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Kβ-to-Kα X-ray intensity ratio studies on the changes of valence electronic structures of Ti, V, Cr, and Co in their disilicide compounds
Nuclear Instruments and Methods in Physics Research B 152 (1999) 417±424
www.elsevier.nl/locate/nimb
Kb-to-Ka X-ray intensity ratio studies on the changes of valence electronic structures of Ti, V, Cr, and Co in their disilicide compounds S. Raj a, H.C. Padhi
a,*
, D.K. Basa b, M. Polasik c, F. Pawøowski
c
a
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India Department of Physics, Utkal University, Bhubaneswar 751004, India Faculty of Chemistry, Nicholas Copernicus University, 87-100 Toru n, Poland b
c
Received 15 October 1998; received in revised form 16 February 1999
Abstract Kb-to-Ka X-ray intensity ratios of Ti, V, Cr, and Co in pure metals and their disilicide compounds have been measured following excitation by 59.54 keV c-rays from a 200 mCi 241 Am point-source. The Kb-to-Ka intensity ratios of all these metals in the disilicide compounds are found to be less than the corresponding ratios for pure metals. Comparison of the measured Kb-to-Ka intensity ratios for the disilicides and pure metals with the multicon®guration Dirac±Fock calculations indicates increase of the 3d electron populations of Ti, V, Cr, and Co in the disilicides from their pure metal values suggesting the rearrangement of electrons between 3d and 4s states of the individual metal atom. This rearrangement is found to be opposite to that observed in our previously reported work on NiSi2 and CuSi2 . Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 32.70.Fw; 32.30.Rj; 32.80.Hd; 31.20.ÿd
1. Introduction The current interest in the valence electronic structure studies of Ti, V, Cr and Co in their disilicide compounds stems from the fact that the transition-metal silicides in the form of metal± semiconductor interfaces have a large number of applications in semiconductor device technology.
* Corresponding author. Tel.: 91-674-5881722,59118; fax: 91-674-581142; e-mail: [email protected]
To better understand the chemical bonding at the interface and to identify potential structural and electronic dierences between bulk silicides and these silicide-like phases, one must examine both bulk and interface silicides. Although spectroscopic techniques such as Auger electron spectroscopy [1,2], ultraviolet photoelectron spectroscopy [3±5] and X-ray photoemission spectroscopy [6] have been used to investigate the basic nature of chemical bond in these complex silicide compounds, a lot more has to be done to understand the charge transfer and electronic
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 2 2 5 - 6
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S. Raj et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 417±424
con®guration rearrangement processes for the transition metals in their compounds. In a number of X-ray spectral studies of 3d transition metals it has been observed that the Kbto-Ka X-ray intensity ratios are dependent on the physical and chemical environments of the metals in the sample. In the earlier studies of 3d transition metal compounds [7±18] the in¯uence of chemical eects has shown dierences in the Kb-to-Ka ratios up to nearly 10%. Such chemical eects can be caused either by a varying 3d electron population or by the admixture of 3p orbitals from the ligand atoms to the 3d orbitals of the metal or both. The main point of the studies presented in this paper has been to show that the in¯uence of the chemical eect on Kb-to-Ka X-ray intensity ratios can be observed and used as a sensitive tool to study the changes of the electronic con®guration of the 3d transition metals in their disilicides. The change of the 3d electron population of the transition metal atom in the disilicide compound modi®es 3p orbitals of the atom stronger than 2p orbitals, what must be followed by the change of the Kb-to-Ka X-ray intensity ratio of the metal atom in the compound. Therefore we have measured the Kb-to-Ka X-ray intensity ratios of Ti, V, Cr, and Co in pure metals and their disilicide compounds. Comparing these results with the multicon®guration Dirac±Fock (MCDF) calculations one can evaluate the changes of the valence electronic con®guration of these 3d transition metals in disilicides, which could provide information on probable mechanisms responsible for these changes. The theoretical calculations presented in this paper have been done using the atomic MCDF package developed by Grant and coworkers [19,20]. The methodology of the calculations used is similar to the one published earlier by Jankowski and Polasik [21] which was found to be quite successful in explaining the experimental results for the Kb-to-Ka ratios of 3d transitionmetals by Perujo et al. [22]. 2. Experimental details The experiments were carried out using high purity disilicide compounds (in powder form)
procured from Alpha, a Johonson Matthey, UK. The powder material is pelletized into the size of 10 mm dia 3 mm thick for ®nal use in the experiments. The pure metal samples in the form of thick discs are procured from Goodfellow, UK. Gamma rays of 59.54 keV from a 200 mCi 241 Am point-source have been used to ionize the target atoms and the emitted X-rays were detected by a 30 mm2 3 mm thick Canberra Si(Li) detecter having a 12.7 lm thick beryllium window. The resolution of the Si(Li) detector was 165 eV [full width at half maximum (FWHM)] for a 5.9 keV X-ray peak. Details of the experimental arrangements can be found in an earlier paper by Bhuinya and Padhi [23]. Pulses from the Si(Li) detector preampli®er were fed to an ORTEC-572 spectroscopy ampli®er and then recorded in a Canberra PC based Model S-100 multichannel analyzer. The gain of the system was maintained at 16 eV/channel. A typical K X-ray spectrum of Ti in TiSi2 is shown in Fig. 1.
Fig. 1. A typical K X-ray spectrum of Ti in TiSi2 . The open circles correspond to experimental data, the continuous curve corresponds to the ®tted spectrum and the dashed line represents the ®tted background.
S. Raj et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 417±424
l=q
3. Data analysis
X i
All the X-ray spectra were carefully analyzed with a multi-Gaussian least-square ®tting programme using a non-linear background subtraction. No low energy tail was included in the ®tting as its contribution to the ratio was shown to be quite small [24]. The Kb-to-Ka intensity ratios were determined from the ®tted peak areas after applying necessary corrections to the data. Corrections to the measured ratios mainly come from the dierence in the Ka and Kb self attenuations in the sample, dierence in the eciency of the Si(Li) detector and air absorption on the path between the sample and the Si(Li) detector window. The eciency of the detector is estimated theoretically as mentioned in an earlier paper by Bhuinya and Padhi [25]. Our theoretically estimated eciency was shown to be in good agreement with the measured eciency [26] and at the energy region of present interest the discrepancy between them was found to be quite small. The self attenuation correction in the sample and the absorption correction for the air path are determined as per the procedure described before [25]. For the estimation of these corrections we have used the mass attenuation coecients compiled in a computer programme XCOM by Berger and Hubbell [27]. The mass attenuation coecients for the compounds are estimated using the elemental values in the following Bragg's-rule formula [28]
419
wi li =qi ;
1
where wi is the proportion by weight of the ith constituent and li =qi is the mass attenuation coecient for the ith constituent in the compound. The systematic errors arising from various instrumental corrections are expected to move in the same direction for the absolute values of Kb-toKa X-ray intensity ratios for 3d transition metals in the disilicide compounds and pure metals. Therefore they should cancel to a large extent when we consider just the observation of the changes between these values. Thus, correction errors should not in¯uence the credibility of the measured chemical eect (on Kb-to-Ka X-ray intensity ratios), which in fact is observed from these changes. So only the statistical errors connected with the nature of the searched phenomenon must be presented to show that the in¯uence of the chemical eect on Kb-to-Ka X-ray intensity ratios can really be observed. That is why the errors quoted for the results given in Table 1 are statistical only. They are calculated by the least-square ®tting programme [29]. 4. Theoretical calculations The Kb-to-Ka ratios for Ti, V, Cr and Co have been theoretically calculated using the MCDF method originally developed by Grant and coworkers and is described in detail in several papers
Table 1 Kb-to-Ka X-ray intensity ratios of Ti, V, Cr and Co in pure metals and disilicide compounds and the normalized Kb-to-Ka ratios with respect to the pure metals. The quoted errors correspond to counting statistics in the measurements Element
Chemical constitution
Kb-to-Ka intensity ratios
Normalized Kb-to-Ka intensity ratios w.r.to the pure metal
Observed increase in the number of 3d electrons
22
Ti
23
V
24
Cr
27
Co
Ti TiSi2 V VSi2 Cr CrSi2 Co CoSi2
0:1265 0:0006 0:1241 0:0005 0:1312 0:0008 0:1298 0:0005 0:1314 0:0008 0:1259 0:0005 0:1335 0:0008 0:1310 0:0005
1.0 0:981 0:009 1.0 0:989 0:010 1.0 0:958 0:010 1.0 0:981 0:010
± 0:7 0:3 ± 0:4 0:3 ± 1:6 0:3 ± 1:0 0:4
420
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[19,20,30±34]. Moreover, all basic ideas of the alternative SAL version of MCDF calculations, which is used in this work, have been presented by Jankowski and Polasik [21]. However, for the sake of clarity, some essential details are very brie¯y recapitulated below. The Hamiltonian for the N-electron atom is taken in the form H
N X
hD i
i1
N X
Cij ;
2
j>i1
where hD i is the Dirac operator for ith electron and the terms Cij account for electron±electron interactions and come from one-photon exchange processes. The latter are a sum of the Coulomb interaction operator and the transverse Breit operator. The atomic state functions with the total angular momentum J and parity p are represented in the multicon®gurational form X cm sU cm J p ; 3 Ws J p m
where U cm J p are con®guration state functions (CSF's), cm s are the con®guration mixing coecients for state s, cm represents all information required to uniquely de®ne a certain CSF. For more details on the MCDF calculations reference may be made to an earlier paper by Jankowski and Polasik [21]. The calculations have been performed for both the Coulomb and Babushkin gauges [35,36].
5. Results and discussion The experimental results for the Kb-to-Ka Xray intensity ratios of Ti, V, Cr and Co for the case of pure metals and in disilicides and the normalized Kb-to-Ka ratios for these metals in disilicides (with respect to the pure metals) are presented in Table 1. As can be seen from this table, the measured Kb-to-Ka intensity ratios for all the disilicide compounds are smaller than those corresponding to pure metal values with minimum change (about 1%) observed for VSi2 and maximum (about 4%) for CrSi2 . The changes of the values of the Kb-to-Ka X-ray intensity ratio for the 3d transition-metal in TiSi2 , VSi2 , CrSi2 and CoSi2 are opposite to those of our earlier results for NiSi2 and CuSi2 [37]. The decrease in the measured Kb-to-Ka ratio is attributed to the increase of 3d electron population of the metal. The changes in the 3d electron population for Ti, V, Cr and Co in disilicides (with respect to those for the pure metals) have been evaluated by comparing the predictions of MCDF calculations with the measured Kb-to-Ka intensity ratios for these 3d transition metals in their disilicides and for pure metals. The results of MCDF calculations on Ti, V, Cr and Co for various valence electronic con®gurations of the 3dmÿr 4sr for r 2; 1; 0) type are presented in Table 2. Moreover in Fig. 2 the results of MCDF calculations have been compared
Table 2 The theoretical MCDF Kb-to-Ka intensity ratios of Ti, V, Cr and Co corresponding to various valence electronic con®gurations Element
Z
Ti
22
V
23
Cr
24
Co
27
Electronic con®guration
3d2 4s2 3d3 4s1 3d4 3d3 4s2 3d4 4s1 3d5 3d4 4s2 3d5 4s1 3d6 3d7 4s2 3d8 4s1 3d9
The Kb-to-Ka intensity ratios Coulomb gauge
Babushkin gauge
0.1308 0.1262 0.1230 0.1322 0.1280 0.1251 0.1333 0.1295 0.1268 0.1356 0.1326 0.1304
0.1334 0.1291 0.1259 0.1345 0.1306 0.1276 0.1354 0.1317 0.1289 0.1370 0.1340 0.1318
S. Raj et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 417±424
Fig. 2. Comparison of the experimental Kb=Ka X-ray intensity ratios for Ti, V, Cr and Co in disilicides and for pure 3d metals with the predictions of MCDF calculations with the results of MCDF calculations for dierent valence electronic con®guration of these 3d transition metals. ``(C)'' denotes Coulomb gauge results; ``(B)'' denotes Babushkin gauge results.
with the measured Kb-to-Ka intensity ratios for Ti, V, Cr and Co in their disilicides and for pure metals. In each case the Coulomb and Babushkin gauge formulae for the electric dipole transitions have been used. It can be noticed that, although
421
the absolute values of the Kb-to-Ka intensity ratios obtained using the Coulomb and Babushkin gauges are quite dierent (see Table 2 and Fig. 2), the changes of the values of the Kb-to-Ka intensity ratio as a result of transition from electronic con®guration of the one type to the other are almost the same. From the present study, two things seem to be very attractive: (i) determining the probable mechanisms responsible for changes of 3d electron populations of Ti, V, Cr and Co metals in their disilicides; (ii) evaluating the absolute electronic con®gurations of these metals in disilicides. The evaluated changes in the number of 3d electrons for Ti, V, Cr and Co in disilicides (presented in Table 1) can be used to estimate the absolute con®gurations of the 3d transition metal in disilicide compounds, if we assume some con®gurations for Ti, V, Cr and Co in pure metal. The electronic con®gurations of the 3d transition metals in their disilicides obtained by assuming the 3dmÿ1 4s1 type con®guration for the pure metal [38,39] (with one exception for Cr, where we have assumed two possibilities) are given in Table 3. Our analysis (based on comparing the measured Kb-to-Ka intensity ratios for the disilicides and pure metals with the predictions of MCDF calculation) indicates that the 3d electron population of Ti in TiSi2 is evidently higher than that of pure Ti metal; the increase of the number of 3d electrons is 0:7 0:3 (see last column of the Table 1). Using this increase of the number of 3d electrons and assuming the 3d3 4s1 con®guration for Ti in a pure metal (as predicted by Berggren [38] and Berggren et al. [39]), we have estimated that the probable electronic con®guration of Ti in TiSi2 is
Table 3 The estimated electronic con®gurations for Ti, V, Cr and Co in disilicides assuming the 3dmÿ1 4s1 type of electronic con®gurations for Ti, V and Co in pure metals and two probable (3d5 4s1 and 3d4 4s2 ) con®gurations for Cr Element
Z
Assumed electronic con®guration in pure metal
Increase of the number of 3d electron
Predicted electronic con®guration in disilicides
Ti V Cr
22 23 24
Co
27
3d3 4s1 3d4 4s1 3d5 4s1 3d4 4s2 3d8 4s1
0:7 0:3 0:4 0:3 1:6 0:3 1:6 0:3 1:0 0:4
3d3:70:3 4s0:30:3 3d4:40:3 4s0:60:3 3d6:60:3 3d5:60:3 4s0:40:3 3d9:00:4 4s0:00:4
422
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3d3:70:3 4s0:30:3 , hence almost all the valence electrons of titanium are in the 3d subshell. This enhanced d electron population will give rise to enhanced metal±metal dAd bonding in TiSi2 . In an earlier density of states (DOS) study (by Weijs et al. [40]) it was stated that the 4s-like wave functions at the titanium site do not contribute to the occupied valence DOS of TiSi2 , which con®rms our present estimation for the electronic con®guration of Ti in TiSi2 . In the case of VSi2 we can see (from Table 1) that rather small decrease (about 1%) in the Kb-toKa X-ray intensity ratio (with respect to pure V) indicates that the 3d electron population of V in VSi2 is only a bit higher than that of pure V metal (the increase of the number of 3d electrons is 0:4 0:3; see Table 1). Based on this change of the 3d electron population of V in VSi2 and assuming the 3d4 4s1 con®guration for V in a pure metal [38], we have estimated the following electronic con®guration 3d4:40:3 4s0:60:3 for V in VSi2 . In the earlier Compton pro®le study of VSi2 Sharma et al. [41] reported an enhanced 3d electron population for V which should give rise to a decrease in the Kb-to-Ka ratio of vanadium in VSi2 , which is consistent with our present results. The most signi®cant decrease (about 4%) in the Kb-to-Ka X-ray intensity ratio (with respect to pure metal) can be observed in the case of CrSi2 (see Table 1). It is easy to ®nd in this case that the 3d electron population of Cr in CrSi2 is drastically higher than that of pure Cr metal, i.e. the increase of the number of 3d electrons is 1:6 0:3 (see Table 1). In the case of Cr we have taken into consideration two dierent electronic con®gurations of Cr in a pure metal (suggested in previous papers) to estimate the valence electronic con®guration of Cr in CrSi2 (based on the increase of the number of 3d electrons evaluated above). If we assume 3d5 4s1 con®guration for Cr in the pure metal we get the valence electron con®guration 3d6:60:3 for Cr in CrSi2 . On the other hand, 3d4 4s2 con®guration predicted by Bauer et al. [42] for a pure Cr gives 3d5:60:3 4s0:40:3 con®guration for Cr in CrSi2 . The electronic structure of CrSi2 was earlier studied by various authors [43,44]. The photoelectron energy distribution curves have been measured in the photon energy range 16±120
eV. Theoretical calculations [43] have also been made to explain the DOS data. However, these studies have not given the orbital electron population of the valence orbitals of Cr in CrSi2 . In the case of CoSi2 the observed (see Table 1) decrease (about 2%) in the Kb-to-Ka X-ray intensity ratio (with respect to pure Co) indicates that the 3d electron population of Co in CoSi2 is evidently higher than that of pure Co metal (the increase of the number of 3d electrons is 1:0 0:4; see Table 1). Using this increase of the number of 3d electrons and assuming the 3d8 4s1 con®guration for Co in a pure metal [38] we have estimated that the probable electronic con®guration of Co in CoSi2 is 3d9:00:4 4s0:00:4 . As regards to CoSi2 the earlier DOS studies [5,45] did not give any information about the orbital electron population of Co in CoSi2 . Our present study, however, gives a valence electronic con®guration of Co in which the 3d subshell has almost all the valence electrons of Co (i.e. about 9 electrons). This reorganization of electrons leaves the 4s orbital of Co almost empty. Because the chemical environment must be a real cause of the increase of the number of 3d electrons for Ti, V, Cr and Co in their disilicides, we have been trying to determine probable mechanisms responsible for these changes. We have taken into consideration two extreme possibilities: (i) the rearrangement between 3d electrons of individual transition metal atom and 4s electrons and/or (ii) the transfer of the 3p electrons of Si atom to the 3d subshell of transition metal atoms. As can be seen from Table 3 there is no need to account the electron transfer mechanism for Ti, V and Co metals with the 3dmÿ1 4s1 con®guration type assumed [38] and for Cr with the 3d4 4s2 con®guration assumed [42]. Therefore, to explain (with above assumptions) evaluated changes of the 3d electron population for all the analysed transition metals it is enough to consider the rearrangement of valence electrons within a given transition metal atom in disilicide. However, if we assume 3d5 4s1 con®guration for a pure Cr metal [38] we get 3d6:60:3 for Cr in CrSi2 , which suggests a possibility of transfer of 0:6 0:3 electrons from 3p subshell of Si to 3d subshell of Cr. Recently, Takao et al. [46] have presented results of molecular orbital calculations for 3d
S. Raj et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 417±424
transition metal disilicides using the discrete variational (DV) X a method for model disilicide clusters. They have concluded that the 3d transition metals are positively charged in their disilicides for early 3d metals and negatively for later metals. This charge transfer is undoubtedly caused by the dierences of electronegativity between particular 3d transition metal element and a silicon atom. We have found the evident quantitative inconsistency between the charge transfer (predicted by Takao et al. [46]) and the changes of the number of the 3d electrons evaluated by us. Moreover, the direction of this charge transfer is dierent for earlier and later 3d transition metals [46], but on the contrary the changes of the number of 3d electrons (see last column of Table 1) evaluated by us, are always in the same direction (i.e. increase of the number of the 3d electrons for the 3d transition metals in their disilicides). This con®rms that the changes of the number of 3d electrons are not consistent with the charge transfer predicted by Takao et al. [46]. Comparing the present results for the disilicide compounds with our earlier results [37] for NiSi2 and CuSi2 we ®nd opposite behaviour in the 3d electron change for Ni and Cu with the other 3d metals in their disilicide compounds. 6. Conclusions In this paper we have presented the experimental results for the Kb-to-Ka X-ray intensity ratios of Ti, V, Cr and Co in pure metals and their disilicide compounds. Comparing these results with the MCDF calculations we have found signi®cant increase of the 3d electron population for Ti, Cr and Co in their disilicides over the corresponding pure metal population and a small change for V in VSi2 . Generally we have found that to explain evaluated changes of the 3d electron population for all the analysed transition metals in their disilicides it is enough to consider the rearrangement between 3d electrons of individual transition metal atom and 4s electrons. However, if we assume 3d5 4s1 con®guration for a pure Cr metal, [38] the electron transfer from 3p orbitals of Si to 3d orbitals of Cr in CrSi2 seems to
423
be possible. Electrons with 4s character are almost absent in Ti, Cr and Co metals of the disilicide compounds and the electrical properties of these compounds are expected to be solely determined by the 3d state electrons as suggested earlier by Weaver et al. [5]. In this respect VSi2 seems to be somewhat exceptional as it does not follow the systematics shown by other disilicides of the present study. The 3d electron changes of Ti, V, Cr and Co in their disilicide compounds are opposite to those of NiSi2 and CuSi2 . The authors believe that the results of this study will be helpful in better understanding of the dependence of the Kb-to-Ka X-ray intensity ratios on changes in the valence electronic con®gurations of 3d transition metals and throw some light on the valence electronic structure of Ti, V, Cr and Co in disilicide compounds. Moreover, the results of this work can stimulate both the experimental and theoretical research with other 3d transition metal compounds (and alloys). Acknowledgements The authors S. Raj and H.C. Padhi are thankful to Council of Scienti®c and Industrial Research, India for the ®nancial support for the work and M. Polasik is thankful to the Polish Committee for Scienti®c Research (KBN). References [1] A. Roth, C.R. Crowell, J. Vac. Sci. Technol. 15 (1978) 1317. [2] P.S. Ho, T.Y. Tan, J.E. Lewis, G.W. Rublo, J. Vac. Sci. Technol. 16 (1979) 1120. [3] P.S. Ho, G.W. Rublo, J.E. Lewis, V.L. Moruzzi, A.R. Williams, Phys. Rev. 22 (1980) 4784. [4] I. Abbati, L. Braicovich, B. De Michelis, O. Bisi, R. Rovetta, Solid State Commun. 37 (1981) 119. [5] J.H. Weaver, A. Franciosi, V.L. Moruzzi, Phys. Rev. B 29 (1984) 3293. [6] N.W. Cheung, P.J. Grunthaner, F.J. Grunthaner, J.W. Mayer, B.M. Ullrich, J. Vac. Sci. Technol. 18 (1981) 917. [7] E. Arndt, G. Brunner, E. Hartmann, J. Phys. B: At. Mol. Phys. 15 (1982) L887. [8] E. Lazzarini, A.L. Lazzarini-Fantola, M. Mandelli Battoni, Radiochem. Acta 25 (1978) 21. [9] B. Paccimazzili, D.S. Urch, Innershell and X-ray physics of atoms and solids, Plenum press, New York, 1981, p. 741.
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www.elsevier.nl/locate/nimb
Kb-to-Ka X-ray intensity ratio studies on the changes of valence electronic structures of Ti, V, Cr, and Co in their disilicide compounds S. Raj a, H.C. Padhi
a,*
, D.K. Basa b, M. Polasik c, F. Pawøowski
c
a
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India Department of Physics, Utkal University, Bhubaneswar 751004, India Faculty of Chemistry, Nicholas Copernicus University, 87-100 Toru n, Poland b
c
Received 15 October 1998; received in revised form 16 February 1999
Abstract Kb-to-Ka X-ray intensity ratios of Ti, V, Cr, and Co in pure metals and their disilicide compounds have been measured following excitation by 59.54 keV c-rays from a 200 mCi 241 Am point-source. The Kb-to-Ka intensity ratios of all these metals in the disilicide compounds are found to be less than the corresponding ratios for pure metals. Comparison of the measured Kb-to-Ka intensity ratios for the disilicides and pure metals with the multicon®guration Dirac±Fock calculations indicates increase of the 3d electron populations of Ti, V, Cr, and Co in the disilicides from their pure metal values suggesting the rearrangement of electrons between 3d and 4s states of the individual metal atom. This rearrangement is found to be opposite to that observed in our previously reported work on NiSi2 and CuSi2 . Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 32.70.Fw; 32.30.Rj; 32.80.Hd; 31.20.ÿd
1. Introduction The current interest in the valence electronic structure studies of Ti, V, Cr and Co in their disilicide compounds stems from the fact that the transition-metal silicides in the form of metal± semiconductor interfaces have a large number of applications in semiconductor device technology.
* Corresponding author. Tel.: 91-674-5881722,59118; fax: 91-674-581142; e-mail: [email protected]
To better understand the chemical bonding at the interface and to identify potential structural and electronic dierences between bulk silicides and these silicide-like phases, one must examine both bulk and interface silicides. Although spectroscopic techniques such as Auger electron spectroscopy [1,2], ultraviolet photoelectron spectroscopy [3±5] and X-ray photoemission spectroscopy [6] have been used to investigate the basic nature of chemical bond in these complex silicide compounds, a lot more has to be done to understand the charge transfer and electronic
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 2 2 5 - 6
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con®guration rearrangement processes for the transition metals in their compounds. In a number of X-ray spectral studies of 3d transition metals it has been observed that the Kbto-Ka X-ray intensity ratios are dependent on the physical and chemical environments of the metals in the sample. In the earlier studies of 3d transition metal compounds [7±18] the in¯uence of chemical eects has shown dierences in the Kb-to-Ka ratios up to nearly 10%. Such chemical eects can be caused either by a varying 3d electron population or by the admixture of 3p orbitals from the ligand atoms to the 3d orbitals of the metal or both. The main point of the studies presented in this paper has been to show that the in¯uence of the chemical eect on Kb-to-Ka X-ray intensity ratios can be observed and used as a sensitive tool to study the changes of the electronic con®guration of the 3d transition metals in their disilicides. The change of the 3d electron population of the transition metal atom in the disilicide compound modi®es 3p orbitals of the atom stronger than 2p orbitals, what must be followed by the change of the Kb-to-Ka X-ray intensity ratio of the metal atom in the compound. Therefore we have measured the Kb-to-Ka X-ray intensity ratios of Ti, V, Cr, and Co in pure metals and their disilicide compounds. Comparing these results with the multicon®guration Dirac±Fock (MCDF) calculations one can evaluate the changes of the valence electronic con®guration of these 3d transition metals in disilicides, which could provide information on probable mechanisms responsible for these changes. The theoretical calculations presented in this paper have been done using the atomic MCDF package developed by Grant and coworkers [19,20]. The methodology of the calculations used is similar to the one published earlier by Jankowski and Polasik [21] which was found to be quite successful in explaining the experimental results for the Kb-to-Ka ratios of 3d transitionmetals by Perujo et al. [22]. 2. Experimental details The experiments were carried out using high purity disilicide compounds (in powder form)
procured from Alpha, a Johonson Matthey, UK. The powder material is pelletized into the size of 10 mm dia 3 mm thick for ®nal use in the experiments. The pure metal samples in the form of thick discs are procured from Goodfellow, UK. Gamma rays of 59.54 keV from a 200 mCi 241 Am point-source have been used to ionize the target atoms and the emitted X-rays were detected by a 30 mm2 3 mm thick Canberra Si(Li) detecter having a 12.7 lm thick beryllium window. The resolution of the Si(Li) detector was 165 eV [full width at half maximum (FWHM)] for a 5.9 keV X-ray peak. Details of the experimental arrangements can be found in an earlier paper by Bhuinya and Padhi [23]. Pulses from the Si(Li) detector preampli®er were fed to an ORTEC-572 spectroscopy ampli®er and then recorded in a Canberra PC based Model S-100 multichannel analyzer. The gain of the system was maintained at 16 eV/channel. A typical K X-ray spectrum of Ti in TiSi2 is shown in Fig. 1.
Fig. 1. A typical K X-ray spectrum of Ti in TiSi2 . The open circles correspond to experimental data, the continuous curve corresponds to the ®tted spectrum and the dashed line represents the ®tted background.
S. Raj et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 417±424
l=q
3. Data analysis
X i
All the X-ray spectra were carefully analyzed with a multi-Gaussian least-square ®tting programme using a non-linear background subtraction. No low energy tail was included in the ®tting as its contribution to the ratio was shown to be quite small [24]. The Kb-to-Ka intensity ratios were determined from the ®tted peak areas after applying necessary corrections to the data. Corrections to the measured ratios mainly come from the dierence in the Ka and Kb self attenuations in the sample, dierence in the eciency of the Si(Li) detector and air absorption on the path between the sample and the Si(Li) detector window. The eciency of the detector is estimated theoretically as mentioned in an earlier paper by Bhuinya and Padhi [25]. Our theoretically estimated eciency was shown to be in good agreement with the measured eciency [26] and at the energy region of present interest the discrepancy between them was found to be quite small. The self attenuation correction in the sample and the absorption correction for the air path are determined as per the procedure described before [25]. For the estimation of these corrections we have used the mass attenuation coecients compiled in a computer programme XCOM by Berger and Hubbell [27]. The mass attenuation coecients for the compounds are estimated using the elemental values in the following Bragg's-rule formula [28]
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wi li =qi ;
1
where wi is the proportion by weight of the ith constituent and li =qi is the mass attenuation coecient for the ith constituent in the compound. The systematic errors arising from various instrumental corrections are expected to move in the same direction for the absolute values of Kb-toKa X-ray intensity ratios for 3d transition metals in the disilicide compounds and pure metals. Therefore they should cancel to a large extent when we consider just the observation of the changes between these values. Thus, correction errors should not in¯uence the credibility of the measured chemical eect (on Kb-to-Ka X-ray intensity ratios), which in fact is observed from these changes. So only the statistical errors connected with the nature of the searched phenomenon must be presented to show that the in¯uence of the chemical eect on Kb-to-Ka X-ray intensity ratios can really be observed. That is why the errors quoted for the results given in Table 1 are statistical only. They are calculated by the least-square ®tting programme [29]. 4. Theoretical calculations The Kb-to-Ka ratios for Ti, V, Cr and Co have been theoretically calculated using the MCDF method originally developed by Grant and coworkers and is described in detail in several papers
Table 1 Kb-to-Ka X-ray intensity ratios of Ti, V, Cr and Co in pure metals and disilicide compounds and the normalized Kb-to-Ka ratios with respect to the pure metals. The quoted errors correspond to counting statistics in the measurements Element
Chemical constitution
Kb-to-Ka intensity ratios
Normalized Kb-to-Ka intensity ratios w.r.to the pure metal
Observed increase in the number of 3d electrons
22
Ti
23
V
24
Cr
27
Co
Ti TiSi2 V VSi2 Cr CrSi2 Co CoSi2
0:1265 0:0006 0:1241 0:0005 0:1312 0:0008 0:1298 0:0005 0:1314 0:0008 0:1259 0:0005 0:1335 0:0008 0:1310 0:0005
1.0 0:981 0:009 1.0 0:989 0:010 1.0 0:958 0:010 1.0 0:981 0:010
± 0:7 0:3 ± 0:4 0:3 ± 1:6 0:3 ± 1:0 0:4
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[19,20,30±34]. Moreover, all basic ideas of the alternative SAL version of MCDF calculations, which is used in this work, have been presented by Jankowski and Polasik [21]. However, for the sake of clarity, some essential details are very brie¯y recapitulated below. The Hamiltonian for the N-electron atom is taken in the form H
N X
hD i
i1
N X
Cij ;
2
j>i1
where hD i is the Dirac operator for ith electron and the terms Cij account for electron±electron interactions and come from one-photon exchange processes. The latter are a sum of the Coulomb interaction operator and the transverse Breit operator. The atomic state functions with the total angular momentum J and parity p are represented in the multicon®gurational form X cm sU cm J p ; 3 Ws J p m
where U cm J p are con®guration state functions (CSF's), cm s are the con®guration mixing coecients for state s, cm represents all information required to uniquely de®ne a certain CSF. For more details on the MCDF calculations reference may be made to an earlier paper by Jankowski and Polasik [21]. The calculations have been performed for both the Coulomb and Babushkin gauges [35,36].
5. Results and discussion The experimental results for the Kb-to-Ka Xray intensity ratios of Ti, V, Cr and Co for the case of pure metals and in disilicides and the normalized Kb-to-Ka ratios for these metals in disilicides (with respect to the pure metals) are presented in Table 1. As can be seen from this table, the measured Kb-to-Ka intensity ratios for all the disilicide compounds are smaller than those corresponding to pure metal values with minimum change (about 1%) observed for VSi2 and maximum (about 4%) for CrSi2 . The changes of the values of the Kb-to-Ka X-ray intensity ratio for the 3d transition-metal in TiSi2 , VSi2 , CrSi2 and CoSi2 are opposite to those of our earlier results for NiSi2 and CuSi2 [37]. The decrease in the measured Kb-to-Ka ratio is attributed to the increase of 3d electron population of the metal. The changes in the 3d electron population for Ti, V, Cr and Co in disilicides (with respect to those for the pure metals) have been evaluated by comparing the predictions of MCDF calculations with the measured Kb-to-Ka intensity ratios for these 3d transition metals in their disilicides and for pure metals. The results of MCDF calculations on Ti, V, Cr and Co for various valence electronic con®gurations of the 3dmÿr 4sr for r 2; 1; 0) type are presented in Table 2. Moreover in Fig. 2 the results of MCDF calculations have been compared
Table 2 The theoretical MCDF Kb-to-Ka intensity ratios of Ti, V, Cr and Co corresponding to various valence electronic con®gurations Element
Z
Ti
22
V
23
Cr
24
Co
27
Electronic con®guration
3d2 4s2 3d3 4s1 3d4 3d3 4s2 3d4 4s1 3d5 3d4 4s2 3d5 4s1 3d6 3d7 4s2 3d8 4s1 3d9
The Kb-to-Ka intensity ratios Coulomb gauge
Babushkin gauge
0.1308 0.1262 0.1230 0.1322 0.1280 0.1251 0.1333 0.1295 0.1268 0.1356 0.1326 0.1304
0.1334 0.1291 0.1259 0.1345 0.1306 0.1276 0.1354 0.1317 0.1289 0.1370 0.1340 0.1318
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Fig. 2. Comparison of the experimental Kb=Ka X-ray intensity ratios for Ti, V, Cr and Co in disilicides and for pure 3d metals with the predictions of MCDF calculations with the results of MCDF calculations for dierent valence electronic con®guration of these 3d transition metals. ``(C)'' denotes Coulomb gauge results; ``(B)'' denotes Babushkin gauge results.
with the measured Kb-to-Ka intensity ratios for Ti, V, Cr and Co in their disilicides and for pure metals. In each case the Coulomb and Babushkin gauge formulae for the electric dipole transitions have been used. It can be noticed that, although
421
the absolute values of the Kb-to-Ka intensity ratios obtained using the Coulomb and Babushkin gauges are quite dierent (see Table 2 and Fig. 2), the changes of the values of the Kb-to-Ka intensity ratio as a result of transition from electronic con®guration of the one type to the other are almost the same. From the present study, two things seem to be very attractive: (i) determining the probable mechanisms responsible for changes of 3d electron populations of Ti, V, Cr and Co metals in their disilicides; (ii) evaluating the absolute electronic con®gurations of these metals in disilicides. The evaluated changes in the number of 3d electrons for Ti, V, Cr and Co in disilicides (presented in Table 1) can be used to estimate the absolute con®gurations of the 3d transition metal in disilicide compounds, if we assume some con®gurations for Ti, V, Cr and Co in pure metal. The electronic con®gurations of the 3d transition metals in their disilicides obtained by assuming the 3dmÿ1 4s1 type con®guration for the pure metal [38,39] (with one exception for Cr, where we have assumed two possibilities) are given in Table 3. Our analysis (based on comparing the measured Kb-to-Ka intensity ratios for the disilicides and pure metals with the predictions of MCDF calculation) indicates that the 3d electron population of Ti in TiSi2 is evidently higher than that of pure Ti metal; the increase of the number of 3d electrons is 0:7 0:3 (see last column of the Table 1). Using this increase of the number of 3d electrons and assuming the 3d3 4s1 con®guration for Ti in a pure metal (as predicted by Berggren [38] and Berggren et al. [39]), we have estimated that the probable electronic con®guration of Ti in TiSi2 is
Table 3 The estimated electronic con®gurations for Ti, V, Cr and Co in disilicides assuming the 3dmÿ1 4s1 type of electronic con®gurations for Ti, V and Co in pure metals and two probable (3d5 4s1 and 3d4 4s2 ) con®gurations for Cr Element
Z
Assumed electronic con®guration in pure metal
Increase of the number of 3d electron
Predicted electronic con®guration in disilicides
Ti V Cr
22 23 24
Co
27
3d3 4s1 3d4 4s1 3d5 4s1 3d4 4s2 3d8 4s1
0:7 0:3 0:4 0:3 1:6 0:3 1:6 0:3 1:0 0:4
3d3:70:3 4s0:30:3 3d4:40:3 4s0:60:3 3d6:60:3 3d5:60:3 4s0:40:3 3d9:00:4 4s0:00:4
422
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3d3:70:3 4s0:30:3 , hence almost all the valence electrons of titanium are in the 3d subshell. This enhanced d electron population will give rise to enhanced metal±metal dAd bonding in TiSi2 . In an earlier density of states (DOS) study (by Weijs et al. [40]) it was stated that the 4s-like wave functions at the titanium site do not contribute to the occupied valence DOS of TiSi2 , which con®rms our present estimation for the electronic con®guration of Ti in TiSi2 . In the case of VSi2 we can see (from Table 1) that rather small decrease (about 1%) in the Kb-toKa X-ray intensity ratio (with respect to pure V) indicates that the 3d electron population of V in VSi2 is only a bit higher than that of pure V metal (the increase of the number of 3d electrons is 0:4 0:3; see Table 1). Based on this change of the 3d electron population of V in VSi2 and assuming the 3d4 4s1 con®guration for V in a pure metal [38], we have estimated the following electronic con®guration 3d4:40:3 4s0:60:3 for V in VSi2 . In the earlier Compton pro®le study of VSi2 Sharma et al. [41] reported an enhanced 3d electron population for V which should give rise to a decrease in the Kb-to-Ka ratio of vanadium in VSi2 , which is consistent with our present results. The most signi®cant decrease (about 4%) in the Kb-to-Ka X-ray intensity ratio (with respect to pure metal) can be observed in the case of CrSi2 (see Table 1). It is easy to ®nd in this case that the 3d electron population of Cr in CrSi2 is drastically higher than that of pure Cr metal, i.e. the increase of the number of 3d electrons is 1:6 0:3 (see Table 1). In the case of Cr we have taken into consideration two dierent electronic con®gurations of Cr in a pure metal (suggested in previous papers) to estimate the valence electronic con®guration of Cr in CrSi2 (based on the increase of the number of 3d electrons evaluated above). If we assume 3d5 4s1 con®guration for Cr in the pure metal we get the valence electron con®guration 3d6:60:3 for Cr in CrSi2 . On the other hand, 3d4 4s2 con®guration predicted by Bauer et al. [42] for a pure Cr gives 3d5:60:3 4s0:40:3 con®guration for Cr in CrSi2 . The electronic structure of CrSi2 was earlier studied by various authors [43,44]. The photoelectron energy distribution curves have been measured in the photon energy range 16±120
eV. Theoretical calculations [43] have also been made to explain the DOS data. However, these studies have not given the orbital electron population of the valence orbitals of Cr in CrSi2 . In the case of CoSi2 the observed (see Table 1) decrease (about 2%) in the Kb-to-Ka X-ray intensity ratio (with respect to pure Co) indicates that the 3d electron population of Co in CoSi2 is evidently higher than that of pure Co metal (the increase of the number of 3d electrons is 1:0 0:4; see Table 1). Using this increase of the number of 3d electrons and assuming the 3d8 4s1 con®guration for Co in a pure metal [38] we have estimated that the probable electronic con®guration of Co in CoSi2 is 3d9:00:4 4s0:00:4 . As regards to CoSi2 the earlier DOS studies [5,45] did not give any information about the orbital electron population of Co in CoSi2 . Our present study, however, gives a valence electronic con®guration of Co in which the 3d subshell has almost all the valence electrons of Co (i.e. about 9 electrons). This reorganization of electrons leaves the 4s orbital of Co almost empty. Because the chemical environment must be a real cause of the increase of the number of 3d electrons for Ti, V, Cr and Co in their disilicides, we have been trying to determine probable mechanisms responsible for these changes. We have taken into consideration two extreme possibilities: (i) the rearrangement between 3d electrons of individual transition metal atom and 4s electrons and/or (ii) the transfer of the 3p electrons of Si atom to the 3d subshell of transition metal atoms. As can be seen from Table 3 there is no need to account the electron transfer mechanism for Ti, V and Co metals with the 3dmÿ1 4s1 con®guration type assumed [38] and for Cr with the 3d4 4s2 con®guration assumed [42]. Therefore, to explain (with above assumptions) evaluated changes of the 3d electron population for all the analysed transition metals it is enough to consider the rearrangement of valence electrons within a given transition metal atom in disilicide. However, if we assume 3d5 4s1 con®guration for a pure Cr metal [38] we get 3d6:60:3 for Cr in CrSi2 , which suggests a possibility of transfer of 0:6 0:3 electrons from 3p subshell of Si to 3d subshell of Cr. Recently, Takao et al. [46] have presented results of molecular orbital calculations for 3d
S. Raj et al. / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 417±424
transition metal disilicides using the discrete variational (DV) X a method for model disilicide clusters. They have concluded that the 3d transition metals are positively charged in their disilicides for early 3d metals and negatively for later metals. This charge transfer is undoubtedly caused by the dierences of electronegativity between particular 3d transition metal element and a silicon atom. We have found the evident quantitative inconsistency between the charge transfer (predicted by Takao et al. [46]) and the changes of the number of the 3d electrons evaluated by us. Moreover, the direction of this charge transfer is dierent for earlier and later 3d transition metals [46], but on the contrary the changes of the number of 3d electrons (see last column of Table 1) evaluated by us, are always in the same direction (i.e. increase of the number of the 3d electrons for the 3d transition metals in their disilicides). This con®rms that the changes of the number of 3d electrons are not consistent with the charge transfer predicted by Takao et al. [46]. Comparing the present results for the disilicide compounds with our earlier results [37] for NiSi2 and CuSi2 we ®nd opposite behaviour in the 3d electron change for Ni and Cu with the other 3d metals in their disilicide compounds. 6. Conclusions In this paper we have presented the experimental results for the Kb-to-Ka X-ray intensity ratios of Ti, V, Cr and Co in pure metals and their disilicide compounds. Comparing these results with the MCDF calculations we have found signi®cant increase of the 3d electron population for Ti, Cr and Co in their disilicides over the corresponding pure metal population and a small change for V in VSi2 . Generally we have found that to explain evaluated changes of the 3d electron population for all the analysed transition metals in their disilicides it is enough to consider the rearrangement between 3d electrons of individual transition metal atom and 4s electrons. However, if we assume 3d5 4s1 con®guration for a pure Cr metal, [38] the electron transfer from 3p orbitals of Si to 3d orbitals of Cr in CrSi2 seems to
423
be possible. Electrons with 4s character are almost absent in Ti, Cr and Co metals of the disilicide compounds and the electrical properties of these compounds are expected to be solely determined by the 3d state electrons as suggested earlier by Weaver et al. [5]. In this respect VSi2 seems to be somewhat exceptional as it does not follow the systematics shown by other disilicides of the present study. The 3d electron changes of Ti, V, Cr and Co in their disilicide compounds are opposite to those of NiSi2 and CuSi2 . The authors believe that the results of this study will be helpful in better understanding of the dependence of the Kb-to-Ka X-ray intensity ratios on changes in the valence electronic con®gurations of 3d transition metals and throw some light on the valence electronic structure of Ti, V, Cr and Co in disilicide compounds. Moreover, the results of this work can stimulate both the experimental and theoretical research with other 3d transition metal compounds (and alloys). Acknowledgements The authors S. Raj and H.C. Padhi are thankful to Council of Scienti®c and Industrial Research, India for the ®nancial support for the work and M. Polasik is thankful to the Polish Committee for Scienti®c Research (KBN). References [1] A. Roth, C.R. Crowell, J. Vac. Sci. Technol. 15 (1978) 1317. [2] P.S. Ho, T.Y. Tan, J.E. Lewis, G.W. Rublo, J. Vac. Sci. Technol. 16 (1979) 1120. [3] P.S. Ho, G.W. Rublo, J.E. Lewis, V.L. Moruzzi, A.R. Williams, Phys. Rev. 22 (1980) 4784. [4] I. Abbati, L. Braicovich, B. De Michelis, O. Bisi, R. Rovetta, Solid State Commun. 37 (1981) 119. [5] J.H. Weaver, A. Franciosi, V.L. Moruzzi, Phys. Rev. B 29 (1984) 3293. [6] N.W. Cheung, P.J. Grunthaner, F.J. Grunthaner, J.W. Mayer, B.M. Ullrich, J. Vac. Sci. Technol. 18 (1981) 917. [7] E. Arndt, G. Brunner, E. Hartmann, J. Phys. B: At. Mol. Phys. 15 (1982) L887. [8] E. Lazzarini, A.L. Lazzarini-Fantola, M. Mandelli Battoni, Radiochem. Acta 25 (1978) 21. [9] B. Paccimazzili, D.S. Urch, Innershell and X-ray physics of atoms and solids, Plenum press, New York, 1981, p. 741.
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