Ge–Sb–S–Se–Te amorphous chalcogenide thin films towards on-chip nonlinear photonic devices

Thanks to their unique optical properties Ge–Sb–S–Se–Te amorphous chalcogenide materials and compounds offer tremendous opportunities of applications, in particular in near and mid-infrared range. This spectral range is for instance of high interest for photonics or optical sensors. Using co-sputtering technique of chalcogenide compound targets in a 200 mm industrial deposition tool, we show how by modifying the amorphous structure of GeSbwSxSeyTez chalcogenide thin films one can significantly tailor their linear and nonlinear optical properties. Modelling of spectroscopic ellipsometry data collected on the as-deposited chalcogenide thin films is used to evaluate their linear and nonlinear properties. Moreover, Raman and Fourier-transform infrared spectroscopies permitted to get a description of their amorphous structure. For the purpose of applications, their thermal stability upon annealing is also evaluated. We demonstrate that depending on the GeSbwSxSeyTez film composition a trade-off between a high transparency in near- or mid-infrared ranges, strong nonlinearity and good thermal stability can be found in order to use such materials for applications compatible with the standard CMOS integration processes of microelectronics and photonics.

www.nature.com/scientificreports/ film samples. Then, all raw spectra were normalized to the chalcogenide film thicknesses in order to get a more accurate comparison between the different thin film samples. Raman scattering spectra were acquired in a micro-Raman spectrometer in the range from 100 to 500 cm −1 using a laser probe at 532 nm wavelength. The acquisition conditions (laser power, magnification and exposure time) were adjusted for each films in order to optimize the signal-to-noise ratio but with a particular emphasis to keep no or very limited impact on the material's structure.
Spectroscopic ellipsometry measurements. Spectroscopic ellipsometry (SE) measurements were performed in the 400-1,700 nm range. Data were collected at three incidence angles (55, 65 and 75°). Analysis of the raw data was performed using WVASE 32 ® software. A 10 nm SiN x layer deposited on a Si substrate was also measured separately in order to take into account any possible influence of capping layer when modelling of chalcogenide films' data. For chalcogenide thin film samples, the film thicknesses, dielectric functions, optical constants (refractive index n and extinction coefficient k) and absorption coefficient α as a function of the photon energy in the 0.73-3.1 eV range were obtained by means of modelling of the SE data with a Cody-Lorentz (CL) model (see also Sect. 3 of the Supplementary Information).
The optical bandgaps of the films were estimated by using the bandgap values obtained from the CL fitting model (E g CL ) as well as by considering the energy for which the absorption reaches 10 4 cm -1 (E g 04 ). Using the M-line technique 28 at two wavelengths (1,313 and 1,548 nm), the effective refractive indices of the films as well as their thicknesses (by prism coupling technic) were also accurately determined (not shown). The obtained values of n at 1,313 and 1,548 nm were compared with those obtained from SE modelling and were used to validate the accuracy of SE results.
In order to get an estimation of the optical nonlinearities of the studied glasses the well-known Sheik-Bahae model was used 29 . This method allows to estimate nonlinear Kerr refractive index n 2 by means of an analytical approach using linear refractive index and optical band gap energy values. This model takes into account contributions from several physical origins: two-photon and Raman transitions, linear Stark and quadratic Stark effects. A divergent term is also added in order to subtract the unphysical behaviour resulting from the formula used to adjust these contributions. For these calculations, we used a Sellmeier fit of the refractive index in the material's transparency range obtained by using the Cody-Lorentz modelling and the optical band gap energy E g 04 . The results of Sellmeier fits were extrapolated to wavelengths beyond spectral range of the ellipsometry measurements.

Results and discussion
As-deposited chalcogenide thin films' composition map. All compositions of the studied chalcogenide thin films are reported on the ternary diagrams of Fig. 1 (see also Sect. 1 of the Supplementary Information). As shown in Fig. 1 the wide composition range of amorphous chalcogenide thin films that can be obtained exhibits a minimal thermal stability of 250 °C and up to higher than 400 °C depending on film composition (see "Methods" and Sect. 2 of the Supplementary Information).
Amorphous structure of chalcogenide thin films by FTIR and Raman spectroscopies. A summary of the bonds nature and main structural motifs detected in the amorphous structure of chalcogenide thin films deposited by co-sputtering is listed hereafter. All the details are given in Sect. 3 of the Supplementary Information. Table 1 summarizes the main structural motifs for each type of chalcogenide thin film compounds of the present study deduced from analysis of the Raman and FTIR vibration modes as well as the references from literature supporting our conclusions. As follows, the main conclusions on the amorphous structure drawn from the analysis of the Raman and FTIR spectra of each film is summarized system by system based on the extended discussion of the Supplementary Information.  Fig. 2a are shown the FTIR and Raman spectra acquired on the Ge 1-x Se x , thin films, with x varying in 0.63-0.74 range. Se enrichment in Ge 1-x Se x films leads to reduction of homopolar inter-tetrahedral Ge-Ge bonds (Ge-Ge ETH ) in favour of Se-Se ones accompanied by an increase of the number of corner-sharing (CS) compared with edge-sharing (ES) GeSe 4/2 tetrahedra in excellent agreement with previous works 30 .
The FTIR and Raman spectra of Ge 1-x S x films are presented in Fig. 2b. In amorphous Ge 1-x S x films, vibrational modes corresponding to GeS 4/2 tetrahedra are observed along with contributions attributed to Ge-Ge and S-S homopolar bonds. The decrease of disorder in the amorphous Ge 1-x S x films evidenced by a hardening of the Raman modes is observed when going from Ge 40 S 60 toward Ge 36 S 64 composition as the amorphous network evolves toward composition being more and more close to that of the stoichiometric GeS 2 glass. The intensity of the visible contribution of the crystalline Si substrate (c-Si) depends on the thickness and transparency of the chalcogenide films at 532 nm.
In Fig. 3c, the FTIR and Raman spectra of the [Ge 37 S 63 ] 1-x [Ge 30 Se 70 ] x thin films upon Sb incorporation show vanishing of the Raman modes of mixed GeS 4-m Se m tetrahedral units at least in favour of an increase number of GeSe 4/2 tetrahedra. This indicates differences between Ge and Sb atoms in chemical bonding affinity with S and Se chalcogen elements. This has a different impact on the amorphous structure depending on the S/Se ratio. Increasing Sb concentration in [Ge 37 S 63 ] 1-x-y [Ge 30 Se 70 ] x Sb y films leads to preferential formation of Ge-Se and Sb-S bonds by detriment to Ge-S and Sb-Se ones in films exhibiting a lack of chalcogen element compared with the stoichiometric compositions in agreement with a previous study 41 . Raman and in a less manner FTIR Compounds' compositions indicated in square bracket correspond to those of the sputtering targets which may differ from those obtained for the deposited films.  52 Te 48 ] x films, which are getting more and more Gerich as x value is increased, Ge-Ge homopolars are found and Ge-Se(Te) bonds represent the vast majority of   Absorbance (a.u.)  Absorbance (a.u.)  The different contribution of vibration modes related to GeSe 2 and GeS 2 amorphous phases are indicated by orange and green dash-lines, respectively. The contribution of the c-Si substrate to the Raman signal appearing on some spectra is also indicated by a red dash-line at 300 cm -1 as well as a shoulder visible after 450 cm -1 that corresponds to a c-Si phonon mode near 520 cm -1 .  52 Te 48 ] x films does not lead to a random and homogeneous distribution of the chalcogen atoms in Ge-centered tetrahedra as observed in Ge-based chalcogenide films. From all the above FTIR and Raman study, one can conclude that the local order and the structure of the (co-)sputtered chalcogenide thin films are shown to largely vary with in particular a very different amount of homopolar bonds and the latter is shown to depend on thin films' atomic composition.
Since the properties of materials being intimely linked to their structure, probing the link between structure and optical properties is an invaluable clue in order to propose design rules aiming at fabricating chalcogenide compounds thin films with optimized properties for the applications in photonics. In the following, the linear and nonlinear optical constants of the films, such as the real and imaginary part of refractive index as well as Kerr nonlinear refractive index, are studied. Raman Shift (cm -1 ) Absorbance (a.u.) 200 300 400 500 Intensity (a.u.) Raman Shift (cm -1 )   www.nature.com/scientificreports/ Optical properties of the chalcogenide thin films. Linear optical constants. The optical constants (refractive index n and extinction coefficient k) of the films were deduced from the spectroscopic ellipsometry measurements from visible to near-IR (NIR) range (see "Methods"). The refractive indices n and Tauc's plots of (α.E) 1/2 vs energy (eV) obtained from the extinction coefficient k (see "Methods") are plotted in Figs. 5, 6, 7 for each composition of the chalcogenide films. First, Figs. 5, 6, 7 call for a general comment. For wavelength range located above inter band absorption range, which corresponds to energies lower than the bandgap energy (see the Tauc's plots of Figs. 5, 6, 7), the refractive indices tend progressively to a kind of plateau. The latter gives therefore an estimation of the refractive indices in the MIR range up to multi-phonons absorption appearing at significantly higher wavelengths than NIR 45,46 . Therefore, the study of optical properties in the visible-NIR range is the best compromise in order to get an estimation of refractive indices from visible to MIR range as well as giving an estimation of the optical band gap energy from absorption measurements or ellipsometry data fitting models (see "Methods" and Sect. 4 of the Supplementary Information).   www.nature.com/scientificreports/ film evidences its highest band gap value among all Ge 1-x Se x (0.63 < x < 0.74) films as seen on the Tauc's plots of Fig. 5a. Besides, the Ge 37 Se 63 composition with the highest refractive index has the lowest band gap energy. The decreasing number of Ge-Ge bonds may be at origin of the strong increase of absorption between Ge 34 Se 66 and Ge 37 Se 63 films. Ge 1-x S x thin films experience a significant increase of their refractive index in the NIR as the Ge fraction is slightly increased from x = 0.64 to x = 0.60 (Fig. 5b). This trend is consistent with previous literature studies 48,49 . This could be attributed to an increasing number of distorted S-Ge-S bonds similarly to the previously mentioned presence of numerous distorted Se-Ge-Se angles in Ge 1-x Se x (0.60 < x < 0.66) 47 related to the increase of the number of Ge-Ge homopolar bonds (see Fig. 2b) with the decrease of the S content. This leads to a reduction of the band gap energy in agreement with previous literature 49 due to creation of new electronic states under the conduction band explaining the progressive shift of absorption toward lower energy (Fig. 5b) 50 . The absorption and refractive index change monotonously as x increases in our Ge 1-x S x films (Fig. 5b). x films also exhibits a minimum for the Ge 33 S 37 Se 30 composition (Fig. 5c). This is not surprising since, similarly to Ge 1-x Se x and Ge 1-x S x films, the amount of Ge-Ge homopolar bonds is expected to be minimum for this near-stoichiometric composition with a Ge/(S + Se) atomic ratio close to 1/2 (Fig. 2c). As a result, the general trends on refractive indices of studied Ge-based chalcogenide films are driven by the ratio between heteropolar and homopolar bonds.
Among the [Ge 40 S 60 ] 1-x [Ge 26 Se 74 ] x thin films, the band gap energy reaches a maximum for the Ge 33 S 37 Se 30 composition as evidenced by the absorption curves in Fig. 5c. This results from the composition of Ge 33 S 37 Se 30 film which is close to the GeCh 2 stoichiometric compound (with Ch referring to S or Se chalcogen element) expected to exhibit almost no homopolar bonds. Note that for a same Ge concentration, replacing S by Se atoms results in a progressive decrease of band gap energy 31 .  Fig. 6, replacing tetravalent Ge atoms, as observed in structural motifs of stoichiometric GeSe 2 glass, by trivalent Sb atoms, as found in amorphous stoichiometric Sb 2 Se 3 compound, within the GeS x Se 1-x films increases the non-chalcogen/chalcogen element ratio required to keep the stoichiometric composition. This has been clearly evidenced for composition tie-lines crossing the stoichiometric GeSe 2 /Sb 2 Se 3 pseudo-binary tie-line as Ge 40-x Sb x Se 60 51 , Ge x Sb 10 Se 90-x 52,53 Ge x Sb 15 Se 85-x or Ge x Sb 20 Se 80-x 53 glasses. These compositions are again those with the smallest amount of homopolar bonds and corresponding to a limit of topological phase transition as reported in Ge 40-x Sb x Se 60 glasses 54,55 . Besides, in bulk glasses as well as in thin films, the decrease of the GeSe 2 /Sb 2 Se 3 ratio was shown to result in an increase of the refractive index and a decrease of the band gap energy since electronic polarizability of Sb-Se bonds is much higher than that of Ge-Se bonds 24,56,57 .
In Fig. 6 are shown the change of optical constants upon Sb addition by means of co-sputtering in the Ge 30 Se 70 ] x films significantly increases films' refractive index and at the same time reducing the optical band gap energy of the material. This effect is attributed to the higher electronic polarizability of Sb atoms in particular when forming Sb-Ch and Sb-Sb bonds. These highly polarizable bonds are revealed to appear in an increasing level as the incorporated Sb amount is increased in films (see amorphous structure analysis detailed in the Amorphous Structure section and in the Supplementary Information). One can also note that among all these compositions, some films exhibit compositions very close to those of well-studied commercial glasses such as for instance AMTIR-3 glass (also commercially called IG5 or IRG-5 or BD-2 or OPTIR-3) of Ge 28 Sb 12 Se 60 composition. Thus, this well-known glass is close to the Ge 26 Sb 13 Se 61 film of the present study and can be used as a point of comparison. In Fig. 6a However, in Fig. 7c upon incorporation of Sb in the ternary Ge 1-2x Se x Te x compound, the refractive index of the films significantly increase while the decrease in bandgap energy is negligible. The explanation is related to the changes in the amorphous structure. First, the increasing concentration of Sb leads to an increase of the material polarizability and thus that of the refractive index. However, at the same time the amount of Te-Ge heteropolar bonds decreases leading to an increase of band gap energy of the material hence counterbalancing the effect of Sb. Therefore in [Ge 1-2x Se x Te x ] 1-y Sb y films, the incorporation of highly polarizable Sb bonds leads to an increase of the refractive index but does not significantly affect the optical band gap energy value. Indeed, the replacement of Ge-Ge homopolars by Sb-Sb bonds upon Sb introduction is expected to impact mostly the density and nature of localized electronic defect states in the material band gap but with no or limited effect on the bandgap energy.
To summarize, the observed trend of refractive indices as a function of chalcogenide thin films' compositions can be reasonably correlated to changes in films' amorphous structure as extensively described in the FTIR/ Raman experiments of the Amorphous Structure section and in Sect. 3 of the Supplementary Information. The electronic polarizability of local chemical environments and bonding configurations in the amorphous is shown to play key role aiming at controlling the refractive indices of the films. In particular, introduction of highly polarizable bonds such as Sb-Sb homopolars, Sb-Ch (with Ch = Te or Se) or Te-X (with X = Sb or Ge) and in a less extent Ge-Ge bonds leads to a significant increase of the refractive index concomitant to a decrease of the optical bandgap energy. The optical band gap energy is also an outmost parameter that should be taken into account for MIR and nonlinear photonic applications regarding for instance transparency window, two-photon absorption (TPA) losses and so on. The band gap energy values of the chalcogenide thin films are listed in Table 2. The obtained band gap energies and refractive index values are found to follow the well-known Moss rule 60 , which relates the refractive index (or optical dielectric constant) to the optical band gap energy of semiconductors 61 .
In order to evaluate the potential interest for applications of such chalcogenide thin films in the emerging field of on-chip nonlinear photonics, determining the Kerr refractive indices, which quantify the nonlinear frequency conversion efficiencies, can give a first interesting insight. For that purpose, in the following the Kerr Scientific RepoRtS | (2020) 10:11894 | https://doi.org/10.1038/s41598-020-67377-9 www.nature.com/scientificreports/ refractive indices of the films are estimated by means of their linear optical constants as well as using estimated values of the band gap energy.
Kerr nonlinear refractive indices n 2 . The n 2 Kerr nonlinear refractive indices were evaluated mainly using the Sheik-Bahae model 29 . We considered this model as enough accurate in order to get qualitative trends on n 2 values (see Sect. 5 of the Supplementary Information). It has to be emphasized that a systematic error of about 20% exists in the experimental determination of non-linear optical coefficients. The Kerr refractive index is strongly correlated with the linear refractive index and the optical band gap energy value. Using the Sheik-Bahae model, the maximum of the third order nonlinear parameter is found for an energy close to 0,534 × E g opt . Therefore, the values of n 2 can vary considerably depending on the wavelength and must be taken into consideration depending on the value of the wavelength that will be used in the application. However, previous experimental work reported a maximum of Kerr index for photon energy values higher than that corresponding to 0,534 × E g opt 53 . Table 2 shows the maximum values of the n 2 Kerr refractive indices for all the films studied in this work and their corresponding wavelengths, as well as the values obtained at 1,550 nm, the standard wavelength for telecommunications. In order to validate the Sheik-Bahae model, this method was also applied to samples of silica and silicon nitride thin film. The n 2 values at 1,550 nm for these two reference materials have been calculated to be 3 × 10 -20 and 2 × 10 -19 m 2 /W, respectively. These values are in excellent agreement with those experimentally measured in the literature 60 . Therefore, one can conclude that in our (co-)sputtered amorphous chalcogenide thin films, the Kerr indices are of two to three orders of magnitude higher than those obtained for silica or silicon nitride materials. These values are also in excellent agreement with previous experimental results reported for similar Ge 1-x-y Sb x Se y , Ge 1-x-y Sb x S y compounds 38,63-65 as well as those deduced using the Sheik-Bahae model for Ge 1-x S x and Ge 1-x-y Sb x Se y glasses 49,66 .
For some of the chalcogenide thin films, the Sheik-Bahae model gives negative Kerr indices at 1,550 nm. In previous works, only positive experimental n 2 values were observed in the transparency window of chalcogenide glasses 61,67 . Nevertheless, negative Kerr indices values were reported in Ge 1-x-y Sb x Se y glasses at 800 nm 66 where absorption starts to be non-negligible. Therefore, we can emphasize that the negative Kerr indices values at 1.55 µm (~ 0.8 eV) are found for co-sputtered films for which the optical band gap energy is below 1.6 eV and as a result when TPA becomes significant.
In literature, n 2 values have been related to the amorphous structure. In particular, a clear correlation was found between n 2 values and the concentration of highly polarizable heteropolar bonds 38 . Herein, one observe a clear correlation between material polarizability, which is proportional to the linear refractive index, and the nonlinearities evidenced by the n 2 values calculated by using the Sheik-Bahae model. Thus, one can relate the enhancement of electronic polarizability and the resulting increase of n 2 values to presence of peculiar local atomic motifs and bonding configurations found in the amorphous structure of films. For instance, the amount of homopolar bonds play a key role in the increase of linear and nonlinear refractive indices as described above.
Finally, in Table 2 are also reported for each films the limit of temperature after which a degradation of the material could be observed by monitoring optical reflectivity at 670 nm upon annealing (see "Methods" and Sect. 2 of the Supplementary Information). This is also very instructive since temperature limit in between 250 and 400 °C were obtained. This emphasizes that a compromise between optical properties and thermal stability has to be found for selection of a particular composition depending on final application as well as taking into account the thermal budget required for integration process flow in devices. Indeed, an annealing after deposition is also expected to significantly affect the optical properties of these Ge-based amorphous chalcogenide thin films due to the structural relaxation. Thus, aging of these metastable amorphous materials will have to be studied in the future aiming at ensuring durability of MIR components that would integrate some of these compounds.

conclusion
To conclude, industrial co-sputtering deposition method is a powerful tool in order to fastly study a wide compositions range of amorphous chalcogenide thin films aiming at ultimately achieving highly nonlinear on-chip MIR components. By means of a systematic study of the amorphous structure correlated with the trend on optical properties (linear optical constants, optical band gap and Kerr nonlinear refractive index) of the as-deposited chalcogenide thin films one can get invaluable clues in order to optimize the materials optical properties towards future applications. The materials' polarizability and thus linear and nonlinear refractive indices increase significantly when moving from light to heavier chalcogen element such as S to Se and toward Te-based chalcogenide compounds but accompanied with a decrease of the thermal stability. The ratio of homopolar/wrong on heteropolar bonds in the amorphous chalcogenide is shown to play a main role on the electronic and thus optical properties of the films. For instance, the introduction of Sb atoms deeply modifies the amorphous structure as well as introducing Sb-Sb homopolar bonds. As a result, the electronic polarizability of the glass is significantly enhanced as evidenced by the significant increase of the material refractive indices. However, this also leads to a detrimental decrease of the material's thermal stability and bandgap. Moreover, we show that the outstanding and state-of-the-art Kerr refractive indices in the MIR range are found for chalcogenide thin films deposited by means of industrial sputtering technique enabling fast transfer to applications. Finally, we demonstrate that a good trade-off between high nonlinearity, good thermal stability and optimized working wavelength in the IR can be found opening wide range of opportunities for future on-chip photonic applications fully compatible with CMOS large-scale integration technologies. Table 2. Summary of chalcogenide thin films obtained by (co-)sputtering deposition: composition, nature of the sputtering targets used for (co-)sputtering deposition, refractive index at 1.55 µm, band gap energy E g 04 and E g CL , n 2 Kerr nonlinear refractive index calculated either by means of the Sheik-Bahae model at 1.55 µm (n 2 ) and maximal n 2 values (n 2 max ) at energy near 0,534 × E g opt eV (the corresponding wavelength value is also indicated into brackets) and a first evaluation of the limit temperature for material's stability (see text for details as well as "Methods" and Sect. 2 of the Supplementary Information). www.nature.com/scientificreports/