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The role of impurities in the quality of paratellurite single crystals
Journal of Crystal Growth 52 (1981) 561—565 © North-Holland Publishing Company
THE ROLE OF IMPURITIES IN THE QUALITY OF PARATELLURITE SINGLE CRYSTALS I. FOLDVARI, K. RAKSANYI, R. VOSZKA, E. HARTMANN and A. PETER Research Laboratory for Crystal Physics, Hungarian Academy of Sciences, Budaörsi
Ut
45, H-1112 Budapest, Hungary
Paratellunte crystals grown by the Czochralski technique often contain Pt inclusions unless extrapure source material is used. Using analytical data obtained on comparable uniform paratellurite crystal samples we have shown that there is a joint incorporation of Pt and the added Fe. In the absence of Fe in the melt no incorporation of Pt occurs. The undesirable interactions of Pt inclusions with dislocations are pointed out.
1. Introduction
material. The impurities may lead to the degradation of the Pt crucible, so increasing the Pt content of the melt. In our investigations the role of Fe ions in this process was studied in detail. A number of analytical methods were developed which were effective under the peculiar conditions of Te02 systems.
Earlier investigations reported by Ant et al. [1] and Uchida et al. [2, 3] have shown that single crystals of paratellurite have excellent acoustooptical properties due to their low ultrasound velocity, high refractive indices and large piezooptic constant. The M2 acousto-optical figure of merit for the shear mode of Te02 is 1.2 X 10-15 53 g~which is the highest known value for optically transparent materials [4]. Its chemical stability, relative hardness and wide optical transmission range also suggest its application in different devices. Deflectors, modulators and tunable optical filters are just a few of the realized possibilities, In order to decrease the dissipation losses the crystals have to be produced free from imperfections. Since the fundamental paper of Liebertz [5] much work was done to improve the quality of paratellurite crystals. Some disturbing defects (gas bubbles, internal strain) in Te02 grown by the conventional Czochralski technique arise from the inadequacies of crystal growth and they could be avoided by selecting appropriate growth conditions [6, 7]. Other imperfections (Pt inclusions, veils of impurity aggregates) were found to depend strongly on the purity of the raw material [8] and could be eliminated by using extrapure material (99.9999%) for crystal growth [9]. This means that some connection should exist between the Pt inclusions and some impurities in the starting
2. Experimental 5N pure starting Te metal (Fe content 7— 10 ~mol/mol) was used. The conventional chemical method for preparing Te02 was improved to obtain uniformly oxidized material the purity of which exceeds that of the starting metal [10] (e.g. Fe content 3—5 ~smol/mol). For these investigations the Te02 material was also doped with Fe203 in concentrations of 10~— 10~mol/mol. 2.1. Analytical methods Few papers can be found in the literature dealing with the determination of Fe and Pt traces in Te02 and most of the methods are of qualitative nature or have low sensitivity. The methods applied were emission spectrography [9, 11] spectrochemical techniques [12, 13], spectrophotometry and electron microprobe analysis [8]. The unreliability of the former methods is mainly due to the matrix effects of Te02. Consequently the prior chemical separation of Fe 561
562
1. Földvári et a!. I frnpurities and quality of paratellurite single crystals
balance-controlled apparatus developed in our laboratory [14] was used. Following Miyazawa
Te0
2 samples
thsso1v~ng HCI
~
7er,ng
~
~~ extrnclrng by 0/PC phase
exfra~t~o9 by
p°~ase
C ‘~i±~.L) 0
[6] a pulling rate of 1.5mm/h along the (110) direction was used. The seed rotation was adjusted between 28—36 rpm depending on the solid-liquidinterfacewaspreferred. cases a flat 2.3. Observation of crystal defects
sq. phase • org. e~s.lractrngby P/BK P ose
Te
~
03 phOse
(P1)
Fig. I. Scheme of chemical separation procedure of Fe and
and Pt is highly recommended. For this end a procedure based on the filtration of inclusions (containing the major part of Pt), the previous removal of Fe by diisopropylether (DIPE) extraction and the subsequent extraction of the bulk Te by methylisobuthylketone (MIBK) was developed. Another part of the Pt remains in the aqueous phase. Fig. I shows the separation procedure. The unexpected appearance of Pt. Te and Fe at certain steps are indicated by parentheses. After preconcentration of the solutions by evaporation the elements were determined by a Spectromom 390 atomic absorption spectrophotometer. In order to increase the sensitivity an integrator and X—Y plotter were attached, thus the sensitivity limit was extended to 2 and 0.4~mol/molin case of Pt and Fe respectively taking into account the preconcentration as well.
The standard method of polarization microscopy was used to observe the internal tension in the crystals. The dislocation structure was developed by etching on the (110) faces. The oriented faces were obtained by cutting and chemical polishing in 15 /o NaOH solution, subsequently an iodine etching (10 ml distilled water + 5 g NaOH + 0.2 g ‘2) was performed at 40—60°Cfor I to 2 mm. .
.
.
.
3. Results and discussion A set of uniform as-grown paratellurite crystals produced by weight control is shown in fig. 2. The diameter of the crystals is 22 ± 1 mm, the length is at about 6 cm (without seed). This type of crystal was used for further investigations.
2.2. Crystal growth The earlier reports clarified the optimal circumstances of crystal growth of paratellurite [5— 9]. None of the authors, however, could avoid the fluctuations of crystal diameter. The changing growing surface during the growth resulted in the deterioration of crystal quality and in an inhomogeneous impurity distribution. For obtaining comparable uniform crystal samples a
Fig. 2. Paratellunic crystals grown by balance control.
I. Földvdri et a!.
/
Impurities and quality of paratellurite single crystals
3.1. Analytical data
563
Table I Distribution of Fe and Pt in Te0
2 crystals ((110) axis) (data in
The chemical separation led to interesting observations. In addition to the sparingly soluble Pt species expected in the precipitate an activated form of Pt soluble in cone. HCI was also found. The opposite situation, an insoluble form of Fe, probably a Pt alloy or inclusion in pt colloids, appeared as well. Table I shows the analytical data of undoped crystals and of those doped with 100 jsmol/mol Fe. Parallel slabs were cut along the (110) growth axes from which a central (c) slab containing the rotation axis and an off-central (o) slab far off it were selected in order to trace the radial distribution of impurities as well. For measurements both types of slabs were cut into 10 uniform pieces perpendicularly to the growth axis. Table I contains the summarized contents detected at different steps of separation (1 and 2 for Fe and 1 and 4 for Pt in fig. 1). The impurity concentration of the crucible residue is also indicated in table 1. The following facts can be seen from table 1: (a) In undoped crystals the distribution of Pt is almost homogeneous and the amount of incor-
~amol/mol) Fe
Pt
__________________
Sample
No.
Undoped
(o)
________________
Doped with
Doped
Fe
with Fe
(c)
(o)
Undoped
(o)
(c)
(o)
3 1 2 0 14 18
2 0 3 2 2 4
23 24 33
8 30 28
_________________________________________
1 2 3 6 7 8 9 10 Residue
4.5 5.0 3.5 3.5 30 2.5 3.0 2.5 2.5 8.3
3.0 3.0 4.5 4.0
7.5 3.5 3.0 2.5
910
410
13.5
5.0
0 2 2 2 3 1
12.5 5.0 15.5 20.0 12.5 12.0 167.3
1 2 1 428
247
(o) off-center, (c) center.
porated Pt is very low in contrast to the high Pt concentration in the residue. (b) A sharp increase in the concentration of both Fe and Pt was observed between the 8th and 9th
Fig. 3. Cross-like internal strain around an inclusion in TeO crystal; magnification 80x.
se4
I. Fö!drdri ci a!. / Impurities and qualit3 of paratellurite c,nqlc Crystals
4
44
44
• 4
*4S~,l~
4
4%,
.
4 •
4.
,
.iO
~4. ~
4
Fig. 4. Interaction of Pt inclusions with dislocations. (a) Starlike ordered dislocations around Pt inclusion; magnification 350~ (h) Network of dislocations around a group of inclusions: magnification 85x.
I. Fdldvdri et al.
I
Impurities and quality of paratellurite single crystals
pieces of the off-central slab of the doped crystal. After the 9th piece the Pt concentration is higher by an order of magnitude than that of the undoped crystal. (c) The concentration jump in the central slab of the doped crystal is situated between the 5th and 6th pieces in the case of both Fe and Pt. (d) The concentration of impurities at the top part of crystals is uncertain probably due to the change in shape of the solid—liquid interface in the transition region between the seed and the prescribed diameter. (e) Contrary to our expectation the Pt content of the crucible residue is rather large in the case of the undoped material as well, 3.2. Interaction of Pt inclusions with other defects The large number of Pt inclusions results in visible gray stripes or non-transparent regions in the crystals [8] which were earlier thought to be reduced forms of Te. Smaller amounts of Pt may also result in the scattering of light and of acoustic waves. From the point of view of acoustooptical devices the interaction of Pt inclusions with some other defects is far more dangerous. In several cases around the inclusions extended internal strained areas were produced (fig. 3) the dimensions of which were larger than 0.1 mm. The figure was obtained with a polarization microscope. The random distribution of dislocations is apparently disturbed by inclusions. Ordered dislocations in a starlike pattern assembled around the Pt inclusions (fig. 4a). Inclusions on the same crystallographic line resulted in a network of dislocations (fig. 4b). These types of ordered dislocation structures strongly diffract the acoustic waves disturbing the acousto-optical interaction inside the crystals.
4 Conclusions From the analytical data the joint incorporation of Pt and Fe into Te02 crystals is fairly
565
evident. The joint incorporation may be due to either a complicated redox reaction or to changes of the physico-chemical properties of the melt containing Fe ions. This fact explains the findings of Bonner et al. [9] that using extrapure starting material inclusion-free crystals can easily be produced. Nevertheless the Pt concentration of the melt (determined from the residue) seems to be independent of the Fe concentration. The harmful interaction of Pt inclusions with dislocations emphasizes the practical importance of preparing inclusion-free crystals. The observation of joint incorporation of Fe and Pt could help the technologists to find a compromise solution to the problem of the enormous price of extrapure source material while keeping in mind the necessary quality of the crystal. Acknowledgement The authors wish to express their gratitude to their assistant staff for the experimental help.
References [11G.
Ant and M. Schweppe. Solid State Commun. 6 (1968) 783. [2] N. Uchida and Y. Omachi, J. AppI. Phys. 40 (1969) [3] N. Uchida and Y. Omachi, J. Appi. Phys. 41 (1970) 2307. [41T. Yano and A. Watanabe, AppI. Phys. Letters 24 (1974) 256. [5] J. Liebertz. Knistall Tech. 4 (1969) 221. [61S. Miyazawa and H. Iwasaki, Japan. J. AppI. Phys. 9 (1970) 441. [7] M. Cerclet, Mater. Res. Bull. 7 (1972) 721.
[8] J.G. Grabmaier, R.D. Plätner and H. Schieber, J. Crystal Growth 20 (1973) 82. [9] W.A. Bonner, S. Shingh. L.G. Van Uitert and A.W. Warner, J. Electron. Mater. 1 (1972) 155. [10] K. Raksányi et al.. to be published. [11] I.P. Alimanin, Ed., Metody Analizo Veskchesto Vysokoi [12] Chistoty F.B. Zajhovskij, (Nauka, Moscow Sovremennye 1965) p. 460. Metody Analiza Metallurgii (Metallurgizdat, Moscow 1955) p. 12. [13] C. Koch, Microchim. Acta 402 (1958). [14] R. Voszka et al., to be published.
THE ROLE OF IMPURITIES IN THE QUALITY OF PARATELLURITE SINGLE CRYSTALS I. FOLDVARI, K. RAKSANYI, R. VOSZKA, E. HARTMANN and A. PETER Research Laboratory for Crystal Physics, Hungarian Academy of Sciences, Budaörsi
Ut
45, H-1112 Budapest, Hungary
Paratellunte crystals grown by the Czochralski technique often contain Pt inclusions unless extrapure source material is used. Using analytical data obtained on comparable uniform paratellurite crystal samples we have shown that there is a joint incorporation of Pt and the added Fe. In the absence of Fe in the melt no incorporation of Pt occurs. The undesirable interactions of Pt inclusions with dislocations are pointed out.
1. Introduction
material. The impurities may lead to the degradation of the Pt crucible, so increasing the Pt content of the melt. In our investigations the role of Fe ions in this process was studied in detail. A number of analytical methods were developed which were effective under the peculiar conditions of Te02 systems.
Earlier investigations reported by Ant et al. [1] and Uchida et al. [2, 3] have shown that single crystals of paratellurite have excellent acoustooptical properties due to their low ultrasound velocity, high refractive indices and large piezooptic constant. The M2 acousto-optical figure of merit for the shear mode of Te02 is 1.2 X 10-15 53 g~which is the highest known value for optically transparent materials [4]. Its chemical stability, relative hardness and wide optical transmission range also suggest its application in different devices. Deflectors, modulators and tunable optical filters are just a few of the realized possibilities, In order to decrease the dissipation losses the crystals have to be produced free from imperfections. Since the fundamental paper of Liebertz [5] much work was done to improve the quality of paratellurite crystals. Some disturbing defects (gas bubbles, internal strain) in Te02 grown by the conventional Czochralski technique arise from the inadequacies of crystal growth and they could be avoided by selecting appropriate growth conditions [6, 7]. Other imperfections (Pt inclusions, veils of impurity aggregates) were found to depend strongly on the purity of the raw material [8] and could be eliminated by using extrapure material (99.9999%) for crystal growth [9]. This means that some connection should exist between the Pt inclusions and some impurities in the starting
2. Experimental 5N pure starting Te metal (Fe content 7— 10 ~mol/mol) was used. The conventional chemical method for preparing Te02 was improved to obtain uniformly oxidized material the purity of which exceeds that of the starting metal [10] (e.g. Fe content 3—5 ~smol/mol). For these investigations the Te02 material was also doped with Fe203 in concentrations of 10~— 10~mol/mol. 2.1. Analytical methods Few papers can be found in the literature dealing with the determination of Fe and Pt traces in Te02 and most of the methods are of qualitative nature or have low sensitivity. The methods applied were emission spectrography [9, 11] spectrochemical techniques [12, 13], spectrophotometry and electron microprobe analysis [8]. The unreliability of the former methods is mainly due to the matrix effects of Te02. Consequently the prior chemical separation of Fe 561
562
1. Földvári et a!. I frnpurities and quality of paratellurite single crystals
balance-controlled apparatus developed in our laboratory [14] was used. Following Miyazawa
Te0
2 samples
thsso1v~ng HCI
~
7er,ng
~
~~ extrnclrng by 0/PC phase
exfra~t~o9 by
p°~ase
C ‘~i±~.L) 0
[6] a pulling rate of 1.5mm/h along the (110) direction was used. The seed rotation was adjusted between 28—36 rpm depending on the solid-liquidinterfacewaspreferred. cases a flat 2.3. Observation of crystal defects
sq. phase • org. e~s.lractrngby P/BK P ose
Te
~
03 phOse
(P1)
Fig. I. Scheme of chemical separation procedure of Fe and
and Pt is highly recommended. For this end a procedure based on the filtration of inclusions (containing the major part of Pt), the previous removal of Fe by diisopropylether (DIPE) extraction and the subsequent extraction of the bulk Te by methylisobuthylketone (MIBK) was developed. Another part of the Pt remains in the aqueous phase. Fig. I shows the separation procedure. The unexpected appearance of Pt. Te and Fe at certain steps are indicated by parentheses. After preconcentration of the solutions by evaporation the elements were determined by a Spectromom 390 atomic absorption spectrophotometer. In order to increase the sensitivity an integrator and X—Y plotter were attached, thus the sensitivity limit was extended to 2 and 0.4~mol/molin case of Pt and Fe respectively taking into account the preconcentration as well.
The standard method of polarization microscopy was used to observe the internal tension in the crystals. The dislocation structure was developed by etching on the (110) faces. The oriented faces were obtained by cutting and chemical polishing in 15 /o NaOH solution, subsequently an iodine etching (10 ml distilled water + 5 g NaOH + 0.2 g ‘2) was performed at 40—60°Cfor I to 2 mm. .
.
.
.
3. Results and discussion A set of uniform as-grown paratellurite crystals produced by weight control is shown in fig. 2. The diameter of the crystals is 22 ± 1 mm, the length is at about 6 cm (without seed). This type of crystal was used for further investigations.
2.2. Crystal growth The earlier reports clarified the optimal circumstances of crystal growth of paratellurite [5— 9]. None of the authors, however, could avoid the fluctuations of crystal diameter. The changing growing surface during the growth resulted in the deterioration of crystal quality and in an inhomogeneous impurity distribution. For obtaining comparable uniform crystal samples a
Fig. 2. Paratellunic crystals grown by balance control.
I. Földvdri et a!.
/
Impurities and quality of paratellurite single crystals
3.1. Analytical data
563
Table I Distribution of Fe and Pt in Te0
2 crystals ((110) axis) (data in
The chemical separation led to interesting observations. In addition to the sparingly soluble Pt species expected in the precipitate an activated form of Pt soluble in cone. HCI was also found. The opposite situation, an insoluble form of Fe, probably a Pt alloy or inclusion in pt colloids, appeared as well. Table I shows the analytical data of undoped crystals and of those doped with 100 jsmol/mol Fe. Parallel slabs were cut along the (110) growth axes from which a central (c) slab containing the rotation axis and an off-central (o) slab far off it were selected in order to trace the radial distribution of impurities as well. For measurements both types of slabs were cut into 10 uniform pieces perpendicularly to the growth axis. Table I contains the summarized contents detected at different steps of separation (1 and 2 for Fe and 1 and 4 for Pt in fig. 1). The impurity concentration of the crucible residue is also indicated in table 1. The following facts can be seen from table 1: (a) In undoped crystals the distribution of Pt is almost homogeneous and the amount of incor-
~amol/mol) Fe
Pt
__________________
Sample
No.
Undoped
(o)
________________
Doped with
Doped
Fe
with Fe
(c)
(o)
Undoped
(o)
(c)
(o)
3 1 2 0 14 18
2 0 3 2 2 4
23 24 33
8 30 28
_________________________________________
1 2 3 6 7 8 9 10 Residue
4.5 5.0 3.5 3.5 30 2.5 3.0 2.5 2.5 8.3
3.0 3.0 4.5 4.0
7.5 3.5 3.0 2.5
910
410
13.5
5.0
0 2 2 2 3 1
12.5 5.0 15.5 20.0 12.5 12.0 167.3
1 2 1 428
247
(o) off-center, (c) center.
porated Pt is very low in contrast to the high Pt concentration in the residue. (b) A sharp increase in the concentration of both Fe and Pt was observed between the 8th and 9th
Fig. 3. Cross-like internal strain around an inclusion in TeO crystal; magnification 80x.
se4
I. Fö!drdri ci a!. / Impurities and qualit3 of paratellurite c,nqlc Crystals
4
44
44
• 4
*4S~,l~
4
4%,
.
4 •
4.
,
.iO
~4. ~
4
Fig. 4. Interaction of Pt inclusions with dislocations. (a) Starlike ordered dislocations around Pt inclusion; magnification 350~ (h) Network of dislocations around a group of inclusions: magnification 85x.
I. Fdldvdri et al.
I
Impurities and quality of paratellurite single crystals
pieces of the off-central slab of the doped crystal. After the 9th piece the Pt concentration is higher by an order of magnitude than that of the undoped crystal. (c) The concentration jump in the central slab of the doped crystal is situated between the 5th and 6th pieces in the case of both Fe and Pt. (d) The concentration of impurities at the top part of crystals is uncertain probably due to the change in shape of the solid—liquid interface in the transition region between the seed and the prescribed diameter. (e) Contrary to our expectation the Pt content of the crucible residue is rather large in the case of the undoped material as well, 3.2. Interaction of Pt inclusions with other defects The large number of Pt inclusions results in visible gray stripes or non-transparent regions in the crystals [8] which were earlier thought to be reduced forms of Te. Smaller amounts of Pt may also result in the scattering of light and of acoustic waves. From the point of view of acoustooptical devices the interaction of Pt inclusions with some other defects is far more dangerous. In several cases around the inclusions extended internal strained areas were produced (fig. 3) the dimensions of which were larger than 0.1 mm. The figure was obtained with a polarization microscope. The random distribution of dislocations is apparently disturbed by inclusions. Ordered dislocations in a starlike pattern assembled around the Pt inclusions (fig. 4a). Inclusions on the same crystallographic line resulted in a network of dislocations (fig. 4b). These types of ordered dislocation structures strongly diffract the acoustic waves disturbing the acousto-optical interaction inside the crystals.
4 Conclusions From the analytical data the joint incorporation of Pt and Fe into Te02 crystals is fairly
565
evident. The joint incorporation may be due to either a complicated redox reaction or to changes of the physico-chemical properties of the melt containing Fe ions. This fact explains the findings of Bonner et al. [9] that using extrapure starting material inclusion-free crystals can easily be produced. Nevertheless the Pt concentration of the melt (determined from the residue) seems to be independent of the Fe concentration. The harmful interaction of Pt inclusions with dislocations emphasizes the practical importance of preparing inclusion-free crystals. The observation of joint incorporation of Fe and Pt could help the technologists to find a compromise solution to the problem of the enormous price of extrapure source material while keeping in mind the necessary quality of the crystal. Acknowledgement The authors wish to express their gratitude to their assistant staff for the experimental help.
References [11G.
Ant and M. Schweppe. Solid State Commun. 6 (1968) 783. [2] N. Uchida and Y. Omachi, J. AppI. Phys. 40 (1969) [3] N. Uchida and Y. Omachi, J. Appi. Phys. 41 (1970) 2307. [41T. Yano and A. Watanabe, AppI. Phys. Letters 24 (1974) 256. [5] J. Liebertz. Knistall Tech. 4 (1969) 221. [61S. Miyazawa and H. Iwasaki, Japan. J. AppI. Phys. 9 (1970) 441. [7] M. Cerclet, Mater. Res. Bull. 7 (1972) 721.
[8] J.G. Grabmaier, R.D. Plätner and H. Schieber, J. Crystal Growth 20 (1973) 82. [9] W.A. Bonner, S. Shingh. L.G. Van Uitert and A.W. Warner, J. Electron. Mater. 1 (1972) 155. [10] K. Raksányi et al.. to be published. [11] I.P. Alimanin, Ed., Metody Analizo Veskchesto Vysokoi [12] Chistoty F.B. Zajhovskij, (Nauka, Moscow Sovremennye 1965) p. 460. Metody Analiza Metallurgii (Metallurgizdat, Moscow 1955) p. 12. [13] C. Koch, Microchim. Acta 402 (1958). [14] R. Voszka et al., to be published.