- Email: [email protected]
The atomic resolution imaging of metallic Ag(111) surface by noncontact atomic force microscope
Applied Surface Science 140 Ž1999. 243–246
The atomic resolution imaging of metallic Agž111/ surface by noncontact atomic force microscope S. Orisaka a
a,)
, T. Minobe a , T. Uchihashi b, Y. Sugawara
a,b
, S. Morita
a
Department of Electronic Engineering, Graduate School of Engineering, Osaka UniÕersity, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan b Joint Research Center for Atom Technology, 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan Received 20 July 1998; accepted 25 August 1998
Abstract Atomic resolution imaging of the AgŽ111. surface is demonstrated with the noncontact atomic force microscope ŽAFM. using frequency modulation ŽFM. detection method in an ultrahigh vacuum at room temperature, for the first time. The constant excitation mode is used to suppress the destruction of the tip apex and sample surface, in which the constant amplitude voltage is supplied to piezoelectric scanner for cantilever oscillation. Trigonal pattern can be clearly seen. ˚ which is in good agreement with the lattice spacing of AgŽ111. Measured distance between the protrusions is 2.8 " 0.1 A, ˚ These results suggest that the noncontact AFM has potential surface. The corrugation height is estimated to be 0.1–0.2 A. for imaging pure metal surfaces with atomic resolution. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.16Ch; 68.35.Bs; 68.35.Dv Keywords: Noncontact atomic force microscopy; AgŽ111.; Metallic surface; Atomic resolution
1. Introduction Recently, several groups including ourselves have reported true atomic resolution imaging using noncontact atomic force microscope ŽAFM. w1–4x. In the noncontact AFM, attractive force gradient acting on tip is used to regulate the tip–sample separation. With a frequency modulation ŽFM. method w5x, the force gradient acting on tip is measured by the frequency shift of the oscillating cantilever due to
)
Corresponding author. Tel.: q81-6-879-7763; Fax: q81-6879-7764; E-mail: [email protected]
the force interaction. In order to apply the noncontact AFM as a scientific tool in a variety of fields such as surface science, it is necessary to observe various surfaces with atomic resolution and clarify the force interaction between tip and sample. Until now, clean semiconductor surfaces such as SiŽ111.7 = 7 w12,6– 9x, InPŽ110. w3,4,8x, SiŽ100.2 = 1 w10x, InAsŽ110. w11x, ionic crystal surface NaClŽ100. w12x and metal oxide surface TiO 2 Ž110. w13x have been observed with atomic resolution. However, there is no report for metallic surface. In the present experiments, we investigate the noncontact AFM imaging on metallic surface. For the first time, we report on atomic resolution imaging of pure metallic surface of AgŽ111..
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 3 4 - 0
244
S. Orisaka et al.r Applied Surface Science 140 (1999) 243–246
2. Experimental We chose Ag epitaxial film on SiŽ111.7 = 7 surface as an atomically flat metallic surface. The growth mode for the thin Ag film on SiŽ111. surface at room temperature is characterized by low island formation and its structure must be either pure islands of Ag on Si surface or pure island of Ag on top of a thin two-dimensional silicide layer on Si surface w14,15x. First, SiŽ111.7 = 7 substrate was prepared by in situ thermal treatments of silicon samples. Ag was then deposited on the sample at room temperature from a hot filament Ag evaporator at 4.8 = 10y1 1 Torr. ˚ Evaporation rate was about 5.7 Armin and the total ˚ The crysevaporated Ag amount was about 85 A. tallinity was checked by the observation of clear LEED patterns generated by the AgŽ111. surfaces w16x. Silver films on SiŽ111.7 = 7 showed only sharp hexagonal spots. We used NC-AFM operating in an ultrahigh vacuum ŽUHV., whose design is similar to Ref. w17x. The cantilever deflection was detected by an optical-fiber interferometer. The FM technique w5x was used to detect the force gradient acting on the tip. As an operating mode, the constant excitation mode was used to suppress the destruction of the tip apex and sample surface, in which the constant amplitude voltage is supplied to piezoelectric scanner for cantilever oscillation w18,19x. Further, absolute-value circuit was inserted between FM demodulator and the feedback controller to avoid the strong mechanical contact between tip and sample due to the reverse of the distance dependence of the feedback signal w19x. The noncontact AFM images were taken in constant force gradient mode, in which the resonant frequency shift D n was controlled to keep at constant level. The experiments were performed in UHV at a base pressure of - 3.3 = 10y1 0 Torr. As a force sensor, we used a conductive Si cantilever with a sharpened tip. Its spring constant and mechanical resonant frequency were about ; 27.4 Nrm and ; 152.3 kHz, respectively. The Si cantilever was cleaned by Ar ion sputtering. 3. Experimental results and discussion Fig. 1a shows a noncontact AFM image of the Ag film evaporated on SiŽ111.7 = 7 surface. The scan
Fig. 1. Ža. Noncontact AFM image of the Ag film evaporated on ˚ ˚ The SiŽ111.7=7 surface. The scan area was 800 A=800 A. frequency shift was set to be D n sy10 Hz. The vibration ˚ Žb. Nanostructure model of Ag amplitude was about A 0 s114 A. film evaporated on SiŽ111.7=7 surface.
˚ = 800 A. ˚ The frequency shift was area was 800 A set to be D n s y10 Hz. The vibration amplitude ˚ Atomically flat islands with was about A 0 s 114 A. ˚ are observed. The sizes between 100 and 300 A heights of the islands were estimated to be about 12 ˚ with respect to the background. Interestto 20 A ingly, the difference in height of the islands is an integer multiple of mono-atomic step height of 2.36 ˚ on AgŽ111. surface. This result means that the A atomically flat islands in Fig. 1a are the pure metallic island of AgŽ111.. The contact potential between tip and sample weakens the distance dependence on the force gradient due to weak distance dependence on the electrostatic force interaction and hence degrades the resolution of the noncontact AFM image. So, compensa-
S. Orisaka et al.r Applied Surface Science 140 (1999) 243–246
245
agreement with lattice spacing of AgŽ111. surface ŽFig. 2b.. Thus, for the first time, atomic resolution imaging on metallic surface was achieved by using noncontact AFM in UHV. From the cross-sectional profile, the measured corrugation height was esti˚ The measured corrugation mated to be 0.1–0.2 A. height are almost equal to that measured by STM w15x, although it depends on imaging parameters such as vibration amplitude A 0 and frequency shift D n . Further, Fig. 3 shows the high-resolution images of two terraces separated with an atomic step. The trigonal pattern which corresponds to the Ag atom can be clearly observed on upper terrace and it can be only slightly observed on lower terrace. However, the image obtained near the atomic step edge shows poorer resolution. The degradation of the resolution near the step edge is probably due to the weak contact between tip and surface, although the irreversible damage of the tip apex and the surface did not occur. That is, in order to make the tip–sample interaction strong enough for atomic resolution imaging on metallic surface, the atom of the tip for oscillating cantilever needs to be positioned very close to the atom of the sample. The distance control between tip and sample can be performed precisely on the flat terraces, but it becomes difficult near the
Fig. 2. Ža. Noncontact AFM imaging on the atomically flat surface ˚ ˚ Trigonal pattern with of AgŽ111.. The scan area was 50 A=50 A. ˚ can be clearly seen. Žb. Structure lattice spacing of 2.8"0.1 A ˚ model of the AgŽ111. surface. Lattice spacing is 2.88 A.
tion of contact potential between tip and surface is essentially important to obtain the atomic resolution image. In the high-resolution imaging on the pure metallic island of AgŽ111., bias voltage of y1.2 V was applied to sample. Fig. 2a shows the noncontact AFM image on the atomically flat surface of AgŽ111.. The scan area ˚ = 50 A. ˚ Trigonal pattern can be clearly was 50 A seen. No image processing was performed in Fig. 2a. The distance between the protrusions of these pattern ˚ which is in good was estimated to be 2.8 " 0.1 A,
Fig. 3. Noncontact AFM image with two terraces separated with ˚ ˚ an atomic step. The scan area was 20 A=20 A.
246
S. Orisaka et al.r Applied Surface Science 140 (1999) 243–246
step edge. Further technical improvements of the precise control of tip–sample distance are required to establish the noncontact AFM as a powerful scientific tool for metallic surface investigation. 4. Conclusions We demonstrated, for the first time, the atomic resolution imaging of the metallic surface of AgŽ111. with the noncontact AFM in a UHV. We observed clear trigonal pattern. Measured distance between the ˚ which was in good protrusions was 2.8 = 0.1 A, agreement with the lattice spacing of AgŽ111. surface. The corrugation height was estimated to be ˚ The trigonal pattern was observed on 0.1–0.2 A. both upper and lower terraces, but it was not observed near the atomic step edge, which seemed to be due to the weak contact between tip and surface without the precise control of tip–sample distance. Acknowledgements This work was supported by a grant-in-aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. References w1x F.J. Giessibl, Science 267 Ž1995. 68. w2x S. Kitamura, M. Iwatsuki, Jpn. J. Appl. Phys. 34 Ž1995. L145.
w3x H. Ueyama, M. Ohta, Y. Sugawara, S. Morita, Jpn. J. Appl. Phys. 34 Ž1995. L10869. w4x Y. Sugawara, M. Ohta, H. Ueyama, S. Morita, Science 270 Ž1995. 1646. w5x T.R. Albrecht, P. Grutter, D. Horne, D. Rugar, J. Appl. Phys. 69 Ž1991. 668. w6x N. Nakagiri, M. Suzuki, K. Okiguchi, H. Sugimura, Surf. Sci. 373 Ž1997. L329. w7x R. Erlandsson, L. Olsson, P. Martensson, Phys. Rev. B 54 Ž1996. R8309. w8x Y. Sugawara, H. Ueyama, T. Uchihashi, M. Ohta, S. Morita, M. Suzuki, S. Mishima, Appl. Surf. Sci. 113r114 Ž1997. 364. w9x T. Uchihashi, Y. Sugawara, T. Tsukamoto, M. Ohta, S. Morita, M. Suzuki, Phys. Rev. B 56 Ž1997. 9834. w10x S. Kitamura, M. Iwatsuki, Jpn. J. Appl. Phys. 35 Ž1996. L668. w11x W. Allers, A. Shwarz, U.D. Shwarz, R. Wiesendanger, Rev. Sci. Instr. 69 Ž1998. 221. w12x M. Bammerlin, R. Luthi, E. Meyer, A. Baratoff, J. Lu, ¨ ¨ M. Guggisberg, Ch. Gerber, L. Howald, H.-J. Guntherodt, Probe ¨ Microsc. 1 Ž1997. 3. w13x K. Fukui, H. Onishi, Y. Iwasawa, Phys. Rev. Lett. 79 Ž1997. 4202. w14x M. Schleberger, D. Fujita, C. Scharfschwerdt, S. Tougaard, J. Vac. Sci. Technol. B 13 Ž1995. 949. w15x G. Meyer, K.H. Rieder, Surf. Sci. 331–333 Ž1995. 600. w16x G. Neuhold, K. Horn, Phys. Rev. Lett. 78 Ž1997. 1327. w17x M. Ohta, Y. Sugawara, S. Morita, H. Nagahara, S. Mishima, T. Okada, J. Vac. Sci. Technol. B 12 Ž1994. 1705. w18x Y. Sugawara, H. Ueyama, T. Uchihashi, M. Ohta, Y. Yanase, T. Shigematsu, M. Suzuki, S. Morita in: J. Michel, T. Kenedy, K. Wada, K. Thonke ŽEds.., Electronic Materials II, MRS Symposia Proceedings No. 442, Materials Research Society, Pittsburgh, 1997, p. 15. w19x H. Ueyama, Y. Sugawara, S. Morita, Appl. Phys. A 66 Ž1998. S295.
The atomic resolution imaging of metallic Agž111/ surface by noncontact atomic force microscope S. Orisaka a
a,)
, T. Minobe a , T. Uchihashi b, Y. Sugawara
a,b
, S. Morita
a
Department of Electronic Engineering, Graduate School of Engineering, Osaka UniÕersity, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan b Joint Research Center for Atom Technology, 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan Received 20 July 1998; accepted 25 August 1998
Abstract Atomic resolution imaging of the AgŽ111. surface is demonstrated with the noncontact atomic force microscope ŽAFM. using frequency modulation ŽFM. detection method in an ultrahigh vacuum at room temperature, for the first time. The constant excitation mode is used to suppress the destruction of the tip apex and sample surface, in which the constant amplitude voltage is supplied to piezoelectric scanner for cantilever oscillation. Trigonal pattern can be clearly seen. ˚ which is in good agreement with the lattice spacing of AgŽ111. Measured distance between the protrusions is 2.8 " 0.1 A, ˚ These results suggest that the noncontact AFM has potential surface. The corrugation height is estimated to be 0.1–0.2 A. for imaging pure metal surfaces with atomic resolution. q 1999 Elsevier Science B.V. All rights reserved. PACS: 61.16Ch; 68.35.Bs; 68.35.Dv Keywords: Noncontact atomic force microscopy; AgŽ111.; Metallic surface; Atomic resolution
1. Introduction Recently, several groups including ourselves have reported true atomic resolution imaging using noncontact atomic force microscope ŽAFM. w1–4x. In the noncontact AFM, attractive force gradient acting on tip is used to regulate the tip–sample separation. With a frequency modulation ŽFM. method w5x, the force gradient acting on tip is measured by the frequency shift of the oscillating cantilever due to
)
Corresponding author. Tel.: q81-6-879-7763; Fax: q81-6879-7764; E-mail: [email protected]
the force interaction. In order to apply the noncontact AFM as a scientific tool in a variety of fields such as surface science, it is necessary to observe various surfaces with atomic resolution and clarify the force interaction between tip and sample. Until now, clean semiconductor surfaces such as SiŽ111.7 = 7 w12,6– 9x, InPŽ110. w3,4,8x, SiŽ100.2 = 1 w10x, InAsŽ110. w11x, ionic crystal surface NaClŽ100. w12x and metal oxide surface TiO 2 Ž110. w13x have been observed with atomic resolution. However, there is no report for metallic surface. In the present experiments, we investigate the noncontact AFM imaging on metallic surface. For the first time, we report on atomic resolution imaging of pure metallic surface of AgŽ111..
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 3 4 - 0
244
S. Orisaka et al.r Applied Surface Science 140 (1999) 243–246
2. Experimental We chose Ag epitaxial film on SiŽ111.7 = 7 surface as an atomically flat metallic surface. The growth mode for the thin Ag film on SiŽ111. surface at room temperature is characterized by low island formation and its structure must be either pure islands of Ag on Si surface or pure island of Ag on top of a thin two-dimensional silicide layer on Si surface w14,15x. First, SiŽ111.7 = 7 substrate was prepared by in situ thermal treatments of silicon samples. Ag was then deposited on the sample at room temperature from a hot filament Ag evaporator at 4.8 = 10y1 1 Torr. ˚ Evaporation rate was about 5.7 Armin and the total ˚ The crysevaporated Ag amount was about 85 A. tallinity was checked by the observation of clear LEED patterns generated by the AgŽ111. surfaces w16x. Silver films on SiŽ111.7 = 7 showed only sharp hexagonal spots. We used NC-AFM operating in an ultrahigh vacuum ŽUHV., whose design is similar to Ref. w17x. The cantilever deflection was detected by an optical-fiber interferometer. The FM technique w5x was used to detect the force gradient acting on the tip. As an operating mode, the constant excitation mode was used to suppress the destruction of the tip apex and sample surface, in which the constant amplitude voltage is supplied to piezoelectric scanner for cantilever oscillation w18,19x. Further, absolute-value circuit was inserted between FM demodulator and the feedback controller to avoid the strong mechanical contact between tip and sample due to the reverse of the distance dependence of the feedback signal w19x. The noncontact AFM images were taken in constant force gradient mode, in which the resonant frequency shift D n was controlled to keep at constant level. The experiments were performed in UHV at a base pressure of - 3.3 = 10y1 0 Torr. As a force sensor, we used a conductive Si cantilever with a sharpened tip. Its spring constant and mechanical resonant frequency were about ; 27.4 Nrm and ; 152.3 kHz, respectively. The Si cantilever was cleaned by Ar ion sputtering. 3. Experimental results and discussion Fig. 1a shows a noncontact AFM image of the Ag film evaporated on SiŽ111.7 = 7 surface. The scan
Fig. 1. Ža. Noncontact AFM image of the Ag film evaporated on ˚ ˚ The SiŽ111.7=7 surface. The scan area was 800 A=800 A. frequency shift was set to be D n sy10 Hz. The vibration ˚ Žb. Nanostructure model of Ag amplitude was about A 0 s114 A. film evaporated on SiŽ111.7=7 surface.
˚ = 800 A. ˚ The frequency shift was area was 800 A set to be D n s y10 Hz. The vibration amplitude ˚ Atomically flat islands with was about A 0 s 114 A. ˚ are observed. The sizes between 100 and 300 A heights of the islands were estimated to be about 12 ˚ with respect to the background. Interestto 20 A ingly, the difference in height of the islands is an integer multiple of mono-atomic step height of 2.36 ˚ on AgŽ111. surface. This result means that the A atomically flat islands in Fig. 1a are the pure metallic island of AgŽ111.. The contact potential between tip and sample weakens the distance dependence on the force gradient due to weak distance dependence on the electrostatic force interaction and hence degrades the resolution of the noncontact AFM image. So, compensa-
S. Orisaka et al.r Applied Surface Science 140 (1999) 243–246
245
agreement with lattice spacing of AgŽ111. surface ŽFig. 2b.. Thus, for the first time, atomic resolution imaging on metallic surface was achieved by using noncontact AFM in UHV. From the cross-sectional profile, the measured corrugation height was esti˚ The measured corrugation mated to be 0.1–0.2 A. height are almost equal to that measured by STM w15x, although it depends on imaging parameters such as vibration amplitude A 0 and frequency shift D n . Further, Fig. 3 shows the high-resolution images of two terraces separated with an atomic step. The trigonal pattern which corresponds to the Ag atom can be clearly observed on upper terrace and it can be only slightly observed on lower terrace. However, the image obtained near the atomic step edge shows poorer resolution. The degradation of the resolution near the step edge is probably due to the weak contact between tip and surface, although the irreversible damage of the tip apex and the surface did not occur. That is, in order to make the tip–sample interaction strong enough for atomic resolution imaging on metallic surface, the atom of the tip for oscillating cantilever needs to be positioned very close to the atom of the sample. The distance control between tip and sample can be performed precisely on the flat terraces, but it becomes difficult near the
Fig. 2. Ža. Noncontact AFM imaging on the atomically flat surface ˚ ˚ Trigonal pattern with of AgŽ111.. The scan area was 50 A=50 A. ˚ can be clearly seen. Žb. Structure lattice spacing of 2.8"0.1 A ˚ model of the AgŽ111. surface. Lattice spacing is 2.88 A.
tion of contact potential between tip and surface is essentially important to obtain the atomic resolution image. In the high-resolution imaging on the pure metallic island of AgŽ111., bias voltage of y1.2 V was applied to sample. Fig. 2a shows the noncontact AFM image on the atomically flat surface of AgŽ111.. The scan area ˚ = 50 A. ˚ Trigonal pattern can be clearly was 50 A seen. No image processing was performed in Fig. 2a. The distance between the protrusions of these pattern ˚ which is in good was estimated to be 2.8 " 0.1 A,
Fig. 3. Noncontact AFM image with two terraces separated with ˚ ˚ an atomic step. The scan area was 20 A=20 A.
246
S. Orisaka et al.r Applied Surface Science 140 (1999) 243–246
step edge. Further technical improvements of the precise control of tip–sample distance are required to establish the noncontact AFM as a powerful scientific tool for metallic surface investigation. 4. Conclusions We demonstrated, for the first time, the atomic resolution imaging of the metallic surface of AgŽ111. with the noncontact AFM in a UHV. We observed clear trigonal pattern. Measured distance between the ˚ which was in good protrusions was 2.8 = 0.1 A, agreement with the lattice spacing of AgŽ111. surface. The corrugation height was estimated to be ˚ The trigonal pattern was observed on 0.1–0.2 A. both upper and lower terraces, but it was not observed near the atomic step edge, which seemed to be due to the weak contact between tip and surface without the precise control of tip–sample distance. Acknowledgements This work was supported by a grant-in-aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. References w1x F.J. Giessibl, Science 267 Ž1995. 68. w2x S. Kitamura, M. Iwatsuki, Jpn. J. Appl. Phys. 34 Ž1995. L145.
w3x H. Ueyama, M. Ohta, Y. Sugawara, S. Morita, Jpn. J. Appl. Phys. 34 Ž1995. L10869. w4x Y. Sugawara, M. Ohta, H. Ueyama, S. Morita, Science 270 Ž1995. 1646. w5x T.R. Albrecht, P. Grutter, D. Horne, D. Rugar, J. Appl. Phys. 69 Ž1991. 668. w6x N. Nakagiri, M. Suzuki, K. Okiguchi, H. Sugimura, Surf. Sci. 373 Ž1997. L329. w7x R. Erlandsson, L. Olsson, P. Martensson, Phys. Rev. B 54 Ž1996. R8309. w8x Y. Sugawara, H. Ueyama, T. Uchihashi, M. Ohta, S. Morita, M. Suzuki, S. Mishima, Appl. Surf. Sci. 113r114 Ž1997. 364. w9x T. Uchihashi, Y. Sugawara, T. Tsukamoto, M. Ohta, S. Morita, M. Suzuki, Phys. Rev. B 56 Ž1997. 9834. w10x S. Kitamura, M. Iwatsuki, Jpn. J. Appl. Phys. 35 Ž1996. L668. w11x W. Allers, A. Shwarz, U.D. Shwarz, R. Wiesendanger, Rev. Sci. Instr. 69 Ž1998. 221. w12x M. Bammerlin, R. Luthi, E. Meyer, A. Baratoff, J. Lu, ¨ ¨ M. Guggisberg, Ch. Gerber, L. Howald, H.-J. Guntherodt, Probe ¨ Microsc. 1 Ž1997. 3. w13x K. Fukui, H. Onishi, Y. Iwasawa, Phys. Rev. Lett. 79 Ž1997. 4202. w14x M. Schleberger, D. Fujita, C. Scharfschwerdt, S. Tougaard, J. Vac. Sci. Technol. B 13 Ž1995. 949. w15x G. Meyer, K.H. Rieder, Surf. Sci. 331–333 Ž1995. 600. w16x G. Neuhold, K. Horn, Phys. Rev. Lett. 78 Ž1997. 1327. w17x M. Ohta, Y. Sugawara, S. Morita, H. Nagahara, S. Mishima, T. Okada, J. Vac. Sci. Technol. B 12 Ž1994. 1705. w18x Y. Sugawara, H. Ueyama, T. Uchihashi, M. Ohta, Y. Yanase, T. Shigematsu, M. Suzuki, S. Morita in: J. Michel, T. Kenedy, K. Wada, K. Thonke ŽEds.., Electronic Materials II, MRS Symposia Proceedings No. 442, Materials Research Society, Pittsburgh, 1997, p. 15. w19x H. Ueyama, Y. Sugawara, S. Morita, Appl. Phys. A 66 Ž1998. S295.