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On the microdistribution of impurities in paratellurite single crystals
Journal of Crystal Growth 60 (1982) 195—198 North-Holland Publishing Company
195
LETFER TO THE EDITORS ON THE MICRODISTRIBUTION OF IMPURITIES IN PARATELLURITE SINGLE CRYSTALS L. MALICSKO Research Laboratory for Crystal Physics, Academy of Sciences of Hungary, P.O. Box 132, H-1502 Budapest, Hundary
and Th. KRAJEWSKI Institute for Solid State Physics and Electron Microscopy, Academy of Sciences of the GDR, P.O. Box 250, 401 Halle/Saale, German Dem. Rep.
Received 15 March 1982; manuscript received in final form 6 August 1982
Paratellurite single crystals grown by the Czochralski technique from extrapure melts and from melts doped with Mg or Fe, respectively, often contain visually imperfect regions. The content and the microdistribution of impurities were studied in a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) on selected samples prepared from visually perfect and imperfect regions of the crystals. Inclusions were found in the imperfect regions of crystals at which an enrichment of the dopant elements and other undesirable accompanying impurities could be detected.
Paratellurite single crystals have excellent acoustooptical properties which are of importance in different devices, such as deflectors, modulators, etc. The acoustooptical applications require very homogeneous and imperfection-free crystals. According to earlier investigations, the deterioration of the crystal quality is caused by the presence of different impurities in the starting materials which lead to an increasing dissolution of the crucible material. Because of the impurities, gas bubbles, inclusions and internal stresses around them appear within the crystals [1,2]. For the present investigation the paratellurite crystals were grown .by the Czochralski technique from extrapure melts and melts doped with 100 /Lmole/mole Mg or Fe in platinum crucibles [1,3]. As reported earlier, the presence of Pt impurities was detected by chemical means in the pure crystals as well as in the Fe doped samples [1]. In the imperfect regions gray colored stripes of inclusions were observed by optical microscopy, which were assumed to be platinum. The aim of the present investigations was to carry out electron beam microanalyses on the optical-microscopically ob0022-0248/82/0000—0000/$02.75
©
served defects in order to obtain further information on the role of the impurities in these defects, and to obtain a better understanding of the quality deterioration of the crystals. From the visually perfect and imperfect regions of the crystals with diameters of about 2.5 cm and lengths of about 6 cm, plates having a size of about 5 >< 5 X 0.5 mm3 were cut along and/or perpendicular to the [110] growth axes. The plates were then mechanically polished, cleaned from the polishing material by applying an ultrasonic field in water, and finally washed in diluted HC1 solution and in different organic solvents. After optical microscopic observations, selected samples were investigated in a JSM-U3 scanning electron microscope equipped with an EDAX-707A energy dispersive X-ray analyzer, and a JSM-35C equipped with a KEVEX-7000 analyzer. In the samples, defects and defect-free places surrounding the defects were analyzed in selected areas with scanning beam and/or with a fixed and well-focussed electron probe having a diameter of about 20 nm. The results were compared. From the peak intensities of the elements detected by
1982 North-Holland
196
L. Malicsk4 Th. Krajewski
I
/
Microdistribution of impurities in paratellurite
spots. In fig. 1 a typical example is shown. Two sphere shaped dark large and some small spots (as marked by arrow 1) as well as thread-like shaped inclusions (as marked by arrow 2) can be seen. .
‘3
*
The fact that these inclusions generally appear only in the lower parts of the crystals, suggests that the formation of these defects is connected with the impurities for which kett is smaller than one In the specimens prepared from the vtsually .
perfect upper parts of the crystals no defects were found by secondary electron (SE) or by backscattered (BE) electron imaging in the SEM. In the
1
•
X-ray energy spectra only the characteristic peaks of the tellurium were detected. Those defects, however which are observable by optical microscopy within or close to the surface of the specimens prepared from the visually imperfect regions of
3 be *
--——-
~
0 5 mm
*
Fig. 1. Transmission optical micrograph of sphere-shaped and thread-like shaped defects (as marked by arrows I and 2) within the imperfect region of a paratellurite crystal doped with g.
crystals could be well identified by the SE or BE imaging The majority of these defects proved to pits on the specimen surfaces; consequently, one may assume that they are gas inclusions (bubbles) within the original crystals. The spectra registered at the defects and at their surroundings have generally shown also characteristic peaks of further elements besides those of the Te. The elements detected in the imperfect regions of the
crystals are listed in the first column of table 1. The second column shows the intervals of the
EDS the concentrations of the elements in atomic percent (at%) could be computed according to a
standardless analysis (SLA)
occurring concentrations computed by SLA in at%. In the table the zero values of concentration refer
[41.
It is a well-known fact that the impurities,
present in the melt are incorporated into the crystal lattice in a concentration which depends on their effective segregation coefficient kett. The con
centration distribution of impurities along the growth axes of the crystals is expected to be uniform only in the case of impurities for which keit ~S equal to unity. According to the previous optical microscopic studies the upper parts of the crystals were generally free from visible defects. The inclusions, similar to those reported earlier [1,2], seemed to be formed rather within the central parts or only near to the bottom of the crystals. The inclusions of different dimensions appear in the optical microscope at transmitted illuminations as dark
Table I Intervals of concentrations (computed by SLA of EDS spectra) . . of detectable elements found in visually imperfect regions both of pure and Mg or Fe doped paratellurite single crystals
-_____________________________________________ Element Te ~
Concentration interval (at%) 6 8—99.5 0.1- 9.2
Al Si
0 —77.5 0 —48.2
~
0
K~ Cl
0 —10.6 0 6.0
Fe
0
17.2
—
1.4
197
L. Malicskô, Th. Krajewski / Microdistribution of impurities in paratellurite
____________________
_______
•
•.
•‘
_______
__________________________________________
.•.
•
••
,
•
~
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~
•~
___
~
•
~
~a
•_
.~
::~•::.:)
~• ______ _________
•
•
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____
.
~•~?~f:
•
~
‘.•
~
N •~
~
••~
•••
•41~~~~• ~
*
*
;•
•
~~;:__.‘
‘
‘
J~\:
~ ~
l~pni ~:
~
~
.~ ~•.
*
Fig. 2. A bubble intersected by the sample surface: (a) secondary electron micrograph; (b) X-ray mapping of the same area showing the enrichment of Pt impurities at the wall of the bubble (registered with the PtM line).
to those elements which did not give detectable peaks of their characteristic lines in either of the places analyzed (3a criterion). From these results it is clear that the visually imperfect regions of the crystals contain more impurities than the visually perfect parts. In connection with table 1, the following remarks are made: The Pt impurities, present in all cases, may originate from the dissolution of the Pt crucible during the growth processes [1].The presence of Cl should be a consequence of the preparation of the samples with HCI during which chlorides could be formed. The impurities, such as Al, Si, S, K, Mg and Fe can originate from the starting materials considering that Mg and Fe were also detected in undoped crystals. Especially, Si, Al and Mg have alreay been found by other authors in Te raw materials [5,6] from which the TeO2 starting material is generally synthesized. Comparing the concentrations of impurities detected on the given defects and on the defect-free places near the defects, one can observe an enrichment of some foreign elements of those listed in
table 1, especially Pt, Al, Si, S and K could be found near the defects. In a few cases the concentration increase at the defects was high enough to make visible the element distribution by X-ray mapping. This is demonstrated by a bubble intersected by the sample surface. On the SE micrograph of the bubble in fig. 2a dark spots at the bubble wall (see arrows) are visible. At the same places in fig. 2b the X-ray mapping using the PtM line shows the accumulation of Pt impurities in these regions. From the results one can conclude that the optical-microscopically visible defects (inclusions) have a close connection with the enrichment of impurities within the crystals. The steady presence of Pt at all defects suggests to assume that the Pt impurities may play a special role in the formation of impurity enrichments resulting in m.tcroscopically visible defects, which lead to the quality deterioration of the crystals. Further investigations to decide whether the impurity elements found form compounds, alloys or precipitates are in progress.
198
L. Malicskô, Th. Krajewski
/
Microdistribution of impurities in paratellurite
References
[11 1. Foldvhri, K. Rakshnyi, R. Voszka, E. Hartmann and A. Peter, J. Crystal Growth 52 (1981) 561. [21 J.G. Grabmaier, R.D. Plättner and M. Schieber, J. Crystal Growth 20 (1973) 82. [3] F. Schmidt and R. Voszka, Crystal Res. Technol. 16 (1981) K 127.
[4]
H. Yakowitz, R.L. Mykleibust and K.F.J. Heinrich,
FRAME — An On-Line Correction Procedure For Quantitative Electron Probe Microanalysis, NatI. Bar. Std., Tech. Note 796 (Washington, DC, 1973). [51 S. Miyazawa and H. Iwasaki, Japan. J. AppI. Phys. 9(1970) 441. [61 M. Cerclet, Mater. Res. Bull. 7 (1972) 721.
195
LETFER TO THE EDITORS ON THE MICRODISTRIBUTION OF IMPURITIES IN PARATELLURITE SINGLE CRYSTALS L. MALICSKO Research Laboratory for Crystal Physics, Academy of Sciences of Hungary, P.O. Box 132, H-1502 Budapest, Hundary
and Th. KRAJEWSKI Institute for Solid State Physics and Electron Microscopy, Academy of Sciences of the GDR, P.O. Box 250, 401 Halle/Saale, German Dem. Rep.
Received 15 March 1982; manuscript received in final form 6 August 1982
Paratellurite single crystals grown by the Czochralski technique from extrapure melts and from melts doped with Mg or Fe, respectively, often contain visually imperfect regions. The content and the microdistribution of impurities were studied in a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) on selected samples prepared from visually perfect and imperfect regions of the crystals. Inclusions were found in the imperfect regions of crystals at which an enrichment of the dopant elements and other undesirable accompanying impurities could be detected.
Paratellurite single crystals have excellent acoustooptical properties which are of importance in different devices, such as deflectors, modulators, etc. The acoustooptical applications require very homogeneous and imperfection-free crystals. According to earlier investigations, the deterioration of the crystal quality is caused by the presence of different impurities in the starting materials which lead to an increasing dissolution of the crucible material. Because of the impurities, gas bubbles, inclusions and internal stresses around them appear within the crystals [1,2]. For the present investigation the paratellurite crystals were grown .by the Czochralski technique from extrapure melts and melts doped with 100 /Lmole/mole Mg or Fe in platinum crucibles [1,3]. As reported earlier, the presence of Pt impurities was detected by chemical means in the pure crystals as well as in the Fe doped samples [1]. In the imperfect regions gray colored stripes of inclusions were observed by optical microscopy, which were assumed to be platinum. The aim of the present investigations was to carry out electron beam microanalyses on the optical-microscopically ob0022-0248/82/0000—0000/$02.75
©
served defects in order to obtain further information on the role of the impurities in these defects, and to obtain a better understanding of the quality deterioration of the crystals. From the visually perfect and imperfect regions of the crystals with diameters of about 2.5 cm and lengths of about 6 cm, plates having a size of about 5 >< 5 X 0.5 mm3 were cut along and/or perpendicular to the [110] growth axes. The plates were then mechanically polished, cleaned from the polishing material by applying an ultrasonic field in water, and finally washed in diluted HC1 solution and in different organic solvents. After optical microscopic observations, selected samples were investigated in a JSM-U3 scanning electron microscope equipped with an EDAX-707A energy dispersive X-ray analyzer, and a JSM-35C equipped with a KEVEX-7000 analyzer. In the samples, defects and defect-free places surrounding the defects were analyzed in selected areas with scanning beam and/or with a fixed and well-focussed electron probe having a diameter of about 20 nm. The results were compared. From the peak intensities of the elements detected by
1982 North-Holland
196
L. Malicsk4 Th. Krajewski
I
/
Microdistribution of impurities in paratellurite
spots. In fig. 1 a typical example is shown. Two sphere shaped dark large and some small spots (as marked by arrow 1) as well as thread-like shaped inclusions (as marked by arrow 2) can be seen. .
‘3
*
The fact that these inclusions generally appear only in the lower parts of the crystals, suggests that the formation of these defects is connected with the impurities for which kett is smaller than one In the specimens prepared from the vtsually .
perfect upper parts of the crystals no defects were found by secondary electron (SE) or by backscattered (BE) electron imaging in the SEM. In the
1
•
X-ray energy spectra only the characteristic peaks of the tellurium were detected. Those defects, however which are observable by optical microscopy within or close to the surface of the specimens prepared from the visually imperfect regions of
3 be *
--——-
~
0 5 mm
*
Fig. 1. Transmission optical micrograph of sphere-shaped and thread-like shaped defects (as marked by arrows I and 2) within the imperfect region of a paratellurite crystal doped with g.
crystals could be well identified by the SE or BE imaging The majority of these defects proved to pits on the specimen surfaces; consequently, one may assume that they are gas inclusions (bubbles) within the original crystals. The spectra registered at the defects and at their surroundings have generally shown also characteristic peaks of further elements besides those of the Te. The elements detected in the imperfect regions of the
crystals are listed in the first column of table 1. The second column shows the intervals of the
EDS the concentrations of the elements in atomic percent (at%) could be computed according to a
standardless analysis (SLA)
occurring concentrations computed by SLA in at%. In the table the zero values of concentration refer
[41.
It is a well-known fact that the impurities,
present in the melt are incorporated into the crystal lattice in a concentration which depends on their effective segregation coefficient kett. The con
centration distribution of impurities along the growth axes of the crystals is expected to be uniform only in the case of impurities for which keit ~S equal to unity. According to the previous optical microscopic studies the upper parts of the crystals were generally free from visible defects. The inclusions, similar to those reported earlier [1,2], seemed to be formed rather within the central parts or only near to the bottom of the crystals. The inclusions of different dimensions appear in the optical microscope at transmitted illuminations as dark
Table I Intervals of concentrations (computed by SLA of EDS spectra) . . of detectable elements found in visually imperfect regions both of pure and Mg or Fe doped paratellurite single crystals
-_____________________________________________ Element Te ~
Concentration interval (at%) 6 8—99.5 0.1- 9.2
Al Si
0 —77.5 0 —48.2
~
0
K~ Cl
0 —10.6 0 6.0
Fe
0
17.2
—
1.4
197
L. Malicskô, Th. Krajewski / Microdistribution of impurities in paratellurite
____________________
_______
•
•.
•‘
_______
__________________________________________
.•.
•
••
,
•
~
~t4’;~~/~i
~
•~
___
~
•
~
~a
•_
.~
::~•::.:)
~• ______ _________
•
•
%
____
.
~•~?~f:
•
~
‘.•
~
N •~
~
••~
•••
•41~~~~• ~
*
*
;•
•
~~;:__.‘
‘
‘
J~\:
~ ~
l~pni ~:
~
~
.~ ~•.
*
Fig. 2. A bubble intersected by the sample surface: (a) secondary electron micrograph; (b) X-ray mapping of the same area showing the enrichment of Pt impurities at the wall of the bubble (registered with the PtM line).
to those elements which did not give detectable peaks of their characteristic lines in either of the places analyzed (3a criterion). From these results it is clear that the visually imperfect regions of the crystals contain more impurities than the visually perfect parts. In connection with table 1, the following remarks are made: The Pt impurities, present in all cases, may originate from the dissolution of the Pt crucible during the growth processes [1].The presence of Cl should be a consequence of the preparation of the samples with HCI during which chlorides could be formed. The impurities, such as Al, Si, S, K, Mg and Fe can originate from the starting materials considering that Mg and Fe were also detected in undoped crystals. Especially, Si, Al and Mg have alreay been found by other authors in Te raw materials [5,6] from which the TeO2 starting material is generally synthesized. Comparing the concentrations of impurities detected on the given defects and on the defect-free places near the defects, one can observe an enrichment of some foreign elements of those listed in
table 1, especially Pt, Al, Si, S and K could be found near the defects. In a few cases the concentration increase at the defects was high enough to make visible the element distribution by X-ray mapping. This is demonstrated by a bubble intersected by the sample surface. On the SE micrograph of the bubble in fig. 2a dark spots at the bubble wall (see arrows) are visible. At the same places in fig. 2b the X-ray mapping using the PtM line shows the accumulation of Pt impurities in these regions. From the results one can conclude that the optical-microscopically visible defects (inclusions) have a close connection with the enrichment of impurities within the crystals. The steady presence of Pt at all defects suggests to assume that the Pt impurities may play a special role in the formation of impurity enrichments resulting in m.tcroscopically visible defects, which lead to the quality deterioration of the crystals. Further investigations to decide whether the impurity elements found form compounds, alloys or precipitates are in progress.
198
L. Malicskô, Th. Krajewski
/
Microdistribution of impurities in paratellurite
References
[11 1. Foldvhri, K. Rakshnyi, R. Voszka, E. Hartmann and A. Peter, J. Crystal Growth 52 (1981) 561. [21 J.G. Grabmaier, R.D. Plättner and M. Schieber, J. Crystal Growth 20 (1973) 82. [3] F. Schmidt and R. Voszka, Crystal Res. Technol. 16 (1981) K 127.
[4]
H. Yakowitz, R.L. Mykleibust and K.F.J. Heinrich,
FRAME — An On-Line Correction Procedure For Quantitative Electron Probe Microanalysis, NatI. Bar. Std., Tech. Note 796 (Washington, DC, 1973). [51 S. Miyazawa and H. Iwasaki, Japan. J. AppI. Phys. 9(1970) 441. [61 M. Cerclet, Mater. Res. Bull. 7 (1972) 721.