JP2006147787A - Light emitting element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element excellent in a light extraction efficiency and to realize a low cost, an improvement in yield and a high efficiency. <P>SOLUTION: The light emitting element is made by sequentially laminating a first clad layer, an active layer and a second clad layer in this order. The second clad layer has a grown pit formed in the front surface at the time of growth, and a thickness of 100 nm or more. Moreover, the thickness of the second clad layer is set to become λ/2n or more wherein a luminescence wavelength in vacuum is λ and the refractive index of the second clad layer is set to n. In the case of a vapor phase epitaxy of the second clad layer, the growth pit is formed in the front surface using the growth temperature as 1,000°C or less, and the vapor phase epitaxy is performed until the thickness is set to 100 nm or more. Moreover, the vapor phase epitaxy of the hydrogen is performed in an atmosphere including a hydrogen. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light emitting element such as a light emitting diode (LED) and a method for manufacturing the same, and more particularly to a technique for improving light extraction efficiency.

  Gallium nitride compound semiconductors are widely used for light emitting diodes (LEDs), laser diodes (LDs), and the like, and particularly LEDs are expected to be used explosively in the field of displays and lighting. Collecting.

  By the way, the refractive index of a gallium nitride compound semiconductor is as large as about 2.5, and when the active layer and the cladding layer of the light emitting element are configured using this, most of the light emitted inside is totally reflected, and the external There is an inconvenience that it is not taken out. Therefore, for example, even if a complete rectangular parallelepiped LED chip is molded with a refractive index of 1.5, only about 50% of the total light emission is extracted outside.

  Therefore, as a technique for improving this, a technique for producing irregularities on the front and back surfaces of the LED chip has been proposed (see, for example, Patent Documents 1 to 4). For example, Patent Document 1 discloses a nitride-based semiconductor including a reflective layer formed on a support substrate, a p-type nitride-based semiconductor layer, a light-emitting layer, and an n-type nitride semiconductor layer sequentially stacked above the reflective layer. There is disclosed a nitride-based semiconductor light-emitting element that is a light-emitting element, and has unevenness formed on a light extraction surface located above the n-type nitride semiconductor layer. In the invention described in Patent Document 1, a light extraction surface having unevenness is formed on the n-type nitride semiconductor layer, and the unevenness is formed by regrowth, polishing, etching, etc. of the n-type nitride semiconductor layer. Yes.

  In Patent Document 2, in a light emitting device in which a gallium nitride compound semiconductor substrate is stacked on a gallium nitride compound semiconductor substrate, the surface of the gallium nitride compound semiconductor substrate facing the surface on which the devices are stacked has irregularities. A gallium nitride-based compound semiconductor device is disclosed, and it is disclosed that the irregularities are formed by dry etching or wet etching.

  Patent Document 3 discloses a semiconductor light emitting device having a light emitting portion made of a nitride-based semiconductor, and the light extraction surface has irregularities so as to improve the extraction efficiency of light emitted from the light emitting portion to the outside. There is disclosed a semiconductor light-emitting element that is provided. The unevenness of the light extraction surface is formed by transferring the shape of the resist pattern by etching, as described in paragraph numbers 0033 to 0035.

Patent Document 4 includes a substrate, a light-emitting layer made of a semiconductor formed on the substrate, and a semiconductor that is formed on the light-emitting layer and has a lattice mismatch to the semiconductor constituting the light-emitting layer. There is disclosed a semiconductor light emitting device comprising a light scattering layer whose surface is roughened by lattice strain.
JP 2003-318443 A JP 2003-69075 A JP 2000-196152 A JP-A-8-102548

  However, as in the inventions described in Patent Documents 1 to 3, the method of forming irregularities by etching or the like increases the number of processes and is disadvantageous in terms of productivity. In addition, there is a risk of damage due to the formation of irregularities, and the characteristics may deteriorate.

  On the other hand, the method described in Patent Document 4 is advantageous in terms of productivity, characteristics, and the like because it does not require uneven processing. However, in order to form irregularities using lattice mismatch, it is necessary to select a semiconductor that is lattice-mismatched with respect to the light-emitting layer, and there is a disadvantage that usable materials are limited. In addition, film forming conditions are severely restricted, and there is a disadvantage that the process becomes complicated.

  As described above, the conventional concavo-convex fabrication technique has various demerits, and improvements are desired. The present invention has been proposed in view of such a conventional situation. That is, an object of the present invention is to provide a light emitting element that is excellent in light extraction efficiency and can realize cost reduction, yield improvement, high efficiency, and the like. It is another object of the present invention to provide a method for manufacturing a light-emitting element that can reduce processes such as lithography and etching and does not damage the process.

  In order to achieve the above object, the light emitting device of the present invention is a light emitting device in which a first cladding layer, an active layer, and a second cladding layer are laminated in this order, and the second cladding layer grows on the surface. It is characterized by having growth pits that are sometimes formed and having a thickness of 100 nm or more.

  In the light emitting device of the present invention, the light extraction efficiency is improved by utilizing the growth pits that are naturally generated in the crystal growth and making them uneven, such as the light extraction surface. The growth pits are naturally generated with the crystal growth of the second cladding layer, and it is not necessary to form irregularities by etching or the like. Further, the deterioration of characteristics can be suppressed without damaging the element.

  However, the technology using the growth pits that occur naturally in crystal growth has a problem that it is difficult to control and it is difficult to improve the efficiency surely, as reported in part. Therefore, as a result of various studies by the present inventors, it has been found that by increasing the thickness of the second cladding layer, the efficiency can be improved reliably and the growth pits are effective for light extraction. . Therefore, in the present invention, when the thickness of the second cladding layer is 100 nm or more, particularly when the emission wavelength in vacuum is λ and the refractive index of the second cladding layer is n, the thickness of the second cladding layer is λ / 2n or more. Thereby, the efficiency improvement by uneven | corrugated formation is implement | achieved reliably, without requiring difficult control.

  The method for manufacturing a light emitting device of the present invention is a method for manufacturing a light emitting device in which a first cladding layer, an active layer, and a second cladding layer are sequentially vapor-grown on a substrate. Growth pits are formed on the surface at a temperature not higher than ° C., and vapor phase growth is performed until the thickness becomes 100 nm or more.

  In the metal organic chemical vapor deposition method, for example, when a second cladding layer made of Mg-doped GaN or the like is grown at a low temperature (1000 ° C. or lower), growth pits are generated starting from threading transitions generated from the substrate interface. These growth pits are composed of {1, -1,1,0,1} planes, and are particularly likely to occur at a growth temperature of 1000 ° C. or less in an atmosphere containing hydrogen.

  In the manufacturing method of the present invention, growth pits that contribute to efficiency improvement are naturally generated under the above-described conditions, so that processes such as lithography and etching are unnecessary, and man-hours are reduced. Further, no damage is caused during the process.

  According to the light emitting element of the present invention, high efficiency can be realized easily and stably, and it can contribute to cost reduction, yield improvement, and high efficiency. Further, according to the method for manufacturing a light emitting element of the present invention, processes such as lithography and etching can be reduced, and no damage is caused during the process.

  Hereinafter, a light emitting device to which the present invention is applied and a method for manufacturing the same will be described in detail with reference to the drawings.

  FIG. 1 shows a basic structure of a light emitting element to which the present invention is applied. In this example, the first cladding layer 3, the active layer 4, and the second cladding layer 5 are sequentially grown on the sapphire substrate 1 via the low-temperature buffer layer 2. The light emitting device of the present embodiment is a light emitting device using a gallium nitride compound semiconductor, and the first cladding layer 3 is formed by crystal growth of, for example, Si-doped GaN, and is an n-type GaN layer. The active layer 4 is a layer having an InGaN / GaN multiple quantum well structure. The second cladding layer 5 is formed by crystal growth of Mg-doped GaN and is a p-type GaN layer.

  The light emitting device of the present invention is characterized in that growth pits 6 are formed on the upper surface of the second cladding layer 5. The growth pits 6 are naturally generated in the second cladding layer 5 during crystal growth with the threading transition generated from the interface of the sapphire substrate 1 as a starting point, and the growth pits 6 are opposite to the light extraction surface or the opposite. By forming on the side surface, the light extraction efficiency can be greatly improved.

  However, when the growth pits 6 are formed in the second cladding layer 5, it is necessary to increase the thickness of the second cladding layer 5. According to the experiments by the present inventors, the thickness of the second cladding layer 5 is preferably 100 nm or more, and more preferably 200 nm or more. By setting the thickness of the second cladding layer 5 to 100 nm or more, an improvement in efficiency was confirmed as compared with the case where the surface was flat.

  In addition, the thickness of the second cladding layer 5 needs to be set appropriately in relation to the emission wavelength. In a semiconductor, irregularities having a wavelength of 1/2 wavelength or more, preferably 1 wavelength or more with respect to the emission wavelength are effective for light extraction. The wavelength at this time is the effective wavelength (wavelength converted by the refractive index) in the semiconductor. Therefore, when the emission wavelength in vacuum is λ and the refractive index of the second cladding layer is n, the thickness of the second cladding layer is preferably λ / 2n or more. More preferably, it is λ / n or more.

  Furthermore, the area of the flat portion is preferably 7/8 or less, more preferably 1/2 or less, of the entire surface due to the formation of the growth pits 6. The large area of the flat part means that the degree of formation of the growth pits 6 is small. Therefore, when aiming at high efficiency by the growth pits 6, it is necessary to reduce the area of the flat part.

By the way, the scattering effect of the growth pit 6 is effective in the range of the wavelength around the growth pit 6. Considering a circle with a radius of about 184 nm from the center of the growth pit 6 (corresponding to the wavelength in the medium when the emission wavelength is 460 nm in vacuum and the refractive index is 2.5), the growth is performed at a density of about 5e + 8 / cm ^2. If the pits 6 are formed, the area of almost half of the surface is covered with the circle, and the effect is great. Even about 1/4 of 1e + 7 / cm ² or more contributes to an increase in light extraction efficiency. When this is formulated, π × d × (λ / n) ² > 1 /, where d is the density of the growth pits 6, λ is the emission wavelength in vacuum, and n is the refractive index of the second cladding layer. 8 is preferable, and it is desirable that π × d × (λ / n) ² > 1/2. When the size of the growth pit 6 is about a radius r, there is an effect in the range of the wavelength from the outer periphery of the pit. Therefore, π × d × (λ / n + r) ² > 1/8, preferably π × d × (λ / n + r) ² > 1/2 may be satisfied.

  The above is the basic structure of the light-emitting element, but in order to be used as the light-emitting element, formation of an electrode or the like is necessary. For example, as shown in FIG. 2, the n-electrode 7 is formed in contact with the first cladding layer 3, and the p-electrode 8 is formed in contact with the second cladding layer 5. Note that the n-electrode 7 is formed, for example, by forming a film of Ti / Al or the like and patterning it with a photolithography technique. The p electrode 8 is formed by forming a film of Ni / Au or the like and patterning it with a photolithography technique.

  The p-electrode 8 formed on the second cladding layer 5 needs to be a transparent electrode when the second cladding layer 5 side is the light extraction surface. On the contrary, when the surface opposite to the second cladding layer 5 is a light extraction surface, the p-electrode 8 needs to be a reflective electrode.

  FIG. 3 shows an example of a light-emitting element mounted on a mount substrate. In this case, the solder connecting metal 9 is formed on the n electrode 7, and the solder connecting metal 10 is also formed on the p electrode 8. The solder connecting metals 9 and 10 are formed with the same height. Then, the sapphire substrate 1 on which the crystal layer and electrodes are formed is turned upside down and flip-chip bonded onto the submount 11. The submount 11 is made of, for example, Si, and has mount electrodes 12 and 13 corresponding to the solder connecting metals 9 and 10.

  After bonding to the submount 11, an energy beam is irradiated from the back side of the sapphire substrate 1, so-called ablation (laser ablation when laser light is used as the energy beam) is caused, and the sapphire substrate 1 is peeled off. Also good.

  As an energy beam used for ablation, a laser beam such as an excimer laser or a YAG laser can be used. For example, in the case of a gallium nitride-based light emitting diode, the crystal layer of the gallium nitride-based compound semiconductor is decomposed into metallic Ga and nitrogen at the interface with the sapphire substrate 1 and is easily separated from the sapphire substrate 1.

  Here, after peeling off the sapphire substrate 1, as shown in FIG. 4, it is also possible to form pits 3a on the exposed surface (the surface of the first cladding layer 3). The pit 3a may be formed by a method of making it non-mirror surface by a chemical or physical method. A specific method is, for example, a method of providing fine irregularities to make a non-mirror surface by etching or polishing. Etching includes wet etching using a mixed acid of phosphoric acid and sulfuric acid, and dry etching such as reactive ion etching (RIE). Polishing may be performed by selecting an appropriate abrasive. For example, by using an abrasive having a Mohs hardness of about 9, the surface of the gallium nitride compound semiconductor layer can be polished to a non-mirror surface.

  Moreover, as shown in FIG. 5, the unevenness | corrugation 1a may be formed in the sapphire substrate 1, and the 1st cladding layer 3, the active layer 4, and the 2nd cladding layer 5 may be formed on this. As shown in FIG. 4 and FIG. 5, by forming irregularities on the surface opposite to the second cladding layer 5 on which the growth pits 6 are formed (surface on the first cladding layer 3 side), the light extraction efficiency is further improved. This can be further improved. In any case, the active layer 4 is flat.

  Next, a method for manufacturing the above-described light emitting element will be described. In order to manufacture a light emitting device, crystal growth of a gallium nitride compound semiconductor on a substrate is a central technology. The gallium nitride compound semiconductor is crystal-grown by metal organic vapor phase epitaxy.

  In the growth sequence of a light emitting element using a gallium nitride compound semiconductor, first, the temperature is raised and thermal cleaning is performed. This thermal cleaning is an operation for cleaning the surface of the sapphire substrate or the like.

  Next, a low-temperature buffer layer 2 made of GaN is formed on the sapphire substrate 1 at a relatively low temperature, and a first cladding layer 3 made of, for example, GaN: Si is crystal-grown thereon. GaN: Si constituting the first cladding layer 3 is n-type GaN, and therefore the first cladding layer 3 is an n-cladding layer. The crystal growth temperature of the first cladding layer 3 is higher than the crystal growth temperature of the low temperature buffer layer 2 described above.

  After the crystal growth of the first cladding layer 3, the active layer 4 (InGaN / GaN multiple quantum well) and the second cladding layer 5 (GaN: Mg) are crystal-grown thereon. The crystal growth temperature of the active layer 4 is lower than the crystal growth temperature of the first cladding layer 3, and the crystal growth temperature of the second cladding layer 5 is higher than the crystal growth temperature of the active layer 4.

  In the second cladding layer 5, it is necessary to spontaneously generate growth pits 6 during crystal growth. For this reason, it is necessary to set the temperature at the time of crystal growth of the second cladding layer 5 to a low temperature. Specifically, by setting the temperature to 1000 ° C. or less, growth pits are generated starting from threading transitions generated from the interface of the sapphire substrate 1. The growth pits 6 are composed of {1, -1, 0, 1} planes, and are easily generated particularly at a growth temperature of 1000 ° C. or less in an atmosphere containing hydrogen.

  Hereinafter, specific examples to which the present invention is applied will be described based on experimental results.

Example 1
A growth sequence in this example is shown in FIG. In the metal organic chemical vapor deposition method, a low-temperature buffer, a Si-doped GaN, a multiple quantum well active layer made of InGaN / GaN, and a light-emitting diode structure made of Mg-doped GaN were grown on a sapphire substrate. The InGaN / GaN multiple quantum well grows at 750 ° C. and the emission wavelength is 460 nm. Mg-doped GaN grew at 900 ° C., which is significantly lower than the growth temperature of Si-doped GaN (1020 ° C.).

  At such a low temperature, growth pits originated from threading dislocations generated from the sapphire substrate interface were generated in Mg-doped GaN. This growth pit consists of {1, -1, 0, 1} planes, and is easily generated at a growth temperature of 1000 ° C. or less in an atmosphere containing hydrogen. Several kinds of structures were produced by changing the film thickness of the Mg-doped GaN.

  Further, Mg-doped GaN was grown to 100 nm at 1020 ° C. by the growth sequence shown in FIG. Thereby, the 2nd clad layer of the structure where the surface is flat was formed, and this was made into the comparative example.

  After the crystal growth, the wafer on which each of the above layers was grown was heated in a nitrogen atmosphere at 800 ° C. for 10 minutes to activate p-type GaN (Mg-doped GaN). Lithography and etching are carried out to form a p-type transparent electrode made of Ni / Au and an n-type electrode made of Ti / Al, each having a pad electrode for wire bonding of about 50 μm, cut into a 300 μm square and wire bonded. . This was molded with an epoxy resin, and the light extraction efficiency was measured. The structure of the light-emitting element used for the measurement is as shown in FIG.

  FIG. 8 shows the difference in light extraction efficiency when the thickness of the Mg-doped GaN (second cladding layer) is changed. When the second cladding layer has a thickness of 100 nm or more, an improvement in efficiency was confirmed as compared with the case where the surface was flat. In particular, it can be seen that 200 nm or more is effective. When the emission wavelength is 460 nm, the length of one wavelength corresponds to 184 nm in a semiconductor with a refractive index of 2.5. Therefore, it can be seen that unevenness of ½ wavelength or more, preferably 1 wavelength or more in the semiconductor is effective for light extraction.

Example 2
In Example 1, the p-type electrode was a high reflectivity electrode made of Ag, and the electrode was flip-chip mounted on the submount. The structure of the manufactured light-emitting element is as shown in FIG. Further, when the light extraction efficiency was measured by molding, the efficiency was about 1.2 times that of Example 1 described above. Silver electrode vapor deposition was performed according to the method of Unexamined-Japanese-Patent No. 11-186598, for example. In such a light emitting element structure, it is important for light extraction that the substrate is transparent with respect to the emission wavelength.

Example 3
After producing a light emitting device having the same structure as in Example 2, the excimer laser was irradiated from the back sapphire side to peel off the sapphire substrate and the low-temperature buffer layer, and as a result, the exposed first cladding layer (Si-doped GaN) The surface was roughened by etching. The structure of the manufactured light-emitting element is as shown in FIG. Further, when the light extraction efficiency was measured by molding, the efficiency was 1.2 times that of Example 2.

Example 4
In Example 2 described above, crystal growth was performed on a sapphire substrate on which irregularities were formed in advance. The structure of the manufactured light-emitting element is as shown in FIG. When this was molded and the light extraction efficiency was measured, the efficiency was 1.1 times that of Example 2.

It is a schematic sectional drawing which shows the basic structure of the light emitting element to which this invention is applied. It is a schematic sectional drawing which shows the structural example of the light emitting element in which the electrode was formed. It is a schematic sectional drawing which shows the structural example of the light emitting element bonded to the submount. It is a schematic sectional drawing which shows the structural example of the light emitting element which peeled the sapphire substrate and formed the unevenness | corrugation. It is a schematic sectional drawing which shows the structural example of the light emitting element which formed the unevenness | corrugation in the sapphire substrate. It is a figure which shows an example of the growth sequence in the case of forming a growth pit. It is a figure which shows an example of the growth sequence in case a growth pit is not formed. It is a figure which shows the difference in the light extraction efficiency by the difference in the thickness of a 2nd clad layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sapphire substrate, 2 Low temperature buffer layer, 3 1st clad layer, 4 Active layer, 5 2nd clad layer, 6 Growth pit, 7 n electrode, 8 p electrode, 9, 10 Solder connection metal, 11 Submount, 12 , 13 Mount electrode

Claims (12)

In the light emitting device in which the first cladding layer, the active layer, and the second cladding layer are laminated in this order,
The light emitting element, wherein the second cladding layer has growth pits formed during growth on the surface and has a thickness of 100 nm or more.   2. The thickness of the second cladding layer is set to λ / 2n or more, where λ is an emission wavelength in vacuum and n is a refractive index of the second cladding layer. Light emitting element.   2. The light emitting device according to claim 1, wherein the first cladding layer, the active layer, and the second cladding layer are formed by vapor phase growth of a gallium nitride compound semiconductor.   The light emitting device according to claim 1, wherein the first cladding layer is uneven.   The light emitting device according to claim 1, wherein a first cladding layer, an active layer, and a second cladding layer are laminated in this order on a substrate.   6. The light emitting device according to claim 5, wherein the first cladding layer, the active layer, and the second cladding layer are grown on a substrate having an uneven surface.   The light emitting device according to claim 1, wherein the second cladding layer is a p-type semiconductor layer.   The light emitting device according to claim 5, wherein the substrate is a sapphire substrate. In a method for manufacturing a light emitting device, in which a first cladding layer, an active layer, and a second cladding layer are sequentially vapor-grown on a substrate,
A method for manufacturing a light emitting device, characterized in that a growth temperature of the second cladding layer is set to 1000 ° C. or lower, growth pits are formed on the surface, and vapor phase growth is performed until the thickness becomes 100 nm or more.   10. The method for manufacturing a light emitting device according to claim 9, wherein at least a part of the second cladding layer is vapor-phase grown in an atmosphere containing hydrogen.   The method for manufacturing a light-emitting element according to claim 9, wherein the first clad layer is roughened after the substrate is peeled off.   10. The method for manufacturing a light-emitting element according to claim 9, wherein the first cladding layer, the active layer, and the second cladding layer are vapor-phase grown on a substrate having an uneven surface.