Patent Application
20100127241
This application relates to electronic devices that have components made with nanowires and methods of manufacturing such electronic devices.
2. Discussion of Related Art
The contents of all references, including articles, published patent applications and patents referred to anywhere in this specification are hereby incorporated by reference.
Flexible and transparent transistors have recently resulted is several noteworthy achievements. Transparent transistors have been fabricated using both polymers and inorganic oxides. Both have significant deficiencies. The former have low mobility the latter does not have the desired flexibility and manufacturability characteristics. These factors severely limit the application potential of the devices.
Carbon nanotubes (NTs), because of their excellent electronic properties, have been explored for applications as active electronic devices. Field Effect Transistors (FETs) with NT conducting channels have been fabricated (S. J. Tans, A. R. M. Verschueren, C. Dekker, “Room-temperature transistor based on a single carbon nanotube”, Nature 393, 49-52 (1998); R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and Ph. Avouris, “Single- and multi-wall carbon nanotube field-effect transistors”, Appl Phys Lett 73, 2447-2449 (1998)). Subsequently, it has been shown that a random network of nanotubes with appropriate density can also act as a conducting channel in a FET configuration (K. Bradley, J-C P. Gabriel, A. Star, and G. Grüner, “Short-channel effects in contact-passivated nanotube chemical sensors”, Appl Phys Lett 83, 3821-3823 (2003); J-C P. Gabriel, “Large Scale Production of Carbon Nanotube Transistors: A Generic Platform for Chemical Sensors”, MRS Proceedings Volume 776, Q12.7; E. S. Snow, J. P. Novak, P. M. Campbell, and D. Park, “Random networks of carbon nanotubes as an electronic material”, Appl Phys Lett 82, 2145-2147 (2003)). This has opened up the avenue for a manufacturable device architecture. Room-temperature fabrication techniques enabling flexible transistors have also been explored (K. Bradley, J-C P Gabriel and G. Gruner, “Flexible Nanotube Electronics”, Nano Lett 3,1353 (2003)). It has been shown that due to the high mobility of carbon nanotubes, a network with low sheet resistance is also transparent in the visible spectral range (Z. Wu, Z. Chen, X. Du, J. M. Logan, J. Sippel, M. Nikolou, K. Kamaras, J. R. Reynolds, D. B. Tanner, A. F. Hebard, and A. G. Rinzler, “Transparent, Conductive Carbon Nanotube Films”, Science 305, 1273-1276 (2004); L. Hu, D. S. Hecht and G. Grüner, “Percolation in Transparent and Conducting Carbon Nanotube Networks”, Nano Letters 4, 2523 (2004)).
Transistors that include carbon nanotubes as part of the transistor have been described in U.S. provisional application 60/544,841 (now pending as U.S. application Ser. No. 10/846,072, filed on May 14, 2004).
These disclosures, however, do not cover the architecture where the conducting channel and other conducting media within the architecture (gate, source and drain contacts) are formed by carbon nanotube networks.
Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.
An electronic device according to an embodiment of this invention has a source electrode, a drain electrode spaced apart from the source electrode and at least one of a conducting material, a dielectric material and a semiconductor material disposed between the source electrode and the drain electrode. At least one of the source electrode, the drain electrode and the semiconductor material has at least one nanowire.
In addition, devices according to embodiments of this invention are manufactured according to the methods of this invention.
The invention is better understood by reading the following detailed description with reference to the accompanying figures in which:
In describing embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
Accordingly, the current invention is directed to electronic devices that have components made with nanowires and the manufacture of such electronic devices. The invention includes two-electrode devices, such as resistors, diodes, capacitors, and inductors. The invention also includes three-electrode devices, such as transistors. Furthermore, each device of the invention can be used in combination with more that one such device of this invention to provide circuits built from a plurality of such components. The invention includes such circuits. Devices according to embodiments of this invention can be made to have a high degree of transparency. However, the invention is not limited to only transparent devices.
In current transistor configurations the gate and also the source and drain are metal electrodes. While this is a manufacturable architecture, neither the gate and/or source/drain electrodes are flexible and/or transparent. In addition there is usually a large interface resistance between the electrodes and the carbon nanotube network. In addition, there is a need for a simple method of fabrication, where the different layers that form the transistors, and the fabrication of the different layers are compatible. The invention satisfies this need, and three components of the device are all formed from the same material.
Transistors in accordance with the present invention include the following four basic elements: a source, a drain, a gate and a conducting channel. As a feature of the present invention, at least one of these four basic elements besides the conducting channel comprise at least one nanowire, for example a carbon nanotube network. As further features of the present invention, two, three or all four of the basic elements can have at least one nanowire.
The fabrication may include pattering using methods such as shadow masking or optical lithography to fabricate devices with appropriate geometry.
The following geometries, and transistor configurations are within the scope of the current invention:
Carbon nanotubes are considered to be one particular type of nanowire according to the current invention. However, this invention is not limited to only carbon nanotubes for the nanowires. The term nanowire is meant to have a broad definition, as follows.
Nanowires, or molecular nanowires are defined as having dimensions less than 500 nm in diameter (the diameter is the average of the cross-sectional width) and have an aspect ratio exceeding 10 (e.g. a 100 nm diameter nanowire must have a length that is equal to or greater than 1 micron). The term “molecular nanowire”, is used herein interchangeably with “molecular nanofibers” and it is intended that when the term “molecular nanowire” is used alone, it is intended to include molecular nanofibers. A network of molecular nanofibers can be made from a variety of known molecular semiconductor nanowires. Set forth below is a listing of known examples of molecular nanowire materials that can be used to make networks of molecular nanowires in accordance with the present invention.
Single element nanowires made from silicon using known procedures may be used to form a nanowire network. Procedures for making such nanowires are set forth in detail in Refs. 1-21. (These references are listed at the end in an appendix. They are a part of the disclosure and are incorporated by references as also indicated above.) Single element nanowires made from germanium may also be used. Details of synthesis are set forth in Refs. 9, 17 and 22-27. Other examples of single element nanowires include selenium and tellurium nanowires, which are made according to known procedures as set forth in Refs. 28-29 and Ref. 30, respectively.
Nanowires made from a combination of Group III-V materials using known procedures may be used to form the network. Examples of Group III-V materials that can be used to form nanowire networks include Ga, In, N, P, As and Sb. Details of examples of synthesis procedures for these nanowires are set forth as follows: GaN (Refs. 8, 31-45); GaP (Refs. 39, 46 and 47); GaAs (Refs. 42 and 48-50); InN (Ref. 51); InP (Refs. 8, 38 and 52-54); and InAs (Ref. 55).
Nanowires made from a combination of Group II-VI materials using known procedures may also be used to form the network. Examples of group II-VI materials that can be used to form nanowire networks include Zn, Cd, Hg, S, Se and Te. Details of examples of synthesis procedures for these nanowires are set forth as follows: ZnS (Refs. 56-60); ZnSe (Refs. 44, 59 and 60); CdS (Refs. 59-72); CdSe (Refs. 59, 60, 65, 68, 69, 71 and 73); CdTe (Refs. 65, 73 and 74); and HgS (Ref. 75).
Nanowires made from metal oxides using known procedures may be used to form the network. Examples of metal oxide nanowires and references to the details for making them are as follows; CdO (Refs. 76-78); Ga2O3 (Refs. 79-88); In2O3 (Refs. 85 and 89-99); MnO (Refs. 100-102); NiO (Ref. 103); PbO (Ref. 104); Sb2O3 (Ref. 25); SnO2 (94 and 105-112); and ZnO (Refs. 113-117).
Nanowires made from metal chalcogenides using known procedure may be used to form the network. Examples of metal chalcogenides that can be used to make nanowires include Mn, Fe, Co, Ni, Cu, Ag, Sn, Pb and Bi. Examples of metal chalcogenide nanowires and references to the details for making them are as follows: AgxMy (Refs. 29 and 118-124); BixMy (Refs. 125-134, 135 and 136-137); CoxMy (Ref. 138); CuxMy (Refs. 139 and140); MnM (Ref. 141); NiM2 (Ref. 142); PbM (Refs. 114 and 143-152); and SnM (Refs. 153 and 154). M is Se, S or Te.
Nanowires made from ternary chalcogenides using known procedures may also be used to form the network. Examples of ternary chalcogenide nanowires and references to the details for making them are as follows: CuInM (Ref. 155); AgSnM (Ref. 156); CdMnM (Ref. 141); and CdZnM (Ref. 157) where M also can be Se, S or Te.
Nanowires (also referred to as nanofibers) made from conducting polymers may be used to form the network. Examples of conducting polymer nanowires and references to the details for making them are as follows: polyaniline (Refs. 82 and 158-167); polypyrrole (Refs. 158, 160 and 168-170); and polythiophene (Refs. 158, 169 and 171-173).
Nanowires of metals and alloys may be made using a variety of techniques.
They include:
Aluminum-Silicon Alloy
Paulose, M.; Grimes, C.; Varghese, O.; Dickey, E. “Self-assembled fabrication of aluminum-silicon nanowire networks.” Applied Physics Letters, Vol. 81, No. 1, 2002.
Gold Nanowire Networks
O'shea, J.; Phillips, M.; Taylor, M.; Moriarty, P.; Brust, M.; Dhanak, V. “Colloidal particle foams: Templates for Au nanowire networks?” Applied Physics Letters, Vol. 81, No. 26, 2002.
Indium Oxide (In2O3)
Lao, J.; Huang, J.; Wang, D.; Ren, Z. “Self-Assembled In203 Nanocrystal Chains and Nanowire Networks.” Advanced Materials, Vol. 16, No. 1, 2004.
Copper Nanowires
Adelung, R. et.al. “Self-Assembled Nanowire Networks by Deposition of Copper onto Layered-Crystal Surfaces.” Advanced Materials, Vol. 14, No. 15, 2002.
Components of the resistor 100 may be constructed from any one or combination of a variety of methods. For example, components of the resistor 100 may be made using printing and/or spraying methods. Both the printing and spraying methods of SWNT film deposition can be patterned. To pattern with a spray technique, standard optical lithography techniques can be used to pattern photoresist on an appropriate substrate, and the SWNTs can be sprayed over the photoresist. Washing away the photoresist yields a patterned SWNT sample. This can be patterned down to 1 μm resolution. To pattern with the printing technique, one can first pattern the PDMS stamp by again using optical lithographic techniques to pattern photoresist on an appropriate substrate and then filling over that with PDMS. The patterned stamp will now yield a patterned nanotube film when printed. The resolution of this is limited by the flexibility of the PDMS stamp, but at least 10 μm can be obtained.
Other manufacturing techniques that may be employed to produce components of the resistor 100 may include the following:
Deposition methods that can be used to form nanowire networks on substrates include the following:
A great variety of nanowires can be made in solution and cast onto a substrate. See Refs. 28, 29, 50, 64, 68, 75, 96, 126, 131, 140, 143, 153 and 174-194 for details of examples of procedures that may be used to make solutions of nanowires. These nanowires can be readily deposited onto an FET device by drop casting. Upon drying the solvent, network structures form. For example, we deposited a polyaniline nanowire network on a silicon wafer cast from a water dispersion using the procedure described in detail in Ref. 164.
Nanowires self-assemble into interconnecting networks when organic solvents containing nanowires are spread onto a water surface. The network can then be transferred from the water surface to a solid substrate by Langmuir-Blodgett techniques. Details of such procedures are set forth in Refs. 195-197.
Using chemical vapor deposition, some nanowires can be directly grown as networks on substrates. Details of an example of CVD procedure for forming a network of nanowires as set forth in Ref. 198.
In a similar fashion to spider web networks, electrospining has been demonstrated to form networks of polymer nanowires/fibers on solid substrates (see Refs. 199 and 200). In a typical process, a polymeric melt or solution is extruded from the orifice of a needle to form a small droplet. In the presence of a strong electric field, charges built up on the surface of the droplet will overcome the surface tension to induce the formation of a liquid jet that is subsequently accelerated toward a grounded target. As the solvent is evaporating, this liquid jet is stretched to many times its original length to produce nanofibers (nanowires) of the polymer. The nanofibers are collected as inter-weaving networks on spinning target.
In accordance to the current invention, the resistor 100 may be constructed on a transparent substrate and may itself be transparent to a sufficient degree to be useful in a variety of electro-optic applications in which it is desirable to have transparent electronic components. In one example, one may manufacture the combination of source electrode 102, drain electrode 104 and conducting channel 106 to have nanowire networks to provide a desired resistance. For example, source electrode 102 and drain electrode 104 may be constructed to be similar to each other, while conducting channel 106 may be constructed to have a nanowire network which differs from source electrode 102 and drain electrode 104. The resistor 100 may be formed on a substrate, for example.
In the liquid gating configuration, the source, drain and conducting channel are connected in a similar manner as other transistor configurations. These components are immersed into an electrolyte along with an electrode. When a voltage is applied to this electrode, it changes the potential of the electrolyte and gates the conducting channel in a manner similar to a traditional transistor. There does not need to be an insulating layer in between the conducting channel and the electrolyte (although there may be) because the interface between the conducting channel and the electrolyte forms a capacitor, thus enabling the conducting channel to be gated.
There may also be a liquid capacitor configuration. In this case, the conducting channel serves as one plate of the capacitor, while the gating electrode and the electrolyte serve as the second plate of the capacitor. It should be noted that just as for traditional transistors and capacitors, any or all of the listed components can be made of nanowires. There has been considerable research into using carbon nanotube bundles as micro electrodes for liquid gating purposes.
Devices according to the current invention, including but not limited to any of the above embodiments, may be very flexible and/or highly transparent as compared to conventional devices. Actual devices may contain a plurality of devices according to the current invention forming various electrical circuits. Materials suitable for the current invention, and methods of manufacture, permit low cost and ease of manufacture according to some embodiments of this invention. Following are a couple of more specific examples according to the current invention. The invention is not limited to only those examples.
A simple spray technology is used to fabricate transparent and highly flexible FETs, in which carbon nanotube networks of different densities deposited on the two sides of a transparent polymer act as the gate and as the conducting channel. The device mobility exceeds that of organic transistors, and the on/off ratio, while adequate, can be improved with optimization. The transparency in the visible range is independent of the operation and no decrease in performance has been found upon bending the device. The simple device architecture together with the ease of fabrication may have a significant impact on the field of plastic electronics.
The devices were prepared on a plastic sheet of polyethylene terephthalate (PET). Unfunctionalized nanotubes are hydrophobic, and thus they stick well to the hydrophobic surface of the PET. The PET sheets we used were simple transparency sheets, although any plastic with a similar surface hydrophobicity can be used as the substrate. For example other suitable substrates include polyethylene, polycarbonate and polystyrene.
To form the gate layer of the FET, a suspension of SWNT was sprayed onto the PET substrate forming a dense nanotube network. The suspension was made from purified HipCo tubes from Rice, in a concentration of 1 mg/mL in a 1% solution of SDS. The suspension was sonicated using a probe sonicator and then centrifuged. The suspension was sprayed onto the PET substrate while the substrate was heated to 100° C. Heating the substrate prevents droplets from forming on the surface, thus inhibiting flocculation of the nanotubes. After several layers of NT are sprayed onto the PET, the substrate is rinsed in distilled water to remove the SDS. Thin strips of gold were evaporated at opposite edges of the substrate on top of the NT network and silver paint was used to connect the gold strips to the back of the substrate. This way, the gate could be contacted through the back of the device.
The insulating layer in our devices consisted of a 1.5 μm layer of Parylene N, evaporated directly onto the dense NT layer. Although there are transparent and flexible dielectrics that have better insulating properties, Parylene N forms a pin-hole free layer and thus insulates well despite the uneven surface of the dense NT network. Other examples of flexible and transparent dielectrics that may be used include polymethyl methacrylate and very thin layers of inorganic oxides.
A suspension of NT in 1% SDS at a concentration of 0.35 mg/mL was used to deposit the NT network for the source-drain channel. To get a thin, homogenous network for the source-drain channel, the NT were adsorbed onto the parylene. A single drop of the suspension is placed on the parylene, and then blown off using an air gun. The device is then rinsed in water to remove the SDS. This process is repeated drop by drop until the desired source-drain channel network is reached. Gold contacts are then evaporated onto the NT network to form the source and the drain. The devices had a channel ratio W/L of approximately 1.2.
The transmittance of the devices was measured using a Beckman Coulter DU 640 Spectrophotometer. At 550 nm, the transparency of the entire device was found to be 70%. Because a different, more transparent plastic substrate may be used, it is interesting to consider the transmittance of the active components of the device. Dividing out the substrate yields a transparency of the gate, insulating layer, and source-drain channel of 80%.
Transistor characteristics were measured using a Keithley 2400 sweeping the gate voltage from +/−35 V at a rate of one sweep per 10 seconds. Comparing the transistor characteristics of two devices with NT networks of different densities in the source-drain channel reveals that a denser network channel leads to overall higher conduction, but a correspondingly lower on/off ratio.
In this configuration a nanotube network together with source and gate electrodes are fabricated using the methods described above. An insulating layer is fabricated on top of the structure and finally a nanotube network gate is deposited. The insulating layer can include Parylene N, evaporated directly onto the dense NT layer. Other exemplary flexible and transparent dielectrics that may be used include PMMA, Y2O3, and barium zirconate titanate (BZT).
In this configuration the nanotube network channel together with the source and gate are fabricated as described above. Using an appropriate patterning technique (shadow masking, optical lithography, ink jet printing , etc) can be used to deposit the gate on the same side of the substrate, next to the conducting channel.
For a device in which nanotube networks make up the gate layer and the source and drain electrodes, a second material, one that is semiconducting, must be used in the conducting channel. Some high performance transparent semiconducting materials include organic materials such as pentacene, and inorganic oxides such as In—Ga—Zn—O. Organic semiconductors can be evaporated or spin-coated onto the insulating layer (or the source and drain electrodes, depending on which transistor configuration is being used). Inorganic oxides can be deposited by pulse laser deposition at room temperature.
If carbon nanotube networks are used as the conducting channel and source and drain electrodes, a second material is needed for the gate. This material must be transparent and suitably conducting. Indium Tin Oxide (ITO), a transparent conducting oxide, and poly(3,4-ethylenedioxythiophene) (PEDOT), a transparent conducting polymer are two examples. The ITO can be evaporated using a CVC 601 Sputtering System. Using standard machine parameters and at a pressure of 2×10−6 Torr, a homogenous layer of ITO can be deposited at room temperature onto any suitable transparent substrate such as glass or polyethylene (PET) or any other polymer. At 90% transparency, ITO has a sheet resistance of 50 Ω/sq. It is often difficult to get a smooth layer of ITO through evaporation, and so a thin layer of PEDOT can be spin-coated on top of the ITO, or a spin-coated layer of PEDOT by itself can be used as the gate layer.
The ease of this technique also allows for a top gating configuration. Source and drain electrodes made from nanotube networks can be sprayed onto a substrate using a shadow mask to form the correct geometry. Next, a rare nanotube network can be spin coated or incubated onto and between the electrodes. Onto this nanotube network, Parylene can be evaporated, or another insulating polymer deposited. And then finally, to form the gate layer, ITO can be evaporated or PEDOT can be spin-coated to complete the device.
The final permutation, using carbon nanotube networks for the gate and the conducting channel, would also require a transparent and conducting material to serve as the source and drain electrodes. ITO could again be used for these electrodes. A shadow mask with an appropriate geometry would be placed either onto the substrate for a top gating configuration, or onto the conducting channel for a bottom gating configuration, and then ITO is simply evaporated.
The fabrication process for an all carbon nanotube transistor follows the same general procedure explained earlier. Although the description of the fabrication process described below describes fabricating an all carbon nanotube transistor in the standard bottom gating configuration, the process can be applied to all of the different device architectures. The only two components needed for the device are a suspension of carbon nanotubes and an insulating polymer.
The suspension of carbon nanotubes is sonicated to break up large bundles, and then centrifuged to remove any remaining bundles. The suspension is then sprayed directly onto the substrate to form a dense nanotube network which will function as the gate. Onto this network, an insulating polymer is deposited. Possible polymer deposition techniques include vapor phase polymerization (Parylene C, N), spin coating (PMMA) or electropolymerization (PmPV). The insulating layer thickness can be adjusted to obtain desired device performance characteristics. For the source-drain channel of the device, a rare network of nanotubes is adsorbed directly onto the insulating polymer.
Finally, using a shadow mask, two dense nanotube networks that act as source and drain are sprayed onto the source-drain channel network. The shadow mask designed with an appropriate source and drain electrode geometry is simply placed on top of the device, and then the suspension of nanotubes is applied through spraying. Current technology allows the fabrication of shadow masks which have a resolution down to 20 μm, and so these source and drain contacts can also have this resolution. The networks comprising the source and drain electrodes should be at least several monolayers thick to ensure adequate differentiation between these functioning electrodes and the rare network acting as the conducting channel and thus ensure a well-defined source-drain channel. Even at several monolayers thickness, these networks will still be around 85% transparent. The precise density of the source and drain networks can be optimized.
To connect to these source and drain electrode networks, standard techniques can be applied. Using a probe station, one can contact the probes directly to the source and drain electrode networks just as one would contact the probes to gold pads on a Si chip. In the case that the device is packaged into a chip carrier, the source and drain network electrodes could have microscopic wires attached through standard wire bonding methods.
Transistors can also be fabricated using the nanotube network as the source and drain, and using other flexible and transparent materials as the gate and the conducting channel. The fabrication routes would follow the routes that are described under 1. above. Configurations where the conducting channel is the nanotube network and the source and drain together with the gate is the other material or materials. Finally, nanotube networks could be used as the gate material.
The following example describes the fabrication of transparent and flexible transistors where both the bottom gate and the conducting channel are carbon nanotube networks of different densities, and Parylene N is the gate insulator. Device mobilities of 1 cm2V−1 s−1 and on/off ratios of 100 are obtained, with the latter influenced by the properties of the insulating layer. Repetitive bending has minor influence on the characteristics, with full recovery after repeated bending. The operation is insensitive to visible light and the gating does not influence the transmission in the visible spectral range.
The quest for flexible and transparent transistors has recently resulted in several noteworthy achievements. Transparent transistors have been fabricated using both polymers (Stutzman, N.; Friend, R. H.; Sirringhaus, H. Science. 2003, 299, 1881; Dimitrakopoulos, C. D.; Purushotharnan, S.; Kymissis, J.; Callegari, A.; Shaw, J. M. Science. 1999, 283, 822; Dimitrakopoluos, C. D.; Malefant, P. R. L. Adv. Mater. 2002, 14, 99) and inorganic oxides (Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Nature, 2003, 432, 488; Nomura, K. ; Ohta, H.; Ueda, K.; Katniya, T.; Hirano, M.; Hosono, H. Science 2003, 300, 1269) These advances, notable in the emerging technology arena that is generally called “plastic electronics,” have received wide publicity. Both, nevertheless, have significant deficiencies. The former have low mobility and the latter do not have the desired flexibility and are not easily manufacturable. These factors severely limit the application potential of the devices. Our method introduces a transistor architecture that has the potential to include only two materials: carbon nanotubes (NTs) and a polymeric gate insulator. This simplicity of structure would ensure a simple manufacturing process.
Carbon nanotubes, because of their excellent electronic properties, have been explored for applications as active electronic devices. Field Effect Transistors (FETs) with NT conducting channels have been fabricated (Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris Ph. Appl. Phys. Lett. 1998, 73, 2447; Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49), and their properties and operation explored (Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 654; Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2004, 4, 35; Bradley, K.; Gabriel, J.-C. P.; Star, A.; Grüner, G. Appl. Phys. Lett. 2003, 83, 3821). Subsequently it has been shown (Gabriel, J.-C. P. Large Scale Production of Carbon Nanotube Transistors: A Generic Platform for Chemical Sensors. MRS Proceedings Volume 776, Q12.7; Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Appl. Phys. Lett. 2003, 82, 2145) that a random network of nanotubes with an appropriate density can also act as a conducting channel in a FET configuration. This has opened up the avenue for a manufacturable device architecture. Room-temperature fabrication techniques enabling flexible transistors (Bradley, K.; Gabriel, J.-C. P.; Gruner, G. Nano Lett. 2003, 3, 1353) have been also explored. It has been shown that due to the high mobility of carbon nanotubes, a network with low sheet resistance is also transparent in the visible spectral range (Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273; Hu, L.; Hecht, D. S.; Grüner, G. Nano Lett. 2004, 4, 2523). We have fabricated, using an extremely simple spray technology, field effect transistors where carbon nanotube networks of different densities provide both the gate and the conducting channel. We find that the devices are highly transparent, that the mobility is superior to that of organic transistors, and that repeated bending does not lead to a substantial effect on the transistor characteristics. The transistor architecture, aside from having a possible impact on a new technology, represents a further step in the advancement of carbon nanotube based transistors.
A schematic illustration of the FET devices that have been fabricated is shown in
The insulating layer in our devices consisted of a 1.5 μm thick layer of Parylene N, evaporated directly onto the dense NT layer. Although there are transparent and flexible dielectrics that have better insulating properties, Parylene N forms a pin-hole free layer and thus insulates well despite the uneven surface of the dense NT network. Parylene can also be deposited at room temperature, ensuring that the PET substrate will not be damaged in the gate deposition process. Accordingly, Parylene is a suitable dielectric.
A similarly prepared suspension of NT in 1% SDS at a concentration of 0.35 mg/ml was used to deposit the NT network for the source-drain channel. To get a thin, homogenous network for the source-drain channel, the NTs were adsorbed onto the parylene. A single drop of the suspension is placed on the parylene, and then blown off using an air gun. The device is then rinsed in water to remove the SDS. This process is repeated drop by drop until the desired source-drain channel network density is reached. Gold contacts are then evaporated onto the NT network to form the source and the drain. The devices had a channel ratio W/L of approximately 1.
AFM images (
The optical transmittance of the devices was measured using a Beckman Coulter DU 640 Spectrophotometer. The transistor characteristics were measured using a Keithley 2400 sweeping the gate voltage from +/−35 V at a rate of 14 V/s and a source-drain bias of 500 mV. Comparing the transistor characteristics of three devices with NT networks of different densities in the source-drain channel reveals that a denser network channel leads to overall higher conduction, but a correspondingly lower on/off ratio.
The optical transparency of a typical example of a device, shown in
Although the devices do not reach saturation in the “on” state, the on/off ratio for the applied voltage range can still be estimated. Device 3 has an on/off ratio of approximately 90. Device 2 has an en/off ratio around 70, while Device 1, with the densest NT network, has an on/off ratio around 7. It is expected that the device with the rarest NT network will have a higher on/off ratio because this device will have fewer all-metallic paths which remain conducting even when the device is in the “off” state. Furthermore, the leakage current through the dielectric is on the order of the “off” current in this device, and so using a better dielectric in order to decrease the leakage current could improve the on/off ratio even more. If we subtract out the leakage current from the off current, the on/off ratio for the rarest device improves to around 400.
Using a standard expression for mobility,
the mobilities of the devices were estimated. In this expression, l represents the length of the channel (i.e., the distance between the source and the drain contacts), w is the width of the channel, d is the thickness of the dielectric layer, k is the dielectric constant of the dielectric, and Vd is the source-drain voltage bias at which the transfer characteristics were measured. To estimate
we measured the slope of the I-Vg curve in the linear region. Though the slopes of the three plots appear similar in
The device with the least dense NT network in the source-drain channel, Device 3, has an estimated mobility of 0.5 cm2V−1 s−1, Device 2 has an estimated mobility of 0.6 cm2V−1 s−1. The device with the more dense NT network, Device I, has an estimated mobility of 1 cm2V−1 s−1. It is understandable that the device with a more dense NT network would have a higher mobility (Y. Zhou, et al. p-Channel, n-Channel Thin Film Transistors and p-n Diodes Based on Single Wall Carbon Nanotube Networks. Nano Lett 4, 2031 (2004)) because in a dense NT network, there are more paths through which the electrons may travel.
To test the devices' flexibility, transistor characteristics measurements were taken before, during and after bending the device to a radial angle of 160°.
We have demonstrated a flexible and transparent transistor architecture where different components are fabricated using carbon nanotube networks. While certain parameters of the devices are comparable to transistors fabricated using room-temperature processes, significant improvements are expected with improved nanotube network characteristics. As is evident form
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. The above-described embodiments of the invention may be modified or varied, and elements added or omitted, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described,
Liquid capacitor devices use a configuration similar to that of the liquid-gated transistor. First, a suspension of carbon nano tubes is produced by sonicating a 0.1 mg/ml mixture of carbon nanotubes in 1% sodium dodecyl sulfide (SDS). The suspension is sonicated for 30 minutes in order to break apart the nanotubes which aggregate due to van der Waals forces. The suspension is then centrifuged at 1400 rpm for 30 minutes to remove the largest bundles from the suspension.
From this stock suspension, more dilute suspensions can be made in order to fabricate nanotube networks using a filtration method. Typically, 100-200 μl of the stock suspension is dispersed into 30 ml of 1% SDS. This suspension is then vacuum filtered onto an alumina filter, yielding a uniform network of small bundles of nanotubes.
This network is then transferred to a strip of PET using a PDMS stamp. Final sheet resistances of these networks is typically around 1 kΩ. A single silver electrode is then painted onto the plastic in order to contact the network. The silver electrode is completely passivated with a thin layer of PDMS.
Such devices can be used as a capacitor in a configuration similar to that of the liquid-gated transistor. This plastic device is inserted into a liquid buffer. A gate electrode is also inserted into the buffer and the entire configuration is the capacitor device. The nanotube network serves as one plate of the capacitor, the gating electrode serves as the other “plate” of the capacitor, and the double layer interface between the nanotube network and the liquid electrolyte serves as the dielectric layer of the capacitor. As the voltage applied to the electrode is changed, the capacitance between the gate electrode and the nanotube network changes, as one would expect for a typical capacitor device.
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This application claims priority to U.S. Provisional Application No. 60/656,571 filed Feb. 25, 2005, the entire contents of which are hereby incorporated by reference.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of NSF Grant No. 040429.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/06610 | 2/27/2006 | WO | 00 | 2/2/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60656571 | Feb 2005 | US |
