Patent Publication Number: US-2012025343-A1

Title: Thermoelectric device having a variable cross-section connecting structure

Description:
BACKGROUND 
     Thermoelectric devices use the Seebeck effect for generating electric power from a temperature gradient across the thermoelectric devices. Conversely, thermoelectric devices use the Peltier effect for creating a temperature gradient between the sides of the thermoelectric devices through use of electric power. 
     The efficiency of a thermoelectric device is measured in terms of ZT, which is the dimensionless figure of merit, defined by, 
     
       
         
           
             
               
                 
                   
                     ZT 
                     = 
                     
                       
                         
                           
                             S 
                             2 
                           
                            
                           σ 
                         
                         k 
                       
                        
                       T 
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     where S is the thermoelectric power, σ is the electrical conductivity, k is the thermal conductivity, and T is the temperature of the thermoelectric device. The thermoelectric power (S), is defined by, 
     
       
         
           
             
               
                 
                   
                     S 
                     = 
                     
                       
                         ∂ 
                         V 
                       
                       
                         ∂ 
                         T 
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     where V is the thermoelectric voltage produced per degree temperature (T) difference. 
     Thermoelectric devices are known to harvest energy that would otherwise be wasted as heat. The efficiency of thermoelectric devices in harvesting heat energy is generally low. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example and not limited in the following figure(s), in which like numerals indicate like elements, in which: 
         FIG. 1  illustrates a cross-sectional side view of a portion of a thermoelectric device, according to an embodiment of the invention; 
         FIG. 2  illustrates a cross-sectional side view of a portion of a thermoelectric device, according to another embodiment of the invention; 
         FIG. 3  illustrates a cross-sectional side view of a portion of a thermoelectric device, according to a further embodiment of the invention; 
         FIG. 4  illustrates a cross-sectional side view of a thermoelectric device, according to a further embodiment of the invention; and 
         FIG. 5  illustrates a flow diagram of a method of fabricating the thermoelectric devices depicted in  FIGS. 1-4 , according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the embodiments are described by referring mainly to examples thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one of ordinary skill in the art, that the embodiments may be practiced without limitation to these specific details. In other instances, well known methods and structures are not described in detail so as not to unnecessarily obscure the description of the embodiments. 
     Disclosed herein is a thermoelectric device that includes at least one n-type section and at least one p-type section. Each n-type section and each p-type section has a first electrode, a second electrode, and one or more connecting structures that connect the first electrode and the second electrode. The n-type section and the p-type section are connected in series electrically, but in parallel thermally, such that the ends of the thermoelectric device may be at the same temperature. The connecting structure includes at least two sections connected in series, which are configured to substantially minimize phonon conduction between the first electrode and the second electrode while having a proportionately lesser limiting effect on the level of electron conduction through the connecting structure. 
     With reference first to  FIG. 1 , there is shown a cross-sectional side view of a portion  100  of a thermoelectric device, according to an embodiment. The portion  100  shown in  FIG. 1  should be understood to represent one of the n-type region and the p-type region of a thermoelectric device, for instance, the thermoelectric device  400  shown in  FIG. 4 . It should be understood that the portion  100  depicted in  FIG. 1  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of a thermoelectric device containing the portion  100 . For instance, the portion  100  may include additional n-type or p-type regions of a thermoelectric device as shown in the thermoelectric device  400  in  FIG. 4 . 
     The portion  100  is configured to either generate electric current from a temperature gradient across the thermoelectric device or to create a temperature gradient across the thermoelectric device through application of an electric current through the thermoelectric device. As depicted in  FIG. 1 , the thermoelectric device  100  includes a first electrode  102 , a second electrode  104  and a plurality of connecting structures  110  connecting the first electrode  102  and the second electrode  104 . Each of the connecting structures  110  includes a first section  112  and a second section  114 . 
     The thermoelectric power varies between different materials and, in general, the thermoelectric power for semiconductors is approximately 100 times larger than for metals. In addition, the magnitude of the thermoelectric power for a semiconductor depends on the doping concentration. The thermoelectric power is typically larger for low doped semiconductors and smaller for highly doped semiconductors. In one regard, therefore, the connecting structures  110  are formed of semiconductor material with appropriate doping to produce a sufficient level of thermoelectric power. 
     According to an embodiment, the first section  112  has a width, which is the dimension that is substantially parallel to the dimension in which the first electrode  102  and the second electrode  104  extend, that substantially limits phonon conduction with a proportionately lesser limiting effect on the level of electron conduction through the first section  112 . More particularly, the width of the first section  112  is smaller than a width that is approximately equivalent to a mean free path of phonons and is larger than a width that is approximately equivalent to a mean free path of electrons for the one or more materials forming the first section  112 . The mean free path of phonons may be defined as the average distance covered by the phonons between collisions, which is dependent upon the material(s) through which the phonons travel, as well as the temperature of the material(s) at which the mean free path of phonons is determined. In addition, the mean free path of electrons may be defined as the average distance covered by the electrons between collisions, which is dependent upon the material(s) through which the electrons travel, as well as the temperature of the material(s) at which the mean free path of electrons is determined 
     Generally speaking, the mean free path of electrons is smaller than the mean free path of phonons for most materials and at most temperatures. In addition, as the ratio of the width of the first section  112  to the width equivalent to the mean free path of phonons decreases, phonon scattering increases. Consequently, greatly increased phonon scattering may suppress phonon conduction completely or nearly completely, reducing thermal conductivity. Conversely, electrical conductivity, which occurs through electron or hole carrier movement/mobility in semiconductors, will be substantially less affected as the width of the first section  112  is greater than the width equivalent to the mean free path of electrons for the material forming the first section  112 . The width of the first section  112  is thus selected to scatter phonons without substantially negatively impacting electron or hole carrier movement/mobility through the first section  112 . 
     In conventional thermoelectric devices that are typically comprised of structures with larger lateral dimensions, electrical conductivity (σ) tracks thermal conductivity (k). In contrast, the first section  112  is able to partially decouple electrical conductivity (σ) from thermal conductivity (k), because in semiconductors, electrical conductivity is primarily due to movement of electrons while thermal conductivity is primarily due to movement of phonons. As the diameter of  112  decreases, the thermal conductivity (k) decreases at a greater rate than electrical conductivity (σ). Consequently, there will be a corresponding increase in efficiency because of the relationship of both to the dimensionless figure of merit (ZT). As such, and as discussed above, the first section  112  has a width that generally results in the movement of phonons to be minimized while still enabling relatively free movement of electrons. 
     The first section  112  has a length that is calculated based on distances that substantially minimize the amount of electrical resistance in the connecting structures  110 . More particularly, the first section  112  has a length that may range from a length equivalent to one or a few mean free paths of phonons for the material forming the first section  112  to a few microns. However, because electrical resistance is directly proportional to the length of the first section  112 , a shorter length of the first section  112  is desirable, in order to reduce electrical resistance. 
     According to an embodiment, the second section  114  has a width that is sized to allow phonon and electron conduction through the second section  114 . More particularly, the second section  114  has a width that may be greater than a width that is equivalent to a mean free path of phonons through the material of the second section  114 . In one regard, the greater width of the second section  114  serves to reduce its electrical resistance and thus, the second section  114  may have a width that is many times larger than the width that is equivalent to a mean free path of phonons through the material of the second section  114 . 
     In addition, the second section  114  has a length that may be minimized in order to maximize electrical conduction in the connecting structure  110 . 
     By virtue of the first section  112  being in series with the second section  114 , the total electrical resistance of the connecting structure  110  may be greatly reduced when compared to a constant cross-section conventional connecting structure of a similar length and a width similar to the first section  112 . The connecting structure  110 , however, may have a comparable, albeit somewhat lesser, ability to scatter phonons as the constant cross-section conventional connecting structure. 
     The first section  112  may be formed of, for instance, silicon, germanium, bismuth telluride, lead telluride, bismuth antimonide, lanthanum chalcogenide and the like, including alloys of one or more of these materials. 
     By way of particular example, the first section  112  and the second section  114  are comprised of silicon. In silicon, the mean free path for phonons is approximately 100 nm while the mean free path for electrons or holes is approximately 10 nm. As such, in this example, the first section  112  has a width that is between 10nm and 100 nm. In addition, the second section  114  has a width that is greater than 100 nm. 
     By way of a further particular example, each of the connecting structures  110  has a first section  112  that is comprised of germanium and a second section  114  that is comprised of silicon with a heterojunction at the interface. The use of multiple materials in this example facilitates methods of fabricating the connecting structures  110  as described in greater detail herein below. 
     According to another example, however, the multiple materials may be made to form an alloy during fabrication of the connecting structures  110 . In this example, germanium may diffuse at a faster rate into silicon than silicon diffuses into germanium. Where different materials are combined into alloys through interdiffusion in the formation process, an added benefit is that phonon scattering increases significantly in the alloys, in this instance a silicon-germanium alloy. Gradual changes in the composition of the connecting structures  110  may be achieved by varying the ratio of the multiple materials, such as, precursors, during deposition of the connecting structures  110 . Furthermore, the strain induced from the different lattice constants of the different materials may also increase phonon scattering. 
     With reference now to  FIG. 2 , there is shown a cross-sectional side view of a portion  200  of a thermoelectric device, according to another embodiment. Similar to the portion  100  depicted in  FIG. 1 , the portion  200  shown in  FIG. 2  should be understood to represent one of the n-type region and the p-type region of a thermoelectric device, for instance, the thermoelectric device  400  depicted in  FIG. 4 . It should be understood that the portion  200  of the thermoelectric device depicted in  FIG. 2  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of a thermoelectric device containing the portion  200 . 
     As depicted in  FIG. 2 , the portion  200  includes a first electrode  102 , a second electrode  104 , and a plurality of connecting structures  210  connecting the first electrode  102  and the second electrode  104 . Each of the connecting structures  210  is comprised of a first section  112 , a second section  114 , and a third section  216 . 
     The connecting structures  210  of the portion  200  performs substantially the same functions as the connecting structures  110  of the portion  100  depicted in  FIG. 1 . As such, the first section  112  of each of the connecting structures  210  has a width that is smaller than a width that is approximately equivalent to a mean free path of phonons and that is greater than a width that is approximately equivalent to a mean free path of electrons for the one or more materials forming the first section  112 . In addition, the second section  114  has a width that is greater than a width that is approximately equivalent to a mean free path of phonons for the one or more materials forming the second section  114 . Similarly to the second section  114 , the third section  216  also has a width that is greater than a width that is approximately equivalent to a mean free path of phonons for the one or more materials forming the third section  216 . 
     With reference to  FIG. 3 , there is shown a cross-sectional side view of a portion  300  of a thermoelectric device, according to a further embodiment. Similar to the portion  100  depicted in  FIG. 1  and the portion  200  shown in  FIG. 2 , the portion  300  shown in  FIG. 3  should be understood to represent one of the n-type region and the p-type region of a thermoelectric device, for instance, the thermoelectric device  400  depicted in  FIG. 4 . It should be understood that the portion  300  of the thermoelectric device depicted in  FIG. 3  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of a thermoelectric device containing the portion  300 . 
     As depicted in  FIG. 3 , the portion  300  includes a first electrode  102 , a second electrode  104  and a plurality of connecting structures  310  connecting the first electrode  102  and the second electrode  104 . Each of the connecting structures  310  is comprised of a first section  112  and a second section  314 . 
     The connecting structures  310  perform substantially the same functions as the connecting structures  110 ,  200  of the sections  100  and  200  depicted in  FIGS. 1 and 2 . The first section  112  of each of the connecting structures  310  has a width that is smaller than a width that is approximately equivalent to a mean free path of phonons and that is greater than a width that is approximately equivalent to a mean free path of electrons for the one or more materials forming the first section  112 . Similarly to the second section  114  depicted in  FIGS. 1 and 2 , a portion of the second section  314  has a width that is greater than a width that is approximately equivalent to a mean free path of phonons for the one or more materials forming the second section  314 . Unlike the second sections  114  depicted in  FIGS. 1 and 2 , however, the second section  314  has a tapered shape with a base positioned on the second electrode  104  and a top that is connected to and has a similar width to the first section  112 . Although the first section  112  and the second section  314  have been depicted as being of the same size at their intersection location  320 , it should be understood that one of the first section  112  and the second section  314  may have a larger width than the other one of the first section  112  and the second section  314  without departing from a scope of the connecting structure  310 . In this instance, a discontinuity may form at the intersection  320  of the first section  112  and the second section  314 . 
     In an alternate embodiment, although not shown, the first section  112  also has a tapered shape, similar to the second section  314 , with a base of the tapered shape being in contact with the first electrode  102 . In this embodiment, the tips of the first section  112  and the second section  314  are in contact with each other and at least one of the tips has a width that is smaller than or approximately equivalent to a mean free path of phonons and that is greater than a width that is approximately equivalent to a mean free path of electrons for the one or more materials forming either or both of the first section  112  and the second section  314 . In addition, a discontinuity may form at the intersection  320  of the tips of the first section  112  and the second section  314 . In this instance, one of the tips may have a width that is greater than a mean free path of phonons for the one or more materials forming that one of the tips. 
     With reference to  FIG. 4 , there is shown a cross-sectional side view of a thermoelectric device  400 , according to an embodiment. It should be understood that the thermoelectric device  400  depicted in  FIG. 4  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the thermoelectric device  400 . For instance, the thermoelectric device  400  may include any number of first electrodes, second electrodes, and connecting structures. 
     As depicted in  FIG. 4 , the thermoelectric device  400  includes a first electrode  102 , a pair of second electrodes  104  and a pair of connecting structures  410 . The first electrode  102  is depicted as being connected to the second electrodes  104  by a pair of p-type and n-type connecting structures  410 . Although individual ones of the p-type and n-type connecting structures  410  have been depicted as connecting the first electrode  102  to respective second electrodes  104 , it should be understood that multiple p-type and n-type connecting structures  410  may connect the first electrode  102  to the second electrodes  104 . 
     Although not explicitly depicted in  FIG. 4 , the connecting structures  410  of the thermoelectric device  400  may have the shapes of any of the connecting structures  110 ,  210 , and  310  depicted in  FIGS. 1-3 . In addition, the thermoelectric device  400  may be provided with a mechanical support in addition to the connecting structures  410 . The mechanical support may include, for instance, an insulator or a retained layer of oxide from a formation process for the thermoelectric device  400 . 
     Turning now to  FIG. 5 , there is shown a flow diagram of a method  500  of fabricating the portions  100 ,  200 , and  300  of a thermoelectric device  400  depicted in  FIGS. 1-4 , according to an embodiment. It should be understood that the method  500  depicted in  FIG. 5  may include additional steps and that some of the steps described herein may be removed and/or modified without departing from a scope of the method  500 . 
     At step  502 , at least one first electrode  102  may be provided. By way of example, the at least one first electrode  102  may be provided by forming the at least one first electrode  102  through any suitable process, such as one or more of growing, chemical vapor deposition, sputtering, evaporating, patterning, bonding, etc. As another example, the at least one first electrode  102  may be prefabricated and the step of providing may include positioning the at least one first electrode  102  with respect to at least one second electrode  104 . 
     At step  504 , one or more segments of connecting structure material may be provided such that as least one of the one or more segments is in contact with the first electrode  102 . By way of example, the one or more segments of connecting structure material are provided by forming the one or more segments of connecting structure material through any suitable formation process, such as, growing, catalyzed or uncatalyzed chemical vapor deposition, physical vapor deposition, molecular-beam deposition, molecular-beam epitaxy, laser ablation, sputtering, selective etching, etc. As another example, the one or more segments of connecting structure material may be prefabricated and the step of providing may include positioning the one or more segments of connecting structure material such that at least one of the one or more segments of connecting structure material is positioned in contact with the first electrode  102 . 
     The one or more segments of connecting structure material are comprised of materials that form the connecting structures  110 ,  210 ,  310 ,  410 . In this regard, one segment of connecting structure material may comprise one or more materials that form the first section  112 , another segment of connecting structure material may comprise one or more materials that form the second section  114 ,  314 , etc. In addition, when a plurality of segments of connecting structure material are provided at step  504 , the segments may be diffused together to increase phonon scattering as discussed above. In any event, the different sections of the connecting structure  110 ,  210 ,  310 ,  410  may be formed to have the variable cross-sections during formation of the connecting structures  110 ,  210 ,  310 ,  410 . 
     Optionally, however, at step  506 , the one or more segments of connecting structure material may be modified if the variable cross sections are not created during step  504 . If performed, the one or more segments of connecting structure material may be modified to form one or more connecting structures  110 ,  210 ,  310 ,  410  having the respective first sections  112  and second sections  114 ,  314  discussed above. The one or more segments of connecting structure material may be modified through any suitable process or combination of processes, such as one or more of, masking, selective etching, oxidation, diffusion, lithography, etc. 
     By way of a particular example, one or more connecting structures  110 ,  210 ,  310 ,  410  may be formed from a plurality of segments of connecting structure material comprised of different materials. In this example, one of the segments of connecting structure material comprises germanium and another of the segments of connecting structure material comprises silicon. The segment of connecting structure material comprising silicon is masked to protect it from ambient oxidation. The segments of connecting structure material are then oxidized and germanium dioxide (GeO 2 ) forms on the segment of connecting structure material comprising germanium, which was not masked. The germanium dioxide on the germanium segment of connecting structure material may then be selectively removed without removing the silicon to form the first section  112 , such that, the first section  112  has a width that is smaller than the second section  114 ,  314 . In addition, or alternatively, the germanium dioxide may not be removed from the germanium segment of the connecting structure material because primary conduction, which includes both heat and electrical conduction, will be through unoxidized regions of the connecting structures. As such, the germanium dioxide may be selectively removed to obtain desired conduction properties through the connecting structures. Moreover, the width of the first section  112  formed of the germanium segment of connecting structure material may be reduced to be smaller than the width that is approximately equivalent to a mean free path of phonons through the first section  112 . 
     In another example, the segments of the connecting structures are again formed by Ge and Si, and the segments are oxidized. However, in this example, the Si segments are not protected by masking. Both the Si and Ge segments are oxidized, but at different rates, so that the width of the different segments is reduced by different amounts. In a further refinement of this example, the oxidized structure is then exposed to a selective etchant, such as water, that removes Ge oxide, but not Si oxide. The above-described oxidation and etching process is repeated to reduce the diameter of the Ge segments much more than the diameter of the Si segments, creating the desired variable cross section of the connecting sections. 
     By way of another particular example, one or more of the connecting structures  110 ,  210 ,  310 ,  410  are formed from a plurality of connecting structure materials comprised of different materials and the first section  112  and the second section  114  are formed through use of the different diffusion rates of the different materials. In this example, one of the segments of connecting structure comprises germanium and another of the segments of connecting structure comprises silicon. Generally speaking, germanium diffuses faster into silicon than silicon diffuses into germanium. This difference in diffusion rates causes net mass transport from the germanium segment of connecting structure to the silicon segment of connecting structure, which causes the initial germanium segment of connecting structure to have a thinner tapered section as compared with the initial silicon segment of connecting structure. 
     At step  508 , at least one second electrode  104  may be provided. By way of example, the at least one second electrode  104  may be provided by forming the at least one second electrode  104  through any suitable process, such as one or more of growing, chemical vapor deposition, sputtering, etching, lithography, etc. Alternately, the at least one second electrode  104  may be provided prior to formation of the connecting structures  110 ,  210 ,  310 , as described in steps  504  and  506 . However, providing the at least one second electrode  104  after the connecting structures  110 ,  210 ,  310  are provided may more readily facilitate the formation of the thermoelectric devices  100 - 400  through processes utilizing catalyzed nanowire growth. For instance, pressure may be varied throughout processes utilizing catalyzed nanowires in order to vary the diameter of the connecting structures  110 ,  210 ,  310 . 
     By way of a further particular example, the method  500  may be used to form a thermoelectric device  400  having connecting structures  410  formed to be n-type and p-type semiconductors as shown in  FIG. 4 . In this example, the thermoelectric device  400  is formed to have a plurality of connecting structures  410 , in which, the one or more connecting structures  410  between a particular pair of electrodes  102 ,  104  are doped to be either p-type or n-type semiconductors and the one or more connecting structures  410  between another particular pair of electrodes  102 ,  104  are doped to be the other of n-type or p-type semiconductors. More particularly, for instance, the p-type connecting structures  410  may be masked while the n-type connecting structures  410  are being provided and the n-type connecting structures  410  may be masked while the p-type connecting structures  410  are being provided to substantially prevent cross-contamination between the p-type and the n-type connecting structures  410 . 
     What has been described and illustrated herein is an embodiment along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.