Optical Properties of Al- and Zr-Doped Rutile Single Crystals Grown by Tilting-Mirror-Type Floating Zone Method and Study of Structure-Property Relationships by First Principle Calculations
High quality and transparent single crystals of undoped rutile TiO2, Al-doped rutile (Al？:？TiO2), and Zr-doped rutile (Zr？:？TiO2) have been grown successfully by tilting-mirror-type floating zone (TMFZ) using travelling solvent floating (TSFZ) technique. The effect of doping on the electronic and optical properties of rutile has been studied experimentally as well as by simulation calculations. The effect of doping on the quality of crystals was also investigated by observing optical micrograph and measuring etch pits density that reveals the presence of defects. Undoped rutile crystals were dark blue and comprised many low-angle grain boundaries. Al+3 and Zr+4 ions pin down the migration of dislocations during the cooling and create oxygen vacancies. Doping of the impurities would improve the electronic and optical properties of rutile. The elastic properties might be changed for doping of the impurities in the rutile crystals. 1. Introduction Rutile (TiO2) has excellent optical, mechanical, and chemical properties, and the demand for rutile single crystals has therefore been increasing to use as polarizer in a variety of optical devices because of their large refractive indices and birefringence [1]. It is of urgent necessity to grow rutile single crystals of high optical quality. The floating zone (FZ) method is one of the most promising techniques, which replace the Verneuil method, to grow rutile single crystals. Rutile single crystals were conventionally manufactured by the Verneuil method for gemstones [2]. However, the quality of Verneuil-grown crystals is usually poor, since the crystals undergo quenching, which induces stress birefringence [3]. The optical quality of the FZ-grown crystals was superior to that of commercially available Verneuil-grown rutile crystals [4]. Floating zone (FZ) method is a powerful technique for growth of single crystals without contaminations because it is a crucible free zone melting method. The segregation control of the dopant is very easy in this method by applying the travelling solvent floating zone technique. Hence, high quality single crystals with required dopant concentration are possible to grow by this technique from both congruently and incongruently melting compounds. The manufacturing use of the FZ method, however, is limited because the diameter of the crystals grown by this method is usually smaller than that by the other melt growth techniques such as the Czochralski, the Bridgman, and the Verneuil methods. The shapes of both the melt-feed and the melt-crystal interfaces are convex in the FZ
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