Memory switching of V2O5TeO2 glasses

Journal of Non-Crystalline Solids 86 (1986) 327-335 North-Holland, Amsterdam

327

M E M O R Y S W I T C H I N G OF V2Os-TeO z GLASSES

Hiroshi HIRASHIMA, Michihisa IDE and Tetsuro YOSHIDA Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, 223 Japan

A dc electric field up to about 106 V / c m was applied to V2Os-TeO 2 glass film of 2 to 20/~m in thickness, and memory switching was observed. The conductivity increased two orders of magnitude or more. Crystallization of these glasses by heat treatment also caused an increase in conductivity of similar magnitude. The threshold field of switching depended on the temperature but not on the film thickness, L, for L _< 5/~m. The dependences of the delay time on the applied field and film thickness were deviated from those predicted from electrothermal switching theory for L ~ 5 ~tm or applied fields > 105 V/cm. It was discussed that in these regions the electronic switching mechanism could be applied.

1. Introduction

Early works showed vanadium tellurite glasses were semiconducting [1], and they switched when a high electrical field was applied [2,3]. Both memory and threshold type switching were reported. Flynn et al. [4,5] and Dhawan et al. [6] reported that these glasses were highly conductive compared with vanadium phosphate glasses or other glasses containing transition metal oxides with the same amount of charge carriers. Various mechanisms, proposed for the switching of semiconducting glasses [7-10], can be divided into two groups; electrothermal and electronic mechanisms. In order to elucidate the switching mechanism, it is important to examine which initiates switching, the increase of conductivity by electrothermal heating or the increase of current by the electronic effect of a high field. The characteristics of switching are known to be influenced by several factors; pre-treatment, addition of transition metal oxides, construction of electrodes, distance between electrodes, applied voltage, ambient temperature, and so on. The electrothermal switching [11] was reported to work when the distance between electrodes was relatively large, for example, larger than 10 /~m for chalcogenide glasses [10,12-14]. Stocker et al. discussed the effects of ambient temperature, distance between electrodes and applied field [14]. The formation of a conductive channel was suggested. The electronic mechanism was suggested to work for thin films of less than several #m in thickness [12]. A single or double injection model was proposed for the electronic switching [10]. Moreover, various types of non-ohmic high field effects [15-19] must be taken into account when the switching mechanism is discussed. 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishin~ Divi~icm~

H. Hirashima et al, / Memory switching of V2Os- TeO 2 glasses

328

Switching of thick films was discussed in most of the previous studies on vanadium tellurite glasses [2,3,20-23]. The effects of microscopic inhomogeneity [20] or crystallization [3,21] on switching of these glasses were suggested. The objective of this paper is to discuss and elucidate the switching mechanism of thin films of vanadium tellurite glasses.

2. Experimental procedure 2.1. Sample preparation and dc conductivity measurement Mixtures of reagent grade V205 and TeO 2 placed in platinum crucibles were melted in an electric heating furnace at 800°C in air. The melt was poured into a stainless steel mold and annealed at 150°C for 4 h. No crystallization or phase separation was observed in the annealed glasses by X-ray diffraction and optical microscopy. The glass transformation temperature and crystallization temperatures were determined from DTA and thermal expansion curves as shown in fig. 1. The annealed glass was cut into plates of thickness about 1 mm, and dc conductivity of the glass plates was measured by the method described in a previous paper [24]. After heat treatment for crystallization, the dc conductivity of crystallized glasses was measured. The crystalline phases were identified by X-ray diffraction.

2.2. Switching Glass films of thickness about 2-20 #m were blown from the molten glass using a quartz glass tube. Because of the low viscosity of the molten glass, it was difficult to prepare glass films in a wide range of thicknesses. Thickness of glass films was measured by a dial gauge. The V - I characteristics of glass

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340

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380

Fig. 1. D T A and thermal e x p a n s i o n curves (50V20 s • 50TeO 2 glass). Tg: glass transf o r m a t i o n temperature; T~r: crystallization temperature.

H. Hirashirna et al. / Memory switching of VeOs-TeO2 glasses f

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Fig. 3. Temperature dependence of dc conductivity (o) of glass before and after heat treatment for crystallization (55V205 • 45TeO2).

films were measured using a sample holder as shown in fig. 2. The sweep rate of the applied voltage was 2-5 V / s . Silver paste was used as electrodes. The delay time of switching was measured using a digital wave memory (TCJ-2000A Transient Converter, Riken Denshi Co., Japan). The time dependence of the current was recorded at a rate of 25-500/~s/word.

3. Results and discussion

3.1. DC conductivity of glasses and crystallized glasses The chemical composition and properties of the glasses are shown in table 1. The dc conductivity of glasses depended on V205 concentration, but the change with composition was not so much as in VzOs-P205 glasses [4-6]. The crystallized glasses which were heat-treated at temperatures higher than the crystallization temperature had a higher conductivity than glass about two orders of magnitude as shown in fig. 3. The crystalline phases determined by X-ray diffraction were V205 and V205 • 2TeO 2. An example of diffraction patterns is shown in fig. 4. The dc conductivity and the activation energy of crystallized glasses were similar to those of crystalline V205 [25], indicating

330

H. Hirashima et al. / Memory switching of V2Os- TeO2 glasses

Table 1 Composition and properties of glass samples No.

Nominal composition

1 2 3

[V205]

[TeOz ]/mol%

55 50 40

45 50 60

T~/oC 1)

Tcr/OC 2)

log o"3)

W / e V 4)

228 232 242

292, 328,404 299, 339 389,430

- 3.46 - 3.58 - 3.85

0.36 0.37 0.40

1) Glass transformation temperature by dilatometry. 2) Crystallization temperature by DTA. 3) de conductivity at 150°C/ohm 1 cm-k 4) Activation energy for conduction. that the dc conductivity of crystalline V205 • 2TeO 2 might be as high as that of

V205. 3.2. V - I characteristics

A n example of V - I curves is shown in fig. 5. The threshold voltage, Vth, was determined b y the point A of the first cycle. Irreversible or m e m o r y type switching is observed. The change between A and B is very fast so that the pen working at a speed of 200 c m / s of the X - Y recorder (RW-101, Rika Denki Co., Japan) cannot trace precisely. The change between B and C was not so fast. The c o n d u c t a n c e at B, GB, was more than 102 GA, and G c was nearly equal to 2G B. The slow increase of the current after the point B could be attributed to the deposition of highly conductive crystals by Joule heating. Therefore, it is considered that the switching becomes threshold type if crystallization does not occur, for example, if the electric field is cut off at point B, or if the distance between electrodes is m u c h larger than several/~m

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2 .~ I degree Fig. 4. X-ray diffraction pattern of crystallized glass (Cu Ka, 55V205 .45TEO2). e: V205; ©: V205

"2TeO

2,

H. Hirashima et al. / Memory switching of V_,Qs- TeO 2 glasses I

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Va/vat Fig. 5. V-1 characteristics of 55V205 . 45TeO 2 glass film (L = 3/~m, 30°C).

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80

Fig. 6. Temperature dependence of the threshold voltage, Vth (55V205 • 45TeO 2 glass, L ~ 7/~m).

and the diffusion of heat becomes large as in the case of ref. [20]. If the increment of conductance during the switching is caused only by the increase of the temperature due to Joule heating, the increase in temperature estimated from the temperature dependence of dc conductivity shown in fig. 3 is about 200 ° C. This could be sufficient to enhance the crystallization, considering that the glass transformation temperature is about 220°C as shown in table 1. The threshold voltage, Vth, decreases with increasing temperature when L --- 7 # m as shown in fig. 6. This indicates that an electrothermal mechanism works. Memory switching is observed at temperatures higher than the

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Fig. 7. The relationship between the threshold field, Eth , and the interelectrode distance, L(55V205 • 45TEO2 glass, 30°C).

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Va/vott Fig. 8. The relationship between the delay time, t d, and the applied voltage, Va (55V205 • 45TeO 2 glass, L --- 5 ~tm, 25°C).

332

H. Hirashima et al. / Memory switching of V2Qs- TeO e glasses

metal-nonmetal transition point of VO 2 as shown in fig. 6, indicating that the switching of this glass has no direct relation with the transition of VO 2 as previously suggested for the switching at temperatures lower than about 340 K of vanadate glasses [22] and crystallized vanadate glasses [26,27]. The logarithm of the threshold field, Eth = V t h / L , is plotted against log L in fig. 7. In the range L >_ 5 /~m, the slope of the line is about - 0 . 8 . In the range L < 5/~m, Eth does not depend on L. It is known that when switching is thermally induced, the slope is in the range from - 0 . 5 to - 1 . 0 , while Eth is constant in the case of electronic switching [12]. Therefore, the switching mechanism may be electrothermal in the range L >_ 5/~m, and electronic in the range L _< 5 /~m. The change of switching mechanism with thickness of glass was also reported for chalcogenide glasses [12].

3.3. Delay time of switching Soon after the field was applied, a short peak of current was observed. This peak can be attributed to the charging current for the space charge layer near the interface between electrode and glass. Such a contact effect was reported for the Ag paste/V2Os-P205 glass interface [28]. The delay time, that is the time elapsing from application of the field to the start of switching, was in the range from 10 .4 to 10 -1 s. The conductance of off-state, Goff, w a s about 10 .4 to 10 .5 ohm -1 and that of on-state or memory state, Gon, was about 10 -2 to 1 0 - 3 ohm -1. The difference between Gon and Goff was about 2 orders of magnitude, which was similar to the difference of dc conductivity between the glass and the crystallized glass as shown in fig. 3. This, together with the stability of on-state, suggests that high conductance of the on-state is caused by the deposition of the crystalline phase after switching. However, the crystalline phase after switching could not be confirmed experimentally owing to the small thickness of glass between the electrodes. The delay time, to, c a n be calculated by the following equations for electrothermal increase of temperature after Stocker et al. [14]: td= toF(Va/Vc),

(1)

t o = CkTaLZ/oWV~ 2 -- C L 2 / x ,

(2)

1

2

F(V'/Vc)= fo [(V,/V~) e ~ - e x ]

-1

dx,

(3)

where V~ is the applied voltage, V~ is the critical voltage equalling Vth, C is the heat capacity of glass, k is the Boltzmann constant, Ta is the ambient temperature, L is the thickness of glass, o is the dc conductivity of glass at T~, W is the activation energy for conduction, and x is the thermal conductivity of glass. The delay time depends on V~ and is proportional to L 2 if switching is electrothermal. The relation between t d and Va is shown in fig. 8 for L --- 5 /~m and Ta -- 25°C. The calculated curve 1 is fitted using suitable values of t o and V~ according to eqs. (1)-(3). The curve fits well, showing that electrother-

H. Hirashirna et al. / Memory switching of V_,Os- TeO 2 glasses

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1.5

Fig. 9. The relationship between the delay time, td, and the interelectrode distance, L (55V205 .45TeO 2 glass, 25°C, Va --- 400 V).

mal switching takes place when Va _< 200 V. However, the dotted line fits better than curve 1 in the range V, > 300 V. Similar results deviating from the electrothermal theory for Va much higher than Vc were reported for chalcogenide glasses [14]. This shows that a simple electrothermal switching model is not valid for high applied fields, but an electronic effect should be considered. The value of V~ obtained from curve 1 is about 160 V for L --- 5 /~m, and is about twice as high as Vth obtained from the V - I curve. The value of t 0, which is about 10 -1 s for L = 5/~m, is also very large in comparison with the value of t o for chalcogenide glass, for example, t o = 5 X 10 -5 s for Ge20As30Tes0 glass of L = 6.8 /~m [14]. The reason for the difference is not yet clear. The logarithm of t d is plotted against log L for constant Va in fig. 9. In the range L >~ 5 /~m, the slope of the line is about 2 as predicted by the electrothermal mechanism (eqs. (1), (2)). However, t a decreases steeply with decreasing L for L _< 5/~m. The electronic process must play an important role in this region. It is also possible that electronic switching occurs when L is less than 5/~m as evidenced from the relationship between log Eth and log L as shown in fig. 7. Concerning the polaron hopping conduction in glasses containing transition metal oxides, non-ohmic exponential V - I characteristics were reported for the applied field higher than 105 V / c m [17-19, 29]. It was theoretically suggested that a Poole-type relation, o 0c exp[E], could be applied to polaronic hopping conduction, because the probability of hopping was increased by lowering of the potential barrier between the nearest neighbors [19]. Therefore, o is considered to depend on exp[E] equalling exp[V,/L], and then to oc 1/exp[V,] for constant L from eq. (2), when V~ becomes high. When V~ is much larger than V~, F(V,/V~) is approximately proportional to (V1/Vc) -2 [14] and t d

334

H. Hirashima et al. / Memory switching of V2Os- TeO 2 glasse~

depends on 1/Va 2 exp[V~] from eq. (1), that is log t d ~ ( V a - 2 log V~). The relation between log t a and Va is approximately linear as shown by the dotted line in fig. 8, when V~ is higher than about 300 V and E is higher than about 105 V / c m as L ~- 5 #m. It is concluded from the discussion above that a high field effect on o must be taken into account in order to elucidate the switching mechanism of V2Os-TeO 2 glass films, and the simple electrothermal switching does not take place when the thickness is less than about 5/~m or the applied field is higher than 105 V / c m .

4. Conclusion Switching characteristics and the switching mechanism of glass films of the system V205-TeO 2 were discussed. (1) Memory switching was observed. After switching, the conductance increased about 2 orders of magnitude or more. The dc conductivity of the glass also increased after crystallization by heat treatment. (2) The threshold voltage decreased with increasing temperature when L -- 7 /zm. The threshold field decreased with increasing thickness of glass in the range L >__5/~m, but it was independent of L in the range L _< 5/~m. (3) In the range L >__5/zm, delay time of switching depended o n L 2 according to the electrothermal switching theory. In the range L _< 5/zm, the dependence of the delay time on V~ and L did not obey the electrothermal theory. In this region, the electronic process was concluded to play an important role.

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