- Email: [email protected]
Ta single-crystal superlattices
Vacuum/volume 41/numbers 4-6/pages 1237 to 1240/1990
0042-207X/90$3.00 + .00 @ 1990 Pergamon Press plc
Printed in Great Britain
A resistometric study of Nb/Ta single-crystal superlattices K H H u a n g , M G Blamire and R E S o m e k h , Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK
We report on resistometric measurements of precisely deposited films of single -crystal Nb /Ta superlattices. These superlattices have been uhv sputter deposited onto a buffer layer of epitaxial NbTa alloy which was itself deposited onto a variety of different sapphire substrate orientations. Using lithographic patterning, comparisons of the resistance ratio and resistivity of small strips were made along the substrates across which a known variation in composition has occurred. The superlattices, which contain near-integral numbers of monolayers, show some additional variation of resistance ratio along the length of substrate in excess of that expected for a fixed interfacial mixing. A preliminary model to illustrate this effect is developed which takes account of intermixing at the interfaces. Anodisation spectroscopy results are presented which provide information about the atomic registry and interfacial roughness of superlattices.
Introduction Metal multilayers have been studied intensively for the last ten years and there is wide interest in the properties of these special films.t If the two metals have the same structure and a similar lattice parameter, high quality single-crystal metallic multilayers can be grown. Some experiments have been aimed at growing perfect films in which each layer of the two elements is made up of an exact integral number of monolayers, but to date insufficient results have been generated to demonstrate success in achieving this goal. There is a need for very precise control of the deposition rate during the whole deposition process and the use of low deposition temperatures to reduce the intermixing at interfaces. Nb and Ta have the same structure and a small lattice misfit ( < 0.1%) and Nb/Ta superlattices have been grown epitaxially on sapphire using molecular-beam-epitaxy (MBE) z'3 and sputtering 4. The crystal structure, intermixing and superconducting properties of these superlattices have been studied by many workers 2-6. In this paper we report on the sputter deposition of Nb/Ta superlattices using precise rate control, and the characterisation of these structures by resistivity, X-ray diffraction and anodisation spectroscopy measurements.
Experimental procedure and sample preparation Nb/Ta superlattices were made by dc magnetron sputtering in an uhv deposition system (based pressure less than I x 10 - 7 Pa). The deposition system and computer-controlled power supplies have been described elsewhere 7'8. Initially a I00 nm epitaxial alloy buffer layer of NbsoTaso was grown at the rate of 3 nm m i n - ~ on four orientations of sapphire substrates (A(1120), R(lI02), M(1010), C(0001)) at 1120 K. The 400 nm superlattices were then deposited on top of the alloy at a temperature of between 820-1020 K at a sputtering pressure
of 3 Pa and a target-substrate distance of 70 mm. The samples were lithographically patterned and CF4 etched to produce a number of 750 + 5 #m long and 25 + 2/~m wide test strips along the length of each slide. A standard four point probe technique was used to measure the resistance of samples in liquid nitrogen and in helium gas close to the superconducting transition. The deposition rate changes slightly with target erosion, and errors of the order of 0.3% occur if this effect is not compensated for. A preliminary series of runs was carried out to determine the variation of deposition rate. It was found that for a new target the deposition rate dropped linearly by about 0.4% over the first five runs. This enabled us to compensate for erosion of the target by means of computer control of the power to the magnetrons during deposition. After calibration, the control of layer thickness throughout a given run is estimated to be 0.1% or better.
Results and discussion The buffer layer characterisation measurements. The constant low resistance ratio R R (ratio of resistivity at 293 K to resistivity at 6 K) for the NbsoTaso alloy allows for a correction of our resistivity measurements of the superlattices. The resistance ratio R R of 100 nm NbsoTaso alloy with post-annealing at 807 K for two hours in vacuum was found to be 3.23 + 0.15; Po (resistivity at 6 K) = 6.35 + 0.5 #f~-cm. At larger thicknesses, our measurements extrapolate to a bulk resistivity P0 of about 5.6 #f~-cm and R R of 3.39 which is within the experimental error of the results of Ogasawara et al 9 for bulk NbTa alloys. We also find that the resistivity is independent of the orientation of the sapphire substrate.
X-ray characteristation. Figure 1 is an X-ray trace (Cu K~ radiation of Philips 0 - 2 0 vertical diffractometer) of a Nb/Ta 1237
K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices
which corresponds well to our X-ray results of the interface sharpness Ax(l~o) < Ax(200) < Ax(Zl 1) < Ax(222)10. These differ slightly from the results of Durbin et al E'5 who do not see such a clear correlation with RRt=IO) > RR(211) > R-R(222) > RR(200) and interface sharpness variation Ax(200)< Ax(I10) <
106 -~ 1°5
AX(211) < Ax(222). Ln
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o
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"~ 103
102 1.6
'
2'.o
'
2.1. '
'
21o
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3:2
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O ( 21T/d ) Figure 1. X-ray intensity vs diffraction vector Q(2~/d) of a Nb/Ta
superlattice (.wavelength 7.3 nm, 64 layers) on A-plane sapphire. Peak at Q = 2.65 A-~ is the substrate reflection.
Figure 2 is a graph of low temperature resistivity vs inverse wavelength and shows Po varying with orientation as po(211)>po(2OO)>po(llO). This confirms the work of Durbin et al 5 who suggest that the quality of the interfaces is dependent on the structure rather than due to inter-diffusion. As the wavelength approaches zero, Po tends to the low temperature resistivity of the alloy namely 5.5/~l)-cm. The second general frame of our resistometric study is the variation of R R across a given slide; this is due to the geometry of the sputtering system leading to a composition gradient across each slide. The composition variation from one end of a (nm)
superlattice with 64 periods (7.3 nm) on A-plane sapphire and it shows at least eleven satellite peaks, indicating very sharp interfaces. Using Durbin et al's calculation 5, we estimate that the intermixing [Ax = (4Dz) ~/2] of our N b / T a (110) films on A-plane sapphire is between 0.14 to 0.18 nm. Even for an exact interface, we could expect the electron density change to occur over about one plane spacing giving a minimum Ax in the region of 0.1 nm. Resistometric measurements. Table 1 gives the values of the
resistance ratio, resistivity and mean free path of N b / T a superlattices on different orientations of sapphire substrate with wavelengths varying from 1 to 10 nm. ( F r o m our measurements the bulk resistivities of Nb and Ta are 14.35/~f~-cm and 15.09 p~-cm.) The high resistance ratios and long mean free paths compared to the layer thickness indicate that the films are structurally good and impurity free. We found that resistance ratios [RRu, k~) for orientation (hkl)] varied with the orientations of the superlattices, with RR(= ~0) > RR(200) > RR(2t ~) > RR(222)
10 5
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1 100 (nm-l) ~x Figure 2. Low temperature resistivity Po vs inverse wavelength showing a correlation of the resistivity of Nb/Ta superlattices with the growth orientation.
Sample
Orientation
Wavelength (nm)
dNb/dTa (nm)
No of layers
RR
Po* (~f~-cm)
Pr* ~f~-cm)
It (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
A R M R A M C A R A R M R M
1.16 1.15 1.16 2.45 2.45 2.5 2.4 4.89 4.5 7.23 6.9 7.1 9.92 9.82
0.56/0.6 0.54/0.61 0.55/0.61 1.18/1.27 1.17/1.33 1.17/1.32 1.08/1.32 2.05/2.84 1.94/2.56 2.9/4.33 2.48/4.42 2.77/4.33 5.16/4.76 5.07/4.75
250 250 250 120 120 120 120 80 80 64 64 64 40 40
4.43 4.15 3.8 5.5 5.2 4.9 4.6 8.9 7.1 12.3 I1 10.8 12.6 11.2
3.64 3.97 4.46 2.94 3.14 3.38 3.67 1.76 2.28 1.27 1.44 1.46 1.24 1.41
16. I 16.5 17 16.1 16.34 16.6 16.9 15.66 16.18 15.6 15.79 15.81 15.59 15.76
11 I0 9 14 13 12 11 26 18 31 28 27 32 28
1238
R-plane
150
Table 1. Resistance ratio, resistivity and mean free path of Nb/Ta multilayers on different orientations of sapphire
* Po resistivity at 6 K, Pr resistivity at 293 K. t mean free path at 6 K (pl = 40 ~12-cm x nm'2).
=
K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices
to the other was about 12 + 1 at%. The major part of this variation of R R can be explained by the differing R R of the Nb and Ta used: in separate single-crystal thin film experiments we found the highest R R for Ta was 70 for a 440nm film, compared with a R R of 130 for a Nb film 175 nm thick. We estimate the R R of our Nb to be four times greater than that of the Ta we used. With this difference in R R for the two pure elements one expects smooth monotonic variation of the superlattice R R with position along a substrate as seen in Figure 3(a). However, data for highest quality films shows a similar variation to that shown in Figure 3(b) in which the R R exhibits a characteristic non-monotonic variation with one or two R R 'peaks'. We found that these peaks occur when there are near-integral numbers of monolayers in each of the layers of Nb and Ta. For the sample shown we see a R R increase about 4.6% superimposed on the smooth variation due to composition variation. Flevaris et al ~ reported on C u - N i multilayers grown with lattice-commensurate wavelengths. Their X-ray results indicated that their layer structure had sharper interfaces if their layers had near-integral numbers of monolayers. In order to gain some insight into this increase of R R we observe, we have developed a simple model to try to obtain a zeroth approximation to a possible cause of this effect. The model first fits the monotonic variations of resistance ratio across each slide and of layer thickness. In the model it is assumed that the films may be visualized as layers of resistive material in parallel, layers of pure Nb and Ta with their respective R R s and a graded interface modeled as a N b - T a alloy. Though this model is crude, by adjusting the thickness of the alloy layer (made up of 25%, 50% and 75% alloy layers)
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Figure 4. Variation of resistance ratio with modulation wavelength, [] are experimental data of Nb/Ta superlattices on A-plane sapphire for composition Nb 50% in the centre. • are values generated using the computer model. The spread of the resistance ratio for a given wavelength is that due to the composition variation across a slide. The broken line is a guide for the eye.
and the R R s of the Nb and Ta a good fit to all the experimental data is achieved. This comparison is shown in Figure 4. The point of this model is that we can also extract an estimate of how the interface layer must change in order to give the observed increases in the R R when the layers are of integral monolayer thickness. The model shows that whilst the effective interface is of order 1 nm thick, the observed increase of R R corresponds to a reduction of the interface region by about 0.2 nm comparable to the atomic plane spacing.
Anodisation spectroscopy measurements.
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Figure 3. RR at different positions along two slides from the same deposition run. Sample (a) • is a superlattice of wavelength 6.9 nm on R-plane sapphire. The variation of RR is only due to changing composition. Sample (b) C~ is a superlattice of wavelength 7.2 nm on A-plane sapphire. For each position the Nb layers change by 0.42 monolayers and Ta by 0.236 monolayers. Positions 2 and I I have near-integral numbers of monolayers (Nb 14.9, Ta 17.0 monolayers and Nb ll.l, Ta 19.1 monolayers repeats) showing an increment of RR near 0.6. The broken lines show variation of RR with different composition.
Anodisation spectroscopy depends on the difference in the rate of voltage-rise of different elements when a constant current is applied to an electrochemical cell containing the specimen under study as the anode; thus the presence of an interface appears as a change in gradient, its abruptness by the sharpness of its gradient change. Using the technique we have shown that it is possible to observe directly individual atomic planes in high quality Nb/Ta epitaxial superlattices 13. The signal corresponding to the atomic plane is superimposed on the superlattice structure associated with the different anodisation rates of the two metals. Figure 5 shows typical results from these experiments: in each case the direct voltage signal (shown in Figure 5(b)) increases monotonically with time and shows a small periodic change in slope as the interfaces between the two elements are passed. This behaviour is much more obvious with the differential signal (Figure 5(a)): the waveform approximates to a square-wave, reflecting the different anodisation rates of the two elements; the Ta shows a higher gradient. Data such as this can be used to provide a measure of the degree of intermixing at each interface between two elements. Preliminary results show that the extent of the agreement is good and also indicate that in general the interface is sharper when a Ta layer is grown on Nb. 1239
K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices
the substrate the resistance ratio is increased and we suggest that this is due to the metal layers being grown which are near integral numbers of atomic layers.
Q
8
Acknowledgements
25
b
~15
3O
We would like to thank Z H Barber, C E Davies, R H Highmore and A L Greet for discussion and reading the manuscript and D Glowacka for help with the figures. We acknowledge Professor D Hull F R S for making available the resources of the Department of Materials Science and Metallurgy. Funding for this project was partly provided by the Science and Engineering Research Council.
20 8.
References
40
I
~=7snm
5
I0
Time (scaled) Figure 5. Anodisation plots of (110) Nb/Ta superlattices showing the direct voltage signal (b), and the differential (a) vs time. Superlattice wavelength = 7.5 nm (l), 3.4 nm (2). Plots are scaled to show the same rate of voltage rise with time. Calibrations derived from EDX are curve (l) T a = 0 . 5 6 n m V - l , N b = 0 . 7 1 n m V - l ; curve (2) T a = 0 . 6 7 n m V -~,Nb=0.80nmV-L
Conclusions With the precise control of deposition which we can achieve, the variation of layer registration over a single substrate has been investigated by using a small area resistometric measurements. These measurements suggest that over some regions of
1240
t L L Chang and B C Giessen (Eds), Synthetic Modulated Structured. Academic, New York (1985). 2 S M Durbin, J E Cunningham and C P Flynn, J Phys F, 12, L75 (1982). 3 y Nishihata, M Nakayama, H Kato, N Sano and H Terauchi, J Appl Phys, 60(10), 3523 (1986). 4 p R Broussard and T H Geballe, Phys Rev, B35, 1664 (1987). 5 S M Durbin, J E Cunningham and C P Flynn, J Phys F, 17, L59 (1987). 6 j L Cohn, J J Lin, F J Lanelas, H He, R Clarke and C Uher, Phys Rev, B38, 2326 (1988). 7 R E Somekh, R J Highmore, K Page, R J Home and Z H Barber, MRS Syrup Proc, 103, 29 (1988). s R E Somekh and Z H Barber, J Phys E, 21, 1029 (1988). 9 T Ogasawara, Y Kubota and K Yasukochi, J Phys Soc Japan, 25, 1307 (1968). l0 R E Somekh, W C Shih, K Dybye, K H Huang and C S Baxter, Proc SPIE, 1140, 453 (1989). 11 N K Flevaris, D Baral and J E HiUard, Appl Phys Lett, 38(12), 992 (1981). t2 M Gurvitch, Phys Rev, B34, 540 (1986). 13 M G Blamire, K H Huang, R E Somekh, E C G Kirk, G W Morris and J E Evetts, Appl Phys Lett, 55(8), 732 (1989).
0042-207X/90$3.00 + .00 @ 1990 Pergamon Press plc
Printed in Great Britain
A resistometric study of Nb/Ta single-crystal superlattices K H H u a n g , M G Blamire and R E S o m e k h , Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK
We report on resistometric measurements of precisely deposited films of single -crystal Nb /Ta superlattices. These superlattices have been uhv sputter deposited onto a buffer layer of epitaxial NbTa alloy which was itself deposited onto a variety of different sapphire substrate orientations. Using lithographic patterning, comparisons of the resistance ratio and resistivity of small strips were made along the substrates across which a known variation in composition has occurred. The superlattices, which contain near-integral numbers of monolayers, show some additional variation of resistance ratio along the length of substrate in excess of that expected for a fixed interfacial mixing. A preliminary model to illustrate this effect is developed which takes account of intermixing at the interfaces. Anodisation spectroscopy results are presented which provide information about the atomic registry and interfacial roughness of superlattices.
Introduction Metal multilayers have been studied intensively for the last ten years and there is wide interest in the properties of these special films.t If the two metals have the same structure and a similar lattice parameter, high quality single-crystal metallic multilayers can be grown. Some experiments have been aimed at growing perfect films in which each layer of the two elements is made up of an exact integral number of monolayers, but to date insufficient results have been generated to demonstrate success in achieving this goal. There is a need for very precise control of the deposition rate during the whole deposition process and the use of low deposition temperatures to reduce the intermixing at interfaces. Nb and Ta have the same structure and a small lattice misfit ( < 0.1%) and Nb/Ta superlattices have been grown epitaxially on sapphire using molecular-beam-epitaxy (MBE) z'3 and sputtering 4. The crystal structure, intermixing and superconducting properties of these superlattices have been studied by many workers 2-6. In this paper we report on the sputter deposition of Nb/Ta superlattices using precise rate control, and the characterisation of these structures by resistivity, X-ray diffraction and anodisation spectroscopy measurements.
Experimental procedure and sample preparation Nb/Ta superlattices were made by dc magnetron sputtering in an uhv deposition system (based pressure less than I x 10 - 7 Pa). The deposition system and computer-controlled power supplies have been described elsewhere 7'8. Initially a I00 nm epitaxial alloy buffer layer of NbsoTaso was grown at the rate of 3 nm m i n - ~ on four orientations of sapphire substrates (A(1120), R(lI02), M(1010), C(0001)) at 1120 K. The 400 nm superlattices were then deposited on top of the alloy at a temperature of between 820-1020 K at a sputtering pressure
of 3 Pa and a target-substrate distance of 70 mm. The samples were lithographically patterned and CF4 etched to produce a number of 750 + 5 #m long and 25 + 2/~m wide test strips along the length of each slide. A standard four point probe technique was used to measure the resistance of samples in liquid nitrogen and in helium gas close to the superconducting transition. The deposition rate changes slightly with target erosion, and errors of the order of 0.3% occur if this effect is not compensated for. A preliminary series of runs was carried out to determine the variation of deposition rate. It was found that for a new target the deposition rate dropped linearly by about 0.4% over the first five runs. This enabled us to compensate for erosion of the target by means of computer control of the power to the magnetrons during deposition. After calibration, the control of layer thickness throughout a given run is estimated to be 0.1% or better.
Results and discussion The buffer layer characterisation measurements. The constant low resistance ratio R R (ratio of resistivity at 293 K to resistivity at 6 K) for the NbsoTaso alloy allows for a correction of our resistivity measurements of the superlattices. The resistance ratio R R of 100 nm NbsoTaso alloy with post-annealing at 807 K for two hours in vacuum was found to be 3.23 + 0.15; Po (resistivity at 6 K) = 6.35 + 0.5 #f~-cm. At larger thicknesses, our measurements extrapolate to a bulk resistivity P0 of about 5.6 #f~-cm and R R of 3.39 which is within the experimental error of the results of Ogasawara et al 9 for bulk NbTa alloys. We also find that the resistivity is independent of the orientation of the sapphire substrate.
X-ray characteristation. Figure 1 is an X-ray trace (Cu K~ radiation of Philips 0 - 2 0 vertical diffractometer) of a Nb/Ta 1237
K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices
which corresponds well to our X-ray results of the interface sharpness Ax(l~o) < Ax(200) < Ax(Zl 1) < Ax(222)10. These differ slightly from the results of Durbin et al E'5 who do not see such a clear correlation with RRt=IO) > RR(211) > R-R(222) > RR(200) and interface sharpness variation Ax(200)< Ax(I10) <
106 -~ 1°5
AX(211) < Ax(222). Ln
1o~
o
T.
"~ 103
102 1.6
'
2'.o
'
2.1. '
'
21o
'
3:2
3.6
O ( 21T/d ) Figure 1. X-ray intensity vs diffraction vector Q(2~/d) of a Nb/Ta
superlattice (.wavelength 7.3 nm, 64 layers) on A-plane sapphire. Peak at Q = 2.65 A-~ is the substrate reflection.
Figure 2 is a graph of low temperature resistivity vs inverse wavelength and shows Po varying with orientation as po(211)>po(2OO)>po(llO). This confirms the work of Durbin et al 5 who suggest that the quality of the interfaces is dependent on the structure rather than due to inter-diffusion. As the wavelength approaches zero, Po tends to the low temperature resistivity of the alloy namely 5.5/~l)-cm. The second general frame of our resistometric study is the variation of R R across a given slide; this is due to the geometry of the sputtering system leading to a composition gradient across each slide. The composition variation from one end of a (nm)
superlattice with 64 periods (7.3 nm) on A-plane sapphire and it shows at least eleven satellite peaks, indicating very sharp interfaces. Using Durbin et al's calculation 5, we estimate that the intermixing [Ax = (4Dz) ~/2] of our N b / T a (110) films on A-plane sapphire is between 0.14 to 0.18 nm. Even for an exact interface, we could expect the electron density change to occur over about one plane spacing giving a minimum Ax in the region of 0.1 nm. Resistometric measurements. Table 1 gives the values of the
resistance ratio, resistivity and mean free path of N b / T a superlattices on different orientations of sapphire substrate with wavelengths varying from 1 to 10 nm. ( F r o m our measurements the bulk resistivities of Nb and Ta are 14.35/~f~-cm and 15.09 p~-cm.) The high resistance ratios and long mean free paths compared to the layer thickness indicate that the films are structurally good and impurity free. We found that resistance ratios [RRu, k~) for orientation (hkl)] varied with the orientations of the superlattices, with RR(= ~0) > RR(200) > RR(2t ~) > RR(222)
10 5
alloy
.-~
4
~
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2
0
i
0
,
,
50
,
,
i
,
,
100
,
,
i
*
M-plane
,
1 100 (nm-l) ~x Figure 2. Low temperature resistivity Po vs inverse wavelength showing a correlation of the resistivity of Nb/Ta superlattices with the growth orientation.
Sample
Orientation
Wavelength (nm)
dNb/dTa (nm)
No of layers
RR
Po* (~f~-cm)
Pr* ~f~-cm)
It (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
A R M R A M C A R A R M R M
1.16 1.15 1.16 2.45 2.45 2.5 2.4 4.89 4.5 7.23 6.9 7.1 9.92 9.82
0.56/0.6 0.54/0.61 0.55/0.61 1.18/1.27 1.17/1.33 1.17/1.32 1.08/1.32 2.05/2.84 1.94/2.56 2.9/4.33 2.48/4.42 2.77/4.33 5.16/4.76 5.07/4.75
250 250 250 120 120 120 120 80 80 64 64 64 40 40
4.43 4.15 3.8 5.5 5.2 4.9 4.6 8.9 7.1 12.3 I1 10.8 12.6 11.2
3.64 3.97 4.46 2.94 3.14 3.38 3.67 1.76 2.28 1.27 1.44 1.46 1.24 1.41
16. I 16.5 17 16.1 16.34 16.6 16.9 15.66 16.18 15.6 15.79 15.81 15.59 15.76
11 I0 9 14 13 12 11 26 18 31 28 27 32 28
1238
R-plane
150
Table 1. Resistance ratio, resistivity and mean free path of Nb/Ta multilayers on different orientations of sapphire
* Po resistivity at 6 K, Pr resistivity at 293 K. t mean free path at 6 K (pl = 40 ~12-cm x nm'2).
=
K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices
to the other was about 12 + 1 at%. The major part of this variation of R R can be explained by the differing R R of the Nb and Ta used: in separate single-crystal thin film experiments we found the highest R R for Ta was 70 for a 440nm film, compared with a R R of 130 for a Nb film 175 nm thick. We estimate the R R of our Nb to be four times greater than that of the Ta we used. With this difference in R R for the two pure elements one expects smooth monotonic variation of the superlattice R R with position along a substrate as seen in Figure 3(a). However, data for highest quality films shows a similar variation to that shown in Figure 3(b) in which the R R exhibits a characteristic non-monotonic variation with one or two R R 'peaks'. We found that these peaks occur when there are near-integral numbers of monolayers in each of the layers of Nb and Ta. For the sample shown we see a R R increase about 4.6% superimposed on the smooth variation due to composition variation. Flevaris et al ~ reported on C u - N i multilayers grown with lattice-commensurate wavelengths. Their X-ray results indicated that their layer structure had sharper interfaces if their layers had near-integral numbers of monolayers. In order to gain some insight into this increase of R R we observe, we have developed a simple model to try to obtain a zeroth approximation to a possible cause of this effect. The model first fits the monotonic variations of resistance ratio across each slide and of layer thickness. In the model it is assumed that the films may be visualized as layers of resistive material in parallel, layers of pure Nb and Ta with their respective R R s and a graded interface modeled as a N b - T a alloy. Though this model is crude, by adjusting the thickness of the alloy layer (made up of 25%, 50% and 75% alloy layers)
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Figure 4. Variation of resistance ratio with modulation wavelength, [] are experimental data of Nb/Ta superlattices on A-plane sapphire for composition Nb 50% in the centre. • are values generated using the computer model. The spread of the resistance ratio for a given wavelength is that due to the composition variation across a slide. The broken line is a guide for the eye.
and the R R s of the Nb and Ta a good fit to all the experimental data is achieved. This comparison is shown in Figure 4. The point of this model is that we can also extract an estimate of how the interface layer must change in order to give the observed increases in the R R when the layers are of integral monolayer thickness. The model shows that whilst the effective interface is of order 1 nm thick, the observed increase of R R corresponds to a reduction of the interface region by about 0.2 nm comparable to the atomic plane spacing.
Anodisation spectroscopy measurements.
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13
15
i
12
Position
Figure 3. RR at different positions along two slides from the same deposition run. Sample (a) • is a superlattice of wavelength 6.9 nm on R-plane sapphire. The variation of RR is only due to changing composition. Sample (b) C~ is a superlattice of wavelength 7.2 nm on A-plane sapphire. For each position the Nb layers change by 0.42 monolayers and Ta by 0.236 monolayers. Positions 2 and I I have near-integral numbers of monolayers (Nb 14.9, Ta 17.0 monolayers and Nb ll.l, Ta 19.1 monolayers repeats) showing an increment of RR near 0.6. The broken lines show variation of RR with different composition.
Anodisation spectroscopy depends on the difference in the rate of voltage-rise of different elements when a constant current is applied to an electrochemical cell containing the specimen under study as the anode; thus the presence of an interface appears as a change in gradient, its abruptness by the sharpness of its gradient change. Using the technique we have shown that it is possible to observe directly individual atomic planes in high quality Nb/Ta epitaxial superlattices 13. The signal corresponding to the atomic plane is superimposed on the superlattice structure associated with the different anodisation rates of the two metals. Figure 5 shows typical results from these experiments: in each case the direct voltage signal (shown in Figure 5(b)) increases monotonically with time and shows a small periodic change in slope as the interfaces between the two elements are passed. This behaviour is much more obvious with the differential signal (Figure 5(a)): the waveform approximates to a square-wave, reflecting the different anodisation rates of the two elements; the Ta shows a higher gradient. Data such as this can be used to provide a measure of the degree of intermixing at each interface between two elements. Preliminary results show that the extent of the agreement is good and also indicate that in general the interface is sharper when a Ta layer is grown on Nb. 1239
K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices
the substrate the resistance ratio is increased and we suggest that this is due to the metal layers being grown which are near integral numbers of atomic layers.
Q
8
Acknowledgements
25
b
~15
3O
We would like to thank Z H Barber, C E Davies, R H Highmore and A L Greet for discussion and reading the manuscript and D Glowacka for help with the figures. We acknowledge Professor D Hull F R S for making available the resources of the Department of Materials Science and Metallurgy. Funding for this project was partly provided by the Science and Engineering Research Council.
20 8.
References
40
I
~=7snm
5
I0
Time (scaled) Figure 5. Anodisation plots of (110) Nb/Ta superlattices showing the direct voltage signal (b), and the differential (a) vs time. Superlattice wavelength = 7.5 nm (l), 3.4 nm (2). Plots are scaled to show the same rate of voltage rise with time. Calibrations derived from EDX are curve (l) T a = 0 . 5 6 n m V - l , N b = 0 . 7 1 n m V - l ; curve (2) T a = 0 . 6 7 n m V -~,Nb=0.80nmV-L
Conclusions With the precise control of deposition which we can achieve, the variation of layer registration over a single substrate has been investigated by using a small area resistometric measurements. These measurements suggest that over some regions of
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