Abstract:
The presently disclosed invention provides for the fabrication of porous anodic alumina (PAA) films on a wide variety of substrates. The substrate comprises a wafer layer and may further include an adhesion layer deposited on the wafer layer. An anodic alumina template is formed on the substrate. When a rigid substrate such as Si is used, the resulting anodic alumina film is more tractable, easily grown on extensive areas in a uniform manner, and manipulated without danger of cracking. The substrate can be manipulated to obtain free-standing alumina templates of high optical quality and substantially flat surfaces PAA films can also be grown this way on patterned and non-planar surfaces. Furthermore, under certain conditions the resulting PAA is missing the barrier layer (partially or completely) and the bottom of the pores can be readily accessed electrically. The resultant film can be used as a template for forming an array of nanowires wherein the nanowires are deposited electrochemically into the pores of the template. By patterning the electrically conducting adhesion layer, pores in different areas of the template can be addressed independently, and can be filled electrochemically by different materials. Single-stage and multi-stage nanowire-based thermoelectric devices, consisting of both n-type and p-type nanowires, can be assembled on a silicon substrate by this method.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) to provisional patent application Ser. No. 60/333,403 filed Nov. 26, 2001; the disclosure of which is incorporated by reference herein. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No. 0205-G-BB853, awarded by MURI/ONR/UCLA, Contract No. N00167-98-K-0024 awarded by the Department of the Navy, Contract No. N00014-96-1-0802 awarded by the Department of the Navy and DARPA, Subcontract No 0205-G-7A114 awarded by MURI, and Grant No DMR-98-04734 awarded by NSF. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to porous anodic alumina films and more specifically to a method and apparatus incorporating porous anodic alumina films as a template for the fabrication of nanostructured devices. 
     BACKGROUND OF THE INVENTION 
     Porous anodic alumina (PAA) films are films generated by electrochemical oxidation of aluminum under selective conditions (electrolyte, temperature and voltage). These films have a unique morphology of a honeycomb array of channels, several nanometers in diameter and several microns in length, which are perpendicular to the surface of the film. At the alumina-aluminum interface however there is a non-porous undulated alumina barrier layer several nanometers thick Since the pore size, the pore length, the inter-pore distance, and the barrier layer thickness can all be controlled by the anodization conditions, PAA films have attracted a lot of interest as a nanotechnology tool. PAA films have found applications as filters, collimators, as templates for nano-patterning and nanowire growth, and as photonic bandgap materials 
     PAA films have several disadvantages associated with them. These disadvantages have precluded the use of PAA films in a wider range of applications. Free-standing anodic films are extremely fragile and cannot sustain stress. Even when the film is attached to the aluminum substrate, the film may fracture since aluminum is a soft metal Such uniform, small-feature and controllable porous structures have being successfully grown only on aluminum, and not on any other substrate. The growing porous film is separated from the underlying metallic aluminum by a scalloped layer of oxide, known as the barrier layer. The barrier layer prevents electrical contact to be established with the bottoms of the pores of the film. 
     The conventional way of fabricating the PAA films starts with an aluminum sheet that goes through several steps of mechanical and electrochemical polishing. Once the surface roughness of the sheet is down to the sub-micron level, the metal is anodized in an acidic bath and the porous alumina is obtained The quality of the starting anodic alumina is usually low in terms of the ordering and uniformity of the pores. Therefore, this initial film is typically etched away and a new PAA film is grown under the same or similar anodization conditions. The pores cannot be provided all the way through the aluminum, since an electrical path through the aluminum is necessary to perform the anodization, and the aluminum substrate functions as an electrode for the anodization process In order to obtain a PAA membrane in which the pores run completely through the film and are open (and accessible) on both sides, it is necessary to etch away the metallic aluminum sustaining the oxide and subsequently also to etch away the barrier layer, or to detach the membrane from the aluminum substrate by one of the available methods to do so. 
     In practice, this process has several disadvantages associated with it. The mechanical polishing steps introduce imperfections and contamination, limit the active area of the film, and limit the throughput of the process. Another disadvantage with the prior art process is that after the removal of the sustaining metal, the free-standing PAA film is very brittle and is hard to manipulate effectively. Further, during the etch steps the surface topography of the film is degraded thereby affecting the optical properties of the film and its use as a mask. 
     It would, therefore, be desirable to provide a method which allows for the fabrication of PAA films on a wide variety of substrates. When a rigid substrate is used, the resulting anodic film is more tractable, easily grown on extensive areas in a uniform manner, and can be manipulated without danger of fracturing. It would be further desirable to provide the film on patterned and non-planar surfaces. It would still further be desirable to provide the PAA film missing the barrier layer (partially or completely) such that the bottom of the pores can be readily accessed electrically such as by a conducting layer on the substrate. Having such a film, an array of nanowires perpendicular to the surface of the film can be deposited into the pores. 
     It would be further desirable to provide the PAA film on a patterned conducting layer such that the resulting anodic film can be provided with one set of pores filled with one type of nanowire material (e.g. n-type material) and another set of pores provided with a different nanowire material (e.g. p-type). It would be further desirable to provide the PAA film missing the barrier layer on a patterned conducting layer such that pores, or nanowires within the pores, can be electrically addressed independently from each other. It would further be desirable to provide the PAA templates such that multiple stages of the templates can be built, and can be stacked to form a multi-stage device. 
     SUMMARY OF THE INVENTION 
     The present new technology described herein allows for the fabrication of PAA films on a wide variety of substrates. The substrate comprises a wafer layer and may further include an adhesion layer deposited on the wafer layer. An alumina template is formed on the substrate. When a rigid substrate such as a conventional silicon wafer is used, the resulting anodic film is more tractable, easily grown on extensive areas in a uniform manner, and manipulated without danger of cracking. PAA films can also be grown this way on patterned and non-planar surfaces. Furthermore, under certain conditions the resulting PAA is missing the barrier layer (partially or completely) and the bottom of the pores can be readily accessed electrically The resultant film can be used as a template for forming an array of nanowires wherein the nanowires are prepared by filling the pores of the template by a different material The nanowires may be formed from various materials within the same template Arrays of nanowires may be stacked on top of each other into a multi-stage architecture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of the presently disclosed process for fabricating a PAA film, 
         FIG. 2A  is a SEM micrograph of a top surface of a PAA film formed by the presently disclosed method; 
         FIG. 2B  is a SEM micrograph of a bottom surface of the PAA film of  FIG. 2A ; 
         FIG. 2C  is an AFM micrograph of a top surface of a PAA film of  FIG. 2A ; 
         FIG. 2D  is an AFM micrograph of a bottom surface of a PAA film of  FIG. 2A ; 
         FIG. 3  is a SEM image showing the presence of nanowires within the template; 
         FIG. 4  is a cross-sectional view of a PAA template filled with nanowires; 
         FIG. 5  is a cross-sectional view of the interface between the barrier layer and a silicon oxide adhesion layer; 
         FIG. 6  is an image of nanowires attached to a substrate after the removal of the template; 
         FIG. 7A  is a side view SEM image of a PAA film grown in a trench between two pieces of other material; 
         FIG. 7B  is a top view SEM image of the PAA film grown in a trench between two pieces of other material, 
         FIG. 7C  is a magnified view of a portion of  FIG. 7A ; 
         FIG. 7D  is a magnified view of a portion of  FIG. 7B ; 
         FIG. 8A  is a diagram of a thermoelectric element arranged as a cooling device; 
         FIG. 8B  is a diagram of a thermoelectric element arranged as a power generating device; 
         FIG. 9A  is a diagram showing the first stage of fabrication of a multicomponent nanowire array; 
         FIG. 9B  is a diagram showing the second stage of fabrication of a multicomponent nanowire array; 
         FIG. 9C  is a diagram showing the third stage of fabrication of a multicomponent nanowire array; 
         FIG. 9D  is a diagram showing the fourth stage of fabrication of a multicomponent nanowire array; 
         FIG. 10  is a diagram of the steps in the assembly of a multi-component, multi-stage thermoelectric device. 
         FIG. 11  is a diagram of a multistage nanowire-based thermoelectric device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Porous anodic alumina (PAA) has received considerable attention as a template for the fabrication of nanostructures. The ordered triangular array of pores of high aspect ratio, whose dimensions can be accurately tuned by the process parameters, has made PAA a suitable host for the fabrication of nanowires of a wide range of materials. Applications of these arrays of nanowires include dense magnetic storage devices, field emission devices, thermoelectric devices, photovoltaic devices, nano-electrodes, sensing devices, photonic components and the study of low-dimensional quantum effects. Several researchers have used PAA as a mask for etching or deposition processes. 
     More recently, it was found that the optical properties of alumina together with the proper positioning of the voids in the film result in a 2-dimensional photonic crystal with a bandgap which can be controlled in the wavelength range of 520-600 nm (for certain polarizations and propagation directions of the light). 
     A new approach for the use of porous alumina films as a template for nanofabrication is presented. In this process the porous films are prepared on silicon substrates, as an example for a technology-relevant rigid substrate, simplifying both the template fabrication and subsequent processing, and improving the quality of the films and their surfaces Structural analysis of the film was carried out. Porous films without a barrier layer separating the substrate from the pore channel were prepared. The aspect ratio of the channels, i.e. the ratio between its length and its diameter, was controlled between ˜10 to ˜1000. Therefore, the film is suitable as a template for the growth of nanorods and nanowires. Prior techniques have produced PAA films on substrates having pores with a maximum aspect ratio of ˜50 and included a barrier layer. Nanowires of various materials (metals, semiconductors, and polymers) were prepared by pressure injection or electrochemical deposition in alumina films 5-10 μm thick with parallel ordered pores 40 nm in diameter. The films were also patterned by lithography, offering new opportunities for area-selective anodization, anodization of non-planar structures, and area-selective growth of nanowires. The new approach offers a straightforward method for the fabrication of arrays of nanostructures and their incorporation into electronic and optical devices 
     The fabrication of PAA films on a silicon wafer which can be used as templates for providing nanowire arrays involves the following steps and is shown in  FIG. 1 . While a specific implementation and process is described, it should be appreciated that similar process steps and materials could also be used. Preparation of the substrate  10  is the first step. While the use of silicon is described, other solid materials, such as III-V type materials, oxides, glasses and polymers, may serve as a substrate as long as their electrically conducting surfaces and their chemically reactive surfaces can be isolated from the electrolytes used in the process. This can be achieved by applying a suitable coating on the substrate or by confining the electrolyte. For example, we have used as wafer  10  glass slides, and silicon wafers whose back side was coated with silicon dioxide. The purpose of the substrate or wafer layer  10  is (1) to give mechanical strength to the device structure and (2) to mold the shape and topography of the PAA film. The PAA device may not, in general, be fabricated directly on the surface of the wafer  10 . This is because of the mechano-chemical constraints imposed by the process The top surface of the substrate (the surface facing the PAA film) needs to strongly adhere to aluminum and to alumina, and it needs to support the strain associated with the volume expansion of the aluminum layer when it is converted to PAA For this reason previous works dealt only with thin, therefore less strained, PAA films on substrates. These films have too low of an aspect ratio to grow nanowires. 
     For this reason, additional layers of material  20  may be deposited on the wafer. The purpose of the layers  20 , so called adhesion layers, is (1) to serve as an adhesion layer that holds together the stack of layers of the device and relieves structural stress, (2) to permit, in case of a conducting layer, the anodization process to react completely with the entire layer of aluminum, (3) to define the structure and properties of the interface between the bottom end of the channels and the substrate, in particular whether the barrier layer will be removed or will remain in the structure (vide infra), (4) to guide the filling of the pores and the formation of nanowires, for example as working electrodes during electrochemical deposition, and (5) to introduce other functionalities to the device, such as sensing and addressing capabilities. For example, an adhesion layer on a silicon wafer may consist of a film of deposited titanium, or a coating of thermal oxide, or a multi-layer structure (SiO 2 /Ti/Pt). At this stage conventional patterning techniques can be used to pattern the layers 
     Nevertheless, omitting the use of the adhesion layer can be advantageous. The PAA film may be fabricated directly on wafer  10 . When the PAA is grown on a silicon wafer with its back side covered with SiO 2  the PAA film will separate from the wafer at the end of the anodization step This is a convenient way to obtain high-quality free-standing PAA films with a surface topography precisely complementary to the topography of the wafer surface For example, atomically flat substrates afford PAA films with extremely flat surfaces, particularly suitable as nanoscale pattern-transfer contact masks. 
     The next step after the substrate has been prepared involves providing a layer of aluminum  30  on the substrate. This step may be best accomplished by thermal evaporation of aluminum on the substrate. Other methods may include electron beam evaporation, rf-sputtering, electrochemical plating or other means as are known by those of reasonable skill in the art. The layer of aluminum may have a thickness of several hundreds of nanometers to several hundreds of microns. Depending on the method by which the aluminum was provided on the substrate, annealing might be required in order to enlarge the aluminum grain size to the level necessary for the proper formation of uniform pores during the anodization step 
     Electrochemical polish of the aluminum surface is performed next. The surface can be polished by various techniques known by those of reasonable skill in the art. A preferred technique is to expose the aluminum surface to an H 3 PO 4 /H 2 SO 4 /CrO 3  solution at 85° C. and 20 volts for 1-2 seconds. 
     The next step comprises anodization of the entire aluminum The conditions for the anodization vary according to the desired feature size. A clear change in the appearance of the film  40  is observed once the aluminum has been completely consumed: As long as aluminum is present, the film has a metallic reflective appearance, while the PAA film itself is transparent Furthermore, features in the current vs time profile can be used to monitor the completion of the anodization. 
     Three cases need to be considered depending on the nature of the top surface of the adhesion layer  20  (at the interface with the aluminum layer  30 ). In case I, this layer is a noble metal which in contact with the electrolyte and under the applied potential will generate oxygen gas. The anodization needs to be stopped as the electrolyte reaches the adhesion layer  20  and the current rises, otherwise gas bubbles will rupture the PAA film  40 . In case II, this layer is an insulator, and the current will approach zero. There is no critical time for terminating the anodization process. In case III, the adhesion layer  20  is a valve metal or other material that gets slowly oxidized under the anodization conditions, after the anodization of the aluminum layer  30 . The most appropriate time to terminate the anodization process needs to be found experimentally depending on the material used and the anodization conditions. 
     The next step is optional. In some instances it may be desirable to widen the pores of the alumina film  40 . This widening of the pores may be accomplished by chemical etching in a solution of H 3 PO 4 . 
     Depending on the nature of the top surface of the adhesion layer  20 , cases I, II and III above, measures may need to be applied to remove the barrier layer  45  at the interface between the PAA film  40  and the adhesion layer  20 . In case I (noble metals), the barrier layer  45  is normally missing from the areas where anodization was carried out till completion, so no further treatment is necessary. However, the treatment that will be discussed in the context of case III may be applied nevertheless. In case II (insulators), the conventional undulated thick barrier layer  45  is usually present at the pore ends, and the measures discussed below will not act as to remove it. Therefore insulators should be used under pores that need to remain blocked. In case III (valve metals, etc), it has been observed that an inverted and thinner than usual barrier layer  45  is obtained. This barrier layer  45  is removed by a substrate-assisted localized etching as will be described below. 
     When the barrier layer  45  is only partially missing it might be necessary to dissolve, etch or remove a thin layer of alumina at the bottom of the pores. The substrate layers are used for the local generation of a chemical agent or a force to rupture the barrier layer  45  without inflicting damage on the rest of the alumina film. For example, cathodic polarization of an adhesion film  20  made of titanium in a potassium chloride solution is used to generate hydrogen gas bubbles and hydroxide ions in the voids under the inverted barrier layer  45 . 
     The resulting film  40  can be used as a template for the fabrication of nanowires  50 . The nanowire material is formed into the pores of the array. Methods for filling the pores include, but are not limited to, electrochemical deposition, chemical vapor deposition, pressure injection of a liquid, and impregnation. 
     This method enables the fabrication of thick (several microns) PAA films on substrates other than aluminum. This technique offers a unique, facile, and versatile approach for the incorporation of anodic alumina films or arrays of nanowires  50  into a variety of environments and devices. 
     As an alternative, the adhesion layers may be deposited on a thick aluminum film and then a wafer may be deposited or otherwise attached to the adhesion layers. 
     In a particular example, the results of which are shown in  FIGS. 2-7 , thick aluminum films (6-12 μm) were obtained by thermal evaporation of A1 (Plasmaterials, 99.999%) on n-type silicon substrates (Wafernet, 1-10 Ωcm) in a custom-built chamber (base pressure: 10 −6  torr). Ti and SiO 2  films were obtained by sputtering (Applied Materials Endura System) Ti and Pt films were obtained by electron-beam evaporation. 
     Electrochemical polishing of the films was carried out in an H 3 PO 4 \H 2 SO 4 \CrO 3  solution at 85° C. and 20 volts The anodization was carried out at constant voltage (50V) in an oxalic acid solution (4 wt %) at 18° C. In both processes a Pt sheet was used as a counter electrode. The resulting alumina film was etched away in an H 3 PO 4 \CrO 3  solution for 16 hours, and the remaining aluminum was reanodized under the same conditions until the metal film was fully oxidized Subsequently the alumina film was dipped for 30 minutes in 5% H 3 PO 4  The alumina at the bottom of the pores was thinned and removed by applying a negative bias (2 25 V) to the template in a 2-electrode cell with a 0.1M KCl solution for 20 minutes. 
     The Bi 2 Te 3  nanowires were fabricated by electrodeposition from a solution of bismuth and tellurium (7 mM and 10 mM, respectively) in 1 M nitric acid in a 3-electrode cell at −10 mV vs. a saturated calomel electrode (SCE) using a PAR Model 273 potentiostat The bismuth nanowires were fabricated by the pressure injection technique or by electrochemical deposition from an aqueous solution of 40 mM bismuth nitrate and 76 mM ethylenediaminetetraacetic acid (EDTA) at −650 mV vs. SCE. 
     Scanning electron microscopy (JEOL 6320FV) and atomic force microscopy (Digital Instruments Nanoscope IIIa, tapping mode) were employed for the structural analysis of the alumina films. 
     The PAA film was fabricated in accordance with the process previously described with respect to  FIG. 1 . The aluminum film was thermally evaporated on a silicon wafer, its back side covered with a silicon oxide layer and its front side coated with a titanium layer. The film was electrochemically polished in a phosphoric acid—sulfuric acid—chromium oxide solution. The porous oxide was formed by anodization in an oxalic acid bath. A prominent change in the appearance of the film and a drop in the current indicated when the aluminum film had been completely oxidized. In order to selectively etch the side of the membrane in contact with the wafer and remove the thin oxide at the end of the pores, the wafer was held under negative bias in an aqueous potassium chloride bath. This process resulted in a high quality PAA film over the full area of the wafer, that was used for further processing, for example: patterning, etching and deposition. In contrast to the conventional PAA films, the films on the silicon substrates can be obtained without an insulating barrier layer at the bottom of the pores, they are very easy to handle due to the mechanical strength of the substrate, and they are suitable for incorporation into larger architectures and devices in the wafer. 
     It was found that the adhesion of the PAA film to the wafer could be controlled by the predeposition of other materials on the wafer. When the aluminum was evaporated on a bare silicon wafer, the alumina detached from the substrate as the anodization endpoint was reached If a titanium layer was sputtered on the substrate before the aluminum film, the alumina adhered permanently to the substrate Since free standing PAA films can be obtained if no adhesion layer is used, both faces of the PAA film can be analyzed. 
     Referring now to  FIGS. 2A and 2B  SEM images of the top side (facing the solution) and the bottom side (facing the wafer) of the PAA film  40  is shown. These images show that the porous structure is continuous through the membrane  40  with a noticeable hexagonal pattern, and that the barrier layer is missing. The AFM images of the surfaces shown in  FIGS. 2C and 2D  show a striking difference between the faces: the bottom side of the film  40  is inherently flat, mirroring the smoothness of the silicon surface, while the top side of the PAA film  40  shows the typical roughness associated with the effects of the etch solutions The flat surface of the PAA film improves its performance as a contact mask for pattern-transfer, compared to PAA films made by other methods, increasing the fidelity of the pattern-transfer process. 
     The alumina-on-silicon system was considered as a template for the fabrication of nanowires Two methods of pore filling and two materials of relevance to thermoelectric applications were employed The first method and material comprised bismuth nanowires, 40 nm in diameter, were prepared by a pressure injection technique. By stripping the filled alumina from the substrate, it was verified through SEM imaging that the nanowires are continuous, sticking out of both ends of the channels.  FIG. 3  shows the bottom (wafer) side of the porous template  40 , partly filled with bismuth nanowires  50  (bright spots in the channels). 
     The second method and material comprised Bi 2 Te 3  nanowires prepared by electrochemical deposition from a nitric acid solution. The titanium layer under the oxide film served as the working electrode from which the nanowires began growing.  FIG. 4  shows a cross section of a bismuth telluride filled template  40 . A high filling factor of continuous nanowires  50  (bright sticks) is observed 
     These two examples demonstrate the accessibility of the pores from either end, despite the fact that the membrane is attached to a substrate. The pores can be filled either by depositing material from the bottom ends (wafer side) up as in the electrochemical deposition, or by inserting material from the top ends (solution side) into the pores. In the same fashion, the obtained nanowires can be contacted physically, mechanically, electrically, thermally and possibly optically from both ends The resulting nanochannel arrays and nanowire arrays can thus be incorporated into electronic and optical devices on the wafer and be further utilized in nano-scale and micro-scale patterning. When a patterned conductor layer is used under the PAA film, it is possible to provide different types of nanowires on different areas within the same template. 
     In another example, the silicon wafer was thermally oxidized. Aluminum was deposited on the wafer, electrochemically polished, and anodized as described in the previous example. The anodization was continued till the current value reached 0.01% of the maximum anodization current.  FIG. 5  shows a cross section of the interface between the scalloped alumina barrier layer  45  and the silicon dioxide layer  20 . This thick barrier layer  45  is resistant to the localized etching process described above. 
     In another example, the silicon wafer was thermally oxidized. Electron-beam evaporation was used to deposit a titanium layer followed by a platinum layer on the front side of the wafer Aluminum was deposited on the wafer, electrochemically polished, and anodized as described in the previous examples The anodization was carried out until a surge in current was observed. No further steps were necessary to remove the barrier layer. Bismuth nanowires were electrochemically deposited in the pores from an aqueous solution of bismuth nitrate and EDTA.  FIG. 6  shows the bismuth nanowires attached to the platinum film on the surface of the wafer after the alumina was etched away 
     The presently disclosed method provides the ability to pattern the PAA film by the fabrication of a series of bars of alumina in between slabs of silicon oxide predeposited on the wafer.  FIGS. 7A-7D  show an example of a 25 μm wide, 5 μm thick, and 1500 μm long PAA strip  40  obtained by the anodization of an aluminum bar between bars of silicon oxide  60 . It is interesting to notice the lateral growth of pores from the sidewalls in addition to the vertical growth of pores from the top surface The different growth rates lead to the curved shape observed in the cross section view of  FIG. 7A . Clearly, the anodization of non-planar features displays an additional complexity, which could be exploited to obtain a new variety of structures 
     Referring now to  FIGS. 8A-B , thermoelectric devices  100  and  101  are shown schematically. The thermoelectric device  100  is arranged to operate as a cooling device. The devices include a leg of n-type material  110 , a leg of p-type material  120  and a junction  130  interconnecting the n-type leg  110  with the p-type leg  120 . Device  100  further comprises a voltage source  140  coupled across the n-type leg  110  and p-type leg  120 . This arrangement results in current flowing from the n-type leg, across junction  130  and through p-type leg  120 . Whenever electrical current flows through two dissimilar materials, depending on the direction of current flow through the materials, the junction of the p-type and n-type material will either absorb or release heat When the thermoelectric device  100  is connected to a voltage source  140  such that the n-type leg  110  is connected to the positive lead of the voltage source and the p-type leg  120  is connected to the negative lead of the voltage source, the following phenomenon occurs. Charge carriers, also known as electrons, in the n-type material are repelled by the negative potential and attracted to the positive potential of the voltage source. Similarly, the positive charge carriers, also known as holes, in the p-type material are repelled by the positive voltage potential and attracted by the negative potential of the voltage source. The charge carriers are carrying heat away from the junction  130  connecting the p-type and n-type material, thus the device is providing a cooling function at the junction connecting the p-type and n-type materials 
     Conversely, when the thermoelectric device  100  is connected to a voltage source such that the p-type leg is connected to the positive lead of the voltage source and the n-type leg is connected to the negative lead of the voltage source the opposite effect takes place. The negative charge carriers (electrons) in the n-type material are repelled by the negative potential and attracted to the positive potential of the voltage source Similarly, the positive charge carriers (holes) in the p-type material are repelled by the positive voltage potential and attracted by the negative potential of the voltage source. The charge carriers are carrying heat to the junction of the p-type and n-type material, thus the device is providing a heating function at the junction of the n-type and p-type materials. 
     Referring now to  FIG. 8B , when a heat source is brought into proximity with junction  160  of device  101 , a voltage differential is provided across p-type leg  120  and n-type leg  110 . In the n-type side of the deice  101 , the heat causes negative charge to flow from the junction  160  to the colder end of the n-type leg  110  of the thermoelectric device. In the p-type side of the device  101 , the heat is causing positive charge to flow from the junction region to the colder end of the p-type leg  120 . In this configuration, the thermoelectric device is converting heat to electrical energy, thus functioning as a power generator. 
     A device comprising a thermoelectric element formed from nanowire arrays and the process for making such a device is shown in  FIGS. 9A-9D . As shown in  FIG. 9A , a silicon substrate  210  is provided as the support for the device. A pair of electrodes  230  is patterned on the silicon substrate  210 . A porous anodic alumina film  220  is provided on the electrodes  230  and substrate  210 . The process for providing such a porous anodic alumina film  220  has been described in detail above. 
     Referring now to  FIGS. 9B and 9C , a plurality of p-type nanowires  222  are provided in the film  220  over one of the electrodes  230 , and as shown in  FIG. 9C  a plurality of n-type nanowires  224  are provided in the film  220  over the other electrode  230 . 
     As shown in  FIG. 9D  a junction  260  is deposited on the top surface of the film  220 . Junction  260  provides an electrical path and a thermal path between the n-type nanowires  222  and the p-type nanowires  224 . The resulting device can perform as a cooling device, similar to the thermoelectric device shown in  FIG. 8A  when a voltage source is provided across the electrodes  230 . The thermoelectric device can also function as a power generator when a heat source is provided to junction  260 . 
     Referring now to  FIGS. 10 and 11 , a multi-stage nanowire-based thermoelectric device is shown. A single stage  300  of the device is the same as the device described above with respect to  FIGS. 9A-9D . However, in order to turn the single thermoelectric device into a multistage thermoelectric device, additional steps are required. As shown in  FIG. 10 , once a single nanowire based thermoelectric device  300  is produced, a thermally conducting electrically insulating material  270  (such as a ceramic) is deposited over the junction  260 . This material extends over the junction  260  and also over the array of p-type nanowires and over the array of n-type nanowires. Another device  300  is provided on top of the material  270  of the first device, using material  270  as a base support. This process is repeated any desired number of times resulting in a multi-stage nanowire based thermoelectric device,  310  as shown in  FIG. 11 . These devices  310  generate a larger temperature gradient than single stage devices  300 . 
     As described above, high quality porous alumina membranes are fabricated on silicon substrates by a novel process Improvements in terms of the effective area of the films and the flatness of the surfaces resulted from the new process. The films may be formed lacking the insulating barrier layer, making the pores accessible from both ends. The adhesion of the porous alumina to the substrate can be modified by intermediate layers, making it possible to obtain both free standing films and films strongly held to the wafer. The films were used as templates for the growth of bismuth and bismuth telluride nanowires. Silicon processing techniques were used for the area-selective growth and patterning of the porous films. In summary, this new approach simplifies the preparation of the porous oxide and allows much more flexibility in the processing of the film, making porous alumina a convenient and versatile tool for the assembly of devices based on nanostructures. Single stage and multistage nanowire-based thermoelectric devices are produced using the present process. 
     Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used Accordingly, it is submitted that the invention should not be limited to the described embodiments but rather should be limited only by the spirit and scope of the appended claims All publications and references cited herein are expressly incorporated herein by reference in their entirety