a resistometric study of nb/ta single-crystal superlattices

4
Vacuum/volume 41/numbers 4-6/pages 1237 to 1240/1990 0042-207X/90$3.00 + .00 Printed in Great Britain @ 1990 Pergamon Pressplc A resistometric study of Nb/Ta superlattices single-crystal K H Huang, M G Blamire and R E Somekh, 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 multilay- ers 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 sput- tering 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 charac- terisation 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 min- ~ on four orientations of sapphire sub- strates (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 compen- sated 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 esti- mated to be 0.1% or better. Results and discussion The buffer layer characterisation measurements. The constant low resistance ratio RR (ratio of resistivity at 293 K to resistiv- ity at 6 K) for the NbsoTaso alloy allows for a correction of our resistivity measurements of the superlattices. The resistance ratio RR 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 RR 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 orienta- tion of the sapphire substrate. X-ray characteristation. Figure 1 is an X-ray trace (Cu K~ radiation of Philips 0-20 vertical diffractometer) of a Nb/Ta 1237

Upload: kh-huang

Post on 15-Jun-2016

217 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: A resistometric study of Nb/Ta single-crystal superlattices

Vacuum/volume 41/numbers 4-6/pages 1237 to 1240/1990 0042-207X/90$3.00 + .00 Printed in Great Britain @ 1990 Pergamon Press plc

A resistometric study of Nb/Ta superlattices

single-crystal

K H Huang, M G Blamire and R E Somekh, 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 multilay- ers 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 sput- tering 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 charac- terisation 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 sub- strates (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 compen- sated 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 esti- mated 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 resistiv- ity 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 orienta- tion of the sapphire substrate.

X-ray characteristation. Figure 1 is an X-ray trace (Cu K~ radiation of Philips 0 -20 vertical diffractometer) of a Nb/Ta

1237

Page 2: A resistometric study of Nb/Ta single-crystal superlattices

K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices

106

-~ 1°5

Ln 1 o ~ o

T. "~ 10 3

10 2

' 2'.o ' ' ' 21o ' 3 :2 1.6 2.1. 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.

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) < AX(211) < Ax(222).

Figure 2 is a graph of low temperature resistivity vs inverse wavelength and shows Po varying with orientation as p o ( 2 1 1 ) > p o ( 2 O O ) > p o ( l l O ) . 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 temper- ature 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

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 Nb/Ta (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 Nb/Ta super- lattices on different orientations of sapphire substrate with wavelengths varying from 1 to 10 nm. (From our measure- ments 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 com- pared to the layer thickness indicate that the films are struc- turally 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)

.-~ 4

~ 3

g. E 2

(nm)

10 5

0 i , , , , i , , , , i ,

0 50 100 150

1 ~x 100 (nm-l)

alloy

A - p l a n e

= R - p l a n e

* M - p l a n e

Figure 2. Low temperature resistivity Po vs inverse wavelength showing a correlation of the resistivity of Nb/Ta superlattices with the growth orientation.

Table 1. Resistance ratio, resistivity and mean free path of Nb/Ta multilayers on different orientations of sapphire

Sample Orientation Wavelength dNb/dTa No of layers RR Po* Pr* I t (nm) (nm) (~f~-cm) ~f~-cm) (nm)

1 A 1.16 0.56/0.6 250 4.43 3.64 16. I 11 2 R 1.15 0.54/0.61 250 4.15 3.97 16.5 I0 3 M 1.16 0.55/0.61 250 3.8 4.46 17 9 4 R 2.45 1.18/1.27 120 5.5 2.94 16.1 14 5 A 2.45 1.17/1.33 120 5.2 3.14 16.34 13 6 M 2.5 1.17/1.32 120 4.9 3.38 16.6 12 7 C 2.4 1.08/1.32 120 4.6 3.67 16.9 11 8 A 4.89 2.05/2.84 80 8.9 1.76 15.66 26 9 R 4.5 1.94/2.56 80 7.1 2.28 16.18 18

10 A 7.23 2.9/4.33 64 12.3 1.27 15.6 31 11 R 6.9 2.48/4.42 64 I 1 1.44 15.79 28 12 M 7.1 2.77/4.33 64 10.8 1.46 15.81 27 13 R 9.92 5.16/4.76 40 12.6 1.24 15.59 32 14 M 9.82 5.07/4.75 40 11.2 1.41 15.76 28

* Po resistivity at 6 K, Pr resistivity at 293 K. t mean free path at 6 K (pl = 40 ~12-cm x nm'2).

1238

Page 3: A resistometric study of Nb/Ta single-crystal superlattices

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 RR can be explained by the differing RR 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 RR of 130 for a Nb film 175 nm thick. We estimate the RR 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 RR

with position along a substrate as seen in Figure 3(a). How- ever, data for highest quality films shows a similar variation to that shown in Figure 3(b) in which the RR exhibits a charac- teristic non-monotonic variation with one or two RR '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 RR increase about 4.6% superimposed on the smooth variation due to composition variation.

Flevaris et al ~ reported on Cu-Ni multilayers grown with lattice-commensurate wavelengths. Their X-ray results indi- cated 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 RR 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 RRs 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)

15

14

13

12

1I

10

"~\NN~N ~ • Sample a

\ \ g Sample b \ \

\ "~-.~ "L.

L ' L . ~.. - . _

, i , i , J , L , i , i

0 2 4 6 8 10 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.

.o

oo

15

10 / /

/ / /

/ I

/ /..s~

/

cl A-plane

• Model

2 4 6 8 l0 12

Wavelength (nrn)

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 wave- length 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 experimen- tal 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. Anodisation spec- troscopy 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 correspond- ing 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 differen- tial 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

Page 4: A resistometric study of Nb/Ta single-crystal superlattices

K H Huang et al: A resistometric study of Nb/Ta single-crystal superlattices

8

Q

b 25 40

I 3O

~15 ~=7s nm 20 8.

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) Ta=0 .67nm V - ~ , N b = 0 . 8 0 n m V - 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 measure- ments. These measurements suggest that over some regions of

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.

Acknowledgements

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 FRS for making available the resources of the Department of Materials Science and Metal- lurgy. Funding for this project was partly provided by the Science and Engineering Research Council.

References

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).

1240