V Nemec

Nazabal

Tomáš Halenkovič

Marion Baillieul

Jan Gutwirth

Petr Němec

Virginie Nazabal
email: virginie.nazabal@univ-rennes1.fr

Amorphous Ge-Sb-Se-Te chalcogenide films fabrication for potential environmental sensing and nonlinear photonics

Keywords: chalcogenide, thin films, glass, amorphous, optical properties, NLO, contact angles, sputtering, Raman

Amorphous Ge-Sb-Se-Te chalcogenide films fabrication

for potential environmental sensing and nonlinear photonics

1. Introduction Amorphous chalcogenides, non-crystalline solids based on the elements of chalcogens (i.e. S, Se, Te), are well known for their unique properties such as low phonon energies resulting in broad transmission window, photoinduced phenomena or high optical nonlinearities without free carriers [START_REF] Ovshinsky | Amorphous and Liquid Semiconductors Imaging in amorphous materials by structural alteration[END_REF][START_REF] Savage | Chalcogenide glasses transmitting in the infrared between 1 and 20 μ -a state of the art review[END_REF][START_REF] Tanaka | Optical Nonlinearity in Photonic Glasses[END_REF]. These make them suitable for potential applications in various fields including phase-change materials [START_REF] Wuttig | Phase-change materials for rewriteable data storage[END_REF], artificial neural networks [START_REF] Tang | Bridging Biological and Artificial Neural Networks with Emerging Neuromorphic Devices: Fundamentals, Progress, and Challenges[END_REF], non-linear photonics [START_REF] Tanaka | Optical Nonlinearity in Photonic Glasses[END_REF], optical sensors [START_REF] Baudet | Development of an evanescent optical integrated sensor in the mid-infrared for detection of pollution in groundwater or seawater[END_REF], mid-IR sources [START_REF] Nguyen | Supercontinuum generation in a chalcogenide all-solid hybrid microstructured optical fiber[END_REF] etc. Indeed, with the addition of a third element such as Sb to the Ge-Se binary system, the glassy material is stabilized by cross-links between more diverse structural entities that create configurational changes in the system, which could promote the broadening of the glass formation domain and influence the physical properties [START_REF] Sharma | Far-infrared investigation of ternary Ge-Se-Sb and quaternary Ge-Se-Sb-Te chalcogenide glasses[END_REF]. In detail, when a small amount of Sb is added to Ge-Se system, a significant decrease in optical loss can be observed [START_REF] Ganjoo | Structural modification of Ge-Se amorphous films with the addition of Sb[END_REF]. Previously, Ge-Sb-Se (herein GSS) based infrared sensor devoted to evanescent wave detection was designed and fabricated [START_REF] Baudet | Development of an evanescent optical integrated sensor in the mid-infrared for detection of pollution in groundwater or seawater[END_REF]. The transducer was made of two superposed amorphous thin layers. The cladding and guiding layers were of Ge 31 Sb 6 Se 62 and Ge 15 Sb 24 Se 61 for the real compositions of the sputtered thin films deposited at 1 × 10 -2 mbar and 20 W and 10 W, respectively and presenting a refractive index contrast of 0.330.02 at 7.7 µm [START_REF] Baudet | Selenide sputtered films development for MIR environmental sensor[END_REF]. Even if these two compositions have allowed the realization of efficient optical waveguides for sensor [START_REF] Baudet | Development of an evanescent optical integrated sensor in the mid-infrared for detection of pollution in groundwater or seawater[END_REF] and nonlinear optical applications [START_REF] Delcourt | Self-phase modulation and four-wave mixing in a chalcogenide ridge waveguide[END_REF], Ge 12.5 Sb 25 Se 62.5 glass targets used for thin film sputtering are less resistant to thermomechanical stresses than Ge 28.1 Sb 6.3 Se 65.6 ones. This induces a high material loss during the various stages of glass target fabrication. Causing higher financial and temporal cost, the authors were interested in finding a pair of compositions that also meet the criteria necessary for the fabrication of a waveguide for integrated optics, while being more robust from thermomechanical point of view. The (GeSe 2 )-(Sb 2 Se 3 ) pseudo-binary system has been previously studied by Olivier et al. [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100-x(Sb2Se3)x glasses[END_REF]. Subsequently, it was found that the particular composition of Ge 19.4 Sb 16.7 Se 63.9 for the glass target , that corresponds to (GeSe 2 ) 70 (Sb 2 Se 3 ) 30 , is suitable for natural water pollution detection sensors in mid-IR region [START_REF] Baillieul | Infrared sensors for aquatic pollutants : synthesis, optimization and qualification[END_REF] as well as for nonlinear photonics devices limiting two photon absorption at 1.55 µm from 0.37 cm • GW -1 for slab waveguide and photosensitivity related to optical nonlinearity effects [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100-x(Sb2Se3)x glasses[END_REF][START_REF] Kuriakose | Demonstration of the spatial self-trapping of a plasmonic wave[END_REF]. Nevertheless, the refractive index contrast of the mid-IR waveguide was lower due to mentioned composition change, with a refractive index contrast of n = 0.210.02 at 7.7 µm between the buffer (Ge 31 Sb 6 Se 63 ) and the guiding (Ge 22 Sb 16 Se 62 ) layers deposited at an Ar pressure of 5 × 10 -3 mbar and RF power at 15 W [START_REF] Baillieul | Infrared sensors for aquatic pollutants : synthesis, optimization and qualification[END_REF]. The linear and nonlinear refractive index contrast would be increased by a substitution of tellurium to selenium. Quaternary Ge-Sb-Se-Te alloys also known as GSST have already attracted the attention of some authors [START_REF] Sharma | Effect of Te on linear and non-linear optical properties of new quaternary Ge-Se-Sb-Te chalcogenide glasses[END_REF][START_REF] Singh | Investigation of dielectric relaxation and a.c. conductivity of third generation multi-component Ge10-xSe60Te30Sbx (0 ≤ x ≤ 6) chalcogenide glasses[END_REF][START_REF] Wang | The phase change memory features high-temperature characteristic based on Ge-Sb-Se-Te alloys[END_REF][START_REF] Zhang | Broadband transparent optical phase change materials for high-performance nonvolatile photonics[END_REF]. First, it is considered to be a promising replacement of Ge 2 Sb 2 Te 5 for highperformance phase-change material applications due to the higher 10-year data retention as reported by Wang et al. [START_REF] Wang | The phase change memory features high-temperature characteristic based on Ge-Sb-Se-Te alloys[END_REF]. Moreover, the substitution of selenium by heavier tellurium atoms causes the decrease of lattice phonon energies broadening the transparency within mid-IR region. Thus, Te-based glasses present extended transmission in the 6-20 μm range and consequently allow the detection of the greenhouse gases absorption peaks especially at longer wavelengths than usual [START_REF] Wang | New far-infrared transmitting Te-based chalcogenide glasses[END_REF][START_REF] Zhang | Purification of Te75Ga10Ge15 glass for far infrared transmitting optics for space application[END_REF]. Besides that, an increasing tellurium concentration causes the rise of the linear refractive index making GSST promising materials for nonlinear optical applications [START_REF] Sharma | Effect of Te on linear and non-linear optical properties of new quaternary Ge-Se-Sb-Te chalcogenide glasses[END_REF]. In this work, the influence of chemical composition and deposition parameters on the optical properties, structure and wettability of the GSST thin films was studied and the potential application of GSST thin film for mid-IR sensing and nonlinear optical (NLO) applications is discussed.

Materials and methods

Glass-ceramics sputtering targets having compositions of Ge 19 Sb 17 Se 64-x Te x (x = 5, 10, 15, 20) were obtained by a conventional melt-quenching technique. The maximum concentration of tellurium in the GSST targets was limited by the feasibility of fabricating sputtering targets of 50 mm diameter and 3.5 mm thickness. The thermal analysis of the bulk targets was performed at 10 °C. min -1 by means of differential scanning calorimetry (DSC, TA Instruments DSC Q20, USA). Bulk material obtained from targets' synthesis was also used for further analysis in order to investigate potential crystallinity of sputtering targets. The X-ray diffraction (XRD) data were recorded with a X'Pert Pro Malvern Panalytical diffractometer (Cu K, 2: 5°-90°, step size: 0.026°, step time: 40s, voltage: 40kV, current: 40mA). Thin films were deposited at a room temperature by rf (13.56 MHz) magnetron sputtering in Ar plasma. The substrates used for the deposition were borosilicate glass (Schott, BK7) and single crystalline silicon <100>. The former was used for spectrometric and X-ray diffraction (XRD) measurements, the later for scanning electron microscopy with energy-dispersive X-ray analyser (SEM-EDS), atomic force microscopy (AFM) and the contact angle measurements. The electrical power applied on targets was 10 and 15 W. Two different Ar pressures were maintained in the deposition chamber, specifically 5 × 10 -3 and 1 × 10 -2 mbar, while the Ar flow was kept constant (i.e. 75 sccm). In order to be able to characterize the surface of the GSST thin films by AFM or contact angle techniques, they were kept in a desiccator in the dark because whatever the composition of the layer, the impact of visible light and UV as well as the moisture on the thin layers is documented and has to be minimized [START_REF] Charpentier | Aging process of photosensitive chalcogenide films deposited by electron beam deposition[END_REF]. In case of both, the contact angle measurements as well as AFM, experiments are carried out with a minimum time delay after the deposition of thin films. Routinely, spectroscopic ellipsometry and AFM are performed with a highest priority compared to other experiments. Topography of fabricated films was measured by amplitude modulated AFM (Solver Next, NT-MDT, Russia). Furthermore, chemical composition of thin films was obtained from EDS analysis using joint SEM-EDS (JEOL IT 300 LA EDS, JEOL Ltd., Japan). EDS (ZAF standardless method) remains a relatively imprecise measurement as evidenced by its expected uncertainty (±1 at. %) especially for thin films where the contribution of the substrate can become significant. However, in similar acquisition conditions and films of the same thickness, the evolutions from one layer to another are generally well provided. The local structure of thin films was studied by μ-Raman scattering spectroscopy (LabRam HR800, Horiba Jobin-Yvon, NJ, USA) coupled with a ×100 microscope (Olympus, Japan) with the excitation wavelength of 785 nm. Thermal population effect at low wavenumbers was minimized by means of reduction of Raman intensity according Shuker and Gammon [START_REF] Shuker | Raman-Scattering Selection-Rule Breaking and the Density of States in Amorphous Materials[END_REF]. Two variable angle spectroscopic ellipsometers (J.A. Woollam Co., Inc., Lincoln, NE, USA) were employed in order to estimate the optical bandgap energy and refractive index dispersion of the fabricated films: first working in the UV-Vis-NIR range (300-2300 nm), the other in mid-IR (~300-6,000 cm -1 ). The fundamental absorption edge was modelled by Cody-Lorentz oscillator model (described elsewhere [START_REF] Ferlauto | Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics[END_REF]) to fit the ellipsometry data in the UV-Vis-NIR region. Within mid-IR range, the Sellmeier model was applied setting extinction coefficient to zero. The thickness of the films on borosilicate glass (Schott, BK7) was about 800 nm (±1 %) as determined by spectroscopic ellipsometry. Surface energy σ of thin films was determined following Owens-Wendt theory. Contact angles were obtained by computer controlled KSV CAM 100 USB video camera (KSV Instruments Ltd., Finland) at resolution of 640 × 480 px from curve fitting based on Young-Laplace equation of the sessile drop shape. Sessile drops of five standard liquids were recorded for 30 seconds immediately after the placing them onto the thin films surface. Equilibrium contact angle θ eq was considered to be established after 30 seconds. All the contact angle measurements were carried out at 20 °C and ~30 % of relative humidity. Thin films used for the contact angle measurements were deposited at the electrical power of 10 W and the Ar pressure of 5 × 10 -3 mbar onto 2-inch silicon wafers. The thickness of these films specifically deposited for contact angle measurements was in the range of ~600-700 nm. Contact angle was measured on the virgin thin films stored for a short time in the light isolated desiccator in order to ensure good reproducibility of the contact angle measurements.

Results and Discussion

Glass target characterisation

The average chemical composition of the targets is in good agreement with theoretical compositions. The DSC measurements performed with GSST glass targets have shown that the glass transition temperature T g was decreasing with an increasing tellurium content from the initial value of 265 °C for tellurium-free composition of Ge 19 Sb 17 Se 64 . Obtained values of T g for Ge 19 Sb 17 Se 64-x Te x with x = 5, 10 and 15 were 247, 232 and 222 °C respectively (±2 °C). The T g variation of about 40 °C with a substitution is observed for almost one quarter of selenium by tellurium. These results agree with the gradual introduction of tellurium atoms in substitution for selenium atoms, the binding energies of the vitreous network decreasing compared to those formed with selenium. This substitution could also have an effect on the crosslinking of the network, but probably to a lesser extent. The change in the difference between the T g and the crystallization onset temperature T x , one of the possible parameters showing the thermal stability of amorphous/glassy material, is also noticeable varying from T x -T g  150°C to T x -T g  90 °C with x = 0 to 15. For the last target with the highest proportion of tellurium, the distinction between its glass transition temperature and crystallization onset temperature is no longer clearly discernible, probably resulting in the formation of a glass-ceramic target. It should be noted that these large diameter bulk targets have very altered conditions of rapid cooling compared to the synthesis of glass of smaller dimensions, which exacerbates the possible formation of crystalline phases. XRD analysis showed that obtained targets were vitreous with one exception. The diffraction peaks in the XRD patterns corresponding to GeSe 2 and Sb 2 Se x Te y (with x + y = 3, x = 1 or 2 and y = 1 or 2) phases appeared for target with nominal composition of Ge 19 Sb 17 Se 44 Te 20 as shown in Fig. 1. Since the average composition of the partially crystalline target remains unchanged from the theoretical composition, the nature of the Ge 19 Sb 17 Se 44 Te 20 glass-ceramic target should marginally influence the composition of the final thin film and its deposition rate, which nevertheless needs to be verified. The very fast quenching speeds during the sputtering process of GSST films on a silicon substrate at room temperature should make it possible to obtain amorphous films even if one of the targets is partially crystallized. The insert in Fig. 1 showing the XRD patterns of four GSST thin films deposited from the individual GSST targets under the same deposition conditions (15 W, Ar pressure of 5×10 -3 mbar) fully confirms assumption described above.

Chemical composition

All the depositions together with indicated electrical power, Ar working pressure and composition determined by EDS are summarized in Table 1. EDS results indicate that some minor differences in composition may take part when the Ar working pressure increases from 5 × 10 -3 mbar to 1 × 10 -2 mbar. With respect to the target composition, it can be seen that the composition of the deposited films tends in general to be close to the nominal values of the target while experiencing an increase of Ge from 1 to 3%, relatively stable values ( +1-2%) for antimony and tellurium, and a rather more significant decrease for selenium of about 3-4%. Of course, these general trends will evolve more or less marginally with the variation of the composition and deposition parameters, which are for the latter relatively little adjustable in this work. Looking in more detail at the composition change of the targets, very similar trends can be noted for the first three target compositions with an average value of 21% for Ge and 18% Sb with related deviation of 1.1% and 0.6%, respectively and for which globally the Se deficit (of about -4%) and the small Te excess (+1-2%) is respected from one target to another. A different behaviour is observed for the fourth, most Te-rich target, where a higher proportion of Sb (excess of +3%) compared to the target and lower concentration of Ge (19% on average, 1% deviation) compared with the other Se-richest films is consistently observed. This may be related to a somewhat enriched antimony concentration of the target itself for this (more difficult to fabricate) composition, but it would still appear that the increasing substitution of Te along the target compositions leads to a gradual increase in the proportion of antimony in the sputtered thin film.

Table 1. Deposition parameters: sputtering targets nominal chemical composition (at. %), electrical power applied on the cathode (W) and Ar pressure in the deposition chamber (× 10 -3 mbar). Properties of the deposited thin films: deposition rate (nm.min -1 ) chemical composition determined by EDS (±1 at. %), linear refractive index n 0 at 1.55 and at 7. The increase of Ar pressure slightly increases the amount of chalcogen elements, and thus the amount of heteropolar bonds in the films obtained when the Ar pressure increases from 5 × 10 -3 to 1 × 10 -2 mbar whatever the composition of the target and the applied power. An increased electrical power from 10 to 15 W at the same Ar pressure seems to increase the amount of heteropolar to a smaller extent [START_REF] Baudet | Structural analysis of RF sputtered Ge-Sb-Se thin films by Raman and X-ray photoelectron spectroscopies[END_REF]. It has been shown in the literature for two Ge-Sb-Se targets with different ratio of Ge/Sb (R Ge/Sb = 0.5 and 4.5) that increasing the Ar pressure increases the content of selenium and decrease the germanium concentration, while the electrical power has a minor effect but can affect the antimony concentration in the case of a high Ge/Sb ratio [START_REF] Baudet | Structural analysis of RF sputtered Ge-Sb-Se thin films by Raman and X-ray photoelectron spectroscopies[END_REF][START_REF] Baudet | Experimental design approach for deposition optimization of RF sputtered chalcogenide thin films devoted to environmental optical sensors[END_REF]. For the GSST targets (R Ge/Sb = 1.2) with substitution of Se by Te, increasing Ar pressure or electrical power seems to increase the number of tellurium atoms (by 1% up to 2%) while the selenium content remains almost the same, perhaps slightly increased (+1%) but only with the effect of pressure and for the first three most selenium rich targets. The concentration of germanium and antimony has an expected behaviour considering the Ge/Sb ratio of these targets, [Sb] is indeed almost unaffected by Ar pressure and electric power while [Ge] tends to decrease slightly by 1-2% with Ar pressure and with electric power to a lesser extent more surprisingly. Therefore, for a given [Se/Te] ratio of the target, small compositional variation due to the change in pressure and electrical power could be responsible for minor changes in structure or optical properties. The work of Baudet et al. [START_REF] Baudet | Experimental design approach for deposition optimization of RF sputtered chalcogenide thin films devoted to environmental optical sensors[END_REF] studying the effect of decisive parameters of the sputtering process on the various properties of thin films in ternary Ge-Sb-Se system devoted to mid-IR sensor development, reports a significant variation of the roughness from 0.2 to 1 nm with a pressure varying from 5 × 10 -3 to 1 × 10 -2 mbar for the target composition richest in germanium considering an electric power lower than 17 W and a film thickness lower than 1 µm. The composition richer in antimony is more stable in terms of roughness with Ar pressure variation. The addition of a few percent of tellurium to Ge 19 Sb 17 Se 64 composition seems to make the roughness of the films relatively insensitive to the change of Ar pressure and to a lesser extent to the change in electrical power. To conclude, the roughness is essentially governed by the percentage of tellurium and could have a detrimental effect on the final optical losses of the photonics system used as a transducer for the IR medium optical sensor. It is therefore important, when selecting the ideal composition of the IR sensor's guiding layer, to make a compromise with the increasing introduction of tellurium between the gradual increase in roughness and the expected increase in both the increase in transmission in the IR domain and the refractive index in order to increase the index contrast and, consequently, the confinement of IR light.

Optical properties

Linear refractive index and optical band-gap energy. The effect of the [Te/Se] target composition ratio and deposition conditions on the optical properties, specifically linear refractive index (n 0 ) and optical bandgap energy (𝐸 𝑔 𝐶𝐿 ), were studied by spectroscopic ellipsometry. These results are summarized in the Table 1. Moreover, the dispersion curves are shown in Figure 3A-D. As seen, the substitution of selenium by heavier tellurium atoms, having higher polarizability, increases the linear refractive index.

At the same time, the optical bandgap energy decreases as expected in accordance with semi-empirical Moss rule [START_REF] Moss | A Relationship between the Refractive Index and the Infra-Red Threshold of Sensitivity for Photoconductors[END_REF]. For the clarity, the dependencies of linear refractive index at 1.55 and 7.7 μm, as well as the optical bandgap energy on the tellurium content are depicted in Figure 4. Values of refractive index at 7.7 μm allow the evaluation of potential suitability of fabricated thin films for mid-infrared sensor application. Baudet et al. successfully applied chalcogenide thin films with composition Ge 12.5 Sb 25 Se 62.5 as a waveguide guiding layer for the evanescent wave sensor for the detection of pollutants in water. The refractive index n 0 at 7.7 μm of these sputtered thin films is 2.77 (±0.01) and the refractive index contrast with the cladding layer is about  = 0.33 at 7.7 μm for an Ar pressure of 1 × 10 -1 mbar during the sputtered films deposition [START_REF] Sharma | Effect of Te on linear and non-linear optical properties of new quaternary Ge-Se-Sb-Te chalcogenide glasses[END_REF]. Among all used targets, the one with the composition Ge 19 Sb 17 Se 59 Te 5 seems to be suitable as a potential replacement of Te-free GSS target for the guiding layer. Thin film with composition Ge 21 Sb 17 Se 55 Te 7 , having values of n 0 at 7.7 μm and bandgap energy 𝐸 𝑔 𝐶𝐿 of 2.81 (±0.01) and 1.37 eV (±0.02) can potentially be used as a guiding layer for the evanescent wave sensors mentioned above [START_REF] Baudet | Development of an evanescent optical integrated sensor in the mid-infrared for detection of pollution in groundwater or seawater[END_REF]. Indeed, the refractive index contrast at 7.7 μm between this thin film and a cladding layer with composition of Ge 28.1 Sb 6.3 Se 65.6 used in such sensors would be 0.32 for the deposition parameters considered in this study (Table 1). Considering the deposition parameters, it can be noted that the linear refractive index decreases with an increasing argon pressure. Similar changes were observed by Baudet et al. in ternary Ge-Sb-Se thin films justified by changes in morphology and/or porosity as well as the roughness of sputtered films related to important variation of pressure between 5 × 10 -3 to 5 × 10 -2 mbar [START_REF] Baudet | Experimental design approach for deposition optimization of RF sputtered chalcogenide thin films devoted to environmental optical sensors[END_REF]. The effect of the later was not proven in quaternary Ge-Sb-Se-Te as the AFM topography shows only small divergence when argon pressure was increased between 5 × 10 -3 to 1 × 10 -2 mbar only. The decrease of linear refractive index and the increase of 𝐸 𝑔 𝐶𝐿 with an increased pressure of argon in the variation range used for this study may be justified by other contributions and would be more related to the changes in composition of the GSST thin film reflected by R variation. As already mentioned, the increase in Ar pressure results in higher proportions of chalcogen atoms (Se and Te) compared to metalloids (Ge, Sb) favouring the heteropolar bond formation. This leads to changes in n 0 and 𝐸 𝑔 𝐶𝐿 as depicted in Figure 4. The electrical power change from 10 to 15 W seem to have less significant effect on optical properties. As follows from the above mentioned, the effect of the minor changes in compositions seem to have rather low effect on the fundamental absorption edge and thus reflects only small changes in the localized/tail states. ). According to classical modelling used to predict the nonlinear coefficients, like Sheik-Bahae method [START_REF] Sheik-Bahae | Semiconductors and Semimetals[END_REF], the bandgap energy of amorphous chalcogenide thin films must be at least higher than ~1.6 eV to obtain a negligible TPA at the 1.55 µm telecommunication wavelength. In selenide or seleno-telluride thin films, one can possibly consider for smaller E g to be able to keep an interesting FOM if the n 2 growth is faster than the TPA increase. The calculated values of nonlinear refractive index at telecommunication wavelength of 1.55 μm and at 7.7 μm mid-IR wavelength using Sheik-Bahae's formalism are plotted in Figure 4A5A. Qualitatively, values of n 2 at 1.55 μm decrease with increasing value of linear refractive index n 0 while the opposite is true for n 2 at 7.7 μm. This is due to the shape of the dispersion function for n 2 within the used model, which becomes negative for approximately ℏ𝜔 ≥ 0.7𝐸 𝑔 when photon energy approaches resonance energy [START_REF] Dory | Ge-Sb-S-Se-Te amorphous chalcogenide thin films towards onchip nonlinear photonic devices[END_REF][START_REF] Tanaka | Nonlinear optics in glasses: How can we analyze?[END_REF] Overall, calculated values of nonlinear refractive index of deposited GSST films, ranging from 16 to 28 m 2 • W -1 (excluding negative values), are higher than reported calculated values of n 2 for GSS thin films which are in the range from 5 to 23 m 2 • W -1 but the two photon absorption (TPA) coefficient () is also expected to be higher [START_REF] Dory | Ge-Sb-S-Se-Te amorphous chalcogenide thin films towards onchip nonlinear photonic devices[END_REF]. Values of  at 1.55 μm for GSST are within the range of ~2-6 × 10 It should be mentioned that the calculated values of n 2 and β in the present work are only order of magnitude estimations as the Sheik-Bahae's formalism is intended for the determination of Kerr coefficient and the two-photon absorption coefficient of direct-gap semiconductors. It should be kept in mind that amorphous chalcogenides are considered as non-direct gap materials and in this model, the tail and localised states related to amorphous materials are not taken into account, which is also the case for the Dinu model [START_REF] Dinu | Dispersion of phonon-assisted nonresonant third-order nonlinearities[END_REF]. Moreover, the presence of uncertainties in the physical parameters used in the Sheik-Bahae's formalism enlarge the error in the estimation. It should be noted that calculated values of n 2 obtained in this work are generally somewhat lower relative to those reported by Dory et al. [START_REF] Dory | Ge-Sb-S-Se-Te amorphous chalcogenide thin films towards onchip nonlinear photonic devices[END_REF]. In spite of this, used formalism allows to have a fairly good predictive vision on the n 2 and  of amorphous chalcogenides by considering their refractive index and optical band-gap indicating trends that were verified experimentally for Ge-Sb-Se bulk glasses or thin films for which an error of experimental measurement of the order of 10-20% is expected [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100-x(Sb2Se3)x glasses[END_REF][START_REF] Halenkovič | Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films[END_REF][START_REF] Lenz | Large Kerr effect in bulk Se-based chalcogenide glasses[END_REF][START_REF] Wang | Systematic zscan measurements of third order nonlinearity of chalcogenide glasses[END_REF] Se 63.9 target using beam self-trapping analysis [START_REF] Kuriakose | Demonstration of the spatial self-trapping of a plasmonic wave[END_REF]. Besides that, values of nonlinear refractive index calculated for GSST films are higher than those obtained for amorphous Ge-Sb-Se thin films with experimental values reaching the maximum value of n 2 at 1.55 μm about 21±3.0 × 10 -18 m 2 • W -1 for amorphous Ge 3 Sb 35 Se 62 [START_REF] Halenkovič | Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films[END_REF]. The highest nonlinearity at 7.7 μm by means of n 2 was found for thin film Ge 19 Sb 20 Se 40 Te 21 (𝐸 𝑔 𝐶𝐿 = 1.05 eV, n 0 = 3.18). This is the consequence the strong TPA absorption at 1.55 µm in these films as depicted in Figure 5B. The trade-off between optical bandgap energy and the refractive index plays the crucial role when evaluating the nonlinear optical properties of materials. Lower optical bandgap energies of GSST relative to GSS thin films would indicate the possible limitation of these materials in terms of application in nonlinear optics at 1.55 μm due to the high multi-photon absorption such as TPA absorption. This results in relatively low values of figure of merit 1.55 µm as shown in Figure 5B.

Wettability

Surface energy σ of thin films plays an important role when considering the potential application of these materials for chemical sensors. It is important to have an idea of the intrinsic surface energy before any functionalization of the chalcogenide film surface and the possible influence related to the tellurium concentration. Obtained contact angles for five standard liquids are summarized in Table 3. These were used for the calculation of values of dispersive 𝛾 𝑠 𝑑 and polar 𝛾 𝑠 𝑝 components together with the total surface energy σ of the GSST thin films using Owens-Wendt theory (Figure 6) -𝛾 𝑠 𝑑 is obtained as a square of an intercept value and 𝛾 𝑠 𝑝 square of a slope value of a linear fit respectively. As seen from Table 3, the values of surface energy for all the four compositions lie between ~37-39 mJ • m -2 . These values are lower than those obtained by Baudet et al. for tellurium-free GSS thin films with the composition of Ge 28.1 Sb 6.3 Se 65.6 and Ge 12.5 Sb 25 Se 62.5 respectively. They obtained the values of about 46 mJ • m -2 for both compositions with negligible polar components (~1 mN • m -1 ) [START_REF] Baudet | Selenide sputtered films development for MIR environmental sensor[END_REF]. Moreover, Lucas et al. studied the contact angles of different liquids on polished bulk discs of Te 20 As 30 Se 50 glass for which the surface roughness is expected to be higher than chalcogenide thin films [START_REF] Lucas | Infrared biosensors using hydrophobic chalcogenide fibers sensitized with live cells[END_REF]. Quantitatively, they obtained the contact angles of 73±3° and 62±3° for water and glycerol respectively. The surface roughness rise usually increases the wettability as observed for these bulk chalcogenide glasses. In case of sputtered GSST films, the wettability just slightly increased when the tellurium content increases from 5 to 15 at. % for most of the used liquids. However, for the thin film with the tellurium content of 20 at. %. the contact angle is enlarged for glycerol and diiodomethane while for the others it is decreased. This causes the decrease of the intercept of the linear fit justified by the important decrease of the dispersive component of surface energy of this film as depicted in Figure 6. In the same time, the small contact angle measured for the distilled water increases the slope of the linear fit enlarging the polar component of the surface energy of this film. The Ge 19 Sb 17 Se 44 Te 20 thin film seems to present an increased polarity of the surface, which might be explained by higher surface oxidation related to presence of tellurium in higher content or influence of surface roughness.

Surface oxidation can be a problem encountered for bulk glass, preform, optical fibers and thin films or waveguides based on chalcogenides. In order to prevent them from oxidation, many strategies are applied such as surface purification, distillation, chemical and mechanical polishing, cladding, coating, surface passivation, etc. Protection against the influence of the atmosphere by a chemical barrier layer can be performed during the manufacture of chalcogenide waveguides in order to limit optical losses, in particular by the deposition of Al 2 O 3 , Si 3 N 4 or SiO 2 on the surface if needed for photonics applications. However, it is first necessary to know well the chalcogenide material by taking care to characterize it before proceeding to the fabrication of the mentioned heterostructure to improve its performances. The nature of the specific surface of the thin films plays a preponderant role in this possible oxidation whatever the physical vapor deposition technique used [START_REF] Charpentier | Aging process of photosensitive chalcogenide films deposited by electron beam deposition[END_REF][START_REF] Baudet | Experimental design approach for deposition optimization of RF sputtered chalcogenide thin films devoted to environmental optical sensors[END_REF][START_REF] Verger | RF sputtered amorphous chalcogenide thin films for surface enhanced infrared absorption spectroscopy[END_REF].

In case of spectroscopic ellipsometry, the three-layer model is typically used consisting of substrate, film layer itself, and the surface layer/roughness. The thicknesses of surface layer were fitted as ~1.6, ~1.7, ~1.6 and ~2.2 nm. Inherently, surface layer/roughness evaluated by ellipsometry is larger than surface roughness obtained from AFM. The thickness values of the surface layer/roughness shown above appear to be comparable to those of the sulphide/selenide thin films, which could lead to the conclusion that the seleno-telluride thin films do not oxidize more than the sulphides/selenides. Moreover, at such small thickness this oxide layer should not contribute significantly to optical losses. Nevertheless, further studies to discriminate between these two factors, i.e. surface oxidation and surface roughness, and to avoid any correlation effect should be conducted in future.

Local structure

The influence of the increasing tellurium content on the structure of thin films deposited at 10 W and Ar working pressure of 5 × 10 -3 mbar is represented by the Raman scattering spectra shown in Figure 7-A. For comparison, the spectra of sputtered thin film from Te-free Ge 19 Sb 17 Se 64 target deposited at 15 W at the same Ar working pressure is also shown.

For films sputtered from Ge 19 Sb 17 Se 59 Te 5 target, the Raman spectra are prima facie dominated by two contributions, the main peak at ~160 cm -1 with an adjacent asymmetric shoulder with the maxima at ~197 cm -1 . The two peaks shift gradually towards the lower wavenumbers with an increasing tellurium content in GSST thin films. As a result, the former is peaking at ~153 cm -1 , the later at ~194 cm -1 in Ge 19 Sb 17 Se 44 Te 20 . The adjacent asymmetric shoulder (from 197 to 194 cm -1 ) seems to find its origin in the symmetric stretching vibrations of Ge-Se bonds, typically at ~198 and ~216 cm -1 , in corner-sharing and edge-sharing [GeSe 4 ] tetrahedra, respectively.

In the Raman spectra of Te-free Ge 19 Sb 17 Se 64 thin film, this peak (~200 cm -1 ) and its shoulder (~214 cm -1 ) are better observed. For this pure selenide film, they are mainly accompanied by an additional shoulder (at ~175 cm -1 ) related to Ge-Ge stretching modes in [GeSe 4-y Ge y ] distorted tetrahedra with y = 1,2,3 and ethane-like Se 3 Ge-GeSe 3 . They are also complemented by an important contribution (at ~190 cm -1 ) of Sb-Se stretching mode of [SbSe 3 ] pyramids and probably vibration modes related to Ge-Sb bonds around 165 cm -1 [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100-x(Sb2Se3)x glasses[END_REF][START_REF] Baudet | Structural analysis of RF sputtered Ge-Sb-Se thin films by Raman and X-ray photoelectron spectroscopies[END_REF]. and finally by a small broad band observed between 250-315 cm -1 with several contributions mainly from (Se-Se) stretching mode from 245 cm -1 to 265 cm -1 coming from -(Se-Se)nlong chains to -(Se-Se)dimers and also  as [GeSe 4 ] [37] (Figure 7-A).

In the case of the GSST thin films, these different bands and shoulders related to [GeSe 4 ], Ge-Ge and Se-Se vibration modes seem to disappear gradually with the [Te] increase to the profit of the main band at lower frequency. In the structure of multicomponent glasses and thin films which contain more than one chalcogen element, mixed entities such as [GeSe 4-x Te x ] and [SbSe 3-x Te x ] can be at the origin of the main broad peak observed at 160 cm -1 as referred by Abdellaoui et al. in quinary Ga 5 Ge 20 Sb 10 Se 65-x Te x (x=10-37.5 at. %) bulk glasses [START_REF] Abdellaoui | Tb3+ doped Ga5Ge20Sb10Se65-xTex (x = 0-37.5) chalcogenide glasses and fibers for MWIR and LWIR emissions[END_REF] and by Gonçalves et al. for ternary Ge 20 Te 80- x Se x bulk glass [START_REF] Gonçalves | Study of the Ge20Te80-xSex glassy structures by combining solid state NMR, vibrational spectroscopies and DFT modelling[END_REF]. The broad band can have also non negligible contributions related to the presence of Ge(Sb)-Ge(Sb) bonds because the GSST films present a deficit in selenium as observed for Te-free Ge 19 Sb 17 Se 64 with vibration modes localised from 150 (mainly related to vibration modes of (Sb-Sb) bond in SbSe 2 -SbSe 2 entities) to 175 cm -1 [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100-x(Sb2Se3)x glasses[END_REF][START_REF] Baudet | Structural analysis of RF sputtered Ge-Sb-Se thin films by Raman and X-ray photoelectron spectroscopies[END_REF][START_REF] Petit | Effect of the substitution of S for Se on the structure of the glasses in the system Ge0.23Sb0.07S0.70-xSex[END_REF].

Even if the GSST films present a deficit of selenium, the presence of Se-Te and even Te-Te can be envisaged, as Se-Se bonds are present in small proportion in Ge 19 Sb 17 Se 64 thin film. It can be noted that vibration modes of (Te-Te) bond from -(Te-Te) nchains to dimer possess vibrations modes in the range from 145 to 165 cm -1 [START_REF] Sen | Physical properties of GexAs2xTe100-3x glasses and Raman spectroscopic analysis of their short-range structure[END_REF] and might slightly contribute to the main broad bands centred at 160 cm -1 while vibration modes of (Te-Se) bonds inside mixed chains or mixed dimers could present vibration modes in the range of 208-266 cm -1 [START_REF] Gonçalves | Study of the Ge20Te80-xSex glassy structures by combining solid state NMR, vibrational spectroscopies and DFT modelling[END_REF][START_REF] Mendoza-Galvan | Structural, optical and electrical characterization of amorphous SeTe thin film alloys[END_REF]. It was proposed in the case of Ge 20 Te 80-x Se x chalcogen rich glass that the substitution of Te to Se in small proportion will lead to the introduction of Te inside the -(Se-Se) nchains before to bond [GeSe 4 ] tetrahedra forming finally mixed entities when the tellurium increases. It could be also the case in the GSST films but it is important to keep in mind that the chemical composition of the Ge 19 Sb 17 Se 64-x Te x targets is stoichiometric and the GSST films present a deficit of selenium compare to the targets. Apart from bonding defects generated by a statistical disorder inherent to amorphous chalcogenides, the selenium deficit limits the chalcogen-chalcogen bonds occurrence while it substantially should increase the proportion of Ge(Sb)-Ge(Sb) bonds. All these mixed entities and homopolar bond modifications will have an important impact on the electronic band structure and localized/tail states of amorphous GSST thin films and on the hyperpolarizability of the entities and bonds present in the amorphous lattice substantially modifying their bandgap E g and nonlinear properties. Moreover, the Raman spectra of GSST films are strongly affected when tellurium is introduced even in small proportion, as it was also observed in stoichiometric Ga 5 Ge 20 Sb 10 Se 65-x Te x glasses much more rapidly than in the case of a system rich in chalcogen [START_REF] Gonçalves | Study of the Ge20Te80-xSex glassy structures by combining solid state NMR, vibrational spectroscopies and DFT modelling[END_REF][START_REF] Mendoza-Galvan | Structural, optical and electrical characterization of amorphous SeTe thin film alloys[END_REF]. Thus, the shift of the main broad band from 160 cm -1 to 153 cm -1 can be mainly explained by the increase of the substitution of Se by Te in the mixed entities as [GeTe 4 ] present a stretching vibration mode centered at 130 cm -1 [START_REF] Gonçalves | Study of the Ge20Te80-xSex glassy structures by combining solid state NMR, vibrational spectroscopies and DFT modelling[END_REF][START_REF] Sen | Physical properties of GexAs2xTe100-3x glasses and Raman spectroscopic analysis of their short-range structure[END_REF][START_REF] Voleská | Structure, electronic, and vibrational properties of glassy Ga11Ge11Te78: Experimentally constrained density functional study[END_REF] and [SbTe 3 ] pyramidal units or defective octahedral present Raman active band at 145 cm -1 [START_REF] Sosso | Raman spectra of cubic and amorphous Ge2Sb2Te5 from first principles[END_REF].

The changes of deposition conditions by means of the increased electrical power and/or argon pressure do not seem to affect the local structure significantly and less and less with tellurium content increase. The pressure and electrical power increase have an effect on  s [GeSe 4 ] intensity with a slight increase and on the position of the main band at 160 cm -1 slightly shifted to higher wavenumber.

Conclusions

GSST thin films were fabricated by rf magnetron sputtering from glass-ceramics targets with nominal composition of Ge 19.4 Sb 16.7 Se 63.9-x Te x (x=5, [START_REF] Baudet | Selenide sputtered films development for MIR environmental sensor[END_REF][START_REF] Sharma | Effect of Te on linear and non-linear optical properties of new quaternary Ge-Se-Sb-Te chalcogenide glasses[END_REF][START_REF] Zhang | Purification of Te75Ga10Ge15 glass for far infrared transmitting optics for space application[END_REF]. The structure of GSST amorphous films according to Raman analysis seems to be built mainly by selenide and mixed entities in variable proportion according to the tellurium concentration such as [GeSe 4-x Te x ] and [SbSe 3-x Te x ] (x = 0, 1, 2) and in all likelihood homopolar Ge(Sb)-Ge(Sb) bonds to compensate for the chalcogen deficiency. The contact angle measurements have shown values of 68-71 ° for water of the GSST thin films resulting in values of surface energy of about ~36-39 mJ • m -2 . The presence of 20 at. % of tellurium increases the polar component of the surface energy of the thin film and decrease the contact angle to 54 ° which might be related to the oxidation of the surface in link with the composition and also roughness increase. Analysis of ellipsometric data revealed the increase of linear refractive index with increasing tellurium concentration while the optical bandgap energy decreases. Moreover, the former seems to decrease with the increasing working pressure of argon from 5 × 10 -3 to 1 × 10 -2 mbar, which is mainly related to the composition changes. The electrical power change from 10 to 15 W seem to have insignificant effect. Among all used targets, the one with the composition Ge 19 Sb 17 Se 59 Te 5 seems to be suitable as a potential replacement of Te-free GSS target for the guiding layer. Indeed, the refractive index contrast at 7.7 μm between a GSST thin film and a cladding layer with composition of Ge 28.1 Sb 6.3 Se 65.6 used in such sensors would be at least equal to 0.32. By adjusting the Te/Se composition, it is possible to control the index contrast between the guiding layer and the buffer layer in order to create an efficient IR sensor in terms of evanescent fields and with compact dimensions. Furthermore, it was shown that thin film with composition Ge 19 Sb 17 Se 56 Te 8 , having nonlinear refractive index n 2 ~28 × 10 -18 m 2 • W -1 , shows the highest FOM at 1.55 µm among all fabricated films. Calculated values of n 2 for GSST films are within the rage from 16 to 28 × 10 -18 m 2 • W -1 (excluding negative values). The potential limitation of these materials when the concentration in tellurium increase may lie in low values of optical bandgap energies resulting in high two-photon absorption at telecommunication wavelength (i.e. 1.55 μm) which will not be the case for mid-IR. To conclude, wide IR transparency in combination with high refractive indices make these materials suitable for potential applications in the field of mid-infrared devices such as optical switches and sensors.

Figure 1 .

 1 Figure 1. XRD patterns of bulk GSST target material with indicated powder diffraction patterns from crystallographic databases of Sb 2 Se 2 Te (a, COD ID 1008844), Sb 2 SeTe 2 (b, COD ID 9007591), and GeSe 2 (c, ICDD 00-042-1104); Insert with XRD patterns of four thin films deposited from individual targets (four compositions) at same conditions -15 W, Ar pressure of 5 × 10 -3 mbar.

3. 2

 2 Topography AFM scans of 1 × 1 μm 2 area of sputtered films having a thickness of 800 nm, represented by figure 2A-D, have shown that the root mean square (RMS) roughness (Sq) of thin films surface increases with an increased tellurium content. Quantitatively, values of Sq increase from ~0.3±0.1 for Ge 19 Sb 17 Se 59 Te 5 (fig. 2A) up to ~0.9±0.1nm for Ge 19 Sb 17 Se 44 Te 20 (fig.2D). This is clearly caused by the presence of large grains for films with tellurium content above 15 at. %.

Figure 2 .

 2 Figure 2. AFM scans of 1 × 1 μm 2 area of sputtered GSST thin films (with thickness of 800 nm) with indicated RMS roughness (Sq, ±0.1 nm), values on z-axis are normalized for each target nominal composition; A -Ge 19 Sb 17 Se 59 Te 5.0 , B -Ge 19 Sb 17 Se 54 Te 10 , C -Ge 19 Sb 17 Se 49 Te 15 , D -Ge 19 Sb 17 Se 44 Te 20 .

Figure 3 .

 3 Figure 3. Spectral dependencies of linear refractive indices of sputtered GSST thin films with indicated nominal compositions of the GSST target and deposition conditions.

Figure 4 . 2 

 42 Figure 4. A -Dependence of linear refractive index at 1.55 μm and the optical bandgap energy 𝐸 𝑔 𝐶𝐿

Figure 5 .

 5 Figure 5. (A) Dependence of nonlinear refractive index n 2 on linear refractive index n 0 at 1.55 μm. (B) Dependence of TPA coefficient β and figure of merit FOM on linear refractive index n 0 at 1.55 μm.

𝐅𝐢𝐠𝐮𝐫𝐞 6 .

 6 Owens-Wendt plot for GSST thin films on silicon wafer with indicated nominal composition (10 W, 0.5 Pa); 𝐴 1 =𝛾 L /2√𝛾 𝐿 𝑑 and 𝐴 2 = √ 𝛾 𝐿 𝑝 𝛾 𝐿 𝑑 , where 𝛾 𝐿 𝑑 and 𝛾 𝐿 𝑝 stands for the dispersive and polar component of the surface tension of the standard liquid 𝛾 𝐿 , respectively.

Figure 7 .

 7 Figure 7. Raman spectra of sputtered GSST thin films : Aindicated theoretical target compositions (* from [13]), and deposition conditions : B -Target Ge 19 Sb 17 Se 54 Te 10 , C -Target Ge 19 Sb 17 Se 54 Te 10

  7 μm (both ±0.01), and optical bandgap energy in eV determined by Cody-Lorentz oscillator model (𝐸 𝑔 𝐶𝐿 , ±0.02 eV) from spectroscopic ellipsometry data analysis.

						Linear
	Theoretical target composition (at. %)	Power (W)	Ar pressure (× 10 -3 mbar)	Deposition rate (nm • min -1 )	Thin film composition (at. %)	refractive index n 0 at at 1.55 7.70	𝑪𝑳 𝑬 𝒈 (eV)
						μm	μm
		10	5	9.9	Ge 22 Sb 17 Se 55 Te 6	2.91	2.84	1.33
	Ge 19 Sb 17 Se 59 Te 5	10 15	10 5	10.4 19.8	Ge 20 Sb 17 Se 56 Te 7 Ge 21 Sb 17 Se 55 Te 7	2.84 2.87	2.78 2.81	1.37 1.37
		15	10	20.0	Ge 19 Sb 17 Se 56 Te 8	2.84	2.78	1.41
		10	5	10.2	Ge 22 Sb 18 Se 50 Te 10	2.97	2.89	1.27
	Ge 19 Sb 17 Se 54 Te 10	10 15	10 5	9.9 21.0	Ge 20 Sb 17 Se 51 Te 12 Ge 21 Sb 18 Se 50 Te 11	2.91 2.96	2.83 2.88	1.33 1.25
		15	10	20.1	Ge 20 Sb 18 Se 51 Te 11	2.91	2.84	1.27
		10	5	10.0	Ge 22 Sb 18 Se 45 Te 15	3.07	2.98	1.11
	Ge 19 Sb 17 Se 49 Te 15	10 15	10 5	10.1 20.4	Ge 20 Sb 18 Se 46 Te 16 Ge 20 Sb 19 Se 45 Te 16	3.00 3.05	2.92 2.96	1.18 1.20
		15	10	20.7	Ge 19 Sb 18 Se 46 Te 17	3.02	2.93	1.21
		10	5	16.0	Ge 20 Sb 20 Se 40 Te 20	3.20	3.09	1.05
	Ge 19 Sb 17 Se 44 Te 20	10 15	10 5	15.6 28.3	Ge 18 Sb 20 Se 40 Te 22 Ge 19 Sb 20 Se 40 Te 21	3.17 3.18	3.06 3.07	1.12 1.05
		15	10	28.4	Ge 18 Sb 20 Se 40 Te 22	3.14	3.04	1.08

  . Consequently, the negative values of n 2 at 1.55 µm were found for films Ge 20 Sb 20 Se 40 Te 20 (𝐸 𝑔 𝐶𝐿 = 1.05 eV, n 0 = 3.20), Ge 19 Sb 20 Se 40 Te 21 (𝐸 𝑔 𝐶𝐿 = 1.05 eV, n 0 = 3.18) and Ge 18 Sb 20 Se 40 Te 22 (𝐸 𝑔 𝐶𝐿 = 1.08 eV, n 0 = 3.14). Similar observation was reported for some chalcogen poor and high Te concentration GSST by Dory et al. [28]. The maximum value of n 2 at 1.55 μm, specifically 28 × 10 -18 m 2 • W -1 , was found for thin film with composition Ge 20 Sb 17 Se 56 Te 7 (𝐸 𝑔 𝐶𝐿 = 1.37 eV, n 0 = 2.84, β = 2.2 × 10 -10 m • W -1 , FOM = 0.08). Moreover, thin film with composition Ge 19 Sb 17 Se 56 Te 8 (𝐸 𝑔 𝐶𝐿 = 1.41 eV, n 0 = 2.84, β = 1.6 × 10 -10 m • W -1 ), having the same value of n 2 shows the highest FOM at 1.55 μm among all fabricated GSST films (i.e. 0.11).

Table 2 .

 2 Comparison of nonlinear optical properties of GSST thin films with various references and with As 2 Se 3 bulk glass; chemical composition of thin films (±1 at. %), linear refractive index n 0 at 1.55 μm (±0.01), optical bandgap energy in eV (𝐸 𝑔 𝐶𝐿 , ±0.02 eV), nonlinear refractive index n 2 (× 10 -18 m 2 • W -1 ), TPA coefficient β (× 10 -10 m • W -1 ) and FOM at 1.55 μm.

	-10

2 ). It should be noted that values of β in tellurium-free GSS having 𝐸 𝑔 𝐶𝐿 >1.60 eV are typically ~0.1 × 10 -10 m • W -1

[START_REF] Halenkovič | Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films[END_REF]

.

Nonlinear optical properties of GSST thin films studied in this work compared to available references are summarized in table

2

.

1 Calculated according Sheik-Bahae with β = 0.1 × 10 -10 m • W -1 2 Tauc gap

  . For instance, Olivier et al. reported experimental values of nonlinear refractive index (n 2 ) at 1.55 μm for bulk sample of Ge 19.4 Sb 16.7 Se 63.9 glass around 10±2.0 × 10 -18 m 2 • W -1 and  around 0.31 cm • GW -1 obtained by direct transmission analysis [12]. Moreover, Kuriakose et al. reported experimental values at 1.55 μm of 8±2 × 10 -18 m 2 • W -1 (n 2 ) and  around 0.37±0.05 cm • GW -1 for chalcogenide slab waveguides of 3 µm thickness prepared from Ge 19.4 Sb 16.7

Table 3 .

 3 Compositions of GSST sputtered films determined by EDS (±1 at. %), RMS roughness (Sq, ±0.1 nm) obtained from AFM scans, contact angles for standard liquids, dispersive 𝛾 𝑠 𝑑 and polar 𝛾 𝑠

	Thin film composition (at. %)	Sq (nm)	Water	ethylene glycol	Contact angle, θ (°) formamide glycerol	diiodomethane	𝛾 𝑠 𝑑 (±2 mN•m -1 )	𝛾 𝑠 𝑝 (±2 mN•m -1 )	σ (±2 mJ•m -2 )
	Ge 22 Sb 17 Se 55 Te 6	0.4	70±3	59±1	61±1	69±1	41±1	29	7	37
	Ge 22 Sb 18 Se 50 Te 10	0.4	71±1	56±1	61±1	67±1	42±1	30	7	39
	Ge 22 Sb 18 Se 45 Te 15	0.6	68±2	54±2	61±2	66±2	39±1	31	8	38
	Ge 20 Sb 20 Se 40 Te 20	0.8	54±2	47±1	51±1	75±1	54±2	22	17	39

𝑝

components of surface energy obtained by Owens-Wendt theory (both ±2 mN • m -1 ) and the values of surface energy σ (±2 mJ • m -2 ) of thin films.

Acknowledgements

Platform Spectroscopy Infrared and Raman (SIR -ScanMAT, Université de Rennes 1) and CMEBA are acknowledged for performed Raman and EDS measurements, respectively. Funding: This work was supported by the CNRS, Brittany region (France), ANR LOUISE (ANR-15-CE04-0001-01), ANR AQUAE (ANR-21-CE04-0011) and Czech Science Foundation (Project No. 19-24516S).


Tomáš Halenkovič
email: tomas.halenkovic@upce.cz

Magdaléna Kotrla

Jan Gutwirth

Virginie Nazabal

Petr Němec
email: petr.nemec@upce.cz

An insight into the photoinduced phenomena in ternary Ge-Sb-Se sputtered thin films

The kinetics of photoinduced changes, namely photobleaching and photodarkening in sputtered ternary Ge 29 Sb 8 Se 63 thin films was studied. Study of time evolution of absorption coefficient Δα(t) upon room-temperature near-bandgap irradiation revealed several types of photoinduced effects. The as-deposited films exhibited a fast photodarkening followed by dominative photobleaching process. Annealed thin films were found to undergo photodarkening only. Local structure studied by Raman scattering spectroscopy showed significant structural changes upon thermal annealing which are presumably responsible for a transition from the photobleaching observed in as-deposited and reversible photodarkening in annealed thin films. Moreover, transient photodarkening process was observed in both asdeposited as well as in annealed thin films. An influence of the initial film thickness and laser optical intensity onto the kinetics of photoinduced changes is discussed.

Introduction

Photostructural changes in amorphous chalcogenide thin films and chalcogenide glasses have been extensively studied for many decades [START_REF] De Neufville | Photostructural transformations in amorphous As2Se3 and As2S3 films[END_REF]. Chemical bond rearrangement induced by the light excitation having the energy near the bandgap of these materials leads to the modifications of amorphous matrix resulting in changes of optical properties [START_REF] Ducharme | Photodarkening profiles and kinetics in chalcogenide glasses[END_REF]. These are referred to a photodarkening (PD) and a photobleaching (PB) depending on a sign of a change [START_REF] Fritzsche | The origin of reversible and irreversible photostructural changes in chalcogenide glasses[END_REF]. The magnitude of PD and PB is related to the composition and the structure of chalcogenide materials. While in the binary arsenic-based chalcogenides only PD occurs, both germaniumbased binaries [START_REF] Kumar | Crossover from photodarkening to photobleaching in a-GexSe100-x thin films[END_REF][START_REF] Halenkovič | Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films[END_REF] and ternaries show rather complex behavior with a sign of the change being a product of a competitive processes of PD and PB [START_REF] Sati | Coexistence of photodarkening and photobleaching in Ge-Sb-Se thin films[END_REF][START_REF] Khan | Role of Ge:As ratio in controlling the light-induced response of a-GexAs35-xSe65 thin films[END_REF].

The PD of exposed annealed films of As 2 S 3 and As 2 Se 3 which may be partially recovered by the consequent annealing was reported as early as in 1970s by De Neufville et al. [START_REF] De Neufville | Photostructural transformations in amorphous As2Se3 and As2S3 films[END_REF]. It is considered that the as-deposited amorphous chalcogenide thin films possess the irreversible photoinduced changes while the annealed films and melt-quenched glasses undergo the reversible ones [START_REF] Pfeiffer | Reversible photodarkening of amorphous arsenic chalcogens[END_REF]. Recently, the waveguide inscription using femtosecond lasers (ultrafast laser inscription, ULI) attracted the attention of some researchers [START_REF] Arriola | Mid-infrared astrophotonics: study of ultrafast laser induced index change in compatible materials[END_REF][START_REF] Torun | Ultrafast Laser Direct-Writing of Self-Organized Microstructures in Ge-Sb-S Chalcogenide Glass[END_REF]. This process includes the nonlinear absorption of sub-bandgap light resulting in changes in refractive index. Results on ULI in Ge-Sb-S glasses by means of structural study suggest that the driving forces of PD during the ULI in chalcogenide glasses are of the same origin as the photostructural changes due to the near-bandgap continuous-wave laser irradiation [START_REF] Torun | Ultrafast Laser Direct-Writing of Self-Organized Microstructures in Ge-Sb-S Chalcogenide Glass[END_REF]. A better comprehension of the processes and variables involved in photoinduced phenomena may play an important role for potential applications, optimization and tailoring properties of these materials in field of photonic integrated circuits, holographic gratings and waveguide inscription [START_REF] Zakery | Optical properties and applications of chalcogenide glasses: a review[END_REF].

The aim of this paper is to provide a study of photoinduced changes in Ge-Sb-Se chalcogenide thin films which are of particular interest in the field of photonic integrated circuits for their potential application in non-linear optics [START_REF] Delcourt | Self-phase modulation and four-wave mixing in a chalcogenide ridge waveguide[END_REF] and optical sensors in midinfrared domain [START_REF] Baillieul | Toward Chalcogenide Platform Infrared Sensor Dedicated to the In Situ Detection of Aromatic Hydrocarbons in Natural Waters via an Attenuated Total Reflection Spectroscopy Study[END_REF][START_REF] Mittal | Perspective on Thin Film Waveguides for on-Chip Mid-Infrared Spectroscopy of Liquid Biochemical Analytes[END_REF]. The nominal composition of Ge 25 Sb 10 Se 65 was chosen based on our previous work related to photosensitivity study on this system [START_REF] Halenkovič | Amorphous Ge-Sb-Se thin films fabricated by co-sputtering: Properties and photosensitivity[END_REF]. It combines favorable optical properties and expected photosensitivity. Relatively high optical bandgap energy causes the orange color of thin films within range of thicknesses under study which in combination with low surface reflectivity allows an easy detection of photoinduced changes. The insight in the photoinduced changes provided in this paper summarizes up to date available references and tries to recognize the challenges in understanding the general principles of these phenomena. The influence of a thickness and the laser optical intensity onto the kinetics of these processes is discussed.

Methods

A sputtering target (ø = 50 mm) of Ge 25 Sb 10 Se 65 chalcogenide glass was synthesized by melt-quenching technique described elsewhere [START_REF] Verger | RF sputtered amorphous chalcogenide thin films for surface enhanced infrared absorption spectroscopy[END_REF]. Thin films with thicknesses d of 350, 500, and 650 nm were deposited by rf-sputtering (13.56 MHz) at 5 × 10 -3 mbar working pressure of argon onto well-cleaned microscope slides (soda-lime float glass, substrate length of 76 mm).

The variance in the thickness was estimated to be < 6 % within the substrate length. Selected thicknesses were found to be optimal for our experiments as the penetration depth of a pump laser described below is ~ 700 and ~ 1800 nm for as-deposited and annealed films, respectively. Composition of thin films was verified by energy-dispersive X-ray spectroscopy (EDS) joint with scanning electron microscope (JEOL IT-300, JEOL Ltd., Japan). X-ray diffraction (XRD) patterns were collected within the range of 2θ from 10° to 80° with a step of 0.02° (Cu K-α, Rigaku, Japan). Thickness of thin films and its initial optical properties, specifically refractive index n and the optical bandgap energy E g , were determined from variable angle spectroscopic ellipsometry (VASE, J. A. Woollam, USA).

Annealing of samples was performed at temperature of 250 °C (i.e. glass transition temperature of bulk T g -50 °C) for 2 hours (heating rate of 2 °C • min -1 , cooling rate of 1 °C • min -1 ) under argon atmosphere. Kinetics of photoinduced changes of both as-deposited and annealed samples at the room temperature was studied by means of in situ transmittance measurements captured by fiber-coupled high-resolution spectrometer (OceanInsight, USA). Samples were placed in a hermetically sealed cell equipped with quartz windows filled with pure argon gas and irradiated by near-bandgap light coming from 589 nm (2.1 eV) continuous-wave DPSS laser source (Shanghai Laser & Optics Century co., Ltd., China) for 180 minutes. The experimental setup for in situ kinetics measurements is depicted in Fig. 1(a). In order to retain the athermal nature of photoinduced changes low to moderate laser optical intensities, specifically 3.0±0.5, 19.0±2.5, 50.0±3.0 and 125±5.0 mW • cm -2 were used.

The changes were expressed as the time evolution of absorption coefficient

Δα(t) calculated as Δα(t) = ln[T 0 • T(t) -1 ] • d -1
, where T 0 is the initial transmittance, T(t) is the transmittance at elapsed time t. Selected value of the initial transmittance at irradiation wavelength was ≈ 18 %, corresponding to the region of exponential absorption edge (Urbach edge, at initial value of absorption coefficient α in order of ~ 10 4 cm -1 ). It is worth to note that the small changes in the thickness are expected during the irradiation (such as photoinduced volume change in As 2 S 3 chalcogenide glass [START_REF] Tanaka | Photoexpansion in As2S3 glass[END_REF]); however, this effect does not affect the value of Δα significantly.

Changes in the reflectance during the irradiation were also neglected.

The magnitude of absorption edge shift by means of a change of E g succeeding the irradiation was determined directly from transmittance measurements using WVASE32 software (J. A. Woollam, USA). It should be emphasized that the evaluation of absolute values of E g and its changes was not the target of this work and so the precision of the estimation using this approach is rather coarse with expected accuracy about ±0.02 eV.

Results and Discussion

Optical properties of thin films

Chemical composition of vitreous target was found to be Ge24Sb11Se66 (±1 at. %). In agreement with a previous work on this system [START_REF] Verger | RF sputtered amorphous chalcogenide thin films for surface enhanced infrared absorption spectroscopy[END_REF], the slight changes in stoichiometry during the sputtering process were observed as the composition of thin films was Ge 29 Sb 8 Se 63 (±1 at. %).

Optical bandgap energy E g of as-deposited thin films was 1.87±0.02 eV and the refractive index n at 1 550 nm is 2.55±0.01 as determined by VASE using Cody-Lorentz oscillator model. An annealing treatment caused bleaching of thin films resulting in optical bandgap energy increase to 1.96±0.02 eV accompanied with slight refractive index decrease down to 2.54±0.01. Both as-deposited as well as annealed ones were found to be amorphous according the XRD measurements.

As shown in Fig. 1(b) the prolonged irradiation caused the change of an opposite sign, i.e. PB in as-deposited films and PD in annealed ones, reasonably of a highest magnitude when the laser optical intensity of 125 mW • cm -2 was used. It should be noted that even after three hours of irradiation the photoinduced change is not fully saturated and so for the measurements at lower laser optical intensities the irradiation time was conserved.

The highest magnitude of PB by means of an increase in E g about 0.04±0.02 eV was found in the sample with the thickness of ~ 350 nm. On the other hand, the highest measure of PD, specifically the decrease in E g by 0.05±0.02 eV, was found in sample with thickness of ~ 650 nm at above mentioned conditions.

Millisecond kinetics of photoinduced changes

Study of time evolution of absorption coefficient Δα(t) revealed several types of photoinduced effects. First, as-deposited films exhibit fast PD which is entirely dominated by PB in longterm. Such behavior has already been observed in some as-deposited selenide films e.g. in evaporated GeSe 2 [START_REF] Barik | Unusual observation of fast photodarkening and slow photobleaching in a-GeSe2 thin film[END_REF] and Ge-As-Se [START_REF] Khan | Role of Ge:As ratio in controlling the light-induced response of a-GexAs35-xSe65 thin films[END_REF], pulsed laser deposited Ge-Sb-Se [START_REF] Olivier | Kinetics of photosensitivity in Ge-Sb-Se thin films[END_REF] or in cosputtered Ga-Sb-Se [START_REF] Halenkovič | Amorphous Ga-Sb-Se thin films fabricated by co-sputtering[END_REF]. Moreover, only PD takes places during the irradiation of annealed films. Finally, transitory change, specifically transient PD (TPD), was observed in both asdeposited as well as in annealed thin films.

As seen in Fig. 2 (top panel), PB in as-deposited thin films is strongly thickness dependent having the magnitude of a process indirectly proportional to the thickness. It is worthy to note that the PB effect in the film with thickness of ~ 350 nm (violet curve) is closest to being saturated when irradiated by 125 mW • cm -2 laser optical intensity. Presumably, the observed PB effect upon prolonged near-bandgap irradiation is of a similar origin as the bleaching upon thermal annealing. Such change is coming from a mutual polymerization of defective entities resulting in ordered structures via homo-to heteropolar bond conversion [START_REF] De Neufville | Photostructural transformations in amorphous As2Se3 and As2S3 films[END_REF]. The excitation generates the charged radiation defects followed by recombination-induced bond rearrangements due to the diffusion of atoms [START_REF] Kaganovskii | Laser Recording in Chalcogenide Glass Films: Driving Forces and Kinetics of the Mass Transfer[END_REF]. Although the photoexcited states have not yet been proven experimentally [START_REF] Tanaka | Light-Induced Phenomena[END_REF], their existence leading to an increased short-and mediumrange order [START_REF] Tichý | On the origin of photo-induced and thermally induced irreversible bleaching of amorphous Ge-Se films[END_REF] is generally accepted explanation of irreversible changes in different properties including absorption edge shift. Other stabilization mechanisms, such as void collapsing [START_REF] Tanaka | Light-Induced Phenomena[END_REF], evaporation of volatile entities, and the photo-enhanced oxidation [START_REF] Spence | Light-induced oxidation and band-edge shifts in thermally evaporated films of germanium chalcogenide glasses[END_REF] may contribute to these changes. As the experiments performed in frame of this work were carried out in a pure argon atmosphere, we do not expect the last mentioned has an important contribution to observed effects. In contrast with PB observed in as-deposited thin films, the PD in annealed thin films as seen in Fig. 2 (bottom panel) is proportionally dependent on the thickness and is a universal process observed in annealed chalcogenide glasses and thin filmsi.e. reversible photodarkening effectwhich may be recovered by thermal annealing. The mechanism of reversible changes is not yet known as no subtle structural changes have yet been experimentally determined. They are likely the result of a similar elemental process as the irreversible ones, i.e. recombination-induced bond rearrangements and atom motions [START_REF] Fritzsche | The origin of reversible and irreversible photostructural changes in chalcogenide glasses[END_REF].

Presumably, the reversible photodarkening is coming from the valence band broadening resulting in the red-shift [START_REF] Tanaka | Light-Induced Phenomena[END_REF].

The origin of fast PD in as-deposited films is not yet fully understood. Sati and Jain [START_REF] Sati | Coexistence of photodarkening and photobleaching in Ge-Sb-Se thin films[END_REF] proposed that it originates in homo-to heteropolar bond conversion at deformed sites such as Transitory changes by means of transient PD (TPD) were easily noticed by on/off experiments found in Fig. 3(c). TPD has already been observed by various authors [START_REF] Khan | Role of Ge:As ratio in controlling the light-induced response of a-GexAs35-xSe65 thin films[END_REF][START_REF] Olivier | Kinetics of photosensitivity in Ge-Sb-Se thin films[END_REF][START_REF] Shimakawa | Transient responses of photodarkening and photoinduced volume change in amorphous chalcogenide films[END_REF][START_REF] Ganjoo | Transient photodarkening in amorphous chalcogenides[END_REF][START_REF] Yan | Effect of Photo-Oxidation on Photobleaching of GeSe2 and Ge2Se3 Films[END_REF][START_REF] Zhang | Photo-induced effects in Ge-As-Se films in various states[END_REF] and may be related to a phenomenon termed as an 'optical stopping effect' by Matsuda et al. back in 1970s [30]. Ganjoo et al. [START_REF] Ganjoo | Transient photodarkening in amorphous chalcogenides[END_REF] observed TPD in annealed evaporated amorphous selenium a-Se and As 2 Se 3 of ~ 500 nm thickness when irradiated by Ar ion laser (λ = 514 nm, optical intensity of 80 mW • cm -2 ). An apparent rise in the transmittance after switching the laser off (λ = 488 and 660 nm, 150 mW • cm -2 ) was also reported for 1 μm thick as-deposited GeSe 2 and Ge 2 Se 3 evaporated films by Yan et al. [START_REF] Yan | Effect of Photo-Oxidation on Photobleaching of GeSe2 and Ge2Se3 Films[END_REF]. Similar behavior was found in sputtered as-deposited Ge-As-Se thin films at different intensities when exposed to 655 nm laser diode [START_REF] Zhang | Photo-induced effects in Ge-As-Se films in various states[END_REF]. Additionally, the magnitude of TPD among the references herein seems to be affected by the optical intensity and the metastable PD or PD depending on composition. The structural origin of TPD is yet to be understood. In situ EXAFS experiments on amorphous selenium indicate that it may be associated to a chalcogen coordination number increase upon irradiation [START_REF] Tanaka | Light-Induced Phenomena[END_REF][START_REF] Kolobov | Structural study of amorphous selenium by in situ EXAFS: Observation of photoinduced bond alternation[END_REF].

Finally, the results on millisecond kinetics of photoinduced changes in as-deposited and annealed Ge-Sb-Se thin films suggest that they are of a very same origin as in Ge-As-Se system [START_REF] Zhang | Photo-induced effects in Ge-As-Se films in various states[END_REF]. Even though the sign and the magnitude of a change may differ with a composition and the structure [START_REF] Halenkovič | Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films[END_REF][START_REF] Khan | Role of Ge:As ratio in controlling the light-induced response of a-GexAs35-xSe65 thin films[END_REF][START_REF] Halenkovič | Amorphous Ge-Sb-Se thin films fabricated by co-sputtering: Properties and photosensitivity[END_REF], the TPD and reversible PD in annealed thin films seem to be a universal feature of photoinduced changes in amorphous chalcogenides.

Structural changes upon annealing and the irradiation

Changes in local structure upon thermal annealing and irradiation were investigated using Raman scattering spectroscopy measurements. As depicted in Fig. 4, the Raman spectra of Ge-Sb-Se thin films are prima facie dominated by the band peaking at ~ 199 cm -1 , which corresponds to A 1 symmetric stretching vibrations of heteropolar Ge-Se bonds in a cornersharing [GeSe 4/2 ] tetrahedra. The companion mode A₁ C assigned to edge-sharing tetrahedral units is located at ~ 214 cm -1 , [START_REF] Němec | Structure and properties of the pure and Pr3+-doped Ge25Ga5Se70 and Ge30Ga5Se65 glasses[END_REF]. Overlapping E 1 mode of [SbSe 3/2 ] pyramidal units around ~ 191 cm -1 may contribute to the Raman intensity of A 1 mode [START_REF] Ivanova | Thermomechanical and structural characterization of GeSe2-Sb2Se3-ZnSe glasses[END_REF]. Peak around ~ 174 cm -1 is considered to be coming from Ge-Ge homopolar bonds in ethane-like [Ge 2 Se 6/2 ] units [START_REF] Jackson | Raman-active modes of a-GeSe2 and a-GeS2: A first-principles study[END_REF].

Vibrational modes of Sb-Sb in [Se 2 Sb-SbSe 2 ] found at ~ 160 cm -1 probably contribute to this peak [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100x(Sb2Se3)x glasses[END_REF].

In order to justify changes in the structure, Raman spectra were deconvoluted using 8 Gaussian peaks. Besides five peaks mentioned above, the rotational modes in selenium polymeric chains (~ 140 cm -1 , [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100x(Sb2Se3)x glasses[END_REF]) and bending vibrational modes of Se-Se at the outrigger (135-145 cm -1 , [START_REF] Baudet | Structural analysis of RF sputtered Ge-Sb-Se thin films by Raman and X-ray photoelectron spectroscopies[END_REF]) are altogether represented by Gaussian peak centered at ~ 134 cm -1 . A 1 vibrational mode of Se n chains in amorphous selenium found at ~ 235 cm -1 , stretching vibrational modes of Se-Se bonds at the outrigger (~ 245 cm -1 ), A 1 vibrational modes of Se-Se dimers and short Se n chains in corner-linked [GeSe 4/2 ], (~ 260-265 cm -1 , [START_REF] Olivier | Structure, nonlinear properties, and photosensitivity of (GeSe2)100x(Sb2Se3)x glasses[END_REF]), and Ge-Ge homopolar bonds' vibrations coming from [Ge-Ge m Se 4-m (m = 1, 2, 3, 4)], found ~ 270 cm -1 [START_REF] Jackson | Raman-active modes of a-GeSe2 and a-GeS2: A first-principles study[END_REF] are collectively represented by Gaussian at ~ 262 cm -1 . Finally, F 1 asymmetric vibrational modes of GeSe 4 tetrahedrons are found at ~ 300 cm -1 , [START_REF] Němec | Structure and properties of the pure and Pr3+-doped Ge25Ga5Se70 and Ge30Ga5Se65 glasses[END_REF].

Upon thermal annealing an apparent change in the ratio between A 1 (green) and A₁ C (blue) peaks occurred. Simultaneous decrease in the intensity of a peak centered at ~ 134 cm -1 (red) and ~ 262 cm -1 (orange) indicates the consumption of selenium atoms as a consequence of increased local order. Other noticeable changes occurred around ~174 cm -1 peak. These changes may be due to a decrease of Ge-Ge homopolar bonds in ethane-like [Ge 2 Se 6/2 ] units.

Changes in intensity of surrounding peaks upon thermal annealing are low and difficult to justify.

Photostructural changes by means of Raman scattering spectroscopy have already been described by several authors. The bond breaking mechanism upon the irradiation followed by bond rearrangement and/or crystallization of structural sites are usually connected with changes in optical properties (i.e. shift of the absorption edge and the refractive index changes upon irradiation) [START_REF] Kumar | Crossover from photodarkening to photobleaching in a-GexSe100-x thin films[END_REF][START_REF] Khan | Role of Ge:As ratio in controlling the light-induced response of a-GexAs35-xSe65 thin films[END_REF][START_REF] Zhang | Observation of photobleaching in Ge-deficient Ge16.8Se83.2 chalcogenide thin film with prolonged irradiation[END_REF]. Noteworthy, even in highly photosensitive composition of As 2 S 3 , the fully saturated PD process results in only up to 1 % of nearest neighbors bonds' alteration at available atomic sites [START_REF] Ducharme | Photodarkening profiles and kinetics in chalcogenide glasses[END_REF]. As only low to moderate laser optical intensities were used in the present study negligible changes in Raman spectra were observed and do not allow a fine analysis of the mechanisms involved. It can be assumed that above mentioned structural changes upon thermal annealing are responsible for a transition from the PB observed in as-deposited and reversible PD in annealed Ge-Sb-Se thin films respectively. In addition, although the experiments were carried out under the pure argon atmosphere the photo-enhanced oxidation should be not completely ruled out from the contribution to the photoinduced changes as the concentration of germanium is ~ 29 at. % [START_REF] Tichý | Photoinduced bleaching of Ge35S65 amorphous film[END_REF].

Conclusions

Comprehensive study of photosensitivity in amorphous Ge 29 Sb 8 Se 63 thin films by means of time evolution of the absorption coefficient Δα(t) revealed several types of photoinduced effects in these materials. Fast PD followed by slow dominative PB process is present in as-deposited films while only PD takes places during prolonged irradiation of annealed films.

The structural changes upon thermal annealing seem to be responsible for a transition from the PB observed in as-deposited and reversible PD in annealed thin films. Transient PD (TPD) was observed in both as-deposited as well as in annealed thin films and, together with a PD in annealed films, seems to be a universal feature of photoinduced changes in amorphous chalcogenides. The processes behind metastable PD and PB in the as-deposited films as well as TPD, which seem to be associated with transitory photoexcited states, remain to be better understood. Furthermore, the throughout study of surface and thickness changes upon the irradiation and annealing as well as long-term stability at normal room conditions in similar systems are also of interest. Such investigations may lead towards the implementation of these materials and photonic processes in them in the design and engineering of photonic integrated circuits by means of tailoring and optimization of optical properties and inscription of treedimensional structures (e.g. waveguides, holographic gratings and diffractive optics elements).

Fig. 1 .

 1 Fig. 1. (a) Experimental setup for irradiation setup for in situ measurements. A pump beam from DPSS laser is aimed onto the sample through the plano-convex lens and its desired optical intensity is adjusted by the neutral density filter(s) (ND). (b) A typical time evolution of absorption coefficient Δα(t) for as-deposited film (black) and annealed film (red) upon room-

Fig. 2 .

 2 Fig. 2. Time evolution of absorption coefficient Δα(t) of as-deposited (top panel) and annealed (bottom panel) Ge-Sb-Se thin films upon room-temperature irradiation depending on thicknesses at different laser optical intensities varying from 3 to 125 mW • cm -2 . (This figure should not be reduced).

  Fig. 3(a) and (b) respectively. The time evolution of fast PD shows a saturation at 125 mW • cm -2 within ~ 245 s and clearly delays with a decreasing intensity. Similar observation was reported for evaporated Ge-As-Se films by Khan et al. [25]. The process of fast PD becomes completely dominated by slow PB process.

Fig. 3 .

 3 Fig. 3. (a) Time evolution of absorption coefficient Δα(t) of as-deposited and (b) annealed Ge-Sb-Se thin films with thickness of ~ 650 nm depending on the laser optical intensity. (c) On/off cycles (240 s) experiment with as-deposited (black line) and annealed (red line) Ge-Sb-Se thin films with the same thickness using laser optical intensity of 50 mW • cm -2 . The shaded area corresponds to the cycle when the laser is switched on.

Fig. 4 .

 4 Fig. 4. (a) Normalized reduced intensity of Raman spectra of nonirradiated as-deposited, and (b) nonirradiated annealed Ge-Sb-S thin films.

Funding

Czech Science Foundation (project No. 22-05179S). Ministry of Education, Youth and Sports of the Czech Republic (project No. LM2018103).

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

Disclosures

The authors declare that they have no competing financial interests.