# Structural investigation of crystallized Ge-Ga-Se chalcogenide glasses

H. Klym, O. Shpotyuk, I. Karbovnyk, Laurent Calvez, A.I. Popov

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H. Klym, O. Shpotyuk, I. Karbovnyk, Laurent Calvez, A.I. Popov. Structural investigation of crystallized Ge-Ga-Se chalcogenide glasses. 12th International Scientific Conference on Functional Materials and Nanotechnologies, FM and NT 2018, Oct 2018, Riga, Latvia. pp.12020, 10.1088/1757-899X/503/1/012020. [hal-02161311](https://hal-02161311)

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# Structural investigation of crystallized Ge-Ga-Se chalcogenide glasses

To cite this article: H Klym et al 2019 IOP Conf. Ser.: Mater. Sci. Eng. 503 012020

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# Structural investigation of crystallized Ge-Ga-Se chalcogenide glasses

H Klym¹,ª, O Shpotyuk²,³, I Karbovnyk⁴, L Calvez⁵ and A I Popov⁶,ᵇ

¹ Lviv Polytechnic National University, 12 Bandera St., Lviv 79646, Ukraine

² Vlokh Institute of Physical Optics, 23 Dragomanova St., Lviv 79005, Ukraine

³ Jan Dlugosz University in Czestochowa, 13/15, Armii Krajowej St., Czestochowa 42200, Poland

⁴ Ivan Franko National University of Lviv, 107 Tarnavskogo St., Lviv 79017, Ukraine

⁵ Equipe Verres et et Céramiques, UMR-CNRS 6226, Institute des Sciences chimiques de Rennes, Université de Rennes 1, Rennes Cedex, France

⁶ Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., Riga, LV-1063, Latvia

E-mail:ªklymha@yahoo.com, ᵇpopov@latnet.lv

**Abstract.** Crystallization transformation in the 80GeSe₂-20Ga₂Se₃ chalcogenide glasses caused by annealing at 380 °C during different duration (25, 50, 80 and 100 hours) are studied using X-ray diffraction and atomic force microscopy methods. It is established that GeGa₄Se phase of low- and high-temperature modification, Ga₂Se₃ phase (α- and γ-modification) and GeSe₂ phases are crystallized during this process. It is shown that annealing duration over 50 h does not lead to further internal structural crystallization, while annealing for 80 h result in processes of surface crystallization.

## 1. Introduction

Development of modern IR photonics brings up the challenges of searching new functional media for effective transfer of wide-spectra electro-magnetic radiation and developing of novel miniaturization technologies of passive and active photonic elements (optical waveguides, resonators, splitters, multiplexers, detectors, signal amplifiers and converters, comparators etc). Among the most promising materials for such applications are special chalcogenide glasses (ChG) – non-oxide glassy-like materials with high content of chalcogens (S, Se, Te) [1-3]. To a great extent, the further success in this field relies on chemical-technological resolutions in development of ChG-based media with unique properties [4-6].

Basic approach in development of functional materials is based on the various methods of technological and post-technological structural modification using external factors such as thermal annealing, high-energy radiation treatment or laser beams [7-9]. It is known that the nearest atomic environment in glass matrix can be adequately studied using numerous experimental methods such as vibration and Raman scattering spectroscopy, X-ray diffraction (XRD), scanning electron microscopy etc. [10].

The aim of this work is the investigation of crystallization processes and features formation of crystalline phases in 80GeSe₂-20Ga₂Se₃ chalcogenide glasses under different durations of thermal annealing above the glass transition temperature using XRD and atomic force microscopy (AFM) methods.

<figure><img src="image_3.png" /></figure>

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## 2. Experimental details

The 80GeSe₂-20Ga₂Se₃ samples were obtained from high purity (99.999%) Ge, Ga and Se components in silica ampoule kept under 10⁻⁶ vacuum. Raw materials were heated from 20 to 850 °C at 2 °C/min and maintained at the highest temperature for 12 h. Then silica tube was cooled in water at room temperature and annealed at 30 °C below the glass transition temperature $T_g$ (370 °C) for 3 h and slowly cooled down to room temperature [11]. Samples with thickness of 1 mm were polished for further investigations [9, 12-14].

The crystallization of the 80GeSe₂-20Ga₂Se₃ ChG was performed with a single step typical heat treatment at $T_g$+10 °C as described in [11,15]. This temperature has been chosen as an optimal temperature of ceramization as it permits to control the generation by simultaneous nucleation and growth of nanoparticles within the glassy matrix according to the heat treatment time. Thus, glass samples were placed in a ventilated furnace where the accuracy of temperature is ± 2 °C for various time varying from 25 to 100 h [12].

The XRD measurements (CuKα₁ radiation) were carried out to determine crystalline phases in the studied glasses. The automatic STOE STADI P diffractometer (“STOE & Cie GmbH, Germany) with a linear position-precision detector was used for XRD measurements. All measurements were conducted in 2θ-step regime, the profiles of peaks being refined using WinPLOTR software [12,15].

The surface morphology of the glasses annealed at 25 and 80 h was studied by means of Solver P47-PRO AFM, the obtained images being processed with Image Analysis program (NT-MDT).

## 3. Results and Discussion

It is possible to assume that basic transformations can be related to inwardly-phase stratification in the 80GeSe₂-20Ga₂Se₃ cut-section on individual components (stoichiometric GeSe₂ and Ga₂Se₃) which set the stage for further reaction forming of GeGa₄Se₈ triple phase [12]. This indicates that the stoichiometric 80GeSe₂-20Ga₂Se₃ cut-section creates terms for crystallization of GeGa₄Se₈ phase due to segregation of preliminary selected GeSe₂ and Ga₂Se₃ phases with additional formation of GeSe₂-enriched residues. Since the stoichiometric GeSe₂ phase is outside glass formation [12] then in future it will be formed in a separate crystalline phase.

The Ge-Ga-Se system (polythermal and isothermal cut-sections, crystal structure of intermediate phase) was studied in [16]. GeGa₄Se₈ (Ga₀.₅Ge₀.₁₃Se) phase exists in two modifications with polymorphic transformation temperature of ~898 K (625 °C). High-temperature modification is crystallized in ZnS structural type (cubic crystal system, F-43m space group, *a* ~5,45 Å), whereas for low-temperature modification (GeGa₂Se₅) structure has not been established exactly, however, lattice parameter *a* is known to be ~5,461 Å [16] indicating affinity in structures of these two modifications. It was also reported about existence of GeGa₂Se₃ but GeGaSe₃ phases [17]. Total structural relationship between these phases and binary Ga₂Se₃ GeSe₂ phases and various modifications are Ga [Se₄] and Ge [Se₄] tetrahedrons.

Combined results obtained by XRD method for 80GeSe₂-20Ga₂Se₃ chalcogenide glasses before and after thermal annealing at 380 °C for 25, 50, 80 and 100 h are shown in Figure 1. In the 80GeSe₂-20Ga₂Se₃ ChG annealed for 25 h and 50 h the well-formed peaks of GeGa₄Se₈, Ga₂Se₃ and GeSe₂ phases appear. The maximal reflection corresponding to GeSe₂ phase is semi-amorphous halo while reflections from GeGa₄Se₈ and Ga₂Se₃ are relatively well formed, especially at maximum of 2θ ~ 28.22° (Figure 1). The intensity at low diffraction angles of 2θ is shown suggesting that the fractal formation in the glass during annealing will not disappear, but become larger.

This process is accompanied by crystallization of GeGa₄Se₈ phase (in the structure of glass from chaotic arrangement of Ga [Se₄] and Ge [Se₄] tetrahedrons is characterized by only short-range order, the transition to the formation of long-range order characteristic of crystalline structure of GeGa₄Se₈ with ZnS structural type ZnS occurs).

Further increasing in annealing duration to 80 and 100 h does not affect the diffraction peaks, reflecting mainly the formation of double and triple GeGa₄Se₈ and Ga₂Se₃ phases, just as for 80GeSe₂-20Ga₂Se₃ glass, annealed for 50 h (Figure 1). Thus, further crystallization process registered by XRD and formation of long-range order during such annealing do not occur. The first maximum of semi-
amorphous halo at ~14.6 °2θ corresponds to the maximum intensity reflection (002) of GeSe₂ phase (monoclinic syngony, P2₁/c space group).

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Experimental XRD patterns for initial 80GeSe2-20Ga2Se3 glasses and after annealing for 25, 50, 80 and 100 h: comparison of experimental results with theoretical lines.</figcaption></figure>

For ChG samples annealed at high duration of 80 and 100 h the additional analysis of Ga₄GeSe₈ and Ga₂Se₃ crystallization phase was performed. Considering the state diagram for the Ga-Se system, temperature of existence of cubic Ga₂Se₃ (high-temperature Ga₀.₆₇Se) begins above T = 730 °C. Below this temperature low-temperature monoclinic modification of Ga₂Se₃ exists. Distinguishing of monoclinic and cubic modification of Ga₂Se₃ phases is possible only on samples with the high degree of crystallization.

Assuming that in studied samples pure high-temperature Ga₂Se₃ phase is crystallized, most likely this is modification of α-Ga₂Se₃ (a ~ 5.44 Å). However, taking into account the fact that the peaks are extended, along with α-modification γ-modification of Ga₂Se₃ is possible (Figure 2). Yet, one should pay attention to the raising of the background on diffraction pattern (indicated by ellipses in Figure 3).

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Comparison of experimental XRD patterns for ChG annealed for 80 h with theoretical reflexes of α-Ga₂Se₃ and γ-Ga₂Se₃ phases</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Raising of the background on experimental XRD patterns for samples annealed for 80 and 100 h.</figcaption></figure>

These raising may correspond to peaks from experimental diffraction pattern for high- and low-temperature modification of Ga₄GeSe₈ (Figure 4) but we can also assume that crystallized high-temperature α-Ga₂Se₃ phase may exist. Amorphous phase of glass is likely to be responsible for the features on the pattern in Figure 4 outlined by ellipses.
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Comparison of experimental XRD patterns for ChG annealed for 80 and 100 h with theoretical reflexes of Ga₄GeSe₈ phase low- and high-temperature modifications.</figcaption></figure>

The width of the peak at 2θ ~ 28,22° indicates the presence of nanoparticles dispersed in the form of nanocrystalline inclusions with the size of 9-10 nm (determined by the Debye-Scherrer equation [12,18]) in the crystal matrix. It should be underlined that the height of this peak in glasses annealed at 80 and 100 h does not change essentially as compared to ChG treated at 50 h. Such behavior speaks in favor of saturated crystallization at longer durations of annealing.

The maxima associated with GeSe₂ phase appear on the XRD patterns of thermally annealed 80GeSe₂-20Ga₂Se₃ glass as well [14] but (in contrast to [19]) they cannot be well distinguished as separate crystalline peaks even for prolonged annealing. It means that GeSe₂ crystals appear only in a small amount.

To better understand these processes on the surface, the glasses were examined by AFM (Figure 5).

<figure><img src="image_8.png" /><figcaption>**Figure. 5.** AFM images of unannealed 80GeSe₂-20Ga₂Se₃ glasses (a) and annealed at 380 °C for 25 h (b) and 80 h (c).</figcaption></figure>

As was shown in [15,20], that surface of base unannealed 80GeSe₂-20Ga₂Se₃ glass is mostly morphologically uniform (Figure 5, a). At various cycles of potential scan, surface irregularities and individual features appear. It can be caused by hitting with microparticles from air since microscope stand is not placed in a vacuum chamber. The elemental analysis of the surface testifies stoichiometric composition of Ge₂₃,₅Ga₁₁,₈Se₆₄,₇ glass [21-23]. However, the thermal annealing of glasses for 25 h
causes obvious changes in the surface morphology (Figure 5, b). After analysing the images, we can conclude that thermal annealing at 380 °C for small durations leads to internal transformation of free volume in the 80GeSe₂-20Ga₂Se₃ glass due to the formation of additional crystalline phase on the surface [15,20,24]. However, after longer treatment (over 50 h), surface crystallization occurs more efficiently. As shown in Figure 5, crystallization of GeSe₂ phase in samples annealed for 80 h at 380 °C is a surface phenomenon. With respect to AFM and SEM data [15], the GeSe₂ crystals in form of wires with 1-3 µm lengths are non-uniformly distributed on sample surface (Figure 5,c).

**4. Conclusions**

It is established that crystallization processes in the 80GeSe₂-20Ga₂Se₃ ChG caused by annealing at 380 °C for 25, 50, 80 and 100 h indicate the formation of GeGa₄Se (high- and low-temperature modification), Ga₂Se₃ (α- and γ-modification) and GeSe₂ crystals with size of 9-10 nm. Increasing duration of thermal treatment for 80GeSe₂-20Ga₂Se₃ glasses leads to obtaining of thermally stable glasses and glass-ceramic media. Longer annealing durations result in surface crystallization of GeSe₂ phase.

**Acknowledgment**

H. Klym thanks to the Ministry of Education and Science of Ukraine for support and Dr. P. Demchenko for the assistance in XRD experiments.

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