Thin film thermoelectric device

Abstract

The present invention realizes a thin film thermoelectric device with which output power can be improved. A thin film thermoelectric device (1) according to the present invention comprises a fine thermoelectric thin film (11), which is a thermoelectric conversion laminated thin film of a semiconductor, on a substrate (10). The fine thermoelectric thin film has a plurality of laminated structures (100), each of which has a first layer (111) that contains a first semiconductor at high purity, and a second layer (112) that has an impurity added to a second semiconductor having a band gap larger than that of the first semiconductor and supplies a carrier to the first layer. The first semiconductor and the second semiconductor contain GaAs or GaN, and the crystal plane orientations of the first semiconductor and the second semiconductor are the same.

Description

Thin-film thermoelectric devices

[0001] The present disclosure relates to thin film thermoelectric devices.

[0002] Toward the realization of a sustainable society, there is a need for stand-alone power sources that can be integrated with small sensors in our daily lives. Thin-film thermoelectric devices, which directly convert environmental temperature differences into electricity, can be integrated with small sensors on a single chip, and are maintenance-free, making them promising for use as stand-alone power sources.

[0003] For example, Non-Patent Document 1 describes a fine thermoelectric thin film that can be used in a thin-film thermoelectric device.

[0004] Ryoto Yanagisawa et al., "Nanostructured planar-type uni-leg Si thermoelectric generators," Appl. Phys. Express, 13, 095001 (2020).

[0005] However, in reality, the output power of thin-film thermoelectric devices is still insufficient, and technological development to improve the output power is required.

[0006] An aspect of the present disclosure aims to realize a thin-film thermoelectric device that can improve output power.

[0007] In order to solve the above problems, a thin-film thermoelectric device according to one embodiment of the present disclosure is a thin-film thermoelectric device including a semiconductor thermoelectric conversion laminated thin film on a substrate, wherein the thermoelectric conversion laminated thin film has a plurality of laminated structures each including a first layer containing a first semiconductor with high purity and a second layer containing a second semiconductor having a larger band gap than the first semiconductor and doped with impurities, and supplying carriers to the first layer, wherein the first semiconductor and the second semiconductor contain GaAs or GaN, and the crystal plane orientations of the first semiconductor and the second semiconductor are the same.

[0008] According to one aspect of the present disclosure, a thin-film thermoelectric device capable of improving output power can be realized.

[0009] 1. A top view of a thin-film thermoelectric device according to Embodiment 1. A partial bird's-eye view of a thin-film thermoelectric device according to Embodiment 1. A cross-sectional view taken along the arrows A-A' in FIG. 1. A cross-sectional view of a fine thermoelectric thin film in a thin-film thermoelectric device according to Embodiment 1. An electron band diagram of a fine thermoelectric thin film having a plurality of laminate structures shown in FIG. 4. A diagram showing a process of providing a bonding electrode to a fine thermoelectric thin film in a thin-film thermoelectric device according to Embodiment 1. A cross-sectional view taken along the arrows B-B' in FIG. 1. A cross-sectional view of a fine thermoelectric thin film in a thin-film thermoelectric device according to Embodiment 2. An electron band diagram of a fine thermoelectric thin film having a plurality of laminate structures shown in FIG. 8. A cross-sectional view of a fine thermoelectric thin film in a thin-film thermoelectric device according to Embodiment 3. An electron band diagram of a fine thermoelectric thin film having a plurality of laminate structures shown in FIG. 10. A diagram showing the structure of a fine thermoelectric thin film of an example according to Embodiment 1. A diagram showing a process of producing a thin-film thermoelectric device of an example. A graph showing the results of measuring the performance of a thin-film thermoelectric device of an example.

[0010] (Introduction) The thin-film thermoelectric device of the present disclosure is composed of fine thermoelectric thin films connected in series, and each fine thermoelectric thin film generates a thermoelectric power in response to an applied temperature difference, thereby enabling the extraction of electrical energy. Therefore, improving the output of the thermoelectric thin films directly leads to improving the performance of the thin-film thermoelectric device.

[0011] One of the most common methods for improving the output of thermoelectric thin films is to use two-dimensional electron gas or two-dimensional hole gas. When two-dimensional electron gas or two-dimensional hole gas is formed and electrons or holes are confined in a narrow region of a few nanometers, a low-dimensional electron density of states or a low-dimensional hole density of states is generated, thereby increasing the thermoelectric power. Another method is to increase the electrical conductivity of the two-dimensional electron gas or two-dimensional hole gas by supplying carriers (electrons or holes) from a donor or acceptor impurity-doped semiconductor region to an intrinsic semiconductor region. As a result, high-power thermoelectric power generation can be expected in two-dimensional electron gas or two-dimensional hole gas regions of a few nanometers in size.

[0012] However, thin-film thermoelectric devices using two-dimensional electron gas or two-dimensional hole gas have not yet been realized. The main reasons for this are the lack of microfabrication technology for thin-film thermoelectric devices using two-dimensional electron gas or two-dimensional hole gas, and the insufficient temperature difference applied to the thermoelectric thin film due to the high thermal conductivity of the thermoelectric thin film caused by the environmental temperature difference. Furthermore, even if the power generation output of only a few nanometers of two-dimensional electron gas or two-dimensional hole gas is high, the power generation output of other parts is not high, so the performance of the thin film as a whole is low.

[0013] Therefore, this disclosure proposes to extract power from the entire thermoelectric thin film by stacking two-dimensional electron gas or two-dimensional hole gas. Furthermore, this disclosure proposes to introduce atoms with large atomic mass differences into the two-dimensional electron gas stacked thin film or the two-dimensional hole gas stacked thin film in order to reduce the thermal conductivity of the material. By introducing atoms with large atomic mass differences into the two-dimensional electron gas stacked thin film or the two-dimensional hole gas stacked thin film, the propagation of lattice vibrations is suppressed, thereby reducing the thermal conductivity of the material.

[0014] Furthermore, this disclosure focuses on gallium arsenide (GaAs), a material that is widely used and has ample knowledge about microfabrication, and proposes a thermoelectric power generation device using a two-dimensional electron gas layered structure thin film or a two-dimensional hole gas layered structure thin film based on the above idea.

[0015] Specifically, the present invention provides a method for manufacturing a two-dimensional electron gas stacked thin film device or a two-dimensional hole gas stacked thin film thermoelectric device by reducing the thermal conductivity of a fabricated AlGaAs / GaAs two-dimensional electron gas stacked thin film or a two-dimensional hole gas stacked thin film by introducing an element (indium, carbon, etc.) that generates an atomic mass difference.

[0016] The basic structure of the thin-film thermoelectric device according to the present disclosure, when the carriers are electrons, is composed of a fine thermoelectric thin film having a layered structure of microfabricated N-type and (intrinsic) aluminum gallium arsenide (AlGaAs) and intrinsic gallium arsenide (GaAs), an ohmic contact such as nickel / gold germanium / nickel / gold, and a titanium / gold wiring electrode.

[0017] The basic structure of the thin-film thermoelectric device according to the present disclosure, when the carriers are holes, is composed of a fine thermoelectric thin film having a layered structure of microfabricated P-type and (intrinsic) aluminum gallium arsenide (AlGaAs) and intrinsic gallium arsenide (GaAs), an ohmic contact such as gold / zinc, and a titanium / gold wiring electrode.

[0018] According to the present disclosure, by substituting a portion of the gallium arsenide in the two-dimensional electron gas stack structure or the two-dimensional hole gas stack structure in the above-described basic structure with another element (e.g., indium, carbon, etc.), the atomic mass difference in the stack structure can be increased, thereby reducing the thermal conductivity of the thermoelectric thin film and increasing the temperature difference generated in the two-dimensional electron gas stack structure thin film or the two-dimensional hole gas stack structure thin film.

[0019] Furthermore, when indium is used as the element to be introduced into a two-dimensional electron gas layered structure thin film or a two-dimensional hole gas layered structure thin film, the band gap of indium gallium arsenide is smaller than that of gallium arsenide, and therefore, an effect of increasing the electron density of the two-dimensional electron gas or the hole density of the two-dimensional hole gas can be expected.

[0020] Furthermore, when carbon is used as an element to be introduced into a two-dimensional electron gas layered structure thin film, it functions as an acceptor impurity in gallium arsenide, and is expected to have the effect of confining the two-dimensional electron gas to a narrower region by sharpening the curvature of the electron band.

[0021] When silicon is used as the element introduced into the two-dimensional hole gas layered structure thin film, it functions as a donor impurity in gallium arsenide, and is expected to have the effect of confining the two-dimensional hole gas in a narrower region by sharpening the curvature of the hole band.

[0022] Furthermore, the present disclosure uses gallium arsenide, which is widely used and has extensive knowledge of microfabrication, and therefore, by performing microfabrication such as creating a hollow structure, it is expected that the temperature difference generated in the thermoelectric thin film will increase.

[0023] Furthermore, the method for manufacturing a thin-film thermoelectric device according to the present disclosure includes reducing the thermal conductivity by introducing a different element into the two-dimensional electron gas stack structure or the two-dimensional hole gas stack structure, and improving the output power of the thin-film thermoelectric device by controlling the electronic band structure. According to the present disclosure, it is possible to manufacture a thin-film thermoelectric device that uses a two-dimensional electron gas or a two-dimensional hole gas to improve the power generation output.

[0024] First Embodiment Hereinafter, one embodiment of the present disclosure will be described in detail.

[0025] (Outline of Thin-Film Thermoelectric Device) Fig. 1 is a top view of a thin-film thermoelectric device 1 according to this embodiment. Fig. 2 is a partial bird's-eye view of the thin-film thermoelectric device 1 according to this embodiment. Fig. 3 is a cross-sectional view taken along line AA' in Fig. 1. Note that Figs. 1 and 2 omit the illustration of the heat dissipation portion 16 shown in Fig. 3 for ease of viewing. The thin-film thermoelectric device 1 is a two-dimensional electron gas laminated thin-film thermoelectric device that supplies electrons as carriers.

[0026] As shown in FIGS. 1 and 2, the thin-film thermoelectric device 1 according to this embodiment is a thin-film thermoelectric device including a fine thermoelectric thin film (thermoelectric conversion laminated thin film) 11 on a substrate 10 .

[0027] The substrate 10 is, for example, a semi-insulating GaAs (100) substrate. A fine thermoelectric thin film 11 is formed on the substrate 10. The structure of the fine thermoelectric thin film 11 will be described in detail later.

[0028] A plurality of fine thermoelectric thin films 11 are arranged vertically and horizontally on the substrate 10. In the example of Fig. 1, the fine thermoelectric thin films 11 have a rectangular shape that is long in one direction when viewed from above, and are arranged in a total of 14, with seven along the short side and two along the long side. The total of 14 fine thermoelectric thin films 11 are connected in series by wiring electrodes 14.

[0029] If a plurality of fine thermoelectric thin films 11 arranged in the short direction are considered to be one group G, then in the example of Fig. 1, two groups G1 and G2 are formed arranged in the long direction. The plurality of fine thermoelectric thin films 11 are connected in series within a group, and are also connected directly between groups G1 and G2.

[0030] Specifically, group G1 includes seven fine thermoelectric thin films 11-1 to 11-7, and group G2 includes seven fine thermoelectric thin films 11-8 to 11-14. Each of the fine thermoelectric thin films 11 in groups G1 and G2 has a proximal end 11n at one end in the longitudinal direction that is close to each other, and a distal end 11f at another end in the longitudinal direction that is opposite to the proximal end 11n.

[0031] A linear wiring electrode 14a is connected to the remote end 11f of the fine thermoelectric thin film 11-1 of group G1, and a Z-shaped wiring electrode 14b connects the proximal end 11n of the fine thermoelectric thin film 11-1 to the remote end 11f of the fine thermoelectric thin film 11-2 located next to the fine thermoelectric thin film 11-1. Similarly, the Z-shaped wiring electrode 14b connects from the proximal end 11n of the fine thermoelectric thin film 11-2 to the remote end 11f of the fine thermoelectric thin film 11-7.

[0032] The proximal end 11n of the fine thermoelectric thin film 11-7 of group G1 is connected to the remote end 11f of the fine thermoelectric thin film 11-8 of group G2 by a U-shaped wiring electrode 14c. The proximal end 11n of the fine thermoelectric thin film 11-8 is connected to the remote end 11f of the fine thermoelectric thin film 11-9 located next to the fine thermoelectric thin film 11-8 by a Z-shaped wiring electrode 14b. Similarly, the Z-shaped wiring electrode 14b connects the proximal end 11n of the fine thermoelectric thin film 11-9 to the remote end 11f of the fine thermoelectric thin film 11-14. A linear wiring electrode 14a is connected to the proximal end 11n of the fine thermoelectric thin film 11-14.

[0033] The proximal end 11n and the distal end 11f of each fine thermoelectric thin film 11 are provided with a junction electrode 13 that is in ohmic contact with the fine thermoelectric thin film 11. Each fine thermoelectric thin film 11 and the wiring electrode 14 are electrically connected via the junction electrode 13 provided on each fine thermoelectric thin film 11.

[0034] 3, a hollow portion 15 is formed by microfabrication in the region of the substrate 10 where the plurality of fine thermoelectric thin films 11 are arranged. By forming the hollow portion 15, each of the fine thermoelectric thin films 11 of groups G1 and G2 has a structure in which only the remote end portion 11f side is in contact with the substrate 10, and the entire surface of the thin film 11 except for the remote end portion 11f is suspended in air. The bonding electrode 13 provided at the proximal end portion 11n and the wiring electrode 14 located above the hollow portion 15 are also suspended in air together with the fine thermoelectric thin film 11.

[0035] In this way, by forming a hollow portion 15 in the substrate 10 and creating a suspended structure in which only the remote end portion 11f of the fine thermoelectric thin film 11 is in contact with the substrate 10, the portion of the fine thermoelectric thin film 11 where the hollow portion 15 is located can be insulated.

[0036] That is, when the substrate 10 is heated, heat is transferred longitudinally through the fine thermoelectric thin film 11 from the remote end 11f in contact with the substrate 10 to the proximal end 11n. At this time, the insulating effect of the hollow portion 15 can increase the temperature difference generated in the fine thermoelectric thin film 11. This increases the thermoelectromotive force obtained from the temperature difference, thereby improving the output power.

[0037] 3, a heat dissipation portion 16 is provided between the two groups G1 and G2. The heat dissipation portion 16 is provided in contact with the wiring electrodes 14 formed on the proximal ends 11n of the fine thermoelectric thin films 11, and is thermally connected to each of the fine thermoelectric thin films 11 included in the two groups G1 and G2. The heat dissipation portion 16 is made of a material with excellent thermal conductivity and is electrically insulated from the wiring electrodes 14. Examples of materials for the heat dissipation portion 16 include Al, GaAs, and Si.

[0038] In this way, by providing the heat dissipation portion 16, the temperature on the near end 11n side of the fine thermoelectric thin film 11 can be lowered, thereby increasing the temperature difference between the far end 11f and the near end 11n of the fine thermoelectric thin film 11. This further increases the thermoelectromotive force obtained from the temperature difference, thereby further improving the output power.

[0039] The arrangement of the multiple fine thermoelectric thin films 11 shown in Figure 1 is merely one example. The more fine thermoelectric thin films 11 connected in series, the greater the output power that can be extracted by thermoelectric conversion. Furthermore, the two groups G1 and G2 formed as shown in Figure 1 can be considered as one block, and multiple blocks can be arranged on the substrate 10, with all the blocks connected in series, or a combination of parallel and series connections can be used to increase the current.

[0040] (Fine Thermoelectric Thin Film) Fig. 4 is a cross-sectional view of the fine thermoelectric thin film 11 of the thin-film thermoelectric device 1 according to this embodiment. As shown in Fig. 4, the thin-film thermoelectric device 1 has the fine thermoelectric thin film 11 fabricated on the aforementioned semi-insulating GaAs (100) substrate 10. The fine thermoelectric thin film 11 has a plurality of laminated structures 100 each having a first layer 111 containing a highly pure first semiconductor and a second layer 112 containing a second semiconductor with a larger band gap than the first semiconductor and doped with impurities, which supplies carriers to the first layer 111.

[0041] In this embodiment, GaAs is exemplified as the first semiconductor, but it may be a GaAs-based material in which other elements are added to GaAs, including InGaAs, etc. Furthermore, the first semiconductor may be a GaN-based material in which other elements are added to GaN, such as InGaN, in addition to GaN.

[0042] In this embodiment, AlGaAs is exemplified as the second semiconductor, but it may be an AlGaAs-based semiconductor in which other elements are added to AlGaAs, such as AlInGaAs. Furthermore, the second semiconductor may be an AlGaN-based semiconductor in which other elements are added to AlGaN, such as AlInGaN.

[0043] It is preferable that the crystal plane orientations of the first semiconductor and the second semiconductor are the same, which allows the thin-film thermoelectric device 1 to be fabricated, including a fine thermoelectric thin film 11 with high crystallinity and low resistance.

[0044] 4, in this embodiment, each stacked structure 100 in the thin-film thermoelectric device 1 has a third layer 113 containing high-purity AlGaAs as the second semiconductor between the first layer 111 and the second layer 112. The third layer 113 is in contact with the first layer 111 and the second layer 112. Although the third layer 113 is provided in all stacked structures 100 in FIG. 4, the stacked structures 100 may include a stacked structure 100 that does not have the third layer 113.

[0045] That is, in the thin-film thermoelectric device 1, at least one of the plurality of stacked structures 100 may have a third layer 113 containing the second semiconductor with high purity between the first layer 111 and the second layer 112.

[0046] Furthermore, in this embodiment, third layers 113 containing the second semiconductor at a high purity are provided on both sides of the second layer 112. That is, the third layers 113 are formed between the second layer 112 and the first layer 111 of the stacked structure 100 in which the second layer 112 is included, and between the second layer 112 and the first layer 111 included in the stacked structure formed on the stacked structure 100 in which the second layer 112 is included.

[0047] That is, in the thin-film thermoelectric device 1 , at least one of the plurality of stacked structures 100 may have a third layer containing the second semiconductor with high purity on both sides of the second layer 112 .

[0048] The first layer 111, the second layer 112, and the third layer 113 are all formed by vapor deposition. For ease of viewing, Fig. 4 shows an example in which the laminate structure 100 is laminated three times, with the third layer 113 on both sides of the second layer 112. The more laminate structures 100 are laminated, the greater the output power that can be extracted by thermoelectric conversion.

[0049] The first layer 111 containing GaAs at a high purity can be expressed as an intrinsic GaAs layer made of intrinsic GaAs. Similarly, the third layer 113 containing AlGaAs at a high purity can be expressed as an intrinsic AlGaAs layer. "Containing at a high purity" refers to a state in which no impurities are doped (non-doped, undoped), or a state in which the contained impurities do not perform the function of carrier supply, i.e., are substantially not contained.

[0050] The first layer 111 is an intrinsic GaAs layer that forms a two-dimensional electron gas. The second layer 112 is an N-type AlGaAs layer that supplies electrons to the two-dimensional electron gas. Although AlGaAs is used for the second layer 112 here, it is sufficient that an element for increasing the band gap is added to the GaAs of the first layer 111, and elements other than Al, such as P or N, may also be added. Furthermore, examples of impurities that supply electrons and are doped into AlGaAs to form the N-type AlGaAs layer include Si and Sn.

[0051] In the above-described stacked structure 100, electrons generated from impurities contained in the second layer 112, which has a larger band gap, are supplied to the first layer 111, which has a smaller band gap than the second layer 112, through the third layer 113, and a two-dimensional electron gas is formed in the first layer 111 near the interface with the third layer 113.

[0052] The technique of adding a small amount of impurity to a semiconductor layer with a large band gap to introduce electrons into the interface with a semiconductor layer with a small band gap and high purity is called "modulation doping."

[0053] The third layer 113 is an intrinsic AlGaAs semiconductor layer that serves to spatially separate the donor impurities contained in the second layer 112 from the two-dimensional electron gas formed in the first layer 111. The third layer 113 is a semiconductor component of the second layer 112 to which no impurities have been added. In this embodiment, the stacked structure 100 including the third layer 113 is exemplified, but the third layer 113 is not necessarily a required layer.

[0054] The thin-film thermoelectric device 1 of this embodiment can also be expressed as follows: That is, it is a thin-film thermoelectric device including a semiconductor fine thermoelectric thin film 11 on a substrate 10, the fine thermoelectric thin film 11 having a first layer 111 containing a first semiconductor with high purity, and a second layer 112 containing a second semiconductor with a larger band gap than the first semiconductor and doped with impurities, which supplies electrons to the first layer 111, and having a plurality of stacked structures 100 in which two-dimensional electron gas is formed in the first layer 111 near the interface with the second layer 112.

[0055] Figure 5 is an electron band diagram of a fine thermoelectric thin film 11 having a plurality of the laminated structures 100 shown in Figure 4. In the electron band diagram of Figure 5, the laminated structure 100 is formed four times, with the basic structure being a periodic structure in which the first layer 111 is 150 nm, the second layer 112 is 40 nm, and the third layer 113 formed on both sides of the second layer 112 is each 2 nm thick. The top surface has the first layer 111 of 10 nm and the second layer 112 below it of 60 nm. The vertical axis in the electron band diagram represents the electron energy (E), and the horizontal axis represents the depth (z) from the surface (top surface) of the fine thermoelectric thin film 11. In Figure 5, line EC represents the conduction band edge energy, and line EF represents the Fermi energy.

[0056] As shown in FIG. 5, a line E Ｆ This portion is two-dimensional electron gas, and it can be seen from FIG. 5 that two-dimensional electron gas is formed.

[0057] (Bonding electrode, wiring electrode) Fig. 6 is a diagram showing a process of providing a bonding electrode 13 on the fine thermoelectric thin film 11 in the thin-film thermoelectric device 1. As shown in Fig. 6, in this embodiment, the fine thermoelectric thin film 11 is formed in a shape in which the side surfaces are inclined inward with increasing distance from the substrate 10, that is, a shape that tapers with increasing distance from the substrate 10. The bonding electrode 13 is formed so as to cover the side surfaces of the fine thermoelectric thin film 11. In the example of Fig. 6, the bonding electrode 13 is formed so as to cover the inclined side surfaces including the upper surface of the fine thermoelectric thin film 11.

[0058] Specifically, the fine thermoelectric thin film 11, which has been formed by vapor deposition on the substrate 10, is wet-etched using a mixed solution of phosphoric acid, hydrogen peroxide, and water to form inclined side surfaces on the fine thermoelectric thin film 11. As a result, the first layer 111, which forms a two-dimensional electron gas, appears on the inclined side surfaces. Then, the bonding electrode 13 is vapor-deposited, thereby making it possible to obtain ohmic contact with all of the two-dimensional electron gas in the fine thermoelectric thin film 11.

[0059] A material is selected for the bonding electrode 13 that can make ohmic contact with the semiconductor that constitutes the fine thermoelectric thin film 11. In the thin-film thermoelectric device 1 of this embodiment, since the first semiconductor of the fine thermoelectric thin film 11 is GaAs-based, the bonding electrode 13 can be made of, for example, nickel / gold germanium / nickel / gold.

[0060] More specifically, for example, nickel 5 nm / gold germanium 50 nm / nickel 30 nm / gold 100 nm are deposited in this order. After deposition, the deposition is heated at 450°C for 90 seconds, whereby germanium in the gold germanium diffuses into the first layer 111 made of intrinsic GaAs of each laminated structure 100 included in the fine thermoelectric thin film 11, thereby reducing the resistance between the two-dimensional electron gas and the bonding electrode 13.

[0061] This makes it possible to obtain ohmic contact and obtain high output power by utilizing the electrical conduction of all two-dimensional electron gases in the multiple layered structures 100 in the fine thermoelectric thin film 11. Here, the 5 nm of nickel present between the fine thermoelectric thin film 11 and the gold germanium plays a role in controlling the diffusion depth of the germanium.

[0062] When the first semiconductor is a GaN-based semiconductor, the bonding electrode 13 may be made of, for example, titanium / aluminum / nickel / gold.

[0063] 7 is a cross-sectional view taken along the line B-B' in FIG. 1. As shown in FIG. 7, the wiring electrode 14 is formed (deposited) so as to contact the top surface of the fine thermoelectric thin film 11 and one of the inclined side surfaces. An example of the wiring electrode 14 is a 30 nm titanium / 400 nm gold electrode. Here, the titanium serves as an adhesive layer, improving the contact between the gold and the substrate 10, and between the gold and the bonding electrode 13. However, since gold is expensive, the material is not limited to Al, Cu, or the like, as long as it can be electrically connected to the bonding electrode 13 and allow current to flow.

[0064] In this embodiment, the fine thermoelectric thin film 11 is assumed to have a total thickness of about 1 μm. However, even if the film thickness is increased and the number of layers is increased to obtain higher output power, it is expected that a two-dimensional electron gas laminated thin film thermoelectric device that can utilize the electrical conductivity of all the two-dimensional electron gas can be fabricated by performing the same microfabrication and obtaining ohmic electrodes as in this embodiment.

[0065] The scope of application includes not only the integration of small sensors into a single chip, but also thin-film thermoelectric devices aimed at reducing power consumption by integrating electronic components that are prone to generating heat, such as power amplifiers, into a single chip.

[0066] The hollow portion 15 in the substrate 10 described above is formed by additionally performing wet etching after the wiring electrodes 14 have been formed.

[0067] (Effect) In the above-mentioned thin-film thermoelectric device 1, when the substrate 10 is heated, heat is conducted from the remote end 11f to the proximal end 11n of the fine thermoelectric thin film 11, and a thermoelectromotive force is generated from the temperature difference generated in the fine thermoelectric thin film 11 during this process. Since the fine thermoelectric thin film 11 has a plurality of stacked structures 100 that form a two-dimensional electron gas, a stacked structure of two-dimensional electron gas is formed, and thermoelectromotive force can be generated from the stacked two-dimensional electron gas. This makes it possible to improve the output power of the thin-film thermoelectric device 1.

[0068] Furthermore, in the thin-film thermoelectric device 1 of this embodiment, the third layer 113 is provided between the first layer 111 and the second layer 112 to spatially separate the donor impurities contained in the second layer 112 from the two-dimensional electron gas formed in the first layer 111. Therefore, the two-dimensional electron gas can be effectively formed.

[0069] Furthermore, the third layer 113 is formed on both sides of the second layer 112, so that two-dimensional electron gas is formed near the interfaces on both sides of the laminated first layer 111. This allows a larger amount of two-dimensional electron gas to be laminated, and the output power of the thin-film thermoelectric device 1 can be further improved.

[0070] [Embodiment 2] Another embodiment of the present disclosure will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0071] 8 is a cross-sectional view of a fine thermoelectric thin film 11A of a thin-film thermoelectric device 1A according to this embodiment. This differs from the thin-film thermoelectric device 1 according to embodiment 1 in that the fine thermoelectric thin film 11 is replaced with a fine thermoelectric thin film 11A.

[0072] 8, the fine thermoelectric thin film 11A includes at least one layer of a laminate structure 100A among the plurality of laminate structures 100 included in the fine thermoelectric thin film 11. In the laminate structure 100A, the first layer 111 is replaced with a first layer 111A including an impurity-doped layer 105 to which an impurity is added.

[0073] The first layer 111A includes a first high-purity layer 111A1 containing a first semiconductor at a high purity, a second high-purity layer 111A2 containing the first semiconductor at a high purity, and an impurity-doped layer 115 sandwiched between the first high-purity layer 111A1 and the second high-purity layer 111A2. The impurity-doped layer 115 is a layer in which the first semiconductor is doped with an impurity having an atomic mass different from that of a main component element of the first semiconductor.

[0074] In this embodiment, GaAs is used as the first semiconductor, and the impurity doped layer 115 is GaAs doped with carbon as an impurity, and is a carbon-doped GaAs layer having a thickness of several nanometers.

[0075] 8 illustrates, for ease of viewing, a configuration in which the laminate structure 100, each having a third layer 113 on both sides of the second layer 112, is laminated three times, one of which is replaced with the laminate structure 100A. However, in the fine thermoelectric thin film 11A, all of the laminate structures 100 may be replaced with the laminate structure 100A.

[0076] FIG. 9 is an electron band diagram of a fine thermoelectric thin film 11A having a plurality of the laminated structures 100A shown in FIG. 8 . In the electron band diagram of FIG. 9 , the laminated structure 100A is formed four times, with the basic structure being a periodic structure in which the first layer 111A is 150 nm thick, the second layer 112 is 40 nm thick, and the third layer 113 formed on both sides of the second layer 112 is 2 nm thick. The top surface is a structure in which the first layer 111 is 10 nm thick and the second layer 112 is 60 nm thick underneath. The vertical axis of the electron band diagram represents electron energy (E), and the horizontal axis represents the depth (z) from the surface (top surface) of the fine thermoelectric thin film 11A. In FIG. 9 , line EC represents the conduction band edge energy, and line EF represents the Fermi energy. The impurity-doped layer 115 in the first layer 111A is 2 nm thick, and the first high-purity layer 111A1 and the second high-purity layer 111A2 each have a thickness of 74 nm.

[0077] As shown in FIG. 9, a line E Ｆ This portion is two-dimensional electron gas, and it can be confirmed from FIG. 9 that two-dimensional electron gas is formed.

[0078] Furthermore, when such a layered structure is fabricated, a reduction in thermal conductivity can be expected in the fine thermoelectric thin film 11A due to the mass difference caused by the introduction of the thin impurity-doped layer 115 into GaAs. To reduce the thermal conductivity, any impurity having an atomic mass different from that of the main component element of GaAs may be used, and the impurity-doped layer 115 may be fabricated by doping with Be, Mg, Zn, or the like instead of carbon.

[0079] 9, it can be seen that the carbon contained in the impurity-doped layer 115, or the carbon in the first layer 111A, acts as an acceptor impurity in the first high-purity layer 111A1 and the second high-purity layer 111A2, thereby sharpening the curvature of the electron band. The sharpening of the curvature of the electron band is expected to have the effect of confining the two-dimensional electron gas in a narrower region.

[0080] This makes it possible to form two-dimensional electron gas even if the first layer 111A that forms the two-dimensional electron gas is made thin, and it is also expected that the density of the two-dimensional electron gas layer will increase by shortening the stacking period of the stacked structure 100A.

[0081] [Embodiment 3] Another embodiment of the present disclosure will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0082] 10 is a cross-sectional view of a fine thermoelectric thin film 11B of a thin-film thermoelectric device 1B according to this embodiment. This differs from the thin-film thermoelectric device 1 according to embodiment 1 in that the fine thermoelectric thin film 11 is replaced with a fine thermoelectric thin film 11B.

[0083] 10 , the fine thermoelectric thin film 11B includes at least one layer of a layer structure 100B among the plurality of layer structures 100 included in the fine thermoelectric thin film 11. In the layer structure 100B, the first layer 111 is replaced with a first layer 111B. The first layer 111B includes, in addition to the main component of the first semiconductor, another element having an atomic mass different from that of the main component element.

[0084] In this embodiment, GaAs is used as the first semiconductor, and the first layer 111B is an InGaAs (indium gallium arsenide) layer.

[0085] 10 illustrates, for ease of viewing, a configuration in which the laminate structure 100, each having a third layer 113 on both sides of the second layer 112, is laminated three times, one of which is replaced with the laminate structure 100B. However, in the fine thermoelectric thin film 11B, all of the laminate structures 100 may be replaced with the laminate structure 100B.

[0086] 11 is an electron band diagram of a fine thermoelectric thin film 11B having a plurality of the laminated structures 100B shown in FIG. 10. In the electron band diagram of FIG. 11, the laminated structure 100B is formed four times, with the basic structure being a periodic structure in which the first layer 111B is 150 nm, the second layer 112 is 40 nm, and the third layer 113 formed on both sides of the second layer 112 is 2 nm thick. The top surface is a structure in which the first layer 111 is 10 nm thick and the second layer 112 is 60 nm thick formed below it. In the electron band diagram, the vertical axis represents electron energy (E), and the horizontal axis represents the depth (z) from the surface (top surface) of the fine thermoelectric thin film 11. In FIG. 9, line EC represents the conduction band edge energy, and line EF represents the Fermi energy.

[0087] As shown in FIG. 11, a line E Ｆ This portion is two-dimensional electron gas, and it can be seen from FIG. 11 that two-dimensional electron gas is formed.

[0088] Furthermore, when such a layered structure is fabricated, the mass difference between the InGaAs of the first layer 111B and the AlGaAs of the second layer 112 is greater than that of the first layer 111 made of GaAs without indium doping. This is expected to reduce the thermal conductivity of the fine thermoelectric thin film 11B. To reduce the thermal conductivity, GaAsSb may be used instead of InGaAs, and GaAsP or GaAsN may be used instead of AlGaAs of the second layer 112.

[0089] 11, it can be seen that the Fermi energy in the two-dimensional electron gas region is larger than the energy at the conduction band edge because the band gap of the InGaAs first layer 111B is smaller than that of GaAs. Therefore, it can be expected that the electron density will be higher than that of the first layer 111 made of GaAs without indium doping.

[0090] [Embodiment 4] Another embodiment of the present disclosure will be described below. For convenience of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0091] In the above-described first to third embodiments, a two-dimensional electron gas laminated thin film thermoelectric device is exemplified in which the second layer 112 in the fine thermoelectric thin film 11 is made of an N-type semiconductor that supplies electrons as carriers to the first layer 111.

[0092] In contrast, the thin-film thermoelectric device of this embodiment is a two-dimensional hole gas stacked thin-film thermoelectric device in which the second layer 112 in the fine thermoelectric thin film 11 is made of a P-type semiconductor that supplies holes as carriers to the first layer 111.

[0093] The second layer 112 made of a P-type semiconductor is, for example, a P-type AlGaAs layer. In the case of the second layer 112 made of a P-type semiconductor, it is sufficient that an element for increasing the band gap is added to the GaAs of the first layer 111, and elements other than Al, such as P or N, may also be added. Furthermore, impurities for supplying holes that are doped into AlGaAs to form the P-type AlGaAs layer can include, for example, C, Be, Zn, etc.

[0094] Holes generated from impurities contained in the second layer 112, which has a larger band gap, pass through the third layer 113 and are supplied to the first layer 111, which has a smaller band gap than the second layer 112, and a two-dimensional hole gas is formed in the first layer 111 near the interface with the third layer 113.

[0095] The thin-film thermoelectric device of this embodiment can be configured in the same manner as the thin-film thermoelectric devices 1 and 1B described in the above-described embodiments 1 and 3, except that the second layer 112 is made of a P-type semiconductor. In the thin-film thermoelectric device of this embodiment, the laminated structure 100 forms a two-dimensional hole gas instead of a two-dimensional electron gas, and the fine thermoelectric thin film 11 is configured by stacking a plurality of such laminated structures 100.

[0096] In the thin-film thermoelectric device 1A described in the above-described second embodiment, silicon, tin, or the like is used as an element to be introduced into the two-dimensional hole gas layer structure thin film instead of carbon, which is used as an element to be introduced into the two-dimensional electron gas layer structure thin film. By using silicon, tin, or the like as an element to be introduced into the two-dimensional hole gas layer structure thin film, it functions as a donor impurity in gallium arsenide, and is expected to have the effect of confining the two-dimensional hole gas in a narrower region by making the hole band curvature steeper.

[0097] 1 and 4, in the above-described thin-film thermoelectric device, similarly to the thin-film thermoelectric device 1 described in embodiment 1, when the substrate 10 is heated, heat is conducted from the remote end 11f to the proximal end 11n of the fine thermoelectric thin film 11, and a thermoelectromotive force is generated from the temperature difference generated in the fine thermoelectric thin film 11 during this process. Since the fine thermoelectric thin film 11 has a plurality of stacked structures 100 that form a two-dimensional hole gas, a stacked structure of two-dimensional hole gas is formed, and thermoelectromotive force can be generated from the stacked two-dimensional hole gas. This can improve the output power of the thin-film thermoelectric device.

[0098] (Summary) A thin-film thermoelectric device according to one aspect of the present disclosure is a thin-film thermoelectric device including a semiconductor thermoelectric conversion laminated thin film on a substrate, wherein the thermoelectric conversion laminated thin film has a plurality of laminated structures each including a first layer containing a first semiconductor with high purity and a second layer containing a second semiconductor having a larger band gap than the first semiconductor and doped with impurities, and supplying carriers to the first layer, wherein the first semiconductor and the second semiconductor contain GaAs or GaN, and the crystal plane orientations of the first semiconductor and the second semiconductor are the same.

[0099] At least one of the plurality of stacked structures may have a third layer between the first layer and the second layer, the third layer containing the second semiconductor with a high purity.

[0100] At least one of the plurality of stacked structures may have third layers containing the second semiconductor at a high purity on both sides of the second layer.

[0101] In order to solve the above problems, a thin-film thermoelectric device according to one embodiment of the present disclosure is a thin-film thermoelectric device including a semiconductor thermoelectric conversion laminated thin film on a substrate, the thermoelectric conversion laminated thin film having a first layer containing a highly pure first semiconductor and a second layer containing a second semiconductor having a larger band gap than the first semiconductor and doped with impurities, which supplies carriers to the first layer, and has a plurality of laminated structures in which two-dimensional electron gas or two-dimensional hole gas is formed in the first layer near the interface with the second layer, the first semiconductor and the second semiconductor containing GaAs or GaN, and the crystal plane orientation of the first semiconductor and the second semiconductor being the same.

[0102] The first layer in at least one of the plurality of stacked structures may have a first high-purity layer and a second high-purity layer containing the first semiconductor at a high purity, and an impurity-doped layer sandwiched between the first high-purity layer and the second high-purity layer, in which the first semiconductor is doped with an impurity having an atomic mass different from that of a main component element of the first semiconductor.

[0103] The first layer in at least one of the plurality of stacked structures may contain, together with the main component of the first semiconductor, another element having an atomic mass different from that of the main component, in which case the other element may be indium.

[0104] The thermoelectric conversion laminated thin film may be provided with a junction electrode that is in ohmic contact with the thermoelectric conversion laminated thin film, the thermoelectric conversion laminated thin film having a shape in which the side surfaces slope inward as they move away from the substrate, and the junction electrode may be formed to cover the side surfaces of the thermoelectric conversion laminated thin film.

[0105] The first semiconductor may be GaAs-based, and the second semiconductor may be AlGaAs-based.

[0106] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure.

[0107] An embodiment of the present invention will now be described.

[0108] FIG. 12 is a diagram showing the structure of a fine thermoelectric thin film 1 according to the first embodiment.

[0109] In the example, a GaAs (001) substrate was used as the substrate 10 by molecular beam epitaxy, and the fine thermoelectric thin film 11 was formed thereon by epitaxial growth according to the following steps.

[0110] First, in order to obtain a clean surface of the substrate 10, an initial layer 20 of non-doped GaAs (300 nm) was formed on the substrate 10. Then, a buffer layer 21 was formed on the initial layer 20. The buffer layer 21 was made of GaAs (10 nm) and Al ０．３ Ga ０．７ GaAs(10 nm) / Al ０．３ Ga ０．７ Twenty superlattice layers of As (10 nm) were formed.

[0111] On the buffer layer 21, non-doped Al ０．３ Ga ０．７ As (2 nm) / non-doped GaAs (150 nm) / non-doped Al ０．３ Ga ０．７ As (2 nm) / Si-doped Al ０．３ Ga ０．７ As (40 nm, dopant concentration: 7 × 10 １７ cm －３ ) was used as a unit structure, and was formed four times.

[0112] Here, the non-doped Al in contact with the buffer layer 21 ０．３ Ga ０．７ Undoped Al except As (2 nm) ０．３ Ga ０．７ As (2 nm) corresponds to the third layer 113, undoped GaAs (150 nm) corresponds to the first layer 111, and Si-doped Al ０．３ Ga ０．７ The As (40 nm) layer corresponds to the second layer 112 .

[0113] Then, non-doped Al is further applied on top of that. ０．３ Ga ０．７ As (2 nm) / non-doped GaAs (150 nm) / non-doped Al ０．３ Ga ０．７ As (2 nm) / Si-doped Al ０．３ Ga ０．７ As (60 nm, dopant concentration: 7×10 １７ cm －３ ) and GaAs (10 nm) were laminated to obtain the fine thermoelectric thin film 11 of the example.

[0114] In the fine thermoelectric thin film 11 of the embodiment, the two-dimensional electron gas is non-doped Al ０．３ Ga ０．７ At the interface between the As (2 nm) layer and the non-doped GaAs (150 nm), the non-doped Al layer in contact with the buffer layer 21 ０．３ Ga ０．７ Two-dimensional electron gas layers were formed at all interfaces except for the interface with As (2 nm). In Fig. 12, the two-dimensional electron gas layers formed at the interfaces are indicated by hatching. In the fine thermoelectric thin film 11 of the example, nine two-dimensional electron gas layers were formed.

[0115] A thin film thermoelectric device 1 of the example was fabricated having such a fine thermoelectric thin film 11 of the example. Figure 13 is a diagram showing the process of fabricating the thin film thermoelectric device 1 of the example.

[0116] 13, first, wet etching was performed on the fine thermoelectric thin film 11 to pattern the fine thermoelectric thin film 11 into a rectangular shape that is long in one direction when viewed from above. In the description of the above embodiment, the fine thermoelectric thin film 11 that is patterned into a rectangular shape that is long in one direction was also referred to as the fine thermoelectric thin film 11, but here it will be referred to as a TE leg 22.

[0117] The etching solution contains H ３ P.O. ４ (85wt%, 2mL), H ２ O ２ (35wt%, 2mL), H ２ The resulting TE leg 22 had a sloped side surface, and the AlGaAs / GaAs interface having the two-dimensional electron gas channel of the fine thermoelectric thin film #11 was exposed.

[0118] Next, as shown in the diagram 1302 in Fig. 13, bonding electrodes 13 of Ni (5 nm) / AuGe (50 nm) Ni (30 nm) / Au (100 nm) were evaporated on both ends of the TE legs 22 to make ohmic contact with the two-dimensional electron gas channel, and annealed at 450°C for 90 seconds. Furthermore, as shown in the diagram 1303 in Fig. 13, these TE legs 22 with bonding electrodes 13 were continuously connected using wiring electrodes 14 of Ti (30 nm) / Au (400 nm).

[0119] FIG. 14 is a graph showing the results of measuring the performance of the thin-film thermoelectric device 1 of the example. FIG. 14 shows the output voltage and the output voltage as a function of the output current. The temperature difference applied to the thin-film thermoelectric device 1 was changed to examine the output voltage and its temperature dependence. In FIG. 14, circles represent the temperature difference (ΔT) of 4.9 K, diamonds represent the temperature difference (ΔT) of 3.3 K, and squares represent the temperature difference (ΔT) of 1.4 K. In the circles, diamonds, and squares, hollows represent the output voltage, and solids represent the output power.

[0120] During the measurements, the temperature of the cold side of the TE leg 22 was maintained at 320 K. When the temperature of the hot side of the TE leg 22 was increased to increase the temperature difference (ΔT), the output voltage and output current increased, and the output power also increased. The maximum value of the output voltage was approximately 14 mV at a temperature difference (ΔT) of 4.9 K.

[0121] The value obtained by this measurement and the absolute value S (absolute value S = 230 μVK) of the electromotive force per kelvin of one TE leg 22 are compared. －１ ) and the output voltage calculated from the number of TE legs 22. This indicates that the temperature difference ΔT was measured accurately and was not affected by the thermal interface resistance.

[0122] 1, 1A, 1B Thin film thermoelectric device 10 Substrate 11, 11A, 11B Fine thermoelectric thin film (thermoelectric conversion laminated thin film) 11f Remote end 11n Proximal end 13 Bonding electrode 14 Wiring electrode 115 Impurity doped layer 111A1 First high purity layer 111A2 Second high purity layer

Claims

1. A thin-film thermoelectric device comprising a semiconductor thermoelectric conversion laminated thin film on a substrate, wherein the thermoelectric conversion laminated thin film has a laminated structure including: a first layer containing a first semiconductor with high purity; and a second layer in which impurities are added to a second semiconductor having a larger bandgap than the first semiconductor and which supplies carriers to the first layer, and has a plurality of such laminated structures, wherein the first semiconductor and the second semiconductor include GaAs or GaN, and a thin-film thermoelectric device in which the crystal plane orientations of the first semiconductor and the second semiconductor are the same.

2. The thin-film thermoelectric device according to claim 1, wherein at least one of the plurality of laminated structures has a third layer containing the second semiconductor with high purity between the first layer and the second layer.

3. The thin-film thermoelectric device according to claim 1, wherein at least one of the plurality of laminated structures has a third layer containing the second semiconductor with high purity on both sides of the second layer.

4. A thin-film thermoelectric device comprising a semiconductor thermoelectric conversion laminated thin film on a substrate, wherein the thermoelectric conversion laminated thin film has a laminated structure including: a first layer containing a first semiconductor with high purity; and a second layer in which impurities are added to a second semiconductor having a larger bandgap than the first semiconductor and which supplies carriers to the first layer, and has a plurality of laminated structures in which a two-dimensional electron gas or a two-dimensional hole gas is formed near the interface between the second layer and the first layer in the first layer, wherein the first semiconductor and the second semiconductor include GaAs or GaN, and a thin-film thermoelectric device in which the crystal plane orientations of the first semiconductor and the second semiconductor are the same.

5. The thin-film thermoelectric device according to any one of claims 1 to 4, wherein the first layer in at least one of the plurality of laminated structures has a first high-purity layer and a second high-purity layer containing the first semiconductor with high purity, and an impurity-added layer in which impurities having an atomic mass different from that of the element of the main component of the first semiconductor are added to the first semiconductor and which is sandwiched between the first high-purity layer and the second high-purity layer.

6. The thin-film thermoelectric device according to any one of claims 1 to 4, wherein the first layer in at least one of the plurality of laminated structures contains, together with the main component of the first semiconductor, another element having an atomic mass different from that of the element of the main component.

7. The thin-film thermoelectric device according to claim 6, wherein the other element is indium.

8. The thin-film thermoelectric device according to any one of claims 1 to 4, comprising a bonding electrode that makes ohmic contact with the thermoelectric conversion laminated thin film, wherein the thermoelectric conversion laminated thin film has a shape in which the side surface inclines inward as it moves away from the substrate, and the bonding electrode is formed so as to cover the side surface of the thermoelectric conversion laminated thin film.

9. The thin-film thermoelectric device according to any one of claims 1 to 4, wherein the first semiconductor is a GaAs-based semiconductor and the second semiconductor is an AlGaAs-based semiconductor.

Images