JPH10189459A - Forming method for boron-doped silicon-germanium mixed crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a boron-doped silicon-germanium mixed crystal having a predetermined boron concentration irrespective of a profile of germanium as silicon-germanium mixed crystal formed by doping boron. SOLUTION: The method for forming boron-doped silicon-germanium mixed crystal by vapor phase growing the mixed crystal and simultaneously doping boron therewith comprises the steps of changing supply amount of hydrogenated germanium gas of germanium material to desired degree in the case of vapor phase growing the mixed crystal, and changing supply amount of diborane gas of boron doping source corresponding to change of the supply amount of the germanium gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a boron-doped silicon-germanium mixed crystal by vapor-growing a silicon-germanium mixed crystal and simultaneously doping the same with boron.

[0002]

In recent years, from the expected of applications such as Si-based high-speed bipolar devices and super lattice element, silicon - germanium (hereinafter, referred to as _{Si (1-X) Ge X} ) mixed crystal has been attracting attention. In order to commercialize such a Si _(1-X) Ge _X mixed crystal, there are various methods for using, for example, a Si substrate as a Si-based base and epitaxially growing a Si _(1-X) Ge _X mixed crystal thereon. Considered and partially implemented.

[0003] Si _(1-X) Ge obtained by such a method
_With respect to the strained superlattice structure of _X / Si, various devices have been developed utilizing electric and optical properties caused by the lattice strain. For example, one of the devices is a base portion of a bipolar transistor. To Si _(1-X)
2. Related Art A heterojunction bipolar transistor (hereinafter abbreviated as HBT) using Ge _x / Si is known.
This Si _(1-x) Ge _x / Si-based HBT can increase the injection of carriers from the emitter to the base as compared with the homojunction HBT, and can increase the h without increasing Rb or τEB.
FE can be secured, fmax = 50 GH
A high-speed bipolar transistor of about z can be realized.

By the way, in order to manufacture an NPN type HBT using the structure of Si _(1-X) Ge _x / Si, it is necessary to use Si
It is necessary to dope boron in _(1-X) Ge _X mixed crystal layer. In general, the formation of a Si _(1-X) Ge _X mixed crystal is performed by gas source MBE, ultra-high vacuum chemical vapor deposition (UHV-CV).
D) or the low pressure chemical vapor deposition (LP-CVD) method. In these various film forming methods, as a method of doping boron, for example, Si
Silicon hydride (S) is a gas for _(1-X) Ge _X mixed crystal film formation.
_{i n H (2n + 2)} ; n ≧ 1) and gas or dichlorosilane (SiH ₂ Cl ₂₎ gas, hydrogenated germanium (Ge
_When reacting with nH _{(2n + 2)} ; n ≧ 1) gas in the reaction furnace, a method of simultaneously supplying diborane (B ₂ H ₆ ) gas into the reaction furnace and supplying it to the reaction is employed. .

At this time, when it is desired to obtain a desired Ge profile by changing the concentration of Ge, conventionally, as shown in FIG. 5, the flow rate (supply amount) of the above-mentioned germanium hydride gas is changed. The reaction is performed while the flow rate (supply amount) of the B ₂ H ₆ gas is constant as shown in the range indicated by A in FIG.

[0006]

[SUMMARY OF THE INVENTION However, in this method, at 630 ° C. or less which is the surface reaction rate-limiting conditions, due to the growth rate of the Si _(1-X) Ge _X mixed crystal layer is reduced When the Ge composition ratio in the Si _(1-X) Ge _X mixed crystal film decreases as indicated by a portion A surrounded by a circle in FIG.
On the contrary, a phenomenon such as an increase in the boron concentration in the film occurs.

When such a phenomenon occurs, for example, the above-described method for forming a Si _(1-X) Ge _X mixed crystal is described as follows.
_(1-X) Ge _X mixed crystal layer when applied to the production of an NPN-type HBT based, base made of Si _(1-X) Ge _X mixed crystal is boron concentration is changed in the layer during the This leads to inconvenience that device characteristics such as hFE vary.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon-germanium mixed crystal doped with boron, which has a constant boron concentration regardless of the profile of germanium. An object of the present invention is to provide a method capable of forming a boron-doped silicon-germanium mixed crystal.

[0009]

SUMMARY OF THE INVENTION According to the present invention, there is provided a boron-doped
In the method for forming a silicon-germanium mixed crystal, a silicon-germanium mixed crystal is vapor-phase grown, and at the same time, a boron-doped silicon-germanium mixed crystal is formed by doping it with boron. During the growth, the supply amount of the germanium hydride gas, which is a germanium source, is changed to a desired degree, and the supply amount of the diborane gas, which is a boron doping source, is changed according to the change in the supply amount of the germanium hydride gas. This change is defined as a means for solving the above problem.

According to the method of forming a boron-doped silicon-germanium mixed crystal, the supply amount of diborane gas is changed in accordance with a change in the supply amount of germanium hydride gas when the silicon-germanium mixed crystal is grown in vapor phase. The B ₂ H ₆ gas supply is changed by the change in the boron concentration in the formed Si _(1-X) Ge _X mixed crystal due to the change in the supply amount of the germanium hydride gas. amount increase of the boron concentration is offset by changing, thereby Si _(1-X) boron concentration in Ge _X mixed crystal is constant.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on an embodiment. In this embodiment, as shown in FIG. 2, first, a Si buffer layer 2 is formed on a Si substrate 1, then a Si _(1-X) Ge _X mixed crystal layer 3 is formed, and then a Si cap layer is formed. 4 is formed.

First, as the Si substrate 1 shown in FIG. 2, a substrate cleaned as follows is prepared. To clean the Si substrate 1, first, the surface of the Si substrate 1 is treated with a sulfuric acid-hydrogen peroxide solution heated to a predetermined temperature to remove organic substances adhering to the substrate surface. Next, the Si substrate 1 was heated with an ammonia-hydrogen peroxide solution heated to a predetermined temperature.
Treat the surface to remove particles adhering to the substrate surface. Thereafter, the surface of the Si substrate 1 is treated with dilute hydrofluoric acid to remove metal contaminants and oxide films on the substrate surface.

After the cleaning process is performed in this manner, the Si substrate 1 is immediately placed in an epitaxial reactor for epitaxial growth, and the Si buffer layer 2 and Si ₍₁₎ are formed on the Si substrate 1 as shown in FIG. _-X) a Ge _X mixed crystal layer 3, Si cap layer 4 are formed in this order. In this epitaxial growth, a low-pressure chemical vapor deposition apparatus of an infrared lamp heating type is used as an epitaxial reactor. When the cleaned Si substrate 1 is placed in the reaction furnace (a low-pressure chemical vapor deposition apparatus), the Si substrate 1 is placed in a hydrogen gas atmosphere at 850 prior to film formation by epitaxial growth.
Heat treatment is performed at a temperature of about 1100 ° C. (preferably 900 ° C.) for about 5 minutes to remove a natural oxide film formed on the surface of the Si substrate 1 after the above-described cleaning treatment.

Next, a film is formed by an epitaxial growth method according to the film forming sequence shown in FIG. In film formation, first, while supplying H ₂ gas into the reaction furnace,
As shown in FIG. 1, the temperature in the reaction furnace is set to 700 ° C., and SiH ₄ gas, which is a silicon hydride gas, is supplied to form a Si buffer layer 2 on the surface of the Si substrate 1 to a thickness of about 30 nm.

Subsequently, H ₂ gas and SiH are introduced into the reactor.
₄ While supplying gas, the temperature inside the reactor
To Then, in this state, the supply of GeH ₄ gas, which is a germanium hydride gas, into the reactor is started. However, at this time, in order to make germanium have a delta-type doping profile, as shown in FIG. 1, the supply amount of the GeH ₄ gas is changed stepwise so as to gradually decrease over time. Then, a few minutes after the start of the supply of the GeH ₄ gas, that is, in this example, as shown in FIG.
The supply of the B ₂ H ₆ gas is started when the supply amount of the eH ₄ gas shifts from the maximum amount to the next largest amount, and thereafter, the supply amount of the B ₂ H ₆ gas is also changed to the supply amount of the germanium hydride gas. It is changed stepwise so as to gradually decrease with the passage of time in accordance with the change in the amount.

Here, when the supply amount of the GeH ₄ gas is reduced in a state of being changed stepwise, Si _(1-X) Ge _X
The film formation speed of the mixed crystal decreases. This is because SiH ₄ and GeH
₄ , the adsorption rate constant on the surface of the substrate (in this example, the Si buffer layer 2) is such that the bonding site of the Si—Ge pair is another Si—Ge pair.
The binding site of the Si pair is larger than the binding site of the Ge—Ge pair, and the surface reaction rate constant of SiH ₄ is Ge—Ge.
Si _(1-X) Ge
_This is because the composition ratio of Ge in the _X mixed crystal greatly affects the film formation rate.

However, if the composition ratio of Ge is reduced during the formation of the Si _(1-X) Ge _X mixed crystal layer 3 as described above, the growth rate of the mixed crystal layer 3 is reduced. As shown in FIG. 1, as the supply amount of GeH ₄ gas changes (decreases), the supply amount of B ₂ H ₆ gas changes (decreases) accordingly. That is, in this embodiment, FIG.
As shown by the middle broken line, if the slope of the change in the supply amount of GeH ₄ gas is Kg and the slope of the change in the supply amount of B ₂ H ₆ gas is Kb, then Kg = Kb.

Here, the relationship between the doping amount of boron in the Si _(1-X) Ge _X mixed crystal and the growth rate of the mixed crystal layer will be described below. The amount of boron deposited in unit time per unit area and the deposition rate _{Db, Si (1-X)} Ge boron concentration in _X mixed crystal _{^{Cb (cm -3), Si (}} 1-X) Ge X
Assuming that the growth rate of the mixed crystal is R (cm / sec), Db is represented by Db = Cb × R (cm ² · sec ⁻¹ ).

Db is determined by the partial pressure of boron during film formation.
It is determined. Therefore, Si_(1-X)Ge_XBoron in mixed crystals
The concentration Cb decreases as the growth rate R increases.
If it decreases, Cb increases. That is, the growth rate R
Tte Si_(1-X)Ge_XThe boron concentration in the mixed crystal changes
It is. Furthermore, Si_(1-X)Ge_XIn mixed crystals, G
eH_FourGas and SiH_FourAnd GeH_FourMixed gas consisting of
Gas ratio [GeH_Four] / ([SiH_Four] + [GeH
_Four]) Determines the Ge composition ratio.
Gas ratio and Si_(1-X)Ge_XThe figure shows the mixed crystal growth rate.
The relationship shown in FIG.

Therefore, in this embodiment, the aforementioned
GeH_FourSlope Kg and B of change in gas supply_TwoH₆
Since the inclination Kb of the change in the gas supply amount is made equal,
As described above, GeH_FourGas supply is low
As the Ge composition ratio decreases, the growth rate R
Becomes smaller, and as a result, Cb increases, _Two
H₆As the gas supply decreases, the Cb increases.
With the addition being offset, Si_(1-X)Ge_XIn mixed crystal
The boron concentration becomes constant. Therefore, this embodiment
In the example, the Si shown in FIG._(1-X)Ge_XFor film formation of mixed crystal 3
The relationship (K) described above in the range indicated by A in FIG.
g = Kb) and B_TwoH₆Since the flow rate is changed, FIG.
As shown by the part A surrounded by the middle circle, germanium (G
e) has a delta doping profile and
Constant concentration of Si_{(1 -X)}Ge_XMixed crystal layer 3 was obtained.
You.

After the Si _(1-X) Ge _X mixed crystal 3 is formed in this way, the supply of GeH ₄ gas is stopped while the H ₂ gas is supplied into the reaction furnace as shown in FIG. 700
Increase to ° C. Then, SiH ₄ and B ₂ H are introduced into the reactor.
₆ gas is supplied to form the Si cap layer 4 to a thickness of about 80 nm. After the film formation is completed in this manner, the Si substrate 1 is taken out of the reaction furnace, and thereby, the boron-doped Si _(1-X) Ge _X mixed crystal layer 3 is formed.
As a result, a layer is obtained in which germanium has a delta-type doping profile, while boron has a constant concentration.

In the above embodiment, Si _(1-X) G
Although the film forming temperature of the e _X mixed crystal layer 3 was set to 625 ° C., the film forming temperature may be in the range of 550 ° C. to 630 ° C. As a gas serving as a Si raw material, silicon hydride other than SiH ₄ (Si _n H _{(2n + 2)} ; n ≧ 1) and dichlorosilane (SiH ₂ Cl ₂ ) can be used. As for the gas to be used, not only GeH ₄ but also germanium hydride Ge _n H _{(2n + 2)} ; n ≧ 1) can be used.

Further, in the above embodiment, in particular, Si
_As an example of the method of forming the _(1-X) Ge _X mixed crystal layer 3, a low pressure chemical vapor deposition method has been described. However, the present invention is not limited to this. An ultra-high vacuum chemical vapor deposition method or the like can also be employed. In the above embodiment, the Ge source gas and B ₂ H ₆
Although the example in which the gas is changed stepwise has been described, the mass flow controller that controls the supply amount of the gas may be adjusted so that the gas is changed not stepwise but continuously.

[0024] In the above embodiment, although so as to grow the Si _(1-X) Ge _X mixed crystal thin film over the entire surface of the Si substrate 1, to form an insulator mask on the Si surface, said insulating mask Alternatively, epitaxial growth may be selectively performed on the Si surface exposed at the opening. In this case, hydrogen chloride (HCl) or chlorine (Cl ₂ ) may be added to the Si source gas, the Ge source gas, and the B ₂ H ₆ gas. Further, in the above-described embodiment, Ge is doped in a delta type to form the profile, but the present invention is not limited to this. For example, as in the case of doping Ge in a trapezoidal shape, It is needless to say that the present invention can be applied to the formation of the portion for changing the Ge concentration.

[0025]

As described above, in the method of forming a boron-doped silicon-germanium mixed crystal of the present invention, when the silicon-germanium mixed crystal is grown in a vapor phase, the supply amount of diborane gas is controlled by the supply of germanium hydride gas. Since the method is changed in accordance with the change in the amount, the change in the boron concentration in the formed Si _(1-X) Ge _X mixed crystal due to the change in the supply amount of the germanium hydride gas is originally considered. , B ₂ H ₆ gas supply amount is changed to offset the increase in boron concentration.
_(1-X) the boron concentration in the Ge _X mixed crystal can be made constant. Therefore, this method is used for Si _(1-X) Ge _X based HB
By applying the present invention to the manufacture of T or Si _(1-X) Ge _X- based MOS, a device having excellent electric characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a film forming sequence for explaining an embodiment of the present invention.

FIG. 2 is a side sectional view of a structure obtained by a film forming sequence shown in FIG.

FIG. 3 is a graph showing a relationship between a gas ratio and a growth rate.

FIG. 4 is a diagram showing concentration profiles of Ge and B of a structure obtained by the film forming sequence shown in FIG.

FIG. 5 is a diagram showing a film forming sequence for explaining an example of the related art.

6 is a diagram showing a concentration profile of Ge and B of a structure obtained by the film forming sequence shown in FIG.

[Explanation of symbols]

Reference Signs List 1 Si substrate 2 Si buffer layer 3 Si
_(1-X) Ge X mixed crystal layer 4 Si cap layer

Claims (1)

[Claims] 1. A method for forming a silicon-germanium mixed crystal by vapor-phase growth of a silicon-germanium mixed crystal at the same time as forming a boron-doped silicon-germanium mixed crystal by doping boron. At this time, the supply amount of the germanium hydride gas as the germanium raw material is changed to a desired degree, and the supply amount of the diborane gas serving as the boron doping source is made to correspond to the change in the supply amount of the germanium hydride gas. A method for forming a boron-doped silicon-germanium mixed crystal, characterized in that the mixed crystal is changed.