Presently, a ZnTe system compound semiconductor single crystal is deemed as a promising crystal which can be applied to a pure green light-emitting element.
In general, the ZnTe system compound semiconductor single crystal is produced by a vapor phase epitaxy method in which a ZnTe polycrystal as a raw material disposed at one end in a quartz ampoule is heated to be sublimed at a temperature around a melting point thereof, and a ZnTe single crystal is deposited on a substrate arranged on an opposite side of the ampoule. This method makes it possible to obtain a rectangular ZnTe single crystal substrate of approximately 20 mm×20 mm at the maximum. Recently, to further enhance the light-emitting characteristic of the crystal as a light-emitting element, it has been devised to increase the electrical conductivity of a crystal, and a method of adding an impurity, such as phosphorus, arsenic or the like, to the crystal is carried out, as the method for increasing the electrical conductivity.
Further, it is also possible to make use of the vertical Bridgeman (VB) method or the vertical gradient freezing (VGF) method to grow a ZnTe system compound semiconductor single crystal. In the VB method and the VGF method, since an impurity can be added during crystal growth, these methods are advantageous in that it is easy to control the electrical conductivity of a crystal through addition of an impurity to the crystal. Further, it is also devised to cover the liquid surface of a raw material melt with an encapsulating material so as to prevent an impurity from being mixed into the melt from above to inhibit forming of a single crystal, and to suppress temperature fluctuation in the melt.
However, in the case of growing a ZnTe system compound semiconductor single crystal by the vapor phase epitaxy method, it is difficult to add a desired impurity during crystal growth, and hence difficult to control the resistivity of the ZnTe system compound semiconductor single crystal. Further, in the vapor phase epitaxy method, the growth rate of a ZnTe crystal is extremely slow, and hence it is difficult to obtain a single crystal having a sufficient size, which results in low productivity.
Moreover, even if a relatively large substrate of approximately 20 mm×20 mm can be obtained by growing the ZnTe system compound semiconductor single crystal, the low productivity makes the substrate very expensive, which offers a barrier to development of devices using the ZnTe system compound semiconductor single crystal.
For the above reasons, production of the ZnTe system compound semiconductor single crystal by the vapor phase epitaxy method is not practical as a method for industrial production.
On the other hand, in production of the ZnTe system compound semiconductor single crystal by the VB method or the VGF method, it is possible to grow a large-sized crystal, but since the crystal is grown by being cooled in the state covered by the encapsulating material, a difference in the coefficient of thermal expansion between the encapsulating material and the growing crystal often causes cracking in the crystal.
Further, in the LEC method, similarly to the VB method and the VGF method, it is also possible to add an impurity, and hence the LEC method is advantageous in that it is easy to control the electrical conductivity of a crystal through addition of an impurity to the crystal. However, almost no example has been known so far in which a large-sized ZnTe compound semiconductor single crystal is grown by the LEC method.
An object of the invention is to provide a method for producing a compound semiconductor single crystal, which is capable of growing a large-sized ZnTe system compound semiconductor single crystal or other kinds of compound semiconductor single crystals while maintaining excellent crystal quality.