JP5238924B2 - Single crystal substrate and method for producing nitride semiconductor single crystal - Google Patents

Description

The present invention relates to a single crystal substrate and a method for producing a nitride semiconductor single crystal using the same, and more particularly, when forming a nitride semiconductor single crystal through an intermediate layer, a higher quality Al _x. The present invention relates to a single crystal substrate on which a nitride semiconductor single crystal having a general formula of Ga _y N (x, y ≧ 0, x + y = 1) can be formed, and a method for manufacturing a nitride semiconductor single crystal using the single crystal substrate. .

  In recent years, group III-V nitride semiconductors typified by gallium nitride (GaN) and aluminum nitride (AlN) are also called so-called wide gap semiconductors, and can emit light in the green, blue, or ultraviolet region. Therefore, research and development on these semiconductors are actively conducted. These single crystals of nitride semiconductors are applied to blue, green, or ultraviolet light emitting diodes (hereinafter referred to as LEDs) used in high-luminance LED full-color displays, and blue, green, or ultraviolet light emitting semiconductor lasers (hereinafter referred to as LED). Application to next-generation device fields such as LD (Laser Diode) is expected.

Among these, as a method for obtaining a nitride semiconductor single crystal represented by the general formula Al _x Ga _y N (x ≧ 0, y> 0, x + y = 1), a CVD apparatus is used on the main surface of the heated base substrate. In general, a single crystal substrate is used as a base substrate in the film forming process. Here, as a single crystal substrate, there is no suitable substrate whose lattice constant matches that of the nitride semiconductor single crystal, and therefore a compound single crystal having a large difference in lattice constant from the nitride semiconductor single crystal, for example, sapphire (single crystal At present, Al ₂ O ₃ ), silicon carbide (SiC), silicon (Si), gallium arsenide (GaAs), or the like is used.

However, in the above-described single crystal substrate, due to the difference in lattice constant between the single crystal substrate and the nitride semiconductor single crystal, the deposited nitride semiconductor single crystal is 10 ⁹ cm ⁻² to 10 ¹⁰ cm ⁻² . A high density of crystal defects (dislocations) has occurred, and there has been a problem of hindering the long life, high output, and high efficiency of LEDs and LDs.

  Therefore, in order to improve the crystallinity and surface morphology of the nitride semiconductor single crystal 2 to a practical level and to form the nitride semiconductor single crystal 2 stably and with good yield, for example, a single crystal substrate made of sapphire A method of forming a nitride semiconductor single crystal 2 on the intermediate layer 3 after forming the intermediate layer 3 on one main surface of the conventional 1 is known (see FIG. 1).

Japanese Patent Laid-Open No. 04-297023

  However, when the intermediate layer 3 is formed on one main surface of the flat single crystal substrate 1 as in the above-mentioned Patent Document 1, the single crystal substrate 1 is warped when the intermediate layer 3 is formed. Since the surface of the intermediate layer 3 is not flat, there is a problem that the film thickness of the nitride semiconductor single crystal 2 formed on the intermediate layer 3 is uneven.

In addition, in the method of forming the intermediate layer 3 on one main surface of the flat single crystal substrate 1 shown in Patent Document 1 described above, it is caused by the difference in lattice constant between the single crystal substrate 1 and the intermediate layer 3. As a result, tensile stress is generated in the intermediate layer 3 and compressive stress is generated in the single crystal substrate 1. Due to these tensile stress and compressive stress, the single crystal substrate 1 after the formation of the intermediate layer 3 was warped. Therefore, a part of the other main surface 4b of the single crystal substrate 1 is lifted from the susceptor of the CVD apparatus and is not heated uniformly, and the temperature distribution on the surface of the intermediate layer 3 is not uniform. In particular, when the nitride semiconductor single crystal 2 represented by the general formula Al _x Ga _y N (x ≧ 0, y> 0, x + y = 1) is formed on the intermediate layer 3, the formed nitridation is performed. There is a problem in that the chemical composition of the semiconductor single crystal 2 varies.

  The present invention has been made in view of the above problems, and its object is to form the shape of the single crystal substrate 1 on which the intermediate layer 3 is formed when the nitride semiconductor single crystal 2 is formed. An object of the present invention is to provide a single crystal substrate 1 that can be preferably used for forming a nitride semiconductor single crystal 2 and a method for manufacturing the nitride semiconductor single crystal 2 using the single crystal substrate 1.

<Basic Structure of Single Crystal Substrate of the Present Invention>
For example, as shown in FIG. 2, the single crystal substrates 1a and 1b according to the present invention are formed by stacking nitride semiconductor single crystals 2 having a general formula of Al _x Ga _y N (x, y ≦ 0, x + y = 1). This is a single crystal substrate 1a used for the purpose, wherein one main surface 4a is formed as a convex surface, and the convex surface is polished to a mirror surface.

<Basic Principle of Single Crystal Substrate of the Present Invention>
By forming one main surface 4a of single crystal substrates 1a and 1b as a convex surface, the lattice constant between single crystal substrates 1a and 1b and intermediate layer 3a is formed when intermediate layer 3a is formed on one main surface 4a. Due to the difference, tensile stress is generated in the intermediate layer 3a, and compressive stress is also generated in the single crystal substrates 1a and 1b. These tensile stresses and the single crystal substrate 1a by compressive stress, warpage occurs in 1b, the warpage and the height difference Delta] h _a convex above is canceled, the surface of the formed intermediate layer 3a is made flat. Therefore, the uneven thickness of the nitride semiconductor single crystal 2 formed on the intermediate layer 3a is reduced, and variation in the Al ratio in the formed nitride semiconductor single crystal 2 is suppressed.

  The present invention has features as described above, but has many other features. These will be described in detail in the description of the examples below.

In the first embodiment, as the single crystal substrate 1a, as shown in FIG. 2, one having a convex surface that is mirror-polished on one main surface 4a is used. The single crystal substrate 1a is made of sapphire (Al ₂ O ₃ ), and has a surface that is off a few degrees from the c-plane as the plane orientation of the main surface. Here, the material of the single crystal substrate 1a may be a material having a larger thermal expansion coefficient than that of the intermediate layer 3a or a material having a larger lattice constant than that of the intermediate layer 3a, and silicon carbide other than sapphire (Al ₂ O ₃ ). (SiC), silicon (Si), and gallium arsenide (GaAs) may be used. Further, the plane orientation of the main surface of the single crystal substrate 1a may be a plane orientation capable of forming the intermediate layer 3a. When a sapphire substrate is used as the single crystal substrate 1a, the a plane, c plane or r plane, or The surface may be off several times from these surfaces.

  The single crystal substrate 1a according to the present example used a mirror having at least one main surface 4a polished to a mirror surface. As a method of polishing to a mirror surface, a known single-side polishing method or double-side polishing method such as tape polishing or lapping polishing may be used.

Here, the one main surface 4a of the single crystal substrate 1a may be a curved surface having a constant radius of curvature. This is because when the intermediate layer 3a is formed, a flat surface can be obtained by applying stress uniformly to the convex surface. The constant radius of curvature is 2.1 × 10 ^{4 to} 1.6 × 10 ⁵ mm, more preferably 8.0 × 10 ^{4 to} 1.6 × 10 ⁵ mm (for example, a single crystal substrate 1a having a diameter of 2 inches). in the case of using a sapphire substrate, a convex height difference Delta] h _a is 2~15μm of one main surface 4a, if more preferably a range of 4-15 .mu.m), when of depositing a nitride semiconductor single crystal 2, This is preferable because a substantially flat surface can be obtained.

In the second embodiment, as shown in FIG. 3, a single crystal substrate 1b having a convex surface that is mirror-polished on one main surface 4a and a concave surface on the other main surface 4b is used. By making the other main surface 4b a concave surface, when the intermediate layer 3a is formed on one main surface 4a, the warp generated in the single crystal substrate 1b and the height difference Δh _{b of} this concave surface are offset, and the other This is because the main surface 4b becomes flatter. Therefore, when the nitride semiconductor single crystal 2 is formed on the intermediate layer 3a, the contact state between the single crystal substrate 1b and the susceptor 14 of the CVD apparatus 10 (shown in FIG. 4) is improved, and the single crystal substrate 1b The temperature distribution inside became uniform. Accordingly, the chemical composition of the general formula Al _x Ga _y N of the nitride semiconductor single crystal 2 to be formed becomes uniform, and the accuracy of the characteristics of the semiconductor element made of the nitride semiconductor single crystal 2 can be improved.

  Here, a concave surface having substantially the same radius of curvature as the convex surface may be formed on the other main surface 4b. This is because when the intermediate layer 3a is formed, the contact between the other main surface 4b and the susceptor 14 of the CVD apparatus 10 is improved by making the other main surface 4b flat with the intermediate layer 3a. .

  As the single crystal substrate 1b according to this example, a substrate obtained by polishing at least one main surface 4a to a mirror surface by the same method as in Example 1 was used. When polishing one main surface 4a to a mirror surface, by controlling processing distortion by means such as additional polishing, heat treatment or etching, one main surface 4a is convex and the other main surface 4b is concave. It is good.

  In the third embodiment, using the single crystal substrate 1b according to the second embodiment (see FIG. 5A), an intermediate layer 3a made of GaN is formed on one main surface 4a having the convex surface of the single crystal substrate 1b. A film was formed (see FIG. 5B). Here, by using GaN as the intermediate layer 3a, the contact state between the other main surface 4b of the single crystal substrate 1b and the susceptor 14 is poor, and the temperature distribution is generated on the main surface of the single crystal substrate 1b. This is because even if the layer 3a is formed, the intermediate layer 3a having a uniform chemical composition and a uniform lattice constant can be formed. Here, the single crystal substrate 1b may be the single crystal substrate 1a according to the first embodiment.

  As the method for forming the intermediate layer 3a, the MOCVD (Metal Organic Chemical Vapor Deposition) method is used, but the HVPE (Hydride Vapor Phase Epitaxy) method may be used. Here, as shown in FIG. 4, the MOCVD CVD apparatus 10 includes a reaction tube 11 formed so as to be hollow, a gas supply source 12 filled with a carrier gas, and a gas supply source 12. A gas jet nozzle 13 for supplying gas to the reaction tube 11 and a susceptor 14 for holding the single crystal substrate 1b were used.

  Then, the single crystal substrate 1b is placed on the upper surface of the susceptor 14, and the source gas and the carrier are supplied through the gas jet nozzle 13 while heating from the other main surface 4b side of the single crystal substrate 1b to a predetermined temperature. A gas was supplied to form an intermediate layer 3a on one main surface 4a of the single crystal substrate 1b. At this time, the intermediate layer 3a is preferably formed to a thickness of 0.5 μm to 4.0 μm. Further, a low temperature buffer layer (not shown) made of GaN may be formed before forming the intermediate layer 3a.

For the single crystal substrate 1b on which the intermediate layer 3a is formed, the other susceptor 14 of the CVD apparatus 10 (shown in FIG. 4) is used in the same way as the film formation process of the intermediate layer 3a. Heating was performed through the main surface 4b until a predetermined temperature was reached, and the nitride semiconductor single crystal 2 was formed on the intermediate layer 3a (FIG. 5C). Here, the nitride semiconductor single crystal 2 is made of gallium nitride (GaN) and aluminum nitride (AlN) and mixed crystals thereof, and has a general formula of Al _x Ga _y N (where x, y ≦ 1, x + y = 1).

  In the fourth example, a specific example of the method for manufacturing the single crystal substrate 1b according to the above-described example and the nitride semiconductor single crystal 2 using the single crystal substrate 1b will be shown in comparison with the comparative example.

[Concrete example]
The single crystal substrate 1b according to the present embodiment has a convex surface formed on one main surface 4a and a concave surface formed on the other main surface 4b. As the single crystal substrate 1b, a sapphire substrate having a diameter of 2 inches and having a mirror surface on one main surface 4a was used. Here, the height differences Δh _a and Δh _b between the convex surface and the concave surface are both 5 μm. This sapphire substrate is placed on the susceptor 14 of the CVD apparatus 10 and heated to 550 ° C. to form a low-temperature buffer layer (not shown) made of GaN having a thickness of 25 to 30 nm, and then the sapphire substrate is heated to 1050 ° C. An intermediate layer 3a made of GaN having a thickness of 3 μm was formed on one main surface 4a. Subsequently, the sapphire substrate was heated to 1070 ° C. to form a nitride semiconductor single crystal 2 made of Al _0.3 Ga _0.7 N with a thickness of 50 nm. At this time, the deviation of the film thickness of the formed nitride semiconductor single crystal 2 was 2.5%, and the variation of the Al ratio in the nitride semiconductor single crystal 2 was 0.5% or less.

[Comparative example]
On the other hand, the single crystal substrate according to the comparative example of the specific example described above has a flat surface formed on one main surface and the other main surface. As the single crystal substrate, a sapphire substrate having a diameter of 2 inches and having a mirror surface on one main surface was used. This sapphire substrate is placed on the susceptor 14 of the CVD apparatus 10, and a nitride semiconductor unit made of a low temperature buffer layer (not shown) and intermediate layer 3a made of GaN and Al _0.27 Ga _0.73 N under the same conditions as in Example 1. Crystal 2 was deposited to 50 nm. At this time, the deviation of the film thickness of the formed nitride semiconductor single crystal 2 was 5%, and the variation in the Al ratio in the nitride semiconductor single crystal 2 was 3%.

It is sectional drawing which shows the manufacturing method of the nitride semiconductor single crystal using the single crystal substrate which concerns on the conventional form. It is sectional drawing which shows the single crystal substrate which concerns on this embodiment and Example 1. FIG. 6 is a cross-sectional view showing a single crystal substrate according to Example 2. FIG. It is the schematic which shows the structure of the CVD apparatus in Example 2 and Example 3. FIG. 6 is a cross-sectional view showing a method for manufacturing a nitride semiconductor single crystal according to Example 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1a, 1b Single crystal substrate 2 Nitride semiconductor single crystal 3, 3a Intermediate layer 4a One main surface 4b The other main surface 10 CVD apparatus 11 Reaction tube 12 Gas supply source 13 Gas ejection nozzle 14 Susceptor

Claims (5)

A single crystal substrate used for forming a nitride semiconductor single crystal having a general formula of Al _x Ga _y N (x, y ≧ 0, x + y = 1),
One main surface is formed as a convex surface,
Furthermore, the convex surface is polished to a mirror surface ,
A single crystal substrate , wherein the other main surface is formed as a concave surface . 2. The single crystal substrate according to claim 1, wherein the one main surface is a curved surface having a constant radius of curvature. The single crystal substrate according to claim 2, wherein the other main surface is a concave surface having substantially the same radius of curvature as the one main surface. The curvature radius is 2.1 × 10 ⁴ ~ 1.6 × 10 ⁵ 4. The single crystal substrate according to claim 2, wherein the single crystal substrate is mm. Using the single crystal substrate according to any one of claims 1 to 4,
After forming an intermediate layer made of GaN on one main surface of the single crystal substrate,
Al in the intermediate layer _x Ga _y A method for producing a nitride semiconductor single crystal, comprising depositing a nitride semiconductor single crystal having a general formula of N (x, y ≧ 0, x + y = 1).

Images