USOO6403874B1 (12) United States Patent (10) Patent No.: US 6,403,874 B1 Shakouri et al. (45) Date of Patent: *Jun. 11, 2002

(54) HIGH-EFFICIENCY HETEROSTRUCTURE OTHER PUBLICATIONS THERMONIC COOLERS N N. W. Ashcroft, et al., Solid State Physics, manual, 1976, pp. (75) Inventors: Ali Shakouri, Santa Cruz; John E. 318-319, 320-321, 362-363. Bowers, Santa Barbara, both of CA D. A. Broido et al., “Effect of Superlattice structure on the (US) th ermoelectriclectric fiIlgure OIf meril:t: , Theline Am erican PhPinySIca 1 Society (Physical Review B.), vol. 51, No. 19, May 15, (73) Assignee: The Regents of the University of 1995, pp. 13797-800. California, Oakland, CA (US) D. A. Broido et al., “Comment of Use of quantum well Superlattices to obtain high figure of merit from nonconven (*) Notice: Subject to any disclaimer, the term of this tional ”, Appl. Phys. Lett. 63, 3230 patent is extended or adjusted under 35 (1993), Applied Physics Letters, vol. 67, No. 8, Aug. 21, U.S.C. 154(b) by 0 days. 1995, pp. 1170–1171. D. A. Broido et al., “Thermoelectric figure or merit of This patent is Subject to a terminal dis- quantum wire Superlattices”, Applied Physics Letters, Jul. 3, claimer. 1995, vol. 67, No. 1, 100-102. (21) Appl. No.: 09/441,787 (List continued on next page.) (22) Filed: Nov. 17, 1999 Primary Examiner—Bruce F. Bell ASSistant Examiner Thomas H Parsons Related U.S. Application Data (74) Attorney, Agent, or Firm-Gates & Cooper LLP (60) Pisional application No. 60/109,342, filed on Nov. 20, (57) ABSTRACT 7 A heterostructure thermionic cooler and a method for mak (51) Int. Cl." ...... H01L 35/34 ing thermionic coolers, employing a barrier layer of Varying conduction bandedge for n-type material, or varying Valence (52) U.S. Cl...... 20 136201 136.203; 1625; bandedge for p-type material, that is placed between two 252/62.3 T. 438/22. 438/As 257/26.25 7/185; layers of material. The barrier layer bandedge is at least kT 257/191; 257/442; 257/443; 257/449; 257/467 higher than the Fermi level of the semiconductor layer, which allows only selected, “hot” electrons, or electrons of (58) Field of Search 2005). ------136,293, 205, high enough energy, acroSS the barrier. The barrier layer is 136/236.1, 238, 239, 240; 252/623 T; 257/467, constructed to have an internal electric field Such that the 185, 191, 196, 26, 442, 443, 449, 930, electrons that make it over the initial barrier are assisted in 15; 438/22, 54, 478, 466 travel to the anode. Once electrons drop to the energy level (56) References Cited of the anode, they lose energy to the lattice, thus heating the lattice at the anode. The barrier height of the barrier layer is U.S. PATENT DOCUMENTS high enough to prevent the electrons from traveling in the 4,353,081 A 10/1982 Allyn et al...... 257/191 reverse direction. 4,694,318 A 9/1987 Capasso et al...... 257/21 5,955,772 A 9/1999 Shakouri et al...... 257/467 51 Claims, 7 Drawing Sheets

30 26 24 d J2 d H 221 GaAs As GaAs GRADED A. Ga, X US 6,403,874 B1 Page 2

OTHER PUBLICATIONS L.W. Whitlow, et al., “Superlative applications to thermo P.J. Lin-Chung, et al., “Thermoelectric figure of merit of electricity”, Journal of Applied Physics, Nov. 1, 1995, vol., composite Superlattice Systems, Physical Review B (con 78, No. 9, 5460-5466. densed matter), vol. 51, No. 19, May 15 1995, pp 1324.4–8. G. D. Mahan, et al., “Thermionic refrigeration”, Journal of A. J. Dekker, “Thermionic Emission”, McGraw-Hill Ency clopedia of Science & Technology, 6' Edition, 1987, vol. Applied Physics, vol. 76, No. 7, Oct. 1, 1994, 4362–6. 18, pp. 272-273. Sofo.J.O., “Thermoelectric figure of merit of Superlattices”, L. D. Hicks, et al., “Effect of quantum-well strucutres on the Applied Physics Letters, vol. 65, No. 21, Nov. 21, 1994, thermoelectric figure of merit”, Physical Review B (con 2690-2. densed matter), vol. 47, No. 19, May 15, 1993, pp. 12727-31. D.M. Rowe, et al., “Multiple Potential Barriers as a Possible L. D. Hicks, et al. “Thermoelectric figure of merit of a Mechanism to Increase the Seebeck Coefficient and Elec one-dimensional conductor”, Physical Review B (con trical Power Factor', Thirteenth International Conference on densed matter), vol. 47, No. 24, Jun. 15, 1993, 16631-4. Thermoelectrics, Kansas City, Mo. USA, Aug. 30-Sep. 1, L. D. Hicks, et al. “Use of Quantum-well Superlattices to 1994). obtain a high figure of merit from nonconventional thermo K. K. Ng, “Complete Guide to Semiconductor Devices.” electric materials”, Applied Physics Letters, vol. 63, No. 23, McGraw-Hill, 1995, pp. 48-55. Dec. 6, 1993, 3230–2. L. D. Hicks, et al., Experimental study of the effect of Mahan, G.D. et al., “Multilayer thermionic refrigerator and quantum-Well Structures on the thermoelectric figure of generator,” Journal of Applied Physics, vol. 83, No. 9, May merit, Phyical Review B (condensed matter), vol. 53.No. 1, 1998, pp. 4683–4689, XP-000956318. 16.R 10493-6. Mahan, G.D. et al., “Multilayer Thermionic Refrigeration,” J. M. Houston, “Theoretical efficiency of the thermionic energy converter”, Journal of Applied Phyics, vol. 30 No. 4, Physical Review Letters, vol. 80, No. 18, May 4, 1998, pp. Apr. 1959, pp. 481-487. 4016–4019, XP-000956428. L.W. Whitlow, et al., “Superlative applications to thermo Shakouri, Ali et al., “Heterostructure integrated thermionic electricity”, Journal of Applied Physics, Nov. 1, 1995, vol. coolers,” Applied Physics Letters, vol. 71, No. 9, Sep. 1, 78, No. 9, 5460-5466. 1997, pp. 1234–1236, XP-000720237. U.S. Patent Jun. 11, 2002 Sheet 1 of 7 US 6,403,874 B1

1 O ? 12 FIG. 1A

1 O FIG. 1B 12

1O

16 FIG. 1C 14 Z% W 12

1 O

FIG. 1D W F % U.S. Patent Jun. 11, 2002 Sheet 2 of 7 US 6,403,874 B1

FIG. 1 E.

3O 26 24 FIG. 2A de .. 22 GaAs GRADED AlGaAs GaAs

28

24 - d - 3O 22 FIG. 2B J2

46 42 38 FIG. 2C V 48 44 36 U.S. Patent Jun. 11, 2002 Sheet 3 of 7 US 6,403,874 B1

62 FIG. 2D 60, 64

68 66 7O

72

78 |y 74

76

FIG. 2E U.S. Patent Jun. 11, 2002 Sheet 4 of 7 US 6,403,874 B1

8O 86 90 92 94 V 34 83

FIG. 4

1O 52 -1 56 56 - U.S. Patent Jun. 11, 2002 Sheet 5 of 7 US 6,403,874 B1

V=0.065V, i =300K, AT= 10K, m/m=004, A = 1.5m /Vs, A=4W/mk

800

Ihickness (um) 3O4 FIG. 6

900 aaka/ 0.5 eyes \nses

904 906 N. Substrate 3. 902 595 Y 904

i 906 U.S. Patent Jun. 11, 2002 Sheet 6 of 7 US 6,403,874 B1

i

0 50 100 150 200 250 300 350 400 I (mA) FIG. 8

Energy axis 1200

12O2 AV kg Fermi energy Barrier height >> kg

FIG. 9A U.S. Patent Jun. 11, 2002 Sheet 7 of 7 US 6,403,874 B1

0.5 InGaAs/InAlAs

O4 12O4

O3 N 02 InGaAs/InP 1208

O. 1 Bulk InGaAs

>/-- - -

Doping (cm) FIG. 9B 1214

1212 i US 6,403,874 B1 1 2 HIGH-EFFICIENCY HETEROSTRUCTURE a need for more reliable electronic coolers. It can also be THERMONIC COOLERS Seen that there is a need for electronic coolers that reach lower temperatures. CROSS REFERENCE TO RELATED APPLICATIONS SUMMARY OF THE INVENTION This application is related to U.S. patent application Ser. The present invention minimizes the above-described No. 08/767,955, filed on on Dec. 17, 1996, entitled “HET problems by using bandgap engineering and modulation EROSTRUCTURE THERMIONIC COOLERS,” by Ali doping to fabricate Small thermionic coolers that operate at Shakouri, et al., now U.S. Pat. No. 5,955,772; and U.S. room temperature. By using proper materials and patent application Ser. No. 09/280,284, filed on Mar. 29, geometries, efficient and Space conserving thermionic cooler 1999, entitled “METHOD FOR MAKING HETERO elements which can reach lower temperatures are fabricated STRUCTURE THERMIONIC COOLERS,” by Ali Shak in a cost-effective manner. Further, these coolers can have a ouri et al., both now U.S. Pat. No. 6,060,331 both of which much faster response. applications are incorporated by reference herein. This 15 The present invention comprises a method and apparatus application also claims priority under 35 U.S.C. S119(e) of for theomionic cooling. The method of the present invention U.S. Provisional Pat. App. No. 60/109,342, filed Nov. 20, comprises growing two Semiconductor layers. The Second 1998, entitled “HIGH-EFFICENCY HETEROSTRUC layer has a variable conduction bandedge as a function of TURE THERMIONIC COOLERS, ” by Ali Shakouri et al., distance (for the case of electron transport) which has a which application is incorporated by reference herein. Fermi level on the order of kT from the bandedge of the Semiconductor layer. This structure allows for Selective STATEMENT REGARDING FEDERALLY thermionic emission of high energy carriers from cathode to SPONSORED RESEARCH AND anode that Suppresses the reverse current, and creates a cold DEVELOPMENT junction at the cathode and a hot junction at the anode. Using This invention was made with Government support under 25 the same device in contact with a hot and a cold bath will Contract No. 442530-25845, awarded by the Office of Naval create a thermionic generator. Research. The Government has certain rights in this inven One object of the present invention is to provide better tion. electronic cooler fabrication techniques. It is a further object of the invention to reduce electronic cooler fabrication costs. BACKGROUND OF THE INVENTION It is a further object of the invention to make more efficient 1. Field of the Invention electronic coolers which reach lower temperatures. It is a This invention relates in general to electronic devices, and further object of the invention to make electronic coolers more specifically to the use of Semiconductor materials to that have a faster response time. fabricate thermionic coolers and generators. These and various other advantages and features of nov 35 elty which characterize the invention are pointed out with 2. Description of Related Art particularity in the claims annexed hereto and form a part The use of electronics to transport heat to and away from hereof. However, for a better understanding of the invention, certain areas has expanded in recent years due to increased its advantages, and the objects obtained by its use, reference packing densities and hostile environments. For cooling should be made to the drawings which form a further part applications, thermoelectric (TE) coolers have been used to 40 hereof, and to accompanying descriptive matter, in which cool areas both in electronic and nonelectronic applications. TE coolers typically include a p-type doped region alterna there are illustrated and described Specific examples of an tively connected to an n-type doped region, which creates apparatus in accordance with the invention. cooling effects at one metal-doped region junction and BRIEF DESCRIPTION OF THE DRAWINGS heating effects at the other metal-doped region junction, 45 depending on the direction of the current through the device. Referring now to the drawings in which like reference However, TE coolers are limited in their overall efficiency numbers represent corresponding parts throughout: by the bulk properties of the materials used in the TE cooler. FIGS. 1A-1E are diagrams of a first embodiment of the Further, the reliability of assemblies of many elements, such present invention; as in TE coolers, is often not Sufficient for many applica 50 FIGS. 2A-2E are graphs of the conduction bandedge of tions. The cost of TE coolerS has not dropped at the same devices made using the present invention; rate as other electronic devices Such as transistor circuits, FIG. 3 shows an alternative structure for the device; lasers and detectors, because TE cooler elements are not FIG. 4 shows a cascaded device using Stages with differ fabricated using high Volume planar integrated circuit tech ent bandedge discontinuities, nology. Further, TE coolers that can generate a large cooling 55 effect tend to be large devices, typically 1 cm x1 cm or larger, FIG. 5 shows a combination of n-doped and p-doped and are Slow cooling devices, and thus, are not acceptable in devices of the present invention; Small electronic devices. FIG. 6 illustrates indicates the potential of Heterostructure Integrated Thermionic (HIT) devices to achieve high cool From the foregoing, it can be seen then that there is a need ing power densities, for improved electronic coolers. It can also be seen then that 60 there is a need for better electronic cooler fabrication tech FIG. 7 displays the surface temperature on top and on the niques. It can also be seen that there is a need for low cost bottom of the device as well as the Substrate temperature far electronic coolers. It can also be seen that there is a need for away from the device as a function of current; more Space efficient electronic coolers. It can also be seen FIG. 8 illustrates the measured cooling on top of the that there is a need for more energy efficient electronic 65 device at various Substrate temperatures, coolers. It can also be seen that there is a need for coolers FIG. 9A illustrates the Fermi levels of a large barrier with faster response times. It can also be seen that there is height device in accordance with the present invention; US 6,403,874 B1 3 4 FIG.9B illustrates the predicted thermoelectric figure-of mionic emission by using bandedge discontinuity between merit for bulk InGaAs and for large barrier InGaAs/InP and various compounds. The accurate epitaxial growth of thin InGaAS/InAIAS Superlattices as a function of doping as used and uniform layers in conjunction with modulation doping in the present invention; and helped to minimize the problem of Space charge which limits FIG. 9C illustrates the predicted net cooling as a function the operation of vacuum thermionic diodes at low tempera of current for the high barrier heterostructure thermionic tureS. coolers of the present invention. The present invention uses thermionic emission in highly degenerate tall barrier Superlattices. This combines the DETAILED DESCRIPTION OF THE advantages of large density of States with Selective emission INVENTION of hot electrons. In the description of the preferred embodiment, reference Thermoelectric Cooler Background is made to the accompanying drawings which form a part Electron conduction in a Solid is affected by the tempera hereof, and in which is shown by-way of illustration the ture and the temperature gradient. This interaction between Specific embodiment in which the invention may be prac the “electrical' current, e.g., the amount of charge trans ticed. It is to be understood that other embodiments may be 15 ported by electrons I, and the “thermal” current, e.g., the utilized as Structural changes may be made without depart amount of heat transported by electrons Q, has been used for ing from the Scope of the present invention. various applications Such as thermoelectric cooling (I to Q) Overview thermoelectric generation (Q to I) and thermal (bolometric) The present invention uses thermionic emission in Semi detectors (AT to V). conductor heterostructures for heat pumping and cooling of The periodicity of crystalline Solids allows a description high power electronic and optoelectronic devices. These of electron movement in a complicated Voltage potential of integrated micro-coolers can improve the efficiency and many atoms, using Some parameterS Such as bandgap, lifetime of electrical and optoelectronic components. The effective mass, etc. In a point-particle picture, localized thermionic coolers could also be used as an additional means Scattering events can be assumed with acoustic and optical for tuning temperature Sensitive devices. 25 phonons, along with various impurities, and coherent Scat The coolers that are commercially available are typically tering events can be neglected. The electron motion can be thermoelectric (TE) coolers, based on the Peltier effect at the adequately described and modeled using the electronic dis junction of two dissimilar materials. TE coolers use material tribution function and the Boltzmann Transport Equation bulk properties, Such as the Seebeck coefficient, electrical, (BTE). and thermal conductivity, and are mostly based on Bismuth Thermoelectric Cooler Modeling (Bi-Te) for room temperature applications. The A Single element TE cooler is composed of two branches, basis of the heterostructure integrated thermionic (HIT) one branch of n-doped and one branch of p-doped material. cooler described herein is to use bandstructure engineering The two branches are connected electrically in Series and to increase the cooling power and efficiency. thermally in parallel. When the current is flowing from n to Recent proposals to use quantum wells, quantum wires, 35 p, e.g., electrons are moving from the p-branch to a metallic and Superlattice Structures to increase the TE cooler figure of contact between the branches and then to the n-branch, the merit can be divided into two categories. The first category heat is absorbed at the junctions p-metal and metal-n. changes the density of electronic States of the cooler mate Electrons in the p-branch have an average transport energy rials to make it more "peaked’ and also more asymmetric Smaller than the Fermi energy, and the ones in the n-branch with respect to the Fermi energy. This increases the electrical 40 have an average transport energy larger than the Fermi power factor, So, and thus the TE cooler figure of merit energy. Metals are considered to have their average transport Z=S’ O/B, where S is the Seebeck coefficient or energy equal to their Fermi energy. thermopower, O is the electrical conductivity, and B is the In a perfect ohmic conduction from the p-branch to metal thermal conductivity. to the n-branch, electrons should absorb energy in the form The Second category uses perpendicular transport of elec 45 of heat to increase their average energy. The same argument trons in Small barrier Superlattices, e.g., where the barrier can be applied to the contacts at the outside ends of the height is on the order of kT, in a way that modifies the branches where the heat is generated. The heat absorption or mobility of low energy electrons with respect to high energy generation occurs at distances very close to the contacts, on electrons. This asymmetry also increases the electrical the order of electron average Velocity times its thermaliza power factor. 50 tion time constant. This heat absorption or generation, called Both methods are expected to only give moderate the Peltier effect, is a reversible thermodynamic phenomena, improvements when various non-ideal effects, Such as the depending on the direction of the current flow. role of barriers and finite level widths, are included in the Thermionic Coolers final models and devices, as shown in papers written by To create a heterostructure integrated thermionic (HIT) Mahan (Appl. Phys. Lett. 65(21) p. 2690, 1994) and Rowe 55 cooler, one uses precise control of layer thickneSS and (13th International Conference on Thermoelectrics, Kansas composition, achieved by molecular beam epitaxy (WBE), City, Mo., 1994, p. 339), which are herein incorporated by metal-organic chemical vapor deposition (MOCVD), or reference. other growth techniques, in conjunction with bandgap engi U.S. Pat. No. 5,955,772 proposed the use of thermionic neering to allow for the design of Specific conduction or emission in heterostructures. Thermionic emission from 60 Valence band profiles within a device. The use of a typically metallic plates into a vacuum or gas filled diode is a key higher bandgap material between two lower bandgap technology for the conversion of heat into electricity at high materials, the two lower bandgap materials comprising the temperatures (>1000 K). If metals with very low work cathode and the anode, will produce a barrier for electrons functions were available and could be placed at Small or holes as they travel from cathode to anode. Thermionic distances apart, the Same principle would make a thermionic 65 emission of carriers over this barrier Selectively removes refrigerator at room temperatures. U.S. Pat. No. 5,955,772 high energy carriers. The Strong electron-electron interac also used Semiconductor heterostructures to tailor the ther tion at the cathode will tend to restore quasi-Fermi distri US 6,403,874 B1 S 6 bution by absorbing heat from the lattice. Electrons that antimonide (GanSb), (Bi-Te), and reach the anode will lose their energy by generating heat. By bismuth Selenide (Bi-Sea) or other ternary or quaternary choosing the appropriate bandedge discontinuities at the materials, where the Subscripts X, y, 1-X, and 1-y denote the cathode and anode, which are typically 0 to 1 eV, the reverse relative amounts of the atomic Species in each ternary or current is Suppressed and cooling is achieved at room quartenary material and range from Zero to one, inclusive. temperatures. Further, layer 12 maybe doped n-type or p-type, or can be a Depending on the growth constraints and lattice mismatch metal layer. between materials, the barrier composition can be graded or FIG. 1B shows device 10 being constructed by adding modulated to produce internal fields and to enhance electron layer 14 on top of layer 12. Layer 14 is a barrier layer for transport properties. In the case of a vacuum diode, the device 10, and for layer 12 consisting of GaAs, layer 14 is problem of Space charge, which is the presence of charged typically a graded AlGaAS layer. Layer 14 creates a slight electrons in the Space between the cathode and the anode, internal electric field throughout the thickness of layer 14 to will create an extra potential barrier for the current going eliminate the Space charge problem around the cathode, from the cathode to the anode, further limits the low which, in this case, is layer 12. Layer 14 is typically grown temperature cooling or power generation applications. Using 15 by MBE or MOCVD techniques, but can be grown in other heterostructure thermionic coolers, close and uniform Spac ways. If layer 14 is n-type, layer 14 has a conduction ing of cathode and anode is less of a problem, and is bandedge that increases as a function of the distance from controlled by accurate crystal growth technologies. the layer 12, e.g., the value of the conduction bandedge at a Furthermore, doping the barrier material (modulation first distance from the layer 12 is more than the value of the doping) can be used to create internal fields, modify the conduction bandedge at a Second, greater distance from the electron flow, and control Space charge effects. layer 12. The layer 14 conduction bandedge increases for By using the thermionic effect, instead of the thermoelec layerS 14 that are doped n-type, corresponding p-type doped tric effect, two immediate advantages are evident. The TE layerS 14 will have a Valence bandedge that is decreasing. cooler materials are restricted to those materials that have The conduction bandedge of the layer 14 can be monotoni high electrical conductivity and thermopower, and low ther 25 cally increasing, Stepped, or piecewise linear, or any other mal conductivity. These materials then only produce cooling shape, So long as at Some point in the layer 14 the conduction or heating at the junction between two materials, with bandedge has a level higher than the conduction bandedge of different Seebeck coefficients. the layer 12. Layer 14 can also be a Strained layer, to Heterostructure integrated thermionic cooling, on the increase the bandedge offset between layer 12 and layer 14. other hand, does not have a requirement for high ther For layers 12 that are SiGe , layer 14 can be a silicon mopower materials. Bandedge discontinuities, also known germanium (SiGe), layer where y>X, or x>y depending as Schottky barriers, between the anode and cathode will on the strain in the SiGe layer. For layers 12 that are perform the cooling or heating as needed. One has to find the InGaASP-1, layer 14 is typically InGaAS, barrier material that has high electrical conductivity and low P, 2, where x2