CN107818900A - A kind of NEA GaAs nano-cone arrays photocathode and preparation method - Google Patents

Abstract

The invention discloses a NEA‑GaAs nanocone array photocathode and a preparation method thereof. The GaAs nanocone array photocathode includes a substrate layer and a nanocone array emission layer located on the surface of the substrate layer, and the substrate is Si or SiC, etc. The insulating film and the nanocone array emission layer are composed of several p-type GaAs nanocones, and the Cs/O activation is performed on the grown nanocone arrays. The effective refractive index of the nanocone array proposed by the present invention is gradual, which effectively reduces the light reflection caused by the discontinuity of the refractive index of the air/GaAs battery interface, and reduces the incident light while improving the quantum efficiency of the GaAs photocathode. Effect of angle on absorption rate.

Description

A kind of NEA-GaAs nanocone array photocathode and its preparation method

technical field

The invention belongs to the field of optoelectronics, and in particular relates to a NEA-GaAs nanocone array photocathode and a preparation method.

Background technique

A photocathode is a photoemissive material that converts light signals into electrical signals by using the external photoelectric effect. As a direct bandgap semiconductor material, GaAs has a good match with the solar spectrum and is an ideal photocathode material. Negative electron affinity, that is, the vacuum energy level on the surface of the cathode is lower than the bottom energy level of the conduction band, which greatly increases the escape probability of photoexcited electrons, so it has unique advantages such as high quantum efficiency, small dark current, and concentrated energy distribution of emitted electrons.

The general GaAs photocathode is made of thin-film materials, which have the advantages of mature growth process and good film-forming quality, but the emissivity of thin-film materials is large and cannot fully absorb the energy of incident light. Whether it is light reflection, absorption or solar spectral response, for thin film solar cells, the design of the light trapping structure is very important, especially in the development trend of the cathode thin film, the light trapping structure is even more important, in addition For the photocathode, the electron transport distance of the material requires the thickness of the material and the light absorption depth of the material requires the thickness of the material to be contradictory.

A nanocone is a light-trapping structure very similar to a nanowire. The cone-shaped structure is a biomimetic structure inspired by the periodic array of protrusions in the moth eye cornea. The effective refractive index of the nanocone structure is graded, which can be regarded as a homogeneous medium with a graded refractive index from the air to the cathode, which effectively reduces the light reflection caused by the discontinuity of the refractive index of the air/GaAs cathode interface. Compared with nanocones and nanowires of the same size, the short-circuit current is 10% higher, and as the incident angle of incident light changes from 0° to 90°, the overall light absorption rate of the nanocone structure has almost no change, so compared with Compared with nanowires, nanocones have better light trapping properties.

At present, there are mainly methods for growing nanocone arrays, such as direct growth method, electrochemical corrosion method, and template method. Among them, the direct growth method uses microwave plasma technology to directly grow nanocone structures. This nanocone array has disadvantages such as poor consistency (orientation, cone angle and distribution), small long-diameter ratio, and large radius of curvature at the top; The aspect ratio of the cone prepared by the corrosion method is too small, the radius of curvature of the top is too large, the preparation process is complicated, the emission stability is poor, the sensitivity is low, and the application prospect is not great; Pyramid stencil, then filled with emissive layer material, and finally etched away the stencil method. The aspect ratio of the vertebral body and the radius of curvature of the tip of this structure are not ideal, and the preparation process is complicated. The existing technology for making nanocones has defects such as complex fabrication, difficulty in large-scale implementation, narrow application range, and poor consistency and controllability. Therefore, we need to find a nanocone growth method suitable for the preparation of GaAs nanocone array photocathode.

Contents of the invention

The purpose of the present invention is to provide a NEA-GaAs nanocone array photocathode and a preparation method.

The technical solution for realizing the object of the present invention is: a NEA-GaAs nanocone array photocathode, including a substrate layer and a nanocone array emission layer located on the surface of the substrate layer;

The emission layer of the nanocone array is composed of several p-type GaAs nanocones, and Cs/O active layers are adsorbed on the surface of the p-type GaAs nanocones.

A method for preparing the above-mentioned NEA-GaAs nanocone array photocathode, the steps are as follows:

Step 1, cleaning the substrate, cleaning the substrate material sequentially with acetone, ethanol and deionized water;

Step 2, preparing Ga droplets on the surface of the cleaned substrate at a preparation temperature of 600°C-700°C, and autocatalytically growing nanocolumns at a temperature of 400°C-600°C; then lowering the temperature to 400°C, Solidify the Ga droplets and grow to form a GaAs film;

Step 3, pre-etch the surface of the sample by injecting hydrogen plasma; the etching conditions are as follows: inject hydrogen gas, the gas pressure is 8 Torr, the sample heating temperature is 800°C-900°C, the bias voltage is 400V, the bias current is 100mA, and the time 25min;

Step 4, turn off the bias voltage in the CVD equipment, turn off the filament current, and re-evacuate to 10 ^-2 Torr;

Step 5, perform plasma etching to form a surface nanocone array, pass a mixed gas of methane and hydrogen during etching, the volume ratio of the mixed gas is 5:100; when using a mixed gas of hydrogen and argon for plasma etching , the mixing volume ratio is 30:50. The sample heating temperature is 800°C-950°C, the pressure is 20Torr, the bias voltage is 400V, the bias current is 120mA, and the time is 2 hours;

Step 6, using a chemical cleaning agent to etch away grease and impurities on the surface of the p-type GaAs nanocones, and using an ultra-vacuum activation process to adsorb a Cs/O activation layer on the surface of the p-type GaAs nanocones array.

Compared with the prior art, the present invention has significant advantages in that:

(1) The GaAs nanocone array of the present invention is made into a photocathode, which can fully absorb the energy of incident light through the emission and refraction between the nanocones, and overcomes the contradictory relationship between the photon absorption depth and the electron diffusion length;

(2) Compared with the nanowire array, due to the unique structure of the nanocone, no matter what angle the light is incident on, the incoming photons will be reflected and refracted between the nanocones, and finally completely absorbed by the nanocone array, forming a "photon The "capture" effect minimizes the loss of energy, thereby greatly improving the quantum efficiency of the photocathode;

(3) The surface nanocone array proposed by the present invention has a high aspect ratio, a small tip curvature radius, and has controllable aspect ratio, cone angle and array density;

(4) The preparation method of the GaAs nanocone array photocathode provided by the present invention uses an ultra-vacuum activation process to adsorb a Cs/O active layer on the surface of the nanocone, which greatly increases the probability of photoelectron escape and improves the quantum efficiency of the photocathode , and the manufacturing process of the nanocone array is simple and operable, which reduces the production cost of the photocathode;

(5) When photons are absorbed inside the nanocone, photoelectrons are excited. Since the nanocone is surrounded by surfaces, photoelectrons escape from the interior of the nanocone to the surroundings, which greatly increases the number of escaped electrons, thereby increasing The photocurrent is increased, and the quantum efficiency of the photocathode is improved.

Description of drawings

Fig. 1 is a schematic diagram of a GaAs nanocone array photocathode according to the present invention.

Fig. 2 (a), Fig. 2 (b), Fig. 2 (c) are the manufacturing process flow of the nanocone array of the present invention, wherein Fig. 2 (a) is the schematic diagram that the substrate is placed on the stainless steel round support, and Fig. 2 ( b) is a schematic diagram of placing a Pt fine metal grid on a substrate, and Fig. 2(c) is a schematic diagram of plasma etching GaAs.

In the figure, 1-incident light; 2-GaAs nanocone array; 3-substrate; 4-GaAs layer; 5-stainless steel tray; 6-Pt fine metal grid; 7-plasma region.

Detailed ways

In conjunction with FIG. 1, a NEA-GaAs nanocone array photocathode includes a substrate layer 3 and a nanocone array emission layer 2 located on the surface of the substrate layer;

The emission layer 2 of the nanocone array is composed of several p-type GaAs nanocones, and the surfaces of the p-type GaAs nanocones are all adsorbed with Cs/O active layers.

The substrate layer is a Si or SiC insulating film, and the p-type GaAs nanocone adopts plasma etching technology to etch an array structure of nanocones on the GaAs emission layer on the surface of the substrate layer.

The aspect ratio of the nanocone is 50-5000, the radius of curvature of the tip is below 5nm, the diameter of the bottom is 200nm-2000nm, the cone angle is 20°-70°, and the density is 10 ⁹ cm ^-2 -10 ⁵ cm ^-2 . The p-type doping concentration is 1×10 ¹⁹ cm ^-3 , and the doping element is Zn.

The present invention also provides a method for preparing the NEA-GaAs nanocone array photocathode, the steps are as follows:

Step 1, cleaning the substrate, cleaning the substrate material sequentially with acetone, ethanol and deionized water;

Step 2, preparing Ga droplets on the surface of the cleaned substrate at a preparation temperature of 600°C-700°C, and autocatalytically growing nanocolumns at a temperature of 400°C-600°C; then lowering the temperature to 400°C, Solidify the Ga droplets and grow to form a GaAs film;

Step 3, pre-etch the surface of the sample by injecting hydrogen plasma; the etching conditions are as follows: inject hydrogen gas, the gas pressure is 8 Torr, the sample heating temperature is 800°C-900°C, the bias voltage is 400V, the bias current is 100mA, and the time 25min;

Step 4, turn off the bias voltage in the CVD equipment, turn off the filament current, and re-evacuate to 10 ^-2 Torr;

Step 5, perform plasma etching to form a surface nanocone array, pass a mixed gas of methane and hydrogen during etching, the volume ratio of the mixed gas is 5:100; when using a mixed gas of hydrogen and argon for plasma etching , the mixing volume ratio is 30:50. The sample heating temperature is 800°C-950°C, the pressure is 20Torr, the bias voltage is 400V, the bias current is 120mA, and the time is 2 hours;

Step 6, use a chemical cleaning reagent to corrode the grease and impurities on the surface of the p-type GaAs nano cone. The chemical cleaning reagent is a mixture of sulfuric acid, hydrogen peroxide and deionized water with a volume ratio of 2:2:1; send it into a high-temperature vacuum system for Heating and purification, the heating temperature is 850°C, so that the p-type GaAs nanocone emission layer obtains an atomically clean surface; the ultra-vacuum activation process is used to adsorb the Cs/O activation layer on the surface of the p-type GaAs nanocone array.

In step 2, Ga(CH ₃ ) ₃ is the gallium source, AsH ₃ is the As source, and ZnCl ₂ is the doping element Zn source, mixed in a molar ratio of 1000:1000:1 as the reaction source, and the reaction source is located at Close to the air inlet of the CVD reaction furnace and 15cm away from the substrate, the substrate is located at the central heating position of the CVD reaction furnace; before heating, the CVD reaction furnace is vacuumed and the furnace tube is cleaned with argon gas; the heating temperature at the position of the substrate is 850 °C, the position where the reaction source is located is heated to 600 °C, and the deposition reaction time is 100-120 min.

When performing surface hydrogen ion etching, the Pt metal wire grid is placed on the surface of the GaAs layer, and the method of split discharge is used to enhance the etching effect on thin film materials with poor conductivity and large area. The diameter of the metal grid is is 0.2mm.

The present invention will be described in detail below in conjunction with the embodiments and the accompanying drawings.

Example

Combined with Figure 2(a), Figure 2(b), and Figure 2(c), the method of growing a GaAs film on the surface of a Si substrate:

First, the Si substrate needs to be cleaned. Select the Si(111) substrate, and use acetone to ultrasonically clean it twice, each time for 4-6 minutes, and then use ethanol to ultrasonically clean it twice, each time for 4-6 minutes. Rinse twice with deionized water, blow dry with nitrogen and place in a vacuum chamber.

Ga droplets were prepared on the cleaned Si(111) substrate at a preparation temperature of 630°C, a Ga beam current of 2×10 ^-7 Torr, and a growth time of 30 min. 1μm nanopillars were autocatalytically grown on Si(111) substrates, the autocatalytic temperature was 550°C, the Ga beam current was 1.5×10 ^-7 Torr, the As beam current was 2×10 ^-6 Torr, and the growth time was 15 min. The temperature was lowered to 400° C. to solidify the Ga droplet, the Ga beam current was 1.5×10 ^-7 Torr, the As beam current was 2×10 ^-6 Torr, and the growth time was 90 min to form the GaAs thin film 4 .

Preparation method of GaAs nanocone:

Put the grown sample on the stainless steel round support 5, put it into the CVD equipment, and inject hydrogen plasma to pre-etch the surface of the sample. The etching conditions are as follows: high-purity hydrogen gas is introduced, the gas pressure is 8Torr, the sample heating temperature is 850°C, the bias voltage is 400V, the bias current is 100mA, and the time is 25min; then the bias voltage in the CVD equipment is turned off, and the filament current is turned off. , re-evacuated to 10 ^-2 Torr; the mixed gas of methane and hydrogen was introduced, and the volume ratio of the mixed gas was 5:100; when the mixed gas of hydrogen and argon was used for plasma etching, the mixed volume ratio was 30:50 . The sample heating temperature is 900°C, the pressure is 20Torr, the bias voltage is 400V, the bias current is 120mA, and the time is 2 hours;

When performing surface hydrogen ion etching 7, the Pt metal wire grid 6 is placed on the surface of the sample, and the method of split discharge is used to enhance the etching effect on thin film materials with poor conductivity and large area. The diameter is 0.2 mm.

Finally, the grease and impurities on the surface of the nanocone are removed by chemical corrosion. The volume ratio of the reagents for chemical cleaning is sulfuric acid, hydrogen peroxide and deionized water in a volume ratio of 2:2:1. Then send it into a high-temperature vacuum system for heating and purification. The heating temperature is 850°C, so that the p-type GaAs nanocone emission layer can obtain an atomically clean surface; and then use the ultra-vacuum activation process to make the surface of the p-type GaAs nanocone emission layer adsorb Cs/O The activation process of the activation layer, Cs and O is that the Cs is continuous, the O source is intermittent, and the surface of the emission layer reaches the negative electron affinity. Finally, the GaAs nanocone array photocathode as shown in Figure 1 is fabricated.

Claims (9)

1. a kind of NEA-GaAs nano-cone arrays photocathode, it is characterised in that including substrate layer and positioned at substrate layer surface Nano-cone array emission layer；

The nano-cone array emission layer is made up of some p-type GaAs nanocones, and p-type GaAs nanometer poppet surfaces are adsorbed with Cs/O Active coating.

2. NEA-GaAs nano-cone arrays photocathode according to claim 1, it is characterised in that the substrate layer is Si Or SiC insulation films.

3. NEA-GaAs nano-cone arrays photocathode according to claim 1 or 2, it is characterised in that the p-type GaAs Nanocone using plasma lithographic technique, the array structure of nanocone is etched on the GaAs emission layers of substrate layer surface.

4. NEA-GaAs nano-cone arrays photocathode according to claim 1, it is characterised in that the draw ratio of nanocone For 50-5000, sophisticated radius of curvature is 20 ° -70 ° in below 5nm, base diameter 200nm-2000nm, cone angle, and density is 10⁹cm^-2-10⁵cm^-2。

5. the GaAs nano-cone array photocathodes according to claim 1 or 4, it is characterised in that p-type doping concentration is 1 ×10¹⁹cm^-3, doped chemical Zn.

A kind of 6. method for preparing NEA-GaAs nano-cone arrays photocathode described in claim 1, it is characterised in that step is such as Under：

Step 1, substrate is cleaned, and backing material is sequentially cleaned using acetone, ethanol and deionized water；

Step 2, Ga drops are prepared on substrate surface after cleaning, preparation temperature is 600 DEG C -700 DEG C, and self-catalysis growth is received Meter Zhu, self-catalysis temperature are 400 DEG C -600 DEG C；Temperature is reduced to 400 DEG C afterwards, solidifies Ga drops and grows to form GaAs Film；

Step 3, it is passed through hydrogen plasma and pre-etching is carried out to sample surfaces；Etching condition is as follows：Hydrogen is passed through, gas pressure is 8Torr, sample heating temperature are 800 DEG C -900 DEG C, are biased as 400V, bias current 100mA, time 25min；

Step 4, bias in CVD equipment is turned off, heater current is turned off, and vacuumizes 10 again^-2Torr；

Step 5, carry out plasma etching and form surface nano tip array, the mixed gas of methane and hydrogen is passed through during etching, The volume ratio of mixed gas is 5:100；When carrying out plasma etching using the mixed gas of hydrogen and argon gas, mixed volume ratio For 30:50.Sample heating temperature biases 400V, bias current 120mA at 800 DEG C -950 DEG C, pressure 20Torr, and the time is 2 small When；

Step 6, with the grease and impurity of Chemical cleaning reagent etching away p-type GaAs nanometer poppet surfaces, activated using ultravacuum Technique is in p-type GaAs nano-cone array adsorption Cs/O active coatings.

7. the preparation method of NEA-GaAs nano-cone arrays photocathode according to claim 6, it is characterised in that step 2 In, Ga (CH₃)₃For gallium source, AsH₃As As sources, ZnCl₂For doped chemical Zn sources, with mole ratio 1000:1000：1 ratio Reaction source is used as after mixing, reaction source is located at close to CVD reacting furnaces air inlet and is located at apart from substrate 15cm position, substrate The center heating location of CVD reacting furnaces；CVD reacting furnaces are first vacuumized before heating and are passed through argon purge boiler tube；Substrate institute is in place Heating-up temperature is put to 850 DEG C, for reaction source position heating-up temperature to 600 DEG C, the deposition reaction time is 100~120min.

8. the preparation method of NEA-GaAs nano-cone arrays photocathode according to claim 6, it is characterised in that carrying out During Surface Hydrogen ion etching, Pt metallic filaments grid mesh is placed on to the surface of GaAs layers, a diameter of 0.2 milli of the metal grid mesh Rice.

9. the preparation method of NEA-GaAs nano-cone arrays photocathode according to claim 6, it is characterised in that chemistry is clear It is that volume ratio is 2 to wash reagent:2:1 sulfuric acid, hydrogen peroxide and deionized water mixed liquor.