Received: January 11, 2012

Advances in Natural Science: Theory & Applications Volume 1

No. 1

2012, 17-28

url: http://www.ansta.eu

AP ansta.eu

GLASS FORMATION IN THE S-Se-Me (Me = Cu, Zn, Ga, Ge, As) SYSTEMS AND PROPERTIES OF THE CHALCOGENIDE S5 Se91 Me4 GLASSES V. Vassilev1 , T. Hristova-Vasileva2 1,2

Department of Non-Ferrous Metals Metallurgy and Semiconductor Technologies University of Chemical Technology and Metallurgy 8 Kliment Ohridski Blvd., 1756, Sofia, BULGARIA 1 e-mail: [email protected]

Abstract: Chalcogenide glasses from the ternary S-Se-Me systems, where Me = Cu, Zn, Ga, Ge, As, were synthesized. The glass-transition temperatures (Tg ), densities (d) and microhardnesses (HV ) of chalcogenide glasses with composition S5 Se91 Me4 were measured. The observed peculiarities in the dependencies property-serial number (N ) of the Me in the Periodic table of elements were systematized. The specifics in the dependencies between the areas of the glassforming regions (S), Tg , d and HV on the one hand and the serial number of the Me, on the other, were analyzed. The reasons for the appearance of a maximum in the investigated dependencies at N =32 (Ge) were discussed, which is logically related to the fact, that the Ge and As chalcogenides are one of the best glassformers. Key Words: analysis

chalcogenide glasses, physicochemical properties, differential-thermal

1. Introduction The chalcogenide glasses (ChG) possess a row of unique properties and many of them are not characteristic for the crystalline semiconductors, as for example, reversible electrical switching and memory, radiation stability, photostructural transformations, simple technology for their obtaining and weak influence of the impurities on their properties, etc. The ChG find wide application in many areas of the engineering, as on their basis are produced: electrographic covers, photo- and x-ray resists, electrical switches and memory elements, optical details for the IR part of the spectra, passive and active

Received:

January 11, 2012

© 2012 Academic Publications, Ltd. url: www.acadpubl.eu

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V. Vassilev, T. Hristova-Vasileva

electronic elements, sensors and sensoric systems, building elements for the integrated optics, devices for reversible information storage with capacity of over a billion bits on one disk, etc. Every year over thousand publications appear in the periodics, devoted to the glassy semiconductor materials. The number of the studied binary and ternary chalcogenide glassforming systems is constantly increasing from 10-12 during the period of 1956-1957 up to over 300 for the period 2007-2010. The glassformation in particular groups of semiconductor systems has to be studied on the basis of parallel analysis of the glassforming regions, the phase fields into the separate subsystems and the properties of the glasses. For this purpose, a “family” of three-element (three-component) systems, obtained by consecutive substitution of one from the elements (or components) with its analogs, has to be investigated. This approach allows us to reveal the objective laws in the composition-property diagrams, which have practical value during the obtaining and the application of the glassy materials, including new ones. It is known, that when the number of the components in a given system is increased, the glassforming ability also increases, since new additional structural units are formed, which impede the formation of crystalline nuclei [1]. Usually one starts with a binary system, composed by two elements or two compounds. The S-Se system is very appropriate as a base binary system, since a wide glassforming concentration region exists in it, which spreads from 50 to 100 at. % Se [2]. The selenium and the sulphur form equilibrium melt in liquid state, built by linear polymer molecules and 8-ring-like monomers. The disordered interlace of the Se- and S-chains with the rings is stimulated by the amorphisation of the structure and by the fact, that the elements S and Se are glassformers. The aim of the present report fits into the general conception for creation of new ChG and searching of objective laws between the composition of glasses with lightly increasing molecular weight, on the one hand, and their properties, on the other. This approach guarantees the obtaining of materials with preliminary given properties. ChG from the S-Se-Me systems, where with Me are marked the metals from the Periodic table with serial number (N ) from 29 to 33, were used as a target. The areas of the glassforming regions (S), the glass-transition temperatures (Tg ), the densities (d) and the microhardnesses (HV ) were chosen for observation.

2. Glassformation in the S-Se-Me (Cu, Zn, Ga, Ge, As) systems 2.1. The S-Se-Cu system The region of glassformation in the S-Se-Cu system is situated in the Se-rich part of the Gibbs concentration triangle, lies partially in the S-Se side and is limited by points with composition Se95 Cu5 , Se68 S20 Cu12 and S50 Se50 – Fig.1,a [3, 4]. The glass-transition temperature (Tg ) of the glasses from the S-Se-Cu system is determined by DTA and depending on the composition varies between 300 and 330

GLASS FORMATION IN THE S-Se-Me (Me = Cu, Zn, Ga, Ge, As)...

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Figure 1: Regions of glassformation in the systems: a - S-Se-Cu [3, 4]; b S-Se-Zn [3]; c - S-Se-Ga [5], d - S-Se-Ge [6-8]; e - S-Se-As [9].

K. Tg increases with the increase of both Se and Cu concentrations [4]. The density (d), measured by hydrostatic method in toluene as immersion fluid, is in the interval 3.40 - 4.25 g cm−3 [4] and significantly increases with the increase of the Se content in the glasses. The Vickers microhardness (HV ) is measured at loading of 10 g and depending on the composition varies in the limits of 23 - 40 kgf mm−2 [4]. 2.2. The S-Se-Zn system The glassforming region in the S-Se-Zn system is presented on Fig. 1,b [3]. The S-Se couple dissolves up to 4 at. % Zn. The main characteristics of the ChG vary in the following limits: Tg = 330-347 K, d =3,34-3,98 g cm−3 and HV =8-19 kgf mm−2 .

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V. Vassilev, T. Hristova-Vasileva 2.3. The S-Se-Ga system

The S-Se couple dissolves up to 6 at. % Ga. The glassforming region in the S-Se-Ga system is very small (Fig. 1,c [5] and the glass-transition temperature of the ChG varies from 340 to 371 K. A trend towards decrease of the Tg at increase of the Ga content is observed. The glasses from this system are susceptible to fast crystallization, since the crystallization temperature Tcr is very near to Tg (∆T = Tcr − Tg ≈ 30 − 50 K). Obviously the addition of Ga leads to increase of the metallic character of the chemical bonds and stimulates the phase transition from amorphous to crystalline state. The density of the glasses varies from 3.54 to 4.13 g cm−3 and the microhardness – from 6 to 12 kgf mm−2 . 2.4. The S-Se-Ge system The glassformation in the S-Se Ge system is investigated by many authors, as in all cases a stepwise increase of the temperature at different heating rates and duration of the temperature steps is used. The cooling of the melt is led at different rates – from low to very high [6-8]. The glassforming region in this system, even very provisionally, can be divided to few regions depending on the synthesis conditions – Fig. 1,d. The ChG with the highest Ge content (from 37 to 45 at. %) fall into region I. Regardless of the high Ge content, the glasses are obtained at low cooling rates – at “turned-off furnace” regime. The glasses, which fall into region II, contain from 32 to 37 at. % Ge and are obtained by sharp cooling in water+ice mixture or liquid nitrogen. The region III includes ChG containing below 30-32 at. % Ge, as this region is contiguous to the binary Ge-S, Ge-Se and S-Se systems. The existence of small region (IV) in region III is determined, in which the glasses stratify. The glass-transition temperature and the microhardness of the investigated ChG from this system vary in very wide gap from 378 to 654 and from 115 to 485 kgf mm−2 , respectively [1]. The density varies from 2,48 to 4,18 g cm−3 . When the Se content is increased the density of the glasses on the GeS-Se section increases, while for the glasses on the GeSe-S section – decreases. The density increases on the GeS2 -GeSe2 section when the GeSe2 content is increased. The lowest values for d are obtained for the ChG with maximum sulphur content, i.e. for the glass with composition S80 Se10 Ge10 . 2.5. The S-Se-As system The glassformation in the S-Se-As system is investigated for the first time by Flaschen et al. [9]. The final temperature of the synthesis is within 700-1000 °C, i.e. much higher than the liquidus temperature and strongly depends on the glasses composition. In most cases the cooling of the melt is performed in “turned-off furnace”, but also cooling on air or in water+ice is possible [10]. The glassforming region is wide and lies on the three sides of the Gibbs concentration triangle – up to 50 % S on the S-Se, from 10 to 65 % As on the S-As side and from ∼40 to 100 % Se on the As-Se side, respectively - Fig. 1,e.


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The glasses from this system have low melting temperatures. The glass-transition temperature changes from 453 K (the Tg of As2 S3 ) to 308 K (the Tg of As2 S3 ) and decreases when the ratio m = [Se]/ {[S] + [Se]} is increased, i.e. the Se content in the glassy alloys. The glasses with compositions equivalent to the compositions of the chemical compounds and the glassy Se possess maximal crystallization trend. The crystallization ability increases in the row As2 S3 →AsSe→As2 S3 →Se. The density and the microhardness of the glasses varies in wide limits - from 2,40 to 4,09 g cm−3 and from 21 to 145 kgf mm−2 [1], respectively, which is related to the large glassforming area. The finding of objective laws in the properties of the ChG from the S-Se-Me systems, where Me = Cu, Zn, Ga, Ge and As (metals with serial atomic numbers from 29 to 33) is complicated due the following reasons: - for the synthesis of the samples different by volume ampoules, different stock quantity and the cooling has been led at different rates; - the systems are investigated by different methods to prove the glassy state, since the scientists have pursued different purposes; - during the selection of the compositions no united methods have been used: at some cases the compositions are with constant Me content and different [S]/[Se] ratio, at others - the [S]/[Se] ratio is constant, but the Me content changes, while at thirds the choice is absolutely random; - in one part of the cases one group of properties is investigated, in second - other properties; - in most papers no correlation between the properties and the composition has been evaluated. The knowing of the shape and the area of the glassforming region, as well as its peculiarities is very important from practical point of view, since it concentrates in compact form the glassforming ability of a given system. Due to the above-stated reasons, the glassforming regions of the five analyzed systems differ by both glassforming borders and by the areas of the regions. With the aim to precise these borders, as well as the glassforming regions of the S-Se-Zn and S-Se-Ga systems, additional samples from them are synthesized and investigated.

3. Experimental procedures The synthesis was led in evacuated quartz tubes, sealed at residual pressure of 0.133 Pa. The common method of direct synthesis from melt was used at the following conditions: dual-stage increase of the temperature up to 150 and 250 ◦ C with rate of 2-3 and 3-4◦C min-1 – for the S-Se system and three-stage – up to 250, 650 and 950◦ C with rate of 2-3, 3-4 and 2-3◦ C min−1 - for the samples from the S-Se-Zn and S-Se-Ga system. The first annealing at 150◦C (for the S-Se alloys) and at 250◦ C (for the ternary alloys) was with duration of 0.5 hours, while the second (at 650◦C) was for 2 h. At the highest synthesis temperature (250◦ C for the binary and 950◦ C for the ternary alloys) a homogenizing annealing was led with duration of 2 h at constant vibration stirring of the melt. After that the melts were quenched in water+ice mixture. As raw materials

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V. Vassilev, T. Hristova-Vasileva Table 1: Composition and condition of the synthesized samples from the S-Se, S-Se-Zn and S-Se-Ga systems No

Composition, at. %

Condition of the sample

S

Se

Zn

Ga

1

40

60

-

-

glass

2

45

55

-

-

glass

3

50

50

-

-

glass+crystal

4

60

40

-

-

crystal

5

54

45

1

-

glass+crystal

6

39

59

2

-

glass+crystal

7

19

76

5

-

glass+crystal

8

9

86

5

-

glass+crystal

9

5

91

4

-

glass

10

5

90

5

-

glass+crystal

11

2

92

4

-

glass+crystal

12

2

96

-

2

glass+crystal

13

2

97

-

1

glass

14

5

91

-

4

glass

15

40

58

-

2

glass+crystal

16

45

54

-

1

glass+crystal

for the synthesis S, Zn and Ga with purity 4N and Se with purity 5N were used. The compositions of the samples, needed for precision of the glassforming borders of the S-Se, S-Se-Zn and S-Se-Ga systems, are shown in Table 1. The synthesized samples were subjected to visual, XRD (apparatus TUR-M61, CuKα -radiation, Ni-filter, θ =5-40◦) and electron-microscopic (transmission electron microscope TEM Philips 3003) analyses. On the basis of the obtained results from these investigations a conclusion for the condition of the samples is made, which is shown in Table 1. The glass-transition temperature was determined by differential thermal analysis (DTA) using apparatus from the F.Paulik-J.Paulik-L. Erdey from the MOM-Hungary factory (weight of the sample 0.3 g, vacuumed Stepanov’s vessels, heating rate of 10◦ C min−1 and reference substance-tempered α-Al2 O3 ; accuracy - ±5◦ C). The density was determined by hydrostatic method (in toluene as immersion fluid; accuracy - ±5◦ C) and


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Figure 2: Regions of glassformation in the S-Se-Zn (a) and S-Se-Ga (b) systems. the microhardness - by Vickers method (microscope MIM-7 and microhardnessmeter PMT-3 at loading of 10 g; accuracy - ±3-5 %).

4. Results and discussion The glassy state was proved by visual analysis (unruffled surface and shell-like crack of freshly revealed surface), XRD (XRD pattern with presence of X-ray amorphous plateau and absence or weak intensity of the diffraction reflections) and SEM (smooth and homogeneous surface of the samples and lack of crystalline areas). On the basis of these results the regions of glassformation in the S-Se-Zn and S-Se-Ga systems were specified – Fig. 2,a,b. The corrections made on the outlines of the glassforming regions in the S-Se-Zn and S-Se-Ga systems were prompted mainly by the changes in the S-Se system, which is side of the Gibbs concentration triangle. The left border of glassformation in this system is shifted towards the sulphur from the point with approximate composition S35 Se65 to the point with composition S50 Se50 . Insignificant corrections were also made in both systems in the Se-rich area - Fig. 2,a,b. The dependence between the area of the glassforming regions and the serial number of the Me is shown on Fig. 3. The S-like shape of this dependence is tightly related to the specifics of the glassformers GeS2 and GeSe2 , respectively – As2 S3 and As2 Se3 . Regardless that the Se by itself is a very good glassformer, the existence of structural fragments in the glassy network, connected to Cu2 S and Cu2 Se (CuS, CuSe), ZnS (ZnSe) Ga2 S3 (Ga2 Se3 ) respectively, harshly restricts the glassformation in the first three systems (the sulphides and the selenides of these three metals have not been obtained in glassy state). On the other hand, with the addition of Cu, respectively of Zn and Ga, the metallic component of the chemical bonds increases and this worsens sharply the glassforming ability of these systems. Besides, the glassforming regions are so small, that one can hardly find correlation between their areas and the serial number of Me. On the other hand the glassforming regions of the systems, in which the Me is Ge and As, are much bigger than these with Cu, Zn and Ga, and the area of the glassforming region of the S-Se-As system is much bigger than the S-Se-Ge

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Figure 3: Dependences of the glassforming area (S) and the glass-transition temperature (Tg ) on the serial number of Me in the S-Se-Me systems

Figure 4: Dependencies of the density (d) and the microhardness (HV ) on the serial number of Me in the S-Se-Me systems

system. This is expected since on the one hand the As- and Ge-chalcogenides are better glassformers than the Se. And on the other hand, besides As2 S3 (As2 Se3 ) and GeS2 (GeSe2 ), in the S-Se-As(Ge) systems also exist AsS(AsSe) and GeS (GeSe), which are excellent glassformers too. Moreover, the fact that the AsS and AsSe are better glassformers than the GeS and GeSe must not be neglected. Two characteristic points are observed in the S(N ), Tg (N ), d(N ) and HV (N ) dependencies. At the first of them (at N = 30; Me=Zn) a stronger change in the slope of the S(N ), d(N ) and HV (N ) dependencies, while at the second (N = 32; Me=Ge) - a well expressed maximum exists in the Tg (N ), d(N ) and HV (N ) dependencies.


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The reason for the presence of the first point in the S(N ), d(N ) and HV (N ) dependencies should be searched in the peculiarities of the element Zn and more precisely in the building in mechanism of its compounds ZnSe and ZnS. The linear structural fragments of ZnSe (ZnS) built in with ease into the linear chains of Se, in which small part of the Se atoms are substituted with S atoms, while the structural fragments of Cu2 Se (Cu2 S) break and close the Se-S chains and the structural fragments Ga2 Se3 (Ga2 S3 ) – partially break and net them. The second extreme point is at the glasses from the S-Se-Ge system and seems completely logical, since the chemical bonds in these ChG are fully covalent. On the left of this maximum are the ChG containing respectively Cu, Zn and Ga, which introduce significant metallic constitute to the chemical bond, which in the final reckoning leads to worsen of the glassformation in these systems. On the right of the second maximum are situated the glasses, containing As. The significantly lower values of Tg , d and HV of the As-containing ChG than the values of the same characteristics of the Gecontaining ChG most probably is not due to the same reasons. At the lowering of the Tg it seems that limiting are the following reasons: on the one hand, this is the presence of ionic component in the chemical bond, even it does not exceeds 5-8 % and on the other hand, these are the weaker As-Se (As-S) bonds compared to the Ge-Se (Ge-S) bonds, respectively 1.8 (2.0) eV – for the first ones and 2.12 (2.40) eV – for the second ones. The influence of the structure of the glasses should also be added to the influence of these two factors: the As-containing ChG possess looser structure compared to the Ge-containing ChG, which will be discussed below. The lower values of d and HV of the As-containing ChG in comparison with the Ge-containing ChG are most probably due to the structure itself and the specifics of the structural units in these ChG, which built it. The GeS(Se)4/2 tetrahedrons in the Ge-containing glasses form denser structure, because of the presence of four bonds of Ge-atoms, which leads to smaller molar volume, respectively higher d and HV values. The AsS(Se)3/2 pyramids in the As-containing glasses contrariwise - create looser structure, since the As-atoms participate in the densification of the structure with three chemical bonds, i.e. the structure of these ChG will be characterized by larger molar volume, as a result of which the d and HV will possess smaller values. As for the path of the S(N ) dependence, it seems quite logical. The addition of Ge, respectively As into the structure of the glasses from the binary S-Se system, leads to harsh increase of the covalent bonds (the covalent content in the chemical bond of the ChG), which immediately reflects on the glassforming ability of the S-Se-ge and S-Se-As systems - the glassforming region area in these two systems sharply increases. The areas of the glassforming regions of these systems are approximately equal, since the reasons, which influence their sizes towards decrease or increase almost compensate each other. For example, if the presence of ionic content in the chemical bond of the AsChG should lead to decrease of the glassforming area, the lower “symmetry” in the near order of these glasses, i.e. the bigger chaos in them will stimulate the glassformation in the S-Se-As system, i.e. will lead to increase of the glassforming region. These two factors will act just the opposite in the S-Se-Ge system. These reasons answer to a great degree the question why the glassforming regions

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in the S-Se-Ge(As) systems are significantly larger by area than these of the S-SeCu(Zn,Ga) systems. At least one argument of considerable importance must be added, related to one of the oldest rules in the chemistry: “similar dissolves in similar”, not withstanding that its reliability does not exceed 40-45 %. In the current case it seems to have important role in the obtaining of glasses from the S-Se-Me systems. The initial components of these systems are characterized by disparate properties, typical for dielectrics (S), semiconductors (Se) and metals (Me). In conformity with the above cited rule it is completely logically the glassforming couple S-Se to dissolve insignificant quantity of the typical metals Cu, Zn and Ga, as result of which thhe glassforming regions of the S-Se-Cu(Zn,Ga) systems are small by area. The influence of the Ge and As elements is more special. The germanium and the black modification of the arsenic are typical semiconductors with band gap ∆E of 0.785 and 1.2 eV, respectively [11]. Besides, the sulphides and the selenides of the Ge and As are one of the best glassformers, due to which it is logical the glassforming regions of the S-Se-Ge(As) systems to have larger areas. When an increase of the relative content of the metal, which as a rule forms the required properties of the ChG, is desired the initial components has to be “similar” to each other, i.e. to be chemical analogues. For example, the initial components have to be binary chalcogenides or oxides, oxychalcogenides, chalcohalides, etc. This approach guarantee, at very high probability, a relatively higher content of the desired metal compared to its quantity in the glasses, obtained in three-element systems. This hypothesis is checked and confirmed during the investigation of many three-component chalcogenide systems [12-20].

5. Conclusions As a result of the comparative analysis of the specialized scientific information and the experiments led the following conclusions are made: i) The possibility for finding of objective laws between certain properties and the composition of glasses from the S-Se-me systems at substitution of the Me with element from the Periodic table with serial number (N ) from 29 to 33 is investigated; ii) A correlation between the areas (S) of the glassforming regions in the S-Se-Me systems, the glass-transitions temperatures (Tg ), the densities (d) and the microhardnesses (HV ) of glasses with composition S5Se91Me4 – on the one hand, and the serial number of Me - on the other, is established. The reasons for the appearance of a maximum in the property-composition diagrams at N =32 are discussed; iii) A new approach for the finding of ChG is proposed, which includes the usage of near by nature chemical compounds as precursors for the synthesis of glasses in binary, ternary or multicomponent systems.


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[16] V.S. Vassilev, S.V. Boycheva, P. Petkov, “Glass formation in the GeSe2 (As2 Se3 )Sb2 Se3 -CdTe”, Materials Letters, 52 (2002) 126-129 [17] V. Vassilev, S. Parvanov, T. Hristova-Vasileva, L. Aljihmani, V. Vachkov, T. Vassileva-Evtimova, “Glassformation in the As2 Te3 -As2 Se3 -SnTe system”, Mat. Letters, 61 (2007) 3676-3678 [18] T. Hristova-Vasileva, V. Vassilev, L. Aljihmani, S. Boycheva, “Glass formation in the As2 Se3 -As2 Te3 -Sb2 Te3 system”, J. Phys. Chem. Sol., 69 (2008) 2540-2543 [19] V. Vassilev, T. Hristova-Vasileva, L. Aljihmani, “Glass formation in the As2 Se3 Sb2 Te3 -CdTe system”, Chalcogenide Letters, 5, 3 (2008) 39-43 [20] V. Vassilev, I. Karadashka, S. Parvanov, system, “New chalcogenide glasses in the Ag2 Te-As2 Se3 -CdTe system”, J. Phys. Chem. Solids, 69 (2008) 1835-1840