JP2011046580A - ZnO-BASED SUBSTRATE AND ZnO-BASED SEMICONDUCTOR ELEMENT - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ZnO-based substrate preventing an alkali metal from reaching a ZnO-based semiconductor layer at least functionally working to act as a semiconductor device without problem and a ZnO-based semiconductor element. <P>SOLUTION: The segregation of the alkali metal to the ZnO-based semiconductor crystal-grown on the ZnO-based substrate is prevented because the ZnO-based substrate is formed such that the concentration of the alkali metal in the ZnO-based substrate is ≤1×10<SP>14</SP>cm<SP>-3</SP>. Even when the concentration of lithium is above 1×10<SP>14</SP>cm<SP>-3</SP>in a ZnO-based substrate, the segregation of the alkali metal to the ZnO-based semiconductor layer formed after the formation of a ZnO film is prevented by forming the ZnO film having thickness of ≥50 nm on the substrate. <P>COPYRIGHT: (C)2011,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 ZnO-based semiconductor element using the same.

  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. Non-Patent Document 3 describes that when single crystal ZnO is used, the field-effect mobility reaches 250 cm ² / V · S, and Non-Patent Document 4 describes that the field-effect mobility is 7 cm with Almorphus ZnO. It has been found that ZnO is also useful as an electronic device material, with reports that it reaches ² / V · S (amorphous Si reaches 1 cm ² / V · S or less).

  ZnO also has a cost advantage. ZnO is already produced and sold by the method of mass production with quartz called hydrothermal synthesis method. This is a huge advantage from an industrial point of view. For this reason, ZnO is expected to cost as much as Si.

  In contrast, SiC and GaN, which are the same wide-gap semiconductors, are sold as substrates, but there are many problems from an industrial standpoint. Both use vapor phase growth methods that use gas as the material source, so compared to the liquid phase growth method that has been proven for Si, GaAs, and sapphire, the material efficiency is poor (unreacted products can be produced in large quantities), and the growth rate It has weaknesses that can not be overcome fundamentally, such as being slow, large lumps or large area boards are not possible.

  Therefore, it has a fatal weak point from the industrial aspect that the cost becomes very high or a large area cannot be obtained. Although there is no such a problem, uniform ZnO can easily perform epitaxial growth indispensable for manufacturing a semiconductor device by using a ZnO substrate, and has a sufficient potential to leap as a semiconductor.

  As described above, ZnO is very promising from the physical and industrial viewpoints, but the ZnO-based substrate has a unique difficulty. An alkali metal (group I element excluding hydrogen) is included as an impurity. When the temperature is raised, ZnO does not become a melt and sublimates. Therefore, it is usually dissolved in a solution of LiOH (lithium hydroxide) and KOH (potassium hydroxide) to perform liquid phase growth. For this reason, the alkali metal which comprises a solution will enter. Group I elements including alkali metals are elements that are difficult to exclude other Group I elements, and it is almost impossible to selectively remove them.

A. Tsukazaki et al., Jpn. J. Appl. Phys. Vol. 44 (2005) L643 A. Tsukazaki et al., Nature Material 4 (2005) 42 Nikkei Electronics 2007-8 / 27 p.52 J. Nishii et al., Jpn. J. Appl. Phys. Vol. 42 (2003) pp.L347-L349

  As described above, the following problem occurs when a ZnO-based thin film is grown on a ZnO-based substrate using an MBE (Molecular Beam Epitaxy) apparatus or the like using a liquid-phase-grown ZnO-based substrate. .

Figure 5 is a stacked structure of the ZnO substrate / MgZnO layer / ZnO film, lithium ZnO substrate exceeds 1 × ¹⁰ 14 ^{cm -3,} it is formed so as to present at a concentration of mid 1 × ¹⁰ 14 ^{cm -3} units . Further, the lithium (Li) concentration of each layer and the secondary ion intensity of Mg and Zn were measured by secondary ion mass spectrometry (SIMS). Li is quantitatively measured by a standard sample. The left vertical axis in FIG. 5 indicates the Li concentration (cm ⁻³ ), the right vertical axis indicates the secondary ion intensity (cps) of Mg or Zn, and the horizontal axis indicates the depth (μm). In the center of the figure, the region where the secondary ion intensity of Mg is high shows the MgZnO film, the right side is the ZnO substrate, and the left side is the ZnO film.

  This is the first time we have found that Li segregates in the MgZnO film. As can be seen from the region where the Li concentration is high in FIG. 5, when there is a ZnO / MgZnO stack, it tends to stay in the MgZnO film. It can be seen from FIG. 5 that Li is lowered to the background level as soon as the ZnO film is laminated.

4A, a Ga (gallium) -doped n-type MgZnO layer 12, an undoped ZnO layer 14, and a nitrogen-doped p-type MgZnO layer 15 are stacked on a ZnO substrate 11, and MgZnO / ZnO. / a ZnO as MgZnO a double sandwiched by ZnO-based semiconductor layer of p-type and n-type to form a heterojunction, lithium exceeds 1 × ¹⁰ 14 ^{cm -3} in the ZnO substrate 11, 1 × ¹⁰ 14 ^{cm -3} units It was formed to exist at a mid concentration.

  As in FIG. 5, the lithium (Li) concentration of each layer and the secondary ion intensity of Mg and Zn were measured by secondary ion mass spectrometry (SIMS). The measurement results are shown in FIG. The meanings of the vertical and horizontal axes are the same as those in FIG. As can be seen from FIG. 4B, Li is segregated in the n-type MgZnO layer 12 and is also present in the undoped ZnO layer 14. Further, Li is segregated again in the upper p-type MgZnO layer 15 through the undoped ZnO layer 14. From these facts, it can be seen that Li is very easily mixed into MgZnO.

  For example, in the case of a light emitting element, a MgZnO / ZnO / MgZnO double heterojunction with a narrow gap sandwiched between wide gaps is basically used, and an MgZnO layer needs to be used in some form even in electronic devices such as transistors. It becomes.

  When these devices are manufactured, not only the impurity Li, which is a kind of alkali metal, is segregated in the MgZnO layer, but also the impurity Li is mixed into the active layer and the channel region, thereby changing the performance of the device. Or lead to deterioration. Therefore, it is necessary to prevent at least the alkali metal such as Li from reaching the ZnO-based semiconductor layer in which the functional function is performed, but the specific configuration has not been known so far.

  The present invention was devised to solve the above-described problems, and prevents alkali metal from reaching a ZnO-based semiconductor layer that performs at least a functional function in order to operate without problems as a semiconductor device. An object of the present invention is to provide a ZnO-based substrate and a ZnO-based semiconductor element that can be used.

In order to achieve the above object, the invention according to claim 1 is a ZnO-based substrate characterized in that the concentration of alkali metal present in the substrate is 1 × 10 ¹⁴ cm ⁻³ or less.

  The invention according to claim 2 is the ZnO-based substrate according to claim 1, wherein the alkali metal is lithium.

  The invention according to claim 3 is the ZnO-based substrate according to claim 1, wherein the main surface on the crystal growth side is a + C plane.

  According to a fourth 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 of the principal surface onto the m-axis / c-axis plane of the substrate crystal axis is m 3. 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.

In the invention according to claim 5, 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 the ZnO-based substrate is satisfied.

  A sixth aspect of the present invention is a ZnO-based semiconductor element characterized in that a ZnO-based semiconductor is crystal-grown on the ZnO-based substrate of any one of the first to fifth aspects.

  The invention according to claim 7 is the ZnO-based semiconductor element according to claim 6, wherein the ZnO-based semiconductor is composed of a ZnO film and an MgZnO film.

  The invention according to claim 8 is the ZnO-based semiconductor element according to claim 7, wherein the Mg composition of the MgZnO film is 35% or less.

The invention according to claim 9 is characterized in that at least a ZnO film having a thickness of 50 nm or more is formed on a ZnO-based substrate having a lithium concentration in the substrate exceeding 1 × 10 ¹⁴ cm ^−3. A semiconductor-based semiconductor device.

The invention according to claim 10 is characterized in that the ZnO-based semiconductor layer formed after the ZnO film has a lithium concentration of 1 × 10 ¹⁴ cm ⁻³ or less. A semiconductor-based semiconductor device.

In the ZnO-based substrate of the present invention, the concentration of the alkali metal present in the substrate is 1 × 10 ¹⁴ cm ⁻³ or less, so that the alkali metal with respect to the ZnO-based semiconductor that is crystal-grown on the ZnO-based substrate. Segregation can be prevented. Further, even if the ZnO-based substrate has a lithium concentration in the substrate exceeding 1 × 10 ¹⁴ cm ⁻³ , the ZnO film formed on the ZnO film is formed after the ZnO film by setting the film thickness of the ZnO film to 50 nm or more. Segregation of alkali metal to the ZnO-based semiconductor layer can be prevented.

It is a figure which shows the result of having formed the laminated body containing a MgZnO layer and a ZnO layer on the ZnO board | substrate containing Li, and measuring by SIMS. It is a figure which shows the result of having formed the ZnO layer on the ZnO board | substrate containing Li, and measuring by SIMS. It is a figure which shows the result of having formed the laminated body shown to (a) on the ZnO board | substrate containing Li, and measuring by SIMS. It is a figure which shows the result of having formed the laminated body shown to (a) on the ZnO board | substrate containing Li, and measuring by SIMS. It is a figure which shows the result of having formed the laminated body containing a MgZnO layer and a ZnO layer on the ZnO board | substrate containing Li, and measuring by SIMS. 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. 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. 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. 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. 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.

In the present _invention, using a Mg _{X Zn 1-X} O board (0 ≦ X <1), in order to prevent segregation of the alkali metal such as Li in MgZnO layer is grown on the substrate, _Mg X Zn ₁ We have found that it is good if the concentration of the alkali metal element in the _-XO substrate is not more than a predetermined concentration. Among the Mg _X Zn _1-X O substrates (0 ≦ X <1), a ZnO substrate with X = 0 is used, and liquid phase growth is performed so that Li, which is a kind of alkali metal element, is contained in the ZnO substrate. It was.

Then, on this ZnO substrate, a ZnO-based semiconductor including an MgZnO layer and a ZnO layer was crystal-grown by MBE or the like and examined. For the Mg _Y Zn _1-Y O layer for crystal growth, the Mg composition ratio was set to 35% or less (Y ≦ 0.35). This is because when the Mg composition ratio exceeds 35%, Mg is often precipitated, resulting in a non-uniform composition. Here, the ZnO-based semiconductor is composed of ZnO or a compound containing ZnO. Specific examples include ZnO, IIA group element and Zn, IIB group element and Zn, or IIA group element and IIB. It means the one containing each oxide of group element and Zn. Further, the alkali metal is a group I element excluding H (hydrogen), and specifically represents any element of Li, Na, K, Rb, Cs, and Fr. To do.

FIG. 1 shows a stacked structure of a ZnO substrate 11, an n-type MgZnO layer 12, an undoped ZnO layer 14, and a p-type MgZnO layer 15, as in FIG. 4A, and lithium is 1 × 10 ¹⁴ cm ^{−3 on the} ZnO substrate 11. Hereinafter, it was formed so as to exist at a concentration of about 1 × 10 ¹³ cm ⁻³ units. Further, the lithium (Li) concentration of each layer, and the secondary ion intensity of Mg (magnesium) and Zn (zinc) were measured by secondary ion mass spectrometry (SIMS). Li is quantitatively measured by a standard sample. In FIG. 1, the left vertical axis represents the Li concentration (cm ⁻³ ), the right vertical axis represents the secondary ion intensity (cps) of Mg or Zn, and the horizontal axis represents the depth (μm). In the center of the figure, the region where the secondary ion intensity of Mg is high shows the n-type MgZnO layer 12, the right side is the ZnO substrate 11, the left side is the undoped ZnO layer 14, and the p-type MgZnO layer 15. As can be seen from comparison with FIG. 4B, no Li segregation occurs in the n-type MgZnO layer 12. In the first place, as can be seen from the fact that SIMS data related to Li in FIG.

FIG. 2 shows the segregation state of Li in the ZnO layer formed on the ZnO substrate. Lithium was formed on a ZnO substrate so as to be present at a concentration of 1 × 10 ¹⁴ cm ⁻³ or less and about 1 × 10 ¹³ cm ⁻³ and measured by SIMS. Impurity Al is also included in the ZnO substrate so that the boundary of the ZnO substrate / ZnO layer can be seen. The meanings of the vertical and horizontal axes are the same as in FIG. The region where the Al concentration is high is the ZnO substrate, and the region where the Al concentration is low is the ZnO layer. As can be seen from FIG. 2, when the lithium concentration in the ZnO substrate is 1 × 10 ¹⁴ cm ⁻³ or less, no segregation of lithium is observed even in the ZnO layer. In this case as well, Li is almost below the detection limit.

As can be seen from a comparison with FIG. 4B, if the ZnO-based substrate is formed so as to have a lithium concentration of 1 × 10 ¹³ cm ⁻³ , that is, 1 × 10 ¹⁴ cm ⁻³ or less. It can be seen that no segregation of Li occurs in the ZnO-based semiconductor layer such as MgZnO or ZnO formed thereon.

On the other hand, FIG. 3 does not define the alkali metal concentration in the ZnO-based substrate, but on the ZnO-based substrate even if the alkali metal concentration in the ZnO-based substrate exceeds 1 × 10 ¹⁴ cm ^−3. Let us consider a structure that prevents alkali metal from being deposited in MgZnO or ZnO constituting the active layer and channel layer formed on the substrate.

3 shows a stacked structure of a ZnO substrate 11, a gallium-doped n-type MgZnO layer 12, an undoped MgZnO layer 13, an undoped ZnO layer 14, and a nitrogen-doped p-type MgZnO layer 15, as shown in FIG. lithium substrate 11 exceeds 1 × ¹⁰ 14 ^{cm -3,} it is formed so as to present at a concentration of mid 1 × ¹⁰ 14 ^{cm -3} units. Further, the lithium (Li) concentration of each layer and the secondary ion intensity of Mg and Zn were measured by secondary ion mass spectrometry (SIMS). Li is quantitatively measured by a standard sample. In FIG. 3, the left vertical axis represents the Li concentration (cm ⁻³ ), the right vertical axis represents the secondary ion intensity (cps) of Mg or Zn, and the horizontal axis represents the depth (μm). In the center of the figure, the region where the secondary ion intensity of Mg is high shows the n-type MgZnO layer 12 and the undoped MgZnO layer 13, the right side is the ZnO substrate 11, the left side is the undoped ZnO layer 14, and the p-type MgZnO layer 15. is there.

  In this figure, no segregation of lithium is observed in the undoped ZnO layer 14 and the p-type MgZnO layer 15. Here, the thickness of the undoped ZnO layer 14 is set to 200 nm. On the other hand, in the structure of FIG. 4 described above, the thickness of the undoped ZnO layer 14 is 50 nm. As described above, it is known that lithium segregation is very likely to occur in the MgZnO layer, whether it is n-type or p-type, so that no lithium segregation occurs from the undoped ZnO layer 14 to the upper layer in FIG. This is because the thickness of the undoped ZnO layer 14 was large. Therefore, even if Li or the like is contained in the ZnO-based substrate, an MgZnO film formed after the ZnO layer can be formed by using a ZnO layer for the semiconductor layer in the middle of the lamination and increasing the film thickness. It is possible to prevent lithium from entering a ZnO-based semiconductor layer such as a ZnO film. In order to prevent lithium from entering the upper layer than the ZnO layer, it is considered that the thickness of the ZnO layer needs to be at least 50 nm or more.

Summing up the above results, even when the alkali metal concentration in the ZnO-based substrate exceeds 1 × 10 ¹⁴ cm ⁻³ , the thickness of the ZnO film for crystal growth after the ZnO-based substrate is set to 50 nm or more, Furthermore, if the alkali metal concentration present in a ZnO-based semiconductor layer such as MgZnO or ZnO that is crystal-grown after the ZnO film is 1 × 10 ¹⁴ cm ⁻³ or less, the active layer or the channel layer that performs a functional function can be obtained. It is possible to prevent the alkali metal from reaching. Further, even if the alkali metal concentration of ZnO-based semiconductor layer to be grown later than the thickness 50nm or more ZnO film is 1 × ¹⁰ 14 ^{cm -3} is as density exceeding 1 × ¹⁰ 14 ^{cm -3,} again A ZnO film having a thickness of 50 nm or more can be provided to prevent alkali metal from being mixed.

Next, the quality condition of the ZnO-based substrate is considered from the crystal structure, and a thin film with good flatness is obtained with a ZnO-based substrate having an alkali metal contamination concentration such as Li of less than 1 × 10 ¹⁴ cm ^−3. Consider obtaining a high-quality substrate that can be formed.

  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. Further, since the ZnO-based compound has a wurtzite structure, there are a + C plane (+ c axis direction plane) and a −C plane on the C plane, and the + C plane (0001) is chemically stronger. It has characteristics.

  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. 8B, when the normal line Z of the substrate main surface 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. 8C, a terrace surface 1a which is a flat surface and step surfaces 1b having regularity at regular intervals are formed in the stepped portions generated by the inclination.

  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. 8C, 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. 8B, 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. 9 and 10 show that the flatness of the growth film changes depending on the inclination angle in the m-axis direction. FIG. 9 shows an example in which a ZnO-based semiconductor is grown on the main surface of a Mg _x Zn _1-x O substrate (for example, Z = 0) having an off-angle with an inclination angle θ of 1.5 degrees.

On the other hand, FIG. 10 shows a case where a ZnO-based semiconductor is grown on the main surface of a Mg _x Zn _1-x O substrate (for example, Z = 0) having an off angle of 3.5 degrees. . 9 and 10 are images scanned in a 1 μm square range using AFM after crystal growth. In the case of FIG. 9, a beautiful film is generated with the steps having the same width, whereas in FIG. 10, the 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). Accordingly, the same can be said for the inclination angle Φ _m in FIG. 7, and therefore, it is optimal that the angle exceeds 0 degrees and is 3 degrees or less (0 <Φ _m ≦ 3).

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. 6, 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 _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. 6, 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). 6 differs from FIG. 6 only in the direction in which the substrate main 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.

  Also, 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 formed on the terrace surface 1c, which is a flat surface shown in FIG. Here, the terrace surface is the C plane (0001), but unlike the case of FIG. 8, the normal line Z is inclined by an angle Φ from the c-axis perpendicular to the terrace surface, as shown in FIG. 7A. It will be.

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 L direction as shown in FIGS. 7A and 7B, but since the M plane is a thermally and chemically stable plane, the tilt angle [Phi _a, the diagonal steps are not kept in 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. 11 shows how the step edge and the 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 7 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 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. 11 shows the angle of θ _S. FIG. 11A shows the case of θ _S = 85 degrees, but the step edge and the step width are not disturbed. FIG. 11B shows the case of θ _S = 78 degrees, but the step edge and step width can be confirmed although there is some disturbance. FIG. 11C 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. 11C, the above-described creeping growth is not performed, and thus a flat film cannot be formed. In the case of FIG. 11C, 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, for the theta _S, not only when the projection axis B of the main surface normals Z is [Phi _a degree inclination in the a-axis direction, even if you are inclined in -a axis direction in FIGS. 7 (a) Since it is equivalent due to symmetry, it must be considered. And the inclination angle and - [Phi] _a, when projecting the stepped portion by the step surfaces the m-axis a-axis plane is expressed as in FIG. 7 (c). 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, the angle unit is expressed in radians (rad), and θ _S is expressed using Φ _m and Φ _a based on FIG. 7 as follows. From FIG. 7, 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 chemically strong + C plane 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, this substrate has an alkali metal concentration of 1 × 10 ¹⁴ cm ⁻³ or less, and when a ZnO-based thin film is laminated, an appropriate device can be produced.

1 ZnO-based substrate 11 ZnO substrate 12 n-type MgZnO layer 13 undoped MgZnO layer 14 undoped ZnO layer 15 p-type MgZnO

Claims (10)

A ZnO-based substrate, wherein the concentration of an alkali metal present in the substrate is 1 × 10 ¹⁴ cm ⁻³ or less.   The ZnO-based substrate according to claim 1, wherein the alkali metal is lithium.   3. The ZnO-based substrate according to claim 1, wherein the main surface on which crystal growth is performed is a + C plane. 4.   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. 3. The ZnO-based substrate according to claim 1, wherein the ZnO-based substrate is formed. 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 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 the ZnO-based substrate is satisfied.   A ZnO-based semiconductor element, wherein a ZnO-based semiconductor is crystal-grown on the ZnO-based substrate according to any one of claims 1 to 5.   The ZnO-based semiconductor element according to claim 6, wherein the ZnO-based semiconductor is composed of a ZnO film and an MgZnO film.   8. The ZnO-based semiconductor device according to claim 7, wherein the Mg composition of the MgZnO film is 35% or less. A ZnO-based semiconductor element, wherein at least a ZnO film having a thickness of 50 nm or more is formed on a ZnO-based substrate having a lithium concentration in the substrate exceeding 1 × 10 ¹⁴ cm ⁻³ . 10. The ZnO-based semiconductor device according to claim 9, wherein the ZnO-based semiconductor layer formed after the ZnO film has a lithium concentration of 1 × 10 ¹⁴ cm ⁻³ or less.