阅读说明：本技术 二维半导体材料中光生载流子的差分反射探测方法 (Differential reflection detection method for photogenerated carriers in two-dimensional semiconductor material ) 是由 姚鹏 李昱江 杨林 王浩枫 于 2021-08-17 设计创作，主要内容包括：本发明涉及一种二维半导体材料中光生载流子的差分反射探测方法,属于超快激光泵浦探测领域。本发明利用二维半导体材料价带电子吸收光子并发生跃迁至导带的原理,在超快激光的激发下,通过测量二维半导体材料的反射光的方式,得到了微观电子的动力学过程。所述超快激光为脉冲持续时间在100飞秒左右、重复频率为80MHz、带宽为10纳米左右的相干光源,保证了光学测量的时间分辨率。本发明具有瞬时响应和飞秒—皮秒级别的时间分辨率。和宏观电学的电流探测手段相比,灵敏度更高,适用于微观探测领域,同时避免了电极材料对测量结果的影响。(The invention relates to a differential reflection detection method for photo-generated carriers in a two-dimensional semiconductor material, belonging to the field of ultrafast laser pumping detection. The invention utilizes the principle that two-dimensional semiconductor material valence band electrons absorb photons and jump to a conduction band, and obtains the dynamic process of microscopic electrons by measuring the reflected light of the two-dimensional semiconductor material under the excitation of ultrafast laser. The ultrafast laser is a coherent light source with the pulse duration of about 100 femtoseconds, the repetition frequency of 80MHz and the bandwidth of about 10 nanometers, and the time resolution of optical measurement is ensured. The invention has transient response and time resolution on the order of femtosecond-picosecond. Compared with a current detection means of macroscopic electricity, the sensitivity is higher, the method is suitable for the field of microscopic detection, and meanwhile, the influence of electrode materials on the measurement result is avoided.)

1. A method for differential reflection detection of photogenerated carriers in a two-dimensional semiconductor material, the method comprising the steps of:

step one, introducing pump light to excite a two-dimensional semiconductor material: with a beam having a suitable wavelength lambda₁The pulse laser is vertically incident to the surface of the two-dimensional semiconductor material, and after the two-dimensional semiconductor material is excited, the electron density in a semiconductor valence band is reduced; the wavelength lambda₁The photon energy of the pulsed laser of (1) is an energy capable of exciting an electron of a valence band of the semiconductor to a conduction band;

step two, introducing a chopper to cut off the pump light at certain time intervals: adding a mechanical chopper into the optical path of the pump light, wherein the fan blades of the chopper continuously cut off the pump light to form discontinuous pulse laser with fixed time intervals;

step three: introducing detection light to generate a differential reflection signal: with a beam having a suitable wavelength lambda₂Is vertically incident to two dimensionsSurface of semiconductor material, the wavelength lambda₂Has photon energy equal to or close to the energy difference between the valence band and the conduction band of the semiconductor, and satisfies lambda₂-λ₁More than or equal to 30 nm; reflection signal R of reflected light of probe light when two-dimensional semiconductor material is not excited by pump light₀The reflected signal R is generated when the two-dimensional semiconductor material is excited by the pump light, so that a differential reflected signal delta R-R is formed₀；

Step four: changing the optical path difference between the pump light and the probe light to obtain a differential reflection signal delta R/R₀And the time difference delta T between the pump light and the probe light and the surface of the two-dimensional semiconductor material.

2. The method of claim 1, wherein the pump light and the probe light are pulsed lasers having a pulse width of about 10nm and a duration of 100 femtoseconds.

3. The method according to claim 1, wherein the excitation light source of the pulsed laser is a titanium sapphire laser with a repetition rate of 80MHz, and the wavelength of the laser pulse is adjustable within a range of 690nm to 1080nm at room temperature.

4. The method for differential reflection detection of photogenerated carriers in a two-dimensional semiconductor material as claimed in claim 1, wherein in the first step, the pump light is focused on the surface of the two-dimensional semiconductor material through a microscope objective lens before being incident on the surface of the two-dimensional semiconductor material.

5. The method for differential reflective detection of photogenerated carriers in a two-dimensional semiconductor material as claimed in claim 1, wherein said two-dimensional semiconductor material is gallium arsenide, transition metal chalcogenides, elemental semiconductor or molybdenum diselenide, and the thickness of the semiconductor material is in the nanometer range.

6. The method for differential reflection detection of photogenerated carriers in a two-dimensional semiconductor material as claimed in claim 1, wherein in step two, two optical lenses are placed before and after the mechanical chopper to ensure that: the chopper is positioned at the focal points of the two lenses; the two lenses do not change the parallelism of the laser.

7. The method for differential reflection detection of photogenerated carriers in a two-dimensional semiconductor material as claimed in claim 1, wherein in step two, the rotational frequency of said mechanical chopper is maintained at 2 KHz.

8. The method for differential reflection detection of photogenerated carriers in a two-dimensional semiconductor material according to claim 1, wherein in the third step, the detection light and the pump light are both vertically incident to the surface of the two-dimensional semiconductor material, and the incident points coincide; in order to measure the reflected light of the detection light, a filter needs to be added in front of the optical detector to filter the pump light.

9. The method for differential reflection detection of photogenerated carriers in a two-dimensional semiconductor material according to any of claims 1 to 8, wherein in step three, the reflection signal R is₀R is a signal for measuring the reflected detection light, and a signal of the pump light is filtered; the absorption change of the two-dimensional semiconductor material to the detection light, the differential reflectivity delta R/R are obtained indirectly by observing the reflection signal of the detection light₀And carrier density N:

wherein the two-dimensional semiconductor thickness is L, alpha₀Refers to the absorption coefficient when the material is not excited, N is the carrier density, N is_satIs the density of carriers when the absorption coefficient drops to 50%.

10. A two-dimensional semiconductor device as defined in claim 9The differential reflection detection method of the photogenerated carriers in the material is characterized in that the fourth step specifically comprises the following steps: when the delta T is 0, the pump light and the probe light simultaneously reach the surface of the two-dimensional semiconductor material; as Δ T increases, the time for the probe light to "arrive later" than the pump light gradually increases; the time difference delta T between the arrival of the pump light and the detection light at the surface of the two-dimensional semiconductor material is realized by continuously changing the optical path difference between the arrival of the pump light and the detection light at the sample; while the differential reflected signal Δ R/R₀The relation between the time difference delta T between the pumping light and the detection light and the time difference delta T between the pumping light and the time difference delta T when the valence band carriers are excited by the pumping light reflects the change of the carrier concentration along with the time.

Technical Field

The invention belongs to the field of ultrafast laser pumping detection, and particularly relates to a differential reflection detection method for photogenerated carriers in a two-dimensional semiconductor material.

Background

As semiconductor device dimensions reach the nanometer scale, moore's law fails to make the development of semiconductor technology, typified by wafer foundry, a bottleneck. Microscopic studies of charge transfer in two-dimensional semiconductor materials and their heterojunctions will have a significant impact on future breakthroughs in semiconductor technology. However, due to the electric charge detection technology, people often cannot capture the movement information of the micro particles in time.

Among semiconductors, electrons located in a conduction band and holes located in a valence band are called carriers, and the carriers are considered as a basis of charge transport because they are mobile. Carriers can be de-excited by different physical mechanisms, such as, for example, in doped semiconductor materials, carriers are provided by doping atoms. Elemental or pure semiconductor materials are not absolutely zero, since thermal excitation also has charge carriers. For both elemental and doped semiconductor materials, photoexcitation can also be used to generate carriers (i.e., so-called photogenerated carriers), primarily because electrons in the valence band can absorb a photon of sufficient energy to reach the conduction band.

Since the recombination process of photogenerated carriers is very fast, often occurring within a few picoseconds, the study of the kinetic processes of carriers under photoexcitation requires experimental methods with very high time resolution. Electronic detection techniques (i.e., by measuring the I-V characteristic between electrodes) that are commonly used in basic physical research have limited temporal resolution. In contrast, the optical technology based on ultrafast laser is considered as a standard tool for studying carrier dynamics under the condition of optical excitation because of its high time resolution.

The patent is intended to design an all-optical detection method of photo-generated carriers in a two-dimensional semiconductor material with femtosecond-nanosecond time resolution and transient response based on light reflection.

Disclosure of Invention

Technical problem to be solved

The invention aims to solve the technical problem of how to provide a differential reflection detection method for photo-generated carriers in a two-dimensional semiconductor material, which has the characteristics of femtosecond-nanosecond time resolution and instantaneous response.

(II) technical scheme

In order to solve the above technical problem, the present invention provides a method for detecting differential reflection of photogenerated carriers in a two-dimensional semiconductor material, which comprises the following steps:

step one, introducing pump light to excite a two-dimensional semiconductor material: with a beam having a suitable wavelength lambda₁The pulse laser is vertically incident to the surface of the two-dimensional semiconductor material, and after the two-dimensional semiconductor material is excited, the electron density in a semiconductor valence band is reduced; the wavelength lambda₁The photon energy of the pulsed laser of (1) is an energy capable of exciting an electron of a valence band of the semiconductor to a conduction band;

step two, introducing a chopper to cut off the pump light at certain time intervals: adding a mechanical chopper into the optical path of the pump light, wherein the fan blades of the chopper continuously cut off the pump light to form discontinuous pulse laser with fixed time intervals;

step three: introducing detection light to generate a differential reflection signal: with a beam having a suitable wavelength lambda₂Is perpendicularly incident to the surface of the two-dimensional semiconductor material, the wavelength lambda₂Has photon energy equal to or close to the energy difference between the valence band and the conduction band of the semiconductor, and satisfies lambda₂-λ₁More than or equal to 30 nm; reflection signal R of reflected light of probe light when two-dimensional semiconductor material is not excited by pump light₀The reflected signal R is generated when the two-dimensional semiconductor material is excited by the pump light, so that a differential reflected signal delta R-R is formed₀；

Step four: changing the optical path difference between the pump light and the probe light to obtain a differential reflection signal delta R/R₀And the time difference delta T between the pump light and the probe light and the surface of the two-dimensional semiconductor material.

Further, the pumping light and the probe light are pulse lasers with a pulse width of about 10nm and a duration of 100 femtoseconds.

Furthermore, the excitation light source of the pulse laser is a titanium sapphire laser with the repetition frequency of 80MHz, and the wavelength of the laser pulse is adjustable within the range of 690nm to 1080nm at room temperature.

Further, in the first step, the pump light needs to be converged on the surface of the two-dimensional semiconductor material through a microscope objective before being incident on the surface of the two-dimensional semiconductor material perpendicularly.

Further, the two-dimensional semiconductor material is gallium arsenide, a transition metal chalcogenide, a semiconductor simple substance or molybdenum diselenide, and the thickness of the semiconductor material is in the nanometer level.

Further, in the second step, two optical lenses are placed in front and at the back of the mechanical chopper to ensure that: the chopper is positioned at the focal points of the two lenses; the two lenses do not change the parallelism of the laser.

Further, in the second step, the rotation frequency of the mechanical chopper is maintained at 2 KHz.

Furthermore, in the third step, the probe light and the pump light are both vertically incident to the surface of the two-dimensional semiconductor material, and incident points are coincident; in order to measure the reflected light of the detection light, a filter needs to be added in front of the optical detector to filter the pump light.

Further, in the third step, the reflected signal R₀R is a signal for measuring the reflected detection light, and a signal of the pump light is filtered; the absorption change of the two-dimensional semiconductor material to the detection light, the differential reflectivity delta R/R are obtained indirectly by observing the reflection signal of the detection light₀And carrier density N:

wherein the two-dimensional semiconductor thickness is L, alpha₀Refers to the absorption coefficient when the material is not excited, N is the carrier density, N is_satIs the density of carriers when the absorption coefficient drops to 50%.

Further, the fourth step specifically includes: when the delta T is 0, the pump light and the probe light simultaneously reach the surface of the two-dimensional semiconductor material; as Δ T increases, the time for the probe light to "arrive later" than the pump light gradually increases; pump lightThe time difference delta T between the detection light and the surface of the two-dimensional semiconductor material is realized by continuously changing the optical path difference between the pumping light and the detection light before reaching the sample; while the differential reflected signal Δ R/R₀The relation between the time difference delta T between the pumping light and the detection light and the time difference delta T between the pumping light and the time difference delta T when the valence band carriers are excited by the pumping light reflects the change of the carrier concentration along with the time.

(III) advantageous effects

The invention provides a differential reflection detection method for photogenerated carriers in a two-dimensional semiconductor material, which is an effective detection method summarized through multiple experimental verifications. The method is suitable for two-dimensional semiconductor materials, and transient optical detection of semiconductor photon-generated carriers is realized by utilizing the characteristic of ultrafast laser femtosecond-level duration. The differential reflection detection method has transient response and time resolution of the order of femtosecond-picosecond. Compared with a current detection means of macroscopic electricity, the sensitivity is higher, the method is suitable for the field of microscopic detection, and meanwhile, the influence of electrode materials on the measurement result is avoided. Is an important research tool in the field of microscopic charge detection.

Drawings

FIG. 1 is a diagram of a molybdenum diselenide optical mirror with a single molecular layer thickness;

FIG. 2 shows the transition of electrons in a single layer of molybdenum diselenide when only pump light excites the single layer of molybdenum diselenide;

FIG. 3 shows transition of electrons in a single layer of molybdenum diselenide when pumping light and probe light excite the single layer of molybdenum diselenide simultaneously;

FIG. 4 shows the differential reflection signal (Δ R/R) of molybdenum diselenide with a single molecular layer thickness under the conditions of the pump light wavelength of 620 nm (10 microwatts) and the probe light wavelength of 790 nm (10 microwatts)₀)。

Detailed Description

In order to make the objects, contents and advantages of the present invention clearer, the following detailed description of the embodiments of the present invention will be made in conjunction with the accompanying drawings and examples.

Based on the above problems, the present invention is intended to design an all-optical detection method for photo-generated carriers in a two-dimensional semiconductor material with femtosecond-nanosecond time resolution and timely response based on light reflection, which comprises the following steps:

step one, introducing pump light to excite a two-dimensional semiconductor material: with a beam having a suitable wavelength lambda₁(photon energy of this wavelength is energy capable of exciting electrons in the valence band of the semiconductor to the conduction band) a pulse laser having a pulse width of about 10nm and a duration of 100 femtoseconds is vertically incident on the surface of the two-dimensional semiconductor material, and the density of electrons in the valence band of the semiconductor is reduced after the two-dimensional semiconductor material is excited.

Step two, introducing a chopper to cut off the pump light at certain time intervals: the mechanical chopper is added into the optical path of the pump light, and the fan blades of the chopper can continuously cut off the pump light to form discontinuous pulse laser with fixed time intervals, wherein the rotation frequency of the mechanical chopper is maintained at 2KHz for realizing the acquisition of differential reflection signals.

Step three: probe light is introduced to produce a differential reflected signal. With a beam having a suitable wavelength lambda₂(the photon energy at this wavelength is equal to or close to the energy difference between the valence and conduction bands of the semiconductor and satisfies lambda₂-λ₁Not less than 30nm) with a pulse width of about 10nm and a duration of 100 femtoseconds is vertically incident to the surface of the two-dimensional semiconductor material. In this case, the probe light reflected by the surface of the two-dimensional semiconductor material has a reflected signal R when the two-dimensional semiconductor material is excited without the pump light due to the presence of the intermittent pump light₀The reflected signal R is generated when the two-dimensional semiconductor material is excited by the pump light, so that a differential reflected signal delta R-R is formed₀. Reflected signal R₀And R are signals measured for the reflected detection light, and signals of the pump light are filtered out. Actually, since the change in the absorption of the probe light by the two-dimensional semiconductor material under the influence of the pump light cannot be obtained by a direct measurement method, it is attempted to indirectly obtain the change in the absorption of the probe light by the two-dimensional semiconductor material by observing the reflected signal of the probe light, and the differential reflectance Δ R/R₀And carrier density N:

wherein the two-dimensional semiconductor thickness is L, alpha₀Refers to the absorption coefficient when the material is not excited, N is the carrier density, N is_satIs the density of carriers when the absorption coefficient drops to 50%.

Step four: changing the optical path difference between the pump light and the probe light to obtain a differential reflection signal delta R/R₀And the time difference delta T between the pump light and the probe light and the surface of the two-dimensional semiconductor material. When the delta T is 0, the pump light and the probe light reach the surface of the two-dimensional semiconductor material; as Δ T increases, the time for the probe light to "arrive later" than the pump light gradually increases. The time difference deltat between the arrival of the pump light and the probe light at the surface of the two-dimensional semiconductor material can be realized by continuously changing the optical path difference between the arrival of the pump light and the arrival of the probe light at the sample. While the differential reflected signal Δ R/R₀The relation between the time difference delta T between the pumping light and the detection light and the time difference delta T between the pumping light and the time difference delta T when the valence band carriers are excited by the pumping light reflects the change of the carrier concentration along with the time.

In the first step, the excitation light source used in the step 1) is a titanium sapphire laser with the repetition frequency of 80 MHz. The wavelength of the laser pulse can be adjusted at room temperature and is within the range of 690nm to 1080 nm; 2) the pump light is vertically incident to the surface of the two-dimensional semiconductor material; 3) before the pump light is vertically incident on the surface of the two-dimensional semiconductor material, the pump light needs to be converged on the surface of the two-dimensional semiconductor material through a microscope objective.

In the second step, the mechanical chopper is placed in the optical path of the pump light, and two optical lenses are required to be placed in front of and behind the chopper to ensure that: the chopper is located at the focal point of the two lenses. And the parallelism of the laser is not changed by the two lenses.

In the third step, 1) the repetition frequency and the pulse duration of the detection light and the pumping light are consistent; 2) the detection light and the pump light are vertically incident to the surface of the two-dimensional semiconductor material, and incident points are overlapped; 3) in order to measure the reflected light of the detection light, a filter needs to be added in front of the optical detector to filter the pump light.

In the fourth step, 1) continuously changing the time difference between the arrival of the pump light and the arrival of the probe light at the sample is the key of the differential reflection detection method. The change in the time difference Δ T between the arrival of the probe light and the arrival of the pump light at the sample can be achieved here by increasing or decreasing the optical path length of the probe light (pump light) before it reaches the sample.

The invention is an effective detection method summarized by a plurality of experimental verifications. The method is suitable for two-dimensional semiconductor materials, and transient optical detection of semiconductor photon-generated carriers is realized by utilizing the characteristic of ultrafast laser femtosecond-level duration. The differential reflection detection method has transient response and time resolution of the order of femtosecond-picosecond. Compared with a current detection means of macroscopic electricity, the sensitivity is higher, the method is suitable for the field of microscopic detection, and meanwhile, the influence of electrode materials on the measurement result is avoided. Is an important research tool in the field of microscopic charge detection.

The following examples further illustrate the invention but are not to be construed as limiting the invention.

Example of implementation

Firstly, transferring molybdenum diselenide with a single molecular layer thickness onto a silicon dioxide substrate (as shown in the figure I), introducing pump light of 620 nanometers, and converging the pump light on the surface of the molybdenum diselenide with the single molecular layer thickness through a microscope objective, wherein pulse laser with the pulse width of about 10nm and the duration of 100 femtoseconds excites partial electrons of a valence band of the molybdenum diselenide to enable the electrons to be transited to a conduction band, so that the electron density of the valence band of the molybdenum diselenide is reduced, as shown in the figure II.

And step two, adding a mechanical chopper into the path of the pump light with the wavelength of 620 nanometers, and continuously cutting off the pump light by a fan blade of the chopper to form discontinuous pulse laser with a fixed time interval, wherein the rotation frequency of the mechanical chopper is maintained at 2KHz for realizing the acquisition of differential reflection signals.

Step three: introducing detection light with 790 nm wavelength, the pulse width of which is about 10nm and the duration of which is 100 femtoseconds, and vertically irradiating the detection light to diselenide with the thickness of a single molecular layer through the same microscope objectiveA molybdenum surface. Under the condition of the co-excitation of the probe light and the pump light, the electron transition condition of the valence band of molybdenum diselenide is shown in fig. three. In this case, molybdenum diselenide with a thickness of a monolayer forms a reflected signal R of the probe light under the simultaneous excitation of the probe light and the pump light, and forms a reflected signal R of the probe light under the excitation of only the probe light₀Thereby forming a differential reflected signal Δ R ═ R-R₀。

Step four: in order to obtain the increase of the time difference Delta T between the arrival of the pump light and the arrival of the probe light at the surface of the two-dimensional semiconductor material, a differential reflection signal Delta R/R₀Continuously changing the optical path length of the 790 nm probe light. The measurement result of the reflected probe light is shown in fig. 4, when Δ T is 0, the signal is strongest, which indicates that the probe light and the pump light reach the surface of the two-dimensional semiconductor material simultaneously, and as the pump light leaves, electrons which are originally excited by the pump light and transited to the conduction band are recombined and then return to the valence band, so that the reflected signal Δ R/R exists₀A gradual decrease in the process. As shown in fig. 4, the time difference Δ T is in the order of picoseconds.

The key points of the invention are as follows:

the invention belongs to the field of ultrafast laser pumping detection, and particularly relates to a differential reflection detection method for photogenerated carriers in a two-dimensional semiconductor material. The ultrafast laser is a coherent light source with the pulse duration of about 100 femtoseconds, the repetition frequency of 80MHz and the bandwidth of about 10 nanometers, and the time resolution of optical measurement is ensured.

Further, 1) the measuring light source is high-frequency ultrafast pulse laser with strong excitation capability; 2) the tested two-dimensional semiconductor material can be common semiconductor material such as gallium arsenide, and can also be transition metal chalcogenide and semiconductor simple substance such as silicon, germanium and the like; 3) the thickness of the semiconductor material is in nanometer order, so that an obvious direct or indirect optical band gap can be formed; 4) the measured optical signals were: the light signal reflected after absorption by the sample through which the light passes is detected.

Furthermore, the phenomenon that the valence band electrons of the two-dimensional semiconductor material absorb photons and jump to the conduction band is applied to the electron dynamics process detection of the two-dimensional semiconductor material.

Further, the incident of the pump light changes the original reflection signal of the probe light, and as the pump light leaves, the change amount of the reflection signal of the probe light becomes smaller with time, and the change amount of the reflection signal of the probe light is positively correlated with the power of the pump light.

Furthermore, the interaction between the two-dimensional semiconductor material and light is utilized, so that the physical characteristics of the sample are not changed due to the contact with the electrode, and the method has strong applicability.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.