Patent Application: US-201214367087-A

Abstract:
a method for enhancement of thermoelectric properties through polarization engineering . internal electric fields created within a material are used to spatially confine electrons for the purpose of enhancing thermoelectric properties . electric fields can be induced within a material by the presence of bound charges at interfaces . a combination of spontaneous and piezoelectric polarization can induce this interfacial charge . the fields created by these bound charges have the effect of confining charge carriers near these interfaces . by confining charge carriers to a channel where scattering centers can be deliberately excluded the electron mobility can be enhanced , thus enhancing thermoelectric power factor . simultaneously , phonons will not be affected by the fields and thus will be subject to the many scattering centers present in the majority of the structure . this allows for simultaneous enhancement of power factor and reduction of thermal conductivity , thus improving the thermoelectric figure of merit , zt . this approach is also compatible with other strategies for reducing thermal conductivity , for example the inclusion of nanostructures .

Description:
in the following description of the preferred embodiment , reference is made to the accompanying drawings which form a part hereof , and in which is shown by way of illustration a specific embodiment in which the invention may be practiced . it is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention . the present invention discloses a method of improving zt beyond bulk values through the use of interfacial properties within a material . by intelligently designing the distribution of charge within a material , the electrical properties can be improved while simultaneously reducing thermal conductivity . this can allow for improved thermoelectric figure of merit , zt . in general , the thermal conductivity and electrical conductivity of a material move in the same direction as parameters are changed . this is because many features , such as dopant atoms or grain boundaries , act as both electron and phonon scattering centers , and thus reduce both electrical and thermal conductivity . in order to break this relationship and improve zt , mechanisms that selectively scatter phonons over electrons must be incorporated into a material . many have proposed the use of nanostructures for this purpose . in general , the structures that most effectively scatter phonons over electrons have sizes comparable to phonon wavelengths , but significantly larger than electron wavelengths . nanostructures in the 1 - 10 nm size range have proven effective for these purposes . [ 1 , 2 ] the present invention proposes an alternative method of obtaining selective scattering of phonons over electrons by spatially separating the electrons into channels within a material while the spatial phonon distribution remains unchanged . scattering centers such as dopant atoms or alloy atoms can then be deliberately excluded from the electron channels while included in most of the material for maximum phonon scattering . the spatial separation of electrons into channels is accomplished by setting up electric fields within a material which will result in sharp potential minima in the conduction band of a material . such potential minima are known to exist at interfaces in highly polar materials , such as algan / gan heterostructures . this strategy has been proven effective at increasing electron mobility in high electron mobility transistor ( hemts ) technology , but has never been proposed for use in thermoelectric materials . high electron mobility transistors fabricated from algan / gan interfaces have achieved electron mobilities as high at 2000 cm 2 / vs , which is four times higher than typical gallium nitride . [ 3 ] the thermal conductivity and seebeck coefficient of such structures has not been investigated prior to this work . fig1 ( a ) is a schematic diagram of a gan crystal grown along the c - axis and fig1 ( b ) is a diagram that shows the orientation of spontaneous polarization p sp charge ( not shown is the effect of piezoelectric polarization ). this orientation has layered gallium and nitrogen atoms . under typical mocvd growth conditions for gan on sapphire , the top surface is ga terminated , i . e ., ga - face , resulting in a ga - polar surface . when a different material , for example , algan or inaln , is grown on top of gan to form a heterointerface , the electronegativity difference between the atoms on each side of the interface causes the spontaneous polarization p sp . for c - plane iii - nitride materials , this effect is much larger than is typical for other iii - v materials . for example , an aln / gan heterointerface has more than 10 times the interface charge in the gaas / algaas system , which is commonly used for modfets . [ 4 ] in addition to spontaneous polarization , piezoelectric polarization can be present at heterointerfaces within a material . this polarization is created when the materials on each side of the interface are piezoelectric materials . since the materials used on each side of the interface often have different lattice constants , both sides of the interface are strained , and therefore piezoelectric electric fields are created . piezoelectric coefficients in iii - nitrides are also typically an order of magnitude larger than most iii - v materials . [ 4 ] the combination of spontaneous and piezoelectric polarizations at these interfaces can create large polarization charges at interfaces for certain alloy compositions . the present invention uses the large polarization charges discussed in the previous section in order to bind charge carriers to interfaces within a material . the structure , band diagram , and carrier distribution for a superlattice of such interfaces are shown in fig2 ( a ), 2 ( b ) and 2 ( c ). specifically , fig2 ( a ) is an example structure of a gan / in 0 . 19 a 1 0 . 81 n superlattice ; fig2 ( b ) is a graph of energy ( ev ) vs . position ( å ) that shows the conduction and valence bands for the structure as calculated by the bandeng simulation software , which is a 1d self consistent schrödinger - poisson solver ; and fig2 ( c ) is a graph of carrier density ( cm − 3 ) vs . position ( å ) that shows the electron distribution within the material , wherein the in 0 . 19 al 0 . 81 n carrier density is set to 4e19 cm − 3 and the gan carrier density is set at 5e18 cm − 3 , and wherein calculations are performed using bandeng . in this example , the material of fig2 ( a ) is strongly n - type . in the band diagram of fig2 ( b ), it can be seen that the conduction band ( upper solid line ) dips below the fermi level ( dashed line ) at every other interface . the band bending which is responsible for these dips is caused by the polarization charges discussed above . it can further be seen in the charge distribution plot of fig2 ( c ) that , at each of the points where the conduction band dips below the fermi level , there is a large spike in electron concentration . if such a structure is properly designed , virtually all of the free carriers within a material can be confined at these interfaces . with the charge confined into relatively narrow channels at certain interfaces , it is now possible to deliberately exclude electron scattering centers from these regions . typically , the dominant scattering mechanisms in these types of systems are ionized impurity scattering and alloy scattering . by keeping the dopant atoms confined to the high bandgap material and a number of nanometers away from the interface , ionized impurity scattering can be drastically reduced due to spatial separation . in addition , it has been shown that alloy scattering of electrons can be drastically reduced in algan / gan systems with the use of an aln spacer . by including a very thin (˜ 0 . 5 nm ) spacer , the charge carriers can be effectively spatially separated from the alloy scattering centers . such a structure is shown in fig3 , which illustrates an algan hemt structure , wherein an aln spacer is included to keep electrons away from alloy scattering centers in the algan . in algan / gan hemts , a combination of these strategies has produced room temperature mobilities as high as 2000 cm 2 / vs , which is about four times higher than bulk gan . [ 3 ] these structures are generally thin films and can be grown by methods such as metal organic chemical vapor depositions ( mocvd ), hydride vapor phase epitaxy ( hvpe ), or molecular beam epitaxy ( mbe ). it is also possible that this same effect could be obtained in bulk materials through mechanisms such as grain boundaries between dissimilar materials . although the example given above describes a layered superlattice structure , it is possible to achieve similar polarization effects using arrays of wire or nanowire structures . this approach is also compatible with the deliberate introduction of nanoparticles for reduction of thermal conductivity or for thermionic energy filtering benefits . this approach is also capable of exploiting the advantages that low dimensional structures have shown in enhancing thermoelectric properties . it was first suggested by hicks and dresselhaus that quantum well structures could modify the density of states in a material and thereby improve the seebeck coefficient . [ 5 ] the confinement in such systems is typically provided by combining materials with differing band gaps to confine charge carriers to the lower bandgap material . high levels of confinement can alternatively be achieved using the polarizations described above . fig4 is a table that shows preliminary results and estimates for polarization - based , single layer gan / aln / algan thermoelectrics and gan / aln / algan superlattice thermoelectrics , grown by mocvd , along with gan and in 0 . 3 ga 0 . 7 n for comparison , wherein data for in 0 . 3 ga 0 . 7 n is taken from [ 7 ]. the sample labeled “ algan hemt ” is a single period of the structure shown in fig3 . this sample exhibits high mobility and low sheet resistance as well as a reasonable seebeck coefficient . the sample labeled “ 17 nm period ” approximates a superlattice of algan hemt structures with a 17 nm period . such structures with have been shown experimentally for use as current spreading layers and maintain high electron mobility , although thermoelectric properties are not explored . [ 6 ] using this 17 nm repeat period and the single layer electrical properties , the electrical conductivity and seebeck coefficient for a gan / aln / algan superlattice is estimated . this results in a power factor which is roughly a factor of two higher than silicon doped gan and ten times higher than published values for ingan . [ 7 ] the thermal properties of such a superlattice are difficult to estimate , but would be expected to be significantly lower than bulk gan due scattering from the additional alloy elements , superlattice boundaries with the material , and the significant strain between the gan and algan layers . future work will include the growth of superlattice structures from various alloys and alloy compositions as well as the characterization of their thermoelectric properties . fig5 is a flowchart illustrating one method of accomplishing the present invention , where the constituent materials are ( al , ga , in ) n or alloys of these materials . block 500 represents forming an initial iii - nitride layer , such as gan , with a growth ( top ) surface oriented along the c - axis . the initial iii - nitride may be an epitaxial gan layer grown on a substrate , such as a sapphire substrate , or a freestanding gan layer , or a bulk gan substrate , etc . the c - axis orientation means that the gan has layered gallium and nitrogen atoms , where a first surface , e . g ., the top surface , is ga terminated , i . e ., ga - face , resulting in a ga - polar surface , and a second surface , e . g ., the bottom surface , is n terminated , i . e ., n - face , resulting in an n - polar surface . block 502 represents the optional step of forming one or more iii - nitride spacer layers or interlayers on the initial iii - nitride layer for example , as noted above , it has been shown that alloy scattering of electrons can be drastically reduced in algan / gan systems with the use of an an spacer . by including a very thin (˜ 0 . 5 nm ) spacer , the high charge carrier concentration areas can be effectively spatially separated from the alloy elements or scattering centers . block 504 represents the step of forming one or more iii - nitride layers on the spacer layers or the initial layer . in this step , when a different material , for example , algan or inaln , is grown on top of gan to form a heterointerface , the electronegativity difference between the atoms on each side of the interface causes what is termed as spontaneous polarization . in addition to spontaneous polarization , piezoelectric polarization can be present at heterointerfaces within a material . this polarization is created when the materials on each side of the interface are piezoelectric materials . since the materials used on each side of the interface often have different lattice constants , both sides of the interface are strained , and therefore piezoelectric electric fields are created . piezoelectric coefficients in iii - nitrides are also typically an order of magnitude larger than most iii - v materials . [ 4 ] the combination of spontaneous and piezoelectric polarizations at these interfaces can create large polarization charges at interfaces for certain alloy compositions . note that the layers formed in block 504 may comprise doped layers . with the charge confined into relatively narrow channels at certain interfaces , it is possible to deliberately exclude electron scattering centers from these regions . typically , the dominant scattering mechanisms in these types of systems are ionized impurity scattering and alloy scattering . by keeping the dopant atoms confined to the high bandgap material and a number of nanometers away from the interface , ionized impurity scattering can be drastically reduced due to spatial separation . generally , the structures formed in steps 502 and 504 are thin films and can be grown by epitaxial methods such as metal organic chemical vapor depositions ( mocvd ), hydride vapor phase epitaxy ( hvpe ), or molecular beam epitaxy ( mbe ). however , it is also possible that these structures could be formed using bulk methods through mechanisms such as grain boundaries between dissimilar materials , where the bulk material is modified to include similar interfaces . note also that these steps 502 and 504 may be repeated to create , in one embodiment , a layered superlattice structure . however , other structures may be formed in other embodiments . for example , it is possible to achieve similar polarization effects by forming arrays of wire or nanowire structures . the approach of this invention is also compatible with the deliberate introduction of nanoparticles for reduction of thermal conductivity or for thermionic energy filtering benefits . in addition , the approach of this invention is also capable of exploiting the advantages that low dimensional structures have shown in enhancing thermoelectric properties , such as quantum well structures that modify the density of states in the material and thereby improve the seebeck coefficient , where the confinement in such structures is typically provided by combining materials with differing band gaps to confine charge carriers to the lower bandgap material . high levels of confinement can alternatively be achieved using the polarizations described above . block 506 represents the end result of the process , namely a composition comprising a thermoelectric material with improved zt that uses electric fields to spatially separate charge carriers and phonons . the electric fields are built in electric fields created by polarization charges present at interfaces with the material . the interfaces may be arranged in a layered superlattice structure , or the interfaces may be achieved using wire or nanowire structures . in this material , charge carrier scattering centers are deliberately excluded from areas in the material with high charge carrier concentration , and phonon scattering centers are deliberately included in areas in the material with low charge carrier concentration . phonon scattering in low electron concentration regions of the material is achieved through alloy scattering induced in the material by binary , ternary , or quaternary alloys in the material . the material may include nanoparticles , where the nanoparticles scatter phonons and further reduce thermal conductivity . the confinement of charge carriers creates sharp features in the material &# 39 ; s density of states , which can be used for the purpose of increasing the seebeck coefficient and enhancing the power factor . this effect is in addition to the electron mobility enhancement achieved by spatially separating electrons from scattering centers . the material may be grown by epitaxial methods , where epitaxial methods are used to create a layered superlattice in the material . the material may be grown by bulk methods , where the material grown by the bulk methods is modified to include interfaces , and the electric fields are built in electric fields created by polarization charges present at the interfaces with the material . the resulting thermoelectric materials , regardless of how prepared , can be processed into thermoelectric modules . such modules can be fabricated through either traditional slicing and placement of individual thermoelements or through thin film processing techniques . these modules will result in higher efficiencies for both solid state cooling and power generation . the terms “ group - iii nitride ” or “ iii - nitride ” or “ nitride ” as used herein refer to any composition or material related to ( al , in , ga ) n semiconductors having the formula al x in y ga z n where 0 ≦ x ≦ 1 , 0 ≦ y ≦ 1 , 0 ≦ z ≦ 1 , and x + y + z = 1 . these terms as used herein are intended to be broadly construed to include respective nitrides of the single species , al , ga , and in , as well as binary , ternary and quaternary compositions of such group iii metal species . accordingly , these terms include , but are not limited to , the compounds of aln , gan , inn , algan , alinn , ingan , and algainn . when two or more of the ( al , ga , in ) n component species are present , all possible compositions , including stoichiometric proportions as well as off - stoichiometric proportions ( with respect to the relative mole fractions present of each of the ( al , ga , in ) n component species that are present in the composition ), can be employed within the broad scope of this invention . further , compositions and materials within the scope of the invention may further include quantities of dopants and / or other impurity materials and / or other inclusional materials . this invention also covers the selection of particular crystal orientations , directions , terminations and polarities of group - iii nitrides . when identifying crystal orientations , directions , terminations and polarities using miller indices , the use of braces , {}, denotes a set of symmetry - equivalent planes , which are represented by the use of parentheses , ( ) the use of brackets , [], denotes a direction , while the use of brackets , & lt ;& gt ;, denotes a set of symmetry - equivalent directions . many group - iii nitride devices are grown along a polar orientation , namely a c - plane { 0001 } of the crystal , which exhibits a quantum - confined stark effect ( qcse ) due to the existence of strong piezoelectric and spontaneous polarizations . other orientations in group - iii nitride devices exhibit decreasing polarization effects , such as devices grown along nonpolar or semipolar orientations of the crystal . the term “ nonpolar ” includes the { 11 - 20 } planes , known collectively as a - planes , and the { 10 - 10 } planes , known collectively as m - planes . such planes contain equal numbers of group - iii and nitrogen atoms per plane and are charge - neutral . subsequent nonpolar layers are equivalent to one another , so the bulk crystal will not be polarized along the growth direction . the term “ semipolar ” can be used to refer to any plane that cannot be classified as c - plane , a - plane , or m - plane . in crystallographic terms , a semipolar plane would be any plane that has at least two nonzero h , i , or k miller indices and a nonzero l miller index . subsequent semipolar layers are equivalent to one another , so the crystal will have reduced polarization along the growth direction . 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[ 8 ] d . brown , phd thesis , “ growth of n - polar gan - based materials and high electron mobility transistors by metal organic chemical vapor deposition ” ( 2010 ). this concludes the description of the preferred embodiment of the present invention . the foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . many modifications and variations are possible in light of the above teaching . it is intended that the scope of the invention be limited not by this detailed description , but rather by the claims appended hereto .