1. Field of the Invention
The present invention relates to a method for growing crystals more particularly semiconductor single crystals such as Si, GaAs, or the like, and more particularly to a method of growing crystals while a magnetic field is being applied to the melt during crystal growth.
2. Description of the Prior Art
Methods of growing a crystal in a magnetic field are industrially important since these methods produce a high quality crystal of Si, GaAs, or the like. One such method is disclosed, for example, in Japanese Patent Publication No. 58-50951. This previously proposed method for growing crystals will be more fully explained in connection with FIG. 1.
In FIG. 1, reference 1 indicates generally a single crystal growing apparatus which is adapted to carry out the above mentioned crystal growing method. Reference 2 refers to a melt of a raw material such as a silicon melt contained in a container 3 consisting, for example, of a crucible made of quartz for accommodating the melt 2. About the outer periphery of the container 3, there is arranged a heating means 4 composed of an electric heater 5 which, for example, takes the form of a zig-zag pattern and forms a cylindrical plane about the outer periphery of the container 3. On the outside of the heating means 4 there is located a jacket 6 which is cooled by means of water or the like. Outside the jacket 6, there is a magnetic field generating means 7 comprising an electromagnet which produces a uniform direct magnetic field and consisting, for example, of a constant conductive electromagnet having an iron core or an electromagnet having a constant conductive solenoid coil. Reference 8 indicates a single crystal seed, and 9 refers to a chuck for pulling the single crystal from the melt as it grows, in keeping with conventional technology.
Pulling the single crystal by use of this apparatus is carried out in the following manner When a direct magnetic field is applied to the melt 2 of raw material in a predetermined direction by the magnetic field generating means 7, the single crystal seed 8 is dipped onto the surface of the raw material melt 2 and pulled while a supporting portion of the single crystal seed 8, specifically, a pulling rotational shaft including the chuck 9 and the crucible 3 are rotated relative to each other whereby a single crystal 10 is grown from the single crystal seed 8.
As described, when a crystal is grown while the direct magnetic field is being applied by the magnetic field generating means 7 to the conductive raw material melt 2, the apparent viscosity of the raw material melt 2 is increased by the magnetofluid effect and consequently the surface tension of the raw material melt 2 is increased and the convection of the raw material melt 2 is decreased. Consequently, variations in temperature and vibrations on the surface of the raw material melt 2 are suppressed. It is therefore possible to obtain a variety of advantages such as growing a single crystal 10 having good crystallinity, the reduction in the amount of materials forming the crucible or container 3, a control of the amount of oxygen to the melt 2, and the like, all of which are accomplished by selecting a suitable intensity of the applied magnetic field.
The aforementioned Japanese Patent Publication discloses that it is important to suppress the ripple of current supplied to the electric heater 5 of the heating means 4 to less than 4% for growing a high quality crystal. However, crystal defects such as a lamination defect or the like may occur in a single crystal grown by the above described method in relatively high density.
It has been found that the occurrence of crystal defects depends on the ripple or pulsation of the current flowing through the electromagnet which generates the magnetic field, and that when the ripple of the current flowing through the electromagnet is large, the intensity of the magnetic field varies with time, and an induced current is generated in the crystal growing apparatus, vibrations occur on the surface of the raw material melt, and consequently crystal defects are produced.
To avoid fluctuations in intensity of the magnetic field caused by the ripple of the direct current flowing through the magnetic field generating means, a battery might be employed as a power supply for the electromagnet. However, a battery is not appropriate as a power supply for the magnetic field which requires an electric power ranging from several kilowatts up to or exceeding 100 kilowatts. Accordingly, a rectifier is used as a direct current power supply source for the magnetic field generating means. The rectifier, for example, may be a silicon rectifier of the three phase, full-wave type, or a six phase, half-wave type, or a double star type with an interphase reactor or the like. Theoretically, the ripple factor or the pulsation ratio can be suppressed but this ripple factor can only be obtained where the rectifier is used in the vicinity of the maxiumum output of the direct power source. If the rectifier is used at a direct current output lower than the value mentioned, the distortion in the output wave form is increased.