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`Europaisches Patentamt
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`European Patent Office
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`Office europeen des brevets
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`111111111111111111111111111111111111111111111111111111111111111111111111111
`.EP 1 378 949 A1
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`EUROPEAN PATENT APPLICATION
`published in accordance with Art. 158(3) EPC
`
`(43) Date of publication:
`07.01.2004 Bulletin 2004/02
`
`(21) Application number: 02705381.8
`
`(22) Date of filing: 20.03.2002
`
`(84) Designated Contracting States:
`AT BE CH CY DE OK ES Fl FR GB GR IE IT L1 LU
`MC NL PTSETR
`
`(30) Priority: 21.03.2001 JP 2001081447
`21.03.2001 JP 2001080806
`
`(71) Applicant: Mitsubishi Cable Industries, Ltd.
`ltami-shi, Hyogo 664-0027 (JP)
`
`(72) Inventors:
`• TADATOMO, Kazuyuki,
`ltami Fac. of MITSUBISHIIND.
`ltami-shi, Hyogo 664-0027 (JP)
`
`(51) lnt Cl.7: H01 L 33/00
`
`(86) International application number:
`PCT/JP2002/002658
`
`(87) International publication number:
`WO 20021075821 (26.09.2002 Gazette 2002/39)
`
`• OKAGAWA, Hiroaki,
`ltami Fac. of MITSUBISHIIND.
`ltami-shi, Hyogo 664-0027 (JP)
`• OUCH I, Yoichiro, ltami Fac. of MITSUBISHIIND.
`ltami-shi, Hyogo 664-0027 (JP)
`• TSUNEKAWA, Takashi,
`ltami Fac. of MITSUBISH IND.
`ltami-shi, Hyogo 664-0027 (JP)
`
`(74) Representative:
`Weber, Thomas, Dr.Dipi.-Chem. et al
`Patentanwalte
`von Kreisler-Selting-Werner,
`Postfach 10 22 41
`50462 Koln (DE)
`
`(54)
`
`SEMICONDUCTOR LIGHT-EMITTING DEVICE
`
`Concaves and convexes 1 a are formed by
`(57)
`processing the surface layer of a first layer 1, and sec(cid:173)
`ond layer 2 having a different refractive index from the
`first layer is grown while burying the concaves and con(cid:173)
`vexes (or first crystal1 0 is grown as concaves and con(cid:173)
`vexes on crystal layerS to be the base of the growth,
`and second crystal 20 is grown, which has a different
`refractive index from the first crystal). After forming
`these concavo-convex refractive index interfaces 1 a
`(1 Oa), an element structure, wherein semiconductor
`crystal layers containing a light-emitting layer A are lam(cid:173)
`inated, is formed. As a result, the light in the lateral di(cid:173)
`rection, which is generated in the light-emitting layer
`changes its direction by an influence of the concavo(cid:173)
`convex refractive index interface and heads toward the
`outside. Particularly, when an ultraviolet light is to be
`emitted using lnGaN as a material of a light-emitting lay(cid:173)
`er, a quantum well structure is employed and all the lay(cid:173)
`ers between the quantum well structure and the low tem(cid:173)
`perature buffer layer are formed of a GaN crystal, re(cid:173)
`moving AIGaN. The quantum well structure preferably
`consists of a well layer made of lnGaN and a barrier lay(cid:173)
`er made of GaN, and the thickness of the barrier layer
`is preferably 6 nm - 30 nm.
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`(-•.)
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`F.IG. 1
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`Printed by Jouve, 75001 PARIS (FA)
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`IPR2017-00539
`Lexington Luminance LLC
`Exhibit 2007
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`EP 1 378 949 A1
`
`Description
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`Technical Field
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`[0001] The present invention relates to a semiconductor light-emitting element (hereinafter to be also referred to
`simply as a light-emitting element). Particularly, this invention relates to a semiconductor light-emitting element having
`a light-emitting layer made of a GaN group semiconductor crystal (GaN group crystal).
`
`Background Art
`
`[0002] The basic element structure of a light-emitting diode (LED) comprises a crystal substrate, and an n-type
`semiconductor layer, a light-emitting layer (including DH structure, MQW structure, SQW structure) and a p-type sem(cid:173)
`iconductor layer sequentially grown thereon, wherein each of then-type layer, the conductive crystal substrate (SiC
`substrate, GaN substrate and the like) and the p-type layer has an external outlet electrode.
`[0003] For example, Fig. 8 shows one exemplary constitution of an element (GaN group LED) comprising a GaN
`group semiconductor as a material of a light-emitting layer, wherein a GaN group crystal layer (n-type GaN contact
`layer (also a clad layer) 102, a GaN group semiconductor light-emitting layer 103 and a p-type GaN contact layer (also
`a clad layer) 1 04) are sequentially laminated on a crystal substrate 101 by crystal growth, and a lower electrode (gen(cid:173)
`erally an n-type electrode) 105 and an upper electrode (generally a p-type electrode) 106 are set. In this specification,
`the layers are mounted on a crystal substrate (downside) and the light goes out upward, for the explanation's sake.
`[0004]
`In LED, an important issue is how efficiently the light produced in the light-emitting layer can be externally
`taken out (what is called, light-extraction efficiency). Therefore, various designs have been conventionally tried such
`as an embodiment wherein an upper electrode 106 in Fig. 8 is rendered a transparent electrode so that the light heading
`upward from the light-emitting layer will not form an obstacle to the outside, an embodiment wherein the light heading
`downward from the light-emitting layer is returned upward by forming a reflective layer and the like.
`[0005] For the light emitted from the light-emitting layer in the vertical direction, the light-extraction efficiency can be
`improved by making the electrode transparent or forming a reflective layer, as mentioned above. However, of the lights
`advancing in the spreading direction of the light-emitting layer (direction shown with a thick arrow in a light-emitting
`layer 103 in Fig. 8, hereinafter to be also referred to as a "lateral direction"), the light that reaches the sidewall within
`the total reflection angle defined by a refractive index differential can be externally emitted, but many other lights repeat
`reflection between, for example, sidewalls, are absorbed in an element, particularly by a light-emitting layer itself,
`attenuated and disappear. Such lights in the lateral direction are enclosed by upper and lower clad layers or a substrate
`(sapphire substrate) and an upper clad layer, or a substrate and an upper electrode (further, a coating substance on
`the outside of the element and the like), and propagated in the lateral direction. The light that propagates in the lateral
`direction occupies a large portion of the entire light amount produced by a light-emitting layer, and in some cases it
`amounts to 60% of the whole.
`[0006] With regard to a flip-chip type LED (light goes out through a substrate) to be mounted with the substrate on
`the upper side, an embodiment is known wherein a side wall of a laminate, which is an element structure, has an angle
`and the side wall is used as a reflection surface toward the substrate side, so that such light in the lateral direction can
`be reflected in the substrate direction. However, cutting 4 facets of a small chip with an angle is a difficult processing,
`posing a problem in costs.
`[0007] Furthermore, the light advancing in the vertical direction is also associated with problems in that a standing.
`wave that repeats reflection between an interface of GaN group semiconductor layer/sapphire substrate and an inter(cid:173)
`face of GaN group semiconductor layer/p-type electrode (or sealing material) is formed and the like, which in turn
`hinder light-extraction efficiency.
`[0008]
`It is a first object of the present invention to provide a light-emitting element having a novel structure capable
`of solving the above-mentioned problem, directing the light in the lateral direction, which is produced in the light-emitting
`layer, to the outside, and further, suppressing the occurrence of the above-mentioned standing wave.
`[0009]
`In addition to the problem of the light-extraction efficiency as mentioned above, the following problem of lower
`output is present when lnGaN is used as a material of a light-emitting layer and the ultraviolet light is to be emitted.
`[001 0] A light-emitting element comprising lnGaN as a light-emitting layer generally provides highly efficient emission.
`This is explained to be attributable to a smaller proportion of carriers captured by the non-radiative center, from among
`the carriers injected into the light-emitting layer, due to the localization of the carriers caused by fluctuation of the In
`composition, which in turn results in a highly efficient emission.
`[0011] When a blue purple light- ultraviolet light having a wavelength of not- more than 420 nm is to be emitted by
`a GaN group light-emitting diode (LED) and a GaN group semiconductor laser (LD); lnGaN (In composition not more
`than 0.15) is generally used as a material of a light-emitting layer, and the structure involved in the emission is a single
`quantum well structure (what is called a DH structure is encompassed because of a thin active layer) or a multiple
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`EP 1 378 949 A1
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`quantum well structure.
`[0012]
`In general terms, the upper limit of.the wavelength of the ultraviolet light is shorter than the end (380 nm-
`400 nm) of the short wavelength of visible light, and the lower limit is considered to be about 1 nm (0.2 nm- 2 nm). In
`this specification, the blue purple light of not more than 420 nm emitted by the above-mentioned lnGaN having an In
`composition of not more than 0.15 is also referred to as an ultraviolet light and a semiconductor light-emitting element
`emitting such ultraviolet light is referred to as an ultraviolet light-emitting element.
`[0013] The ultraviolet light GaN can produce has a wavelength of 365 nm. Therefore, in the case of a ternary system
`wherein lnGaN essentially contains In composition and free of A 1 composition, the lower limit of the wavelength of the
`ultraviolet light which can be generated is longer than the aforementioned 365 nm.
`[0014] When compared to blue and green light-emitting elements having a light-emitting layer having a high In com(cid:173)
`position, the ultraviolet light-emitting element has a shorter wavelength. Thus, the In composition of the light-emitting
`layer needs to be reduced. As a consequence, the effect of the localization of the aforementioned fluctuation of the In
`composition decreases and the proportion thereof to be captured in the non-radiative recombination center increases,
`which prevents a high output. Under the circumstances, dislocation density, which causes the non radiative recombi-
`nation center, has been actively reduced. As a method for reducing the dislocation density, ELO method (lateral growth
`method) can be mentioned, and high output and long life have been achieved by reducing the dislocation density (see
`reference (Jpn. J. Appl. Phys. 39 (2000) pp. L647) etc.).
`[0015]
`In a GaN group light-emitting element, a light-emitting layer (well layer) is sandwiched between clad layers
`(barrier layers) made of a material having a greater band gap. According to a reference (Hi roo Yonezu, Hikari Tsushin
`Soshi Kogaku, Kougakutosho Ltd., p. 72); a guidance of setting the difference in the band gap to generally not less
`than "0.3 eV" . has been provided.
`[0016] From the above-mentioned.background, when lnGaN having a composition capable of emitting ultraviolet
`light is to be used as a light-emitting layer (well layer), the clad layer (a single quantum well structure contains not only
`a clad layer but also a barrier layer) used to sandwich the light-emitting layer is AIGaN having a greater band gap in
`view of enclosure of the carrier.
`[0017]
`In addition, when a quantum well structure is to be constituted, the barrier layer needs to have a thickness of
`a level producing a tunnel effect, which is generally about 3-6 nm.
`[0018] For example, Fig. 9 shows one embodiment of a conventional light-emitting diode using ln0.05Ga0.95N as a
`material of a light-emitting layer, which has an element structure wherein an n-type GaN contact layer 202, an n-type
`A10.1Ga0.9N clad layer 203, an ln0.05Ga0.95N well layer (light-emitting layer) 204, a p-type A10.2Ga0 .8N clad layer 205
`and a p-type GaN contact layer 206 are sequentially laminated on a crystal substrate S1 0 via a buffer layer 201, by
`crystal growth, and a lower electrode (generally an n-type electrode) P1 0 and an upper electrode (generally p-type
`electrode) P20 are formed.
`[0019] However, the ELO method is problematic in that the methods for growing a GaN layer to be a base, forming
`a mask layer and re-growing are necessary, and growth in a number of times is necessary, thus increasing the number
`of steps. In addition, because a re-growth interface exists, it has a problem that, although dislocation density reduces,
`the output does not increase easily.
`[0020] The present inventors have studied conventional element structures in an attempt to use lnGaN as a material
`of the light-emitting layer and achieve higher output of ultraviolet light, and found that AIGaN layer. is behind the distortion
`relative to the lnGaN light-emitting layer, which results from the difference in the lattice constant.
`[0021]
`It has been also found that, when a barrier layer is made thin in the quantum well structure, Mg is diffused
`from the p-type layer formed thereon to a light-emitting layer and forms a non-radiative center, thus problematically
`preventing high output of an ultraviolet light-emitting element.
`[0022] A second object of the present invention is to achieve high output, and further, a long life, by optimizing the
`structure of the element, when lnGaN is used as a material of a light-emitting layer of the light-emitting element of the
`present invention and ultraviolet light is to be emitted.
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`Disclosure of the Invention
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`so
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`[0023] Accordingly, the present invention is characterized by the following.
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`(1) A semiconductor light-emitting element having an element structure comprising a first crystal layer processed
`to have concaves and convexes on its surface, a second crystal layer directly formed thereon or formed via a
`buffer layer, burying the concaves and convexes, the second crystal layer being made from a semiconductor ma-
`terial having a different refractive index from the aforementioned crystal layer, and a semiconductor crystal layer
`comprising a light-emitting layer laminated thereon.
`(2) The semiconductor light-emitting element of the above-mentioned (1 ), wherein the second crystal layer and
`the semiconductor crystal layer thereon are made of a GaN group semiconductor crystal.
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`EP 1 378 949 A1
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`(3) The semiconductor light-emitting element of the above-mentioned (2), wherein the first crystal layer is a crystal
`substrate and the second crystal layer has grown while substantially forming a facet structure from the concaves
`and convexes processed on the surface of the crystal substrate.
`(4) The semiconductor light-emitting element of the above-mentioned (3), wherein the concaves and convexes
`processed on the surface of the crystal substrate have a stripe pattern, and the longitudinal direction of the stripe
`is a <11-20> direction or a <1-1 00> direction of a GaN group semiconductor grown while burying them.
`(5) The semiconductor light-emitting element of the above-mentioned (1) or (4), wherein the concaves and con(cid:173)
`vexes have a cross sectional shape of a rectangular wave, a triangular wave or a sine curve.
`(6) The semiconductor light-emitting element of the above-mentioned (1 ), wherein the difference between the
`refractive index of the first crystal layer and that of the second crystal layer at the wavelength of a light emitted
`from the light-emitting layer is not less than 0.05.
`(7) The semiconductor light-emitting element of the above-mentioned (1 ), wherein the light-emitting layer is made
`of an lnGaN crystal having a composition capable of generating an ultraviolet light.
`(8) The semiconductor light-emitting element ofthe above-mentioned (1 ), wherein the light-emitting layer is aquan-
`tum well structure comprising a well layer made of lnGaN and a barrier layer made of GaN.
`(9) The semiconductor light-emitting element of the above-mentioned (1 ), wherein the first crystal layer is a crystal
`substrate, the second crystal layer is grown, via a low temperature buffer layer, on the concaves and convexes
`processed on the surface ofthe crystal substrate while burying the con caves and convexes, the light-emitting layer
`is a quantum well structure comprising a well layer made of lnGaN and a barrier layer made of GaN, and all the
`layers between the quantum well structure and the low temperature buffer layer are made of a GaN crystal.
`(1 0) The semiconductor light-emitting element of the above-mentioned (8) or (9), wherein the barrier layer has a
`thickness of 6 nm- 30 nm.
`(11) A semiconductor light-emitting element having an element structure comprising a crystal layer surface to be
`a base for crystal growth, a first GaN group semiconductor crystal grown on said surface to form concaves and
`convexes, a second GaN group semiconductor crystal having a different refractive index from the first GaN group
`semiconductor crystal, which is grown while covering at least a part of said concaves and convexes, a third GaN
`group semiconductor crystal grown until it flattens the above-mentioned concaves and convexes, and a semicon(cid:173)
`ductor crystal layer having a light-emitting layer laminated thereon.
`(12) The semiconductor light-emitting element of the above-mentioned (11 ), wherein the crystal layer surface to
`be a base for crystal growth has a structure or has been subjected to a surface treatment, which dimensionally
`limits a crystal growth area, and said limitation causes growth of the first GaN group semiconductor crystal into
`concaves and convexes, while forming substantial facet structure or a pseudo-facet structure.
`(13) The semiconductor light-emitting element of the above-mentioned (12), wherein the structure or surface treat(cid:173)
`ment limiting the crystal growth area is concaves and convexes processed on the crystal layer surface to be a
`base for crystal growth, or a mask pattern capable of causing a lateral growth, which is formed on the surface of
`the crystal layer to be a base for crystal growth, or a surface treatment capable of suppressing GaN group crystal
`growth, which is applied to a specific area of the surface of the crystal layer to be a base for crystal growth.
`(14) The semiconductor light-emitting element of the above-mentioned ( 11 ), having an element structure wherein
`the second GaN group semiconductor crystal is grown covering, in a membrane, at least a convex part of the
`concaves and convexes made of the first GaN group semiconductor crystal, the third GaN group semiconductor
`crystal covering same is grown until it flattens the above-mentioned concaves and convexes, and the semicon(cid:173)
`ductor crystal layer having the light-emitting layer is laminated thereon, and wherein the second GaN group sem(cid:173)
`iconductor crystal has a'multilayer membrane structure.
`(15) The semiconductor light-emitting element of the above-mentioned (11 ), wherein the light-emitting layer is
`made of an lnGaN crystal having a composition capable of generating an ultraviolet light.
`(16) The semiconductor light-emitting element of the above-mentioned (11 ), wherein the light-emitting layer is a
`quantum well structure comprising a well layer made of lnGaN and a barrier layer made of GaN.
`(17) The semiconductor light-emitting element of the above-mentioned (16), wherein the barrier layer has a thick(cid:173)
`ness of 6 nm- 30 nm.
`(18) The semiconductor light-emitting element of the above-mentioned (11 ), wherein the above-mentioned con(cid:173)
`caves and convexes have a stripe pattern, and the longitudinal direction of the stripe is a <11-20> direction or a
`<1-1 00> direction of the first GaN group semiconductor crystal.
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`[0024]
`In the following, the embodiment of the above-mentioned (1.) is referred to as "embodiment (I)" and the em-
`bodiment of the above-mentioned (11) is referred to as "embodiment (II)" and explained.
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`EP 1 378 949 A1
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`Brief Description of the Drawings
`
`[0025]
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`Fig. 1 is a schematic drawing showing an example of the structure of the light-emitting element of.the present
`invention, wherein hatching is partially applied to show the boundary of areas (the same in the following Figures).
`Fig. 2 is a schematic drawing showing one example of the crystal growth method for forming a concave-convex
`refractive index interface in the embodiment (I) of the present invention.
`Fig. 3 is a schematic drawing showing a method of processing the surface of a crystal substrate to form concaves
`and convexes having a slant in the embodiment (I) of the present invention.
`Fig. 4 is a schematic drawing showing one example of the crystal growth method for forming a concavo-convex
`refractive index interface in the embodiment (II) of the present invention.
`Fig. 5 is a schematic drawing showing another example ofthe crystal growth method for forming a concavo-convex
`refractive index interface in the embodiment (II) of the present invention.
`Fig. 6 is a schematic drawing showing a variation of the crystal growth methods shown in Figs. 4 and 5.
`Fig. 7 is a schematic drawing showing another example of the crystal growth method for forming a concave-convex
`refractive index interface in the embodiment (II) of the present invention.
`Fig. 8 is a schematic drawing showing the structure of a conventional GaN group light-emitting element.
`Fig. 9 is a schematic drawing showing one example of a conventional light-emitting diode using ln0.05Ga0.95N as
`a material of a light-emitting layer.
`
`Detailed Description of the Invention
`
`[0026] Since the problem of the present invention has the most important significance for a light-emitting element,
`the light-emitting element of the present invention is most preferably in the form of an LED. While the materials are not
`limited, an LED using a GaN group material (GaN group LED), wherein the usefulness of the present invention becomes
`particularly remarkable as shown below, is taken as an example, and such light-emitting element is explained.
`[0027] The light-emitting element enhances, in any embodiment, the light-extraction efficiency by the action and
`effect of a concavo-convex refractive index interface formed downward of the light-emitting layer. The light-emitting
`element can be further grouped into the above-mentioned embodiment (I) and embodiment (II) based on how this
`concavo-convex refractive index interface is formed.
`[0028]
`In the above-mentioned embodiment (I), concaves and convexes are processed on a crystal substrate and
`buried with a semiconductor crystal (particularly GaN group crystal) to constitute a concave-convex refractive index
`interface.
`[0029]
`In the above-mentioned embodiment (II), a GaN group crystal is grown in the concaves and convexes, which
`are buried with a different GaN group crystal to constitute a concave-convex refractive index interface.
`[0030] First, the above-mentioned embodiment (I) is explained. Fig. 1 (a) shows a GaN group LED as an example
`of the structure of the light-emitting element of embodiment (I), wherein concaves and convexes 1 a are processed on
`the surface of the first crystal layer (hereinafter to be also referred to as the "first layer'') 1, and a second crystal layer
`(hereinafter to be also referred to as the "second layer") 2 made of a material having a refractive index different from
`that of the aforementioned crystal layer is grown directly or via a buffer layer, while burying the concaves and convexes.
`As a result, an interface having a different refractive index is formed in a concavo-convex form. Still thereon is laminated
`a semiconductor crystal layer (n-type contact layer 3, light-emitting layer A, p-type contact layer 4) by crystal growth,
`and electrodes P1 and P2 are formed to give an element structure. While the element structure in this Figure is a simple
`DH structure, an exclusive contact layer, an exclusive clad layer and the like may be formed, and the light-emitting
`layer may be an SQW structure or MQW structure, possibly having any structure as a light-emitting element.
`[0031] Due to the above-mentioned constitution, the light produced in the light-emitting layer A, which propagates
`in the lateral direction, is influenced by a concavo-convex refractive index interface 1 a, which causes a kind of mode
`conversion (change in the light direction to the direction of surface emission), and advances in a direction other than
`the lateral direction. As a result, the amount of the light heading toward the extraction surface increases, and the light
`absorption layer in the element decreases. As a result, the light-extraction efficiency is improved.
`[0032] As stated in the description of the prior art, the light that advances in the direction other than toward the
`extraction outlet of the light (e.g., downward direction and lateral direction) has been conventionally made to head
`toward the outlet solely by simple reflection of the light by an end surface.
`[0033]
`In contrast, in the present invention, a GaN group semiconductor layer region formed on a substrate by 'epi(cid:173)
`taxial growth is regarded [a waveguide that propagates light in the lateral direction], and along the waveguide, a con(cid:173)
`cava-convex refractive index interface is formed at a position capable of affecting the light directed to the lateral direc(cid:173)
`tion, thereby causing a kind of mode conversion (or diffused reflection) to direct the light toward a different direction.
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`EP 1 378 949 A1
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`[0034] The present invention takes note of the fact that the light propagating in the lateral direction propagates in
`the lateral direction as an electromagnetic wave having an electric field widely expanded to the upper and lower layers
`thereof, with the light-emitting layer as a center. The thickness of the light-emitting layer is approximately 10 nm-1 00
`nm for the active layer of a typical DH structure. The light in the lateral direction propagates not only within such a thin
`active layer but also in the lateral direction as a wave having a wide distribution width reaching the crystal substrate.
`As shown in Fig. 1 (a), therefore, when a concavo-convex refractive index interface 1 a is formed within the light dis(cid:173)
`tribution in the lateral direction, the wave of the light in the lateral direction is influenced and a certain amount thereof
`can be directed to a different direction by a sort of mode conversion (or by producing a diffused reflection), which in
`turn increases the amount of light that goes outside. These concaves and convexes also function as a reflection surface
`that causes upwardly diffused reflection' for the light emitted from the light-emitting layer toward the concaves and
`convexes per se.
`[0035] Moreover, these concaves and convexes also function to lower the reflectance, in the perpendicular direction,
`of the interface of GaN group semiconductor layer/sapphire substrate. Thus, it is possible to suppress occurrence of
`a standing wave in the vertical direction, thereby to allow a large amount of light to enter the sapphire substrate, increase
`the amount of light to be extracted from the sapphire substrate, and enhance the light-extraction efficiency, particularly
`when extracting the light from the substrate side.
`[0036]
`In embodiment (I), the concaves and convexes to be processed on the surface of a first layer are those formed
`by the surface itself of the first layer. They are different from the concaves and convexes formed when a mask layer
`made from Si02 and the like, used for the conventionally known lateral growth method, is applied to a flat surface.
`[0037]
`In addition; the above-mentioned constitution makes it possible to preferably reduce the dislocation density
`of the' GaN group crystal grown on a crystal substrate. By this constitution, dislocation density can be reduced by a
`one time growth without using a mask layer for ELO.
`[0038] Namely, by the ELO method using a mask, a GaN film is grown on a base, once taken out from the growth
`apparatus to form a mask, then returned to a growth apparatus for re-growth. In contrast, by a growth method comprising
`formation of concaves and convexes on a crystal substrate, the growth does not need to be stopped once a crystal
`substrate after concave- convex processing is set' in a growth apparatus. As a result, a re-growth interface is void and
`a crystal having fine crystallinity can be produced.
`[0039] According to the above-mentioned constitution of the present invention, moreover, since a GaN group crystal
`layer is grown without using a mask, the problems of staining with impurities due to decomposition of the mask and
`degradation of crystal quality are obliterated.
`[0040] These actions and effects afford a fine crystal having less dislocation, thus strikingly increasing the emission
`output. In addition, since the dislocation density that causes degradation reduces, a longer life can be obtained.
`[0041] The whole location pattern of concaves and convexes may be any as long as it can exert an influence on the
`wave of light in the lateral direction, and may be a pattern wherein dot-like concave parts (or convex parts) are se-
`quenced on the surface (standard plane) of the first layer, or a stripe like concavo-convex pattern wherein linear or
`curved concave grooves (or convex ridges) are sequenced at certain intervals. A pattern of convex ridges arranged in
`a lattice shape can be said to be an arrangement of angular concave parts. Of these, a stripe like concavo-convex
`pattern can exert a strong influence on the light in the lateral direction.
`[0042] The cross sectional shape of the concaves and convexes may be a rectangular (including trapezoid) wave
`as shown in Fig. 2 (a), a triangular wave and a sine curve as shown in Fig. 3 (c), a combined wave of these and the like.
`[0043] For the specification of the details of the concaves and convexes, the concavo-convex structure for crystal
`growth, which is formed for the'reduction of dislocation density of the GaN group crystal to be mentioned later, may be
`referred to.
`[0044] Furthermore, for the concaves and convexes to exert an influence on the light in the lateral direction, the
`concaves and convexes are preferably within a specific distance from the light-emitting layer. This distance is about
`0.5 11m - 20 11m, particularly preferably 1 11m- 10 11m, shown by kin Fig. 1 (a), and includes the distance between the
`upper surface of the substrate and the undersurface of the light-emitting layer in general LEOs. Thus, when an element
`structure is constructed by using a crystal substrate of the element as a first layer, forming concaves and convexes
`thereon, and growing a second layer to bury them, the concaves and convexes sufficiently exert an influence on the
`light in the lateral direction.
`[0045] The materials of the light-emitting element may be conventionally known materials such as GaAs group, lnP
`group, GaN group and the like. The usefulness of the present invention becomes highly remarkable in a GaN group
`light-emitting element (at least a material of a light-emitting layer being a GaN group semiconductor) having a major
`problem of reduction of dislocation density of crystal. In a GaN group light-emitting element, reduction of dislocation
`density of GaN group crystal is an essential assumption for forming an element. In the present invention, reduction of
`dislocation density of GaN group crystal is achieved by providing a growth method using a useful concavo-convex
`structure as in the following. Because the concavo-convex structure can serve both as the concaves and convexes of
`the above-mentioned refractive index interface, the usefulness of the concaves and convexes can be enhanced as
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`EP 1 378 949 A1
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`compared to concaves and convexes formed only for the purpose of refractive index interface. In the following, a GaN
`group crystal growth method using such concave-convex structure is explained.
`[0046] The GaN group crystal growth method using a concave-convex structure comprises, as shown in Fig. 2 (a),
`processing concaves and convexes 1 a on the surface of a crystal substrate (first layer) 1, and growing, as shown in
`Fig. 2 (b) , GaN group crystals 21, 22 from the concave parts and convex parts thereof while substantially forming a
`facet structure, whereby, as shown in Fig. 2 (c), said concaves and conve