JP2008218655A - Nitride semiconductor forming substrate, nitride semiconductor using the substrate, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the problem of cracks and warpage caused by thermal distortion and thin film distortion caused by difference in lattice constant and thermal expansion coefficient between materials during growth of a nitride semiconductor thin film on a silicon substrate. To provide a nitride semiconductor thin film forming substrate suitable for forming a nitride semiconductor thin film.
A second porous silicon layer having a relatively high porosity and a first porous silicon layer having a relatively low porosity removed from an oxide film are sequentially provided on one surface of a silicon single crystal substrate. A first porous silicon layer having a relatively low porosity is formed by anodizing a nitride semiconductor forming substrate and a silicon single crystal substrate on one side in an electrolyte at a relatively low current density. Forming a second porous silicon layer having a relatively high porosity between the silicon single crystal substrate and the first silicon layer by anodizing at a relatively high current density in the electrolytic solution; A method for manufacturing a substrate for forming a nitride semiconductor, comprising: a step of removing an oxide film after forming the first porous layer silicon layer and the second porous silicon layer.
[Selection] Figure 1

Description

  The present invention relates to a substrate for forming a nitride semiconductor, and more specifically, using a substrate for forming a nitride semiconductor that hardly causes cracking or warping even when a nitride semiconductor film is formed thereon, and the substrate. The present invention relates to a nitride semiconductor and a manufacturing method thereof.

  Conventionally, nitride semiconductors such as GaN are formed on substrates having different lattice constants because lattice-matched substrates are expensive. At present, sapphire, which is more expensive than silicon, is generally used as a substrate for GaN formation, and this is mainly due to the following reasons.

That is, c-plane GaN (thermal expansion coefficient parallel to c-axis 5.59 × 10 ⁻ on parallel c-plane sapphire (thermal expansion coefficient 7.5 × 10 ⁻⁶ [K ⁻¹ ] parallel to c-axis)) ^In the case of growth of ⁶ [K ⁻¹ ]), since the thermal expansion coefficient of sapphire is larger than that of GaN, in-plane compressive stress is generated in GaN when the temperature is lowered after crystal growth. In general, since the in-plane compressive stress is less likely to cause cracks than the tensile stress, a GaN thick film of 4 μm or more can be formed. In addition, there is a region where crystal defects due to the difference in lattice constant are concentrated near the interface between the substrate and the GaN layer, but the device operation layer can be moved away from the region where defects are concentrated by forming a thick film, which adversely affects device operation. It is possible to avoid the influence of crystal defects that give

On the other hand, when silicon is used as the substrate, since the thermal expansion coefficient (2.4 × 10 ⁻⁶ [K ⁻¹ ]) is smaller than that of GaN, tensile stress is generated in GaN when the temperature is lowered after crystal growth. This tensile stress is liable to generate cracks and has a large difference in expansion coefficient between them, which causes extreme concave warpage, making it difficult to form a thick GaN film and reducing crystal defects.

  In order to avoid these problems and to use cheaper silicon as a substrate, in Patent Document 1, the porous structure of the porous silicon layer has a pore diameter of 1 nm to 10 μm, a thickness of 3 nm to 10 μm, and a porous structure. By controlling the degree to 10% to 90%, it is said that the lattice mismatch can be relaxed and good crystallinity can be maintained.

  On the other hand, in Patent Document 2, the pore size of the porous silicon layer is 3 nm or more and 10 nm or less, the thickness is 0.1 μm or more and 10 μm or less, and the porosity is more than 0 or 0.7 or less, It is said that the number of cracks is reduced by relieving thermal stress.

  However, in both techniques of Patent Documents 1 and 2, the structural brittleness of the porous silicon layer necessary for thermal stress relaxation can be reduced by reducing the pore diameter and decreasing the porosity in order to suppress lattice mismatch. There was a problem that it was not able to earn enough, and a sufficient effect was not exhibited for suppressing cracks and warpage. On the other hand, if the porosity is too large, the surface of the compound semiconductor thin film is roughened and the crystallinity is lowered with the change in the pore structure of the porous silicon layer. Therefore, there is a problem that it is difficult to achieve both lattice mismatch relaxation and thermal stress relaxation.

  Patent Document 3 discloses that a porous silicon layer for compound semiconductor epitaxial growth has a two-layer structure in which the crystal growth surface side has a low porosity and the inside has a high porosity. However, this porous silicon layer is only used to separate the substrate and the compound semiconductor thin film from the substrate, and is not even suggested for use in thermal stress relaxation.

Further, Patent Document 4 discloses that the porosity of the porous silicon layer formed on the surface layer of the substrate is continuously or stepwise from the surface layer to the inside in order to alleviate the lattice mismatch with the compound semiconductor (Si-Ge). It is disclosed that the surface of the porous layer is recrystallized into a crystalline layer of Si-Ge in a reducing atmosphere under a reducing atmosphere, and the lattice strain generated in the recrystallized layer is relaxed by the internal porous silicon layer. However, in general, recrystallization requires high-temperature treatment in an atmosphere of a reducing gas such as hydrogen, and surface roughness occurs due to the presence of moisture, so advanced equipment and know-how are required, and the cost increases. There was a problem. Furthermore, the above-mentioned patent document does not disclose the use of the above technique for thermal stress relaxation.
JP2002-270515 JP 2000-106348 A JP2005-129876 JP 2003-282464 A

  The present invention has been made in view of the above circumstances, and distortion of the thin film and heat generated from the difference in lattice constant and thermal expansion coefficient between materials during the growth of the nitride semiconductor thin film on the silicon substrate. An object of the present invention is to provide a substrate for forming a nitride semiconductor thin film suitable for forming a nitride semiconductor thin film, in which the problem of cracks and warpage due to stress is reduced.

  As a result of intensive investigations to solve the above problems, the inventors of the present application made a silicon single crystal substrate combined with a silicon layer having a different porous structure as a substrate, and an oxide film from the surface thereof. As a result, it was found that a very excellent substrate for forming a nitride semiconductor can be obtained, and the present invention has been completed.

  That is, according to the present invention, a second porous silicon layer having a relatively high porosity and a first porous silicon layer having a relatively low porosity removed from an oxide film are sequentially provided on one surface of a silicon single crystal substrate. This is a nitride semiconductor forming substrate.

  The present invention also provides a step of forming a first porous silicon layer having a relatively low porosity by anodizing one surface of a silicon single crystal substrate at a relatively low current density in the electrolyte. Forming a second porous silicon layer having a relatively high porosity between the silicon single crystal substrate and the first porous silicon layer by anodizing at a relatively high current density in the first step, This is a method for manufacturing a nitride semiconductor forming substrate including a step of removing an oxide film after forming a porous silicon layer and a second porous silicon layer.

  Furthermore, the present invention relates to a nitride semiconductor using the nitride semiconductor forming substrate and a method for manufacturing a nitride semiconductor thin film using the method for manufacturing the nitride semiconductor forming substrate.

  The nitride semiconductor forming substrate of the present invention grows a nitride semiconductor such as GaN on the substrate and forms a thin film, thereby causing lattice distortion of the nitride semiconductor, cracks and warpage associated with thermal stress, etc. Therefore, the performance of the nitride semiconductor device on the silicon substrate can be improved.

  Further, in the method for manufacturing a nitride semiconductor forming substrate of the present invention and the method for manufacturing a nitride semiconductor thin film using the same, recrystallization treatment under a reducing atmosphere is unnecessary, and the manufacturing cost is reduced. As an excellent method, the method is highly applicable to the production of nitride semiconductors.

  In the present specification, “relatively” means the size when compared with the corresponding counterpart. For example, “a relatively porous second porous silicon layer” means that the porosity is higher than that of the corresponding first porous silicon layer. Similarly, “relatively low current density” for the former stage anodization means that the current density is lower than the corresponding latter stage anodization. In the present invention, the second porous silicon layer includes not only a single porous silicon layer but also a plurality of different porous silicon layers. In this case, the average porosity is means. Moreover, the porosity in this specification means the ratio of the pore volume to the volume of the porous silicon layer.

  The substrate for forming a nitride semiconductor of the present invention can be obtained, for example, by anodizing a silicon single crystal substrate in an electrolytic solution using a silicon single crystal substrate with an electrode attached to the back surface as an anode and platinum as a cathode.

  That is, in the anodic oxidation, first, a first porous silicon layer having a relatively low porosity is formed by electrolysis on one surface of a silicon single crystal substrate at a relatively low current density. Next, the second porous silicon having a relatively high porosity is formed on the silicon single crystal substrate by anodizing the silicon single crystal substrate on which the first porous silicon layer is formed in place of the relatively high current density. A layer can be formed.

  This anodic oxidation can be performed using, for example, a hydrofluoric acid and alcohol, preferably a mixed solution of hydrofluoric acid and ethyl alcohol as an electrolytic solution. The ratio of hydrofluoric acid to alcohol is not particularly limited, but the ratio of 30% by mass of hydrofluoric acid to alcohol is preferably 1: 1 to 2: 1.

  As the silicon single crystal substrate used as a raw material in the present invention, a substrate doped with boron, phosphorus, antimony, arsenic or other elements can be used. The thickness is preferably 0.3 to 2 mm. When the thickness is less than 0.3 mm, the warpage of the substrate is too large, which is inconvenient. When the thickness is more than 2 mm, the number of slicable substrates is less than that of the silicon single crystal ingot, which is inefficient.

  On the other hand, the porosity and thickness of the first porous silicon layer and the second porous silicon layer formed on the silicon crystal substrate by anodic oxidation can be controlled by changing the current density and time. Specifically, the higher the current density, the higher the porosity due to anodic oxidation, and the longer the time, the thicker the film thickness. In actual implementation, these relationships may be confirmed experimentally, and conditions may be determined based on this.

The first porous silicon layer on the silicon single crystal substrate preferably has a porosity of 0.1 to 3% and a thickness of 0.1 to 2 μm. As an example of the electrolysis conditions for forming the first porous silicon layer satisfying such conditions of porosity and film thickness, the current density is 0.1 to 1 mA / cm ² , preferably 1 mA / cm ² . Can be mentioned.

  The first porous silicon layer thus obtained has a lower porosity and a smaller thickness is suitable for growing a nitride semiconductor thin film. For example, when the porosity is more than 3%, the crystal growth surface is rough due to the change in the pore structure during the growth of the nitride semiconductor thin film, and cracks are generated in the thin film. On the other hand, if the thickness is out of the above range, it is inconvenient because cracks occur due to the structural change of the second porous silicon layer during GaN growth.

The second porous silicon layer has a porosity of 20 to 50%, preferably 30 to 50%, and a thickness of 10 to 60 μm, preferably 20 to 25 μm. Examples of electrolysis conditions for forming the second porous silicon layer having such a porosity and film thickness include a current density of 1 to 50 mA / cm ² .

  By adjusting the porosity and thickness of the second porous silicon layer to the above ranges, the second porous silicon layer can exert an effect on relaxation of lattice strain and thermal stress, and can reduce cracks and warpage. If the thickness of the second silicon layer is less than 10 μm, thermal stress relaxation becomes insufficient and cracks and warpage occur, and if it exceeds 60 μm, the pore structure changes significantly and the nitride semiconductor thin film is cracked and peeled off. Inconvenient.

The second porous silicon layer is a semi-insulator having a resistance of 10 ⁶ Ωcm or more and 10 ⁷ Ωcm or less. For example, in the use of a high-frequency transistor using a GaN crystal, the porous layer is a resistance layer. It is useful because it functions as

  The above-mentioned second porous silicon layer may have a structure in which at least one layer of high porosity and low porosity are alternately laminated within the above-described range of porosity. Such a structure can be easily formed by changing the current density for a certain time during the anodic oxidation for forming the second porous silicon layer. That is, when forming a highly porous layer, the current density may be increased, and when forming a low porosity layer, the current density may be decreased. And the low-porosity layer which exists in the 2nd porous silicon layer which is high porosity suppresses the excessive structural change of a 1st porous silicon layer, and is formed on a 1st porous silicon layer. The deterioration of the crystal quality of the nitride semiconductor thin film can be prevented.

  Further, the second porous silicon layer has a structure in which the second porous silicon layer is changed continuously or stepwise from a high porosity portion to a low porosity portion, or from a low porosity portion to a high porosity portion in the vertical direction. It may be. Such a structure can also be easily formed by changing the current density continuously or stepwise during the anodic oxidation for forming the second porous silicon layer, and has the same effect as described above. Obtainable.

  As described above, the silicon single crystal substrate having the first and second porous silicon layers formed on the surface is further subjected to the treatment for removing the oxide film from the surface of the porous silicon layer and hydrogen termination. . This treatment is for removing an oxide film that is problematic in the growth of nitride semiconductor thin films, and forming a porous silicon layer surface structure that is difficult to oxidize, in order to obtain a surface structure suitable for nitride semiconductor crystal growth. It is important to.

  This treatment can be performed using, for example, hydrofluoric acid, and is particularly preferably performed with about 50 to 40% by mass of hydrofluoric acid. At this time, when hydrofluoric acid having a low concentration is used, porous surface oxidation with water as a diluent solvent and dissolution with hydrogen fluoride are repeated, which is inconvenient because the surface of the porous silicon layer becomes rough.

  FIG. 1 shows a schematic view of one embodiment of the nitride semiconductor forming substrate of the present invention after the above processing is completed. In the figure, 1 is a first porous silicon layer from which an oxide film is removed and hydrogen-terminated, 2 is a second porous silicon layer, and 3 is a silicon crystal substrate.

  As shown in FIG. 1, the substrate for forming a nitride semiconductor of the present invention has a second porous silicon layer 2 and a first porous silicon layer 1 which is oxide-removed and hydrogen-terminated on a silicon crystal substrate 3. Are sequentially provided. The second porous silicon layer 2 has a laminated structure of a high porous layer 2c, a low porous layer 2b, and a high porous layer 2a from the substrate 1 side. By adopting such a laminated structure, the low porous layer 2b can increase the strength of the second porous silicon layer and prevent deformation.

  In order to grow and form the nitride semiconductor thin film on the nitride semiconductor forming substrate of the present invention thus obtained, a known technique can be used, and there is no particular limitation. Known techniques include atmospheric pressure metalorganic vapor phase epitaxy (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), sublimation and liquid phase epitaxy.

  FIG. 2 schematically shows a state in which a GaN crystal is grown on the nitride semiconductor formation substrate of the present invention. The GaN crystal growth step shown in FIG. 2 is performed by atmospheric pressure metal organic chemical vapor deposition (MOCVD), and after an AlN layer 4 is grown and formed as an initial layer on the first porous silicon layer 1. GaN layer 5 is grown and formed.

The nitride semiconductor-forming substrate of the present invention can grow and form various nitride semiconductor thin films. Among these, as preferred examples, the following general formula B _X Al _Y Ga _{(1-X— Y-Z)} In _Z N
(Where 0 ≦ X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1)
The compound represented by these can be mentioned.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in more detail, this invention is not restrict | limited at all by these Examples.

Example 1
Next, a first porous silicon layer having a thickness of 0.46 μm and a second porous silicon layer having a thickness of 24 μm formed on the silicon single crystal substrate below the first silicon layer on one surface of the silicon single crystal substrate. An example of a method for producing a GaN layer on a silicon single crystal substrate on which two types of porous silicon layers comprising:

Boron is doped at 5 × 10 ¹⁸ atoms / cm ³ in a 30% by mass hydrofluoric acid-ethyl alcohol mixed solution (capacity ratio 1: 1), which is an electrolytic solution, and a specific resistance value is 0.01 Ωcm. Using silicon (111) as an anode and a platinum electrode as a cathode, current was passed at a current density of 1 mA / cm ² for 4 minutes to form a first porous silicon layer on P-type silicon. This first porous silicon layer had a porosity of 1% and a thickness of 0.46 μm. The porosity was measured by a nitrogen gas adsorption method.

Thereafter, further 8 minutes at a current density of 21 mA / cm ^2, 4 minutes at a current density of 1 mA / cm ^2, by energizing a current density 21 mA / cm ² 8 min, the silicon single crystal substrate top surface under the first porous silicon layer Then, a second porous silicon layer was formed, and a silicon substrate for forming a nitride semiconductor of the present invention was produced. This second porous silicon layer had a porosity of 50% and a thickness of 24 μm.

  Furthermore, the silicon substrate on which the first and second porous silicon layers were formed was immersed in 50 wt% HF solution for 1 minute to remove the oxide film. As a result, a silicon substrate having a first porous silicon layer whose surface was hydrogen-terminated was obtained.

Next, the produced silicon substrate was set in a reactor of a MOCVD apparatus, and film formation was performed. First, while flowing a mixed carrier gas of nitrogen and hydrogen (nitrogen 10 L / min, hydrogen 7.5 L / min), the silicon substrate temperature was set to 1150 ° C., and the surface of the second porous silicon layer was thermally cleaned. Next, the temperature of the silicon substrate was set to 1120 ° C., trimethylaluminum (TMA) was supplied into the reaction furnace at 17 μmol / min, and NH ₃ was supplied at 2.5 L / min, and a thickness of 50 nm was formed on the first porous silicon layer. An AlN thin film was grown.

Subsequently, the temperature of the silicon substrate was 1120 ° C., trimethylgallium (TMG) was supplied at 69 μmol / min, and NH ₃ was supplied at 2.5 L / min to grow a GaN thin film having a thickness of 1 μm on AlN.

  The grown GaN surface was mirror-like, and the surface was observed using a normalsky microscope (100 times), but no cracks or bits were observed. Further, when a fan-shaped substrate having a regular triangle shape of about 2 cm, a radius of about 2 cm, and a central angle of 60 degrees was measured with a laser length measuring instrument, the warp after forming GaN was 2.6 μm.

Comparative Example 1
A thickness of 50 nm is formed on the silicon substrate in the same manner as in Example 1 except that a silicon substrate (raw material silicon plate) having a cleaned plane orientation (111) is used instead of the porous silicon substrate of the present invention. The AlN thin film and the GaN thin film having a thickness of 1 μm were sequentially grown.

  The grown GaN surface was specular, but cracks were observed when observed using a normalsky microscope (100 times). Further, when the substrate was measured with a laser length measuring device in the same manner as in Example 1, the warpage after forming GaN was 10 μm.

  In the present invention, a nitride semiconductor thin film having excellent surface flatness and crystallinity can be grown on a silicon substrate, and the performance of the nitride semiconductor device on the silicon substrate can be improved.

It is drawing which showed typically the one aspect | mode of the board | substrate for nitride semiconductor formation of this invention. 2 is a drawing schematically showing a state after GaN is crystal-grown on the nitride semiconductor formation substrate of FIG. 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ...... The 1st porous silicon layer from which the oxide film was removed 2 ...... The 2nd porous silicon layer 3 ...... Silicon substrate 4 ...... AlN thin film 5 ...... GaN thin film

Claims (16)

  A second porous silicon layer having a relatively high porosity and a first porous silicon layer having a relatively low porosity removed from an oxide film are sequentially provided on a silicon single crystal substrate. A substrate for forming a nitride semiconductor.   The substrate for forming a nitride semiconductor according to claim 1, wherein the second porous silicon layer is formed by alternately stacking at least one layer having a high porosity and a layer having a low porosity.   The second porous silicon layer is continuously or stepwise changed in a vertical direction from a high porosity portion to a low porosity portion, or from a low porosity portion to a high porosity portion. The substrate for forming a nitride semiconductor according to claim 1.   The substrate for forming a nitride semiconductor according to any one of claims 1 to 3, wherein the porosity of the first porous silicon layer is 0.1 to 3%.   The substrate for forming a nitride semiconductor according to any one of claims 1 to 4, wherein the average porosity of the second porous silicon layer is 20% to 50%.   The nitride according to any one of claims 1 to 5, wherein the first porous silicon layer has a thickness of 0.1 to 2 µm, and the second porous silicon layer has a thickness of 10 to 60 µm. Semiconductor forming substrate.   The substrate for forming a nitride semiconductor according to any one of claims 1 to 6, wherein the silicon single crystal substrate has a thickness of 0.3 to 2 mm.   A nitride semiconductor comprising a nitride semiconductor thin film formed on the semiconductor forming substrate according to any one of claims 1 to 7. The nitride semiconductor thin film has the following formula: B _X Al _Y Ga _(1-XYZ) In _Z N
(Where 0 ≦ X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1)
The nitride semiconductor according to claim 8, which is formed of a compound represented by the formula:   Forming a first porous silicon layer having a relatively low porosity by anodizing a silicon single crystal substrate at a relatively low current density in the electrolyte; a relatively high current in the electrolyte Forming a second porous silicon layer having a relatively high porosity between the silicon single crystal substrate and the first porous silicon layer by anodizing at a density; the first porous silicon layer; (2) A method for manufacturing a substrate for forming a nitride semiconductor, comprising a step of removing an oxide film after forming a porous silicon layer. The method for manufacturing a substrate for forming a nitride semiconductor according to claim 10, wherein a relatively low current density is 0.1 to 1 mA / cm ² . 12. The method for manufacturing a nitride semiconductor forming substrate according to claim 10, wherein the relatively high current density is 1 to 50 mA / cm ² .   The method for manufacturing a nitride semiconductor forming substrate according to any one of claims 10 to 12, wherein the electrolytic solution is a mixed solution of hydrofluoric acid and alcohol.   The oxide film removing step is performed by immersing the silicon single crystal substrate on which the first porous silicon layer and the second porous silicon layer are formed in 50 to 40% by mass of hydrofluoric acid. A method for producing a nitride semiconductor forming substrate according to any one of claims 13 to 14.   15. A nitride semiconductor thin film formed by forming a nitride semiconductor thin film on a nitride semiconductor formation substrate formed by the manufacturing method according to claim 10. Manufacturing method. The nitride semiconductor thin film has the following formula: B _X Al _Y Ga _(1-XYZ) In _Z N
(Where 0 ≦ X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1)
The method for producing a nitride semiconductor thin film according to claim 15, wherein the nitride semiconductor thin film is formed of a compound represented by the formula:

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