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Structural and optical properties of annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films
Optical Materials 37 (2014) 718–723
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Structural and optical properties of annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films I.P. Studenyak a,1, Yu.Yu. Neimet a,⇑, Y.Y. Rati a,1, D. Stanko b,2, M. Kranjcˇec b,2, S. Kökényesi c,3, L. Daróci c,3, R. Bohdan c,3 a b c
Department of Applied Physics, Faculty of Physics, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine Faculty of Geotechnics, University of Zagreb, 7 Hallerova Aleja, 42000 Varazˇdin, Croatia Department of Experimental Physics, Faculty of Science and Technology, University of Debrecen, 18/a Bem Sq., 4026 Debrecen, Hungary
a r t i c l e
i n f o
Article history: Received 16 May 2014 Accepted 28 August 2014 Available online 18 September 2014 Keywords: Thin film Cones XRD SEM Annealing Illumination
a b s t r a c t (Ag3AsS3)0.6(As2S3)0.4 thin films were deposited upon a quartz substrate by rapid thermal evaporation. Structural studies of the as-deposited, annealed and illuminated films were performed using XRD, scanning electron and atomic force microscopies. Surfaces of all the films were found to be covered with Agrich crystalline micrometer sized cones. Thermal annealing leads to mechanical deformation of part of the cones and their detachment from the base film surface while the laser illumination leads to the new formations appearance on the surface of thin films. The spectroscopic studies of optical transmission spectra for as-deposited, annealed and illuminated thin films were carried out. The optical absorption spectra in the region of its exponential behaviour were analysed, the dispersion dependences of refractive index as well as their variation after annealing and illumination were investigated. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A wide variety of thin films, which are used as active or passive components in various devices for microelectronics, optics, micromechanics, and other advanced technologies, can be prepared by physical vapour deposition (PVD) techniques [1]. PVD thin films should be tightly adherent to the substrate surface to ensure performance and reliability of devices. Usually, PVD thin films prepared under appropriate conditions are firmly bonded to thick substrates. As a result, any change in length along the film plane, which is not matched, will generate a stress in the film. Excessive stresses in thin films may cause defect formation and delamination at the film–substrate interface, mechanical damage in the film (film fracture), adhesion failures, and defect formation in the substrate. Elimination and release of stress can also cause formation of hillocks, whiskers, holes, and other defects which affect physical properties of the films [1]. In the present paper, we report on the formation of crystalline Ag-containing cones from an Ag–As–S chalcogenide composite ⇑ Corresponding author. Tel.: +380 (95)6773500. E-mail addresses: [email protected] (I.P. Studenyak), mladen.kranjcec@yahoo. com (M. Kranjcˇec), [email protected] (S. Kökényesi). 1 Tel.: +380 (3122)32012. 2 Tel.: +385 (42)213583. 3 Tel.: +36 (52)316012. http://dx.doi.org/10.1016/j.optmat.2014.08.019 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
material using simple thermal evaporation technique on a cold substrate. Ag-doped Ag–As–S chalcogenide glasses and films have found many current and potential applications, such as solid electrolytes for batteries, electrochemical sensors, photoresists, optical waveguides, diffraction elements, Fresnel lenses, optical recording materials, surface patterns for different applications, formed by a laser beam, and other optical and optoelectronic elements [2]. Recently we have demonstrated high values of electrical (mostly ionic) conductivity in superionic Ag3AsS3–As2S3 glasses and composites [3,4]. This, in combination with a strong photosensitivity, attracts a great interest towards fabrication and studies of Ag3AsS3–As2S3 thin films. Thus, the present paper is aimed at the deposition, structural and optical studies of annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films which includes XRD, SEM, AFM as well as the spectroscopic studies of transmission spectra, absorption spectra, refractive indices and their dispersion curves under the influence of annealing and illumination. 2. Experimental Synthesis of (Ag3AsS3)0.6(As2S3)0.4 composite material which consists of crystalline Ag3AsS3 and glassy As2S3 [4] was carried out at a temperature of 700 °C for 24 h with subsequent melt homogenization for 72 h. (Ag3AsS3)0.6(As2S3)0.4 thin films were prepared by rapid thermal evaporation from the corresponding
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composite material at near 1350 °C in vacuum (3 103 Pa) using a VU-2M setup. The composite material was initially placed in a perforated tantalum evaporator for preventing the material falling out onto a glass substrate kept at room temperature. The film thickness was measured using an Ambios XP-1 profile meter. Xray studies were performed using a DRON-3 diffractometer (conventional h 2h scanning technique, Bragg angle 2h ranging from 10° to 60°, Cu Ka Ni filtered radiation). Structural properties of the thin films under investigation were studied using scanning electron microscopy (SEM: Hitachi S-4300) and atomic force microscopy (AFM: Nanoscope 3100). Energy-dispersive X-ray spectroscopy (EDX) was used to ensure the film chemical composition. Annealing was performed for 1 h at 50 °C and 100 °C in vacuum. Optical transmission spectra T(k) of the thin films were studied at room temperature using a MDR-3 grating monochromator. Normal illumination of the films through the 50 50 lm copper mesh for 1 h as well as holography pattern recording for 40 min were performed using green diode pumped laser (532 nm, 20 mW). Another green diode pumped laser (532 nm, 100 mW) was used for the illumination of the samples for 1, 3, and 5 min with following transmission spectra recording.
3. Results and discussion 3.1. Structural studies Analysed by SEM, a characteristic surface view of (Ag3AsS3)0.6 (As2S3)0.4 thin films is given in Fig. 1. Microscopy studies revealed the presence of cones on the top of a fresh evaporated film surface, which remind the whiskers, discovered rather long ago [5]. The average height of the cones is found to be 2.5 lm which is well seen from the AFM image in Fig. 2(a), and is also confirmed by the profile meter. As estimated by the profile meter, the thickness of the base film layer is about 0.5 lm. Meanwhile, the average base diameter of the cones is about 2 lm. This value is in a good agreement with the detailed SEM study of the film under investigation. The average aspect ratio (height over diameter) of the cones was
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estimated from Fig. 1 and found to be 1.2. SEM-image (Fig. 2a) enabled us to estimate the average density of the cones at the surface of the as-deposited film which equals approximately 14 103 per mm2. Recently we reported on the nanocrystalline layer which appeared on the bulk (Ag3AsS3)0.6(As2S3)0.4 composite surface after it had been heated to 450 K [3]. Simultaneously, this material was found to be phase separated, or, more exactly, having smithite (AgAsS2) crystalline inclusions in the amorphous As2S3 matrix [4]. Hence, the most probably, performed quick high temperature evaporation of the (Ag3AsS3)0.6(As2S3)0.4 composite results in the phase segregation. Fig. 3 shows X-ray diffraction patterns of the fresh and annealed (Ag3AsS3)0.6(As2S3)0.4 thin films. The presence of peaks and less pronounced local maxima at the patterns shows that both studied thin films very likely have crystalline inclusions in their structures. Comparing the crystalline patterns of silver, high-temperature aAg2S, and smithite (AgAsS2) crystals with the obtained XRD patterns of thin films under investigation gives us the possibility to find out the nature of those inclusions. From the Fig. 3 one can see broad amorphous maxima in the 15–27° 2h-region in both investigated samples. The presence of crystalline peaks was revealed in both thin film patterns. The Xray diffraction experiment for the as-deposited thin film enable us to treat a pronounced peak at 2h 38.2°, and a small maximum at 2h 44.2° as Ag crystalline peaks. Fig. 3 also reveals signatures of smithite (in the vicinity of 2h 29°) and a-Ag2S (2h 34°) crystals in the X-ray diffraction pattern of the fresh (Ag3AsS3)0.6 (As2S3)0.4 thin film. Opposite to the as-deposited (Ag3AsS3)0.6 (As2S3)0.4 thin film, the diffraction pattern of the annealed at 100 °C for 1 h thin film exhibits a clearly pronounced narrow peak of high-temperature a-Ag2S crystalline phase (2h 37.9°) which is very close to that of pure silver, and a broader maxima at 2h 34°. Somewhat less pronounced smithite (AgAsS2) maxima in the vicinity of 2h 29° and 2h 31.5° enable us to consider the annealed film or even the fresh one to have the smithite crystalline inclusions in its structure, as it was proved for the bulk (Ag3AsS3)0.6 (As2S3)0.4 composite in [4].
Fig. 1. Scaled SEM images of the as-deposited (a) and annealed (Ag3AsS3)0.6(As2S3)0.4 composite thin films: 50 °C, 1 h (b), and 100 °C, 1 h (c) annealing in vacuum.
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Fig. 2. AFM images of the as-deposited (Ag3AsS3)0.6(As2S3)0.4 thin film (a), and the film with recorded holographic pattern on its surface (b).
film as well. An excess of silver in the EDX results on cones and the corresponding peaks in XRD pattern of the fresh (Ag3AsS3)0.6 (As2S3)0.4 thin film enable us to ascribe the cones, obtained in the present work, to silver crystals or crystalline Ag-rich structures. Fig. 1(b) and (c) presents SEM images of (Ag3AsS3)0.6(As2S3)0.4 thin films thermally annealed in vacuum at 50 °C and 100 °C for 1 h. The thermal annealing process can be seen to result in the cone destruction and detachment from the base film surface which is enhanced at the annealing temperature increase (Fig. 1b and c). On the other hand, the film surface and the cones of the film annealed at 50 °C for 1 h were covered with new formations, whereas for the film annealed at 100 °C for 1 h only the cones and areas in their vicinity are changed while the base surface remains flat. The detachment of the cones is most likely caused by the presence of additional thermal strain due to the film temperature increase related to the annealing process. The second reason is an arising of new outgoing formations which changes the surface of the film, an intrinsic energy distribution in thin film volume, and, probably, reorients local bonds between constituent molecules and atoms in the material. For the (Ag3AsS3)0.6(As2S3)0.4 thin film annealed at 100 °C one can observe formations (Fig. 1) which, as follows from the XRD data (Fig. 3), can be ascribed to a-Ag2S crystals. Moreover, as we can see from small scale pictures in Fig. 1, the cones are embedded with their basements into the base film which can be taken into the consideration as the additional reason for the presence of arsenic and sulphur in the EDX compositional spectra of the cones (Table 1).
3.2. Optical absorption Fig. 4 shows the optical transmittance of the as-deposited (Ag3AsS3)0.6(As2S3)0.4 thin film as well as those annealed at both 50 °C and 100 °C. One can observe a considerable increase of transmittance by almost 30% in the annealed films spectra. This fact is characteristic for a structural ordering, which occurs in the film structure as a consequence of thermal annealing. One should note that annealing at 50 °C for 1 h causes the transmission onset to shift towards shorter wavelengths. On the other hand, annealing at the increased to 100 °C temperature involves an opposite reaction, i.e. causes the transmission onset to shift slightly towards longer wavelengths. The spectra of the annealed films at the onset of transmission are seen to be less smeared or, in the other words, the slopes of the plots are seen to be sharper. Although a reduction Fig. 3. X-ray diffraction patterns of the as-deposited and annealed at 100 °C for 1 h (Ag3AsS3)0.6(As2S3)0.4 composite thin films.
Combined with SEM, EDX technique enabled us to ensure that the films formed are As-enriched and have a deficiency of sulphur percentage in comparison with the initial bulk composite, as follows in Table 1. Simultaneously, local EDX analysis estimated an excess of silver in the atomic composition of the cones. The presence of small amounts of arsenic and sulphur in the cones shown by the EDX can be explained by the fact that the minimum estimation area covers not only the cone itself, but some part of the base
Table 1 The elemental content (in at.%) of the as-deposited (Ag3AsS3)0.6(As2S3)0.4 thin films and corresponding initial bulk composite. The composition in atomic percentage is given for the cones formed on the fresh thin film surface as well. Elements (at. %)
Initial bulk composite
As-deposited thin film
Cones
Ag As S
29 22.6 48.4
29 32 39
62.8 18 19.2
Fig. 4. Optical transmittance of the (Ag3AsS3)0.6(As2S3)0.4 composite thin films: asdeposited (1), and annealed for 1 h at 50 °C (2), and at 100 °C (3). The inset shows the optical transmittance for as-deposited (1), and illuminated for 1 (2), 3 (3), and 5 (4) min by green laser (k = 532 nm).
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of smearing is observed for the annealed at 100 °C film, this film remains still more ordered comparing to the as-deposited fresh one. It should be noted that, by analogy with annealing, the film was found to increase its transmittance almost by 30% as a result of the 532 nm laser illumination for 1 min (Fig. 4). The transmittance falls down after the increase of illumination time up to 3 and 5 min. Nevertheless it remains increased in comparison with non-illuminated sample. The spectral dependences of the absorption coefficient as well as dispersion dependences of the refractive index were derived from the spectrometric studies of interference transmission spectra [6]. It should be noted that two spectral regions can be taken into consideration in amorphous films at the fundamental absorption edge at high absorption levels – a Tauc region and an exponential region [7]. In such a case both the Tauc gap Eopt and the energy g position of an exponential absorption edge Eag at a fixed absorption coefficient a (for example, in Ref. [8] the a value was chosen to be a = 104 cm1), characterizing the exponential part of the absorption edge, were used. The absorption edge spectra of the as-deposited, annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films in the range of their exponential behaviour are presented in Fig. 5. In Refs. [9,10] we used the optical pseudogap value Eg (energy position of the exponential absorption edge at a = 103 cm1) based on a possibility of comparison with Eg values of corresponding bulk materials, for which it is much more difficult to reach high absorption levels. In our case we used the Eag values taken at a = 5 104 cm1 for the characterization of the absorption edge spectral position, as far as the applying of Eg value seems to be unreasonable due to the strong absorption edge smearing. Determined in such a way Eag values are presented in Table 2, where Eopt values calcug lated for Tauc region are listed for comparison as well. It is shown that the increase of Eag is identical for annealing and illumination processes in (Ag3AsS3)0.6(As2S3)0.4 thin films. Thus, the annealing at 50 °C leads to Eag increase by 0.134 eV, while annealing at 100 °C causes the decrease of Eag by 0.042 eV, however the Eag value is still larger than that of as-deposited thin film (Table 2). The similar situation is observed after the illumination of thin films by green laser light (k = 532 nm) for 1 min which leads to the increase of Eag by 0.126 eV. Further extension of illumination time causes the Eag decrease, though at t = 5 min this value is still larger than that of as-deposited thin film (Table 2). It is seen from Fig. 5 that the optical absorption edge spectra in the range of their exponential behaviour in the thin films under investigation can be fitted by the equation [11]:
hm E 0 ; aðhm; TÞ ¼ a0 exp EU
ð1Þ
where EU is the Urbach energy (a reciprocal of the absorption edge slope EU1 = D(ln a)/D(hm)), a0 and E0 are the convergence point coordinates of the Urbach bundle. It is well-known that the Urbach energy EU characterises the smearing of optical absorption edge (in fact the EU is a degree of the system disordering). It was shown in Ref. [12] that both temperature and structural disordering affect the absorption edge shape in amorphous materials, i.e. the Urbach energy EU is described by equation:
EU ¼ ðEU ÞT þ ðEU ÞX ;
ð2Þ
where (EU)T and (EU)X are the contributions of temperature disordering and structural disordering to EU, respectively. The temperature disordering arises due to the thermal vibrations of atoms and structural elements. The structural disordering in amorphous materials is related to the absence of long-range order, the presence of structural inhomogeneities, i.e. defects, pores, etc. It should be noted that annealing at 50 °C leads to the Urbach energy EU decreasing. Meanwhile, the annealing at 100 °C leads to some increasing of
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Fig. 5. Absorption edge spectra of the (Ag3AsS3)0.6(As2S3)0.4 composite thin films: (a) as-deposited (1), and annealed for 1 h at 50 °C (2), and at 100 °C (3); (b) asdeposited (1), and illuminated for 1 (2), 3 (3), and 5 (4) min by green laser. The insets show the dependences of energy position of absorption edge (1) and the Urbach energy (2) on annealing temperature (a) and illumination time (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
EU, however this value is still smaller in comparison with that of the as-deposited thin film (Table 2). The last fact is an evidence of some structure ordering at annealing, but the annealing of this type of thin films is recommended to be carried out at 50 °C. It is shown that illumination leads to the Urbach energy decreasing and, respectively, to the structure ordering, which is, however, larger than that of the film after annealing (Table 2). The dispersion dependences of the refractive index for the asdeposited, annealed, and illuminated thin films were obtained from the interference transmittance spectra (Fig. 6). The slight dispersion of the refractive index is observed in the transparency region while it increases when approaching to the optical absorption edge region. The refractive index of the annealed at 50 °C thin film is smaller by more than 15% in comparison with the as-deposited film. One observes the refractive index increases by more than 9% with annealing temperature increase to 100 °C (Fig. 6, Table 2). After 1 min laser illumination the refractive index decreases by more than 15%, and subsequently increases by more than 11% with the illumination time increases to 5 min (Fig. 6, Table 2). Comparison of the refractive index values of the films annealed at 50 °C and illuminated for 1 min gives the evidence of the fact that observed changes under the influence of external factors are almost identical.
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Table 2 Optical parameters of the as-deposited, annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films. Thin film
n (1500 nm)
Eag (5 104 cm1) (eV)
Eopt (eV) g
EU (meV)
As-deposited Annealed 50 °C, 1 h Annealed 100 °C, 1 h Illuminated 1 min Illuminated 3 min Illuminated 5 min
3.08 2.60 2.84 2.61 2.78 2.92
2.085 2.219 2.177 2.211 2.172 2.164
1.708 1.644 1.684 1.864 1.639 1.571
457 421 432 403 393 396
Fig. 6. Refractive indices dispersions of the (Ag3AsS3)0.6(As2S3)0.4 composite thin films: as-deposited (1), and annealed for 1 h at 50 °C (2) and 100 °C (3). The inset shows the refractive indices dependence on annealing temperature.
Fig. 7. Profile formed on the (Ag3AsS3)0.6(As2S3)0.4 film surface due to laser illumination (k = 532 nm) through the 50 50 lm copper mesh. The inset shows the thin film surface by SEM due to laser illumination through the mesh (squares correspond to illuminated areas).
3.3. Laser illumination and holographic recording Thin (Ag3AsS3)0.6(As2S3)0.4 films are found to be strongly sensitive to the monochromatic green laser light influence. The film was illuminated through the 50 50 lm copper mesh by a normally incident horizontally polarised green laser light (532 nm, 20 mW) for 1 h. New formations were observed to appear on the surface of both the base film and the cones due to the laser illumination (Fig. 7, inset SEM image). At the same time the illuminated areas are found to lift up under the influence of the laser light which can be seen from the illuminated film profile analysis (Fig. 7). It should be noted that at the dark non-illuminated areas, shadowed by the mesh, only the cones are covered by additional structures. This can be seen if to compare the last one with the fresh film (Fig. 1a). At the same time both the cones and the base film at the illuminated areas covered by the additional structures are observed. This can be understood in the way that the laser light affects not only the illuminated areas in the squares inside the mesh, but also the whole film area under the mesh due to the ions transport processes in the thin film volume. The above mentioned processes can be true for holographic patterns recording as well (Fig. 8), where light and dark areas are distinguished by the presence of the surface cover. Phase holographic gratings were recorded on the surface of the (Ag3AsS3)0.6(As2S3)0.4 thin films with a period of 5 lm. SEM images of the holographic patterns are reproduced in Fig. 8. The holographic recording by the green horizontally polarised laser light (532 nm, 20 mW) for 40 min causes a growth of new formations of various shapes and smaller sizes in comparison with the big initial cones at the areas of interferometry maxima illumination. It should be mentioned, that these formations start to disappear under the influence of a strong (40 kV) e-beam irradiation in SEM. The formation and dis-
Fig. 8. SEM images of the (Ag3AsS3)0.6(As2S3)0.4 thin film with holographic patterns recorded on its surface.
appearance of the new formations due to the laser light impact are most probably related to the Ag+ ions movements attracted by the laser light as it was shown earlier [13,14]. At the same time this effect was not observed (or was negligibly small) when treating by the e-beam the formations grown at the big cones. This can testify the fact that the small formations grown after illumination are of the same nature as initial main big cones, i.e. Ag-enriched ones. AFM image of the holographic pattern formed on the (Ag3AsS3)0.6(As2S3)0.4 thin film are shown in Fig. 2(b). It should be noted that the AFM confirms bigger roughness of the illuminated film in comparison with the as-deposited one. This fact is
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supported by the SEM images of the corresponding holographic pattern (Fig. 8).
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respectively, the decrease of structural disordering in the (Ag3AsS3)0.6(As2S3)0.4 thin films. Acknowledgements
4. Conclusions (Ag3AsS3)0.6(As2S3)0.4 chalcogenide thin films were prepared by rapid thermal evaporation in vacuum. Subsequently, the films were annealed at 50 °C and 100 °C for 1 h in vacuum as well as were illuminated for 1, 3, and 5 min by laser with k = 530 nm. SEM and AFM imaging of the as-deposited, annealed and illuminated films revealed numerous micrometer-sized cones on their surfaces. The presence of crystalline peaks was revealed in the XRD patterns of both fresh and annealed thin films. The EDX analysis showed an excess of silver in the obtained cones, which, together with the pronounced peak of Ag in the XRD pattern, enabled us to ascribe the last one to the cones. Annealing at 50 °C and 100 °C was shown to result in a mechanical deformation of part of the cones and their detachment from the base film surface. Another result of annealing is probable appearance of crystalline a-Ag2S on the surface of the cones and the base film. It was shown that laser illumination causes the appearance of the new formations on the surface of the base film and the cones. The optical transmission spectra of the annealed and illuminated films have shown an increase of transmission for all samples, whereas the largest change of transmittance appeared to be a result of annealing at 50 °C and illumination for 1 min. The spectral dependences of the absorption coefficient as well as dispersion dependences of the refractive index were derived from the spectrometric studies of interference transmission spectra. In the range of the exponential behaviour of the optical absorption edge the energy position of exponential absorption edge Eag and the Urbach energy EU in the as-deposited, annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films were determined. It was shown that annealing and illumination lead to the increase of theEag , and to the decrease of the refractive index n. Moreover, the comparison of the Eag and n values after annealing at 50 °C and after illumination for 1 min give the evidence of the fact that the observed changes under the influence of external factors are almost identical. Additionally, it is revealed that annealing and illumination cause the Urbach energy EU decrease and,
The authors are grateful to the TAMOP Grant (TAMOP 4.2.2.A11/1/KONV-2012-0036 project) which is co-financed by the European Union and European Social Fund. Yuriy Neimet (Contract Number 51301014) is strongly grateful to the International Visegrad Fund scholarship for the funding of the project. References [1] Y. Pauleau, Materials Surface Processing by Directed Energy Techniques, Elsevier, Oxford, 2006. [2] E. Bychkov, A. Bychkov, A. Pradel, M. Ribes, Percolation transition in Ag-doped chalcogenide glasses: comparison of classical percolation and dynamic structure models, Solid State Ionics 113 (1998) 691–695. [3] I. Studenyak, Yu. Neimet, C. Cserhati, S. Kökényesi, E. Kazakevicˇius, T. Šalkus, A. Kezˇionis, A. Orliukas, Structural and electrical investigations of (Ag3AsS3)x(As2S3)1x superionic glasses, Cent. Eur. J. Phys. 10 (2012) 206–209. [4] I.P. Studenyak, Yu.Yu. Neimet, M. Kranjcˇec, A.M. Solomon, A.F. Orliukas, A. Kezˇionis, E. Kazakevicˇius, T. Šalkus, Electrical conductivity studies in (Ag3AsS3)x(As2S3)1x superionic glasses and composites, J. Appl. Phys. 115 (2014) 033702–1033702-5. [5] R.S. Wagner, W.C. Ellis, Vapor–liquid–solid mechanism of single crystal growth, Appl. Phys. Lett. 4 (1964) 89–90. [6] R. Swanepoel, Determination of the thickness and optical constants of amorphous silicon, J. Phys. E:Sci. Instrum. 16 (1983) 1214–1222. [7] R. Bindemann, O. Paetzold, The influence of thermal disorder on the absorption edge in the Tauc region of a–Si:H, Phys. Stat. Sol. (B) 160 (1990) K183–K188. [8] S. Kökényesi, J. Csikai, P. Raics, I.A. Szabo, S. Szegedi, A. Vitez, Comparison of photo- and deuteron-induced effects in amorphous chalcogenide layers, J. Non-Cryst. Sol. 326&327 (2003) 209–214. [9] I.P. Studenyak, M. Kranjec, M.M. Pop, Urbach absorption edge and disordering processes in As2S3 thin films, J. Non-Cryst. Solids 357 (2011) 3866–3869. [10] I.P. Studenyak, M. Kranjec, V.Yu. Izai, A.A. Chomolyak, M. Vorohta, V. Matolin, C. Cserhati, S. Kökényesi, Structural and temperature-related disordering studies of Cu6PS5I superionic thin film, Thin Solid Films 520 (2012) 1729– 1733. [11] F. Urbach, The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids, Phys. Rev. 92 (1953) 1324–1326. [12] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Disorder and the opticalabsorption edge of hydrogenated amorphous silicon, Phys. Rev. Lett. 47 (1981) 1480–1483. [13] N. Yoshida, K. Tanaka, Ag migration in Ag–As–S glasses induced by electronbeam irradiation, J. Non-Cryst. Sol. 210 (1997) 119–129. [14] T. Kawaguchi, Photoinduced metastability in Ag-containing chalcogenide glasses, J. Non-Cryst. Sol. 345&346 (2004) 265–269.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Structural and optical properties of annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films I.P. Studenyak a,1, Yu.Yu. Neimet a,⇑, Y.Y. Rati a,1, D. Stanko b,2, M. Kranjcˇec b,2, S. Kökényesi c,3, L. Daróci c,3, R. Bohdan c,3 a b c
Department of Applied Physics, Faculty of Physics, Uzhhorod National University, 3 Narodna Sq., 88000 Uzhhorod, Ukraine Faculty of Geotechnics, University of Zagreb, 7 Hallerova Aleja, 42000 Varazˇdin, Croatia Department of Experimental Physics, Faculty of Science and Technology, University of Debrecen, 18/a Bem Sq., 4026 Debrecen, Hungary
a r t i c l e
i n f o
Article history: Received 16 May 2014 Accepted 28 August 2014 Available online 18 September 2014 Keywords: Thin film Cones XRD SEM Annealing Illumination
a b s t r a c t (Ag3AsS3)0.6(As2S3)0.4 thin films were deposited upon a quartz substrate by rapid thermal evaporation. Structural studies of the as-deposited, annealed and illuminated films were performed using XRD, scanning electron and atomic force microscopies. Surfaces of all the films were found to be covered with Agrich crystalline micrometer sized cones. Thermal annealing leads to mechanical deformation of part of the cones and their detachment from the base film surface while the laser illumination leads to the new formations appearance on the surface of thin films. The spectroscopic studies of optical transmission spectra for as-deposited, annealed and illuminated thin films were carried out. The optical absorption spectra in the region of its exponential behaviour were analysed, the dispersion dependences of refractive index as well as their variation after annealing and illumination were investigated. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A wide variety of thin films, which are used as active or passive components in various devices for microelectronics, optics, micromechanics, and other advanced technologies, can be prepared by physical vapour deposition (PVD) techniques [1]. PVD thin films should be tightly adherent to the substrate surface to ensure performance and reliability of devices. Usually, PVD thin films prepared under appropriate conditions are firmly bonded to thick substrates. As a result, any change in length along the film plane, which is not matched, will generate a stress in the film. Excessive stresses in thin films may cause defect formation and delamination at the film–substrate interface, mechanical damage in the film (film fracture), adhesion failures, and defect formation in the substrate. Elimination and release of stress can also cause formation of hillocks, whiskers, holes, and other defects which affect physical properties of the films [1]. In the present paper, we report on the formation of crystalline Ag-containing cones from an Ag–As–S chalcogenide composite ⇑ Corresponding author. Tel.: +380 (95)6773500. E-mail addresses: [email protected] (I.P. Studenyak), mladen.kranjcec@yahoo. com (M. Kranjcˇec), [email protected] (S. Kökényesi). 1 Tel.: +380 (3122)32012. 2 Tel.: +385 (42)213583. 3 Tel.: +36 (52)316012. http://dx.doi.org/10.1016/j.optmat.2014.08.019 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
material using simple thermal evaporation technique on a cold substrate. Ag-doped Ag–As–S chalcogenide glasses and films have found many current and potential applications, such as solid electrolytes for batteries, electrochemical sensors, photoresists, optical waveguides, diffraction elements, Fresnel lenses, optical recording materials, surface patterns for different applications, formed by a laser beam, and other optical and optoelectronic elements [2]. Recently we have demonstrated high values of electrical (mostly ionic) conductivity in superionic Ag3AsS3–As2S3 glasses and composites [3,4]. This, in combination with a strong photosensitivity, attracts a great interest towards fabrication and studies of Ag3AsS3–As2S3 thin films. Thus, the present paper is aimed at the deposition, structural and optical studies of annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films which includes XRD, SEM, AFM as well as the spectroscopic studies of transmission spectra, absorption spectra, refractive indices and their dispersion curves under the influence of annealing and illumination. 2. Experimental Synthesis of (Ag3AsS3)0.6(As2S3)0.4 composite material which consists of crystalline Ag3AsS3 and glassy As2S3 [4] was carried out at a temperature of 700 °C for 24 h with subsequent melt homogenization for 72 h. (Ag3AsS3)0.6(As2S3)0.4 thin films were prepared by rapid thermal evaporation from the corresponding
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composite material at near 1350 °C in vacuum (3 103 Pa) using a VU-2M setup. The composite material was initially placed in a perforated tantalum evaporator for preventing the material falling out onto a glass substrate kept at room temperature. The film thickness was measured using an Ambios XP-1 profile meter. Xray studies were performed using a DRON-3 diffractometer (conventional h 2h scanning technique, Bragg angle 2h ranging from 10° to 60°, Cu Ka Ni filtered radiation). Structural properties of the thin films under investigation were studied using scanning electron microscopy (SEM: Hitachi S-4300) and atomic force microscopy (AFM: Nanoscope 3100). Energy-dispersive X-ray spectroscopy (EDX) was used to ensure the film chemical composition. Annealing was performed for 1 h at 50 °C and 100 °C in vacuum. Optical transmission spectra T(k) of the thin films were studied at room temperature using a MDR-3 grating monochromator. Normal illumination of the films through the 50 50 lm copper mesh for 1 h as well as holography pattern recording for 40 min were performed using green diode pumped laser (532 nm, 20 mW). Another green diode pumped laser (532 nm, 100 mW) was used for the illumination of the samples for 1, 3, and 5 min with following transmission spectra recording.
3. Results and discussion 3.1. Structural studies Analysed by SEM, a characteristic surface view of (Ag3AsS3)0.6 (As2S3)0.4 thin films is given in Fig. 1. Microscopy studies revealed the presence of cones on the top of a fresh evaporated film surface, which remind the whiskers, discovered rather long ago [5]. The average height of the cones is found to be 2.5 lm which is well seen from the AFM image in Fig. 2(a), and is also confirmed by the profile meter. As estimated by the profile meter, the thickness of the base film layer is about 0.5 lm. Meanwhile, the average base diameter of the cones is about 2 lm. This value is in a good agreement with the detailed SEM study of the film under investigation. The average aspect ratio (height over diameter) of the cones was
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estimated from Fig. 1 and found to be 1.2. SEM-image (Fig. 2a) enabled us to estimate the average density of the cones at the surface of the as-deposited film which equals approximately 14 103 per mm2. Recently we reported on the nanocrystalline layer which appeared on the bulk (Ag3AsS3)0.6(As2S3)0.4 composite surface after it had been heated to 450 K [3]. Simultaneously, this material was found to be phase separated, or, more exactly, having smithite (AgAsS2) crystalline inclusions in the amorphous As2S3 matrix [4]. Hence, the most probably, performed quick high temperature evaporation of the (Ag3AsS3)0.6(As2S3)0.4 composite results in the phase segregation. Fig. 3 shows X-ray diffraction patterns of the fresh and annealed (Ag3AsS3)0.6(As2S3)0.4 thin films. The presence of peaks and less pronounced local maxima at the patterns shows that both studied thin films very likely have crystalline inclusions in their structures. Comparing the crystalline patterns of silver, high-temperature aAg2S, and smithite (AgAsS2) crystals with the obtained XRD patterns of thin films under investigation gives us the possibility to find out the nature of those inclusions. From the Fig. 3 one can see broad amorphous maxima in the 15–27° 2h-region in both investigated samples. The presence of crystalline peaks was revealed in both thin film patterns. The Xray diffraction experiment for the as-deposited thin film enable us to treat a pronounced peak at 2h 38.2°, and a small maximum at 2h 44.2° as Ag crystalline peaks. Fig. 3 also reveals signatures of smithite (in the vicinity of 2h 29°) and a-Ag2S (2h 34°) crystals in the X-ray diffraction pattern of the fresh (Ag3AsS3)0.6 (As2S3)0.4 thin film. Opposite to the as-deposited (Ag3AsS3)0.6 (As2S3)0.4 thin film, the diffraction pattern of the annealed at 100 °C for 1 h thin film exhibits a clearly pronounced narrow peak of high-temperature a-Ag2S crystalline phase (2h 37.9°) which is very close to that of pure silver, and a broader maxima at 2h 34°. Somewhat less pronounced smithite (AgAsS2) maxima in the vicinity of 2h 29° and 2h 31.5° enable us to consider the annealed film or even the fresh one to have the smithite crystalline inclusions in its structure, as it was proved for the bulk (Ag3AsS3)0.6 (As2S3)0.4 composite in [4].
Fig. 1. Scaled SEM images of the as-deposited (a) and annealed (Ag3AsS3)0.6(As2S3)0.4 composite thin films: 50 °C, 1 h (b), and 100 °C, 1 h (c) annealing in vacuum.
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Fig. 2. AFM images of the as-deposited (Ag3AsS3)0.6(As2S3)0.4 thin film (a), and the film with recorded holographic pattern on its surface (b).
film as well. An excess of silver in the EDX results on cones and the corresponding peaks in XRD pattern of the fresh (Ag3AsS3)0.6 (As2S3)0.4 thin film enable us to ascribe the cones, obtained in the present work, to silver crystals or crystalline Ag-rich structures. Fig. 1(b) and (c) presents SEM images of (Ag3AsS3)0.6(As2S3)0.4 thin films thermally annealed in vacuum at 50 °C and 100 °C for 1 h. The thermal annealing process can be seen to result in the cone destruction and detachment from the base film surface which is enhanced at the annealing temperature increase (Fig. 1b and c). On the other hand, the film surface and the cones of the film annealed at 50 °C for 1 h were covered with new formations, whereas for the film annealed at 100 °C for 1 h only the cones and areas in their vicinity are changed while the base surface remains flat. The detachment of the cones is most likely caused by the presence of additional thermal strain due to the film temperature increase related to the annealing process. The second reason is an arising of new outgoing formations which changes the surface of the film, an intrinsic energy distribution in thin film volume, and, probably, reorients local bonds between constituent molecules and atoms in the material. For the (Ag3AsS3)0.6(As2S3)0.4 thin film annealed at 100 °C one can observe formations (Fig. 1) which, as follows from the XRD data (Fig. 3), can be ascribed to a-Ag2S crystals. Moreover, as we can see from small scale pictures in Fig. 1, the cones are embedded with their basements into the base film which can be taken into the consideration as the additional reason for the presence of arsenic and sulphur in the EDX compositional spectra of the cones (Table 1).
3.2. Optical absorption Fig. 4 shows the optical transmittance of the as-deposited (Ag3AsS3)0.6(As2S3)0.4 thin film as well as those annealed at both 50 °C and 100 °C. One can observe a considerable increase of transmittance by almost 30% in the annealed films spectra. This fact is characteristic for a structural ordering, which occurs in the film structure as a consequence of thermal annealing. One should note that annealing at 50 °C for 1 h causes the transmission onset to shift towards shorter wavelengths. On the other hand, annealing at the increased to 100 °C temperature involves an opposite reaction, i.e. causes the transmission onset to shift slightly towards longer wavelengths. The spectra of the annealed films at the onset of transmission are seen to be less smeared or, in the other words, the slopes of the plots are seen to be sharper. Although a reduction Fig. 3. X-ray diffraction patterns of the as-deposited and annealed at 100 °C for 1 h (Ag3AsS3)0.6(As2S3)0.4 composite thin films.
Combined with SEM, EDX technique enabled us to ensure that the films formed are As-enriched and have a deficiency of sulphur percentage in comparison with the initial bulk composite, as follows in Table 1. Simultaneously, local EDX analysis estimated an excess of silver in the atomic composition of the cones. The presence of small amounts of arsenic and sulphur in the cones shown by the EDX can be explained by the fact that the minimum estimation area covers not only the cone itself, but some part of the base
Table 1 The elemental content (in at.%) of the as-deposited (Ag3AsS3)0.6(As2S3)0.4 thin films and corresponding initial bulk composite. The composition in atomic percentage is given for the cones formed on the fresh thin film surface as well. Elements (at. %)
Initial bulk composite
As-deposited thin film
Cones
Ag As S
29 22.6 48.4
29 32 39
62.8 18 19.2
Fig. 4. Optical transmittance of the (Ag3AsS3)0.6(As2S3)0.4 composite thin films: asdeposited (1), and annealed for 1 h at 50 °C (2), and at 100 °C (3). The inset shows the optical transmittance for as-deposited (1), and illuminated for 1 (2), 3 (3), and 5 (4) min by green laser (k = 532 nm).
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of smearing is observed for the annealed at 100 °C film, this film remains still more ordered comparing to the as-deposited fresh one. It should be noted that, by analogy with annealing, the film was found to increase its transmittance almost by 30% as a result of the 532 nm laser illumination for 1 min (Fig. 4). The transmittance falls down after the increase of illumination time up to 3 and 5 min. Nevertheless it remains increased in comparison with non-illuminated sample. The spectral dependences of the absorption coefficient as well as dispersion dependences of the refractive index were derived from the spectrometric studies of interference transmission spectra [6]. It should be noted that two spectral regions can be taken into consideration in amorphous films at the fundamental absorption edge at high absorption levels – a Tauc region and an exponential region [7]. In such a case both the Tauc gap Eopt and the energy g position of an exponential absorption edge Eag at a fixed absorption coefficient a (for example, in Ref. [8] the a value was chosen to be a = 104 cm1), characterizing the exponential part of the absorption edge, were used. The absorption edge spectra of the as-deposited, annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films in the range of their exponential behaviour are presented in Fig. 5. In Refs. [9,10] we used the optical pseudogap value Eg (energy position of the exponential absorption edge at a = 103 cm1) based on a possibility of comparison with Eg values of corresponding bulk materials, for which it is much more difficult to reach high absorption levels. In our case we used the Eag values taken at a = 5 104 cm1 for the characterization of the absorption edge spectral position, as far as the applying of Eg value seems to be unreasonable due to the strong absorption edge smearing. Determined in such a way Eag values are presented in Table 2, where Eopt values calcug lated for Tauc region are listed for comparison as well. It is shown that the increase of Eag is identical for annealing and illumination processes in (Ag3AsS3)0.6(As2S3)0.4 thin films. Thus, the annealing at 50 °C leads to Eag increase by 0.134 eV, while annealing at 100 °C causes the decrease of Eag by 0.042 eV, however the Eag value is still larger than that of as-deposited thin film (Table 2). The similar situation is observed after the illumination of thin films by green laser light (k = 532 nm) for 1 min which leads to the increase of Eag by 0.126 eV. Further extension of illumination time causes the Eag decrease, though at t = 5 min this value is still larger than that of as-deposited thin film (Table 2). It is seen from Fig. 5 that the optical absorption edge spectra in the range of their exponential behaviour in the thin films under investigation can be fitted by the equation [11]:
hm E 0 ; aðhm; TÞ ¼ a0 exp EU
ð1Þ
where EU is the Urbach energy (a reciprocal of the absorption edge slope EU1 = D(ln a)/D(hm)), a0 and E0 are the convergence point coordinates of the Urbach bundle. It is well-known that the Urbach energy EU characterises the smearing of optical absorption edge (in fact the EU is a degree of the system disordering). It was shown in Ref. [12] that both temperature and structural disordering affect the absorption edge shape in amorphous materials, i.e. the Urbach energy EU is described by equation:
EU ¼ ðEU ÞT þ ðEU ÞX ;
ð2Þ
where (EU)T and (EU)X are the contributions of temperature disordering and structural disordering to EU, respectively. The temperature disordering arises due to the thermal vibrations of atoms and structural elements. The structural disordering in amorphous materials is related to the absence of long-range order, the presence of structural inhomogeneities, i.e. defects, pores, etc. It should be noted that annealing at 50 °C leads to the Urbach energy EU decreasing. Meanwhile, the annealing at 100 °C leads to some increasing of
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Fig. 5. Absorption edge spectra of the (Ag3AsS3)0.6(As2S3)0.4 composite thin films: (a) as-deposited (1), and annealed for 1 h at 50 °C (2), and at 100 °C (3); (b) asdeposited (1), and illuminated for 1 (2), 3 (3), and 5 (4) min by green laser. The insets show the dependences of energy position of absorption edge (1) and the Urbach energy (2) on annealing temperature (a) and illumination time (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
EU, however this value is still smaller in comparison with that of the as-deposited thin film (Table 2). The last fact is an evidence of some structure ordering at annealing, but the annealing of this type of thin films is recommended to be carried out at 50 °C. It is shown that illumination leads to the Urbach energy decreasing and, respectively, to the structure ordering, which is, however, larger than that of the film after annealing (Table 2). The dispersion dependences of the refractive index for the asdeposited, annealed, and illuminated thin films were obtained from the interference transmittance spectra (Fig. 6). The slight dispersion of the refractive index is observed in the transparency region while it increases when approaching to the optical absorption edge region. The refractive index of the annealed at 50 °C thin film is smaller by more than 15% in comparison with the as-deposited film. One observes the refractive index increases by more than 9% with annealing temperature increase to 100 °C (Fig. 6, Table 2). After 1 min laser illumination the refractive index decreases by more than 15%, and subsequently increases by more than 11% with the illumination time increases to 5 min (Fig. 6, Table 2). Comparison of the refractive index values of the films annealed at 50 °C and illuminated for 1 min gives the evidence of the fact that observed changes under the influence of external factors are almost identical.
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Table 2 Optical parameters of the as-deposited, annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films. Thin film
n (1500 nm)
Eag (5 104 cm1) (eV)
Eopt (eV) g
EU (meV)
As-deposited Annealed 50 °C, 1 h Annealed 100 °C, 1 h Illuminated 1 min Illuminated 3 min Illuminated 5 min
3.08 2.60 2.84 2.61 2.78 2.92
2.085 2.219 2.177 2.211 2.172 2.164
1.708 1.644 1.684 1.864 1.639 1.571
457 421 432 403 393 396
Fig. 6. Refractive indices dispersions of the (Ag3AsS3)0.6(As2S3)0.4 composite thin films: as-deposited (1), and annealed for 1 h at 50 °C (2) and 100 °C (3). The inset shows the refractive indices dependence on annealing temperature.
Fig. 7. Profile formed on the (Ag3AsS3)0.6(As2S3)0.4 film surface due to laser illumination (k = 532 nm) through the 50 50 lm copper mesh. The inset shows the thin film surface by SEM due to laser illumination through the mesh (squares correspond to illuminated areas).
3.3. Laser illumination and holographic recording Thin (Ag3AsS3)0.6(As2S3)0.4 films are found to be strongly sensitive to the monochromatic green laser light influence. The film was illuminated through the 50 50 lm copper mesh by a normally incident horizontally polarised green laser light (532 nm, 20 mW) for 1 h. New formations were observed to appear on the surface of both the base film and the cones due to the laser illumination (Fig. 7, inset SEM image). At the same time the illuminated areas are found to lift up under the influence of the laser light which can be seen from the illuminated film profile analysis (Fig. 7). It should be noted that at the dark non-illuminated areas, shadowed by the mesh, only the cones are covered by additional structures. This can be seen if to compare the last one with the fresh film (Fig. 1a). At the same time both the cones and the base film at the illuminated areas covered by the additional structures are observed. This can be understood in the way that the laser light affects not only the illuminated areas in the squares inside the mesh, but also the whole film area under the mesh due to the ions transport processes in the thin film volume. The above mentioned processes can be true for holographic patterns recording as well (Fig. 8), where light and dark areas are distinguished by the presence of the surface cover. Phase holographic gratings were recorded on the surface of the (Ag3AsS3)0.6(As2S3)0.4 thin films with a period of 5 lm. SEM images of the holographic patterns are reproduced in Fig. 8. The holographic recording by the green horizontally polarised laser light (532 nm, 20 mW) for 40 min causes a growth of new formations of various shapes and smaller sizes in comparison with the big initial cones at the areas of interferometry maxima illumination. It should be mentioned, that these formations start to disappear under the influence of a strong (40 kV) e-beam irradiation in SEM. The formation and dis-
Fig. 8. SEM images of the (Ag3AsS3)0.6(As2S3)0.4 thin film with holographic patterns recorded on its surface.
appearance of the new formations due to the laser light impact are most probably related to the Ag+ ions movements attracted by the laser light as it was shown earlier [13,14]. At the same time this effect was not observed (or was negligibly small) when treating by the e-beam the formations grown at the big cones. This can testify the fact that the small formations grown after illumination are of the same nature as initial main big cones, i.e. Ag-enriched ones. AFM image of the holographic pattern formed on the (Ag3AsS3)0.6(As2S3)0.4 thin film are shown in Fig. 2(b). It should be noted that the AFM confirms bigger roughness of the illuminated film in comparison with the as-deposited one. This fact is
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supported by the SEM images of the corresponding holographic pattern (Fig. 8).
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respectively, the decrease of structural disordering in the (Ag3AsS3)0.6(As2S3)0.4 thin films. Acknowledgements
4. Conclusions (Ag3AsS3)0.6(As2S3)0.4 chalcogenide thin films were prepared by rapid thermal evaporation in vacuum. Subsequently, the films were annealed at 50 °C and 100 °C for 1 h in vacuum as well as were illuminated for 1, 3, and 5 min by laser with k = 530 nm. SEM and AFM imaging of the as-deposited, annealed and illuminated films revealed numerous micrometer-sized cones on their surfaces. The presence of crystalline peaks was revealed in the XRD patterns of both fresh and annealed thin films. The EDX analysis showed an excess of silver in the obtained cones, which, together with the pronounced peak of Ag in the XRD pattern, enabled us to ascribe the last one to the cones. Annealing at 50 °C and 100 °C was shown to result in a mechanical deformation of part of the cones and their detachment from the base film surface. Another result of annealing is probable appearance of crystalline a-Ag2S on the surface of the cones and the base film. It was shown that laser illumination causes the appearance of the new formations on the surface of the base film and the cones. The optical transmission spectra of the annealed and illuminated films have shown an increase of transmission for all samples, whereas the largest change of transmittance appeared to be a result of annealing at 50 °C and illumination for 1 min. The spectral dependences of the absorption coefficient as well as dispersion dependences of the refractive index were derived from the spectrometric studies of interference transmission spectra. In the range of the exponential behaviour of the optical absorption edge the energy position of exponential absorption edge Eag and the Urbach energy EU in the as-deposited, annealed and illuminated (Ag3AsS3)0.6(As2S3)0.4 thin films were determined. It was shown that annealing and illumination lead to the increase of theEag , and to the decrease of the refractive index n. Moreover, the comparison of the Eag and n values after annealing at 50 °C and after illumination for 1 min give the evidence of the fact that the observed changes under the influence of external factors are almost identical. Additionally, it is revealed that annealing and illumination cause the Urbach energy EU decrease and,
The authors are grateful to the TAMOP Grant (TAMOP 4.2.2.A11/1/KONV-2012-0036 project) which is co-financed by the European Union and European Social Fund. Yuriy Neimet (Contract Number 51301014) is strongly grateful to the International Visegrad Fund scholarship for the funding of the project. References [1] Y. Pauleau, Materials Surface Processing by Directed Energy Techniques, Elsevier, Oxford, 2006. [2] E. Bychkov, A. Bychkov, A. Pradel, M. Ribes, Percolation transition in Ag-doped chalcogenide glasses: comparison of classical percolation and dynamic structure models, Solid State Ionics 113 (1998) 691–695. [3] I. Studenyak, Yu. Neimet, C. Cserhati, S. Kökényesi, E. Kazakevicˇius, T. Šalkus, A. Kezˇionis, A. Orliukas, Structural and electrical investigations of (Ag3AsS3)x(As2S3)1x superionic glasses, Cent. Eur. J. Phys. 10 (2012) 206–209. [4] I.P. Studenyak, Yu.Yu. Neimet, M. Kranjcˇec, A.M. Solomon, A.F. Orliukas, A. Kezˇionis, E. Kazakevicˇius, T. Šalkus, Electrical conductivity studies in (Ag3AsS3)x(As2S3)1x superionic glasses and composites, J. Appl. Phys. 115 (2014) 033702–1033702-5. [5] R.S. Wagner, W.C. Ellis, Vapor–liquid–solid mechanism of single crystal growth, Appl. Phys. Lett. 4 (1964) 89–90. [6] R. Swanepoel, Determination of the thickness and optical constants of amorphous silicon, J. Phys. E:Sci. Instrum. 16 (1983) 1214–1222. [7] R. Bindemann, O. Paetzold, The influence of thermal disorder on the absorption edge in the Tauc region of a–Si:H, Phys. Stat. Sol. (B) 160 (1990) K183–K188. [8] S. Kökényesi, J. Csikai, P. Raics, I.A. Szabo, S. Szegedi, A. Vitez, Comparison of photo- and deuteron-induced effects in amorphous chalcogenide layers, J. Non-Cryst. Sol. 326&327 (2003) 209–214. [9] I.P. Studenyak, M. Kranjec, M.M. Pop, Urbach absorption edge and disordering processes in As2S3 thin films, J. Non-Cryst. Solids 357 (2011) 3866–3869. [10] I.P. Studenyak, M. Kranjec, V.Yu. Izai, A.A. Chomolyak, M. Vorohta, V. Matolin, C. Cserhati, S. Kökényesi, Structural and temperature-related disordering studies of Cu6PS5I superionic thin film, Thin Solid Films 520 (2012) 1729– 1733. [11] F. Urbach, The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids, Phys. Rev. 92 (1953) 1324–1326. [12] G.D. Cody, T. Tiedje, B. Abeles, B. Brooks, Y. Goldstein, Disorder and the opticalabsorption edge of hydrogenated amorphous silicon, Phys. Rev. Lett. 47 (1981) 1480–1483. [13] N. Yoshida, K. Tanaka, Ag migration in Ag–As–S glasses induced by electronbeam irradiation, J. Non-Cryst. Sol. 210 (1997) 119–129. [14] T. Kawaguchi, Photoinduced metastability in Ag-containing chalcogenide glasses, J. Non-Cryst. Sol. 345&346 (2004) 265–269.