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Activation treatment effects on characteristics of BeO layer and secondary electron emission properties of an activated Cu–Be alloy
Applied Surface Science 355 (2015) 19–25
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Activation treatment effects on characteristics of BeO layer and secondary electron emission properties of an activated Cu–Be alloy Bin Wang, Erdong Wu, Yongli Wang, Liangyin Xiong, Shi Liu ∗ Materials for Special Environment Department, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
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Article history: Received 8 March 2015 Received in revised form 3 June 2015 Accepted 29 June 2015 Available online 11 July 2015 Keywords: Thickness and composition distribution of BeO layer Oxidation duration Oxidation temperature Pre-heat treatment Oxygen pressure
a b s t r a c t Dependence of characteristics of surface BeO layer and secondary electron emission properties of a Cu–Be alloy on activation conditions were studied by measurements of X-ray photoelectron spectroscopy and secondary electron yield of the alloy based on varying conditions of an optimized activation treatment. The results showed that the thickness and composition distribution of the surface BeO layer of the alloy were closely related to the pre-heat treatment and the affecting factors (oxidation duration, oxidation temperature, and oxygen pressure) of the oxidation process in the activation treatment. The improper treatment leaded to a too thick or too thin surface BeO layer, the presence of metallic Be, and poor distribution of metallic and oxidized Cu in the layer, hence significantly reduced the secondary electron emission yield of the activated Cu–Be alloy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction High, stable, and durable secondary electron emission is essentially necessary for application of dynodes of electron multipliers and photomultiplier tubes. The activated Cu–Be alloys have been widely used as an alloy type of secondary electron emission (SEE) material for these applications owing to their good emission properties. The activation process plays a key role on the secondary electron yield (ı) of the Cu–Be alloys, and has been partially investigated by means of scanning electron microscopy (SEM), Augur electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) in the earlier time of 1950s–1980s [1–5]. However, further development and study on the Cu–Be alloys have been rather limited in recent years. The toxicity of Be compounds is probably a reason, but the most important is that the main efforts of the researchers nowadays have been concentrated on the development of new materials with higher SEE properties. Some novel SEE materials, such as MgO/Au cermet films [6,7], hydrogen terminated and Cs terminated chemical vapor deposition (CVD) diamond [8,9], have been developed in recent years, and shown promising SEE properties. Nevertheless, up to now, the activated Cu–Be alloys are still used as a major SEE material due to their high SEE coefficients. In comparison with another widely used SEE material of Mg alloys, the Cu–Be alloys are more stable at high temperatures
∗ Corresponding author. Tel.: +86 024 2397 1968; fax: +86 024 2397 1968. E-mail address: [email protected] (S. Liu). http://dx.doi.org/10.1016/j.apsusc.2015.06.189 0169-4332/© 2015 Elsevier B.V. All rights reserved.
and have some other advantages. On the other hand, unlike that of the Pb used as an electronic packing material which in most cases is exposed to the environment in working conditions, the Cu–Be alloys would be sealed in the electron multiplier when working as dynode. Moreover, the typical content of Be in the Cu–Be alloys is very low (Cu–2.75Be), and the toxicity is only involved in the fabrication and waste disposal of the material. Therefore, the impact of toxicity of the alloy on human and environment is controllable and the further study on improvement of this important SEE material is necessary. The formation of oxide layer during activation has a significant influence on the secondary electron emission properties of the alloy materials. For instance, it is observed that the maximum SEE yield is achieved when the thickness of the MgO film on a Si substrate is close to the penetration depth of the primary electrons under low electric potential [10]. For the Cu–Be alloys, it is known that the characteristics of the BeO layer, i.e. its thickness, composition, and topography, depend on the activation conditions. However, although the activation process includes pre-heat treatment and oxidation treatment involving several factors, such as temperature, duration, and pressure, only the factors of oxygen pressure and oxidation temperature in the oxidation have been studied, whereas the oxidation duration and pre-heat treatment have seldom been considered. On the other hand, we note that the relationship between the thickness, composition, and topography of the BeO layer and the activation conditions of the Cu–Be alloys have not been thoroughly investigated in previous studies. Therefore, we have carried out an investigation on the activation process of the Cu–Be alloys
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and paid more attention to the oxidation duration and pre-heat treatment of the process as well. In the investigation, the four factors in the activation process, i.e. the pre-heat treatment, oxidation duration, oxygen pressure, and oxidation temperature, respectively, have been examined separately for clarification of the role of each factor on the process and for more understanding of the formation mechanism of the surface BeO layer. On the other hand, the dependence of the characteristics of the BeO layer on activation conditions and the impact of the BeO layer on the secondary electron yield (ı) of the alloys have been explored. The relevant literatures on activation have suggested that the condition for the activation treatment of Cu–Be alloys would be in the following range: oxidation temperature from 500 ◦ C to 900 ◦ C; oxygen pressure below 10 Pa; and the oxidation duration between 2 min and several hours [1–5]. The condition for the preheat treatment has not been particularly suggested, but appears to be in the same range except for under a vacuum rather than under an oxygen pressure environment. Based on the above conditions, we have performed a comprehensive search for optimization of the activation conditions of a Cu–Be alloy based on the measurement of the corresponding secondary electron yield (ı) of the alloy. An optimized condition has been selected from nearly a hundred experiments. In this paper, the effects of the activation process on the properties of the SEE of the activated Cu–Be alloy are exhibited and discussed based on comparison of the characteristics and performance of the surface BeO layer formed by separately varying the four affecting factors of the optimized activation condition. The results show that the characteristics of the BeO layer and in particular its thickness and composition (residual metallic and oxidized Cu and metallic Be) distribution, is closely related to all four factors of the activation process, and the oxidation duration and pre-heat treatment take the same importance as the oxidation temperature and oxygen pressure and significantly affect the SEE properties of the activated alloy.
2. Experiments The Cu–2.75Be (copper alloy containing 2.75 wt.% beryllium with a purity of 99.5%) plates with a dimension of 10 mm × 10 mm × 0.2 mm were used for the samples. The surface of the samples was polished with diamond powder and rinsed in anhydrous alcohol to remove surface contaminants and oxides. The activation process of all the samples was carried in a high vacuum system. The high vacuum system comprises a bakeable, stainless steel vacuum chamber with internal heaters and thermocouples, an oil-free high vacuum turbo pump, and a gas flowmeter, capable of 7 × 10−6 Pa base pressure. Based on the optimized activation condition (denoted as OA), the samples were divided into four groups, corresponding to varying the affecting factors of preheat treatment time (PH), oxidation duration (OD), oxygen pressure (OP), and oxidation temperature (OT), respectively. A subgroup in PH was specially denoted as PO, defining an activation condition for pre-heat treatment only without further oxidation treatment. For each affecting factor, the insufficient and excessive treatments were denoted as “−” and “+”, respectively, after the symbols of the factor. The details of the activation conditions of the samples in the four groups were listed in Tables 1–4, respectively. The preheat treatment was performed at a fixed temperature of 600 ◦ C and a high vacuum of about 3 × 10−4 Pa for selected time. The oxidation treatment was conducted at selected temperatures and oxygen pressures for selected time. After the activation, the secondary electron yield (ı) of the samples was measured by a test assembly similar to that described in [10]. The assembly contained a vacuum system, an electron source
Table 1 Variation in pre-heat treatment time (tp /h), ım is the maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are further activated at oxygen pressure 0.65 Pa and 700 ◦ C for 15 min, except for PO and PO+, which only have pre-heat treatment without further oxidation treatment. Sample
Pre-heat treatment (h)
ım
PH− OA PH+ PO PO+
0 1 2 1 2
7.0 9.2 8.2 6.3 7.5
Thickness (nm) ± ± ± ± ±
0.4 0.4 0.4 0.4 0.4
43 23 17 12 16
± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
Table 2 Variation in oxidation duration (t/min), ım is maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are activated at oxygen pressure 0.65 Pa and 700 ◦ C with 1 h pre-heat treatment. Sample
Oxidation duration (min)
ım
Thickness (nm)
OD− OA OD+
3 15 30
6.4 ± 0.4 9.2 ± 0.4 6.0 ± 04
18 ± 0.5 23 ± 0.5 33 ± 0.5
Table 3 Variation in oxygen pressure (P/Pa), ım is maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are activated at 700 ◦ C for 15 min with 1 h pre-heat treatment. Sample
Oxygen pressure (Pa)
ım
Thickness (nm)
OP− OA OP+
10−4 0.65 5
7.2 ± 0.4 9.2 ± 0.4 3.6 ± 0.4
19 ± 0.5 23 ± 0.5 32 ± 0.5
system and a target testing system. Once the primary current, Ip , was measured from electron gun and secondary current, Is , was measured with the Faraday cup, the secondary electron yield (ı) was calculated based on the expression of ı = Is /Ip . The ı comprised both the secondary electron (with energies below 50 eV) and the reflected primaries (with almost all the incident energy). The sample bias voltage, Vb , was −65 V. The maximum secondary electron yield (ım ) of each sample derived based on a curve between ı and primary electron energy was listed in the relevant tables. The surface products and depth profiles of the activated samples were estimated by X-ray photoelectron spectroscopy (XPS) with Mg K␣ (1253.6 eV) and Al K␣ (1486.6 eV) twin anode X-ray using an ESCALAB250 system. The Ar+ -ion repetitive bombarded etching was applied to obtain the quantification and valence states of the elements along the film depth. The sputtering rate was 0.1 nm/s for the Ar+ -ion gun of 3 kV and 2 A. 3. Results and discussion The profiles of the depth concentration of the samples in the four groups derived from XPS are plotted against sputtering time and shown in Figs. 1–5, respectively. As the BeO layer on the surface is responsible for the emission of secondary electron, the performance of the activated Cu–Be alloys is closely related to
Table 4 Variation in oxidation temperature (T/◦ C), ım is maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are activated at oxygen pressure 0.65 Pa for 15 min with 1 h pre-heat treatment. Sample
Oxidation temperature (◦ C)
ım
Thickness (nm)
OT− OA OT+
600 700 800
8.6 ± 0.4 9.2 ± 0.4 8.0 ± 0.4
13 ± 0.5 23 ± 0.5 >35
B. Wang et al. / Applied Surface Science 355 (2015) 19–25
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Fig. 1. Quantitative depth profiles of the activated Cu–Be alloy: (a) sample PH−; (b) sample OA; (c) sample PH+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
characteristics of the BeO layer. The XPS analysis shows that the BeO layers occur on the surface of all activated samples, agreeing well with the previous results. According to Barr [11,12], the initial growth of beryllium oxide (BeO) on the surface forms islands of BeO in a sea of Cu–Be alloy, while the continuous growth of BeO eventually covers almost entire outer surface of the Cu–Be alloy. Although the surface composition and thickness of the BeO layer of the samples are different from each other, the content of the formed
BeO generally decreases from surface to the interior and eventually disappears inside of the samples. According to the observed atomic ratio of O/Be, the metallic Be starts to appear on the surface or the superficial layer of the surface, and basically increases along the depth to the content of the base alloy. On the other hand, the variation trend of the metallic Cu content has some similar with that of the metallic Be. During the SEE process, electrical conductivity of the BeO layer is considered to be another important factor, since at
Fig. 2. Quantitative depth profiles of the Cu–Be alloy: (a) cleaned alloy sample without any treatment; (b) sample PO; (c) sample PO+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer. Note that the scale of the etch time is only 1/3 of that in Fig. 1.
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Fig. 3. Quantitative depth profiles of activated Cu–Be alloy: (a) sample OD−; (b) sample OA; (c) sample OD+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
least a certain level of electrical conductivity is necessary to replenish new electrons to the electron deficient surface. If the content of Cu is more than 20 at.% in the BeO layer, the electrical conductivity of the BeO layer has little effect on electron supply [6]. Therefore, the thickness of the BeO layer on the surface of the activated Cu–Be alloy has been defined as the thickness of the layer from surface to the interior in which the Cu content reaches 20 at.%, and listed
in the third column of the four tables. Due to the importance of the states, content, and distribution of the residual Cu in the BeO layer on SEE properties of the alloy, the XPS valence band structures of Cu 2p on the surface of the excessively activated Cu–Be alloy are particularly shown in Fig. 6 for understanding of the effects of the activation process on the SEE properties of the activated Cu–Be alloy.
Fig. 4. Quantitative depth profiles of activated Cu–Be alloy: (a) sample OP−; (b) sample OA; (c) sample OP+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
B. Wang et al. / Applied Surface Science 355 (2015) 19–25
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Fig. 5. Quantitative depth profiles of activated Cu–Be alloy: (a) sample OT−; (b) sample OA; (c) sample OT+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
3.1. Pre-heat treatment time According to Table 1 and Fig. 1, the thickness of the BeO layer of the samples in group PH decreases with increase of pre-heat treatment time from 0 (PH−, without pre-heat treatment) to 2 h (PH+, with 2 h pre-heat treatment). Fig. 1 shows that the surface of the three samples in group PH mainly consists of BeO. The Be,
Cu, C, and copper oxides can occasionally be seen, but only in small amounts. It has been suggested that when the surface compositions are similar, as the thickness decreases, the secondary electron yield of the BeO layer increases to a maximum value at a thickness of about 20 nm [13]. Beyond this thickness the yield reduced towards that of the base alloy. The variation trend of ım of these samples in Table 1 agrees with the above suggestion, where
Fig. 6. The XPS spectra of Cu2p for the alloys after excessive activation treatment: (a) sample OA; (b) sample PH+; (c) sample OD+; (d) sample OP+; (e) sample OT+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The intensities of the Cu2p spectra of each sample have been calibrated with the Be1s spectrum of the sample etched for 15 s for comparison of the relative contents of Cu on the surface of the samples. The intensities of spectra (b), (c), and (e) have been reduced 4, 4, and 2.5 times, respectively, due to the scale limits of these spectra.
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the 1 h pre-heat treatment derives the highest secondary electron yield. The sample without pre-heat treatment (Sample PH−) exhibits the thickest BeO layer in all the examined samples. Due to the inferior electrical conductivity of BeO, a too thick BeO layer is not in favor of electron supply from the base alloy (an electron reservoir) to the electron-emitting BeO surface. On the other hand, the XPS analysis indicates that the content of O is less than that of Be on the surface of the sample. Owning to the shortage of O, the excessive metallic Be atoms on the surface will not be oxidized. The secondary electron yield of the activated Cu–Be alloys can be reduced by presence of metallic Be, copper oxides and other contaminants which have much lower ım , in particular, the metallic Be has an extremely low yield value of only 0.68 [14]. As shown in Fig. 2(a), there is generally no effective BeO layer on the cleaned sample without treatment. Therefore, during the oxidation treatment, the active O atoms at a high temperature of 700 ◦ C would easily enter the surface and migrate to the interior, and result in formation of a thick BeO layer on the surface of the sample. Therefore, the lack of a pre-heat treatment before oxidation would lead to a reduced ım . In contrast, the 1 h pre-heat treatment under vacuum for sample OA generates a very thin Be oxide film on the surface (see Fig. 2(b) for sample PO with 1 h pre-heat treatment only). The very thin Be oxide film on the surface will prevent the O atoms from extensive migration to the interior and result in an accumulation of O atoms and built-up of a greater O concentration gradient in the surface layer during the subsequent oxidation treatment. Consequently, the oxidation with a proper pre-heat treatment delivers a BeO layer with a moderate thickness and an enhancement of O in the layer with the ratio of O/Be approximately equal to 1. The pre-treatment leads to a significantly improved ım for sample OA, so that the sample delivers the highest ım in the three samples in PH group. For sample PH+, the 2 h pre-heat treatment generates a thicker Be oxide film on the surface (see Fig. 2(c) for sample PO+ with 2 h pre-heat treatment only), which will prevent the O atoms from effective migration to the interior during the subsequent oxidation treatment, and results in an even thinner BeO layer. The SEE process, involving the steps of secondary electrons generation (the inner electrons getting energy from the primary electrons), transportation to the surface, overcoming the surface barrier potential and escaping from the surface, has been extensively discussed by Bruining [15], Grais [16], Dionne [17], and many other researchers. For a too thin BeO layer, the primary electrons could penetrate the BeO layer and enter the base alloy. Therefore, part of the secondary electrons would be excited in the base alloy. The secondary electrons excited in the base alloy are much harder to move towards the surface and escape to the vacuum than those in the BeO layer due to the multiple collisions with a great number of free electrons in the alloy and losing energy. Therefore, a too thin BeO layer will lead to a shortage of secondary electrons emission, hence deliver smaller ım . On the other hand, compared to the surface of the BeO layer in sample OA, the outer surface of the BeO layer in sample PH+ appears to have a small amount of Cu. The appearance of copper oxides will also lower ım . The effects of the content and distribution of the residual Cu in the surface layer on the SEE properties of the alloy will be further analyzed later in a separate section. 3.2. Oxidation duration As shown in Fig. 3, the thickness of the BeO layer of the samples in group OD increases with oxidation duration (Table 2), indicating that the BeO layer grows with the extension of oxidation. The thickness of BeO layer increases from sample OD− up to sample OD+, whereas the ım value increases to a maximum at the optimum thickness in sample OA, and the yield reduces beyond this
thickness. As mentioned above, the too large or too small thickness of the BeO layer will result in lower secondary electron emission yield. On the other hand, compared to the surface of the BeO layer in sample OA with an optimized O/Be ratio of around 1, the surface of the BeO layer of the sample in OD group changes from a remarkable shortage of O atoms for sample OD− to a considerable excess of O atoms for sample OD+. Associated with the shortage of O atoms, the presence of excessive metallic Be atoms on the surface of sample OD− will lead to a lower ım . Whereas, the presence of excessive O atoms and small amount of Cu on the surface of sample OD+ will result in formation of copper oxides, which would also deliver smaller ım (also refer to Section 3.5). Therefore, the oxidation duration vary ım by affecting the thickness of the BeO layer, the presence of excessive metallic Be and the formation of copper oxides in the layer. 3.3. Oxygen pressure Fig. 4 and Table 3 for samples in group OP indicate that the thickness of the surface BeO layer of the sample generally increases with higher oxygen pressure during oxidation. The relationship between the thickness of the BeO layer and the ım follows the same trend of oxidation time. On the other hand, the oxygen pressure also changes the surface composition of the BeO layer, which has a significant influence on the secondary electron yield of the sample. The thickness of the BeO layer in sample OP− is only slightly less than that in sample OA, and both of them are close to 20 nm. However, the ım of sample OA is considerably better than that of sample OP−. Fig. 4 shows that the ratio of O/Be on the surface BeO layer of sample OA is much more closer to 1 than that of OP−, which is likely to be the cause of the larger ım of sample OA. For sample OP+, its ım has become significantly smaller than that of sample OA, which could be associated to its thicker BeO layer. The thick BeO layer appears to be attributed a significant lack of Cu in the layer, which will be further discussed in Section 3.5. On the other hand, the XPS analysis on sample OP+ shows that the content of O atoms is always remarkably lower than that of Be on the BeO layer, suggesting that there should be a significant amount of excessive metallic Be atoms on the layer. Therefore, both the excessive thickness of the BeO layer and the presence of metallic Be atoms in the layer would contribute to the significantly low ım of sample OP+. 3.4. Oxidation temperature The relationship between oxidation temperature of the activation process and the surface state of Cu–Be alloy has previously been studied by Fujii et al. [4]. Generally, the changing trend between oxidation temperature and ım derived from our experiment is in agreement with theirs. However, they have paid more attention to the influence of the oxidation temperature on the surface morphology of the activated Cu–Be alloys, whereas this work has been concentrated in the impact of oxidation temperature on the thickness and composition of the surface BeO layer. From Table 4 and Fig. 5 for samples in group OT, it can be seen that the thickness of the surface BeO layer increases with higher temperature during the oxidation process. The thickness of the BeO layer in sample OT− is remarkably smaller than that of sample OA and the optimized thickness of 20 nm, leads to a smaller ım . On the other hand, the thickness of sample OT+ is far greater than that of sample OA. As mentioned above, the too large thickness of the BeO layer in sample OT+ also results in decrease of ım . The effect of oxidation temperature on the O/Be ratio on the surface layer is less significant, according to the XPS analysis. However, the O/Be ratio of the BeO layers in both sample OT− and sample OT+ are still considerably smaller than 1, indicating the existence of metallic Be
B. Wang et al. / Applied Surface Science 355 (2015) 19–25
in the BeO layers. On the other hand, there appears to have substantial amount of residual Cu in the surface layer of sample OT+, which could also affect ım . The effects of oxidation temperature on the content and distribution of the residual Cu in the BeO layer and the relevant SEE properties of the alloy will be discussed in the next section. 3.5. Residual Cu in BeO layer As Cu is primarily detected on the surface of the samples after excessive activation treatments, the XPS valence band structures of Cu 2p on the surface of these samples are particularly analyzed. As shown in Fig. 6, the XPS spectra of Cu 2p for these samples indicate that Cu more or less exists on the surface of all these samples. For the optimized sample, there is nearly no Cu on the outer surface, and the content of metallic Cu gradually increases toward the inner surface of the sample. This kind of Cu distribution on the surface layer appears to be ideal for an enhanced performance of the SEE material. In comparison with that of the optimized sample, the XPS spectra of Cu 2p (a shift of the bond energy from 933 eV to ∼935 eV) for the excessively pre-heat treated sample (PH+) and the sample oxidized for excessive duration (OD+) in Fig. 6(b) and (c), respectively, indicate the existence of a substantial amount of copper oxides (refer to the caption of Fig. 6 on the intensity scales of these spectra) on the outer surface of these samples, which will be detrimental to the secondary electron yield of the samples. Whereas, the XPS spectra of Cu 2p for the samples oxidized at excessively high oxygen pressure (OP+) and the sample oxidized at excessively high temperature (OT+) in Fig. 6(d) and (e), respectively, suggest that Cu on the outer surface of these samples is primarily in the metallic state. The appearance of the metallic Cu on the outer surface can be associated with the formation process of the BeO layer on the surface of the samples under these two activation conditions. Unlike the activation conditions of excessive pre-heat treatment time and oxidation time, which could only increase the extent of the oxidation on the surface of the samples, the condition of excessively high oxygen pressure or temperature will speed up the formation of BeO layer on the surface of the samples. The quickly formed BeO layer on the surface of the samples could prevent the small amount of residual metallic Cu from further oxidation. On the other hand, as shown in Fig. 6(d) and (e), these two different activation treatments have shown remarkably different effects on the distribution of Cu on the surface of the samples. The excessively high oxygen pressure appears to lead to a significant deficiency of Cu on the surface layer of the sample. Although a small amount of Cu is retained on the outer surface of the sample, the content of Cu has remarkably decreased toward the inner surface of the sample. Besides the large thickness of the Be oxide layer, this kind of Cu distribution on the surface layer of the sample is opposite to that of the optimized sample, and could be an important affecting factor for the worse secondary electron yield of the alloy. In contrast, the excessively high temperature appears to dilute the segregation of Be on the surface of the sample, and lead to a significant surplus of Cu in the surface layer of the sample, which would also be detrimental to the secondary electron yield of the alloy. However, such a detrimental effect could be partially counterbalanced by the favorable distribution of Cu in the surface layer (similar to that of the optimized sample), therefore the secondary electron yield of this sample have not shown significant deterioration from that of the optimized sample. 4. Conclusions (1) The thickness and composition distribution of the surface BeO layer of the Cu–Be alloy are closely related to both pre-heat
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treatment and affecting factors (oxidation duration, oxidation temperature, and oxygen pressure) of the oxidation process in the activation treatment. (2) The thickness of the surface BeO layer has great influence on the secondary electron yield of the activated Cu–Be alloy. The best secondary electron yield corresponds to an alloy with a moderate thickness of BeO layer. A lack of pre-heat treatment results in very thick BeO layer, whereas an excessive pre-heat treatment lead to thinner BeO layer in the alloy. The thickness of the BeO layer in the activated alloy increases with longer duration, higher temperature and greater oxygen pressure of the oxidation process. (3) The lack of pre-heat treatment and oxidation duration and improper temperature and oxygen pressure of the oxidation process also result in presence of unoxidized Be in the surface BeO layer and decrease of secondary electron yield of the activated Cu–Be alloy. (4) The extension of pre-heat treatment and oxidation duration lead to formation of Cu oxides on the surface BeO layer and reduce the SEE properties of the alloy. The excessively high oxygen pressure or temperature speed up formation of BeO layer and prevent the residual metallic Cu from further oxidation. The excessively high oxygen pressure lead to a poor distribution and significant deficiency of residual Cu in the BeO layer, whereas the excessively high temperature leads to a surplus of Cu in the BeO layer, which also reduce the SEE properties of the alloy. Acknowledgment The authors are grateful to Mr. Yongqiang Wu, for supplying technical support and counseling. References [1] A.H. Sommer, Activation of silver–magnesium and copper–beryllium dynodes, J. Appl. Phys. 29 (1958) 598. [2] M.B. Pongratz, Cleaning and activation of beryllium–copper electron multiplier dynodes, Rev. Sci. Instrum. 43 (1972) 1714. [3] Y. Fujii, F. Kanematsu, Activation treatment study of Cu–Be alloy and its secondary electron emission characteristics, J. Mass Spectrom. Soc. Jpn. 18 (1970) 994. [4] Y. Fujii, F. Kanematsu, T. Koshikawa, E. Sugata, Surface analysis of Cu–Be dynode, J. Vac. Sci. Technol. 17 (1980) 1221. [5] F.E. Ruttenberg, Surface composition and morphology vs secondary-electron yield of Be–Cu dynodes, J. Vac. Sci. Technol. 12 (1975) 1043. [6] J.C.C. Fan, V.E. Henrich, Preparation and properties of sputtered MgO/Au, MgO/Ag, and MgO/Ni cermet films, J. Appl. Phys. 45 (1974) 3742. [7] V.E. Henrich, J.C.C. Fan, Effects of cesiation on secondary-electron emission from MgO/Au cermets, J. Appl. Phys. 45 (1974) 5484. [8] A. Shih, Secondary electron emission from diamond surfaces, J. Appl. Phys. 82 (1997) 1860. [9] E.J. Yater, A. Shih, R. Abrams, Electron transport and emission properties of diamond, J. Vac. Sci. Technol. A 16 (1998) 913. [10] L. Jeonghee, J. Taewon, Y. SeGi, J. Sunghwan, H. Jungna, Y. Whikun, D. Jeonb, J.M. Kim, Thickness effect on secondary electron emission of MgO layers, Appl. Surf. Sci. 174 (2001) 62. [11] T.L. Barr, ESCA studies of metals and alloys: Oxidation, migration and dealloying of Cu-based systems, Surf. Interface Anal. 4 (1982) 185. [12] T.L. Barr, X-ray photoelectron spectroscopy as a method for monitoring segregation in alloys: Cu–Be, Chem. Phys. Lett. 43 (1976) 89. [13] A. Dallos, E. Shapiro, B. Shaw, Secondary emission of beryllia on beryllium, IEEE Trans. Electron Dev. 11 (1992) 2611. [14] M. Sulemani, E.B. Pattinson, The SEE yield changes in slowly oxidised Be with surface characterisation by AES, J. Phys. D 13 (1980) 693. [15] H. Bruining, Physics and Applications of Secondary Emission, McGraw-Hill, New York, 1954. [16] K.I. Grais, A.M. Bastawros, A study of secondary electron emission in insulators and semiconductors, J. Appl. Phys. 53 (1982) 5239. [17] F. Gerald, Dionne, Origin of secondary electron emission yield curve parameters, J. Appl. Phys. 46 (1975) 3347.
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Activation treatment effects on characteristics of BeO layer and secondary electron emission properties of an activated Cu–Be alloy Bin Wang, Erdong Wu, Yongli Wang, Liangyin Xiong, Shi Liu ∗ Materials for Special Environment Department, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 8 March 2015 Received in revised form 3 June 2015 Accepted 29 June 2015 Available online 11 July 2015 Keywords: Thickness and composition distribution of BeO layer Oxidation duration Oxidation temperature Pre-heat treatment Oxygen pressure
a b s t r a c t Dependence of characteristics of surface BeO layer and secondary electron emission properties of a Cu–Be alloy on activation conditions were studied by measurements of X-ray photoelectron spectroscopy and secondary electron yield of the alloy based on varying conditions of an optimized activation treatment. The results showed that the thickness and composition distribution of the surface BeO layer of the alloy were closely related to the pre-heat treatment and the affecting factors (oxidation duration, oxidation temperature, and oxygen pressure) of the oxidation process in the activation treatment. The improper treatment leaded to a too thick or too thin surface BeO layer, the presence of metallic Be, and poor distribution of metallic and oxidized Cu in the layer, hence significantly reduced the secondary electron emission yield of the activated Cu–Be alloy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction High, stable, and durable secondary electron emission is essentially necessary for application of dynodes of electron multipliers and photomultiplier tubes. The activated Cu–Be alloys have been widely used as an alloy type of secondary electron emission (SEE) material for these applications owing to their good emission properties. The activation process plays a key role on the secondary electron yield (ı) of the Cu–Be alloys, and has been partially investigated by means of scanning electron microscopy (SEM), Augur electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) in the earlier time of 1950s–1980s [1–5]. However, further development and study on the Cu–Be alloys have been rather limited in recent years. The toxicity of Be compounds is probably a reason, but the most important is that the main efforts of the researchers nowadays have been concentrated on the development of new materials with higher SEE properties. Some novel SEE materials, such as MgO/Au cermet films [6,7], hydrogen terminated and Cs terminated chemical vapor deposition (CVD) diamond [8,9], have been developed in recent years, and shown promising SEE properties. Nevertheless, up to now, the activated Cu–Be alloys are still used as a major SEE material due to their high SEE coefficients. In comparison with another widely used SEE material of Mg alloys, the Cu–Be alloys are more stable at high temperatures
∗ Corresponding author. Tel.: +86 024 2397 1968; fax: +86 024 2397 1968. E-mail address: [email protected] (S. Liu). http://dx.doi.org/10.1016/j.apsusc.2015.06.189 0169-4332/© 2015 Elsevier B.V. All rights reserved.
and have some other advantages. On the other hand, unlike that of the Pb used as an electronic packing material which in most cases is exposed to the environment in working conditions, the Cu–Be alloys would be sealed in the electron multiplier when working as dynode. Moreover, the typical content of Be in the Cu–Be alloys is very low (Cu–2.75Be), and the toxicity is only involved in the fabrication and waste disposal of the material. Therefore, the impact of toxicity of the alloy on human and environment is controllable and the further study on improvement of this important SEE material is necessary. The formation of oxide layer during activation has a significant influence on the secondary electron emission properties of the alloy materials. For instance, it is observed that the maximum SEE yield is achieved when the thickness of the MgO film on a Si substrate is close to the penetration depth of the primary electrons under low electric potential [10]. For the Cu–Be alloys, it is known that the characteristics of the BeO layer, i.e. its thickness, composition, and topography, depend on the activation conditions. However, although the activation process includes pre-heat treatment and oxidation treatment involving several factors, such as temperature, duration, and pressure, only the factors of oxygen pressure and oxidation temperature in the oxidation have been studied, whereas the oxidation duration and pre-heat treatment have seldom been considered. On the other hand, we note that the relationship between the thickness, composition, and topography of the BeO layer and the activation conditions of the Cu–Be alloys have not been thoroughly investigated in previous studies. Therefore, we have carried out an investigation on the activation process of the Cu–Be alloys
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and paid more attention to the oxidation duration and pre-heat treatment of the process as well. In the investigation, the four factors in the activation process, i.e. the pre-heat treatment, oxidation duration, oxygen pressure, and oxidation temperature, respectively, have been examined separately for clarification of the role of each factor on the process and for more understanding of the formation mechanism of the surface BeO layer. On the other hand, the dependence of the characteristics of the BeO layer on activation conditions and the impact of the BeO layer on the secondary electron yield (ı) of the alloys have been explored. The relevant literatures on activation have suggested that the condition for the activation treatment of Cu–Be alloys would be in the following range: oxidation temperature from 500 ◦ C to 900 ◦ C; oxygen pressure below 10 Pa; and the oxidation duration between 2 min and several hours [1–5]. The condition for the preheat treatment has not been particularly suggested, but appears to be in the same range except for under a vacuum rather than under an oxygen pressure environment. Based on the above conditions, we have performed a comprehensive search for optimization of the activation conditions of a Cu–Be alloy based on the measurement of the corresponding secondary electron yield (ı) of the alloy. An optimized condition has been selected from nearly a hundred experiments. In this paper, the effects of the activation process on the properties of the SEE of the activated Cu–Be alloy are exhibited and discussed based on comparison of the characteristics and performance of the surface BeO layer formed by separately varying the four affecting factors of the optimized activation condition. The results show that the characteristics of the BeO layer and in particular its thickness and composition (residual metallic and oxidized Cu and metallic Be) distribution, is closely related to all four factors of the activation process, and the oxidation duration and pre-heat treatment take the same importance as the oxidation temperature and oxygen pressure and significantly affect the SEE properties of the activated alloy.
2. Experiments The Cu–2.75Be (copper alloy containing 2.75 wt.% beryllium with a purity of 99.5%) plates with a dimension of 10 mm × 10 mm × 0.2 mm were used for the samples. The surface of the samples was polished with diamond powder and rinsed in anhydrous alcohol to remove surface contaminants and oxides. The activation process of all the samples was carried in a high vacuum system. The high vacuum system comprises a bakeable, stainless steel vacuum chamber with internal heaters and thermocouples, an oil-free high vacuum turbo pump, and a gas flowmeter, capable of 7 × 10−6 Pa base pressure. Based on the optimized activation condition (denoted as OA), the samples were divided into four groups, corresponding to varying the affecting factors of preheat treatment time (PH), oxidation duration (OD), oxygen pressure (OP), and oxidation temperature (OT), respectively. A subgroup in PH was specially denoted as PO, defining an activation condition for pre-heat treatment only without further oxidation treatment. For each affecting factor, the insufficient and excessive treatments were denoted as “−” and “+”, respectively, after the symbols of the factor. The details of the activation conditions of the samples in the four groups were listed in Tables 1–4, respectively. The preheat treatment was performed at a fixed temperature of 600 ◦ C and a high vacuum of about 3 × 10−4 Pa for selected time. The oxidation treatment was conducted at selected temperatures and oxygen pressures for selected time. After the activation, the secondary electron yield (ı) of the samples was measured by a test assembly similar to that described in [10]. The assembly contained a vacuum system, an electron source
Table 1 Variation in pre-heat treatment time (tp /h), ım is the maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are further activated at oxygen pressure 0.65 Pa and 700 ◦ C for 15 min, except for PO and PO+, which only have pre-heat treatment without further oxidation treatment. Sample
Pre-heat treatment (h)
ım
PH− OA PH+ PO PO+
0 1 2 1 2
7.0 9.2 8.2 6.3 7.5
Thickness (nm) ± ± ± ± ±
0.4 0.4 0.4 0.4 0.4
43 23 17 12 16
± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
Table 2 Variation in oxidation duration (t/min), ım is maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are activated at oxygen pressure 0.65 Pa and 700 ◦ C with 1 h pre-heat treatment. Sample
Oxidation duration (min)
ım
Thickness (nm)
OD− OA OD+
3 15 30
6.4 ± 0.4 9.2 ± 0.4 6.0 ± 04
18 ± 0.5 23 ± 0.5 33 ± 0.5
Table 3 Variation in oxygen pressure (P/Pa), ım is maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are activated at 700 ◦ C for 15 min with 1 h pre-heat treatment. Sample
Oxygen pressure (Pa)
ım
Thickness (nm)
OP− OA OP+
10−4 0.65 5
7.2 ± 0.4 9.2 ± 0.4 3.6 ± 0.4
19 ± 0.5 23 ± 0.5 32 ± 0.5
system and a target testing system. Once the primary current, Ip , was measured from electron gun and secondary current, Is , was measured with the Faraday cup, the secondary electron yield (ı) was calculated based on the expression of ı = Is /Ip . The ı comprised both the secondary electron (with energies below 50 eV) and the reflected primaries (with almost all the incident energy). The sample bias voltage, Vb , was −65 V. The maximum secondary electron yield (ım ) of each sample derived based on a curve between ı and primary electron energy was listed in the relevant tables. The surface products and depth profiles of the activated samples were estimated by X-ray photoelectron spectroscopy (XPS) with Mg K␣ (1253.6 eV) and Al K␣ (1486.6 eV) twin anode X-ray using an ESCALAB250 system. The Ar+ -ion repetitive bombarded etching was applied to obtain the quantification and valence states of the elements along the film depth. The sputtering rate was 0.1 nm/s for the Ar+ -ion gun of 3 kV and 2 A. 3. Results and discussion The profiles of the depth concentration of the samples in the four groups derived from XPS are plotted against sputtering time and shown in Figs. 1–5, respectively. As the BeO layer on the surface is responsible for the emission of secondary electron, the performance of the activated Cu–Be alloys is closely related to
Table 4 Variation in oxidation temperature (T/◦ C), ım is maximum secondary electron yield, and thickness is for BeO layer of the samples. The samples are activated at oxygen pressure 0.65 Pa for 15 min with 1 h pre-heat treatment. Sample
Oxidation temperature (◦ C)
ım
Thickness (nm)
OT− OA OT+
600 700 800
8.6 ± 0.4 9.2 ± 0.4 8.0 ± 0.4
13 ± 0.5 23 ± 0.5 >35
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Fig. 1. Quantitative depth profiles of the activated Cu–Be alloy: (a) sample PH−; (b) sample OA; (c) sample PH+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
characteristics of the BeO layer. The XPS analysis shows that the BeO layers occur on the surface of all activated samples, agreeing well with the previous results. According to Barr [11,12], the initial growth of beryllium oxide (BeO) on the surface forms islands of BeO in a sea of Cu–Be alloy, while the continuous growth of BeO eventually covers almost entire outer surface of the Cu–Be alloy. Although the surface composition and thickness of the BeO layer of the samples are different from each other, the content of the formed
BeO generally decreases from surface to the interior and eventually disappears inside of the samples. According to the observed atomic ratio of O/Be, the metallic Be starts to appear on the surface or the superficial layer of the surface, and basically increases along the depth to the content of the base alloy. On the other hand, the variation trend of the metallic Cu content has some similar with that of the metallic Be. During the SEE process, electrical conductivity of the BeO layer is considered to be another important factor, since at
Fig. 2. Quantitative depth profiles of the Cu–Be alloy: (a) cleaned alloy sample without any treatment; (b) sample PO; (c) sample PO+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer. Note that the scale of the etch time is only 1/3 of that in Fig. 1.
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Fig. 3. Quantitative depth profiles of activated Cu–Be alloy: (a) sample OD−; (b) sample OA; (c) sample OD+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
least a certain level of electrical conductivity is necessary to replenish new electrons to the electron deficient surface. If the content of Cu is more than 20 at.% in the BeO layer, the electrical conductivity of the BeO layer has little effect on electron supply [6]. Therefore, the thickness of the BeO layer on the surface of the activated Cu–Be alloy has been defined as the thickness of the layer from surface to the interior in which the Cu content reaches 20 at.%, and listed
in the third column of the four tables. Due to the importance of the states, content, and distribution of the residual Cu in the BeO layer on SEE properties of the alloy, the XPS valence band structures of Cu 2p on the surface of the excessively activated Cu–Be alloy are particularly shown in Fig. 6 for understanding of the effects of the activation process on the SEE properties of the activated Cu–Be alloy.
Fig. 4. Quantitative depth profiles of activated Cu–Be alloy: (a) sample OP−; (b) sample OA; (c) sample OP+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
B. Wang et al. / Applied Surface Science 355 (2015) 19–25
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Fig. 5. Quantitative depth profiles of activated Cu–Be alloy: (a) sample OT−; (b) sample OA; (c) sample OT+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The dash line marks the thickness of the BeO layer, and the magnified profile within the BeO layer is displayed on the top right corner of each diagram.
3.1. Pre-heat treatment time According to Table 1 and Fig. 1, the thickness of the BeO layer of the samples in group PH decreases with increase of pre-heat treatment time from 0 (PH−, without pre-heat treatment) to 2 h (PH+, with 2 h pre-heat treatment). Fig. 1 shows that the surface of the three samples in group PH mainly consists of BeO. The Be,
Cu, C, and copper oxides can occasionally be seen, but only in small amounts. It has been suggested that when the surface compositions are similar, as the thickness decreases, the secondary electron yield of the BeO layer increases to a maximum value at a thickness of about 20 nm [13]. Beyond this thickness the yield reduced towards that of the base alloy. The variation trend of ım of these samples in Table 1 agrees with the above suggestion, where
Fig. 6. The XPS spectra of Cu2p for the alloys after excessive activation treatment: (a) sample OA; (b) sample PH+; (c) sample OD+; (d) sample OP+; (e) sample OT+. The etch time of 100 s corresponds to the depth of about 10 nm on the surface of the sample. The intensities of the Cu2p spectra of each sample have been calibrated with the Be1s spectrum of the sample etched for 15 s for comparison of the relative contents of Cu on the surface of the samples. The intensities of spectra (b), (c), and (e) have been reduced 4, 4, and 2.5 times, respectively, due to the scale limits of these spectra.
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the 1 h pre-heat treatment derives the highest secondary electron yield. The sample without pre-heat treatment (Sample PH−) exhibits the thickest BeO layer in all the examined samples. Due to the inferior electrical conductivity of BeO, a too thick BeO layer is not in favor of electron supply from the base alloy (an electron reservoir) to the electron-emitting BeO surface. On the other hand, the XPS analysis indicates that the content of O is less than that of Be on the surface of the sample. Owning to the shortage of O, the excessive metallic Be atoms on the surface will not be oxidized. The secondary electron yield of the activated Cu–Be alloys can be reduced by presence of metallic Be, copper oxides and other contaminants which have much lower ım , in particular, the metallic Be has an extremely low yield value of only 0.68 [14]. As shown in Fig. 2(a), there is generally no effective BeO layer on the cleaned sample without treatment. Therefore, during the oxidation treatment, the active O atoms at a high temperature of 700 ◦ C would easily enter the surface and migrate to the interior, and result in formation of a thick BeO layer on the surface of the sample. Therefore, the lack of a pre-heat treatment before oxidation would lead to a reduced ım . In contrast, the 1 h pre-heat treatment under vacuum for sample OA generates a very thin Be oxide film on the surface (see Fig. 2(b) for sample PO with 1 h pre-heat treatment only). The very thin Be oxide film on the surface will prevent the O atoms from extensive migration to the interior and result in an accumulation of O atoms and built-up of a greater O concentration gradient in the surface layer during the subsequent oxidation treatment. Consequently, the oxidation with a proper pre-heat treatment delivers a BeO layer with a moderate thickness and an enhancement of O in the layer with the ratio of O/Be approximately equal to 1. The pre-treatment leads to a significantly improved ım for sample OA, so that the sample delivers the highest ım in the three samples in PH group. For sample PH+, the 2 h pre-heat treatment generates a thicker Be oxide film on the surface (see Fig. 2(c) for sample PO+ with 2 h pre-heat treatment only), which will prevent the O atoms from effective migration to the interior during the subsequent oxidation treatment, and results in an even thinner BeO layer. The SEE process, involving the steps of secondary electrons generation (the inner electrons getting energy from the primary electrons), transportation to the surface, overcoming the surface barrier potential and escaping from the surface, has been extensively discussed by Bruining [15], Grais [16], Dionne [17], and many other researchers. For a too thin BeO layer, the primary electrons could penetrate the BeO layer and enter the base alloy. Therefore, part of the secondary electrons would be excited in the base alloy. The secondary electrons excited in the base alloy are much harder to move towards the surface and escape to the vacuum than those in the BeO layer due to the multiple collisions with a great number of free electrons in the alloy and losing energy. Therefore, a too thin BeO layer will lead to a shortage of secondary electrons emission, hence deliver smaller ım . On the other hand, compared to the surface of the BeO layer in sample OA, the outer surface of the BeO layer in sample PH+ appears to have a small amount of Cu. The appearance of copper oxides will also lower ım . The effects of the content and distribution of the residual Cu in the surface layer on the SEE properties of the alloy will be further analyzed later in a separate section. 3.2. Oxidation duration As shown in Fig. 3, the thickness of the BeO layer of the samples in group OD increases with oxidation duration (Table 2), indicating that the BeO layer grows with the extension of oxidation. The thickness of BeO layer increases from sample OD− up to sample OD+, whereas the ım value increases to a maximum at the optimum thickness in sample OA, and the yield reduces beyond this
thickness. As mentioned above, the too large or too small thickness of the BeO layer will result in lower secondary electron emission yield. On the other hand, compared to the surface of the BeO layer in sample OA with an optimized O/Be ratio of around 1, the surface of the BeO layer of the sample in OD group changes from a remarkable shortage of O atoms for sample OD− to a considerable excess of O atoms for sample OD+. Associated with the shortage of O atoms, the presence of excessive metallic Be atoms on the surface of sample OD− will lead to a lower ım . Whereas, the presence of excessive O atoms and small amount of Cu on the surface of sample OD+ will result in formation of copper oxides, which would also deliver smaller ım (also refer to Section 3.5). Therefore, the oxidation duration vary ım by affecting the thickness of the BeO layer, the presence of excessive metallic Be and the formation of copper oxides in the layer. 3.3. Oxygen pressure Fig. 4 and Table 3 for samples in group OP indicate that the thickness of the surface BeO layer of the sample generally increases with higher oxygen pressure during oxidation. The relationship between the thickness of the BeO layer and the ım follows the same trend of oxidation time. On the other hand, the oxygen pressure also changes the surface composition of the BeO layer, which has a significant influence on the secondary electron yield of the sample. The thickness of the BeO layer in sample OP− is only slightly less than that in sample OA, and both of them are close to 20 nm. However, the ım of sample OA is considerably better than that of sample OP−. Fig. 4 shows that the ratio of O/Be on the surface BeO layer of sample OA is much more closer to 1 than that of OP−, which is likely to be the cause of the larger ım of sample OA. For sample OP+, its ım has become significantly smaller than that of sample OA, which could be associated to its thicker BeO layer. The thick BeO layer appears to be attributed a significant lack of Cu in the layer, which will be further discussed in Section 3.5. On the other hand, the XPS analysis on sample OP+ shows that the content of O atoms is always remarkably lower than that of Be on the BeO layer, suggesting that there should be a significant amount of excessive metallic Be atoms on the layer. Therefore, both the excessive thickness of the BeO layer and the presence of metallic Be atoms in the layer would contribute to the significantly low ım of sample OP+. 3.4. Oxidation temperature The relationship between oxidation temperature of the activation process and the surface state of Cu–Be alloy has previously been studied by Fujii et al. [4]. Generally, the changing trend between oxidation temperature and ım derived from our experiment is in agreement with theirs. However, they have paid more attention to the influence of the oxidation temperature on the surface morphology of the activated Cu–Be alloys, whereas this work has been concentrated in the impact of oxidation temperature on the thickness and composition of the surface BeO layer. From Table 4 and Fig. 5 for samples in group OT, it can be seen that the thickness of the surface BeO layer increases with higher temperature during the oxidation process. The thickness of the BeO layer in sample OT− is remarkably smaller than that of sample OA and the optimized thickness of 20 nm, leads to a smaller ım . On the other hand, the thickness of sample OT+ is far greater than that of sample OA. As mentioned above, the too large thickness of the BeO layer in sample OT+ also results in decrease of ım . The effect of oxidation temperature on the O/Be ratio on the surface layer is less significant, according to the XPS analysis. However, the O/Be ratio of the BeO layers in both sample OT− and sample OT+ are still considerably smaller than 1, indicating the existence of metallic Be
B. Wang et al. / Applied Surface Science 355 (2015) 19–25
in the BeO layers. On the other hand, there appears to have substantial amount of residual Cu in the surface layer of sample OT+, which could also affect ım . The effects of oxidation temperature on the content and distribution of the residual Cu in the BeO layer and the relevant SEE properties of the alloy will be discussed in the next section. 3.5. Residual Cu in BeO layer As Cu is primarily detected on the surface of the samples after excessive activation treatments, the XPS valence band structures of Cu 2p on the surface of these samples are particularly analyzed. As shown in Fig. 6, the XPS spectra of Cu 2p for these samples indicate that Cu more or less exists on the surface of all these samples. For the optimized sample, there is nearly no Cu on the outer surface, and the content of metallic Cu gradually increases toward the inner surface of the sample. This kind of Cu distribution on the surface layer appears to be ideal for an enhanced performance of the SEE material. In comparison with that of the optimized sample, the XPS spectra of Cu 2p (a shift of the bond energy from 933 eV to ∼935 eV) for the excessively pre-heat treated sample (PH+) and the sample oxidized for excessive duration (OD+) in Fig. 6(b) and (c), respectively, indicate the existence of a substantial amount of copper oxides (refer to the caption of Fig. 6 on the intensity scales of these spectra) on the outer surface of these samples, which will be detrimental to the secondary electron yield of the samples. Whereas, the XPS spectra of Cu 2p for the samples oxidized at excessively high oxygen pressure (OP+) and the sample oxidized at excessively high temperature (OT+) in Fig. 6(d) and (e), respectively, suggest that Cu on the outer surface of these samples is primarily in the metallic state. The appearance of the metallic Cu on the outer surface can be associated with the formation process of the BeO layer on the surface of the samples under these two activation conditions. Unlike the activation conditions of excessive pre-heat treatment time and oxidation time, which could only increase the extent of the oxidation on the surface of the samples, the condition of excessively high oxygen pressure or temperature will speed up the formation of BeO layer on the surface of the samples. The quickly formed BeO layer on the surface of the samples could prevent the small amount of residual metallic Cu from further oxidation. On the other hand, as shown in Fig. 6(d) and (e), these two different activation treatments have shown remarkably different effects on the distribution of Cu on the surface of the samples. The excessively high oxygen pressure appears to lead to a significant deficiency of Cu on the surface layer of the sample. Although a small amount of Cu is retained on the outer surface of the sample, the content of Cu has remarkably decreased toward the inner surface of the sample. Besides the large thickness of the Be oxide layer, this kind of Cu distribution on the surface layer of the sample is opposite to that of the optimized sample, and could be an important affecting factor for the worse secondary electron yield of the alloy. In contrast, the excessively high temperature appears to dilute the segregation of Be on the surface of the sample, and lead to a significant surplus of Cu in the surface layer of the sample, which would also be detrimental to the secondary electron yield of the alloy. However, such a detrimental effect could be partially counterbalanced by the favorable distribution of Cu in the surface layer (similar to that of the optimized sample), therefore the secondary electron yield of this sample have not shown significant deterioration from that of the optimized sample. 4. Conclusions (1) The thickness and composition distribution of the surface BeO layer of the Cu–Be alloy are closely related to both pre-heat
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treatment and affecting factors (oxidation duration, oxidation temperature, and oxygen pressure) of the oxidation process in the activation treatment. (2) The thickness of the surface BeO layer has great influence on the secondary electron yield of the activated Cu–Be alloy. The best secondary electron yield corresponds to an alloy with a moderate thickness of BeO layer. A lack of pre-heat treatment results in very thick BeO layer, whereas an excessive pre-heat treatment lead to thinner BeO layer in the alloy. The thickness of the BeO layer in the activated alloy increases with longer duration, higher temperature and greater oxygen pressure of the oxidation process. (3) The lack of pre-heat treatment and oxidation duration and improper temperature and oxygen pressure of the oxidation process also result in presence of unoxidized Be in the surface BeO layer and decrease of secondary electron yield of the activated Cu–Be alloy. (4) The extension of pre-heat treatment and oxidation duration lead to formation of Cu oxides on the surface BeO layer and reduce the SEE properties of the alloy. The excessively high oxygen pressure or temperature speed up formation of BeO layer and prevent the residual metallic Cu from further oxidation. The excessively high oxygen pressure lead to a poor distribution and significant deficiency of residual Cu in the BeO layer, whereas the excessively high temperature leads to a surplus of Cu in the BeO layer, which also reduce the SEE properties of the alloy. Acknowledgment The authors are grateful to Mr. Yongqiang Wu, for supplying technical support and counseling. References [1] A.H. Sommer, Activation of silver–magnesium and copper–beryllium dynodes, J. Appl. Phys. 29 (1958) 598. [2] M.B. Pongratz, Cleaning and activation of beryllium–copper electron multiplier dynodes, Rev. Sci. Instrum. 43 (1972) 1714. [3] Y. Fujii, F. Kanematsu, Activation treatment study of Cu–Be alloy and its secondary electron emission characteristics, J. Mass Spectrom. Soc. Jpn. 18 (1970) 994. [4] Y. Fujii, F. Kanematsu, T. Koshikawa, E. Sugata, Surface analysis of Cu–Be dynode, J. Vac. Sci. Technol. 17 (1980) 1221. [5] F.E. Ruttenberg, Surface composition and morphology vs secondary-electron yield of Be–Cu dynodes, J. Vac. Sci. Technol. 12 (1975) 1043. [6] J.C.C. Fan, V.E. Henrich, Preparation and properties of sputtered MgO/Au, MgO/Ag, and MgO/Ni cermet films, J. Appl. Phys. 45 (1974) 3742. [7] V.E. Henrich, J.C.C. Fan, Effects of cesiation on secondary-electron emission from MgO/Au cermets, J. Appl. Phys. 45 (1974) 5484. [8] A. Shih, Secondary electron emission from diamond surfaces, J. Appl. Phys. 82 (1997) 1860. [9] E.J. Yater, A. Shih, R. Abrams, Electron transport and emission properties of diamond, J. Vac. Sci. Technol. A 16 (1998) 913. [10] L. Jeonghee, J. Taewon, Y. SeGi, J. Sunghwan, H. Jungna, Y. Whikun, D. Jeonb, J.M. Kim, Thickness effect on secondary electron emission of MgO layers, Appl. Surf. Sci. 174 (2001) 62. [11] T.L. Barr, ESCA studies of metals and alloys: Oxidation, migration and dealloying of Cu-based systems, Surf. Interface Anal. 4 (1982) 185. [12] T.L. Barr, X-ray photoelectron spectroscopy as a method for monitoring segregation in alloys: Cu–Be, Chem. Phys. Lett. 43 (1976) 89. [13] A. Dallos, E. Shapiro, B. Shaw, Secondary emission of beryllia on beryllium, IEEE Trans. Electron Dev. 11 (1992) 2611. [14] M. Sulemani, E.B. Pattinson, The SEE yield changes in slowly oxidised Be with surface characterisation by AES, J. Phys. D 13 (1980) 693. [15] H. Bruining, Physics and Applications of Secondary Emission, McGraw-Hill, New York, 1954. [16] K.I. Grais, A.M. Bastawros, A study of secondary electron emission in insulators and semiconductors, J. Appl. Phys. 53 (1982) 5239. [17] F. Gerald, Dionne, Origin of secondary electron emission yield curve parameters, J. Appl. Phys. 46 (1975) 3347.