Study of cesium and oxygen adsorption on surface of GaAlAs photocathode in ultra-high vacuum chamber

Optik 130 (2017) 806–812

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Original research article

Study of cesium and oxygen adsorption on surface of GaAlAs photocathode in ultra-high vacuum chamber Xinlong Chen a,∗ , Guanghua Tang a , Shumeng Wang a , Benkang Chang b a b

Nanjing Electronic Devices Institute, Nanjing 210016, China School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

a r t i c l e

i n f o

Article history: Received 4 September 2016 Accepted 31 October 2016 Keywords: Cesium and oxygen adsorption GaAlAs photocathode Electron affinity Spectral response

a b s t r a c t The mechanism of cesium (Cs) and oxygen (O) adsorption on GaAlAs photocathode has been investigated. The models of Cs and O adsorption on GaAlAs surface are studied, and the electron affinity changing with Cs coverage on GaAlAs surface is calculated based on Topping model. The experiments of Cs activation on GaAlAs and GaAs are performed, and the Cs, O activation experiments with different heat cleaning temperatures are performed on GaAlAs photocathodes. The photocurrent and spectral response curves in the experiments are measured and analyzed. The results show that the Topping calculation of electron affinity changing with Cs coverage on GaAlAs surface is in consistent with the result of experiment, and the electron affinity nearly reaches the bottom of conduction band after the Cs activation. The GaAlAs phtocathode treated by 700 ◦ C heat cleaning could obtain a good photoemission performance after Cs, O activation. © 2016 Elsevier GmbH. All rights reserved.

1. Introduction As we known, negative electron affinity (NEA) semiconductor photocathodes have already been widely used in low-lightlevel image intensifiers, ultraviolet detection, photon-enhanced thermionic solar cell, and potential electron sources for the next-generation electron accelerators due to their high quantum efficiency, low energy spread, and high spin polarization [1–4]. Different with GaAs, InGaAs, and GaN photocathodes, GaAlAs photocathode is a potential photocathode for the nextgeneration electron accelerators and the marine detection due to the long lifetime and the controlled threshold wavelength [5,6]. For most III–V s emiconductor photocathodes, the activation of Cs and O plays an important role on the preparation of photocathode [7]. The surface could approach or achieve zero electron affinity after Cs activation, and obtain negative electron affinity by Cs and O activation. The photoelectrons generated in the body of photocathode would cross the Cs-O active layer and emit into vacuum. High quality Cs-O layer is conductive to make more photoelectrons escape to vacuum, the surface electron escape probability is often used to evaluate the effect of Cs-O active layer. So far, there are plenty of reports about GaAs photocathode in theory and experiment, and detailed studies about the adsorption of Cs and O on GaAs surface [8–11]. In contrast to GaAs photocathode, there are few studies about photoemission performances of GaAlAs photocathodes, much less the adsorption mechanism of Cs and O on GaAlAs surface.

∗ Corresponding author. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.ijleo.2016.10.124 0030-4026/© 2016 Elsevier GmbH. All rights reserved.

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Fig. 1. GaAlAs(100) surface model after Cs adsorption.

In this paper, we focus on investigating the Cs and O adsorption mechanism of GaAlAs(100) photocathode in the ultra-high vacuum (UHV) chamber. The models of Cs and O adsorption on GaAlAs surface are built and the electron affinity changing with Cs coverage on GaAlAs surface is calculated based on Topping model. The experiments of Cs adsorption are performed on GaAlAs and GaAs surface. The Cs, O activation experiments with different heat cleaning temperatures are performed on GaAlAs photocathodes. The photocurrent and spectral response curves in the experiments are measured and analyzed. 2. Theoretical calculation The ‘yo-yo’ activation technology was often used to prepare NEA photocathode, which Cs source was kept continuous and O source was introduced periodically [12]. There were lots of studies about GaAs(100) surface reconstruction model, which focus on GaAs(100) ␤2 (2 × 4) surface [13–15]. In contrast, the studies about GaAlAs(100) reconstruction were less. Yu et al. studied the properties of GaAlAs(100) ␤2 (2 × 4) surface [16,17]. The p-type GaAlAs material is often zinc (Zn) doping or beryllium (Be) doping. The GaAlAs surface models built in this paper are Zn doping. The atoms on the polar p-GaAlAs(100) surface have dangling bonds, which could react with Cs and form covalent bond. As we known, Cs is the most effective material to drop the surface electron affinity of semiconductor. For the GaAlAs(100) ␤2 (2 × 4) reconstruction surface, there exists several Cs adsorption positions, Cs atoms would rest on these positions and form the first dipole layer with the surface atoms. The GaAlAs(100) surface after Cs adsorption is shown in Fig. 1, where we presume the Zn atom replace the primary Ga atom. In the process of Cs adsorption, the Cs on the GaAlAs surface loses 6 s outer valence electrons easily and turns to Cs+ ion, which would combine with dangling bonds on the surface. In the p-GaAlAs surface, the Zn-centered cluster geometry has a big electronegative. The Cs+ ion and Zn-centered cluster geometry form the first dipole layer, namely GaAlAs(Zn)-Cs, which would arouse the variation of electric potential and lower the surface electron affinity. When the Cs coverage on GaAlAs surface achieves a certain extent, the polarization and depolarization of dipoles would reach a balance. In this stage, the surface electron affinity reaches minimum and the GaAlAs photocathode has the best photoemission performance in the process of Cs adsorption. Following more coverage of Cs on the surface, the reaction of Cs-Cs would make the premier dipoles depolarized. The excess Cs atoms will form two dimension metal islands, which would cause the increasing of surface electron affinity and prevent the emitting of photoelectrons generated in the photocathode body. For NEA photocathode, the surface dipoles induced by the atom adsorption will change the ionization energy of semiconductor. In comparison with Ga, Al, As and Zn atoms, the Cs atom has the minimum electronegative, but Cs has a large polarizability. The electron affinity changing induced by Cs adsorption follows the Topping model [18,19],

  = −

endip ε0

1+

3/2

9ˇndip

−1

4ε0

(1)

where,  is moment of dipole induced by atom adsorption, e is electron charge,  is atomic planar density,  is surface coverage, ndip is product of  and  of 1 ML adsorbent, ε0 is dielectric constant, ˇ is polarizability of adsorbed atom. The moment of dipole is,  = q(rCs + rsub )

(2)

where, rCs and rsub are covalent radius of Cs and substrate atoms respectively. q is the charge transferred to substrate atoms from Cs atoms, which could be obtained by the Pauling theory [20], q = 0.16e|Xsub − XCs | + 0.035e(Xsub − XCs )2

(3)

where, Xsub and XCs are electronegative of substrate and Cs atoms. According to Eq. (3), the contribution of Zn could be ignored because the proportion of Zn atoms in the GaAlAs surface is small. In the calculation, the electronegative of Ga, Al,

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Fig. 2. Topping calculation and experimental data of electron affinity change with Cs coverage on GaAs(100) surface [19].

Fig. 3. Topping calculation and DFT calculation of electron affinity change with Cs coverage on GaAlAs(100) surface.

As, and Cs is 1.6, 1.5, 2.0 and 0.7 respectively, and the covalent radius of Ga, Al, As, and Cs is 1.24 Å, 1.26 Å, 1.21 Å and 2.32 Å respectively [21]. The lowering of surface electron affinity is affected mainly by two factors, one is the contribution of surface dipoles, and the other is the energy band bend induced by the surface fermi level pining. The bend of surface energy band is about a third to energy gap of some semiconductors such as GaAs. The surface electron affinity change caused by the Cs adsorption could be calculated according to Eq. (1). In the calculation, ˇ is 1.81 × 10−39 C m2 /V,  is 7.3 × 1014 cm−2 . Liu reported the Topping calculation and experimental data of electron affinity change with Cs coverage on GaAs(100) surface, as shown in Fig. 2 [19]. It could be found that Topping calculation agrees with the experimental data, especially in the condition of low Cs coverage. Based on density functional theory (DFT), Yu et al. calculated the electron affinity change with Cs coverage on GaAlAs(100) surface by Material Studio, the Al fraction in the surface model is 0.5[17], the DFT calculation is shown in Fig. 3. Topping calculation of electron affinity change with Cs coverage on GaAlAs(100) surface is also shown in Fig. 3. It could be found that the electron affinity of GaAlAs decreases as increasing of Cs coverage to a certain extent, which is similar to Cs adsorption on GaAs surface. The electron affinity would increase, when Cs coverage is over 0.8ML, as shown in Fig. 3. According to the calculation, it is not hard to conclude that the electron affinity of GaAlAs photocathode nearly reaches the bottom of conduction band after the first Cs activation. 3. Experiments In the experiments of this paper, four 10 × 10 mm reflection-mode GaAlAs photocathode samples and one 10 × 10 mm reflection-mode GaAs photocathode sample were used. The samples were cut from the materials grown on the high quality n-type GaAs(100)-oriented substrate by metal organic chemical vapor deposition (MOCVD). The epitaxial layers of them were p-type Zn doping. The emission and buffer layers of GaAlAs photocathode samples are GaAlAs(Al fraction is almost 0.63) and GaAlAs (Al fraction is almost 0.79) layers respectively. The emission and buffer layers of GaAs photocathode samples are GaAs and GaAlAs(Al fraction is almost 0.63) layers respectively. The emission layers of GaAlAs and GaAs photocathode samples are doped in the form of exponential, and the doping concentration ranges from 1019 cm−3 to 1018 cm−3 , the doping

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Fig. 4. Photocurrent curves of GaAlAs and GaAs samples by the Cs activation.

Fig. 5. Spectral response curves of GaAlAs photocathode by Cs activation and Cs, O curves. Curve 1 is spectral response curve of GaAlAs photocathode by Cs activation; Curve 2 is spectral response curve of GaAlAs photocathode by Cs, O activation.

concentration at near surface is 1018 cm−3 . Among four GaAlAs samples, three samples have same structures completely, while the other one has a thinner emission layer and a 100 nm thick GaAs protective layer on emission layer. Before activation, the GaAlAs samples were treated firstly in a 4:1:100 solution of sulfuric acid to hydrogen peroxide to de-ionized water for 2 min, while the GaAs sample was treated in hydrofluoric acid for 10 min. After that, the samples were transferred into the UHV chamber with a base pressure less than 10−7 Pa. One GaAlAs sample and GaAs sample were heated under the temperature about 650 ◦ C for 20 min for removing the surface contamination such as carbon and oxides. When the samples cooled down to room temperature, the Cs activation was performed on the GaAlAs and GaAs samples respectively. In the activation, the Cs evaporation currents for two photocathodes were same and constant, and the photocurrents were monitored by a multi-information measurement system [22]. After the activation, the spectral response curves of samples were measured by the on-line spectral response measurement system. The Cs activated photocurrent curves of GaAlAs and GaAs samples are shown in Fig. 4. In Fig. 4, the take-off time of two photocurrent curves are approximate, and peak photocurrents are appearing at an adjacent time. We could conclude that the coverage of Cs on GaAlAs and GaAs surfaces is much the same when the photocurrents achieve the peak in the process of Cs activation. In order to study the effect of surface cleaning level on Cs, O activation, the other three GaAlAs photocathode samples were treated by different heating temperature, which were 600 ◦ C, 650 ◦ C and 700 ◦ C respectively. As we known, the chemical cleaning could only remove partial oxides on the surface, while the heat cleaning would make the photocathode sample obtain an atomically clean surface. The chemical cleaning for three samples were as the same as the GaAs sample mentioned above. In the process of Cs, O activation, the Cs source was kept continuous and the O source was introduced periodically 4. Results and discussion The spectral response curve of GaAlAs photocathode by Cs activation is shown in Fig. 5, which is Curve 1. Curve 2 in Fig. 5 is spectral response curve of GaAlAs photocathode by Cs, O activation, which is quoted from Ref. [6]. For reflectionmode GaAlAs photocathode, the short-wave light is mainly absorbed in near surface, and the excitation electrons have a big

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Fig. 6. GaAlAs(100) surface model after Cs and O adsorption.

Fig. 7. Spectral response curve of GaAs photocathode by Cs activation.

energy. These electrons would emit into vacuum in a large probability after the Cs activation, even if the electron affinity of GaAlAs doesn’t decrease to the bottom of conduction band. The long-wave light has a longer adsorption depth, the generated electrons have thermalized before transporting to the surface charge region, which causes some low energy electrons could not pass through the surface barrier due to a high surface electron affinity. The Cs and O alternated activation could increase the photoemission performance obviously. In Fig. 5, we could find that the cutoff wavelength of Curve 2 is longer than that of Curve 1, the reason is that the photocathode activated by Cs and O has a negative electron affinity. However, we could conclude that the electron affinity of the caesiated GaAlAs surface is close to zero electron affinity. The low energy electrons generated by long wavelength light still have a large probability to emit into vacuum. In the process of Cs, O activation, the O2 molecule would decompose into O atom firstly, and adsorb on the caesiated GaAlAs surface. The O atoms could diffuse into the Cs cover layer and ionize due to a smaller size, as shown in Fig. 6. The O−2 and Cs+ ions would form the second dipole layer, namely Cs-O-Cs layer, which would help to decrease the surface electron affinity of GaAlAs photocathode. The introduced O atoms make Cs atoms ionize easily, the dipole depolarization caused by the reaction of Cs-Cs would not happen. Besides, the O adsorption make premier Cs atoms ionize, which make surface appear added room because of the Cs+ ion is smaller than the excess Cs atom mentioned above. Therefore, more Cs would adsorb on GaAlAs surface. With the proceeding of Cs-O alternate activation, more Cs-O-Cs dipoles are formed, which make surface electron affinity further decrease. However, the excess O would damage the active layer and decrease the performance of photocathode. So, an appropriate Cs/O proportion is a key factor to obtain a high performance GaAlAs photocathode. The spectral response curve of GaAs photocathode by Cs activation is shown in Fig. 7. The cutoff wavelength of caesiated GaAs photocathode is a little shorter than that of Cs, O activated photocathode, which could be found from the reported literatures [7]. The phenomenon is similar to that of GaAlAs photocathode mentioned above. The experiments of caesiated GaAlAs and GaAs photocathodes are in accordance with the Topping theory. As we known, an atomically clean surface is an essential condition of preparing photocathode, which mainly depends on the heat cleaning process. The maximum temperature is an important factor in the heat cleaning process. The evaporation of As atoms from GaAlAs surface would happen when the heat temperature is too high[23]. But, the too low heat temperature could not remove the oxides on GaAlAs surface. So, the maximum temperature of GaAlAs sample is set to be 700 ◦ C in this

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Fig. 8. Spectral response curves of GaAlAs photocathodes treated by different temperature.

Table 1 Performance parameters of GaAlAs photocathodes treated by different temperature. Heat temperature (◦ C)

Spectral response @532 nm (mA/W)

Quantum efficiency @532 nm

700 650 600

110 89 74

26% 21% 17%

paper. The spectral response curves of three GaAlAs samples respectively treated by 700 ◦ C, 650 ◦ C and 600 ◦ C are shown in Fig. 8. The related performance parameters of three photocathodes are shown in Table 1, the quantum efficiency is obtained by transforming the spectral response according to the following equation[24], Y (h ) ≈ 1.24S /

(4)

where Y(hv) is quantum efficiency and S␭ is the spectral response value of the corresponding wavelength . Quantum efficiency is the an important characteristic parameter to evaluate the performance of photocathodes, which reflects photoelectric transformation capability of photocathode. It could be found that the GaAlAs phtocathode treated by 700 ◦ C has a largest quantum efficiency at 532 nm, which shows the sample has a cleanest surface among three samples after heat cleaning. The carbon (C) and oxides on surface would affect the formation of Cs-O active layer. The C atoms could segregate the substrate atoms and Cs, O atoms, which stop the formation of dipoles. The photoelectrons could not emit into vacuum from the regions covered by C atoms, namely the regions are invalid for photoemission. Furthermore, the Cs atoms would react with the oxides, which is also disadvantageous for the formation of dipole layers. However, the photocathode treated by a suitable heat temperature could obtain the best photoemission performance.

5. Conclusion In this paper, we have investigated the mechanism of Cs and O on the GaAlAs photocathode in the UHV chamber. The models of Cs and O adsorption on GaAlAs(100) surface are built and analyzed. The electron affinity changing with Cs coverage on GaAlAs(100) surface is calculated based on Topping model and analyzed by comparing with GaAs photocathode. The experiments of Cs adsorption have been performed on GaAlAs and GaAs surface. The Cs, O activation experiments with different heat cleaning temperature have been performed on GaAlAs photocathodes. The spectral response curves of GaAlAs photocathodes activated by Cs and O are measured by the multi-information measurement system. The results show that Cs and Zn-centered cluster geometry form the first dipole layer GaAlAs(Zn)-Cs, and Cs and O would form the second dipole layer Cs-O-Cs. Topping calculation of electron affinity changing with Cs coverage approaches to the DFT calculation. According to the calculation and experiments, we could obtain that the electron affinity of GaAlAs photocathode nearly reaches the bottom of conduction band after the first Cs activation, and the Cs coverage on GaAlAs and GaAs surfaces is much the same. The GaAlAs phtocathode treated by 700 ◦ C heat cleaning could obtain a good photoemission performance. Moreover, some results and discussions of this study could be extended to other III–V NEA photocathodes.

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