CN112908806B

CN112908806B - Electron source package

Info

Publication number: CN112908806B
Application number: CN202110058615.XA
Authority: CN
Inventors: 刘磊; 陆菲菲; 田�健; 张杨星月
Original assignee: Nanjing University of Science and Technology
Current assignee: Nanjing University of Science and Technology
Priority date: 2021-01-16
Filing date: 2021-01-16
Publication date: 2022-09-20
Anticipated expiration: 2041-01-16
Also published as: CN112908806A

Abstract

The invention provides an electron source package, which comprises a substrate, a sealing cover and an electron source, wherein a power supply is arranged in the substrate, the sealing cover is arranged on the substrate and forms a closed space with the substrate, and the electron source is arranged on the closed space substrate; the electron source comprises a substrate, a nano wire harness, an electrode ring, an anti-reflection layer and a protective layer, wherein the substrate is arranged on the substrate and is grounded, the nano wire harness is arranged on the substrate, the electrode ring is arranged above the nano wire harness and is electrically connected with a power supply of the substrate, the anti-reflection layer is arranged on the electrode ring, and the protective layer is arranged on the anti-reflection layer.

Description

Electron source package

Technical Field

The invention relates to a vacuum electron source packaging technology, in particular to an electron source package.

Background

With the advent of nanotechnology, nanomaterials play an increasingly important role in many fields. The nanowire array structure is a novel quasi-one-dimensional nanostructure with novel physical or chemical properties different from those of thin film materials. When photons contact the array with the surface nanoscale, a light absorption effect is generated, photons which are not absorbed but penetrate are absorbed finally due to reflection or refraction and are difficult to escape, a photon capture effect is formed, the absorption rate of the photons is improved, and the quantum efficiency of the photocathode can be further improved.

Although the nanowire array structure is adopted to improve the quantum efficiency, in order to better meet the emission requirement of an electron source, the concentration ratio of electron beams emitted by the cathode of the AlGaN nanowire array can be further improved. At present, AlGaN nanowires adopt a single Al component, incident light irradiates on a nanowire array to generate electrons, and the single Al component has larger transport resistance to the electrons in a photoelectric emission material, so that the AlGaN nanowire array with the single component cannot further improve the photoelectric emission performance of a photocathode. In addition, the nanowire array with a single Al component has a disadvantage that when a single nanowire is formed into an array, electrons escaping from the side surfaces of the nanowire are secondarily absorbed by adjacent nanowires to generate an electron shielding effect. Electrons escaping from the side surface can be collected by the collecting side positioned on the top surface only when being emitted in a corresponding angle, otherwise the electrons can be captured by the adjacent nano wires and cannot escape from the array structure, so that the collecting efficiency of the nano wire array photoelectric cathode is reduced, and even the electron beam is diffused, so that the nano wire array photoelectric cathode cannot meet the requirement of an electron source and the like on the concentration ratio of the electron beam.

At present, the growth mode proposed for the heterojunction AlxGa1-xN/GaN nanowire array structure is mainly an etching method, but the etching can damage the Al component content on the side wall of the nanowire array, and the heterojunction AlxGa1-xN/GaN nanowire array obtained by etching has larger diameter and larger surface damage degree, so that the heterojunction AlxGa1-xN/GaN nanowire array with higher growth quality is urgently required to be obtained by improving the growth method.

Disclosure of Invention

The invention aims to provide an electron source package, which comprises a substrate, a sealing cover and an electron source, wherein a power supply is arranged in the substrate, the sealing cover is arranged on the substrate and forms a closed space with the substrate, and the electron source is arranged on the closed space substrate; the electron source comprises a substrate, a nano wire harness, an electrode ring, an anti-reflection layer and a protective layer, wherein the substrate is arranged on the substrate and is grounded, the nano wire harness is arranged on the substrate, the electrode ring is arranged above the nano wire harness and is electrically connected with a power supply of the substrate, the anti-reflection layer is arranged on the electrode ring, and the protective layer is arranged on the anti-reflection layer.

Further, the substrate surface comprises a number of raised structures, each nanowire of the nanowire bundle being grown on a respective raised structure.

Furthermore, each nanowire of the nanowire bundle comprises a p-type GaN nanowire, a heterojunction AlxGa1-xN nanowire and a Cs/O active layer, wherein the p-type GaN nanowire grows on the substrate, the heterojunction AlxGa1-xN nanowire grows on the p-type GaN nanowire, and the Cs/O active layer covers the surfaces of the p-type GaN nanowire and the heterojunction AlxGa1-xN nanowire.

Compared with the prior art, the invention has the following advantages:

(1) the heterojunction AlxGa1-xN/GaN nanowire array is manufactured into a photocathode, when light enters the nanowire array at a proper angle, a light absorption effect occurs when photons contact the surface nanoscale array, the photons which are not absorbed but penetrate are finally absorbed due to reflection or refraction, and are difficult to escape, so that a 'photon capture effect' is formed, and the absorption rate of the photons is improved; (2) the content change of the Al component x forms a strong built-in electric field, incident photons irradiate the nanowire, excited photoelectrons can be rapidly and directionally conveyed to the top of the nanowire through the GaN nanowire and the heterojunction AlxGa1-xN/GaN nanowire, the conveying resistance of the photoelectrons in the nanowire is greatly reduced due to the built-in electric field, the probability of the photoelectrons emitted from the side wall is reduced, and the quantum efficiency of electron beam emission of the heterojunction AlxGa1-xN/GaN nanowire array photocathode is further enhanced; (3) the method for growing the heterojunction AlxGa1-xN/GaN nanowire on the p-type GaN nanowire by the MOCVD method of growing from bottom to top improves the defect that the diameter of the nanowire can only reach the micron level caused by the traditional etching method, and can ensure that the Al component on the side wall of the nanowire is consistent with the Al component in the center of the nanowire; (4) the p-type GaN nanowire grows on the substrate firstly, so that an important size base for controlling the growth process of the subsequent AlxGa1-xN nanowire section can be provided, and the problem of a highly-conglomerated nanowire structure or a quasi-film structure caused by directly growing the AlxGa1-xN nanowire on the substrate without a GaN nanowire template under the low nitrogen flow rate is solved; (5) an external electric field formed by bias voltage between the electrode ring and the heterojunction AlxGa1-xN/GaN nanowire array cathode can change the motion track of electrons emitted by the nanowire side wall, and the emitted electrons are drawn by the external electric field to approach the anode collecting side of the electrode ring, so that the concentration of electron beams emitted by an electron source is improved; (6) the application of an anti-reflection layer on the upper surface of the electrode ring can improve the light absorption characteristic of the cathode of the electron source.

The invention is further described below with reference to the accompanying drawings.

Drawings

Fig. 1 is a schematic diagram of an electron source package according to an embodiment.

Fig. 2 is a schematic diagram of the effect of a built-in electric field and an applied electric field on particles according to an embodiment, in which (a) is a schematic diagram of the movement of particles in the built-in electric field, and (b) is a schematic diagram of the movement of particles in the applied electric field.

FIG. 3 is a schematic diagram of an application of an electron source package according to an embodiment.

FIG. 4 is a schematic diagram of an exemplary two-nanowire growth environment.

FIG. 5 is a schematic diagram of a second embodiment of a nanowire structure.

In fig. 6, (a) - (c) are respectively a comparison of quantum efficiency, collection efficiency and electron collection ratio of the reflective NEA nanowire array photocathode, (d) - (f) are respectively a comparison of quantum efficiency, collection efficiency and electron collection ratio of the tri-hetero junction AlxGa1-xN/GaN nanowire array photocathode of the example, (g) is a schematic of the "secondary absorption" problem existing in the nanowire array, and (h) and (i) are respectively a schematic of the reflective NEA nanowire array photocathode and the percentages of the number of emitted electrons and the number of collected electrons at the incident face, the exit face and the top face of the heterojunction AlxGa1-xN/GaN nanowire array photocathode of the present invention.

Detailed Description

Example one

Referring to fig. 1 to 3, a first embodiment of the present invention provides an electron source package for photoelectric emission of external photoelectric effect, comprising a substrate 100, a sealing cap 200, and an electron source. The substrate 100 is used for carrying an electron source, the sealing cap 200 cooperates with the substrate 100 to form a closed space, and the electron source emits electrons under the external photoelectric effect. Specifically, the substrate 100 includes a substrate housing and a power supply circuit disposed in the substrate housing, and the substrate housing plays a role of protecting the power supply circuit and also plays a role of supporting the electron source. The sealing cover 200 is a transparent cover, which plays a role in sealing to form a vacuum environment, and the transparent material facilitates the subsequent devices to collect overflowed electrons.

Further, the power supply circuit includes a bias voltage circuit.

Further, the electron source includes a substrate 800, a nanowire bundle, an electrode ring 600, an anti-reflection layer 500, and a protective layer 400. The substrate 800 is disposed on the substrate 100 and grounded, the nanowire bundle is disposed on the substrate 800, the electrode ring 600 is disposed above the nanowire bundle and electrically connected to the power supply of the substrate 100, the anti-reflection layer 500 is disposed on the electrode ring 600, and the protection layer 400 is disposed on the anti-reflection layer 500. The substrate 800 is used for growing the nanowires 700, a plurality of nanowires 700 form a nanowire bundle, and the nanowires 700 emit electrons under the external photoelectric effect. The electrode ring 600 generates an electric field to enable photogenerated carriers formed by photon energy absorbed inside the nanowires to migrate to the emitting layer at the top of the nanowires, so as to reduce the possibility of emitting to the side surfaces of the nanowires, the anti-reflection layer 500 is used for improving the light absorption characteristic of the cathode of the electron source, and the protective layer 400 is used for protecting other components of the electron source. Preferably, the electrode ring 600 is an Indium Tin Oxide (ITO) anode, and the anti-reflection layer 500 is a two-dimensional g-GaN nanosheet structure with anti-reflection property, which can improve the light absorption characteristics of the cathode of the electron source. The application of the two-dimensional g-GaN nanosheet structure with anti-reflection property on the upper surface of the Indium Tin Oxide (ITO) anode proposed in this example can improve the light absorption characteristics of the cathode of the electron source.

Further, a light source 300 is provided inside the sealing cap 200.

Further, with reference to fig. 2, in this embodiment, an external electric field is applied between the cathode and the anode of the encapsulated nanowire array, so that the original emission trajectory of the side-emitting electrons is changed, which can help solve the problem of concentration divergence of the electron beam emitted by the cathode of the electron source, where the line with an arrow in fig. 2 represents the movement trajectory and direction of the particle. Further, referring to fig. 3, the electron source package according to this embodiment is disposed inside the virtual coil, and is connected to the subsequent deflector, particle collector, and other devices through the multiport flange.

Example two

Referring to fig. 4 and 5, a second embodiment of the present invention is shown, which is different from the first embodiment in that a structure and a growth environment of a nanowire are provided. The nanowires 700 are grown on a substrate 800 in this embodiment. The surface of the substrate 800 includes a number of raised structures 820, and each nanowire 700 in the nanowire bundle is grown on a respective raised structure 820. The raised structure 820 can grow nanowires, which can be arranged and grown in fixed positions in order, and the grown array has a fixed period and a controllable diameter; meanwhile, the method for growing the nanowires on the protruding structure can avoid the formation of a dense array, electrons emitted from the side surface have more possibility to move to the collecting electrode at the top, and the probability of repeated absorption by adjacent nanowires is reduced.

Further, each nanowire 700 of the nanowire bundle is a heterojunction AlxGa1-xN/GaN nanowire, including a p-type GaN nanowire 721, a heterojunction AlxGa1-xN nanowire 722, and a Cs/O active layer 723, as shown in fig. 5. Wherein the p-type GaN nanowire 721 is grown on the substrate 800, the heterojunction AlxGa1-xN nanowire 722 is grown on the p-type GaN nanowire 721, and the Cs/O active layer 723 covers the p-type GaN nanowire 721 and the surface of the heterojunction AlxGa1-xN nanowire 722. A GaN coating is grown on the substrate 800 to facilitate the growth of the p-type GaN nanowires 721. The heterojunction AlxGa1-xN nanowire 722 is formed by stacking several layers with gradually decreasing Al composition from each layer close to the p-type GaN nanowire 721 to the outside.

Further, the heterojunction AlxGa1-xN nanowire 722 is grown on the p-type GaN nanowire 721, and the heterojunction AlxGa1-xN nanowire is continuously grown on the grown GaN nanowire, so that the problem of a highly-aggregated nanowire structure or a quasi-film structure caused by the fact that the AlxGa1-xN nanowire is directly grown on a substrate without a GaN nanowire template or the GaN nanowire is too dense under the low nitrogen flow rate is solved. And a predetermined heterojunction component structure can be formed in the radial direction of the nanowire better, so that the obvious bending, dislocation or nonuniformity of a periodic structure of the heterojunction nanowire is reduced.

Furthermore, the diameter of the nano-wire is 10-100nm, the length of the nano-wire is 500 nm-1 μm, and the center distance between adjacent nano-wires is 2 μm. The p-type doping concentration is 1 x 1019cm-3, the doping element is Mg, the length of the p-type GaN nanowire is 100-200nm, the heterojunction AlxGa1-xN nanowire 722 consists of n AlxGa1-xN thin layers with different components from bottom to top, wherein x is more than 0 and less than or equal to 1, the value range of n is more than 2 and less than 10, and the thickness of each layer of the heterojunction AlxGa1-xN nanowire 722 is 10-100 nm. In this embodiment, the heterojunction AlxGa1-xN nanowire 722 includes AlxGa1-xN thin layers of different compositions, namely al0.9ga0.1n, al0.8ga0.2n, al0.7ga0.3n, al0.6ga0.4n, al0.5ga0.5n, al0.4ga0.6n, al0.3ga0.7n, al0.2ga0.8n, and al0.1ga0.9n, from bottom to top in the direction of the legend. The band gap of the semiconductor material is changed by adjusting the content of the Al component x in the AlxGa1-xN nanowire array, and a strong built-in electric field is formed in the AlGaN nanowire array, as shown in (a) in FIG. 2, excited photoelectrons can be rapidly and directionally conveyed to the top of the nanowire through the GaN nanowire and the AlxGa1-xN nanowire continuously under the action of the electric field, so that the probability of the photoelectrons emitted from the side wall is reduced, the quantum efficiency and the collection efficiency of the cathode of the AlxGa1-xN nanowire array are further enhanced, and the problem that the electrons escaping from the side surface of the AlGaN nanowire array with uniform electrons are shielded by adjacent nanowires is solved.

The field-assisted heterojunction AlxGa1-xN/GaN nanowire array electron source in the embodiment solves the problem of low quantum efficiency of a GaN film photocathode, and the nanowire array structure is adopted, so that light beams incident to the surface of the nanowire array at a proper angle can be fully absorbed through reflection and refraction among nanowires, and light beams which are not absorbed and penetrate can be finally absorbed due to reflection or refraction, so that the photocurrent is increased, and the photon absorption rate is improved.

EXAMPLE III

In conjunction with fig. 6, for the third embodiment of the present invention, a method for fabricating the nanowires of the second embodiment is provided, which includes a method for generating an aligned nano-SiO 2 mask on the surface of the substrate, a method for growing a heterojunction AlxGa1-xN nanowire array, and a method for fabricating an electron source cathode of the heterojunction AlxGa1-xN/GaN nanowire array.

Step S100, a method for generating an orderly-arranged nano-scale SiO2 mask on the surface of a substrate is provided:

step S101, using a volume ratio of 1: 1: 2, cleaning a sapphire Al2O3 substrate by using acetone, alcohol and deionized water, and growing a layer of GaN film on a sapphire Al2O3(0001) surface by using an MOCVD method, wherein the thickness of the layer is 1.5 mm;

step S102, depositing a layer of SiO2 on the GaN film by a radio frequency sputtering method, wherein the thickness is 40 nm; an array of circular holes with a fixed center distance of 2 μm and an aperture of about 50nm was fabricated on a SiO2 mask by electron beam lithography and reactive ion etching.

Step S200, the method for growing the heterojunction AlxGa1-xN nanowire array comprises the following steps:

step S201, placing a substrate and a reaction source into a horizontal MOCVD reactor for zone heating under the pressure of 76Torr, introducing ammonia gas and argon gas into the horizontal MOCVD reactor when the temperature of the region where the substrate and the reaction source are located reaches 950 ℃, keeping the temperature of the MOCVD reactor to 980 ℃ for deposition reaction, wherein trimethyl gallium (TMG) is a gallium source, MgCl2 is a doping element magnesium source, the mixture is used as the reaction source after the mixture is mixed according to the molar ratio of 1000:1, the flow rates are both 10mmol/min, the growth is carried out for 8 minutes on a patterned substrate, and the length of the obtained p-type GaN nanowire is 150nm, and the diameter is 50 nm;

step S202, placing the prepared p-type GaN nanowire into a 350 ℃ corrosive liquid for 15min, wherein the corrosive liquid is a NaOH solution with the mass fraction of 5% so as to remove the residual SiO2 mask plate on the surface of the p-type GaN nanowire;

step S203, opening an Al source shutter to start the growth of the p-type heterojunction AlxGa1-xN nanowire, controlling the flux ratio of Al/(Al + Ga) every 5min by controlling the flux ratio of the Al and the Ga, reducing the flux ratio by reducing the flow rate of ammonia gas to 18mmol/min and controlling the substrate temperature to be in the range of 895 + 960 ℃, and growing the p-type AlxGa1-xN nanowire with the Al component from bottom to top decreasing from 4.5mmol to 22.5mmol by increasing the flux ratio of the Ga, wherein the length of each layer is about 50 nm; the total length of the finally obtained p-type GaN/AlxGa1-xN nanowire is 600nm, and the diameter of the p-type GaN/AlxGa1-xN nanowire is 50 nm.

Step S300, the method for further manufacturing the p-type heterojunction AlxGa1-xN/GaN nanowire array grown on the substrate into the heterojunction AlxGa1-xN/GaN nanowire array electron source cathode comprises the following steps:

step S301, firstly, putting the grown AlxGa1-xN nanowire array into a carbon tetrachloride solution (CCl4), soaking for 5 minutes, then putting the nanowire array into an acetone solution, and soaking for 5 minutes to remove organic pollutants on the surface of the nanowire array;

step S302, placing the AlxGa1-xN nanowire array into a chemical inorganic cleaning agent for cleaning, wherein the cleaning agent is 1: 4: preparing concentrated sulfuric acid H2SO4 or hydrofluoric acid HF according to the proportion of 100: hydrogen peroxide H2O 2: the mixed solution of deionized water was immersed in the mixed solution for 8 minutes, followed by rinsing with deionized water. And thirdly, cleaning, and then conveying the obtained product into a 850 ℃ high-temperature vacuum system for heating and purifying, so that the AlxGa1-xN nanowire array has an atomic-scale clean surface. Then, a Cs/O activation layer is formed on the p-type AlxGa1-xN nanowire array by adopting a Cs source continuous and O source intermittent activation process under the pressure of 10-10Torr through an ultrahigh vacuum system, and the surface of the emission layer reaches negative electron affinity at the moment;

step S303, packaging the heterojunction AlxGa1-xN/GaN nanowire array in an ultrahigh vacuum system device (the vacuum degree is kept at 10-10 Torr). After the heterojunction AlxGa1-xN/GaN nanowire array was packaged as a cathode, an 80nm thick transparent Indium Tin Oxide (ITO) electrode was deposited as a top transparent anode at 1000nm from the top of the nanowire in an ultra-high vacuum environment. And (3) bonding a two-dimensional g-GaN nanosheet anti-reflection layer on the upper surface of the transparent Indium Tin Oxide (ITO) anode to finally prepare the field-assisted heterojunction AlxGa1-xN/GaN nanowire array electron source.

In connection with fig. 6 (a) - (c), the electrons exiting from the uniform top surface of the nanowire actually account for a very small fraction of the total number of exiting electrons, and most of the electrons will exit from the incident side surface of the nanowire, so the highest collection efficiency is 11.67%, and the total electron collection ratio is only 52.03% at the highest. As shown in (d) - (f) of fig. 6, the quantum efficiency and the collection efficiency of the heterojunction AlxGa1-xN/GaN nanowire array electron source cathode reach 22.73% and 14.13%, respectively, and are increased compared with the uniform nanowire array structure, especially the electron collection ratio is as high as 62.15%, and the improvement of the photoelectric properties is mainly benefited from the obvious increase of the number of emitted electrons on the top surface of the heterojunction AlxGa1-xN/GaN nanowire array electron source cathode. In connection with (g) of fig. 6, it can be observed that only the electron current emitted from the top surface of the nanowire can be fully collected, and the electrons escaping from the side surface can be collected by the collecting side located on the top surface only when the electrons are emitted within a corresponding angle, and otherwise the electrons will be captured by the adjacent nanowire and cannot escape from the array structure. From the results of (h) and (i) in fig. 6, it can be found that the heterojunction AlxGa1-xN/GaN nanowire array electron source cathode can significantly improve the percentage of the electron current emitted from the top surface of the nanowire, greatly improve the collection efficiency of the photocathode, and can better make up for the deficiency of the collection efficiency of the uniform nanowire array photocathode.

In the embodiment, the growth method of the heterojunction AlxGa1-xN/GaN nanowire array electron source cathode nanowire is grown on the p-type GaN nanowire by the MOCVD method from bottom to top, so that the defect that the diameter of the nanowire can only reach the micron level caused by the traditional etching method is overcome, and meanwhile, the content of the Al component on the side wall of the nanowire is consistent with that of the Al component positioned at the center of the nanowire. In addition, the p-type GaN nanowire is firstly grown on the substrate, so that an important size base for controlling the growth process of the subsequent AlxGa1-xN nanowire section can be provided, and the problem of a high-coalescence nanowire structure or a quasi-film structure caused by directly growing the AlxGa1-xN nanowire on the substrate without a GaN nanowire template under low nitrogen flow rate is solved.

Claims

1. An electron source package is characterized by comprising a substrate (100), a sealing cover (200) and an electron source, wherein a power supply is arranged in the substrate (100), the sealing cover (200) is arranged on the substrate (100) and forms a closed space with the substrate (100), and the electron source is arranged on the closed space substrate (100); the electron source comprises a substrate (800), a nanowire bundle, an electrode ring (600), an anti-reflection layer (500) and a protective layer (400), wherein the substrate (800) is arranged on the substrate (100) and is grounded, the nanowire bundle is arranged on the substrate (800), the electrode ring (600) is arranged above the nanowire bundle (700) and is electrically connected with a power supply of the substrate (100), the anti-reflection layer (500) is arranged on the electrode ring (600), and the protective layer (400) is arranged on the anti-reflection layer (500).

2. The package of claim 1, wherein the surface of the substrate (800) comprises a number of raised structures (820), each nanowire (700) of the nanowire bundle being grown on a respective raised structure (820).

3. The package of claim 1 or 2, wherein each nanowire (700) of the nanowire bundle comprises a p-type GaN nanowire (721), a heterojunction AlxGa1-xN nanowire (722), a Cs/O active layer (723), wherein the p-type GaN nanowire (721) is grown on the substrate (800), the heterojunction AlxGa1-xN nanowire (722) is grown on the p-type GaN nanowire (721), and the Cs/O active layer (723) covers the surfaces of the p-type GaN nanowire (721) and the heterojunction AlxGa1-xN nanowire (722).

4. The package according to claim 3, wherein the heterojunction AlxGa1-xN nanowire (722) is formed by stacking a plurality of layers, and the Al composition of each layer gradually decreases from the position close to the p-type GaN nanowire (721) to the outside, wherein x is more than 0 and less than or equal to 1, n is more than 2 and less than 10, and the thickness of each layer is 10-100 nm.

5. The package of claim 3, wherein the surface of the substrate (800) is covered with a GaN layer (810), and the plurality of raised structures (820) are divided into a plurality of hexagonal cells (810).

6. The package according to any of claims 1 to 5, wherein the light source (300) is arranged in the closed space, the light source (300) being connected to a power supply in the substrate (100).