electronics

Article High Photocurrent Density and Continuous Emission Characterization of a Multi-Alkali Antimonide Photocathode

Jun Dai 1, Yikun Ding 1, Cunjun Ruan 1,2,* , Xiangyan Xu 3,* and Hulin Liu 3 1 School of Electronic and Information Engineering, Beihang University, Beijing 100191, China; [email protected] (J.D.); [email protected] (Y.D.) 2 Beijing Key Laboratory for Microwave Sensing and Security Applications, Beihang University, Beijing 100191, China 3 Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China; [email protected] * Correspondence: [email protected] (C.R.); [email protected] (X.X.)

 Received: 27 October 2020; Accepted: 17 November 2020; Published: 24 November 2020 

Abstract: High photocurrent density cathodes that enable small cross-section electron beams are required for high-power terahertz vacuum devices. Multi-alkali antimonide photocathodes may be well suited for generating sub-mm electron beam sources. This paper involves the repeatability, stability, uniformity, and linearity experiments of the multi-alkali antimonide photocathodes electron emission operations under a continuous-wave 450 nm laser with a bias voltage of 5000 V. The effect of heat, electric contact, and cathode surface roughness to emission characterizations is analyzed. The methods to maintain the high-current-density emission and avoid the fatigue of the photocathode are verified. The emission can be repeated with increased optical power. The stable photocurrent density of near 1 A/cm2 and maximum current density of near 1.43 A/cm2 is recorded. The continuous photocurrent density is significantly improved compared to the current density reported in traditional applications. It is found that the current curves measuring at different areas of the photocathode differ greatly after the laser power of 800 mW. The increase in current for some areas may be attributed to the conductive current caused by built-in electric fields between the emission rough area and its adjacent areas.

Keywords: continuous high current density emission; nonuniformity emission; nonlinear emission; built-in electric field

1. Introduction Vacuum electronic devices are widely used in microwave and terahertz (THz) practical applications, as they can provide efficient high-power electromagnetic output [1]. As vacuum devices evolved from MHz to THz, high current electron beams with sub-mm cross-sections are required due to the physics mechanisms of frequency and geometry correlation. The development of THz power chips, including THz vacuum that could translate optical source to THz band and THz vacuum amplifiers that could amplify THz signals, have motivated us to develop high current density cathodes with ultra-small size [2]. In addition, emerging research in high power THz radiation source based on planar antenna integrated vacuum also requires high current density electron emission source [3,4]. Multi-alkali antimonide photocathode has been considered to be the strongest candidate for a high-brightness electron source. It has a comparatively shorter response time (approximately hundreds of femtoseconds) and can be operated with quantum efficiency above 15% over a wide spectral range, which enables both high frequency and high current density operations for THz vacuum devices

Electronics 2020, 9, 1991; doi:10.3390/electronics9121991 www.mdpi.com/journal/electronics Electronics 2020, 9, 1991 2 of 10 and radiation sources [5]. The methods to develop the multi-alkali antimonide photocathode has been extensively researched and reviewed over the past decades [6–8]. The fabrication technique to achieve a high-quantum-efficiency multi-alkali antimonide photocathode has followed a similar and empirical process [9]. The photocathode growth process in this paper also using the process only with some difference in the vacuum pressure, temperature, and evaporation control. Although the physics of the process is far from clear, much progress has been achieved through experiments for photocathodes applying in a wide range of applications in the detectors and electron guns [10]. As a detecting material, photocathodes are mainly used in either continuous mode or pulse mode under weak illumination [11]. In most of the detecting applications, the photocathodes are designed to operate below 1 µA/cm2, which is a very low current density [12]. When used as an electron gun material, photocathodes are mainly used in short pulse or ultrashort pulse mode because continuous emission is always difficult and costly [13]. However, in traditional applications, the spectrum sensitivity and quantum efficiency are emphasized [14]. Little attention has been focused on the applications that require the high-repetition-rate emission or continuous emission at a high current density, which often results in fatigue of the photocathode. Sommer has pointed out that the stability of the photocurrent at a higher absolute value will be affected by the ion bombardment [15]. It has been revealed that the heat emanating may raise the temperature of the photocathode, leading to a possible loss in the output current [4]. Simulations show that the local heating problem can be solved using a more thermally conductive substrate, such as a diamond [16]. Poor electric contact designs will cause electrolytic decomposition of the photocathode [17]. A previous study on high brightness emission has shown that a strong electric field is needed to overcome the big emittance caused by the surface roughness of the photocathode [18]. For high current density emissions in THz applications, the uniformity and linearity caused by the surface roughness are also significant to avoid the fluctuation in THz current. This study aims to explore the repeatability,stability,uniformity,and linearity of the alkali antimonide photocathode with small areas operating at high current density in CW mode through experiments. The effect and tolerance of the photocathode growth process are also studied. The photocathode growth technology was optimized to support the operations at high current density following the suggestions to avoid cathode damage and promote quantum efficiency [16]. The performances of the photocathode were characterized by measuring the anode current as a function of laser power. The continuous emission performances of the photocathodes in different areas were measured and discussed. The new findings of the nonlinear emission characterization are analyzed theoretically.

2. Materials and Experimental Details Traditional technology to fabricate multi-alkalis antimonide photocathode usually relies on a sequential growth of (K), Antimony (Sb), Sodium (Na), and Cesium (Cs) under ultra-high vacuum conditions. To support high current density emission operations, we made two changes to the traditional photocathode growth technology. First, a diamond wafer is utilized as the cathode substrate due to its high thermal conductivity. As there are great difficulties in sealing the diamond substrate with a metal tube body, we simply put a polycrystal diamond substrate inner the tube body by connecting it with a metal holder, as shown in Figure1. Second, an aluminum (Al) film was coated on the diamond to provide electric contact. The primary technical indicators for the THz devices listed in the introduction are the current density generated in a small area, which is indicated to be from 3 µm 3 µm to 300 µm 300 µm [2]. We also want to reduce the strict requirements of the photocathode × × growth process, for example, the global uniformity and small surface roughness. A photocathode with bad surface consistency is supposed to be explored. So, the photocathode chemical elements are designed to evaporate in a horizontal direction. The chemical elements in the traditional chemical vapor growth process deposit in the vertical direction of the ground plane. This fabrication setup may enhance the cathode surface roughness in the fringe areas of the substrate wafer, as well as uniformity difference. Electronics 2020, 9, x FOR PEER REVIEW 3 of 10 fabrication setup may enhance the cathode surface roughness in the fringe areas of the substrate wafer, as well as uniformity difference. During the fabrication of the photocathode, the photocathode and the collecting anode are always connected to form a path with the micro-current meter and high-temperature wire. A tungsten lamp as a stable light source is placed outside the photocathode table and shines on the photocathode through a window. The tungsten lamp current is maintained unchanged during the process to make sure the light intensity is constant. The extracted photocurrent generated by the photocathode represents the sensitivity characteristics of the photocathode, and the manufacturing process of the cathode is then adjusted and controlled according to the change of the photocurrent to achieve the optimized photocathode fabrication and highest emission current. At the beginning of the fabrication procedures, the alkali and Antimony metals went through a degassing process. Then, the tube was evacuated to 1.0 × 10−6 Pa. The temperature of the fabrication chamber was set to near 200 °C. The evaporation begins with the K process. K evaporation was performed with control through the feedback from the photocurrent of the cathode composition film. K source can be shut down temporarily until the maximum of extracted current was achieved. The following Sb source was evaporated with the same process control. Then, K and Sb were alternately deposited until the photocurrent reaches the peak to establish a K-Sb film. The Na process needs the current to be increased to its extremum point and then decreased to half of the peak current. Then, K and Sb were alternately deposited once again with current monitoring to form a transparent Na2KSb film through chemical reactions. Theoretical analysis has shown that the interface between the p-type Na2KSb film and n-type cesium antimonide compound can bring down the electron affinity below the bottom of the conduction band of Na2KSb. The drift field that arises at the heterointerface enables almost all the photogenerated within the cathode to reach the surface [19]. To enhance the emission current through the multilayer structure growth, considerations must be taken to adjust the evaporation and doping concentrations on both p-type and n-type compound. Therefore, the correct doping of Cs and Sb is essential for the growth of the dipole layer. To perform the growth, the temperature was optimized to 160 °C. Then, the alternating evaporations of Cs and Sb over the transparent Na2KSb film was used. The whole fabrication was completed with the roasting process to eliminate the redundant alkali metals. Finally, a transmission-type multi-alkalis antimonide photocathode was deposited on a 0.3 mm thick diamond substrate in a assembly. The vacuum diode tube, marked with a dotted line in Figure 1, was applied for the measurement of emission performance. The photocathode film was kept parallel to the optical glass window at a distanceElectronics 2020of ,19 ,mm 1991 and mounted on an electric holder. The anode was aligned coaxially to3 ofthe 10 photocathode surface. The distance between the photocathode and its nearest anode area is 2 cm.

FigureFigure 1. 1. SchematicSchematic diagram diagram of of the the electron electron emission emission experiment experiment setup. setup.

TheDuring experimental the fabrication setup of for the the photocathode, electron emission the photocathode test consistsand of a thephotocathode collecting anode vacuum are tube, always a circularconnected aperture to form plate, a path a with Thorlabs the micro-current PM100 power meter meter and high-temperatureconsole, an energy wire. measurement A tungsten lampsensor, as quartza stable lenses, light source and power is placed supply, outside as theillustrated photocathode in Figure table 1. and A continuous-wave shines on the photocathode blue laser through with a a window. The tungsten lamp current is maintained unchanged during the process to make sure the light intensity is constant. The extracted photocurrent generated by the photocathode represents the sensitivity characteristics of the photocathode, and the manufacturing process of the cathode is then adjusted and controlled according to the change of the photocurrent to achieve the optimized photocathode fabrication and highest emission current. At the beginning of the fabrication procedures, the alkali and Antimony metals went through a degassing process. Then, the tube was evacuated to 1.0 10 6 Pa. The temperature of the fabrication × − chamber was set to near 200 ◦C. The evaporation begins with the K process. K evaporation was performed with control through the feedback from the photocurrent of the cathode composition film. K source can be shut down temporarily until the maximum of extracted current was achieved. The following Sb source was evaporated with the same process control. Then, K and Sb were alternately deposited until the photocurrent reaches the peak to establish a K-Sb film. The Na process needs the current to be increased to its extremum point and then decreased to half of the peak current. Then, K and Sb were alternately deposited once again with current monitoring to form a transparent Na2KSb film through chemical reactions. Theoretical analysis has shown that the interface between the p-type Na2KSb film and n-type cesium antimonide compound can bring down the electron affinity below the bottom of the conduction band of Na2KSb. The drift field that arises at the heterointerface enables almost all the photogenerated electrons within the cathode to reach the surface [19]. To enhance the emission current through the multilayer structure growth, considerations must be taken to adjust the evaporation and doping concentrations on both p-type and n-type compound. Therefore, the correct doping of Cs and Sb is essential for the growth of the dipole layer. To perform the growth, the temperature was optimized to 160 ◦C. Then, the alternating evaporations of Cs and Sb over the transparent Na2KSb film was used. The whole fabrication was completed with the roasting process to eliminate the redundant alkali metals. Finally, a transmission-type multi-alkalis antimonide photocathode was deposited on a 0.3 mm thick diamond substrate in a vacuum tube assembly. The vacuum diode tube, marked with a dotted line in Figure1, was applied for the measurement of emission performance. The photocathode film was kept parallel to the optical glass window at a distance of 1 mm and mounted on an electric holder. The anode was aligned coaxially to the photocathode surface. The distance between the photocathode and its nearest anode area is 2 cm. The experimental setup for the electron emission test consists of a photocathode vacuum tube, a circular aperture plate, a Thorlabs PM100 power meter console, an energy measurement sensor, quartz lenses, and power supply, as illustrated in Figure1. A continuous-wave blue laser with a Electronics 2020, 9, x FOR PEER REVIEW 4 of 10 wavelengthElectronics 2020 ,of9, 1991450 nm was used for excitation of the photocathode. Quartz lenses and circular4 of 10 aperture plates are used for enhancing the input optical intensity and aligning visible light beam to thewavelength center of of the 450 photocathode nm was used illumination for excitation areas. of the The photocathode. emission areas Quartz are all lenses circular and circularwith a diameter aperture Φ × ofplates = are 0.5 used mm. for It enhancingis bigger thethan input 300 opticalμm 300 intensity μm which and aligning is enough visible for light the beamrequirement to the center of the of applications.the photocathode Laser illumination powers were areas. all measured The emission near areas the window are all circular of the with vacuum a diameter tube using of Φ =Thorlabs0.5 mm. S405It is biggerpower thansensor 300 whoseµm maximum300 µm which detecting is enough power for is 5 the W. requirement The actual incident of the applications. laser power to Laser the × glasspowers was were about all measuredhalf of the near measured the window power. of theThe vacuumdiameter tube of the using laser Thorlabs beam S405is bigger power than sensor the diameterwhose maximum of the circular detecting aperture, power iswhich 5 W. The causes actual ha incidentlf of the laserpower power to be to blocked the glass by was the about plate. half The of powerthe measured illuminated power. at Thethe photocathode diameter of the is laser proportional beam is bigger to the than measured the diameter power, of yet the it circular is less than aperture, half ofwhich the causesmeasured half power. of the power The direct to be blockedcurrent bypowe ther plate. supply The was power used illuminated to provide at thea biased photocathode voltage betweenis proportional photocathode to the measured and anode power, at 5000 yet V. it isThe less value than halfof the of thecollected measured anode power. current The will direct be monitoredcurrent power using supply a passive was used QILIGANG to provide B5C1 a biased ammeter voltage with between an accuracy photocathode of 0.01 mA, and anodewhich atis 5000made V. byThe Chanan value of Group the collected Co. Ltd. anode (Guangzhou, current will China). be monitored using a passive QILIGANG B5C1 ammeter with an accuracy of 0.01 mA, which is made by Chanan Group Co. Ltd. (Guangzhou, China). 3. Results and Discussion 3. Results and Discussion 3.1. Repeatability and Stability 3.1. Repeatability and Stability As mentioned in the introduction, for high-repetition-rate emission or continuous emission, the thermalAs storage mentioned may in be the the introduction,main reason for for the high-repetition-rate deterioration of the emission photocathode or continuous because it emission, leads to thethe decomposition thermal storage of may the be surface the main compound. reason for The the th deteriorationermal storage of comes the photocathode from the joule because heat, itcarrier leads recombinationto the decomposition heat, and of the optical surface absorption compound. heat The accumulation. thermal storage Before comes using from diamond the joule as heat, substrate carrier material,recombination we have heat, fabricated and optical four absorption tubes heatusing accumulation. a borosilicate Before glass using substrate diamond whose as substratethermal conductivitymaterial, we haveis about fabricated 1.3 W/m four∙K. The tubes glass using substrate a borosilicate serves glass as the substrate optical window whose thermal of the testing conductivity tubes asis aboutwell. The 1.3 Wbest/m stableK. The current glass substratedensity from serves limit as thetests optical we got window was beyond of the 30 testing mA/cm tubes2 at 1500 as well. V. · WhenThe best we stable further current increased density the fromincident limit power tests of we CW got laser was beyondillumination, 30 mA the/cm current2 at 1500 of V.the When cathode we tubefurther rapidly increased reached the incidentits plateau power and ofthen CW descended laser illumination, fleetly. During the current one of thethe cathodelimit tests, tube the rapidly glass substratereached its began plateau to crack and then as the descended current reached fleetly. its During peak. one Therefore, of the limit the thermal tests, the storage glass substrate may affect began the repeatabilityto crack as the and current stability reached of itsthe peak. emission Therefore, photoc theurrent thermal when storage operating may aff ectat thehigh-repetition-rate repeatability and emissionstability ofor the continuous emission photocurrentemission mode, when which operating will lead at high-repetition-rate to accuracy problems emission in generating or continuous THz signalemission and mode, radiation. which will lead to accuracy problems in generating THz signal and radiation. The repeatability experiments with three seconds of exposure time are carried out at a a randomly randomly chosen linear area near the center of the the diamond diamond wafer. The The bias bias voltage voltage of of the the testing testing vacuum vacuum diode diode tubetube is maintained to to be 5000 V during the experiments.experiments. No No current current drift was observed during the exposure. The The mean mean photocurrent photocurrent line line of of three three curren currentt lines lines measured measured at at different different power is shown in Figure 22.. TheThe onlyonly errorerror happenshappens at at 800 800 mW, mW, 1200 1200 mW, mW, and and 1400 1400 mW mW shows shows the the standard standard deviation deviation is isunder under 2.9%, 2.9%, which which indicates indicates the the high high repeatability repeatability of of the the emission emission current.current. The maximum current density measure at 1.8 mA near 1 A/cm A/cm2,, was was recorded recorded as stable.

FigureFigure 2. 2. PlotsPlots of of the the current current measured measured at at different different laser powers.

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To findfind out out the the heat heat tolerance tolerance of the of photocathode the photoc usingathode a diamondusing a substrate,diamond stabilitysubstrate, experiments stability ofexperiments electron emission of electron with emission the optical wi powerth the optical at 1600 power mW were at 1600 carried mW out. were As carried we can out. see fromAs we Figure can see3., 2 highfrom continuousFigure 3., high emission continuous current emi densityssion atcurrent near 1 density A/cm couldat near be 1 maintainedA/cm2 could after be maintained the current drop.after Thethe current current drop. drop usuallyThe current happened drop usually in the time happened slot of 20–30 in the min time after slot theof 20–30 beginning min after of the the experiments. beginning Theof the decay experiments. performance The ofdecay the photocathodeperformance of is the common photocathode among theis common different among photocathodes the different with diphotocathodesfferent measurement with different environments measurement [2,12]. At enviro beginningnments of the [2,12]. test, theAt photocathodebeginning of will the undergo test, the a processphotocathode of a sharp will temperatureundergo a process increase. of a Asharp bad thermaltemperature conductivity increase. ofA thebad substrate thermal conductivity leads to a long of timethe substrate to reach aleads balance to a betweenlong time heat to reach accumulation a balance and between dissipation. heat accumulation The drop at diandfferent dissipation. times in The the twodrop measurements at different times can in be the attributed two measurements to instabilities can in be the attributed laser source. to instabilities The loss ofin thethe performancelaser source. 6 inThe the loss vacuum of the ofperformance 1.0 10− Pain the is mainly vacuum due of to1.0 the × 10 Cs−6 desorptionPa is mainly from due theto the surface. Cs desorption The observed from × currentthe surface. decay The suggested observed that current the diamond decay suggested substrate th inat the the tube diamond doesn’t substrate dissipate in the the heat tube eff ectively.doesn’t Thedissipate reason the is heat that theeffectively. interior The of the reason testing is tubethat the is a interior vacuum of where the testing heatis tube conducted is a vacuum very slowly.where Besides,heat is conducted the substrate very is heldslowly. by aBesides, Kovar alloythe substrat structure.e is Though held by the a Kovar diamond alloy has structure. thermal conductivityThough the neardiamond 2000 has W/m thermalK, the thermalconductivity conductivity near 2000 of KovarW/m∙K, alloy the isthermal near 50 conductivity W/m K, which of makesKovar italloy diffi iscult near to · · spread50 W/m the∙K, heatwhich from makes inside it difficult to the outside to spread of thethe tubeheat veryfrom soon.inside If to the the diamond outside of substrate the tube is very also soon. used asIf the an opticaldiamond window, substrate the is situation also used could as an be opti muchcal better.window, The the heat situation can be conductedcould be much faster, better. and other The coolingheat can technologies be conducted are faster, convenient and other to be applied.cooling So,tech thenologies technique are ofconvenient the sealing-in to be between applied. diamond So, the andtechnique metal of must the besealing-in tackled inbetween the future. diamond and metal must be tackled in the future.

Figure 3. Plots of the current measured at 1600 mW with a five-minutefive-minute interval.

3.2. Uniformity and Linearity The uniformity and linearity measurements of th thee photocathode film film are performed extensively in didifferentfferent areas.areas. Figure4 4.. showsshows thethe selectedselected continuouscontinuous emissionemission characterizationcharacterization fromfrom thethe typicaltypical areas. The marked areas in the circle on the left of Figure 4 4 represent represent theirtheir adjacentadjacent areas.areas. TheThe redred lineline 2 shows thatthat thethe maximum maximum current current density density is 1.43 is 1.43 A/cm A/cm(at2 2.8 (at mA), 2.8 mA), which which is close is toclose the currentto the current density reporteddensity reported with Cs3 withSb photocathode Cs3Sb photocathode [12]. When [12]. the When optical the power optical is belowpower 300 is below mW, it 300 is not mW, obvious it is not to seeobvious the emission to see the diff erenceemission between difference different between areas. different The emission areas. current The emission measured current between measured different areasbetween with different the optical areas power with belowthe optical 100 mWpower shows below a di100fference mW shows below a 8%,difference which indicatesbelow 8%, that which the non-uniformityindicates that the of non-uniformity the photocathode of the won’t photocat be a bighode problem won’t when be a big the problem photocathode when the is operated photocathode under weakis operated illumination. under weak It can illumination. be seen that theIt can emission be seen current that the performances emission current are very performances different especially are very whendifferent the especially laser power when is above the la 800ser mW. power This is non-uniformity above 800 mW. at This high non-uniformity current emission at from high di currentfferent areasemission may from be caused different by fabricationareas may technology.be caused by Generally, fabrication the techno sequentiallogy. depositionGenerally, tothe fabricate sequential the multi-alkalisdeposition to antimonide fabricate the photocathode multi-alkalis is antimonide an unreliable photocathode process because is an the unreliable chemical process reaction because during the cathodechemical growth reaction is very during sensitive the cathode to many parametersgrowth is very such assensitive temperature, to many pressure, parameters and deposition such as ratestemperature, [9]. Any pressure, little change and indepositio the parametersn rates [9]. could Any a littleffect thechange homogeneity in the parameters of the cathode could affect chemical the compositionshomogeneity of and the surface cathode states. chemical In our compositions cathode growth and surface setup, thestates. evaporation In our cathode directions growth of allsetup, the the evaporation directions of all the multi-alkalis and antimonide metal sources are difficult to

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confine. The non-uniformity behaviors are also the results of the intentional design in the fabrication setup. It should be noted that the current characteristics in Area_1, Area_2, and Area_3 are exhibiting a current enhancement while Area_6 and Area_7 are exhibiting a saturation-like behavior at higher optical power. Area_1, Area_2, and Area_3 almost locate at the same place. The currents measured at these areas also indicate the repeatability of nonlinear emission. Area_4 and Area_5 both exhibit linear behavior. Actually, Area_4 and Area_5 also represent most of the emission areas. The emission scheme for different areas photocathode is the same. The unique difference is the uniformity of the cathode related to the whole processing and treatment. It is easy to understand the emission current rises linearly for most of the areas when the light power increases in the condition of the constant anode voltage. With more incident to the photocathode, the emission current is consequently Electronicselevated.2020 It, 9is, 1991also reasonable that some central areas, for example, Area_6, show a loss of output6 of 10 linearity owing to the effect of heat accumulation. The fatigue of the cathode is due to the frequent measurements in the above repeatability and stability experiments. Area_7 also represents a random multi-alkalis and antimonide metal sources are difficult to confine. The non-uniformity behaviors are area with a big sheet resistance caused by cathode nonuniformity. also the results of the intentional design in the fabrication setup.

Figure 4. Plots of the current monitored at different laser powers and different areas. Figure 4. Plots of the current monitored at different laser powers and different areas. It should be noted that the current characteristics in Area_1, Area_2, and Area_3 are exhibiting a currentIt is enhancementinteresting to whilesee the Area_6 current and lines Area_7 of some are areas exhibiting are beyond a saturation-like the linear behaviorlines after at 800 higher mW. opticalThe current power. increase Area_1, effect Area_2, has and also Area_3 been observ almosted locate in the at thephotomultiplier same place. The tubes. currents The reasons measured are atusually these areascomplicated also indicate and can the be repeatability categorized of as nonlinear follows: thermionic emission. Area_4 emission and current, Area_5 field both emission exhibit linearcurrent, behavior. ionization Actually, current Area_4 from andgases. Area_5 Thermionic also represent emission most current of the is emissiona typical kind areas. of The dark emission current. schemeIt is difficult for diff toerent generate areas photocathodemA level dark is current the same. within The uniquesuch a dismallfference area. is The the uniformityfield between of thethe cathodecathode related and anode to the is whole about processing 2500 V/cm. and This treatment. is very Itlow is easycompared to understand to the threshold the emission of the current field risesemission. linearly Ionization for most current of the areasfrom whengases theis a light serious power problem increases for pulse in the operations condition ofbecause the constant a large anodecurrent voltage. is produced With morewhen photon the positive incident ions to strike the photocathode, the photocathode. the emission If there currentis an ionization is consequently current, elevated.the increase It is of also the reasonablecurrent could that exist some in central any emission areas, for areas. example, At least, Area_6, it will show occur a at loss the of emission output linearityzones with owing the tosame the radius. effect of The heat experiments accumulation. don’t The approve fatigue of ofall the these cathode kinds isof due effects. to the Also, frequent in our measurementsprevious experiments, in the above we repeatability have measured and stability the performance experiments. of Area_7the photocathode also represents on a Sapphire random arearegarding with a the big variation sheet resistance of laser caused spot size by as cathode a function nonuniformity. of laser power using the same laser. The linear resultsIt is help interesting eliminate to other see the unknown current lineseffects of from some the areas laser are source beyond [20]. the linear lines after 800 mW. The currentThe cathode increase fabrication effect has and also testing been observed can only in be the performed photomultiplier in the ultrahigh tubes. The vacuum reasons environment are usually −6 complicated(~10 Pa). The and presence can be categorized of air will lead as follows: to the oxid thermionication of emission the photocathode current, field immediately. emission current, So, it is ionizationvery difficult current to obtain from the gases. evolution Thermionic of the comp emissionosition current of the is cathode a typical with kind time. of darkThe metal current. pedestal It is diofffi thecult testing to generate vacuum mA tube level also dark prevents current withinthe imag suching a characteristics, small area. The the field elemental between distribution, the cathode and and anodethe structural is about 2500analysis V/cm. of Thisthe iscathode very low surface. compared The toanalysis the threshold of the emission of the field performance emission. Ionization has been currentrelying from on theoretical gases is a seriousspeculation problem and forexperimental pulse operations verification. because a large current is produced when the positive ions strike the photocathode. If there is an ionization current, the increase of the current could exist in any emission areas. At least, it will occur at the emission zones with the same radius. The experiments don’t approve of all these kinds of effects. Also, in our previous experiments, we have measured the performance of the photocathode on Sapphire regarding the variation of laser spot size as a function of laser power using the same laser. The linear results help eliminate other unknown effects from the laser source [20]. The cathode fabrication and testing can only be performed in the ultrahigh vacuum environment 6 (~10− Pa). The presence of air will lead to the oxidation of the photocathode immediately. So, it is very difficult to obtain the evolution of the composition of the cathode with time. The metal pedestal of the testing vacuum tube also prevents the imaging characteristics, the elemental distribution, and the structural analysis of the cathode surface. The analysis of the emission performance has been relying on theoretical speculation and experimental verification. Electronics 2020, 9, 1991 7 of 10 Electronics 2020, 9, x FOR PEER REVIEW 7 of 10

For high current density operations, when the electrons are emitted from the photocathode the innerinner areas areas of of the the photocathode photocathode become become positively positively ch charged.arged. Consequentially, Consequentially, only the most energetic energetic electrons can overcome the resultant positive potent potentialial and be emitted. This This effect effect prevents a high emission current. So, So, fine fine electronic electronic contact contact needs needs to to be be designed near the the emission areas areas to provide compensating electrons for the positively charged areas immediately. One practical conductive ring contact configurationconfiguration isis shownshown in in Figure Figure5a. 5a. The The photocathode photocathode and and electronic electronic contact contact are are deposited deposited on onthe the same same surface surface of the of substrate. the substrate. Owing toOwing the sheet to the resistance sheet ofresistance the alkali of antimonide the alkali photocathode antimonide × 6 × 7 photocathodeis about 1 10 is6 aboutΩ/sq 1 to 110 Ω10/sq7 Ωto/ sq,1 10 a largeΩ/sq, potential a large potential difference differ betweenence between the inner the area inner of area the × × ofphotocathode the photocathode and the and conductive the conductive ring is formedring is fo [21rmed]. This [21]. large This potential large potential difference difference could cause could a causecurrent a current increase increase from the from adjacent the adjacent areas ofareas the of emission the emission area. area. It has It beenhas been recorded recorded that, that, when when the thephotomultiplier tube is illuminated is illuminated by an by intense an intense pulse pulse laser laser with with a repetitive a repetitive frequency frequency of kHz of and kHz a andwidth a width of 10 nanoseconds,of 10 nanoseconds, the time the widthtime width of the of current the current pulse pulse will change will change to microsecond-level to microsecond-level after after5 min. 5 min. The The broadening broadening of theof the time time width width can can be be eliminated eliminated by by improving improving the the designdesign of electronic contact. When When the the built-in built-in electric electric field field becomes becomes sufficiently sufficiently large, large, the the electric breakdown could happen which which will will lead lead to to a asudden sudden increase increase in emission in emission current. current. The Theelectric electric breakdown breakdown will be will very be likelyvery likely to cause to cause an irreversible an irreversible electrolytic electrolytic decom decompositionposition of the of cesium the cesium antimonide antimonide compound. compound. This decompositionThis decomposition effect e ffisect easily is easily recognizable recognizable by a bychange a change of the of photocathode’s the photocathode’s color. color.

(a) (b)

Figure 5. ((aa)) Top Top view of the electric breakdown between positively charged photocathode emission area and electric contact which are on the same surface of the substrate; ( b)) side view of the built-in electric fieldfield between positivelypositively chargedcharged photocathode photocathode emission emission area area and and electrical electrical contact contact designed designed in inour our experiments. experiments.

However, no color change is found during the ex experiments.periments. The The high high current current density density operations operations can be well repeated over at least 2020 times.times. Besid Besides,es, the the electric contact configuration configuration used in the fabrication of this this photocathode photocathode is is different different from the ring structure. structure. In In the the ring ring contact contact configurations, configurations, thethe contact contact and and photocathode photocathode are are deposited deposited on on the the same same plane. plane. In In our our layered layered fabrication fabrication structures, structures, the aluminum contact contact film film is coated on the substrate.substrate. Then, Then, the the photocathode is is grown grown on the thin contact film. film. This This structure structure can can guarantee guarantee the theelectr electronsons could could be supplied be supplied to the to emission the emission areas. areas.Even ifEven there if might there mightbe positively be positively charged charged inside the inside photocathode, the photocathode, the built-in the built-infield is very field low is very because low thebecause vertical-direction the vertical-direction resistance resistance of the photocathode of the photocathode is very low. is very The low.low potential The low potentialdifference di betweenfference thebetween inner thearea inner of the area photocathode of the photocathode and the conducti and theve conductive film almost film does almost not affect does notthe aemissionffect the performances.emission performances. Lastly, a possible reason for the current enhanc enhancementement is is the the big big roughness roughness of of the the photocathode. photocathode. This roughness causes a local distortion in the electricelectric field.field. Previous research has shown that the roughness has a strong effect effect on emission emittance.emittance. The The surface roughness roughness is is thought to to limit the intrinsicintrinsic emittance emittance of of alkali-antimo alkali-antimonidenide cathodes cathodes to to a a50% 50% larger larger valu valuee than than the the thermal thermal limit limit [22]. [22 A]. simulationA simulation using using VSim VSim has has studied studied the the electron electron emis emissionsion performance performance from from a a ridge ridge period period of of 394 394 nm, nm, ridge height of of 194 nm, and a width of the ridge flatflat top of 79 nm. The results have shown that the quantum yield from the surface of th thee ridges is higher than from the flatflat surface [[23].23]. This enhanced field in the vacuum helps to lower the vacuum energy level and permit photoelectrons up cross the surface barriers, leading to an increase of the emission current [24]. It should be noted that the field

Electronics 2020, 9, 1991 8 of 10

Electronicsfield in the 2020 vacuum, 9, x FOR helpsPEER REVIEW to lower the vacuum energy level and permit photoelectrons up cross8 of the 10 surface barriers, leading to an increase of the emission current [24]. It should be noted that the field enhancement in the vacuum can be the dominating reason for the increase of the emission current enhancement in the vacuum can be the dominating reason for the increase of the emission current because it doesn’t show any difference below 800 mW. Normally, roughness also causes reflection because it doesn’t show any difference below 800 mW. Normally, roughness also causes reflection and and scattering of the laser [25]. However, the thickness of the multi-alkali antimonide photocathode scattering of the laser [25]. However, the thickness of the multi-alkali antimonide photocathode is is below 100 nm, which makes the reflection and scattering contribute a little to the current below 100 nm, which makes the reflection and scattering contribute a little to the current enhancement. enhancement. For the flat cathode surface, the local electric field normal is perpendicular to the surface and the For the flat cathode surface, the local electric field normal is perpendicular to the surface and the conductive thin metal film on the substrate, as shown in Figure5b. The transverse field inside the conductive thin metal film on the substrate, as shown in Figure 5b. The transverse field inside the cathode is zero. For the surface with a small roughness, the transverse fields are also weak. If the cathode is zero. For the surface with a small roughness, the transverse fields are also weak. If the roughness of the cathode surface structure is big enough, the local electric field normal will differ roughness of the cathode surface structure is big enough, the local electric field normal will differ greatly from the global normal, which is depicted in Figure6. As we have discussed in Figure5a, greatly from the global normal, which is depicted in Figure 6. As we have discussed in Figure 5a, the the transverse fields inner the photocathode emission areas could be strong enough to induce the transverse fields inner the photocathode emission areas could be strong enough to induce the current current increase from its adjacent areas [26]. When the incident of the laser power is low, the emission increase from its adjacent areas [26]. When the incident of the laser power is low, the emission current current is also low, and the built-in electric field of the rough emission areas is too low to induce is also low, and the built-in electric field of the rough emission areas is too low to induce current from current from adjacent areas. When the emission current increases. the current density also increases, adjacent areas. When the emission current increases. the current density also increases, which causes which causes a big field intensity. Therefore, the current will then be enhanced. However, in this case, a big field intensity. Therefore, the current will then be enhanced. However, in this case, the heat the heat accumulation caused by the transverse sheet resistance cannot be ignored. accumulation caused by the transverse sheet resistance cannot be ignored.

Figure 6. SideSide view view built-in built-in electric electric field field between between posi positivelytively charged charged photocathode emission area and electric contact in our experiments.

To summarize, the the current current enhancement enhancement may may be be attributed attributed to to many many effects. effects. The The enhancement enhancement in thein the built-in built-in field field caused caused by by the the roughness roughness may may be be the the dominant dominant reason. reason. The The roughness roughness can can be attributed toto thethe traditionaltraditional photocathode photocathode growth growth procedures procedures which which we we have have already already discussed discussed at the at thebeginning beginning of this of section.this section. A previous A previous ultrahigh ultrahigh vacuum vacuum atomic forceatomic microscope(UHV-AFM) force microscope(UHV-AFM) study on studybi-alkali on antimonide bi-alkali antimonide photocathodes photocathodes grown by thegrow similarn by traditionalthe similar procedures traditional showedprocedures that theshowed final thatrms the surface final roughness rms surface is roughness about 25 nm is about over a 25 100 nm nm over spatial a 100 period nm spatial [27]. period In recent [27]. years, In recent the growth years, themechanism growth mechanism of sequentially of sequenti grown bi-alkalially grown antimonide bi-alkali antimonide K2CsSb photocathodes K2CsSb photocathodes has been studied has been by studiedsynchrotron by synchrotron X-ray techniques X-ray [ 28techniques]. The randomly [28]. The distributed randomly roughness distributed does roughness cause problems does tocause the problemsuniformity to problems the uniformity and increase problems the and emittance. increase If the we emittance. look at the If perspective we look at the of high perspective quantum of yieldhigh quantumor emission yield current or emission density, thecurrent roughness density, has the advantages roughness compared has advantages to the flat compared surface. In to the the future, flat surface.controlled In roughnessthe future, technologycontrolled roughness needs to be technology studied by needs X-ray to techniques. be studied Theby X-ray current techniques. enhancement The currenteffect also enhancement inspired us toeffect design also and inspired optimize us theto desi artificialgn and rough optimize structures the artificial of the substrate rough structures for practical of theapplications substrate [for29]. practical applications [29]. The experimentalexperimental evidenceevidence of of high high current current density density alkali alkali antimonide antimonide photocathode photocathode has laidhas alaid very a verygood good foundation foundation for us for to furtherus to further explore explore the possibility the possibility of continuously of continuously tunable tunable CW THz CW sources. THz sources.Electron Electron emission emission from a surface from a withsurface a controlled with a controlled roughness roughness can be utilized can be utilized to improve to improve the current the currentdensity, density, though therethough must there be must a tradeo be ffa withtradeoff the energywith the spread energy and spread emittance and emittance of the initial of the electrons. initial electrons.It is foreseeable It is foreseeable that the current that the density current will density be upgraded will be upgraded to an astonishing to an astonishing high value, high for value, example for examplebeyond 10 beyond A/cm2 10in CWA/cm mode.2 in CW Similar mode. to Similar the Uni-traveling-carrier to the Uni-traveling-carrier photodiode photodiode (UTC-PD) based(UTC-PD) THz basedemitter THz configuration, emitter configuration, we could replace we could the replace solid-state the solid-state photodiode photodiode with the photocathode with the photocathode emission THzemission vacuum THz photodiode. vacuum photodiode. The vacuum The photodiode vacuum photodiode enables bigger enables carrier bigger mobility carrier and mobility allows more and allows more carriers to transport. No electron diffusion problems exist. A detailed calculation shows that, for a photocathode with a conversion factor of 60 mA/W, the current is expected to be ~30 mA biased at 40 V by focusing the 500 mW laser into an area of 0.01 mm2 [2]. This possible required area is far smaller than the testing emission area, which is circular with a diameter of 0.5 mm. When the

Electronics 2020, 9, 1991 9 of 10 carriers to transport. No electron diffusion problems exist. A detailed calculation shows that, for a photocathode with a conversion factor of 60 mA/W, the current is expected to be ~30 mA biased at 40 V by focusing the 500 mW laser into an area of 0.01 mm2 [2]. This possible required area is far smaller than the testing emission area, which is circular with a diameter of 0.5 mm. When the impedance between the photoconductive vacuum tube and the high-gain antenna is matched, the THz radiation is expected to be beyond 20 mW, even if the conversion efficiency is 2%.

4. Conclusions In conclusion, we have developed a high current density alkali antimonide photocathode and experimentally validated the repeatability, stability of the photocathode at different laser powers. The current enhancement effect has inspired us to further improve the current emission density through artificial rough substrate structures. Also, the linear zone of high current density emission can be used for THz photomixing modulation. The experimental evidence has laid a very good foundation for us to further explore the possibility of continuously tunable CW THz sources. The multi-alkali photocathode has the potential to provide high current density, small cross-section electron beams with long lifetimes, and lower cathode temperatures for practical use in THz vacuum devices.

Author Contributions: Writing—Original Draft Preparation, J.D., Writing—Review and editing, J.D., C.R. and X.X., methodology, J.D., X.X., H.L., data curation, J.D., validation, Y.D., and C.R., supervision, C.R., project administration, C.R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 61831001, and General Assembly Pre-Research Fund (6140862010116HK01001). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Dhillon, S.S.; Vitiello, M.S.; Linfield, E.H.; Davies, A.G.; Hoffmann, M.C.; Booske, J.; Paoloni, C.; Gensch, M.; Weightman, P.; Williams, G.P.; et al. The 2017 terahertz science and technology roadmap. J. Phys. D Appl. Phys. 2017, 50, 043001. [CrossRef] 2. Diamant, G.; Halahmi, E.; Kronik, L.; Levy, J.; Naaman, R.; Roulston, J. Integrated circuits based on nanoscale vacuum . Appl. Phys. Lett. 2008, 92, 262903. [CrossRef] 3. Dai, J.; Ruan, C.; Ding, Y.; Zhang, X. High Power Terahertz Source Based on Planar Antenna Integrated Vacuum Photodiode. In Proceedings of the 2019 PhotonIcs & Electromagnetics Research Symposium—Spring (PIERS-Spring), Xiamen, China, 17–20 December 2019; pp. 3064–3067. 4. Levy, J. Electron Emission Device of High Current Density and High Operational Frequency. U.S. Patent No. 8143566, 27 March 2012. 5. Cultrera, L.; Lee, H.; Bazarov, I. Alkali antimonides photocathodes growth using pure metals evaporation from effusion cells. J. Vac. Sci. Technol. Nanotechnol. Microelectron. Mater. Process. Meas. Phenom. 2016, 34, 011202. [CrossRef] 6. Sommer, A.H. Brief History of Photoemissive Materials; Kaufmann, K.J., Ed.; International Society for Optics and Photonics: San Diego, CA, USA, 1993; p. 2. 7. Sommer, A.H. The element of luck in research—Photocathodes 1930 to 1980. J. Vac. Sci. Technol. A Vac. Surf. Film. 1983, 1, 119–124. [CrossRef] 8. Stovall, J. A Review of Radio-Frequency Photocathode Electron Sources. In Proceedings of the 1992 Linear Accelerator Conference (LINAC’92), Ottawa, ON, Canada, 24–28 August 1992; paper TU2-03. pp. 285–289.

9. Erjavec, B. vapour dynamics during Na2KSb (Cs) photocathode growth. Vacuum 1992, 43, 1171–1175. [CrossRef] 10. Townsend, P.D. Photocathodes—Past performance and future potential. Contemp. Phys. 2003, 44, 17–34. [CrossRef] 11. Takahashi, A.; Nishizawa, M.; Inagaki, Y.; Koishi, M.; Kinoshita, K. New femtosecond streak camera with temporal resolution of 180 fs. In Proceedings of the SPIE, San Diego, CA, USA, 11 November 1994; Volume 2116, pp. 275–284. 12. Lee, C. High current density photoemissive electron source. Appl. Phys. Lett. 1984, 44, 565–566. [CrossRef] Electronics 2020, 9, 1991 10 of 10

13. Nishimori, N.; Nagai, R.; Sawamura, M.; Hajima, R. Development of a Multialkali Photocathode Dc Gun for a Smith-Purcell Terahertz Free-Electron Laser. Particles 2018, 1, 12. [CrossRef]

14. Erjavec, B. Alkali vapour pressure variations during Na2KSb(Cs) photocathode synthesis. Vacuum 1994, 45, 617–622. [CrossRef] 15. Sommer, A.H. Stability of Photocathodes. Appl. Opt. 1973, 12, 90. [CrossRef][PubMed] 16. Liu, Z.; Sun, Y.; Pianetta, P. High current density GaN/CsBr heterojunction photocathode with improved photoyield. Appl. Phys. Lett. 2007, 4. [CrossRef] 17. Ding, Z.; Gaowei, M.; Sinsheimer, J.; Xie, J.; Schubert, S.; Padmore, H.; Muller, E.; Smedley, J. In-situ

synchrotron x-ray characterization of K2CsSb photocathode grown by ternary co-evaporation. J. Appl. Phys. 2017, 121, 055305. [CrossRef] 18. Smedley, J.; Attenkofer, K.; Bhandari, H.; Ding, Z.; Frisch, H.J.; Gaowei, M.; Muller, E.M.; Padmore, H.A.; Schubert, S.G.; Sinsheimer, J.; et al. Sputter Growth of Alkali Antimonide Photocathodes: An in Operando Materials Analysis. In Proceedings of the International Conference, Richmond, VA, USA, 3–8 May 2015; pp. 1965–1967. 19. Natarajan, A.; Kalghatgi, A.T.; Bhat, B.M.; Satyam, M. Role of the cesium antimonide layer in the

Na2KSb/Cs3Sb photocathode. J. Appl. Phys. 2001, 90, 6434–6439. [CrossRef] 20. Dai, J.; Ruan, C.; Xu, X.; Liu, H.; Ding, Y.High current density photocathode for CW terahertz photoconductive vacuum devices. Vacuum 2020, 180, 109587. [CrossRef] 21. Wright, A.G. The Photomultiplier Handbook; Oxford University Press: Oxford, UK, 2017; ISBN 0-19-252808-4. 22. Schubert, S.; Wong, J.; Feng, J.; Karkare, S.; Padmore, H.; Ruiz-Osés, M.; Smedley, J.; Muller, E.; Ding, Z.; Gaowei, M.; et al. Bi-alkali antimonide photocathode growth: An X-ray diffraction study. J. Appl. Phys. 2016, 120, 035303. [CrossRef] 23. Dimitrov, D.; Bell, G.; Ben-Zvi, I.; Karkare, S.; Padmore, H.; Smedley, J.; Smithe, D.; Zhou, C. 3D Modeling and Simulations of Electron Emission From Photocathodes With Controlled Rough Surfaces. In Proceedings of the 2nd North American Particle Accelerator Conference, København, Denmark, 14–19 May 2017; p. THPOA42. 24. Wieland, M.J.; Kampherbeek, B.J.; Addessi, P.; Kruit, P. Field emission photocathode array for multibeam electron lithography. Microelectron. Eng. 2001, 57–58, 155–161. [CrossRef] 25. Putnam, W.P.; Hobbs, R.G.; Keathley,P.D.; Berggren, K.K.; Kärtner, F.X. Optical-field-controlled photoemission from plasmonic nanoparticles. Nat. Phys. 2017, 13, 335–339. [CrossRef] 26. Dimitrov, D.; Bell, G.; Ben-Zvi, I.; Feng, J.; Karkare, S.; Padmore, H.; Smedley, J.; Smithe, D.; Veitzer, S. Modeling Cathode Roughness, , and Field Enhancement Effects on Electron Emission. In Proceedings of the 8th International Particle Accelerator Conference, Copenhagen, Denmark, 14–19 May 2017; p. THPAB066. 27. Schubert, S.; Ruiz-Osés, M.; Ben-Zvi, I.; Kamps, T.; Liang, X.; Muller, E.; Müller, K.; Padmore, H.; Rao, T.; Tong, X.; et al. Bi-alkali antimonide photocathodes for high brightness accelerators. APL Mater. 2013, 1, 032119. [CrossRef] 28. Ruiz-Osés, M.; Schubert, S.; Attenkofer, K.; Ben-Zvi, I.; Liang, X.; Muller, E.; Padmore, H.; Rao, T.; Vecchione, T.; Wong, J.; et al. Direct observation of bi-alkali antimonide photocathodes growth via in operando x-ray diffraction studies. APL Mater. 2014, 2, 121101. [CrossRef] 29. Hwang, S.-T.; Gil, B.; Yun, A.J.; Kim, J.; Kim, J.; Woo, H.; Park, K.; Park, B. Highly effective III-V solar cells by controlling the surface roughnesses. Curr. Appl. Phys. 2020, 20, 899–903. [CrossRef]

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