Source: http://www.google.com/patents/US7785922?dq=7,752,326
Timestamp: 2014-03-15 12:06:49
Document Index: 601007208

Matched Legal Cases: ['Application No. 60', 'Application No. 60', 'art 2500', 'art 2500', 'art 2500', 'art 2500', 'art 2500', 'art 2500', 'art 2500', 'art 2500', 'Application No. 60', 'art 2600', 'Application No. 06851310']

Patent US7785922 - Methods for oriented growth of nanowires on patterned substrates - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsThe present invention is directed to systems and methods for nanowire growth and harvesting. In an embodiment, methods for nanowire growth and doping are provided, including methods for epitaxial oriented nanowire growth using a combination of silicon precursors, as well as us of patterned substrates...http://www.google.com/patents/US7785922?utm_source=gb-gplus-sharePatent US7785922 - Methods for oriented growth of nanowires on patterned substratesAdvanced Patent SearchPublication numberUS7785922 B2Publication typeGrantApplication numberUS 11/641,946Publication dateAug 31, 2010Filing dateDec 20, 2006Priority dateApr 30, 2004Also published asUS20080038521Publication number11641946, 641946, US 7785922 B2, US 7785922B2, US-B2-7785922, US7785922 B2, US7785922B2InventorsVirginia RobbinsOriginal AssigneeNanosys, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (106), Non-Patent Citations (57), Referenced by (2), Classifications (21), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetMethods for oriented growth of nanowires on patterned substratesUS 7785922 B2Abstract The present invention is directed to systems and methods for nanowire growth and harvesting. In an embodiment, methods for nanowire growth and doping are provided, including methods for epitaxial oriented nanowire growth using a combination of silicon precursors, as well as us of patterned substrates to grow oriented nanowires. In a further aspect of the invention, methods to improve nanowire quality through the use of sacrificial growth layers are provided. In another aspect of the invention, methods for transferring nanowires from one substrate to another substrate are provided.
1. A method for producing nanowires, comprising:
(a) creating or generating at least one void in a catalyst-repelling material on a substrate material;
(b) depositing a metallic film directly on the catalyst-repelling material and the substrate material at the site of the at least one void;
(c) heating the metallic film to a temperature wherein the metallic film melts, thereby generating one or more metallic nucleating particles, wherein the nucleating particles migrate from the catalyst-repelling material to the site of the at least one void and deposit on the substrate; and
(d) heating the nucleating particles to a temperature of greater than about 400� C. and contacting the nucleating particles with a precursor gas mixture at a pressure greater than about 0.5 torr to create an alloy droplet, whereby nanowires are grown at the site of the alloy droplet.
2. The method of claim 1, wherein the creating or generating in (a) comprises layering a catalyst-repelling material on a crystallographic substrate material.
3. The method of claim 2, wherein the substrate comprises silicon, and the creating or generating in (a) comprises layering a catalyst-repelling material on the silicon substrate.
4. The method of claim 2, wherein the growing occurs epitaxially.
5. The method of claim 1, wherein the creating or generating in (a) comprises layering a catalyst-repelling material which comprises SiO2, or anodic alumina.
6. The method of claim 1, wherein the growing is out of a plane of the substrate material.
7. The method of claim 1, wherein the heating in (d) is to a temperature of about 450� C. to about 700� C.
8. The method of claim 1, wherein the depositing in (b) comprises depositing a metallic film comprising a metal that reacts with the precursor gas mixture of step (d) to form a eutectic from which Si precipitates.
9. The method of claim 8, wherein the depositing in (b) comprises depositing a Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In film.
10. The method of claim 9, wherein the depositing in (b) comprises depositing a Au or Al film.
11. The method of claim 1, wherein the contacting with a precursor gas mixture comprises contacting with a gas mixture comprising SiH4, Si2H6, SiCl4 or SiH2Cl2.
12. The method of claim 1, wherein the contacting comprises performing plasma enhanced sputter deposition.
13. The method of claim 1, wherein the contacting occurs at a pressure of between about 5 to about 200 torr.
14. The method of claim 13, wherein the contacting occurs at about 45 torr.
15. The method of claim 1, wherein the depositing in (b), and the contacting in (d) occur in separate reaction chambers.
16. Nanowires produced by the process of claim 1.
17. An electronic circuit comprising nanowires produced by the process of claim 1.
18. The method of claim 1, wherein the creating or generating in (a) comprises creating or generating voids in an oriented pattern.
19. The method of claim 1, wherein the depositing in (b) comprises depositing a 3-50 nm thick metallic film.
20. The method of claim 1, wherein the heating in (c) comprises heating to a temperature of about 500� C. to about 900� C. at a pressure of about 5*10−8 to about 10−7 torr.
21. A method for producing nanowires, comprising:
(a) creating or generating at least one void in a catalyst-repelling material on a silicon substrate;
(b) depositing a metallic film directly on the catalyst-repelling material and the silicon substrate at the site of the at least one void;
(c) heating the metallic film to a temperature wherein the metallic film melts, thereby generating one or more metallic nucleating particles, wherein the nucleating particles migrate from the catalyst-repelling material to the site of the at least one void and deposit on the silicon substrate; and
(d) heating the metallic nucleating particles to a temperature of greater than about 400� C. and contacting the metallic nucleating particles with a precursor gas mixture at a pressure greater than about 0.5 torr to create an alloy droplet, whereby nanowires are grown at the site of the alloy droplet.
22. The method of claim 21, wherein the creating or generating in (a) comprises layering a catalyst-repelling material which comprises SiO2, or anodic alumina.
23. The method of claim 21, wherein the growing occurs epitaxially.
24. The method of claim 21, wherein the growing is out of a plane of the silicon substrate.
25. The method of claim 21, wherein the heating in (d) is to a temperature of about 450� C. to about 700� C.
26. The method of claim 21, wherein the depositing in (b) comprises depositing a Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In film.
27. The method of claim 26, wherein the depositing in step (b) comprises depositing a Au or Al film.
28. The method of claim 21, wherein the contacting with a precursor gas mixture comprises contacting with SiH4, Si2H6, SiCl4 or SiH2Cl2.
29. The method of claim 21, wherein the contacting comprises performing plasma enhanced sputter deposition.
30. The method of claim 21, wherein the contacting occurs at a pressure of about 5 to about 200 torr.
31. The method of claim 30, wherein the contacting occurs at about 45 torr.
32. The method of claim 21, wherein the depositing in (b), and the contacting in (d) occur in separate reaction chambers.
33. Nanowires produced by the process of claim 21.
34. An electronic circuit comprising nanowires produced by the process of claim 21.
35. The method of claim 21, wherein the creating or generating in (a) comprises creating or generating voids in an oriented pattern.
36. The method of claim 21, wherein the depositing in (b) comprises depositing a 3-50 nm thick metallic film.
37. The method of claim 21, wherein the heating in (c) comprises heating to a temperature of about 500� C. to about 900� C. at a pressure of about 5*10−8 to about 10−7 torr.
CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 60/754,519, filed Dec. 29, 2005, the disclosure of which is incorporated by reference herein in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH Portions of this invention may have been made with United States Government support under a grant from the National Science Foundation, Grant No. IIP-0620589. As such, the United States Government may have certain rights in this invention.
SUMMARY OF THE INVENTION The present invention provides methods for producing nanowires. In one embodiment, one or more nucleating particles are deposited on a substrate material. Then, the nucleating particles are heated to a first temperature and then contacted with a first precursor gas mixture to create a liquid alloy droplet and initiate nanowire growth. The alloy droplet is then heated to a second temperature and then contacted with a second precursor gas mixture, whereby nanowires are grown at the site of the alloy droplet. The substrate material utilized in the processes of the present invention may be crystallographic or amorphous. Suitably, the substrate material comprises crystallographic silicon, either polycrystalline or single crystalline. In other embodiments, the substrate may be amorphous SiO2, Si3N4, or alumina.
Additional methods for producing nanowires (e.g., Si nanowires) are also provided by the present invention. For example, such methods include depositing one or more nucleating particles (e.g., a metal catalyst such as gold nanoparticles) on a substrate material (or nucleating nanoparticles on a substrate surface (e.g., by heating a gold film coating layer on the surface)). The nucleating particles are then heated to a first temperature at which a first precursor gas decomposes to form a eutectic phase with the nucleating particles and then the nucleating particles are contacted with the first precursor gas mixture, wherein the first precursor gas mixture comprises a first precursor gas comprising at least one atomic species (e.g., Cl) that assists in orienting the growing nanowires (e.g., by etching as described in more detail below). The nucleating particles are then contacted with a second precursor gas mixture after initiation of nanowire growth, wherein the second precursor gas mixture includes a precursor gas that decomposes to form a eutectic phase with the nucleating particles at a second temperature which is lower than the first temperature, and heating the nucleating particles to the second temperature.
The above method can be reversed such that the process of nanowire growth is initiated with a precursor gas at the lower temperature, and then nanowire growth is continued at a higher temperature using a second precursor gas (e.g., a gas having a reactive etchant species to aid in nanowire orientation such as chlorine). The first precursor gas utilized is preferably SiCl4 or SiH2Cl2 which contains Si and Cl atoms upon disassociation at the first temperature. The Si atoms provide for nanowire growth and the Cl atoms allow for growth of the wires in a <111> orientation when grown on a crystallographic substrate as a result of etching of the native oxide layer on the silicon substrate. Once nanowire growth has been initiated, a second precursor gas mixture including a precursor gas such as SiH4 or Si2H6 can be introduced which decomposes to form a eutectic phase with the nucleating particles at a lower temperature than the first precursor gas. The disassociated Si atoms from SiH4 or Si2H6 at the second temperature continue the growth of the Si nanowires. Thus, nanowire growth can continue with the free Si atoms at a lower temperature than that at which nanowire growth is initiated, e.g., allowing growth of the oriented wires to a desired length while minimizing diffusion of the metal catalyst into the growing nanowires.
In certain suitable embodiments of the methods of the present invention, the first temperature to which the nucleating particles is heated is higher than the second temperature to which the alloy droplet is heated. Suitably, the first temperature is at least about 50� C. higher than the second temperature. The nucleating particles used in the practice of the present invention will suitably be a metal catalyst and will comprise a metal that reacts with both the first precursor gas mixture and the second precursor gas mixture (i.e., decomposed first and second precursor gas mixtures) to form a eutectic from which Si may precipitate. Suitable metal catalysts comprise Au, Pt, Fe, Ti, Ga, Ni, Sn or In and in certain such embodiments, may be a Au colloid or Au film.
The first precursor gas mixture and the second precursor gas mixture utilized in the processes of the present invention will suitably comprise SiH4, Si2H6, SiCl4 or SiH2Cl2, and may further comprise B2H6, trimethyl boron (TMB), POCl3 or PH3 (e.g., as dopant materials). Additional embodiments of the processes of the present invention may further comprise contacting the growing nanowires with one or more additional precursor gas mixtures comprising SiH4, Si2H6, SiCl4 or SiH2Cl2 and further comprising B2H6, TMB, POCl3 or PH3 to grow the nanowires to a desired length. The precursor gases used in the processes of the present invention may also suitably be introduced via plasma enhanced sputter deposition.
In an additional embodiment, methods for harvesting and transferring nanowires are disclosed that include providing a substrate material having one or more nanowires attached to a top surface. A transfer substrate is then provided, oriented above the top surface of the substrate. Pressure is applied to the transfer substrate, such that the transfer substrate is brought in contact with the one or more nanowires. One or more of the nanowires are then transferred from the substrate to the transfer substrate, and the transfer substrate is then removed. In this embodiment, the transfer substrate can be a flexible polymer and a probe can be used to apply pressure. In embodiments the probe may be heated or the substrate may be heated.
In a further embodiment, the present invention provides methods for producing nanowires on patterned substrates. In suitable embodiments, the methods comprise layering a catalyst-repelling material on a substrate material to at least partially cover the substrate material. One or more nucleating particles are then deposited on the substrate material. The nucleating particles are then heated to a first temperature and contacted with a first precursor gas mixture to create a liquid alloy droplet to initiate nanowire growth. The alloy droplet is then heated to a second temperature and the alloy droplet is then contacted with a second precursor gas mixture, whereby nanowires are grown at the site of the alloy droplet. Suitably, the substrate material will comprise a crystallographic material, such as Si, and the nucleating particles will comprise metallic films or colloids, such as films or colloids comprising Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In. Examples of catalyst-repelling material include, but are not limited to, SiO2, and anodic alumina. As noted throughout, the temperature and precursor gases can be varied throughout the growth process, suitably starting at a higher temperature initially, and then using a lower temperature for continued nanowire growth. Examples of precursor gas mixtures for use in the methods of the present invention include, but are not limited to, SiH4, SiCl4 and SiH2Cl2. The present invention also provides nanowires produced by the methods of the present invention and electronic devices comprising such nanowires.
Additional methods for producing nanowires on patterned substrates are also provided. Suitably, a catalyst-repelling material is applied on a substrate material to at least partially cover the substrate material. Then, one or more nucleating particles are applied on the substrate material. The nucleating particles are then heated (e.g., to a temperature of above about 400� C.) and then contacted with a precursor gas mixture (e.g., at a pressure of above about 0.5 torr) to create an alloy droplet, whereby nanowires are grown at the site of the alloy droplet. Suitably, the catalyst-repelling material (e.g., SiO2 or anodic alumina) comprises at least one void that does not cover the substrate material (e.g., a silicon or other crystallographic substrate). In exemplary embodiments, nucleating particles are deposited on the catalyst-repelling material (e.g., in the form of a film or colloid and then heated), wherein the nucleating particles deposit on the substrate material at the site of the at least one void.
In exemplary embodiments, the methods of the present invention comprise heating to a temperature of about 450� C. to about 700� C., at a pressure of about 5 to about 200 torr, suitably about 45 torr. Exemplary nucleating particles include those disclosed throughout, such as metals comprising Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In. Exemplary precursor gas mixtures include those disclosed throughout, such as gas mixtures comprising SiH4, Si2H6, SiCl4 or SiH2Cl2.
The present invention also provides additional methods for producing nanowires. For example, one or more nucleating particles (e.g., metallic colloids or films) are be applied on a substrate material. Then, the nucleating particles are heated to a temperature of greater than about 400� C. (suitably, between about 450� C. to about 700� C.), and contacted with a precursor gas mixture at a pressure greater than about 0.5 torr (suitably between about 5 to about 200 torr, more suitably about 45 torr) to create a liquid alloy droplet, whereby nanowires are grown at the site of the alloy droplet.
Exemplary nucleating particles include those disclosed throughout, such as metals comprising Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In. Exemplary precursor gas mixtures include those disclosed throughout, such as gas mixtures comprising SiH4, Si2H6, SiCl4 or SiH2Cl2.
FIG. 23A is a transmission electron micrograph that shows a substrate with e-field oriented nanowires prior to transfer, according to an embodiment of the invention.
FIG. 23B is a transmission electron micrograph that shows nanowires remaining on substrate following transfer, according to an embodiment of the invention.
FIG. 23C is a transmission electron micrograph that shows nanowires on transfer substrate following transfer, according to an embodiment of the invention.
FIG. 24 is a diagram showing oriented nanowire growth using a patterned substrate in accordance with one embodiment of the present invention.
FIG. 25 is a flowchart of a method for preparing nanowires on a patterned substrate accordance with one embodiment of the present invention.
FIG. 26 is a flowchart of a method for preparing nanowires in accordance with one embodiment of the present invention.
A wide range of types of materials for nanowires, nanorods, nanotubes and nanoribbons can be used, including semiconductor material selected from, e.g., Si, Ge, Sn, Se, Te, B, C (including diamond), P, B�C, B�P(BP6), B�Si, Si�C, Si�Ge, Si�Sn and Ge�Sn, SiC, BN/BP/BAs, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN2, CaCN2, ZnGeP2, CdSnAs2, ZnSnSb2, CuGeP3, CuSi2P3, (Cu,Ag)(Al,Ga,In,Tl,Fe)(S,Se,Te)2, Si3N4, Ge3N4, Al2O3, (Al,Ga,In)2 (S,Se,Te)3, Al2CO, and an appropriate combination of two or more such semiconductors.
It should be understood that the spatial descriptions (e.g., �above�, �below�, �up�, �down�, �top�, �bottom,� etc.) made herein are for purposes of illustration only, and that devices of the present invention can be spatially arranged in any orientation or manner. In addition, there may also be intervening layers or materials present in such devices to facilitate processing.
FIG. 11B shows a nanowire 110 doped according to a core-shell structure. As shown in FIG. 11B, nanowire 110 has a doped surface layer 112, which can have varying thickness levels, including being only a molecular monolayer on the surface of nanowire 110.
In suitable embodiments, the substrate material on which the nanowires are grown is a crystallographic substrate. The term �crystallographic substrate� includes any substrate material which comprises atoms situated in a repeating or periodic array over large atomic distances, typically on the order of 10 or more angstroms (Å). Such crystallographic substrates may be polycrystalline or may comprise single crystals. Suitably, the crystallographic substrate utilized in the processes of the present invention is silicon (Si). Other suitable crystallographic materials included, but are not limited to, germanium (Ge), gallium arsenide (GaAs), gallium nitride (GaN), sapphire, quartz, and silicon germanium (SiGe). In other embodiments of the present invention, the substrate material may comprise an amorphous material. Suitable amorphous substrate materials which may be used in the practice of the present invention include, but are not limited to SiO2, Si3N4 and alumina. In additional embodiments, substrate materials are composed of multiple layers, such as a silicon on insulator (SOI) surface.
As outlined in FIG. 2, in certain embodiments, the methods of the present invention comprise first depositing nucleating particles on a substrate material. Nucleating particles that may be used in the practice of the present invention include metal catalysts and can be any metal that reacts with both the first precursor gas mixture and the second precursor gas mixture (i.e., decomposed forms of the first and second precursor gas mixtures) to form a eutectic phase. Such a mixture has a minimum melting point at which all components are in solution. Upon addition and decomposition of precursor gas molecules (e.g., silicon) a saturation point on the eutectic phase diagram is reached such that semiconductor particles (e.g., Si) begin to precipitate out of the metal solution, thereby creating a growing nanowire. Continuous addition of precursor gas, as it decomposes, will continue to saturate the eutectic, thereby generating additional material for nanowire growth.
In suitable embodiments, the nucleating particles will be metal catalysts and can comprise any of the transition metals from the Periodic Table, including, but not limited to, copper, silver, aluminum gold, nickel, palladium, platinum, cobalt, rhodium, iridium, iron, ruthenium, tin, osmium, manganese, chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium and gallium, including mixtures of one or more of these metals. In preferred embodiments of the present invention, the metal catalyst can comprise a gold (Au) colloid (i.e., a Au nanoparticle) or Au film. In certain such embodiments, 60 nanometer (nm) diameter gold colloids can be used. The target is to achieve a uniform deposition of gold nanoparticles with density between 2-4 particles per square micrometer (μm). A key is minimized gold particle cluster formation. The clusters can result in undesired larger diameter nanowire growth. Spin coating and self assembly methods can be explored for the deposition (see e.g., U.S. patent application Ser. No. 10/674,060, filed Sep. 30, 2003, which incorporated by reference herein in its entirety).
Heating these Si precursor gases above the temperature at which the thermal energy is sufficient to break the bond energies between the gaseous molecules generates free Si atoms. (e.g., Si�H bond: 93 kcal/mole, Si�Cl bond: 110 kcal/mole, Si�Si bond; 77 kcal/mole, see M. T. Swihart and R. W. Carr, J. Phys Chem A 102:1542-1549 (1998).) Provided that this temperature is also high enough to liquefy the metal catalyst, the free Si atoms will diffuse into the metal and generate a eutectic phase. Dissociation temperatures for SiH4 and Si2H6, and SiCl4 and SiH2Cl2 gases are between about 300� C. to 500� C. (for Si2H6 and SiH4), and above about 800� C. (for SiCl4 and SiH2Cl2) respectively. In the instances of SiCl4 or SiH2Cl2, Cl atoms are also generated. Decomposition of SiCl4 or SiH2Cl2 into Si and Cl in the presence of a carrier gas (e.g., H2, H2Ar) forms HCl.
In all embodiments of the present invention, the precursor gas mixtures used during any of the nanowire growth processes may further comprise one or more doping gases. Examples of suitable doping gases that may be used in the practice of the present invention include, but are not limited to, B2H6, trimethyl boron (TMB), POCl3 and PH3.
In one embodiment of the present invention, the first precursor gas can comprise SiCl4 and suitably a carrier gas, such as He, Ar, or other inert gas. Heating this gas mixture to a sufficiently high temperature, e.g., above about 800� C., generates free Si and Cl atoms. In suitable such embodiments, the first precursor gas may comprise one or more dopant gases selected from those described throughout the application. The first precursor gas mixture is passed over the nucleating particles, suitably metal-catalyst particles (e.g., gold nanoparticles) deposited on the substrate material at a total pressure between about 20 to about 50 Torr, while the nucleating particles are heated up to a temperature of about 800� C. In other embodiments of the present invention, the gas pressure may be increased or decreased, thereby requiring a modification in the temperature required to dissociate the precursor gas mixture.
In suitable embodiments, the various precursor gas mixtures that are introduced in any of the processes of the present invention may be introduced via Plasma Enhanced Sputter Deposition (or Plasma Enhanced Chemical Vapor Deposition (PECVD)) and the processes performed at lower temperatures. (See Hofmann et al., �Gold Catalyzed Growth of Silicon Nanowires by Plasma Enhanced Chemical Vapor Deposition,� J. Appl. Phys. 94:6005-6012 (2003).) Decomposition of SiCl4 or SiH2Cl2 into Si and Cl in the presence of a carrier gas (e.g., H2, H2Ar) forms HCl. This creates a competition between etching with HCl and growth from the Si vapor. Chlorine aids in removal of interfacial oxide on Si substrates leading to the oriented NW growth. Loss of metal catalyst (e.g., Au) can occur either by etching or thermal evaporation of AuCl that can form. Use of PECVD to grow NWs at temperatures below about 800� C., with the addition of chlorine gas from SiCl4 or SiH2Cl2, and/or the addition of chlorine gas from a source separate from the source of SiCl4 or SiH2Cl2, the reactive species of Si and Cl can be independently controlled in the plasma to enhance or suppress etching as needed to promote nanowire growth. Sputter deposition can be accomplished via any method known to the ordinarily skilled artisan, for example, diode, radio frequency and direct current deposition.
In another embodiment, the present invention provides processes for producing nanowires which does not require a starting metal catalyst, as outlined in FIG. 3. FIG. 3 is a flowchart of a method for preparing nanowires using a combination of Si precursors which does not require a starting metal catalyst, according to an embodiment of the invention.
This process of the present invention does not require the use of a metal catalyst to provide a nucleation site for the nanowire, and therefore eliminates the problems and concerns that arise due to metals diffusing into the growing nanowires. A similar process has been described by De Salvo et al. for the production of nanocrystals in the form of nanodots (�How far will Silicon nanocrystals push the scaling limits of NVMs technologies?,� IEEE Proceeding, Session 26, p. 1 (2003)), but has not been extended to the production of nanowires as in the present invention.
Following hydroxylation, the substrate material is then contacted with one or more precursor gas mixtures to allow nucleation and initiation of nanowire growth. Any precursor gas mixture known to the ordinarily skilled artisan can be used in the processes of the present invention, and suitably can comprise dopants. Examples of precursor gases useful in the practice of the present invention comprise, but are not limited to, SiH4, Si2H6, SiCl4 and SiH2Cl2, preferably SiH4 or Si2H6, which nucleates particles on the surface of the substrate, and in suitable embodiments may further comprise dopants such as, but not limited to, B2H6, TMB, POCl3 and PH3. The temperature for dissociation and nucleation of the nanowires is dependent upon the dissociation temperature of the precursor gas mixture as discussed throughout. In suitable embodiments, this temperature is at least about 300� C., but is optimized based on the dissociation temperature of the precursor gas mixture as discussed throughout. In certain such embodiments, the first precursor gas mixture will comprise SiH4.
Following nucleation and initiation of growth, the substrate material is then contacted with one or more second precursor gas mixtures as described throughout, and suitably can comprise SiH4, Si2H6, SiCl4 or SiH2Cl2, preferably SiCl4 or SiH2Cl2, and may further comprise B2H6, TMB, POCl3 or PH3. In certain embodiments, the second precursor gas mixture will comprise SiCl4 or SiH2Cl2. Use of such precursor gases will allow for growth in a <111> orientation when grown on a crystallographic substrate as a result of etching from the dissociated Cl as discussed above. In other embodiments of the invention, as shown in step 312 of FIG. 3, any number of precursor gases may be introduced to the nanowires during the initiation and growth processes, as long as one or more of the precursor gases is capable of nucleating particles on the surface of the substrate, and one or more precursor gas(es) aids in orienting the nanowires during the growth process (e.g., via etching). Provided further that as long as the temperature at which the wires contact the metal catalyst is above the dissociation temperature of the gas mixture, the wires will continue to grow. In other embodiments free H, Cl or Si atoms can be added to the growing nanowires as discussed throughout. As discussed throughout, the processes of the present invention can be used to produce nanowires that comprise various dopants and different regions of these dopants throughout the length of the nanowire.
FIG. 5 is a flowchart of method 500 for doping nanowires, according to an embodiment of the invention. Method 500 is similar to method 400, except that instead of synthesizing nanowires as was done in method 400 a similar approach to that of method 400 is used to dope nanowires. Method 500 begins in step 510. In step 510, a dopant precursor material is positioned at one end of a synthesis vessel, which is at temperature Ti. Example dopant precursor materials can include, but are not limited to Mg, B, and PO4.
FIG. 6 is a flowchart of method 600 to reduce surface states from dangling bonds on a nanowire structure, according to an embodiment of the invention. In the growth of nanowires, such as Si nanowires, dangling Si bonds often form at the interface between a Si nanowire and a dielectric that inhibit nanowire performance. Hydrogen passivation is often used in Si nanowire processing to reduce surface states from dangling bonds to address this problem. However, the hydrogen plasma can induce surface damage during processing. Method 600 describes an approach to use a sacrificial layer to protect the nanowire, while passivation is used to reduce surface states of dangling bonds.
In another embodiment, the present invention provides processes by which nanowires can be modified directly on a substrate for device preparation. Preferably, the nanowires used in these processes will be substantially vertical nanowires. Vertical nanowires encompass nanowires that are substantially perpendicular to the surface of the substrate on which they are grown or deposited. Suitably, the vertical nanowires will be oriented such that they are between about 45� and about 90� with respect to the horizontal plane of the substrate, more suitably about 60� to about 90�, and most suitably about 80� to about 90� with respect to the horizontal plane of the substrate. Such nanowires can be produced using any suitable nanowire growth process known in the art, including those disclosed herein. While any substrate material disclosed herein can be used as a nanowire growth platform, suitably, the substrate material will be single-crystalline or polycrystalline, such that growth from the substrate will generate oriented, straight, single crystal dimension wires (suitably epitaxially oriented nanowires). In other embodiments, the nanowires can be horizontal, such as disclosed in U.S. Provisional Patent Application No. 60/632,337, filed Dec. 2, 2004, the disclosure of which is incorporated by reference herein in its entirety. In further embodiments, after processing of the nanowires on the growth substrate, the nanowires can be removed from the substrate by coating the nanowires with a polymer to form a composite, and then removing the nanowires from the substrate.
Any on-substrate processing known or required by those skilled in the art can be performed on the substantially vertical nanowires. By providing nanowires that are separate, oriented and vertical, device processing of the wires is simplified and improved. In certain embodiments, a dielectric layer can be grown or deposited directly on the wires. The dielectric layer can be formed by oxidizing the nanowires, or otherwise forming the dielectric layer. Polymer dielectrics for use in the present invention include for example, polyimides, fluorinated polyimides, polybenzimidazoles and others. Dielectrics for use in the invention include SiO2, Ta2O5, TiO2, ZrO2, HfO2, Al2O3, and similar materials. Nitride dielectrics include AlN and SiN. As the wires are substantially separate, each wire can be fully coated with a dielectric material without the concern of sections of overlapping wire not receiving coating. In addition, further processing can include oxidation or nitridization of the nanowires. Nitridation of nanowires can be accomplished with processes similar to those employed in oxidation of nanowires. These materials can be applied to nanowires by chemical vapor deposition (CVD), solution phase over-coating, or by spin-coating the appropriate precursor onto the substrate. Other known techniques can be employed, for example sputtering and others.
FIG. 9 is a diagram showing nanowire processing following transfer in accordance with one embodiment of the present invention. As shown in FIG. 9, the polymer-nanowire composites 810 can subsequently be transferred to an additional substrate 902 where the nanowires can be metallized 904 to form electrical conductivity to device regions (e.g., gain, source, gait). In such embodiments, nanowires 804 can be coupled between a source electrode 906 and a drain electrode 908 over a portion of the gate electrode 910. In other embodiments, source and drain electrodes can be added and ohmic contacts can be generated on the wires. As the wires are further �anchored� by the metal contacts, gate isolation and metal processing steps as known in the art can be used to finalize the nanowire preparation. Such processing allows for wafers that can comprise multiple semiconductor devices on the same base substrate. In other embodiments, such processing can occur directly on the growth substrate 802, followed by removal of the nanowire composite, such that all, or substantially all, nanowire processing is prepared on the initial growth substrate 802.
Making reliable ohmic contacts with nanowires is difficult due to small contact areas and complicated interface states. Interfacial chemistry and physics between metal contacts and silicon are important technical areas regarding ohmic contacts. A key to success is the precise control of the metallization process and surface cleaning process prior to metallization. Suitable metallization schemes include Ti�Au, Ni and Al by electron beam (e-beam) evaporation. Various further processes, including ion gun cleaning, or HF etching can be employed to remove the surface dielectrics prior to metallization of source-drain electrodes.
Methods 1000, 1200, 1300, 1500 and 1600 provided below describe methods to address this problem. In particular, method 1000 involves the use of a sacrificial portion of a nanowire to provide more effective removal of the nanowires.
Method 1000 begins in step 1010. In step 1010 a desired portion of a nanowire is grown. In one embodiment, an Au colloid is used to grow the desired portion of the nanowire. In step 1020 a sacrificial portion of a nanowire is grown that has different properties from the desired portion. FIG. 11 illustrates a nanowire with a desired and sacrificial portion of a nanowire, according to an embodiment of the invention. In particular, FIG. 11 shows a nanowire including three parts�sacrificial portion 1110, desired portion 1120 and stub 1130, which is described below, that has been grown on substrate 1140.
In step 1040 the sacrificial portion of the nanowire is differentially removed. The sacrificial portion of the nanowire can be differentially removed by using a wet etchant with a etching rate that is significantly higher for the materials within the sacrificial than for the materials within the desired portions of the nanowire. For example, hydrofluoric peroxide acetic acid (1HF:2H2O2:3CH3COOH) can be used to remove a sacrificial portion that contains SiGe, when the desired portion is Si. When using this etchant, the etchant removes the SiGe alloy and stops efficiently at the Si surface of the desired portion. Other etchants can be used, as would be known by persons skilled in the relevant arts based on the teachings herein.
In an alternate embodiment, where the sacrificial portion of the nanowire is p-doped using a boron NWS as a dopant and the desired portion of the nanowire is n-doped example etchants can include, by are not limited to potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH) and ethylene diamine/pyrocatechol/water (EDP). These etchants etch the sacrificial portion of the nanowire at a rate ranging from 27:1 to greater than 500:1 compared to the etch rate of the desired portion, when Si nanowires are used, for example. The range depends on the specific etchant and the temperature, as would be known by individuals skilled in the art.
In another embodiment of the invention, a method of harvesting a nanowire includes growing a desired portion of the nanowire; growing a sacrificial portion of the nanowire with different properties from the desired portion of the nanowire; differentially removing the sacrificial portion of the nanowire; and removing a growth stub from the desired portion of the nanowire. This method can further include protecting the desired portion of the nanowire. In this method an Au alloy, such as, for example, AuGe or AuSiGe, can be used to grow the sacrificial portion of the nanowire. In this method the sacrificial portion of the nanowire can include SiGe and the desired portion can include Si. In this method differentially removing the sacrificial portion of the nanowire can further include using a wet etchant to selectively chemically etch to remove the sacrificial portion of the nanowire. In an embodiment, the wet etchant can be Hydroflouric Peroxide Acetic Acid (1HF:2H2O2:3CH3OOH).
In an embodiment of the invention, a method of harvesting a nanowire with a first material with a first orientation, includes establishing a substrate of a second material with a second orientation; growing the nanowire of the first material with the first orientation on the substrate of a second material with the second orientation; protecting the nanowire of the first material with the first orientation; and selectively wet etching based on orientation the substrate of the second material with the second orientation to free the nanowire of the first material with the first orientation. In this method the first material can be Si and the second material can be Si and the first crystal orientation is <111> and the second orientation is <100>. In the method selectively wet etching the substrate of the second material with the second orientation includes using Potassium Hydroxide (KOH) or tetramethylammonium hydroxide (TMAH).
In step 1920 the transfer surface is pressed against nanowires affixed to a nanowire growth substrate. In step 1930 the transfer surface is positioned above a second substrate. In step 1940 pressure is applied uniformly to the backside of the transfer surface to release the nanowires. In an alternative embodiment, pressure can be applied only to the patterned areas of the transfer surface. In step 1950 method 1900 ends.
The processes of the present invention can be used to transfer nanowires to select regions of transfer substrate 2106. Only regions where contact is made between transfer substrate 2106 and nanowires 2104, will the nanowires be transferred. Such embodiments of the present invention are referred to herein as a �tapping� method of nanowire transfer. In such embodiments, the probe tip can be moved around the transfer substrate, �tapping� the nanowires below to facilitate transfer from the substrate 2102 to the transfer substrate 2106. In other embodiments, the probe can be held stationary and the substrate and transfer substrate moved beneath it so as to control where nanowire transfer occurs, and the orientation of the nanowires on the transfer substrate. As discussed above, in such embodiments, either, or both, substrate 2102 and probe 2108/probe tip 2110 can be heated. Such embodiments of the present invention allow orientation of nanowires directly on transfer substrate 2106 by selectively transferring wires that have already been oriented on substrate 2102 using such methods as described herein (e.g., Langmuir Blodget, e-field, epitaxial growth, horizontal growth, etc.), or orienting the wires on the transfer substrate 2106 can also be achieved by transferring individual wires, or groupings of wires, and positioning (e.g., rotating) the transfer substrate 2106 such that the wires are oriented on the transfer substrate as they are transferred.
The nanowires produced by the processes of the present invention can also be incorporated in applications requiring a single semiconductor device, and to multiple semiconductor devices. For example, the nanowires produced by the processes of the present invention are particularly applicable to large area, macro electronic substrates on which a plurality of semiconductor devices is formed. Such electronic devices can include display driving circuits for active matrix liquid crystal displays (LCDs), organic LED displays, field emission displays. Other active displays can be formed from a nanowire-polymer, quantum dots-polymer composite (the composite can function both as the emitter and active driving matrix). The nanowires produced by the processes of the present invention are also applicable to smart libraries, credit cards, large area array sensors, and radio-frequency identification (RFID) tags, including smart cards, smart inventory tags, and the like.
Oriented Growth of Nanowires on Patterned Substrates
In another embodiment, the present invention provides methods for producing nanowires on patterned substrates. FIG. 24 shows a diagram of oriented nanowire growth using a patterned substrate in accordance with one embodiment of the present invention. It should be noted that FIG. 24 is not to scale and is simply provided to illustrate certain aspects of oriented nanowire growth on patterned substrates in accordance with this embodiment of the present invention. FIG. 25 shows a flowchart 2500 describing a method for producing nanowires utilizing patterned substrates in accordance with one embodiment of the present invention.
As shown in FIG. 24, a substrate material 2402, suitably a crystallographic substrate, such a silicon or other semiconductor material, is provided. In step 2502 of flowchart 2500, a catalyst-repelling material 2404 is applied on substrate material 2402 to at least partially cover the substrate material 2402. As used herein, the terms �applying� or �applied� refers to any suitable method for preparing a catalyst-repelling material 2404 on a substrate material 2402, and includes, layering depositing, spraying, coating, etc. As used herein the phrase �at least partially cover the substrate material� means that the catalyst-repelling material 2404 covers at least 1% of the surface area of substrate material 2404. Suitably, the catalyst-repelling material 2404 will cover at least about 10%, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 75%, at least about 80%, at least about 90%, or at least about 95% of the surface area of substrate material 2402.
As used herein the term �catalyst-repelling material� includes any material which does not allow a catalyst material to substantially bond, affix, interact, or attach thereto. Catalyst-repelling materials for use in the present invention suitably do not allow nucleating catalyst particles to bond or attach to their surface, thereby creating a repelling effect whereby nucleating particles do not, or cannot, attach, and as such are driven to areas of the substrate material which are not covered by the catalyst-repelling material. In other embodiments, nucleating catalyst particles are driven from the catalyst repelling material, or leave, for example during application (evaporation) or heating of the catalyst material. Examples of catalyst-repelling materials include, but are not limited to SiO2 and anodic alumina. As shown in FIG. 24, in suitable embodiments, catalyst-repelling material 2404 will be patterned or otherwise prepared such that the material at least partially covers substrate material 2402.
In suitable embodiments, catalyst-repelling material 2404 comprises voids 2410 in the material that expose substrate material 2402 below. The term �voids� as used herein includes, holes, openings, cracks, or other patterns that expose portions of substrate material 2402, while continuing to at least partially cover substrate material 2402. Suitably, voids 2410 are spaced throughout the catalyst-repelling material 2404 such that when nanowires are grown at the sites of the voids, they are spaced far enough from each other such that they don't contact or disturb other growing nanowires. Any suitable orientation of voids 2410 can be used. In addition, voids 2410 can be any desirable shape, for example, circular, square, random, etc. In suitable embodiments, voids 2410 are randomly spaced across catalyst-repelling material 2404 to create a variety of shapes and spacings. In other embodiments, voids 2410 can be evenly spaced, or in an oriented pattern, throughout catalyst-repelling material 2404, for example as a checkerboard, or other application-specific pattern. Voids 2410 can either be created in catalyst repelling material 2404 simply by forming them during initial application (i.e., by forming them around a void opening or �mold�), or they can be generated, for example as shown in step 2514, in flowchart 2500 of FIG. 25. For example, voids 2410 can be generated in step 2514 by removing catalyst-repelling material 2404 to expose substrate material 2402 via any suitable method, for example, by etching, cutting, scraping, drilling, or similar method. Catalyst-repelling material 2404 can be any suitable thickness that will prevent nucleating particles from contacting the substrate surface in areas which are desired to be covered. Suitably, catalyst-repelling material 2404 will be on the order of several nanometers to a few microns in thickness, though thicker material can also be used.
In step 2504 of flowchart 2500, one or more nucleating particles 2406 are then applied to the substrate material 2402. When nucleating particles 2406 contact catalyst-repelling material 2404, the particles are repelled and move to open voids 2410 where substrate material 2402 is exposed, or leave the substrate all together (e.g., during an evaporation deposition process). Nucleating particles that initially were in contact with the substrate material following application (i.e., contact at the sites of the voids) remain on the substrate material. As described herein, nucleating particles are suitably metallic catalysts that react with the decomposed precursor gas mixture(s) to form a eutectic from which Si precipitates, such as described throughout. For example, Au, Al, Pt, Fe, Ti, Ga, Ni, Sn or In can be deposited. Nucleating particles 2406 can be deposited as colloid droplets directly on the surface of substrate surface 2404. In additional embodiments, colloid droplets can be deposited on catalyst-repelling material 2404, at which time, assuming they are in a liquid state, they will move back into solution and may subsequently migrate from the catalyst-repelling material to the voids 2410 where they will contact substrate material 2402.
In additional embodiments, nucleating particles comprise metallic films, such as gold (Au), aluminum (Al) or other metal films as described throughout. Depositing a metallic film on catalyst-repelling material 2404 causes the metallic catalyst to migrate to open voids 2410 in the material where it enters the voids and then contacts substrate material 2402, or leave the substrate all together (e.g., during an evaporation deposition process). Nucleating particles that initially were in contact with the substrate material following application (i.e., contact at the sites of the voids) remain on the substrate material. In cases of both metallic colloids and metallic films, the catalyst material migrates to open areas of the patterned catalyst-repelling material 2404, or leaves the catalyst-repelling material, assuming that the catalyst is in a liquid state. Catalyst material can either be deposited in a liquid state, or can be deposited in a solid state and then, as shown in step 2512 of flowchart 2500, heated to a temperature where it melts and can flow/migrate or leave/evaporate. At this elevated temperature, metallic catalyst that is in contact with catalyst-repelling material 2404 can migrate to open voids 2410 and contact substrate material 2402. Alternatively, at the elevated temperature, the metallic catalyst in contact with the catalyst-repelling material leaves (e.g., evaporates) while the metallic catalyst that is in contact with the underlying substrate (i.e., in the void) coalesces into a metallic particle that can be alloyed with the substrate. Migration and/or coalescence of the nucleating particles can occur just prior to, or at the beginning, of the nanowire growth process.
In suitable embodiments, nucleating particles 2406 are applied/deposited onto catalyst-repelling material 2404 and substrate material 2402 by evaporating a metallic film (e.g., Al, Au or other suitable material) onto the substrate. Suitably, application of the nucleating particles (e.g., by evaporating a metallic film) occurs at room temperature (e.g., about 20-28� C.) and at a reduced pressure (i.e., in a vacuum at a pressure of less than about 10−7 torr, for example, between about 5*10−8 to about 10−7 torr). In further embodiments, application of the nucleating particles (e.g., via film evaporation) can be performed at an elevated temperature and reduced pressure. For example, the film can be evaporated at a temperature of greater than about 600� C., e.g., greater than about 650� C., greater than about 700� C., greater than about 750� C., greater than about 760� C., greater than about 770� C., about 775� C., about 780� C., about 790� C., or about 800� C.
The thickness of the metallic film deposited on the catalyst-repelling material 2404 and substrate 2402 will suitably be on the order of a few nanometers to 10's of nanometers thick, e.g., about 3-50 nm thick, suitably about 5-10 nm thick. Following application of the film (e.g., evaporation to form the metallic film), the film is then heated (e.g., step 2512 in flowchart 2500) to remove it from the catalyst-repelling material 2404, either via evaporation and/or by flowing into the voids in catalyst-repelling material 2404, thereby allowing the nucleating particles to coalesce and alloy with the substrate 2402. Suitably, the film is heated to between about 500� C. to about 900� C. to cause it to flow into voids 2410. For example, the film is suitably heated to between about 600� C. to about 800� C., to between about 650� C. to about 800� C., to between about 700� C. to about 800� C., about 725� C., about 750� C., about 760� C., about 770� C., about 775� C., about 780� C., about 790� C., or about 800� C. In suitable embodiments, heating step 2512 occurs at a reduced pressure, for example, between about 5*10−8 to about 10−7 torr. Following heating of the film, the substrate is then suitably cooled, and then transferred to a CVD reactor or other suitable apparatus to grow nanowires. Generally, the application of the nucleating particles (e.g., deposition of nanoparticles or evaporation of a catalyst film, followed by subsequent heating) and the contacting of the nanowires with a precursor gas mixture (i.e., growth) occur in separate reaction chambers, though they can occur in the same chamber. Suitably, the application of the nucleating particles occurs in an high vacuum chamber, while the growth occurs in a separate CVD reactor.
Once the nucleating particles 2406 are deposited on substrate material 2402, either directly, following evaporation from the catalyst-repelling material, or after migrating from catalyst-repelling material 2404, the nucleating particles 2406 are heated in step 2506 and contacted with a precursor gas mixture in step 2508 of flowchart 2500 (e.g., in a CVD reactor) to create a liquid alloy droplet 2412, whereby nanowire 2408 growth occurs at the site of the liquid alloy droplet in step 2510. Suitable growth conditions, including temperatures and times, are described herein. Suitable precursor gases include those described herein, and include gases comprising, but not limited to, SiH4, SiCl4 and SiH2Cl2. The use of catalyst-repelling material 2404, in addition to aiding in deposition of nucleating particles, also helps to keep nucleating particles 2406 from migrating during nanowire growth. If nucleating particles 2406 are heated in the absence of catalyst-repelling material 2404, they can often migrate on substrate material 2402 and coalesce into larger nucleating particles. This can compromise nanowire diameter and structure. In addition to problems with coalescence, catalyst-repelling material 2404 also helps to keep nucleating particles 2406 and growing nanowires 2408 properly spaced and oriented, thereby reducing tangling and other complications.
As shown in flowchart 2500 of FIG. 25, suitably, a single precursor gas is used to grow nanowires. In further embodiments, a second precursor gas can be utilized, as described throughout. For example, following contacting with a first precursor gas, alloy droplet 2412 can be heated to a second temperature, and contacted with a second precursor gas mixture, to continue nanowire growth at the site of the alloy droplet. Suitable gases for use as the second precursor gas include those described herein, including, but not limited to, SiH4, Si2H6, SiCl4 and SiH2Cl2. In further embodiments, a third, fourth, fifth, etc., precursor gas mixture can be provided to continue growing the nanowire(s). In such embodiments, the temperature of the growing nanowires 2408 and alloy droplets 2412 is maintained at a suitable temperature to allow precursor gas dissociation and nanowire growth. Thus, in suitable embodiments, the present invention provides for a continuously varying growth process in which the temperature of the nanowire growth and the precursor gases used can be continuously switched throughout the growth process until the final nanowire composition and characteristics (i.e., length, diameter) are achieved. Suitably, the first temperature used in the nanowire growth methods will be higher than the second temperature, for example about 50� C. higher. Any suitable method can be used to introduce the gases for the nanowire growth process. For example, plasma enhanced sputter deposition can be used to introduce the precursor gas mixtures. Rapid control of the chamber and substrate chamber can be achieved through any method known in the art. For example, a cold wall in an ultra high vacuum (UHV) reactor can be used.
In embodiments in which a single precursor gas mixture is utilized to grow nanowires (as well as where two or more precursor gas mixtures are utilized), the pressure and temperature of the contacting/growth conditions will suitably be greater than about 400� C., for example, between about 450� C. to about 700� C., and greater than about 0.5 torr, for example, between about 5 torr to about 200 torr. For example, suitable growth temperatures for use in the practice of the present invention are between about 475� C. to about 675� C., about 500� C. to about 650� C., about 550� C. to about 650� C., about 575� C. to about 625� C., or about 580� C., about 590� C., about 600� C., about 610� C., or about 620� C. Suitable precursor gas mixture pressures for use in the practice of the present invention are between about 5 torr to about 175 torr, about 10 torr to about 150 torr, about 20 torr to about 150 torr, about 40 torr to about 150 torr, about 45 torr, about 50 torr, about 55 torr, about 60 torr, about 65 torr, about 70 torr, about 75 torr, about 80 torr, about 85 torr, about 90 torr, about 95 torr, about 100 torr, about 105 torr, about 110 torr, about 115 torr or about 120 torr.
As described throughout, the present invention also provides nanowires produced by such processes of the present invention, and electronic circuits comprising such nanowires.
By combining patterned substrates and in suitable embodiments, varying temperature/precursor gas growth conditions, the nanowires produced according to the methods of the present invention are substantially vertical, oriented nanowires. Suitably, the methods produce epitaxially oriented nanowires that grow substantially normal to the plane of the substrate material. By controlling the deposition of nucleating particles, thereby controlling their migration on the substrate material, nanowire thickness is controlled. The use of a single precursor gas mixture (e.g., a single temperature and pressure) as well as use of varied growth conditions (two or more precursor gas mixtures at different temperatures and pressures) substantially aid in producing nanowires that do not exhibit taper throughout their length, but rather show substantial uniformity throughout. Suitably, nanowires produced using the various methods of the present invention exhibit a degree of taper that is less than about 0.1 nm taper/μm nanowire length. In further embodiments, nanowire taper can also be controlled, eliminated or substantially eliminated by introducing an etchant gas into the reaction prior to, during, or after nanowire growth. For example, as disclosed in U.S. Provisional Patent Application No. 60/857,450, filed Nov. 7, 2006 (the disclosure of which is incorporated herein by reference in its entirety), HCl can be introduced to control, eliminate or substantially eliminate nanowire tapering during growth.
In further embodiments, the present invention provides methods for producing nanowires. Suitably, a catalyst-repelling material is applied on a silicon substrate, to at least partially cover the silicon substrate, wherein the catalyst-repelling material comprises at least one void that does not cover the silicon substrate. One or more metallic nucleating particles are then applied on the catalyst-repelling material, wherein the metallic nucleating particles deposit and coalesce on the silicon substrate, via selective evaporation from the catalyst-repelling material and/or migration to the at least one void. The metallic nucleating particles are then heated to a first temperature and contacted with a first precursor gas mixture to create a liquid alloy droplet to initiate nanowire growth. The alloy droplet is then heated to a second temperature and contacted with a second precursor gas mixture, whereby nanowires are grown at the site of the alloy droplet. Suitably the nucleating particles will be metallic catalysts, such as metallic films or colloids (e.g., Au or Al films or colloids). The use of crystallographic substrates, such as Si <111> substrates (as well as other crystal orientations and substrate materials), allows for substantially oriented (suitable epitaxially oriented), vertical nanowires with substantially constant diameter and little taper. As described herein, suitably the catalyst-repelling material comprises SiO2, or anodic alumina. Examples of precursor gases include those described herein, such as SiH4, SiCl4 and SiH2Cl2. In additional embodiments, further precursor gas compositions and conditions can be used to continuously vary the growth process.
Additional methods for producing nanowires are also provided. For example, in and additional embodiment, a catalyst-repelling material is applied on a silicon substrate to at least partially cover the silicon substrate, wherein the catalyst-repelling material comprises at least one void that does not cover the silicon substrate. One or more metallic nucleating particles are then applied on the catalyst-repelling material, wherein the metallic nucleating particles deposit and coalesce on the silicon substrate, via selective evaporation from the catalyst-repelling material and/or migration to the at least one void. The metallic nucleating particles are then heated (e.g., to a temperature of greater than about 400� C.). The metallic nucleating particles are then contacted with a precursor gas mixture (e.g., at a pressure greater than about 0.5 torr) to create an alloy droplet, whereby nanowires are grown at the site of the alloy droplet.
Suitable conditions, including temperature and pressure conditions, for applying nucleating particles are described throughout. For example, the application process can comprise evaporating a metallic film either at room temperature (e.g., about 20� C.-28� C.), or at an elevated temperature (e.g., greater than about 600� C.) and at a reduced pressure, for example, between about 5*10−8 to about 10−7 torr. Suitably the application of the nucleating particles and the contacting with a precursor gas mixture occur in different reaction chambers.
Exemplary catalyst repelling materials, nucleating particles and precursor gas mixtures are described throughout. Suitably, the heating is to a temperature of about 450� C. to about 700� C., and the pressure is between about 5 torr and about 200 torr, suitably about 45 torr. In suitable embodiments, the step of applying the nucleating particles further comprises heating to a temperature wherein the metallic film melts (e.g., about 450-900� C.), thereby generating more metallic nucleating particles on the catalyst-repelling material, wherein the metallic nucleating particles deposit and coalesce on the substrate, via selective evaporation from the catalyst-repelling material and/or migration to the at least one void.
In still further embodiments, the present invention provides methods for producing nanowires utilizing a single precursor gas mixture. For example as shown in flowchart 2600 in FIG. 26, in step 2602, one or more nucleating particles is applied on a substrate material. In step 2604, the nucleating particles are then heated to a temperature of greater than about 550� C. In step 2606, the nucleating particles are then contacted with a precursor gas mixture at a pressure of greater than about 0.5 torr to create a liquid alloy droplet, whereby nanowires grow at the site of the alloy droplet in step 2608.
Exemplary substrate materials (e.g, Si), nucleating particles (e.g., Au or Al) and precursor gas mixtures (e.g., SiH4) are described throughout. Suitably, the nucleating particles are heated to a temperature of about 600� C. to about 700� C., e.g., about 600� C., and then contacted with a precursor gas mixture at between about 5 torr to about 200 torr, suitably about 45 torr. As discussed throughout, the application process can comprise evaporating a metallic film either at room temperature (e.g., about 20� C.-28� C.), or at an elevated temperature (e.g., greater than about 600� C.) and at a reduced pressure, for example, between about 5*10−8 to about 10−7 torr. Suitably the application of the nucleating particles and the contacting with a precursor gas mixture occur in different reaction chambers.
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Lett (1997) 71:611-613.Referenced byCiting PatentFiling datePublication dateApplicantTitleUS8058117 *Oct 23, 2009Nov 15, 2011Tsinghua UniversityMethod of synthesizing silicon wiresUS8609333Oct 1, 2008Dec 17, 2013University Of Southern CaliforniaDetection of methylated DNA and DNA mutations* Cited by examinerClassifications U.S. Classification438/99, 438/760, 257/E51.04, 438/669International ClassificationH01L51/40Cooperative ClassificationH01L21/02653, H01L21/02573, H01L21/02645, H01L21/02532, C30B25/04, C30B29/62, H01L21/02603, H01L21/0262European ClassificationC30B29/62, H01L21/02K4E3S8, C30B25/04, H01L21/02K4C5M6, H01L21/02K4C1A3, H01L21/02K4E3S3S, H01L21/02K4E3C, H01L21/02K4C3CLegal EventsDateCodeEventDescriptionJun 12, 2013ASAssignmentEffective date: 20130611Owner name: NANOSYS, INC., CALIFORNIAFree format text: RELEASE BY SECURED PARTY;ASSIGNOR:NANOSYS, INC.;REEL/FRAME:030599/0907Apr 25, 2013ASAssignmentEffective date: 20121221Owner name: NANOSYS, INC., CALIFORNIAFree format text: SECURITY AGREEMENT;ASSIGNOR:PRVP HOLDINGS, LLC;REEL/FRAME:030285/0829Mar 20, 2007ASAssignmentOwner name: NANOSYS, INC., CALIFORNIAFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:ROBBINS, VIRGINIA;REEL/FRAME:019034/0925Effective date: 20070227RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google