Concentration Profiles for Irregular Surfaces from X-ray Photoelectron Angular Distributions I?.J. Baird, C. S. Fadley," S. K. Kawamoto, and Madhu Mehta Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822
Robustiano Alvarez and J. A. Silva Department of Agronomy and Soil Science, University of Hawaii, Honolulu, Hawaii 96822
Core-level x-ray photoelectron angular distribution measurements have been performed for solid Specimens with highly irregular surface contours and are shown to provide a general method for obtaining qualitative concentration profiles for the atomic species near a surface. At conditions of low or grazing electron exit angle, slgniflcant increases were observed In the relative intensltles of near-surface atoms for two types of specimens. The first is unldlrectlonally-polished aluminum metal with oxide- and carbon-containlng-overlayers, and the second, pellets of Ai203 powder with solution-adsorbed SI and Cam
The enhancement of surface sensitivity in x-ray photoelectron spectroscopy (XPS, ESCA) achieved by utilizing grazing angles of electron escape from solid specimens has been the subject of several prior exploratory investigations (1-8). I t has been demonstrated that for specimens with relatively flat surfaces, such as highly polished metals or materials deposited on glass substrates, surface sensitivity can be increased by a t least an order of magnitude by using low escape angles (1-6). More detailed analyses of such angular distribution (AD) data can also be used to provide information concerning atomic concentration profiles relative to the surface, surface layer thicknesses, and electron attenuation lengths (3-6). The presence of a significant amount of surface contour irregularity or roughness renders the analysis of XPS AD data more difficult, however, because the true electron escape angles from various surface points are not directly measureable, and also shading of certain surface regions may occur for both x-ray incidence and electron exit. In principle, characteristic roughness dimensions need only be somewhat larger than electron attenuation lengths (that is, 210-100 A) in order to influence angular distributions. Such roughness effects have been discussed theoretically by Fadley et al. ( 4 , 5 ) and have recently been observed experimentally by Baird et al. (7), in particular for diffraction grating surfaces with periodic profiles. As a result of these studies, it is possible to conclude that surface profile variations can dramatically alter the form of the surface sensitivity variation with electron emission angle. Furthermore, for several periodic roughness profiles, it is predicted that the amount of surface enhancement possible for low exit angles will be less than for an equivalent flat-surface specimen. As the surfaces of most specimens presently used in XPS studies (including a number of types of considerable practical important) exhibit some sort of roughness as judged by these criteria, it becomes important to ask how much surface enhancement can be achieved for such specimens a t low exit angles. We have thus carried out an experimental study on two types of samples with irregular roughness contours, and have attempted to derive qualitative concen-
tration profile information from the core-level XPS AD'S obtained. EXPERIMENTAL The x-ray photoelectron spectrometer used was a Hewlett-Packard 5950A specially modified to permit varying the electron exit angle relative to the surface-average plane of the specimen (9). This angle B we define such that B = 90' corresponds to exit perpendicular to the average plane. Quartz-crystal monochromatized AI K a radiation was used for excitation, with a resultant full width a t half maximum intensity value for Au 4f7/2 in Au of 0.80 eV a t 0 = 38.5O (an angle very near that for which minimum line widths are achieved because of dispersion compensation). The pressure inside the spectrometer during analysis was Torr. Angular distribution data were accumulated automatically by means of a stepping motor linked to a Hewlett-Packard 2100A computer. All intensities reported are based on peak areas or ratios of these areas, and thus are implicitly corrected for any changes in peak width occurring with variations of 8 for this dispersion-compensated optical arrangement (9). The use of peak area ratios furthermore has been shown to correct for instrument response function changes with 8; ratio AD'S are thus more directly relatable to specimen properties alone ( 4 , 5 , 7,8). The two types of specimens investigated were as follows. Unidirectionally-Polished Aluminum. Nominally-flat aluminum substrates were polished with relatively coarse 400 grit S i c abrasive using a single direction of motion. This treatment is assumed to have left irregular but very nearly parallel polishing grooves of approximately 10 000 8, in depth (10). These specimens were then exposed to atmospheric oxidation for -15 min to form a covering oxide layer of -15 8, thickness ( I I ) , and finally cleaned thoroughly with acetone and methyl alcohol just before placement in the spectrometer. For these specimens, the intensities of the following core peaks were measured as a function of 0 for 8 variation both parallel to and perpendicular to the polishing grooves: 0 Is, C Is,A1 2p(oxide) and A1 2p(metal). The chemically-shifted oxide and metal peaks were easily resolvable with a separation of -2.5 eV. The angle 8 was varied over its full 180' range for each groove orientation in two separate 90' scans that are labeled B+ and 8-. Powdered A1203 with Adsorbed Si and Ca. High purity A1203 powder of 2-5 w diameter was immersed in an aqueous solution containing 7 5 parts per million by weight (ppm) of Si (as Si(OH4)) and 200 ppm Ca (as CaC12) and allowed to reach equilibrium. (This treatment was intended to simulate the action of certain fertilizers in soils (12).) The powder was then washed three times with large volumes of ethyl alcohol (or, for some specimens, deionized distilled water) to remove all but very strongly adsorbed species, and finally dried by heating in atmosphere at 50 'C for -8 h. Specimens for XPS characterization were prepared by pressing the powder into pellets. Untreated A1203 powder that was subjected to all preparative steps except the solution adsorption was also studied as a reference. Core-level peak intensities were measured a t 8 = 38.5' and 5' for both types of specimens. The peaks detectable by X P S were 0 Is, C Is, A1 2s, and A1 2p for all specimens and Si 2s, Si 2p, and Ca 2p for the treated specimens.
RESULTS AND DISCUSSION Polished Aluminum. The angular dependence of the peak ratios A1 Bp(oxide)/Al2p(metal), C ls/Al 2p(metal), 0 ls/Al 2p(metal), and 0 ls/Al Bp(oxide) are presented in Figures 1-3. The first three ratios are referenced to the substrate-associated Ai 2p(metal) peaks; the last is beANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976 * 843
--
b
-
50-
-
4 -- (
0 0
- I I Groovar, 8, - II Grooves, 8- 1 Grooves, 8 + -1
\
GIOOV~S, 8.-
P
&
AlZp (me)
AI 2 ~ ( 0 x 1
C I s /A 12p (ma)
Flgure 1. Normalized core-level peak area ratios for AI Pp(oxide)/Al 2p(metal) for an aluminum specimen subjected to unidirectionalpolishing are plotted vs. 8 Data obtained for 8 variation both perpendicular to and parallel to the polishing direction is shown. 8+ and 8- represent the two halves of a full 180' scan of 8. Normalization involved dividing by the average ratio at 90' of AI Pp(oxide)/Al Pp(meta1) = 1.35
tween two peaks that would both be expected to be derived predominantly within the oxide layer. Data for 8 variation both parallel to and perpendicular to the polishing grooves are shown. All ratio values are normalized by dividing by an average value for that ratio a t 8 = 90'. T o within experimental error, the 90' ratios for a given pair of peaks were identical for both parallel and perpendicular orientations, as expected from the spatial symmetry of the two experiments with respect to electron emission into the analyzer. The included angle between x-ray incidence and electron exit for this spectrometer is only 72O; also the A1 K a attenuation length in aluminum is -90000 %, (13), and thus much larger than the typical roughness dimensions for these specimens. Therefore, to a good approximation, x-ray shading does not need to be considered in discussing these data. Figure 1 presents A1 Bp(oxide)/Al 2p(metal) data and shows in a direct way the relative enhancement of the surface oxide signal. I t is clear that there is a marked difference between the amounts of surface enhancement possible for the parallel and perpendicular orientations. The net enhancement achieved with the parallel orientation is approximately a factor of 4.2 whereas that with the perpendicular orientation is only a factor of 1.7, a net difference of a factor of 2.5, This difference can be qualitatively explained in terms of the special character of the roughness profile of these specimens. In the case of perpendicular orientation, decreasing 8 to low values should leave unshaded electron-emitting surfaces only on the sloping sides and tops of the polishing grooves. Because these slopes are, in general, perpendicular to the electron emission direction, the true angles of emission relative to the surface a t these unshaded points will tend to be larger than the experimental angle 8. For parallel orientation, however, decreasing 8 to low values yields electron emission directions that are very nearly parallel to the grooves, so that the true emission angle a t an arbitrary surface point can be quite small, and will probably be approximately equal to 8. Thus, surface enhancement is predicted to be much more pronounced for low 8 values with parallel orientation, in agreement with experiment. I t is worth stressing, however, that 844
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
ob
'
2b
io 'e
'
4
io
'
do
'
'
Figure 2. Normalized core-levell peak area ratios for C ls/Al 2p (metal), 0 l s / A l Pp(metal), and 0 l s / A l Pp(oxide)for a unidirectionally-polished aluminum specimen 8 variation was carried out parallel to the polishing direction. The average ratios at 90' used for normalization were C ls/Al Pp(rneta1) = 3.00, 0 ls/Ai Pp(rneta1) = 15.0, and 0 ls/Ai Pp(oxide) = 13.7
I
20
,
I
I
I
40
e
60
I
I
I
80
Figure 3.1 Normalized core-level peak area ratios for C l s / A l 2p (metal), 0 lslAl Pp(meta1) and 0 ls/Al 2p(oxide) for a unidirectionally-polishedaluminum specimen 0 variation was carried out perpendicular to the polishing direction. The average ratios at 90' used for normalization were C ls/Al Pp(meta1) = 3.10, 0 ls/Al Pp(meta1) = 16.3, and 0 ls/Al Pp(oxide) = 13.5
surface enhancement is possible for both orientations. The angular variations of the ratios C ls/Al 2p(metal), 0 ls/Al Pp(metal), and 0 ls/Al 2p(oxide) are shown in Figures 2 and 3 for parallel and perpendicular orientations, respectively. The C ls/Al Pp(meta1) and 0 ls/Al Sp(meta1) data for both orientations show much different behavior, with the C 1s enhancement relative to the substrate being approximately twice as large as that for 0 1s. This result is consistent with carbon being present as an outermost contaminant layer, as would be expected for these specimens. The 0 ls/Al 2p(oxide) ratios, however, show only very
small deviations from unity and, if anything, decrease slightly at small 8 values. This behavior indicates that essentially all of the oxygen was present in the oxide layer, as any adsorbed contaminant oxygen lying above this would be expected to yield a low 6' enhancement ratio greater than unity for 0 Is/Al 2p(oxide). The C ls/Al Zp(meta1) and 0 ls/Al Pp(meta1) data in Figures 2 and 3 also again clearly demonstrate the greater surface enhancements possible at low 6' for the parallel orientation. The C ls/Al 2p(metal) enhancement possible between 90' and 10' is approximately 6 times greater for the parallel orientation; for 0 ls/Al 2p(metal), it is roughly 4 times greater. These data thus clearly indicate that surface roughness can significantly affect XPS angular distributions, and that it will generally act to decrease the amount of surface enhancement possible at low 8, as has been predicted theoretically in prior studies (4, 5 ) . However, sufficient surface enhancement is possible for either orientation of these specimens to permit qualitative concentration profiling of the species present in a manner identical in principle to that used previously for aluminum specimens with much more carefully prepared, nearly flat, surfaces (4, 5 ) . The intentional use of unidirectional polishing and AD measurements parallel to that direction is also suggested as a method for increasing possible surface enhancements by as much as a factor of 5 or more. A1203 Powder. The results of these two-angle measurements are summarized in Figure 4 and Table I. Figure 4 shows XPS spectra in the region of the A1 2s,2p and Si 2s,2p core peaks as obtained from a specimen treated with a Si- and Ca-containing solution. The Si peaks are quite intense, indicating that a significant amount has remained on the powder even after washing. Also, there is a small, but unambiguous, enhancement of the relative intensities of the Si peaks when the emission angle is lowered from 38.5' to 5'. This enhancement indicates that the Si is on the average nearer the surface than the substrate-derived Al, as would be expected for a surface-adsorbed species. (Washing with deionized water or ethyl alcohol gave very similar results.) Table I summarizes analogous two-angle results for all of the peaks observed in both untreated and treated A1203 specimens. For a given peak k , the intensity ratio k/Al2s a t 38.5' is given, as well as the relative enhancement ratio between 5' and 38.5' as defined by [k/A1 2s]p/[k/Al 2s]38.p. For both types of specimens, the A1 2p enhancement ratios are within experimental error of unity, as they should be, since A1 2p originates in the same atom as the reference A1 2s and also has nearly the same kinetic energy. The 0 Is enhancement ratios for all specimens are also very near unity, indicating that essentially all 0 is associated with the substrate A1203. For C Is, on the other hand, the enhancement ratios are much larger with values of 3.0 and 2.6.; this finding suggests that C is present in an outermost contaminant layer, as expected from the mode of specimen preparation and analysis. There was, furthermore, no distinguishable 0 1s fine structure due to chemical shifts, a result consistent with a relatively well-defined chemical state. The 0 ls/Al 2p ratio was also essentially identical for both treated and untreated specimens, indicating relatively little modification of the near-surface stoichiometry by the adsorption. Thus, distinct surface species containing oxygen do not appear to play a major role in altering the 0 1s enhancement ratio. The enhancement ratios for the specimens treated with Si and Ca have been written in decreasing order in Table I. The Ca 2p, Si 2s, and Si 2p ratios associated with atoms in the treatment solution are above unity (1.8, 1.2, and 1.2, re-
I
P
I
I
I
I
A12r
A
,
0
I
I
h
1
I
150
50
100
e - BINDING ENERGY l e v )
Flgure 4. Si 2s,2p and AI 2s,2p spectra at 6' = 5' and 38.5' for a powdered A1203 specimen exposed to aqueous solution adsorption of Si and Ca
Table I. Core-Level Intensities Relative to A1 2s at 0 = 38.5' and Relative Intensity Enhancement Ratios between 0 = 5' and 38.5' for an Untreated Specimen of A1203 Powder and a Similar Specimen Exposed to Solution Adsorption of Si and Ca Untreated A1203
A1203
+ Si t Ca
[klAl2~]p/ [kl [klAl2~]38.50 [klAl 2~138.50 A1 2~138.50
k
c 1s Ca 2p Si 2s Si 2p A1 2s A1 2p 0 1s
0.13
... ... ...
1 0.74 12.16
3.0 f 0.4
...
... ... 1 1.1 f 0.1 0.9 f 0.1
0.29 0.03 0.56 0.62 1 0.82 15.32
[klAl2s]p/
I klAl2sI38.50 2.6 f 0.2 1.8 f 0.5 1.2 f 0.1 1.2 f 0.1 1 0.9 f 0.1 0.9 f 0.1
spectively), indicating that these species are, to a significant degree, surface-adsorbed. The larger value for the Ca 2p ratio may also suggest that Ca is on the average nearer the surface than Si, although the quoted error limits do not permit this to be concluded with certainty. The largest ratio for the treated specimens is 2.6 for C Is, indicating that C occurs in an outermost contaminant layer. These two-angle results for powdered specimens thus also permit qualitative concentration profiling for all species observed, and indicate that Ca and Si are, to a large degree, surface adsorbed. Similar results have also been obtained for A1203 powder specimens treated with solutions containing a number of other combinations of salts of Si, Ca, or P (12). Extensions of this procedure to a variety of studies involving powder specimens would thus appear to be possible. In addition to concentration profiles for systems in which intentional surface chemical alteration has occurred, a useful type of information that might be obtained for some cases is the degree to which the bulk stoichiometry of a compound may have been unintentionally modified near the surface. As general rules for the qualitative analysis of low-angle enhancement ratio data with a substrate peak as ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
845
reference, the following would be appropriate for a set of peaks that do not differ too much in kinetic energy (and thus also attenuation length): 1) A larger enhancement ratio indicates that a given species is distributed nearer the surface. 2) A ratio > unity indicates some degree of concentration near the surface. 3) A ratio very near unity suggests a species homogeneously distributed in the substrate. 4) A ratio < unity suggests a species whose maximum concentration may be somewhat below the surface of the substrate. For a group of peaks with a lar,ge kinetic energy range, the interpretation of subtle changes in enhancement ratio may be difficult, however. For example, the 0 Is ratio is consistently slightly less than unity in the present data as well as in that of other similar studies of A1203 powder (121, and the normalized 0 ls/Al 2p(oxide) ratio for the aluminum specimen of Figure 3 is also somewhat less than unity a t 10'. These effects could be due to the lower attenuation length for 0 Is photoelectrons (kinetic energy = 955 eV) as compared to the reference A1 2s photoelectrons (kinetic energy = 1365 eV), which would cause more attenuation of 0 Is in any overlayers present on the oxide; such differences in attenuation would be amplified a t lower angles. Alternatively, however, these enhancement ratios slightly less than unity could be due to a slight reduction in the O/Al concentration ratios near the surface. One test for the energy-dependent attenuation length effect might consist of enhancement ratio measurements for the low-lying band of 0 2s states observed in the valence region of A1203, which possesses a kinetic energy greater than that of A1 2s. This band, however, is much lower in intensity than 0 Is and thus difficult to measure with sufficient precision, and
also is subject to certain ambiguities of interpretation because of the possibility of 0 2s-derived levels for other near-surface species which could occur a t somewhat different energies.
LITERATURE CITED
4.
(1) C. S.Fadley and S. L. Bergstrom, Phys. Lett. A, 35, 375 (1971). (2) C. S.Fadley and S.A. L. Bergstrom, in "Electron Spectroscopy", D. A. Shirley, Ed., North Holland Publishing Co., 1972, p 233. (3) W. A. Fraser, J. V. Florio, W. N. Delgass, and W. D. Robertson, Surf. Sci., 36,661 (1973). (4) C. S. Fadley, R. J. Baird, W. Siekhaus, T. Novakov, and S. L. Bergstrom, J. Electron Spectrosc., 4, 93 (1974). (5) C. S. Fadley, J. Electron Spectrosc., 5, 725 (1974). (6) J. Brunner and H. Zogg. J. Electron Spectrosc., 5, 81 1 (1974). (7) R. J. Baird, C. S.Fadley, S.K. Kawamoto, and M. Mehta, Chem. Phys. Lett., 34, 49 (1975). ( 8 ) C. S.Fadley, to appear in Faraday Society Discussion, NO. 60. (9) R . J. Baird and C. S. Fadley. abstract for the 30th Northwest Regional Meeting, ACS, Honolulu, Hawaii, June 12-13, 1975, and unpublished results. (10) The AB Metal Digest, Vol. 11, No. 213 (1973), Buehler Ltd., Evanston, Ill. (11) J. E. Boggioand R. C. Plumb, J. Chem. Phys., 44, 1081 (1966). (12) R. Alvarez, Ph.D. Thesis, Department of Agronomy and Soil Science, University of Hawaii, 1975, and R . Aivarez, C. S.Fadley, J. A. Silva, and G. Uehara, unpublished results. (13) B. L. Henke and M. Tester in "Advances in X-ray Analysis". Vol. 18, Plenum Press, New York, 1975.
A.
RECEIVEDfor review September 3,1975. Accepted January 12, 1976. This work was presented in part a t the Faraday Discussion on Electron Spectroscopy of Solids and Surfaces, Vancouver, Canada, July 15-18, 1975. Those portions of the study performed in the Chemistry Department, University of Hawaii, were also supported by the National Science Foundation (Grant GP 38640X) and the University of Hawaii Research Council.
Digital, Photon-Counting Fluorescence Polarometer I?.J. Kelly and W. B. Dandliker" Department o f Biochemistry, Scripps Clinic and Research Foundation, La Jolla, Calif. 92037
Donald E. Wllliamson Cordis Corporation, Miami, Fla. 33 127
The design and construction features of a digital, photoncounting instrument for the measurement of fluorescence polarization (fluorescence poiarometer) are described. The instrument also performs the functions of a highly sensitive, stable fluorometer. The data from photon counts is processed in a digital computer and is displayed digitally. The design features of the instrument make It possible to achieve very high stability and sensltlvlty, while maintaining low levels of illumination of the sample. The standard error of estimate in making polarization measurements is fO.OO1 polarization unit. As a fluorometer, the response is linear over a wide dynamic range and useful measurements M fluorescein. can be made down to about 5 X
The dependence of the polarization of fluorescence upon rotational relaxation times provides information on the size, shape, and segmental motion of macromolecules in SO846
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
lution. In addition, fluorescence polarization methods lend themselves to the measurement of the kinetic and equilibrium parameters of reactions over extremely wide ranges of concentration when a fluorescent or fluorescence-labeled molecule is involved ( I ). Also, polarization measurements can be readily adapted to give sensitive methods of assay for many substances present only a t a few ng/ml(2). Instrumentation is a crucial factor in making fluorescence polarization measurements of sufficient precision to realize the full potential of the method in the kinetic, equilibrium, and assay areas. A number of fluorescence polarometers (instruments for measuring fluorescence polarization) with a variety of electronic and optical arrangements have been described in the past decade. In our early instruments ( 3 ) , the fluorescence intensity was compared to that of a reference beam by means of either an Aryton shunt and galvanometer or of a synchronous chopper and tuned amplifier. Monnerie and NBel ( 4 ) employed a galvanometer readout and servoed the photomultiplier with a signal from
