JP2009062226A - ZnO-BASED SUBSTRATE AND PROCESSING METHOD OF ZnO-BASED SUBSTRATE - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ZnO-based substrate having a good quality surface suitable for crystal growth and to provide a processing method of a ZnO-based substrate. <P>SOLUTION: In the ZnO-based substrate, a principal plane is formed so that the excitation peak energy of 2p3/2 inner-shell electrons of Zn atom is in the range of 1,021.75 eV-1,022.25 eV when the principal plane of the side of a Mg<SB>X</SB>Zn<SB>1-X</SB>O (0≤X<1) substrate on which the crystal growth is conducted is spectrally diffracted by X-ray photoelectron. As the final processing of the surface of the side of the Mg<SB>X</SB>Zn<SB>1-X</SB>O substrate on which the crystal growth is conducted, an oxidation processing is conducted. In this way, in a ZnO-based substrate a damage-free principal plane can be prepared and a surface suitable for crystal growth is formed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a ZnO-based substrate suitable for crystal growth of a ZnO-based thin film or the like and a method for processing a ZnO-based substrate.

  ZnO-based semiconductors are expected to be applied to ultraviolet LEDs, high-speed electronic devices, surface acoustic wave devices and the like that are used as light sources for illumination, backlights, and the like. Although ZnO-based semiconductors have attracted attention for their multifunctionality, light emission potential, and the like, they have hardly grown as semiconductor device materials. The biggest difficulty is that acceptor doping is difficult and P-type ZnO cannot be obtained.

However, as seen in Non-Patent Document 1 and Non-Patent Document 2, in recent years, P-type ZnO can be obtained due to technological advances, and light emission has been confirmed. However, these achievements are valuable after showing the usefulness of ZnO, but it is a special substrate called ScAlMgO ₄ and uses an insulating substrate, and the large area called pulse laser deposition. Using unsuitable methods is disadvantageous for industrial development.

  The best way to solve these problems is to use a ZnO substrate. For ZnO-based devices, ZnO substrates are already commercially available, which is an advantage over GaN. Looking at this point alone, such as the half-width of the X-ray diffraction peak, which is already possible for a ZnO substrate of 3 inches, looks very promising.

  However, in the case of manufacturing a device that exhibits a new function by stacking not only dopants but also films having different compositions as in many compound semiconductors, the problem is the surface of the substrate. In compound semiconductors, thin film growth is often performed by vapor phase growth because of its good controllability, but material atoms and molecules supplied from the vapor phase crystallize by picking up information only on the surface of the landing substrate. Therefore, even if the quality is high as a bulk, it does not make any sense unless the surface is sufficiently high quality.

As for the quality of this surface, the flatness is usually considered in most cases. If the flatness of the substrate surface is poor, the flatness of the thin film to be laminated also deteriorates, resulting in resistance when carriers move through the thin film, and the surface roughness increases as the upper layer of the laminated structure increases. In particular, the etching depth is not uniform and anisotropic crystal plane growth occurs due to surface roughness, which makes it difficult to perform a desired function as a semiconductor device. .
A. Tsukazaki et al., JJAP44 (2005) L643 A. Tsukazaki et al NtureMaterial4 (2005) 42

  On the other hand, in addition to flatness, a substrate cleaning process for obtaining a clean surface is performed as a process for improving the quality of the substrate surface. This substrate cleaning is performed to remove deposits such as particles attached to the substrate surface and an oxide film, and wet etching, sputtering, thermal cleaning, or the like is used. However, if the cleaning process is excessively performed to completely remove deposits, etc., the substrate surface may be damaged, affecting the chemical bonding of atoms constituting the substrate, and conversely degrading the quality of the substrate surface. Therefore, a problem similar to the problem caused by the poor flatness of the substrate surface occurs, and it is difficult to exert a desired function as a semiconductor device.

  Therefore, in order to improve the quality of the substrate surface, it is necessary to improve the flatness of the substrate surface, but the substrate cleaning process is appropriately performed so as not to affect the chemical bonding of the ZnO-based substrate surface. It is also necessary.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a ZnO-based substrate having a high-quality surface suitable for crystal growth and a processing method for the ZnO-based substrate.

In order to achieve the above object, the invention according to claim 1 is characterized in that when the main surface on the side of crystal growth of the Mg _X Zn _1-X O substrate (0 ≦ X <1) is analyzed by X-ray photoelectrons, Zn The ZnO-based substrate is characterized in that an excitation peak energy of 2p3 / 2 inner-shell electrons of atoms exists in a range of 1021.75 eV to 1022.25 eV.

  The invention according to claim 2 is the ZnO-based substrate according to claim 1, characterized in that the main surface on the crystal growth side is a + C plane.

  According to a third aspect of the present invention, the principal surface on the crystal growth side has a C plane, and the projection axis obtained by projecting the normal line of the principal surface onto the m-axis c-axis plane of the substrate crystal axis is m 2. The ZnO-based substrate according to claim 1, wherein the substrate is inclined within a range of 3 degrees or less in the axial direction.

According to a fourth aspect of the present invention, the projection axis obtained by projecting the normal line of the main surface onto the a-axis c-axis plane of the substrate crystal axis is Φ _a degrees in the a-axis direction, and the normal line of the main surface is the main surface. The projection axis projected on the m-axis c-axis plane in FIG. 5 is tilted by Φ _m degrees in the m-axis direction, and the Φ _a is 70 ≦ {90− (180 / π) arctan (tan (πΦ _a / 180) / tan (πΦ _m / 180)) ≦ 110
The ZnO-based substrate according to claim 1, wherein:

According to a fifth aspect of the present invention, the final treatment on the main surface on the side of crystal growth of the Mg _X Zn _1-X O substrate (0 ≦ X <1) is an oxidation treatment. A substrate processing method.

  The invention according to claim 6 is the method for treating a ZnO-based substrate according to claim 5, wherein an acidic wet etching treatment having a pH of 3 or less is performed before the oxidation treatment.

  In the ZnO-based substrate of the present invention, the excitation peak energy of 2p3 / 2 core electrons in Zn atoms when the main surface on the crystal growth side is excited by X-ray photoelectron spectroscopy is in the range of 1021.75 eV to 1022.25 eV. Therefore, the state of the main surface is not chemically changed, and the surface is optimal for crystal growth. Further, since the final treatment on the main surface on the crystal growth side of the ZnO-based substrate is an oxidation treatment, the same effect as described above can be obtained.

In the present invention, an Mg _X Zn _1-X O substrate (0 ≦ X <1) is used, and the crystal growth side surface of this substrate is a surface suitable for crystal growth. We have found that a high-quality ZnO-based substrate can be obtained. Of the Mg _X Zn _1-X O substrates (0 ≦ X <1), the following consideration was made using a ZnO substrate with X = 0.

  First, as a method of cleaning the surface of the ZnO substrate, a method of sputtering the surface with Ar ions is often used, so how the state of the ZnO substrate surface changes by sputtering with Ar ions is shown. FIG. 2 shows the state of each crystal plane of the ZnO substrate examined by XPS (X-ray Photoelectron Spectroscopy), and the excitation peak energy of 2p3 / 2 core electrons of Zn atoms was measured. .

  The + C plane, −C plane, and M plane of the ZnO substrate were cut out, and each plane was measured by XPS immediately after the mirror polishing process was completed. The horizontal axis represents the binding energy or binding energy (Binding Energy unit: eV), the vertical axis represents the measured binding energy intensity, and the vertical axis represents the normalized value based on the highest point of the binding energy intensity. As shown in FIG. 2, immediately after the polishing process is completed, the binding energy peaks all coincide.

  On the other hand, FIG. 3 to FIG. 5 show the curves obtained by measuring the surfaces immediately after completion of the polishing process with XPS on the crystal planes of the + C plane, M plane, and −C plane of the ZnO substrate with solid lines, and Ar ions after the completion of the polishing process. A curve obtained by measuring the surface after sputtering by 30 nm with XPS is represented by a dotted line. Each shows the excitation peak energy of 2p3 / 2 inner-shell electrons of Zn atoms, FIG. 3 shows data on the + C plane, FIG. 4 shows data on the M plane, and FIG. 5 shows data on the -C plane. In addition, As-received described in FIGS. 3 to 5 means that each surface is mirror-polished and then left as it is without any other processing, and immediately after the polishing process described in FIG. It has the same meaning.

  Regarding the M plane in FIG. 4 and the −C plane in FIG. 5, the surface after sputtering is shifted to a lower energy side than the peak before sputtering. Since sputtering using Ar ions is a technique that gives a physical impact to the surface, this energy shift indicates that the chemical bond between Zn and O is not in an ordinary bonding state, and the chemical bond between Zn and O is broken. it is conceivable that.

  As shown in FIGS. 3 to 5, the peak energy shift amount is −C plane> M plane> + C plane in descending order, and the + C plane has almost no change. If the shift amount is considered to be the breakage of the chemical bond between Zn and O, it can be seen that the + C plane is most resistant to damage and is advantageous for device fabrication.

  However, as shown in FIG. 1, even the + C plane, which is resistant to physical impact, differs from chemical etching. Each curve of S2, A represented by a solid line in FIG. 1, T represented by a broken line, and R represented by a two-dot chain line is data of + C plane, and a curve of SP represented by a one-dot chain line is , -C plane data. In all cases, the excitation peak energy of 2p3 / 2 core electrons of Zn atoms was measured by XPS. SP, as with the dotted curve in FIG. 5, is obtained by performing XPS measurement after performing sputtering with Ar ions after polishing of the −C plane, and the peak P1 of the binding energy is 1021.7 eV.

  On the other hand, the data after hydrochloric acid etching after polishing the + C surface is S2, and the peak of S2 is shifted to near the peak of the SP curve where the chemical bond between Zn and O is most severed. Curve A is the result of oxidation treatment such as exposing the ZnO substrate to oxygen plasma (radical oxygen) (ashing) from the state of S2, and the entire distribution curve including the peak shifts to the high energy side (green) shift).

  After the oxidation treatment as described above, the peak shifted from the S2 state and approached the initial state R before the hydrochloric acid etching, so that the chemical bond between Zn and O was broken in the S2 state. This is thought to suggest that.

Next, XPS measurement data after performing annealing in the environment of 1 × 10 ⁻⁷ Pa at 900 ° C. for 30 minutes from the state of A is curve T, but there is almost no peak shift. The annealing gives a strong reducing action for the oxide, that is, an action of removing oxygen, but there is almost no peak shift between A and T. Therefore, before crystal growth on the ZnO substrate, the annealing of the substrate is performed. Performing the oxidation treatment as the final treatment repairs or stabilizes the chemical bond between Zn and O on the crystal growth side surface of the substrate, and a high-quality substrate surface can be obtained. Further, from FIG. 1 and the above description, it is understood that the excitation peak energy E of 2p3 / 2 inner-shell electrons of Zn atoms is preferably about 1022 ± 0.25 eV (1021.75 eV ≦ E ≦ 1022.25 eV).

  By the way, it is considered that the acidity of the acidic wet etching by the hydrochloric acid etching or the like increases, which contributes to cleaning for obtaining a clean surface of the ZnO substrate surface. In particular, deposits such as silica and particles are removed from the substrate surface. In order to achieve this, we have found that the etching solution must have a predetermined acidity, which is detailed in Japanese Patent Application No. 2007-171132, which will be described here again.

  For a ZnO substrate, a flat and clean surface suitable for epitaxial growth cannot be obtained only by ordinary polishing such as a clean surface is obtained by wet etching. In order to obtain a surface suitable for epitaxial growth, CMP (Chemical Mechanical Polishing) well known in the planarization process is used.

In the CMP method, for example, chemical mechanical polishing is performed while supplying an alkaline aqueous polishing slurry in which colloidal silica is dispersed between a polishing pad such as a rotary single-side polishing apparatus and a workpiece such as a ZnO substrate. Colloidal silica (a small particle of SiO _{2 having} a diameter of about 5 nm) used as an abrasive aggregates unless it is in an alkaline solution. Therefore, an alkaline aqueous polishing slurry is used as described above. Then, silica that is a component of the abrasive adheres to the surface of ZnO, and Zn (OH) _X that is a hydroxide of Zn is formed on the surface of the ZnO substrate by being exposed to the alkaline aqueous solution in the slurry. .

  Among the above, silica adhesion appears as Si diffusion during the subsequent crystal growth of the ZnO-based thin film. On the other hand, the formation of hydroxide on the surface of the ZnO substrate has an adverse effect in the form of defects in the crystal film formed on the ZnO substrate and an increase in pit density. Therefore, a predetermined process is required to remove silica, hydroxide, particles, and the like.

  It is known that the zeta potential of a solid in a solution depends on pH (pH) representing alkalinity or acidity. For example, Tadahiro Omi's “Ultrascreen ULSI Technology” Baifukan shows an example of the pH dependence of the zeta potential, and the pH dependence of the zeta potential varies with the type of substance. In addition, from the description of the above-mentioned reference, it can be seen that, as for particle adhesion, if the zeta potential of the adhering particle and the solid surface is the same polarity, repulsive force acts between each other, and particle adhesion hardly occurs.

  Next, FIG. 9 shows the pH dependence of the zeta potential in the + C plane of the ZnO substrate and the zeta potential in colloidal silica. Z1 (curve represented by a black square) represents the pH dependence of the zeta potential on the + C plane of the ZnO substrate, Z2 (curve represented by ◯) represents the pH dependence of the zeta potential in colloidal silica, and Z3 (curve represented by ●). ) Shows the pH dependence of the zeta potential in the aforementioned PSL. In FIG. 9, the vertical axis represents zeta potential (mV), and the horizontal axis represents pH. Here, PSL (polystyrene latex) used as a standard substance for measurement of particle size, turbidity, and the like was substituted as particles.

  If the zeta potential of the adhering particle and the solid surface is the same polarity, repulsive force works between each other, and particle adhesion is unlikely to occur. On the side (5.5 or more), colloidal silica, PSL and ZnO have the same zeta potential, and as described above, particle adhesion should not occur, but in the case of ZnO, adhesion is not simple.

The reason for this will be described with reference to FIG. The left and right vertical axes in FIG. 8 indicate the redox potential, and the horizontal axis indicates the pH. The area between the parallel dotted lines a and b shows an equilibrium diagram in water. In addition, the hatched region in the equilibrium diagram in water represents the stable region of Zn (OH) ₂ . Moreover, the line represented by the number in a figure shows various equilibrium states. For example, the number 3 is Zn ²⁺ + 2H ₂ O = HZnO ² + + 3H ⁺ , and the number 4 is HZnO ²⁻ = ZnO ₂ ² + H ⁺ , indicating ion equilibrium. Also, numeral 5 is Zn (OH) ₂ + 2H + 2e ⁻ = Zn + 2H ₂ O and metal / hydroxide equilibrium, 6 is Zn ²⁺ + 2H ₂ O = Zn (OH) ₂ + 2H ⁺ , and 7 is Zn (OH) _2. = HZnO ² + + H ⁺ , 8 represents the equilibrium of Zn (OH) ₂ = ZnO ₂ ² + 2H ⁺ and aqueous solution / hydroxide. 10 is HZnO ² + 3H ⁺ + 2e ⁻ = Zn + 2H ₂ O, 11 is ZnO ₂ ² + 4H ⁺ + 2e ⁻ = Zn + 2H ₂ O, which represents an aqueous solution / metal equilibrium.

As can be seen from FIG. 8, there is a stable region of Zn (OH) ₂ on the alkali side with a pH of 5.5 or higher. Zn hydroxide Zn (OH) ₂ is a gel-like substance, so to speak, the surface of the ZnO substrate is in a state of being sticky, and plays a role like an adhesive. Therefore, even when the pH is 5.5 or higher and the zeta potential of the ZnO substrate surface and the zeta potential of colloidal silica are the same polarity, there is a stable region of Zn (OH) ₂ at a pH of 5.5 or higher. OH) ₂ acts as an adhesive and repels easily and cannot be separated.

  When the surface in this state was measured using XPS (X-ray photoelectron spectroscopy), the data shown in FIGS. 6 and 7 were obtained. FIG. 7 shows the difference in the inner electronic state of oxygen (O) on the surface of the ZnO substrate, and shows the binding energy of the 1s inner electron orbit. FIG. 6 shows the difference in inner electronic state of zinc (Zn) on the surface of the ZnO substrate, and shows the binding energy of the 2p3 / 2 inner shell electron orbit. 6 and 7, the horizontal axis represents binding energy, and the vertical axis represents strength.

  6 and 7 show the case where the curve of S1 is not subjected to the substrate treatment, but in particular, the presence of OH groups can be clearly confirmed by the swelling of the peak on the high energy side shown in FIG. In this state, the pit density indicating the crystal defects of the thin film grown on the ZnO substrate increases. FIG. 10 shows the MgZnO thin film formed on the ZnO substrate and the crystal defect density (pit density) of the MgZnO thin film was examined. The pit density was obtained by capturing a dark field image obtained by observing the surface of the MgZnO thin film with an optical microscope, and counting the rounded white dots appearing in the imaging as pits. This process is performed while changing the growth temperature of the MgZnO thin film, and this is plotted in FIG.

Data when a conventional ZnO substrate polished by CMP is used is shown by an inverted white triangle (▽). As shown in the figure, it is necessary to raise the growth temperature in order to reduce the pit density. In the range of the substrate temperature of 750 ° C. to 850 ° C., the pit density is concentrated above 1 × 10 ⁶ cm ⁻² .

Therefore, in order to prevent Zn (OH) _{2 from} being stably present on the surface of the ZnO substrate and to prevent the adhesion of colloidal silica, the surface of the ZnO substrate is less than pH 5.5 (on the acidic side from pH 5.5) as shown in FIG. ) At least. If the pH is less than 5.5, the zeta potential polarity of the ZnO substrate and colloidal silica exists in the same polarity region, so it can be said that the adhesion is unlikely to occur, but in the middle, the zeta potential of the ZnO substrate and colloidal silica. Some regions have different polarities, and considering not only colloidal silica but also other particles such as PSL, the zeta potentials of the ZnO substrate, colloidal silica, and PSL are all the same polarity. As a region where Zn (OH) ₂ is not generated, a value of pH 3 or less is derived from FIG. Therefore, it is desirable that the surface of the ZnO substrate is in a state of pH 3 or more acidic than pH 3.

  On the other hand, after the surface of the ZnO substrate was wet-etched with a nitric acid solution having a pH of 3 or less (pH 3 or more acidic than pH 3), XPS measurement was performed on the surface of the ZnO substrate. The curves of S2 in FIGS. 6 and 7 represent the difference in the inner electronic state of zinc (Zn) and the difference in the inner electronic state of oxygen (O) on the surface of the ZnO substrate subjected to the substrate treatment. 6 and 7 show that the peak shape is particularly changed in FIG. 7 and that the signal derived from the OH group is weak, so that the OH group is greatly reduced. Thus, in order to make the ZnO substrate surface almost free of OH groups, the final treatment of the substrate surface is preferably performed by acidic wet etching at pH 3 or lower.

Further, an MgZnO thin film was formed on the ZnO substrate subjected to the acidic wet etching substrate treatment at pH 3 or lower as described above, and the pit density in the MgZnO thin film was observed. The pit density was calculated by changing the growth temperature of the MgZnO thin film. This is the data of the black circles (●) shown in FIG. 10 and is considerably included in the region described as having substrate processing. When compared in the growth temperature range of 750 ° C. to 850 ° C., it can be seen that the pit density is considerably reduced as compared with the case without the substrate treatment. When the growth temperature is 750 ° C. or higher, all the data with substrate processing (●) is 1 × 10 ⁶ pieces / cm ² or less, and the data without substrate processing (reverse white triangle: ▽) There is a clear difference.

  Furthermore, the P line indicated by the dotted line shows the lower limit of the pit density that changes depending on the state of the ZnO substrate surface. As can be seen from the comparison with this line, the comparison is made with the same value with a small pit density. When the substrate treatment is performed, the growth temperature of the thin film formed on the substrate can be lowered. Therefore, an etching treatment with a pH of 3 or less is desirable in order to prevent the generation of hydroxide and to prevent the adhesion of particles with the same zeta potential.

  As described above, an etching treatment with a pH of 3 or less is appropriate for removing deposits that cause problems on the substrate surface. However, an acidic solution with a pH of 3 or less has a very high acidity. Thus, the chemical bond between Zn and O on the substrate surface is likely to be broken. Therefore, after performing the CMP polishing process, an acidic wet etching process with a pH of 3 or less is performed, and then an oxidation process such as ashing described in the explanation of FIG. 1 is performed to restore the chemical bonding state on the surface of the ZnO substrate. The damage on the substrate surface can be recovered. As described above, a high-quality ZnO substrate surface can be obtained by combining wet etching with a pH of 3 or less and oxidation treatment.

Next, the quality condition of the surface of the crystal growth side of the Mg _X Zn _1-X O substrate (0 ≦ X <1) is considered from the crystal structure, and there is no deposit such as silica or particles, and the substrate surface is damaged. Consider obtaining a high-quality substrate surface that can form a thin film with good flatness.

  The ZnO-based compound has a hexagonal crystal structure called wurzeite like GaN. Expressions such as the C plane and the a-axis can be expressed by a so-called Miller index. For example, the C plane is expressed as a (0001) plane. When a ZnO-based thin film is grown on a ZnO-based material layer, a C-plane (0001) plane is usually performed. However, when a C-plane just substrate is used, the method of the wafer main surface as shown in FIG. The line direction Z coincides with the c-axis direction. However, it is known that even when a ZnO-based thin film is grown on a C-plane just ZnO substrate, the flatness of the film is not improved. In addition, unless the cleavage plane of the bulk crystal is used, the normal direction of the main surface of the wafer does not coincide with the c-axis direction, and if the C-plane just substrate is used, the productivity becomes worse.

  Therefore, the normal direction Z of the main surface of the ZnO substrate 1 (wafer) does not coincide with the c-axis direction, and the normal direction Z is inclined from the c-axis of the wafer main surface so as to have an off angle. As shown in FIG. 11B, when the normal line Z of the main surface of the substrate is inclined by θ degrees only in the m-axis direction from the c-axis, for example, an enlarged view of the surface portion (for example, T1 region) of the substrate 1 As shown in FIG. 11C, a terrace surface 1a that is a flat surface and step surfaces 1b that are regular at regular intervals are formed in the stepped portions that are formed by inclining.

  Here, the terrace surface 1a becomes the C surface (0001), and the step surface 1b corresponds to the M surface (10-10). As shown in the figure, the formed step surfaces 1b are regularly arranged while maintaining the width of the terrace surface 1a in the m-axis direction. As shown in FIG. 11C, the c-axis perpendicular to the terrace surface 1a is inclined by θ degrees from the Z-axis. Further, the step lines 1e serving as the step edges of the step surface 1b are arranged in parallel while taking the width of the terrace surface 1a while maintaining a relationship perpendicular to the m-axis direction.

  Thus, if the step surface is an M-plane equivalent surface, the ZnO-based semiconductor layer crystal-grown on the main surface can be a flat film. On the main surface, a stepped portion is generated by the step surface 1b. Since the atoms flying to the stepped portion are coupled to the two surfaces of the terrace surface 1a and the step surface 1b, the step surface 1b is more than the case of flying to the terrace surface 1a. However, atoms can bond strongly and trap incoming atoms stably.

  Flying atoms diffuse in the terrace during the surface diffusion process, but stable by creeping growth where crystal growth proceeds by trapping at the stepped portion with strong bonding force and the kink position formed by this stepped portion and incorporating it into the crystal Growth takes place. As described above, when a ZnO-based semiconductor layer is stacked on a substrate whose normal to the main surface of the substrate is inclined at least in the m-axis direction, the ZnO-based semiconductor layer has a crystal growth centered on the step surface 1b and is flat. A film can be formed.

  By the way, the step lines 1e are regularly arranged in the m-axis direction, and the m-axis direction and the step line 1e are perpendicular to each other, which is necessary for producing a flat film. If the distance between the lines 1e and the line are disturbed, the above-described creeping growth is not performed, and a flat film cannot be produced.

On the other hand, if the inclination angle (off angle) θ is too large in FIG. 11B, the step height t of the step surface 1b may become too large, and the crystal does not grow flat. It is necessary to limit the angle to a certain angle. 12 and 13 show that the flatness of the growth film changes depending on the inclination angle in the m-axis direction. FIG. 12 shows an example in which a ZnO-based semiconductor is grown on the main surface of an Mg _X Zn _1-X O substrate having an off-angle with an inclination angle θ of 1.5 degrees. On the other hand, FIG. 13 shows a case where a ZnO-based semiconductor is grown on the main surface of a Mg _X Zn _1-X O substrate having an off angle of 3.5 degrees with an inclination angle θ. 12 and 13 are images scanned in a 1 μm square range using AFM after crystal growth. In the case of FIG. 12, a beautiful film is generated with the steps having the same width, but in the case of FIG. 13, unevenness is scattered and the flatness is lost. From the above, it is desirable that the angle be in the range exceeding 0 degree and 3 degrees or less (0 <θ ≦ 3). Therefore, since the same is true of the tilt angle [Phi _m in FIG. 16, in the range exceeding 0 ° and 3 ° or less (0 <Φ _m ≦ 3) it is optimal.

As described above, it is most desirable that the normal direction Z of the main surface of the substrate is inclined only from the c-axis to the m-axis direction, and the inclination angle is in the range exceeding 0 degree and not more than 3 degrees. More practically, it is difficult to limit to the case of cutting by inclining only in the m-axis direction, and as a production technique, it is necessary to allow inclination to the a-axis and set the tolerance. For example, as shown in FIG. 15, the normal Z of the substrate principal surface is inclined by an angle Φ from the c-axis of the substrate crystal axis, and the normal Z is an orthogonal coordinate system of the c-axis, the m-axis, and the a-axis of the substrate crystal axis. _Let us consider a case in which the projection axis projected onto the c-axis m-axis plane is inclined by an angle Φ _m toward the m-axis, and the projection axis projected onto the c-axis a-axis plane is inclined by an angle Φ _a toward the a-axis.

As shown in FIG. 15, the state in which the substrate main surface normal Z is inclined is more easily understood, and the relationship between the orthogonal coordinate system of the c-axis, m-axis, and a-axis and the normal Z is shown in FIG. a). 15 differs from FIG. 15 only in the direction in which the substrate principal surface normal Z is inclined. The meanings of Φ, Φ _m , and Φ _a are the same as those in FIG. The projection axis A projected onto the c-axis m-axis plane and the projection axis B projected onto the c-axis a-axis plane in the m-axis a-axis orthogonal coordinate system are shown.

  In addition, the direction of the projection axis obtained by projecting the substrate principal surface normal Z onto the a-axis m-axis plane of the orthogonal coordinate system of the c-axis m-axis a-axis which is the substrate crystal axis is represented as the L direction. At this time, a step surface 1d is generated on the terrace surface 1c which is a flat surface shown in FIG. Here, the terrace surface is the C plane (0001). Unlike FIG. 11, the normal line Z is inclined at an angle Φ from the c-axis perpendicular to the terrace surface, as shown in FIG. Become.

Since the normal direction of the substrate main surface is inclined not only in the m-axis direction but also in the a-axis direction, the step surface comes out obliquely and the step surface is aligned in the L direction. This state appears as a step edge arrangement in the m-axis direction as shown in FIGS. 16 (a) and 16 (b). However, since the M plane is a thermally and chemically stable plane, of the inclination angle [Phi _a, oblique step is not maintained in the clean, can uneven step surfaces 1d, it is disturbed to a sequence of step edges, it is no longer possible to create the flat film on the main surface. The inventors have found that the M-plane is thermally and chemically stable and has been described in detail in Japanese Patent Application No. 2006-160273.

FIG. 17 shows how the step edge and step width change when the normal line Z on the growth surface (main surface) has an off-angle in the a-axis direction in addition to the off-angle in the m-axis direction. Show. Figure 16 m-axis direction of the off angle [Phi _m described in (a) was fixed to 0.4 degrees, and compared varied so as to increase the off-angle [Phi _a in the a-axis direction. This was realized by changing the cut-out surface of the Mg _X Zn _1-X O substrate.

When the off-angle Φa in the _a- axis direction is changed so as to increase, the angle θ _S formed between the step edge and the m-axis direction also changes in the increasing direction, and FIG. 17 shows the angle of θ _S. FIG. 17A shows the case where θ _S = 85 degrees, but the step edge and the step width are not disturbed. FIG. 17B shows the case of θ _S = 78 degrees, but the step edge and step width can be confirmed although there is some disturbance. FIG. 17C shows the case of θ _S = 65 degrees, but the disturbance is severe and the step edge and step width cannot be confirmed. If the ZnO-based semiconductor layer is epitaxially grown on the surface state of FIG. 17C, the above-mentioned creeping growth is not performed, and thus a flat film cannot be formed. In the case of FIG. 17C, this corresponds to 0.15 degrees when converted to the inclination Φa in the _a- axis direction. From the above data, it can be seen that a range of 70 degrees ≦ θ _S ≦ 90 degrees is desirable.

In this way, the oblique step is not kept clean, the step surface is uneven, and the angle at which the step edge arrangement is disturbed is θ _S = 70 °, for example, Φ _m = 0.5 °. If, which correspond to 0.1 degrees in terms of inclination [Phi _a in the a-axis direction.

Incidentally, regarding θ _S , not only when the projection axis B of the principal surface normal Z is inclined by Φ _a degrees in the a-axis direction, but also when it is inclined in the −a-axis direction in FIG. Since it is equivalent due to symmetry, it must be considered. When this inclination angle is set to −Φ _a and the stepped portion due to the step surface is projected onto the m-axis a-axis plane, it is expressed as shown in FIG. Here, the condition of the angle θ _i formed by the m-axis and the step edge also satisfies the above-mentioned 70 degrees ≦ θ _i ≦ 90 degrees. Since the relationship θ _S = 180 degrees−θ _i is established, the maximum value of θ _S is 180 degrees−70 degrees = 110 degrees, and finally the range of 70 degrees ≦ θ _S ≦ 110 degrees is flat. This is a condition that allows the film to grow.

Next, assuming that the unit of angle is radians (rad) and θ _S is expressed using Φ _m and Φ _a based on FIG. 16, the following is obtained. From FIG. 16, the angle α is expressed as α = arctan (tanΦ _a / tanΦ _m ),
θ _S = (π / 2) −α = (π / 2) −arctan (tanΦ _a / tanΦ _m ).
Here, since θ _S is converted from radians to degrees (deg), θ _S = 90− (180 / π) arctan (tanΦ _a / tanΦ _m ),
70 ≦ {90− (180 / π) arctan (tanΦ _a / tanΦ _m )} ≦ 110 Here, as is well known, tan represents a tangent and arctan represents an arctangent. Note that θ _S = 90 degrees is a case where there is no inclination in the a-axis direction and only in the m-axis direction. Further, when the units of the angles of Φ _m and Φ _a are not radians but Φ _m degrees and Φ _a degrees, the above inequality is expressed as follows.

70 ≦ {90− (180 / π) arctan (tan (πΦ _a / 180) / tan (πΦ _m / 180)) ≦ 110
As described above, the substrate main surface is formed so that the + C plane that is strong against physical impact of the ZnO-based substrate is used, and the off angle between the c-axis and the substrate main surface normal in the + C plane has the above relationship. By doing so, a flat thin film can be laminated. In addition, when acid wet etching is performed after polishing the main surface of the substrate, it is performed with a solution having a pH of 3 or less, and after etching, if oxidation treatment such as ashing is performed as a final treatment, Surface deposits can be removed, surface damage can be eliminated, and a ZnO-based substrate having an extremely high quality crystal growth main surface can be formed.

Finally, FIG. 14 shows an example of a ZnO-based semiconductor element in which a ZnO-based thin film is stacked on the ZnO-based substrate of the present invention. FIG. 14 shows an example of an ultraviolet LED using an Mg _Y Zn _1-Y O film (0 ≦ Y <1) containing a p-type impurity. In order to form a crystal growth surface as a main surface having a + C plane of the ZnO substrate 12 and to form a normal surface of this main surface so as to be slightly inclined from the c-axis to the m-axis direction, and to provide a clean surface of the main surface, CMP is performed. Polishing was performed, followed by ashing. On this ZnO substrate 12, an undoped ZnO layer 13 and a nitrogen-doped p-type MgZnO layer 14 were grown in order, and then a p-electrode 15 and an n-electrode 11 were formed. As shown in the figure, the p-electrode 15 is composed of a multilayer metal film of Au (gold) 152 and Ni (nickel) 151, and the n-electrode 11 is composed of In (indium). The growth temperature of the nitrogen-doped MgZnO layer 14 was set to about 800 ° C.

It is a figure which shows the binding energy intensity distribution of the 2p3 / 2 inner-shell electron in a Zn atom when performing XPS measurement after performing predetermined processing to the C surface of a ZnO substrate. It is a figure which shows the binding energy intensity distribution of the 2p3 / 2 inner-shell electron in a Zn atom when XPS measurement is performed immediately after completion | finish of grinding | polishing processing about each surface of + C surface, -C surface, and M surface of a ZnO substrate. It is a figure which shows the binding energy intensity distribution of 2p3 / 2 inner-shell electrons in a Zn atom when XPS measurement is performed before and after sputtering on the + C plane of the ZnO substrate. It is a figure which shows the binding energy intensity distribution of 2p3 / 2 inner-shell electrons in a Zn atom when XPS measurement is performed before and after sputtering on the M plane of the ZnO substrate. It is a figure which shows the binding energy intensity distribution of 2p3 / 2 inner-shell electrons in a Zn atom when XPS measurement is performed before and after sputtering on the -C plane of a ZnO substrate. It is a figure which shows the inner-shell electronic state difference of zinc of ZnO. It is a figure which shows the inner-shell electronic state difference of the oxygen of ZnO. It is a figure which shows the pH-potential balance relationship of ZnO. It is a figure which shows the pH dependence of the zeta potential of ZnO, colloidal silica, and PSL. It is a figure which shows the relationship between the pit density in the MgZnO thin film at the time of forming a MgZnO thin film on a ZnO substrate, and the substrate temperature at the time of growth. It is a figure which shows the ZnO board | substrate surface in case the board | substrate principal surface normal line Z has an off angle only in the m-axis direction. Is a diagram showing the Mg X _Zn 1-X _O deposition surface on a substrate principal surface of the substrate normal line has an off-angle in the m-axis direction Substrate principal surface normal is a diagram showing the Mg X _Zn 1-X _O deposition surface on a substrate having an off-angle in the m-axis direction. It is a figure which shows an example of the ZnO-type semiconductor element comprised using the ZnO-type board | substrate of this invention. It is a figure which shows the relationship between a substrate main surface normal line, and the c-axis, m-axis, and a-axis which are a substrate crystal axis. It is a figure which shows the inclination state of the normal line of a ZnO substrate surface, and the relationship between a step edge and an m-axis. Off-angle in the a-axis direction of the substrate main surface normal is a diagram showing a different Mg X _Zn 1-X _O substrate surface condition.

Explanation of symbols

1 ZnO substrate

Claims (6)

When the main surface on the crystal growth side of the Mg _X Zn _1-X O substrate (0 ≦ X <1) is dispersed by X-ray photoelectrons, the excitation peak energy of 2p3 / 2 core electrons of Zn atoms is 1021. A ZnO-based substrate characterized by existing in a range of 75 eV to 1022.25 eV.   2. The ZnO-based substrate according to claim 1, wherein the main surface on the crystal growth side is a + C plane.   The principal surface on the crystal growth side has a C-plane, and the projection axis obtained by projecting the normal of the principal surface onto the m-axis c-axis plane of the substrate crystal axis is tilted within 3 degrees in the m-axis direction. The ZnO-based substrate according to claim 1, wherein the substrate is a ZnO-based substrate. A projection axis obtained by projecting the normal of the main surface onto the a-axis c-axis plane of the substrate crystal axis is Φ _a degrees in the a-axis direction, and a projection of the normal of the main surface projected onto the m-axis c-axis plane of the main surface The axis is inclined by Φ _m degrees in the m-axis direction, and the Φ _a is 70 ≦ {90− (180 / π) arctan (tan (πΦ _a / 180) / tan (πΦ _m / 180)) ≦ 110
The ZnO-based substrate according to claim 1, wherein: A processing method for a ZnO-based substrate, characterized in that the final treatment on the main surface on the crystal growth side of the Mg _X Zn _1-X O substrate (0 ≦ X <1) is an oxidation treatment.   6. The method for treating a ZnO-based substrate according to claim 5, wherein an acidic wet etching treatment having a pH of 3 or less is performed before the oxidation treatment.

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