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Thickness effect on secondary electron emission of MgO layers
Applied Surface Science 174 (2001) 62±69
Thickness effect on secondary electron emission of MgO layers Jeonghee Leea, Taewon Jeonga, SeGi Yua, Sunghwan Jina, Jungna Heoa, Whikun Yia, D. Jeonb, J.M. Kima,* a
National Creative Research Initiatives, Center for Electron Emission Source, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea b Department of Physics, Myong Ji University, Yongin Kyunggi-do 449-728, South Korea Received 20 October 2000; accepted 31 December 2000
Abstract Two series of MgO thin layers having various thicknesses were prepared on the Si substrate by electron-beam evaporation and by spin coating of MgO precursor solutions. We found that the magnitude of the secondary electron emission (SEE) yield of the MgO ®lms strongly depends on the ®lm thickness and the sample bias voltage. We ascribed it to the electric ®eld through the insulating MgO layer, which allowed fast supply of electrons from the Si substrate to the surface. The mechanism of electron supply can be explained either as an acceleration through the MgO layer that becomes partially conductive upon primary electrons bombardment (radiation induced conductivity), or as a tunneling through the non-irradiated region of the insulating layer where the primary electrons cannot reach deeply into the sample with a certain penetration depth. The maximum SEE yield of the each MgO ®lm on the Si substrate was observed when the penetration depth of primary electrons was close to the thickness of the MgO ®lm, if the applied electric potential to the sample was low. Under a strong electric potential, the relationship between the penetration depth of primary electrons and the thickness of MgO ®lms is not observed. It suggests the existence of the non-irradiated region, where electron supply is allowed by electron tunneling. Therefore, the magnitude of SEE yield for the thin insulating layer is strongly related to the detailed mechanism of electron supply, which is determined by the thickness of the insulating layer and the applied bias voltage to the sample during the SEE process. # 2001 Published by Elsevier Science B.V. PACS: 79.20.Hx Keywords: Secondary electron; Penetration depth; Thin insulating ®lm; MgO
1. Introduction Microchannel plates (MCPs), which utilize electron multiplication by secondary electron emission (SEE), have been extensively studied due to the diverse demands from conventional particle detectors to a recently developed ®eld emission display (FED) * Corresponding author. E-mail address: [email protected] (J.M. Kim).
incorporated with an MCP [1,2]. It is well known that insulating materials exhibit higher SEE yield than semiconductors or metals [3±7]. The wide energy band gap in an insulator prevents internal secondary electrons (SEs) from losing energy through collision with conduction electrons, resulting in a large escape depth of the SEs and a large SEE yield (d). Generally, the mechanism of SEE can be simpli®ed by the following processes: (i) the primary electrons penetrate into a certain depth of an insulating layer; (ii)
0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 0 1 5 - 0
J. Lee et al. / Applied Surface Science 174 (2001) 62±69
through collision, the energy of the primary electrons is transferred to the bound electrons of the insulator leading to a release of electrons; (iii) the released electrons migrate to the surface and escape into the vacuum as secondary electrons. During the SEE process, electrical conductivity is considered to be an important factor, since at least a certain level of electrical conductivity is necessary to replenish new electrons to the electron de®cient surface. A build-up of positive charges at the electronemitting surface is originated from the emission when the number of SEs escaped into the vacuum is larger than that of input primary electrons (d > 1). The SEE on insulators has been studied widely for the last few decades, and a number of interests were focused on the insulating property related to charging effect [3±7]. Several techniques were employed to overcome the charging effect; pulse mode technique [8,9], preparation of cermet system [10], and application of a thin layer [11,12]. Magnesium oxide (MgO) [8±14] is a representative material showing a high SEE yield before the recent discovery of the surface-treated diamond whose SEE yield is found to be as high as 130 [15±17]. It was reported that the magnitude of SEE yield in MgO has a wide range of values from 2 to 22. Although the origin of the low SEE yield is presumed by poor surface treatment with a lack of high vacuum condition, the large variation of the previously reported values primarily comes from the charging effect. Besides the charging effect, the large difference of SEE yield may also be originated from the crystal quality of MgO itself, for example, existence of impurities or defects. Cazaux et al. explained that the large decrease in SEE yield for polycrystalline insulators is caused by a reduced mean free path of secondaries through scattering and trapping at defects [13]. Even though there are other kinds of dif®culties on measuring the SEE yield in insulators, the charging of a sample during SEE process gives a major in¯uence. In this paper, we report the SEE yields of MgO ®lms with respect to their thickness in order to investigate the surface charging effect and its releasing mechanism during SEE process. Films were prepared either by electron-beam evaporation or by spin coating of MgO precursor solutions [18,19]. Our experiments were performed at two different bias voltages on a MgO specimen, i.e. ÿ45 and ÿ1000 V, to apply the
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electric ®eld to the insulating layer. Most of previously reported SEE yields in literature adopted the measurement setup applying a variable negative bias potential to the sample with a ®xed primary electron energy for the variation of net primary electron energy [11]. In those cases, the maximum SEE yield was obtained when the sample bias voltage was very negative. For example, the sample bias voltage becomes ÿ1000 V for the net primary electron energy of 500 eV, if a ®xed energy of primary electrons with 1500 eV is used. This situation is similar to the measurement at V b ÿ1000 V in this work. The variation of a ®xed sample bias voltage during SEE process brings a partial understanding of electron supplying mechanism of an insulating layer on a conductive substrate. 2. Experimental A series of MgO ®lms with thickness of 100, 300, Ê were prepared by elec500, 700, 1000, and 2000 A tron-beam evaporation. The size of a Si wafer (phosphorus doped n-type (1 0 0) and 1±30 O cm of resistivity) was about 2 cm 2 cm. Substrate temperature, O2 ¯ow rate, and deposition rate during MgO deposition were maintained to be 1008C, Ê /s, respectively, for all samples. The 2 sccm, and 2 A thickness of the MgO ®lm during deposition was controlled by monitoring the deposition rate and Ê ®lm thickness, then the thickness of the 2000 A was con®rmed by scanning electron microscopy (SEM). Another series of MgO ®lms were obtained by spin coating of MgO precursor solutions. The solution was prepared by dissolving magnesium acetate (Mg(CH3CO2)24H2O) in 1,3-propanediol completely at 1008C. The above solution was cooled down below 508C, then magnesium methoxide (9.5% in methanol) was slowly added under vigorous mixing. Continuous overnight stirring at this temperature led to an evaporation of methanol, and resulted in 0.65 M Mg ion concentration. For the preparation of thinner thickness of MgO ®lms, consecutive dilutions of the last solution were carried out with the same volume of ethanol in order to prepare 0.33, 0.16, 0.08, 0.04, and 0.02 M of Mg ion solutions, respectively. Each MgO ®lm was deposited on the Si substrate by spin coating of each solution at 4000 rpm for 40 s, and by subsequent thermal-treatment at 4508C for 2 h in air.
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Fig. 1. Schematic diagram of the measurement system for SEE yield. The primary electron's energy (Ep) was varied from 0 to 3000 eV by applying variable kinetic energy of the primary electrons (Ea) from 50 to 3000 eV at V b ÿ45 V or from 1000 to 4000 eV at V b ÿ1000 V. The distance from the sample holder to the chamber ground is L.
The schematic diagram of SEE measurement is shown in Fig. 1, where bombardment of primary electrons onto the target sample induces emission of SEs. The SEE yield is de®ned as the ratio of the emitted secondary electrons (is) to the input primary electrons (ip). The target current (it) ¯ows to replenish electrons to the positively charged surface in the case where more electrons are emitted from the target sample than the input primary electrons (d > 1). The SEE yield was obtained by measuring ip and it, and using the equation of ip is it , which led to d is =ip ip ÿ it =ip 1 ÿ it =ip . The electron beam size on the target was controlled to be about 1 mm diameter. The magnitudes of the primary electron current (ip) as a function of primary electron energy were measured by applying 100 V to the sample holder before the measurement of SEE yield and they are found to be around 0.26 mA. The continuous electron beam was used, thus the electron dose used in this experiment is 0.26 mA/mm. In order to repel all secondary electrons from the surface, a negative bias voltage (Vb) was applied on the sample through the sample holder during SEE measurement. We used two different sample bias voltages, i.e. V b ÿ45 and ÿ1000 V. The primary electron energy
(Ep) is de®ned as Ea eV b , where Ea is the initial kinetic energy of the primary electrons from an electron gun (Kimball Physics, EFG-7). A series of data were collected sequentially on one spot area of the each MgO ®lms. The time required for stabilization of measuring current at each Ep was around 2±5 s. 3. Results and discussion The SEE yields of the electron-beam evaporated MgO ®lms having various thickness were plotted in Fig. 2 as a function of the primary electron energy at two different sample bias voltages, i.e. V b ÿ45 and ÿ1000 V. As the thickness increases at V b ÿ45 V (Fig. 2(a)), the maximum value of the SEE yield (dmax) decreases, resulting the highest yield from Ê ®lm, and the primary electron's energy the 100 A for the maximum SEE yield (Epmax) shifts to a higher energy. However, the SEE behavior at V b ÿ1000 V is different from that at V b ÿ45 V showing that the highest yields were obtained from the medium thick Ê thickness ®lms. We ascribe samples, i.e. 300±700 A these results to the surface charge accumulation on insulating materials. During the SEE process, a thick
J. Lee et al. / Applied Surface Science 174 (2001) 62±69
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Fig. 2. The SEE yield curves for the MgO ®lms prepared by electron beam evaporation. The energy of the primary electrons was varied from 0 to 3000 eV by varying electron gun energy with a ®xed sample bias voltage of (a) ÿ45 V and (b) ÿ1000 V.
MgO layer suffers from providing electrons from the MgO/Si interface to the electron-emitting MgO/ vacuum surface. Therefore, the magnitude of SEE yield is mostly governed by the capability of electron supply through the MgO layer. One way to increase the capability of electron supply is to decrease the MgO layer thickness. In order to understand the behavior of SEE at varied thickness, we follow the explanation of `radiation induced conductivity (RIC)' introduced by Melchinger and Hofman [20]. The electron-irradiated area is expected to have a higher electrical conductivity than the non-irradiated area [20,21]. Primary electron bombardment causes continuous excitation of valence electrons in an insulator into the conduction band. The resultant mobile electrons increase the electrical conductivity. This socalled `radiation induced conductivity', g, depends on the dose rate D according to the power law with the material parameter D [20,22,23]: 8 D 10 IPE EPE ; (1) g g0 DD g0 FRr where IPE is the primary electron current (in A), EPE the primary electron energy (in keV), F the area of primary electron beam (in cm2), R the penetration depth of the primaries (in cm), and r the density of the material (in g/cm3). The g0 in Eq. (1) describes the RIC at a reference dose rate (1 rad/s). The penetration
depth of the primary electrons can be approximated by the following equation [24]: R
1:15 10ÿ5 EPE 1G : r
(2)
In order to predict the degree of increased conductivity induced by electron irradiation, the RIC of sapphire (sapphire is selected due to its availability of parameters with g0 4:0 10ÿ15 /Om, r 3:95 g/cm3, and D 0:5) [20] is calculated. Applying our experimental conditions, i.e. ip 0:26 mA and Ep 1 keV, the RIC is almost four order of magnitude higher than the conductivity of the non-irradiated sapphire. Thus, this amount of conductivity increase might be useful to evaluate our MgO system. Considering our experimental system of MgO on Si, an important parameter controlling the available depth of the region having RIC is the energy of the primary electrons. Higher the energy of primary electrons, deeper penetration depth into the MgO layer. As the thickness of the MgO layer increases at a ®xed primary electron energy, the distance of non-irradiated region in the MgO layer increases. Under this circumstance, the electron supply from the Si substrate (an electron reservoir) to the electron-emitting MgO/ vacuum surface is hampered by the existence of the non-irradiated region within the MgO layer. Therefore, the behavior of SEE is generally controlled by
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J. Lee et al. / Applied Surface Science 174 (2001) 62±69
Fig. 3. (a) Schematic illustration of SEE process of a thick insulating layers having a thickness `t'. Primary electrons penetrate into the MgO layer to the depth of `c', then the distance of `r' is left as non-irradiated region. The shaded region generated by the primary electron irradiation is expected to have a relatively high conductivity. (b) Probable potential pro®les established on the MgO layer. The potential slope within the region `c' is not stiff due to the relatively conductive property by RIC. The insulating region `r' is responsible for the major potential drop. The potential slope at this region depends strongly on the sample bias voltage (Vb). The higher bias potential is applied to the sample, the higher electric ®eld is established on the region r.
two parameters, i.e. the energy of the primary electrons and the thickness of the MgO layer. In case that the penetration depth is smaller than the thickness of the MgO layer due to a relatively low primary electron energy, the MgO layer is divided into two regions, i.e. `relatively conducting region (c)' by RIC and `highly resistive region (r)' having the original resistivity of the non-irradiated MgO (see Fig. 3(a)). The thickness of a MgO layer (t), the penetration depth of primary electrons (c), and the distance of nonirradiated region (r) are schematically illustrated in Fig. 3(a) with the potential pro®les in Fig. 3(b). Based on the huge difference of the electrical conductivity between the two regions according to the calculation by Eq. (1), which is under continuous electron ¯owing at each Vb, two different potential slopes could be drawn. In fact, the boundary between two regions cannot be clearly divided, thus a smooth variation of slope is used. The highly resistive region (r) sustains a stronger electrical ®eld than the less resistive region (c). Thus, the electron supply through these two regions is divided into two mechanisms, i.e. `tunneling' and `electric ®eld acceleration,' depending on the electrical resistance of the region. The electron tunneling mechanism is more dominant at the highly resistive region (r) and the electric ®eld acceleration is more dominant at the less resistive region (c).
It should be emphasized that the establishment of two different potential slopes within a MgO ®lm is set only under primary electron bombardment. As long as an irradiation lasts, two different electric ®elds within a MgO ®lm are maintained as a dynamic equilibrium state. The surface potential of the MgO ®lm speci®ed as `a' in Fig. 3(b) might be set in this dynamic equilibrium state. Although the position of the surface potential in Fig. 3 is drawn on the equi-potential slope, it could be moved to further positive direction as a result of more secondary electron emission with respect to incoming primary electrons. The penetration depth is calculated using Eq. (2) with G 0:35 [25]. Although there have been some arguments about the value of G, the value of 0.35 is more relevant to describe our system properly [2,22,23]. In the case of V b ÿ45 V, the dmax for the various thickness of MgO ®lms is occurred when the thickness of the MgO ®lm is close to the penetration depth, that is, c is close to t in Fig. 3. For example, Ê ®lm occurs at Epmax 1000 eV the dmax for the 300 A Ê where the calculated penetration depth (R) is 315 A (where r 3:65 g/cm3 for MgO). This coincidence between R and t at Epmax extends to all other thickness, which is summarized in Table 1. From this result, we can understand that the SEE process in a thin insulating layer on a conductive substrate is maintained by
J. Lee et al. / Applied Surface Science 174 (2001) 62±69 Table 1 The thickness of MgO layer (t) versus the calculated penetration depth (R) at V b ÿ45 and ÿ1000 Va Thickness of Ê) MgO layer (A (t in Fig. 3)
V b ÿ45 V
V b ÿ1000 V
Epmax (eV)
Ê) R (A
Epmax (eV)
Ê) R (A
100 300 500 700 1000
500 1000 1500 2000 3000
124 315 545 803 1388
500 600 600 600 1300
124 158 158 158 450
a
It is notable to ®nd out the coincidence between the thickness of MgO layer (t) and the calculated penetration depth (R) using Eq. (2) with Epmax at V b ÿ45 V. The distinction of behavior at V b ÿ1000 V is explained in the text and Fig. 4.
continuous electron supply from the insulator/substrate interface to the electron-emitting surface, and that the electron supply is improved by the increased conductivity through RIC. The coincidence between R and t using Epmax at V b ÿ45 V cannot be any more reliable at V b ÿ1000 V, but this disagreement supports the previously suggested structure of two different regions. The distinction between V b ÿ45 and ÿ1000 V is originated from the stronger electric ®eld within the insulating MgO layer, more speci®cally within the highly resistive region (r). If a distance from a sample to chamber ground is L and a symmetrical equi-potential condition is assumed, an approximate electric ®eld over the space between the sample and the chamber ground is Vb/L. Noticing that the term
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L is constant, the electric ®eld of the region r at V b ÿ1000 V is about 20 times higher than that at V b ÿ45 V. The magnitudes of Epmax at V b ÿ1000 V are smaller compared to those at V b ÿ45 V for each MgO ®lms (see Table 1). It implies the existence of the region r, where the electrons are supplied by electron tunneling mechanism under a stronger electric ®eld. The almost similar values of Epmax (600 eV) for Ê MgO ®lms at V b ÿ1000 V the 300, 500, and 700 A are explained as following. Upon the primary electron's penetration depth obtained from the experimental Epmax, the distance of the relatively conducting Ê for all of those three ®lms. region (c) is about 160 A Then, the distance of the rest of the region c, i.e. the Ê for the 300, region r, would be 140, 340, and 540 A Ê ®lms, respectively. The very close 500, and 700 A values of dmax (5.3) for those three ®lms indicates that the energy to overcome the barrier of the region r for electron tunneling is comparable up to the distance Ê at V b ÿ1000 V. This is schematically of the 540 A shown in Fig. 4. This explanation is supported by the Ê ®lm. The primary electron's result of the 1000 A penetration depth obtained from Epmax (1300 eV) is Ê , leading that the distance of the region r is 450 A Ê . The magnitude of this number is very close to 550 A Ê obtained from the 700 A Ê ®lm. From the value 540 A this result, we may anticipate that the maximum distance for electron tunneling at V b ÿ1000 V is Ê. about 550 A The similar behavior of thickness effect was observed from the MgO ®lms prepared by the spin
Fig. 4. Schematic diagram of electric ®eld establishment on the MgO ®lms having three different thicknesses. The penetration depth of the primary electrons is same for all, but the thickness of the non-irradiated region (r) is varied depending on the whole MgO layer. The energy to overcome the region r for electron tunneling is expected to be comparable for all three thicknesses at V b ÿ1000 V. The thick lines represent the electric potential pro®les.
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Fig. 5. The SEE yield curves of MgO layers prepared by spin coating using MgO precursor solutions at Vb of (a) ÿ45 V and (b) ÿ1000 V. The preparation method is described in the text. The higher concentrated solution produced the thicker layer of MgO on Si. The `1 M (d)' stands for `double coating' with 1 M solution. The other measurement condition was same as Fig. 2.
coating method using the Mg ion precursor solutions. The SEE yield curves are shown in Fig. 5. We comment just a few distinctions between two series of samples. First, the magnitudes of the SEE yield at both of V b ÿ45 and ÿ1000 V for the 0.02 M from the spin coating method are low. We ascribed it to the very thin thickness of the MgO layer, which is thinner than Ê , where most of SEs are produced from the bulk 100 A of the Si substrate rather than in the MgO layer, since the SEE yield of Si is lower than that of MgO. Another possibility for the low yield curve is that the surface is not completely covered by MgO due to the low concentration of Mg ions, thereby exposing bare Si surface. Second, the overall SEE yields of the spin coated MgO ®lms are a little higher than those prepared by electron beam evaporation. The dmax values at V b ÿ1000 V are about 5 for the ®lms made by electron beam evaporation, whereas the values over 6 are observed for the ®lms by spin coating. This high dmax for the spin coated MgO layers could be understood by an increased surface roughness, which is supported by the measurement of the surface roughness using atomic force microscopy (AFM) [19]. Except a few subtle differences, the general feature of SEE yield curves under thickness variation is similar to those of two kinds of MgO ®lms. From the ®nding that the MgO thickness is close to the penetration depth of Epmax at V b ÿ45 V, the thick-
ness of the 0.16, 0.65, and 1 M could be estimated Ê , respectively. approximately to be 120, 300, and 550 A 4. Conclusion Two series of MgO thin ®lms were prepared by electron beam evaporation and by spin coating of MgO precursor solutions. We found that the thickness of a MgO ®lm is an important factor to determine the electron supply from the Si substrate (an electron reservoir) to the electron-emitting surface. As the MgO layer becomes thick, the SEE yield decreases and Epmax shifts to higher energy. The dif®culty of the electron supply from the Si substrate was overcome by applying a highly negative bias potential to a sample. The electrical ®eld established within the insulating MgO layer can be divided into two regions depending on their relative resistance. A relatively conductive region is formed by primary electron irradiation. The mechanisms of the electron supply are distinguished as an electron acceleration in the relatively conductive region and an electron tunneling in the highly resistive region. Under a weak electric potential (e.g. V b ÿ45 V), the maximum SEE yield of each of the MgO ®lms on the Si substrate was obtained when the penetration depth of the primary electron is close to the thickness of the MgO layer. On the other hand,
J. Lee et al. / Applied Surface Science 174 (2001) 62±69
under a strong electric potential (V b ÿ1000 V), an extra thickness of MgO ®lm is allowed to supply electrons by electron tunneling mechanism. The magnitude of SEE yield for a thin insulating layer should be carefully measured, because it is strongly related to the structure of electron supplying mechanism determined by the thickness of the insulating layer. The behavior of SEE and the thickness effect of the MgO ®lms prepared from spin coating method are similar to the ones prepared from the electron beam evaporation method. Acknowledgements Authors thank to Korean Ministry of Science and Technology for supporting the National Creative Research Initiative Program. We are grateful to the members of Analytical Engineering Laboratory in Samsung Advanced Institute of Technology for their help in performing useful analyses. References [1] W. Yi, S.H. Jin, T.W. Jeong, J.H. Lee, S. Yu, Y.S. Choi, J.M. Kim, Appl. Phys. Lett. 77 (2000) 1716. [2] W. Yi, T.W. Jeong, S.H. Jin, S. Yu, J.H. Lee, J.M. Kim, Rev. Sci. Instrum. 71 (2000) 4165. [3] K. Kanaya, S. Ono, F. Ishigaki, J. Phys. D: Appl. Phys. 11 (1978) 2425.
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[4] J. Schou, in: R.A. Baragiola (Ed.), Ionization of Solids by Heavy Particles, Plenum Press, New York, 1993, p. 351. [5] P. Ganachaud, C. Attard, R. Renoud, Phys. Stat. Sol. 199 (1997) 175. [6] J.P. Ganachaud, A. Mokrani, Surf. Sci. 334 (1995) 329. [7] T. Takuma, M. Yashima, T. Kawamoto, IEEE. Trans. Dielect. Elect. Insulat. 5 (1998) 497. [8] I.L. Krainsky, G.G. Lesny, Rev. Sci. Instrum. 69 (1998) 1916. [9] Y. Ushio, T. Banno, N. Matuda, Y. Saito, S. Baba, A. Kinbara, Thin Solid Films 167 (1988) 299. [10] V.E. Henrich, J.C.C. Fan, Appl. Phys. Lett. 23 (1973) 7. [11] J.J. Scholtz, R.W.A. Schmitz, B.H.W. Hendriks, S.T. de Zwart, App. Surf. Sci. 111 (1997) 259. [12] H. Seiler, Z. Angew. Phys. 22 (1967) 249. [13] J. Cazaux, K.H. Kim, O. Jbara, G. Salace, J. Appl. Phys. 70 (1991) 960. [14] J. Dresner, B. Goldstein, J. Appl. Phys. 47 (1976) 1038. [15] E. Yater, A. Shih, R. Abrams, Phys. Rev. B 56 (1997) R4410. [16] A. Shih, J. Yater, P. Pehrsson, J. Butler, C. Hor, R. Abrams, J. Appl. Phys. 82 (1997) 1860. [17] I.L. Krainsky, V.M. Asnin, J.A. Dayton Jr., Appl. Surf. Sci. 111 (1997) 265. [18] J. Lee, T.W. Jeong, S.G. Yu, H.W. Son, S.H. Jin, W. Yi, Y.S. Choi, D. Jeon, J.M. Kim, in: Proceedings of the AVS 46th International Symposium, 25 October 1999, Appl. Surf. Sci. Div. [19] J. Lee, T.W. Jeong, S.G. Yu, S.H. Jin, J.N. Heo, W. Yi, J.M. Kim, in: Proceedings of the 13th International Vac. Microelect. Conference, 14 August 2000. [20] A. Melchinger, S. Hofmann, J. Appl. Phys. 78 (1995) 6224. [21] J. Cazaux, J. Appl. Phys. 85 (1999) 1137. [22] L. Nunes de Oliverira, B. Gross, J. Appl. Phys. 46 (1975) 3132. [23] B. Gross, G.M. Sessler, J.E. West, J. Appl. Phys. 45 (1974) 2841. [24] H. Seiler, Electron Beam Interactions with Solids, SEM, Chicago, 1982, p. 33. [25] J.R. Young, Phys. Rev. 103 (1956) 292.
Thickness effect on secondary electron emission of MgO layers Jeonghee Leea, Taewon Jeonga, SeGi Yua, Sunghwan Jina, Jungna Heoa, Whikun Yia, D. Jeonb, J.M. Kima,* a
National Creative Research Initiatives, Center for Electron Emission Source, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea b Department of Physics, Myong Ji University, Yongin Kyunggi-do 449-728, South Korea Received 20 October 2000; accepted 31 December 2000
Abstract Two series of MgO thin layers having various thicknesses were prepared on the Si substrate by electron-beam evaporation and by spin coating of MgO precursor solutions. We found that the magnitude of the secondary electron emission (SEE) yield of the MgO ®lms strongly depends on the ®lm thickness and the sample bias voltage. We ascribed it to the electric ®eld through the insulating MgO layer, which allowed fast supply of electrons from the Si substrate to the surface. The mechanism of electron supply can be explained either as an acceleration through the MgO layer that becomes partially conductive upon primary electrons bombardment (radiation induced conductivity), or as a tunneling through the non-irradiated region of the insulating layer where the primary electrons cannot reach deeply into the sample with a certain penetration depth. The maximum SEE yield of the each MgO ®lm on the Si substrate was observed when the penetration depth of primary electrons was close to the thickness of the MgO ®lm, if the applied electric potential to the sample was low. Under a strong electric potential, the relationship between the penetration depth of primary electrons and the thickness of MgO ®lms is not observed. It suggests the existence of the non-irradiated region, where electron supply is allowed by electron tunneling. Therefore, the magnitude of SEE yield for the thin insulating layer is strongly related to the detailed mechanism of electron supply, which is determined by the thickness of the insulating layer and the applied bias voltage to the sample during the SEE process. # 2001 Published by Elsevier Science B.V. PACS: 79.20.Hx Keywords: Secondary electron; Penetration depth; Thin insulating ®lm; MgO
1. Introduction Microchannel plates (MCPs), which utilize electron multiplication by secondary electron emission (SEE), have been extensively studied due to the diverse demands from conventional particle detectors to a recently developed ®eld emission display (FED) * Corresponding author. E-mail address: [email protected] (J.M. Kim).
incorporated with an MCP [1,2]. It is well known that insulating materials exhibit higher SEE yield than semiconductors or metals [3±7]. The wide energy band gap in an insulator prevents internal secondary electrons (SEs) from losing energy through collision with conduction electrons, resulting in a large escape depth of the SEs and a large SEE yield (d). Generally, the mechanism of SEE can be simpli®ed by the following processes: (i) the primary electrons penetrate into a certain depth of an insulating layer; (ii)
0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 0 1 5 - 0
J. Lee et al. / Applied Surface Science 174 (2001) 62±69
through collision, the energy of the primary electrons is transferred to the bound electrons of the insulator leading to a release of electrons; (iii) the released electrons migrate to the surface and escape into the vacuum as secondary electrons. During the SEE process, electrical conductivity is considered to be an important factor, since at least a certain level of electrical conductivity is necessary to replenish new electrons to the electron de®cient surface. A build-up of positive charges at the electronemitting surface is originated from the emission when the number of SEs escaped into the vacuum is larger than that of input primary electrons (d > 1). The SEE on insulators has been studied widely for the last few decades, and a number of interests were focused on the insulating property related to charging effect [3±7]. Several techniques were employed to overcome the charging effect; pulse mode technique [8,9], preparation of cermet system [10], and application of a thin layer [11,12]. Magnesium oxide (MgO) [8±14] is a representative material showing a high SEE yield before the recent discovery of the surface-treated diamond whose SEE yield is found to be as high as 130 [15±17]. It was reported that the magnitude of SEE yield in MgO has a wide range of values from 2 to 22. Although the origin of the low SEE yield is presumed by poor surface treatment with a lack of high vacuum condition, the large variation of the previously reported values primarily comes from the charging effect. Besides the charging effect, the large difference of SEE yield may also be originated from the crystal quality of MgO itself, for example, existence of impurities or defects. Cazaux et al. explained that the large decrease in SEE yield for polycrystalline insulators is caused by a reduced mean free path of secondaries through scattering and trapping at defects [13]. Even though there are other kinds of dif®culties on measuring the SEE yield in insulators, the charging of a sample during SEE process gives a major in¯uence. In this paper, we report the SEE yields of MgO ®lms with respect to their thickness in order to investigate the surface charging effect and its releasing mechanism during SEE process. Films were prepared either by electron-beam evaporation or by spin coating of MgO precursor solutions [18,19]. Our experiments were performed at two different bias voltages on a MgO specimen, i.e. ÿ45 and ÿ1000 V, to apply the
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electric ®eld to the insulating layer. Most of previously reported SEE yields in literature adopted the measurement setup applying a variable negative bias potential to the sample with a ®xed primary electron energy for the variation of net primary electron energy [11]. In those cases, the maximum SEE yield was obtained when the sample bias voltage was very negative. For example, the sample bias voltage becomes ÿ1000 V for the net primary electron energy of 500 eV, if a ®xed energy of primary electrons with 1500 eV is used. This situation is similar to the measurement at V b ÿ1000 V in this work. The variation of a ®xed sample bias voltage during SEE process brings a partial understanding of electron supplying mechanism of an insulating layer on a conductive substrate. 2. Experimental A series of MgO ®lms with thickness of 100, 300, Ê were prepared by elec500, 700, 1000, and 2000 A tron-beam evaporation. The size of a Si wafer (phosphorus doped n-type (1 0 0) and 1±30 O cm of resistivity) was about 2 cm 2 cm. Substrate temperature, O2 ¯ow rate, and deposition rate during MgO deposition were maintained to be 1008C, Ê /s, respectively, for all samples. The 2 sccm, and 2 A thickness of the MgO ®lm during deposition was controlled by monitoring the deposition rate and Ê ®lm thickness, then the thickness of the 2000 A was con®rmed by scanning electron microscopy (SEM). Another series of MgO ®lms were obtained by spin coating of MgO precursor solutions. The solution was prepared by dissolving magnesium acetate (Mg(CH3CO2)24H2O) in 1,3-propanediol completely at 1008C. The above solution was cooled down below 508C, then magnesium methoxide (9.5% in methanol) was slowly added under vigorous mixing. Continuous overnight stirring at this temperature led to an evaporation of methanol, and resulted in 0.65 M Mg ion concentration. For the preparation of thinner thickness of MgO ®lms, consecutive dilutions of the last solution were carried out with the same volume of ethanol in order to prepare 0.33, 0.16, 0.08, 0.04, and 0.02 M of Mg ion solutions, respectively. Each MgO ®lm was deposited on the Si substrate by spin coating of each solution at 4000 rpm for 40 s, and by subsequent thermal-treatment at 4508C for 2 h in air.
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Fig. 1. Schematic diagram of the measurement system for SEE yield. The primary electron's energy (Ep) was varied from 0 to 3000 eV by applying variable kinetic energy of the primary electrons (Ea) from 50 to 3000 eV at V b ÿ45 V or from 1000 to 4000 eV at V b ÿ1000 V. The distance from the sample holder to the chamber ground is L.
The schematic diagram of SEE measurement is shown in Fig. 1, where bombardment of primary electrons onto the target sample induces emission of SEs. The SEE yield is de®ned as the ratio of the emitted secondary electrons (is) to the input primary electrons (ip). The target current (it) ¯ows to replenish electrons to the positively charged surface in the case where more electrons are emitted from the target sample than the input primary electrons (d > 1). The SEE yield was obtained by measuring ip and it, and using the equation of ip is it , which led to d is =ip ip ÿ it =ip 1 ÿ it =ip . The electron beam size on the target was controlled to be about 1 mm diameter. The magnitudes of the primary electron current (ip) as a function of primary electron energy were measured by applying 100 V to the sample holder before the measurement of SEE yield and they are found to be around 0.26 mA. The continuous electron beam was used, thus the electron dose used in this experiment is 0.26 mA/mm. In order to repel all secondary electrons from the surface, a negative bias voltage (Vb) was applied on the sample through the sample holder during SEE measurement. We used two different sample bias voltages, i.e. V b ÿ45 and ÿ1000 V. The primary electron energy
(Ep) is de®ned as Ea eV b , where Ea is the initial kinetic energy of the primary electrons from an electron gun (Kimball Physics, EFG-7). A series of data were collected sequentially on one spot area of the each MgO ®lms. The time required for stabilization of measuring current at each Ep was around 2±5 s. 3. Results and discussion The SEE yields of the electron-beam evaporated MgO ®lms having various thickness were plotted in Fig. 2 as a function of the primary electron energy at two different sample bias voltages, i.e. V b ÿ45 and ÿ1000 V. As the thickness increases at V b ÿ45 V (Fig. 2(a)), the maximum value of the SEE yield (dmax) decreases, resulting the highest yield from Ê ®lm, and the primary electron's energy the 100 A for the maximum SEE yield (Epmax) shifts to a higher energy. However, the SEE behavior at V b ÿ1000 V is different from that at V b ÿ45 V showing that the highest yields were obtained from the medium thick Ê thickness ®lms. We ascribe samples, i.e. 300±700 A these results to the surface charge accumulation on insulating materials. During the SEE process, a thick
J. Lee et al. / Applied Surface Science 174 (2001) 62±69
65
Fig. 2. The SEE yield curves for the MgO ®lms prepared by electron beam evaporation. The energy of the primary electrons was varied from 0 to 3000 eV by varying electron gun energy with a ®xed sample bias voltage of (a) ÿ45 V and (b) ÿ1000 V.
MgO layer suffers from providing electrons from the MgO/Si interface to the electron-emitting MgO/ vacuum surface. Therefore, the magnitude of SEE yield is mostly governed by the capability of electron supply through the MgO layer. One way to increase the capability of electron supply is to decrease the MgO layer thickness. In order to understand the behavior of SEE at varied thickness, we follow the explanation of `radiation induced conductivity (RIC)' introduced by Melchinger and Hofman [20]. The electron-irradiated area is expected to have a higher electrical conductivity than the non-irradiated area [20,21]. Primary electron bombardment causes continuous excitation of valence electrons in an insulator into the conduction band. The resultant mobile electrons increase the electrical conductivity. This socalled `radiation induced conductivity', g, depends on the dose rate D according to the power law with the material parameter D [20,22,23]: 8 D 10 IPE EPE ; (1) g g0 DD g0 FRr where IPE is the primary electron current (in A), EPE the primary electron energy (in keV), F the area of primary electron beam (in cm2), R the penetration depth of the primaries (in cm), and r the density of the material (in g/cm3). The g0 in Eq. (1) describes the RIC at a reference dose rate (1 rad/s). The penetration
depth of the primary electrons can be approximated by the following equation [24]: R
1:15 10ÿ5 EPE 1G : r
(2)
In order to predict the degree of increased conductivity induced by electron irradiation, the RIC of sapphire (sapphire is selected due to its availability of parameters with g0 4:0 10ÿ15 /Om, r 3:95 g/cm3, and D 0:5) [20] is calculated. Applying our experimental conditions, i.e. ip 0:26 mA and Ep 1 keV, the RIC is almost four order of magnitude higher than the conductivity of the non-irradiated sapphire. Thus, this amount of conductivity increase might be useful to evaluate our MgO system. Considering our experimental system of MgO on Si, an important parameter controlling the available depth of the region having RIC is the energy of the primary electrons. Higher the energy of primary electrons, deeper penetration depth into the MgO layer. As the thickness of the MgO layer increases at a ®xed primary electron energy, the distance of non-irradiated region in the MgO layer increases. Under this circumstance, the electron supply from the Si substrate (an electron reservoir) to the electron-emitting MgO/ vacuum surface is hampered by the existence of the non-irradiated region within the MgO layer. Therefore, the behavior of SEE is generally controlled by
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Fig. 3. (a) Schematic illustration of SEE process of a thick insulating layers having a thickness `t'. Primary electrons penetrate into the MgO layer to the depth of `c', then the distance of `r' is left as non-irradiated region. The shaded region generated by the primary electron irradiation is expected to have a relatively high conductivity. (b) Probable potential pro®les established on the MgO layer. The potential slope within the region `c' is not stiff due to the relatively conductive property by RIC. The insulating region `r' is responsible for the major potential drop. The potential slope at this region depends strongly on the sample bias voltage (Vb). The higher bias potential is applied to the sample, the higher electric ®eld is established on the region r.
two parameters, i.e. the energy of the primary electrons and the thickness of the MgO layer. In case that the penetration depth is smaller than the thickness of the MgO layer due to a relatively low primary electron energy, the MgO layer is divided into two regions, i.e. `relatively conducting region (c)' by RIC and `highly resistive region (r)' having the original resistivity of the non-irradiated MgO (see Fig. 3(a)). The thickness of a MgO layer (t), the penetration depth of primary electrons (c), and the distance of nonirradiated region (r) are schematically illustrated in Fig. 3(a) with the potential pro®les in Fig. 3(b). Based on the huge difference of the electrical conductivity between the two regions according to the calculation by Eq. (1), which is under continuous electron ¯owing at each Vb, two different potential slopes could be drawn. In fact, the boundary between two regions cannot be clearly divided, thus a smooth variation of slope is used. The highly resistive region (r) sustains a stronger electrical ®eld than the less resistive region (c). Thus, the electron supply through these two regions is divided into two mechanisms, i.e. `tunneling' and `electric ®eld acceleration,' depending on the electrical resistance of the region. The electron tunneling mechanism is more dominant at the highly resistive region (r) and the electric ®eld acceleration is more dominant at the less resistive region (c).
It should be emphasized that the establishment of two different potential slopes within a MgO ®lm is set only under primary electron bombardment. As long as an irradiation lasts, two different electric ®elds within a MgO ®lm are maintained as a dynamic equilibrium state. The surface potential of the MgO ®lm speci®ed as `a' in Fig. 3(b) might be set in this dynamic equilibrium state. Although the position of the surface potential in Fig. 3 is drawn on the equi-potential slope, it could be moved to further positive direction as a result of more secondary electron emission with respect to incoming primary electrons. The penetration depth is calculated using Eq. (2) with G 0:35 [25]. Although there have been some arguments about the value of G, the value of 0.35 is more relevant to describe our system properly [2,22,23]. In the case of V b ÿ45 V, the dmax for the various thickness of MgO ®lms is occurred when the thickness of the MgO ®lm is close to the penetration depth, that is, c is close to t in Fig. 3. For example, Ê ®lm occurs at Epmax 1000 eV the dmax for the 300 A Ê where the calculated penetration depth (R) is 315 A (where r 3:65 g/cm3 for MgO). This coincidence between R and t at Epmax extends to all other thickness, which is summarized in Table 1. From this result, we can understand that the SEE process in a thin insulating layer on a conductive substrate is maintained by
J. Lee et al. / Applied Surface Science 174 (2001) 62±69 Table 1 The thickness of MgO layer (t) versus the calculated penetration depth (R) at V b ÿ45 and ÿ1000 Va Thickness of Ê) MgO layer (A (t in Fig. 3)
V b ÿ45 V
V b ÿ1000 V
Epmax (eV)
Ê) R (A
Epmax (eV)
Ê) R (A
100 300 500 700 1000
500 1000 1500 2000 3000
124 315 545 803 1388
500 600 600 600 1300
124 158 158 158 450
a
It is notable to ®nd out the coincidence between the thickness of MgO layer (t) and the calculated penetration depth (R) using Eq. (2) with Epmax at V b ÿ45 V. The distinction of behavior at V b ÿ1000 V is explained in the text and Fig. 4.
continuous electron supply from the insulator/substrate interface to the electron-emitting surface, and that the electron supply is improved by the increased conductivity through RIC. The coincidence between R and t using Epmax at V b ÿ45 V cannot be any more reliable at V b ÿ1000 V, but this disagreement supports the previously suggested structure of two different regions. The distinction between V b ÿ45 and ÿ1000 V is originated from the stronger electric ®eld within the insulating MgO layer, more speci®cally within the highly resistive region (r). If a distance from a sample to chamber ground is L and a symmetrical equi-potential condition is assumed, an approximate electric ®eld over the space between the sample and the chamber ground is Vb/L. Noticing that the term
67
L is constant, the electric ®eld of the region r at V b ÿ1000 V is about 20 times higher than that at V b ÿ45 V. The magnitudes of Epmax at V b ÿ1000 V are smaller compared to those at V b ÿ45 V for each MgO ®lms (see Table 1). It implies the existence of the region r, where the electrons are supplied by electron tunneling mechanism under a stronger electric ®eld. The almost similar values of Epmax (600 eV) for Ê MgO ®lms at V b ÿ1000 V the 300, 500, and 700 A are explained as following. Upon the primary electron's penetration depth obtained from the experimental Epmax, the distance of the relatively conducting Ê for all of those three ®lms. region (c) is about 160 A Then, the distance of the rest of the region c, i.e. the Ê for the 300, region r, would be 140, 340, and 540 A Ê ®lms, respectively. The very close 500, and 700 A values of dmax (5.3) for those three ®lms indicates that the energy to overcome the barrier of the region r for electron tunneling is comparable up to the distance Ê at V b ÿ1000 V. This is schematically of the 540 A shown in Fig. 4. This explanation is supported by the Ê ®lm. The primary electron's result of the 1000 A penetration depth obtained from Epmax (1300 eV) is Ê , leading that the distance of the region r is 450 A Ê . The magnitude of this number is very close to 550 A Ê obtained from the 700 A Ê ®lm. From the value 540 A this result, we may anticipate that the maximum distance for electron tunneling at V b ÿ1000 V is Ê. about 550 A The similar behavior of thickness effect was observed from the MgO ®lms prepared by the spin
Fig. 4. Schematic diagram of electric ®eld establishment on the MgO ®lms having three different thicknesses. The penetration depth of the primary electrons is same for all, but the thickness of the non-irradiated region (r) is varied depending on the whole MgO layer. The energy to overcome the region r for electron tunneling is expected to be comparable for all three thicknesses at V b ÿ1000 V. The thick lines represent the electric potential pro®les.
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Fig. 5. The SEE yield curves of MgO layers prepared by spin coating using MgO precursor solutions at Vb of (a) ÿ45 V and (b) ÿ1000 V. The preparation method is described in the text. The higher concentrated solution produced the thicker layer of MgO on Si. The `1 M (d)' stands for `double coating' with 1 M solution. The other measurement condition was same as Fig. 2.
coating method using the Mg ion precursor solutions. The SEE yield curves are shown in Fig. 5. We comment just a few distinctions between two series of samples. First, the magnitudes of the SEE yield at both of V b ÿ45 and ÿ1000 V for the 0.02 M from the spin coating method are low. We ascribed it to the very thin thickness of the MgO layer, which is thinner than Ê , where most of SEs are produced from the bulk 100 A of the Si substrate rather than in the MgO layer, since the SEE yield of Si is lower than that of MgO. Another possibility for the low yield curve is that the surface is not completely covered by MgO due to the low concentration of Mg ions, thereby exposing bare Si surface. Second, the overall SEE yields of the spin coated MgO ®lms are a little higher than those prepared by electron beam evaporation. The dmax values at V b ÿ1000 V are about 5 for the ®lms made by electron beam evaporation, whereas the values over 6 are observed for the ®lms by spin coating. This high dmax for the spin coated MgO layers could be understood by an increased surface roughness, which is supported by the measurement of the surface roughness using atomic force microscopy (AFM) [19]. Except a few subtle differences, the general feature of SEE yield curves under thickness variation is similar to those of two kinds of MgO ®lms. From the ®nding that the MgO thickness is close to the penetration depth of Epmax at V b ÿ45 V, the thick-
ness of the 0.16, 0.65, and 1 M could be estimated Ê , respectively. approximately to be 120, 300, and 550 A 4. Conclusion Two series of MgO thin ®lms were prepared by electron beam evaporation and by spin coating of MgO precursor solutions. We found that the thickness of a MgO ®lm is an important factor to determine the electron supply from the Si substrate (an electron reservoir) to the electron-emitting surface. As the MgO layer becomes thick, the SEE yield decreases and Epmax shifts to higher energy. The dif®culty of the electron supply from the Si substrate was overcome by applying a highly negative bias potential to a sample. The electrical ®eld established within the insulating MgO layer can be divided into two regions depending on their relative resistance. A relatively conductive region is formed by primary electron irradiation. The mechanisms of the electron supply are distinguished as an electron acceleration in the relatively conductive region and an electron tunneling in the highly resistive region. Under a weak electric potential (e.g. V b ÿ45 V), the maximum SEE yield of each of the MgO ®lms on the Si substrate was obtained when the penetration depth of the primary electron is close to the thickness of the MgO layer. On the other hand,
J. Lee et al. / Applied Surface Science 174 (2001) 62±69
under a strong electric potential (V b ÿ1000 V), an extra thickness of MgO ®lm is allowed to supply electrons by electron tunneling mechanism. The magnitude of SEE yield for a thin insulating layer should be carefully measured, because it is strongly related to the structure of electron supplying mechanism determined by the thickness of the insulating layer. The behavior of SEE and the thickness effect of the MgO ®lms prepared from spin coating method are similar to the ones prepared from the electron beam evaporation method. Acknowledgements Authors thank to Korean Ministry of Science and Technology for supporting the National Creative Research Initiative Program. We are grateful to the members of Analytical Engineering Laboratory in Samsung Advanced Institute of Technology for their help in performing useful analyses. References [1] W. Yi, S.H. Jin, T.W. Jeong, J.H. Lee, S. Yu, Y.S. Choi, J.M. Kim, Appl. Phys. Lett. 77 (2000) 1716. [2] W. Yi, T.W. Jeong, S.H. Jin, S. Yu, J.H. Lee, J.M. Kim, Rev. Sci. Instrum. 71 (2000) 4165. [3] K. Kanaya, S. Ono, F. Ishigaki, J. Phys. D: Appl. Phys. 11 (1978) 2425.
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