- Email: [email protected]
Optical studies of chemisorption on metals
Surface Science48 (1975) 214-228 0 North-Holland Publishing Company
OPTICAL STUDIES OF CHEMISORPTION
ON METALS
R.C. O’HANDLEY * and D.K. BURGE Michelson Laboratory, Naval Weapons Center, China Lake, California 93555, U.S.A.
Optical techniques, particularly ellipsometry, have been highly successful in the study of chemisorption on semiconductors. Such studies on metal surfaces are more difficult because the short screening lengths limit the chemisorption-induced perturbation to a surface-layer only a few ingstr6ms thick. Some success has been achieved using differential reilectance spectroscopy. However this necessitates (1) two independent reflectance measurements or the use of KK analysis to obtain complex optical constants, and (2) assumed or independently-measured values for the thickness of the surface layer in order to completely specify the adsorption-induced changes. The advent of extremely precise in situ modulated ellipsometers has made the optical study of gas-metal interactions less difficult, in particular eliminating the necessity of KK analysis. We briefly describe such a system here, the automated polarization-modulated ellipsometer. When this instrument is coupled with a recently developed approximation technique for obtaining surface-layer thickness and optical constants from only two ellipsometric measurements, the complete optical characterization of many metal-absorbate systems is straightforward. Recent experimental results indicating a chemisorption-induced surface state within 2.4 eV of the Fermi level for oxygen on silver are discussed.
1. Introduction Chemisorption is a subject of wide interest both for its application to gas-solid interactions and heterogeneous catalysis as well as for the basic insight it gives concerning the electronic and structural properties of solid surfaces. Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) are two of the more widely used techniques that have been indispensable in determining chemisorbed species and their structural phases at surfaces. Optical measurements, particularly ellipsometry, so useful in studying bulk electronic properties, have also contributed significantly to our understanding of surface electronic properties in chemisorption situations. However, these optical studies have been limited often to measurements at one wavelength and mostly to adsorbate/semiconductor systems. The early work of Archer [I] concentrated on the physical chemistry of adsorption processes on silicon. The most extensive ellipsometric studies are those of Bootsma, Meyer et al. [2,3]. This group investigated physical and chemical adsorption on specific Si and Ge crystal faces. An important result of their work was understanding * Present address: New Jersey
07960,
Materials U.S.A.
Research
Center,
Allied Chemical
Corporation,
Morristown,
R.C. O’Handley, D.K. Burge/Chemisorption
on metals
215
the perturbing effects of chemisorbed species on the electronic structure, and, therefore, on the optical properties, of clean semiconductor surfaces [3]. It is now appreciated that even though a substance may be a dielectric (Z 3 n - ik, k = O}:in the condensed state, its presence at a solid surface often causes a perturbation sufficient to enhance the optical absorption. Such is not the case with physisorbed layers which appear optically as thin dielectric surface films [ 2-41.
Ellipsometric studies of chemisorption on metals are more difficult, and therefore less abundant [4-81. This is because the short screening length in metals limits the charge perturbation due to the presence of the adsorbate, to a layer of a few %rgstroms at the surface. Many [5,6] of the existing investigations on metals are limited in scope to the physical chemistry of the adsorption process, using data taken at a single waveleng~. Nevertheless, the chemisorbed layer at a metal surface can be treated formally as a thin surface film [7,9-l I] allowing the determination of its optical response as a function of photon energy. We refer to this as surface optical spectroscopy (SOS). Kolb and McIntyre [7] have obtained striking spectral results for adsorbed oxygen layers on gold using differenti~ reflectance spectroscopy [ 1O]_ Subsequent work ]lO,l l] has led to better understand~g of these surface optical spectra. More recently the use of differential reflectance spectroscopy and a sensitive light-pipe reflectometer [ 121 have made possible the spectral characterization of optically absorbing, chemisorption-induced surface layers on MO [ 131 and W [ 141. While reflectance spectroscopy is more straightforward and has greater spectral range than elljpsomet~, it has two drawbacks. It requires (1) two reflectance measurements with polarized light or a Kramers-Kronig (KK) analysis of the (unpolarized) reflectance data, and (2) an auxiliary measurement of (or assumed value for) the surface-layer thickness in order to completely characterize the surface. 1.2. Purpose It is the purpose of this paper to describe recent experimental and theoretical advances that have made less difficult the SOS of thin optically absorbing layers induced on mtals by chemisorption. The experimental technique used is ellipsometry because it gives two optical parameters for the surface, thus eliminating the need for KK analysis. The basic ma~ematic~ equations necessary to understand e~~ipsometric measurements of surfaces and surface layers are given in section 2. The extremely precise polarization-modulated ellipsometer used in the measurements is briefly described in section 3. Section 4 outlines the formalism of an approximation technique that allows the determination of three surface film parameters (complex optical response % or Z, and layer thickness, 6) for certain systems from only the two ehipsometric
216
R.C. 0 ‘Handley, D.K. Burge/Chemisorption
on metals
parameters of the filmed surface and knowledge of the substrate. In section 5 results are presented for oxygen chemisorption on evaporated Ag. Concluding remarks are in section 6.
2. Theory Some formulae necessary to ellipsometry film covered surfaces are given. 2.1. Ellipsometry
then equations
for
of simple surfaces
The parameters equation tan($)
are first presented,
$J and A measured in ellipsometry
are defined by the complex
= rp/rs ,
exp(iA)
(11
where rp and rs are the complex parallel and perpendicular amplitude reflection coefficients for the sample. We assume local, frequency-dependent, optical response functions of the form %=n-ik
(2)
F= E2 = e1 - ie2.
(3)
and
The Fresnel formulae, ,O = P
iii’COS
pi
ii _ -iii
~j
COS~j
-
njCOS 9j
(4)
+ ~j COS~j
COS ~j
rs - niCOS ~j
+ ~j
+
’
COS ~j
njCOS
~j
(5)
’
connect the complex amplitude reflection coefficient for the two media, i and j, with the indices of refraction and angles of incidence for those media, Because of Snell’s law, eqs. (4) and (5) effectively relate the reflection coefficients to the (complex) index of refraction of the second (jth) medium for a given angle of incidence, pi. Therefore, in a two-phase system (ambient/substrate) the real and imaginary parts of the substrate optical response can be related to the ellipsometric parameters by the equations [IS] n2 - k2 = el = sin2@1
1 + tan2@,
cos2(2$)
- sin2(2$)
[I +sir1(2$)cos(A)]~
sin2(A)
n: ’
(6)
R.C. O’Handley,
D.K. Burge/Chemisorption
sin(4$)
2nk = e2 = sin2 $I tan2 @I
sin(A)
[l + sin(2$)c0s(A)]~
on metals
n: ,
211
(7)
where @I is the real angle of incidence in medium 1 (ambient) and nl is the real refractive index of that medium. Thus for clean substrates, the determination of the optical constants by ellipsometry is a straightforward process. For a surface covered with an optically absorbing layer, the situation is obviously more complicated. We now work in a three-phase system: (1) ambient, (2) surface layer, and (3) substrate. Assuming the clean substrate has already been characterized, i.e., Z3 or n3 are known, there still remain three parameters to be determined, the complex response of the surface layer, T, or s2, and its thickness, d. The Fresnel formulae can no longer be used directly in eq. (1). They must first be put into the expressions for the reflection coefficients of a three-phase system, in our case a film-covered substrate I2 + ri:
J.p,s =
‘P,S
exp(-iD)
1 + r12 r23 exp(-iD) P,S P>S
’
(8)
where D = (4nn2d/h) cos I#I~
(9)
is the phase thickness of the surface layer and X is the wavelength of the light. Thus the simple relations between the ellipsometric parameters and the optical constants shown in eqs. (6) and (7) no longer apply to filmed substrates. More complicated relations, containing the additional parameter, d, now obtain. Therefore, in general, a third independent measurement, beyond $ and A, is required to completely determine the three-phase problem. In some cases the film thickness can be measured by a separate technique. When this is not possible, the auxiliary measurement can be a differential reflectance measurement [16], (6 Irl)/lrl, where 6 Jr1 is the reflectance change during chemisorption, (r-l2 = lrp12 sin201 + lr,12 cos201,
(101
and (Yis the azimuth of polarization of the incident light, usually 45”. Thus, the determination of the optical properties and thickness of a thin absorbing layer is a formidable task, particularly if several measurements are to be made either as a function of film growth or as a function of energy. AlI of the calculations referred to in this section can be carried out by McCrackin’s Fortran program for ellipsometric calculations [ 171.
3. Ellipsometry Some comments about standard ellipsometric paractice are first made, then polarization-modulated ellipsometry is discussed.
218
R.C. O’HandIey, D. K. BurgejC~e~isar~ri#n
on metals
Standard ellipsometers most often measure $ and A by a null technique. A polarized beam of light is reflected from a sample whose optical properties are to be determined. A polarizer, compensator, and analyzer are placed in the path of the beam and oriented in such a way as to null the intensity at the detector. Their azimuths at null can be related directly to the ellipsometric parameters of the sample. Measurements by this method are tedious; consequently, several automatic ellipsometers have been developed [ 1618-2 11. 3.2. Polarization-modulation ~~~ipsor~~etry More recently a new generation of automated elhpsometers has gained widespread recognition. These instruments do not use a null technique, and consequently require little or no movement of optical elements. Instead, the light beam is modulated. This can be accomplished by the rotation of a polarizer [22], by the use of a periodically driven Faraday cell [ 231, or by a piezoelectric modulator [24]. All of these systems combine the automation, speed and precision necessary for chemisorption studies. The third system, a polarization-modulated ellipsometer is now discussed in more detail. It is an extension of a system developed originally by Jasperson and Schnatteriy 1251. A polarized light beam is incident upon a c~st~line-quartz-driven fused quartz h
I’
6= a
CRYSTAL
QUARTZ
\
FUSED
sinkt)
QUARTZ
Fig. 1. Composite oscillator, driven such as to introduce a periodic relative phase shift, 6, between the components of incident polarized light that lie along its iandjaxes. The modulator is oriented 45” off the s and i axes of the sample so that both modulated light components, 1 and j, mix with each of the complex amplitude reflection coefficients of the sample, rs and rP.
R.C. O’Handiey, D.K. Burgel Chemisorption
on metals
219
Fig. 2. Ellipsometer schematic. The components are labeled as follows: spectrometer (SPEC), linear poiarizer (P), modulator (M), modulator driver unit (OSC), calibration circular/linear analyzer (Q’ or A'), sample (S) in ultrahigh vacuum chamber with residual gas analyzer (RGA), Iinear analyzer (A), photomultiplier (DET), lock-in detectors (LI I00 kHz, LI 50 kiiz), low pass filter (LPF), coupler/converter (C), digital voltmeter (DV), and programmable calculator (PC). modulator as shown in fig. 1. The periodic strain wave introduces a time-dependent phase shift, 6, between the polarized light components along the modulator axes. The polarization modulated Tight is then reflected from the sample (fig. 2) and anaiyzed before entering a detector. The intensity at the detector possesses a complicated harmonic content due to the sinusoidal modulation. Odd and even harmonics of the intensity contain independent information directly related to the ellipsometric parameters of the sample. Because of the modulation and alignment, the ellipsometric parameters are measured in such a way that the instrument is particularly suited to studying highly reflecting metals. The instrument requires no adjustments of its components once aligned and calibrated and is, therefore, rapid, easily automated and extremely precise and stable. In fact, the precision, an order of magnitude better than that of standard ellipsometers [24], corresponds to a sensitivity to a change of 0.04 A in the thickness of an absorbing surface layer (Z2 s 3.04 - 0.5i) on silver (g3 1 0.06 - 3.751). All of these qualities are important in chemisorption studies.
220
R. C. O’Handlev, D.K. Burge/Chemisorption
on metals
While automated ellipsometers can eliminate the tedium of the manual instruments, they ordinarily would be inadequate to the task of making a third measurement without inter~pting their automatic operating mode [9,26]. Exceptions to this are the ellipsometers of Paik and Bockris [26] and of Cahan and Spanier [ 161. In the next section an approximation technique is described that makes a third measurement superfluous in the optical characterizations of many chemisorbed layers on metals. Using this technique, automatic ellipsometers can operate in their normal mode of collecting $ and A data as a function of time or energy. These two parameters then suffice to determine 2 (or r> a&d for the surface layer under investigation.
4. Approximation
technique
In section 2 it was mentioned that a measurement of( I&-l)fir] is often made in order to supplement the two ellipsometric parameters, and allow determination of the three surface layer parameters. The reflectance for this three-phase system is related [though eqs. (I), (4), (5) and (8)-(lo)] to the observed ellipsometric parameters and d, However, if it is known a pr&+i that the surface optical constants fall within certain limits [9], it is a good approximation to cafmfate in the two-phase mdef the reflectance (or change in reffectance~ from the two measured e~ipsometric parameters. We call this the effective reflectance, Ireffl, and calculate it simply from eqs. (6) and (7), then (4) and (5), and finally (IO). (In this case the surface layer thickness does not enter the calculation.) The effective reaectance is then used instead of the measured reflectance to determine the third unknown in the problem. In practice, McCrackin’s [ 171 program for ellipsometric calculations is used. The measured values Ji and A (as well as X, &, nl, and ji3) are suf~cient to generate (not approximate) a set of possible solutions {nz, k2, d). These solutions all satisfy the observed changes in the ellipsometric parameters during chemisorption. Such a set of solutions is shown in fig. 3 for a hypothetical experiment in which a 10 a absorbing layer (Zz = 3.04 - 0.5) has grown on a silver surface. Other parameters used are h=54618,91=63.90,andnl= 1. The dashed line indicates the true solution which remains unknown to the investigator in this hypothetical experiment. Not only is there a continuum of solutions for the observed $J and A, but two branches exist. Each of these branches spans an overlapping range of layer thicknesses. Thus, it is not possible with a measurement of, or assumed value for, d to determine the solution un~bi~ously [9,26]. The exact reflectance, iri, can be calculated for each of the possible solutions shown in fig. 3. This is plotted in fig. 4 as a function of the layer thickness. The degeneracy of the solutions, for known d, is evident here also. Again the exact solution, lrl = 0.98452, d = 10 A is indicated by dashed lines. At this point the approximation jr,,, I SCIP[ is introduced. The effective reflec-
R.C. O’Handley, D.K. Burgelchemisorption
4.0
221
on metals
l-
loI8
2 2.0
0.9
1
b.
Fig. 3. Solutions to eq. (1) for a surface layer on silver,%3 = 0.06 - 3.751, J, = 44.434”, A = 126.599”, nr = 1, h = 5461 A, angle of incidence = 63.9”. The branch at the left goes with the lefthand scale for k2. while that at the right goes with the righthand scale.
0.987
1
II111111
I
II
I
11,111
-
LARGE
k2 BRANCH LARGE
g
rt2 BRANCH
0.986
d
Y 2
Y ii 2 0.965
0.9841 Fig. 4. Reflectance, lr I, as a function of surface layer thickness, d, calculated of solutions shown in fig. 3. Dashed line indicates the exact reflectance (and thickness) that would result from a perfect measurement of relative reflected and a three-phase calculation. Dot-dashed line indicates effective reflectance layer thickness) calculated from JI and A in the two-phase model.
from the two sets corresponding layer intensity change (and corresponding
222
R. C. O’Handley, D. K. Burge/Chemisorption
on metals
Table 1 Comparison of results from exact and approximate methods for characterizing an absorbing film on silver; same parameters as fig. 3 __-._---___ . .. -_~.-...-...-~ .-.-___ .-...-. -.--.- -.-Exact __
This method -__l_
Difference
0.98448
-0
Irl
0.98452
w
3.04
3.13
3%
kz
0.50
0.5 1
2%
9.5 -.--~-____-
5%
d GU
10.0
a
b
a Computed Irt. b Computed Ir,ff /.
0.005 ;I T, 51 “m ‘-O.oof -
- 0.002
-
0.001
= T_*
z ?.-
20&l -0.002 -
- 0.000
Fig. 5. Computed relative difference between true reflectance and effective reflectance as a function of surface layer index, n2, for a highly reflecting substrate iia = 0.06 - 3.75i. The optical absorption of the layer, k2, is 0.2 in the set of curves at the top of the figure, labeled (a) and 0.6 in the lower set of curves, labeled (b). The scale for set (a) is at the left of the figure, that for fb) at the right. Other parameters: Q,= 63.9”, nr = 1 .O, h = 5461 A.
R.C. 0 ‘Handley, D.K. BurgefChemisorption
0.002 ,_
I
I
(a) "2 = 3.0 =
0.001I-
T-5
5
k
on metals
I
223
I
./408
..A./ ..*
..e ..*
..Ae ..H /.- ..) ........3oi ..*.....,........... ..............e**.. .-*-.-.-Mw_mw------__-______
l0.000
201 ---10s
-0.001
-o.0020,0
I
I
0.2
0.4
.
k2
I 0.6
I 0.8
Fig. 6. Relative error as a function of fiim absorption, k2, for the same highly reflecting substrate and other parameters used in fig. 5. The upper set of curves, labeled (a), correspond to n2 = 3.0 and the lower set, labeled (b), are for n2 = 1.5. tance is calculated in the two-phase model, from the measured ellipsometric parameters. This value, (reff ( = 0.98448 (dot-dashed line, fig. 4), allows determination of the branch and layer thickness with remarkable accuracy in this case. A modest fciliarity with the situation is sufficient to exclude the large k2 solution at d > 100 8. (Such solutions are also present when Irl is measured.) Returning to fig. 3, the rest of the solution, n2 and k,, is then determined (dot-dashed line) from the branch and thickness just found in fig. 4. The solutions obtained by this approximation method are compared with the exact results in table 1. Again, it should be mentioned that this entire process can be carried out by computer [ 171. The key to the utility of the method is the smallness of the relative error involved in the approximation:
(11) The function on the left-hand side of the inequality is displayed in figs. 5 and 6 for a range of surface-layer optical constants on silver. Choosing 0.001 as an upper limit to the acceptable error (this value is comparable to the precision of the best reflectometer) still permits a wide range of applicability to various chemisorbed species on highly reflecting metals. The validity of the method for important oxidation, adsorption and corrosion situations on a variety of materials is discussed in detail in ref. 9.
R. C. 0 ‘Handley, D. K. Burge/Chemisorption
224
on metals
5. Results Surface optical spectra for oxygen chemisorption on silver are now presented. They were obtained by in situ polarization-modulation ellipsometry (section 3) combined with the above (section 4) approximation technique. The silver films were vapor deposited onto supersmooth fused quartz flats in UHV at room temperature. Films prepared in this manner are polycrystalline with [ 1111 or [ 2 111 fiber texture. Fig. 7 shows typical changes in $J and A at h = 5461 .& for a silver film brought quasi-statically to atmospheric pressure with oxygen. The chanber pressure was recycled to establish reversibility. At about 10-l Torr, an irreversible step-like change due to oxygen chemisorption is observed in the ellipsometric parameters. (The reversible changes, seen mostly in A, are due to physisorbed H,O [4] .) The pressure (10-l Torr) is in reasonable agreement with that for the onset of near-instantaneous
I
44.60
I
I
”
-
I
1
”
G .
z 3
z
A _
.^ -. ti
44.40
132
-
‘c
1281 10-4
I 10-Z PRESSURE
I 0 ITOR!,
Fig. 7. Ellipsometric parameters, rl, and A, as a function of the pressure for a typical stabilized silver film. The film was cycled from high vacuum to atmospheric pressure by back-tilling wit11 oxygen (open circles), back to vacuum (solid circles), and finally back to air with oxygen (triangles). Parameters measured at h = 5461 A and angle of incidence = 63.9”.
R.C. O’ffandley, D.K. Burge/Chemisorption on metals
225
Fig. 8. Auger electron spectra taken on a silver film before and after pressure cycling similar to that described in fig. 7. The principal change observed is the presence of considerable oxygen on the surface after exposure to about 1 Torr of oxygen.
adsorption calculated from kinetic theory and a measured value for the sticking coefficient of oxygen on silver [ 11 I] faces (- 10A3) [27]. Fig. 8 shows Auger electron spectra taken in a companion experiment before and after oxygen chemisorp tion. The circumst~ces were similar to those obt~ning in fig. 7. While small traces of sulfur appear, considerable additional oxygen is clearly the most significant change on the surface. Fig. 9 shows the spectral dependence of the real and imaginary parts of the effective (calculated in the two-phase approximation) dielectric response function before and after a typical chemisorption sequence. It is interesting to note that most of the change occurs in the absorptive part of Z. The surface plasmon on silver appears unaffected by oxygen chemisorption [ZS] . The dashed lines show free-electron theoretic results for comparison. In the visible, where or >> 1, we can write fl
=
1 -(w&)2
E2 = (~p/~)2(l~~T)
(12)
, f
(13)
The dashed curves were generated using Rw, = 9.65 eV and r = I X IO-l4 sec. The poor correspondence between experiment and theory for e2 is typical and is usually accounted for by allowing for a frequency-dependent relaxation time [29]. These data (fig. 9) can be used with the method of section 4 to calculate the complex optical response and thickness of the surface layer produced in this manner.
R.C. O’Handley, D.K. BurgejChemisorption
226
on metals
-8
1.0
-12 ‘1
0.8
-18
0.8
-20
0.4
‘2
-24 ----______ -28
0.2 0.0
3.5 2.0
2.5ENERGY
,e”?
Fig. 9. Effective dielectric constants ~1 and Ed, as a function of photon energy for a silver fflm before and after oxygen chemisorption. These parameters are calculated in the two-phase model from eqs. (6) and (7). Free-electron theoretic predictions are also displayed for comparison.
I , ---A--A__A__~__-e__A____~h-
,
A
a
I
2.0
I
I
2.5 ENERGY
3.0 (eV)
Fig. 10. Surface optical response, ~“1and E: and thickness of chemisorption-induced surface layer on silver as a function of energy. These parameters were obtained from $ and A by the approximation method described in the text.
The results are shown in fig. IO. A faint optical absorption is seen at 2.4 eV. This is attributed to an optical electron transition between the Fermi level and a surface state which is induced on the silver by the otherwise nonabsorbing chemisorbed oxygen. More involved interpretations of surface optical spectra have been explored
R.C. O’Handley, D.K. BurgefChemisorption
on metals
227
by McIntyre [ 1 l] and by Rubloff et al. [ 141. It is interesting that this structure (fig. 10) is not evident from a casual inspection of fig. 9. Also shown in fig. 10 is the thickness calculated at each photon energy. The average, 3.1 A, is the thickness of the surface layer whose optical response (assumed to be spatially homogeneous) differs from that of the bulk and is given by ES and ~5.
6. Conclusion It would be desirable to repeat the above experiments using single crystal surfaces and include simultaneous AES and LEED studies. Parallel photoemission experiments would also be helpful in determining whether the observed surface state lies above or below the Fermi energy. SOS, either by differential reflectance or ellipsometry, offers a unique insight into surface electronic structure in metals and its dependence on chemisorption.
References [l] R.J. Archer, in: Ellipsometry in the Measurement of Surface and Thin Films, Eds. E. Passagha, R.R. Stromberg and J. Kruger, Natl. Bur. Std. (U.S.) Misc. Pub. 256 (U.S. Govt. Printing Office, Washington, D.C., 1964) p. 255. [2] G.A. Bootsma and F. Meyer, Surface Sci. 13 (1969) 110; F. Meyer, Surface Sci. 27 (1971) 107. [3] G.A. Bootsma and F. Meyer, Surface Sci. 14 (1969) 52; F. Meyer and G.A. Bootsma, Surface Sci. 16 (1969) 221; F. Meyer, E.E. De Kluizenaar, and G.A. Bootsma, Surface Sci. 27 (1971) 88. [4] R.C. O‘handley, D.K. Burge, S.N. Jasperson and E.J. Ashley, Surface Sci. (submitted for publication). [S] T. Smith, J. Opt. Sot. Am. 58 (1968) 1069. [6] R.H. Muller, R.F. Steiger, G.A. Somorjai and J.M. Morabito, Surface Sci. 16 (1969) 234. [7] D.M. Kolb and J.D.E. McIntyre, Surface Sci. 28 (1971) 321. [8] T. Smith, J. Appl. Phys. 43 (1972) 2964. [9] R.C. O’Handley, Surface Sci. 46 (1974) 24,704. [lo] J.D.E. McIntyre and D.E. Aspnes, Surface Sci. 24 (1971) 417. [ll] J.D.E. McIntyre, in: Advances in Electrochemistry and Electrochemical Engineering, Vol. 9, Ed. R.H. Muller (Wiley-Interscience, New York, 1973) p. 61; Surface Sci. 37 (1973) 658. [12] G.W. Rubloff, J. Anderson and P.J. Stiles, Surface Sci. 37 (1973) 75. [13] J. Anderson, B.W. Rubloff and P.J. Stiles, Solid State Commun. 12 (1973) 825. [14] G.W. Rubloff, J. Anderson, M.A. Passler and P.J. Stiles, Phys. Rev. Letters 32 (1974) 667; J. Anderson, G.W. Rubloff, M.A. Passler and P.J. Stiles, Phys. Rev. B 10 (1974) 2401. [15] M. Born and E. Wolf, Principles of Optics, 2nd ed. (Pergamon, New York, 1964) p. 620. [16] B.D. Cahan and R.F. Spanier, Surface Sci. 16 (1969) 166. [ 171 F.L. McCrackin, A Fortran Program for Analysis of Ellipsometric Measurements, Natl. Bur. Std. Tech. Note 479 (U.S. Govt. Printing Office, Washington, D.C., 1969). [18] H. Takasaki, J. Opt. Sot. Am. 51 (1961) 463; Appl. Opt. 5 (1966) 759.
228 [IS]
R.C. O’Handley, D.K. BurgejChemkorption
on metals
J.L. Ord and B.L. Wills, Appl. Opt. 6 (1967) 1673. 120) T. Yamaguchi, S. Yoshida and A. Kinbara, Japan. J. Appl. Phys. 8 (1969) 559. [21] R. Greef, Rev. Sci. lnstr. 41 (1970) 532. 1221 D.E. Aspnes, Opt. Commun. 8 (1973) 222. [23] J. Monin and G.-A. Boutry, Nouv. Rev. Optique 4 (1973) 159. [24] S.N. Jasperson, D.K. Burge and R.C. O’Handley, Surface Sci. 37 (1973) 548. (251 S.N. Jasperson and S.E. Schnatterly, Rev. Sci. Instr. 40 (1969) 761. 1261 W.-K. Paik and J. O’M Bockris, Surface Sci. 28 (1971) 61. [27] A.W. Dweydari and C.H.B. Mee, Phys. Status Solidi (a) 17 (1973) 247. [ 281 Chemisorption can significantly affect the observation of the surface plasmon. See for example, T.A. Colcott and A.U. MacRae, Phys. Rev. 178 (1969) 966. (291 M.-L. Theye, Phys. Rev. B2 (1970) 3060; S.R. Nagel and S.E. Schnattedy, Phys. Rev. B9 (1974) 1299.
OPTICAL STUDIES OF CHEMISORPTION
ON METALS
R.C. O’HANDLEY * and D.K. BURGE Michelson Laboratory, Naval Weapons Center, China Lake, California 93555, U.S.A.
Optical techniques, particularly ellipsometry, have been highly successful in the study of chemisorption on semiconductors. Such studies on metal surfaces are more difficult because the short screening lengths limit the chemisorption-induced perturbation to a surface-layer only a few ingstr6ms thick. Some success has been achieved using differential reilectance spectroscopy. However this necessitates (1) two independent reflectance measurements or the use of KK analysis to obtain complex optical constants, and (2) assumed or independently-measured values for the thickness of the surface layer in order to completely specify the adsorption-induced changes. The advent of extremely precise in situ modulated ellipsometers has made the optical study of gas-metal interactions less difficult, in particular eliminating the necessity of KK analysis. We briefly describe such a system here, the automated polarization-modulated ellipsometer. When this instrument is coupled with a recently developed approximation technique for obtaining surface-layer thickness and optical constants from only two ellipsometric measurements, the complete optical characterization of many metal-absorbate systems is straightforward. Recent experimental results indicating a chemisorption-induced surface state within 2.4 eV of the Fermi level for oxygen on silver are discussed.
1. Introduction Chemisorption is a subject of wide interest both for its application to gas-solid interactions and heterogeneous catalysis as well as for the basic insight it gives concerning the electronic and structural properties of solid surfaces. Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) are two of the more widely used techniques that have been indispensable in determining chemisorbed species and their structural phases at surfaces. Optical measurements, particularly ellipsometry, so useful in studying bulk electronic properties, have also contributed significantly to our understanding of surface electronic properties in chemisorption situations. However, these optical studies have been limited often to measurements at one wavelength and mostly to adsorbate/semiconductor systems. The early work of Archer [I] concentrated on the physical chemistry of adsorption processes on silicon. The most extensive ellipsometric studies are those of Bootsma, Meyer et al. [2,3]. This group investigated physical and chemical adsorption on specific Si and Ge crystal faces. An important result of their work was understanding * Present address: New Jersey
07960,
Materials U.S.A.
Research
Center,
Allied Chemical
Corporation,
Morristown,
R.C. O’Handley, D.K. Burge/Chemisorption
on metals
215
the perturbing effects of chemisorbed species on the electronic structure, and, therefore, on the optical properties, of clean semiconductor surfaces [3]. It is now appreciated that even though a substance may be a dielectric (Z 3 n - ik, k = O}:in the condensed state, its presence at a solid surface often causes a perturbation sufficient to enhance the optical absorption. Such is not the case with physisorbed layers which appear optically as thin dielectric surface films [ 2-41.
Ellipsometric studies of chemisorption on metals are more difficult, and therefore less abundant [4-81. This is because the short screening length in metals limits the charge perturbation due to the presence of the adsorbate, to a layer of a few %rgstroms at the surface. Many [5,6] of the existing investigations on metals are limited in scope to the physical chemistry of the adsorption process, using data taken at a single waveleng~. Nevertheless, the chemisorbed layer at a metal surface can be treated formally as a thin surface film [7,9-l I] allowing the determination of its optical response as a function of photon energy. We refer to this as surface optical spectroscopy (SOS). Kolb and McIntyre [7] have obtained striking spectral results for adsorbed oxygen layers on gold using differenti~ reflectance spectroscopy [ 1O]_ Subsequent work ]lO,l l] has led to better understand~g of these surface optical spectra. More recently the use of differential reflectance spectroscopy and a sensitive light-pipe reflectometer [ 121 have made possible the spectral characterization of optically absorbing, chemisorption-induced surface layers on MO [ 131 and W [ 141. While reflectance spectroscopy is more straightforward and has greater spectral range than elljpsomet~, it has two drawbacks. It requires (1) two reflectance measurements with polarized light or a Kramers-Kronig (KK) analysis of the (unpolarized) reflectance data, and (2) an auxiliary measurement of (or assumed value for) the surface-layer thickness in order to completely characterize the surface. 1.2. Purpose It is the purpose of this paper to describe recent experimental and theoretical advances that have made less difficult the SOS of thin optically absorbing layers induced on mtals by chemisorption. The experimental technique used is ellipsometry because it gives two optical parameters for the surface, thus eliminating the need for KK analysis. The basic ma~ematic~ equations necessary to understand e~~ipsometric measurements of surfaces and surface layers are given in section 2. The extremely precise polarization-modulated ellipsometer used in the measurements is briefly described in section 3. Section 4 outlines the formalism of an approximation technique that allows the determination of three surface film parameters (complex optical response % or Z, and layer thickness, 6) for certain systems from only the two ehipsometric
216
R.C. 0 ‘Handley, D.K. Burge/Chemisorption
on metals
parameters of the filmed surface and knowledge of the substrate. In section 5 results are presented for oxygen chemisorption on evaporated Ag. Concluding remarks are in section 6.
2. Theory Some formulae necessary to ellipsometry film covered surfaces are given. 2.1. Ellipsometry
then equations
for
of simple surfaces
The parameters equation tan($)
are first presented,
$J and A measured in ellipsometry
are defined by the complex
= rp/rs ,
exp(iA)
(11
where rp and rs are the complex parallel and perpendicular amplitude reflection coefficients for the sample. We assume local, frequency-dependent, optical response functions of the form %=n-ik
(2)
F= E2 = e1 - ie2.
(3)
and
The Fresnel formulae, ,O = P
iii’COS
pi
ii _ -iii
~j
COS~j
-
njCOS 9j
(4)
+ ~j COS~j
COS ~j
rs - niCOS ~j
+ ~j
+
’
COS ~j
njCOS
~j
(5)
’
connect the complex amplitude reflection coefficient for the two media, i and j, with the indices of refraction and angles of incidence for those media, Because of Snell’s law, eqs. (4) and (5) effectively relate the reflection coefficients to the (complex) index of refraction of the second (jth) medium for a given angle of incidence, pi. Therefore, in a two-phase system (ambient/substrate) the real and imaginary parts of the substrate optical response can be related to the ellipsometric parameters by the equations [IS] n2 - k2 = el = sin2@1
1 + tan2@,
cos2(2$)
- sin2(2$)
[I +sir1(2$)cos(A)]~
sin2(A)
n: ’
(6)
R.C. O’Handley,
D.K. Burge/Chemisorption
sin(4$)
2nk = e2 = sin2 $I tan2 @I
sin(A)
[l + sin(2$)c0s(A)]~
on metals
n: ,
211
(7)
where @I is the real angle of incidence in medium 1 (ambient) and nl is the real refractive index of that medium. Thus for clean substrates, the determination of the optical constants by ellipsometry is a straightforward process. For a surface covered with an optically absorbing layer, the situation is obviously more complicated. We now work in a three-phase system: (1) ambient, (2) surface layer, and (3) substrate. Assuming the clean substrate has already been characterized, i.e., Z3 or n3 are known, there still remain three parameters to be determined, the complex response of the surface layer, T, or s2, and its thickness, d. The Fresnel formulae can no longer be used directly in eq. (1). They must first be put into the expressions for the reflection coefficients of a three-phase system, in our case a film-covered substrate I2 + ri:
J.p,s =
‘P,S
exp(-iD)
1 + r12 r23 exp(-iD) P,S P>S
’
(8)
where D = (4nn2d/h) cos I#I~
(9)
is the phase thickness of the surface layer and X is the wavelength of the light. Thus the simple relations between the ellipsometric parameters and the optical constants shown in eqs. (6) and (7) no longer apply to filmed substrates. More complicated relations, containing the additional parameter, d, now obtain. Therefore, in general, a third independent measurement, beyond $ and A, is required to completely determine the three-phase problem. In some cases the film thickness can be measured by a separate technique. When this is not possible, the auxiliary measurement can be a differential reflectance measurement [16], (6 Irl)/lrl, where 6 Jr1 is the reflectance change during chemisorption, (r-l2 = lrp12 sin201 + lr,12 cos201,
(101
and (Yis the azimuth of polarization of the incident light, usually 45”. Thus, the determination of the optical properties and thickness of a thin absorbing layer is a formidable task, particularly if several measurements are to be made either as a function of film growth or as a function of energy. AlI of the calculations referred to in this section can be carried out by McCrackin’s Fortran program for ellipsometric calculations [ 171.
3. Ellipsometry Some comments about standard ellipsometric paractice are first made, then polarization-modulated ellipsometry is discussed.
218
R.C. O’HandIey, D. K. BurgejC~e~isar~ri#n
on metals
Standard ellipsometers most often measure $ and A by a null technique. A polarized beam of light is reflected from a sample whose optical properties are to be determined. A polarizer, compensator, and analyzer are placed in the path of the beam and oriented in such a way as to null the intensity at the detector. Their azimuths at null can be related directly to the ellipsometric parameters of the sample. Measurements by this method are tedious; consequently, several automatic ellipsometers have been developed [ 1618-2 11. 3.2. Polarization-modulation ~~~ipsor~~etry More recently a new generation of automated elhpsometers has gained widespread recognition. These instruments do not use a null technique, and consequently require little or no movement of optical elements. Instead, the light beam is modulated. This can be accomplished by the rotation of a polarizer [22], by the use of a periodically driven Faraday cell [ 231, or by a piezoelectric modulator [24]. All of these systems combine the automation, speed and precision necessary for chemisorption studies. The third system, a polarization-modulated ellipsometer is now discussed in more detail. It is an extension of a system developed originally by Jasperson and Schnatteriy 1251. A polarized light beam is incident upon a c~st~line-quartz-driven fused quartz h
I’
6= a
CRYSTAL
QUARTZ
\
FUSED
sinkt)
QUARTZ
Fig. 1. Composite oscillator, driven such as to introduce a periodic relative phase shift, 6, between the components of incident polarized light that lie along its iandjaxes. The modulator is oriented 45” off the s and i axes of the sample so that both modulated light components, 1 and j, mix with each of the complex amplitude reflection coefficients of the sample, rs and rP.
R.C. O’Handiey, D.K. Burgel Chemisorption
on metals
219
Fig. 2. Ellipsometer schematic. The components are labeled as follows: spectrometer (SPEC), linear poiarizer (P), modulator (M), modulator driver unit (OSC), calibration circular/linear analyzer (Q’ or A'), sample (S) in ultrahigh vacuum chamber with residual gas analyzer (RGA), Iinear analyzer (A), photomultiplier (DET), lock-in detectors (LI I00 kHz, LI 50 kiiz), low pass filter (LPF), coupler/converter (C), digital voltmeter (DV), and programmable calculator (PC). modulator as shown in fig. 1. The periodic strain wave introduces a time-dependent phase shift, 6, between the polarized light components along the modulator axes. The polarization modulated Tight is then reflected from the sample (fig. 2) and anaiyzed before entering a detector. The intensity at the detector possesses a complicated harmonic content due to the sinusoidal modulation. Odd and even harmonics of the intensity contain independent information directly related to the ellipsometric parameters of the sample. Because of the modulation and alignment, the ellipsometric parameters are measured in such a way that the instrument is particularly suited to studying highly reflecting metals. The instrument requires no adjustments of its components once aligned and calibrated and is, therefore, rapid, easily automated and extremely precise and stable. In fact, the precision, an order of magnitude better than that of standard ellipsometers [24], corresponds to a sensitivity to a change of 0.04 A in the thickness of an absorbing surface layer (Z2 s 3.04 - 0.5i) on silver (g3 1 0.06 - 3.751). All of these qualities are important in chemisorption studies.
220
R. C. O’Handlev, D.K. Burge/Chemisorption
on metals
While automated ellipsometers can eliminate the tedium of the manual instruments, they ordinarily would be inadequate to the task of making a third measurement without inter~pting their automatic operating mode [9,26]. Exceptions to this are the ellipsometers of Paik and Bockris [26] and of Cahan and Spanier [ 161. In the next section an approximation technique is described that makes a third measurement superfluous in the optical characterizations of many chemisorbed layers on metals. Using this technique, automatic ellipsometers can operate in their normal mode of collecting $ and A data as a function of time or energy. These two parameters then suffice to determine 2 (or r> a&d for the surface layer under investigation.
4. Approximation
technique
In section 2 it was mentioned that a measurement of( I&-l)fir] is often made in order to supplement the two ellipsometric parameters, and allow determination of the three surface layer parameters. The reflectance for this three-phase system is related [though eqs. (I), (4), (5) and (8)-(lo)] to the observed ellipsometric parameters and d, However, if it is known a pr&+i that the surface optical constants fall within certain limits [9], it is a good approximation to cafmfate in the two-phase mdef the reflectance (or change in reffectance~ from the two measured e~ipsometric parameters. We call this the effective reflectance, Ireffl, and calculate it simply from eqs. (6) and (7), then (4) and (5), and finally (IO). (In this case the surface layer thickness does not enter the calculation.) The effective reaectance is then used instead of the measured reflectance to determine the third unknown in the problem. In practice, McCrackin’s [ 171 program for ellipsometric calculations is used. The measured values Ji and A (as well as X, &, nl, and ji3) are suf~cient to generate (not approximate) a set of possible solutions {nz, k2, d). These solutions all satisfy the observed changes in the ellipsometric parameters during chemisorption. Such a set of solutions is shown in fig. 3 for a hypothetical experiment in which a 10 a absorbing layer (Zz = 3.04 - 0.5) has grown on a silver surface. Other parameters used are h=54618,91=63.90,andnl= 1. The dashed line indicates the true solution which remains unknown to the investigator in this hypothetical experiment. Not only is there a continuum of solutions for the observed $J and A, but two branches exist. Each of these branches spans an overlapping range of layer thicknesses. Thus, it is not possible with a measurement of, or assumed value for, d to determine the solution un~bi~ously [9,26]. The exact reflectance, iri, can be calculated for each of the possible solutions shown in fig. 3. This is plotted in fig. 4 as a function of the layer thickness. The degeneracy of the solutions, for known d, is evident here also. Again the exact solution, lrl = 0.98452, d = 10 A is indicated by dashed lines. At this point the approximation jr,,, I SCIP[ is introduced. The effective reflec-
R.C. O’Handley, D.K. Burgelchemisorption
4.0
221
on metals
l-
loI8
2 2.0
0.9
1
b.
Fig. 3. Solutions to eq. (1) for a surface layer on silver,%3 = 0.06 - 3.751, J, = 44.434”, A = 126.599”, nr = 1, h = 5461 A, angle of incidence = 63.9”. The branch at the left goes with the lefthand scale for k2. while that at the right goes with the righthand scale.
0.987
1
II111111
I
II
I
11,111
-
LARGE
k2 BRANCH LARGE
g
rt2 BRANCH
0.986
d
Y 2
Y ii 2 0.965
0.9841 Fig. 4. Reflectance, lr I, as a function of surface layer thickness, d, calculated of solutions shown in fig. 3. Dashed line indicates the exact reflectance (and thickness) that would result from a perfect measurement of relative reflected and a three-phase calculation. Dot-dashed line indicates effective reflectance layer thickness) calculated from JI and A in the two-phase model.
from the two sets corresponding layer intensity change (and corresponding
222
R. C. O’Handley, D. K. Burge/Chemisorption
on metals
Table 1 Comparison of results from exact and approximate methods for characterizing an absorbing film on silver; same parameters as fig. 3 __-._---___ . .. -_~.-...-...-~ .-.-___ .-...-. -.--.- -.-Exact __
This method -__l_
Difference
0.98448
-0
Irl
0.98452
w
3.04
3.13
3%
kz
0.50
0.5 1
2%
9.5 -.--~-____-
5%
d GU
10.0
a
b
a Computed Irt. b Computed Ir,ff /.
0.005 ;I T, 51 “m ‘-O.oof -
- 0.002
-
0.001
= T_*
z ?.-
20&l -0.002 -
- 0.000
Fig. 5. Computed relative difference between true reflectance and effective reflectance as a function of surface layer index, n2, for a highly reflecting substrate iia = 0.06 - 3.75i. The optical absorption of the layer, k2, is 0.2 in the set of curves at the top of the figure, labeled (a) and 0.6 in the lower set of curves, labeled (b). The scale for set (a) is at the left of the figure, that for fb) at the right. Other parameters: Q,= 63.9”, nr = 1 .O, h = 5461 A.
R.C. 0 ‘Handley, D.K. BurgefChemisorption
0.002 ,_
I
I
(a) "2 = 3.0 =
0.001I-
T-5
5
k
on metals
I
223
I
./408
..A./ ..*
..e ..*
..Ae ..H /.- ..) ........3oi ..*.....,........... ..............e**.. .-*-.-.-Mw_mw------__-______
l0.000
201 ---10s
-0.001
-o.0020,0
I
I
0.2
0.4
.
k2
I 0.6
I 0.8
Fig. 6. Relative error as a function of fiim absorption, k2, for the same highly reflecting substrate and other parameters used in fig. 5. The upper set of curves, labeled (a), correspond to n2 = 3.0 and the lower set, labeled (b), are for n2 = 1.5. tance is calculated in the two-phase model, from the measured ellipsometric parameters. This value, (reff ( = 0.98448 (dot-dashed line, fig. 4), allows determination of the branch and layer thickness with remarkable accuracy in this case. A modest fciliarity with the situation is sufficient to exclude the large k2 solution at d > 100 8. (Such solutions are also present when Irl is measured.) Returning to fig. 3, the rest of the solution, n2 and k,, is then determined (dot-dashed line) from the branch and thickness just found in fig. 4. The solutions obtained by this approximation method are compared with the exact results in table 1. Again, it should be mentioned that this entire process can be carried out by computer [ 171. The key to the utility of the method is the smallness of the relative error involved in the approximation:
(11) The function on the left-hand side of the inequality is displayed in figs. 5 and 6 for a range of surface-layer optical constants on silver. Choosing 0.001 as an upper limit to the acceptable error (this value is comparable to the precision of the best reflectometer) still permits a wide range of applicability to various chemisorbed species on highly reflecting metals. The validity of the method for important oxidation, adsorption and corrosion situations on a variety of materials is discussed in detail in ref. 9.
R. C. 0 ‘Handley, D. K. Burge/Chemisorption
224
on metals
5. Results Surface optical spectra for oxygen chemisorption on silver are now presented. They were obtained by in situ polarization-modulation ellipsometry (section 3) combined with the above (section 4) approximation technique. The silver films were vapor deposited onto supersmooth fused quartz flats in UHV at room temperature. Films prepared in this manner are polycrystalline with [ 1111 or [ 2 111 fiber texture. Fig. 7 shows typical changes in $J and A at h = 5461 .& for a silver film brought quasi-statically to atmospheric pressure with oxygen. The chanber pressure was recycled to establish reversibility. At about 10-l Torr, an irreversible step-like change due to oxygen chemisorption is observed in the ellipsometric parameters. (The reversible changes, seen mostly in A, are due to physisorbed H,O [4] .) The pressure (10-l Torr) is in reasonable agreement with that for the onset of near-instantaneous
I
44.60
I
I
”
-
I
1
”
G .
z 3
z
A _
.^ -. ti
44.40
132
-
‘c
1281 10-4
I 10-Z PRESSURE
I 0 ITOR!,
Fig. 7. Ellipsometric parameters, rl, and A, as a function of the pressure for a typical stabilized silver film. The film was cycled from high vacuum to atmospheric pressure by back-tilling wit11 oxygen (open circles), back to vacuum (solid circles), and finally back to air with oxygen (triangles). Parameters measured at h = 5461 A and angle of incidence = 63.9”.
R.C. O’ffandley, D.K. Burge/Chemisorption on metals
225
Fig. 8. Auger electron spectra taken on a silver film before and after pressure cycling similar to that described in fig. 7. The principal change observed is the presence of considerable oxygen on the surface after exposure to about 1 Torr of oxygen.
adsorption calculated from kinetic theory and a measured value for the sticking coefficient of oxygen on silver [ 11 I] faces (- 10A3) [27]. Fig. 8 shows Auger electron spectra taken in a companion experiment before and after oxygen chemisorp tion. The circumst~ces were similar to those obt~ning in fig. 7. While small traces of sulfur appear, considerable additional oxygen is clearly the most significant change on the surface. Fig. 9 shows the spectral dependence of the real and imaginary parts of the effective (calculated in the two-phase approximation) dielectric response function before and after a typical chemisorption sequence. It is interesting to note that most of the change occurs in the absorptive part of Z. The surface plasmon on silver appears unaffected by oxygen chemisorption [ZS] . The dashed lines show free-electron theoretic results for comparison. In the visible, where or >> 1, we can write fl
=
1 -(w&)2
E2 = (~p/~)2(l~~T)
(12)
, f
(13)
The dashed curves were generated using Rw, = 9.65 eV and r = I X IO-l4 sec. The poor correspondence between experiment and theory for e2 is typical and is usually accounted for by allowing for a frequency-dependent relaxation time [29]. These data (fig. 9) can be used with the method of section 4 to calculate the complex optical response and thickness of the surface layer produced in this manner.
R.C. O’Handley, D.K. BurgejChemisorption
226
on metals
-8
1.0
-12 ‘1
0.8
-18
0.8
-20
0.4
‘2
-24 ----______ -28
0.2 0.0
3.5 2.0
2.5ENERGY
,e”?
Fig. 9. Effective dielectric constants ~1 and Ed, as a function of photon energy for a silver fflm before and after oxygen chemisorption. These parameters are calculated in the two-phase model from eqs. (6) and (7). Free-electron theoretic predictions are also displayed for comparison.
I , ---A--A__A__~__-e__A____~h-
,
A
a
I
2.0
I
I
2.5 ENERGY
3.0 (eV)
Fig. 10. Surface optical response, ~“1and E: and thickness of chemisorption-induced surface layer on silver as a function of energy. These parameters were obtained from $ and A by the approximation method described in the text.
The results are shown in fig. IO. A faint optical absorption is seen at 2.4 eV. This is attributed to an optical electron transition between the Fermi level and a surface state which is induced on the silver by the otherwise nonabsorbing chemisorbed oxygen. More involved interpretations of surface optical spectra have been explored
R.C. O’Handley, D.K. BurgefChemisorption
on metals
227
by McIntyre [ 1 l] and by Rubloff et al. [ 141. It is interesting that this structure (fig. 10) is not evident from a casual inspection of fig. 9. Also shown in fig. 10 is the thickness calculated at each photon energy. The average, 3.1 A, is the thickness of the surface layer whose optical response (assumed to be spatially homogeneous) differs from that of the bulk and is given by ES and ~5.
6. Conclusion It would be desirable to repeat the above experiments using single crystal surfaces and include simultaneous AES and LEED studies. Parallel photoemission experiments would also be helpful in determining whether the observed surface state lies above or below the Fermi energy. SOS, either by differential reflectance or ellipsometry, offers a unique insight into surface electronic structure in metals and its dependence on chemisorption.
References [l] R.J. Archer, in: Ellipsometry in the Measurement of Surface and Thin Films, Eds. E. Passagha, R.R. Stromberg and J. Kruger, Natl. Bur. Std. (U.S.) Misc. Pub. 256 (U.S. Govt. Printing Office, Washington, D.C., 1964) p. 255. [2] G.A. Bootsma and F. Meyer, Surface Sci. 13 (1969) 110; F. Meyer, Surface Sci. 27 (1971) 107. [3] G.A. Bootsma and F. Meyer, Surface Sci. 14 (1969) 52; F. Meyer and G.A. Bootsma, Surface Sci. 16 (1969) 221; F. Meyer, E.E. De Kluizenaar, and G.A. Bootsma, Surface Sci. 27 (1971) 88. [4] R.C. O‘handley, D.K. Burge, S.N. Jasperson and E.J. Ashley, Surface Sci. (submitted for publication). [S] T. Smith, J. Opt. Sot. Am. 58 (1968) 1069. [6] R.H. Muller, R.F. Steiger, G.A. Somorjai and J.M. Morabito, Surface Sci. 16 (1969) 234. [7] D.M. Kolb and J.D.E. McIntyre, Surface Sci. 28 (1971) 321. [8] T. Smith, J. Appl. Phys. 43 (1972) 2964. [9] R.C. O’Handley, Surface Sci. 46 (1974) 24,704. [lo] J.D.E. McIntyre and D.E. Aspnes, Surface Sci. 24 (1971) 417. [ll] J.D.E. McIntyre, in: Advances in Electrochemistry and Electrochemical Engineering, Vol. 9, Ed. R.H. Muller (Wiley-Interscience, New York, 1973) p. 61; Surface Sci. 37 (1973) 658. [12] G.W. Rubloff, J. Anderson and P.J. Stiles, Surface Sci. 37 (1973) 75. [13] J. Anderson, B.W. Rubloff and P.J. Stiles, Solid State Commun. 12 (1973) 825. [14] G.W. Rubloff, J. Anderson, M.A. Passler and P.J. Stiles, Phys. Rev. Letters 32 (1974) 667; J. Anderson, G.W. Rubloff, M.A. Passler and P.J. Stiles, Phys. Rev. B 10 (1974) 2401. [15] M. Born and E. Wolf, Principles of Optics, 2nd ed. (Pergamon, New York, 1964) p. 620. [16] B.D. Cahan and R.F. Spanier, Surface Sci. 16 (1969) 166. [ 171 F.L. McCrackin, A Fortran Program for Analysis of Ellipsometric Measurements, Natl. Bur. Std. Tech. Note 479 (U.S. Govt. Printing Office, Washington, D.C., 1969). [18] H. Takasaki, J. Opt. Sot. Am. 51 (1961) 463; Appl. Opt. 5 (1966) 759.
228 [IS]
R.C. O’Handley, D.K. BurgejChemkorption
on metals
J.L. Ord and B.L. Wills, Appl. Opt. 6 (1967) 1673. 120) T. Yamaguchi, S. Yoshida and A. Kinbara, Japan. J. Appl. Phys. 8 (1969) 559. [21] R. Greef, Rev. Sci. lnstr. 41 (1970) 532. 1221 D.E. Aspnes, Opt. Commun. 8 (1973) 222. [23] J. Monin and G.-A. Boutry, Nouv. Rev. Optique 4 (1973) 159. [24] S.N. Jasperson, D.K. Burge and R.C. O’Handley, Surface Sci. 37 (1973) 548. (251 S.N. Jasperson and S.E. Schnatterly, Rev. Sci. Instr. 40 (1969) 761. 1261 W.-K. Paik and J. O’M Bockris, Surface Sci. 28 (1971) 61. [27] A.W. Dweydari and C.H.B. Mee, Phys. Status Solidi (a) 17 (1973) 247. [ 281 Chemisorption can significantly affect the observation of the surface plasmon. See for example, T.A. Colcott and A.U. MacRae, Phys. Rev. 178 (1969) 966. (291 M.-L. Theye, Phys. Rev. B2 (1970) 3060; S.R. Nagel and S.E. Schnattedy, Phys. Rev. B9 (1974) 1299.