Liquid phase epitaxy of high-Tc superconductors

Journal of Crystal Growth 129 (1993) 421—428 North-Holland

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o~

CRYSTAL GROWTH

Liquid phase epitaxy of high-1~superconductors C. Klemenz and H.J. Scheel Crystal Growth Group, Institute of Micro- and Optoelectronics, Swiss Federal Institute of Technology, Chemin de Bellertie 34, CH-1007 Lausanne, Switzerland Received 7 May 1992; manuscript received in final form 5 January 1993

Extremely flat surfaces of high-I~superconductors (HTSCs) are needed for physical measurements and for tunnel device applications because of the very short coherence lengths of HTSCs. Unprecedented materials problems have so far prevented such flat layers from being obtained. The present work describes partially established prerequisites of liquid phase epitaxy (LPE) of HTSCs, so that layer-by-layer growth and thus single-crystalline epitaxial layers of NdBCO could be achieved for the first time. Surface flatness is proven by differential interference contrast microscopy and Talystep measurements.

1. Introduction The recently discovered high-temperature superconductivity (HTSC), i.e. superconductivity above the boiling point of liquid nitrogen (77 K), stimulated thousands of scientists worldwide. Numerous applications based on tunnelling were envisaged assuming that a timely solution of the material and crystal growth problems could be found. However, crystal growers are faced here with unprecedented problems which are not widely recognized by physicists and device engineers [11: The first is based on the very short coherence lengths of the HTSC, depending on the specific compound and orientation, less than 20 A or even less than 10 A. This means that for several basic measurements and applications, representative (of the bulk composition) HTSC surfaces of quasi-atomic flatness are required. Other problems [1] arise from the complexity of HTSC compounds which are made of four to six constituents (stoichiometry; defects), their thermal and chemical instabilities (limits to process parameters; phase diagrams not yet available), their pronounced anisotropy, the complex phase transformations of YBa2Cu3O7_~(“123” or —

—

YBCO) compounds, the size and distribution of 0022-0248/93/$06.OO © 1993

—

flux-pinning defects for applications requiring high critical current densities (IC)’ and the non-availability of suitable substrates [2]. Highly textured and epitaxial HTSC layers with high critical temperatures T~and high J~values have been achieved in uncounted laboratories worldwide. By far the greatest emphasis has been on physical vapour deposition (PVD) techniques like sputtering, laser ablation, molecular beam epitaxy (MBE), etc., but also metalorganic chemical vapour deposition (MOCVD) has yielded excellent T~and J~values [1]. However, compact single-crystalline epitaxial layers with the required surface flatness have not been achieved so far despite enormous efforts. This fact can be explained by the above-mentioned problems and by a set of critical conditions required for the layer-by-layer growth mechanism, which would allow extremely flat surfaces to be obtained: very low supersaturation, very small misfit at epitaxial growth temperature (depending on the layer thickness), similarity of thermal expansion coefficients of HTSC and substrate, and precise control of substrate misorientation [3]. From thermodynamics and growth kinetics, liquid phase epitaxy (LPE) should result in extremely perfect layers with quasi-atomically flat surfaces when supersaturation and substrate con-

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422

C. Klemenz, Hi. Scheel

/ LPE of high-T,. superconductors

ditions are sufficiently met. The potential of LPE was demonstrated with epitaxial layers of cornplex rare-earth garnet compositions for magneticbubble and magneto-optic applications [4], for the first semiconductor lasers [5],for the achievement of equilibrium surfaces [3] and of multilayer structures of p- and n-GaAs, which are atomically flat [6]. This was proven by optical differential interference contrast (Nomarski) microscopy in combination with scanning tunnelling microscopy. LPE of HTSC was attempted in a few laboratories for YBCO, Bi2Sr2CaCu2O~ (2212), and Tl-1223/-1324. These activities are summarized in table 1 along with results of the present work, Clearly, systematic efforts are required in order to achieve progress towards the potential limits of LPE. In the present work the prerequisites for LPE of “123” are discussed, and it is shown how they

can be achieved. Preliminary LPE experiments yielded the first single-crystalline epitaxial layers the surface morphology of which was determined by Nomarski microscopy and Talystep measurements.

2. Prerequisites for LPE of HTSC Phase diagrams, especially the primary crystallization fields (PCFs) and the solubility curves, are of crucial importance for LPE (and crystal growth) of HTSC due to their limited thermal stability. However, the published PCFs of YBCO in the system Y203—BaO—CuO show a wide scatter [17] so that additional investigations were required [18]. Fig.1 shows the superposition of two sections of the diagrams YBCO—BaCu2O3 (Ba1Cu5O5) and NdBCO—BaCu2O3 (Ba2Cu~O8)

Table 1 Results of liquid phase epitaxy HTSC

Solvent

Substrate

Cooling and growth rate

T~ (K)

Surface

Reference

YBCO YBCO

KCI

LaGaO~(001)

0.5°C/mm 0.5 fLm/min

78

Smooth, shiny

SEM

Belt et al. [7]

2212: Pb

KCI+ PbO

LaGaO3 (100) SrTiO3 (100) LaGaO3 (100)

Gradient transport 0.6 A/s

75 75 75

Mirror-like Mirror-like Smooth continuous

scm SEM

Balestrino et a!. [8,9]

—

2212

2212 in KCI 1:4

NdGaO3 (001)

3—6°C/h

40

Lamellar grains oriented parallel to each other

SEM

Narayanan et a!. [101

2212

2212 in KCI 1:7

MgO (100)

1 /zm/min

<50

Platy grains, size 10—50 ~zm

SEM

Takeya and Takei [11]

2212 (?) Tl—1223/—l324

Bi2Ca3Sr3Cu4O,, MgO (001) Tl4Ba4Ca1Cu6O~ MgO (001)

0.5—2°C/mm 0.3—1°C/mm 4 ~zm/min

81 Ill

Flat terrace-like Flat terrace-like

SEM SEM

Liu et al. [12,131

YBCO 2212

BaO—CuO BLO3—CuO

Sapphire (1102) Sapphire (1102)

15°C/h 19°C/h

85 80

Large singlecrystalline films

SEM

Yue et a!. [141

2212

2212 in KCI 1:7

MgO (100)

NM

82

Fairly fiat, layered structure

SEM

Shin and Ozaki [15]

YBCO YBCO

Ba~Cu7O~, Ba~Cu7O1(}

MgO (100) Sapphire

NM NM

61 61

Large polygonal grains, needle-like crystals

SEM

Peng et a!. [16]

YBCO NdBCO

BaCuO,—CuO BaCuO~—CuO

LaGaO3 (001) LaGaO3 (001)

0.16—0.33°C/h nm 30—50 A/s nm

Single-crystalline epitaxial, facet with growth steps and cracks

DIC+ This work Talystep

NM

=

not mentioned: nm

=

(?)

not measured.

—

C. Klemenz, H.J. Scheel

/ LPE of high-T~.superconductors

order to achieve continuous growth without inclusions and surface dendrites. Furthermore, the supersaturation has to be small enough to prevent spontaneous three-dimensional nucleation, to prevent step bunching (described by the kine-

Temperature (°C) 1300 ~O 3+L

1200 1100

Nd4~aCuOç~ C

L,q~d

-.

_.-t--_.

‘~__t~~.

1000

.

C~

~ —

—

-~Z-~..—

900

I23~ CW

800

v~&~r-

—

—~

~

BaC~O

~

I

80 123’

60 40 20 Composition (weight %)

423

0 “BaCu.P3”

Fig. 1. Tentative pseudobinary phase diagrams of YBCO— BaCu2O3 (dotted lines) and NdBCO—BaCu2O3 (dashed lines) in air with data from ref. [18] and from the present work,

from our data and from those of ref. [19]. NdBCO, compared to YBCO, has two advantages for crystal growth, namely: a higher thermal stability reaching nearly 1100°Cin air, and a much wider PCF allowing concentrations up to 25 wt% for single-phase growth. This is also shown in the solubility curves of YBCO and NdBCO in fig. 2. Our data were obtained from crystal growth and LPE experiments. The scatter of the data may be explained with the difficulties of HTSC [17] and the corrosion of the alumina crucibles. Since all commercial crucibles are heavily corroded by the BaCuO2—CuO melts [201,thus leading to contamination of the HTSC (e.g., Al on Cu site), new crucibles have been developed by plasma spraying and corrosion studies [21]. An alternative approach to reduce the corrosion problem consisted of optimizing the BaCuO2— CuO ratio of the solvent for minimum corrosion and distribution coefficient [22]. Similarly, the creeping of liquid out of the crucible can be reduced. An important prerequisite for achieving very flat surfaces in LPE is the precise control of supersaturation o’. As a function of solute concentration, hydrodynamic conditions, and substrate surface area, the concept of the maximum stable growth rate [23,24] has to be followed in

matic wave theory), and to achieve the epitaxial layer-by-layer growth mode, i.e. to prevent island growth. This latter aspect requires, in addition to low u, a very low misfit between substrate and film (~a= Asubstrate atiim) at growth temperature (according to experiences with garnets with a 12.4 A, z~a is less than 0.01 A, to avoid cracking and other problems [4b]). Technically, the precise control of supersaturation requires a corresponding precision of temperature control and programming, which can be achieved by spe—

cific thermopile temperature sensors [25] in combination with homogenizing of the solution by forced convection. The misfit condition (and thermal expansion similarity) is one of the substrate requirements which are discussed elsewhere [1,2]. Until such optimized substrates are developed, we used LaGaO3 and NdGaO3 as best compromises in the present study.

Temperature(°C)

1100 ./ 2~

1050

A //

1000

~‘

/ I / ~

/

// /

950 900

~‘

iC~C~2o3Oux(o~~,w~~rk) —

—o

NdBC()

_____________

850~ 10 2030 Composition (weight % “123”) . .. Fig. 2. Solubility curves for YBCO and NdBCO with Ba3Cu5O8 flux (from ref. [19]) and with BaCu2O3 flux (our work), in air.

C. Klemenz, H.J. Scheel

424

/ LPE of high- 1~.superconductors

3. Experimental procedure

Temperature (°c)

The LPE experiments were performed in a modified commercial chamber furnace with Kantal Al heating elements on three sides. Openings at the top for observation were covered by goldplated quartz glass plates. Fig. 3 shows a cross section of the growth chamber with mechanical means for substrate rotation and vertical translation for dipping. Of specific importance is the crucible position on firebricks separating it from the bottom of the chamber. The optimized position of the crucible should minimize nucleation in the lower region of the melt as well as at the top of the melt. The starting chemicals were annealed before weighing to remove volatiles, and Ba02 was analysed for its Ba content. We used Ba02 (Fluka

~

_____

Lift & Rotation

~mp

Observation ->

_________________

_________________

I

/////J

________________

____________

Alumina Ho!der~

//

Pt/Rh Wires ~ubstrate

—

‘

Alumina (Yttrla)

7 Crucible

rHigh-Tempe!re ‘1 23” Solution

A______________

Pt/Rh 6% - pPt/Rh

_____~\

300/

1100

iooo 900 800 0

5

10 15 Time (hours)

50

Fig. 4. Temperature program for a sequence of LPE experiments of NdBCO. The corresponding temperatures for LPE of YBCO are about 90°Clower.

puriss, p.a. > 99.5%), CuO (Preussag 5N), Y203 and Nd203 (Johnson-Matthey 4N and 3N, respectively). The chemicals were well mixed and pressed by hand into the cylindrical alumina or yttria cru3 size. The crucible in the furnace heated cibles ofwas 34 to 40 cmup followed by soaking and equilibration according to the temperature protemperature was done for various concentrations (shown gram of infig.fig. 4. The 2) byequilibration observation toofthe theliquidus liquid surface for crystallization and disappearance of crystals, and confirmed by dipping of test substrates into the slightly supersaturated solutions. The temperatures measured near the crucible top sidewall (see fig. 3) and the concentrations were used to establish the solubility curves shown in

~

___________

Successive I LPE experiments

I

~/

____________________

_____________________________________ ____________________

KanthalAl Heating Elements_J

Fig. 3. Schematic cross section of LPE growth furnace with substrate lift and rotation,

to 352. rpm fig. Rotation for steeply of substrates inclined wasand used vertical with up posito tions of the substrate. In the course of this work, 100 rpm for nearly horizontal substrates, and up with improved nucleation control, the cooling rate was successively reduced, so that finally all deposition seemed to occur on the LaGaO 3(001) substrates. Growth occurred onto the (001) surfaces, but also sidewise along (100/010). Growth rates were about 30 A/s along [001] and 50 A/s along (100). A specific problem was the removal of residual flux after withdrawal of the substrate: high substrate rotation rates of 700 rpm to spin

C. Klemen:. lii. Schee/ I LPE of hi~’h—T supercooducwr.~

off the liquid were not very successful. However, slow substrate withdrawal, in order to suppress dynamic wetting effects, was more successful, Cspecially when layer surface perfection was improved.

4.

425

Results

After the definition and the partial establishment of the prerequisites for LPE of 123 cornpounds, the growth experiments have l’or the first

/

i/

S

S

Fig. 5. The tirst stages of LPE of Ndl3CO on L.aGaO~with “mirror-like’ surfaces thea naked Nomarski photographs show (a) 2) andto(h) nearl~eye. compact NdBCO surtace o.sith grain epitaxial faceted growth islands of mainly (001) orientation (2080 x 1400 ~sin boundaries, cracks and inclusions (520 \ 350 jsm2 ).

42~i

(

.

Kh’own:

/I..Ieheel

0! of /ii~’h-I

LI

time allo~edto control nucleation and supersatu— ration in these systems. The very l’irst experiments of YBCO on SrTiO~substrates showed either surface dendrites or nucleation and growth of individual crystallites up to 1 mm size with (1(H)) or (001) orientation. A similar epitaxial

\~

mpcrl onduetoro

growth of NdBCO ci’vstallites on LaGaO.(00 I substrates is shown n fig. 5a. ‘Fhe space between the crvstallites of niainly (001) orientation and typical 200 jim size is filled with flux. Fig. Sb shows a NdE3CO layer about 70 jim thick, which is nearly compact. Grain boundaries and a few

~**~r~ ‘~

a

a

a

‘

/1’

ijs,/

/1//

~!/‘i,~~1/

~f/

Fig. 6. Nomarski photographs (5211 X 350 ii in 1 of single—crystalline epiiaxial Nd B( ‘0 la~ ers ~sith (a) step heights ol less than lOt) A. fI tix d rople t~.and a fess maj r cracks and (h ) a seque nec 0! 01 aerosteps 55 iih step heights t~eissceo 11)1) a id 1111)1) A a iid a few

tins

cracks.

C. Klemenz, Hf. Scheel

A 16000

~~“:

8000

1

If

.

/ LPE of high-Ta

.‘

0 I

—

100

—

200

427

steps. However, the central terrace of fig. 6b seems to be free of microsteps. one defines the relative roughness of a representative surface with macroscopic dimensions as —

O

superconductors

~ (horizontal area — vertical area) ~ horizontal area

26 ‘

300

0

A

then this value is 0.9995 for the surface of fig. 6b, in contrast to R 0.9—0.95 for the best PVDgrown layers, as can be calculated for any characterized surface. As an example we may use the STM Hawley et al. with growthinvestigations islands of 200ofnm diameter and[27] typically —

3000 .

.

2000 .

1000

,.

0

.

.

b .

0

.

20

40

~O

Fig. 7. Talystep traces of different resolutions of the surface shown in fig. 6b: (a) confirms the regular train of macrosteps of mean 800 A height and up to 17 Am distance; (b) shows a region with terraces and microsteps, and in the center one terrace of 16 Am width without any [001] step of nearly 12 A (which should be visible). In these figures the horizontal scales are extended by ~ since the step fronts made an angle of 450 with the Talystep trace.

cracks are clearly visible, and the individual crystallite (001) surfaces are relatively flat. With improved experimental conditions, especially the position of the crucible in the furnace, and with a cooling rate of 0.38°C/h, the desired layer-bylayer growth could be achieved on a LaGaO3(001) substrate of 0.3°misorientation. This is presented in fig. 6 for a NdBCO layer of 0.5 mm thickness. In fig. 6a, a quite flat surface with steps of less than 100 A can be seen, along with a few cracks and droplets of solidified flux. Macrosteps, besides a few cracks, are the dominating feature of fig. 6b. The regularity of the train of steps mdicates the single-crystalline character of this epitaxial NdBCO surface. The surface profile measured by Talystep is shown in fig. 7a. In this region, the macrosteps are between 100 and 1000 A high and up to 17 jim apart. In fig. 7b, it can be recognized, in correspondence to fig. 6b, that most of the terraces show very fine step structures (microsteps) which are monosteps or double

ca. 20 steps of 1.2 nm height, which gives R 0.90; see also ref. [1]. The present surface of fig. 6a might be still flatter than fig. 6b, but was not measured due to the flux droplets.

5. Conclusions Crystal growth and epitaxy of the complex and not very stable HTSC confront crystal growers with numerous unprecedented problems. These can only be overcome when they are recognized and appreciated, and when they are solved or at least reduced, which may require significant efforts. The present work has shown that by reducing some of the crucial problems for LPE, the layer-by-layer growth mode in epitaxy of HTSC and thus single-crystalline 123 layers could be achieved for the first time. Space does not allow us to discuss here why it would be quite difficult to achieve similarly flat surfaces by PVD, whereas LPE seems most promising, both from earlier experiences with garnets and semiconductors and from present results, to achieve still flatter surfaces. However, even with LPE the conditions have to be further optimized, especially the substrate problem, before large-scale, quasi-atomically flat 123 surfaces without cracks can be expected. LPE results of other groups presented in table 1 have shown that in diluted systems very low growth rates, less than 1 A/s, can be obtained. This opens the possibility to achieve LPE-grown super-

428

C. Klemenz, H.J. Scheel

/ LPE of high-Ta

lattice structures with quasi-atomically flat surfaces and interfaces, which would be of high interest for physical studies and specific HTSC applications.

superconductors

[8] G. Balestrino, V. Foglietti, M. Marinelli, E. Milani, A. Paoletti and P. Paroli, IEEE Trans. Magnetics MAG-27 (1991) 1589. . . . . [9] G. Balestrino, V. Foglietti, M. Marinelli, E. Milani, A. Paoletti, P. Paroli and G. Luce, Solid State Commun. 76 (1990) 503. [10] S. Narayanan, K.K. Raina, R.K. Pandey and CD. Bran-

Acknowledgements The authors thank the Swiss National Science Foundation (projects FN 21-25’576.88, FN 2131’199.9) for support, and Thomson—CSF for collaboration on substrate development and HTSC phase-diagram problems. Furthermore, we thank Professor F.K. Reinhart for support and encouragement, and Mr. R. Rochat for constructing the substrate lift and rotation mechanism.

References [1] H.J. Seheel, M. Berkowski and B. Chabot, J. Crystal Growth 115 (1991) 19. [2] H.J. Scheel, M. Berkowski and B. Chabot, Physica C 185—189 (1991) 2095. [31H.J. Schee!, App!. Phys. Letters 37 (1980) 70. [4] (a) R.C. Linares, J. Crystal Growth 3/4 (1968) 443; (b) G. Winkler, Magnetic Garnets (Vieweg, Braunschweig, 1981). [5] (a) H. Nelson, RCA Rev. 24 (1963) 603; (b) I. Hayashi, MB. Panish and F.K. Reinhart, J. App!. Phys. 42 (1971) 1929. [6] H.J. Scheel, G. Bmnnig and H. Rohrer, J. Crystal Growth 60 (1982) 199. [7] R.F. Belt, J. Ings and G. Diercks, App!. Phys. Letters 56 (1990) 1805.

dIe, Mater. Letters 11(1991) 212. [Ii] H. Takeya and H. Takei, Japan. J. AppI. Phys. 28 (1989) L229. [12] R.S. Liu, Y.T. Huang, PT. Wu and J.J. Chu, Japan. J. App!. Phys. 27 (1988) L1470. [13] R.S. Liu, Y.T. fluang, J.M. Liang and PT. Wu, Physica C 156 (1988) 785. [14] AS. Yue, W.S. Liao and H.J. Choi, preprint 16.1.1992. [15] J.S. Shin and H. Ozaki, Physica C 173 (1991) 93. [16] L.H. Perng, T.S. Chin, K.C. Chen and C.H. Lin, Supercond. Sci. Technol. 3 (1990) 233. [17] H.J. Schee! and F. Licci, Thermochim. Aeta 174 (1991)

115. [18] H.J. Scheel and P. Holba, J. Crystal Growth, submitted. [19] K. Oka and H. Unoki, J. Crystal Growth 99 (1990) 922. [20] H.J. Scheel, W. Sadowski and L. Sche!lenberg, Supercond. Sci. Technol. 2 (1989) 17. [21] M. Berkowski, P. Bowen, T. Liechti and H.J. Scheel, J. Am. Ceram. Soc. 75 (1992) 1005. [22] F. Licci, C. Frigeri and H.J. Scheel, J. Crystal Growth 112 (1991) 606. [23] H.J. Scheel and D. Elwell, J. Crystal Growth 12 (1972) 153. [24] D. Elwell and H.J. Scheel, Crystal Growth from HighTemperature Solutions (Academic Press, L.ondon, 1975) chs. 4, 6, 7. [25] H.J. Scheel and C.H. West, J. Phys. E (Sci. Instr.) 6 (1973) 1178. [26] H.J. Scheel, in: Proc. E-MRS Meeting, Strasbourg, June 1987, Vol. 16 (1987) p. 175. [27] M. Hawley, ID. Raistrick, J.G. Beery and R.J. Hou!ton, Science 251 (1991) 1587.