Correlation of crystal perfection and growth parameters in czochralski-grown Cu3Au single crystals

396

Journal of Crystal Growth 85 (1987) 396-410 North-Holland, Amsterdam

CORRELATION OF CRYSTAL PERFECTION AND GROWTH IN CZOCHRALSKI-GROWN Cu3Au SINGLE CRYSTALS *

PARAMETERS

Y.K. C H A N G Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

and W. U E L H O F F , A. F A T F A H a n d G. H A N K E lnstitut )"fir Festkgrperforschung, Kernforschungsanlage, D-5170 Jiilich, Fed. Rep. of Germany

Received 22 April 1987

The effects of variation of the growth parameters on the quality of Czochralski-grown Cu3Au single crystals have been investigated. An assessment of alterations in the mosaic spread of these crystals was made by means of "r-ray diffractometry. Neither initial seeding with Cu or Cu3Ao single crystals nor pulling along [100], [110], or [111] orientations was found to have a significant influence on the crystal perfection. Variations of either the crystal diameter or rotation rate did have a definite effect on the mosaic spread, however, and it was also found that the pulling rate clearly produced a systematic and profound effect on the crystal quality. The critical pulling rate for which constitutional supercooling occurred was determined to be 7 to 8 mm/h.

1. Introduction A l l o y a n d c o m p o u n d single crystals are in increasing d e m a n d for use in m a t e r i a l s p r o p e r t i e s studies, a n d accordingly, s y s t e m a t i c investigations o f the g r o w t h of b i n a r y alloy crystals are warr a n t e d in o r d e r to i m p r o v e the crystal quality. In the p r e s e n t study, the b i n a r y m a t e r i a l C u 3 A u was c h o s e n as the subject of a s y s t e m a t i c investigation d u e to its k n o w n p h y s i c a l properties, its m a n a g e a b i l i t y in existing g r o w t h systems, a n d the desirability o f o b t a i n i n g b e t t e r q u a l i t y single crystals for use in a n u m b e r of research p r o g r a m s . C o p p e r a n d gold f o r m a c o n t i n u o u s d i s o r d e r e d solid solution at elevated t e m p e r a t u r e s , a n d the p h a s e dia g r a m for this system is n o w well k n o w n [1]. Several o r d e r e d p h a s e s such as C u 3 A u , C u A u , a n d C u A u 3 , can f o r m at lower temperatures, however. T h e C u 3 A u phase, with a characteristic * Research sponsored by the Division of Materials Sciences, US Department of Energy under contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc.

o r d e r - d i s o r d e r t r a n s f o r m a t i o n at 3 9 0 ° C a n d a negative e n t h a l p y of mixing, has been the subject of n u m e r o u s investigations over the p a s t half century. The d i s o r d e r e d alloy has a face-centered cubic structure that is c h a n g e d u p o n cooling to a simple cubic form. This solid-solid t r a n s f o r m a t i o n changes the crystal s y m m e t r y , a n d this a l t e r a t i o n c a n be d e t e c t e d as superlattice lines in the diffraction pattern. A d d i t i o n a l l y , the c h a n g e in a t o m i c a r r a n g e m e n t p r o d u c e s changes in a n u m b e r of the p h y s i c a l p r o p e r t i e s of the material. A m o n g these effects are a l t e r a t i o n s in the F e r m i surface, m a g netic susceptibility, thermoelectric power, H a l l effect, specific heat, a n d electrical resistivity. A s a result of these effects, the o r d e r e d alloy c o n t i n u e s to be of c o n s i d e r a b l e theoretical a n d e x p e r i m e n t a l interest. W h i l e C u 3 A u single crystals have t r a d i t i o n a l l y b e e n p r e p a r e d b y the B r i d g m a n m e t h o d , in the p r e s e n t work a c o m p u t e r - c o n t r o l l e d Czochralski system, d e v e l o p e d previously [2-4] for growing n e a r l y perfect p u r e metal single crystals, was emp l o y e d in s t u d y i n g the crystal g r o w t h process. T h e

0 0 2 2 - 0 2 4 8 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

Y.K. Chang et aL / Correlation of perfection and growth parameters in CZ-grown Cu3Au

effects of several controllable growth parameters (i.e., seed materials, necking, seed orientation, crystal diameter, rotation rate, and pulling rate) on the resulting crystal perfection were studied, and the mosaic spread, which is a measure of crystal imperfection, was systematically assessed by 7-ray diffractometry. The experimental details, including the materials preparation, growth procedure, and specimen examination are given in the following section. Section 3 presents the results of the characterization of Cu3Au crystal perfection and its relation to the growth parameters.

(,0s

Position

~li

Seed Neck Thermocouple Crystal T~" emil ~ Interface Meni scus 0 ~...~_ ~_~~) Melt RF 0 Crucible ¢,0c -------- ~--

2.,-':-x-1~3 Carl

2. Experimental 2.1. Materials preparation

Charges consisting of roughly 450 g of copper-gold ( - 9 9 . 9 9 9 % purity) were prepared either by arc casting in a water-cooled copper hearth under an argon atmosphere or by vacuuminduction melting directly in the graphite crucible used for pulling the single crystal. The graphite crucible was 27 mm in depth and 44 mm in ID and had a 6 mm wall thickness. The ratio of the crystal diameter to the ID of the crucible should be sufficiently small to insure that the growth rate is not greatly influenced by decrease in the melt level. The inner wall of the graphite crucible was first polished with tissue paper to improve the surface smoothness and then treated in boiling concentrated hydrochloric acid. Finally, the entire crucible was cleaned in distilled water followed by RF baking to remove possible surface contamination. 2.2. Growth system

The crystal growth system consisted of a sophisticated computer-controlled Czochralski apparatus that has been described in detail elsewhere [2-4]. A schematic drawing of the system is shown in fig. 1. A 12 kW RF generator equipped with a precision power ramping feature powered the system and the crystal growth was carried out in a vacuum of (2-8) x 10 -6 Torr. The bulk melt temperature was determined by means of a pair of

397

_--_ -

0

--,--

__-

-

0

Measurement And

ControlSystem

Fig. 1. Schematic diagram of the Czochralski crystal growth system used for preparing Cu 3Au single crystals.

thermocouples located opposite each other in the melt (0.1 K resolution), and the position of the pulling rod was measured by an electronic digital counter. Both the seed and the crucible could be independently rotated in either direction. During the crystal pulling operation, the meniscus and crystal-melt interface in the crucible were monitored with a closed circuit TV camera, and an image of the crystal was electronically decoded to obtain both the crystal diameter and the position of the crystal-melt interface to within an accuracy of 0.03 mm. With the aid of a PDP-11 computer, other quantities during growth such as the interface height, melt temperature gradient at the interface, heat flow, growth rate, growth angle, and crystal length could be determined. 2.3. Growth procedures 2.3.1. Growth rate

The kinetics of the crystal-melt interface determine the ultimate characteristics of a grown crystal. Cellular structures (or cell formations) are often observed in crystals prepared from alloys or impure melts. Rutter and Chalmers [5] first suggested that this cellular structure arose from the

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu ~Au

398

formation of a region of "constitutionally supercooled" liquid in front of the crystal-melt interface. It has been shown by Tiller et al. [6] that for convectionless melts this constitutional supercooling can be avoided if the growth rate v at the interface satisfies the following relationship: G / v > mC~(1 - k o ) / k o D , where G is the temperature gradient in the melt, m is the liquidus slope, Q is the solute concentration, k 0 is the equilibrium segregation coefficient, and D is the diffusion constant of the solute in the melt. For Cu3Au , m = 4 ° C / a t % , Q = 25at%, k 0 = 0 . 8 , a n d D is assumed to be 4 × 1 0 5cm2/s. To avoid constitutional supercooling in Cu3Au, the growth rate should be less than 3 m m / h for a temperature gradient of 5 o C / r a m . Therefore, the pulling rate was maintained at a practical value of 3 m m / h which is the lower limit of the growth system. The pulling rate was increased in steps during certain runs in order to investigate the effect of pulling rate on the quality of the resulting crystals and on the onset of constitutional supercooling. The actual growth rate is, of course, equal to the sum of the pulling rate and the rate of the crystal-melt interface drop. 2.3.2. Seeding Nearly perfect

copper

single crystals with

orientations of [100], [110], and [111] were used as seeds for growing Cu3Au single crystals. During the growth process, the seed was moved toward the Cu3Au melt very slowly in order to minimize thermal shocks that might damage the seed. Once the seed made contact with the melt, a subsequent 5 to 10 min delay permitted thermal equilibrium to be established before the pulling operation was initiated. The Cu3Au melt initially tends to bulge at the C u - C u 3 A u interface because of the difference in the melting temperatures of Cu and Cu3Au. This bulge gradually recedes, however, as a result of dissolution of the seed tip in the Cu 3Au melt as the growth proceeds. 2. 3.3. Necking process A necking process that results in a thin neck is important for retarding dislocation propagation during crystal growth. Dash [7-10] has successfully prepared dislocation-free silicon crystals by adopting an extremely thin seed that is produced by shrinking the seed to a "bottle neck". During growth, the crystal diameter is normally controlled by varying the melt temperature and a typical profile during necking is shown in fig. 2. The melt temperature can be raised and lowered at proper rates to regulate the rate of the diameter change. The position of the crystal-melt interface naturally moves down and up as the crystal diameter 12

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Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu 3Au

399

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400

}(K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu3A u

decreases and increases, and, in order to keep an effective growth rate below the critical value where constitutional supercooling might occur, the crystal diameter must be reduced slowly. All of the seeds were narrowed to a diameter of 1 m m or less in the present case. Additionally, although a long neck can, in principle, lead to better crystal quality, the neck length here was limited to no more than 10 m m to avoid excessive crystal vibration. 2.3. 4. Diameter control and rotation rate Normally, the melting temperature of the charge is determined in situ at the beginning of each run by observing the transition plateau of the thermocouple output as the melt begins to solidify. The temperature is then increased and maintained at about 40 to 50 ° C above the melting temperature, depending on the desired crystal diameter. Melt flow caused by free or forced convection has an effect on both the crystal perfection and crystal diameter [11] and this effect has been examined in the present case by varying the rotation rates of the seed and crucible. 2.4. Examination by y-ray diffractometry The perfection of C u a A u single crystals was examined by means of y-ray diffractomet~. Because of its shorter wavelength of 0.0265 A, the y-ray effectively "sees through" large crystals better than X-rays which primarily scan the surface region. Sets of rocking curves for certain low-index reflections were generally measured in volume elements that were equally spaced along the crystal under examination. The intrinsic half-width of the y-ray diffractometer was 12 arc sec as determined from measurements on subgrain-free copper crystals. A beam slit with dimensions of 0.5 × 5 m m 2 was utilized with the longer side of the slit arranged vertically. The crystal to be examined was mounted either vertically or horizontally on a goniometer. It was found that the set of rocking curves measured for a horizontally mounted crystal usually revealed more details regarding the mosaic spread than the vertical set, as shown in fig. 3 Since the crystal quality in the initial section is good, there is little difference regarding the mosaic spread measured either vertically or horizontally

in this region. (Note that the position of a volume element in one set of rocking curves may differ from the other by as much as 1 m m because of the uncertainty in determining both the position of the beam slit and that of the crystal.) In fig. 3, beginning between 46 and 51 mm, where the rotation rates were altered, the mosaic spread deterrnined from the horizontal measurement was much wider and more complex than that determined from the corresponding measurement made vertically. This indicates that the crystal grains tend to elongate along the growth direction at random but small angles and that grains developed by perturbed growths (described in the next section) can be healed. Accordingly, for the best assessment of the crystal quality, the more sensitive horizontal arrangement was employed in the present case.

3. Results and discussion

3.1. Seeding and necking High-quality copper single crystals with a mosaic spread of less than 16 arc sec were initially used as seeds for the Cu3Au growth since the Cu lattice constant is comparable to that of Cu3Au (only differing by 3.61% at room temperature). Subsequently grown Cu3Au crystals were employed as seeds in some cases. Seven Cu3Au single crystals were produced and their growth conditions are summarized in table 1. Crystals 2, 3, and 4 were prepared by using Cu3Au seeds, while the others were using copper seeds. These copper seeds were cut by the electrochemical method. It was found from the y-ray diffraction results that using either copper or Cu3Au as a seed material had no apparent effect on the perfection of the resulting crystals. All of the crystals except crystal 1 were prepared using a necking process. A higher pulling rate of 4.2 m m / h for crystal 1 was also used for testing the capability of the growth system. Consequently, the first crystal in spite of the use of a nearly perfect seed, had a broad mosaic spread and displayed a y-ray rocking curve whose halfwidth was 0.5 ° to 1.6 ° The perfection of the

Y.K. Chang et aL / Correlation of perfection and growth parameters in CZ-grown Cu 3Au

401

Table 1 Summary of Cu3Au crystal growth Crystal No.

Growth direction

Seed

1

111

2 3 4 5 6 7

111 111 111 111 100 110

Cu Cu3Au-1 Cu3Au-2 Cu3Au-I Cu Cu Cu

Necking

No Yes Yes Yes Yes Yes Yes

Pull rate (mm/h) 4.2 3 3 3 3, 4, 5, 6, 7, 8, 9, 10 3, 4, 5, 6, 7, 8, 3 3 3 3,10,3

remaining crystals normally degraded during the initial stages of necking, greatly improved immediately after the necking process, and often continued to improve after further stable growth.

3.2. Growth orientation and facet formation Initially five Cu3Au single crystals were grown along the [111] direction. Subsequently, the other two principal symmetry directions, [100] and [110], were also chosen as growth axes. No significant difference in the crystal quality was found in relation to the growth orientation. Unlike the [111] crystals, however, both the [100] and [110] crystals had distinct facet strips on the otherwise cylindrical surfaces. Such facets were not observed on either pure Cu or pure Au single crystals pulled using the same technique. It should be noted that faceting has been observed previously on (111) and (100) poles of small spherical Cu, Au, and C u - A u alloy particles prepared by solidification of a drop of melt on a graphite surface [12-14]. On the surface of the [100] Cu3Au crystal 6, four (111) facet strips appear at the tapered portion of the neck where the surface normal is at an angle of about 55° from the growth direction (see fig. 4). When the surface normal approaches 90 o from the growth direction, these (111) facet strips gradually disappear and four (100) facet strips begin to merge on the crystal surface. These are alternatively interspersed with the previous (111) facet strips at an angle of 45 ° about the [100] growth direction.

Crystal diameter (mm)

Rotation (rpm) COs

60 c

30 30 30 30 15 30 30 15 0

- 30

6

- 30 - 30 - 30 5 - 30 - 30 5 0

5 6 6 6, 10 6 6

The width of a facet strip is proportional to the crystal diameter and inversely proportional to the growth rate. The former factor is geometric and independent of the thermal conditions, and the latter effect is thermodynamic in nature and is related to the growth kinetics. For example, when the pulling rate of crystal 6 was increased from an initial value of 3 m m / h to 4, 5, 6, 7, and 8 m m / h , for every 10 mm growth in crystal length the width of the (100) facet strip narrowed from about 1.5 to 0.2 mm. When the pulling rate returned to 3 m m / h from 8 m m / h , however, the width of the

[100] Cu3Au-6 - -

~

Neck

(111)Facets Crystal (100)Facets

i I

J

Interlace

Melt Fig. 4. Facet formation near the neck region of crystal 6 observed during growth.

402

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu ~Au

(100) facet strip gradually increased to its original value. Because of the availability of all three low-index planes on the surface of a [110] crystal, the [110] growth direction was chosen for the study of facet formation. F o u r (111) and two (100) facets were observed on the surface of crystal 7 (fig. 5), and this orientational dependence clearly shows that the surface energy ~, for C u 3 A u is a function of crystallographic orientation in contrast to pure Cu or Au. There are "cusps" in the y-plot at orientations corresponding to low-index planes [15]. This crystal 7 was grown at a pulling rate of 3 m m / h with a seed rotation rate of ~ = 30 rpm and a crucible rotation rate of ~ = - 3 0 rpm for the initial 48.7 m m as shown in fig. 6. (The crystal position was determined with a digital position counter that recorded the distance traveled by the pull rod. For a constant diameter, the actual crystal length equals the sum of the crystal position and the distance the melt level d r o p p e d from its original level.) Over the section immediately following

the neck, only four (111) facet strips parallel to the crystal axis were observed with an extended angle of - 1 5 ° + 3 ° ( - 0 . 8 m m in width). The beginning of the next section, however, where the rotation rates were reduced to ~0~= 15 rpm and ~0c = 5 rpm, two (100) facet strips began to appear with an extended angle of 24 ° _+ 3 °. The existing (111) facet strips also widened to about 33 ° _+ 3 °. These results indicate that the formation of facets is depressed by fast stirring. At the crystal position 69.2 m m as shown in fig. 6, both the seed and crucible rotations were abruptly stopped, and the crystal diameter began to shrink - apparently as a result of local temperature disturbances in the melt (see fig. 5). (An increase in the melt temperature was recorded via a thermocouple immersed in the melt near the edge of the crucible and 13 m m from the pulled crystal.) All of the facets vanished at this point because the surface normal was tilted away from a 90 ° angle and the crystal underwent an increase in its growth rate for several millimeters. W h e n the crystal diameter returned to a

Fig. 5. Facet strips o n crystal 7 observed d u r i n g growth. The wide strip is a (111) facet and the n a r r o w strip is a (100) facet. This picture was t a k e n while the c o u n t e r p o s i t i o n (of the interface) was at 74.4 mm, with both r o t a t i o n s stopped.

403

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu 3Au

110

Position

mm 0

Cu3Au_ 7

~g Rotation ~ Rate

~

c o l u m n a r structure can be observed on the surface at this point, a n d all of the facets are suppressed. This c o l u m n a r structure is evidence of the occurrence of constitutional supercooling. At 90 m m as indicated in the figure, the pulling rate was lowered to 3 m m / h for the r e m a i n d e r of the crystal growth.

rpm

[1111

-~COs=30 C~¢=-30

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mm

mm/h

20

Rate

30 40

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80

90

100

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c o n s t a n t value at a b o u t 78 m m as shown in fig. 6, b o t h the (111) a n d (100) facet strips reappeared with a n g u l a r widths of 2 5 ° + 2 ° a n d 2 1 ° + 2 ° , respectively. This slight decrease in facet width p r o b a b l y results from crystal imperfections introduced by the s u d d e n halt in the stirring a n d the c o n c u r r e n t increase in growth rate. Beginning at the position of 80.9 m m in fig. 6, the pulling rate was increased to 10 m m / h a n d

Tz

Fig. 7. Cu3Au single crystal 5 is shown with its various pulling rates. Because of the large diameter beyond position 70 mm, the actual crystal position displays an increasing difference from the position given by the counter.

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu sAu

404

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Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu 3Au

The facet strips then reappeared with widths of 2 6 ° + 1 ° and 18°_+ 3 ° for the (111) and (100) facets, respectively. The widths are comparable to the earlier values obtained just before the pulling rate was increased.

100

f Position mm

3.3. Effects of diameter changes

405

Cu3Au- 6

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0-

3.3.1. Cu ~Au crystal 5: in the growth region from 6 to 10 mm

10

Following the necking process, the diameter of crystal 5 (see fig. 7) was maintained at 6 mm for nearly 50 mm of the ensuing crystal length (starting at the crystal position indicated at 20 ram) and was then increased to 10 mm. This change in diameter altered the crystal quality momentarily as shown in the measured y-ray rocking curves of figs. 8 and 9. Beginning approximately at the crystal position of 70 mm, where the crystal diameter began to increase rapidly from 6 to 10 mm, the y-ray peak intensity dropped and the halfwidth broadened for a section about 5 mm in length as shown in fig. 9. The crystal quality was largely recovered when a constant diameter was re-established. In order to enlarge the crystal diameter from 6 to 10 m, the temperature of the melt was lowered from 2 8 ° C above the melting point of the charge to 13°C above (see fig. 2) during a time span of 1 h. The resulting crystal quality could be improved if the rate of melt temperature drop is reduced in order to decrease the rate of the diameter change.

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70-80-90-I00

57

--

73

110

3.3.2. Cu~Au crystal 6: fluctuations Near the completion of the necking process crystal 6 experienced a diameter fluctuation as a result of an unintentional power variation. This portion of the crystal can be visually identified from the crystal position that extends from about 36 to about 40 mm as shown in fig. 10. A series of rocking curves for the (200) reflection measured in 12 volume elements equally spaced from 35 to 57 mm in the horizontal position is shown in fig. 11. The half-width initially has a value of 60 arc sec, but, beginning at 39 mm, the rocking curve not only increases in width several-fold but also exhibits a loss in peak intensity. There is a gradual recovery in the crystal perfection, however, after

120

--

Fig. 10. Cu3Au single crystal 6 is shown with its pulling rates.

the crystal diameter reaches a constant value. At the position of about 57 mm, the crystal perfection has nearly recovered to its original condition before the fluctuation.

3.3.3. Cu3Au crystal 7: abrupt shrinking A sudden arrest of both the seed and crucible rotations from ~0~= 15 rpm and ~oc = 5 rpm near the crystal position of 69.2 mm for Cu3Au crystal

406

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu ~Au

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mm

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800

ANGLE, a r c

see

Fig. II. A series of y-ray rocking curves of the (200) reflection measured from 35 to 57 mm at 2 mm intervals on crystal 6 in order to reveal the effect of diameter fluctuations on the crystal perfection.

7 drastically altered the melt flow pattern and r e s u l t e d in a m e l t t e m p e r a t u r e rise i n a n d n e a r t h e meniscus. The crystal diameter experienced a sharp

d e c r e a s e as s h o w n in fig. 6. Fig. 12 d i s p l a y s t h e measured rocking curves taken before and after this abrupt diameter change, the sharp spectrum

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ANGLE, arc sec Fig. 12. A series of y-ray rocking curves of the (200) reflection measured on crystal 7 before and after the sudden change in diameter due to the stopping of rotations.

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu 3A u

broadening and intensity attenuation are a result of the change of rotation and concurrent increase in the effective growth rate. 3. 4. Rotation The purpose of rotating the seed and crucible is to alter the melt flow pattern in such a way that b o t h heat and mass transport are conducive to the desired crystal growth conditions. The influence of rotation on the crystal perfection was selectively studied during the growth of crystal 7 by varying both the seed and crucible rotation rates. A counter rotation of cos = 30 rpm and o~c = - 3 0 rpm, as initially chosen for all of the crystals except Cu 3Au crystal 5 (table 1). This was later changed to an isorotation of 0~ = 15 r p m and ~oc = 5 rpm after the crystal had been pulled to 48.7 m m in length. There were no noticeable diameter deviations or melt temperature fluctuations related to this change. A series of (220) reflection rocking curves measured in volume elements for every millimeter from 47 to 57 m m is shown in fig. 13. A trend of increasing mosaic spread evidently occurred im-

407

mediately after the change of rotation rates. The width of the mosaic spread, obtained from the rocking curves and plotted in fig. 14, expands monotonically from 180 arc sec prior to the change in rotation rate and reaches a plateau of 740 arc sec at the position of 55 mm. This indicates that rotation rates can have an influence on crystal perfection even when these rates produce no noticeable diameter and melt temperature alteration. At 69.2 mm, both the seed and crucible rotations were stopped, and a sharp drop in the crystal diameter and serious damage to the crystal perfection occurred. 3.5. Pulling rate The initial choice of a pulling rate of 3 m m / h proved to be excellent and a measured half-width in the rocking curve of less than 60 arc sec (for horizontal mount; 30 arc sec for vertical mount) was obtained at this rate. In order to investigate the effect of pulling rate on crystal quality (particularly on the constitutional supercooling), pull-

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Fig. 13. A series of "~-ray rocking curves of the (220) reflection m e a s u r e d from 47 to 57 m m at 1 m m intervals o n crystal 7 to reveal the effect of r o t a t i o n rate change (occurred near 49 ram) on the crystal perfection.

Y.K. Chang et aL / Correlation of perfection and growth parameters in CZ-grown Cu3 A u

408

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every 10 m m of travel (see fig. 7). Surface striations that were a c o n s e q u e n c e of the r o t a t i o n were o b s e r v a b l e until the pulling rate reached a value of 8 m m / h . F r o m this p o i n t on, a l o n g i t u d i n a l colu m n a r structure a p p e a r e d on the surface. This indicates that c o n s t i t u t i o n a l s u p e r c o o l i n g occurs

ing rates were varied d u r i n g the growth of crystals 5, 6, a n d 7. A f t e r the crystal d i a m e t e r was e x p a n d e d from 6 to 10 m m a n d the s a m p l e had g r o w n for 10 m m at 3 m m / h , the pulling rate of crystal 5 was increased to 10 m m / h at a step of 1 m m / h for

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Y.K. Chang, et al. / Correlation of perfection and growth parameters in CZ-grown Cu 3Au

when the pulling rate exceeds 7 m m / h . Fig. 15 shows a series of rocking curves for the (111) reflection as measured from volume elements beginning at 75 m m and at an interval of 5 m m throughout the rest of crystal 5. The growth of the mosaic spread with an increase in pulling rate is evident. In addition, the peak position of the mosaic spread continuously shifts to one side by as much as 2° near the end of the crystal. A series of rocking curves of the (200) reflec-

409

tion measured over the full length of crystal 6 after the necking process is shown in fig. 16. The broadening of mosaic spread observed in the middle of the crystal for the 3 m m / h pulling rate range was discussed earlier. The pattern of mosaic spread broadening versus pulling rate is similar to that found for crystal 5. During the growth of the last 15 m m of the specimen, the pulling rate was lowered to 3 m m / h , and the rocking curves for this region show a clear increase in crystal quality.

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410

Y.K. Chang et al. / Correlation of perfection and growth parameters in CZ-grown Cu 3Au

Most of this recovery, however, originated from a different " g r a i n " of the crystal,

c o m m e n t s d u r i n g the p r e p a r a t i o n of this m a n u script.

4. S u m m a r y

References

The effects of several growth parameters such as seeding material, growth direction, crystal diameter, r o t a t i o n rate, a n d pulling rate on the crystal perfection of Czochralski-grown C u 3 A u single crystals have been investigated by m e a n s of "t-ray diffractometry. The choice of either C u or C u 3 A u single crystals as a seed did not p r o d u c e a noticeable difference in the perfection of the final C u 3 A u crystals. T h e growth direction a p p a r e n t l y did n o t affect the crystal perfection except for facet formation. Variation in the crystal diameter d u r i n g growth causes the mosaic spread to increase. The diameter change is not a direct cause of the crystal imperfection, however, b u t is rather a c o n s e q u e n c e of other kinetic growth changes. Since the r o t a t i o n rate affects the t e m p e r a t u r e d i s t r i b u t i o n in the melt, it influences the crystal quality indirectly as does the crystal diameter. Finally, the pulling rate was f o u n d to have a systematic a n d significant effect on the crystal quality. C o n s t i t u t i o n a l supercooling in Cu 3Au was observed when the p u l l i n g rate reached a value between 7 and 8 m m / h .

[l] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958), p. 199. [2] H. Fehmer and W. Uelhoff, J. Crystal Growth 13/14 (1972) 257. [3] W. Uelhoff, J. Crystal Growth 65 (1983) 278. [4] W. Uelhoff, A.W.A. van der Hart, A. Fattah, G. Hanke, D. Jedamzik, B. Knook, G.J. van der Berg and H. Wenzl, A Model for Macroscopic Czochralski Growth: Theoretical and Experimental Investigations, Report No. Jii1-1554 (Nov. 1978), IFF, KFA, Jiilich, Fed. Rep. of Germany. [5] J.W. Rutter and B. Chalmers, Can. J. Phys. 31 (1953) 15. [6] W.A. Tiller, K.A. Jackson, J.W. Rutter and B. Chalmers. Acta Met. 1 (1953) 428. [7] W.C. Dash, in: Growth and Perfection of Crystals, Eds. R.H. Doremus, B.W. Roberts and D. Turnbull (Wiley, New York, 1958) p. 361. [8] W.C. Dash, J. Appl. Phys. 29 (1958) 736. [9] W.C. Dash, J. Appl. Phys. 30 (1959) 459. [10] W.C. Dash, J. Appl. Phys. 31 (1960) 736. [11] J.R. Carruthers, in: Crystal Growth: A Tutorial Approach, Eds. W. Bardsley, D.T.L. Hurle and J.B. Mullin (North-Holland, Amsterdam, 1979) p. 157. [12] K.D. Stock, M. Schneegans and E. Menzel, Z. Metallk. 71 (1980) 293. [13] M. Schneegansand E. Menzel, J. Crystal growth 63 (1983) 309. [14] M. Schneegans and E. Menzel, J. Crystal Growth 63 (1983) 315. [15] D.P. Woodruff, The Solid-Liquid Interface (Cambridge University Press, London, 1973)p. 3.

Acknowledgments T h e authors gratefully acknowledge H.F. Wenzl, L.A. Boatner, a n d M.M. A b r a h a m for helpful