EP0750795A1

EP0750795A1 - Thin film devices

Info

Publication number: EP0750795A1
Application number: EP96901090A
Authority: EP
Inventors: Victor Institute of Biophysics EROKHIN; Paulo Institute of Biophysics FACCI; Claudio Institute of Biophysics NICOLINI
Original assignee: TECHNOBIOCHIP
Current assignee: TECHNOBIOCHIP
Priority date: 1995-01-13
Filing date: 1996-01-15
Publication date: 1997-01-02
Also published as: WO1996021952A1; AU4495196A

Abstract

Nanometer scale Cds particles are formed by exposing cadmium arachidate Langmuir-Blodgett bilayers to an atmosphere of H2S. This results in particles of less than 90 A in size in a film which is capable of exhibiting monoelectric effects. By synthesising clusters of these nanometer scale particles on the tip of a sharp metal stylus, tool has been produced which enables monoelectric phenomena to be measured at room temperature and thus without the need for a scanning tunnelling microscope.

Description

THIN FILM DEVICES

This invention relates to thin films and structures containing them.

SECTION 1

STM STUDY OF STRUCTURE AND ELECTRICAL PROPERTIES OF

NANOMETER SCALE CdS PARTICLES IN LANGMUIR-BLODGETT FILMS

Ultra small CdS particles were formed by exposing deposited cadmium arachidate Langmuir-Blodgett bilayers to atmosphere of H²S. STM images of the resultant films reveals the presence of particles with sizes of about 40 - 60 A. Also voltage-current characteristics were measured by STM on the structure "tip-tunnelling gas - CdS particle - tunnelling gap - graphite substrate". Steps in voltage- current characteristics indicate the appearance of single electron process (Coulomb blockade) at room temperature.

Unusual behaviour of Voltage-Current (V/I) characteristics in systems containing two electrodes and ultra small conductive particles between them, separated by two tunnelling junctions, is very interesting and has no analogies in other systems^1,2. Theory explains the phenomenon through the appearance of "Coulomb blockade" due to passing of electrons through the intermediate granule^3,4. The presence of an electron in the granule changes strongly the energy of the particle {due to its small capacity) . This energy has to be overcome by external voltage in order to provide the possibility for one more electron to be tunnelled to the granule. Increasing the voltage one can observe discrete increase in the current in correspondence to those voltage values which overcome the Coulomb blockage potential . The increase in current takes place every time the voltage increases of:

ΔV=e/C (1) where e is the electron charge and C is a capacity of the granule

The process is called monoelectronic because it is possible to distinguish current steps in V/I curve due to unitary increase in the number of electrons in the granule. Step-like behaviour of current was observed in several works at low temperature^{1 5} The value of the temperature is very important for observing such phenomena and the following equation must hold true to allow the monitoring of steps in V/I characteristics⁶ e²/2C > kT (2) Thus, the temperature at which monoelectron phenomena can be observed is limited by the capacity of the granule and therefore by its dimensions. Rough estimations, assuming spherical shape, give 90 A as limiting value of the granule radius: for bigger radii Coulomb blockade cannot take place at room temperature.

Technologically, the fabrication of structures suitable for observation of monoelectron phenomena, such as "electrode" - tunnelling gap - nanometric granule - tunnelling gap - electrode" is very difficult. Using STM for realising such structures can be very fruitful, because, when the particle is formed, it can be found with STM and the tip, placed just above the granule, acting as the upper electrode. Few works whicn perform these measurements are reported m literature?

A method, resulting in the formation of CdS inside arachidic acid film by exposing initial cadmium arachidate Langmuir-Blodgett (LB) film to H-S atmosphere, was suggested⁸ During such treatment, protons of H₂S become to be bond to the acid head groups and sulphur atoms, bond to cadmium, form CdS phase. The formation of CdS was checked by optical absorbance⁸ and electron diffraction⁹. Sizes of the formed particles were estimated to be about 100 A if the initial film contained 20 - 30 monolayers

It is interesting to mention, that the spacing of the LB film itself was decreased from 49 0 A to 39 0 A, as was measured by X-ray diffraction⁹ - 3 -

The aim of this work was to investigate by STM a bilayer of cadmium arachidate after exposing it to H₂S, to find granules of nanometer sizes and to measure with the STM tip local V/I characteristics on "graphite - tunnelling gap - CdS particle - tunnelling gap - tip" structure.

A bilayer of cadmium arachidate was transferred onto the graphite surface according to standard procedure¹⁰. For providing the chemical reaction the sample was placed into a chamber, containing H₂S, for 30 minutes.

STM measurements were performed using a device (MM- MDT) , allowing also to measure local V/I characteristics. For the measurements of V/I characteristics STM tip was placed over the desired point (CdS particle, identified on previously obtained image) in constant current mode. When the tip was above this point, feedback was switched off and the tip - substrate voltage was swept from -0.5 to 0.5 V. The configuration for measurements of V/I characteristics is presented in Figure 1. STM image of cadmium arachidate bilayer after exposure to H₂S atmosphere is presented in Figure 2. CdS particles are well distinguishable in the picture. Sizes and shapes of the particles are not equal one another, but, in general, sizes are in the range 40 - 60 A (nevertheless it is possible to find also particles with sizes outside the range) . The surface of the particles is rather flat. This fact becomes understandable if we suppose that CdS particles are small monocrystals . The hypothesis is also in agreement with light absorption data, showing the existence of the original CdS band structure in the particles after the reaction with H₂S⁸, and with electron diffraction data, demonstrating that the lattice spacing value of the particles is the same as in bulk crystal'. The surface of LB film after the reaction becomes rough due to the disturbance caused by the particles formation process. This fact is in good agreement with the decrease of the film spacing (bilayer thickness)⁵. The decrease implies the declination of hydrocarbon chains from the normal direction to the film plane and so the increase of the area per molecule in the film plane. As a result of it, the total area of the film should increase, while the physical area remains the same (geometrical areas of the substrate) . The mentioned contradiction seems to be responsible for the increased roughness of the LB film.

The value of the particle sizes is less than the one measured by electron diffraction method. The difference is likely due to the fact that in⁹ the initial film of cadmium arachidate was 10 - 15 bilayers (growth of CdS crystal can involve atoms also from different film planes) , while here we have only one bilayer.

Distribution of the particles inside the film is not regular. Some areas contain several particles but there are regions were no particles were observed.

Analysis of the image in Figure 2 leads to suppose that monoelectron phenomena should be detectable in such material, because particle sizes are small enough to make equation (2) valid.

V/I characteristics of the system "graphite substrate - tunnelling gap - CdS particle - tunnelling gap - STM tip" is presented in Figure 3. The characteristic was obtained by placing the tip above the CdS particle, the position of which was determined from previously acquired image. Despite some noise, steps in V/I characteristics are well distinguishable. Steps in the characteristics are equidistant and correspond to the value of voltage of about 0.2 V. Taking into account that the particles have in-plane dimensions of 40 - 60 A and their surface is flat, we can conclude, that the most probable shape of them is disk-like one and the thickness of the disk is a couple of lattice unit cells of CdS.

As a conclusion, nanometer scale CdS particles were formed by exposing Cadmium Arachidate LB film to H₂S atmosphere. There sizes measured by STM were found to be small enough to allow monoelectron phenomena. These phenomena were observed at room temperature. Rather big noise level points out that measurements were performed near boundary conditions of the validity of equation (2) . Analysis of all experimental data allows to make conclusion about the disk-like shape of the particles as the most probable one. Thus, such treatment of the cadmium arachidate films results in the creation of a new material, where nanometer scale monocrystal semiconductor particles are embedded into insulating LB matrix. This material displays new kinds of phenomena, particularly, but probably not only, - monoelectron ones, which allow to study fundamental properties of systems with decreased number of dimensions and from technological point of view can permit the construction of new types of devices, such as monoelectron transistors.

SECTION 2

ROOM TEMPERATURE SINGLE ELECTRON DEVICE

A method for realising room temperature single electron junctions which do not require any STM investigation for localising the quantum dot is now described.

Ultra small CdS clusters have been directly synthesised on the very tip of a sharp metal stylus. Voltage-current characteristics measured with such stylus, brought in the proximity of another electrode, display Coulomb blockade and Coulomb staircase pointing out junction capacitances in the 10'¹⁹F Range.

For several years single electron phenomena has been attracting the interest of researchers both from basic and applied viewpoints¹'¹⁰. When a quantum dot is placed in between two electrodes and is spaced asymmetrically from them by two tunnelling gaps, it is possible to observe a quantised charging of the dot resulting in a quantised increase of the current flowing through the system upon bias voltage. The conditions for achieving such a situation imply that the capacitance C of the structure is small enough to make the charging energy of the junction (e /2C) exceeding the thermal energy kT. At a fixed temperature there is, therefore, a limiting value of the dot size which allows the appearance of single electron phenomena. In particular, room temperature monoelectron junctions require typically dots not larger than some nanometers in size. From the applied point of view, these phenomena will likely be the basis for future devices as long as the scale of integration will reach the 10 nm limit⁹. The increasing interest for monoelectron phenomena has stimulated researchers to develop techniques both for forming suitable quantum dots and measuring the electrical behaviour out of a single granule. As a consequence of these efforts, room temperature monoelectron phenomena have been recently reported⁹'¹¹ on nanoclusters of different nature, and synthesised in different ways involving physical and chemical approaches. Once these quantum dots have been formed, however, a tool is needed for finding them on the surface where they are placed. This tool has been so far the scanning tunnelling microscope^5,6,9"¹¹.

Any further step toward the development of such kind of junctions for basic and technological aims should try to simplify the set up required to deal with single electron phenomena at room temperature. Our attempt to satisfy this general requirement involved the development of a method for synthesising nanoclusters capable of displaying single electron phenomena, directly on the tip of a sharp metal stylus. Few (up to 6) Langmuir-Blodgett (LB) monolayers of cadmium arachidate formed at the surface of a commerical trough were deposited onto sharp tungsten styli. The styli have been obtained from a 0.5 mm thick tungsten wire by electrochemical etching. These tips covered with LB films were subsequently exposed to an atmosphere of H₂S for forming CdS nanoclusters imbedded in a matrix of protonated fatty acids. Such procedure of nanometer scale CdS particle formation has been reported in literature for LB films deposited onto flat surfaces. Typical sizes of CdS dots have been found to be in 3-5 nm range^13■". The treated tips have been mounted on an experimental set up (see Figure 4) capable of bringing them in the proximity of another electrode (we used plates of highly oriented pyrolytic graphite) as in STM measurements. Basically, the tip was connected to a ID piezo-mover which allows to keep a desired tunnelling gap. After landing the tip in a random place, voltage current characteristics were measured by switching off the feedback system, sweeping the bias voltage and recording the current flowing through the system. Locking a rather small current value it is presumably that a suitable asymmetric double junction should be achieved.

The measured characteristics display, irrespective the place were the tip was landed, trends like those reported in Figure 5. Such kinds of characteristics have been obtained in about 60% of the prepared samples. These kinds of features are a typical indication of the appearance of single electron phenomena and have been recently reported and discussed both for cryogenic^5,9 and room temperature⁹ measurements on different kinds of quantum dots. The analysis of the experimental curve in the framework of the Coulomb blockade theory gives a capacitance value of 1.5 10^"19F (estimated from a voltage step of 0.54.+0.03 V) which is consistent with the appearance of room temperature single electron phenomena (540 meV with respect to the thermal energy, 26 meV) and is moreover of the same order of magnitude of that estimated by simple calculations based on the typical size of the CdS nanoclusters synthesised by Langmuir-Blodgett fatty acids salts precursors. FIGURE LEGENDS

Figure 1 A scheme of the experimental setup for measuring V/I characteristics with STM tip. Figure 2 STM image of cadmium arachidate LB film after the reaction with H₂S. The image was acquired with Pt/Ir (90% - 10%) tip m constant current mode; tunnelling voltage was 2 V, tunnelling current was 0.7 nA. Image sizes are 576 x 576 A². flat regions represent CdS particles. Figure 3 Voltage current characteristics of the system

"graphite - tunnelling gap - CdS granule - tunnelling gap - STM tip" . The curve is a result of averaging of 16 acquisitions at the same point. The curve contains 500 experimental points and errors (5 pA) do not exceed the line thickness. Figure 4 Simplified scheme of the experimental set up used to realise the single electron phenomenon measurements. Figure 5 Voltage-current characteristics of the single electron element obtained using a stylus covered with 2 bilayers of cadmium arachidate precursor. The inset is the differential conductance obtained by calculating the first derivative of voltage-current characteristic.

REFERENCES TO SECTION 1

1. T.A Fulton, and G.J. Dolan, J. Phys . Rev. Lett., 59, 109 (1987) .

2. M.H. Doveret, D. Esteve, and C. Urbina, Nature, 360, 547 - 553, (1992) .

3. D.V. Averin, and K.K. Likharev, K.K. , in Quantum Effects in Small Disordered Systems (eds. Altshuler, B.L., Lee, P.A., and Webb, R.A.) (Elsevier, Amsterdam, 1991) . 4. G. Schδn, . and A.D. Zaikin, Phys. Rep., 198, 237 (1990) .

5. K. Mullen, E. Ben-Jacob, R.C. Jaklevic, and Z. Schuss, Phys, Rev. B, 37, 98-105 (1988) .

6. D.V. Averin, and K.K. Likharev, Zh, Eksp. Teor. Fiz., 90, 733 (1986) .

7. R. Wilkins, E. Ben-Jacob, and R.D. Jaklevic, Phys. Rev. Lett., 63, 801 (1989)

8. E.S. Smotkin, C. Lee, A.J. Bard, A. Campion, M.A. Fox, T.E. Mallouk, S.E. Webber, and J.M. White, Chem. Phys. Lett., 152, 265 - 268 (1988) .

9. V. Erokhin, L. Feigin, G. Ivakin, V. Klechkovskaya, Yu. Lvov, and N. Stiopina, Makromol. Chem., Makro ol . Sy p., 46, 359-363 (1991) .

10. I.P. Peterson, and G.J. Russell, Thin Solid Films, 134, 143 - 152, (1985) .

REFERENCES TO SECTION 2

1. H.R. Zeller and I. Giaever, Phys. Rev. 181, 789

(1969) .

2. J. Lambe and R.C. Jaklevic, Phys. Rev. Lett., 22, 1371 (1969) .

3. D.V. Averin and K.K. Likharev, J Low Temp. Phys., 62, 345 (1986) .

4. P.J.M. van Bentum, H. van Kempen, L.E.C. van de Leemput and P.A.A. Teunissen, Phys. Rev. Lett., 60 369 (1988) .

5. P.J.M. van Bentum, R.T.M. Smokers and H. van Kempen, Phys. Rev. Lett., 60, 2543 (1988) .

6. R. Wilkins, E. Ben-Jacob and R.C. Jaklevic, Phys. Rev. Lett., 63, 801 (1989) . 7. S.T. Ruggiero and J.B. Barner, Z. Phys. B - Condensed Matter, 85, 333 (1991) .

8. L.P. Kouwenhoven, N.C. van der Vaart, A.T. Johnson, W. Kool, C.J.P.M. Harmans, G.J. Williamson, A.A.M. Staring and C.T. Foxon, Z. Phys. B - Condensed Matter, 85, 367 (1991) .

9. C. Schόnenberger, H. van Houten, H.C. Donkersloot, A.M.T. van der Putten and L.G.J. Fokkink, Physica Scripta, T45, 289 (1992) .

10. M.H. Devoret, Daniel Esteve and C. Urbina, Nature, 360, 547 (1992) .

11. V. Erokhin, P. Facci, S. Carrara and C. Nicolini, J. Phys. - Applied Physics submitted, (1994) .

12. G. Roberts, Langmuir-Blodgett Films (Plenum Press, New York, 1990) . 13. E.S. Smotkin, C. Lee, A,J. Bard, A. Campion, M.A.

Fox, T.I. Mallouk, C.I. Webber and J.M. White, Chem. Phys. Lett., 152, 265 (1988) . 14. P. Facci, V. Erokhin, A. Tronin and C. Nicolini, J. Phys. Chem., in press, (1994)

Claims

- 11 -CLAIMS

1. A Langmuir-Blodgett film capable of exhibiting monoelectronic effects, which comprises particles less than 900 nm (90 A) in size.

2. A film according to claim 1, wherein the particles are of a heavy metal sulphide.

3. A film according to claim 2, wherein the heavy metal is cadmium.

4. A film according to any preceding claim, which is a bilayer.

5. A film according to claim 1, obtainable by exposing a calcium arachidate bilayer to H₂S.

6. A film according to any preceding claim, wherein the predominant particle size is in the range 400 to 600 nm (40 to 60 A)

7. A structure comprising a film according to any preceding claim on a graphite substrate.

8. A structure comprising two electrodes and a film according to any preceding claim disposed between the electrodes.

9. A thin film comprising a nanometer scale particle formed on a sharp tip of conductive material.

10. A thin film according to claim 9 in which the tip comprises a stylus.

11. A thin film according to claim 9 in which the conductive material comprises tungsten.

12. A thin film according to claim 9 in which the particle is a heavy metal sulphide.

13. A thin film according to claim 12 in which the heavy metal is cadmium.

14. A thin film according to claim 9 in which the tip is formed by electrochemical etching.

15. A thin film according to claim 9 in which the tip is formed by micromachining.

16. A thin film according to claim 9 in which the particle size is in the range 3-5 nm.

17. Apparatus for locating a thin film according to any of claims 9 to 16 comprising an electrode mounted on a ID mover and spaced from the thin film.

18. Apparatus according to claim 17 in which the electrode is selectively connected in a feedback loop with the thin film.