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High resolution electrical characterisation of Ag-conducting heterogeneous chalcogenide glasses
Solid State Ionics 181 (2010) 1205–1208
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
High resolution electrical characterisation of Ag-conducting heterogeneous chalcogenide glasses A.A. Piarristeguy a,⁎, M. Ramonda b, N. Frolet a, M. Ribes a, A. Pradel a a b
Institut Charles Gerhardt Montpellier, Equipe Physicochimie des Matériaux Désordonnés et Poreux (PMDP), CC1503, Université Montpellier II, F-34095 Montpellier Cedex 5, France Laboratoire de Microscopie en Champ Proche (LMCP), Université de Montpellier II, F-34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 2 February 2010 Received in revised form 6 June 2010 Accepted 25 June 2010 Keywords: Ag chalcogenide glasses Phase separation Near field microscopy Conductivity
a b s t r a c t Electric force microscopy (EFM) and conductive atomic force microcopy (C-AFM) are introduced to perform nanoscale electrical characterization of phase separated Agx(Ge0.25Se0.75)100 − x glasses. Changes in the relative permittivity are found for both phases when the silver content is changed. Furthermore, the sensitivity of the C-AFM technique revealed current variations of a few pico-amperes in the Ag-rich phase for the different glass compositions. This result confirms that the increase in conductivity of the Ag–Ge–Se samples in the region of high ionic conduction (x N 8–10 at.%) arises from an increase in conductivity of the Ag-rich phase and not from an increase in amount of Ag-rich phase with a fixed composition and conductivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Near field microscopy methods are powerful sensing techniques for the characterization of the electrical conductivity variations in highlyto-medium resistivity samples. In particular, the conductive atomic force microcopy (C-AFM) technique has been applied in many research and manufacturing areas for the analysis of a wide range of materials, such as, thin silicon oxide dielectric films [1], the diamond-like carbon films used in the data-storage industry [2], and piezoelectric and ferroelectric materials for applications in both MEMS and microelectronics [3]. C-AFM has also proven to be very useful for ionic conductors [4–10] and also for chalcogenides where there is much interest in phase change materials [11]. Several experimental techniques, such as, optical microscopy [12] and modulated differential scanning calorimetry (MDSC) [13,14] show that Agx(Ge0.25Se0.75)100 − x glasses, interesting for the development of electrical memory devices [15], are phase separated. Mitkova et al. [13] reported the presence of two vitreous transition temperatures (Tg) by MDSC and tentatively interpreted the results in terms of phase separation of the Se-rich glasses into a Ag2Se phase and a Se-deficient backbone, GetSe1 − t with t = 0.25[1 − (x / 100)] / [1 − (3x / 200)]. Unfortunately, these experiments only give information on the global properties of glasses and cannot provide direct answers about, for example, the composition or the conductivity of each phase with increasing silver content.
⁎ Corresponding author. Tel.: + 33 4 67 14 45 27; fax: + 33 4 67 14 42 90. E-mail address: [email protected] (A.A. Piarristeguy). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.06.050
There is broad recognition that Ag in Ge–Se glasses readily photodiffuses [16]. Therefore, the application of spectroscopies that use a beam of photons or electrons to probe the local composition of Ag-based glasses has always been problematic; one runs the risk of altering the local structure around Ag by the probe if the flux of the incident beam is high. Recently, the application of near field microscopy methods to several Ag-conducting chalcogenide glasses [17–19] demonstrated the possibility of imaging the electrical contrast in phase separated materials by electrical force microscopy (EFM) without local structure modification. For Agx(Ge0.25Se0.75)100 − x glasses, these results helped in understanding the origin of the 7−8 order change in magnitude of the conductivity of the system with increasing Ag content: the glasses are phase-separated into Ag-rich and Ag-poor phases and percolation of the Ag-rich phase leads to a sudden jump in the conductivity at about x ~ 8–10at.% [17,19]. In this work, we demonstrate the ability and the complementarities of EFM and C-AFM microscopies to give insight into the intrinsic electrical properties of each phase present in bulk Agx(Ge0.25Se0.75)100 − x glasses and their composition dependence.
2. Experimental The investigated samples were Agx(Ge0.25Se0.75)100 − x with x = 5, 10, 15 and 20 at.%. Bulk glasses were prepared from high-purity (4 N) elements by the melt-quenching technique using an ice-water bath as previously described [20]. No crystalline phase was detected by X-ray diffraction. The glass samples are named Agx according to the concentration of Ag (in at.%) present.
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Fig. 1. (a) Schematic illustration of the experimental setup used for electrostatic force microscopy and (b) an equivalent circuit model for the overall capacitance of the system between the tip and the sample.
The EFM and C-AFM measurements were performed using a Veeco Nanoscope Dimension 3100 microscope equipped with an extended TUNA module. The EFM was carried out using the conventional frequency modulation technique at the first cantilever frequency (60 kHz) in Lift-Mode (30 nm above the sample surface, the amplitude of the vibrating lever (Alift) was lower than the amplitude set-point (Alift ~ 1/3 ASP)). Both measurements were performed under ambient conditions using a commercial coated metal PtIr5 tip. The spring constant of the cantilever was around 2 N/m. Scans were performed
on the surfaces of freshly fractured glass to avoid contamination by oxidation. 3. Results and discussion To help in interpreting the EFM data the overall capacitance of the system between the tip and the sample was modeled by an equivalent simple circuit formed by two serial capacitors (capacitive plan–plan model), one corresponding to the tip-sample region (with a permittivity
Fig. 2. EFM micrographs for the two same glasses of the Agx(Ge0.25Se0.75)100 − x system: (a) x = 5 at.%, topological, (b) x = 5 at.%, applied voltage = − 5 V, (c) x = 15 at.%, topological, (d) x = 15 at.%, applied voltage = − 5 V.
A.A. Piarristeguy et al. / Solid State Ionics 181 (2010) 1205–1208
1207
the frequency shift Δf in an EFM experiment is proportional to the fo ∂Felec electrical force gradient (Δf = − 2k , where fo and k are the resonant ∂z frequency and the stiffness of the lever, respectively) [21] we find that Δf is proportional to the square of the applied voltage through the following expression, Δf = −
fo ε0 S 2 V 2k D + dεo 3
ð1Þ
εsample
Fig. 3. Parabolic curves obtained from EFM measurements for (a) the Ag-rich phase and (b) the Ag-poor phase of Agx(Ge0.25Se0.75)100 − x glasses with x = 5, 15, and 20 at.%.
ε0) and the other corresponding to the sample (with a permittivity εsample) (Fig. 1). Taking into account the electrical force equation for a voltage V (Felec = 12 ∂C V 2 , where C is the capacitance) and the fact that ∂z
where S is the sample surface which senses the polarization of the tip, d is the penetration depth of the electric field and D corresponds to the tip to sample distance. The concavity of the parabola in Eq. (1) gives information on the sample permittivity (εsample). Fig. 2 shows the topography and EFM images of two glasses: Ag5 and Ag15. The images clearly show the phase separation already reported for this family of glasses. Interestingly the connected phase for Ag5, i.e. the light Agpoor phase is the embedded phase in Ag15 while the embedded phase for Ag5, i.e. the dark Ag-rich phase becomes the connected phase for Ag15. A series of EFM experiments helped in building parabolas for Ag5, Ag15, and Ag20 glasses. Two sets of curves were obtained for each glass, one for the Ag-rich phase, and the second one for the Agpoor phase (Fig. 3). These plots show that the concavity increases with silver content and hence with the permittivity according to the simple plan–plan model of Eq. (1). Since the permittivity of a phase depends on its composition, these results demonstrate that the composition of each phase changes with x for Agx(Ge0.25Se0.75)100 − x glasses. C-AFM experiments were then carried out in order to investigate the electrical behavior of both phases more thoroughly. Fig. 4 shows a C-AFM current image acquired for the Ag20(Ge0.25Se0.75)80 glass when different dc bias was applied to the sample (horizontal lines). The electrical contrast clearly shows phase separation in agreement with the EFM image shown in Fig. 2. Furthermore, it is clear that the current flowing through each phase is different and whatever the applied dc bias, hardly any current flows through the Ag-poor phase. This indicates that the conductivity of this phase is very small or negligible and below the limitation of detection of the equipment. By comparison, the current through the Ag-rich phase increases when the applied dc bias increases. The raw data were used to plot I–V curves for a negative dc sample bias for the Ag-rich phase of the Agx(Ge0.25Se0.75)100 − x glasses with x =10, 15, and 20 at.% (Fig. 5). In the region of measurements where a linear dependence of the current with voltage was observed the slope increases
Fig. 4. C-AFM current image for the Ag20(Ge0.25Se0.75)80 glass.
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The EFM contrast electrically evidenced a phase separation in Ag–Ge–Se glasses. The EFM experiments, through a simple modeled electrical circuit, showed an increase in the relative permittivity of each present phase when the silver content of the glasses increased. In addition, C-AFM studies allowed characterizing the conductive phase (or Ag-rich phase) behavior: an increase in the conductivity of this phase when the silver content of the glass increased was observed. This result indicated that the increase in conductivity of the Ag–Ge–Se samples in the region of high ionic conduction (xN 8–10at.%) was produced by an increase in the conductivity of the Ag-rich phase. The evolution of the conductivity with the silver content evaluated from C-AFM data is consistent with that calculated from previous measurements performed by complex impedance spectroscopy, i.e. an increase of ~1 order of magnitude when the silver content changes from 10 to 20 at.%. These promising results will contribute to an understanding of the local conductivity behavior in ion conducting glasses, materials that are used as solid electrolytes in electrical devices. Fig. 5. C-AFM I–V curves measured for the Ag-rich phase of Agx(Ge0.25Se0.75)100 − x glasses with x = 10, 15 and 20 at.%.
as the glass changes from Ag10 to Ag20. Resistances R of about 97– 750 GΩ were evaluated from linear regression. If the contact is assumed to be circular and Ohmic in nature then the relation between the resistance R and the resistivity ρ is given by the basic spreading resistance formula R ~ ρ / 4r where r is the radius of the contact [22]. Hence, for the tip radius of ~10 nm, conductivity values for the Ag-rich phase between ~3 × 10−6 Ω− 1 cm− 1 and ~0.3 × 10−6 Ω−1 cm− 1 were calculated for the glasses. Even though the conductivity values obtained by C-AFM measurements are about one order of magnitude smaller than those commonly estimated by conventional measurements performed by complex impedance spectroscopy [23], the evolution of the conductivity follows the same trend with silver content of the glass. Also, additional information is obtained by the C-AFM experiment, i.e. the increase in conductivity of the Ag–Ge–Se samples in the high ionic conductivity region (x N 8–10at.%) does not result from an increase in amount of Ag-rich phase with a fixed composition and conductivity but from an increase in conductivity of Ag-rich phase. This indicates that the composition of the Ag-rich phase continually changes as silver is added to the base glass. This is the first time that this type of information has been assessed in Ag-conducting glasses since the methods usually used to probe the composition are based upon the use of an electron or photon beam which leads to diffusion of the Ag ions that are then to be discarded in these photosensitive glasses. Furthermore, this type of information is not accessible from the other techniques such as MDSC that have been used in previous work on phase separated glasses [13,14]. 4. Conclusion This work demonstrated the complementary of EFM and C-AFM experiments to perform a nanoscale electrical characterization of ion conducting chalcogenide glasses.
Acknowledgement The work was supported in the framework of the Project ANR-05BLAN-0058-01. References [1] X. Blasco, M. Nafrı́a, X. Aymerich, J. Pétry, W. Vandervorst, Microelectron. Reliab. 45 (5–6) (2005) 811. [2] N.S. Tambe, B. Bhushan, Scr. Mater. 52 (2005) 751. [3] P. Fiorenza, V. Raineri, Appl. Phys. Lett. 88 (2006) 212112. [4] A. Layson, S. Gadad, D. Teeters, Electrochim. Acta 48 (2003) 2207. [5] R. O'Hayre, M. Lee, F. Prinz, J. Appl. Phys. 95 (2004) 8382. [6] A.J. Bhattacharyya, J. Fleig, Y. Guo, J. Maier, Adv. Mater. 17 (2005) 2630. [7] A. Schirmeisen, A. Taskiran, H. Fuchs, H. Bracht, S. Murugavel, B. Roling, Phys. Rev. Lett. 98 (2007) 225901. [8] O. Douhéret, L. Lutsen, A. Swinnen, M. Breselge, K. Vandewal, L. Goris, J. Manca, Appl. Phys. Lett. 89 (2006) 032107. [9] I. Yee Phanga, T. Liuc, W. Zhangd, H. Schönherra, G.J. Vancso, Eur. Polym. J. 43 (10) (2007) 4136. [10] R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (6) (2005) 1077. [11] R. Pandian, B.J. Kooi, G. Palasantzas, J.T.M. De Hosson, A. Pauza, Appl. Phys. Lett. 91 (2007) 152103. [12] M. Gutenev, A. Tabolin, A. Rykova, Fiz. Khim. Stakla 17 (1991) 36. [13] M. Mitkova, Y. Wang, P. Boolchand, Phys. Rev. Lett. 83 (1999) 3848. [14] Y. Wang, M. Mitkova, D.G. Georgiev, S. Mamedov, P. Boolchand, J. Phys. Condens. Matter 15 (2003) S1573. [15] M. Kozicki, M. Mitkova, US Patent App. 11/276,097 (2006). [16] A.V. Kolobov, S.R. Elliott, Adv. Phys. 40 (1991) 625. [17] V. Balan, A.A. Piarristeguy, M. Ramonda, A. Pradel, M. Ribes, J. Optoelect. Adv. Mat. 8 (6) (2006) 2112. [18] A.A. Piarristeguy, M. Ramonda, N. Kuwata, A. Pradel, M. Ribes, J. Non-Cryst. Solids 177 (2006) 3157. [19] A.A. Piarristeguy, M. Ramonda, M.A. Ureña, A. Pradel, M. Ribes, J. Non-Cryst. Solids 353 (2007) 1261. [20] A.A. Piarristeguy, G.J. Cuello, P. Yot, A. Pradel, M. Ribes, J. Phys. Conden. Matter 20 (2008) 155106. [21] P. Girard, M. Ramonda, D. Saluel, J. Vac. Sci. Technol. B 20 (2002) 1348. [22] P. De Wolf, E. Brazel, A. Erickson, Mater. Sci. Semicond. Process. 4 (2001) 71. [23] M.A. Ureña, A.A. Piarristeguy, M. Fontana, B. Arcondo, Solid State Ion. 176 (2005) 505.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
High resolution electrical characterisation of Ag-conducting heterogeneous chalcogenide glasses A.A. Piarristeguy a,⁎, M. Ramonda b, N. Frolet a, M. Ribes a, A. Pradel a a b
Institut Charles Gerhardt Montpellier, Equipe Physicochimie des Matériaux Désordonnés et Poreux (PMDP), CC1503, Université Montpellier II, F-34095 Montpellier Cedex 5, France Laboratoire de Microscopie en Champ Proche (LMCP), Université de Montpellier II, F-34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 2 February 2010 Received in revised form 6 June 2010 Accepted 25 June 2010 Keywords: Ag chalcogenide glasses Phase separation Near field microscopy Conductivity
a b s t r a c t Electric force microscopy (EFM) and conductive atomic force microcopy (C-AFM) are introduced to perform nanoscale electrical characterization of phase separated Agx(Ge0.25Se0.75)100 − x glasses. Changes in the relative permittivity are found for both phases when the silver content is changed. Furthermore, the sensitivity of the C-AFM technique revealed current variations of a few pico-amperes in the Ag-rich phase for the different glass compositions. This result confirms that the increase in conductivity of the Ag–Ge–Se samples in the region of high ionic conduction (x N 8–10 at.%) arises from an increase in conductivity of the Ag-rich phase and not from an increase in amount of Ag-rich phase with a fixed composition and conductivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Near field microscopy methods are powerful sensing techniques for the characterization of the electrical conductivity variations in highlyto-medium resistivity samples. In particular, the conductive atomic force microcopy (C-AFM) technique has been applied in many research and manufacturing areas for the analysis of a wide range of materials, such as, thin silicon oxide dielectric films [1], the diamond-like carbon films used in the data-storage industry [2], and piezoelectric and ferroelectric materials for applications in both MEMS and microelectronics [3]. C-AFM has also proven to be very useful for ionic conductors [4–10] and also for chalcogenides where there is much interest in phase change materials [11]. Several experimental techniques, such as, optical microscopy [12] and modulated differential scanning calorimetry (MDSC) [13,14] show that Agx(Ge0.25Se0.75)100 − x glasses, interesting for the development of electrical memory devices [15], are phase separated. Mitkova et al. [13] reported the presence of two vitreous transition temperatures (Tg) by MDSC and tentatively interpreted the results in terms of phase separation of the Se-rich glasses into a Ag2Se phase and a Se-deficient backbone, GetSe1 − t with t = 0.25[1 − (x / 100)] / [1 − (3x / 200)]. Unfortunately, these experiments only give information on the global properties of glasses and cannot provide direct answers about, for example, the composition or the conductivity of each phase with increasing silver content.
⁎ Corresponding author. Tel.: + 33 4 67 14 45 27; fax: + 33 4 67 14 42 90. E-mail address: [email protected] (A.A. Piarristeguy). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.06.050
There is broad recognition that Ag in Ge–Se glasses readily photodiffuses [16]. Therefore, the application of spectroscopies that use a beam of photons or electrons to probe the local composition of Ag-based glasses has always been problematic; one runs the risk of altering the local structure around Ag by the probe if the flux of the incident beam is high. Recently, the application of near field microscopy methods to several Ag-conducting chalcogenide glasses [17–19] demonstrated the possibility of imaging the electrical contrast in phase separated materials by electrical force microscopy (EFM) without local structure modification. For Agx(Ge0.25Se0.75)100 − x glasses, these results helped in understanding the origin of the 7−8 order change in magnitude of the conductivity of the system with increasing Ag content: the glasses are phase-separated into Ag-rich and Ag-poor phases and percolation of the Ag-rich phase leads to a sudden jump in the conductivity at about x ~ 8–10at.% [17,19]. In this work, we demonstrate the ability and the complementarities of EFM and C-AFM microscopies to give insight into the intrinsic electrical properties of each phase present in bulk Agx(Ge0.25Se0.75)100 − x glasses and their composition dependence.
2. Experimental The investigated samples were Agx(Ge0.25Se0.75)100 − x with x = 5, 10, 15 and 20 at.%. Bulk glasses were prepared from high-purity (4 N) elements by the melt-quenching technique using an ice-water bath as previously described [20]. No crystalline phase was detected by X-ray diffraction. The glass samples are named Agx according to the concentration of Ag (in at.%) present.
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Fig. 1. (a) Schematic illustration of the experimental setup used for electrostatic force microscopy and (b) an equivalent circuit model for the overall capacitance of the system between the tip and the sample.
The EFM and C-AFM measurements were performed using a Veeco Nanoscope Dimension 3100 microscope equipped with an extended TUNA module. The EFM was carried out using the conventional frequency modulation technique at the first cantilever frequency (60 kHz) in Lift-Mode (30 nm above the sample surface, the amplitude of the vibrating lever (Alift) was lower than the amplitude set-point (Alift ~ 1/3 ASP)). Both measurements were performed under ambient conditions using a commercial coated metal PtIr5 tip. The spring constant of the cantilever was around 2 N/m. Scans were performed
on the surfaces of freshly fractured glass to avoid contamination by oxidation. 3. Results and discussion To help in interpreting the EFM data the overall capacitance of the system between the tip and the sample was modeled by an equivalent simple circuit formed by two serial capacitors (capacitive plan–plan model), one corresponding to the tip-sample region (with a permittivity
Fig. 2. EFM micrographs for the two same glasses of the Agx(Ge0.25Se0.75)100 − x system: (a) x = 5 at.%, topological, (b) x = 5 at.%, applied voltage = − 5 V, (c) x = 15 at.%, topological, (d) x = 15 at.%, applied voltage = − 5 V.
A.A. Piarristeguy et al. / Solid State Ionics 181 (2010) 1205–1208
1207
the frequency shift Δf in an EFM experiment is proportional to the fo ∂Felec electrical force gradient (Δf = − 2k , where fo and k are the resonant ∂z frequency and the stiffness of the lever, respectively) [21] we find that Δf is proportional to the square of the applied voltage through the following expression, Δf = −
fo ε0 S 2 V 2k D + dεo 3
ð1Þ
εsample
Fig. 3. Parabolic curves obtained from EFM measurements for (a) the Ag-rich phase and (b) the Ag-poor phase of Agx(Ge0.25Se0.75)100 − x glasses with x = 5, 15, and 20 at.%.
ε0) and the other corresponding to the sample (with a permittivity εsample) (Fig. 1). Taking into account the electrical force equation for a voltage V (Felec = 12 ∂C V 2 , where C is the capacitance) and the fact that ∂z
where S is the sample surface which senses the polarization of the tip, d is the penetration depth of the electric field and D corresponds to the tip to sample distance. The concavity of the parabola in Eq. (1) gives information on the sample permittivity (εsample). Fig. 2 shows the topography and EFM images of two glasses: Ag5 and Ag15. The images clearly show the phase separation already reported for this family of glasses. Interestingly the connected phase for Ag5, i.e. the light Agpoor phase is the embedded phase in Ag15 while the embedded phase for Ag5, i.e. the dark Ag-rich phase becomes the connected phase for Ag15. A series of EFM experiments helped in building parabolas for Ag5, Ag15, and Ag20 glasses. Two sets of curves were obtained for each glass, one for the Ag-rich phase, and the second one for the Agpoor phase (Fig. 3). These plots show that the concavity increases with silver content and hence with the permittivity according to the simple plan–plan model of Eq. (1). Since the permittivity of a phase depends on its composition, these results demonstrate that the composition of each phase changes with x for Agx(Ge0.25Se0.75)100 − x glasses. C-AFM experiments were then carried out in order to investigate the electrical behavior of both phases more thoroughly. Fig. 4 shows a C-AFM current image acquired for the Ag20(Ge0.25Se0.75)80 glass when different dc bias was applied to the sample (horizontal lines). The electrical contrast clearly shows phase separation in agreement with the EFM image shown in Fig. 2. Furthermore, it is clear that the current flowing through each phase is different and whatever the applied dc bias, hardly any current flows through the Ag-poor phase. This indicates that the conductivity of this phase is very small or negligible and below the limitation of detection of the equipment. By comparison, the current through the Ag-rich phase increases when the applied dc bias increases. The raw data were used to plot I–V curves for a negative dc sample bias for the Ag-rich phase of the Agx(Ge0.25Se0.75)100 − x glasses with x =10, 15, and 20 at.% (Fig. 5). In the region of measurements where a linear dependence of the current with voltage was observed the slope increases
Fig. 4. C-AFM current image for the Ag20(Ge0.25Se0.75)80 glass.
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A.A. Piarristeguy et al. / Solid State Ionics 181 (2010) 1205–1208
The EFM contrast electrically evidenced a phase separation in Ag–Ge–Se glasses. The EFM experiments, through a simple modeled electrical circuit, showed an increase in the relative permittivity of each present phase when the silver content of the glasses increased. In addition, C-AFM studies allowed characterizing the conductive phase (or Ag-rich phase) behavior: an increase in the conductivity of this phase when the silver content of the glass increased was observed. This result indicated that the increase in conductivity of the Ag–Ge–Se samples in the region of high ionic conduction (xN 8–10at.%) was produced by an increase in the conductivity of the Ag-rich phase. The evolution of the conductivity with the silver content evaluated from C-AFM data is consistent with that calculated from previous measurements performed by complex impedance spectroscopy, i.e. an increase of ~1 order of magnitude when the silver content changes from 10 to 20 at.%. These promising results will contribute to an understanding of the local conductivity behavior in ion conducting glasses, materials that are used as solid electrolytes in electrical devices. Fig. 5. C-AFM I–V curves measured for the Ag-rich phase of Agx(Ge0.25Se0.75)100 − x glasses with x = 10, 15 and 20 at.%.
as the glass changes from Ag10 to Ag20. Resistances R of about 97– 750 GΩ were evaluated from linear regression. If the contact is assumed to be circular and Ohmic in nature then the relation between the resistance R and the resistivity ρ is given by the basic spreading resistance formula R ~ ρ / 4r where r is the radius of the contact [22]. Hence, for the tip radius of ~10 nm, conductivity values for the Ag-rich phase between ~3 × 10−6 Ω− 1 cm− 1 and ~0.3 × 10−6 Ω−1 cm− 1 were calculated for the glasses. Even though the conductivity values obtained by C-AFM measurements are about one order of magnitude smaller than those commonly estimated by conventional measurements performed by complex impedance spectroscopy [23], the evolution of the conductivity follows the same trend with silver content of the glass. Also, additional information is obtained by the C-AFM experiment, i.e. the increase in conductivity of the Ag–Ge–Se samples in the high ionic conductivity region (x N 8–10at.%) does not result from an increase in amount of Ag-rich phase with a fixed composition and conductivity but from an increase in conductivity of Ag-rich phase. This indicates that the composition of the Ag-rich phase continually changes as silver is added to the base glass. This is the first time that this type of information has been assessed in Ag-conducting glasses since the methods usually used to probe the composition are based upon the use of an electron or photon beam which leads to diffusion of the Ag ions that are then to be discarded in these photosensitive glasses. Furthermore, this type of information is not accessible from the other techniques such as MDSC that have been used in previous work on phase separated glasses [13,14]. 4. Conclusion This work demonstrated the complementary of EFM and C-AFM experiments to perform a nanoscale electrical characterization of ion conducting chalcogenide glasses.
Acknowledgement The work was supported in the framework of the Project ANR-05BLAN-0058-01. References [1] X. Blasco, M. Nafrı́a, X. Aymerich, J. Pétry, W. Vandervorst, Microelectron. Reliab. 45 (5–6) (2005) 811. [2] N.S. Tambe, B. Bhushan, Scr. Mater. 52 (2005) 751. [3] P. Fiorenza, V. Raineri, Appl. Phys. Lett. 88 (2006) 212112. [4] A. Layson, S. Gadad, D. Teeters, Electrochim. Acta 48 (2003) 2207. [5] R. O'Hayre, M. Lee, F. Prinz, J. Appl. Phys. 95 (2004) 8382. [6] A.J. Bhattacharyya, J. Fleig, Y. Guo, J. Maier, Adv. Mater. 17 (2005) 2630. [7] A. Schirmeisen, A. Taskiran, H. Fuchs, H. Bracht, S. Murugavel, B. Roling, Phys. Rev. Lett. 98 (2007) 225901. [8] O. Douhéret, L. Lutsen, A. Swinnen, M. Breselge, K. Vandewal, L. Goris, J. Manca, Appl. Phys. Lett. 89 (2006) 032107. [9] I. Yee Phanga, T. Liuc, W. Zhangd, H. Schönherra, G.J. Vancso, Eur. Polym. J. 43 (10) (2007) 4136. [10] R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (6) (2005) 1077. [11] R. Pandian, B.J. Kooi, G. Palasantzas, J.T.M. De Hosson, A. Pauza, Appl. Phys. Lett. 91 (2007) 152103. [12] M. Gutenev, A. Tabolin, A. Rykova, Fiz. Khim. Stakla 17 (1991) 36. [13] M. Mitkova, Y. Wang, P. Boolchand, Phys. Rev. Lett. 83 (1999) 3848. [14] Y. Wang, M. Mitkova, D.G. Georgiev, S. Mamedov, P. Boolchand, J. Phys. Condens. Matter 15 (2003) S1573. [15] M. Kozicki, M. Mitkova, US Patent App. 11/276,097 (2006). [16] A.V. Kolobov, S.R. Elliott, Adv. Phys. 40 (1991) 625. [17] V. Balan, A.A. Piarristeguy, M. Ramonda, A. Pradel, M. Ribes, J. Optoelect. Adv. Mat. 8 (6) (2006) 2112. [18] A.A. Piarristeguy, M. Ramonda, N. Kuwata, A. Pradel, M. Ribes, J. Non-Cryst. Solids 177 (2006) 3157. [19] A.A. Piarristeguy, M. Ramonda, M.A. Ureña, A. Pradel, M. Ribes, J. Non-Cryst. Solids 353 (2007) 1261. [20] A.A. Piarristeguy, G.J. Cuello, P. Yot, A. Pradel, M. Ribes, J. Phys. Conden. Matter 20 (2008) 155106. [21] P. Girard, M. Ramonda, D. Saluel, J. Vac. Sci. Technol. B 20 (2002) 1348. [22] P. De Wolf, E. Brazel, A. Erickson, Mater. Sci. Semicond. Process. 4 (2001) 71. [23] M.A. Ureña, A.A. Piarristeguy, M. Fontana, B. Arcondo, Solid State Ion. 176 (2005) 505.