Interface shape transitions in czochralski grown YAG crystals

Mat. Res. Bull. Vol. 13, pp. 675-680, 1978. Pergamon P r e s s , Inc. Printed in the United States.

INTERFACE

SHAPE TRANSITIONS

IN CZOCHRALSKI

GROWN YAG CRYSTALS

Y. Miyazawa, Y. Mori, S. Homma and K. Kitamura National Institute for Researches in Inorganic Materials Kurakake, Sakura-mura, Niihari-gun, Ibaraki, 300-31, Japan

(Received May 18, 1978; Communicated by M. Nakahira)

ABSTRACT In order to understand the abrupt interface shape change w h i c h often occurs during the Czochralski growth of many oxide crystals, the relationship between this critical diameter and the rotation rate of the crystal in the case of YAG (Y3AIsO12) was carefully investigated and e x p e r i m e n t a l l y established. A recent theory could explain the obtained results very well.

Introduction In Czochralski crystal growth, it is very important to understand the interaction between forced convection effects, due to crystal rotation, and thermal convection in the melt. It is well known that the heat flow in oxide Czochralski growth often undergoes an abrupt change. This becomes evident as a sudden crystal w e i g h t reduction. Such effects have well been documented for the growth of garnet crystals by three different papers(l-3). Quantitative analyses have recently been proposed by Carruthers (4) and Brice and W h i f f i n (5). The former developed the relationship between forced convection and thermal convection flows which q u a n t i t a t i v e l y explains the interface shape transition behavior reported for GGG (Gd3GasO12) crystals (i), and he pointed out that the important growth variables were not always specified by the experimenters. The latter showed that the interface shape transition effects could be described in terms of an instability in the Couette flow region between the rotating column under the crystal and the crucible wall. A l t h o u g h their analysis explained experimental data very well, two unknown constants had to be determined empirically. The interface shape transitions for YAG crystals were reported by Zydzik (3), however, no quantitative relationship between the critical inversion diameter and the crystal rotation rate were given.

575

676

Y. MIYAZAWA, et al.

Vol. 13, No. 7

In this report, the same type of experiments as for GGG (i) was p e r f o r m e d for YAG to compare w i t h C a r r u t h e r s analysis (4). A t t e n t i o n was also directed toward the interface shape after the transition. Experimental The pulling aparatus used for these experiments was made by Kokusai Electric Co. for oxide crystals. The r.f. generator had m a x i m u m output power, 40 kw, and o s c i l l a t i o n frequency, 100 khz. This aparatus has been m o d i f i e d for automatic diameter control (ADC). The ADC method used for the experiments was a popular crystal w e i g h i n g system. The load cell w h i c h was used to m e a s u r e the crystal w e i g h t was made by Ohkura Electric Co., and it had a very high sensitivity of 0.01 gr. Hence it was possible to monitor the interface shape transitions in these experiments. The oxide m a t e r i a l s used were 4-nine pure Y203 or Dy203 and A1203 obtained from Shinetsu Chemical Co. and Sumitomo Chemical Co., respectively. The oxides w e r e heated at 1300°C for 2 hours to e l i m i n a t e m o i s t u r e prior to mixing. The s t o i c h i o m e t r i c a l l y m i x e d powders were pressed at i000 kg/cm 2 in a rubber bag in a hydrostatic press, heated at 1300°C for 12 hours. A cylindrical i r r i d i u m crucible, 47 m m in diamete r , 48.5 m m in depth and 1.5 m m in thickness, was used to contain the melt. N2 gas was flowed through the chamber at a rate of 1 litter/min. After 3 hours of soaking, the melt was held at an a p p r o p r i a t e seeding temperature~ The both radial and vertical temperature d i s t r i b u t i o n in the m e l t near the seeding point was m e a s u r e d by m o v i n g an Ir/Ir-40%Rh thermocouple. YAG crystals were grown on an axis, at the pulling rate of 2-8 m m / h o u r and the crystal rotation rate of 40-180 rpm. In order to d e t e r m i n e the r e l a t i o n s h i p b e t w e e n the critical inversion diameter and the crystal rotation rate, a crystal, about 5 mm in diameter, was pulled for 10 mm long. Then the r.f. generator output power was d e c r e a s e d at a c o n s t a n t rate so that the diameter of crystal was increased. When the crystal d i a m e t e r r e a c h e d to the critical inversion diameter, the crystal w e i g h t suddenly began to decrease. After the crystal w e i g h t started to increase again, the crystal was quickly d e t o u c h e d from the melt in order to preserve the s o l i d - l i q u i d interface shape and to measure the critical inversion diameter. A typical change in w e i g h t recorded by the load cell during the t r a n s i t i o n is shown in FIG. i. Up to 3 gr weight loss was recorded. For the rotation rate higher than 100 rpm, the sudden reduction of crystal w e i g h t was not detected. The rate of diameter change o b s e r v e d by the naked eye, however, p r o v i d e d a vague indication as to w h e t h e r the transition had occurred. The critical inversion diameter was d e t e r m i n e d by growing the crystal till well after the transition, o b s e r v i n g its l o n g i t u d i n a l section and directly m e a s u r i n g the d i a m e t e r along the d i s c o n t i n u i t y w h i c h is d e s c r i b e d below. In order to check the change for the interface shape after the transition, a YAG crystal was grown at the rotation rate of 60 rpm, first by the same p r o c e d u r e s d e s c r i b e d above untill the transition, and then by applying the ADC method.

Vol. 13, No. 7

I

YAG CRY'STAIZ

677

I i0g FIG. 1 The weight change recorded by the load cell during the interface inversion.

w < o0 >O

f

TIME

10

I

min

I

Results A typical photograph of the shape of the crystal which was detouched from the melt after the transition is shown in FIG. 2. A DyAG (Dy3AlsO12) crystal, grown by the same procedures, is also shown for discussion stated below. These are of lower rotation rates for w h i c h the inversions were detected as the weight reduction. A longitudinal section of a YAG crystal as viewed between the crossed nicols is shown in FIG. 3 as a typical example grown at the rotation rate higher than 100 rpm. It was not possible to detect sudden weight reduction as demonstrated by the attached diagram. But the rate of increase in diameter changed d r a s t i c a l l y at x-x, the shoulder of the crystal, while the heater power decreased at constant rate. The d i s c o n t i n u i t y along x-x clearly indicates that the interface transition occurred there. The striations in the crystal grown by applying the ADC method are viewed between the crossed nicole and shown in FIG. 4. It FIG. 2 The longitudinal secis evident that the shape of the tion of YAG and DyAG crystals interface was still fairly convex showing the interface shape toward the melt after the transiafter the interface inversion tion. The r e l a t i o n s h i p between the critical inversion diameter and the crystal rotation rate for two different pulling rate is shown in FIG. 5. The critical inversion diameter decreases with increasing rotation rate in both cases.

678

I---

Y. M I Y A Z A W A , et ~.

I

Vol. 18, Mo. 7

lO g i--'r"

-,m---i ,,:IZ I--" >r,f

>-

r

TIME

I

10 min

I

FIG. 3 Above: A longitudinal section of YAG crystal under a polarlzing microscope showing the change in interface shape. The rotation rate is 140 rpm. The pulling rate is 8 mm/hr. Below: The weight change recorded by the load cell.

TIME

!

10 min

I

FIG. 4 Above: A longitudinal section of YAG crystal under a polarizing microscope showing the change in interface shape. The rotation rate is 60 rpm. The pulling rate is 8 mm/hr. Below: The weight change recorded by the load cell.

Vol. 13, No. 7

YAG CRYSTALS

679

25

FIG. 5 Relation b e t w e e n crystal rotation rate and diameter at w h i c h the s o l i d - l i q u i d interface inversion occurs. Dashed curve is from equation (i).

A 0

~ o z

2o

II l

0

O

8

•

2 mm/hr

mm/hr

0

i

••

k

O %

,~15.

'•. 8 EQUATION ( I )

"'.

lO0°c/cm O~-.

--. 0

10 ,

40

i

J

60

0

80

100

120

140

160

180

CRYSTAL ROTATION RATE (rpm)

Discussion For the r o t a t i o n rate higher than 100 rpm, sudden w e i g h t red u c t i o n could not be detected. This was because the inversion diameter was so small that the amount of r e m e l t i n g was not enough to be d e t e c t e d by the load cell. The load cell picks up the noise due to the rotation, the level of w h i c h is larger at a higher r o t a t i o n rate. Therefore, the sensitivity of the load cell could not be fully utilized. A c c o r d i n g to C a r r u t h e r s (4), the critical inversion diameter d, can be expressed as d =

[ g ~ AT

R 3 7r-2

]i/4

~-i/2

(i)

where g is the g r a v i t a t i o n a l acceleration, ~ is the m e l t thermal e x p a n s i o n coefficient, R is the crucible radius, AT is the radial t e m p e r a t u r e d i f f e r e n c e and ~ is the crystal rotation rate. In the present w o r k for YAG, the radial t e m p e r a t u r e gradient was m e a s u r e d to be about 100°C/cm. The only u n k n o w n factor in equation (I) is ~. S u b s t i t u t i n g ~=i0-3/°C w h i c h was used by C a r r u t h e r s (4) for GGG, the interface t r a n s i t i o n curve shown in FIG. 5 is obtained. The a g r e e m e n t w i t h the e x p e r i m e n t a l transition data in FIG. 5 is very good c o n s i d e r i n g the e x p e r i m e n t a l error and the c r u d e n e s s of the assumption. It is surprising that the a g r e e m e n t is good for the very wide range of the crystal rotation rate. This result strongly supports that equation (i) is e s s e n t i a l l y correct.

680

Y. MIYAZAWA, et al.

Vol. 13, No. 7

It is evident from FIG. 5 that the higher pulling rate makes the inversion diameter smaller. The same tendency was also reported by Takagi et al. (i) for GGG. The reason is not clear yet. It may be probable that at the higher pulling rate, the liberation rate of the latent of heat is larger, which makes the radial temperature gradient smaller. One may find in the equation (i) that this would result in the smaller inversion diameter explaining the experimental results. Particular remarks should be made here regarding the flatness of the interface after the transition. For YAG crystals, whose diameter is over 10 mm, the rotation rate of 150 rpm was required to obtain a flat interface (6), while the interface shape transition occured as low a rotation rate as 40 rpm. It may seem a contradictory fact for those who are preoccupied with the idea that the transition always results in the formation of flat interface. However, it has to be made clear that the interface shape transition which is caused by the heat flow transition in the melt does not necessarily result in a flat or concave interface toward the melt which was described by the previous workers (1,2). As shown in FIG. 2, after the transition, the shape of the interface is still clearly convex toward the melt for YAG crystals, while it is concave for DyAG crystals. FIG. 4 shows a reduced convexity of the post-transition interface. The degree of convexity for a given diameter is controlled by the rotation rate. Therefore it may be natural to have 150 rpm rotation rate in order to hold a flat interface. Conclusion Carruthers' analysis (4) for the interface shape transition can explain the present experimental results for YAG crystals very well. After the transitions, the shape of the interface for YAG crystals is still fairly convex toward the melt, while for DyAG crystals it is concave. It is concluded that the interface shape transition phenomenon does not necessarily mean a flat or concave interface toward the melt. Acknowledgment We thank Dr.

S. Kimura

for helpful

discussions.

References i. K. Takagi, (1976).

T. Fukazawa

2. B. Cockayne, 259 (1976). 3. G. zydzik,

and M. Ishii,

J. Crystal

B. Lent and J. M. Roslington,

Mat.

Res.

4. J. R. Carruthers,

Bull.

10,

J. Crystal

701

J. Mat.

Science

(1975).

Growth 36,

212

5. J. C. Brice and P. A. C. Whiffin, (1977).

J. Crystal

6. C.D. Brandle (1972).

J. Crystal,

and A. J. Valentino,

Growth 32,

(1977). Growth 38,

245

Growth 12, 3

89 ii,