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Electron-induced segregation to surfaces of the superionic conductor Naβ-alumina studied by AES and XPS
314
Surface Science 119 (1982) 314-330 North-Holland Publishing Company
ELECTRON-INDUCED SEGREGATION TO SURFACES OF THE SUPERIONIC CONDUCTOR Nap-ALUMINA STUDIED BY AES AND XPS A. LIVSHITS Department
of Physics, Ben Gurion University of the Negeu, Beer-Sheun.
Israel
and M. POLAK
*
Departments of Materials Engineering and Nuclear Engineering, Beer-Sheva. Israel Received
14 October
1981; accepted
for publication
31 March
Ben-Gurion
University of the Negeo,
1982
Pronounced changes in sodium surface concentration and chemical state were observed after electron bombardment of cleaved single crystal Na&alumina, which is known to exhibit unusually fast Na+ transport. In particular, a distinct increase in the sodium Auger signal started at a certain energy and current density of the primary electron beam and was always accompanied by a large surface charging. Thus, migration of Na+ under the influence of the induced electric fields was found to be the major process leading to the sodium segregation at the Nap-alumina cleavage surface. Possible mechanisms for the segregation process are discussed. XPS measurements of the bombarded surface revealed the appearance of neutral sodium atoms as well as sodium-oxygen multilayer surface compounds. A simple method to obtain the electric potential distribution at the surface (“charging-map”) is demonstrated. The sodium distribution on the Nap-alumina surface after e-bombardment was found to be reflected in the corresponding charging-map.
1. Introduction Superionic conductors constitute a unique group of solids characterized by extremely fast ion transport, even at room temperature, orders of magnitude faster than ordinary ionic solids. One of these materials, Nap-alumina exhibits a two-dimensional, relatively high conductivity (- 10e2 Q-’ cm-‘) with sodium cations as the charge carriers [ 11. It is an inherently non-stoichiometric compound having excess sodium, which can be described by the formula (1 + X) Na,O . 11 Al,O,, where X ranges from about 0.2 to 0.4 [2]. Bulk properties of Nap have been intensively studied [3] using, for example, * To whom all correspondence
should be addressed.
0039-6028/82/0000-0000/$02.75
0 North-Holland
A. Ltvshits, M. P&k
/ Eleciron-induced segregation
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diffraction techniques [2], nuclear magnetic resonance [4,5], tracer-diffusion and ionic conductivity measurements [6,7]. It was found that the highly mobile sodium ions are confined to relatively spacious basal planes 11.3 A apart (“conduction planes”), separated by spinel-like closely packed blocks of aluminum and oxygen (symmetry D6 h) [2]. Being through widely separated oxygen atoms, the binding between the spine1 blocks is weak and crystals are readily cleaved along the sodium-rich conduction planes. It makes this unique material attractive for surface studies. In particular, since the conduction plane lying next to the top plane is 11.3 A deep in the crystal, most of the sodium KLL Auger electrons detected (escape depth - 13 A) should originate from the top, cleavage-exposed, conduction plane. It is the purpose of this paper to report the first results of such studies concerning the response of the sodium ions in Nab-alumina to electron bombardment of the cleaved single crystal surfaces. Auger electron spectroscopy (AES) including scanning experiments, and X-ray photoelectron spectroscopy (XPS) were used in a complementary way. After a relatively heavy electron bombardment causing severe negative charging, a milder beam had to be used to monitor the induced composition changes Mechanisms for a sodium migration including their surface distribution. process leading to segregation of sodium at the surface are discussed in section 4. Since surface charging appeared to play a significant role in the processes observed, a scanning mode of operation was used to obtain “charging-maps” by monitoring the locally shifted secondary-emission energy. The results are compared to the response of a conventional ionic solid, soda silica glass, when bombarded by an identical electron beam. Beam induced variations in the chemical state of sodium were studied by XPS. Due to extra-atomic relaxation, chemical shifts in the Na Auger electron energy are known to be larger than the photoelectron shifts [8,9]. Indeed, most of the chemical-state information was deduced from shifts in the X-ray excited Auger lines. Under certain electron-beam conditions, a decay of the Na AES signal with bombardment time was observed. The results are discussed in terms of electron stimulated desorption (ESD) of sodium from the Nap surface.
2. Experimental Single crystal slabs of Nap-alumina, about 1 mm thick, were air-cleaved from a large single crystal (grown at Union Carbide). The orientation was confirmed by X-ray diffraction to be parallel to the conduction planes. A Physical Electronics Model 549 XPS/SAM instrument with a single cylindrical mirror analyzer (CMA) for AES-SAM experiments, and a double CMA for the XPS measurements. was used. The electron beam used for
316
A. Liushits, hf. Polak / Electron-induced segregation
bombardment and subsequent AES measurements made an angle of about 60’ with the crystal surface. A mild, 5OOV, Ar ion-sputtering was in some cases used to prepare a clean, carbon-free surface (base pressure - 3 X lo-” Torr). The same mild ion beam was also used to obtain in-depth profiles of sodium, aluminum and oxygen before and after electron bombardment. The major effects started at distinct primary-beam energy and current density and were accompanied by an unstable negative charging of several hundred volts manifested through large energy shifts of the Auger spectrum. In order to monitor the electron-beam induced effects under heavy bombardment, spectra were recorded using reduced beam energy and current density. These standard monitoring conditions of 2 kV and 0.3 A cm-’ usually gave stable Auger spectra, and when applied to a fresh surface produced only relatively slow variations attributed to ESD of sodium. During the recording time of the Auger spectrum following heavy bombardment, this effect was negligible. The same beam parameters were used for the scanning Auger and charge-scanning experiments giving a stable and reproducible mapping. For XPS measurements (using Mg Ken X-rays) several adjacent spots were first electron bombarded to produce a large enough affected area, and then the sample was oriented before the double CMA by monitoring the typical absorbed-current image of the bombarded surface. A small positive charging of 1 to 3 eV occurred during the XPS measurements since Na/3 is an insulator with respect to conduction of electrons. The sodium lines were referenced to the 1s line of carbon (binding energy of 284.6 eV) when present, or to the Al(2p) line of Al,O, at 74.5 eV [lo]. 3. Results Pronounced variations in Nap-alumina surface composition were found to depend primarily on the energy and current density of the impinging beam and on the bombardment time. Fig. 1 shows an initial Auger spectrum of Nap obtained with an “ineffective” 0.17 A cm-*, 5 kV electron beam. A rough quantitative estimate based on the peak-to-peak amplitudes of sodium, aluminum and oxygen lines gives concentration values close to the bulk stoichiometry in Na&alumina (Na: Al : 0 = 1: 9 : 14). Considering the particular layered structure of this crystal and the use of standard Auger sensitivity factors this result seems to be accidental. (According to a detailed XPS quantitative analysis [l l] each of the two cleavage-exposed surfaces, not surprisingly, contains about half the number of sodium ions in a normal bulk conduction plane.) The spectrum (fig. 1) served as a proper reference for the heavier bombardment experiments. 3.1. AES and charge mapping
after heavy electron bombardment
When high current densities, around 1 A cm-*, and primary-beam energies starting from 3.5 keV were used, drastic changes occurred in the Auger
A. Livshits,
hf. Polak / Electron-indeed
segregative
317
dN dE
hiLMM I 200
GKLL
NaKLL
*KLL
I 400
I 600
1 800
ELECTRON Fig. 1. An Auger electron crystal. The primary-beam
I woo
“KLL , 1200
I 1400
ENERGY, eV
spectrum from the cleavage face (the (0001) plane) of Nab-alumina current density was 0.17 A cm-’ at an energy of 5 keV.
spectrum. Typically, the signal intensity of sodium considerably increased, while aluminum and oxygen signals decreased. The results of a 3 min electron bombardment with a 5 kV, 1 A cm- ’ beam are shown in fig. 2b (fig. 2a is equivalent to fig. 1). As can be seen, sodium, the least abundant element in Nap, became the most abundant surface element at the expense of aluminum and oxygen. The process was accompanied by a large negative charging during the heavy bombardment, of which about 200 eV were left when recording the spectrum under the standard beam conditions (fig. 2b). To further elucidate the role of the negative surface charging in the process leading to Na-enrichment, precise mapping of the charging distribution at and around the bombarded area seemed to be desirable. It was performed by scanning the surface with the 2 kV standard beam, while each time monitoring the secondary electrons emitted with a certain kinetic energy according to the local charging. In fig. 3a we present a typical “charging-map” obtained after 3.5 kV, 1 A cm-2,- 1 min bombardment. The scanning speed was set for each energy to 15 s per frame, and the charging-maps were stable and reproducible. The large intensity of the secondary emission entering the CMA allowed the use of considerably reduced detection sensitivities. Under these conditions, the flux of Auger electrons was well below the detection limit, so that the resultant mapping presents pure equipotential lines only. The field strength at each point on the surface can be calculated from a charging-map Such as given in fig. 3a. An almost uniform field of - 2 x lo5 V
318
A. Livshits, M. Pofak / E~ectro~-in~~ce~ segregation
cm-’ is present to the right of the center, and a field of - 7 X lo5 V cm-’ to the left. Auger spectra taken under standard beam conditions at several points :b)
after 5 k'd, IAc~-~
electron
b~bardment
Na
(All
KINETIC ENERGY,eV Fig. 2. Effects of a 3 min heavy electron bombardment Auger spectra. before (a) and after (b) the bombardment, conditions used (2 kV, 0.3 A cm-2).
on Nab-alumina cleavage face. Both were recorded under the standard beam
across the charged surface revealed a direct correlation between the charging level and the amount of sodium, so that the charging-map reflects also the actual sodium distribution on the surface. Repeated 2 kV bombardments at points extending from the center of the char~ng-map to the right and beyond the charged area caused gradual variations in sodium signal intensity, which were reflected also in a subsequent charging-map (fig. 3b). In particular, bombardments outside the charged area in fig. 3a forced out charge (fig. 3b) as well as sodium. In order to obtain the precise unperturbed Na distribution immediately after heavy bombardment, rather than measuring point spectra, fast line scans of sodium seemed to be advantageous. Fig. 3c shows such line scans of sodium Each line represents the taken after 7 kV, 0.4A cm -‘, 1 min bombardment. Na distribution on the surface at a distinct charging level. Again, the amount of sodium correlates with the level of local charging. Clearly, in quantitative measurements, such as the time-dependence of the sodium accumulation on the surface, one has to take into account all the segregated sodium, which is not necessarily confined to the bombarded surface alone but may extend also beyond it. This was done for two bombardment times, 30 and 60 S, where Na Auger spectra were taken at points extending
319
I 990,&
KINETIC
I
I
I
II
lo20,0&lo20
ENERGY (eV)
Fig. 3. Charging-maps (a, b) (magnification 400X) and Na Auger line scans (c) (magnification 1000X) taken after heavy electron bombardment. The equipotential lines in (a) and (b) correspond to 650, 450, 250, 150 and 50 V, beginning at the innermost contour. The bar represents 20 pm beam.
A. Livshits,
320
_+-~,
50 s
$
‘x‘x \
IO
DISTANCE
segregation
bombardmen
beam radius 0
M. Polak / Electron-induced
20
\ \ 30
40
FROM CHARGING-MAP
\
\ 50
CENTER
(pm ) Fig. 4. Line distribution of sodium after 3.5 kV, 1 A cm-’ with the standard beam parameters.
bombardment
for 30 and 60 s, obtained
from the center of the corresponding charging-map along a horizontal line to the edge (fig. 4). If one wishes to obtain a more accurate Na distribution, the size of the electron beam used for data collection has to be taken into account. Assuming radially symmetric Na distribution leads to 20 pm and 40 pm radii for the 30 s and 60 s bombardment times, respectively. The beam radius, 10 pm, obtained also in this straightforward calculation, fits the actual value, and the calculated distribution profiles expected for Na (dashed lines in fig. 4) fit the experimental points. The product of the surface area (s) of segregated sodium and the Auger intensity coming from it (I), is proportional to the total number of Na atoms (Na,) in each case (In general, Na, - jFZ ds). The curves in fig. 4 give Nar(60 s)/Na,(30 s) = 2/l. The variations in surface segregation of sodium with the current density of the impinging beam are shown in fig. 5 for two beam energies. An equal bombardment time of 1 min was used in all measurements. As can be seen, the segregation as well as the surface negative charging are first enhanced by increasing the current density, until a maximum is reached around 2 A cmM2 after which a relatively slow decrease in sodium and charging level occurs. The decrease in Na seems to be due to a different mechanism, which according to other data presented at the end of this section, is attributed to electron stimulated desorption of sodium. At all current densities the segregation process was more pronounced under 7 kV bombardment than under 5 kV. 3.2. XPS measurements There spectrum
after e-bombardment
is a clear distinction before (fig. 1) and
between the shape of the Na Auger electron after heavy electron bombardment (fig. 2b),
A. Liushits, M. Polak / Electron-induced
CURRENT
segregation
321
DENSITY, A~rn-~
Fig. 5. Sodium signal intensity and charging level as function of beam current after 7 kV bombardments; (0) Na after 5 kV bombardments; (m) charging bombardments.
density: (0) Na level after 5 kV
indicating a possible modification in the sodium chemical state and bonding. Also, the surface composition changed considerably (fig. 2). In order to examine the changes in chemical state of sodium, X-ray excited electron spectra were recorded immediately after electron bombardment done at several spots covering a large enough area to be easily detected by XPS. (Still, the area imaged by the analyzer included some fresh, unbombarded surface.) The Auger part of the Na spectrum exhibited the largest chemical shifts and is shown in fig. 6. Two major spectral features appeared after bombardment: a considerable and asymmetric broadening of the single line observed before bombardment, and a new narrow component about 6.5 eV higher in kinetic energy (fig. 6). Also, in agreement with the AES results (e.g., fig. 2) the total area of the peak increased significantly (- 75%). 3.3. In-depth
sodium profiles
The in-depth distribution of sodium after electron bombardment, which was determined by alternate ion sputtering and XPS measurements, is compared to the profile of the unbombarded surface in fig. 7. The time required to remove the sodium-rich layer after electron bombardment was much longer than the time needed for removal of sodium at the top of the freshly cleaved surface. Clearly, a multilayer sodium-rich region was produced by the bombardment (details will be given in the next section). As can be seen, there is an appreciable increase in the Na/Al ratio after e-bombardment, by about a factor of 4 at the maxima (fig. 7). The actual increase is even larger since only part of the area imaged by the analyzer was e-bombarded.
322
A. Lioshits, M. P&k
/ Electron-induced
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No (KLuLm)after c
No (KL,,Lz3) before electron bombardment
I
J
I
I
964
62
k
986
966
KINETIC
I
I
990
992
I
I
996
994
996
ENERGY,eV
Fig. 6. High-resolution X-ray excited heavy electron bombardment.
Auger
spectra
of sodium
in Nab-alumina
before
and after
f+$znbadment I I
\ I 1 I
i
i
20
7%
2, cl= ZQ - -
IO-
\ \ \ I \\ \ I \ \ \ \ I
before e-banbordment
!I \ \ \
\
\ ‘1
0
!!?7--3
5
IO
15
---+7___r_&_ 20 25 30
ION BOMBARDMENT TIME (min 1 Fig. 7. XPS in-depth an electron-bombarded
sputtering profile of sodium/aluminum in air-cleaved Nap-alumina, sample. Defocused Art beam: 500 V, 5 X IO-’ Torr.
and in
A. Livshits,
M. Polak / Electron-induced
segregation
323
Ihe lack of any sign in the profiles to the periodicity of the sodium-rich conduction planes in the Nafi structure, should be due to atomic mixing, even during the relatively mild, 500 V, sputtering used, and to preferential sputtering of the loosely bound sodium [ 111. 3.4. Electron stimulated
desorption
In those experiments where sodium accumulation was not induced, a relatively slow, charging-free decay of the sodium Auger signal was observed under certain bombardment conditions. Fig. 8 shows the variation with time of the sodium signal during a 2 kV electron bombardment using two different current densities. Both sets of data fit an exponential decay (dashed lines) with characteristic times T = 90 and 1000 s for current densities J = 1.6 and 0.3 A cmP2, respectively. Such first-order kinetics are expected for electron stimulated desorption (ESD) as a dominant process [ 12,131. Indeed, the same desorption cross section, e/rJ = 10m21 cm2, was derived from the data for both current densities. It is about one order of magnitude smaller than the cross-section for desorption of sodium from thin films of soda silica glass [14]. The decay observed in sodium signal starting at a current density of about 2 A
5.0
O-Current
denwty Q3Acm-z
l-Cwenl demlly I GAcm-’
ELECTRON-BOMBARDMENT Fig. 8. Na (I-
L)/L
TIME (set)
AES line intensity versus e-bombardment with respect to the final value I,, derived
time plotted as the fractional from the best-fit (dashed line).
change
A. L&hits,
M. Polak / Electron-induced
segregatron
325
distortions in the impinging beam [ 181. This effect will also influence the charging level maintained at the surface at the end of the electron bombardment. Therefore, in order to establish quantitatively each data point, given for example in fig. 5, several measurements were made, each involving the same bombardment of a fresh Nap surface. It should be noted that although it is evident that the beam induced electric to the Nap surface, an field is the driving force for the Na+ migration appreciable local heating, due to the typical high power density of the impinging beam, is expected to affect the diffusion rates. Indications of electron-beam induced migration of ions were reported in several cases [ 14,19,20]. For example, AES studies of thin films of soda silica glass [ 141 revealed Nat migration in the opposite direction to that occurring in the present case, namely diffusion from the surface into the solid. It was caused by a small positive surface charging resulting from 1.5 to 10 kV electron bombardment at an angle of 45”. In experiments made in our laboratory with this material, electron bombardment at a 60” angle of incidence (the same as for Nab) did induce negative charging under the same beam parameters as used in the Nap case, but no sodium accumulation was observed at the glass surface. The X-ray excited Auger spectra of sodium after bombardment (fig. 6) indicate the appearance of several chemical states of sodium. Thus, the broad line centered around 990 eV covers a wide range of chemical shifts including that reported for a sodium oxide [21]. Indeed, according +r) both Auger and XP spectra the bombarded surface is predominantly composed of sodium and oxygen (hydrogen cannot be detected by these methods). The smaller, narrower peak, about 6.5 eV higher in kinetic energy is most probably due to neutral sodium atoms. Reported energy shifts between KLL Auger lines from metallic sodium and from cationic sodium depend on the nature of the neighbouring anion. For example, the chemical shift between Na metal and Na in a thin oxide layer on top of the metal was reported to be about 7 eV [9]. This shift is primarily due to enhanced extra-atomic relaxation in the metallic state. It can be expected to depend on the state of dispersion of the neutral sodium atoms. The picture that emerges from these observations is that at least part of the migrating sodium ions are reduced at the surface to neutral atoms; part of these atoms react with residual gases in the vacuum chamber (e.g., H,O) to give sodium surface compounds, revealed by the broad complex Na Auger line (fig. 6). It should be noted that electron microscopy of Ag&alumina revealed appearance of a metallic silver precipitate during bombardment, and X-ray microprobe experiments on Nap-alumina showed variations in the Na Kcr X-ray intensity which could be due to similar phenomena [2a,22]. Also, some evidence for sodium ion redistribution in Na /%-alumina under the influence of the energetic alpha particle beam used in RBS was recently reported [23]. Effects of electron bombardment on thin NaCl
A. Livshits,
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M. P&k
/ Electron-induced
segregation
films were found to involve surface dissociation giving neutral chlorine and sodium [24]. No sodium migration was reported for the 2 kV bombardment applied. Indications of appearance of a metallic phase after long 1 kV bombardment were reported for soda silica glass [ 131. Appearance of metallic sodium on the Nap surface must change its secondary emission, and thus affect the degree of charging and the migration process. Metallic sodium is known to exhibit secondary emission yields considerably smaller than 1 over the entire primary energy range with a,,,,, = 0.82 at 300 eV [25]. This behavior can be a major factor in the efficient charge trapping observed once sodium segregates to the surface during high-energy bombardment. Thus, a 2 kV beam, which normally did not induce any negative charging, was sufficient to maintain a relatively stable charging level of several hundred volts (see the charging-map in fig. 3a). Furthermore, almost the same charge distribution (and Na) was found to persist on the Nap surface under UHV conditions for hours, during which the surface was not bombarded. This relationship between sodium accumulation on the surface and charge trapping is evidenced also by the results of soda silica bombardment. Thus, although during heavy bombardment, such as the one applied to Nap, a large negative charging had developed at the glass surface, it completely disappeared when the beam energy was reduced, and indeed no sodium accumulation did occur. The thickness of the segregated sodium-rich layer can be estimated from the attenuated Auger signals of the underlying aluminum and oxygen atoms. However, in both cases, only a lower limit can be estimated, since, typically, the Al signal after heavy electron bombardment was below the noise level, and part of the residual oxygen signal was due to sodium-oxygen compounds present at the bombarded surface according to XPS results, and possibly also to adsorbed oxygen-containing molecules. Because of uncertainties in the electron escape-depth, X, and its dependence on the kinetic energy, first we calculate the reduced thickness, d/X, which is the geometric thickness, d, of the layer expressed in multiples of the escape-depth. From the attenuation in the signals of aluminum and oxygen after 7 kV, 1.5 A cmp2, 3 min bombardment, minimum values of d,,/X,,
= 1.4
and
dmi,/Xo, = 2.2,
respectively, are derived. The noise level was taken as an upper limit for the attenuated aluminum intensity. The escape-depth and its dependence on the electron kinetic energy are known to be strongly affected by the matrix in which the electrons are inelastically scattered. In the present case it is composed of neutral sodium atoms as well as of sodium compounds. For energies fits the general equation X = kE”, above - 300 eV the mean escape-depth where the exponent n varies between 0.54 and 0.81 depending on the material considered [26]. Using n = 0.75 [lo] and n = 0.6 [27], estimated dmin values of 34 and 36 A, respectively, are obtained from the aluminum data; 25 and 29 A,
A. Livshits, M. Polak / Electron-induced segregation
321
respectively, are derived from the oxygen data. Due to the above-mentioned complication with the oxygen data the estimate based on the aluminum data (- 35 A) should be taken as the minimum thickness. The overall layer thickness can be approximated from the sputtering profiles (fig. 7). Since the sputtering rate is not known, it can be only roughly estimated according to reported rates for other materials [IO]. Thus, from the profiles in fig. 7, the sodium-rich segregated layer is found to be about 60 * 20 A thick. It should be noted that the sodium ions arriving at the bombarded cleavage face may first occupy vacant sodium sites, which were formed when the crystal was cleaved 1111. Inspection of the experimental results seems to shed some light on the mechanism involvd in the observed field assisted sodium migration towards the cleavage face of Nap. Normally, the fast diffusion of the sodium cations (associated with the superionic conductivity) is confined to the area of the conduction planes through interstitially pair jumps with a very low activation energy of - 0.17 eV [2,6]. If the electric field is directed along the conduction planes one expects fast Na+ flow according to this mechanism. Although the induced electric field during the electron bombardment of the cleavage face might have some component along the conduction planes, a direct observation of the Na+ fast flow in these planes would require the performing of experiments in which the electron beam impinges on the crystal edge face, where the planes are terminated, and monitor the concentration changes there [28]. Some hypothetical paths for the present case of Na+ migration towards a bombarded cleavage surface are: (i) Surface diffusion. Electron bombardment of nearly sodium-free surfaces, which were produced during long 500 V sputtering of large area, induced, at least in qualitative terms, the same effects as bombardment of freshly cleaved surfaces. This result suggests that sodium ions migrate to the surface from deeper layers in the crystal. On the other hand, the perturbation (fig. 3b) in the equipotential lines (fig. 3a) caused by the mild, 2 kV, subsequent bombardments outside the Na-covered surface, was accompanied by a similar change in the sodium distribution; this rearrangement of sodium, caused by the mild bombardment, once it appears on the external surface after heavy bombardment, is probably due to field assisted surface diffusion. (ii) Near-surface Na+ planar diffusion. Steps and other surface irregularities are, most likely, produced during cleavage of the Nap crystal. The cations may migrate towards the charged area along near-surface conduction planes, until they arrive at the external surface through such defects. (iii) Na+ migration through defects in the spine1 blocks. According to previous studies the structure of Nap is highly defective and one out of ten unit cells contains, on the average, an Al 3+ Frenkel defect in the closely packed spine1 block [2]. Sodium ions may migrate through the defective spine1 block to the charged surface from conduction planes which are deeper in the crystal. (iv) Na+ flow through fault planes created by field induced sodium aggre-
328
A. Lioshits, M. Polak / Electron-induced
segregation
gates within the crystal. The process starts with fast flow of sodium ions driven by the electric-field component parallel to the conduction planes. It results in accumulation of large quantities of sodium beneath the bombarded surface. Thus, the high internal stress produced, may cause faults through which excess sodium would emerge to the charged external surface. In view of the results obtained recently [28] and some optical-microscope observations of the bombarded crystal, this two-stage mechanism seems to be the most probable one. It is well known that ion sputtering creates a large number of near-surface vacancies and interstitials. Also, the surface roughness produced during sputtering is known to reduce the emission of secondary electrons and thus to enhance the surface negative charging. These two factors are expected to enhance the sodium migration to the surface. An opposite effect is expected from possible sputter destruction of the particular layered structure, responsible for the fast ionic transport in Nap-alumina.
5. Summary and conclusions Several complex phenomena occur at the cleavage surface of Nap-alumina subjected to electron bombardment. All the processes involve the highly mobile sodium cations and include a field-enhanced Na+ migration to the bombarded surface, Nat reduction to neutral sodium atoms, part of which seem to react with residual gases in the test chamber, and electron stimulated desorption of sodium with typically slower rates than the Na+ migration. The migration process results in an appreciable segregation of sodium at the surface, which depends on the electron-beam energy, current density and time of bombardment. These factors strongly affect the secondary emission yield, and, hence, also the local-field strength responsible for the migration. The two-stage segregation mechanism (iv) suggested in the previous section is now under further investigation. The difficulty of recording Auger spectra during heavy bombardment because of the large and unstable negative charging was usually circumvented by subsequently applying a milder monitoring beam for Auger and residualcharge mapping. Thus, a charging-map and corresponding Auger line scans provide, in a complementary way, a detailed, quantitative picture of the elemental distribution on such inhomogeneously charged surfaces. It appears that the relatively stable charge trapped at the Nap-alumina surface is associated with the presence of the new sodium-rich overlayers. The observed charge and sodium distributions were typically somewhat asymmetric around the bombarded area. This as well as other effects are probably influenced by the cleaved crystal surface topography [ 111. Additional experiments, involving studies of the detailed time-dependence as well as the angular-dependence of the effects, in which the orientation of the
A. Liushits, M. Polak / Electron-jnduced segregation
329
impinging electron beam is varied with respect to the conduction planes, have been undertaken to further elucidate and quantitatively evaluate the role of the two-dimensional high mobility of the sodium ions. In preliminary experiments, a substantially different response was observed during electron bombardment of the Nap--alumina crystal edge face 1281. The fast Na+ flow observed in that case, appears to be a direct manifestation of superionic conductivity. Finally, in view of the findings that H,O molecules severely inhibit the ionic conductivity by associating with the mobile cations in the conduction planes [29,30], the adsorption of H,O on the Nab-alumina surfaces and its impact on the Na+ migration will be investigated.
References [I] Y.F.Y. Yao and J.T. Kummer, J. Inorg. Nucl. Chem. 29 (1967) 2453. [2] (a) W.L. Roth, General Electric Corporate Research and Development report 74-CRD-054 (1974); (b) W.L. Roth, F. Reidinger and S.J. Laplaca, in ref. [3], p. 223. [3] G.D. Mahan and W.L. Roth, Eds., Superionic Conductors (Plenum, New York, 1976). [4] I. Chung, H.S. Story and W.L. Roth, J. Chem. Phys. 63 (1975) 4903. [5J M. Polak and R.W. Vaughan, J. Chem. Phys. 71 (1979) 3141; A. Highe, M. Polak and R.W. Vaughan, in: Proc. Intern. Conf. on Fast Ion Transport in Solids, Wisconsin, 1979, Eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (NorthHolland, Amsterdam, 1979) p. 305. [6] K.K. Kim, J.N. Mundy and W.K. Chen, J. Phys. Chem. Solids 40 (1979) 743. [7] D. Wolf. J. Phys. Chem. Solids 40 (1979) 757. [8] SD. Kowalczyk, L. Ley. F.R. McFeely, R.A. Pollak and D.A. Shirley, Phys. Rev. B8 (1973) 3583. [9] S.D. Kowalczyk, L. Ley, F.R. McFeely, R.A. Pollak and D.A. Shirley, Phys. Rev. B9 (1974) 381. [IO] CD. Wagner, W.M. Riggs and L.E. Davis, in: Handbook of X-Ray Photoelectron Spectroscopy, Ed. J.F. Moulder (Physical Electronics Industries Inc., Edina, MN, 1978). [ 111 A. Livshits and M. Polak. Solid State Ionics. to be published. [ 121 D. Menzel, Surface Sci. 47 (1975) 370. [13] P.H. Dawson, Nuovo Cimento Suppl. 5 (1967) 612. (141 F. Ohuchi, M. Ogino, P.H. Hollaway and C.G. Pantano, Surface Interface Anal. 2 (1980) 85. [ 151 N. Rey Whetten, in: Methods of Experimental Physics, Vol. 4, Part A, Eds. V.W. Hughes and H.L. Schultz (Academic Press, New York, 1967) p. 69. 1161 M. Grundner and J. Halbritter, J. Appi. Phys. 51 (1980) 5396. [ 171 F. Pellerin and C. Le Gressus, Surface Sci. 87 (1979) 203. [18] D.E. Newbury and H. Yakowitz, in: Practical Scanning Electron Microscopy (Plenum, New York, 1976) ch. 6. [19] N.J. Chou, CM. Osburn, Y.J. Van der Meulen and R. Hammer, Appl. Phys. Letters 22 (1973) 380. [20] P.T. Dawson, O.S. Heavens and A.M. Pollard. J. Phys. Cl 1 (1978) 2183. [21] A. Barrie and F.J. Street, J. Electron Spectrosc. Related Phenomena 7 (1975) 1. [22] W.L. Roth, in: Proc. 5th Materials Research Symp. (National Bureau of Standards, 1972) Publ. No. 367. [23] R.E. Benenson, W.L. Roth, V.K. Tikku and P.J. Cong, Solid State Ionics 5 (1981) 295. 1241 A. Friedenberg and Y. Shapira, Surface Sci. 87 (1979) 581.
330 [25] [26] [27] [28] [29] [30]
A. L&hits,
M. P&k
/ Electron-induced
segregation
J. Woods, Proc. Phys. Sot. (London) B67 (1954) 843. M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. H. Gant and W. Month. Surface Sci. 105 (1981) 217. M. Polak and A. L&hits, Appl. Surface Sci.. to be published. D. Kline, H. Story and W.L. Roth. J. Chem. Phys. 57 (1972) 5180. J.B. Bates, R. French, H. Engstrom, J.C. Wang and T. Kaneda, Solid State Ionics
1 (1980) IS.
Surface Science 119 (1982) 314-330 North-Holland Publishing Company
ELECTRON-INDUCED SEGREGATION TO SURFACES OF THE SUPERIONIC CONDUCTOR Nap-ALUMINA STUDIED BY AES AND XPS A. LIVSHITS Department
of Physics, Ben Gurion University of the Negeu, Beer-Sheun.
Israel
and M. POLAK
*
Departments of Materials Engineering and Nuclear Engineering, Beer-Sheva. Israel Received
14 October
1981; accepted
for publication
31 March
Ben-Gurion
University of the Negeo,
1982
Pronounced changes in sodium surface concentration and chemical state were observed after electron bombardment of cleaved single crystal Na&alumina, which is known to exhibit unusually fast Na+ transport. In particular, a distinct increase in the sodium Auger signal started at a certain energy and current density of the primary electron beam and was always accompanied by a large surface charging. Thus, migration of Na+ under the influence of the induced electric fields was found to be the major process leading to the sodium segregation at the Nap-alumina cleavage surface. Possible mechanisms for the segregation process are discussed. XPS measurements of the bombarded surface revealed the appearance of neutral sodium atoms as well as sodium-oxygen multilayer surface compounds. A simple method to obtain the electric potential distribution at the surface (“charging-map”) is demonstrated. The sodium distribution on the Nap-alumina surface after e-bombardment was found to be reflected in the corresponding charging-map.
1. Introduction Superionic conductors constitute a unique group of solids characterized by extremely fast ion transport, even at room temperature, orders of magnitude faster than ordinary ionic solids. One of these materials, Nap-alumina exhibits a two-dimensional, relatively high conductivity (- 10e2 Q-’ cm-‘) with sodium cations as the charge carriers [ 11. It is an inherently non-stoichiometric compound having excess sodium, which can be described by the formula (1 + X) Na,O . 11 Al,O,, where X ranges from about 0.2 to 0.4 [2]. Bulk properties of Nap have been intensively studied [3] using, for example, * To whom all correspondence
should be addressed.
0039-6028/82/0000-0000/$02.75
0 North-Holland
A. Ltvshits, M. P&k
/ Eleciron-induced segregation
315
diffraction techniques [2], nuclear magnetic resonance [4,5], tracer-diffusion and ionic conductivity measurements [6,7]. It was found that the highly mobile sodium ions are confined to relatively spacious basal planes 11.3 A apart (“conduction planes”), separated by spinel-like closely packed blocks of aluminum and oxygen (symmetry D6 h) [2]. Being through widely separated oxygen atoms, the binding between the spine1 blocks is weak and crystals are readily cleaved along the sodium-rich conduction planes. It makes this unique material attractive for surface studies. In particular, since the conduction plane lying next to the top plane is 11.3 A deep in the crystal, most of the sodium KLL Auger electrons detected (escape depth - 13 A) should originate from the top, cleavage-exposed, conduction plane. It is the purpose of this paper to report the first results of such studies concerning the response of the sodium ions in Nab-alumina to electron bombardment of the cleaved single crystal surfaces. Auger electron spectroscopy (AES) including scanning experiments, and X-ray photoelectron spectroscopy (XPS) were used in a complementary way. After a relatively heavy electron bombardment causing severe negative charging, a milder beam had to be used to monitor the induced composition changes Mechanisms for a sodium migration including their surface distribution. process leading to segregation of sodium at the surface are discussed in section 4. Since surface charging appeared to play a significant role in the processes observed, a scanning mode of operation was used to obtain “charging-maps” by monitoring the locally shifted secondary-emission energy. The results are compared to the response of a conventional ionic solid, soda silica glass, when bombarded by an identical electron beam. Beam induced variations in the chemical state of sodium were studied by XPS. Due to extra-atomic relaxation, chemical shifts in the Na Auger electron energy are known to be larger than the photoelectron shifts [8,9]. Indeed, most of the chemical-state information was deduced from shifts in the X-ray excited Auger lines. Under certain electron-beam conditions, a decay of the Na AES signal with bombardment time was observed. The results are discussed in terms of electron stimulated desorption (ESD) of sodium from the Nap surface.
2. Experimental Single crystal slabs of Nap-alumina, about 1 mm thick, were air-cleaved from a large single crystal (grown at Union Carbide). The orientation was confirmed by X-ray diffraction to be parallel to the conduction planes. A Physical Electronics Model 549 XPS/SAM instrument with a single cylindrical mirror analyzer (CMA) for AES-SAM experiments, and a double CMA for the XPS measurements. was used. The electron beam used for
316
A. Liushits, hf. Polak / Electron-induced segregation
bombardment and subsequent AES measurements made an angle of about 60’ with the crystal surface. A mild, 5OOV, Ar ion-sputtering was in some cases used to prepare a clean, carbon-free surface (base pressure - 3 X lo-” Torr). The same mild ion beam was also used to obtain in-depth profiles of sodium, aluminum and oxygen before and after electron bombardment. The major effects started at distinct primary-beam energy and current density and were accompanied by an unstable negative charging of several hundred volts manifested through large energy shifts of the Auger spectrum. In order to monitor the electron-beam induced effects under heavy bombardment, spectra were recorded using reduced beam energy and current density. These standard monitoring conditions of 2 kV and 0.3 A cm-’ usually gave stable Auger spectra, and when applied to a fresh surface produced only relatively slow variations attributed to ESD of sodium. During the recording time of the Auger spectrum following heavy bombardment, this effect was negligible. The same beam parameters were used for the scanning Auger and charge-scanning experiments giving a stable and reproducible mapping. For XPS measurements (using Mg Ken X-rays) several adjacent spots were first electron bombarded to produce a large enough affected area, and then the sample was oriented before the double CMA by monitoring the typical absorbed-current image of the bombarded surface. A small positive charging of 1 to 3 eV occurred during the XPS measurements since Na/3 is an insulator with respect to conduction of electrons. The sodium lines were referenced to the 1s line of carbon (binding energy of 284.6 eV) when present, or to the Al(2p) line of Al,O, at 74.5 eV [lo]. 3. Results Pronounced variations in Nap-alumina surface composition were found to depend primarily on the energy and current density of the impinging beam and on the bombardment time. Fig. 1 shows an initial Auger spectrum of Nap obtained with an “ineffective” 0.17 A cm-*, 5 kV electron beam. A rough quantitative estimate based on the peak-to-peak amplitudes of sodium, aluminum and oxygen lines gives concentration values close to the bulk stoichiometry in Na&alumina (Na: Al : 0 = 1: 9 : 14). Considering the particular layered structure of this crystal and the use of standard Auger sensitivity factors this result seems to be accidental. (According to a detailed XPS quantitative analysis [l l] each of the two cleavage-exposed surfaces, not surprisingly, contains about half the number of sodium ions in a normal bulk conduction plane.) The spectrum (fig. 1) served as a proper reference for the heavier bombardment experiments. 3.1. AES and charge mapping
after heavy electron bombardment
When high current densities, around 1 A cm-*, and primary-beam energies starting from 3.5 keV were used, drastic changes occurred in the Auger
A. Livshits,
hf. Polak / Electron-indeed
segregative
317
dN dE
hiLMM I 200
GKLL
NaKLL
*KLL
I 400
I 600
1 800
ELECTRON Fig. 1. An Auger electron crystal. The primary-beam
I woo
“KLL , 1200
I 1400
ENERGY, eV
spectrum from the cleavage face (the (0001) plane) of Nab-alumina current density was 0.17 A cm-’ at an energy of 5 keV.
spectrum. Typically, the signal intensity of sodium considerably increased, while aluminum and oxygen signals decreased. The results of a 3 min electron bombardment with a 5 kV, 1 A cm- ’ beam are shown in fig. 2b (fig. 2a is equivalent to fig. 1). As can be seen, sodium, the least abundant element in Nap, became the most abundant surface element at the expense of aluminum and oxygen. The process was accompanied by a large negative charging during the heavy bombardment, of which about 200 eV were left when recording the spectrum under the standard beam conditions (fig. 2b). To further elucidate the role of the negative surface charging in the process leading to Na-enrichment, precise mapping of the charging distribution at and around the bombarded area seemed to be desirable. It was performed by scanning the surface with the 2 kV standard beam, while each time monitoring the secondary electrons emitted with a certain kinetic energy according to the local charging. In fig. 3a we present a typical “charging-map” obtained after 3.5 kV, 1 A cm-2,- 1 min bombardment. The scanning speed was set for each energy to 15 s per frame, and the charging-maps were stable and reproducible. The large intensity of the secondary emission entering the CMA allowed the use of considerably reduced detection sensitivities. Under these conditions, the flux of Auger electrons was well below the detection limit, so that the resultant mapping presents pure equipotential lines only. The field strength at each point on the surface can be calculated from a charging-map Such as given in fig. 3a. An almost uniform field of - 2 x lo5 V
318
A. Livshits, M. Pofak / E~ectro~-in~~ce~ segregation
cm-’ is present to the right of the center, and a field of - 7 X lo5 V cm-’ to the left. Auger spectra taken under standard beam conditions at several points :b)
after 5 k'd, IAc~-~
electron
b~bardment
Na
(All
KINETIC ENERGY,eV Fig. 2. Effects of a 3 min heavy electron bombardment Auger spectra. before (a) and after (b) the bombardment, conditions used (2 kV, 0.3 A cm-2).
on Nab-alumina cleavage face. Both were recorded under the standard beam
across the charged surface revealed a direct correlation between the charging level and the amount of sodium, so that the charging-map reflects also the actual sodium distribution on the surface. Repeated 2 kV bombardments at points extending from the center of the char~ng-map to the right and beyond the charged area caused gradual variations in sodium signal intensity, which were reflected also in a subsequent charging-map (fig. 3b). In particular, bombardments outside the charged area in fig. 3a forced out charge (fig. 3b) as well as sodium. In order to obtain the precise unperturbed Na distribution immediately after heavy bombardment, rather than measuring point spectra, fast line scans of sodium seemed to be advantageous. Fig. 3c shows such line scans of sodium Each line represents the taken after 7 kV, 0.4A cm -‘, 1 min bombardment. Na distribution on the surface at a distinct charging level. Again, the amount of sodium correlates with the level of local charging. Clearly, in quantitative measurements, such as the time-dependence of the sodium accumulation on the surface, one has to take into account all the segregated sodium, which is not necessarily confined to the bombarded surface alone but may extend also beyond it. This was done for two bombardment times, 30 and 60 S, where Na Auger spectra were taken at points extending
319
I 990,&
KINETIC
I
I
I
II
lo20,0&lo20
ENERGY (eV)
Fig. 3. Charging-maps (a, b) (magnification 400X) and Na Auger line scans (c) (magnification 1000X) taken after heavy electron bombardment. The equipotential lines in (a) and (b) correspond to 650, 450, 250, 150 and 50 V, beginning at the innermost contour. The bar represents 20 pm beam.
A. Livshits,
320
_+-~,
50 s
$
‘x‘x \
IO
DISTANCE
segregation
bombardmen
beam radius 0
M. Polak / Electron-induced
20
\ \ 30
40
FROM CHARGING-MAP
\
\ 50
CENTER
(pm ) Fig. 4. Line distribution of sodium after 3.5 kV, 1 A cm-’ with the standard beam parameters.
bombardment
for 30 and 60 s, obtained
from the center of the corresponding charging-map along a horizontal line to the edge (fig. 4). If one wishes to obtain a more accurate Na distribution, the size of the electron beam used for data collection has to be taken into account. Assuming radially symmetric Na distribution leads to 20 pm and 40 pm radii for the 30 s and 60 s bombardment times, respectively. The beam radius, 10 pm, obtained also in this straightforward calculation, fits the actual value, and the calculated distribution profiles expected for Na (dashed lines in fig. 4) fit the experimental points. The product of the surface area (s) of segregated sodium and the Auger intensity coming from it (I), is proportional to the total number of Na atoms (Na,) in each case (In general, Na, - jFZ ds). The curves in fig. 4 give Nar(60 s)/Na,(30 s) = 2/l. The variations in surface segregation of sodium with the current density of the impinging beam are shown in fig. 5 for two beam energies. An equal bombardment time of 1 min was used in all measurements. As can be seen, the segregation as well as the surface negative charging are first enhanced by increasing the current density, until a maximum is reached around 2 A cmM2 after which a relatively slow decrease in sodium and charging level occurs. The decrease in Na seems to be due to a different mechanism, which according to other data presented at the end of this section, is attributed to electron stimulated desorption of sodium. At all current densities the segregation process was more pronounced under 7 kV bombardment than under 5 kV. 3.2. XPS measurements There spectrum
after e-bombardment
is a clear distinction before (fig. 1) and
between the shape of the Na Auger electron after heavy electron bombardment (fig. 2b),
A. Liushits, M. Polak / Electron-induced
CURRENT
segregation
321
DENSITY, A~rn-~
Fig. 5. Sodium signal intensity and charging level as function of beam current after 7 kV bombardments; (0) Na after 5 kV bombardments; (m) charging bombardments.
density: (0) Na level after 5 kV
indicating a possible modification in the sodium chemical state and bonding. Also, the surface composition changed considerably (fig. 2). In order to examine the changes in chemical state of sodium, X-ray excited electron spectra were recorded immediately after electron bombardment done at several spots covering a large enough area to be easily detected by XPS. (Still, the area imaged by the analyzer included some fresh, unbombarded surface.) The Auger part of the Na spectrum exhibited the largest chemical shifts and is shown in fig. 6. Two major spectral features appeared after bombardment: a considerable and asymmetric broadening of the single line observed before bombardment, and a new narrow component about 6.5 eV higher in kinetic energy (fig. 6). Also, in agreement with the AES results (e.g., fig. 2) the total area of the peak increased significantly (- 75%). 3.3. In-depth
sodium profiles
The in-depth distribution of sodium after electron bombardment, which was determined by alternate ion sputtering and XPS measurements, is compared to the profile of the unbombarded surface in fig. 7. The time required to remove the sodium-rich layer after electron bombardment was much longer than the time needed for removal of sodium at the top of the freshly cleaved surface. Clearly, a multilayer sodium-rich region was produced by the bombardment (details will be given in the next section). As can be seen, there is an appreciable increase in the Na/Al ratio after e-bombardment, by about a factor of 4 at the maxima (fig. 7). The actual increase is even larger since only part of the area imaged by the analyzer was e-bombarded.
322
A. Lioshits, M. P&k
/ Electron-induced
segregation
No (KLuLm)after c
No (KL,,Lz3) before electron bombardment
I
J
I
I
964
62
k
986
966
KINETIC
I
I
990
992
I
I
996
994
996
ENERGY,eV
Fig. 6. High-resolution X-ray excited heavy electron bombardment.
Auger
spectra
of sodium
in Nab-alumina
before
and after
f+$znbadment I I
\ I 1 I
i
i
20
7%
2, cl= ZQ - -
IO-
\ \ \ I \\ \ I \ \ \ \ I
before e-banbordment
!I \ \ \
\
\ ‘1
0
!!?7--3
5
IO
15
---+7___r_&_ 20 25 30
ION BOMBARDMENT TIME (min 1 Fig. 7. XPS in-depth an electron-bombarded
sputtering profile of sodium/aluminum in air-cleaved Nap-alumina, sample. Defocused Art beam: 500 V, 5 X IO-’ Torr.
and in
A. Livshits,
M. Polak / Electron-induced
segregation
323
Ihe lack of any sign in the profiles to the periodicity of the sodium-rich conduction planes in the Nafi structure, should be due to atomic mixing, even during the relatively mild, 500 V, sputtering used, and to preferential sputtering of the loosely bound sodium [ 111. 3.4. Electron stimulated
desorption
In those experiments where sodium accumulation was not induced, a relatively slow, charging-free decay of the sodium Auger signal was observed under certain bombardment conditions. Fig. 8 shows the variation with time of the sodium signal during a 2 kV electron bombardment using two different current densities. Both sets of data fit an exponential decay (dashed lines) with characteristic times T = 90 and 1000 s for current densities J = 1.6 and 0.3 A cmP2, respectively. Such first-order kinetics are expected for electron stimulated desorption (ESD) as a dominant process [ 12,131. Indeed, the same desorption cross section, e/rJ = 10m21 cm2, was derived from the data for both current densities. It is about one order of magnitude smaller than the cross-section for desorption of sodium from thin films of soda silica glass [14]. The decay observed in sodium signal starting at a current density of about 2 A
5.0
O-Current
denwty Q3Acm-z
l-Cwenl demlly I GAcm-’
ELECTRON-BOMBARDMENT Fig. 8. Na (I-
L)/L
TIME (set)
AES line intensity versus e-bombardment with respect to the final value I,, derived
time plotted as the fractional from the best-fit (dashed line).
change
A. L&hits,
M. Polak / Electron-induced
segregatron
325
distortions in the impinging beam [ 181. This effect will also influence the charging level maintained at the surface at the end of the electron bombardment. Therefore, in order to establish quantitatively each data point, given for example in fig. 5, several measurements were made, each involving the same bombardment of a fresh Nap surface. It should be noted that although it is evident that the beam induced electric to the Nap surface, an field is the driving force for the Na+ migration appreciable local heating, due to the typical high power density of the impinging beam, is expected to affect the diffusion rates. Indications of electron-beam induced migration of ions were reported in several cases [ 14,19,20]. For example, AES studies of thin films of soda silica glass [ 141 revealed Nat migration in the opposite direction to that occurring in the present case, namely diffusion from the surface into the solid. It was caused by a small positive surface charging resulting from 1.5 to 10 kV electron bombardment at an angle of 45”. In experiments made in our laboratory with this material, electron bombardment at a 60” angle of incidence (the same as for Nab) did induce negative charging under the same beam parameters as used in the Nap case, but no sodium accumulation was observed at the glass surface. The X-ray excited Auger spectra of sodium after bombardment (fig. 6) indicate the appearance of several chemical states of sodium. Thus, the broad line centered around 990 eV covers a wide range of chemical shifts including that reported for a sodium oxide [21]. Indeed, according +r) both Auger and XP spectra the bombarded surface is predominantly composed of sodium and oxygen (hydrogen cannot be detected by these methods). The smaller, narrower peak, about 6.5 eV higher in kinetic energy is most probably due to neutral sodium atoms. Reported energy shifts between KLL Auger lines from metallic sodium and from cationic sodium depend on the nature of the neighbouring anion. For example, the chemical shift between Na metal and Na in a thin oxide layer on top of the metal was reported to be about 7 eV [9]. This shift is primarily due to enhanced extra-atomic relaxation in the metallic state. It can be expected to depend on the state of dispersion of the neutral sodium atoms. The picture that emerges from these observations is that at least part of the migrating sodium ions are reduced at the surface to neutral atoms; part of these atoms react with residual gases in the vacuum chamber (e.g., H,O) to give sodium surface compounds, revealed by the broad complex Na Auger line (fig. 6). It should be noted that electron microscopy of Ag&alumina revealed appearance of a metallic silver precipitate during bombardment, and X-ray microprobe experiments on Nap-alumina showed variations in the Na Kcr X-ray intensity which could be due to similar phenomena [2a,22]. Also, some evidence for sodium ion redistribution in Na /%-alumina under the influence of the energetic alpha particle beam used in RBS was recently reported [23]. Effects of electron bombardment on thin NaCl
A. Livshits,
326
M. P&k
/ Electron-induced
segregation
films were found to involve surface dissociation giving neutral chlorine and sodium [24]. No sodium migration was reported for the 2 kV bombardment applied. Indications of appearance of a metallic phase after long 1 kV bombardment were reported for soda silica glass [ 131. Appearance of metallic sodium on the Nap surface must change its secondary emission, and thus affect the degree of charging and the migration process. Metallic sodium is known to exhibit secondary emission yields considerably smaller than 1 over the entire primary energy range with a,,,,, = 0.82 at 300 eV [25]. This behavior can be a major factor in the efficient charge trapping observed once sodium segregates to the surface during high-energy bombardment. Thus, a 2 kV beam, which normally did not induce any negative charging, was sufficient to maintain a relatively stable charging level of several hundred volts (see the charging-map in fig. 3a). Furthermore, almost the same charge distribution (and Na) was found to persist on the Nap surface under UHV conditions for hours, during which the surface was not bombarded. This relationship between sodium accumulation on the surface and charge trapping is evidenced also by the results of soda silica bombardment. Thus, although during heavy bombardment, such as the one applied to Nap, a large negative charging had developed at the glass surface, it completely disappeared when the beam energy was reduced, and indeed no sodium accumulation did occur. The thickness of the segregated sodium-rich layer can be estimated from the attenuated Auger signals of the underlying aluminum and oxygen atoms. However, in both cases, only a lower limit can be estimated, since, typically, the Al signal after heavy electron bombardment was below the noise level, and part of the residual oxygen signal was due to sodium-oxygen compounds present at the bombarded surface according to XPS results, and possibly also to adsorbed oxygen-containing molecules. Because of uncertainties in the electron escape-depth, X, and its dependence on the kinetic energy, first we calculate the reduced thickness, d/X, which is the geometric thickness, d, of the layer expressed in multiples of the escape-depth. From the attenuation in the signals of aluminum and oxygen after 7 kV, 1.5 A cmp2, 3 min bombardment, minimum values of d,,/X,,
= 1.4
and
dmi,/Xo, = 2.2,
respectively, are derived. The noise level was taken as an upper limit for the attenuated aluminum intensity. The escape-depth and its dependence on the electron kinetic energy are known to be strongly affected by the matrix in which the electrons are inelastically scattered. In the present case it is composed of neutral sodium atoms as well as of sodium compounds. For energies fits the general equation X = kE”, above - 300 eV the mean escape-depth where the exponent n varies between 0.54 and 0.81 depending on the material considered [26]. Using n = 0.75 [lo] and n = 0.6 [27], estimated dmin values of 34 and 36 A, respectively, are obtained from the aluminum data; 25 and 29 A,
A. Livshits, M. Polak / Electron-induced segregation
321
respectively, are derived from the oxygen data. Due to the above-mentioned complication with the oxygen data the estimate based on the aluminum data (- 35 A) should be taken as the minimum thickness. The overall layer thickness can be approximated from the sputtering profiles (fig. 7). Since the sputtering rate is not known, it can be only roughly estimated according to reported rates for other materials [IO]. Thus, from the profiles in fig. 7, the sodium-rich segregated layer is found to be about 60 * 20 A thick. It should be noted that the sodium ions arriving at the bombarded cleavage face may first occupy vacant sodium sites, which were formed when the crystal was cleaved 1111. Inspection of the experimental results seems to shed some light on the mechanism involvd in the observed field assisted sodium migration towards the cleavage face of Nap. Normally, the fast diffusion of the sodium cations (associated with the superionic conductivity) is confined to the area of the conduction planes through interstitially pair jumps with a very low activation energy of - 0.17 eV [2,6]. If the electric field is directed along the conduction planes one expects fast Na+ flow according to this mechanism. Although the induced electric field during the electron bombardment of the cleavage face might have some component along the conduction planes, a direct observation of the Na+ fast flow in these planes would require the performing of experiments in which the electron beam impinges on the crystal edge face, where the planes are terminated, and monitor the concentration changes there [28]. Some hypothetical paths for the present case of Na+ migration towards a bombarded cleavage surface are: (i) Surface diffusion. Electron bombardment of nearly sodium-free surfaces, which were produced during long 500 V sputtering of large area, induced, at least in qualitative terms, the same effects as bombardment of freshly cleaved surfaces. This result suggests that sodium ions migrate to the surface from deeper layers in the crystal. On the other hand, the perturbation (fig. 3b) in the equipotential lines (fig. 3a) caused by the mild, 2 kV, subsequent bombardments outside the Na-covered surface, was accompanied by a similar change in the sodium distribution; this rearrangement of sodium, caused by the mild bombardment, once it appears on the external surface after heavy bombardment, is probably due to field assisted surface diffusion. (ii) Near-surface Na+ planar diffusion. Steps and other surface irregularities are, most likely, produced during cleavage of the Nap crystal. The cations may migrate towards the charged area along near-surface conduction planes, until they arrive at the external surface through such defects. (iii) Na+ migration through defects in the spine1 blocks. According to previous studies the structure of Nap is highly defective and one out of ten unit cells contains, on the average, an Al 3+ Frenkel defect in the closely packed spine1 block [2]. Sodium ions may migrate through the defective spine1 block to the charged surface from conduction planes which are deeper in the crystal. (iv) Na+ flow through fault planes created by field induced sodium aggre-
328
A. Lioshits, M. Polak / Electron-induced
segregation
gates within the crystal. The process starts with fast flow of sodium ions driven by the electric-field component parallel to the conduction planes. It results in accumulation of large quantities of sodium beneath the bombarded surface. Thus, the high internal stress produced, may cause faults through which excess sodium would emerge to the charged external surface. In view of the results obtained recently [28] and some optical-microscope observations of the bombarded crystal, this two-stage mechanism seems to be the most probable one. It is well known that ion sputtering creates a large number of near-surface vacancies and interstitials. Also, the surface roughness produced during sputtering is known to reduce the emission of secondary electrons and thus to enhance the surface negative charging. These two factors are expected to enhance the sodium migration to the surface. An opposite effect is expected from possible sputter destruction of the particular layered structure, responsible for the fast ionic transport in Nap-alumina.
5. Summary and conclusions Several complex phenomena occur at the cleavage surface of Nap-alumina subjected to electron bombardment. All the processes involve the highly mobile sodium cations and include a field-enhanced Na+ migration to the bombarded surface, Nat reduction to neutral sodium atoms, part of which seem to react with residual gases in the test chamber, and electron stimulated desorption of sodium with typically slower rates than the Na+ migration. The migration process results in an appreciable segregation of sodium at the surface, which depends on the electron-beam energy, current density and time of bombardment. These factors strongly affect the secondary emission yield, and, hence, also the local-field strength responsible for the migration. The two-stage segregation mechanism (iv) suggested in the previous section is now under further investigation. The difficulty of recording Auger spectra during heavy bombardment because of the large and unstable negative charging was usually circumvented by subsequently applying a milder monitoring beam for Auger and residualcharge mapping. Thus, a charging-map and corresponding Auger line scans provide, in a complementary way, a detailed, quantitative picture of the elemental distribution on such inhomogeneously charged surfaces. It appears that the relatively stable charge trapped at the Nap-alumina surface is associated with the presence of the new sodium-rich overlayers. The observed charge and sodium distributions were typically somewhat asymmetric around the bombarded area. This as well as other effects are probably influenced by the cleaved crystal surface topography [ 111. Additional experiments, involving studies of the detailed time-dependence as well as the angular-dependence of the effects, in which the orientation of the
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impinging electron beam is varied with respect to the conduction planes, have been undertaken to further elucidate and quantitatively evaluate the role of the two-dimensional high mobility of the sodium ions. In preliminary experiments, a substantially different response was observed during electron bombardment of the Nap--alumina crystal edge face 1281. The fast Na+ flow observed in that case, appears to be a direct manifestation of superionic conductivity. Finally, in view of the findings that H,O molecules severely inhibit the ionic conductivity by associating with the mobile cations in the conduction planes [29,30], the adsorption of H,O on the Nab-alumina surfaces and its impact on the Na+ migration will be investigated.
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