Thermal stability of Pballoy Josephson junction electrode materials. VII. Concentrationrange of single εphase PbBi films used in counterelectrodesMasanori Murakami, H.C. W. Huang, J. Angilello, and B. L. Gilbert Citation: Journal of Applied Physics 54, 738 (1983); doi: 10.1063/1.332030 View online: http://dx.doi.org/10.1063/1.332030 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/54/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Thermal stability of Pballoy Josephson junction electrode materials. VIII. Effects of Au addition to PbBicounterelectrodes J. Appl. Phys. 54, 743 (1983); 10.1063/1.332031 Thermal stability of Pballoy Josephson junction electrode materials. VI. Effects of film edges on the straindistribution of PbBi counterelectrodes J. Appl. Phys. 53, 3560 (1982); 10.1063/1.331135 Thermal stability of Pballoy Josephson junction electrode materials. IV. Effects of crystal structure of PbBi counterelectrodes J. Appl. Phys. 53, 337 (1982); 10.1063/1.329936 Thermal stability of Pballoy Josephson junction electrode materials. V. Effects of repeated cycling between 298and 4.2 K of PbBi counter electrode J. Appl. Phys. 53, 346 (1982); 10.1063/1.329893 Lead alloy Josephson junctions with PbBi counterelectrodes Appl. Phys. Lett. 36, 334 (1980); 10.1063/1.91483

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Thermal stability of Pb-alloy Josephson junction electrode materials. VII. Concentration range of single €-phase Pb-Bi films used in counterelectrodes

Masanori Murakami, H.-C. W. Huang,a) J. Angilello, and B. L. Gilbert IBM Thomas J. Watson Research Center. Yorktown Heights. New York 10598

(Received 22 July 1982; accepted for publication 21 September 1982)

From the viewpoint of mechanical stability upon repeated thermal cycling between 298 and 4.2 K, Pb-Bi films with single E-phase have been found to be desirable as the counterelectrode material of Pb-alloy Josephson junction devices. The Bi concentration range of the E-phase was studied by asing x-ray diffraction and electron microprobe techniques for Pb-Bi films with various Bi contents prepared by evaporation from an alloy source. The film composition was determined by controlled-potential coulometry. From the lattice constants and phase identification experiments, the Bi content range of the single E-phase was determined to be 27.5-31.8 wt. % ( ± 0.3 wt. %) at room temperature. The lower limit ofthe Bi content of the E-phase agrees reasonably well with those reported previously, but the upper limit of the Bi content is -1 wt. % higher than that of Preece and King, and -2 wt. % lower than that of Predel and Schwermann. Although Predel and Schwermann indicated that there would be a eutectoid at 27.5 wt. % Bi and 227 K, no evidence of phase separation of the E-phase was obtained upon cooling as low as 4.2 K, or upon heating to 350 K. It was found by electron microprobe analysis that Bi-rich regions (/3-phase) as large as 100 p.m in size segregated in the Pb-Bi film surface when the Bi concentration is within the E + f3 phase region.

PACS numbers: 62.20.Dc, 68.60. + q, 81.40.Lm, 74.50. + r

I. INTRODUCTION

The E-phase Pb-Bi films have been used as a countere­lectrode material for experimental Pb-alloy Josephsonjunc­tions, because significant improvements in the junction qua­lity and device thermal cyclability between 298 and 4.2 K have been obtained through their use. 1,2 The imprOVed ther­mal cyclability was considered to be primarily due to more resistance of the E-phase films (hcp structure) to strain relax­ation by dislocation glide at temperatures below room tem­perature. 1 Experimentally, it was demonstrated that the E­

phase films supported more strain elastically than the a-phase films (fcc structure) upon cooling from 298 to 4.2 K (Ref. 3). In addition, the fiber structure of the E-phase Pb-Bi films was found to be unfavorable for dislocation glide: dislo­cation glide planes are approximately perpendicular to the substrate so that they contain only a small component of the planar strain. 3

The phase boundaries of the E-phase at and below room temperature reported for bulk materials differ consider­ably.4-6 In addition, the single E-phase Pb-Bi covers only a small range ofBi content (27-32 wt. % Bi).4-6 Pb-Bi films used for the counterelectrode materials are prepared by eva­porating an alloy source. 1,2 Since the vapor pressures of Pb and Bi vary with evaporating temperature, the film composi­tions vary with evaporation rate (i.e., evaporation source temperature). Thus, an exact Bi composition range of E­

phase Pb-Bi films should be determined. The purpose of the present paper is to determine the Bi

concentration range of the single E-phase Pb-Bi films by us­ing an x-ray diffraction technique, controlled-potential cou­lometry, and electron microprobe. A technique to control

-) Present address: IBM General Technology Division. Hopewell Junction. N.Y. 12533.

the Bi contents of the Pb-Bi films that are prepared by eva­porating an alloy source has been developed7 based on Hertz-Knudsen-Langmuir theory.s Using this technique, Pb-Bi films with various Bi contents were prepared. Since the devices of interest are heated up to - 350 K or cooled down to as low as 4.2 K, the work focused on the phase boundaries below 350 K.

II. EXPERIMENTAL PROCEDURES AND RESULTS

A. Sample preparation

The details of the sample preparation procedure were given in previous papers. 1,2,7 Here, a brief description of it is given.

Pb-Bi films were prepared by evaporating a single alloy source. The source was prepared from Pb and Bi pellets with purities better than 99.99%. Pb pellets were etched in a solu­tion of acetic acid and hydrogen peroxide to remove surface oxide, and then kept in a carbon crucible which was im­mersed in alcohol to minimize surface oxidation. Then, pre­calculated amounts ofBi pellets were added into the crucible to obtain the desired source composition. After draining al­cohol, the source was outgassed and premelted by rf-induc­tion heating in the film deposition system of _10- 7 Torr of vacuum.

The Pb-Bi source was evaporated at vacuum of 1-3 X 10- 7 Torr. Films were deposited onto oxidized (Ill) Si substrates with 2. 54-cm diameter. The distance between the substrate and the source was kept 36.5 cm apart to obtain uniform film thickness throughout the substrate. The sub­strate was kept at - 273 K by circulating ice water through the substrate holder. The film thickness and the deposition rate were measured by a calibrated quartz crystal monitor placed near the substrates. The film thickness was typically

738 J. Appl. Phys. 54 (2). February 1983 0021-8979/83/020738-05$02.40 © 1983 American Institute of Physics 738

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1.0 p,m to provide enough material for chemical analysis with greater accuracy.

The composition of Pb-Bi film prepared by the present method depends on the deposition rate and the source his­tory.7 (The brief description on the relation between the film composition and the deposition rate and source composition will be given later.) By controlling the source composition and deposition rate, Pb-Bi films with Bi concentrations ranging from 7-40% Bi (wt. %) were prepared. The film compositions were analyzed by controlled-potential coulo­metry. A modified analysis procedure of Su and Campbe1l9

was adopted. The Pb-Bi films were first stripped from the Si substrates with 1-2 ml of 6N HN03 • After removing the Si substrate, 1-2 ml of 70% HCl04 was added. The solution was then fumed to dryness on a hot plate. On cooling to room temperature, the residue of Pb and Bi perchlorates was dis­solved in a mixture of 10 ml of I M tartaric acid and 10 ml of 1.25 M perchloric acid. The solution was then transferred, by rinsing with 5 ml of deionized water, to a coulometer cell which contained 8 ml of triple-distilled mercury as a work­ing electrode. The Pb + 2 and Bi + 3 ions were both reduced at - 0.5 V as amalgams into the mercury pool, after which

they were successfully anodically oxidized at - 0.16 V and + 0.2 V using Agi AgCl as a reference electrode. The meth­

od was checked against a known mixture of Pb and Bi and shown to be accurate to ± 0.2%, when a sample weight of 2-3 mg or greater was used.

B. X-ray diffraction study

X-ray diffraction intensity measurements were carried out at ambient temperature using a computer-controlled Phillips x-ray diffractometer. The CuKa radiation (operated at 40 K V and 20 rnA) was used and monochromatic diffrac­tion intensities were obtained by using a single-bent graphite monochrometer. The diffracted intensities were step­scanned for sufficient time at intervals of 20 = 0.01 or 0.02· for a predetermined diffraction angle range. The diffractom­eter was calibrated using a standard W powder sample be­fore and after measurements.

Typical diffraction patterns of (a) Pb-19.0% Bi, (b) Pb-25.2% Bi, (c) Pb-31.7% Bi, and (d) Pb-33.0% Bi film are shown in Fig. 1. All peaks observed in the diffraction pat­terns correspond to either a, E, or p-phase. Peaks denoted by al-a5 correspond to those from a-phase [al: (111), a2: (200), a3: (220), a4: (311), a5: (222)], peaks denoted by EI-E6correspond to those from E-phase [El: (002),E2: (101), E3: (102), E4: (110), E5: (200), E6: (201)], and peaks denoted by P 1-P 6 correspond to thoseofp-phase IP 1- (014),P 2: (105), P3: (113),P4: (204),P5: (116),P6, (216)]. In Fig. l(a) single a(fcc)-phase is observed, in Fig. l(b) two phases of a + E

(hcp) are observed, in Fig. l(c) single E-phase is observed, and in Fig. ltd) two phases of E + P (rhombohedral) are observed. The Bi concentration of the films in which only single E­

phase was observed was in the range 28.0-31.8%. The films with 28.0 or 31.8% Bi were heated up to 350 K or cooled down to 260 K in high-IO or low-temperature cameras II at­tached to the x-ray diffractometers. After annealing for -1 day, no evidence of the phase separation was obtained. Also,

739 J. Appl. Phys., Vol. 54, No.2, February 1983

.~~~~~~. ··~--l

(0) I

a2 a5 a4

10

16 al

E2

10 ~ ~

en z IU I ~ ~

z Q

102

r ~ u ! « I a: IL IL i3 10

2 ' 10 ~

-,. _ ....... ___ -r----_.,.-------.------,--,

E4 (d) I

10

30 40 50 60 70 DIFFRACTION ANGLE 28 (OEG)

FIG. I. Diffraction patterns of (a) Pb-19.0% Bi, (b) Pb·25.2% Bi, (c) Pb· 31.7% Bi, and (d) Pb·33.0% Bi.

a Pb-29.2% Bi film was annealed for 1 week at 200 K in the low-temperature camera and, then, the diffraction pattern was taken at that temperature. No change in the diffraction pattern was observed.

In Fig. 2, the Pb-Bi phase diagram around the E-phase is shown that was constructed based on the present x-ray re­sults. (The phase diagram at higher temperatures is from Ref. 5.) The triangles, circles, and squares indicate the re­spective a + E, E, and E + P phases observed in the diffrac­tion patterns.

The phase boundaries of the E-phase can be determined accurately by measuring the lattice parameters of the E­

phase Pb-Bi. When the alloy composition is within two­phase regions, the lattice constant of the E-phase does not depend on the alloy composition. When the alloy composi­tion is within the single-phase region, the lattice constant of the E-phase Pb-Bi changes with the alloy composition. The lattice constants of the E-phase Pb-Bi films in the single­phase region or the two-phase regions were obtained from the measured diffraction patterns. As seen in Fig. 1, the E­

phase Pb-Bi film has a fiber structure such that the c axis is close to parallel to the film surface. Thus, by the present Bragg-Brentano method, the lattice constants of the a axis

Murakami et al. 739

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g / w / a::

:::> I t:t 400 a+E I E -a:: /' ILl I / a.

f ~ E+{3 ILl 66 0

~

300 M 6 66 <lJl)()0

60

2002~2---2~4---2~6--~2~8-0-3~0--~3L2---3L4--~36

Bi (wt'lfol

FIG. 2. Pb-Bi phase diagram constructed based on the present x-ray experi­ment.

can be determined more accurately than that of the c axis from diffraction peaks with zero I indices, i.e., (110), (200), (210), (3(0), (220), and (310). The lattice constants of the a axis determined using Nelson-Riley's extrapolation meth­od 12 are shown in Fig. 3. It is seen that there are three re­gions. In regions I and III, lattice constants do not change with the Bi concentration in Pb-Bi films. These invariant lattice constants indicate that the regions I and III are in the two-phase regions. In region II, the lattice constants increase linearly with the Bi concentration. This monotonic change of the lattice constants are indicative that the region II is a single-phase region. Referring to the phase diagram shown in Fig. 2, regions I, II, and III correspond to the a + E, E, and E + fJphases, respectively, and the Bi content of the single E­phase Pb-Bi is determined to be in the range of 27.5-31.8% Bi ( ± 0.3 %), as indicated by two vertical solid lines from the present lattice constant measurement.

]0.3508

o !Z 0.3507

~ ~ 0.3506 8 ~ 0.3505 F !ii ..J 0.3504

L..---,_--,-_--"-__ -.l....._L_L- L_ 22 24 26 28 30 32 34 36 38

Bi (wI %1

FIG. 3. Lattice constants of f"-phase Pb-Bi films with various Bi contents.

740 J. Appl. Phys., Vol. 54, No.2, February 1983

The determination of the lattice constants of the c axis was carried out using the presently determined a axis values. Since the diffraction peaks from high I index were not ob­served in the diffraction patterns, the accuracy of the lattice constant of the c axis is not higher than that of the a axis. The typical value of the c axis is c = 1.622 nm in the single E­phase.

c. Electron microprobe analysis

Distributions of Pb and Bi atoms in Pb-Bi films with various Bi contents were examined by using an electron mi­croprobe that has a wavelength-dispersive analysis capabi­lity. The microscope was typically operated at 10 KV with beam current of 0.5 J.lA, and the beam size was smaller than 1 f.lm. The Pb-Ma and Bi-Ma radiations were collected for imaging when the electron beam scanned the film surface.

For Pb-Bi films within E-phase region, Pb and Bi atoms were observed to be distributed uniformly through the sam­ple and no local segregation of Bi or Pb were observed. For Pb-Bi films within a + E-phases, no local segregation of Bi or Pb was observed. This is due to the fact that the concen­tration resolution of x-ray mapping is not high enough to distinguish between a (- 17% Bi) and E-phases. Thus, the boundary between a + E and E-phases could not be deter­mined by this technique. However, for films within the E + fJ phase, local segregation of the fJ-phase was observed. The Bi concentration in thefJ-phase is about three times higher than that in the E-phase. Examples of x-ray mapping taken for a Pb-32.1 % Bi film are shown in Figs. 4(a) and 4(b). Figure 4(a) is a micrograph taken using Bi-Ma radiation. The density of the white dots represents that of the Bi in the film. In this figure, Bi-rich regions as large as -100 J.lm in size are ob­served. Figure 4(b) is a micrograph taken for the same sam­ple area using Pb-Ma radiation. The density of the white dots corresponds to that of Pb. It is noted that the Bi-rich region is depleted of Pb. Consequently, this region corre­sponds to the fJ-phase and the matrix corresponds to the E­phase.

A backscattered electron micrograph taken for the same sample area is shown in Fig. 4(c). It is interesting to note that hillocks (protrusions from the film surface) are ob­served on the Bi-rich region. The hillocks are believed to be formed to relax the compressive strain introduced upon

a b c S0l'm

FIG. 4. Electron microprobe photographs ofPb-32.1 % Bi films taken using Bi-Ma radiation (a) and Pb-Ma radiation (b). Figure (c) shows backscattered electron micrographs of the same area.

Murakami et a/. 740

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heating from 273 K (the substrate temperature during the film deposition) to ambient temperature due to a thermal coefficients mismatch between the film and the Si substrate. Apparently, p-phase Pb-Bi film is more susceptible to hil­lock formation.

III. DISCUSSIONS

A. Comparison with other phase diagrams

The present experimental results were compared with the reported Pb-Bi phase diagrams studied for bulk material. Several studies relating to the phase diagram of the Pb-Bi system have been carried out by various experimental tech­niques.5,6,13 Previously reported phase diagrams over the re­gion T < 380 K and Bi < 30% are classified into two types. In one type, the phase diagram shows the existence of a eutec­toid at 27.5% Bi and 227 K,5 i.e., it was indicated that the possibility might exist that the single E-phase would decom­pose into two (a and P ) phases below 227 K. However, the eutectoid was determined by extrapolating the data points obtained above room temperature. So far, no experimental evidence has been obtained to indicate the decomposition of the E-phase on cooling below 227 K. Another type of phase diagram indicates that the eutectoid actually does not exist at temperatures between 4.2 K and room temperature. Such a Pb-Bi phase diagram was recently reported by Preece and King.6 There is a reasonable degree of agreement between the present results and this phase diagram. However, the present results are not adequate to resolve the difference between the two types of phase diagrams, as it is almost pos­sible to anneal the specimens below 227 K until equilibrium phases are obtained. For example, the diffusivity at 200 K is approximately 10-29 cm2/sec, ifthe volume diffusion is con­sidered to be of importance. 14 It takes almost an infinite time for atoms to move to form lO-nm-size Bi clusters with suffi­cient volume fraction to be detected by the x-ray diffraction technique. However, it may thus be concluded that, even though the eutectoid may exist at a low temperature, the E­

phase will not decompose into two phases during the cooling process from 300 to 4.2 K in our typical thermal cycling experiment.

In the present lattice constant measurements, a single E­

phase region near room temperature was determined to be in the range of 27.5-31.8%. This Bi concentration range is close to those determined by Preece and King6 (27.3-30.8%), but about 2% narrower than that (28-34%) deter­mined by Predel and Schwermann.5

B. Sample preparation technique for E-phase Pb-Bi counterelectrode

Currently, Pb-Hi counterelectrode films are prepared from a single-alloy source using an rf-induction heater. The composition of such an alloy-evaporated film is generally expected to be different from the source composition. S To prepare Pb-Bi films with a single E-phase, a good control of film composition is required, since in the present experiment the Bi content range of the E-phase Pb-Bi was found to be only ~4.3%. Recently, a film preparation method to con-

741 J. Appl. Phys., Vol. 54, No.2, February 1983

trol the film composition was developed by Huang and Gil­bert,7 and their method is discussed here.

The film and source composition is related by Hertz-Knudsen-Langmuir equationS as

X~, =KX~, x v Xl' Pb Pb

where X ~b andX ~i are atomic fraction ofPb and Bi in Pb-Bi vapor, respectively, and X~b and X~i are those in Pb-Bi liquid. These concentrations correspond to those of the film and the source, respectively, assuming unity sticking coeffi­cients. The parameter K is the composition parameter de­fined by

K = P~i + V2:BiX~iP~i' . rBi, (1)

PPb rPb

where P ~b and P ~i are partial vapor pressures of Pb and Bi over their own melt, and rPb and rBi are activity coefficients ofPb and Bi, respectively. In order to have the vapor compo­sition, i.e., film composition, be the same as the source com­position, the K value has to equal to 1. When K> 1, the Bi content in the film monotonically decreases as evaporation proceeds at fixed source temperature due to preferential de­pletion ofBi from the source. In the reverse case, i.e., K < 1, the Bi content in the film monotonically increases due to preferential enrichment of Bi in the source. To control the film concentration, the optimum evaporation condition to achieve K = 1 should be found.

The parameter K is mainly determined by the ratio of the P~b to P~i over its own melt as expected from Eq. (1), which, in tum, is determined by the evaporation (source) temperature. The optimum source temperature to achieve K = 1 was, first, calculated as a function of source composi­tion, and then, experiments were carried out to prove the prediction. Films of 1-2 /-lm thickness were collected onto Si substrates at a series of predetermined evaporation time in­tervals at a given source temperature. For Pb-27% Bi and Pb-29% Bi source composition, the film compositions were found not to change through source lifetime when the source temperature was 1080 ± 10 K and 1022 ± 10 K, respective­ly. These optimum temperatures were in good agreement with predicted values using Eq. (1). The film compositions were Pb-28.2% Bi and 29.2% ( ± 0.3%) Bi for alloy sources of Pb-27 % Bi and 29% Bi, respectively. The slightly higher Bi film compositions were explained to be due to the differ­ence in the sticking coefficients of Pb and Bi onto the Si substrates at 273 K. However, these film compositions were within the E-phase range, and the compositions did not change during the source lifetime. This method is recom­mended to be used where E-phase Pb-Bi counterelectrode materials are prepared by evaporating an alloy source.

IV. SUMMARY

The Bi content range of E-phase Pb-Bi films was deter­mined at room temperature by using x-ray diffraction, elec­tron microprobe, and controlled-potential coulometry for 1-/-lm-thick Pb-Bi films prepared by evaporation from an alloy source onto oxidized Si substrates. The Bi content range was

Murakami et al. 741

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determined to be 27.5-31.8% ( ± 0.3%). The lower bound­ary agreed reasonably well with the previously reported val­ues,5.6 but the upper boundary was - I % higher than that of Preece and King,5 and - 2 % lower than that of Predel and Schwermann.6 The E-phase was found to be stable upon heating to 350 K, and upon cooling down, to as low as 4.2 K. Regions of /3-phase as large as -100 pm in size were ob­served in Pb-Bi films when the average film concentration was within the E + /3 phase region. Also, the /3-phase was found to be susceptible to hillock formation at room tem­perature. Finally, a technique to control the film composi­tion accurately was described.

ACKNOWLEDGMENTS

The authors would like to thank C. J. Kircher for useful discussion, R. Schad for electron microprobe analysis, A.

742 J. Appl. Phys., Vol. 54, No.2, February 1983

Segmuller for x-ray automation, and V. Tom for sample preparation.

IS. K. Lahiri, S. BasaYaiah, and C. J. Kircher, App!. Phys. Lett. 36, 334 (1980).

2J. H. Greiner, C. J. Kircher, S. P. K1epner, S. K. Lahiri, A. J. Warnecke, S. Basayaiah, E. T. Yeh, J. M. Baker, P. R. Brosious, H.-C.W. Huang, M. Murakami, and I. Ames, IBM J. Res. Dey. 24,195 (1980).

'J. H. Basson, M. Murakami, and H. Booyens, J. App!. Phys. 53, 337 (1982).

4M. Hansen and K. Anderko, Constitution 0/ Binary Alloys (McGraw-Hili, New York, 1958), p. 324.

'B. Predel and W. Schwermann, Z. Metallk. 58, 553 (1967). 6c. M. Preece and H. W. King, Scr. Metall. 3,859 (1969). 'H.-C. W. Huang and B. L. Gilbert, Thin Solid Films 91,201 (1982). "See, for example, L. Maissel and R. Giang, Handbook a/Thin Film Tech­nology (McGraw-Hill, New York, 1970).

9y. S. Su and D. E. Campbell, Ann. Chim. Acta 47,261 (1969). 10M. Murakami and T. S. Kuan, Thin Solid Films 66,381 (1980). "M. Murakami, Acta. Metal!. 26, 175 (1978). 12J. B. Nelson and D. P. Riley, Proc. Phys. Soc. 57,160 (1945). uD. Solomon and W. Morris Jones, Philos. Mag. 11,1090 (1931). I4A. Seeger and H. Mehrer, Phys. Status Solidi 29, 231 (1968).

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