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Surface structure analysis of oxidized Fe(100) by low energy ion scattering
179
Surface Science 182 (1987) 179-199 North-Holland, Amsterdam
SURFACE STRUCTURE ANALYSIS OF OXIDIZED BY LOW ENERGY ION SCATTERING J.M. VAN ZOEST, Qsisch
Lahoratorium,
T.J. VINK
J.M. FLUIT Rijksuniuersiteit
Utrecht, Princetonplein
5, 3584 CC Utrecht. The Netherlrrnds
* and B.A. VAN HASSEL
Vun ‘t Hoff Lcrhorutorium ooor Fysische en Colloidchemle, The Netherlunds Received
Fe(100)
12 June 1986: accepted
for publication
Paduulaan
31 October
8, 3584 CH Utrecht.
1986
The surface structures of differently prepared oxide layers on Fe(100) have been analysed using low energy ion scattering with time-of-flight analysis. The intensity of scattered 5 keV Nc+ ions and neutrals was measured as a function of elevation and azimuth of the incoming beam at a constant scattering angle. producing photograms of clean as well as oxygen covered Fe(100) surfaces. The formation of the Fe(lOO) p(1 X1)-0 surface structure at full monolayer coverage was confirmed by measuring azimuthal distributions of reflected Ne + ions and neutrals at specular reflection and a scattering angle of 45O. The oxygen atoms are found to be located in the fourfold symmetrical hollows of the substrate surface. From the 0.. recoil intensity as a function of the elevation angle of incidence at specular reflection it was calculated that the oxygen atoms are located at a distance of 0.56 & 0.05 A above the first substrate layer of iron atoms. A disordered oxide layer grown on Fe(100) at room temperature reconstructs at elevated temperatures leaving a well-ordered monolayer of oxygen on top of the surface. The surface structure of this oxygen monolayer appears to be identical with respect to the surface properties found for a single monolayer of oxygen on Fe(lOO).
1. Introduction The oxidation of Fe(lOO) has been studied extensively with a large variety of techniques. This paper deals with the influence of temperature on the surface structure of thin oxide films on Fe(lOO), while most investigations only concentrate on oxide layers formed at room temperature. For this purpose low energy ion scattering (LEIS) will appear to be eminently suitable. LEIS with time-of-flight analysis was used by Marchut et al. [I], for the analysis of a clean unreconstructed Fe(lOO) surface. They demonstrate a relatively simple technique for preliminary analysis of the surface structure. The interaction of a * To whom correspondence
should
be addressed
0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
180
J.M. wn Zoest et ul. / LEIS surface structure una!vsi.r of Fe(lOO)-0
clean Fe(lOO) surface with gasphase oxygen was studied by Legg et al. [2], using LEED and AES. A LEED analysis of the atomic structure of the first monolayer of oxygen on Fe(lOO) revealed that the oxygen atoms are located inside the fourfold symmetrical hollows of the substrate surface, with the first substrate interlayer spacing expanded by 7.5% with respect to the bulk. Earlier work by the same authors [3] suggests that the clean Fe(lOO) surface is not reconstructed. This study will focus on the surface structure determination of a set of differently prepared oxide layers on Fe(lOO). A preceding analysis by Vink et al. (41 concerning the preparation and subsequent characterization of these oxide surfaces is often used as a reference in the present paper. It seems therefore relevant to give a short abstract of its conclusions. From ellipsometry, AES and LEED measurements it was concluded that an oxide layer on Fe(lOO) grows to a limited thickness at room temperature and rearranges at crystal temperatures above 573 K. The oxide layer formed at room temperature appeared to be disordered, whereas the annealed surface exhibited a LEED pattern of p(1 x 1) symmetry. This drastic change in surface composition and surface structure is further examined in this paper, using LEIS. It will be shown that after reconstruction a well-ordered monolayer of chemisorbed oxygen is present on the surface, while in the subsurface region an incorporated oxide phase is formed [4].
2. Experimental 2.1. The apparatus The LEIS measurements were performed in a Varian UHV system, described previously by Bronckers [5], of which the experimental facilities were improved by application of mass selection of the primary beam and detection of neutral particles. Only a short description is given here. The base pressure of about 6 X lo-lo Torr is reached by means of a turbomolecular pump and a liquid nitrogen trap with a Ti sublimation unit. The pressure increases to about 1 X 10m9 Torr due to the neon projectile gas. The chamber containing the ion gun is also pumped by a turbomolecular pump. The ion gun consists of an electron impact source, a lens system, deflection plates, a Wien filter for mass selection followed by another lens system, and a final aperture of one millimeter diameter. For the experiments described here, a 5 keV “Ne+ beam was chosen with an ion flux of 4 X lo-* A m -2 at the target. The iron crystal was mounted in a target manipulator [6]. With this manipulator the elevation angle of incidence J, could be varied between 0” and 350“ (accuracy O.l”). The azimuthal angle of incidence + was variable over 330” (accuracy 0.2”). For the analysis and detection of scattered
J.M. uan Zoest et al. / LEIS surjace structure un&sis
of Fe(lOO)-0
181
ions a double flat-plate energy analyzer (ESA) was used. The analyzer can be rotated with an accuracy of 2” in the plane defined by the incoming beam and the surface normal in the range of scattering angles 9 from 0” to 135”. The energy resolution of the ESA is about 4%. An extra channeltron was added to the analyzer in order to count neutralized ions, which are scattered by the surface. Energy selection of neutrals was achieved by means of a time-of-flight (TOF) technique. The resolution of the TOF spectrometer is about 8%. This is mainly because of the short flight path (I = 0.222 m), which is limited by the dimensions of the UHV chamber. The crystal was heated by placing an infrared heater in front of it. The Fe(lOO) crystal had been used before for experiments with ellipsometry, AES and LEED in another UHV apparatus [7]. It was then depleted from bulk impurities by cycles of heating and 400 eV Ar+ ion bombardment. Before the LEIS experiments were performed the crystal was polished again. In vacuum it was cleaned by cycles of heating above 450 K during 10 min and 5 keV Net ion bombardment at a small elevation angle, while the crystal was continuously rotated over the azimuthal angle. The crystal structure of the clean Fe(lOO) surface was monitored by measuring azimuthal and polar distributions of scattered Net ions. In this vacuum chamber no other facilities were present for measuring the absolute oxygen coverage, e.g. AES and LEED. 2.2. Low energy ion scattering
(LEIS)
LEIS is frequently applied for the structure analysis of metal single crystal surfaces with possible adsorbates. In our group the LEIS method was used for a study of oxygen adsorption on Cu(100) [8]. Here the method is applied for studying the surface structure of differently prepared oxide layers on Fe(lOO). With this technique only a few surface layers are probed, due to beam attenuation as the ion beam penetrates the crystal. If ions are detected the depth information is restricted to one or two layers, since ions reflected from deeper layers become neutralized. The clean and oxygen-covered iron single crystals were continuously or intermittently bombarded with 5 keV neon ions. Collisions between the projectile ions and the target atoms may be treated to good approximation as purely binary and elastic. The relations that express the relative energy of reflected and recoiled particles after collision are well-known: El/E,
=
cos 9 f (A* - sin* 9)l’* (A + 1)
and E,/E’,
= [4A,‘( A + 1)2] cos* {,
1 ’
(1) (2)
182
J. M. vun Zoest et al. / LEIS
surfacestructureana!pw of Fe(lOO)-0
with A = m,/m,, where, : energy of projectile and recoil particle after collision, : primary energy of the projectile, E* m1,m2: mass of projectile and recoil particle, : branching-off angle in the laboratory system for projectile and recoil a71 particle given. Peak positions in the energy spectra measured at a given scattering angle are described well by these formulae. Deviations are attributed to multiple scattering and electronic stopping. Since we deal with oxygen-covered iron surfaces, we expect to find the following contributions of reflected and recoiled particles: (a) neon reflected by iron and neon reflected by oxygen, (b) iron recoil and oxygen recoil particles. In this study the number of neon ions and neon neutrals, which are scattered by iron atoms into the solid angle of the detector, will be denoted as the Net (Fe and Ne ]Fe reflection intensity respectively. For the recoils the subscript ret is used. The theoretical values of the energy E, and E, with respect to E, for this situation are given in fig. 1 for scattering angles up to about 60”. On the right hand side two measured energy spectra are given for 6 = 45O. In the energy spectrum of the positive ions (solid curve) the Ne+ ]Fe, Or:, and Nef 10 signals are present, whereas in the energy spectrum of the negative ions (dashed curve) the O,, signal is visible. These spectra were measured in the [OOl] surface direction. The relative energy of the Nei ]Fe reflection was measured for different scattering angles and is indicated by the black dots in fig. 1. This illustrates the validity of the binary collision approximation. From the calculated curve *‘Ne 1160 in fig. 1 it is clear that in case of neon reflection on oxygen the scattering angle is restricted to a maximum value of 53”. We have chosen 8 = 45” for measuring the energy spectra given in fig. 1 in order to have sufficient separation between the Or:, and Ne+ 10 reflection peak. At larger angles peak broadening occurs. The contributions of iron recoils in the Ne+ ]O reflection peak is considered to be small (5 5%) because of cross section differences. In principle four ion signals are available for the surface characterization, viz. the reflection signals Ne+ ]Fe, Ne+ (0 and the recoil signals O,:, and 0,. These signals may be recorded for two purposes: (i) In order to obtain depth information. In this case the signals must be measured as a function of time during ion-induced desorption. This was succesfully applied previously [4], where the Ne+ ]Fe reflection and Net 10 reflection signals were recorded. (ii) For analyzing the surface structure. In this case the signals must be measured as a function of the elevation angle 4 and the azimuthal angle +. For a correct interpretation of the latter spectra, the desorption induced by E,, E,
J. M. uan Zoesr et al. / LEIS surface structure analysis of Fe(lOO)-0
-if/
ION 5 k&J
Net-
YIELD
183
(Arb.U.1
Fe (100)
-Oad
w” 0.5 , w
j-
"Ne 1'6O
I. 0 LABORATORY
SCATTERING
ANGLE
il
Fig. 1. The energy E after the collision (relative to the initial energy Ee of the Ne+ projectile) as a function of the laboratory scattering angle (or recoil angle) for a collision of Ne with Fe or 0. The energy spectra of positive ions (solid curve) and negative ions (dashed curve) for an oxygen covered Fe(100) surface at the scattering angle of 9 = 45” were measured at specular reflection in the [OOl] azimuthal direction. The angular resolution of the energy analyzer is indicated by the shaded strip around 9 = 45 ‘.
the ion beam has to be taken into account and peak shifts as a function of the azimuthal and elevation angle must be small with respect to the analyzer resolution. It turned out that only for the Ne+ 1Fe reflection the peak shifts in the energy spectra were small enough to measure useful photograms. A photogram is obtained by measuring the Ne+ (Fe or Ne 1Fe reflection intensity as a function of the azimuthal angle C#B and the elevation angle $ at a fixed scattering angle 9. In fact azimuthal spectra are measured for discrete values of the elevation angle, in the range 0” -Z # -C 9. A spectrum is given by five shades of grey. The highest intensity is represented by a black dot, the lowest by a white one. The scaling of all photograms given in this paper is a linear scaling between zero and maximum intensity for every horizontal line (i.e. for every azimuthal distribution). However, no photograms for the Net [Fe reflection intensity of the oxygen-covered surfaces can be obtained due to ion-induced desorption. If a desorption of 10% of a monolayer of oxygen is accepted, the measuring time is limited to about half a minute. We therefore present NeC JFe reflection yields in the form of one single azimuthal spectrum, when dealing with oxygen-covered surfaces. Specular reflection, which
184
I_
30
J&
I 3
2
1 FLIGHT
TIME
_
I 4
Fig. 2. The TOF spectra for a clean Fe surface (solid curve) and an oxygen-covered Fe surface (dashed curve) at a scattering angle of 9 = 45O were measured at specular reflection in the [OOl] azimuthal direction. The time window which was used in the experiments has been indicated.
means that the elevation angle of incidence is equal to the angle of ejection 19- 4, was chosen in order to avoid mutual shadowing and blocking of first layer iron atoms at a scattering angle of 19= 45”. If neutrals are detected, photograms can be obtained for both clean and oxygen-covered surfaces, as the necessary ion dose is a factor of 100-1000 lower. In fig. 2 the TOF spectra for a clean and an oxygen-covered surface, measured under the same experimental conditions as in fig. 1, are presented. The spectra are given on an absolute scale. A broadening of the peak towards longer flight time (lower energy) is observed for an oxygen-covered Fe(lOO) surface. This is due to the contribution of Or,, and Ne 10 reflection. None of the peaks are separated from each other, since the resolution of the spectrometer is insufficient. Nevertheless it will turn out that the neutrals are suited for a structure analysis of oxidized Fe(lOO) surfaces. In order to select the Ne 1Fe reflection intensity in the TOF spectrum a time window was applied. The centre of the window of adjustable width (typical 0.1 ps) was located at the time corresponding to the highest yield, as indicated in fig. 2.
3. Results and discussion 3.1. The clean Fe(lO0) surface In this section the clean Fe(lOO) surface is studied with photograms. A photogram for the NeC [Fe reflection intensity
the help of of the clean
J.M. oan Zoest et al. / LEIS surface structure analysis of Fe(lOO)-0
185
Fe(lOO) surface measured at a scattering angle of 9 = 125O is shown in fig. 3. Reduced intensities due to mutual shadowing of first and second layer Fe atoms in the main crystallographic directions are clearly visible. A schematic view of the atomic orientation in the first and second Fe layer is shown in fig. 4. For clarity some theoretically expected shadow and blocking cones for a non-reconstructed Fe(lOO) surface have been depicted in fig. 3. The shadow and blocking cone dimensions have been calculated following Martynenko [9]. The Firsov potential used by Martynenko, however, was multiplied by a factor 0.8 according to Mbller et al. [lo]. From fig. 3 it can be concluded that there is a good agreement between the calculated and the measured shadow (at 4 c 62.5O) and blocking cones (at # > 62.5”). However, surface relaxation of the interlayer spacing cannot be determined accurately by means of such a photogram. Most of the following experiments have been performed at a scattering angle of 8 = 45”. A photogram for the Ne+ 1Fe reflection intensity of the clean Fe(lOO) surface measured at that scattering angle is shown in fig. 5. A photogram for the Ne ]Fe reflection intensity of the clean Fe(lOO) surface
1
LOO11
LOi31 AZIMUTHAL
to:,11
[OIlI ANGLE
co101
9
Fig. 3. A photogram for the Ne’ IFe reflection intensity of a clean Fe(100) surface. measured at a scattering angle of 9 = 125”. The Ne+ [Fe reflection intensities are indicated with five shad< :s of grey; the darker dots corresponding to higher intensities. Some theoretically expected shadow and blocking cones are indicated (see text).
186
Fe (100
1
t 2.87
-22,87A-
(017)
PLANE
Fig. 4. A schematic top and side view of the atoms in the first and second Fe layer of a Fc(100) crystal. The large circles represent the Fe atoms in the first layer and the small circles represent the Fe atoms in the second layer. The main crystallographic directions are indicated.
measured under the same conditions is given in fig. 6. The main difference between these two photograms is the pronounced minimum in the [OOl] azimuthal direction around specular reflection when the Ne /Fe reflection signal is measured. For convenience, equivalent surface directions will not be given. The interpretation of this minimum will be discussed in the following section.
3.2. The oxygen-covered
or oxidized Fe(lO0) surface
As mentioned in the introduction we focused our attention on four differently prepared oxygen-covered or oxidized surfaces: (i) The Fe(100) surface after an oxygen exposure of 15 L (150 s at Po_ = 1 X lop7 Torr) at room temperature.
J. M. uan Zoest et al. / LEIS surface structure am+
cob11
ro; 31 AZIMUTHAL
Fig
6.
A photogram
;0311
roll1 ANGLE
of Fe(lOO)-0
187
[0103
9
for the Ne JFe reflection intensity of a clean Fe(100) surface. scattering angle of 8 = 45 o
measured
at a
188
J.M. cmn
Zoestet al. / LEIS surJuce structureana!ym of Fe(lOO)-0
(iv) The oxide layer mentioned under (iii) after annealing at 700 K for 15 min and cooling to room temperature. The oxygen exposure of 15 L at room temperature results in a coverage of approximately one monolayer of chemisorbed oxygen. On the apparatus there was unfortunately no technique for measuring the absolute oxygen coverage. By Vink et al. [4] the oxygen-related signals, using LEIS, were measured as a function of time in order to obtain high-resolution depth information from the surfaces mentioned above. These ion-induced desorption profiles showed a decrease of the oxygen related signals as a function of time. Due to the observed ion-induced desorption our measuring time had to be limited to half a minute at an ion flux of 4 X lop2 A me2. The azimuthal distributions of the Net ]Fe reflection intensity from the differently prepared oxygen-covered surfaces were obtained within this limited measuring time. They are shown on the same scale in figs. 7a-7e and are normalized to the Ne+ ]Fe reflection intensity in the [OOl] surface direction from the clean Fe(lOO) surface. The ion-induced desorption of oxygen during such a measurement was approximately 10% of a monolayer. Before giving a detailed discussion on the individual spectra, a few general statements can be made. In fig. 7a the azimuthal distribution of the Ne+ ]Fe reflection from the clean Fe(lOO) surface is given which will serve as a reference for the discussion of the subsequent spectra. When the Fe(lOO) surface is covered with a monolayer of oxygen or saturated with oxygen at room temperature, a relative lack of azimuthal anisotropy is observed (figs. 7b and 7d). This is probably due to disorder of the substrate surface [4,11,12] and neutralization of the Ne+ ions by the adsorbed oxygen atoms. When both surfaces are annealed at 700 K the surfaces become well-ordered as can be concluded from the azimuthal distributions depicted as figs. 7c and 7e. Both spectra are almost identical, which suggests that the structure of the topmost layers of both surfaces are the same. The spectra 7a, 7c and 7e are discussed in more detail now, starting with the azimuthal distribution of the Net ]Fe reflection intensity from the clean Fe(lOO) surface, as shown in fig. 7a. We observe that the Net ]Fe reflection intensity in the [OOl] azimuthal direction is about twice as high as the Ne+ 1Fe reflection intensity in the [031] azimuthal direction. This can be explained with the help of fig. 4. In the [OOl] azimuthal direction it is possible for the Ne+ ions to reflect from the first two Fe layers whereas in the [031] azimuthal direction and at the elevation angle of incidence of 4 = 22.5”, it is only possible to reflect from the first Fe layer as the atoms of the second Fe layer are shadowed by the first layer. The maxima close to the [OOl] and [Oil] azimuthal directions can be attributed to focusing of the Ne’ ions onto the second Fe layer. When the monolayer of oxygen prepared at room temperature is annealed and subsequently cooled to room temperature the azimuthal distribution of
J. M. uan Zoest et al. / LEIS surface structure ana!ysrs of Fe(lOO)-0
:I= 0
1
co111
Lo131 AZIMUTHAL
ANGLE
[031]
[O’O’
189
1
‘+’
Fig. 7. The azimuthal distributions of the Ne+ IFe (a-e) and the Ne(Fe (a’-e’) reflection intensity from the clean, oxygen-covered and oxidized Fe(100) surfaces, measured at an elevation angle of incidence of $ = 22S” (ions) and LJJ= 22O (neutrals) at a scattering angle of 9 = 45O. Please note the different angular scale units. (a, a’): The clean Fe(100) surface: (b, b’): a monolayer of oxygen on the Fe(100 surface, prepared at room temperature; (c, c’): the monolayer of oxygen on the Fe(100) surface, prepared at room temperature, after annealing and subsequent cooling to room temperature; (d, d’): the Fe(100) surface after saturation with oxygen at room temperature; (e, e’): the oxide layer mentioned under (d) and (d’) after annealing and subsequent cooling to room temperature.
the Ne+ (Fe reflection intensity shown in fig. 7c is observed. The Net (Fe reflection intensity is no longer independent of the azimuthal angle. The maxima in this azimuthal distribution can be attributed to focusing of Ne+ ions onto the second Fe layer. As the focusing of Ne+ ions onto the second Fe layer is not observed in the azimuthal distribution of the Ne+ (Fe reflection intensity from the monolayer of oxygen prepared at room temperature it can be concluded that due to annealing the monolayer of oxygen becomes well-
ordered. So, the monolayer of oxygen prepared at room temperature on Fe(lOO) is disordered. The azimuthal distribution of the annealed monolayer of oxygen can be very well explained with a monolayer of oxygen in which the oxygen atoms are located in the fourfold symmetrical hollows of the substrate. This surface structure was reported previously by Legg et al. [2] who carried out a LEED structure analysis of the Fe(lOO) p(1 X 1)-O structure. The vertical position of the oxygen atoms with respect to the first Fe layer will be discussed in section 3.3. Oxygen atoms located in the fourfold symmetrical hollows of the substrate should effectively shadow the second Fe layer when the Net (Fe reflection intensity is measured in the [OOI] azimuthal direction. This is in fact what we observe. With respect to the clean Fe(lOO) surface the Ne+ (Fe reflection intensity in the [OOl] azimuthal direction is a factor of two smaller. An alternative explanation of this decrease of the Ne’ IFe reflection intensity is that the neutralization probability for the Ne+ ions reflected from the second Fe layer has become larger due to the adsorbed oxygen atoms in the fourfold symmetrical hollows. A discrimination between these two possibilities can be made by comparing the azimuthal distributions of the Ne+ 1Fe and of the Ne ]Fe reflection intensity from the same oxygen-covered or oxidized surface. This will be shown at the end of this section. After the substrate surface was saturated with oxygen at room temperature, annealed and subsequently cooled to room temperature the same azimuthal distribution of the Ne+ (Fe reflection intensity was measured (fig. 7e) as in the case of the annealed monolayer of oxygen. The structure of the chemisorbed oxygen atoms in the first Fe layer is therefore the same for both annealed oxygen-covered surfaces and corresponds to a monolayer of oxygen of which the oxygen atoms are located in the fourfold symmetrical hollows of the substrate. In order to examine the effect of neutralization on the measurements of the Ne+ 1Fe reflection intensity, azimuthal distributions of the Ne )Fe reflection intensity were measured. These measurements have also been performed to extend our information on the structure of these oxygen-covered or oxidized surfaces beyond the first and the second Fe layer. Due to the large neutral fraction, measurements of the Ne ]Fe reflection intensity could be performed with low dosis. This allowed us to obtain complete photograms of the oxygen-covered or oxidized surfaces since ion-induced desorption could be neglected. The azimuthal distributions of the Ne (Fe reflection intensity measured at an elevation angle of incidence of + = 22” and at a scattering angle of 8 = 45” are shown in figs. 7a’-7e’. The data were not scaled to the target current since it could not be measured accurately. Instead the azimuthal distributions have been normalized to the Ne 1Fe reflection intensity in the [013] azimuthal direction. In the [013] azimuthal direction the Ne ]Fe reflection intensity is due to scattering of the Ne ’ ions by the first Fe layer only. ions by the first Fe layer in the [013] azimuthal The scattering of Net
J.M. uan Zoest et al. / LEIS surface structure analysis
of Fe(lOO)-0
191
direction is not influenced by the adsorbed oxygen atoms; this will be evidenced in section 3.3. The azimuthal distribution of the Ne 1Fe reflection intensity obtained from the clean Fe(lOO) surface is shown in fig. 7a’. The most important difference with respect to the azimuthal distribution of the Ne+ ]Fe reflection intensity (fig. 7a) is the presence of high Ne 1Fe reflection intensities close to the [OOl] azimuthal direction. These high Ne (Fe reflection intensities are probably due to focusing of Ne+ ions onto the second and third Fe layer. These high focusing intensities are not observed when the Ne+ IFe reflection intensity is measured because of the large neutralization probability for scattering by the third Fe layer. In the [OOl] azimuthal direction the Ne ]Fe reflection intensity is 2.5 times larger as in the [013] azimuthal direction (reflection from the first Fe layer). The Ne ]Fe reflection intensity in the [OOl] azimuthal direction is apparently not only due to the Net ions reflected from the first two Fe layers, but also to Ne+ ions reflected from the third Fe layer. After an oxygen exposure of 15 L at room temperature, the azimuthal distribution of the Ne 1Fe reflection intensity depicted in fig. 7b’ is measured. The maxima in this azimuthal distribution are caused by focusing effects on deeper Fe layers which are possible despite the disordered monolayer of chemisorbed oxygen atoms. Due to neutralization these focusing effects on deeper iron layers are not observed in the azimuthal distribution of the Ne+ I Fe reflection intensity. The Ne I Fe reflection intensity in the [OOl] azimuthal direction has diminished with respect to the clean Fe(lOO) surface due to shadowing and blocking of the second Fe layer by the adsorbed oxygen atoms. After annealing of this disordered monolayer of oxygen and subsequent cooling to room temperature the azimuthal distribution of the Ne I Fe reflection intensity shown in fig. 7c’ was measured. This azimuthal distribution is almost identical to the one obtained from the clean Fe(lOO) surface. The Ne I Fe reflection intensity has slightly diminished in the azimuthal direction close to the [OOl] azimuthal direction and in the [OOl] azimuthal direction itself. This azimuthal distribution from the annealed monolayer of oxygen atoms shows that focusing of Ne+ ions onto the third Fe layer is still possible. When the oxygen atoms are located in the fourfold symmetrical hollows of the Fe(lOO) surface shadowing of the second Fe layer in the [OOl] azimuthal direction is expected. The Ne )Fe reflection intensity in this azimuthal direction, however, has not diminished to the level of only the first Fe layer (such as measured in the [013] azimuthal direction). A possible explanation for this apparent inconsistency is that the Ne + ions are focused on the third Fe layer by the adsorbed oxygen atoms. As a consequence the decrease of the Ne I Fe reflection intensity in the [OOl] azimuthal direction due to the shadowing of the second Fe layer by the adsorbed oxygen atoms is counter-balanced by focusing of the Net ions on the third Fe layer.
192
J. M. uan Zoest et al. / LEIS surfme structure una!ysis of Fe(lOO)-0
The azimuthal distribution of the Ne ]Fe reflection intensity from the Fe(lOO) surface which has been saturated with oxygen at room temperature is given in fig. 7d’. A significant dependence of the Ne 1Fe reflection intensity on the azimuthal angle is measured. This structure is caused by Ne+ ions reflected from the second and deeper Fe layers. All these Net ions are neutralized, which is evidenced by the fact that the Ne+ ]Fe reflection intensity from this oxide layer is independent of the azimuthal angle. The Ne ]Fe reflection intensity in the [OOl] azimuthal direction has diminished with respect to the clean Fe(lOO) surface due to shadowing and blocking of the second Fe layer by the adsorbed oxygen atoms. The azimuthal distribution of the Ne (Fe reflection intensity from the substrate surface which has been saturated with oxygen, annealed and subsequently cooled to room temperature is depicted in fig. 7e’. The high focusing intensities close to the [OOl] azimuthal direction have decreased with respect to those observed for the clean Fe(lOO) surface (fig. 7a’). From this decrease it can be concluded that the first Fe layer still focuses Ne’ ions onto the second Fe layer, but focusing of Ne + ions on the third Fe layer is not possible anymore. This can be due to oxygen atoms which are located in the octahedral hollows between the first and the third Fe layer. The Ne JFe reflection intensity in the [OOl] azimuthal direction has an intensity which is almost as high as the Ne ]Fe reflection intensity from the first Fe layer. This is due to the expected shadowing of the second Fe layer by the adsorbed oxygen atoms in the fourfold symmetrical hollows of the substrate. The shadowing of the second Fe layer by the adsorbed oxygen atom is no longer compensated by focusing of the Ne+ ions on the third Fe layer (as in the case of the annealed monolayer of oxygen) since the third Fe layer is shadowed by the oxygen atoms in the octahedral hollows. The azimuthal distributions of the Ne [Fe reflection intensity shown in figs. 7b’ and 7e’ are almost identical. A disordered monolayer of oxygen results apparently in nearly the same azimuthal distribution as a well-ordered monolayer of oxygen with also oxygen atoms in deeper layers. The azimuthal distributions of the Ne+ JFe reflection intensity from both annealed oxide layers (figs. 7c and 7e) could be explained in two ways: the Ne+ 1Fe reflection intensity in the [OOl] direction had decreased by neutralization of the Ne+ ions reflected from the second Fe layer or it had decreased due to shadowing of the second Fe layer by the adsorbed oxygen atoms in the fourfold symmetrical hollows. In the azimuthal distribution of the Ne (Fe reflection intensity from the Fe(lOO) surface that has been saturated with oxygen, annealed and cooled to room temperature a decrease of the reflection intensity in the [OOl] azimuthal direction is observed with respect to the clean Fe(lOO) surface. Therefore it can be concluded that the Net (Fe reflection intensity in the mentioned azimuthal direction from the annealed oxygencovered or oxidized surfaces has decreased due to shadowing of the second Fe layer by the adsorbed oxygen atoms.
J. M. uan Zoest et al. / LEIS surface strucfure ardysis
of Fe(lOO)-0
3.3. The vertical position of oxygen in the fourfold symmetrical sut-face
193
site on the Fe(lO0)
‘The preceding azimuthal distributions of the Ne+ (Fe and the Ne (Fe reflection intensity from the annealed oxygen-covered or oxidized surfaces are well explained with a monolayer of chemisorbed oxygen of which the oxygen atoms are located in the fourfold symmetrical hollows of the substrate. It was also possible to determine the distance between the plane of these oxygen atoms and the plane of the first Fe layer. This information was obtained by measuring the OP recoil intensity as a function of the elevation angle of incidence. During the measurement of the O- recoil intensity we have not only varied the elevation angle of incidence 4 but also the scattering angle 9. The O- recoil intensity appeared to be maximal under specular conditions (9 = 24) for scattering angles up to 45 O. The measurements were performed in the [Oil] and [OOl] azimuthal directions. In fig. 8 the collision geometry is given at a low elevation angle of incidence in the [Oil] azimuthal direction, assuming that there is a complete coverage with oxygen of all the fourfold symmetrical hollows of the Fe(lOO) surface. From fig. 8a a low O- recoil intensity is expected at low elevation angles of incidence since each oxygen atom is in the shadow cone of its nearest neighbour in the [Oil] azimuthal direction. The collision geometry at a critical elevation angle of incidence I/$ is shown in fig. 8b. At that critical elevation angle of incidence an incident Ne+ ion on the edge of the shadow cone of the nearest-neighbour oxygen atom has
0
0
0
0
Fig. 8. A side view of the [Oli] plane from the Fe(100) surface with a complete coverage of all the fourfold symmetrical hollows with oxygen. The Fe atoms and the adsorbed oxygen atoms in the fourfold site are represented with large and small circles respectively. The shadow cones for the 5 keV Ne+ ions scattered by the oxygen atoms were calculated following Marchut et al. [l]. (a) Collision geometry at an elevation angle of I$ < 4;. (b) Collision geometry at the critical elevation angle #t.
194
J.M. cun Zest
et d. / LEIS surface structure cmalp-is of Fe(lOO)-0
an impact parameter with the oxygen atom which is just small enough to scatter the oxygen atom through a scattering angle {. Therefore we expect a steep increase of the number of detected O- recoils at $‘,. The effect is substantially enhanced by a “focusing effect”, the incoming intensity being concentrated just outside the shadow cone. During a measurement of the OP recoil intensity as a function of the elevation angle of incidence there is of course a considerable ion-induced desorption of the oxygen atoms. A representative collision geometry after a certain desorption period is given in fig. 9. At a low elevation angle of incidence there is no longer the situation in which the oxygen atoms are shadowed by next-nearest-neighbour oxygen atoms. Instead we have at a low elevation angle of incidence the situation in which the oxygen atoms are shadowed by the Fe atoms in the first Fe layer (fig. 9a). The collision geometry at a critical elevation angle of incidence Ir/f is shown in fig. 9b. At this critical elevation angle of incidence again a steep increase of the O- recoil intensity is expected. Three measurements of the O- recoil intensity as a function of the elevation angle of incidence 4 have been made consecutively for the annealed monolayer of oxygen in the [Oil] azimuthal direction and are shown in fig. 10. During the first measurement of the OP recoil intensity as a function of the elevation angle of incidence, approximately 40% from the monolayer of oxygen atoms is desorbed due to the Net bombardment. In order to compare the three measurements, the measured O- recoil intensities (IoFCC) have been divided by the maximum O,, intensity ( Io,L._aV) from that measurement. In all
Fig. 9. A side view of the [Oli] plane for an incomplete monolayer of oxygen (due to ion-induced desorption). The Fe atoms and the oxygen atoms in the fourfold site are represented with large and small circles respectively. The shadow cone for the 5 keV Ne + ions scattered by the Fe atom was calculated following Marchut et al. [l]. (a) Collision geometq at an elevation angle of + < 4:. (b) Collision geometry at the critical elevation angle $f.
J. M. uan Zoest et al. / LEIS surface structure analysis of Fe(lOO)-0
195
measurements a steep increase of the OP recoil intensity as a function of the elevation angle of incidence is observed. With respect to the first measurement the steep slopes of the second and third measurement have shifted towards smaller elevation angles of incidence. As explained above, this shift is due to the desorption of nearest-neighbour oxygen atoms. The angle at half height of the O- recoil intensity is taken as the critical angle of incidence [13]. In order to determine the distance between the plane of the oxygen atoms and the plane of the first Fe layer (y), the critical angle of the measurements 2 and 3 from fig. 10 is used. From this figure it is found that +f = 6.9” k 0.5”. At this critical elevation angle of incidence an incident Ne+ ion on the edge of the shadow cone of the preceding Fe atom of the first Fe layer has an impact
j-
)
L
0"
L
II
_______
IO0 ELEVATION
20” ANGLE
$
Fig. 10. Three measurements of the O- recoil intensity as a function of the elevation angle of incidence $. These measurements (0,~ A, respectively first, second and third measurement) were performed consecutively for the annealed monolayer of oxygen in the [Oil] azimuthal direction and have been scaled on their own maximum.
parameter p with the adsorbed oxygen atom which is just small enough to scatter it through a scattering angle I (fig. 9b). In order to calculate y, the impact parameter p and the radius of the shadow cone as a function of the distance behind the Fe atom of the first layer R(L) have to be known. As shown in the photogram of fig. 3 the calculated shadow cones of Fe are in good agreement with the measured shadow cones. Therefore we used the same expression for the radius of the shadow cone as a function of the distance behind the Fe atom in this case. The impact parameter p of the incident Ne+ ion with the oxygen atom was calculated in the manner of Mashkova and Molchanov [14] with the approximation to the Thomas-Fermi potential used by Marchut et al. [l]. The result of this calculation was p = 0.02 A ({ = 13.8O). Using this impact parameter and the expression for the radius of the shadow cone and the critical elevation angle of incidence it was calculated that the oxygen atom is located 0.56 + 0.05 A above the plane of the first Fe layer. The critical angle $‘, has to result in a vertical position of the oxygen atom y’, relative to its nearest neighbour, which is not significantly different from zero. This is, however, in the [Oil] azimuthal direction not the case. The critical angle of +k = 10.6” + 0.5” results in a relative vertical position of I” = - 0.14 + 0.05 A. This discrepancy can be attributed to the first layer Fe atom which is lying between the two oxygen atoms. These iron atoms can deflect the incident Ne’ ion on the edge of the shadow cone of its nearestneighbour oxygen atom through a small angle. As a consequence, the critical angle 4: is larger than expected. In the [OOl] azimuthal direction, the critical angle is not influenced by the deflection of the Ne + ions from the first Fe layer. A measurement of the 0 recoil intensity as a function of the elevation angle of incidence in the [OOl] azimuthal direction is depicted in fig. 11. The measured critical angle of +f = 9.8” i_ 0.5O results in a vertical position of the oxygen atom, relative to its nearest neighbour, of _p’= + 0.05 I 0.05 A, which is indeed not significantly different from zero. When the measurements of the O- recoil intensity as a function of the elevation angle of incidence were performed for a Fe(lOO) surface, which had been saturated with oxygen at room temperature, annealed and cooled to room temperature, the same critical angles and shift of the steep slope were observed as in the case of the annealed monolayer of chemisorbed oxygen. So the distance between the plane of the adsorbed oxygen atoms and the plane of the first Fe layer is the same for both annealed oxygen-covered or oxidized surfaces. The vertical position of the oxygen atom in the fourfold symmetrical hollow of the substrate is substantiated by the photogram of the Ne 1Fe reflection intensity shown in fig. 12. This is a photogram of the Fe(lOO) surface which had been saturated with oxygen at room temperature,* annealed and subsequently cooled to room temperature. At the elevation angle of incidence of 4 = 11” in the [013] azimuthal direction one sees clearly the shadowing of the
J.M. van Zoest et al. / LEIS surface structure ana!ysis of Fe(lOO)-0
ELEVATION
ANGLE
197
+
Fig. 11. The measurement of the O- recoil intensity as a function of the elevation angle of incidence 4, which was performed for the annealed monolayer of oxygen in the [OOI] azimuthal direction.
first Fe layer by the adsorbed oxygen atoms. At the elevation angle of incidence of 4 = 35” the corresponding blocking of the first Fe layer is observed. These shadow and blocking dips can only occur, when the oxygen atoms are located above the plane of the first Fe layer in the fourfold symmetrical hollows. These shadow and blocking dips are of course absent in the photogram of the clean Fe(lOO) surface as shown in fig. 6. It can be calculated that at the elevation angle of incidence 4 = 22O and in the [013] and corresponding azimuthal directions the first Fe layer is not shadowed or blocked by the adsorbed oxygen atoms at 8 = 45 O. Normalization of the azimuthal distributions of the Ne IFe reflection intensity on the Ne [Fe reflection intensity in the [013] azimuthal direction, such as done in section 3.2, is therefore justified.
198
J. M. run Zoest et ul. / LEIS swfocr structure clnuo’s~s of Fe(lOO)-0
45” 1
-1301;
7.--
--
io7 3: AZIMUTHAL
-
--r--
-
i@l!]
--- r ro311
ANGLE
-
1 --- -iOlOj
-
-
‘$J
Fig 12. A photogram for the Ne IFe reflection intensity of a Fc(100) surface which was saturated wit11 oxygen, annealed and cooled to room temperature, measured at a scattering angle of Q= 4S”. The additional shadow and blocking cones ($J =ll” and I&= 35O, respectively). with respect to the dean Fe(lO0) surface, are due to the presence of the adsorbed oxygen atoms in the fourfold
Legg et p(1 x 1)-O interesting tioned. and
symmetrical
hollows of the substrate.
al. [Z] reported on the LEED structure analysis of the Fe(lOO) structure and found y = 0.48 A for the oxygen position. It is most that both this quantitative LEED analysis by the authors menLEIS as a “real space” technique come to the same result.
4. Conclusions From the present surface structure determination of differently prepared oxygen-covered or oxidized Fe(lOO) surfaces, the following conclusions can be made. _ A monolayer of oxygen adsorbed on Fe(lOO) at room temperature appears to be disordered in terms of surface structure. _ Beyond a monolayer cove!age, i.e., when the oxide layer on Fe(lOO) is grown to a limiting thickness (15 A from ref. [4]) at room temperature, the topmost layers remain disordered. _ Annealing of the unordered monolayer of oxygen present on the Fe(lOO) surface results in a well defined p(1 x 1) surface structure, where the oxygen atoms are situated in the fourfold symmetrical hollows of the substrate surface. The adsorbed oxygen atoms are located at a distance of 0.56 & 0.05 A above the first substrate layer of iron atoms.
.J.M uan Zoest et al. / LEIS surface structure analysis of Fe(lOO)-0
199
_ As concluded before [4], an oxide layer grown on Fe(lOO) at room temperature reconstructs at elevated temperatures leaving a monolayer of oxygen on top of the surface beneath which a different oxide phase is present. The surface structure of this oxygen monolayer appears to be identical with respect to the surface properties found for a single monolayer of oxygen on Fe(lOO) (which are given above). The results obtained from the reconstructed oxide surface (given in the last conclusion) are especially relevant with respect to a recent study [15] on the interaction of hydrogen with this particular surface. It appeared that the monolayer of oxygen, left on top of the surface after reconstruction, is inactive to hydrogen whereas the incorporated oxide can be removed under identical reaction conditions.
Acknowledgements The authors thank Professor Dr. A. Niehaus for helpful discussions and for a critical reading of this manuscript. This work was performed as part of the research programme of both the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Stichting Scheikunding Onderzoek in Nederland (SON) with financial support from the Nederlandse Organisatie voor ZuiverWetenschappelijk Onderzoek (ZWO).
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15]
L.. Marchut, T.M. Buck, G.H. Wheatley and C.J. McMahon, Jr., Surface Sci. 141 (1984) 549. K.O. Legg, F. Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. B16 (1977) 5271. K.O. Legg, F. Jona, D.W. Jepsen and P.M. Marcus, J. Phys. Cl0 (1977) 937. T.J. Vink, J.M. der Kinderen, O.L.J. Gijzeman, J.W. Geus and J.M. van Zoest, Appl. Surface Sci. 26 (1986) 357. R..P.N. Bronckers, Thesis, Utrecht (1981). R..P.N. Bronckers, Th.M. Hupkens, A.G.J. de Wit and W.C.N. Post, Nucl. Instr. Methods 179 (1981) 125. E.G. Keim, F. Labohm, O.L.J. Gijzeman, G.A. Bootsma and J.W. Geus, Surface Sci. 112 (1981) 52. Th.M. Hupkens, Nucl. Instr. Methods B9 (1985) 277, 285. Yu.V. Martynenko, Radiation Effects 20 (1973) 211. J. MFller, H. Niehus and W. Heiland, Surface Sci. 166 (1986) Llll. M. Watanabe, M. Miyamura, T. Matsudaire and M. On&i, in: Proc. ICSS-2, Kyoto, 1974, p. 501. G.W. Simmons and D.W. Dwyer, Surface Sci. 48 (1975) 373. H. Niehus and G. Comsa, Surface Sci. 140 (1984) 18. E.S. Mashkova and V.A. Molchanov, in: Modem Problems in Condensed Matter Sciences, Veal. 11, Eds. V.M. Agranovich and A.A. Maradudin (North-Holland, Amsterdam, 1985). T.J. Vink, J.M. der Kinderen, O.L.J. Gijzeman and J.W. Geus, Appl. Surface Sci. 26 (1986) 367.
Surface Science 182 (1987) 179-199 North-Holland, Amsterdam
SURFACE STRUCTURE ANALYSIS OF OXIDIZED BY LOW ENERGY ION SCATTERING J.M. VAN ZOEST, Qsisch
Lahoratorium,
T.J. VINK
J.M. FLUIT Rijksuniuersiteit
Utrecht, Princetonplein
5, 3584 CC Utrecht. The Netherlrrnds
* and B.A. VAN HASSEL
Vun ‘t Hoff Lcrhorutorium ooor Fysische en Colloidchemle, The Netherlunds Received
Fe(100)
12 June 1986: accepted
for publication
Paduulaan
31 October
8, 3584 CH Utrecht.
1986
The surface structures of differently prepared oxide layers on Fe(100) have been analysed using low energy ion scattering with time-of-flight analysis. The intensity of scattered 5 keV Nc+ ions and neutrals was measured as a function of elevation and azimuth of the incoming beam at a constant scattering angle. producing photograms of clean as well as oxygen covered Fe(100) surfaces. The formation of the Fe(lOO) p(1 X1)-0 surface structure at full monolayer coverage was confirmed by measuring azimuthal distributions of reflected Ne + ions and neutrals at specular reflection and a scattering angle of 45O. The oxygen atoms are found to be located in the fourfold symmetrical hollows of the substrate surface. From the 0.. recoil intensity as a function of the elevation angle of incidence at specular reflection it was calculated that the oxygen atoms are located at a distance of 0.56 & 0.05 A above the first substrate layer of iron atoms. A disordered oxide layer grown on Fe(100) at room temperature reconstructs at elevated temperatures leaving a well-ordered monolayer of oxygen on top of the surface. The surface structure of this oxygen monolayer appears to be identical with respect to the surface properties found for a single monolayer of oxygen on Fe(lOO).
1. Introduction The oxidation of Fe(lOO) has been studied extensively with a large variety of techniques. This paper deals with the influence of temperature on the surface structure of thin oxide films on Fe(lOO), while most investigations only concentrate on oxide layers formed at room temperature. For this purpose low energy ion scattering (LEIS) will appear to be eminently suitable. LEIS with time-of-flight analysis was used by Marchut et al. [I], for the analysis of a clean unreconstructed Fe(lOO) surface. They demonstrate a relatively simple technique for preliminary analysis of the surface structure. The interaction of a * To whom correspondence
should
be addressed
0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
180
J.M. wn Zoest et ul. / LEIS surface structure una!vsi.r of Fe(lOO)-0
clean Fe(lOO) surface with gasphase oxygen was studied by Legg et al. [2], using LEED and AES. A LEED analysis of the atomic structure of the first monolayer of oxygen on Fe(lOO) revealed that the oxygen atoms are located inside the fourfold symmetrical hollows of the substrate surface, with the first substrate interlayer spacing expanded by 7.5% with respect to the bulk. Earlier work by the same authors [3] suggests that the clean Fe(lOO) surface is not reconstructed. This study will focus on the surface structure determination of a set of differently prepared oxide layers on Fe(lOO). A preceding analysis by Vink et al. (41 concerning the preparation and subsequent characterization of these oxide surfaces is often used as a reference in the present paper. It seems therefore relevant to give a short abstract of its conclusions. From ellipsometry, AES and LEED measurements it was concluded that an oxide layer on Fe(lOO) grows to a limited thickness at room temperature and rearranges at crystal temperatures above 573 K. The oxide layer formed at room temperature appeared to be disordered, whereas the annealed surface exhibited a LEED pattern of p(1 x 1) symmetry. This drastic change in surface composition and surface structure is further examined in this paper, using LEIS. It will be shown that after reconstruction a well-ordered monolayer of chemisorbed oxygen is present on the surface, while in the subsurface region an incorporated oxide phase is formed [4].
2. Experimental 2.1. The apparatus The LEIS measurements were performed in a Varian UHV system, described previously by Bronckers [5], of which the experimental facilities were improved by application of mass selection of the primary beam and detection of neutral particles. Only a short description is given here. The base pressure of about 6 X lo-lo Torr is reached by means of a turbomolecular pump and a liquid nitrogen trap with a Ti sublimation unit. The pressure increases to about 1 X 10m9 Torr due to the neon projectile gas. The chamber containing the ion gun is also pumped by a turbomolecular pump. The ion gun consists of an electron impact source, a lens system, deflection plates, a Wien filter for mass selection followed by another lens system, and a final aperture of one millimeter diameter. For the experiments described here, a 5 keV “Ne+ beam was chosen with an ion flux of 4 X lo-* A m -2 at the target. The iron crystal was mounted in a target manipulator [6]. With this manipulator the elevation angle of incidence J, could be varied between 0” and 350“ (accuracy O.l”). The azimuthal angle of incidence + was variable over 330” (accuracy 0.2”). For the analysis and detection of scattered
J.M. uan Zoest et al. / LEIS surjace structure un&sis
of Fe(lOO)-0
181
ions a double flat-plate energy analyzer (ESA) was used. The analyzer can be rotated with an accuracy of 2” in the plane defined by the incoming beam and the surface normal in the range of scattering angles 9 from 0” to 135”. The energy resolution of the ESA is about 4%. An extra channeltron was added to the analyzer in order to count neutralized ions, which are scattered by the surface. Energy selection of neutrals was achieved by means of a time-of-flight (TOF) technique. The resolution of the TOF spectrometer is about 8%. This is mainly because of the short flight path (I = 0.222 m), which is limited by the dimensions of the UHV chamber. The crystal was heated by placing an infrared heater in front of it. The Fe(lOO) crystal had been used before for experiments with ellipsometry, AES and LEED in another UHV apparatus [7]. It was then depleted from bulk impurities by cycles of heating and 400 eV Ar+ ion bombardment. Before the LEIS experiments were performed the crystal was polished again. In vacuum it was cleaned by cycles of heating above 450 K during 10 min and 5 keV Net ion bombardment at a small elevation angle, while the crystal was continuously rotated over the azimuthal angle. The crystal structure of the clean Fe(lOO) surface was monitored by measuring azimuthal and polar distributions of scattered Net ions. In this vacuum chamber no other facilities were present for measuring the absolute oxygen coverage, e.g. AES and LEED. 2.2. Low energy ion scattering
(LEIS)
LEIS is frequently applied for the structure analysis of metal single crystal surfaces with possible adsorbates. In our group the LEIS method was used for a study of oxygen adsorption on Cu(100) [8]. Here the method is applied for studying the surface structure of differently prepared oxide layers on Fe(lOO). With this technique only a few surface layers are probed, due to beam attenuation as the ion beam penetrates the crystal. If ions are detected the depth information is restricted to one or two layers, since ions reflected from deeper layers become neutralized. The clean and oxygen-covered iron single crystals were continuously or intermittently bombarded with 5 keV neon ions. Collisions between the projectile ions and the target atoms may be treated to good approximation as purely binary and elastic. The relations that express the relative energy of reflected and recoiled particles after collision are well-known: El/E,
=
cos 9 f (A* - sin* 9)l’* (A + 1)
and E,/E’,
= [4A,‘( A + 1)2] cos* {,
1 ’
(1) (2)
182
J. M. vun Zoest et al. / LEIS
surfacestructureana!pw of Fe(lOO)-0
with A = m,/m,, where, : energy of projectile and recoil particle after collision, : primary energy of the projectile, E* m1,m2: mass of projectile and recoil particle, : branching-off angle in the laboratory system for projectile and recoil a71 particle given. Peak positions in the energy spectra measured at a given scattering angle are described well by these formulae. Deviations are attributed to multiple scattering and electronic stopping. Since we deal with oxygen-covered iron surfaces, we expect to find the following contributions of reflected and recoiled particles: (a) neon reflected by iron and neon reflected by oxygen, (b) iron recoil and oxygen recoil particles. In this study the number of neon ions and neon neutrals, which are scattered by iron atoms into the solid angle of the detector, will be denoted as the Net (Fe and Ne ]Fe reflection intensity respectively. For the recoils the subscript ret is used. The theoretical values of the energy E, and E, with respect to E, for this situation are given in fig. 1 for scattering angles up to about 60”. On the right hand side two measured energy spectra are given for 6 = 45O. In the energy spectrum of the positive ions (solid curve) the Ne+ ]Fe, Or:, and Nef 10 signals are present, whereas in the energy spectrum of the negative ions (dashed curve) the O,, signal is visible. These spectra were measured in the [OOl] surface direction. The relative energy of the Nei ]Fe reflection was measured for different scattering angles and is indicated by the black dots in fig. 1. This illustrates the validity of the binary collision approximation. From the calculated curve *‘Ne 1160 in fig. 1 it is clear that in case of neon reflection on oxygen the scattering angle is restricted to a maximum value of 53”. We have chosen 8 = 45” for measuring the energy spectra given in fig. 1 in order to have sufficient separation between the Or:, and Ne+ 10 reflection peak. At larger angles peak broadening occurs. The contributions of iron recoils in the Ne+ ]O reflection peak is considered to be small (5 5%) because of cross section differences. In principle four ion signals are available for the surface characterization, viz. the reflection signals Ne+ ]Fe, Ne+ (0 and the recoil signals O,:, and 0,. These signals may be recorded for two purposes: (i) In order to obtain depth information. In this case the signals must be measured as a function of time during ion-induced desorption. This was succesfully applied previously [4], where the Ne+ ]Fe reflection and Net 10 reflection signals were recorded. (ii) For analyzing the surface structure. In this case the signals must be measured as a function of the elevation angle 4 and the azimuthal angle +. For a correct interpretation of the latter spectra, the desorption induced by E,, E,
J. M. uan Zoesr et al. / LEIS surface structure analysis of Fe(lOO)-0
-if/
ION 5 k&J
Net-
YIELD
183
(Arb.U.1
Fe (100)
-Oad
w” 0.5 , w
j-
"Ne 1'6O
I. 0 LABORATORY
SCATTERING
ANGLE
il
Fig. 1. The energy E after the collision (relative to the initial energy Ee of the Ne+ projectile) as a function of the laboratory scattering angle (or recoil angle) for a collision of Ne with Fe or 0. The energy spectra of positive ions (solid curve) and negative ions (dashed curve) for an oxygen covered Fe(100) surface at the scattering angle of 9 = 45” were measured at specular reflection in the [OOl] azimuthal direction. The angular resolution of the energy analyzer is indicated by the shaded strip around 9 = 45 ‘.
the ion beam has to be taken into account and peak shifts as a function of the azimuthal and elevation angle must be small with respect to the analyzer resolution. It turned out that only for the Ne+ 1Fe reflection the peak shifts in the energy spectra were small enough to measure useful photograms. A photogram is obtained by measuring the Ne+ (Fe or Ne 1Fe reflection intensity as a function of the azimuthal angle C#B and the elevation angle $ at a fixed scattering angle 9. In fact azimuthal spectra are measured for discrete values of the elevation angle, in the range 0” -Z # -C 9. A spectrum is given by five shades of grey. The highest intensity is represented by a black dot, the lowest by a white one. The scaling of all photograms given in this paper is a linear scaling between zero and maximum intensity for every horizontal line (i.e. for every azimuthal distribution). However, no photograms for the Net [Fe reflection intensity of the oxygen-covered surfaces can be obtained due to ion-induced desorption. If a desorption of 10% of a monolayer of oxygen is accepted, the measuring time is limited to about half a minute. We therefore present NeC JFe reflection yields in the form of one single azimuthal spectrum, when dealing with oxygen-covered surfaces. Specular reflection, which
184
I_
30
J&
I 3
2
1 FLIGHT
TIME
_
I 4
Fig. 2. The TOF spectra for a clean Fe surface (solid curve) and an oxygen-covered Fe surface (dashed curve) at a scattering angle of 9 = 45O were measured at specular reflection in the [OOl] azimuthal direction. The time window which was used in the experiments has been indicated.
means that the elevation angle of incidence is equal to the angle of ejection 19- 4, was chosen in order to avoid mutual shadowing and blocking of first layer iron atoms at a scattering angle of 19= 45”. If neutrals are detected, photograms can be obtained for both clean and oxygen-covered surfaces, as the necessary ion dose is a factor of 100-1000 lower. In fig. 2 the TOF spectra for a clean and an oxygen-covered surface, measured under the same experimental conditions as in fig. 1, are presented. The spectra are given on an absolute scale. A broadening of the peak towards longer flight time (lower energy) is observed for an oxygen-covered Fe(lOO) surface. This is due to the contribution of Or,, and Ne 10 reflection. None of the peaks are separated from each other, since the resolution of the spectrometer is insufficient. Nevertheless it will turn out that the neutrals are suited for a structure analysis of oxidized Fe(lOO) surfaces. In order to select the Ne 1Fe reflection intensity in the TOF spectrum a time window was applied. The centre of the window of adjustable width (typical 0.1 ps) was located at the time corresponding to the highest yield, as indicated in fig. 2.
3. Results and discussion 3.1. The clean Fe(lO0) surface In this section the clean Fe(lOO) surface is studied with photograms. A photogram for the NeC [Fe reflection intensity
the help of of the clean
J.M. oan Zoest et al. / LEIS surface structure analysis of Fe(lOO)-0
185
Fe(lOO) surface measured at a scattering angle of 9 = 125O is shown in fig. 3. Reduced intensities due to mutual shadowing of first and second layer Fe atoms in the main crystallographic directions are clearly visible. A schematic view of the atomic orientation in the first and second Fe layer is shown in fig. 4. For clarity some theoretically expected shadow and blocking cones for a non-reconstructed Fe(lOO) surface have been depicted in fig. 3. The shadow and blocking cone dimensions have been calculated following Martynenko [9]. The Firsov potential used by Martynenko, however, was multiplied by a factor 0.8 according to Mbller et al. [lo]. From fig. 3 it can be concluded that there is a good agreement between the calculated and the measured shadow (at 4 c 62.5O) and blocking cones (at # > 62.5”). However, surface relaxation of the interlayer spacing cannot be determined accurately by means of such a photogram. Most of the following experiments have been performed at a scattering angle of 8 = 45”. A photogram for the Ne+ 1Fe reflection intensity of the clean Fe(lOO) surface measured at that scattering angle is shown in fig. 5. A photogram for the Ne ]Fe reflection intensity of the clean Fe(lOO) surface
1
LOO11
LOi31 AZIMUTHAL
to:,11
[OIlI ANGLE
co101
9
Fig. 3. A photogram for the Ne’ IFe reflection intensity of a clean Fe(100) surface. measured at a scattering angle of 9 = 125”. The Ne+ [Fe reflection intensities are indicated with five shad< :s of grey; the darker dots corresponding to higher intensities. Some theoretically expected shadow and blocking cones are indicated (see text).
186
Fe (100
1
t 2.87
-22,87A-
(017)
PLANE
Fig. 4. A schematic top and side view of the atoms in the first and second Fe layer of a Fc(100) crystal. The large circles represent the Fe atoms in the first layer and the small circles represent the Fe atoms in the second layer. The main crystallographic directions are indicated.
measured under the same conditions is given in fig. 6. The main difference between these two photograms is the pronounced minimum in the [OOl] azimuthal direction around specular reflection when the Ne /Fe reflection signal is measured. For convenience, equivalent surface directions will not be given. The interpretation of this minimum will be discussed in the following section.
3.2. The oxygen-covered
or oxidized Fe(lO0) surface
As mentioned in the introduction we focused our attention on four differently prepared oxygen-covered or oxidized surfaces: (i) The Fe(100) surface after an oxygen exposure of 15 L (150 s at Po_ = 1 X lop7 Torr) at room temperature.
J. M. uan Zoest et al. / LEIS surface structure am+
cob11
ro; 31 AZIMUTHAL
Fig
6.
A photogram
;0311
roll1 ANGLE
of Fe(lOO)-0
187
[0103
9
for the Ne JFe reflection intensity of a clean Fe(100) surface. scattering angle of 8 = 45 o
measured
at a
188
J.M. cmn
Zoestet al. / LEIS surJuce structureana!ym of Fe(lOO)-0
(iv) The oxide layer mentioned under (iii) after annealing at 700 K for 15 min and cooling to room temperature. The oxygen exposure of 15 L at room temperature results in a coverage of approximately one monolayer of chemisorbed oxygen. On the apparatus there was unfortunately no technique for measuring the absolute oxygen coverage. By Vink et al. [4] the oxygen-related signals, using LEIS, were measured as a function of time in order to obtain high-resolution depth information from the surfaces mentioned above. These ion-induced desorption profiles showed a decrease of the oxygen related signals as a function of time. Due to the observed ion-induced desorption our measuring time had to be limited to half a minute at an ion flux of 4 X lop2 A me2. The azimuthal distributions of the Net ]Fe reflection intensity from the differently prepared oxygen-covered surfaces were obtained within this limited measuring time. They are shown on the same scale in figs. 7a-7e and are normalized to the Ne+ ]Fe reflection intensity in the [OOl] surface direction from the clean Fe(lOO) surface. The ion-induced desorption of oxygen during such a measurement was approximately 10% of a monolayer. Before giving a detailed discussion on the individual spectra, a few general statements can be made. In fig. 7a the azimuthal distribution of the Ne+ ]Fe reflection from the clean Fe(lOO) surface is given which will serve as a reference for the discussion of the subsequent spectra. When the Fe(lOO) surface is covered with a monolayer of oxygen or saturated with oxygen at room temperature, a relative lack of azimuthal anisotropy is observed (figs. 7b and 7d). This is probably due to disorder of the substrate surface [4,11,12] and neutralization of the Ne+ ions by the adsorbed oxygen atoms. When both surfaces are annealed at 700 K the surfaces become well-ordered as can be concluded from the azimuthal distributions depicted as figs. 7c and 7e. Both spectra are almost identical, which suggests that the structure of the topmost layers of both surfaces are the same. The spectra 7a, 7c and 7e are discussed in more detail now, starting with the azimuthal distribution of the Net ]Fe reflection intensity from the clean Fe(lOO) surface, as shown in fig. 7a. We observe that the Net ]Fe reflection intensity in the [OOl] azimuthal direction is about twice as high as the Ne+ 1Fe reflection intensity in the [031] azimuthal direction. This can be explained with the help of fig. 4. In the [OOl] azimuthal direction it is possible for the Ne+ ions to reflect from the first two Fe layers whereas in the [031] azimuthal direction and at the elevation angle of incidence of 4 = 22.5”, it is only possible to reflect from the first Fe layer as the atoms of the second Fe layer are shadowed by the first layer. The maxima close to the [OOl] and [Oil] azimuthal directions can be attributed to focusing of the Ne’ ions onto the second Fe layer. When the monolayer of oxygen prepared at room temperature is annealed and subsequently cooled to room temperature the azimuthal distribution of
J. M. uan Zoest et al. / LEIS surface structure ana!ysrs of Fe(lOO)-0
:I= 0
1
co111
Lo131 AZIMUTHAL
ANGLE
[031]
[O’O’
189
1
‘+’
Fig. 7. The azimuthal distributions of the Ne+ IFe (a-e) and the Ne(Fe (a’-e’) reflection intensity from the clean, oxygen-covered and oxidized Fe(100) surfaces, measured at an elevation angle of incidence of $ = 22S” (ions) and LJJ= 22O (neutrals) at a scattering angle of 9 = 45O. Please note the different angular scale units. (a, a’): The clean Fe(100) surface: (b, b’): a monolayer of oxygen on the Fe(100 surface, prepared at room temperature; (c, c’): the monolayer of oxygen on the Fe(100) surface, prepared at room temperature, after annealing and subsequent cooling to room temperature; (d, d’): the Fe(100) surface after saturation with oxygen at room temperature; (e, e’): the oxide layer mentioned under (d) and (d’) after annealing and subsequent cooling to room temperature.
the Ne+ (Fe reflection intensity shown in fig. 7c is observed. The Net (Fe reflection intensity is no longer independent of the azimuthal angle. The maxima in this azimuthal distribution can be attributed to focusing of Ne+ ions onto the second Fe layer. As the focusing of Ne+ ions onto the second Fe layer is not observed in the azimuthal distribution of the Ne+ (Fe reflection intensity from the monolayer of oxygen prepared at room temperature it can be concluded that due to annealing the monolayer of oxygen becomes well-
ordered. So, the monolayer of oxygen prepared at room temperature on Fe(lOO) is disordered. The azimuthal distribution of the annealed monolayer of oxygen can be very well explained with a monolayer of oxygen in which the oxygen atoms are located in the fourfold symmetrical hollows of the substrate. This surface structure was reported previously by Legg et al. [2] who carried out a LEED structure analysis of the Fe(lOO) p(1 X 1)-O structure. The vertical position of the oxygen atoms with respect to the first Fe layer will be discussed in section 3.3. Oxygen atoms located in the fourfold symmetrical hollows of the substrate should effectively shadow the second Fe layer when the Net (Fe reflection intensity is measured in the [OOI] azimuthal direction. This is in fact what we observe. With respect to the clean Fe(lOO) surface the Ne+ (Fe reflection intensity in the [OOl] azimuthal direction is a factor of two smaller. An alternative explanation of this decrease of the Ne’ IFe reflection intensity is that the neutralization probability for the Ne+ ions reflected from the second Fe layer has become larger due to the adsorbed oxygen atoms in the fourfold symmetrical hollows. A discrimination between these two possibilities can be made by comparing the azimuthal distributions of the Ne+ 1Fe and of the Ne ]Fe reflection intensity from the same oxygen-covered or oxidized surface. This will be shown at the end of this section. After the substrate surface was saturated with oxygen at room temperature, annealed and subsequently cooled to room temperature the same azimuthal distribution of the Ne+ (Fe reflection intensity was measured (fig. 7e) as in the case of the annealed monolayer of oxygen. The structure of the chemisorbed oxygen atoms in the first Fe layer is therefore the same for both annealed oxygen-covered surfaces and corresponds to a monolayer of oxygen of which the oxygen atoms are located in the fourfold symmetrical hollows of the substrate. In order to examine the effect of neutralization on the measurements of the Ne+ 1Fe reflection intensity, azimuthal distributions of the Ne )Fe reflection intensity were measured. These measurements have also been performed to extend our information on the structure of these oxygen-covered or oxidized surfaces beyond the first and the second Fe layer. Due to the large neutral fraction, measurements of the Ne ]Fe reflection intensity could be performed with low dosis. This allowed us to obtain complete photograms of the oxygen-covered or oxidized surfaces since ion-induced desorption could be neglected. The azimuthal distributions of the Ne (Fe reflection intensity measured at an elevation angle of incidence of + = 22” and at a scattering angle of 8 = 45” are shown in figs. 7a’-7e’. The data were not scaled to the target current since it could not be measured accurately. Instead the azimuthal distributions have been normalized to the Ne 1Fe reflection intensity in the [013] azimuthal direction. In the [013] azimuthal direction the Ne ]Fe reflection intensity is due to scattering of the Ne ’ ions by the first Fe layer only. ions by the first Fe layer in the [013] azimuthal The scattering of Net
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direction is not influenced by the adsorbed oxygen atoms; this will be evidenced in section 3.3. The azimuthal distribution of the Ne 1Fe reflection intensity obtained from the clean Fe(lOO) surface is shown in fig. 7a’. The most important difference with respect to the azimuthal distribution of the Ne+ ]Fe reflection intensity (fig. 7a) is the presence of high Ne 1Fe reflection intensities close to the [OOl] azimuthal direction. These high Ne (Fe reflection intensities are probably due to focusing of Ne+ ions onto the second and third Fe layer. These high focusing intensities are not observed when the Ne+ IFe reflection intensity is measured because of the large neutralization probability for scattering by the third Fe layer. In the [OOl] azimuthal direction the Ne ]Fe reflection intensity is 2.5 times larger as in the [013] azimuthal direction (reflection from the first Fe layer). The Ne ]Fe reflection intensity in the [OOl] azimuthal direction is apparently not only due to the Net ions reflected from the first two Fe layers, but also to Ne+ ions reflected from the third Fe layer. After an oxygen exposure of 15 L at room temperature, the azimuthal distribution of the Ne 1Fe reflection intensity depicted in fig. 7b’ is measured. The maxima in this azimuthal distribution are caused by focusing effects on deeper Fe layers which are possible despite the disordered monolayer of chemisorbed oxygen atoms. Due to neutralization these focusing effects on deeper iron layers are not observed in the azimuthal distribution of the Ne+ I Fe reflection intensity. The Ne I Fe reflection intensity in the [OOl] azimuthal direction has diminished with respect to the clean Fe(lOO) surface due to shadowing and blocking of the second Fe layer by the adsorbed oxygen atoms. After annealing of this disordered monolayer of oxygen and subsequent cooling to room temperature the azimuthal distribution of the Ne I Fe reflection intensity shown in fig. 7c’ was measured. This azimuthal distribution is almost identical to the one obtained from the clean Fe(lOO) surface. The Ne I Fe reflection intensity has slightly diminished in the azimuthal direction close to the [OOl] azimuthal direction and in the [OOl] azimuthal direction itself. This azimuthal distribution from the annealed monolayer of oxygen atoms shows that focusing of Ne+ ions onto the third Fe layer is still possible. When the oxygen atoms are located in the fourfold symmetrical hollows of the Fe(lOO) surface shadowing of the second Fe layer in the [OOl] azimuthal direction is expected. The Ne )Fe reflection intensity in this azimuthal direction, however, has not diminished to the level of only the first Fe layer (such as measured in the [013] azimuthal direction). A possible explanation for this apparent inconsistency is that the Ne + ions are focused on the third Fe layer by the adsorbed oxygen atoms. As a consequence the decrease of the Ne I Fe reflection intensity in the [OOl] azimuthal direction due to the shadowing of the second Fe layer by the adsorbed oxygen atoms is counter-balanced by focusing of the Net ions on the third Fe layer.
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J. M. uan Zoest et al. / LEIS surfme structure una!ysis of Fe(lOO)-0
The azimuthal distribution of the Ne ]Fe reflection intensity from the Fe(lOO) surface which has been saturated with oxygen at room temperature is given in fig. 7d’. A significant dependence of the Ne 1Fe reflection intensity on the azimuthal angle is measured. This structure is caused by Ne+ ions reflected from the second and deeper Fe layers. All these Net ions are neutralized, which is evidenced by the fact that the Ne+ ]Fe reflection intensity from this oxide layer is independent of the azimuthal angle. The Ne ]Fe reflection intensity in the [OOl] azimuthal direction has diminished with respect to the clean Fe(lOO) surface due to shadowing and blocking of the second Fe layer by the adsorbed oxygen atoms. The azimuthal distribution of the Ne (Fe reflection intensity from the substrate surface which has been saturated with oxygen, annealed and subsequently cooled to room temperature is depicted in fig. 7e’. The high focusing intensities close to the [OOl] azimuthal direction have decreased with respect to those observed for the clean Fe(lOO) surface (fig. 7a’). From this decrease it can be concluded that the first Fe layer still focuses Ne’ ions onto the second Fe layer, but focusing of Ne + ions on the third Fe layer is not possible anymore. This can be due to oxygen atoms which are located in the octahedral hollows between the first and the third Fe layer. The Ne JFe reflection intensity in the [OOl] azimuthal direction has an intensity which is almost as high as the Ne ]Fe reflection intensity from the first Fe layer. This is due to the expected shadowing of the second Fe layer by the adsorbed oxygen atoms in the fourfold symmetrical hollows of the substrate. The shadowing of the second Fe layer by the adsorbed oxygen atom is no longer compensated by focusing of the Ne+ ions on the third Fe layer (as in the case of the annealed monolayer of oxygen) since the third Fe layer is shadowed by the oxygen atoms in the octahedral hollows. The azimuthal distributions of the Ne [Fe reflection intensity shown in figs. 7b’ and 7e’ are almost identical. A disordered monolayer of oxygen results apparently in nearly the same azimuthal distribution as a well-ordered monolayer of oxygen with also oxygen atoms in deeper layers. The azimuthal distributions of the Ne+ JFe reflection intensity from both annealed oxide layers (figs. 7c and 7e) could be explained in two ways: the Ne+ 1Fe reflection intensity in the [OOl] direction had decreased by neutralization of the Ne+ ions reflected from the second Fe layer or it had decreased due to shadowing of the second Fe layer by the adsorbed oxygen atoms in the fourfold symmetrical hollows. In the azimuthal distribution of the Ne (Fe reflection intensity from the Fe(lOO) surface that has been saturated with oxygen, annealed and cooled to room temperature a decrease of the reflection intensity in the [OOl] azimuthal direction is observed with respect to the clean Fe(lOO) surface. Therefore it can be concluded that the Net (Fe reflection intensity in the mentioned azimuthal direction from the annealed oxygencovered or oxidized surfaces has decreased due to shadowing of the second Fe layer by the adsorbed oxygen atoms.
J. M. uan Zoest et al. / LEIS surface strucfure ardysis
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3.3. The vertical position of oxygen in the fourfold symmetrical sut-face
193
site on the Fe(lO0)
‘The preceding azimuthal distributions of the Ne+ (Fe and the Ne (Fe reflection intensity from the annealed oxygen-covered or oxidized surfaces are well explained with a monolayer of chemisorbed oxygen of which the oxygen atoms are located in the fourfold symmetrical hollows of the substrate. It was also possible to determine the distance between the plane of these oxygen atoms and the plane of the first Fe layer. This information was obtained by measuring the OP recoil intensity as a function of the elevation angle of incidence. During the measurement of the O- recoil intensity we have not only varied the elevation angle of incidence 4 but also the scattering angle 9. The O- recoil intensity appeared to be maximal under specular conditions (9 = 24) for scattering angles up to 45 O. The measurements were performed in the [Oil] and [OOl] azimuthal directions. In fig. 8 the collision geometry is given at a low elevation angle of incidence in the [Oil] azimuthal direction, assuming that there is a complete coverage with oxygen of all the fourfold symmetrical hollows of the Fe(lOO) surface. From fig. 8a a low O- recoil intensity is expected at low elevation angles of incidence since each oxygen atom is in the shadow cone of its nearest neighbour in the [Oil] azimuthal direction. The collision geometry at a critical elevation angle of incidence I/$ is shown in fig. 8b. At that critical elevation angle of incidence an incident Ne+ ion on the edge of the shadow cone of the nearest-neighbour oxygen atom has
0
0
0
0
Fig. 8. A side view of the [Oli] plane from the Fe(100) surface with a complete coverage of all the fourfold symmetrical hollows with oxygen. The Fe atoms and the adsorbed oxygen atoms in the fourfold site are represented with large and small circles respectively. The shadow cones for the 5 keV Ne+ ions scattered by the oxygen atoms were calculated following Marchut et al. [l]. (a) Collision geometry at an elevation angle of I$ < 4;. (b) Collision geometry at the critical elevation angle #t.
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et d. / LEIS surface structure cmalp-is of Fe(lOO)-0
an impact parameter with the oxygen atom which is just small enough to scatter the oxygen atom through a scattering angle {. Therefore we expect a steep increase of the number of detected O- recoils at $‘,. The effect is substantially enhanced by a “focusing effect”, the incoming intensity being concentrated just outside the shadow cone. During a measurement of the OP recoil intensity as a function of the elevation angle of incidence there is of course a considerable ion-induced desorption of the oxygen atoms. A representative collision geometry after a certain desorption period is given in fig. 9. At a low elevation angle of incidence there is no longer the situation in which the oxygen atoms are shadowed by next-nearest-neighbour oxygen atoms. Instead we have at a low elevation angle of incidence the situation in which the oxygen atoms are shadowed by the Fe atoms in the first Fe layer (fig. 9a). The collision geometry at a critical elevation angle of incidence Ir/f is shown in fig. 9b. At this critical elevation angle of incidence again a steep increase of the O- recoil intensity is expected. Three measurements of the O- recoil intensity as a function of the elevation angle of incidence 4 have been made consecutively for the annealed monolayer of oxygen in the [Oil] azimuthal direction and are shown in fig. 10. During the first measurement of the OP recoil intensity as a function of the elevation angle of incidence, approximately 40% from the monolayer of oxygen atoms is desorbed due to the Net bombardment. In order to compare the three measurements, the measured O- recoil intensities (IoFCC) have been divided by the maximum O,, intensity ( Io,L._aV) from that measurement. In all
Fig. 9. A side view of the [Oli] plane for an incomplete monolayer of oxygen (due to ion-induced desorption). The Fe atoms and the oxygen atoms in the fourfold site are represented with large and small circles respectively. The shadow cone for the 5 keV Ne + ions scattered by the Fe atom was calculated following Marchut et al. [l]. (a) Collision geometq at an elevation angle of + < 4:. (b) Collision geometry at the critical elevation angle $f.
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measurements a steep increase of the OP recoil intensity as a function of the elevation angle of incidence is observed. With respect to the first measurement the steep slopes of the second and third measurement have shifted towards smaller elevation angles of incidence. As explained above, this shift is due to the desorption of nearest-neighbour oxygen atoms. The angle at half height of the O- recoil intensity is taken as the critical angle of incidence [13]. In order to determine the distance between the plane of the oxygen atoms and the plane of the first Fe layer (y), the critical angle of the measurements 2 and 3 from fig. 10 is used. From this figure it is found that +f = 6.9” k 0.5”. At this critical elevation angle of incidence an incident Ne+ ion on the edge of the shadow cone of the preceding Fe atom of the first Fe layer has an impact
j-
)
L
0"
L
II
_______
IO0 ELEVATION
20” ANGLE
$
Fig. 10. Three measurements of the O- recoil intensity as a function of the elevation angle of incidence $. These measurements (0,~ A, respectively first, second and third measurement) were performed consecutively for the annealed monolayer of oxygen in the [Oil] azimuthal direction and have been scaled on their own maximum.
parameter p with the adsorbed oxygen atom which is just small enough to scatter it through a scattering angle I (fig. 9b). In order to calculate y, the impact parameter p and the radius of the shadow cone as a function of the distance behind the Fe atom of the first layer R(L) have to be known. As shown in the photogram of fig. 3 the calculated shadow cones of Fe are in good agreement with the measured shadow cones. Therefore we used the same expression for the radius of the shadow cone as a function of the distance behind the Fe atom in this case. The impact parameter p of the incident Ne+ ion with the oxygen atom was calculated in the manner of Mashkova and Molchanov [14] with the approximation to the Thomas-Fermi potential used by Marchut et al. [l]. The result of this calculation was p = 0.02 A ({ = 13.8O). Using this impact parameter and the expression for the radius of the shadow cone and the critical elevation angle of incidence it was calculated that the oxygen atom is located 0.56 + 0.05 A above the plane of the first Fe layer. The critical angle $‘, has to result in a vertical position of the oxygen atom y’, relative to its nearest neighbour, which is not significantly different from zero. This is, however, in the [Oil] azimuthal direction not the case. The critical angle of +k = 10.6” + 0.5” results in a relative vertical position of I” = - 0.14 + 0.05 A. This discrepancy can be attributed to the first layer Fe atom which is lying between the two oxygen atoms. These iron atoms can deflect the incident Ne’ ion on the edge of the shadow cone of its nearestneighbour oxygen atom through a small angle. As a consequence, the critical angle 4: is larger than expected. In the [OOl] azimuthal direction, the critical angle is not influenced by the deflection of the Ne + ions from the first Fe layer. A measurement of the 0 recoil intensity as a function of the elevation angle of incidence in the [OOl] azimuthal direction is depicted in fig. 11. The measured critical angle of +f = 9.8” i_ 0.5O results in a vertical position of the oxygen atom, relative to its nearest neighbour, of _p’= + 0.05 I 0.05 A, which is indeed not significantly different from zero. When the measurements of the O- recoil intensity as a function of the elevation angle of incidence were performed for a Fe(lOO) surface, which had been saturated with oxygen at room temperature, annealed and cooled to room temperature, the same critical angles and shift of the steep slope were observed as in the case of the annealed monolayer of chemisorbed oxygen. So the distance between the plane of the adsorbed oxygen atoms and the plane of the first Fe layer is the same for both annealed oxygen-covered or oxidized surfaces. The vertical position of the oxygen atom in the fourfold symmetrical hollow of the substrate is substantiated by the photogram of the Ne 1Fe reflection intensity shown in fig. 12. This is a photogram of the Fe(lOO) surface which had been saturated with oxygen at room temperature,* annealed and subsequently cooled to room temperature. At the elevation angle of incidence of 4 = 11” in the [013] azimuthal direction one sees clearly the shadowing of the
J.M. van Zoest et al. / LEIS surface structure ana!ysis of Fe(lOO)-0
ELEVATION
ANGLE
197
+
Fig. 11. The measurement of the O- recoil intensity as a function of the elevation angle of incidence 4, which was performed for the annealed monolayer of oxygen in the [OOI] azimuthal direction.
first Fe layer by the adsorbed oxygen atoms. At the elevation angle of incidence of 4 = 35” the corresponding blocking of the first Fe layer is observed. These shadow and blocking dips can only occur, when the oxygen atoms are located above the plane of the first Fe layer in the fourfold symmetrical hollows. These shadow and blocking dips are of course absent in the photogram of the clean Fe(lOO) surface as shown in fig. 6. It can be calculated that at the elevation angle of incidence 4 = 22O and in the [013] and corresponding azimuthal directions the first Fe layer is not shadowed or blocked by the adsorbed oxygen atoms at 8 = 45 O. Normalization of the azimuthal distributions of the Ne IFe reflection intensity on the Ne [Fe reflection intensity in the [013] azimuthal direction, such as done in section 3.2, is therefore justified.
198
J. M. run Zoest et ul. / LEIS swfocr structure clnuo’s~s of Fe(lOO)-0
45” 1
-1301;
7.--
--
io7 3: AZIMUTHAL
-
--r--
-
i@l!]
--- r ro311
ANGLE
-
1 --- -iOlOj
-
-
‘$J
Fig 12. A photogram for the Ne IFe reflection intensity of a Fc(100) surface which was saturated wit11 oxygen, annealed and cooled to room temperature, measured at a scattering angle of Q= 4S”. The additional shadow and blocking cones ($J =ll” and I&= 35O, respectively). with respect to the dean Fe(lO0) surface, are due to the presence of the adsorbed oxygen atoms in the fourfold
Legg et p(1 x 1)-O interesting tioned. and
symmetrical
hollows of the substrate.
al. [Z] reported on the LEED structure analysis of the Fe(lOO) structure and found y = 0.48 A for the oxygen position. It is most that both this quantitative LEED analysis by the authors menLEIS as a “real space” technique come to the same result.
4. Conclusions From the present surface structure determination of differently prepared oxygen-covered or oxidized Fe(lOO) surfaces, the following conclusions can be made. _ A monolayer of oxygen adsorbed on Fe(lOO) at room temperature appears to be disordered in terms of surface structure. _ Beyond a monolayer cove!age, i.e., when the oxide layer on Fe(lOO) is grown to a limiting thickness (15 A from ref. [4]) at room temperature, the topmost layers remain disordered. _ Annealing of the unordered monolayer of oxygen present on the Fe(lOO) surface results in a well defined p(1 x 1) surface structure, where the oxygen atoms are situated in the fourfold symmetrical hollows of the substrate surface. The adsorbed oxygen atoms are located at a distance of 0.56 & 0.05 A above the first substrate layer of iron atoms.
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199
_ As concluded before [4], an oxide layer grown on Fe(lOO) at room temperature reconstructs at elevated temperatures leaving a monolayer of oxygen on top of the surface beneath which a different oxide phase is present. The surface structure of this oxygen monolayer appears to be identical with respect to the surface properties found for a single monolayer of oxygen on Fe(lOO) (which are given above). The results obtained from the reconstructed oxide surface (given in the last conclusion) are especially relevant with respect to a recent study [15] on the interaction of hydrogen with this particular surface. It appeared that the monolayer of oxygen, left on top of the surface after reconstruction, is inactive to hydrogen whereas the incorporated oxide can be removed under identical reaction conditions.
Acknowledgements The authors thank Professor Dr. A. Niehaus for helpful discussions and for a critical reading of this manuscript. This work was performed as part of the research programme of both the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Stichting Scheikunding Onderzoek in Nederland (SON) with financial support from the Nederlandse Organisatie voor ZuiverWetenschappelijk Onderzoek (ZWO).
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15]
L.. Marchut, T.M. Buck, G.H. Wheatley and C.J. McMahon, Jr., Surface Sci. 141 (1984) 549. K.O. Legg, F. Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. B16 (1977) 5271. K.O. Legg, F. Jona, D.W. Jepsen and P.M. Marcus, J. Phys. Cl0 (1977) 937. T.J. Vink, J.M. der Kinderen, O.L.J. Gijzeman, J.W. Geus and J.M. van Zoest, Appl. Surface Sci. 26 (1986) 357. R..P.N. Bronckers, Thesis, Utrecht (1981). R..P.N. Bronckers, Th.M. Hupkens, A.G.J. de Wit and W.C.N. Post, Nucl. Instr. Methods 179 (1981) 125. E.G. Keim, F. Labohm, O.L.J. Gijzeman, G.A. Bootsma and J.W. Geus, Surface Sci. 112 (1981) 52. Th.M. Hupkens, Nucl. Instr. Methods B9 (1985) 277, 285. Yu.V. Martynenko, Radiation Effects 20 (1973) 211. J. MFller, H. Niehus and W. Heiland, Surface Sci. 166 (1986) Llll. M. Watanabe, M. Miyamura, T. Matsudaire and M. On&i, in: Proc. ICSS-2, Kyoto, 1974, p. 501. G.W. Simmons and D.W. Dwyer, Surface Sci. 48 (1975) 373. H. Niehus and G. Comsa, Surface Sci. 140 (1984) 18. E.S. Mashkova and V.A. Molchanov, in: Modem Problems in Condensed Matter Sciences, Veal. 11, Eds. V.M. Agranovich and A.A. Maradudin (North-Holland, Amsterdam, 1985). T.J. Vink, J.M. der Kinderen, O.L.J. Gijzeman and J.W. Geus, Appl. Surface Sci. 26 (1986) 367.