On possible binary molecular structure in the AsTeSe amorphous system

Journal of Non-Crystalline Solids, 15 (1974) 279-288. © North-Holland Publishing Company

ON POSSIBLE BINARY MOLECULAR STRUCTURE IN THE AsTeSe AMORPHOUS SYSTEM J.M. MACKOWSKI Institut de Physique Nucl(aire, Universit~ Claude Bernard Lyon-L 43, Bd du 11 novembre 1918, 69621 Villeurbanne, France J.J. SAMUELI Nuclgtudes Paris, France and

P. KUMURDJIAN CE.A., Montrouge, Prance Received 31 July 1973

We present here experimental results on sixty different compositions of the ternary arsenictellurium-seleniumsystem. The parameters considered here are: electrical conductivity and its thermal dependence, and transformation temperatures deduced from differential thermal analysis measurements. The synthesis of results leads to the vitrification area for glasses obtained by air quenching and gives rise to an analysis of the influence of the relative concentration of two elements with respect to the third considered as a parameter. DTA studies permit the separation of two different areas in the vitrification surface, corresponding to materials involving or not involving crystallization processes. We have investigated the possible existence of binary or ternary molecular structures in the vitrification region. The results presented here lead to the conclusion that no ternary molecular structure occurs in the As-'Fe-Se system. Nevertheless it seems possible to conclude that one binary molecular structure can be built around As:Se3.

1. Introduction Amorphous semiconductor glasses have been developed for some years and have been the subject of many papers on switching and memory effects. Pearson [1 ] investigated the first ternary system, AsTeI. Since that time, alloys based on As Te Se Ge, Si Ga P S, binary, ternary, quarternary and greater, have been prepared and used in devices involving a threshold switching. The phase-separation mechanism has been well explained (Takamori [2, 4], Savage [3]), allowing the memory effect to be

J.M. Mackowskiet aL, AsTeSeamorphoussystem

280

understood, but the switching has not yet been entirely elucidated. As2Te3-As2Se 3type glasses have been studied by Bagley [5], Kinser [6], Roilos [7], Kolomiets [8] and have been described by Dewald [9] as switching-type glasses.

2. Experimentalprocedure 2.1. Glass preparation Glasses are elaborated in a rocking furnace at 750°C by heating and mixing appropriate constituents in an evacuated silica tube. After about 12 h heating, the tubes are drawn out from the furnace and air-quenched. Starting materials present spectroscopic quality (5 N) and are conditioned in a desoxygenated and dehydrated argon atmosphere. Before sealing, pumping is carried out for 10 h at 10 -6 torr. This experiment leads to the determination of the frontiers delimiting homogeneous, vitreous and polycrystallized areas (see fig. 1), measurements are achieved with 40 g airquenched samples. The exploration step is I0 at % and this requires the study of about 60 compositions in order to cover the ternary diagram.

® []

Amorphous

(~ /~X •, ~ 10 +/_~L~90

Crystalline

22 ~ ' ~* 0

/+'.7-./\,

501 + 7\: 60

*.

9 0 ~ ' + " ~

+/~+~+-+'. ÷ . .

@

10

®)

20

,'

'

",.

30

\

". yk~)~,

\~/÷\\ \ \ 5 0

"~,s,',J

\t~\\~40

\ , . . . . .e.i W4 \ ~k%_\ \ \ ~ \ \ \ 10 (~) ' '' , 40

50

60

70

80

90

@

Fig. 1. Glass-formingregions in As Te Se system determined by air-quenching.

J.M. Mackowski et al., AsTeSe amorphous system

281

2.2. Measurement details

Flat square samples (about 1 cm 2) were ground to a thickness of about 0.5 mm for conductivity measurements. 50 mm 2 circular gold contacts were evaporated on the opposite sides of the sample. Once ohmic contacts are in place, measurements below 107 ~2 can be made with a Fairchild multimeter and above 107 ~2 with a Hewlett Packard high resistance meter. Measurements are field independent for 10-2< E < 102 V/cm, whatever the direction of the field may be. No self-heating is to be taken into account. The range of temperatures extends from 30°C to 200°C. For high resistive samples, the value of resistance at 30°C is extrapolated. (Results at 30°C are altered by a leakage current in cables and connectors.) The characteristic activation energy of the glass was derived from conductivity measurements. In the same manner, values at 30°C were extrapolated for high resistive samples. DTA is an ideal tool for investigating heat-induced transformations in the glasses. The softening point (Tg), temperature of crystallization (Tc) and temperature of fusion (Tf) were determined by using this technique. DTA measurements were achieved with a Setaram M4 microthermal analyser. The heating rate was 5°C/min, under argon pressure.

3. Results and discussion 3.1. Evidence o f structure

The molecular structure of s.c. glasses has been extensively discussed [10]. Classical investigation techniques do not work for amorphous s.c. and such structures have to be investigated through other accessible physical parameters. Therefore, we looked for known molecular compositions which could be synthetized in the Te As Se system. Molecules based on the binaries of the diagram should necessarily look like Asx Sey, Asv Tew, Seu Tez. Each point on the ternary diagram was projected on the different bases in order to make such possible structures obvious. We assume that each composition is represented in the ternary system, the third element being the diluting agent interspersed with the molecular structure. Our results were compared with published values. They are quite close to those of Mort and Davis [11] and Kinser et al. [6], as far as the resistivity and activation energy of the As2Se3-As2Te 3 pseudobinary are concerned. Kolomiets [8] published different results which have since been interpreted by Kinser as resulting from an insufficient thermal melting process.

J.M. Mackowski et aL, AsTeSe amorphous system

282 w

I

I

I

1015

1016 --

1013

10 14

E t~

~ 10 tt

0 1012

\

10 9

10 ~

10 7

10 s s] = 60

105

I06

20 I0

-103

I 0

®

I 40

I

I I 80 At;

104 0

®

Fig. 2. Variation of the electrical resistivity versus As/Te ratio for different selenium concentrations.

®

"10~030 DO ~ 20

itlJ

40

80

At

®

Fig..3. Variation of the electrical resistivity versus Se/Te ratio for different arsenic concentrations.

3.2. Electrical resisitivity As previously indicated, figs. 2, 3 and 4 show variations of resistivity versus possible binaries of the ternary mixture, the third element being the parameter. We must consider the different possibilities: As-Te, parameter Se- We investigate here the potential existence of a binary Asx Tey diluted in selenium. Fig. 2 shows that for a given concentration of Se, the resistivity decreases monotonically versus Te concentration. The more important the concentration in Se is, the more important is the resistivity. No singular point can be detected; consequently, the existence of a molecular structure built on As-Te is very unlikely. Se-Te, parameter A s - We now discuss the possible existence of binary Sex Tey diluted in As. Once more, a monotonic decrease of resistivity is reported when

J.M. Mackowski et aL, AsTeSe amorphous system

S~

AS2se 3

As 3 Se2 At ~

283

A~

Fig. 4. Variation of the electrical resisitivity versus Se/As ratio for different tellurium concentrations. tellurium is introduced for a given concentration of As (fig. 3). Many authors have considered the direct substitution Se-Te, Te taking the place of Se isomorphously. Arsenic, on the other hand, accounts for a limited number of cross-links. Our results suggest that the decreasing resistivity should be explained by increasing polymer chains, favoured by the introduction of Te which opens cycle structures. Tellurium included in chains is responsible for conductivity especially when it is placed at the end of the chains. Arsenic creates essentially cross-linker bonds, and accounts for conduction only to a small extent. A s - S e , parameter T e - The possible existence of a binary Asx Sey diluted in Te is finally investigated. This time, extrema appear on resistivity plots. First, for zero tellurium concentration, a minimum is observed on As 2 Se 3 which is a defined molecular component, along with a maximum around As 3 Se 2. Increasing the tellurium concentration up to 40% preserves the aspect of the curves but attenuates

284

J.M. Mackowski et aL, AsTeSe amorphous system

extrema. For each tellurium concentration, the resistivity decreases when the arsenic concentration increases in the glass. This can be due to the fact that long chains of Se and Te are broken by As which, on the other hand, creates more cross-linking bonds. If this is true, a reduced specific volume and an increased number of crosslinking bonds should cause glass stability to increase as the concentration in As increases. We shall check this assumption further. A defined minimum of the resistivity at As 2 Se 3 stoichiometry, whatever the Te concentration may be, does make the molecular character of glasses obvious. It is just as if the defined compound As 2 Se 3 were diluted in tellurium. When one departs from this stoichiometry, heterogeneous regions appear to be caused by excess As and Se atoms, which partially account for dilution. 3. 3. Energy o f activation

Proceeding in the same way, we observe that the activation energy (see figs. 5 - 7 ) behaves like the electric conduction. Activation energy, practically independent of As concentration (fig. 6), decreases monotonically with Te and Se concentration (fig. 5). These results agree with those of/Vlott and Davis [11 ]. Considering As-Se (fig. 7) for a given concentration in tellurium, we see that oscillating behaviours appear again as in resistivity plots. However, extrema are no longer located at the defined compounds. This can be explained by the fact that the energy of activation depends more upon traps and edge densities than upon kinds of bonds. One can also notice that the dependence of the energy of activation is valid only for concentrations in tellurium of less than 50%. AE is practically independent of the Te concentration above this value.

1

z~E (eV)

i

I

i

I

I

i

I 80

=

3

=

As

I 0

~

] l 20

=

I I 40

I 60

t

Te

Fig. 5. Variation of activation energy versus As/Te ratio for different selenium concentrations.

AM. Mackowski et al., AsTeSe amorphous system

2' ~

285

A

I

s/= 6~ [As/= 50

Se

, 0

I

,

20

I

J

40

1

J

60

I

I

Te

80

Fig. 6. Variation of activation energy versus Se/Te ratio for different arsenic concentrations.

~E (eV) 3_

~ - 0

, ~ . . ~ ~ /re]- 50

I re] - 2O [Te]= 40

i

Se ' 0

I

I I 20

I J 40

J I 60

I 80

I

As

Fig. 7. Variation of activation energy versus Se/As ratio for different tellurium concentrations.

3.4. Differential thermal analysis results DTA measurements realised on ternary Te As Se glasses produced evidence that glasses could or could not crystallise according to their composition. Complete results can be found in an unpublished report [ 12]. Here, we shall only be concerned with the results of the first endothermic transformation which characterises the vitreous transition. Tg was plotted versus different ratios (see figs. 8 - 1 0 ) . As-Te, parameter S e - Fig. 8 shows variations of T~ for constant selenium concentration versus T e - A s ratio. Tg decreases as the Te and Se concentrations increase. However, when the concentration is lower than 20%, the heavy tellurium atoms

286

J.M. Mackowski et aL, AsTeSe amorphous system

oc

I

Tg

[Se]- 20

'

I

[

I

t

'

'

oC F ' Tg

200

[ '

~ '

I

200

150

I [As] - 60

150~

100

'

50

100L

~'~ 30

As 0

t

0

L

I

20

,

r

40

,

[

J

60

I

,

Se 0

Te

80

Fig. 8. Variation of glass transition temperature versus As/Te ratio for different selenium concentrations.

°C

'

I

[

'

I

I

20

I

40

J

I

60

,

I

,

re

80

Fig. 9. Variation of glass transition temperature versus Se/Te ratio for different arsenic concentrations.

'

I

rg

200

J 0

I

'

f re ] =1o

/ ~ 2 ~ 0

_

t

Se o 0

I 20

f

r 40

J I 60

I

I

I

As

80

Fig. 10. Variation of glass transition temperature versus Se/As ratio for different tellurium concentrations. favour long-range disorder and increase the vitreous transition temperature. This result is to be compared to the results o f Das [13] who noticed an increase of Tg temperature in S e - T e binaries for weak tellurium concentrations. S e - T e , p a r a m e t e r A s - Fig. 9 shows Tg plotted versus S e - T e ratio for different concentrations o f arsenic. The temperature o f vitreous transition depends on As concentration and decreases proportionally to tellurium injection. Beyond an As concentration of 50%, the temperature o f vitreous transition becomes practically

J.M. Mackowski et al., AsTeSe amorphous system

287

independent of the T e - S e ratio. In conclusion, when the number of cros,~ lked bonds is important, the vitreous matrix is reticulated enough so that the vibration mode is not modified by the injection of heavy tellurium atoms. A s - S e , parameter T e - In fig. 10, Tg is plotted versus the As-Se ratio, [Te] being the parameter. For [Te] = 0, one finds again the characteristic As-Te binary with one extremum centered on As 2 Se 3. On the other hand, as soon as the tellurium concentration is different from zero, the Tg plots do not clearly reflect the molecular aspect which was previously observed. To explain this, it could be proposed that Tg is strongly related to cross-linking bonds, while resistivity is related to covalent bonds or dangling bonds of chains. For [Te = 0] the vitreous transition temperature increases with As concentration until As 2 Se 3 is reached. Nevertheless, beyond this concentration the vitreous transition temperature decreases as noted by Arai [14]. This could demonstrate that all arsenic atoms do not form cross-linking bonds. When one observes DTA diagrams it is clear that for As 2 Se 3 there is a true fusion at 260°C which gives evidence for molecular structure of the glass, whereas As 3 Se 2 fusion is spread along as it should be for a solid solution, probably of the form xAs2Se 3 + yAs.

4. Discussion The study of electric resisitivity results allowed us to formulate the hypothesis of a molecular structure in glasses: These assumptions suggest to us that the amorphous state consists of low range ordered regions (crystallites, molecular chains) and disordered regions. Disordered regions form high resistivity areas which strongly affect the resistivity. It is logical to think that these regions grow when moving off from stoichiometry. At stoichiometry, unlike crystalline s.c., resistivity is at a minimum. The material is then relatively homogeneous and the conductivity is high. When introducing selenium in excess, the disordered regions surrounding As2Se 3 are insulating (amorphous Se) and resistivity increases regularly up to amorphous selenium resistivity. If As is now introduced in excess, in the same manner, the appearance of disordered regions makes the resistivity increase to the resistivity of As3Se 2, which can be written x(As2Se3) + y(As). Arsenic excess is so important that crystallization occurs. This crystallization is the reason for the observed drop in resistivity beyond As3Se 2. The consideration of DTA diagrams and the different representations we plotted allow us to determine the specific role of the different elements. (a) Arsenie favours formation of cross-linking bonds and consequently helps to make the vitreous transition temperature higher. It is most likely that all atoms do not link similarly. It could be admitted that As exhibits different valence states inside the same glass, these valence states being a function of the concentration of As and of other different elements constituting the glass. (b) Se is the element of the ternary chains. The development of chains is directly

288

J.M. Mackowski et al., AsTeSe amorphous system

related to Se concentration. High resistive and strongly covalent glasses are observed, but, on the other hand, the melting temperature drops. Its action is similar to sulfur's. (c) Tellurium presents metallic character. Its action is twofold: at low concentration, it allows Se to form Sen chains from Se8 rings. As soon as the concentration increases beyond 20% it favours devitrification, the resulting glass being less stable and less resistive. (d) For zero Te concentration, the stable structure As2Se 2 identified by Uelsman [15] does not exhibit any particular behaviour. However (fig. 10) a slight maximum in the transition temperature can be noticed.

5. Conclusion Starting from our experimental results, we tried to interpret Te As Se ternary glasses structures which appear to exhibit short-range ordering. This order could consist of molecular structures embedded in a surrounding amorphous state. If they exist, these molecules should be built on the As Se binary. Starting from this hypothesis, we shall try to ascertain the existence of such a structure through specific techniques, such as X-ray diffraction techniques or the ESCA method.

Acknowledgements The authors are pleased to acknowledge the excellent technical assistance of Mrs. B. Cimma, J. Dupuis and P. Ganau and the helpful discussions with Professor J. Tousset.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10] [11] [12] [13] [14] [15]

A.D. Pearson, J. Non-Crystalline Solids 2 (1970) 1. T. Takamori, R. Roy and G.J. McCarthy, J. Appl. Phys. 42 (1971) 2577. J.A. Savage, J. Mater. Sci. 6 (1971) 964. T. Takamori et al., Mater. Res. Bull. 5 (1970) 529. B.G. Bagley, H.E. Bait, J. Non-Crystalline Solids 2 (1970) 155. D.L. Kinser, L.K. Wilson, H.R. Sanders and D.J. Hill, J. Non-Crystalline Solids 8 - 1 0 (1972) 823. M.N. Roilos, J. Non-Crystalline Solids 6 (1971) 5. B.T. Kolomiets and T.F. Mazets, J. Non-Crystalline Solids 3 (1970) 46. J.F. Dewald, A.D. Pearson, W.R. Northover and V.F. Pecic, Electrochem. Soc. Meet. Los Angeles (May 1962). J.M. Mackowski, J. Samueli and P. Kumurdjian, J. Non-Crystalline Solids 8 - 1 0 (1972) 985. N.F. Mott and E.A. Davis, Electronic Processes in Non-Crystalline Materials (Clarendon Press, Oxford, 1971). J.M. Mackowski et al., Rapport LYCEN/7238 (1972). G.C. Das and M.B. Bever, J. Non-Crystalline Solids 7 (1972) 251. K. Arai and S. Saito, Japanese J. Appl. Phys. 10 (1971) 1669. J. Uelsman, Ann. China. Phys. 116 (1966) 122.