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Potential sputtering of hydrogen atoms on various surfaces with slow highly-charged ions
Current Opinion in Solid State and Materials Science 6 (2002) 169–179
Potential sputtering of hydrogen atoms on various surfaces with slow highly-charged ions Yasunori Yamazaki a,b , *, Kenro Kuroki a,c a
Institute of Physics, University of Tokyo, Komaba, Meguro-ku, Tokyo 153 -8902, Japan b RIKEN, 2 -1 Hirosawa, Wako, Saitama 351 -0198, Japan c NRIPS, 6 -3 -1 Kashiwanoha, Kashiwa, Chiba 277 -0882, Japan
Abstract Several recent findings on a new sputtering mechanism of hydrogen with slow highly-charged ions are discussed. The sputtering yields of protons were proportional to q |5 for q&10 independent of the surface condition for both untreated and well-defined surfaces, where q is the charge state of the ion. This q |5 dependence started to level off for q & 10. The yield for the Si(100)131–H surface was ten times larger than that for the Si(100)231–H surface although the stoichiometric hydrogen abundance of the former is only twice that of the latter. The key quantity to govern the yield is proposed to be surface roughness, which also influences the energy distribution of sputtered protons. These findings were consistently explained with a pair-wise potential sputtering model involving two successive electron transfers which follow the classical over barrier mechanism. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Interaction of slow highly-charged ions (HCIs) with metal, semiconductor, and insulator surfaces has been intensively studied in the last two decades [*1– 6,**7,*8,**9] because of the exotic nature of collision dynamics and the possible application to high-sensitivity surface analysis and surface modification. Charge state evolution above and below the surface with multiple electron transfers, including hollow atom formation, has been a key subject of research. Amongst various interesting phenomena introduced with slow HCIs, this article focuses on proton sputtering, which can be explained with a new mechanism involving a double electron transfer to a slow HCI above the surface. It will be shown that this new mechanism is particularly effective for light atoms like hydrogen on the first surface layer, the yield of which could be as high as 1.The yield does not depend very much on the HCI kinetic energy, which allows researchers to make a damage-free analysis with extremely high sensitivi*Corresponding author. Tel.: 181-3-5454-6521; fax: 181-3-54546433. E-mail addresses: [email protected] (Y. Yamazaki), [email protected] (Y. Yamazaki).
ty. Needless to say hydrogen impurity in solids influences material properties drastically. However, sensitive analysis has been quite difficult, primarily because neither X-rays nor Auger electrons are emitted (see e.g., Ref. [10]). ERDA is known to be a powerful technique to probe hydrogen in matter, although it does not usually provide atomic resolution, and often causes radiation damage on substrates. HCIs are characterized by two parameters, i.e., the charge state q, and the potential energy eq , the energy to be released when the HCI is neutralized. To get an idea about the order of magnitude of the potential energy, eq is plotted in Fig. 1 for several ions as a function of charge state q. Very crudely, collision processes with large impact parameters are governed by q, while those with small impact parameters are governed by eq . For an ion with its kinetic energy around several keV/ u, multiple binary collisions with target atoms are the major process of energy deposition (nuclear stopping domain), which causes not only radiation damage to the target but also ejection of atoms / ions (kinetic sputtering). In the case of highly charged ion impacts: (1) the potential energy becomes another source of deposited energy, and (2) multiple electron transfer processes from the target to the projectile play important roles in particle emission. The latter process is the major subject of the present review.
1359-0286 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 02 )00046-3
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tunneling process. On the other hand, when it is equal to or lower than the workfunction of the metal, electrons are transferred classically beyond the barrier (which will hereafter be described as the classical over barrier (COB) condition). Because the transition rate under the COB condition is much higher than that under tunneling condition [11], electron transfer phenomena in slow HCIsurface collisions are more or less governed by the period when the COB condition is satisfied [**7]. Equating the depth of the saddle point to be the workfunction W, the critical distance d c , where the first electron transfer takes place, is obtained as ] œ2q d c | ]]. W
(2)
For a typical metal having W|4–5 eV, d c is of the order of nm. Since the electron transfer takes place resonantly, the binding energy of the transferred electron is similar to that of the valence electron, i.e. the electron is transferred to high Rydberg states of the HCI. The principal quantum number n c in which the first electron is transferred is given by Fig. 1. Potential energy of Ar, Kr, Xe and U ions as a function of charge state q.
2. Fundamental process in HCI–surface interaction— classical over barrier model When a slow HCI approaches a metal surface, it is accelerated toward the surface due to its image force, then at a certain distance, it captures target electrons, and finally it jumps into the surface releasing all its potential energy via Auger and X-ray emissions. A schematic view of this scenario is given in Fig. 2. The active electron to be transferred is subject to the following interactions: the interaction with the HCI, with the image of the HCI, and with the image of the electron itself, a sketch of which is drawn in Fig. 3(a). The interaction potential is given by → q q 1 → V( r e , R) 5 2 ]]] → 1 ]]] → 2] → → u r e 2Ru u r e 1Ru 4uzu →
(1)
where r e (x, 0, z) is the electron position with z-axis → perpendicular to the surface, and R(0, 0, d) is the position of the ion (atomic units are used unless otherwise noted.). → → Fig. 3(b) is a 3D representation of V( r e , R), which shows a saddle point along the line connecting the HCI and its image charge. The potential wall at the saddle point works as a barrier to block target electrons to be transferred to the HCI. The position of the barrier and its height are evaluated to be d /(8q)1 / 2 and 2(2q)1 / 2 /d, respectively, i.e., the barrier gets lower as d gets smaller. When the barrier is higher than the workfunction of the metal, valence electrons can only be transferred to the HCI via a
q n c | ]]]]] ]]]] ] œ2W(1 1œq / 8)
(3)
The solid line in Fig. 4 shows n c for W50.2 as a function of q. The dashed line corresponds to n5q, which more or less reproduces n c for q&20. The HCI captures the second electron as soon as the transfer condition is satisfied for q21 (see Eq. (3)), i.e., electrons are successively transferred into high Rydberg states of the HCI as it gets closer to the surface. The atomic state so formed, with many electrons occupying highly excited levels whilst still keeping inner shell hole(s), is called a hollow atom. It is noted that the radii of the excited states are comparable to d c , and accordingly the electrons can move back and forth between the ion and the target when they are in resonance. In this respect, the hollow atom in front of the surface could better be described as a ‘dynamic hollow molecule’. Lifetimes of innershell vacancies of the hollow atom are often longer than the time interval between the hollow atom formation and the collision with the surface (typical213 ly &10 s), i.e. a hollow atom with inner shell vacancies collides violently with the target surface, where these vacancies are filled via quasi-resonant charge transfer and / or inter- and intra-Auger transitions [**7]. The collision process of an HCI with a surface is then divided into two successive stages: (1) a soft above-surface collision accompanying multiple electron transfers among outershells and (2) a hard collision at or below the surface involving innershell transitions (hereafter referred to as Stage I and Stage II, respectively). As is evident from the discussion above, Stage II is the period when a major part of the potential energy is released from the HCI and deposited to the target. On the other hand, in the Stage I, the area
Y. Yamazaki, K. Kuroki / Current Opinion in Solid State and Materials Science 6 (2002) 169–179
Fig. 2. A schematic view of atomic processes taking place during the interaction of a slow highly charged ion with a metal / semiconductor surface.
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172 Y. Yamazaki, K. Kuroki / Current Opinion in Solid State and Materials Science 6 (2002) 169–179
→
→
Fig. 3. (a) Potential energy of the active electron; (b) 3D representation of V( r , R). An HCI with q510 is fixed at d5a.u.
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Fig. 4. The solid line shows n c predicted with Eq. (3). The dashed line shows n5q, which more or less reproduces n c for q & 20.
around the entrance point of the HCI experiences a strong charge-up due to multiple electron transfer, which is reneutralized by valence electrons with a time constant tre .tre 21 is expected to be | v pl for metals, where vpl is the plasma angular frequency. For insulators, tre is |10 a.u., which is estimated from a typical band width or a hopping matrix element between the nearest neighbor atoms. It is expected that the mutual Coulomb repulsion among charged target atoms prepared in the Stage I causes particle emission when reneutralization of the surface is slow.
3. Proton sputtering from untreated surfaces with slow highly charged ions Fig. 5 shows a schematic drawing of the experimental set-up used to measure the proton sputtering yield with
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slow HCIs. Slow HCIs extracted from an electron beam ion source (EBIS) [12] were introduced in the collision region where an electric field was applied, and eventually hit a target prepared on the Cu mesh [**9]. When the HCI hits the target, secondary electron(s) as well as secondary ion(s) are ejected. The electric field was used to accelerate secondary ions of positive charge toward the MCP, and to reflect secondary electrons back to the mesh, a part of which pass through the openings of the mesh and are detected by the channeltron located downstream of the mesh. The mass of the secondary ions was determined with the arrival time difference between the secondary electron and the secondary ion. In the present set-up, any macroscopic charge-up of the target was negligibly small because the HCI intensity was less than |10 3 cps, i.e. |10 216 A. Fig. 6 shows the proton sputtering yields from an untreated CuO surface (i.e. covered with some layers of water, hydrocarbons etc.) for Ne q1 , Ar q1 , Kr q 1 and Xe q1 ions [13]. It is seen that the proton yields increase steeply as q increases (~q |5 ). A similar q-dependence was also observed for H 1 2 sputtering [14]. Such a strong dependence is compared with other sputtering mechanism observed with HCI on insulators. In the case of LiF, the yields were ~eq , which was interpreted in terms of a multiple electronic defect production process induced by sequential electron transfer to the projectile [*15]. Fig. 6 further indicates that the slope of the proton yields gradually levels off for q * 10. Energy distributions of sputtered protons normal to the surface have been determined with a high resolution TOF spectroscopy. Figs. 7 (a) and (b) show the energy distributions of sputtered protons for (a) 600 eV Ar 61 and 500
Fig. 5. The experimental set-up used to measure the potential sputtering yield of protons from an untreated surface [9].
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Fig. 6. Proton sputtering yields from an untreated surface for 500 eV Ne q 1 (q 5 4–8), Ar q 1 (q 5 4–13), Kr q 1 (q 5 5–17) and Xe q 1 (q 5 7– 24) [13].
eV Ar 121 ions, and (b) 4.8 keV Ar 61 and 4.8 keV Ar 121 ions respectively. It is seen that: 1. the energy distribution was highly nonthermal with a peak at |10 eV, which depended neither on q nor on the kinetic energy 2. high energy tails grew as the kinetic energy of HCI got higher but did not depend very much on q 3. the yields for q512 were almost two orders of magnitudes larger than those for q56 The peak energy of the sputtered proton is comparable to the Coulomb repulsion energy between H 1 and X 1 ions after the breaking of a H–X bond because chemical bond lengths are typically |2 a.u. The fact that the 10 eV peak intensity depends strongly on q but barely on the kinetic energy indicates that the proton sputtering phenomena observed here are neither induced kinematically nor in-
duced by a Coulomb explosion of a charged hemisphere produced during the Stage II [16]. On the other hand, the behavior of the high energy tails followed at least qualitatively that of the kinematically sputtered particles. Because of the contribution of kinematic sputtering, the observed q-dependence of the proton yield gets weaker as the incident kinetic energy of HCI gets higher [**9]. Summarizing the observations above, the following scenario emerges as a mechanism of the proton sputtering: When an HCI approaches a chemical bond on a surface, electrons in the chemical bond are transferred to the HCI above the surface. Then two ions in the same bond repel each other along its bond direction, resulting in emission of one of the atoms away from the surface in the bond. Such a sputtering mechanism is expected to be particularly effective for light elements like hydrogen, firstly because it quickly leaves the bond area and so is less likely to be reneutralized, and secondly because it can gain enough kinetic energy before the partner is reneutralized. The latter condition is important for the leaving ion to overcome a possible attractive force, which might recover after the reneutralization of the partner. The sputtering mechanism described above is called a pair-wise potential sputtering. A simulation has been performed based on the scenario described above [**17] employing the COB process as the ruling mechanism of the electron transfer [**7]. The solid curve in Fig. 6 shows the result of the simulation, which satisfactorily reproduces both the q-dependence and the order of magnitude of the yield. The key mechanism causing the strong q-dependence is understood as follows: An active electron to be transferred forms a molecular orbital and is shared by both the bond atom and the HCI. The transfer ratio is estimated by the ratio of the phase space volume, i.e. 1:n 2 , which is approximately 1:q 2 because the COB model predicts n|q (see Fig. 4). Considering that two electron removal from the same bond and maintenance of a such condition is a minimum requirement of effective ion emission, the probability of
Fig. 7. The energy distribution of sputtered protons for (a) 600 eV Ar 61 and 500 eV Ar 121 ions, and (b) 4.8 keV Ar 61 and 4.8 keV Ar 121 ions [9].
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proton sputtering is expected to be proportional to q g with g |4 or even larger. The simulation also succeeded in reproducing the saturation of the q dependence for higher charge states (q * 10), which is explained as due to the delayed approach to the ionization equilibrium [**17].
4. Proton sputtering from well-defined Si(100)231– H, 331–H, and 131–H surfaces with slow highlycharged ions To study the proton sputtering phenomena for welldefined surfaces, H-terminated Si(100)231–H, 331–H, and 131–H surfaces were prepared as targets. Fig. 8 shows a schematic drawing of the UHV chamber used for the experiment. In this configuration, pulsed HCI beams were used to sort out the mass of the secondary ions with the TOF technique. The Si(100)231–H surface was prepared by exposing the Si(100)231 clean surface to atomic hydrogen keeping the substrate temperature at 600 K. The Si(100)331–H and Si(100)131–H surfaces were prepared by exposing the Si(100)231–H surface to atomic hydrogen at 400 K and at room temperature, respectively. Fig. 9 shows an example of the TOF spectrum of secondary ions sputtered from the Si(100)231–H surface with 3 keV Xe 18 ions, the major components of which were proton, Si 1 and Si 2 O(H)1 . The Si 1 peak skewed to the shorter TOF side, which corresponds to energetic Si 1 emission induced by kinetic sputtering [18,19]. Fig. 10 shows the sputtering yields of proton and Si 1
Fig. 8. A schematic drawing of the UHV chamber, which consists of a sample preparation chamber (upper), and a detection chamber (lower).
Fig. 9. A TOF spectrum of secondary ions sputtered from the Si(100)23 1–H surface with 3 keV Xe 81 ion [19].
from the Si(100)231–H surface with 4 keV Xe 81 as a function of incidence angle u [19]. The Si 1 yield depends strongly on u, which is the characteristic feature of kinetic sputtering [20]. The yield for u .608 is bigger than is shown here because the emission energy of Si 1 was larger for larger u, and accordingly Si 1 ions were not fully collected on the 2DPSD (see Fig. 8). The solid line shows cos 22u dependence, which more or less reproduces the observation. On the other hand, the proton yield was independent of u, proving that the proton sputtering is not induced by binary collisions which cause the kinematic sputtering. Fig. 11 shows the sputtering yields of proton and Si 1 from the Si(100)231–H surface with Xe 81 as a function of the charge state q. The dashed line draws ~q 5 , which roughly reproduced the proton yield, i.e. the q-dependence of the proton yield is almost the same as those for untreated surfaces (see Fig. 6). This observation supports the idea that the basic mechanism of the proton sputtering
Fig. 10. Sputtering yields of proton and Si 1 as a function of incidence angle u from the Si(100)231–H surface with 4 keV Xe 81 ions [19]. The solid line shows cos 22u.
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Fig. 11. Sputtering yields of proton and Si 1 from a Si(100)231–H surface as a function of q [19]. The dashed line shows ~q 5 .
is the same between the well-defined H-terminated Si surfaces and untreated surfaces. On the other hand, the Si 1 yield did not show any visible dependence on q. Similar behavior was reported for proton and Cs 1 emissions for 18 keV Ar q1 (q51–11) ions bombarding an untreated CsI surface [21]. Fig. 11 also shows that the sum of Si 1 and proton yields is roughly constant for q,8, and then
increases as a function of q for q * 9. This observation is consistent with a report by de Zwart et al. [22]. It was found that total secondary ion yields started to increase at q59 when an amorphized Si(100) surface was bombarded with 20 keV Ar q 1 (1,q,9) ions. The absolute proton yields for the Si(100)331–H and Si(100)131–H surfaces for 3 keV Xe 81 ions were |2.03 10 24 / ion and | 1.3310 23 / ion, respectively, which are compared with |1.2310 24 / ion, the yield for the Si(100)231–H surface. TDS measurements showed that the saturation coverages of hydrogen for these surfaces were 1 ML, 1.5 ML, and 2 ML (stoichiometrically 1:4 / 3:2). It is seen that the yield for the Si(100)–(131)H surface was outstandingly large as compared with the other two. A key to understanding this remarkable observation was found in STM (scanning tunnelling microscopy) images of these surfaces (see Figs. 12(a), (b) and (c)) [23] which proved that the Si(100)231–H and Si(100)331–H surfaces are atomically flat, although the Si(100)131–H surface is very rough. Such a rough surface is supposed to be formed due to a bond-breaking H-termination (H–Si5 Si–H&H–Si–H, H–Si–H) and an etching reaction (H– Si–H12H&SiH 4 ) when the Si(100)131–H surface is exposed to atomic hydrogen at room temperature [23]. It is easy to imagine that a doubly charged bond on an atomically rough surface is much more difficult to re-
Fig. 12. STM images of (a) Si(100)231–H, (b) Si(100)331–H, and (c) Si(100)131–H surfaces together with schematic drawings of each surface [23].
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Fig. 13. Two dimensional distribution of sputtered proton from (a) Si(100)231–H, (b) Si(100)331–H, and (c) Si(100)131–H surfaces with 3 keV Xe 81 ions [19].
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neutralize than that on an atomically flat surface, which can explain why the proton yield for the Si(100)131–H surface was much larger than those for other well-defined surfaces studied. Further, the reneutralization of Si may also be slower for a rough surface, leading to a prediction that the emission energy of proton is larger for the rough surface because the repulsive force between proton and Si 1 lasts longer. In order to confirm the above explanation, two dimensional distribution of sputtered protons were measured for: (a) Si(100)231–H, (b) Si(100)331–H, and (c) Si(100)131–H surfaces bombarded with 3 keV Xe 81 , which is shown in Fig. 13. The distribution width for the Si(100)131–H surface was much broader than those of the Si(100)231–H and Si(100)331–H surfaces, which is in good agreement with the prediction.
5. Comparison with electron induced Auger stimulated desorption The proton sputtering mechanism discussed above has some similarity with the so-called Auger stimulated desorption (ASD), which is known to be effective in electron- and photon-induced desorptions (ESD and PSD), where the creation of an innershell vacancy followed by Auger transitions plays an essential role [*24]. It has been recognized that a simple process like excitation or ionization of a bond electron to a dissociative state, which is very important for dissociation of isolated molecules, is often ineffective for atoms or molecules on a surface because of quick reneutralization from surrounding atoms. In other words, suppression of reneutralization is the key mechanism for ESD/ PSD to take place. The ASD process realizes this suppression via double ionization of anion atoms through an ionization of an innershell followed by an Auger electron emission. The reneutralization of the doubly ionized anion is usually suppressed because surrounding atoms have positive charges from the beginning, i.e. a repulsive force lasts for a considerable time which induces a positive ion emission. A similar observation was also reported for Ar q1 ion (q51,2) impacts on LiF. F 1 yields for Ar 21 ions were a couple of orders of magnitudes larger than those for Ar 1 ions, which was proposed to be induced with the ASD process [25], where a 2p electron of F 2 is captured into a 3p state of an Ar 1 ion transferring excess energy to another 2p electron of F. In the case of Ar 1 , only one electron removal from F 2 is energetically allowed, which can lead to the production of F 0 but not F 1 . In the HCI induced proton sputtering observed here, not a single electron removal from an innershell but a double electron transfer from a chemical bond is the key process to induce secondary ion emission. A study of F 1 sputtering mechanism with HCI is in progress [26].
6. Summary A proton sputtering mechanism induced with a highly charged ion is discussed. Both for untreated and welldefined surfaces, the proton yields are proportional to q 25 , which is successfully explained by a pair-wise potential sputtering model induced by successive electron transfers to the HCI governed by the classical over barrier process. The interaction depth of slow HCI with a surface can be limited just at the first few layers, i.e. slow HCI can be an ideal probe to study hydrogen on a surface with negligible damage to its substrate. This is not the case for other techniques which employ target atom recoil processes like ERDA, SIMS, etc. Further studies are in progress to test the applicability of the pair-wise potential sputtering model to other secondary ions induced with slow HCIs.
Acknowledgements The authors are deeply indebted to K. Komaki, J. Burgdoerfer, K. Ueda and Y. Murata for vivid and fruitful discussions on the dynamics of proton sputtering with HCIs.
References Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest. [*1] Briand J-P, de Billy L, Charles P, Essabaa S. Phys Rev Lett 1990;65:159. [2] Winter H. Europhys Lett 1992;18:207. [3] Kurz H et al. Phys Rev Lett 1992;69:1140. [4] Das J, Morgenstern R. Phys Rev A 1993;47:R755. [5] Meyer FW et al. Phys Rev A 1993;48:4479. [6] Grether M et al. Phys Rev A 1995;52:426. [**7] Burgdoerfer J, Lerner P, Meyer FW. Phys Rev A 1991;44:5674. [*8] Ninomiya S, Yamazaki Y, Koiki F, Masuda H, Azuma T, Komaki K, Kuroki K, Sekiguchi M. Phys Rev Lett 1997;78:4557. [**9] Kakutani K, Azuma T, Yamazaki Y, Komaki K, Kuroki K. Jpn J Appl Phys 1995;34:L580. [10] Oura K, Lifshits VG, Saranin AA, Zotov AV, Katayama M. Surf Sci Rep 1999;35:1. [11] Deutscher S, Yang X, Burgdoerfer J. Phys Rev A 1997;55:466. [12] Okuno K. Jpn J Appl Phys 1989;28:1124. [13] Kuroki K, Takahira T, Tsuruta Y, Okabayashi N, Azuma T, Komaki K, Yamazaki Y. Physica Scripta 1999;T80:557. [14] Kakutani K, Azuma T, Yamazaki Y, Komaki K, Kuroki K. Nucl Instrum Methods 1995;B96:541. [*15] Neidhart T, Pichler F, Aumayr F, Winter HP, Schmid M, Varga P. Phys Rev Lett 1995;74:5280. [16] Bitensky I, Parilis E, Della-Neglla E, Beyec YeL. Nucl Instrum Methods 1992;B72:380. [**17] Burgdoerfer J, Yamazaki Y. Phys Rev A 1996;54:4140.
Y. Yamazaki, K. Kuroki / Current Opinion in Solid State and Materials Science 6 (2002) 169–179 [18] Okabayashi N, Kuroki K, Tsuruta Y, Azuma T, Komaki K, Yamazaki Y. Physica Scripta 1999;T80:555–6. [19] Kuroki K, Okabayashi N, Torii H, Komaki K, Yamazaki Y, to be published. [20] Sigmund P. Phys Rev 1969;184:383. [21] Della-Negra S, Depauw J, Joret H, Beyec VLe, Schweikert EA. Phys Rev Lett 1988;60:948.
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[22] deZwart ST, Fried T, Boerma DO, Hoekstra R, Drentje AG, Boers AL. Surf Sci 1986;177:L939. [23] Boland JJ. Phys Rev Lett 1990;65:3325. [*24] Knotek ML, Feibelman PL. Phys Rev Lett 1978;40:964. [25] Varga P, Diebold U, Wutte D. Nucl Instrum Methods 1991;B58:417. [26] Okabayashi N et al., to be submitted.
Potential sputtering of hydrogen atoms on various surfaces with slow highly-charged ions Yasunori Yamazaki a,b , *, Kenro Kuroki a,c a
Institute of Physics, University of Tokyo, Komaba, Meguro-ku, Tokyo 153 -8902, Japan b RIKEN, 2 -1 Hirosawa, Wako, Saitama 351 -0198, Japan c NRIPS, 6 -3 -1 Kashiwanoha, Kashiwa, Chiba 277 -0882, Japan
Abstract Several recent findings on a new sputtering mechanism of hydrogen with slow highly-charged ions are discussed. The sputtering yields of protons were proportional to q |5 for q&10 independent of the surface condition for both untreated and well-defined surfaces, where q is the charge state of the ion. This q |5 dependence started to level off for q & 10. The yield for the Si(100)131–H surface was ten times larger than that for the Si(100)231–H surface although the stoichiometric hydrogen abundance of the former is only twice that of the latter. The key quantity to govern the yield is proposed to be surface roughness, which also influences the energy distribution of sputtered protons. These findings were consistently explained with a pair-wise potential sputtering model involving two successive electron transfers which follow the classical over barrier mechanism. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Interaction of slow highly-charged ions (HCIs) with metal, semiconductor, and insulator surfaces has been intensively studied in the last two decades [*1– 6,**7,*8,**9] because of the exotic nature of collision dynamics and the possible application to high-sensitivity surface analysis and surface modification. Charge state evolution above and below the surface with multiple electron transfers, including hollow atom formation, has been a key subject of research. Amongst various interesting phenomena introduced with slow HCIs, this article focuses on proton sputtering, which can be explained with a new mechanism involving a double electron transfer to a slow HCI above the surface. It will be shown that this new mechanism is particularly effective for light atoms like hydrogen on the first surface layer, the yield of which could be as high as 1.The yield does not depend very much on the HCI kinetic energy, which allows researchers to make a damage-free analysis with extremely high sensitivi*Corresponding author. Tel.: 181-3-5454-6521; fax: 181-3-54546433. E-mail addresses: [email protected] (Y. Yamazaki), [email protected] (Y. Yamazaki).
ty. Needless to say hydrogen impurity in solids influences material properties drastically. However, sensitive analysis has been quite difficult, primarily because neither X-rays nor Auger electrons are emitted (see e.g., Ref. [10]). ERDA is known to be a powerful technique to probe hydrogen in matter, although it does not usually provide atomic resolution, and often causes radiation damage on substrates. HCIs are characterized by two parameters, i.e., the charge state q, and the potential energy eq , the energy to be released when the HCI is neutralized. To get an idea about the order of magnitude of the potential energy, eq is plotted in Fig. 1 for several ions as a function of charge state q. Very crudely, collision processes with large impact parameters are governed by q, while those with small impact parameters are governed by eq . For an ion with its kinetic energy around several keV/ u, multiple binary collisions with target atoms are the major process of energy deposition (nuclear stopping domain), which causes not only radiation damage to the target but also ejection of atoms / ions (kinetic sputtering). In the case of highly charged ion impacts: (1) the potential energy becomes another source of deposited energy, and (2) multiple electron transfer processes from the target to the projectile play important roles in particle emission. The latter process is the major subject of the present review.
1359-0286 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 02 )00046-3
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tunneling process. On the other hand, when it is equal to or lower than the workfunction of the metal, electrons are transferred classically beyond the barrier (which will hereafter be described as the classical over barrier (COB) condition). Because the transition rate under the COB condition is much higher than that under tunneling condition [11], electron transfer phenomena in slow HCIsurface collisions are more or less governed by the period when the COB condition is satisfied [**7]. Equating the depth of the saddle point to be the workfunction W, the critical distance d c , where the first electron transfer takes place, is obtained as ] œ2q d c | ]]. W
(2)
For a typical metal having W|4–5 eV, d c is of the order of nm. Since the electron transfer takes place resonantly, the binding energy of the transferred electron is similar to that of the valence electron, i.e. the electron is transferred to high Rydberg states of the HCI. The principal quantum number n c in which the first electron is transferred is given by Fig. 1. Potential energy of Ar, Kr, Xe and U ions as a function of charge state q.
2. Fundamental process in HCI–surface interaction— classical over barrier model When a slow HCI approaches a metal surface, it is accelerated toward the surface due to its image force, then at a certain distance, it captures target electrons, and finally it jumps into the surface releasing all its potential energy via Auger and X-ray emissions. A schematic view of this scenario is given in Fig. 2. The active electron to be transferred is subject to the following interactions: the interaction with the HCI, with the image of the HCI, and with the image of the electron itself, a sketch of which is drawn in Fig. 3(a). The interaction potential is given by → q q 1 → V( r e , R) 5 2 ]]] → 1 ]]] → 2] → → u r e 2Ru u r e 1Ru 4uzu →
(1)
where r e (x, 0, z) is the electron position with z-axis → perpendicular to the surface, and R(0, 0, d) is the position of the ion (atomic units are used unless otherwise noted.). → → Fig. 3(b) is a 3D representation of V( r e , R), which shows a saddle point along the line connecting the HCI and its image charge. The potential wall at the saddle point works as a barrier to block target electrons to be transferred to the HCI. The position of the barrier and its height are evaluated to be d /(8q)1 / 2 and 2(2q)1 / 2 /d, respectively, i.e., the barrier gets lower as d gets smaller. When the barrier is higher than the workfunction of the metal, valence electrons can only be transferred to the HCI via a
q n c | ]]]]] ]]]] ] œ2W(1 1œq / 8)
(3)
The solid line in Fig. 4 shows n c for W50.2 as a function of q. The dashed line corresponds to n5q, which more or less reproduces n c for q&20. The HCI captures the second electron as soon as the transfer condition is satisfied for q21 (see Eq. (3)), i.e., electrons are successively transferred into high Rydberg states of the HCI as it gets closer to the surface. The atomic state so formed, with many electrons occupying highly excited levels whilst still keeping inner shell hole(s), is called a hollow atom. It is noted that the radii of the excited states are comparable to d c , and accordingly the electrons can move back and forth between the ion and the target when they are in resonance. In this respect, the hollow atom in front of the surface could better be described as a ‘dynamic hollow molecule’. Lifetimes of innershell vacancies of the hollow atom are often longer than the time interval between the hollow atom formation and the collision with the surface (typical213 ly &10 s), i.e. a hollow atom with inner shell vacancies collides violently with the target surface, where these vacancies are filled via quasi-resonant charge transfer and / or inter- and intra-Auger transitions [**7]. The collision process of an HCI with a surface is then divided into two successive stages: (1) a soft above-surface collision accompanying multiple electron transfers among outershells and (2) a hard collision at or below the surface involving innershell transitions (hereafter referred to as Stage I and Stage II, respectively). As is evident from the discussion above, Stage II is the period when a major part of the potential energy is released from the HCI and deposited to the target. On the other hand, in the Stage I, the area
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Fig. 2. A schematic view of atomic processes taking place during the interaction of a slow highly charged ion with a metal / semiconductor surface.
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→
→
Fig. 3. (a) Potential energy of the active electron; (b) 3D representation of V( r , R). An HCI with q510 is fixed at d5a.u.
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Fig. 4. The solid line shows n c predicted with Eq. (3). The dashed line shows n5q, which more or less reproduces n c for q & 20.
around the entrance point of the HCI experiences a strong charge-up due to multiple electron transfer, which is reneutralized by valence electrons with a time constant tre .tre 21 is expected to be | v pl for metals, where vpl is the plasma angular frequency. For insulators, tre is |10 a.u., which is estimated from a typical band width or a hopping matrix element between the nearest neighbor atoms. It is expected that the mutual Coulomb repulsion among charged target atoms prepared in the Stage I causes particle emission when reneutralization of the surface is slow.
3. Proton sputtering from untreated surfaces with slow highly charged ions Fig. 5 shows a schematic drawing of the experimental set-up used to measure the proton sputtering yield with
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slow HCIs. Slow HCIs extracted from an electron beam ion source (EBIS) [12] were introduced in the collision region where an electric field was applied, and eventually hit a target prepared on the Cu mesh [**9]. When the HCI hits the target, secondary electron(s) as well as secondary ion(s) are ejected. The electric field was used to accelerate secondary ions of positive charge toward the MCP, and to reflect secondary electrons back to the mesh, a part of which pass through the openings of the mesh and are detected by the channeltron located downstream of the mesh. The mass of the secondary ions was determined with the arrival time difference between the secondary electron and the secondary ion. In the present set-up, any macroscopic charge-up of the target was negligibly small because the HCI intensity was less than |10 3 cps, i.e. |10 216 A. Fig. 6 shows the proton sputtering yields from an untreated CuO surface (i.e. covered with some layers of water, hydrocarbons etc.) for Ne q1 , Ar q1 , Kr q 1 and Xe q1 ions [13]. It is seen that the proton yields increase steeply as q increases (~q |5 ). A similar q-dependence was also observed for H 1 2 sputtering [14]. Such a strong dependence is compared with other sputtering mechanism observed with HCI on insulators. In the case of LiF, the yields were ~eq , which was interpreted in terms of a multiple electronic defect production process induced by sequential electron transfer to the projectile [*15]. Fig. 6 further indicates that the slope of the proton yields gradually levels off for q * 10. Energy distributions of sputtered protons normal to the surface have been determined with a high resolution TOF spectroscopy. Figs. 7 (a) and (b) show the energy distributions of sputtered protons for (a) 600 eV Ar 61 and 500
Fig. 5. The experimental set-up used to measure the potential sputtering yield of protons from an untreated surface [9].
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Fig. 6. Proton sputtering yields from an untreated surface for 500 eV Ne q 1 (q 5 4–8), Ar q 1 (q 5 4–13), Kr q 1 (q 5 5–17) and Xe q 1 (q 5 7– 24) [13].
eV Ar 121 ions, and (b) 4.8 keV Ar 61 and 4.8 keV Ar 121 ions respectively. It is seen that: 1. the energy distribution was highly nonthermal with a peak at |10 eV, which depended neither on q nor on the kinetic energy 2. high energy tails grew as the kinetic energy of HCI got higher but did not depend very much on q 3. the yields for q512 were almost two orders of magnitudes larger than those for q56 The peak energy of the sputtered proton is comparable to the Coulomb repulsion energy between H 1 and X 1 ions after the breaking of a H–X bond because chemical bond lengths are typically |2 a.u. The fact that the 10 eV peak intensity depends strongly on q but barely on the kinetic energy indicates that the proton sputtering phenomena observed here are neither induced kinematically nor in-
duced by a Coulomb explosion of a charged hemisphere produced during the Stage II [16]. On the other hand, the behavior of the high energy tails followed at least qualitatively that of the kinematically sputtered particles. Because of the contribution of kinematic sputtering, the observed q-dependence of the proton yield gets weaker as the incident kinetic energy of HCI gets higher [**9]. Summarizing the observations above, the following scenario emerges as a mechanism of the proton sputtering: When an HCI approaches a chemical bond on a surface, electrons in the chemical bond are transferred to the HCI above the surface. Then two ions in the same bond repel each other along its bond direction, resulting in emission of one of the atoms away from the surface in the bond. Such a sputtering mechanism is expected to be particularly effective for light elements like hydrogen, firstly because it quickly leaves the bond area and so is less likely to be reneutralized, and secondly because it can gain enough kinetic energy before the partner is reneutralized. The latter condition is important for the leaving ion to overcome a possible attractive force, which might recover after the reneutralization of the partner. The sputtering mechanism described above is called a pair-wise potential sputtering. A simulation has been performed based on the scenario described above [**17] employing the COB process as the ruling mechanism of the electron transfer [**7]. The solid curve in Fig. 6 shows the result of the simulation, which satisfactorily reproduces both the q-dependence and the order of magnitude of the yield. The key mechanism causing the strong q-dependence is understood as follows: An active electron to be transferred forms a molecular orbital and is shared by both the bond atom and the HCI. The transfer ratio is estimated by the ratio of the phase space volume, i.e. 1:n 2 , which is approximately 1:q 2 because the COB model predicts n|q (see Fig. 4). Considering that two electron removal from the same bond and maintenance of a such condition is a minimum requirement of effective ion emission, the probability of
Fig. 7. The energy distribution of sputtered protons for (a) 600 eV Ar 61 and 500 eV Ar 121 ions, and (b) 4.8 keV Ar 61 and 4.8 keV Ar 121 ions [9].
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proton sputtering is expected to be proportional to q g with g |4 or even larger. The simulation also succeeded in reproducing the saturation of the q dependence for higher charge states (q * 10), which is explained as due to the delayed approach to the ionization equilibrium [**17].
4. Proton sputtering from well-defined Si(100)231– H, 331–H, and 131–H surfaces with slow highlycharged ions To study the proton sputtering phenomena for welldefined surfaces, H-terminated Si(100)231–H, 331–H, and 131–H surfaces were prepared as targets. Fig. 8 shows a schematic drawing of the UHV chamber used for the experiment. In this configuration, pulsed HCI beams were used to sort out the mass of the secondary ions with the TOF technique. The Si(100)231–H surface was prepared by exposing the Si(100)231 clean surface to atomic hydrogen keeping the substrate temperature at 600 K. The Si(100)331–H and Si(100)131–H surfaces were prepared by exposing the Si(100)231–H surface to atomic hydrogen at 400 K and at room temperature, respectively. Fig. 9 shows an example of the TOF spectrum of secondary ions sputtered from the Si(100)231–H surface with 3 keV Xe 18 ions, the major components of which were proton, Si 1 and Si 2 O(H)1 . The Si 1 peak skewed to the shorter TOF side, which corresponds to energetic Si 1 emission induced by kinetic sputtering [18,19]. Fig. 10 shows the sputtering yields of proton and Si 1
Fig. 8. A schematic drawing of the UHV chamber, which consists of a sample preparation chamber (upper), and a detection chamber (lower).
Fig. 9. A TOF spectrum of secondary ions sputtered from the Si(100)23 1–H surface with 3 keV Xe 81 ion [19].
from the Si(100)231–H surface with 4 keV Xe 81 as a function of incidence angle u [19]. The Si 1 yield depends strongly on u, which is the characteristic feature of kinetic sputtering [20]. The yield for u .608 is bigger than is shown here because the emission energy of Si 1 was larger for larger u, and accordingly Si 1 ions were not fully collected on the 2DPSD (see Fig. 8). The solid line shows cos 22u dependence, which more or less reproduces the observation. On the other hand, the proton yield was independent of u, proving that the proton sputtering is not induced by binary collisions which cause the kinematic sputtering. Fig. 11 shows the sputtering yields of proton and Si 1 from the Si(100)231–H surface with Xe 81 as a function of the charge state q. The dashed line draws ~q 5 , which roughly reproduced the proton yield, i.e. the q-dependence of the proton yield is almost the same as those for untreated surfaces (see Fig. 6). This observation supports the idea that the basic mechanism of the proton sputtering
Fig. 10. Sputtering yields of proton and Si 1 as a function of incidence angle u from the Si(100)231–H surface with 4 keV Xe 81 ions [19]. The solid line shows cos 22u.
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Fig. 11. Sputtering yields of proton and Si 1 from a Si(100)231–H surface as a function of q [19]. The dashed line shows ~q 5 .
is the same between the well-defined H-terminated Si surfaces and untreated surfaces. On the other hand, the Si 1 yield did not show any visible dependence on q. Similar behavior was reported for proton and Cs 1 emissions for 18 keV Ar q1 (q51–11) ions bombarding an untreated CsI surface [21]. Fig. 11 also shows that the sum of Si 1 and proton yields is roughly constant for q,8, and then
increases as a function of q for q * 9. This observation is consistent with a report by de Zwart et al. [22]. It was found that total secondary ion yields started to increase at q59 when an amorphized Si(100) surface was bombarded with 20 keV Ar q 1 (1,q,9) ions. The absolute proton yields for the Si(100)331–H and Si(100)131–H surfaces for 3 keV Xe 81 ions were |2.03 10 24 / ion and | 1.3310 23 / ion, respectively, which are compared with |1.2310 24 / ion, the yield for the Si(100)231–H surface. TDS measurements showed that the saturation coverages of hydrogen for these surfaces were 1 ML, 1.5 ML, and 2 ML (stoichiometrically 1:4 / 3:2). It is seen that the yield for the Si(100)–(131)H surface was outstandingly large as compared with the other two. A key to understanding this remarkable observation was found in STM (scanning tunnelling microscopy) images of these surfaces (see Figs. 12(a), (b) and (c)) [23] which proved that the Si(100)231–H and Si(100)331–H surfaces are atomically flat, although the Si(100)131–H surface is very rough. Such a rough surface is supposed to be formed due to a bond-breaking H-termination (H–Si5 Si–H&H–Si–H, H–Si–H) and an etching reaction (H– Si–H12H&SiH 4 ) when the Si(100)131–H surface is exposed to atomic hydrogen at room temperature [23]. It is easy to imagine that a doubly charged bond on an atomically rough surface is much more difficult to re-
Fig. 12. STM images of (a) Si(100)231–H, (b) Si(100)331–H, and (c) Si(100)131–H surfaces together with schematic drawings of each surface [23].
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Fig. 13. Two dimensional distribution of sputtered proton from (a) Si(100)231–H, (b) Si(100)331–H, and (c) Si(100)131–H surfaces with 3 keV Xe 81 ions [19].
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neutralize than that on an atomically flat surface, which can explain why the proton yield for the Si(100)131–H surface was much larger than those for other well-defined surfaces studied. Further, the reneutralization of Si may also be slower for a rough surface, leading to a prediction that the emission energy of proton is larger for the rough surface because the repulsive force between proton and Si 1 lasts longer. In order to confirm the above explanation, two dimensional distribution of sputtered protons were measured for: (a) Si(100)231–H, (b) Si(100)331–H, and (c) Si(100)131–H surfaces bombarded with 3 keV Xe 81 , which is shown in Fig. 13. The distribution width for the Si(100)131–H surface was much broader than those of the Si(100)231–H and Si(100)331–H surfaces, which is in good agreement with the prediction.
5. Comparison with electron induced Auger stimulated desorption The proton sputtering mechanism discussed above has some similarity with the so-called Auger stimulated desorption (ASD), which is known to be effective in electron- and photon-induced desorptions (ESD and PSD), where the creation of an innershell vacancy followed by Auger transitions plays an essential role [*24]. It has been recognized that a simple process like excitation or ionization of a bond electron to a dissociative state, which is very important for dissociation of isolated molecules, is often ineffective for atoms or molecules on a surface because of quick reneutralization from surrounding atoms. In other words, suppression of reneutralization is the key mechanism for ESD/ PSD to take place. The ASD process realizes this suppression via double ionization of anion atoms through an ionization of an innershell followed by an Auger electron emission. The reneutralization of the doubly ionized anion is usually suppressed because surrounding atoms have positive charges from the beginning, i.e. a repulsive force lasts for a considerable time which induces a positive ion emission. A similar observation was also reported for Ar q1 ion (q51,2) impacts on LiF. F 1 yields for Ar 21 ions were a couple of orders of magnitudes larger than those for Ar 1 ions, which was proposed to be induced with the ASD process [25], where a 2p electron of F 2 is captured into a 3p state of an Ar 1 ion transferring excess energy to another 2p electron of F. In the case of Ar 1 , only one electron removal from F 2 is energetically allowed, which can lead to the production of F 0 but not F 1 . In the HCI induced proton sputtering observed here, not a single electron removal from an innershell but a double electron transfer from a chemical bond is the key process to induce secondary ion emission. A study of F 1 sputtering mechanism with HCI is in progress [26].
6. Summary A proton sputtering mechanism induced with a highly charged ion is discussed. Both for untreated and welldefined surfaces, the proton yields are proportional to q 25 , which is successfully explained by a pair-wise potential sputtering model induced by successive electron transfers to the HCI governed by the classical over barrier process. The interaction depth of slow HCI with a surface can be limited just at the first few layers, i.e. slow HCI can be an ideal probe to study hydrogen on a surface with negligible damage to its substrate. This is not the case for other techniques which employ target atom recoil processes like ERDA, SIMS, etc. Further studies are in progress to test the applicability of the pair-wise potential sputtering model to other secondary ions induced with slow HCIs.
Acknowledgements The authors are deeply indebted to K. Komaki, J. Burgdoerfer, K. Ueda and Y. Murata for vivid and fruitful discussions on the dynamics of proton sputtering with HCIs.
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