PROCESS FOR PRODUCING NANOCLUSTERS OF SILICON AND/OR GERMANIUM EXHIBITING A PERMANENT MAGNETIC AND/OR ELECTRIC DIPOLE MOMENT

Abstract

A process for producing nanoclusters of silicon and/or germanium exhibiting a permanent magnetic and/or electric dipole moment for adjusting the work function of materials, for micro- and nano-electronics, for telecommunications, for “nano-ovens”, for organic electronics, for photoelectric devices, for catalytic reactions and for fractionation of water.

Claims

1. A process for producing nanoclusters of silicon and/or germanium exhibiting a permanent magnetic and/or electric dipole moment by means of a capacitive- or inductive-coupling plasma reactor by implementing a pulsed plasma-enhanced chemical vapour deposition (PECVD), chemical vapour deposition (CVD), microwave plasma chemical vapour deposition, cathodic arc or magnetron sputtering, atmospheric pressure microplasma deposition, dielectric barrier discharge (DBD), pyrolysis or laser ablation process; the process being carried out with the following parameters: a pressure comprised between 0.01 mbar and atmospheric pressure; a direct-current voltage, radiofrequency or microwave discharge at 13.56 MHz or 2.45 GHz or their harmonics; a driving power comprised between 0.01 mW/cm.sup.3 and 20 mW/cm.sup.3; a gas temperature between ambient temperature and 350° C.; a time period between the start of generation and the end of generation of a plasma in the reactor comprised between 0.01 second and 20 seconds; and the nanoclusters are deposited under the effect of a DC electrical voltage comprised between 0 V and 1,000 V.

2. The process according to claim 1, characterized in that it comprises depositing nanoclusters on a material; parameters being defined so as to adjust the work function of this material.

3. The process according to claim 1, characterized in that the nanoclusters have a size less than or equal to 4 nm.

4. The process according to claim 2, characterized in that the plasma is generated during a time period that is a function of the desired level of adjustment of the work function of the material.

5. The process according to claim 1, characterized in that production of the nanoclusters of silicon and/or germanium is obtained starting from a gaseous mixture based on silane and/or germane, mixed with argon, helium and/or hydrogen, or starting from a gaseous mixture based on tetrafluorosilane.

6. The process according to claim 1, characterized in that the nanoclusters are deposited on a material in at least one homogeneous layer or leaving a part of the surface of the material uncovered.

7. The process according to claim 1, characterized in that during a plasma deposition on a material in a reactor, the process comprises the steps of: generating a plasma; stopping the plasma at the end of a predetermined time period, allowing the creation and preservation of the nanoclusters of silicon and/or germanium exhibiting a permanent magnetic and/or electric dipole moment; and applying at least one direct-current voltage so as to modify the orientation of the nanostructures being deposited on the material so as to adjust the work function of the material to be treated.

8. The process according to claim 7, characterized in that the adjustment consists of: applying a direct-current voltage; and varying this direct-current voltage in space or time, so as to obtain a variation of the work function along the material.

9. The process according to claim 7, characterized in that the adjustment consists of applying in the reactor at least two direct-current voltages, the directions of which are orthogonal.

10. The process according to claim 9, characterized in that it comprises a step of varying in space or time at least one of these two voltages so as to obtain a variation of the work function along the material.

11. The process according to claim 7, characterized in that the time period between stopping the generation of the plasma and application of a direct-current voltage is comprised between 1 ms and 2,000 ms.

12. The process according to claim 1, characterized in that it comprises the creation of several individual layers.

13. The process according to claim 12, characterized in that during pulsed plasma-enhanced chemical vapour deposition (PECVD) in a reactor, the time period between two pulses is greater than or equal to the time necessary to empty the reactor.

14. The process according to claim 1, characterized in that the deposition step comprises a first phase of deposition of the nanoclusters on a transporter element comprising a sol-gel, a liquid, or a sol-gel liquid matrix; and a second phase of deposition on a material.

15. The process according to claim 1, characterized in that the deposition is carried out on a metal or a metal oxide forming cathodes or electrodes in a thermionic device, a thermoelectric device or an electron emission device.

16. The process according to claim 1, characterized in that the deposition is carried out on a catalytic material intended to be utilized as catalyst during a chemical reaction.

17. The process according to claim 1, characterized in that it comprises depositing an all-silicon or all-germanium magnetic thin film by applying a magnetic or electric field with transitory effect during the deposition so as to align the magnetic dipole moments of the nanoclusters.

18. The process according to claim 17, characterized in that several layers composed of alternating ferromagnetic and non-magnetic layers are produced with different directions of magnetization, these directions being defined by virtue of the application of the transitory-effect magnetic or electric fields with different directions during deposition of the nanoclusters.

19. A device comprising a support and nanoclusters deposited on the surface of the support or in the support according to claim 1.

20. The device according to claim 19, characterized in that the nanoclusters have respective dipole moments the axes of which are substantially aligned in one and the same direction.

21. The device according to claim 19, characterized in that it consists of an all-silicon or all-germanium memory, in which the dipole orientation of the nanoclusters is used for coding.

22. A utilization of the device according to claim 19, for generating a terahertz (THz) emission, in which an oscillating electric field is applied on the device.

23. A utilization of the device according to claim 19, for generating a terahertz (THz) emission, in which a short charge pulse is applied on the device.

24. A utilization of the device according to claim 19, for generating a terahertz (THz) emission, in which a thermal heating is produced on the device.

25. A utilization of the device according to claim 19, for heating the support, in which a remote or external alternating electric field is applied to the nanoclusters.

26. The utilization according to claim 25, characterized in that in order to excite the nanoclusters effectively and to avoid direct excitation of water molecules, the external alternating electric field has an excitation frequency comprised between 1 Hz and 1 GHz.

27. An application of the process according to claim 19 for the design of nano-furnaces for annealing devices or treating diseases.

Description

[0120] Other characteristics and advantages of the invention will become apparent on reading the detailed description of implementations and embodiments that are in no way limitative, in light of the attached figures in which:

[0121] FIG. 1 is a diagrammatic view of a plasma-enhanced chemical vapour deposition reactor for deposition of non-tetrahedral nanostructures of silicon and/or germanium on a material, for example a lanthanum hexaboride (LaB.sub.6) substrate;

[0122] FIG. 2 is a graphic representation illustrating a typical cycle of deposition of nanostructures;

[0123] FIG. 3 is a diagrammatic view of a configuration of electrodes for applying several direct-current voltages of equal or different values, after extension of the plasma and during the deposition of the nanostructures on the material;

[0124] FIG. 4 is a diagrammatic view illustrating the displacement of the material during the deposition.

[0125] The embodiments which will be described hereinafter are in no way limitative; it is possible in particular to implement variants of the invention comprising only a selection of characteristics described hereinafter in isolation from the other features described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0126] In particular, all the variants and all the embodiments described are intended to be combined together in all combinations where there is no objection thereto from a technical point of view.

[0127] In the figures, the elements common to several figures retain the same reference.

[0128] The purpose of the present invention is to modify the work function of a material. It was noted that the non-tetrahedral nanostructures, nanoclusters or quantum dots of silicon and/or germanium constituted an excellent candidate. To this end, a device and a mode of operation were defined to generate such elements, and provision was made to deposit them in the form of thin layers on the surface of the substrate of a given material.

[0129] It is known that a dipole surface layer modifies the work function of the material on which this layer is deposited. The present invention goes further, as it makes it possible to control precisely the work function modification value (in particular by controlling the thickness and the constitution of the layer, and the orientation of the nanostructures in the layer), and to vary the work function of the material along the surface on which the layer of nanostructures is placed. Furthermore, these nanostructures are non-toxic and inexpensive as they are produced starting from inexpensive materials such as silicon for example, and can be generated in a plasma-enhanced chemical vapour deposition reactor.

[0130] FIG. 1 shows very diagrammatically a reactor 1 comprising an enclosure 2 in which are arranged a first electrode 3 and a second electrode 4 connected to ground. Heating elements 5 are provided to maintain a desired temperature. The substrate of a given material 6 on which a layer of nanostructures is intended to be deposited is arranged on the second electrode 4. Provision is made to generate between the two electrodes 3 and 4, a plasma 7 of nanoclusters of silicon and/or germanium exhibiting a permanent magnetic and/or electric dipole moment which will be deposited on the surface of the material 6 facing the first electrode 3.

[0131] In order to create this plasma 7, a recipient 8 containing a gaseous mixture 9, for example, of silane and/or germane and argon is connected to the first electrode 3 via an injection pipe 10. A valve 11 is provided in the pipe 10 to control the injection and the injection flow. The electrode 3 has, for example, the shape of a shower head and contains a pierced inner face to allow the gaseous mixture 9 to pass through to the plasma generation zone between the two electrodes.

[0132] A radiofrequency generator 12 is also shown, capable of generating a radiofrequency discharge signal during the injection of the gaseous mixture so as to create the plasma 7.

[0133] A pump 13 is connected to the enclosure 2 to evacuate the gaseous content of the enclosure and adjust the desired pressure level in the enclosure 2.

[0134] A temperature sensor 14 and a pressure sensor 15 make it possible to measure the temperature and pressure respectively within the enclosure 2.

[0135] In operation, as a typical but non-limitative example, a primary vacuum is firstly formed in the enclosure 2 having a volume of approximately 0.02 m.sup.3. Then, a radiofrequency discharge of 13.56 MHz is applied at low pressure between the two electrodes 3 and 4. The surfaces of the electrodes are circular and have a diameter of 7 cm. The inter-electrode distance is 4 cm. The first electrode 3 has, for example, the shape of a shower head allowing the injection of the gaseous mixture 9 of silane and/or germane diluted in a neutral gas (for example, argon or/and helium) to which hydrogen can be added in order to produce nanostructures according to the invention. The flow of the gaseous mixture 9 is 10 cm.sup.3/min. The pump 13, connected to the enclosure 2 by an electronically-controlled butterfly valve (not shown), is used to continuously extract the gaseous mixture from the enclosure and to maintain a constant low pressure of 0.12 mbar in the reactor. Under these conditions, the time required for completely eliminating the residual nanostructures around the material, between the electrodes, after a radiofrequency discharge pulse is estimated at 1.5 second. This time period determines the typical time interval of 6 seconds between two trigger pulses of the plasma discharge during deposition of the nanostructures.

[0136] During the injection, the gaseous mixture is typically heated at 150° C. The enclosure is also brought to the same temperature of 150° C. At such a gas temperature, the formation of “first generation” nanoparticles, which take approximately 6 seconds before starting to agglomerate, is guaranteed. Advantageously, the discharge time (time period during which the plasma is generated) is set at 2 seconds, much less than said 6 seconds, so as to allow in this way the creation of nanostructures having dimensions according to the invention and to avoid the formation of agglomerated particles.

[0137] In other words, as soon as the plasma is switched on, dissociation of the silane molecules (SiH.sub.4) leads to the formation of SiH.sub.3, recognized as the main core starting from which the formation of silicon nanoparticles begins in low-pressure non-thermal plasmas. The electrical properties of the plasma and of the discharge are utilized to monitor the different phases of growth. The agglomeration phase is not reached, so that only the individual nanostructures are deposited on the material.

[0138] In order to carry out the deposition, the plasma is generated in pulsed fashion by controlling the instant of creation and end of creation of the plasma. According to the invention, several milliseconds after stopping the plasma, a continuous electric field is generated between the two electrodes so as to impose an orientation on the nanostructures before they are placed on the substrate material. The work function of the substrate material on which a layer of nanostructures has been deposited has an increase or a reduction in the work function according to the orientation of the DC electric field. The orientation of the nanostructures during their deposition on the surface of the material thus makes it possible to precisely adjust the value of the work function, which will be lower or higher than the work function of the material before the deposition.

[0139] FIG. 2 represents a typical example of some command signals of the process of deposition. Each deposition cycle starts with the generation 16 of the plasma during a well determined time period. Then a waiting time period comprised between 1 and 200 ms is complied with before generating, following the signal 17 in FIG. 2, a direct-current voltage between the two electrodes 3 and 4 so as to modify the orientation of the nanostructures that are deposited on the surface of the material. In the typical but non-limitative example, the next cycle starts 6 seconds after the beginning of the first cycle, which leaves approximately 1.5 second for evacuation of the residual nanostructures of the first generation in the enclosure.

[0140] According to the invention, the direct-current voltage applied makes it possible to modify the orientation of the nanostructures on the surface of the material. This direct-current voltage can advantageously be adjusted, after extension of the plasma and during deposition of the nanostructures. The adjustment can be continuous or in stages. It is possible to refine the adjustment of the work function. It can then be envisaged to create different work function levels along the surface of the material. It is possible in particular to envisage implementing a voltage gradient, i.e. different direct-current voltage values along the surface of the material. To this end, in FIG. 3 it is envisaged to arrange several sub-electrodes 18, 19, 20 and 21 (which can originate from subdivision of the first electrode), each fed with a different voltage such as for example respectively 0 V, 30 V, 60 V and 90 V. The level of direct-current voltage for each sub-electrode is a function of the desired work function level in the part of the material facing the sub-electrode in question.

[0141] These direct-current voltage values can be progressive, so as to define a continuous gradient, stages of values, or not progressive, so as to define any variation whatever in the form of the work function along the material. According to the invention, provision is also made to apply at least two direct-current voltages that are for example oriented orthogonally so as to have a finer control over the orientation of nanostructures during their deposition. One can be applied vertically between the two electrodes 3 and 4, the other can be applied horizontally between the electrodes 22 and 23. The direct-current voltages applied are preferably comprised between 0 V and 1,000 V; preferably approximately 200 V.

[0142] In FIG. 4 according to the invention, provision is made to displace the material with respect to at least the first electrode during deposition. This configuration makes it possible in particular to be able to modify the layer thickness and thus, the work function, on different parts of the material. In this configuration, either the electrode is fixed and the material is displaced, or vice-versa. The electrode used to apply the direct-current voltage is advantageously the first electrode 3 utilized to apply the discharge after switching off the plasma, but this can be another electrode.

[0143] The present invention relates to the production and deposition of one or more layers of nanoclusters of silicon and/or germanium exhibiting a permanent magnetic and/or electric dipole moment at the surface or in the matrix of a given material or in a liquid. The objective being to reduce or increase the work function of this material according to the requirements of the application in question, so as to improve the performance and/or reduce the costs. The materials can be presented in the form of mass (3D), thin layer (2D), nanowire (1D), or nanoparticle (0D). The applications in question include, non-limitatively, thermionic emission, thermoelectric devices, band bending in micro- and nanoelectronics, new all-silicon memories, due to magnetic properties of these nanostructures; telecommunications based on terahertz devices; nano-furnaces for annealing devices or for the treatment of cancer or other diseases such as Alzheimer's disease; organic electronics; photoelectric devices; catalytic reactions and fractionation of water. Furthermore, the layer of non-tetrahedral nanostructures of silicon and/or germanium prevents the evaporation of the atoms of volatile materials and thus protects the material against any chemical degradation, such as for example oxidation. Moreover, the deposition of non-tetrahedral nanostructures of silicon and/or germanium can have environmental advantages when it is utilized for the modification of the work function of toxic materials.

[0144] Of course, the invention is not limited to the examples that have just been described, and numerous adjustments may be made to these examples without departing from the scope of the invention.