DOI: 10.1021/cg100398k
Reduced Etch Pit Density of Rutile (TiO2) Single Crystals by Growth Using a Tilting-Mirror-Type Infrared Heating Image Furnace
2010, Vol. 10 3929–3930
Md. Abdur Razzaque Sarker,*,† Satoshi Watauchi,† Masanori Nagao,† Takashi Watanabe,‡ Isamu Shindo,‡ and Isao Tanaka† †
Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan, and ‡Crystal Systems Co., 9633-1 Kobuchisawa, Hokuto, Yamanashi 408-0044, Japan
Received March 25, 2010; Revised Manuscript Received July 8, 2010
ABSTRACT: Rutile single crystals were grown by the floating zone (FZ) method using a tilting-mirror-type image furnace. The tilting angle (θ) of the mirrors during the growth was changed from 0 to 10 and 20 to examine the effects of θ on crystal quality. The etch pit density (EPD) at the center of the crystals was low compared with that at the periphery. The EPDs at the periphery and center of the crystals grown at θ = 0 were (11-16) 104 and 5 104 cm-2, respectively. Both values significantly decreased with an increase in θ. In the crystal grown at θ = 20, the EPDs at the periphery and center were 7.9 104 and 1.5 104 cm-2, respectively. In particular, at the center of the crystals, the EPD for θ = 20 was 30% of that for θ = 0. This result suggests that the quality of rutile crystals can be improved by tilting the focusing mirrors used in crystal growth.
1. Introduction
2. Experimental Section
In the crystal growth by the floating zone (FZ) method, the molten zone is maintained mainly by the balance between the surface tension and the gravity of the melt. The FZ method is ideal for the crystal growth of crucible-reactive materials without any contamination because it is a crucible-free zone melting method. However, in the conventional FZ method using an infrared-heating image furnace, the solid-liquid interface during growth is usually convex for many materials because the molten zone is heated from the surface. A more convex crystal-melt interface might make the melt more unstable, especially during the growth of crystals with a larger diameter by the conventional FZ method. In the FZ method, the shape of the solid-liquid interface affects the density of crystal defects such as etch pits.1 In our recent study, it was found that the solid-liquid interface shape during the FZ growth of rutile crystals can be controlled by the tilting-mirror-type floating zone (TMFZ) method. The convexity h/r (h, height of the interface; r, radius of grown crystal) of the crystal-melt interface in the molten zone decreased from 0.55 at the tilting angle 0 to 0.19 at 20. Moreover, the molten zone was more stabilized at larger tilting angle.2 We expect that the crystal quality might be improved by the TMFZ method, but this was not clear. In the current study, we investigate the effects of tilting the mirrors on the defects of rutile single crystals. Rutile single crystals were grown at three different tilting angles. Each crystal was characterized by determining the etch pit density (EPD) because Hirthe et al. showed that each EPD was known to be a good measure of the dislocation density.3 Kinoshita et al.4 showed that etch pits arise in the presence of low-angle grain boundaries. The effects of tilting angle on the quality of rutile single crystals will be discussed. We also show whether the newly developed TMFZ technique is useful for controlling crystal defects.
As the starting material, rutile phase TiO2 powder of high purity (99.99%) was used for the feed preparation. The TiO2 powder was packed in a rubber tube using a long glass bar, shaped like a rod, and sealed. After sealing, the rod was pressed up to 3 108 Pa using a cold isostatic pressing machine (Nikkiso Co., Ltd.; model CL3-22-60). A hole on the rod was drilled, and the rod was tied with a Pt-Rh wire for hanging and then sintered at 1600 C for 12 h in oxygen gas flow. The sintered rod was typically 10-11 mm in diameter and 70-80 mm in length. The growth apparatus was a newly developed tilting-mirrortype image furnace (Crystal Systems Co.; model TLFZ-4000-H-VPO) with four mirrors. The tilting angle (θ), as shown Figure 1, can be changed up to 30. The upper part of each mirror is pruned away to facilitate mirror tilting. Therefore, light heating shows no horizontal mirror symmetry even at θ = 0, although the alignment of the molten zone and lamps shows horizontal mirror symmetry. The following growth parameters were used for each growth experiment; growth rate, 5 mm/h; feed supply rate, 5 mm/h; upper-shaft rotation rate, 3 rpm; lower-shaft rotation rate, 50 rpm; growth
*Corresponding author. E-mail:
[email protected]. r 2010 American Chemical Society
Figure 1. Schematic illustration of the tilting mirror type floating zone method. The definition of tilting angle (θ) is given. Published on Web 07/26/2010
pubs.acs.org/crystal
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Sarker et al.
Figure 3. Distributions of etch pit density as a function of radius position (X) of crystals grown at different θ values.
Figure 2. Optical microphotographs of etch pits on (100) of rutile single crystals grown at (a) θ = 0 and (b) θ = 20. [(A), (C) photographs at the center of the crystals; (B), (D) those at the periphery]. direction, Æ001æ using a rutile seed crystal; and growth atmosphere, CO2 flow (2 L/min). After seeding, the growth parameters were optimized manually during the first 2 h to achieve the optimally stabilized molten zone. The shaft rotation rates described above were empirically good to stabilize the molten zone. The θ values used were 0, 10, and 20. Rutile crystals of 9.5-10.5 mm in diameter and 4550 mm in length were obtained. For each growth, the molten zone became stable without manual operation until separation after the growth length reached 10 mm. Therefore, the analyzed region from 20 mm growth after the seeding point could be sufficient for the EPD analysis, although we did not measure the growth length dependence of EPDs. The grown crystals were cut perpendicular to the growth direction in the range 22-25 mm from the seeding point. Then the crystals were cut parallel to (100) and the surface was polished like a mirror. The polished samples were soaked in a mixture of (NH4)2SO4 and H2SO4 solutions (1:1 weight ratio) for 3 h at 300 C to etch their surfaces. The etch pits were observed by optical microscopy.
reduced by TMFZ growth. As the etch pits are associated with dislocations3 and grain boundaries,4 the quality of rutile crystals can be improved by tilting the mirrors. In our previous report, the convexity of the solid-liquid interface between the melt and the grown crystal decreased with increasing θ.2 A less convex interface was realized at θ = 20 than at θ = 0, which shows the same alignment of mirrors as that in the conventional experiments. An extremely convex interface is unfavorable for the growth of crystals with a small defect density because of the large number of defects caused by thermal stress.6 Kitamura et al. found that line defects propagate to the edge of a crystal if the solid-liquid interface is more convex toward the melt.1 Kinoshita et al. showed that low angle grain boundaries are markedly reduced when the solid-liquid interface during crystal growth is almost flat.4 A slightly convex interface toward the melt is better for improving the quality of the grown crystals.7 The slightly convex interface was achieved by tilting the mirror at θ = 20.2 The behavior of EPDs observed by our present experiment is consistent with these reported results. Therefore, we can conclude that the tilting angle of the mirrors used in crystal growth is an important factor for controlling defects in rutile crystals grown by the FZ method using an infrared heating image furnace. 4. Summary
3. Results and Discussion Figure 2 shows optical microphotographs of the samples on the (100) surface after chemical etching. The observed etch pits are clearly few in the crystal grown at θ = 20 compared with that at θ = 0, and etch pits are fewer at the center than the periphery. It was found that the number of etch pits depends on the radius position of the grown crystals and θ in the crystal growth. Figure 3 shows the EPDs on the (100) surface as functions of the radius position (X) of the crystals grown at various θ values. For all θ values, the EPDs at the center are lower than those at the periphery. A similar distribution of etch pits has been reported in Sn-doped PbTe single crystals grown by the Bridgman method.4 In our case, the observed EPDs of the crystal grown at θ = 0 ranged from (5 to 16) 104 cm-2, as shown in Figure 3. These values are not far from the reported EPDs of 7 104 cm-2 on the (110) plane for the rutile single crystals grown by the FZ method using a conventional image furnace with double ellipsoidal mirrors.5 As θ increased, the EPDs at both the periphery and center systematically decreased. The EPD at the center of the rutile crystals grown at θ = 20 is minimum with 1.5 104 cm-2, whereas that of the rutile crystals grown at θ = 0 is 5 104 cm-2. So EPD was reduced by 70% with TMFZ growth. The EPDs at the periphery for θ = 0 are (11-16) 104 cm-2, and those for θ = 20 are 7.9 104 cm-2. EPDs in the periphery are also
In this study, we showed that the etch pit density of rutile single crystals grown by the tilting-mirror-type floating zone (TMFZ) method significantly decreases with an increase in tilting angle. It is clear that the crystal quality can be improved by tilting the mirrors because the etch pits are associated with dislocations and grain boundaries. This is caused by the less convex crystal-melt interface based on our previous report about the θ dependence of the interface shape. Finally, TMFZ is an excellent movable-mirrors growth method for crystals with improved quality. Acknowledgment. This work was partially supported by a Sasagawa Science Foundation Program (No. 21-332) of the Japan Science Society (JSS) and a Grant-in-Aid for Scientific Research (C) (No. 20550173) of Japan Society for the Promotion of Science (JSPS).
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