Method and apparatus of the continuous preparation of epitaxial layers of semiconducting III-V compounds from vapor phase

Abstract

This invention relates to a method for the continuous preparation of epitaxial layers of semiconducting III-V compounds on suitable substrates of the single crystal from a gas mixture thereof. The plurality of substrates are moved in parallel with the flow of said gas mixture in a reaction furnace which is given a temperature gradient in the same direction as the flow of said gas mixture, the temperature decreasing with distance in the direction of the gas flow. The semiconducting compounds are effectively recovered as epitaxial layers on the substrates, and the composition of the mixed crystal of III-V semiconductors can be changed continuously to result in the graded composition, without timewise changing the composition of said gas mixture. Through the gas mixture entering into the reaction furnace is of constant composition, the composition of the gas mixture is gradually changed downstream of the gas flow in the reaction furnace as a result of effective recovery of the semiconducting compounds on the substrates.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method of continuously preparing on a III-V semiconductor substrate the epitaxial layers of one of the III-V semiconductors which include III-V binary semiconductors consisting of a III-B element of the Periodic Table and a V-B element (for example, GaAs, GaP, InP, etc.) and especially their mixed crystals (for example, InAs.sub.1-x P.sub.x, , GaAs.sub.1-x P.sub.x, In.sub.1-x Ga.sub.x P, etc. where 0 < x < 1).

The continuous vapor growth method, particularly the method of continuously growing epitaxial layers on a single crystal stubstrate, was suggested already in the past with respect to Si and Ge. For instance, a method of continuously growing epitaxial layers of Si from a gas mixture of hydrogen and SiCl.sub.4 is described in Japaneses Patent Publication No. SHO-43/5564 by the present applicant. Another patent publication, Japanese Patent Publication No.SHO-43/21369, reveals a method wherein epitaxial layers of Si are continuously prepared from a gas mixture of hydrogen and SiCl.sub.4, while the continuous growth of the epitaxial layers is expedited by the formation of an oxide film on the surface of the epitaxially grown silicon layer from CO.sub.2 gas.

However, not only III-V semiconductors but also other compound semiconductors at large are unlike Si and Ge. They consist of two or more elements. Since control is required to maintain a stoichiometric ratio during the growth of these semiconductors, the continuous growth of the epitaxial layers of these compound semiconductor is very difficult and is seldom accomplished.

The growth of epitaxial layers of III-V semiconductors is generally carried out by a batch process, which is not continuous. For example, there are the following reports.

I. Japanese Patent Publication No.SHO-40/24704, "A Method for the Deposition of Epitaxial Film of III-B - V-B element compounds," reveals a method of preparing epitaxial layers by flowing a hydrogen gas in the storage of a volatile compound of a III-B element and the source of a V-B element or the storage of a volatile compound of a V-B element and bringing the reaction mixture from these two flows in contact with a substrate, the method being applicable not only to III-V binary semiconductors but also to III-V mixed crystal semiconductors. However, all that is mentioned therein refers to a batch process. Nothing is mentioned with respect to the continuous growth of epitaxial layers.

II. Japanese Publication No. SHO-42/7492, "A Method of Manufacturing Epitaxial Films," describes a method of depositing an epitaxial film having a graded energy gap of III-V mixed crystal, wherein a gas mixture similar to that of the above (I) is used and the composition of two kinds of reacting gases of the same group of the Periodic Table is changed timewise by changing the flow rate of the afore-mentioned hydrogen gas or the temperature of the afore-mentioned source or storage tank, to thereby grow an epitaxial film having a graded energy gap. However, the description also refers to a batch process. It is impossible to continuously grow an epitaxial film while changing the composition of the reacting gases timewise by this method. It is inevitable that a batch process must be adopted which is discontinuous or semi-continuous.

III. The method (II) above is described in a more concrete way in John W. Burd: Transactions of the Metallurigcal Society of AIME Vol. 245, March (1969), p. 571. As to gas mixtures, the mixtures of H.sub.2 /HC1/GaAs and P, H.sub.2 /HC1/Ga and AsH.sub.3 /PH.sub.3 and H.sub.2 /AsC1.sub.3 /PC1.sub.3 /Ga are mentioned. However, a batch process only is described.

IV. A method described in Tooru Hara et al: Oyobutsuri, Volume 37, No. 11(1968), p. 1064, refers to the gas mixture from H.sub.2 /AsC1.sub.3 / PC1.sub.3 /Ga, that is, the gas mixture from the reaction products of a hydrogen gas saturated with AsC1.sub.3 and PC1.sub.3 with Ga. It is especially mentioned in this literature that the value of x in GaAs.sub.1-x P.sub.x varies according to the temperature of the substrate. It is also mentioned that if the temperature of the GaAs substrate is kept at 815.degree.C, GaAs.sub.1-x P.sub.x grows having a value of x equal to the ratio of As and P in the, therefore, the value of x can be controlled by the composition of the gas. However, nothing is mentioned in regard to how the value of x can be controlled for a continuous epitaxial growth. Furthermore, according to this literature, a decreasing temperature gradient with distance from the Ga source zone to the GaAs substrate zone, i.e., in the direction of the gas flow is provided. Such a system unavoidably has a drawback in that besides the growth of GaAs.sub.1-x P.sub.x on the GaAs substrate, GaAs.sub.1-x P.sub.x is also deposited on the inner wall of the reaction tube on the side of the Ga source zone from the GaAs substrate in the aforementioned temperature gradient zone, so that GaAs.sub.1-x P.sub.x is not effectively recovered on the afore-mentioned substrate. This drawback is common also to the afore-mentioned (I), (II) and (III).

V. U.S. Pat. No. 3,441,453, "Method for Making Graded Composition Mixed Compound Semiconductor Materials" by R.W. Conrad et al describes a kind of batch process, wherein the composition of the gas mixture is of constant composition and a substrate is moved in the reaction furnace having a decreasing temperature gradient with distance in the direction of gas flow, thereby gradually changing the temperature of said substrate so as to deposit the graded composition compound semiconductor. This process also has a drawback as aforementioned in method (IV). Description of the Invention

This invention has for an object epitaxial layers of a III-V mixed crystal semi-conductor on a plurality of III-V compound semiconductor substrates while at the same time having the composition of the mixed crystal semiconductor (for example, the value of x in GaAs.sub.1-x P.sub.x) change continuously, without changing timewise the composition of said gas mixture.

A characteristic of this invention is that in the method of growing an epitaxial layer of a III - V ternary compound semiconductor represented by a general formula AB.sub.1-x C.sub.x on a III - V binary compound semiconductor from a gas mixture, wherein AB.sub.1-x C.sub.x stands for both A.sup.III B.sup.V.sub.1-x C.sup.V.sub.x and B.sup.III.sub.1-x C.sup.III.sub.x A.sup.V, A.sup.III being a III-B element, A.sup.V a V-B element, B.sup.V and C.sup.V two different V-B elements, B.sup.III and C.sup.III two different III-B elements and the value of x between 0 and 1, the value of x is continuously changed to result in the graded composition without changing timewise the composition of said gas mixture entering into the reaction furnace, which is given a temperature gradient, decreasing with distance in the direction of the gas flow, by continuously moving the plurality of said III-V binary compound semiconductor substrates, upstream of said gas flow in the case of the substrate which has a lower melting point, and downstream of said gas flow in the case of the substrate which has a higher melting point than said ternary compound semiconductor, respectively. If the binary compound semiconductor AB has a lower melting point than the semiconductor AC, the composition of the gas mixture in the reaction furnace is richer in the constituent C and poorer in the constituent B in the higher temperature zone and vice versa in the lower temperature zone of the reaction furnace. If the semiconductor AB has a higher melting point than the semiconductor AC, the change of the composition of the gas mixture is reversed.

As AB.sub.1-x C.sub.x compounds in the afore-mentioned characteristics of this invention, there are GaP.sub.1-x N.sub.x (0<x <<1), A1As.sub.1-x P.sub.x, GaAs.sub.1-x P.sub.x, InAs.sub.1-x P.sub.x (0<x<1 for the foregoing three), Ga.sub.1-y A1.sub.y, As.sub.1-x P.sub.x, In.sub.1-y Ga.sub.y As.sub.1-x P.sub.x (0<x <1, 0<y<1 for the foregoing two), GaSb.sub.1-x As.sub.x, InSb.sub.1-x As.sub.x (0<x <1 for the foregoing two), InSb.sub.1-x Bi.sub.x (0<x<<1), In.sub.1-x Ga.sub.x N, Ga.sub.1-x Al.sub.x P, In.sub.1-x Al.sub.x P, In.sub.1-x Ga.sub.x P, Ga.sub.1-x B.sub.x As, Ga.sub.1-x Al.sub.x As, In.sub.1-x Al.sub.x As, In.sub.1-x Ga.sub.x As, Ga.sub.1-x Al.sub.x Sb, In.sub.1-x A1.sub.x Sb, In.sub.1-x Ga.sub.x Sb (0<x<1 for the foregoing eleven), etc.

The gas mixture is produced from various reaction products of many known gas systems such as an H.sub.2 /AsC1.sub.3 /PC1.sub.3 /Ga, an H.sub.2 /AsH.sub.3 /PH.sub.3 /HC1/Ga, an H.sub.2 /AsC1.sub.3 /PC1.sub.3 /GaAs, and H.sub.2 /AsC1.sub.3 /PC1.sub.3 /GaAs/GaP and so on.

The above and other objects of this invention along with the features and advantages thereof will become more fully apparent as the description proceeds with reference to the accompanying drawing. Some embodiments of the present invention are illustrated in the drawing, but the invention is in no way limited thereto.

The figure is a vertical cross section of an example of the continuous epitaxial growth furnace for III-V semi-conductors used for the present invention.

The present invention will be described in detail herebelow mainly with respect to GaAs and GaAs.sub.1-x P.sub.x as examples.

In the figure, 1 denotes the horizontal vapor phase reaction chamber, 2 and 3 the resistance heating furnaces for heating the horizontal vapor phase reaction chamber 1 and giving it a temperature gradient in the horizontal direction, 4 a gas mixture of a carrier gas, at least one volatile compound of a III-B element and at least one element or compound selected from among the V-B elements and their volatile compounds, and 5 the exhaust gas.

6 denotes a platinum tape heater for producing a heating and mixing zone 7 to prevent the condensation of the gas mixture 4. In case the gas mixture 4 is, for example, composed of a hydrogen gas, Ga CH.sub.3,.sub.3, AsH.sub.3 and PH.sub.3 to heat the mixing zone 7. 8 and 9 denote a preheater and afterheater respectively. Each of these heating furnaces 2, 3, 8, and 9 is of a split type. 10 denotes a fused quartz reaction tube, 11 a plate of carbon or quartz for the purpose of transporting the substrate 12 for epitaxial growth into the horizontal reaction chamber 1, and 13 and 14 two flows hydrogen gas. Two flows of gas are introduced into the reaction chamber 1 through the slits 15 and 16 above the substrates, and is discharged together with the exhaust gas 5.

17 and 18 denote the slits which serve to separate the flow of hydrogen gas from inert gases 19 and 20 (for example, Ar, N.sub.2, etc.) which are exhausted to the inlet and outlet 21 and 22 respectively through which the substrates to 23 and 24 into clean benches (not shown in the drawing). The arrow 25 indicates the direction of movement of the plate 11, but the movement may be reversed.

The characteristic features of the continuous epitaxial growth furnace shown in the figure are that the shower of an inert gas (19, 23 and 20, 24) prevents the outer atmosphere from finding its way into the quartz reaction tube 10 and the slits 17 and 18 prevent the hydrogen gas 13, 14 from flowing out to the outlet and inlet 21 and 22 and that the hydrogen gas introduced in through the slits 15 and 16 is exhausted together with the exhaust gas 5, so that the gas mixture 4 flows only in the horizontal vapor phase reaction chamber 1 and does not get in contact with substrates 12-1 being sent into said reaction chamber 1 and substrates 12-2 sent out from said reaction chamber 1. For continuous growth of epitaxial layers of a III-V semiconductor, especially a III-V mixed crystal semi-conductor, it is necessary to effect control such that the resistance furnace 2 shall have a temperature which is about uniform (though the temperature of the left part is slightly higher) and the heating furnace 3 shall have a temperature gradient that the temperature is higher in the left.

In case it is desired to do vapor phase etching as a pretreatment before the subsrate 12 enters the vapor phase reaction chamber 1, a very small quantity of HC1 gas may be added to hydrogen gas 14.

This and the next examples are described in order to clarify the characteristic features of the continuous epitaxial growth of III-V compound semiconductors.

Example 1

This example relates to a method of continuously growing epitaxial layers of GaAs.

The continuous epitaxial growth furnace shown in the figure was used. The substrate 12 was used a wafer having a (100) face of the crystallographic plane cut from n-type GaAs grown by the horizontal Bridgeman method. The substrate was doped with Te of approximately 1 .times. 10.sup.18 cm.sup..sup.-3 and was chemically etched after mirror polishing. The region of the electric furnace 2 was kept at 755.degree.C - 750.degree.C, and the region of the electric furnace 3 at 750.degree.C - 730.degree.C. The region of the electric furnace 3 was given a temperature gradient of 1.degree.- 2.degree.C per cm. The temperature gradient is decreasing with distance in the direction of the gas flow in the region of the electric furnaces 2 and 3. The gas mixture 4 used was the reaction product of hydrogen saturated with AsC1.sub.3 and Ga placed at 850.degree.C. The temperature of the gas mixing zone 7 was controlled at 800.degree.- 900.degree.C. The basic parameters for epitaxial growth, i.e., the proportions of the components in the gas mixtures, the working range of the flow rates of the gas mixtures, the pressures of the various gas streams, etc., were similar to the prior art reference (IV). When the speed of movement of the substrate 12 was about 5 cm/hour in the direction of the arrow 25, the thickness of the epitaxial layer in the epitaxial wafer sent out one after another to the left of the growth furnace shown in the figure was 15.mu..+-.1.mu.. The electron concentration of the surface layer of the grown layers was approximately 1 .times. 10.sup.15 cm.sup..sup.-3 and it changed to approximately 3 .times. 10 .sup.14 cm.sup..sup.-3 as it progressed further into the interior. This may be due to the change of an identified impurity concentration.

What deserves special attention with respect to this Example is that the growth layers of the epitaxial wafer prepared by the continuous epitaxial growth have a uniform thickness and that there is little difference between those on different substrates. If the growth furnace shown in the figure is used and only one piece of substrate 12-3 is placed in the region of the furnace 3 in the reaction chamber 1 to be grown there without moving, other substrates being removed -- that is to say, if grown by a batch process -- then GaAs is deposited on the inner wall of the quartz tube 10 between the substrate 12-3 and the mixing zone 7, namely the region of the electric furnace 2 and the region of the electric furnace 3 on the left side of the substrate 12-3. If the gas is then stopped and the substrate 12-3 is taken out, and the next batch, i.e. a fresh substrate 12-3, is put in and growth effected again, then growth takes place also on the grains of GaAs deposited on the inner wall of the quartz tube, with the result that the quantity deposited on the inner wall increases and consequently the quantity grown on the new substrate 12-3 becomes less than that grown on the preceding substrate, even if the quantity of the gas mixture supplied is kept constant. This means that the batch process has a drawback in that the rate of growth varies depending on the number of times growth has been made. This also means that the gas mixture is lost as a condensation product on the inner wall of the quartz tube. This problem indicates the difficulty of the epitaxial growth of compound semi-conductors.

As described in detail in this Example, GaAs which would have been deposited on the inner wall of the quartz tube as mentioned above is instead effectively recovered on the GaAs substrate 12 by continuous epitaxial growth according to this invention. The afore-mentioned loss is consequently reduced and besides the rate of growth does not vary timewise. Although the rate of growth somewhat changes depending on the positon of the substrate as it moves on, all the substrates undergo the same growth course in the continuous epitaxial growth, so that epitaxial wafers of little scatter in quality are obtained. It is also permissible to place dummy wafers on the slate 11 at the time the movement is started and recover GaAs on the dummy wafers instead of depositing it on the inner wall of the quartz tube.

EXAMPLE 2

In place of Ga in Example 1, GaAs with an electron concentration of about 1 .times. 10.sup.12 cm.sup..sup.-3 at 300.degree.K was used. As the gas mixture 4, the reaction product of hydrogen saturated with AsC1.sub.3 and GaAs held at 850.degree.C was used. The other conditions were the same as in Example 1. In this case, too, epitaxial layers of the n-type GaAs could be grown continuously without any difficulty.

Epitaxial layers of the p-type GaAs can easily be obtained by using GaAs doped with Zn in place of Ga.

The same result will also be obtained by using a gas mixture of, for example, the reaction product of H.sub.2 /HC1/Ga and a gas mixture of H.sub.2 /AsH.sub.3 or As.

Continuous epitaxial growth of GaAs.sub.1-x P.sub.x (0<x<1) can be effected by adding a gas such as P or PH.sub.3 or PC1.sub.3 in the gas mixture of the afore-mentioned Examples 1 and 2.

EXAMPLE 3

This example relates to a method of continuously growing epitaxial layers of GaAs.sub.1-x P.sub.x on the (111) face of the n-type GaP. The concentration of Te in the substrates was approximately 5 x 10.sup.17 cm.sup..sup.-3. direction of movement of the substrates 12 was opposite to the direction of the arrow 25. The region of the electric furnace 2 was kept at 885.degree.C - 820.degree.C, the region of the electric furnace 3 was kept at 820.degree.C - 815.degree.C, the region of the electric furnace 2 was given a temperature gradient of 3.degree.- 10.degree.C per cm. The temperature gradient is decreasing with distance in the direction of the gas flow in the region of the electric furnaces 2 and 3. As the gas mixture 4, the mixture of the reaction product of H.sub.2/ AsCl.sub.3 /GaAs, the reaction product of H.sub.2 /PC1.sub.3 /GaP, and the gas mixture of H.sub.2 /H.sub.2 Se was used. The temperatures of the GaAs source and GaP source were about 800.degree.C and 900.degree.C, respectively. The temperature of the gas mixing zone was controlled at 900.degree.-950.degree.C. The speed of movement of substrates 12 was about 10 cm/hour in the opposite direction to the direction of the arrow 25.

The basic parameters for epitaxial growth were similar to the prior art reference (IV). The ratio of As and P in the gas mixture was kept at about 6:4, and the temperature and the flow rate of hydrogen gas were always kept constant. The epitaxial layers obtained consisted of a layer of about 25.mu. of GaAs.sub.1-x P.sub.x in which the value of x changed continuously from the proximity of 1 to approximately 0.4 and a layer of about 45.mu. in which the value of x was 0.39 .+-. 0.02.Though the gas mixture entering into the reaction furnace is of constant composition, that is, the ratio of As and P in the gas mixture is kept at about 6:4, the composition of the gas mixture in the reaction furnace is richer in the constituents of the binary III-V semiconductor of the higher melting point, i.e., P at the higher temperature and poorer at the lower temperature, because the GaAs.sub.1-x P.sub.x crystal with the value of x in the proximity of 1 is deposited at the higher temperatures such as 885.degree.C, that is, the crystal which is much richer in GaP is effectively deposited from the gas mixture wherein the ratio of As and P is about 6:4 in the left part of the furnace 2, and as a result, the content of P is changed at the lower temperature.

The method of continuously growing epitaxial layers of the n-type GaAs and the p-type GaAs has been explained with reference to Examples 1 and 2, and the method of continuously growing epitaxial layers of GaAs.sub.1-x P.sub.x was explained with reference to Example 3. In Example 3, epitaxial layers of GaAs.sub.1-x P.sub.x in which the value of x was varied continuously were continuously grown without timewise changing the ratio of As and P in the gas mixture 4 in the figure.

In the afore-mentioned Examples, GaAs was taken up as an example of III-V binary semiconductors and GaAs.sub.1-x P.sub.x as an example of III-V mixed crystal semiconductors. However, the method of this invention is by no means limited to these III-V semiconductors. It can continuously grow epitaxial layers of the afore-mentioned various III-V semiconductors on the afore-mentioned various substrates. That is to say, the combination of the gas mixture 4, temperature gradient of the electric furnace 2 or 3 and the direction of movement of the substrates 12 shown in the figure will make this method easily applicable to the continuous epitaxial growth of many other III-V semiconductors. For substrates 12, Ge and Si may also be used instead of III-V semiconductors.

Claims

1. A method for the continuous preparation of the epitaxial layers of a III-V ternary compound semiconductor represented by a general formula AB.sub.1-x C.sub.x on a III-V binary compound semiconductor from a gas mixture, wherein Ab.sub.1-x C.sub.x stands for both A.sup.III B.sup.V.sub.1-x C.sup.V.sub.x and B.sup.III.sub.1-x C.sup.III.sub.X A.sup.V, A.sup.III and A.sup.v being a III-B element and a V-B element, respectively, and B.sup.III and C.sup.III and B.sup.V and C.sup.V being two different III-B elements and V-B elements, respectively, and the value of x being between 0 and 1, wherein said III-V ternary compound semiconductor with a graded composition is grown without changing timewise the composition of said gas mixture entering into the reaction furnace, which is given a temperature gradient decreasing with distance in the direction of the gas flow, and by gradually changing the temperature of said III-V binary compound semiconductor, and which is characterized by continuously changing the value of x to result in the graded composition by continuously moving the plurality of said III-V binary compound semiconductor substrates, upstream of said gas flow in the situation when the substrate has a lower melting point, and downstream of said gas flow in the situation when the substrate has a higher melting point than said ternary compound semiconductor, respectively, thus effectively growing said epitaxial layers on said substrates thereby gradually decreasing the constituent C and increasing the consituent B in the gas mixture in the reaction furnace downstream of said gas flow in the situation when the compound AB has a lower melting point than the compound AC and vice versa in the situation when the compound AB has a higher melting point than the compound AC.

2. A method as claimed in claim 1 wherein said III-V ternary compound semiconductor is GaAs.sub.1-x P.sub.x and GaP is utilized for said III-V binary compound semiconductor substrate.

3. A method as claimed in claim 2 which is characterized in that said gas mixture is produced from the reaction products of a hydrogen gas saturated with AsC1.sub.3 with GaAs and the reaction products of a hydrogen gas saturated with PC1.sub.3 with GaP.

4. A method as claimed in claim 1 which is characterized in that dummy wafers are placed in the reaction furnace before the movement of said substrates is started, thereby effectively depositing said ternary compound semiconductor on said wafers.