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Scanning-tunneling-microscope modifications of Cu(Ag)-chalcogenide glasses
Journal of Non-Crystalline Solids 227–230 Ž1998. 784–788
Scanning-tunneling-microscope modifications of Cuž Ag/ -chalcogenide glasses M. Ohto, K. Tanaka
)
Department of Applied Physics, Faculty of Engineering, Hokkaido UniÕersity, Sapporo 060, Japan
Abstract Chalcogenide glasses containing Cu and Ag were investigated using a scanning tunneling microscope ŽSTM.. Cu–As–Se and Ag–As–Se glasses exhibited different phenomena. When voltages applied to STM tips were low Ž< V < F 1 V., tunneling spectra in Ag–As–Se changed with scan speed of the tip voltages, while such a feature was not observed in Cu–As–Se. At high voltages Ž< V < G 3 V., geometrical surface modifications appeared in both glasses, but the shapes were different. In Ag–As–Se hills or holes were created, and in Cu–As–Se deformations were induced under negative tip voltages. Origins of these observations are discussed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscope ŽSTM.; Chalcogenide glasses; Cu–As–Se; Ag–As–Se; Micro-fabrication
1. Introduction Recently, scanning tunneling microscopes ŽSTMs. have been employed as a tool for surface inspection and also as a machine for fabricating with nanometer dimensions w1,2x. Chalcogenide glasses appear to be a material suitable for photo-electro micro-fabrications because of their unique properties. Utsugi w3,4x has demonstrated high-resolution STM etching of Ag–Se films, in which there is Agq ionic conduction. In conventional chalcogenide glasses such as As 2 SŽSe. 3 , micron-scale modifications have been induced by irradiation with light and electron beams w5,6x.
)
Corresponding author. Fax: q81-011 716 6175; e-mail: [email protected].
Despite these interesting features, systematic studies of micro-fabrication of chalcogenide glasses have not been reported at present. Specifically, STM studies have been limited in scope. In the present work, therefore, microscopic properties of CuŽAg. –As–Se glasses are investigated using a conventional STM. Interesting features in scanning tunneling spectroscopy ŽSTS. and surface modifications are demonstrated, and underlying mechanisms are discussed.
2. Experiments Two kinds of glasses were prepared as samples. One series consisted of the mixed ion-hole conductors Ag x ŽAs 0.4 Se 0.6 .100yx where x s 15, 20, 25, 35; the other of electronic conductors
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 7 9 - 3
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
Cu x ŽAs 0.4 Se 0.6 .100yx where x s 0, 10, 20, 30. These glasses were prepared through the conventional melt-quenching method, and freshly-cleaved surfaces were inspected using an STM. STM observations, STS measurements and surface modifications were performed using a commercial STM ŽDigital Instruments, Nanoscope E. in air at room temperature. Mechanically-sharpened PtIr wires were used as STM tips. STM images were observed under a constant-current mode with a tip bias of ; y0.2 V and a tunneling current of ; 200 pA. STS spectra were measured in a voltage range of y1 to 1 V, and the voltage-scan rate was varied from 1.2 to 40 Vrs. The STS measurements were repeated 100 times, and the averages were displayed. Surface modifications were induced by applying pulse voltages to the STM tip after fixing its position. The applied voltages were "2 to "10 V and the durations were 0.1 ms to 1 s.
3. Results 3.1. Scanning tunneling spectroscopy Fig. 1 shows differential conductance spectra of Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 glasses obtained under fast Ž40 Vrs. and slow Ž1.2 Vrs. voltage scans. The features observed in
Fig. 1. The STS spectra of Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 . Solid lines and dashed lines show the spectra obtained in the fast scan Ž40 Vrs. and the slow scan Ž1.2 Vrs..
785
Ag 35 As 26 Se 39 and in Cu 30 As 28 Se 42 appear to be very different. In Ag 35 As 26 Se 39 , we see that the differential conductance in the fast scan Žsolid line. is larger than that in the slow scan Ždashed line., specifically under positive tip voltages. In contrast, in Cu 30 As 28 Se 42 , the two spectra are mostly the same over the entire range of the voltage. The slow scan just gives a more noisy spectrum, probably due to mechanical instability. Similar scan-speed dependences of STS spectra have been observed in all glasses that were prepared. In the Ag–As–Se system, the scan-speed dependence becomes less in glasses with smaller Ag contents, which possess smaller ionic conductivity Žsee Fig. 3.. In contrast, no composition dependence is detected in the Cu–As–Se system.
3.2. Surface modifications Fig. 2 shows surface modifications of Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 induced by a voltage pulse V s y5V with a width of 1 s. Cross sectional views along the lines marked by arrows in Ža. and Žb. are also shown in ŽaX . and ŽbX .. We see that the structures fabricated in Ag 35 As 26 Se 39 and Cu 30 As 28 Se 42 differ. In Ag 35 As 26 Se 39 , only a hill Ž; 200 nm in diameter and ; 20 nm in height. exists at the center of the image. In contrast, in Cu 30 As 28 Se 42 , the structure consists of a hill Ž; 300 nm in diameter and ; 30 nm in height. which is surrounded by a valley with a depth of ; 10 nm. Note that volumes of the hill and the valley are comparable. To investigate whether the modifications are geometrical or just reflections of chemical modifications, two kinds of experiments have been performed using STS and an atomic force microscope. The STS measurements demonstrated that the modified structures exhibit the same tunneling spectra as the unmodified parts of the sample. Moreover, the surface modifications were also detected by using an atomic force microscope ŽBurleigh, ARIS-3300.. These two observations indicate that the modifications are geometrical. Details of the voltage dependence have been investigated. When the tip voltage is y3 to y5 V for
786
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
Fig. 2. The STM images of surface modifications in Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 created by applying a voltage pulse Ž V s y5 X X V, 1 s.. Ža . and Žb . are cross-sectional views.
Fig. 3. The compositional dependence of electrical conductivity ŽOhto and Tanaka, unpublished.. Solid circles and squares show ionic and hole conductivity of Ag–As–Se glasses, and open squares show hole conductivity of Cu–As–Se glasses. Lines are guides to the eye.
Ag 35 As 26 Se 39 and y3 to y10 V for Cu 30 As 28 Se 42 , similar structures to those shown in Fig. 2 are fabricated. However, in Ag 35 As 26 Se 39 , when V F y6 V, a hole is created. On the other hand, in both glasses, when the tip voltage is positive and longer than q3 V, holes are created. When < V < - 3 V, no geometrical modifications are detected. Composition dependence of the modifications appears also interesting. In all the Ag–As–Se glasses examined, similar features in structure and voltage dependence were observed. On the contrary, in Cu– As–Se glasses, a surface modification as shown in Fig. 2 Žb,bX . becomes smaller with a decrease in Cu content. That is, in Cu 20 As 32 Se 48 , the size of the central hill was, typically, 50 nm in diameter and 5 nm in height. In Cu 10 As 36 Se 54 , surface modifications were not observed. In As 2 Se 3 glass, STM
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
imaging was impossible, probably due to small electrical conductivity ŽFig. 3..
4. Discussion 4.1. Scan-speed dependence Before discussing the scan-speed dependence manifested in Ag–As–Se glasses ŽFig. 1., we first summarize related features. First, it is known that the Ag–As–Se glasses of interest exhibit an Agq ionic conductivity which is larger than the electronic conductivity ŽFig. 3., and that the Ag content substantially affects the energy position of the top of the valence band w7x. Secondly, it is understood that STS spectra in the positive and negative tip-voltage regions give information of the valence- and conduction-band edges, respectively w8x. Taking this knowledge into account, we assume that the scan-speed dependence observed in the Ag– As–Se glasses subjected to the positive tip voltages is induced by microscopic Agq-ion migration. That is, in the fast scan, STS spectra are assumed to be obtained before Agq ions migrate. Thus, an intrinsic electronic density-of-states of the glass surface can be obtained. However, in the slow scan, Agq ions can move under electric fields produced by the STM tip during the measurements. When the tip voltage is positive Agq ions move away, and when it is negative Agq ions gather. The changes in the Ag content appear to affect the electronic density-of-states in the sample surface, specifically in the valence-band side. This effect is attributed to the fact that the valence and the conduction band of Ag–As–Se glasses are known to consist of Ag–Se states and As–Se antibonding states w7x. As a result, the STS spectrum in the positive tip voltages Žthe valence-band side. has a greater dependence upon the voltage scan-speed. In contrast, Cu–As–Se glasses exhibit no ionic conduction w9x, and accordingly STS spectra are independent of the scan speed. 4.2. Hill formation We account for the hole creation described in Section 3.2 by assuming a field evaporation w1,2,10x.
787
The principal feature is well understood, and accordingly, in the following, we will concentrate upon the hills fabricated by negative voltage pulses. Fig. 2 demonstrates that the shapes of the modifications produced in the Ag- and Cu-chalcogenide glasses differ, which implies different underlying mechanisms. The hills in the Ag–As–Se glasses are assumed to be caused by field desorption w10,11x. That is, Ag– As–Se fragments are absorbed on the tip, and these can be transferred to the sample surface under the negative voltage pulses. This model is supported by the following observations: When the voltage pulses were applied repeatedly, the size of the hills becomes smaller, and ultimately, no deformations appeared. The modification mechanism of the Cu–As–Se glasses is still speculative. Tunneling electron induced fluidity and electrostatic forces may be responsible. We first note that the created structures are similar to those induced in As 2 SŽSe. 3 exposed to electron beams w6x. The electron-beam-induced deformation has been understood by assuming fluidity and electrostatic force, and we may envisage a similar mechanism here. That is, when a voltage pulse of y5 V is applied to a tip, electrons having energy of ; 5 eV are injected into the glass surface, where they can generate electron-hole pairs. Then, through some processes, presently speculative, the fluidity at the sample surface may increase. Simultaneously, the tip exerts electrostatic force upon the sample surface, and in response the surface deforms. However, many features remain unresolved. For instance, why does the peculiar modification appear only in the Cu-alloys, and not in the Ag-alloys? As shown in Fig. 3, the electrical conductivities of these glasses are comparable. It is mentioned here that no similar deformations appear in As 2Te 3 glass, which exhibits a comparable electrical conductivity. Accordingly, it seems difficult to find a reason only in the electrical conductivity. Alternatively, a reason may lie in the chemical nature of Cu–Se bonds, but no convincing explanation can be put forward here. In addition, we also cannot explain why the modification appears only under negative tip voltages. The polarity dependence may be related to the fact that in chalcogenide glasses holes are more mobile than electrons w12x, but that relationship remains speculative at present.
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M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
5. Conclusions Scanning tunneling spectra probed under positive tip voltages and surface modifications induced by negative tip voltages substantially different between the two glassy systems. The scan-speed dependence of the tunneling spectra for Ag–As–Se glasses can be related to the Agq migration. However, origins of the peculiar surface modification observed in Cu– As–Se glasses when exposed to a negatively-biased tip are speculative.
Acknowledgements The authors would like to thank Dr. Kitao for providing Ag–As–Se glasses. The present work is partially supported by funds from the Ministry of Education and Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.
References w1x T.T. Tsong, C.-S. Chang, Jpn. J. Appl. Phys. 34 Ž1995. 3309. w2x S. Hosaka, S. Hosoki, T. Hasegawa, H. Koyanagi, T. Shintani, M. Miyamoto, J. Vac. Sci. Technol. B 13 Ž1995. 2813. w3x Y. Utsugi, Nature 347 Ž1990. 747. w4x Y. Utsugi, Jpn. J. Appl. Phys. 32 Ž1993. 2969. w5x H. Hisakuni, K. Tanaka, Science 270 Ž1995. 974. w6x K. Tanaka, Appl. Phys. Lett. 70 Ž1997. 261. w7x M. Itoh, J. Non-Cryst. Solids 210 Ž1997. 178. w8x R.J. Hamers, in: H.-J. Guntherodt, R. Wiesendanger ŽEds.., ¨ Scanning Tunneling Microscopy I, Chap. 5, 2nd edn., Springer, Berlin, 1994. w9x Y.G. Vlasov, E.A. Bychkov, Solid State Ionics 14 Ž1984. 329. w10x K. Bessho, S. Hashimoto, Appl. Phys. Lett. 65 Ž1994. 2142. w11x M.S. Gupalo, I.L. Yarish, V.M. Zlupko, Yu. Suchorski, J. Vac. Sci. Technol. B 15 Ž1997. 491. w12x N.F. Mott, E.A. Davis, Electron Processes Non-Crystalline Materials, 2nd edn., Clarendon, Oxford, 1979.
Scanning-tunneling-microscope modifications of Cuž Ag/ -chalcogenide glasses M. Ohto, K. Tanaka
)
Department of Applied Physics, Faculty of Engineering, Hokkaido UniÕersity, Sapporo 060, Japan
Abstract Chalcogenide glasses containing Cu and Ag were investigated using a scanning tunneling microscope ŽSTM.. Cu–As–Se and Ag–As–Se glasses exhibited different phenomena. When voltages applied to STM tips were low Ž< V < F 1 V., tunneling spectra in Ag–As–Se changed with scan speed of the tip voltages, while such a feature was not observed in Cu–As–Se. At high voltages Ž< V < G 3 V., geometrical surface modifications appeared in both glasses, but the shapes were different. In Ag–As–Se hills or holes were created, and in Cu–As–Se deformations were induced under negative tip voltages. Origins of these observations are discussed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscope ŽSTM.; Chalcogenide glasses; Cu–As–Se; Ag–As–Se; Micro-fabrication
1. Introduction Recently, scanning tunneling microscopes ŽSTMs. have been employed as a tool for surface inspection and also as a machine for fabricating with nanometer dimensions w1,2x. Chalcogenide glasses appear to be a material suitable for photo-electro micro-fabrications because of their unique properties. Utsugi w3,4x has demonstrated high-resolution STM etching of Ag–Se films, in which there is Agq ionic conduction. In conventional chalcogenide glasses such as As 2 SŽSe. 3 , micron-scale modifications have been induced by irradiation with light and electron beams w5,6x.
)
Corresponding author. Fax: q81-011 716 6175; e-mail: [email protected].
Despite these interesting features, systematic studies of micro-fabrication of chalcogenide glasses have not been reported at present. Specifically, STM studies have been limited in scope. In the present work, therefore, microscopic properties of CuŽAg. –As–Se glasses are investigated using a conventional STM. Interesting features in scanning tunneling spectroscopy ŽSTS. and surface modifications are demonstrated, and underlying mechanisms are discussed.
2. Experiments Two kinds of glasses were prepared as samples. One series consisted of the mixed ion-hole conductors Ag x ŽAs 0.4 Se 0.6 .100yx where x s 15, 20, 25, 35; the other of electronic conductors
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 7 9 - 3
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
Cu x ŽAs 0.4 Se 0.6 .100yx where x s 0, 10, 20, 30. These glasses were prepared through the conventional melt-quenching method, and freshly-cleaved surfaces were inspected using an STM. STM observations, STS measurements and surface modifications were performed using a commercial STM ŽDigital Instruments, Nanoscope E. in air at room temperature. Mechanically-sharpened PtIr wires were used as STM tips. STM images were observed under a constant-current mode with a tip bias of ; y0.2 V and a tunneling current of ; 200 pA. STS spectra were measured in a voltage range of y1 to 1 V, and the voltage-scan rate was varied from 1.2 to 40 Vrs. The STS measurements were repeated 100 times, and the averages were displayed. Surface modifications were induced by applying pulse voltages to the STM tip after fixing its position. The applied voltages were "2 to "10 V and the durations were 0.1 ms to 1 s.
3. Results 3.1. Scanning tunneling spectroscopy Fig. 1 shows differential conductance spectra of Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 glasses obtained under fast Ž40 Vrs. and slow Ž1.2 Vrs. voltage scans. The features observed in
Fig. 1. The STS spectra of Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 . Solid lines and dashed lines show the spectra obtained in the fast scan Ž40 Vrs. and the slow scan Ž1.2 Vrs..
785
Ag 35 As 26 Se 39 and in Cu 30 As 28 Se 42 appear to be very different. In Ag 35 As 26 Se 39 , we see that the differential conductance in the fast scan Žsolid line. is larger than that in the slow scan Ždashed line., specifically under positive tip voltages. In contrast, in Cu 30 As 28 Se 42 , the two spectra are mostly the same over the entire range of the voltage. The slow scan just gives a more noisy spectrum, probably due to mechanical instability. Similar scan-speed dependences of STS spectra have been observed in all glasses that were prepared. In the Ag–As–Se system, the scan-speed dependence becomes less in glasses with smaller Ag contents, which possess smaller ionic conductivity Žsee Fig. 3.. In contrast, no composition dependence is detected in the Cu–As–Se system.
3.2. Surface modifications Fig. 2 shows surface modifications of Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 induced by a voltage pulse V s y5V with a width of 1 s. Cross sectional views along the lines marked by arrows in Ža. and Žb. are also shown in ŽaX . and ŽbX .. We see that the structures fabricated in Ag 35 As 26 Se 39 and Cu 30 As 28 Se 42 differ. In Ag 35 As 26 Se 39 , only a hill Ž; 200 nm in diameter and ; 20 nm in height. exists at the center of the image. In contrast, in Cu 30 As 28 Se 42 , the structure consists of a hill Ž; 300 nm in diameter and ; 30 nm in height. which is surrounded by a valley with a depth of ; 10 nm. Note that volumes of the hill and the valley are comparable. To investigate whether the modifications are geometrical or just reflections of chemical modifications, two kinds of experiments have been performed using STS and an atomic force microscope. The STS measurements demonstrated that the modified structures exhibit the same tunneling spectra as the unmodified parts of the sample. Moreover, the surface modifications were also detected by using an atomic force microscope ŽBurleigh, ARIS-3300.. These two observations indicate that the modifications are geometrical. Details of the voltage dependence have been investigated. When the tip voltage is y3 to y5 V for
786
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
Fig. 2. The STM images of surface modifications in Ža. Ag 35 As 26 Se 39 and Žb. Cu 30 As 28 Se 42 created by applying a voltage pulse Ž V s y5 X X V, 1 s.. Ža . and Žb . are cross-sectional views.
Fig. 3. The compositional dependence of electrical conductivity ŽOhto and Tanaka, unpublished.. Solid circles and squares show ionic and hole conductivity of Ag–As–Se glasses, and open squares show hole conductivity of Cu–As–Se glasses. Lines are guides to the eye.
Ag 35 As 26 Se 39 and y3 to y10 V for Cu 30 As 28 Se 42 , similar structures to those shown in Fig. 2 are fabricated. However, in Ag 35 As 26 Se 39 , when V F y6 V, a hole is created. On the other hand, in both glasses, when the tip voltage is positive and longer than q3 V, holes are created. When < V < - 3 V, no geometrical modifications are detected. Composition dependence of the modifications appears also interesting. In all the Ag–As–Se glasses examined, similar features in structure and voltage dependence were observed. On the contrary, in Cu– As–Se glasses, a surface modification as shown in Fig. 2 Žb,bX . becomes smaller with a decrease in Cu content. That is, in Cu 20 As 32 Se 48 , the size of the central hill was, typically, 50 nm in diameter and 5 nm in height. In Cu 10 As 36 Se 54 , surface modifications were not observed. In As 2 Se 3 glass, STM
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
imaging was impossible, probably due to small electrical conductivity ŽFig. 3..
4. Discussion 4.1. Scan-speed dependence Before discussing the scan-speed dependence manifested in Ag–As–Se glasses ŽFig. 1., we first summarize related features. First, it is known that the Ag–As–Se glasses of interest exhibit an Agq ionic conductivity which is larger than the electronic conductivity ŽFig. 3., and that the Ag content substantially affects the energy position of the top of the valence band w7x. Secondly, it is understood that STS spectra in the positive and negative tip-voltage regions give information of the valence- and conduction-band edges, respectively w8x. Taking this knowledge into account, we assume that the scan-speed dependence observed in the Ag– As–Se glasses subjected to the positive tip voltages is induced by microscopic Agq-ion migration. That is, in the fast scan, STS spectra are assumed to be obtained before Agq ions migrate. Thus, an intrinsic electronic density-of-states of the glass surface can be obtained. However, in the slow scan, Agq ions can move under electric fields produced by the STM tip during the measurements. When the tip voltage is positive Agq ions move away, and when it is negative Agq ions gather. The changes in the Ag content appear to affect the electronic density-of-states in the sample surface, specifically in the valence-band side. This effect is attributed to the fact that the valence and the conduction band of Ag–As–Se glasses are known to consist of Ag–Se states and As–Se antibonding states w7x. As a result, the STS spectrum in the positive tip voltages Žthe valence-band side. has a greater dependence upon the voltage scan-speed. In contrast, Cu–As–Se glasses exhibit no ionic conduction w9x, and accordingly STS spectra are independent of the scan speed. 4.2. Hill formation We account for the hole creation described in Section 3.2 by assuming a field evaporation w1,2,10x.
787
The principal feature is well understood, and accordingly, in the following, we will concentrate upon the hills fabricated by negative voltage pulses. Fig. 2 demonstrates that the shapes of the modifications produced in the Ag- and Cu-chalcogenide glasses differ, which implies different underlying mechanisms. The hills in the Ag–As–Se glasses are assumed to be caused by field desorption w10,11x. That is, Ag– As–Se fragments are absorbed on the tip, and these can be transferred to the sample surface under the negative voltage pulses. This model is supported by the following observations: When the voltage pulses were applied repeatedly, the size of the hills becomes smaller, and ultimately, no deformations appeared. The modification mechanism of the Cu–As–Se glasses is still speculative. Tunneling electron induced fluidity and electrostatic forces may be responsible. We first note that the created structures are similar to those induced in As 2 SŽSe. 3 exposed to electron beams w6x. The electron-beam-induced deformation has been understood by assuming fluidity and electrostatic force, and we may envisage a similar mechanism here. That is, when a voltage pulse of y5 V is applied to a tip, electrons having energy of ; 5 eV are injected into the glass surface, where they can generate electron-hole pairs. Then, through some processes, presently speculative, the fluidity at the sample surface may increase. Simultaneously, the tip exerts electrostatic force upon the sample surface, and in response the surface deforms. However, many features remain unresolved. For instance, why does the peculiar modification appear only in the Cu-alloys, and not in the Ag-alloys? As shown in Fig. 3, the electrical conductivities of these glasses are comparable. It is mentioned here that no similar deformations appear in As 2Te 3 glass, which exhibits a comparable electrical conductivity. Accordingly, it seems difficult to find a reason only in the electrical conductivity. Alternatively, a reason may lie in the chemical nature of Cu–Se bonds, but no convincing explanation can be put forward here. In addition, we also cannot explain why the modification appears only under negative tip voltages. The polarity dependence may be related to the fact that in chalcogenide glasses holes are more mobile than electrons w12x, but that relationship remains speculative at present.
788
M. Ohto, K. Tanakar Journal of Non-Crystalline Solids 227–230 (1998) 784–788
5. Conclusions Scanning tunneling spectra probed under positive tip voltages and surface modifications induced by negative tip voltages substantially different between the two glassy systems. The scan-speed dependence of the tunneling spectra for Ag–As–Se glasses can be related to the Agq migration. However, origins of the peculiar surface modification observed in Cu– As–Se glasses when exposed to a negatively-biased tip are speculative.
Acknowledgements The authors would like to thank Dr. Kitao for providing Ag–As–Se glasses. The present work is partially supported by funds from the Ministry of Education and Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.
References w1x T.T. Tsong, C.-S. Chang, Jpn. J. Appl. Phys. 34 Ž1995. 3309. w2x S. Hosaka, S. Hosoki, T. Hasegawa, H. Koyanagi, T. Shintani, M. Miyamoto, J. Vac. Sci. Technol. B 13 Ž1995. 2813. w3x Y. Utsugi, Nature 347 Ž1990. 747. w4x Y. Utsugi, Jpn. J. Appl. Phys. 32 Ž1993. 2969. w5x H. Hisakuni, K. Tanaka, Science 270 Ž1995. 974. w6x K. Tanaka, Appl. Phys. Lett. 70 Ž1997. 261. w7x M. Itoh, J. Non-Cryst. Solids 210 Ž1997. 178. w8x R.J. Hamers, in: H.-J. Guntherodt, R. Wiesendanger ŽEds.., ¨ Scanning Tunneling Microscopy I, Chap. 5, 2nd edn., Springer, Berlin, 1994. w9x Y.G. Vlasov, E.A. Bychkov, Solid State Ionics 14 Ž1984. 329. w10x K. Bessho, S. Hashimoto, Appl. Phys. Lett. 65 Ž1994. 2142. w11x M.S. Gupalo, I.L. Yarish, V.M. Zlupko, Yu. Suchorski, J. Vac. Sci. Technol. B 15 Ž1997. 491. w12x N.F. Mott, E.A. Davis, Electron Processes Non-Crystalline Materials, 2nd edn., Clarendon, Oxford, 1979.