JP5419116B2 - Bulk crystal growth method - Google Patents

Description

  The present invention relates to a bulk single crystal / polycrystal growth technique by a growth method using sublimation recrystallization (hereinafter abbreviated as “sublimation method”).

By adding impurities to crystals with high purity, for example, electrical conductivity characteristics, optical characteristics, thermal conductivity characteristics, mechanical characteristics, chemical resistance characteristics, environmental resistance characteristics, heat resistance, radiation resistance characteristics, etc. There is a crystal manufacturing technology that provides functionality.
Particularly in semiconductor materials, electrical conductivity control by adding impurities has been performed for a long time, and electrical and electronic devices that make use of various characteristics of each material such as single element semiconductors and compound semiconductors have been developed and manufactured.

  For example, silicon carbide (SiC), a group IV-IV compound semiconductor, has excellent thermal and chemical characteristics and a large forbidden bandwidth compared to Si semiconductors. It attracts attention as a new semiconductor material for high temperature and high frequency devices.

  SiC is an intrinsic semiconductor, and pure crystals that are not mixed with impurities have poor conductivity. Therefore, in order to give p-type and n-type semiconductor characteristics to SiC, it is common to introduce conductivity by introducing group III and group V elements as impurities, for example. There is a method of injecting these impurities from the outside after the crystal is grown, but generally, a method of introducing an impurity simultaneously with the raw material at the time of crystal growth and forming it as a crystal mixed with impurities is often carried out.

Large bulk single crystal growth for the purpose of hexagonal SiC wafer production is performed by a sublimation method (modified Lerry method: see Non-Patent Document 1) in which a raw material is heated and sublimated to recrystallize and grow on a seed crystal. Is common. FIG. 1 shows an example of an apparatus for growing a SiC single crystal by a sublimation method. The apparatus mainly includes a crucible 1, a lid 2, a seed crystal 3, a SiC raw material 4, a single crystal 5, a pedestal 6, a gas introduction port 7, a gas exhaust port 8, and a heating coil 9. The crucible 1 is mainly composed of columnar and cylindrical graphite, and is performed in a semi-sealed space in which the upper portion of the crucible is closed with a graphite lid 2. The SiC raw material 4 is loaded in the lower part of the crucible, and the seed crystal 3 is placed and fixed on a pedestal 6 sufficiently protruded from the lid 2 so as to face the SiC raw material 4.

  The SiC raw material 4 is usually SiC powder obtained by the Atchison method or chemical synthesis. As the seed crystal 3, an SiC single crystal obtained by the Atchison method or the Lely method, or an SiC single crystal grown from the Atchison crystal or the Lely crystal by the sublimation method is used. The growth is performed in a high-purity argon gas (Ar gas) atmosphere introduced from the gas inlet 7, and nitrogen gas (N 2 gas), for example, is introduced from the gas inlet 7 as an impurity. The introduced gas is exhausted from the gas exhaust port 8. While heating the crucible with a heating coil 9 such as a high-frequency furnace, resistance heating furnace, infrared furnace, etc., while measuring the temperature at the upper end of the crucible (seed crystal temperature: Ta) and the temperature at the lower end (raw material temperature: Tb) with a color thermometer Control. At this time, the seed crystal temperature and the raw material temperature are controlled to 2000 to 2500 ° C., and the temperature gradient between the raw material and the seed crystal (Tb-Ta) is controlled to 0 to 20 ° C./cm. The growth is started by depressurizing the inside of the growth apparatus after heating to the control temperature, and maintaining the constant pressure at 1 to 100 Torr.

A single crystal 5 is grown on the seed crystal 3 by performing growth under the above conditions using this apparatus. The rate of crystal growth can be increased by lowering the pressure in the crystal device, or increasing the growth temperature or increasing the temperature gradient.
However, in the conventional sublimation method, it has been found that a large number of crystal defects are generated in the grown single crystal 5 and the crystallinity is deteriorated.
Journal of Crystal Growth 43 (1978) 209-212 Journal of Crystal Growth 230 (2001) 239-246

  An object of the present invention is to suppress generation of crystal defects in a sublimation method and obtain a high-quality bulk crystal.

In the sublimation method, impurities are introduced in a gasified form and added to the growing crystal. However, if the amount of added impurities is too large, that is, if the limit impurity addition amount is exceeded, the original crystal composition is disturbed. Crystal quality decreases.
The growth rate is just zero when crystal growth is just started, while the opposite is true at the end. In this way, crystal growth always has an arbitrary growth rate and a transition region up to zero growth rate. If you enter a region where changes in growth rate are unavoidable while maintaining any growth conditions that achieve the target impurity addition concentration in the grown crystal, the impurity concentration in the crystal becomes extremely high and the limit impurity addition amount Will be exceeded. Therefore, in such a growth rate transition region, crystal defects are generated due to excessive impurities.
Therefore, in order to solve the above-described problems, the present invention controls the impurity addition amount so as not to exceed the limit impurity addition amount in the growth rate transition region.

According to the present invention, the addition of impurities is stopped during the transition region of the crystal growth rate that starts immediately after the start of the growth and between the transition region of the crystal growth rate that reaches immediately before the end of the growth and immediately before the end of the growth. Thus, generation of crystal defects can be suppressed and a high-quality bulk crystal can be obtained.

Hereinafter, the present invention will be described in detail based on examples.
(Example)
A SiC single crystal was grown on the seed crystal with the crystal growth apparatus shown in FIG. Nitrogen gas was mixed with argon gas and introduced so that the conductivity of the crystal became n-type during growth. In order to ensure that the impurity concentration of nitrogen does not exceed 1 × 10 ¹⁹ cm ^{−3 in} the growth region 10 immediately above the seed crystal 3 which is a transition region of the crystal growth rate, the thickness is about 100 μm without mixing nitrogen gas. Grown up. Thereafter, nitrogen gas was introduced at a concentration of 50% so that the impurity concentration of nitrogen became 8 × 10 ¹⁸ cm ⁻³ after the growth rate was stabilized. When the crystal was grown in this growth rate stable region 11 to a thickness of about 25 mm, the growth was completed. At this time, the introduction of nitrogen gas was stopped so that the nitrogen impurity concentration was 1 × 10 ¹⁹ cm ⁻³ or more in the growth rate transition region 12 at the end of growth.

After that, the crystal growth apparatus was opened, the raw materials were newly replaced, and the growth apparatus was loaded again, and the second growth was continued. At this time, as in the first time, in the growth region 13 immediately above the growth crystal, which is a transition region of the crystal growth rate, the thickness is increased without mixing nitrogen gas so that the impurity concentration of nitrogen does not exceed 1 × 10 ¹⁹ cm ^−3. It was grown about 100 μm. Thereafter, nitrogen gas was introduced at a concentration of 50% so that the impurity concentration of nitrogen became 8 × 10 ¹⁸ cm ⁻³ after the growth rate was stabilized. When the crystal was grown in this growth rate stable region 14 to a thickness of about 25 mm, the growth was completed. At this time, the introduction of nitrogen gas was stopped so that the impurity concentration of nitrogen would not be 1 × 10 ¹⁹ cm ⁻³ or more in the growth rate transition region 15 at the end of growth.

The crystal was cut into a plate having a thickness of 1 mm in a direction perpendicular to and parallel to the growth direction to prepare a sample for evaluation. The sample for evaluation was polished to have a flat and mirror-finished surface, and the processing damage was removed from the surface. Evaluation was performed by observation of etch pits of defects using molten KOH and X-ray topography to analyze crystal defects. As a result, no generation of defects was observed at the interface between the seed crystal and the growth crystal, and the interface between the first growth crystal and the second growth crystal, and no increase in dislocation density was observed.
If the second crystal growth is unnecessary, the second and subsequent crystal growths can be omitted. If necessary, the third and subsequent crystal growth can be performed in the same manner following the second crystal growth.

(Comparative example)
A SiC single crystal was grown on the seed crystal with the crystal growth apparatus shown in FIG. During the growth, nitrogen gas was introduced so that the conductivity of the crystal was n-type. In the growth region 10 immediately above the seed crystal 3, which is a transition region of the crystal growth rate, nitrogen gas is supplied at a concentration of 50% so that the nitrogen impurity concentration in the crystal becomes 8 × 10 ¹⁸ cm ⁻³ in the growth rate stable region 11. The introduced amount was grown to a thickness of about 100 μm, and the crystal was subsequently grown to a thickness of about 25 mm in the growth rate stable region 11. Thereafter, the growth was terminated, but the amount of nitrogen gas introduced was not changed even in the growth rate transition region 12 at the end of the growth.

After that, the crystal growth apparatus was opened, the raw materials were newly replaced, and the growth apparatus was loaded again, and the second growth was continued. At this time, as in the first time, in the growth region 13 immediately above the growth crystal, which is the transition region of the crystal growth rate, the nitrogen impurity concentration in the crystal in the growth rate transition region 14 is 50 × 10 ¹⁸ cm ^−3. Nitrogen gas was introduced at a concentration of% to grow about 100 μm in thickness, and then a crystal was grown about 25 mm in thickness in the growth rate stable region 14. Thereafter, the growth was terminated, but the amount of nitrogen gas introduced was not changed even in the growth rate transition region 15 at the end of the growth.

The crystal was cut into a plate having a thickness of 1 mm in a direction perpendicular to and parallel to the growth direction to prepare a sample for evaluation. The sample for evaluation was polished to have a flat and mirror-finished surface to remove damage caused by processing from the surface. Evaluation was performed by observation of etch pits of defects using molten KOH and X-ray topography to analyze crystal defects. As a result, a large number of crystal defects such as dislocations within the basal plane and stacking faults are newly generated at the interface between the seed crystal and the grown crystal and the interface between the first grown crystal and the second grown crystal. It turned out to be worse. When the impurity concentration in the vicinity of these interfaces was measured, it was found that the impurity concentration of nitrogen was added at least 1 × 10 ¹⁹ cm ⁻³ or more in the portion corresponding to the growth rate transition region. This is probably because an impurity concentration higher than the design was added because the amount of impurities taken into the crystal increased in a region where the growth rate was slow.

(Applications other than SiC)
Although the present invention has been described above by taking SiC as an example, the present invention can also be applied to bulk crystal growth of III-V group compounds such as AlN and II-VI group compounds such as ZnSe obtained by the sublimation method. In this case, the limit impurity addition amount is 1 × 10 ¹⁹ cm ^{−3 in} SiC, but the limit impurity addition amount is appropriately determined according to the material in consideration of the generation of crystal defects.

It is a schematic sectional drawing of the single crystal growth apparatus explaining a prior art example. It is a schematic sectional drawing of a single crystal growth apparatus.

1 crucible 2 lid 3 seed crystal 4 SiC raw material 5 single crystal with many crystal defects 6 pedestal 7 gas introduction port 8 gas exhaust port 9 heating coil 10 growth rate transition region 11 at the start of growth 11 growth rate stable region 12 at the end of growth Growth rate transition region 13 Growth rate transition region 14 at the start of the second growth Growth rate stable region 15 Growth rate transition region at the end of the second growth

Claims (1)

When a SiC bulk crystal to which impurity nitrogen is added is produced by the sublimation method, the growth ends between the transition region of the crystal growth rate starting immediately after the start of the growth and from the transition region of the crystal growth rate immediately before the end of the growth. Immediately before, the addition of impurities is stopped, and the amount of impurities added between them is controlled so as not to exceed the limit impurity addition amount of 1 × 10 ¹⁹ cm ^−3. A method for growing a bulk crystal, comprising adding an impurity in a stable growth rate region.

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