Study on Inclusions in Large Sapphire Optical Crystal Grown by SAPMAC Method

Transcription

1 Chinese Journal of Aeronautics 19(2006) S31-S35 Chinese Journal of Aeronautics Study on Inclusions in Large Sapphire Optical Crystal Grown by SAPMAC Method WANG Gui-gen, ZHANG Ming-fu*, ZUO Hong-bo, HE Xiao-dong, HAN Jie-cai Center for Composite Structure, Harbin Institue of Technology, Harbin , China Received 10 August 2006; accepted 6 November 2006 Abstract The sapphire (Al 2 O 3 ) single crystal is a kind of excellent infrared transmission window materials. A large-sized sapphire (Φ225 mm 205 mm, 27.5 kg) was grown by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at cooled center). Several kinds of inclusion in the large sapphire crystal were investigated by means of an optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The experimental results show that most inclusions are consisted of solid metallic and non-metallic particles as well as gas pores caused by the impurity of alumina as the raw material, the thermal dissociation of aluminum oxide melt and the reaction of the melt to the crucible material (Mo) at high temperatures. It is also found that in different crystal regions the inclusions are of varied sizes, morphology and chemical compositions. Finally, the measures to reduce and eliminate the inclusions are proposed to improve the crystal quality. Keywords: sapphire single crystal; inclusions; bubbles; SAPMAC method 1 Introduction * Due to a series of remarkable mechanical and physical properties, sapphire single crystals are widely used in several modern high-technology applications, from military and commercial optical systems to high power laser components and semi-conductor substrates [1]. In some cases, for fabricating devices such as infrared transmission window and dome, large-size and high-quality sapphire single crystals are indispensable [2]. However, in sapphire crystal growth process from high temperature melt, the inclusions of bubbles and foreign particles, especially for large-sized crystal, are often observed [3]. The appearance of inclusions causes Tyndall scattering centers, which results in sapphire *Corresponding author. Tel.: address: wanggghit@163.com Foundation item: National Defensive Preliminary Research Funds of China ( ) optical transmission drop and even crystal cracking [4,5]. In this paper, the inclusions types, size and distribution were observed in the grown sapphire crystal by means of OM, SEM and EPMA; and formation mechanisms of them were also analyzed. 2 Experimental Procedures 2.1 Specimen preparation The sapphire single crystal was grown by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at cooled center). The growth conditions are displayed in Table 1. Detail growth procedures have been described in elsewhere [6]. The specimens (10 mm 10 mm 3 mm) were cut perpendicularly to growth direction from different regions of large sapphire crystal, and then they were finely polished.

2 S32 WANG Gui-gen et al. / Chinese Journal of Aeronautics 19(2006) S31-S35 Table 1 The experimental set-up characteristics Parameters Settings -1 Pulling velocity/(mm h ) Heater type and material Resistive, tungsten Crucible material Molybdenum Screen material Molybdenum Ambient atmosphere pressure/pa Microstructure observation The chemical interaction between molten alumina and crucible material is guessed to be another important cause of gases (MoO, MoO2 and MoO3) appeared in the melt[8]. As to gas bubbles concentration, distribution and dimension, there are many influence factors including crystal growth method, pulling rate, crystallization interface shape, and various flows hydrodynamics in the crystal growth system[8-10]. The shape and distribution of the inclusions in the sapphire crystal were observed by OM (Olympus Gx51). The chemical compositions of the inclusions were analyzed by energy dispersive X-ray spectroscopy (EDX) in Japan Hitachi-S4700 SEM. 3 Results and Discussion 3.1 Large sapphire crystal Large-sized (Φ225 mm 205 mm, 27.5 kg) a-plane (11-20) sapphire single crystal was grown by SAPMAC method, which is shown in Fig.1. It is obviously seen that the inclusions, including gaseous bubbles and foreign solid particles, constitute a kind of common defect in sapphire. 3.2 Gas inclusions Gaseous inclusions, such as voids and bubbles, are major defects in large sapphire crystal. Practically, there are no available techniques to grow bubbles-free large sapphire crystal[3]. There are two aspects of the problem concerned with bubbles in sapphire crystal. One is gas and bubbles enrichment in high temperature melt, the other is bubbles capture by the grown crystal[1]. From the phase diagram of Al-Al2O3, the following decomposition reactions occur above the melting point Al2O3 2AlO (g) +1/2 O2 (g) (1) Al2O3 Al2O (g) +O2 (g) (2) The composed gases such as AlO, O2 and Al2O dissolve in the melt, and then the supersaturated gases are trapped into the crystal boule in the form of bubbles[7]. Moreover, during the sapphire crystal grown process by SAPMAC method, the presence of growth crucible complicates bubbles formation. Fig.1 Large sapphire crystal grown by SAPMAC method. (a) Φ225 mm 205 mm, 27.5 kg; (b) the top region of sapphire crystal; (c) the bottom region of sapphire crystal. The bubbles distribution is shown in Fig.1 and Fig.2. At the initial stage of crystal growth, thermal field fluctuations led to visible bubbles and thread pipeline pores formation in the crystal boule region near to the seed crystal. It can be seen from Fig.1(b)

3 WANG Gui-gen et al. / Chinese Journal of Aeronautics 19(2006) S31-S35 S33 and Fig.2 (a). The phenomenon is caused mainly by high crystal growth velocity and convex crystallization front [11]. The bubbles will be largely eliminated, if seed crystal is sufficiently pre-heating, the length of seed crystal immersed is controlled well, the temperature of melt is aptly high during seeding process, and efficient wetting between seed crystal and melt is guaranteed [12]. low temperature gradient. As far as the presence of some small-dimension bubbles concerned, it is because much gas can not escape from crystal axis center, and is captured by melt, thus causes bubbles aggregation and growth. In addition, high concentration and large-sized (up to several centimeters) pores and bubbles (Fig.1 (c) and Fig.2 (c)), were formed in the center part of crystal boule tail region. It was due to crystal-melt interface shape s change along with crystal growth and melt shrinkage during solidification process [12]. 3.3 Solid inclusions Fig.2 The OM micrographs of gas bubbles of sapphire crystal. (a) the top central region; (b) the iso-diameter central region; (c) the bottom region. However, no apparent bubbles and pores, just only few dispersive ones appeared in iso-diameter crystal region following shoulder-expanding process as Fig.2(b) shown. It also shows SAPMAC method has merits of growing large-sized and high quality sapphire crystal because of its low growth speed and These are the second type of inclusions. Certain impurities in alumina melt were present in sufficient quantities, which exceeded solid solubility limit, and formed solid inclusions. Their distribution is shown in Fig.3. Compared with sapphire boule bottom and periphery region, there are fewer solid inclusions in boule center region (Fig.3(a), Fig.3(b) and Fig.3(c)). It was because convex interface impelled melt impurity moving from crystal center region to the periphery and bottom portions [13,14]. In order to further study the inclusions in crystal, SEM were performed, which experimental results are shown in Fig.4. The main inclusions observed by EPMA are composed of C, Si and some metallic elements such as Ca, Cu, which is indicated in Table 2. The rectangle inclusion in Fig.4(a) contains mainly % carbon. As to the inclusions with farinose shape (Fig.4(b) and Fig.4(c)), they are composed of a major impurity element of C, Si and a few metallic ones (Ca, Fe, Mg), except for main composition of Al and O. The impurities come from alumina raw material, and may be also introduced during the samples preparation process [15]. Another kind of inclusions exhibits erose shape (Fig.4(d)), in which Mo impurity is present. It is indicated that container material produces metal-particle inclusions, as a result of complicated chain of chemical reaction mentioned above volatile oxides formation, gas transport in the melt and oxides decomposition with metal particles formation. So it is necessary to

4 S34 WANG Gui-gen et al. / Chinese Journal of Aeronautics 19(2006) S31-S35 select high purity raw material, choose proper cooling rate and avoid crucible superheating[7,13,14,16]. Fig. 4 The SEM micrographs of the solid inclusions in sapphire crystal boule. (a) the rectangle inclusion; (b), (c) the farinose inclusions; (d) the erose inclusion. Table 2 Fig.3 The OM micrographs of the solid inclusions in sapphire crystal boule. (a) the top region; (b) the iso-diameter region; (c) the bottom region. The constituents of the solid inclusions Constituent /at% a b Inclusions type c d O Al C Si Ca Cu Fe 6.21 Mg 0.21 Zn 0.25 P 2.28 Cl 1.21 Ti 0.18 Mo

5 WANG Gui-gen et al. / Chinese Journal of Aeronautics 19(2006) S31-S35 S35 4 Conclusions Gas bubbles and foreign particles are two main kinds of inclusions in large sapphire crystal grown by SAPMAC method. A majority of bubbles appear in crystal region near to seed crystal and center part of crystal boule bottom region; there are almost no visible gas bubbles in crystal iso-diameter region. As to solid inclusions, they are present mostly in crystal periphery and bottom regions, other crystal region shows good optical quality. It also shows SAPMAC method s merits of growing large and high quality sapphire crystal. Preparing high purity raw material, controlling crystal growth velocity, optimizing thermal field and keeping good vacuum in growth furnace, the inclusions in large sapphire boule can be eliminated and reduced largely. References [1] Capper P. Bulk crystal growth in electronic, optical and optoelectronic materials. New York: Wiley & Sons, [2] Schmid F, Khattak C P, Rogers H H, et al. Current status of very large sapphire crystal growth for optical application. SPIE 1999; 3705: [3] Saito M. Gas-bubble formation of ruby single crystals by floating zone method with an infrared radiation convergence type heater. J Cryst Growth 1985; 71: [4] Cockayne B, Chesswas M, Gasson D B. Singe-crystal growth of sapphire. J Cryst Growth 1967; 2:7-11. [5] Theodore F, Duffer T, Santailler J L, et al. Crack generation and avoidance during the growth of sapphire domes from an element of shape. J Cryst Growth 1999; 204: [6] Han J C, Zuo H B, Meng S H, et al. The preparation of large sapphire crystal by SAPMAC method. CN , [in Chinese] [7] Fukuda T, Okano Y, Kodama N, et al. Growth of bubble-free Ti-doped Al 2 O 3 single crystal by the Czochralski method. Cryst Res Technol 1995; 30(2): [8] Musatov M. Causes of the formation of bubbles in corundum crystals. Izv Akad Nauk SSSR 1977; 15: [9] Bunoiu O, Nicoara I, Santailler J L, et al. On the void distribution and size in shaped sapphire crystals. Cryst Res Technol 2005; 40(9): [10] Bunoiu O, Defoort F, Santailler J L, et al. Thermodynamic analyses of gases formed during the EFG sapphire growth process. J Cryst Growth 2005; 275:e1707-e1713. [11] Xu C H, Zuo H B, Meng S H, et al. Study on SAP-MAC-grown large sapphire crystal growth process by numerical simulation analysis. J Synth Cryst 2006; 35(5): [in chinese] [12] Wang C L. The gases and bubbles in sapphire single crystal. Bulletin of the Chinese Ceramic Society 1983; 2: [in chinese] [13] Song C, Hang Y, Xia C T, et al. Characteristics of large-sized ruby crystal grown by temperature gradient technique. Opt Mater 2005; 27: [14] Wang H B, Zhang Q, Chai Y, et al. Analysis of inclusions in Titanium-doped sapphire. J Inorg Mater 1996; 11: [in chinese] [15] Klapper H. Structural defects in crystals and techniques for their detection. Mater Sci Forum 1998; : [16] Chernov A A. Modern crystallography III: crystal growth. Switzerland: Springer-Verlag, Biography: WANG Gui-gen Born in 1981, he received B.S. from Anhui University of Technology and Science in 2002, and then became a master-doctor combined program graduate student up to now. His main research field is single crystal material growth and its property modification. wanggghit@163.com