JP2010192580A - Thermoelectric conversion element and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelectric conversion element that can be produced with comparative ease, which has high electrical conducting properties and has low thermal conductivity and a high thermoelectric conversion efficiency, as well as, to provide a method of manufacturing the element. <P>SOLUTION: On an SiO<SB>2</SB>substrate 21, an Si film 22, into which a p-type or n-type impurity is introduced, is formed into a thickness, for instance, from 3 to 160 nm. After that, ion implantation equipment is used to form a wall 23 made of the SiO<SB>2</SB>onto the Si film 22 into a stripe shape, and then the surface of the Si film 22 is oxidized to form a SiO<SB>2</SB>film 24; by so doing, a silicon nano-wire is formed that is made of long and thin Si surrounded by the SiO<SB>2</SB>; and after repeatedly carrying out these steps to form a laminated structure, it is cut into a predetermined size to make a semiconductor block. A p-type semiconductor block and an n-type semiconductor block formed in this way are combined, to form a thermoelectric conversion element. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thermoelectric conversion element that converts heat into electricity and a method for manufacturing the same.

  Thermoelectric conversion elements that convert heat into electricity have been developed, and as one of the measures to prevent global warming, the heat discharged from data centers, factories, or homes is converted into electricity using thermoelectric conversion elements and re-generated. Proposed to use.

  The thermoelectric conversion element is formed by joining two types of conductors or semiconductors having different compositions. When the two junctions of these conductors or semiconductors are kept at different temperatures, an electromotive force is generated between the junctions due to the Seebeck effect. In a general thermoelectric conversion element, two conductors or semiconductors having different compositions are joined as one cell, and a large electromotive force is obtained by connecting a plurality of cells in series.

Japanese translation of PCT publication No. 2002-540636

Allon I. Hochbaum et al., Enhanced thermoelectric performance of rough silicon nanowires, NATURE Vol. 451/10 January 2008

By the way, the thermoelectric figure of merit Z (K ⁻¹ ) serving as an index of the thermoelectric conversion efficiency is expressed by the following equation (1).

Z = α ² / ρκ (1)
Here, α is the Seebeck coefficient (V / K), ρ is the electrical resistivity (Ωm), and κ is the thermal conductivity (W / mK).

  As can be seen from the equation (1), in order to increase the thermoelectric conversion efficiency, the electrical conductivity is high (that is, the value of the electrical resistivity ρ is small) and the thermal conductivity is low (that is, the thermal conductivity κ is low). Thermoelectric conversion element material (small value) is required. However, there is a correlation between the electrical conductivity and thermal conductivity of the material itself. In general, a substance having high electrical conductivity has high thermal conductivity.

It is known that a relatively large electromotive force can be obtained by using a BiTe-based metal (for example, Bi ₂ Te _2.85 Se _{0.15 as} an n-type element material and Bi _0.5 Sb _1.5 Te ₃ as a p-type element material) as a material of a thermoelectric conversion element. It has been. However, since there is a concern that a material containing a heavy metal as a main component may give a large load to the environment, it is preferable to form a thermoelectric conversion element using a material that does not contain a heavy metal as a main component.

  On the other hand, a method of forming silicon nanowires in an aqueous solution using an electroless etching method has been proposed, and the thermoelectric figure of merit Z may be about 0.6 in a silicon nanowire having a diameter of 50 nm prepared by this method. It has been reported. It is considered that a thermoelectric conversion element having high thermoelectric conversion efficiency can be obtained by forming a thermoelectric conversion element by bundling a large number of silicon nanowires. However, this requires a complicated manufacturing process.

  In view of the above, an object is to provide a thermoelectric conversion element that can be manufactured relatively easily, has high electrical conductivity, low thermal conductivity, and high thermoelectric conversion efficiency, and a method for manufacturing the same.

  According to one aspect, in a method of manufacturing a thermoelectric conversion element having a p-type semiconductor block, an n-type semiconductor block, and a connection portion that electrically connects the p-type semiconductor block and the n-type semiconductor block, Forming a silicon film into which a p-type impurity or an n-type impurity is introduced, and implanting oxygen into the silicon film to form a first silicon oxide film in a stripe shape, and the silicon film is formed into a plurality of regions. And a step of oxidizing the surface of the silicon film to form a second silicon oxide film, and forming a silicon wire surrounded by the first silicon oxide film and the second silicon oxide film. A method of manufacturing a thermoelectric conversion element, comprising: a step of cutting the substrate on which the silicon wire is formed to form the p-type semiconductor block or the n-type semiconductor block It is provided.

  In the present application, the silicon wire refers to wire-like silicon, and includes silicon nanowires having a nanometer diameter (or width).

  According to the above aspect, oxygen is implanted into the silicon film to form a first silicon oxide film in a stripe shape, and the surface of the silicon film is oxidized to form a second silicon oxide film. Separated into a plurality of elongated silicon (silicon wires). By producing a silicon wire by such a method, a semiconductor block composed of a large number of silicon wires can be easily formed. Since silicon wires have a high thermoelectric figure of merit, thermoelectric conversion elements formed of semiconductor blocks including a large number of silicon wires have good thermoelectric conversion efficiency. In addition, since heavy metals are not used as the main raw material, the burden on the environment is small.

Drawing 1 is a mimetic diagram showing the structure of the thermoelectric conversion element concerning an embodiment. FIG. 2 is a schematic diagram showing one cell of the thermoelectric conversion element. Drawing 3 (a) and (b) is a sectional view and a top view (the 1) showing a manufacturing method of a thermoelectric conversion element concerning an embodiment. 4A and 4B are a cross-sectional view and a plan view (part 2) illustrating the method for manufacturing the thermoelectric conversion element according to the embodiment. 5A and 5B are a cross-sectional view and a plan view (part 3) illustrating the method for manufacturing the thermoelectric conversion element according to the embodiment. 6A and 6B are a cross-sectional view and a plan view (part 4) illustrating the method for manufacturing the thermoelectric conversion element according to the embodiment. 7A and 7B are a cross-sectional view and a plan view (No. 5) illustrating the method for manufacturing the thermoelectric conversion element according to the embodiment.

  Hereinafter, embodiments will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram illustrating a structure of a thermoelectric conversion element according to an embodiment.

  The thermoelectric conversion element 10 according to the present embodiment has a structure in which a plurality of p-type semiconductor blocks 11 and a plurality of n-type semiconductor blocks 12 are sandwiched between two heat transfer plates 14a and 14b. The p-type semiconductor blocks 11 and the n-type semiconductor blocks 12 are alternately arranged and are electrically connected in series by conductors 13 provided on the surfaces of the heat transfer plates 14a and 14b. In addition, electrodes 15a and 15b for taking out electric power are provided at both ends of an assembly of the p-type semiconductor block 11 and the n-type semiconductor block 12 connected in series.

FIG. 2 is a schematic diagram showing one cell of the thermoelectric conversion element 10. Each of the p-type semiconductor block 11 and the n-type semiconductor block 12 has a structure in which a number of elongated thread-like silicons are bundled. A p-type impurity (B (boron), Al (aluminum), Ga (gallium), In (indium), etc.) is introduced into the silicon of the p-type semiconductor block 11, and an n-type impurity (P (Phosphorus), As (arsenic), Sb (antimony), etc.). Hereinafter, the thread-like silicon in the semiconductor blocks 11 and 12 is referred to as a silicon nanowire (silicon wire) 16. The impurity concentration in the silicon nanowire 16 of the p-type semiconductor block 11 and the n-type semiconductor block 12 is, for example, about 10 ¹⁵ to 10 ²¹ cm ⁻³ .

The silicon nanowires 16 have different thicknesses (widths) at positions in the length direction. The silicon nanowires 16 are separated from each other by an insulating silicon oxide (SiO ₂ ) 17 interposed therebetween. However, the silicon nanowires 16 do not have to be completely separated, and adjacent silicon nanowires 16 may be partially connected to each other.

  In the thermoelectric conversion element 10 having such a structure, when a temperature difference is given to the two heat transfer plates 14a and 14b, a potential difference is generated between the p-type semiconductor block 11 and the n-type semiconductor block 12 due to the Seebeck effect. A current can be taken out from the electrodes 15a and 15b.

In order to obtain high thermoelectric conversion efficiency, the p-type semiconductor block 11 and the n-type semiconductor block 12 need to have low electrical resistivity ρ and high thermal conductivity κ. In this embodiment, impurities are introduced into the silicon nanowire 16 at a concentration of about 10 ¹⁵ to 10 ²¹ cm ⁻³ , and the electrical resistivity ρ per silicon nanowire 16 is about 10 to 10 ⁻⁶ Ωcm. is there. Further, in the present embodiment, the semiconductor blocks 11 and 12 are formed of a large number of silicon nanowires 16 that are thin and whose thickness (width) changes at positions in the length direction. For this reason, the phonons transmitted through the silicon nanowires 16 are scattered in a complicated manner at the interface between the silicon nanowires 16 and the silicon oxide 17 and the thermal conductivity is lowered. Thereby, the thermoelectric figure of merit Z of thermoelectric conversion element 10 becomes large, and heat can be efficiently converted into electricity.

  Hereinafter, the manufacturing method of the thermoelectric conversion element which concerns on embodiment is demonstrated. Initially, the manufacturing method of the p-type semiconductor block 11 is demonstrated with reference to FIGS.

First, as shown in a sectional view in FIG. 3A and a plan view in FIG. 3B, a SiO ₂ substrate 21 having a surface made of SiO ₂ is prepared. In order to reduce the thermal conductivity of the semiconductor blocks 11 and 12, it is effective to scatter phonons transmitted through the silicon nanowires 16. For this purpose, the surface of the SiO ₂ substrate 21 is not completely flat, and is deposited on this surface. It is preferable that there is an unevenness of 5 to 40% of the thickness of the Si portion.

Next, as shown in a sectional view in FIG. 4A and a plan view in FIG. 4B, for example, on the SiO ₂ substrate 21 by CVD (Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy). A Si film (p-type Si film) 22 into which a p-type impurity having a thickness of 3 to 160 nm is introduced is formed. The impurity concentration of the Si film 22 is about 10 ¹⁵ to 10 ²¹ cm ⁻³ as described above.

When the p-type Si film 22 is formed by the CVD method, for example, the pressure in the chamber is maintained at 0.25 atm (about 2.53 × 10 ⁴ Pa) or less. Then, a mixed gas of SiCl ₄ and B ₂ H ₆ is introduced into the chamber and reacted at a temperature of 120 ° C. to grow a p-type Si film 22 on the substrate 20 (SiO ₂ film 21).

When the p-type Si film 22 is formed by the MBE method, for example, Si ₂ H ₆ (disilane gas) or a solid Si source is used, the temperature is 500 ° C., the pressure is 5 × 10 ⁻⁷ Torr (6.65 × 10 ^−5). A silicon film is grown on the substrate 20 (SiO ₂ film 21) under the condition of Pa). The dopant heats an individual solid dopant element and supplies it as atomic vapor. If an atom heavier than Si such as Ga or In is used as a dopant, phonon scattering can be further increased and thermal conductivity can be more efficiently reduced.

Next, as shown in a cross-sectional view in FIG. 5A and a plan view in FIG. 5B, oxygen is implanted into the Si film 22 using an ion implantation apparatus, and SiO formed by combining Si and oxygen. _Two walls (first silicon oxide film) 23 are formed in a stripe shape. The interval between the SiO ₂ walls 23 is, for example, 3 to 100 nm, and the width of the SiO ₂ wall 23 is, for example, 1/8 layer to 100 nm. Note that the 1/8 layer is a layer in which oxygen is bonded to the silicon monolayer at a rate of 1/8 (one oxygen atom per 8 Si atoms), and it is said that insulation can be secured. The minimum thickness.

The width of SiO ₂ wall 23 need not be uniform, varies in thickness 5-40% of the range of the width, for example, SiO ₂ wall 23 of SiO ₂ wall 23 in order to scatter phonons efficiently Preferably it is. Further, the acceleration voltage at the time of ion implantation may be set according to the thickness of the Si film 22, for example, 2.5 to 120 keV. However, the lower end of the SiO ₂ wall 23 only needs to reach the lower SiO ₂ film 21, and the acceleration voltage need not be precisely controlled.

Next, as shown in a cross-sectional view in FIG. 6A and a plan view in FIG. 6B, the surface of the Si film 22 is oxidized at a temperature of 700 to 1000 ° C. in an oxygen-containing atmosphere. A SiO ₂ film (second silicon oxide film) 24 having a thickness of about 100 nm to 100 nm is formed. At this time, the oxidation may be performed so that the thickness of the Si film 22 remaining after the oxidation becomes, for example, 3 to 100 nm. Unless the flatness is particularly controlled, irregularities are naturally formed in the natural oxide film (SiO ₂ film) formed on the surface of the Si film 22. However, it is preferable to select the formation conditions such that the unevenness falls within a range of about 5 to 40% of the thickness of the Si film deposited thereon.

Thus, a large number of silicon wires 16 separated by the SiO ₂ films 21 and 24 and the SiO ₂ walls 23 (corresponding to the silicon oxide 17 in FIG. 2) are formed in the Si film 22.

Thereafter, a step of forming a Si film 22 doped with p-type impurities on the SiO ₂ film 24 to a thickness of 3 to 160 nm (see FIG. 4), a step of forming the SiO ₂ wall 23 (see FIG. 5), And the step of forming the SiO ₂ film 24 (see FIG. 6) are repeated several to several tens of times, and finally, the entire periphery of the laminated structure is left in the atmosphere until oxidation or a natural oxide film is formed. Thus, a laminated structure is formed as shown in a sectional view in FIG. 7A and a plan view in FIG.

Thereafter, the laminated film made of SiO ₂ and Si is cut by a substrate cutting device (such as a dicer), and the p-type semiconductor block 11 having a size of, for example, 1 mm × 1 mm × 10 mm (the length direction of the silicon nanowire) (see FIG. 2).

The n-type semiconductor block 12 is formed in the same manner as the above-described p-type semiconductor block. However, pentavalent impurities such as P, As, or Sb are used as impurities introduced into the Si film 22. For example, when forming a Si film by the CVD method, a gas such as PH ₃ or AsH ₃ is mixed with SiCl ₄ gas. Further, when forming a Si film by the MBE method, atoms heavier than Si such as As or Sb are used as a dopant.

  After the p-type semiconductor block 11 and the n-type semiconductor block 12 having a predetermined size are formed in this way, as shown in FIG. 1, the p-type semiconductor block is formed on the heat transfer plate 14a on which the conductor pattern 13 is formed. 11 and the n-type semiconductor block 12 are arranged side by side, and the semiconductor blocks 11 and 12 and the conductor pattern 13 of the heat transfer plate 14a are mechanically and electrically connected by an adhesive (high thermal conductive adhesive). At this time, the semiconductor blocks 11 and 12 are arranged such that the length direction of the silicon nanowire 16 is the height direction.

  Next, the heat transfer plate 14b is disposed on the semiconductor blocks 11 and 12, and the conductor pattern of the heat transfer plate 14b and the semiconductor blocks 11 and 12 are mechanically and electrically joined by an adhesive (high thermal conductive adhesive). To do. In addition, lead electrodes 15a and 15b are formed on both ends of the assembly of the semiconductor blocks 11 and 12 by, for example, silver paste. Thus, the thermoelectric conversion element according to the present embodiment is completed.

In the thermoelectric conversion element according to the present embodiment, since a current flows through each silicon nanowire 16 separated by the SiO ₂ film and the SiO ₂ wall, the silicon nanowire 16 and the silicon oxide 17 (SiO ₂ films 21, 24 and Phonons are scattered at the interface with the SiO ₂ wall 23), and the thermal conductivity becomes lower than the electrical conductivity. Thereby, a high thermoelectric figure of merit is realized.

  In the present embodiment, the silicon nanowire 16 is formed by a simple process of silicon deposition and oxidation. Therefore, manufacture is easy. Further, these silicon nanowires 16 are formed integrally with the silicon oxide 17. Therefore, the process of bundling the individual silicon nanowires 16 is unnecessary, and the manufacturing is easy.

  According to this method, thermoelectric conversion materials using silicon nanowires can be easily and in large quantities by a simple process using abundant, inexpensive and safe materials.

DESCRIPTION OF SYMBOLS 10 ... Thermoelectric conversion element, 11 ... p-type semiconductor block, 12 ... n-type semiconductor block, 13 ... Conductor, 14a, 14b ... Heat-transfer plate, 15a, 15b ... Electrode, 16 ... Silicon wire, 17 ... Silicon oxide, 20 ... Substrate, 21 ... SiO ₂ substrate, 22 ... Si film, 23 ... SiO ₂ wall, 24 ... SiO ₂ film.

Claims (5)

In a method of manufacturing a thermoelectric conversion element having a p-type semiconductor block, an n-type semiconductor block, and a connection portion that electrically connects the p-type semiconductor block and the n-type semiconductor block.
Forming a silicon film doped with p-type impurities or n-type impurities on a substrate;
Implanting oxygen into the silicon film to form a first silicon oxide film in a stripe shape, and dividing the silicon film into a plurality of regions;
Oxidizing the surface of the silicon film to form a second silicon oxide film, and forming a silicon wire surrounded by the first silicon oxide film and the second silicon oxide film;
And a step of cutting the substrate on which the silicon wire is formed to form the p-type semiconductor block or the n-type semiconductor block.   The step of forming the silicon film, the step of forming the first silicon oxide film, and the step of forming the second silicon oxide film are repeated a plurality of times to form a laminated structure on the substrate, and then The method for manufacturing a thermoelectric conversion element according to claim 1, wherein a step of cutting the substrate is performed.   The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the substrate has a natural oxide film made of silicon oxide on a surface thereof. In a thermoelectric conversion element having a p-type semiconductor block, an n-type semiconductor block, and a connection portion that electrically connects the p-type semiconductor block and the n-type semiconductor block,
A thermoelectric conversion element, wherein at least one of the p-type semiconductor block and the n-type semiconductor block includes a plurality of silicon wires into which impurities are introduced and an oxide disposed between the silicon wires.   The thermoelectric conversion element according to claim 4, wherein the width of the silicon wire is in a range of 3 nm to 100 nm.

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