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Czochralski growth of thulium gallium garnets: Tm3(TmxGa2−x)Ga3O12
600
LETTER
Journal
TO THE EDITORS
CZOCHRALSKI A. BAERMANN,
GROWTH W. GUSE
Mi,lerulogisch-Petrogrtcphisches Received
of Crystal Growth 57 (1982) 600-602 North-Holland Publishing Company
10 August
OF THULIUM
GALLIUM
GARNETS:
Tm,(Tm,Ga,_,)Ga,O,,
and H. SAALFELD
Institut, Unioersitiit Humburg, 2 Hamburg 13. Fed. Rep. of Gerntcrq
198 I; manuscript
received in final form 14 January
1982
Large, optically-clear single crystals of Tms(Tm,Ga,_,)Ga,O,, (x=0.09-0.15) have been grown using the Czochralski method for the first time. The growth direction was (OOI]. X-ray topography and optical methods were used to check the quality of the single crystals. Lattice constants in comparison to the melt stoichiometry are discussed, spectrographic and refractive data are presented.
In Mossbauer spectroscopy with synchroton radiation, thulium gallium garnets (TGG) can be used as monochromator crystals. For this purpose there was a need of large TGG single crystals which could be obtained by the Czochralski technique. Due to its physical properties, TGG has been of high interest for many scientific programs. Despite its importance, however, the production of thulium gallium garnets has been reported by only a few authors. In all cases, only very small crystals (mm size) were produced by the flux method [l] and thin layers by solid solution reaction [2]. For our purpose it was necessary to grow large (cm size) thulium gallium garnets, which should be produced by the Czochralski method. This paper describes an investigation into the growth of TGG. Lattice constants and some properties of TGG are presented. Starting materials used were 4n gallium oxide (Alusuisse, Switzerland) and 5n thulium oxide (Alfa Division, Massachusetts, USA). The oxides were isostatically pressed into cylinders of about 150 g and sintered at 1700 K in plain air to allow filling of the crucible in one run. Growing garnets by the Czochralski method is a well known experimental procedure for many crystal growers and therefore there is no need to describe this method again. For details, see e.g. ref. [3]. The pulling chamber described in ref. [3] is 0022-0248/82/0000-0000/$02.75
0 1982 North-Holland
similar to the one we used to grow TGG. To minimize the weight loss of iridium [4], the crystal growth was performed under controlled gas atmosphere (240 l/h) of 95% nitrogen and 5% carbon dioxide. Lacking a suitable seed, the pulling process was started using a small Ir pipe. Crystal growth mainly occurred in [OOl]. The pull rate leading to crystals with lowest dislocation density was 3.5 mm/h with a crystal rotation of 16 rpm. Fig. 1 shows a polished crosssection (cut nearby the neck) of a TGG single crystal grown in [OOl] with a length of 30 mm and a biggest diameter in the cylindrical part of 15 mm. All crystals were coloured slightly green and were optically transparent.
Fig. I. Optically transparent crystal cut nearby the neck.
cross-section
of a TGG
single
601
A. Baermann et al. / Czochralski growth of thulium gallium garnets
The quality of the crystals was examined by microscopy and X-ray topography. Crystal slices of 1 mm thickness cut perpendicularly to the growth direction were viewed between crossed polarizers. All slices showed regions of stress birefringence caused by facets and in only a few cases by dislocations. Several slices were etched by a technique described in ref. [5]. The dislocation density was found to be < lOcm-*. A wafer of 1 mm thickness cut perpendicularly to [OOl] was examined by X-ray reflection topography. The topographs were taken with CuKcv radiation and a slightly non-parallel double crystal configuration. [OOl] and [llO] wafers were studied with the 888 reflection preceded by a 5 11 reflection from a (111) cut germanium crystal. Apart from facets, X-ray topographs showed a non-concentric development of the growth bands due to the angle between [OOl] and the growth direction up to 15”. Only a few dislocations were found. The high melting point of TGG (2 100 K) compared to other garnets like GGG (2000 K) favours the formation of gallium suboxides and as a result of chemical reactions, the growth of small Ir particles [6]. Therefore the grown crystals were covered by many Ir particles, which may be divided into two classes due to their probable formation: (1) Ir particles from the melt surface accumulated in a spiral arrangement. (2) The remaining part of the crystal surface is covered by smaller Ir particles which might be condensed directly from the gaseous phase. Only a comparable small Ir portion was found to be incorporated in the TGG crystal itself. All single crystals produced so far still show a slight spiral growth, although many runs were made to optimize the crystal growth. Special difficulties in growing TGG arose from two facts: (1) The relatively high melting point of TGG. (2) The relatively small radius of RE ion Tm3+ (0.994 A [7]). The first strongly influences the formation of Ir particles. The second is. responsible for an especially high increase of octahedral site occupation by Tm3+, which causes difficulties in keeping the stoichiometry of the melt. Both facts favour the unstable crystal growth of TGG.
Further progress in crystal growth (e.g. to influence the location of facets) might be possible. For our purpose, the construction of monochromization devices, the quality of the grown almost dislocation-free TGG single crystals is satisfying. A lattice constant of a, = 12.225 A for fluxgrown TGG crystals was reported by Schieber 1967 [l]. In 1971 Botdorf and McCarthy [8] measured a lattice constant of a, = 12.228 A. The TGG crystals described in this paper, which were grown from stoichiometric melts, showed a lattice constant of a, = 12.2540 ? 0.0018 A (measured by an automatic single crystal diffractometer and data refined by a LSQ program). Brandle and Barns [9] reported on a defined increase of octahedral site occupation by rare earth ions with decreasing ionic radii. Dy,Ga,O,,, for example, contained an excess of dysprosium resulting in an x-value 0.05, and they predicted that Lu,Ga,O,, should contain the largest amount of octahedrally-arranged RE ions leading to an x-value of approximately 0.15-0.20. Using the equations of Brandle and Barns the moles of rare earth on octahedral sites are determinable. For thulium we calculated an x-value of 0.12 using Shannon’s radii [7]. This value is in good agreement to the measured value of x = 0.14 (electron microprobe analysis). The calculated and measured x-values exhibit the shift of the congruent melting composition away from the stoichiometric 3 : 5 ratio. Fig. 2 shows an optical transmission spectrum
60
I%1
0e
50 LO
I 30 20
10
Fig. 2. Optical transmission 300-2400 nm.
spectrum
h lnml
of TGG in the region of
A. Baermann
602
320
400
500 T-
660
1000
_
et al. / Czochralski
_~_.
2000
growth
of thulium
gallium
garnets
~_
Fig. 3. Dispersion curve of TGG (320-2100 nm). Open circles demonstrate values measured by the prism method.
ranging from 300-2400 nm. In the area from 400 to 750 nm, there are a few sharp absorption bands resulting from 4f electron levels. The slightly green colour finds its explanation in the sum of these sharp absorption bands. The refractive index was measured in the range from 320 nm to - 2100 nm by a refractive index spectrograph. Additional data are given in the range from 400 to 660 nm measured by the more exact prism method. These values are shown by open circles in fig. 3. In fig. 4 the dispersion curves of Tm,Ga,O,, (the small excess of Tm,O, in the garnet can be neglected) are given in comparison with some other garnets. The curves demonstrate clearly the dependence of the refractive index upon RE ionic radii. Density measurements correspond to the calculated values and show the influence of host lattice expansion. A density of Dexp = 7.636 g/cm3 was found for Tm,(Tm,,,Ga,,,,)Ga,O,,. The calculated density comes to Dca, = 7.634g/cm3. The Vickers mIcrohardness on [OOl] revealed an average value of 1202 +_80 kP/mm*. Large, almost dislocation-free thulium gallium garnets were successfully grown using the Czochralski method for the first time. Crystal growth was strongly affected by the formation of Ir particles and an increasingly high octahedral
Fig. 4. Dispersion of 400-650 nm.
curves of some RE Ga garnets
in the region
site occupation of Tm3’ . The grown single crystals are the basis for the construction of monochromization devices. The authors wish to thank the Deutsche Forschungsgemeinschaft for financial support and the Philips Forschungslabor Hamburg for providing X-ray topographs and the use of the refractive index spectrograph. W.G. wishes to thank Dr.D. Mateika for helpful discussions.
References 111M. Schieber, Kristall Tech. 2 (1967) 55. 121L. Suchow and M. Kokta, J. Solid State Chem. 5 (1972) 85. 131 W. Guse and D. Mateika, [41 D. Mateika
J. Crystal Growth 22 (1974) 237. and Ch. Rusche, J. Crystal Growth 42 (1977)
440. [51 D.C. Miller,
J. Electrochem. Sot., Solid-State Sci. Technol. 120 (1973) 678. D.C. Miller and J.W. Nielsen, J. Crystal [61 C.D. Brandle, Growth 12 (1972) 195. 171 R.D. Shannon, Acta Cryst. A32 (1976) 75 1. in: JCPDS International Centre for PI Botdorf and McCarthy, Diffraction Data (1980). 191 C.D. Brandle and R.L. Barns, J. Crystal Growth 26 (1974) 169.
LETTER
Journal
TO THE EDITORS
CZOCHRALSKI A. BAERMANN,
GROWTH W. GUSE
Mi,lerulogisch-Petrogrtcphisches Received
of Crystal Growth 57 (1982) 600-602 North-Holland Publishing Company
10 August
OF THULIUM
GALLIUM
GARNETS:
Tm,(Tm,Ga,_,)Ga,O,,
and H. SAALFELD
Institut, Unioersitiit Humburg, 2 Hamburg 13. Fed. Rep. of Gerntcrq
198 I; manuscript
received in final form 14 January
1982
Large, optically-clear single crystals of Tms(Tm,Ga,_,)Ga,O,, (x=0.09-0.15) have been grown using the Czochralski method for the first time. The growth direction was (OOI]. X-ray topography and optical methods were used to check the quality of the single crystals. Lattice constants in comparison to the melt stoichiometry are discussed, spectrographic and refractive data are presented.
In Mossbauer spectroscopy with synchroton radiation, thulium gallium garnets (TGG) can be used as monochromator crystals. For this purpose there was a need of large TGG single crystals which could be obtained by the Czochralski technique. Due to its physical properties, TGG has been of high interest for many scientific programs. Despite its importance, however, the production of thulium gallium garnets has been reported by only a few authors. In all cases, only very small crystals (mm size) were produced by the flux method [l] and thin layers by solid solution reaction [2]. For our purpose it was necessary to grow large (cm size) thulium gallium garnets, which should be produced by the Czochralski method. This paper describes an investigation into the growth of TGG. Lattice constants and some properties of TGG are presented. Starting materials used were 4n gallium oxide (Alusuisse, Switzerland) and 5n thulium oxide (Alfa Division, Massachusetts, USA). The oxides were isostatically pressed into cylinders of about 150 g and sintered at 1700 K in plain air to allow filling of the crucible in one run. Growing garnets by the Czochralski method is a well known experimental procedure for many crystal growers and therefore there is no need to describe this method again. For details, see e.g. ref. [3]. The pulling chamber described in ref. [3] is 0022-0248/82/0000-0000/$02.75
0 1982 North-Holland
similar to the one we used to grow TGG. To minimize the weight loss of iridium [4], the crystal growth was performed under controlled gas atmosphere (240 l/h) of 95% nitrogen and 5% carbon dioxide. Lacking a suitable seed, the pulling process was started using a small Ir pipe. Crystal growth mainly occurred in [OOl]. The pull rate leading to crystals with lowest dislocation density was 3.5 mm/h with a crystal rotation of 16 rpm. Fig. 1 shows a polished crosssection (cut nearby the neck) of a TGG single crystal grown in [OOl] with a length of 30 mm and a biggest diameter in the cylindrical part of 15 mm. All crystals were coloured slightly green and were optically transparent.
Fig. I. Optically transparent crystal cut nearby the neck.
cross-section
of a TGG
single
601
A. Baermann et al. / Czochralski growth of thulium gallium garnets
The quality of the crystals was examined by microscopy and X-ray topography. Crystal slices of 1 mm thickness cut perpendicularly to the growth direction were viewed between crossed polarizers. All slices showed regions of stress birefringence caused by facets and in only a few cases by dislocations. Several slices were etched by a technique described in ref. [5]. The dislocation density was found to be < lOcm-*. A wafer of 1 mm thickness cut perpendicularly to [OOl] was examined by X-ray reflection topography. The topographs were taken with CuKcv radiation and a slightly non-parallel double crystal configuration. [OOl] and [llO] wafers were studied with the 888 reflection preceded by a 5 11 reflection from a (111) cut germanium crystal. Apart from facets, X-ray topographs showed a non-concentric development of the growth bands due to the angle between [OOl] and the growth direction up to 15”. Only a few dislocations were found. The high melting point of TGG (2 100 K) compared to other garnets like GGG (2000 K) favours the formation of gallium suboxides and as a result of chemical reactions, the growth of small Ir particles [6]. Therefore the grown crystals were covered by many Ir particles, which may be divided into two classes due to their probable formation: (1) Ir particles from the melt surface accumulated in a spiral arrangement. (2) The remaining part of the crystal surface is covered by smaller Ir particles which might be condensed directly from the gaseous phase. Only a comparable small Ir portion was found to be incorporated in the TGG crystal itself. All single crystals produced so far still show a slight spiral growth, although many runs were made to optimize the crystal growth. Special difficulties in growing TGG arose from two facts: (1) The relatively high melting point of TGG. (2) The relatively small radius of RE ion Tm3+ (0.994 A [7]). The first strongly influences the formation of Ir particles. The second is. responsible for an especially high increase of octahedral site occupation by Tm3+, which causes difficulties in keeping the stoichiometry of the melt. Both facts favour the unstable crystal growth of TGG.
Further progress in crystal growth (e.g. to influence the location of facets) might be possible. For our purpose, the construction of monochromization devices, the quality of the grown almost dislocation-free TGG single crystals is satisfying. A lattice constant of a, = 12.225 A for fluxgrown TGG crystals was reported by Schieber 1967 [l]. In 1971 Botdorf and McCarthy [8] measured a lattice constant of a, = 12.228 A. The TGG crystals described in this paper, which were grown from stoichiometric melts, showed a lattice constant of a, = 12.2540 ? 0.0018 A (measured by an automatic single crystal diffractometer and data refined by a LSQ program). Brandle and Barns [9] reported on a defined increase of octahedral site occupation by rare earth ions with decreasing ionic radii. Dy,Ga,O,,, for example, contained an excess of dysprosium resulting in an x-value 0.05, and they predicted that Lu,Ga,O,, should contain the largest amount of octahedrally-arranged RE ions leading to an x-value of approximately 0.15-0.20. Using the equations of Brandle and Barns the moles of rare earth on octahedral sites are determinable. For thulium we calculated an x-value of 0.12 using Shannon’s radii [7]. This value is in good agreement to the measured value of x = 0.14 (electron microprobe analysis). The calculated and measured x-values exhibit the shift of the congruent melting composition away from the stoichiometric 3 : 5 ratio. Fig. 2 shows an optical transmission spectrum
60
I%1
0e
50 LO
I 30 20
10
Fig. 2. Optical transmission 300-2400 nm.
spectrum
h lnml
of TGG in the region of
A. Baermann
602
320
400
500 T-
660
1000
_
et al. / Czochralski
_~_.
2000
growth
of thulium
gallium
garnets
~_
Fig. 3. Dispersion curve of TGG (320-2100 nm). Open circles demonstrate values measured by the prism method.
ranging from 300-2400 nm. In the area from 400 to 750 nm, there are a few sharp absorption bands resulting from 4f electron levels. The slightly green colour finds its explanation in the sum of these sharp absorption bands. The refractive index was measured in the range from 320 nm to - 2100 nm by a refractive index spectrograph. Additional data are given in the range from 400 to 660 nm measured by the more exact prism method. These values are shown by open circles in fig. 3. In fig. 4 the dispersion curves of Tm,Ga,O,, (the small excess of Tm,O, in the garnet can be neglected) are given in comparison with some other garnets. The curves demonstrate clearly the dependence of the refractive index upon RE ionic radii. Density measurements correspond to the calculated values and show the influence of host lattice expansion. A density of Dexp = 7.636 g/cm3 was found for Tm,(Tm,,,Ga,,,,)Ga,O,,. The calculated density comes to Dca, = 7.634g/cm3. The Vickers mIcrohardness on [OOl] revealed an average value of 1202 +_80 kP/mm*. Large, almost dislocation-free thulium gallium garnets were successfully grown using the Czochralski method for the first time. Crystal growth was strongly affected by the formation of Ir particles and an increasingly high octahedral
Fig. 4. Dispersion of 400-650 nm.
curves of some RE Ga garnets
in the region
site occupation of Tm3’ . The grown single crystals are the basis for the construction of monochromization devices. The authors wish to thank the Deutsche Forschungsgemeinschaft for financial support and the Philips Forschungslabor Hamburg for providing X-ray topographs and the use of the refractive index spectrograph. W.G. wishes to thank Dr.D. Mateika for helpful discussions.
References 111M. Schieber, Kristall Tech. 2 (1967) 55. 121L. Suchow and M. Kokta, J. Solid State Chem. 5 (1972) 85. 131 W. Guse and D. Mateika, [41 D. Mateika
J. Crystal Growth 22 (1974) 237. and Ch. Rusche, J. Crystal Growth 42 (1977)
440. [51 D.C. Miller,
J. Electrochem. Sot., Solid-State Sci. Technol. 120 (1973) 678. D.C. Miller and J.W. Nielsen, J. Crystal [61 C.D. Brandle, Growth 12 (1972) 195. 171 R.D. Shannon, Acta Cryst. A32 (1976) 75 1. in: JCPDS International Centre for PI Botdorf and McCarthy, Diffraction Data (1980). 191 C.D. Brandle and R.L. Barns, J. Crystal Growth 26 (1974) 169.