JPH0521877B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing a Bi ₁₂ GeO ₂₀ (hereinafter referred to as BGO) single crystal. BGO single crystal has a large electro-optical effect and is useful as a material for voltage sensor elements and image storage elements. Conventional technology Conventionally, BGO single crystal has a crystal composition close to Bi ₂ O ₃ :
It has been grown by the rotary pulling method from a melt with a composition of GeO ₂ =6:1. However, the BGO single crystal grown using this method is
The bismuth component is incorporated into the crystal in excess, and as a result, it exhibits much greater light absorption in the wavelength range of 380 to 500 nm than the theoretical properties of BGO, which has a pure stoichiometric composition. Moreover, the degree of light absorption sensitively reflects the growth conditions, so if the growth conditions change during crystal growth, the characteristic values of the single crystal (light transmittance, etc.)
Variations occur. In addition, light absorption may be different between a region called the core (or facet) and other regions in the grown crystal, or distortion may occur at the boundary between the two regions. BGO like this
The non-uniformity of the optical properties of a single crystal and the presence of intracrystal strain are
Strict limits have been placed on device reliability and device manufacturing yield. Purpose of the Invention The present invention aims to eliminate the drawbacks of the conventional BGO single crystal manufacturing method.The purpose of the present invention is to have light transmittance close to the ideal properties of pure BGO, uniform optical properties, and low strain within the crystal. The purpose of this study is to provide a method for producing BGO single crystals. Structure of the Invention As a result of intensive research to achieve the above object, the present inventors have found that the excess bismuth component in BGO is directly related to the bismuth component concentration in the melt, and the bismuth component concentration in the melt is kept within a specific range. It is possible to reduce the excess bismuth component incorporated into the crystal, and also to eliminate non-uniformity in the degree of light absorption in each region of the crystal, resulting in excellent device reliability and yield. I was able to find out. The present invention was completed based on this knowledge. The gist of the present invention is a method for growing a Bi ₁₂ GeO ₂₀ single crystal from a separation liquid by a melt solidification method, in which the composition of the melt is changed to GeO ₂ /GeO 2 with an excess of germanium.
(Bi ₂ O ₃ +GeO ₂ ) is characterized by being grown from a melt that maintains the molar fraction in the range of 0.18 to 0.28.
In the method for producing Bi ₁₂ GeO ₂₀ single crystal. If the molar fraction is less than 0.18, the effect of making the germanium component excessive is small, and fluctuations in the Bi component in the crystal cannot be fully controlled. Moreover, when it exceeds 0.28, it approaches too much the eutectic point on the side where the germanium component is excessive, and precipitation of multiple phases containing germanium becomes visible in the crystal. Therefore, it needs to be in the range of 0.18 to 0.28. Ideally, it is better to control it between 0.25 and 0.28.
Methods for growing a single crystal from a melt in the present invention include a pulling method, a Bridgeman method, a zone melting method,
Any melt solidification method such as a floating zone method may be used. However, the pulling method, zone melting method, and floating zone method are preferred from the viewpoint of easily achieving high quality grown crystals. As the starting purity raw material used in the present invention, commercially available
Bi ₂ O ₃ or GeO ₂ with a purity of 99.99% may be used, but it is preferable to use raw materials with as high a purity as possible. In the case of the floating zone method, these starting material powders
Bi ₂ O ₃ :GeO ₂ =6:1 (stoichiometric composition) is mixed, formed into a rod shape and sintered to obtain a raw material rod. Also,
In order to make the melt composition in the melt zone excessive in germanium, the excess germanium (GeO ₂ / (Bi ₂ O ₃
+GeO ₂ ) molar fraction = 0.18 to 0.28 (preferably about 0.25), prepare a pellet-like sintered body (preferably with a volume comparable to that of the molten zone), place it on a seed crystal in advance, and heat it to melt it. Growth begins after a molten zone is formed. The crystal growth rate in the present invention is 0.1 to 15
mm/hour, preferably 0.5 to 4.0 mm/hour. The growth atmosphere can be oxygen, nitrogen, or air. In the floating zone method, it is preferable to use an after-heater or the like to prevent cracking of the grown crystals due to rapid cooling. Further, the grown crystal is preferably annealed at about 780 to 820°C. Example Commercially available high purity (99.9999%) Bi ₂ O ₃ and GeO ₂ (purity
99.99%) raw material powders are mixed at a molar ratio of 6:1,
Rubber press molded into a rod shape using hydrostatic pressure of 1 ton/cm ² .
A raw material rod was created by sintering in oxygen at 810°C. Also,
In order to make the composition of the melting zone excessive in germanium, GeO ₂ /(Bi ₂ O ₃ + GeO ₂ ) was added in a similar process.
Sintered pellets having a composition with a mole fraction of 0.26 were prepared. Next, the raw material rod was loaded into a condensing floating zone method single crystal production device using a spheroidal mirror, a separately prepared BGO seed crystal was loaded, and a sintered pellet was placed on top of the seed crystal. The sintered pellets are heated by increasing the lamp output while flowing nitrogen gas at a rate of 2 liters per minute as an atmospheric gas, and when the sintered pellets are melted, they are joined to the raw material rod to form a stable molten zone, and then the floating zone method is applied. Crystal growth operations were performed according to conventional methods. The growth conditions are that the rotation speed of the raw material rod and the seed crystal is 40 times/min in opposite directions, and the crystal growth rate is 3 mm/min.
It was hot at the time. The obtained BGO single crystal was heated for about 800 h for 20 hours.
C. to obtain a slightly yellowish transparent crystalline material. When the obtained BGO single crystal was optically evaluated,
390 ~ than those grown using the conventional pulling method.
It was found that the transmittance was significantly higher in the wavelength region of 500 nm.

[Table] Furthermore, it was observed that the difference in optical properties between the core region and other regions was much smaller than in conventional BGO crystals. Effects of the Invention According to the method of the present invention, it has high light transmittance,
Moreover, a BGO single crystal with homogeneous optical properties can be obtained. When manufacturing sensors and image storage devices that utilize the electro-optic effect of BGO single crystals, single crystals grown using conventional methods have had reliability problems due to non-uniform optical properties and strain within the crystal. However, according to the present invention, higher quality BGO single crystals can be grown, and voltage sensor elements, image storage elements, etc. with excellent characteristics can be obtained at a high yield.

Claims (1)

[Claims] 1. A method of growing a Bi ₁₂ GeO ₂₀ single crystal from a melt by a melt solidification method, in which the composition of the melt is changed to a molar fraction of GeO ₂ /(Bi ₂ O ₃ +GeO ₂ ) with an excess of germanium. A method for producing a Bi ₁₂ GeO ₂₀ single crystal, characterized in that it is grown from a melt kept in the range of = 0.18 to 0.28. 2. The manufacturing method according to claim 1, wherein the melt solidification method is a pulling method, a Bridgeman method, a zone melting method, or a floating zone method.