JPH0779086B2 - Crystal growth method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] Among crystal growth methods, the present invention relates to a pretreatment method for a substrate to be grown, which is made of a compound semiconductor for the purpose of forming a buffer layer as thin as possible on the surface of the substrate to be grown. Heat treatment to a film thickness of 15
~ 30 Å thermal oxide film is generated, the substrate to be grown is heated by irradiating the thermal oxide film with a molecular beam of vaporized atoms to remove the substrate surface layer having a thickness of 30 to 60 Å including the thermal oxide film. , Then
It is characterized in that a step of growing crystals on the substrate to be grown is included.

[Field of Industrial Application] The present invention relates to, among crystal growth methods, a pretreatment method for a substrate to be grown.

When manufacturing a semiconductor device, an epitaxy method of epitaxially growing a semiconductor film along a crystal substrate is an indispensable basic technique, but in particular, a III-V group compound semiconductor layer is epitaxially grown to produce a high-performance semiconductor device. Research is being actively conducted, for example, GaAs / AlGaAs / InP.
An ultra-high speed device and an optical device using a heterojunction structure such as / InGaAs system are being developed.

A molecular beam crystal growth (MBE) method has been developed as a method for epitaxially growing such a compound semiconductor layer, and this is known as a growth method capable of forming a steep junction. However, in order to obtain a high-quality crystal growth layer,
The quality of the surface of the crystal substrate which is the basis of this is extremely important.

[Prior Art] FIG. 5 shows an outline of a gas source molecular beam crystal growth apparatus to which the MBE method is applied. 1 is a vacuum growth container, 2 is a substrate to be grown (wafer), 3 is a Mo (molybdenum) stage. Reference numeral 4 is a heater, 5 is a molecular beam source cell, 6 is a shutter, 7 is a liquid nitrogen shroud, and 8 is a gate valve. Of these, the growth substrate 2
Is held on the Mo stage 3 and heated by the heater 4, and is heated to 600 to 700 ° C. when the growth substrate 2 is a GaAs substrate, for example. Moreover, a plurality of molecular beam source cells are provided,
For example, when growing a GaAs device such as HEMT, an As molecular beam source cell, a Ga molecular beam source cell and other molecular beam source cells are arranged to face the substrate 2 to be grown.

When molecular beam epitaxial growth is performed on a growth substrate using such a crystal growth apparatus, pretreatment for cleaning the growth surface of the growth substrate is performed. For example, when the growth substrate is a GaAs substrate, In a vacuum, the substrate to be grown is heated at about 300-400 ℃ to remove the moisture adhering to the surface, and immediately before growth, it is heated at about 600 ℃ for 3-5 minutes to keep the surface of the substrate natural. The oxide film is removed, and then the required crystal growth layer is grown. The conventional pretreatment method is to remove such moisture and natural oxide film (film thickness of about 10Å).

[Problems to be Solved by the Invention] However, although the above pretreatment method can remove water and oxygen, it does not remove carbon (C) adhering to the substrate surface as hydrocarbons or carbon dioxide. Can not. Such hydrocarbons and carbon dioxide fly from the inside of the vacuum growth container or the atmosphere and adhere.

Thus, when carbon is attached to the surface of the GaAs substrate and a crystal growth layer is epitaxially grown on that surface, carbon atoms mix into GaAs and act as acceptors (p-type),
An interface state is formed between the GaAs substrate and the crystal growth layer, which impairs device characteristics.

Conventionally, in order to avoid the effect of this interface state, a thick buffer layer having a film thickness of about 1 μm is formed. However, providing this thick buffer layer increases the man-hours for crystal growth processing, and thus lowers the throughput accordingly. And cause deterioration of device characteristics.

The present invention proposes a pretreatment method of a crystal growth method for the purpose of forming a buffer layer as thin as possible on a substrate surface.

[Means for Solving Problems] The purpose thereof is as shown in the diagram for explaining the principle of FIG.
The growth substrate 10 made of a compound semiconductor is heat-treated to form a film
A thermal oxide film 12 of 15 to 30 Å is generated, and the growth substrate is heated while irradiating the thermal oxide film with a molecular beam of vaporized atoms, and an oxide film substrate transition of 30 to 60 Å including the thermal oxide film. This is accomplished by a crystal growth method that includes the steps of removing layer 11 and then crystal growing on the substrate to be grown.

[Operation] That is, according to the present invention, a thin thermal oxide film is formed, and the substrate to be grown is heated and removed while the substrate surface layer including the thermal oxide film is irradiated with a molecular beam of vaporized atoms. Then, the carbon atoms attached to the surface are removed, and a clean and well-crystallized surface appears. If a crystal growth layer is epitaxially grown on the surface, a growth layer with good crystal quality can be obtained.

[Examples] Hereinafter, examples will be described in detail with reference to the drawings.

When the above-mentioned substrate 10 to be grown is a GaAs substrate, the film thickness is 15 to 30 Å
The thermal oxide film 12 of is a Ga ₂ O ₃ · As ₂ O ₃ film containing a large amount of Ga ₂ O ₃ , and the oxide film substrate transition layer 11 having a thickness of 15 to 30 Å, which is almost the same as the thermal oxide film 12,
It is a layer of GaAs with a large amount. Therefore, in the present invention, the film thickness of 15 to 30
It removes a thin Å thermal oxide film and an underlying oxide film substrate transition layer of 15-30 Å film thickness with poor crystal quality.

Now, in order to remove the carbon atoms attached to the surface, it is sufficient to thermally oxidize that portion to remove the thermal oxide film, but it is desirable to form this thermal oxide film as thin as possible and remove it. This is because, if the thick thermal oxide film is removed, the surface becomes rough, and the surface becomes rough, and if it is remarkable, the quality of the crystal growth layer will deteriorate. For that purpose, the oxide film thickness (vertical axis) with respect to the heating temperature of the GaAs substrate was obtained experimentally.
FIG. 2 shows the relationship between the heating time and the heating time (horizontal axis). From the figure, it can be seen that the oxide film thickness is saturated during the heating time of 10 to 30 minutes, and that the film thickness increases more as the heating temperature increases up to the heating time of 10 minutes. Therefore, it is considered that the lower the heating temperature is, the better the reproducibility of the oxide film thickness is. Moreover, the low temperature heating is suitable for thinning the oxide film thickness, and the heating temperature 300
At ˜350 ° C., the oxide film formation is almost saturated in 5 to 10 minutes. When the surface condition was measured under these heating conditions, it was found that the oxide film thickness (depth of oxygen existence) was 15 to 30Å and the oxide film substrate transition layer (layer with disordered crystal) was 15 to 39Å.

On the other hand, the observation by the reflection type high-energy electron diffraction (RHEED) method revealed that the thermal oxide film is evaporated and removed by the heat treatment in vacuum at 620 to 640 ° C or higher for 3 to 5 minutes. I'm running.

Next, based on this data, an example in which a selectively doped GaAs / n-AlGaAs heterostructure as shown in FIG. 3 is grown on a GaAs substrate using the MBE method will be described. In FIG. 3, 20 is a semi-insulating GaAs substrate, 21 is a GaAs buffer layer, 22
Is an AlGaAs spacer layer, 23 is an n-AlGaAs electron supply layer, and 24 is n
-GaAs contact layer, which is a heterostructure for HEMTs. First, the GaAs substrate 20 is placed on a hot plate heated to 350 ° C. and heated in the atmosphere for 10 minutes to form a thermal oxide film having a film thickness of 15 to 30 Å on the substrate surface.

Next, this GaAs substrate 20 is attached to a Mo stage, inserted into the gas source molecular beam crystal growth apparatus (see FIG. 5), and a substrate preparation chamber (on the right side of the gate valve 8 and not shown). Heat at 400 ° C for 10 minutes to remove water. Next, it is transferred to the vacuum growth container 1 and heated to 680 ° C. (substrate heating temperature during crystal growth) while irradiating with As molecular beam.
Hold for minutes to remove the substrate surface layer including the thermal oxide film with a thickness of 30 to 60Å. The purpose of irradiating this As molecular beam is 680 ℃
This is because As is likely to evaporate, so that As is suppressed.

Then, while maintaining the temperature at 680 ° C., the selectively-doped GaAs / n-AlGaAs heterostructure shown in FIG. 3 is grown. This is HE
A crystal growth layer for MT is grown by changing the film thickness of the GaAs buffer layer, and the data of the film thickness and other growth layers are as follows.

GaAs buffer layer 21; thickness 0.1,0.2,0.4μmAl _x Ga _1-x As spacer layer 22; thickness _{60Å, x = 0.3n-Al x} Ga 1-x As electron supply layer 23;
Film thickness 900Å, x = 0.3 Impurity concentration 10 ¹⁸ / cm ³ n-GaAs contact layer 24; Film thickness 100Å, Impurity concentration 10 ¹⁸ / cm ³ Thus, the thickness of the buffer layer 21 is 0.1, 0.2, 0.4 μm. Although changing, the thickness of the conventional buffer layer is 0.6 μm
Therefore, the film thickness of the buffer layer 21 is reduced to 0 by the conventional pretreatment.
We have also created the data that changed to 1,0.2,0.4,0.6μm.

The data measured by the sample having this crystal structure is shown in FIG. 5, which shows the electron mobility (second vertical axis) of the secondary electron gas and the sheet electron with respect to the film thickness of the buffer layer (vertical axis). In the relationship diagram of concentration (right vertical axis), Hall measurement is performed at 77 ° k.

In FIG. 5, black circles and black triangles represent electron mobility (black circles), sheet electron density (black triangles), white circles and white triangles in the above sample to which the present invention is applied, and white circles and white triangles represent electron mobility (conventional pretreatment method). (White circles) and sheet electron density (white triangles). From this, when the pretreatment method according to the present invention is applied, the electron mobility is 85000 cm when the thickness of the buffer layer is 0.2 μm.
² / Vs, sheet electron concentration is 5 × 10 ¹¹ / cm ² , which is comparable to the conventional buffer layer thickness of 0.6 μm. Moreover, when the thickness of the buffer layer is reduced, the thickness of the entire crystal growth layer is reduced, and the crystal defects generated in the growth process are reduced, which has the advantage that the defect density is reduced to about 1/2. The surface morphology (surface roughness) is also good for crystal growth without problems. By the way, if a buffer layer having a thickness of 0.2 μm is formed by the conventional pretreatment method, the electron mobility is 48000 cm ² / Vs, and the sheet electron concentration is 3.3 × 10 ¹¹
Only a value of / cm ² is obtained, which is interpreted to be due to the effect of electron traps at the interface between the substrate and the crystal growth layer.

Therefore, according to the present invention, interface traps are reduced, and a good crystal growth layer can be obtained even if a thin buffer layer is formed. For example, in an electronic device such as HEMT, the substrate bias effect is removed during operation. be able to. As described above, when the present invention is applied to provide a thin buffer layer, the number of man-hours for crystal growth treatment is reduced, throughput is improved, and device characteristics are improved.

The above is an example in which the GaAs substrate is used as an example and is heated while irradiating the As molecular beam of atoms that easily evaporate, but the present invention can be applied to other compound semiconductor substrates, for example, InP substrate and InSb substrate. The irradiation molecular beam of is phosphorus (P) or antimony (Sb).

[Effects of the Invention] As is clear from the above description, according to the pretreatment of the crystal growth method according to the present invention, a buffer layer having a small film thickness is provided, the throughput is improved, and high-quality crystal growth is achieved. The layers are provided to contribute to improving the performance of electronic devices and optical devices.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention, FIG. 2 is a relational diagram of oxide film thickness and heating time with respect to heating temperature, FIG. 3 is a sectional view of a heterostructure to which the present invention is applied, and FIG. FIG. 3 is a relationship diagram of the buffer layer film thickness, electron mobility and sheet electron concentration in the embodiment of FIG. 3, and FIG. 5 is a schematic diagram of a gas source molecular beam crystal growth apparatus. In the figure, 1 is a vacuum growth container, 2 is a substrate to be grown, 3 is a Mo stage,
4 is a heater, 5 is a molecular beam source cell, 8 is a gate valve, 10
Is a growth substrate, 11 is an oxide film substrate transition layer, 12 is a thermal oxide film,
20 is a semi-insulating GaAs substrate, 21 is a GaAs buffer layer, 22 is AlGa
As spacer layer, 23 is n-AlGaAs electron supply layer, 24 is n-Ga
The As contact layer is shown.

Claims (2)

[Claims] 1. A growth substrate made of a compound semiconductor is heat-treated to form a thermal oxide film having a film thickness of 15 to 30 Å, and the growth substrate is heated while irradiating the thermal oxide film with a molecular beam of vaporized atoms. Then, a step of removing the substrate surface layer having a film thickness of 30 to 60 Å including the thermal oxide film and then performing crystal growth on the substrate to be grown is included. 2. The crystal growth method according to claim 1, wherein the compound semiconductor is GaAs and the molecular beam of vaporized atoms is an As molecular beam.