JP3314666B2 - Nitride semiconductor 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 nitride semiconductor (In _X Al _Y Ga _{1 -XYN} , 0 ≦ X,
The present invention relates to a device comprising 0 ≦ Y, X + Y ≦ 1) and a method for growing a nitride semiconductor constituting the device.

[0002]

2. Description of the Related Art A nitride semiconductor is used as a material for a high-brightness blue LED and a pure green LED by the present applicant.
It has just been put to practical use in ED displays and traffic signals. The LEDs used in these various devices are n
Between a p-type nitride semiconductor layer and a p-type nitride semiconductor layer, a single-quantum-well (SQW)
It has a double hetero structure in which an active layer made of GaN is sandwiched. Wavelengths such as blue and green are determined by the IGaN of the InGaN active layer.
It is determined by increasing or decreasing the n composition ratio. Blue LED
Is an emission wavelength of 450 nm and a half width of 20 n at 20 mA.
m, luminous intensity 2 cd, optical output 5 mW, external quantum efficiency 9.1%
It is. On the other hand, the green LED is also at 20 mA,
The emission wavelength is 525 nm, the half width is 30 nm, the luminous intensity is 6 cd, the optical output is 3 mW, and the external quantum efficiency is 6.3%.

[0003] The present applicant has recently used this material to
World's first laser oscillation at room temperature under lus current
First published {for example, Jpn.J.Appl.Phys.35 (1996) L7
4, Jpn.J.Appl.Phys.35 (1996) L217}. This laser element
Has an active layer with a multiple quantum well structure using InGaN.
Pulse width 2μs, pulse width
Under the condition of a cycle of 2 ms, the threshold current is 610 mA and the threshold current density is
8.7 kA / cm^Two, 410 nm oscillation. Improved
Laser element is also described in Appl.Phys.Lett. 69 (1996) 1477.
We announced. This laser device is a p-type nitride semiconductor
With a structure in which a ridge stripe is formed in part of the layer
Pulse width 1μs, pulse period 1ms, duty
-With a ratio of 0.1%, a threshold current of 187 mA and a threshold current density of 3
kA / cm ^Two, 410 nm oscillation. Further, the applicant
Announced for the first time continuous oscillation at room temperature. {Example
For example, Nikkei Electronics December 2, 1996 issue
, Appl.Phys.Lett.69 (1996) 3034, Appl.Phys.Lett.69
(1996) 4056 et al.], This laser device has a threshold
Value current density 3.6 kA / cm^Two, Threshold voltage 5.5V, 1.
At 5 mW output, it shows continuous oscillation for 27 hours.

[0004]

As described above, the light emitting device using the nitride semiconductor has already been put to practical use as an LED, but there are still insufficient points, and further improvement in the light emitting output is desired. In addition, LDs are currently under intensive research with a view to practical use, and it is desired not only to improve the output but also to extend the life. If the light emission output of a light emitting device such as an LED or LD can be improved, the light receiving efficiency of a light receiving device such as a solar cell or an optical sensor having a similar structure can be improved at the same time. Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a novel structure of a nitride semiconductor device, and mainly provide an LED, an LD,
The purpose is to improve the output.

[0005]

According to the nitride semiconductor device of the present invention, a first nitride semiconductor layer containing a p-type impurity is formed on an active layer, and the first nitride semiconductor layer is formed on the first nitride semiconductor layer. A second nitride semiconductor layer whose p-type impurity concentration gradually decreases as the distance from the first nitride semiconductor layer increases, and an average of the second nitride semiconductor layer is formed on the second nitride semiconductor layer. A third nitride semiconductor layer including a p-type impurity in an amount larger than the p-type impurity concentration is provided. In the present invention, the active layer and the first nitride semiconductor layer do not have to be formed in contact with each other, and the first nitride semiconductor layer and the second nitride semiconductor layer are formed in contact with each other. The second nitride semiconductor layer and the third nitride semiconductor layer need not be formed in contact with each other.

Further, in the nitride semiconductor device according to the present invention, the second nitride semiconductor layer is formed of a multilayer film in which a plurality of nitride semiconductor layers are stacked, and the p-type impurity of the multilayer film is reduced stepwise. It is characterized by having.

Further, the nitride semiconductor device according to the present invention is characterized in that the first nitride semiconductor layer is a superlattice layer formed by laminating two types of nitride semiconductor layers having different compositions from each other. Further, the nitride semiconductor device of the present invention has a first aspect.
Nitride semiconductor layer is _{Al X Ga 1-X N (} 0 ≦ X ≦ 1)
And the third nitride semiconductor layer is formed of Al _x Ga _1-x
N (0 ≦ X ≦ 0.3). Further
The nitride semiconductor device according to the present invention is characterized in that the second nitride semiconductor
The body layer has the same composition as the third nitride semiconductor layer
It is characterized by. Further, the nitride semiconductor device of the present invention
The p-type impurity concentration of the first nitride semiconductor layer is 1 × 10
¹⁸ / cm ³ or more and 1 × 10 ²¹ / cm ³ or less
And features. Further, the nitride semiconductor device of the present invention
The p-type impurity concentration of the third nitride semiconductor layer is 1 × 10
¹⁸ / cm ³ or more and 1 × 10 ²¹ / cm ³ or less
And features. Further, the nitride semiconductor device of the present invention
The active layer has a single quantum well structure including a nitride semiconductor layer containing at least In or a multiple quantum well structure.

[0008]

FIG. 1 is a schematic sectional view showing the structure of a nitride semiconductor device according to one embodiment of the present invention, and specifically shows the structure of an LED device. The element structure is such that a buffer layer 2 made of GaN, an n-side contact layer 3 (also serving as an n-side cladding layer) made of GaN doped with Si, and a single quantum well structure having a thickness of 30 Å are formed on a substrate 1 made of sapphire. An active layer 4 made of InGaN, a first p-side nitride semiconductor layer 5 made of Mg-doped AlGaN, and a second p-side nitride semiconductor layer 5 made of GaN doped with Mg in an inclined manner.
And a third p-side nitride semiconductor layer 7 made of GaN doped with a higher average Mg concentration than the second p-side nitride semiconductor layer 6.
On almost the entire surface of the third p-side nitride semiconductor layer 7, a p-electrode 8 made of a light-transmitting metal thin film is formed.
A pad electrode 9 for bonding is formed at the corner of the. On the other hand, an n-electrode 10 is formed on the surface of the n-side contact layer 3 exposed by etching from the p-side nitride semiconductor layer side.

FIG. 2 shows this LED element as a SIMS
The data analyzed by (secondary ion mass spectrometer) are shown. Mg indicates the concentration, and In indicates the secondary ionic strength. That is, the In peak indicates the position of the active layer,
This indicates that Mg is distributed more on the p layer side than the active layer. In this figure, the LED element is sputtered with Cs ions from the uppermost layer, and the elements that come out are analyzed. The horizontal axis shows the depth, and the vertical axis shows the Mg concentration and In intensity. Thus, the device of the present invention has the nitride semiconductor layer adjusted so that the Mg concentration gradually decreases as the distance from the active layer increases.

[0010] The device of the present invention has a second p-side nitride semiconductor layer 6 to which a p-type impurity is graded doped, on the first nitride semiconductor layer 5 containing a p-type impurity. This second
In the p-side nitride semiconductor layer, the output of the light-emitting element can be improved by the graded doping of the p-side impurity. That is, a third p-side nitride semiconductor doped with a p-type impurity which acts as a contact layer at a high concentration, and a p-type impurity located at a position closer to the active layer than the third p-side nitride semiconductor layer. And a first p-side nitride semiconductor doped with a higher concentration of p-type impurities at a position closer to the active layer than the second nitride semiconductor. With the provision, carriers injected from the contact layer side can be easily stored in the active layer, so that the output of the entire device can be improved.

The active layer 4 has a single quantum well structure including a nitride semiconductor layer containing at least In or a multiple quantum well structure. The well layer has a thickness of 100 angstrom or less, more preferably 70 angstrom or less.
It is desirable to constitute in _{X Ga 1-X N (0} <X ≦ 1),
The barrier layer has a band gap energy higher than that of the well layer, ie, In _Y Ga _{1 -Y} N (0 ≦ Y <) or Al _{X ′} G.
It is desirable that a _{1-X ′} N (0 <X ′ ≦ 1) be formed to a thickness of 200 Å or less, more preferably 150 Å or less.

The first p-side nitride semiconductor layer 5 may be formed of a nitride semiconductor layer containing a p-type impurity, and may or may not be particularly in contact with the active layer. As the semiconductor, a nitride semiconductor having a band gap energy larger than that of the active layer is selected. For example, as described above, Al _x Ga ₁ -xN (0
≦ X ≦ 1) is preferably grown. On the other hand, the concentration of the p-type impurity to be doped is not less than 1 × 10 ¹⁸ / cm ^{3 and not} more than 1 × 10 ²¹ / cm ³ , more preferably not less than 5 × 10 ¹⁸ / cm ³ and 5 × 10 5 / cm ^3.
Adjusted to ²⁰ / cm ^3. As a p-type impurity, for example, Mg,
Group II elements such as Zn, Cd, Ca, Be, and Sr are preferably doped. Further, the first nitride semiconductor layer may be a superlattice layer in which two types of nitride semiconductor layers having different compositions are stacked. When a superlattice layer is used, the thickness of the nitride semiconductor layer constituting the superlattice layer is adjusted to 100 Å or less, more preferably 70 Å or less, and most preferably 50 Å or less. With a superlattice layer, the crystallinity of the nitride semiconductor layer is improved, and the output is further improved. In the case of a superlattice layer, a p-type impurity may be doped into both layers,
Any one of the layers may be doped.

The second nitride semiconductor layer 6 is desirably formed in contact with the first nitride semiconductor layer 5, but need not be particularly formed in contact therewith. For example, first and second
An undoped nitride semiconductor layer having a thickness of several hundred angstroms or less can be grown between the nitride semiconductor layer and the nitride semiconductor layer. It is preferable that the impurity is adjusted so as to be continuously reduced in the vicinity of the third nitride semiconductor layer 6. Can also be grown. Although the composition of the nitride semiconductor layer is not particularly limited,
Has the same composition as that of the nitride semiconductor layer. The thickness of the second nitride semiconductor layer is adjusted to 2 μm or less, more preferably 1 μm or less, and most preferably 0.5 μm or less. Further, the second nitride semiconductor layer may have a multilayer structure (including a superlattice) of a nitride semiconductor, and the p-type impurity concentration of the nitride semiconductor layer forming the multilayer film may be gradually reduced. .

The third nitride semiconductor layer 7 is desirably a contact layer for forming a p-electrode.
If the value is set to Al _x Ga ₁ -xN (0 ≦ X ≦ 0.3) having a value of 0.3 or less, a favorable ohmic is obtained with the p electrode. The p-type impurity concentration of the third nitride semiconductor layer 7 is 1 × 10 ¹⁸ / cm ³ or more and 1 × 10 ²¹ , similarly to the first nitride semiconductor layer 5.
/ Cm ³ or less, more preferably 5 × 10 ¹⁸ / cm ³ or more,
It is desirable to adjust to × 10 ²⁰ / cm ³ . It is preferable that the thickness of the third nitride semiconductor layer be adjusted to be smaller than that of the second nitride semiconductor layer. That is, the thickness of the third p-type nitride semiconductor layer acting as a contact layer is reduced,
Since the contact resistance is reduced by doping the p-type impurity with a high concentration, Vf (forward voltage) tends to decrease.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for manufacturing a nitride semiconductor device according to the present invention using the MOCVD method will be described below.

Example 1 A substrate having a sapphire (0001) surface as a main surface was prepared, and a buffer layer made of GaN was formed at 200 ° C. at 500 ° C. using TMG (trimethylgallium) and ammonia as a source gas. Grow with.

Next, the temperature is raised to 1050 ° C. and TM
An n-type GaN layer doped with 1 × 10 ¹⁹ / cm ^{3 of} Si is grown to a thickness of 5 μm using G, ammonia, and a monosilane gas as an impurity gas.

Next, at a temperature of 800 ° C., a well layer made of undoped In _0.4 Ga _0.6 N is grown to a thickness of 25 Å as an active layer using TMI (trimethyl indium), TMG and ammonia.

Then, the temperature was raised to 1050 ° C., TMG, ammonia, and Cp 2 Mg (cyclopentadienyl magnesium) as an impurity gas, and Mg was added to 1 × 10 ²⁰ / cm 3.
^A first nitride semiconductor layer made of ^3- doped p-type Al _0.3 Ga _0.7 N is grown to a thickness of 200 Å. This first nitride semiconductor layer functions as a layer for confining carriers.

After the growth of the first nitride semiconductor layer, the raw material gas is stopped, and then TMG, ammonia and Cp2Mg are flowed again.
A second nitride semiconductor layer made of N is grown to a thickness of 0.18 μm. However, the flow rate of Cp2Mg is adjusted by an MFC (mass flow controller) so that the flow rate gradually decreases during the growth, so that the flow rate of Cp2Mg becomes zero by the time the second nitride semiconductor is completely grown.

After the growth of the second nitride semiconductor layer, a third nitride semiconductor layer doped with Mg at 1 × 10 ²⁰ / cm ³ is grown to a thickness of 300 Å using TMG, ammonia and Cp 2 Mg.

The wafer on which the nitride semiconductor has been grown as described above is placed in a reaction vessel in a nitrogen atmosphere at 700.degree.
Annealing is performed at a temperature of ° C. to further reduce the resistance of the layer doped with the p-type impurity. After annealing, the wafer is taken out of the reaction container, and the wafer is etched from the third nitride semiconductor layer side of the uppermost layer by the RIE apparatus to expose the surface of the n-side contact layer on which the n-electrode is to be formed. A full-surface electrode made of Ni / Au is formed with a thickness of 200 Å on almost the entire surface of the third nitride semiconductor layer as the uppermost layer, and a pad electrode made of Au is formed with a thickness of 1 μm on a part of the whole electrode. I do. On the other hand, an n-electrode made of W and Au is formed on the exposed surface of the n-side contact layer.

The wafer on which the electrodes were formed as described above was separated into chips of 350 μm square, and light was emitted.
At 0 mA, Vf 3.2 V, emission wavelength 525 nm,
The light output was 3.5 mW and the external quantum efficiency was 7.3%, which was about 1.2 times higher than that of a conventional green LED without gradient doping.

[Embodiment 2] In Embodiment 1, after the first nitride semiconductor layer is grown, the source gas is stopped, and then TM
G, ammonia and Cp2Mg were flowed at 1050 ° C.
GaN layer doped with 1 × 10 ¹⁹ / cm ³
0 Å growth, then changing the flow rate of Cp2Mg, growing a GaN layer doped with 5 × 10 ¹⁸ / cm ^{3 of} Mg to 500 Å, and then growing 1 × 10
A GaN layer doped with ¹⁸ / cm ^{3 is} grown to 500 Å, and a GaN layer not doped with Mg is finally
The second nitride semiconductor layer having a total film thickness of 0.2 μm is grown by growing the thickness by 00 Å. Others are Example 1.
As a result, an LED element having substantially the same characteristics as those of Example 1 was produced.

[Embodiment 3] FIG. 3 is a schematic sectional view showing the structure of one laser device according to the present invention. Hereinafter, a third embodiment of the present invention will be described with reference to FIG.

A GaN substrate 100 is prepared by growing a single crystal of GaN with a thickness of 120 μm on a substrate having a sapphire (0001) plane as a main surface via a buffer layer of GaN. With this GaN substrate 100 grown on sapphire, it was set in a reaction vessel, and the temperature was increased to 1050 ° C.
Ga doped with 1 × 10 ¹⁸ / cm ³ of Si on a substrate 100
An n-side buffer layer 11 of N is grown to a thickness of 4 μm. This n-side buffer layer is a buffer layer grown at a high temperature, for example, sapphire, Si
C, on a substrate made of a material different from a nitride semiconductor such as spinel,
It is distinguished from the buffer layer 2 in which AlN or the like is directly grown to a thickness of 0.5 μm or less.

(N-side cladding layer 12 = strained superlattice layer) Subsequently, n-type Al _0.3 Ga _0.7 N doped with Si at 1 × 10 ¹⁹ / cm ³ using TMA, TMG, ammonia and silane gas at 1050 ° C. The first layer is grown to a thickness of 40 angstroms, the silane gas and TMA are stopped, and the second layer of undoped GaN is grown to a thickness of 40 angstroms. Then, a strained superlattice layer is composed of a first layer + a second layer + a first layer + a second layer +... 100 layers are alternately stacked, each having a total thickness of 0.8 μm. The n-side cladding layer 12 is grown.

(N-side light guide layer 13) Subsequently, the silane gas is stopped and the n-side light guide layer 13 made of undoped GaN is grown at 1050 ° C. to a thickness of 0.1 μm. This n-side light guide layer acts as a light guide layer of the active layer,
It is desirable to grow GaN or InGaN, usually 100 Å to 5 μm, more preferably 2 Å.
It is desirable to grow with a film thickness of 00 Å to 1 μm. This layer can also be an undoped strained superlattice layer. In the case of a strained superlattice layer, the band gap energy is larger than that of the active layer and smaller than that of the n-side cladding layer.

(Active Layer 14) Next, TMG is used as a raw material gas.
The active layer 14 is grown using TMI and ammonia.
The active layer 14 maintains the temperature at 800 ° C.
A well layer made of n _0.2 Ga _0.8 N is grown to a thickness of 25 Å. Next, a barrier layer made of undoped In _0.01 Ga _0.95 N is grown to a thickness of 50 angstroms at the same temperature only by changing the molar ratio of TMI. This operation was repeated twice, and finally, a multiple quantum well structure (M
A QW) active layer is grown. The active layer may be undoped as in this embodiment, or may be an n-type impurity and / or a p-type impurity.
Type impurities may be doped. The impurity may be doped into both the well layer and the barrier layer, or may be doped into either one.

(P-side cap layer 15 = first nitride semiconductor layer) Next, the temperature was increased to 1050 ° C., and TMG, TM
Using A, ammonia, and Cp2Mg (cyclopentadienylmagnesium), and having a band gap energy larger than that of the p-side light guide layer 16, Mg is 1 × 10 ²⁰ / cm ^3.
A p-side cap layer 17 made of doped p-type Al _0.3 Ga _0.7 N is grown to a thickness of 300 Å.
When the p-side cap layer is grown to a thickness of 0.5 μm or less, more preferably 0.1 μm or less, the output is improved because the p-side cap layer acts as a barrier for confining carriers in the active layer. . This p-type cap layer 1
Although the lower limit of the film thickness of No. 5 is not particularly limited, it is desirable to form the film with a film thickness of 10 Å or more.

(P-side light guide layer 16 = second nitride semiconductor layer) After the growth of the p-side cap layer 15, TMG, Cp2
Using Mg and ammonia, as in Example 1, 10
Ga having a band gap energy smaller than that of the p-side cap layer 15 at 50 ° C.
A p-side light guide layer 16 made of N is grown to a thickness of 0.1 μm. This layer acts as a light guide layer for the active layer.

(P-side cladding layer 17)
P-type Al _0.3 Ga doped with Mg at 1 × 10 ²⁰ / cm ³
_A third layer of _0.8 N is grown to a thickness of 40 Å, followed by stopping only TMA and undoping Ga.
A fourth layer of N is grown to a thickness of 40 Å. This operation is repeated 100 times to form a p-side cladding layer 17 composed of a strained superlattice layer having a total film thickness of 0.8 μm.

(P-side contact layer 18 = third nitride semiconductor layer) Finally, at 1050 ° C., p-type GaN doped with 2 × 10 ²⁰ / cm ^{3 of} Mg is formed on the p-side cladding layer 17. The p-side contact layer 18 is grown to a thickness of 150 angstroms. The p-side contact layer 18 is a p-type
n _X Al _Y Ga _{1 -XYN} (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), preferably GaN doped with Mg
Then, the most preferable ohmic contact with the p electrode 21 can be obtained. Further, a nitride semiconductor having a small band gap energy is used as a p-side contact layer in contact with the p-side cladding layer 17 having a strained superlattice structure containing p-type Al _Y Ga _1-Y N, and the film thickness is as thin as 500 Å or less. As a result, the carrier concentration of the p-side contact layer 18 substantially increases, a favorable ohmic contact with the p-electrode is obtained, and the threshold current and voltage of the device decrease.

The wafer on which the nitride semiconductor has been grown as described above is placed in a reaction vessel in a nitrogen atmosphere at 700.degree.
Annealing is performed at a temperature of ° C. to further reduce the resistance of the layer doped with the p-type impurity.

After annealing, the wafer is taken out of the reaction vessel, and as shown in FIG. 3, the uppermost p-side contact layer 18 and the p-side cladding layer 17 are etched by an RIE apparatus to have a stripe width of 4 μm. Ridge shape. As described above, by forming the layer above the active layer into a stripe-shaped ridge, light emission of the active layer is concentrated below the stripe ridge, and the threshold value is reduced. In particular, it is preferable that a layer of the p-side cladding layer 17 or more composed of the strained superlattice layer has a ridge shape.

After the formation of the ridge, a p-electrode 21 of Ni / Au is formed in a stripe shape on the outermost surface of the ridge of the p-side contact layer 18, and the outermost nitride semiconductor layer other than the p-electrode 21 is formed of SiO _2. An insulating film 25 is formed, and a p pad electrode 22 electrically connected to the p electrode 21 via the insulating film 25 is formed.

As described above, the wafer on which the p-electrode is formed is transferred to the polishing apparatus, the sapphire substrate is removed by polishing, and the surface of the GaN substrate 10 is exposed. An n-electrode 23 made of Ti / Al is formed on almost the entire exposed GaN substrate surface.

After the electrodes are formed, the GaN substrate is cleaved on the M plane (a plane corresponding to the side surface of a hexagonal prism when the nitride semiconductor is approximated by a hexagonal system), and the cleavage plane is formed of a dielectric material composed of SiO ₂ and TiO _2. A multilayer film is formed, and finally, in a direction parallel to the p-electrode,
The bar is cut to form a laser element.

This laser chip was placed on a heat sink face-up (in a state where the substrate and the heat sink faced each other), and each electrode was wire-bonded to perform laser oscillation at room temperature. At 0.0 kA / cm ² and a threshold voltage of 4.0 V, continuous oscillation at an oscillation wavelength of 405 nm was confirmed, and a life of 1000 hours or more was shown.

[0040]

As described above, in the nitride semiconductor device of the present invention, the nitride semiconductor layer on the active layer, which is heavily doped with p-type impurities, and the nitride semiconductor layer which is heavily doped with p-type impurities, By interposing a layer in which a p-type impurity is graded, an output can be greatly improved. The element of the present invention can be used not only for light-emitting devices such as LEDs and LDs but also for many electronic devices using nitride semiconductors such as other light-receiving devices.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of one LED element according to one embodiment of the present invention.

FIG. 2 is a distribution diagram showing a p-type impurity concentration of the LED element of FIG. 1;

FIG. 3 is a schematic sectional view showing the structure of an LD device according to another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Buffer layer 3 ... n-side contact layer 4 ... Active layer 5 ... 1st p-side nitride semiconductor layer 6 ... 2nd p-side nitride semiconductor Layer 7: Third p-side nitride semiconductor layer 8: P electrode 9: Pad electrode 10: N electrode

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-233530 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 33/00

Claims (8)

(57) [Claims] 1. A first nitride semiconductor layer containing a p-type impurity is formed on an active layer, and the first nitride semiconductor layer containing p-type impurities is formed on the first nitride semiconductor layer as the distance from the first nitride semiconductor layer increases.
A second nitride semiconductor layer having a gradually reduced impurity concentration, and a second nitride semiconductor layer formed on the second nitride semiconductor layer.
Larger than the average p-type impurity concentration of the nitride semiconductor layer of
A nitride semiconductor device comprising a third nitride semiconductor layer containing a type impurity. 2. The semiconductor device according to claim 1, wherein the second nitride semiconductor layer comprises a multilayer film in which a plurality of nitride semiconductor layers are stacked, and the multilayer film has a stepwise decrease in p-type impurities. Item 3. The nitride semiconductor device according to item 1. 3. The method according to claim 1, wherein the first nitride semiconductor layer is a superlattice layer formed by laminating two types of nitride semiconductor layers having different compositions from each other. Nitride semiconductor device. 4. The method according to claim 1, wherein the first nitride semiconductor layer is made of Al. _X Ga
_1-X N (0 ≦ X ≦ 1), and the third nitride half
The conductor layer is Al _X Ga _1-X N (0 ≦ X ≦ 0.3)
4. The method according to claim 1, wherein
Item 6. The nitride semiconductor device according to item 1. 5. The semiconductor device according to claim 1, wherein the second nitride semiconductor layer is a third nitride semiconductor layer.
Characterized by having the same composition as the semiconductor layer
The nitride semiconductor according to any one of claims 1 to 4,
element. 6. A p-type impurity in the first nitride semiconductor layer
The concentration is 1 × 10 ¹⁸ / Cm ³ Above 1 × 10 ²¹ / Cm
³ 6. The method according to claim 1, wherein:
The nitride semiconductor device according to claim 1. 7. The p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 ¹⁸ / cm ³ or more and 1 × 10 ²¹ / cm.
7. The nitride semiconductor device according to claim 1, wherein the number is ³ or less. 8. 8. The device according to claim 1, wherein the active layer has a single quantum well structure including a nitride semiconductor layer containing at least In or a multiple quantum well structure. Nitride semiconductor device.