The present invention relates to a ZnO-based substrate suitable for crystal growth of a ZnO-based thin film and the like, and to a method of treating the ZnO-based substrate.
Electronics engineering is a field where semiconductor materials, particularly silicon materials, play an active role. In recent years, however, it is becoming more difficult to achieve device configurations to satisfy required functions due to property limitations of silicon materials themselves. Such limitations include operational incapability at high temperature equal to or above 150° C. and lack of breakdown voltage capability in a high electric field, for example.
In the meantime, oxides and organic materials represented by high-temperature superconductor YBCO, an ultraviolet-emitting material ZnO, an organic EL and the like are drawing attention as next generation materials owing to a variety of species and functional versatility. These substances have a possibility of realizing functions that have not been achieved due to restriction imposed by silicon as the material. For example, ZnO is drawing attention because of its versatility, high light emitting potential, and so forth.
To manufacture a device using ZnO, a sapphire substrate has heretofore been used as a crystal growth substrate for growing a crystalline thin film of ZnO. However, a sapphire substrate has a large degree of lattice mismatch with ZnO as the thin film material, which accounts for as high as about 18%. For this reason, a conventional thin film is likely to have numerous lattice defects, an increase in mosaicness, and the like, and therefore it has been difficult to form a high-quality single-crystal thin film. High light emitting potential and other features of ZnO are exerted only in the form of a high-quality single-crystal thin film. Therefore, a device, which is formed of a ZnO thin film deposited on a sapphire substrate, is incapable of fully exerting intrinsic performances of ZnO. Hence, the sapphire substrate is not always the most suitable substrate for growth.
With this taken into consideration, a proposal has been made on the use of ScAlMgO4 (SCAM) crystals or the like as a crystal growth substrate because of their smaller lattice mismatch with ZnO. However, a ScAlMgO4 substrate is a special substrate which is not suitable for industrial applications without modifications. The most desirable solution in the industrial perspective is to use ZnO substrates.
Non-patent Document 1: Ulrike Diebold, et al., Applied Surface Science 237(2004) p. 336-342.
Non-patent Document 2: Chevtchenko, S. A. et al., Applied Physics Letters 89(2006) p. 182111-182113.
However, concerning the ZnO substrate, a normal polishing operation, such as forming a clean surface by wet etching, alone does not make it possible to obtain a flat and clean surface suitable for epitaxial growth (see Non-patent Documents 1 and 2, for example). CMP (chemical mechanical polishing) which is well known as a planarization process is used in order to obtain a surface suitable for epitaxial growth.
In a method using the CMP, chemical mechanical polishing is executed while supplying an alkaline aqueous polishing slurry, which includes dispersed colloidal silica, for example, to a space between a polishing pad such as a rotary one-side polishing apparatus and a process object such as a ZnO substrate. The colloidal silica used as an abrasive (small SiO2 granules having diameters less than about 5 nm) agglutinates unless the solvent is alkaline. Therefore, the alkaline aqueous polishing slurry is used as described above. However, when polishing with the colloidal silica, the silica which is an ingredient of the abrasive adheres to the ZnO surface. Moreover, since the ZnO substrate is exposed to the alkaline aqueous solution, zinc hydroxide Zn(OH)2 is formed on the surface of the ZnO substrate.
Of those, adhesion of silica appears in the form of Si diffusion afterwards at the time of crystal growth of a ZnO-based thin film.
As shown in
Moreover, the formation of the hydroxide on the surface of the ZnO substrate generates defects on a crystalline film formed on the ZnO substrate, and causes adverse effects in the form of an increase in a pit density.
In addition, Zn(OH)2, which is the hydroxide of Zn, forms a hydrate which is a gel amorphous substance. Therefore, when particles of the above-described silica or other impurity particles are taken into the gel Zn(OH)x, the particles are apt to remain on the ZnO substrate, and may also cause problems of: diffusion of the silica from the surface of the ZnO substrate to the crystal growth film as described above; and difficulty in cleaning the substrate surface.
The present invention has been made to solve the above-described problems. An object thereof is to provide a ZnO-based substrate having a surface suitable for crystal growth, and to provide a method of processing the ZnO-based substrate.
For the purpose of fulfilling the object, the invention as recited in claim 1 is a zinc-oxide-based substrate in which almost no hydroxide groups exist on a crystal growth-side surface of a MgxZn1-xO substrate (0≦x<1).
The invention as recited in claim 2 is the zinc-oxide-based substrate as recited, in claim in which: the crystal growth-side surface is a main surface of the substrate having a +C plane; and a line normal to the main surface of the substrate is inclined from a c-axis toward an m-axis, the c-axis and m-axis being crystal axes of the substrate.
The invention as recited in claim 3 is the zinc-oxide-based substrate as recited in any one of claims 1 and 2, in which a defect density is equal to or below 1×106 cm−2 when a surface of a MgyZn1-yO substrate (0≦y<1) crystal-grown on the MgxZn1-xO substrate (0≦x<1) is observed by use of optical means.
The invention as recited in claim 4 is a method of processing a zinc-oxide-based substrate, in which a final treatment to be applied on the crystal growth-side surface of the MgxZn1-xO substrate (0≦x<1) is acidic wet etching at pH 3 or lower.
A ZnO-based substrate of the present invention is formed in a way that almost no OH groups exist on a crystal growth-side surface. Accordingly, it is possible to prevent production of a hydroxide of Zn. Moreover, when a ZnO thin film or a MgZnO thin film is formed on the ZnO-based substrate, it is possible to reduce a crystal defect density of any of these thin films to an extremely low level. Further, as the hydroxide of Zn is not produced, it is also possible to reduce adhesion of particles of silica and the like, and thereby to form a surface suitable for crystal growth.
Meanwhile, by applying an acidic wet etching process pH 3 or lower as a final process on a surface of a ZnO-based substrate, it is possible to eliminate almost all OH groups from the crystal growth-side surface. Hence, an effect similar to the above-described effect is achieved. Moreover, since it is possible to set the zeta potentials of ZnO and particles of silica or the like to the same polarity by the above-described etching, it is possible to further suppress generation of adhesion.
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1 MgxZn1-xO SUBSTRATE
In the present invention, a MgxZn1-xO substrate (0≦x<1) is used, and a configuration is worked out for forming a crystal growth-side surface of this substrate into a surface suitable for crystal growth which does not allow adhesion of particles of silica or the like or generation of zinc hydroxide Zn(OH)2. The following studies are carried out by using a ZnO substrate which represents one of the above-mentioned MgxZn1-xO substrates in a case of x=0.
First, a guideline for preventing particle adhesion is zeta potential of a solid in a solution.
Incidentally, it is known that the zeta potential depends on pH which indicates acidity or alkalinity. Well-known examples will be shown about Si below to explain a relationship between zeta potential and pH, as well as a relationship between zeta potential and particle adhesion.
Next,
Meanwhile, P2 (a curve indicated with ∘) shows a relationship in concentration (cm−3) between PSL in an ultrapure water solution at pH 6 and adsorbed PSL adhering to the surface of the Si substrate. In the case of P1, since the pH is set to 3.3 as shown in
On the other hand, the pH is set to 6 in the case of P2. Accordingly, as shown in
Next,
When viewing only this drawing, it seems that, because the zeta potentials of colloidal silica, PSL and ZnO have the same polarity when the pH is on the alkaline side from pH 5.5 (at pH 5.5 or higher) and no particle adhesion occurs. Nevertheless, the case of ZnO is not simple, and adhesion occurs as shown in
a) is an image obtained by: subjecting the +C plane of the ZnO substrate to a CMP treatment; then etching this +C plane to etching for 10 minutes using a HF solution at pH of 4.5; and subsequently taking a picture of the C+ plane in a field of 2 μm×2 μm with an AFM (atomic force microscope). In an area surrounded by a dot line in the drawing, particles seem to be buried.
A reason for this will be explained based on
As apparent from
Data in
In both of
Data, which is obtained when using a ZnO substrate polished by conventional CMP, are indicated with inverted white triangles (∇). As shown in the drawing, it is necessary to raise the growth temperature in order to reduce the pit density. Moreover, in a range of the substrate temperature from 750° C. to 850° C., the pit densities are concentrated above the order of 1×106 cm−2.
With this taken into consideration, in order to avoid stable existence of Zn(OH)2 on the surface of the ZnO substrate and to prevent adhesion of colloidal silica to the surface, it is necessary to at least set the surface of the ZnO substrate at pH less than 5.5 (on the acid side from pH 5.5). When pH is below 5.5, it may be said that adhesion does not occur easily because there still exists an area where the zeta potential polarities of the ZnO substrate and colloidal silica are the same. However, there is also an area where the zeta potential polarities of the ZnO substrate and colloidal silica are opposite to each other. Moreover, considering avoidance of adhesion of not only colloidal silica but also other particles such as PSL particles, a numerical value, pH less than 3, is derived from
In the meantime, the surface of the ZnO substrate is subjected to a wet etching process by using a HCl (hydrochloric acid) solution at pH 3 or lower (at pH 3 or on the acidic side from pH 3), and then the wet-etched surface of the ZnO substrate is measured by XPS. Curves S2 in
Moreover, the MgZnO thin film is formed on the ZnO substrate which is treated by acidic wet etching at p H3or lower as described above, and an observation is made on the pit density of the MgZnO thin film. A result is shown in
In addition, a calculation is made on their pit densities while changing the growth temperature of the MgZnO thin film, and the calculated pit densities are plotted. These are the data already indicated with the black circles () in
Meanwhile, a line P indicated with a dotted line shows a lower limit value of the pit density that varies depending on the condition on the surface of the ZnO substrate. As apparent from comparison using this line, it is possible for the treated substrate to make the growth temperature of the thin film to be formed on the substrate than the untreated substrate, as long as the pit density of the treated substrate is equal to that of the untreated substrate. Accordingly, it is desirable to achieve the etching process at pH 3 or lower in order to prevent adhesion of particles while preventing production of a hydroxide and setting the zeta potentials of the substrate and particles to the same polarity.
Next, descriptions will be provided for examples of the substrate processes on a crystal growth-side surface of the MgxZn1-xO substrate (0≦x<1). In any of the following examples, a main surface of the crystal growth-side surface of the MgxZn1-xO substrate is formed as shown in
Here, descriptions will be provided for the reason for inclining the line normal to the main surface of the substrate at least from the c-axis toward the m-axis.
However, in the case of a bulk crystal, a direction of the line normal to the main surface of a wafer does not coincide with the direction of the c-axis unlike in the case shown in
Here, the terrace surfaces 1a correspond to the C plane (0001) and the stepped surfaces 1b correspond to an M plane (10-10). As shown in the drawing, the respective stepped surfaces 1b thus formed are arranged regularly in the direction of the m-axis while maintaining widths of the terrace surfaces. That is, the c-axis perpendicular to the terrace surfaces 1a and the normal line Z to the principal surface of the substrate forms the off angle at θ degrees.
As described above, by forming the stepped surface as a surface corresponding to the M plane, it is possible to form the flat film in terms of a ZnO-based semiconductor layer which is crystal-grown on the main surface. Although the step portions are formed on the main surface by the stepped surfaces 1b, atoms coming to the step portions are bound to two surfaces of the terrace surface 1a and the stepped surface 1b so that the coming atoms can be stably trapped.
Although the coming atoms are dispersed inside terraces during a surface diffusion process, stable growth is achieved by lateral growth, in which crystal growth progresses as the atoms are trapped by the step portion having a strong binding force or by a kinked position formed by this step position and are incorporated into the crystal. Incidentally, the stepped surfaces 1b must be chemically stable in order to maintain the condition of the stepped surfaces 1b having the even intervals and regularity as shown in
In particular, it is necessary to use the acidic wet etching equal to or below pH 3 during the substrate processing. Accordingly, in order to ensure stability of the crystal growth surface, it is desirable to configure the main surface on the crystal growth side of the MgxZn1-xO substrate so as to have the +C plane and to configure the c-axis of the main surface so as to have the off angle in the direction of the m-axis. Moreover, the ZnO substrate is used as the MgxZn1-xO substrate that applies x=0. It is possible to use HCl (hydrochloric acid), HF (hydrofluoric acid) , HNO3 (nitric acid) and the like as an acidic solution for the above-described wet etching. However, HCl is particularly preferable.
First, the main surface of the ZnO substrate is subjected to polishing by using colloidal silica to obtain a surface having a stepped structure including one to two molecular layers. If there is a significant process damage at this stage, removal etching for a damaged layer is subsequently executed by using an acid having the pH equal to or below 1. Thereafter, Zn(OH)2 is removed by etching using an acid having the pH set in a range of 2±0.5.
First, the main surface of the ZnO substrate is subjected to polishing by using colloidal silica to obtain a surface having a stepped structure including one to two molecular layers. If there is a significant process damage at this stage, etching is executed by using an acid having the pH equal to or below 1 and then etching is executed by using an acid containing fluorine and having the pH set in a range of 3.5±1 to dissolve silica on the ZnO substrate. Finally, etching is executed by using an acid having the pH set in a range of 2±0.5.
First, the main surface of the ZnO substrate is subjected to polishing by using colloidal silica to obtain a surface having a stepped structure including one to two molecular layers. If there is not a significant process damage at this stage, etching is executed by using an acid having the pH equal to or below 3. A time period for this etching varies depending on the pH but it is necessary to spend at least 15 second or longer.
In any of the above-described examples, reduction of the OH groups is confirmed as shown in
Number | Date | Country | Kind |
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2007-171132 | Jun 2007 | JP | national |
2007-337435 | Dec 2007 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2008/061711 | 6/27/2008 | WO | 00 | 12/24/2009 |