JP2012028476A - Method for manufacturing light emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor element capable of simultaneously achieving reduction in a forward voltage and improvement in an electrostatic withstand voltage.SOLUTION: A semiconductor light emitting element as an embodiment includes an n-side contact layer, an active layer and a p-side contact layer, in the order. A first superlattice structure, which includes a first barrier layer comprising GaN or InGaN and a first well layer comprising InGaN, and a second superlattice structure, which includes a second barrier layer comprising GaN or InGaN and a second well layer comprising InGaN, are provided between the n-side contact layer and the active layer, in the order from an n-side contact layer side. Further, an average In mixed crystal ratio in the second superlattice structure is smaller than that in the first superlattice structure.

Description

  The present invention relates to a semiconductor light emitting device such as a light emitting diode (LED) or a laser diode (LD).

  Conventionally, a semiconductor light emitting device having a superlattice structure between an n-type contact layer and an active layer is known (for example, Patent Document 1).

JP 2005-197292 A

  However, the conventional semiconductor light emitting device has a problem that it is difficult to achieve both reduction of the forward voltage and improvement of the electrostatic withstand voltage.

In other words, in order to reduce the forward voltage of the semiconductor light emitting device, it is necessary to make it easy to flow current inside the device. However, if current flows easily inside the element, the current that flows inside the element increases when a predetermined voltage is applied, so that the electrostatic withstand voltage decreases. Thus, the reduction of the forward voltage and the improvement of the electrostatic withstand voltage are inherently contradictory characteristics, and it is not easy to make them compatible.
SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor element that can achieve both reduction of forward voltage and improvement of electrostatic withstand voltage.

  A semiconductor light emitting device according to an embodiment includes an n-side contact layer, an active layer, and a p-side contact layer in this order. Between the n-side contact layer and the active layer, in order from the n-side contact layer side, a first superlattice structure including a first barrier layer made of GaN or InGaN and a first well layer made of InGaN, and GaN or And a second superlattice structure including a second barrier layer made of InGaN and a second well layer made of InGaN. Furthermore, the average In mixed crystal ratio of the second superlattice structure is smaller than the average In mixed crystal ratio of the first superlattice structure.

  The semiconductor light emitting device according to another embodiment includes an n-side contact layer, an active layer, and a p-side contact layer in this order. Between the n-side contact layer and the active layer, in order from the n-side contact layer side, a first superlattice structure including a first barrier layer made of GaN and a first well layer made of InGaN, and a first superlattice structure made of GaN. And a second superlattice structure including a second barrier layer and a second well layer made of InGaN. The thickness of the first well layer and the thickness of the second well layer are substantially the same, and the thickness of the first barrier layer and the thickness of the second barrier layer are substantially the same. Furthermore, the In mixed crystal ratio of the second well layer is smaller than the In mixed crystal ratio of the first well layer.

FIG. 1 is a diagram for explaining the structure of the semiconductor light emitting device according to the first embodiment. FIG. 2 is a diagram for explaining the structure of the semiconductor light emitting device according to the first embodiment.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. However, the form shown below exemplifies a semiconductor light emitting element for embodying the technical idea of the present invention, and does not limit the present invention to the following. In addition, the dimensions, materials, shapes, relative arrangements, and the like of the component parts described in the embodiments are not intended to limit the scope of the present invention to that only, unless otherwise specified. Only. Note that the size, positional relationship, and the like of the members shown in each drawing may be exaggerated for clarity of explanation.

[Embodiment 1]
FIG. 1 is a sectional view of the semiconductor light emitting device according to this embodiment. The semiconductor light emitting device of this embodiment includes a substrate 1, a buffer layer 2, an n-side contact layer 5, an active layer 13, and a p-side contact layer 17 in this order. Between the n-side contact layer 5 and the active layer 13, a first superlattice structure including a first barrier layer 11a made of GaN or InGaN and a first well layer 11b made of InGaN in this order from the n-side contact layer 5 side. A body 11 and a second superlattice structure 12 including a second barrier layer 12a made of GaN or InGaN and a second well layer 12b made of InGaN are provided. Furthermore, the average In mixed crystal ratio of the second superlattice structure 12 is smaller than the average In mixed crystal ratio of the first superlattice structure 11.

  As a result, the forward voltage of the semiconductor light emitting element can be reduced with good reproducibility and the electrostatic withstand voltage can be improved. Details will be described below.

  In general, the crystallinity of InGaN is inferior to that of GaN, and the crystallinity of InGaN is inferior as the In mixed crystal ratio increases. In addition, since crystallinity is inherited in epitaxial growth, for example, even if GaN is grown on InGaN with poor crystallinity, GaN with good crystallinity cannot be grown immediately on InGaN. It is considered that the crystallinity of GaN having a film thickness is inferior to the crystallinity of GaN in the upper region. Therefore, in the semiconductor light emitting device, as the thickness of InGaN and / or the In mixed crystal ratio of InGaN increases, the crystallinity of InGaN itself and / or other layers such as GaN grown thereafter decreases and the electrical resistance decreases. As a result, the forward voltage decreases. The details of the mechanism by which the current easily flows due to the decrease in crystallinity is unknown, but it is probably because the current flows through crystal defects such as dislocations.

  However, if the crystallinity is reduced in order to reduce the forward voltage, the output of the semiconductor light emitting element is reduced. Furthermore, if the current flows easily, the current flowing through the semiconductor light emitting element increases when a predetermined voltage is applied, and as a result, the electrostatic withstand voltage, which is an important characteristic, decreases.

  Therefore, in the present embodiment, by making the average In mixed crystal ratio of the second superlattice structure 12 smaller than the average In mixed crystal ratio of the first superlattice structure 11, the second superlattice structure 12 The crystallinity is improved over the crystallinity of the first superlattice structure 11 (in other words, the crystallinity of the first superlattice structure 11 is lowered than the crystallinity of the second superlattice structure 4. ing.). In the first superlattice structure 11, the crystallinity of the first superlattice structure 11 is made lower than the crystallinity of the second superlattice structure 12, thereby facilitating the flow of current in the region, thereby reducing the forward voltage as a semiconductor light emitting device. I am letting. On the other hand, by improving the crystallinity of the second superlattice structure 12 as compared with the first superlattice structure 11, the electrostatic withstand voltage can be improved without reducing the output as the semiconductor light emitting device. it can.

  Here, not improving the crystallinity of the first superlattice structure 11 far from the active layer 13 but improving the crystallinity of the second superlattice structure 12 close to the active layer 13 maintains the output. On the other hand, it is considered that the forward voltage is lowered and the electrostatic withstand voltage is improved. That is, since the active layer 13 is disposed on the second superlattice structure 12, good crystallinity of the second superlattice structure 12 is inherited. As a result, it is considered that the electrostatic withstand voltage can be effectively improved without decreasing the output.

  Whether the average In mixed crystal ratio of the second superlattice structure 12 is smaller than the average In mixed crystal ratio of the first superlattice structure 11 can be confirmed by SIMS (secondary ion mass spectrometry) or the like. . For example, if the In level of the second superlattice structure 12 is lower than the In level of the first superlattice structure 11, the average In mixed crystal ratio of the second superlattice structure 12 is the average of the first superlattice structure 11. It can be said that it is smaller than the In mixed crystal ratio.

  Hereinafter, main components of the present embodiment will be described.

(Substrate 1)
The substrate 1 may be any material as long as a semiconductor layer can be grown on the upper surface thereof, and the material and the like are not limited. Examples of the substrate 1 include sapphire, spinel, SiC, ZnS, ZnO, GaAs, Si, and GaN. In general, sapphire is often used as a substrate. The substrate 1 can be finally thinned or removed by polishing or the like.

(Buffer layer 2)
When using a different substrate such as sapphire, it is preferable to provide the buffer layer 2 which is a low temperature growth layer. By interposing the buffer layer 2, the crystallinity of the semiconductor layer grown thereon is improved. On the other hand, when GaN, for example, is used as the substrate instead of the heterogeneous substrate, the buffer layer 2 is not necessarily provided. The buffer layer 2 can be finally removed.

(N-side contact layer 5)
The n-side contact layer 5 is a layer where the n-electrode 18 is finally provided. Although the material etc. are not limited, For example, it can be set as n-type GaN or AlGaN. In general, an n-type layer made of Si-doped GaN is often used as the n-side contact layer.

  The material and configuration of the n-electrode 18 are not limited. As the n electrode 18, for example, Ti / Rh / W / Au provided in a part of the n-side contact layer 5 can be used.

(First superlattice structure 11)
The first superlattice structure 11 includes a first barrier layer 11a made of GaN or InGaN and a first well layer 11b made of InGaN. That is, the first barrier layer 11a and the first well layer 11b are paired and a plurality of layers are sequentially stacked. However, the first layer (the layer closest to the n-side contact layer 5) and the last layer (the layer farthest from the n-side contact layer 5) are not limited, and both are the first barrier layer 11a and the first well layer 11b. Good.

  By making the first superlattice structure 11 a superlattice structure instead of a bulk, it is considered that the above-described effect in the present embodiment can be obtained with good reproducibility. That is, by making the first superlattice structure 11 have a superlattice structure, the first superlattice can be achieved at a certain level without causing an unnecessary decrease in crystallinity (for example, generation of cracks) due to the effect of the critical film thickness. The crystallinity of the entire structure 11 can be reduced. Further, instead of the first superlattice structure 11, it seems that bulk (single layer) InGaN having the average In mixed crystal ratio of the first superlattice structure 11 may be formed. It is difficult to grow bulk InGaN with a high reproducibility while keeping the mixed crystal ratio constant. Therefore, it is essential that the first superlattice structure 11 has a superlattice structure, not a bulk.

  The film thickness of the first well layer 11b is preferably larger than the film thickness of the first barrier layer 11a. As a result, the crystallinity of the first superlattice structure 11 as a whole can be easily reduced as a result of the crystallinity of the first well layer 11b and the layer grown thereon being reduced. The forward voltage can be effectively reduced.

  The thickness of the first barrier layer 11a is preferably 0.3 nm to 6 nm, more preferably 0.3 nm to 3 nm, and still more preferably 0.5 nm to 1.5 nm. The film thickness of the first well layer 11b is preferably 0.5 nm to 8 nm, more preferably 0.5 nm to 4 nm, and still more preferably 1 nm to 2.5 nm. Thereby, the said effect can be acquired more easily.

  Since the first barrier layer 11a is a barrier layer and the first well layer 11b is a well layer, even if the first barrier layer 11a is made of InGaN, the In mixed crystal of the first barrier layer 11a. It goes without saying that the ratio is smaller than the In mixed crystal ratio of the first well layer 11b.

  The film thickness of the first superlattice structure 11 is preferably larger than the film thickness of the second superlattice structure 12. Specifically, the film thickness of the first superlattice structure 11 is preferably 2 to 20 times, more preferably 3 to 15 times, and even more preferably 5 times the film thickness of the second superlattice structure 12. -10 times. Thereby, reduction of the forward voltage and improvement of the electrostatic withstand voltage can be realized with good reproducibility.

In order to obtain the above characteristics with good reproducibility, it is preferable that the first barrier layer 11a and the first well layer 11b are not substantially doped with n-type impurities (undoped). However, it is also possible to dope n-type impurities to such an extent that at least one of the first barrier layer 11a and the first well layer 11b does not impair the characteristics. In this specification, “undoped” means 1 × 10 ¹⁷ / cm ³ or less.

(Second superlattice structure 12)
The second superlattice structure 12 includes a second barrier layer 12a made of GaN or InGaN and a second well layer 12b made of InGaN. That is, the second barrier layer 12a and the second well layer 12b are paired and a plurality of layers are sequentially stacked. However, the first layer (the layer closest to the first superlattice structure 11) and the last layer (the layer farthest from the first superlattice structure 11) are not limited, and both are the second barrier layer 12a and the second layer. The well layer 12b may be used.

  By making the second superlattice structure 12 a superlattice structure instead of a bulk, it is considered that the above-described effects in this embodiment can be obtained with good reproducibility. That is, it seems that instead of the second superlattice structure 12, bulk (single layer) InGaN having the average In mixed crystal ratio of the second superlattice structure 12 may be formed. It is difficult to grow bulk InGaN with a high reproducibility while keeping the mixed crystal ratio constant. Therefore, it is essential that the first superlattice structure 11 has a superlattice structure, not a bulk.

  The thickness of the second barrier layer 12a is preferably larger than the thickness of the second well layer 12b. As a result, the crystallinity of the second superlattice structure 12 as a whole can be easily improved as a result of improving the crystallinity of the second barrier layer 12a and the layer grown thereon, so that the entire semiconductor light emitting device The electrostatic withstand voltage can be effectively improved.

  The thickness of the second barrier layer 12a is preferably 0.5 nm to 8 nm, more preferably 0.5 nm to 4 nm, and still more preferably 1 nm to 2.5 nm. The film thickness of the second well layer 12b is preferably 0.3 nm to 6 nm, more preferably 0.3 nm to 3 nm, and still more preferably 0.5 nm to 1.5 nm. Thereby, the said effect can be acquired more easily.

  Since the second barrier layer 12a is a barrier layer and the second well layer 12b is a well layer, even if the second barrier layer 12a is made of InGaN, the In mixed crystal of the second barrier layer 12a. It goes without saying that the ratio is smaller than the In mixed crystal ratio of the second well layer 12b.

  In order to obtain the above characteristics, it is preferable that the second barrier layer 12a and the second well layer 12b are not substantially doped with n-type impurities (undoped). However, n-type impurities can be doped to such an extent that the characteristics are not impaired in at least one of the second barrier layer 12a and the second well layer 12b.

(Active layer 13)
The structure of the active layer 13 is not limited, but is preferably a single quantum well structure (SQW) having a single well layer or a multiple quantum well structure (MQW) having a plurality of well layers and barrier layers. When the active layer 13 is MQW, the stacking order is not particularly limited, and the active layer 13 may be stacked from a well layer or a barrier layer, or may end with a well layer or a barrier layer.

(P-side contact layer 17)
The p-side contact layer 17 is a layer where the p-electrode 19 is finally provided. Although the material etc. are not limited, For example, it can be p-type GaN or AlGaN. In general, a p-type layer made of Mg-doped GaN is often used as the p-side contact layer.

  The material and configuration of the p-electrode 19 are not limited. As the p-electrode 19, for example, a translucent electrode 19 a made of ITO provided on substantially the entire surface of the p-side contact layer 17 and Ti / Rh / W / Au provided on a part of the translucent portion. And a pad electrode 19b.

  In the present embodiment, the substrate 1 to the p-side contact layer 17 are in direct contact with the adjacent layer or superlattice structure, but other layers may be provided therebetween. However, the first superlattice structure 11, the second superlattice structure 12, and the active layer 13 are preferably in direct contact with each other. Thereby, the reduction of the forward voltage and the improvement of the electrostatic withstand voltage can be obtained with good reproducibility.

[Embodiment 2]
The semiconductor light emitting device of this embodiment includes a substrate 1, an n-side contact layer 5, an active layer 13, and a p-side contact layer 17 in this order. Between the n-side contact layer 5 and the active layer 13, a first superlattice structure 11 including a first barrier layer 11a made of GaN and a first well layer 11b made of InGaN in this order from the n-side contact layer 5 side. And a second superlattice structure 12 including a second barrier layer 12a made of GaN and a second well layer 12b made of InGaN.

  Here, the thickness of the first well layer 11b and the thickness of the second well layer 12b are substantially the same, and the thickness of the first barrier layer 11a and the thickness of the second barrier layer 12a are substantially the same. The In mixed crystal ratio of the second well layer 12b is smaller than the In mixed crystal ratio of the first well layer 11b. Thereby, the crystallinity of the first superlattice structure 11 can be made lower than the crystallinity of the second superlattice structure 12, so that the forward voltage of the semiconductor light emitting element can be lowered. Furthermore, since the crystallinity of the second superlattice structure 12 can be improved more than the crystallinity of the first superlattice structure 11, the electrostatic withstand voltage is improved without reducing the output as the semiconductor light emitting device. be able to.

  About the mechanism and each component which can acquire the said effect, it is the same as that of the content demonstrated in Embodiment 1. FIG. Therefore, it will not be repeated here.

  In the present embodiment, the In mixed crystal ratio of the first well layer 11b is preferably 0.05 or more and less than 0.15, and the In mixed crystal ratio of the second well layer 12b is 0.01 or more and less than 0.05, More preferably, the In mixed crystal ratio of the first well layer 11b may be 0.05 or more and less than 0.10, and the In mixed crystal ratio of the second well layer 12b may be 0.02 or more and less than 0.05. Thereby, reduction of the forward voltage and improvement of the electrostatic withstand voltage can be realized with good reproducibility.

  In this specification, the film thickness “substantially the same” means that one film thickness is within ± 20% of the other film thickness.

<Example 1>
The semiconductor light emitting device of this example will be described with reference to FIG. The semiconductor light-emitting device of this example is composed of a sapphire substrate 1 having irregularities (not shown) formed on a semiconductor growth surface, a buffer layer 2 having a thickness of 20 nm made of Al _0.2 Ga _0.8 N, and Si-doped GaN. The first n-side layer 3 having a thickness of 2 μm, the second n-side layer 4 having a thickness of 4 nm made of AlN, the third n-side layer 5 (n-side contact layer) having a thickness of 2 μm made of Si-doped GaN, and the film made of undoped GaN 150 nm thick fourth n-side layer 6, Si-doped GaN 10 nm-thickness 5 n-side layer 7, undoped GaN 150-nm thick sixth n-side layer 8, Si-doped GaN 30 nm-thickness 7 n-side layer 9, first 8n side layer 10 of thickness 5nm of undoped GaN, the first barrier layer 11a and the undoped _{an in} 0.06 _{Ga 0.9} thickness 0.8nm of undoped GaN First superlattice structure 11 which was repeated 27 times a first well layer 11b having a thickness 1.7nm of N as a pair, the second barrier layer 12a and the undoped In thickness 0.8nm of undoped GaN The second superlattice structure 12, which ends with the second barrier layer 12a after repeating this three times as a pair of the first well layer 12b made of _0.03 Ga _0.97 N and having a thickness of 1.7 nm, Si doping A multi-layered structure in which a 5-nm-thick well layer composed of In _0.3 Ga _0.7 N and a 5-nm-thick barrier layer composed of undoped GaN are repeated as a pair, starting with a 5-nm-thick barrier layer composed of GaN. An active layer 13 having a quantum well structure, a first p-side layer 14 made of Mg-doped AlGaN with a thickness of 15 nm, a second p-side layer 15 made of undoped GaN with a thickness of 150 nm, an Mg-doped GaN ( A third p-side layer 16 having a thickness of 150 nm made of Mg low concentration and a fourth p-side layer 17 having a thickness of 15 nm made of Mg-doped GaN (high Mg concentration) (p-side contact layer) are sequentially provided.

  The forward voltage at 20 mA of the semiconductor light emitting device of this example was about 2.80 V, which was about 0.05 V lower than that of Comparative Example 1. Furthermore, the electrostatic withstand voltage was about 1150 V, which was about 150 V higher than that of Comparative Example 1. From these results, it can be seen that by providing the first superlattice structure 11 and the second superlattice structure 12, it is possible to achieve both a reduction in forward voltage and an improvement in electrostatic withstand voltage.

  Here, the first n-side layer 3, the second n-side layer 4, the fourth n-side layer 6 to the eighth n-side layer 10, and the first p-side layer 14 to the third p-side layer 16 are not necessarily required. However, it is preferable to provide these because it is easy to achieve both a decrease in forward voltage and an improvement in electrostatic withstand voltage while maintaining the output.

<Comparative Example 1>
In Example 1, instead of the first superlattice structure 11 and the second superlattice structure 12, a 0.8 nm-thickness barrier layer made of GaN and a thickness 1 made of In _0.06 Ga _0.94 N are used. A semiconductor light emitting device similar to that of Example 1 was fabricated except that a superlattice structure terminated with a similar barrier layer was formed after repeating this 30 times with a pair of .7 nm well layers as a pair.

  The forward voltage at 20 mA of the semiconductor light emitting device of this comparative example was about 2.85V. Furthermore, the electrostatic withstand voltage was about 1000V.

  The light emitting device manufacturing method according to the present invention includes various light emitting device manufacturing methods such as illumination light sources, various indicator light sources, in-vehicle light sources, display light sources, liquid crystal backlight light sources, sensor light sources, and traffic lights. Can be used.

DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Buffer layer 3 ... First n-side layer 4 ... Second n-side layer 5 ... Third n-side layer (n-side contact layer)
6 ... 4n-side layer 7 ... 5n-side layer 8 ... 6n-side layer 9 ... 7n-side layer 10 ... 8n-side layer 11 ... first superlattice structure 11a ... first barrier layer 11b ... first well layer 12 ... second superlattice structure 12a ... second barrier layer 12b ... second well layer 13 ... active layer 14 .... First p-side layer 15 ... second p-side layer 16 ... third p-side layer 17 ... fourth p-side layer (p-side contact layer)
18 ... n electrode 19 ... p electrode

Claims (5)

A semiconductor light emitting device comprising an n-side contact layer, an active layer, and a p-side contact layer in order,
A first superlattice structure including a first barrier layer made of GaN or InGaN and a first well layer made of InGaN in order from the n-side contact layer side between the n-side contact layer and the active layer; A second superlattice structure including a second barrier layer made of GaN or InGaN and a second well layer made of InGaN, and
An average In mixed crystal ratio of the second superlattice structure is smaller than an average In mixed crystal ratio of the first superlattice structure.
A semiconductor light emitting device comprising an n-side contact layer, an active layer, and a p-side contact layer in order, Between the n-side contact layer and the active layer, in order from the n-side contact layer side, a first superlattice structure including a first barrier layer made of GaN and a first well layer made of InGaN, and GaN A second superlattice structure including a second barrier layer made of and a second well layer made of InGaN,
The thickness of the first well layer and the thickness of the second well layer are substantially the same, and the thickness of the first barrier layer and the thickness of the second barrier layer are substantially the same. And
The semiconductor light emitting device, wherein the In mixed crystal ratio of the second well layer is smaller than the In mixed crystal ratio of the first well layer.   3. The semiconductor light emitting element according to claim 1, wherein a film thickness of the first superlattice structure is larger than a film thickness of the second superlattice structure.   4. The semiconductor light emitting device according to claim 1, wherein a thickness of the first well layer is larger than a thickness of the first barrier layer. 5.   5. The semiconductor light emitting device according to claim 1, wherein a thickness of the second barrier layer is larger than a thickness of the second well layer.

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