JP2012017236A - METHOD FOR PRODUCING MgO THIN FILM OF PLANE ORIENTATION (111) - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve problems that it is not successful to obtain a flat (111) plane in an atomic scale by cutting a bulk single crystal MgO and polishing, and a flat MgO (111) plane in an atomic scale is not obtained even by a film-forming method.SOLUTION: In a method for depositing an MgO thin film on a substrate by a laser ablation deposition method using an MgO sintered compact or single crystal as a target, the method for producing an MgO thin film of plane orientation (111) is characterized by using a single crystal substrate to which deposition of a single crystal NiO (111) thin film layer is carried out to the surface flatness in an atomic scale as the substrate and carrying out deposition of the MgO (111) thin film to the surface flatness in an atomic scale by making lamination accumulate by making "Mg-O" layer into one unit on this NiO (111) thin film layer and carrying out epitaxial growth. The MgO (111) thin film carries out epitaxial growth using "Mg-O" layer as one unit.

Description

The present invention relates to a method for producing a magnesium oxide (MgO) thin film, in particular MgO (hereinafter referred to as (111))
The present invention relates to a method for producing a thin film (referred to as “MgO (111)”) on a flat surface on an atomic scale by a laser ablation deposition method.

Examples of the substrate for depositing a thin film having a cubic (111) crystal system include yttria-stabilized zirconia (YSZ), strontium titanate (SrTiO ₃ ), and magnesium oxide (MgO).
, Magnesium aluminate (MgAl ₂ O ₄ ), yttrium oxide, (LaAlO ₃ ) _0.3 − (SrAl _0.5
Ta _0.5 O ₃ ) _0.7 (LSAT), silicon, etc. are used. In order to form an oxide single crystal substrate surface that is flat on the atomic scale and has good crystallinity, a method of polishing the surface of the single crystal substrate is usually used.

MgO, which is a typical oxide insulator, is commercially available as a reliable insulator substrate having a polished surface with a (100) plane orientation. However, when the MgO (111) plane is cut out from a single crystal and mechanically polished, the flatness of the surface is extremely poor and defects such as surface defects are likely to occur.
It is difficult to obtain an electrostatically unstable MgO (111) flat surface by such a method.
A transmission diffraction image reflecting surface irregularities appears in the ED image, and no Kikuchi line appears (Non-Patent Document 1).

As a technique for solving this problem, when a 3C—SiC film is provided on the surface of the MgO substrate whose surface is any one of the (100) plane, the (111) plane, and the (110) plane, the MgO substrate is replaced with hydrogen. 10 in the atmosphere
A method has been proposed in which atomic steps are formed on the surface of an MgO substrate by heating to 00 ° C. or higher and 1600 ° C. or lower and holding for a predetermined time (Patent Document 1).

Attempts to vapor-phase growth of a flat MgO (111) thin film on an atomic scale on a substrate other than MgO include 6H-SiC (0001) (Non-Patent Document 2), GaN (0002) (Non-Patent Document 3), Al ₂ O ₃ (0
001) (Non-Patent Documents 4 and 5), Ag (111) (Non-Patent Document 6) and other substrates have been used, but the step difference on the atomic scale was confirmed by the cross-sectional profile of the atomic force microscope. In the past, there was no strong vibration due to reflection high-energy electron diffraction (RHEED) during growth, and no layer-by-layer growth on the atomic scale was confirmed.

MgO (111) is also used as a buffer layer of a ferroelectric thin film element (Patent Documents 2 and 3).
. The MgO (111) surface is known to have activity for molecular adsorption and catalytic reaction, and its use as a decomposition catalyst for methanol at low temperatures has been reported (Non-patent Document 7).

The present inventors have previously described a p-type oxide semiconductor that can be used as a p-type layer of a hole injection electrode such as a display device such as an organic EL display, a light emitting diode (LED), a laser diode (LD), or an ultraviolet detector. Has developed a method for forming a flat NiO (111) thin film on an atomic scale on a single crystal substrate, and applied for a patent (Patent Document 4, Non-Patent Document 8). Like MgO, it takes a rock salt structure. NiO is also difficult to obtain a (111) flat surface by cutting out a single crystal.
A NiO (111) ultra-flat surface can be obtained on a single crystal substrate by the method shown above. However, in this method, when trying to form a flat MgO (111) surface on the atomic scale, MgO
This is not possible because the single crystal substrate undergoes a solid solution reaction.

JP 2007-149739 JP-A-6-342920 JP-A-10-120494 JP 2004-91253 A (Patent No. 4,014,473)

Lazarov et al., PHYSICAL REVIEW B 71, 115434 (2005) Goodrich et al., APPLIED PHYSICS LETTERS 90, 042910 (2007) Craft et al., APPLIED PHYSICS LETTERS 88, 212906 (2006) Susaki et al., Applied Physics Express 2 091403 (2009) Susaki, ANNUAL REPORT OF THE MURATA SCIENCE FOUNDATION, No.23, A81129 (2009) Kiguchi et al., PHYSICAL REVIEW B 68, 115402 (2003) Hu et al., J. Phys. Chem. C 111, 12038 (2007) H. Ohta, M. Hirano, K. Nakahara, H. Maruta, T. Tanabe, M. Kamiya, T. Kamiya, and H. Hosono, Appl. Phys. Lett. 83, 1029 (2003)

The flat surface on the atomic scale of the substrate or the thin film deposited on the substrate is extremely important in fabricating a device structure having a very thin film thereon. For example, when a very thin channel layer is constructed on this, the roughness of the underlying surface is the same order of magnitude as the thickness of the channel layer.
The effect of scattering on the unevenness of the underlying surface is increased, and the channel layer does not operate as designed.

As for MgO, a substrate having a (100) polished surface is commercially available. On the other hand, when the (111) surface of MgO is flattened on an atomic scale, the surface consisting of all cations (Mg ²⁺ ) or all anions (O ²⁻ ) will be exposed, and electrostatically It is extremely unstable. In fact, it has not been successful to cut out bulk single crystal MgO and obtain a flat (111) plane on an atomic scale by polishing.

The present inventors have reported a method of depositing MgO (111) on an Al ₂ O ₃ (0001) substrate having a flat surface on an atomic scale (Non-Patent Document 4). As shown in FIG. The average roughness is 0.2n
The roughness increased as the film thickness increased beyond about m, and the thickness was about 0.6 nm at a thickness of 80 nm, and a flat MgO (111) surface on an atomic scale was not obtained.

At present, Al ₂ O ₃ is an oxide substrate used for depositing thin films having a crystal system of trigonal or hexagonal.
Regarding (0001), YSZ (111), MgAl ₂ O ₄ (111), and ZnO (0001), there are commercially available products that are flat on the atomic scale. Since the lattice constant of these substrates is significantly different from that of MgO, it cannot be used as a substitute for the MgO (111) flat surface.

The MgO (111) plane can be used as a substrate for deposition of trigonal and hexagonal thin films in the crystal system, and MgO has a lattice constant different from that of commercially available substrate materials other than MgO. Because it is different
If MgO (111) is used as a substrate or a buffer layer, it is possible to introduce a lattice strain that has not been tried so far into the deposited thin film.

Although the electrical and chemical activity due to the electrical instability inherent to the (111) surface of the rock salt structure has attracted theoretical attention, the MgO (111) thin film has a flat surface on the atomic scale. The realization of has been hampered by the large electrical instability of MgO. Therefore, MgO (1
11) There has been no empirical evaluation of surface activity, and there has been no progress in the use of electrically and chemically active catalysts.

The present invention relates to (1) a method of depositing an MgO thin film on a substrate using an MgO sintered body or a single crystal as a target by a laser ablation deposition method.
Using a single crystal substrate on which an O (111) thin film layer is formed on a flat surface on an atomic scale, an “Mg—O” layer is deposited as a unit on the NiO (111) thin film layer and grown epitaxially. A method for producing a MgO thin film with a plane orientation (111), characterized in that an MgO (111) thin film is formed on an atomic scale so as to be flat on the surface.

The present invention also provides (2) the MgO thin film according to (1) above, wherein the MgO (111) thin film is formed at a substrate temperature of 500 to 900 ° C. and an oxygen partial pressure of 10 ^{−4 to 1} Pa. Production method,
It is.

In the present invention, (3) the number of layers of “Mg—O” is 1 to 500,
1) A method for producing an MgO thin film.

In the present invention, (4) the number of layers of “Mg—O” is 1 to 10, and the RHEED intensity oscillation is monitored during the deposition of the “Mg—O” layer, thereby stopping the deposition at the desired number of layers. (1) The method for producing an MgO thin film according to (1) above.

The present invention is also (5) the method for producing an MgO thin film according to the above (1), wherein the root mean square roughness R _RMS of the MgO (111) thin film does not exceed 0.25 nm.

The present inventors produce an MgO thin film using a MgO sintered body or a single crystal as a target by a laser ablation deposition method on a single crystal NiO (111) thin film having a flat surface on an atomic scale formed on a single crystal substrate. Thus, for the first time, it became possible to deposit “Mg—O” in a layered (layer-by-layer) manner on an atomic scale, and it was found that an MgO (111) thin film can be produced on a flat surface on an atomic scale. “Mg—O” means an MgO (111) thin film,
It is the thinnest film that maintains electrical neutrality, and refers to a layer composed of two planes: a plane in which only Mg exists and a plane in which only O adjacent thereto exists.

As the single crystal substrate, a heat-resistant single crystal substrate, for example, stabilized zirconia such as YSZ or the like is preferably used. MgO is directly deposited on YSZ at room temperature, and later, 1,300 ° C.
The method of annealing by heating with is not feasible because MgO and YSZ react. When MgO is deposited at an intermediate temperature, a film with a flat surface is formed to some extent, but a flat surface on the atomic scale cannot be obtained, and no RHEED intensity vibration is observed. When MgO is deposited on a NiO thin film that is not flat on the atomic scale, an MgO surface that is flat on the atomic scale cannot be obtained.
There is no HEED intensity vibration.

Note that MgO and NiO are the same in terms of electrical instability of the (111) surface of the rock salt structure, but in the case of NiO, Ni, which is a transition metal, can take various valences. It can be expected to take an electron configuration that relaxes qualitative properties. In that respect, the MgO (111) surface is more unstable than NiO, combined with strong ionic bonding and a very large band gap. Single crystal NiO (111) thin film has a function as a buffer layer when depositing highly unstable MgO (111) surface, but when NiO (111) thin film is converted into an ultra-flat single crystal film by annealing treatment, This substrate material enables layer-by-layer growth of a more unstable MgO (111) thin film that could not be realized in the past, and the reason why a flat film on the atomic scale is deposited is not clear.

Furthermore, the NiO (111) surface causes in-plane reconstruction of atomic positions to partially alleviate the instability, whereas the MgO (111) surface produced by the method of the present invention is highly unstable. Nevertheless, no in-plane reconstruction occurs. Although the reason for this is not clear, the present MgO (111) surface that does not cause restructuring has great utility as a template for the growth of other films.

The atomic scale flat MgO (111) thin film produced by the method of the present invention is particularly useful as a buffer layer for the growth of other thin films. In particular, since the crystallinity of the present MgO (111) thin film is extremely higher than that of other thin films, it is useful in that high crystallinity can be expected even in a thin film grown thereon.

In addition, the meaning of “surface flat on the atomic scale” used in this specification is as follows. The surface of the oxide single crystal substrate usually has polishing marks due to optical polishing in the manufacturing process, and the crystal itself is damaged. Such a substrate is placed in air or in vacuum.
When surface diffusion is caused by heating to 00 ° C. or higher, a super flat surface can be obtained. A structure reflecting the crystal structure appears on the surface of the ultra flattened oxide single crystal substrate. That is, a structure composed of a terrace having a width of about several hundreds of nanometers and a step having a height of about a sub-nanometer (nm) is generally called “atomic scale surface flatness”. The same applies to the thin film deposited on the substrate.

The terrace portion is composed of atoms arranged in a plane, and is a completely flattened surface if the presence of some defects is ignored. The presence of the step does not result in a completely planarized surface throughout the structure. If this structure is expressed in terms of roughness R _RMS by the mean square roughness measurement method, R _RMS is 1.0 nm or less. R _RMS is a value calculated by, for example, scanning a 1 μm square range with an atomic force microscope.

It is impossible by a method of cutting out MgO (111) surface from a single crystal and mechanically polishing it, and conventional Al ₂ O ₃ (0001), YSZ (111), MgAl ₂ O ₄ (111), ZnO ( We have succeeded in producing a highly crystalline MgO (111) thin film having a flat surface on an atomic scale, which was not obtained using a substrate having a flat surface on an atomic scale such as (0001).

The schematic diagram which shows the process of the manufacturing method of the MgO (111) thin film of this invention. The drawing substitute photograph which shows the atomic force microscope image of the MgO (111) thin film and reference system which were obtained in the Example. The drawing substitute photograph (A) which shows the RHEED image of the NiO (111) thin film in an Example, and a MgO (111) thin film, and the graph (B) which shows the RHEED intensity vibration at the time of MgO (111) deposition. Graph showing the _{Al 2 O 3 (0001) MgO} (111) deposited by the PLD method on the substrate surface roughness of the thin film having a planar surface on an atomic scale.

FIG. 1 is a conceptual diagram showing steps for carrying out the method of the present invention. In the method of the present invention,
Rather than depositing an MgO thin film directly on a substrate whose surface has been polished by laser ablation deposition, a single crystal NiO composed of a flat terrace and sub-nanometer (nm) steps on an atomic scale on the surface of the single crystal substrate A substrate on which a (111) thin film is formed is used. As a method for forming such a single crystal NiO (111) thin film, the method described in the above-mentioned Patent No. 4,014,473 specification and drawings can be used.

As the substrate, a heat-resistant single crystal, such as an oxide single crystal substrate, a Si substrate, a SiC substrate, or a CaF ₂ substrate, is used. Examples of the oxide single crystal substrate include stabilized zirconia such as YSZ, sapphire, MgO, and ZnO. In particular, YSZ is preferable because the reaction with NiO due to annealing hardly occurs and the crystallinity does not deteriorate.

That is, first, a NiO single crystal thin film is deposited on an ultra flattened single crystal substrate, preferably a YSZ (111) substrate (A in FIG. 1). As the deposition method, a pulse laser deposition method, a sputtering method, a CVD method, an MO-CVD method, an MBE method, or the like can be used. The substrate temperature during deposition is 100 ° C. or lower. The lower limit temperature is 0 ° C. The substrate temperature during deposition is more preferably 10 to 50 ° C. The root mean square roughness R _RMS of the substrate surface is preferably 1.0 nm or less. R _RMS can be calculated, for example, by scanning a 1 μm square with an atomic force microscope.

In the NiO thin film deposited on the YSZ substrate, only three-dimensionally deposited particles are observed, and a flat terrace and molecular layer step structure on the atomic scale are not observed, but this is annealed at a high temperature. The annealing temperature is preferably 600 ° C to 1500 ° C.

When annealing, it is preferable to cover the NiO thin film surface with a YSZ single crystal substrate or the like. Alternatively, two substrates on which NiO thin films are deposited may be stacked and annealed with the film portion sandwiched inside (B in FIG. 1). The atmosphere during annealing is preferably air or oxygen gas. Next, when two sheets are stacked, they are peeled back to one sheet (C in FIG. 1). Thereby, a single crystal NiO (111) (hereinafter referred to as “NiO (111) / YSZ (111)”) having a flat surface on an atomic scale is formed.

Next, MgO is deposited on NiO (111) / YSZ (111) by laser ablation (D in FIG. 1). The substrate temperature during deposition is preferably 500 to 900 ° C., and the oxygen partial pressure is preferably 10 ^{−4 to 1} Pa. If the substrate temperature is lower than 500 ° C., the RHEED vibration cannot be seen, and if it exceeds 900 ° C., remarkable solid solution of MgO in the NiO layer is expected, which is not preferable.
When the oxygen partial pressure is less than 10 ⁻⁴ , precipitation of metallic Ni occurs at a high temperature, and when it exceeds 1 Pa, RHEE.
D vibration is not visible again, which is not preferable. As the target, an MgO sintered body or an MgO single crystal is used. The distance between the target and the substrate is arbitrary, but is appropriately selected according to the scale of the apparatus. By the above method, the film is deposited so as to have a desired film thickness according to the purpose.

By this method, up to 10 layers with one unit of “Mg—O” are grown in layers (layer-by-layer), and the surface is flat on an atomic scale that does not exist in single crystal polishing (111) A surface is obtained. Further, even when the number of layers of “Mg—O” is increased to 10 to 500, the flatness not exceeding the mean square roughness R _RMS of 0.5 nm, preferably not exceeding 0.25 nm, and the crystallinity comparable to the single crystal can be obtained. It is done. When an MgO (111) thin film is used as a buffer layer, the film thickness is 1 nm to 2000 nm, preferably about 1 nm to 200 nm, but the MgO (111) thin film obtained by the method of the present invention has a large film thickness. However, it does not become rough gradually.
Accordingly, it is possible to realize flatness not exceeding 0.5 nm, preferably 0.25 nm when the film thickness is 1 nm to 200 nm.

Further, since the RHEED intensity vibration can be observed up to about 10 cycles, “M” is used up to about 10 cycles.
It was found that the number of “gO” layer units (atomic scale film thickness) can be controlled by monitoring the RHEED intensity vibration. Since the size of the portion irradiated with the electron beam of RHEED is on the order of millimeters, the film surface is completely flat (zero-order spot intensity maximum) in that region, that is, on a very macro scale, and the film surface is halfway. While the atoms are occupied (the intensity decreases), the RHEED intensity oscillation shows that the atoms are spread again and the complete flatness is restored (zero order spot intensity is maximized again). So RHE
By counting the ED intensity vibration and finishing the deposition at the number of vibrations corresponding to the desired number of layers, the thickness of the film can be accurately determined on the spot, so when making a barrier layer such as a magnetic tunnel junction with MgO This method can be applied to.

An MgO (111) thin film was prepared by the following process.
1. NiO thin films were deposited on two YSZ (111) substrates at room temperature by pulsed laser ablation. A YSZ (111) single crystal substrate (manufactured by Shinko Co., Ltd., 10 mm square) was heated to 1300 ° C. in the atmosphere to produce an atomic flat surface. Ultra-high vacuum container for laser ablation (Pascal)
The YSZ single crystal substrate was installed in the company, and the temperature was kept at room temperature. 1 × 10 ^{−3 in the} container
Introduce oxygen gas of Pa, irradiate the NiO sintered body target with KrF excimer laser light (Laser luminescence device manufactured by Lambda Physics Co., Ltd.), and apply the NiO thin film on the substrate facing away from the target by 50 mm. Deposited. The film thickness was 20 nm. Next, the substrate on which the NiO thin film was deposited was taken out of the vacuum container.

2. Two sheets of NiO thin film deposited on top of each other with the NiO thin film part sandwiched inside 1
After annealing at 300 ° C. in the air for 1 hour, it was cooled to room temperature. When the two substrates are peeled off, a flat single crystal NiO (111) surface on the atomic scale can be formed on the substrate. Atomic force microscope image shown in FIG. 2 (b) and rough cross section between C and D in the mirror image It was confirmed by the height profile. R _RMS was 0.13 nm.

3. Mg on NiO (111) / YSZ (111) by the same pulse laser ablation method as above 1.
O was deposited. However, the substrate temperature was 600 ° C. and the target was MgO single crystal. 500
A film having a thickness less than or equal to layers was repeatedly prepared as a sample.

When MgO is deposited on NiO (111) / YSZ (111), the atomic force microscope image shown in FIG. 2 (c) shows a step-terrace structure on the atomic scale, and between E and F in the mirror image. Cross section roughness (
Looking at the (Height) profile, it can be seen that one step consists of 7 units of “Mg—O” units. R _RMS was 0.17 nm. As shown in FIG.
When the number of deposited layers of the “gO” unit was increased and viewed with an atomic force microscope, “Mg—O”
10 units (10 layers), 100 units (100 layers), 500 units (500 laye
R _RMS does not exceed 0.25 nm even when the film becomes thicker than rs), and is suppressed to an extremely small value.

As shown in FIG. 3A, the RHEED image of this MgO (111) thin film has a sharp Kiku.
Chi lines appear, reflecting significant surface flatness and high crystallinity. (B) in FIG. 3 shows Mg
It is a graph which shows the RHEED intensity vibration at the time of O (111) deposition, a horizontal axis | shaft shows deposition time (second), and a vertical axis | shaft shows the intensity | strength (au) of zero order spot. “1ML: 32s” in the inset shows that the period corresponding to one unit layer of “Mg—O” is 32 seconds. As shown in FIG. 3A, NiO
The RHEED image of the (111) thin film shows sharp Kikuchi lines, reflecting a remarkable surface flatness and high crystallinity. Further, as shown in FIG. 3 (B), an intensity vibration is observed in the intensity of the zero-order spot of RHEED during the deposition of the MgO (111) thin film, and the layer growth is performed for each “Mg—O” unit. I understood that.

The MgO (111) surface can be used as a substrate for deposition of trigonal and hexagonal thin films with a crystal system, and the MgO (111) surface obtained by the method of the present invention is particularly suitable for other thin films. As a buffer layer for deposition, it is used in the field of electronic devices such as TFT (thin film transistor), LD (laser diode), LED (light emitting diode), MTJ (magnetic tunnel junction). The MgO (111) surface is known to have activity for molecular adsorption and catalytic reaction.
It can also be expected as a catalyst material for methanol decomposition at low temperatures.

Claims (5)

In a method of depositing an MgO thin film on a substrate using an MgO sintered body or a single crystal as a target by a laser ablation deposition method, a single crystal NiO (111) thin film layer is formed as a substrate on a flat surface on an atomic scale. Using a crystal substrate, “Mg” is formed on the NiO (111) thin film layer.
-O "layer is deposited as a unit in a stack and epitaxially grown to form a MgO (111) thin film on an atomic scale with a flat surface. (111) M
A method for producing a gO thin film. The method for producing an MgO thin film according to claim 1, wherein the MgO (111) thin film is formed at a substrate temperature of 500 to 900 ° C and an oxygen partial pressure of 10 ^{-4 to 1} Pa. The method for producing an MgO thin film according to claim 1, wherein the number of layers of “Mg—O” is 1 to 500. The number of layers of “Mg—O” is 1 to 10, and the deposition is stopped at a desired number of layers by monitoring the RHEED intensity oscillation during the deposition of the “Mg—O” layer.
A method for producing the described MgO thin film. 2. The method for producing an MgO thin film according to claim 1, wherein the root mean square roughness _RMS of the MgO (111) thin film does not exceed 0.25 nm.