Oxide single crystal and method of manufacturing thereof

Abstract: To provide an oxide single crystal of large size having the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and containing Ge as a constituent element and a method of manufacturing thereof. The oxide single crystal is obtained by a manufacturing method comprising the steps of preparing a melt of starting materials containing GeO 2 and growing said oxide crystal from said melt of starting materials in single-crystal growing atmosphere, which is characterized in that said starting materials contain GeO 2 in a stoichiometrically excess amount, or said single-crystal growing atmosphere is a gas having an oxygen partial pressure greater than 2 × 10 -1 atm.

Description: BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates to an oxide single crystal and a method of manufacturing thereof. More particularly, the oxide single crystal has the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and contains Ge as a constituent element and the method comprises the steps of preparing a melt of starting materials containing Ge, and growing said oxide crystal from said melt of starting materials in single-crystal growing atmosphere. The oxide single crystal is used as a material for piezoelectric devices (because of its piezoelectric properties) and a solid laser host material. Related Art of the Invention Since the discovery of Ca 3 Ga 2 Ge 4 O 14 compound by B. V. Mill et al., a number of compounds of the same structure have been found as the result of substitution of constituents with cations. Their examples are La 3 Ga 5 SiO 14 , La 3 Ga 5.5 Nb 0.5 O 14 , La 3 Ga 5.5 Ta 0.5 O 14 , and La 3 Ga 5 ZrO 14 which are formed by substitution of Ca with La or substitution of Ge with Ga and Si, Ga and Nb, Ga and Ta, or Ga and Zr. They have been found to have electromechanical coupling coefficients greater than that of - quartz. They have also been found to have a crystal axis along which the temperature coefficient of freaquency is zero. For these reasons, they are regarded as a promising material for the piezoelectric devices of mobile communications equipment of next generation. The technology in this field has advanced to such an extent that a La 3 Ga 5 SiO 14 single crystals (an ingot having about 76 cm (3 inches) in diameter) can be produced easily by Czochralski method. It is known that the La,Ga-based compound mentioned above is surpassed in electromechanical coupling coefficients by those Ca 3 Ga 2 Ge 4 O 14 compounds in which the Ca site is replaced by an alkali metal ion or alkaline earth metal ion. Nevertheless, they are not yet available in the form of large single crystal because of many difficulties involved. For example, they have a melting point of 1300°C or above which exceeds the vaporizing point of GeO 2 as one of the starting materials. This leads to a product of poor quality due to its composition differing from that of the starting materials, cracking in the crystal, and gaseous impurity phases. Randles et al discloses in "Disordered oxide crystal hosts for diode pumped lasers", published in journal of crystal growth, volume 128, No. 1-4, March 1993, Amsterdam, NL, pages 1016 - 1020, XP000349498 a method for growing oxide crystals having the features of the preamble of claim 1. SUMMARY OF THE INVENTION It is in object of the present invention to provide an oxide single crystal of large size having the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and containing Ge as a constituent element and an improved method of manufacturing thereof. This object is achieved with a method having the features of claim 1 and a single crystal having the features of claim 6. Preferable embodiments form the subject matter of the respective subclaims. Because GeO 2 begins to evaporate at 1250° C or above in air (in which the oxygen partial pressure is CC.2 x 10 4 Pa (2 x 10 -1 atm)) and an oxide of Ca 3 Ga 2 Ge 4 O 14 crystal structure containing Ge as a constituent element has a melting point higher than this temperature, the evaporation of GeO 2 from this oxide at its melting point is inevitable. The present invention to cope with this situation is designed to grow the single crystal from a melt of the starting material containing GeO 2 in an excess amount which would make up for the evaporation loss of GeO 2. The excess amount is less than 1 wt% in view of the fact that the evaporation of GeO 2 from the melt of starting materials is less than 1wt% during crystal growth. To suppress the evaporation of GeO 2 , the present invention is also designed to grow the single crystal in an atmosphere in which the oxygen partial pressure is higher than that (2 × 10 4 Pa (2 × 10 -1 atm)) of air. The present invention produces the following effects. Production of an oxide single crystal of large size having the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and containing Ge as a constituent element by growth from a melt of starting materials without change in composition due to evaporation loss of GeO 2 from the melt. Suppression of evaporation of GeO 2 . Production of a high-quality oxide single crystal of large size having the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and containing Ge as a constituent element. These and other objects of the invention will become more apparent in the detailed description and examples which follow. PREFERRED EMBODIMENTS OF THE INVENTION The following two examples demonstrate methods for manufacturing Sr 3 Ga 2 Ge 4 O 14 single crystals as an oxide single crystal which has the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and contains Ge as a constituent element. (Example 1) SrCO 3 , Ga 2 O 3 , and GeO 2 as starting materials were weighed in a molar ratio of 3:1:4. To them was further added 0.75 wt% of GeO 2 . These components were mixed by dry process and then press-formed and placed in a platinum crucible being 50 mm in diameter, 50 mm high, and 1.5 mm wall thickness. With radio-frequency heating, a single crystal (20 mm in diameter) was grown by Czochralski method at a crystal pulling rate of 1 mm/hour. The atmosphere for crystal growth was oxygen (with the oxygen partial pressure being 2 × 10 4 Pa (2 × 10 -1 atm)), and the crystal was turned continuously at a rate of 10 rpm during crystal pulling. During crystal growth, evaporation of a small amount of GeO 4 from the melt of raw materials was noticed. Nevertheless, there was obtained a high-quality single crystal, 60 mm in length, free of bubbles and cracks. The results of powder X-ray diffractometry indicate that there exists only the single phase of Sr 3 Ga 2 Ge 4 O 14 in the axis parallel to the direction of crystal pulling and that the crystal has an invariable lattice constant. (Example 2) SrCO 3 , Ga 2 O 3 , and GeO 2 as starting materials were weighed in a molar ratio of 3:1:4. These components were mixed by dry process and then press-formed and placed in a platinum crucible being 50 mm in diameter, 50 mm high, and 1.5 mm wall thickness. With radio-frequency heating, a single crystal (20 mm in diameter) was grown by Czochralski method at a crystal pulling rate of 1 mm/hour. The atmosphere for crystal growth was oxygen (with the oxygen partial pressure being 10 5 Pa (1 atm)), and the crystal was rotated continuously at a rate of 10 rpm during crystal pulling. During crystal growth, evaporation of a small amount of GeO 2 from the melt of raw materials was noticed. Nevertheless, there was obtained a high-quality single crystal, 60 mm long, free of bubbles and cracks. The results of powder X-ray diffractometry indicate that there exists only the single phase of Sr 3 Ga 2 Ge 4 O 14 in the axis parallel to the direction of crystal pulling and that the crystal has an invariable lattice constant. The crystal-growing atmosphere in Example 1 should preferably contain an inert gas such as argon and nitrogen. Although the above-mentioned examples demonstrate Sr 3 Ga 2 Ge 4 O 14 single crystals, the present invention also covers other oxide single crystals having the crystal structure of Ca 3 Ga 2 Ge 4 O 14 and containing Ge as a constituent element in which the Ca site is replaced by an alkali metal ion or alkaline earth metal ion, for example.

Claims: A method of manufacturing an oxide crystal having a crystal structure of Ca 3 Ga 2 Ge 4 O 14 and containing Ge as a constituent element, comprising the steps of; 1) preparing a melt of starting materials containing GeO 2 , and 2) growing said oxide crystal from said melt of starting materials in single-crystal growing atmosphere, characterized in that said starting materials contain GeO 2 in a stoichiometrically excess amount, and/or said single-crystal growing atmosphere is a gas having an oxygen partial pressure greater than that (2 × 10 4 Pa (2 × 10 -1 atm)) of air. A method according to Claim 1, characterized in that said starting materials contain GeO 2 in a stoichiometrically excess amount. A method according to Claim 2, characterized in that said starting materials contain GeO 2 in a stoichiometrically excess amount less than 1.0 wt%. A method according to one of Claims 1 to 3, characterized in that said single-crystal growing atmosphere is a gas having an oxygen partial pressure greater than that (2 × 10 4 Pa (2 × 10 -1 atm)) of air. A method according to Claim 4, characterized in that said gas contain an inert gas. An oxide single crystal obtainable by the method according to one of Claims 1 to 5. An oxide single crystal according to Claim 6, characterized in that a Ca site of Ca 3 Ga 2 Ge 4 O 14 is replaced by an alkali metal ion or alkaline earth metal ion. An oxide single crystal according to Claim 7, characterized in that said oxide single crystal is Sr 3 Ga 2 Ge 4 O 14 .

Inventors: FUKUDA TSUGUO, KUMATORIVA MAKOTO, TAKAGI HIROSHI, FUKUDA, TSUGUO, KUMATORIVA, MAKOTO, TAKAGI, HIROSHI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: C30B15
IPC codes: C30B29, H01L21, C30B15