Abstract:
A thermoelectric device formed of nanowires on the nm scale. The nanowires are preferably of a size that causes quantum confinement effects within the wires. The wires are connected together into a bundle to increase the power density.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 60/292,052, filed May 18, 2001. 
    
    
     STATEMENT AS TO FEDERALLY-SPONSORED RESEARCH 
     The invention described herein was made in the performance of work under a NASA 7-1407 contract, and is subject to the provisions of Public Law 96-517 (U.S.C. 202) in which the contractor has elected to retain title. 
    
    
     BACKGROUND 
     Thermoelectric devices may operate to create energy based on a temperature difference, or conversely, create a temperature difference based on applied energy. Thermoelectric devices may be used in harsh environments to create electrical power. 
     Miniaturized solid-state devices may be of interest for various miniaturized applications. 
     SUMMARY 
     The present invention recognizes that smaller thermoelectric devices may be used to create increased conversion efficiency. A number of individual “nano wires” are formed. These wires may be connected in bundles to create a composite positive device formed of many individual small elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects will now be described in detail with reference to the accompanying drawings, wherein: 
         FIGS. 1A and 1B  show representative curves for a thermoelectric material; 
         FIG. 2  shows a representative device with thermoelectric wires; 
         FIG. 3  shows the way in which the thermoelectric wires can be interconnected; 
         FIG. 4  shows another embodiment in which connections are carried out both on the top and bottom portions; 
         FIG. 5  shows other circuitry being formed on the same substrate as the thermoelectric device. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment is shown in  FIG. 2 . In the embodiment, a plurality of thermoelectric legs  200  are formed. Each of the thermoelectric legs is in the form a thin nanometer scale wire, that is a leg having an outer diameter or outer extent which is of the scale of nanometers: less than one micron in maximum outer extent. More preferably, the outer extent of the wire is less than 500 nm in outer extent, and even more preferably is 100 nm or less. 
     The thermoelectric elements are formed within a “template”  205 , which is a device made of a specified material formed with a number of holes  210  therein. Any porous material with consistent size pores can be used as the template. The holes forming the pores extend through the device. The pores are filled with thermoelectric material, which become the legs of the thermoelectric device. A connection to the legs is made. 
     A state-of-the-art thermoelectric material may form such a miniaturized thermoelectric power source based on a Bi 2 Te 3  alloy, although other thermoelectric materials can be used. 
       FIG. 1A  shows a representative material conversion efficiency for an application of power generation using waste heat recovery using such devices, as a function of the figure of merit, ZT, of the material. For temperature differentials in the range of 20 to 200 degrees Kelvin, the graph of  FIG. 1   a  shows that conversion efficiency between 1 and 15 percent may be possible.  FIG. 1B  shows the specific power obtained in watts/cm 3 . It may be possible, as shown, to obtain in excess of 1000 watts/cm 3 , depending on the temperature differential and other factors.  FIG. 1B  specifically illustrates that a greater power per cc is obtained when the leg thickness of the thermoelectric generator becomes smaller. This has lead to the conclusion that smaller legs may be desirable. However, current manufacturing techniques have often been limited. For example, bulk devices may be limited to 100 thermoelectric devices per square cm. Devices using micro techniques may be limited to 100,000 devices per square cm. The present system may have 10 times this, at least one million elements per square cm, and as many as one billion elements per square cm. 
     The present application teaches formation of vertically integrated thermoelectric microdevices that use a combination of thick film electrochemical deposition and integrated circuit processing techniques. In the embodiment, the legs are produced that are no thicker than 50 to 100 microns. Even for small temperature differences such as 10–20 degrees K., specific power outputs in the 1–10 watts/cm 3  range become achievable. 
     The present system may operate using templates of porous alumina. Such templates are commercially available, or can be formed using semiconductor techniques. Templates which have holes on the order of 100 nm by 60 um can be commercially purchased. 
     One aspect of the present system recognizes that the efficiency of the thermoelectric device is increased by decreasing the leg diameter to a size at which quantum confinement effects will occur. A preferred system, therefore, increases the performance efficiency by enhancing the charge carrier mobility using Quantum confinement effects. The thermoelectric devices may be for example of molecular scale in diameter, for example on the order of magnitude of 10 nm diameter (more specifically, anything less than around 100 nm). In addition, these elements may have a high aspect ratio. For example, it may be desirable for the height of the nanowire to be three orders of magnitude greater than the diameter of the nanowire. One example is a nanowire of the order of 10 nm in diameter by 20 to 50 um tall. This may maintain a large ΔV at low heat fluxes, thus further increasing the generator capability. 
     It may be difficult to contact individual wires of this size. Accordingly, in this embodiment, the wires are contacted in bundles  215  as shown in  FIG. 2 . The bundles can be, for example, a bundle of multiple wires collectively measuring approximately 10 microns. Each bundle may have a fill factor of approximately 50 percent, representing the ratio between opening areas that have been filled with thermoelectric material, and closed areas representing areas of the template. 
     The formation of the device can be made by filling the pores with thermoelectric material, for example using a growth technique. Each pore may be over filled, which hence overgrows the material onto the top of the template. This is then followed by polishing the template back to a level of fill. 
     After this polishing, the wire bundles are contacted by electrochemically depositing a metallic material such as nickel, on top of the wires. This electrochemically grown metal cap on each wire allows the metal to spread and contact adjacent metal caps. 
     As noted above, the templates may be porous templates formed of alumina, having pores of 100 nm or in another embodiment, pores of 40 nm. 
       FIG. 3  shows an embodiment, with Bi 2 Te 3  nanowires 300 of 100 nm by 40 microns long, although the figure is not shown to scale. The thermoelectric material overgrows the top portion  305  of the template, forming caps which contact one another, such as at the contact area  310  between the caps  311  and  312 . These wires may have a measured Seebeck coefficient of −125 uV/K. The caps contacting one another may form the connections of the bundles. A separation area  320  separates between between adjacent bundles. 
     The inventors found that even though several wires are contacted in parallel to form a group, the Quantum effects are not effected by this contact and each wire remains isolated from the other wires except at the area of the contact. The alumina template is left in place. 
       FIG. 4  shows how multiple wires can be grown in groups by masking the template and patterning the bottom contact. The electrochemical growth begins at the bottom, and then the groups of wires are contacted with an electrochemically grown metal cap.  FIG. 4  shows n type and p type nanowires being grown within the template. Wires within a group are contacted in parallel. The groups are contacted in series to make a thermoelectric device in the same way that thermoelectric Microdevices are fabricated. The bottom metallic layer is used for the electrochemical growth and for the series connection of the bundles. 
     The above has described forming the thermoelectric material within templates. However, other techniques may include photoresist masking and microfabrication. 
     Another embodiment is shown in  FIG. 5 . In this embodiment, semiconductor techniques are used to form a chip  499  which includes both a thermoelectric device  500 , and other operative structures  510  on the chip. For example, the  FIG. 5  device shows a whole system on a chip, including the functions of sense, compute, actuate, control, communicate and power on the single chip. The system shows power output lines  501  from the thermoelectric device which are used to drive a sensor part  520 , a microcomputer or microprocessor part  522 , and a communication part  524 . All of this is formed on the same semiconductor substrate. 
     In addition, this system may be used for any thermoelectric application, including but not limited to a micro cooler, or a microchip or generator based on thermoelectric conversion. The device can be used in sensors, detectors, microprobes and micro Rovers. This may also be used in consumer electronics, automotive applications, wearable and flexible power generating device structures as well as biological and medicinal applications. 
     Although only a few embodiments have been disclosed in detail above, other modifications are possible. For example, the above has described nanoscale wires which are effectively one-dimensional devices. Nanoscale layers, effectively two-dimensional devices, are also contemplated. Also, and zero dimensional devices, effectively Quantum dots, are contemplated.