Patent Application
20130029278
The present invention relates to a method for synthesizing group II-VI compound semiconductor polycrystals as raw materials of group II-VI compound semiconductor single crystals, and particularly relates to a method for synthesizing CdTe family compound semiconductor polycrystal, particularly CdTe polycrystal.
A compound semiconductor polycrystal has to be synthesized from a group II element and a group VI element at first for synthesizing a group II-VI compound semiconductor single crystal. Conventionally, a group II-VI compound semiconductor was synthesized by putting a group II single element and a group VI single element in a semi-airtight pBN inner vessel, sealing the inner vessel, setting the inner vessel in an airtight outer vessel of a quartz ampoule, vacuum sealing the outer vessel, and heating the outer vessel in an electric furnace up to a melting point or more so as to melt and react the raw elements.
Patent Document 1 and Patent Document 2, for example, disclose such a kind of synthesizing method of polycrystals.
Patent Document 1: JP Patent No. 2517803
Patent Document 2: Tokukai 2003-277197A
When synthesizing a group II-VI compound semiconductor polycrystal as a raw material of a group II-VI compound semiconductor single crystal, it is desirable to use a larger vessel so as to synthesize a large amount of polycrystal efficiently at a time for reducing synthesizing costs. However, in a case where an airtight quartz ampoule is used as disclosed in Patent Documents 1 and 2, there is a problem that the larger the size of the quartz ampoule, the more difficult to handle the quartz ampoule and easier to break the quartz ampoule due to increase of pressure during the synthesis, which result in a low yield.
In addition, in the synthesis method using a quartz ampoule, the quartz ampoule was disposed because a synthesized polycrystal has to be taken out by breaking the ampoule after synthesis. Thus it also causes difficulty to reduce the costs.
It is an object of the present invention to provide a method for synthesizing group II-VI compound semiconductor polycrystals without using a quartz ampoule as a synthesis vessel for the polycrystals, which contributes to a size up of a synthesis vessel and reduction of the synthesis costs while the yield is not lowered.
To achieve above object, a method for synthesizing group II-VI compound semiconductor polycrystals according to claim 1 includes steps of:
putting two or more raw material elements in a semi-airtight pBN inner vessel and putting the inner vessel in a semi-airtight heat-resistant outer vessel,
placing the outer vessel in a high-pressure furnace having a heating means,
evacuating the high-pressure furnace and filling the high-pressure furnace with an inert gas at a predetermined pressure,
heating the inner vessel and the outer vessel by the heating means so as to melt and promote a reaction of the raw material elements in the inner vessel, and
cooling the reacted raw material elements so as to promote a growth of a polycrystal.
According to the method above explained, it becomes possible to upsize a vessel for synthesis of polycrystals because it becomes not necessary to use a quartz ampoule, which is difficult to upsize, as a vessel for synthesis. Furthermore, when using a quartz ampoule as a vessel for synthesis of polycrystals, the quartz ampoule may burst during the synthesis process and result in a failure of the synthesis, which causes decreasing of a yield. However, the present invention provides a synthesizing method without using a quartz ampoule and contributes cost reduction by virtue of the reuse of a vessel.
The invention of claim 2 is a method for synthesizing group II-VI compound semiconductor polycrystals of claim 1, in which the outer vessel is made of graphite and a surface of which is covered with a coating layer.
It becomes easy to handle the outer vessel because the outer vessel is made of graphite. Also it becomes possible to restrict leakage of gasified raw materials from the outer vessel during heating by coating the surface of the outer vessel even when the vessel is made of graphite.
The invention of claim 3 is a method for synthesizing group II-VI compound semiconductor polycrystals of claim 1 or 2, in which the inert gas pressure in the high-pressure furnace before heating is 0.5 MPa or more.
It becomes possible to reduce leakage of gasified raw materials from the vessel during the heating by setting the inert gas pressure in the high-pressure furnace before heating at 0.5 MPa or more.
The invention of claim 4 is a method for synthesizing group II-VI compound semiconductor polycrystals of claim 3, in which the group II-VI compound semiconductor polycrystal is a CdTe polycrystal, and a temperature raising rate at the heating step is 100 to 2000 degrees C. per hour.
It becomes possible to eliminate remaining of non-reacted materials and accomplish the synthesis of CdTe polycrystals completely by setting the temperature raising rate at the heating at 100 to 2000 degrees C. per hour.
According to the present invention, it becomes possible to synthesize group II-VI compound semiconductor polycrystals without using a quartz ampoule as a synthesis vessel and thus it becomes possible to upsize a vessel for synthesis, which results in reduction of a synthesis cost.
An exemplary embodiment of the present invention will be explained below with reference to the drawings.
The pressurizing device 4 may include, for example, a high pressure gas-container. The pressure in the high-pressure furnace 1 may be set at a desirable pressure by introducing the gas into the furnace 1 from the high pressure gas-container via a pressure reducing valve and a pressure controller 5.
According to the crystal synthesizing device of an embodiment, a synthesis vessel 6 is placed around the center of the high-pressure furnace 1 and the vessel 6 is surrounded by a heater 7. The synthesis vessel 6 has a double-container structure of an inner vessel 6a and an outer vessel 6b. The inner vessel 6a is composed of a cylindrical crucible having an opening at an uppermost portion and a disc-shaped lid having a diameter almost the same as an outer diameter at the uppermost opening portion of the crucible, both of which are made of pBN.
The outer vessel 6b is composed of a cylindrical body having a diameter slightly larger than that of the inner vessel 6a and an opening at an uppermost portion and a disc-shaped lid having a diameter of almost the same as an outer diameter at the uppermost opening portion of the cylindrical body, both of which are made of graphite. Because the outer vessel 6b made of graphite is gas-permeable, it is preferable to coat its surface by glass fiber type coating agent. It contributes to reduced gas leakage from the outer vessel 6b.
In addition, it is structured such that the lid of the outer vessel 6b can be fixed on the cylindrical body in semi-airtight state by a screw that is not shown in the drawing. Preferably a height of an inner space of the outer vessel 6b is designed to be the same height as the total height of the inner vessel 6a. By virtue of the design, it becomes possible to prevent the lid of the inner vessel 6a from moving caused by an inner gas pressure because the lid of the outer vessel 6b holds down the lid of the inner vessel 6a from upper side when the lid of the outer vessel 6b is fixed on the cylindrical body. It is also preferable to provide a lid opening/closing mechanism to open and close the lids so as to perform gas exchange of the synthesis vessel at the same time of the gas exchange of the high-pressure furnace before synthesis.
For preparing a CdTe polycrystal, for example, with the crystal synthesizing device of
After that the inside of the high-pressure furnace 1 is heated by supplying electric power to the heater 7.
The high-pressure furnace 1 is finally heated up to 1092 degrees C., as a melting point of CdTe, and remained in this state in predetermined period of time so as to react the Cd and Te completely to form CdTe. After that the inside of the furnace is cooled to promote growth of a CdTe polycrystal. The synthesis vessel 6 is then taken out, the lids of the inner vessel 6a and outer vessel 6b are removed and the CdTe polycrystal is taken out. According to the present invention, it is not necessary to break both of the inner vessel 6a and the outer vessel 6b and both of them can be reused.
A CdTe polycrystal was synthesized using the crystal synthesizing device explained in an exemplary embodiment having a pBN crucible of 6 inches in diameter and 40 cm in height. A Cd block and a Te block as raw materials, 20 kg in total weight, were put in the pBN crucible, the high-pressure furnace 1 was vacuumed and filled with an inert gas at a predetermined pressure (0.5 MPa, for example), the temperature was raised up to 1092 degrees C. over a period of 1 hour, kept the state in two hours and cooled inside of the furnace to the room temperature over five hours to grow the CdTe polycrystal, according to the method above explained. The synthesizing experiments were repeated in 120 times. Both of the inner vessel 6a and the outer vessel 6b did not break at all during the 120 times experiments.
As shown in
A Cd block and a Te block, 20 kg in total weight, were put in a pBN inner vessel 6a of 6 inches in diameter and 40 cm in length as shown in
It can be seen by the comparative examples 1 and 2 that, when a conventional synthesis method in which a quartz ampoule was used as an outer vessel 6b was applied, the quartz ampoule may sometimes burst and a frequency of the incident becomes high as the size of the ampoule becomes large. Thus the limitation of synthesis weight by the conventional method using the quartz ampoule was about 10 kg at a once.
According to an exemplary embodiment and comparative examples 1 and 2, it can be understood that the CdTe polycrystal can be grown by utilizing a graphite vessel instead of a quartz ampoule as an outer vessel 6b. In addition, an incident that the graphite vessel burst did not occur during the synthesis and thus the graphite vessel can be reused. Further, the pBN inner vessel 6a having a diameter of 20 cm or larger can be used and thus a polycrystal can be synthesized at least 20 kg or more at a time of synthesis, which resulting in an increasing of synthesis efficiency. Although an initial manufacturing cost of the graphite vessel is higher than that of the quartz ampoule, the total costs for synthesizing polycrystals can be reduced because several tens times of repeated usage are possible and single-use waste, which is generated when using a quartz ampoule, is eliminated.
An exemplary embodiment of the present invention was explained in details by way of an example and thus it is not restrictive. For example, the inner pressure of the high-pressure furnace 1 before heating was set 0.5 MPa in an embodiment. However, the inner pressure is not limited to 0.5 MPa but may be preferably 0.5 MPa to 1 MPa.
Temperature raising rate is not limited to an embodiment, either. It may be 100 to 2000 degrees C. per hour. It was explained that the high-pressure furnace 1 was filled with an inert gas at a predetermined pressure in an embodiment. However, it can be structured such that a reservoir portion is provided in the high-pressure furnace 1 so as to produce vapor of one of the raw materials (Cd, for example).
The present invention is applied to synthesize the CdTe polycrystal in the above explanation. However, the present invention can be applied to synthesize group II-VI compound semiconductor polycrystals other than CdTe polycrystal.
1 high-pressure furnace
2
a,
2
b piping
3 vacuum pump
4 pressurizing device
5 pressure controller
6 synthesis vessel (inner vessel 6a, outer vessel 6b)
7 heater
8 Cd block
9 Te block
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-074010 | Mar 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/055748 | 3/11/2011 | WO | 00 | 9/28/2012 |
