UDC 543.4

RAMAN SPECTROSCOPY STUDIES OF Ge –As –Se CHALCOGENIDE

GLASSES IN THE NANOPHASE SEPARATION REGION

L. O. Revutska1, a, K. V. Shportko2, O. P. Paiuk2, A. V. Stronski2, J. Baran3, A. O. Gubanova4

1National Technical University of Ukraine “Igor Sikorkiy Kyiv Polytechnical Institute”, Kyiv, Ukraine2V. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, Kyiv, Ukraine

3Institute of Low Temperatures and Structure Research, PAS, Wroclaw, Poland4Kamianets-Podilsky National University, Kamianets-Podilsky, Ukraine

AbstractIn this study the Raman spectroscopy is employed to investigate the compositional dependencies in the spectra ofGe-As-Se chalcogenide glasses with systematic increase of the Ge-content. Obtained Raman data show that studiedchalcogenides are nano-structurized materials. Raman spectra of Ge-As-Se samples showed that the backbones of thestudied samples consist of AsSe3/2 pyramidal units, edge- and corner-shared Ge(Se1/2)4 tetrahedral units. Systematiccompositional changes in studied glasses result in the evolution of the observed Raman bands. Such dependences ofcharacteristic constituent Raman bands’ intensities showed that Ge-As-Se samples contain different nanophases whoseconcentration is changing along chosen compositional cross-section.

Keywords: chalcogenide glasses, Raman spectroscopy

Introduction

Chalcogenide glasses (CG) are widely used in versa-tile technological applications such as infrared opticalelements, acousto-optic and all-optical switching de-vices, holography recording media etc. Chalcogenideglasses and films due to their unique properties: trans-parency IR region, quasi-stability, photoinduced phe-nomena, ion-conductivity of doped chalcogenide glassesand films, etc. serve as a base of their numerous applica-tions [1, 2, 3]. Chalcogenide glasses have transmissionwindows in the near and middle IR range (0.7-0.18𝜇m), excellent solubility of metals, rare-earth elements,high non-linear optical properties, which makes possiblefabrication of various optical elements, elements of in-tegrated optics, waveguides on their base for IR region,to use them in information technologies and devices ofoptoelectronics, integrated and fiber optics [2, 3].

Information on the short-range order structure ofchalcogenide glasses is particularly valuable in order toestablish useful correlations between their structuraland macroscopic properties [4]. Raman spectroscopyhas shown to be a powerful tool in studying the struc-ture of amorphous and crystalline materials [5, 6, 7].Recently, the analysis of Raman spectra of binary ar-senic sulfide chalcogenide glasses, 𝐴𝑠𝑥𝑆1−𝑥, evidencedthe presence of phase separation effects for x<0.25 [8]and the occurrence of intrinsic nanoscale phase separa-tion for x=0.4 [9]. Previous studies of 𝐺𝑒𝑥𝐴𝑠𝑦𝑆𝑒1−𝑥−𝑦

glasses employing Raman spectroscopy have been per-formed in [10, 11, 12]. Authors [10] measured Ramanspectra of amorphous Ge-As-Se and classified two dis-tinct regimes based on the dominant peaks in the Ra-

aliubov.revutska@gmail.com

man spectra: low Ge (0 ≤ 𝐺𝑒 ≤ 15) and high Ge(22 ≤ 𝐺𝑒 ≤ 39). Raman spectra of Ge-As-Se glasseswith a similar concentrations of germanium were pre-sented in [11]. Quantitative analysis of these spectrabased on their decompositions and relative ratio of thedifferent chemical bonds derived from the decomposedRaman spectra. Authors confirmed that, while the glassnetwork mainly consists of chemical-ordered 𝐺𝑒(𝑆𝑒1/2)4and 𝐴𝑠𝑆𝑒3/2 structural units, the concentrations ofthe structural units evolve with a change in Ge andAs concentrations. In [12] authors focused on glasses𝐺𝑒𝑥𝐴𝑠𝑦𝑆𝑒100−𝑥−𝑦 with 33 ≤ 𝑥 ≤ 39. The structure ofthese glasses is dominated by tetrahedral units of germa-nium. Homo-polar and semi-polar «defect» bonds, suchas Ge–Ge, Ge–As and As–As bonds, also occurred in thestructural units. It is interesting to study amorphousGe-As-Se samples that lie within the nanophase sepa-ration region. Better understanding of the mechanismof compositional nanophase evolution in Ge-As-Se sys-tem can help in the optimization of the sensitivity andrelief formation processes of composite nanomultilayerstructures based on Ge-As-Se glasses. Such compositenanomultilayer structures are perspective for the directrecording of optical elements [13, 14, 15, 16].

The aim of this study is to perform the analysis ofthe compositional evolution of Raman spectra of theamorphous chalcogenide glasses upon systematic in-crease of the Ge-content. In our study we have chosengroup of chalcogenide glasses which includes non-phase-change Ge-As-Se glasses: As2Se3, Ge0, 1As2Se2, 9 (Ge 2at.%), Ge0, 3As1, 9Se2, 8 (Ge 6 at.%), Ge0, 5As1, 8Se2, 7(Ge 10 at.%). Results of the detailed qualitative andquantitative analysis of the compositional evolution ofRaman spectra might yield the better understanding

Fig. 1. Raman spectra of amorphous Ge-As-Se: As2Se3,Ge0, 1As2Se2, 9,Ge0, 3As1, 9Se2, 8, Ge0, 5As1, 8Se2, 7(HH polarization, normalized on area).

the structure of amorphous samples by establishing theinterpretation and assignment of the Raman bands tothe vibration of the structure units.

Methods

Sample preparationChalcogenide Ge-As-Se with systematic increase of

Ge doping (As2Se3, Ge0, 1As2Se2, 9, Ge0, 3As1, 9Se2, 8,Ge0, 5As1, 8Se2, 7) samples were obtained in the formof glass. Component elements (Ge, As, Se) of 6N pu-rity were melted in evacuated (p ∼ 10−5 Torr) andsealed in silica ampoules at 800 − 850∘𝐶 for 8 hoursand subsequently quenched in air.

Raman measurementsHH- and HV-polarized Raman spectra of Ge-As-Se

glasses were measured in the spectral range from 50 to400 𝑐𝑚−1 at room temperature using a FRA-106 Ramanattachment to Bruker IFS 88 applying the diode pumpNd:YAG laser of ca. 100 𝑚𝑊 power. Obtained Ra-man spectra were analyzed in the home-made softwareCoRa [17].

Results

The HH- and HV-polarized Raman spectra of Ge-As-Se glasses presented in Fig. 1 and Fig. 2 ex-hibit main asymmetric band around 230 𝑐𝑚−1, shoul-der at 198 𝑐𝑚−1 and a wide band structure around100 𝑐𝑚−1. Observed bands in the Raman spectraof Ge-As-Se glasses can be explained in the terms ofvibrational modes As2Se3 and GeSe2 glasses studiedin [2, 18, 19, 20] The strongest band at 226 𝑐𝑚−1 inall spectra, is ascribed to AsSe3/2 pyramidal units [20].The band at 237 𝑐𝑚−1 is attributed to the As4Se3 enti-ties [20]. Additional weaker band can be revealed at 258𝑐𝑚−1, it is assigned to the presence of Se-Se bonds [18].The intensity of the less intensive peak at around 198𝑐𝑚−1 follows the increase of Ge content. Furthermore,within the main band two bands at 226 and 237 𝑐𝑚−1

can be resolved in spectra of samples with higher Gecontent. The bands around 110 𝑐𝑚−1 become visibleby gaining its intensity upon increasing the Ge doping.

Fig. 2. Raman spectra of amorphous Ge-As-Se: As2Se3,Ge0, 1As2Se2, 9, Ge0, 3As1, 9Se2, 8, Ge0, 5As1, 8Se2, 7(HV polarization, normalized on area).

Discussion of the resultsRaman spectra were fitted using a series of Gaussians

peaks with width appropriate for glasses. The peakwidths, heights and center frequencies were optimizedto fit the experimental data. The frequency assignmentsof known structural units in As-Se and Ge-Se glasseswere used to perform the peak-fitting analyses and tocompare the relative contribution of each structural unitin the spectrum of amorphous Ge-As-Se glasses. Thetentative Gaussian decomposition of obtained Ramanspectra was fitted for a quantitative analysis of theircompositional dependencies. It should be noted thatalthough we decomposed the broad bands into severalGaussian components, it is quite conceivable that alarger number of vibrational modes contribute to theoverall spectrum. Upon fitting the Raman spectra weallowed a maximum 2–3 𝑐𝑚−1 displacement of the peakfrom the position of the peaks known from the referencedata.

The broad band structure in the range of 60–180𝑐𝑚−1 was described using the set of Gaussians at 85,110, 137, 150, 157 and 188 𝑐𝑚−1 (Fig. 3). One oscillator(marked as A, Fig. 3) was used for modelling 198 𝑐𝑚−1

band in the glasses with Ge-content. To describe thestrongest broad band we used five oscillators at 213,226, 237, 245, 258 and 276 𝑐𝑚−1 (marked as B, C, D,E, F and G, Fig. 3).

The Raman spectra of As2Se3 glasses are character-ized by a strong band at 226 𝑐𝑚−1 (marked as C, Fig. 3),it is assigned to the vibrations of the AsSe3/2 pyramidalunits [20]. In the obtained spectra we have detected de-creasing intensity of this band. In the higher frequencyregion, there are two weaker bands at 245 and 258 𝑐𝑚−1

(marked as E and F, Fig. 3). The band at 245 𝑐𝑚−1 canbe assigned to the motion of As–Se bonds in As4Se4structural units. The band at 258 𝑐𝑚−1 correspondsto the Se–Se vibrations in the Se-chains. Consideringthat Ge–Ge clusters can be formed in the selenium-richglasses [21], formation of a small amount of Se-Se clus-ters in these germanium containing glasses seems tobe understandable. The lack of selenium leads to theexistence of unsatisfied Ge and As valences, which must

Fig. 3. Fit of Raman spectra Ge0, 5As1, 8Se2, 7 glasswith Gauss functions (HH polarization).

be satisfied by formation of Ge–Ge, AS–AS and/orGe-As «wrong» or «defect» bonds [12]. The bands lo-cated in the 100˘200 𝑐𝑚−1 region (Fig. 3) confirm thepresence of structural units containing As–As bondsand Ge–As vibrations [12, 19]. The band at 276 𝑐𝑚−1

(marked as G, Fig. 3) can be assigned to the vibrationalmode of –Se–Se– bridges between AsSe3/4 pyramidalunits and As4Se4 or As4Se3 cages [20]. For the sam-ple Ge0, 1As2Se2, 9 (Ge 2 at.%), the Raman spectrumshows an additional strong band located at 198 𝑐𝑚−1

(marked as A, Fig. 3). This feature follows the growthof Ge content and can be attributed to the formationof corner-shared Ge(Se1/2)4 tetrahedral units [22, 23].The band at 213 𝑐𝑚−1 (marked as B, Fig. 3) is assignedto edge-shared Ge(Se1/2)4 tetrahedral units. The peakat 237 𝑐𝑚−1 (marked as D, Fig. 3), which becomesclearly noticeable with an increase in Ge content to6 at. %, may be attributed to As4Se3 structural unitcontaining As–As homo-polar bonds, it is typical forSe-deficit glasses [20]. The 245 𝑐𝑚−1 band (markedas E, Fig. 3) may be related to the presence of As-Sebond vibrations of As4Se3 molecule [19]. A peak at 258𝑐𝑚−1 (marked as F, Fig. 3) can be attributed to theSe–Se bonds appears in bonds in free Se [11]. Composi-tional dependences of characteristic constituent bandsintensity were obtained from simulated data and arepresented in the Fig. 4 and Fig. 5. We observed increaseof the intensity of the bands modeled by oscillators A,B, D and E, as well as intensity decrease of the bandsmodelled by oscillators C and F. The behavior of theband at 245 𝑐𝑚−1 (marked as E, Fig. 3) is typical forSe-deficit structures which contain homo-polar As–Asbonds. Compositional dependences show that the inten-sities of Se-related bonds decrease with increasing Geconcentration, which qualitatively reflect the changesin the vibrational density of states of studied Ge–As–Seglasses [24]. As shown in [11], the number of Ge–Ge orAs–As homo-polar bonds increases in the samples withhigher Ge or As contents [11]. Our results show that thenumber of As–As homo-polar bonds increases with theincrease of the Ge content in the samples along the stud-ied compositional line. Thus, Raman data show thatGe-As-Se glasses contain different nanophases whose

Fig. 4. Compositional dependence of fitted Ramanspectra (HH polarization).

Fig. 5. Compositional dependence of fitted Ramanspectra (HV polarization).

concentration is changing along chosen compositionalcross-section.

For amorphous materials the depolarization ratio, 𝜌,is defined as the ratio of 𝐼𝐻𝑉 /𝐼𝑉 𝑉 [2] and ranges from0 to 0.75. Raman bands with 𝜌 approaching 0.75 aredepolarized and those for which 𝜌 < 0.70 are polar-ized. Information regarding the symmetries of the localactive vibration modes can be obtained through thedependence of the depolarization ratio on the Ramanshift.

𝐼𝐻𝑉 is the scattered intensity when the incident lightis polarized horizontal to the scattering plane and thescattered light is polarized perpendicular to the scat-tering plane. 𝐼𝑉 𝑉 is the scattered intensity when bothincident and scattered light are polarized horizontal tothe scattering plane.

Within the frames of molecular model, the weak in-termolecular interaction is supposed and 𝜌 value inthe anti-symmetrical modes range must be equal to0.75 [2, 25]. But in our case, 𝜌 value in Raman depolar-ization spectra of As2Se3 glasses is about 0.63 (withinthe area of main bands). It can be supposed that thisis connected with intermolecular interactions [2, 25].

ConclusionsCompositional dependences of Ge–As–Se glasses Ra-

man spectra indicate that intensity of the bands corre-sponding to molecular fragments with Ge-related andhomopolar As–As bonds is increased with the growth ofGe content. The intensity of the bands correspondingto presence of AsSe3/2 pyramidal units and Se–Se bondsis decreased with higher Ge content.

Thus, Raman data show that Ge–As–Se glasses con-tain different nanophases whose concentration is chang-ing along chosen compositional cross-section.

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