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Quantitative Auger electron spectroscopy of superconducting YBaCuO single crystals and thin films
Volume
I 1, number I ,2
MATERIALS
LETTERS
April 1991
Quantitative Auger electron spectroscopy of superconducting Y-Ba-Cu-0 single crystals and thin films E.W. Seibt and A. Zalar I Institut ftir Technische Physik, Kernforschungszentrum Karlsruhe GmhH, Poscfach 3640. D-7500 Karlsruhe I. Federal Republic ofGerman, Received
17 December
1990
Quantitative Auger electron spectroscopy (AES) measurements of stoichiometric YBa2Cu30, single-crystal superconductors have been performed to determine the relative elemental Auger sensitivity factors used for internal standards. This quantitative method was applied to investigate the compositional structures ofYBa,Cu#_, thin films sputtered onto ( 1OO)-oriented SrTiO,, MgO, and A1,O1 substrate materials. For YBa2Cu30,_, films with these typical substrates the resulting stoichiometric structures and epitaxial growth behaviour are fairly different. Interdiffusion effects at the superconductor-substrate interfaces observed by application of an AES depth profiling technique are remarkable for YBa2Cu,0,_ ~films sputtered onto AllO, substrates. The experimental results presented here demonstrate that AES can be a very useful method in the quantitative analysis study of highT, superconductors.
1. Introduction The determination of the interface and bulk compositions of high-T, superconductors is a subject of considerable interest in studying the homogeneity and the elemental distribution on surfaces and subsurfaces as well as the grain properties of superconducting thin films or ceramics with desirably high transport-current carrying capabilities [ l-31. Because of its relatively high spatial and depth resolutions, Auger electron spectroscopy (AES ) [ 4,5 ] has been used to investigate the actual compositions of Y-Ba-Cu-0 single crystals and thin films sputtered onto different substrate materials. The key problem of quantitative Auger measurements (AES multiplex spectra or depth profiles) is the difficulty encountered in calibration of the specific Auger signals in terms of elemental concentrations. For pure elements the relationship is given by the relative Auger sensitivity factors ( RSFs) related to a silver standard. However, for compounds the calibration procedure is more complicated and is ’ On leave from the Institute for Electronics and Vacuum Technique. Teslova 30, 61000 Ljubljana, Yugoslavia. 0167-577x/9
I /$ 03.50 0 I99
I - Elsevier Science Publishers
B.V.
performed here by means of stoichiometrically known Y ,BazCu~O, single crystals in order to obtain reference materials as standards for which the relations of compositions are similar to those of the compound samples under investigation. The analytical work presented here concentrates on two aspects of AES investigations applied to YBa-Cu-0 superconductors using the PHI 600 Auger scanning multiprobe system: ( 1) to perform a calibration procedure in determining the RSF values for a Y ,Ba$Zu,O, single crystal used as an internal standard and (2) to measure the changes in compositional structure relating to layer growth inhomogeneities and interdiffusion effects on superconductorsubstrate interfaces.
2. Experimental Single crystals with nominal molar compositions of Y ,BaZCu307_, were grown by a self-flux method using the starting materials of Y203, BaC03 and 010 powders for l-2-3-phase samples (sample no. I ). Typical heat treatments were applied between 990 and 1010°C in air. Slow cooling rates were used ( North-Holland
)
1
Volume
11,number I,2
MATERIALS LETTERS
which ranged from 0.1 to 1“C/h in order to obtain more extended single crystals, e.g., with maximum sizes of 5 x 5 x 2 mm3 [ 61. The stoichiometric structure of the single crystal used as an internal standard was generally determined by X-ray powder diffraction and EDX measurements. Epitaxial Y-Ba-Cu-0 films have been grown in situ on single-crystalline ( lOO)-oriented SrTiO, (sample no. 2), MgO (sample no. 3), and A&O3 (sample no. 4) substrates by using techniques of rf sputtering from a single, non-stoichiometric, sintered oxide target at substrate temperatures between 800 and 850°C. The transition temperature T, was measured by inductivity and resistivity methods. SEM and high-resolution AES (PHI 600 scanning Auger multiprobe; Perkin-Elmer) were used to investigate the surface morphology and the elemental distribution of the samples. Especially for quantitative AES measurements a depth profiling technique was applied in combination with a small-spot duoplasmatron ion source (Perkin-Elmer) for 6 keV argon ions [ 7 1. The main experimental parameters used here were for the electron beams: 10 keV, 500 nA, incident angle 0~45”; for Ar+ ions: 6 keV, 2 A/ m2, incident angle (Y= 4 Iof and a sputter rate of about 30 nm/min.
3. Quantitative analysis method The quantitative AES analysis can be performed with good accuracy by comparison of the Auger signals from an unknown compound A to the signals from a compound standard A0 of known stoichiometry using the method of Auger RSF values under identical experimental conditions. The usual quantification formalism for the atomic concentration C, of the ith element of a homogeneous compound is based on the following equation taken from refs.
[VI:
where the running index j includes all elements existing in the compound. Z, is the measured peak-topeak Auger amplitude of the ith element. cyi= I/ (RSF)i are the inverse relative elemental Auger sen2
April 199 1
sitivity factors; f; are the electron-energy- and beamdependent scale factors. The matrix factor MU includes three main error sources which affect this quantitative technique, namely ( 1) differences in surface topography and density of samples, (2) matrix effects on electron escape depths and backscattering factors, and (3) chemical effects on peak shapes. As internal standards single-crystal oxides were used with stoichiometrically known compositions determined by X-ray powder di~raction measurements. The measured intensities of AES signals from the standards were quantified by determination of the RSF values using an iterative process of fitting of the elemental concentrations. Then the RSF values together with eq. (1) were employed to determine the stoichiometric structure of an unknown sample within an accuracy of typically 5- 10%. In this way, if the actual sample concentrations are close to the standards, backscattering factors, electron escape depths, and chemical effects are fairly similar in both samples, i.e. MU= I.
4. Results and discussion The quantification procedure is presented in fig. la; it indicates the fitted Auger depth profile of the Y ,BaZCu307 single-crystal standard sample. Fig. 1b shows the typical growth of a I-2-3-phase Y,Ba2Cu307 single crystal forming thin crystallites at cooling rates of about 0.1 “C/h. The Auger transition signals in eV used in these AES measurements were: oxygen (IUL) 510, barium (MNN) 584/600, copper (LMM) 920, and yttrium (LMM) 1746. The results of the RSF values determined at a primary electron energy of 10 keV are summa~zed in table 1. It is noteworthy that there is a significant difference between the measured RSF values of the compound and those of pure elements which means that the Auger relaxation processes are very sensitive to the existence of various binding states in oxide superconductors [ 10 1. With the quantification method including the Auger sensitivity factors applied, AES depth profile measurements of YBa2Cu30,_, thin films sputtered onto ( lOO)-oriented SrTiO,, MgO, and A1203 substrates were carried out under equal experimental
Volume I 1,number I ,2
MATERIALS LETTERS
April 1991
ium, copper, and oxygen concentration gradients as well as on the concentration levels of the substrate components in sub-micron YBa$Zu,O,_ t films (film thicknesses between 0.5 and 0.8 pm). 4. I. I’Ba2Cu.Ju7_ ,./SrTiOj A typical Auger depth profile plot of a YBaZCu307__.r film (about 0.5 urn thick) sputtered onto SrTiO, substrate is presented in fig. 2a indicating a nearly homogeneous course of the single components yttrium, barium, copper. and oxygen. Fig, 2b shows the epitaxial growth of the Y-Ba-Cu0 surface with a multitude of small single crystallites ( ~3 urn) grown in the melt. In combination with
Fig. 1 (a) Determination by AES depth profiling technique of the relative Auger sensitivity factors (RSFs) of a Y ,BaQ,O, single crystal of known stoichiometry. (b) Growth of l-2-3 phase forming thin Y ,BazCu,O, crystallites at cooling rates of 0. I “C/ h (magnification 75x ). Table 1 Relative Auger sensitivity factors ( 10 keV) of Y ,Ba&ItsO, Y (7.7 at%) a1 Ba(154atl) Cu (23.1 at%) II.)?(53.8 at%)
0.100~0.00~ 0.125f0.005 0.105+0.005 0.170+0.003
(0.038) h, (0.082) (0.215) (0.350)
‘) Atomic concentration values are calculated from the stoichiometric structure determined by X-ray powder diffraction measurements. b, RSF values for pure elements related to silver.
conditions to determine quantitatively changes in surface and bulk compositions. The high-resolution AES technique has been shown to be capable of providing fine-scale information on the yttrium, bar-
Fig. 2. (a) AES depth profile plot of a YBa2Cuj0,_, thin film sputtered onto SrTiOl substrate. (b) SEM micrograph of the epitaxial growth of the Y-Ba-Cu-0 tilm surface with a multitude of single crystallites (magni~cation I~OOX ).
3
Volume 1I, number I,2
MATERIALS LETTERS
April 1991
the YBa2Cu307_-x single-crystal RSF values the actual stoichiometric structure of this YBazCusO,_, film averaged over the film thickness was found to be Y:Ba:Cu:0=0.9:2.0:2.9:6.7. The obvious,deficiency of oxygen (fluctuations up to 5 at.%) is associated here with a lowering of T, within 200/b(T, (midpoint) = 76 K). For the substrate elements, strontium and titanium, no effect of segregation or diffusion towards grain boundaries or interfaces has been observed in the Y-Ba-Cu-0 layer within a detection limit of about 1 at.%. However, the measured interface latitude was relatively broad (about 150, nm) in contrast to MgO substrates. 4.2. YBa2Cu30,_,/Mg0 The corresponding depth profile plot (see fig. 3a) shows a relatively sharp transition at the interface (about 50 nm) with a fairly low measured rate of Mg diffusion ( G 1 at.%) into the Y-Ba-Cu-0 layer. Only small changes in the concentration gradients have been observed, e.g., for copper less than 4 at.%. The structural composition averaged over the whole YBa-Cu-0 layer was determined here to be Y:Ba:Cu:O=I.0:2.0:2.9:7.0. Inductively measured T, values ranging from 88 to 90 K confirmed also the nearly ideal stoichiometric composition of these Y-Ba-Q-0 films sputtered onto MgO substrates. In contrast to films with SrTi03, the surface of the Y-Ba-Cu-O/MgO film does not show a sufficient epitaxially grown structure (see fig. 3b) in spite of a multitude of single crystals grown at the surface.
The enhanced aluminum diffusion (see fig. 4a) occurring through the whole Y-Ba-Cu-0 layer up to its surface is due to preferential segregation of aluminum at the grain boundaries. Additionally, strong changes can be observed for copper (up to 25 at.%) and oxygen ( x 10 at.%) whereas the concentration of aluminum still reaches values of about 10 at.% in the middle of the Y-Ba-Cu-0 layer depth, Furthermore, aluminum is substituted in the Y-Ba-Cu0 film at copper sites, thus resulting in a relatively strong T, reduction (T, (midpoint) ~50 K). An 4
Fig.3. (a) AES depth profile plot of a YBa$&O,_,
film sputtered onto MgO substrate. (b) SEM micrograph of the film surface ( ma~i~cation I0000x ) .
SEM picture of the Y-Ba-Cu-0 surface (see fig. 4b) shows an equal distribution of grains without any epitaxial orientation, but with relatively small grain diameters of about OS pm.
5. Conclusions Acceptable quantitative AES results have been obtained by using YBa&&O, single-crystal standards of known stoichiometry (the stoichiometry is given by X-ray diffraction measurements) to determine the Auger RSF values needed for the quantification of the measured AES depth profiles. To eliminate matrix correction effects, the proposed AES quantification based on elemental RSF comparisons strictly
Volume 11, number I ,2
MATERIALS LETTERS
April 1991
sputtered onto SrTi03, MgO, and A&O3 substrates are strongly changed for films with A&O, due to an extended segregation effect of aluminum at the grain boundaries which leads to relatively large reductions in r,. Films with SrTi03 and MgO substrates show a nearly ideal stoichiometric composition as well as a sufficient epitaxially grown structure and should be promising candidates for superconductors with relatively high transport-current carrying capabilities.
Acknowledgement The authors would like to acknowledge the technical assistance by Mr. J. Pytlik and thank Mrs. B. Runtsch for ~~-measurements.
References
Fig. 4. (a) AES depth profile plot of a YBa2Cu@_, film sputtered onto AI,OJ substrate. (b) SEM micrograph of the film surface (magnification 10000 X ) .
requires that the inst~mentai and sample conditions are the same as those used for standards. The compositions of YBa2Cu30,_, thin films
[ I] D.K. Christen, C.E. Klabunde, H.R. Kerchner, S.T. Seknla, R. Feenstra and J.D. Budai, Physica C 162-I 64 ( 1989) 653. [2] H. Hohler, H. Neeb and C. Heiden, Physica C 162-164 (1989) 607. [ 31 H. Ohlsen, L. Stolt, J. Hudner and E. Johansson, Physica C 162-164(1989)621. [4] R. Berjoan, Rev. Phys. Appl. 25 ( 1990) 17. [5] M. Hrovat, S. Bemik, A. Zalar and E.W. Seibt. Vacuum 40 (1990) 197. [6] Th. Wolf, W. Goldacker, B. Obst, G. Roth and R. Fliikiger, J. Crystal Growth 96 (1989) 1010. [ 71 E.W. Seibt and A, Zalar, Mater. Letters 7 ( 1988) 256. [ 81 J.T. Grant, Surface Interface Anal. 14 ( 1989) 27 1. [9]T. Sekine, K. Hirata and A. Mogami, Surface Sci. 125 (1983) 565. [IO] E.W. Seibt and A. Zalar, EVC2 Conference, May 21-25, 1990, Trieste, Italy.
I 1, number I ,2
MATERIALS
LETTERS
April 1991
Quantitative Auger electron spectroscopy of superconducting Y-Ba-Cu-0 single crystals and thin films E.W. Seibt and A. Zalar I Institut ftir Technische Physik, Kernforschungszentrum Karlsruhe GmhH, Poscfach 3640. D-7500 Karlsruhe I. Federal Republic ofGerman, Received
17 December
1990
Quantitative Auger electron spectroscopy (AES) measurements of stoichiometric YBa2Cu30, single-crystal superconductors have been performed to determine the relative elemental Auger sensitivity factors used for internal standards. This quantitative method was applied to investigate the compositional structures ofYBa,Cu#_, thin films sputtered onto ( 1OO)-oriented SrTiO,, MgO, and A1,O1 substrate materials. For YBa2Cu30,_, films with these typical substrates the resulting stoichiometric structures and epitaxial growth behaviour are fairly different. Interdiffusion effects at the superconductor-substrate interfaces observed by application of an AES depth profiling technique are remarkable for YBa2Cu,0,_ ~films sputtered onto AllO, substrates. The experimental results presented here demonstrate that AES can be a very useful method in the quantitative analysis study of highT, superconductors.
1. Introduction The determination of the interface and bulk compositions of high-T, superconductors is a subject of considerable interest in studying the homogeneity and the elemental distribution on surfaces and subsurfaces as well as the grain properties of superconducting thin films or ceramics with desirably high transport-current carrying capabilities [ l-31. Because of its relatively high spatial and depth resolutions, Auger electron spectroscopy (AES ) [ 4,5 ] has been used to investigate the actual compositions of Y-Ba-Cu-0 single crystals and thin films sputtered onto different substrate materials. The key problem of quantitative Auger measurements (AES multiplex spectra or depth profiles) is the difficulty encountered in calibration of the specific Auger signals in terms of elemental concentrations. For pure elements the relationship is given by the relative Auger sensitivity factors ( RSFs) related to a silver standard. However, for compounds the calibration procedure is more complicated and is ’ On leave from the Institute for Electronics and Vacuum Technique. Teslova 30, 61000 Ljubljana, Yugoslavia. 0167-577x/9
I /$ 03.50 0 I99
I - Elsevier Science Publishers
B.V.
performed here by means of stoichiometrically known Y ,BazCu~O, single crystals in order to obtain reference materials as standards for which the relations of compositions are similar to those of the compound samples under investigation. The analytical work presented here concentrates on two aspects of AES investigations applied to YBa-Cu-0 superconductors using the PHI 600 Auger scanning multiprobe system: ( 1) to perform a calibration procedure in determining the RSF values for a Y ,Ba$Zu,O, single crystal used as an internal standard and (2) to measure the changes in compositional structure relating to layer growth inhomogeneities and interdiffusion effects on superconductorsubstrate interfaces.
2. Experimental Single crystals with nominal molar compositions of Y ,BaZCu307_, were grown by a self-flux method using the starting materials of Y203, BaC03 and 010 powders for l-2-3-phase samples (sample no. I ). Typical heat treatments were applied between 990 and 1010°C in air. Slow cooling rates were used ( North-Holland
)
1
Volume
11,number I,2
MATERIALS LETTERS
which ranged from 0.1 to 1“C/h in order to obtain more extended single crystals, e.g., with maximum sizes of 5 x 5 x 2 mm3 [ 61. The stoichiometric structure of the single crystal used as an internal standard was generally determined by X-ray powder diffraction and EDX measurements. Epitaxial Y-Ba-Cu-0 films have been grown in situ on single-crystalline ( lOO)-oriented SrTiO, (sample no. 2), MgO (sample no. 3), and A&O3 (sample no. 4) substrates by using techniques of rf sputtering from a single, non-stoichiometric, sintered oxide target at substrate temperatures between 800 and 850°C. The transition temperature T, was measured by inductivity and resistivity methods. SEM and high-resolution AES (PHI 600 scanning Auger multiprobe; Perkin-Elmer) were used to investigate the surface morphology and the elemental distribution of the samples. Especially for quantitative AES measurements a depth profiling technique was applied in combination with a small-spot duoplasmatron ion source (Perkin-Elmer) for 6 keV argon ions [ 7 1. The main experimental parameters used here were for the electron beams: 10 keV, 500 nA, incident angle 0~45”; for Ar+ ions: 6 keV, 2 A/ m2, incident angle (Y= 4 Iof and a sputter rate of about 30 nm/min.
3. Quantitative analysis method The quantitative AES analysis can be performed with good accuracy by comparison of the Auger signals from an unknown compound A to the signals from a compound standard A0 of known stoichiometry using the method of Auger RSF values under identical experimental conditions. The usual quantification formalism for the atomic concentration C, of the ith element of a homogeneous compound is based on the following equation taken from refs.
[VI:
where the running index j includes all elements existing in the compound. Z, is the measured peak-topeak Auger amplitude of the ith element. cyi= I/ (RSF)i are the inverse relative elemental Auger sen2
April 199 1
sitivity factors; f; are the electron-energy- and beamdependent scale factors. The matrix factor MU includes three main error sources which affect this quantitative technique, namely ( 1) differences in surface topography and density of samples, (2) matrix effects on electron escape depths and backscattering factors, and (3) chemical effects on peak shapes. As internal standards single-crystal oxides were used with stoichiometrically known compositions determined by X-ray powder di~raction measurements. The measured intensities of AES signals from the standards were quantified by determination of the RSF values using an iterative process of fitting of the elemental concentrations. Then the RSF values together with eq. (1) were employed to determine the stoichiometric structure of an unknown sample within an accuracy of typically 5- 10%. In this way, if the actual sample concentrations are close to the standards, backscattering factors, electron escape depths, and chemical effects are fairly similar in both samples, i.e. MU= I.
4. Results and discussion The quantification procedure is presented in fig. la; it indicates the fitted Auger depth profile of the Y ,BaZCu307 single-crystal standard sample. Fig. 1b shows the typical growth of a I-2-3-phase Y,Ba2Cu307 single crystal forming thin crystallites at cooling rates of about 0.1 “C/h. The Auger transition signals in eV used in these AES measurements were: oxygen (IUL) 510, barium (MNN) 584/600, copper (LMM) 920, and yttrium (LMM) 1746. The results of the RSF values determined at a primary electron energy of 10 keV are summa~zed in table 1. It is noteworthy that there is a significant difference between the measured RSF values of the compound and those of pure elements which means that the Auger relaxation processes are very sensitive to the existence of various binding states in oxide superconductors [ 10 1. With the quantification method including the Auger sensitivity factors applied, AES depth profile measurements of YBa2Cu30,_, thin films sputtered onto ( lOO)-oriented SrTiO,, MgO, and A1203 substrates were carried out under equal experimental
Volume I 1,number I ,2
MATERIALS LETTERS
April 1991
ium, copper, and oxygen concentration gradients as well as on the concentration levels of the substrate components in sub-micron YBa$Zu,O,_ t films (film thicknesses between 0.5 and 0.8 pm). 4. I. I’Ba2Cu.Ju7_ ,./SrTiOj A typical Auger depth profile plot of a YBaZCu307__.r film (about 0.5 urn thick) sputtered onto SrTiO, substrate is presented in fig. 2a indicating a nearly homogeneous course of the single components yttrium, barium, copper. and oxygen. Fig, 2b shows the epitaxial growth of the Y-Ba-Cu0 surface with a multitude of small single crystallites ( ~3 urn) grown in the melt. In combination with
Fig. 1 (a) Determination by AES depth profiling technique of the relative Auger sensitivity factors (RSFs) of a Y ,BaQ,O, single crystal of known stoichiometry. (b) Growth of l-2-3 phase forming thin Y ,BazCu,O, crystallites at cooling rates of 0. I “C/ h (magnification 75x ). Table 1 Relative Auger sensitivity factors ( 10 keV) of Y ,Ba&ItsO, Y (7.7 at%) a1 Ba(154atl) Cu (23.1 at%) II.)?(53.8 at%)
0.100~0.00~ 0.125f0.005 0.105+0.005 0.170+0.003
(0.038) h, (0.082) (0.215) (0.350)
‘) Atomic concentration values are calculated from the stoichiometric structure determined by X-ray powder diffraction measurements. b, RSF values for pure elements related to silver.
conditions to determine quantitatively changes in surface and bulk compositions. The high-resolution AES technique has been shown to be capable of providing fine-scale information on the yttrium, bar-
Fig. 2. (a) AES depth profile plot of a YBa2Cuj0,_, thin film sputtered onto SrTiOl substrate. (b) SEM micrograph of the epitaxial growth of the Y-Ba-Cu-0 tilm surface with a multitude of single crystallites (magni~cation I~OOX ).
3
Volume 1I, number I,2
MATERIALS LETTERS
April 1991
the YBa2Cu307_-x single-crystal RSF values the actual stoichiometric structure of this YBazCusO,_, film averaged over the film thickness was found to be Y:Ba:Cu:0=0.9:2.0:2.9:6.7. The obvious,deficiency of oxygen (fluctuations up to 5 at.%) is associated here with a lowering of T, within 200/b(T, (midpoint) = 76 K). For the substrate elements, strontium and titanium, no effect of segregation or diffusion towards grain boundaries or interfaces has been observed in the Y-Ba-Cu-0 layer within a detection limit of about 1 at.%. However, the measured interface latitude was relatively broad (about 150, nm) in contrast to MgO substrates. 4.2. YBa2Cu30,_,/Mg0 The corresponding depth profile plot (see fig. 3a) shows a relatively sharp transition at the interface (about 50 nm) with a fairly low measured rate of Mg diffusion ( G 1 at.%) into the Y-Ba-Cu-0 layer. Only small changes in the concentration gradients have been observed, e.g., for copper less than 4 at.%. The structural composition averaged over the whole YBa-Cu-0 layer was determined here to be Y:Ba:Cu:O=I.0:2.0:2.9:7.0. Inductively measured T, values ranging from 88 to 90 K confirmed also the nearly ideal stoichiometric composition of these Y-Ba-Q-0 films sputtered onto MgO substrates. In contrast to films with SrTi03, the surface of the Y-Ba-Cu-O/MgO film does not show a sufficient epitaxially grown structure (see fig. 3b) in spite of a multitude of single crystals grown at the surface.
The enhanced aluminum diffusion (see fig. 4a) occurring through the whole Y-Ba-Cu-0 layer up to its surface is due to preferential segregation of aluminum at the grain boundaries. Additionally, strong changes can be observed for copper (up to 25 at.%) and oxygen ( x 10 at.%) whereas the concentration of aluminum still reaches values of about 10 at.% in the middle of the Y-Ba-Cu-0 layer depth, Furthermore, aluminum is substituted in the Y-Ba-Cu0 film at copper sites, thus resulting in a relatively strong T, reduction (T, (midpoint) ~50 K). An 4
Fig.3. (a) AES depth profile plot of a YBa$&O,_,
film sputtered onto MgO substrate. (b) SEM micrograph of the film surface ( ma~i~cation I0000x ) .
SEM picture of the Y-Ba-Cu-0 surface (see fig. 4b) shows an equal distribution of grains without any epitaxial orientation, but with relatively small grain diameters of about OS pm.
5. Conclusions Acceptable quantitative AES results have been obtained by using YBa&&O, single-crystal standards of known stoichiometry (the stoichiometry is given by X-ray diffraction measurements) to determine the Auger RSF values needed for the quantification of the measured AES depth profiles. To eliminate matrix correction effects, the proposed AES quantification based on elemental RSF comparisons strictly
Volume 11, number I ,2
MATERIALS LETTERS
April 1991
sputtered onto SrTi03, MgO, and A&O3 substrates are strongly changed for films with A&O, due to an extended segregation effect of aluminum at the grain boundaries which leads to relatively large reductions in r,. Films with SrTi03 and MgO substrates show a nearly ideal stoichiometric composition as well as a sufficient epitaxially grown structure and should be promising candidates for superconductors with relatively high transport-current carrying capabilities.
Acknowledgement The authors would like to acknowledge the technical assistance by Mr. J. Pytlik and thank Mrs. B. Runtsch for ~~-measurements.
References
Fig. 4. (a) AES depth profile plot of a YBa2Cu@_, film sputtered onto AI,OJ substrate. (b) SEM micrograph of the film surface (magnification 10000 X ) .
requires that the inst~mentai and sample conditions are the same as those used for standards. The compositions of YBa2Cu30,_, thin films
[ I] D.K. Christen, C.E. Klabunde, H.R. Kerchner, S.T. Seknla, R. Feenstra and J.D. Budai, Physica C 162-I 64 ( 1989) 653. [2] H. Hohler, H. Neeb and C. Heiden, Physica C 162-164 (1989) 607. [ 31 H. Ohlsen, L. Stolt, J. Hudner and E. Johansson, Physica C 162-164(1989)621. [4] R. Berjoan, Rev. Phys. Appl. 25 ( 1990) 17. [5] M. Hrovat, S. Bemik, A. Zalar and E.W. Seibt. Vacuum 40 (1990) 197. [6] Th. Wolf, W. Goldacker, B. Obst, G. Roth and R. Fliikiger, J. Crystal Growth 96 (1989) 1010. [ 71 E.W. Seibt and A, Zalar, Mater. Letters 7 ( 1988) 256. [ 81 J.T. Grant, Surface Interface Anal. 14 ( 1989) 27 1. [9]T. Sekine, K. Hirata and A. Mogami, Surface Sci. 125 (1983) 565. [IO] E.W. Seibt and A. Zalar, EVC2 Conference, May 21-25, 1990, Trieste, Italy.