Introduction

Chalcogenide glasses (in bulk, fiber, and thin film forms) are fascinating materials due to their excellent properties and wide applications1 such as photolithography, holography, for optical switching, laser written waveguides, photonic crystals, optical phase change memories, etc.2,3,4,5,6,7,8. In recent years amorphous chalcogenides and their thin films are broadly used for optical lenses, demultiplexers, filters, deflectors, Bragg mirrors, couplers, waveguides9, for the fabrication of large area photodiode arrays, solar selective coatings, solar cells, photoconductors, sensors, phase change memories, etc.1,10,11,12,13,14.

The thin films of amorphous chalcogenides are fabricated using numerous deposition techniques. Currently, the most commonly used deposition methods are pulsed laser deposition15,16, radio-frequency magnetron sputtering17,18,19, thermal evaporation20, chemical vapor deposition21,22, etc. Nowadays, these methods are used to prepare high-quality thin films from different binary, ternary, or quaternary chalcogenide glass systems.

Expecting that germanium can be an alternative to arsenic (as a typical, but toxic glass network former), the chalcogenide glasses from the binary Ge-Se system were widely studied to reveal their physical properties (transparency window in IR, (non)linear refractive index, etc.). Moreover, the incorporation of antimony to Ge-Se glasses helps to enhance the glass forming ability and tailor their physical properties23,24.

Another approach exploits Ga as a prospective glass network former in Ga-Sb-S25, Ga-Sb-Se26, Ga-La-S27,28, or Ga-Na-S29 systems. Among them, considerable attention was recently paid to Ga-Sb-S glasses focusing on their doping with rare earth ions30,31. It is well known that the substitution of sulfur by selenium strongly broadens the transmission window of chalcogenide glasses in the infrared range. Therefore, Lecomte et al. devoted lately their research to study Ga-Sb-Se glasses transparent up to 18 µm26. Moreover, Lu et al. studied Ga-Sb-Se chalcogenide system in their bulk as well as thin film forms for the phase change characteristics. According to their results, Ga-Sb-Se materials exhibit outstanding data retention ability, small density change during crystallization, and fast crystallization speed which indicates their great potential for phase change memory applications32,33. Therefore, the Ga-Sb-Se chalcogenide glasses and thin films can be considered as a promising alternative for germanium containing ones; however, these materials and their properties are reported in the literature only rarely.

Previous studies evidenced that the laser desorption ionization (LDI) or laser ablation (LA) Time-of-Flight mass spectrometry (TOFMS) is a useful technique for the generation and identification of clusters of binary, ternary or quaternary chalcogenide glasses or their thin films, present in the plasma as a consequence of high energy pulsed laser irradiation34,35,36.

Here, we investigate Ga-Sb-Se amorphous chalcogenide thin films. The thin layers were fabricated via rf magnetron co-sputtering, which allows to obtain thin films with various composition by only adjusting electrical power applied on individual cathodes. This method of thin films fabrication thus enables a cost-effective way to study compositional dependencies of material’s properties. Moreover, co-sputtering allows synthesizing amorphous thin films with compositions inaccessible as glasses in bulk form, i.e. films with compositions outside the glass-forming region of corresponding bulk counterparts. The fabricated films were characterized in detail using LA TOFMS analysis. Further, the clusters generated from parafilm coated and non-coated Ga-Sb-Se thin films were compared. For selected binary and ternary species, their structure was predicted via density functional theory calculations. The resulting information provides a better understanding of the materials fragments present in the plasma, generated via the pulsed laser irradiation of the solid-state material.

Results and Discussion

The surface characterization of the fabricated thin films was carried out by using SEM. The morphology of the thin layers surface is of good quality as shown by an example of SEM micrograph given in Supplementary Fig. S1. The chemical composition data of thin films, as obtained by EDS when averaging at three measured positions (Table 1), indicate that by using rf power 15–25 and 5–15 W for Ga2Se3 and Sb2Se3, respectively, the co-sputtered films cover a broad range of compositions: 7.6–31.7 at. % of Ga and 5.2–31.2 at. % of Sb, while the content of Se remains fairly constant (61.2–63.3 at. %) and in good accordance with nominal composition (60 at. %). XRD patterns revealed that all the deposited films are amorphous. The thickness of the thin films is in the range 560–780 nm (±2 nm).

Table 1 Thin films identification, rf sputtering power used on individual cathodes, and chemical composition of the fabricated Ga-Sb-Se thin films determined using EDS analysis (±1 at. %).
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Initially, the Ga-Sb-Se thin films of different compositions (samples labeled as A-G, Tab. 1) fabricated via rf magnetron co-sputtering were characterized by LA TOFMS in both, positive and negative ion modes. The mass spectra were recorded by systematically increasing the laser energy and the threshold energies for the ionization were determined for each sample. The ‘optimal’ mass spectra with sufficient mass resolution and the highest number of clusters were recorded at 160 and 140 a.u. for positive and negative ion modes, respectively, and used for further data processing. For the positive ion mode, the threshold energy where the ionization gets started was found to be 120 a.u., while for the negative ion mode it was 110 a.u. It was observed that for parafilm coated thin films the threshold energy was higher than for the non-coated thin films.

The effect of laser fluence was studied for all the samples (A-G) by recording mass spectra at different laser energies. As mentioned above, after reaching threshold laser energy of 120 a.u. in positive ion mode, the intensity of the mass spectra peaks climbed until 160 a.u. Afterward, the intensity decreased with further increase in laser power. This behavior generally occurs due to the decomposition of the ions at very high laser energies. The stoichiometry of the plasma species was determined by comparison of theoretical and experimental isotopic patterns (Supplementary Fig. S2). For all the thin film samples, the lowest mass ion assigned was Sb+ (quadrupole ion trap does not allow the detection of ions below m/z 100, Ga+ (69.723) and Se+ (78.96) ions maybe formed but they were not detected in the mass spectra). In addition to this one, two other unary Ga2+ and Se2+ clusters were detected. Many binary GaxSez+ and SbySez+ clusters were also identified. The comparison of mass spectra for all the samples in range m/z 100–400 is given in Fig. 1. In this m/z range of mass spectra mostly unary and binary together with few ternary clusters were detected. Qualitatively, the clusters obtained in this mass range are the same for all the samples; however, their intensities are varied according to the chemical composition of the sample. For all the samples, the lowest mass binary clusters identified (correspond to general formula GaxSez+ and SbySez+) were Ga2Se+ and SbSe+, while the highest mass clusters Ga19Se28+ and Sb5Se4+ were detected for samples A and F, respectively. It is worthy to note that no GaxSby+ species were detected. On the other hand, many ternary GaxSbySez+ clusters were observed. As examples, the F and C samples mass spectra in range m/z 250–1200 and 800–2500 are given in Figs 2 and 3, respectively. It was observed that the intensity of the peaks corresponding to the high mass clusters is lower than those detected in the low mass range m/z 100–400. The list of clusters detected by LA TOFMS in positive ion mode for all the samples is given in Supplementary Table S1. For simplicity, the number of binary GaxSez+, SbySez+, and ternary GaxSbySez+ clusters detected during LA of each thin film (parafilm coated and non-coated) sample is plotted and shown in Supplementary Fig. S3(a–c). For non-coated thin films, the highest number of GaxSez+, SbySez+, and ternary GaxSbySez+ clusters was identified for sample B, E, and F, respectively. For samples A-D, the highest intensity clusters are gallium selenide (Ga3Se+ or Ga3Se2+), while for the sample E, F, and G, the antimony selenide clusters (Sb3Se2+ or Sb3Se+) are of highest intensity. Several low-intensity binary GaxSez+, SbySez+, and ternary GaxSbySez+ clusters were also detected, but due to the insufficient mass resolution, the stoichiometry of these species was not determined.

Figure 1
figure 1

Comparison of mass spectra obtained from Ga-Sb-Se thin films in 100–400 m/z range. Conditions: positive ion mode, laser energy 160 a.u. For the sake of clarity, the only description of spectra for two measured samples is given.

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Figure 2
figure 2

Mass spectrum obtained from non-coated thin film F. Conditions: positive ion mode, laser energy 160 a.u.

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Figure 3
figure 3

Mass spectrum obtained from parafilm non-coated thin film C. Conditions: positive ion mode, laser energy 160 a.u.

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Secondly, the parafilm coated thin films were examined via LA at similar experimental conditions and the results were compared with the non-coated ones. Many new, especially high mass clusters were identified in case of parafilm coated films. The comparison of mass spectra obtained from parafilm coated and non-coated thin film (sample B) is given in Fig. 4. Generally, one can observe that in most of the cases the intensity of the peaks obtained for non-coated thin films is higher than for coated ones under identical experimental conditions. Similarly to the non-coated thin films, Sb+ is the lowest mass species identified for all the samples, while the highest mass cluster was Ga19Se28+ which is detected in samples B, D, E, and F. The highest number of GaxSez+ (23) and SbySez+ (9) clusters detected for samples B and C, E, G, respectively (Supplementary Fig S3a,b). Many new ternary GaxSbySez+ clusters were also generated from parafilm coated thin films especially for the samples C-G, which have larger content of antimony (≥17.5 at. %). The highest number of ternary clusters (38) was identified for the parafilm coated thin film G (Supplementary Fig. S3c). An important part of the mass spectrum (m/z 800–5000) obtained for parafilm coated thin film G is presented in Supplementary Fig. S4. A series of peaks such as Ga5Se7, Ga7Se10, Ga9Se13, Ga11Se16, etc. are prominently detected and the difference between these main peaks is Ga2Se3 (c.f. Fig. 4) with the largest cluster of the series as Ga19Se28+.

Figure 4
figure 4

Comparison of mass spectra obtained for sample B with and without the parafilm coating. Conditions: positive ion mode, laser energy 160 a.u.

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Considering the MS results in all m/z ranges, the highest intensity peaks for samples A and B (<17.5 at. % of Sb) are Ga3Se2+ and Ga3Se+, while for the samples C-G (≥17.5 at. % of Sb), the most intensive are Sb3Se2+ and Sb3Se+ peaks. An overview of all the species detected for coated as well as non-coated Ga-Sb-Se thin films is summarized in Supplementary Table S1. Few other clusters were also observed in MS data, but the stoichiometry of these species was not determined due to the insufficient mass resolution.

Furthermore, for both coated and non-coated Ga-Sb-Se thin films, it was found that the intensities of most of the peaks were varied according to the chemical composition of the thin films. For example, thin films with a higher amount of Ga (samples A, B) have high intensity of GaxSez+ clusters, while the intensities decreased as the amount of Sb increased (samples C-G) and vice-versa for SbySez+ clusters. To exemplify this, we have selected some clusters which are identified in all the samples and plotted their intensities versus chemical composition (Supplementary Figs S5 and S6). The intensities of the Ga2+ and Ga3Se+ species is the highest for samples A and B and then decreases for samples C-G as the amount of Sb particularly increased. In contrary, the intensities of Sb+ and SbSe+ increased; they are highest for sample G where the content of Ga is lowest (7.6 at. %) and Sb is largest (31.2 at. %). It should be noticed that no significant intensity difference was observed in the case of Se2+ and ternary GaSbSe+ clusters.

All the thin film samples (coated and non-coated) were also examined via LA TOFMS in negative ion mode; however, the spectra are not that ‘rich’ like in positive ion mode with respect to the number of species detected. The effect of laser energy was studied by recording mass spectra at different laser power. It was observed that the intensity of the peaks grew with the increased laser energy until 140 a.u.; with a further rise of laser energy, the intensity dropped. The ‘optimal’ spectra with sufficient mass resolution and the highest number of peaks were recorded at a laser energy of 140 a.u. The lowest mass species identified (at these experimental conditions) is Se2− in all the samples and the highest mass clusters detected was Ga17Se26− for parafilm coated thin film B. Considering the mass spectra in all m/z ranges, the highest intensity peak observed for all the thin films is GaSe2−. An example of a negative ion mode mass spectra in the range of m/z 100–400 for sample E is given in Fig. 5. The complete list of negatively charged clusters identified during LA of parafilm coated and uncoated Ga-Sb-Se thin films is given in Supplementary Table S2. Most of the negatively charged clusters detected are found to be different than positively charged ones. Many new and higher mass negatively charged clusters were detected for parafilm coated thin films as compared to non-coated ones. A comparison of negative ion mode mass spectra obtained from parafilm coated and non-coated thin film F is given in Supplementary Fig. S7.

Figure 5
figure 5

Mass spectrum obtained for parafilm non-coated thin film (sample E). Conditions: negative ion mode, laser energy 140 a.u.

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Effect of parafilm coatings of thin films on mass spectra

LA is basically a destructive method which may generally break the original structure of the studied materials. In the case of amorphous Ga-Sb-Se films, we identified many species generated during LA. As already reported in35,37, some of the species might originate from real glass network structure, which makes generally possible to obtain structural information for solid state materials like glasses and their thin films. In our previous study38, we have shown that by dispersing the powdered chalcogenide material into a parafilm solution or in polymers (polyvinylpyrrolidone, polyethylene glycol, etc.), resulting mass spectra contained a significantly higher number of identified species present in the plasma plume. Moreover, the parafilm came out to be the most efficient material (among investigated ones) in terms of highest number of clusters detected in mass spectra of binary arsenic chalcogenides (As2Ch3, Ch = S, Se, Te).

Therefore, in this work, we have used a parafilm to coat Ga-Sb-Se thin films. As a result, many new clusters were identified in both, positive and negative ion modes using the technique depicted above. In positive ion mode, for some thin films, more than 30 new species were detected. The largest number of new clusters (40) was identified for a film with the highest content of antimony (sample G). In negative ion mode, many new clusters were also detected with a maximum of 16 new clusters, again for film G.

One can speculate that among detected new clusters, mostly higher mass clusters can be considered as larger fragments of the original glass network structure. Therefore, one can suggest that to some extent parafilm might reduce the fragmentation of larger parts of the structures into lower fragments. As a result, some more complex structures in the form of higher mass clusters are generated when using parafilm. But, in that case, the number of low mass clusters would be decreased; nevertheless this has not been observed. Therefore, another interpretation might be that the parafilm can play some role within the reactions of highly energetic low mass species present in the plasma plume. These lower mass constituents then might undergo laser ablation synthesis (LAS) and form heavier clusters.

Both theories are equivalent for the results. Higher mass clusters are formed (i) because of lower fragmentation under the presence of parafilm or (ii) parafilm is inducing synthesis of higher mass clusters from lower ones, it is difficult to distinguish here. We have therefore studied the synthesis of binary GaxSez clusters also from the mixture of elements. The mass spectra obtained from Ga:Se mixture and Ga-Sb-Se thin films were compared (Fig. 6). Evidently, main clusters Ga5Se7+, Ga7Se10+, Ga9Se13+, Ga11Se16+, etc. are also detected during LAS from the mixture of elements but there are also many other clusters. One can speculate that the possible mechanism of the formation of the clusters might be step-by-step synthesis from low mass clusters reacting with selenium and/or gallium (Fig. 7).

Figure 6
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Comparison of clusters generated from a thin film (Sample B) with those from Ga-Se mixture. Conditions: positive ion mode, laser energy 160 and 170 a.u., respectively.

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Figure 7
figure 7

Possible mechanism of high mass GamSen clusters generation. Conditions: positive ion mode, laser energy 170 a.u. The stoichiometry of clusters is given as: Ga,Se.

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Indeed, the presence of parafilm probably leads to complex processes including several features resulting in the observation of many species identified in mass spectra. In the absence of the parafilm, some clusters may dissipate fast and are not observed, while under the presence of parafilm, they are stabilized and a higher number of clusters maybe detected. It cannot be excluded that under the presence of parafilm higher mass clusters are generated via LAS reactions from lower mass fragments.

However, there is also another possibility. As demonstrated in39,40,41, laser desorption ionization from the surface of parafilm leads to increased sensitivity. Concluding, all above depicted explanations might be possible, they can proceed simultaneously in lower or higher extent and it is difficult to decide which one prevails.

Structure of GaxSez, SbySez, and GaxSbySez clusters

TOFMS does not provide direct structural information and thus it is difficult to elucidate the structure of the generated clusters from the mass spectra itself. On the other hand, the structures of many gallium42,43,44,45, antimony46,47,48, and selenium49,50 clusters are known. Apart from the aforementioned structures, only a few structures of the binary gallium selenide and antimony selenide clusters have been reported51,52. However, there are extensive laser ablation mass spectrometric results showing that in binary systems Ga-Se and Sb-Se over one hundred GaxSez and SbySez clusters were detected53,54.

The structure of most of the clusters generated in this work was not previously reported. Moreover, the number of possible structural isomers highly increases with the growing number of atoms in the clusters.

The structural patterns for selected binary and ternary clusters generated during LA of Ga-Sb-Se amorphous thin films were computed by density functional theory (DFT) optimization. A few possible structural isomers which are in the ground state with local minima in the potential energy surface are shown in Figs 8 and 9. The structures with the lowest energy are shown here if more than one stable structures of the same stoichiometry were found. From the calculated data it was observed that for small GaxSez, SbySez, and GaxSbySez (x, y, z = 1–3) clusters, the molecular geometry does not change much with the charge. The bond lengths and angles of the structure for monocationic, neutral and monoanionic entities differ by only a few percents. As we obtained extensive data in positive ion mode during LA TOFMS, we further provided the calculated molecular data for singly charged positive clusters.

Figure 8
figure 8

Calculated structural isomers of Ga3Se+ (a), Sb3Se+ and Sb3Se2+ (b). Spin, bond energy in eV, bond lengths in pm, and angles in degree are given. Gray, silver and red balls represent gallium, antimony, and selenium atoms, respectively.

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Figure 9
figure 9

Example of calculated structural isomers of monocationic GaSbySez with y and z = 1–2. Spin, bond energy in eV, bond lengths in pm, and angles in degree are given. Gray, silver and red balls represent gallium, antimony, and selenium atoms, respectively.

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The DFT optimized structures of selected binary and ternary clusters are given in Figs 8 and 9. Optimizations were done first at the lower level of theory using LDA approximation with TZ2P basis set55, frozen core, and scalar relativistic ZORA56,57,58 methods followed by a higher level of theory with OPBE59 functional and all electron QZ4P basis sets55 with scalar relativistic ZORA approximation in ADF package60,61 (Version 2017). The unrestricted approach was used in the case of open-shell structures. The frequencies are calculated by using Gaussian software package62. The frequencies are computed from the square roots of the force constants. The optimized structures correspond to the “ground state” with local minimum in the potential energy surfaces with no imaginary frequency. The structures with the lowest energy are shown here if more than one stable structures of the same stoichiometry were found. The typical distance from adjacent gallium and selenium nuclei lies between 230–280 pm, while the distance between gallium-gallium nuclei is 270–290 pm. The distance between antimony and selenium nuclei is found between 230–260 pm and for Sb-Sb nuclei it is 260–280 pm. Detailed calculations are needed to generalize the structural features of all the generated clusters; however, this is beyond the scope of this study.

Conclusions

LA of Ga-Sb-Se amorphous thin films prepared via co-sputtering method from Ga2Se3 and Sb2Se3 polycrystalline targets generates many unary, binary and ternary clusters (Gax+/−, Sby+/−, Sez+/−, GaxSez+/−, SbySez+/−, and GaxSbySez+/−). As deposited Ga-Sb-Se thin films with seven different chemical compositions were examined and in each case, approximately 50 clusters were identified in positive ion mode. When parafilm coated Ga-Sb-Se thin films were studied, many new and high mass clusters were identified. The highest number (40) of new clusters was detected in positive ion mode for the thin film containing the largest amount of antimony. This indicates that parafilm is playing an important role during the LA process either reducing the destruction of the original amorphous network structure during the interaction of laser pulses with amorphous material or promoting the laser ablation synthesis of heavier species from low mass ones. Also, it is possible that detection from the parafilm surface is more sensitive. To get insight into the structure of the observed species, some DFT calculations were performed.

Concluding, LA TOFMS is found to be an important and useful analytical technique to study the amorphous chalcogenide thin films in terms of identification of species present in the plasma phase when the materials are exposed to the laser pulses. The stoichiometry of the species might help for partial structural characterization of thin films. Lastly, the parafilm as a material increasing the number of identified species in the plasma can be used to widen the applicability of the LA for other materials and thin films.

Experimental

Chemicals and materials

Gallium, selenium, methanol, acetone, and acetonitrile were purchased from Sigma-Aldrich (Steinheim, Germany). Polycrystalline 2″ 99.999% Ga2Se3 and Sb2Se3 sputtering targets were purchased from ALB Materials, Inc., Henderson, NV, USA. Xylene (a mixture of isomers) was purchased from Mikrochem Spol. s.r.o. (Pezinok, Slovak Republic) and parafilm from Pechiney Plastic Packaging (Chicago, IL, USA). Micro-90 (cleaning agent) was from Kratos (Manchester, UK). Silicon wafers used as substrates for the thin films depositions were purchased from ON SEMICONDUCTORS (Czech Republic). Deionized water was distilled once in glass and then double distilled from a quartz apparatus Heraeus Quarzschmelze (Hanau, Germany) to produce ultrapure water. All the other reagents were of analytical grade purity.

Methods

Ga-Sb-Se thin films

Amorphous thin films from Ga-Sb-Se system were fabricated using conventional rf (13.56 MHz) magnetron co-sputtering technique. The chemical composition of the thin films was varied by changing the rf power (up to 25 W) on the two cathodes (Table 1). The experimental conditions of the co-sputtering deposition process are described elsewhere17.

Ga:Se mixture

Gallium-selenium mixture was prepared as suspension (~1 mg/mL) in acetonitrile by mixing liquid gallium with selenium powder in a molar ratio of 1:1 and applying ultrasonication (15 min). This suspension was used for MS analysis.

Scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction

A scanning electron microscope (SEM) with an energy dispersive X-ray analyzer (EDS, TESCAN VEGA 3 EasyProbe) was used for the study of surface morphology and determination of chemical composition of fabricated materials. The uncertainty of EDS measurements for the studied films is ±1 at. %. Typically, the EDS measurements were performed at 3 spots per sample and averaged. X-ray diffraction (XRD) technique (D8-Advance diffractometer, Bruker AXS, Germany) was exploited to determine the structure of co-sputtered thin films using Bragg–Brentano θ–θ geometry with CuKα radiation and a secondary graphite monochromator. The diffraction angles were measured at room temperature from 5 to 65° (2θ) within 0.02° steps.

LA TOF mass spectrometry

The cleaning of the target plate was performed according to the Shimadzu target cleaning protocol. It includes initial cleaning with water, followed by sonication in Micro-90 cleaning agent solution for 15–20 min and then rinsing several times with water, acetone, and methanol. Finally, the target was air dried and kept overnight under vacuum in MS instrument and then used for measurements.

Thin films were fixed on the target plate using an adhesive tape. A part of the thin film was coated manually with a parafilm solution prepared by dissolving a piece of parafilm (1 cm × 1 cm) in xylene (1 ml). The thickness of the parafilm layer on studied films was approximately 100 μm. The coated and non-coated parts of the thin film were then examined using LA TOF mass spectrometry. The absorption coefficient of used parafilm at the wavelength of laser exploited for LA TOFMS (337 nm) is ~95 ± 20 cm−1 as calculated from its transmission spectrum.

AXIMA Resonance mass spectrometer from Kratos Analytical Ltd. (Manchester, UK) was used to record the mass spectra. The instrument details are given elsewhere38. Mass spectra were acquired in both, positive and negative ion modes by accumulating the data from at least 2000 laser shots. The peaks with sufficient mass resolution and intensity values higher than 1 mV (3 sigma of noise level) are considered relevant. For both modes, each m/z range was calibrated externaly using red phosphorus clusters63, while the accuracy achieved was below ±20 mDa.

Theoretical isotopic patterns were calculated using Launchpad software (Kompact version 2.9.3, 2011) from Kratos Analytical Ltd. (Manchester, UK). The examples of possible cluster structures were computed via DFT calculations using scalar relativistic ZORA approximation in ADF package (Version 2017)60,61.