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The auger parameter, its utility and advantages: a review
Journal of Electron Spectroscopy and Related Phenomena, 47 (1988) 283-313 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE AUGER PARAMETER, REVIEW*
ITS UTILITY
AND ADVANTAGES:
283
A
C.D. WAGNER 29 Starview Drive, Oakland, CA 94618 (U.S.A) A. JOSH1 Lockheed Palo Alto Research Laboratory, 3251 Hanover Street, Palo Alto, CA 94304 (U.S.A.) (Received 9 September 1987)
ABSTRACT Photoelectron lines and X-ray excited Auger lines often exhibit differences in chemical shifts which are a function of the chemical environment of the atom. In particular, for elements which are conductors with inner shell vacancies, the doubly ionized final state results in substantial differences in screening energy, which reflects largely in the Auger line shifts. The Auger parameter, involving measurements of both Auger and photoelectron line energies, provides abundant chemical state information without the necessity or concern for charge correction and work function measurements. Since the change in Auger parameter is equal to twice the change in extraatomic relaxation energy, it is also a direct measure of the electronic interaction with the surrounding atoms. Data are presented showing systematic variations in Auger parameter values of several cations with the type and number of participating anions. The physical state of the element/ compound, whether solid, gas or in solution, is also shown to profoundly influence the Auger parameter. Finally a number of special situations such as condensed, implanted and adsorbed atoms at solid surfaces and the role of lattice structure in selected oxide systems on the Auger parameter are discussed.
INTRODUCTION
Electron spectroscopy for chemical analysis (ESCA) [ 1 ] has a special place among surface analytical techniques because of its ability to furnish nondestructively direct information of electronic states in the atom. In addition, the technique provides quantitative analysis of solid surfaces which is of significant fundamental and practical importance in understanding material behavior. The chemical state information is commonly derived from precise measurements of photoelectron energies, and to an increasing extent, from measurements of Auger electron lines. Chemical shifts in X-ray excited Auger lines are usually larger than those of the photoelectron lines. * Submitted as part of the celebration of the 70th birthday of Professor Kai Siegbahn.
036%2048/88/$03.50
0 1988 Elsevier Science Publishers B.V.
284
In some cases, natural peak widths of Auger lines are larger than photoelectron lines resulting in less accurate measurements in line shifts. However, the Auger lines resulting from Auger transitions with a final core level vacancy have at least one sharp and intense component and are measured to gain additional chemical state information. In many instances, examination of both Auger and photoelectron line shifts provides more information than either one alone. The Auger parameter, an empirical measure devised in 1975, is being found increasingly useful as evidenced by publication of over 100 articles involving its theoretical and experimental considerations and applications. This paper reviews the concept of the Auger parameter, its utility and discusses selected applications. CONCEPT OF THE AUGER PARAMETER
The concept of the Auger parameter has its origin in a survey by Wagner evaluating the Auger and photoelectron line positions in various compounds [ 21. The Auger and photoelectron data were compared for a number of sodium salts; it was seen that in the salts of NaF, NaCl, NaBr and NaI the difference between the kinetic energy of KLL Auger electrons and the photoelectrons varied from 574.0 to 576.9 eV, a 2.9 eV range. This difference is independent of reference energies and charging effects. It was also seen that the polarizability of the sodium for halides increases in the order F < Cl < Br < I. The study was followed by a comparison of the spectra of eight metals and their oxides [ 31. The data revealed large differences between the chemical shifts in Auger lines and photoelectron lines (see Table 1) . The ratio of Auger shift to photoelectron shift appears to vary in the 2-10 range, with many values at about 3. It is apparent that the Auger shifts considerably exceed the photoTABLE 1 Photoelectron and Auger energy shifts” in selected elements [ 31
Element
Photoelectronb BE shift (eV)
Auger electron KE shift (eV)
Mg Zn Ga Ge As Cd In Sn
- 1.2 -0.5 -2.0 -3.2 -3.0 -0.4 -0.8 -1.4
6.2 4.2 6.2 6.7 6.4 5.5 2.6 3.9
“Shift corresponds to energy in the elemental form less that in the oxide form. bThe average is taken of two values: 2p,,, and 3dslz, or 3d,,, and 4d.
285
electron shifts if two conditions are fulfilled at the same time: (1) the element is a conductor, and ( 2) the initial vacancy is in the inner shell. The polarization energy has been shown [ 41 to depend quadratically on the total hole charge. Since the photoelectron emission results in a singly ionized state and Auger emission in a doubly ionized final state, it then follows that differences in polarization energy are a source of systematic differences between Auger and photoelectron chemical shifts. An alternative scheme has been advanced by Shirley [ 51, employing “extraatomic relaxation”. This scheme is primarily a theoretical approach, using Hartree-Fock orbital energies, and a relaxation energy correction. A second article [ 61 relates relaxation effects on Auger energies, plus two-electron integrals from Hartree-Fock calculations. The correction amounts to approximately 2-10 eV and this shift can be attributed to extra-atomic relaxation resulting from the polarization of neighboring molecules in the solid. Shirley has also described the role of dynamic and static relaxation of outer orbitals. The experimental work with copper, zinc, sodium, and lithium metals has been detailed by Shirley and co-workers [ 7-101. In the most recent article, the derivation of the theoretical background is presented, and the compounds, salts, and oxides of sodium and zinc, examined for the first time. The difference in the values for the Auger line and photoelectron line for the different compounds has been noted. The Auger parameter concept was presented at the Faraday Society meeting in 1975 [ 111. Since the chemical shifts of photoelectron and Auger electrons are different, the difference between their kinetic energies constitutes a special spectral property, and its numerical value is unique for each chemical state. In its simplest terms, the Auger parameter is the kinetic energy of an Auger electron minus the kinetic energy of a photoelectron. In the example shown in Fig. 1 for sodium hydrogen phosphate, the Auger parameter, a, is the difference in energies of Nals and Na KLL lines. The Auger parameter concept was based on the following ideas: (1) There is a fixed difference between two line energies (Auger and photoelectron) of the same element in the same sample. (2 ) Charge corrections to the individual peak measurements are unnecessary because they simply cancel during the estimation of the Auger parameter. ( 3 ) Work function corrections are also unnecessary, and vacuum level data can be compared to Fermi level data directly. The larger Auger chemical shift leads to a value for the Auger parameter that is a major fraction of the Auger electron kinetic energy, E( Auger), and much more accurately measurable. For the state change going from the isolated atom to the elemental conductive state, the shift in kinetic energy of the photoelectron is dE(PE,,)
= -de,
+RF
(K+ )
286
(AUGER
PARAMETER
c z z
1200
1100
1000
900
800
700
BINDiNG I 0
I 100
I 300
I 200
I 400
I 500 KINETIC
600
500
ENERGY I 600
400
300
200
100
0
(eV) I 700
ENERGY
I 800
I 900
1 1000
I 1100
I 1200
(eV)
Fig. 1. ESCA spectrum of sodium hydrogen phosphate, showing the Auger parameter.
and the kinetic energy shift of the Auger electron is dE ( Auger,, e) = --de, +3R.y (K+ ) Where dc, is the energy of the electron shell in the ground elemental conductive state, e and Rz is the extra-atomic relaxation or polarization energy for single-hole (K+ ) atom. Subtraction of the photoelectron kinetic energy from the Auger kinetic energy results in the change in the Auger parameter A%-, =2Ry
(K+)
for a single-hole state and Aa,-, =1/2R:
(L+L+
)
for a 2-hole state. It was stated [ 12,131 that the average chemical shift in the Auger line is a factor of three times the photoelectron shift, and that the average agrees with theory. For elements such as Na, Mg, Cu, Zn, Ag, and Cd with small photoelectron shifts and much larger Auger shifts, the large Auger shifts as well as the Auger parameter are the logical spectral features for investigation. In 1977 Gaarenstroom and Winograd [ 141 evolved the idea of the modified
Auger parameter, ar=E(Auger)
(Y+ hv, using Wagner’s [ 111 definition
of the Auger parameter
-E(PE)
andsinceE(PE)=hv-En(PE) tr+hv=E(Augner)
+En(PE)
for a given set of E (Auger) and En (photoelectron) values, the Auger parameter is in exact reciprocal relationship (Fig. 1) with the photon energy (the higher the X-ray energy, the lower is the Auger parameter value and vice versa). AUGERPARAMETERANDSCREENINGENERGY The Auger parameters themselves have been both theoretically calculated and measured. A derivation of the Auger parameter was given by Thomas [ ES], who investigated the fundamental assumptions concerning the next level of higher order terms involved. Those contributions are probably small except when comparison is made between atoms in very dissimilar environments. Nominally, Jar equals twice the change in relaxation energy of the core ionization process, but deviations arise because of the higher order terms. In the early days of Auger spectroscopy, Aksela, using electron beam for primary excitation, conducted numerous studies of metals in the gas phase. In 1977 Varynen et al. [ 161 first studied magnesium, zinc, and cadmium in both the gas and solid phase. For these systems they superimposed Auger spectra and were able to determine the chemical shifts in gas and solid directly. This study led to additional investigations into the energetics of a large number of elements [ 171, series F-Ar, Cu-Kr and Pd-Ba. Electron binding energies and Auger electron energies in free atoms were calculated. Utilizing the measured molecular gas phase and solid state Auger spectra, Auger parameter changes were estimated. Determination of the screening energy can be accomplished by noting the change in the Auger parameter between the isolated atomic state (no screening energy) and the chemical state in question. Table 2 shows experimental data for Auger parameter chemical shifts between gaseous atomic and hydride states on the one hand and conductive solids on the other. For those elements for which no data on atomic states are available, the hydrides (with bonding to poorly polarizable hydrogen) offer the nearest basis for comparison. The increase in the Auger parameter in the conductive states ranges from 5.6to 10.7 eV. The screening energy for a single-hole state ranges from 2.8 to 5.4 eV, which is in accord with the relationship Aa=2AR”“. Typically monoatomic gases, gaseous hydrides and fluorides have minimum screening energy, in contrast to metals with conduction bands of phosphides and sulfides that possess high screening energies.
288 TABLE 2 Extremes of Auger parameter chemical shifts Gas
Reference
Solid state
Reference
Ad
Ne (9) Ar (g) Xe (9) Na (9)
140,411 [421 1481 144,451 145,161 1421 1491 1311 1511 152,161 1541 1561 1581 145,601
Ne (in Fe) Ar (in Fe) Xe (in Fe, C, or diamond)
[III [III [ 11,27,43] 1461 1471 1181 1501 1211 1511 [ 14,53 1 1551 1341 1591 [I41
6.5 6.7 5.6 9.7 10.6 9.1 8.7 9.6 10.7 10.3 8.1 8.7 8.8 8.4
Mg (9) SiF, (9) PH, (9)
I-M (9) cu(id Zn Cd Geb (d Ad-L” (9) Ag k) Cd (9)
Na (s) Mg (s) Si (s) GaP (s) NiS (s) Cu (8) En (s) Ge (s) As (s) Ag (s) Cd (s)
“36.0 eV subtracted from L2M&f5 to give value for L3M5M5 163.7 eV subtracted from 3s to give value for 3d,,,. bAa =2AR”” where R’” is the extra-atomic relaxation or screening energy. USE OF HIGHER ENERGY X-RAYS
A limitation of the most commonly employed Al or Mg Ka! X-rays is the small number of elements which produce measurable Auger lines. With Al Xrays, these values can be measured for only twenty four elements: (KU) Ne, Na, Mg; (LMM) Cu, Zn, Ga, Ge, As, Se; (M4NJVs) Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba and ( N60505) Au, Hg, Tl, Pb and Bi. All the others have transitions that are valence type Auger, or they are core type that are energetically not possible. Higher energy X-ray sources offer the potential to excite new core level lines. In 1976 Castle et al. [ 191 used Si Ka radiation to detect the Alls photoelectrons and KLL Auger electrons. Keski-Rahkonen and Krause [ 203 used the Ag La line at 2984.3 eV, and the Rh La line at 2696.8 eV (gas sampies) . Wagner used Au Mcu X-rays [ 211 and West and Castle used Zr La Xrays [ 221 at very nearly the same energy (2122.9 eV vs. 2042.4 eV). Then Yates and West [ 231 and Edge11et al. [ 241 used a silver anode, with a monochromatizing crystal. A titanium anode affords the Ti Ka line at 4510.9 eV and Nishikida and Ikeda [ 251 used Cr Ka X-ray energy at 5414.7 eV. All of these higher energy anodes have the disadvantages, however, of wide lines, and extraneous lines other than the characteristic energy. However, these have their utility in many special situations. The silver anode, for example, produces useful Auger transitions from the following elements: Al, Si, P, S, Cl,
289
Br, Kr, Rb, Sr, Y, Zr, Nb, MO, Ru, Rh, Hf, Ta, W, Re, OS, Ir, Pt, Au, Hg, Tl, Pb, and Bi. Higher-energy data from elements Al, Si, P, S, Cl, and Pb are included in this review, but the Auger parameter data for Se, Br, MO, Sn, Te, Cs, Ba, Ta, W, Pt, and Au are omitted, which for the most part are rather fragmentary. Recognizing the need for multiple X-ray sources in studying a variety of materials, a spectrometer manufacturer has produced a “quadranode” source [ 261 with four individually selectable X-rays. The anode source additionally offers simul~neous excitation of two X-ray source radiations, thereby enhancing the Auger intensities. ELEMENTAL AUGER PARAMETER AND CHEMICAL STATE PLOTS
Over the past fifteen years, Auger parameters for a number of elements in various physical and chemical states have been examined and compiled [ 27,281. Some of the data were not included in these compilations where the Auger parameter values appeared unreliable. The unreliability stems from a variety of factors including the instruments, experimental conditions and charge referencing techniques that were used in collecting the photoelectron and Auger electron data. The Auger parameter data are presented in one-dimensional plots as in Fig.2, and in the form of two-dimensional chemical state plots as in Fig. 3 [ 121. These latter plots provide detailed information on photoelectron and Auger energies as well as the modifed Auger parameter. On this graph, the kinetic energies of Auger electrons are on the ordinate and the binding energies of the photoelectrons, plotting on the reverse, are on the abscissa [ 291. Each chemical state then occupies a unique position on the two-dimensional grid. An Auger parameter grid is also drawn as a family of parallel lines with a slope of + 1. All points on any one of these lines have the same value of the Auger parameter. Errors in charge referencing introduce uncertainty in data points parallel to the Auger parameter grid. For this reason, labels of compost are shown as rectangles with the long dimension parallel to the grid lines. Since the function plotted is (cx+hv), rather than (a) itself, no notation of the photon energy is required and the information can be utilized with ESCA data obtained using any X-ray source [ 291.
The Auger and photoelectron line positions of sodium in various sodium containing compounds have been measured in detail and the data summarized in Fig. 2. The (Y+ hv values correspond to the KLL Auger and 1sphotoelectron lines of sodium. The value of the Auger parameter is the highest for solid sodium, due to its conduction electrons and associated screening energy. Gaseous
290 SODIUM
SALTS
AND
FROZEN
AQUEOUS
Frozen Aqueous Solutions
a+
hv. eV (KLL, 1s) 2066 -
2065 -
SOLUTIONS
Salts Na
Na3Sb
2064 -
2063 Nal X_, 2062 -
2061 -
NaBr* NaCI* -,\ -4
\ \
NaNO 3*_. Na2SOi
2060 -
‘\
NaF*-
N.\ --,//
-
Na2SiF6 2059 -
2058 -
2057 Na (g) _--_2056 -
Fig. 2. Auger parameter of sodium salts, solids [ 11,30*,46,84] and gas [ 44,451
sodium, on the other hand, can provide no screening energy and has a 9.7 eV lower Auger parameter. For the various solid compounds the cy+ hv values vary from 2059.4 eV to 2066.1 eV, a 6.7 eV range. In the NaF-NaI series, the difference increases progressively from 2060.0 eV to 2062.7 eV as the small fluoride ion is replaced by the large, polarizable iodide ion. The shift observed in the Auger parameter is thus a measure of polarizability of the environment of the affected atom. Many of these salts were converted into their frozen aqueous solutions [ 301. When they were converted, their Auger parameters clustered together with a spread of only 0.5 eV as shown in Fig. 2. The various solvated cations have similar nearest neighbors, and have similar screening energies. It is interesting to note that for the anions, the photoelectron binding energies did not vary much between the salts and solutions.
291
1620
r
1618
1616
1614
> U !z
1612
? w 2
1610
L z G -I
1608 !Jz
,” 2
1606
1604
1602
1600
L
108
106 2p3,2
5 eV
added
to
gas
phase
104 BINDING kinetic
102
100
ENERGY energies
98
96
(eV) to
simulate
Fermi-level
Fig. 3. Chemical state plot of silicon compounds, solid [ 38,50,63,641 and gases
referencing
[481.
Magnesium Magnesium, like sodium, exhibits a small shift in its photoelectron line and a large one in its Auger line, almost 8 eV between metal and MgF, (Fig. 4).
292 MAGNESIUM
01 + hu, eV (KLL, 1s)
2489
States
Mg2Cu / RMg3Bi2
Mg
2488
2487 MgBr2*6H20 2486 MgC12
- 6H20
MgO
2485 / 2484
Olivine
-
Phlogopite k-
2483
2482
2481
2480
2478
-_---
_---
Mg(g)
Fig. 4. Auger parameter of magnesium compounds, solids [ 61,621 and gas [ 16,451.
The change in Auger parameter of metal-MgFz is the largest among the compounds presented. MgC12 and MgO have similar Auger parameter values. The intermetallic compounds Mg,Cu and Mg,B& have Auger parameters similar in value to metallic magnesium, indicating similar high screening energy.
Silicon The chemical state plot (Fig. 3 ) shows the 2p,,, KLL and Auger parameter values for silicon and its selected compounds. Several of its gas phase com-
293
pounds (four of those cited, out of a total of 24) are also included on the plot for comparison. It is noteworthy that the silicides (conductors) PdSi and V$i have larger Auger parameters while SiO, and S&N4 (insulators) have smaller Auger parameters when compared to silicon (semiconductor). The gases examined, which include some large molecules, have lower Auger parameters, indicating lower screening energy than any of the solids. Phosphorus Figure 5 shows the chemical state information for phosphorus and its compounds. As with silicon, all the gas phase compounds [ 181 have lower Auger parameters than the solids. Also, GaP (a semiconductor) has a slightly higher Auger parameter than red phosphorus. It has a total range, from GaP to PHB, of almost 9 eV. Sulfur The chemical state plot for sulfur, shown in Fig. 6, includes measurements from elemental sulfur, metal sulfides, sulfur-oxygen compounds and a considerable number [ 281 of gas phase compounds. Once again, all gas phase compounds have lower Auger parameters than any of the solids, and H2S has the lowest Auger parameter of them all. These data, indicating that atoms in insulating solids have Auger parameter values at least equal to or greater than those of atoms in the most polarizable gas phase compound, suggest that the screening energy in an insulating solid is derived from electrons associated with atoms more distant than nearest neighbors. Of particular interest are compounds SFG, SO, and CSz that were examined in the gas phase [ 31,321 as well as in the condensed phase [ 181. Note the increase in Auger parameter from the gas phase to solid phase of 2.0 eV for SFG, 3.3 eV for SO, and 3.8 eV for CS,. SFG, acquiring the least screening energy upon condensation, has a sulfur atom surrounded by six fluorine atoms, limiting the proximity of foreign molecular orbitals. CS, has peripheral sulfur atoms, so condensed neighboring molecules can exert maximal effect. S02, having two oxygen atoms surrounding the sulfur atom is an intermediate case. It becomes clear that near-neighbor atoms in a solid that are not involved in the molecular orbital system of the ionized atom can still provide substantial screening energy, amounting to l-2 eV for photoelectron transitions, and four times as much (4-8 eV) for the doubly charged final ion in the Auger transition. Chlorine The chemical state plot in Fig. 7 shows the Auger parameter data for various chlorine compounds in gas phase. The parameter increases with the substitution of chlorine in each of the series CCIFs--CC& and CH&l--CC14. Substitution of higher alkanes in the series CH&l--t-C&H&l also results in an
294 PHOSPHORUS
1858
1856
1854 2 .% 5
1852
: U F
1850
g z -I 2
1848 z m 4 1846
1844
1842
1840
140
138 2p3,2
5 eV
added
to
gas
phase
kinetic
136 BINDING
energies
134 ENERGY
to
132
130
(eV)
simulate
Fermi-level
referencing
Fig. 5. Chemical state plot of phosphorus compounds, solids [ 50,561 and gases [ 49,561.
z g
2: 0 0 s
N
N 0 a=
KL23L23 N z
8 0
N
MODIFIED
ENERGY
N
KINETIC
N
PARAMETER
N a=
N
AUGER
(eV)
(eV)
\
\
\
\
\
\
ki Q, t
N
296 CHLORINE
2378
2377
2376
2 -
2375
-I
2
2372
2371
2370
2369
2P 3,2 -.-e---
-
‘*********.* All
the
ENERGY
Compounds
CCIF3--CC12F2--CC13F--Ccl4
Compounds
CC14--CHC13--CH2C12--CH3CI
Compounds data
BINDING
are
from
(eV)
CH3CI-C2H5CI--n-C3H7CI--i-C3H7CI--t-C4HgCI gas
phase
and
are
referenced
to
Fig. 7. Chemical state plot of chlorine gas phase compounds [ 681.
the
vacuum
level
297
increase in Auger parameter. Bonding with the poorly polarizable hydrogen atom and formation of HCl results in the lowest value for the Auger parameter. Copper Formation of cuprous compounds with Se, S and 0 results in progressively larger deviation of the Auger parameter from the metallic state, as shown in Fig. 8. The 2psj2 peak of copper shows no significant chemical shift in the
$M&
2P,,,)
States
cu 1851
1850
lcu2s
Cu2Se, CulnSe2---
-cu20
1849
1848 CUCI
CuCN 1847
1846
1845 1 1844
1843
1842
1841 ------cu(g) 1840
Cuprous
t
states
only
are
cited;
cupric
had
multiplet
Fig. 8. Auger parameter of copper compounds, solids [ 59,69-721
splitting
and gas [ 511.
298
cuprous state and the entire Auger parameter shift is primarily due to the Auger line shift. Zinc Figure 9 shows the Auger parameter shifts for the halide series (group VII) and 0, S, Se and Te (group VI) elements. With fluorine being the least polarizable of these elements, the fluoride exhibits the lowest and the telluride the highest Auger parameter. Zinc compounds show almost no shift in the photoZINC
CY+ hv.
eV
States (qy4j.
ZP3,2)
2014-
2013
Zn
-
ZnTe
2012 -
ZnSe ZnS
Zn12 2011 ZnBr2 2010 -
ZnO ZnC12
ZnAl 20,+ ZnS04
2009 ZnF2 2008
-
2007
-
2006
-
2005
-
2004
-------s-m
Zn(g)
2003-
Fig.
9.
Auger parameter of zinc compounds, solids [ 14,53,73,74] and gas [ 16,521.
299
electron line, but shifts of several electron volts are observed in the Auger parameter values for these compounds. Gallium
Compounds of gallium with group V and group VI elements are shown in Fig. 10. Again a systematic trend is observed for each group, with the least polarizable compounds giving the lowest Auger parameter. Germanium
Figure 11 shows the chemical state plot for Ge and its compounds. Gas phase compounds GeF, and GeH, are observed to have the lowest Auger parameter values. The value for GeBr, is the highest of the gas phase compounds explored, and is equal to the value for solid germanium dioxide. The series GeF,--GeBr,, GeH,--GeBr, and GeH,--Ge (Me) 4 show the expected trends. Arsenic
The gas phase compound ASH, has the lowest Auger parameter value, and the As (Me) 3, and especially arsabenzene, have the highest values. Figure 12 shows a representative selection of compounds from Bahl et al. [ 341 in solid compounds. GALLIUM
ff+ hv, eV (L3M5M5,
States -
3d)
1087Ca
Ga
1086 CaAs _ 1085 -
Ca2Se3
1084 -
1083 :
CaP
CaN
Ga203
1082-
Fig. 10. Auger parameter of gallium solid compounds [ 75-781.
38
36 3d 5,2
5 eV
added
to
gas
phase
kinetic
34 BINDING
energies
32
30
ENERGY
to
28
(eV)
simulate
Fermi-level
referencing
Fig. 11. Chemical state plot of germanium compounds, solids [ 55,79,80] and gases [ 541.
,” 0
r: N
2 c
L3”45M45 N 2 !I! m
KINETIC Li 0 i
(eV)
MODIFIED
ENERGY
AUGER
;j c” : co
PARAMETER
Lz m
(eV)
z 0
302
Silver, cadmium, indium and antimony
Photoelectron lines of silver and cadmium show insignificant shifts, but the Auger lines show up to 5 eV shifts. Figures 13-15 indicate systematic Auger parameter shifts for silver, cadmium and indium in compounds formed with group VI and VII elements. Figure 16 for antimony shows similar behavior in compounds with elements in groups I and VI. The dilute alloys of silver with aluminum and magnesium have slightly higher values than elemental silver. SILVER
ff + hv. (M,+N5N5s 727
726
eV States
3d,,,)
Mg97Ag3 A’95Ag5 Ag
Ag
Ag2= Ag2Se
725
Ag20* 724
723 -
*Agl
AgO” AgOOCCF3
*AgF “AgF2 Ag2S04
722 -
721 -
720 -
719 -
718 -
__--__
_ ---
Ag(g)
717 *6.0
eV added
to value
of M5N5N5
to give
value
for
M4M5M5
Fig. 13. Auger parameter of silver compounds, solids [ 11,14*,59,82,83]
and gas [ 581.
303 CADMIUM (Y+ hv,
eV States
(M4N5N5’ 789 r
3d5/23 *Cd
788 CdTe* 787 CdSe* CdS Cd0
Cdl 2 786 _ CdBr2.4H20
CdTe03 CdS04
785 - 2CdC12. 5H20 Cd(OH12
CdF2 784 -
783 -
782 -
781 --------__
Cd(g)
780 “6.7
eV added
to value
of M5N5N5
to give
value
for
M4N5N5
Fig. 14. Auger parameter of cadmium compounds, solids [ 14*,63,85-871 and gas [ 45,603.
Iodine Auger parameter values of iodine and its compounds with various metals are shown in Fig. 17. Among the salts, LiI exhibited the lowest parameter. The I2 gas has a higher Auger parameter value than monoatomic iodine, as expected. Lead This was the only representative of the series (N,O,O,, 4f,,2). Compounds of lead with elements of groups VI and VII and selected complex lead oxides and hydroxides are shown in Fig. 18. PbFz is found to have the lowest Auger parameter value. PbOp has a slightly lower value than PbO.
304 INDIUM
0 + hv,
(M4N5”‘5,
eV
States
3d5,2) In
854
853
E
ln2Te3
-
InSb In2Se3
CulnSe2
852
InP ‘n2S3
lnl3 851
lnt3r3 IdI3
E 850
ln2’3 In(0l-l)
InF3
3
849
848 i
Fig. 15. Auger parameter of indium solid compounds [ 11,12,27,72,88-901. ANTIMONY Q + hv,
(M4N5N5,
992
eV States
3d5,2) Sb Sb2S3
991
Cs3Sb
Sb2S5
K 3Sb Na3Sb
990
Sb203 989
988
KSbFs 987
_
Fig. 16. Auger parameter of antimony solid compounds [ 11,84,90].
305 IODINE
a + hv, (M4N5N5,
eV 3d5,2)
States
1139
1138
*Agl *CUl Li
1133
--____---
I,(g)
-______----
l(g)
k
1132 I
*l 1.5 eV
value
L
I
for
added to the M4N5N 5
value
of M5N5N 5 to provide
Fig. 17. Auger parameter of iodine compounds, solids [ 14*,27,92] and gases [93,94].
LEAD CY + hv,
eV
(M4N5N5n3d5,2) 233
232
States
Pb PbTe PbSe PbS Pb12 PbBr2
231
Pb02 PbO
230
229
PbCrO4
PbC12 -
PbF2
Fig. 18. Auger parameter of lead solid compounds [ 95,961.
PbTi03 Pb(N0 I2 Pb(OH J3 2 PbSi03
306 SELECTED SPECIAL CASES
There are a number of special cases where the Auger parameters have been most helpful in understanding the screening energy changes resulting from modifications in the atomic or molecular environment. These involve valence band transitions, photoelectron and Auger core level shifts and surface core level shifts, changes in crystallinity and absorption and implantation phenomena.
Ammonia and water
Data are available on two other molecules in both solid and gas phase: ammonia and water (see Table 3). Although the Auger transitions are valencetype (CVV) transitions, so that changes in bonding can affect the line distribution, it appears that valid comparisons can be made. For water, the dominant (lbllbl) peak appears in both spectra [ 33,35],23.7-24.0 eV distant from the ( 2a,lbl) peak, so hydrogen bonding does not seem to affect the energies of these peaks. The Auger parameter for solid water [ 331 is found to be 2.3 eV larger than that for gaseous water [ 35,401. For ammonia, again, the distribution (le3a1) peak is almost the same for the gas and the solid [ 36, 371, and the parameter for the solid [ 361 is 4.4 eV larger than that for the gas [ 371. It is clear that the proximity of other molecules is important in modifying the kinetic energies of emitted Auger and photoelectrons. Knowledge of these energies will be useful in determining the environment of atoms in thin films, adsorbed species, and aggregates approaching atomic dimensions. TABLE 3 Auger parameters of ammonia and water
NinNH, NinNH, Change
(g) (s)
0 in H20 (g) OinHzO (s) Change
Photoelectron BE (ls)“(eV)
Auger electron KE (KLL)“(eV)
405.55 (v) 398.8
365.46(v) 376.6
539.9(v) 532.9
498.2(v) 507.9
a(v) Energy references to vacuum level.
a+hv 771.01 775.4 4.39 1038.1 1040.8 2.7
Reference 1371 [361 135,401 [331
307
Phenylsilicone and methylsilicone resins Figure 19 shows the chemical state plot for Si-0 compounds. Included in this category are the silicon polymers, silicon dioxide, aluminosilicates and silicates [ 381. The Auger parameters for dimethylsilicone polymer and the methysilicone resin ( MeSiO,,,) were at 1711.8 eV, while the phenylsilicone resin ( PhSiO,,,) was 1.0 eV higher due to its greater polarizability. The observation of the oxygen Auger parameter at the same time confirmed that the oxygen was perturbed also, but to a lesser degree. The silicon is bonded directly to the phenyl, while the oxygen is bonded to silicon and only indirectly to the phenyl group. Octahedral alumina versus tetrahedral alumina The octahedral aluminum oxides, hydroxides, and aluminosilicates are in the region of aluminum Auger parameters of 1461.4-1462.0 eV (Fig. 20). On the other hand, albite and natrolite, and the synthetic zeolites possess tetrahedral crystal form and they occupy a region 1460.3-1460.8 eV, about 1.2 eV lower in parameter. In a parallel study, West and Castle [ 221 have observed different Auger parameter values for tetrahedral aluminum. Using 1s and KLL lines, they related the parameter values to variable polarizabilities of the oxygen atom in the aluminosilicates, as measured by the refractive indices. Auger parameter shifts of xenon in adsorbed and implanted forms It would be expected that there should be effects of the nature of the substrate on line energies from thin films or from very small aggregates. Data are available on xenon, adsorbed on and implanted in host matrices, as shown in Fig. 21. The Auger parameter is the largest for xenon implanted in conductors, such as iron and graphite. Slightly less screening energy is available to adsorbed films of conductors, platinum, and molybdenum. When the support is oxidized, conduction electrons are not available, and screening energy is smaller still, but is still appreciable, as shown by the much smaller value for gaseous xenon. Distance-dependent relaxation shifts of photoemission and Auger energies for Xe on Pd(001) An elegant experiment of rare gas adsorption on metal substrates was performed by Kaindl et al. [ 391. They adsorbed one monolayer of xenon on a crystal face of palladium, and recorded the xenon 4cE,,,line and the NO0 group of lines. Figure 22 summarizes the results. The shift from gaseous xenon of each of these lines for the first adsorbed monolayer is 2.14 and 6.57 eV. The
308 COMPARISON
OF SILICONE AND
WITH
SILICATES,
SILICON
ALUMINOSILICATES,
DIOXIDE
.VI
1606
Iii
ALUMINO-SILICATES SILICONES
103 Si2p
BINDING
102 ENERGY
(eV1
ms
Dimethylsilicone
ox
Silicon
mr
Methylsilicone
Resin
cr
o-Cristobalite
pr
Phenylsilicone
Resin
t
Talc
9
o-Quartz
k
Kaolinite
”
Vycor
P
Pyrophyllite
9
Silica
sp
Spodumene
w
Wollastonite
Gel
Polymer
Dioxide
Fig. 19. Chemical state plot of selected silicon-oxygen compounds [ 381,
309 COMPARISON
OF
OCTAHEDRAL
TES, OXIDES AND OCTRAHEDRAL
AND
TETRAHEDRAL
ALUMINOSILICATES
TETRAHEDRAL
HYDROXIDES,
Al2p
BINDING
sa
Sapphire
sp
Spodumene
CY
o-Alumiha
p
Pyrophylli
Y
Y-Alumina
k
ENERGY
A
(eV)
Molecular
Sieve
X
Molecular
Sieve
Type
X
Kaolinite
Y
Molecular
Sieve
Type
Y
te
bo
Boehmite
a
Albite
2
H
ba
Bayerite
n
Natrolite
HS
Hydroxysodalite
gi
Cibbsite
Type
A
Zeolon
Fig. 20. Chemical state plot of selected aluminum-oxygen compounds [ 381.
ratio is almost exactly three (3.07)) as expected from simple theory. When a second layer was added, the shift in the first layer increased slightly because of the extra screening energy from the first layer, but the intensity of the lines diminished. The shifts registered by the second layer were smaller, 1.49 and
310 AUGER
PARAMETER
ADSORBED
SHIFTS
AND
OF XENON
IMPLANTED
IN
FORMS
(Y + h v, eV
(M4N5N5, 3d,,,) 1215
1214
Xe Xe Xe Xe Xe
States
imp imp imp ads ads
in in in on on
Fe C C* MO Pt
1213 Xe ads on Moo2
Xc(g)
*C
in diamond
---
-
-
form
Fig. 21 Auger parameter shifts of xenon in adsorbed and implanted forms, solids [ 11,27,43,97,98] andgas [99]. @Es)
(AE)
4d -----2 I
Auger
-
Au (evl
L I I I
3.2 1
1
----I
L
I
2 LAYERS
NO0
2
I
4.5
I
1 I
2 i
I I 1 LAYER
Xe
1
I
I
(001)
SHIFT
IN
I
I 4
2
imsg Pd
q.4i
I I
KINETIC
ENERGY
I 6
(eV)
Fig. 22. Distance-dependent relaxation shifts of photoelectron Auger energies for Xe on Pd (001)
[391.
311
4.69 eV respectively, as expected because of the greater distance from the metal, but the ratio was still a factor of three (3.15).
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THE AUGER PARAMETER, REVIEW*
ITS UTILITY
AND ADVANTAGES:
283
A
C.D. WAGNER 29 Starview Drive, Oakland, CA 94618 (U.S.A) A. JOSH1 Lockheed Palo Alto Research Laboratory, 3251 Hanover Street, Palo Alto, CA 94304 (U.S.A.) (Received 9 September 1987)
ABSTRACT Photoelectron lines and X-ray excited Auger lines often exhibit differences in chemical shifts which are a function of the chemical environment of the atom. In particular, for elements which are conductors with inner shell vacancies, the doubly ionized final state results in substantial differences in screening energy, which reflects largely in the Auger line shifts. The Auger parameter, involving measurements of both Auger and photoelectron line energies, provides abundant chemical state information without the necessity or concern for charge correction and work function measurements. Since the change in Auger parameter is equal to twice the change in extraatomic relaxation energy, it is also a direct measure of the electronic interaction with the surrounding atoms. Data are presented showing systematic variations in Auger parameter values of several cations with the type and number of participating anions. The physical state of the element/ compound, whether solid, gas or in solution, is also shown to profoundly influence the Auger parameter. Finally a number of special situations such as condensed, implanted and adsorbed atoms at solid surfaces and the role of lattice structure in selected oxide systems on the Auger parameter are discussed.
INTRODUCTION
Electron spectroscopy for chemical analysis (ESCA) [ 1 ] has a special place among surface analytical techniques because of its ability to furnish nondestructively direct information of electronic states in the atom. In addition, the technique provides quantitative analysis of solid surfaces which is of significant fundamental and practical importance in understanding material behavior. The chemical state information is commonly derived from precise measurements of photoelectron energies, and to an increasing extent, from measurements of Auger electron lines. Chemical shifts in X-ray excited Auger lines are usually larger than those of the photoelectron lines. * Submitted as part of the celebration of the 70th birthday of Professor Kai Siegbahn.
036%2048/88/$03.50
0 1988 Elsevier Science Publishers B.V.
284
In some cases, natural peak widths of Auger lines are larger than photoelectron lines resulting in less accurate measurements in line shifts. However, the Auger lines resulting from Auger transitions with a final core level vacancy have at least one sharp and intense component and are measured to gain additional chemical state information. In many instances, examination of both Auger and photoelectron line shifts provides more information than either one alone. The Auger parameter, an empirical measure devised in 1975, is being found increasingly useful as evidenced by publication of over 100 articles involving its theoretical and experimental considerations and applications. This paper reviews the concept of the Auger parameter, its utility and discusses selected applications. CONCEPT OF THE AUGER PARAMETER
The concept of the Auger parameter has its origin in a survey by Wagner evaluating the Auger and photoelectron line positions in various compounds [ 21. The Auger and photoelectron data were compared for a number of sodium salts; it was seen that in the salts of NaF, NaCl, NaBr and NaI the difference between the kinetic energy of KLL Auger electrons and the photoelectrons varied from 574.0 to 576.9 eV, a 2.9 eV range. This difference is independent of reference energies and charging effects. It was also seen that the polarizability of the sodium for halides increases in the order F < Cl < Br < I. The study was followed by a comparison of the spectra of eight metals and their oxides [ 31. The data revealed large differences between the chemical shifts in Auger lines and photoelectron lines (see Table 1) . The ratio of Auger shift to photoelectron shift appears to vary in the 2-10 range, with many values at about 3. It is apparent that the Auger shifts considerably exceed the photoTABLE 1 Photoelectron and Auger energy shifts” in selected elements [ 31
Element
Photoelectronb BE shift (eV)
Auger electron KE shift (eV)
Mg Zn Ga Ge As Cd In Sn
- 1.2 -0.5 -2.0 -3.2 -3.0 -0.4 -0.8 -1.4
6.2 4.2 6.2 6.7 6.4 5.5 2.6 3.9
“Shift corresponds to energy in the elemental form less that in the oxide form. bThe average is taken of two values: 2p,,, and 3dslz, or 3d,,, and 4d.
285
electron shifts if two conditions are fulfilled at the same time: (1) the element is a conductor, and ( 2) the initial vacancy is in the inner shell. The polarization energy has been shown [ 41 to depend quadratically on the total hole charge. Since the photoelectron emission results in a singly ionized state and Auger emission in a doubly ionized final state, it then follows that differences in polarization energy are a source of systematic differences between Auger and photoelectron chemical shifts. An alternative scheme has been advanced by Shirley [ 51, employing “extraatomic relaxation”. This scheme is primarily a theoretical approach, using Hartree-Fock orbital energies, and a relaxation energy correction. A second article [ 61 relates relaxation effects on Auger energies, plus two-electron integrals from Hartree-Fock calculations. The correction amounts to approximately 2-10 eV and this shift can be attributed to extra-atomic relaxation resulting from the polarization of neighboring molecules in the solid. Shirley has also described the role of dynamic and static relaxation of outer orbitals. The experimental work with copper, zinc, sodium, and lithium metals has been detailed by Shirley and co-workers [ 7-101. In the most recent article, the derivation of the theoretical background is presented, and the compounds, salts, and oxides of sodium and zinc, examined for the first time. The difference in the values for the Auger line and photoelectron line for the different compounds has been noted. The Auger parameter concept was presented at the Faraday Society meeting in 1975 [ 111. Since the chemical shifts of photoelectron and Auger electrons are different, the difference between their kinetic energies constitutes a special spectral property, and its numerical value is unique for each chemical state. In its simplest terms, the Auger parameter is the kinetic energy of an Auger electron minus the kinetic energy of a photoelectron. In the example shown in Fig. 1 for sodium hydrogen phosphate, the Auger parameter, a, is the difference in energies of Nals and Na KLL lines. The Auger parameter concept was based on the following ideas: (1) There is a fixed difference between two line energies (Auger and photoelectron) of the same element in the same sample. (2 ) Charge corrections to the individual peak measurements are unnecessary because they simply cancel during the estimation of the Auger parameter. ( 3 ) Work function corrections are also unnecessary, and vacuum level data can be compared to Fermi level data directly. The larger Auger chemical shift leads to a value for the Auger parameter that is a major fraction of the Auger electron kinetic energy, E( Auger), and much more accurately measurable. For the state change going from the isolated atom to the elemental conductive state, the shift in kinetic energy of the photoelectron is dE(PE,,)
= -de,
+RF
(K+ )
286
(AUGER
PARAMETER
c z z
1200
1100
1000
900
800
700
BINDiNG I 0
I 100
I 300
I 200
I 400
I 500 KINETIC
600
500
ENERGY I 600
400
300
200
100
0
(eV) I 700
ENERGY
I 800
I 900
1 1000
I 1100
I 1200
(eV)
Fig. 1. ESCA spectrum of sodium hydrogen phosphate, showing the Auger parameter.
and the kinetic energy shift of the Auger electron is dE ( Auger,, e) = --de, +3R.y (K+ ) Where dc, is the energy of the electron shell in the ground elemental conductive state, e and Rz is the extra-atomic relaxation or polarization energy for single-hole (K+ ) atom. Subtraction of the photoelectron kinetic energy from the Auger kinetic energy results in the change in the Auger parameter A%-, =2Ry
(K+)
for a single-hole state and Aa,-, =1/2R:
(L+L+
)
for a 2-hole state. It was stated [ 12,131 that the average chemical shift in the Auger line is a factor of three times the photoelectron shift, and that the average agrees with theory. For elements such as Na, Mg, Cu, Zn, Ag, and Cd with small photoelectron shifts and much larger Auger shifts, the large Auger shifts as well as the Auger parameter are the logical spectral features for investigation. In 1977 Gaarenstroom and Winograd [ 141 evolved the idea of the modified
Auger parameter, ar=E(Auger)
(Y+ hv, using Wagner’s [ 111 definition
of the Auger parameter
-E(PE)
andsinceE(PE)=hv-En(PE) tr+hv=E(Augner)
+En(PE)
for a given set of E (Auger) and En (photoelectron) values, the Auger parameter is in exact reciprocal relationship (Fig. 1) with the photon energy (the higher the X-ray energy, the lower is the Auger parameter value and vice versa). AUGERPARAMETERANDSCREENINGENERGY The Auger parameters themselves have been both theoretically calculated and measured. A derivation of the Auger parameter was given by Thomas [ ES], who investigated the fundamental assumptions concerning the next level of higher order terms involved. Those contributions are probably small except when comparison is made between atoms in very dissimilar environments. Nominally, Jar equals twice the change in relaxation energy of the core ionization process, but deviations arise because of the higher order terms. In the early days of Auger spectroscopy, Aksela, using electron beam for primary excitation, conducted numerous studies of metals in the gas phase. In 1977 Varynen et al. [ 161 first studied magnesium, zinc, and cadmium in both the gas and solid phase. For these systems they superimposed Auger spectra and were able to determine the chemical shifts in gas and solid directly. This study led to additional investigations into the energetics of a large number of elements [ 171, series F-Ar, Cu-Kr and Pd-Ba. Electron binding energies and Auger electron energies in free atoms were calculated. Utilizing the measured molecular gas phase and solid state Auger spectra, Auger parameter changes were estimated. Determination of the screening energy can be accomplished by noting the change in the Auger parameter between the isolated atomic state (no screening energy) and the chemical state in question. Table 2 shows experimental data for Auger parameter chemical shifts between gaseous atomic and hydride states on the one hand and conductive solids on the other. For those elements for which no data on atomic states are available, the hydrides (with bonding to poorly polarizable hydrogen) offer the nearest basis for comparison. The increase in the Auger parameter in the conductive states ranges from 5.6to 10.7 eV. The screening energy for a single-hole state ranges from 2.8 to 5.4 eV, which is in accord with the relationship Aa=2AR”“. Typically monoatomic gases, gaseous hydrides and fluorides have minimum screening energy, in contrast to metals with conduction bands of phosphides and sulfides that possess high screening energies.
288 TABLE 2 Extremes of Auger parameter chemical shifts Gas
Reference
Solid state
Reference
Ad
Ne (9) Ar (g) Xe (9) Na (9)
140,411 [421 1481 144,451 145,161 1421 1491 1311 1511 152,161 1541 1561 1581 145,601
Ne (in Fe) Ar (in Fe) Xe (in Fe, C, or diamond)
[III [III [ 11,27,43] 1461 1471 1181 1501 1211 1511 [ 14,53 1 1551 1341 1591 [I41
6.5 6.7 5.6 9.7 10.6 9.1 8.7 9.6 10.7 10.3 8.1 8.7 8.8 8.4
Mg (9) SiF, (9) PH, (9)
I-M (9) cu(id Zn Cd Geb (d Ad-L” (9) Ag k) Cd (9)
Na (s) Mg (s) Si (s) GaP (s) NiS (s) Cu (8) En (s) Ge (s) As (s) Ag (s) Cd (s)
“36.0 eV subtracted from L2M&f5 to give value for L3M5M5 163.7 eV subtracted from 3s to give value for 3d,,,. bAa =2AR”” where R’” is the extra-atomic relaxation or screening energy. USE OF HIGHER ENERGY X-RAYS
A limitation of the most commonly employed Al or Mg Ka! X-rays is the small number of elements which produce measurable Auger lines. With Al Xrays, these values can be measured for only twenty four elements: (KU) Ne, Na, Mg; (LMM) Cu, Zn, Ga, Ge, As, Se; (M4NJVs) Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba and ( N60505) Au, Hg, Tl, Pb and Bi. All the others have transitions that are valence type Auger, or they are core type that are energetically not possible. Higher energy X-ray sources offer the potential to excite new core level lines. In 1976 Castle et al. [ 191 used Si Ka radiation to detect the Alls photoelectrons and KLL Auger electrons. Keski-Rahkonen and Krause [ 203 used the Ag La line at 2984.3 eV, and the Rh La line at 2696.8 eV (gas sampies) . Wagner used Au Mcu X-rays [ 211 and West and Castle used Zr La Xrays [ 221 at very nearly the same energy (2122.9 eV vs. 2042.4 eV). Then Yates and West [ 231 and Edge11et al. [ 241 used a silver anode, with a monochromatizing crystal. A titanium anode affords the Ti Ka line at 4510.9 eV and Nishikida and Ikeda [ 251 used Cr Ka X-ray energy at 5414.7 eV. All of these higher energy anodes have the disadvantages, however, of wide lines, and extraneous lines other than the characteristic energy. However, these have their utility in many special situations. The silver anode, for example, produces useful Auger transitions from the following elements: Al, Si, P, S, Cl,
289
Br, Kr, Rb, Sr, Y, Zr, Nb, MO, Ru, Rh, Hf, Ta, W, Re, OS, Ir, Pt, Au, Hg, Tl, Pb, and Bi. Higher-energy data from elements Al, Si, P, S, Cl, and Pb are included in this review, but the Auger parameter data for Se, Br, MO, Sn, Te, Cs, Ba, Ta, W, Pt, and Au are omitted, which for the most part are rather fragmentary. Recognizing the need for multiple X-ray sources in studying a variety of materials, a spectrometer manufacturer has produced a “quadranode” source [ 261 with four individually selectable X-rays. The anode source additionally offers simul~neous excitation of two X-ray source radiations, thereby enhancing the Auger intensities. ELEMENTAL AUGER PARAMETER AND CHEMICAL STATE PLOTS
Over the past fifteen years, Auger parameters for a number of elements in various physical and chemical states have been examined and compiled [ 27,281. Some of the data were not included in these compilations where the Auger parameter values appeared unreliable. The unreliability stems from a variety of factors including the instruments, experimental conditions and charge referencing techniques that were used in collecting the photoelectron and Auger electron data. The Auger parameter data are presented in one-dimensional plots as in Fig.2, and in the form of two-dimensional chemical state plots as in Fig. 3 [ 121. These latter plots provide detailed information on photoelectron and Auger energies as well as the modifed Auger parameter. On this graph, the kinetic energies of Auger electrons are on the ordinate and the binding energies of the photoelectrons, plotting on the reverse, are on the abscissa [ 291. Each chemical state then occupies a unique position on the two-dimensional grid. An Auger parameter grid is also drawn as a family of parallel lines with a slope of + 1. All points on any one of these lines have the same value of the Auger parameter. Errors in charge referencing introduce uncertainty in data points parallel to the Auger parameter grid. For this reason, labels of compost are shown as rectangles with the long dimension parallel to the grid lines. Since the function plotted is (cx+hv), rather than (a) itself, no notation of the photon energy is required and the information can be utilized with ESCA data obtained using any X-ray source [ 291.
The Auger and photoelectron line positions of sodium in various sodium containing compounds have been measured in detail and the data summarized in Fig. 2. The (Y+ hv values correspond to the KLL Auger and 1sphotoelectron lines of sodium. The value of the Auger parameter is the highest for solid sodium, due to its conduction electrons and associated screening energy. Gaseous
290 SODIUM
SALTS
AND
FROZEN
AQUEOUS
Frozen Aqueous Solutions
a+
hv. eV (KLL, 1s) 2066 -
2065 -
SOLUTIONS
Salts Na
Na3Sb
2064 -
2063 Nal X_, 2062 -
2061 -
NaBr* NaCI* -,\ -4
\ \
NaNO 3*_. Na2SOi
2060 -
‘\
NaF*-
N.\ --,//
-
Na2SiF6 2059 -
2058 -
2057 Na (g) _--_2056 -
Fig. 2. Auger parameter of sodium salts, solids [ 11,30*,46,84] and gas [ 44,451
sodium, on the other hand, can provide no screening energy and has a 9.7 eV lower Auger parameter. For the various solid compounds the cy+ hv values vary from 2059.4 eV to 2066.1 eV, a 6.7 eV range. In the NaF-NaI series, the difference increases progressively from 2060.0 eV to 2062.7 eV as the small fluoride ion is replaced by the large, polarizable iodide ion. The shift observed in the Auger parameter is thus a measure of polarizability of the environment of the affected atom. Many of these salts were converted into their frozen aqueous solutions [ 301. When they were converted, their Auger parameters clustered together with a spread of only 0.5 eV as shown in Fig. 2. The various solvated cations have similar nearest neighbors, and have similar screening energies. It is interesting to note that for the anions, the photoelectron binding energies did not vary much between the salts and solutions.
291
1620
r
1618
1616
1614
> U !z
1612
? w 2
1610
L z G -I
1608 !Jz
,” 2
1606
1604
1602
1600
L
108
106 2p3,2
5 eV
added
to
gas
phase
104 BINDING kinetic
102
100
ENERGY energies
98
96
(eV) to
simulate
Fermi-level
Fig. 3. Chemical state plot of silicon compounds, solid [ 38,50,63,641 and gases
referencing
[481.
Magnesium Magnesium, like sodium, exhibits a small shift in its photoelectron line and a large one in its Auger line, almost 8 eV between metal and MgF, (Fig. 4).
292 MAGNESIUM
01 + hu, eV (KLL, 1s)
2489
States
Mg2Cu / RMg3Bi2
Mg
2488
2487 MgBr2*6H20 2486 MgC12
- 6H20
MgO
2485 / 2484
Olivine
-
Phlogopite k-
2483
2482
2481
2480
2478
-_---
_---
Mg(g)
Fig. 4. Auger parameter of magnesium compounds, solids [ 61,621 and gas [ 16,451.
The change in Auger parameter of metal-MgFz is the largest among the compounds presented. MgC12 and MgO have similar Auger parameter values. The intermetallic compounds Mg,Cu and Mg,B& have Auger parameters similar in value to metallic magnesium, indicating similar high screening energy.
Silicon The chemical state plot (Fig. 3 ) shows the 2p,,, KLL and Auger parameter values for silicon and its selected compounds. Several of its gas phase com-
293
pounds (four of those cited, out of a total of 24) are also included on the plot for comparison. It is noteworthy that the silicides (conductors) PdSi and V$i have larger Auger parameters while SiO, and S&N4 (insulators) have smaller Auger parameters when compared to silicon (semiconductor). The gases examined, which include some large molecules, have lower Auger parameters, indicating lower screening energy than any of the solids. Phosphorus Figure 5 shows the chemical state information for phosphorus and its compounds. As with silicon, all the gas phase compounds [ 181 have lower Auger parameters than the solids. Also, GaP (a semiconductor) has a slightly higher Auger parameter than red phosphorus. It has a total range, from GaP to PHB, of almost 9 eV. Sulfur The chemical state plot for sulfur, shown in Fig. 6, includes measurements from elemental sulfur, metal sulfides, sulfur-oxygen compounds and a considerable number [ 281 of gas phase compounds. Once again, all gas phase compounds have lower Auger parameters than any of the solids, and H2S has the lowest Auger parameter of them all. These data, indicating that atoms in insulating solids have Auger parameter values at least equal to or greater than those of atoms in the most polarizable gas phase compound, suggest that the screening energy in an insulating solid is derived from electrons associated with atoms more distant than nearest neighbors. Of particular interest are compounds SFG, SO, and CSz that were examined in the gas phase [ 31,321 as well as in the condensed phase [ 181. Note the increase in Auger parameter from the gas phase to solid phase of 2.0 eV for SFG, 3.3 eV for SO, and 3.8 eV for CS,. SFG, acquiring the least screening energy upon condensation, has a sulfur atom surrounded by six fluorine atoms, limiting the proximity of foreign molecular orbitals. CS, has peripheral sulfur atoms, so condensed neighboring molecules can exert maximal effect. S02, having two oxygen atoms surrounding the sulfur atom is an intermediate case. It becomes clear that near-neighbor atoms in a solid that are not involved in the molecular orbital system of the ionized atom can still provide substantial screening energy, amounting to l-2 eV for photoelectron transitions, and four times as much (4-8 eV) for the doubly charged final ion in the Auger transition. Chlorine The chemical state plot in Fig. 7 shows the Auger parameter data for various chlorine compounds in gas phase. The parameter increases with the substitution of chlorine in each of the series CCIFs--CC& and CH&l--CC14. Substitution of higher alkanes in the series CH&l--t-C&H&l also results in an
294 PHOSPHORUS
1858
1856
1854 2 .% 5
1852
: U F
1850
g z -I 2
1848 z m 4 1846
1844
1842
1840
140
138 2p3,2
5 eV
added
to
gas
phase
kinetic
136 BINDING
energies
134 ENERGY
to
132
130
(eV)
simulate
Fermi-level
referencing
Fig. 5. Chemical state plot of phosphorus compounds, solids [ 50,561 and gases [ 49,561.
z g
2: 0 0 s
N
N 0 a=
KL23L23 N z
8 0
N
MODIFIED
ENERGY
N
KINETIC
N
PARAMETER
N a=
N
AUGER
(eV)
(eV)
\
\
\
\
\
\
ki Q, t
N
296 CHLORINE
2378
2377
2376
2 -
2375
-I
2
2372
2371
2370
2369
2P 3,2 -.-e---
-
‘*********.* All
the
ENERGY
Compounds
CCIF3--CC12F2--CC13F--Ccl4
Compounds
CC14--CHC13--CH2C12--CH3CI
Compounds data
BINDING
are
from
(eV)
CH3CI-C2H5CI--n-C3H7CI--i-C3H7CI--t-C4HgCI gas
phase
and
are
referenced
to
Fig. 7. Chemical state plot of chlorine gas phase compounds [ 681.
the
vacuum
level
297
increase in Auger parameter. Bonding with the poorly polarizable hydrogen atom and formation of HCl results in the lowest value for the Auger parameter. Copper Formation of cuprous compounds with Se, S and 0 results in progressively larger deviation of the Auger parameter from the metallic state, as shown in Fig. 8. The 2psj2 peak of copper shows no significant chemical shift in the
$M&
2P,,,)
States
cu 1851
1850
lcu2s
Cu2Se, CulnSe2---
-cu20
1849
1848 CUCI
CuCN 1847
1846
1845 1 1844
1843
1842
1841 ------cu(g) 1840
Cuprous
t
states
only
are
cited;
cupric
had
multiplet
Fig. 8. Auger parameter of copper compounds, solids [ 59,69-721
splitting
and gas [ 511.
298
cuprous state and the entire Auger parameter shift is primarily due to the Auger line shift. Zinc Figure 9 shows the Auger parameter shifts for the halide series (group VII) and 0, S, Se and Te (group VI) elements. With fluorine being the least polarizable of these elements, the fluoride exhibits the lowest and the telluride the highest Auger parameter. Zinc compounds show almost no shift in the photoZINC
CY+ hv.
eV
States (qy4j.
ZP3,2)
2014-
2013
Zn
-
ZnTe
2012 -
ZnSe ZnS
Zn12 2011 ZnBr2 2010 -
ZnO ZnC12
ZnAl 20,+ ZnS04
2009 ZnF2 2008
-
2007
-
2006
-
2005
-
2004
-------s-m
Zn(g)
2003-
Fig.
9.
Auger parameter of zinc compounds, solids [ 14,53,73,74] and gas [ 16,521.
299
electron line, but shifts of several electron volts are observed in the Auger parameter values for these compounds. Gallium
Compounds of gallium with group V and group VI elements are shown in Fig. 10. Again a systematic trend is observed for each group, with the least polarizable compounds giving the lowest Auger parameter. Germanium
Figure 11 shows the chemical state plot for Ge and its compounds. Gas phase compounds GeF, and GeH, are observed to have the lowest Auger parameter values. The value for GeBr, is the highest of the gas phase compounds explored, and is equal to the value for solid germanium dioxide. The series GeF,--GeBr,, GeH,--GeBr, and GeH,--Ge (Me) 4 show the expected trends. Arsenic
The gas phase compound ASH, has the lowest Auger parameter value, and the As (Me) 3, and especially arsabenzene, have the highest values. Figure 12 shows a representative selection of compounds from Bahl et al. [ 341 in solid compounds. GALLIUM
ff+ hv, eV (L3M5M5,
States -
3d)
1087Ca
Ga
1086 CaAs _ 1085 -
Ca2Se3
1084 -
1083 :
CaP
CaN
Ga203
1082-
Fig. 10. Auger parameter of gallium solid compounds [ 75-781.
38
36 3d 5,2
5 eV
added
to
gas
phase
kinetic
34 BINDING
energies
32
30
ENERGY
to
28
(eV)
simulate
Fermi-level
referencing
Fig. 11. Chemical state plot of germanium compounds, solids [ 55,79,80] and gases [ 541.
,” 0
r: N
2 c
L3”45M45 N 2 !I! m
KINETIC Li 0 i
(eV)
MODIFIED
ENERGY
AUGER
;j c” : co
PARAMETER
Lz m
(eV)
z 0
302
Silver, cadmium, indium and antimony
Photoelectron lines of silver and cadmium show insignificant shifts, but the Auger lines show up to 5 eV shifts. Figures 13-15 indicate systematic Auger parameter shifts for silver, cadmium and indium in compounds formed with group VI and VII elements. Figure 16 for antimony shows similar behavior in compounds with elements in groups I and VI. The dilute alloys of silver with aluminum and magnesium have slightly higher values than elemental silver. SILVER
ff + hv. (M,+N5N5s 727
726
eV States
3d,,,)
Mg97Ag3 A’95Ag5 Ag
Ag
Ag2= Ag2Se
725
Ag20* 724
723 -
*Agl
AgO” AgOOCCF3
*AgF “AgF2 Ag2S04
722 -
721 -
720 -
719 -
718 -
__--__
_ ---
Ag(g)
717 *6.0
eV added
to value
of M5N5N5
to give
value
for
M4M5M5
Fig. 13. Auger parameter of silver compounds, solids [ 11,14*,59,82,83]
and gas [ 581.
303 CADMIUM (Y+ hv,
eV States
(M4N5N5’ 789 r
3d5/23 *Cd
788 CdTe* 787 CdSe* CdS Cd0
Cdl 2 786 _ CdBr2.4H20
CdTe03 CdS04
785 - 2CdC12. 5H20 Cd(OH12
CdF2 784 -
783 -
782 -
781 --------__
Cd(g)
780 “6.7
eV added
to value
of M5N5N5
to give
value
for
M4N5N5
Fig. 14. Auger parameter of cadmium compounds, solids [ 14*,63,85-871 and gas [ 45,603.
Iodine Auger parameter values of iodine and its compounds with various metals are shown in Fig. 17. Among the salts, LiI exhibited the lowest parameter. The I2 gas has a higher Auger parameter value than monoatomic iodine, as expected. Lead This was the only representative of the series (N,O,O,, 4f,,2). Compounds of lead with elements of groups VI and VII and selected complex lead oxides and hydroxides are shown in Fig. 18. PbFz is found to have the lowest Auger parameter value. PbOp has a slightly lower value than PbO.
304 INDIUM
0 + hv,
(M4N5”‘5,
eV
States
3d5,2) In
854
853
E
ln2Te3
-
InSb In2Se3
CulnSe2
852
InP ‘n2S3
lnl3 851
lnt3r3 IdI3
E 850
ln2’3 In(0l-l)
InF3
3
849
848 i
Fig. 15. Auger parameter of indium solid compounds [ 11,12,27,72,88-901. ANTIMONY Q + hv,
(M4N5N5,
992
eV States
3d5,2) Sb Sb2S3
991
Cs3Sb
Sb2S5
K 3Sb Na3Sb
990
Sb203 989
988
KSbFs 987
_
Fig. 16. Auger parameter of antimony solid compounds [ 11,84,90].
305 IODINE
a + hv, (M4N5N5,
eV 3d5,2)
States
1139
1138
*Agl *CUl Li
1133
--____---
I,(g)
-______----
l(g)
k
1132 I
*l 1.5 eV
value
L
I
for
added to the M4N5N 5
value
of M5N5N 5 to provide
Fig. 17. Auger parameter of iodine compounds, solids [ 14*,27,92] and gases [93,94].
LEAD CY + hv,
eV
(M4N5N5n3d5,2) 233
232
States
Pb PbTe PbSe PbS Pb12 PbBr2
231
Pb02 PbO
230
229
PbCrO4
PbC12 -
PbF2
Fig. 18. Auger parameter of lead solid compounds [ 95,961.
PbTi03 Pb(N0 I2 Pb(OH J3 2 PbSi03
306 SELECTED SPECIAL CASES
There are a number of special cases where the Auger parameters have been most helpful in understanding the screening energy changes resulting from modifications in the atomic or molecular environment. These involve valence band transitions, photoelectron and Auger core level shifts and surface core level shifts, changes in crystallinity and absorption and implantation phenomena.
Ammonia and water
Data are available on two other molecules in both solid and gas phase: ammonia and water (see Table 3). Although the Auger transitions are valencetype (CVV) transitions, so that changes in bonding can affect the line distribution, it appears that valid comparisons can be made. For water, the dominant (lbllbl) peak appears in both spectra [ 33,35],23.7-24.0 eV distant from the ( 2a,lbl) peak, so hydrogen bonding does not seem to affect the energies of these peaks. The Auger parameter for solid water [ 331 is found to be 2.3 eV larger than that for gaseous water [ 35,401. For ammonia, again, the distribution (le3a1) peak is almost the same for the gas and the solid [ 36, 371, and the parameter for the solid [ 361 is 4.4 eV larger than that for the gas [ 371. It is clear that the proximity of other molecules is important in modifying the kinetic energies of emitted Auger and photoelectrons. Knowledge of these energies will be useful in determining the environment of atoms in thin films, adsorbed species, and aggregates approaching atomic dimensions. TABLE 3 Auger parameters of ammonia and water
NinNH, NinNH, Change
(g) (s)
0 in H20 (g) OinHzO (s) Change
Photoelectron BE (ls)“(eV)
Auger electron KE (KLL)“(eV)
405.55 (v) 398.8
365.46(v) 376.6
539.9(v) 532.9
498.2(v) 507.9
a(v) Energy references to vacuum level.
a+hv 771.01 775.4 4.39 1038.1 1040.8 2.7
Reference 1371 [361 135,401 [331
307
Phenylsilicone and methylsilicone resins Figure 19 shows the chemical state plot for Si-0 compounds. Included in this category are the silicon polymers, silicon dioxide, aluminosilicates and silicates [ 381. The Auger parameters for dimethylsilicone polymer and the methysilicone resin ( MeSiO,,,) were at 1711.8 eV, while the phenylsilicone resin ( PhSiO,,,) was 1.0 eV higher due to its greater polarizability. The observation of the oxygen Auger parameter at the same time confirmed that the oxygen was perturbed also, but to a lesser degree. The silicon is bonded directly to the phenyl, while the oxygen is bonded to silicon and only indirectly to the phenyl group. Octahedral alumina versus tetrahedral alumina The octahedral aluminum oxides, hydroxides, and aluminosilicates are in the region of aluminum Auger parameters of 1461.4-1462.0 eV (Fig. 20). On the other hand, albite and natrolite, and the synthetic zeolites possess tetrahedral crystal form and they occupy a region 1460.3-1460.8 eV, about 1.2 eV lower in parameter. In a parallel study, West and Castle [ 221 have observed different Auger parameter values for tetrahedral aluminum. Using 1s and KLL lines, they related the parameter values to variable polarizabilities of the oxygen atom in the aluminosilicates, as measured by the refractive indices. Auger parameter shifts of xenon in adsorbed and implanted forms It would be expected that there should be effects of the nature of the substrate on line energies from thin films or from very small aggregates. Data are available on xenon, adsorbed on and implanted in host matrices, as shown in Fig. 21. The Auger parameter is the largest for xenon implanted in conductors, such as iron and graphite. Slightly less screening energy is available to adsorbed films of conductors, platinum, and molybdenum. When the support is oxidized, conduction electrons are not available, and screening energy is smaller still, but is still appreciable, as shown by the much smaller value for gaseous xenon. Distance-dependent relaxation shifts of photoemission and Auger energies for Xe on Pd(001) An elegant experiment of rare gas adsorption on metal substrates was performed by Kaindl et al. [ 391. They adsorbed one monolayer of xenon on a crystal face of palladium, and recorded the xenon 4cE,,,line and the NO0 group of lines. Figure 22 summarizes the results. The shift from gaseous xenon of each of these lines for the first adsorbed monolayer is 2.14 and 6.57 eV. The
308 COMPARISON
OF SILICONE AND
WITH
SILICATES,
SILICON
ALUMINOSILICATES,
DIOXIDE
.VI
1606
Iii
ALUMINO-SILICATES SILICONES
103 Si2p
BINDING
102 ENERGY
(eV1
ms
Dimethylsilicone
ox
Silicon
mr
Methylsilicone
Resin
cr
o-Cristobalite
pr
Phenylsilicone
Resin
t
Talc
9
o-Quartz
k
Kaolinite
”
Vycor
P
Pyrophyllite
9
Silica
sp
Spodumene
w
Wollastonite
Gel
Polymer
Dioxide
Fig. 19. Chemical state plot of selected silicon-oxygen compounds [ 381,
309 COMPARISON
OF
OCTAHEDRAL
TES, OXIDES AND OCTRAHEDRAL
AND
TETRAHEDRAL
ALUMINOSILICATES
TETRAHEDRAL
HYDROXIDES,
Al2p
BINDING
sa
Sapphire
sp
Spodumene
CY
o-Alumiha
p
Pyrophylli
Y
Y-Alumina
k
ENERGY
A
(eV)
Molecular
Sieve
X
Molecular
Sieve
Type
X
Kaolinite
Y
Molecular
Sieve
Type
Y
te
bo
Boehmite
a
Albite
2
H
ba
Bayerite
n
Natrolite
HS
Hydroxysodalite
gi
Cibbsite
Type
A
Zeolon
Fig. 20. Chemical state plot of selected aluminum-oxygen compounds [ 381.
ratio is almost exactly three (3.07)) as expected from simple theory. When a second layer was added, the shift in the first layer increased slightly because of the extra screening energy from the first layer, but the intensity of the lines diminished. The shifts registered by the second layer were smaller, 1.49 and
310 AUGER
PARAMETER
ADSORBED
SHIFTS
AND
OF XENON
IMPLANTED
IN
FORMS
(Y + h v, eV
(M4N5N5, 3d,,,) 1215
1214
Xe Xe Xe Xe Xe
States
imp imp imp ads ads
in in in on on
Fe C C* MO Pt
1213 Xe ads on Moo2
Xc(g)
*C
in diamond
---
-
-
form
Fig. 21 Auger parameter shifts of xenon in adsorbed and implanted forms, solids [ 11,27,43,97,98] andgas [99]. @Es)
(AE)
4d -----2 I
Auger
-
Au (evl
L I I I
3.2 1
1
----I
L
I
2 LAYERS
NO0
2
I
4.5
I
1 I
2 i
I I 1 LAYER
Xe
1
I
I
(001)
SHIFT
IN
I
I 4
2
imsg Pd
q.4i
I I
KINETIC
ENERGY
I 6
(eV)
Fig. 22. Distance-dependent relaxation shifts of photoelectron Auger energies for Xe on Pd (001)
[391.
311
4.69 eV respectively, as expected because of the greater distance from the metal, but the ratio was still a factor of three (3.15).
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