WO1994016465A1

WO1994016465A1 - Superlattice structures particularly suitable for use as thermoelectric cooling materials

Info

Publication number: WO1994016465A1
Application number: PCT/US1993/008134
Authority: WO
Inventors: Theodore C. Harman
Original assignee: Massachusetts Institute Of Technology
Priority date: 1993-01-12
Filing date: 1993-08-30
Publication date: 1994-07-21
Also published as: EP0679289A4; US5415699A; AU5124493A; JPH08505736A; EP0679289A1; CA2152772A1

Abstract

A superlattice comprising alternating layers of (PbTeSe)m and (BiSb)n (where m and n are the number of PbTeSe and BiSb monolayers per superlattice period, respectively) having engineered electronic structures for improved thermoelectric cooling materials (and other uses) may be grown by molecular beam epitaxial growth. Preferably, for short periods, n + m ∫ 50. However, superlattice films with 10,000 or more such small periods may be grown. For example, the superlattice may comprise alternating layers of (PbTe1-zSez)m and (BixSb1-x)n. According to a preferred embodiment, the superlattice comprises a plurality of layers comprising m layers of PbTe0.8Se0.2 and n layers of Bi0.9Sb0.1, where m and n are preferably between 2 and 20.

Description

SUPERLATTICE STRUCTURES PARTICULARLY

SUITABLE FOR USE AS

THERMOELECTRIC COOLING MATERIALS

U.S. Government Interest

This invention was developed pursuant to research sponsored in part by the U.S. Government under Department of the U.S. Air Force Contract No. F19628-90-C-0002, and the U.S. Government may have certain rights in the invention.

Field of the Invention

The invention relates to superlattice structures and specifically to such structures which are particularly useful as thermoelectric cooling materials. Background of the Invention Superlattice structures, in general, are known and typically comprise a composite made of alternating ultrathin layers of different materials. Typically, the superlattice has an energy band structure which is different than, but related to, the energy band structure of the component materials. By the appropriate choice of materials (and other factors discussed below), a superlattice having a desired energy band structure and other characteristics can be produced.

Superlattices have many uses, including, but not limited to, use in the field of thermoelectric cooling materials. Whall and Parker have suggested using a superlattice prepared by molecular beam epitaxy (MBE) to enhance the thermoelectric figure of merit of a thermoelectric material. They expressed particular interest in silicon and SiGe alloys, but also mentioned the possibility of using PbTe, InAs and the transition silicides (e.g. CoSi₂). Specifically, a strained-layer superlattice consisting of 20 or more layers of Si and SiGe composition was suggested.

The use of MBE to grow superlattice structures (e.g., GaAs and AlAs) is disclosed in U.S. Patent No. 4,261,771 issued to Dingle, et al. As disclosed in Dingle et al., the general technique for fabricating a superlattice by MBE comprises the following standard steps: (1) obtaining a substrate (e.g., from a commercial source); (2) cleaning a major surface of the substrate using standard preparation procedures; (3 placing the substrate in an evacuable metal chamber; (4) reducing the chamber pressure; (5) loading shuttered effusion cells (ovens) with the requisite source materials fo growth; (6) heating the substrate to about 600 degrees C, to cause desorption o contaminants from the growth surface and then adjusting the substrate temperature to that desired for growth; (7) with the shutters closed, heating the ovens until the source materials vaporize; and (8) opening selected shutters to effect growth until the desired layer thickness is attained.

According to Dingle et al., the fabrication process can be described as forming a new composition of matter of A (e.g., GaAs) and B (e.g., AlAs or Ge) by directing a periodically pulsed molecular beam at a substrate. During the first part of each period an A-beam is directed at the substrate for a time effective to grow material A having a thickness of n monolayers and during the second part of each period directing a B- beam at the substrate for a time effective to grow material B having a thickness of m monolayers.

Therefore, the fabrication of a superlattice by MBE, or other known epitaxial growth techniques, is generally known. However, the choice of materials and the relative amounts of the materials which make up the superlattice are predominant factors in determining the characteristics of the superlattice. For use as a thermoelectric material, it is desirable to choose the materials, and their relative amounts, so that the thermoelectric figure of merit is maximized.

The thermoelectric cooling figure of merit (ZT) is a measure of the effectiveness of a cooling material and is related to material properties by the following equation:

where S, σ, K, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature, respectively. The Seebeck coefficient (S) is a measure of how readily electrons (or holes) can change energy in a temperature gradient as they move across a thermoelement, and is related to the strength of interaction of charge carriers with the lattice and the available energy states. The highest useful Seebeck coefficients are found in semiconductor materials with low crystal symmetry. In theory to maximize ZT, one would try to maximize S, σ and T and minimize K. However, i practice, this is not so simple. For example, as a material is doped to increase it electrical conductivity (σ), bandfilling tends to lower S and the electron component κ of K increases. In most materials, ZT is maximized at doping levels approaching 10¹ cm^"3. Since increasing (or decreasing) one parameter may adversely increase (o decrease) another parameter, it is important to select carefully the component material to provide a high ZT. Currently, the best thermoelectric cooling materials have a Z of approximately 1, at 300 K. The figure of merit ZT is related to the thermoelectric materials factor (b^*) where:

b' - μ m^'3'² / κ_L; (2)

where μ is the carrier mobility, m* is the density of states effective mass and κ_L is the lattice thermal conductivity. The precise relationship between b^* and ZT is complex. However, for simplicity, it may be approximated as follows. If it assumed that b" = b'T⁵/² and that there is one-band conduction, then ZT increases monotonically as b" increases. A superlattice provides the opportunity to enhance ZT for a number of reasons.

For example, it is known that the Seebeck coefficient increases as the period of a quasi- two-dimensional superlattice decreases. The electrical conductivity may be enhanced by means of modulation doping, which has been shown to increase the carrier mobilities in Si/SiGe strained-layer superlattices. Furthermore, the lattice thermal conductivity of a small-period superlattice is expected to be substantially lower than the average o the component materials because of augmented Umklapp phonon-phonon scattering process (in which the total phonon vector is not conserved, but rather changes by 2n times the reciprocal lattice vector) due to phonon-interface scattering effects.

Multistage thermoelectric modules, which are used, for example, to cool different types of infrared imaging arrays (and for a number of other well known applications), are well known. Typically, however, they are limited to cold-sink temperatures greater than 160 K. Thermoelectric cooling materials, for example, Bi₂ Te₃ and BiSb wer researched 30 to 40 years ago. Unfortunately, the best currently known productio thermoelectric material, Bi₂ Te₃, is not capable of efficient heat removal belo approximately 180 K and has a ZT less than 1. Summary of the Invention

It is an object of the present invention to overcome these and other drawback of the prior art.

It is another object of the invention to provide a new superlattice structure whic is made of a novel combination of materials and which has a new and distinct energ band structure.

It is another object of the invention to provide a new thermoelectric coolin material which has a thermoelectric cooling figure of merit greater than 1.7 and preferably, approximately 3.2.

In order to accomplish these and other objects of the present invention, there i provided a short-period superlattice structure comprising alternating layers o component materials which already have reasonably good thermoelectric characteristics

The selection of materials may include consideration of many variables such as ban structure, electron affinity, lattice constant match, coefficient of expansion match, an inherent thermoelectric properties (e.g., at 300 K) for the components of th superlattice. The superlattice (SL) can be engineered to have an optimal band gap fo the particular application. For thermoelectric cooling materials, an energy band gap o between 0 and 8kT, and preferably 4kT, is desired, where k is Boltzmann's Constant an

T is temperature. The optimum band gap will result in the enhancement of transpor effects of the desirable multivalley ^conduction band and the elimination o minimization of the degrading effects of the extraneous T-valence band. Preferably, th superlattice is a short-period superlattice alloy structure having two components whic are not psuedomorphic but have slightly different crystal structures (e.g. face-centere cubic versus rhombohederal).

A short period superlattice (SPS) is a compositionally modulated, periodi structure, consisting of alternate layers of two distinct phases (materials). The material may consist of one semiconductor and one semimetal, two semiconductors or tw semimetals, but should have a superlattice period in thickness shorter than the electro (hole for p-type) carrier mean free path. Also, the SPS should have a potential provide by the layered structure, which significantly modifies the band structure of eithe component material. Furthermore, the regular array of phase interfaces should alter th phonon spectra so that the lattice thermal conductivity will be lower than that of eithe component. Therefore, the basic physical properties of the SPS (such as thermopowe electrical resistivity and thermal conductivity) are different from either of the origin materials and favorable for a high thermoelectric figure of merit ZT.

Preferably, the superlattice is made of component materials which have a larg number of relatively heavy atoms. A large number of relatively heavy atoms i advantageous for the achievement of a low lattice thermal conductivity (κ_L). Preferabl the materials comprise elements chosen from the lower center section of the Periodi Table. Elements from the lower center section of the Periodic Table tend to hav relatively large carrier mobilities (μ), which is advantageous for high electric conductivities (σ).

According to one aspect of the invention, a superlattice is provided having on or more of the following characteristics: an energy band gap of about 0-8kT; a densit of states effective mass of greater than 0.05, a lattice thermal conductivity less than 1 mWcm^K^"1 and a b^* greater than 40,000 cπrΛCWW¹ (at 300 K). For example, t provide a superlattice with a ZT of approximately 3.2, it may have the followin approximate 300 K properties: κ_L = 0.0020 W/cm K, ,. = 0.0022 W/cm K, S = 35 V/K and σ = 370 Ω^{' 1}.

For example, a superlattice according to the present invention may be made o alternating layers of two distinct materials where one material comprises Bi, a Bi allo or Bi and (at least one material selected from the group consisting of Pb, Sn, Ge, T

Se, S, Sb, As or P) and where the other material comprises materials selected from th group consisting of Pb, Sn, Ge, Te, Se, S, Sb, As or P.

For example, but without limitation, the superlattice may be made of alternatin layers of L^_yM_yN_jO^ and Bi^.,,; where L is a material selected from the group of Europium (Eu) or Strontiu

(Sr), M is a metal selected from Group IV (preferably Pb, Sn or Ge), N is a non-met selected from Group VI (preferably Te, Se or S), O is a non-metal selected from Grou VI (preferably Te, Se or S), R is a Group V material (preferably Sb, As or P); an where 0_<x_< 1, 0_<y_< 1 and 0_<z_<.l. Specifically, the superlattice preferably comprises alternating layers of PbTe₁._zS and B^Sb^. According to a preferred embodiment, the superlattice comprises m laye of PbTe₀₈Se₀₂ and n layers of B^ ₉Sb₀₁₅ where m and n are preferably between 2 an 20. However, the invention is not so limited. Other combinations and amounts materials may be used. A superlattice made of alternating layers of BiSb (e.g. Bi^ ₉Sb_{0 Λ}) and PbTeSe (e.

PbTe₀₈Se₀^) (or similar materials) is believed to be particularly advantageous for sever reasons. For example, at low temperatures BiSb can have ZT values comparable to th of Bi₂Te₃ alloys at room temperature and PbTeSe is also a good thermoelectric materi which may be lattice-matched to BiSb. Additionally, both PbTeSe and BiSb hav multivalley conduction bands with relatively large density-of-states effective masses an favorable carrier-scattering mechanisms, which tend to yield unusually high Seebec coefficients.

A short-period superlattice enhances ZT for several reasons. The short-rang disorder and the multiplicity of interfaces increases the phonon scattering over the entir spectrum of phonon frequencies. Additionally, a large number of different heavy atom

(e.g. five for PbTeSe/BiSb) will act as point defects and scatter mainly high frequenc phonons. Furthermore, phonon-electron interactions will scatter the phonons mainly low frequencies. The combined three effects will substantially reduce the lattic component of the thermal conductivity (κ_L) and thereby increase ZT. Moreover, a short-period SL can be engineered to have an optimal band ga e.g., a value between the value of BiSb and the value for PbTeSe, which will result i the enhancement of ZT because of the minimization of the harmful effects of the valence band of BiSb. Additionally, both modulation doping effects and the long-rang order of a good-quality SL enhance the electron carrier mobility and hence increas electrical conductivity, which will also increase ZT. Bi₂Te₃ alloys are considered by many to be the best conventional thermoelectri materials and have a rhombohederal crystal structure with a large c/a ratio (~7) However, they have a large lattice constant mismatch with most other goo thermoelectric materials. This may be undesirable in some instances. Preferably, th average lattice constants of the two components of the SL structure of the presen invention are matched in the growth plane, but not necessarily in the directio perpendicular to the growth plane, thereby producing coherent growth and excellen thermoelectric transport properties due to the minimization of defects at the interfaces

According to one aspect of the invention, a superlattice is synthesized from thos materials by epitaxial growth, for example, by using molecular beam epitaxy o organometallic vapor-phase epitaxy (OMVPE). Depending on the choice of materials other techniques such as liquid-phase epitaxy or sputtering may also be desired. Brief Description of the Drawings

Fig. 1 is a schematic illustration of the monolayers of two distinct materials of superlattice according to the present invention.

Fig. 2 is a schematic illustration of preparing superlattice chips for assembly int bulk thermoelements.

Fig. 3 is a schematic illustration of a plurality of superlattice chips assembled a a bulk thermoelement. Fig. 4 is an example of a thermoelectric cooling module according to on embodiment of the present invention. Detailed Description of the Preferred Embodiments

The present invention relates to novel superlattice structures which may be use for various purposes including, but without limitation, thermoelectric cooling, electroni devices and electrooptic devices. For example, a superlattice according to the presen invention may be made of alternating layers of a first material and a second material where the first material comprises Bi or Bi alloy (e.g., Bi and a material selected fro the group consisting of Pb, Sn, Ge, Te, Se, S, Sb, As or P) and where the secon material comprises materials selected from the group consisting of Pb, Sn, Ge, Te, Se S, Sb, As or P. For example, but without limitation, the superlattice may be made o alternating layers of L_j.yM_yN^.., and Bi^^; where L is a rare metal selected from the group of Europium (Eu) or Strontiu (Sr),

M is a metal selected from Group IV (preferably Pb, Sn or Ge), N is a non-meta selected from Group VI (preferably Te, Se or S), O is a non-metal selected from Grou VI (preferably Te, Se or S), R is a Group V (preferably Sb, As or P); and wher

0_<x_< 1, 0_<y_< 1 and 0_<z_< 1.

A superlattice comprising alternating layers of (PbTeSe)_m and (BiSb)_n (where and n are the number of PbTeSe and BiSb monolayers per superlattice period respectively) having engineered electronic structures for improved thermoelectric coolin materials (and other uses) may be grown by molecular beam epitaxial growth.

Preferably, for short-periods, n+m<50. However, superlattice films with 10,000 or more such small periods may be grown. For example, the superlattice may compris alternating layers of (PbTe₁._zSe_z)_m and (Bi^-Sb^),,. According to a preferred embodiment, the superlattice comprises a plurality of layers comprising m layers o PbTe₀₈Se₀₂ and n layers of Bi^Sbo _Λ, where m and n are preferably between 2 and 20.

However, the invention is not so limited. Other combinations and amounts of similar materials may be used. According to a preferred embodiment, molecular beam epitaxy (MBE) may be used to grow PbTe₀₈Se₀₂ and Bio₉Sb₀₁ on (111) BaF₂ substrates at substrate temperatures in the range of 100 to 250°C. One advantage of using a superlattice for thermoelectric cooling elements is that the energy gap of the L-point bands as well as the other bands can be engineered. A schematic arrangement of the monolayers of the superlattice is shown in Fig. 1. In the limit of layer thicknesses of the order of a few monolayers, the energy gap of the superlattice E_g ^sl can be approximated by the equation:

E = {E^ d_A + E^ d_B}/(d_A+ d_B), (3)

where E^ and E^ are the band gap energies of the two materials (e.g., PbTeSe and

BiSb), respectively, and d_A and d_B are the layer thicknesses of the materials (e.g., PbTeSe and BiSb), respectively. According to a preferred embodiment, if it is desired that E_g ^sl « 0.1 eV (which corresponds approximately to 4kT at 300K), then since E^ and E_gg are 0.30 eV and -0.05 eV, respectively, the BiSb layer should be somewhat thinne than the PbTeSe layer. Additionally, an even number of monolayers of PbTeSe i preferred to maintain stoichiometry. Therefore, if, for example, a superlattice perio of 9 monolayers is small enough to produce the desired band changes, a superlattic consisting of 4 monolayers of PbTeSe and 5 monolayers of BiSb should have optima thermoelectric properties. The thickness of a monolayer in the [111] direction is 0.369 and 0.3929 nm for PbTeSe and BiSb, respectively. For a period of 9 monolayers (a specified above), d_A = 1.4768 nm and d_B = 1.9645 n , and Eq. (3) yields E^sl = 0.10 eV The PbTeSe/BiSb superlattice is also advantageous because it is lattice-matche in the (111) plane. For example, the Bio₉Sb_{0 x} and PbTe₀₈Se₀₂ alloys are lattice-matche at the expected superlattice growth temperature of about 250 °C. However, it is onl necessary to lattice match in the growth plane for coherent growth and minimization o defects at the interfaces.

The L-band extrema of BiSb is very favorable for high ZT, since it is highl anisotropic. Its low-effective-mass directions correspond to a high electron mobility (μ) and its high-mass directions cause a relatively large density-of-states effective mass (m*) The high value of m* leads to a high Seebeck coefficient because, for a fixed carrie concentration, the Fermi level energy decreases with increasing m*. Also, there ar three equivalent L bands with three valley extrema belonging to the I_^minima, whic is favorable for high ZT because m* is directly proportional to the number of valleys

The T and H or ∑ valence bands of BiSb are less favorable because of their relativel low carrier mobilities and nearly isotropic nature (single valley). The deleterious effect of the T and H valence bands can be minimized or eliminated by the use of short perio superlattice (e.g., PbTeSe/BiSb), so that the inherently high ZT value of the conduction and valence bands of BiSb can be realized.

The L bands of PbTeSe are also favorable since they have good anisotropy an four equivalent valleys. Both PbTeSe and BiSb have low lattice thermal conductivitie (κ_L) at 300 K. From a physics point of view, the BiSb/PbTeSe superlattice ha enhanced thermoelectric properties. In addition, from a crystal growth point of view th system is ideal for thermoelectric materials development. For example, the PbTe-PbS pseudobinary system forms a continuous series of solid solutions. The liquidus an solidus part of the phase diagram form a minimum at the composition of PbTe₀₈Se₀₂ which is advantageous for the preparation of compositionally homogenous MBE source materials.

The superlattice provides a unique opportunity to create made-to-order energy gaps, phonon spectra and Fermi surfaces by controlling the energy and bandwidth of the subbands, which are determined by the barrier and well thicknesses and the barrier height. The barrier and well thicknesses are adjustable MBE growth parameters, whereas the barrier heights can be varied by choosing closely related pseudobinary alloys, such as PbEuTeSe, PbSrTeSe, etc. Many parameters of the growth process will be subject to optimization. These include, for example, impurity doping, layer thickness, deposition temperatures, growth rate, beam vapor pressures, etc.

The relationship between the molecular beam flux monitoring pressure and the epitaxial growth rate for both PbTeSe and BiSb has been calculated and measured experimentally to be the following:

G_r( m/hr) = P(Torr)/ 2 x 10^"6

where G_r is the epitaxial growth rate and P is the beam flux monitoring vapor pressure. The above equation is applicable in the 1 to 3 μm/hr. The calculated growth rates compare reasonably well with those obtained by cleaving the MBE grown layers and using an optical microscope to measure the layer thickness. The measured growth rate is calculated as the ratio of the layer thickness to the growth time.

Epitaxial growth methods in general are well known and can be used for growing the superlattices. For example, the superlattice may be grown by MBE on cleaved or polished single-crystal BaF₂ substrates. BaF₂ has a [lll]-oriented cleavage plane and a crystal lattice constant of 6.200 A at room temperature, which is relatively close to that of both PbTeSe and BiSb (in its trigonal orientation). Thus, BaF₂ is a viable substrate for the BiSb/PbTeSe superlattice. Other substrates may be used however. After growth, the superlattice layers can be easily removed from the BaF₂ substrate by any one of several known techniques. Preferably, the superlattice will have carrier concentrations less than lxl0¹⁷cm and bulk carrier mobilities for each component of the layered structure on large are polished substrates. The relatively low carrier concentrations are preferable for at leas the BiSb component of the superlattice for high Seebeck coefficients (>350μV/K a 300K). The high carrier mobilities are preferred for high electrical conductivity ( > 350μ at 300 K).

The interdiffusion coefficient of PbTeSe/BiSb should be small because the fiv different atoms are large and diffuse substitutionally rather than interstitially Nevertheless, knowledge of the diffusion coefficient is important for setting upper limit on the epitaxial growth temperatures and on thermal process times and temperatures

Secondary-ion-mass spectroscopy may also be used to assess impurity cross contamination. Modulation doping of the superlattice where the wide-gap PbTeS layers are doped with impurities and the narrow-gap BiSb layers are left undoped ma be desired. As shown, for example, in Fig. 2., the MBE-grown layered structures may b assembled into bulk thermoelements for cooling modules by removing the substrate 10, cutting the chips into a predetermined size having a length L and a width W (e.g. 2x mm), stacking and bonding (e.g. by soldering 30) the chips into small cubically-shaped thermoelectric cooling elements 20 having a thickness T, as shown, for example, i Fig. 3. The process of forming thermoelements, per se, is well known.

From these cubes, thermoelectric cooling modules can be fabricated. Fo example, n-type superlattice structures 20 may be used as the n-type leg 41 of a multi¬ stage thermoelectric module, as shown for example in Fig. 4. Thermoelectric modules using this new type of material may be used for various purposes, including, for example, commercial refrigeration and air-conditioning equipment, the development of cryogenic coolers for infrared thermal imaging, superconductive electronics, and other quantum device applications.

While multiple n-type and p-type thermoelement pairs in each stage of multistage unit have been required for cryogenic coolers, it may not be necessary to develop a new p-type leg for high-performance modules. For example, in view of the superiority of the n-type thermoelements, the p-type leg 42 of Fig. 4 may be replace by a passive thermoelement, e.g., one that has close to zero Seebeck coefficient bu which has a high ratio of electrical to thermal conductivity. It is known that the Seebec coefficient is zero in superconductors but the ratio of the electrical to therma conductivity is infinite. Consequently, a thermoelectric module or stage made with th new n-type superlattice thermoelement and a superconductor has a ZT precisely tha of the n-type superlattice thermoelement. A multistage cooler using n-type superlattic thermoelements can be made with stages operating above ~ 120 K having p-type leg of conventional Bi₂Te₃ or BiSb alloys (or other suitable material) and still have a effective ZT of 2.1 and stages operating below -120 K having superconductor legs an have an effective ZT of 3.2. Thus, a matching p-type superlattice leg may not be neede to provide a 300 to 77 K thermoelectric cooler.

The realization of a ZT of 3.2 over the entire multistage cooler temperatur range is adequate to accomplish thermoelectric coohng from room temperature to 77 K.

It will also greatly expand the commercial refrigeration and cooling applications fo thermoelectric coolers. Many other uses and advantages will also be readily apparen to one of ordinary skill in the art.

While PbTeSe/BiSb was discussed as a preferred combination of materials, various other combinations of materials are within the scope of the invention. Specifically, other materials which exhibit characteristics similar to the advantageous characteristics discussed above are within the scope if the invention. In general, it is preferred that at least one of the layers comprise PbTeSe, PbTe, SnSe, SnTe, TeSe, Bi₂Te₃, Bi, BiSb or their alloys. Without limiting the invention, the following are some specific examples of combinations of materials which may also be used:

Bi and Bi₂Te₃ B Sbo.π and Bi

B¹o.87ϊ> >o.i2.A^so.oι and

Bi₂Te₃ and PbTe

SbTe and BiTe (or another Bi alloy)

AgSbTe and BiSb (or another Bi alloy) CdSb and BiSb (or another Bi alloy) While the foregoing is a description of an example of the preferred embodiments of the present invention, various alternatives will be readily apparent to one of ordinary skill in the art. The invention is only limited by the claims appended hereto.

Claims

1. A superlattice comprising a plurality of layers of a first materi comprising N_z0_1-Z and a second material comprising Bi; where N is a Group VI non-metal selected from the group consisting Te, Se and S; O is a Group VI non-metal selected from the group consisting of Te, S and S; and O≤z≤l

2. The superlattice of claim 1 having an energy gap between OkT and lOkT where k is Boltzman's Constant and T is temperature.

3. The superlattice of claim 1 having at least one of the following properties a density of states effective mass less than 0.05; a lattice thermal conductivity less tha

10 mWcm^K^"1 and a materials factor (b^*) greater than 40,000 cm^W^V^S^-1;

4. The superlattice of claim 1 having thermoelectric figure of merit greate than approximately 1.7.

5. The superlattice of claim 1 having a thermoelectric figure of merit greate than approximately 2.0.

6. The superlattice of claim 1 having a thermoelectric figure of merit greate than approximately 2.5.

7. The superlattice of claim 1 having a thermoelectric figure of merit greate than approximately 3.0.

8. The superlattice of claim 1 having a thermoelectric figure of meri approximately equal to 3.2.

9. The superlattice of claim 1 wherein the first and second materials and th number of layers of the first and second materials within a period of the superlattice ar selected such that E_g ^sl is approximately OkT to 8 kT, where k is Boltzman's Constant T is Temperature and

E_g ^sl = {E^ d_A + E^ d_B}/(d_A+ d_B); where E_g ^sl is the energy gap of the superlattice, E^ and E_gB are the energ gaps of the first and second materials, respectively, and d_A and d_B are the laye thicknesses of the first and second materials, respectively.

10. A superlattice comprising a plurality of layers of: L_j._yM_yNA., and

where L is a material selected from the group consisting of Europium an Strontium; M is a Group IV metal selected from the group consisting of Pb, Sn and Ge

N is a Group VI non-metal selected from the group consisting of Te, Se and S; O is

Group VI non-metal selected from the group consisting of Te, Se and S; R is a Grou

V material; O≤x≤l; O≤y≤l; and O≤Z≤l

11. The superlattice of claim 10 having an energy gap between OkT and lOkT where k is Boltzman's Constant and T is temperature.

12. The superlattice of claim 10 having at least one of the followin properties: a density of states effective mass less than 0.05; a lattice thermal conductivit less than 10 mWcm^K^*1 and a materials factor (b^*) greater than 40,000 cm^W ^'

13. The superlattice of claim 10 having thermoelectric figure of merit greate than approximately 1.7.

14. The superlattice of claim 10 having a thermoelectric figure of meri greater than approximately 2.0.

15. The superlattice of claim 10 having a thermoelectric figure of meri greater than approximately 2.5.

16. The superlattice of claim 10 having a thermoelectric figure of meri greater than approximately 3.0.

17. The superlattice of claim 10 having a thermoelectric figure of meri approximately equal to 3.2.

18. The superlattice of claim 10 comprising alternating layers of L^._y M_y N_z O_x _z and Bi_x Ri^.

19. The superlattice of claim 10 wherein the L^ M_y N_z O^ and Bi_x R _x ar grown on a substrate and are lattice matched in a plane perpendicular to the directio of growth.

20. The superlattice of claim 10 wherein y is approximately equal to 1, N i Te and O is Se.

21. The superlattice of claim 20 wherein M is Pb and z is approximately equ

22. The superlattice of claim 21 wherein R is Sb.

23. The superlattice of claim 22 wherein x is approximately equal to 0.9.

24. A superlattice comprising a predetermined number of alternating layer of two component materials, where one of the component materials comprises TeSe an the other component material comprises Bi.

25. A short-period superlattice comprising at least one period comprising layers of PbTeSe and n layers of BiSb where m is less than n.

26. The superlattice of claim 25 where m = 4 and n = 5.

27. The superlattice of claim 25 wherein 2 < m < 20

2 < n < 20

28. A short-period superlattice comprising: a plurality of epitaxially grown layers of materials A and B, where A and B are substantially lattice matched in a direction prependicular to the direction of growth and are selected to provide a thermoelectric figure of merit greater than 1.7.

29. The short-period superlattice of claim 28 wherein the materials A and B and the number of layers of materials A and B within a period of the superlattice are selected such that E_g ^sl is approximately OkT to 8 kT, where k is Boltzman's Constant, T is Temperature and E_g ^SI = {E^ d_A + E_gB d_B}/(d_A+ d_B); where E_g ^sl is the energy gap of the superlattice, E^ and E^ are the energy gaps of materials A and B, respectively, and d_A and d_B are the layer thicknesses of materials A and B, respectively.

30. A thermoelectric module comprising: a plurality of thermoelectric chips which are stacked and bonded, wherein said thermolectric chips comprise a superlattice according to claim 1.

31. A thermoelectric device comprising: an n-type leg; a second leg operatively connected to said n-type leg for thermoelectric cooling; wherein said n-type leg comprises a thermoelectric module according to claim 30.

32. A thermoelectric device according to claim 31 wherein said second leg comprises a p-type leg.

33. A thermoelectric device according to claim 31 wherein said second leg comprises a superconductor material.