Sensor element

Abstract

We provide a new sensor element that can measure temperature, magnetic field, and electromagnetic wave frequency. [Solution] This sensor element comprises a bulk semiconductor layer 101 made of a crystal of a direct-bandgap type first semiconductor, and a plurality of quantum dots 102 formed on the bulk semiconductor layer 101. The energy range in which the density of states of the second semiconductor is distributed is set to be lower in energy than the energy range in which the density of states of the first semiconductor is distributed. Furthermore, the plurality of quantum dots 102 are covered with a semiconductor layer 103 made of a third semiconductor having a larger band gap than the second semiconductor.

Description

[Technical Field]

[0001] This invention relates to a sensor element. [Background technology]

[0002] Sensors that utilize the magnetic response properties of solid materials such as semiconductors include temperature sensors, magnetic field Hall sensors, and frequency sensors.

[0003] For example, a typical temperature sensor is the resistance temperature sensor. This temperature sensor utilizes the fact that the resistance of a solid changes with temperature, and metals are often used as the solid (Non-Patent Document 1).

[0004] Another commonly used magnetic field sensor is the Hall sensor. This sensor utilizes the Hall effect in semiconductors. The output voltage of this sensor changes with the strength of the magnetic field (Non-Patent Literature 2).

[0005] One device that detects the frequency of specific electromagnetic waves is called a microwave counter. This device measures the frequency of an input microwave by using a microwave with a precisely determined frequency as a reference (Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Basic knowledge of electronic components, "Basic knowledge of temperature sensors, Basics of temperature sensors," [Accessed August 26, 2020], KOA Corporation, Product Information, (https: / / www.koaglobal.com / product / library / sensor / basic). [Non-Patent Document 2] Hall sensor, technical explanation, "#02 Summary of Hall Sensors", [Retrieved August 26, 2019], Asahi Kasei Electronics Corporation, product information, (https: / / www.akm.com / jp / ja / products / hall-sensor / tutorial / hall-sensors / ). [Non-Patent Document 3] Know & Learn, Learning Information, "Basics of Measurement | Measuring Time and Frequency", TechEyesOnline, [Searched August 26, 2019], (https: / / www.techeyesonline.com / article / tech-column / detail / Basic-Column-009 / #:~:text=%E5%91%A8%E6%B3%A2%E6%95%B0%E3%82%AB%E3%82%A6%E3%83%B3%E3%82%BF%E3%81%AF%E5%86%85%E9%83%A8%E3%81%AB,%E3%81%AA%E3%82%8A%E3%81%BE%E3%81%99%EF%BC%88%E5%9B%B31%EF%BC%89%E3%80%82). [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Conventional technologies all use solid materials, but they measure either temperature, magnetic field, or electromagnetic wave (microwave) frequency.

[0008] This invention was made to solve the above-mentioned problems and aims to provide a new sensor element that can measure temperature, magnetic field, and electromagnetic wave frequency. [Means for solving the problem]

[0009] The sensor element according to the present invention comprises a bulk semiconductor layer composed of a direct-bandgap type first semiconductor and a plurality of quantum dots composed of a direct-bandgap type second semiconductor formed on the bulk semiconductor layer, wherein the energy range in which the density of states of the second semiconductor is distributed is at a lower energy level than the energy range in which the density of states of the first semiconductor is distributed. [Effects of the Invention]

[0010] As described above, according to the present invention, since multiple quantum dots are formed on a bulk semiconductor layer, a new sensor element is provided that can measure temperature, magnetic field, and electromagnetic wave frequency. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a cross-sectional view showing the configuration of a sensor element according to an embodiment of the present invention. [Figure 2] Figure 2 is an explanatory diagram illustrating the effect of a magnetic field on the electronic state. [Figure 3] Figure 3 shows the band diagram of a typical direct bandgap semiconductor. [Figure 4] Figure 4 is a characteristic diagram showing the relationship between the g-factor and temperature of a bulk semiconductor. [Figure 5] Figure 5 is a characteristic diagram showing the density of states of a semiconductor. [Figure 6] Figure 6 is a distribution diagram showing the distribution of electrons in a semiconductor in the density of states space. [Figure 7] Figure 7 is a characteristic diagram showing the relationship between the g-factor and temperature of a bulk semiconductor. [Figure 8] Figure 8 is a perspective view showing an example of a quantum dot configuration. [Figure 9] Figure 9 is a characteristic diagram showing the density of states of a quantum dot where carriers have been formed. [Figure 10] Figure 10 is a characteristic diagram showing the density of states of a composite structure of quantum dots and bulk semiconductors. [Figure 11] Figure 11 is a perspective view showing the theoretical structure of the quantum dot used in the analysis of the exciton coupling energy. [Figure 12] FIG. 12 is a characteristic diagram showing the analysis result of exciton binding energy. [Figure 13] FIG. 13 is an explanatory diagram for explaining the state in which carriers confined in quantum dots are distributed in the bulk due to the increase in temperature. [Figure 14] FIG. 14 is a characteristic diagram showing the calculation result of the temperature dependence of the g factor. [Figure 15] FIG. 15 is a perspective view showing a configuration in which electrodes are arranged on the left and right of a quantum dot and an electric field is applied. [Figure 16] FIG. 16 is a characteristic diagram showing the state in which the exciton binding energy Eb of a quantum dot changes by arranging electrodes on the left and right of the quantum dot and applying an electric field in the lateral direction. [Figure 17] FIG. 17 is a cross-sectional view showing the configuration of the sensor element in Example 1. [Figure 18] FIG. 18 is a characteristic diagram showing the relationship between microwave absorption and magnetic field in the sensor element of Example 1. [Figure 19] FIG. 19 is a cross-sectional view showing the configuration of the sensor element in Example 2. [Figure 20] FIG. 20 is a characteristic diagram showing the relationship between microwave absorption and microwave frequency in the sensor element of Example 2.

MODE FOR CARRYING OUT THE INVENTION

[0012] Hereinafter, a sensor element according to an embodiment of the present invention will be described with reference to Figure 1. This sensor element comprises a bulk semiconductor layer 101 composed of a crystal of a direct-bandgap type first semiconductor, and a plurality of quantum dots 102 formed on the bulk semiconductor layer 101. The energy range in which the density of states of the second semiconductor is distributed is set to be on the lower energy side than the energy range in which the density of states of the first semiconductor is distributed. The first semiconductor can be, for example, GaAs. The second semiconductor can also be, for example, GaAs. Furthermore, the plurality of quantum dots 102 are covered with a semiconductor layer 103 composed of a third semiconductor having a larger band gap than the second semiconductor. The third semiconductor can be, for example, AlGaAs.

[0013] Here, the number of quantum dots 10² per square centimeter can be greater than or equal to the reciprocal of the square of the exciton diffusion distance. For example, in the case of GaAs, the exciton diffusion distance is about 0.2 μm, and the number of quantum dots 10² per square centimeter is 2.5 × 10¹⁶. 9 This can be done.

[0014] The sensor element according to the above-described embodiment can perform temperature measurement, magnetic field measurement, and electromagnetic wave frequency measurement.

[0015] The following provides a more detailed explanation.

[0016] Let's consider the effect of applying a magnetic field to an electronic state in a solid or gas. Generally, an electronic state can contain electrons with both upward and downward spins (Figure 2). When a magnetic field B0 is applied to this electronic state, the energy of the electronic state separates into two, with electrons of different spins existing in each. This separation energy depends on the magnitude of the applied magnetic field and a coefficient called the g-factor, which represents the magnitude of the interaction between the electronic state and the magnetic field.

[0017] These two distinct states absorb electromagnetic waves with energy equal to their separation energy. Furthermore, the g-factor is known to vary depending on the material or medium in which the electrons are present, as well as on environmental factors such as temperature and pressure.

[0018] Let's consider the g-factor of a typical semiconductor (bulk semiconductor). Figure 3 is the band diagram of a representative direct bandgap semiconductor. The g-factor of the state near the bottom of the conduction band in this band diagram is given by the following equation (1) (Reference 1).

[0019]

number

[0020] In equation (1), P is a constant determined by the material. Equation (1) includes the band gap E g This is included. If the band gap changes, the g factor changes.

[0021] Now, let's consider the temperature dependence of the g factor. As mentioned earlier, the g factor depends on the band gap, and it is known that this band gap changes with temperature, according to equation (2) below (Reference 2).

[0022]

number

[0023] In equation (2), E g (0), α, and β are constants determined by the material; in the case of GaAs, they are 1.519 eV and 10.6 × 10⁻¹⁰ eV, respectively. -4 The eV / K is 671K. g From the temperature change, it is predicted that the g factor will change with temperature, as shown in Figure 4.

[0024] The analysis described above is a theoretical prediction and is known to be inconsistent with experimental results (Reference 3). The energy structure of the semiconductor is as shown in Figure 3, and when this density of states is expressed, it becomes the state shown in Figure 5. This represents the number of states that can exist in the semiconductor per unit energy as a function of energy, and electrons in the semiconductor are distributed in this density of states space according to a function determined by temperature. At low temperatures, the distribution becomes narrower as shown in Figure 6, but it becomes more widespread as the temperature rises. When this effect is taken into account, the temperature dependence of the g factor becomes as shown in Figure 7, which is consistent with the experimental results.

[0025] Here, we consider a structure in which quantum dots are coupled to the bulk semiconductor described above. As shown in Figure 8, quantum dots are structures several times larger than the exciton Bohr radius of the semiconductor (approximately 10 nm for GaAs). In the example shown in Figure 8, quantum dots made of GaAs are surrounded by a layer of AlGaAs. Since the band gap of GaAs is smaller than the band gap of AlGaAs, when carriers such as electrons, holes, and excitons are formed in the quantum dots, they are confined within the quantum dots.

[0026] Carriers confined within a quantum dot cannot move spatially, and therefore possess properties similar to electrons bound to an atom. The density of states of this quantum dot is as shown in Figure 9. Although there are multiple states, the number of states can be adjusted by the size of the quantum dot, and it is also possible to have only one state. The density of states of this composite structure of quantum dots and bulk semiconductor is a combination of Figures 5 and 9, as shown in Figure 10. The exciton coupling energy Eb in Figure 10 plays an important role in the operation of the sensor element according to Embodiment 1.

[0027] The exciton binding energy is the energy at which the electrons and holes forming the exciton attract each other via Coulomb attraction. To estimate this exciton binding energy, it is necessary to analyze only the quantum dot. Since the quantum dots actually fabricated are not cubes but mainly thin rectangular prisms, the analysis was performed using the theoretical structure shown in Figure 11. The analysis results are shown in Figure 12. As shown in Figure 12, the exciton binding energy is in the lateral direction (L) of the quantum dot. x ,L y ) changes with increasing size (Reference 4).

[0028] From the results described above, the following can be said about the carrier distribution in the composite structure of quantum dots and bulk semiconductors.

[0029] First, in the low-temperature region, carriers are confined to quantum dots, but as the temperature increases and exceeds the exciton binding energy, they begin to be distributed in the bulk (Figure 13). Therefore, the g-factor has the characteristics of quantum dots in the low-temperature region and the characteristics of the bulk in the high-temperature region. Since the state in which carriers are confined to quantum dots does not change with temperature like the bulk alone, the temperature dependence of the g-factor is expected to follow the characteristics shown in Figure 4.

[0030] Figure 14 shows the results of calculating the temperature dependence of the g factor. It can be seen that the temperature dependence of the g factor changes with the exciton coupling energy Eb of the quantum dot. Focusing on the result for Eb = 9 meV, the g factor is -0.456 at a temperature of 122 K. Using this g factor, the calculation results for the electromagnetic wave frequency corresponding to energy separation with respect to the magnetic field are shown in Tables 1 and 2 below.

[0031] [Table 1]

[0032] As shown in Tables 1 and 2, it can be seen that the frequency of absorbed electromagnetic waves changes with temperature and magnetic field. Also, when Eb = 12 meV, the g-factor changes to -0.474 at a temperature of 214 K, and the frequency of absorbed electromagnetic waves also changes. This is the principle by which the sensor element according to the embodiment operates as a sensor. A key feature of these characteristics is that the g-factor takes a minimum value at a certain temperature. Near this minimum value, the g-factor does not change much even with some temperature fluctuations. Therefore, a stable g-factor can be obtained, and the sensor element according to the embodiment can obtain a stable output.

[0033] Furthermore, consider placing electrodes on the left and right sides of the quantum dot (Figure 15) and applying an electric field. Applying an electric field in the lateral direction makes it possible to change the exciton coupling energy Eb of the quantum dot, as shown in Figure 16. This result is a calculation result for a quantum dot with a thickness of 10 nm and a lateral size of 50 nm × 50 nm. Eb can be changed by the electric field. This means that, as shown in the result in Figure 14, the temperature dependence of the g factor (of the sensor element according to the embodiment) can be changed by the electric field.

[0034] The following will provide a more detailed explanation using examples.

[0035] [Example 1] First, Example 1 will be described. In Example 1, as shown in Figure 17, a bulk semiconductor layer 101 made of GaAs with a thickness of 100 nm was used as a sample, and multiple quantum dots 102 were formed on its surface. The bulk semiconductor layer 101 was grown on a substrate 111 via a buffer layer 104 made of AlGaAs. The quantum dots 102 had a height of 10 nm and a width of 50 nm × 50 nm. The multiple quantum dots 102 were covered with a semiconductor layer 103 made of AlGaAs.

[0036] For the sample of Example 1 described above, first, the sample is irradiated with laser 131 to form electron-hole pairs (excitons) in each quantum dot 102. In this state, the sample is irradiated with microwave 132 having a frequency of 13.2 GHz, and the temperature is set to 214 K. Also, a microwave detector 121 is arranged to estimate the amount of microwave absorption of the sample. An external magnetic field B0 133 is applied to the sample, and the external magnetic field B0 133 is changed to analyze the amount of microwave absorption. The number per square centimeter (dot density) of the plurality of quantum dots 102 is set to be not less than the reciprocal of the square of the exciton diffusion distance.

[0037] When the dot density is low, excitons that cannot be confined in each quantum dot 102 exist, and the g-factor has the properties of the bulk even at low temperatures, resulting in a degradation in the performance as a sensor. In the case of GaAs, since the diffusion distance is about 0.2 μm (Reference 5), the dot density needs to be 2.5×10 9 (per cm 2 ) or more. In the sample of Example 1, the dot density of the plurality of quantum dots 102 is 5×10 9 (per cm 2 ).

[0038] The calculation results of microwave absorption in the sample of Example 1 are shown in FIG. 18. The absorption of the microwave increases when the external magnetic field B0 133 is near 2 T. The width of the absorption peak is determined by the lifetime of the electron spin. It was found that the sensor element of Example 1 can sense the magnetic field.

[0039] Also, when the magnetic field is fixed at 2 T and the temperature is changed, the absorption of the microwave increases near 122 K. Therefore, the sensor element of Example 1 can function as a temperature sensor. Also, if the magnetic field and temperature are fixed, the sensor element of Example 1 can detect a microwave with a frequency of 13.2 GHz by changing the frequency of the microwave.

[0040] [Example 2] Next, Example 2 will be described. In Example 2, as shown in Figure 19, electrodes 122a, 122b and a bias power supply are added to the configuration of Example 1. Electrodes 122a and 122b make it possible to apply a lateral electric field to the quantum dot 102. The width of the quantum dot 102 is 50 nm, and Eb is 12 meV with an electric field of 0 kV / cm, but when an electric field of 3 kV / cm is applied, Eb becomes 9 meV. In Example 2, the external magnetic field was fixed at 2 T, and the temperature was changed by the electric field to analyze microwave absorption. Figure 20 shows the analysis results plotted on a coordinate system with microwave absorption on the vertical axis and frequency on the horizontal axis. It was found that the absorption frequency changes when the electric field is changed. This means that the sensor element according to Example 1 functions as a detector for a specific microwave frequency.

[0041] As described above, according to the embodiment of the present invention, since multiple quantum dots are formed on a bulk semiconductor layer, a new sensor element capable of measuring temperature, magnetic field, and electromagnetic wave frequency can be provided.

[0042] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented within the technical concept of the present invention by those with ordinary skill in the art. [Explanation of symbols]

[0043] 101...Bulk semiconductor layer, 102...Quantum dot, 103...Semiconductor layer.

Claims

1. A bulk semiconductor layer composed of a direct-bandgap type first semiconductor, A plurality of quantum dots formed on the bulk semiconductor layer, which are composed of a direct-bandgap type second semiconductor, Equipped with, The energy range in which the density of states of the second semiconductor is distributed is lower than the energy range in which the density of states of the first semiconductor is distributed.

2. In the sensor element according to claim 1, A sensor element in which the number of the aforementioned plurality of quantum dots per square centimeter is equal to or greater than the reciprocal of the square of the diffusion distance of the exciton.

3. In the sensor element according to claim 1 or 2, The sensor element is characterized by a plurality of quantum dots covered by a semiconductor layer composed of a third semiconductor having a larger band gap than the second semiconductor.

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