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US005436467A
Unlted States Patent [19]
[11] Patent Number:
Elsner et al.
[45]
[54] i?i?g‘?cégéumumwmmu
2-17s9ss
p
[21] Appl No _ 185 562
[ 1
7/1990 Japan ................................. .. 257/930
12px '
both of La 16112, Calif. 92037
51
Jul. 25, 1995
FOREIGN PATENT DOCUMENTS
[76] Inventors: Norbert B. Elsner, 5656 Soledad Rd.;
Saeid Ghamaty, 7922 Ave. Kirjah,
[22] Filed:
Date of Patent:
5,436,467
‘
Primary Examiner-Mahshid D. Saadat
Attorney, Agent, or Firm-John R. Ross
Jan. 24, 1994
Int. Cl.6 ................... .. H0
HD1111:
[57]
1 -
-
ABSTRACT
A multi-layer superlattice quantum well thermoelectric
[52] US. (:1. .................................... .. 257/15- 257/616-
material “sing maten'als f°r the layers having the same
257/930; 62/302’; 136/203:
crystalline structure. A preferred embodiment is a
[58] Field of Search ............. .. 257/613, 614, 616, 625,
Superlat?ce 0f Si and SiGe’ b°th °f whi°h have a cubic
257/930, 15; 62/32; 135/203
structure. Another preferred embodiment is a superlat
tice of B-C alloys, the layers of which would be di?‘er
[56]
References Cited
U.S. PATENT DOCUMENTS
3,780,425 12/1973
ent stoichometric forms of B-C but in all cases the crys
talline structure would be alpha rhombohedral.
Penn et al. ........................ .. 257/930
4,835,059 5/1989 Kodato .............................. .. 257/930
10 Claims, 2 Drawing Sheets
US. Patent
4
July 25, 1995
»
I
Sheet 1 of 2
5,436,467
PRIOR ART
3
,
SW5
2 \IRLATTICE "AERIAL
1
BULK MATERIAL
0
.20
v
40
so
so
LAYER THICKNESS
100
US. Patent
July 25, 1995
Sheet 2 of 2
‘
5,436,467
5,436,467
1
2
?cient or the thermal conductivity. Harmon of Lincoln
SUPERLA'I'I‘ICE QUANTUM WELL
THERMOELECI‘RIC MATERIAL
Labs, operated by MIT has claimed to have produced a
superlattice of layers of (Bi,Sb) and Pb(Te,Se). He
claims that his preliminary measurements suggests ZTs
This invention relates to thermoelectric devices and 5 of 3 to 4. However, the present inventors have demon
in particular to materials for such devices.
strated that high ZT values can de?nitely be achieved
with Si/SIQSGGQZ superlattice quantum wells. Most of
BACKGROUND OF THE INVENTION
the efforts to date with superlattices have involved
alloys which are known to be good thermoelectric ma-_
Thermoelectric cooling and heating has been known
for many years; however, its use has not been cost com
terials for cooling, many of which are dif?cult to manu
petitive except for limited applications because of the
lack of thermoelectric materials having the needed ther
facture as superlattices because the stoichiometry of the
alloys have to be very carefully controlled which is
very dif?cult in vapor deposition techniques such as
moelectric properties.
A good thermoelectric material is measured by its
“?gure of merit” or Z, de?ned as
'
15
molecular beam epitaxy.
What is needed are thermoelectric materials with
improved ZT values which permit a simpli?ed manu
facturing process.
where S is the Seebeck coef?cient, p is the electrical
resistivity, and K is the thermal conductivity. The See
SUMMARY OF THE INVENTION
The present invention provides a superlattice quan
beck coef?cient is further de?ned as the ratio of the
tum well thermoelectric material using elements for the
open-circuit voltage to the temperature difference be
tween the hot and cold junctions of a circuit exhibiting
layers all of which have the same crystalline structure.
the Seebeck effect, or
This makes the vapor disposition process easy because
the exact ratio of the materials in the quantum well
25
layers is not critical. The inventors have demonstrated
that materials produced in accordance with this inven
Therefore, in searching for a good thermoelectric mate
tion provide ?gures of merit more than six times that of
rial, we look for materials with large values of S, and
prior art thermoelectric materials. A preferred embodi
low values of p0 and K.
ment is a superlattice of Si, as the barrier material, and
Thermoelectric materials currently in use today in
SiGe, as the conducting material, both of which have
clude the materials listed below with their ?gures of
the same cubic structure. Another preferred embodi
merit shown:
ment is a superlattice of BC alloys, the layers of which
would be different stoichiometric forms of B-C but in
Peak zeta.
35 all cases the crystalline structure would be alpha rhom
bohedral.
Thermoelectric Material
Z (at temperature shown)
ZT
Lead telluride
Bismuth telluride
Silicon germanium
0.9 X l0-3/'K. at 500' K.
3.2 x 10—3/'K. at 300' K.
0.8 x 10—3/'1c at 1100' x.
0.9
1.0
09
Workers in the thermoelectric have been attempting too
improve the ?gure of merit for the past 20—30 years
with not much success. Most of the effort has been
directed to reducing the lattice thermal conductivity
(K) without adversely affecting the electrical conduc
tivity. Experiments with superlattice quantum well ma
terials have been underway for several years. These
materials were discussed in an paper by Gottfried H.
Dohler which was published in the November 1983
issue of Scienti?c American. This article presents an
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the theoretical thermoelec
tric properties of superlattice materials.
FIG. 2 is simple drawing showing an apparatus for
making superlattice materials.
FIG. 3 shows a thermoelectric eggcrate arrangement.
FIG. 4 shows how the thermoelectric elements are
45 cut from the substrate on which grown.
FIG. 5 shows a portion of a thermoelectric device to
show current ?ow through the elements.
DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS
excellent discussion of the theory of enhanced electric
conduction in superlattices. These superlattices contain
lutionary thermoelectric material, with ZT values in
alternating conducting and barrier layers. These super
excess of 6. This material is a superlattice material of
The present inventor has demonstrated that a a revo
lattice quantum well materials are crystals grown by
very thin layers of silicon and silicon germanium. The
depositing semiconductors in layers whose thicknesses 55 principal advantage of this superlattice over many oth
is in the range of a few to up to about 100 angstroms.
ers that have been suggested is that Si and Ge both have
Thus, each layer is only a few atoms thick. There has
been speculation that these materials might be useful as
the same'crystalline-structure so that when the layers
thermoelectric materials. (See articles by Hicks, et al
and Harman published in Proceedings of 1992 1st Na
tional Thermoelectric Cooler Conference Center for
Night Vision & Electro Optics, US. Army, Fort Bel
tices.
are grown the atoms fit together to form the superlat
Making the n-Type Superlattice
SIQ8GCQ2/Si layers were grown on (100) Si substrates
voir, Va. FIG. 1 has been extracted from the Hicks
by codeposition from two electron beam evaporation
paper and is included herein as prior art. It projects
sources in a molecular beam epitaxy. The ?uxes from
theoretically very high ZT values as the layers are 65 the Si and Ge electron beam sources were separated,
made progressively thinner.) The idea being that these
sensed and controlled to yield a total deposition rate of
materials might provide very great increases in electric
5 A/sec. Prior to deposition, substrates were chemically
conductivity without adversely affecting Seebeck coef
cleaned, then argon sputtered in situ and annealed at
5,436,467
3
4
800°—850° C. Above 500° C., annealing and growth
temperatures were measured directly by infrared py
rometry.
Si_gGe_7/Si
8Ge0_7/Si are compared in Table I with the properties of
bulk material with the same ratios of Si and Ge:
TABLE I
Electrical
Seebeck
Carrier Band
Power
Resitivity
Coef.
Conc.
Gap
Factor
Figure of
Z(Abs Temp)
a
n
Eg
az/p
Merit
(r = 300“ K.)
2
ZT
Sample
Superlattice
Quantum Well
Bulk
.
tested thermoelectric properties of samples of Sig,
(mo-cm)
(pV/oC) (l/cm) (eV)
((/1000)
0.26
-260
1016
1.25
260
s X 10-3
1.5
1
-130
101°
1.05
17
.33 x 10-3
0.1
Then the layers could be alternatively deposited on
top of each other to make Si0_gGe0_7/Si superlattices
The band gap (Eg) for this Si/Si0.sG60.2 multilayer
with each layer being about 50 A thick.
15 quantum well was determined to be 1.25 eV. This value
The actual deposition con?guration is illustrated
demonstrated that a quantum well was formed with Si
schematically in FIG. 1. Five substrates 2 are mounted
on the bottom of platen 4 which rotates at a rate of l
revolution per second. The platen is 50 cm in diameter
and the substrate wafers are each 125 mm in diameter.
Two deposition sources 6 and 8 are mounted on a
source ?ange 7 such that their deposition charges are
about 20 cm from the axis 5. Deposition source 6 is pure
silicon and deposition source 8 is a germanium doped to
and SiqgGeol materials because the band gap of Si is 1.1
and the band gap of the Si0_gGe0_2 alloy is 1.05 eV. The
ZT of the Si/Sio.8Gco_2 material at room temperature is
about 1.5. At higher temperature, such as 500° C., the
ZT would be approximately 2.5 and at 1000° C. the ZT
would be 6.5, providing Z remained constant with tem
perature.
Similar High Temperature Lattices
~1016 carriers per cc. The rotating platen is positioned 25
23 cm above the sources. An Airco Temescal electron
beam is used for evaporation. We use one 150 cc source
of pure silicon and a 40 cc source of germanium. We
temperatures for very long periods (i.e., above about
of about 0.254 cm. which is the thickness needed for a
generated on bulk B-C alloys one should be able to
The Si/SiGe superlattice is not stable at very high
500° C.); therefore, there is a need for a similar superlat
alternate the beams so layers of silicon only and silicon
tice which can be operated at these high temperatures.
and germanium are deposited. Dopants may be mixed 30 Boron and Boron-Carbon alloys are also expected to
with the germanium.
perform as excellent P type quantum well materials.
The apparatus is computer controlled to evaporate
The same alpha rhombohedral crystal structure exists
the sources alternatively at intervals appropriate to
over a wide range of composition from B4C to B110. As
achieve the desired thicknesses while the platen rotates
the B content is increased in going from B4C to B11C
above. Layer thicknesses are monitored by two electro 35 the B atoms substitute for C atoms. As a result of this
luminescent deposition meters 9 at the side of platen 4.
progressive change in composition without a change in
Layers will continue to build on the substrates until we
structure it should be possible to grow eptaxial layers of
have a wafer with about‘ 250,000 layers and a thickness
various B-C compositions on one another. From data
preferred thermoelectric device. The wafer is then 40 fabricate a quantum well device by using compositions
diced into chips as indicated in FIG. 2.
close to BuC as the insulating layer and compositions
Making the p-Type Material
close to B4C as the more conducting layer that is the
are electrically shorted. The n and p couples are electri
temperatures and not be subject to degradation by adja
cent layers diffusing together with time. This annihila
quantum well. Also pure alpha boron which is rhombo
We make the p-type material exactly as discussed
hedral could be used as the insulating layer. The BC
above except we use a p-type dope for the SiGe layers. 45 alloys are of further interest because these alloys exhibit
These layers are boron doped with 1016 carriers per cc.
extremely low diffusion rates at temperatures at which
they would be used as thermoelectric materials. For
Making the Thermoelectric Element
example, the B-C alloys of interest melt at temperatures
The p-type and the n-type chips are formed into an
in excess of 2400° C. yet they will only be operated up
egg crate con?guration in a manner standard in the
to about 1100" C. By using materials such as B-C alloys
industry. Metal contacts are applied then all n and p legs
the quantum well layers will remain intact at elevated
cally isolated by lapping the surface until the insulating
egg crate is visible and a series circuit of n and p ele
tion of the two adjacent layers via diffusion is of serious
ments is produced. In FIG. 3 elements shown are n-type 55 concern with the lower melting alloys such as Si/SiGe,
thermoelectric elements 10, p-type thermoelectric ele
ments 12, aluminum electrical connectors 14, eggcrate
electrical barrier 16 and molybdenum diffusion barrier
18. FIG. 4 shows how the chips are cut from the silicon
substrate. FIG. 5 is another view showing how the n 60
elements 10 and p elements 12 are connected to produce
electric power from hot and cold sources. Arrows 30
show current ?ow. Insulators are shown as 22 and elec
PbTe based alloys, and (Bi,Sb)2(Se,Te)3 based alloys
and limits their usefulness in high temperature applica
tions.
While the above description contains many speci?cit
ies, the reader should not construe these as limitations
on the scope of the invention, but merely as exempli?ca
tions of preferred embodiments thereof. For example,
the SiGe ratio could be any composition between about
trical conductors are shown as 14.
5 percent Ge to about 95 percent Ge; however, the
65 preferred composition is between about 10 percent Ge
Test Results
and about 40 percent Ge. Those skilled in the art will
Materials produced in accordance with the teachings
envision many other possible variations are within its
of this invention have been tested by the inventors. The
scope. Accordingly the reader is requested to determine
5
5,436,467
the scope of the invention by the appended claims and
their legal equivalents, and not by the examples which
6
5. A thermoelectric material as in claim 4 wherein
said conducting material is doped with phosphorous.
have been given.
6. A thermoelectric material as in claim 4 wherein
said conducting material is doped with boron.
I claim:
5
7. A thermoelectric material as in claim 1 wherein
1. A thermoelectric material comprised of:
said
crystal structure is alpha rhombohedral.
a plurality of alternating layers of at least two differ
8. A thermoelectric material as in claim 4 wherein at
ent semiconducting materials, one of said material
least one of said two different materials is an alloy of
de?ning a barrier material and another of said ma
boron and carbon.
terials de?ning a conducting material,
9. A thermoelectric material as in claim 1 wherein at
said barrier material and said conducting material
least two of said at least two different materials are both
alloys of boron and carbon.
having the same crystalline structure,
10. A thermoelectric material comprised of:
said barrier material and said conducting material
at least 100 alternating layers of at least two different
having band gaps, the band gap of said barrier
semiconducting materials, one of said materials
material being higher than the band gap of 'said 15
de?ning a barrier material and another of said ma
conducting material,
said conducting material being doped to create con
ducting properties,
said layer arrangement of said at least two different 20
materials creating a superlattice and quantum wells
within said layers of said conducting material.
2. A thermoelectric material as in claim 1 wherein
said crystal structure is cubic.
3. A thermoelectric material as in claim 2 wherein 25
said barrier material is silicon and said conducting mate
rial is silicon germanium.
terials de?ning a conducting material, each layer of
at least 50 layers of said conducting material being
less than 100 A thick,
said barrier material and said conducting material
having the same crystalline structure,
said barrier material and said conducting material
having band gaps, the band gap of said barrier
material being higher than the band gap of said
conducting material,
said conducting material being doped to create con
ducting properties,
4. A thermoelectric material as in claim 3 wherein the
concentration of germanium in said conducting material 30
is between 10 percent and 40 percent.
said layer arrangement of said at least two different
materials creating a superlattice and quantum wells
within said layers of said conducting material.
*
35
45
50
55
65
t
*
t
i