Apparatus and method for measuring quantum dot spin energy level using mechanical mode

Abstract

The application provides a method, device and preparation method for measuring quantum dot spin energy level by mechanical mode, which aims to prepare a carbon nanotube micro-nano resonator by micro-nano processing technology, and measure the spin energy level by coupling the mechanical mode of carbon nanotube vibration with the electron spin energy level in the quantum dot. The carbon nanotube is regulated by a gate voltage to form a quantum dot structure, and the mechanical mode formed when the carbon nanotube vibrates can be coupled with the electron spin energy level in the quantum dot, the frequency offset of the mechanical mode is read by the change of the spin state, and thus the measurement of the spin energy level in the quantum dot is realized. The device has simple structure, small size and is compatible with the micro-nano processing technology, and provides a new idea for measuring the spin energy level in the semiconductor quantum dot.

Description

Technical Field

[0001] This invention relates to the fields of quantum dots, micro / nano electromechanical system resonators, and electron spin level measurement. Specifically, it relates to a method and apparatus for measuring quantum dot spin levels using a mechanical mode. Background Technology

[0002] Semiconductor quantum dots primarily utilize surface electrodes to modulate the charge density at certain locations within the two-dimensional electron gas of the semiconductor, thereby forming zero-dimensional structures with a small number of electrons clustered in specific regions. A single quantum dot system typically consists of the quantum dot itself, a source, a drain, and a control gate. The quantum dot itself is connected to the source and drain via potential barriers, but is not directly connected to the gate, only exhibiting a capacitive effect. Electron spin plays a crucial role in high-precision magnetic induction detection and quantum computing; the single electron spin within a quantum dot, as a fundamental qubit in quantum information processing, is currently being studied in various systems.

[0003] Traditional methods for measuring spin energy levels in quantum dots employ transport measurements at extremely low temperatures, specifically measuring the relationship between the current generated when electrons are transferred from the source to the drain under a given bias voltage and the electrode voltage. The advantage of this approach is its simple circuitry and direct readout of the electrical signal. However, transport measurements also have a significant drawback: they are highly sensitive to the magnitude of the current signal. When the current generated by electron tunneling is much smaller than the ammeter's maximum measurement sensitivity, effective current signal data cannot be obtained.

[0004] Based on this, the present invention provides a method and apparatus for measuring the spin energy level of a quantum dot using a mechanical mode. By coupling the spin of electrons in the quantum dot with the mechanical mode of a micro / nanocomputer system resonator, and reading the resonant signal of the mechanical mode, the spin energy level in the quantum dot is measured. Summary of the Invention

[0005] To achieve the above-mentioned objectives, this invention provides an apparatus and method for measuring the spin energy level of a quantum dot using a mechanical mode.

[0006] A method for measuring the spin energy level of quantum dots using a mechanical mode first excites the carbon nanotube 4 to vibrate by applying an RF signal to the source electrode, while simultaneously applying a DC voltage to the gate electrode to modulate the resonant frequency of the carbon nanotube and adjust the electrochemical potential of the quantum dot. The tunneling current is detected to determine whether electron tunneling has occurred in the quantum dot. Then, an external magnetic field is applied in the X direction parallel to the carbon nanotube to modulate the spin state of the electrons in the quantum dot, causing further splitting of the degeneracy energy levels of the spin states. Zeeman splitting occurs between the different spin states.

[0007] ΔE=gμ B B (1)

[0008] Where ΔE is the spin level splitting factor, g is the Landé factor, and μ B =58μeVT -1 is the Bohr magneton, B is the magnetic flux density, and for carbon nanotube quantum dots, the g factor is taken as 2 in this formula;

[0009] To obtain spin level splitting with a target value of ΔE, the external magnetic field B to be applied is calculated according to formula (1). At the same time, the gate voltage is changed to control the resonant frequency of the carbon nanotube. When a certain spin energy level of the electron in the quantum dot matches the mechanical mode of the carbon nanotube, a coupling phenomenon will occur. Let the resonant frequency of the mechanical mode at this time be f0. Mode splitting is observed on its phase diagram. Then, the change of electron spin in the quantum dot will cause the frequency change of the mechanical mode of the carbon nanotube. Thus, an amplitude modulation or frequency modulation signal of f0 is applied to the source, and the signal passing through the quantum dot is measured at f0 using a lock-in amplifier at the drain. The resonant signal is demodulated, thereby obtaining the frequency shift of the mechanical mode. The frequency of the mechanical mode shifts from f0 to f1, and the frequency shift is Δf. Different spin energy levels of the electron in the quantum dot correspond to different Zeeman energies. When the mechanical mode of the carbon nanotube is coupled with different spin energy levels, the obtained frequency shift Δf is also different. Thus, the change of electron spin energy level in the quantum dot is converted into the frequency shift of the mechanical mode of the carbon nanotube, thereby realizing the measurement of spin energy level.

[0010] The X direction is from source 1 to drain 2, and the Y direction is from the silicon dioxide substrate to the silicon substrate, perpendicularly upward.

[0011] As a preferred embodiment, the device used includes: a silicon substrate 6, a silicon dioxide substrate 5 above the silicon substrate 6, a source electrode 1 and a drain electrode 2 on the left and right sides of the upper surface of the silicon dioxide substrate 5 respectively, a groove is provided in the middle of the silicon dioxide 5 of the source electrode 1 and the drain electrode 2, a gate electrode 3 is provided at the bottom of the groove, a gap is provided between the two sides of the gate electrode 3 in the X direction and the edge of the groove, the two ends of the carbon nanotube 4 are located on the surface of the source electrode 1 and the drain electrode 2 respectively, and the middle part of the carbon nanotube 4 is suspended directly above the gate electrode 3, and a quantum dot 7 is formed by regulating the gate voltage.

[0012] As a preferred method, this method can operate at a temperature of 100mK, with the target spin level splitting ΔE being 100μeV, and the external magnetic field B calculated according to formula (1) is 0.86T.

[0013] As a preferred embodiment, the source 1, drain 2, and gate 3 are all composite electrodes of 5nm Ti and 45nm Au, and the width of gate 3 is 200nm.

[0014] As a preferred embodiment, the depth of the groove where the gate is located in the Y direction is 360nm, the width in the X direction is 1μm, and the distance from both sides of the gate in the X direction to the edge of the groove is 400nm.

[0015] The present invention also provides an apparatus for measuring the spin energy level of a quantum dot using a mechanical mode, as used in the method described above.

[0016] This invention also provides a method for fabricating a device for measuring the spin energy level of a quantum dot using a mechanical mode, comprising the following steps:

[0017] ① Pre-treatment: including wafer dicing and ultrasonic cleaning of the Si substrate and SiO2 substrate surfaces;

[0018] ② Coating: Spin coat the clean Si substrate and SiO2 substrate with photoresist on a spin coater, and immediately place them on a heating stage for pre-baking treatment;

[0019] ③ First photolithography: The Si substrate and SiO2 substrate with photoresist spin-coated are subjected to electron beam exposure treatment, and then the photoresist layer is successively developed and fixed to obtain a groove pattern with a width of 1μm.

[0020] ④ Dry etching: The substrate with the groove pattern is dry etched by an inductively coupled plasma etching machine. The purpose is to transfer the groove pattern from the photoresist layer to the SiO2 layer. By controlling the parameters in the dry etching process, a groove with an etching depth of 360nm and a width of 1μm is obtained.

[0021] ⑤ Photoresist coating: After the previous step, clean the substrate, including ultrasonic cleaning in acetone, anhydrous ethanol and deionized water in sequence; then perform a spin coating of photoresist.

[0022] ⑥ Second photolithography: Similarly, the Si substrate and SiO2 substrate coated with photoresist are subjected to electron beam exposure, and then developed and fixed in sequence to obtain a gate trench pattern with a width of 200nm and an exposed source and drain pattern.

[0023] ⑦ Evaporated electrode: After the second photolithography, 5nm of Ti and 45nm of Au are sequentially deposited on the surface using an electron beam evaporation coating system, thereby obtaining an electrode with a thickness of 50nm.

[0024] ⑧ Metal stripping: The substrate with the deposited electrode is ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in sequence to remove the remaining photoresist and strip away the metal on the photoresist to obtain the electrode structure.

[0025] ⑨ Transferred carbon nanotubes: Using a polymethyl methacrylate-assisted transfer method, the target carbon nanotubes are transferred above the gate to form a carbon nanotube suspension structure, thereby obtaining the device.

[0026] The beneficial effects of this invention are as follows: This invention provides a method and apparatus for measuring the spin energy level of quantum dots using mechanical modes. The purpose is to fabricate carbon nanotube micro / nano resonators using micro / nano fabrication technology, and to measure the spin energy level by coupling the mechanical modes of carbon nanotube vibration with the electron spin energy levels in the quantum dots. The carbon nanotubes are used to form a quantum dot structure through gate voltage modulation. Simultaneously, the mechanical modes formed during the vibration of the carbon nanotubes can couple with the electron spin energy levels in the quantum dots. The frequency shift of the mechanical mode is read by changing the spin state, thereby realizing the measurement of the spin energy level in the quantum dots. This device has a simple structure, small size, and is compatible with micro / nano fabrication technology, providing a new approach for measuring the spin energy level in semiconductor quantum dots. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the overall structure of the device of the present invention.

[0028] Figure 2 This is a front cross-sectional view of the device of the present invention.

[0029] Figure 3 This is a schematic diagram of the energy relationship of quantum dots.

[0030] Figure 4 This is a schematic diagram of the frequency shift of the mechanical mode of the carbon nanotube olefin harmonic oscillator.

[0031] Figure 5 This is a flowchart illustrating the manufacturing process of the device of the present invention.

[0032] 1 is the source, 2 is the drain, 3 is the gate, 4 is a carbon nanotube; 5 is a silicon dioxide substrate; 6 is a silicon substrate, and 7 is a quantum dot. Detailed Implementation

[0033] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0034] The tunneling rate of electrons jumping into and out of a quantum dot depends on the relative magnitudes of the quantum dot's electrochemical potential and the source / drain electrochemical potentials. The quantum dot's electrochemical potential can be modulated by the gate voltage, while the source / drain electrochemical potentials are determined by the Fermi level and can be modulated by the bias voltage between the source and drain. When the quantum dot's electrochemical potential is mismatched with the source / drain electrochemical potentials, tunneling current cannot be formed, i.e., Coulomb blockage. When the three potentials are matched, tunneling occurs, and a Coulomb peak can be observed; the magnitude of the Coulomb peak is related to the tunneling rate. Figure 3The diagram shows the energy relationship of quantum dots. In the case shown, electron tunneling cannot be achieved, and the number of electrons in the quantum dots is fixed.

[0035] like Figure 2 As shown, this embodiment provides a method for measuring the spin energy level of a quantum dot using a mechanical mode. First, an RF signal is applied to the source to excite the carbon nanotube 4 to vibrate. Simultaneously, a DC voltage is applied to the gate, thereby controlling the resonant frequency of the carbon nanotube and adjusting the electrochemical potential of the quantum dot. The tunneling current is detected to determine whether electron tunneling has occurred in the quantum dot. Then, an external magnetic field is applied in the X direction parallel to the carbon nanotube. This magnetic field controls the spin state of the electrons in the quantum dot, causing further splitting of the degenerate energy levels of the spin states. Zeeman splitting occurs between the different spin states.

[0036] ΔE=gμ B B (1)

[0037] Where ΔE is the spin level split, g is the Landé factor, and μ B =58μeVT -1 is the Bohr magneton, B is the magnetic flux density, and for carbon nanotube quantum dots, the g factor is taken as 2 in this formula;

[0038] To obtain spin level splitting with a target value of ΔE, the required external magnetic field B is calculated according to formula (1). Simultaneously, the gate voltage is changed to regulate the resonant frequency of the carbon nanotube. When a certain spin level of the electron in the quantum dot matches the mechanical mode of the carbon nanotube, coupling occurs. At this time, the resonant frequency of the mechanical mode is f0, and mode splitting is observed on the phase diagram of the carbon nanotube. Furthermore, the change in the electron spin level in the quantum dot will cause a frequency change in the mechanical mode of the carbon nanotube. Therefore, an amplitude modulation or frequency modulation signal of f0 is applied to the source, and a lock-in amplifier is used at f0 to measure the signal passing through the quantum dot. The resonant signal is demodulated, thereby obtaining the frequency offset of the mechanical mode. Figure 4 As shown, the mechanical mode frequency shifts from f0 to f1, with a frequency shift of Δf. Different spin energy levels of electrons in the quantum dot correspond to different Zeeman energies. When the mechanical mode of the carbon nanotube is coupled with different spin energy levels, the resulting frequency shift Δf is also different. Thus, the change in the electron spin energy level in the quantum dot is transformed into the frequency shift of the carbon nanotube mechanical mode, thereby realizing the measurement of the spin energy level.

[0039] The X-direction is from source 1 to drain 2, and the Y-direction is from the silicon dioxide substrate to the silicon substrate, perpendicularly upwards.

[0040] like Figure 1As shown, this embodiment also provides a device for measuring the spin energy level of quantum dots using a mechanical mode, including: a silicon substrate 6, a silicon dioxide substrate 5 above the silicon substrate 6, a source 1 and a drain 2 on the left and right sides of the upper surface of the silicon dioxide substrate 5 respectively, a groove is provided in the middle of the silicon dioxide 5 of the source 1 and the drain 2, a gate 3 is provided at the bottom of the groove, a gap is provided between the two sides of the gate 3 in the X direction and the edge of the groove, the two ends of the carbon nanotube 4 are located on the surface of the source 1 and the drain 2 respectively, the middle part of the carbon nanotube 4 is suspended directly above the gate 3, and a quantum dot 7 is formed by regulating the gate voltage.

[0041] This embodiment can operate at a temperature of 100mK. When the target spin level splitting ΔE is 100μeV, the external magnetic field B calculated according to formula (1) is 0.86T.

[0042] Source 1, drain 2, and gate 3 are all composite electrodes of 5nm Ti and 45nm Au, and the width of gate 3 is 200nm.

[0043] The depth of the groove where the gate is located is 360nm in the Y direction and the width is 1μm in the X direction. The distance from the gate to the edge of the groove on both sides in the X direction is 400nm.

[0044] This embodiment also provides a method for fabricating a device for measuring the spin energy level of a quantum dot using a mechanical mode, comprising the following steps:

[0045] ① Pre-treatment: This includes wafer dicing and ultrasonic cleaning of the surfaces of Si and SiO2 substrates; such as... Figure 5 As shown in ①;

[0046] ② Photoresist Coating: Photoresist is spin-coated onto clean Si and SiO2 substrates using a spin coater, and immediately placed on a heating stage for pre-baking. Figure 5 As shown in ②;

[0047] ③ Single-step photolithography: A Si substrate and a SiO2 substrate coated with photoresist are spin-coated and then subjected to electron beam exposure. Subsequently, development and fixing processes are performed sequentially to obtain a groove pattern with a width of 1 μm on the photoresist layer; for example... Figure 5 As shown in ③;

[0048] ④ Dry etching: Dry etching of the substrate with groove patterns is performed using an inductively coupled plasma etching machine. The purpose is to transfer the groove pattern from the photoresist layer to the SiO2 layer. By controlling the parameters during the dry etching process, grooves with an etching depth of 360 nm and a width of 1 μm are obtained; for example... Figure 5 As shown in ④;

[0049] ⑤ Photoresist Coating: After the previous step, clean the substrate, including sequential ultrasonic cleaning in acetone, anhydrous ethanol, and deionized water; then perform another spin-coating of photoresist; such as... Figure 5As shown in ⑤;

[0050] ⑥ Second photolithography: Similarly, the Si substrate and SiO2 substrate coated with photoresist are subjected to electron beam exposure, followed by development and fixing to obtain a gate trench pattern with a width of 200nm and exposed source / drain patterns; such as Figure 5 As shown in ⑥;

[0051] ⑦ Evaporated Electrode: After the second photolithography, an electron beam evaporation deposition system is used to sequentially deposit 5nm of Ti and 45nm of Au across the entire surface, thus obtaining an electrode with a thickness of 50nm; for example... Figure 5 As shown in ⑦;

[0052] ⑧ Metal stripping: The substrate with the deposited electrodes is ultrasonically cleaned sequentially in acetone, anhydrous ethanol, and deionized water to remove the remaining photoresist and strip away the metal on the photoresist to obtain the electrode structure; for example... Figure 5 As shown in Figure ⑧;

[0053] ⑨ Transferred carbon nanotubes: Using a polymethyl methacrylate-assisted transfer method, the target carbon nanotubes are transferred above the gate to form a suspended carbon nanotube structure, thereby obtaining the device of the present invention. For example... Figure 5 As shown in Figure 9.

[0054] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for measuring the spin energy level of a quantum dot using a mechanical mode, characterized in that: First, by applying an RF signal to the source, the carbon nanotube (4) is excited to vibrate. At the same time, a DC voltage is applied to the gate to regulate the resonant frequency of the carbon nanotube and the electrochemical potential of the quantum dot. The tunneling current is detected to determine whether electron tunneling occurs in the quantum dot. Then, an external magnetic field is applied in the X direction parallel to the carbon nanotube. The spin state of the electron in the quantum dot is regulated by the magnetic field, causing the degenerate energy levels of the spin state to split further, and Zeeman splitting occurs between different spin states. ΔE=gμ B B (1) Where ΔE represents spin level splitting, g is the Landé factor, and μ B =58μeVT -1 is the Bohr magneton, B is the magnetic flux density, and for carbon nanotube quantum dots, the g factor is taken as 2 in this formula; To obtain spin level splitting with a target value of ΔE, the external magnetic field B to be applied is calculated according to formula (1). At the same time, the gate voltage is changed to control the resonant frequency of the carbon nanotube. When a certain spin energy level of the electron in the quantum dot matches the mechanical mode of the carbon nanotube, a coupling phenomenon will occur. Let the resonant frequency of the mechanical mode at this time be f0. Mode splitting is observed on its phase diagram. Then, the change of electron spin in the quantum dot will cause the frequency change of the mechanical mode of the carbon nanotube. Thus, an amplitude modulation or frequency modulation signal of f0 is applied to the source, and the signal passing through the quantum dot is measured at f0 using a lock-in amplifier at the drain. The resonant signal is demodulated, thereby obtaining the frequency shift of the mechanical mode. The frequency of the mechanical mode shifts from f0 to f1, and the frequency shift is Δf. Different spin energy levels of the electron in the quantum dot correspond to different Zeeman energies. When the mechanical mode of the carbon nanotube is coupled with different spin energy levels, the obtained frequency shift Δf is also different. Thus, the change of electron spin energy level in the quantum dot is converted into the frequency shift of the mechanical mode of the carbon nanotube, thereby realizing the measurement of spin energy level. The X direction is from the source (1) to the drain (2), and the Y direction is from the silicon dioxide substrate (5) to the silicon substrate (6) in a vertical upward direction.

2. The method for measuring quantum dot spin energy levels using a mechanical mode according to claim 1, characterized in that: The device used includes: a silicon substrate (6) and a silicon dioxide substrate (5) above the silicon substrate (6). The upper surface of the silicon dioxide substrate (5) has a source (1) and a drain (2) on the left and right sides respectively. The silicon dioxide (5) of the source (1) and drain (2) is provided with a groove in the middle. The bottom of the groove is provided with a gate (3). There is a gap between the two sides of the gate (3) in the X direction and the edge of the groove. The two ends of the carbon nanotube (4) are located on the surface of the source (1) and the drain (2) respectively. The middle part of the carbon nanotube (4) is suspended directly above the gate (3). Quantum dots (7) are formed by regulating the gate voltage.

3. The method for measuring quantum dot spin energy levels using a mechanical mode according to claim 1, characterized in that: This method can operate at a temperature of 100 mK. When the target spin level splitting ΔE is 100 μeV, the external magnetic field B calculated according to formula (1) is 0.86 T.

4. The method for measuring quantum dot spin energy levels using a mechanical mode according to claim 2, characterized in that: The source (1), drain (2), and gate (3) are all composite electrodes of 5nm Ti and 45nm Au, and the gate (3) has a width of 200nm.

5. The method for measuring quantum dot spin energy levels using a mechanical mode according to claim 2, characterized in that: The depth of the groove where the gate is located is 360nm in the Y direction and the width is 1μm in the X direction. The distance from the gate to the edge of the groove on both sides in the X direction is 400nm.

6. The apparatus for measuring the spin energy level of a quantum dot using a mechanical mode, as used in the method of claim 2, 4, or 5.

7. The method for fabricating a device for measuring quantum dot spin energy levels using a mechanical mode according to claim 6, characterized in that... Includes the following steps: ① Pre-treatment: including wafer dicing and ultrasonic cleaning of the Si substrate and SiO2 substrate surfaces; ② Coating: Spin coat the clean Si substrate and SiO2 substrate with photoresist on a spin coater, and immediately place them on a heating stage for pre-baking treatment; ③ First photolithography: The Si substrate and SiO2 substrate with photoresist spin-coated are subjected to electron beam exposure treatment, and then the photoresist layer is successively developed and fixed to obtain a groove pattern with a width of 1μm. ④ Dry etching: The substrate with the groove pattern is dry etched by an inductively coupled plasma etching machine. The purpose is to transfer the groove pattern from the photoresist layer to the SiO2 layer. By controlling the parameters in the dry etching process, a groove with an etching depth of 360nm and a width of 1μm is obtained. ⑤ Photoresist coating: After the previous step, clean the substrate, including ultrasonic cleaning in acetone, anhydrous ethanol and deionized water in sequence; then perform a spin coating of photoresist. ⑥ Second photolithography: Similarly, the Si substrate and SiO2 substrate coated with photoresist are subjected to electron beam exposure, and then developed and fixed in sequence to obtain a gate trench pattern with a width of 200nm and an exposed source and drain pattern. ⑦ Evaporated electrode: After the second photolithography, 5nm of Ti and 45nm of Au are sequentially deposited on the surface using an electron beam evaporation coating system, thereby obtaining an electrode with a thickness of 50nm. ⑧ Metal stripping: The substrate with the deposited electrode is ultrasonically cleaned in acetone, anhydrous ethanol and deionized water in sequence to remove the remaining photoresist and strip away the metal on the photoresist to obtain the electrode structure. ⑨ Transferred carbon nanotubes: Using a polymethyl methacrylate-assisted transfer method, the target carbon nanotubes are transferred above the gate to form a carbon nanotube suspension structure, thereby obtaining the device.

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