Czochralski growth of neodymium-doped yttrium orthoaluminate

Czochralski growth of neodymium-doped yttrium orthoaluminate

Journal of Crystal Growth 20 (1973) 7 1—72 North-Holland Publishing Co. CZOCHRALSKI GROWTH OF NEODYMIUM-DOPED YTTRIUM ORTHOALUMINATE P. KORCZAK and ...

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Journal of Crystal Growth 20 (1973) 7 1—72

North-Holland Publishing Co.

CZOCHRALSKI GROWTH OF NEODYMIUM-DOPED YTTRIUM ORTHOALUMINATE P. KORCZAK and C. B. STAFF AEG-Teletlinken Forschungsinstitur, 6 Frank/urt am Main 71, Germany Received 12 April 1973; revised manuscript received 26 June 1973 Single crystals of neodymium-doped yttrium orthoaluminate have been grown by the Czochralski method. Growth parameters and the neodymium distribution coefficient are discussed and compared with those of yttrium aluminium garnet. ParticLilar attention is placed on the occurrence of cracking and twinning in yttrium orthoaluminate.

Yttrium orthoaluminate (YAIO3) has received increasing interest as laser host material, having the advantage of a higher distribution for have neo1’2), although nocoefficient actual values dymium than YAG yet been reported. Additionally the anisotropic optical nature of YAIO 3 allows optimisation of laser properties by suitable choice of3).the crystallographic orientation lasera rod Asof yettheonly few axis details have been published con cerning single crystal growth and related problems of cracking and twinning in yttrium oithoaluminate1 ~ The purpose of our work was to investigate in greater detail the growth behavior of this laser material. We have grown Nd-doped YAIO 3 single crystals by the Czochralski-pulling technique. The component powders Y203, Al203 and Nd203 (supplied by KochLight Lab. and Rare Earth Prod, of quality 4—5 N) were melted in an iridium crucible under inert nitrogen atmosphere.

was observed. From carefully orientated seeds in the directions <001> and <010> (orthorhombic c- and bsingle good optical quality(fig. up I). to 7axis cmresp.) in length andcrystals 2 cm inof diameter were grown Growth parameters covered the ranges of 2—10 mm/h pulling rate and 10—30 rpm rotation rate for doping levels of 0.6—1.0 at~/,neodymium. These transparent,

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Neodymium-doped yttrium orthoaluminate single crys-

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red-brown crystals generally exhibit a core of less than I mm diameter, somewhat smaller than that of corn8). parable YAG crystals An effective distribution coefficient for Nd in YAIO 3 of k = 0.65 was determined by electron microprobe analysis (Nd L~xemission), to be approximately four times greater than that for Nd in YAG*. The axial gradient doping a 10 ~< increase in over the concentration total length ofcorresponds the crystal.toNeodymium contents in the core region are about l5% higher than in the surrounding area. However, these distribu8), as tions are more homogeneous than those of YAG one would expect from a higher value of k for Nd in

In agreement with Weber et al.i) we observed pyrometrically the melting point of undoped YAIO3 to be 1870 ±10 °C.Since X-ray powder patterns of crushed boules exhibited only the perovskite 5), YAIO3 congruent melting.phase Thiswe is assume, as Abell et al. contrary to observations of Antonov et al.6) and the phase diagram published by Mizuno et al.7), which clearly shows a decomposition of YAIO 3 below the melting point, During preliminary work to obtain seed material by dipping an iridium rod into the melt preferential crystal growth in the <001> direction (orthorhombic c-axis)

measurements the distribution for NdAccording in YAGto isour 0.16, in good agreement with coefficient recent publications8’9). *

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P. KORCZAK AND C. B. STAFF

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Fig. 2.

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Microt~~inriing and crack line in~ttriumorihoaluminate.

In addition, samples from cracked crystals were examined optically after etching for 10 mm in boiling l-13P04. Cracking originating from microtwinning following the { I l0} twin plane is clearly evident as seen in fig. 2. This {l l0} type twinning can be understood from interchange1between 0) the nearly identical a and h lattice We therefore parametersbelieve that faster pulling rates enhance the formation of microtwins and this coupled with anisotropic thermal expansion behaviour leads to cracking of the boules. However, it remains that even with restrictions in pulling rates of under 5 mm/h, YA1O 3 can be grown significantly faster than YAG.

YA1O3. We can therefore reasonably expect to apply higher pulling rates to YAIO3, without introducing detrimental optical defects into the crystal originating from constitutional supercooling. However, the faster grown boules (more than 5 mm/h) exhibit an increasing tendency to crack during cooling, independent of the cooling rate.

The authors wish to thank Dr. H. Hoppert (CRL Electronic Bauelemente, Germany) for his work on dilatometric measurements. We are also grateful to Dr. M. Reese for providing chemical analyses and M. Gauntlett for careful X-ray investigations on crystal samples. References

We investigated this cracking problem by dilatome-

tric measurements up to 1550°Con orientated single crystal samples (in orthorhombic a-, b- and c-axes). No anomalous thermal expansion behaviour or phase instabilities were detected. This is in contradiction to a recent observation on polycrystalline samples, where 5). phase was detected by X-ray methods We decomposition observed, however, that between room temperature and 900 °Cthe thermal expansion coefficient in the c-axis (11.7 x 106/°C) is more than twice of its value in the a- and b-axis (5.1 x l06/°C and 4.2x 106/°C,respectively clearly indicating anisotropic expansion in the bulk material.

I) M. J. Weber, M. Bass and K. Andringa, AppI. Phys. Letters 342. 2) 15 K. (1969) S. Bagdasarov, A. A. Kaminskii and G. I. Rogov, Soviet Phys.-Dokl. 14 (1969) 346. 3) 4) 5)

G. A. Massey, IEEE J. Quantum Electron. QE-8 (1972) 669. W. Class, J. Crystal Growth 3/4 (1968) 241. J. S. Abell, (1972) 1088. I. R. Harris and B. Cockayne, J. Mater. Sci. 7

6) V. A. Antonov, P. A. Arsenev, I. G. Linda and V. L. Farshtendiker, Phys. Status Solidi (a) 15 (1973) K 63. 7) M. Mizuno and T. Noguchi, Rept. Govt. md. Res. Inst. Nagoya 16(1967) 210. 8) R. F. Belt, R. C. Puttbach and D. A. Lepore, J. Crystal Growth 13/14 (1972) 268. 9) R. R. Monchamp, J. Crystal Growth 11(1971) 310. 10) 5. Geller and E. A. Wood, Acta Cryst. 9 (1956) 563.