Method of Chemically Vapor Depositing a Thin Film of GaAs

The present invention relates to a vapor deposition method for the GaAs thin film by which it has been made possible particularly to manufacture the wafer with a small dispersion in the distribution of the carrier density of the film of deposited n-type crystals.

In general, for the vapor deposition for the GaAs thin film, a vapor deposition apparatus shown in FIG. 4 is used, wherein a high-frequency induction heating coil (5) is provided around the outer circumference of a reaction tube (1) with an introductory port (2) of source gases at the upper end and an exhaust port (3) of gases at the lower end through a cooling jacket (4) and a carbon susceptor (6) in the shape of truncated hexagonal pyramid is arranged in the reaction tube (1). With this apparatus GaAs substrates (7) are fitted onto the pyramid faces of the susceptor (6) and, allowing the susceptor to rotate in the direction of arrow mark, source gases are introduced into the reaction tube (1) from the introductory port (2) and allowed to eject from the exhaust port (3) at the lower end.

In this way, the substrates (7) are heated to a predetermined temperature by the heating coil (5) and, through the thermal decomposition of stock gases near the surface of the substrates (7), the crystals of GaAs are allowed to deposit onto the substrates (7).

As the source gases, organic gallium, for example, trimethyl gallium [Ga(CH.sub.3).sub.3 ] and arsine gas (AsH.sub.3) are used. The deposition conditions to allow the high resistance GaAs layer to deposit are generally that the supplying ratio (V/III) of the source gases of Ga(CH.sub.3).sub.3 and AsH.sub.3 lies between 10 and 20 and the temperature of the substrate is near 650.degree. C. Moreover, for the formation of n-type film (a carrier density of not less than 1.times.10.sup.16 cm.sup.-3), gas having an impurity possible to become the source for the supply of electrons as an ingredient element is to be added to both stock gases. As the impurity, sulfur is used most frequently and, as the gas containing this, hydrogen sulfide gas (H.sub.2 S) is used. By varying the flow rate of this gas, the concentration of electrons, that is, the carrier density of n-type crystals is controlled.

For example, for the deposition of epitaxial wafer used for field effect transistor, high resistance crystal film (hereinafter abbreviated as buffer layer) is allowed to deposit onto the GaAs substrate in a thickness of 2 to 3 .mu.m and n-type crystal film (hereinafter abbreviated as doping layer) is allowed to deposit thereon in a thickness of about 0.5 .mu.m. The buffer layer is made from non-dope crystals added no impurity intentionally and is allowed to deposit making the supplying ratio (V/III) 10 to 20 since the carrier density becomes lowest and the resistivity becomes lowest within a range of the supplying ratio (V/III) of AsH.sub.3 and Ga(CH.sub.3).sub.3 of 10 to 20. In succession H.sub.2 S is added to these gases and the doping layer is allowed to deposit at the same deposition temperature as that in case of the buffer layer.

Although the carrier density of the depositing doping layer can be controlled by the flow rate of H.sub.2 S to be added to source gases as described above, it depends also on other deposition conditions. Namely, the carrier density of the doping layer varys also with the supplying ratio (VIII) of the source gases of AsH.sub.3 and Ga(CH.sub.3).sub.3 and further with the flow rate of Ga(CH.sub.3).sub.3 or the temperature of the substrate. As a result, in the crystal deposition of the wafer with a large area, there has been a problem that the dispersion is caused in the distribution of the carrier density of the face of wafer due to the temperature distribution on susceptor, the difference in the decomposition ratio of AsH.sub.3 resulting from the location on susceptor, or the like.

As a result of various investigations in view of the situation, a vapor deposition method of wafer has been developed, wherein the dispersion in the distribution of the carrier density of the face of wafer is small in the treatment of wafer with a large area by means of the vapor deposition. Namely, the invention is characterized in that, in the method wherein arsine gas and organic gallium gas are allowed to react through thermal decomposition by heating the GaAs substrate in a mixed stream of both gases and further H.sub.2 S is added to both gases to allow the doping layer to deposit, the temperature of the substrate is raised to 700.degree. to 800.degree. C. and kept thereat to allow the doping layer to deposit, wherein supplying ratio (V/III) of source gases is made between 10 and 20.

FIG. 1 is an illustration diagram showing the relationship between the temperature of the substrate and the carrier density of the doping layer deposited.

FIG. 2 is an illustration diagram showing the relationship between the temperature of the substrate and the dispersion in the distribution of the carrier density of the doping layer deposited.

FIG. 3 is an illustration diagram showing one example of the time chart in the vapor deposition method of the invention.

FIG. 4 is an illustration diagram showing one example of the vapor deposition apparatus. In the last diagram, 1 is a reaction tube, 2 is an introductory port of source gases, 3 is an exhaust port, 4 is a cooling jacket, 5 is a high-frequency induction heating coil, 6 is a susceptor, and 7 is a GaAs substrate.

According to the invention, a vapor deposition apparatus, for example, the apparatus shown in FIG. 4 is employed, and, at the time when the doping layer is allowed to deposit through the thermal decomposition reaction by using arsine gas and organic gallium gas and further by adding hydrogen sulfide gas the substrate is kept at 700.degree. to 800.degree. C. for the deposition.

In the invention, by raising and keeping the temperature of the substrate at 700.degree. to 800.degree. C. for the deposition of doping layer onto the GaAs substrate, the dispersion in the distribution of the carrier density of doping layer decreases to within 5%. The reason why the distribution of the carrier density of doping layer descreases by keeping the temperature of the GaAs substrate at high temperature is not clear, but, as the temperature becomes high, the carrier density is lowered and the dispersion in the distribution of the carrier density becomes also small. Here, the reason why the temperature of the substrate was confined to 700.degree. to 800.degree. C. is due to the facts that the dispersion in the distribution of the carrier density cannot be suppressed within 5% if the temperature of the substrate is lower than 700.degree. C., and the disadvantages appear in the crystallinity of wafer obtained and in others if the temperature exceeds 800.degree. C.

Employing the apparatus shown in FIG. 4 and using AsH.sub.3 and Ga(CH.sub.3).sub.3 for source gases, the buffer layer was allowed to deposit in a thickness of about 2 .mu.m onto the GaAs substrate having a diameter of 2 inches. Thereafter, the temperature of the substrate was raised to respective temperatures of 600.degree., 650.degree., 710.degree. and 750.degree. C. and, adding H.sub.2 S to both gases described above, the doping layer having a thickness of about 0.5 .mu.m was allowed to deposit to manufacture the epitaxial wafer for field effect transistor.

Besides, the molar fraction of H.sub.2 S to whole carrier gas was made 1.83.times.10.sup.-6, the molar fraction of Ga(CH.sub.3).sub.3 was made 1.23.times.10.sup.-4, and the molar fraction of AsH.sub.3 was made 1.23.times.10.sup.-3.

Of the wafer with a diameter of 2 inches thus deposited the carrier density within a diameter of 40 mm on the doping layer and the dispersion in the distribution of the carrier density within a diameter of 40 mm at respective locations within a diameter of 2 inches were measured. The results thereof are shown in FIG. 1 and FIG. 2.

FIG. 1 shows the relationship between the temperature of the substrate and the carrier density and FIG. 2 shows the temperature of the substrate and the dispersion in the distribution of the carrier density. As evident from the diagrams, it can be seen that, with an increase of the temperature of the substrate, the carrier density decreases and the dispersion in the distribution of the carrier density becomes also small simultaneously and that the carrier density is 8.0.times.10.sup.16 cm.sup.-3 and the dispersion in the distribution of the carrier density is suppressed lower than 5% at a temperature of the substrate of 700.degree. to 800.degree. C.

Besides, in the manufacture of epitaxial wafer for field effect transistor as described above, an example of the time chart by which the buffer layer is allowed to deposit onto the substrate making the temperature of the substrate 650.degree. C. and, following this, the doping layer is allowed to deposit onto the buffer layer raising the temperature of the substrate to 710.degree. C. is shown in FIG. 3. Namely, AsH.sub.3 is supplied constantly at a rate of molar fraction of 1.23.times.10.sup.-3 and the substrate is heated. When the temperature of the substrate reached to 650.degree. C., Ga(CH.sub.3).sub.3 is supplied for 60 minutes at a rate of molar fraction of 1.23.times.10.sup.-4 to allow the buffer layer to deposit onto the substrate. Then, the supply of Ga(CH.sub.3).sub.3 is stopped and the temperature of the substrate is raised from 650.degree. C. to 710.degree. C. Thereafter, by supplying Ga(CH.sub.3).sub.3 again at a rate of molar fraction of 1.23.times.10.sup.-4 and, at the same time, by supplying H.sub.2 S for 10 minutes at a rate of molar fraction of 2.00.times.10.sup.-6, the doping layer may be allowed to deposit onto the buffer layer.

According to the invention, in the vapor deposition method wherein the doping layer is allowed to deposit onto the GaAs substrate, by raising the temperature of the substrate to 700.degree. to 800.degree. C. and by keeping thereat during the deposition, the dispersion in the distribution of the carrier density within a diameter of 40 mm on the doping layer of wafer with a diameter of 2 inches can be suppressed lower than 5%. Therefore, the invention exerts a remarkable effect in that the elements for electronic devices can be made out efficiently.