научная статья по теме CHALLENGE TO GROW REFRACTORY RARE-EARTH SESQUIOXIDES: APPLICATION OF THE MICRO-PULLING-DOWN METHOD Химия

Текст научной статьи на тему «CHALLENGE TO GROW REFRACTORY RARE-EARTH SESQUIOXIDES: APPLICATION OF THE MICRO-PULLING-DOWN METHOD»

HEOPTAHHHECKHE MATEPHAMbI, 2007, moM 43, № 7, c. 824-829

UDC 548.55

Challenge to Grow Refractory Rare-Earth Sesquioxides: Application of the Micro-Pulling-Down Method

© 2007 r. A. Novoselov, J. H. Mun, R. Simura, A. Yoshikawa, T. Fukuda

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

e-mail: anvn@tagen.tohoku.ac.jp Received 17.11.2006

The RE2O3 (RE = Y, Lu, Sc) sesquioxides are promising host materials for solid-state lasers owing to their low phonon energy and high thermal conductivity. Our results demonstrate that the micro-pulling-down method is a viable approach to the single-crystal growth of refractory rare-earth sesquioxides with melting points over 2400°C. The method yields chemically homogeneous single-crystal rods of high crystallinity. We also present thermal conductivity data for Yb-doped Y2O3, Lu2O3, and Tm-doped Y2O3.

INTRODUCTION

To produce a laser, three main elements are needed: a laser host, active ion, and optical pump source. These elements are interdependent and must be selected carefully to achieve the desired performance [1]. Advances in InGaAs high-power laser diodes emitting between 900 and 990 nm have renewed interest in the Yb3+ activator ion, which first attracted attention in the mid-1970s with the advent of flash-lamp pumped Yb-doped Y3Al5O12 (YAG) [2]. Yb3+ possesses many advantages because of its simple electronic structure, involving only two states, the 2F5/2 excited state and 2F7/2 ground state, associated with spin-orbit coupling. Since there is only one 2F5/2 excited state, there is no excited-state absorption, cross-relaxation, upconversion, or any other internal mechanism that might otherwise impede population inversion and reduce the effective laser cross section in crystals without other impurities. The unique advantage of Yb3+ is its broad absorption band (about ten times broader than that of Nd3+), which is ideally suited for diode pumping. The low thermal load in Yb3+-doped crystals is another feature important for highpower applications [3, 4].

Since the laser performance of Yb-doped YAG strongly depends on temperature [5], new single-crystal hosts, combining the attractive mechanical and optical properties of YAG and surpassing it in thermal properties, are needed. These requirements are met by the rare-earth sesquioxides Y2O3, Lu2O3, and Sc2O3. They possess high thermal conductivity (13.6, 12.5, and 16.5 W/(m K) in undoped yttria, lutetia, and scandia, respectively), which notably exceeds that of un-doped YAG: 11 W/(m K). Doping with 3 at % ytterbium reduces the thermal conductivity of yttria and scandia to 7.7 and 6.6 W/(m K), respectively, whereas the thermal conductivity of lutetia decreases only slightly, to 11.0, in contrast to that of YAG, which drops to 6.8 W/(m K) [6]. The rare-earth sesquioxides under consideration are characterized by a wide transparency

region, e.g., from 0.23 to 8 |m in Y2O3, which is wider than that of YAG (0.24-6 |m). The effective phonon energy in Y2O3, Lu2O3, and Sc2O3 is quite low in comparison with other oxides. A low phonon energy means low rates of nonradiative transitions between metasta-ble electronic levels of the luminescent ions incorporated in the lattice and, therefore, high radiative probabilities and high quantum efficiencies of their optical transitions.

The crystal structure of Y2O3, Lu2O3, and Sc2O3 is another attractive feature of these oxides. In general, isotropic crystals are very attractive for optical applications and can be grown easily because there is no anisotropy in their thermal properties. At room temperature, Y2O3, Lu2O3, and Sc2O3 have the C-type (bixby-ite) structure (bcc, sp. gr. Ia3, 32 cations and 48 oxygens (16 formula units) per unit cell) [7]: each rare-earth cation is situated in the center of a distorted cube, with six of the eight cube corners occupied by oxygens. The unit cell contains 24 sites of C2 symmetry and 8 sites of C3i symmetry. The lattice parameter is about 10 A and depends on the cation: 9.846 (Sc2O3), 10.391 (Lu2O3) and 10.602 A (Y2O3) [8]. The cubic structure is stable up to the melting point, except for Y2O3, which undergoes a transition to the high temperature, hexagonal structure near the melting point [9].

At the same time, there is one, but important problem with these very attractive laser hosts: there is still no mature process for the growth of bulk single crystals of refractory rare-earth sesquioxides. A variety of crystal growth techniques have been applied to date, such as Verneuil, Czochralski, edge-defined film-fed growth, Bridgman, heat exchanger, floating zone, and laser heated pedestal processes, but the grown crystals were of limited size or insufficient optical quality [6, 10-15]. Note also recent work on transparent Yb-doped Y2O3 ceramics prepared by vacuum sintering-the only method so far that enables production of large-scale laser

media of acceptable quality based on rare-earth sesqui-oxides [16].

In this paper we show that single crystals of refractory rare-earth sesquioxides can be grown by the micro-pulling-down (|-PD) method, which allows the production of relatively large single-crystal rods of high optical quality, well suited for laser applications [17].

EXPERIMENTAL

To grow crystals of refractory rare-earth sesquioxides by the |-PD method, some changes to the standard experimental setup were made. The high melting point (above 2400°C) of the materials in question restricts the choice of the crucible material. The crucible must be stable at temperatures well above the melting point of the growing material and must be nonreactive with the melt. Therefore, platinum and iridium, routinely used in | -PD experiments, are unsuitable for our purposes. In view of this, we used a crucible of rhenium, which melts at 3180°C [11]. Rhenium is sufficiently resistant to molten rare-earth sesquioxides; its only drawback is its very high reactivity with oxidizing agents that may be present in the growth atmosphere and with ceramic insulation. For this reason, all growth experiments were conducted in high-purity (99.999%) argon mixed with 3-4 vol % H2, which was found to prevent the crucible from oxidation. A rhenium crucible 46 mm in height and 30 mm in outer diameter, covered with a rhenium lid, was put on a rhenium afterheater mounted on a zir-conia pedestal. To obtain a nearly isothermal temperature distribution, the crucible was surrounded by zirco-nia thermal insulation located in a vertical quartz tube and was heated using an rf generator. The raw material was melted and allowed to pass through a capillary mi-cronozzle in the crucible bottom. Crystal growth was initiated by bringing a seed into contact with the crucible tip and slowly pulling it down with a constant velocity. The meniscus and growing crystal were observed using a CCD camera. The crystal diameter was kept constant by adjusting the applied rf power and growth rate. The obtained phase was identified by powder X-ray diffraction (XRD) at room temperature in air on a Rigaku Rint Ultima diffractometer, operated at 40 kV and 40 mA (20 = 10°-80°, step size of 0.02°). Crystallinity was investigated by X-ray rocking curve (XRC) analysis of the 222 diffraction peak, using a multipurpose advanced thin-film X-ray system ATX-E and extrahigh-resolution Rigaku diffractometer (highpower rotating-anode X-ray source, incident-beam four-crystal Ge (220) monochromator, multilayer X-ray mirror). Crystallinity was evaluated from the full width at half maximum (FWHM) of the peak. Chemical composition was checked by electron probe microanalysis (EPMA) with a JEOL JXA-8621MX. The cation distribution was measured along the growth axis using a 10-|m electron probe.

Thermal conductivity was calculated from the thermal diffusivity, heat capacity, and density data obtained in

this study. Thermal diffusivity was measured by a laser flash method with an Nd glass laser (1060 nm) and InSb IR detector under vacuum [18]. Each crystal was cut into 4.2 x 5 x 0.5 mm wafers, and their surface was sputter-coated with gold to prevent laser light penetration. Heat capacity was determined by differential scanning calorimetry between room temperature and 823 K [19]. The measurements were carried out in a high-purity (99.999%) argon atmosphere at a flow rate of 20 ml/min, using 30-mg samples. The heat capacity of a-Al2O3 was used as a reference value.

RESULTS AND DISCUSSION

First, we used Y2O3 in our |-PD growth experiments because it combines good host crystal properties with a relatively low price of high-purity (4N) powder. Availability of a seed crystal is an important point in crystal growth. Fiber crystals grown using seeds cut from Czochralski boules are most commonly used as seeds in |-PD experiments. In the case of the refractory rare-earth sesquioxides, however, no seed crystals were available, so a piece of high-purity yttria ceramic was used as a seed to grow the first yttria crystals (Fig. 1). Those crystals were used, without removing the ceramic, for seeding in subsequent experiments with Y2O3 and Lu2O3, and (Yb-Y - x)2O3 (x = 0, 0.005, 0.01, 0.05, 0.08, 0.15, 0.20, 0.30, 0.50) single crystals were grown. The best quality crystals were obtained at a pulling rate of 0.1 mm/min. The crystals were 4.2 mm in diameter and 10-20 mm in length (Fig. 2). There was some instability in crystal diameter, but their surface was rather smooth. Rhenium is well wetted by molten yttria, and the melt may flow out of the crucible, making it difficult to control the growth process. The Yb-doped crystals were gray-blue, with a color intensity dependent on Yb content. In earlier studies, the blue coloration of Yb-doped YAG crystals grown in a neutral (argon or nitrogen) atmosphere, which might well be considered reducing at high temperatures, was ascribed to the reduction Yb3+ —- Yb2+ [20]. In our case, the growth atmosphere was strongly reducing, and the appearance of Yb2+ ion

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