking, etc. The glass-forming region in the pseudoternary As2Se3– Ag4SSe–SnTe system was determined by electron microscopy and by X-ray and electron diffractions revealing the glassy nature and the uniformity of the synthesized samples [10]. The glasses are located up to 45 mol% SnTe and
V. Vassilev et al. / Materials Letters 58 (2004) 3802–3806
3803
immersion fluid; their compactness (d) was calculated by the equation #1 ( )" X X M i xi X M i xi Mi xi ð1Þ d¼q qi q i i i
25 mol% Ag4SSe in the binary As2Se3–SnTe and As2Se3– Ag4SSe systems, respectively (Fig. 1). The maximum solubility of the Ag4SSe in the glassy alloys reaches about 30 mol%. The obtained chalcogenide glasses were investigated with respect to their thermal (temperatures of the glass-transition, crystallization and melting) and mechanical (density, microhardness and compactness) properties. These multicomponent chalcogenide glasses are characterized by good glass-forming ability, thermal stability (glass-transition temperature above 370 K), hardness of 0.76–0.89 GPa and density between 4.48 and 5.40 g/cm3, which are satisfactory for many practical applications. The aim of the present work is to deposit amorphous thin films from the bulk As2Se3–Ag4SSe–SnTe glasses by thermal vacuum evaporation and to investigate their structure, morphology, topography and stress with respect to the composition.
where M i , x i and q i are the molar weight, the fraction and the density of each component. Thin As2Se3–Ag4SSe–SnTe films were deposited from the synthesized bulk glasses by thermal vacuum evaporation in a standard high-vacuum installation bHochvacuumQ B 30.2. The process conditions were as following: 1.33103 Pa residual pressure in the chamber; 0.12 m source– substrate distance; the maximum evaporation temperature was varied in the region 1200–1300 K depending on the composition of the glasses. The evaporation time was determined from the desired thickness of the films. An inductively heated covered ribbon evaporator from Ta was used, which allows the preparation of uniform and homogeneous films from sublimating complex materials. The chalcogenide amorphous films were deposited on silicon crystalline substrates for scanning electron microscopy (SEM) and atomic force microscopy (AFM) investigations. The surface roughness of the thin films was examined by an atomic force microscope bNanoScope IIQ in a tapping mode. Their morphology, structure and thickness were investigated by a scanning electron microscope bHitachiQ S-4000. The film stress was evaluated by the bending method using silicon micromachined cantilevers, on which the chalcogenide films were deposited. These cantilever substrates consist of seven beams with a thickness of 45 Am, a width between 0.7 and 2 mm and a length between 2 and 8 mm, and this configuration allows accurate measurement of stress in a wide range. The deflection of the cantilever beams was measured by the depth of the focus of an optical microscope. The cantilever fabrication was described in details in Ref. [11].
2. Experimental
3. Results and discussions
Bulk (As2Se3)x (Ag4SSe)y (SnTe)z glasses, where x+y+ z=100 mol%, situated in the glass-forming region on two tie-lines with concentrations z=10 mol% and z=20 mol% (Fig. 1) were prepared. The compositions of the glassy alloys are listed in Table 1. As2Se3, Ag4SSe and SnTe preliminary synthesized from As, Se, Ag, S, Te and Sn (5N purity) were used as starting materials to guarantee a definite composition of the multicomponent alloys. Conventional direct monotemperature synthesis was carried out in evacuated (~103 Pa) and sealed quartz ampoules using a rotary furnace. Stepwise heating regimes were employed taking into account the physicochemical properties of the starting components [10]. The density of the samples (q) was measured by a hydrostatic method using toluene as
The preparation of homogeneous amorphous thin films from complex chalcogenide systems with a composition
Fig. 1. Glass-forming region of the As2Se3–Ag4SSe–SnTe system (.) glass, (o) crystal, (e) glass+crystal).
Table 1 Composition, thickness, rms roughness and height difference of (As2Se3)x (Ag4SSe)y (SnTe)z thin films, where m=x/(x+y) Composition (mol%) As2Se3
Ag4SSe
SnTe
81 68 58 72 64 52
9 22 32 8 16 28
10 10 10 20 20 20
m
Thickness (nm)
Max. height (nm)
rms roughness (nm)
0.9 0.8 0.6 0.9 0.8 0.7
288 386 324 426 378 318
5.92 5.22 2.79 3.65 14.48 11.56
0.54 0.58 0.31 0.34 1.29 0.74
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Fig. 2. SEM images of the (As2Se3)58(Ag4SSe)32(SnTe)10 thin film (a) top-view image and (b) cross-section.
close to that of the bulk material was carried out under appropriate conditions maintained during the vacuum thermal deposition process: evaporation temperature range 700–800 K, low substrate temperature (b300 K) and low evaporation rate. It can be supposed that the relatively low apparent evaporation rate of 0.12 kg/m2 s guarantees simultaneous evaporation of the structural units building up the As2Se3–Ag4SSe–SnTe glassy network, owing to their close values of the evaporation enthalpy. This leads to low deposition rates (around 0.7 nm/s in our case) which ensure simultaneous condensation of the structural units onto the substrate and consequently the close composition with the bulk materials. On the other hand, the evaporation under high vacuum (1.33103 Pa) ensures the creation of a molecular flow with a steady distribution of the flux on the entire substrate surface, resulting in smooth coatings with uniform thickness. This approach has been successfully used in other cases for deposition of multicomponent amorphous thin films with a definite composition [12]. Typical SEM images of As2Se3–Ag4SSe–SnTe thin films are shown in Fig. 2. The top-view SEM picture (Fig. 2a) reveals uniform, homogeneous, featureless and smooth surface of an amorphous phase. Neither defects nor traces of initial nucleation nor any liquation separation are visible on the film surfaces. However, after longer exposure with high energy electron beam during the SEM observations local districts of initial and progressive nucleation are observed, known as mechanism of bisland crystallizationQ
(Fig. 3). The cross-section view in Fig. 2b shows that the films possess also an internal amorphous compact structure. The lack of features and voids in depth of the films and also at the interface with the substrate surface is indicative for coatings with good quality and adhesion. The values of the film thickness measured from the cross-section SEM images are given in Table 1. The surface images of the investigated thin films taken by AFM were very similar and those of (As2Se3)58(Ag4 SSe)32(SnTe)10 samples are given as an example in Fig. 4 in two different presentations; the calculated data for the maximum height and rms roughness of all films under study are listed in Table 1. The results from topographic analyses are in good agreement with the information obtained by the SEM investigation and confirm that vacuum thermal evaporated thin films possess relatively uniform surfaces with a high degree of smoothness. A slight tendency of increasing the rms roughness with increasing of the SnTe content in the glassy alloy is observed. The film stress (r) was measured by bending cantilever method and was calculated by the help of the Stoney’s equation: r¼
E D2 ; 6ð1 mÞ Rd
ð2Þ
where d is the film thickness, R the radius of the curvature of the substrate, E and m the Young’s modulus and Poisson’s ratio of the substrate, respectively, and D the substrate
Fig. 3. SEM images of a sample showing initial nucleation after longer electron irradiation [in this case (As2Se3)81(Ag4SSe)9(SnTe)10]: (a) top-view image and (b) cross-section.
V. Vassilev et al. / Materials Letters 58 (2004) 3802–3806
3805
Fig. 5. Relationship between the stress and the composition of the (As2Se3)x (Ag4SSe)y (SnTe)z thin films [m=x/(x+y)].
sional structure as a result of the placement of Ag atoms in the microvoids of the glassy network. It is known from the literature that the glassy As2Se3 structure is composed from strongly bonded layers consisting of two-dimensional bonded AsSe3/2 structural units [1]. At higher concentrations of Ag4SSe additional bonds in the glassy network appear resulting in more rigidity structure, and consequently increasing the stress. This topology transition from layered (two-dimensional) to cross-bonded (three-dimensional) structure corresponds to mean coordination numbers Z=2.27 and Z=2.56 and has also been shown up as a peculiarity in the compositional dependences of the density and the compactness of the As2Se3–Ag4SSe–SnTe glasses (Fig. 6). Fig. 4. AFM images of the (As2Se3)58(Ag4SSe)32(SnTe)10 thin film (a) topview image and (b) three-D image.
thickness. In our case d was on the order of 290–430 nm, while D was 45 Am; i.e., the thickness of the films was much smaller than that of the substrate, which allowed us to apply the approximated Stoney’s equation for the stress determination. The magnitude and the sign of the stress are functions of the film composition and structure, as well as of the mechanical and thermomechanical properties. The variation of the stress with the film composition is presented in Fig. 5. It is obvious that the film tensile stress significantly decreases with increasing the concentration of the glassformer As2Se3 in the region 0.6VmV0.8 at constant content of SnTe, and decreases monotonically or remains almost constant when mz0.8. The increase of SnTe content in the As2Se3-rich condensates (mz0.8) leads to reduction of the stress values. Taking into consideration that the structural units building up the network of the bulk glasses are preserved in the corresponding thin films, we could suppose that the addition of Ag4SSe up to 10–20 mol% leads to densification and stabilization of the AsSe3/2 two-dimen-
Fig. 6. Relationships of the density and the compactness of the As2Se3– Ag4SSe–SnTe glasses versus the composition, according to the results published in Ref. [10].
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The incorporation of atoms with larger atomic radius, like Sn and Te, in the vitreous rigid network (0.6Vm) considerably increases the tensile stress in the films.
University of Chemical Technology and Metallurgy under project No. 10053. The authors would like to thank G. Georgiev and K. Ludolph (University of Kassel) for the SEM and AFM measurements.
4. Conclusions Thin amorphous films from the As2Se3–Ag4SSe–SnTe system were prepared by thermal vacuum evaporation from the synthesized bulk glasses. The coatings are characterized with smooth, featureless and uniform surfaces as shown by morphological and topological studies. These chalcogenide films possess tensile stress revealed by cantilever bending measurements. It was established that the addition of more than 20 mol% Ag4SSe in the glassy alloy leads to higher stress values and changes drastically the structural network of the As2Se3 glass former. The introduction of small amounts of SnTe up to its solubility in the glassy mixtures reduces the rigidity and consequently the stress in the films. A topological threshold is observed in the relationships between the stress and the composition of the condensates corresponding to change in the film structure.
Acknowledgements V. V. and L. A. gratefully acknowledge the financial support of the Ministry of Education and Science, Scientific Investigations Fund under contract TN-1102 and the
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