JP5488562B2 - Manufacturing method of nitride semiconductor substrate - Google Patents

Description

The present invention relates to a method for manufacturing a nitride semiconductor substrate, and more particularly, a nitride semiconductor, for example, (In _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, 0 ≦ X + Y ≦ 1). The present invention relates to a method for manufacturing a nitride semiconductor substrate capable of manufacturing a self-standing substrate with small off-angle variation.

  Nitride-based semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN), and gallium aluminum nitride (GaAlN) have a large forbidden band and direct transition between bands, so visible light and ultraviolet light Development of applications for short-wavelength light emitting devices is progressing.

The nitride semiconductor substrate has a very high vapor pressure of nitrogen, so that bulk crystal growth from the melt is extremely difficult. For this reason, as a manufacturing method mainly used for a nitride semiconductor substrate, as shown in FIG. 4, on a different substrate 21 different from a nitride semiconductor such as a sapphire substrate, a silicon substrate, or a gallium arsenide substrate, After the nitride semiconductor layer 22 is heteroepitaxially grown mainly using a vapor phase growth method (FIG. 4A), the heterogeneous substrate 21 is removed using a technique such as peeling, polishing or etching (FIG. 4B). )), The so-called “self-supporting substrate” is obtained by polishing or the like on the front and back surfaces of the nitride semiconductor layer 22 formed on the heterogeneous substrate 21 (FIG. 4C). In addition, the arrow in FIG. 4 has shown the C-axis in the main surface orientation of a crystal | crystallization, for example, sapphire, GaN, etc.
Thus, as a specific method for producing a self-standing substrate, for example, a method described in Patent Document 1 is known.

For the method of growing the nitride semiconductor layer, in order to hetero-epitaxially growing a nitride semiconductor layer on a different heterogeneous substrate which is a nitride semiconductor as described above, due to the large lattice constant difference, large distortion crystals initial growth, ¹⁰⁹ Dislocation density of -10 ¹⁰ cm ^-2 is included. These defects significantly reduce the reliability of light emitting devices such as LDs (Laser diodes) and LEDs (Light emitted diodes), so it is necessary to reduce the dislocation density.

In recent years, growth techniques such as ELO (epitaxial lateral overgrowth), FIELO (facet initiated epitaxial lateral overgrowth), and pendeo epitaxy have been reported as methods for reducing the density of such defects. In these growth techniques, a mask patterned with SiO ₂ or the like is formed on GaN grown on a substrate such as sapphire, and further a GaN crystal is selectively grown from the window portion of the mask. By covering GaN with lateral growth, the propagation of dislocations from the underlying crystal is prevented. With the development of these growth techniques, the dislocation density in GaN can be drastically reduced to about 10 ⁷ cm ⁻² .

Furthermore, various methods have been proposed in which a GaN layer having a reduced dislocation density is epitaxially grown thickly on a dissimilar substrate such as a sapphire substrate, peeled off from the underlying layer after growth, and used as a self-supporting GaN substrate. For example, it has been proposed to form a GaN layer on a sapphire substrate using the above-described ELO technique, and then remove the sapphire substrate by etching or the like to obtain a GaN free-standing substrate.
VAS (Void-assisted Separation: for example, Non-Patent Document 1) method, DEEP (Dislocation Elimination by the Epi-growth with inverted-Pyramidal pits: For example, Non-Patent Document 2) method, and the like are disclosed. VAS grows GaN on a substrate such as sapphire via a TiN thin film with a network structure, thereby forming a void at the interface between the base substrate and the GaN layer, and simultaneously enabling peeling and low dislocation of the GaN substrate. It is a thing. Further, DEEP grows GaN using a mask such as SiN patterned on a GaAs substrate that can be removed by etching or the like to form a plurality of pits deliberately surrounded by facet surfaces on the crystal surface. By accumulating dislocations at the bottom, other regions are reduced in dislocation.

JP 2002-57119 A Y. Oshima et.al., Jpn.J.Appl.Phys., Vol.42 (2003), pp.L1-L3 K. Motoki et.al., Jpn.J.Appl.Phys., Vol.40 (2001), pp.L140-L143

However, since the nitride semiconductor layer 22 manufactured by the above-described conventional method has a defect density difference between the front and back surfaces, the nitride semiconductor after the crystal lattice is distorted and the internal stress is generated and peels from the base substrate 21. In any way, the layer 22 warps so that the back surface 22b has a convex shape (a shape in which the central portion protrudes) on the back surface side (FIG. 4B). Even if the nitride semiconductor layer 22 warped as described above is flattened by polishing or the like, the nitride semiconductor layer 22 is not oriented in a uniform direction at each position within the surface 22a. There arises a problem that the off-angle of the substrate 23 varies in the plane (FIG. 4C). The “off angle” here means “the angle of deviation of the normal of each position on the surface from the main plane orientation” (or “the angle between the surface and the main crystal plane”), “The off-angle varies” means that the angle of deviation of the normal of each position on the surface from the main surface orientation is not uniform, and the angle of deviation varies depending on the position in the surface. .
If the in-plane off-angle varies, it greatly affects the variation in the emission wavelength of the light-emitting element device formed thereon, and the yield is significantly reduced.
Further, even if epitaxial growth is performed on the nitride semiconductor free-standing substrate 23 (FIG. 4C) having an off-angle variation in the plane as a seed crystal, the orientation is inherited, so that the epitaxial layer is grown thickly. The free-standing substrate obtained by slicing will also vary in off-angle. Further, even when grown on a nitride semiconductor substrate having a uniform plane orientation, it is warped again due to a defect density difference between the front and back surfaces due to a decrease in threading dislocations, etc., which causes off-angle variation.

  An object of the present invention is to solve the above-described problems and to provide a method for manufacturing a nitride semiconductor substrate capable of manufacturing a nitride semiconductor substrate with small off-angle variation.

In order to solve the above problems, the present invention is configured as follows.
According to a first aspect of the present invention, a nitride semiconductor substrate is formed by forming a nitride semiconductor layer on a sapphire substrate and producing a self-supporting nitride semiconductor substrate using the nitride semiconductor layer separated from the sapphire substrate. In the method, the surface of the sapphire substrate is previously set so as to cancel out the inward inclination of the C-axis of the nitride semiconductor layer due to the warp due to the defect density difference between the front and back surfaces of the separated nitride semiconductor layer. The nitride semiconductor substrate manufacturing method is characterized in that the C axis on the entire surface of the substrate is inclined radially outward.

  According to the present invention, a nitride semiconductor free-standing substrate with small off-angle variation can be obtained. Moreover, the yield in the case where semiconductor light emitting devices such as LEDs and LDs are fabricated on the obtained nitride semiconductor free-standing substrate can be significantly improved.

  Hereinafter, embodiments of a method for manufacturing a nitride semiconductor substrate according to the present invention will be described with reference to the drawings.

FIG. 1 is a process diagram schematically showing the steps of the method for manufacturing a nitride semiconductor substrate according to the present embodiment.
In the method for manufacturing a nitride semiconductor substrate according to the present embodiment, first, a C-plane sapphire substrate 1 is prepared as a base substrate, and a nitride semiconductor layer 2 is formed on the C-plane sapphire substrate 1 (FIG. 1A).
The nitride semiconductor layer 2 is, for example, an In _X Al _Y Ga _1-XY N (0 ≦ X, 0 ≦ Y, 0 ≦ X + Y ≦ 1) layer or the like. As a method for forming the nitride semiconductor layer 2, a metal organic vapor phase epitaxy method (MOVPE method), a hydride vapor phase epitaxy method (HVPE method), or a vapor phase epitaxy method combining these is used.

  Next, the nitride semiconductor layer 2 formed on the sapphire substrate 1 is separated from the sapphire substrate 1 (FIG. 1B). As the separation method (removal method), methods such as polishing and etching may be used in addition to peeling (illustrated example).

Finally, the front surface 2a and the back surface 2b of the separated nitride semiconductor layer 2 are polished (leaving the broken line portion in FIG. 1B), and a self-supporting nitride semiconductor substrate 3 is manufactured by performing a cleaning process or the like. (FIG. 1 (c)). In addition, the arrow in FIG. 1 is the C axis (main plane orientation of the crystal).
Even if only one isolated nitride semiconductor layer 2 is obtained from the separated nitride semiconductor layer 2, a plurality of nitride semiconductor free-standing substrates can be obtained by epitaxially growing the nitride semiconductor layer 3 thickly and slicing. May be. Note that the “self-standing substrate” of nitride semiconductor is a substrate having a strength that allows substrate processing such as transportation, and the thickness of the substrate is preferably 200 μm or more, for example.

  The manufacturing method of the above embodiment is characterized in that the direction and magnitude of the C-axis inclination on the surface of the sapphire substrate 1 that is the base for forming the nitride semiconductor layer 2 are defined within a predetermined range. .

  As shown in FIG. 4A, the surface (C-plane) of a sapphire substrate used conventionally has a small off angle and the C-axis is almost perpendicular to the surface, and is nitrided epitaxially on the sapphire substrate. The physical semiconductor layer is also formed so that the orientation is inherited and the C-axis is uniformly aligned. However, since the nitride semiconductor layer formed on the sapphire substrate has a small defect density on the front surface and a large defect density on the back surface, internal stress always acts so that the surface of the nitride semiconductor layer warps in a concave shape. . For this reason, as shown in FIG. 4B, the separated nitride semiconductor layer is distorted by the internal stress caused by the difference in defect density and the surface warps into a concave shape, and is subjected to flat processing by polishing or the like. In the nitride semiconductor free-standing substrate, as shown in FIG. 4C, the C-axis is inclined inward in the radial direction, and the off-angle varies in the plane.

Therefore, in the present embodiment, in consideration of the inclination of the C-axis of the nitride semiconductor layer 2 in the radial direction due to warping caused by the defect density difference between the front and back surfaces of the separated nitride semiconductor layer 2, this C-axis The C-axis of the surface of the sapphire substrate 1 has an inclination outward in the radial direction in advance so that the C-axis in the nitride semiconductor layer 2 after separation is substantially uniformly aligned by offsetting the inclination inward in the radial direction. I try to make it.
That is, as shown in FIG. 1, the inclination of the C-axis of the sapphire substrate 1 is inclined to the same extent in such a direction as to cancel the inclination of the C-axis due to the warp caused by the defect density difference of the nitride semiconductor layer 2 after separation. Then, even if warpage occurs in the nitride semiconductor layer 2 after separation, the C-axis is substantially uniform, and the surface of the nitride semiconductor free-standing substrate 3 is polished by polishing the surface of the concave nitride semiconductor layer 2. The variation in the off angle is reduced. Thus, by defining the direction and magnitude of the off-angle of the C-axis of the sapphire substrate 1, variation in the inclination of the C-axis of the nitride semiconductor layer 2 after separation is controlled, and as a result, variation in off-angle is small. Thus, a nitride semiconductor free-standing substrate 3 having a uniform C axis is obtained.
Although depending on the growth conditions of the nitride semiconductor layer 2, the variation of the C-axis inclination (inward in the radial direction) caused by the difference in defect density in the nitride semiconductor layer 2 is about 0.3 ° to 0.7 °. On the other hand, when the off-angle variation of the sapphire substrate 1 is tilted by 0.3 ° to 1 ° in the opposite direction (outward in the radial direction of the substrate), the off-angle variation of the nitride semiconductor substrate 3 is a practical problem. It is possible to control to below 0.3 °.

The inclination of the C-axis radially inward due to warping caused by the difference in defect density of the nitride semiconductor layer 2 after separation is small at the center of the nitride semiconductor layer 2 and increases toward the outer periphery.
Therefore, in order to uniformly align the C-axis of the nitride semiconductor layer 2 after separation, the C-axis of the sapphire substrate 1 is radially outward from the half of the radius of the sapphire substrate 1 surface. Is preferably smaller than 0.3 °. Moreover, it is preferable that the inclination of the C axis of the sapphire substrate 1 is inclined outward in the radial direction on the outside including the half of the radius of the surface of the sapphire substrate 1, and further, the outside including the half of the radius of the surface of the sapphire substrate 1. Then, it is preferable that the inclination of the C-axis of the sapphire substrate 1 in the radial direction outward increases.

Next, examples of the present invention will be described.
[Example 1]
A GaN epitaxial layer is grown on the sapphire substrate using the Void-Assisted Separation Method (VAS method), and then the sapphire substrate is removed to obtain a self-supporting GaN substrate for evaluation. It was.
The VAS method is a method of performing crystal growth by sandwiching a titanium nitride thin film having a network structure between a sapphire substrate and a GaN growth layer, and details thereof are described in Non-Patent Document 1, for example. .

Below, the manufacturing method of the GaN free-standing substrate of a present Example is demonstrated.
First, an undoped GaN layer was grown to a thickness of 300 nm on a sapphire C-plane substrate having a diameter of 2 inches by MOVPE using trimethyl gallium (TMG) and NH ₃ as raw materials. Next, a Ti thin film having a thickness of 20 nm is deposited on the GaN epitaxial substrate, and this is put in an electric furnace, and in an atmosphere of a mixed gas of 20% NH ₃ and 80% H ₂ 1050 Heat treatment was performed for 20 minutes. As a result, a part of the undoped GaN layer is etched to generate high-density voids and change to void-formed GaN layers, and the Ti thin film is nitrided to form high-density submicron holes on the surface. It changed into the hole formation TiN layer formed in this.
This substrate was placed in an HVPE furnace, and GaN was deposited to a total thickness of 800 μm. The Ga metal boat was heated to 900 ° C., the substrate side was set to 1100 ° C., and a mixed gas of 5% hydrogen and 95% nitrogen was used as a carrier gas. HCl gas and Ga were reacted as source gases to generate GaCl, and ammonia gas was supplied at the same time, and the flow rate was adjusted so that the V / III ratio was 10 at the start of growth.
Under these conditions, the GaN nuclei grew in a three-dimensional island shape on the TiN layer, and then the crystals grew laterally and joined to each other, and the surface was flattened. This state was confirmed by observing the substrate surface and cross-section taken out of the furnace with different growth times under a microscope. We continued to grow by extending the growth time. In the process of cooling the HVPE apparatus after completion of the GaN crystal growth, the GaN layer naturally separated from the sapphire base substrate with the void layer as a boundary, and a GaN free-standing substrate having a thickness of 800 μm was obtained.

The GaN free-standing substrate was produced for a conventional sapphire substrate (off-angle variation of 0.1 ° or less) and a sapphire substrate of the example.
In the sapphire substrate of the example, the variation of the C-axis inclination (the off-angle to the outer side in the radial direction) is the difference between the maximum value and the minimum value of the C-axis radial outward inclination at each point on the surface of the sapphire substrate. The dispersion of the sapphire substrate is less than 0.3 ° in the radial direction of the C-axis of the sapphire substrate. The sapphire substrate whose C-axis inclination of the sapphire substrate is inclined outward in the radial direction and the inclination of the C-axis outward in the radial direction is increased outward, including the half of the radius of the sapphire substrate surface. Was used.
A GaN free-standing substrate was manufactured under the same manufacturing conditions as described above except that the off-angle variation of the sapphire substrate used was different, and the off-angle variation of the obtained GaN free-standing substrate was measured.
As a result, when a normal sapphire substrate (off-angle variation of 0.1 ° or less) was used, the off-angle variation of the GaN free-standing substrate was 0.45 °, and the sapphire substrate of this example was used. In this case, the variation in the off-angle of the GaN free-standing substrate was 0.23 ° or less.
Here, the variation in off-angle is defined as “maximum off-angle−minimum off-angle” among a plurality of off-angles in the substrate surface, and the variation in off-angle in the substrate surface was obtained. Specifically, an X-ray diffractometer is used to measure the off-angle, and as shown in FIG. 2, at a position of 10 points at intervals of 5 mm on a straight line passing through the center of a 2-inch (50.8 mm) substrate. The off-angle was measured to determine the variation in off-angle.
Depending on the growth conditions, the variation in the off-axis angle of the sapphire substrate must be selected in a timely manner, but the variation in the off-angle of the sapphire substrate (radially outward) falls within the range of 0.3 ° to 1 °. As a result, it has been found that, for most growth conditions, the variation in the off-angle of the grown nitride semiconductor substrate can be suppressed to within 0.3 °, which has no practical problem.

[Example 2]
On the GaN substrate produced in Example 1, a growth layer having the following light emitting element structure was formed. FIG. 3 is a structural cross-sectional view showing the nitride-based semiconductor light-emitting device according to the second embodiment.
The semiconductor layer of Example 2 has a quantum well structure. A multilayer film for a light emitting diode was produced by metal organic chemical vapor deposition (MOCVD). Trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI), and biscyclopentadienylmagnesium (Cp ₂ Mg) were used as organometallic raw materials. Ammonia (NH ₃ ) and silane (SiH ₄ ) were used as gas raw materials. Moreover, hydrogen and nitrogen were used as carrier gas.

The semiconductor light emitting device of this example was manufactured as follows.
First, using the GaN free-standing substrate 11 (with different variations in off-angle) 11 obtained in Example 1 above, Si is added at a carrier concentration of 1 × 10 ¹⁹ cm at 1050 ° C. on the GaN free-standing substrate 11. ^{A −3} doped n-type GaN layer 12 was grown to a thickness of 4 μm.
Next, a multi-quantum well in which GaN barrier layers 13 (4 layers) having a thickness of 10 nm and In _0.1 Ga _0.9 N well layers 14 (3 layers) having a thickness of 3 nm are alternately stacked as active layers at 800 ° C. An InGaN-based active layer 15 having a structure (MQW) was grown. A p-type Al _0.1 Ga _0.9 N clad layer 16 and a p-type GaN contact layer 17 were formed in this order on the top.
Further, a positive electrode 19 was formed on the p-type GaN contact layer 17 and a negative electrode 18 was formed on the back surface of the GaN free-standing substrate 11, and then fabricated as a chip.
After that, the chip is selected from the same 10 points as the off-angle measurement points shown in FIG. 2, and the emission wavelength of each chip is measured by EL (Electro Luminescence) measurement. Asked. Here, “the maximum value of the emission wavelength−the minimum value of the emission wavelength” was defined as the variation in the emission wavelength.

Table 1 shows the results of measuring the variation in emission wavelength for all the samples prepared in this example. As shown in Table 1, a sample with a small variation in off-angle gave a result that the variation in emission wavelength was small.

It is process drawing which shows schematically the process of the manufacturing method of the nitride semiconductor substrate which concerns on embodiment of this invention. It is a top view of the board | substrate for demonstrating the dispersion | variation in the off angle in a board | substrate surface. It is the schematic which shows the cross-section of the nitride semiconductor light-emitting device formed using the GaN self-supporting substrate produced in the Example. It is process drawing which shows typically the manufacturing process which produces the nitride semiconductor self-supporting board | substrate on the conventional dissimilar board | substrate.

1 Sapphire substrate (C-plane sapphire substrate)
2 Nitride semiconductor layer 3 Nitride semiconductor substrate (nitride semiconductor free-standing substrate)
11 GaN free-standing substrate 15 InGaN-based active layer

Claims (1)

In a method of manufacturing a nitride semiconductor substrate, a nitride semiconductor layer is formed on a sapphire substrate, and a self-supporting nitride semiconductor substrate is produced using the nitride semiconductor layer separated from the sapphire substrate.
So as to cancel the inclination of the radially inward in the C-axis of the nitride semiconductor layer due to warping caused by a defect density difference on the front and back surfaces of the separated the nitride semiconductor layer, the entire surface in advance the sapphire substrate surface A method for manufacturing a nitride semiconductor substrate, wherein the C-axis is inclined radially outward.

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