JP2889463B2 - Oriented ferroelectric thin film device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an epitaxial MgO
Is used as a buffer layer, and an epitaxial or oriented ferroelectric thin film is formed on a single crystal Si (100) substrate.
Alternatively, the present invention relates to an oriented ferroelectric thin film element that can be used when an element such as an optical integrated circuit is formed on a Si semiconductor substrate.

[0002]

2. Description of the Related Art Conventionally, an oxide ferroelectric thin film has a number of properties such as ferroelectricity, piezoelectricity, pyroelectricity, and electro-optic effect of a ferroelectric material. Many applications such as infrared pyroelectric elements, acousto-optical elements, and electro-optical elements are expected. Among these applications, it is indispensable to produce a single crystal thin film in order to reduce the optical loss in the thin film optical waveguide structure and obtain the polarization characteristics and electro-optic effects comparable to those of a single crystal. Therefore, BaTiO ₃ , PbTi
_{O 3, Pb 1-x La} x (Zr y Ti 1-y) 1-x / 4 O
₃ (PLZT), LiNbO ₃ , KNbO ₃ , Bi ₄ T
An epitaxial ferroelectric thin film such as i ₃ O ₁₂ is formed on an oxide single crystal substrate by a method such as Rf-magnetron sputtering, ion beam sputtering, laser ablation (laser deposition), and metal organic chemical vapor deposition (MOCVD). Various attempts have been made to form them.

However, for integration with a semiconductor element, it is necessary to form a ferroelectric thin film on a semiconductor substrate. Epitaxial growth of a ferroelectric thin film on a Si substrate is difficult due to high growth temperature, interdiffusion between Si and the ferroelectric, oxidation of Si, and the like. For these reasons, it is necessary to form a buffer layer on the semiconductor substrate as a buffer layer, which serves as a buffer layer, which epitaxially grows on the semiconductor substrate at a low temperature, assists the epitaxial growth of the ferroelectric thin film, and also functions as a diffusion barrier. Furthermore, in an FET device in which an insulator is formed between a ferroelectric and a semiconductor, the presence of such a buffer layer can prevent charge injection from the semiconductor during polarization of the ferroelectric, and It is easy to maintain the polarization state of the dielectric. Also,
The refractive index of ferroelectrics is generally lower than that of Si, but if a buffer layer with a refractive index lower than that of ferroelectrics can be obtained, laser light can be confined in a ferroelectric thin film optical waveguide, and optical integration A circuit can be manufactured on a Si semiconductor integrated circuit.

[0004] On the other hand, M
Japanese Patent Application Laid-Open No. 61-185808 discloses that a ferroelectric compound is epitaxially grown on a substrate on which gAl ₂ O ₄ (100) or MgO (100) is epitaxially grown.
As shown in Japanese Patent Application Publication No.
The crystallographic relationship with O (100) is not disclosed. In fact, in subsequent studies, the (100) -oriented M
When gO is formed on a Si (100) single crystal, MgO
O has been revealed that the (100) plane is only parallel to the Si (100) plane, and the in-plane orientation is random oriented polycrystalline MgO (P. Tiwari et a).
l. , J. et al. Appl. Phys. 69, 8358 (19
91)). Thereafter, MgO, which is often used as a substrate for ferroelectrics and high-temperature superconductors, due to its lattice constant and thermal stability,
Have been demonstrated for the first time to be able to grow epitaxially on Si (DK Fork et a
l. , Appl. Phys. Lett. 58, 22
94 (1991)), and a superconducting thin film using the same has been proposed.

[0005]

As described above, it is difficult to epitaxially grow a ferroelectric thin film on a single-crystal Si substrate by the conventional technique. Therefore, an object of the present invention is to solve this problem. That is, an object of the present invention is to provide a single crystal Si (10
0) An object of the present invention is to provide an oriented ferroelectric thin film element having an epitaxial or oriented ferroelectric thin film formed on a substrate. Another object of the present invention is to provide an oriented ferroelectric thin film element that can be used when an element such as a high-performance nonvolatile memory, a capacitor, or a light modulation element is manufactured on a semiconductor substrate.

[0006]

Means for Solving the Problems The present inventors have grown MgO on a Si (100) substrate by vapor phase epitaxy, and when MgO is used as a buffer layer, the ferroelectric thin film becomes Si (100). The inventors have discovered that epitaxial growth can be performed on a substrate, and have completed the present invention.
The oriented ferroelectric thin film element of the present invention is composed of single crystal Si (10
0) An epitaxial MgO buffer layer is formed on a substrate, and an epitaxial or oriented perovskite ABO ₃ type ferroelectric thin film is further formed thereon ,
The single crystal Si (100) substrate and epitaxial MgO
The crystallographic relationship of the buffer layer is MgO (100) // Si
(100), in-plane orientation MgO [001] // Si [00
1] .

Hereinafter, the present invention will be described in detail. In the present invention, the crystallographic relationship in the oriented ferroelectric thin film is
For example, for BaTiO ₃ , BaTiO ₃ (00
1) // MgO (100) // Si (100), in-plane orientation B
aTiO ₃ [010] // MgO [001] // Si [00
1], and a structure in which the polarization direction of the tetragonal ferroelectric is perpendicular to the substrate surface can be formed. Regarding the crystallographic relationship between MgO and Si, although the lattice misalignment is 22.5%, the relationship between the crystal orientations of MgO and Si is MgO.
(100) // Si (100), in-plane orientation MgO [00
1] // Si [001]. Observation of the interface between MgO and Si with a high-resolution transmission electron microscope reveals that about MgO: S
A structure considered to be lattice matching of i = 4: 3 is formed, and it is recognized that the interface is a steep interface without generation of SiO ₂ or the like. Considering the lattice matching of 4: 3, MgO (100) // Si (10
0), 3.4% smaller lattice misalignment than in the case of epitaxial growth of MgO [011] // Si [001],
Despite the apparently large lattice mismatch, M
The stress in the film is relaxed as compared with the case of gO [011] // Si [001], and the film quality of the epitaxial thin film of MgO [001] // Si [001] is improved. Therefore, on a Si (100) single crystal, MgAl ₂ O ₄ (10
0) or when a ferroelectric compound is epitaxially grown on a substrate on which MgO (100) has been epitaxially grown, MgO and MgO (/) in MgO (100) // Si (100)
The relationship of the in-plane crystal orientation of i is MgO [011] // Si [0
[01], but not MgO [001] // Si [001].

In the present invention, an epitaxial MgO buffer layer is formed on a single crystal Si (100) substrate.
As the film forming conditions, a film forming temperature of room temperature to 1200 ° C. and a film forming rate of 0.01 to 10.0 Å / sec are employed, whereby an epitaxial MgO buffer layer can be formed. The thickness of MgO is 10
10 ⁵ angstroms of the range is preferred. As described above, the epitaxial Mg on the single crystal Si (100) substrate
By forming the O buffer layer, it becomes possible to form an epitaxial or oriented perovskite ABO ₃ type ferroelectric thin film thereon. Specifically, Ba
_{TiO 3, PbTiO 3, Pb 1} -x La x (Zr y Ti
_1-y ) _{1-x / 4} O ₃ (PLZT), LiNbO ₃ , KNb
An oriented perovskite ABO ₃ type ferroelectric thin film such as O ₃ or Bi ₄ Ti ₃ O ₁₂ is formed. Its film thickness is 0.
It is preferably in the range of 01 to 10 μm. These deposition methods include excimer laser deposition,
Rf-magnetron sputtering, ion beam sputtering, electron beam evaporation, flash evaporation,
Vapor growth methods such as ion plating, molecular beam epitaxy (MBE), ionized cluster beam epitaxy, chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), and plasma CVD Can be used effectively.

[0009] In the present invention, the single-crystal Si (10
0) crystallographic relation between the substrate and the epitaxial MgO buffer layer, MgO (100) // Si ( 100), the in-plane orientation MgO [001] // Si [001 ] Although Ru Der, said epitaxial MgO buffer layer The crystallographic relationship of the epitaxial or oriented perovskite ABO ₃ type ferroelectric thin film is ABO ₃ (001) // MgO (100) or ABO ₃ (100) // MgO (100), and the in-plane orientation A
BO ₃ [010] // MgO [001] or ABO
₃ [001] // MgO [001] is preferred.

[0010]

Since the oriented ferroelectric thin film element of the present invention has the above-described structure, epitaxial growth of a ferroelectric thin film on a single crystal Si (100) substrate becomes possible. That is, the epitaxial MgO buffer layer assists the epitaxial growth of the ferroelectric thin film and also functions as a diffusion barrier. At this time, MgO (10
0) // Si (100), MgO [011] // Si [00
3.4) lattice irregularity smaller than the epitaxial growth of [1].
% Of MgO—Si interface, it is considered that a lattice matching of 4: 3 is formed, and MgO [01] despite having an apparently large lattice mismatch of 22.5%.
1] // Si [001], the stress in the film is relaxed, and better quality MgO [001] // Si [001] epitaxial growth becomes possible. Furthermore, since the orientation of the ferroelectric thin film can be controlled, a large remanent polarization value and a large electro-optic constant can be obtained. In an FET device in which an insulator is formed between a ferroelectric and a semiconductor, the ferroelectric Injection of charges from the semiconductor during polarization of the body can be prevented, and the polarization state of the ferroelectric can be easily maintained.
Also, the refractive index of a ferroelectric is generally smaller than that of Si,
If a buffer layer with a refractive index smaller than that of a ferroelectric can be obtained, laser light can be confined in a ferroelectric thin-film optical waveguide, and an optical integrated circuit can be fabricated on a Si semiconductor integrated circuit. Become.

[0011]

【Example】

Example 1 An epitaxial MgO buffer layer was formed on a single-crystal Si substrate by an excimer laser deposition method in which a target surface was instantaneously heated by a UV laser pulse to perform evaporation. The laser used was a XeCl excimer laser (wavelength 308 nm) with a pulse period of 4H.
z, pulse length 17 ns, energy 130 mJ (energy density on the target surface 1.3 J / cm ² ). The distance between the target and the substrate is 50 mm. As the target, metallic Mg was used because MgO has no absorption at a wavelength of 308 nm. Since MgO has a high binding energy of 10 eV or more, Mg is easily oxidized by introducing O ₂ during film formation. The Si substrate was heated by a halogen lamp.

As a single crystal Si substrate, n-type or p-type
A mold, a (100) plane, and a 6 × 6 mm wafer were used. These substrates were etched with a HF solution after solvent cleaning. Further, the substrate was rinsed with deionized water and ethanol, and finally spin-dried with ethanol under a nitrogen flow. Immediately after the spin drying, the substrate was introduced into a deposition chamber, and heated at a constant temperature, a background pressure of 3 × 10 ⁻⁷ Torr, and 500 ° C. or higher, to remove (sublimate) the H passivation layer on the Si surface. And then MgO at 200 to 700 ° C., 1 × 10 ⁻⁶ to 1 × 10 ^−3.
At Torr O ₂ of the conditions was the formation of 40 to 300 angstroms of MgO.

When analyzed by X-ray diffraction, the MgO film formed on the Si substrate turned into an epitaxial film of (100) plane unidirectional orientation under a wide range of conditions.
It was confirmed that a good quality thin film could be obtained under the conditions of 2 ° C. and 2 × 10 ⁻⁶ to 1 × 10 ⁻⁵ Torr O ₂ . In order to identify the relationship between the in-plane crystal orientations of MgO and Si, X-ray diffraction
A scan was performed. The phi scan on the (202) plane, which has a 45 ° angle with respect to the (100) plane in the cubic crystal, was performed using MgO (100) / Si (100).
Shows a sharp peak having a 90 ° rotation period with respect to MgO, and this position coincided with the Si peak position.
From these, the crystallographic relationship between MgO and Si is
Despite the lattice misalignment being 22.5%, the relationship between the crystal orientations of MgO and Si is that MgO (100) // Si (1
00), the in-plane orientation was MgO [001] // Si [001].

When the interface between MgO and Si is observed with a high-resolution transmission electron microscope, a structure which is considered to have a lattice matching of about MgO: Si = 4: 3 is formed.
_There was no generation of ₂ etc. and the interface was steep. Considering the lattice matching of 4: 3, when MgO: Si = 4: 3, 3.4.
%, And the stress in the film was relaxed despite having a large lattice mismatch, so that high quality MgO [001] // Si
It is considered that an epitaxial thin film of [001] was obtained.

On the other hand, BaTiO, a typical ferroelectric,
_Although the lattice constants of ₃ (tetragonal system, perovskite structure) and Si (cubic system, diamond structure) are greatly different and the lattice mismatch is 26.4%, BaTiO ₃ (00
1) When considering the lattice matching in the in-plane rotation of the plane and the Si (100) plane by 45 °, that is, the orientation of BaTiO ₃ [110] // Si [001], the lattice mismatch is 4.0%.
And relatively small. Therefore, BaTiO ₃ was directly grown on GaAs. Upon film formation, during the first 100 laser pulses, and the various O ₂ pressure as described above, was continued deposited by subsequent 1.0mTorrO _2. First 10
BaTiO ₃ depending on the O ₂ pressure between 0 pulses and the film forming temperature
The crystallinity changed, but the obtained one was (110) /
It was a (101) oriented polycrystalline film. Thus, epitaxy was not determined by simple lattice matching.

When BaTiO ₃ was grown directly on Si, epitaxy was not observed as described above, but at 600 to 800 ° C., 1 × 10 ^-4 to 1 × 10 ^-2 Torr.
BaTiO having a thickness of 500 to 2,000 Å grown in situ on an MgO buffer layer under the condition of rO ₂
Sample No. ₃ had a lattice mismatch with MgO of 5.2%, but all were epitaxially grown. Analysis of the X-ray diffraction pattern shows that BaTiO ₃ has c-axis orientation, and the relationship between the crystal orientations of BaTiO ₃ and MgO / Si identified by the phi scan shows that BaTiO ₃ (00
1) // MgO (100) // Si (100), BaTiO
₃ [010] // MgO [001] // Si [001]. In FIG. 1, 1 is a single crystal Si substrate (diamond structure), 2 is an MgO thin film (NaCl structure), 3
Indicates an epitaxial BaTiO ₃ thin film (perovskite structure).

BaT observed with a scanning electron microscope
The surface of iO ₃ was extremely smooth. Further, when the surface of BaTiO ₃ was observed in an area of 1 × 1 μm ^{2 with} an atomic force microscope, it was confirmed that the optically polished glass (50 to 1 μm) was used.
(Surface roughness of 50 angstroms). From this, it can be expected that this BaTiO ₃ film leads to good low optical attenuation characteristics as an optical waveguide in terms of its surface smoothness. In addition, Cr / 20
B in 00 Angstroms -BaTiO _3/400 Å -MgO / Si capacitors structure
When the polarization characteristics of aTiO ₃ are measured, P
The -E characteristic shows a hysteresis loop, and BaTiO ₃
Has a polarization axis of single crystal S, as estimated by structural analysis.
It was found that the ferroelectric phase (tetragonal) was oriented perpendicular to the i-substrate.

Example 2 An epitaxial MgO buffer layer was formed on Si in the same manner as in Example 1 above. Si substrate is n-type or p-type
A mold, a (100) plane, and a 6 × 6 mm wafer were used. These substrates were etched, rinsed,
After drying, the substrate was immediately introduced into the deposition chamber, at a constant temperature and a background pressure of 3 × 1.
Heating was performed at 0 ^-7 Torr and at 500 ° C. or higher to remove (sublimate) the H passivation layer on the Si surface. Subsequently, MgO was heated to about 30 ° C. at 600 ° C. and 1 × 10 ⁻⁵ Torr O _2.
0 Å of MgO was deposited, and MgO and S
The relationship of the in-plane crystal orientation of i is MgO (100) // Si (1
00), an epitaxial thin film of MgO [001] // Si [001] was obtained. 650 ° C, 1 × 10 ^-2 Torr
PbTiO ₃ with a thickness of 2000 Å grown in situ on the MgO buffer layer under the condition of O ₂ is a
Axial growth was performed. The relationship between the crystal orientations of PbTiO ₃ and MgO / Si identified by the X-ray diffraction pattern is PbT
iO ₃ (100) // MgO (100) // Si (100)
Met.

PbT observed by scanning electron microscope
The surface of iO ₃ was extremely smooth, which can be expected to lead to good low optical attenuation characteristics as an optical waveguide. Also,
Similarly, _{Pb 1-x La x (Zr} y Ti 1-y) 1-x / 4 O
₃ (PLZT) could also be epitaxially grown on Si by using an epitaxial MgO buffer layer.

Example 3 An MgO buffer layer was formed on a Si (100) single crystal substrate by an electron beam evaporation method. Using MgO as target, distance between target and substrate is 130m
m, and the electron beam current was 5 to 20 mA. The Si single crystal substrate is heated by a halogen lamp and the substrate temperature is 3
00 ° C to 700 ° C. As an Si single crystal substrate, an n-type or p-type 6 × 6 mm wafer having a (100) plane was used. After cleaning these Si single crystal substrates with a solvent, etching was performed with an HF-based solution. Finally, spin drying with ethanol was performed under a nitrogen flow. After spin drying, immediately introduce the Si single crystal substrate into the deposition chamber, and after reaching the background pressure,
The Si single crystal substrate was heated, and when a certain substrate temperature was reached, an MgO film was formed to form a 500 Å-thick MgO film. Diffracted by X-ray diffraction,
MgO deposited by changing the deposition rate by changing the electron beam current has a deposition rate of 0.5 angstroms / sec.
It was found that under the conditions of a film forming temperature of 610 ° C., a film forming rate of 0.2 Å / sec, and a film forming temperature of 440 ° C., the film had a (100) plane unidirectional orientation.

Immediately after the MgO buffer layer was formed on the Si single crystal substrate, PZT was formed on MgO by a sol-gel method. First, the production of PZT is performed using Ti (OiC
₃ H ₇₎ at ₄ and _{Zr (O-i-C 3} H 7) 4 a predetermined molar ratio of 2-methoxyethanol: CH ₃ OCH ₂ CH ₂
OH (abbreviated as ROH), followed by Pb (C
H ₃ COO) ₂ was converted to Pb: (Zr + Ti) = 1.0: 1.
It was blended and dissolved so as to have a molar composition ratio of 0. Thereafter, the mixture was distilled at 125 ° C. for a certain period of time to obtain a metal complex P
b (Zr, Ti) O ₂ (OR) ₂ was formed and the by-product CH ₃ COOCH ₂ CH ₂ OCH ₃ was removed. Next, Pb: H ₂ O: NH ₄ OH =
RO of H ₂ O: NH ₄ OH so as to be 1: 1: 0.1
The H solution was added and refluxed for several hours to partially hydrolyze the metal alkoxide. Thereafter, the solution was concentrated under reduced pressure to finally obtain a stable precursor solution having a Pb concentration of 0.6M. All of the above operations were performed in an N ₂ atmosphere. This precursor solution was mixed with the Si (1) on which the MgO buffer layer was formed.
00) The substrate was spin-coated at 2000 rpm in a N ₂ atmosphere at room temperature. Substrate was spin-coated, after heating at 300 ° C. in an O ₂ atmosphere, 650
Heated to ° C. and crystallized. Thereby, the film thickness of 1000
An Angstrom thin film was obtained. The obtained tetragonal PZT (Zr: Ti = 50: 50) is (100)
It was tetragonal PZT having a (001) orientation on the plane single orientation MgO and a polarization axis [001] oriented perpendicular to the substrate surface.

[0022]

[0023]

[0024]

Further, in Examples 1 to 3 , the excimer
Although the laser deposition method or the electron beam evaporation method was used, the film formation process is not limited to this, and Rf-magnetron sputtering, ion beam sputtering, flash evaporation, ion plating, molecular Beam Epitaxy (MB
E), ionized cluster beam epitaxy, chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MO)
Similarly, vapor phase growth methods such as CVD) and plasma CVD are effective for producing the MgO layer in the present invention.

[0026]

According to the present invention, a ferroelectric thin film can be epitaxially grown on a single-crystal Si (100) substrate which is less expensive than a conventionally used single-crystal oxide substrate. Become. Therefore, the oriented ferroelectric thin film device of the present invention is useful for manufacturing nonvolatile memories, capacitors, FET devices, and the like, and can be used for manufacturing devices such as optical integrated circuits on a Si semiconductor substrate. Can be.

[Brief description of the drawings]

FIG. 1. Epitaxial BaTiO ₃ thin film and Mg
It is explanatory drawing which shows the relationship of the crystal orientation with respect to the single crystal Si substrate of the O thin film.

[Explanation of symbols]

1 ... monocrystalline Si substrate, 2 ... MgO thin film, 3 ... epitaxial BaTiO ₃ thin film

Claims (3)

(57) [Claims] 1. A single crystal Si (100) epitaxial MgO buffer layer on a substrate is formed, which is further the epitaxial or orientation of the perovskite ABO ₃ type ferroelectric thin film on the form, the single-crystal Si ( 10
0) Crystallography of substrate and epitaxial MgO buffer layer
Relationship is MgO (100) // Si (100), in-plane orientation
An oriented ferroelectric thin film element comprising MgO [001] // Si [001] . 2. The method according to claim 1, wherein the epitaxial MgO buffer layer has a film forming temperature of room temperature to 1200 ° C.
2. The oriented ferroelectric thin film element according to claim 1, formed at a film forming rate of 0.0 angstroms / sec. 3. The epitaxial MgO buffer layer and an epitaxial or oriented perovskite ABO _3.
The crystallographic relation of the ferroelectric thin film is ABO ₃ (001) /
/ MgO (100) or ABO ₃ (100) // MgO
(100), in-plane orientation ABO ₃ [010] // MgO [0
2. The oriented ferroelectric thin film element according to claim 1, wherein the element is ABO ₃ [001] // MgO [001].