JPH035397A - Vapor growth method for thin oxide crystal film - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a method for vapor phase growth of oxide crystal thin films.

(Prior Art) Recently, oxide superconductors that exhibit superconducting properties at high temperatures have been discovered and are attracting attention. Typical oxide superconductors include:
Those represented by YBa2 Cu307-J, or those in which a part of Y is replaced with a lanthanum-based element (hereinafter referred to as YBaCuO-based) are well known. In addition, Bi25r2CaCu208 and B i2 S r
2Ca2Cu30.

Bismuth-based materials such as (hereinafter referred to as B15rCaC
(referred to as uO-based) is also attracting attention as a promising product. These oxide superconductors have a critical temperature Tc exceeding 100K, and various applications have been proposed for ultrahigh-speed logic devices that operate at liquid nitrogen temperature (77K).

In order for these 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, flat single-crystal thin films are essential for applications in ultra-high-speed logic devices.

Up until now, the method for forming oxide superconductor thin films has been using organic metals, which is a type of chemical vapor deposition (CVD) method.
The OCV D method is being actively researched as a promising method. This is because in the MOCVD method, the supply amount of each elemental raw material gas constituting the oxide superconductor can be precisely controlled independently. In this case, as the substrate, an S r T i O3 crystal substrate or an MgO crystal substrate has been used, which exhibits relatively good lattice matching with the oxide superconductor and also has a linear expansion coefficient similar to that of the oxide superconductor. It has been reported that when a YBaCuO-based superconductor thin film is grown on a 5rTi03 substrate, it exhibits superconducting properties only at a high growth temperature of 800° C. or higher. In addition, the obtained thin film has a (001) plane of S r
grown parallel to the (001) plane of the T i O3 substrate,
It is said to exhibit a so-called C-axis orientation.

However, so far, all YBaCuO-based oxide superconductor thin films obtained using 5rTi03 substrates are polycrystalline. Furthermore, the flatness of the thin film surface is also very poor. This is thought to be caused by irregularities and lattice defects on the substrate surface. In particular, a major cause of this is thought to be that heating the substrate to a high temperature causes the release of oxygen due to the decomposition of crystals on the substrate surface before film deposition, which impairs the flatness of the substrate surface and increases the number of defects. Therefore, it is impossible to form a practical logic element using such a thin film.

The MgO substrate is selected because its linear expansion coefficient is close to that of the oxide superconductor, but even when a B15rCaCuO-based oxide superconductor thin film is formed on the (100) plane of the MgO crystal, flatness remains was very poor, and only polycrystalline materials were obtained.

In addition to the above, YBaCuO using a LaGaO3 crystal substrate
There have also been reports on the formation of thin films of oxide superconductors.

Although this crystal substrate also has a relatively good lattice matching with YBaCuO and a coefficient of linear expansion close to that of YBaCuO, a single crystal has not been obtained and the surface flatness is also poor.

(Problems to be Solved by the Invention) As described above, a high quality oxide superconductor thin film is desired for the application of oxide superconductors to devices, but so far, single crystals have not been able to achieve the desired surface flatness. Nothing good has been achieved.

Similar problems occur not only in the case of oxide superconductor thin films but also in the case of vapor phase growth of ferroelectric crystal thin films, for example.

The present invention has been made in view of these points, and an object of the present invention is to provide a vapor phase growth method capable of obtaining an oxide crystal thin film with excellent surface flatness.

[Structure of the Invention] (Means for Solving the Problem) The present invention accommodates a substrate in a reaction vessel, introduces a raw material gas into the reaction vessel, and grows an oxide crystal thin film on the substrate in a vapor phase. The method is characterized in that a thin crystalline film made of the same substance as the substrate is grown in vapor phase on the substrate as a base film, and then a desired oxide crystalline thin film is grown in vapor phase by changing the raw material gas.

(Function) According to the present invention, since the base crystal thin film is deposited on the substrate surface until just before the deposition of the desired oxide crystal thin film begins, the substrate is heated to a high temperature in a state where film deposition is not performed as in the conventional case. not be exposed to.

As a result, a situation in which the flatness of the underlying layer deteriorates or crystal defects increase when the desired oxide crystal thin film starts to be deposited is prevented, and therefore a single-crystal oxide crystal thin film with good flatness is obtained. can be obtained.

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

FIG. 1 schematically shows the state of deposition of an oxide superconductor thin film according to an example. Substrate 1 is an MgO single crystal substrate, and an MgO base film is first formed on this by MOCVD.
An O crystal thin film 2 is formed, and B15rC is continuously formed on this film.
An aCuO-based oxide superconductor thin film 3 is formed.

FIG. 2 shows the MOCVD apparatus used in this example. The main body of the apparatus includes a reaction vessel 21 and an electric furnace 24, and a substrate 23 is placed on a susceptor 22 inside the reaction vessel 21. The susceptor 22 is, for example, a graphite surface coated with silicon carbide. 25 is a pressure gauge, 26
is the exhaust pump. The organometallic raw materials are stored in raw material containers 271 to 277, respectively, and raw material gas is taken out as needed. That is, the Ar gas from the Ar gas cylinder 28 flows into the mass flow controller 30. ~30
7, and the raw material gas sent out from the raw material containers 27 is controlled by the three-way valve 31.7, which is driven by a solenoid valve. Reaction vessel 2 via ~317
It is set to be introduced in 1. Also, 0° gas is 0
2 gas cylinder 29 to mass flow controller 30
The flow rate is adjusted by 8 and introduced into the reaction vessel 21 via the three-way valve 318. Mass flow・
All the solenoid valves of the controller 30 are controlled by a sequence controller, and the opening/closing timing and holding time of the three-way valve 31 can be programmed in advance.

The organometallic raw material for magnesium (M g ) is Mg (DP
M)2, bismuth (Bi), strontium (S
r), calcium (Ca) and copper (Cu) organometallic raw materials are Bi (C, H6) and Sr (
DPM)2. Ca(DPM)2 and Cu(DPM)2
It is. Among the raw material containers 27, the raw material containers for Bi, Sr, Ca, and Cu are at 150° C. and 120° C., respectively.

Heated to 230°C and 200°C. The raw material gas piping system is all made of stainless steel, and is kept at about 260°C by a heater to prevent raw material vapor from condensing.

Specifically, the process of forming the B15rCaCuO-based oxide thin film will be described below. As described above, the substrate 23 is (10
0) orientation is prepared, and after cleaning the surface by chemical etching, the MgO crystal substrate is placed in a reaction vessel 21. Thereafter, an Ar gas cylinder 2 is placed inside the reaction vessel 21.
8 to high-purity A gas (the gas piping at this time is not shown in FIG. While watching the total 25, increase the pressure inside the reaction vessel 21 by 5 to 7.
Adjust to a range of 6 Torr. After that, high-purity 02 gas is supplied from the 02 gas cylinder 29 to the reaction vessel 21, and the susceptor 22 and the substrate 23 are heated to 600°C in the electric furnace 24.
The substrate surface is cleaned by heating to a predetermined temperature in the range of ~850°C.

The supply of 02 gas was stopped by quickly switching the three-way valve 318 to discharge the gas to the exhaust path side, thereby reducing temporary flow rate fluctuations due to flow path interruption. While cleaning the substrate surface, Ar gas whose flow rate was adjusted from the Ar gas cylinder 28 via the mass flow controller 30 was supplied to Mg(DPM) 2 , Bi
(C6Hs)3.

S r (DPM)2 , Ca (DPM)2 , Cu
(DPM) 2 is fed into each raw material container 27 at a rate of 50 cm'/min, and the obtained steam is sent downstream through the piping system. At this time, the three-way valve 31 allows these raw material gases to be discharged not to the reaction vessel side but to the exhaust path side. This is the preliminary stage before growth begins.

Next, the three-way valve 31 is switched to deposit an MgO crystal thin film as a base thin film. The deposition conditions were as follows. The substrate is heated to 600°C, and the pressure inside the reaction vessel 21 is 10To.
rr, and the flow rate of 0□ gas to the reaction vessel 21 is 50 cI.
It was set to T13/min. A deposited film of about 1 μm can be obtained with a deposition time of 1 hour.

After this, crystal growth is performed by continuously changing the raw material gas, but a test sample is prepared in which the process is stopped until the formation of the base thin film described above, and the crystallinity of the base thin film is examined by X-ray diffraction and electron beam analysis. Examined by diffraction. As a result, the lattice constant was a -4,212, same as that of the substrate, and it was confirmed that a single crystal thin film was epitaxially grown. Further, the surface unevenness of the underlying thin film was about 50, indicating excellent flatness.

After the base thin film is deposited in this way, the three-way valve 31 is switched to discharge the raw material gas for forming the base thin film into the exhaust path, and instead supply the raw material gas for the B i S rcacuo oxide superconductor thin film to the reaction vessel 21 . The deposition conditions at this time are as follows. The substrate temperature was 600° C., the pressure inside the reaction vessel was 10 Torr, and the amount of 02 gas supplied to the reaction vessel was 50 cm 3 /min. With a deposition time of 30 minutes, 0,
A 5 μm thin film was obtained.

The B15rCaCuO thin film obtained in this way was confirmed to be a single crystal on the entire surface by X-ray diffraction, and the lattice constant was a
-5.4 people, b-26 people, c-37 people. Moreover, the surface of the thin film had about 100 irregularities and was excellent in flatness. No diffusion reaction was observed at the interface between the underlying thin film and the oxide superconductor thin film.

Moreover, the obtained B15rCaCuO thin film has a critical temperature of 10
0 showed superconducting properties.

As a comparative example, a B15rCaCuO thin film was formed on an MgO crystal substrate under the same conditions without forming a base thin film. Film thickness 0.5am formed at substrate temperature 800'C
The B15rCaCuO thin film exhibited superconducting properties at a critical temperature of 65%, but it is polycrystalline and the surface roughness is 1000%.
It was about the size of a human being.

Next, an example in which a YBaCuO thin film is formed on an MgO crystal substrate will be described.

The MOCVD apparatus used was the same as in the previous example. The organometallic raw materials of Y, Ba and Cu each include:
Y(DPM)s, Ba(DPM)2 and Cu(
DPM) 2, and their raw material containers are
Heated to 140°C, 250°C and 150°C. The organometallic raw material for Mg is the same as in the previous example. MgO used
The crystal substrate had a (100) orientation, and a MgO crystal thin film was formed as a base thin film through a preliminary step as in the previous example.

After forming the base thin film, the three-way valve was switched to deposit a YBaCuO thin film. The deposition conditions for the YBaCuO thin film were a substrate temperature of 600°C, a reaction vessel internal pressure of 10 Torr, and a
The flow rate of 2 gas and Ar gas into the reaction vessel is 50 cm3.
/ minute. With a film deposition time of 1 hour, approximately 1 μm of Y
A BaCuO thin film was obtained.

The YBaCuO thin film obtained in this way was examined by X-ray diffraction and electron beam diffraction, and it was found that the (100) plane of the YBaCuO thin film was epitaxially grown parallel to the (100) plane of the MgO crystal that was the base thin film, and the entire surface of the substrate was grown. It was confirmed that it was a single crystal. Also, the surface unevenness is 50
The flatness was also good with a thickness of Å or less. Superconducting critical temperature is 89
It was there.

The present invention is not limited to the above embodiments. For example, when using 5rTiO, LaGaO3, LaAgo, etc. as the substrate crystal, the same crystal thin film as these substrate materials is similarly grown in the vapor phase as the base film to continuously form the desired oxide superconductor thin film. It is good to grow. B15rCaCuO
For thin films, use PB, SB, etc. added, YBaCu
In the case of an O thin film, the present invention can be similarly applied to a film in which a part of Y is replaced with La or the like. Moreover, the present invention
Not limited to superconductor thin films, for example, BaTiO3, LiN
bO3, LiTa0. .

This method is effective when growing dielectric oxide crystal thin films such as P b T i O 3 in a vapor phase.

[Effects of the Invention] As described above, according to the present invention, when forming an oxide crystal thin film by vapor phase growth, a crystal thin film of the same material as the substrate is grown as a base film on a substrate in advance, and then By growing a desired oxide crystal thin film using this method, a high-quality crystalline oxide crystal thin film with excellent flatness can be obtained.

[Brief explanation of drawings]

FIG. 1 shows B15rCaCuO according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view showing the state of deposition of the oxide superconductor thin film. FIG. 2 is a diagram showing the configuration of the MOCVD apparatus used to form the thin film. 11...MgO crystal substrate, 12...MgO crystal thin film, 1B ---B i S r Ca Cu O
thin film, 21 =-reaction vessel, 22... susceptor, 23
...Substrate, 24...Electric furnace, 25...Pressure gauge, 2
6... Exhaust pump, 27... Raw material container, 28...
Ar gas cylinder, 29...O2 gas cylinder, 30...
・Mass flow controller 31...Three-way valve.

Claims (1)

[Claims] (1) A substrate is housed in a reaction vessel, and when a raw material gas is introduced into the reaction vessel to grow an oxide crystal thin film on the substrate in a vapor phase, a crystal thin film made of the same material as the substrate is grown on the substrate. A method for vapor phase growth of an oxide crystal thin film, which is characterized in that a base film is grown in a vapor phase, and then a necessary oxide crystal thin film is grown in a vapor phase by changing the raw material gas.