Raman evidence for the oxidation of amorphous As2(SexS1–x)3 film surfaces under visible light

Raman spectra of thermally evaporated As2Se3 and Se-rich (above 50 at. %) ternary As–Se–S films measured at high (above 1 MW cm−2) excitation power density reveal new narrow peaks emerging during the measurements which are attributed to crystalline arsenolite As2O3. The latter is formed on the As–Se–S film surface due to thermal decomposition of the film and oxidation of arsenic in ambient air. Contrary to As2S3, for which the photoassisted oxidation of the film surface requires UV light, for narrower-gap As–Se–S films this effect occurs under illumination by visible light.


Introduction
Amorphous arsenic chalcogenides are extensively studied due to a variety of photostructural effects resulting in drastic changes of their properties under illumination by light of appropriate energy, intensity, and polarization [1][2][3][4].The photoinduced effects make these materials suitable for applications in all-optical switching [5,6], waveguides [7], memory devices [8], holographic gratings [6], microstructured optical fibres [9], photoresists [10], substrates for surface-enhanced Raman scattering (SERS) [11], etc.For doped arsenic chalcogenides, illumination by above-bandgap light of sufficient intensity can lead to segregation of a crystalline phase formed by the dopant atoms and atoms of arsenic [12] or chalcogen [13][14][15] due to a photoinduced drastic decrease of the amorphous material viscosity and strongly enhanced diffusion of atoms.Photochemical reactions may occur on the surface of amorphous chalcogenide samples due to the interaction with oxygen from the ambient air (photooxidation) which is facilitated by illumination with ultraviolet (UV) light, strongly absorbed within the surface layer [16,17].For As 2 S 3 films, in particular, this was shown to result in photooxidation of the surface which was confirmed by x-ray diffraction (XRD) [16,17], energy-dispersive x-ray spectroscopy (EDX) [16,17], and x-ray photoelectron spectroscopy (XPS) [18].In our earlier study [19], we presented direct evidence for the oxidation of amorphous As 2 S 3 surface under UV laser irradiation based on Raman spectroscopy results.
UV irradiation of amorphous arsenic selenide is also known to result in the oxidation of its surface [20].Moreover, as As 2 Se 3 is a narrower-gap material compared to As 2 S 3 , oxidation of As 2 Se 3 surface was also observed upon illumination by less energetic light from the green spectral range (514.5 nm) and the formation of arsenolite crystallites was confirmed by scanning electron microscopy (SEM) and EDX [21] as well as XRD [22].
Here we intend to apply Raman spectroscopy to study possible oxidation of amorphous As 2 Se 3 and seleniumrich As 2 (Se x S 1-x ) 3 films by visible light.Raman spectroscopy is expected to serve simultaneously as a technique providing the source of irradiation by the excitation light and a means to detect the products of the oxidation reaction.

Experimental
1.2-1.5 μm thick As-Se-S films were prepared from pre-synthesized As 2 (Se x S 1-x ) 3 glasses (x = 0, 0.25, 0.5, 0.75, and 1) by thermal evaporation from a tantalum Knudsen cell at 770 °C-830 °C and 1.3 × 10 -6 mbar at a rate of 6 nm s −1 onto silicon and silicate glass substrates kept at room temperature.No post-preparation annealing of the films was performed.
To check the film uniformity and smoothness, atomic force microscopy (AFM) measurements were carried out using an Agilent AFM5420 with an additional piezoelectric scanner from nPoint Inc. and a commercial silicon NGS-01A tip from NT-MDT (curvature radius 10 nm) operating in intermittent mode.
Micro-Raman scattering measurements were carried out using a LabRAM spectrometer (Horiba Scientific).Excitation was provided by a Cobolt Fandango solid-state laser (λ exc = 514.7 nm) with a maximal output power of 50 mW and a set of filters providing attenuation down to 0.001%.The incident laser beam was focused by a 50× objective (numeric aperture 0.5), leading to the laser spot diameter of 1.3 μm on the sample surface.The scattered light was detected by a cooled CCD camera.The instrumental resolution was better than 2.5 cm -1 .The laser power density P exc was initially reduced down to values which ensured stability of the Raman spectra and integrity of the film surface in the laser spot.Afterwards P exc was gradually increased to observe the dynamics of photoinduced changes in the Raman spectra during the measurements.The details of the laser power density values are given in table S1 (Supplementary Information).For each measurement (unless intentionally planned) a new spot was chosen on the film surface in order to exclude the photoinduced changes related to previous measurements.All measurements were carried out at room temperature.
Micro-Raman spectra were measured using a Horiba XPloRa Plus spectrometer equipped with a cooled CCD camera, the excitation being provided by a Cobold 08-DPL laser (λ exc = 532 nm) with a maximal output power of 25 mW and a set of filters providing attenuation down to 0.1%.The incident laser beam was focused by a 10x objective (numeric aperture 0.25), leading to the laser spot diameter of 2.6 μm on the sample surface.This enabled a stepwise variation of the excitation power density from 0.4 to 400 kW cm −2 .

Results
AFM surface images of the As 2 (Se x S 1-x ) 3 films (an example is shown in figure 1) confirm that the prepared films have rather smooth and homogeneous surfaces.The measured AFM data were processed using Gwyddion software [23].The average surface roughness R a determined from the AFM measurements is within 0.8-1.2nm (Suplementary Information, table S2).This value slightly varies with the film composition and is in agreement with those obtained from the AFM studies of amorphous arsenic chalcogenide films prepared earlier by a similar technique [13,14,19,24].
Micro-Raman spectra of the As-Se-S films are shown in figure 2. The spectra of samples prepared on silicon and silicate glass substrates reveal no differences, except for the substrate features.The intense broad maxima observed in the spectra measured at a relatively low P exc = 100 kW cm −2 provide clear evidence for the amorphous character of the films.The spectrum of the end-point As 2 Se 3 film (figure 2(a)) is dominated by a broad asymmetric maximum near 220 cm -1 (As-Se bond stretching vibrations of corner-sharing AsSe 3 pyramidal units (commonly referred to as AsSe 3/2 ) in the amorphous film network) in good agreement with earlier data for amorphous As 2 Se 3 [13,15,[25][26][27][28].The other end-point compound, amorphous arsenic sulphide As 2 S 3 (not shown) is characterised by a broad dominating maximum near 340 cm -1 (As-S vibrations of AsS 3 pyramidal structures) and two broad weaker bands near 180 cm -1 and 230 cm -1 related to the vibrations of homopolar As-As vibrations [29][30][31][32].Vibrations of this kind were traditionally attributed to As-rich As 4 S 4 and As 4 S 3 structural units present in the amorphous film network [29][30][31], but later targeted studies showed that As 4 S 4 and As 4 S 3 units cannot be solely responsible for these bands [32] and currently it is considered that the homopolar As-As bonds are not only present in the cage-like As 4 S 4 units, but are also formed by overcoordinated As atoms in the glass network [33].Schematic diagrams of the mentioned structural units in the As 2 S 3 glass network are shown in Supplementary Information (figure S2).Note that a weak broad Raman feature with a maximum near 480 cm -1 is characteristic for both amorphous As 2 Se 3 and As 2 S 3 .For the latter it is reasonably assigned to the superposition of vibrations of S-S bonds in S n polymeric chains, S 8 rings, and disulfide bonds (S 2 As─S─S─AsS 2 ), respectively [34].Meanwhile, for As 2 Se 3 it was suggested in earlier studies to originate from second-order Raman scattering [35], although this concept is quite questionable for amorphous materials.Attempts were made to assign it to Se-Se vibrations [25], but the high frequency of the band contradicts such explanation.Therefore, in most cases authors avoid discussion of the nature of this weak band in the Raman spectra of non-crystalline As 2 Se 3 .The Raman spectra of the ternary As-Se-S films with intermediate compositions contain features typical for both end-point compounds which correspond to the vibrations of relevant structural groups.For these ternary films, the band near 225 cm -1 includes contributions from both the As 2 Se 3 -related maximum and the weaker As 2 S 3 -related maximum.Still, as the latter is less prominent, the relative intensities of the broad maxima near 225 and 340 cm -1 in the spectra of the ternary As-Se-S films correlate with the ratio of Se and S content in the film composition.Such behaviour is in agreement with earlier studies for non-crystalline As-Se-S materials [24,[38][39][40][41].A more detailed analysis of the Raman features of this amorphous ternary system was performed in our recent study [24].
When P exc increases above 1 MW cm −2 (for the ternary As-Se-S films under study with x 0. 5) and above 3 MW cm −2 (for As 2 Se 3 ), new narrow peaks near 268, 369, and 560 cm -1 emerge in the spectra.Peaks at such frequencies are characteristic for the Raman spectra of crystalline As 2 O 3 (arsenolite), for which the narrow bands of A 1 symmetry at 370-371 and 560-561 cm -1 as well as a F 2 symmetry peak at 268-269 cm -1 were reported [36,37].The spectral positions and relative intensities of the peaks are in a good agreement with those observed in figure 2. One may conclude that photoinduced oxidation of the As 2 Se 3 and As-Se-S films at sufficient intensity of the 514.7 nm laser light leads to the formation of arsenolite crystallites on the film surface.The new peaks emerge in the Raman spectra of the films under investigation within less then a minute (figure 3) which means that the photooxidation on the film surface is quite fast.As the laser beam was tightly focused during the measurement, the photoinduced changes were spatially limited to the laser spot area.Earlier we showed that similar As 2 O 3 -related features are observed in the Raman spectra of amorphous As 2 S 3 films under UV laser irradiation [19].The Raman spectra of the sulphur-rich As 2 (Se 0.25 S 0.75 ) 3 films (figure 2(d)) do not exhibit any As 2 O 3 -related peaks even at the highest power densities employed.It should be also noted that the observed sharp features cannot be related to the silicon glass or silicon substrates, the spectra of which measured at similar conditions are shown in Supplementary Information (figure S1).
The irreversible character of the local photooxidation of the film surface is confirmed by Raman measurements performed at a lower P exc = 100 kW cm −2 from the same spot after the irradiation by laser light of the same wavelength λ exc = 514.7 nm, but at a higher power density P exc = 1.4 MW cm −2 .The As 2 O 3 peaks are distinctly revealed at the background of the broad maxima of the amorphous As-Se-S film (figure 4).

Discussion
It can be seen from figure 2 that the Raman spectra of As 2 (Se x S 1-x ) 3 samples on silicon substrates measured at increasing P exc contain the Si feature at 521 cm -1 , which was not observed at lower-P exc measurements from the same spot.We estimated the absorption depth (1/α) of the λ exc = 514.7 nm laser for each alloy composition based on the available data on their absorption coefficient α [42].The corresponding data are provided in Supplementary Information (table S2).Note that the absorption depth is the length (or depth), at which the laser beam intensity does not diminish completely, but decreases by factor e ≈ 2.71828, hence the intense feature of the silicon substrate at 521 cm -1 can be revealed at thicknesses above the absorption depth, in particular, at elevated P exc values.
However, in the case of the data presented in figures 2 and 3 when the intensity of the silicon substrate peak increases with P exc much faster than the features of the film under study, a possible explanation could be the film thinning-down in the laser spot, thereby enabling the exciting laser light to penetrate through the film and reach the substrate, producing a detectable Raman signal.Indeed, at high laser power densities (P exc 600 kW cm −2 ) illumination of the As-Se-S film during the Raman measurement by the tightly focused λ exc = 514.7 nm laser beam results in a circular pit and a rim (protrusion) around it formed on the film surface that is shown in Supplementary Information (figure S3).Photoinduced mass transport upon illumination at λ exc , P exc , and duration values of the same order was earlier reported for amorphous arsenic chalcogenides, revealing the formation of similar pits in the laser spot with depth up to 500-600 nm as well as noticeable protrusions with lateral size up to several micrometers [4, 13-15, 28, 43].Despite a temptation to ascribe the drastic changes of the amorphous film surface to illumination-induced heating, the formation of the pit and the surrounding circular protrusion are generally attributed to a nonthermal mechanism related to photosoftening (photofluidisation) [1,3,4,28,43].Local structural changes in amorphous chalcogenides upon illumination are considered as relaxation events in the vicinity of the atom having absorbed a photon.Non-radiative recombination results in multiple bond rearrangement by initiation of electron-hole pairs via transient self-trapped excitons [44,45].
Although at the microscopic level the material structure is constantly changed with perpetual bond breaking and rearrangement, at the macroscopic level it is in a 'saturated' dynamic state with constant physical characteristics.A variety of acts of formation, changing, and vanishing of local and collective energy barriers occurring under illumination lead to multiple local fluidisation events [3,44,45].The photofluidisation is revealed as a drastic decrease of the material viscosity and a strongly enhanced diffusion under intense illumination.The lateral mass transport from heavily illuminated towards less illuminated areas can result in the formation of a surface relief on quite a large scale [4].In amorphous arsenic chalcogenide films doped with Group II or Group III elements the photoenhanced diffusion facilitates aggregation of the dopant atoms with the atoms of the amorphous film network and formation of II-VI or III-V nanocrystals in the illuminated area [12,14,15,28,43].However, in this case we have no intentional doping and oxygen atoms are not present (at least, in noticeable amounts) in the film composition, hence the observed formation of arsenolite crystal signatures under intense illumination should be related to a different mechanism.
The mechanism of the photoinduced formation of As 2 O 3 crystallites on an arsenic chalcogenide film surface is related to a thermal effect of illumination when most of the light is absorbed within the top layer of the film resulting in the material heating [17][18][19]21].For As 2 S 3 this effect is revealed under UV irradiation when most of the UV light is absorbed within the top 0.05 μm layer of the film and the material is heated up to 740 °C [17].Meanwhile, in our case the As 2 O 3 -related peaks are revealed at an increasing power density of the λ exc = 514.7 nm light only for the As 2 Se 3 films and ternary As-Se-S films with Se content x 0.5.These materials have much narrower bandgaps than As 2 S 3 and thereby a higher optical absorption coefficient in the green spectral range, therefore absorption of the laser light in a thin surface layer enables breakdown of bonds in the glass network and diffusion of atoms results in the formation of elemental arsenic [21] As 2 Se 3 → xAs + As 2-x Se 3 and its subsequent vaporisation and photooxidation by ambient oxygen in the presence of ambient water vapour as a catalyst 4As + 3O 2 → 2As 2 O 3 .
Eventually, the conditions for the photochemical reaction on the film surface (photooxidation) can also be dependent of the substrate characteristics.For a substrate with higher thermal conductivity one could expect the photooxidation to be less probable or achieved at higher P exc values or longer durations because of faster dissipation of the absorbed energy.
To our knowledge, this is the first evidence of the visible-light photooxidation of amorphous As 2 Se 3 as well as ternary As-Se-S films by Raman spectroscopy.Earlier, the formation of crystalline As 2 O 3 on the surface of amorphous arsenic selenide was observed by XPS [46], AFM [47], SEM and EDX [21], and x-ray diffraction [22].For ternary As-Se-S photooxidation of the film surface with the formation of arsenolite crystals was observed by SEM only under UV irradiation [48,49].
It is worth noting that photooxidation reactions on the surface of arsenic chalcogenide materials can also lead to the formation of more complicated compounds.In particular, studies of degradation of aged artwork containing orpiment (crystalline As 2 S 3 ) yellow pigment, performed using Raman spectroscopy, x-ray absorption near-edge structure (XANES), tomographic transmission x-ray microscopy (TXM), and scanning electron microscopy (SEM), confirmed not only oxidation of arsenic to As 2 O 3 , but also formation of compounds with arsenic in a higher oxidation state (+5) [50][51][52].Further oxidation of arsenic is strongly facilitated by water from the ambient air, in the presence of which arsenite AsO and H 2 AsO 4 -) ions with pentavalent As [52].
Formation of arsenates can be strongly assisted by the presence of dopants in the arsenic chalcogenide, for instance, Pb [52].Our recent Raman study of (As 1-x Bi x ) 2 S 3 glasses [53] confirmed that AsO 4 3-ions are formed under visible-light illumination only at a noticeable amount of bismuth (x 0.14) while for the glasses with lower Bi content no signs of oxidation were observed.On the other hand, it is known that doping amorphous As 2 Se 3 with metals prevents (in case of Cu dopant) or inhibits (doping with Ag) the photooxidation reactions on the surface [21].This can be related to the modification of the glass network upon doping with monovalent impurities and rearrangement of chemical bonds involving arsenic atoms which makes them more tightly bound within the network.
In our case, for amorphous As 2 Se 3 and Se-rich As-Se-S films we did not reveal any Raman features which could be responsible for the formation of arsenates, hence one can assume that, at least for the P exc range up to 3 MW cm −2 , illumination by 514.7 nm light does not lead to oxidation of arsenic on the film surface to its highest (+5) oxidation state.
We did not observe any evidence for the formation of selenium clusters upon the photodecomposition of As 2 Se 3 and Se-rich ternary As-Se-S films although such reaction could generally be assumed [21].Evidently, Se clusters could hardly be observed because the Raman frequency of the most intense peak of nanocrystalline trigonal Se (237 cm -1 [54]) falls within the range where the broad band of amorphous As 2 Se 3 is revealed (see figure 2).Neither did we observe any features of crystalline SeO 2 , which is known to be characterised by an ample set of narrow peaks from 47 to 938 cm -1 [55].Most likely, under intense illumination selenium atoms in amorphous As 2 Se 3 (together with sulphur for the Se-rich As-Se-S films) are rearranged in the amorphous film structure forming chalcogen chains and rings in addition to those which are known to exist in the amorphous chalcogenide network [26,34].For ternary As-Se-S films one should not exclude the possibility of formation of binary Se x S y compounds of various compositions which are known to possess clearly defined characteristic Raman features [56].However, comparison of the observed frequencies of the new peaks emerging in our spectra at high P exc values with the most intense features reported for Se x S y [56] did not show any coincidences.
Since the experiment was performed on air, one should also consider that intense illumination could also facilitate formation of compounds of nitrogen with arsenic, selenium or sulphur on the film surface.Arsenic nitride crystal had not been reported earlier until a quite recent study [57] where it was obtained under very high pressure and temperature conditions (300,000 times the atmospheric pressure and about 1200 °C).We believe that we can exclude such a possibility in our case.For selenium nitride Se 4 N 2 , vibrational peaks of Se-N bonds are known, based on the infrared spectra, to appear at much higher frequencies, near 800 cm -1 [58].Another selenium nitride, Se 4 N 4 , is reported to be highly explosive [59] and, therefore, could hardly be formed under illumination by a tightly focused laser beam.Neither did we find any correlation of the new peaks observed in our experiment to those reported for S 4 N -and S 3 N -ions [60] or cyclic S 4 N 3 + cations [61].Thus, in our opinion, one may conclude that at such conditions photochemical reaction of arsenic chalcogenide film with nitrogen is hardly probable.

Conclusions
Amorphous As-Se-S films of various compositions were prepared by thermal evaporation.AFM surface topography showed the film surface roughness of 0.8-1.2nm.Raman measurements performed under 514.7 nm laser excitation revealed that at increasing laser power density P exc above 1 MW cm −2 (for the Se-rich As-Se-S films under study) and above 3 MW cm −2 (for As 2 Se 3 ) new sharp intense peaks appear near 268, 369, and 560 cm -1 that were not revealed at lower P exc values.The new Raman peaks are unambiguously identified as arsenolite (As 2 O 3 ) features.Arsenolite is formed on the As 2 Se 3 and Se-rich As-Se-S film surface under intense illumination with strong absorption of the above-bandgap light by the film surface layer and thereby thermally induced decomposition of the arsenic chalcogenide in the illuminated area and oxidation of the vaporized arsenic atoms by oxygen from the ambient air.For As 2 S 3 and As-Se-S films with predominant sulphur content this effect is not revealed under 514.7 nm laser light irradiation because of broader bandgap values and lower absorption coefficients.

Figure 2 .
Figure 2. Micro-Raman spectra of an As 2 Se 3 film on a silicate glass substrate (a) as well as As 2 (Se 0.75 S 0.25 ) 3 , As 2 (Se 0.5 S 0.5 ) 3 , and As 2 (Se 0.25 S 0.75 ) 3 films on silicon substrates (b), (c), (d) measured at the excitation with λ exc = 514.7 nm at different P exc values.Asterisks mark Raman peaks related to As 2 O 3 .Vertical dotted lines correspond to the reference data [36, 37] for the frequencies of the most intense Raman peaks of As 2 O 3 .

Figure 3 .
Figure 3. Micro-Raman spectra of an As 2 (Se 0.5 S 0.5 ) 3 film on a silicon substrate, measured at the excitation with λ exc = 514.7 nm at P exc = 1.4 MW cm −2 and different acquisition times.asterisks mark raman peaks related to As 2 O 3 .

Figure 4 .
Figure 4. Micro-Raman spectrum of an As 2 (Se 0.5 S 0.5 ) 3 film on a silicon substrate, measured at the excitation with λ exc = 514.7 nm at P exc = 100 kW cm −2 after irradiation by laser light with λ exc = 514.7 nm at P exc = 1.4 MW/cm 2 for 30 s. Asterisks mark Raman peaks related to As 2 O 3 .