- Email: [email protected]
Cr superlattices
PHYSICA[ ELSEVIER
Physica B 221 (1996)411 415
Resonant X-ray reflectivity study of Fe/Cr superlattices Jiaming Bai a'b, Eric E. Fullerton b, Pedro A. M o n t a n o b'c'* aBrooklyn College of the City University o f New York, New York, USA b Materials Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Bldg. 223, Argonne, IL 60439, USA c University oflllinois, Chicago, IL, USA
Abstract We have measured the composition profile on an Fe/Cr superlattice using glancing incidence X-ray reflectivity. Resonant reflectivity measurements were carried out by tuning the X-ray energy around the respective K-edge of Fe and Cr. We were able to obtain excellent fits to the data and get consistent geometry and composition parameters from the reflectivity measurements at six different X-ray energies. We obtained valuable information on the interface composition of the superlattice and observed also a slight variation in composition at the bottom and top interfaces. The information obtained using this method allows a determination of not only the electron density but also the composition profiles of the multilayers. This non-destructive technique is a promising tool for the determination of the chemical composition of thin film.
1. Introduction X-ray scattering techniques provide a powerful tool to study electron density profiles in thin films. These techniques have become more powerful with the advent of synchrotron radiation sources which allows tunability of the X-rays. Over the last few years, glancing angle X-ray reflectivity measurements are frequently used to investigate the microstructures of thin films on thick substrates [1-4]. The specular reflectivity measurements provide useful structural information such as sublayer thickness, effective electron density and absorption coefficient (which are respectively related to the real and imaginary parts of the complex dielectric constant), as well as information on interfacial roughness. However, the conventional reflectivity measurement is not an element sensitive technique, it can only be used to detect the electron density but not the atomic composition of the sample.
On the other hand, the electron density and the linear absorption are strong energy dependent variables near the absorption edges of the materials. Resonant X-ray reflectivity contains more information than ordinary reflectivity and allows a determination of the atomic composition from the X-ray dispersion behavior of the sample [5]. This element of contrast expands the experimental capabilities of X-ray reflectivity measurements, as was recently applied to the determination of the composition profile of permalloy thin films [5]. We report in this paper resonant reflectivity measurements on an Fe/Cr superlattice. The measurements were carried out by tuning the X-ray energy below and above, close and away from the respective K-edges of Fe and Cr. We describe in the following paragraphs the experimental procedures and results of our measurements. 2. Experimental
*Correspondence address: Materials Sciences Division, Argonne National Laboratory, 9700 S. Cass Avenue, Bldg. 223, Argonne, IL 60439, USA.
A (1 0 0)-oriented [Fe(20 A)/Cr(11 A)] 2o superlattice was grown by DC magnetron sputtering onto an
0921-4526/96/$15.00 @ 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 9 5 9 - 0
412
J, Bai et al./Physica B 221 (1996) 411-415
epitaxially polished single-crystal MgO(l 0 0) substrate. The sputtering chamber had a base pressure of < 5 x 10-STorr. The 2" planar magnetron sputtering guns were operated in an Ar pressure of 3 naTorr and a target-substrate distance of 9 cm. A 100A Cr buffer layer was initially deposited at a substrate temperature of 600°C to establish the epitaxial orientation with the substrate and provide a template for the growth of the superlattice. The MgO(1 0 0) surface (a0 = 4.213 A) has an ao/',v/2 = 2.98A square surface net which is only 3.3% lattice mismatched with the Cr(1 00) surface upon a 45 ° rotation about the surface normal (i.e. Cr[0 1 0]//MgO[0 1 1]). The substrate was then cooled to 250°C and the superlattice is grown by sequential deposition of the Fe and Cr layers. This procedure results in the growth of epitaxial Fe/Cr(1 0 0) superlattices which exhibited oscillatory interlayer magnetoresistance and record magnetoresistance values [6,7]. The reflectivity measurements were conducted on beam line X-6B at the National Synchrotron Light Source at BNL. The experimental X-ray energies were 5.8, 6.1, 6.94, 7.16, 7.3, and 8 KeV. The Fe and Cr K-edge energies are 7.112 and 5.989 keV, respectively. Horizontal and vertical slits were placed before the monochromator to define the beam size prior to monochromatization of the radiation. Inside the experimental hutch a second slit was placed for defining the monochromatic beam, a beam monitor was placed after this slit to measure the photon flux before scattering from the sample. A third slit was placed after the beam monitor (ionization chamber) for trimming to the slit scattering, but causing no reduction in the collimated photon beam. The sample was mounted on a high precision goniometer attached to a four circle Huber diffractometer. After the sample a fourth slit was placed to cut down any scattering not originating in the sample. The last slit was placed before a NaI scintillation detector and was set wide enough to accept all the specularly reflected scattering. The distance between sample and detector was over a meter, consequently the signal due to fluorescence from the sample was negligible. At very low angles the X-ray intensity is very large and this will saturate and damage the detector. To avoid this problem at very low angles we used a set of calibrated aluminum absorbers to reduce the beam intensity reaching the detector. In X-ray specular reflectivity measurements one must determine the angle of incidence with great accuracy. The alignment of the sample and diffractometer was carried out using a methodology similar to that described in the literature [5]. The diffuse contribution to the scattering was subtracted from all the spectra. The experimental data were analyzed using a non-linear least square fitting program. The best fit was obtained when the reflectivity spectra for the different energies gave consistent structural parameters (layers thickness and interfacial roughness).
All the measurements were carried out at room temperature.
3. Results and discussion
The X-ray waves reflected from an interface between two media el and E2 can be represented by E2 = Ae-ik2~(z z2) + Beik~A= --2),
(1) E t = Ce ikt'(z zl) + D e ik''(= ~'),
where kjz = kox/'~:; - cos2 0o. The amplitudes A, D, C, D are related [8] by A = C + rFe2ik~=dD,
(2) B = r F C + eeik~dD,
where d = Z 2 - Z ~
and r is the Fresnel formula
r = (k2z - klz)/(k2z + k~z), To calculate the reflectivity
from a multilayer, Eq. (2) can be treated iteratively, the final reflectivity will be given by B / A . The form factor F is given by (e
E =
i(k,=+ k~)h(x, y)):,y
(3)
( c - i ( k . -k2z)h(x, v)):,r '
where h ( x , y ) is the two-dimensional profile function of the roughness and ( )xr means in-plane averaging. For an ideal interface, h(x,y) = 0 for all x , y , and F = 1. Assuming the ergodicity of the profile function h(x, y), the in-plane average can be written as nx
(e itk~+k~)ht~.)'))~y=
t
e i(k~=+k~):co(z)dz,
(4)
:(
where z is relative to the average position of the interface and re(z) is the height distribution function of the interface. It can be related to the derivative of the in-plane average of the effective electron density profile by 1 o(~)
-
-
-
dp
(5)
P2 - - P l d z
In this way, the reflectivity is related to the effective electron density without using the Born approximation. The relation between the real dielectric coefficient and the effective electron density is e,} = 1 - 26j,
6j = 0i )~2/2re"
Now we consider an interface between two metal layers, e.g. Cr and Fe. The total electron density is the sum of those from Cr and Fe atoms assuming the same spatial function for the composition profile [-9]. In our analysis, a simple Gaussian model suffices to describe the
413
J. Bai et al./Physica B 221 (1996) 411-415
structure of the interface (it includes roughness and diffusion). The data curves at different X-ray energies exhibit strong energy dispersive effects. Fig. 1 shows the linear absorption coefficients and effective electron densities for Fe and Cr. The letter a through f indicate the energy selected for our measurements. Fig. 2 shows model calculations with and without absorption effects. The strong effect of the K-edge absorption on the reflectivity spectra is evident. It is clear that absorption effects are extremely important for analysis of the data. This is clearly illustrated in Fig. 2; at 7.3 and 6.94 KeV when absorption is not included there is practically no difference between the Fe and Cr contributions. Resonance effects allow to differentiate the contribution to the reflectivity from elements with similar effective electron densities. We show in Fig. 3 refleetivity measurements at various energies as well as the fitting curves. The data curves at different X-ray energies exhibit strong energy dependence effects. This can be seen from the curve variation near the critical angle and the changes in the shape of the
a
300
b
c
Fe/Cr Sample A, Model Ca|culation (no absomtlon))
,.
r ....
= ....
~
, ....
E
0.0
J ....
, ....
=
0.0
0.2
i ....
, ....
,
•
8
0.3
0Key
0A C=(1/A)
0.5
0.8
0.7
Fe/Cr Sampte A, Model Calculation (With absorption))
de
, , , ~ , , , i , i , ,
I .... ~
Cr
E
=
8
.
0
K
e
v
E" 250
e- 200 0
150
o
loo e--I
50
~
~
, , , I , , i , i i
:
oo
i
, I ; i , , I , L l , : l , ,
oood
5500
65oo
l i l l = l
7ooo)
75oo
:
E
Fe
' '
' 1 ' '
. . . .
I
'
]: r
:
80oo i
]:
/
[
. . . .
~
'
3.5 0
._~ g3.0
C (
Q>
2.5
._> LU
2.0 5600
i
0.2
. . . .
i
03
. . . .
i
0.4 qz{ltA1
. . . .
~
0.5
. . . .
r
. . . .
o.e
i
0.7
. . . .
l
0.8
Fig. 2. Model calculations of the reflectivity using the parameters from Fig. 1 and the nominal layer thickness. The top figure is calculated with no X-ray absorption and the bottom including absorption effects.
::
"13 E
E
,
0.0
60oo
65oo
7000
75o0
6000
X-ray energy (eV)
Fig. 1. Linear absorption coefficients (top) and effective electron densities (bottom) as function energy for Fe and Cr (a = 5.8 KeV, b = 6.1 KeV, c = 6.94KeV, d = 7.16 KeV, e 7.3 KeV, f = 8.0 KeV).
oscillations envelope, as well as the superlattice Bragg's peaks. These differences are due to the energy dependence of the effective electron densities and linear absorption coefficients, o f f ' a n d f " . Each element in the sample responds differently to the X-ray energy variations while the total energy dependent effective electron densities and linear absorption coefficients are simply the sum of those of the components, weighted by their atomic weight percentages. In other words, we can calculate the effective electron densities and linear absorption coefficients at the various energies. So with a single group of composition and geometry parameters, we calculated the reflectivity and fit it to the experimental data curves at all
414
J. Bai et al./'Physica B 221 (1996) 411 415
10
TM
•
'
. i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
t
.
.
.
.
Table 1 List of parameters obtained from the analysis of the measurements (errors are given in parentheses)
.
1 0 ~5 f
1 0 ~4 10,3
Layer thickness
10~2 1011
Cr buffer Cr/Fe bi-layers Cr layers Top Fe layer Top Cr layer
101° 10 s 10 B 1 O7
.#
10 s
~
10 4
107 (3) 31.3 (1) 11.4 (4 23 (2) 16.6 (1.2)
._~ 10 s
Interface thickness/roughess
MgO/Cr buffer Cr buffer/Fe Fe/Cr Cr/Fe Top Fe/top Cr Top Cr/air
10 3 10 ~ 10 ~ 10 o 10-1 10-2
7 (3) 3.7 (1.0) 1.7 (3) 2.9 (2) 2 (1) 4.3 (1.5)
10-3
Percent oxygen in top layers
10 4 lO-S
Top Fe layer Top Cr layer
10-6 lO-r 10-8
,
,
,
L
. . . .
0.0
m
. . . .
0.0
i
. . . .
0.2
i
. . . .
0.3
~
. . . .
i
0.4
. . . .
0.5
%
11 (1) 43 (2)
i
0.6
Qz (l/A)
Fig. 3. Reflectivity measurements and fit (continuous lines) for the Fe/Cr superlattice: (a) 5.8 KeV; (b) 6.1 KeV; (c) 6.94 KeV; (d) 7.16 KeV; (e) 7.3 KeV; 8 KeV. The Fe and Cr K-edge energies are 7.112 and 5.898 KeV, respectively.
e~
o
. . . . -50
i 0
. . . .
= . . . . 50
, . . . . 1130
t,, 150
,,
i I I' 200
*'
i . . . . 700
Fig. 4 is the schematic diagram of our fitting model. The parameters are given in Table 1. While the composition parameters are closely related to the shape of the oscillations envelope, the geometrical parameters are linked to the periods of the oscillations. At the top interface there is clear evidence of oxide formation. There is enough oxygen diffusion to affect the top iron layer. We carried out surface EXAFS measurements on this sample and found evidence of metal-oxygen bonds at the top Fe and Cr layers. Details of the EXAFS work will be reported elsewhere. In conclusion, the resonant X-ray reflectivity is a promising non-destructive method to measure the composition profile of multilayers and thin films. With this method, we were able to measure the composition profile of an Fe/Cr superlattice, obtaining important information on the interface structure.
]
750
Z (Angstrom)
Acknowledgements
Fig. 4. Composition profile of the Fe/Cr superlattice. Work was performed under D O E contract W-31-109Eng-38. six different energies, thus deriving the composition profile of the Fe/Cr superlattice. The resonant effect near the K-edges of Fe and Cr gives strong element sensitivity and allows a more unequivocal analysis of the reflectivity measurement.
References [1] R. Rohlsberger et al., SPIE, Vol. 1160, X-Ray/EUV Optics for Astronomy and Microscopy, Vol. 26 (1989).
J. Bai et al./Physica B 221 (1996) 411~15
[2] S.C. Woronick et al., J. Appl. Phys. 66 (1989) 3566. [3] I.M. Tidswell, B.M. Ocko, P.S. Pershan, S.R. Wasserman, G.M. Whitesides and J.D. Axe, Phys. Rev. B 41 (1990) 1111. [4] P. Boher, P. Houdy and C. Schiller, J. Appl. Phys. 68 (1990) 6133. [5] J. Bai, M. Tomkiewicz and P.A. Montano, Z. Phys. B 97 (1995) 465 and references therein.
415
[6] E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers and S.D. Bader, Phys. Rev. B 48 (1993) 15755. [7] E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers and S.D. Bader, Appl. Phys. Lett. 63 (1993) 1699. [8] B. Vidal and P. Vincent, Appl. Opt. 23 (1984) 1794 [9] J. Crank, The Mathematics of Diffusion (Clarendon Press, Oxford, 1975).
Physica B 221 (1996)411 415
Resonant X-ray reflectivity study of Fe/Cr superlattices Jiaming Bai a'b, Eric E. Fullerton b, Pedro A. M o n t a n o b'c'* aBrooklyn College of the City University o f New York, New York, USA b Materials Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Bldg. 223, Argonne, IL 60439, USA c University oflllinois, Chicago, IL, USA
Abstract We have measured the composition profile on an Fe/Cr superlattice using glancing incidence X-ray reflectivity. Resonant reflectivity measurements were carried out by tuning the X-ray energy around the respective K-edge of Fe and Cr. We were able to obtain excellent fits to the data and get consistent geometry and composition parameters from the reflectivity measurements at six different X-ray energies. We obtained valuable information on the interface composition of the superlattice and observed also a slight variation in composition at the bottom and top interfaces. The information obtained using this method allows a determination of not only the electron density but also the composition profiles of the multilayers. This non-destructive technique is a promising tool for the determination of the chemical composition of thin film.
1. Introduction X-ray scattering techniques provide a powerful tool to study electron density profiles in thin films. These techniques have become more powerful with the advent of synchrotron radiation sources which allows tunability of the X-rays. Over the last few years, glancing angle X-ray reflectivity measurements are frequently used to investigate the microstructures of thin films on thick substrates [1-4]. The specular reflectivity measurements provide useful structural information such as sublayer thickness, effective electron density and absorption coefficient (which are respectively related to the real and imaginary parts of the complex dielectric constant), as well as information on interfacial roughness. However, the conventional reflectivity measurement is not an element sensitive technique, it can only be used to detect the electron density but not the atomic composition of the sample.
On the other hand, the electron density and the linear absorption are strong energy dependent variables near the absorption edges of the materials. Resonant X-ray reflectivity contains more information than ordinary reflectivity and allows a determination of the atomic composition from the X-ray dispersion behavior of the sample [5]. This element of contrast expands the experimental capabilities of X-ray reflectivity measurements, as was recently applied to the determination of the composition profile of permalloy thin films [5]. We report in this paper resonant reflectivity measurements on an Fe/Cr superlattice. The measurements were carried out by tuning the X-ray energy below and above, close and away from the respective K-edges of Fe and Cr. We describe in the following paragraphs the experimental procedures and results of our measurements. 2. Experimental
*Correspondence address: Materials Sciences Division, Argonne National Laboratory, 9700 S. Cass Avenue, Bldg. 223, Argonne, IL 60439, USA.
A (1 0 0)-oriented [Fe(20 A)/Cr(11 A)] 2o superlattice was grown by DC magnetron sputtering onto an
0921-4526/96/$15.00 @ 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 9 5 9 - 0
412
J, Bai et al./Physica B 221 (1996) 411-415
epitaxially polished single-crystal MgO(l 0 0) substrate. The sputtering chamber had a base pressure of < 5 x 10-STorr. The 2" planar magnetron sputtering guns were operated in an Ar pressure of 3 naTorr and a target-substrate distance of 9 cm. A 100A Cr buffer layer was initially deposited at a substrate temperature of 600°C to establish the epitaxial orientation with the substrate and provide a template for the growth of the superlattice. The MgO(1 0 0) surface (a0 = 4.213 A) has an ao/',v/2 = 2.98A square surface net which is only 3.3% lattice mismatched with the Cr(1 00) surface upon a 45 ° rotation about the surface normal (i.e. Cr[0 1 0]//MgO[0 1 1]). The substrate was then cooled to 250°C and the superlattice is grown by sequential deposition of the Fe and Cr layers. This procedure results in the growth of epitaxial Fe/Cr(1 0 0) superlattices which exhibited oscillatory interlayer magnetoresistance and record magnetoresistance values [6,7]. The reflectivity measurements were conducted on beam line X-6B at the National Synchrotron Light Source at BNL. The experimental X-ray energies were 5.8, 6.1, 6.94, 7.16, 7.3, and 8 KeV. The Fe and Cr K-edge energies are 7.112 and 5.989 keV, respectively. Horizontal and vertical slits were placed before the monochromator to define the beam size prior to monochromatization of the radiation. Inside the experimental hutch a second slit was placed for defining the monochromatic beam, a beam monitor was placed after this slit to measure the photon flux before scattering from the sample. A third slit was placed after the beam monitor (ionization chamber) for trimming to the slit scattering, but causing no reduction in the collimated photon beam. The sample was mounted on a high precision goniometer attached to a four circle Huber diffractometer. After the sample a fourth slit was placed to cut down any scattering not originating in the sample. The last slit was placed before a NaI scintillation detector and was set wide enough to accept all the specularly reflected scattering. The distance between sample and detector was over a meter, consequently the signal due to fluorescence from the sample was negligible. At very low angles the X-ray intensity is very large and this will saturate and damage the detector. To avoid this problem at very low angles we used a set of calibrated aluminum absorbers to reduce the beam intensity reaching the detector. In X-ray specular reflectivity measurements one must determine the angle of incidence with great accuracy. The alignment of the sample and diffractometer was carried out using a methodology similar to that described in the literature [5]. The diffuse contribution to the scattering was subtracted from all the spectra. The experimental data were analyzed using a non-linear least square fitting program. The best fit was obtained when the reflectivity spectra for the different energies gave consistent structural parameters (layers thickness and interfacial roughness).
All the measurements were carried out at room temperature.
3. Results and discussion
The X-ray waves reflected from an interface between two media el and E2 can be represented by E2 = Ae-ik2~(z z2) + Beik~A= --2),
(1) E t = Ce ikt'(z zl) + D e ik''(= ~'),
where kjz = kox/'~:; - cos2 0o. The amplitudes A, D, C, D are related [8] by A = C + rFe2ik~=dD,
(2) B = r F C + eeik~dD,
where d = Z 2 - Z ~
and r is the Fresnel formula
r = (k2z - klz)/(k2z + k~z), To calculate the reflectivity
from a multilayer, Eq. (2) can be treated iteratively, the final reflectivity will be given by B / A . The form factor F is given by (e
E =
i(k,=+ k~)h(x, y)):,y
(3)
( c - i ( k . -k2z)h(x, v)):,r '
where h ( x , y ) is the two-dimensional profile function of the roughness and ( )xr means in-plane averaging. For an ideal interface, h(x,y) = 0 for all x , y , and F = 1. Assuming the ergodicity of the profile function h(x, y), the in-plane average can be written as nx
(e itk~+k~)ht~.)'))~y=
t
e i(k~=+k~):co(z)dz,
(4)
:(
where z is relative to the average position of the interface and re(z) is the height distribution function of the interface. It can be related to the derivative of the in-plane average of the effective electron density profile by 1 o(~)
-
-
-
dp
(5)
P2 - - P l d z
In this way, the reflectivity is related to the effective electron density without using the Born approximation. The relation between the real dielectric coefficient and the effective electron density is e,} = 1 - 26j,
6j = 0i )~2/2re"
Now we consider an interface between two metal layers, e.g. Cr and Fe. The total electron density is the sum of those from Cr and Fe atoms assuming the same spatial function for the composition profile [-9]. In our analysis, a simple Gaussian model suffices to describe the
413
J. Bai et al./Physica B 221 (1996) 411-415
structure of the interface (it includes roughness and diffusion). The data curves at different X-ray energies exhibit strong energy dispersive effects. Fig. 1 shows the linear absorption coefficients and effective electron densities for Fe and Cr. The letter a through f indicate the energy selected for our measurements. Fig. 2 shows model calculations with and without absorption effects. The strong effect of the K-edge absorption on the reflectivity spectra is evident. It is clear that absorption effects are extremely important for analysis of the data. This is clearly illustrated in Fig. 2; at 7.3 and 6.94 KeV when absorption is not included there is practically no difference between the Fe and Cr contributions. Resonance effects allow to differentiate the contribution to the reflectivity from elements with similar effective electron densities. We show in Fig. 3 refleetivity measurements at various energies as well as the fitting curves. The data curves at different X-ray energies exhibit strong energy dependence effects. This can be seen from the curve variation near the critical angle and the changes in the shape of the
a
300
b
c
Fe/Cr Sample A, Model Ca|culation (no absomtlon))
,.
r ....
= ....
~
, ....
E
0.0
J ....
, ....
=
0.0
0.2
i ....
, ....
,
•
8
0.3
0Key
0A C=(1/A)
0.5
0.8
0.7
Fe/Cr Sampte A, Model Calculation (With absorption))
de
, , , ~ , , , i , i , ,
I .... ~
Cr
E
=
8
.
0
K
e
v
E" 250
e- 200 0
150
o
loo e--I
50
~
~
, , , I , , i , i i
:
oo
i
, I ; i , , I , L l , : l , ,
oood
5500
65oo
l i l l = l
7ooo)
75oo
:
E
Fe
' '
' 1 ' '
. . . .
I
'
]: r
:
80oo i
]:
/
[
. . . .
~
'
3.5 0
._~ g3.0
C (
Q>
2.5
._> LU
2.0 5600
i
0.2
. . . .
i
03
. . . .
i
0.4 qz{ltA1
. . . .
~
0.5
. . . .
r
. . . .
o.e
i
0.7
. . . .
l
0.8
Fig. 2. Model calculations of the reflectivity using the parameters from Fig. 1 and the nominal layer thickness. The top figure is calculated with no X-ray absorption and the bottom including absorption effects.
::
"13 E
E
,
0.0
60oo
65oo
7000
75o0
6000
X-ray energy (eV)
Fig. 1. Linear absorption coefficients (top) and effective electron densities (bottom) as function energy for Fe and Cr (a = 5.8 KeV, b = 6.1 KeV, c = 6.94KeV, d = 7.16 KeV, e 7.3 KeV, f = 8.0 KeV).
oscillations envelope, as well as the superlattice Bragg's peaks. These differences are due to the energy dependence of the effective electron densities and linear absorption coefficients, o f f ' a n d f " . Each element in the sample responds differently to the X-ray energy variations while the total energy dependent effective electron densities and linear absorption coefficients are simply the sum of those of the components, weighted by their atomic weight percentages. In other words, we can calculate the effective electron densities and linear absorption coefficients at the various energies. So with a single group of composition and geometry parameters, we calculated the reflectivity and fit it to the experimental data curves at all
414
J. Bai et al./'Physica B 221 (1996) 411 415
10
TM
•
'
. i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
t
.
.
.
.
Table 1 List of parameters obtained from the analysis of the measurements (errors are given in parentheses)
.
1 0 ~5 f
1 0 ~4 10,3
Layer thickness
10~2 1011
Cr buffer Cr/Fe bi-layers Cr layers Top Fe layer Top Cr layer
101° 10 s 10 B 1 O7
.#
10 s
~
10 4
107 (3) 31.3 (1) 11.4 (4 23 (2) 16.6 (1.2)
._~ 10 s
Interface thickness/roughess
MgO/Cr buffer Cr buffer/Fe Fe/Cr Cr/Fe Top Fe/top Cr Top Cr/air
10 3 10 ~ 10 ~ 10 o 10-1 10-2
7 (3) 3.7 (1.0) 1.7 (3) 2.9 (2) 2 (1) 4.3 (1.5)
10-3
Percent oxygen in top layers
10 4 lO-S
Top Fe layer Top Cr layer
10-6 lO-r 10-8
,
,
,
L
. . . .
0.0
m
. . . .
0.0
i
. . . .
0.2
i
. . . .
0.3
~
. . . .
i
0.4
. . . .
0.5
%
11 (1) 43 (2)
i
0.6
Qz (l/A)
Fig. 3. Reflectivity measurements and fit (continuous lines) for the Fe/Cr superlattice: (a) 5.8 KeV; (b) 6.1 KeV; (c) 6.94 KeV; (d) 7.16 KeV; (e) 7.3 KeV; 8 KeV. The Fe and Cr K-edge energies are 7.112 and 5.898 KeV, respectively.
e~
o
. . . . -50
i 0
. . . .
= . . . . 50
, . . . . 1130
t,, 150
,,
i I I' 200
*'
i . . . . 700
Fig. 4 is the schematic diagram of our fitting model. The parameters are given in Table 1. While the composition parameters are closely related to the shape of the oscillations envelope, the geometrical parameters are linked to the periods of the oscillations. At the top interface there is clear evidence of oxide formation. There is enough oxygen diffusion to affect the top iron layer. We carried out surface EXAFS measurements on this sample and found evidence of metal-oxygen bonds at the top Fe and Cr layers. Details of the EXAFS work will be reported elsewhere. In conclusion, the resonant X-ray reflectivity is a promising non-destructive method to measure the composition profile of multilayers and thin films. With this method, we were able to measure the composition profile of an Fe/Cr superlattice, obtaining important information on the interface structure.
]
750
Z (Angstrom)
Acknowledgements
Fig. 4. Composition profile of the Fe/Cr superlattice. Work was performed under D O E contract W-31-109Eng-38. six different energies, thus deriving the composition profile of the Fe/Cr superlattice. The resonant effect near the K-edges of Fe and Cr gives strong element sensitivity and allows a more unequivocal analysis of the reflectivity measurement.
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