SHORT COMMUNICATION

Energy Calibration Secondary Standards for X-ray Photoelectron Spectrometers

Stephen Evans Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, Dyfed, SY23 1 NE, UK

The selection of elementary calibration standards for X-ray photoelectron spectrometers is discussed ; it is suggested that Zn and Ga may be more satisfactory routine single-element standards than Cu, Ag or Au. Binding energies for the 2p3,, and 3d photoelectron peaks from elemental Zn, Ga (and Ge) and for the most intense LMM Auger peaks from Zn and Ga are reported and relevant energy separations compared with published L a X-ray energies. The instrumental voltage scale was calibrated to a precision of 0.0035% (one standard deviation, SD) by reference to published data for Cu and Au and the SD of the measured peak energies were generally Q0.03eV. Taking residual voltage scaling uncertainties into account, the largest energy separations in the data for Zn and Ga should be accurate to 0.05 eV (one SD).

There has in recent years been renewed interest in the problems associated with the accurate energy calibration of X-ray photoelectron spectrometers. A ‘round-robin’ survey’ revealed that many instruments were yielding accuracies far below those attainable, while serious dis- crepancies in reported standard binding energies ( B E ) stimulated careful redeterminations for the commonly used Group 1B rnetal~.”~ In addition, increased aware- ness of the existence of surface core-level shifts (SCS) has led to discussions of their possible effects on energy calibration, both in theory4 and in practice.’

Although other methods have recently been advo- cated,6 many workers use standard BE differences for the routine checking of instrument calibration.’ This has the major practical advantage that no measuring equip- ment of certified accuracy is required. However, although the relevant BE differences for the Group 1B metals have now been established with sufficient a c ~ u r a c y , ~ . ~ none of these metals alone is an ideal cali- brant.

Instrumental calibration errors may be categorised’ under three headings: voltage scaling, zero offset and random. The accuracy of the voltage scaling is para- mount; zero offset errors (effectively, changes in the spectrometer work function) normally cancel when reference is made (as is our usual practice) to a standard peak of known BE at the time of each BE determination. Random errors are minimised if sharp intense peaks are used ; the precision attainable depends greatly on the techniques available. Anthony and Seah3 claim a SD of 0.005 eV, our instrument and data system yield typically 0.02 eV while with an analogue XY recorder results are unlikely to be better than 0.05 eV. If the known BE used to calibrate the scale are not adequately separated in energy, these random uncertainties can lead to voltage scaling errors substantially in excess of the random uncertainty in peak location. Thus, for example, the use of Cu2p3/, and CuLMM, the only two really suitable Cu peaks (see below), to set up the scale with a random uncertainty of 0.05 eV could lead to an error in the BE for a Zn2p peak, referred to Au4f, of almost 0.2eV. Consequently, the known BE should span as large an

energy range as possible. An intermediate known BE is valuable as a check on serious instrument malfunction or operator error; the scales of most common instrument types have recently been shown normally to remain accurately linear5. The use of symmetrical peaks is pre- ferred, because the measured maximum of an asym- metrical peak shifts as the instrumental resolution is changed.

Cu is probably the least unsatisfactory of the Group 1B elements, using the 2p3/,-3s or 3p separation in the first instance and the most intense component of the LMM Auger structure as an intermediate check. The 3s peak is nevertheless both weak and broad (full width at half maximum, FWHM, ca. 2.4 eV), and it is not easy rapidly to obtain consistent measurements of its peak maximum. The more intense 3p peak consists of a partly- resolved spin-orbit doublet, of overall FWHM - 4.4 eV; changes in instrumental resolution thus have a sig- nificant effect on the measured 3p3/, BE.’ Neither peak is a very suitable as a calibrant.

No element fully complies with the ideal, but Zn, Ga and Ge initially appeared worth evaluating as possible calibrants. The 3d peaks from these elements are only slightly spin-orbit split (-0.6 eV), so that the two com- ponents are not resolved using Mg Ka X-rays, and, unlike the Cu3d peak, they are not significantly broad- ened by valence effects. Their overall FWHM are 1.3- 1.5 eV, cf. -2.4 eV for both Cu3d and Cu3s. It might, however, be thought that these 3d peaks should not be used for calibration purposes because, as for Cu3p3/,, the observed maximum will vary with the instrumental resolution. However, the effect will be much smaller over the usual range of instrumental settings because the spin-orbit splitting is so much less, and difficulties are unlikely unless an X-ray monochromator is in use. The 2p3l2 signals are sharp (FWHM 1.2-1.4 eV) and symmetrical, and occur to higher BE than that for Cu; the 2p3/,-3d separations are 1012-1188 eV compared with only -598 eV for Cu 2p3I2 - LMM or -848 eV for CU 2p3/2-A~ 4f,/2.

Comparisons involving two different metals, however, bring additional difficulties. Cleaning by ion bombard-

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SURFACE AND INTERFACE ANALYSIS, VOL. 7, NO. 6, 1985 299

S. EVANS

ment may sputter one element into the other, if they are physically contiguous, and any resulting alloy formation may in principle affect peak energies.' If, however, the samples are physically separate, additional measure- ments will be required to ensure that the spectrometer work function has not changed between measurements which must inevitably then be separated in time. A single-element calibrant is clearly more convenient for rapid routine checking purposes.

Because the escape depth of low kinetic energy ( K E ) photoelectrons is very small, the material selected should in principle have a small SCS? althnough SCS effects in practice, using polycrystalline samples, were not found to be detectable for Cu2p3/, using a spectrometer with a relatively wide acceptance angle.3 SCS measure- ments have recently been reviewed by Eastman and Himpsel.' Transition elements (including Group 1 B) show large SCS, related to the number of valence d- electrons, so that4 the Cu 2p3I2 peak, for example, might be shifted by as much as 0.1 eV relative to the higher K E peaks. Semiconductors such as Si and Ge also show large SCS (20.5 eV), but SCS are expected to be small for sp metals. Al, for example, shows an SCS of 0.06 eV? values for Zn and Ga have not been reported.

The ease and reproducibility of preparation of cali- brants are also important. Zn, Ga and Ge form oxides more readily than do Cu, Ag and Au, but this is not a significant drawback. The partial pressure of O2 in XPS instruments is very low, and the thin protective oxide layers are readily removed by Ar-ion bombardment. Small quantities of residual oxide can be tolerated more readily than for Cu, because the chemical shifts for the oxides are large and the oxide peaks hardly overlap the elemental peaks. The low melting points of Zn and Ga in particular are an advantage to the extent that they obviate any necessity for annealing following ion bom- bardment.

The present measurements were made with Mg K a , , , radiation using an AEI/ Kratos ES200A electron spec- trometer operating in the fixed-retardation-ratio (FRR) mode, and equipped with a microcomputer-based data system." The linearity of all potentials applied to the analyser was checked using a Solartron A240 digital voltmeter following Bird and Swift.2 No deviations greater than 0.02V were detected in potentials up to 1200 V. These measurements did not, however, establish an absolute energy scale because (i) in the FRR mode both retarding and deflection potential change .with elec- tron energy, and (ii) the outputs are at high impedance and were detectably loaded by the meter.

For each set of energy separations measured, the relevant peaks were accumulated for a number of sweeps in sequence ABC. ..CBA using a channel width of

(usually) 0.05 eV; energy maxima were obtained by quadratic least squares fits to the net peak Several sets of measurements were made in each deter- mination; the standard deviations (SD) obtained are indicated in the tables.

Samples were mounted on a standard 4-sided probe tip; Ar-ion bombardment, where used, was at 700 eV, 2-5 FA using a Physical Electronics 04-131 ion gun.

Initially, measurements on Cu and Au were made to ensure that the instrument calibration settings were com- patible with the absolute values of Bird and Swift2 and Anthony and Seah? as neither a high-energy X-ray source (e.g. AgLa)' nor a suitable absolute voltage stan- dard' was available. These measurements were made on a sample consisting of alternate contiguous transverse trips of Cu and Au silver-soldered to a 15 x 7 mm Cu backing. This sample yielded Au4f, Cu2p and CuLMM peaks of high intensity; it was polished before insertion and heated in vacuum to 400°C to remove oxide and carbonaceous contamination. It was not Ar-ion bom- barded.

The peak energy separations obtained for Cu and Au are compared with standard values in Table 1. The instrumental energy scale was set to obtain the closest possible agreement. The data indicate the residual uncer- tainty in the voltage scaling to be ca. 0.03 eV, in 848 eV, i.e. 0.0035%.

Measurements on Ge were conducted on a sample from British Drug Houses (BDH) - 8 x 4 mm in size, polished and mounted at 90" to the Cu/Au sample. The 2p3/2 and 3d peaks were recorded for Ge Ar-ion bombar- ded at room temperature and also at 400 "C. The latter showed the same BE as those bombarded at room tem- perature and subsequently annealed. Cu/ Au peaks were recorded alternatively with those of Ge at each stage for comparison of absolute BE. A negative bias (-40V) was applied to the probe during these measurements to increase the intensity of the Ge 2p,,, signal. The back- ground to this peak sloped quite steeply; a straight-line background function was subtracted, leaving a sym- metrical net peak. The least squares fits to the maxima were conducted on the net data.

Ga samples were prepared by spreading liquid Ga (Metals Research Ltd.) over a freshly-abraded Cu plate. Surface-tensional forces were sufficient to prevent loss of Ga even over 90" rotation of the probe. As inserted, some oxide signals were detected. Despite the liquid surface of the sample, a few minutes of Ar-ion bombard- ment sufficed to remove most of this contamination. Ga can alloy with Cu, but under normal conditions attack was negligibly slow; samples could be reused after months of storage. However, on one occasion heating to 400 "C in vacuum resulted in the formation of a solid

Table 1. Energy separations and absolute BE for Cu and Au

Peak energy separations Absolute BE

Source C U ? ~ , ~ , - LMM Cu2p3,,-3s CU~P,/Z -Au4f,l, cu2p3/2 Cu LMM Au4f71,

Ref. 2 597.70 (6) 810.21 (6) 848.68 (6) 932.66 (6) 334.96 (4)a 83.98 (2) Ref. 3 597.72 (2) not recorded 848.67 (2) 932.67 (2) 334.95 (1) 84.00 (1)

This work 597.72 (3) 810.25 848.67 (3) not determined

a Taking Mg K ~ ~ , , ~ = 1 2 5 3 . 6 0 eV. Large standard deviation resulting from width and weakness of 3s peak.

Figures in parentheses indicate the standard deviation of the last digit cited.

300 SURFACE AND INTERFACE ANALYSIS, VOL. 7, NO. 6, 1985

ENERGY CALIBRATION FOR XPS

Table 2. Absolute BE for Zn, Ga and Ge

Element 2P3/2 LMM 3d (unresolved)

Zn 1021.70 261.50 9.98 Ga 11 16.67 185.52 18.69 Ge/Ar+ 121 7.28' not 29.45 Ge/ann. 1217.38 recorded 29.65

a Up to 1217.38 eV observed after prolonged bombardment; for all other data no dependence of peak position on Ar-ion bombardment time was detected. Notes: (1) Standard deviation 60.03 eV except where indicated. (2) Calculated taking Cu 2p3,,=932.67 eV and Cu LMM=334.95eV; see Table 1 . (3) Oxide peaks were never greater than ca. 15% of the3otal signal and often much less; within this range no systematic dependence of the elemental peak energy with oxide content could be detected.

alloy. Strong Cu peaks developed and significant chemical shifts were apparent. Ga samples spread on glass slides were not satisfactory, because the Ga separ- ated in vacuum into discrete globules spread over the surface.

Measurements on Ga were alternated with measure- ments on the other samples. The Ga samples remained liquid throughout the measurements reported here. Sep- arate experiments using a cooled probe had revealed no detectable BE shifts between solid Ga and liquid Ga.13

The Zn samples comprised polished pieces of sheet Zn (BDH), mounted on the 4-sided tip with a Ga or Ge sample. A third side of the tip was cleaned and polished to provide Cu reference spectra. These samples were Ar-ion bombarded in turn at -50 "C and the 2p3l2, main LMM (Zn and Ga only) and 3d peaks recorded for each surface in turn several times.

All the above measurements were made with the elec- tron takeoff angle 0 (measured with respect to the surface normal) in the range 15-45'. In order to examine the possibility4 that SCS might have affected the measure- ments some data were also collected at higher values of 0 for both Ge and Cu. Angles up to -70-85" were possible in separate experiments using a single-sided probe tip with the rotation axis along the sample surface. Only marginal effects on the 2p3/, - 3d energy separation in Ge were observed. For Cu( 11 1) an effect of 0.06 eV between 2p3/2 and 3s was predicted4 but none was detec- ted for our polycrystalline sample. However, a sig- nificant effect (-0.1 eV) was observed for the principal Cu LMM Auger peak. Loss of extra-atomic relaxation at the outermost atomic layer should reduce the mean K E of this signal at high 8, as was observed; extra- atomic relaxation is much larger for Auger electron emission than for photoelectron emission because of the double-hole final states involved in the Auger process." When Auger peaks are used for calibration, low 8 values are thus desirable, and low K E Auger peaks should be avoided.

Absolute BE for Zn, Ga and Ge, obtained by com- parison with Cu, are given in Table 2. The 3d peaks of Zn and Ga can each be curve-resolved into two peaks with the expected 2 :3 intensity ratio, separated by -0.6 eV. The curve-resolved 3d,,, maxima occur 0.2* 0.05 eV to lower BE than the unresolved maxima cited in the table.

Table 3. Comparison of BE differences with L a X-ray energies"

Element X-ray15 2p3/,-3d XPS'

cu 929.7 (2) 929.90b Zn 1011.7(2) 101 1.72 Ga 1097.92 (1 0) 1097.98

Ge 1 188.00 (9) 1 187.8g-93 (Ar+) 1187.73 (ann.)

a Estimated as accurate to ~ 0 . 0 5 eV SD, comprising 0.0035% uncertainty in voltage scaling and 0.03 eV SD in peak loca- tion-see text.

Bird and Swift' report 929.84 eV; the 3d peak is broad and asymmetric.

The 2p3/,-3d XPS energy separations measured in this work should14 correspond exactly with the energies of the La1,, X-rays resulting from 3d+2p transitions in the elements; within the framework of a one-electron model both experiments measure the difference in energy between the 'P3/, and 'D5/2,3/2 states of the ions. These X-ray energies (Table 3) are however presently known to a lower accuracy than the XPS results reported here. The SD of the Zn and Ga measurements reported in Table 2 are all G0.03 eV, and the voltage scaling uncertainty was estimated above as 0.0035% ; quadratic summation yields an estimate of the overall accuracy (one SD) of the XPS energy separations in Table 3 of 0.05 eV. The XPS and X-ray data for Zn and Ga agree very closely, but the XPS data for Ge indicate a lower energy than that given by the X-ray measurements. This may be attributed to the high (negative) SCS for Ge.' As discussed above, the 2p3/2 peak (at only -35 eV K E ) is expected to shift to higher K E by much more than the 3d peak at -1223 eV. However, the K E of the Ge 2p3/, photoelectrons is so low that the possibility of an apparent shift arising from scale non-linearity due to stray magnetic fields or the application of the -40 V bias cannot easily be eliminated.

It is interesting that ion-bombardment causes a sig- nificant (-0.2eV) shift of the Ge peaks to lower BE (Table 2), implying a pinning of the Fermi level towards the lower (valence band) edge of the band gap (0.7 eV). Ion-bombarded Ge thus shows strong p-type behaviour.

It is clear that Ge is not a suitable calibration material. Zn and Ga, on the other hand, do appear suitable for the calibration of XPS instruments, the present results complementing existing data for Cu, Ag and Au. If only one element is to be used in a routine check, Zn and Ga offer advantages over other single-element standards such as Cu. The present data permit single-element calibration to an accuracy of ca. 0.05 eV (SD) over an 1100 eV range in K E for apparatus capable of locating peaks within <0.04 eV (SD). An independent absolute redetermination of the BE reported here, to an accuracy comparable with that of Anthony and Seah3 would be valuable in improving further the accuracy attainable with a single-element calibration.

Acknowledgements

1 thank Mr. A. G. Hioms and Dr. L. A. Ash for experimental assistance and the SERC for support.

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Received 4 December 1984; accepted 10 May 1985

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