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Growth and defects of In-doped LEC GaP crystals
Journal of Crystal Growth 94 (1989) 373—380 North-Holland, Amsterdam
373
GROWTH OF BGO SINGLE CRYSTALS USING A DIRECTIONAL SOLIDIFICATION TECHNIQUE F. ALLEGRETTI
*
Dipartimenlo di Chiniica Fisica ed Eletirochimica, Via Golgi 19, 1-20133 Milano, Italy
B. BORGIA
*
Dipartimento di Fisica, Piazza A. Moro 2, 1-00185 Rome, Italy
R. RIVA
*
Dipartimento di Chimica Fisica ed Elettrochimica, Via Golgi 19, 1-20133 Milano, Italy
F. DE NOTARISTEFANI
*
Dipartimento di Fisica, Piazza A. Moro 2, 1-00185 Rome, Italy
and S. PIZZINI
*
Dipartimento di Chimica Fisica ed Elettrochimica, Via Golgi 19, 1-20133 Milano, Italy Received 15 July 1988
Bismuth germanate (Bi
4Ge3O12, BGO) crystals are currently used as scintillation detectors in high energy physics, as well as in sanitary physics experiments, for their superior performance with respect to NaI(Tl) detectors. At the present, most of the commercially available crystals are obtained by the Czochralski pulling technique, albeit Bridgman grown crystals seem to present excellent characteristics, such to having been chosen for the LEP3 experiment at CERN in Geneve. In order to prove that these superior properties 3) derive single from crystals a minor using aamount verticalofBridgman dislocations furnace and explicitally other structural designed defects, for this we scope have grown and bya suitably numberchanging of large selected parameters like the temperature gradient in the solid and the growth rate. The results show that crystals with a very (25 x 25 >
I. Introduction In spite of the fact that commercial crystals grown with the Czochralski or Bridgman technique [1—7]present comparable properties in terms of energy resolution (see table 1), the Bndgman technique offers several advantages compared to the Czochralski technique.
*
Also at INFN Sezione di Roma.
It is in fact possible to limit the deviations from the stoichiometry, which arise from differential sublimation of bismuth and germanium oxides from the free liquid surface during the growth thanks to the lower surface to volume ratio which could be achieved with the Bndgman technique and to improve the material utilization, thanks to the growth of crystals already shaped in view of their final utilization. However, both in CZ crystals and in Bridgman grown ingots several structural defects could still
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
374
F. Allegretti et al.
/
Growth of BGO single crystals using directional solidification
Table 1 Comparison between the scintillation properties of Czochralski and Bndgman grown crystals
_____________________________________________ Manufacturer Harshaw Crismatec NKK Shanghai NKK Shanghai Inst. Ceramics Scholz Philips Crismatec
Size (mm) 15 x 15 X 200 15 x 15x200 113<11 X 200 10~10x20 15x15x200 15x15 x200 20x20x60 20x20x240
Growth technique
Energy Ref. resolution
Czochralski 24.7 Czochralski 23.5 Czochralski 16.6 Bridgman 18.7 Bridgman 17.3 Czochralski 17.3 Bridgman 13.6 Czochralski 20.0 137Cs source (Er The energy resolution is measured with a keV).
[8] [8] [8] [8] [8] [8] [11] [12] = 662
be systematically observed, like bubbles, halos, microprecipitates of Pt and diffuse haze regions [6—8],which induce degradation of the scintillation efficiency. The aim of this paper is to describe a novel methodology which is a variant of the vertical Bridgman technique and which in principle should allow the growth of nearly perfect and strain free crystals due to its capability of controlling very carefully the imposed growth rate and to maintain the solidified material at approximately constant temperature up to the end of the solidification cycle,
2. Experimental 2.1 Furnace design The furnace used was explicitly designed for growing 24 x 24 X 200 mm3 crystals and consists of 11 Superkanthal coil shaped resistors embedded in a high alumina content sintered ceramic body, which in turn is enclosed in an stainless steel frame. The heating element has an internal diameter of 110 mm and a total height of 250 mm, and it is powered by 11 independent Eurotherm mod. 6358 units, driven by a microprocessor control unit Eurotherm mod. 6433, for a total power of 2.5 kW. The temperature of each heating zone is monitored with a Pt/Pt—Rh 10% (Pt) thermocouple and each heating zone could be ramped at any
desired temperature with an increase rate between 0.1 and 999.9 ° C/mm or held at constant temperature for a time lasting up to 99.99 h. The growth chamber is a quartz tube (outside diameter 100 mm) with a total height of 810 =
mm, with a movable quartz pedestal inside which is the support of the Pt crucible used for BGO growing and which also works as an JR transparent window to dissipate the latent heat of solidification. A quartz cylinder, filled with quartz wool is suitably housed on the top of the furnace to minimize the heat losses and to favour the estabilishment of the desired temperature profile in the chamber, without excessive distorsions in the last heating zone. A hole on the bottom of this quartz vessel permits the passage of a Pt thermocouple lined with a Pt tube (outside diameter 3 mm), used to determine the temperature profile of the furnace and to measure, eventually, the temperature of the liquid BGO in the crucible during the growth cycle. The temperature on the bottom of the crucible is measured as well with a Pt thermocouple, which is pressed by means of a spring on the bottom of the crucible in order to improve the thermal contact. A schematic drawing of the furnace is reported in fig. 1, which shows the water cooled flanges used to mantain a controlled atmosphere in the growth chamber and to hold in vertical position the quartz pedestal as well as the top insulating cover. As crucibles, pure Pt crucibles were used throughout, which were manufactured by cold shaping and arc welding 0.2 mm thick Pt foils. =
2.2. Materials As starting materials, bismuth germanate powders of different origin were used, after having set up the growth procedure with a material kindly delivered by the Academia Sinica, Shanghai Institute of Ceramics, which is already proved to be of excellent quality and which is obtained by melting a sintered powder of BGO, freezing it and crushing it to powder.
F Allegretti et aL
/
Growth of BGO single crystals using directional solidification
L.
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___
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__
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_____
I~I
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_____
____
______
___
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Fig. 1. Schematic picture of the growth furnace.
j~’
375
376
F. A Ilegretti et al.
/
Growth of BGO single crystals using directional solidification
A Johnson-Matthey (JM) material (batch No. S 94064 AC) was proved to yield yellow crystals and plenty of bubbles, and therefore it was discarded for further experiments, while the BGO powder delivered by Preussag (batch No. 6109/0102) was found instead to be suitable as an alternative to the chinese material. A material obtained from the Cadarache Research Center (France) (batch No. H 86) was found to be of high purity but not direcly suited for growing BGO crystals, as it presented the same problems of the JM material, albeit to a lesser extent. All the experiments reported in the next section were therefore conducted using either the Chinese or the Preussag materials, With the aim of trying a classification of the various materials in term of their suitability for use in Bridgman growth experiments, we carried out a preliminary test directed to find a possible relationship between the deviation from the stoichiometry of the raw material and the quality of the final crystal, using a standard chemical method for the analysis of bismuth (EDTA titration), having however care to inhibit the volatility of bismuth chloride during the preparation of the sample by adding a sufficient amount of sulphuric acid. The results of these experiments, reported in table 2, show that in all cases the deviations from the stoichiometry are not negligible but that the experimentally found differences between the vanous samples are not large enough to permit their classification in terms of suitability for good qua!ity single crystal growth. It seems, however, confirmed that also in the case of Bridgman grown crystals a deviation from the stoichiometry less
Table 2 Average values of the bismuth deficiency in BGO powders of vanous origin
than 1% is sufficient for inducing pratically zero yields, like in CZ crystals [9]. As a result of a further series of experiments performed in order to determine the material losses during the growth cycle (frequently called responsible of the crystal quality deterioration), we could demonstrate that a thermal treatment at 1070°C induces a material loss which in turn causes slight deviations from the stoichiometry (see table 3). These experiments, typically, were performed by pouring ca. 40 g of BGO powder into a Pt crucible having about the same cross-section as
limits of detectability by X-ray diffraction phase analysis, indicating an important sensitivity of the Bridgman process to the presence of traces of extraneous oxides. This sensitivity should be attributed to the practical absence of convective
_____________________________________________ Material
Deviation from stoichiometry (average) (%)
Remarks
Pellets JM (batch S 94046 AC) —0.68 (1) Powder JM (batch S 94046 AC) —1.14 (2) Academia Sinica —0.37 (3) Preussag —0.68 (4) Cadarache —0.93 (5) (1) The material is completely soluble in boiling HCI: the crystals obtained from this matenal are yellow in colour and have plenty of bubbles (see text). (2) The material leaves an insoluble residual after dissolution in boiling HC1 (3) The material yields perfectly transparent crystals under the proper growth conditions (see text). (4) Like under (3). (5) The material yields crystals yellow in colour and rich in bubbles under the same growth conditions at which Chinese and Preussag powders yield good crystals.
the crucibles used for the crystal growth experiments, and heating it at a temperature slightly above the melting temperature in an electrically heated furnace. The samples withdrawn after selected times were then analyzed for bismuth, using the procedure indicated above. Also in this case, however, the results do not present any evidence either of a quantitative difference between powders of different origin. We believe therefore that the major difference between the various materials examined is their content of non-reacted oxides or of phases different from the Bi4Ge3O12 one, which are, however, below the
T bl 3 a e deficiencies induced by differential . . . . Bismuth sublimation losses at 10700 C of a BGO sample .
.
.
.
Initial value
After 20 mm
After 80 mm
—0.93%
—1.91%
—2.12%
F Allegretti et at
/
Growth of BGO single crystals using directional solidification
motions in the liquid charge during the growth and therefore to the permanence of local composition differences, which could easily lead to conditions of constitutional supercooling and which could be avoided instead when using the Czochralski technique. 2.3 Crystal growth procedures
A typical growth cycle is performed according to the following procedure. The Pt crucible is preliminarly charged with the BGO powder, taking care to use a Pt funnel fixed on the top of the crucible which contains the proper excess of powder to ensure the complete filling of the crucible when the charge is molten. Then, after having positioned the crucible in the furnace, with the seed centered midway the first heating zone, the entire furnace is flushed with pure dry oxygen at a rate of about 15 1/h, while the heating of the charge is initiated with a temperature increase rate of 100°C/h up to 350°C and then at the same rate up to 970°C, with a stop of about 2 h at 350 °C. The final heating up to the melting point (1050 °C) and then to 1070°C is carried out at lower rate, having care that a portion of the seed, typically 50%, remains at a temperature slightly below the melting point of BGO, typically at 1037°C.
Fig. 2. Typical etch pits on {110} surface. Marker represents 10 pm.
377
When the melting process has ended and the liquid has reached the right temperature, the charge is held at constant temperature for about 4 h before beginning the solidification cycle. This is carried out by cooling down the second heating zone at a constant rate (0.8—0.3°C/h, see table 4) and then, sequentially, also the further heating zones up to the completion of the solidification. The cycle is ended by cooling down the crystal at a rate of 5°C/h up to 900°C and then of 15°C/h up to 600°C. The cooling down to room temperature is eventually carried out at the furnace natural cooling rate. During the solidification, the temperature of the bottom of the crucible is continuously recorded, using a 3530 Orion data system. 2.4 Analysis of the microstructure of the crystals
The knowledge of the crystal microstructure is of customary importance for establishing the right relationships between the kind of structural imperfections and its physical properties. Therefore, attention has been paid to setting up a suitable method for determining by a chemical etching procedure the dislocation density, this last being a quantitative measure of the stress relieved by plastic deformation during the growth cycle. The etch pit density has been determined by a method set up in our laboratory, which consists in the preliminary polishing of the surface of a slice in a sequence of mild chemical etching and of mechanical polishing, using a diamond paste (1 ~tm size) and then an alumina paste (0.05 ~tm size). After polishing, the surface is etched with a solution of 1M HC1 in ethylic ether at room temperature, under continuous stirring for about 10 mm. After careful rinsing with distilled water, the samples are dried and then examined with a scanning microscope where the etch pits appear well developed and square in shape on the {100} surface and triangular on the {110} and (111} surfaces, as expected (see fig. 2). To demonstrate that these etch pits are in one-to one relationship with the dislocations, the same sample was submitted to sequential polishing and etching tests, and then the invariance in the position of the etch pits has been demonstrated on
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/
Growth of BGO single crystals using directional solidification
specific regions of the sample, which resulted in easily distinguishable, thanks (as an example) to the presence of well-developed grain boundaries.
-
3 Experimental results and discussion The experimental results relative to a first series of 13 runs of which eight turned out to be useful! for this analysis, are reported in table 4, where the most prominent process parameters and the macroscopic features are shown as well It appears that the quality of the crystal is very sensitive to the imposed growth rate as well as to the nature of the BGO powder used and that, in good agreement with the literature, the most important defects in bad crystals are vertical rows of microscopic bubbles, which normally nucleate in correspondence to anomalous interfaces which develope during the growth and which are typically sligthly concave upwards and often slightly coloured (see fig. 3). These interfaces are symptomatic of a local excess of impurities resulting from a sudden growth rate change and disappear when the average growth rate is maintained close to 0.6 mm/h or less. The nucleation of bubbles could be avoided completely when using as the starting materials both the Chinese or the Preussag materials under the proper growth rates, indicated in table 4, but
Fig. 3. Vertical rows of bubbles and concave interfaces in a BGO crystals.
not when using the JM material under the same optimized growth conditions. It must be noted that the bubble nucleation process is certainly heterogeneous in nature, as it originates in correspondence to an interface, but that it must be thought of as due to the segregation of the oxygen [10] dissolved in the liquid when the liquid freezes, under conditions of local supersaturation, if the segregation coefficient K c(solid)/c(liquid) (where c is the oxygen concentration) is less than 1. The dislocation density has been measured on three different crystals, of which one of Chinese origin, taken as an example of a crystal grown with the Bnidgman technique, but surely sub=
Table 4 Relationships between the growth parameters and the properties of BGO crystals Crystal code
dT/dt ‘> (0 C/h)
Growth rate (mm/h)
dT/dx ~ (0 C/mm)
Remarks
P3 P14 P5 P6 P8 P10 P11 P13
1.7 1.0 0.8 0.8 0.6 0.6 0.6 0.3
2.08 (ave. 1.67) 0.71 1.04 (ave. 0.81) 0.83 (ave. 0.87) 0.66 0.64 0.64 0.54 (ave. 0.43)
0.8 1.40 0.76 0.96 0.92 0.96 0.96 0.56
Polycrystal Single crystal, bubbles Single crystal, interfaces and bubbles (JM) Single crystal, rows of bubbles Single crystal, rows of bubbles (JM) Good crystal Good crystal Good crystal, except on top
~ Consigned rate of cooling down of each heating zone. 5) Consigned rate of displacement of the solidification interface. The figures between parentheses are the average values of the growth rate, as determined from the experimental duration of the solidification cycle. ~ Consigned values of the temperature gradient in correspondence with the solidification interface region.
F. Allegretti et al.
/ Growth of BGO single crystals
TableS Dislocation density of three BGO crystals 2) Variance Sample Na (cm~ Seed 1200 500 P10 790 163 P13 (top) 17800 2665
Ba Cs 20
,/Co
/
00
2
3 Energy (MeV)
Fig. 4. Energy dependece of the light output of a BGO crystal. T=18°C.
Co
Na
o 0
1.5 20 1/E~(MevY Fig. 5. Energy resolution of a 20 cm long BGO crystal.
0.5
1.0
(0.2—2.6 MeV). The best energy resolution obtamed, as shown in fig. 5, is 16% (FWHM/E).
4. Conclusion It has been shown in the last section that our Bridgman grown crystals still present some macroscopic defects like bubbles and haze, which however could be virtually suppressed by optimizing the growth conditions. On clear and bubble free crystals only dislocations are the residual defects, the density of them being of the order of 800 cm2. The latter is a figure which well compares with that measured on the crystals of Chinese origin (see table 5) as well as with that already reported in ref. [11] for crystals quite comparable in size with ours. Apparently, however, in our case the density of dislocations is quite uniform within the entire sample, while in the crystal of ref. [11] a dislocation density profile exists, with an excess of dislocations at the crystal edges (100—1000 cm2 in the A
~a
1
Na
I
/
1000
379
~
mitted to a strong temperature gradient during the cooling cycle. Incidentally, a qualitative difference between our best crystals and the Chinese ones is the lower brittleness observed in our crystals, which has not been measured quantitatively, however. The results are reported in table 5 which shows that the dislocation density is lower than that of the Chinese crystal in the best crystal grown and definitely sensitive to the crystal growth parameters, as it increases in the case of a crystal presenting bubbles and anomalous interfaces or microcrystalline regions, like it is the case of the top of the ingot P13. Energy resolution and linearity against the energy of different radioactive sources have been measure on a 20 x 20 X 200 mm3 crystal. Figs. 4 and 5 show the results of these measurements. The crystal response (fig. 4) is shown to be linear over one order of magnitude in energy
200(
using directional solidification
with the dislocation density
found in CZ crystals (10~cm2 according to ref. [11]) shows that it is instead comparable with that observed in the degenerated regions of Bridgman crystals (see table 5). It seem therefore confirmed that Bridgman grown ingots present superior properties with respect to the crystal structure, which in turn should
380
F. Aliegretti et at.
/
Growth of BGO single crystals using directional solidification
beneficially affect both their radiation hardness [11] and their sensitivity to impurity contamination. The present experimental results do not produce significant evidence of a difference between crystals obtained by the different variants of the Bnidgman technique, albeit that the large brittleness of the Chinese crystals and the existence of differential stresses in the crystals grown by the Bnidgman—Stockbarger technique [11] suggest that the static Bridgman technique is ideally suited for the growth of stain-free crystals. Acknowledgments This work has been entirely supported by INFN The authors are, however, greatly indebted to dr. M. Rustioni of Enichem SpA for analytical results and to Professor F. Demartin of the Istituto di Strutturistica of the University of Milan for his precious assistance in XR structural measurements. funds.
References [1] K. Takagi, T. Oi, T. Fukazawa, M. lshii and S. Akiyama, J. Crystal Growth 52 (1981) 584. [2] F. Schmmd, C.P. Khattak and M.B. Smith, J. Crystal Growth 70 (1984) 446. [3] H. Chongfan, F Shiji, L. Jingying, S. Quanshun, S. Dingzhong and Z. Tiangun, Progr. Crystal Growth Characterization 11(1985) 253. [4] A. Horowitz and G. Kramer. J. Crystal Growth 79 (1986) [5] 296. X. Zhilin, Bull. Bismuth Inst. 51(1987)1. [6] G. Gevay, Progr. Crystal Growth Characterization 15 (1987) 145. [7] A. Horowitz and G. Kramer, J. Crystal Growth 78 (1986) 121. [8] Ch. Bieler, D. Burkart, J. Marks, M. Riebesell, H. Spitzer, K. Wittenburg and G.G. Winter. NucI. lnstr. Methods A234 (1985) 435. [9] E. Lorentz, Recent Progress in BGO Development for High Resolution Calorimetry, Report 1984 MPEPAE/Exp-El-129. [10] Ya. E. Geguzmn and AS. Dzuha, J. Crystal Growth 52 (1981) 337. [11] L.A.H. van Hoof and W.J. Bartels, Mater. Res Bull. 20 (1985) ~l9. [12] J. Marechal, private communication, 1987.
373
GROWTH OF BGO SINGLE CRYSTALS USING A DIRECTIONAL SOLIDIFICATION TECHNIQUE F. ALLEGRETTI
*
Dipartimenlo di Chiniica Fisica ed Eletirochimica, Via Golgi 19, 1-20133 Milano, Italy
B. BORGIA
*
Dipartimento di Fisica, Piazza A. Moro 2, 1-00185 Rome, Italy
R. RIVA
*
Dipartimento di Chimica Fisica ed Elettrochimica, Via Golgi 19, 1-20133 Milano, Italy
F. DE NOTARISTEFANI
*
Dipartimento di Fisica, Piazza A. Moro 2, 1-00185 Rome, Italy
and S. PIZZINI
*
Dipartimento di Chimica Fisica ed Elettrochimica, Via Golgi 19, 1-20133 Milano, Italy Received 15 July 1988
Bismuth germanate (Bi
4Ge3O12, BGO) crystals are currently used as scintillation detectors in high energy physics, as well as in sanitary physics experiments, for their superior performance with respect to NaI(Tl) detectors. At the present, most of the commercially available crystals are obtained by the Czochralski pulling technique, albeit Bridgman grown crystals seem to present excellent characteristics, such to having been chosen for the LEP3 experiment at CERN in Geneve. In order to prove that these superior properties 3) derive single from crystals a minor using aamount verticalofBridgman dislocations furnace and explicitally other structural designed defects, for this we scope have grown and bya suitably numberchanging of large selected parameters like the temperature gradient in the solid and the growth rate. The results show that crystals with a very (25 x 25 >
I. Introduction In spite of the fact that commercial crystals grown with the Czochralski or Bridgman technique [1—7]present comparable properties in terms of energy resolution (see table 1), the Bndgman technique offers several advantages compared to the Czochralski technique.
*
Also at INFN Sezione di Roma.
It is in fact possible to limit the deviations from the stoichiometry, which arise from differential sublimation of bismuth and germanium oxides from the free liquid surface during the growth thanks to the lower surface to volume ratio which could be achieved with the Bndgman technique and to improve the material utilization, thanks to the growth of crystals already shaped in view of their final utilization. However, both in CZ crystals and in Bridgman grown ingots several structural defects could still
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
374
F. Allegretti et al.
/
Growth of BGO single crystals using directional solidification
Table 1 Comparison between the scintillation properties of Czochralski and Bndgman grown crystals
_____________________________________________ Manufacturer Harshaw Crismatec NKK Shanghai NKK Shanghai Inst. Ceramics Scholz Philips Crismatec
Size (mm) 15 x 15 X 200 15 x 15x200 113<11 X 200 10~10x20 15x15x200 15x15 x200 20x20x60 20x20x240
Growth technique
Energy Ref. resolution
Czochralski 24.7 Czochralski 23.5 Czochralski 16.6 Bridgman 18.7 Bridgman 17.3 Czochralski 17.3 Bridgman 13.6 Czochralski 20.0 137Cs source (Er The energy resolution is measured with a keV).
[8] [8] [8] [8] [8] [8] [11] [12] = 662
be systematically observed, like bubbles, halos, microprecipitates of Pt and diffuse haze regions [6—8],which induce degradation of the scintillation efficiency. The aim of this paper is to describe a novel methodology which is a variant of the vertical Bridgman technique and which in principle should allow the growth of nearly perfect and strain free crystals due to its capability of controlling very carefully the imposed growth rate and to maintain the solidified material at approximately constant temperature up to the end of the solidification cycle,
2. Experimental 2.1 Furnace design The furnace used was explicitly designed for growing 24 x 24 X 200 mm3 crystals and consists of 11 Superkanthal coil shaped resistors embedded in a high alumina content sintered ceramic body, which in turn is enclosed in an stainless steel frame. The heating element has an internal diameter of 110 mm and a total height of 250 mm, and it is powered by 11 independent Eurotherm mod. 6358 units, driven by a microprocessor control unit Eurotherm mod. 6433, for a total power of 2.5 kW. The temperature of each heating zone is monitored with a Pt/Pt—Rh 10% (Pt) thermocouple and each heating zone could be ramped at any
desired temperature with an increase rate between 0.1 and 999.9 ° C/mm or held at constant temperature for a time lasting up to 99.99 h. The growth chamber is a quartz tube (outside diameter 100 mm) with a total height of 810 =
mm, with a movable quartz pedestal inside which is the support of the Pt crucible used for BGO growing and which also works as an JR transparent window to dissipate the latent heat of solidification. A quartz cylinder, filled with quartz wool is suitably housed on the top of the furnace to minimize the heat losses and to favour the estabilishment of the desired temperature profile in the chamber, without excessive distorsions in the last heating zone. A hole on the bottom of this quartz vessel permits the passage of a Pt thermocouple lined with a Pt tube (outside diameter 3 mm), used to determine the temperature profile of the furnace and to measure, eventually, the temperature of the liquid BGO in the crucible during the growth cycle. The temperature on the bottom of the crucible is measured as well with a Pt thermocouple, which is pressed by means of a spring on the bottom of the crucible in order to improve the thermal contact. A schematic drawing of the furnace is reported in fig. 1, which shows the water cooled flanges used to mantain a controlled atmosphere in the growth chamber and to hold in vertical position the quartz pedestal as well as the top insulating cover. As crucibles, pure Pt crucibles were used throughout, which were manufactured by cold shaping and arc welding 0.2 mm thick Pt foils. =
2.2. Materials As starting materials, bismuth germanate powders of different origin were used, after having set up the growth procedure with a material kindly delivered by the Academia Sinica, Shanghai Institute of Ceramics, which is already proved to be of excellent quality and which is obtained by melting a sintered powder of BGO, freezing it and crushing it to powder.
F Allegretti et aL
/
Growth of BGO single crystals using directional solidification
L.
I
___
—~QQ-
_-i__
__
‘iii
_____
I~I
I~I I~. ==
_____
____
______
___
____
I
I’l.
I
iiiI~L
Fig. 1. Schematic picture of the growth furnace.
j~’
375
376
F. A Ilegretti et al.
/
Growth of BGO single crystals using directional solidification
A Johnson-Matthey (JM) material (batch No. S 94064 AC) was proved to yield yellow crystals and plenty of bubbles, and therefore it was discarded for further experiments, while the BGO powder delivered by Preussag (batch No. 6109/0102) was found instead to be suitable as an alternative to the chinese material. A material obtained from the Cadarache Research Center (France) (batch No. H 86) was found to be of high purity but not direcly suited for growing BGO crystals, as it presented the same problems of the JM material, albeit to a lesser extent. All the experiments reported in the next section were therefore conducted using either the Chinese or the Preussag materials, With the aim of trying a classification of the various materials in term of their suitability for use in Bridgman growth experiments, we carried out a preliminary test directed to find a possible relationship between the deviation from the stoichiometry of the raw material and the quality of the final crystal, using a standard chemical method for the analysis of bismuth (EDTA titration), having however care to inhibit the volatility of bismuth chloride during the preparation of the sample by adding a sufficient amount of sulphuric acid. The results of these experiments, reported in table 2, show that in all cases the deviations from the stoichiometry are not negligible but that the experimentally found differences between the vanous samples are not large enough to permit their classification in terms of suitability for good qua!ity single crystal growth. It seems, however, confirmed that also in the case of Bridgman grown crystals a deviation from the stoichiometry less
Table 2 Average values of the bismuth deficiency in BGO powders of vanous origin
than 1% is sufficient for inducing pratically zero yields, like in CZ crystals [9]. As a result of a further series of experiments performed in order to determine the material losses during the growth cycle (frequently called responsible of the crystal quality deterioration), we could demonstrate that a thermal treatment at 1070°C induces a material loss which in turn causes slight deviations from the stoichiometry (see table 3). These experiments, typically, were performed by pouring ca. 40 g of BGO powder into a Pt crucible having about the same cross-section as
limits of detectability by X-ray diffraction phase analysis, indicating an important sensitivity of the Bridgman process to the presence of traces of extraneous oxides. This sensitivity should be attributed to the practical absence of convective
_____________________________________________ Material
Deviation from stoichiometry (average) (%)
Remarks
Pellets JM (batch S 94046 AC) —0.68 (1) Powder JM (batch S 94046 AC) —1.14 (2) Academia Sinica —0.37 (3) Preussag —0.68 (4) Cadarache —0.93 (5) (1) The material is completely soluble in boiling HCI: the crystals obtained from this matenal are yellow in colour and have plenty of bubbles (see text). (2) The material leaves an insoluble residual after dissolution in boiling HC1 (3) The material yields perfectly transparent crystals under the proper growth conditions (see text). (4) Like under (3). (5) The material yields crystals yellow in colour and rich in bubbles under the same growth conditions at which Chinese and Preussag powders yield good crystals.
the crucibles used for the crystal growth experiments, and heating it at a temperature slightly above the melting temperature in an electrically heated furnace. The samples withdrawn after selected times were then analyzed for bismuth, using the procedure indicated above. Also in this case, however, the results do not present any evidence either of a quantitative difference between powders of different origin. We believe therefore that the major difference between the various materials examined is their content of non-reacted oxides or of phases different from the Bi4Ge3O12 one, which are, however, below the
T bl 3 a e deficiencies induced by differential . . . . Bismuth sublimation losses at 10700 C of a BGO sample .
.
.
.
Initial value
After 20 mm
After 80 mm
—0.93%
—1.91%
—2.12%
F Allegretti et at
/
Growth of BGO single crystals using directional solidification
motions in the liquid charge during the growth and therefore to the permanence of local composition differences, which could easily lead to conditions of constitutional supercooling and which could be avoided instead when using the Czochralski technique. 2.3 Crystal growth procedures
A typical growth cycle is performed according to the following procedure. The Pt crucible is preliminarly charged with the BGO powder, taking care to use a Pt funnel fixed on the top of the crucible which contains the proper excess of powder to ensure the complete filling of the crucible when the charge is molten. Then, after having positioned the crucible in the furnace, with the seed centered midway the first heating zone, the entire furnace is flushed with pure dry oxygen at a rate of about 15 1/h, while the heating of the charge is initiated with a temperature increase rate of 100°C/h up to 350°C and then at the same rate up to 970°C, with a stop of about 2 h at 350 °C. The final heating up to the melting point (1050 °C) and then to 1070°C is carried out at lower rate, having care that a portion of the seed, typically 50%, remains at a temperature slightly below the melting point of BGO, typically at 1037°C.
Fig. 2. Typical etch pits on {110} surface. Marker represents 10 pm.
377
When the melting process has ended and the liquid has reached the right temperature, the charge is held at constant temperature for about 4 h before beginning the solidification cycle. This is carried out by cooling down the second heating zone at a constant rate (0.8—0.3°C/h, see table 4) and then, sequentially, also the further heating zones up to the completion of the solidification. The cycle is ended by cooling down the crystal at a rate of 5°C/h up to 900°C and then of 15°C/h up to 600°C. The cooling down to room temperature is eventually carried out at the furnace natural cooling rate. During the solidification, the temperature of the bottom of the crucible is continuously recorded, using a 3530 Orion data system. 2.4 Analysis of the microstructure of the crystals
The knowledge of the crystal microstructure is of customary importance for establishing the right relationships between the kind of structural imperfections and its physical properties. Therefore, attention has been paid to setting up a suitable method for determining by a chemical etching procedure the dislocation density, this last being a quantitative measure of the stress relieved by plastic deformation during the growth cycle. The etch pit density has been determined by a method set up in our laboratory, which consists in the preliminary polishing of the surface of a slice in a sequence of mild chemical etching and of mechanical polishing, using a diamond paste (1 ~tm size) and then an alumina paste (0.05 ~tm size). After polishing, the surface is etched with a solution of 1M HC1 in ethylic ether at room temperature, under continuous stirring for about 10 mm. After careful rinsing with distilled water, the samples are dried and then examined with a scanning microscope where the etch pits appear well developed and square in shape on the {100} surface and triangular on the {110} and (111} surfaces, as expected (see fig. 2). To demonstrate that these etch pits are in one-to one relationship with the dislocations, the same sample was submitted to sequential polishing and etching tests, and then the invariance in the position of the etch pits has been demonstrated on
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Growth of BGO single crystals using directional solidification
specific regions of the sample, which resulted in easily distinguishable, thanks (as an example) to the presence of well-developed grain boundaries.
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3 Experimental results and discussion The experimental results relative to a first series of 13 runs of which eight turned out to be useful! for this analysis, are reported in table 4, where the most prominent process parameters and the macroscopic features are shown as well It appears that the quality of the crystal is very sensitive to the imposed growth rate as well as to the nature of the BGO powder used and that, in good agreement with the literature, the most important defects in bad crystals are vertical rows of microscopic bubbles, which normally nucleate in correspondence to anomalous interfaces which develope during the growth and which are typically sligthly concave upwards and often slightly coloured (see fig. 3). These interfaces are symptomatic of a local excess of impurities resulting from a sudden growth rate change and disappear when the average growth rate is maintained close to 0.6 mm/h or less. The nucleation of bubbles could be avoided completely when using as the starting materials both the Chinese or the Preussag materials under the proper growth rates, indicated in table 4, but
Fig. 3. Vertical rows of bubbles and concave interfaces in a BGO crystals.
not when using the JM material under the same optimized growth conditions. It must be noted that the bubble nucleation process is certainly heterogeneous in nature, as it originates in correspondence to an interface, but that it must be thought of as due to the segregation of the oxygen [10] dissolved in the liquid when the liquid freezes, under conditions of local supersaturation, if the segregation coefficient K c(solid)/c(liquid) (where c is the oxygen concentration) is less than 1. The dislocation density has been measured on three different crystals, of which one of Chinese origin, taken as an example of a crystal grown with the Bnidgman technique, but surely sub=
Table 4 Relationships between the growth parameters and the properties of BGO crystals Crystal code
dT/dt ‘> (0 C/h)
Growth rate (mm/h)
dT/dx ~ (0 C/mm)
Remarks
P3 P14 P5 P6 P8 P10 P11 P13
1.7 1.0 0.8 0.8 0.6 0.6 0.6 0.3
2.08 (ave. 1.67) 0.71 1.04 (ave. 0.81) 0.83 (ave. 0.87) 0.66 0.64 0.64 0.54 (ave. 0.43)
0.8 1.40 0.76 0.96 0.92 0.96 0.96 0.56
Polycrystal Single crystal, bubbles Single crystal, interfaces and bubbles (JM) Single crystal, rows of bubbles Single crystal, rows of bubbles (JM) Good crystal Good crystal Good crystal, except on top
~ Consigned rate of cooling down of each heating zone. 5) Consigned rate of displacement of the solidification interface. The figures between parentheses are the average values of the growth rate, as determined from the experimental duration of the solidification cycle. ~ Consigned values of the temperature gradient in correspondence with the solidification interface region.
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/ Growth of BGO single crystals
TableS Dislocation density of three BGO crystals 2) Variance Sample Na (cm~ Seed 1200 500 P10 790 163 P13 (top) 17800 2665
Ba Cs 20
,/Co
/
00
2
3 Energy (MeV)
Fig. 4. Energy dependece of the light output of a BGO crystal. T=18°C.
Co
Na
o 0
1.5 20 1/E~(MevY Fig. 5. Energy resolution of a 20 cm long BGO crystal.
0.5
1.0
(0.2—2.6 MeV). The best energy resolution obtamed, as shown in fig. 5, is 16% (FWHM/E).
4. Conclusion It has been shown in the last section that our Bridgman grown crystals still present some macroscopic defects like bubbles and haze, which however could be virtually suppressed by optimizing the growth conditions. On clear and bubble free crystals only dislocations are the residual defects, the density of them being of the order of 800 cm2. The latter is a figure which well compares with that measured on the crystals of Chinese origin (see table 5) as well as with that already reported in ref. [11] for crystals quite comparable in size with ours. Apparently, however, in our case the density of dislocations is quite uniform within the entire sample, while in the crystal of ref. [11] a dislocation density profile exists, with an excess of dislocations at the crystal edges (100—1000 cm2 in the A
~a
1
Na
I
/
1000
379
~
mitted to a strong temperature gradient during the cooling cycle. Incidentally, a qualitative difference between our best crystals and the Chinese ones is the lower brittleness observed in our crystals, which has not been measured quantitatively, however. The results are reported in table 5 which shows that the dislocation density is lower than that of the Chinese crystal in the best crystal grown and definitely sensitive to the crystal growth parameters, as it increases in the case of a crystal presenting bubbles and anomalous interfaces or microcrystalline regions, like it is the case of the top of the ingot P13. Energy resolution and linearity against the energy of different radioactive sources have been measure on a 20 x 20 X 200 mm3 crystal. Figs. 4 and 5 show the results of these measurements. The crystal response (fig. 4) is shown to be linear over one order of magnitude in energy
200(
using directional solidification
with the dislocation density
found in CZ crystals (10~cm2 according to ref. [11]) shows that it is instead comparable with that observed in the degenerated regions of Bridgman crystals (see table 5). It seem therefore confirmed that Bridgman grown ingots present superior properties with respect to the crystal structure, which in turn should
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Growth of BGO single crystals using directional solidification
beneficially affect both their radiation hardness [11] and their sensitivity to impurity contamination. The present experimental results do not produce significant evidence of a difference between crystals obtained by the different variants of the Bnidgman technique, albeit that the large brittleness of the Chinese crystals and the existence of differential stresses in the crystals grown by the Bnidgman—Stockbarger technique [11] suggest that the static Bridgman technique is ideally suited for the growth of stain-free crystals. Acknowledgments This work has been entirely supported by INFN The authors are, however, greatly indebted to dr. M. Rustioni of Enichem SpA for analytical results and to Professor F. Demartin of the Istituto di Strutturistica of the University of Milan for his precious assistance in XR structural measurements. funds.
References [1] K. Takagi, T. Oi, T. Fukazawa, M. lshii and S. Akiyama, J. Crystal Growth 52 (1981) 584. [2] F. Schmmd, C.P. Khattak and M.B. Smith, J. Crystal Growth 70 (1984) 446. [3] H. Chongfan, F Shiji, L. Jingying, S. Quanshun, S. Dingzhong and Z. Tiangun, Progr. Crystal Growth Characterization 11(1985) 253. [4] A. Horowitz and G. Kramer. J. Crystal Growth 79 (1986) [5] 296. X. Zhilin, Bull. Bismuth Inst. 51(1987)1. [6] G. Gevay, Progr. Crystal Growth Characterization 15 (1987) 145. [7] A. Horowitz and G. Kramer, J. Crystal Growth 78 (1986) 121. [8] Ch. Bieler, D. Burkart, J. Marks, M. Riebesell, H. Spitzer, K. Wittenburg and G.G. Winter. NucI. lnstr. Methods A234 (1985) 435. [9] E. Lorentz, Recent Progress in BGO Development for High Resolution Calorimetry, Report 1984 MPEPAE/Exp-El-129. [10] Ya. E. Geguzmn and AS. Dzuha, J. Crystal Growth 52 (1981) 337. [11] L.A.H. van Hoof and W.J. Bartels, Mater. Res Bull. 20 (1985) ~l9. [12] J. Marechal, private communication, 1987.