JP2008254997A - Single crystal zinc oxide substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high-quality single crystal ZnO substrate at a cost lower than the prior art. <P>SOLUTION: The method for producing a single crystal ZnO substrate comprises (a) the step of providing a semiconductor substrate comprising an SiO<SB>2</SB>insulating layer 2 and a single crystal silicon layer 1 provided on the SiO<SB>2</SB>insulating layer 2 to constitute the surface of the semiconductor substrate, (b) the step of oxidizing the single crystal silicon layer 1 from the surface side to allow the single crystal silicon layer 1 to remain unoxidized in a thickness of 3-7 nm from the top of the insulating layer 2, (c) the step of removing the formed SiO<SB>2</SB>layer 4, (d) the step of supplying a carrier gas and a hydrocarbon gas to the residual single crystal layer 1' with heating to convert the whole layer to a single crystal SiC layer 5, (e) the step of forming a 0.1-5 μm-thick single crystal ZnO layer 6 by chemical vapor deposition on the surface of the single crystal SiC layer 5, (f) the step of annealing the single crystal ZnO layer 6, and (g) the step of forming a single crystal ZnO layer 6' on the surface of the annealed single crystal ZnO layer 6 by chemical vapor deposition to increase the thickness of the single crystal ZnO layer 6. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a single crystal zinc oxide substrate, and more particularly to a single crystal zinc oxide substrate used for construction of a semiconductor device.

  In the semiconductor manufacturing field, wide-gap semiconductors (referred to as semiconductors with large band gaps) are materials responsible for the next generation development of optical semiconductors such as light emitting diodes in the blue, purple and ultraviolet regions, and low-loss power devices. There are great expectations. One of the main wide gap semiconductors is a group II oxide typified by zinc oxide (ZnO). In particular, ZnO is a direct transition type, has a large band gap of 3.37 eV, and excitons. Since the binding energy is as extremely high as 60 meV, there is a great potential for providing a highly efficient light-emitting device with a short wavelength.

  On the other hand, in order for a device to be developed from a certain semiconductor material, commercialized, and widely used, it is important that the semiconductor material can be manufactured with high quality and relatively low cost. Although ZnO is abundant in the crust and is cheap in that respect, it has been supplied to date as a single crystal ZnO substrate with a relatively low dislocation defect density that can be used as a material for constructing semiconductor devices. The only thing that has been achieved is the growth of single crystal ZnO on a sapphire substrate. However, the lattice mismatch between sapphire and ZnO crystal is as large as about 18%. For this reason, the single crystal ZnO thin film on the sapphire substrate is likely to generate grain boundaries and fluctuations in the lattice plane direction, and it is difficult to form a high quality single crystal ZnO thin film. This is one of the obstacles in promoting the development of light emitting devices. In addition, since the sapphire substrate is very expensive, a single crystal ZnO substrate manufactured using the sapphire substrate must be very expensive, and this also means that a device based on the single crystal ZnO substrate is used. This is a major impediment to development.

  In Japanese Patent No. 3183939 (Patent Document 1), a thin film made of silicon carbide (SiC) is formed on the surface of a single crystal silicon substrate in the same apparatus without exposing the single crystal silicon substrate to the atmosphere. A method for producing a single crystal ZnO thin film by forming a single crystal ZnO thin film thereon is disclosed. According to the method described in the document, the single crystal silicon substrate is heated to about 1350 ° C., and a hydrocarbon gas in a carrier gas (hydrogen, etc.) is supplied to the single crystal silicon substrate. Then, a SiC thin film of 0.2 to 3 μm, preferably 2 to 3 μm is formed, and then single crystal ZnO is formed thereon.

  However, according to this method, the present inventor confirmed that the SiC thin film cannot be obtained as a single crystal in the formation stage of the SiC thin film, or even if it is obtained, the crystal is disordered and only a very many defect can be obtained. (Data not shown). For this reason, the single crystal ZnO grown on the obtained SiC thin film also has many defects, and the quality suitable for practical use is not reached.

On the other hand, as a substrate for reducing the leakage current from a silicon layer in a semiconductor device and reducing so-called parasitic capacitance and improving the performance of a transistor, an insulating layer [a silicon dioxide (SiO ₂ ) layer or the like under a single crystal silicon layer An SOI (silicon-on-insulator) substrate is known. As a method of forming a single crystal SiC layer using an SOI substrate, the SOI substrate is heated in a hydrocarbon gas to modify the surface single crystal silicon layer into a single crystal SiC layer, and SiC is epitaxially grown on the layer. There has been proposed a method for this (Patent Document 2).

Further, the SOI substrate is heated in a hydrocarbon gas atmosphere to modify the surface single-crystal silicon layer into a single-crystal SiC layer. In some cases, SiC is epitaxially grown on this surface to grow a single-crystal SiC layer. A method of manufacturing a single crystal ZnO substrate has been proposed, characterized in that ZnO is epitaxially grown thereon to form a single crystal ZnO layer (Patent Document 3, unpublished at the time of filing this application). However, as a result of detailed studies, it is difficult to obtain a high-quality SiC layer if the method of Patent Document 3 is performed as it is, and a single crystal ZnO substrate having a low dislocation defect density cannot be obtained by this method. The present inventors have found that.
Therefore, there is still a need for a technique that can produce a high-quality single crystal ZnO substrate at a relatively low cost.

On the other hand, the single crystal silicon layer on the surface of the SOI substrate is thinned to about 5 nm by sacrificial oxidation, removal by hydrofluoric acid, etc., and this is heated in a hydrocarbon gas atmosphere to convert the single crystal silicon layer on the surface into a single crystal SiC layer. It has also been reported that a high-quality single-crystal SiC substrate can be obtained by modifying the layer into layers and then stacking SiC by epitaxial growth to increase the thickness (see Non-Patent Document 1).
Japanese Patent No. 3183939 JP 2006-228773 A Japanese Patent Application No. 2005-297687 Abstracts of “Symposium on Industry-Academia-Government Collaboration”, pages 1-11, Osaka Prefecture University Industry-Academia-Government Collaboration Organization, February 28, 2006

In the above background, an object of the present invention is to provide a method for producing a high-quality single crystal ZnO substrate at a lower cost than in the past.
Another object of the present invention is to provide a single crystal ZnO substrate which is cheaper and higher quality than the conventional one by such a method.

The inventor has disclosed a substrate having a single crystal silicon layer on the surface and a SiO ₂ insulating layer provided therebelow, by oxidizing the surface of the single crystal silicon layer and removing the thin layer with a thickness of about several nm. Then, by supplying the hydrocarbon gas in the carrier gas to this under heating and converting the entire layer to SiC, a single crystal SiC layer with extremely good flatness is obtained, and on this flat SiC layer It has been found that by using chemical vapor deposition (CVD), a single crystal ZnO layer having a very low dislocation defect density can be easily formed under a predetermined heating condition. The present invention has been completed based on this finding and further studies.

That is, the present invention provides the following.
1. A method for producing a single crystal ZnO substrate, comprising:
(A) providing a semiconductor substrate comprising a SiO ₂ insulating layer and a single-crystal silicon layer constituting the surface provided on the insulating layer;
(B) oxidizing the single crystal silicon layer of the semiconductor substrate from the surface side, leaving the single crystal silicon layer on the insulating layer by a thickness of 3 to 7 nm;
(C) removing the surface-side SiO ₂ layer produced by the oxidation in step (b) from the remaining single crystal silicon layer;
(D) supplying a hydrocarbon gas together with a carrier gas while heating the remaining single crystal silicon layer to convert the entire remaining single crystal silicon layer into a single crystal SiC layer When,
(E) forming a single crystal ZnO layer having a thickness of 0.1 to 5 μm on the surface of the single crystal SiC layer by chemical vapor deposition;
(F) annealing the single crystal ZnO layer formed in step (e) at a predetermined temperature; and (g) performing single vapor chemical vapor deposition on the surface of the single crystal ZnO layer annealed in step (f). Forming a crystalline ZnO layer to increase the overall thickness of the single crystalline ZnO layer;
A method for manufacturing a single crystal ZnO substrate.
2. The manufacturing method according to 1 above, wherein step (d) is performed at a substrate temperature of 1250 ° C. or higher and lower than 1405 ° C.
3. 3. The production method according to any one of 1 and 2 above, wherein the hydrocarbon gas supply amount relative to the carrier gas supply amount in step (d) is 0.1 to 0.8% by volume.
4). 4. The production method according to any one of 1 to 3 above, wherein the carrier gas is hydrogen.
5. The method according to any one of 1 to 4 above, wherein the formation of the single crystal ZnO layer in steps (e) and (g) is by supplying a zinc source gas and an oxygen source gas to the substrate while heating the substrate. .
6). The production method according to 6 above, wherein the zinc source gas comprises di (lower) alkylzinc.
7). 7. The method according to 5 or 6 above, wherein the oxygen source gas is selected from the group consisting of oxygen, carbon dioxide, water, carbon monoxide, nitrogen dioxide, and nitric oxide.
8). 8. The production method according to any one of 1 to 7 above, wherein the formation of the single crystal ZnO layer in step (e) is performed at a substrate temperature of 300 ° C. or higher and lower than 800 ° C.
9. 9. The manufacturing method according to any one of 1 to 8 above, wherein the annealing in step (f) is performed at a substrate temperature of 500 ° C. or higher and lower than 1000 ° C.
10. 10. The manufacturing method according to any one of 1 to 9 above, wherein the formation of the single crystal ZnO layer in step (g) is performed at a substrate temperature of 400 ° C. or higher and lower than 900 ° C.
11. 11. The manufacturing method according to any one of 1 to 10 above, wherein the semiconductor substrate comprises a base layer supporting the semiconductor substrate under the SiO ₂ layer.
12 12. The production method as described in 11 above, wherein the base layer is a single crystal or polycrystalline silicon layer.
13. The base layer, the base layer and the SiO ₂ layer, from the base layer to the single crystal SiC layer, or from the base layer, including all layers of the single crystal ZnO layer formed in step (e); The manufacturing method according to 11 or 12 above, further including a step of removing a part of the thickness of the single crystal ZnO layer formed from the interface side with the single crystal SiC layer.
14 14. The manufacturing method as described in 13 above, wherein the partial thickness corresponds to a portion from the interface with the single crystal SiC layer to a thickness within 10 μm.
15. 15. The method according to 13 or 14 above, further comprising a step of forming a light reflecting layer on the back surface of the substrate left by the removal.
16. 14. The production method as described in 14 above, wherein the light reflecting layer is selected from the group consisting of aluminum, silver or alloys thereof.
17. 16. The production method as described in 16 above, wherein at least the part from the base layer to the SiO ₂ layer is removed.
18. 18. The method according to 17 above, further comprising the step of forming a solderable metal layer on the back surface of the light reflecting layer.
19. 18. The production method as described in 18 above, wherein the solderable metal comprises a metal selected from the group consisting of copper, nickel and alloys thereof.
20. A single crystal ZnO substrate obtained by the production method according to any one of 1 to 19 above.
21. 20. The single crystal ZnO substrate according to the above 20, wherein the single crystal ZnO layer has a dislocation defect density of 10 ⁷ / cm ² or less.

The present invention configured as described above enables formation of a high-quality single-crystal ZnO layer (the surface is a (0001) plane). This is not only because the lattice constant difference between single crystal ZnO and single crystal SiC is smaller than the lattice constant difference with sapphire, but as another factor, in the present invention, the single crystal on the SiO ₂ insulating layer A layer obtained by forming the silicon layer into an ultra-thin film of 3 to 7 nm and converting it to SiC gives an extremely flat SiC surface as compared with the case where the thicker single crystal silicon layer is converted to SiC (the present invention). Confirmed by the person). That is, the flatness of this ultra-thin SiC surface (becomes the (111) plane of 3C-SiC) is significantly roughened in proportion to the layer thickness when SiC is further epitaxially grown thereon to increase the layer thickness. The inventor has also found that the surface roughness increases by about 1 nm (RMS) every time the layer thickness increases by 10 nm, compared to the initial surface roughness of about 0.2 nm (RMS). Therefore, although the layer obtained by converting the single crystal silicon layer on the SiO ₂ insulating layer into an ultra-thin film of 3 to 7 nm and converting it to SiC does not have strength due to its extremely thin thickness, an SiC layer is further added thereto. Stacking and using as it is for the growth of single crystal ZnO without increasing the thickness and strength contributes to obtaining a single crystal ZnO layer of excellent quality. In addition, this ultra-thin SiC layer is easily stretchable to some extent, and the SiO ₂ insulating layer that supports the layer from the bottom is amorphous and has flexibility under heating, so that it prevents some stretching of the SiC layer. Absent. For these reasons, the lattice mismatch between the ultra-thin SiC and the single crystal ZnO grown thereon can be relaxed by a slight shrinkage of the crystal lattice of the SiC layer. This, combined with the high flatness of the surface of the ultra-thin SiC layer, significantly reduces the dislocation defect density of the single-crystal ZnO grown on it to provide a high-quality single-crystal ZnO layer. A big contribution. Thus, the present invention makes it possible to manufacture a high-quality ZnO single crystal substrate having a dislocation defect density of 10 ⁷ / cm 2 or less.

Further, the present invention uses an SOI substrate in which a SiO ₂ insulating layer and a single crystal silicon layer are laminated on a base layer made of single crystal or polycrystalline silicon, and thus can be obtained at a much lower price than a sapphire substrate. Therefore, a high-quality single crystal ZnO substrate can be provided at a much lower cost than before. In addition, since such a silicon-based SOI substrate has a much larger diameter than a substrate made of a single crystal ZnO layer formed on the sapphire surface, it can be obtained at a lower price. In addition, a high-quality single crystal ZnO substrate can be manufactured at low cost.

  Furthermore, when a semiconductor substrate having a non-light transmissive base layer (single crystal silicon, polycrystalline silicon, etc.) in the present invention is used as a material, the entire light transmission is achieved by removing the non-light transmissive base layer. It is possible to inexpensively manufacture a single crystal ZnO substrate that is compatible.

  In addition, the formation of the light reflecting layer according to the present invention includes, when a light emitting diode is configured on a single crystal ZnO substrate obtained thereby, out of light emitted from the light emitting layer in all directions (solid angle 4π), By changing the direction of the light going backward (inside the device to which the light emitting diode is attached) to the front (solid angle 2π), the light extraction efficiency is increased, thereby preventing the temperature of the light emitting diode from rising. Make it possible. In addition, as the reflective layer, use of aluminum, silver, or an alloy thereof is extremely effective because it is a metal having high reflection efficiency for visible light including a blue wavelength region. In addition, when a metal layer that can be soldered is further provided on the reflective layer, it becomes possible to use this as an electrode while improving the light extraction efficiency. It is possible to increase without impairing the efficiency of light generation and extraction.

The semiconductor substrate used for the production of the single crystal ZnO substrate of the present invention is not particularly limited as long as it is an SOI substrate in which the surface single crystal silicon layer is supported by the SiO ₂ insulating layer. A preferred example of an SOI substrate that is relatively inexpensive and readily available is one having a SiO ₂ insulating layer under the surface of a single crystal silicon substrate and a silicon crystal layer as a base layer that supports the SiO ₂ insulating layer (Si / SiO ₂ / Si). In this case, in the present invention, any substrate on which a single crystal silicon layer is formed on an insulating layer can be used. Therefore, even if the base layer located under and supporting the SiO ₂ layer is single crystal silicon. Crystalline silicon may be used.

[Sacrificial oxidation of single-crystal silicon layer on the surface]
FIG. 1 (a) conceptually illustrates a cross section of an example of the SOI substrate that is currently available at a relatively low cost (note that the thickness of each layer in the figure does not correlate with the actual thickness). In this substrate, a single crystal silicon layer 1 of approximately 45 nm is typically formed on the SiO ₂ insulating layer 2 and constitutes the (upper) surface of the semiconductor substrate. Under the SiO ₂ insulating layer 2, there is a silicon crystal layer (single crystal or polycrystal) which is a base layer that supports the insulating layer 2. SiO ₂ insulating layer 2 Oxidation of single crystal silicon layer 1 on the upper surface may be performed by any method known in the art as sacrificial oxidation. An oxidation process is performed on the single crystal silicon layer 1 on the surface of the SOI substrate so as to leave an unoxidized single crystal silicon layer 1 'on the SiO ₂ insulating layer 2 with a thickness of 3 to 7 nm (FIG. 1B). See). For this purpose, each condition of the oxidation step may be appropriately set in a manner commonly used by those skilled in the art, and such adjustment is a daily work item for those skilled in the art. After setting the conditions, the material substrate may be processed under the same conditions. For example, this is performed by flowing oxygen gas at 1000 cc / min for about 110 minutes at a substrate temperature of 1000 ° C. under atmospheric pressure or by flowing oxygen gas at 1000 cc / min for about 40 minutes at a substrate temperature of 1100 ° C. Can do. The temperature, oxygen supply amount, and processing time may be adjusted as appropriate according to the thickness of the surface single crystal silicon layer. When single crystal silicon is oxidized to form a SiO ₂ layer, the layer thickness increases about 2.25 times. Therefore, since the increment of the thickness after the thickness is oxidized can be calculated from the thickness to be oxidized in the single crystal silicon layer, this may be used as an index in the condition setting.

[Removal of sacrificial oxide layer]
The removal of the SiO ₂ sacrificial layer 4 on the surface caused by the above treatment may be performed using any of the well-known methods commonly used for removing SiO ₂ on the surface of single crystal silicon in the semiconductor industry. Typically, for example, it can be performed by immersing in hydrofluoric acid (having an HF concentration of, for example, 1 to 50%). The immersion time depends on the thickness of the SiO ₂ sacrificial layer and the HF concentration of hydrofluoric acid. When the thickness of the SiO ₂ sacrificial layer is 100 nm and the HF concentration of hydrofluoric acid is 10%, the immersion time is 10 at room temperature. It may be about minutes. Further, instead of hydrofluoric acid, buffer hydrofluoric acid (a solution obtained by adding ammonia to an HF aqueous solution, commercially available) may be used. After cleaning, a substrate having a clean thin single crystal silicon layer 1 'of 3 to 7 nm on the surface is obtained (see FIG. 1 (c)).

[Conversion of single crystal silicon layer to single crystal SiC layer]
The conversion of the single crystal silicon layer 1 ′ having a thickness of 3 to 7 nm on the surface obtained by the above treatment into a single crystal SiC layer is carried out while heating the substrate to a temperature of 1250 ° C. or higher and lower than 1405 ° C. Can be carried out by contacting with hydrogen gas) and hydrocarbon gas. Only when the single crystal silicon layer 1 ′ is very thin as 3 to 7 nm and is on the SiO ₂ insulating layer 2, an extremely high quality single crystal SiC layer 5 (3C-SiC. The surface is the (111) plane) is easy. (See FIG. 1 (d)). If the monocrystalline silicon layer remains thicker than this, even if it is treated with hydrocarbon / carrier, a flat SiC layer cannot be obtained, and even if the treatment time is prolonged, SiC grows in a granular form and becomes polycrystalline. There is a tendency to become. The substrate temperature during the treatment is more preferably 1290 to 1390 ° C., still more preferably 1320 to 1380 ° C., for example, about 1350 ° C. The hydrocarbon gas to be used may be appropriately selected from those that are easily available, and is not particularly limited, but it is usually convenient to use propane. The treatment may be performed under atmospheric pressure to vacuum, but is preferably performed under atmospheric pressure. The volume ratio of hydrogen gas to the amount of carrier gas is preferably 0.1 to 0.8%, more preferably 0.2 to 0.7%, and usually, for example, around 0.5%. Just set it up. The supply amounts of the carrier gas and the hydrocarbon gas may be appropriate, and may be, for example, about 10,000 cc / min and about 50 cc / min, respectively, under atmospheric pressure.

[Formation of single-crystal ZnO layer]
For the formation of the single crystal ZnO layer by chemical vapor deposition on the single crystal SiC layer 5 formed as described above, appropriate materials well known to those skilled in the art can be used as a zinc source and an oxygen source, and particularly limited. Not. Examples of the zinc source include di (lower) alkylzinc (herein, “lower” means one having 1 to 6 carbon atoms). Specific examples include dimethyl zinc, diethyl zinc, diisopropyl zinc, dibutyl zinc, di (sec-butyl) zinc and the like. Examples of the oxygen source include oxygen, carbon dioxide, water, carbon monoxide, nitrogen dioxide, and nitric oxide. The formation of the single crystal ZnO layer is performed according to the following steps (1) to (3).

(1) Formation of single-crystal ZnO buffer layer The formation of a single-crystal ZnO layer (the surface is the (0001) plane) has a predetermined thickness within the range of 0.1 to 5 μm (setting may be appropriate) ), The substrate temperature may be set as appropriate, but is preferably 300 ° C. or higher, more preferably 350 ° C. or higher, particularly preferably 500 ° C. or higher, and lower than 1200 ° C., more preferably lower than 800 ° C. More preferably, it is kept below 750 ° C., particularly preferably below 700 ° C. (this single crystal ZnO layer forming step is referred to as “buffer layer forming step”) (see FIG. 1 (e)). A suitable example of the substrate temperature in the buffer layer forming step is, for example, about 500 ° C. Performing the treatment in such a temperature range is important in preventing the surface form of the single crystal ZnO layer (buffer layer 6) formed first on the SiC layer 5 from being disturbed.

  The buffer layer forming step may be performed under atmospheric pressure to vacuum. For example, when the treatment is performed under vacuum, the supply amount of the ZnO source (for example, di (lower) alkylzinc) is preferably 0.1 to 10 cc / min, and the supply amount of the oxygen source is 10 to 1000 cc / min. It is preferable to do this. For example, the supply amount of the ZnO source may be about 1 cc / min, that of the oxygen source may be about 100 cc / min. Further, when the treatment is performed under vacuum, the growth pressure is preferably 0.01 to 10 Pa, for example, about 0.1 Pa.

(2) Annealing The buffer layer 6 is then subjected to annealing in order to adjust the disturbance that may exist in the single crystal ZnO crystal (FIG. 1 (f)). The substrate temperature during annealing may be set as appropriate, but is preferably 500 ° C. or more, more preferably 600 ° C. or more, and less than 1405 ° C., preferably less than 1200 ° C., more preferably less than 1000 ° C., and particularly preferably 800 ° C. Is less than. A suitable example of the substrate temperature during annealing is 650 ° C. Annealing is preferably performed in an oxygen atmosphere. Annealing may be performed at any pressure from atmospheric pressure to vacuum. When annealing is performed under vacuum, the supply amount of oxygen gas is preferably 100 to 10000 cc / min, and the pressure is 0.1 to 100 Pa. Annealing is suitable for a short time treatment at a high temperature. For example, it is preferable to use a lamp heating type heat treatment furnace. Note that the SiO ₂ layer derived from the Si / SiO ₂ / Si type SOI substrate has an effect of suppressing the occurrence of warpage of the substrate during annealing (particularly when the temperature is lowered), which is also a high-quality single crystal ZnO substrate. Is particularly advantageous in the formation of

(3) Formation of single-crystal ZnO thick film After the buffer layer 6 is annealed, a thick film is formed by further forming a single-crystal ZnO layer by chemical vapor deposition (this step is referred to as a “thick film formation step”). (FIG. 1 (g)). By this step, a thick single crystal ZnO layer 6 ′ [surface is (0001) plane] including the buffer layer 6 formed first is formed. In the thick film forming step, the single crystal ZnO layer can be grown by appropriately setting the substrate temperature (see FIG. 1 (g)). The substrate temperature is preferably 400 ° C. or more, more preferably 500 ° C. or more, and preferably less than 1405 ° C., more preferably less than 900 ° C., still more preferably less than 800 ° C., still more preferably less than 700, particularly preferably It is less than 600 ° C.

  Similar to the buffer layer forming step, the thick film forming step may be performed under atmospheric pressure to vacuum. For example, when the treatment is performed under vacuum, the supply amount of the ZnO source (for example, di (lower) alkylzinc) is preferably 1 to 100 cc / min, and the supply amount of the oxygen source is 100 to 10000 cc / min. Is preferred. When processing is performed under vacuum, the growth pressure is preferably 0.1 to 100 Pa, for example, about 1.0 Pa. The thickness of the single crystal ZnO layer to be formed in the thick film forming step may be appropriately set according to the various specific uses, and is not particularly limited. However, the thickness of 100 to 300 μm is suitable for the substrate. It is usually preferable to provide sufficient production efficiency.

Although not essential, the single crystal ZnO layer 6 ′ formed in the buffer layer forming step and the thick film forming step may then be annealed if desired. In the case of annealing, the conditions are the same as those described above for the annealing of the buffer layer. Also in this case, the SiO ₂ layer has an effect of suppressing the occurrence of warpage of the substrate during annealing (particularly when the temperature is lowered), and is particularly advantageous in the formation of a high-quality single crystal ZnO substrate.

A single crystal ZnO layer 6 ′ having a dislocation defect density of 10 ⁷ / cm ² or less, preferably 10 ⁶ / cm ² or less can be formed through the processing steps under a series of conditions in the above appropriate range.

A single crystal ZnO substrate (ZnO / SiC / SiO ₂ / Si substrate) manufactured using an SOI substrate made of Si / SiO ₂ / Si as a starting material can be used as it is, but a portion from the back side to a desired position May be removed. For removal, chemical mechanical polishing or any other appropriate method known to those skilled in the art may be used. In the case where each layer is sequentially removed, for example, the following method can be used.

[Removal of silicon base layer]
A single crystal ZnO substrate (ZnO / SiC / SiO ₂ ) that is transparent to light by removing the opaque silicon crystal layer 3 (base layer: the thickness usually depends on the diameter of the wafer used, 200 to 800 μm). ) Can be obtained (see FIG. 2 (h)). The removal of the silicon crystal layer 3 can be performed using a KOH aqueous solution. For example, it is preferably 5 to 50% by weight, more preferably 20% by weight in a KOH aqueous solution, and 60 to 95 ° C., more preferably 80 ° C. Can be carried out by dipping in When a 20 wt% KOH aqueous solution (80 ° C.) is used for an 8-inch diameter SOI substrate (silicon layer thickness: about 800 μm), the immersion time of the substrate is about 800 minutes (etching rate: about 1 μm / min) ). Prior to the treatment with the aqueous KOH solution, the silicon layer may be polished by polishing (for example, chemical mechanical polishing) until the silicon layer has a thickness of about 100 μm, for example.

[Removal of SiO ₂ insulating layer]
In addition to the silicon crystal layer, the SiO ₂ insulating layer 2 may be further removed. Since the back surface of the single crystal ZnO substrate obtained by removing the SiO ₂ insulating layer 2 becomes a SiC surface (FIG. 2 (i)) and becomes conductive, it becomes possible to directly bond an electrode to the back surface. The method for removing the SiO ₂ layer is the same as that described above for removing the SiO ₂ sacrificial layer. The thickness of the SiO ₂ insulating layer in a commercially available SOI substrate is about 150 nm, and if it is immersed in 10% hydrofluoric acid, it can be removed in about 15 minutes.

[Removal of SiC layer]
If desired, the SiC layer 5 may also be removed, whereby a substrate made of only single-crystal ZnO is obtained (see FIG. 2 (j)). For example, dry etching can be used to remove the SiC layer 5. For example, the dry etching is performed by supplying CF ₄ gas to the SiC layer 5 at a supply rate of 0.1 to 100 cc / min (for example, 10 cc / min) in a vacuum and performing arc discharge using parallel plates or the like. be able to. In this case, the vacuum value of the chamber may be 10 to 10000 Pa, preferably about 100 Pa. Note that the SiC layer may be removed by polishing (for example, chemical mechanical polishing) instead of dry etching.

[Partial removal of single crystal ZnO layer]
If desired, the back surface side of the single crystal ZnO layer 6 ′ may be removed by a certain thickness. The thickness to be removed is preferably stopped within 10 μm on condition that the portion that was originally the buffer layer 6 is included. The removal may be performed in excess of 10 μm, but a sufficiently high-quality back surface will already appear if the removal is within 10 μm, and there is no particular advantage in removing it beyond that. The removal of the back surface side of the single crystal ZnO layer 6 ′ may be performed by chemical mechanical polishing, for example.

[Formation of reflective layer and electrode layer]
A reflective layer may be formed on the back surface of the single crystal ZnO substrate after removing at least the opaque layer (silicon crystal layer 3). The reflective layer forming material is preferably a metal, particularly preferably aluminum, silver or an alloy thereof. For example, sputtering may be used to form the reflective layer made of metal, and the method is well known and commonly used by those skilled in the art, and the operating conditions and the like may be set as appropriate. When a metal layer is formed on the back surface from which at least the SiO ₂ insulating layer 2 has been removed, the metal layer can be used as an electrode layer as well as a reflective layer. Moreover, since soldering is enabled on the surface of the metal layer which consists of aluminum, silver, or these alloys, the metal layer suitable for this can further be formed. Representative examples of such metals include copper, nickel or alloys thereof. As long as soldering is possible, it is also allowed that metals other than these metals are contained simultaneously. For the formation of the metal layer for enabling soldering, for example, sputtering may be used as in the formation of the reflective layer.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not intended to be limited to the examples.

(1) Formation and removal of oxide film on surface layer of SOI substrate A commercially available SOI substrate [UNIBOND (registered trademark), manufactured by Shin-Etsu Semiconductor Co., Ltd., diameter 200 mm, surface single crystal silicon layer thickness 45 nm] was procured. This was put into an electric furnace, and O ₂ gas was allowed to flow for 180 minutes at a supply rate of 10,000 cc / min at 1 atm while being heated to 1000 ° C. The SOI substrate was taken out from the electric furnace and immersed in an aqueous HF solution (5% by weight) for 10 minutes at room temperature. The substrate is washed with ultrapure water by a conventional method and dried. The thickness of the single crystal silicon layer on the SiO ₂ insulating layer was 5 nm (measured by spectroscopic ellipsometry).

(2) Modification of surface Si layer to SiC layer The SOI substrate subjected to the above treatment is placed in a lamp heating type processing furnace, and propane is diluted with hydrogen at 1350 ° C. The entire surface single crystal silicon layer is converted to a SiC layer by holding the film in 0.5 volume%) for 15 seconds. The thickness of the obtained single crystal SiC layer was 7 nm.

(3) Formation of ZnO buffer layer on SiC layer The modified substrate is put in a CVD apparatus, diethyl zinc (1 cc / min) is used as a Zn source, and oxygen gas (10 cc / min) is used as an oxygen source. A treatment was performed at a temperature of 500 ° C. and an atmospheric pressure of 0.1 Pa for 5 minutes to grow a ZnO film having a thickness of about 0.3 μm on the single crystal SiC surface.

(4) Annealing Next, while heating the substrate to 650 ° C. in a lamp-type heat treatment furnace, annealing was performed at 1 atm for 1 minute while supplying oxygen gas at 10,000 cc / min.

(5) Formation of thick ZnO layer on ZnO buffer layer Subsequent to (3) above, substrate temperature is 600 ° C. using diethyl zinc (10 cc / min) as a Zn source and oxygen gas (100 cc / min) as an oxygen source. Then, treatment is performed at a pressure of 1 Pa for 240 minutes to grow a ZnO single crystal layer to a thickness of about 200 μm on the ZnO buffer layer. Thereby, a ZnO single crystal substrate having a dislocation defect density of 1 × 10 ⁷ / cm ² is obtained.

  In the step of forming the ZnO buffer layer (sample ZnO-1) formed in (3) above, the buffer layer (sample ZnO-2) after the annealing in (4) above, and the thick film ZnO layer in (5) above The crystallinity was measured by X-ray diffraction for each of the ZnO layers (sample ZnO-3) at the stage where the thickness was grown to about 4 μm. The results are shown in FIG. In the figure, the vertical axis indicates the half width. The figure shows that after the buffer layer is formed, the crystallinity is rapidly improved by annealing, and the crystallinity is further improved by the subsequent lamination of ZnO. 4 and 5 are photographs substituted for drawings, showing cross sections of samples ZnO-1 and ZnO-3, respectively. From these figures, it can be confirmed that a uniform and flat ZnO layer is formed by stacking ZnO single crystals on the buffer layer after annealing.

(6) Removal of silicon layer, etc. For the ZnO single crystal substrate (ZnO / SiC / SiO ₂ / Si) obtained as described above, from the Si layer, the Si layer and the SiO ₂ layer, and from the Si layer to the SiC layer by the following method Or even part of the ZnO layer is optionally removed.
(A) Removal of Si layer: The ZnO single crystal substrate is immersed in a 20 wt% KOH aqueous solution at 80 ° C. (etching rate: about 1 μm / min). Alternatively, the Si layer is removed by chemical mechanical polishing.
(B) Removal of SiO ₂ layer insulating layer: The single crystal ZnO substrate after removal of the Si layer is immersed in a 10% by weight HF aqueous solution at room temperature (removal rate of about 10 nm / min). Alternatively, the SiO ₂ insulating layer is removed by chemical mechanical polishing, or both methods are used in combination.
(C) Removal of SiC layer: The SiC layer is formed by introducing CF ₄ gas into a vacuum (for example, 100 Pa) at 10 cc / min by dry etching and causing arc discharge by parallel plates. Alternatively, the SiC layer is removed by chemical mechanical polishing, or both methods are used in combination.
(D) The back side surface of the ZnO layer is removed by chemical mechanical polishing by a thickness of about 8 μm.

  According to the present invention, a single crystal ZnO substrate having a very low dislocation defect density can be manufactured. In addition, it is possible to manufacture a single-crystal ZnO substrate having an extremely low dislocation defect density, in which all layers are light-transmitting single crystal ZnO substrates and a reflective layer and / or an electrode layer are provided on the back surface. In addition, it is possible to manufacture a single crystal ZnO substrate that is extremely inexpensive and has a large diameter and extremely low dislocation defect density.

Conceptual diagram showing the manufacturing process of a ZnO substrate from an SOI substrate Schematic showing the manufacturing process of the ZnO substrate with the back side layer removed The graph which shows the X-ray-diffraction measurement result of sample ZnO-1, ZnO-2, and ZnO-3 Drawing substitute photograph showing cross section of sample ZnO-1 Drawing substitute photograph showing cross section of sample ZnO-3

Explanation of symbols

1, 1 ′ = single crystal silicon layer, 2 = SiO ₂ insulating layer, 3 = silicon crystal layer, 4 = SiO ₂ sacrificial layer, 5 = single crystal SiC layer, 6, 6 ′, 6 ″ = single crystal ZnO layer

Claims (21)

A method for producing a single crystal ZnO substrate, comprising:
(A) providing a semiconductor substrate comprising a SiO ₂ insulating layer and a single-crystal silicon layer constituting the surface provided on the insulating layer;
(B) oxidizing the single crystal silicon layer of the semiconductor substrate from the surface side, leaving the single crystal silicon layer on the insulating layer by a thickness of 3 to 7 nm;
(C) removing the surface-side SiO ₂ layer produced by the oxidation in step (b) from the remaining single crystal silicon layer;
(D) supplying a hydrocarbon gas together with a carrier gas while heating the remaining single crystal silicon layer to convert the entire remaining single crystal silicon layer into a single crystal SiC layer When,
(E) forming a single crystal ZnO layer having a thickness of 0.1 to 5 μm on the surface of the single crystal SiC layer by chemical vapor deposition;
(F) annealing the single crystal ZnO layer formed in step (e) at a predetermined temperature; and (g) performing single vapor chemical vapor deposition on the surface of the single crystal ZnO layer annealed in step (f). Forming a crystalline ZnO layer to increase the overall thickness of the single crystalline ZnO layer;
A method for manufacturing a single crystal ZnO substrate.   The method according to claim 1, wherein step (d) is performed at a substrate temperature of 1250 ° C or higher and lower than 1405 ° C.   The production method according to claim 1, wherein the hydrocarbon gas supply amount with respect to the carrier gas supply amount in step (d) is 0.1 to 0.8% by volume.   The production method according to claim 1, wherein the carrier gas is hydrogen.   The production according to any one of claims 1 to 4, wherein the formation of the single crystal ZnO layer in steps (e) and (g) is by supplying a zinc source gas and an oxygen source gas to the substrate while heating the substrate. Method.   The production method according to claim 6, wherein the zinc source gas comprises di (lower) alkylzinc.   The method according to claim 5 or 6, wherein the oxygen source gas is selected from the group consisting of oxygen, carbon dioxide, water, carbon monoxide, nitrogen dioxide, and nitric oxide.   The manufacturing method according to any one of claims 1 to 7, wherein the formation of the single crystal ZnO layer in step (e) is performed at a substrate temperature of 300 ° C or higher and lower than 800 ° C.   The manufacturing method according to any one of claims 1 to 8, wherein the annealing in step (f) is performed at a substrate temperature of 500 ° C or higher and lower than 1000 ° C.   The method according to any one of claims 1 to 9, wherein the formation of the single crystal ZnO layer in step (g) is performed at a substrate temperature of 400 ° C or higher and lower than 900 ° C. 11. The manufacturing method according to claim 1, wherein the semiconductor substrate includes a base layer that supports the semiconductor substrate under the SiO ₂ layer.   The manufacturing method according to claim 11, wherein the base layer is a monocrystalline or polycrystalline silicon layer. The base layer, the base layer and the SiO ₂ layer, from the base layer to the single crystal SiC layer, or from the base layer, including all layers of the single crystal ZnO layer formed in step (e); The method according to claim 11 or 12, further comprising the step of removing a part of the thickness of the single-crystal ZnO layer formed from the interface side with the single-crystal SiC layer.   The manufacturing method according to claim 13, wherein the partial thickness corresponds to a portion from the interface with the single crystal SiC layer to a thickness within 10 μm.   The manufacturing method of Claim 13 or 14 which further includes the step of forming a light reflection layer in the back surface of the board | substrate left by the said removal.   The manufacturing method according to claim 14, wherein the light reflecting layer is selected from the group consisting of aluminum, silver, or an alloy thereof. It is designed to remove at least the base layer to the SiO ₂ layer, the manufacturing method of claim 16.   The manufacturing method according to claim 17, further comprising forming a solderable metal layer on the back surface of the light reflecting layer.   The method according to claim 18, wherein the solderable metal comprises a metal selected from the group consisting of copper, nickel and alloys thereof.   A single crystal ZnO substrate obtained by the manufacturing method according to claim 1. 21. The single crystal ZnO substrate according to claim 20, wherein the single crystal ZnO layer has a dislocation defect density of 10 ⁷ / cm ² or less.