Mobility of Ag ions in AgAsS glasses

Pergamon

Solid State Communications, Vol. 92, No.ll, pp. 895-tdgX.1994 Elsevier Science Ltd Printed in Great Britain {M}3X-11}9Xt)94){10657- 1 tX}3X-109X/94 $7.00+.00 MOBILITY OF Ag IONS IN Ag-As-S GLASSES Y. Miyamoto, M. Itoh and K. Tanaka Department of Applied Physics, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received 4th August 1994 by A. Okiji)

The drift mobility and the carrier density of Ag ions in Ag-As-S glasses with the Ag content of 15 - 45 at.% have been measured over a temperature range of 20 - 100 "C using a time-of-flight method. With an increase in the Ag content, the mobility increases exponentially, while the carrier density remains nearly constant. These two results are in quantitative agreement with electrical conductivity data. The observations are discussed in light of structural models.

The mobility of Ag ions was measured at 20 - 100 °C by using a time-of-flight technique schematically illustrated in Fig.2 [14,15]. In this technique, which may be referred to as a voltage-reversal method [16], the sample is first polarized by applying a d c electric field. As a consequence, mobile Ag ions will accumulate near the negative electrode. On reversing the polarity, the accumulated Ag ions will move toward the opposite electrode, and the transient ionic current l(t) can be detected in an external circuit. When the Ag ions reach the opposite electrode, the ionic current decreases. The time when the decrease begins can be defined as the transit time t,. The mobility # of Ag ions may then be calculated from t, as

It is known that chalcogenide glasses containing a large amount of metals such as Ag and Li exhibit remarkable ionic conduction [1,2,3]. In some cases, the glass has a higher conductivity than the crystal, exhibiting the so-called super-ionic conduction [4]. In other cases, the chalcogenide glasses possess the electron(hole)ion mixed-conduction [5], reflecting smaller bandgap energies than ,,~ 2 eV. These materials may therefore be promising to ionic and photo-electro-ionic applications [6,7], nonetheless fundamental properties of the ionic transport have not necessarily been understood satisfactorily [2,3,8]. One of the reasons may be due to ambiguons knowledge of glassy structures [3,8]. In addition, we should note that most of previous studies have dealt with the ionic conductivity [1,4], and investigations of the mobility and the carrier density ate rather limited. To understand ionic transports in Ag-chalcogenide glasses in more detail, the mobility and the carrier density in Ag-As-S glasses are studied for the first time. It is known that Ag-As-S glasses with the Ag content higher than 15 at.% exhibit the mixed ion-hole conduction, in which the ionic conductivity is higher by 1 - 4 orders of magnitude than the hole conductivity [5,9]. It may also be worth mentioning that the materials exhibit interesting photoinduced phenomena accompanying Ag migra, tion, while the mechanisms remain speculative [7,10]. Samples investigated were melt-quenched bulk glasses with the compositions Ag2S-As2Ss, in which the Ag content was 15, 25 and 35 at.%, and Ag~sAslsS40. These compositions are indicated in Fig. 1 together with glassforming regions reported by three different groups [11, 12,13], which appear to be strongly dependent upon preparation procedures. The melt-quenched ingots were polished using alumina powder into thin flakes with a thickness d of 150 #m. Then, Ag coplanar electrodes with a width W of 1 mm and a gap distance L of 0.5 mm were deposited onto the polished surfaces by vacuum evaporation. In some compositions, samples with a gap distance reduced to 0.15 mm were also examined to confirm the reliability of obtained data.

~,

= L2/v.

t,,

(I)

where V is an applied voltage, which is fixed at O.1 V, unless otherwise specified. On the other hand, the mobile carrier density n may be estimated from 1

1

/o" l(t)dt,

(2)

where q is assumed to be the unit charge. Figure 2 shows transient ionic currents typical of the Ag-As-S glasses investigated. The current shows an initial rise, followed with a relatively-constant flow, which is terminated with a rapid decrease. The current level at the constant flow largely depends on the sample composition and temperature, ranging around 10 nA - 50 ~A. When the voltage is reversed again, similar current shapes have been observed. However, after several reversal cycles, the sample tends to show no signals, which may be caused by some kinds of phase separation. It may be interesting to note here that the ionic current flow appears to be non-dispersive in a similar way to that observed in the electronic transport in amorphous Se and hydrogenated Si films [17,18]. Figure 3 shows the compositional dependence of # and n in Ag-As-S glasses as a function of the Ag con895

896

MOBILITY OF Ag IONS

Ag

As

S

As253

Vol. 92, No. I I

the present values are consistent with the electrical conductivity. Figure 4 compares a and qnl~. Tile conductivity data are cited from two different sources [1,9]. We see that a and qny show a satisfactory agreement in absolute magnitude, which may imply reliability of the present results. However, qn# appears to exhibit stronger temperature dependence than a, and the reason remains to be studied. The composition dependence shown in Fig.3 manifests two interesting features. First, the mobility obtained here is higher by three orders of magnitude than that calculated from the diffusion coefficient D using Einstein's relation. For instance, in Ag.~sAs.~sS~0,the time-of-flight mobility at room temperature is 10-6 cm2/Vs, while the mobility derived from the diffusion coefficient is 10-s cm~/Vs [19]. Thus, it may be thought that the Haven ratio f , which is de-

fined as, Fig. 1. The compositions (solid circles) investigated in the present study and glass-forming regions in the AgAs-S ternary system. Solid, dotted and dashed lines are, respectively, due to Refs. [11], [12] and [13]. tent. Some results obtained through the voltage reversals of several times and using a few samples are plotted in order to show the degree of data scattering, which seems inevitable at present. It is mentioned here that, in Ag2sAs2sSs0 at room temperature, # and n have not changed when V is varied at 0.1 - 1.0 V. We see in Fig.3 that, with an increase in the Ag content, # increases exponentially, while n appears to be almost constant. It is also seen that # and n are thermally activated, the features implying Arrhenius-type dependences. The activation energies of the two quantities can be evaluated as 0.4 + 0.2 eV, irrespective of the Ag content. The fairly-large experimental error is mainly due to the scattered data. Since the electrical conductivity a can be written down as a = qnla, it seems important to examine if

is 10-a. The magnitude is substantially smaller than that (0.3 - 0.4} previously reported for glassy samples [3]. We should note, however, that in most of previous studies/J is estimated from a through some assumptions, and the procedure remains to be re-examined. Second, the ratio of the mobile ion density n to the density of Ag atoms contained in the bulk glasses is, for instance, 10-3 at room temperature. The ratio appears to be quite small, and the reason is worth to be considered. Structural studies suggest that most of Ag atoms in Ag-As-S glasses appear to be tightly bonded

Ha

v:lO0"C o :80"C

~ o

O

o

I

I

0

,,

\

"~ ~.~.

5

I

g

~ i0I~

~"

It

3 ~; tm~e (hour)

B

o: R.T.

rl

2

zg

~:s0"c

,,o-e.,\ 1

(3)

f = Dql#kT,

6

"18

Fig. 2. Tt:,nsient currents in an Ag2sAs2sSs0 sample at some temperatures indicated, t, denotes tile transit time. A schematic illustration of the time-of-flight method is also provided.

80

o

1's ~

~'s Ls

1018 Ag content (at.%)

Fig. 3. The mobility and the ionic carrier density in Ag-As-S gla~ses as functions of the Ag content and temperature. Tile scattering of the data are due to sampleto-sample variations and repeated measurements using single specimens.

Vol. 92, No. ll

MOBILITY OF Ag IONS

897

Fig. 4. Electrical conductivities in Ag-As-S glasses calculated from the present mobility-density data (symbols) and those obtained from dc (solid lines) [1] and ac (dashed lines) [9] measurements. The present samples are obtained from the same ingots with those employed in the ac measurenmnts. The sample composition is expressed with the Ag content.

account upon the glass-forming regions of chalcogenidc glasses, pres,mes that Ag atoms are cov,'dently bonded with four S atoms [21]. Taking these structural ideas into account, wc may assnme that most of Ag atoms cannot movc ea.~ily, and instead only a few (,-- I019 cm -3) interstitial-like Ag atoms, which cannot be detected with structurM experiments, can drift through the covalentlybonded gla.~sy network. Then, the ratio of 10-3 can be understood a.~ representing the number ratio between the defective and the bonded Ag atoms. This speculation may also provide additional explanation for the small f number. That is, in diffusion measurements, thermally-activated motion of the threeor four-fold coordinated Ag ato,ns can be seen, since the measure,neat monitors diffused atomic profiles. It seems difficult to detect the fast motion of a small number of Ag ions, because of limited sensitivity of profile monitoring. In contrast, electrical measurements coukl be sensitive to the fast motion of the low-coordinated ions. In s,mmary, we have measured the transient ionic currents in Ag-As-S glasses using the time-of-flight technique. The mobility is higher than that inferred from the diffusion coefficient by three orders of magnitude, while the mobile ion density is substantially smaller titan the Ag atomic density. Further studies concerning electrical aatd thermal activations are now in progress.

and hardly to move. For instance, an extended-X-rayabsorption-fine-structure study of Ag-As2S3 glasses suggests that Ag atoms are bonded with three S atoms [20]. The formal-valence-shell model, which can provide an

The authors would like to thank Dr. T. I{awaguchi for providing Ag4sAstsS40 samples. The present work is partially supported by Izumi Science and Technology Foundation.

10-2 i(3-3

I0-/. { 10-5

10-7 v :A915

10-8

26

3.0 ~0~/T(K -I)

34

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in Non-Crystalline Maleriais 2nd ed., Clarendon, Oxford, (1979).

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