Device, evaluating method, and evaluating apparatus

Abstract

This device comprises: a multilayer structure in which a substrate, a sample layer formed on the substrate from a material that exhibits a transverse thermoelectric effect, an insulating layer formed on the sample layer, and a heater / reflector layer formed on the insulating layer are formed; two outer sample pads that are made of the same material as the sample layer, are each connected to the sample layer, and each have an exposed upper surface; and two central sample pads that are made of the same material as the sample layer, are each connected to a part of the sample layer closer to the center thereof than the part to which the outer sample pads are connected, and each have an exposed upper surface.

Description

Device, evaluation method, and evaluation apparatus

[0001] The present disclosure relates to a device, an evaluation method, and an evaluation apparatus.

[0002] As disclosed in Patent Document 1, the dimensionless figure of merit ZT expressed by the following formula is used to evaluate the figure of merit of a thermoelectric material: ZT=S ２ In Patent Document 1, a measurement sample is prepared by joining a substance to be measured whose thermoelectric properties are to be measured with a reference substance whose thermoelectric properties, at least the Seebeck coefficient and the electrical conductivity, are known. According to the technology disclosed in Patent Document 1, the temperature distribution on the surface of the measurement sample is measured using an infrared camera while applying an alternating current to the measurement sample using lock-in thermography so that the temperature distribution of the measurement target substance changes periodically; from the obtained thermal image, a temperature change signal that changes over time at the same frequency as the current is selectively extracted by Fourier analysis, and a first amplitude image and a first phase image derived solely from the temperature change signal derived from the Peltier effect generated at the junction of the measurement sample are generated; from the same thermal image, a temperature change signal derived from the Peltier effect obtained from the first amplitude image and the first phase image is subtracted by Fourier analysis, thereby generating a second amplitude image and a second phase image derived solely from the temperature change signal derived from Joule heat generated in the measurement sample; and using the temperature change profiles of amplitude and phase in a certain range including the junction of the measurement sample, the thermal conductivity and Seebeck coefficient of the measurement target substance are calculated.

[0003] Japanese Patent Publication No. 2022-101744

[0004] The measurement sample disclosed in Patent Document 1 is intended to measure temperature changes resulting from the Peltier effect occurring at the junction between a measurement target substance and a reference substance. For thin-film samples, which have been actively developed in recent years, there has been a demand for a device, evaluation method, and evaluation apparatus that can easily and quickly determine a dimensionless figure of merit while maintaining high accuracy.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a device, an evaluation method, and an evaluation apparatus that can easily and quickly determine a dimensionless figure of merit for a thin-film sample while maintaining high accuracy.

[0006] The device according to the present disclosure is characterized by comprising a laminated structure including a substrate, a sample layer formed on the substrate from a material that exhibits a lateral thermoelectric effect, an insulating layer formed on the sample layer, and a heater / reflector layer formed on the insulating layer, two outer sample pads made of the same material as the sample layer, each connected to the sample layer and with an exposed top surface, and two central sample pads made of the same material as the sample layer, each connected to a portion of the sample layer closer to the center than the portion to which the outer sample pads are connected, and with an exposed top surface.

[0007] Other features of the present disclosure are set forth below.

[0008] For thin film samples, the dimensionless figure of merit can be determined easily and quickly while maintaining high accuracy.

[0009] 13 is a perspective view showing an example of the configuration of a device. FIG. 14 is a cross-sectional view showing another example of a sample layer. FIG. 15 is a cross-sectional view showing another example of a sample layer. FIG. 16 is a flowchart of an evaluation method. FIG. 17 is a perspective view showing an example of a measurement for obtaining thermal conductivity. FIG. 18 is a diagram showing an example of a physical model. FIG. 19 is a perspective view showing an example of a measurement for obtaining transverse thermoelectric power. FIG. 20 is a perspective view showing an example of a measurement of resistance value. FIG. 21 is a perspective view showing scanning of laser light along a laminated structure. FIG. 22 is a perspective view of a device according to an embodiment. FIG. 23 is a cross-sectional view of the device of FIG. 12. FIG. 24 is a diagram showing the state of a first measurement. FIG. 25 is a diagram showing fixed parameters set to obtain thermal conductivity. FIG. 26 is a diagram plotting observation results. FIG. 27 is a diagram showing the state of a second measurement. FIG. 28 is an experimental result showing the magnetic field dependence of ΔT1f.

[0010] A device, an evaluation method, and an evaluation apparatus according to embodiments will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0011] 1 is a perspective view showing an example of a device 10, which is a measurement sample. The device 10 includes a substrate 12. According to one example, the length of the substrate 12 in the x direction is 5 mm and the length in the y direction is 10 mm. The substrate 12 is provided to support and maintain the structure thereon. The material of the substrate 12 is not particularly limited, but examples thereof include MgO, sapphire, and SrTiO ３ It can be said that:

[0012] On the substrate 12 is a sample layer 14 made of a material that exhibits a transverse thermoelectric effect. The sample layer 14 is made of any material that exhibits a transverse thermoelectric effect, which generates an electromotive force in a direction perpendicular to an applied temperature gradient. Materials that exhibit the transverse thermoelectric effect have been actively researched in recent years, and many materials can be used as the sample layer 14.

[0013] An insulating layer 16 is disposed on the sample layer 14. The insulating layer 16 can be made of an electrically insulating material. In one example, the insulating layer 16 can be made of a material with high electrical insulation performance. In one example, the insulating layer 16 is Al—O or Si—O. A heater-reflecting layer 18 is disposed on the insulating layer 16. In one example, the heater-reflecting layer 18 is made of metal. In one example, the heater-reflecting layer 18 has a conversion coefficient (thermoreflectance coefficient C ) of reflectance change to temperature change for the wavelength of the laser used. ＴＲ The material can be made to have a large thermoreflectance coefficient C ＴＲ Examples of materials with a large thermoreflectance coefficient C include Au, Al, and Mo. When the laser wavelength is about 500 nm, Au can be used for the heater / reflector layer 18, and when the laser wavelength is about 800 nm, Al or Mo can be used. ＴＲ As long as the S / N ratio can be sufficiently high and the temperature change caused by the lateral thermoelectric effect of the sample layer 14 is small, another material can be used. ＴＲ Therefore, a green laser with a wavelength of 520 nm can be used.

[0014] The laminated portion of the substrate 12, the sample layer 14, the insulating layer 16, and the heater / reflector layer 18 is referred to as a laminated structure 20. According to one example, the laminated structure 20 may be formed linearly in a plan view. FIG. 1 illustrates that the laminated structure 20 is formed one-dimensionally in the y direction.

[0015] A plurality of pads are formed on the substrate 12. FIG. 1 illustrates two outer sample pads 14a and 14b made of the same material as the sample layer 14, and two central sample pads 14c and 14d made of the same material as the sample layer 14. The outer sample pads 14a and 14b are connected to both ends of the sample layer 14. In another example, the outer sample pads 14a and 14b are connected to non-end portions of the sample layer 14. The central sample pads 14c and 14d are connected to the middle portion of the sample layer 14, which is a non-end portion. The central sample pads 14c and 14d are connected closer to the center of the sample layer 14 than the portion to which the outer sample pads 14a and 14b are connected. In one example, the outer sample pads 14a and 14b and the central sample pads 14c and 14d are formed in the same process as the sample layer 14. The upper surfaces of the outer sample pads 14a and 14b and the central sample pads 14c and 14d are exposed, allowing stable contact with probes.

[0016] 1, four heater-reflection pads are also formed on the substrate 12 using the same material as the heater-reflection layer 18. The four heater-reflection pads include two heater-reflection pads 18a, 18b connected to both ends of the heater-reflection layer 18 and having exposed top surfaces, and two heater-reflection pads connected to the middle portion, i.e., the non-end portions, of the heater-reflection layer 18 and having exposed top surfaces. In another example, all four heater-reflection pads can be connected to the non-end portions of the heater-reflection layer 18. In one example, the four heater-reflection pads are formed in the same process as the heater-reflection layer 18.

[0017] FIG. 2 is a cross-sectional view showing a modified example of the sample layer 14. FIGS. 2A and 2B show examples in which the thicknesses of the sample layers 14A and 14B vary along the y direction. The thickness of the sample layer 14A in FIG. 2A varies continuously along the y direction. The thickness of the sample layer 14B in FIG. 2B varies discontinuously along the y direction. The upper surface of the sample layer 14A is inclined, while the upper surface of the sample layer 14B is stepped. In this way, the thickness of the sample layer can be varied along the longitudinal direction of the sample layer.

[0018] FIG. 3 is a cross-sectional view showing another modified example of the sample layer 14. The composition of the sample layers shown in FIGS. 3A and 3B varies along the y direction. The composition of sample layer 14C in FIG. 3A varies continuously along the y direction. The composition of the sample layer in FIG. 3B varies discontinuously along the y direction. Specifically, sample layers 14D, 14E, 14F, 14G, and 14H, each with a different composition, are arranged adjacent to one another, causing the composition of the entire sample layer to vary discontinuously along the y direction. In this way, the composition of the sample layer can be varied along the longitudinal direction of the sample layer.

[0019] 4 is a flowchart showing an example of an evaluation method according to an embodiment. First, in step S10, a device is set in an evaluation apparatus. Next, the process proceeds to step S12. In step S12, it is determined whether or not there is a one-dimensional distribution in the thickness or composition of the sample layer. According to one example, an operator inputs the presence or absence of this one-dimensional distribution into a computer.

[0020] If the result of the determination in step S12 indicates that the thickness or composition of the sample layer does not have a one-dimensional distribution, the process proceeds to step S14, in which three physical properties are measured in succession in one region of the device. In one example, a first measurement is performed to obtain thermal conductivity, a second measurement is performed to obtain transverse thermoelectric power, and a third measurement is performed to obtain electrical conductivity.

[0021] First Measurement The first measurement is to obtain the thermal conductivity in the thickness direction of the sample layer. Figure 5 illustrates an example of a measurement to obtain thermal conductivity. In one example, thermal conductivity is obtained using a method known as the 2ω method. First, the probes of the AC power supply 30 are brought into contact with the heater / reflection pads 18a and 18b, and an AC current of frequency f is applied to the heater / reflection layer 18. This causes Joule heat in the heater / reflection layer 18 to generate a temperature change in the heater / reflection layer 18 that varies with a frequency of 2f. The temperature change on the surface of the heater / reflection layer 18 is then obtained using the thermoreflectance method.

[0022] The thermoreflectance method is an optical temperature measurement method that utilizes the temperature dependency of the reflectance of light from an object. The thermoreflectance method can obtain the temperature change in the irradiated area on the surface of the heater / reflective layer 18 by measuring the intensity of reflected light. The change in surface temperature and reflectance can be expressed as follows:

[0023]

[0024] where R is the reflectance, ΔR is the change in reflectance, and C ＴＲ is the thermoreflectance coefficient, that is, the conversion coefficient between temperature change and reflectance change, and ΔT is the amount of temperature change.

[0025] FIG. 5, which explains the measurement of thermal conductivity, illustrates temperature measurement using the thermoreflectance method. According to one example, the temperature measuring device 40 includes a laser light source, a light receiving element, and a lock-in amplifier. The temperature measuring device 40 irradiates the heater / reflecting layer 18 with incident light 42, which is continuous wave laser light, and receives the reflected light 44. The temperature measuring device 40 detects the temperature change, which fluctuates at a frequency 2f, twice the frequency of the AC current, using the lock-in amplifier. Then, the frequency f of the AC current is swept, and the phase component Φ of the temperature change is measured. ２ｆ The thermal conductivity κ of the sample in the film thickness direction is calculated by fitting the frequency response of the sample to a physical model.

[0026] Fig. 6 shows an example of a physical model of the 2ω method. In Fig. 6, κ is the thermal conductivity in the thickness direction, C is the volumetric heat capacity, d is the thickness, Q is the heat density due to Joule heating, q is the heat flux, T is the temperature, and G is the interfacial thermal conductance. The governing equation describing this is a one-dimensional heat diffusion equation, and specifically, it is expressed by the following equation:

[0027]

[0028] Q is expressed by the following equation:

[0029]

[0030] Here, the boundary condition is T ４ｂ = 0, q １ｔ = 0, and the interface conductance G is taken into consideration at the boundary between each layer as shown in the following equation. ｉｂ is the heat flux at the bottom of the i-th layer, q ｊｔ is the heat flux at the surface of the jth layer, G ｊｉ is the interface thermal conductance between the i-th layer and the j-th layer, T ｉｂ is the temperature at the bottom of the i-th layer, T ｊｔ is the temperature at the surface of the jth layer.

[0031]

[0032] By solving Equation 2 under the conditions of Equation 3 and Equation 4, the temperature T １ｔ The response to the frequency 2f is obtained where T １ｔ is a complex number, and the amplitude A ２ｆ and phase Φ ２ｆ The physical quantity used for fitting to the experimental value is Φ ２ｆ The thermal conductivity of the heater and reflective layer, κ １ , volumetric heat capacity C １ , film thickness d １ , the thermal conductivity of the insulating layer κ ２ , volumetric heat capacity C ２ , film thickness d ２ , the volumetric heat capacity of the sample layer C ３ , film thickness d ３ , the thermal conductivity of the substrate κ ４ , volumetric heat capacity C ４ , and the interfacial thermal conductance G２１ , G ３２ , G ４３ is treated as a fixed parameter, and the experimentally obtained Φ ２ｆ The thermal conductivity of the sample layer can be calculated by fitting the frequency 2f dependence data of κ to the derived theoretical curve. ２ must be obtained separately using a thin-film thermal conductivity measurement method such as the 2ω method or the time-domain thermoreflectance (TDTR) method. ２ The thermal resistance d per unit area of ​​the insulating layer 16 is measured to a degree that ２ / κ ２ is high (i.e., d ２ High κ ２ (low). ４ , C ４ can be measured by a combination of standard techniques such as laser flash calorimetry, differential scanning calorimetry, and Archimedes' method. １ , d ２ , d ３ It is desirable to measure κ by X-ray reflectivity or cross-sectional observation using a transmission electron microscope. １ , C １ , C ２ , C ３ is the bulk value, and G ２１ , G ３２ , G ４３ is a typical interfacial thermal conductance value for the metal-insulator interface.

[0033] The thermal conductivity of the sample layer in the thickness direction can be obtained by methods other than the 2ω method. For example, the thermal conductivity can be obtained by the frequency domain thermoreflectance method described in the following document. This method uses periodic laser heating as the heating method and measures the temperature response in the frequency domain. AJ Schmidt et al., Rev. Sci. Instrum. 80, 094901 (2009)

[0034] As another example, thermal conductivity can be obtained by the time-domain thermoreflectance method described in the following document. This method uses impulse laser heating as the heating method and measures the temperature decay in the time domain: DG Cahill, Rev. Sci. Instrum. 75, 5119 (2004) https: / / scienceedge.com / j / learning_center / basic_principles_of_time-domain_thermoreflectance(TDTR).html

[0035] As shown above, thermal conductivity can be calculated using a variety of methods. Thermal conductivity can be obtained using a variety of techniques that use the thermoreflectance method as a temperature measurement method. That is, a sample layer made of a material that exhibits a transverse thermoelectric effect is heated transiently, the temperature response of the sample layer is obtained using the thermoreflectance method, and the thermal conductivity of the sample layer is calculated by fitting it to a physical model. Here, transient heating of the sample layer refers to periodic heating. Although Joule heating is used as an example of transient heating, the sample layer may also be transiently heated using other methods.

[0036] Second Measurement The second measurement is a measurement for obtaining transverse thermoelectric power. Figure 7 shows an example of a measurement for obtaining transverse thermoelectric power. First, the probe of the AC power supply 30 is brought into contact with the outer sample pads 14a and 14b, and an AC current of frequency f is applied to the sample layer 14. This causes a temperature change in the sample layer 14 that varies with frequency f due to the transverse thermoelectric effect of the sample layer 14. Then, the temperature change on the surface of the heater / reflecting layer 18 is obtained by the thermoreflectance method using the temperature measuring device 40. The temperature measuring device 40 measures the temperature change ΔT, which varies with the same frequency f as the AC current. １ｆ The temperature change ΔT is detected by a lock-in amplifier. １ｆ and the thermal conductivity κ of the sample in the film thickness direction, which has already been obtained, based on the following formula, the transverse thermoelectric power S Ｔ Calculate.

[0037]

[0038] Here, jc is the current density, d is the thickness of the sample layer 14, and T is the absolute temperature. In this way, the temperature change due to the transverse thermoelectric effect that occurs when an AC current is applied to the sample layer 14 is obtained using the thermoreflectance method, and the transverse thermoelectric power of the sample layer 14 is calculated from the temperature change and thermal conductivity. In the second measurement, the thermal resistance per unit area of ​​the insulating layer 16 is preferably low. However, as mentioned above, to enable the first measurement, the thermal resistance per unit area of ​​the insulating layer 16 is preferably high. Therefore, the thermal resistance per unit area of ​​the insulating layer 16 should be high enough to enable the first measurement, yet low enough to enable the second measurement. Therefore, the thermal resistance per unit area of ​​the insulating layer 16 should be within a certain range that satisfies both these requirements.

[0039] Third Measurement The third measurement is a measurement to obtain electrical conductivity. FIG. 8 shows an example of a measurement to obtain electrical conductivity. In this example, the resistance value R of the sample layer 14 is measured using the four-terminal method. The probes of the current source 50 are brought into contact with the outer sample pads 14a and 14b, respectively, and the probes of the voltmeter 52 are brought into contact with the central sample pads 14c and 14d, respectively. By separating the current application terminals and the voltage measurement terminals, the influence of contact resistance is eliminated, and the resistance R between the voltage measurement terminals is measured. The electrical conductivity σ is calculated from the measured resistance R, the separately obtained sample film thickness d, sample width w, and distance l between the voltage measurement terminals, according to the following equation:

[0040]

[0041] After the acquisition of the three physical property values ​​in step S14 of Fig. 4 is completed, the measurement process is terminated in step S18, and the dimensionless figure of merit of the horizontal thermoelectric conversion is calculated in step S20. The figure of merit of the horizontal thermoelectric conversion is calculated from the acquired three physical property values, namely, thermal conductivity, horizontal thermoelectric power, and electrical conductivity. The dimensionless figure of merit of the horizontal thermoelectric conversion Z Ｔ T is calculated using the following formula:

[0042]

[0043] Here, S Ｔ is the transverse thermoelectric power, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. ＴFor the electrical conductivity σ and the thermal conductivity κ, the values ​​obtained in the first to third measurements are used, and for the absolute temperature T, the temperature T of the device being monitored during the first to third measurements is used, for example.

[0044] In this way, the dimensionless figure of merit for the lateral thermoelectric conversion of the thin-film sample layer 14 can be evaluated. As described above, the device 10 is configured to enable measurements to acquire the lateral thermopower, thermal conductivity, and electrical conductivity. This allows for faster measurements than, for example, measuring the lateral thermopower, thermal conductivity, and electrical conductivity individually using three different devices. Furthermore, by evaluating the dimensionless figure of merit through a series of measurements on a single device, uncertainty in measurement accuracy due to variations among multiple devices can be avoided.

[0045] 4, if it is determined that the thickness or composition of the sample layer has a one-dimensional distribution, the process proceeds to step S16. In step S16, based on the measurements of the three types of physical properties described above, a laser beam is scanned along the longitudinal direction of the layered structure 20 using the thermoreflectance method. This allows for the acquisition of multiple thermal conductivities and corresponding multiple transverse thermoelectric powers according to the position of the sample layer 14. For example, the thermal conductivity κ is calculated for each of different positions a, b, c, d, and e of the sample layer 14. ａ , κ ｂ , κ ｃ , κ ｄ , κ ｅ and transverse thermoelectric power S Ｔａ , S Ｔｂ , S Ｔｃ , S Ｔｄ , S Ｔｅ To calculate the dimensionless figure of merit at each location, information on the electrical conductivity is also required.

[0046] If the electrical conductivity of the sample layer is uniform and does not have a one-dimensional distribution, the resistance measured by the four-terminal method described above can be used to determine the electrical conductivity at each position (a, b, c, d, e). On the other hand, if the composition of the sample layer has a one-dimensional distribution and the electrical conductivity also has a one-dimensional distribution, or if the electrical conductivity has a one-dimensional distribution for another reason, the electrical conductivity can be obtained using the following method. As shown in Figure 9, an AC current with a frequency f is applied to the sample layer 14, causing a temperature change in the sample layer 14 that changes with a frequency 2f due to Joule heating. While applying this temperature change to the sample layer 14, the temperature change on the surface of the heater / reflectance layer 18 is obtained using the thermoreflectance method. The temperature change component, which fluctuates at a frequency 2f, twice the frequency of the AC current, is detected using a lock-in amplifier. Laser light is irradiated onto the heater / reflectance layer 18 while scanning it in the longitudinal direction, and the amplitude component A of the temperature change is measured. ２ｆ Obtain the one-dimensional distribution of

[0047] Amplitude component A of temperature change ２ｆ The square of the current density j ｃ ２ A normalized by ２ｆ / j ｃ ２ corresponds to the resistance value of the laser light irradiation part. Resistance value A between voltage terminals ２ｆ / j ｃ ２ By using the correspondence between the average value of A and the resistance value obtained by the four-terminal method, ２ｆ / j ｃ ２ The distribution of electrical conductivity can be obtained from the distribution of the amplitude component A ２ｆ The resistance value is calculated from the one-dimensional distribution of the electrical conductivity, and the distribution of the electrical conductivity is obtained by comparing it with the resistance value obtained by the four-terminal method.

[0048] The electrical conductivity thus obtained and the thermal conductivity κ ａ , κ ｂ , κ ｃ , κ ｄ , κ ｅ and transverse thermoelectric power S Ｔａ , S Ｔｂ , S Ｔｃ , S Ｔｄ , S Ｔｅ The dimensionless figure of merit Z along the longitudinal direction of the sample layer 14 is calculated asＴ This method allows the evaluation of the distribution of T. Since this method allows the evaluation of dimensionless figures of merit for multiple sample layers in one device, it has a higher throughput than measuring one dimensionless figure of merit in one device.

[0049] Even if the thickness or composition of the sample layer 14 does not have a one-dimensional distribution, the laser beam can be scanned along the longitudinal direction of the layered structure 20. In this case, the distribution of the lateral thermoelectric performance in the longitudinal direction of the sample layer 14 can be evaluated. In this case, for example, the manufacturing variation of the sample layer 14 can be evaluated.

[0050] FIG. 10 shows a modified device. While the device in FIG. 1 has four pads connected to the heater-reflecting layer 18, the device in FIG. 10 has only two pads, heater-reflecting pads 18a and 18b, connected to the heater-reflecting layer 18. For example, when measuring the thermal conductivity shown in FIG. 5, electrical resistance can be measured using a four-terminal method using the four pads connected to the heater-reflecting layer 18 to determine the AC current power applied to the heater-reflecting layer 18. On the other hand, if such measurement is not necessary, the two pads connected to the heater-reflecting layer 18 can be omitted, as shown in FIG. 10. Additionally, the device structure can be modified as desired as long as the measurements disclosed herein are possible. For example, the shape of the layered structure 20, which includes the substrate 12, sample layer 14, insulating layer 16, and heater-reflecting layer 18, is not linear in plan view and can have a different shape. The planar shape of the layered structure 20 can be any shape as long as it can achieve approximately uniform current density between the voltage measurement terminals of the four-terminal method and approximately uniform current density in the laser light irradiation area, while keeping in mind that current density is likely to be non-uniform at the edges of the layered structure. Such a planar shape can be various shapes such as a square, a U-shape, a ring shape, etc.

[0051] FIG. 11 shows an example of an evaluation device. This evaluation device is a dedicated device that enables the aforementioned evaluation method. This evaluation device includes a stage 60 configured to support the device 10. The evaluation device also includes a temperature measurement unit 61 for measuring the temperature of the device. The temperature measurement unit 61 is an arbitrary thermometer that measures the absolute temperature of the device 10 on the stage 60. The temperature change acquisition unit 62, which acquires the temperature change of the device using the thermoreflectance method, corresponds to the aforementioned temperature measurement instrument 40. According to one example, the temperature change acquisition unit 62 includes at least a laser light source, a light receiving unit, and a lock-in amplifier. The AC current application unit 64 is a device that applies an AC current to the device on the stage. The AC current application unit 64 applies an AC current to the heater / reflecting layer 18 when acquiring thermal conductivity, and to the sample layer 14 when acquiring transverse thermoelectric power. The resistance measurement unit 66 is used to measure resistance using a four-terminal method. The resistance measurement unit 66 has two voltage measurement lines and two current measurement lines and measures the resistance of the device on the stage using the four-terminal method.

[0052] The evaluation device includes a computer 70. The computer 70 performs the above-described arithmetic processing. According to one example, the computer 70 performs a fitting process for calculating the thermal conductivity κ, a arithmetic process for calculating the transverse thermoelectric power, and a process for calculating a dimensionless figure of merit from three physical property values. According to one example, all of the above-described arithmetic processing can be performed by the computer 70. In this case, the computer 70 can function as a figure of merit calculation unit for calculating a figure of merit for transverse thermoelectric conversion from the thermal conductivity, transverse thermoelectric power, and electrical conductivity, a thermal conductivity calculation unit for calculating the thermal conductivity of the device, a transverse thermopower calculation unit for calculating the transverse thermopower of the device, and an electrical conductivity calculation unit for calculating the electrical conductivity of the device. According to another example, the arithmetic processing can be shared between the computer 70 and a cloud server. According to yet another example, part of the arithmetic processing can be performed by the temperature change acquisition unit 62, and the remaining calculations can be performed by the computer 70 or the cloud server.

[0053] This evaluation device can sequentially acquire temperature response using the thermoreflectance method, acquire temperature change due to the transverse thermoelectric effect, and measure resistance. The series of measurements for calculating the dimensionless figure of merit may be performed with the stage 60 fixed or while the stage 60 is moving. The stage 60 can be replaced with another component that supports, grips, or fixes the device 10. In one example, the device 10 can be supported, clamped, or chucked at multiple points. The series of measurements for calculating the dimensionless figure of merit can be completed using this single evaluation device. In one example, the evaluation device has a user interface 72. The user interface 72 allows the user to set various settings related to measurements and calculations and view calculation results. The user interface 72 includes, for example, a display, a keyboard, and a mouse.

[0054] Example: A device capable of measuring thermal conductivity, transverse thermoelectric power, and electrical conductivity was actually manufactured. Figure 12 shows the external appearance of the device. This device includes a sample layer 14 formed linearly on a substrate 12, outer sample pads 14a and 14b connected to the sample layer 14, and central sample pads 14c and 14d. A heater / reflector layer 18 is provided so as to cross the sample layer 14 in a plan view. The longitudinal direction of the heater / reflector layer 18 is parallel to the x-axis, and the longitudinal direction of the sample layer 14 is parallel to the y-axis.

[0055] 13 is a cross-sectional view taken along the dashed line in FIG. 12. The substrate 12 is MgO (111). The substrate 12 is heated at 600° C. for 1 hour. The sample layer 14 is a Co ２０ Fe ６０ B ２０The sample layer 14 was formed by UHV magnetron sputtering at room temperature. The insulating layer 16 was 50 nm thick and made of AlO. The heater / reflector layer 18 was 100 nm thick and made of Au, with a 1 nm thick layer of Cr deposited underneath for adhesion. The heater / reflector layer 18 functions as a transducer that converts light into heat. The insulating layer 16 and heater / reflector layer 18 were formed by ion beam sputtering at room temperature. The sample layer 14, insulating layer 16, and heater / reflector layer 18 were each microfabricated using photolithography and Ar ion milling. Note that the dimensions described here are nominal values.

[0056] A first measurement was performed using the 2ω method to obtain thermal conductivity. FIG. 14 shows the first measurement. First, an AC current with a frequency f was applied to the heater-reflecting layer 18 from the AC power supply 30. Then, a temperature change with a frequency of 2f occurred in the heater-reflecting layer 18 due to Joule heat in the heater-reflecting layer 18. This temperature change was obtained using the thermoreflectance method. Specifically, the temperature change of the heater-reflecting layer 18 was obtained using a temperature measuring device 40 including a laser light source 40a, a lens 40b, a beam splitter 40c, a lens 40d, a light receiving element 40e, and a lock-in amplifier 40f.

[0057] The physical model of the 2ω method is the four-layer model shown in Figure 6. That is, a one-dimensional heat conduction equation is defined as in the previously explained equations 2 and 3, and the boundary condition is T ４ｂ = 0, q １ｔ = 0, and the interface thermal conductance G was taken into consideration. The surface temperature T of the heater / reflector layer 18 obtained as a result of cyclic heating was measured by the thermoreflectance method. １ｔ and this is expressed as Aexp(iφ). Here, A is the amplitude and φ is the phase. FIG. 15 is a diagram showing fixed parameters set to obtain the thermal conductivity. The fixed parameters include the following values: Thickness d, thermal conductivity κ, and volumetric heat capacity C of the heater / reflecting layer 18 Thickness d, thermal conductivity κ, and volumetric heat capacity C of the insulating layer 16 Thickness d and volumetric heat capacity C of the sample layer 14 Thermal conductivity κ and volumetric heat capacity C of the substrate 12 Interface thermal conductance G ２１ , G ３２ , G４３ These fixed parameters were obtained from literature values ​​and from separate experiments. The values ​​shown as [1] in Figure 15 are from DRLide, CRC Handbook of Chemistry and Physics, 85 th ed. (2005). The value indicated by [2] in Figure 15 was obtained with reference to J. Paterson et al., J. Appl. Phys. 127, 245105 (2020). The value indicated by [3] in Figure 15 was obtained with reference to C. Monachon and L. Weber, Adv. Eng. Mater. 17, 68 (2015). The value indicated by [4] in Figure 15 was obtained with reference to H. Jang et al., Phys. Rev. Applied 13, 024007 (2020). The desired value is the thermal conductivity κ of the sample layer 14, which was treated as a fitting parameter.

[0058] To obtain the frequency 2f dependence data of the phase φ, the phase φ was observed while varying the frequency f of the cyclic heating. Figure 16 is a plot of the observation results. The thermal conductivity κ of the sample layer 14 was calculated by fitting the obtained solid theoretical curve to the plotted experimental data. The value of the thermal conductivity κ of the sample layer 14 obtained by fitting was 9.8±0.4 Wm －１ K －１ It was.

[0059] Next, a second measurement was carried out to obtain transverse thermoelectric power. This measurement was conducted using the anomalous Etchingshausen effect (AEE), a phenomenon in which heat flow occurs in the cross product direction of magnetization and current when a current is applied. Figure 17 shows the second measurement. The probes of the AC power supply 30 were brought into contact with the outer sample pads 14a and 14b, and an AC current of frequency f was applied to the sample layer 14. The transverse thermoelectric effect of the sample layer 14 then produces a temperature change in the sample layer 14 that varies with frequency f. The temperature change ΔT on the surface of the heater / reflector layer 18, which fluctuates at the same frequency as the AC current, was measured using the thermoreflectance method with the temperature measuring device 40. １ｆ was detected by a lock-in amplifier.Ｔ is expressed by the following formula:

[0060]

[0061] Here, Π T is the AEE coefficient, T is the absolute temperature, and κ ｘｘ is the thermal conductivity of the sample layer, d is the thickness of the sample layer 14, and j ｃ is the current density, and ΔT １ｆ ＡＥＥ is the temperature change due to the AEE effect. ｃ and temperature T are known values, and κ ｘｘ is the value measured earlier. Therefore, ΔT １ｆ ＡＥＥ By finding S Ｔ This means that we can derive

[0062] FIG. 18 shows the ΔT １ｆ The AEE changes direction depending on the direction of magnetization. In other words, when a magnetic field is applied, the temperature change ΔT １ｆ From the experimental data plotted in Figure 18, the temperature change ΔT １ｆ It was confirmed that the tendency for the value of the magnetic field to change was observed. Figure 18 also shows the measured M-H curve. The plotted experimental data roughly matches the M-H curve, which indicates that the AEE effect can be measured effectively. This confirms that the AEE effect can be measured, but in order to finally extract the AEE component, the differential signal ΔT １ｆ ＡＥＥ , that is, ΔT of the AEE component １ｆ was calculated using the following formula:

[0063]

[0064] As a result of the measurements and calculations, ΔT １ｆ ＡＥＥ The value of ΔT was 4.45 mK. １ｆ ＡＥＥ and the value of the thermal conductivity κ of the sample layer 14 obtained in the first measurement (κ ｘｘ ) is 9.8±0.4 Wm​－１ K －１ and the transverse thermoelectric power S Ｔ Substituting this into the formula, the following results were obtained: Ｔ =1.9±0.1μVK －１ That is, in this example, the transverse thermoelectric power S Ｔ The value is 1.9 μVK －１ It was found that...

[0065] Next, a third measurement was carried out to obtain the electrical conductivity. The electrical conductivity was determined by the four-terminal method shown in FIG. 8. The electrical conductivity σ obtained as a result of the measurement ｙｙ is 8.43 x 10 ５ Ω －１ m －１ It was.

[0066] The values ​​of thermal conductivity, transverse thermoelectric power, and electrical conductivity thus obtained are used as the dimensionless figure of merit Z of transverse thermoelectric conversion. Ｔ By substituting into the equation for T, the dimensionless figure of merit Z in this example is Ｔ The value of T is (0.97±0.1) × 10 －４ Thus, it was found that the dimensionless figure of merit Z Ｔ Although this example is a demonstration experiment focusing on AEE among multiple horizontal thermoelectric conversion phenomena, the dimensionless figure of merit Z Ｔ T can be found.

[0067] 10 Device, 12 Substrate, 14 Sample layer, 14a, 14b Outer sample pads, 14c, 14d Central sample pads, 16 Insulating layer, 18 Heater / reflector layer, 18a, 18b Heater / reflector pads

Claims

1. A device comprising a substrate, a sample layer formed of a material that exhibits a transverse thermoelectric effect on the substrate, an insulating layer formed on the sample layer, and a heater-reflection layer formed on the insulating layer, having a laminated structure; two outer sample pads made of the same material as the sample layer, each connected to the sample layer and having an exposed upper surface; and two central sample pads made of the same material as the sample layer, each connected to a portion of the sample layer closer to the center than the portion to which the outer sample pads of the sample layer are connected and having an exposed upper surface.

2. The device according to claim 1, further comprising two heater-reflection pads made of the same material as the heater-reflection layer, each connected to the heater-reflection layer and having an exposed upper surface.

3. The device according to claim 1, wherein the layer thickness of the sample layer varies along the longitudinal direction of the sample layer.

4. The device according to claim 1, wherein the composition of the sample layer varies along the longitudinal direction of the sample layer.

5. An evaluation method comprising: non-steadily heating a sample layer formed of a material that exhibits a transverse thermoelectric effect, obtaining the temperature response of the sample layer by the thermoreflectance method, and calculating the thermal conductivity of the sample layer by fitting with a physical model; obtaining the temperature change due to the transverse thermoelectric effect generated when an alternating current is applied to the sample layer by the thermoreflectance method, and calculating the transverse thermoelectric power of the sample layer from the temperature change and the thermal conductivity; and obtaining the electrical conductivity of the sample layer.

6. The evaluation method according to claim 5, further comprising calculating a performance index of transverse thermoelectric conversion from the thermal conductivity, the transverse thermoelectric power, and the electrical conductivity.

7. The evaluation method according to claim 5, wherein the acquisition of the temperature response, the acquisition of the temperature change, and the acquisition of the electrical conductivity are sequentially performed by one evaluation device.

8. The evaluation method according to claim 5, comprising: obtaining the temperature change of a heater-reflection layer formed on the sample layer by the thermoreflectance method while applying a temperature change to the sample layer; irradiating a laser beam while scanning it in the longitudinal direction of the heater-reflection layer to obtain a one-dimensional distribution of the amplitude component of the temperature change; obtaining the resistance value from the one-dimensional distribution of the amplitude component and comparing it with the resistance value obtained by the four-terminal method to obtain the distribution of the electrical conductivity.

9. A temperature change acquisition unit that acquires a temperature change of a device by a thermoreflectance method, an alternating current application unit that applies an alternating current to the device, a resistance value measurement unit that has two voltage measurement lines and two current measurement lines and measures the resistance value of the device by a four-terminal method, a temperature measurement unit that measures the absolute temperature of the device, and a performance index calculation unit that calculates a performance index of a lateral thermoelectric conversion. An evaluation apparatus comprising:

10. A thermal conductivity calculation unit that calculates the thermal conductivity of the device, a lateral thermoelectric power calculation unit that calculates the lateral thermoelectric power of the device, and an electrical conductivity calculation unit that calculates the electrical conductivity of the device. The performance index calculation unit calculates the performance index from the thermal conductivity, the lateral thermoelectric power, and the electrical conductivity. The evaluation apparatus according to claim 9.

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