Effects of Temperature Gradient on Growth of Srb4o7 Crystals by the Micro-Pulling-Down Method

Trans. Mat. Res. Soc. Japan 42[5] 123-126 (2017)

Effects of temperature gradient on growth of SrB4O7 crystals by the micro-pulling-down method

Sho Inaba, Takaaki Machida, Harutoshi Asakawa, and Ryuichi Komatsu Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan e-mail: [email protected]

SrB4O7 (SBO) shows superior transparency in the vacuum ultraviolet (UV) region, a high radiation damage threshold, and high nonlinearity, and SBO crystals have therefore attracted great interest as materials for low-cost, high-power UV light sources. To use SBO crystals as laser sources, it is essential to grow transparent SBO crystals. Here we show the effects of temperature gradient G on the growth of SBO crystals by the micro-pulling-down method. At low G, SBO crystal fibers were obtained. The density of striations and the size of voids also decreased with decreasing G. These results demonstrate that a low G is essential for growing completely transparent SBO crystal fibers. In addition, we considered the effects of G on the oscillatory growth of SBO crystals in relation to latent heat removal. Key words: Strontium tetraborate, Micro-pulling-down method, Crystal growth, Striation, Void

1. INTRODUCTION 2. EXPERIMENTAL Micromachining technology requires low-cost, The μ-PD furnace system used in this study was the high-power ultraviolet (UV) light sources for various same as that in our previous studies [13, 14]. Fig. 1A applications, such as semiconductor lithography and laser shows the setup of our μ-PD furnace to grow SBO crystals. ablation [1]. Borate crystals, including β-BaB2O4 [2], The Pt crucible was heated by applying a voltage. To CsLiB6O10 [3], and Li2B4O7 [4], are promising as maintain the temperature of the SBO melt, the Pt crucible nonlinear optical crystals even in the deep UV region. The was surrounded with a lid (black arrowhead) and wall nonlinear properties of SrB4O7 (SBO) crystals in the deep (white arrowhead) of refractory materials. The UV region include wavelength conversion of femtosecond temperature near the nozzle of the Pt crucible was second-harmonic generation pulses down to 125 nm [5, 6]. confirmed by a thermocouple (black-and-white Furthermore, SBO crystals show superior transparency in arrowheads). SBO crystal powder was used as the starting the vacuum UV region, a high radiation damage threshold, material. <010> crystal bars from SBO crystals, which and high nonlinearity [7-9]. Hence, it is crucial to develop were obtained by the CZ method described in our previous a growth method to obtain completely transparent single study, were cut and used as seed crystals [11, 12]. Each SBO crystals. seed crystal was brought into contact with the melt drop at SBO crystals have been grown by the Czochralski (CZ) the nozzle and was then pulled down in the direction of method [10, 11]. In our previous study, we revealed that the white arrow. The growing crystal was observed in situ reducing the growth rate produces SBO crystals without by using an eyepiece lens (10×) installed beside the hot cloudiness [12]. However, completely transparent SBO zone. The crystal fibers were grown in air at a pulling rate crystals were still not obtained owing to the incorporation of 0.02 mm/min. of macroscopic opaque defects by the CZ method. Wide To control temperature gradient G in the μ-PD furnace, SBO crystals are not expected to be used as nonlinear the diameter Dh of the hole on the lid and the thickness Tw devices. of the wall were adjusted. The thermocouple Recently, we grew macroscopically transparent SBO (black-and-white arrowhead) was set below the nozzle crystal fibers by the micro-pulling-down (μ-PD) method, instead of the SBO crystals (Fig. 1B). Then, the which can produce crystal fibers easily [13]. To grow thermocouple was pulled along the direction of the black defect-free SBO crystal fibers, we focused on controlling arrow, and temperature was measured at distance x from the thermal diffusion, because the removal of the latent the bottom of the nozzle (Fig. 1C). The temperature heat of crystallization is generally the rate-determining profiles corresponded approximately to these in the SBO process in melt growth. In this study, we investigated the crystals. Data near x = 0 mm were fitted with a linear effects of the temperature gradient from the solid-liquid function. interface to the inside of the SBO crystals on the growth of SBO crystals by the μ-PD method.

123 124 Effects of temperature gradient on growth of SrB4O7 crystals by the micro-pulling-down method

Table 1. Temperature gradient near the nozzle and structure of the μ-PD furnace.

We first checked structure of all the grown crystals by powder XRD, and the results showed that these crystals were the SBO. Our previous study revealed that even when the composition in starting materials slightly changed, other phases such as SrB6O10 and SrB2O4 appeared [12]. Hence, we could confirm that the crystals grown by our system had the stoichiometric composition. Next, we visually determined the morphology of the SBO crystals grown at various G values (Fig. 2). For G = 97.8 and 104.4 °C/mm, the SBO crystals were irregular and turbid (Fig. 2A). With decreasing G, the SBO crystals became transparent although they were still irregular (Fig. 2B). Finally, when G = 56.4 °C/mm, transparent SBO crystal fibers were obtained (Fig. 2C). These results demonstrate that it is essential to use a low G to obtain SBO crystal fibers.

Fig. 1. Schematics of the μ-PD furnace. (A) μ-PD furnace. The black arrowhead shows the lid, the white arrowhead shows the wall of the refractory materials, and the black-and-white arrowheads show the thermocouples. The white arrow indicates the pulling-down direction of the seed crystals. (B) Temperature measured just below the nozzle of the μ-PD furnace. (C) Temperature profile (open circles) for Dh = 4 mm and Tw = 6 mm (see Table 1). The solid curve is a visual guide. The black dashed line shows temperature gradient G at x = 0.5 mm. Fig. 2. Effects of temperature gradient G on the external shape and transparency of SBO crystals. G = (A) 98 °C/mm, (B) 81 °C/mm, and (C) 56 °C/mm. The white 3. RESULTS AND DISCUSSION arrow indicates the pulling-down direction. The scale bar in (B) also applies to (A). Table 1 shows G for values of Dh and Tw. G was small, when Dh and Tw were small. Sho Inaba et al. Trans. Mat. Res. Soc. Japan 42[5] 123-126 (2017) 125

Table 1. Temperature gradient near the nozzle and To examine why the SBO crystals became transparent The errors of Lst in Fig. 4 correspond to the standard structure of the μ-PD furnace. at low G, we observed the surface of the SBO crystal deviations. Although the errors of Lst were large, Lst fibers by optical microscopy (Fig. 3). The SBO crystals increased with decreasing G. This result demonstrates that, contained voids (white arrowheads, Fig. 3A). The borate when SBO crystals are grown by the μ-PD method, G melt contained about 1000 ppm H2O as a volatile governs the oscillatory growth of SBO crystals. impurity; thus, these voids were formed by the evaporation of H2O [15]. Growth stripes, which are called striations (black arrowheads), were observed, irrespective of G. Generally, striations appear because of periodic changes in the normal growth rates, and networks of dislocations exist in We first checked structure of all the grown crystals by striations [15, 16]. Based on these insights, we concluded powder XRD, and the results showed that these crystals that the normal growth rate VG of SBO crystals fluctuated were the SBO. Our previous study revealed that even periodically under our growth conditions, although the when the composition in starting materials slightly average VG of the SBO crystals corresponded to the changed, other phases such as SrB6O10 and SrB2O4 pulling-down rate. Figs. 3B and C show the effect of G on appeared [12]. Hence, we could confirm that the crystals distance Lst between striations. Lst at G = 56 °C/mm was grown by our system had the stoichiometric composition. longer than that at G = 81 °C/mm. Next, we visually determined the morphology of the SBO crystals grown at various G values (Fig. 2). For G = 97.8 and 104.4 °C/mm, the SBO crystals were irregular and turbid (Fig. 2A). With decreasing G, the SBO crystals became transparent although they were still irregular (Fig. 2B). Finally, when G = 56.4 °C/mm, transparent SBO crystal fibers were obtained (Fig. 2C). These results demonstrate that it is essential to use a low G to obtain Fig. 4. Effects of temperature gradient G on the distance SBO crystal fibers. between striations, Lst.

We observed the cores of the SBO crystals grown at various G. Voids elongated in the pulling-down direction (white arrowheads) were observed, irrespective of G (Fig. 5). Because VG was the same as the floating rate of voids under our growth conditions, the elongated voids were incorporated, as in our previous study [13]. With decreasing G, the number of elongated voids decreased. In SBO crystals grown at G = 104 °C/mm, dendritic voids were observed, although the reason for this was unclear. When G = 81 °C/mm, elongated voids were periodically included (dashed lines) in the SBO crystals (Fig. 5B). The distance between the dashed lines, which show the edges of the voids in the pulling-down direction, was about 10 μm, and it corresponded to Lst (Fig. 4). This result demonstrates that the incorporation of voids was induced by oscillatory growth. When G = 56 °C/mm, the SBO crystals contained spherical and elongated voids (Fig. 5C). In addition, with decreasing G, the occupancy of voids in the pulling-down direction became small. These results suggest that growing SBO crystals at low G is essential for Fig. 1. Schematics of the μ-PD furnace. (A) μ-PD furnace. obtaining completely transparent SBO crystals. The black arrowhead shows the lid, the white arrowhead shows the wall of the refractory materials, and the black-and-white arrowheads show the thermocouples. The white arrow indicates the pulling-down direction of the seed crystals. (B) Temperature measured just below the nozzle of the μ-PD furnace. (C) Temperature profile (open Fig. 3. Micrographs of striations on SBO crystal surfaces circles) for Dh = 4 mm and Tw = 6 mm (see Table 1). The at temperature gradient G = (A and B) 81 °C/mm and (C) solid curve is a visual guide. The black dashed line shows 56 °C/mm. (A) and (B) are stereomicroscope and optical Fig. 2. Effects of temperature gradient G on the external temperature gradient G at x = 0.5 mm. microscope images, respectively. The white arrow shows shape and transparency of SBO crystals. G = (A) the pulling-down direction. The black-and-white 98 °C/mm, (B) 81 °C/mm, and (C) 56 °C/mm. The white arrowheads show striations and voids, respectively. L arrow indicates the pulling-down direction. The scale bar st 3. RESULTS AND DISCUSSION corresponds to the distance between striations. in (B) also applies to (A). Table 1 shows G for values of Dh and Tw. G was small, when Dh and Tw were small. We investigated the dependence of Lst on G (Fig. 4). 126 Effects of temperature gradient on growth of SrB4O7 crystals by the micro-pulling-down method

future work, we will directly observe the dislocations on the striations by X-ray topography and transmission electron microscopy.

4. CONCLUSIONS In this study, we investigated effects of temperature gradient G on the growth of SBO crystals by the μ-PD method. The external form, the surface, and the core of SBO crystals grown at different G were observed by optical microscopy. Then, we found that at low G, SBO crystal fibers are obtained, and growing SBO crystals at low G is crucial to obtain SBO crystals that contain no striations and voids. In addition, we considered the effects of G on the oscillatory growth of SBO crystals in relation to latent heat removal.

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