CN101539519B - Method and apparatus for generating charged excitons in undoped quantum wells by photoexcitation - Google Patents

Abstract

The invention relates to a method and a device for generating electriferous excitors in a quantum well by undoped photo excitation, in particular to a method and a device for generating electriferous excitors. Laser with the energy higher than the forbidden band width of a ZnSe quantum well is used for exciting a zinc selenide well layer in a hetero-structure of zinc selenide/beryllium telluride so as to generate electrons and cavities. The device comprises a low-temperature thermostat, a laser, a spectrometer, and the like, and an optical path which consists of a reflective mirror, a lens and a prism is put in front of the laser, wherein the low-temperature thermostat is put in front of the prism, and the laser turned by the prism can be irradiated into the low-temperature thermostat; the other lens is put in the rear of the prism and can converge radiant light (namely fluorescence PL) of a zinc selenide/beryllium telluride sample in the low-temperature thermostat; and optical fibersand the spectrometer are put at the rear of the lens, wherein the spectrometer is provided with a CCD detector, and both the spectrometer and the CCD detector are connected to a computer so as to automatically complete spectrum detecting duties. The device has simple method, loose experiment condition and no influence on the structural quality of crystals.

Description

Method and apparatus for generating charged excitons in undoped quantum wells by photoexcitation

technical field

The invention relates to a method and a device for generating charged excitons, in particular to a method and a device for generating charged excitons in a non-doped quantum well by photo-induced excitation.

Background technique

Both zinc selenide (ZnSe) and beryllium telluride (BeTe) are important wide bandgap compound semiconductors with strong ionicity and large binding energy. The zinc selenide/beryllium telluride (ZnSe/BeTe) heterostructure composed of the two has good structural quality, low dislocation density and point defects, and the lattice mismatch at the interface is less than 0.5%. ZnSe/BeTe has a type II energy band structure, which makes the excited electrons and holes in the ZnSe layer spatially separated, and can form a two-dimensional electron gas in the ZnSe layer, resulting in the appearance of charged excitons. These characteristics make the ZnSe/BeTe II quantum well (or superlattice) structure a good candidate material for studying condensed matter phenomena.

Charged excitons are one of the important forms of condensation phenomena and are an important topic for understanding many optical processes involving spin states in intentionally doped or unintentionally doped structures. According to the disclosed methods for generating charged excitons, there are mainly doping through structures in I-type quantum wells, unintentional doping of impurities and defects in structures, and tunneling particle injection. American academic journal "Physical Review B" (2002, volume 65, page 115310, "Optical method for the determination of carrier density in modulation-doped quantum wells") discloses a preparation method for generating charged excitons by a structural doping method ( G.V.Astakhov, et al., 65(2002) 115310), but these impurity ions not only affect the structural quality of the crystal, but also cause the deformation of the energy band structure and even affect the quantum well, and also complicate the growth process due to doping In addition, American academic journal "Physical Review B" (1996, volume 54, page 10609, "Exciton and trion spectral lineshape in the presence of an electron gas in GaAs/AlAs quantum wells") discloses tunneling particle injection method to generate charged excitons (A. Manassen, et al., 54 (1996) 10609), but because the tunneling particle injection is affected by the reflection of the barrier layer and various defects in the barrier layer, the charged excitons The efficiency of particle formation is affected, and two excitation light sources must also be used simultaneously.

Contents of the invention

In order to overcome the defects and deficiencies of the prior art, the present invention proposes a method and a device for generating charged excitons in non-doped quantum wells by using photoexcitation.

A method for generating charged excitons in a non-doped quantum well by photoexcitation, the steps are as follows:

1. Cut the zinc selenide/beryllium telluride (ZnSe/BeTe) sample into a rectangular wafer, decontaminate the surface with acetone solution, rinse with ultra-pure water, and dry with high-purity nitrogen, and then paste the sample on a round copper wafer. Chip;

2. Fix the copper sheet pasted with the ZnSe/BeT sample on the sample holder, and then place it in an optical cryostat filled with cryogenic liquid helium (He); the optical cryostat is connected to the air pump, A heating wire and a temperature detection device are installed next to the sample; the temperature of the sample is controlled between about 1-150K by separately adjusting the pumping speed of the aspirator or the current in the heating wire;

3. Vertically project a laser beam with an energy of 2.8-4.0eV and an excitation density of 0.001-200W/ ^cm2 on the upper surface of the sample;

4. The fluorescence generated by the sample can be output to the spectrometer through the optical fiber after being converged by the convex lens;

5. After being separated by the spectrometer, the fluorescence spectrum of the charged excitons is obtained by the CCD detector.

The zinc selenide/beryllium telluride (ZnSe/BeTe) sample structure in the step 1 can be a single quantum well, multiple quantum wells or a superlattice structure.

The thickness of the ZnSe quantum well in the zinc selenide/beryllium telluride (ZnSe/BeTe) sample in the step 1 is about 10-80ML.

ML is the abbreviation of the English word monolayer, that is, molecular layer. For BeTe or ZnSe, 1ML is about 0.28nm.

In the above method, if the temperature in the optical cryostat filled with liquid helium is to reach below 4.2K, it needs to be adjusted by pumping air through an air pump; when the temperature is above 4.2K, it needs to be controlled by adjusting the current in the heating wire temperature.

The sample ZnSe/BeTe used in the present invention is grown by using GaAs as the substrate by the method of molecular beam epitaxy (MBE). The preparation method of the sample is as follows.

A method for preparing type II quantum wells by molecular beam epitaxy, the steps are as follows:

1. Fix the (001) oriented gallium arsenide (GaAs) substrate on the molybdenum (Mo) sample holder with indium (In);

2. Pass liquid nitrogen to cool the growth chamber. After confirming that the vacuum degree of the III-V growth chamber is below 1×10-10Torr, transfer the sample to the III-V growth chamber through the magnetic transfer rod; The K-cell container of the As solid source and the K-cell container equipped with the Ga solid source are heated to the set temperature of 300°C, 100°C and 750°C respectively;

3. Adjust the temperature of the As source K-cell container from 100°C to 295°C, then set the temperature of the sample tray and the Ga source K-cell container to 550°C and 915°C respectively, and start to heat up At the same time, the baffle of the As source K-cell container is opened, and the As molecular beam is irradiated on the substrate to compensate for the evaporation of As on the substrate surface caused by the increase of the substrate temperature, and to make the evaporation of As on the substrate surface and Attachment reaches equilibrium;

4. After the Ga source K-cell container rises to 915°C, then raise the temperature of the sample holder to 620°C. During the heating period, it can be observed in the [1ī0] direction by a reflection high-energy electron diffraction (RHEED instrument). When clear stripes appear, it means that the oxide on the substrate surface has been removed at this temperature and a clean and orderly substrate surface has been obtained;

5. Generation of GaAs buffer layer: Open the baffle of the Ga source K-cell container whose temperature has risen to 915°C, and the Ga molecular beam is irradiated on the substrate; at this time, the Ga and As molecular beams are irradiated on the substrate surface at the same time, The growth of the GaAs buffer layer starts, the growth time is 24~120 minutes, and the thickness of the GaAs buffer layer can reach 200~1000nm. At this time, turn off the Ga source and lower the temperature of the Ga source K-cell container from 915°C to 750°C. ℃, and then lowered to 300 ℃, and it can be observed by the RHEED instrument that the substrate at this time has a clean, flat and orderly surface;

6. After stabilizing for 5 minutes, gradually lower the substrate temperature from 620°C to 580°C. The cooling method is stepwise, that is, the temperature is set to drop 5°C each time, and the next time is set when the set temperature is reached, divided into 8 minutes. Complete the cooling for the first time; after the cooling starts, when there is no abnormal change in the image observed by the RHEED instrument, turn off the As source, and adjust the temperature of the As source from 295°C to 100°C; gradually lower the substrate temperature from 580°C to 500°C , the cooling method is stepwise, 10°C each time; after that, the substrate temperature is gradually lowered from 500°C to 300°C, and the entire cooling process is divided into 10 times, and the cooling interval is 20°C;

7. Observe that there is no abnormal change in the image of the RHEED instrument. At this time, the RHEED instrument can be turned off. After confirming that the vacuum degree of the III-V growth chamber has become below 7.5×10-9Torr, turn off the power supply for heating the substrate. and prepare to transfer the substrate to the II-VI growth chamber through the high vacuum transfer pipeline;

8. After confirming that the vacuum degree of the II-VI growth chamber is below 1×10-10 Torr and the temperature of the Zn, Be, Te, Se and Mg sources in the II-VI growth chamber are respectively heated to 150, 820, 150, After 50 and 200°C, the substrate is transferred from the III-V growth chamber to the II-VI growth chamber through an ultra-high vacuum transmission pipeline; the substrate is heated to 300°C~350°C, and the Zn, Be, Te , Se and Mg sources are heated up to 307, 1065, 320, 202 and 322.5°C respectively;

9. Formation of the BeTe buffer layer: when the temperatures of the Be and Te sources reach 1065 and 320°C respectively, stabilize for 30 minutes, open the baffles of the Be and Te sources, and irradiate the Be and Te molecular beams on the surface of the substrate. Begin to grow BeTe buffer layer at this moment; Carry out real-time monitoring with RHEED instrument (or according to the growth rate of each solid source measured in advance before growing, set, control with computer. The same below), when the growth thickness of BeTe buffer layer is about When it is 5ML, the growth is over, then turn off the Be source first, and then turn off the Te source to make the surface Te-rich;

10. Growth of the Zn0.77Mg0.15Be0.08Se isolation layer: first open the baffle of the Zn source, and then open the baffles of the Se, Be and Mg sources to grow the Zn0.77Mg0.15Be0.08Se isolation layer. Zn molecular beams, Se molecular beams, Be molecular beams and Mg molecular beams are irradiated on the surface of the substrate at the same time. When the growth thickness of the isolation layer is 200~1000nm, the growth ends, and the Se, Be, and Mg sources are turned off first, about 5~10 After a few seconds, turn off the Zn source;

11. Growth of ZnSe potential well layer: Turn on the Zn and Se sources and irradiate them on the surface of the substrate at the same time. The temperatures of the Zn and Se solid sources are kept at 307 and 202°C respectively. When the thickness of the ZnSe layer grows to 10~ At 80ML, the growth is over, first turn off the Se source, and then turn off the Zn source after about 5 to 10 seconds, so as to form a Zn-rich surface layer;

12. Growth of BeTe barrier layer: keep the temperatures of Be and Te sources at 1065 and 320°C respectively, turn on the Te source, and then turn on the Be source after 5 to 10 seconds to start growing the BeTe barrier layer. When the thickness of the BeTe layer grows When the growth is about 10ML, the growth is over, first turn off the Be source, and then turn off the Te source after about 5 to 10 seconds to form a Te-rich surface layer. At this time, the temperature of the Te source can be lowered to 150°C; the above growth process can be performed at A chemical bond in the form of Zn-Te is obtained at the interface between ZnSe and BeTe

13. Growth of ZnSe potential well layer: turn on the Zn source, and then turn on the Se source after 5-10 seconds to grow the ZnSe potential well layer. The temperatures of the Zn and Se solid sources are kept at 307 and 202°C respectively. When it is 10~80ML, the growth is over, and the Se source and the Zn source are turned off successively; the above growth process can obtain a chemical bond in the form of Te-Zn at the interface between BeTe and ZnSe;

14. Repeat the growth process of the isolation layer in step 11 to obtain a Zn0.77Mg0.15Be0.08Se isolation layer with a thickness of 200~1000nm; at the end of the growth, turn off the Se, Be, and Mg sources first, and then turn off the Zn after about 10 seconds source, so that a layer of Zn can be grown on the top of the sample as a covering layer to protect the sample from corrosion; The temperature of the source was set to 100, 50, 820 and 150 °C respectively and allowed to cool down; when the substrate temperature dropped to 150 °C and the temperature of the Se source was confirmed to drop below 100 °C, the sample was taken out of the growth chamber, namely A complete quantum well material in the form of ZnSe/BeTe/ZnSe can be obtained.

Through the above growth process, we can obtain a quantum well structure in the form of ZnSe/BeTe/ZnSe (that is, there are two potential well layers and one potential barrier layer).

The above-mentioned ML is an abbreviation of the English word monolayer, that is, molecular layer. For BeTe or ZnSe, 1ML is about 0.28nm.

The RHEED instrument used above is a reflection high energy electron diffractometer.

In the present invention, the thickness of the ZnSe quantum well has a great influence on the formation efficiency of charged excitons, and its thickness is about 10-80ML. Lasers can be cw or pulsed. The incident laser beam can be perpendicular to the sample surface or at an angle. In addition, the fluorescence intensity of charged excitons increases with the decrease of temperature and the increase of excitation intensity.

The working principle of the inventive method is as follows:

At low temperature, the ZnSe well layer in the heterostructure ZnSe/BeTe is excited by a laser with energy greater than the forbidden band width of the ZnSe quantum well to generate electrons and holes, because the lowest energy values of the electrons and holes are in different quantum wells. layer (that is, when the electrons are in the ZnSe layer and the holes are in the BeTe layer, their energy is the lowest), so the spatial separation of the charges occurs, so that the electrons in the ZnSe layer have a higher concentration relative to the holes and thus Negatively charged excitons (ie, exciton complexes consisting of two electrons and a hole) are generated.

A device used in the above method is composed of a laser, a reflector, two convex lenses, a prism, an optical cryostat, a ZnSe/BeTe sample, an optical fiber, a spectrometer, a CCD detector and a computer. It is characterized in that the optical path composed of reflective mirror, convex lens and prism is placed in front of the laser, which can reflect, focus and turn the outgoing laser light of the laser; an optical cryostat is placed in front of the prism, and the laser light diverted by the prism can enter the optical cryostat Middle; another convex lens is placed behind the prism to receive the fluorescence (PL) spectrum of the ZnSe/BeTe sample in the cryostat; the optical fiber and the spectrometer are placed behind the convex lens, the spectrometer has a CCD detector, and the two are connected On the computer, the spectrum detection task can be automatically completed. The prism is a semi-reflective and semi-transparent prism.

The method of the present invention does not require doping in the crystal sample, and does not need to inject particles into the sample through the tunneling effect. Large variation range) can generate charged excitons. Simultaneously because what the present invention uses is non-doped sample, so do not need to consider the structure quality problem that influences crystal because of doping and the complexity that causes growth process because of doping, also need not need two exciting light sources and must Consider the influence of barrier width and other factors on the generation of charged excitons.

Charged excitons are fermions, which are also one of the important forms of condensation phenomena. Studying the various spectral and electrical properties of charged excitons under different experimental conditions is an important topic for understanding many physical processes including spin states and high-density states. At the same time, it also has important applications in optical communications, quantum computers (including quantum information storage and processing), and optoelectronic devices.

Description of drawings

Figure 1 is a schematic diagram of the device of the present invention.

Among them: 1. Laser, 2. Mirror, 3. Convex lens, 4. Prism, 5. Optical cryostat, 6. ZnSe/BeTe sample, 7. Convex lens, 8. Optical fiber, 9. Spectrometer, 10 , CCD detector, 11, computer.

Fig. 2 is a schematic diagram of the crystal structure and energy band of the zinc selenide/beryllium telluride sample of the present invention.

The left side is the cross-sectional view of the ZnSe/BeTe structure (sample), and the right side is its corresponding energy band structure. C.B and V.B represent the conduction band and valence band, respectively, and DT represents the spatially direct transition (i.e., type I transition) in the ZnSe layer.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but is not limited thereto.

Embodiment 1: (method embodiment)

A method for generating charged excitons in a non-doped quantum well by photoexcitation, the steps are as follows:

1. Cut the zinc selenide/beryllium telluride (ZnSe/BeTe) sample into a rectangular wafer, decontaminate the surface with acetone solution, rinse with ultra-pure water, and dry with high-purity nitrogen, and then paste the sample on a round copper wafer. Chip;

2. Fix the copper sheet pasted with the ZnSe/BeTe sample on the sample holder, and then place it in an optical cryostat filled with cryogenic liquid helium; the optical cryostat is connected to the air pump, and the sample is installed There is a heating wire and a temperature detection device; the temperature of the sample is controlled at about 1.4K by adjusting the pumping speed of the aspirator;

3. Vertically project a laser beam with an energy of 2.85eV and an excitation density of 0.1W/ ^cm2 on the upper surface of the sample;

4. The fluorescence generated by the sample can be output to the spectrometer through the optical fiber after being converged by the convex lens;

5. After being separated by the spectrometer, the fluorescence spectrum of the charged excitons is obtained by the CCD detector.

The zinc selenide/beryllium telluride (ZnSe/BeTe) sample structure in the step 1 is a single quantum well; the thickness of the ZnSe quantum well in the sample is 20ML.

Embodiment 2: (method embodiment)

Embodiment 2 of the method of the present invention is the same as Embodiment 1, except that the temperature of the zinc selenide/beryllium telluride sample is controlled at about 20K by adjusting the current in the heating wire; the energy of the laser beam is 3.06eV, and the excitation density is 5W/cm ² . The sample structure of zinc selenide/beryllium telluride (ZnSe/BeTe) is multiple quantum wells, and the thickness of the ZnSe quantum wells in the sample is 28ML.

Embodiment 3: (method embodiment)

Embodiment 3 of the method of the present invention is the same as Embodiment 1, except that the temperature of the zinc selenide/beryllium telluride sample is controlled at about 70K by adjusting the current in the heating wire; the energy of the laser beam is 3.35eV, and the excitation density is 90W/cm ² . The zinc selenide/beryllium telluride (ZnSe/BeTe) sample structure is a superlattice structure, and the thickness of the ZnSe quantum well in the sample is 40ML.

Embodiment 4: (method embodiment)

Embodiment 4 of the method of the present invention is the same as Embodiment 1, except that the temperature of the zinc selenide/beryllium telluride sample is controlled at about 140K by adjusting the current in the heating wire; the energy of the laser beam is 3.8eV, and the excitation density is 180W/cm ² . The zinc selenide/beryllium telluride (ZnSe/BeTe) sample structure is a superlattice structure, and the thickness of the ZnSe quantum well in the sample is 55ML.

Embodiment 5: (device embodiment)

Embodiment 2 of the present invention is shown in Figure 1, consists of a laser 1, a reflector 2, two convex lenses 3 and 7, a prism 4, an optical cryostat 5, a zinc selenide/beryllium telluride sample 6, an optical fiber 8, and a spectrometer 9 , CCD detector 10 and computer 11, it is characterized in that, reflective mirror 2, convex lens 3, prism 4 make up the optical path and place before laser 1, can reflect, focus and turn the outgoing laser light of laser 1; Place light in front of prism 4 In the cryostat 5, the laser light diverted by the prism 4 can be incident on the optical cryostat 5; another convex lens 7 is placed behind the prism 4 to receive the ZnSe/BeTe sample 6 in the optical cryostat 5. Fluorescence (PL) spectrum; optical fiber 8 and spectrometer 9 are placed behind the convex lens 7, spectrometer 9 has a CCD detector 10, both are connected on the computer 11, can automatically complete the spectrum detection task. The prism 4 is a semi-reflective and semi-transparent prism.

Claims (3)

1. A method for generating charged excitons in non-doped quantum wells with photoinduced excitation, wherein the charged excitons are negatively charged excitons, which are exciton complexes composed of two electrons and a hole, The method steps are as follows: 1) Cut the zinc selenide/beryllium telluride sample into a rectangular wafer, decontaminate the surface with acetone solution, rinse with ultra-pure water, and dry with high-purity nitrogen, and then paste the sample on a circular copper sheet; 2) Fix the copper sheet pasted with the ZnSe/BeTe sample on the sample holder, and then place it in an optical cryostat filled with cryogenic liquid helium; the optical cryostat is connected to the air pump, and the sample is installed There is a heating wire and a temperature detection device; the temperature of the sample is controlled between 1 and 150K by individually adjusting the pumping speed of the aspirator or the current in the heating wire; 3) Vertically project a laser beam with an energy of 2.8-4.0eV and an excitation density of 0.001-200W/ ^cm2 on the upper surface of the sample; 4) The fluorescence generated by the sample is converged by the convex lens and then output to the spectrometer through the optical fiber; 5) After being separated by the spectrometer, the fluorescence spectrum of the charged excitons is obtained by the CCD detector. 2. a kind of method as claimed in claim 1 produces charged excitons in non-doped quantum well with photoinduced excitation, it is characterized in that the zinc selenide/beryllium telluride sample structure in step 1) can be single quantum Wells can also be multiple quantum wells or superlattice structures. 3. a kind of method as claimed in claim 1 produces charged excitons in non-doped quantum wells with photoinduced excitation, it is characterized in that the ZnSe quantum wells in the zinc selenide/beryllium telluride sample in step 1) The thickness is 10-80ML.

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