JP3301601B2 - Nitride semiconductor light emitting device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting diode (LE).
D), a laser diode (LD), a super luminescent diode (SLD) nitride is used such as a semiconductor _{(In a Al b Ga 1-} ab N, 0 ≦ a, 0 ≦ b, a + b ≦ 1)
A light-emitting element comprising:

[0002]

2. Description of the Related Art Nitride semiconductors are blue LEDs and green LEDs.
It has already been put to practical use. These LEDs have a double hetero structure in which n-type and p-type nitride semiconductors are stacked on a sapphire substrate, and the active layer has a nitride semiconductor layer having a quantum well structure. The nitride semiconductor light-emitting elements constituting the LED are classified into two types, that is, the case where the sapphire substrate side is the light emission observation surface side and the case where the nitride semiconductor layer side is the light emission observation surface side. In general, since a nitride semiconductor has positive and negative electrodes on the same surface side, when the sapphire substrate side is used as a light emission observation surface, for example, when connecting the electrodes to a support such as a lead frame, a chip size is required. However, there is a drawback that the handleability becomes poor,
Actively utilizing the properties of transparent sapphire,
There is an advantage that light extraction efficiency is improved. On the other hand, when the nitride semiconductor side is used as the light emission observation surface, the chip size can be reduced, and the handleability is much better than the former case, but light leaking to the sapphire substrate side is, for example, a lead frame. There is a disadvantage that light extraction efficiency is deteriorated due to absorption by the adhesive. In general, commercially available LEDs are selected for the latter having better handling properties.

[0003]

In the latter case, a technique has been proposed in which a light reflecting film is formed on the sapphire substrate surface in order to reflect light leaking to the sapphire substrate side.
This technique was not yet satisfactory. Also, in a technology of forming a reflecting mirror inside a semiconductor layer such as a surface emitting laser element, if a reflecting mirror is provided on the sapphire substrate side, the distance from the active layer is too large, and it is difficult to use the reflecting mirror as a resonator. There is a tendency.

The present invention has been made in view of such circumstances, and a purpose of the present invention is to improve the light extraction efficiency of a light emitting element by mainly effectively reflecting light leaking to the substrate side. Another object of the present invention is to realize a laser device having an effective reflecting mirror serving as a resonator inside a semiconductor layer.

[0005]

According to the present invention, there is provided a light emitting device comprising a nitride semiconductor formed on a heterogeneous substrate made of a different material from a nitride semiconductor, and a nitride semiconductor formed on a surface of the first nitride semiconductor. Has a property that it does not grow or hardly grows, and a first reflecting mirror that reflects light emitted from the active layer is partially formed. A plurality of nitride semiconductor layers including at least an active layer are stacked on the second nitride semiconductor grown to reach the surface of the substrate.

Further, in the light emitting device according to the present invention, a second reflecting mirror is partially formed on the second nitride semiconductor layer, and a second reflecting mirror is formed through a window of the second reflecting mirror. A third nitride semiconductor grown to reach the surface of a mirror is used as a substrate, and a plurality of nitride semiconductor layers including at least an active layer are grown on the substrate.

It is preferable that the second reflector is formed on a second nitride semiconductor layer corresponding to a position of a window of the first reflector. Note that, in the present invention, the second nitride semiconductor and the nitride semiconductor layer including the heterogeneous substrate, the first reflecting mirror, the underlayer, and the active layer above the second nitride semiconductor are not necessarily formed in contact with each other. However, for example, a semiconductor layer made of a material different from that of the nitride semiconductor may be grown between them.

In the light emitting device according to the present invention, the area of the reflecting mirror is larger than the area of the window.

[0009]

FIG. 1 is a schematic cross-sectional view showing a structure of a nitride semiconductor wafer obtained in each step of growing a second nitride semiconductor layer from a heterogeneous substrate in a light emitting device of the present invention. FIG.

In order to grow a substrate made of the second nitride semiconductor 3, first, as shown in FIG.
On the base layer 2 made of the first nitride semiconductor is grown, and the first reflecting mirror 100 is partially formed on the base layer 2. The heterogeneous substrate 1 is made of a material different from a conventionally proposed nitride semiconductor, and a material on which the nitride semiconductor can be grown via a buffer layer, for example, is selected. Among them, in addition to sapphire that has been put to practical use, spinel (MgAl ₂ O ₄ ), ZnO, GaAs, Si, S
Oxide-based substrates and the like that lattice-match with iC and nitride semiconductors have been proposed.

The underlayer 2 is formed, for example, at a low temperature of 200 ° C. to 900 ° C. through a buffer layer made of Al _x Ga ₁ -xN (0 ≦ x ≦ 1) at a higher temperature than the buffer layer. Can grow by growing. In the present invention, the underlayer includes the nitride semiconductor including the buffer layer. That is, the underlayer may be composed of a plurality of nitride semiconductor layers. However, since the underlayer is grown on a heterogeneous substrate, crystal defects are extremely large due to factors such as a difference in thermal expansion coefficient between the heterogeneous substrate and the underlayer, lattice mismatch, and the like. For example, the number of threading dislocations is 10 ⁹ / cm. There are ^two or more, and it does not become a nitride semiconductor substrate. As the most preferred underlayer, undoped or GaN having an n-type impurity concentration of 1 × 10 ¹⁷ / cm ³ or less is grown via a buffer layer. Note that a buffer layer made of a semiconductor different from a nitride semiconductor such as ZnO can be grown between the underlayer and the heterogeneous substrate.

First reflecting mirror 10 formed on underlayer 2
0 has the effect of reflecting the light emitted from the active layer upward, and the effect of reducing the crystal defects of the second nitride semiconductor layer that grows laterally from the window of the reflector to the surface of the reflector. Having. The reflecting mirror may have any shape, such as a stripe shape, a dot shape, a grid shape, or the like, as long as it is formed partially on the underlayer, but is preferably formed in a stripe shape. As a material for the reflector, any material may be used as long as the nitride semiconductor does not grow or hardly grows on the surface of the reflector, and reflects the light emitted from the active layer toward the active layer. However, for example, SiO
_{2, SixNy, TiO 2, TixNy} , multilayer film of dielectric such as ZrO ₂ can be selected. These dielectrics are, for example, λ
/ 4n (λ: emission wavelength, n: refractive index of the dielectric) acts as a reflector by being laminated. Also P
For example, a metal such as t, Ni, Cr, Ag, or the like having a property of reflecting light emitted from the active layer with a silver-white metal and making it difficult for the nitride semiconductor to grow on the surface may be used. The reflecting mirror is
It is desirable to select a material in which the dielectric multilayer film, metal, or the like has a melting point that can withstand the growth temperature of the second nitride semiconductor.

The first reflecting mirror 100 is formed on a base layer made of a first nitride semiconductor grown on a heterogeneous substrate 1 as shown in FIG. It can also be formed. For example, when sapphire is used for the dissimilar substrate 1, growing the first nitride semiconductor 2 on sapphire is desirable for growing the second nitride semiconductor 3 having fewer crystal defects. On the other hand, when a substrate lattice-matched with a nitride semiconductor or a substrate having a lattice constant close to that of the nitride semiconductor is used, the first reflecting mirror 100 can be formed directly in contact with a different kind of substrate.

Next, as shown in FIG. 1B, a second nitride
The semiconductor 3 is composed of a window of an underlayer on which the reflecting mirror is formed.
Lengthen. The reflecting mirror 100 has a nitride semiconductor grown on its surface.
The second nitride semiconductor 3
It grows out of the window, and as shown in FIG.
It grows laterally above the mirror 100. Continue to grow
Then, as shown in FIG. 1C, in the horizontal and vertical directions,
The growing second nitride semiconductor is approximately above the center of the reflector.
The portions are connected to form a nitride semiconductor substrate. Like this
When the nitride semiconductor 3 is grown,
Growing laterally due to the reflector being covered by the reflector
Crystal defects of the second nitride semiconductor extend from the underlying layer.
Will not come. In addition, crystal defects extending from the window part
Stops in the middle of the nitride semiconductor layer, so the second nitride semiconductor
Very few crystal defects appear on the surface after body layer growth
For example, 10⁸Pieces / cm^TwoBelow, and even 10⁷Pieces / cm^TwoLess than
Below. The second nitride semiconductor 3 is undoped or
Means that the n-type impurity concentration is 1 × 10 ¹⁷/cm^ThreeThe following GaN
Length is the best way to produce a substrate with good crystallinity.
Good.

As a further preferred embodiment, as shown in FIG. 1D, a first nitride semiconductor layer is further formed on the second nitride semiconductor layer.
A second reflecting mirror 101 is formed in the same manner as the first reflecting mirror, and a third nitride semiconductor layer 4 is grown on the second reflecting mirror 101 in the same manner. By forming the second reflecting mirror 101, the third reflecting mirror grown on the second reflecting mirror 101 is formed.
The crystal defects of the nitride semiconductor 4 are further reduced. This is because the second nitride semiconductor layer 3 serving as a base has few crystal defects. Preferably, as shown in (d), by forming the position of the second reflecting mirror 101 on the surface of the second nitride semiconductor layer 3 corresponding to the window of the first reflecting mirror,
When viewed in a plan view from the nitride semiconductor layer side, the entire structure is covered with a reflecting mirror, so that the light extraction efficiency is further improved. Further, for example, when a crystal defect of the second nitride semiconductor 3 appears in the window, the window is further laterally grown on the second reflector to further cover the window with the second reflector. The crystal defects of the third nitride semiconductor layer are further reduced. That is, it is most preferable that the second reflecting mirror 101 is formed on the first nitride semiconductor layer 3 where crystal defects appear on the surface. However, the third reflecting mirror may be formed at random.

The operation of the reflector according to the present invention is as follows.
The nitride semiconductor layer grown from the window of the reflector and grown laterally on the surface of the reflector has very few crystal defects.
Therefore, by using the nitride semiconductor as a substrate,
The crystal defects of the plurality of nitride semiconductor layers including the active layer grown on the substrate are reduced as in the case of the nitride semiconductor substrate. Therefore, when a light-emitting element is manufactured, the crystal has no dislocation in the active layer, and the element has a long life. In addition, it is improved in all aspects such as reverse breakdown voltage and current characteristics of leak current. In addition, since the reflecting mirror also has the function of reflecting the light emitted from the active layer to the active layer side, the amount of light leaking to the substrate side is reduced, and the light extraction efficiency is improved in the light emitting element having the semiconductor side as the light emission observation surface. Therefore, by making the area of the reflecting mirror larger than the area of the window portion, the reflecting portion becomes larger and the light extraction efficiency is improved, and the number of crystal defects extending from the window portion is reduced, thereby realizing a more preferable light emitting element. it can. Preferably, as described above, by arranging the reflecting mirrors in two or more stages in the vertical direction, the reflecting mirror covers substantially all surfaces on a plane, so that the light extraction efficiency is further improved. I do. In the case of a laser device, the first reflecting mirror 100 or the second reflecting mirror 101
Is inside the semiconductor layer and acts as one of the resonators whose distance from the active layer is short, so that a surface emitting laser element can be realized with a nitride semiconductor.

[0017]

[Embodiment 1] FIG. 2 is a schematic sectional view showing the structure of an LED element according to Embodiment 1. Below, based on this figure,
Example 1 will be described. First, a first nitride semiconductor layer 2 is grown on a heterogeneous substrate 1 made of sapphire by MOVPE. The first nitride semiconductor layer 2 includes, in order from the heterogeneous substrate side, a buffer layer made of GaN grown at 500 ° C. and a G layer grown at 1050 ° C. on the buffer layer.
aN.

Next, the wafer is taken out of the reaction vessel,
With the VD device, S is formed on the entire surface of the first nitride semiconductor layer 2.
A dielectric multilayer film composed of iO ₂ and SiN is formed to have a single film thickness of λ /
4n are alternately formed to form a dielectric multilayer film.

After forming the dielectric multilayer film, a mask is formed at a predetermined position on the dielectric multilayer film, and the dielectric multilayer film is selectively etched to have a stripe width of 10 μm and a stripe interval (window) of 2 μm. And the first reflecting mirror 100. When such a first reflecting mirror 100 is formed of a dielectric multilayer film, first, a dielectric multilayer film is formed on the entire surface of the first nitride semiconductor layer, and then the dielectric multilayer film is selectively etched. The technique for forming a predetermined shape is to form a mask at a predetermined position on the nitride semiconductor layer, form a dielectric multilayer film thereon, and then remove the mask by a lift-off method to remove the dielectric multilayer film. Compared with the method of leaving only the film, a dielectric multilayer film having a uniform film thickness can be easily formed. Further, since the surface of the nitride semiconductor layer in the window is also etched, it is easy to observe etch pits, crystal defects, and the like appearing on the surface. The advantage of this method is the same in the case of the second reflecting mirror 101.

Next, after forming the first reflecting mirror 100, the wafer is returned to the MOVPE reaction vessel, and the second nitride semiconductor layer 3 made of undoped GaN is deposited at 1050 ° C. for 20 minutes.
It is grown to a thickness of μm.

After the growth of the second nitride semiconductor layer 3, the wafer is taken out of the reaction vessel, and a dielectric multilayer film made of SiO ₂ and SiN is formed again on the entire surface of the second nitride semiconductor layer 3 by the CVD apparatus. After that, the stripe width is set to 10 μm and the window portion is set to 2 μm by selective etching.
And However, the formation position of the second reflecting mirror 101 is, as shown in FIG. 2, the first reflecting mirror 100 and the second reflecting mirror 101.
Are formed so as to be parallel, so that the window of the first reflecting mirror 100 is closed when viewed from a plane.

Next, the wafer is returned into the reaction vessel, and
At 50 ° C., a third nitride semiconductor layer 4 made of undoped GaN is grown to a thickness of 20 μm.

Subsequently, an n-side contact layer 11 made of n-type GaN doped with Si is grown at 1050 ° C. to a thickness of 4 μm, and an n-side cladding layer 12 made of GaN having a Si concentration lower than that of the n-side contact layer is formed thereon. Growing by 1 μm.

Next, at 800 ° C., undoped In _0.1 Ga having a single quantum well structure having a thickness of 30 Å.
_An active layer 13 of _0.9 N is grown, and 105
At 0 ° C., a p-side cladding layer 14 made of Mg-doped p-type Al _0.1 Ga _0.9 N is grown to 0.1 μm, and finally, a p-side contact layer 15 made of Mg-doped p-type GaN is grown to _0.5 μm.
m.

After the reaction is completed, the wafer is taken out of the reaction vessel, annealed in a nitrogen atmosphere at 700 ° C.
After further reducing the resistance of the mold layer, etching is performed from the p-side contact layer 15 side to expose the surface of the n-side contact layer 11 as shown in FIG. Thereafter, a light-transmitting ohmic p-electrode 16 is formed on almost the entire surface of the uppermost p-side contact layer, and a bonding p-pad electrode 17 is formed thereon. On the other hand, on the surface of the n-side contact layer 18 exposed earlier, the n-electrode 1 made of W / Al
8 is formed.

Finally, after the sapphire substrate was polished and thinned, it was separated into chips of 350 μm square to obtain a blue LED element. The output was 50% or more as compared with a conventional LED element without a reflecting mirror. And the life of the device has improved several times. In addition, the withstand voltage in the reverse direction was improved by 50% or more as compared with the conventional device. It can be inferred that, because the second and third nitride semiconductors serve as the substrate, the crystal defects of the element itself are reduced, and the withstand voltage in the reverse direction and the element life are improved.

Embodiment 2 FIG. 3 is a schematic sectional view showing the structure of an LD device according to Embodiment 2 of the present invention, and specifically shows the structure of a surface emitting laser device. The second embodiment will be described below with reference to FIG.

As in the first embodiment, a GaN buffer layer and an undoped G
First nitride semiconductor layer 2 made of aN, striped first reflector 100 made of a dielectric multilayer, second nitride semiconductor layer 3 made of undoped GaN, striped made of dielectric multilayer A second reflecting mirror 101 and a third nitride semiconductor layer 4 made of undoped GaN are sequentially stacked.

Subsequently, the n-type n-type GaN
After growing the side contact layer 21 by 4 μm, an undoped Al _0.15 Ga _0.85 N layer having a thickness of 25 Å,
An n-side cladding layer 22 composed of a superlattice having a total thickness of 0.4 μm is grown by alternately stacking Si-doped GaN layers having a thickness of 25 Å.

Next, a barrier layer made of undoped In _0.01 Ga _0.95 N of 40 Å and a well layer made of undoped In _0.2 Ga _0.8 N of 40 Å are alternately laminated. An active layer 23 having a multiple quantum well structure (MQW) of 440 angstroms is grown.

Next, a 25 angstrom undoped Al _0.15 Ga _0.85 N layer and a 25 angstrom Mg
Alternately laminated with doped GaN layers to a total film thickness of 0.4 μm
The p-side cladding layer 24 composed of the super lattice layer is grown.

After the growth of the p-side cladding layer, the wafer is taken out of the reaction vessel, and a circular protective film made of SiO ₂ is formed on the surface of the p-type cladding layer. However, the position of the mask is smaller than the second reflecting mirror 101, and is formed so as to be directly above the reflecting mirror.

After the formation of the protective film, the wafer is again transferred into the reaction vessel, and a current blocking layer 26 of Si-doped n-type Al _0.1 Ga _0.9 N is formed on the surface of the p-side cladding layer 24 where the protective film is not formed. It is formed with a thickness of 0.4 μm. This current blocking layer may be doped with a p-type impurity, such as Zn or Cd, which does not easily become a p-type even when doped with a p-type impurity, to form a high-resistance i-type nitride semiconductor layer.
High resistance i-type Al with mixed crystal ratio larger than cladding layer
GaN can also be formed.

After the formation of the current blocking layer 26, the wafer is taken out of the reaction vessel, and the protective film is dissolved and removed.
A p-side contact layer 25 of type GaN is grown.

After the reaction is completed, annealing is performed to further reduce the resistance of the p-layer. As in the first embodiment, a part of the n-side contact layer 22 is exposed by etching, and the n-electrode 28 is formed on the exposed n-side contact layer 22. On the other hand, after forming a p-electrode 27 on the surface of the p-side contact layer, the wafer was separated into chips to obtain a laser device having a structure as shown in FIG.
m laser light was observed from the sapphire substrate side.

Embodiment 3 FIG. 4 is a schematic sectional view showing a structure of an LD device according to Embodiment 3 of the present invention, which also shows a structure of a surface emitting laser device. Hereinafter, a third embodiment will be described with reference to FIG.

In the first embodiment, a GaN buffer layer and an undoped GaN
After forming the first reflecting mirror 100 in the form of a stripe made of a first nitride semiconductor layer 2 and a dielectric multilayer film, when growing the second nitride semiconductor layer 3, 5 × 1 of Si is used.
A GaN doped with 0 ¹⁶ / cm ³ is grown to a thickness of 60 μm. The reason why the thickness of the second nitride semiconductor layer is set to 60 μm or more is to remove the heterogeneous substrate later and use the second nitride semiconductor layer as a substrate. If the thickness is less than 60 μm, the second nitride semiconductor layer is broken during removal of the heterogeneous substrate,
Device fabrication tends to be difficult. Further, the n-type impurity is doped in a small amount because the exposed second nitride semiconductor layer 3 itself is used as a contact layer after the foreign substrate is removed. Further, in the case where the second nitride semiconductor layer and the third nitride semiconductor layer serving as the substrate are doped with an n-type impurity, as described above, the concentration is preferably 1 × 10 ¹⁷ / cm ³ or less. If the number is larger than that, the number of crystal defects in the nitride semiconductor layer increases, and it is difficult to obtain a substrate having good crystallinity.

After growing the second nitride semiconductor layer 3, the first embodiment
The second reflector 101 is formed in the same manner as described above. After the formation of the second reflecting mirror, a third nitride semiconductor layer 4 made of undoped GaN is grown by 15 μm.

Next, In _0.05 Ga _0.95 N doped with Si
The crack preventing layer 20 is grown to a thickness of 0.15 μm. When the crack prevention layer 20 is a nitride semiconductor containing In, this layer becomes a buffer layer, and it is difficult for cracks to be formed in the nitride semiconductor layer containing Al to be grown later. It goes without saying that this crack prevention layer may be included in the laser device of the second embodiment.

Thereafter, as in the second embodiment, the n-side cladding layer 22 made of a superlattice, the active layer 23 of MQW, the p-side cladding layer 24 made of a superlattice, the current blocking layer 26, and the p-side contact layer 25 are formed. Laminate.

After completion of the reaction, annealing is performed to further reduce the resistance of the p-layer, and then polished from the sapphire side to form a heterogeneous substrate 1, a first nitride semiconductor layer 2, a first reflecting mirror 101
Is removed. Thereafter, a ring-shaped n-electrode was provided on the exposed surface of the second nitride semiconductor layer 2, and the wafer was separated into chips to obtain a laser device having a structure as shown in FIG. Shows continuous oscillation at 4
Laser light of 10 nm was observed from the second nitride semiconductor layer 3 side. When forming the n-electrode 28, the surface of the second nitride semiconductor layer is heavily doped with n-type impurities by using a technique such as ion implantation in order to improve the ohmic contact of the electrode contact surface. You may.

[0042]

As described above, in the light emitting device of the present invention, the crystal defects of the nitride semiconductor layer grown from the window of the reflector and grown laterally on the surface of the reflector are extremely small. Therefore, the crystal defects of the plurality of nitride semiconductor layers including the active layer grown on the substrate are reduced, and the reliability of the device is improved. Further, since the reflecting mirror reflects the light emitted from the active layer toward the active layer at a short distance of several tens μm or less, the light extraction efficiency of the light emitting element is improved. Therefore, if this reflecting mirror is actively used, a surface emitting laser element can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view partially showing the structure of a wafer for explaining each step in manufacturing a light emitting device of the present invention.

FIG. 2 is a schematic cross-sectional view illustrating a structure of a light emitting element of Example 1.

FIG. 3 is a schematic cross-sectional view illustrating a structure of a light emitting element of Example 2.

FIG. 4 is a schematic cross-sectional view illustrating a structure of a light-emitting element of Example 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Different substrate 2 ... 1st nitride semiconductor layer 3 ... 2nd nitride semiconductor layer 4 ... 3rd nitride semiconductor layer 11, 21 ... n side contact layer 12 , 22 ... n-side cladding layer 13, 23 ... active layer 14, 24 ... p-side cladding layer 15, 25 ... p-side contact layer 20 ... crack preventing layer 26 ... current blocking Layers 16, 27: p-electrode 17: p-pad electrode 18, 28: n-electrode 100: first reflecting mirror 101: second reflecting mirror

Continuation of the front page (56) References JP-A-10-308558 (JP, A) JP-A-11-163402 (JP, A) JP-A-11-163401 (JP, A) JP-A-11-4048 (JP) JP-A-11-191637 (JP, A) JP-A-10-312971 (JP, A) JP-A-11-126948 (JP, A) JP-A-11-186178 (JP, A) 10-321529 (JP, A) JP-A-10-326921 (JP, A) JP-A-3-133182 (JP, A) JP-A 5-190476 (JP, A) WO 97/11518 (WO, A1) (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 33/00 H01S 5/00-5/50

Claims (4)

(57) [Claims] A nitride semiconductor does not grow or hardly grows on a surface of a base layer made of a first nitride semiconductor formed on a heterogeneous substrate made of a material different from that of a nitride semiconductor. And a first reflecting mirror that reflects light emitted from the active layer is partially formed,
A plurality of nitride semiconductor layers including at least an active layer are laminated on the substrate, using the second nitride semiconductor grown from the window of the reflector to the surface of the reflector as a substrate. A nitride semiconductor light-emitting device, comprising: 2. A second reflecting mirror is partially formed on the second nitride semiconductor layer, and extends from a window of the second reflecting mirror to a surface of the second reflecting mirror. 2. The nitride semiconductor light emitting device according to claim 1, wherein a plurality of nitride semiconductor layers including at least an active layer are grown on the grown third nitride semiconductor as a substrate. . 3. The device according to claim 2, wherein the second reflector is formed on a second nitride semiconductor layer corresponding to a position of a window of the first reflector. Nitride semiconductor light emitting device. 4. The nitride semiconductor light emitting device according to claim 1, wherein an area of the reflecting mirror is larger than an area of the window.