Cs-NF3 activation method for improving quantum efficiency of gallium arsenide photoelectric cathode

Abstract

The invention discloses a Cs-NF3 activation method for improving the quantum efficiency of a gallium arsenide photoelectric cathode. The method comprises the following steps: step 1, carrying out single cesium activation treatment; step 2, carrying out Cs-NF3 co-deposition activation treatment; step 3, carrying out Cs-NF3 excessive activation treatment; in the activation process, the whole cathode surface is vertically irradiated by white light of a halogen tungsten lamp; the activation process needs to be completed in an ultrahigh vacuum environment. The activation process is simple and convenient, only a switch Cs source single variable needs to be controlled, the vacuum degree in the activation process is higher, and the gallium arsenide photoelectric cathode which is higher in quantum efficiency and good in stability as traditional Cs-NF3 activation can be obtained.

Description

Technical Field

[0001] This invention belongs to the field of gallium arsenide photocathode activation technology, and in particular relates to a Cs-NF3 activation method for improving the quantum efficiency of gallium arsenide photocathodes. Background Technology

[0002] Gallium arsenide (GaAs) photocathodes are a crucial component of modern low-light night vision devices, converting weak light signals into electrical signals. They have wide applications in low-light image intensifiers, transmission electron microscopes, and novel solar cells; they also play a key role in high-energy physics and electron beam lithography. Current photocathode applications demand the fabrication of photocathodes with the highest possible quantum efficiency and stability. However, the decay of cathode quantum efficiency over time remains a significant technical challenge for practical application. Therefore, fabricating highly stable and quantum-efficient GaAs photocathodes is of paramount importance. The activation method under ultra-high vacuum is a critical step in determining the performance of NEA GaAs photocathodes, largely influencing their overall quality. The timing and sequence of source activation, as well as the current ratio of different sources, are important factors affecting the final quantum efficiency and stability of the photocathode.

[0003] Currently available Cs-NF3 activated transmission-type GaAs photocathodes exhibit strong immunity to environmental gases (such as O2, CO, CO2, etc.) and a low rate of decay in cathode quantum efficiency. A common activation method is Cs-NF3 co-deposition activation (Chanlek N, Herbert DJ, Jones MR, et al. High stability of negative electron affinity gallium arsenide photocathodes activated with Cs and NF3[J]. Journal of Physics D: Applied Physics, 2015, 48 (37): 375102-375102.), which consists of two steps: Step 1: Single cesium activation; Step 2: Cs-NF3 co-deposition activation. In the specific implementation steps, Step 1: The cesium source is turned on, the photocurrent gradually increases, and then decreases after reaching its peak. Step 2: When the current drops to 50%~90% of the cesium peak photocurrent, the NF3 source is turned on, and the cesium source remains on, causing the current to rise. Once the photocurrent reaches its peak and begins to decline, both the cesium and NF3 sources are immediately turned off. While this activation process can obtain a GaAs photocathode with negative electron affinity, the resulting photocathode's emission performance still has room for improvement. Furthermore, the vacuum level of the experimental chamber during the activation process is relatively poor; the ultra-high vacuum system typically used in experiments has a vacuum level of 7*10⁻⁶. -7 Below the order of Pa.

[0004] Therefore, it is necessary to find an activation method that can improve the quantum efficiency of the cathode and has a smaller impact on the vacuum level during the activation process. Summary of the Invention

[0005] The purpose of this invention is to solve the problems mentioned in the background art and to propose a Cs-NF3 activation method to improve the quantum efficiency of gallium arsenide photocathodes.

[0006] To achieve the objective of this invention, this invention provides a Cs-NF3 activation method for improving the quantum efficiency of gallium arsenide photocathodes, the method comprising:

[0007] Step 1: Perform single cesium activation processing;

[0008] Step 2: Perform Cs-NF3 co-deposition activation treatment;

[0009] Step 3: Perform Cs-NF3 overactivation treatment;

[0010] During the activation process of steps 1 to 3, the entire cathode surface is vertically illuminated with white light from a halogen tungsten lamp;

[0011] The activation process must be completed in an ultra-high vacuum environment, with a vacuum level of not less than 10. -7 On the order of Pa.

[0012] Further, step 1 includes: turning on the cesium source, the photocurrent gradually increases, and then decreases after the photocurrent reaches its peak.

[0013] Further, step 2 involves Cs-NF3 co-deposition activation treatment; including:

[0014] Step 2-1: When the photocurrent drops to 50% to 90% of its peak value, open the NF3 intake valve and keep the NF3 valve open, then close the cesium source; the photocurrent will start to rise again, and then drop again after reaching its peak value.

[0015] Step 2-2: When the photocurrent drops to 70%~90% of the previous peak value, turn on the cesium source and keep the NF3 source on. The photocurrent gradually changes from decreasing to increasing.

[0016] Steps 2-3: After keeping the cesium source on for 5 minutes, turn it off. After the photocurrent forms a new peak, the photocurrent drops sharply.

[0017] Step 2-4: Repeat steps 2-2 and 2-3 until the photocurrent reaches its peak and then begins to decrease during the cesium source activation period, at which point step 3 is executed.

[0018] Further, step 3 involves Cs-NF3 overactivation treatment; including:

[0019] Step 3-1: When the current rises to its peak value and then drops to 70%~90% of the current peak value, turn off the cesium source and keep the NF3 source on. The photocurrent will immediately increase. Wait for the current to rise to a new peak value and then start to decrease.

[0020] Step 3-2: When the photocurrent drops to 70%~90% of the previous peak value, turn on the cesium source and wait for the photocurrent to rise to a new peak value before it starts to decrease.

[0021] Step 3-3: Repeat steps 3-1 and 3-2 until the peak photocurrent no longer increases compared to the previous peak. Then, turn off the cesium source and the NF3 source in sequence to end the activation process.

[0022] Furthermore, before performing the activation step, a preparatory step is performed: chemical cleaning and high-temperature heating of the GaAs photocathode.

[0023] Furthermore, the purpose of chemical cleaning methods is to remove grease from the surface of GaAs photocathodes;

[0024] The steps include immersing the sample in carbon tetrachloride, acetone, alcohol, and deionized water in sequence for ultrasonic cleaning;

[0025] Next, chemical etching is performed using HF solution;

[0026] The etched product was then placed in a 1:1 HCl:isopropanol mixed solution for etching.

[0027] The sample was then rinsed with deionized water; finally, the cleaned GaAs photocathode was dried.

[0028] Furthermore, the high-temperature purification step involves placing the chemically cleaned sample into an ultra-high vacuum system and heating it for 60 minutes at a temperature of 550~1000℃, while maintaining a vacuum level of no less than 10. -7 On the order of Pa.

[0029] Furthermore, the cesium source is a solid source for reducing cesium chromate using zirconium aluminum alloy powder packaged in a nickel tube, and the NF3 source is a high-purity gaseous source.

[0030] The cesium source is automatically controlled by a computer, which adjusts the current of the external current source to control the amount of gas released when the cesium source is powered on. The NF3 source uses a micro-leakage valve to control the amount of gas released.

[0031] Compared with existing technologies, the significant advancements of this invention are as follows: 1. This invention employs an activation method with continuous cesium source current and alternating NF3 source current, which exhibits higher quantum efficiency compared to the traditional Cs-NF3 co-deposition activation method; 2. The activated GaAs photocathode of this invention is subjected to attenuation testing in an ultra-high vacuum environment by vertically irradiating the entire cathode surface with white light from a halogen tungsten lamp and applying a certain voltage, and the photocurrent attenuation curve is observed. This demonstrates that this invention possesses the same stability as the traditional Cs-NF3 co-activation method; 3. The activation process of this invention is simple, requiring only the control of a single variable, the switching of the Cs source, and the higher vacuum level during activation, resulting in less influence from residual gas in the vacuum environment.

[0032] To more clearly illustrate the functional characteristics and structural parameters of the present invention, further explanation is provided below in conjunction with the accompanying drawings and specific embodiments. Attached Figure Description

[0033] Figure 1 This is a flowchart of the present invention.

[0034] Figure 2 The photocurrent curve for activating the GaAs photocathode in this invention.

[0035] Figure 3 The photocurrent curves for activating GaAs photocathodes using the traditional Cs-NF3 method are shown.

[0036] Figure 4 A comparison of the quantum efficiency of GaAs photocathodes activated by the present invention and those activated by conventional methods. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] like Figure 1 As shown, an activation method for improving the quantum efficiency of GaAs photocathodes involves vertically illuminating the entire cathode surface with white light from a halogen tungsten lamp during the activation process. The specific steps are as follows:

[0039] Before formally implementing the specific steps of this application, preliminary steps are performed: chemical cleaning and high-temperature heating of the GaAs photocathode; the purpose of the chemical cleaning method is to remove grease from the surface of the GaAs photocathode; the steps include sequentially immersing the sample in carbon tetrachloride, acetone, alcohol, and deionized water for ultrasonic cleaning; next, chemical etching is performed with HF solution for 10 minutes; then, the etched product is placed in a 1:1 HCl:isopropanol mixed solution for 2 minutes; then, the sample is rinsed with deionized water for more than one minute; finally, the cleaned GaAs photocathode is dried.

[0040] The high-temperature purification step is as follows: the chemically cleaned sample is placed in an ultra-high vacuum system and heated for 60 minutes at a temperature of 550~1000℃, with the vacuum level of the ultra-high vacuum system not lower than 10. -7 On the order of Pa.

[0041] Step 1: Perform single cesium activation processing;

[0042] Step 1 specifically includes: turning on the cesium source, the photocurrent gradually increases, and then decreases after reaching its peak value;

[0043] Step 2 involves Cs-NF3 co-deposition activation treatment; the cesium source activation time is fixed during this step; Step 2 includes:

[0044] Step 2-1: When the photocurrent drops to 50% to 90% of its peak value, open the NF3 intake valve and keep the NF3 valve open, then close the cesium source; the photocurrent will start to rise again, and then drop again after reaching its peak value.

[0045] Step 2-2: When the photocurrent drops to 70%~90% of the previous peak value, turn on the cesium source and keep the NF3 source on. The photocurrent gradually changes from decreasing to increasing.

[0046] Steps 2-3: After keeping the cesium source on for 5 minutes, turn it off. After the photocurrent forms a new peak, the photocurrent drops sharply.

[0047] Step 2-4: Repeat steps 2-2 and 2-3 until the photocurrent reaches its peak and begins to decrease during the cesium source activation period, at which point step 3 is executed.

[0048] Step 3: Perform Cs-NF3 overactivation treatment; during this step, the timing of source activation is determined based on the degree of overactivity of the cesium and NF3 sources.

[0049] Step 3-1: When the photocurrent can rise to its peak value and then decrease to 70%~90% of the current peak value, turn off the cesium source and keep the NF3 source on. The photocurrent will immediately increase. Wait for the current to rise to a new peak value and then start to decrease.

[0050] Step 3-2: When the photocurrent drops to 70%~90% of the previous peak value, turn on the cesium source and wait for the photocurrent to rise to a new peak value before it starts to decrease.

[0051] Step 3-3: Repeat steps 3-1 and 3-2 until the peak photocurrent no longer increases compared to the previous peak. Then, turn off the cesium source and the NF3 source in sequence to end the activation process.

[0052] All of the above reactions are carried out in an ultra-high vacuum system, and the vacuum level of the ultra-high vacuum system is not less than 10. -7 On the order of Pa. During activation, the entire cathode surface is vertically irradiated with white light from a halogen tungsten lamp. The cesium source is a solid-state source for reducing cesium chromate using zirconium-aluminum alloy powder encapsulated in a nickel tube, and the NF3 source is a high-purity gaseous source. During the activation process, the cesium source is controlled by computer-aided control, i.e., the magnitude of the external current source is adjusted to control the amount of gas released from the cesium source, while the amount of gas released from the NF3 source is manually controlled using a micro-leakage valve.

[0053] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0054] Example 1:

[0055] Activation process Figure 1 As shown.

[0056] Before activation, the GaAs photocathode material undergoes chemical cleaning and high-temperature purification.

[0057] The chemical cleaning process involves first ultrasonically cleaning the sample in sequence with carbon tetrachloride, acetone, anhydrous ethanol, and deionized water to remove grease; then etching the sample in HF solution for 10 minutes, rinsing it with deionized water, followed by chemical etching in a 1:1 HCl: IPA mixed solution for 10 minutes; finally, thoroughly rinsing the sample with deionized water and drying it.

[0058] The high-temperature purification process involves placing the chemically cleaned sample into a vacuum with a vacuum level of not less than 10°C. -7 The sample was heated for 60 minutes in an ultra-high vacuum system on the order of Pa at a temperature of 950°C. After the sample cooled naturally to room temperature, it was placed in the activation position to begin Cs-NF3 activation.

[0059] During activation, a halogen tungsten lamp is used to vertically illuminate the cathode surface. The on / off state of the cesium source and NF3 source is determined by real-time measurement and observation of the photocurrent generated by the cathode. The cesium source used for activation is a solid source of zirconium aluminum alloy powder reducing cesium chromate, and the NF3 source is a high-purity gaseous source. The amount of gas released by the cesium source is controlled by adjusting the current of the external current source, while the NF3 source needs to be manually opened via a micro-leakage valve. Since the amount of gas released by different cesium and NF3 sources may vary, the cesium source current used during activation will also vary. After changing the cesium source, a suitable Cs-NF3 ratio should be obtained experimentally. In this example, the cesium source current is 3.9 amperes during activation, and activation is performed using a continuous NF3 source and an intermittent cesium source. The activation steps are as follows:

[0060] (1) When the cesium source is turned on, the photocurrent gradually increases, and then decreases after reaching its peak value;

[0061] (2) When the photocurrent drops to 85% of its peak value, turn on the NF3 source and keep the NF3 source on, then turn off the cesium source; the photocurrent starts to rise; and then drops again after the photocurrent reaches its peak value.

[0062] (3) When the peak photocurrent of the first NF3 source is turned on drops to 85% of the current peak, the cesium source is turned on and the NF3 source valve is kept open, and the photocurrent gradually changes from decreasing to increasing.

[0063] (4) When the photocurrent drops to 90% of the peak current, turn on the cesium source and turn it off after 5 minutes. The photocurrent will immediately begin to decrease. During this stage, ensure that the vacuum level of the ultra-high vacuum system is not lower than 5*10 -7 Pa order of magnitude;

[0064] (5) Repeat steps (3) and (4) until the photocurrent can rise to a peak value and start to decline within 5 minutes when the cesium source is turned on. Then perform the following steps: change the cesium source alternation method to: wait for the photocurrent when the cesium source is turned on to drop to 90% of the peak value of the photocurrent and then turn off the cesium source; and wait for the photocurrent when the NF3 source is turned on alone to rise to the peak value.

[0065] (6) When the photocurrent of the NF3 source drops to 90% of the current peak value, turn on the cesium source and keep the NF3 source on. Wait for the photocurrent to rise to the peak value and then begin to decrease. During this stage, ensure that the vacuum level of the ultra-high vacuum system is not lower than 5*10 -7 Pa order of magnitude;

[0066] (7) When the photocurrent drops to 90% of the photocurrent peak, turn off the cesium source and continue to wait for the photocurrent when the NF3 source is turned on alone to rise to the peak value and then begin to decrease. During this stage, ensure that the vacuum level of the ultra-high vacuum system is not lower than 2*10 - 7 Pa order of magnitude;

[0067] (8) Repeat steps (6) and (7) until the peak current of the photocurrent no longer increases, then turn off the cesium source and the NF3 source in turn to end the activation process.

[0068] Comparative experiments were conducted using the same GaAs photocathode and under the same conditions of chemical cleaning, high-temperature purification, and activation. Figure 3 The photocurrent curve of the GaAs photocathode activated by the present invention; Figure 2 The image shows the photocurrent curves for activating a GaAs photocathode using a traditional Cs-NF3 co-deposition method without alternation; the horizontal axis represents time in minutes, and the vertical axis represents photocurrent in microamps. Figure 2 and Figure 3 As can be seen from this, the maximum peak value of the photocurrent of the present invention is greater than that of the traditional activation method.

[0069] Figure 4 This is a comparison of the quantum efficiency curves of GaAs photocathodes activated by two methods. Curve 1 is the quantum efficiency curve of the cathode obtained by the present invention, and curve 2 is the quantum efficiency of the cathode obtained by the conventional method. It can be seen that the quantum efficiency of the cathode obtained by the present invention is higher than that of the conventional activation method.

[0070] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A Cs-NF3 activation method for improving quantum efficiency of a GaAs photocathode, characterized by, The method comprises: Step 1, performing single cesium activation treatment; Step 2, performing Cs-NF3 co-deposition activation treatment; Step 3, performing Cs-NF3 over-activation treatment; During the activation process of steps 1 to 3, the entire cathode surface is vertically irradiated with halogen tungsten lamp white light; The activation process is completed in an ultra-high vacuum environment, the vacuum degree of which is not less than 10 -7 Pa order of magnitude.

2. The method of claim 1, wherein, Step 1 includes: turning on the cesium source, and gradually increasing the photocurrent; when the photocurrent reaches the peak value, it then decreases.

3. The method of claim 1, wherein, Step 2, performing Cs-NF3 co-deposition activation treatment; Comprise: Step 2-1, when the photocurrent decreases to 50% to 90% of the peak value, open the NF3 inlet valve, keep the NF3 valve open, and close the cesium source; the photocurrent starts to rise again, and decreases again after the photocurrent rises to the peak value; Step 2-2, when the photocurrent decreases to 70%~90% of the previous peak value, turn on the cesium source, and keep the NF3 source open, the photocurrent gradually changes from decreasing to increasing; Step 2-3, keep the cesium source open for 5 minutes, then close the cesium source, and the photocurrent suddenly decreases after forming a new peak value; Step 2-4, repeat steps 2-2 and 2-3 until the photocurrent can reach the peak value and turn to decrease during the cesium source is turned on, then execute step 3.

4. The method of claim 1, wherein, Step 3, performing Cs-NF3 over-activation treatment; Comprise: Step 3-1, when the photocurrent can rise to the peak value and then decrease to 70%~90% of the current peak value, close the cesium source and keep the NF3 source open, the photocurrent immediately increases, and waits for the current to rise to a new peak value and turn to decrease; Step 3-2, when the photocurrent decreases to 70%~90% of the previous peak value, turn on the cesium source, and wait for the photocurrent to rise to a new peak value and turn to decrease; Step 3-3, repeat steps 3-1 and 3-2 until the peak current of the photocurrent does not increase compared with the previous peak value, then close the cesium source and the NF3 source in turn, and end the activation process.

5. The method of claim 1, wherein, Before executing the activation step, a pre-step is executed: chemical cleaning and high-temperature heating of the GaAs photocathode.

6. The method of claim 5, wherein, The purpose of the chemical cleaning method is to remove oil on the surface of the GaAs photocathode; The steps include sequentially immersing the sample in carbon tetrachloride, acetone, alcohol, and deionized water for ultrasonic cleaning; Secondly, chemical etching is performed with HF solution; Then, the etched product is placed in a mixed solution of HCl:isopropyl alcohol with a ratio of 1:1 for etching; Then, the sample is washed with deionized water; finally, the cleaned GaAs photocathode is dried.

7. The method of claim 5, wherein, The high-temperature purification step is: placing the chemical cleaned sample into an ultra-high vacuum system for heating for 60 minutes, the heating temperature is 550-1000℃, the vacuum degree of the ultra-high vacuum system is not less than 10 -7 Pa order of magnitude.

8. The method of claim 3 or 4, wherein, The cesium source is a solid-state source of cesium chromate reduced by zirconium-aluminum alloy powder packaged in a nickel tube, and the NF3 source is a high-purity gaseous source; The cesium source is automatically controlled by a computer to adjust the size of the external current source current and control the size of the cesium source power-on gas emission, and the NF3 source is a micro-leakage valve to control the gas emission size.

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