阅读说明：本技术 外加电场辅助GaN纳米线阵列光电阴极及制备方法 (Additional electric field assisted GaN nanowire array photocathode and preparation method thereof ) 是由 刘磊 陆菲菲 田�健 张杨星月 于 2019-09-12 设计创作，主要内容包括：本发明提供了一种外加电场辅助GaN纳米线阵列光电阴极,包括银环氧树脂电极、柔性聚甲基丙烯酸甲酯层,高度对齐的p型GaN纳米线阵列和透明氧化铟锡电极；柔性聚甲基丙烯酸甲酯层设置于银环氧树脂电极和透明氧化铟锡电极之间,且柔性聚甲基丙烯酸甲酯层内部真空,p型GaN纳米线阵列设置于柔性聚甲基丙烯酸甲酯层内,p型GaN纳米线阵列外部设置Cs/O激活层,透明氧化铟锡电极和p型GaN纳米线阵列顶部接触,形成肖特基结,透明氧化铟锡电极和银环氧树脂电极之间的偏置电压引入外部场,控制偏置电压的大小和方向改变场强。(The invention provides an externally-applied electric field assisted GaN nanowire array photocathode, which comprises a silver epoxy resin electrode, a flexible polymethyl methacrylate layer, a highly-aligned p-type GaN nanowire array and a transparent indium tin oxide electrode, wherein the silver epoxy resin electrode is arranged on the flexible polymethyl methacrylate layer; the flexible polymethyl methacrylate layer is arranged between the silver epoxy resin electrode and the transparent indium tin oxide electrode, the interior of the flexible polymethyl methacrylate layer is vacuum, the p-type GaN nanowire array is arranged in the flexible polymethyl methacrylate layer, a Cs/O (Cs/O) activation layer is arranged outside the p-type GaN nanowire array, the transparent indium tin oxide electrode is contacted with the top of the p-type GaN nanowire array to form a Schottky junction, an external field is introduced by bias voltage between the transparent indium tin oxide electrode and the silver epoxy resin electrode, and the field intensity is changed by controlling the size and the direction of the bias voltage.)

1. An external electric field assisted GaN nanowire array photocathode is characterized by comprising a silver epoxy resin electrode (1), a flexible polymethyl methacrylate layer (2), a highly aligned p-type GaN nanowire array (3) and a transparent indium tin oxide electrode (4); wherein

The flexible polymethyl methacrylate layer (2) is arranged between the silver epoxy resin electrode (1) and the transparent indium tin oxide electrode (4), the interior of the flexible polymethyl methacrylate layer (2) is vacuum,

the p-type GaN nanowire array (3) is arranged in the flexible polymethyl methacrylate layer (2),

a Cs/O active layer (5) is arranged outside the p-type GaN nanowire array (3),

the transparent indium tin oxide electrode (4) is contacted with the top of the p-type GaN nanowire array (3) to form a Schottky junction,

external fields are introduced by bias voltage between the transparent indium tin oxide electrode (4) and the silver epoxy resin electrode (1), and the field intensity is changed by controlling the magnitude and direction of the bias voltage.

2. The photocathode of claim 1, wherein the effective area of the fabricated additional photocathode for an applied electric field is 1 x 1mm²。

3. The photocathode of claim 1, wherein the prepared p-type GaN nanowires are vertically aligned, have a length of 4-5 μm, a diameter of about 100nm, and a pitch of 10-100 nm.

4. The photocathode of claim 1, wherein the transparent indium tin oxide electrode (4) deposited on top of the p-type GaN nanowire array (3) is 80nm thick.

5. The photocathode of claim 1, wherein the p-type GaN nanowire array (3) has a doping element of Mg with a doping concentration in the range of 10¹⁶～10¹⁹cm^-3。

6. A method of making an applied electric field assisted GaN nanowire array photocathode of any preceding claim, comprising the steps of:

step 1, using a catalyst-induced CVD method on SiO in large areas₂Growing p-type GaN nanowire on Si substrate, wherein SiO₂Coating on Si;

step 2, chemically cleaning the obtained p-type GaN nanowire array (3), then thermally cleaning in an ultrahigh vacuum environment, and then activating by Cs/O (hydrogen sulfide/oxygen) to form a negative electron affinity state surface;

step 3, in an ultrahigh vacuum environment, spin-coating a flexible polymethyl methacrylate layer (2) to completely wrap the nanowires;

step 4, in an ultrahigh vacuum environment, immersing the vertically arranged p-type GaN nanowire array (3) into a diluted hydrofluoric acid solution, and immersing SiO₂Removing the/Si substrate;

step 5, treating the flexible polymethyl methacrylate layer (2) by oxygen plasma in an ultrahigh vacuum environment to expose the top surface and the bottom surface of the nanowire;

step 6, adhering the silver epoxy resin electrode (1) to the p-type GaN nanowire array (3) in an ultrahigh vacuum environment;

and 7, depositing a transparent indium tin oxide electrode (4) with the thickness of 80nm on the top of the nanowire in an ultrahigh vacuum environment to serve as a top transparent electrode.

7. The photocathode of claim 6, wherein SiO₂The thickness of the/Si substrate is about 2-3 μm, SiO₂The thickness is 800 nm.

8. The photocathode of claim 6, wherein the flexible polymethylmethacrylate layer (2) formed in step 3 is wrapped around the entire p-type GaN nanowire array (3) with a vacuum environment inside.

9. The photocathode of claim 6, wherein after the p-type GaN nanowire array (3) after the chemical cleaning in step 2 is sent into an ultrahigh vacuum environment for Cs/O activation, all the applied electric field assisted photocathode packaging processes are performed in an ultrahigh vacuum chamber.

Technical Field

The invention relates to a discharge tube technology, in particular to an external electric field assisted GaN nanowire array photocathode and a preparation method thereof.

Background

The GaN photocathode is widely applied to astronomical observation, aerospace, missile early warning and corona detection. When the size of the material reaches the nanometer level, different photoelectric characteristics are caused due to quantum size effect and surface effect, and the GaN nanowire array (NWAs) is the research subject of application in the fields of photoelectric detection, photocatalysis, photovoltaics, lasers and sensors at present. Vacuum devices based on GaN NWAs Negative Electron Affinity (NEA) photocathodes have a high priority in terms of sensitivity and signal-to-noise ratio compared to solid state devices, such as photovoltaic devices like uv detectors, and therefore more attention must be paid to vacuum devices based on GaN NWAs.

As a main structural unit of the vacuum device, the performance of the GaN NWAs photocathode determines the quality of the vacuum device. The research on the photoelectric emission characteristics of the GaN NWAs photocathode shows that the theoretical quantum efficiency of the GaN NWAs photocathode is obviously higher than that of a GaN plane photocathode. Although NWAs has satisfactory photon absorption capacity, the experimental quantum efficiency of GaAs NWAs photocathodes is still in a low stage [ gao guan. Donghua Rich worker university, 2016 ]. The difference between the experimental results and the theoretical predictions is due to the shielding effect between the nanowires. Electrons emitted from the surface of the nanowire will be partially obstructed by the neighboring nanowire, resulting in a reduction in photocurrent. To remedy this drawback, two possible solutions are proposed: (a) introducing a built-in electric field by axial exponential doping; (b) an external electric field is introduced (bias applied) to enhance the emission and collection of electrons. The method mainly researches the structural design and the preparation method of the additional electric field auxiliary GaNNWAs photocathode in the scheme (b).

Disclosure of Invention

The invention aims to provide an external electric field assisted GaN nanowire array photocathode and a preparation method thereof.

The technical scheme for realizing the photocathode comprises the following steps: an external electric field assisted GaN nanowire array photocathode comprises a silver epoxy resin electrode, a flexible polymethyl methacrylate layer, a highly aligned p-type GaN nanowire array and a transparent indium tin oxide electrode; the flexible polymethyl methacrylate layer is arranged between the silver epoxy resin electrode and the transparent indium tin oxide electrode, the interior of the flexible polymethyl methacrylate layer is vacuum, the p-type GaN nanowire array is arranged in the flexible polymethyl methacrylate layer, a Cs/O (Cs/O) activation layer is arranged outside the p-type GaN nanowire array, the transparent indium tin oxide electrode is contacted with the top of the p-type GaN nanowire array to form a Schottky junction, an external field is introduced by bias voltage between the transparent indium tin oxide electrode and the silver epoxy resin electrode, and the field intensity is changed by controlling the size and the direction of the bias voltage.

Furthermore, the effective area of the manufactured auxiliary photocathode of the external electric field is 1 multiplied by 1mm²。

Furthermore, the prepared p-type GaN nanowires are vertically arranged, the length is 4-5 μm, the diameter is about 100nm, and the distance is 10-100 nm.

Further, the transparent indium tin oxide electrode deposited on the top of the p-type GaN nanowire array is 80nm thick.

Furthermore, the doping element of the p-type GaN nanowire array is Mg, and the doping concentration range is 10¹⁶～10¹⁹cm^-3。

The technical scheme for realizing the method of the invention is as follows: a preparation method of an additional electric field assisted GaN nanowire array photocathode comprises the following steps:

step 1, using a catalyst-induced CVD method on SiO in large areas₂Growing p-type GaN nanowire on Si substrate, wherein SiO₂Coating on Si;

step 2, chemically cleaning the obtained p-type GaN nanowire array, then thermally cleaning in an ultrahigh vacuum environment, and then activating by Cs/O (carbon monoxide/oxygen) to form a negative electron affinity state surface;

step 3, in an ultrahigh vacuum environment, spin-coating a flexible polymethyl methacrylate layer to completely wrap the nanowires;

step 4, in an ultrahigh vacuum environment, immersing the vertically arranged p-type GaN nanowire array into a diluted hydrofluoric acid solution, and then soaking SiO₂Removing the/Si substrate;

step 5, treating the flexible polymethyl methacrylate layer by oxygen plasma in an ultrahigh vacuum environment to expose the top surface and the bottom surface of the nanowire;

step 6, adhering the silver epoxy resin electrode to the p-type GaN nanowire array in an ultrahigh vacuum environment;

and 7, depositing a transparent indium tin oxide electrode with the thickness of 80nm on the top of the nanowire in an ultrahigh vacuum environment to serve as a top transparent electrode.

Further, SiO₂The thickness of the/Si substrate is about 2-3 μm, SiO₂The thickness is 800 nm.

Further, the flexible polymethyl methacrylate layer formed in the step 3 is wrapped around the whole p-type GaN nanowire array, and the inside of the flexible polymethyl methacrylate layer is in a vacuum environment.

Further, after the p-type GaN nanowire array subjected to the chemical cleaning in the step 2 is sent to an ultrahigh vacuum environment for Cs/O activation, all the additional electric fields assist the packaging process of the photocathode to be carried out in the ultrahigh vacuum cavity.

Compared with the prior art, the invention has the following advantages: (1) the invention adopts the NWAs structure to ensure that electrons are easy to diffuse to the surfaces of the surrounding Cs/O layers, thereby reducing the surface potential barrier; (2) the light trapping structure formed by the p-type GaN nanowire array 3 enables incident light to be reflected back and forth in NWAs and finally absorbed completely; (3) an external electric field formed by bias voltage between the transparent indium tin oxide electrode and the silver epoxy resin electrode can change the motion trail of electrons escaping from the side wall of the nanowire, and the external electric field power pulls the escaping electrons to approach the transparent indium tin oxide electrode collecting side tightly connected with the top of the nanowire, so that the electron collecting probability of the GaN NWAs photocathode is improved.

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

Drawings

FIG. 1 is a structural diagram of the package of the GaN NWAs photocathode assisted by an applied electric field.

FIG. 2 is a schematic diagram of the trajectory deflection of photoelectrons escaping from the side wall of the nanowire in the GaN NWAs photocathode under the action of an external electric field assisted by the external electric field.

FIG. 3 is a schematic diagram of the fabrication process of the GaN NWAs photocathode assisted by an applied electric field.

FIG. 4 is a schematic diagram of a process for growing GaN NWAs by the catalyst-assisted CVD method of the present invention.

FIG. 5 is a schematic diagram showing the effect of external electric field intensity on the quantum efficiency and collection efficiency of a photocathode according to the present invention, wherein (a) the effect of quantum efficiency is shown, and (b) the effect of collection efficiency is shown.

Detailed Description

With reference to fig. 1, an applied electric field assisted GaN NWAs photocathode includes: a silver epoxy electrode 1, a flexible Polymethylmethacrylate (PMMA) layer 2, a highly aligned p-type GaN NWAs3 and a transparent Indium Tin Oxide (ITO) electrode 4. The silver epoxy resin electrode 1 is positioned at the bottom of the GaN NWAs3, the ITO transmission layer 4 is tightly connected with the top of the GaN NWAs to form a Schottky junction, a bias voltage is input between the ITO electrode 4 and the silver epoxy resin electrode 1, and an external electric field regulated and controlled by the bias voltage is formed inside the GaN NWAs; the PMMA layer 2 is formed outside the GaN NWAs, and a vacuum cavity is reserved inside the PMMA layer; the GaN nanowire is externally provided with a NEA surface by a Cs/O active layer 5.

Furthermore, the effective area of the manufactured auxiliary photocathode of the external electric field is 1 multiplied by 1mm²。

Furthermore, the prepared GaN nanowires are vertically arranged, the length is 4-5 μm, the diameter is about 100nm, and the distance is 10-100 nm.

Further, the ITO electrode 4 deposited on top of the NWAs was 80nm thick.

Furthermore, in the p-type GaN NWAs, the doping element is Mg, and the doping concentration range is 10¹⁶～10¹⁹cm^-3。

With reference to fig. 3, the method for preparing an additional electric field assisted GaN NWAs photocathode of the present invention comprises the following steps:

step 1, using thin Ni film 6 as catalyst, in SiO using horizontal CVD reactor₂Growing highly aligned p-type GaN NWAs3 on the substrate 8 and the Si substrate 7, wherein the obtained GaN NWAs are vertically arranged;

step 2, chemically and thermally cleaning the GaN NWAs prepared in the step 1 to obtain an atomic-level clean surface;

step 3, useUltra-high vacuum (10)^-8Torr) the activation process adsorbs the Cs/O activation layer 5 on the surface of the p-type GaN NWAs, the thickness is a single atomic layer, and the aim is to make the GaN NWAs have an NEA surface;

step 4, in an ultrahigh vacuum environment, spin-coating a PMMA layer 2 around the activated GaN NWAs, wherein a vacuum cavity is arranged inside the PMMA layer 2, and the thickness of the PMMA layer is 20 nm-50 nm, so that good insulation of the top electrode and the bottom electrode is ensured;

step 5, in the ultra-high vacuum environment, the vertically arranged GaN NWAs are immersed in diluted hydrofluoric acid (HF: H)₂O1: 10) solution, SiO was removed₂a/Si substrate so that GaN NWAs can be easily bonded to silver epoxy 1;

step 6, processing the PMMA layer 2 by using an oxygen plasma technology in an ultrahigh vacuum environment to expose the top and the bottom of the nanowire so as to facilitate the subsequent tight adhesion of the GaN NWAs and the electrode;

step 7, bonding silver epoxy resin 1 at the bottom of GaN NWAs3 in an ultrahigh vacuum environment, wherein the silver epoxy resin (1) is also used as a bottom electrode;

and 8, depositing the transparent ITO electrode 4 on the top of the GaN NWAs by using a sputtering method in an ultrahigh vacuum environment, wherein the ITO electrode 4 and the top of the nanowire form a Schottky junction, and the ITO electrode 4 is used as an electron collecting side and has the thickness of 80 nm.

And 9, applying bias voltage between the silver epoxy resin electrode 1 and the top ITO electrode 4 to form an external electric field between the GaN NWAs, and finally preparing the GaN NWAs photocathode assisted by the external electric field.

Further, referring to fig. 4, the specific process of growing GaN NWAs by using the catalyst-induced CVD method in step 1 includes the following steps:

step 11, firstly, using a mixture of 1:1, cleaning a Si substrate 7 by sulfuric acid and hydrogen peroxide, corroding and cleaning the substrate by hydrofluoric acid with the mass percentage of 5%, and then thermally growing amorphous SiO on the (001) surface of the Si substrate₂Layer 8, layer thickness 800nm, complete SiO₂The thickness of the/Si substrate is about 2-3 μm;

step 12, using acetone, ethanol and deionization before GaN NWAs growthUltrasonic cleaning of SiO with water₂a/Si substrate for 10 minutes, using a thin Ni film 6 as a growth catalyst for NWAs, and deposited on SiO by electron beam evaporation₂The thickness of the Ni/Si substrate is 5-15 nm, and then the Ni/Si substrate is placed into a 5% NaOH solution to be soaked for 3 minutes, so that a Ni nanoparticle 9 lattice is obtained on the substrate;

step 13, growing p-type GaN NWAs in a 60mm horizontal CVD reactor, vacuumizing, introducing argon to clean a reaction cavity, and adding GaCl₃(99.999％)，MgCl₃(99.999%) and NH₃(99.999%) as gallium source, doped magnesium source and nitrogen source, GaCl₃And MgCl₃The mol ratio is 1000: 1 in a CVD reaction chamber;

and step 13, the substrate is positioned at the central heating position of the reaction cavity, ammonia gas and argon gas are introduced, the flow rates are respectively 30 and 200ml/min, the growth temperature during the deposition reaction is controlled to be 950 ℃, and the Ni nanowire particles 9 on the top of the GaN NWAs are removed by using a mixed acid solution of 5% hydrochloric acid, phosphoric acid and hydrofluoric acid, so that the clean p-type GaN NWAs3 is prepared.

Further, the method for preparing the p-type GaN NWAs obtained by the growth into the NEAGaN NWAs photocathode comprises the following steps:

step 21: chemically cleaning the obtained p-type GaN NWAs, and conveying the cleaned sample to a heating position of an ultrahigh vacuum activation system, wherein when the vacuum degree of the ultrahigh vacuum system is not lower than 1 × 10^-7Pa, carrying out thermal cleaning, and after the thermal cleaning is finished, starting activation when the sample is naturally cooled to about 60 ℃;

step 22, activating a nickel tube type Cs source and an O source which are hollow flat oval nickel-chromium alloy tubes with holes, wherein all the holes are distributed in the same plane direction of the oval, active ingredients in the Cs source nickel tube are cesium chromate and zirconium-aluminum alloy powder, active ingredients in the O source nickel tube are barium peroxide, when the nickel tube is heated, Cs and O enter an ultrahigh vacuum activation system through small holes, and the flow depends on the heating current of the alloy tubes;

and step 23, activating the GaN NWAs cathode by adopting a traditional yo-yo process, keeping the O source current unchanged when cesium and oxygen alternate, and changing the Cs flow by adjusting the Cs source current so as to change the Cs/O ratio, thereby finally preparing the NEAGaN NWAs photocathode.

With reference to fig. 5(a), the abscissa is the intensity of the external electric field, the ordinate is the emergent light current, the carrier concentration inside the nanowire can be changed by applying the external electric field, and when the external electric field is 0-0.1V/μm, the number of photoelectrons escaping from the nanowire is increased to a certain extent at any incident angle. Referring to fig. 5(b), wherein the abscissa is the external electric field intensity and the ordinate is the collected photocurrent, the trajectory of the laterally emitted photoelectrons can be changed by applying the external electric field to deflect the photoelectrons toward the top transparent electrode, thereby increasing the collection probability, and the result shows that the highest collection probability is obtained at an electric field intensity of 0.5V/μm and an incident angle of 70 °.

The improvement in performance of vacuum devices depends to a large extent on the performance of the photocathode. The development of the growth process, the structural design, the preparation process and the vacuum technology of the photocathode correspondingly improves the performance of the photocathode. The additional electric field assisted GaN NWAs photocathode provided by the invention improves the traditional photocathode in structural design, adopts the structure of the NWAs to ensure that electrons are easy to diffuse to the surfaces of the surrounding Cs/O layers, and reduces the surface potential barrier; meanwhile, the incident light is reflected back and forth in the NWAs and is finally and completely absorbed by the light trapping structure formed by the p-type GaN nanowire array 3; when light beams carrying certain photon energy penetrate through the transparent ITO electrode to enter the NWAs, photons with photon energy larger than a band gap are converted into photoelectrons by the GaN material through an external photoelectric effect and are emitted from the side wall and the top surface of the nanowire, an external electric field applied between the ITO electrode and the silver epoxy resin electrode can also well overcome the shielding effect brought by the GaN NWAs photocathode, the movement time of the photoelectrons emitted at different angles and energy in an array gap can be reduced, the movement track can be changed by the action of the electric field force and the photoelectrons are pulled to the charge collection electrode again (the arrow direction in figure 2 is the electron movement direction), and therefore the actual efficiency of the GaN NWAs photocathode is improved as much as possible.