Patent Grant
RE39778
1. Field of the Invention
The present invention relates to a method of preparing a group III-V compound semiconductor crystal, and particularly a carbon-doped group III-V compound semiconductor crystal.
2. Description of the Background Art
Conventionally, there are various prior art methods for preparing a carbon-doped group III-V compound semiconductor crystal, as set forth in the following described publications.
Japanese Patent Laying-Open No. 64-79087 (referred to as “prior art 1” hereinafter) discloses a method of preparing a carbon-doped GaAs single crystal according to the gradient freeze method for horizontal Bridgman method (HB method).
The carbon of graphite boat 51 reacts with oxygen supplied from As2O3, Ga2O and the like remaining in quartz ampoule 55 to result in the formation of gas of CO, CO2 and the like which is doped into the growing GaAs crystal. It is described that the doping amount of carbon can be controlled according to the total amount of oxygen in the sealed quartz ampoule 55, the synthesization reaction condition, or single crystal growth condition, and the like.
The Journal of the Japanese Association of Crystal Growth, 1991, Vol. 18, No. 4, pp. 88-95 (referred to as “prior art 2” hereinafter) discloses a method of preparing a carbon-doped GaAs single crystal by the vertical gradient freeze method (VGF method).
Here, boron oxide containing 200 ppm of water spreads around only the periphery of the upper surface of GaAs melt 62. The center area of the upper surface of GaAs melt 62 is exposed to the ambient. According to the method of prior art 2, the upper surface of the melt must be exposed to the ambient to control the stoichiometry of the GaAs melt. The vapor pressure in quartz ampoule 65 is controlled by arsenic 67.
According to this method, the carbon concentration of the crystal depends on the carbon concentration of the raw material. U.S. Pat. No. 4,999,082 (referred to as “prior art 3” hereinafter) discloses a method of preparing a carbon-doped GaAs single crystal by the vertical Bridgman method. (VB method)
Referring to
Japanese Patent Laying-Open No. 3-252399 (referred to as “prior art 4” hereinafter) discloses a method of preparing a semi-insulating GaAs substrate. Prior art 4 is characterized in that the impurity which becomes the acceptor is doped so as to result in 1˜3×1015 atoms/cm3 after subtracting the concentration of the impurity which becomes the donor in a GaAs crystal.
Japanese Patent Laying-Open No. 2-74597 (referred to as “prior art 5” hereinafter) discloses a chromium-doped semi-insulating GaAs single crystal and a method of preparing the same. This prior art 5 is characterized in that carbon is contained in a concentration nc that satisfies both the relations of:
1×1015cm−3≦nc<nsi and nsi−nc≦4.4×1015cm−3
for the residual Si concentration nsi remaining in the single crystal, with the resistivity of at least 106Ω·cm.
The above-described prior art methods have various disadvantages. In prior art 1, boron oxide is not used. Therefore, impurity contamination can be expected. Furthermore, since the amount of the carbon source cannot be controlled in this method, it is difficult to control the carbon concentration.
In prior art 2, carbon cannot be doped during the crystal growth since a carbon source is not used. There is a problem that the carbon concentration cannot be adjusted during crystal preparation. Furthermore, a part of the carbon in the GaAs melt reacts with oxygen, which is generated as a result of the water in the boron oxide decomposing, to be lost as CO gas. As a result, there is a problem that the carbon concentration in the GaAs crystal is lowered.
In prior art 3, it is difficult to control the carbon concentration since the carbon source is located outside the crucible. Furthermore, impurity contamination can be expected since boron oxide is not used.
In prior art 4, carbon is recited as the impurity serving as the acceptor. However, only the doping of zinc and copper is disclosed as the example. There is no description of carbon doping.
Prior art 5 describes a chromium-doped semi-insulating GaAs single crystal containing carbon. However, this prior art 5 is silent about the method of doping carbon.
In view of the foregoing, an object of the present invention is to provide a method of preparing, with high reproducibility, a carbon-doped group III-V compound semiconductor crystal having favorable electrical characteristics and having impurities removed therefrom, in which the amount of doped carbon can easily be adjusted during crystal growth.
One aspect of the present invention provides a method of preparing a group III-V compound semiconductor crystal. This method of preparing a group III-V compound semiconductor crystal having carbon doped therein includes the steps of: filling a crucible or boat with compound raw material, solid carbon, and boron oxide; sealing the crucible or boat filled with compound raw material, solid carbon, and boron oxide in an airtight vessel formed of a gas impermeable material; heating and melting the compound raw material in a sealed state in the airtight vessel; and solidifying the melted compound raw material to grow a carbon-doped compound semiconductor crystal.
Since the crucible or boat is filled with compound raw material, solid carbon, and boron oxide according to the present invention, the boron oxide softened by heating is brought into contact with at least a portion of the solid carbon in the state where the compound raw material is melted.
According to the present invention, the carbon concentration in the initial raw material does not have to be adjusted since carbon can be doped into the material during crystal growth. Good controllability of the carbon concentration is obtained. In other words, the target carbon concentration can be obtained with high reproducibility. By using boron oxide which has an impurity removal effect, the contamination of impurities in the crystal can be suppressed to obtain a crystal of favorable electrical characteristics.
Quartz or pBN (pyrolytic boron nitride) and the like can be enumerated as the gas impermeable material.
Preferably, the boron oxide contains water. This is because the water in boron oxide is essential to remove impurities. Furthermore, it is considered that the water in the boron oxide effects the incorporation of carbon into the crystal. The boron oxide preferably contains 10-500 wt ppm of water.
In the present invention, the amount of solid carbon to be filled into the crucible is preferably larger than the amount of carbon to be doped into the compound semiconductor crystal. This is to promote reaction using an excessive amount of carbon since the reaction rate of solid carbon is extremely low. Furthermore, an additional amount of carbon must be supplied to make up for the consumption of part of the solid carbon in gas generation of the carbon compound. Thus, by using solid carbon of an amount larger than the total amount of carbon doped into the crystal, the advantage of the present invention works effectively. Specifically, the amount of solid carbon must be at least ten times, preferably at least 100 times larger than the weight of the carbon doped into the crystal.
In the present invention, it is preferred that the solid carbon is subjected to a heat treatment under reduced pressure before being filled in the crucible or boat. By this process, any impurity element remaining in the carbon is removed to result in a crystal of higher purity. The pressure in applying the heat treatment to the carbon is preferably from 1 Torr to 1×10−8 Torr. The appropriate temperature of the heat treatment is 500° C.-2000° C. The above-described effect can be obtained by carrying out the heat treatment for at least one hour. It was found that a greater effect can be obtained as the time for the heat treatment becomes longer. However, there is very little further change in the effect when the time for the heat treatment exceeds 12 hours. Considering that the cost for production is increased as the time for the heat treatment becomes longer, the time period for the heat treatment of not more than 12 hours is appropriate.
In the present invention, it is preferable to keep the compound raw material in its melted state for a certain time period before it is solidified for crystal growth. By this process, the impurities of Si and the like in the GaAs polycrystalline raw material can be removed by gettering with boron oxide. Although Si of approximately 1×1016cm−3 is included as impurities in the raw material synthesized by the HB method, the amount of Si in the GaAs subjected to the above-described process is less than 1×1015cm−3, which is below the detection limit of an analyzer. Si of an amount over 1×1015cm−3 was detected from those samples not subjected to the above-described process.
Thus, carbon can be sufficiently melted in the GaAs melt from the solid carbon by the above-described process. This process also provides the advantage that the temperature of the GaAs melt is stabilized, and the carbon concentration and impurity concentration in the melt can be made uniform.
The above-described effect can be obtained when the holding time period in the melted state of raw material is at least 3 hours. Further favorable characteristics can be obtained stably when the holding time is at least 6 hours. Although a greater effect can be obtained as the holding time becomes longer, the degree of change in the effect gradually becomes smaller when the holding time period exceeds 36 hours. There is very little further change in the effect when the holding time exceeds 72 hours. Considering that the cost for production becomes higher as the holding time is increased, the holding time is preferably not more than 72 hours, further preferably not more than 36 hours.
In the present invention, powder carbon can be used as the solid carbon. Powder carbon is advantageous in promoting the reaction due to its greater specific surface area. Increase in the reaction speed allows carbon to be doped efficiently in the crystal.
Also, the amount of carbon to be doped into the crystal can easily be adjusted according to the grain size, the weight, and the like, of the powder being used. For example, powder of a smaller grain size has a greater specific surface area to increase the reaction speed, whereby the amount of doped carbon is increased. Therefore, the grain size of the powder carbon is preferably smaller. More specifically, the average grain size is preferably not more than 100 μm, more preferably not more than 50 μm. When powder carbon is used, the powder carbon spreads in the boron oxide which is softened by heating in the state in which the compound raw material is melted.
In the present invention, fiber carbon, as well as powder carbon, can be used as the solid carbon. Fiber carbon is advantageous in that the diameter of the fiber is small and a greater surface area can be obtained to result in a faster reaction speed. It is therefore possible to dope carbon into the crystal efficiently. Also, the amount of carbon doped into the crystal can easily be adjusted according to the diameter or weight of the fiber that is used. Uniform distribution of the carbon concentration can be obtained from the shoulder to the tail of the prepared crystal when fiber carbon is used. The diameter of the fiber carbon is preferably smaller. Specifically, the average diameter is preferably not more than 50 μm, more preferably not more than 10 μm. Usage of fiber carbon allows carbon to spread in the boron oxide which is softened by heating in the state in which the compound raw material is melted. Also, the carbon can float above the boron oxide and thereby be exposed to the ambient.
In the present invention, bulk carbon can be used as the solid carbon, in addition to powder carbon and fiber carbon. Bulk carbon is advantageous in that the amount of carbon to be doped in the crystal can easily be adjusted by the weight and configuration of the carbon used. Uniform distribution of carbon concentration can be obtained from the shoulder to the tail of the prepared crystal when bulk carbon is used.
Bulk carbon is preferably used in a disk shape that is smaller than the inner diameter of the crucible. The amount of doped carbon can easily be controlled by the diameter of the disk. The bulk solid carbon is preferably a sintered compact of carbon powder. The reaction speed is particularly high for the sintered compact of powder having high porosity. Sintered carbon powder is advantageous in distributing carbon uniformly in the crystal. When bulk solid carbon is used, a state can be obtained in which at least a portion of the bulk solid carbon is immersed in the softened boron oxide.
In the present invention, the crucible or boat is preferably formed of pBN (pyrolytic boron nitride). Depending upon the constituent elements of the crucible or boat, there is a possibility that boron oxide or carbon reacts with the crucible and induces contamination of the raw material melt. pBN is most appropriate as the material of the crucible or boat to suppress reaction with boron oxide or carbon.
The present invention is particularly effective as a method of doping carbon into a GaAs crystal.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
Crucible 1 was inserted in a quartz ampoule 5 together with solid arsenic. Ampoule 5 was sealed under reduced pressure with a quartz cap 6.
Respective conditions of Example 1 are shown in the following Table 1.
Referring to
The condition mentioned above was maintained for approximately 36 hours. Then, heater 8 was moved upwards at the rate of 4 mm/hour, whereby solidification started from the portion of seed crystal 7. Thus, a single crystal was grown. The characteristics of the obtained single crystal are shown in the following Table 2.
In the present specification, the “shoulder” and “tail” of a crystal correspond to the relevant portions shown in FIG. 5.
The role of solid arsenic (As) sealed under reduced pressure in the quartz ampoule in the present example is set forth in the following. The dissociation pressure at the melting point of GaAs is approximately 1 atm. When GaAs is melted, the airtight vessel is filled with As vapor of approximately 1 atm at the temperature of the melting point. This As vapor is generated as a result of the GaAs melt being decomposed. Therefore, the composition of the GaAs melt is shifted from the original composition of GaAs=1:1 to Ga rich composition. By sealing solid arsenic in the quartz ampoule in addition to GaAs, the shift from the composition of Ga:As=1:1 caused by decomposition of the GaAs melt can be suppressed.
Quartz ampoule 5 was heated at the rate of approximately 120° C./hour by a heater 8 in a vertical furnace 50, as shown in FIG. 2. During the process of heating, boron oxide 4 was softened and melted. Also, GaAs polycrystalline raw material 2 was melted. At this time point, boron oxide 4 was present as a film 4a having a thickness of not more than 1 mm between pBN crucible 1 and GaAs melt 2. The remainder of boron oxide 4 covered the upper surface of the GaAs melt 2 as a boron oxide layer 4b having a thickness of approximately 5 mm. The carbon fiber 23 was partially dispersed in boron oxide layer 4b on GaAs melt 2, and partially floated. Furthermore, a portion of carbon fiber 23 was present also at the proximity of the interface between GaAs melt 2 and boron oxide layer 4b.
Then, the condition mentioned above was maintained for approximately 12 hours. Then, heater 8 was moved upwards at the rate of 3 mm/hour, whereby solidification started from the portion of seed crystal 7. Thus, a single crystal was grown. The characteristics of the obtained single crystal are shown in the following Table 4.
A carbon-doped GaAs single crystal was grown using 20 mg of carbon fiber similar to that of Example 2. The other conditions of the experiment were identical to those of Example 2, and their description will not be repeated. The characteristics of the obtained single crystal are shown in the following Table 5.
A carbon-doped GaAs single crystal was grown using 7.5 mg of carbon fiber similar to those of Examples 2 and 3. The other conditions were identical to those of Examples 2 and 3, and their description will not be repeated. The characteristics of the obtained single crystal are shown in the following Table 6.
It is appreciated from Examples 2, 3 and 4 that the carbon concentration in the crystal can easily be adjusted by just adjusting the amount of solid carbon to be doped according to the present invention.
This crucible 1 was inserted in a quartz ampoule 5 together with solid arsenic. Quartz ampoule 5 was sealed under reduced pressure using quartz cap 6. Respective conditions of example 5 are indicated in the following Table 7.
The above-described quartz ampoule 5 was heated at the rate of approximately 200° C./hour by heater 8 in a vertical furnace 50. During the process of heating, boron oxide 4 was softened and melted. Also, GaAs polycrystalline raw material 2 was melted. At this time point, boron oxide 4 was present as a film 4a having a thickness of less than 1 mm between pBN crucible 1 and GaAs melt 2. The remainder of boron oxide 4 covered the upper surface of GaAs melt 2 as a boron oxide layer 4b having a thickness of approximately 6 mm. Carbon disk 43 had its bottom surface in contact with raw material melt 2, and its top surface exposed to the ambient. The side surface thereof was surrounded by boron oxide layer 4b.
The condition mentioned above was maintained for approximately 6 hours. Then, heater 8 was moved upwards at the rate of 4 mm/hour, whereby solidification started from the portion of seed crystal 7. Thus, a single crystal was grown. The characteristics of the obtained single crystal are shown in the following Table 8.
In a semi-insulating GaAs crystal, the resistivity is one of the most important characteristics. It is preferable that variation in resistivity is smaller. Furthermore, since this resistivity value depends on the carbon concentration in the GaAs crystal, variation in the carbon concentration in the crystal should be as small as possible.
In the above-described examples where carbon fiber or bulk carbon was used as the solid carbon, the carbon was doped substantially uniformly from the shoulder to the tail of the crystal. It is appreciated that carbon fiber and bulk carbon are preferable as solid carbon sources. The shape of bulk carbon is not limited to the disk shape shown in Example 5, and any shape can be used. Also, bulk carbon is preferably a sintered compact of carbon powder.
Comparison of the effect of the present invention depending upon differences in the type of solid carbon is shown in the following Table 9.
Comparison of the carbon concentration in a GaAs crystal between the present invention and the prior art is shown in the following Table 10.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 8-107009 | Apr 1996 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4083748 | Gault | Apr 1978 | A |
| 4404172 | Gault | Sep 1983 | A |
| 4478675 | Akai | Oct 1984 | A |
| 4521272 | Gault | Jun 1985 | A |
| 4678534 | Tada et al. | Jul 1987 | A |
| 4923561 | Chemans et al. | May 1990 | A |
| 4946544 | Ejim | Aug 1990 | A |
| 4999082 | Kremer et al. | Mar 1991 | A |
| 5041186 | Nishio et al. | Aug 1991 | A |
| 5131975 | Bourret-Courchesne | Jul 1992 | A |
| 5145550 | Tada et al. | Sep 1992 | A |
| 5186784 | Rau et al. | Feb 1993 | A |
| 5256381 | Tada et al. | Oct 1993 | A |
| 5259916 | Rau et al. | Nov 1993 | A |
| 5342475 | Yoshida et al. | Aug 1994 | A |
| 5387544 | Hayafuji | Feb 1995 | A |
| 5400742 | Nishio et al. | Mar 1995 | A |
| 5415125 | Fujita et al. | May 1995 | A |
| 5454346 | Uchida et al. | Oct 1995 | A |
| 5471945 | Nishio et al. | Dec 1995 | A |
| 5515810 | Yamashita | May 1996 | A |
| Number | Date | Country |
|---|---|---|
| 0 417 843 | Mar 1991 | EP |
| 0529963 | Aug 1992 | EP |
| 0529963 | Mar 1993 | EP |
| 59-054699 | Mar 1984 | JP |
| 60-210599 | Oct 1985 | JP |
| 01037833 | Feb 1989 | JP |
| 64-37833 | Feb 1989 | JP |
| 64-79087 | Mar 1989 | JP |
| 02034597 | Feb 1990 | JP |
| 2-74597 | Mar 1990 | JP |
| 3-252399 | Nov 1991 | JP |
| 04-104989 | Apr 1992 | JP |
| 06-128096 | May 1994 | JP |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 08843124 | Apr 1997 | US |
| Child | 09824965 | US |
