Identification of amorphous zones in the GeTeSb system

Materials Science and Engineering. A 132 ( 1991 ) 273-276

273

Identification of amorphous zones in the GeTeSb system P. Lebaudy, J. M. Saiter, J. Grenet, M. Belhadji* and C. Vautier Facult~ des Sciences de Rouen, Laboratoire L.E. ('.A.P., B.P. 118, 76134 Mont-Saint-A ignan Cedex (France)

(ReceivedMarch 5. 1990; in revisedform July 17.1990)

Abstract We studied the phase diagram of the Ge-Te-Sb ternary system in order to establish the different compositions which lead to an amorphous material. The system was investigated by differential thermal analysis, differential scanning calorimetry and by X-ray diffraction at fixed temperature. We have determined the eutectic compositions of the system and tested the ability of these compositions to give an amorphous material. Only a small range of compositions, around the Ge~sTe82Sb3 eutectic compound, gives an amorphous material after quenching from the melt; this range of composition is widened when a vapour deposition technique is used. One of these compositions has two glass transition temperatures and two crystallization peaks, showing the existence of two amorphous states in the sample.

1. Introduction Chalcogenide glasses and their alloys are of great interest because of their possible use in the preparation of both electrical and optical storage memories [1-8]. Nevertheless, a material suitable for competitive industrial production of such memories has not yet been found. The major reason is the complexity of the problem, for example in G e - T e - A s - S i and Ge-Te-AI systems, as Boonell and coworkers [3, 9] and Thomas et al. [4] have shown, the electric properties are influenced by thermal and structural effects. In the same way, the Ge40Se60_xTe~ system exhibits a difference in conductivity of three orders of magnitude between the thin film and bulk forms [5, 6]. In alloys such as SixTe,~0_ x [7] and Ge~Te,~0_, [8], a threshold switch effect is observed, and in A s - T e - G e glasses [10], a memory switch effect is observed. The main optical properties used in chalcogenide optical memories is the optical contrast in transmission and/or reflexion between two phases of the material. Very often the contrast appears between the amorphous and the corresponding crystalline form as for the Ge-Te sys*Permanent address: Universit6 ES-SENIA, Oran, Algdrie. 0921-5093/91/$3.50

tem for which the values of this contrast are higher than 200% in transmission and 40% in reflexion [8]. In all the systems studied, the common feature is the presence of a phase separation in the amorphous chalcogenide alloys during annealing. The first observation of this phase separation, was made by calorimetric and optical microscopy studies in the P b - S e - A s - G e system [1 1] and then in the Se-Bi and As3SbH~Se8 systems by Meyers and Bekes [12]. The double glass transition temperatures, which are characteristic of phase separation, are often found in chalcogenide glass systems including tellurium, such as Te-Se-Ge [13], A123Te77 [14], Ge20Tes0 and Si2,,Tes0 [15]. This is the major reason for which the chemical aspects of glass formation of telluride systems have been widely studied [8, 16]. For Ge-Te alloys, two glass transition temperatures (Tgl, Tg2)and two crystallization temperatures ( Tc~, To2) are generally observed in the sequence Tgl< T~l< Tg2< Tc2 [7]. This is explained by the rejection of germanium atoms into the surrounding glass during the primary crystallization; the composition of the alloy therefore changes during heating, and the glass transition temperature shifts from TgI to Tg2. The crystallization of the remaining amorphous matrix is observed at Tc2. During the secondary © ElsevierSequoia/Printed in The Netherlands

274

crystallization phase, nucleation takes place preferentially on the grain boundaries of tellurium crystallites still present. In order to have a demixing system made of two tellurium-based amorphous phases, we introduced a third element into the Ge-Te alloys. We chose antimony because of its metallic character which is expected to enhance the formation of the two demixed phases. As eutectic compositions are prone to give amorphous materials [8], in order to determine these invariant compositions, a systematic study of the Ge-Te-Sb ternary system was made using differential thermal analysis (DTA) measurements. Then we tried to vitrify the alloys around the eutectic compositions in two different ways: by cold water quenching from the liquid state to obtain bulk samples, and by flash deposition to obtain thin solid films.

2. Experimental details About 100 samples of Ge-Te-Sb alloys of different compositions (the values are reported in Fig. 1 ) were studied. Each sample of the expected composition was obtained from a mixture of the three elements (99.999% purity) in a granular form. The mixture was introduced into a quartz ampoule and sealed in a vacuum of 10 -3 Torr. Then the ampoules were placed in a horizontally rotating oven and annealed at 1000 °C for 3 h. Finally, they were air quenched. The composition of the bulk alloys obtained after quenching were checked by atomic absorption (Perkin-Elmer 2380). The melting temperatures were measured

by DTA, and the glass transition and recrystallization temperatures were measured by differential scanning calorimetry (DSC, Perkin-Elmer system 4). The amorphicity, the number of crystalline phases and their distribution were checked at room temperature by X-ray diffraction using a Seeman-Bohlin chamber.

3. Binary systems Before studying the ternary phase diagram it is necessary to give the main characteristics of the different binary systems. The Ge-Sb system is characterized by a eutectic reaction (El) at 590°C for a composition of 17 at.% Ge [17]. In the Ge-Te system, a definite compound GesoTes0 (C1) with congruent melting temperature at 725 °C and two eutectic reactions, at 725 °C for 49.85 at.% Te (E2) and at 385 °C for 85 at.% Te (E3), are observed [17]. Finally, the Sb-Te system exhibits a congruent melting compound Sb2Te 3 at 622 °C (C2) which divides the system into two parts. The tellurium-rich zone is characterized by an invariant at 424 °C and 89 at.% Te (eutectic reaction E4), and the antimony-rich zone by another invariant at 540 °C corresponding to the eutectic E5 with 29 at.% Te [17]. The GeTe and Sb2Te3 compositions are the only congruent melting compounds of these binary systems. The system along the line joining these two compounds was also examined. Our results are shown in Fig. 2 (full points), along with those obtained by Legendre et al. [18] and Abri-

T

°c

700 l 600 , ~ 500

400

-C ~B

i I

3OO Sb

Ge

Fig. 1. o, location in the G e - T e - S b ternary system of the compositions studied in order to establish the phase diagram; *, location of the compositions tested in order to obtain an amorphous material.

o Sb2Te 3

lb

2b at%

3'o Ge

4b

50 Ge Te

Fig. 2. Phase diagram of the pseudobinary GeTe-Sb2Te 3 system: - - , refs. 18-20; e, our results. Composition A, Ge2Sb2Tes; B, GeSbSFea C GeSb4Te7 .

275

kosov and Danilova-Dobryakova [19, 20] (full line). These different results are in good agreement and confirm the validity of our experimental method. This pseudobinary system exhibits three compounds with incongruent melting, GeTe4Sb 7, GeTe4Sb _, and Ge2TesSb 2 which decompose at 606, 616 and 630 °C respectively, and a eutectic reaction between SbzTe 3 and GeTevSb4 at 595 °C.

4. Liquidus surface Figures 3(a) and 3(b) show the liquidus surface and the results obtained at room temperature from X-ray analysis. Two ternary eutectic reactions are observed for the Tex2Gel~Sb3 (E6)and Te2~GesSbc,4 (E7) compounds at 390 and 530 °C respectively. We should point out that the points A1 and A2 in Fig. 3(a), marking the interception of eutectic valleys, do not represent ternary eutectic reactions, and neither E6 nor E7 can be considered as deep eutectic valleys.

(a)

re

Sb2~Te3 Sb

In order to obtain amorphous materials from the liquid state, we used an alternative method of preparation. The ingot previously obtained by the method described in Section 2 was ground down to a fine powder. The powder was placed in a capillary tube, sealed under vacuum, annealed up to 1000 °C and quenched in cold water. For the vapour deposition method, we used a flash technique, in which the powder falls at a constant rate from a vibrating spoon onto a tantalum heater kept at 1000 °C. In Fig. 1 we showed the location of the compositions tested. The results reported in Table 1 show the compositions for which we obtained a bulk amorphous material. For an increasing quantity of antimony (1-5 at.%) and a constant quantity of tellurium (82 at.%) we observe that the values of the glass transition temperatures decrease while the crystallization temperatures increase. For 6 at.% Sb, the material is partially crystallized (the crystallization enthalpy AHc= 2.4 J g i is very small compared with that measured for the other compositions AH c = 40 J g ~) while for a greater quantity of antimony the material is completely crystallized. These results can be compared with those obtained for another ternary system, Ge-Te-As [ 10, 21 ]. Indeed, both systems show major similarities in their respective phase diagrams. However, in the case of Ge-Te-As, the composition range which leads to an amorphous material is greater than that observed for the TABLE 1

Ge

Glass transition 7[, and crystallization T. temperatures for different compo~tions of G e - T e - S b alloy,~ Composition (at.°/,,)

(b)

Sb2~Te

/b Sb

5. Amorphous zones: results and discussion

Ge

Fig. 3. (a) Liquidus surface of the G e - T e - S b system (temperature in degrees celsius); (b) crystalline phases at room temperature obtained by X-ray diffraction.

Ge

Te

5 13 14 l0 12

86 84.5 84.5 83 82

13 14 15 16 17 8

82 82 82 82 82 8ll

Transition temperature °C/

Sb 9 2.5 1.5 7 6 5 4 3 2 1 12

G

7~

r,

75

108 ll6

157 164

181 196

120

168

181

113 115 119 123 127

165 174 185 193 200

195

Notes

Crystallized

123

Crystallized Partially crystallized

CrystaUized

The temperatures were measured by DSC (heating rate 10°Cmin ~).

276 30

i Tc, ¢15 E

rc2

TG2

b

A o

90

130

170 tem peratu°c re 210

250

Fig. 4. (a) DSC curve obtained for GezsTe~2Sb~ (heating rate 10°Cmin-~), the curve shows clearly the two peaks of crystallization; (b) expanded view of the Tgzone.

result supposes the existence of two amorphous forms in the bulk material. Further experiments on this amorphous material are now in progress. The areas of compositions for which amorphous alloys were obtained are shown in Fig. 5. For bulk materials, only neighbouring compositions around the eutectic E6 lead to an amorphous material, while no amorphous phase was found for compositions near the second eutectic E7. For thin films this area is weakly extended, and a second zone of amorphization is observed around the binary eutecfic E4 of the Sb-Te system.

References 1 J. Colmenero and J. M. Barandiaran, J. Non-Cryst. Solids, 30(1979) 263. 2 J. P. de Neufville, in J. Stuke and W. Brenig (eds.), Proc.

Te

Int. Conf. on Amorphous and Liquid Semiconductors, 3 4

Sb2Te~3 ~~

" ~Ge

Te

Fig. 5. Glass-forming region in the Ge-Te-Sb system: e water quenching; n extending by vapour deposition.

5 6 7 8 9 10

Ge-Te-Sb system. This agrees with the good glass~forming nature of arsenic, and the poor glass-forming nature of antimony when these elements are included in chalcogenide alloys [22]. Moreover, one glass transition temperature and two crystallization temperatures are observed for alloys containing a small quantity of antimony and a large quantity of tellurium (84.5 at.%). In this case we could suppose that the influence of tellurium is predominant and the results can be compared with those obtained for A1-Te [14], Si-Te and Ge-Te [15]. The first peak of crystallization is caused by a precipitation of tellurium during the first stage of crystallization. We should note the composition Ge15Tes2Sb 3 for which two glass transition temperatures Tg1= 119 °C, Tg2 = 123 °C and two crystallization peaks T~l = 185 °C and To2= 199°C are observed with the sequence T~l < T~2< Tel < T~2 (Fig. 4). This

11 12 13 14 15 16 17 18 19 20 21 22

Taylor and Francis, London, 1974, p. 135. J. R. Boonell and C. B. Thomas, Solid State Electron., 15 (1972) 1261. C. B. Thomas, A. F. Fray and J. Boonell, Philos. Mag., 26 (1972)617. H. Kahnt and A. Feltz, J. Non-Cryst. Solids, 86 (1986) 41. H. Kahnt and A. Feltz, J. Non-Cryst. Solids, 86 (1986) 33. S. Asokan, G. Parthasarathy and E. S. R. Gopal, J. NonCryst. Solids, 86 (1986) 48. J. Cornet, Ann. Chim., 10 (1975) 239. J. R. Boonell and J. A. Savage, J. Mater. Sci., 7 (1972) 1235. K. Tanaka, Y. Okada, M. Sugi, S. Azima and M. Kiruchi, J. Non-Cryst. Solids, 12 (1973) 100. C.T. Moynihan and P. B. Macedo, J. Non-Cryst. Solids, 6 (1971)322. M. B. Myers and J. S. Bekes, J. Non-Cryst. Solids, 8-10 (1972) 809. D. S. Sarrach and J. P. de Neufville, J. Non-Cryst. Solids, 22(1976) 245. G. Parthasarathy, A. K. Bandyopadhyay, E. S. R. Gopal and G. N. Subbana, J. Mater. Sci. Lett., 3 (1984) 97. S. Asokan, G. Parthasarathy and E. S. R. Gopal, J. Mater Sci. Lett., 4 (1985) 502. J.P. de Neufville, J. Non-Cryst. Solids, 8-10 (1972) 85. M. Hansen, Constitution of Binary Alloy, McGraw-Hill, New York, 1958, pp. 773,776, 1177. B. Legendre, C. Hancheng, S. Bordas and M. T. Clavaguera-Mora, Thermochim. Acta, 78 (1984) 14. N. K. Abrikosov and G. T. Danilova-Dobryakova, lzv. Akad. Nauk. SSSR, Neorg. Mat., 1 (1965) 204. N. K. Abrikosov and G. T. Danilova-Dobryakova, lzv. Akad. Nauk. SSSR, Neorg. Mat., 6 (1970) 475. Z. U. Borisova, Glassy Semiconductors, Plenum Press, New York, 1981, p. 394. A. Winter, VerresR~fract., 36 (1982) 259.