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Differences in stability and repeatability between GaAs and GaAlAs photocathodes
Optics Communications 380 (2016) 320–325
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Differences in stability and repeatability between GaAs and GaAlAs photocathodes Yuan Xu a,b, Yijun Zhang a,n, Cheng Feng a, Feng Shi c, Jijun Zou d, Xinlong Chen a, Benkang Chang a a
School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Department of Electronic and Electrical Engineering, Nanyang Institute of Technology, Nanyang 473004, China c Science and Technology on Low-Light-Level Night Vision Laboratory, Xi'an 710065, China d Engineering Research Center of Nuclear Technology Application (East China Institute of Technology), Ministry of Education, Nanchang 330013, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 29 May 2016 Accepted 13 June 2016
For the applications in vacuum photodetectors and photoinjectors, a crucial limiting factor for conventional GaAs photocathodes is the limited lifetime, depending on the Cs–O activation layer vulnerable to the harmful residual gases. In order to develop a type of GaAs-based photocathode with good stability and repeatability, Cs/O activation and multiple recesiation experiments under the same preparation condition were performed on reflection-mode exponential-doped GaAs and GaAlAs photocathodes grown by metalorganic vapor phase epitaxy, and quantum efficiency and photocurrent decay were measured after activation and recesiation. The experimental results show that the photoemission characteristics on cathode degradation and repeatability are different between GaAs and GaAlAs photocathodes. In an unsatisfactory vacuum system, the operational lifetime for GaAlAs photocathode is nearly twice longer than that for GaAs photocathode after Cs/O activation under a high intensity illumination. After multiple recesiations, the quantum efficiency and operational lifetime for GaAlAs photocathode remain nearly unchanged, while those for GaAs photocathode become lower and lower with the increase of recesiation cycles, which reflects the superiority in stability and repeatability for GaAlAs photocathode in contrast to GaAs photocathode operating in the poor vacuum environment. & 2016 Elsevier B.V. All rights reserved.
Keywords: GaAs photocathode GaAlAs photocathode Degradation Recesiation Quantum efficiency
1. Introduction In the last decades, negative-electron-affinity (NEA) photocathodes have played a critical role in the development of photocathode technology. Because of the high quantum efficiency, prompt response time, low thermal emittance and narrow energy spread, GaAs and GaAs-based photocathodes have found important applications in the fields of vacuum detectors for photoelectric imaging detection and polarized electron sources for high energy physics research [1–4]. For underwater photodetection, GaAs-based photocathodes with favorable blue–green response are the key underpinning technology in vacuum detectors [5]. In the application of particle accelerators, high-power 532 nm lasers are commonly used to irradiate photocathodes to produce high brightness electron beams [6]. In view of these requirements, GaAlAs photocathodes could be promising candidates, with the ease of epitaxial growth by matching with GaAs lattice on one n
Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.optcom.2016.06.034 0030-4018/& 2016 Elsevier B.V. All rights reserved.
hand, and the tunability of response threshold by varying aluminum composition on the other hand. For instance, unlike broadband GaAs photocathodes, GaAlAs photocathodes operating in the transmission-mode can obtain a narrowband spectrum without any narrowband filters [7,8]. NEA photocathodes are usually prepared through the adsorption of low-work-function species on the p-type semiconductor following the specific procedure under the ultra-high vacuum (UHV) condition. Currently, an important practical limitation of NEA photocathodes of concern is the limited lifetime of the NEA surface which requires that the NEA preparation is repeated [9– 11]. Despite some similarities in the activation process of GaAs and GaAlAs photocathodes [12,13], there are few reports on comparison of the difference in photoemission degradation between them under the same preparation conditions. From the experimental point of view, many of the photocathode performance usually depends on the experimental apparatus and crafts. In this paper, reflection-mode (r-mode) NEA GaAs and GaAlAs photocathodes grown by the metalorganic vapor phase epitaxy (MOVPE) technique are prepared. Meanwhile, the differences in stability and
Y. Xu et al. / Optics Communications 380 (2016) 320–325
321
Fig. 1. Schematic structures of the r-mode GaAs and GaAlAs cathode samples.
repeatability between GaAs and GaAlAs photocathodes are investigated by comparison of photocurrent decay and quantum efficiency variation.
2. Experimental The 2-inch-diameter r-mode GaAs and GaAlAs photocathode samples were grown on the Si-doped (100)-oriented GaAs substrate in the MOVPE reactor with heavily p-type Zn doping, and the epitaxial structures are shown in Fig. 1. The epitaxial layers for both GaAs and GaAlAs samples are of p–p type heterostructure, including a uniform-doped 0.5 μm-thick buffer layer and an exponential-doped 1.6 μm-thick active layer. In the buffer layer, the dopant concentration was 1 1019 cm 3, while in the active layer, the dopant concentration was distributed quasi-exponentially from 1 1019 to 1 1018 cm 3, which would help to improve photoemission capability [14]. Besides, the buffer layer of the two samples not only improves the crystal quality of active-layer but also reflects the back-diffusion photoelectrons to the surface [15]. In our experiments, the cut GaAs and GaAlAs samples of 10 10 mm square were ready for activation respectively. Prior to activation, the photocathode samples underwent a two-step surface cleaning process to achieve the atomically clean surfaces. First, the GaAs and GaAlAs samples were chemically cleaned in a solution of 4:1:100 H2SO4 :H2O2:H2 O for 2 min [16]. After the chemical treatment, the samples were mounted on the holders, and transferred into the preparation chamber with a base pressure of 10 7 Pa to perform the heat cleaning at 650 °C for 20 min. After cooling to the room temperature, the activation process for the cleaned samples was started using a Cs/O codeposition technique [16,17], wherein the cesium and oxygen flux was adjusted by the heating current through the dispensers. In the activation process, the Cs/O flux ratio employed to GaAs and GaAlAs samples was adjusted to 1.67/1.6, which corresponds to the ratio of dispenser heating current. A halogen tungsten lamp and a bias voltage of 200 V were applied to the photocathodes to collect the photo–electron emission, and the distance between the cathode and anode was about 5 mm. Until the peak photocurrent reached to its maximum, the Cs/O activation was terminated successively, and then the photocurrent decay was measured for the activated GaAs and GaAlAs samples under intensive illumination of 100 lx white light. After the first degradation, the GaAs and GaAlAs samples were recesiated to restore the photoemission capability by introducing the fresh Cs flux, and then the quantum efficiency curves of the recesiated samples were measured in situ immediately. After that,
the photocurrent decay was measured under the same 100 lx illumination. The recesiation on the degraded cathode samples were performed for a total of three times. After every recesiation, the quantum efficiency curves and photocurrent decay were immediately measured in situ. In all the processes of photocathode degradation, the preparation chamber was maintained at a relatively poor vacuum level of around 1 10 7 Pa, and the main residual gases in the chamber were H2, H2O, CO and CO2.
3. Results and discussion 3.1. Degradation after Cs/O activation For GaAs and GaAlAs samples activated by Cs/O codeposition, the typical photocurrent changes with time during the Cs/O activation process are shown in Fig. 2. In the first Cs deposition process, the photocurrent growth rate for GaAlAs sample is less than that for GaAs sample. In the next process of Cs/O co-deposition, the photocurrent for both samples increases substantially with the continuous alternation cycles. It is noted that there are more Cs/O alternation cycles and higher peak photocurrent for GaAs sample. The ultimate peak photocurrent of GaAs sample was more than twelvefold higher than that of GaAlAs sample. Because of its larger band gap, GaAlAs sample would absorb insufficient white light, especially the visible and near-infrared light. Besides, the surface
Fig. 2. Photocurrent curves of GaAs and GaAlAs photocathode samples during the Cs/O alternate activation process.
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Y. Xu et al. / Optics Communications 380 (2016) 320–325
Fig. 3. Photocurrent decay of GaAs and GaAlAs cathode samples after Cs/O activation under illumination of 100 lx.
electron escape probability decreases with the high Al content for GaAlAs photocathode [12]. The normalized photocurrent decay curves after Cs/O activation under 100 lx illumination are shown in Fig. 3. The lifetime of NEA photocathodes is determined by time when photocurrent is 1/e of the initial value. The measured 1/e lifetime results after activation for GaAs and GaAlAs samples are listed in Table 1. Obviously, GaAlAs sample with a lower peak photocurrent is more durable and can achieve a lifetime that is about twice longer than that of GaAs sample. For the activated NEA photocathodes, the rapid degradation could be caused by the desorption of Cs with the rising temperature under the high intensity illumination on one hand, and the adsorption of residual gas in the unsatisfactory vacuum environment on the other hand. Fig. 4. Experimental quantum efficiency curves of the original and recesiated (a) GaAs and (b) GaAlAs cathode samples.
3.2. Degradation after multiple recesiations Applying the fresh Cs flux affords an effective way to restore the photoemission capability after degradation [10,11]. Accordingly, three recesiations after degradation were performed on GaAs and GaAlAs samples respectively to verify the repeatability. After the first recesiation, the photocurrent was restored to approximately 90% of its initial value for GaAs sample and 60% for GaAlAs sample. With the increase of the number of recesiations, the recovery of photocurrent for GaAs sample became worse. After the third recesiation, the photocurrent was only restored to about 70% of its initial value, while that for GaAlAs sample was restored to 50% of its initial value. The measured quantum efficiency curves of original and recesiated GaAs and GaAlAs samples are shown in Fig. 4. Because of the different band gap of the emission layer, the cutoff wavelength for GaAlAs sample is limited to the blue–green response region. It is clear to see that the reproducibility of quantum efficiency through multiple recesiations between GaAs and GaAlAs samples is completely different, wherein the quantum efficiency over the entire Table 1 Photocurrent decay lifetime of GaAs and GaAlAs samples after Cs/O activation. Cathode type
Initial photocurrent/μA
1/e lifetime/min
GaAs GaAlAs
46.8 3.6
45 93
waveband for GaAs sample dropped gradually after every recesiation. As for GaAlAs sample, the quantum efficiency over the entire waveband after the first recesiation is lower than that after the original Cs/O activation, while after the subsequent recesiation, the quantum efficiency curves remain substantially unchanged, which demonstrates that GaAlAs sample can obtain a better repeatability than the conventional GaAs sample under the same UHV condition. Meanwhile, an operational lifetime as long as possible after every recesiation should be especially important to save the time consumption and manpower for photocathodes operating in the demountable vacuum system. The photocurrent decay under intensive illumination of 100 lx after every recesiation was measured, as shown in Fig. 5. The 1/e lifetime results after activation for GaAs and GaAlAs samples are listed in Table 2. For both samples after the first recesiation, the photocurrent becomes lower and the lifetime is decreased nearly by half. However, after the second and third recesiation, the lifetime of GaAlAs sample is almost the same as that after the first recesiation, while the case for GaAs sample is different. After the succeeding recesiation, the lifetime of GaAs sample becomes shorter and shorter. It is found that whether after Cs/O activation or after subsequent recesiation, the operational lifetime of GaAlAs sample could be more than two times that of GaAs sample, which shows that GaAlAs photocathode could be more robust when operating in the demountable vacuum system through multiple recesiations.
Y. Xu et al. / Optics Communications 380 (2016) 320–325
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P0 is related to the surface cleanliness and profile of surface barrier. Generally, the three parameters characterizing cathode performance can be obtained by fitting experimental quantum yield curves based on the quantum efficiency formula [19]. According to the r-mode exponential-doped quantum yield formula [20], we can simulate the theoretical quantum efficiency curves consistent with experimental curves for GaAs and GaAlAs samples after original activation and multiple recesiations, as shown in Fig. 6. The obtained values of cathode performance parameters are listed in Table 3. It is seen from Table 3 that the due to the invariable material intrinsic property, the LD and Sv remain unchanged for the recesiated photocathodes. By comparison of P0 between original and recesiated samples, it is found that with the increase of recesiation times, P0 becomes smaller and smaller for GaAs sample, while the case is different for GaAlAs sample. Although the values of P0 of the recesiated GaAlAs sample are smaller than those of the activated one, there are no remarkable changes in P0 with the increase of recesiation times. Since the escape of photoexcited electrons reaching the surface
Fig. 5. Photocurrent decay of (a) GaAs and (b) GaAlAs cathode samples after multiple recesiations under illumination of 100 lx.
3.3. Evaluation of photocathode performance As is well known, the photoemission process from NEA photocathodes is in terms of three successive steps—photoelectron excitation, electron diffusion, and travel across the surface [18]. For various photocathodes in practice, the quantum efficiency is distinctly a very important parameter utilized to evaluate the cathode performance. Some parameters, e.g. electron diffusion length LD, back interface recombination velocity Sv, and surface electron escape probability P0 would significantly affect the quantum efficiency [19]. The LD and Sv are usually related to the crystal quality, while the Table 2 Photocurrent decay lifetime of GaAs and GaAlAs samples after multiple recesiations. Cathode type
Recesiation cycles
Initial photocurrent/μA
1/e lifetime/min
GaAs
1st 2nd 3rd
43.2 37.4 32.7
25 20 14
GaAlAs
1st 2nd 3rd
2.1 1.9 1.8
44 40 38
Fig. 6. Experimental (solid lines) and theoretical (dashed lines) quantum efficiency curves of GaAs and GaAlAs samples. Curves 1, 2, 3, and 4 are corresponding to the quantum efficiency curve after original Cs/O activation, the first, second, and third recesiation, respectively.
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Table 3 Simulated performance parameters for original and recesiated GaAs and GaAlAs samples. 1
Cathode type
Curve No.
LD/μm
Sv/cm s
GaAs
1 2 3 4
3.0 3.0 3.0 3.0
106 106 106 106
0.45 0.38 0.28 0.22
GaAlAs
1 2 3 4
1.6 1.6 1.6 1.6
106 106 106 106
0.40 0.30 0.28 0.28
P0
is closely related to the surface barrier, we should investigate the variation of the surface barrier profile. As proved by a series of activation experiments, the NEA formation mechanism for the GaAs-based photocathodes could be explained by the double dipole model [9,21–24]. The proposed surface barrier profile consists of barriers I and II, arising from the Cs–O activation layer, as shown in Fig. 7. It is clear to see that the traversing resistance for photoelectrons reaching the surface would rely on the surface barriers. As we know, the lifetime for the activated NEA photocathodes would be attenuated with the increase of the illumination intensity [20], and the heating as a result from the high intensity illumination would sublimate some of Cs from NEA surface and change the original Cs/O ratio [6]. More importantly, the residual gases such as CO2, H2O, CO, and so on in the UHV chamber would adsorb on the cathode surface. Once the oxygen from the O-containing gases is attached to the surface, it would change the chemistry of the surface Cs–O activation layer on one hand, and form substrate oxides on the other hand. Consequently, the dipoles in the activation layer region for the degraded photocathode are weaken, which results in the overall increase of the height and width of the surface barriers, which is displayed by the dashed line in Fig. 7. During the recesiation process, the vacuum level is lowered once more and the barrier II can be recovered after applying
supplementary Cs atoms onto the degraded surface, as displayed by the dotted line in Fig. 7. Whereas, the recesiation process cannot remove the inner formed substrate oxides, and thus the barrier I can hardly be restored [10]. In other words, the quantum efficiency of the recesiated photocathode cannot recover its initial value just after Cs/O activation. Compared with GaAs photocathode, GaAlAs photocathode achieves a lower recovery of quantum efficiency after the first recesiation, because that there exist additional stable Al–O bonds on GaAlAs surface by adsorption of residual O-containing gases, and the Al–O bonds are difficult to be broken by applying supplementary Cs. After the succeeding recesiation processes, GaAlAs photocathode can obtain relatively consistent quantum efficiency and lifetime, which demonstrates a good repeatability for GaAlAs photocathode. For GaAs photocathode, the performance in terms of quantum efficiency and lifetime becomes worse and worse as the recesiation times increases. By comparison of degradation and recesiation results between GaAs and GaAlAs photocathodes, it is inferred that the photoemission capability of GaAlAs photocathode is not so sensitive to Cs/O ratio. As a result of the harmful adsorption of gas molecules containing O combined with partial Cs loss under the high intensity illumination, the original chemistry of the Cs–O activation layer would be changed. As for GaAs photocathode, it is well known that the O flux has to be carefully matched to the Cs flux, and a slight deviation from the optimal Cs/O ratio will cause a large drop of quantum efficiency [16]. Therefore, GaAlAs photocathode can obtain better repeatability and longer lifetime under unfavorable vacuum environment in contrast to GaAs photocathode.
4. Conclusions In summary, we have investigated the differences in stability and repeatability between GaAs and GaAlAs photocathodes under the same preparation condition. Both MOVPE-grown samples underwent the same surface cleaning and Cs/O activation process. Moreover, multiple recesiation experiments were performed on the two cathode samples, and quantum efficiency and photocurrent decay were measured after activation and recesiation. The results show that GaAlAs photocathode with NEA surface state under high intensity illumination can obtain a longer operational lifetime than GaAs photocathode after whether Cs/O activation or multiple recesiation. More importantly, differing from GaAs photocathode, GaAlAs photocathode exhibits a coincident photoemission capability and operational lifetime after multiple recesiations. It is concluded that GaAlAs photocathode could be more robust to resist the damage of residual gases than GaAs photocathode.
Acknowlwdgments This work was supported by the National Natural Science Foundation of China (Grant nos. 61301023 and 91433108), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20133219120008), and Science and Technology on Low-Light-Level Night Vision Laboratory Foundation of China (Grant no. J20150702).
Fig. 7. Schematic variation of surface barrier for the r-mode exponential-doped NEA photocathode after Cs/O activation, degradation and recesiation. Ec is the conduction band minimum, Ev is the valence band maximum, EF is the Fermi level, and Evac is the vacuum level.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Differences in stability and repeatability between GaAs and GaAlAs photocathodes Yuan Xu a,b, Yijun Zhang a,n, Cheng Feng a, Feng Shi c, Jijun Zou d, Xinlong Chen a, Benkang Chang a a
School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Department of Electronic and Electrical Engineering, Nanyang Institute of Technology, Nanyang 473004, China c Science and Technology on Low-Light-Level Night Vision Laboratory, Xi'an 710065, China d Engineering Research Center of Nuclear Technology Application (East China Institute of Technology), Ministry of Education, Nanchang 330013, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 29 May 2016 Accepted 13 June 2016
For the applications in vacuum photodetectors and photoinjectors, a crucial limiting factor for conventional GaAs photocathodes is the limited lifetime, depending on the Cs–O activation layer vulnerable to the harmful residual gases. In order to develop a type of GaAs-based photocathode with good stability and repeatability, Cs/O activation and multiple recesiation experiments under the same preparation condition were performed on reflection-mode exponential-doped GaAs and GaAlAs photocathodes grown by metalorganic vapor phase epitaxy, and quantum efficiency and photocurrent decay were measured after activation and recesiation. The experimental results show that the photoemission characteristics on cathode degradation and repeatability are different between GaAs and GaAlAs photocathodes. In an unsatisfactory vacuum system, the operational lifetime for GaAlAs photocathode is nearly twice longer than that for GaAs photocathode after Cs/O activation under a high intensity illumination. After multiple recesiations, the quantum efficiency and operational lifetime for GaAlAs photocathode remain nearly unchanged, while those for GaAs photocathode become lower and lower with the increase of recesiation cycles, which reflects the superiority in stability and repeatability for GaAlAs photocathode in contrast to GaAs photocathode operating in the poor vacuum environment. & 2016 Elsevier B.V. All rights reserved.
Keywords: GaAs photocathode GaAlAs photocathode Degradation Recesiation Quantum efficiency
1. Introduction In the last decades, negative-electron-affinity (NEA) photocathodes have played a critical role in the development of photocathode technology. Because of the high quantum efficiency, prompt response time, low thermal emittance and narrow energy spread, GaAs and GaAs-based photocathodes have found important applications in the fields of vacuum detectors for photoelectric imaging detection and polarized electron sources for high energy physics research [1–4]. For underwater photodetection, GaAs-based photocathodes with favorable blue–green response are the key underpinning technology in vacuum detectors [5]. In the application of particle accelerators, high-power 532 nm lasers are commonly used to irradiate photocathodes to produce high brightness electron beams [6]. In view of these requirements, GaAlAs photocathodes could be promising candidates, with the ease of epitaxial growth by matching with GaAs lattice on one n
Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.optcom.2016.06.034 0030-4018/& 2016 Elsevier B.V. All rights reserved.
hand, and the tunability of response threshold by varying aluminum composition on the other hand. For instance, unlike broadband GaAs photocathodes, GaAlAs photocathodes operating in the transmission-mode can obtain a narrowband spectrum without any narrowband filters [7,8]. NEA photocathodes are usually prepared through the adsorption of low-work-function species on the p-type semiconductor following the specific procedure under the ultra-high vacuum (UHV) condition. Currently, an important practical limitation of NEA photocathodes of concern is the limited lifetime of the NEA surface which requires that the NEA preparation is repeated [9– 11]. Despite some similarities in the activation process of GaAs and GaAlAs photocathodes [12,13], there are few reports on comparison of the difference in photoemission degradation between them under the same preparation conditions. From the experimental point of view, many of the photocathode performance usually depends on the experimental apparatus and crafts. In this paper, reflection-mode (r-mode) NEA GaAs and GaAlAs photocathodes grown by the metalorganic vapor phase epitaxy (MOVPE) technique are prepared. Meanwhile, the differences in stability and
Y. Xu et al. / Optics Communications 380 (2016) 320–325
321
Fig. 1. Schematic structures of the r-mode GaAs and GaAlAs cathode samples.
repeatability between GaAs and GaAlAs photocathodes are investigated by comparison of photocurrent decay and quantum efficiency variation.
2. Experimental The 2-inch-diameter r-mode GaAs and GaAlAs photocathode samples were grown on the Si-doped (100)-oriented GaAs substrate in the MOVPE reactor with heavily p-type Zn doping, and the epitaxial structures are shown in Fig. 1. The epitaxial layers for both GaAs and GaAlAs samples are of p–p type heterostructure, including a uniform-doped 0.5 μm-thick buffer layer and an exponential-doped 1.6 μm-thick active layer. In the buffer layer, the dopant concentration was 1 1019 cm 3, while in the active layer, the dopant concentration was distributed quasi-exponentially from 1 1019 to 1 1018 cm 3, which would help to improve photoemission capability [14]. Besides, the buffer layer of the two samples not only improves the crystal quality of active-layer but also reflects the back-diffusion photoelectrons to the surface [15]. In our experiments, the cut GaAs and GaAlAs samples of 10 10 mm square were ready for activation respectively. Prior to activation, the photocathode samples underwent a two-step surface cleaning process to achieve the atomically clean surfaces. First, the GaAs and GaAlAs samples were chemically cleaned in a solution of 4:1:100 H2SO4 :H2O2:H2 O for 2 min [16]. After the chemical treatment, the samples were mounted on the holders, and transferred into the preparation chamber with a base pressure of 10 7 Pa to perform the heat cleaning at 650 °C for 20 min. After cooling to the room temperature, the activation process for the cleaned samples was started using a Cs/O codeposition technique [16,17], wherein the cesium and oxygen flux was adjusted by the heating current through the dispensers. In the activation process, the Cs/O flux ratio employed to GaAs and GaAlAs samples was adjusted to 1.67/1.6, which corresponds to the ratio of dispenser heating current. A halogen tungsten lamp and a bias voltage of 200 V were applied to the photocathodes to collect the photo–electron emission, and the distance between the cathode and anode was about 5 mm. Until the peak photocurrent reached to its maximum, the Cs/O activation was terminated successively, and then the photocurrent decay was measured for the activated GaAs and GaAlAs samples under intensive illumination of 100 lx white light. After the first degradation, the GaAs and GaAlAs samples were recesiated to restore the photoemission capability by introducing the fresh Cs flux, and then the quantum efficiency curves of the recesiated samples were measured in situ immediately. After that,
the photocurrent decay was measured under the same 100 lx illumination. The recesiation on the degraded cathode samples were performed for a total of three times. After every recesiation, the quantum efficiency curves and photocurrent decay were immediately measured in situ. In all the processes of photocathode degradation, the preparation chamber was maintained at a relatively poor vacuum level of around 1 10 7 Pa, and the main residual gases in the chamber were H2, H2O, CO and CO2.
3. Results and discussion 3.1. Degradation after Cs/O activation For GaAs and GaAlAs samples activated by Cs/O codeposition, the typical photocurrent changes with time during the Cs/O activation process are shown in Fig. 2. In the first Cs deposition process, the photocurrent growth rate for GaAlAs sample is less than that for GaAs sample. In the next process of Cs/O co-deposition, the photocurrent for both samples increases substantially with the continuous alternation cycles. It is noted that there are more Cs/O alternation cycles and higher peak photocurrent for GaAs sample. The ultimate peak photocurrent of GaAs sample was more than twelvefold higher than that of GaAlAs sample. Because of its larger band gap, GaAlAs sample would absorb insufficient white light, especially the visible and near-infrared light. Besides, the surface
Fig. 2. Photocurrent curves of GaAs and GaAlAs photocathode samples during the Cs/O alternate activation process.
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Fig. 3. Photocurrent decay of GaAs and GaAlAs cathode samples after Cs/O activation under illumination of 100 lx.
electron escape probability decreases with the high Al content for GaAlAs photocathode [12]. The normalized photocurrent decay curves after Cs/O activation under 100 lx illumination are shown in Fig. 3. The lifetime of NEA photocathodes is determined by time when photocurrent is 1/e of the initial value. The measured 1/e lifetime results after activation for GaAs and GaAlAs samples are listed in Table 1. Obviously, GaAlAs sample with a lower peak photocurrent is more durable and can achieve a lifetime that is about twice longer than that of GaAs sample. For the activated NEA photocathodes, the rapid degradation could be caused by the desorption of Cs with the rising temperature under the high intensity illumination on one hand, and the adsorption of residual gas in the unsatisfactory vacuum environment on the other hand. Fig. 4. Experimental quantum efficiency curves of the original and recesiated (a) GaAs and (b) GaAlAs cathode samples.
3.2. Degradation after multiple recesiations Applying the fresh Cs flux affords an effective way to restore the photoemission capability after degradation [10,11]. Accordingly, three recesiations after degradation were performed on GaAs and GaAlAs samples respectively to verify the repeatability. After the first recesiation, the photocurrent was restored to approximately 90% of its initial value for GaAs sample and 60% for GaAlAs sample. With the increase of the number of recesiations, the recovery of photocurrent for GaAs sample became worse. After the third recesiation, the photocurrent was only restored to about 70% of its initial value, while that for GaAlAs sample was restored to 50% of its initial value. The measured quantum efficiency curves of original and recesiated GaAs and GaAlAs samples are shown in Fig. 4. Because of the different band gap of the emission layer, the cutoff wavelength for GaAlAs sample is limited to the blue–green response region. It is clear to see that the reproducibility of quantum efficiency through multiple recesiations between GaAs and GaAlAs samples is completely different, wherein the quantum efficiency over the entire Table 1 Photocurrent decay lifetime of GaAs and GaAlAs samples after Cs/O activation. Cathode type
Initial photocurrent/μA
1/e lifetime/min
GaAs GaAlAs
46.8 3.6
45 93
waveband for GaAs sample dropped gradually after every recesiation. As for GaAlAs sample, the quantum efficiency over the entire waveband after the first recesiation is lower than that after the original Cs/O activation, while after the subsequent recesiation, the quantum efficiency curves remain substantially unchanged, which demonstrates that GaAlAs sample can obtain a better repeatability than the conventional GaAs sample under the same UHV condition. Meanwhile, an operational lifetime as long as possible after every recesiation should be especially important to save the time consumption and manpower for photocathodes operating in the demountable vacuum system. The photocurrent decay under intensive illumination of 100 lx after every recesiation was measured, as shown in Fig. 5. The 1/e lifetime results after activation for GaAs and GaAlAs samples are listed in Table 2. For both samples after the first recesiation, the photocurrent becomes lower and the lifetime is decreased nearly by half. However, after the second and third recesiation, the lifetime of GaAlAs sample is almost the same as that after the first recesiation, while the case for GaAs sample is different. After the succeeding recesiation, the lifetime of GaAs sample becomes shorter and shorter. It is found that whether after Cs/O activation or after subsequent recesiation, the operational lifetime of GaAlAs sample could be more than two times that of GaAs sample, which shows that GaAlAs photocathode could be more robust when operating in the demountable vacuum system through multiple recesiations.
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P0 is related to the surface cleanliness and profile of surface barrier. Generally, the three parameters characterizing cathode performance can be obtained by fitting experimental quantum yield curves based on the quantum efficiency formula [19]. According to the r-mode exponential-doped quantum yield formula [20], we can simulate the theoretical quantum efficiency curves consistent with experimental curves for GaAs and GaAlAs samples after original activation and multiple recesiations, as shown in Fig. 6. The obtained values of cathode performance parameters are listed in Table 3. It is seen from Table 3 that the due to the invariable material intrinsic property, the LD and Sv remain unchanged for the recesiated photocathodes. By comparison of P0 between original and recesiated samples, it is found that with the increase of recesiation times, P0 becomes smaller and smaller for GaAs sample, while the case is different for GaAlAs sample. Although the values of P0 of the recesiated GaAlAs sample are smaller than those of the activated one, there are no remarkable changes in P0 with the increase of recesiation times. Since the escape of photoexcited electrons reaching the surface
Fig. 5. Photocurrent decay of (a) GaAs and (b) GaAlAs cathode samples after multiple recesiations under illumination of 100 lx.
3.3. Evaluation of photocathode performance As is well known, the photoemission process from NEA photocathodes is in terms of three successive steps—photoelectron excitation, electron diffusion, and travel across the surface [18]. For various photocathodes in practice, the quantum efficiency is distinctly a very important parameter utilized to evaluate the cathode performance. Some parameters, e.g. electron diffusion length LD, back interface recombination velocity Sv, and surface electron escape probability P0 would significantly affect the quantum efficiency [19]. The LD and Sv are usually related to the crystal quality, while the Table 2 Photocurrent decay lifetime of GaAs and GaAlAs samples after multiple recesiations. Cathode type
Recesiation cycles
Initial photocurrent/μA
1/e lifetime/min
GaAs
1st 2nd 3rd
43.2 37.4 32.7
25 20 14
GaAlAs
1st 2nd 3rd
2.1 1.9 1.8
44 40 38
Fig. 6. Experimental (solid lines) and theoretical (dashed lines) quantum efficiency curves of GaAs and GaAlAs samples. Curves 1, 2, 3, and 4 are corresponding to the quantum efficiency curve after original Cs/O activation, the first, second, and third recesiation, respectively.
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Table 3 Simulated performance parameters for original and recesiated GaAs and GaAlAs samples. 1
Cathode type
Curve No.
LD/μm
Sv/cm s
GaAs
1 2 3 4
3.0 3.0 3.0 3.0
106 106 106 106
0.45 0.38 0.28 0.22
GaAlAs
1 2 3 4
1.6 1.6 1.6 1.6
106 106 106 106
0.40 0.30 0.28 0.28
P0
is closely related to the surface barrier, we should investigate the variation of the surface barrier profile. As proved by a series of activation experiments, the NEA formation mechanism for the GaAs-based photocathodes could be explained by the double dipole model [9,21–24]. The proposed surface barrier profile consists of barriers I and II, arising from the Cs–O activation layer, as shown in Fig. 7. It is clear to see that the traversing resistance for photoelectrons reaching the surface would rely on the surface barriers. As we know, the lifetime for the activated NEA photocathodes would be attenuated with the increase of the illumination intensity [20], and the heating as a result from the high intensity illumination would sublimate some of Cs from NEA surface and change the original Cs/O ratio [6]. More importantly, the residual gases such as CO2, H2O, CO, and so on in the UHV chamber would adsorb on the cathode surface. Once the oxygen from the O-containing gases is attached to the surface, it would change the chemistry of the surface Cs–O activation layer on one hand, and form substrate oxides on the other hand. Consequently, the dipoles in the activation layer region for the degraded photocathode are weaken, which results in the overall increase of the height and width of the surface barriers, which is displayed by the dashed line in Fig. 7. During the recesiation process, the vacuum level is lowered once more and the barrier II can be recovered after applying
supplementary Cs atoms onto the degraded surface, as displayed by the dotted line in Fig. 7. Whereas, the recesiation process cannot remove the inner formed substrate oxides, and thus the barrier I can hardly be restored [10]. In other words, the quantum efficiency of the recesiated photocathode cannot recover its initial value just after Cs/O activation. Compared with GaAs photocathode, GaAlAs photocathode achieves a lower recovery of quantum efficiency after the first recesiation, because that there exist additional stable Al–O bonds on GaAlAs surface by adsorption of residual O-containing gases, and the Al–O bonds are difficult to be broken by applying supplementary Cs. After the succeeding recesiation processes, GaAlAs photocathode can obtain relatively consistent quantum efficiency and lifetime, which demonstrates a good repeatability for GaAlAs photocathode. For GaAs photocathode, the performance in terms of quantum efficiency and lifetime becomes worse and worse as the recesiation times increases. By comparison of degradation and recesiation results between GaAs and GaAlAs photocathodes, it is inferred that the photoemission capability of GaAlAs photocathode is not so sensitive to Cs/O ratio. As a result of the harmful adsorption of gas molecules containing O combined with partial Cs loss under the high intensity illumination, the original chemistry of the Cs–O activation layer would be changed. As for GaAs photocathode, it is well known that the O flux has to be carefully matched to the Cs flux, and a slight deviation from the optimal Cs/O ratio will cause a large drop of quantum efficiency [16]. Therefore, GaAlAs photocathode can obtain better repeatability and longer lifetime under unfavorable vacuum environment in contrast to GaAs photocathode.
4. Conclusions In summary, we have investigated the differences in stability and repeatability between GaAs and GaAlAs photocathodes under the same preparation condition. Both MOVPE-grown samples underwent the same surface cleaning and Cs/O activation process. Moreover, multiple recesiation experiments were performed on the two cathode samples, and quantum efficiency and photocurrent decay were measured after activation and recesiation. The results show that GaAlAs photocathode with NEA surface state under high intensity illumination can obtain a longer operational lifetime than GaAs photocathode after whether Cs/O activation or multiple recesiation. More importantly, differing from GaAs photocathode, GaAlAs photocathode exhibits a coincident photoemission capability and operational lifetime after multiple recesiations. It is concluded that GaAlAs photocathode could be more robust to resist the damage of residual gases than GaAs photocathode.
Acknowlwdgments This work was supported by the National Natural Science Foundation of China (Grant nos. 61301023 and 91433108), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20133219120008), and Science and Technology on Low-Light-Level Night Vision Laboratory Foundation of China (Grant no. J20150702).
Fig. 7. Schematic variation of surface barrier for the r-mode exponential-doped NEA photocathode after Cs/O activation, degradation and recesiation. Ec is the conduction band minimum, Ev is the valence band maximum, EF is the Fermi level, and Evac is the vacuum level.
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