Materials Science and Engineering, BI2 (1992) 219-222 219

Electronic switching in Ge-Bi-Se-Te glasses

Syed Rahman and G. Sivarama Sastry Physics Department, Osmania University, Hyderabad (India)

(Received December 19, 1990; in revised form August 14, 1991 )

Abstract

Thin Ge2c~BixSe7o_ ~Te m (x = 7, 9, 1 1 and 13) films were studied for their switching properties. It is shown that the switch- ing characteristics can be varied by controlling the film thickness and concentration of bismuth. It was observed that films with x >/9 exhibited electronic switching. The results were analysed on the basis of a chemically ordered network model (CONM), where the formation of a sufficient number of heteropolar bonds is favoured over the formation of homopolar bonds.

1. Introduction

It is generally observed that most atomic impurities have little influence on the electrical properties of amorphous semiconductors, including chalcogenides.

This is usually explained by Mott's 8-N rule [1], according to which the covalent bonding requirements of the impurities are satisfied and hence they do not form donors or acceptors. Chalcogenide glasses exhibit many useful electrical properties, including threshold and memory switching [2-4]. These electrical prop- erties are influenced by structural effects associated with thermal effects and can be related to thermally induced transitions [5, 6]. In chalcogenide glass sys- tems, glasses exhibiting no exothermic crystallization reaction above the glass transition temperature Tg appear to be of the threshold switching type, and the glasses exhibiting an endothermic crystallization reac- tion above Tg show a memory type of switching [7, 8]. Memory switches come from the boundaries of glass- forming regions where glasses are more prone to crys- tallization. Threshold switches are made near the centre of the glass-forming region where the glasses are stable and show no tendency to crystallize when heated or cooled slowly [9].

It is very well established that some chalcogenide glasses, when prepared in the form of thin films, exhib- ited switching properties [10, 11]; these properties can be strongly modified by making compositional changes in these materials and offer the advantage of being fabricated with relative ease. Many glasses based on the Si-Te and Ge-Te systems exhibit the switching phe- nomenon [12, 13]. In this paper we report switching properties observed in Ge-Bi-Se-Te thin films, in which the bismuth content was varied from 7 to 13 at.%.

2. Experimental details

Bulk glasses in Ge20BixSel00_xTem system (7 ~< x ~< 13) were prepared by the melt quench tech- nique. Appropriate amounts of the constituent ele- ments (99.99% pure) were weighed, and sealed in. quartz tubes evacuated to about 10-s Torr. The sealed quartz ampoules were then baked in a high tempera- ture furnace at about 1000 °C for 48 h. The ampoules were frequently rotated to obtain homogeneous mixing of the starting materials. The ampoules were then quenched in an ice-water mixture. For all samples, scooped off from different bulk ingots, the absence of any diffraction peak in the X-ray powder pattern confirmed that they were amorphous and homo- geneous. It was confirmed by the X-ray fluorescence technique that the specific chemical ratios of the con- stituents agreed with the weighed amounts of source materials. The thermal behaviour of the bulk glass sam- ples was investigated using a Du Pont 1090 differential scanning calorimeter at a heating rate of 10 °C min- Figure 1 illustrates the differential scanning calor- imetry thermogram of bulk Ge20BigSe6~Te m glass.

The bulk glass samples obtained by the above method were evaporated from a tantalum boat in a vacuum better than 10 -5 Torr onto ultrasonically cleaned glass substrates maintained at ice temperature. The composition of the films was checked using atomic absorption spectrometry and was found to be identical with that of the bulk samples within the limits of accu- racy of the analysis. The amorphous nature of the films was confirmed using a Phillips X-ray diffractometer (Fig. 2). Aluminium electrodes were deposited onto either side of the experimental film to make,an elec- trical contact. The sandwich structure of the specimen

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T~ Tc

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Fig. 1. Differential scanning calorimetry thermogram Ge20BigS% j Telo bulk glass.

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Fig. 2. X-ray diffraction of GeBiSeTe thin film.

trace from measurement o f thickness

bottom electrode

top electrode substrate

hin Layer

Fig. 3. Sandwich arrangement of the specimens for measuring the switching characteristics.

for measuring the switching characteristics is shown in Fig. 3. The current flows through the film from top to bottom. The thickness of the film was measured by multiple-beam interferometry.

3. Experimental results

The variation of current I in Ge20Bi7Se63Te10 films with the applied voltage V is shown in Fig. 4. At first log I varies linearly with log V and then becomes non- linear. At a certain voltage V B depending on the film thickness the films break down irreversibly with the emission of light. Though the wavelength of the emitted light was not measured, it was observed visually.

Figures 5 and 6 show the variation of l o g / of Ge20BigSe~tTel0 and Ge2oBi13Se57Te10 films. It is seen

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Ge20 Bi7 Se63TelO

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- 7

- 8

vst vBz %3 - -c I I I ~ l t I

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Fig. 4. Variation of log I with log V in Ge2.BiTSe~3Tejo film.

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:-2o50 1 / 1 A

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. . . . .-~ Vrl V-r2 VT3 --7 ~ I I [ I I I

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Fig. 5. Variation of log I with log V in Ge~0Bi~jS% ~Te.~ film.

that log I varies linearly with log V and the dependence fits a relation of the type I = A V a where A and a are constants. The films show switching characteristics at a certain voltage V T which is found to be dependent on the film thickness, as can be seen from Figs. 5 and 6. The value of a is approximately unity below VT, indi- cating that the behaviour is almost ohmic up to the threshold value and becomes greater than unity beyond VT.

4. Discussion

The switching phenomenon has been explained on the basis of two types of theories: (i) thermally initiated

S. Rahman and G. Sivarama Sastry / Electronic switching in glasses 221

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6

- 7

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log V (volts)

Fig. 6. Variation of log I with log V in Gez.Bi13S%7Tel. film.

[14, 15] owing to thermal instability and (ii) electroni- cally initiated [16, 17] owing to the breakdown of the electronic equilibrium as a result of applied field or current. Boer and Ovshinsky [15] have shown that the switching phenomenon is initiated by a thermal process followed by an electronic process. Buckley and Holm- berg [18] have observed threshold switching in chal- cogenide thin films in a sandwich structure and concluded that the switching phenomenon is a bulk effect.

In the present study the films were subjected to continuous voltage loading. The films with 7 at.% Bi showed irreversible breakdown after a certain voltage. However, for the films with 9, 11 and 13 at.% Bi, the log I vs. log V plots were exactly retraceable under repeated cycles of voltage loading. The films which exhibited switching only showed destructive and irre- versible breakdown when voltages well above the threshold value were applied. The breakdown voltage was nearer the threshold voltage as the film thickness decreased. The films with 9, 11 and 13 at.% Bi which showed switching also emitted light during breakdown, just as the films with 7 at.% Bi. At breakdown, the light was emitted along the plane of the film and the dura- tion of the light pulse was very short. The film remained amorphous in nature with no filament forma- tion during breakdown.

From the above results, it is observed that the addi- tion of bismuth above a certain concentration leads to the appearance of the switching phemomenon in GeBiSeTe films. The origin of switching may be either thermal or electronic. If thermal effects are causing the switching, then it is possible that bismuth may be facili- tating the process of crystallization and hence the formation of a conducting channel. For thermal switch- ing to occur, the material should have strongly temper- ature-dependent electrical conductivity. However, the results of electrical conductivity of GeBiSeTe thin films [19] indicate that an electronic mechanism is more probable. Adler and coworkers [20, 21] have proposed

that the capacity of chalcogenides to switch without immediate crystallization can be understood in terms of the electronic band structure. According to these authors, the likely stability condition for switching is that the highest occupied band in chalcogenides should result from interaction between the non-binding electrons, i.e. the lone pair of chalcogen atoms. This allows intense electronic excitation without disrupting the band structure of the materials. The results of Vezzoli [22-24] also seem to favour an electronic mechanism for switching in chalcogenides.

In Ge20Se8. glass, only Ge-Se and Se-Se bonds are assumed to be present. When bismuth is incorporated in GeSeTe glass, bismuth is expected to combine pre- ferably with selenium because the bond energy of the Bi-Se bond (40.7 kcal m o l ] ) is larger than that of the Bi-Ge bond (31 kcal mol- l ), followed by the decrease in the concentration of the Se-Se bonds [25]. Tellurium combines with germanium rather than bismuth, con- sidering that the bond energy is 37.5 kcal mol- ~ for the Ge-Te bond and 29.9 kcal mol-~ for the Bi-Te bond. The concentration of Ge-Se bonds is almost constant over the whole composition range, whereas that of the Bi-Se bonds increases and that of Se-Se bonds de- creases monotonously with increasing bismuth content up to 9 at.%, where Se-Se and Te-Te bonds vanish.

Under this reasoning, the addition of bismuth (greater than or equal to 9 at.%) in the GeBiSeTe glass system produces a large number of heteropolar bonds which contribute to the observed switching in thin films with 9-13 at.% Bi.

The pre-switching and post-switching plots are not suggestive of a thermally governed effect.

5. Conclusions

The electronic switching phenomenon was observed with emission of radiation in Ge20Bi~SeT0_ ~Tel0 (x/> 9) glassy thin films. The results were discussed on the basis of a chemically ordered network model (CONM).

Acknowledgment

One of the authors (SR) wishes to thank the CSIR, New Delhi, for providing financial assistance.

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