ISSN 10637826, Semiconductors, 2012, Vol. 46, No. 7, pp. 943–947. © Pleiades Publishing, Ltd., 2012. Original Russian Text © O.L. Kheifets, E.F. Shakirov, N.V. Melnikova, A.L. Filippov, L.L. Nugaeva, 2012, published in Fizika i Tekhnika Poluprovodnikov, 2012, Vol. 46, No. 7, pp. 966–970.
AMORPHOUS, VITREOUS, AND ORGANIC SEMICONDUCTORS
Influence of the Composition on the Electrical Properties of Amorphous Chalcogenides AgGe1 + xAs1 – xS3 O. L. Kheifets^, E. F. Shakirov, N. V. Melnikova, A. L. Filippov, and L. L. Nugaeva Ural Federal University, ul. Mira 19, Yekaterinburg, 620000 Russia ^email:
[email protected] Submitted December 13, 2011; accepted for publication December 19, 2011
Abstract—This article is devoted to the synthesis and examination of the electrical properties of amorphous chalcogenides AgGe1 + xAs1 – xS3 (x = 0.1, 0.4–0.6, 0.9) at low temperatures. The studies are performed in order to obtain materials with improved characteristics (an increase in the fraction of the ionic transfer, a decrease in the temperatures of its emergence, and an increase in conductivity). The synthesized compounds are electron–ion conductors. An increase in the Ge fraction leads to an increase in the temperature corre sponding to the onset of ionic transport and to a decrease in conductivity. DOI: 10.1134/S1063782612070123
1. INTRODUCTION Studies of rapid ionic transport in solids (superi onic conductivity) started to be intensely developed in the last decades of the last century in connection with the wide use of superionic conductors in current sources. The development of modern cryoelectronics requires the development of new semiconductor materials with low onset temperatures for both the electron and ionic transfer of the electric charge. The search for new compounds for cryogenic microelec tronics in the still poorly studied class of complex chal cogenides, involving ionic semiconductors with a low ion current, is an interesting and urgent problem [1–3]. From the practical viewpoint, materials with high ionic conductivity are of interest for use as the func tional electrodes of various devices. For example, cur rently, these materials are already used in smallscale powerintensive current sources and in various sensors of compositions with ionic conductivity, which are stable in the required temperature range and in the given medium. The electrical conductivity of vitreous and amor phous solid electrolytes often exceeds the electrical conductivity of crystalline solid electrolytes of the same composition by several orders of magnitude. It is assumed that the conductivity of both electrolytes is caused by migration of the same ions. The interrela tion of processes of the ionic transport and the struc ture of glasses is of great interest. In the context of the search for materials that can be used as electrodes with the optimal electron–ion conductivity or an electrolyte medium for the current source, the influence of nonstoichiometry on the ionic conductivity of AgGeAsS3 was studied (an ionic conduc tor with fraction of ionic conductivity of 0.992 [1–3]).
In this study, we synthesized and attested multi component AgGe1 + xAs1 – xS3 chalcogenides with x = 0, 0.1, and 0.4–0.9. The electrical properties (the impedance, the temperature dependences of conduc tivity and permittivity, the time dependence of resistiv ity) of the synthesized compounds with x = 0.1, 0.4–0.6, and 0.9 at 20–300 K are studied. The influence of the composition of compounds on their electric properties is analyzed. 2. EXPERIMENTAL METHODS AND ATTESTATION OF THE SAMPLES All compounds were synthesized using cell tech nology in the stepheating mode over 14 days. The
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Intensity, arb. units 700 600 500
x=0 0.1
400
0.4 0.5 0.6
300 200
0.7 0.8
100 0
0.9 0
10
20
30
40
50
60
70
80
90 2θ
Fig. 1. Xray diffraction patterns of the AgGe1 + xAs1 – xS3 compounds (x = 0, 0.1, and 0.4–0.9).
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1 μm
(a)
10 μm
(b)
10 μm
(c)
Fig. 2. Microstructure of the samples (a) AgGe1.9As0.1S3, (b) AgGe1.1As0.9S3, and (c) AgGeAsS3.
120 Im Z, kΩ
80 Im Z, kΩ
x = 0.5 x = 0.6
x = 0.1 373 K
40
300 K
80 200 kHz
fb
40
0
0
80
40
0
120 Re Z, kΩ
0
80
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120 Re Z, kΩ
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x = 0.9 290 K
6
Im Z, kΩ
Im Z, MΩ
8
4 2 0
x = 0.9 373 K
120 80
fb
40
0
2
4
6
8 Re Z, MΩ
0
0
40
80
120
160 Re Z, kΩ
Fig. 3. Impedance hodographs Z of the AgGe1 + xAs1 – xS3 samples (x = 0.1, 0.5, 0.6, and 0.9). SEMICONDUCTORS
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−1
log(σ, S/m)
tanδ 4 3 2
1.6 kHz x=0 x = 0.1 x = 0.9
−3 −5 −7
1 −9 10
104
10 log(f, Hz)
Fig. 4. Dependence of the dielectric tangent loss on fre quency f for the AgGe1.6As0.4S3 sample.
maximal synthesis temperature was 1000°C. Vitreous compounds were obtained by quenching via immer sion of cells containing the melt into ice water from 790°C. The Xray structural attestation was performed using Shimadzu XRD 6000, Shimadzu 7000, and Bruker D8 Xray diffractometers (Tomsk Materials Science Center; Institute of SolidState Chemistry, Ural Branch, Russian Academy of Sciences; and the Ural Federal University). The surface of the samples was studied using scanning electron microscopy. The Xray diffraction analysis showed that the AgGe1 + xAs1 – xS3 materials (x = 0, 0.1, 0.4–0.9) are Xray amorphous. The Xray diffraction patterns of the samples are presented in Fig. 1. Fragments of the Xray diffraction patterns in the angle range, where first two halos are observed, are typical of the amor phous compounds of the Ag–Ge–As(Sb)–S(Se) sys tem [4, 5]. According to the data of microscopic stud ies, the AgGe1 + xAs1 – xS3 glasses with x = 0 and 0.1 are nonuniform with spherical inclusions, while materials with x ≥ 0.4 are uniform (Fig. 2). The samples for studying the electric properties were pellets ~(0.5–3) mm in height with a crosssec tional area of 10–30 mm2. The electrical properties of compounds were stud ied by impedance spectroscopy in the temperature range T = 78–400 K and frequency range f = 0.2– 200 kHz. The measurements were performed using an RLC2000 impedance analyzer. The electron and ionic components of conductivity were determined using the polarization Wagner method [6]. The tem perature was measured using a copper–Copel thermo couple accurate to 0.5 K. SEMICONDUCTORS
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−3.5 log(σ, S/m)
3
11 103/T, K−1 200 kHz x = 0.5
−4.0
−4.5
0
10
20
30 103/T, K−1
−3.0
log(σ, S/m)
0
200 kHz x = 0.4 x = 0.6
−3.5 −4.0 −4.5 −5.0
3
4
5
6
7
8 103/T, K−1
Fig. 5. Temperature dependences of electrical conductivity σ for the AgGe1 + xAs1 – xS3 compounds (x = 0, 0.1, 0.4–0.6, and 0.9).
3. EXPERIMENTAL RESULTS To reveal the influence of effects at the sample– electrode interface, the roomtemperature impedance (Z) was measured in the cells with copper electrodes. Figure 3 presents the impedance hodographs for the cell with the sample. The electrical characteristics depend on the frequency (Fig. 4). The relaxation times for some compounds were estimated from the frequency dependences of the dielectric loss tangent (tanδ). For the AgGe1.6As0.4S3 compound, the relaxation time is 6.4 × 10–5 s. This
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ε
ε
6000 80 40
4000
0 80
2000
120
160 T, K
x=0 x = 0.1 x = 0.9
200
240
0 50
100 150 200 250 300 350 400 T, K
1400 x = 0.5 200 kHz
ε
1350 1300 1250 1200
0
50
100
150
200
250
300
350 T, K
70
ε
60 50
x = 0.6 x = 0.4 200 kHz
40 30 20 100
150
200
250
300 T, K
Fig. 6. Temperature dependences of permittivity ε for AgGe1 + xAs1 – xS3 compounds (x = 0, 0.1, 0.4–0.6, and 0.9).
time is typical of ionrelaxation polarization, which is caused by the excess transfer of weakly bound ions under the effect of an electric field to distances on the scale of sizes that describe the structure. Ionrelax
ation polarization, the time of establishment of which is ~(10–8–10–4) s, manifests itself in some crystal sub stances in the presence of impurities in them in the form of ions with a loosely packed crystal lattice or in amorphous materials. The higher the frequency, the higher the conductivity and the lower the permittivity, which is associated with weaker polarization effects at the electrode–sample interface. The electrical characteristics of the materials are measured in a frequency range where the influence of electrode processes can be neglected, i.e., at frequen cies exceeding the boundary ones. The temperature dependences of conductivity σ are presented in Fig. 5. The studies were performed at various ac frequencies. The temperature dependences of conductivity of all the samples at all frequencies are approximated by exponential functions with expo nents depending on the temperature range. The regions of activation energy change and the activation energies are presented in Table 1. The transition from low activation energies of the carriers to higher ones can be caused by the emergence of ionic conductivity. The onset temperature of ionic transport was esti mated from the temperature corresponding to the onset of a rapid increase in conductivity. The study of permittivity allowed us to refine the onset temperature of the ionic transfer. Typical tem perature dependences of permittivity ε for the com pounds under study are presented in Fig. 6. The temperature regions for the onset of ionic transport and the values of conductivity σ in the AgGe1 + xAs1 – xS3 materials are presented in Table 1. The substitution of As atoms by Ge atoms led to an increase in the onset temperature and to a decrease in conductivity. Conductivity decreases as the sizes of the spherical silverenriched inclusions decrease, and upon transition to homogeneous glasses. The study of the time dependence of conductivity at a constant potential difference in the cell containing the copper electrodes allowed us to evaluate the elec tron component of conductivity and the polarization time, i.e., the time from the instant of dc switchon to the instant when the resistance reaches a constant value.
Table 1. xDependences of the onset temperature for the ionic transfer Ti and activation energy of conductivity σ at 300 K in the AgGe1 + xAs1 – xS3 materials Compound AgGeAsS3 AgGe1.1As0.9S3 AgGe1.4As0.6S3 AgGe1.5As0.5S3 AgGe1.6As0.4S3 AgGe1.9As0.1S3
Ti, K
Activation energy, eV
σ, μS/m
120–150 150–200 160–220 190–230 170–240 220–250
0.13 (190–300 K) 0.15 (200–300 K) 0.4 (220–300 K) 0.14 (230–300 K) 0.47 (240–300 K) 0.2 (240–300 K)
1400 130 61.4 34 73 23
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fraction of the ionic component of conductivity calcu lated from these measurements is presented in Table 2.
ρ, MΩ m 6
4 x = 0.4 x = 0.6
2
0
0
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100
150
200 Time, min
Fig. 7. Time dependences of resistivity ρ for the AgGe1.4As0.6S3 and AgGe1.6As0.4S3 compounds.
The time dependences of the current, resistivity, and conductivity studied at various voltages applied to the cell has a form characteristic of solid electrolytes (Fig. 7). The time instant t = 0 corresponds to switch ing on the constant voltage applied to the sample. The
4. CONCLUSIONS Studies of AgGe1 + xAs1 – xS3 chalcogenides show that, at a high cooling rate of the melt with 0.4 ≤ x ≤ 0.9, vitreous homogeneous materials are formed; while at x = 0 and x = 0.1, an amorphous host with spherical Agenriched microinclusions is formed, and the frac tion of the ionic transport substantially increases (compared with homogeneous glasses), while the tem peratures of the onset of ionic transport become lower. ACKNOWLEDGMENTS This study was supported in part by the Federal Targeted Program “Scientific and ScientificandPed agogical Staff of Innovative Russia” for 2009–2013. We thank colleages of the Tomsk Materials Science Center for Collective Use and the Institute of Solid State Chemistry of the Ural Branch, Russian Academy of Sciences, for help in the Xray diffraction studies. REFERENCES
Table 2. Fraction of the ionic conductivity and times of current establishment in the AgGe1 + xAs1 – xS3 materials Compound
Fraction of ionic conductivity, % (dc voltage)
Times of current establishment, min
99.7 96 (1 V) 96 (5 V) 78 (5 V) 80 (9 V) 95 (12 V) 99 (5 V) 85 (1 V)
5 5 5–10 20–23.5 41.5–50 23.5 55 47
AgGeAsS3 AgGe1.1As0.9S3 AgGe1.4As0.6S3 AgGe1.5As0.5S3
AgGe1.6As0.4S3 AgGe1.9As0.1S3
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1. E. R. Baranova, V. L. Kobelev, O. L. Kobeleva, N. V. Mel nikova, V. B. Zlokazov, L. Ya. Kobelev, and M. V. Per filyev, Solid State Ionics 124, 255 (1999). 2. V. B. Zlokazov, N. V. Melnikova, E. R. Baranova, M. V. Perfilyev, and L. Ya. Kobelev, Elektrokhimiya 28, 1523 (1992). 3. E. R. Baranova, V. L. Kobelev, O. L. Kobeleva, L. L. Nu gaeva, Z. B. Zlokazov, and L. Ya. Kobelev, Solid State Ionics 146, 415 (2002). 4. N. V. Melnikova, O. L. Kheifets, and A. N. Babushkin, ISJAEE, No. 5, 56 (2007). 5. M. Krbal, S. Stehlik, T. Wagner, V. Zima, L. Benes, and M. Frumar, J. Phys. Chem. Sol. 68, 958 (2007). 6. C. Wagner, Z. Electrochem. Berichte Bunseng.: Phys. Chem. B 60, 4 (1956).
Translated by N. Korovin
