JPH08102560A - Surface treatment for strontium titanate substrate - Google Patents

Abstract

PURPOSE: To finely control epitaxial growth with a unit of atomic layer by annealing a strontium titanate substrate at a high temperature in an oxygen atmosphere and forming the topmost surface structure of TiO2 . CONSTITUTION: A strontium titanate substrate, hereinafter referred to as STO substrate, is annealed at a high temperature of approximately 1000±200 deg.C in an oxygen atmosphere. Thus, the topmost surface of the STO substrate is formed of approximately 100% TiO2 . When The STO substrate is heated to nearly 1000 deg.C, Ti, which is the element of the substrate, stays on the surface of the substrate. However, Sr jumps out from the surface of the substrate, and as a result, SrO is not allowed to remain on the topmost surface of the substrate. Such mechanism allows the TiO2 plane to be formed on the topmost surface of the substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method for a strontium titanate (SrTiO ₃ ) substrate (hereinafter referred to as an STO substrate) for epitaxially growing a high temperature superconducting thin film or a ferroelectric thin film.

[0002]

2. Description of the Related Art When a thin film of SrTiO ₃ or the like is epitaxially grown on an STO substrate, a process for cleaning the substrate surface is usually performed before film formation. As a method for cleaning the surface, conventionally purchased STO is used. A method in which a [001] substrate is washed with an organic solvent or the like, then sputter-etched (Ar sputtered, etc.) in vacuum, and further annealed is generally adopted.

[0003]

By the way, according to the above-mentioned conventional method, the TiO ₂ surface and the SrO surface are mixed on the outermost surface of the STO substrate even after the surface treatment (see FIG. 3). This is a factor that makes it impossible to precisely control the subsequent epitaxial growth.

That is, when a thin film of SrTiO ₃ or the like is grown on an STO substrate in atomic layer units with good controllability,
The outermost surface of the O board is said that it is good to complete TiO ₂ side or SrO surface, TiO ₂ on the outermost surface of the substrate
When the planes and the SrO planes are mixed, the imperfections are reflected in the crystal growth process, and the influence thereof becomes a hindrance factor for precisely controlling the epitaxial growth in atomic layer units.

The present invention has been made in view of such a situation, and the termination structure on the outermost surface of the STO substrate is almost completely T-shaped.
It is an object of the present invention to provide a treatment method capable of forming an iO ₂ surface.

[0006]

In order to achieve the above object, in the surface treatment method of the present invention, an STO substrate is annealed at a high temperature in an oxygen atmosphere to obtain an outermost surface structure terminated with a TiO ₂ surface. Characterized by:

Here, the temperature of the high temperature annealing adopted in the present invention is appropriately in the range of 750 ° C. to 1300 ° C., and the atmospheric oxygen pressure during annealing is not particularly limited.
It may be at atmospheric pressure (1 atm), pressurized or depressurized.

[0008]

The STO [001] substrate is placed under oxygen atmosphere for 10
When annealed at a high temperature of around 00 ℃ ± 200 ℃, STO
The outermost surface of the substrate is terminated with almost 100% TiO ₂ surface.
This is because when the STO substrate is heated to around 1000 ° C., Ti, which is a constituent element of the substrate, remains on the substrate surface, but Sr jumps out from the substrate surface, and as a result, SrO does not exist on the outermost surface of the substrate. It is thought to be due to a mechanism such as.

In consideration of the above points, the temperature range of high temperature annealing for carrying out the surface treatment method of the present invention is
The lower limit is near the minimum temperature required for Sr to jump out from the substrate surface, and the upper limit is Ti due to high temperature heating.
Must be set to a temperature at which it does not jump out of the substrate surface, and its specific numerical value is 750 ° C. to 1 as described above.
300 ° C is suitable.

[0010]

EXAMPLE First, a purchased STO [001] substrate (untreated) was used as a sample, and the sample was placed in a processing furnace and heated to 1000 ° C. under an oxygen atmosphere at atmospheric pressure (1 atm) for 10 hours. High temperature annealing, and the surface of the sample after this treatment
CAICSS (Coaxial Impact Collision Ion Scatte)
ring spectroscopy). Further, when the STO [001] substrate in the as-purchased state was used as a sample and similarly measured by CAICISS, the TO
F spectra SP1 and SP3 were obtained.

From the measurement results, the TOF of the untreated sample was
In the spectrum SP3, both signals of Ti and Sr are observed, and the TiO ₂ surface and the SrO surface are mixed on the outermost surface, whereas in the TOF spectrum SP1 of the sample annealed at high temperature, the Sr signal is extremely high. It was found that it was small and the outermost surface was a TiO ₂ surface of almost 100%. From this, it was confirmed that the surface treatment method of the present invention is an excellent method suitable for making the outermost terminal structure of the STO substrate a nearly perfect TiO ₂ surface.

Then, after the Sr thin film was epitaxially grown on the surface of the sample subjected to the above-mentioned surface treatment by the MBE method or the like, and the outermost surface structure of the sample was measured by CAICISS, the TOF spectrum SP2 shown in FIG. 1 was obtained. From this measurement result, it was found that the outermost surface of the substrate was a perfect SrO surface. Therefore, it was confirmed that by making the outermost surface of the STO substrate 100% TiO ₂ surface, it becomes possible to grow a thin film of SrTiO _{3 on} the surface of the substrate while precisely controlling it in atomic layer units.

Here, according to the TOF spectrum measurement by CAICISS, the outermost surface structure of the STO substrate is 100% Ti.
The reason why it can be evaluated whether or not it is the O ₂ surface will be described below. First, since SrTiO ₃ has a perovskite structure, the surface of the STO substrate is irradiated with He ⁺ from the [111] direction (incident angle α = 3 as shown in FIG. 2).
5 °) and the scattering intensity was measured, and if the surface first atomic layer of the STO substrate was a perfect TiO ₂ surface, the Sr of the second layer was hidden in the shadow cone of Ti, and only the Ti signal was observed. It On the contrary, when the surface first atomic layer is a perfect SrO plane, the second layer Ti is hidden in the Sr shadow cone and only the Sr signal is observed. On the other hand, TiO ₂ surface and SrO
When the surfaces are mixed, the TOF spectrum of CAICISS has both Sr and Ti peaks. Therefore, such TOF by CAICISS
By performing spectrum measurement, the structural integrity (100% TiO ₂ surface) of the outermost surface of the STO substrate can be evaluated.

The high temperature annealing for carrying out the surface treatment method of the present invention is preferably a heat treatment at a temperature of 1000 ° C. for about 10 hours, but as described above, the annealing temperature is 750 ° C. to 1300 ° C. Within the range, the method of the present invention can be carried out, and the annealing treatment time may be appropriately changed depending on the level of the annealing temperature to be set.

Furthermore, the method of the present invention is not limited to the case where the SrTiO ₃ thin film is epitaxially grown, and Bi ₂ Sr ₂ C is also used.
Of course, it can be applied to the pretreatment for growing another high temperature superconducting thin film such as uO ₆ or a ferroelectric thin film on the STO substrate.

[0016]

As described above, according to the surface treatment method of the present invention, an STO [001] substrate whose outermost surface is terminated by almost 100% TiO ₂ surface can be stably manufactured. When epitaxially growing a high-temperature superconducting thin film or a ferroelectric thin film on an STO substrate, it becomes possible to precisely control the growth mode. As a result, the quality of these thin films is improved, and eventually it becomes possible to develop various high-performance electronic devices.

[Brief description of drawings]

FIG. 1 is a TOF spectrum showing the result of evaluation of the terminal structure on the outermost surface of the STO substrate by CAICISS.
Is the T of the STO [001] substrate processed by the method of the present invention.
OF spectrum, SP2 is ST processed by the method of the present invention
TOF spectrum of a sample obtained by epitaxially growing an Sr thin film on an O [001] substrate, SP3 is untreated STO
TOF spectrum of [001] substrate

FIG. 2 is an explanatory diagram of the evaluation method by CAICISS.

FIG. 3 is a diagram schematically showing a state where the TiO ₂ surface and the SrO surface are mixed on the outermost surface of the STO substrate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Ishiyama 1 Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto Prefecture Kyoto Sanjo Factory Sanjo Factory (72) Inventor Shin Shinohara 1 Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto Prefecture Kyoto Shimadzu Sanjo Factory

Claims (1)

[Claims] 1. A method for surface treatment of a substrate for epitaxially growing a high temperature superconducting thin film or a ferroelectric thin film, comprising:
A surface treatment method for a strontium titanate substrate, which comprises obtaining an outermost surface structure terminated by a TiO ₂ surface by annealing the strontium titanate substrate at a high temperature in an oxygen atmosphere.