JPH0511080B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a manufacturing technology for compound semiconductor single crystals, for example, by liquid confinement pulling method (LEC method).
This article relates to an effective technology that can be used to produce GaAs single crystals with low carbon concentrations. [Conventional technology] Compound semiconductor single crystals such as GaAs, GaP, and InP are
It has characteristics that cannot be obtained with Si single crystals, and is used as a substrate for electronic and optical devices. Among them, GaAs has attracted attention in recent years as a substrate for high-speed ICs because it can be undoped to produce a semi-insulating substrate. This GaAs single crystal for ICs is manufactured, for example, by the LEC method using direct synthesis. Incidentally, in order to obtain semi-insulating crystals with good reproducibility, it is necessary to reduce residual impurities in the crystals as much as possible. Various impurities include those that originally exist in the raw material and those that are mixed in during the handling of the raw material or during the setting of the raw material and parts in the furnace. However, one of the impurities that poses a particularly big problem is carbon. This is due to the fact that in a normal pulling furnace, a large amount of graphite parts are used for the crucible's susceptor, heater, heat insulating material, and the like. These graphite parts are usually made by mixing coke powder and a binder, shaping, firing, graphitizing, and then processing graphite material, which is then subjected to a high-purity treatment. Ru. However, since these graphite materials are originally sintered bodies, the porosity (ratio of voids) therein is approximately 10 to 15%, and these voids are almost interconnected open voids. For this reason, the actual specific surface area of graphite parts is very large, and
In addition to easily adsorbing CO ₂ into graphite parts, the components of the binder used in the sintered body may volatilize into the outside air as hydrocarbons when graphite parts are heated. come. Because the various gases mentioned above flow out during crystal growth, conventionally, these graphite parts were set in a pulling furnace without being vacuumed or vacuumed.
The various gases adsorbed in graphite parts were released by reducing the pressure or by heating with a heater while pressurizing with an inert gas such as _N2 , and then the crystals were pulled up. However, in such conventional methods, the graphite parts are heated in the furnace, which not only contaminates the inside of the furnace, but also causes substances emitted from the high-temperature part to be adsorbed again to the low-temperature part, or to cause single crystal Since it is heated using a heater for growth, it is difficult to heat most of the graphite insulation material to a sufficient temperature.
Therefore, degassing is insufficient, and during actual long-term single crystal growth, a certain amount of oxygen, CO ₂ gas, various hydrocarbons, etc. that were not sufficiently removed from the graphite parts remain in the furnace. It will be full. These gases, for example, react with graphite to become CO ₂ and CO, and together with the gas that has already flowed out from the graphite parts as CO ₂ , they dissolve into the sealant, and _some of them are melted. Ga in liquid
It is thought that the carbon reacts with the carbon and becomes carbon, which is mixed into the melt. Therefore, as mentioned above, the method of baking graphite parts in a pulling furnace cannot prevent gas from being released into the furnace, and there is no way to prevent carbon from entering the melt. Very inconvenient.
However, due to the large size of graphite parts, only the above-mentioned method has been used until now. Therefore, 5×10 ¹⁵ to 3×10 ¹⁶ cm ^-3 in the crystal
It was common for some carbon to be mixed in. In order to prevent this type of carbon from being mixed in, there is a method, for example, of applying a strong magnetic field parallel to the surface of the raw material melt to suppress convection within the melt to reduce the amount of carbon mixed in as much as possible. The amount of carbon in the crystal was reduced by using heaters and heat insulating parts coated with carbon (Nikkei Micro Devices, published by Nikkei McGraw-Hill).
(November 1986 issue, pp. 99-110), the maximum amount of carbon in the crystal is about 3×10 ¹⁵ cm ^-3 , and 2
It has not yet been possible to reduce the amount to below ×10 ¹⁵ cm ^-3 . This invention was made with attention to the above-mentioned problems, and its purpose is to reduce the carbon concentration in the single crystal after growth even in a pulling device in which graphite parts are used in the hot zone. The object of the present invention is to sufficiently reduce the resistivity of the crystal and its dispersion, and to improve thermal stability. [Means for Solving the Problems] In order to achieve the above object, the present invention puts the graphite parts constituting the crystal growth apparatus into a vacuum heating apparatus before starting crystal pulling, and heats the graphite parts under reduced pressure.
After heating to a temperature above the melting point of the GaAs single crystal, cooling to room temperature in vacuum or inert gas, and placing the graphite parts together with raw materials in a crystal growth apparatus, crystal growth begins. A GaAs single crystal having a carbon concentration of less than 2×10 ¹⁵ cm ^-3 is obtained. [Operation] According to the above-described means, impurities (especially carbon and its compounds) contained in graphite parts are sufficiently degassed in the vacuum heating device, so that, for example, impurities contained in graphite parts are removed in the furnace of the crystal pulling device. Even if the parts are heated above the melting point of the grown crystal after it is installed and the pulling starts, the gas degassed from the graphite parts will hardly be mixed into the furnace atmosphere. Carbon concentration can be significantly reduced. Hereinafter, preferred embodiments of the present invention will be described. [Example] FIG. 1 shows an example of a vacuum heating apparatus used for carrying out the present invention. In the figure, reference numeral 1 denotes a device body made of a heat insulating material, etc. A support stand 2 for placing parts is provided at the center of the bottom wall of the device body 1, and above the device body 1,
A top lid 3 that can seal the inside of the container is placed. A heating body 4 such as a carbon electrode is arranged around the support base 2 so as to surround the object placed thereon. Furthermore, an exhaust pipe 5 connected to an external vacuum pump and a gas feed pipe 6 connected to a gas supply device such as a gas cylinder are connected to the side wall of the apparatus main body 1. Furthermore, temperature sensors 7a and 7b such as thermocouples are attached so that the temperature of the area inside the heating body 4 can be detected. Although not shown, a cooling water pipe is provided in the main body 1 or the upper lid 3, and after heating is completed, the main body 1
The structure is such that the temperature inside can be quickly lowered. Next, an embodiment of the crystal manufacturing method of the present invention using the vacuum heating device described above will be described. First, a heater made of graphite, a heat insulating material, and other graphite parts 10 constituting the hot zone of the crystal pulling device are placed on the support stand 2 of the vacuum heating device, and the top lid 3 is placed on the top. Then, the vacuum pump is activated to draw out the air inside the device body 1 to create a vacuum level of 1×10 ^-3 Torr or more, and then power is supplied to the heating element 4 to remove the graphite component 10 on the support base 2. is heated, and while the temperature is detected by temperature sensors 7a and 7b, the temperature is raised to a temperature higher than the melting point of the single crystal to be grown (for example, 1238° C. in the case of GaAs). This heating state is maintained until the degree of vacuum measured by the vacuum pump's ionization vacuum gauge (measuring range 1 x 10 ^-3 to 1 x 10 ^-7 Torr) hardly decreases (usually takes 2 to 5 hours). Let's do it. In this way, after sufficient heating and degassing under high vacuum, the temperature of the vacuum heating device is quickly lowered to room temperature while maintaining the vacuum or by supplying an inert gas such as N ₂ gas from the gas supply pipe. do. Then, the graphite component 10 is taken out from the apparatus main body 1 and immediately placed in a high-pressure furnace of a crystal pulling apparatus together with a raw material or a crucible containing a raw material and a sealing agent such as B ₂ O ₃ . Thereafter, N ₂ gas or Ar gas is supplied into the high-pressure furnace of the crystal pulling apparatus to pressurize it, and then the single crystal is heated above its melting point to begin pulling the single crystal. Applying the above example, if the graphite parts constituting the hot zone are heated in a vacuum heating device separate from the single crystal pulling device and then installed in the pulling device, the graphite parts with a large number of pores inside Impurities such as oxygen, CO ₂ or binder used during sintering that have been adsorbed in the parts have been sufficiently degassed, so these graphite parts are removed along with the crucible when pulling the single crystal. Even when heated, impurities adsorbed in the pores do not fill the high-pressure furnace. As a result, compared to single crystals produced using conventional methods,
The carbon concentration in the crystal is significantly reduced and the normal FT
-The carbon concentration can be reduced to the extent that it cannot be detected by carbon concentration measurement using the IR method (Fourier transform infrared spectroscopy). Furthermore, the method of the present invention is easier to implement than conventional methods that aim to reduce carbon concentration by applying a strong magnetic field to the raw material melt in a crystal pulling device or using parts coated with AlN. Moreover, the carbon concentration can be sufficiently reduced with good reproducibility. Next, the results of actually pulling a single crystal by the LEC method using the vacuum heating apparatus shown in FIG. 1 and applying the present invention will be described. First, baking of graphite parts is 6.2×
The heating was carried out at 1600° C. in a vacuum of 10 ⁻⁴ to 7.5×10 ⁻⁵ Torr and maintained for 5 hours. The raw materials gallium and arsenic are each
A high purity product of 99.99999wt.% was used, and a predetermined amount of these was charged into a pBN (pyrolytic boron nitride) crucible with a diameter of 6 inches, and then B ₂ O ₃ as a sealant was placed on top of the crucible. This crucible is placed in the high-pressure furnace of the pulling equipment and heated with a heater to directly synthesize the raw materials gallium and arsenic, and then GaAs
The seed crystal was immersed in the melt and pulled up at a speed of 9 mm/hour. During crystal growth, the inside of the furnace was filled with Ar gas at 20 atm, and after the growth was completed, the crystal was slowly cooled to room temperature over about 8 hours. Thereafter, the GaAs single crystal was taken out and the carbon concentration was measured using the FT-IR method. The detection limit concentration of the experimental equipment used by the inventor is 2.0×
10 ¹⁵ cm ^-3 . Table 1 below shows the measurement results. For comparison, the results of measuring the carbon concentration of crystals grown by the conventional method without baking graphite parts are also shown.

【table】

[Table] From Table 1, the carbon concentration of crystal lots (lot codes E, F, G, H) obtained by the conventional method without baking graphite parts is 2.7 to 6.0×
10 ¹⁵ cm ^-3 , and none of them were less than 2×10 ¹⁵ cm ^-3 , whereas all of the crystal lots (A, B, C, D) obtained by applying the present invention were It can be seen that the carbon concentration is below the detection limit of 2×10 ¹⁵ cm ^-3 . In the above example, graphite parts with many pores were heated and degassed, but parts other than graphite parts may also be baked at the same time to remove surface impurities. Of course. As explained above, in the case of pulling a single crystal by the liquid-sealed pulling method, the graphite parts constituting the hot zone of the crystal pulling device are placed in a vacuum heating device before the start of crystal pulling. , after heating to a temperature above the melting point of the GaAs single crystal under reduced pressure, cooling to room temperature in vacuum or inert gas, and then placing the graphite parts together with the raw materials in the high-pressure furnace of the crystal pulling equipment. Since the crystal growth has started, the impurities contained in the graphite parts are sufficiently degassed by the vacuum heating equipment, so these parts are installed in the high-pressure furnace of the crystal pulling equipment. Even if the crystal is heated above the melting point of the grown crystal after the start of pulling, the gas degassed from the graphite parts will hardly enter the furnace atmosphere, thereby significantly reducing the carbon concentration in the grown crystal. be able to. As a result, the variation in resistivity in the crystal is reduced, and the change in resistivity before and after heat treatment is small, that is, a crystal with high thermal stability can be obtained. Although this invention has been described above as being applied to the growth of GaAs single crystals by the direct synthesis method using the LEC method, the present invention is not limited thereto.
It can also be used to grow other compound semiconductor single crystals, and even polycrystalline GaAs, which is the raw material for pulling single crystals.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional front view showing an example of a vacuum heating apparatus used in carrying out the present invention. 1...Device main body, 2...Support stand, 3...Top lid,
4...Heating body, 5...Exhaust pipe, 7a, 7b...Temperature sensor, 10...Graphite parts.

Claims (1)

[Claims] 1 Place the graphite parts that make up the crystal growth device into a vacuum heating device before starting crystal growth,
The graphite parts are heated to a temperature higher than the melting point of the GaAs crystal under reduced pressure, cooled to room temperature in a vacuum or in an inert gas, and then placed in a crystal growth apparatus together with the raw materials, and then a single crystal is grown. 1. A method for producing a GaAs single crystal, characterized in that a GaAs single crystal having a carbon content of less than 2×10 ¹⁵ cm ^-3 is obtained.