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IL NUOVO CIMENTO VOL. 19 D, N. 5 Maggio 1997

Dielectric relaxation in glassy Se

_{80 2x}

Te

20

Sb

x

(*)

D. K. GOEL(1), C. P. SINGH(1) and A. KUMAR(2) (1_{) Department of Physics, V.S.S.D. College - Kanpur, India} (2_{) Department of Physics, H.B.T.I. - Kanpur, India}

(ricevuto il 26 Settembre 1995; revisionato il 10 Gennaio 1997; approvato il 27 Gennaio 1997)

Summary. — Frequency and temperature dependences of dielectric constant (e 8)

and dielectric loss (e 9) are studied for glassy Se80 2xTe20Sbxalloys where 0 GxG20.

The measurements are made in the audio-frequency range (1 kHz to 10 kHz) and in the temperature range ( 30 7C to 60 7C). The results indicate that the dielectric dispersion occurs in the above frequency and temperature range. An analysis of the results shows that the dielectric losses are dipolar in nature and can be understood in terms of hopping of charge carriers over a potential barrier. The increase of dielectric loss with Sb concentration can be understood in terms of increased defects due to Sb incorporation as also evidenced by other measurements.

PACS 72.22.Gm – Dielectric loss and relaxation. PACS 72.80.Ng – Disordered solids.

1. – Introduction

Se-Te alloys have gained much importance because of their higher photosensitivity, greater hardness, higher crystallization temperature, and smaller aging effects as compared to pure Se. The addition of a third element in binary chalcogenide glasses is found to be useful in obtaining stable glassy alloys due to cross-linked structure, thus increasing the glass transition and crystallization temperatures. The effects of incorporation of Sb on the electrical properties of glassy Se-Te alloys have been studied by various workers [1-5]. In general, it is observed that the d.c. conductivity increases, the activation energy for d.c. conduction decreases, the thermoelectric power decreases, and the photoconductive decay becomes slower on incorporation of Sb to the binary Se80Te20 alloy. To explain the above results it is generally assumed that the addition of Sb in the Se-Te system leads to a cross-linking of the Se-Te chains which enhances the disorder in the system and hence leads to a deeper penetration of the localized states into the energy gap. As the dielectric relaxation studies are important to understand the nature and the origin of dielectric losses which, in turn, may be

(*) The authors of this paper have agreed to not receive the proofs for correction.

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D.K.GOEL,C.P.SINGHandA.KUMAR 706

Fig. 1. – Temperature dependence of dielectric constant and dielectric loss in glassy Se80Te20.

useful in the determination of structure and the defects in solids, the present paper reports the dielectric relaxation studies in glassy Se_{80 2x}Te20Sbx.

Temperature and frequency dependences of dielectric constant (e 8) and dielectric loss (e 9) are studied in the frequency range 1 kHz to 10 kHz and temperature range 30 7C to 607C. The results indicate that dielectric dispersion does exist in the present alloys, dispersion being stronger at higher concentration of Sb. The variation of dielectric parameters with the percentage of Sb in Se-Te-Sb system is discussed in terms of the increased defects in these alloys.

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DIELECTRIC RELAXATION IN GLASSYSe_{80 2x}Te20Sbx 707

Fig. 2. – Same as fig. 1 but for Se75Te20Sb5.

2. – Experimental

Glassy alloys of Se_{80 2x}Te20Sbx ( 0 GxG20) were prepared by a quenching technique. 5 N purity materials were weighed in accordance with their atomic percentages and then sealed in quartz ampoules (length A 5 cm, internal diameter A 8 mm) in a vacuum A 1025Torr. The sealed ampoules were kept inside a furnace where they were rocked frequently at 600 7C for 10 h. Quenching was done by dropping the ampoules in ice cooled water and the glassy nature was verified by X-ray diffraction.

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The glassy alloys thus prepared were ground to a fine powder and pellets (diameter A 6 mm and thickness A 1 mm) were obtained by compressing the powder in a die at a load of 3–4 tons.

For the dielectric measurements, the pellets were mounted in between two steel electrodes of a metallic sample holder. The temperature was measured by mounting a thermocouple very near to the sample. A vacuum A 1023_{Torr was maintained over} the entire range of temperature. e 8 and e9 were measured at three different

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frequencies (1 kHz, 5 kHz and 10 kHz) by maintaining a constant temperature inside the sample holder. Temperature was varied from 30 7C to 607C.

Parallel capacitance and conductance were measured simultaneously using a GR 1620 AP capacitance measuring assembly and then e 8 and e9 were calculated. Three terminal measurements were performed to avoid the stray capacitances.

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3. – Results and discussions

The temperature dependence of e 8 and e9 is studied at three fixed frequencies (1, 5 and 10 kHz) for glassy Se_{80 2x}Te20Sbx (x 40, 5, 10, 15 and 20). Figures 1-5 show the results for all the glassy alloys studied. It is clear from these figures that dielectric dispersion exists in these glasses, e 8 and e9 both increase with the increase of temperature, the increase being higher at low frequencies.

e 9 is found to follow a power law with frequency in all the glassy alloys studied, i.e. e 94Avm_{. Figure 6 confirms this behaviour in case of glassy Se}

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Fig. 6. – Frequency dependence of dielectric loss in glassy Se75Te20Sb5.

vs. ln v curves are found to be straight lines at various temperatures. Similar results

have been obtained for other glassy alloys also. The power m, calculated from the slopes of ln e 9 vs. lnv curves, is found to be negative at all temperatures in all the glassy alloys.

The temperature dependence of e 8 at various frequencies for all the glassy alloys in the Se_{80 2x}Te20Sbx system indicated that e 8 varies exponentially with temperature as the ln e 8 vs. 1OT curves are found straight lines in all the cases (results not shown here). This type of temperature dependence is generally observed in molecular solids where Debye theory [6] for the viscosity dependence of relaxation time holds good. According to this theory, e 8 should increase exponentially with temperature as found in the present case also. However, the peaks in the e 9 vs. lnv curves have not been observed in the present samples as expected in case of dipolar-type relaxation. Such peaks may be absent if there is a wide distribution of relaxation times as expected in the present glassy systems.

The dipolar model for dielectric dispersion has recently been proposed by Guintini

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Fig. 7. – Composition dependence of dielectric constant and dielectric loss in glassy Se_{80 2x}Te20Sbx.

potential barrier between charged defect states. Each pair of sites is assumed to form a dipole which has a relaxation time depending on its activation energy [9, 10].

According to the above model [7], e 9 at a particular frequency in the temperature range where dielectric dispersion occurs, is given by

e 9(v)4 (e02 eQ) 2 p 2_N

g

ne 2 e0

h

2 kTtm0WM24vm, (1)

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DIELECTRIC RELAXATION IN GLASSYSe_{80 2x}Te20Sbx 713 where m 424 kT WM . (2)

Here, n is the number of electrons that hop, N is the concentration of localized sites, e0 and e_Q are the static and optical dielectric constants, respectively. WM is the energy required to move the electron from a site to infinity.

According to eq. (1), e 9 should follow a power law with frequency, i.e. e94Avm_, where m should be negative and linear with T as given by eq. (2).

In the present case also, e 9 follows a power law with frequency and the values of m are negative. The linear relation with temperature can, however, not be ascertained from the present results due to narrow temperature range of measurements. The above model is, therefore, only a possible explanation to be confirmed by future measurements in wide temperature range.

The composition dependence of dielectric parameters (e 8 and e9) is shown in fig. 7. It is clear from this figure that e 8 and e9 both increase with the increase of Sb concentration in Se_{80 2x}Te20Sbx. As the dielectric loss in these glasses depends upon the total number of localized sites, the increase of e 9 with the increase of Sb indicates the increase in the density of defects on addition of Sb to Se80Te20binary system.

It is generally assumed [3] that the addition of Sb to the Se-Te system leads to a cross linking of the Se-Te chains enhancing the disorder in the system which in turn may increase the density of defect states. The present results also support this argument.

4. – Conclusions

Temperature and frequency dependences of dielectric constant and dielectric loss are studied in glassy Se_{80 2x}Te20Sbx( 0 GxG20). Dielectric dispersion is found to occur in these alloys near room temperature in the audio frequency range. Dielectric constant and loss, at particular frequency and temperature, increase with the increase of Sb concentration. A detailed analysis of the results showed that dielectric dispersion is dipolar in nature. A possible explanation is given in terms of the hopping of charge carriers over a potential barrier between charged defect states. The increase of e 9 with Sb concentration can be understood in terms of the increased defect states on addition of Sb to Se-Te system.

R E F E R E N C E S

[1] MEHRAR. M., GURINDERand MATHURR. C., International Conference on Semiconductor

Materials, New Delhi, December 1988, unpublished.

[2] SAKAIH., SHIMAKAWAK., INAGAKIY. and ARIZUMIT., Jpn. J. Appl. Phys., 13 (1974) 500. [3] SHIMAKAWAK., YOSHIDAA. and ARIZUMIT., J. Non-Cryst. Solids, 16 (1974) 258.

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[4] NAGELSP., Phys. Status Solidi A, 59 (1980) 505. [5] JOPEJ. K., Ind. J. Pure Appl. Phys., 20 (1982) 774.

[6] DEBYE P., Polar Molecules (The Chemical Catalogue Company, New York) 1929, Chapt. 5. [7] GUINTINIJ. C., ZANCHETTA J. V., JULIEN D., EHOLIE R. and HOUENOU P., J. Non-Cryst.

Solids, 45 (1981) 57.

[8] ELLIOTTS. R., Philos. Mag., 36 (1977) 1291.

[9] STEARNA. E. and EYRINGH., J. Chem. Phys., 5 (1937) 113.

[10] GLASSTONES., LAIDLERK. J. and EYRINGH., The Theory of Rate Processes (McGraw Hill Publ. Co., New York) 1941.