The effect of the surface enhanced polariton field on the tunneling current of a STM

8 May 1995

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PHYSICS

LETTERS A

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ELSEVIER

Physics Letters A 200 (1995) 438-444

The effect of the surface enhanced polariton field on the tunneling current of a STM Igor Smolyaninov a Institute of

a, Anatoly Zayats a, Ole Keller b

Russian Academy of Sciences, Troitsk, Moscow Region 142092, Russian Federation b Institute of Physics, University of Aalborg, Pontoppidanstmde 103, DK-9220 Aalborg last, Denmark Spectroscopy,

Received 20 February 1995; accepted for publication 3 March 1995 Communicated by V.M. Agranovich

Abstract ( SPs) , excited on the external (fast mode) and internal (slow mode) surfaces of a The role of the surface polaritons is directly demonstrated by the angular excitation spectra of the induced tunneling current. Possible mechanisms which might give rise to the SP induced change in the tunneling current are discussed. An electromagnetic field of SPs within a tunneling gap is estimated to be surface enhanced in order to explain the value of the induced tunneling current. Images of the polariton induced signal distribution over the sample surface taken by means of different SP modes are compared in order to obtain information on the structure of the interna surface of the film. The influence

of surface polaritons

gold film, on the tunneling current of a STM is investigated.

1. Introduction A few years ago the scanning tunneling microscopy technique for the first time has been used to investigate the interaction of tunneling electrons with surface polaritons (SP) [ l-31. By excitation of the surface polaritons confined to the external surface of a silver film, an additional SP induced tunneling current was observed, and it was assumed that this current originates in a rectification of the optical field due to the nonlinearity of the Z-V curve of the tunneling junction. The distribution of the polariton induced current over the silver film surface was also measured, and a correlation between this distribution and the topography of the film was found. For a thin metal film deposited on a dielectric substrate it is often possible to examine optically the two metal surfaces by generating SPs at each of the sur-

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faces [ 41. Polaritons confined to the internal metaldielectric interface are usually called slow surface polaritons (SSPs) in order to distinguish them from the polaritons localized on the external surface (fast surface polaritons) . If the metal film is sufficiently thick the SP modes of the two interfaces are decoupled and SPs can be excited separately at each interface by varying the angle of incidence keeping the excitation wavelength fixed. The electromagnetic field associated with the SPs is concentrated at the interface region and thus very sensitive to the defect structure, the surface roughness etc. [ 4,5]. The aim of this work is to study the effect of surface polaritons on the tunneling current of a STM in order to make clear the physical picture of this phenomenon. Possible mechanisms which might give rise to the polariton induced current are discussed. The possibility of STM imaging of the internal interface using the

I. Smolyaninou et al. /Physics

current induced studied.

by the slow surface polariton

Letters A 200 (1995) 438-444

439

is also movable mirror

piezo-tube

2. Experimental

results

The experiments on light induced tunneling currents were performed with a STM operated in air and described elsewhere [ 61. To excite surface polaritons the metal film is deposited on a glass prism and the exciting light is incident through the prism (Kretschmann configuration) [ 71. Two structures have been investigated, namely, a Au film with a thickness of about 800 A deposited on a prism with refractive index n = 1.56 (this structure allows us to excite surface polaritons at the metal-air interface), and a two-layer structure consisting of a Au film (600 A) deposited on a magnesium fluoride layer (d = 1700 A, n = 1.28) on top of a prism (n = 1.64). The last structure gives the possibility to excite two surface polariton modes, viz., a fast surface polariton at the metal-air interface and the slow SP mode at the metal-fluoride interface. The light from a He-Ne laser (A = 6328 A, P = 10 mW) has been used for the SP excitation. For comparison, optical reflection measurements were made of the angular excitation spectra of the polaritons. By fitting these angular spectra to the calculated reflection coefficients of the layered structure, the dielectric constants of the gold films have been obtained. The scanning tunneling microscope was working in the constant current mode (tunneling current of about 0.5 nA at a tip-sample voltage of about 50 mV) , and the STM measurements have been performed with the unfocused laser light directed through a prism onto the Au film near the STM tip (Fig. 1) . The set-up gives the possibility to scan the angle of incidence just by moving the mirror. The intensity of the light incident on the Au film is estimated to be about 0.1-0.2 W/cm*. The laser radiation was modulated by a chopper with a variable frequency (from 1.5 to 4 kHz). Since the modulation frequency was out of the frequency band of the feedback system, the feedback loop did not affect changes of the tunneling current induced by the light. A surface polariton induced tunneling current of the order of 10 pA has been selected with a lock-inamplifier and recorded simultaneously with the feedback signal which directly reveals the topography of

Fig. 1. The combined

30

30

STM and surface-wave

40 Angle

40

spectroscopic

50 60 of Incidence

50 60 70 Angle of Incidence

set-up.

70

60

Fig. 2. Angular excitation spectrum of the surface polaritons (circles) and angular dependence of the induced tunneling current (squares) for the Au/prism (a) and Au/fluoride/prism (b) structures. The solid line is the reflectivity calculated with a Au film refractive index n = 0.13 + 2.7i (a) and n = 0.13 + 2.751 (b), respectively.

the sample. To eliminate thermal drift and instabilities occurring during measurements of the angular dependences of the polariton induced tunneling current, at every chosen angle of incidence we recorded topography maps

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I. Smolyaninov et al./Physics

TOPOGRAPHY

Letters A 200 (1995) 438-444

POLARITON INDUCED SIGNAL

Fig. 3. 3.5 x 1.2 pm* topographical and induced tunneling current image of an 800 A gold film evaporated scale of the topographical image is 1200 A from black to white. ( 1.2x 1.2 ,um2) and maps of the induced changes of the tunneling current. Afterwards the data were averaged over the map region. The angular dependences of the induced tunneling current and the reflection coefficient are shown in Fig. 2a for the first sample investigated. It is seen that the light induced tunneling current is significant only in the angle of incidence range where the surface polariton is excited. The topography of the sample (Fig. 3) is the usual one for a micron-size inhomogeneous film (the topography grey scale corresponds to a height of 1200 A from white to black). The SP induced image reveals analogous features and apparently results from the topography. But the topography is reflected in the SP induced images in a nonlinear manner. The spatial variation of the induced tunneling current can be un-

onto the prism. The vertical

derstood in terms of the lateral distribution of the surface polariton field. Both valleys and hiUs of a topography scatter the surface polaritons resulting in analogous changes of the induced current. The strength of the surface polariton field, which influences the tunneling, is determined not only by a scattering by the film inhomogeneity but also by the SP field enhancement at defects, by the polariton localization, etc., as a result, the signal variation across the SP induced image depends on the defect distribution as well as the defect sizes and dielectric parameters. In fact, the lateral distribution of the SP field depends on the surface structure and the inner defect structure of the metal film. Since the fast surface polariton field is located in the vicinity of the air-metal surface the SP image is influenced mainly by the surface topography.

I. Smolyaninou et al. /Physics

TOPOGRAPHY

Fig. 4. 1.2x 1.2 ym* topographical and surface angles of incidence (compare with Fig. 2b).

Letters A 200 (1995) 438-444

441

75=

polariton

induced images of the Au film in the two-layer

The situation is different for the slow surface polaritons which are expected to be more sensitive to the properties of the internal metal-dielectric interface. An angular excitation spectrum of the fast and slow SP branches for a two-layer structure and the related induced current are presented in Fig. 2b. As in the case above, the angular dependence of the induced tunneling current qualitatively follows the SP excitation spectra. Both the fast and slow surface polariton branches are visible in the tunneling current. The angular dependent light induced images recorded from the same place of the sample are shown in Fig. 4. The low resolution of these images (30x 30 pixels) result-

structure

recorded

at different

ing from the necessity to reduce the time of the measurements smears out the fine lateral structure of the film images. Nevertheless, in the topographical image one can see three large regions of the film having different heights. The region close to the right edge of the image is probably contaminated since the associated tunneling current is quite noisy. This strong noise in turn leads to an angle independent background signal. Even though the tunneling current in the region at the bottom of the topographical image is much less noisy, it was not possible to detect a polariton induced signal at this region. In the top-left region of the image a prominent angular dependence of the SP induced tun-

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neling current is observed at the angles of incidence where the slow and fast polaritons are excited. The Au film in the two-layer structure under investigation (Fig. 5) is flatter (the grey scale of the topographical image is 200 8, from white to black) than the one presented in Fig. 3. This results in the less pronounced lateral structure of the fast SP induced image but increases the probability to see the structure of the internal interface due to the reduction of the background caused by the topography. As discussed above, both fast and slow SPs are subject to scattering by the surface and internal defects, so the SP images induced by them reflect the total defect structure of the film. The film image recorded with the fast SP is more related to the surface topography of the film than the slow SP image. For the latter the prominent border between the dark and bright regions which is not correlated with the topographical surface structure is clearly seen in the top part of the image. The crosssectional view of the topography and the slow SP induced image in this region is presented in the insert of Fig. 5 (the direction of the cross-section is indicated by the line at the images). The sharpness of the border has been estimated from the edge profile in the SSP image to be equal to about 30 nm. This sharpness can be considered as an indication of the resolution of our technique.

3. Discussion Several effects which may contribute to the polariton induced current were discussed in Ref. [ 21. Thus, besides the main effect, i.e. the rectification of the polariton field by the nonlinearity of the I-V characteristic of the tunneling junction, several other effects were mentioned, namely, thermal expansion of the sample and tip, thermovoltage effects and photoemission. All these effects were estimated to be small compared with the main one in the Kretschmann geometry. (In the Kretschmann configuration only a small part of the tip is exposed to the light, compared to experiments, where the light is shone from the tip side into the tunneling junction, like for example in Ref. [ 81.) Since we used an unfocused laser beam in our experiments, all the thermal effects are anticipated to be even smaller in our case than in Ref. [ 21. As for the

Letters A 200 (1995) 438-444

main effect, the origin of the polariton induced current is essentially the same for the fast and slow surface polaritons. It was assumed in Refs. [ 1,2] that the polariton induced current arises from the rectification of the high frequency field via the nonlinear static I-V characteristic, i.e. v2 d21 Ip = $dlf’

(1)

where Zr,is the polariton induced current, VPis the polariton induced voltage, and V, is the tunneling voltage. The authors of Ref. [ 21 were able to explain their experimental data if VP was set to approximately 500 mV. This means that the polariton electric field should be about Epotaimn N 0.5 V/O.5 nm = 1 V/nm, where 0.5 nm is chosen as the characteristic tunneling distance. Such a large amplitude can be accepted if there is a polariton field enhancement of several orders of magnitude near the tunneling tip. Nevertheless, the validity of the use of the static Z-V characteristics seems to be doubtful for the explanation of high frequency processes in the model above [2]. Since the problem of electron motion through the tunneling junction in the presence of a high frequency polariton field is closely related to the problem of the Kapitza pendulum (a pendulum whose point of support oscillates with a high frequency) [ 91, it seems more reasonable to use this well known approach to describe the experimental situation. The effective potential energy for the Kapitza pendulum is c/,,=a+*MJ+&,

(2)

where U is the unperturbed potential in the absence of the high frequency force f cos ( wt) , and m is the mass of the pendulum. Even the vertically upward position of the pendulum can be stabilized in the new effective potential. Qualitatively, one would expect that the effective potential for an electron in a polariton field is determined by a similar expression, i.e. lJ& = U(r)

+ W(r)

= U(r)

+

e2EZ(r) 2m02

’

(3)

where U(r) is the potential distribution for tunneling electrons, e is the electron charge, and Er,( I) is the polariton field.

I. Smolyaninou et al. /Physics

TOPOGRAPHY

-

5

Letters A 200 (1995) 438-444

SLOW SP INDUCED IMAGE

-I

FAST

443

SP INDUCED IMAGE

!-

j-1

Fig. 5. Top: 2.1 x0.7 pm2 topographical, slow and fast surface polariton induced images of a 600 .& gold film in the two-layer Au/fluoride/prism structure. The vertical scale of the topographical image is 200 8, from black to white. Bottom: cross-sectional view of ( 1) topography and (2) slow polar&on induced image (see the text).

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I. Smolyaninou et al. /Physics

This result can be understood a quantum mechanical treatment. Schriidinger equation

ifi!!!!! = dt

(P

-eAj2 2m

+U(r>

easily also from Thus, if in the

* >

for the electron wave function I,/J,one considers A to be the vector potential of the polariton field and U(r) to be the potential configuration, remembering that E,, = -6’A/dt one realizes that the same correction SU( I) to the effective potential arises from the dc part of the term e2A2/2m. The polariton induced potential 6U(r) acts in the region of the tunneling barrier as well as in the film slab. Using the same value Ep = 1 V/nm for the polariton field, one obtains 6Z.ZN 10 mV, a value which is sufficient to explain the polariton induced current observed in Ref. [ 21 (see Figs. 3 and 4 in Ref. [ 21) . In our experiments the polariton induced potential is estimated to be 6U N (Zr/Z)U N 1 mV, where the tunneling voltage and current is U N 50 mV and Z N 500 pA, respectively, and the polariton induced tunneling current is Zr N 10 pA. This additional potential can be induced in our model by the polariton field Ep = 300 mV/nm (see Eq. (3) ) . Taking into account the power of SP excitation and the experimental geometry the total electromagnetic field enhancement is estimated to be lo5 to explain this value of Ep. Such values of the field enhancement are usually observed in surface enhanced Raman scattering experiments and can be caused by different mechanisms [ lo]. Since surface enhanced Raman scattering is observed with similar metal films one may conclude that analogous physical processes are responsible for the polariton induced tunneling current.

Letters A 200 (1995) 438-444

Acknowledgement The authors are indebted to Dr. Torben Skettrup from the Technical University of Denmark, Lyngby, for providing the two-layer structure used in our experiments and to Professor V.M. Agranovich for helpful discussions.

References [l]

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R. Moller, U. Albrecht, J. Boneberg, B. Koslowski, P. Leiderer and K. Dransfeld, J. Vat. Sci. Technol. B 9 ( 1991) 506. C. Baur, B. Koslowski, R. Miiller and K. Dransfeld, in: Near field optics, eds. D.W. Pohl and D. Coujon (Kluwer, Dordrecht, 1993) p. 325. N. Kroo, J.P. Thost, M. Viilcker, W. Krieger and H. Walther, Europhys. L&t. 15 (1991) 289. E Abel& and T. Lopez-Rios, in: Surface polaritons, eds. V.M. Agranovich and D.L. Mills (North-Holland, Amsterdam, 1982) p. 239. H. Raether, in: Surface polaritons, eds. V.M. Agranovich and D.L. Mills (North-Holland, Amsterdam, 1982) p. 331. VS. Edelman, Sov. Phys. Prib. Tekh. Eksp. No. 4 (1989) 149. E. Kretschmann, Z. Phys. 241 (1981) 313. N.M. Amer, A. Skumanich and D. Ripple, Appl. Phys. Lett. 49 (1986) 137. L.D. Landau and E.M. Lifshitz, Mechanics (Pergamon, Oxford, 1960). A.G. Malshukov, in: The chemical physics of solvation, Part C, eds. R.R. Dogonadze, E. Kalman, A.A. Komyshev and J. Ulstrup (Elsevier, Amsterdam, 1988) p. 433; Phys. Rep. 194 (1990) 343.