JP4985579B2 - Group III nitride compound semiconductor and gallium nitride free-standing substrate manufacturing method - Google Patents

Description

The present invention relates to a method for producing a group III nitride compound semiconductor. In the present application, the group III nitride compound semiconductor is a semiconductor represented by Al _x Ga _y In _1-xy N (where x, y, and x + y are all 0 or more and 1 or less), n-type / p-type, etc. For which any element is added. Furthermore, it is intended to include those in which a part of the composition of the group III element and the group V element is substituted with B or Tl, or P, As, Sb or Bi.

Group III nitride compound semiconductor light emitting devices are widely used as green, blue, or ultraviolet light emitting devices. Many HEMTs using Group III nitride compound semiconductors have also been proposed. These are usually formed by epitaxially growing a group III nitride compound semiconductor on a dissimilar substrate such as sapphire, silicon, or silicon carbide. Group III nitride compound semiconductors have good crystallinity for epitaxial growth stacked in the c-axis direction, and the fastest growth is obtained.
Patent Document 1 is a publication of a prior application filed by the present applicant regarding a technique for forming a surface different from the c-plane as a main surface of a group III nitride compound semiconductor.
JP 2006-036561 A

However, it has been pointed out that a group III nitride compound semiconductor generates a piezoelectric field in the c-axis direction. As a result, for example, in a light emitting layer having a multiple quantum well structure, the result is that the high-concentration portions of electrons and holes are different, and the light emission efficiency is reduced.
The present inventors have completed the present invention from a new idea as a technique for forming a surface different from the c-plane as the main surface of a group III nitride compound semiconductor.

The invention according to claim 1 is a method for producing a group III nitride compound semiconductor in which a group III nitride compound semiconductor is epitaxially grown on a sapphire substrate.
The normal vector of the main surface of the sapphire substrate is a direction rotated from 0.2 degrees to 5 degrees in the c-axis direction from the a-axis,
Forming concavo-convex having a side surface with an angle of 20 degrees or less formed on the sapphire substrate with c-plane or c-plane
The surface of the other sapphire substrate is group III-nitrided so that only the side with the larger normal angle between the normal vector and the a-axis of the sapphire substrate is exposed. Cover with a mask material that does not grow epitaxially compound semiconductors,
By epitaxially growing a group III nitride compound semiconductor from an exposed c-plane of the sapphire substrate or an angle formed with the c-plane that is not covered by the mask material and having an angle of 20 degrees or less, the main surface becomes the m-plane and 0. A method for producing a group III nitride compound semiconductor comprising obtaining a group III nitride compound semiconductor crystal having an angle of 2 degrees or more and 5 degrees or less.

Here, the fact that only the surface having the same direction of the normal vector in the c-plane or the side surface having an angle of 20 degrees or less formed with the c-plane is exposed means the following.
For example, when a c-plane is formed on the sapphire substrate and the c-plane is formed as the side surface of the concavo-convex surface, the group III nitride compound semiconductor grows from the c-plane having the front and back relationship. The group III nitride compound semiconductor that grows from the surface of the sapphire substrate and the group III nitride compound semiconductor that grows from the side surface on the back side of the unevenness of the sapphire substrate have a growth direction different by 180 degrees. Then, even if grown to cover the unevenness, for example, the group III nitride compound semiconductor grown from the front side surface of the sapphire substrate and the group III nitride grown from the back surface side of the sapphire substrate With a compound compound semiconductor, even if crystals grow, discontinuous surfaces with different polarities remain at the interface.
Therefore, only one of the front side surface and the back side surface of the sapphire substrate is exposed. This is performed with all the irregularities of the sapphire substrate. In this way, after growing so as to cover the unevenness, the crystal bonding surfaces grown from different side surfaces have the same polarity, and thus do not become discontinuous surfaces.
Further, among the side surfaces of the unevenness, the side surface having the same normal vector and the larger angle formed by the normal vector and the a-axis of the sapphire substrate means the following. As shown in FIG. 1, it is assumed that the normal vector of the main surface is tilted counterclockwise from the a-axis to the c-axis side by δ (deg)> 0. Assume that the angle display is positive in the counterclockwise direction, and the normal vectors of the two side surfaces of the unevenness are 90−θ (deg) and −90 + θ (deg) symmetrically with respect to the normal vector of the main surface. The angles formed with the a-axis are 90−θ + δ (deg) and −90 + θ + δ (deg), respectively. Of these, the one with the larger absolute value is 90−θ + δ (deg). This is true regardless of whether the sign of θ is positive or negative.

The invention according to claim 2 is characterized in that an angle formed between the concavo-convex exposed side surface of the sapphire substrate, which is not covered with a mask material, and the c-plane is 0.5 degrees or less.
The invention according to claim 3 is characterized in that, in the formation of the group III nitride compound semiconductor, the single crystal layer is formed at a temperature of 1000 ° C. or higher after forming the buffer layer at a temperature of less than 800 ° C. .
The invention according to claim 4 is characterized in that the buffer layer is formed before the mask material is formed.
The invention according to claim 5 is characterized in that the group III nitride compound semiconductor is gallium nitride (GaN).
In the invention according to claim 6, the epitaxial growth is performed by metal organic vapor phase epitaxy (MOVPE) using trimethylgallium and ammonia, and the V / III ratio of the ammonia supply amount to the trimethylgallium supply amount is uneven. The V / III ratio is high until the recesses are filled, the V / III ratio is low until the epitaxial growth film covers the uneven projections, and the V / III ratio is increased after the epitaxial growth film covers the projections. It is characterized by being raised.
The invention according to claim 7 is a method in which after the epitaxial growth by the method for producing a group III nitride compound semiconductor according to claim 6, a gallium nitride single crystal is further grown to a thickness of 200 μm or more by a halide vapor phase growth method, and thereafter A method of manufacturing a gallium nitride free-standing substrate, wherein cleavage is caused to separate from a sapphire substrate when cooling.

An uneven surface is provided on the sapphire substrate, and only the surface having the same direction of the normal vector among the side surfaces having an angle of 20 degrees or less formed with the c surface or the c surface is exposed. At this time, since the side surface is not a low index surface of the sapphire substrate, the c-plane of the sapphire substrate eventually becomes a stepped surface that is exposed with a fine step. Then, the group III nitride compound semiconductor epitaxially grown thereon is formed on the c-plane of the sapphire substrate at each step. That is, the c-plane of the group III nitride compound semiconductor is formed so as to be parallel to the c-plane of the sapphire substrate exposed stepwise.
Since the main surface of the sapphire substrate used here is slightly off from the a-plane, the m-plane of the group III nitride compound semiconductor to be formed is formed parallel to the a-plane of the sapphire substrate. .
Thus, the surface of the group III nitride compound semiconductor parallel to the main surface of the sapphire substrate to be used is an m-plane that is turned off in the direction in which the + c plane becomes the side surface of the step. When OFF is formed in this direction, lateral growth substantially parallel to the main surface is greatly promoted, and an m-plane epitaxial growth layer with extremely excellent flatness can be formed.

For example, according to the present invention, when a group III nitride compound semiconductor is obtained as a thick film crystal and separated from the sapphire substrate, an almost nonpolar GaN substrate having a thickness direction substantially perpendicular to the c-axis direction can be obtained. Since group III nitride compound semiconductors are easy to cleave in the m-plane, for example, when a thick film crystal is formed and then cooled to room temperature, cleavage occurs in the vicinity of the interface with the sapphire substrate, and a thick self-supporting substrate can be obtained. (Claim 7).
Further, a group III nitride compound semiconductor film or a group III nitride compound semiconductor substrate having a main surface slightly off from the m-plane can be obtained, and a group III nitride compound semiconductor layer is further laminated thereon. By doing so, it is possible to easily form a semiconductor element or the like that does not cause distortion in the stacking direction (film thickness direction).
Further, a group III nitride compound semiconductor thick film substrate having a main surface slightly off from the m-plane by crystal growth of a group III nitride compound semiconductor in a thicker film by a known method ( It is also possible to obtain a self-supporting substrate).

  If the off-angle is less than 0.2 degrees, lateral growth is not promoted, flatness is deteriorated, and adjacent epitaxial growth portions are not completely merged and voids remain. On the other hand, when the off-angle exceeds 5 degrees, the main surface becomes a surface that differs greatly from the m-plane. For example, when a semiconductor element is formed by repeating epitaxial growth on such a semiconductor film, the influence of the piezoelectric field in the c-axis direction increases in the thickness direction of each stacked layer.

  For the sapphire substrate to be used, the off direction of the main surface is obtained by rotation around the m-axis. The rotation around the c-axis may be less than 0.5 degrees. Thereby, since the m-axis of the sapphire substrate is substantially parallel to the main surface, the a-axis of the group III nitride compound semiconductor formed thereon according to the present invention is substantially parallel to the main surface.

An arbitrary processing method can be used as a method of forming irregularities on the sapphire substrate surface. For example, the unevenness may be formed by etching using an etching mask. It is desirable that the irregularities formed on the sapphire substrate do not form a desired c-plane or a plane other than a plane that forms an angle of 20 degrees or less with the c-plane. For example, grooves are formed in a number of stripes parallel to the m-axis of the sapphire substrate. It is good to form.
In this case, it is preferable that the width of the concave portion formed by etching is 1 to 10 μm, the width of the convex portion remaining without being etched is 1 to 10 μm, and the height difference between the concave portion and the convex portion is about 1 to 10 μm. If the groove is too narrow, the crystal raw material is difficult to reach, and high-quality crystals cannot be expected by epitaxial growth in a narrow region. On the other hand, if the width of the convex portion is too wide, it takes time to cover the convex portion, and it becomes difficult to completely flatten the convex portion.
As the mask material that covers the surface other than the c-plane or a plane that forms an angle of 20 degrees or less with the c-plane formed during the epitaxial growth, any material that does not allow the group III nitride compound semiconductor to grow epitaxially can be used. For example, SiO ₂ can be easily used. Although the formation method is also arbitrary, when SiO ₂ is used, the mask can be formed with high accuracy by electron beam evaporation or plasma CVD.

To increase the V / III ratio, for example, the molar ratio of ammonia and trimethylgallium is 800 or more, more preferably 1000 or more.
To lower the V / III ratio, for example, the molar ratio of ammonia to trimethylgallium is 600 or less, more preferably 300 or less.

Hereinafter, a specific embodiment of the present invention will be described with reference to the drawings.
A sapphire substrate 10 having a main surface forming an off angle of 1 degree with the a-plane was prepared, and striped irregularities whose longitudinal direction was parallel to the m-axis direction were formed. FIG. As shown in A, the bottom 10v of the groove is formed by etching perpendicular to the sapphire substrate 10, and the direction of the normal vector, which is in the relation of the front and back, is 180 degrees different from the surface 10t of the unetched convex portion 2 Two side surfaces 10c-1 and 10c-2 were formed. Here, the direction perpendicular to the paper surface is the m-axis.
Among these, from the side surfaces 10c-1 and 10c-2, the group III nitride compound semiconductor can be epitaxially grown so that the + c axis of the group III nitride compound semiconductor coincides with the c-axis direction of sapphire. is there. If the epitaxial growth from the side surfaces 10c-1 and 10c-2 coexists, the group III nitride compound semiconductor having polarity in the c-axis direction grows from different directions, so that the bonding surface becomes a discontinuous surface, which is not preferable.
Here, the normal vector of the side surface 10c-1 forms an angle of 90-θ + δ (deg) with the a axis shown in FIG. 1, and the normal vector of the side surface 10c-2 is the a axis shown in FIG. Note that it is preferable to use the side surface 10c-1 having a large absolute value of the angle formed by the a-axis and the normal vector. This is because the + c-axis direction of the group III nitride compound semiconductor formed is directed from the inside of the sapphire substrate 10 to the outside with respect to the main surface of the sapphire substrate 10, whereas the side surface 10c-2 is formed. The + c-axis direction of the group III nitride compound semiconductor is the direction from the outside to the inside of the sapphire substrate 10 with respect to the main surface of the sapphire substrate 10. Here, in the case of the epitaxial growth of the group III nitride compound semiconductor from the side surface 10c-2, the c-plane where the growth is most remarkable is formed in a shape of a stripe-shaped recess and can be formed at a position higher than the protrusion. There is no sex. On the other hand, during the epitaxial growth of the group III nitride compound semiconductor from the side surface 10c-1, the c-plane where growth is most remarkable reaches a position where at least a part thereof is higher than the stripe-shaped convex portion. The convex portion can be covered quickly by the growth in the + c-axis direction which is the lateral direction.
Thus, in order to expect the growth of the group III nitride compound semiconductor in the + c-axis direction, it is preferable that the side surface 10c-1 is exposed and the side surface 10c-2 is covered with a mask.
FIG. In A, the width of the stripe-shaped convex part and the width of the concave part were both 2 μm. That is, the stripe has a period of 4 μm. The height difference between the top surface of the convex portion and the bottom surface of the concave portion was 2 μm.
The two side surfaces 10c-1 and 10c-2 having a groove angle, that is, a front-back relationship, were formed perpendicular to the main surface of the sapphire substrate 10 having an off angle of 1 degree with respect to the a-plane. This angle does not have to be an exact 90 degrees (that is, a plane that forms an off angle of 1 degree with the c-plane), and may be 70 degrees or more and 110 degrees or less. That is, the two side surfaces 10c-1 and 10c-2 may be 69 degrees or more and 111 degrees or less (also a rotation angle around the m-axis) with respect to the a-plane of the sapphire substrate.
When SiO ₂ is used as a mask during etching, it can be controlled to 80 to 90 degrees.

Next, an epitaxial growth mask 20 made of SiO ₂ was formed by electron beam (EB) evaporation. At this time, FIG. As indicated by the thick arrow B, the direction of the sapphire substrate 10 may be rotated so that the SiO ₂ deposition direction is swept from the direction perpendicular to the main surface of the sapphire substrate 10 to the direction corresponding to the side surface 10c-2. . At this time, the epitaxial growth mask 20 made of SiO ₂ was not formed on the side surface 10c-1. An epi-growth mask 20 made of SiO ₂ having a thickness of 500 nm was deposited on the surfaces 10t, 10v, and 10c-2 of the sapphire substrate 10 thus subjected to uneven processing (FIG. 2.B).
Next, the foreign matter partially adhering to the side surface 10c-1 to be exposed was treated with a diluted nitric acid solution to which hydrofluoric acid was added. For example, commercially available hydrofluoric acid, concentrated sulfuric acid, and water may be mixed at a ratio of 3: 2: 200. In this way, an epitaxial growth substrate was obtained in which the surface other than the uneven side surface 10c-1 having an angle of 20 degrees or less with the c-plane of the sapphire substrate 10 was covered with the epitaxial growth mask 20 made of SiO ₂ (FIG. 2.C).

An epitaxial growth substrate in which an angle formed with the c-plane of the sapphire substrate 10 (rotational angle about the m-axis) is 20 degrees or less, except for the uneven side surface 10c-1, which is covered with an epitaxial growth mask 20 made of SiO ₂ is mounted on the MOCVD apparatus. (Reposted in FIG. 3.A and FIG. 2.C).
Epitaxial growth was basically performed as follows. First, the surface of the sapphire substrate 10 was baked at 1100 ° C. Next, the substrate temperature was set to 600 ° C., and a low-temperature buffer layer made of GaN was formed. Thereafter, the GaN layer 30 was formed at 1100 ° C.

The GaN layer 30 was formed as follows. First, after the growth of the GaN layer 30 from the uneven side surface 10c-1 whose angle formed with the c-plane of the sapphire substrate 10 is 20 degrees or less (FIG. 3.B), the groove is filled (FIG. 3.C). Kept the V / III ratio high at normal pressure. That is, ammonia was 7.5 L / min (336 mmol / min), trimethylgallium was 285 μmol / min, and the V / III ratio was 1180.
The V / III ratio was lowered (ammonia supply amount was 1/5) under reduced pressure until the groove portion was filled and the upper surface of the convex portion was almost covered and flattened (FIG. 3.D). That is, ammonia was 1.5 L / min (67 mmol / min), trimethylgallium was 285 μmol / min, and the V / III ratio was 236.
After the upper surface of the convex portion was almost covered and flattened (FIG. 3.E), the V / III ratio was increased at normal pressure (the supply amount of ammonia was restored). That is, ammonia was 7.5 L / min (336 mmol / min), trimethylgallium was 285 μmol / min, and the V / III ratio was 1180.
The crystal thus obtained was completely covered with stripe-shaped irregularities, and a GaN film in which the main surface was a flat surface having an off angle of 1 degree with the m-plane could be formed. When the flatness of the obtained crystal surface was promoted by AFM, the RMS was 1.0 nm.
FIG. D and FIG. E describes that there is a large step on the growth surface of the GaN layer 30, which indicates that the microstructure of the growth surface of the GaN layer 30 is stepped. . That is, the size relationship between the unevenness provided on the sapphire substrate 10 and the step of the GaN layer 30 is shown in FIG. D and FIG. Not shown in E.
To the last, the growth surface of the GaN layer 30 forms an off angle of 1 degree from the m-plane, so that the fine structure merely shows that the step between the m-plane and the narrow + c-plane is repeated. FIG. D and FIG. E is a deformed figure.

According to the above embodiment, the epitaxial growth is performed so as to cover the upper part of the epi-growth mask 20 by lateral growth with the side surface 10c-1 formed at intervals of 4 μm as nuclei (growth start region). The period at which the epitaxial crystals from the two adjacent side surfaces 10c-1 merge together at the upper portion of the epi growth mask 20 is also 4 μm apart.
That is, according to the present invention, since only the side surface 10c-1 is used as the nucleus for epitaxial growth, a high-quality crystal having the same polarity as the entire substrate surface can be obtained.

When the sapphire substrate 10 whose main surface is a normal vector in the direction rotated by 3 degrees from the a-axis to the c-axis is used to form irregularities, the one side surface 10c-1 becomes a substantially accurate c-plane. It was processed into. That is, the angle formed by the side surface 10c-1 and the main surface of the sapphire substrate was 87 degrees. Other than this, a low-temperature buffer layer made of GaN was formed at 600 ° C. as in Example 1, and a GaN layer 30 was formed at 1100 ° C.
The surface of the GaN layer 30 was completely planarized. When the height of the surface was evaluated with an AFM image, the RMS was calculated to be 0.8 nm.

[Comparative Example]
For comparison, a sapphire substrate having a normal vector of the main surface that is rotated in the c-axis direction from 0 degrees to 0.5 degrees from the a plane is used to form striped irregularities as in Example 1. FIG. The mask was formed such that the side surface 10c-2 of A, that is, the side surface in which the direction of the normal vector from the c surface was the direction from the outside to the inside with respect to the main surface of the sapphire substrate 10, was exposed. Other than this, when GaN was epitaxially grown in the same manner as in Example 1, the stripe-shaped irregularities could not be completely covered. Moreover, when the height of the surface was evaluated by the AFM image, RMS was 5 nm or more.

On the GaN layer 30 formed on the sapphire substrate 10 obtained in Example 1, a thick film GaN was further formed by HVPE. That is, ammonia (NH ₃ ) was used as a group V material, and GaCl obtained by reacting Ga and HCl was used as a group III material, and GaN was grown to 250 μm at a substrate temperature of 1100 ° C. The supply amount was 50 sccm (standard cubic centimeter) for HCl, 2000 sccm for NH ₃ , and 5000 sccm for the carrier gas (N ₂ ).
After crystal growth, when cooled to room temperature over 200 minutes, the thick GaN layer and the sapphire substrate 10 were separated, and a thick GaN substrate having a main surface slightly off from the m-plane was obtained. This is because during the cooling, the interface between the thick GaN layer and the sapphire substrate 10 is striped unevenness, so stress based on the difference in thermal expansion coefficient is concentrated, and the m-plane of the GaN layer is cleaved in the vicinity. Separated. The obtained GaN substrate was a high-quality crystal without cracks or fine cracks.

  According to the present invention, there is provided a group III nitride compound semiconductor film whose main surface is not c-plane for forming a group III nitride compound semiconductor element.

Explanatory drawing which shows the relationship between the normal line of the main surface of a sapphire substrate, the normal line of the formed unevenness | corrugation, and the a-axis and c-axis of sapphire in this invention. The first half (sectional drawing) of the process figure which shows the manufacturing method of the GaN layer 30 which concerns on an Example. The latter half (sectional drawing) of the process figure which shows the manufacturing method of the GaN layer 30 which concerns on an Example.

Explanation of symbols

10: Sapphire substrate whose main surface forms an off-angle with the a-plane 10t: Top surface of unevenness formed on the sapphire substrate 10v: Bottom surface of unevenness formed on the sapphire substrate 10c-1: On the sapphire substrate 10 used for epitaxial growth Formed uneven side surface 10c-2: Unused side surface formed on sapphire substrate 10 not used for epitaxial growth 20: Epi growth mask made of SiO ₂ 30: GaN layer

Claims (7)

In a method for producing a group III nitride compound semiconductor, wherein a group III nitride compound semiconductor is epitaxially grown on a sapphire substrate,
The normal vector of the principal surface of the sapphire substrate is a direction rotated from 0.2 degrees to 5 degrees in the c-axis direction from the a-axis,
Forming concavo-convex having a c-plane or a side with an angle of 20 degrees or less formed on the sapphire substrate;
Among the side surfaces, the surface of the other sapphire substrate is group III so that the normal vector is the same and only the side surface with the larger angle formed by the normal vector and the a-axis of the sapphire substrate is exposed. Cover with a mask material that nitride compound semiconductor does not grow epitaxially,
By epitaxially growing a group III nitride compound semiconductor from an exposed c-plane of the sapphire substrate or an angle formed with the c-plane that is not covered by the mask material and having an angle of 20 degrees or less, the main surface is the m-plane. A method for producing a Group III nitride compound semiconductor, comprising obtaining a Group III nitride compound semiconductor crystal having an angle of 0.2 degrees to 5 degrees. 2. The group III nitride compound according to claim 1, wherein an angle formed between a c-plane and an exposed side surface of the sapphire substrate that is not covered with the mask material is 0.5 degrees or less. 3. method of manufacturing a semiconductor body. 3. The formation of the group III nitride compound semiconductor includes forming a buffer layer at a temperature of less than 800 degrees and then forming a single crystal layer at a temperature of 1000 degrees or more. A method for producing a group III nitride compound semiconductor as described in 1. 4. The method for producing a group III nitride compound semiconductor according to claim 3, wherein the buffer layer is formed before the mask material is formed. The method for producing a group III nitride compound semiconductor according to any one of claims 1 to 4, wherein the group III nitride compound semiconductor is gallium nitride (GaN). The epitaxial growth is performed by metal organic vapor phase epitaxy (MOVPE) using trimethylgallium and ammonia,
Regarding the V / III ratio of ammonia supply to trimethylgallium supply,
The V / III ratio is increased until the concave and convex portions are filled,
The V / III ratio is low until the epitaxially grown film covers the uneven portions.
6. The method for producing a group III nitride compound semiconductor according to claim 5, wherein the V / III ratio is increased after the convex and concave portions are covered with the epitaxial growth film. After the epitaxial growth by the method for producing a group III nitride compound semiconductor according to claim 6, a gallium nitride single crystal is further grown to a thickness of 200 μm or more by a halide vapor phase growth method and then cleaved when cooled. And separating the sapphire substrate from the sapphire substrate.

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