Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2000-06-19
2003-03-18
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S010140, C266S234000, C266S239000, C266S241000, C366S136000, C366S137000, C366S147000, C366S149000, C366S191000, C366S274000
Reexamination Certificate
active
06533838
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a refining process and an apparatus for metallic gallium (Ga), and it further refers to a high purity Ga suitable for obtaining a compound semiconductor such as a GaAs single crystal.
BACKGROUND OF THE INVENTION
Among the compound semiconductors, the Group III-V compounds, particularly GaAs single crystals, are widely used as the substrates of electronic devices and optical devices such as high speed ICs and photoelectronic integrated circuits because they not only have superior high electron mobility which is about five times as high as that of an elemental semiconductor such as silicon, but also exhibit excellency in, for example, high frequency characteristics, magnetic conversion functions, photoreceptor functions and light emitting functions.
Wafers of GaAs single crystal are manufactured through various processes. The basic process thereof must comprise a step of growing GaAs crystals from a melt of Ga—As and a step of slicing them into wafers. A wafer (semi-insulating GaAs substrate) thus prepared is then subjected to selective ion injection or various types of epitaxial growth processes to finally obtain the desired semiconductor device element.
In using a GaAs single crystal (GaAs ingot) as a semi-insulating substrate, it is an indispensable requirement that the single crystal stably maintains a specific resistance (referred to hereinafter as resistivity) of 1×10
7
&OHgr;·cm or higher. Although it is most desirable to obtain an intrinsic GaAs single crystal completely free from impurities and lattice defects, it is practically difficult to produce such an intrinsic GaAs single crystal of high purity because of the unavoidable crystal defects and residual impurities. As a reason for causing such difficulties, there can be mentioned the presence of impurities that accompany the raw material for Ga (gallium) used in the step of growing a GaAs crystal from the Ga—As melt.
In growing a GaAs crystal from a Ga—As melt, generally used is the LEC (Liquid Encapsulated Czocralski) process. This process comprises covering the surface of a Ga—As melt placed inside a crucible with B
2
O
3
, and pulling up a seed crystal of GaAs through the B
2
O
3
layer while rotating the melt and applying pressure in an inert gas atmosphere. In carrying out this process, various improvements are devised to reduce the incorporation of impurities into the GaAs single crystal as much as possible, such as using a crucible made of PBN (Pyrolytic Boron Nitride) or controlling the gaseous atmosphere.
In spite of the improvements made on the constitution of the apparatus and on the process conditions, the probability of incorporating the impurities into the GaAs single crystal increases if the concentration of impurities incorporated in the starting melt from which the GaAs crystal is grown remains high. That is, it is still difficult to obtain a high quality GaAs single crystal if the purity of the raw materials for Ga and As remains low. Among the impurities which accompany the raw materials for Ga and As, there certainly are impurity elements having a low segregation index that are less incorporated into the growing crystal and reside in the melt; however, from the viewpoint of improving the yield in producing GaAs single crystals, it is still undesirable to result in a melt containing impurity elements at high concentration. Accordingly, the concentration of impurities in the raw materials for Ga and As is preferably as low as possible and it is further desirable to previously recognize the type and content of each impurity present in the raw material.
Concerning the raw materials for Ga and As for use in producing GaAs single crystals, it is relatively easy to find a commercially available high purity As (arsenic) having a purity of 7N (seven nines; stands for a 99.99999% purity, and is sometimes used hereinafter to express the purity). However, the case for raw Ga materials is not so simple. Any raw Ga material contains, to some extent, a variety of impurities in various forms depending on its origin, and, the quantity of the impurities fluctuates in general. It is therefore difficult to stably obtain a raw Ga material free from impurities which are inconvenient for the production of GaAs single crystals. Furthermore, with the present day analytical technology (glow discharge mass spectrometer) for analyzing the content of the impurity elements present in metallic Ga, it is difficult to obtain reliable results for each of the components incorporated at a level of 0.01 ppm or lower. It can be understood therefrom that it is even difficult to know the exact concentration of each of the impurity elements contained in trace quantities in the raw Ga material to be used for the production of GaAs single crystals.
In addition to the aforementioned GaAs single crystals, compound semiconductors using Ga include GaP, GaN, etc. Because a GaP single crystal has excellent photoreceptor and light emitting functions, it is used as a substrate for optical devices such as light emitting devices. A GaP single crystal wafer is produced by first synthesizing a polycrystalline GaP, pulling up the polycrystalline GaP as a GaP single crystal and by means of a process similar to, for example, the aforementioned LEC process, and slicing the resulting GaP single crystal ingot. A light emitting device can be finally obtained by performing liquid layer expitaxy. To obtain a light emitting device of high luminance in this case, the incorporation of impurities in the GaP single crystal substrate must be suppressed to the lowest limit. Particularly harmful are the impurities which increase the concentration of the carriers on synthesizing the polycrystalline GaP and lowers the resistivity. Similar to the case of GaAs, furthermore, the incorporation of such harmful impurities is believed to be originated from the raw Ga material in many cases.
As a process for refining metallic gallium to remove impurities from the raw materials, conventionally known processes include acid processing, electrolytic smelting, zone melting, pulling up crystals, recrystallization by melting and solidification, etc. Among these processes, the recrystallization process comprising melting and solidification is advantageous in that it enables refining using a relatively simple installation and operation. In solidifying a liquid of a raw gallium material containing an impurity, there is known a phenomenon as such that the impurity concentration of the crystal becomes lower than that of the residual liquid. The principle of this process is based on this phenomenon.
For the process of refining gallium by utilizing the phenomenon above, proposals for improving the process conditions and operations can be found in, for example, JP-A-Sho62-270494 (the term “JP-A-” as referred herein signifies “an unexamined published Japanese patent application”), JP-A-Sho63-242996, JP-A-Hei2-50926, JP-A-Hei2-50927, JP-B-Hei2-53500 (the term “JP-B-” as referred herein signifies “an examined published Japanese patent application”) JP-A-Hei6-136467, etc.
OBJECT OF THE INVENTION
At present, in producing compound semiconductors such as GaAs and GaP, it is practically impossible to obtain a highly pure metallic gallium having a purity of 6N or 7N or even higher and also provided with reliable analytical data for each of the impurity contents. Since this caused a problem in producing high quality compound semiconductors such as GaAs and GaP, a first object of the present invention is to overcome this problem.
Among the prior art technologies for producing high purity gallium, the recrystallization process using melting and solidification comprises separating the crystalline gallium (solid phase) containing impurities at a low concentration level from the residual liquid (liquid phase) containing impurities at a higher concentration, and is based on the concept of separating the solid phase from the liquid phase differing in impurity concentration. In order to separate the high purity solid phase from the liqui
Kato Hidekazu
Ohgami Takashi
Okuda Kanichi
Tayama Kishio
Yamamura Takeharu
Dowa Mining Co. Ltd.
King Roy
McGuthry-Banks Tima
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