Patent Publication Number: US-2016240737-A1

Title: Light-emitting device and production method therefor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flip-chip type Group III nitride semiconductor light-emitting device using a Group III nitride semiconductor substrate and exhibiting improved light extraction performance. The present invention also relates to a production method therefor. 
     2. Background Art 
     Conventionally, in the flip-chip type Group III nitride semiconductor light-emitting device, a concave and convex structure is formed on a rear surface of a sapphire substrate (a surface opposite to the surface on which a semiconductor layer is formed) to improve light extraction performance. Moreover, when the flip-chip type light-emitting device is resin sealed, the sapphire substrate is covered with a resin material, and there is a reflection at an interface between the resin material and the sapphire substrate, resulting in deterioration of light extraction performance. Therefore, an antireflection film is formed on the rear surface of the sapphire substrate to reduce reflection between the rear surface of the sapphire substrate and the resin material, thereby improving the light extraction performance. 
     A sapphire substrate has been widely used as a growth substrate of the Group III nitride semiconductor light-emitting device. Recently, a GaN substrate has come to be used. 
     Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2001-217467 
     Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2006-128202 
     In the flip-chip type Group III nitride semiconductor light-emitting device using the GaN substrate, the GaN substrate comes into contact with the sealing resin when resin sealed. However, more light is reflected at the interface between the resin and the GaN substrate because of a large relative refractive index difference between the resin material and GaN. There was a problem that the light extraction performance is not sufficiently improved simply by forming the concave and convex structure on the rear surface of the GaN substrate or the antireflection film. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the present invention is to improve light extraction performance in a flip-chip type Group III nitride semiconductor light-emitting device using a Group III nitride semiconductor substrate. 
     In one aspect of the present invention, there is provided a flip-chip type light-emitting device in which a Group III nitride semiconductor layer is disposed on one surface of a Group III nitride semiconductor substrate, and other surface of the substrate is covered with a sealing material, the light-emitting device comprising: 
     an uneven structure with ridges and recesses formed on a surface opposite to the semiconductor layer side of the substrate; and 
     an antireflection film formed continuously on the uneven structure and side surfaces of the substrate along the ridges and recesses of the uneven structure without being filled in recesses, which is made of a material having a refractive index smaller than that of the substrate and larger than that of the sealing material to prevent reflection between the substrate and the sealing material by the interference of light; and 
     wherein areas of the side surfaces of the substrate at the uneven structure side are inclined with respect to a vertical direction of the main surface of the substrate, and other areas of the side surfaces of the substrate are vertical to the main surface of the substrate; and 
     wherein the antireflection film is formed on the inclined side surface areas of the substrate and not formed on the vertical side surface areas of the substrate. 
     In the present specification, unless otherwise specified, the refractive index is a value at the peak emission wavelength. 
     The sealing material to seal the light-emitting device includes silicone resin, epoxy resin, and glass. 
     The antireflection film may be formed of any material having a refractive index smaller than that of the substrate and larger than that of the sealing material. When a GaN substrate is employed, the antireflection film may be formed of, for example, HfO 2 , ZrO 2 , AlN, SiN, TiO 2 , and Ta 2 O 5 . 
     The antireflection film may be a single layer or a plurality of layers. 
     The thickness of the antireflection film is, preferably, 80 nm to 100 nm. Such a thickness can further prevent reflection and improve the transmittance, thereby improving the light extraction performance of the light-emitting device. 
     The standard deviation of the thickness of the antireflection film is, preferably, not more than 10 nm. Such a uniform thickness can improve the light extraction performance. The antireflection film having such a uniform thickness can be formed, for example, through ALD. 
     In the other aspect of the present invention, there is provided a method for producing a flip-chip type light-emitting device in which a Group III nitride semiconductor layer is disposed on one surface of a Group III nitride semiconductor substrate, and other light output surface of the substrate is covered with a sealing material, the method for producing the light-emitting device comprising the steps of forming an isolation trench on the light output surface of the substrate to separate a wafer for each device; forming an uneven structure with ridges and recesses on the light output surface of the substrate; and forming an antireflection film continuously along the ridges and recesses of the uneven structure and the side surfaces of the substrate through ALD, which is made of a material having a refractive index smaller than that of the substrate and larger than that of the sealing material to prevent reflection between the substrate and the sealing material by the interference of light. 
     In the step of forming an isolation trench, the isolation trench may be formed by laser scanning, dry etching, and dicer cutting. Particularly, laser scanning is preferable because a deep isolation trench can be formed without destroying the substrate. 
     In the step of forming an uneven structure, the uneven structure may be formed, for example, by wet etching. 
     In the present invention, an antireflection film having a uniform thickness is formed along the ridges and recesses on the rear surface of the substrate and the side surfaces of the substrate, thereby improving the light extraction performance of the light-emitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various other objects, features, and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: 
         FIG. 1  shows a structure of a light-emitting device according to Embodiment 1; 
         FIGS. 2A to 2F  are sketches showing processes for producing the light-emitting device according to Embodiment 1; 
         FIG. 3  is a graph showing the comparison of the light outputs between the light-emitting device according to Embodiment 1 and the light-emitting device according to Comparative Example; and 
         FIG. 4  is a graph showing the relationship between the angle average of transmittance and the thickness of Al 2 O 3  layer. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     A specific embodiment of the present invention will next be described with reference to the drawings. However, the present invention is not limited to the embodiment. 
     Embodiment 1 
       FIG. 1  shows a structure of a flip-chip type light-emitting device according to Embodiment 1. As shown in  FIG. 1 , the light-emitting device according to Embodiment 1 comprises a GaN substrate  10 ; a Group III nitride semiconductor layer  11  disposed on a surface of the GaN substrate  10 , in which an n-type layer  11   a , a light-emitting layer  11   b , and a p-type layer  11   c  are sequentially deposited from the GaN substrate  10  side; a p-electrode  13 ; an n-electrode  14 ; and an antireflection film  15 . The light-emitting device according to Embodiment 1 has a flip-chip type (face-down type) structure which reflects light emitted from the light-emitting layer  11   b  by the p-electrode  13 , transmits the light through the GaN substrate  10 , and extracts the light from a rear surface  10   a  of the GaN substrate  10 . 
     The GaN substrate  10  has a c-plane main surface. On a surface having Ga polarity (hereinafter, referred to as a surface) of two surfaces of the GaN substrate  10 , a semiconductor layer  11  is formed. Moreover, on the rear surface  10   a  of the GaN substrate  10  (a surface opposite to the semiconductor layer  11  side, a surface having N polarity), an uneven structure  16  is formed. 
     The uneven structure  16  has a structure in which a plurality of fine projections and grooves (ridges and recesses or convexes and concaves) are randomly and two-dimensionally arranged. Each of the projections or grooves has a cone or pyramid shape. Such uneven structure  16  is obtained by wet etching the rear surface  10   a  of the GaN substrate  10 . The method for forming an uneven structure will be described later. An angle between the main surface of the GaN substrate  10  and side surfaces of ridges and recesses falls within a range of 110° to 130°. The depth of the uneven structure is 0.1 μm to 10 μm. The light confined in the GaN substrate  10  can be extracted from the rear surface  10   a  of the GaN substrate  10  by the uneven structure  16 , thereby improving the light extraction performance. 
     The uneven structure  16  may be a moth-eye structure in which the projections are periodically arranged and the arrangement period of the projections is equal to or smaller than the emission wavelength. 
     Of the side surfaces  10   b  of the GaN substrate  10 , areas  10   b   1  of the rear surface  10   a  side of the GaN substrate  10  (the uneven structure  16  side) are inclined so that the cross section parallel to the main surface of the GaN substrate  10  is decreased toward the rear surface  10   a  of the GaN substrate  10 . The inclination angle falls within a range of 140° to 160° to the main surface of the GaN substrate  10 . Of the side surfaces  10   b  of the GaN substrate  10 , areas  10   b   2  of the semiconductor layer  11  side of the GaN substrate are vertical to the main surface of the GaN substrate  10  (at an angle of 80° to 90° to the main surface of the GaN substrate, considering errors). A partial area of the side surfaces  10   b  of the GaN substrate  10  is inclined because an isolation trench  20  was formed on the rear surface  10   a  of the GaN substrate  10  to easily separate a wafer for each device in the method for producing the light-emitting device according to Embodiment 1. The details will be described in the method for producing the light-emitting device later. 
     The semiconductor layer  11  is formed of Group III nitride semiconductor, in which an n-type layer  11   a , a light-emitting layer  11   b , and a p-type layer  11   c  are sequentially deposited from the GaN substrate  10  side on the surface of the GaN substrate  10 . The n-type layer  11   a  has a structure in which an n-type contact layer, an ESD layer, and an n-type cladding layer are sequentially deposited from the GaN substrate  10  side. The light-emitting layer  11   b  has a MQW structure in which a well layer and a barrier layer are repeatedly deposited. The p-type layer  11   c  has a structure in which a p-type cladding layer and a p-type contact layer are sequentially deposited from the light-emitting layer  1   b  side. 
     The p-electrode  13  is formed so as to cover almost the entire surface of the p-type layer  11   c . The p-electrode  13  is a reflective electrode which reflects the light emitted from the light-emitting layer  11   b  to the GaN substrate  10 . The p-electrode  13  may be formed of, for example, Ag, Al, or alloy mainly containing Ag or Al. 
     A part of the semiconductor layer  11  is etched, and a trench is formed so as to have a depth reaching the n-type layer  11   a  from the surface of the p-type layer  11   c  (the surface opposite to the light-emitting layer  11   b  side). The n-electrode  14  is formed on the n-type layer  11   a  exposed at the bottom surface of the trench. 
     The structures of the semiconductor layer  11 , the p-electrode electrode  13 , and the n-electrode  14  are not limited to the above. Any structure employed as a flip-chip type structure of the conventional Group III nitride semiconductor light-emitting device may be employed. 
     The antireflection film  15  is continuously formed along the rear surface  10   a  and the side surfaces  10   b  of the GaN substrate  10 . However, in the area of the side surfaces  10   b , the antireflection film  15  is formed only on the inclined areas  10   b   1  of the side surfaces  10   b  of the rear surface  10   a  side of the GaN substrate  10 , and not formed on the remaining areas (vertical areas  10   b   2  of the side surfaces  10   b  of the semiconductor layer  11  side of the GaN substrate  10 ). Moreover, the antireflection film  15  is formed along the ridges and recesses of the uneven structure  16  without being filled on the rear surface  10   a  side of the GaN substrate  10 . 
     The antireflection film  15  is formed to prevent reflection at the interface between the rear surface  10   a  of the GaN substrate  10  and the sealing material and at the interface between the side surfaces  10   b  of the GaN substrate  10  and the sealing material and thereby to improve the light extraction performance. Reflection is prevented by the interference of light. The reflected lights interfere with each other to weaken each other by that the refractive index is set to an intermediate value between the refractive indices of the GaN substrate  10  and the sealing material and the thickness of the antireflection film  15  is set to a specific value, thereby preventing light reflection. Moreover, the antireflection film  15  is formed to cover the uneven structure  16 , thereby improving the durability or chemical resistance of the uneven structure  16  formed on the rear surface  10   a  of the GaN substrate  10 . 
     As the material sealing the light-emitting device according to Embodiment 1, a resin material such as silicone resin and epoxy resin, or glass is employed. 
     The antireflection film  15  has a very uniform thickness of 100 nm, and the standard deviation is not more than 10 nm. This is because the antireflection film  15  is formed by ALD. More preferably, the standard deviation is not more than 5 nm. The antireflection film  15  formed on the uneven structure  16  and the antireflection film  15  formed on the areas  10   b   1  on the side surfaces  10   b  of the GaN substrate  10  have the same thickness. 
     The thickness of the antireflection film  15  is not limited to the above, and any other thickness may be employed as long as reflection is reduced by the interference of light. Preferably, the thickness of the antireflection film  15  is 80 nm to 100 nm. Within this range, the antireflection film  15  has a high light transmittance in the emission wavelength range of the Group III nitride semiconductor light-emitting device (for example, peak wavelength of 400 nm to 500 nm, especially 440 nm to 460 nm). More preferably, the thickness of the antireflection film  15  is 85 nm to 95 nm. 
     In Embodiment 1, the antireflection film  15  is made of Al 2 O 3  having a refractive index of 1.65, and any material may be used as long as the refractive index is smaller than that of the GaN substrate  10  and larger than that of the sealing material. The refractive index of GaN is 2.4, and when the sealing material is resin, the refractive index of resin is approximately 1.5. Therefore, the refractive index of the antireflection film  15  may be more than 1.5 and less than 2.4, more preferably, 1.6 to 2.3, and further preferably, 1.7 to 2.2. For example, the antireflection film  15  may be formed of HfO 2 , ZrO 2 , AlN, SiN, TiO 2 , and Ta 2 O 5  other than Al 2 O 3 . 
     Next will be described the processes for producing the light-emitting device according to Embodiment 1 with reference to  FIG. 2 . 
     Firstly, a semiconductor layer  11  is formed by forming an n-type layer  11   a , a light-emitting layer  11   b , and a p-type layer  11   c  sequentially on a surface having Ga polarity of a GaN substrate  10  having a +c-plane main surface through MOCVD (refer to  FIG. 2A ). 
     The raw material gases employed for MOCVD are as follows: ammonia (NH 3 ) as a nitrogen source, trimethylgallium (Ga(CH 3 ) 3 ) as a Ga source, trimethylindium (In(CH 3 ) 3 ) as an indium source, trimethylaluminum (Al(CH 3 ) 3 ) as an aluminum source, silane (SiH 4 ) as an n-type dopant gas, and bis(cyclopentadienyl)magnesium (Mg(C 5 H 5 ) 2 ) as a p-type dopant gas, and H 2  or N 2  as a carrier gas. 
     Subsequently, a part of the semiconductor layer  11  is dry etched from the surface of the p-type layer  11   c  (the surface opposite to the light-emitting layer  11   b  side) to form a trench having a depth reaching the n-type layer  11   a . A p-electrode  13  is formed so as to cover almost the entire surface of the p-type layer  11   c , and an n-electrode  14  is formed on the n-type layer  11   a  exposed at the bottom of the trench (refer to  FIG. 2B ). An isolation trench is also formed at the same time. 
     Next, the GaN substrate  10  is thinned by grinding the rear surface  10   a  of the GaN substrate  10  (refer to  FIG. 2C ). Thus, the thickness of the GaN substrate  10  is 50 μm to 200 μm. Mechanical grinding, CMP grinding, or a combination thereof is employed. Thinning the GaN substrate  10  facilitates a step of separating a wafer for each device later. 
     Subsequently, an isolation trench  20  is formed by laser scanning on an area to separate the wafer for each device, of the rear surface  10   a  of the GaN substrate  10  (refer to  FIG. 2D ). The depth of the isolation trench  20  is half the thickness of the GaN substrate  10 . Forming the isolation trench  20  facilitates a step of separating the wafer for each device later. Moreover, the cross section of the isolation trench  20  is formed in a V shape, and the side surfaces of the isolation trench  20  are inclined to the main surface of the GaN substrate  10  at an angle of 140° to 160°. The side surfaces of the isolation trench  20  correspond to the arears  10   b   1  of the side surfaces  10   b  of the GaN substrate  10  (refer to  FIG. 1 ). 
     The depth of the isolation trench  20  is not necessarily half the thickness of the GaN substrate  10 , and is preferably 0.2 to 0.7 times, more preferably 0.3 to 0.6 times the thickness of the GaN substrate  10 . 
     The isolation trench  20  may be formed by dry etching, dicer cutting other than laser scanning. However, laser scanning is preferable as in Embodiment 1 because no chips or cracks occur in the GaN substrate  10 , and a deep isolation trench  20  can be formed. A nano-second laser may be used, for example, with a wavelength of 255 nm, a pulse width of 20 ns to 40 ns, a pulse frequency of 10 Hz to 20 Hz, and an energy per pulse of 0.06 to 0.12 V. 
     Next, the rear surface  10   a  of the GaN substrate  10  is wet etched by TMAH(Tetramethylammonium Hydroxide). TMAH is a solution having a concentration of 25% and a temperature of 60° C., and etching was performed for 60 minutes. Wet etching of GaN by TMAH has face orientation dependency. Therefore, the rear surface  10   a  of the GaN substrate  10  having N-polarity is wet etched so that fine ridges and recesses remain, to form the uneven structure  16  (refer to  FIG. 2E ). The ridges and recesses become fine by wet etching, thereby improving the light extraction performance, and facilitating the formation of the uneven structure  16 . 
     A strong alkaline aqueous solution such as KOH and NaOH, or phosphoric acid may be used other than TMAH as a wet etching solution to form the uneven structure  16 . The uneven structure  16  may be formed by dry etching, and by both wet etching and dry etching. For example, a two-stage uneven structure may be formed, in which fine ridges and recesses are formed by wet etching and large-scale ridges and recesses are formed by dry etching. 
     Next, through ALD (Atomic Layer Deposition), an antireflection film  15  made of Al 2 O 3  is formed along the ridges and recesses of the uneven structure  16  on the rear surface of the GaN substrate so as not to be filled in the recesses, and along the side surfaces of the isolation trench  20  (refer to  FIG. 2F ). In ALD, TMA and H 2 O were used as a precursors gas, the temperature was 50° C. to 300° C., and the pressure was 1×10 3  Pa or less. The antireflection film  15  was formed so as to have a thickness of 80 nm to 100 nm. 
     Since Al 2 O 3  atomic layers can be formed one by one using ALD, the antireflection film  15  can have a uniform thickness. Moreover, the thickness of the antireflection film  15  can be precisely controlled in units of atomic layer thickness. Therefore, the antireflection film  15  can be homogenously formed along the ridges and recesses of the uneven structure  16 . Moreover, by using ALD, the antireflection film  15  can be formed so as to cover the side surfaces of the isolation trench  20  as well as the rear surface  10   a  of the GaN substrate  10 . The thickness of the antireflection film  15  on the side surfaces of the isolation trench  20  is equal to that on the rear surface  10   a  of the GaN substrate  10 , and a uniform thickness is achieved. 
     Subsequently, a scribe line is formed by moving a dicer or scriber along the trench for separating the wafer for each device (at a position facing the isolation trench  20 ) on the +c-plane main surface of the GaN substrate  10  to separate the wafer for each device at the isolation trench  20  and the scribe line position by applying stress. At this time, the areas that have already exposed as the side surfaces of the isolation trench  20  become the areas  10   b   1  of the side surfaces  10   b  of the GaN substrate  10 , which are inclined in a direction perpendicular to the main surface of the GaN substrate  10 . The areas that are newly exposed by separating the wafer for each device become the areas  10   b   2  of the side surfaces  10   b  of the GaN substrate  10 , which are perpendicular to the main surface of the GaN substrate  10 . Through the steps described above, the light-emitting device shown in  FIG. 1  is produced. Since the isolation trench  20  is formed on the areas for separating the wafer for each device on the rear surface  10   a  of the GaN substrate  10 , the wafer can be easily separated for each device. 
     From the above, in the light-emitting device according to Embodiment 1, the antireflection film  15  is formed along the ridges and recesses of the uneven structure  16  on the rear surface  10   a  of the GaN substrate  10  and along the areas  10   b    1  of the side surfaces  10   b  of the GaN substrate  10 , and deviation in thickness of the antireflection film  15  is very small. Thus, the light reflection is effectively prevented at the interface between the rear surface  10   a  of the GaN substrate  10  and the sealing material, thereby improving the light extraction performance. 
     Experiment Results 
     The results of the experiments on the light-emitting device according to Embodiment 1 will be next described. 
       FIG. 3  is a graph showing the comparison of the light outputs between the light-emitting device according to Embodiment 1 and the light-emitting device according to Comparative Example 1. The light-emitting device according to Comparative Example has the same as that of the light-emitting device according to Embodiment 1 except for that the antireflection film  15  was omitted from the light-emitting device according to Embodiment 1. 
     As shown in  FIG. 3 , the light output of the light-emitting device according to Embodiment 1 was improved by 6.6% than that of the light-emitting device according to Comparative Example. It was found from this result that reflection was effectively prevented by the antireflection film  15 , and thereby the light extraction performance was improved. 
       FIG. 4  is a graph showing the relationship between the angle average of transmittance and the thickness of Al 2 O 3  layer in a model. The angle average of transmittance is defined as the average of transmittance with respect to various incident angles of a light. The model has a structure in which the Al 2 O 3  layer having a refractive index of 1.65 and the sealing material having a refractive index of 1.5 are sequentially deposited on the GaN layer having a refractive index of 2.4. When light is incident from the rear surface of the GaN layer (the surface opposite to the Al 2 O 3  layer side) of the model, the angle averages of transmittance were obtained by simulation by varying the thicknesses of the Al 2 O 3  layer. The angle average was taken for the incident angles from 0° to 90°. 
     As shown in  FIG. 4 , there was a peak where the angle average of transmittance is highest when the Al 2 O 3  layer has a thickness of 100 nm. It was found from the simulation results that in the light-emitting device according to Embodiment 1, the thickness of the antireflection film  15  is, preferably, in the vicinity of 90 nm, 80 nm to 100 nm, and more preferably, 85 nm to 95 nm. 
     Variations 
     In the present invention, the substrate is not limited to a GaN substrate, and a substrate made of any material may be employed as long as the substrate is formed of Group III nitride semiconductor. The conductive type of the substrate may be either n-type, p-type, or intrinsic. When the n-type Group III nitride semiconductor substrate is employed, Si or Ge may be used as an n-type impurity and Mg may be used as a p-type impurity. 
     In Embodiment 1, the antireflection film  15  was a single layer. However, it may comprise a plurality of layers. In that case, various structures conventionally used as a multi-layer antireflection film may be employed as the antireflection film of the present invention. However, the antireflection film  15  is preferably a single layer as in Embodiment 1, in view of the balance between the easiness of production and the improvement of transmittance. Since the characteristics can be changed depending on the layer structure in the case where the antireflection film comprises a plurality of layers, transmittance may be increased and material selection may be expanded than in the case where the antireflection film comprises a single layer. 
     The light-emitting device of the present invention can be employed as a light source of an illumination apparatus or a display apparatus.