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Structural and optical characterization of Ag photo-doped thin As40S60 − x Sex films for nonlinear applications

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IOP PUBLISHING

JOURNAL OF OPTICS

J. Opt. 12 (2010) 065601 (9pp)

doi:10.1088/2040-8978/12/6/065601

Structural and optical characterization of Ag photo-doped thin As40S60−x Sex films for non-linear applications J Tasseva, R Todorov1 , Tz Babeva and K Petkov Central Laboratory of Photoprocesses ‘Academician Jordan Malinowski’, Bulgarian Academy of Sciences, Academician Georgi Bonchev Street, Block 109, 1113 Sofia, Bulgaria E-mail: [email protected]

Received 8 April 2010, accepted for publication 18 May 2010 Published 10 June 2010 Online at stacks.iop.org/JOpt/12/065601 Abstract This paper deals with the structure and the optical properties of thin As40 S60−x Sex films doped opt with silver. The refractive index n and the optical band gap E g were calculated from the transmittance and reflectance spectra. The results showed that the photo-doping leads to increase in the refractive index by about 0.25–0.27. An effect of thickness expansion was observed in the photo-doped layers. The non-linear refractive index, γ , and the two-photon absorption coefficient, β , were evaluated by applying a formula developed by Sheik-Bahae. Each of the films studied exhibits a highly non-linear refractive index at the telecommunication wavelength, 70–850 times higher than that measured for fused silica. From the Raman spectra of thin As40 S30 Se30 it might be concluded that under dissolution, the silver interacts with both sulfur and selenium. The surface of the thin films was investigated by using a white light interferometric profiler. It was found that the increase in the thickness of the silver layer results in roughening of the surface of the photo-doped films. Keywords: optical properties, chalcogenide thin films, silver diffusion, non-linear refractive

index, Raman spectroscopy (Some figures in this article are in colour only in the electronic version)

conventional melt quenching technique; (ii) photo-doping of bulk glasses or thin chalcogenide films [14–19]. The structure and physical properties of thin films from As2 S(Se)3 /Ag systems have been investigated for many years [20–24]. The optimal concentration of Ag in the photo-doped As2 S3 layers is found to be 30–32 at%, which corresponds to the composition As2 S3 Ag2.2−2.4 [25]. According to [21], the rate of diffusion of Ag in thin films from the Asx S100−x system is enhanced when the sulfur content increases due to changes in the free volume. The most favourable host matrix for silver among all the compositions studied was reported to be the thin As33 S67 layer. It is shown that thin As–S–Ag films have similar structure to bulk quenched glasses of the same composition [1, 14]. The opportunity for dissolving large amounts of Ag in chalcogenide thin films creates the possibility of varying their properties over a wide range. The addition of silver into chalcogenide glasses leads to drastic changes in the physical and chemical

1. Introduction The photo-induced reaction between the chalcogenide glasses and various metals, such as silver, copper and zinc, has been extensively studied over the years [1–3]. The interest in the effect of silver photo-diffusion or photo-doping of bulk glasses or thin films from the As–S(Se) and Ge–S(Se) systems has been stimulated by their numerous practical applications in manufacturing various optical elements, such as waveguides [3, 4], diffraction gratings [5, 6], microlenses [7, 8], and phase change recording media [9]. This is what determines the position of chalcogenide glasses in light wave technology and creates a demand for the characterization and control of their properties [10–13]. Two methods for production of silver containing chalcogenide glasses are known: (i) synthesis by the 1 Author to whom any correspondence should be addressed.

2040-8978/10/065601+09$30.00

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Figure 1. Spectral dependence of the transmission of (curve 1) a double-layered structure As40 S30 Se30 /Ag and (curves 2–6) a photo-doped film.

Figure 2. Variation of the reflectance in the absorbing region of the spectra (λ = 480 nm) with irradiation time for samples of As40 S30 Se30 , deposited on 25 and 80 nm Ag layers.

properties of the materials [26, 27]. For instance, in thin films from Asx S(Se)100−x systems (for x = 30, 33, 40 at%) large increases of about 0.4–0.7 in the refractive index were observed following Ag photo-dissolution [28–31]. This could be ascribable to the easily polarizable electron clouds of the silver atoms. Kosa et al [32] showed by Z -scan measurements that silver doping of As2 S3 glass may possibly alter the sign of the non-linear refractive index. The interest in the As–S–Se system is due to the large glass-forming region and high optical non-linearity [33]. The structure and physicochemical, linear and non-linear optical properties of thin As33 S67−x Se x films photo-doped with silver have been widely investigated [34–38], since it has been shown that at the stoichiometric composition of the As–S system, photo-dissolved silver reaches a maximum without disrupting the amorphous state of the host matrix [39]. On the other hand, following the photo-diffusion process in thin As40 S60−x Se x , no destruction of pyramidal structural units was found by Raman spectroscopy [40]. Furthermore, some authors relate the deviation from the linear dependence of some physical properties of thin As40 S60−x Se x films to changes in the average molar volume [41, 42]. These make the thin films from the As40 S60−x Se x system an appropriate host matrix for silver. Ultimately, a great advantage of the photo-stimulated dissolution of metal in chalcogenide glasses is the provision of the possibility for enhancing the refractive index and decreasing the optical band gap, thus increasing the nonlinear refractive index of the layers. In this paper we study the influence of the Ag concentration on the surface, structural and optical properties of thin films from the system As40 (S + Se)60 .

initially deposited silver coatings by RF (radio-frequency) sputtering. Thin films of compositions As40 S60 , As40 S30 Se30 and As40 Se60 , and thickness 700 nm, were deposited onto previously sputtered silver layers with thicknesses of 25 and 80 nm (the Ag/chalcogenide thin film thickness ratios were ∼0.03 and 0.1, respectively). Following the deposition process, the double-layered structure, chalcogenide/silver, was illuminated in portions through the chalcogenide side until a homogeneous Ag doped layer was obtained. The samples were exposed in air to a mercury lamp (20 mW cm−2 ). The composition of the thin films obtained was determined with a scanning electron microscope, Jeol Superprobe 733 (Japan), with an x-ray microanalyser. The experiments were performed at an accelerating voltage of 20 kV, current of 1.4 nA and scanning time of 200 s for each spectrum. The transmittance, T and reflectance, R , were measured using a UV–visible–NIR spectrophotometer, Cary 5E (Australia), in the region 400–2000 nm with accuracies of ±0.1% and ±0.5%, respectively. The Raman measurements were performed using a Horiba Jobin Yvon confocal Raman spectrometer with a diameter of the confocal pinhole of 100 μm, an excitation line of 660 nm and a magnification of the microscope objective of 100×. The spatial resolution is 1 cm−1 . The surface morphology of the samples was studied using a white light interferometric (WLI) surface profiler, MicroXAM S/N 8038, with vertical and lateral resolution of 1 nm and 1 μm, respectively.

2. Experimental details

The process of photo-diffusion of silver in thin films was monitored by means of spectrophotometric measurements. The variation of the transmission spectra with irradiation time and the dependence of the reflectance at 480 nm (the absorbing region for chalcogenide films), from both film and substrate sides, on the irradiation time are shown in figure 1 and figure 2, respectively.

3. Results and discussion 3.1. Characterization of the process of photo-doping of thin As40 S60−x Sex films

Thin As–S–Se films were deposited on Si, graphite and optical BK-7 glass substrates, in a high vacuum of 10−3 Pa, by thermal evaporation of previously weighed quantities of bulk materials. The photo-induced diffusion of Ag into the chalcogenide system was performed using substrates with 2

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Figure 3. Surface profiles for thin Ag–As40 S60−x Sex layers collected using the WLI profiler: ((a), (d), (g)) undoped; ((b), (e), (h)) Ag doped, with thickness of the metal layer 25 nm; ((c), (f), (i)) doped, with thickness of the Ag layer 80 nm; the top row is for As40 S60 , the middle for As40 S30 Se30 , and the bottom for As40 Se60 .

the changes in reflectance being more pronounced in the first 30 min of the light irradiation. Table 1 presents the chalcogenide layer/silver layer thickness ratio as well as the composition, measured by means of x-ray microanalysis, of the photo-doped thin chalcogenide films with an experimental error of 1–1.5% and compositions calculated from the thicknesses. According to [47], silver/chalcogenide layer thickness ratios of 0.03 and 0.1 result in respectively 5 and 12 at% concentration of Ag in photo-doped thin As30 Se70 films. It is seen in table 1 that the discrepancy between the calculated and experimentally determined concentrations of silver is greater for higher thickness ratios. The effect of the electron induced modification in Ag–As–Se glasses described by Yoshida [48] and manifested by accumulation of silver in the area analysed is a possible reason for the observed discrepancies. The surface morphology of the layers from the system As40 S60−x Sex /Ag was studied by using a white light interferometric profiler. Surface profiles of the undoped and doped layers are displayed in figure 3, and the RMS (root mean square) roughness, calculated from five measurements at different points of the samples, in figure 4. As is seen in figure 3, the increase of the silver layer thickness leads to an increase of the surface roughness of the photo-doped layers, thus being in accord with the results from the SEM of Ag/As33 S66−x Sex , published elsewhere [37]. According to [38], the reason for the increase of the surface roughness is that silver growth enhances the formation of nano/mesoscale inhomogeneities on the surface. The Ag doped As2 S3 and As2 Se3 films are approximately twice rougher compared to Ag/As40 S30 Se30 layers (figure 4). The smaller

It is seen from figure 1 that before exposure to light, the double-layered structure is opaque; the transmittance T is close to 0% (curve 1). The exposure leads to silver dissolution in chalcogenide glass film and as a result T increases gradually with illumination time (curves 2–6). The time dependence of the reflectance from the substrate side Rb has the opposite trend (figure 2). Prior to illumination of the samples Rb is high: 90 and 35% for the silver thicknesses of 80 and 25 nm, respectively. Considering the sample architecture (substrate/metal/chalcogenide film) the reflectance is expected to be high when it is measured from substrate side. As silver dissolution progresses in the chalcogenide film, the reflectance decreases, reaching 13–15% after 1 h. The production of homogeneous layers was controlled by means of measurements of reflectance spectra of the air and substrate sides of the samples in the transparent region, Rf and Rb , respectively. Rf and Rb were the same after 60 min of illumination. The homogeneity of the thin films fabricated by such a method has been approximately confirmed by secondary-ion mass spectroscopy [43, 44]. It is seen in figure 2 that the reflectance changes more drastically in the first half an hour of the irradiation. According to [45], the tracer diffusion coefficient, D , of silver in some chalcogenide and chalcohalide glasses is in the range of 10−8 –10−12 cm2 s−1 . Bychkov et al [46] found that the concentration profile of silver changes over time via the so called error function. √ Considering that the mass diffusion length is L d = 2 Dt , we calculated that the time needed for silver to penetrate through the chalcogenide layer with a thickness of 700 nm and reach its surface is approximately 20 min for D = 10−12 cm2 s−1 . This is a possible reason for 3

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Table 1. Calculated and measured compositions of thin Ag doped chalcogenide layers. Ag/chalcogenide layer (thickness ratio)

Composition calculated from the thickness ratio

Composition of photo-doped thin films determined via x-ray microanalysis

Ag/As40 S60 (0.033) Ag/As40 S60 (0.104) Ag/As40 S30 Se30 (0.035) Ag/As40 S30 Se30 (0.112) Ag/As40 Se60 (0.035) Ag/As40 Se60 (0.113)

As38.7 S58.0 Ag3.3 As36.2 S54.3 Ag9.5 As38.0 S28.4 Se28.4 Ag5.2 As35.9 S27.0 Se27.0 Ag10.1 As38.6 Se58.0 Ag3.4 As36.0 Se53.9 Ag10.1

As28.4 S67.9 Ag3.7 As28.1 S60.0 Ag11.9 As31.9 S40.8 Se22.1 Ag5.2 As29.2 S34.9 Se18.8 Ag17.1 As39.1 Se55.1 Ag5.7 As34.3 Se49.1 Ag16.6

Table 2. Optical parameters of thin Ag–As–S–Se films.

As40 S60

dAg = 25 nm dAg = 80 nm As40 S30 Se30 dAg = 25 nm dAg = 80 nm As40 Se60 dAg = 25 nm dAg = 80 nm

Undoped Undoped Ag doped Ag doped Undoped Undoped Ag doped Ag doped Undoped Undoped Ag doped Ag doped

Exposure time (h)

d (nm)

n at λ = 1.55 μm

γ (m2 W−1 ) ×10−18 γ /γfused Si

FOM

E gopt B (cm−1/2 eV−1/2 ) (eV)

0 3 5 5 0 3 4 4 0 3 0.25 2

762 742 745 850 709 700 714 767 705 693 807 808

2.32 2.43 2.53 2.57 2.46 2.60 2.63 2.70 2.69 2.80 2.66 3.06

1.96 1.93 2.08 4.52 3.58 3.96 2.08 8.62 6.32 7.19 19.1 23.2

0 0 0 0 0 0 0 0 0 0 1 33

765 869 833 621 747 828 798 738 817 858 694 530

71 70 76 165 131 145 76 315 231 262 697 847

2.40 2.36 2.23 1.80 2.08 2.00 1.91 1.73 1.82 1.76 1.58 1.32

Figure 5. Dispersion of the refractive index of thin (Ag)–As40 S30 Se30 films.

Figure 4. RMS versus silver concentration in photo-doped As–S–Se films.

changes in RMS of the photo-doped As40 S30 Se30 layer can be associated with the deviation from the statistical substitution of Se and S in the As40 S60−x Se x system, which could explain the observations of non-linear compositional dependences of the optical and thermal properties and the decrease in photo-induced changes of the thickness for the intermediate compositions [41, 42, 49].

program developed by Konstantinov [51]. The spectral dependence, n(λ), of the refractive indices of as-evaporated thin As40 S30 Se30 films, exposed to saturation and doped with silver, are plotted in figure 5. The refractive index of the silver doped layer is evidently ∼0.25 higher than that of the undoped one. In table 2 some optical parameters of undoped and photodoped films from the system As40 (S + Se)60 are listed. The introduction of silver into the system enhances the refractive index—for the Ag doped As40 S60 layer (in the case of an 80 nm silver layer as the metal source) n is 10.8% higher than that of the undoped one. Furthermore the calculated values for the thin film thicknesses show that the photo-doping

3.2. Optical properties of As40 S60−x Sex films The refractive index, n , and thickness, d , of thin films were calculated from the interference extrema in the transmission spectra using Swanepoel’s method [50] and a computer 4

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systems [32, 57]. In the simple model, γ can be expressed as h¯ c E p h¯ ω γ =K G (4) opt opt4 2 Eg 2n 2 E g

leads to volume expansion of the layer. This expansion effect could be attributable to the increased length of the Ag–S bonds which are of more ionic character compared to the As– S(Se) bonds. The decrease of the exposure time with the increase of the selenium content in thin films is probably due to enhancement of the diffusion coefficient, D . At high values of the linear absorption coefficient α , where the condition αd 1 is fulfilled, α can be calculated from the equation T = (1 − R)2 exp(−αd) (1)

where E p = 21 eV, K is found to be 3.1 × 10−8 in units such opt that E p and E g are measured in eV, and γ is measured in m2 W−1 , h¯ is the reduced Planck’s constant, c the speed of light in the vacuum of free space and G 2 is a universal function:

G 2 (x) = 3

−2 + 6x − 3x 2 − x 3 − 34 x 4 − 34 x 5 + 2(1 − 2x) 2 (1 − 2x) 64x 6 (5)

where T is the transmittance, R is the reflectance and d is the thickness of the film. The analysis of the strong absorption region (104 cm−1 α 105 cm−1 ) has been carried out using the following well-known quadratic equation, often referred to as Tauc’s law [52]:

(αhν) = B(hν − E gopt )2

where is the Heaviside step function. n 2 and γ are related by cn γ (SI). n 2 (esu) = (6) 40π In the same approximation, the two-photon absorption, βNL , originally defined by α = α + IβNL (α being the intensitydependent absorption coefficient), can be expressed as Ep βNL = K F (2h ω/E gopt ) (7) opt3 2 ¯ 2 n Eg

(2)

where B is a material parameter, which is in an inverse proportion to the width of the localized states bands in the opt density of states diagram, hν is the photon energy and E g is opt called Tauc’s gap. The optical gap E g for indirect transitions can be derived from the intercept of the linear part of the αhν 1/2 versus hν plot with the energy axis. As seen in table 2, the photo-diffusion of silver in the chalcogenide layer is associated with shrinkage of the optical band gap. The narrowest band gap material (1.32 eV) is found to be Ag doped As40 Se60 (dAg ∼ 80 nm). According to Shimakawa et al [1], silver is incorporated in the amorphous chalcogenide network, forming small species of Ag2 Se and/or fourfold-coordinated Ag atoms. These may add to the density of the defects (i.e. localized states), thus increasing the width of the localized states bands. This assumption is confirmed by the lowest value of the factor B (530 cm−1/2 eV−1/2 ) compared to those, calculated for the remaining of the films. opt The minimization of the optical band gap, E g could be advantageous in the context of maximizing the non-linear index of refraction in glasses [33, 53]. If we consider the nonlinear response of the chalcogenide medium to intense light opt with photon energies h¯ ω < E g , we know that two-photon absorption must be involved in the interband transitions [54]. One of the associated effects is the inducing of a non-linear refractive index, n 2 (esu), or γ (m2 W−1 ). It is known [55] that the intensity-dependent refractive index n can be expressed as

n = n + γ I = n +

n2 |E|2 2

where

(2x − 1)3/2 for 2x > 1 (8) 2x 5 and F2 (2x) = 0 otherwise. That means that two-photon absorption occurs only for photon energies higher than at least half of the optical band gap. It is shown [58] that the two-photon absorption possibly accompanying the highly nonlinear refractive index, γ , could prevent the optical switching effect, thus seriously limiting the applicability of any highly third-order non-linear material in all-optical switches. This limitation is quantitatively expressed by a simple criterion, based on the figure-of-merit (FOM), F . For the effective operation of any non-linear device, the following inequality should be fulfilled: F2 (2x) =

F=

2βNL λ < 1. γ

(9)

In figure 6 the dispersion of γ and FOM for a chalcogenide Ag doped As40 S30 Se30 layer are shown. It is seen that the opt FOM values are low at extreme points of γ (at ∼0.55 E g and opt ∼E g ). This is relevant to the application of the materials in non-linear optics, since low values of the FOM indicate low two-photon absorption values. Calculations at λ = 1.55 μm (hω = 0.8 eV) are reported in table 2. At the telecommunication wavelength each of the films studied exhibits a highly non-linear refractive index—70–850 times higher than that measured for fused silica (γ = 2.74 × 10−20 m2 W−1 at λ = 1.53 μm [59]; table 2). The requirement for effective operation of a certain material as a device at that specific wavelength (FOM < 1) is observed for all the films except for the Ag doped As40 Se60 films, FOM being 33 and 1, respectively for the thinner and thicker silver layers used as the metal source.

(3)

where n is the linear, weak-field refractive index, I denotes the intensity, E is the strength of the applied optical field and n 2 gives the rate at which the refractive index increases with increasing optical intensity. For prediction of the non-linear refractive index we have applied a formula, developed by Sheik-Bahae et al [56] for crystalline semiconductors and successfully applied to the glasses and thin films from the As33 S67−x Sx and Ag–As–S 5

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Figure 6. Dispersion of the (a) non-linear refractive index, γ , and (b) figure-of-merit of thin Ag/As40 S30 Se30 films.

In [36] the increase in the value of n 2 was related to an increase in the dispersion energy, E d , from the dispersion model of Wemple and DiDomenico [60]. In terms of the single-oscillator model, the authors use the following dependence to describe the dispersion of the linear refractive index: Ed E0 n2 = 1 + 2 (10) E 0 − (hν)2

layer’s structure. Raman spectra of thin films of As–S–Se–Ag are presented in figure 7. Several Raman scattering bands are measured for the asdeposited undoped As40 S60 film (figure 7(a)). The most intense one is located in the frequency range 275–435 cm−1 , producing a maximum at ∼334 cm−1 . The band in the 275–435 cm−1 range in the spectrum of glasses with composition As40 S60 is reported to be composed of three components [62]: an IR active mode at 316 cm−1 , corresponding to the anti-symmetric stretching vibrations of As–S bonds in AsS3/2 pyramids, a symmetric stretching Raman active mode at 340 cm−1 and an IR active mode at 380 cm−1 , resulting from the antisymmetric vibrations in As–S–As chains. In the spectrum of crystalline β -As4 S4 three strong lines are reported, at 342, 351 and 361 cm−1 [63]. The shoulder at ∼300 cm−1 is probably a result of Raman scattering from the silicon substrate and has been independently observed by other authors [64]. In the lowfrequency range 100–270 cm−1 of the spectrum of As40 S60 several Raman scattering bands can be seen, the most intense producing maxima at 184 and 230 cm−1 and corresponding to the vibrations of homopolar As–As bonds in realgar, As4 S4 , or S–S bonds [63, 64]. Upon illumination of the sulfide film, the shoulder at ∼300 cm−1 disappears, the maximum at 334 cm−1 shifts to 344 cm−1 , and the band at ∼184 cm−1 drastically weakens. The spectrum of the irradiated As40 S60 layer is close to those reported and theoretically calculated for bulk samples [40, 65]. Hence, it can be concluded that illumination increases the concentration of AsS3/2 structural units on account of decreasing the number of AsS2/2 ones. The spectrum of the as-deposited thin As40 Se60 film (figure 7(c)) features an intense band at 190–300 cm−1 , including a shoulder at ∼223 and an indistinct maximum at 252 cm−1 . According to [66] the Raman scattering in this frequency region for Se-rich glasses from the system Asx Se100−x is due to the presence of AsSe3 pyramidal units and Se8 rings. Furthermore, it is possible that the highintensity band observed in the spectra of As2 Se3 is a result of the convolution of other bands, at 205, 237 and 280 cm−1 , featuring the presence of realgar-like structures, As4 Se3 , and −Se−Se bridges among AsSe3 pyramids, respectively [66]. In the spectrum of the as-deposited arsenic selenide layer a scattering band at 366 cm−1 is detected, which might attest to the presence of Se4 clusters [67]. Under illumination of the As2 Se3 film, the intensity of the band at 252 cm−1 decreases

where hν is the photon energy, E 0 is the single-oscillator energy. Equation (10) shows that larger refractive index values arise from smaller E 0 and/or greater E d values. The magnitude of E d strongly depends on the effective coordination number of the cation nearest neighbour of the anion, Nc :

E d = β Nc Z a Ne

(11)

where Z a is the formal chemical valency of the anion, Ne is the effective number of valence electrons per anion, and β is a two-valued constant with either an ionic or a covalent value (β = 0.26 ± 0.03 eV and β = 0.37 ± 0.04 eV, respectively). The coordination number of arsenic cations in As2 Se3 is about 3. In [1] it is shown that Ag atoms in the glasses from As–Se–Ag system are fourfold coordinated. The addition of silver in the glassy As2 Se3 leads to an increase of Nc and correspondingly the value of E d arises. According to the formula proposed in [61], the non-linear optical susceptibility, χ (3) , can be calculated from the following equation:

χ

(3)

E0 Ed =A 4π(E 02 − h¯ 2 ω2 )

4 (12)

where A = 1.7 × 10−10 (for χ (3) in esu).

n2 =

12πχ (3) , n0

(13)

n 0 being the limit of the refractive index dispersion as hω approaches zero. It is seen from equations (12) and (13) that the non-linear refractive index is in direct proportion to the fourth power of the dispersion energy. 3.3. The structure of thin As40 S60−x Sex films The interpretation of the dependences of the optical properties on the composition of the thin film requires knowledge on the 6

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Figure 7. Raman spectra of thin films from the Ag–As–S–Se systems.

and a new band at 225 cm−1 is simultaneously differentiated. Besides this, the band at 366 cm−1 is reduced and one at 448 cm−1 appears. In the spectrum of the thin As40 S30 Se30 layer (figure 7(b)) two intense Raman scattering bands are measured—between 190 and 290 cm−1 giving a wide peak at 234 cm−1 and between 295 and 405 cm−1 with a maximum at 343 cm−1 , these being close to the scattering bands of the extreme compositions As40 Se60 and As40 S60 , respectively. According to [68] the low-frequency band is related to As–Se vibrations, and the high-frequency one to As–S vibrations in the mixed AsS3−n Sen (n = 0, 1, 2, 3) pyramids. The low-frequency shift of the scattering bands with the increasing of the selenium content in the pyramids is produced by the accumulation of the mode mass of the material. In [40] the most intense Raman scattering for spectra of As40 S20 Se40 films was observed at 246 and 354 cm−1 . According to the authors, the positions of these bands are very sensitive to the chaotic replacement of chalcogens of one type by chalcogens of another type in the pyramidal AsS3−n Sen structural units. Silver inclusion in the thin As2 S3 layer produces a high-frequency shift of the band at 336 cm−1 towards 348 cm−1 for 3.7 at% Ag and 371 cm−1 for 11.9 at% Ag (figure 7(b)). Furthermore, the amplitude of the low-frequency band (230 cm−1 ) in the spectrum of the undoped sulfide layer is reduced by the augmentation of the silver concentration. There has been some speculation [36] that under Ag dissolution in thin As33 S67 film the silver consumes the free S from the As–S–As matrix, forming S–Ag–S bonds between the AsS3 pyramids, or AgS3 pyramids. These reorganizations lead to the appearance of a new band at 376 cm−1 in the Raman spectrum of the layers. We may assume that the band at 371 cm−1 in the spectrum of the doped sulfide layer for 11.9 at% Ag concentration (figure 7(a)) is due to the formation of those very structures. The diffused silver atoms would break the bonds between the arsenic and sulfur in the AsS3 pyramids, bonding with a part of the sulfur atoms, thus transforming the AsS3 units into structures of realgar, AsS2 . It is known that in the Raman

spectrum of crystal As4 S4 an intense peak at 361 cm−1 should be detected [63]. Hence, the higher arsenic concentration and, therefore, higher concentration of realgar structural units in our layers compared to those reported in [36] would cause the shift of the band at 376 cm−1 towards lower frequencies (371 cm−1 , our results). The inclusion of 5.8 at% of silver in the arsenic selenide layer reduces the contribution of the substrate to the Raman signal; the low-intensity band at 300 cm−1 detected in the spectrum of the undoped layer disappears on introducing small amounts of silver (figure 7(c)). The maximum at 252 cm−1 in the 190–300 cm−1 band is shifted towards lower frequencies (227 cm−1 ), thus approaching the band at 225 cm−1 from the spectrum of the undoped, illuminated As2 Se3 layer. The band at 227 cm−1 is attributed to symmetric stretching vibrations in AsSe3 pyramids [44]. The latter is shifted towards higher frequencies, 244 cm−1 , producing a shoulder around 214 cm−1 on increasing the silver content above 17 at%. According to [47], the decrease in the Se–Se bridging units between AsSe3 pyramids may possibly shift the vibrations of the Se rings (256 cm−1 ) towards frequencies corresponding to the Sen chain vibrations (238 cm−1 ) which is supported by the shift of the band at 256–248 cm−1 in the Raman spectrum of As30 Se70 film photo-doped with 12 at% silver. The most intense Raman scattering bands for crystal AgAsSe2 are observed at 214 and 250 cm−1 [44]. According to [69], the Raman spectrum of Ag2 Se has a strong band in the frequency range 100–200 cm−1 with a maximum at about 160 cm−1 . It is difficult to find any indication of the existence of Ag2 Se in the measured spectra of photo-doped As–Se–Ag films. As a result of the Ag doping of As40 S30 Se30 film, the amplitude of the band at 304 cm−1 is reduced and gradually fades away for 17 at% silver concentration (figure 7(b)). That could be related to a decrease in the As–S bond concentration. Besides this, the bands at 234 and 343 cm−1 are shifted towards higher frequencies, 247 and 360 cm−1 , respectively. Such alterations of both Raman scattering bands are similar to those considered above for the case of Ag doping of thin As2 S3 and 7

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As2 Se3 films. Such behaviour is in accord with the Raman spectra of bulk Ag photo-doped As40 S40 Se20 and As40 S20 Se40 layers [40, 70]. Hence, it might be inferred that under diffusion in a thin As40 S30 Se30 layer, the silver (at the concentrations studied) interacts with both sulfur and selenium. The approach of the low-intensity band to 250 cm−1 can be attributed to the formation of Se–Ag–Se bonds between AsSe3 pyramids or As3 Se6 units, or AgSe3 pyramids [36]. The Raman spectra of the photo-doped thin Ag/As40 S60−x Se x films indicate that silver atoms have probably been incorporated in the glassy network in the form of AgAsS(Se)2 structural units. The formation of such structures leads to an increase of the effective dispersion energy (equations (10) and (11)). In [36] it is found for thin Ag25 (As33 S67 )75 film with composition close to AgAsS2 that the dispersion energy is 5.7 eV higher than that of the undoped layer. The calculated value for E d [71] for thin Ag2 S film is 32.5 eV.

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4. Conclusion The photo-doping of thin films from the As40 S60−x Sex system with silver was investigated. The duration of the light illumination needed for obtaining homogeneous silver doped layers was determined by spectrophotometric measurements. Smaller changes of the surface roughness due to doping occur for thin film with the composition As40 S30 Se30 because of the extreme in the average molar volume compared to the binary systems. The refractive index increases with increasing silver and selenium concentrations in thin films. The difference between the refractive indices of the undoped and silver doped thin films is 0.25–0.27, which is similar to the changes observed in the As33 S67−x Sex system [36, 37]. Applying the formula proposed by Sheik-Bahae et al [56], it was found that there was a non-linear refractive index increase of 70–800 times due to doping. The increase in the linear and non-linear refractive indices was explained on the basis of the proposed model, from Wemple and DiDomenico [60]. By means of Raman spectroscopy it was found that silver creates AgAsS(Se)2 and AgS(Se)3 structural units in the glass network.

Acknowledgments The authors would like to acknowledge the Centre for Industrial and Engineering Optics (IEO) and the School of Physics and Facility for Optical Characterization and Spectroscopy (FOCAS), both at Dublin Institute of Technology, for technical support with using the WLI profiler and performing the Raman measurements.

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