CN107895681B - Photocathode and preparation method thereof - Google Patents
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- 239000002344 surface layer Substances 0.000 claims abstract description 52
- 239000004065 semiconductor Substances 0.000 claims abstract description 50
- 239000000463 material Substances 0.000 claims abstract description 46
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
- H01J1/02—Main electrodes
- H01J1/34—Photo-emissive cathodes
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
- H01J9/02—Manufacture of electrodes or electrode systems
- H01J9/12—Manufacture of electrodes or electrode systems of photo-emissive cathodes; of secondary-emission electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract
The invention discloses a photocathode, which comprises a substrate, a p-type AlN buffer layer formed on the substrate, a p-type Al xGa1‑x N emitting layer formed on the p-type AlN buffer layer and a narrow forbidden band semiconductor surface layer formed on the p-type Al xGa1‑ x N emitting layer; the material of the narrow-band-gap semiconductor surface layer is a semiconductor material with a band gap of less than or equal to 2.3eV at room temperature, and the range of x in the p-type Al xGa1‑x N emitting layer is more than or equal to 0 and less than 1. The photocathode grows a narrow-band-gap semiconductor surface layer on the p-type Al xGa1‑x N emitting layer, atoms of the narrow-band-gap semiconductor surface layer and atoms of the p-type Al xGa1‑x N emitting layer are combined in a covalent bond mode, the improvement of the surface energy band structure of an Al xGa1‑x N material is facilitated, the work function of the surface of the photocathode is reduced, and the probability of electron tunneling through the surface of the photocathode is improved.
Description
Technical Field
The invention relates to the technical field of photoelectric cathode electron sources. And more particularly, to a photocathode and a method for preparing the same.
Background
The photocathode is a core component for photoelectric conversion in a micro-light detection device, the performance of the photocathode is improved year by year in recent years, and the types of photocathode materials in different wave bands are gradually increased. The traditional photocathode has the advantages of high quantum efficiency, high response speed, small dark current, concentrated energy of emitted electrons and the like, and the peak electron emission capacities of the spectral responses of the reflective and transmissive GaAs photocathodes reach 379.9 mA and 245.2mA/W respectively.
The traditional photocathode mainly adopts a cesium (Cs) atom adsorption mode to reduce the work function of the photocathode, the GaAs photocathode can form a negative electron affinity surface, electrons excited from a valence band to a conduction band have a larger probability of escaping into vacuum through a surface potential barrier, and therefore, the photocathode has higher electron emission capability. However, the adsorption thickness of Cs atoms is only a single atomic layer, and Cs atoms are easily desorbed from the cathode surface at high temperature or when the emission current density is large, resulting in reduced cathode performance. Meanwhile, the electron emission level of the photocathode has strict requirements on the gas atmosphere of the vacuum chamber, CO 2, CO, H 2 O and the like can inhibit the electron emission capability of the cathode, and the electron emission capability of the cathode can not be recovered again, so that the traditional photocathode can not be applied to devices and instruments requiring a high-current electron source.
Therefore, the photocathode can only be applied to an environment with lower illumination intensity, and under the condition of higher illumination intensity, the electron emission capability of the cathode can be drastically reduced, even the electron emission capability is lost, so that the photocathode cannot be applied to other devices and instruments with high current as an electron source. So the photocathode is only applied to a low-light-level image enhancement device and a part of free electron lasers at present, and is not applied to other vacuum devices and instruments requiring an electron source.
In order to solve the problem that the traditional photocathode cannot be used in a strong illumination condition, in 2010, an activation-free GaN photocathode is proposed in the United states, the transportation efficiency and the surface work function of photoelectrons in a cathode emission layer are improved by adjusting doping atoms and doping concentration, the cathode emission layer comprises a p-type GaN emission layer, a Si delta doping layer GaN layer and an n-type GaN surface layer, the larger the thickness of the surface layer, the lower the electron emission performance of the cathode, and when photon energy is 5eV, the quantum efficiency of the cathode reaches 0.1%. The 55 th research of China electric science proposes an activation-free GaN photocathode with a p-i-n structure, and the electron emission of the cathode is realized by adjusting the energy band structure of a cathode emission layer and an n-type GaN surface layer by using the doping concentration. Compared with the traditional photocathode, the photocathode without Cs activation has higher service life and stability, but the GaN crystal is a wide-bandgap semiconductor, the conductivity of the material is smaller, the conductivity is poorer, and after a large number of electrons are emitted from the surface of the cathode, the cathode is positively charged, and the electrons on the cathode electrode cannot be timely supplemented into a cathode emission layer, so that the photocathode becomes an important factor for limiting the cathode performance.
Therefore, although the activation-free photocathode can be suitable for strong illumination, especially when high-power laser is used as a light source, the emission current of the photocathode is far higher than that of the traditional photocathode, and meanwhile, the photocathode has stable emission performance and long service life, and is an ideal electron source with simple packaging process and convenient modulation and use, but the quantum efficiency of the activation-free photocathode is far lower than that of the traditional photocathode.
Therefore, it is desirable to provide a high current density photocathode that can be applied to high light conditions and that can enhance the strong light response of the quantum efficiency of the photocathode.
Disclosure of Invention
It is an object of the present invention to provide a photocathode.
Another object of the present invention is to provide a method for manufacturing a photocathode.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a photocathode comprising a substrate, a p-type AlN buffer layer formed on the substrate, a p-type Al xGa1-x N emission layer formed on the p-type AlN buffer layer, and a narrow bandgap semiconductor surface layer formed on the p-type Al xGa1-x N emission layer; the material of the narrow-band-gap semiconductor surface layer is a semiconductor material with a band gap of less than or equal to 2.3eV at room temperature, and the range of x in the p-type Al xGa1-x N emitting layer is more than or equal to 0 and less than 1. Compared with the traditional photocathode Cs-O adsorption activation process, the invention grows the narrow-band-gap semiconductor surface layer on the p-type Al xGa1-x N emission layer, and the atoms of the narrow-band-gap semiconductor surface layer and the atoms of the p-type variable-doped Al xGa1-x N emission layer are combined in a covalent bond mode, so that the invention is beneficial to improving the surface energy band structure of an Al xGa1-x N material, reducing the work function of the photocathode surface and improving the probability of electron tunneling through the photocathode surface, and the obtained photocathode has the advantages of high stability, strong ion bombardment resistance, high emission current density and the like. At present, no literature report similar to the content of the patent is available in the aspects of photocathode and preparation technology thereof.
Preferably, the material of the narrow bandgap semiconductor surface layer may be an n-type, intrinsic or p-type group III-V crystal material; further, the III-V crystal in the n-type, intrinsic or p-type III-V crystal material may be AlAs, gaAs, gaP, inAs, inP or a crystal of a III-V multi-compound or the like. The preferable surface layer material in the invention can change the surface energy band structure of the cathode material more effectively, reduce the potential barrier of the cathode surface and improve the electron emission capability of the cathode.
Preferably, the material of the narrow bandgap semiconductor surface layer can Be p-type III-V crystal material, the doping atoms are Zn or Be and the like, and the doping concentration is less than or equal to 1 multiplied by 10 20cm-3. The doping concentration range can more effectively improve the conductivity of the p-type narrow bandgap semiconductor surface layer.
Preferably, the material of the narrow bandgap semiconductor surface layer can be n-type III-V crystal material, the doping atoms are Si, sn, ge, C, te or S, and the doping concentration is less than or equal to 1×10 20cm-3. The doping concentration range can more effectively improve the conductivity of the p-type narrow bandgap semiconductor surface layer.
Preferably, the thickness h of the narrow bandgap semiconductor surface layer is in the range 0<h-200 nm. The invention only needs to have a narrow forbidden band semiconductor surface layer, wherein the enhancement effect of the heterostructure on electron transport can be reduced when the thickness of the narrow forbidden band semiconductor surface layer is larger than 200nm, and the improvement on the potential barrier of the cathode surface can be reduced. In certain embodiments of the present invention, the narrow bandgap semiconductor surface layer thickness may be, for example: 0.5 to 100nm, 0.5 to 50nm, 0.5 to 10nm, 10 to 200nm, 10 to 100nm, 10 to 50nm, 50 to 200nm, 50 to 100nm, etc., more preferably 0.5 to 10nm, and even more preferably the results obtained by the embodiments are most excellent.
Preferably, the doping atoms of the p-type AlN buffer layer can be Mg, and the doping mode can be uniform doping, and the doping concentration is less than or equal to 1 multiplied by 10 19cm-3. The method is beneficial to improving the capability of directionally supplementing electrons for the buffer layer in the Mg doping concentration range, and the doping concentration is more than 1 multiplied by 10 19cm-3, so that the ionization efficiency of doping atoms can be reduced, and the growth quality of crystals is influenced.
Preferably, the thickness of the p-type AlN buffer layer can be 10-1000 nm. The thickness of the buffer layer is smaller than 10nm, so that the crystal growth quality of the emitting layer is reduced, and the thickness of the buffer layer is larger than 1000nm, so that incident light is attenuated in the AlN buffer layer. In some embodiments of the present invention, the thickness of the p-type AlN buffer layer may be, for example, ：10～700nm、10～600nm、10～500nm、10～400nm、10～300nm、10～200nm、10～100nm、10～50nm、10～30nm、30～1000nm、30～700nm、30～600nm、30～500nm、30～400nm、30～300nm、30～200nm、30～100nm、30～50nm、50～1000nm、50～700nm、50～600nm、50～500nm、50～400nm、50～300nm、50～200nm、50～100nm、100～1000nm、100～700nm、100～600nm、100～500nm、100～400nm、100～300nm、100～200nm、200～1000nm、200～700nm、200～600nm、200～500nm、200～400nm、200～300nm、300～1000nm、300～700nm、300～600nm、300～500nm、300～400nm、400～1000nm、400～700nm、400～600nm、400～500nm、500～1000nm、500～700nm、500～600nm、600～1000nm、600～700nm、700～1000nm, more preferably 50 to 700nm, and still more preferably the best results.
Preferably, the doping atoms of the p-type Al xGa1-x N emitting layer are Mg, the doping mode is gradient doping, the number of concentration gradients is m, m is more than or equal to 1, the doping concentration is N 1、N2、…、Nm-1、Nm from the narrow bandgap semiconductor surface layer to the p-type AlN buffer layer, and 1× 15cm-3≤N1≤N2≤…≤Nm-1≤Nm≤1×1020cm-3 is satisfied. The emission layer can be uniformly doped or variably doped, and the doping concentration range can improve the transport efficiency of electrons to the surface of the cathode on the basis of ensuring the growth quality and conductivity of crystals.
Preferably, the p-type Al xGa1-x N emitting layer comprises N p-type Al xGa1-x N sublayers, wherein N is more than or equal to 1, the Al component of each p-type Al xGa1-x N sublayer in the direction from the narrow bandgap semiconductor surface layer to the p-type AlN buffer layer is x 1、x2、…、xn-1、xn in sequence, and 0-x 1≤x2≤…≤xn-1≤xn <1 is satisfied. According to the invention, the Al component of the p-type Al xGa1-x N emitting layer is gradually increased from the surface layer to the buffer layer, so that the formed energy band structure change is beneficial to improving the transport efficiency of electrons in the photocathode to the surface direction of the photocathode.
Preferably, the thickness of the p-type Al xGa1-x N emitting layer can be 10-300 nm. The thickness of the emitting layer is smaller than 10nm, so that the absorption efficiency of incident light can be reduced, and the transportation of electrons can be influenced when the thickness of the emitting layer is larger than 300nm. In some embodiments of the present invention, the p-type Al xGa1-x N emissive layer thickness may be, for example, ：10～200nm、10～170nm、10～150nm、10～130nm、10～100nm、10～80nm、10～45nm、45～300nm、45～200nm、45～170nm、45～150nm、45～130nm、45～100nm、45～80nm、80～300nm、80～200nm、80～170nm、80～150nm、80～130nm、80～100nm、100～300nm、100～200nm、100～170nm、100～150nm、100～130nm、130～300nm、130～200nm、130～170nm、130～150nm、150～300nm、150～200nm、150～170nm、170～300nm、170～200nm、200～300nm, etc., more preferably 45 to 80nm, and even more preferably the most effective.
Preferably, the substrate may be a sapphire substrate. The thickness of the substrate in the present invention is not particularly limited and may be appropriately selected according to the purpose.
In addition, the shapes of the substrate, the p-type AlN buffer layer, the p-type AlxGa1-xN emission layer, and the narrow bandgap semiconductor surface layer of the present invention are not particularly limited and may be appropriately selected depending on the intended purpose.
The preparation method of the photocathode comprises the following steps:
Forming a p-type AlN buffer layer on a substrate, forming a p-type Al xGa1-x N emission layer on the p-type AlN buffer layer, and forming a narrow bandgap semiconductor surface layer on the p-type Al xGa1-x N emission layer to obtain a photocathode material; and activating the photocathode material to obtain the photocathode.
Preferably, the p-type AlN buffer layer may be formed by MOCVD (Metal-organic Chemical Vapor Deposition Metal organic chemical vapor deposition) or MBE (Molecular Beam Epitaxy molecular beam epitaxy) epitaxial growth.
Preferably, the p-type Al xGa1-x N emitting layer can be formed by MOCVD or MBE epitaxial growth.
Preferably, the formation mode of the narrow bandgap semiconductor surface layer may be MOCVD, MBE, VPE (Vapor Phase Epitaxy vapor phase epitaxy), LEP (Liqiud Phase Epitaxy liquid phase epitaxy) or epitaxial growth process such as thermal evaporation.
Preferably, the activation mode of the photocathode material is annealing activation. The photocathode can be obtained without Cs-O adsorption activation and annealing activation. The annealing activation in the present invention is a conventional technical means, and will not be described herein.
Preferably, the preparation method of the photocathode specifically comprises the following steps:
1) On a double-sided polished sapphire substrate, growing a p-type AlN buffer layer by adopting an MOCVD or MBE epitaxial growth mode;
2) Growing a p-type Al xGa1-x N emission layer on the p-type AlN buffer layer by adopting an MOCVD or MBE epitaxial growth mode and a p-type doping process of a semiconductor material;
3) Adopting MOCVD, MBE, VPE, LEP or thermal evaporation and other epitaxial growth processes to grow a narrow forbidden band semiconductor surface layer on the p-type Al xGa1-x N emission layer to obtain a photocathode material;
4) Placing the photocathode material into a vacuum system for heat treatment to remove impurity atoms on the surface of the cathode;
5) And (5) annealing and activating the photocathode material to obtain the photocathode.
How to enable the photocathode to be applied to strong illumination conditions and have strong current emission capability is a main technical problem overcome by the invention. The traditional photocathode is activated by adopting a Cs-O adsorption mode, and has the defects of lower binding energy of Cs atoms on the surface of the cathode and poor stability although the quantum efficiency is higher, so that the photocathode activated by Cs-O is not suitable for working under strong light conditions and cannot be used as a high-current density electron source.
In order to overcome the technical problems, the invention grows an ultrathin narrow-bandgap semiconductor surface layer on the surface of the cathode material to replace the traditional Cs-O activation method, thereby not only enhancing the built-in electric field intensity in the area near the surface of the cathode and enhancing the electron transport performance, but also effectively reducing the vacuum energy level on the surface of AlGaN crystal, so that the photocathode has the capability of large-current density electron emission. In addition, the ultrathin narrow-bandgap semiconductor surface layer is combined with the AlGaN crystal in a covalent bond mode, and is more stable than the adsorption of Cs atoms on the surface of the AlGaN crystal, so that the cathode has the capability of bearing electron emission with high current density. In order to enhance the transport capacity of electrons in the Al xGa1-x N emitting layer towards the surface of the cathode, the emitting layer adopts a variable component variable doping structure design, a built-in electric field direction is formed in the emitting layer and points to the surface of the cathode from the inside, drift movement is generated on the basis of the diffusion movement of the electrons towards the surface of the cathode, the diffusion length of the electrons is improved, and finally the electron emitting capacity of the cathode is enhanced. That is, the technical scheme of the invention is a unified whole, each technical feature is not independent linearly, and different technical features can mutually influence each other, so that the realization of the final technical effect of the invention is necessarily dependent on an integral which is not detachable and organically integrates all the technical features, but is not the simple addition of a plurality of technical features.
Any range recited in the present invention includes any numerical value between the end values and any sub-range formed by any numerical value between the end values or any numerical value between the end values unless specifically stated otherwise.
The beneficial effects of the invention are as follows:
(1) The invention adopts an ultrathin narrow-band-gap semiconductor layer to replace the traditional activation mode of the adsorption of the Cs-O atoms of the photocathode, and the obtained photocathode has the advantages of high stability, strong ion bombardment resistance, high emission current density and the like.
(2) The atoms of the narrow forbidden band semiconductor layer on the surface of the photocathode and the atoms on the surface of the emission layer form covalent bonds, so that the bonding energy of the active atoms on the surface of the cathode emission layer is greatly improved, and the capability of the cathode of the invention for bearing electron emission with high current density is further improved.
(3) The Al component of the p-type AlGaN emission layer is gradually reduced from inside to surface, the structure can gradually reduce the energy band structure in the emission layer from inside to surface, and effectively reduces the vacuum energy level of the AlGaN crystal surface, so electrons can move to the cathode surface in the emission layer in two movement modes of diffusion and drift, the diffusion length of the electrons is increased, and finally the quantum efficiency of the photocathode is effectively improved.
Drawings
The following describes the embodiments of the present invention in further detail with reference to the drawings.
Fig. 1 shows a schematic structural diagram of a photocathode in the present invention: FIG. 1 (a) is a schematic view showing the optical structure of a photocathode and the Al composition in the present invention; FIG. 1 (b) is a schematic diagram showing the concentrations of the photo-cathode optical structure and the Al xGa1-x N emitting layer in the present invention; wherein: a 1-sapphire substrate, a 2-p type AlN buffer layer, a 3-p type Al xGa1-x N emitting layer and a 4-narrow bandgap semiconductor surface layer.
Fig. 2 shows a schematic diagram of the optical structure of the photocathode and Al composition in example 1 and comparative example 1 of the present invention: FIG. 2 (a) is a schematic view showing the optical structure of the photocathode and the Al composition in comparative example 1 of the present invention; FIG. 2 (b) is a schematic view showing the optical structure of the photocathode and the Al composition in example 1 of the present invention; wherein: 201-sapphire substrate, 202-p type AlN buffer layer, 203-p type GaN emission layer, 204-intrinsic GaAs surface layer.
Fig. 3 shows a schematic view of the optical structure and Al composition of the photocathode in embodiment 2 of the present invention, wherein: 301-sapphire substrate, 302-p type AlN buffer layer, 303-p type GaN emission layer, 304-intrinsic type InP surface layer.
Fig. 4 shows a schematic view of the optical structure and Al composition of the photocathode in embodiment 3 of the present invention, wherein: a 401-sapphire substrate, a 402-p type AlN buffer layer, a 403-p type Al 0.9Ga0.1 N emitting layer, a 404-p type Al 0.65Ga0.35 N emitting layer, a 405-p type GaN emitting layer, a 406-p type variable component variable doped Al xGa1-x N emitting layer and a 407-intrinsic type GaAs surface layer.
Fig. 5 shows quantum efficiency curves of photocathodes in examples 1, 2 and 3 of the present invention.
Fig. 6 shows a quantum efficiency curve of the photocathode of comparative example 2 of the present invention.
Detailed Description
In order to more clearly illustrate the present invention, the present invention will be further described with reference to preferred embodiments and the accompanying drawings. Like parts in the drawings are denoted by the same reference numerals. It is to be understood by persons skilled in the art that the following detailed description is illustrative and not restrictive, and that this invention is not limited to the details given herein.
Fig. 1 shows a schematic structure of a photocathode in the present invention, which comprises a sapphire substrate 1, a p-type AlN buffer layer 2 formed on the sapphire substrate 1, a p-type Al xGa1-x N emission layer 3 formed on the p-type AlN buffer layer 2, and a narrow bandgap semiconductor surface layer 4 formed on the p-type Al xGa1-x N emission layer 3.
In fig. 1, the p-type AlN buffer layer is uniformly doped, and the doping atoms are Mg;
The p-type Al xGa1-x N emission layer is doped with Mg in a gradient doping mode, the number of doping concentration gradients is m and is larger than or equal to 1, the concentration of the Al xGa1-x N emission layer from top to bottom is N 1、N2、…、Nm-1、Nm respectively, and 1X 10 15cm-3≤N1≤N2≤…≤Nm-1≤Nm≤1×1020cm-3 is satisfied, as shown in a figure 1 (b);
The p-type variable component variable doped Al xGa1-x N emitting layer can be composed of N p-type Al xGa1-x N sublayers, wherein N is more than or equal to 1, the Al components of the p-type Al xGa1-x N sublayers from top to bottom are x 1、x2、…、xn-1、xn respectively, and 0-x 1≤x2≤…≤xn-1≤xn < 1 is satisfied, as shown in fig. 1 (a).
Example 1
A photocathode, whose optical structure and Al composition structure are schematically shown in FIG. 2 (b), comprises a sapphire substrate 201, a p-type doped AlN buffer layer 202 formed on the sapphire substrate 201, a p-type GaN emission layer 203 formed on the p-type doped AlN buffer layer 202, and an intrinsic GaAs surface layer 204 formed on the p-type GaN emission layer 203.
The preparation method comprises the following steps:
1) A p-type AlN buffer layer 202 with the thickness of 200nm is grown on a double-sided polished sapphire substrate 201 with the thickness of 0.46mm by using an MOCVD epitaxial growth mode, and the doping atoms are Mg, and the doping concentration is 1 multiplied by 10 16cm-3;
2) Using an MOCVD epitaxial growth mode and a p-type doping process of a semiconductor material to grow a p-type Al xGa1-x N emission layer on the p-type AlN buffer layer 202, wherein the number of sub-layers of the p-type Al xGa1-x N emission layer is n=1, the Al component is 0, the doping concentration gradient degree m=1, namely the Al xGa1-x N emission layer is a GaN emission layer 203, the thickness is 80nm, the doping atoms are Mg, and the doping concentration is 1×10 18cm-3;
3) And 2) respectively placing the photocathode material obtained in the step 2) into acetone and ethanol for ultrasonic cleaning for 5 minutes to remove organic matters stained on the surface of the cathode material in the transportation process. Then, the resultant was subjected to a heat-cleaning at 800℃for 20 minutes in a vacuum system to remove the compound on the surface C, O of the cathode material, thereby obtaining an atomically clean surface.
4) Growing a GaAs crystal by using an LEP growth mode, namely growing an intrinsic GaAs surface layer 204 with the thickness of about 5nm on the p-type GaN emission layer 203 obtained in the step 3);
5) And (3) performing annealing activation on the photocathode material obtained in the step (4).
Through the above steps, a photocathode assembly is manufactured.
And testing the quantum efficiency of the activated photocathode. Fig. 5 is a graph of quantum efficiency of a cathode, with photon energy in horizontal coordinates and quantum efficiency of a photocathode in vertical coordinates. It can be seen from fig. 5 that the quantum efficiency of the cathode is only 10 -3～10-2 orders of magnitude, which is lower than that of the Cs/O activated photocathode with the quantum efficiency of 10 -1 orders of magnitude, but under the irradiation of a laser with the wavelength of 266nm (photon energy of 4.66 eV) and the power of 1W, the continuous emission current of the photocathode can reach 0.92mA, and the stability is far higher than that of the Cs/O activated photocathode.
Comparative example 1
A photocathode, whose optical structure and Al composition structure are schematically shown in FIG. 2 (a), comprises a sapphire substrate 201, a p-type AlN buffer layer 202 formed on the sapphire substrate 201, and a p-type GaN emission layer 203 formed on the p-type AlN buffer layer 202.
The preparation method is the same as in example 1, and only the difference is that: and (3) performing annealing activation on the photocathode material obtained in the step (3) in the step (5), wherein the step (4) is not included.
Through the steps, the photoelectric cathode component is manufactured,
And testing the quantum efficiency of the activated photocathode, wherein the cathode does not emit current in a testing range, and the cathode quantum efficiency is 0.
Comparative example 2
A photocathode, whose optical structure and Al composition structure are schematically shown in FIG. 2 (a), comprises a sapphire substrate 201, a p-type AlN buffer layer 202 formed on the sapphire substrate 201, and a p-type GaN emission layer 203 formed on the p-type AlN buffer layer 202.
The preparation method is the same as in example 1, and only the difference is that: excluding the step 4), annealing the photocathode material obtained in the step 3) in the step 5), and then activating by using a Cs/O activation mode.
Through the above steps, a photocathode assembly is manufactured.
And testing the quantum efficiency of the activated photocathode. Fig. 6 is a graph of quantum efficiency of a cathode, with photon energy in horizontal coordinates and quantum efficiency of a photocathode in vertical coordinates. The quantum efficiency of the photocathode reaches 23.7% when the photon energy is 5.167 eV. Although the quantum efficiency of the Cs/O photocathode reaches 10 -1 orders of magnitude, which is higher than that of the photocathode of the narrow bandgap semiconductor surface of example 1, as shown in fig. 5, the stability of the cathode is very poor under the irradiation of a laser with a power of 1W at 266nm (photon energy of 4.66 eV), and the cathode emission current rapidly decays to 0, so that it is not suitable as an electron source of a high-current vacuum device.
Example 2
A photocathode, whose optical structure and Al composition structure are schematically shown in FIG. 3, comprises a sapphire substrate 301, a p-type AlN buffer layer 302 formed on the sapphire substrate 301, a p-type GaN emission layer 303 formed on the p-type AlN buffer layer 302, and an intrinsic type InP surface layer 304 formed on the p-type GaN emission layer 303.
The preparation method comprises the following steps:
1) A p-type AlN buffer layer 302 with the thickness of 200nm is grown on a double-sided polished sapphire substrate 301 with the thickness of 0.46mm by using an MOCVD epitaxial growth mode, and the doping atoms are Mg, and the doping concentration is 1 multiplied by 10 16cm-3;
2) Using MOCVD epitaxial growth mode and p-type doping process of semiconductor material to grow p-type Al xGa1-x N emission layer on p-type AlN buffer layer 302, wherein the number of sub-layers of p-type Al xGa1-x N emission layer is n=1, al component is 0, doping concentration gradient degree m=1, i.e. Al xGa1-x N emission layer is GaN emission layer 303, thickness is 80nm, doping atom is Mg, doping concentration is 1× 18cm-3;
3) And 2) respectively placing the photocathode material obtained in the step 2) into acetone and ethanol for ultrasonic cleaning for 5 minutes to remove organic matters stained on the surface of the cathode material in the transportation process. Then, the resultant was subjected to a heat-cleaning at 800℃for 20 minutes in a vacuum system to remove the compound on the surface C, O of the cathode material, thereby obtaining an atomically clean surface.
4) An InP crystal is grown using LEP growth, i.e., an intrinsic InP surface layer 304 having a thickness of about 5nm is grown on the p-type GaN emitter layer 303;
5) And (3) performing annealing activation on the photocathode material obtained in the step (4).
And testing the quantum efficiency of the activated photocathode. Fig. 5 is a graph of quantum efficiency of a cathode, with photon energy in horizontal coordinates and quantum efficiency of a photocathode in vertical coordinates. It can be seen from fig. 5 that the quantum efficiency of the cathode is only 10 -3～10-2 orders of magnitude, which is lower than that of the Cs/O activated photocathode with the quantum efficiency of 10 -1 orders of magnitude, but under the irradiation of a laser with the wavelength of 266nm (photon energy of 4.66 eV) and the power of 1W, the continuous emission current of the photocathode can reach 1.5mA, and the stability is far higher than that of the Cs/O activated photocathode.
Example 3
A photocathode, whose optical structure and Al composition structure are schematically shown in FIG. 3, comprises a sapphire substrate 401, a p-type AlN buffer layer 402 formed on the sapphire substrate 401, a p-type Al 0.9Ga0.1 N emitting layer 403 formed on the p-type AlN buffer layer 402, a p-type Al 0.65Ga0.35 N emitting layer 404 formed on the p-type Al 0.9Ga0.1 N emitting layer 403, a p-type GaN emitting layer 405 formed on the p-type Al 0.65Ga0.35 N emitting layer 404, and an intrinsic GaAs surface layer 407 formed on the p-type GaN emitting layer 405.
The preparation method comprises the following steps:
1) A p-type AlN buffer layer 402 with the thickness of 200nm is grown on a double-sided polished sapphire substrate 401 with the thickness of 0.46mm by using an MOCVD epitaxial growth mode, the doping atoms are Mg, and the doping concentration is 1 multiplied by 10 16cm-3;
2) Using MOCVD epitaxial growth mode and p-type doping process of semiconductor material to grow p-type Al xGa1-x N emission layer on p-type AlN buffer layer 402, the doping concentration gradient number m=1, doping atom is Mg, doping concentration is 1× 18cm-3;
The number of sub-layers n=3 of the p-type Al xGa1-x N emission layer, the Al composition of the 3 sub-layers is 0.9, 0.65 and 0, respectively, the corresponding thicknesses are 5nm, 5nm and 35nm, respectively, i.e. the Al xGa1-x N emission layer comprises a p-type Al 0.9Ga0.1 N emission layer 403 with a thickness of 5nm, a p-type Al 0.65Ga0.35 N emission layer 404 with a thickness of 5nm and a p-type GaN emission layer 405 with a thickness of 35 nm;
3) And 2) respectively placing the photocathode material obtained in the step 2) into acetone and ethanol for ultrasonic cleaning for 5 minutes to remove organic matters stained on the surface of the cathode material in the transportation process. Then, the resultant was subjected to a heat-cleaning at 800℃for 20 minutes in a vacuum system to remove the compound on the surface C, O of the cathode material, thereby obtaining an atomically clean surface.
4) A GaAs crystal is grown using LEP growth, i.e., an intrinsic GaAs surface layer 406 having a thickness of about 5nm is grown on the p-type GaN emitter layer 405;
5) And (3) performing annealing activation on the photocathode material obtained in the step (4).
And testing the quantum efficiency of the activated photocathode. Fig. 5 is a graph of quantum efficiency of a cathode, with photon energy in horizontal coordinates and quantum efficiency of a photocathode in vertical coordinates. It can be seen from fig. 5 that the quantum efficiency of the cathode is only 10 -3～10-2 orders of magnitude, which is lower than that of the Cs/O activated photocathode with the quantum efficiency of 10 -1 orders of magnitude, but under the irradiation of a laser with the wavelength of 266nm (photon energy of 4.66 eV) and the power of 1W, the continuous emission current of the photocathode can reach 2.3mA, and the stability is far higher than that of the Cs/O activated photocathode.
It should be understood that the foregoing examples of the present invention are provided merely for clearly illustrating the present invention and are not intended to limit the embodiments of the present invention, and that various other changes and modifications may be made therein by one skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.
Claims (5)
1. A photocathode, characterized in that the photocathode comprises a substrate, a p-type AlN buffer layer formed on the substrate, a p-type Al xGa1-x N emission layer formed on the p-type AlN buffer layer, and a semiconductor surface layer formed on the p-type Al xGa1-x N emission layer;
Wherein the material of the semiconductor surface layer is a semiconductor material with the forbidden bandwidth less than or equal to 2.3eV at room temperature, the range of x in the p-type Al xGa1-x N emitting layer is more than or equal to 0 and less than 1,
The thickness h of the semiconductor surface layer is 0<h-200 nm,
The material of the semiconductor surface layer is an n-type, intrinsic or p-type III-V crystal material,
When the material of the semiconductor surface layer is p-type III-V group crystal material, the doping atoms are Zn or Be, the doping concentration is less than or equal to 1 multiplied by 10 20cm-3,
When the material of the semiconductor surface layer is an n-type III-V crystal material, the doping atoms are Si, sn, ge, C, te or S, and the doping concentration is less than or equal to 1 multiplied by 10 20cm-3.
2. The photocathode of claim 1, wherein the doping atoms of the p-type AlN buffer layer are Mg, and the doping concentration is equal to or less than 1×10 19 cm-3.
3. The photocathode of claim 1, wherein the doping atoms of the p-type Al xGa1-x N emission layer are Mg, the number of concentration steps is m, and m is equal to or greater than 1, and the doping concentration is N 1、N2、…、Nm-1、Nm in the direction from the semiconductor surface layer to the p-type AlN buffer layer, and satisfies 1×10 15 cm-3≤N1≤N2≤…≤Nm-1≤Nm≤1×1020 cm-3.
4. The photocathode of claim 1, wherein the p-type Al xGa1-x N emission layer comprises N p-type Al xGa1-x N sublayers, wherein N is not less than 1, the Al composition of each p-type Al xGa1-x N sublayer in the direction from the semiconductor surface layer to the p-type AlN buffer layer is x 1、x2、…、xn-1、xn in sequence, and 0 not more than x 1≤x2≤…≤xn-1≤xn < 1 is satisfied.
5. The preparation method of the photocathode is characterized by comprising the following steps of: forming a p-type AlN buffer layer on a substrate, forming a p-type Al xGa1-x N emitting layer on the p-type AlN buffer layer, and forming a semiconductor surface layer on the p-type Al xGa1-x N emitting layer to obtain a photocathode material; activating the photocathode material to obtain a photocathode, wherein x in the p-type Al xGa1-x N emitting layer is more than or equal to 0 and less than 1,
The material of the semiconductor surface layer is a semiconductor material with the forbidden band width less than or equal to 2.3eV at room temperature,
The thickness h of the semiconductor surface layer is 0<h-200 nm,
The activation mode is annealing activation.
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