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Quantitative surface structure analysis by low-energy ion scattering
314
Nuclear
QUANTITATIVE
SURFACE STRUCTURE
Instruments
ANALYSIS
and Methods
in Physics Research B2 (1984) 374-383 North-Holland. Amsterdam
BY LOW-ENERGY
ION SCATT’ERING
M. AONO N~i~o~~i Znsiilute
fur Research in inorganic Mate~iais, Namiki i -I, Sukuru, Niihari, Z~~aki 305, Japan
A review is presented of low-energy ion scattering spectroscopy (1%) as a tool for surface atomic structure analysis. Especially, quantitative surface atomic structure analysis by 1% is highlighted. An important difference between ISS and Rutherford backscattering spectroscopy (RBS), a specialization of ISS for quantitative surface atomic structure analysis, and a general feature of the shadow cone in the energy range of ISS are first discussed as a basis for the descriptions of particular examples of ISS studies which follow. The examples are concerned with the atomic structure analysis of clean surfaces, surfaces with adsorbates, and surface defects.
1. infusion
2. ISS in comparison with RBS
Ion scattering spectroscopy is one of the most powerful techniques for surface atomic structure analysis because of its simple principles, In this technique, a “classical” particle - an ion - is used, and simple “classical” concepts - shadowing and blocking - are utilized. As a result, ion scattering spectroscopy provides direct information on the relative positions of atoms in a surface region. Ion scattering spectroscopy is classified into low-energy ion scattering spectroscopy (ISS) [l-4] and high-energy ion scattering spectroscopy or Rutherford backscattering spectroscopy (RBS) [5]. The apparent difference between ISS and RBS is only that the ion energy is ISS in - keV, while that in RBS is - MeV. However, this quantitative difference gives rise to an important qualitative difference between ISS and RBS. In this paper, a review of ISS will be presented in such a manner that the excellent ability of ISS as a tool for surface atomic structure analysis is emphasized. Especially, quantitative surface atomic structure analysis by ISS will be highlighted. Surface chemical composition analysis by ISS will not be discussed in this paper. It is also not the purpose of this paper to present a comprehensive list of the literature on ISS; for that purpose, we can refer to more comprehensive reviews 12-41. In the next section, ISS and RBS will be compared with each other in order to clarify the difference between these almost independently advancing ion scattering techniques. In sect. 3, a specialization of ISS, which makes quantitative surface atomic structure analysis possible, will be described. In sect. 4, a general feature of the shadow cone will be discussed. Examples of actual KS studies will be shown and discussed in sect. 5.
ISS is characterized by the thick shadow cone (radius - 1 A) and very high ion neutralization probability (comparable to 100% for inert-gas ions) which result in extremely high surface sensitivity, while RBS is characterized by the very thin shadow cone (radius - 0.1 A) and negligible ion neutralization probability. The most important difference between ISS and RBS in connection with surface atomic structure analysis is schematically depicted in fig. 1. In both ISS [fig. 1 (a)] and RBS [fig. l(b)], when the ion incidence angle measured from the surface, cy, is decreased from 90”, atom A in the first layer shadows atom B in the second layer. As (Y is decreased more, in RBS, atom C in the first
0168-583X/84/$03.00 0 Elsevier Science Publishers (Noah-Holland Physics Publishing Division)
B.V.
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4
0
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0
la)
0
0
ISS
-MeV
L
0
0
0
Ibl
D
0
0
RBS
Fig. 1. Schematic figure depicting an important difference between (a) ISS and (b) RBS in connection with surface atomic structure analysis.
315
M. Aono / Quantitative surface structure analysis layer shadows atom D in the third layer. This shadowing effect C -+ D cannot be observed in ISS since atom D in the third layer is “invisible” in surface-sensitive ISS. Instead of this, in ISS, atom C in the first layer shadows atom E in the same first layer because of the thick shadow cone. This shadowing effect C --) E cannot be observed in RBS since such a shadowing effect occurs only at unpractically small a because of the very thin shadow cone. These arguments for the shadowing effect also apply for the blocking effect. In this way, it is found that RBS determines surface atomic positions relative to subsurface atomic positions (it is hard to determine the relative positions of surface atoms directly), while ISS determines the relative positions of surface atoms ~~depende~t~y of surface atomic positions (it is impossible to obtain information on subsurface atomic positions).
3. A specialization of IS!3 ICISS In surface atomic structure analysis by ISS, the shadowing and blocking effects are both useful. However, the coexistence of the two effects is sometimes troublesome, especially in the analysis of complicated surface atomic structures, since it is not easy to assign observed various intensity drops to the shadowing or blocking effect without ambiguity. It is therefore desired to make experiments in such a condition that only one of the two effects works. If we take the experimental
tb) 1SS
scattering angle 0, close to 180”, the blocking effect rarely occurs since ions backscattered along such trajectories that are close to their incident trajectories are selectively observed. This specialization of ISS is called impact-collision ion scattering spectroscopy (ICISS) [6,7]. Because of its simplicity, ICISS can analyze the surface atomic structure more straightforwardly than ordinary ISS in which 8, is arbitrary; ICISS shows its ability in the analysis of complicated surface atomic structures (see sect. 5.1). A more important advantage of ICISS is that because of 0, = 180°, those scattered ions which have made impact collisions (almost head-on collisions) with atoms are selectively observed. Namely, ICISS “sees” the close vicinity of the center of each atom. Because of this, as illustrated in fig. 2(a), at the onset of a shadowing effect in which atom A begins to shadow atom B, the edge of the shadow cone of atom A necessarily passes the center of atom B to a good approximation. To recognize this characteristic of ICISS is a bre~throu~ since it clears the way to analyze the surface atomic structure quantitatively (see sect. 5.1). Note that in ordinary ISS in which 6, is arbitrary, there is no special relation between the edge of the shadow cone of atom A and the position of atom B even at the onset of the shadowing effect as illustrated in fig. 2(b).
4. Universal shadow cone expressions The shape of the shadow cone can be determined experimentally by ICISS described in the previous section. Consider a chain of atoms at a crystal surface shown in fig. 3. When the ion incidence angle measured from the surface, ru, is decreased, the intensity of ions scattered from the atomic chain suddenly drops to zero because of a shadowing effect. Fig. 3 illustrates the situation just at the onset of the shadowing effect in the ICISS condition; for simplicity, the incident and scattering trajectories of each ion observed in the ICISS condition are drawn as if they were coincident with each other [actually, they are slightly different from each
Fig. 2. Schematic figure depicting the difference between (a) ICISS (8, = 180”) and (b) ordinary ISS (8, + 1800). In ICISS, the center of each atom is “seen” in contrast to ordinary ISS, and hence at the onset of the shadowing effect, the edge of the shadow cone of a shadowing atom A passes the center of a concealed atom B to a good approximation. Because of this, ICISS can analyze the surface atomic structure quantitatively.
Fig. 3. Schematic figure illustrating how the shape of the shadow cone can be determined quantitatively by ICISS. Ref.
[71. VI. LOW ENERGY
SURFACE
INTERACTIONS
316
M. Aono / Quantitative surface structure analysis lb1 liC10011-1x1
loI TiCIlllI-1x1
He+
R,(A) 1
-.i
,*--
------\.____
Fig. 6. Shape of the shadow cone of Ti for He+ of 1.0 keV determined by ICES. Ref. [7]. Fig. 4. Atomic structures of the (111) and (001) surfaces of TIC with a NaCl structure. Ref. [7].
other even if 8, is taken just at 180” as we can understand from fig. 2(a)]. At the onset of the shadowing effect in the ICISS condition, the edge of the shadow cone of each atom passes the center of its neighboring atom to a good approximation as in fig. 3 (see section 3). Therefore, by measuring the critical value of a corresponding to the onset of the shadowing effect, ac, the radius of the shadow cone at the distance L = d cos a, is found to be R, = d sin a,, where d is the atomic distance; see fig. 3. By taking other azimuths (if necessary, by taking other surfaces with different indices), we have other atomic chains with different d’s, and hence we can determine Rs’s at different L’s. The shape of the shadow cone of Ti for He+ at E, (primary ion energy) = 1 keV has been determined [7] by this method using the (111) and (001) surfaces of
ICISS
INTENSITY
OF Ti
-
c
tic+ (E,=l.O
TiC(lllI; TiC(0011
10
kcV1
ri2il
; I1101
20
a
(deg)
Fig. 5. Intensity of the KISS spectral peak due to surface titanium atoms of Tic plotted against the ion incidence angle measured from the surface, a. Ref. [7].
Tic. The structure of the surfaces are shown in figs. 4(a) and (b) [8,9]; since either surface has a 1 x 1 structure, the Ti-Ti distances indicated in the figures are readily calculated from the bulk lattice constant. Fig. 5 shows selected results of the KISS experiments for the surfaces; the intensity of He+ ions scattered from the surface titanium atoms is plotted as a function of a. As a decreases, the intensity suddenly drops to zero at a critical angle a, which depends on the Ti-Ti distance d (3.06 and 5.30 A for the right and left curves, respectively) because of the shadowing effects; an increase in the intensity just before the shadowing effect begins is due to the focusing effect (each atom is bathed in concentrated ion flux just outside the shadow cone). Using a,‘s determined from fig. 5 (and other data not shown), we can draw the shape of the shadow cone of Ti for 1 keV He+ as shown in fig. 6 by dots. Shown in figs. 7(a)-(d) are possible trajectories of He+ (E, = 1 keV) scattered by Ti calculated for Bohr, Born-Mayer, and Thomas-Fermi potentials (for the Thomas-Fermi potential, Sommerfeld and Moliere approximations were used). The shadow cones for the Bohr and Born-Mayer potentials, figs. 7(a) and (b), are too thin and thick, respectively, compared with the experimental result (fig. 6) while the shadow cone for the Thomas-Fermi potential, fig. 7(c) or (d) (the Sommerfeld and Moliere approximations gives essentially the same result), agrees well with the experimental result. This indicates that the Thomas-Fermi potential is a good approximation to the ion-atom interaction potential in the energy range of ISS, although further studies are necessary for many other ion-atom combinations. Recently, “universal expressions” [lo-121 for the shadow cone, which agree with the experimental result mentioned above, have been calculated. From the expressions, the shadow cone radius R, at any distance L can be estimated immediately for any ion-atom combination and ion energy E, (the expressions in ref. 10 do not cover a region corresponding to usual ISS experimental conditions, but those in ref. 12 cover the region). The “universal shadow cone expressions” are very useful to help plan ISS experiments and analyze the results.
M. Aono / Quantitative ki)
BORH
ICI THOMAS-FERMI-SOMMERFELD
POTENTIAL
lb)
I
:
\
BORN-MAYER
POTENTIAL
\
_1 I
371
surface structure analysis
\
POTENTIAL
\
\
Id)
THOMAS-FERMI-MOLIiRE
POTENTIAL
Fig. 7. Trajectories of He+ ions (primary energy E, = 1.0 keV) scattered by Ti calculated by assuming (a) Bohr, (b) Born-Mayer, (c) Thomas-Fermi-Sommerfeld, and (d) Thomas-Fermi-Moliere potentials. Ref. [7].
5. Examples of 1% studies 5. I. Clean surfaces
The simplest quantitative surface atomic structure analysis is to measure the magnitude of “rumpling relaxation” of a compound surface, i.e., opposite (inward and outward) displacements of two kinds of constituent surface atoms. The magnitude of the rumpling relaxation of the TiC(OO1) surface has been measured [13] by ICISS (1 keV He+) in the following way. Fig. 8(a) shows the intensity of He+ ions scattered from surface carbon atoms at the TiC(OO1) surface in the ICISS condition as a function of the ion incidence angle measured from the surface, cu; the measurement was made in the (100) azimuth in which surface titanium and carbon atoms are arranged alternately [see fig. 4(b)]. As a decreases, the intensity suddenly drops to zero because every surface carbon atom is concealed by the shadow cone of its neighboring surface titanium atom as shown in fig. 8(b); an increase in the intensity just before the shadowing effect begins is due to the focusing effect. From fig. 8(a), the shadowing critical angle is found to be OL,(C; Ti) = 24” [in the notation a,(A; B), A and B are shadowing and concealed atoms,
respectively]. Since we already know the shape of the shadow cone of Ti for 1 keV He+ as given in fig. 6, we can draw fig. 8(b), which shows the situation just at the onset of the shadowing effect (a, = 24’); broken curves show the shadow cone of the surface titanium atom drawn along the two opposite (100) azimuths. In the ICISS condition, the edge of the shadow cone of the surface titanium atom passes the center of the surface carbon atom to a good approximation at the onset of the shadowing effect. Therefore, we can readily determine the position of the surface carbon atom as the intersection of the edges of the two shadow cones drawn in fig. 8(b). The position of the surface carbon atom thus determined almost coincides with its idea1 position. However, we should estimate the error margin of the result. The biggest source error is the ambiguity in determining a, from the intensity curve in fig. 8(a). In the above discussion, a, was estimated to be an (Y corresponding to the half-maximum intensity. However, this is an underestimate of a, for the following reason. The reason why the observed intensity drop due to the shadowing effect is not very sharp is that both the titanium (shadowing) and carbon (concealed) atoms are vibrating around their equilibrium positions. Namely, the obVI. LOW ENERGY SURFACE INTERACTIONS
M. Aono / Quantitative surface structure analysis
378
r
I
I
ICISS
I
INTENSITY
OF
1.0
I Cl
I
C
FOR
I
TiC(OOl1
AZIMUTH kr’f
He*
,
1
i
I
I
/
I
I
IO
20
30
40
50
60
70
60
a
Idegl
(b) TiClOO
-
Fig. 8. (a) Intensity of He+ ions scattered from carbon atoms at the TiC(OO1) surface [see fig. 4(b)] as a function of the ion incidence angle measured from the surface, a; the measurement was made in the (100) azimuth in which surface carbon and titanium atoms are arranged alternately as shown in (b). (b) The position of the surface carbon atom determined from the experimental result shown in (a) (marked by +). Ref. f13].
served
intensity
distribution
curve
function.
is smoothed
by some
gaussian
Even so, if the intensity enhancement due to the focusing effect were absent, an (Y corresponding to the half-maximum intensity would be equal to the correct (xc. Actually, however, because of the intensity enhancement which occurs on the high-a side of a,, such an estimation leads to an underestimate of (Y,. According to computer simulations in various cases 1141, we can state the following empirical rule: Roughly speaking, an a corresponding to the 70%maximum intensity is approximately equal to the correct a,. This rule is rough but surely improves the accuracy of analysis. In the case of fig. 8(a), aI, estimated according to this rule is 26’ as indicated by a vertical broken line. The position of the surface carbon atom determined from this a, is - 0.1 A below the level of the surface titanium atom. It is thus found that the TiC(OO1) surface exhibits a very small if any rumpling relaxation (5 0.1 A).
When the observed intensity drop due to the shadowing effect is fairly sharp as in fig. 5, a, estimated at the half-m~mum intensity is practicaily satisfactory. A number of ISS studies have been made for surfaces of simple metals [15-341 but the main interest of most of the studies has not been to analyze the atomic structure of the surfaces but to study basic physics of low-energy ion scattering such as multiple scattering [17,19,26,2’7,31,32], inelastic scattering f15,16,30], and ion-surface electron exchange [18,20-25,28,29] by using the surfaces. An example of atomic structure analysis of simplemetal surfaces by ISS is a recent study [34] of the Au(llO)-2 x 1 surface. In this study, a beam of K’ ions was used. Alkali ions such as K+ are characterized by their low ionization potentials, and hence their neutralization probability is very low. Therefore, the use of alkali ions [18,25,26,31,35,36] is, in effect, similar to using a time-of-flight analyzer [20,23,37,38] or a stripping (reionization) cell [l&39] in inert-gas-ion ISS; the latter two techniques detect not only ions but “neutralized ions”. Namely, by the use of alkali ions, multiple scattering has significant contribution to measured spectra. The Au(ll0) surface changes from a 2 x 1 structure at room temperature to a 1 X 1 structure at elevated temperatures. Plotted in figs. 9(a) and (b) by dots are ISS spectra for the 1 x 1 (900 K) and 2 X 1 (300 K) surfaces, respectively; @ denotes the azimuthal angle measured from [liO]. In every spectrum, the low- and high-energy peaks are due to single and multiple scatterings, respectively. The histograms in figs. 9(a) and (b) show the results of computer simulations for a perfect bulk-termination model (1 X 1 structure) and an unrelaxed missing-row model (2 X 1 structure), respectively. In both the figures, agreement between the experimental result and the computer simulation is unsatisfactory, especially at + = 27.5’. From these results, it is concluded that (1) the 1 x 1 surface at elevated temperatures may not be in the perfect bulk-termination structure, and (2) for the 2 x 1 surface at room temperature, the unrelaxed missing-row model is not a very probable model (a distorted hexagonal overlayer model is also rejected). According to a recent glancing-incidence X-ray diffraction experiment [40], a relaxed missing-row model is derived for the 2 x 1 structure. As for covalent semiconductor surfaces, the Si(OOl)-2 X 1 surface, for example, has been studied 1411 by ICISS. Fig. 10(a) shows the intensity of He+ ions scattered from the Si(OOl)-2 X 1 surface in the ICISS condition as a function of the polar angle measured from the surface, a, and the azimuthal angle measured from [ 1 IO], +, of the ion incidence direction; the primary energy of He’ ions was 1 keV. The intensity variations observed in fig. 10(a) are due to various shadowing effects (accompanied with a focusing effect) and anisotropic ion neutralization probability, although the latter
M. Aono / Quantitative
surface structure analysis
_b
60
a
319
MISSING ROW STRUCTURE
BULK TERMINATION STRUCTURE
T=300K
r
360
390
420
450 460 ENERGY
5fO (eV)
540
570
600
360
390
420
450
490
ENERGY
510
540
570
600
(eV)
Fig. 9. ISS spectra (K+ ions) for the (a) 1 X 1 (900 K) and (b) 2 X 1 (300 K) structures of the Au(ll0) surface (dots); $I denotes the azimuthal angle measured from the [liO] direction. The histograms show the results of theoretical computer simulations. Ref. [34].
is minor; no blocking effect has to be considered because of the ICES condition. The intensity curve for a grazing angle of a = 4’ in fig. 10(a) is magnified in part in fig. 10(b). the drop in intensity observed at + = O” ([llo]), 32” (azimuth S), and 58” (azimuth s’) are unambiguously identified to be due to shadowing effects since an intensity rise due to a focusing effect is clearly observed on either side of each of the intensity drops as indicated by broken curves. Fig. 10(b) contains the following information: (i) Shadowing effects are observed in [llO] and in azimuths S and S’ which deviate from [OlO] by 6 = 13”, and (ii) the magnitudes of the intensity drops due to the shadowing effects are AZ([llO]) = 63% and AZ(S or S’) = 15%. On the basis of these experimental facts, we can infer the structure of the Si(OOl)-2 x 1 surface. The experimental fact (i) rejects the unreconstructed 1 X 1 structure shown in fig. 11(a) as expected; shadowing effects for this structure should occur in [llO] and [OlO] (indicated by arrows) in
contradiction to the experimental fact (i). The experimental fact (i) is decisive evidence for that the surface is “dimerized” like fig. 11(b); in this case, shadowing effects occur in [llO] and in azimuths which deviate from [OlO] by some angle 6 (indicated by arrows) being consistent with the experimental fact (i). By examining fig. 11(b) in detail, we find that this dimer structure is also consistent with the experimental fact (ii). The fact that 8 in fig. 11(b) is 13” [experimental fact (i)] indicates that the intradimer atomic distance parallel to to the surface is 2.4 A ( f 0.1 A). Corresponding theoretical values reported, 2.3 A [42] and 2.2 A [43], agree with this experimental value.
5.2. Surfaces
with adrorbates
A number of ISS studies have been reported for surfaces with adsorbates, e.g., O/Ni(llO) [44,45], VI. LOW ENERGY
SURFACE
INTERACTIONS
M. Aono / Quantitative
surface structure analysis 14)
UNRECONSTRCTED
Ilxl)
STRUCTURE
I
BOUNDARY
rl l
l
l
IO101
CIIOI
15
0
15 30 @ ldeg)
45
60
Fig. 10. (a) Intensity of He+ ions scattered from the Si(OOl)-2 X 1 surface in the ICISS condition as a function of polar angle a (measured from the surface) and azimuthal angle $I (measured from [IlO]) of the ion incidence direction. (b) The intensity curve for a = 4’ in (a) magnified in part. Ref. [41].
O/NiWW
O/Cu(llO) O,CO/Si(lll)
O/Ag(llO) [48], O/W(llO) [51], O/Cu(lOO) 1521, Ag/Si(lll) [54], and O/TiC(lOO) [55].
[46,47],
l
l
I
LAYER
6
016 o SECOND
Jf
l 0
LAYER
Fig. 11. (a) Unreconstructed 1 X 1 and (b) asymmetrically marked 2 X 1 structures of the Si(OO1) surface. Ref. [41].
_r
r
FIRST
I 0
[SO], 1531,
di-
As an example, ISS spectra measured for the O/Ni(llO)-2 X 1 surface are shown in figs. 12(a) and (b) 1441. In the figures, t9( = 60”) indicates the scattering angle, $I shows the azimuthal angle measured from the [Ol] direction, and # denotes the ion incidence angle measured from the surface. In every spectrum, the highand low-energy peaks are due to nickel and oxygen atoms, respectively. Fig. 12(c) shows the structure of the O/Ni(llO)-2 X 1 surface inferred from the experimental results of figs. 12(a) and (b), where open and hatched circles show nickel atoms in the first and second layers, respectively, and cross-hatched circles show adsorbed oxygen atoms. As fig. 12(a) shows, when I/J is decreased in the [Ol] direction (+ = O’), the Ni/O intensity ratio drops by about a factor of two between # = 22.5” and 4 = 10”. This is consistent with the structure shown in fig. 12(c); that is, the adsorbed oxygen atoms (protruding - 0.4-0.8 A) partially shadow the nickel atoms in the first layer in the [Ol] direction. The reason why the Ni/O intensity ratio in the [Ol] direction (+ = 0”) [fig. 12(a)] is larger than that in the [lo] direction (9 = 90°) [fig. 12(b)] by about a factor three is that the nickel atoms in the second layer significantly contribute to ion scattering in the [Ol] direction.
381
IU. Aono / Quantitatiue surface structure analysis
I
(a)
KISS
t I
ICI
INTENSITY He+
[lo01
AZIMUTH
I
u,
E/E,
Ti FOR
TiCIOOlI-1x1
lb)+O,(-3L) \
I?
05
OF
IkeV
-
lb)
Ial
ANNEALED
AT -1600°C
*t 7.;e_____
-
I 0
&iTi;Cl=22.1°
10
I
I
20
30
a
E/E,,-
Fig. 12. (a) and (b), ISS spectra for the O/Ni(llO)-2X1 surface. (c), A structural model of the surface which is consistent with (a) and (b). Ref. [44].
5.3. Surface
defects
The influence of various surface defects on the ISS spectrum has been discussed extensively [37,56]. In what follows, a recent study [57], in which the structure, concentration, and chemical activity of surface atomic vacancies at the TiC(OO1) surface have been studied by ICISS, will be shown as an example. The TiC(OO1) surface, which has a structure as shown in fig. 4(b), was first annealed at a high temperature - 1600°C in order to remove surface defects. In fig. 13, curve (a) shows the intensity of He+ ions scattered from titanium atoms at the TiC(OO1) surface as a function of
I
LO
c
ldeg)
Fig. 13. Intensity of He+ ions scattered from titanium atoms at the TiC(OO1) surface [see fig. 4(b)] in the KISS condition as a function of the ion incidence angle measured from the surface, a; the three curves correspond to different surface conditions (see text). The measurements were made in the [lOOIazimuth in which surface titanium and carbon atoms are arranged alternately. Ref. [57].
the ion incidence angle measured from the surface, a; the measurement was made in the ICISS condition in the [lOO] azimuth in which surface titanium and carbon atoms are arranged alternately [see fig. 4(b)]. The drop in the intensity observed in the neighborhood of a, = 22” is due to a shadowing effect illustrated in fig. 14(a); all the surface titanium atoms “l”, “2”, . . . are concealed by the shadow cones of their neighboring surface carbon atoms “l”‘, “2”‘, . . . . It is known for Tic that carbon atoms are preferentially sputtered by energetic light ions [58]. Using this characteristic, carbon vacancies were created at the TiC(OO1) surface. That is, the surface was bombarded by He+ ions of 1 keV (- 0.3 PA/cm*) for 10 min. By this ion bombardment, a 1 X 1 LEED pattern, which had been observed initially, was destroyed. The sample was then annealed at various temperatures for - 15 s. When the annealing temperature TA was higher than - lOOO”C, the surface exhibited a clear 1 x 1 LEED pattern. In fig. 13, curve (b) shows an experimental result measured after the annealing at T, = 1400°C. In this curve (b), a shadowing effect observed in the neighborhood of (Y,= 22” results from the same mechanism as the shadowing effect observed in curve (a). However, the intensity does not drop to zero as a result of this shadowing effect but shows a shoulder indicated by hatching. This clearly indicates that surface VI. LOW ENERGY
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INTERACTIONS
382
M. Aono / Quantitaiive surface structure analysis TiClOOll-1x1
the atoms were “frozen” at their displaced positions. Therefore, the thermal vibration of surface atoms can be regarded as one kind of surface defect. The thermal vibration amplitude of surface atoms has been measured by ICISS for the TiC(lll) surface [59].
6. Summary
Fig. 14. Two different shadowing effects in the [lOO] azimuth of the TiC(OO1) surface; in every case, the situation at the shadowing critical angle in the ICISS condition is shown. Ref. [57].
carbon vacancies were present as traces of the preferential sputtering of carbon atoms; namely, as illustrated in fig. 14(a), if there is a surface carbon vacancy shown by a square, its neighboring surface titanium atom “3” cannot be concealed. As (Ydecreases more, this “surviving” surface titanium atom “3” is after all concealed by the shadow cone of the next-nearest surface titanium atom “2” as illustrated in fig. 14(b). From the critical angle of the latter shadowing effect, a, = 15.4’, and the shape of the shadow cone of Ti for 1 keV He+ shown in fig. 6, we find that the surface titanium atoms adjacent to the surface carbon vacancy are not displaced within an error margin of - kO.1 A; this may be related to the fact that carbon atoms in Tic occupy small interstitial spaces of almost closely packed titanium atoms. From the relative intensity of the shoulder in curve (b) of fig. 13, we can estimate the concentration of the surface carbon vacancies, although the estimation is not necessarily simple since the relative intensity of the shoulder depends on the primary energy of He+ ions. In the particular case of curve (b), the concentration of the surface carbon vacancies is estimated to be (10 + 2)%. It is known that the TiC(OO1) surface (without the surface carbon vacancies) is considerably inert. In fact, to have monolayer adsorption of oxygen, the surface has to be exposed to oxygen more than - 1000 L (L = 10e6 Torr . s) [9]. In order to analyze the chemical activity of the surface carbon vacancies, the surface corresponding to curve (b) in fig. 13 was exposed to various amounts of oxygen, and similar measurements were made. Curve (c) in fig. 13 shows the experimental result after a small oxygen exposure of - 3 L. The shoulder observed in curve (b) has dramatically disappeared. This indicates that the surface carbon vacancies capture oxygen atoms into the vacancy holes exhibiting a very high chemical activity. In low-energy ion scattering, the collision time is of the order of 10-‘5-10-‘6 s, while the period of atomic thermal vibration is of order - lo-l3 s. In other words, ISS “ views” the thermal vibration of surface atoms as if
ISS is one of the most powerful techniques for surface atomic structure analysis. The accuracy of 1% in quantitative surface atomic structure analysis, - + 0.1 A, is not very good, but ISS is a reliable technique because of its simple principles. A serious disadvantage of ISS is damage to the surface caused by ion bombardment; when we use inert-gas ions and detect only ions using an electrostatic or magnetic analyzer, we must use a very high ion flux because of the high neutralization probability of the ions. This problem appears to be solved by the use of alkali ions whose neutralization probability is very low, a time-of-flight analyzer which can detect both ions and “neutralized ions”, or a stripping cell which reionizes “neutralized ions”. However, these methods seem to abandon the most important advantage of ISS: by utilizing high ion neutralization probability, troublesome multiple scattering effects are reduced, and hence a simple “classical” concept, i.e., the shadowing effect, is effectively utilized. In order to reduce the damage to the surface, it is hoped to develop such an energy analyzer that can measure the whole of an ISS spectrum or a considerable part of it at the same time without scanning energy. Such a technique would be able to shorten the time of measurement by more than one order of magnitude.
References [l] D.P. Smith, J. Appl. Phys. 38 (1967) 340. [2] T.M. Buck, in: Methods of surface analysis, ed., A.W. Czanderna (Elsevier, Amsterdam, 1975). 131 E. Taglauer and W. Heiland, Appl. Phys. 9 (1976) 261. 141 W. Heiland, Appl. Surface Sci. 13 (1982) 282. [51 L.C. Feldman, in: Surface Science: Recent progress and perspectives, ed., R. Vanselow (CRC Press, Cleaveland, 1980). [61M. Aono, C. Oshima, S. Zaima, S. Otani and Y. Ishizawa, Jpn J. Appl. Phys. 20 (1981) L829. [71M. Aono, Y. Hou, R. Souda, C. Oshima, S. Otani, Y. Ishizawa, K. Matsuda and R. Shim& Jpn J. Appl. Phys. 21 (1982) L670. PI C. Oshima, M. Aono, S. Zaima, Y. Shibata and S. Kawai, J. Less-Common Metals 82 (1981) 69. 191C. Oshima, M. Aono, T. Tanaka, s. Kawa, S. Zaima and Y. Shibata, Surf. Sci. 102 (1981) 312. WI O.S. Oen, Proc. 7th Int. Conf. Atomic Collision in Solids, Moscow (1977) p. 124.
M. Aono / Quantitative [ll] [12] [13]
[14] [15] [16] 1171 [18] [19] [20]
K. Komaki, A. Ootsuka and F. Fujimoto, Jpn J. Appl. Phys. 21 (1982) L521. O.S. Oen, Surf. Sci. 131 (1983) L407. M. Aono, R. Souda, C. Oshima, S. Otani and Y. Ishizawa, to be published; a preliminary report is given in M. Aono, Oyo Butsuri (Appl. Phys.) 51 (1982) 887. R. Souda, M. Aono and C. Oshima, to be published. W. Eckstein, V.A. Molchanov and H. Verbeek, Nucl. Instr. and Meth. 149 (1978) 599. D.J. O’Connor and R.J. MacDonald, Rad. Eff. 45 (1980) 205. E. Taglauer and W. Heiland, Surf. Sci. 33 (1972) 27. E. Taglauer, W. Englert, W. Heiland and d.P. Jackson, Phys. Rev. Lett. 45 (1980) 740. W. Heiland, H.G. Schaffler and E. Taglauer, Surf. Sci. 35 (1973) 381. T.M. Buck, Y.-S. Chen and G.H. Wheatley, Surf Sci. 47 (1975) 244.
[21] R.L. Erickson and D.P. Smith, Phys. Rev. Lett. 34 (1975) 297. [22] T.W. Rusch and R.L. Erickson, J. Vat. Sci. Technol. 13 (1976) 374. [23] T.M. Buck, G.H. Wheatley, G.L. Miller, D.A.H. Robinson and Y.-S. Chen, Nucl. Instr. and Meth. 149 (1978) 591. [24] S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Beers, Surf. Sci. 99 (1980) 631; 99 (1980) 652. [25] S.H. Overbury, Surf. Sci. 112 (1981) 23. [26] H. Niehaus and E. Preuss, Surf. Sci. 119 (1982) 349. [26] H.F. Helbig, M.W. Linder, G.A. Morris and S.A. Steward, Surface Sci. 114 (1982) 251. [27] R.J. MacDonald and D.J. O’Connor, Surf. Sci. 124 (1983) 423. [29] A.G. de Wit, J.M. van Zoest and J.M. Fluit, Surf. Sci. 126 (1983) 589. [30] R.J. MacDonald and D.J. O’Connor, Surf. Sci. 124 (1983) 433. [31] S.H. Overbury, P.C. Stair and P.A. Agron, Surf. Sci. 125 (1983) 377. [32] J.A. Yarmoff and R.S. Williams, Surf. Sci. 127 (1983) 461. [33] R.P.N. Bronckers and A.G.J. de Wit, Surf. Sci. 104 (1981) 384. [34] S.H. Overbury, W. Heiland, D.M. Zehner, S. Datz and R.S. Thoe, Surf. Sci. 109 (1981) 239. [35] W. Englert, E. taglauer and W. Heiland, Surf. Sci. 117 (1982) 124.
383
surface structure analysis
[36] T. von dem Hagen, M. Hou and E. Bauer, Surf. Sci. 117 (1983) 134. [37] S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Boers, Surf. Sci. 100 (1980) 315. [38] T.M. Buck, G.H. Wheatley and L.K. Verheij, Surf. Sci. 90 (1979) 635. [39] S.B. Luitjens, A.J. Algra, E.P.Th.M. Suurmeijer and A.L. Boers, J. Phys. E:4 (1980) 663. [40] I.K. Robinson, Phys. Rev. Iett. 50 (1983) 1145. [41] M. Aono, Y. Hou, C. Oshima and Y. Ishizawa, Phys. Rev. Lett. 49 (1982) 49. [42] D.J. Chadi, Phys. Rev. Lett. 43 (1979) 43; J. Vat. Sci. Technol. 16 (1979) 1290. [43] M.T. Yin and M.L. Cohen, Phys. Rev. B24 (1981) 2303. [44] W. Heiland and E. Taglauer, J. Vat. Sci. Technol. 9 (1972) 620. [45] J.A. van den Berg, L.K. Verheij and D.G. Armour, Surf. Sci. 91 (1980) 218. [46] H.H. Brongersma and J.B. Theeten, Surf. Sci. 54 (1976) 519. [47] D.J. Godfrey and D.P. Woodruff, surf. Sci. 95 (1979) 76. [48] W. Heiland, F. Iberl, E. Taglauer and D. Mentzel, Surf. Sci. 53 (1975) 383. [49] H. Niehaus and E. Bauer, Surf. Sci. 47 (1975) 222. [50] S. Prigge, H. Niehaus and E. Bauer, Surf. Sci. 65 (1977) 141. [51] A.G.J. de Wit, R.P.N. Bronckers, Th.M. Hupkens and J.M. Fluit, Surf. Sci. 82 (1979) 177. [52] D.J. Godfrey and D.P. Woodruff, Surf. Sci. 105 (1981) 459. [53] M. Saitoh, F. Shoji, K. Oura and T. Hanawa, Jpn J. Appl. Phys. 19 (1980) L421. [54[ J. Onsgaad, W. Heiland and E. Taglauer, Surf. Sci. 99 (1980) 112. [55] C. Oshima, M. Aono, T. Tanaka, S. Kawai, S. Zaima and Y. Shibata, Surf. Sci. 102 (1981) 312. [56] A.J. Algra, S.B. Luitjens, E.P.Th.M. Suurmeijer and A.L. Boers, Surf. Sci. 100 (1980) 329. [57] M. Aono, Y. Hou, R. Souda, C. Oshima, S. Otani and Y. Ishizawa, Phys. Rev. Lett. 50 (1983) 1293. [58] M. Kaminsky, R. Nielsen and P. Zschack, J. Vat. Sci. Technol. 20 (1982) 1304. [59] R. Souda, M. Aono, C. Oshima, S. Otani and Y. Ishizawa, Surf. Sci. 128 (1983) L236.
VI. LOW ENERGY
SURFACE
INTERACTIONS
Nuclear
QUANTITATIVE
SURFACE STRUCTURE
Instruments
ANALYSIS
and Methods
in Physics Research B2 (1984) 374-383 North-Holland. Amsterdam
BY LOW-ENERGY
ION SCATT’ERING
M. AONO N~i~o~~i Znsiilute
fur Research in inorganic Mate~iais, Namiki i -I, Sukuru, Niihari, Z~~aki 305, Japan
A review is presented of low-energy ion scattering spectroscopy (1%) as a tool for surface atomic structure analysis. Especially, quantitative surface atomic structure analysis by 1% is highlighted. An important difference between ISS and Rutherford backscattering spectroscopy (RBS), a specialization of ISS for quantitative surface atomic structure analysis, and a general feature of the shadow cone in the energy range of ISS are first discussed as a basis for the descriptions of particular examples of ISS studies which follow. The examples are concerned with the atomic structure analysis of clean surfaces, surfaces with adsorbates, and surface defects.
1. infusion
2. ISS in comparison with RBS
Ion scattering spectroscopy is one of the most powerful techniques for surface atomic structure analysis because of its simple principles, In this technique, a “classical” particle - an ion - is used, and simple “classical” concepts - shadowing and blocking - are utilized. As a result, ion scattering spectroscopy provides direct information on the relative positions of atoms in a surface region. Ion scattering spectroscopy is classified into low-energy ion scattering spectroscopy (ISS) [l-4] and high-energy ion scattering spectroscopy or Rutherford backscattering spectroscopy (RBS) [5]. The apparent difference between ISS and RBS is only that the ion energy is ISS in - keV, while that in RBS is - MeV. However, this quantitative difference gives rise to an important qualitative difference between ISS and RBS. In this paper, a review of ISS will be presented in such a manner that the excellent ability of ISS as a tool for surface atomic structure analysis is emphasized. Especially, quantitative surface atomic structure analysis by ISS will be highlighted. Surface chemical composition analysis by ISS will not be discussed in this paper. It is also not the purpose of this paper to present a comprehensive list of the literature on ISS; for that purpose, we can refer to more comprehensive reviews 12-41. In the next section, ISS and RBS will be compared with each other in order to clarify the difference between these almost independently advancing ion scattering techniques. In sect. 3, a specialization of ISS, which makes quantitative surface atomic structure analysis possible, will be described. In sect. 4, a general feature of the shadow cone will be discussed. Examples of actual KS studies will be shown and discussed in sect. 5.
ISS is characterized by the thick shadow cone (radius - 1 A) and very high ion neutralization probability (comparable to 100% for inert-gas ions) which result in extremely high surface sensitivity, while RBS is characterized by the very thin shadow cone (radius - 0.1 A) and negligible ion neutralization probability. The most important difference between ISS and RBS in connection with surface atomic structure analysis is schematically depicted in fig. 1. In both ISS [fig. 1 (a)] and RBS [fig. l(b)], when the ion incidence angle measured from the surface, cy, is decreased from 90”, atom A in the first layer shadows atom B in the second layer. As (Y is decreased more, in RBS, atom C in the first
0168-583X/84/$03.00 0 Elsevier Science Publishers (Noah-Holland Physics Publishing Division)
B.V.
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0
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0
la)
0
0
ISS
-MeV
L
0
0
0
Ibl
D
0
0
RBS
Fig. 1. Schematic figure depicting an important difference between (a) ISS and (b) RBS in connection with surface atomic structure analysis.
315
M. Aono / Quantitative surface structure analysis layer shadows atom D in the third layer. This shadowing effect C -+ D cannot be observed in ISS since atom D in the third layer is “invisible” in surface-sensitive ISS. Instead of this, in ISS, atom C in the first layer shadows atom E in the same first layer because of the thick shadow cone. This shadowing effect C --) E cannot be observed in RBS since such a shadowing effect occurs only at unpractically small a because of the very thin shadow cone. These arguments for the shadowing effect also apply for the blocking effect. In this way, it is found that RBS determines surface atomic positions relative to subsurface atomic positions (it is hard to determine the relative positions of surface atoms directly), while ISS determines the relative positions of surface atoms ~~depende~t~y of surface atomic positions (it is impossible to obtain information on subsurface atomic positions).
3. A specialization of IS!3 ICISS In surface atomic structure analysis by ISS, the shadowing and blocking effects are both useful. However, the coexistence of the two effects is sometimes troublesome, especially in the analysis of complicated surface atomic structures, since it is not easy to assign observed various intensity drops to the shadowing or blocking effect without ambiguity. It is therefore desired to make experiments in such a condition that only one of the two effects works. If we take the experimental
tb) 1SS
scattering angle 0, close to 180”, the blocking effect rarely occurs since ions backscattered along such trajectories that are close to their incident trajectories are selectively observed. This specialization of ISS is called impact-collision ion scattering spectroscopy (ICISS) [6,7]. Because of its simplicity, ICISS can analyze the surface atomic structure more straightforwardly than ordinary ISS in which 8, is arbitrary; ICISS shows its ability in the analysis of complicated surface atomic structures (see sect. 5.1). A more important advantage of ICISS is that because of 0, = 180°, those scattered ions which have made impact collisions (almost head-on collisions) with atoms are selectively observed. Namely, ICISS “sees” the close vicinity of the center of each atom. Because of this, as illustrated in fig. 2(a), at the onset of a shadowing effect in which atom A begins to shadow atom B, the edge of the shadow cone of atom A necessarily passes the center of atom B to a good approximation. To recognize this characteristic of ICISS is a bre~throu~ since it clears the way to analyze the surface atomic structure quantitatively (see sect. 5.1). Note that in ordinary ISS in which 6, is arbitrary, there is no special relation between the edge of the shadow cone of atom A and the position of atom B even at the onset of the shadowing effect as illustrated in fig. 2(b).
4. Universal shadow cone expressions The shape of the shadow cone can be determined experimentally by ICISS described in the previous section. Consider a chain of atoms at a crystal surface shown in fig. 3. When the ion incidence angle measured from the surface, ru, is decreased, the intensity of ions scattered from the atomic chain suddenly drops to zero because of a shadowing effect. Fig. 3 illustrates the situation just at the onset of the shadowing effect in the ICISS condition; for simplicity, the incident and scattering trajectories of each ion observed in the ICISS condition are drawn as if they were coincident with each other [actually, they are slightly different from each
Fig. 2. Schematic figure depicting the difference between (a) ICISS (8, = 180”) and (b) ordinary ISS (8, + 1800). In ICISS, the center of each atom is “seen” in contrast to ordinary ISS, and hence at the onset of the shadowing effect, the edge of the shadow cone of a shadowing atom A passes the center of a concealed atom B to a good approximation. Because of this, ICISS can analyze the surface atomic structure quantitatively.
Fig. 3. Schematic figure illustrating how the shape of the shadow cone can be determined quantitatively by ICISS. Ref.
[71. VI. LOW ENERGY
SURFACE
INTERACTIONS
316
M. Aono / Quantitative surface structure analysis lb1 liC10011-1x1
loI TiCIlllI-1x1
He+
R,(A) 1
-.i
,*--
------\.____
Fig. 6. Shape of the shadow cone of Ti for He+ of 1.0 keV determined by ICES. Ref. [7]. Fig. 4. Atomic structures of the (111) and (001) surfaces of TIC with a NaCl structure. Ref. [7].
other even if 8, is taken just at 180” as we can understand from fig. 2(a)]. At the onset of the shadowing effect in the ICISS condition, the edge of the shadow cone of each atom passes the center of its neighboring atom to a good approximation as in fig. 3 (see section 3). Therefore, by measuring the critical value of a corresponding to the onset of the shadowing effect, ac, the radius of the shadow cone at the distance L = d cos a, is found to be R, = d sin a,, where d is the atomic distance; see fig. 3. By taking other azimuths (if necessary, by taking other surfaces with different indices), we have other atomic chains with different d’s, and hence we can determine Rs’s at different L’s. The shape of the shadow cone of Ti for He+ at E, (primary ion energy) = 1 keV has been determined [7] by this method using the (111) and (001) surfaces of
ICISS
INTENSITY
OF Ti
-
c
tic+ (E,=l.O
TiC(lllI; TiC(0011
10
kcV1
ri2il
; I1101
20
a
(deg)
Fig. 5. Intensity of the KISS spectral peak due to surface titanium atoms of Tic plotted against the ion incidence angle measured from the surface, a. Ref. [7].
Tic. The structure of the surfaces are shown in figs. 4(a) and (b) [8,9]; since either surface has a 1 x 1 structure, the Ti-Ti distances indicated in the figures are readily calculated from the bulk lattice constant. Fig. 5 shows selected results of the KISS experiments for the surfaces; the intensity of He+ ions scattered from the surface titanium atoms is plotted as a function of a. As a decreases, the intensity suddenly drops to zero at a critical angle a, which depends on the Ti-Ti distance d (3.06 and 5.30 A for the right and left curves, respectively) because of the shadowing effects; an increase in the intensity just before the shadowing effect begins is due to the focusing effect (each atom is bathed in concentrated ion flux just outside the shadow cone). Using a,‘s determined from fig. 5 (and other data not shown), we can draw the shape of the shadow cone of Ti for 1 keV He+ as shown in fig. 6 by dots. Shown in figs. 7(a)-(d) are possible trajectories of He+ (E, = 1 keV) scattered by Ti calculated for Bohr, Born-Mayer, and Thomas-Fermi potentials (for the Thomas-Fermi potential, Sommerfeld and Moliere approximations were used). The shadow cones for the Bohr and Born-Mayer potentials, figs. 7(a) and (b), are too thin and thick, respectively, compared with the experimental result (fig. 6) while the shadow cone for the Thomas-Fermi potential, fig. 7(c) or (d) (the Sommerfeld and Moliere approximations gives essentially the same result), agrees well with the experimental result. This indicates that the Thomas-Fermi potential is a good approximation to the ion-atom interaction potential in the energy range of ISS, although further studies are necessary for many other ion-atom combinations. Recently, “universal expressions” [lo-121 for the shadow cone, which agree with the experimental result mentioned above, have been calculated. From the expressions, the shadow cone radius R, at any distance L can be estimated immediately for any ion-atom combination and ion energy E, (the expressions in ref. 10 do not cover a region corresponding to usual ISS experimental conditions, but those in ref. 12 cover the region). The “universal shadow cone expressions” are very useful to help plan ISS experiments and analyze the results.
M. Aono / Quantitative ki)
BORH
ICI THOMAS-FERMI-SOMMERFELD
POTENTIAL
lb)
I
:
\
BORN-MAYER
POTENTIAL
\
_1 I
371
surface structure analysis
\
POTENTIAL
\
\
Id)
THOMAS-FERMI-MOLIiRE
POTENTIAL
Fig. 7. Trajectories of He+ ions (primary energy E, = 1.0 keV) scattered by Ti calculated by assuming (a) Bohr, (b) Born-Mayer, (c) Thomas-Fermi-Sommerfeld, and (d) Thomas-Fermi-Moliere potentials. Ref. [7].
5. Examples of 1% studies 5. I. Clean surfaces
The simplest quantitative surface atomic structure analysis is to measure the magnitude of “rumpling relaxation” of a compound surface, i.e., opposite (inward and outward) displacements of two kinds of constituent surface atoms. The magnitude of the rumpling relaxation of the TiC(OO1) surface has been measured [13] by ICISS (1 keV He+) in the following way. Fig. 8(a) shows the intensity of He+ ions scattered from surface carbon atoms at the TiC(OO1) surface in the ICISS condition as a function of the ion incidence angle measured from the surface, cu; the measurement was made in the (100) azimuth in which surface titanium and carbon atoms are arranged alternately [see fig. 4(b)]. As a decreases, the intensity suddenly drops to zero because every surface carbon atom is concealed by the shadow cone of its neighboring surface titanium atom as shown in fig. 8(b); an increase in the intensity just before the shadowing effect begins is due to the focusing effect. From fig. 8(a), the shadowing critical angle is found to be OL,(C; Ti) = 24” [in the notation a,(A; B), A and B are shadowing and concealed atoms,
respectively]. Since we already know the shape of the shadow cone of Ti for 1 keV He+ as given in fig. 6, we can draw fig. 8(b), which shows the situation just at the onset of the shadowing effect (a, = 24’); broken curves show the shadow cone of the surface titanium atom drawn along the two opposite (100) azimuths. In the ICISS condition, the edge of the shadow cone of the surface titanium atom passes the center of the surface carbon atom to a good approximation at the onset of the shadowing effect. Therefore, we can readily determine the position of the surface carbon atom as the intersection of the edges of the two shadow cones drawn in fig. 8(b). The position of the surface carbon atom thus determined almost coincides with its idea1 position. However, we should estimate the error margin of the result. The biggest source error is the ambiguity in determining a, from the intensity curve in fig. 8(a). In the above discussion, a, was estimated to be an (Y corresponding to the half-maximum intensity. However, this is an underestimate of a, for the following reason. The reason why the observed intensity drop due to the shadowing effect is not very sharp is that both the titanium (shadowing) and carbon (concealed) atoms are vibrating around their equilibrium positions. Namely, the obVI. LOW ENERGY SURFACE INTERACTIONS
M. Aono / Quantitative surface structure analysis
378
r
I
I
ICISS
I
INTENSITY
OF
I Cl
I
C
FOR
I
TiC(OOl1
AZIMUTH kr’f
He*
,
1
i
I
I
/
I
I
IO
20
30
40
50
60
70
60
a
Idegl
(b) TiClOO
Fig. 8. (a) Intensity of He+ ions scattered from carbon atoms at the TiC(OO1) surface [see fig. 4(b)] as a function of the ion incidence angle measured from the surface, a; the measurement was made in the (100) azimuth in which surface carbon and titanium atoms are arranged alternately as shown in (b). (b) The position of the surface carbon atom determined from the experimental result shown in (a) (marked by +). Ref. f13].
served
intensity
distribution
curve
function.
is smoothed
by some
gaussian
Even so, if the intensity enhancement due to the focusing effect were absent, an (Y corresponding to the half-maximum intensity would be equal to the correct (xc. Actually, however, because of the intensity enhancement which occurs on the high-a side of a,, such an estimation leads to an underestimate of (Y,. According to computer simulations in various cases 1141, we can state the following empirical rule: Roughly speaking, an a corresponding to the 70%maximum intensity is approximately equal to the correct a,. This rule is rough but surely improves the accuracy of analysis. In the case of fig. 8(a), aI, estimated according to this rule is 26’ as indicated by a vertical broken line. The position of the surface carbon atom determined from this a, is - 0.1 A below the level of the surface titanium atom. It is thus found that the TiC(OO1) surface exhibits a very small if any rumpling relaxation (5 0.1 A).
When the observed intensity drop due to the shadowing effect is fairly sharp as in fig. 5, a, estimated at the half-m~mum intensity is practicaily satisfactory. A number of ISS studies have been made for surfaces of simple metals [15-341 but the main interest of most of the studies has not been to analyze the atomic structure of the surfaces but to study basic physics of low-energy ion scattering such as multiple scattering [17,19,26,2’7,31,32], inelastic scattering f15,16,30], and ion-surface electron exchange [18,20-25,28,29] by using the surfaces. An example of atomic structure analysis of simplemetal surfaces by ISS is a recent study [34] of the Au(llO)-2 x 1 surface. In this study, a beam of K’ ions was used. Alkali ions such as K+ are characterized by their low ionization potentials, and hence their neutralization probability is very low. Therefore, the use of alkali ions [18,25,26,31,35,36] is, in effect, similar to using a time-of-flight analyzer [20,23,37,38] or a stripping (reionization) cell [l&39] in inert-gas-ion ISS; the latter two techniques detect not only ions but “neutralized ions”. Namely, by the use of alkali ions, multiple scattering has significant contribution to measured spectra. The Au(ll0) surface changes from a 2 x 1 structure at room temperature to a 1 X 1 structure at elevated temperatures. Plotted in figs. 9(a) and (b) by dots are ISS spectra for the 1 x 1 (900 K) and 2 X 1 (300 K) surfaces, respectively; @ denotes the azimuthal angle measured from [liO]. In every spectrum, the low- and high-energy peaks are due to single and multiple scatterings, respectively. The histograms in figs. 9(a) and (b) show the results of computer simulations for a perfect bulk-termination model (1 X 1 structure) and an unrelaxed missing-row model (2 X 1 structure), respectively. In both the figures, agreement between the experimental result and the computer simulation is unsatisfactory, especially at + = 27.5’. From these results, it is concluded that (1) the 1 x 1 surface at elevated temperatures may not be in the perfect bulk-termination structure, and (2) for the 2 x 1 surface at room temperature, the unrelaxed missing-row model is not a very probable model (a distorted hexagonal overlayer model is also rejected). According to a recent glancing-incidence X-ray diffraction experiment [40], a relaxed missing-row model is derived for the 2 x 1 structure. As for covalent semiconductor surfaces, the Si(OOl)-2 X 1 surface, for example, has been studied 1411 by ICISS. Fig. 10(a) shows the intensity of He+ ions scattered from the Si(OOl)-2 X 1 surface in the ICISS condition as a function of the polar angle measured from the surface, a, and the azimuthal angle measured from [ 1 IO], +, of the ion incidence direction; the primary energy of He’ ions was 1 keV. The intensity variations observed in fig. 10(a) are due to various shadowing effects (accompanied with a focusing effect) and anisotropic ion neutralization probability, although the latter
M. Aono / Quantitative
surface structure analysis
_b
60
a
319
MISSING ROW STRUCTURE
BULK TERMINATION STRUCTURE
T=300K
r
360
390
420
450 460 ENERGY
5fO (eV)
540
570
600
360
390
420
450
490
ENERGY
510
540
570
600
(eV)
Fig. 9. ISS spectra (K+ ions) for the (a) 1 X 1 (900 K) and (b) 2 X 1 (300 K) structures of the Au(ll0) surface (dots); $I denotes the azimuthal angle measured from the [liO] direction. The histograms show the results of theoretical computer simulations. Ref. [34].
is minor; no blocking effect has to be considered because of the ICES condition. The intensity curve for a grazing angle of a = 4’ in fig. 10(a) is magnified in part in fig. 10(b). the drop in intensity observed at + = O” ([llo]), 32” (azimuth S), and 58” (azimuth s’) are unambiguously identified to be due to shadowing effects since an intensity rise due to a focusing effect is clearly observed on either side of each of the intensity drops as indicated by broken curves. Fig. 10(b) contains the following information: (i) Shadowing effects are observed in [llO] and in azimuths S and S’ which deviate from [OlO] by 6 = 13”, and (ii) the magnitudes of the intensity drops due to the shadowing effects are AZ([llO]) = 63% and AZ(S or S’) = 15%. On the basis of these experimental facts, we can infer the structure of the Si(OOl)-2 x 1 surface. The experimental fact (i) rejects the unreconstructed 1 X 1 structure shown in fig. 11(a) as expected; shadowing effects for this structure should occur in [llO] and [OlO] (indicated by arrows) in
contradiction to the experimental fact (i). The experimental fact (i) is decisive evidence for that the surface is “dimerized” like fig. 11(b); in this case, shadowing effects occur in [llO] and in azimuths which deviate from [OlO] by some angle 6 (indicated by arrows) being consistent with the experimental fact (i). By examining fig. 11(b) in detail, we find that this dimer structure is also consistent with the experimental fact (ii). The fact that 8 in fig. 11(b) is 13” [experimental fact (i)] indicates that the intradimer atomic distance parallel to to the surface is 2.4 A ( f 0.1 A). Corresponding theoretical values reported, 2.3 A [42] and 2.2 A [43], agree with this experimental value.
5.2. Surfaces
with adrorbates
A number of ISS studies have been reported for surfaces with adsorbates, e.g., O/Ni(llO) [44,45], VI. LOW ENERGY
SURFACE
INTERACTIONS
M. Aono / Quantitative
surface structure analysis 14)
UNRECONSTRCTED
Ilxl)
STRUCTURE
I
BOUNDARY
rl l
l
l
IO101
CIIOI
15
0
15 30 @ ldeg)
45
60
Fig. 10. (a) Intensity of He+ ions scattered from the Si(OOl)-2 X 1 surface in the ICISS condition as a function of polar angle a (measured from the surface) and azimuthal angle $I (measured from [IlO]) of the ion incidence direction. (b) The intensity curve for a = 4’ in (a) magnified in part. Ref. [41].
O/NiWW
O/Cu(llO) O,CO/Si(lll)
O/Ag(llO) [48], O/W(llO) [51], O/Cu(lOO) 1521, Ag/Si(lll) [54], and O/TiC(lOO) [55].
[46,47],
l
l
I
LAYER
6
016 o SECOND
Jf
l 0
LAYER
Fig. 11. (a) Unreconstructed 1 X 1 and (b) asymmetrically marked 2 X 1 structures of the Si(OO1) surface. Ref. [41].
_r
r
FIRST
I 0
[SO], 1531,
di-
As an example, ISS spectra measured for the O/Ni(llO)-2 X 1 surface are shown in figs. 12(a) and (b) 1441. In the figures, t9( = 60”) indicates the scattering angle, $I shows the azimuthal angle measured from the [Ol] direction, and # denotes the ion incidence angle measured from the surface. In every spectrum, the highand low-energy peaks are due to nickel and oxygen atoms, respectively. Fig. 12(c) shows the structure of the O/Ni(llO)-2 X 1 surface inferred from the experimental results of figs. 12(a) and (b), where open and hatched circles show nickel atoms in the first and second layers, respectively, and cross-hatched circles show adsorbed oxygen atoms. As fig. 12(a) shows, when I/J is decreased in the [Ol] direction (+ = O’), the Ni/O intensity ratio drops by about a factor of two between # = 22.5” and 4 = 10”. This is consistent with the structure shown in fig. 12(c); that is, the adsorbed oxygen atoms (protruding - 0.4-0.8 A) partially shadow the nickel atoms in the first layer in the [Ol] direction. The reason why the Ni/O intensity ratio in the [Ol] direction (+ = 0”) [fig. 12(a)] is larger than that in the [lo] direction (9 = 90°) [fig. 12(b)] by about a factor three is that the nickel atoms in the second layer significantly contribute to ion scattering in the [Ol] direction.
381
IU. Aono / Quantitatiue surface structure analysis
I
(a)
KISS
t I
ICI
INTENSITY He+
[lo01
AZIMUTH
I
u,
E/E,
Ti FOR
TiCIOOlI-1x1
lb)+O,(-3L) \
I?
05
OF
IkeV
-
lb)
Ial
ANNEALED
AT -1600°C
*t 7.;e_____
-
I 0
&iTi;Cl=22.1°
10
I
I
20
30
a
E/E,,-
Fig. 12. (a) and (b), ISS spectra for the O/Ni(llO)-2X1 surface. (c), A structural model of the surface which is consistent with (a) and (b). Ref. [44].
5.3. Surface
defects
The influence of various surface defects on the ISS spectrum has been discussed extensively [37,56]. In what follows, a recent study [57], in which the structure, concentration, and chemical activity of surface atomic vacancies at the TiC(OO1) surface have been studied by ICISS, will be shown as an example. The TiC(OO1) surface, which has a structure as shown in fig. 4(b), was first annealed at a high temperature - 1600°C in order to remove surface defects. In fig. 13, curve (a) shows the intensity of He+ ions scattered from titanium atoms at the TiC(OO1) surface as a function of
I
LO
c
ldeg)
Fig. 13. Intensity of He+ ions scattered from titanium atoms at the TiC(OO1) surface [see fig. 4(b)] in the KISS condition as a function of the ion incidence angle measured from the surface, a; the three curves correspond to different surface conditions (see text). The measurements were made in the [lOOIazimuth in which surface titanium and carbon atoms are arranged alternately. Ref. [57].
the ion incidence angle measured from the surface, a; the measurement was made in the ICISS condition in the [lOO] azimuth in which surface titanium and carbon atoms are arranged alternately [see fig. 4(b)]. The drop in the intensity observed in the neighborhood of a, = 22” is due to a shadowing effect illustrated in fig. 14(a); all the surface titanium atoms “l”, “2”, . . . are concealed by the shadow cones of their neighboring surface carbon atoms “l”‘, “2”‘, . . . . It is known for Tic that carbon atoms are preferentially sputtered by energetic light ions [58]. Using this characteristic, carbon vacancies were created at the TiC(OO1) surface. That is, the surface was bombarded by He+ ions of 1 keV (- 0.3 PA/cm*) for 10 min. By this ion bombardment, a 1 X 1 LEED pattern, which had been observed initially, was destroyed. The sample was then annealed at various temperatures for - 15 s. When the annealing temperature TA was higher than - lOOO”C, the surface exhibited a clear 1 x 1 LEED pattern. In fig. 13, curve (b) shows an experimental result measured after the annealing at T, = 1400°C. In this curve (b), a shadowing effect observed in the neighborhood of (Y,= 22” results from the same mechanism as the shadowing effect observed in curve (a). However, the intensity does not drop to zero as a result of this shadowing effect but shows a shoulder indicated by hatching. This clearly indicates that surface VI. LOW ENERGY
SURFACE
INTERACTIONS
382
M. Aono / Quantitaiive surface structure analysis TiClOOll-1x1
the atoms were “frozen” at their displaced positions. Therefore, the thermal vibration of surface atoms can be regarded as one kind of surface defect. The thermal vibration amplitude of surface atoms has been measured by ICISS for the TiC(lll) surface [59].
6. Summary
Fig. 14. Two different shadowing effects in the [lOO] azimuth of the TiC(OO1) surface; in every case, the situation at the shadowing critical angle in the ICISS condition is shown. Ref. [57].
carbon vacancies were present as traces of the preferential sputtering of carbon atoms; namely, as illustrated in fig. 14(a), if there is a surface carbon vacancy shown by a square, its neighboring surface titanium atom “3” cannot be concealed. As (Ydecreases more, this “surviving” surface titanium atom “3” is after all concealed by the shadow cone of the next-nearest surface titanium atom “2” as illustrated in fig. 14(b). From the critical angle of the latter shadowing effect, a, = 15.4’, and the shape of the shadow cone of Ti for 1 keV He+ shown in fig. 6, we find that the surface titanium atoms adjacent to the surface carbon vacancy are not displaced within an error margin of - kO.1 A; this may be related to the fact that carbon atoms in Tic occupy small interstitial spaces of almost closely packed titanium atoms. From the relative intensity of the shoulder in curve (b) of fig. 13, we can estimate the concentration of the surface carbon vacancies, although the estimation is not necessarily simple since the relative intensity of the shoulder depends on the primary energy of He+ ions. In the particular case of curve (b), the concentration of the surface carbon vacancies is estimated to be (10 + 2)%. It is known that the TiC(OO1) surface (without the surface carbon vacancies) is considerably inert. In fact, to have monolayer adsorption of oxygen, the surface has to be exposed to oxygen more than - 1000 L (L = 10e6 Torr . s) [9]. In order to analyze the chemical activity of the surface carbon vacancies, the surface corresponding to curve (b) in fig. 13 was exposed to various amounts of oxygen, and similar measurements were made. Curve (c) in fig. 13 shows the experimental result after a small oxygen exposure of - 3 L. The shoulder observed in curve (b) has dramatically disappeared. This indicates that the surface carbon vacancies capture oxygen atoms into the vacancy holes exhibiting a very high chemical activity. In low-energy ion scattering, the collision time is of the order of 10-‘5-10-‘6 s, while the period of atomic thermal vibration is of order - lo-l3 s. In other words, ISS “ views” the thermal vibration of surface atoms as if
ISS is one of the most powerful techniques for surface atomic structure analysis. The accuracy of 1% in quantitative surface atomic structure analysis, - + 0.1 A, is not very good, but ISS is a reliable technique because of its simple principles. A serious disadvantage of ISS is damage to the surface caused by ion bombardment; when we use inert-gas ions and detect only ions using an electrostatic or magnetic analyzer, we must use a very high ion flux because of the high neutralization probability of the ions. This problem appears to be solved by the use of alkali ions whose neutralization probability is very low, a time-of-flight analyzer which can detect both ions and “neutralized ions”, or a stripping cell which reionizes “neutralized ions”. However, these methods seem to abandon the most important advantage of ISS: by utilizing high ion neutralization probability, troublesome multiple scattering effects are reduced, and hence a simple “classical” concept, i.e., the shadowing effect, is effectively utilized. In order to reduce the damage to the surface, it is hoped to develop such an energy analyzer that can measure the whole of an ISS spectrum or a considerable part of it at the same time without scanning energy. Such a technique would be able to shorten the time of measurement by more than one order of magnitude.
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M. Aono / Quantitative [ll] [12] [13]
[14] [15] [16] 1171 [18] [19] [20]
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383
surface structure analysis
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VI. LOW ENERGY
SURFACE
INTERACTIONS