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Enhancement of growth rate for BSO crystals by improving thermal conditions
Materials Research Bulletin, Vol. 31. No. 11, DD.1341-1354, 1996 Copyright 0 1998 ‘EiseviexS&&e Ltd Printed in the USA. All rights reserved 0025-54OW96 $15.00 +.OO
Pergamon
PI1 SO0215408(96)00134-l
ENHANCEMENT
OF GROWTH RATE FOR BSO CRYSTALS THERMAL CONDITIONS
Senlin Fu and Hiroyuki
BY IMPROVING
Ozoe*
Institute of Advanced Material Study, Kyushu University, Fukuoka 816, Japan (Refereed) (Received June 24, 1996; Accepted June 28, 1996)
ABSTRACT
Effects of various thermal conditions on growth of single crystal of BilZSi020 (BSO) with a floating zone method were systematically investigated. The following are the main results: (1) Change of heating source from an infrared heater to a CO*-laser system could have increased the temperature gradient in the grown crystal near the solid-liquid interface about 3 to 4 times, which gives rise to a dramatic increase of the critical transparent growth rate more than 3 times. (2) A single crystal seed with high thermal conductivity (e.g., A&03) can dramatically increase the critical transparent growth rates. (3) For a source rod of the multi-sintered powder rod, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2-3%/cm to less than OS%/cm. KEYWORDS: A. optical materials, A. oxides, B. crystal growth, C. X-ray diffraction, D. crystal structure INTRODUCTION
Undoped and doped photorefractive bismuth silicon oxide, BiiGSi020 (BSO), is of considerable current interest for various optical signal processing applications [l-5]. BSO crystals are usually grown by the Czochralski method [6,7]. The principal source of contamination in growing BSO is the Pt container [6], although it is extremely inert to many other oxide melts. Such contamination can effectively be avoided by using a floating zone (FZ) method, which offers the possibility of producing single crystal fibers directly from the
*To whom correspondence
should be addressed. 1341
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Vol. 3 1. No. 11
source powder rod [8]. However, the growth rate in producing BSO crystals is limited by its small thermal conductivity (0.1782 W/mK at 1073 K [9]). An enhancement in crystal growth rate is profitable, not only for increasing industrial efficiency but also for reducing the possible evaporation of heated material. Thermogravimetric analysis (TGA) has revealed that the rate of evaporation loss for the bismuth oxide (Bi203) at the melting of BilzGeOzo (BGO) is about 0.02 mg/cm’ h [lo] and the melting point of BSO (1168 K [9]) is close to that of BGO (1203 K [lo]). Furthermore, a high growth rate is profitable for the uniform distribution of the doped element in the matrix [ 1I]. In this article, the effects of various thermal conditions on growth of BSO crystals by a FZ method were systematically investigated. These results appear to be useful in producing other oxide crystals by a FZ method. EXPERIMENTAL
PROCEDURES
The three different FZ devices employed in the present work are shown in Figure 1. Figure la shows an infrared light with a shutter, which was used to heat the molten zone. This device was employed in our earlier work [S] and thus we refer to it as a former FZ (F.FZ) device. Figure lb shows a COz-laser system with shutters, which we refer to as a normal FZ (N.FZ) device. Figure lc shows a COZ-laser system with shutters and an infrared light with a shutter. This device is used to meal the grown crystal, thus we refer to it as an improved FZ (I.FZ) device. The moving and rotating system for each device is the same as that described in ref. 8. Bismuth oxide (a-B&03) of 99.99% purity and silicon oxide (SiOz) of 99.99% purity, made by Johnson Matthey Inc., were thoroughly mixed together in a stoichiometric ratio of 6:l. They were mixed with about 5 wt% deionized water at room temperature and then placed into a cylindrical rod 6.5 mm in diameter and 68 mm in length. After drying for one day at room temperature, the rod was sintered either (a) at 500°C for 8 hours (singlesintering process) or (b) first at 500°C for 8 hours, then at 515°C for 7 hours, 53O’C for 5 hours, 550°C for 5 hours, 580°C for 5 hours, and, finally, 618°C for 5 hours (multi-sintering process, see ref. 12). This sintered source rod had a density of approximately 50% of the theoretical value. The quantitative analysis of the X-ray diffraction pattern [ 121 revealed that the amounts of the y-Bi&i020 in the sintered source rods were 53.83 wt% for the single sintering and 96.98 wt% for the multi-sintering. All of the BSO crystals, except for those specified, were produced from the source rod sintered by the multi-sintering process. The back reflection Laue pattern and metalloscopy were employed to identify single crystals. Crystal structure was measured with X-ray diffraction. The diameter fluctuation of the grown crystal was determined by both the micrometer and the metalloscopy. RESULTS AND DISCUSSION Figure 2 shows the temperature distributions in grown crystals for three different heating sources: the infrared heater with the light shutter (Fig. la ), the COz-laser system, including ZnSe lenses, without light shutters (not shown), and the CO1laser system with light shutters (Fig. lb). Here, a thermocouple (R-type: R-13% Rh-Pt, size: 2 x 40.2 mm) was employed as a seed in order to measure directly the temperature in the grown crystal rod (4 2.2-2.3 mm). The line curves in Figure 2 represent the temperature Effect of Heating Sources.
(W
I
Shutter
Laser Beam
l_--Single Crystal 4
AZ
(
C)
EIliptical Mirror
Lamp
FIG. 1 Schematic diagrams of three floating zone devices employed in this work, (a) infrared-heated one, (b) CO2 laser-heated one, and {c) improved one.
(a )
\
Single crystal
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Vol. 31, No. 11
distributions obtained by fitting the experimental points. By differentiating these fitting equations, we can obtain the temperature gradient in the grown crystal near the solid-liquid interface, as listed in Table 1. However, the temperature in the grown crystal near the solidliquid interface dropped rather rapidly, thus a simple mathematical equation is difficult to fit the experimental points in this range even though a good fitting curve can be obtained for other ranges, as shown in Figure 2. Therefore, we have taken the average value between the temperature gradient calculated directly from the experimental points and that calculated by differentiating the fitting equation as the real temperature gradient in the grown crystal near the solid-liquid interface (Table 1). From Table 1, it can be seen that the temperature gradient in the crystal grown near the solid-liquid interface (G,,) was increased from 88.0Wmm for the “IR heater” source to 320Wmm for the “Laser-S’ heating source. The critical transparent and cracking rates for growth of a BSO crystal for the three heating sources are shown in Figure 3. As shown in this figure, the laser with light shutters (Laser-S) increases the critical transparent growth rate significantly (about 3 to 4 times greater than that for the IR heater), probably because it dramatically increases the temperature gradient in the grown crystal near the solid-liquid interface. The critical transparent growth rate for each diameter, 1.5, 3.0, and 5.0 mm, can be found from the points or lines in Figure 3 and plotted against the temperature gradient in the grown crystal near the solid-liquid interface (G,,, in Table 1) in Figure 4. The critical transparent growth rate appears to be directly proportional to the temperature gradient. This is because the critical growth rate is mainly controlled by the temperature gradient in the grown crystal
l
CO,-laser
system:
with shutters
6
1
2
3
Crystal
4
Length
5
6
7
L
8
[ mm ]
FIG. 2 Temperature distributions in grown crystals for the different heating sauces, infrared heater, CO2 laser system and CO, iaser system with shutters.
Vol. 31, No. 11
Temperature
BISMUTH SILICON OXIDE
Gradients
Temperature gradient G‘caf.(“C/mm) Cl-2 (“C/mm) G,_, (“C/mm) C;,, (‘C/mm) G (IW(“C/mm)
TABLE 1 in the Grown Crystals Near the Solid-Liquid Different Heating Sources.
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Interface for Three
IR heater
Laser
Laser-S
50.1 148.0 104.0 126.0 88.0
159.1 156.0 120.0 138.0 148.5
268.9 372.0 370.0 371 .o 320.0
IR heater = the infrared heater with a light shutter. Laser = the two-C02-lasers system without light shutters. Laser-S = the two-CO*-lasers system with light shutters. G refers to the temperature gradient in the grown crystal near the soiid-liquid interface. G,,, = G calculated by differentiating the fitting equation; G,_z = G calculated from the measuring points at the crystal lengths of 0 and 0.25 mm; Cl_., = G calculated from the measuring points at the crystal lengths of 0 and 0.5 mm; G,, = (GI-z + GI-$2; Go,. = (G,, + G,, Y2.
near the solid-liquid interface, as discussed in ref. 8. For a BSO crystal of + 2 3 mm, the result in Figure 3 appears to show that the critical cracking growth rate was largely increased by employing the “Laser-S’ heating source. The reason for this may be that a more uniform radial temperature profile in the grown crystal can be obtained due to the large increase in total heat flux along the axial direction, which gives rise to less cracking in the produced crystal. Effect of Seeds. Figure 5 shows the relations between critical transparent and cracking growth rates and the diameter of the grown crystal when BSO, YAG, andA1203 crystal rods (all with Q 2.0 mm) were employed as seeds. All of these results were obtained by using the COJaser system with shutters as a heating source. The critical cracking growth rate was dramatically increased by employing the A1203 crystal as a seed. This can be attributed to the thermal conductivity of A1203 crystal (13.0 WlmK at 800 K [13] or 10.3 W/mK at 1073 K, according to the extension of the curve, fitted the values at 300, 400, 500, and 800 K [ 131) is much larger than that of BSO crystal (0.1782 W/mK at 1073 K [9]). The thermal conductivity of YAG crystal is 12.0 W/mK at 300 K [ 141 or 3.1 W/mK at 1073 K, according to the extension of the thermal conductivity-temperature curve in the range of 3-300 K [ 151. However, there appears to be no distinct difference for the critical cracking growth rate between these three seeds, and all are much smaller than those of critical transparent growth rates. Thus the crystal growth rate could not be enhanced with this scheme. To overcome this difficulty, we designed an improved FZ device, shown in Figure lc, in which an infrared heater was employed to anneal the grown crystal. The objective of the light shutter set at the upside of the infrared heater is to avoid the decrease of the temperature gradient in the grown crystal near the solid-liquid interface. Multiple experiments revealed that the best height between the laser beam and the infrared heater, AZ, is 18-21 mm, and the best heating power for a @2.7-2.8 mm crystal is about 1.35 kW, that for a 4 5.0-5.3 mm crystal is about 1.88 kW. In general, an increase in diameter should increase the heating power of the infrared light. Figure 6 shows the critical transparent (tra.) and cracking (era.) growth rates for the Al203 seed by employing the improved FZ device (LFZ). Those by employing
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Las.
0
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. ..A...
-A_
2 3 4 1 Diameter of Grown Crystal
5 [
6
mm ]
FIG. 3 Critical transparent (u-a.) and cracking (cm) growth rates for BSO crystals by employing three different heating sources, infrared heater (IR), CO, laser system (Las.) and CO? laser system with shutters (Las-S).
the normal FZ device (N.FZ, Fig. lb) were copied from Figure 5 and plotted in Figure 6 for comparison. The critical cracking growth rates were dramatically increased by employing the improved FZ device, although the critical transparent growth rate was decreased slightly. Thus, a transparent BSO crystal without cracking can be produced at a very high growth rate by employing the improved FZ device. Property Variation
with Growth Rate.
Diameterfluctuation. Figure 7 shows the relationship between the growth rate and the diameter fluctuation of the BSO crystal rod ($I2.3 mm) produced from three different source rods, i.e., the powder rods sintered by a single-sintering or a multi-sintering process and the grown BSO crystal (with a uniform diameter). For a source rod of the multi-sintered powder rod or the grown crystal, the diameter fluctuation of the newly grown BSO crystal decreases dramatically with the growth rate. For example, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2-3%/cm to less than O.S%/cm for the former source rod. The reason for this decrease may be as follows. As similarly reported in ref. 16, theoretical analysis of the crystal growth process reveals the following equation according to the energy equilibrium:
Vol. 31, No. 11
BISMUTH SILICON OXIDE
A
50
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d = 3.0 mm
100 150 Temperature
200 250 300 Gradient [ K/mm
350 ]
FIG. 4 Critical transparent growth rates are proportional to the temperature gradient in the grown crystal near the solid-liquid interface for growth of BSO crystals of three different diameters,d= 1.5,3.0and5.0mm.
A = 0%- 0~)/(L, VPs)
(1)
where A is the cross-section area of the grown crystal. Q, is the heat transferred horn the solid-liquid interface to the grown crystal and then to the surrounding in a unit time and can be considered as a constant if the surrounding temperature is kept unchanged. Q, is the heat transferred from the melt to the solid-liquid interface in a unit time and changes depending on the heating power and the cross-section area of the molten zone near the solid-liquid interface. L,,, is the latent heat from solidification, and pS is the density of the grown crystal. V is the crystal growth rate and can be considered as a constant if the moving device for crystal growth is stable enough. Thus, by differentiating equation 1 about time, we obtain
AA= -CA&)/& VPS)
(2)
Equation 2 shows the fluctuation of the cross-section area of the grown crystal, A,4 is directly proportional to the fluctuation of the heat transfer rate, Ag, , which results from the fluctuation in the heating power and the instability of the molten zone. For a cylindrical crystal rod, the relationship between the diameter fluctuation of the grown crystal Ad/d (where d is the diameter of the grown crystal) and the cross-section fluctuation of the grown crystal is
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+2(y)+($) Therefore,
for the same fluctuation
of the heat transfer rate, A&,
(3)
an increase of growth
rate, V, can decrease the value of AA, according to equation 2, which gives rise to a decrease of the diameter fluctuation of the grown crystal according to equation 3, as shown in Figure 7. The diameter fluctuation of the BSO crystal produced from the grown crystal with a uniform diameter is much smaller than that produced from the multi-sintered powder rod for any growth rate. For a source rod obtained by a single-sintering process, the minimum value was attained at a growth rate of 38.8 mm/h. This is because the shape of molten zone in growth from a powder rod is not so stable as that from a grown crystal rod, because the density of the sintered powder rod is only 50% of that of the grown crystal and thus the melt in the molten zone may be easily necked in or puffed out by negative or positive pressure in the upper source rod near the melt-solid interface, as shown in ref. 8. For a source rod of the single-sintered powder rod, the solid-state reaction is insufficient (only 53.83 wt% “IBi$Si020 was obtained), thus bubbles may easily occur in the molten zone due to tiuther reaction. Therefore the diameter fluctuation of the crystal produced from the single-sintered powder rod is usually larger than that from the multi-sintered powder rod, as shown in Figure 7. These bubbles may occur more easily when the growth rate is high, because the rate of this reaction increases with the growth rate. As a result, the diameter fluctuation of
120 100
60
Diameter of Grown Crystal
[ mm ]
RG. 5 Critical transparent (tra.) and cracking (cm) growth rates for BSO crystals by employing three different single-crystal rods (Id 2.0 mm) of BSO, YAG and Al,O, as seeds. The heating source was CO,-laser system with shutters.
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*=
‘5 0
BISMUTH SILICONOXIDE
1349
20-
0 2.5
I 3.0
I 3.5
I 4.0
I 4.5 Diameter of Grown Crystal
I 5.0 5.5 [ mm ]
FIG. 6 Critical transparent (tra.) and cracking (cm.) growth rates for BSO crystals by employing the normal floating zone device (N. FZ, Fig. l(b)) and the improved floating zone device (I. Fz, Fig. l(c)). The single-crystal rod of A&O, (o 2.0 mm) was employed as a seed. the BSO crystal produced from the single-sintered powder rod increased with growth rate when the growth rate was larger than 38.8 mm/h, as shown in Figure 7. In view of these reasons, the best process for growing single-crystal rods and fibers of BSO appears to be as follows. 1.
2. 3.
Obtain a source powder rod with a high concentration of Y-BirZSi020by the multisintering process in order to keep the uniform shape of the source rod during the sintering process (see ref. 12). Grow a transparent crystal without cracking from this sintered source rod at a high growth rate and obtain a grown crystal rod with a uniform diameter. Grow a crystal rod or fiber with the desired diameter from the grown crystal rod in step 2.
Usually, we use a moderate growth rate in the third step in order to reduce possible thermal stress. Figure 8 shows the metalloscopy of a representative crystal rod of $0.82 mm produced with this process. The growth rate at the third step is 38.8 mm/h. It appears there is no clear diameter fluctuation. Crystal structure. Figure 9 is the X-ray diffraction pattern (at 25°C of powdered BSO crystal rod ($3.5 mm) grown from a sapphire (AlzOj, + 2.0 mm) seed at 50 mm/h with the
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0
20
40
Vol. 31, No. 11
60
Growth Rate
80 [
100
120
mm/h ]
FIG. 7 Diameter fluctuations of grown BSO crystals (a 2.3 mm) by employing three different materials, i.e., the powder rods sintered with a single-sintering or a multi-sintering process (see text) and the grown BSO crystal rod (with a uniform diameter) as source rods.
t growth
Metalloscopy of a representative described in Section 3.3.
direction
FIG. 8 BSO crystal rod (6 0.82 mm) produced
with the process
I-I-m
i--
I---
i:
I-----
4(099
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OOl
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50 Growth Rate
VoL31,No.
11
100 [ mm/h ]
FIG. 10 Lattice constants, transmittances (in (a)) and half-peak breadths (in (b), see text) for BSO cq~cta! rods (PI3.5mm) produced by the former FZ (F. FZ, Fig. l(a)), the normal FZ(N. Ez, Fig. l(b)) with a Pt-wire (6 0.5mm) seed, and the improved FZ (I. FZ Fig l(c)) with a sapphire (A&O,, 0 2.0 mm) seed.
Vol. 31, No.
improved
BISMUTH
11
FZ (I.FZ)
device.
SILICON OXIDE
A Cu target was employed
1353
to give a radiation
of heat
=
1.540562 A. The small peaks at each of the main peaks (from &) were due to $& radiation and can be easily deleted from the main peaks. The diffraction pattern for 29 < 60” is similar to that in ref. 8. The pattern in Figure 9 shows the grown crystal has a pure bodycentered structure and is similar to the reported powder diffraction file for BSO [ 171, except that the peaks at 28 > 110” such as (10,4,0), . ... (11,6,3) in Figure 9 were not shown in ref. 17. This pattern gives a lattice constant of 10.1066( 18) A. Similarly, the lattice constants of the BSO crystals ($ 3.5 mm) grown with the former FZ and the normal FZ devices by employing a Pt wire of Cp0.5 mm as seed were determined and plotted in Figure la (see F.FZ-Pt and NFZ-Pt). From comparison, those grown from the sapphire seed with the improved FZ device (I.FZ-Sap) were also plotted. Figure lob shows the half-peak breadths (obtained from the average of peaks (11,6,3), (11,5,0), and (9,7,2) in the X-ray diffraction patterns) for the BSO crystals produced by the three different methods. Here “CTa”, “CCa”, “CTb”, “CCb”, “CT?‘, and “CCC” are the corresponding critical transparent (CT) and cracking (CC) rates for growth of a @3.5 mm crystal by the “F.FZ-Pt” (“CTa” and “CCa”), “N.FZ-Pt” (“CTb” and “CCb”), and “I.FZ-Sap” (“CTc” and “Ccc”) methods, respectively. Comparison revealed no clear variation in lattice constant and halfpeak breadth if the growth rate is less than both the critical transparent (CT) and cracking (CC) growth rates. A small increase of lattice constant and half-peak breadth with growth rate between CC and CT may be attributed to thermal stress. By employing the “l.FZ-Sap” method, the lattice distortion of the produced crystal is much smaller even if the growth rate is as large as 80 mm/h. This is because both the critical transparent and cracking growth rates have been dramatically increased, as shown in Figure 6. Transmittance. Transmittances of the BSO crystals (9 3.5 mm) produced by the “F.FZ-Pt”, “N.FZ-Pt,” and “I.FZ-Sap” methods are shown in Figure 10a. The transmittance obtained from the average value of infrared transmission spectrum at the wave number of 4000-3000 cm-’ . The tested samples were crystal plates with a thickness of 2.00 + 0.01 mm, cut from the grown crystal rods. As with the results for lattice constant and half-peak breadth measurements, there is no clear variation in transmittance if the growth rate is less than both the critical transparent and cracking growth rates and each of the grown crystals has a good transmission spectrum. With the “I.FZ-Sap” method, the transmittance of the produced crystal was much higher even if the growth rate is as large as 80 mm/h.
CONCLUSIONS Various thermal conditions in producing single-crystal rods and fibers of Bi12Si020 (BSO) with a floating zone method were obtained by improving the growing device. The effects of these thermal conditions on growth rate and property of produced BSO crystals were systematically investigated. The main results are: 1.
Change of heating source from an infrared heater to a CO*-laser system can increase the temperature gradient in the grown crystal near the solid-melt interface about from 88 “C/mm to 320°C /mm, which gives rise to an increase in the critical transparent growth rates for growth of single crystals of all diameters by more than 3 times. The critical cracking rowth rate for a BSO crystal of $I < 3 mm was also greatly increased.
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A single crystal seed with high thermal conductivity
(e.g., A1203) can dramatically increase the critical transparent growth rates. However, for a normal floating zone device (i.e., heating source is a CO*-laser system without the infrared heater), the critical cracking growth rate was increased only slightly. A great increase of the critical cracking growth rate requires an improved floating zone device, which employs an infrared heater to anneal the grown crystals. For a source rod of the multi-sintered powder rod, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2-3%/cm to less than 0.5%/cm. This diameter fluctuation can be decreased further if a grown BSO crystal with a uniform diameter is employed as the source rod. REFERENCES
1. T. Takamori and D. Just, J. Appl. Phys. 67,848 (1990). 2. V. Tassev, G. Diankov, and M. Gospodinov, Mater. Res. Bull. 30, 1263 (1995). 3. Y. Zhu, T. Takada, and D. Tu, J. Phys. D: Appl. Phys. 28, 1468 (1995). 4. S.L. Sochava, K. Buse and E. Kratzig, Phys. Rev. B 51,4684(1995). 5. D. Nesheva, Z. Aneva, and Z. Levi, .I Phys. Chem. Solids 56,241 (1995). 6. T. Takamori and J.J. Boland, J. Mater. Sci. Lett. 10,972 (1991). 7. M. Harris, J. Larkin, J. E. Cormier, and A.F. Armington, J. Cryst. Growth 137, 128 (1994). 8. S. Fu and H. Ozoe, .I Appl. Phys. 77,5968 (1995). 9. C. Lin and S. Motakef, .I Cyst. Growth 128,834 (1993). 10. D.H.H. Quon, S. Chehab, J. Aota, A.K. Kuriakose, S.S.B. Wang, M.Z. Saghir and H.L. Chen, J. Cryst. Growth 134,266 (1993).
11. S. Fu, J. Jiang, J. Chen, and Z. Ding, .I Mater. Sci. 28, 1659 (1993). 12. S. Fu and H. Ozoe, “Solid-Phase Reaction in Synthesis of Bi&iOso Source Rods for SingleCrystal Growth in a Floating Zone,” J. Phys. Chem. Solid 56, in press. 13. Japan Society of Thermal Physical Properties, Thermal Physical Properties Handbook, Youkendou Inc., Tokyo (1990). (in Japanese) 14. A.I. Zagumennyi, G.B. Lutts, P.A. Popov, N.N. Sirota and LA. Shcherbakov, Laser Phys. 3, 1064 (1993). 15. G.A. Slack and D.W. Oliver, Phys. Rev. B 4, 592 (1971). 16. K.-Th. Wilke, Kristall Ziichtung, Harri Deutsch, Thun-Frankfurt/Main
(1988). (in German) 17. Nat. Bur. Stand. (US), Powder Diffraction File 37, 179, No. 485 (I 987).
Pergamon
PI1 SO0215408(96)00134-l
ENHANCEMENT
OF GROWTH RATE FOR BSO CRYSTALS THERMAL CONDITIONS
Senlin Fu and Hiroyuki
BY IMPROVING
Ozoe*
Institute of Advanced Material Study, Kyushu University, Fukuoka 816, Japan (Refereed) (Received June 24, 1996; Accepted June 28, 1996)
ABSTRACT
Effects of various thermal conditions on growth of single crystal of BilZSi020 (BSO) with a floating zone method were systematically investigated. The following are the main results: (1) Change of heating source from an infrared heater to a CO*-laser system could have increased the temperature gradient in the grown crystal near the solid-liquid interface about 3 to 4 times, which gives rise to a dramatic increase of the critical transparent growth rate more than 3 times. (2) A single crystal seed with high thermal conductivity (e.g., A&03) can dramatically increase the critical transparent growth rates. (3) For a source rod of the multi-sintered powder rod, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2-3%/cm to less than OS%/cm. KEYWORDS: A. optical materials, A. oxides, B. crystal growth, C. X-ray diffraction, D. crystal structure INTRODUCTION
Undoped and doped photorefractive bismuth silicon oxide, BiiGSi020 (BSO), is of considerable current interest for various optical signal processing applications [l-5]. BSO crystals are usually grown by the Czochralski method [6,7]. The principal source of contamination in growing BSO is the Pt container [6], although it is extremely inert to many other oxide melts. Such contamination can effectively be avoided by using a floating zone (FZ) method, which offers the possibility of producing single crystal fibers directly from the
*To whom correspondence
should be addressed. 1341
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Vol. 3 1. No. 11
source powder rod [8]. However, the growth rate in producing BSO crystals is limited by its small thermal conductivity (0.1782 W/mK at 1073 K [9]). An enhancement in crystal growth rate is profitable, not only for increasing industrial efficiency but also for reducing the possible evaporation of heated material. Thermogravimetric analysis (TGA) has revealed that the rate of evaporation loss for the bismuth oxide (Bi203) at the melting of BilzGeOzo (BGO) is about 0.02 mg/cm’ h [lo] and the melting point of BSO (1168 K [9]) is close to that of BGO (1203 K [lo]). Furthermore, a high growth rate is profitable for the uniform distribution of the doped element in the matrix [ 1I]. In this article, the effects of various thermal conditions on growth of BSO crystals by a FZ method were systematically investigated. These results appear to be useful in producing other oxide crystals by a FZ method. EXPERIMENTAL
PROCEDURES
The three different FZ devices employed in the present work are shown in Figure 1. Figure la shows an infrared light with a shutter, which was used to heat the molten zone. This device was employed in our earlier work [S] and thus we refer to it as a former FZ (F.FZ) device. Figure lb shows a COz-laser system with shutters, which we refer to as a normal FZ (N.FZ) device. Figure lc shows a COZ-laser system with shutters and an infrared light with a shutter. This device is used to meal the grown crystal, thus we refer to it as an improved FZ (I.FZ) device. The moving and rotating system for each device is the same as that described in ref. 8. Bismuth oxide (a-B&03) of 99.99% purity and silicon oxide (SiOz) of 99.99% purity, made by Johnson Matthey Inc., were thoroughly mixed together in a stoichiometric ratio of 6:l. They were mixed with about 5 wt% deionized water at room temperature and then placed into a cylindrical rod 6.5 mm in diameter and 68 mm in length. After drying for one day at room temperature, the rod was sintered either (a) at 500°C for 8 hours (singlesintering process) or (b) first at 500°C for 8 hours, then at 515°C for 7 hours, 53O’C for 5 hours, 550°C for 5 hours, 580°C for 5 hours, and, finally, 618°C for 5 hours (multi-sintering process, see ref. 12). This sintered source rod had a density of approximately 50% of the theoretical value. The quantitative analysis of the X-ray diffraction pattern [ 121 revealed that the amounts of the y-Bi&i020 in the sintered source rods were 53.83 wt% for the single sintering and 96.98 wt% for the multi-sintering. All of the BSO crystals, except for those specified, were produced from the source rod sintered by the multi-sintering process. The back reflection Laue pattern and metalloscopy were employed to identify single crystals. Crystal structure was measured with X-ray diffraction. The diameter fluctuation of the grown crystal was determined by both the micrometer and the metalloscopy. RESULTS AND DISCUSSION Figure 2 shows the temperature distributions in grown crystals for three different heating sources: the infrared heater with the light shutter (Fig. la ), the COz-laser system, including ZnSe lenses, without light shutters (not shown), and the CO1laser system with light shutters (Fig. lb). Here, a thermocouple (R-type: R-13% Rh-Pt, size: 2 x 40.2 mm) was employed as a seed in order to measure directly the temperature in the grown crystal rod (4 2.2-2.3 mm). The line curves in Figure 2 represent the temperature Effect of Heating Sources.
(W
I
Shutter
Laser Beam
l_--Single Crystal 4
AZ
(
C)
EIliptical Mirror
Lamp
FIG. 1 Schematic diagrams of three floating zone devices employed in this work, (a) infrared-heated one, (b) CO2 laser-heated one, and {c) improved one.
(a )
\
Single crystal
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distributions obtained by fitting the experimental points. By differentiating these fitting equations, we can obtain the temperature gradient in the grown crystal near the solid-liquid interface, as listed in Table 1. However, the temperature in the grown crystal near the solidliquid interface dropped rather rapidly, thus a simple mathematical equation is difficult to fit the experimental points in this range even though a good fitting curve can be obtained for other ranges, as shown in Figure 2. Therefore, we have taken the average value between the temperature gradient calculated directly from the experimental points and that calculated by differentiating the fitting equation as the real temperature gradient in the grown crystal near the solid-liquid interface (Table 1). From Table 1, it can be seen that the temperature gradient in the crystal grown near the solid-liquid interface (G,,) was increased from 88.0Wmm for the “IR heater” source to 320Wmm for the “Laser-S’ heating source. The critical transparent and cracking rates for growth of a BSO crystal for the three heating sources are shown in Figure 3. As shown in this figure, the laser with light shutters (Laser-S) increases the critical transparent growth rate significantly (about 3 to 4 times greater than that for the IR heater), probably because it dramatically increases the temperature gradient in the grown crystal near the solid-liquid interface. The critical transparent growth rate for each diameter, 1.5, 3.0, and 5.0 mm, can be found from the points or lines in Figure 3 and plotted against the temperature gradient in the grown crystal near the solid-liquid interface (G,,, in Table 1) in Figure 4. The critical transparent growth rate appears to be directly proportional to the temperature gradient. This is because the critical growth rate is mainly controlled by the temperature gradient in the grown crystal
l
CO,-laser
system:
with shutters
6
1
2
3
Crystal
4
Length
5
6
7
L
8
[ mm ]
FIG. 2 Temperature distributions in grown crystals for the different heating sauces, infrared heater, CO2 laser system and CO, iaser system with shutters.
Vol. 31, No. 11
Temperature
BISMUTH SILICON OXIDE
Gradients
Temperature gradient G‘caf.(“C/mm) Cl-2 (“C/mm) G,_, (“C/mm) C;,, (‘C/mm) G (IW(“C/mm)
TABLE 1 in the Grown Crystals Near the Solid-Liquid Different Heating Sources.
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Interface for Three
IR heater
Laser
Laser-S
50.1 148.0 104.0 126.0 88.0
159.1 156.0 120.0 138.0 148.5
268.9 372.0 370.0 371 .o 320.0
IR heater = the infrared heater with a light shutter. Laser = the two-C02-lasers system without light shutters. Laser-S = the two-CO*-lasers system with light shutters. G refers to the temperature gradient in the grown crystal near the soiid-liquid interface. G,,, = G calculated by differentiating the fitting equation; G,_z = G calculated from the measuring points at the crystal lengths of 0 and 0.25 mm; Cl_., = G calculated from the measuring points at the crystal lengths of 0 and 0.5 mm; G,, = (GI-z + GI-$2; Go,. = (G,, + G,, Y2.
near the solid-liquid interface, as discussed in ref. 8. For a BSO crystal of + 2 3 mm, the result in Figure 3 appears to show that the critical cracking growth rate was largely increased by employing the “Laser-S’ heating source. The reason for this may be that a more uniform radial temperature profile in the grown crystal can be obtained due to the large increase in total heat flux along the axial direction, which gives rise to less cracking in the produced crystal. Effect of Seeds. Figure 5 shows the relations between critical transparent and cracking growth rates and the diameter of the grown crystal when BSO, YAG, andA1203 crystal rods (all with Q 2.0 mm) were employed as seeds. All of these results were obtained by using the COJaser system with shutters as a heating source. The critical cracking growth rate was dramatically increased by employing the A1203 crystal as a seed. This can be attributed to the thermal conductivity of A1203 crystal (13.0 WlmK at 800 K [13] or 10.3 W/mK at 1073 K, according to the extension of the curve, fitted the values at 300, 400, 500, and 800 K [ 131) is much larger than that of BSO crystal (0.1782 W/mK at 1073 K [9]). The thermal conductivity of YAG crystal is 12.0 W/mK at 300 K [ 141 or 3.1 W/mK at 1073 K, according to the extension of the thermal conductivity-temperature curve in the range of 3-300 K [ 151. However, there appears to be no distinct difference for the critical cracking growth rate between these three seeds, and all are much smaller than those of critical transparent growth rates. Thus the crystal growth rate could not be enhanced with this scheme. To overcome this difficulty, we designed an improved FZ device, shown in Figure lc, in which an infrared heater was employed to anneal the grown crystal. The objective of the light shutter set at the upside of the infrared heater is to avoid the decrease of the temperature gradient in the grown crystal near the solid-liquid interface. Multiple experiments revealed that the best height between the laser beam and the infrared heater, AZ, is 18-21 mm, and the best heating power for a @2.7-2.8 mm crystal is about 1.35 kW, that for a 4 5.0-5.3 mm crystal is about 1.88 kW. In general, an increase in diameter should increase the heating power of the infrared light. Figure 6 shows the critical transparent (tra.) and cracking (era.) growth rates for the Al203 seed by employing the improved FZ device (LFZ). Those by employing
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0
Vol. 3 1, No. 11
. ..A...
-A_
2 3 4 1 Diameter of Grown Crystal
5 [
6
mm ]
FIG. 3 Critical transparent (u-a.) and cracking (cm) growth rates for BSO crystals by employing three different heating sources, infrared heater (IR), CO, laser system (Las.) and CO? laser system with shutters (Las-S).
the normal FZ device (N.FZ, Fig. lb) were copied from Figure 5 and plotted in Figure 6 for comparison. The critical cracking growth rates were dramatically increased by employing the improved FZ device, although the critical transparent growth rate was decreased slightly. Thus, a transparent BSO crystal without cracking can be produced at a very high growth rate by employing the improved FZ device. Property Variation
with Growth Rate.
Diameterfluctuation. Figure 7 shows the relationship between the growth rate and the diameter fluctuation of the BSO crystal rod ($I2.3 mm) produced from three different source rods, i.e., the powder rods sintered by a single-sintering or a multi-sintering process and the grown BSO crystal (with a uniform diameter). For a source rod of the multi-sintered powder rod or the grown crystal, the diameter fluctuation of the newly grown BSO crystal decreases dramatically with the growth rate. For example, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2-3%/cm to less than O.S%/cm for the former source rod. The reason for this decrease may be as follows. As similarly reported in ref. 16, theoretical analysis of the crystal growth process reveals the following equation according to the energy equilibrium:
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BISMUTH SILICON OXIDE
A
50
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d = 3.0 mm
100 150 Temperature
200 250 300 Gradient [ K/mm
350 ]
FIG. 4 Critical transparent growth rates are proportional to the temperature gradient in the grown crystal near the solid-liquid interface for growth of BSO crystals of three different diameters,d= 1.5,3.0and5.0mm.
A = 0%- 0~)/(L, VPs)
(1)
where A is the cross-section area of the grown crystal. Q, is the heat transferred horn the solid-liquid interface to the grown crystal and then to the surrounding in a unit time and can be considered as a constant if the surrounding temperature is kept unchanged. Q, is the heat transferred from the melt to the solid-liquid interface in a unit time and changes depending on the heating power and the cross-section area of the molten zone near the solid-liquid interface. L,,, is the latent heat from solidification, and pS is the density of the grown crystal. V is the crystal growth rate and can be considered as a constant if the moving device for crystal growth is stable enough. Thus, by differentiating equation 1 about time, we obtain
AA= -CA&)/& VPS)
(2)
Equation 2 shows the fluctuation of the cross-section area of the grown crystal, A,4 is directly proportional to the fluctuation of the heat transfer rate, Ag, , which results from the fluctuation in the heating power and the instability of the molten zone. For a cylindrical crystal rod, the relationship between the diameter fluctuation of the grown crystal Ad/d (where d is the diameter of the grown crystal) and the cross-section fluctuation of the grown crystal is
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+2(y)+($) Therefore,
for the same fluctuation
of the heat transfer rate, A&,
(3)
an increase of growth
rate, V, can decrease the value of AA, according to equation 2, which gives rise to a decrease of the diameter fluctuation of the grown crystal according to equation 3, as shown in Figure 7. The diameter fluctuation of the BSO crystal produced from the grown crystal with a uniform diameter is much smaller than that produced from the multi-sintered powder rod for any growth rate. For a source rod obtained by a single-sintering process, the minimum value was attained at a growth rate of 38.8 mm/h. This is because the shape of molten zone in growth from a powder rod is not so stable as that from a grown crystal rod, because the density of the sintered powder rod is only 50% of that of the grown crystal and thus the melt in the molten zone may be easily necked in or puffed out by negative or positive pressure in the upper source rod near the melt-solid interface, as shown in ref. 8. For a source rod of the single-sintered powder rod, the solid-state reaction is insufficient (only 53.83 wt% “IBi$Si020 was obtained), thus bubbles may easily occur in the molten zone due to tiuther reaction. Therefore the diameter fluctuation of the crystal produced from the single-sintered powder rod is usually larger than that from the multi-sintered powder rod, as shown in Figure 7. These bubbles may occur more easily when the growth rate is high, because the rate of this reaction increases with the growth rate. As a result, the diameter fluctuation of
120 100
60
Diameter of Grown Crystal
[ mm ]
RG. 5 Critical transparent (tra.) and cracking (cm) growth rates for BSO crystals by employing three different single-crystal rods (Id 2.0 mm) of BSO, YAG and Al,O, as seeds. The heating source was CO,-laser system with shutters.
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*=
‘5 0
BISMUTH SILICONOXIDE
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20-
0 2.5
I 3.0
I 3.5
I 4.0
I 4.5 Diameter of Grown Crystal
I 5.0 5.5 [ mm ]
FIG. 6 Critical transparent (tra.) and cracking (cm.) growth rates for BSO crystals by employing the normal floating zone device (N. FZ, Fig. l(b)) and the improved floating zone device (I. Fz, Fig. l(c)). The single-crystal rod of A&O, (o 2.0 mm) was employed as a seed. the BSO crystal produced from the single-sintered powder rod increased with growth rate when the growth rate was larger than 38.8 mm/h, as shown in Figure 7. In view of these reasons, the best process for growing single-crystal rods and fibers of BSO appears to be as follows. 1.
2. 3.
Obtain a source powder rod with a high concentration of Y-BirZSi020by the multisintering process in order to keep the uniform shape of the source rod during the sintering process (see ref. 12). Grow a transparent crystal without cracking from this sintered source rod at a high growth rate and obtain a grown crystal rod with a uniform diameter. Grow a crystal rod or fiber with the desired diameter from the grown crystal rod in step 2.
Usually, we use a moderate growth rate in the third step in order to reduce possible thermal stress. Figure 8 shows the metalloscopy of a representative crystal rod of $0.82 mm produced with this process. The growth rate at the third step is 38.8 mm/h. It appears there is no clear diameter fluctuation. Crystal structure. Figure 9 is the X-ray diffraction pattern (at 25°C of powdered BSO crystal rod ($3.5 mm) grown from a sapphire (AlzOj, + 2.0 mm) seed at 50 mm/h with the
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0
20
40
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60
Growth Rate
80 [
100
120
mm/h ]
FIG. 7 Diameter fluctuations of grown BSO crystals (a 2.3 mm) by employing three different materials, i.e., the powder rods sintered with a single-sintering or a multi-sintering process (see text) and the grown BSO crystal rod (with a uniform diameter) as source rods.
t growth
Metalloscopy of a representative described in Section 3.3.
direction
FIG. 8 BSO crystal rod (6 0.82 mm) produced
with the process
I-I-m
i--
I---
i:
I-----
4(099
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100 [ mm/h ]
FIG. 10 Lattice constants, transmittances (in (a)) and half-peak breadths (in (b), see text) for BSO cq~cta! rods (PI3.5mm) produced by the former FZ (F. FZ, Fig. l(a)), the normal FZ(N. Ez, Fig. l(b)) with a Pt-wire (6 0.5mm) seed, and the improved FZ (I. FZ Fig l(c)) with a sapphire (A&O,, 0 2.0 mm) seed.
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improved
BISMUTH
11
FZ (I.FZ)
device.
SILICON OXIDE
A Cu target was employed
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to give a radiation
of heat
=
1.540562 A. The small peaks at each of the main peaks (from &) were due to $& radiation and can be easily deleted from the main peaks. The diffraction pattern for 29 < 60” is similar to that in ref. 8. The pattern in Figure 9 shows the grown crystal has a pure bodycentered structure and is similar to the reported powder diffraction file for BSO [ 171, except that the peaks at 28 > 110” such as (10,4,0), . ... (11,6,3) in Figure 9 were not shown in ref. 17. This pattern gives a lattice constant of 10.1066( 18) A. Similarly, the lattice constants of the BSO crystals ($ 3.5 mm) grown with the former FZ and the normal FZ devices by employing a Pt wire of Cp0.5 mm as seed were determined and plotted in Figure la (see F.FZ-Pt and NFZ-Pt). From comparison, those grown from the sapphire seed with the improved FZ device (I.FZ-Sap) were also plotted. Figure lob shows the half-peak breadths (obtained from the average of peaks (11,6,3), (11,5,0), and (9,7,2) in the X-ray diffraction patterns) for the BSO crystals produced by the three different methods. Here “CTa”, “CCa”, “CTb”, “CCb”, “CT?‘, and “CCC” are the corresponding critical transparent (CT) and cracking (CC) rates for growth of a @3.5 mm crystal by the “F.FZ-Pt” (“CTa” and “CCa”), “N.FZ-Pt” (“CTb” and “CCb”), and “I.FZ-Sap” (“CTc” and “Ccc”) methods, respectively. Comparison revealed no clear variation in lattice constant and halfpeak breadth if the growth rate is less than both the critical transparent (CT) and cracking (CC) growth rates. A small increase of lattice constant and half-peak breadth with growth rate between CC and CT may be attributed to thermal stress. By employing the “l.FZ-Sap” method, the lattice distortion of the produced crystal is much smaller even if the growth rate is as large as 80 mm/h. This is because both the critical transparent and cracking growth rates have been dramatically increased, as shown in Figure 6. Transmittance. Transmittances of the BSO crystals (9 3.5 mm) produced by the “F.FZ-Pt”, “N.FZ-Pt,” and “I.FZ-Sap” methods are shown in Figure 10a. The transmittance obtained from the average value of infrared transmission spectrum at the wave number of 4000-3000 cm-’ . The tested samples were crystal plates with a thickness of 2.00 + 0.01 mm, cut from the grown crystal rods. As with the results for lattice constant and half-peak breadth measurements, there is no clear variation in transmittance if the growth rate is less than both the critical transparent and cracking growth rates and each of the grown crystals has a good transmission spectrum. With the “I.FZ-Sap” method, the transmittance of the produced crystal was much higher even if the growth rate is as large as 80 mm/h.
CONCLUSIONS Various thermal conditions in producing single-crystal rods and fibers of Bi12Si020 (BSO) with a floating zone method were obtained by improving the growing device. The effects of these thermal conditions on growth rate and property of produced BSO crystals were systematically investigated. The main results are: 1.
Change of heating source from an infrared heater to a CO*-laser system can increase the temperature gradient in the grown crystal near the solid-melt interface about from 88 “C/mm to 320°C /mm, which gives rise to an increase in the critical transparent growth rates for growth of single crystals of all diameters by more than 3 times. The critical cracking rowth rate for a BSO crystal of $I < 3 mm was also greatly increased.
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Vol. 3 1, No. 11
A single crystal seed with high thermal conductivity
(e.g., A1203) can dramatically increase the critical transparent growth rates. However, for a normal floating zone device (i.e., heating source is a CO*-laser system without the infrared heater), the critical cracking growth rate was increased only slightly. A great increase of the critical cracking growth rate requires an improved floating zone device, which employs an infrared heater to anneal the grown crystals. For a source rod of the multi-sintered powder rod, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2-3%/cm to less than 0.5%/cm. This diameter fluctuation can be decreased further if a grown BSO crystal with a uniform diameter is employed as the source rod. REFERENCES
1. T. Takamori and D. Just, J. Appl. Phys. 67,848 (1990). 2. V. Tassev, G. Diankov, and M. Gospodinov, Mater. Res. Bull. 30, 1263 (1995). 3. Y. Zhu, T. Takada, and D. Tu, J. Phys. D: Appl. Phys. 28, 1468 (1995). 4. S.L. Sochava, K. Buse and E. Kratzig, Phys. Rev. B 51,4684(1995). 5. D. Nesheva, Z. Aneva, and Z. Levi, .I Phys. Chem. Solids 56,241 (1995). 6. T. Takamori and J.J. Boland, J. Mater. Sci. Lett. 10,972 (1991). 7. M. Harris, J. Larkin, J. E. Cormier, and A.F. Armington, J. Cryst. Growth 137, 128 (1994). 8. S. Fu and H. Ozoe, .I Appl. Phys. 77,5968 (1995). 9. C. Lin and S. Motakef, .I Cyst. Growth 128,834 (1993). 10. D.H.H. Quon, S. Chehab, J. Aota, A.K. Kuriakose, S.S.B. Wang, M.Z. Saghir and H.L. Chen, J. Cryst. Growth 134,266 (1993).
11. S. Fu, J. Jiang, J. Chen, and Z. Ding, .I Mater. Sci. 28, 1659 (1993). 12. S. Fu and H. Ozoe, “Solid-Phase Reaction in Synthesis of Bi&iOso Source Rods for SingleCrystal Growth in a Floating Zone,” J. Phys. Chem. Solid 56, in press. 13. Japan Society of Thermal Physical Properties, Thermal Physical Properties Handbook, Youkendou Inc., Tokyo (1990). (in Japanese) 14. A.I. Zagumennyi, G.B. Lutts, P.A. Popov, N.N. Sirota and LA. Shcherbakov, Laser Phys. 3, 1064 (1993). 15. G.A. Slack and D.W. Oliver, Phys. Rev. B 4, 592 (1971). 16. K.-Th. Wilke, Kristall Ziichtung, Harri Deutsch, Thun-Frankfurt/Main
(1988). (in German) 17. Nat. Bur. Stand. (US), Powder Diffraction File 37, 179, No. 485 (I 987).