JP2005314121A - Single crystal sapphire substrate, its producing method, and semiconductor light-emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form irregularities or recesses ranging from micrometer order to sub-micrometer order on the surface of a sapphire substrate with good productivity by using porous anodically oxidized alumina without using expensive equipment having a complicated optical system, such as the conventional stepper. <P>SOLUTION: In the production of a single crystal sapphire substrate having a plurality of irregularities or recesses on its main surface, the irregularities or recesses are formed by performing etching using the porous anodically oxidized alumina formed on the single crystal sapphire substrate as a mask. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a single crystal sapphire substrate for a semiconductor device, a method for manufacturing the same, and a semiconductor light emitting device. In particular, the semiconductor light emitting device is composed of a GaN-based semiconductor crystal including LED devices such as blue LEDs, ultraviolet LEDs, and white LEDs. About things.

  GaN-based compound semiconductors are attracting attention not only for optical devices but also for high-frequency devices for communication and environmentally resistant devices. For epitaxial growth of GaN-based compound semiconductor crystals, sapphire is generally used as a crystal growth substrate. Originally, a single crystal sapphire substrate is exposed to a specific crystal plane on the surface, then polished to a mirror surface state by mechanical polishing and chemical polishing, and a group III nitride compound used for a light emitting device or the like thereon A semiconductor element is deposited.

  However, only by chemical polishing, there is still a lattice mismatch between the crystal growth substrate and the GaN-based compound semiconductor crystal, which causes dislocations (see Patent Document 1).

For example, when a GaN film or an AlN film acting as a buffer layer is formed by epitaxial growth by low-temperature CVD, and an AlxGayInzN film containing Al as an essential component is epitaxially grown on the GaN film or AlN film, dislocations are greatly increased. The dislocation density of such an AlxGayInzN film reaches, for example, 109 cm ² .

  When the dislocation density is high as described above, this constitutes a light absorption center, and the characteristics of the device deteriorate. In particular, it becomes a serious problem in an optical device such as a laser diode that requires high efficiency. In addition, since such dislocations cause deterioration of the pn junction, reduction of dislocations becomes a serious problem even when an electronic device is manufactured.

  Therefore, various studies have been made to minimize dislocations caused by the difference in lattice constant between the crystal growth substrate and the deposited crystal.

  As a result, it has become clear that a substrate having stripe-shaped grooves on the surface of a film-forming substrate is advantageous for epitaxial growth of a gallium nitride semiconductor compound (see Patent Document 1 and Patent Document 2).

  When a gallium nitride-based compound is epitaxially grown on this substrate, a film including a region having a small dislocation density can be formed by growing in the lateral direction from the convex portion of the stripe-shaped groove.

  An LED element manufactured using a crystal layer having a low dislocation density manufactured in this manner can increase the light emission efficiency by increasing the amount of light generated in the light emitting layer of the LED, that is, by increasing its internal quantum efficiency. .

  Further, by forming irregularities or depressions on the surface of the sapphire substrate and epitaxially growing a gallium nitride compound thereon to produce an LED element, the extraction efficiency of light emitted from the light emitting layer of the LED, so-called external quantum efficiency, is improved. It has been found that the luminous efficiency can be increased by increasing the thickness (see Patent Document 3 and Non-Patent Document 1).

  As a method for forming such fine irregularities or depressions on a sapphire substrate, an optical lithography technique has been generally used. In particular, a reduction exposure apparatus (stepper) using an ultraviolet light source has been mainly used as a lithography technique in recent years, and the wavelengths of the light source are 436 nm for ultra high pressure mercury g-line, 365 nm for i line, and 248 nm for KrF excimer laser. The resolution has been improved by shortening. Furthermore, an ArF excimer laser (wavelength 193 nm) with a short wavelength was also developed. However, these steppers require very complicated optical systems, and the equipment price is very expensive, making it difficult to reduce costs. For this reason, it is necessary to establish a lithography technique that uses simpler equipment, has high mass productivity, and forms irregularities or depressions on the order of microns to submicrons on the surface of the sapphire substrate at low cost.

  Therefore, as a submicron lithography method, a nanolithography method such as a nanoimprint method has been proposed instead of the photolithography method using a stepper (see Patent Document 4).

  According to such a nanolithography method, it is possible to form irregularities or depressions at a relatively low cost compared to a photolithography method using a stepper, but even in that case, the cost of the subsequent dry etching process is still high. Remains.

  In order to solve such problems, the present inventors have found that a method of etching a sapphire substrate using a porous anodized alumina coating as a mask is useful as a microfabrication method for forming irregularities or depressions of micron to submicron order on the surface of sapphire. I thought.

  As a porous material having a uniform pore diameter, a conventional porous anodized alumina film is known (see Patent Document 5, Patent Document 6, and Non-Patent Document 2).

However, there has been no example of using this as a mask for etching sapphire, and there has been no example of using porous anodized alumina in the manufacturing process of a semiconductor light emitting device.
JP2001-210598 JP2001-274093 JP 2002-280611 JP 2002-289560 JP 10-121292 A JP 2000-1393 Nikkei Electronics March 31, 2003 issue p.128-p.133 Applied Physics Vol.72, No.10 (2003)

  Thus, the present inventor considered that a method of etching a sapphire substrate using a porous anodized alumina coating as a mask was useful as a microfabrication method for forming irregularities or depressions on the order of microns to submicrons on the surface of sapphire. However, porous anodized alumina is generally chemically unstable and has a problem that performance as a mask material for etching is insufficient as it is.

  Further, as disclosed in Patent Document 6 as a prior art, as a method for producing a porous diamond body, there is known a method in which a porous anodized alumina film is placed on a diamond substrate as a mask and dry etching is performed. However, in this case, the porous anodized alumina film is not strictly flat and has warpage and undulation, so it is not in close contact with the diamond substrate, and the regular structure of the mask is transferred as it is because the etchant wraps around the back side of the mask. There was a problem that it was easy to become an irregular shape.

  Further, when wet etching is performed by this method, the problem of wraparound is further increased, and there is a problem that practical etching cannot be performed.

  Therefore, as a result of researches to solve the above problems, the present inventors have found conditions for producing a chemically stable porous anodized alumina coating. In addition, since the porous anodized alumina film produced in this way is formed in close contact with the sapphire substrate, the regular shape of the mask can be transferred to the sapphire substrate and etched by using this as a mask. I found out that I can.

  That is, the present invention provides a method for producing a single crystal sapphire substrate having a plurality of irregularities or depressions on a main surface, and etching the porous anodized alumina formed on the single crystal sapphire substrate as a mask. It is the manufacturing method of the single crystal sapphire substrate which forms.

  Further, the method for forming the porous anodized alumina includes a step of forming an aluminum film on a main surface of a single crystal sapphire substrate having a smooth surface, and a plurality of depressions on the surface of the aluminum film with a predetermined interval and arrangement. And a step of forming a porous anodized alumina having a plurality of pores at a predetermined interval and arrangement by anodizing the aluminum film.

  Furthermore, in the step of forming a plurality of depressions on the surface of the aluminum film, a mold having a plurality of protrusions corresponding to the plurality of depressions is pressed against the surface of the aluminum film.

  Furthermore, the material of the mold having the plurality of protrusions is sapphire, SiC, or diamond.

  Further, the porous anodized alumina is etched using an acid containing at least one of phosphoric acid, sulfuric acid, and oxalic acid, thereby penetrating the pores of the porous anodized alumina to the sapphire substrate. To do.

  Furthermore, the single crystal sapphire substrate on which the porous anodized alumina is formed is heat-treated in an oxidizing atmosphere at 300 ° C. or higher or in an inert atmosphere.

  Further, as a method for etching the single crystal sapphire substrate, a dry etching method including at least an RIE method is used.

  Further, as the etching method for the single crystal sapphire substrate, wet etching is performed using hot phosphoric acid, a mixed acid of hot phosphoric acid and hot sulfuric acid, or hot molten potassium hydroxide.

  Further, a plurality of depressions or depressions are formed on the main surface of the single crystal sapphire substrate by the etching, and at the same time, etching is performed until the porous anodized alumina disappears, and the main surface other than the recesses is flattened. And

  Furthermore, the single crystal sapphire substrate having a plurality of irregularities or depressions on the main surface is manufactured by any one of the methods described above.

  Furthermore, the single crystal sapphire substrate having the plurality of irregularities or depressions is characterized in that the plurality of irregularities or depressions are cylindrical and perpendicular to the main surface.

  Further, in the single crystal sapphire substrate having the plurality of irregularities or depressions, the plurality of irregularities or depressions are arranged in a polygonal shape such as a triangle, a quadrangle, a hexagon, or are arranged in a straight line, or a straight line It is characterized in that it is densely arranged in a shape and is substantially striped.

Furthermore, in the single crystal sapphire substrate having the plurality of irregularities or depressions, the plurality of irregularities or depressions are arranged at intervals of 10 nm to 10 μm, and the angle formed between the side surface of the irregularities or depressions and the main surface is 10 ° or more. Further, the single crystal sapphire substrate having the plurality of depressions or depressions has etch pits on at least the bottom surface of the plurality of depressions.

  Further, the main surface is any one of sapphire C surface within ± 2 °, A surface within ± 2 °, R surface within ± 2 °, M surface within ± 2 °, or M surface within ± 2 °. It is characterized by being.

  Furthermore, a second crystal layer made of a semiconductor material having a refractive index different from that of the single crystal sapphire substrate is grown on the main surface of the single crystal sapphire substrate having a plurality of unevenness or depressions as described above. And a semiconductor light emitting device having an element structure in which a semiconductor crystal layer including a light emitting layer is stacked.

  Further, in the semiconductor light emitting device, the second crystal layer is composed of a buffer layer of AlXGaYN (X + Y = 1, X ≧ 0, Y ≧ 0) and a GaN-based semiconductor crystal layer thereon. .

  Furthermore, in the single crystal sapphire substrate according to any one of the above, the repetition period of the unevenness or the depression is 1/20 or more and 20 times or less of the emission wavelength of the GaN-based semiconductor light emitting device formed on the substrate. It is characterized by that.

  Furthermore, the semiconductor light emitting element is characterized in that a second crystal layer is grown from a main surface of the single crystal sapphire substrate having the plurality of irregularities or depressions while substantially forming a facet structure.

  Furthermore, the semiconductor light emitting element is characterized in that the difference between the refractive index of the single crystal sapphire substrate and the refractive index of the second crystal layer at the wavelength of the light emitted from the light emitting layer is 0.05 or more. .

  Further, the semiconductor light emitting element is grown on the main surface of the single crystal sapphire substrate having the plurality of irregularities or depressions so that the first GaN-based semiconductor crystal covers the irregularities or depressions and forms irregularities. A second GaN-based semiconductor crystal having a refractive index different from that of the first GaN-based semiconductor crystal is grown so as to cover at least part of the unevenness, and further, the third GaN-based semiconductor crystal flattens the unevenness. It has an element structure in which a semiconductor crystal layer including a light emitting layer is stacked thereon.

  Further, the semiconductor light emitting element is grown on the main surface of the single crystal sapphire substrate having the plurality of irregularities or depressions so that the first GaN-based semiconductor crystal covers the irregularities or depressions and forms irregularities. A second GaN-based semiconductor crystal grows by covering at least the protrusions of the unevenness in a film shape, and further grows until the third GaN-based semiconductor crystal flattens the unevenness, covering the same, and a light emitting layer thereon And the second GaN-based semiconductor crystal has a multilayer structure.

  Furthermore, in the semiconductor light-emitting device formed by forming a GaN-based semiconductor layer using a single crystal sapphire substrate having a plurality of irregularities or depressions on the main surface as described above, the repetition period of the irregularities or depressions is It is characterized by being 1/20 or more and 20 times or less of the emission wavelength of the semiconductor light emitting device.

  As described above, the present invention uses a porous anodized alumina to form irregularities on the surface of a sapphire substrate from micron to submicron order without using expensive equipment having a complicated optical system such as a conventional stepper. It can be formed efficiently and has excellent mass productivity.

  Furthermore, a semiconductor light-emitting element with high light extraction efficiency can be manufactured using the sapphire substrate formed with irregularities formed in this way.

  Hereinafter, an embodiment using the GaN-based material (GaN-based LED) will be described as an example with reference to the drawings.

  FIG. 1 is a view showing a sapphire substrate having a plurality of irregularities or depressions on the main surface according to the present invention, (a) is a sectional view thereof, and (b) is a plan view thereof.

  In the present invention, a single crystal sapphire substrate 1 having a plurality of irregularities or depressions 2 on the main surface was produced by the following procedure. That is, as shown in FIG. 2, an aluminum film 3 is formed on the main surface of a single crystal sapphire substrate 1 having a smooth surface by vapor deposition or sputtering, and the surface of the aluminum film 3 is formed as shown in FIG. After the mold 4 is pressed and a plurality of depressions 5 are formed at a predetermined interval and arrangement as shown in FIG. 4, porous anodization having a plurality of pores at a predetermined interval and arrangement as shown in FIG. Alumina 6 was formed, and the sapphire substrate was etched using the porous anodized alumina 6 as a mask to form a plurality of irregularities or depressions 2.

  At this time, the formation of the plurality of depressions 5 on the surface of the aluminum film 1 is performed by pressing the mold 4 having a plurality of protrusions made of a material harder than aluminum such as sapphire, SiC, diamond, and the like onto the surface of the aluminum film 3. This is performed by so-called imprint lithography. The formation of the plurality of depressions on the surface of the aluminum film 3 may be performed by a method other than imprint lithography, for example, pattern formation and etching by a normal photolithography method or electron beam lithography method.

  Further, as shown in FIG. 5, the pores of the porous anodized alumina 6 used for etching sapphire must penetrate from the surface to the sapphire surface. For this purpose, the porous anodized alumina 6 produced by anodic oxidation of alumina as described above is etched using an acid containing at least one of phosphoric acid, sulfuric acid, and oxalic acid, and the porous anodized alumina 6 The through hole was made into a through hole up to the sapphire substrate. In addition to this, etching can be used to adjust the diameter of the pores to an appropriate size.

  The porous anodized alumina 6 produced in this way is chemically unstable as it is, and when this is used as a mask to etch a sapphire substrate, the etching rate of the porous anodized alumina itself during etching However, the performance as an etching mask is insufficient. Therefore, the single crystal sapphire substrate on which the porous anodized alumina 6 was formed was chemically stabilized by heat treatment in an oxidizing atmosphere of 300 ° C. or higher or in an inert atmosphere. At this time, the heat treatment was performed in an oxidizing atmosphere or an inert atmosphere at 300 ° C. or higher. The porous anodized alumina 6 was chemically stabilized at 300 ° C. or higher, and the performance sufficient as an etching mask was obtained. It is because it will be able to have.

  Thereafter, etching was performed using the porous anodized alumina 6 thus produced as a mask to produce a single crystal sapphire substrate having a plurality of irregularities or depressions 2 on the main surface. At this time, as the etching method of the sapphire substrate, a dry etching method such as an RIE method is used, or a wet etching method using hot phosphoric acid, a mixed acid of hot phosphoric acid and hot sulfuric acid, or hot molten potassium hydroxide is used. Using. Note that in the case of dry etching by RIE, a chlorine-based gas containing at least one of chlorine, fluorine, boron, and bromine, or a mixed gas of these gases was used.

  At this time, etching is performed until the porous anodized alumina almost disappears so as not to hinder the epitaxial growth of GaN on the single crystal sapphire substrate having a plurality of unevenness or depressions 2 thereafter. Flattened.

  The plurality of irregularities or depressions 2 on the main surface thus produced are cylindrical and have a feature that they are perpendicular to the main surface. Furthermore, the plurality of irregularities or depressions 2 on the main surface are arranged in a polygonal shape such as a triangle, a quadrangle, a hexagon, etc., or are arranged in a straight line, or are arranged in a straight line closely and are substantially striped. A plurality of irregularities or depressions 2 on the main surface were arranged at intervals of 10 nm to 10 μm. The arrangement and spacing of the plurality of irregularities or depressions 2 are determined so as to maximize the light extraction efficiency for each wavelength of the LED element manufactured using the depressions or depressions. In addition, the angle formed by the plurality of irregularities or depressions 2 and the main surface is preferably 10 ° or more and less than 90 °, but this angle is suitable for efficiently scattering light and optimizing light extraction efficiency. This is because.

  In addition, the plurality of irregularities or depressions 2 produced in this way has a characteristic of having etch pits on at least the bottom surface under appropriate conditions. This is particularly noticeable in the case of wet etching.

  Further, the repetition period of the plurality of irregularities or depressions 2 is 1/20 or more and 20 times or less, more preferably 1/10 or more and 10 times or less of the emission wavelength of the GaN-based semiconductor light emitting device formed thereon. Is desirable. This is because the repetition period of the plurality of irregularities or depressions 2 is close to the emission wavelength of the light emitting element, thereby increasing the effect of light diffraction or scattering and increasing the light extraction efficiency from the light emitting element.

  The main surface satisfies any of sapphire C surface within ± 2 °, A surface within ± 2 °, R surface within ± 2 °, M surface within ± 2 °, or M surface within ± 2 °. This is preferably within the range necessary for epitaxial growth of GaN-based semiconductors with appropriate crystallinity. At this time, the main surface may be slightly inclined from each surface orientation.

  As described above, according to the manufacturing method of the present invention, the sapphire substrate is etched using the porous anodized alumina 6 as a mask, so that the equipment is simpler and more cost-effective than a very complicated optical system such as a stepper. In particular, it is possible to produce a plurality of irregularities or depressions on the order of submicron to micron on a sapphire substrate at low cost.

  Next, a nitride-based semiconductor light-emitting device using a sapphire substrate having a plurality of irregularities or depressions 2 of the present invention, particularly a semiconductor light-emitting device with high light extraction efficiency will be described.

  When a nitride semiconductor is epitaxially grown on the main surface of the sapphire substrate having the plurality of irregularities or depressions 2, the nitride semiconductor film has a flat surface by combining the growth in the vertical direction and the growth in the horizontal direction of the nitride semiconductor. Is formed. A light emitting element is manufactured using the substrate on which the nitride semiconductor is formed.

  An example of the produced LED is shown in FIG. An n-type GaN contact layer 8, an n-type AlGaN cladding layer 9, and a GaN-based semiconductor light emitting layer (MQW structure) are disposed on the main surface of the single crystal sapphire substrate 1 having a plurality of irregularities or depressions 2 through an AlGaN low-temperature buffer layer 7. 10. A p-type AlGaN cladding layer 11 and a p-type GaN contact layer 12 are formed, an upper electrode (usually a p-type electrode) 13 is formed thereon, and a lower electrode (usually an n-type is formed on the n-type GaN contact layer 8). Electrode) 14 was formed.

  The main surface having the plurality of irregularities or dents 2 forms a light scattering layer, whereby the light traveling downward from the light emitted from the light emitting layer 10 is scattered by the light scattering layer of the main surface, and the sapphire substrate 1 The light can be taken out from the side. Thereby, the light emitted from the light emitting layer 10 is not repeatedly reflected and attenuated in the GaN layer, and the light extraction efficiency of the entire LED can be improved.

  Moreover, it is preferable that the refractive index of the single crystal sapphire substrate 1 and the refractive index of the n-type GaN contact layer 8 as the second crystal layer are different at the wavelength of light emitted from the GaN-based semiconductor light emitting layer 10, The difference in refractive index is more preferably 0.05 or more. This is because there is a difference in refractive index, and more preferably, a difference of 0.05 or more facilitates irregular reflection at the interface and improves light extraction efficiency.

  As the buffer layer 7, either GaN-based or AlN-based including AlXGaYN (X + Y = 1, X ≧ 0, Y ≧ 0) is used, and GaN-based is used as a semiconductor crystal layer formed thereon. At this time, even when a GaN film is grown on a single crystal sapphire substrate having a plurality of irregularities or depressions, by using AlXGaYN (X + Y = 1, X ≧ 0, Y ≧ 0) as a buffer layer, the crystalline property is increased. A good GaN film can be grown.

  As a GaN crystal growth mode, the n-type GaN contact layer 8 or the like as the second crystal layer is grown from the main surface of the single crystal sapphire substrate 1 while substantially forming a facet structure. Is preferred. This is because dislocations in the GaN crystal can be bent laterally by forming a facet structure, so a GaN film with higher crystallinity can be formed and the luminous efficiency of the LED can be further increased. It is.

  As yet another embodiment, although not shown, the first GaN-based semiconductor crystal is grown on the main surface of the single crystal sapphire substrate so as to cover a plurality of projections or depressions and to form projections and depressions. A second GaN-based semiconductor crystal having a refractive index different from that of the first GaN-based semiconductor crystal is grown so as to cover at least part of the unevenness or depression of the first GaN-based semiconductor crystal, and the third GaN-based semiconductor crystal further flattens the unevenness. An element structure in which a semiconductor crystal layer including a light emitting layer is stacked thereon can be formed. With such a structure, it is possible to efficiently cause light scattering from the interface including the above-described irregularities, and to form a semiconductor light emitting device with high light extraction efficiency.

  Alternatively, on the main surface of the single crystal sapphire substrate, the first GaN-based semiconductor crystal grows so as to cover the plurality of irregularities or depressions, and at least the projections of the plurality of depressions or depressions are formed into a film shape. A second GaN-based semiconductor crystal is grown over the substrate, and a third GaN-based semiconductor crystal is grown over the surface until the unevenness is flattened, and a semiconductor crystal layer including a light emitting layer is stacked thereon. It may have an element structure, and the second GaN-based semiconductor crystal may have a multilayer structure. This multilayer film structure includes an MWQ structure. This makes it possible to form a semiconductor light emitting device with high light emission efficiency, and to realize both high light emission efficiency and high light extraction efficiency at the same time.

  The repetition period of the irregularities or depressions of the semiconductor light-emitting element formed by forming a GaN-based semiconductor layer using a single crystal sapphire substrate having a plurality of irregularities or depressions 2 on the main surface is the light emission of the semiconductor light-emitting element. It is desirable that the wavelength is 1/20 or more and 20 times or less, more preferably 1/10 or more and 10 times or less. This is because the effect of light diffraction or scattering is increased and the light extraction efficiency from the light emitting element is increased because the repetition period of the plurality of irregularities or depressions is close to the emission wavelength of the light emitting element.

  Normally, this semiconductor light emitting element is fabricated on the sapphire substrate 1 having the plurality of irregularities or depressions 2 and cuts together with the sapphire substrate such as dicing, and is packaged to serve as a semiconductor light emitting element. However, the semiconductor light emitting element portion may be peeled from the sapphire substrate to obtain a more efficient LED. Even in such a case, the sapphire substrate having the plurality of depressions or depressions has a structure having a cavity between the plurality of depressions or depressions and the nitride semiconductor film by selecting the growth condition of the nitride semiconductor for correction. Thus, the semiconductor light emitting device can be easily separated from the sapphire substrate.

  Furthermore, the sapphire substrate 1 having a plurality of irregularities or depressions according to the present invention has a low dislocation nitride semiconductor layer of 100 μm on the sapphire substrate having the plurality of depressions or depressions 2 for use other than the manufacture of a semiconductor light emitting device. Even when the nitride semiconductor with low dislocations is peeled off from the sapphire substrate after being formed to have the above thickness, peeling is also easy when an independent nitride semiconductor substrate is manufactured.

  Furthermore, the sapphire substrate having a plurality of irregularities or depressions 2 of the present invention can be used not only as a substrate for a semiconductor light emitting device but also as a substrate for an electronic device using a nitride semiconductor.

  Furthermore, the sapphire substrate having a plurality of irregularities or depressions 2 of the present invention can be used as an optical component, for example, as a member for a diffraction grating or a photonic crystal.

  In this example, a method for manufacturing a sapphire substrate having a plurality of depressions or depressions for a semiconductor light emitting device will be described.

  First, the sapphire material was pulled up by the EFG method so that the main surface became the C surface. In addition to the EFG method, the sapphire crystal growth method may be performed by a method such as Czochralski, Cairo porous method, TGT method, or HEM method. In this embodiment, the main surface of the sapphire substrate is the C plane, but the plane orientation of the main surface is not only the C plane, but also a plane rotated 30 degrees from the A plane, M plane, R plane, and M plane. Etc. Further, it may be off-angled within ± 2 ° from these surfaces.

  Next, this material was appropriately cut, ground, polished, and washed to produce a sapphire substrate having a smooth surface for forming a nitride semiconductor film having a thickness of about 0.4 mm.

  Next, an aluminum film having a thickness of 5 μm was formed on the main surface of the single crystal sapphire substrate having a smooth surface by vacuum evaporation. Next, a mold having a plurality of protrusions made of SiC was pressed against the surface of the aluminum film to form a plurality of depressions in a hexagonal packing array at 1 μm intervals.

At this time, the SiC mold was fabricated by EB (electron beam) lithography. For pressing the mold, an imprint technique was applied, and a depression was formed on the surface of the aluminum film by applying a pressure of about 2000 kg / cm ^{2 with} a hydraulic press.

  Next, the single crystal sapphire substrate on which the aluminum film having the plurality of projections and depressions was formed was anodized to form a plurality of pores corresponding to the plurality of projections and depressions. For anodic oxidation, 0.3 mol of phosphoric acid was used, and a DC voltage of 400 V was applied for 15 hours. At this time, cooling was performed so that the temperature of the electrode was 30 ° C. or lower.

  Next, in order to penetrate the pores of the porous anodized alumina thus prepared from the surface to the sapphire surface, the porous anodized alumina was etched at 30 ° C. for 1 hour using 5% phosphoric acid. It was. At this time, the diameter of the pore was about 420 nm, which was the same as the wavelength of the blue LED.

  The porous anodized alumina produced in this way is chemically unstable as it is, and when this is used as a mask to etch a sapphire substrate, the etching rate of the porous anodized alumina itself during etching is low. Since it is fast, its performance as an etching mask is insufficient. Therefore, the single crystal sapphire substrate on which porous anodized alumina was formed was chemically stabilized by heat treatment in an oxidizing atmosphere at 300 ° C. or higher.

  Thereafter, etching was performed using the porous anodized alumina thus prepared as a mask to prepare a single crystal sapphire substrate having a plurality of irregularities or depressions on the main surface. At this time, a wet etching method using hot phosphoric acid at 200 ° C. was used as an etching method for the sapphire substrate.

  At this time, etching is performed until the porous anodized alumina almost disappears so that the GaN epi-growth on the single-crystal sapphire substrate having a plurality of unevenness or depressions is not lost, and the main surface other than the recesses is flattened. I made it.

  Next, a nitride-based semiconductor light-emitting device using a sapphire substrate having a plurality of irregularities or depressions according to the present invention, particularly a semiconductor light-emitting device with high light extraction efficiency will be described.

  First, a plurality of irregularities or depressions were formed on the C-plane sapphire substrate by the process of Example 1. After cleaning the substrate, the substrate was mounted on a MOVPE apparatus, and the temperature was raised to 1100 ° C. in a nitrogen gas main component atmosphere to perform thermal cleaning. The temperature was lowered to 500 ° C., trimethylgallium (hereinafter referred to as TMG) was flown as a Group 3 raw material of the periodic rate table, and ammonia was flowed as an N raw material to grow an AlGaN low-temperature buffer layer having a thickness of 30 nm.

  Subsequently, the temperature was raised to 1000 ° C., and TMG and ammonia as raw materials and silane as a dopant were flown to grow an n-type GaN layer (contact layer). The growth of the GaN layer at this time was a growth in which the entire GaN layer was embedded without forming a cavity in the recess.

  Subsequently, an n-type AlGaN cladding layer, an InGaN light emitting layer (MQW structure), a p-type AlGaN cladding layer, and a p-type GaN contact layer were formed in this order. Further, etching processing for exposing the n-type contact layer, electrode formation, and element separation were performed to obtain an LED element.

  The present invention relates to a single crystal sapphire substrate for a semiconductor light emitting device, a manufacturing method thereof, and a semiconductor light emitting device, and can contribute to manufacturing a sapphire substrate for a semiconductor light emitting device at low cost with simple equipment. .

The single-crystal sapphire substrate which has several unevenness | corrugations or hollows of this invention is shown, (a) is sectional drawing, (b) is a top view. It is a figure which shows the state which vapor-deposited the aluminum film on the sapphire substrate. It is a figure which shows a mode that a mold is pressed on the surface of an aluminum film by the imprint method. It is a figure which shows the state which formed the several hollow in the surface of the aluminum film. It is a figure which shows the state which anodized the aluminum film on a sapphire substrate, and formed the porous anodic oxidation alumina. It is a figure which shows the semiconductor light-emitting device of this invention.

Explanation of symbols

1: Sapphire substrate 2: Depression 3: Aluminum film 4: Mold 5: Depression 6: Porous anodic alumina 7: AlGaN low temperature buffer layer 8: n-type GaN contact layer 9: n-type AlGaN cladding layer 10: GaN-based semiconductor light emission Layer (MQW structure)
11: p-type AlGaN cladding layer 12: p-type GaN contact layer 13: upper electrode 14: lower electrode

Claims (23)

In the method for producing a single crystal sapphire substrate having a plurality of irregularities or depressions on the main surface, the irregularities or depressions are formed by etching using porous anodized alumina formed on the single crystal sapphire substrate as a mask. A method for producing a single-crystal sapphire substrate. The method for forming porous anodized alumina includes a step of forming an aluminum film on a main surface of a single crystal sapphire substrate having a smooth surface, and forming a plurality of depressions on the surface of the aluminum film at a predetermined interval and arrangement. 2. A single crystal sapphire according to claim 1, comprising the steps of: anodizing the aluminum film to form porous anodized alumina having a plurality of pores at a predetermined interval and arrangement. A method for manufacturing a substrate. 3. The process for forming a single crystal sapphire substrate according to claim 2, wherein in the step of forming a plurality of depressions on the surface of the aluminum film, a mold having a plurality of protrusions corresponding to the plurality of depressions is pressed against the surface of the aluminum film. Method. 4. The method for manufacturing a single crystal sapphire substrate according to claim 3, wherein a material of the mold having the plurality of protrusions is any one of sapphire, SiC, and diamond. The porous anodized alumina is etched using an acid containing at least one of phosphoric acid, sulfuric acid, and oxalic acid, thereby penetrating the pores of the porous anodized alumina to the sapphire substrate. Item 5. A method for producing a single crystal sapphire substrate according to any one of Items 2 to 4. The single crystal sapphire substrate according to any one of claims 1 to 5, wherein the single crystal sapphire substrate on which the porous anodized alumina is formed is heat-treated in an oxidizing atmosphere or an inert atmosphere at 300 ° C or higher. Production method. The method for manufacturing a single crystal sapphire substrate according to claim 1, wherein a dry etching method including at least an RIE method is used as an etching method for the single crystal sapphire substrate. The wet etching is performed using hot phosphoric acid, a mixed acid of hot phosphoric acid and hot sulfuric acid, or hot molten potassium hydroxide as an etching method for the single crystal sapphire substrate. Manufacturing method of single crystal sapphire substrate. A plurality of irregularities or depressions are formed on the main surface of the single crystal sapphire substrate by the etching, and at the same time, etching is performed until the porous anodized alumina disappears to flatten the main surface other than the recesses. The manufacturing method of the single-crystal sapphire substrate in any one of Claims 1-8. A single crystal sapphire substrate having a plurality of irregularities or depressions on a main surface, which is manufactured by the method according to claim 1. The single crystal sapphire substrate according to claim 10, wherein the plurality of irregularities or depressions on the main surface are cylindrical and perpendicular to the main surface. A plurality of irregularities or depressions on the main surface are arranged in a polygonal shape such as a triangle, a quadrangle, a hexagon, etc., arranged in a straight line, or arranged in a dense line and substantially in a stripe shape The single crystal sapphire substrate according to claim 10, wherein The plurality of irregularities or depressions on the main surface are arranged at intervals of 10 nm to 10 μm, and the angle formed between the side surface of the depression and the main surface is 10 ° or more and 90 ° or less. The single crystal sapphire substrate according to any one of? The single crystal sapphire substrate according to claim 10, wherein etch pits are provided on at least the bottom surface of the plurality of irregularities or depressions on the main surface. The main surface is either within C surface ± 2 ° of sapphire, within ± 2 ° of A surface, within ± 2 ° of R surface, within ± 2 ° of M surface, or within 30 ° ± 2 ° of M surface. The single crystal sapphire substrate according to claim 10. A second crystal layer made of a semiconductor material having a refractive index different from that of the single crystal sapphire substrate is grown on the main surface of the single crystal sapphire substrate having a plurality of irregularities or depressions according to claim 10. A semiconductor light emitting device having an element structure in which a semiconductor crystal layer including a light emitting layer is stacked therein. The second crystal _layer, the _{Al X Ga Y N (X +} Y = 1, X ≧ 0, Y ≧ 0) buffer layer with claim 16, characterized in that it consists of a semiconductor crystal layer of GaN based thereon The semiconductor light emitting element as described. 18. The single crystal according to claim 16, wherein a repetition period of the unevenness or depression of the sapphire substrate is 1/20 or more and 20 times or less of an emission wavelength of a light emitting layer formed on the substrate. Sapphire substrate. The second crystal layer is grown from a main surface of the single crystal sapphire substrate having the plurality of irregularities or depressions while substantially forming a facet structure. Semiconductor light emitting device. The difference between the refractive index of the single-crystal sapphire substrate and the refractive index of the second crystal layer at the wavelength of light emitted from the light-emitting layer is 0.05 or more. The semiconductor light-emitting device described in 1. On the main surface of the single crystal sapphire substrate having the plurality of irregularities or depressions, a first GaN-based semiconductor crystal grows so as to form irregularities covering the irregularities or depressions, and covers at least a part of the irregularities. A second GaN-based semiconductor crystal having a refractive index different from that of the first GaN-based semiconductor crystal is grown, and further a third GaN-based semiconductor crystal is grown until the unevenness is flattened, and a light emitting layer thereon 21. The semiconductor light-emitting element according to claim 17, wherein the semiconductor light-emitting element has an element structure in which semiconductor crystal layers containing the element are stacked. On the main surface of the single crystal sapphire substrate having the plurality of irregularities or depressions, a first GaN-based semiconductor crystal grows to form irregularities covering the irregularities or depressions, and at least the convex portions of the irregularities are film-like A second GaN-based semiconductor crystal grows over the substrate, and further covers the third GaN-based semiconductor crystal until the unevenness is flattened, and a semiconductor crystal layer including a light emitting layer is laminated thereon. The semiconductor light-emitting device according to claim 21, wherein the second GaN-based semiconductor crystal has a multilayer structure. A semiconductor light emitting device formed by forming a GaN-based semiconductor layer using a single crystal sapphire substrate having a plurality of irregularities or depressions on the main surface, wherein the repetition period of the irregularities or depressions is an emission wavelength of the semiconductor light emitting device 23. The semiconductor light emitting element according to claim 17, wherein the semiconductor light emitting element is 1/20 or more and 20 times or less.

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