JP2015192138A - Light-emitting element, epitaxial substrate for light-emitting element, method of manufacturing epitaxial substrate for light-emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-emitting element in which sufficient luminous efficiency is ensured even during high drive current.SOLUTION: A light-emitting element includes an active layer having a multiquantum well structure formed by laminating a well layer 31 composed of InGaN(0<x≤1) and a barrier layer 32 composed of GaN repeatedly and alternately. A plurality of pits P are formed in the well layer while being dispersed to penetrate the well layer, and the barrier layer 32 has a portion to be buried in the pit P formed in the well layer 31 immediately below.

Description

  The present invention relates to a light emitting device, and more particularly to a light emitting device having a multiple quantum well structure.

  A light-emitting diode (LED) having a light-emitting layer made of a group 13 nitride (group III nitride) can be used as a long-life, high-efficiency white light source, and in recent years, various conventional light sources have been replaced with LED light sources. Progressing. Low-brightness LED light sources such as backlights and light bulbs are already in widespread use, and are being developed for high-brightness applications such as projectors and automobile headlights.

  When the InGaN layer is included in the recombination layer that is the light emitting region of the LED, the growth temperature is generally lowered to increase the In composition in the InGaN layer. This is because when the In composition is high, it is easily decomposed in a high-temperature reducing atmosphere. In addition, it is already known that a region where the In composition is locally high exists in the InGaN layer due to a phenomenon in which indium (In) aggregates during film formation (see, for example, Patent Document 1).

  In manufacturing an LED using a group 13 nitride, if electrons injected from the n-type region leak into the p-type region beyond the active layer, the luminous efficiency is lowered. Has been made. For example, a structure in which an electron blocking layer using AlGaN or the like having a larger band gap energy than GaN is disposed between the p-GaN layer and the active layer to suppress leakage of electrons to the p-type region is already known. (For example, refer to Patent Document 2).

In addition, a method of forming a quantum box by etching a nitride semiconductor layer containing indium with H ₂ gas is already known (for example, see Patent Document 3).

On the other hand, in LEDs, it is already known that a phenomenon called “efficiency droop” occurs in which the light emission efficiency decreases when driven at a high driving current (for example, a current density of 200 A / cm ² or more) (for example, non Patent Document 1).

JP 2001-196632 A JP 2009-218235 A Japanese Patent No. 3773713

“Efficiency droop in nitride-based light-emitting diodes” Joachim Piprek, physica status solidi (a)., Volume 207, Issue 10, pages 2217-2225, October 2010.

  In order to realize an LED with sufficient luminous efficiency even at a high driving current, it is necessary to suppress the occurrence of efficiency droop. The efficiency droop is considered to be one of the causes that the leakage of electrons beyond the active layer from the n-type region to the p-type region more significantly occurs as the current density of the driving current is increased. Yes. This means that the provision of an electron blocking layer as disclosed in Patent Document 3 is not sufficient to cope with efficient droop.

  In order to suppress a decrease in light emission efficiency at a high driving current, it is considered effective to increase the probability that holes are injected into the multiple quantum well layer even at a high driving current.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a light emitting element in which sufficient light emission efficiency is ensured even at a high driving current.

In order to solve the above-mentioned problem, the invention of claim 1 is a light emitting device, in which a well layer made of In _x Ga _1-x N (0 <x ≦ 1) and a barrier layer made of GaN are alternately and alternately stacked. An active layer having a multiple quantum well structure, and a plurality of pits are dispersedly formed in the well layer so as to penetrate the well layer, and the barrier layer is formed immediately below the well. It has the part embedded in the said some pit formed in the layer, It is characterized by the above-mentioned.

  A second aspect of the present invention is the light emitting device according to the first aspect, wherein the existence ratio of the plurality of pits in the well layer is 2% or more and 25% or less.

  A third aspect of the present invention is the light emitting device according to the second aspect, wherein the existence ratio of the plurality of pits in the well layer is 8% or more and 25% or less.

According to a fourth aspect of the present invention, there is provided an epitaxial substrate for a light emitting device, comprising: a base substrate made of GaN; and In _x Ga _1-x N (0 <x ≦ 1) epitaxially formed on the base substrate. An active layer having a multi-quantum well structure formed by alternately and alternately stacking well layers and barrier layers made of GaN, wherein the well layer has a plurality of pits penetrating the well layer The barrier layer has a portion embedded in the plurality of pits formed in the well layer immediately below.

The invention according to claim 5 is a method of manufacturing an epitaxial substrate for a light emitting device, comprising an active layer forming step of forming an active layer on a base substrate made of GaN, wherein in the active layer forming step, In A well layer forming step for epitaxially forming a well layer made of _x Ga _1-x N (0 <x ≦ 1) and an annealing process for annealing the surface of the well layer with an atmosphere gas containing nitrogen gas and hydrogen gas are performed. Thus, an annealing treatment step for dispersing and forming a plurality of pits on the surface of the well layer and a barrier layer formation step for epitaxially forming a barrier layer made of GaN on the well layer after the annealing treatment are repeated in this order. Thus, the active layer having a multiple quantum well structure is formed.

  The invention of claim 6 is the method for manufacturing an epitaxial substrate for a light emitting device according to claim 5, wherein, in the annealing treatment step, the existence ratio of the plurality of pits in the well layer is 2% or more and 25% or less. The annealing treatment is performed so that

  The invention according to claim 7 is the method for manufacturing an epitaxial substrate for a light-emitting element according to claim 6, wherein, in the annealing treatment step, the existence ratio of the plurality of pits in the well layer is 8% or more and 25% or less. The annealing treatment is performed so that

  According to the first to seventh aspects of the present invention, in the light emitting device in which the active layer has a multiple quantum well structure, a plurality of pits are dispersedly formed in the well layer so that the hole-electron recombination region is formed in the active layer. Can be dispersed throughout. As a result, the efficiency droop can be suppressed while ensuring the light emission intensity, so that a light emitting element having sufficient light emission efficiency even at a high driving current can be obtained.

1 is a diagram schematically showing a configuration of an epitaxial substrate 10. FIG. 1 is a diagram schematically showing a configuration of a light-emitting element 20 manufactured using an epitaxial substrate 10. FIG. 1 is a cross-sectional TEM (transmission electron microscope) image of an example of an epitaxial substrate 10. It is a figure which illustrates the cross-sectional profile of the board | substrate for pit evaluation obtained by AFM measurement.

  The group numbers in the periodic table shown in this specification are based on the 1 to 18 group number designations according to the 1989 International Union of Pure Applied Chemistry (IUPAC) revised inorganic chemical nomenclature. , Group 13 refers to aluminum (Al), gallium (Ga), indium (In), and the like, and Group 15 refers to nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and the like.

<Outline of light emitting element>
FIG. 1 is a diagram schematically showing a configuration of an epitaxial substrate 10 used for manufacturing a light emitting device according to the present embodiment. FIG. 2 is a diagram schematically showing the configuration of the light emitting element 20 manufactured using the epitaxial substrate 10.

  The epitaxial substrate 10 is roughly formed of an n-type GaN layer 2 and an active layer (light emitting layer) 3 having a multiple quantum well structure (MQW structure) on a GaN base substrate 1 by a known epitaxial growth method such as MOCVD. The p-type cladding layer 4 and the p-type GaN layer 5 are stacked in this order (epitaxially formed). In the following description, the case where each layer is formed using the MOCVD method will be described.

  In addition, the light emitting element 20 is formed on the epitaxial substrate 10 in a so-called mother substrate state by using a known photolithography technique, etching technique, and film forming technique, and the anode electrode portion 6 and the cathode electrode in each light emitting element 20. After the electrode pattern to be the part 7 is formed, it is separated into a predetermined element size by a dicer or the like.

  As the GaN base substrate 1, a substrate having a c-plane, that is, a (0001) plane as a main surface and a flat surface at the atomic level is used. There is no particular limitation on the size of the GaN base substrate 1, but from the viewpoint of easy handling, a substrate having a diameter of about several inches and a thickness of about several hundred μm to several mm is preferable.

The n-type GaN layer 2 is a layer made of GaN doped with, for example, an n-type dopant such as Si. For example, the n-type GaN layer 2 may be formed from 1050 ° C. to TMG (trimethylgallium) gas as Ga source gas, silane gas as Si dopant source gas, ammonia gas as N source gas, nitrogen gas and hydrogen gas as carrier gases. It is preferable to be formed to a thickness of about 1 μm to 10 μm at a forming temperature of 1150 ° C. Moreover, it is preferable that the doping concentration of Si is about 2 × 10 ¹⁸ / cm ³ to 2 × 10 ¹⁹ / cm ³ . In the present specification, including the following, the formation temperature means the heating temperature for the GaN base substrate 1 when the target layer is formed.

The active layer 3 is a layer responsible for light emission in the light emitting element 20, and includes a well layer (first unit layer) 31 made of In _x Ga _1-x N (0 <x ≦ 1) and a barrier layer made of GaN (second layer). (Unit layer) 32 is a layer having a multiple quantum well structure (MQW structure) formed by alternately and repeatedly layering.

  The active layer 3 is preferably formed at a forming temperature of 700 ° C. to 850 ° C. Specifically, the well layer 31 is made of TMG gas as Ga source gas, TMI (trimethylindium) gas as In source gas, ammonia gas as N source gas, and nitrogen gas as carrier gas, about 2 nm to 10 nm. The barrier layer 32 is preferably formed to a thickness of about 5 nm to 15 nm using TMG gas as Ga source gas, ammonia gas as N source gas, and nitrogen gas as carrier gas. Is preferred. Further, the number of repetitions N of the well layer 31 and the barrier layer 32 is preferably 3 to 10.

  More specifically, although not shown in FIGS. 1 and 2, in the epitaxial substrate 10 (and the light emitting element 20) according to the present embodiment, each well layer 31 has an upper surface (laminated layer). A plurality of pits (indentations) P (see FIG. 3) opened to the side in the upper direction are provided in a dispersed manner. The pits P are formed by performing an annealing process after the well layer 31 is formed and before the barrier layer 32 is formed. Details of the pits P and the pits P provided in the active layer 3 will be described later.

The p-type cladding layer 4 is a layer made of Al _y Ga _1-y N (0 <y ≦ 1) doped with a p-type dopant such as Mg. The p-type cladding layer 4 uses TMG gas as Ga source gas, TMA (trimethylaluminum) gas as Al source gas, Cp ₂ Mg (Biz (cyclopentadienyl) magnesium) gas as Mg dopant source gas, ammonia gas It is preferable that the film is formed to a thickness of about 5 nm to 60 nm at a forming temperature of 950 ° C. to 1050 ° C. using N source gas and nitrogen gas and hydrogen gas as carrier gases. The Mg doping concentration is preferably about 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²⁰ / cm ³ . Note that providing the p-type cladding layer 4 is not an essential aspect, and an aspect in which the p-type GaN layer 5 is provided adjacent to the active layer 3 may be employed.

The p-type GaN 5 layer is a layer made of GaN doped with a p-type dopant such as Mg. The p-type GaN 5 layer is 900 ° C. to 1050 ° C. using TMG gas as Ga source gas, Cp ₂ Mg gas as Mg dopant source gas, ammonia gas as N source gas, nitrogen gas and hydrogen gas as carrier gases. The film is preferably formed to a thickness of about 50 nm to 300 nm at the forming temperature. The Mg doping concentration is preferably about 1 × 10 ¹⁹ / cm ³ to 1 × 10 ²⁰ / cm ³ .

  When the p-type cladding layer 4 and the p-type GaN layer 5 are doped with Mg, the epitaxial substrate 10 after both layers are formed is placed in a nitrogen atmosphere at a temperature of 750 ° C. to 950 ° C. for 5 minutes to An activation treatment is performed by heating for 30 minutes.

  The anode electrode portion 6 provided in the light emitting element 20 is preferably formed as an Ni / Au laminated film on the upper surface of the p-type GaN layer 5. The anode electrode layer 6 can be formed by patterning at a predetermined position of the epitaxial substrate 10 before singulation by a photolithography process and a vacuum deposition method.

  The thicknesses of the Ni film and the Au film constituting the Ni / Au laminated film serving as the anode electrode layer 6 are preferably about 0.01 μm to 0.05 μm and 0.05 μm to 0.5 μm, respectively.

  In addition, after forming the Ni / Au multilayer film to be the anode electrode portion 6, in order to improve the ohmic contact characteristics, heat treatment at a temperature of 500 ° C. or higher in a nitrogen atmosphere is performed for several minutes. preferable.

  The cathode electrode portion 7 provided in the light emitting element 20 is preferably formed as a Ti / Al / Ni / Au multilayer film. The cathode electrode portion 7 exposes a part of the n-type GaN layer 2 at a predetermined position of the epitaxial substrate 10 before singulation by a photolithography process and RIE (reactive ion etching), and then exposes the exposed portion. Further, it can be formed by patterning by a photolithography process and a vacuum deposition method. The cathode electrode portion 7 may be referred to as a cathode electrode pad 7.

  The thicknesses of the Ti film, Al film, Ni film, and Au film constituting the Ti / Al / Ni / Au multilayer film that becomes the cathode electrode portion 7 are 0.01 μm to 0.05 μm, 0.1 μm to 1 μm, and 0, respectively. It is preferably about 0.02 μm to 0.1 μm, 0.05 μm to 0.3 μm.

  In addition, after the formation of the Ti / Al / Ni / Au multilayer film serving as the cathode electrode portion 7, in order to improve the ohmic contact characteristics, heat treatment at a temperature of 900 ° C. or higher in a nitrogen atmosphere is performed. The heat treatment at 1000 ° C. is preferably performed for several tens of seconds to several minutes.

  In the light emitting element 20 having the above-described configuration, emission of light having an emission wavelength in the range of 450 nm to 460 nm can be obtained by passing a current between the anode electrode portion 6 and the cathode electrode portion 7. A specific emission wavelength is determined according to the composition of the well layer 31.

In the light emitting element 20, the band gap energy of Al _y Ga _1-y N constituting the p-type cladding layer 4 is larger than the band gap energy of GaN constituting the p-type GaN layer 5. The mold cladding layer 4 functions as an electron blocking layer.

<Pit formation and its effects>
Next, with regard to the pits P provided in the well layer 31 of the active layer 3, the formation mode and operational effects will be described.

FIG. 3 is a cross-sectional TEM (transmission electron microscope) image of an example of the epitaxial substrate 10 according to the present embodiment. In the epitaxial substrate 10 illustrated in FIG. 3, the well layer 31 is formed of In _0.1 Ga _0.9 N, the barrier layer 32 is formed of GaN, and the repetition number N is 5. In the TEM image of FIG. 3, a linear region that extends in the left-right direction as viewed in the drawing and looks blacker than its upper and lower portions is the well layer 31. And the discontinuous part and narrow part of the said linear area | region which exist in the middle of the well layer 31 which concerns in FIG. The pits P usually exist in a form that penetrates the well layer 31. The reason why the contrast in the region inside the pit P is thin is that the GaN forming the barrier layer 32 formed immediately above is buried in the pit P. In other words, the upper and lower barrier layers 32 are in contact with each other at the location where the pits P are formed. The upper surface of the barrier layer 32 (interface with the adjacent layer in the upper direction in the stacking direction) is flat. Further, the thickness of the barrier layer 32 described above does not include a portion embedded in the pit P. The pits P have a planar size of about several tens of nanometers and are uniformly distributed in each well layer 31.

  Such pits P are formed by performing an annealing process after the well layer 31 is formed and before the barrier layer 32 is subsequently formed, as described above. Specifically, in the annealing process, the formation of the well layer 31 having a predetermined thickness is completed (the supply of TMG and TMI gas is stopped) in the process of forming the epitaxial substrate 10 using a predetermined MOCVD apparatus, and then the GaN underlayer is formed. The heating state for the substrate 1 is realized by flowing at least nitrogen gas and hydrogen gas into the MOCVD apparatus as annealing atmosphere gases while maintaining the same heating state as when the well layer 31 is formed. The annealing atmosphere gas may further contain ammonia gas. Immediately after the annealing treatment, the atmospheric gas is switched to a gas type and flow rate suitable for forming the barrier layer 32 to form the barrier layer 32. At that time, the heating state of the GaN base substrate 1 is continuously maintained.

The formation of the pits P by such annealing treatment is that the regions having a high In composition are scattered in the well layer 31 made of In _x Ga _1-x N as disclosed in Patent Document 1. The property and the property that a region having a high In composition is more easily etched in a high-temperature reducing atmosphere. That is, the formation of the pits P by performing the annealing process subsequent to the formation of the well layer 31 is realized by preferentially etching a region of the well layer 31 having a higher In composition ratio than the surroundings. It is what is done.

  Usually, the longer the annealing time, the larger the pit presence ratio (pit ratio) in the well layer 31 tends to be.

  In the light emitting element 20 according to the present embodiment, by providing the pits P in the well layer 31 as described above, it is possible to suppress the efficiency droop at the time of a high driving current (decrease in light emission efficiency). This is because, in general, in a light-emitting element in which layers are configured in the order of an n-type layer, an active layer having a multiple quantum well structure, and a p-type layer like the light-emitting element 20, the hole moving speed is low. Where light emission (recombination of holes and electrons) tends to occur in a region near the p-type layer in the active layer, the pits P are dispersed in the well layer 31 in the light-emitting element 20 according to the present embodiment. As a result, a state in which holes easily move in the active layer 3 is realized, and as a result, the recombination region of holes and electrons is dispersed throughout the active layer 3 having the MQW structure. is there. For example, in the light-emitting element 20 provided with the pits P under suitable conditions, the degree of decrease in the light emission efficiency is approximately assured while maintaining the light emission intensity as compared with the light-emitting elements having a common configuration except that the pits P are not provided. It is reduced to 1/2 or less. That is, suppression of efficiency droop is realized.

  The excessive formation of the pits P is not preferable because it makes it difficult for recombination of holes and electrons to occur and lowers the emission intensity itself.

  As described above, according to the present embodiment, in the light emitting device in which the active layer has a multiple quantum well structure, a plurality of pits are dispersedly formed in the well layer, so that the recombination region of holes and electrons is formed. It can be dispersed throughout the active layer. As a result, the efficiency droop can be suppressed while ensuring the light emission intensity, so that a light emitting element having sufficient light emission efficiency even at a high driving current can be obtained.

  Twenty-two types of epitaxial substrates 10 with different annealing conditions were produced, and light-emitting elements 20 were produced using the respective epitaxial substrates 10. For the obtained light-emitting element 20, the light emission intensity was measured, and the light emission efficiency reduction rate was calculated.

  First, in forming the epitaxial substrate 10, a c-plane GaN substrate having a diameter of 4 inches and a thickness of 500 μm was prepared as the GaN base substrate 1. The GaN base substrate 1 is placed in a MOCVD furnace (organometallic vapor phase growth furnace) and heated to 1100 ° C. in a hydrogen / nitrogen mixed atmosphere, and then TMG gas, ammonia gas, and silane gas are used with nitrogen and hydrogen as carrier gases. The n-type GaN layer 2 was grown to a thickness of 3 μm.

Next, after the substrate temperature was lowered to 750 ° C., the active layer 3 was formed. In the middle of the process, after the formation of the well layer 31, an annealing process was performed. As the active layer 3, five well layers 31 of In _0.10 Ga _0.90 N and 2 nm thick well layers 31 and 10 nm thick barrier layers 32 were alternately formed.

  The well layer 31 was formed by flowing TMG gas, TMI gas, and ammonia gas using nitrogen gas having a flow rate of 16 slm as a carrier gas. The flow rate of ammonia gas was 6 slm. Hydrogen gas was not used. On the other hand, in the subsequent annealing treatment, the components and flow rates of the annealing atmosphere gas were changed to the following five levels.

(Condition 1) Nitrogen gas 16 slm, hydrogen gas 0 slm, ammonia gas 6 slm;
(Condition 2) Nitrogen gas 16 slm, hydrogen gas 1 slm, ammonia gas 5 slm;
(Condition 3) Nitrogen gas 16 slm, hydrogen gas 3 slm, ammonia gas 3 slm;
(Condition 4) Nitrogen gas 16 slm, hydrogen gas 5 slm, ammonia gas 1 slm;
(Condition 5) Nitrogen gas 16 slm, hydrogen gas 6 slm, ammonia gas 0 slm.

  Furthermore, in condition 1, the annealing time was changed to four levels of 0 seconds (no annealing treatment), 600 seconds, 1200 seconds, and 3600 seconds (Samples 1-1 to 1-4). In condition 2, the annealing time was changed to four levels of 60 seconds, 120 seconds, 240 seconds, and 480 seconds (Samples 2-1 to 2-4). Under condition 3, the annealing time was changed to six levels of 15 seconds, 30 seconds, 60 seconds, 150 seconds, 180 seconds, and 210 seconds (Samples 3-1 to 3-6). In condition 4, the annealing time was changed to five levels of 30 seconds, 60 seconds, 90 seconds, 120 seconds, and 150 seconds (Samples 4-1 to 4-5). In condition 5, the annealing time was changed to three levels of 5 seconds, 10 seconds, and 15 seconds (Samples 5-1 to 5-3).

  The barrier layer 32 was formed by flowing TMG gas and ammonia gas using nitrogen gas with a flow rate of 16 slm as a carrier gas. The flow rate of ammonia gas was 6 slm. Hydrogen gas was not used.

Further, after the formation of the active layer 3, the substrate temperature is again raised to 1080 ° C., and nitrogen and hydrogen are used as carrier gases, and TMA gas, TMG gas, ammonia gas, and Cp ₂ Mg gas are mixed at a Mg concentration of 5 × 10 ¹⁸ / supplied so that the cm _^3, Al _{0.10 Ga 0.90} to form a p-type cladding layer 4 composed of N to a thickness of 20 nm, followed by nitrogen and hydrogen as the carrier gas, TMG gas, ammonia A gas, Cp ₂ Mg gas, was supplied so that the Mg concentration was 1 × 10 ¹⁹ / cm ^3, and the p-type GaN layer 5 was formed to a thickness of 80 nm. After forming the p-type GaN layer 5, the temperature was lowered to room temperature in a nitrogen atmosphere, and the epitaxial substrate 10 was taken out of the MOCVD furnace. Subsequently, the obtained epitaxial substrate 10 was heat-treated at 800 ° C. for 10 minutes in a nitrogen atmosphere by using a rapid annealing furnace (RTA), thereby performing an activation treatment of Mg.

  Next, the light emitting element 20 was created using each obtained epitaxial substrate 10.

  First, a part of the n-type GaN layer 2 was exposed to the epitaxial substrate 10 by a photolithography process and RIE. The approximate dimensions of the non-etched area were set to 0.3 mm × 0.3 mm.

  Subsequently, a Ti / Al / Ni / Au multilayer film as the cathode electrode pad 7 was patterned on the exposed portion of the n-type GaN layer 2 by a photolithography process and a vacuum deposition method. The thickness of each film was set to 15 nm, 220 nm, 40 nm, and 75 nm in order from the Ti film. Thereafter, a heat treatment at 700 ° C. in a nitrogen atmosphere was performed for 30 seconds in order to improve the ohmic contact characteristics.

  Further, the Ni / Au laminated film as the anode electrode layer 6 was patterned by a photolithography process and a vacuum deposition method. The thickness of each film was 20 nm and 200 nm. Thereafter, in order to improve the ohmic contact characteristics, a heat treatment at 500 ° C. in a nitrogen atmosphere was performed for 5 minutes.

  Then, the light emitting element 20 was obtained by separating into pieces with a dicer.

  With respect to all 22 types of light emitting elements 20 obtained, the light emission intensity was measured. Specifically, the emission intensity is measured with an optical power meter in a state where a current is passed between the anode electrode pad 6 and the cathode electrode pad 7 via a manual prober, and the degree of efficiency droop is expressed from the result. The reduction rate of luminous efficiency as an index was determined.

The light emission intensity was measured in two ways: when the light emitting element 20 was driven at a current density of 20 A / cm ² and when the light emitting element 20 was driven at 200 A / cm ² . In the following, a current density of 20 A / cm ² is referred to as a reference current density, and a current density of 200 A / cm ² that is 10 times that is referred to as a 10 times current density. Further, the light emission intensity when the light emitting element 20 is driven at the reference current density is Wa, and the light emission intensity when the light emitting element 20 is driven at a 10 times current density is Wb. Note that the current density value of 20 A / cm ² is set from the point that the luminous efficiency is almost the maximum in any sample.

  The rate of decrease in luminous efficiency is obtained by standardizing Wa and Wb to luminous intensity per reference current density (actually, Wa is a value as it is and Wb is divided by 10), and the difference between the two values obtained is The ratio to the emission intensity Wa at the reference current density was obtained. The luminous efficiency reduction rate obtained in this way is the light emission intensity when the light emitting element 20 is driven at a current density that is a real number multiple (10 times in this embodiment) of the reference current density. The rate of decrease of the intensity from the real number value is shown. The larger the value, the lower the light emission efficiency than when driving at the reference current density. In other words, the efficiency droop is suppressed as the luminous efficiency decrease rate is smaller.

  Further, as in the case where the epitaxial substrate 10 is manufactured, after the MQW structure is formed under all 22 conditions, the well layer 31 is formed on each MQW structure under the same conditions as when the MQW structure is formed. And 22 kinds of pit evaluation substrates were prepared by performing annealing treatment. And the surface shape of the well layer 31 after the annealing process by AFM (atomic force microscope) was measured for the substrate for pit evaluation, and the pit ratio was calculated based on the result. Measurement by AFM was performed on a 3 μm square region.

  The pit ratio was specified based on a cross-sectional profile at an arbitrary position (scanning position) in the shape measurement range by AFM. Specifically, the ratio of the portion belonging to the range where the depth difference from the deepest portion is 0.3 nm or less with respect to the entire range of the cross-sectional profile was obtained as the pit ratio.

  FIG. 4 is a diagram illustrating a cross-sectional profile related to the pit evaluation substrate of Sample 3-3. In FIG. 4, the cross-sectional profile is shown with the deepest part as the height reference point. FIG. 4 confirms that the pits P are formed at intervals of about several hundred μm. The pit ratio of Sample 3-3 shown in FIG. 4 was 10%.

  Conditions for annealing treatment at the time of forming the well layer 31 (flow rate of annealing atmosphere gas and annealing treatment time), pit ratio in the substrate for pit evaluation, emission intensity per reference current density, and reduction in emission efficiency for each condition Table 1 shows a list of rates. Sample 1-1 in Table 1 corresponds to the case where the annealing treatment is not performed. Moreover, the pit ratio calculated | required based on the board | substrate for pit evaluation can be considered to be comparable also in the light emitting element 20 which has the MQW structure produced on the same conditions.

  As shown in Table 1, in the case of samples 1-2 to 1-4 in which the annealing treatment was performed using only nitrogen gas and ammonia gas as the annealing atmosphere gas without including hydrogen gas, the annealing treatment was performed regardless of the annealing treatment time. There was no significant difference between the luminous efficiency reduction rate and the case of the sample 1-1 that did not perform (luminous efficiency reduction rate 55%). On the other hand, in the case of other samples including hydrogen in the annealing atmosphere gas, the rate of decrease in luminous efficiency was smaller than that in the case of Sample 1-1.

  Such a result indicates that annealing the surface of the well layer 31 in an annealing atmosphere containing at least nitrogen gas and hydrogen gas to form the pits P is effective in suppressing the efficiency droop. Specifically, in view of the results of the sample 2-1, it can be said that the effect of suppressing the efficiency droop in the light emitting element 20 can be obtained by performing the annealing process so that at least the pit ratio is 2% or more.

  In particular, from the results shown in Table 1, it can be seen that as the pit ratio increases, the rate of decrease in luminous efficiency decreases and the decrease in efficiency droop is suppressed. For example, when the annealing process is performed so that the pit ratio is 8% or more, the luminous efficiency reduction rate is reduced to 20% or less.

  However, on the other hand, it can also be found that if the pit ratio becomes too large, the emission intensity itself tends to decrease. From this point of view, it can be said that the annealing treatment is preferably performed in a range where the pit ratio is 25% or less. When satisfying such a range, a value of 80 mW to 105 mW, which is approximately the same as or higher than the light emission intensity of 90 mW in the case of the sample 1-1 not subjected to the annealing process, is obtained as the light emission intensity when driven at the reference current density. In addition, the efficiency droop is suppressed.

  That is, from the results shown in Table 1, it can be said that the annealing treatment is preferably performed under the condition that the pit ratio is 2% or more and 25% or less, more preferably 8% or more and 25% or less. In this case, it is possible to suitably suppress the efficiency droop while keeping the light emission intensity when driven at the reference current density at least equal to or slightly smaller than the light emission intensity when the annealing treatment is not performed.

  Further, from Table 1, as the flow rate of hydrogen gas in the annealing atmosphere gas is larger and the flow rate of ammonia gas is smaller, the pit ratio is increased even in a short annealing time, and the luminous efficiency reduction rate tends to be reduced accordingly. It is also found that there is. From this result, it can be said that as the flow rate ratio of hydrogen gas in the annealing atmosphere gas is larger than the flow rate ratio of ammonia gas, it is possible to effectively suppress the droop in a short annealing time.

  When actually manufacturing and using the light emitting element 20, an annealing process is performed after adjusting the processing conditions (flow rate ratio and processing time of the annealing atmosphere gas) so that the pit ratio satisfies the above-described range. A light emitting element may be obtained.

DESCRIPTION OF SYMBOLS 1 Ground substrate 2 n-type GaN layer 3 Active layer 4 p-type cladding layer 5 p-type GaN layer 6 Anode electrode part 7 Cathode electrode part (cathode electrode pad)
DESCRIPTION OF SYMBOLS 10 Epitaxial substrate 20 Light emitting element 31 Well layer 32 Barrier layer P Pit

Claims (7)

A light emitting device,
An active layer having a multi-quantum well structure in which a well layer made of In _x Ga _1-x N (0 <x ≦ 1) and a barrier layer made of GaN are alternately and repeatedly stacked;
A plurality of pits are dispersedly formed in the well layer so as to penetrate the well layer,
The barrier layer has a portion embedded in the plurality of pits formed in the well layer immediately below,
A light emitting element characterized by the above. The light emitting device according to claim 1,
The proportion of the plurality of pits in the well layer is 2% or more and 25% or less,
A light emitting element characterized by the above. The light emitting device according to claim 2,
The ratio of the plurality of pits in the well layer is 8% or more and 25% or less.
A light emitting element characterized by the above. An epitaxial substrate for a light emitting device,
A base substrate made of GaN;
A multiple quantum well structure formed by repeatedly laminating a well layer made of In _x Ga _1-x N (0 <x ≦ 1) and a barrier layer made of GaN formed epitaxially on the base substrate. An active layer, and
With
A plurality of pits are dispersedly formed in the well layer so as to penetrate the well layer,
The barrier layer has a portion embedded in the plurality of pits formed in the well layer immediately below,
An epitaxial substrate for a light emitting device characterized by the above. A method of manufacturing an epitaxial substrate for a light emitting device,
An active layer forming step of forming an active layer on a base substrate made of GaN;
With
In the active layer forming step,
A well layer forming step of epitaxially forming a well layer made of In _x Ga _1-x N (0 <x ≦ 1);
An annealing treatment step of dispersing and forming a plurality of pits on the surface of the well layer by performing an annealing treatment to anneal the surface of the well layer with an atmosphere gas containing nitrogen gas and hydrogen gas,
A barrier layer forming step of epitaxially forming a barrier layer made of GaN on the well layer after the annealing treatment;
Are repeated in this order to form the active layer having a multiple quantum well structure.
A method for producing an epitaxial substrate for a light-emitting element. It is a manufacturing method of the epitaxial substrate for light emitting elements according to claim 5,
In the annealing treatment step, the annealing treatment is performed so that the existence ratio of the plurality of pits in the well layer is 2% or more and 25% or less.
A method for producing an epitaxial substrate for a light-emitting element. It is a manufacturing method of the epitaxial substrate for light emitting elements of Claim 6,
In the annealing treatment step, the annealing treatment is performed so that the existence ratio of the plurality of pits in the well layer is 8% or more and 25% or less.
A method for producing an epitaxial substrate for a light-emitting element.