JPH0813717B2 - Silicon molecular beam growth method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon molecular beam growth method, and more particularly to phosphorus doping in a silicon growth method using a gas source.

(Prior Art) In the silicon molecular beam epitaxial growth method, a method of heating solid silicon with an electron beam has been conventionally used as a method of generating a silicon molecular beam. In such solid source silicon molecular beam epitaxial growth, doping into the epitaxial film is performed by simultaneously supplying an extremely small amount of dopant from the Knudsen cell containing the dopant during growth. However, in this case, it is necessary that the vapor pressure of the dopant can be appropriately controlled by the heating temperature of the Knudsen cell. Arsenic, phosphorus and antimony are often used as n-type dopants in silicon crystals. However, since the vapor pressure of arsenic and phosphorus was too high, doping with a conventional Knudsen cell was impossible. Therefore, in the conventional silicon molecular beam epitaxial growth method, only antimony can be used as the n-type dopant. Since antimony has a lower solid solubility limit and is easier to diffuse than arsenic and phosphorus, for example, an emitter layer of an npn bipolar transistor that requires steep and n-type high-concentration doping is formed by using a silicon molecular beam epitaxial growth method. It was impossible.

(Problems to be Solved by the Present Invention) A problem to be solved by the present invention is to enable high concentration doping of phosphorus with good controllability in a silicon molecular beam epitaxial growth method.

(Means for Solving the Problems) The present invention uses a gas source silicon molecular beam growth method in which a silane-based gas is used as a source gas as a silicon molecular beam growth method, and further uses a phosphine obtained by diluting phosphorus with a silane-based gas. The present invention provides a doping method with good controllability of phosphorus in the silicon molecular beam growth method, which is characterized in that it is supplied simultaneously in the form of growth.

(Operation) If a silicon source molecular beam epitaxy method of a gas source method is used as a silicon growth method, a substance in a gas state which cannot be used in the conventional solid source silicon molecular beam epitaxy method can be used as a dopant. If the dopant can be supplied in a gas state, the doping concentration can be controlled by controlling the flow rate, and it becomes possible to use phosphorus as a dopant, which was not available in the conventional solid source molecular beam growth method due to the vapor pressure. Phosphorus becomes a gas at room temperature when it forms a phosphine molecule made of three hydrogen atoms bonded to one phosphorus atom.
Therefore, this phosphine can be used as a source of phosphorus. However, the solid solubility limit of phosphorus in silicon crystals is ~ 2
× 10 ²⁰ cm ^-3 , and the number of phosphine molecules required is about 1 to / 10 ^{4 of the} silicon atoms used for growth even when considering high concentration doping up to the solid solubility limit. . Therefore, the flow rate of phosphine used for growth needs to be controlled in an extremely minute amount and accurately. However, it is actually difficult to accurately control such a minute flow rate. To overcome this difficulty, phosphine is used as a dopant gas diluted with silane, which is a source gas for growth in the gas source silicon molecular beam growth method, to about several percent. In this case, the flow rate to be controlled is larger than that when pure phosphine is used, the flow rate control is easy, and more accurate phosphine flow rate control is possible. In gas source silicon molecular beam epitaxial growth, the degree of vacuum during growth is 10 ^-5 Torr or less, and under such a high vacuum, the gas is not in thermal equilibrium with the substrate temperature. The probability of collision between gas molecules is also very low. Therefore, this is different from the growth process in the normal chemical vapor reaction growth method in which gas molecules are dissociated in the gas phase and reach the substrate. In gas source silicon molecular beam epitaxial growth, all gas molecules reach the substrate without being decomposed in the gas phase. The gas molecules contribute to the growth by receiving thermal energy and dissociatively adsorbed on the substrate. Dissociative adsorption on the substrate surface occurs due to the reaction between chemically active dangling bonds on the silicon surface and gas molecules. This situation also applies to the phosphine molecule of the dopant gas and the silane molecule that dilutes it. Hydrogen is often used when diluting phosphine. However, when hydrogen is used in the gas source silicon molecular beam epitaxial growth method, active dangling bonds on the surface of the substrate are saturated with hydrogen, and the growth hardly occurs. On the other hand, when silane is used for phosphine dilution, silane itself can be used as a growth source gas, and there is an advantage that it does not adversely affect growth. Further, in the gas source silicon molecular beam epitaxial growth method, the temperature of the gas molecules in the vapor phase is lower than the substrate temperature because the degree of vacuum during growth is good. Further, since collisions between gas molecules are almost negligible, a process in which phosphine reacts with silicon in the vapor phase to reach the substrate in the form of a compound does not occur unlike the ordinary chemical vapor deposition method. In the gas source silicon molecular beam epitaxial growth, all the phosphines reach the substrate as they are, and are efficiently incorporated as a dopant into the epitaxial film by the dissociation reaction on the substrate surface. Therefore, high-concentration doping, which cannot be realized by the ordinary chemical vapor deposition method, becomes possible.

(Example) The present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic diagram of a gas source silicon molecular beam growth apparatus for explaining an embodiment of the present invention. A 4-inch p-type Si (100) wafer 1 was used as the substrate. This substrate is loaded into the gas source silicon molecular beam growth apparatus 2. This silicon substrate is heated at 900 ° C. for 10 minutes by the heater 3 on the back side of the substrate in the ultrahigh vacuum silicon molecular beam growth apparatus. This process results in a clean Si (100) surface. The cleanliness of the surface was confirmed by observing the characteristic 2 × 1 surface superstructure on the clean Si (100) surface in the diffraction pattern of the reflection high-energy electron diffraction device composed of the high-speed electron gun 4 and the fluorescent screen 5. did. The silicon substrate is maintained at 630 ° C. on this clean surface, and disilane, which is a source gas for silicon growth, is supplied from the gas cell 6. For phosphorus topping, 5% phosphine diluted with silane is used. This phosphine gas is supplied from the phosphine gas cylinder 7 to the sub chamber 8. At the same time, disilane is also supplied from the disilane gas cylinder 9 to the rust chamber, and these are mixed in the sub chamber and irradiated from the gas cell toward the substrate. The flow rate of disilane gas supplied to the sub-chamber was 41 sccm, and the flow rate of silane-diluted phosphine was changed in the range of 0 to 0.5 sccm by the needle valve 10. The substrate is partially irradiated with this. The flow rate of the gas emitted from the gas cell toward the substrate is ~ 1scc.
m.

FIG. 2 shows the result of Hall measurement of the carrier concentration of the 0.4 μm thick epitaxial film formed on the Si (100) substrate by the above method. It can be seen that the conductivity type of the epitaxial film is n-type as expected when phosphorus is doped, and its carrier concentration is proportional to the flow rate of phosphine in the range of 10 ¹⁹ to 10 ²⁰ cm ^-3 . That is, it is understood that the high-concentration doping of phosphorus controlled by the flow rate of phosphine is realized.

Although disilane is used as the silicon raw material and silane is used as the phosphine diluent gas in the above examples, any silane-based gas may be used.

(Effects of the Invention) As described in detail above, according to the present invention, since phosphorus can be doped as an n-type dopant in the silicon molecular beam epitaxial growth method, high concentration doping can be realized. Further, since the silane-based gas is used as the dilution gas for the phosphine used as the phosphorus raw material, the phosphine can be used efficiently.

[Brief description of drawings]

FIG. 1 is a schematic view of a gas source type silicon molecular beam epitaxial growth apparatus for explaining an embodiment of the present invention, and FIG. 2 shows carrier concentration and phosphine flow rate for explaining an embodiment of the present invention. It is the figure which showed the relationship.
In the figure, 1 is a Si (100) substrate, 2 is a gas source silicon molecular beam growth apparatus, 3 is a substrate heater, 4 is a high-speed electron gun for reflection high-energy electron diffraction, 5 is a fluorescent screen, 6 is a gas cell, and 7 is silane. A phosphine gas cylinder diluted to 5%, 8 is a sub-chamber, 9 is a disilane gas cylinder, and 10 is a needle valve.

Claims (1)

[Claims] 1. When doping phosphorus in silicon molecular beam growth using a silane-based gas as a silicon source gas,
A method for growing silicon molecular beams, characterized in that a phosphine gas diluted with a silane-based gas is introduced into the growth chamber at the same time as the silicon source gas.