Fine structure analysis of Si KL2,3V Auger spectra of Si, SiC and SiO2

Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 21–25

Fine structure analysis of Si KL2,3V Auger spectra of Si, SiC and SiO2 Tomoyuki Yamamoto a,∗ , Chikai Sato b , Masato Mogi b , Isao Tanaka c , Hirohiko Adachi c a

Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano-nishibiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan b NISSAN ARC LTD., 1 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan c Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 29 August 2003; received in revised form 2 December 2003; accepted 2 December 2003

Abstract High-resolution Si KL2,3 V Auger spectra of Si, SiC and SiO2 are measured to investigate changes in the electronic structure of the valence band in these materials. Significant differences between the lineshapes of these spectra are observed. First-principles electronic structure calculations are also carried out, which reproduce the lineshapes of the KL2,3 V Auger spectra of Si, SiC and SiO2 observed experimentally. The core–hole effects on the theoretical spectral lineshapes are also taken into account in the present calculations. © 2003 Elsevier B.V. All rights reserved. Keywords: KL2,3 V Auger spectrum; Electronic structure; First-principles calculation; Core–hole effect

1. Introduction Auger electron spectroscopy (AES) is mainly used for elemental analysis of surfaces and has been employed with good success in the field of surface science. On the other hand, chemical state analysis or electronic structure analysis has not widely been done by AES. In principle, chemical state analysis with the AES technique is possible by measuring the chemical shift of the core level Auger spectrum. In practice, it is not so reliable to use the chemical shift of the core Auger peak for chemical state analysis of insulating materials, because peak shifts due to sample charging take place. Moreover, only the charge state on the atom of interest can be obtained by the chemical shift, but no detailed information on the electronic structure of the valence band is given. It is reported that high-resolution AES spectra, including the valence electron transitions, are sensitive to changes in the electronic structure of the valence band [1]. There are two types of transitions involving valence electrons, i.e., core–core–valence (CCV) and core–valence–valence (CVV) Auger transitions. In both cases, spectral fine struc∗ Corresponding author. Tel.: +81-75-753-5454; fax: +81-75-753-5447. E-mail address: [email protected] (T. Yamamoto).

0368-2048/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2003.12.001

tures reflect the fine structures of the valence band. Therefore, electronic structure analysis is possible by using AES valence measurements. For analyzing these CCV and CVV Auger spectra theoretically, the final state rule [2–6] was proposed, which can well explain the experimental spectra. However, the AES technique has still been employed mostly for elemental analysis of the surface, and electronic structure analysis has not been done sufficiently by CCV and/or CVV Auger measurements. Several detailed studies on the KLV Auger spectra have been reported for metallic materials, e.g., Mg [7,8], Al [9,10] and Si [10–12], and for metallic compounds, e.g., Mg–Ni [7], Mg–Cu [7,8,13], Mg–Zn [7,8,13], but not for the standard compounds of these elements like carbides and oxides. Among these materials, Si has extensively been studied. High-resolution KLV Auger measurements and calculations were reported by Fowles et al. [11], and ab initio calculations done with the Green’s function formalism were reported very recently by Chang and Shirley [14]. To the best of our knowledge, only one experimental KLV Auger spectrum for SiO2 [15] and no measurements for SiC have been reported. In the present work, high-resolution KL2,3 V Auger spectra for a series of standard Si compounds, Si, SiC and SiO2 , have been studied. We used Zr L␣ X-rays to ionize the Si K shell effectively and a high energy-resolution hemispherical electron analyzer. Significant differences

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T. Yamamoto et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 21–25

were found between the lineshapes of these observed spectra, suggesting that there were differences in the electronic structure of the valance band in these materials. First-principles model cluster calculations were also carried out to analyze the spectral lineshapes of these materials in detail by using the DV-X␣ method [16]. Based on the final state rule for the CCV Auger transitions, we also performed calculations on the theoretical lineshapes of Si KL2,3 V Auger spectra taking account of the core–hole effects. There were only slight differences between the theoretical lineshapes calculated with and without the core–hole effects, though the calculations done with the core–hole effects showed better agreement with the experimental Si KL2,3 V Auger spectra of Si, SiC and SiO2 .

graphic structures, in which an Si atom was put at the center of the clusters and the third nearest neighboring atoms were taken into account. We assumed Si and C to be electronically neutral in the case of Si and SiC, whereas formal charges of the ions, Si4+ and O2− , were adopted for SiO2 . The number of random sampling points for obtaining resonance and overlap integrals was 500 per atom, and the basis sets were 1s–3d for Si and 1s–2p for C and O, respectively. The coefficient of the X␣ potential, ␣, was fixed at 0.7 for all atoms. We obtained the partial density of state (PDOS) of Si 3s and 3p orbitals by broadening the populations of all molecular orbitals (MO) in the valence band through the use of the Gaussian distribution function with a width of 3 eV, in which the orbital populations were calculated by using Mulliken’s population analysis method.

2. Experimental procedures 4. Results and discussion The measured Si KL2,3 V Auger spectra of Si, SiC and SiO2 are shown in Fig. 1a–c, respectively. All the spectra except that of SiO2 accompany the plasmon-loss spectra due to the Si KL2,3 V Auger emissions, as marked by asterisks. The present Si KL2,3 V spectrum of metallic Si agrees well with the high-resolution experiment [11] previously reported, although the energy-resolution of the earlier report was slightly better than that of our present measurement. Observed spectrum of SiO2 in Fig. 1c shows higher energy-resolutions and a better S/N ratio than recent measurement [17]. To the best of our knowledge, an experimental spectrum for SiC has not been reported. In order to see the fine structures of the Si KL2,3 V Auger spectra more clearly, the background and the plasmon-loss spectra due to the KL2,3 V Auger electron emissions were subtracted,

(a) Si *

Intensity

The Si KL2,3 V Auger spectra of Si, SiC and SiO2 were measured with an ESCA-5800 (Physical Electronics Inc.) equipped with a high energy-resolution hemispherical electron analyzer. We used Zr L␣ X-rays with an energy of 2042.4 eV as an excitation source for Si K shell ionization, which was done at 300 W. The X-rays were irradiated perpendicularly onto the sample surface, and the emitted Auger electrons were collected at an angle of 45◦ normal to the sample surface. Pressure in the experimental chamber was kept at 1–3 × 10−6 Pa during the measurements. For the measurement of the Si KL2,3 V Auger spectrum of metallic Si, Si(1 0 0) specimens were cut from a p-type single crystal to a size of 1 cm × 1 cm. For SiC, commercially available high purity powder with the zinc-blend type of structure, i.e., ␤-SiC, prepared by Soekawa Chemical Co. Ltd., was pressed into pellets with a diameter and a thickness of 5 and 0.1 mm, respectively. An ␣-quartz structured high purity powder pressed into the same-size pellets as SiC was used for SiO2 . All samples were cleaned by cycles of Ar-ion etching before the Auger spectrum measurements. Sample cleanliness was monitored by the photoelectron spectra before and after the measurements. Only small amounts of C and O contaminants were found on the sample surfaces. Pass energy of the electron analyzer was set to 23.5 eV. For the energy calibration of the spectrum, the energies were shifted so that the observed Si 2p photoelectron peaks coincided with those reported previously for Si [17], SiC [18] and SiO2 [19].

(b) SiC *

(c) SiO2

3. Calculation procedures First-principles calculations were performed by using the DV-X␣ code [16]. The materials examined showed similar local crystallographic structures around Si, i.e., Si was present on the tetrahedral sites. We built the model clusters, SiSi4 Si12 Si12 , SiC4 Si12 C12 and (SiO4 Si4 O12 )12− , for Si, SiC and SiO2 , respectively, corresponding to their crystallo-

1700

1720

1740

Energy (eV) Fig. 1. Observed Si KL2,3 V Auger spectra of (a) Si; (b) SiC; and (c) SiO2 . Plasmon-loss spectra due to the emission of Si KL2,3 V Auger electrons are indicated by asterisks in (a) and (b). Note that no plasmon-loss spectra are found for SiO2 in this energy range.

T. Yamamoto et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 21–25

A

0

Energy shift (eV)

(a) Si

Intensity

B (b) SiC

A B

(c) SiO2 A B 1700

which are shown in Fig. 2a–c. All of these spectra are composed mainly of two peaks, denoted as peaks A and B on the higher and lower energy sides, respectively. In the spectrum of SiO2 shown in Fig. 2c, there is a satellite peak C on the higher energy side of peak A. In order to obtain precise peak energies, all the observed spectra shown in Fig. 2a–c are decomposed by using the Gaussian distribution function, as shown in Fig. 2a–c by the dashed lines. Peak energies thus obtained are summarized in Table 1. The energies of the most intense peaks in these spectra, i.e., peak A, are shifted by a change in the chemical environment of Si of interest, which is the so-called chemical shift of Auger lines, although these spectra are not core Auger lines, but valence Auger lines. Fig. 3 shows the relationship between the energy of the main peak, peak A, and the calculated Mulliken charge on the central Si atom in the model cluster. As energy calibrations are always necessary for insulating samples, special care must be taken for the calibrations when we discuss the chemical shift of Auger lines. In the present work, good linearity is obtained between the energy of peak A and the Mulliken charge as shown in Fig. 3, which has Table 1 Peak energies and relative energies of the observed Si KL2,3 V Auger spectra of Si, SiC and SiO2

Si SiC SiO2

EB

1734.9 1732.0 1723.6

1727.8 1723.4 1710.6

13

14

Fig. 3. Relationship between calculated total number of electrons on the central Si atom in the model cluster and experimental energy shift, EA , of the most intense peak (peak A).

1740

Fig. 2. Observed Si KL2,3 V Auger spectra of (a) Si; (b) SiC; and (c) SiO2 after subtracting the background and plasmon-loss spectra from the spectra shown in Fig. 1.

EA

-10

Total number of electrons on central Si

Energy (eV)

Kinetic energy (eV)

-5

12 C

1720

23

Relative energy (eV) EC

EA

EB

EC

1729.3

0 −2.9 −11.3

−7.1 −8.6 −13.0

+5.7

These values are obtained by deconvoluting the observed spectra with the Gaussian distribution function after subtracting the background and plasmon-loss spectra. EA , EB and EC are the energies of peaks A, B and C, respectively. EA is the relative energy of peak A to that of Si, and EB and EC are the relative energies of peaks B and C to that of peak A of each material.

implications for the chemical state analysis by means of the chemical shift in the valence Auger spectrum, although we have only three points to plot in the present study. The theoretical profile, A(E), of the Si KL2,3 V Auger spectrum is expressed by the combined use of the calculated partial density of state of Si 3s and 3p, D3s (E) and D3p (E), respectively, and the atomic Auger transition probabilities for 1s2p3s and 1s2p3p transitions, P1s2p3s and P1s2p3p , respectively, for the Si atom as A(E) = P1s2p3s D3s (E) + P1s2p3p D3p (E)

(1)

Here we calculated PDOS, D3s (E) and D3p (E), with the DV-X␣ code [16] and the transition probabilities, P1s2p3s and P1s2p3p , were taken from [16], in which the ratio between these transition probabilities, i.e., P1s2p3s /P1s2p3p , is 0.23. These transition probabilities were calculated within the first-principles theory by using relativistic Dirac–Hartree– Slater wave functions [20]. Theoretical Auger profiles of Si, SiC and SiO2 are shown in Fig. 4a–c, respectively, together with the experimental ones and in Fig. 5a–c, in which Si 3s and 3p contributions to the theoretical spectra are indicated. As shown in these figures, the experimental spectral lineshapes are qualitatively well reproduced by the present calculations. In the case of the Si KL2,3 V Auger spectra of the series of Si compounds employed here, 3p PDOS is dominant and the 3s contribution to the spectral lineshapes is relatively smaller, as shown in Fig. 5. The results for metallic Si show a spectral shape similar to the recent ab initio calculations done with the Green’s function formalism by Chang and Shirley [14], although they considered the angular momentum of the Auger electrons. The experimental energy separation between peaks A and B, i.e., EB in Table 1, becomes larger in the order of Si, SiC and SiO2 , which is also well reproduced by the present calculations as shown in Fig. 4. Based on the final state rule for the CCV Auger transitions [2–6], the core–hole effect should be taken into account. Therefore, we also carried out calculations that included core–hole effects. In the case of the Si KL2,3 V Auger

T. Yamamoto et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 21–25

B expt. calc. (gs) calc.(fs)

Intensity (arb. unit)

1700

1720 Energy (eV)

(b)

1740

A

SiC B

expt. calc.(gs) calc.(fs)

1700

Intensity (arb. unit)

transition, the core–hole should be put on the Si 2p orbital under the final state rule [2–6]. Then, the PDOS of Si 3s and 3p components with the Si 2p core–hole are calculated in a same manner as for the case without the core–hole. Resultant theoretical Si KL2,3 V Auger spectra are shown in Fig. 4 together with the experimental data and calculated results without the core–hole and in Fig. 5 to indicate Si 3s and 3p contributions to the theoretical spectra. As shown in Fig. 4c, the satellite peak C on the higher energy side of the main peak B appeared in the case of SiC and SiO2 , when the core–hole effect was taken into account. This strongly supports the experimental results for SiO2 , although no clear satellite peak is seen in the experimental spectrum of SiC to compare with our present calculations. As shown in Figs. 4 and 5, slight changes are observed between the calculated spectra with and without the core–hole, although an earlier report [21] showed striking core–hole effects on the valence band DOS of metallic Si. However, in that work an empirical method was used to introduce the core–hole [21], whereas the core–hole is directly introduced in our present calculations, i.e., by full first-principles calculations. Our present first-principles calculations suggest that the core–hole effects on the electronic structure of the valence band of the compounds employed here are not significant. However, there still remain small discrepancies between the experimental and calculated spectral shapes, i.e., the calculated spectra show (a) a smaller intensity of peak B in Si and (b) a larger intensity of peak B in SiO2 , even after the core–hole effects are taken into account. In the present calculations, we employed Mulliken’s population analysis method to obtain the PDOS of Si 3s and 3p components, which may have caused the discrepancies noted here. Further calculations, using a more accurate method of calculating the PDOS, are necessary to obtain full agreement between the experimental and theoretical lineshapes.

A

Si

(a)

(c)

1720 Energy (eV)

1740

A

SiO2 B

C

expt. calc.(gs) calc.(fs)

1700

1720 Energy (eV)

1740

Fig. 4. Comparison of experimental and calculated Si KL2,3 V Auger spectra of (a) Si; (b) SiC; and (c) SiO2 . Shown from top to bottom are the experimental and calculated spectra with and without the core–hole. The experimental spectra are the same ones as in Fig. 2. Energies of the calculated spectra are shifted so as to coincide with the experimental spectra.

B

3p 3s

with core-electron-hole

without core-electron-hole

A

A

Intensity (arb. unit)

Intensity (arb. unit)

without core-electron-hole

without core-electron-hole

A

A

B

B

Intensity (arb. unit)

Intensity (arb. unit)

24

3p 3s with core-electron-hole A

(a)

-20 0 Relative MO energy (eV)

3s

(b)

with core-electron-hole

A

3p

3p

3s

3s

B

B

B 3p

3p

C

-20 0 Relative MO energy (eV)

3s

(c)

C

-20 0 Relative MO energy (eV)

Fig. 5. Calculated Si 3s (dotted line) and 3p (dashed line) components in the Si KL2,3 V Auger spectra of (a) Si; (b) SiC; and (c) SiO2 . The solid line is the sum of these two. The energy of the theoretical spectrum is the relative molecular orbital (MO) energy to the highest occupied molecular orbital (HOMO).

T. Yamamoto et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 21–25

5. Conclusions

[3] [4] [5] [6]

High-resolution Si KL2,3 V Auger spectra of metallic Si, SiC and SiO2 were measured. The valence level chemical shift of the most intense line of a series of Si KL2,3 V Auger spectra was found and was well explained by the calculated total charges of the Si atom of interest. Significant differences between the spectral lineshapes of these spectra were observed, which were qualitatively reproduced by our present first-principles calculations without any consideration of core–hole effects. Calculated spectra with the core–hole showed better agreement with the experimental spectra than those without, especially in the case of SiO2 . This type of measurement and the first-principles calculations for CCV Auger spectrum can be a powerful tool for direct analysis of the valence band electronic structure of the surface and of materials adsorbed on the surface.

[10]

Acknowledgements

[16]

The authors would like to thank Drs. Y. Shichi and Y. Inoue of NISSAN ARC LTD. for their fruitful discussions. This work was partially supported by the Computational Materials Science Unit in Kyoto University by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] D.E. Ramaker, Crit. Rev. Solid State 17 (1991) 211. [2] D.E. Ramaker, Phys. Rev. B25 (1985) 7341.

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