Core level and valence band xray photoelectron spectroscopy of gold oxideCarolyn Rubin Aita and Ngoc C. Tran Citation: Journal of Vacuum Science & Technology A 9, 1498 (1991); doi: 10.1116/1.577652 View online: http://dx.doi.org/10.1116/1.577652 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/9/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Deduction of the chemical state and the electronic structure of Nd2Fe14B compound from X-ray photoelectronspectroscopy core-level and valence-band spectra J. Appl. Phys. 116, 163917 (2014); 10.1063/1.4900732 Carbon contamination and oxidation of Au surfaces under extreme ultraviolet radiation: An x-ray photoelectronspectroscopy study J. Vac. Sci. Technol. B 30, 041603 (2012); 10.1116/1.4737160 Algorithm for automatic x-ray photoelectron spectroscopy data processing and x-ray photoelectron spectroscopyimaging J. Vac. Sci. Technol. A 23, 741 (2005); 10.1116/1.1864053 Detailed xray photoelectron spectroscopy valence band and core level studies of select metals oxidations J. Vac. Sci. Technol. A 10, 2383 (1992); 10.1116/1.577970 Xray photoelectron emission measurements of the valence band density of states and core levels of CuAlS2 Appl. Phys. Lett. 23, 453 (1973); 10.1063/1.1654955

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Oxidation and corrosion studies by valence band photoemission P. M. A. Sherwood Department a/Chemistry. Willard Hall. Kansas State University. Manhattan. Kansas 66506

(Received 31 August 1990; accepted 26 November 1990)

A combination of valence band x-ray photoelectron spectroscopy (VXPS) interpreted by Xa calculations, and x-ray diffraction is discussed as a means of understanding the surface chemistry associated with oxidation and its prevention. The cluster models used to interpret VXPS for oxide systems are discussed, and the spectrum of cuprous oxide is examined in detail. It can be seen that good agreement can be obtained with the valence band spectra of oxidation films and corrosion inhibitor films. Examples are given that show how VXPS can reveal chemical features that cannot be obtained from core x-ray photoelectron spectroscopy.

I. INTRODUCTION

A detailed understanding of the surface chemistry associat­ed with oxidation and its prevention is needed to tackle the complex problems associated with corrosion. In the past twenty years a powerful array of surface analytical tech­niques have become available for the examination of such systems. Despite this advance, corrosion and oxidation sys­tems present analysis problems. This is because: the systems of interest are not normally single crystals, thus eliminating the use of a number of powerful surface analysis methods; the surfaces often contain trace impurities not easily detect­ed; and the surface can be altered by the application of the normal methods for achieving the best ultrahigh vacuum (UHV) conditions. In particular hydrated surface layers-a normal feature of corrosion systems-are normally dehy­drated and altered when true UHV conditions are obtained. This is particularly unfortunate since it is clear that hydrated and oxyhydroxide type layers playa very important role in corrosion processes.

The author has been interested in the application of sur­face science methods, especially x-ray photoelectron spec­troscopy (XPS), to the study of electrode surface films and corrosion layers for a number of years. l XPS [or electron spectroscopy for chemical analysis (ESCA)] displays valu­able core chemical shifts allowing the chemical species on corroded surfaces to be examined,2 but there are often diffi­culties in distinguishing subtle chemical differences.

Valence band photoemission using x-ray, ultraviolet (UV), and synchrotron radiation has been extensively ex­amined over the years, but these studies have often concen­trated on metals and semiconductors and have often been linked to their interpretation by band structure calculations. Little analytical use has been made of this region. In recent years the author3

-l2 has found that considerable use can be

made of valence band photoemission as an aid to identifying subtle chemical differences in various analytical applica­tions. The success of this approach has been aided by the interpretation of the spectra by Xa calculations. In this pa­per the use of a combination of valence band XPS (VXPS) interpreted by Xa calculations and aided by x-ray diffraction is discussed with an emphasis on the type of cluster needed to appropriately model oxide solids.

II. CALCULATION DETAILS

The Xa calculations discussed here are not trivial, but can be performed on computers that can be reasonably located in the laboratory. For example most of the calculations were performed on an IBM PS/2 Model 80 using highly opti­mized double precision Fortran code with the processor op­erating in 32-bit code running under XENIX. These calcula­tions are now being performed on an IBM RISC/6000 system which can lead to substantial improvements in speed (10-45 times) compared with the Model 80. Both systems could be reasonably part of a surface analysis laboratory budget, being only a fraction of the cost of the typical surface science instrument.

III. RESULTS AND DISCUSSION

A. The need and choice of calculations

Valence band spectra can show considerable differences between different compounds. which is not surprising since the spectra involve energy levels directly involved in chemi­cal bonding. The challenge is to understand and identify these differences. One obvious approach is to generate the spectra ofa series of standards, but this approach is not with­out its difficulties since the surface of many standard com­pounds is not representative of the bulk. The confidence with which VXPS can be used for analytical purposes is greatly enhanced if one can use a reliable and efficient calculation method to predict the spectra.

An enormous number of calculation methods are avail­able, but there is a need to balance exactness of the approach (usually associated with a corresponding increase in calcula­tion complexity and expense) with the need to have a reason­ably predicted spectrum available at the time that the experi­mental data are collected. The author has found that multiple scattered wave Xa calculations (MSXA) l.1 repre­sent a reasonable approximation and provide accurate pre­dictions of VXPS data for oxide and corrosion systems.

The Xa calculations yield energy levels. For each energy level a percentage of the occupancy of the level correspond­ing to each atom in the molecule is given, together with the percentage contribution from each type of atomic orbital for each atom. Calculated spectra can be generated by adding

1493 J. Vac. Sci. Technol. A 9 (3), May/Jun 1991 0734-2101/91/031493-05$01.00 @ 1991 American Vacuum Society 1493

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1494 P. M. A. Sherwood: Oxidation and corrosion studies

together a number of component peaks corresponding to the calculated energy levels (or some combination of closely spaced calculated energy levels). Each peak has a position corresponding to the calculated energy level and an intensity (area) corresponding to the number of electrons in the ener­gy level multiplied by the atomic population for the level, adjusted by the appropriate atomic photoelectric cross sec­tions (often using the values by Scofield 14 ). Thus a level with two electrons in it made up entirely of s electron density with a 50% atom A contribution and 500/£: atom B contribu­tion would have an area corresponding to 2 X [(0. Sa) + (O.5b) 1, where a and b are the s photoelec­tron cross sections of atoms A and E, respectively. The val­ues are also adjusted for compound stoichiometry (e.g., for an FeO~ cluster used to represent Fe2 0, the oxygen to iron stoichiometry is taken as I: 1. 5 rather than 1:6). Each com­ponent in the calculated spectrum is either a single energy level or a group of closely spaced energy levels and corre­sponds to a 50% Gaussian-Lorentzian product function, 15

with each peak having the same full width at half-maximum and an area generated as described above.

B. The cluster model

The first consideration is whether the use ofa cluster mod­el is appropriate for calculating solid valence bands, and ifso what size of cluster is to be used. In the case of corrosion and oxidation systems, most oxide and hydroxide species can be represented by clusters where the "muffin-tin" approxima­tion is an appropriate model (MSXA calculations use a model where each atom is represented by a sphere surround­ed by an outer sphere containing all the atoms-the best approach is when the interatomic region volume within the outer sphere is minimized). An important consideration is the size of cluster needed to suitably reproduce the valence band features. In some cases the choice is simple, such as in t he case of discrete ions such as the phosphate or sulfate ions. In other cases the choice will depend upon the packing in the crystal structure. In general a minimum cluster shouid be the cluster that represents the symmetry around the atom of

T.'\lIl L \. Parameter, used and fealures on the X-alpha ~alculations.

It Value" Copper 0.706'17 Oxygen 0.74447

OUler IntersphereO.B659 (CuO: ) 0.72780 (Cu,O;

1494

highest photoelectric cross section. Thus in the case of most metal oxide systems this will be the cluster representing the symmetry about the metal. Where there is a substantial dif­ference in cross section for x radiation, e.g., iron versus oxy­gen, clusters of this type work well e.g.,o FeO: ,but even when the cross sections are more closely matched e.g., alu­minum versus oxygen the choice of clusters of this type may still be appropriate, thus we find that an AIO~ cluster gives better agreement with the a-All OJ VXPS data than either an A120~ or an Alz O:~ - cluster. 12

In some oxide structures the oxygen is closely surrounded by many metal atoms and consideration of a larger cluster becomes necessary if significant weak features in the spec­trum are to be predicted. To illustrate this point calculations of the cuprous oxide spectrum will be considered. Cuprous oxide is of considerable interest because of its structural rela­tionship to high Tc superconductor materials. Its VXPS data has been extensively discussed and cluster calculations considered as a means of modeling the solid. 16. 17

In an earlier studyl we used a CuO~ cluster as a model to describe the satellite features in the Cu 2p core region. Using this model (and the parameters given in Table l) the calcu­lated valence band spectrum shown in Fig. 1 (c) is obtained. At first sight the agreement with the previously published experimental VXPS data lb is good, but on closer examina­tion one notices some significant discrepancies. Thus the cal­culated spectrum for the CuO~- cluster is significantly broader than the experimental data [Fig I (a) 1, and a signif­icant high binding energy shoulder around 8 eV is missing. This shoulder can be shown to contain considerable 0 2p intensity (which is of low photoelectric cross section in XPS studies) since it increases into a substantial peak when UV radiation is used (when the 0 2p has a substantial photoelec­tric cross section )-see Fig. 5 in Ref. 16. Cuprous oxide provides a good case for the examination of a larger cluster since while the CuO~ cluster provides a good model for the two oxygen atoms that surround the metal. atom in a linear manner, it does not account for the tetrahedron of metal atoms that surround each oxygen atom. To consider the

Maximum.' Value: Outer 4 Copper 2 Oxygen 1 Cluster: CuO; Cu, O~

Symmetry: D , 1~,

Cu-O hondlengths: 1.850 A 1.850 A Copper Sphere Radiuy 1.210 A 1. 21 0 A Oxygen Sphere Radiu" 1.030 A 1.030 A Watson Sphere Radiu" 1.850 A 4.730 A Outer Sphere Radius: 2.880 A 4.730 A Virial Ratio ( -2T/V): 1.000400 1.000295 Con\'ergen~e: When the difference in potentials at the beginning and end of the iteration were less than 10 'of

the potential at the slart of the iteration. This gives energy levels that differed by less than 10 " Ry between the last two iterations.

Core electrons: 'Thawed" so that they retained atomic character while being fully induded in the iterative process. Cu Is. 2s. 2p electrons and 0 Is electrons were treated as core electrons.

J. Vac. Sci. Technol- A, Vol- 9, No_ 3, May/Jun 1991

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1495 P. M. A. Sherwood: Oxidation and corrosion studies

rn ..., .... ~ ;::l

,0 ... ro

;>, ..., .... rn ~ Q.) ..., ~ ......

I I

~ I

8 5 2

Cuprous Oxide

Expt.

(a)

Trans. State Calc.

(b)

Trans. State Calc.

(c)

Binding Energy (e V)

FIG. I. (a) XPS valence band region forCu, 0 from Ref. 16. (b) Calculated

XPS valence band region for Cu,O using transition state Xa calculations

for a CU40~ cluster. (c) Calculated XPS valence band region for Cu,O

using transition state Xa calculations for a CuO; cluster.

symmetry about both the metal and the oxygen the larger cluster CU4 O~ was considered. This cluster contains a cen­tral oxygen atom tetrahedrally surrounded by four copper atoms with each copper atom being connected to an oxygen atom. The cluster has linear O-Cu-O units and a tetrahedral OCu4 unit. The parameters chosen for this unit are shown in Table I and the calculated spectrum is shown in Fig. I (b). It can be seen that the spectrum is of comparable width to that found in the experiment, and the calculation also predicts a weaker peak at higher binding energy which corresponds largely to oxygen 2p intensity (69% 0 2p on the central oxygen atom). Both Cuoi and CU4 O~· calculations were full transition state calculations which means that half an electron is removed from each energy level to account for ionization in a series of different calculations, one for each energy level.

Obviously the choice of larger clusters greatly increases the amount of computer time required, and a compromise has to be made. However larger clusters should be consid­ered if there is a substantially different environment about different component atoms in the solid. We find that ZnO requires a Zn4 0 4 cluster I I rather than the simpler ZnO~ cluster in order to identify the weaker spectral features. The choice of high symmetry clusters greatly speeds the calcula­tion; even simple clusters with no symmetry can lead to ex­tended calculation times and other complications.

J. Vac. Sci. Technol. A, Vol. 9, No.3, May/Jun 1991

1495

C. X-ray diffraction studies

X-ray diffraction (XRD) provides a useful method for verification of crystal structure for model compounds used in conjunction with a VXPS study. While one can never be sure that the surface represents the bulk, the combination of a model compound characterized by XRD and a VXPS study of the model compound interpreted by MSXA calcu­lations can lead to confident identification of VXPS of known compounds. For example in our laboratory we have distinguished between different aluminum oxides/oxyhy­droxides by powder XRD and have been able to show that the VXPS is significantly different for the different oxides. 12

We can also predict the VXPS data of these aluminum com­pounds by MSXA calculations.

D. The effect of contaminants and multicomponent spectra

Initially one is concerned that the spectra of contamin­ants, especially hydrocarbons, will substantially alter the va­lence band spectra. We find that hydrocarbons have little effect on the VXPS of many oxides and oxide/hydroxides since the carbon 2p photoelectron cross section is usually small compared to that of the metal. We have used variable takeoff angle VXPS to enhance the spectrum due to surface species and again generally find contamination to be oflittle importance. Since VXPS measures photoelectrons of the highest kinetic energy for a given photon source, then the spectrum is less sensitive to surface impurities (escape depth increases with photoelectron kinetic energy).

In the case of multicomponent species in a corrosion sys­tem there is the need to extract overlapping valence band spectra. We are currently using an approach based upon syn­thesizing possible spectra from added calculated spectra of possible known components and by using factor analysis. In such cases considerable information can generally also be obtained by a combination of variable takeoff angle VXPS allowing different component features to be separated, infor­mation form core XPS, and other experiments.

E. Corrosion and oxidation examples

We have examined a number of systems using this ap­proach/ 12 and in particular we find:

(i) It is sometimes possible to identify two different oxida­tion states where core XPS leads to ambiguous results. For example SnO and Sn02 have similar Sn 3d core chemical shifts but a significantly different valence band spectrum. I" The difference in the valence band spectrum is well ex­plained by MSXA calculations. 9 It is interesting to find that the VXPS data are dominated by the small, but high photo­electric cross section, contribution of Sn 4d character.

(ii) It is possible to distinguish between oxide and oxyhy­droxide. While core XPS generally shows a clear shift in the o Is region between oxide and hydroxide it is not possible to clearly conclude that all the 0 Is intensity in the hydroxide region is due to oxide, since water can also often contribute to this region. Thus attempts to calculate stoichiometry of species in corrosion systems using this method alone must be treated with caution. Fortunately the VXPS can show differ-

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1496 P. M. A. Sherwood: Oxidation and corrosion studies

ences, for example we have been able to distinguish between oxide and oxyhydroxide of FeC III) and interpret the differ­ences by MSXA calculations. b

(iii) It is possible to identify surface complex formation associated with corrosion inhibitors. A great deal of effort has been directed at examining the role of corrosion inhibi­tors, but unfortunately the activity of these inhibitors may well depend upon subtle chemical changes in the surface film. We have been concerned with the possibility of reaction between corrosion inhibitors and the oxide corrosion layer, and have identified differences in the VXPS data which al­IO\\/s us, for example, to distinguish between phosphate ions and etidronate ions. In particular we find that an etidronate inhibitor can interact with the oxide corrosion layer to form phosphate. 411 A clear difference is seen in the VXPS even though the P 2p chemical shift for these two groups is identi­cal. Figure 2 shows the VXPS for the two calcium salts of these ions indicating the difference. These different spectra are well predicted by MSXA calculations as shown in Fig. 3.

(iv) Obtaining analytical information from the 0 2s re­gion. We lind that the 0 2s region has considerable analyti­cal potential. In oxide systems the 0 2s orbitals usually lie at a substantially different energy from the metal orbitals and are essentially nonbonding. Nevertheless they show some bonding and thus show a "chemical shift" which is not un­like the core 0 Is region, but with the bonding involvement leading to larger shifts.b When organic systems are exam­ined the 0 2s region behaves in a completely different man­ner to the 0 Is region indicating substantial shifts due to the considerable interaction between C 2s and 0 2s electrons.

(v) Calibration is not necessary. Interpretation of core XPS data is complicated by the need to calibrate the spectra.

(a)

• 0- ., .. ..., ... ~~ .. f'l"

.'.f.'

(b)

, , , .. ,.. ... ,

'1--;

15

~; . ;. .; .

': ".

10 5

, \

"

Calcium Etidronate

Expt .

Calcium Phosphate

Expt.

o -5

1'1(; 2. (a) XI'S \ aknc~ hand region for cakium ctidronat~. (h) XI'S \a­

lence hand region for cakium phosphate.

J. Vac. Sci. Technol. A, Vol. 9, No.3, May/Jun 1991

1496

Calcium Etidronate Calcium Phosphate

111 12

t'. f, ..

:'0'~. Expt. .. ' ..

CaIen.

6 15 12 9

Rinding Energy (eV)

... ., ".

Fl(;. ~. XI'S valence band regions of calcium etidronate and calcium phos­phate compared with calculated spectra generated by Xa calculations.

Differential charging can lead to substantial complications, especially in substantially corroded samples. VXPS data re­quire no calibration, since normally differences in peak posi­tions are determined and most corrosion systems have an intense 0 2s feature to assist ready alignment. Comparison with calculated spectra is always done on the basis of relative positions. Core satellite features (which often are associated with differences in the valence band region) also show differ­ences in binding energy with respect to the principal core peak, and these features can be usefully examined with the aid of MSXA and other calculations.'

IV. CONCLUSIONS

VXPS interpreted by MSXA calculations can clearly play a significant role in the analysis of oxidation and corrosion . The calculation enhances confidence in the use ofVXPS and assists in the interpretation of mixed systems. The calcula­tions require a cluster model for the solid, and the choice of this cluster size depends upon the solid geometry and the relative photoelectric cross sections for the component atoms. Ideally the cluster that reflects the geometry about both the metal and the oxygen atoms in a metal oxide should be chosen, though in appropriate cases a simple cluster re­flecting the geometry about the metal atom is sufficient.

ACKNOWLEDGMENTS

This work was supported by the National Science Foun­dation (CHE-8922538). I am grateful to Mike Ndefru, Sa­jan Thomas, Ian Welsh and Yaoming Xie for their involve­ment in the valence band studies.

I I' M. A. Sherwood. Chcm. Soc. Rev. 14. I (19BS) .

• 1. E. Castle. Surf Sci. 68. 5H3 (1977).

's. Thomas, 1'. M. A. Sherwood. N. Singh. A. AI-Sharif. and M. 1. O'Shea, Ph),. Rev. B 39. 6641J ( 19R9).

'I. D. Wchh and 1'. M. A. Sherwood. I'roc. Elcctrochcm. Soc. 89. 417 ( 1999)

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1497 P. M. A. Sherwood: Oxidation and corrosion studies

'Y. Xie and P. M. A. Sherwood, Chern. Mater. 1,427 (1989). 'I. D. Welsh and P. M. A. Sherwood, Phys. Rev. B 40,6386 (1989). 7 P. M. A. Sherwood. Auger and X·ray Photoelectron Spectroscopy, 2nd ed., edited by D. Briggs and M. P. Seah (Wiley, New York, 1990), vol. 1, Appendix 3.

'Y. Xie and P. M. A. Sherwood, Chern. Mater. 2, 293 (1990). "P. M. A. Sherwood. Phys. Rev. B41, 10151 (1990). lOy. Xie and P. M. A. Sherwood, Chern. Mater. (in press). II 1. D. Welsh and P. M. A. Sherwood (to be published). "s. Thomas and P. M. A. Sherwood (to be published).

J. Vac. Sci. Techno!. A, Vol. 9, No.3, May/Jun 1991

II D. A. Case. Ann. Rev. Phys. Chern. 33, 151 (1982). 14 J. H. Scofield. J. Electron Spectrosc. 8,129 (1976).

1497

"P. M. A. Sherwood. in Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy. edited by D. Briggs and M. P. Seah (Wiley. New York. 1983). Appendix 3.

1/, J. Ghijsen. L. H. Tjeng, 1. van Elp, H. Eskes, 1. Westerink. and G. A. Sawatzky, Phys. Rev. B 38, 1\322 (1988).

17 A. P. Kaduwela, J. D. Head, W. K. Kuhn, and G. Andermann. J. Elec­tron. Spectrosc. 49, 183 (1989).

"c. L. Lau and G. K. Wertheim, 1. Vac. Sci. Techno!. 15.622 (1978).

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Core level and valence band x-ray photoelectron spectroscopy of gold oxide Carolyn Rubin Aitaa ) and Ngoc C. Tranb

)

Materials Department, Universityo/Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201

(Received 19 September 1990; accepted 5 November 1990)

Reactive sputter deposition was used to grow a phase-separated 0.40 Au/0.56 AU2 OJ /0.04 Au­hydroxide cermet film and x-ray photoelectron spectroscopy was used to examine the Au 4fand 0 Is core electron binding energy of each phase. The valence band structure of AU2 0 3 is reported here for the first time, and consists of an 0 2p electron-derived peak centered at 5.5 eV and a density of states that approaches zero at the Fermi energy, characteristic of a semiconductor. The results are compared to other third long period conducting and semiconducting oxides.

I. INTRODUCTION

Auric oxide (Au 2 0 1 ), with a volume free-energy of forma­tion of + 39 kcallmol, I cannot be synthesized in bulk under conditions of equilibrium thermodynamics. We have, how­ever, used reactive sputter deposition to produce ceramic­metal composite (cermet) films in which one of the compo­nents is Auz 0,,2 and used x-ray photoelectron spectroscopy to determine the chemical shift in Au 4/core electron bind­ing energy in this compound. Here, a corresponding shift in the 0 Is electron binding energy is reported. Furthermore, the valence band structure of AU2 0, is reported for the first time, and discussed in relationship to the oxides of other third long period metals. ' -

o

II. EXPERIMENTAL

An Au/Au-oxide cermet film was grown on a SH Ill) substrate by sputtering a 99.99%,76 mm diam Au target in a radio-frequency (rn -excited 50% Ne-SO% O2 discharge. 2

The substrate was chemically cleaned using a chelating pro­cedure before being placed in the chamber and sputter cleaned in an Ar discharge before deposition. The sputtering chamber base pressure was < 5 X 10 7 Torr. The sputtering gases, 99.996% pure Ne and 99.99% pure O2 , were ad­mitted separately into the chamber using mass flow control­lers. The total gas pressure was 1 X 10 2 Torr. The depo­sition was carried out at - 1 kV peak-to-peak cathode voltage with a grounded anode. With a shutter covering the substrate. the target was sputtered for 10 min in Ne and then for 10 min in 50'7c Ne-50% O2 , When an Au target is sput­tered in an O-bearing discharge. Au-O molecular complexes are formed at and sputtered from the target surface. 7

<) The delivery of these complexes to the substrate is essential for Au-O bonding in the film. 2'1 The purpose of the presputters, therefore, is to allow target surface reactions that produce these species to reach dynamic equilibrium. The shutter was then opened and a 70-nm thick film was deposited at the rate of 1.2 nm/min.

A Perkin-Elmer PHI Model 5400 electron spectroscopy for chemical analysis (ESCA) system equipped with a 15 keV Mg Ka radiation source was used for core level and valence band x-ray photoelectron spectrometry. The instru­ment was calibrated using the Au 4];, 2 electron binding en­ergy of a gold foil standard at 84.0 ± 0.2 eV.

With respect to other film properties, the crystallography, resistivity, and optical reflectivity at 500 nm incident photon wavelength were reported in Ref. 2. To summarize here, the film showed a single broad x-ray diffraction (XRD) peak attributable to (Ill> planes of the Au lattice. The integrated intensity of the peak was l/lOOth of its value for an Au film of the same thickness. The electrical resistivity of the film was 7.5 ± 1.1 X 103 ,un em, - 3 orders of magnitude greater than that of a pure Au film of the same thickness. The optical reflectivity at 500 nm is 26 ± 4%, - 1/2 of its value for an Au film of the same thickness, and of the bulk material. The cermet film had a bronze color in reflected light.

III. Au 4' AND 0 15 CORE ELECTRON BINDING

ENERGY

The Au 4/ electron spectrum of the cermet, shown in Fig. 1, is more complicated than for a pure Au film, shown in the insert, and is resolved into five Gaussian peaks with binding energy and relative integrated intensity recorded in Table I. These peaks can be related to contributions from metallic Au and Au bonded to 0 in two different oxidation states using the following information: (I) the Au 4/7!2 and 4/'12 electron binding energy for metallic Au is 84.0 and 87.7 eV, respectively, (2) the energy splitting between the Au 4/7/2 and 4/Vl total angular momentum (J) states corresponding to the same Au oxidation state is 3.7 eV, and (3) the integrated intensity ratio of the 7/2:5/2 states is equal to the ratio their multiplicities (2J + I), {[2(7/2) + 1):[2(512) + I J} = 4:3.

With respect to Fig. 1, peak A is attributed to the 4/712 state in metallic Au. Peaks Band D, whose integrated inten­sities I( B) and I (D) are in the ratio of 4:3, are attributed to the 4f712 and 41;/2 states of Au bonded to 0 in a phase de­noted here "Au-oxide /." Peak £ is attributed to the 4/'12 state of Au bonded 0 in a phase denoted here "Au-oxide 2." Peak C is the sum of contributions from the 4/'12 state in metallic Au, of intensity is 3/(A)/4, and the 4/712 state in Au-oxide 2, of intensity 4/( E) 13. The fact that the measured intensity of peak C equals the calculated intensity [3/( A)/4} + [4/( £)/3) supports the correctness of its as­signment.

Based on the chemical shift observed for the anodic oxida­tion products of Au, 10 Au-oxide 1 was identified as auric

1498 J. Vac. Sci. Technol. A 9 (3), May/Jun 1991 0734-2101/91/031498-03$01.00 ,,,",1991 American Vacuum Society 1498

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1499 C. R. Aita and N. C. Tran: Core level and valence band XPS of AuO 1499

92.4 89.6 86.8 84.0 81.2

10

525

Binding Energy reV]

FIG. I. The Au 4/ and 0 Is electron binding energy spectra of a 0.40 Au/0.56 Au, 0,/0.04 Au-hydroxide cermet film. The intensity scale factor

of the Au 4/spectrum is 3X that of the 0 Is spectrum. The Au 4/electron spectrum of an Au film is shown in the insert.

oxide, Au2 0." and Au-oxide 2, as a hydrated form, possibly Au(OH) 3' In addition to the electrochemical study cited above, Au 4/ electron spectra of the form shown in Fig. 1 have been obtained from an Au surface that had been sput­tered in O2 , II and from an Au surface that had been etched in CF4 and subsequently hydrolyzed in air.12 The atom frac­tion of Au in each phase of the cermet was calculated from a ratio of the Au 4.h12 electron peak intensities, and the phase composition of the cermet was determined to be 0.40 Au/0.56 AU2 0 3 /0.04 Au-hydroxide.

The 0 Is electron binding energy peak of the cermet, shown in Fig. 1, is centered at 530.1 eV, and based on electro­chemical data 10 is attributed to 0 bonded to Au in AU2 0, .

TABLE 1. XPS Au 4/eledron binding energy inaO.40 Au/0.56Au,O,/0.04

Au-hydroxide cermet film.

Binding energy Peak (eV) ReI. intensity J, assignment

A 84.0 60 7/2 Au B 85.9 100 7/2 Au,O, e 87.7 53 5/2 Au +

7/2 Au-hydroxide D 89.6 75 5/2 Au,O, E 91.4 l1 5/2 Au-hydroxide

J. Vac. Sci. Technol. A, Vol. 9, No.3, May/Jun 1991

The small shoulder at - 532.5 eV is attributed to 0 in water and/or hydroxyl groups.IO.12 The ratio of Au/O atoms in the AU2 0 3 phase of the cermet was calculated from the ratio of the integrated intensities of the Au 4/7!2 peak in AU2 0 3

(peak B) to the 0 Is peak, adjusted to the same scale and divided by their sensitivity factors,13 and found to be 0.6, in good agreement with stoichiometry.

On the basis of XRD and x-ray photoelectron spectrosco­py (XPS) data, the proposed structure of the film is that ofa phase-separated material consisting of a mixture of Au and AU2 0" with a small amount of Au hydroxide. The O-bear­ing phases have no long range order detectable by XRD, and the Au phase is microcrystalline.

IV. Au2 0 3 VALENCE BAND STRUCTURE

Figure 2 shows the upper part of the valence band of the cermet, from the Fermi level (E F) at zero binding energy to 10eV below E F • The valence band spectrum obtained from a metallic Au film is shown for comparison in the insert in Fig. 2. The Au 5d states dominate the metallic Au spectrum from \.5-8 eV, with Au 6s states at the Fermi edge. 14 The cermet valence band spectrum is a composite of contributions from metallic Au and the O-bearing phases. The metallic Au com­ponent was separated out, assuming the same atomic density for Au in the pure metal film and in the metallic component of the cermet. The difference spectrum, shown in Fig. 3, represents the valence band of AU2 0." "contaminated" with - 7% Au hydroxide. This spectrum consists of a single peak centered at -S.5eV below En with shoulders at -2 and 8 eV, and is similar in form to the valence band of a-Pt02" ReOJ 4 and Na, WOJ (SOdium-tungsten bronzes)s valence band spectra have similar structures centered at 5.5-7 eV below EF which are attributed to 0 2p-derived states. The high energy binding shoulder is possibly due to the 30" orbital of OH ,keeping in mind that the fine structure in the cer­met-min us-gold valence band difference spectrum shown in Fig. 3 is somewhat speculative because the cross section for

10

'"""' rIl ..., '2 ~

~

~ .c 'fil ~ Q) ..., ~ .......

10 5 0

10 0= E,.

Binding Energy [eV]

FIG. 2. The valence band of a 0.40 Au/0.56 Au,OJO.04 Au-hydroxide

cermet film. The valence band of an Au film is shown in the insert.

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1500 C. R. Aita and N. C. Tran: Core level and valence band XPS of AuO 1500

8 'rIl' 7 /\ .... '2 6 ;:J ~ 5 I-.

~ 4 I '\

>, 3 1° ~ ...,

'00 0 \

0 i:: 2

\ <lJ .... i:: 1

>-<

0 10 8 6 4 2 O' EF

Binding Energy [eV]

FIl;. J. The ,aknee hand of the Au, 0, phase of the cerml'l. including 7"1r

Au hydroxide. The intensity scale factor is the same in Figs. 2 and 3.

Au Sd electrons is much larger than for 0 2p electrons. How­ever, unlike Reo." Na, Wo." and other third long period conducting oxides. Ir02 and OS02' which have filled metal d-O 2p~ states that form a split-off band centered just below Er with a significant number of states at Er." it can be seen from Fig. 3 that the number of valence band states in AU 2 0." like PtOc 1 approaches zero at E I , characteristic of a semi­conductor.

ACKNOWLEDGMENTS

The authors thank Professor M. G. Lagally, University of Wisconsin-Madison, for use of the XPS facility, We also

J. Vac. Sci. Technol. A, Vol. 9, No.3, May/Jun 1991

thank the referee of this paper for pertinent comments which have been incorporated into the text. This work was support­ed under USARO Grant No. DAAL-03-89-K-0022 and through a gift from Johnson Controls, Inc. to the Wisconsin Distinguished Professorship of CRA.

,", Author to whom correspondence should be addressed. h' Permanent address: Auger/ESCA Laboratory, Vniversity of Wisconsin­

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(Pergamon. Oxford. London. 1971), Vol. 5. pp. 362-263. 'A Hecq, M. Vandy, and M. Hecq. J. Chern. Phys. 72. 2876 (1980). 'B, L. Bentz and W. W. Harrison, Int. J. Mass Spectrom.lon Phys. 37,167

(1981 ). 'Ie. R. Aila. J. App/. Phys. 61, 5182 (1987). lilT. Dickinson, A. F. Povey. and P. M. A. Sherwood, J. Chern. Soc. Fara­

day Trans. I 71. 298 ( 1975), II J. J. Pireaux, M. Liehr. P. A. Thiry, J. P. Delrue, and R. Caudano. Surf.

Sci. 141.221 (1984). "J. H. Linn and W, E. Swartz, App/. Spectrosc. 39. 755 (1985). 1\ D. Briggs and M. P. Seah. Practical Surface Analysis by Augerand X-Ray

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