Studies of binding energies of various components in bismuthbasedcuprate superconductors through secondary ion mass spectrometry and xray photoelectron spectroscopyP. Rajasekar, N. Ray, S. D. Dey, S. K. Bandyopadhyay, P. Barat et al. Citation: J. Appl. Phys. 77, 343 (1995); doi: 10.1063/1.359327 View online: http://dx.doi.org/10.1063/1.359327 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v77/i1 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Studies of binding energies of various components in bismuth-based cuprate superconductors through secondary ion mass spectrometry and x-ray photoelectron spectroscopy

P. Rajasekar, N. Ray, and S. D. Dey Saha Institute of Nuclear Physics, l/AI$ Bidhannagar, Calcutta-700064, India

S. K. Bandyopadhyay, P. Barat, and Pintu Sen Variable Energy Cyclotron Centre, 1JM Bidhannagar; Calcutta 700064, India

P. Chakrabortya) and F. Caccavale Universita di Padova, Dipartimento di Fisica, Ka Marzolo 8, 35131 Padova, Italy

R. Bertoncello Universita di Padova, Dipartimento di Chemica Inorganica, Metallorganica ed Analitica, Via Loredan 4, 35131 Padova, Italy :

(Received 2 August 1994; accepted for publication 16 September 1994)

Surface analysis of bismuth-based high-T, superconductors, like BizSrzCaCuzOs+8 (Bi-2212) and E%WWWho+s ( Bi-2223) has been carried out by secondary ion mass spectrometry and x-ray photoelectron spectroscopy techniques. The results have been compared with that of similar measurements on BizOs, CuO, and CaCO, samples so as to have detailed information about the surface binding energy as well as the chemical nature of the various individual components that exist inside these high-T, compounds. 0 1995 American Institute of Physics.

I. INTRODUCTION

The high-T, cuprates have some common characteristics-a conducting Cu02 layer and a charge res- ervoir layerrW4 (like Bi-0 in Bi-based cuprates). Knowledge of the binding energy of Cu-0, Bi-0, etc., is very important to have an understanding of the mechanism of the conduc- tion.

Secondary ion-mass spectrometry (SIMS) measurements on the kinetic energy distributions of the various secondary ion species, emitted from a sample under ion bombardment provide the surface binding energies of the individual com- ponents of the sample material. A number of papers,‘-lo in- cluding a reviewr’ on secondary ion emission from various high-T, superconductors, was reported earlier.

Among the other characterization techniques, a spectro- scopic method like x-ray photoelectron spectroscopy (XPS) has been widely utilized in this field. It is a powerful tool for exploring the nature of bonding in molecules and also is a direct source of information on the energy and nature of molecular orbitals of a given compound. Several comprehen- sive papers on this aspect were cited in the review paper.”

From the present study, in a continuation of our earlier studies on SIMS of high-T, materials,778710’11 information about the surface binding energies and the chemical compo- sitions of the various components in the samples Bi-2212 and Bi-2223 have been extracted through SIMS and XPS measurements of these samples. Similar measurements on samples like CuO, BizO,, and CaCO, have also been per- formed so as to compare their results with that of Bi-2212 and Bi-2223 in order have more ‘detailed information about the structure of these two superconducting compounds.

%n leave from Saha Institute of Nuclear Physics, Calcutta, India.

II. EXPERIMENT Polycrystalline samples of Bi-2212 and Pb-doped Bi-

2223 were prepared starting from the nitrates of Bi, Sr, Ca, and Cu by usual solid state reaction.12’13 B&O,, CaCO,, and CuO were pelletized and sintered at 850 “C before they were used as targets.

The dc resistances for the Bi-2212 and Bi-2223 samples were measured by the four-probe method and T, were found to be 65 and 112 K, respectively.

The energy distribution of secondary ions, emitted from the various samples, were measured using a CAMECA ims 4, ion microscope. A 14.5 keV Csf primary beam (30 nA), focused to about 10 pm in diameter, was rastered over an area of 250 pmX250 ,um and the secondary negative ions, emitted from a central circular area (20 ,um diam), were collected by the spectrometer.

The spectrometer is a double focusing system which em- ploys an electrostatic sector in combination with a magnetic prism. The achromatic mass separation is achieved through an electrostatic lens, termed “energy lens.” The energy width of the secondary ions is changed by varying the open- ing aperture of the energy lens. The resolution of the second- ary ion energy spectrum is directly related to the energy ac- ceptance of the mass spectrometer which in the present case was set at 5 eV. The energy distributions of secondary ions were obtained by varying the sample potential from 4625 to 4375 V around the nominal value, i.e., 4500 V. (This is equivalent to same as varying the target voltage from -125 to +125 V when the sample is maintained at ground poten- tial.)

However, the above procedure does not allow to make energy distribution measurements possible for the insulating samples (such as B&O, and CaCOa in the present case), be- cause of the surface-charge build up on these samples due to

J. Appl. Phys. 77 (i), 1 January 1995’ ,, 0021-8979/95/77(1)1343/7/$6.00 Q 1995 American institute of Physics 343

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primary beam impact, causing the samples with nonequipo- tential surfaces. The surface-charge compensation on a sample is made by employing an electron flood gun which operates only if the sample surface rema@s at a constant (nominal) voltage, enabling the electron flood to reach the sample surface with almost zero energy. Energy distributions for both secondary positive and negative ions were measured for Bi-2212 and Bi-2223 samples, but for CuO sample, the distributions of only secondary negative ions were made.

As has already been mentioned the secondary negative ions were detected using a 14.5 keV Cs+ primary beam, whereas the secondary positive ions were detected using 0; primary ions (8 keV), keeping all experimental parameters unchanged, but in the latter case, however, oxygen and any oxide species were not monitored because of the possible interference of these species with probing oxygen ions. The working pressure in the target chamber during analysis was 8X lo-‘a Torr.

In XPS experiments, the excitation energy is high enough to ionize inner electron shells as well, and therefore, information about the core electrons may be obtained in ad- dition to the valence-band spectra, the important information being the binding energy E, (always measured with respect to the Fermi level EF) of a core electron related to an atom.

XPS spectra were recorded ‘in a Perkin Elmer PHI 5600-U spectrometer equipped with a spherical capacitor analyzer, using nonmonocmomatized Mg Ka radiation (1253.6 eV). The working pressure was less than 1X10m9 Torr. The spectrometer was calibrated by assuming the bind- ing energy (BE) of the Au4fTn line at 83.9 eV with respect to the Fermi level. In order to avoid surface charging an electron flood gun was employed. As an internal reference for these charging effects, the Cls peak of hydrocarbon con- tamination has been assumed at 284.8 eV.r4

Survey scans (58 eV pass energy, 1 eV step; 0.5 s step-’ dwell time) were obtained in the BE range between 0 and 1100 eV. Detailed scans were recorded for the individual peak regions in the high-resolution mode (23 eV pass energy, 0.1 eV step; 1.0 s step-’ dwell timej.

3 3

u Tl F

1 o6

1 o5

1 o4

1 o3

102

10’ 40 60

Energy (eV)

FIG. 1. Kinetic energy distribution of secondary ions emitted from FIG. 3. Kinetic energy distribution of secondary ions emitted from Cs+-bombarded Bi-2212. Csf-bombarded CuO.

c ‘3

Ti .- >

Sample Bi-2223 (

-. - ~0‘ -~9- Bi

- BiO

Energy (eV)

FIG. 2. Kinetic energy distribution of secondary ions emitted from Cs+-bombarded Bi-2223.

The standard deviation in the BE values of the XPS lines is 0.15 eV. After a Shirley-type background subtraction,” the raw spectra were fitted using a nonlinear least-square fitting program adopting Gaussian-Lorentzian peak shapes.16 The atomic compositions were evaluated using sensitivity factors as determined from theoretical photoionization cross sections and asymmetry parameters calculated using the Hartree- Fock-Slater one electron central potential model.17

Ill. RESULTS AND DISCUSSION

A. SIMS analysis

The kinetic energy distributions for secondary negative ions are presented in Figs. l-3. The intensities of secondary ions (shown in logarithmic scale) have been replotted in lin- ear scale (not shown) and then from the best fitting of the experimental curves on a computer, the true nature of distri- butions have been determined. Table I shows how the high- energy tails of the energy distributions for various species are characterized.

As seen from the Table I, the high-energy part of the energy distribution for the individual species emitted from

Sample CuO

106~--‘---‘---T-i-;. n ‘,-’ -‘-6. to5

104

1 OS

102

10’ 0 20 40 60 80 100

. . Energy (eV)

344 J. Appl. Phys., Vol. 77, No. ‘I, I January 1995 Rajasekar et a/.

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TABLE I. The high-energy dependence, E-“, of the energy distributions-of secondary ions emitted from Cs+-bombarded Bi-2212, Bi-2223, and CuO (from SIMS).

Sample Emitted species Yield dependence

in the high-energy tail

Bi-2212

Bi-2223

CUO

0- N(E)=E-= Cu-, Bi- IV(E)~E-~.~ Sr- N(E)KE-~.~ CUT LV(E)~~E-‘~~ BiO-, CuO-, CaO-, SrO- N(EpE-3.4 0- N(EpE-‘,7 Cum, Bi- N(E).=E-*.’ Sr- LV(.E)~E-~~~ al; N(E)=E-3.’ BiO-, CuO-, CaO-, SrO- N(EpE-3.0 o-, cu- N(E)KE-~.~ al,) cuo- N(E)c+Y-~ cue;’ N(E)wP.4 cue; N(E)=E-” (where n>7)

various targets is well described by the E-’ dependence, as predicted by the relation N(E)=El(E + U)3,‘8 based on the linear collision cascade. The above formula for secondary ion yield predicts a Em2 dependence for large E and a maxi- mum at E,,i = Uf 2, where U is the surface binding energy or the heat of atomization Ha.

However, it is found for oxides, in particular, that the surface binding energy is less than the heat of atomization. We, for the present case, consider the approximate relation E,zz-= U for the compounds, as proposed by Kosyachkov,” as it seems to be reasonably acceptable for a qualitative descrip- tion of surface binding energies for the high-T, supercon- ducting compounds.” Therefore, from the energy distribu- tions (shown in Figs. l-3), as measured by the SIMS process, surface binding energies of the various emitted spe-

ties can be roughly estimated from the respective peak po- sitions (E,) of those species.

As seen from Figs. 1 and 2, the E,n of Sr- in case of Bi-2223 is shifted towards lower energy than in case of Bi- 2212. This is in consonance with the fact that in Bi-2223 the occurrence of anti-site disorder (Bi occupying Sr site) is more than that in Bi-2212.

The high-energy dependencies of the type E-” of the ions (Table I) reveal an increase in Vz’ with the number of atoms in the ion, so that the more complex the ion, the higher will be the value of ‘n,’ i.e., the faster the decline in intensity with energy. A steeper fall off is predicted on the assumption that dimers, trimers, and higher-order clusters are produced by the binding of two, three, or more monomer atoms as they leave the surface almost simultaneously and from almost the same point of surface in response to a collision cascade pro- duced by a single incident ion.

In order for clusters to be formed in the sudden release of many atoms, the atoms must have low relative kinetic energies, lower than the cluster binding energy. The statistics of this process lead to predictions of E-4.5 and EM7 tails2’ for dimers and trimers, considering the monomer energy dis- tributions to be E -*, appropriate to a linear collision cascade.

However, the contradictory model which assumes that the emission of small clusters, like dimers, trimers, etc. takes place ‘as such’ from the solid seems to be more likely be- cause of the fact that smaller clusters, created by coalescence outside the surface, will be in highly vibrational excited states unless they undergo collisions with other atoms to re- move their internal energy. The vibrational excitation makes it seem very likely that fragmentation of dimers and trimers would occur, leading to disagreement with the above statis- tics. The distribution of Cu, in all the cases has been found to follow approximately Ee3 dependence instead of Em”.’ dependence and this could be attributed to the above possi-

9

7

3

2

0 ’ I I I I 1 1 I

1000 000 600 400 200 0

Binding Energy (eV)

FIG. 4. General survey of the XPS spectrum for Bi-2212.

J. Appl. Phys., Vol. 77, No. 1, 1 January 1995 Rajasekar et a/. 345

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‘r 4

3

2

I 1

Contamination )

1 I I I I I I , I 1 I 0 1000 800 600 400 200 0

Binding Energy (eV)

FIG. 5. General survey of the XPS spectrum for Bi-2223.

bility supporting therefore the ‘as such’ emission model, as Similar arguments can be given for the emission of also confirmed by our previous studies with metallic as well CuO, complexes from CuO, but in this case the energy dis- as high-T, superconducting compounds.7*‘0121 The emission tribution has much steeper fall than that for CuO; in the of various monoxides, like BiO-, CaO-, CuO-, SrO-, etc. high-energy region, as expected from the statistics’ for the follows the similar behavior as copper dimers. coalescence to form higher-order clusters.

The high-energy tail of the distribution for the heavier copper oxide, like CuO; molecule, emitted from CuO (Table I), follows close to Em7 dependence, supporting the forma- tion of this complex molecule by ‘vacuum recombination’ process which also seems to be quite reasonable, as the mo- lecular complex like CuO; cannot be associated within the CuO structure.

B. XPS analysis

The general survey of the XPS spectra for Bi-2212, Bi- 2223, Bi20s, CuO, and CaCOs are presented in Figures 4-8.

800 600 400 200 0

Binding Energy (eV)

FIG. 6. General survey of the XPS spectrum for Bi,O, .

346 J. Appl. Phys., Vol. 77, No. 1, 1 January 1995 Rajasekar et al.

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0 1000 800 600 400 200 0

(a) Bindmg Energy (eV)

(b)

1000 800 600 400 200 0

Blnding Energy (eV)

FIG. 7. (a) General survey of the XPS spectrum for CuO (before sputtering). (b) General survey of the XPS spectrum for CuO (after sputtering).

XPS spectra for each sample (except CaCO,) have been XPS spectra of the various individual peaks are not, how- taken both before and after 5 min of sputtering with 3 keV ever, shown here. Results of the XPS measurements on the A? beam. various samples are discussed in the following.

Figures 4-6 show the XPS spectra for the sputter- cleaned Bi-2212, Bi-2223, and Bi,O, samples, respectively. Figures 7(a) and 7(b) present those for CuO before and after sputtering and Fig. 8 gives the XPS spectra for only the unsputtered CaCOs sample. No attempt was made to sputter clean the CaCOs sample in order to avoid the sputter-induced chemical changes in the surface of this sample, which is more sensitive to the chemical changes in comparison to the other samples.

1. Bi-2212

The XPS peaks for Bi4f.&BE) and Bi4f7,,(BE) in the case of Bi-2212 sample (Fig. 4) do not seem to correspond to those from metallic bismuth, as the separation (A) between these two peaks in the present case is different from what is expected from metallic bismuth.

Spectrum for each ‘sample after sputter cleaning showed reproducible signals of the bulk species with absence of sur- face contaminants. Although a general survey has been pre- sented here for each sample, individual peaks of particular interest were elaborated in greater detail. The elaborated

The true (BE) value for the Bi4f7,2(158.5 eV), after making a shift by 0.5 eV from the experimental peak posi- tion due to surface charges (estimated from standard Cls peak at 284.6 eV as the reference for binding energy) seems to be very much comparable to that Bi4fTj2 from Bi,O, com- pound r-159.5 eV as seen in the Fig. 6; literature value for

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8

W 6

h w 5 ?

1000 800 600 400 200 0

Blnding Energy (eV)

FIG. 8. General survey of the XPS spectrum for C&O3 (uasputtered). Cls contains two peaks. Lower BE peak is due to some carbon contamination. Higher BE peak is due to the (CO,) group.

Bi4f7&BE) for Bi,0,=158.8 to 159.8 eV], indicating that the BE of Bi is the same in Bi-2212 as it is in Bi203, which implies the ionic character of the Bi layer in Bi-2212. The valence state of Bi in Bi-2212 is also 3f as it is in Bi,O,. This means that extra oxygen in the Bi layer which causes incommensurate modulation in Bi-2212,22.“3 does not bring any valence or binding energy change in Bi, i.e., Bi-0 bonding is nearly ionic and covalent bond contribution to B&--O bond is small. Only extra holes are created in CuOz layer.

and insulating. From the careful analysis of the Sr3dsn peak (after correcting for the surface charge), it seems that this peak corresponds to some mixed oxides of Sr together with Bi or Cu.

2. Bi-2223

From Ols(BE) peak position (Fig. 4), indication of other oxides, like SrO, CaO, and copper oxide (CuO and/or Cu,O), in addition to Biz03, is also evident. The oxidation state of copper (whether as CuO or Cu20), present in Bi-2212, is not, however, confirmed from the XPS (BE) peak position of 01s or of CU~P~,~ as obtained from Bi-2212.

The kinetic energy (KE) of CuLMM Auger electron

All the above arguments given for sample Bi-2212 are also valid for the various XPS peaks obtained from Bi-2223, except the stoichiometric composition. The stoichiometric compositions for these two samples obtained from the XPS analysis are presented in Table II.

(from Bi-2212), as estimated from the binding energy of the CuLMM Auger peak is found to be in much closer agree- ment with that (CuLMM Auger) from Cu20. (literature value 918.1 eV), in comparison to that from CuO or metallic cop- per, indicating thereby the presence of Cu+ state in Bi-2212 which might have resulted due to sputtering (the same effect is observed in CuO after sputtering). From the consideration of the a parameter for copper [defined as BE(Cu2p3n) +KE(CuLMM Auger)], the indication of Cu20 (not CuO or metallic copper) is also evident, since the Q parameter for copper, estimated from the experimental peak positions of Cu2~,~ XPS (BE) and CuLMM Auger, in the case of Bi- 2212, is 1847.7 eV which is close to that for CuzO (literature value is 1849.2 eV).

3. cllo The general XPS survey for the CuO sample, before and

after sputtering, is presented in Figs. 7(a) and 7(b). The shake up indicated in between two XPS peaks of Cu2p3, and Cu2pm for the CuO sample before sputtering [Fig. 7(a)] is characteristic of CuO {i.e., Cu++ state). The absence of

TABLE II. The stoichiometric compositions for Bi-2212 and Bi-2223 (Tom XPS).

Concentration (at. %)

Ca2p3n (BE) peak position seems to be comparable to that for CaCO,. (Fig. 8) or CaO (literature value), i.e., in Bi-2212 the BE of Ca is the same as that in CaC03 or CaO. This is in agreement with the fact that the Ca layer is ionic

Sample

Bi-2212

Bi-2223

Element Measured Calculated

01s 48.32 58.8 cu2p 16.83 11.8 ca2p 6.42 5.9 Sr3d 19.49 11.8 Bi4f 8.94 11.8 01s 49.93 57.6 &2P 14.91 15.7 Ca2p 10.51 10.5 Sr3d 17.71 10.5 BMf 6.94 10.5

348 J. Appl. Phys., Vol. 77, No. 1, 1 January 1995 Rajasekar et al.

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TABLE III. The atomic concentrations of Cu2p in CuO, before and after sputtering (from XPS).

Measured concentration (at. W)

Sample

cue

Element Before sputtering

01.9 56.0 (=P 44.0

After sputtering

25.5 74.5

this shake up indicates.,the presence of the Cu+ state (i.e., Cr.@). The concentration (in atomic percent) of Cu seems to be more (from 44% to 74%) in CuO after sputtering (Table III). The higher copper contribution in CuO on sputtering could be due to the presence of some metallic copper, re- leased due to the sputtering of the sample and/or due to the presence of some Cu,O, arising out of conversion from CuO to Cu,O due to preferential sputtering of oxygen from CuO. This possibility is also reflected in CuLMM Auger peak which indicates the,predominant presence of Cu20 on sput- tering of CuO sample. The Q parameter for copper estimated from the experimental peak positions of Cu2~a,~ XPS and CuLMM Auger for the CuO sample (after sputtering) sup- ports more strongly the presence of CuzO in CuO on sput- tering.

IV. CONCLUSIONS .i

The kinetic energy distributions of various ions in the SIMS analysis seem to follow E-f’ with increase in ‘IZ ’ value with the increase in the number of atoms in the ion. The E, of Sr in the case of Bi-2223 is shifted towards lower energy. Lower binding energy of Sr causes the anti-site disorder which leads to the occupation of the Sr site by Bi. Emission of small clusters, like metallic dimers, trimers, and various monoxide oxide ions is explained as due to preferred ion emission, whereas the emission of heavier molecular ions, like CuOy , CuO,) etc. are due to ‘vacuum recombination’ process.

From the XPS studies, the binding nature of Cu-0 layer is found to be unusual, bearing resemblance to that of Cu,O. The BE of Ca in Bi-2212 and in Bi-2223 is found to be same as it is in CaO or in CaCO,. The BE of Bi in both Bi-2212 (and in Bi-2223) and B&O, seems to be the same. The cop- per in Bi-2212 (and in Bi-2223) appears to exist in the Cuf state which could be due to sputtering. Ca is found to be the same as it is in CaO or CaCO,, supporting the insulating

behavior of Ca layer. The presence of Cuf state in CuO has been ,explained as due to the presence of metallic copper released during sputtering and/or due to presence of Cu,O, arising out of conversion of CuO to Cu2p by the preferential sputtering of oxygen.

ACKNOWLEDGMENTS

One of the authors (P. Chakraborty) is grateful to Profes- sor P. Mazzoldi of the Physics Department, University of Padova, Italy for his constant encouragement and support. The partial financial support provided to the same author by the International Center for Theoretical Physics, Trieste, Italy is also gratefully acknowledged. This work was partially supported by Italian Research Council (CNR), contract PF MSTA No. 93.01207.PF68.

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