JPH06151304A - Compound semiconductor wafer - Google Patents

Abstract

PURPOSE:To obtain a compound semiconductor wafer which is covered with a protective film to obtain a surface whose cleanness is very high in the case of epitaxial growth. CONSTITUTION:The outermost surface of a substrate 12 to be subjected to epitaxial growth is covered with a chlorine adsorption layer 13 formed by terminating dangling bonds of gallium arsenide surface atoms with chlorine atoms. Impurities like carbon based impurities in the air react with chlorine of the chlorine adsorption layer 13 abundant in reactivity and are adsorbed by chlorine. But the impurities do not adhere to the substrate 12 When the temperature becomes 200 to 300 deg.C in the course of raising temperature in epitaxial growth, the chlorine adsorption layer 13 formed on the substrate 12 surface is desorbed, and at the same time, the adsorbed impurities also are desorbed. As the result of the desorption, the cleanness of the substrate 12 surface becomes very high, and epitaxial growth is performed in the state that the temperature is further raised.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compound semiconductor wafer made of gallium arsenide, and more particularly to a compound semiconductor wafer suitable as a substrate for epitaxial growth of electronic devices such as field effect transistors.

[0002]

2. Description of the Related Art Conventionally, compound semiconductor wafers made of gallium arsenide (GaAs) semi-insulating crystals are
A GaAs single crystal ingot is produced by a pulling method or the like, and is formed by a wafer cut from this single crystal ingot. The surface of the cut-out wafer is processed by polishing, etching, and polishing so as to have a predetermined thickness. Then, in order to make the surface mirror-finished, mechanochemical polishing with a solution containing sodium hypochlorite is performed.

The thus mirror-polished wafer is further processed as shown in the process chart of FIG. That is, surface impurities are removed by acid treatment (HCl boil) or treatment with an alkaline solution, further washed with running water, organically treated and dried, and packaged in a nitrogen gas (N ₂ ) atmosphere. After this, the wafer is transferred and epitaxially grown on the surface of the wafer to form an electronic device, for example.

As shown in the sectional view of FIG. 9, the compound semiconductor wafer formed as described above has gallium (Ga) generated during washing with running water on the surface of the GaAs substrate 1.
An extremely thin natural oxide film (Ga ₂ O ₃ / As ₂ O ₃ ) 2 having a thickness of several nm of arsenic (As) is formed. Reference numeral 3 denotes an impurity adsorption layer formed on the surface of the natural oxide film 2. Prior to removing the impurity adsorption layer 3 before performing epitaxial growth, an acid-based etching solution (H
₂ SO ₄ : H ₂ O ₂ : H ₂ O = 5: 1: 1).

As is well known, since the active layer is formed in the epitaxial growth, the cleanliness of the surface of the GaAs wafer before the epitaxial growth is very important, and the impurity adsorption layer 3 is formed on the surface of the GaAs substrate 1. If they exist, they are captured during growth and become a leak source when the electronic device operates. Therefore, methods such as thermal cleaning and ion sputtering have been used for cleaning the wafer surface immediately before epitaxial growth.

However, since these cleaning processes are carried out at a high temperature of about 650 ° C., the composition ratio of GaAs on the wafer surface deviates from the desired one, and face holes are generated, which adversely affects the epitaxial growth. .

On the other hand, as shown in the process flow chart in FIG. 10 and the sectional view in FIG. 11 so that the treatment can be carried out at a relatively low temperature, an ultraviolet ray of 1.42 eV or more is generated during acid treatment (HCl boil). It has been attempted to form a surface protective film for the As-rich GaAs layer 4 and the As-rich natural oxide film 5 on the surface of the GaAs substrate 1 by irradiating it with (UV light).

The surface protective film thus formed volatilizes at a relatively low temperature.
The s-layer 4 and the As-rich natural oxide film 5 are 45
It is not formed of only an As layer and an As oxide film that volatilize at a low temperature of about 0 ° C., and it is difficult to say that it is a completely volatile surface protective film because it contains a Ga oxide film. There is a possibility that a problem in high temperature processing may occur in order to perform proper cleaning. Further, when exposed to the air, carbon-based impurities and the like are adsorbed on the surface protective film, and these enter the substrate 1 during the temperature rise of the wafer, and there is a problem that a p-type conductive layer is formed.

Further, FIG. 12 shows a process chart, and FIG.
As shown in the cross-sectional view in FIG. 3, it has been attempted to form a natural oxide film 6 on the surface of the GaAs substrate 1 and then to perform ozone (O ₃ ) treatment under irradiation of ultraviolet light, but it is not sufficient. Absent.

[0010]

As described above, in the epitaxial growth in which the active layer is formed, it is very important that the surface of the compound semiconductor wafer on which the epitaxial growth is performed has a very high cleanliness. For this reason, a compound semiconductor wafer having a protective film formed so that a surface cleaned by various methods is obtained immediately before epitaxial growth has been shown, but high temperature treatment is required and impurities are adsorbed. And it wasn't enough. The present invention has been made in view of such circumstances, and an object thereof is to provide a compound semiconductor wafer coated with a protective film so as to obtain a surface having extremely high cleanliness during epitaxial growth.

[0011]

A compound semiconductor wafer of the present invention is formed of a gallium arsenide semi-insulating crystal substrate, an outermost surface on which this substrate is epitaxially grown, and chlorine atoms covering this outermost surface. And a dangling bond of gallium arsenide atoms on the surface of the substrate is terminated by chlorine atoms. A gallium arsenide oxide film is formed between the surface gallium arsenide and the adsorption layer,
Furthermore, a gallium arsenide layer mainly containing arsenic and an oxide film of gallium arsenide mainly containing arsenic are formed between the gallium arsenide on the substrate surface and the adsorption layer.

[0012]

The compound semiconductor wafer configured as described above has a structure in which the outermost surface of the substrate on which epitaxial growth is performed is covered with an adsorption layer formed of chlorine atoms. Impurities such as carbon-based impurities in the atmosphere react and adsorb, and the impurities do not adhere to the substrate. When the temperature of the substrate during the epitaxial growth is 200 to 300 ° C., the adsorption layer formed on the surface of the substrate is desorbed, and at the same time as the desorption, impurities adsorbed on the adsorption layer are desorbed. Also desorb.
Due to this desorption, the substrate surface becomes extremely clean,
Epitaxial growth is carried out with the temperature further increased. In this way, it is possible to obtain a compound semiconductor wafer coated with a protective film so as to obtain a surface having extremely high cleanliness in performing epitaxial growth.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

First, a first embodiment will be described with reference to FIGS. 1 is a sectional view, FIG. 2 is a process step diagram, FIG. 3 is a diagram showing an adsorption amount of chlorine, and FIG. 4 is a diagram showing an impurity concentration profile of a compound semiconductor wafer on which epitaxial growth is performed. FIG. 5 is a characteristic diagram showing IV characteristics of a compound semiconductor wafer which has been subjected to mesa etching after epitaxial growth.

In FIG. 1, 11 is a compound semiconductor wafer made of GaAs, and a GaAs substrate 12
On the surface where the epitaxial growth of
The dangling bond (dangling bond) of the GaAs surface atoms
A chlorine adsorption layer 13 in which chlorine (Cl) atoms are bonded to a ring bond is formed as a protective film of the substrate 12. The chlorine adsorption layer 13 is composed of 1 to 2 atomic layers of Cl.

As described above, the chlorine adsorption layer 1 is formed on the surface of the substrate 12.
The GaAs wafer 11 on which No. 3 has been formed is manufactured through the following steps. That is, first, a wafer is cut from a GaAs single crystal ingot, and the surface of the cut wafer is processed by polishing, etching, and polishing so as to have a predetermined thickness. Further, the wafer having a predetermined thickness is mirror-polished by mechanochemical polishing with a solution containing sodium hypochlorite to form a GaAs wafer for forming the substrate 12.

Subsequently, processing is performed according to the processing steps shown in FIG. First, surface impurities on the wafer surface are removed by acid treatment with HCl boil. The HCl boil is immersed in HCl heated to 110 ° C. for about 15 minutes to remove the natural oxide film of the contaminated surface impurities and to bond Cl atoms to the dangling bonds of GaAs atoms on the substrate 12. Cl is adsorbed on the surface of the substrate 12.

After the surface impurities are removed, the substrate 12 is washed with running water.
A chlorine adsorption layer ¹³ having a surface chlorine adsorption amount of 10 ¹³ to 10 ¹⁵ at / cm ² and a layer thickness of 1 to 2 atomic layers of Cl is formed. For washing with running water, use a tank with an internal volume of about 4 l
It is carried out for about 5 minutes under the flow rate of 1 / min. The result is that the horizontal axis shows the flowing water time and the vertical axis shows the chlorine adsorption amount. The longer is, the Cl adsorbed on the surface of the substrate 12 is dissolved in pure water and reduced.

In the determination of the cleaning time, in the initial stage of determining the process conditions, the amount of chlorine adsorbed on the substrate 12 treated for each flowing water time is applied to the surface of the substrate 12 by the excitation X-ray under the condition of total reflection. , Total reflection X-ray fluorescence (TR
XF: Total Reflection of Fl
It is detected by using the energy spectrum of the uorecent X-rays). And the cleaning time is 10 ¹³
A chlorine adsorption of -10 ¹⁵ at / cm ² is obtained, for example determined under a flow rate of 2 l / min for 4-8 min.
The chlorine adsorption layer 13 having 1 to 2 atomic layers of Cl is formed on the surface 2.

If the washing with running water is too long and the amount of chlorine adsorbed is too small to be out of the predetermined range, the amount of HCl
After the boiling process is performed, washing with running water is performed. If the amount of adsorbed chlorine is out of the predetermined range, the washing with running water is continued to bring it into the predetermined range. And T
The measurement and confirmation of the amount of adsorbed chlorine by RXF is omitted after the cleaning conditions are determined, and is carried out by, for example, sampling when the manufacturing conditions change or when the lot changes as necessary.

After the predetermined cleaning, the GaAs wafer 11 of this embodiment is formed by being dried in the drying process. As a result, the GaAs wafer 11 is made of a crystal having a high ionicity, and the surface of the GaAs substrate 12 that is more active than silicon (Si) is coated with Cl having a high reactivity, and this Cl is reacted with carbon-based impurities in the atmosphere. It will be protected. After this GaAs
The wafer 11 is transferred to the epitaxial growth step, and the surface is epitaxially grown.

Next, the operation of the GaAs wafer 11 having the above structure will be described with reference to the epitaxial growth process.

In the epitaxial growth process, the GaAs wafer 1 is added to the molecular beam epitaxial growth equipment (MBE equipment).
1 was loaded and the inside of the MBE device was evacuated to a high vacuum state of 10 ⁻¹⁰ Torr level, and then heated and Ga at 650 ° C.
An epitaxial layer having a thickness of about 2 μm was formed on the surface of the As wafer 11. At the same time, the surface diffraction pattern of the GaAs wafer 11 in the process of heating and raising the temperature to about 650 ° C. is taken as a reflection type high-speed electron beam diffraction (RHEED: Reflect).
tion of High Energy Elect
Oron Diffraction), the diffraction pattern became clear from about 200 ℃,
It was observed that a clean surface was obtained at about 300 ° C., and an epitaxial layer was formed at 650 ° C. on the substrate surface of the GaAs wafer 11 which became this clean surface.

[0024] There is no risk of high temperature processing problems.
The surface of the substrate 12 was cleaned at a relatively low temperature of 300 ° C. because the chlorine adsorption layer 13 formed on the surface of the substrate 12 was desorbed at this temperature, and at the same time as this desorption, the chlorine adsorption layer 13 was formed on the chlorine adsorption layer 13. This is because the carbon-based impurities supplied from the adsorbed atmosphere were also desorbed. The temperature was raised to 650 ° C. in steps of 50 ° C. from 100 ° C., and each temperature state was maintained for 10 minutes.

For cleaning the surface of the substrate 12, the impurity concentration profile of the GaAs wafer 11 of the present invention on which the epitaxial layer shown in FIG. 4 is formed is measured by secondary ion mass spectrometry (SIMS).
As a result of measurement by Spectroscopy), it was confirmed that it was below the detection limit like Si shown by a solid line a and carbon (C) shown by a solid line b, and the same was true for iron (Fe) and Cl. For comparison, GaAs formed by the conventional method of organically treating after HCl boil is formed.
An epitaxial layer is formed on the substrate of the wafer,
At the same time, looking at the impurity concentration profile, at the interface between the epitaxial layer and the substrate, a peak of 10 ¹⁷ at / cm ³ was seen for Si shown by the broken line c and 10 ¹⁸ at / cm ^{3 for} C shown by the broken line d. The peak of was seen.

Further, a substrate of a GaAs wafer formed by irradiating with ultraviolet light of 1.42 eV or more in another HCl boil shown in the conventional example, or ozone (O ₃ ) treatment under irradiation of ultraviolet light. An epitaxial layer was also formed on the substrate of the GaAs wafer formed by the execution, and at the same time, the impurity concentration profile was observed, but peaks were observed for Si and C although the peak concentration was lowered at the interface between the epitaxial layer and the substrate. It was

As described above, according to this embodiment, the GaAs wafer 11 has the surface of the substrate 12 protected and protected by the chlorine adsorption layer 13 made of highly reactive Cl. The chlorine adsorption layer 13 is desorbed during the heating immediately before the epitaxial growth, and at the same time, the carbon impurities adsorbed on the chlorine adsorption layer 13 are also desorbed, so that the surface of the substrate 12 is extremely clean. It becomes the surface. Then, high-purity epitaxial growth is performed on the surface having high cleanliness, and the impurity concentration at the interface between the epitaxial layer and the substrate 12 becomes low.

Therefore, when a predetermined epitaxial growth is performed using the GaAs wafer 11 to form an electronic device such as a field effect transistor which requires a high-purity buffer layer, the breakdown voltage is improved, and the epitaxial layer and the substrate 12 are Since there is no conductivity at the interface of, the characteristic deterioration due to parallel conduction is small.

Further, FIG. 5 shows that the current I-voltage V characteristic between the mesas of the GaAs wafer 11 having the epitaxial layer formed on the substrate 12 and subjected to the mesa etching was measured. As shown by the solid line e, it was a good one with no kink. In the same figure, the case of using a conventional GaAs wafer is shown by a solid line f. Further, the present example was also stable and favorable in manufacturing yield.

In this embodiment, the chlorine adsorption layer 13 formed on the surface of the substrate 12 has a chlorine adsorption amount of 10 ¹³ to 10 ¹⁵ at / cm ² so that the layer thickness is 1 to 2 atomic layers of Cl. The reason is that if the amount of adsorbed chlorine is more than this, excess Cl reacts with moisture in the atmosphere to form particulate precipitates on the substrate 1.
2 is not desirable on the surface, and if less than this, a portion not protected by Cl is generated on the surface of the active GaAs substrate 12 and the above effect cannot be obtained.

Further, in order to form the chlorine adsorption layer 13 on the surface of the substrate 12, in the above-mentioned one, the removal of the oxide film on the wafer surface is performed by HCl boil, and at the same time, GaA
Although the dangling bond of the s surface atom is terminated by the Cl atom, the oxide film is removed by chlorine plasma without being limited to this, and at the same time, the dangling bond of the GaAs surface atom is terminated by the Cl atom. You may The same action and effect can be obtained by forming the chlorine adsorption layer 13 composed of 1 to 2 atomic layers of Cl after terminating with Cl atoms.

Next, a second embodiment will be described with reference to FIG. FIG. 6 is a cross-sectional view, 14 is a compound semiconductor wafer made of GaAs, and is a GaAs substrate 12.
On the surface where the epitaxial growth of
The natural oxide film 15 is formed, and the natural oxide film 1 is further formed.
A chlorine adsorption layer 16 is formed on the outermost surface on the surface 5. The chlorine adsorption layer 16 is composed of 1 to 2 atomic layers of Cl.

Thus, the natural oxide film 1 is formed on the surface of the substrate 12.
5 and a GaAs wafer 14 on which a chlorine adsorption layer 16 is formed
Is created through the following steps. That is, the substrate 12 having a mirror-finished surface is formed by the mechanochemical polishing process. Subsequently, the natural oxide film contaminated by the treatment of NH ₄ F or NaOH solution or the like is removed, and then, the natural oxide film 15 is formed again on the surface of the substrate 12 by washing with running water.

H under the same conditions as in the first embodiment
The native oxide film 15 is removed by Cl boil or chlorine plasma so that a part of it is left. The substrate 12 on which the native oxide film 15 is partially left by such processing
The chlorine adsorption layer 16 is formed on the outermost surface of the.

In the case of such a structure, a relatively high temperature as in the conventional case is required for cleaning during the epitaxial growth, but the chlorine adsorption layer 16 and the like are desorbed during heating and the surface is removed. Are cleaned, epitaxial growth with high purity can be performed, and the impurity concentration at the interface between the epitaxial layer and the substrate 12 becomes low.

After the surface of the substrate 12 is mirror-finished, the contaminated natural oxide film is removed by using an acid-based etching solution containing sulfuric acid and hydrogen peroxide (H ₂ SO ₄ : H ₂ O ₂ : H ₂ O =
5: 1: 1), and
After that, the same operation and effect can be obtained even if the same processing as in the second embodiment described above is performed to leave a part of the natural oxide film 15 and form the chlorine adsorption layer 16 on the outermost surface of the substrate 12. Is obtained.

Next, a third embodiment will be described with reference to FIG. FIG. 7 is a sectional view, and 17 is a compound semiconductor wafer made of GaAs, which is a GaAs substrate 12.
On the surface where the epitaxial growth of
As-rich GaAs layer 18 and As-rich oxide film 19
And a chlorine adsorption layer 20 is formed on the outermost surface of the oxide film 19. The chlorine adsorption layer 20 is composed of 1 to 2 atomic layers of Cl.

The GaAs wafer 17 having the As-rich GaAs layer 18, the As-rich oxide film 19 and the chlorine adsorption layer 20 formed on the surface of the substrate 12 in this manner is manufactured through the following steps. That is, the substrate 12 having a mirror-finished surface is formed by the mechanochemical polishing process. Subsequently, the natural oxide film contaminated by the treatment of HCl boil while irradiating the UV light of 1.42 eV or more is removed to form the As-rich GaAs layer 18 and the As-rich oxide film 19.

H under the same conditions as in the first embodiment
The As-rich oxide film 19 is removed by Cl boil or chlorine plasma so that a part thereof remains. By such a treatment, the chlorine adsorption layer 20 is formed on the outermost surface of the substrate 12 where a part of the As-rich oxide film 19 remains.

In such a structure, the chlorine-adsorbing layer 20 and the highly volatile protective film composed of the As-rich GaAs layer 18 and the As-rich oxide film 19 are formed on the surface of the substrate 12. Therefore, it is possible to obtain a highly clean surface at a relatively low temperature. Then, high-purity epitaxial growth in which impurities such as C are extremely mixed can be performed, and the impurity concentration at the interface between the epitaxial layer and the substrate 12 becomes low.

The present invention is not limited to each of the above-mentioned embodiments, but can be carried out by appropriately modifying it within the scope of the invention.

[0042]

As is clear from the above description, according to the present invention, the outermost surface of the substrate on which the epitaxial growth is performed is covered with the adsorption layer formed of chlorine atoms. It is possible to obtain a compound semiconductor wafer coated with a protective film so as to obtain a high surface.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of the present invention.

FIG. 2 is a process step diagram according to the first embodiment shown in FIG.

FIG. 3 is a diagram showing an adsorption amount of chlorine with respect to flowing water time in FIG.

FIG. 4 is a diagram showing an impurity concentration profile of a compound semiconductor wafer which has been epitaxially grown according to the first embodiment.

FIG. 5 is a characteristic diagram showing IV characteristics of a compound semiconductor wafer subjected to mesa etching after epitaxial growth according to the first example.

FIG. 6 is a cross-sectional view showing a second embodiment of the present invention.

FIG. 7 is a sectional view showing a third embodiment of the present invention.

FIG. 8 is a process chart of a conventional first example.

FIG. 9 is a cross-sectional view showing a first conventional example.

FIG. 10 is a process step diagram of a second conventional example.

FIG. 11 is a cross-sectional view showing a second conventional example.

FIG. 12 is a process chart of a conventional third example.

FIG. 13 is a cross-sectional view showing a second conventional example.

[Explanation of symbols]

 12 ... Substrate 13 ... Chlorine adsorption layer

Claims (4)

[Claims] 1. A gallium arsenide semi-insulating crystal substrate, an outermost surface on which the substrate is epitaxially grown, and an adsorption layer formed of chlorine atoms which covers the outermost surface. Compound semiconductor wafer. 2. The compound semiconductor wafer according to claim 1, wherein dangling bonds of gallium arsenide atoms on the surface of the substrate are terminated by chlorine atoms. 3. The compound semiconductor wafer according to claim 1, wherein an oxide film of gallium arsenide is formed between the gallium arsenide on the surface of the substrate and the adsorption layer. 4. A gallium arsenide layer mainly containing arsenic and an oxide film of gallium arsenide mainly containing arsenic are formed between the gallium arsenide on the surface of the substrate and the adsorption layer. 1. The compound semiconductor wafer according to 1.