Growth of single crystals of lanthanum aluminate

Growth of single crystals of lanthanum aluminate

ournal of Crystal Growth 109 (1991) 467—471 ‘~orth-HolIand 467 Growth of single crystals of lanthanum aluminate (Tr.W. Berkstresser, A.J. Valentino ...

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ournal of Crystal Growth 109 (1991) 467—471 ‘~orth-HolIand

467

Growth of single crystals of lanthanum aluminate (Tr.W. Berkstresser, A.J. Valentino and C.D. Brandle ~ T&T Bell Laboratories, Murray Hill, New Jersey 07974, USA

Single crystals of lanthanum aluminate have application as a substrate for deposition of thin films of the high temperature s~perconductorssuch as YBa 2Cu3O7. Single crystals have been grown by the Czochralski technique in the (100> pseudo-cubic c ystallographic orientations in sizes from 16 mm weighing 180 g to 59 mm diameter weighing 2500 g. The design of the crystal g owth furnace used for the Czochralski growth of lanthanum aluminate required some modifications from the geometry suitable for II e growth of lower melting temperature oxides.

1 Introduction

to space group R3m with unit cell dimensions of a 5.357 A and a 60.1°.Cell lattice constants of Czochralski grown single crystals were determined on a Philips APD 3720 automated powder diffractometer scanning over the 29 range of 60° to 120° to improve the accuracy of the diffraction angle measurement. Single crystal samples of the LaA1O3 were ground, then mixed with CeO2 which served as the internal standard for X-ray powder diffraction, then the powder bonded to a glass slide with a silicone grease. An example of the powder X-ray diffraction pattern for pure LaAlO3 is given in fig. 1 with rhombohedral cell indexing of the major diffraction peaks over the 10° to 90° 26 range. Geller and Bala [3] observed for LaA1O3 a gradual change from rhombohedral to the cubic unit cell with increasing temperature, but could detect no further structural changes above 350°C. Wood [41observed a phase change at 435°C, but via DTA found no appreciable thermal effects at this transition temperature. Fay and Brandle [5] reported visual observations upon the phase transition in LaA1O3 and agree with Geller and Bala [3] that the change is gradual, but indicate the presence of the rhombohedral phase up to 512°C. Dilatometry, high temperature X-ray diffraction (XRD), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) have been performed on samples of LaAlO3 to determine the =

Electrical conduction in the high temperature superconducting oxides such as YBa2Cu3O7 occi trs in the a—b crystallographic plane; thus high quality superconducting thin films are grown on a si. bstrate oriented so that growth is perpendicular tc the c-axis. LaAIO3 is a perovskite-like material w th a lattice constant about 2% lower than that of Y Ba2Cu3O7 and dielectric properties which make it preferable for high frequency device applications. The deposition of YBa2Cu3O7 superconthctor upon LaA1O3 has been studied by Simon et a) [1], who found the film characteristics to be comparable to that measured when SrTiO3 was us ~d as the substrate. The characteristics of superconducting transmission lines fabricated on Y13a2cu307 films grown upon LaA1O3 substrates have been examined by Nuss et al. [21. The epita:ial films on LaA1O3 were observed to be of a be ;ter quality than films deposited upon SrTiO3 substrates.

2. Crystallography of LaAIO3 The crystallographic data on LaAIO3 has been reçorted by Geller and Bala [3] on samples prepased by standard ceramic techniques. They report that LaAlO3 is rhombohedral, and belonging 0O2~-0248/9l/$03.50© 1991



Elsevier Science Publishers By. (North-Holland)

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G. W. Berkstresser et aL

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Growth of single crystals of lanthanum aluminate

-

(211)

~21T(

20.0 1.00

uJ I-

25.0

30.0

35.0

40.0

45.0

50.0

55.0

85.0

90.0

-

(310) (1121

LTi~,,

55.0

60.0

65.0 70.0ANGLE, 1(4~1)1~l1L40:) 80.0 DIFFRACTION 275.0 0 (DEGREES)

Fig. 1. Rhombohedral unit cell indexing of major diffraction peaks in X-ray powder diffractometer scan of LaAlO1 over 28 range of 20° to 90°.

presence of any phase transitions [6]. XRD between 25 and 600°C of a (211)r (r denotes the rhombohedral unit cell) oriented single crystal wafer reveal a gradual structural change from the rhombohedral to the cubic unit cell. This is evident from the merging of the (022)~and (422)r diffraction peaks near 500°C as shown in fig. 2. Via DSC a gradual heat capacity change (i.e. a second phase transition) wasis found 400 andorder 500°C.Our recent work in goodbetween agreernent with the data of both Wood [4] and of Fay and Brandle [5].

3. Twinning of LaAIO3 The high temperature form of LaAIO3 is the cubic perovskite structure which belongs to point group m3m, space group Pm3m, while the low temperature rhombohedral form belongs to point group 3m, space group R3m. Thus, the suppressed symmetry set which results from the phase transformation is the set of mirror planes (m) per-

pendicular to the cube axes. The mirror planes parallel to the optic axis remain, as does the three fold axis. An inversion three fold axis arises due to the doubling of the original cubic unit cell to form the rhombohedral cell. Thus, one can expect twin planes to be mirror planes perpendicular to the (100) and (110) directions of the original cubic unit cell. 211)r To demonstrate the orientation slice was fabricated from aof10these mm twins, a ( LaAIO diameter 3 crystal. Since this is an oriented single crystal, only those reflections belonging to the (211)r class would be observed with the diffractometer system. The resulting pattern showed only two sets of reflections, the (211)r set and the (422)r set. Each of these sets consisted of a doublet that is best illustrated by the (422)~ set as shown in fig. 3. Fig. 4 shows the relative orientation of the rhombohedral unit cell to that of the simple pseudo-cubic perovskite cell and the minor image twin cell (a~,a~,a~).One may observe the twin boundary being parallel to the (10O)~of the pseudo-cubic cell. The two planes which result in the doublets of the (211)r wafer XRD scan are

G. W. Berkstresser et al.

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/ Growth of single crystals of lanthanum aluminate

469

500 C CUB

2.45

-

0.05

-

~ 2.45

400

~ >-



02

2.45

C,)

z Ui

I-

U __

005 300

080

\

/(0(

01/

Fig. 4. Relative orientation of rhombohedral and pseudo-cubic

2.45 200 0 80~

unit cells and the planes responsible for the doublet reflection shown in the X-ray diffractometer scan of fig. 3.

005

only the (011) reflections. equal, the unit cell structure shown in fig. 4 shows 0 05 68

69

70

4. Czochralski crystal growth

71

29

Fi:;. 2. XRD data for 022 and 422 lines and calculated rhornbohedral angle at 25, 200, 300, 400. and 500°C for

Fay and Brandle [5] grew single crystal LaAlO3 via the Czochralski technique, and we have also

LaAIO3 crystal.

used a similar procedure in the present work. A cross section of a typical crystal growth furnace is illustrated by the shading of the (211)r and the nearly parallel (O11)r of the twin cell. Although th~~ plane spacing of the (022)r and the (202)r are

QUARTZ TUBE

_____________________________________________

GRANULAR Zr02

Zr02 FELT

Zr02 TUBE RF C0IL—~O 0 IRLID

z Ui 22)

19.6

69.8

700 I

1022)

70.2 ~ ~ 70.4 ~, 26

706

70.8

71.0

Fig. 3. X-ray diffractometer scan in the region of the (422), rhombohedral cell orientation) reflections for LaAIO3.

SUPPORT Zr02 Zr02 BASE

0 .0

0

Zr02 RING

0 0

0 0

Ir CRUCIBLE

0

0

0

..

0

Zr02PLATE LINER Zr02 SETTER

Fig. 5. Cross-section of Czochralski growth furnace.

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Growth ofsingle crystals oflanthanum aluminate

shown in fig. 5. Iridium crucibles 50 mm diameter by 50 mm high with 1.5 mm thick walls and bottom were used for growth of crystals less than 25 mm diameter. Larger diameter crystals were grown in proportionately scaled crucible and growth furnace geometries. The initial crucible charge was sized to fill the crucibles with liquid to within 5 to 10 nml of the top to achieve both radial and axial temperature gradients in the liquid phase necessary for successful starting of the crystal growth. The charge for the growth of LaA1O3 was a mixture of 99.999% purity La203 and A1203. Immediately prior to preparation of a charge the La203 was dried at 1200°C for a period of 8 to 12 h. The initial growths produced crystals with a reddish brown color, rather than the expected colorless crystal. Selecting a lot of La203 which contained less group II and transition metal atoms improved the crystal color to a light tan. This improvement in quality is similar to that reported by Fay and Brandle [5] when more pure La203 was used, except that they report a colorless material could be grown. We have found that for all crystals grown exposure to short or long wavelength ultraviolet light will darken the surface of a crystal to a reddish brown color in only a few hours. The melting point of LaA1O3 was measured by Fay and Brandle [5] to be 2075 to 2080°C,and by Bondar and Vinogradava [7] to be 2100°C. Since iridium melts at 2410°C, the growth of LaA1O3 must be approached with some caution in the design of the crystal growth furnace. For example, prior to the initiation of the crystal growth we have measured liquid surface temperatures of 2150 to 2200°C using a pyrometers set for a surface emissivity of 0.85. Thus a crucible wall temperature on the order of 2300°C was quite possible. With RF induction heating of a crucible the bulk of the energy is deposited at the top and bottom outside edges of the end of the cylindrical surface [8]. Therefore, sufficient zirconia insulation is provided to allow sufficient heat transport into the liquid phase without imposing too high a temperature gradient in the crucible wall between the top and the level of the liquid contained in the crucible. The basic crystal growth furnace design for a

50 mm diameter crucible is shown in fig. 5, and has proved adequate for materials melting in the 1700 to 2000°C range. The initial crystal growth run utilized an Ir rod to solidify a polycrystalline button at the melt surface. Growth was continued from this polycrystalline start, and soon several single crystals dominated the growth and an unoriented single crystal seed could be fabricated. Several more growth runs were necessary to prepare seeds with the growth axis oriented along the rhombohedral unit cell (110) direction. Often these perovskite like compounds are discussed in terms of a pseudo-cubic cell, then the above orientation transforms into a (100)~direction. For application to lanthanum aluminate the zirconia ceramic placed directly above the iridium crucible lid was thicker than used in lower temperature crystal growth. In addition the top of the furnace was closed up with a zirconia disk having a 25 mm diameter hole and a 10 mm wide radial slot, forming a keyhole shape. This arrangement was adequate for the seeding operation. But, once the crystal diameter had expanded to 15 nmi and the crystal began to protrude above the crucible lid, direct radiation transfer from the melt surface to the cooler parts of the furnace greatly reduced. Inspection of the crystals revealed that at about the same time in the growth, internal flaws began to appear. These defects are apparently due to the formation of (001)~facets on the growth interface, an event consistent with a decrease in the temperature gradient at the growth interface. To increase the radiation transfer heat flux from the melt and crystal surfaces the inside diameter of the zirconia ceramic above the crucible lid was increased as was the opening at the top of the growth furnace. These changes resulted in reduced facet formation and subsequent improvement in the quality of the crystal during the latter stages of growth. From the lattice constant data summarized in table 1, we have found close agreement between the lattice parameters reported by Geller and Bala [3] and the Czochralski grown crystals. Variance in the rhombohedral lattice dimension is not seen between the first grown and last grown portion of the crystal as evident from results for typical growth runs reported in table 1. A sample of

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Growth ofsingle crystals oflanthanum aluminate

5. Conclusions

‘able 1 Lattice constant of LaAIO

3 Czochralski grown crystals; corni arison with ceramic LaAIO3 samples; space group R3m -

(‘rystal G&B ~ 3LA-10 3LA-10 3 LA-12 3LA-12

3LA-16 3 LA-16 3 .A-19 3 .A-19 P ate

(A)

Position

a

a (deg)

28 (deg)

Seed Tail Seed Tail

5.357 5.3573 5.3574 5.3553 5.3553

60.1 60.11 60.11 60.13 60.12

Seed

5.3566 5.3567 5.3566

60.11 60.10 60.10

20—90 1090 10—90 60—120 60-120 60—120

Tail

5.3571

60.11

Seed Tail

Sinter

471

5.3553

60.11

60—120 60—120 60—120

60—120

Single crystal LaA1O3 has been reported in the literature to be a desirable substrate for deposition of high temperature superconductors to fabricate devices designed to function at high frequencies. The Czochralski process has been successfully adapted to the growth of LaA1O3 single crystals up to 59 mm diameter and weighing 2500 g. The high melting point of this compound, 2100°C,has required some modifications in the crystal growth furnace to control the amount of radiative heat loss from the melt surface.

~ Geiler and Bala [3).

References si ntered material prepared from a mechanical mixtt:re of La203 and Al203 reacted at 1800 to 1950°C a so agrees well with the Czochralski grown ciystals. The LaAlO3 crystal growth initially conc&ntrated upon development of 16 mm diameter boules for the fabrication of 1 cm square wafers. This size crystal was successfully grown in lengths of 100 mm, with the crystals weighing approxim itely 130 g. We did not experience any unusual problems in the growth of the larger diameter

[1) R.W. Simon, C.E. Platt, K.P. Daly, A.E. Lee and M.K. Wagner, AppI. Phys. Letters 53 (1988) 2677. 121 T.E. M.C. Harvey, Nuss, P.M. RE.Berkstresser, Howard, B.L. Strauglin. C.D.Mankiewich, Brandle, G.W. K.W. Goossen and P.R. Smith, App!. Phys. Letters 54 (1989) 2265. [3) S. Geller and V.8. Bala, Acta Cryst. 9 (1956) 1019. [4] E.A. Wood, Am. Mineralogist 36 (1951) 768. [5) H. Fay and C.D. Brandle, in: Crystal Growth (Pergamon, Oxford, 1967) p. 51. [6) H.M. O’Bryan. P.K. Gallager, G.W. Berkstresser and CD. Brandle, J. Mater. Red 5 (1990) 183. [7] l.A. Bondar and NV. Vinogradava, lzv. Akad. Nauk SSSR, Ser. Khim. 5 (1964) 785; l.A. Bondar and N.y.

U A103 crystals. The most recent sizes have been 59 mm in diameter by 100 mm in length and wcighing about 2500 g.

Vinogradava, in: Phase Diagrams for Ceramists (Am. Ceram. Soc., 1969) fig. 2340. [8] P.M. Gresho and J.J. Derby, J. Crystal Growth 85 (1987) 40.