Characterization of microtips for scanning tunneling microscopy

Characterization of microtips for scanning tunneling microscopy

L539 Surface Science 202 (1988) L539-L549 North-Holland, Amsterdam SURFACE SCIENCE LETTERS CHARACTERIZATION OF MICROTIPS FOR SCANNING TUNNELING M...

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L539

Surface Science 202 (1988) L539-L549 North-Holland, Amsterdam

SURFACE

SCIENCE

LETTERS

CHARACTERIZATION OF MICROTIPS FOR SCANNING TUNNELING MICROSCOPY Vu Thien BINH Departement de Physique des Materiaux F-69622 Villeurbanne. France

(UA CNRS),

UniversitP Claude Bernard

- Lyon I,

and J. MARIEN Institut de Chimie, Departement de Physico-Chimie Unioersitd de Liege, B-4000 LiSge, Belgique Received

16 December

1987; accepted

des Surfaces,

for publication

Departement

de Chimie,

13 April 1988

Progress in STM, in particular for the interpretation of the tunnel signals, is related to the possibility to sharpen and to regenerate in situ the tip which is used as the probe. Three techniques are now available to produce tips with controlled geometry at the atomic level. By the use of FIM to characterize the geometry, and FEM and FEM-FES to study the tunneling properties and electronic structure, we show that microtips produced by a technique based on the pseudo-stationary profile principle suited the best.

1. Introduction and formulation of the problem In scanning tunneling microscopy, the tunnel signal between the surface of a sample and the tip probe depends not only on the local surface atomic and electronic structure of the sample, but is also related to the morphology of the tip [l-6]. In order to differentiate contributions from the sample surface and from the tip, in particular with regard to investigations of nonperiodic structures or non-ideal surfaces, it is essentially to have at our disposal a well-defined tip: that means known position of the active tunneling area (at the apex of the tip, for example) and known atomic arrangement and electronic structure. In order to achieve atomic lateral resolution, the tip must have an apex with a radius at least below 10 A; ideally it should terminate in a single atom. Finally, the STM tip must be reproducible and should be regenerated in situ by a reliable and relatively simple technique which should not need a continuous control during the tip formation; we call this a “blind” method. 0039-6028/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Presently, and to our knowledge, three principal technique are available to produce such a microtip [7]: (1) the deposition technique which was introduced by Fink [8], (2) the build-up technique [7-91 and (3) the pseudo-stationary profile technique [9]. Details and characterizations of monatomic tips obtained by the first technique are largely presented in ref. [8]. The second and third technique allow us to produce and regenerate the microtips systematically and in situ by a thermal treatment; the details for their production are presented elsewhere [9]. The subject of this paper is to characterise W microtips obtained by the build-up and pseudo-stationary profile (PSP) technique. We used field ion microscopy (FIM) to determine their geometry, field electron microscopy (FEM) and field electron spectroscopy (FEM-FES) to study their tunneling properties and electronic structure.

2. Micro-tips obtained by the build-up technique 2.1. Principle of the technique The initially nearly hemispherical field emitter tip surface deforms into a polyhedral shape in a process known as “build-up”, when the emitter is heated in the presence of a high electric field [lO,ll]. The build-up process consists essentially of a local rearrangement by surface diffusion of the atoms of some low index facets, leading to an enlargement of these planes (fig. 1). This local migration is essentially due to the presence of a gradient of the electric field between the center of the facet and the vicinal regions. The facet enlargement will stop by itself when two neighbouring facets meet each other, that means when they are separated by a one-atom boundary line. For a tungsten tip, it is well known that the build-up phenomena leads to the enlargement of the (011) and (112) planes [lo]. Thus, if the tip axis is in the (111) direction then build-up facet

build-lrp facet

thermal

profile

I

build-up

profile

Fig. 1. Schematic representation of the build-up process, the enlargement of the facets are due to surface diffusion under the gradient of the electric field over the facets.

Vu Thien Binh, J. Marien / Characterization of microtipsfor STM

Fig. 2. FIM diagram of a tungsten tip with (111) axis. The dark spot near the center micrograph is a probe hole drilled at the chamxel plate’s center (77 K, He, 9.2 kv).

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the build-up “equilibrium” geometry shows at the apex a microfacet resulting from the meeting of the three (112) build-up planes. 2.2. Observations by field ion microscopy Starting from a field evaporated tungsten tip with (111) axis, whose FIM diagram is presented in fig. 2, the build-up process leads to a geometry of

Fig. 3. FIM diagram of a build-up (111) W tip. (a) This diagram shows a three-atom microfacet which results from the meeting of the three { 112) build-up facets with one-atom-ledge boundaries (77 K Ne, 9.2 kv). (b) The blurred region represents local protusion, or bump, relative to the surrounding areas (77 K, Ne, 14.5 kV).

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three atoms at the apex of the tip (fig. 3a). This microtip is the result of the meeting of the three enlarged (112) one-atom-ledge boundary facets. This is an equilibrium geometry. In order to determine local protuberances of this microtip, we use the properties of the local best image voltage. Namely, for one imaging gas the “best image field” has a value for which it is ionized at a critical distance of about 0.4 nm above the surface of the tip [12]. With regard to surfaces where local radii of curvature are not the same and for one applied voltage to the tip, the local electric fields obtained are different from one point to another. The FIM diagrams will then present zones with atomic resolution (local best image voltage}, as well as “blurred” regions; the last ones correspond to higher local surface curvature areas (figs. 3b and 6d). In the case of a build-up tip, fig. 3b shows that the protrusion area is rather extended over all the tip apex surface, even if it is localised around the junction zones of the three build-up facets

Fig. 4. I-V characteristics of a build-up (111) W tip. The straight line indicates a normal Fowler-Nordheim behavior (I = A V2 exp( - B/Y) where A and B are parameters which depend on the tip geometry and surface work function)

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(112). This means that the three-atom microfacet not really protrudes regard to the support tip, and should be screened by it, in most cases.

Is43

with

2.3. Field electron characteristics Plots of the tunneling currents versus the applied voltages (I-V characteristics) indicate a normal Fowler-Nordheim behavior, that means a straight line for log( i/V’) versus l/K An example is presented in fig. 4. The total energy distribution (TED) of field-emitted electrons from the apex of the build-up tip are measured by a Kuyatt-Plummer type spherical deflection energy analyser which has been adapted to a field emission microscope [13,14]. The total energy distribution obtained for thermal (fig. 5a) and build-up (fig. 5b) tips are shown. No anomalous structure is observed for the build-up tip, apart from an increase of the spectrum width. The two TED’s are characteristic of W(lll), and are in agreement with other measurements 1131. In particular, we observed in both cases a break (at 0.75 eV below the Fermi

Volts Fig. 5. Total energy distribution of field emitted electrons from W(111) (77 K). (a) Clean thermai equilibrium surface. (b) Build-up morphology.

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-1

(111) BUlid-up

1L 4

,.I

' 5

I

\

6

!d

morphology

..J

I

I

I

8

9

Volts Fig. 5. Continued.

level) in the exponential tail of the energy distribution. A similar curve is also obtained for the field evaporated W(111) tip.

3. Microtips obtained from the pseudo-stationary

profile (PSP) technique

3.1. Principle of the technique Under the simultaneous action of surface diffusion and evaporation (or corrosion) the tip geometry evolves from the initial geometry to reach a constant geometry called pseudo-stationary profile [15]. That means that the profile, and in particular the tip radius value, result from an equilibrium between surface diffusion and evaporation or corrosion rate. Such technique is routinely used to obtain FEM tips in situ and under ultra-high vacuum [16,17]. In order to obtain W microtips, processes using heat treatments in UHV and in the presence of oxygen atmosphere and electric field are used, details of such processes are presented elsewhere [9]. It appears from our first results

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that the obtaining of the microtips is independent of the initial geometry of the tip (pseudo-stationary profile principle [15]). Owing to this “blind” method, a reproducible microtip can be obtained by only setting the parameters temperature, electric field, and oxygen pressure. The PSP technique allows then in situ sharpening and regeneration of the microtips.

Fig. 6. FIM diagrams of a microtip obtained by the PSP technique. The following patterns correspond to the monatomic apex of the microtip (77 K, Ne, 5.3 kV) (a); then, successive removal by field evaporation reveals: the three atoms constituting the second layer of the (111) microtip (77 K, Ne, 6.2 kV) (b); the atomic arrangement of the base of the microtip (77 K, He, 12.7 kV) (c). FIM diagram for the determination of the position of the microtip on the support tip with the local best image process: the blurred area at the apex region of the tip indicates the position of the microtip (77 K, Ne, 11.8 kV) (d). (Note: On FIM diagrams the spacings between the atoms depend on the geometrical factor of the tip and in particular the radius of the tip; an increase in the spacing is an indication of the sharpness of the tip apex. Compare for example the three atoms of fig. 3 and fig. 6b.)

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3.2. Obse~ou~ions by field ion microscopy The geometry of such pseudo-stationary profile microtip has been analysed by field ion microscopy with progressive field evaporation in order to visualise the gee-dimensions structure. The successive field ion diagrams of the microtip show at first a monatomic apex (fig. 6a); then the successive underlying microplane structures: a three-atom second layer (fig. 6b), a seven-atom third layer and so on, until the base of the microtip (figs. 6c and 6d). This leads to the conclusion that the equilibrium emitter profile is made up of a microtip erected on top of a support tip (fig. 7). A geometrical estimation using the FIM diagrams of fig. 6 gives values of - 5 nm for the dimensions of the microtip base and height. The dimensions of the microtip seem to be independent of the value of the radius of the support tip, which could be settled from ten to hundred of nanometers. This microtip is always localised around the (111) axis of the support tip. 3.3. Field electron c~urucierist~cs If we plot log(I,/V*) versus l/V, the results cannot be fitted by a straight line (fig. 8). The increase of the tunnel current with regard to the applied field is smaller than for normal Fowler-Nordheim behavior. The deviation from the straight line is att~buted to the presence of space charge. This interpreta-

tip

QXiS

(111) t

Fig. 7. Schematic

drawing

of a microtip

obtained

by the PSP technique.

Vu Thien Binh, J. Marien / Characterization of microtips for STM

8

10

12

14

16

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18

lo4 /V Fig. 8. Z-V characteristics of a microtip obtained by the PSP technique. This plot indicates the presence of space charge during field emission. (The dashed straight line is here only to have a linear reference.)

tion is consistent with the theoretical dependence of the current density on applied voltage given by Dyke and Dolan [18] showing a deviation from a straight line in presence of space charge. This interpretation is also coherent with the values of the measured total currents, in relation with the space charge current density (J > 106 A/cm2 [18]) and the field emission area localised at the level of one or few atoms at the apex of the microtip. TED of the field emitted electrons from the PSP microtips show visible differences with TED of tips with greater radius, in particular by the presence of localised peaks in the energy region from 0.3 to 0.8 eV below the Fermi level (fig. 9). Interpretation of these peaks is still open, their origin could be either from surface states due to the atomic dimension of the emitting area

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4

5

I

6

9

8

volts Fig, 9. TED

of field emitted electrons

from a (111) technique.

(cluster behavior), or from electron-electron emission processes.

microtip

(77 K) obtained

interactions

by the PSP

during tunneling or

4. Conclusions

Progress in STM, in particular for the interpretation of the tunnel signals, is related to the possibility to sharpen and regenerate in situ its tip-probe in a reproducibly manner. Nowadays, three techniques are available to produce microtips at atomic scale. They are (i) the deposition technique, (ii) the build-up technique, and (iii) the pseudo-stationary profile (PSP) technique. Two of them, the build-up and the PSP techniques are based on the obtaining of characteristic equilibrium profiles under defined conditions. Only the PSP technique allows the production of microtips which are really erected from the support tip, giving then the insurance that the active tunneling area will be localised at the apex of the microtip and in the axis of

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the tip. Moreover, in most cases, this geometry is favourable to the maintenance of the same active tunneling area even if the scanned surface is non-periodic or corrugated. Besides, the morphology of our microtip insures mechanical stability of the probe in STM. In principle, PSP microtips could be obtained in other materials than tungsten.

Acknowledgements The authors acknowledge the Belgium National Fund of Scientific Research, the French National Center of Scientific Research (CNRS), and the Commissariat GCnCral aux Relations Internationales de la Communaute Franqaise de Belgique for their financial support.

References [l] [2] [3] [4] [5] [6] [7]

[8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18]

G. Birmig, H. Rohrer, Ch. Gerber and E. Weibel, Appl. Phys. Letters 40 (1982) 178. J. Tersoff and D.R. Hamann, Phys. Rev. B 31 (1985) 805. N.L. Lang, Phys. Rev. Letters 55 (1985) 230. E. StoII, A. Baratoff, A. SeIloni and P. CamevaIIi, J. Phys. C 17 (1984) 167. MS. Chung, T.E. Feuchtwang and P.H. Cutler, Surface Sci. 187 (1987) 559. Y. Kuk and P.J. Silverman, Appl. Phys. Letters 48 (1986) 1597. Vu Thien Binh and J. Marien, 3rd French-German Symposium on Field Emission and their Applications, Rouen, France, September 1987; Vu Thien Binh, Workshop on Mathematical Treatments and Image Processings of STM, Luminy, France, October 1987. H.W. Fink, IBM J. Res. Develop. 30 (1986) 460. Vu Thien Bit& to be published. PC. Bettler and F.M. Charbonnier, Phys. Rev. 119 (1960) 85. L.W. Swanson and L.C. Crouser, J. Appl. Phys. 40 (1969) 4741. E.W. MuIIer and T.T. Tsong, in: Field Ion Microscopy, Principles and Applications (Elsevier, New York, 1969). C.E. Kuyatt and E.W. Plummer, Rev. Sci. Instr. 43 (1972) 108. N. Rihon, Thesis, Universid de Li&ge, Belgique (1978). Vu Thien Binh and R. Uzan, Surface Sci. 179 (1987) 540. Vu Thien Binh, A. Piquet, H. Roux, R. Uzan and M. Drechsler, J. Phys. E (Sci. Instr.) 9 (1976) 377. Vu Thien Binh, in: Semiconductor Interfaces, Formation and Properties, Springer Proc. Phys. 22 (1987) 126. W.P. Dyke and W.W. Dolan, Advan. Electron. Electron Phys. 8 (1956) 89.