JPH03109292A - Vapor growth method for oxide thin film - Google Patents

Abstract

PURPOSE:To improve the purity of a single crystal by installing the substrate crystal constituted of a specific surface in a reaction furnace and introducing org. metal vapor and O2 or an oxidizing agent which can supply O2 into the furnace and thereby causing pyrolysis. CONSTITUTION:The substrate crystal 20 which is cleaned by chemical etching and is constituted of the face that the low-index face is inclined by 2 to 5 deg. toward a (100) direction from (011) face or (110) face is imposed on a sample imposing base 2. Gaseous Ar is supplied to the reaction furnace 1 to substitute the in-furnace atmosphere; thereafter, the gaseous O2 is supplied from a gaseous O2 cylinder 16 into the reaction furnace 1 where the substrate crystal 20 is heated to a prescribed temp. by an electric furnace 3. The gaseous Ar is passed to containers 10, 11, 12 for org. metal raw materials after stopping of the supply of the gaseous O2 and the org. metal vapors are supplied into the reaction furnace 1 by operating 3-way valves 13, 14, 15. The supply of the vapors to the reaction furnace 1 is stopped after the prescribed time and the gaseous O2 is supplied into the reaction furnace 1 upon lapse of some time. This stage is repeated several tens times, by which the thin film of the oxide superconductor consisting of the single crystal is obtd. at a prescribed thickness over the entire surface.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method for vapor phase growth of oxide thin films by metal organic chemical vapor deposition (MOCVD).

(Prior art) In recent years, YBa2CuO7-a, Bi2Sr
2 Oxide superconductors such as CaCu and Os have been discovered and are attracting attention. Previously known Nb
3 In superconductors made of intermetallic compounds such as Ga,
The critical temperature (Tc), which is an indicator of its superconducting properties, is at most 2
The superconductor's applications have been severely limited because it exhibits superconducting properties only when cooled with an expensive liquid helium (4.2K) coolant. On the other hand, the Tc of the oxide superconductor mentioned above exceeds 100 K, and it exhibits superconducting properties when cooled with liquid nitrogen (77 K), which is industrially produced at low cost. ,7
New uses are beginning to be proposed, including applications to electronic devices such as ultra-high-speed logic elements that operate at 700 nm.

In order for oxide superconductors to be used industrially, it is essential that oxide crystals with controlled composition and few defects can be produced with good reproducibility. In particular, a flat thin film composed of oxide i11 crystal is essential for application to electronic devices such as ultrahigh-speed logic elements.

Conventionally, physical methods such as sputtering and electron beam evaporation have been used to fabricate oxide superconductor thin films. Using these methods, thin films can be obtained using relatively simple equipment. However, with these methods, it is difficult to independently and precisely control the supply amount of the constituent elements of the oxide superconductor thin film, and furthermore, the supply quantity of the constituent elements depends on the shape of the sputtering target and evaporation source. However, it has been difficult to deposit an oxide superconductor with a desired composition with good reproducibility.

Therefore, in order to produce oxide superconductor thin films, CVD (Chemical Vapor Dep.
MOCV D (Metalorgan
c CVD) method development is actively underway. This is a chemical vapor deposition method that is different from the physical method described above, and the MOCVD method allows for independent and precise control of the amount of raw materials containing the constituent elements of the oxide superconductor, making it possible to control the composition of the thin film. The purpose is to improve.

However, the following problems have been reported regarding thin films deposited by the conventional MOCVD method.

That is, when the growth temperature is 600° C. or lower, the deposited thin film is composed of a polycrystalline layer or an amorphous layer different from the superconducting layer, and does not exhibit superconducting properties as it is.

After the thin film is deposited, the thin film exhibits superconducting properties only by performing oxidation and crystallization heat treatment in air or pure oxygen at a high temperature of 800° C. or higher.

However, the thin film obtained in this way is polycrystalline, and the superconductor layer has a different crystal composition from the as-deposited crystal composition due to the crystallization heat treatment process at a higher temperature, so the surface flatness of the thin film obtained is Its properties are extremely poor, and it cannot be used in ultrahigh-speed electronic devices.

If an attempt is made to omit post-deposition oxidation and crystallization heat treatment using the conventional method, the desired superconducting layer cannot be obtained unless the deposition temperature is at least 800° C. or higher.

However, even in this case, the flatness of the thin film surface was extremely poor. Also, if the deposition temperature and post-deposition heat treatment temperature are high,
Undesirable problems such as deterioration of film quality due to reaction between the substrate and the thin film also occur.

As described above, in the conventional MOCVD method, the desired superconductor layer cannot be obtained unless high-temperature heat treatment is performed in oxygen or air after thin film deposition, and the deposition temperature must be increased if high-temperature heat treatment is to be omitted. This indicates that oxidation of the thin film does not proceed sufficiently at low temperatures. Therefore, there is a problem in that a desired superconductor crystal layer cannot be obtained at a lower temperature unless the degree of oxidation is increased in some way.

Furthermore, when depositing an oxide superconductor by the conventional MOCVD method, a mixed vapor of organometallic raw materials and oxygen gas are simultaneously introduced into a reactor. However, in this method, even if the amount of oxygen supplied is increased, oxygen reacts in the organometallic vapor gas phase and is consumed as compounds other than the desired deposit, so oxygen is not sufficiently supplied to the surface of the thin film being deposited. Hateful. Furthermore, the degree to which oxygen reacts in the metal organic vapor gas phase becomes more pronounced during low-temperature deposition.

In contrast, the present inventors have developed an oxide high-temperature superconductor thin film with good surface flatness at low temperatures by alternately and repeatedly introducing organic metal raw material vapor and oxygen gas into the reactor during thin film deposition. We found that it is possible to obtain with good reproducibility.

However, as a result of observing X-ray diffraction images of thin films deposited by this method, the oxide high temperature superconductor is almost completely C-axis oriented (the C-axis of the oxide superconductor thin film crystal is perpendicular to the substrate surface). However, the a-axis parallel to the thin film surface (
and b-axis), it was found that they were distributed in directions perpendicular to each other.

That is, although the deposited thin film is epitaxially grown, it is not a single crystal but contains many twin crystals.

(Problems to be Solved by the Invention) As described above, it has become possible to deposit a thin film with a flat surface by the conventional MOCVD method, but it is possible to eliminate twin crystals distributed throughout the thin film, making it possible to control the entire surface of the thin film. There was also the problem that single crystallization could not be achieved with good reproducibility.

The present invention has been made based on the above-mentioned circumstances, and an object of the present invention is to provide a method capable of producing a twin-free, single-crystal oxide thin film over the entire surface with good controllability and reproducibility.

[Structure of the Invention] (Means for Solving the Problems) The method of vapor phase growth of an oxide thin film of the present invention accommodates a substrate crystal in a reactor, and generates an organic metal vapor containing a metal element constituting the oxide. and oxygen or an oxidizing agent capable of supplying oxygen, and thermally decompose them to grow an oxide thin film on the substrate crystal in a vapor phase. It is characterized by being composed of a surface that is tilted away from the surface.

In the present invention, the low index plane that serves as a reference for the plane constituting the surface of the substrate crystal is usually used when growing an oxide high temperature superconductor in the vapor phase using the MOCVD method (0
01) plane or (110) plane. The plane constituting the surface of the substrate crystal is, for example, a (001) plane or a (110) plane.
It is desirable that the surface is inclined by 2 to 5 degrees from a low index surface such as a plane.

(Function) When depositing an oxide thin film on a substrate crystal whose surface is a low-index plane as in the past, two types of crystal grains grow with the same probability. , twins are formed. On the other hand, if a substrate crystal whose surface is tilted from the low-index plane of the crystal constituting the substrate as in the present invention is used, one of the crystal grains constituting the twin described above can be used. The probability that only crystal grains grow on the substrate increases.

As a result, the entire surface of the thin film can be made into a single crystal.

In the present invention, the reason why it is desirable that the plane constituting the surface of the substrate crystal be a plane inclined by 2 to 5 degrees from the low index plane is as follows. In other words, with current crystal manufacturing technology, it is difficult to control and tilt the surface of the substrate crystal at an angle of less than 2 degrees from the low-index plane, and if the angle is less than 2 degrees, only one type of crystal grain will be on the substrate. The probability of growth is not high. On the other hand, if the angle exceeds 5 degrees, growth on the substrate becomes disadvantageous for both types of crystal grains due to reasons such as approaching a low-index plane different from the reference low-index plane, and a single crystal is obtained. I can't.

(Example) Hereinafter, an example in which a YBa 2 Cu O7-a thin film was grown in vapor phase on a substrate crystal will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing the MOCVD apparatus used in this example. A sample mounting table 2 whose graphite surface is coated with silicon carbide (S t C) is installed in the reactor 1 , and a substrate crystal 20 is placed on this sample mounting table 2 .

The reaction furnace 1 is externally heated by an electric furnace 3. The pressure inside the reactor 1 is reduced by a rotary pump 4, and the pressure inside the reactor is monitored by a pressure gauge 5.

Organic metal raw materials for yttrium (Y), barium (Ba), and copper (Cu) include Y (02C, □H1
9) 3 (=Y (DPM) 3 ), Ba (0
2CIIH19) 2 (=Ba (DPM) 2
), Cu (02CIIH19) 2 (-Cu (
DPM) 2) is used. The organometallic raw materials are stored in stainless steel raw material containers 10 and IL 12 kept at 140°C, 250°C, and 150°C, respectively.

Pure oxygen sealed in an oxygen gas cylinder 16 is used as the oxygen source. All piping is made of stainless steel, and the piping system is heated to 260°C by a heater to prevent raw material vapor from condensing.
It is kept warm at 'C. High-purity arcon gas from the argon gas cylinder 6 is introduced into the mass flow controller 7.8.9 through the piping system and the flow rate is adjusted.
0.11.12 to generate the respective organometallic vapors, which are supplied to the reactor 1 via three-way valves 13, 14, and 15. Pure oxygen is introduced from an oxygen gas cylinder 16 through a piping system into a mass flow controller 17, the flow rate of which is adjusted, and supplied to the reactor 1 via a three-way valve 18. The three-way valves 13, 14, 15, 18 are electrically opened and closed by pneumatically actuated solenoid valves.

By switching the three-way valves 13, 14, 15, 18, the raw material vapor and oxygen are allowed to flow into either the piping to the reactor 1 or the exhaust path 19. All of the solenoid valves that make up the three-way valves 13, 14, 15, and 18 are controlled by a sequence controller, and their opening/closing timing and holding time can be programmed in advance.

Y B a 2 Cu 07 using this vapor phase growth apparatus
-a The vapor phase growth of the thin film is performed as follows.

First, the following operations are performed as a preliminary step before starting the vapor phase growth of a thin film. Strontium titanate (SrTt03) substrate crystal 20 whose surface was cleaned by chemical etching
Place the sample on the sample mounting table 2. High purity argon gas is supplied from the argon gas cylinder 6 to the reactor 1 to replace the atmosphere inside the reactor. While operating the rotary pump 4 and observing the pressure gauge 5, the pressure inside the reactor 1 is adjusted to 5 to 76 Torr.
Adjust within the range of r. High-purity oxygen gas is supplied from the oxygen gas cylinder 16 to the reaction furnace 1, and the electric furnace 3 heats the processing table 2 and the substrate crystal 20 at a predetermined temperature in the range of 600 to 850°C, so that the surface of the substrate crystal 20 is heated. Cleanse. The supply of oxygen gas to the reactor 1 is stopped by quickly switching the three-way valve 18 to allow the oxygen gas supplied from the oxygen gas cylinder 16 to pass through the exhaust path 19. By quickly switching the three-way valve 18 in this manner, temporary flow fluctuations due to flow path blockage can be reduced. Substrate crystal 20
While the surface of
1. Introduce into L2 at a flow rate of 50 3/min, 3-way valve 13.
14 and 15 to release steam into the exhaust path 19.

Next, a process for vapor phase growth of a thin film will be explained with reference to FIG. ■Switch three-way valves 13, 14, and 15 at the same time to supply organic metal raw material vapor to reactor 1. After 90 seconds have elapsed, the three-way valves 13, 14, and 15 are simultaneously switched to discharge the steam to the exhaust path I9, and the supply of organometallic vapor to the reactor 1 is stopped. (2) Switch the three-way valve 18 within at least 3 seconds to supply the oxygen gas that was flowing through the exhaust path 19 to the reactor 1.

After supplying oxygen gas for 90 seconds at a flow rate of 300c+n3/min, the three-way valve 18 is switched to stop the supply of oxygen gas to the reactor 1. By repeating the above steps 20 times, an oxide thin film with a thickness of approximately 5,000 wafers is deposited.

The surface of the substrate crystal is a (001) plane or a (00
1) Various angles θ shown in Table 1 from the plane to the (100) direction
We used a device that consists of a surface that is tilted by a certain angle. No matter which substrate crystal was used, all of the obtained thin films had a flat surface with about 200 surface irregularities. Furthermore, X-ray diffraction measurements revealed that no matter what substrate crystal was used, no particles other than YBa2Cu, O2, and crystal grains were found, and the (001) plane of the YBa2Cu307-J crystal was 5rTi.
It was found that O was arranged parallel to the substrate crystal plane, that is, it was epitaxially grown. However, it was found that when each substrate crystal was used, there were differences in the occurrence of twin crystals as shown in Table 1.

As is clear from Table 1, the substrate crystal surface is accurately (0
01), many twins occur. On the other hand, the substrate crystal surface changes from the (001) plane to the (100) plane.
When the crystal is composed of a plane tilted by 2 to 5 degrees in the direction, a twin-free single-crystal thin film can be grown. However, when the tilt angle θ from the (001) plane of the substrate crystal surface exceeds 5 degrees, twins occur and a-axis oriented crystals also begin to appear.

Table 1 O: None, Δ: Slightly, ×: Many In addition, in order to investigate in which direction the tilt angle of the substrate is effective, (100),
A YBa2Cu30t-a thin film was grown in the same manner as above using substrates tilted 3 degrees toward the (110) and (111) planes, and substrates tilted 3 degrees from the (001) plane without any particular direction. .

As a result, all of the deposited thin films were twin-free. From this, it was found that the same effect as described above can be obtained no matter which direction the substrate crystal surface is inclined from the (001) plane.

The critical temperature, which is an index of the thin film thus obtained, varies slightly, but is on average 4 to 5 inches higher than that of a thin film formed on (001).

Furthermore, when depositing oxides or metals to form electronic device structures, twin grains react differently with the deposit depending on the orientation difference of each crystal grain, but with the method of the present invention, the thin film becomes a single crystal. Therefore, it was unlikely that such a phenomenon would occur.

Next, vapor phase growth of yJH was performed using the apparatus shown in FIG. 1 and using a method different from that of the previous example regarding the switching timing of the three-way valve.

First, the following operations are performed as a preliminary step before starting the vapor phase growth of a thin film. Argon gas whose flow rate is adjusted from the argon gas cylinder 6 via the mass flow controllers 7 to 9 is introduced into each raw material container 10-12 at a flow rate of 50 cm3/min, and the steam is exhausted by operating the three-way valves 13 to 15. Route 1
Release it at 9.

Next, a process for vapor phase growth of a thin film will be explained with reference to FIG. ■Switch the 3-way valve 13 to Y (DPM
) 3 steam is supplied to the reactor 1. After 90 seconds,
The three-way valve 13 is switched to discharge steam to the discharge path 19. Within 3 seconds after switching the 3-way valve 13, the 3-way valve 18 is switched to remove the oxygen gas flowing into the exhaust path 19 from the reactor 1.
supply to After supplying oxygen gas for 90 seconds at a flow rate of 300 cm'/min, the three-way valve 18 is switched to stop the supply of oxygen gas to the reactor 1. Similarly, ■Ba(DP
M) z and oxygen gas alternately, ■Cu (D P M)
2 and oxygen gas are alternately supplied to the reaction tube 1. By repeating the operations from ■ to ■ above, YBa2Cu, 0
□-6 Grow thin film.

The growth temperature was 600° C., and the flow rates of organometallic vapor and oxygen gas and their integral supply time were the same as in the previous example for comparison. In addition, the supply time of organometallic vapor and oxygen gas per supply was the same as in the above example, and was changed to 1 minute (30 repetitions) and 5 minutes (6 repetitions).

A thin film with a thickness of about 8,000 wafers was obtained with a growth time of 3 hours. Furthermore, the substrate temperature was varied from 600° C. to 500° C. to examine how the substrate temperature affects the thin film properties. Further, the oxygen supply time was varied from 1 to 10 times the organometallic supply time.

As a result of trying the above experiment, we found that
The oxide thin film deposited at 0° C. contained no twins, had good surface flatness, and exhibited a higher superconducting critical temperature, as in the previous example.

When the growth temperature was lowered, thin films containing almost no twins were obtained up to 550°C. That is, it was found that an epitaxial thin film could be obtained at a lower temperature of 50'C than in the above example. Also, if the oxygen supply time is increased by 10 times,
It was possible to improve the superconducting critical temperature to a higher temperature side by about 5 to IOK. Furthermore, Cu.

It was found that extending the oxygen supply time, taking into account the difficulty of oxidizing B a s Y, improves the superconducting critical temperature by several orders of magnitude.

The gist of the present invention is not limited to the embodiments described above, and can be implemented with various modifications.

For example, in the above embodiment, 5iTrO is used as the substrate crystal material.
, and the sono surface was tilted several places from the (001) plane, but the substrate crystal material was L a Ga Os or L
aA10* or the like can be used, and a (100) plane, (010) plane, (110) plane, etc. can be used as a reference low index plane. Furthermore, the oxide thin film deposited on the substrate is not limited to the Y-Ba-Cu-0 based oxide high temperature superconductor thin film, but may also include Bi-8r-Ca-Cu-0 based oxide high temperature superconducting thin film, PbTi0 ．． , PbZrx Tf+-x 03 , BaTiO3, or other ferroelectric thin films may be used. When depositing the latter oxide,
MgO, CaF2, etc. can also be used as the substrate crystal material.

[Effects of the Invention] As detailed above, according to the method of the present invention, an oxide thin film made of single crystal can be produced over the entire surface.

[Brief explanation of drawings]

FIG. 1 shows the MOCV[
>Schematic configuration diagram of the device, FIG. 2 is a timing diagram showing switching operation of the three-way valve of the MOCVD device in an embodiment of the present invention, and FIG. 3 is a diagram of the MOCVD device in another embodiment of the present invention.
FIG. 3 is a timing diagram showing a switching operation of a three-way valve of the device. DESCRIPTION OF SYMBOLS 1...Reaction furnace, 2...Sample mounting stand, 3...Electric furnace, 4...Rotary pump, 5...Pressure gauge, 6...
- Argon gas cylinder, 7.8.9... Mass flow controller, 10.11.12... Raw material container, 13.
14.15...3-way valve, 16...oxygen gas cylinder,
17... Mass flow controller, 18... Three-way valve, 19... Exhaust path, 20... Substrate crystal.

Claims (1)

[Scope of Claims] (1) A substrate crystal is housed in a reactor, and organometallic vapor containing a metal element and oxygen or an oxidizing agent capable of supplying oxygen are introduced, and these are thermally decomposed. 1. A method for vapor phase growth of an oxide thin film on a substrate crystal, characterized in that the surface of the substrate crystal is comprised of a plane tilted from a low index plane of the crystal. (2) The method for vapor phase growth of an oxide thin film according to claim (1), wherein the low index plane is a (001) plane or a (110) plane. (3) Claim (1) characterized in that the plane constituting the surface of the substrate crystal is a plane inclined by 2 to 5 degrees from the low index plane.
Or the method for vapor phase growth of an oxide thin film according to (2).