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Dynamical resonant charge transfer of fast C−, O−, F− ions and water covered Si(111) surface
Vacuum 137 (2017) 23e30
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Dynamical resonant charge transfer of fast C, O, F ions and water covered Si(111) surface Feifei Xiong, Lei Gao, Yuefeng Liu, Jianjie Lu, Pinyang Liu, Shunli Qiu, Xiyu Qiu, Yanling Guo, Ximeng Chen, Lin Chen* School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 12 November 2016 Accepted 6 December 2016 Available online 9 December 2016
The experiment of 6.5e22.5 keV C, O and F ions scattering on water covered Si(111) surface has been performed. It is found that the positive-ion fraction is very low and increases monotonically as a function of perpendicular exit velocity and exit angle. In particular, the negative-ion fraction increases monotonically with perpendicular exit velocity for specular scattering, and for a given incident energy the angle dependence of the fraction is nonmonotonic. We interpret the observed positive ions in terms of inelastic binary collision, and adopt a modified resonant charge transfer model to calculate the bellshaped negative-ion fraction. We find that the neutral yield at short ion-surface distance is nonzero and obeys well an exponential dependence. It strongly indicates that a dynamical equilibrium for negative ion formation is never achieved at short distances, and the band gap effect on charge transfer can be neglected to a large extent in this relatively high energy region. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Charge transfer on solid surfaces plays a crucial role not only from a fundamental physical interest, but also for practical applications, i.e., secondary ion mass spectroscopy and low energy ion scattering (LEIS). Semiconductor, is one of the indispensable materials for modern science and technology [1]. The fascinating physical and chemical properties of semiconductor surfaces are mainly dominated by their electronic structures. Charge transfer during ion scattering on semiconductor surface could also be affected by the surface electronic structure properties. The most direct way to probe charge transfer process is the ionsurface scattering technique [2e5]. In the scattering of keV-energy atoms or ions on metal surfaces, resonant charge transfer (RCT) is the dominant process, which involves one electron transition between an atomic level and a surface [6e13]. For negative-ion formation, the affinity level of the projectile is downward shifted due to the image potential effects [14], and is below the Fermi level of the metal surface at short distances, then a tunneling transition of an electron from occupied levels of the valence band to the anion level of the projectile occurs and forms a negative ion. Departing
* Corresponding author. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.vacuum.2016.12.008 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
from the surface, the initially formed negative ion quickly decays via resonant ionization and becomes an atom when its affinity level lies above the Fermi level again. Thus the final negative-ion fraction depends on the width of the affinity level, the surface work function, and the interaction time the particle spends near the surface where RCT processes occur [15]. Most research on negative-ion formation have been confined to the interactions between atoms/ions and metallic surfaces [6e13,16e24]. In comparison, there are only few studies on semiconductor surfaces [25e32]. The formation of H is particularly simple for its s character state, which has been studied experimentally and theoretically on metallic surfaces [6e10,16e18] as well as silicon surfaces [29e32]. In contrast, the formation of F is significantly more complex than H for its multielectron aspect [19,20]. Although electronegative ions like O, F and Cl, play significant roles in etching and growth studies for semiconductor manufacture, the study of these ions scattering on silicon is scarce. From the point of view of materials, clean Si has special electronic structure, i.e., the existence of a narrow band gap (~1.1 eV) and localized dangling-bond surface states in the gap region. These features directly affect charge transfer process on the surface. H and O fractions are of the same magnitude as on an Al surface reported by Esaulov et al. in the scattering of hydrogen and oxygen ions with clean Si(111), which was explained by the nonresonant charge transfer involving localized dangling-bond surface states
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F. Xiong et al. / Vacuum 137 (2017) 23e30
[29,30]. On the other hand, the narrow band gap is located at an energy position close to the bulk valence band region, about 5 eV below the vacuum level. The neutralization of Liþ ion scattering on clean and hydrogen covered Si(111) has clearly demonstrated the significant band gap effect [26] where hydrogen adsorbates removed the dangling-bond surface states. The above mentioned studies of positive ions scattering on silicon surface mainly concentrate in the low energy region (0.5e4 keV), while related investigations for some tens of keV energies are scarce. The situation may be different for high incident energies because the dynamical charge equilibrium may not be achieved at short ion-surface distance [6]. Moreover, negative ions with low electron affinities are very suitable for studying charge transfer that can probe the electronic structure of the silicon surface. We have examined the doping and the band gap effects in the scattering of some tens of keV energies negative carbon and fluorine ions on water covered Si(100) surfaces [33,34], in which positive and negative-ion fractions were measured and the observed bell-shaped negative fluorine ion fraction was well interpreted by using a complete theoretical framework [34]. However, we do not know whether these experimental phenomena can be observed for water covered Si(111) and what is the difference of the results between Si(111) and Si(100) surfaces. We also do not know whether this modified theoretical model is fully applicable to other collision systems. To answer these questions, we have performed the measurements of C, O and F scattering on water covered Si(111) surface at the same experimental conditions as our previous work. In this work, we measured the positive-ion and negative-ion fractions for 6.5e22.5 keV C, O and F ions scattering on water covered Si(111). Indeed, we find similar variation of the fractions as our previous results [33e36]: (1) the positive-ion fraction is small and increases with perpendicular exit velocity and exit angle; (2) the negative-ion fraction also increases monotonically with perpendicular exit velocity for specular scattering; (3) the variation of negative-ion fractions with exit angle is nonmonotonic and presents a bell shape. We find that the fractions of Si(111) are all smaller than that of Si(100) for a given projectile. The difference of the data between Si(111) and Si(100) surfaces has been well discussed. For positive ion production, it is interpreted by the molecular orbital (MO) promotion model. For negative ion formation, it is found that the modified resonant charge transfer model we built [34] can also reproduce the experimental data of Si(111). It is further confirmed that, for the Si(111) surface, a dynamical charge equilibrium is never achieved at short distances, and the band gap effect is inefficient in electron transfer process in this relatively high energy range.
Fig. 1. Schematic diagram of the experimental setup.
passed through two separated slits and a parallel-plate electrostatic deflector, and were simultaneously counted by a one-dimensional position sensitive microchannel plate (PSMCP) detector. We appropriately choose the biased voltage of the deflector to ensure that all scattered particles are well collected by the detector. The detector consists of two MCPs mounted in a chevron configuration placed above a resistive anode. The detector efficiency for scattered particles with different charge states was assumed to be identical for the same energies [37e39]. In situ preparation of the surface was performed by many cycles of small angle sputtering with 3 keV Arþ ions and subsequent annealing at about 773 K (15 min) using electron bombardment to prevent sputtering damage. Our setup is not yet equipped with a low-energy-electron-diffraction (LEED) system that would allow us to check the state of the surface. We check the surface cleanliness by measuring the time-of-flight scattering and recoiling spectroscopy (TOF-SARS) with a scattering angle of 20 and an incident angle of 5 . The scattered and recoil particles are recorded by a MCP detector. The measured TOF spectra are shown in Fig. 2. It is observed that the recoiled intensities of adsorbates such as H and O disappear after many cycles of preparation. However, a small amount of residual impurities will be observed again in the TOF spectra during our measurements. This may be mainly caused by the chemisorption of H2O molecules from background gases in our chamber.
2. Experiment The schematic diagram of the experimental setup is shown in Fig. 1. The details have been described elsewhere [33e36]. Briefly, the C, O and F ions with energies from 6.5 up to 22.5 keV were produced in a cesium-sputtered ion source. After analyzing the mass to charge ratio by a 45 bending magnet, the extracted ion beam was collimated by two slits. Then a pair of electrostatic plates placed between the two slits were used to separate the negative component from neutrals and steer the ion beam to pass through the third collimators. These collimators guaranteed the angular divergence of the incident ion beam less than 0.28 . A lightly doped p-type Si(111) wafer with a resistivity of 1.0 Ucm was used as a target mounted on a precision manipulator in a ultrahigh-vacuum (UHV) chamber, with a typical pressure of better than 3.0 107 Pa. The scattering angle was fixed at 38 and the incident angle a was varied from 4 to 31 measured with respect to the surface plane. The reflected particles from the surface
Fig. 2. Time of flight (TOF) spectra of 3 keV Arþ scattering on a Si(111) surface with a scattering angle of 20 and an incident angle of 5 . Ar(S) represents the scattered Ar particles. Si(DR), O(DR) and H(DR) represent the recoil Si, O, and H particles from the surface, respectively.
F. Xiong et al. / Vacuum 137 (2017) 23e30
Our previous work has demonstrated that the ratio between the measured recoil intensities of H and O is about 2 as the surface was kept under UHV conditions [35], and the amount of H2 adsorption from the residual gases in the chamber is much smaller than that of water vapor [33]. From the TOF spectra, we have found that the relative intensity of hydrogen (oxygen) rapidly increased to a saturated value within an hour [35]. Water molecules adsorbed on Si(111) in dissociation forms at room temperature, and the saturation of dangling-bond surface states for H and OH needs only 0.2 monolayer of water [40]. In view of the vacuum condition of 3.0 107 Pa, the surface can be completely covered with water vapor in a few minutes (1 L ¼ 106 Torr s). As mentioned in the introduction, this completely passivated surface is well suited to delineate the narrow band gap effect of Si(111). We used low beam current during the measurements to prevent damage of the water passivated surface. The negative-ion (positive-ion) fraction is defined as Fe/ þ ¼ N(Ae/þ)/N(Total), where N(Total) is the total number of scattered particles neglecting the recoil particles. It is noted that, the PSMCP detector can collect all energetic scattered and recoil particles. The recoil particles are almost atoms and would reduce the ion fractions that has been discussed in our previous studies [34,35]. In the present work, we cannot do the TOF analysis to evaluate the influence of recoil particles because of the absence of the pulsed beam for some tens of keV-energy negative ions. But we did the simulation for 21.5 keV F in specular scattering on clean and water covered Si(111) surfaces. The simulation was performed using the Kalypso software package [41]. The Si(111) target used in this work consists four atomic layers, and it was very long in the direction of ion path to allow continuous collisions. For water covered Si(111), the top layer atoms of Si(111) were approximately replaced with H and O with the ratio of 2:1. The interaction potential of F-Si (F-H and F-O) was represented by uncorrected Ziegler-Biersack-Littmark (ZBL) universal screening function potential [42]. The reflected particles were collected with an altitudinal angle of 19 ± 2 , and with a plane angle width of ± 2 . The simulation used approximately 1.2 105 projectiles for both clean and water covered Si(111) surfaces. In Fig. 3, two main peaks located at 16 keV and 18.5 keV correspond to the single and double scattering of F on Si. A small peak located at 17 keV corresponds to the recoil Si. In the
25
simulation, the recoil H and O particles from the water covered Si(111) surface were not observed. The main component of the reflected flux should be the scattering F, and its fraction is about 95.5% and 90.8% for clean and water covered surfaces, respectively. Thus the simulated results excluding the recoil particles could be reduced by a factor of about 1.1, which is consistent with that reported by Esaulov et al. where their corrected results are reduced by a factor of 1e1.3 [30]. 3. Charge transfer model Scattering of electronegative atoms/ions (i.e., O, F, Cl) off solid surfaces is generally accomplished by highly efficient electron capture and loss processes, and thus the scattered particle flux is largely composed of anions and neutrals [22,43e45]. However, in our case positive ions have been observed. Positive-ion production is a local process, whereas negative-ion formation is governed by a nonlocal process as the projectile approaches or departs from the surface. For negative ions scattering on Si, efficient neutralization occurs in the incoming path and negative projectiles first become energetic atoms prior to form positive ions. The observed positive ions must be produced in inelastic binary collisions with surface Si or O atoms at short distances where the electron excitation and direct ionization processes occur. To produce positive ions, direct single ionization process takes place via Coulomb repulsion that is well described by the binary encounter approximation [46e48]. For the electron excitation process, we briefly point out that the possible electron excitation for C (O, F) Si collision system is caused by 4fs MO promotion, and for C (O, F) - O is caused by 4fs or 3ds MO promotion which is associated with the C (O, F) 2p level [49]. The final positive-ion fraction is determined by the production probability and subsequent neutralization process, and the final survival probability of positive ions can be expressed by,
P þ ð∞Þ ¼ P þ ðz0 Þ exp
vc
(1)
v⊥:out
where P þ ðz0 Þ represents the initial positive-ion production probaaz0 bility at z0, vc ¼ t0 ea is the characteristic velocity proportional to the neutralization rate, and v⊥,out is the perpendicular exit velocity of the scattered positive ions. Here, z0 is the starting point of the effective position of the outgoing trajectory where efficient neutralization occurs. The more details of positive ion production can be found in our previous work [33,34]. For negative-ion formation, the conventional understanding assumes that the memory effect of the initial charge state of the projectile is completely lost [50]. In the scattering of negative ions on metal-like surfaces, it is reasonable to describe RCT processes only along the outgoing trajectory. The expression for negative-ion 0 1 Z ∞ 1 @ fraction pout ¼ exp v⊥out GðzÞdzA is in good agreement with z0
a lot of monotonic experimental results [6,18,19]. G(z) is the total transition probability. However, this expression cannot give a full description for the nonmonotonic results in our case. In this respect, a nonzero neutral fraction is assumed at the starting point of the trajectory integration [33,34], can be expressed by,
" i h p0 v⊥;in ¼ 1 pþ ðz0 Þ 1 exp Fig. 3. The simulated energy spectra of reflected particles for 21.5 keV fluorine atoms in specular scattering on clean and water covered Si(111) surfaces.
gz0 v⊥;in
!# (2)
where g is a constant corresponding to electron transition rate, and v⊥;in is the incident velocity normal to the surface.
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F. Xiong et al. / Vacuum 137 (2017) 23e30
After considering the positive-ion production, the calculation is done only along the outgoing trajectory. The calculation is performed by using two rate equations,
dn⊥ ðzÞ 1 ¼ dz v⊥;out
function of perpendicular exit velocity and exit angle. In Fig. 4, positive-ion fractions are measured for 6.5e22.5 keV C, O and F in specular (19 /19 ) scattering on water covered Si(111) surface.
h i G⊥ ðzÞf n⊥ ðzÞ þ N⊥ G⊥ ðzÞð1 f Þ 1 P þ ðzÞ n⊥ ðzÞ n⁄ ⁄ ðzÞ
(3)
h i G⁄ ⁄ ðzÞf n⁄ ⁄ ðzÞ þ N⁄ ⁄ G⁄ ⁄ ðzÞð1 f Þ 1 P þ ðzÞ n⊥ ðzÞ n⁄ ⁄ ðzÞ
(4)
þt0 eaz P þ ðzÞ
and
dn⁄ ⁄ ðzÞ 1 ¼ dz v⊥;out
þt0 eaz P þ ðzÞ
For these two equations, the first terms on the right side correspond to negative-ion formation, and second and third terms represent the production of neutrals. The parameter f is the FermieDirac distribution function [14], n⊥ ðzÞ and n⁄ ⁄ ðzÞ is the total neutral population in the m ¼ 0 and m ¼ ±1 states, respectively. The m ¼ 0 state is oriented normal and the m ¼ ±1 states parallel to the surface. The degeneracies are N⊥ ¼ 2 and N⁄ ⁄ ¼ 4. Finally, we obtain the negative-ion fraction ½1 P þ ð∞Þ n⊥ ð∞Þ n⁄ ⁄ ð∞Þ. More details of negative-ion formation have been discussed in our previous work [33,34]. 4. Results We measured the perpendicular exit velocity and exit angle dependences of fractions in the collisions of C, O and F ions with water covered Si(111) surface. The perpendicular exit velocity is calculated with the binary collision approximation from the interaction energy of the C (O, F)-Si collision partner. Results of positive-ion fraction are shown in Figs. 4e5 as a
Fig. 4. (Color online) Positive-ion fractions as a function of perpendicular exit velocity, measured for 6.5e22.5 keV C, O and F in specular scattering on water covered Si(111) surface. The black solid, red dashed and blue dash dotted curves are the fitting results.
For oxygen and fluorine, the positive-ion fraction Fþ is much smaller than F, while for carbon, the fraction Fþ is very close to F and larger than that of oxygen and fluorine. The fraction Fþ increases with perpendicular exit velocity which is similar to that measured for C and F in specular scattering on water covered Si(100) surface in our previous work [34]. In Fig. 5, the fraction Fþ for oxygen and fluorine increases as a function of exit angle b, and this trend is consistent with the previous results of 1e4 keV oxygen ions scattering on Mg, Al, and Ag surfaces [21], of 1e4 keV fluorine ions scattering on Mg [22] and 4 keV oxygen ions scattering on Si(111) [30], as well as the results of C and F in specular scattering on water covered Si(100) surface in our previous work [34]. The fraction Fþ (maximum value of 2.55% and 1.98% for oxygen and fluorine) is much smaller than Fe. In Fig. 6, we plot the negative-ion fractions measured for C, O and F ions scattering on water covered Si(111) as a function of perpendicular exit velocity, corresponding to incident energies from 6.5 to 22.5 keV for specular scattering (19 /19 ). The negativeion fraction Fe increases monotonically with perpendicular exit velocity, which is similar to that measured for C and F in specular scattering on water covered Si(100) surface in our previous work
Fig. 5. (Color online) Positive-ion fractions as a function of exit angle with respect to the surface plane, measured for 22.5 keV O [35] and 21.5 keV F scattering on water covered Si(111) surface. The black solid and blue dashed curves are the fitting results.
F. Xiong et al. / Vacuum 137 (2017) 23e30
Fig. 6. (Color online) Negative-ion fractions as a function of perpendicular exit velocity, measured for 6.5e22.5 keV C, O and F in specular scattering on water covered Si(111) surface. The black solid, red dashed and blue dash dotted curves are the calculated results including the contribution of positive ions.
[34]. The fraction Fe for carbon (0.59%e4.8%) is much smaller than that of oxygen (2.5%e24%) and fluorine (5.3%e27%). Results of O and F fractions as a function of exit angle b for 22.5 and 21.5 keV incident energies are shown in Fig. 7. As expected, we also find that the fraction Fe presents a bell shape which is the same as that presented in our previous work [34]. Fe first increases with increasing exit angle and decreases as exit angle increases. It is obvious that the maximum value of F is around 19 , corresponding to specular scattering (19 /19 ). A much low C beam current prevents us to measure its angle dependence. The data of the angle dependence corresponding to 22.5 keV O ions scattering on water covered Si(111) are from Ref. [35]. 5. Discussion
27
from the inelastic binary collisions with the surface atoms. The positive-ion fraction measured is determined by the production probability and subsequent neutralization process. We also display the fitting positive-ion fractions in Fig. 4, which are well described by Eq. (1) with P þ ðz0 Þ ¼ 0:154 and vc ¼ 0:0819 (~1.8 105 m/s) for Cþ ions, P þ ðz0 Þ ¼ 0:0656 and vc ¼ 0:0537 (~1.2 105 m/s) for Oþ ions, and P þ ðz0 Þ ¼ 0:0536 and vc ¼ 0:0668 (~1.5 105 m/s) for Fþ ions, respectively. The probability of 0.154 for carbon ions is larger than that for oxygen and fluorine ions. It indicates that it is easier to produce Cþ ions and agrees with the fact that carbon has a lower first ionization energy (11.26 eV) as compared to oxygen and fluorine (13.62 and 17.42 eV) [51]. The exit angle dependence of Oþ and Fþ fraction (shown in Fig. 5) can also be well described by Eq. (1) with P þ ðz0 Þ ¼ 0:0373 and vc ¼ 0:0367 (~0.8 105 m/s) for Oþ ions, P þ ðz0 Þ ¼ 0:0276 and vc ¼ 0:033 (~0.07 105 m/s) for Fþ ions. The production probability of 0.0373 for oxygen ions is larger than that for fluorine ions. It also indicates that it is easier to produce Oþ ions. In general, the positive-ion fraction depends exponentially upon 1 ), the larger the velocity, the higher the positive-ion fraction. (v⊥;out The formation probability of positive ions and survival probability are both large for high perpendicular exit velocities. For a given incident energy, the ions can penetrate into deeper layers of the surface at small exit angles, where neutralization is enhanced. The Auger and/or resonance neutralization are efficient as the projectile spends more time near the surface. Therefore the positive-ion fraction decreases with the decrease of exit angle. It is noted that, the ionization energy levels of positive ions are upward shifted with the help of image potential effects, thus the initially formed positive ions may be neutralized by picking up electrons from the residual OH and H on the surface [52] (see Section 5.2). In addition, there is a difference in the fitting values of the az0 characteristic velocity vc ¼ t0 ea between the velocity and angle dependences, which indicates that z0 may be different for two cases. The appropriate treatment of the starting point of the trajectory integration has been unavailable at present. In Refs. [14,53], Auger neutralization and resonant neutralization occur efficiently at typical distances of 3e4 a.u. Thus, we use z0 ¼ 3.0 a.u. in the calculations, which is consistent with others [20,54].
5.1. Positive-ion fraction 5.2. Negative-ion fraction As described in Section 3, the observed positive ions must come
Fig. 7. (Color online) Negative-ion fractions as a function of exit angle with respect to the surface plane, measured for 21.5 keV F and 22.5 keV O [35] scattering on water covered Si(111) surface. The black solid, blue dashed curves are the calculated results including the contribution of positive ions.
To understand well the formation of negative ions on Si(111), the energy band of H2O adsorbed Si(111) is shown in Fig. 8, as well as the energy position of C, O and F affinity levels (located at 1.269 eV, 1.462 eV and 3.399 eV with respect to the vacuum level) [51]. As mentioned in Section 2, the surface in our case is mainly covered with water vapor. Water molecules are adsorbed on Si(111) in the dissociation forms of OH and H, which can saturate all the dangling bonds on the surface. The localized dangling-bond surface states in the gap region can provide a non-resonant electron transfer channel [29,30], but they completely disappear. For water covered surface, the calculated local density of state (LDOS) as OH and H adsorbed together on Si(111) surface displays five distinct features at 33.8, 18.5, 14.8, 12.1 and 9.9 eV with respect to the vacuum level [52,55]. It is found that the upper valance band region from 9 up to 5 eV corresponding to Si-self states is not affected by water adsorption [52,56], and the conduction-band states are emptied after water dissociation adsorption [55]. Compared with clean Si(111), the surface work function decreased after water adsorption [57]. Ranke's group has obtained a small decrease of the work function of about 0.1eV for Si(111) after small dose of water, and about 0.2 eV for large dose [57]. In comparison, hydrogen adsorption can also reduce the work function. For the saturated hydrogen covered Si(111) surface, its
28
F. Xiong et al. / Vacuum 137 (2017) 23e30
the bottom of the band gap to interact with the valence band states. Thus, the band gap effect disappears. As a consequence, we can qualitatively explain the velocity dependence of the negative-ion fractions, as shown in Fig. 6. For a high incident energy (corresponding to high perpendicular exit velocity), the nonadiabatic condition ensures that the affinity level easily arrives at the bottom of the narrow band gap at short distances. The affinity level then overlaps the valance band states with relatively higher DOS and is in resonance with the valance band to form a negative ion, thus the formation probability is higher. In addition, the outgoing negative ion spends a shorter duration time near the surface, and it leads also to a greater survival probability for negative ions. As discussed above, we use Eqs. (3) and (4) to reproduce the negative-ion fractions. In Eqs. (3) and (4) the width of the negativeion states [20,61] is, Fig. 8. (Color online) Schematic of the band structure of H2O-Si(111) (7 7) [52,56] as well as the energy positions of the C, O and F affinity levels in front of the surface. As a representation, the solid line represents the image shift of the O affinity level as the ion-surface distance decreases. For adiabatically slow collisions, an avoided crossing structure of energy level appears as the ion-surface distance decreases due to the band gap effect. The first red dashed curve represents the higher lying resonance, and the second red dashed curve represents the lower lying resonance. However, for fast collisions, the effects of the narrow band gap disappear and the dominant charge transfer is nonadiabatic (see text).
work function is around 4.35e4.54 eV [26]. To better explain the experimental results, we take the value of about 4.48 eV in the calculation. As the distance between the projectile and the surface decreases, the affinity levels of negative ions shift down due to the image potential effects, and the outermost electron of the projectile can transfer to the conduction band states of the surface through a resonance ionization process. In the absence of dangling-bond surface states, electron available for negative ion formation must come from the valance band of the surface. For adiabatically slow collisions, the existence of the narrow band gap (~1.1 eV) might block electron transfer, and the formation of negative ions could be inefficient. As shown in Fig. 8, the affinity level of O ions is found to follow the image charge variation in the large distance region, while in the small distance region the situation is very complex, an avoided energy level crossing structure appears [24] and follows weaker interactions with the valance band states which can be described by an analogy related to non-resonant charge transfer in ion-atom collisions in the gas phase. However, the band gap effect on negative-ion formation is not observed in our previous work related to fast C and F ions scattering on water covered Si(100) surfaces with two different doping concentrations [34]. This situation is very similar to electron transfer on metal surfaces in the presence of the L band gap [58]. The collision is so fast that the electron does not have enough time to probe the existence of the band gap. Indeed, the band gap effect on charge transfer is to a large extent erased in the scattering of Liþ and fast hydrogen on Ag [7,59]. Moreover, Esaulov et al. did not observe the significant band gap effect for F scattering on Ag(111) [7]. Winter et al. also did not find the significant band gap effect for negative ions (i.e., O, S, F, and Cl) and Cu(111) collisions [60]. Thus, in our case, the band gap effect can be erased for C, O and F collision with water covered Si(111) surface. The affinity level of oxygen (1.462 eV) is located between carbon (1.269 eV) and fluorine (3.399 eV), as shown in Fig. 8. As the ion-surface distance decreases, the affinity level is downward shifted following the image charge variation. It is below the Fermi level at a critical value of 2.1 a. u. (2.3, 6.3 a. u.) for C (O, F). In our case, i.e., some tens of keV incident energies, the affinity level of negative ions lies quite close to the Fermi level and can easily arrive
Gjmj ¼ △jmj exp ajmj ðz þ 0:85Þ
(5)
where z is measured from the image plane. Multielectron and orientation effects have been found for negative-ion states (C(4S), O(2P), and F(1S)) [62,63]. However, the transition rate for m ¼ 0 state is much more efficient than m ¼ ±1 states [62,63]. In Table 1, the parameters used to describe the negative-ion states of C, O, and F are listed, which are all cited from the literature [61,62]. In the calculation, to get the best fit, the value of a0 is slightly different from Ref. [61]. We use g ¼ 0.017 (0.7 1015/s) for carbon and oxygen. For fluorine, g ¼ 0.028 (1.15 1015/s), is larger than that of carbon and oxygen. In fact, the value of g is in a reasonable range as mentioned in Ref. [64]. The calculation results are well consistent with the experimental data, as shown in Figs. 6 and 7. It indicates that the incident velocity and initially formed positiveions play an important role in negative-ion formation. The monotonic increase of the negative-ion fraction in Fig. 6 can " !# be
explained
by
½1 P þ ðz0 Þ exp
gz0 v⊥;in
1 þ v⊥;out
.
For
a ¼ b ¼ 19 , the exponential item increases with increasing velocity. In general, the bell-shaped negative-ion fractions in Fig. 7 can be qualitatively explained as follows: on the one hand, the perpendicular exit velocity is low for small exit angles (<19 ), the initially formed negative ions will spend a long duration time near the surface. The efficient neutralization occurs and the final negativeion fraction is mainly determined by the decay probability associ 1 ated with exp v⊥;out where the smaller exit angles correspond to smaller negative-ion fractions. On the other hand, the perpendicular exit velocity is high for large exit angles (>19 ), and there is only a short time for the exit neutrals to pick up electrons from the surface to form a negative ion. The final negative-ion fraction is mainly associated with the formation probability of ! ½1 P þ ðz0 Þ exp
z0 vg⊥;in
where the larger exit angles (smaller
incident angles) correspond to smaller negative-ion fractions. Moreover, it indicates that the dynamical equilibrium is never achieved for high perpendicular exit velocities.
Table 1 Parameters describing the distance dependence of the negative-ion states (Eq. (5)), and the electron transition rate g. D is given in eV and a is in a.u.1. Atom 4
C ( S) O(2P) F(1S)
D0
a0
D1
a1
g (angle)
g (velocity)
5.11 4.60 12.0
0.48 0.70 0.51
0.50 1.30 2.9
0.49 0.71 1.05
/ 0.017 0.028
0.017 0.017 0.028
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5.3. Comparison with other data In Fig. 7, the negative-ion fraction presents a bell shape as a function of exit angle. The fraction increases when the exit angles are smaller than 19 , which is consistent with the previous results [6,17,21,29e31]. As discussed in Section 5.2, the increased survival probability of negative ions can be qualitatively attributed to the higher perpendicular exit velocities of negative ions moving away from the surface. However, the decreased fractions at large exit angles (>19 ) do not agree with the previous results where the fractions continue to increase. We emphasize here that, the nonmonotonic curve of negative-ion fraction as a function of exit angle is in contradiction with the conventional understanding. The conventional understanding is that the increasing of the perpendicular exit velocity corresponding to the decreasing of the dwelling time the particles spend near the surface, which is in favor for the survival of negative ions and would cause a higher negative-ion fraction for large exit angles. As a result, the bell-shaped negative-ion fractions of oxygen and fluorine have been well described by the calculations using the modified resonant charge transfer model, as shown in Fig. 7. The bell shape of negative-ion fraction has been observed for 3e4 keV F atoms scattering on Ag [22], for 22.5 keV O scattering on water covered Si(111) [35] and for 22.5 keV F scattering on water covered Si(100) [33,34], as well as 22.5 keV F scattering on a highly oriented pyrolytic graphite (HOPG) surface [36]. Under the same experiment conditions, our F fraction results on water covered Si(111) are much larger than that on HOPG, which can be attributed to the different DOS in the top valance band region. Considering the similarity of the surface work function between HOPG (about 4.6 eV) and water covered Si(111), we think that the influence of the work function on negative-ion formation is not obvious. It is interesting that the band structures of HOPG [65] shows equivalently a band gap of ~1 eV, i.e., the DOS near the Fermi level is very low. As discussed before, the band-gap effects on charge transfer can be neglected in our energy region. Therefore, the initial negative-ion yield is strongly dominated by the surface DOS. The DOS of HOPG is less than 0.1 states/eV within the 2-eV range below the Fermi level [66]. In contrast, for the unreconstructed Si(111) surface, the total DOS is as high as 5 states/eV below the Fermi level [67] in the top valance band region. Therefore, the initial negative-ion formation probability is higher on Si(111) than that on HOPG. In addition, the positive-ion fraction on HOPG (about 10%) is larger than that on Si(111), it may indicate that the neutralization of Fþ on HOPG is less efficient than that on Si(111). This can be also easily understood in terms of the lower DOS of the valence band of HOPG. The repulsive potential of carbon nuclei is weaker than that of silicon nuclei, thus the incident fluorine ions can approach much closer to HOPG than Si(111), and the excitation and/or ionization processes for the projectiles are enhanced at a close distance. Hence, there are not enough neutrals left to form negative ions. At the same energies, the maximum value of F fraction on water covered Si(111) (~28%) is slightly smaller than that on water covered Si(100) (34%) reported in Refs. [33,34]. The similar results have also been observed for negative carbon and oxygen ions. In Fig. 9, the C, O, and F fractions of water covered Si(111) are smaller than that of water covered Si(100). The difference of negative-ion fraction between Si(111) and Si(100) can also be ascribed to the different DOS below the Fermi level. As we know, there are many similarities for the calculated DOS of clean Si(111) and Si(100) surfaces [67]. For Liþ scattering on clean Si, similar neutral fractions were observed for different crystal orientation [26]. It indicates that the localized dangling-bond surface states of Si(111) and Si(100) have the same effect on charge
Fig. 9. Negative-ion fractions as a function of incident energy, measured for fluorine, oxygen and carbon negative ions in specular scattering on water covered Si(111) and Si(100) [34] surfaces.
transfer. Zeng's group have calculated the DOS of clean Si(100) and Si(111), and found that the total DOS of Si(111) is slightly smaller than that of Si(100), particularly in the range below the Fermi level [67]. In addition, the energy level diagram of clean Si(111) shows two dominant surface states located at 0.2 eV and 0.8 eV below the Fermi level, while there is only one surface state located at 0.8 eV below the Fermi level for clean Si(100) [68,69]. For water covered surfaces, the localized dangling-bond surface states completely disappear for Si(111) surface, while they have not disappeared completely for Si(100) surface [40,70,71]. On water covered Si(100) surface, isolated dangling bonds have been observed and are about one percent of Si atoms deduced from scanning tunneling microscopy (STM) images [70,71]. In a word, the DOS in the top valence band region of water covered Si(111) may be still smaller than that of water covered Si(100). As a consequence, the C, O, and F fractions from water covered Si(111) are smaller than that from water covered Si(100), as shown in Fig. 9, and the bell-shaped curve for Si(111) is lower than that for Si(100) [34] as shown in Fig. 7. In Fig. 9, for C, O, and F scattering on Si(111), F fraction is the largest and the C fraction is the smallest in the energy range investigated. It can be qualitatively attributed to the smallest affinity level of C that is located at 1.269 eV with respect to the vacuum level and the largest affinity level of F that is located at 3.399 eV [51]. 6. Conclusions We presented new experimental results of negative carbon, oxygen, fluorine ions scattering on water covered Si(111) surface. These results are very similar to that on water covered Si(100) in our previous work. Based on the MO promotion model, the positive-ion production was associated with electronic excitation, ionization, and Auger and/or resonance neutralization. In particular, for oxygen and fluorine, the bell-shaped negative-ion fractions as a function of exit angle cannot be completely understood using the conventional rate equation that is suitable to the case of adiabatic slow collisions. In our case, for fast collisions, the affinity level of negative ion (C, O and F) lies quite close to the Fermi level. With the help of the image potential effect, it can easily arrive to the bottom of the band gap and interact with the valence band states, which strongly indicates that the band gap effect on charge transfer can be neglected to a large extent in this relatively high energy range.
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As a consequence, we have adopted a complete theoretical framework to fit the final positive-ion fractions, and found that the first ionization energy of the projectile and the perpendicular exit velocity play important roles in positive-ion production. Then we use the modified resonant charge transfer model to calculate negative-ion fractions. A good agreement between experiment and calculation has been obtained, which further confirms the nonadiabatic electron transfer processes. Under the same scattering configurations, we compare the experimental results for different collision systems. We find that the negative-ion fractions for H2O-Si(111) are slightly smaller than that for H2O-Si(100), which indicates that the electron affinities of the projectiles and surface's DOS play crucial roles in negative ion production. In summary, the relevant studies on semiconductor surfaces are still inadequate and much effort should be devoted to this subject. This experimental study has improved our understanding of charge transfer at semiconductor surfaces for multielectron projectiles at relatively high energies. The dynamical resonant charge transfer may be important to specific surface reactions, molecular devices and electron emission at semiconductors. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No.11405078, No.11474140, No.11104124, and No.11505087). References [1] P. Meredith, C.J. Bettinger, M. Irimia-vladu, A.B. Mostert, P.E. Schwenn, Rep. Prog. Phys. 76 (2013) 034501. [2] L. Chen, J. Shen, J.J. Jia, T. Kandasamy, K. Bobrov, L. Guillemot, J.D. Fuhr, M.L. Martiarena, V.A. Esaulov, Phys. Rev. A 84 (2011) 052901. [3] L. Chen, Y.L. Guo, J.J. Jia, H.Q. Zhang, Y. Cui, J.X. Shao, Y.Z. Yin, X.Y. Qiu, X.Y. Lv, G.Z. Sun, J. Wang, Y.F. Chen, F.Y. Xi, X.M. Chen, Phys. Rev. A 84 (2011) 032901. [4] H. Zhou, L. Chen, D. Feng, Y.L. Guo, M.C. Ji, G.Y. Wang, W. Zhou, Y. Li, L. Zhao, X.M. Chen, Phys. Rev. A 85 (2012) 014901. [5] L. Chen, X.Y. Lv, J.J. Jia, M.C. Ji, P. Zhou, G.Z. Sun, J. Wang, Y.F. Chen, F.Y. Xi, Y. Cui, J.X. Shao, X.Y. Qiu, Y.L. Guo, X.M. Chen, J. Phys B: At. Mol. Opt. Phys. 44 (2011) 045203. [6] M. Maazouz, A.G. Borisov, V.A. Esaulov, J.P. Gauyacq, L. Guillemot, S. Lacombe, D. Tellet-Billy, Phys. Rev. B 55 (1997) 13869. [7] L. Guillemot, V.A. Esaulov, Phys. Rev. Lett. 82 (1999) 4552. [8] A.G. Borisov, A.K. Kazansky, J.P. Gauyacq, Phys. Rev. B 59 (1999) 10935. [9] H. Chakraborty, T. Niederhausen, U. Thumm, Phys. Rev. A 70 (2004) 052903. [10] B. Obreshkov, U. Thumm, Phys. Rev. A 87 (2013) 022903. [11] A.G. Borisov, A. Mertens, S. Wethekam, H. Winter, Phys. Rev. A 68 (2003) 012901. [12] T. Kravchuk, V.A. Esaulov, A. Hoffman, R.C. Monreal, Nucl. Instrum. Methords Phys. Res. B 232 (2005) 27. n, E.C. Goldberg, [13] C. Meyer, F. Bonetto, R. Vidal, E.A. García, C. Gonzalez, J. Ferro Phys. Rev. A 86 (2012) 032901. [14] H. Winter, Phys. Rep. 367 (2002) 387. [15] A.G. Borisov, V.A. Easulov, J. Phys. Condens. Matter 12 (2000) R177. [16] F. Wyputta, R. Zimny, H. Winter, Nucl. Instrum. Methords Phys. Res. B 58 (1991) 379. [17] M. Maazouz, R. Baragiola, A. Borisov, V.A. Esaulov, S. Lacombe, J.P. Gauyacq, L. Guillemot, D. Teillet-Billy, Surf. Sci. 364 (1996) L568. [18] A.G. Borisov, D. Teillet-Billy, J.P. Gauyacq, Phys. Rev. Lett. 68 (1992) 2842. [19] N. Lorente, A.G. Borisov, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 429 (1999) 46. [20] S. Ustaze, L. Guillemot, V.A. Esaulov, P. Nordlander, D.C. Langreth, Surf. Sci. 415 (1998) L1027. [21] M. Maazouz, L. Guillemot, T. Schlatholter, S. Ustaze, V.A. Esaulov, Nucl.
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Dynamical resonant charge transfer of fast C, O, F ions and water covered Si(111) surface Feifei Xiong, Lei Gao, Yuefeng Liu, Jianjie Lu, Pinyang Liu, Shunli Qiu, Xiyu Qiu, Yanling Guo, Ximeng Chen, Lin Chen* School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 12 November 2016 Accepted 6 December 2016 Available online 9 December 2016
The experiment of 6.5e22.5 keV C, O and F ions scattering on water covered Si(111) surface has been performed. It is found that the positive-ion fraction is very low and increases monotonically as a function of perpendicular exit velocity and exit angle. In particular, the negative-ion fraction increases monotonically with perpendicular exit velocity for specular scattering, and for a given incident energy the angle dependence of the fraction is nonmonotonic. We interpret the observed positive ions in terms of inelastic binary collision, and adopt a modified resonant charge transfer model to calculate the bellshaped negative-ion fraction. We find that the neutral yield at short ion-surface distance is nonzero and obeys well an exponential dependence. It strongly indicates that a dynamical equilibrium for negative ion formation is never achieved at short distances, and the band gap effect on charge transfer can be neglected to a large extent in this relatively high energy region. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Charge transfer on solid surfaces plays a crucial role not only from a fundamental physical interest, but also for practical applications, i.e., secondary ion mass spectroscopy and low energy ion scattering (LEIS). Semiconductor, is one of the indispensable materials for modern science and technology [1]. The fascinating physical and chemical properties of semiconductor surfaces are mainly dominated by their electronic structures. Charge transfer during ion scattering on semiconductor surface could also be affected by the surface electronic structure properties. The most direct way to probe charge transfer process is the ionsurface scattering technique [2e5]. In the scattering of keV-energy atoms or ions on metal surfaces, resonant charge transfer (RCT) is the dominant process, which involves one electron transition between an atomic level and a surface [6e13]. For negative-ion formation, the affinity level of the projectile is downward shifted due to the image potential effects [14], and is below the Fermi level of the metal surface at short distances, then a tunneling transition of an electron from occupied levels of the valence band to the anion level of the projectile occurs and forms a negative ion. Departing
* Corresponding author. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.vacuum.2016.12.008 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
from the surface, the initially formed negative ion quickly decays via resonant ionization and becomes an atom when its affinity level lies above the Fermi level again. Thus the final negative-ion fraction depends on the width of the affinity level, the surface work function, and the interaction time the particle spends near the surface where RCT processes occur [15]. Most research on negative-ion formation have been confined to the interactions between atoms/ions and metallic surfaces [6e13,16e24]. In comparison, there are only few studies on semiconductor surfaces [25e32]. The formation of H is particularly simple for its s character state, which has been studied experimentally and theoretically on metallic surfaces [6e10,16e18] as well as silicon surfaces [29e32]. In contrast, the formation of F is significantly more complex than H for its multielectron aspect [19,20]. Although electronegative ions like O, F and Cl, play significant roles in etching and growth studies for semiconductor manufacture, the study of these ions scattering on silicon is scarce. From the point of view of materials, clean Si has special electronic structure, i.e., the existence of a narrow band gap (~1.1 eV) and localized dangling-bond surface states in the gap region. These features directly affect charge transfer process on the surface. H and O fractions are of the same magnitude as on an Al surface reported by Esaulov et al. in the scattering of hydrogen and oxygen ions with clean Si(111), which was explained by the nonresonant charge transfer involving localized dangling-bond surface states
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[29,30]. On the other hand, the narrow band gap is located at an energy position close to the bulk valence band region, about 5 eV below the vacuum level. The neutralization of Liþ ion scattering on clean and hydrogen covered Si(111) has clearly demonstrated the significant band gap effect [26] where hydrogen adsorbates removed the dangling-bond surface states. The above mentioned studies of positive ions scattering on silicon surface mainly concentrate in the low energy region (0.5e4 keV), while related investigations for some tens of keV energies are scarce. The situation may be different for high incident energies because the dynamical charge equilibrium may not be achieved at short ion-surface distance [6]. Moreover, negative ions with low electron affinities are very suitable for studying charge transfer that can probe the electronic structure of the silicon surface. We have examined the doping and the band gap effects in the scattering of some tens of keV energies negative carbon and fluorine ions on water covered Si(100) surfaces [33,34], in which positive and negative-ion fractions were measured and the observed bell-shaped negative fluorine ion fraction was well interpreted by using a complete theoretical framework [34]. However, we do not know whether these experimental phenomena can be observed for water covered Si(111) and what is the difference of the results between Si(111) and Si(100) surfaces. We also do not know whether this modified theoretical model is fully applicable to other collision systems. To answer these questions, we have performed the measurements of C, O and F scattering on water covered Si(111) surface at the same experimental conditions as our previous work. In this work, we measured the positive-ion and negative-ion fractions for 6.5e22.5 keV C, O and F ions scattering on water covered Si(111). Indeed, we find similar variation of the fractions as our previous results [33e36]: (1) the positive-ion fraction is small and increases with perpendicular exit velocity and exit angle; (2) the negative-ion fraction also increases monotonically with perpendicular exit velocity for specular scattering; (3) the variation of negative-ion fractions with exit angle is nonmonotonic and presents a bell shape. We find that the fractions of Si(111) are all smaller than that of Si(100) for a given projectile. The difference of the data between Si(111) and Si(100) surfaces has been well discussed. For positive ion production, it is interpreted by the molecular orbital (MO) promotion model. For negative ion formation, it is found that the modified resonant charge transfer model we built [34] can also reproduce the experimental data of Si(111). It is further confirmed that, for the Si(111) surface, a dynamical charge equilibrium is never achieved at short distances, and the band gap effect is inefficient in electron transfer process in this relatively high energy range.
Fig. 1. Schematic diagram of the experimental setup.
passed through two separated slits and a parallel-plate electrostatic deflector, and were simultaneously counted by a one-dimensional position sensitive microchannel plate (PSMCP) detector. We appropriately choose the biased voltage of the deflector to ensure that all scattered particles are well collected by the detector. The detector consists of two MCPs mounted in a chevron configuration placed above a resistive anode. The detector efficiency for scattered particles with different charge states was assumed to be identical for the same energies [37e39]. In situ preparation of the surface was performed by many cycles of small angle sputtering with 3 keV Arþ ions and subsequent annealing at about 773 K (15 min) using electron bombardment to prevent sputtering damage. Our setup is not yet equipped with a low-energy-electron-diffraction (LEED) system that would allow us to check the state of the surface. We check the surface cleanliness by measuring the time-of-flight scattering and recoiling spectroscopy (TOF-SARS) with a scattering angle of 20 and an incident angle of 5 . The scattered and recoil particles are recorded by a MCP detector. The measured TOF spectra are shown in Fig. 2. It is observed that the recoiled intensities of adsorbates such as H and O disappear after many cycles of preparation. However, a small amount of residual impurities will be observed again in the TOF spectra during our measurements. This may be mainly caused by the chemisorption of H2O molecules from background gases in our chamber.
2. Experiment The schematic diagram of the experimental setup is shown in Fig. 1. The details have been described elsewhere [33e36]. Briefly, the C, O and F ions with energies from 6.5 up to 22.5 keV were produced in a cesium-sputtered ion source. After analyzing the mass to charge ratio by a 45 bending magnet, the extracted ion beam was collimated by two slits. Then a pair of electrostatic plates placed between the two slits were used to separate the negative component from neutrals and steer the ion beam to pass through the third collimators. These collimators guaranteed the angular divergence of the incident ion beam less than 0.28 . A lightly doped p-type Si(111) wafer with a resistivity of 1.0 Ucm was used as a target mounted on a precision manipulator in a ultrahigh-vacuum (UHV) chamber, with a typical pressure of better than 3.0 107 Pa. The scattering angle was fixed at 38 and the incident angle a was varied from 4 to 31 measured with respect to the surface plane. The reflected particles from the surface
Fig. 2. Time of flight (TOF) spectra of 3 keV Arþ scattering on a Si(111) surface with a scattering angle of 20 and an incident angle of 5 . Ar(S) represents the scattered Ar particles. Si(DR), O(DR) and H(DR) represent the recoil Si, O, and H particles from the surface, respectively.
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Our previous work has demonstrated that the ratio between the measured recoil intensities of H and O is about 2 as the surface was kept under UHV conditions [35], and the amount of H2 adsorption from the residual gases in the chamber is much smaller than that of water vapor [33]. From the TOF spectra, we have found that the relative intensity of hydrogen (oxygen) rapidly increased to a saturated value within an hour [35]. Water molecules adsorbed on Si(111) in dissociation forms at room temperature, and the saturation of dangling-bond surface states for H and OH needs only 0.2 monolayer of water [40]. In view of the vacuum condition of 3.0 107 Pa, the surface can be completely covered with water vapor in a few minutes (1 L ¼ 106 Torr s). As mentioned in the introduction, this completely passivated surface is well suited to delineate the narrow band gap effect of Si(111). We used low beam current during the measurements to prevent damage of the water passivated surface. The negative-ion (positive-ion) fraction is defined as Fe/ þ ¼ N(Ae/þ)/N(Total), where N(Total) is the total number of scattered particles neglecting the recoil particles. It is noted that, the PSMCP detector can collect all energetic scattered and recoil particles. The recoil particles are almost atoms and would reduce the ion fractions that has been discussed in our previous studies [34,35]. In the present work, we cannot do the TOF analysis to evaluate the influence of recoil particles because of the absence of the pulsed beam for some tens of keV-energy negative ions. But we did the simulation for 21.5 keV F in specular scattering on clean and water covered Si(111) surfaces. The simulation was performed using the Kalypso software package [41]. The Si(111) target used in this work consists four atomic layers, and it was very long in the direction of ion path to allow continuous collisions. For water covered Si(111), the top layer atoms of Si(111) were approximately replaced with H and O with the ratio of 2:1. The interaction potential of F-Si (F-H and F-O) was represented by uncorrected Ziegler-Biersack-Littmark (ZBL) universal screening function potential [42]. The reflected particles were collected with an altitudinal angle of 19 ± 2 , and with a plane angle width of ± 2 . The simulation used approximately 1.2 105 projectiles for both clean and water covered Si(111) surfaces. In Fig. 3, two main peaks located at 16 keV and 18.5 keV correspond to the single and double scattering of F on Si. A small peak located at 17 keV corresponds to the recoil Si. In the
25
simulation, the recoil H and O particles from the water covered Si(111) surface were not observed. The main component of the reflected flux should be the scattering F, and its fraction is about 95.5% and 90.8% for clean and water covered surfaces, respectively. Thus the simulated results excluding the recoil particles could be reduced by a factor of about 1.1, which is consistent with that reported by Esaulov et al. where their corrected results are reduced by a factor of 1e1.3 [30]. 3. Charge transfer model Scattering of electronegative atoms/ions (i.e., O, F, Cl) off solid surfaces is generally accomplished by highly efficient electron capture and loss processes, and thus the scattered particle flux is largely composed of anions and neutrals [22,43e45]. However, in our case positive ions have been observed. Positive-ion production is a local process, whereas negative-ion formation is governed by a nonlocal process as the projectile approaches or departs from the surface. For negative ions scattering on Si, efficient neutralization occurs in the incoming path and negative projectiles first become energetic atoms prior to form positive ions. The observed positive ions must be produced in inelastic binary collisions with surface Si or O atoms at short distances where the electron excitation and direct ionization processes occur. To produce positive ions, direct single ionization process takes place via Coulomb repulsion that is well described by the binary encounter approximation [46e48]. For the electron excitation process, we briefly point out that the possible electron excitation for C (O, F) Si collision system is caused by 4fs MO promotion, and for C (O, F) - O is caused by 4fs or 3ds MO promotion which is associated with the C (O, F) 2p level [49]. The final positive-ion fraction is determined by the production probability and subsequent neutralization process, and the final survival probability of positive ions can be expressed by,
P þ ð∞Þ ¼ P þ ðz0 Þ exp
vc
(1)
v⊥:out
where P þ ðz0 Þ represents the initial positive-ion production probaaz0 bility at z0, vc ¼ t0 ea is the characteristic velocity proportional to the neutralization rate, and v⊥,out is the perpendicular exit velocity of the scattered positive ions. Here, z0 is the starting point of the effective position of the outgoing trajectory where efficient neutralization occurs. The more details of positive ion production can be found in our previous work [33,34]. For negative-ion formation, the conventional understanding assumes that the memory effect of the initial charge state of the projectile is completely lost [50]. In the scattering of negative ions on metal-like surfaces, it is reasonable to describe RCT processes only along the outgoing trajectory. The expression for negative-ion 0 1 Z ∞ 1 @ fraction pout ¼ exp v⊥out GðzÞdzA is in good agreement with z0
a lot of monotonic experimental results [6,18,19]. G(z) is the total transition probability. However, this expression cannot give a full description for the nonmonotonic results in our case. In this respect, a nonzero neutral fraction is assumed at the starting point of the trajectory integration [33,34], can be expressed by,
" i h p0 v⊥;in ¼ 1 pþ ðz0 Þ 1 exp Fig. 3. The simulated energy spectra of reflected particles for 21.5 keV fluorine atoms in specular scattering on clean and water covered Si(111) surfaces.
gz0 v⊥;in
!# (2)
where g is a constant corresponding to electron transition rate, and v⊥;in is the incident velocity normal to the surface.
26
F. Xiong et al. / Vacuum 137 (2017) 23e30
After considering the positive-ion production, the calculation is done only along the outgoing trajectory. The calculation is performed by using two rate equations,
dn⊥ ðzÞ 1 ¼ dz v⊥;out
function of perpendicular exit velocity and exit angle. In Fig. 4, positive-ion fractions are measured for 6.5e22.5 keV C, O and F in specular (19 /19 ) scattering on water covered Si(111) surface.
h i G⊥ ðzÞf n⊥ ðzÞ þ N⊥ G⊥ ðzÞð1 f Þ 1 P þ ðzÞ n⊥ ðzÞ n⁄ ⁄ ðzÞ
(3)
h i G⁄ ⁄ ðzÞf n⁄ ⁄ ðzÞ þ N⁄ ⁄ G⁄ ⁄ ðzÞð1 f Þ 1 P þ ðzÞ n⊥ ðzÞ n⁄ ⁄ ðzÞ
(4)
þt0 eaz P þ ðzÞ
and
dn⁄ ⁄ ðzÞ 1 ¼ dz v⊥;out
þt0 eaz P þ ðzÞ
For these two equations, the first terms on the right side correspond to negative-ion formation, and second and third terms represent the production of neutrals. The parameter f is the FermieDirac distribution function [14], n⊥ ðzÞ and n⁄ ⁄ ðzÞ is the total neutral population in the m ¼ 0 and m ¼ ±1 states, respectively. The m ¼ 0 state is oriented normal and the m ¼ ±1 states parallel to the surface. The degeneracies are N⊥ ¼ 2 and N⁄ ⁄ ¼ 4. Finally, we obtain the negative-ion fraction ½1 P þ ð∞Þ n⊥ ð∞Þ n⁄ ⁄ ð∞Þ. More details of negative-ion formation have been discussed in our previous work [33,34]. 4. Results We measured the perpendicular exit velocity and exit angle dependences of fractions in the collisions of C, O and F ions with water covered Si(111) surface. The perpendicular exit velocity is calculated with the binary collision approximation from the interaction energy of the C (O, F)-Si collision partner. Results of positive-ion fraction are shown in Figs. 4e5 as a
Fig. 4. (Color online) Positive-ion fractions as a function of perpendicular exit velocity, measured for 6.5e22.5 keV C, O and F in specular scattering on water covered Si(111) surface. The black solid, red dashed and blue dash dotted curves are the fitting results.
For oxygen and fluorine, the positive-ion fraction Fþ is much smaller than F, while for carbon, the fraction Fþ is very close to F and larger than that of oxygen and fluorine. The fraction Fþ increases with perpendicular exit velocity which is similar to that measured for C and F in specular scattering on water covered Si(100) surface in our previous work [34]. In Fig. 5, the fraction Fþ for oxygen and fluorine increases as a function of exit angle b, and this trend is consistent with the previous results of 1e4 keV oxygen ions scattering on Mg, Al, and Ag surfaces [21], of 1e4 keV fluorine ions scattering on Mg [22] and 4 keV oxygen ions scattering on Si(111) [30], as well as the results of C and F in specular scattering on water covered Si(100) surface in our previous work [34]. The fraction Fþ (maximum value of 2.55% and 1.98% for oxygen and fluorine) is much smaller than Fe. In Fig. 6, we plot the negative-ion fractions measured for C, O and F ions scattering on water covered Si(111) as a function of perpendicular exit velocity, corresponding to incident energies from 6.5 to 22.5 keV for specular scattering (19 /19 ). The negativeion fraction Fe increases monotonically with perpendicular exit velocity, which is similar to that measured for C and F in specular scattering on water covered Si(100) surface in our previous work
Fig. 5. (Color online) Positive-ion fractions as a function of exit angle with respect to the surface plane, measured for 22.5 keV O [35] and 21.5 keV F scattering on water covered Si(111) surface. The black solid and blue dashed curves are the fitting results.
F. Xiong et al. / Vacuum 137 (2017) 23e30
Fig. 6. (Color online) Negative-ion fractions as a function of perpendicular exit velocity, measured for 6.5e22.5 keV C, O and F in specular scattering on water covered Si(111) surface. The black solid, red dashed and blue dash dotted curves are the calculated results including the contribution of positive ions.
[34]. The fraction Fe for carbon (0.59%e4.8%) is much smaller than that of oxygen (2.5%e24%) and fluorine (5.3%e27%). Results of O and F fractions as a function of exit angle b for 22.5 and 21.5 keV incident energies are shown in Fig. 7. As expected, we also find that the fraction Fe presents a bell shape which is the same as that presented in our previous work [34]. Fe first increases with increasing exit angle and decreases as exit angle increases. It is obvious that the maximum value of F is around 19 , corresponding to specular scattering (19 /19 ). A much low C beam current prevents us to measure its angle dependence. The data of the angle dependence corresponding to 22.5 keV O ions scattering on water covered Si(111) are from Ref. [35]. 5. Discussion
27
from the inelastic binary collisions with the surface atoms. The positive-ion fraction measured is determined by the production probability and subsequent neutralization process. We also display the fitting positive-ion fractions in Fig. 4, which are well described by Eq. (1) with P þ ðz0 Þ ¼ 0:154 and vc ¼ 0:0819 (~1.8 105 m/s) for Cþ ions, P þ ðz0 Þ ¼ 0:0656 and vc ¼ 0:0537 (~1.2 105 m/s) for Oþ ions, and P þ ðz0 Þ ¼ 0:0536 and vc ¼ 0:0668 (~1.5 105 m/s) for Fþ ions, respectively. The probability of 0.154 for carbon ions is larger than that for oxygen and fluorine ions. It indicates that it is easier to produce Cþ ions and agrees with the fact that carbon has a lower first ionization energy (11.26 eV) as compared to oxygen and fluorine (13.62 and 17.42 eV) [51]. The exit angle dependence of Oþ and Fþ fraction (shown in Fig. 5) can also be well described by Eq. (1) with P þ ðz0 Þ ¼ 0:0373 and vc ¼ 0:0367 (~0.8 105 m/s) for Oþ ions, P þ ðz0 Þ ¼ 0:0276 and vc ¼ 0:033 (~0.07 105 m/s) for Fþ ions. The production probability of 0.0373 for oxygen ions is larger than that for fluorine ions. It also indicates that it is easier to produce Oþ ions. In general, the positive-ion fraction depends exponentially upon 1 ), the larger the velocity, the higher the positive-ion fraction. (v⊥;out The formation probability of positive ions and survival probability are both large for high perpendicular exit velocities. For a given incident energy, the ions can penetrate into deeper layers of the surface at small exit angles, where neutralization is enhanced. The Auger and/or resonance neutralization are efficient as the projectile spends more time near the surface. Therefore the positive-ion fraction decreases with the decrease of exit angle. It is noted that, the ionization energy levels of positive ions are upward shifted with the help of image potential effects, thus the initially formed positive ions may be neutralized by picking up electrons from the residual OH and H on the surface [52] (see Section 5.2). In addition, there is a difference in the fitting values of the az0 characteristic velocity vc ¼ t0 ea between the velocity and angle dependences, which indicates that z0 may be different for two cases. The appropriate treatment of the starting point of the trajectory integration has been unavailable at present. In Refs. [14,53], Auger neutralization and resonant neutralization occur efficiently at typical distances of 3e4 a.u. Thus, we use z0 ¼ 3.0 a.u. in the calculations, which is consistent with others [20,54].
5.1. Positive-ion fraction 5.2. Negative-ion fraction As described in Section 3, the observed positive ions must come
Fig. 7. (Color online) Negative-ion fractions as a function of exit angle with respect to the surface plane, measured for 21.5 keV F and 22.5 keV O [35] scattering on water covered Si(111) surface. The black solid, blue dashed curves are the calculated results including the contribution of positive ions.
To understand well the formation of negative ions on Si(111), the energy band of H2O adsorbed Si(111) is shown in Fig. 8, as well as the energy position of C, O and F affinity levels (located at 1.269 eV, 1.462 eV and 3.399 eV with respect to the vacuum level) [51]. As mentioned in Section 2, the surface in our case is mainly covered with water vapor. Water molecules are adsorbed on Si(111) in the dissociation forms of OH and H, which can saturate all the dangling bonds on the surface. The localized dangling-bond surface states in the gap region can provide a non-resonant electron transfer channel [29,30], but they completely disappear. For water covered surface, the calculated local density of state (LDOS) as OH and H adsorbed together on Si(111) surface displays five distinct features at 33.8, 18.5, 14.8, 12.1 and 9.9 eV with respect to the vacuum level [52,55]. It is found that the upper valance band region from 9 up to 5 eV corresponding to Si-self states is not affected by water adsorption [52,56], and the conduction-band states are emptied after water dissociation adsorption [55]. Compared with clean Si(111), the surface work function decreased after water adsorption [57]. Ranke's group has obtained a small decrease of the work function of about 0.1eV for Si(111) after small dose of water, and about 0.2 eV for large dose [57]. In comparison, hydrogen adsorption can also reduce the work function. For the saturated hydrogen covered Si(111) surface, its
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F. Xiong et al. / Vacuum 137 (2017) 23e30
the bottom of the band gap to interact with the valence band states. Thus, the band gap effect disappears. As a consequence, we can qualitatively explain the velocity dependence of the negative-ion fractions, as shown in Fig. 6. For a high incident energy (corresponding to high perpendicular exit velocity), the nonadiabatic condition ensures that the affinity level easily arrives at the bottom of the narrow band gap at short distances. The affinity level then overlaps the valance band states with relatively higher DOS and is in resonance with the valance band to form a negative ion, thus the formation probability is higher. In addition, the outgoing negative ion spends a shorter duration time near the surface, and it leads also to a greater survival probability for negative ions. As discussed above, we use Eqs. (3) and (4) to reproduce the negative-ion fractions. In Eqs. (3) and (4) the width of the negativeion states [20,61] is, Fig. 8. (Color online) Schematic of the band structure of H2O-Si(111) (7 7) [52,56] as well as the energy positions of the C, O and F affinity levels in front of the surface. As a representation, the solid line represents the image shift of the O affinity level as the ion-surface distance decreases. For adiabatically slow collisions, an avoided crossing structure of energy level appears as the ion-surface distance decreases due to the band gap effect. The first red dashed curve represents the higher lying resonance, and the second red dashed curve represents the lower lying resonance. However, for fast collisions, the effects of the narrow band gap disappear and the dominant charge transfer is nonadiabatic (see text).
work function is around 4.35e4.54 eV [26]. To better explain the experimental results, we take the value of about 4.48 eV in the calculation. As the distance between the projectile and the surface decreases, the affinity levels of negative ions shift down due to the image potential effects, and the outermost electron of the projectile can transfer to the conduction band states of the surface through a resonance ionization process. In the absence of dangling-bond surface states, electron available for negative ion formation must come from the valance band of the surface. For adiabatically slow collisions, the existence of the narrow band gap (~1.1 eV) might block electron transfer, and the formation of negative ions could be inefficient. As shown in Fig. 8, the affinity level of O ions is found to follow the image charge variation in the large distance region, while in the small distance region the situation is very complex, an avoided energy level crossing structure appears [24] and follows weaker interactions with the valance band states which can be described by an analogy related to non-resonant charge transfer in ion-atom collisions in the gas phase. However, the band gap effect on negative-ion formation is not observed in our previous work related to fast C and F ions scattering on water covered Si(100) surfaces with two different doping concentrations [34]. This situation is very similar to electron transfer on metal surfaces in the presence of the L band gap [58]. The collision is so fast that the electron does not have enough time to probe the existence of the band gap. Indeed, the band gap effect on charge transfer is to a large extent erased in the scattering of Liþ and fast hydrogen on Ag [7,59]. Moreover, Esaulov et al. did not observe the significant band gap effect for F scattering on Ag(111) [7]. Winter et al. also did not find the significant band gap effect for negative ions (i.e., O, S, F, and Cl) and Cu(111) collisions [60]. Thus, in our case, the band gap effect can be erased for C, O and F collision with water covered Si(111) surface. The affinity level of oxygen (1.462 eV) is located between carbon (1.269 eV) and fluorine (3.399 eV), as shown in Fig. 8. As the ion-surface distance decreases, the affinity level is downward shifted following the image charge variation. It is below the Fermi level at a critical value of 2.1 a. u. (2.3, 6.3 a. u.) for C (O, F). In our case, i.e., some tens of keV incident energies, the affinity level of negative ions lies quite close to the Fermi level and can easily arrive
Gjmj ¼ △jmj exp ajmj ðz þ 0:85Þ
(5)
where z is measured from the image plane. Multielectron and orientation effects have been found for negative-ion states (C(4S), O(2P), and F(1S)) [62,63]. However, the transition rate for m ¼ 0 state is much more efficient than m ¼ ±1 states [62,63]. In Table 1, the parameters used to describe the negative-ion states of C, O, and F are listed, which are all cited from the literature [61,62]. In the calculation, to get the best fit, the value of a0 is slightly different from Ref. [61]. We use g ¼ 0.017 (0.7 1015/s) for carbon and oxygen. For fluorine, g ¼ 0.028 (1.15 1015/s), is larger than that of carbon and oxygen. In fact, the value of g is in a reasonable range as mentioned in Ref. [64]. The calculation results are well consistent with the experimental data, as shown in Figs. 6 and 7. It indicates that the incident velocity and initially formed positiveions play an important role in negative-ion formation. The monotonic increase of the negative-ion fraction in Fig. 6 can " !# be
explained
by
½1 P þ ðz0 Þ exp
gz0 v⊥;in
1 þ v⊥;out
.
For
a ¼ b ¼ 19 , the exponential item increases with increasing velocity. In general, the bell-shaped negative-ion fractions in Fig. 7 can be qualitatively explained as follows: on the one hand, the perpendicular exit velocity is low for small exit angles (<19 ), the initially formed negative ions will spend a long duration time near the surface. The efficient neutralization occurs and the final negativeion fraction is mainly determined by the decay probability associ 1 ated with exp v⊥;out where the smaller exit angles correspond to smaller negative-ion fractions. On the other hand, the perpendicular exit velocity is high for large exit angles (>19 ), and there is only a short time for the exit neutrals to pick up electrons from the surface to form a negative ion. The final negative-ion fraction is mainly associated with the formation probability of ! ½1 P þ ðz0 Þ exp
z0 vg⊥;in
where the larger exit angles (smaller
incident angles) correspond to smaller negative-ion fractions. Moreover, it indicates that the dynamical equilibrium is never achieved for high perpendicular exit velocities.
Table 1 Parameters describing the distance dependence of the negative-ion states (Eq. (5)), and the electron transition rate g. D is given in eV and a is in a.u.1. Atom 4
C ( S) O(2P) F(1S)
D0
a0
D1
a1
g (angle)
g (velocity)
5.11 4.60 12.0
0.48 0.70 0.51
0.50 1.30 2.9
0.49 0.71 1.05
/ 0.017 0.028
0.017 0.017 0.028
F. Xiong et al. / Vacuum 137 (2017) 23e30
29
5.3. Comparison with other data In Fig. 7, the negative-ion fraction presents a bell shape as a function of exit angle. The fraction increases when the exit angles are smaller than 19 , which is consistent with the previous results [6,17,21,29e31]. As discussed in Section 5.2, the increased survival probability of negative ions can be qualitatively attributed to the higher perpendicular exit velocities of negative ions moving away from the surface. However, the decreased fractions at large exit angles (>19 ) do not agree with the previous results where the fractions continue to increase. We emphasize here that, the nonmonotonic curve of negative-ion fraction as a function of exit angle is in contradiction with the conventional understanding. The conventional understanding is that the increasing of the perpendicular exit velocity corresponding to the decreasing of the dwelling time the particles spend near the surface, which is in favor for the survival of negative ions and would cause a higher negative-ion fraction for large exit angles. As a result, the bell-shaped negative-ion fractions of oxygen and fluorine have been well described by the calculations using the modified resonant charge transfer model, as shown in Fig. 7. The bell shape of negative-ion fraction has been observed for 3e4 keV F atoms scattering on Ag [22], for 22.5 keV O scattering on water covered Si(111) [35] and for 22.5 keV F scattering on water covered Si(100) [33,34], as well as 22.5 keV F scattering on a highly oriented pyrolytic graphite (HOPG) surface [36]. Under the same experiment conditions, our F fraction results on water covered Si(111) are much larger than that on HOPG, which can be attributed to the different DOS in the top valance band region. Considering the similarity of the surface work function between HOPG (about 4.6 eV) and water covered Si(111), we think that the influence of the work function on negative-ion formation is not obvious. It is interesting that the band structures of HOPG [65] shows equivalently a band gap of ~1 eV, i.e., the DOS near the Fermi level is very low. As discussed before, the band-gap effects on charge transfer can be neglected in our energy region. Therefore, the initial negative-ion yield is strongly dominated by the surface DOS. The DOS of HOPG is less than 0.1 states/eV within the 2-eV range below the Fermi level [66]. In contrast, for the unreconstructed Si(111) surface, the total DOS is as high as 5 states/eV below the Fermi level [67] in the top valance band region. Therefore, the initial negative-ion formation probability is higher on Si(111) than that on HOPG. In addition, the positive-ion fraction on HOPG (about 10%) is larger than that on Si(111), it may indicate that the neutralization of Fþ on HOPG is less efficient than that on Si(111). This can be also easily understood in terms of the lower DOS of the valence band of HOPG. The repulsive potential of carbon nuclei is weaker than that of silicon nuclei, thus the incident fluorine ions can approach much closer to HOPG than Si(111), and the excitation and/or ionization processes for the projectiles are enhanced at a close distance. Hence, there are not enough neutrals left to form negative ions. At the same energies, the maximum value of F fraction on water covered Si(111) (~28%) is slightly smaller than that on water covered Si(100) (34%) reported in Refs. [33,34]. The similar results have also been observed for negative carbon and oxygen ions. In Fig. 9, the C, O, and F fractions of water covered Si(111) are smaller than that of water covered Si(100). The difference of negative-ion fraction between Si(111) and Si(100) can also be ascribed to the different DOS below the Fermi level. As we know, there are many similarities for the calculated DOS of clean Si(111) and Si(100) surfaces [67]. For Liþ scattering on clean Si, similar neutral fractions were observed for different crystal orientation [26]. It indicates that the localized dangling-bond surface states of Si(111) and Si(100) have the same effect on charge
Fig. 9. Negative-ion fractions as a function of incident energy, measured for fluorine, oxygen and carbon negative ions in specular scattering on water covered Si(111) and Si(100) [34] surfaces.
transfer. Zeng's group have calculated the DOS of clean Si(100) and Si(111), and found that the total DOS of Si(111) is slightly smaller than that of Si(100), particularly in the range below the Fermi level [67]. In addition, the energy level diagram of clean Si(111) shows two dominant surface states located at 0.2 eV and 0.8 eV below the Fermi level, while there is only one surface state located at 0.8 eV below the Fermi level for clean Si(100) [68,69]. For water covered surfaces, the localized dangling-bond surface states completely disappear for Si(111) surface, while they have not disappeared completely for Si(100) surface [40,70,71]. On water covered Si(100) surface, isolated dangling bonds have been observed and are about one percent of Si atoms deduced from scanning tunneling microscopy (STM) images [70,71]. In a word, the DOS in the top valence band region of water covered Si(111) may be still smaller than that of water covered Si(100). As a consequence, the C, O, and F fractions from water covered Si(111) are smaller than that from water covered Si(100), as shown in Fig. 9, and the bell-shaped curve for Si(111) is lower than that for Si(100) [34] as shown in Fig. 7. In Fig. 9, for C, O, and F scattering on Si(111), F fraction is the largest and the C fraction is the smallest in the energy range investigated. It can be qualitatively attributed to the smallest affinity level of C that is located at 1.269 eV with respect to the vacuum level and the largest affinity level of F that is located at 3.399 eV [51]. 6. Conclusions We presented new experimental results of negative carbon, oxygen, fluorine ions scattering on water covered Si(111) surface. These results are very similar to that on water covered Si(100) in our previous work. Based on the MO promotion model, the positive-ion production was associated with electronic excitation, ionization, and Auger and/or resonance neutralization. In particular, for oxygen and fluorine, the bell-shaped negative-ion fractions as a function of exit angle cannot be completely understood using the conventional rate equation that is suitable to the case of adiabatic slow collisions. In our case, for fast collisions, the affinity level of negative ion (C, O and F) lies quite close to the Fermi level. With the help of the image potential effect, it can easily arrive to the bottom of the band gap and interact with the valence band states, which strongly indicates that the band gap effect on charge transfer can be neglected to a large extent in this relatively high energy range.
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As a consequence, we have adopted a complete theoretical framework to fit the final positive-ion fractions, and found that the first ionization energy of the projectile and the perpendicular exit velocity play important roles in positive-ion production. Then we use the modified resonant charge transfer model to calculate negative-ion fractions. A good agreement between experiment and calculation has been obtained, which further confirms the nonadiabatic electron transfer processes. Under the same scattering configurations, we compare the experimental results for different collision systems. We find that the negative-ion fractions for H2O-Si(111) are slightly smaller than that for H2O-Si(100), which indicates that the electron affinities of the projectiles and surface's DOS play crucial roles in negative ion production. In summary, the relevant studies on semiconductor surfaces are still inadequate and much effort should be devoted to this subject. This experimental study has improved our understanding of charge transfer at semiconductor surfaces for multielectron projectiles at relatively high energies. The dynamical resonant charge transfer may be important to specific surface reactions, molecular devices and electron emission at semiconductors. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No.11405078, No.11474140, No.11104124, and No.11505087). References [1] P. Meredith, C.J. Bettinger, M. Irimia-vladu, A.B. Mostert, P.E. Schwenn, Rep. Prog. Phys. 76 (2013) 034501. [2] L. Chen, J. Shen, J.J. Jia, T. Kandasamy, K. Bobrov, L. Guillemot, J.D. Fuhr, M.L. Martiarena, V.A. Esaulov, Phys. Rev. A 84 (2011) 052901. [3] L. Chen, Y.L. Guo, J.J. Jia, H.Q. Zhang, Y. Cui, J.X. Shao, Y.Z. Yin, X.Y. Qiu, X.Y. Lv, G.Z. Sun, J. Wang, Y.F. Chen, F.Y. Xi, X.M. Chen, Phys. Rev. A 84 (2011) 032901. [4] H. Zhou, L. Chen, D. Feng, Y.L. Guo, M.C. Ji, G.Y. Wang, W. Zhou, Y. Li, L. Zhao, X.M. Chen, Phys. Rev. A 85 (2012) 014901. [5] L. Chen, X.Y. Lv, J.J. Jia, M.C. Ji, P. Zhou, G.Z. Sun, J. Wang, Y.F. Chen, F.Y. Xi, Y. Cui, J.X. Shao, X.Y. Qiu, Y.L. Guo, X.M. Chen, J. Phys B: At. Mol. Opt. Phys. 44 (2011) 045203. [6] M. Maazouz, A.G. Borisov, V.A. Esaulov, J.P. Gauyacq, L. Guillemot, S. Lacombe, D. Tellet-Billy, Phys. Rev. B 55 (1997) 13869. [7] L. Guillemot, V.A. Esaulov, Phys. Rev. Lett. 82 (1999) 4552. [8] A.G. Borisov, A.K. Kazansky, J.P. Gauyacq, Phys. Rev. B 59 (1999) 10935. [9] H. Chakraborty, T. Niederhausen, U. Thumm, Phys. Rev. A 70 (2004) 052903. [10] B. Obreshkov, U. Thumm, Phys. Rev. A 87 (2013) 022903. [11] A.G. Borisov, A. Mertens, S. Wethekam, H. Winter, Phys. Rev. A 68 (2003) 012901. [12] T. Kravchuk, V.A. Esaulov, A. Hoffman, R.C. Monreal, Nucl. Instrum. Methords Phys. Res. B 232 (2005) 27. n, E.C. Goldberg, [13] C. Meyer, F. Bonetto, R. Vidal, E.A. García, C. Gonzalez, J. Ferro Phys. Rev. A 86 (2012) 032901. [14] H. Winter, Phys. Rep. 367 (2002) 387. [15] A.G. Borisov, V.A. Easulov, J. Phys. Condens. Matter 12 (2000) R177. [16] F. Wyputta, R. Zimny, H. Winter, Nucl. Instrum. Methords Phys. Res. B 58 (1991) 379. [17] M. Maazouz, R. Baragiola, A. Borisov, V.A. Esaulov, S. Lacombe, J.P. Gauyacq, L. Guillemot, D. Teillet-Billy, Surf. Sci. 364 (1996) L568. [18] A.G. Borisov, D. Teillet-Billy, J.P. Gauyacq, Phys. Rev. Lett. 68 (1992) 2842. [19] N. Lorente, A.G. Borisov, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 429 (1999) 46. [20] S. Ustaze, L. Guillemot, V.A. Esaulov, P. Nordlander, D.C. Langreth, Surf. Sci. 415 (1998) L1027. [21] M. Maazouz, L. Guillemot, T. Schlatholter, S. Ustaze, V.A. Esaulov, Nucl.
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