CN111289832B - Method for measuring and changing trend of circularly polarized light induced current of three-dimensional topological insulator Bi2Te3 along with temperature - Google Patents
Method for measuring and changing trend of circularly polarized light induced current of three-dimensional topological insulator Bi2Te3 along with temperature Download PDFInfo
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Abstract
The invention provides a method for measuring, calculating and changing a three-dimensional topological insulator Bi2Te3The method for changing the trend of circularly polarized current along with temperature comprises the following steps: step S1: preparation of three-dimensional topological insulators Bi with different thicknesses2Te3A film; step S2: in three-dimensional topological insulator Bi2Te3Depositing a pair of circular electrodes on the surface of the film to obtain a test sample; step S3: measuring the variable-temperature circularly polarized light induced current of test samples with different thicknesses by adopting a variable-temperature circularly polarized light induced current measuring system to obtain a three-dimensional topological insulator Bi2Te3The films were left to polarize circularly polarized light at different temperatures. The experimental device for measuring and calculating provided by the method is simple in structural design, easy to operate, beneficial to popularization and application, accurate in test result, and capable of changing the Bi of the three-dimensional topological insulator simply, quickly and effectively based on the method provided by the demonstration2Te3The circularly polarized light of (2) causes a trend of current change with temperature.
Description
Technical Field
The invention belongs to the field of topological insulator polarization optics and spintronics, and particularly relates to a method for measuring and changing the trend of circularly polarized light induced current of a three-dimensional topological insulator Bi2Te3 along with temperature.
Background
The three-dimensional topological insulator is a novel quantum material state with a spin momentum locked Dirac electron state on the surface, and has good application potential in the fields of spin electronics and quantum computing. Bi2Te3Is a typical three-dimensional topological insulator, and therefore, has attracted much attention.
The three-dimensional topological insulator leads to the locking of the spin momentum of the topological surface state due to strong spin orbit coupling and time reversal symmetry, and provides an ideal platform for the spintronics. Under the condition of no external bias voltage and under the irradiation of a beam of circularly polarized light, the distribution of photogenerated carriers on a k-space is unbalanced according to the energy conservation and the angular momentum conservation, so that measurable directional current is formed in a real space. This current is a circularly polarized light induced current. Circularly polarized light induced current is an effective method for studying band spin splitting. Moreover, circularly polarized light induced current is an effective means for developing novel spin optoelectronic devices. The three-dimensional topological insulator has larger circularly polarized light induced current due to the surface state of the spin momentum locking. How to effectively regulate and control circularly polarized light induced current has important significance for developing novel spin optoelectronic devices.
Disclosure of Invention
The present invention contemplates that changing the temperature may be an efficient and feasible way of manipulating the circularly polarized light induced current. However, at present, the three-dimensional topological insulator Bi2Te3The trend with temperature is still unclear, as is the method of changing its trend with temperature. A method for measuring and changing the temperature variation trend of circularly polarized light induced current of a three-dimensional topological insulator Bi2Te3 is provided.
The invention specifically adopts the following technical scheme:
a method for measuring and calculating the trend of circularly polarized light induced current of a three-dimensional topological insulator Bi2Te3 along with temperature is characterized by comprising the following steps:
step S1: preparation of three-dimensional topological insulators Bi with different thicknesses2Te3A film;
step S2: in three-dimensional topological insulator Bi2Te3Depositing a pair of circular electrodes on the surface of the film to obtain a test sample;
step S3: measuring the variable-temperature circularly polarized light induced current of test samples with different thicknesses by adopting a variable-temperature circularly polarized light induced current measuring system to obtain a three-dimensional topological insulator Bi2Te3Film at different temperaturesCircularly polarized light under the condition of low power consumption;
the temperature-changing circularly polarized light current measurement system comprises: excitation light source to and set gradually along the light path of excitation light source: the device comprises an attenuation sheet, a chopper, a polarizer, a quarter-wave plate and a Dewar flask with a light-transmitting window; the Dewar flask is connected with a temperature control instrument; the test sample is arranged in the Dewar flask, and the electrode is led out of the Dewar flask through a lead and is connected with the data acquisition device; the quarter-wave plate is connected with the stepping motor.
Preferably, in step S1, the three-dimensional topological insulator Bi2Te3The film grows on the insulating Si substrate along the (111) crystal plane.
Preferably, the material of the Si substrate is undoped, with the fermi level in the middle of the band gap.
Preferably, in step S1, the three-dimensional topological insulator Bi2Te3The film grows on SrTiO along the (111) crystal face3On a substrate.
Preferably, the three-dimensional topological insulator Bi2Te3The thickness of the film is 3-30 nm.
Preferably, in step S2, the circular electrodes have a pitch of 1-3 mm and a diameter of 0.5-1 mm.
Preferably, the step S3 specifically includes the following steps:
step S31: smoothly adhering a test sample on the AlN substrate by using low-temperature glue, and adhering 2 gold-plated ceramic plates on the AlN substrate by using the low-temperature glue; adhering the AlN substrate with the test sample on a sample seat of the Dewar flask by using low-temperature glue, connecting a circle center electrode on the test sample with a gold wire serving as a lead wire with a gold-plated ceramic wafer on the AlN substrate, and connecting the gold-plated ceramic wafer with a binding post of the Dewar flask by using a silver wire; vacuumizing the Dewar flask by a mechanical pump, and keeping the vacuum degree below 1 Pa;
step S32: a 1064-nanometer laser is used as an excitation light source, and the laser emitted by the laser sequentially passes through the attenuation sheet, the chopper, the polarizer, the quarter-wave plate and the Dewar flask window and is incident at the midpoint of the connecting line of the two circular electrodes of the test sample at an incidence angle theta; the diameter of the light spot is smaller than the distance between the two circular electrodes; under illumination, the test sample generates photocurrent, and the photocurrent enters a low-noise preamplifier and a lock-in amplifier and finally enters a data acquisition card; the chopper is connected with the phase-locked amplifier and is used for inputting a reference signal;
step S33: setting the temperature of the temperature controller at 77K, filling liquid nitrogen into the Dewar flask, and placing a temperature control rod to stabilize the temperature;
step S34: controlling a stepping motor to drive the quarter-wave plates to rotate from 0 degree to 360 degrees, taking 5 degrees as a step length, and collecting photocurrent at each quarter-wave plate rotation angle through a data acquisition cardJ(ii) a When the quarter-wave plate is at 0 degree, the fast axis direction is parallel to the polarizer direction;
step S35: changing the temperature of the test sample by the temperature controller, repeating the step S34, and measuring the photocurrent at different temperaturesJ；
Step S36: photocurrent at each temperature by the following formula (1)JAnd (3) fitting:
whereinJ C Is the current caused by the excitation of circularly polarized light, namely circularly polarized light induced current,L 1 andL 2 the current caused by the linearly polarized light,J 0 is background current, mainly comes from photovoltaic effect, thermoelectric effect and Danpei effect,φobtaining a circularly polarized light current-induced curve of the test sample along with the temperature change for the included angle between the fast axis of the quarter-wave plate and the polarization direction of the polarizer;
step S37: and replacing samples with different thicknesses, and repeating the steps S31-S36 to obtain the circularly polarized light induced current of the samples with different thicknesses along with the change of the temperature.
Preferably, the power of the 1064 nm laser in step S32 is 10-200 mW; the working frequency of the chopper is 60-1000 Hz; the range of the incident angle theta is 10-50 degrees; the temperature setting range of the temperature controller in the step S35 is 77K-300K.
And a method for changing the trend of circularly polarized light induced current of a three-dimensional topological insulator Bi2Te3 along with temperature, which is characterized by comprising the following steps: by changing the three-dimensional topological insulator Bi2Te3The thickness of the substrate and the type of the substrate change the trend of the circularly polarized light induced current along with the temperature.
Preferably, the substrate is an undoped Si substrate or SrTiO3A substrate.
Compared with the prior art, the innovation points of the invention and the preferred scheme thereof are as follows:
1. the provided measuring and calculating experiment device is simple in structural design, easy to operate, beneficial to popularization and application and accurate in test result.
2. The method based on the demonstration can simply, conveniently, quickly and effectively change the three-dimensional topological insulator Bi2Te3The circularly polarized light of (2) causes a trend of current change with temperature.
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The invention is described in further detail below with reference to the following figures and detailed description:
FIG. 1 is a schematic optical path diagram of a measurement system of the present invention;
FIG. 2 is a three-dimensional topological insulator Bi of 5 nm thickness grown on a Si substrate in one embodiment of the present invention2Te3The change curve of the photocurrent of the film under the excitation of light with the wavelength of 1064 nanometers at 300K along with the rotation angle of the quarter-wave plate is shown. The solid line is a curve obtained by linear fitting according to the formula (1), and the hollow origin is data measured by experiments.
FIG. 3 shows three-dimensional topological insulators Bi of different thicknesses grown on Si substrate according to one embodiment of the present invention2Te3Comparing the curves of the circularly polarized light induced current and temperature variation measured by the sample;
FIG. 4 shows an example of an embodiment of the present invention grown on SrTiO3Three-dimensional topological insulator Bi with different thicknesses on substrate2Te3Comparing the curves of the circularly polarized light induced current and temperature variation measured by the sample;
FIG. 5 shows an example of a three-dimensional topological insulator Bi for photogenerated spin-polarized carriers in a Si substrate2Te3Injection ofA schematic diagram of (a);
FIG. 6 shows a three-dimensional topological insulator Bi grown on a Si substrate in an embodiment of the present invention2Te3The energy band schematic diagrams of the upper surface and the lower surface, and the schematic diagrams of the generation of circularly polarized light induced current and the influence of substrate spin injection electrons on the circularly polarized light induced current;
FIG. 7 shows an embodiment of the present invention grown on Si substrate to a thickness of 7 nm and 20 nm and on SrTiO3Three-dimensional topological insulator Bi with thickness of 5 nm and 20 nm on substrate respectively2Te3Electron mobility of the sample is plotted as a function of temperature.
Detailed Description
In order to make the features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail as follows:
in this embodiment, the design measures the three-dimensional topological insulator Bi2Te3The method for changing the trend of circularly polarized light induced current along with temperature specifically comprises the following process steps:
step 1, growing three-dimensional topological insulators Bi with different thicknesses on an insulating i substrate growing along a (111) crystal plane by using molecular beam epitaxy equipment2Te3A film;
step 2: in three-dimensional topological insulator Bi through electron beam evaporation2Te3Depositing a pair of round titanium/gold electrodes on the surface of the film;
and step 3: measuring the variable-temperature circularly polarized light induced current of the samples with different thicknesses obtained in the steps 2 and 3 by adopting a variable-temperature circularly polarized light induced current measuring system; wherein, alternating temperature circular polarized light current measurement system includes: excitation light source to and set gradually along the light path of excitation light source: the device comprises an attenuation sheet, a chopper, a first small hole, a polarizer, a quarter-wave plate second small hole and a Dewar flask with a light-transmitting window; wherein the Dewar flask is connected with a temperature controller for temperature regulation; the test sample is arranged in the Dewar flask, and the electrode is led out of the Dewar flask through a lead and is connected with a data acquisition device comprising a preamplifier, a lock-in amplifier and a computer; the quarter-wave plate is connected with a stepping motor and used for controlling the accurate rotation of the quarter-wave plate. The chopper is also connected to the lock-in amplifier for inputting the reference signal.
And 4, step 4: using molecular beam epitaxy equipment to grow SrTiO along (111) crystal plane3Growing three-dimensional topological insulators Bi with different thicknesses on a substrate2Te3A film;
and 5: adopting the methods of the steps 2 and 3 to the sample obtained in the step 4 to obtain SrTiO3Three-dimensional topological insulator Bi grown on substrate2Te3The films were left to polarize circularly polarized light at different temperatures.
In this example, the three-dimensional topological insulator thickness in step 1 was 3, 5, 7, 20 nm, respectively, and the sample size was 2 mm × 12 mm. In the step 4, the thicknesses of the three-dimensional topological insulator are respectively 5 nm, 7 nm and 20 nm, and the size of the sample is 2 mm multiplied by 5 mm.
Preferably, in the present embodiment, the pair of circular electrodes in step 2 is located at the rectangle Bi2Te3On the midline of the long side of the film, the diameter of a pair of circular electrodes is 0.25 mm, and the electrode distance is 1.5 mm.
In this embodiment, step 3 specifically includes the following steps:
step 31: the sample is smoothly pasted on an AlN substrate by low-temperature glue, and 2 gold-plated ceramic wafers are pasted on the AlN substrate by the low-temperature glue. The AlN substrate with the sample attached thereto was attached to the sample holder of the Dewar flask with the light-transmittable window by means of a low-temperature adhesive. The electrodes on the sample were then connected to the gold-plated ceramic wafer on the AlN substrate using gold wires as leads, and then the gold-plated ceramic wafer was connected to the terminal posts of the dewar flask using silver wires. The dewar flask was evacuated with a mechanical pump, maintaining the vacuum below 1 Pa.
Step 32: a 1064 nm laser was used as the excitation light source. The laser emitted by the laser sequentially passes through the attenuation sheet, the chopper, the polarizer, the quarter-wave plate and the window of the Dewar flask and is incident on the midpoint of the connecting line of the two electrodes of the sample at an incidence angle theta. The diameter of the light spot is smaller than the distance between the two electrodes. Under illumination, a photocurrent is generated on the sample and enters the low noise preamplifier and the lock-in amplifier. And finally, entering a data acquisition card for data acquisition.
Step 33: the temperature of the temperature controller was set at 77K, enough liquid nitrogen was poured into the dewar, and then a temperature control rod was placed to stabilize the temperature.
Step 34: the stepping motor is controlled by a computer to drive the quarter-wave plate to rotate from 0 degree to 360 degrees, 5 degrees is taken as a step length, and the photocurrent at each quarter-wave plate rotation angle is collected by a data acquisition cardJ. When the quarter-wave plate is at 0 degree, the fast axis direction is parallel to the polarizer direction.
Step 35: changing the temperature of the sample by the temperature controller, repeating step 34, and measuring the photocurrent at different temperaturesJ。
Step 36: photocurrent at each temperature by the following formula (1)JAnd (3) fitting:
whereinJ C Is the current caused by the excitation of circularly polarized light, namely circularly polarized light induced current,L 1 andL 2 the current caused by the linearly polarized light,J 0 is background current, mainly comes from photovoltaic effect, thermoelectric effect and Danpei effect,φis the included angle between the fast axis of the quarter-wave plate and the polarization direction of the polarizer. This makes it possible to obtain a circularly polarized light current curve of the sample as a function of temperature.
Step 37: and replacing samples with other thicknesses, and repeating the steps 31-36 to obtain circularly polarized light induced currents of the samples with different thicknesses along with temperature change.
In step 32, the 1064-nanometer laser power is 50 mW; the working frequency of the chopper in the step 32 is 229 Hz; in step 32, the incident angle theta is 30 degrees; the range of the temperature change in step 35 is 77K-300K, and a set of photoelectric flow data is measured every 20K.
Specifically, the measurement optical path of the variable temperature circularly polarized light induced current adopted in the present embodiment is as shown in fig. 1. The laser used in this embodiment is a solid-state laser with a wavelength of 1064 nm. The light from the laser passes through the attenuation sheet, the chopper, the polarizing sheet and the quarter wave plate in sequence, penetrates through the window of the Dewar flask, and irradiates the midpoint of the connecting line of the two electrodes on the sample. The incident angle of the laser was 30 degrees. The rotation angle of the quarter-wave plate is controlled by a stepping motor. The temperature of the dewar is controlled by a temperature controller. The generated photo-generated current enters a current preamplifier to be amplified, then enters a phase-locked amplifier, and finally enters a computer to be collected by a data acquisition card arranged on the computer.
FIG. 2 is a three-dimensional topological insulator Bi of 5 nm thickness grown on a Si substrate2Te3The change curve of the photocurrent of the film under the excitation of light with 1064 nm wavelength at 300K along with the rotation angle of the quarter-wave plate. The solid line is a curve obtained by linear fitting according to the formula (1), and the hollow origin is data measured by experiments. Curve lineJ C To the circularly polarized light current-causing data obtained by fitting,L 1 andL 2 to obtain data of the linearly polarized light induced current by fitting,J 0 data for background current obtained by fitting.
Three-dimensional topological insulator Bi with different thicknesses on Si substrate samples2Te3After depositing the titanium gold electrode on the sample, obtaining three-dimensional topological insulators Bi with different thicknesses growing on different substrates according to the formula (1) according to the steps 31-372Te3The circularly polarized light induced current of the sample is plotted as a function of temperature, as shown in fig. 3 and 4. The three-dimensional topological insulator Bi grown on the Si substrate can be seen2Te3Sample and SrTiO3Three-dimensional topological insulator Bi grown on substrate2Te3The samples exhibited completely different trends with temperature. The circularly polarized light induced current of the sample with the thickness of 3, 5 and 7 nm grown on the Si substrate firstly increases and then decreases along with the decrease of the temperature, and the circularly polarized light induced current of the sample with the thickness of 20 nm has the current sign opposite to that of the sample with the thickness of 3, 5 and 7 nm at room temperature. This is because the upper surface state of the 20 nm sample is oxidized more seriously, so that the measured circularly polarized light current in the sample is dominant in the lower surface state, and the circularly polarized light current of the samples with the thickness of 3, 5, 7 nm is dominant in the upper surface stateThe states dominate, so their circularly polarized currents have opposite signs. However, growth on SrTiO3Bi with thickness of 5 and 7 nm on three-dimensional topological insulator on substrate2Te3Sample of Bi with a thickness of 20 nm in which circularly polarized light current monotonously increases with decreasing temperature2Te3The circularly polarized current of the sample decreases with decreasing temperature and finally exhibits the opposite sign.
Bi grown on Si substrate2Te3Sample and SrTiO3Three-dimensional topological insulator Bi grown on substrate2Te3The completely different trend of the circularly polarized light current of the sample with the temperature is due to the different dominant mechanisms of the circularly polarized light current of the samples with different thicknesses and different substrates under different temperatures. For the sample grown on the Si substrate, the circularly polarized light induced current is influenced by the spin injection of the Si substrate. And SrTiO3The sample grown on the substrate has no spin injection effect of the substrate, and the influence of the thermoelectric effect is more obvious for the sample with larger thickness. Therefore, their circularly polarized light current effect shows different variation trends with the change of temperature.
FIG. 5 shows a graph consisting of i/Bi2Te3Energy band distribution at interface and injection of spin-polarized carriers of Si substrate into Bi2Te3Schematic in thin films. Wherein the conduction band order
=0.81 eV, valence band offset
=0.14 eV is measured by XP. Since Bi of this example2Te3The film is thin and therefore, a large portion of the light is still transmitted to the Si substrate. The Si substrate absorbs these circularly polarized light, creating an optical transition, i.e. an electron transition from the valence band to the conduction band, creating a spin polarized carrier. These spin-polarized carriers are injected into Bi2Te3In the film, as indicated by the curved arrow located lowermost in fig. 5. Since Bi2Te3The spin diffusion length of the thin film electrons is short, and therefore, these injected spin-polarized electrons mainly affect the lower surface state of the sample.
FIG. 6 shows Bi grown on a Si substrate2Te3The energy band structures of the upper surface and the lower surface of the film and the principle of circularly polarized light induced current generation are schematically shown. Where 1 and 2 represent the first and second surface states, respectively, CB1 represents the first conduction band, and VB1 and VB2 represent the first and second valence bands, respectively. Under excitation of circularly polarized light, the resulting transitions will contribute to circularly polarized light induced current, namely (1) electron transition to 1 at VB1, (2) electron transition to 2 at 1, and (3) electron transition to 2 at CB 1.
Electrons on VB1 transition to 1, generating photogenerated electrons on 1 and photogenerated holes on VB1, noting that the circularly polarized light contributed by the photogenerated electrons on 1 isJ e1 The circularly polarized light of VB1 due to the photogenerated holes isJ h1 . The electron at 1 transits to 2, the electron at CB1 transits to 2, so that photogenerated holes are generated at 1 and CB1, photogenerated electrons are generated at 2, and the circularly polarized light induced current contributed by the photogenerated holes at 1 and CB1 isJ h2 2 circularly polarized light with photo-generated electron contribution asJ e2 . Since the spins and momentum of the surface states are locked, the direction of motion of the 1 spin up electron and the 2 spin down electron is opposite, and thus, they areJ e1 AndJ e2 is opposite in direction. At the same time, the user can select the desired position,J h1 andJ h2 the current direction of (2) is also opposite. Allowing for faster relaxation of electrons in high energy states and resultingJ e1 AndJ h1 the momentum of the electrons of (i.e., k is greater), and therefore,J e1 andJ h1 will be greater thanJ e2 AndJ h2 . Thus, circularly polarized light causes current to flow fromJ e1 AndJ h1 and (4) leading. According to the transition selection rule, in the sameUnder the excitation of circularly polarized light, the spin direction of the electron generated by the transition from valence band to conduction band in i and Bi2Te3The spin polarization direction of the electrons generated by the VB1 to 1 transition in the film is the same. These injected spin-polarized carriers will therefore recombine with holes in the valence band, thereby reducing the amount of charge carriers injectedJ h1 So that the circularly polarized light on the lower surface will have a reduced current. From the experiment of front-back incidence, it is shown that the circularly polarized light current of the sample of this example is dominated by the upper surface state. The currents are opposite due to circularly polarized light in the upper surface state and the lower surface state. Therefore, a decrease in the circularly polarized light induced current of the lower surface state will enhance the total circularly polarized light induced current. As can be seen from this example, Bi grown on a Si substrate2Te3Film ratio of SrTiO3Bi grown on a substrate2Te3The circularly polarized light induced current of the film is much stronger because the spin injection of the Si substrate enhances the signal of the circularly polarized light induced current.
For three-dimensional topological insulator Bi grown on Si substrate2Te3The circularly polarized light induced current of the sample, especially the sample with the thickness of 3, 5 and 7 nm, increases with the decrease of the temperature, because the electron mobility increases with the decrease of the temperature, as shown in fig. 7, thereby the circularly polarized light induced current increases. And the circularly polarized light induced current decreases again with further decrease in temperature because the spin injection of the Si substrate decreases with decrease in temperature, thereby decreasing the total circularly polarized light induced current.
For in SrTiO3Three-dimensional topological insulator Bi grown on substrate2Te3In the sample, as can be seen from fig. 4, when the film thickness of the sample is 5 nm and 7 nm, the circularly polarized light current shows a tendency to increase with decreasing temperature, because the electron mobility increases with decreasing temperature, and in particular, refer to fig. 6. However, for a sample with a thickness of 20 nm, as the temperature decreases, the circularly polarized light current decreases first, and finally the opposite sign shows an increasing tendency. This is because the thermoelectric current generated in the vertical direction of the sample having a thickness of 20 nm is smaller than that of the samples having a thickness of 5 nm and 7 nm which are thinnerMuch stronger. Specifically, since laser irradiation of the upper surface state produces a certain heating effect, a temperature gradient exists between the upper surface and the substrate, and the influence of this gradient on circularly polarized light induced current increases with the increase in thickness under the same conditions. The existence of the temperature gradient will generate a vertical thermal current, thereby generating a vertical thermal field. Under the action of the thermal electric field, spin current in vertical direction is generated, and the spin polarization direction of the spin current is in plane. In addition, due to the inverse spin hall effect, this spin current is converted into a charge current, which is referred to as an inverse spin hall current. The direction of this anti-spin hall current is exactly the same as or opposite to that of the circularly polarized light induced current. When the direction of the counter-spin Hall current is opposite to that of the circularly polarized current, the counter-spin Hall current and the circularly polarized current cancel each other and even have opposite signs. As the temperature decreases, the temperature difference between the upper surface of the sample and the substrate increases, so that the vertical pyroelectric effect increases, and the resulting inverse spin hall current increases, and therefore, the opposite sign of the circularly polarized light current appears with decreasing temperature, and the circularly polarized light current increases inversely with further decreasing temperature.
In summary, the present embodiment changes the three-dimensional topological insulator Bi2Te3The thickness and the substrate can change the trend of the circularly polarized light current changing along with the temperature. For a sample growing on the Si substrate, the degree of the injection influence of the Si substrate is changed by changing the thickness, so that the purpose of changing the trend of the circularly polarized light induced current along with the temperature is realized. For SrTiO3Three-dimensional topological insulator Bi grown on substrate2Te3The strength of the thermal field in the vertical direction is changed by changing the thickness of the sample, and the change trend of the circularly polarized light current along with the temperature is changed by utilizing the influence of the reverse spin Hall current induced by the thermal field on the circularly polarized light current.
The present invention is not limited to the above preferred embodiments, and any other methods for measuring and changing the trend of circularly polarized light induced current with temperature of the three-dimensional topological insulator Bi2Te3 can be obtained from the teaching of the present invention, and all equivalent changes and modifications made according to the claims of the present invention shall fall within the scope of the present invention.
Claims (8)
1. Three-dimensional topological insulator Bi is calculated2Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized by comprising the following steps of:
step S1: preparation of three-dimensional topological insulators Bi with different thicknesses2Te3A film;
step S2: in three-dimensional topological insulator Bi2Te3Depositing a pair of circular electrodes on the surface of the film to obtain a test sample;
step S3: measuring the variable-temperature circularly polarized light induced current of test samples with different thicknesses by adopting a variable-temperature circularly polarized light induced current measuring system to obtain a three-dimensional topological insulator Bi2Te3Circularly polarized light induced current of the film at different temperatures;
the temperature-changing circularly polarized light current measurement system comprises: excitation light source to and set gradually along the light path of excitation light source: the device comprises an attenuation sheet, a chopper, a polarizer, a quarter-wave plate and a Dewar flask with a light-transmitting window; the Dewar flask is connected with a temperature control instrument; the test sample is arranged in the Dewar flask, and the electrode is led out of the Dewar flask through a lead and is connected with the data acquisition device; the quarter-wave plate is connected with a stepping motor;
by changing the three-dimensional topological insulator Bi2Te3The thickness of the substrate and the type of the substrate change the trend of the circularly polarized light current along with the temperature;
the substrate is an undoped Si substrate or SrTiO3A substrate.
2. The reckoned three-dimensional topological insulator Bi according to claim 12Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: in step S1, the three-dimensional topological insulator Bi2Te3The film grows on the insulating Si substrate along the (111) crystal plane.
3. The reckoned three-dimensional topological insulator Bi as claimed in claim 22Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: the material of the Si substrate is not doped, and the Fermi level is in the middle of a band gap.
4. The reckoned three-dimensional topological insulator Bi according to claim 12Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: in step S1, the three-dimensional topological insulator Bi2Te3The film grows on SrTiO along the (111) crystal face3On a substrate.
5. The reckoned three-dimensional topological insulator Bi according to any one of claims 1 to 42Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: the three-dimensional topological insulator Bi2Te3The thickness of the film is 3-30 nm.
6. The reckoned three-dimensional topological insulator Bi according to any one of claims 1 to 42Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: in step S2, the distance between the circular electrodes is 1-3 mm, and the diameter is 0.5-1 mm.
7. The reckoned three-dimensional topological insulator Bi according to claim 12Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: the step S3 specifically includes the following steps:
step S31: smoothly adhering a test sample on the AlN substrate by using low-temperature glue, and adhering 2 gold-plated ceramic plates on the AlN substrate by using the low-temperature glue; adhering the AlN substrate with the test sample on a sample seat of the Dewar flask by using low-temperature glue, connecting a circle center electrode on the test sample with a gold wire serving as a lead wire with a gold-plated ceramic wafer on the AlN substrate, and connecting the gold-plated ceramic wafer with a binding post of the Dewar flask by using a silver wire; vacuumizing the Dewar flask by a mechanical pump, and keeping the vacuum degree below 1 Pa;
step S32: a 1064-nanometer laser is used as an excitation light source, and the laser emitted by the laser sequentially passes through the attenuation sheet, the chopper, the polarizer, the quarter-wave plate and the Dewar flask window and is incident at the midpoint of the connecting line of the two circular electrodes of the test sample at an incidence angle theta; the diameter of the light spot is smaller than the distance between the two circular electrodes; under illumination, the test sample generates photocurrent, and the photocurrent enters a low-noise preamplifier and a lock-in amplifier and finally enters a data acquisition card; the chopper is connected with the phase-locked amplifier and is used for inputting a reference signal;
step S33: setting the temperature of the temperature controller at 77K, filling liquid nitrogen into the Dewar flask, and placing a temperature control rod to stabilize the temperature;
step S34: controlling a stepping motor to drive the quarter-wave plates to rotate from 0 degree to 360 degrees, taking 5 degrees as a step length, and collecting photocurrent at each quarter-wave plate rotation angle through a data acquisition cardJ(ii) a When the quarter-wave plate is at 0 degree, the fast axis direction is parallel to the polarizer direction;
step S35: changing the temperature of the test sample by the temperature controller, repeating the step S34, and measuring the photocurrent at different temperaturesJ；
Step S36: photocurrent at each temperature by the following formula (1)JAnd (3) fitting:
whereinJ C Is the current caused by the excitation of circularly polarized light, namely circularly polarized light induced current,L 1 andL 2 the current caused by the linearly polarized light,J 0 in the case of a background current, the current,φobtaining a circularly polarized light current-induced curve of the test sample along with the temperature change for the included angle between the fast axis of the quarter-wave plate and the polarization direction of the polarizer;
step S37: and replacing samples with different thicknesses, and repeating the steps S31-S36 to obtain the circularly polarized light induced current of the samples with different thicknesses along with the change of the temperature.
8. The reckoned three-dimensional topological insulator Bi according to claim 72Te3The method for changing the trend of circularly polarized light induced current along with temperature is characterized in that: in the step S32, the power of the 1064 nm laser is 10-200 mW; the working frequency of the chopper is 60-1000 Hz; the range of the incident angle theta is 10-50 degrees; the temperature setting range of the temperature controller in the step S35 is 77K-300K.
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