Fitted parameters of the two different t-mode AlGaAs/GaAs photocathode samples.
Aiming to enhance the photoemission capability in the waveband region of interest, a graded bandgap structure was applied to the conventional transmission-mode AlGaAs/GaAs photocathodes based on energy bandgap engineering, wherein the composition in AlxGa1−xAs window layer and the doping concentration in GaAs active layer were gradual. According to Spicer’s three-step model, a photoemission theoretical model applicable to the novel transmission-mode AlxGa1−xAs/GaAs photocathodes was deduced so as to guide the cathode structural design. Then the cathode material was grown by the metalorganic chemical vapor deposition technique, and the epitaxial cathode material quality was evaluated by the means of scanning electron microscope, electrochemical capacitance-voltage, X-ray diffraction and spectrophotometry. Through a series of specific processes, the cathode material was made into the multilayered module, possessing a glass/Si3N4/AlxGa1−xAs/GaAs structure. After the surface treatment including heat cleaning and Cs▬O activation for the cathode module, the image intensifier tube comprising the activated cathode module, microchannel plate, and phosphor screen was fabricated by indium sealing. The spectral response test results confirm the validity of the novel structure for the enhancement of blue-green photoresponse.
- AlGaAs/GaAs photocathode
- graded bandgap
- photoemission model
- material epitaxy
- tube fabrication
Since negative-electron-affinity (NEA) GaAs photocathode was proposed as a type of excellent photoemitter by Scheer and Laar , GaAs-based photocathodes have found widespread applications in photodetectors, accelerators, electron microscopes, photon-enhanced thermionic emission devices, and other fields [2, 3, 4, 5]. In view of the high visible spectral response, good spectral extensibility to the near infrared (NIR) region and low dark current, NEA GaAs, GaAsP, and InGaAs photocathodes are important components in the vacuum photodetectors, for example, low-light-level (LLL) image intensifiers, photomultiplier tubes, and streak tubes . In the modern light sources based on free electron lasers or energy recovery linacs, GaAs-based photocathodes serve as high brightness electron sources with the unique virtues of large current density driven by visible lasers, high spin polarization, low thermal emittance, and narrow energy distribution . In recent years, a spin-polarized transmission electron microscope combining electron microscopy and accelerator technology using GaAs-GaAsP strained superlattice photocathodes was developed to observe dynamically a magnetic field images with high spatial and temporal resolutions . Moreover, with the aid of the ultrahigh speed pulse laser, GaAs photocathodes can satisfy the requirements of fast response speed and large emission current density aiming to THz frequency vacuum devices .
As is well known, GaAs photocathodes can operate in the transmission-mode (t-mode) and the reflection-mode (r-mode), respectively, depending on the difference in the direction of the incident light [10, 11], as shown in Figure 1. For the t-mode operation, the incident light is irradiated on the substrate surface, and the photoelectrons are extracted from the opposite surface side, whereas for the r-mode operation, the incident light and photoelectrons are located on the same emission surface side. Due to the difference in absorption length of longwave and shortwave photons, the shapes of spectral response curves for GaAs photocathodes working in the two modes are different . Differing from r-mode GaAs photocathodes, t-mode ones are difficult to achieve high spectral response in a broadband region from ultraviolet to NIR spectrum. Usually in the practical applications, the researches on t-mode photocathodes are more concerned. For example, the image intensifiers and related imaging systems, t-mode photocathodes conform to the optical imaging structure . Besides, as polarized electron sources in photoinjector apparatus, t-mode photocathodes are more popular than r-mode ones, because the laser spot size can be reduced through the short focus lens placed on the photocathode backside, which would not hinder the path of the electron beam and is more conducive to achieve a super-high-brightness electron beam [13, 14].
As proposed by Spicer and Herreragomez , the photoemission process from photocathodes consists of electron excitation by incident light absorption, electron transport toward surface, and electron escape across the surface barrier into vacuum. For t-mode photocathodes, some important cathode parameters such as electron diffusion length, interface recombination velocity, and surface escape probability are crucial to the photoemission performance, especially the shortwave photosensitivity . Enhancing the blue-green response of t-mode GaAs photocathodes as far as possible, would not only be beneficial to the detection in sandy or desert terrain for image intensifiers , but also increase the current density driven by 532 nm laser for electron sources . Although the external electric field biased across the photocathodes can improve the photoemission capability, the limitations of this approach are the difficulty in making thin electrode pattern and the increased dark current with the strong field [19, 20]. In view of this adverse case, internal built-in electric fields through energy band engineering design could be an alternative approach. In our research, a complex structure composed of the composition-graded structure and the doping-graded structure is proposed to prepare high efficient t-mode AlGaAs/GaAs photocathodes. Furthermore, the photoemission model, cathode structure design, cathode material epitaxy, and vacuum tube fabrication are investigated through the integrated analysis of theory and experiments. Finally, the effectivity of the designed novel structure is verified by comparison with the common photocathodes.
2. Graded bandgap structure
For the t-mode GaAs photocathode, the AlGaAs and GaAs materials are usually used as the window layer and the active layer, which determine the shortwave cutoff and longwave cutoff, respectively. A built-in electric field in the interior of the photocathode material can be realized by the variation of dopant or composition according to energy band engineering design [21, 22]. Based on this concept, a novel structure is proposed to improve photoelectron emission capability, wherein a composition-graded structure and a doping-graded structure are employed to the AlxGa1−xAs window layer and GaAs active layer, respectively [23, 24], as shown in Figure 2. To form a built-in constant electrical field in the GaAs active layer of the photocathode, the p-type dopant concentration can follow the exponential variation, and the doping formula is expressed by 
In the AlxGa1−xAs window layer, the bandgap is decreased from the substrate interface to the GaAs interface due to the composition-graded structure. Because of the high p-type doping concentration, the valence bands of the AlxGa1−xAs/GaAs heterojunction are aligned, as shown in Figure 2. The graded Al composition in the window layer results in a built-in electric field
3. Photoemission model derivation and simulation
3.1. Photoemission model derivation
As is well known, the one-dimensional continuity equation can afford a useful avenue to establish the photoemission model of t-mode or r-mode III–V group photocathodes, which takes account of the spatial photon adsorption, spatial carrier distribution, and interface electron recombination [10, 11]. As shown in Figure 2, the photoelectrons generated in the AlxGa1−xAs layer are able to move into the GaAs layer and contribute to the total emitted electrons. For the composition-graded AlxGa1−xAs layer, some physical properties, for example, electron mobility (
Because of the aforesaid variable physical properties regarding to Al composition, the continuity equation of electron transport in the AlxGa1−xAs window layer is quite complex. For simplicity, the AlxGa1−xAs layer is treated to be of a series of sublayers with different Al compositions. As shown in Figure 2, the AlxGa1−xAs window layer can be considered to be of
In Eqs. (8) and (9),
As for the GaAs active layer, the excess electrons consist of electrons contributed from the AlxGa1−xAs window layer and electrons generated in the GaAs active layer. Under the second-stage built-in electric field, the photoelectron transport process in GaAs active layer follows the one-dimensional continuity equation as described by
By solving Eq. (13) via the boundary conditions Eq. (14) and the electron concentration
Meanwhile, it is noted that the quantum efficiency has a close relation with the reflectivity
In Eqs. (16)–(18),
3.2. Quantum efficiency simulation
As to the t-mode AlGaAs/GaAs photocathodes, the optical properties between the g-composition and u-composition structures should be different. For simplified calculation, the composition-graded AlxGa1−xAs window layer is assumed to be of five sublayers with the fixed Al composition in each sublayer. The five Al composition values are assumed to be 0.9, 0.675, 0.45, 0.225, and 0, respectively, distributed from the AlGaAs/Si3N4 interface to AlGaAs/GaAs interface. For the u-composition AlGaAs/GaAs photocathode, the Al composition in the AlGaAs window layer is assumed to be 0.7. The optical properties including the reflectivity
By using the deduced quantum efficiency models which take into account the reflectivity varying with the wavelength, the quantum efficiency curves of the t-mode AlxGa1−xAs/GaAs photocathode with those unique graded bandgap structures are simulated, wherein the window layer is of the g- or u-composition structure, and the active layer is of the e- or u-doping structure, respectively. Figure 5 exhibits the superiority of the AlxGa1−xAs/GaAs photocathode with g-composition window layer and e-doping active layer. In Figure 5, some structural parameters such as the Al composition in the u-composition window layer, the Al composition distribution in each sublayer of AlxGa1−xAs window layer, and the thicknesses of Si3N4, AlGaAs and GaAs layers are identical to those in Figure 4. In the GaAs active layer, the doping concentration for e-doping structure is exponentially varied from 1 × 1019 to 1 × 1018 cm−3, and that for the u-doping structure is 1 × 1019 cm−3. In addition, the surface electron escape probability
It is seen clearly from Figure 5 that the t-mode g-composition and e-doping photocathode can obtain the highest quantum efficiency in the spectrum region from 400 to 900 nm in contrast to other photocathodes. The quantum efficiency in the shortwave region, that is, blue-green region are enhanced greatly for the two former photocathodes with the g-composition structure. In the g-composition AlxGa1−xAs window layer, the photoelectrons excitated by shortwave light would be promoted toward the GaAs active layer under the g-composition induced electric field. Then, these shortwave photoelectrons are successively boosted toward the emission surface under the built-in electric field formed by the e-doping structure. As shown in Figure 5, the e-doping structure for the g-composition AlxGa1−xAs/GaAs photocathodes can slightly enhance the quantum efficiency, which is not like the case for the u-composition AlGaAs/GaAs photocathodes. The possible reason is that the g-composition AlxGa1−xAs layer can also absorb some extra longwave photons, which are originally absorbed by the GaAs active layer. In other words, more enough absorption space for longwave photons can be provided by the g-composition structure. While for the u-composition AlGaAs/GaAs photocathodes, the case is different. The GaAs active layer just absorbs the longwave photons, and the transport efficiency for these generated photoelectrons can just be improved by the doping-induced electric field.
To guide the structural design of t-mode graded bandgap AlxGa1−xAs/GaAs photocathode, the changes of quantum efficiency with the active layer thickness and the window layer thickness are analyzed, as shown in Figure 6. Figure 6(a) shows the changes of quantum efficiency curves with the active layer thickness
Considering that the built-in electric field in the window layer is inversely proportional to window layer thickness, the effect of the window layer thickness on quantum efficiency in the shortwave region, especially in the blue-green waveband for g-composition photocathodes, is more pronounced than that for the u-composition ones. Figure 6(b) shows the quantum efficiency changing with the window layer thickness
4. Epitaxial growth and quality characterization
4.1. Epitaxial growth of photocathode materials
In modern epitaxial growth techniques, the metalorganic chemical vapor deposition (MOCVD) technique is suitable for growing the complex ultrathin multilayer materials with the composition-graded or doping-graded structures. To confirm the actual effect of the g-composition and e-doping structure on the quantum efficiency of t-mode AlGaAs/GaAs photocathodes, the 2-inch-diameter AlxGa1−xAs/GaAs epilayers with two different structures were grown on the low-defect n-type GaAs (100) substrates in the horizontal low-pressure MOCVD reactor from AIXTRON. As shown in Figure 7(a), the multiple epitaxial layers consist of four AlGaAs/GaAs heterostructures, which follow the “inverted structure” technology [32, 33]. In Figure 7(a), the AlGaAs stop layer serves as an etching-resistance layer, and the GaAs cap layer serves as an oxidation-blocking layer. The detailed structures of the two types of cathode materials are shown in Figure 7(b) and (c). The difference between the two samples is the structure of window layer, wherein one is of g-composition AlxGa1−xAs layer, and the other is of u-composition Al0.7Ga0.3As layer. Note that, as a result of the current epitaxial limitation, the GaAs active layer exhibits a quasi-exponential doping structure with the p-type dopant concentration varying from 1 × 1019 to 1 × 1018 cm−3.
During the epitaxial growth process of the multiple layers, the group III sources are the trimethylgallium (TMGa) and trimethylaluminum (TMAl), the group V source was the AsH3, the dopant source was the diethylzinc (DEZn), and the carrier gas was the H2 gas. Additionally, the growth process was monitored in situ using the LayTech EpiRAS-200 spectrometer. The parameters of the epitaxial growth process are as follows: the growth rate was about 2.5 μm/h, the V/III flux ratio was adjusted at 10–15, the Al composition was controlled by the flow ratio of TMGa to TMAl, and the growth temperature was set as 680°C and 710°C for GaAs and AlxGa1−xAs, respectively.
4.2. Quality characterization of photocathode materials
To understand the profile structure of the multilayered photocathode samples, the cross-sectional photographs of the multilayered structure for the two cathode material samples were measured by the scanning electron microscope (SEM) from Hitachi. It is clearly seen from Figure 8 that differing from the case for u-composition sample, no sharp borderline exists at the interface of the AlxGa1−xAs window layer and GaAs active layer for the g-composition sample. This seamless interface would greatly reduce the interface electron recombination. It is noted that many cracks in Figure 8(b) are caused by the inappropriate cleavage, which cannot reflect the true quality of the epitaxy. From the SEM photographs, it is judged that the vertically multilayered constructions of the epitaxial cathode materials agree well with the structural design.
The depth distribution of carrier concentration in the multilayered p-type AlGaAs/GaAs materials was measured by the electrochemical capacitance-voltage (ECV) system from Bio-Rad. As shown in Figure 9, a series of sublayers forming the graded doping structure can be realized by the MOCVD technique. The carrier concentration of no more than 8 × 1018 cm−3 in the GaAs active layer shows a gradient distribution. For the AlxGa1−xAs window layer in Figure 9(a), the carrier concentration decreases with the increase in Al composition, which exactly reflects the composition-graded structure.
To investigate the crystalline quality of the epitaxial cathode materials, the X-ray diffraction (XRD) curves were measured by the X’Pert Pro MRD system. As shown in Figure 10, the rightmost peak represents the GaAs material, which is the superposition of the diffraction peaks of the GaAs cap layer, active layer, and substrate. The only one diffraction peak indicates that the crystalline perfection of the GaAs epilayers is consistent with the GaAs substrate. The left two diffraction peaks for the u-composition sample represent the AlGaAs window layer and stop layer, respectively. In the g-composition sample, there is no diffraction peak denoting the window layer, and a series of diffraction peaks exist nearby the peak of the GaAs layer, which are caused by the g-composition AlxGa1−xAs epilayer. The slightly narrower full width at half maximum of the GaAs diffraction peak indicates that the GaAs active layer in the g-composition sample has a better crystalline quality.
5. Device fabrication and spectral response
5.1. Transmission-mode cathode module fabrication
Following the recipe of fabricating glass-sealed t-mode AlGaAs/GaAs photocathodes [32, 33], the epitaxial cathode materials cutted from the 2-inch-diameter epitaxial wafer were fabricated into the multilayered t-mode cathode module. The schematic process flow for fabricating t-mode AlGaAs/GaAs photocathode modules is shown in Figure 11. First, the GaAs cap layer was removed by chemical etching to expose the AlGaAs window layer, and by plasma enhanced chemical vapor deposition (PECVD), a thin antireflective layer of 100 nm-thick Si3N4 was deposited on the exposed window layer surface. Then, the 7056 glass, serving as the incident window and support layer, was bonded on the Si3N4 antireflection layer by thermocompression. Following that, through selective etching process, the GaAs substrate and AlGaAs stop layer were etched away to expose the GaAs active layer to prepare the NEA surface . Finally, the Cr-Ni ring electrode applied to bias on the cathode was prepared by the physical vapor deposition (PVD), such as magnetron sputtering method. After these processing steps, the multilayered cathode module with a glass/Si3N4/AlGaAs/GaAs structure was finished. In addition, to eliminate etching-induced damage at the active layer surface, the polishing treatment was implemented, which slightly decreased the thickness of the GaAs active layer.
The optical property curves of the t-mode cathode modules with two different structures were measured by utilizing the Shimadzu UV-3600 spectrophotometer, which possesses three detectors working from ultraviolet to NIR waveband. The optical properties were measured based on the double optical path method, and light was incident on the surface of glass faceplate in a normal direction. Figure 12 shows the experimental reflectivity and transmissivity curves of the two different multilayered module samples. It is found that, just as the simulated results in Figure 4, the reflectivity curve in the region of 400–800 nm for g-composition structure is relatively smoother than that for u-composition structure. In other words, the smooth reflectivity curve verifies the composition-graded structure in the AlxGa1−xAs window layer from another aspect. Thereby, the characterization results regarding the cross-sectional photographs, carrier concentration distributions, X-ray diffraction peaks, and optical properties all reflect the special design structure.
5.2. Activation of photocathode surface
Prior to activation, the 18-mm-diameter cathode modules experienced the chemical cleaning and vacuum annealing to obtain an atomic level clean surface. The heat treatment with a suitable temperature under ultrahigh vacuum (UHV) condition is particularly important for the activation, and the quadrupole mass spectrometer (QMS) was adopted to monitor the change of residual gas components during the programmed temperature rose and fell. Figure 13 shows the changes of mainly concerned residual gas components for the two t-mode cathode module samples. Through detecting the gas presence of the QMS traces at
After the sample cooled to room temperature, the Cs▬O activation to form the NEA state at the cathode surface was performed in the UHV chamber with a base pressure of 10−9 Pa. The Cs and O sources used in the activation are solid dispensers easily controlled by direct current, and the flux is proportional to the operating current . During the activation, the Cs source was on all the time, and the O source was switched on and off . The operating current of Cs and O dispensers was regulated by program control current supply, and the photocurrent induced by a white light source was monitored in real time by the computer-controlled test system . The initial Cs supply caused the gradual increase of the photocurrent. With the continuous Cs flux, when the photocurrent dropped to 80% of its peak, the O source was open. In subsequent alternate activation cycles, the O source was closed when the photocurrent reached its peak and was open again when the photocurrent dropped to 80% of the peak. The operating current ratio of Cs source to O source for both samples was regulated as the same 1.65/1.8. Until the photocurrent peak no longer increased, the O source and Cs source were closed successively, and the activation process was finished. To further improve the photoemission performance, the second heat treatment with a lower temperature was employed to the samples . After that, the samples were activated again using the same co-deposition activation. As seen from Figure 14, the second activation can dramatically enhance the final cathode performance. Meanwhile, the final photocurrent peaks of the two samples are approximately the same.
5.3. Tube package and spectral response test
After the two-step Cs▬O activation process, the cathode module in the UHV activation chamber was transferred to the UHV seal vacuum chamber and indium sealed into an image intensifier tube, wherein the t-mode AlGaAs/GaAs cathode module was equipped in association with the filmed microchannel plate (MCP), phosphor screen, output window, ceramics, and Kovar sealing parts . The schematic structure and the photograph of the LLL proximity focused image intensifier are shown in Figure 15. As shown in Figure 15(a), the proximity focused image intensifier is capable of enhancing a LLL image from several thousands to tens of thousands of times. The input LLL image is converted into photoelectrons by the AlGaAs/GaAs photocathode, and then the number of photoelectrons is multiplied several thousands of times by the MCP coated with a thin ion barrier film which can prevent ion feedback. Lastly, the multiplied photoelectrons bombard the phosphor screen and are converted into photons. Thus, the input LLL image is intensified and appears as the output image on the phosphor screen. In addition to the function of direct eye observation, the LLL image intensifier can be coupled with CCD/CMOS array by the fiber optic taper to realize video output and remote monitoring [38, 39].
The sealed image intensifiers were extracted from the seal vacuum chamber into ambient air, and the spectral response curves were measured by the spectral response testing instrument . Through the spectral response values corresponding to the wavelength, the quantum efficiency values corresponding to the wavelength for the two different cathode samples were obtained . In the spectral region of 600–750 nm, the quantum efficiency exceeds 40%. As shown in Figure 16, it is found that in contrast to the u-composition structure, the g-composition structure is especially useful to the enhancement of shortwave quantum efficiency, which conforms to the original intention of our design concept. By fitting the experimental optical property and quantum efficiency data based on the theoretical photoemission model, the internal cathode parameters difficult to be measured directly can be obtained. The thickness values of each layer calculated by fitting the experimental reflectivity and transmittivity curves are listed in Table 1. It is seen that the Al composition in the g-composition window layer is not distributed uniformly, and the sublayers with low Al composition are relatively thinner compared to those with high Al composition. For the two samples, the thicknesses of the GaAs active layer are smaller than the design values, which are caused by the polishing treatment after the fabrication of cathode modules.
|Cathode sample||Al composition in each AlGaAs sublayer|
By means of fitting the experimental quantum efficiency curves, we can obtain some performance parameters, for example, interface recombination velocity
In this chapter, we have carried out systematically theoretical and experimental researches on t-mode AlGaAs/GaAs photocathodes, with regard to bandgap structure design, photoemission model derivation, epitaxial growth, surface activation, device fabrication, and performance evaluation. Compared with the common t-mode AlGaAs/GaAs photocathode, the graded bandgap t-mode AlxGa1−xAs/GaAs photocathode with a g-composition and e-doping structure can achieve higher quantum efficiency in the shortwave response region, particularly the blue-green spectral region of interest. In addition, this g-composition structure is helpful to mitigate the interface recombination and enhance the absorption of the longwave light, which leads to the enhanced photoemission capability. This work has reference significance for the design of other graded bandgap III-V group photocathodes.
This work was supported by the National Natural Science Foundation of China (grant nos. 61771245 and 61301023) and Science and Technology on Low-Light-Level Night Vision Laboratory Foundation of China (grant no. J20150702). The authors would like to thank Dr. Feng Cheng for her efforts in the theoretical photoemission model, and the staff from Science and Technology on Low-Light-Level Night Vision Laboratory for their assistance in the fabrication of t-mode cathode modules and image intensifiers.