JP3314620B2 - Nitride semiconductor light emitting device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a nitride semiconductor (In _X A).
The present invention relates to a light emitting device such as a light emitting diode (LED) or a laser diode (LD) composed of 1 _Y Ga _{1 -XYN} (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and particularly to a light emitting device excellent in light emission output.

[0002]

2. Description of the Related Art A nitride semiconductor is known as a material for a light-emitting device that emits light in the ultraviolet to red region, and blue LEDs and green LEDs have just been put to practical use with this material. Currently, commercially available blue and blue-green LEDs made of a nitride semiconductor have an active layer made of InGaN having n
It has a double heterostructure sandwiched between a p-type nitride and a p-type nitride.
The output is sufficient for mW and LED. However, in this LED, the light emission of the active layer is obtained by the donor-acceptor pair light emission by the acceptor impurity and the donor impurity added to the active layer, so that the half width of the emission spectrum is wide. If the half width is large, the emission color looks whitish and the visibility is good, so the brightness of the LED itself is high, but the color purity is poor.

In addition, the present applicant has announced an LED that emits blue to green light by InGaN interband emission without doping the active layer with impurities (for example, Jpn. J. Appl. Phys. Vol.
34 1995 pp. L1332-L1335). This LED has a double heterostructure having an active layer of a single quantum well structure made of non-doped InGaN, and has a light emission output of 5 mW for a blue LED having a peak wavelength of 450 nm and a green LED of 520 nm.
The ED is 3 mW, and the half width of the emission spectrum is 20 to
Since there is only 30 nm, an LED with high color purity and very high output was realized.

[0004]

As described above, blue-green LEDs have been almost completed in a practical range, but in order to realize a light-emitting device such as an LD, the output of the light-emitting device must be further increased. Need to be increased. Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to further increase the output of a light emitting device using a nitride semiconductor.

[0005]

As a result of repeated experiments on a nitride semiconductor multilayer structure using an active layer having a quantum well structure such as a single quantum well structure or a multiple quantum well structure, we have found that a unique structure has been obtained. The inventors have newly found that a light-emitting element having a light-emitting spectrum has higher light-emitting output, and have accomplished the present invention. That is, the nitride semiconductor light-emitting device of the present invention, on a _{substrate, Al x Ga 1-x N} (0 <x <1) and the n-type contact layer made _{of, Al y Ga 1-y N} (0 <y <
And n-type light confinement layer comprising _{1), In z Ga 1-} z N (0
≦ z ≦ 1), and an active layer having a quantum well structure having a well layer with a thickness of 100 ° or less made of a nitride semiconductor containing at least indium, and a p-type layer.
A ridge-shaped nitride semiconductor laser, the surface of the active layer has irregularities of 5 Å or more, the emission spectrum of the nitride semiconductor laser,
It is characterized by having a plurality of emission peaks that are not equally spaced.

Further, the interval between the plurality of emission peaks is 1
The interval is meV to 100 meV.
In the present invention, it does not necessarily mean that the interval between all adjacent light emission peaks is within the above range.

Further, the light emitting device of the present invention is characterized in that a cladding layer having a band gap larger than that of an active layer is grown on an active layer having a well layer made of a nitride semiconductor having a quantum well structure containing at least indium. A semiconductor light emitting device. Further, it is preferable that the active layer and the cladding layer are in contact with each other due to lattice mismatch.

[0008]

FIG. 1 is a schematic sectional view showing one structure of a light emitting device according to one embodiment of the present invention, and specifically shows the structure of a laser device. This laser element
On a substrate 101 made of spinel, a buffer layer 102 made of GaN, an n-type contact layer 103 made of n-type GaN, an n-type optical confinement layer 104 made of n-type AlGaN, an n-type GaN or an n-type InGaN A light guide layer 105, a well layer made of InGaN, an active layer 106 having a multiple quantum well structure in which a barrier layer having a larger band gap than the well layer is stacked, and light made of p-type GaN or p-type InGaN Guide layer 107, p-type A
p-type optical confinement layer 108 of lGaN, p-type GaN
And a p-type contact layer 109 composed of n-type contact layers 10 on almost the entire surface of the p-type contact layer 109.
3 is provided with a negative electrode 21 parallel to the positive electrode.

FIG. 2 shows a spectrum when each pulse current is applied to this laser element. In FIG. 2, (a) is 28
0 mA (immediately after the threshold), (b) is 295 mA, (c) is 3
(D) shows the emission spectrum at 340 mA. (B), (c), and (d) show spectra during oscillation.

(A) shows the spectrum immediately after the oscillation. In this state, a large number of small emission peaks (Fabry-Perot mode) appear before and after the main emission peak near about 404.2 nm, and the state is immediately after the laser oscillation. You can see that. This is the so-called longitudinal mode spectrum. When the current value is increased, as shown in (b), the spectrum becomes a single mode and shows laser oscillation around 404.2 nm. When the current is further increased, as shown in (c), 403.3 nm (3.075 eV), 403.6 n
m (3.072 eV), 403.9 nm (3.070 eV)
V), 404.2 nm (3.068 eV), 404.4
nm (3.066 eV), in addition to the main emission peak, an emission peak having a large intensity is 1 meV to 100 me.
Appear irregularly at V intervals. Further, in (d), a new peak clearly appears in addition to the above-mentioned peak. These spectral intervals are not equidistant but distinctly different from the longitudinal mode spectra.

In general, in the case of a semiconductor laser, when laser oscillation occurs, a small emission peak due to the longitudinal mode of the laser light appears before and after the main emission peak. The emission spectrum in this case consists of emission peaks at substantially equal intervals. In a red semiconductor laser, the interval between the emission peaks is about 0.5.
2 nm. About 0.05n for blue semiconductor laser
m (1 meV) or less (however, the longitudinal mode of the blue semiconductor laser was first measured by the present applicant when the resonator length was 600 μm). That is, FIG.
In the state shown in (b), the behavior of a normal laser element is shown. However, as shown in (c) and (d), a number of non-equidistant peaks that clearly differ from the emission peak in the longitudinal mode of the conventional laser device appear. This is Figure 2
It can also be seen by comparing the spectra at different current values. In the light emitting device of the present invention, such an emission spectrum is generated, thereby increasing the output.

Although it is not clear why the output of the light emitting element increases when such a peak occurs, for example, the following is presumed. FIG. 3 shows an n-type light guide layer 105 and an active layer 106 in the light emitting device of FIG.
FIG. 4 is a schematic cross-sectional view showing an enlarged interface between the substrate and a p-type light guide layer 107. When the active layer has a quantum well structure,
The thickness of the well layer is adjusted to 100 angstrom or less, preferably 70 angstrom or less, and most preferably 50 angstrom or less. On the other hand, the barrier layer
It is adjusted to not more than Å, preferably not more than 100 Å. In the light-emitting element of the present invention, when a single thin film having a thickness of several tens of angstroms is stacked, both the well layer and the barrier layer are not grown in a uniform thickness, and the uneven layer is formed in multiple layers. Are also overlapped. As shown in FIG. 3, when a double hetero structure in which such an active layer having irregularities is sandwiched by a cladding layer having a band gap larger than that of the active layer is realized, electrons and holes injected into the active layer become Is also confined, and confined in both the vertical and horizontal directions along with the vertical direction of the cladding layer. Therefore, the carriers are confined in a three-dimensional InGaN quantum box or quantum disk having a difference of about 10 to 70 angstroms, and a quantum effect different from the conventional quantum well structure appears. Therefore, light emission based on a large number of quantum levels is observed even at room temperature, and the light emission spectrum is 1 meV to 10 meV.
A number of non-equally spaced emission peaks are observed at 0 meV intervals. Another reason is that three-dimensional InGaN
Since carriers are confined in a smaller quantum box, the exciton effect appears remarkably, and a large number of emission peaks are observed.

FIG. 4 is a perspective view partially showing the state of the surface of the active layer immediately after the growth of the active layer. The state of the crystal on the surface of the active layer is measured by an AFM (Atomic Force Microscope). As shown in FIG. 4, 1
Innumerable irregularities are generated at a height of 0 to 60 angstroms, and the active layer is in contact with the cladding layer in this state.
In the light emitting device of the present invention, by providing irregularities on the surface of the active layer as described above, the small emission peak as described above tends to appear at intervals of 1 meV to 100 meV, and as a result, the emission output is improved. . In the case of a quantum well structure, it is desirable that the difference in unevenness be 5 Å or more, more preferably 10 Å or more, and most preferably 15 Å or more. The lower limit is not particularly limited as long as it is equal to or less than the thickness of the active layer.

One of the reasons why a large number of irregularities occur in the active layer having the well layer made of InGaN is as follows.
In-plane non-uniformity of the In composition is considered. That is, since a region having a large In composition and a region having a small In composition are formed in a single well layer, many irregularities are generated on the surface of the well layer. InGaN is a material that makes it difficult for mixed crystals to grow,
InN and GaN tend to phase separate. Therefore I
A region having a non-uniform n composition is formed. Then, electrons and holes are localized in the region having a high In composition, and exciton light emission or biexciton light emission is performed, so that the output of the LED is improved and a number of peaks are generated. Also, in the laser element, by oscillating this biexciton laser, a number of peaks appear as many quantum disks and quantum boxes, and the threshold value of the laser element is lowered by the number of peaks, thereby improving the output. . The exciton is a pair in which electrons and holes are attached by weak Coulomb force.

In the light emitting device of FIG. 1, an active layer having a well layer made of InGaN is sandwiched between cladding layers having a larger band gap than the active layer. In this way, ternary mixed crystal InGaN is converted to binary mixed crystal or ternary mixed crystal Al
In _{X Ga 1-X N (0} ≦ X ≦ 1) interposed in a structure, it is theoretically impossible to interface the lattice matching between the active layer and the cladding layer. Conventionally, it has been common sense to grow semiconductor crystals by lattice matching. However, in the light emitting device of the present invention,
By intentionally bringing the interface of the active layer into a lattice mismatch state, the active layer is distorted due to a lattice constant mismatch with the cladding layer and a difference in thermal expansion coefficient, and the effect of the quantum box described above remarkably appears due to this distortion. As a result, the output of the light emitting element is improved.

What has been described about the active layer of the laser device of the present invention is clearly shown in the energy band diagram of FIG. FIG. 5A shows an energy band of an active layer having a multiple quantum well structure, and FIG. 5B shows an enlarged energy band of a single well layer surrounded by a circle in FIG. 5A. As described above, the in-plane non-uniformity of the In composition in the well layer means that the single InG
InGaN regions having different band gaps exist in the width of the aN well layer. Therefore, electrons in the conduction band once fall into an InGaN region having a large In composition, and recombine therefrom with holes in the valence band, thereby emitting hν energy. This means that electrons and holes are formed in the well layer width In.
It is localized in the composition-rich region to form localized excitons, which helps to reduce the threshold of the laser. The lower threshold and the higher output are due to this localized exciton effect. A large number of peaks appear because, in addition to the localized excitons, light emission between a large number of quantum levels is produced by the effect of the three-dimensionally confined quantum box.

[0017]

Next, the light emitting device of the present invention will be described in detail with reference to specific examples. FIG. 6 is a schematic sectional view showing the inside of the reaction vessel of the MOVPE apparatus used to obtain the light emitting device of the present invention. A method for obtaining the light emitting device shown in FIG. 1 using this device will be described. In FIG. 6, reference numeral 30 denotes a reaction vessel made of, for example, stainless steel; a susceptor 32 in which the substrate 50 and the tray 34 are placed;
2 and a heater 33 for heating the substrate 50 are provided. Further, a nozzle 35 for supplying a source gas in a direction parallel or inclined toward the substrate is provided close to the susceptor 32, and a conical tube 36 made of, for example, quartz is provided above the substrate. First, a nitride semiconductor is supplied through a vacuum pump 37 inside the chamber 30.
Then, the substrate 50 placed on the susceptor 32 via the tray 34 is heated to a high temperature by the heater 33, and at the same time, the raw material gas is supplied from the nozzle 35 together with the carrier gas, and the raw material gas is supplied onto the substrate. Grow by breaking down. During the supply of the raw material gas, a pressurized gas composed of an inert gas such as nitrogen or hydrogen is constantly flowed from the top into the conical tube 36 toward the substrate to prevent the raw material gas from being diffused by thermal convection of the substrate. are doing. Note that the general formulas such as In _x Ga _1-x N and Al _y Ga _1-y N shown in this specification merely indicate the composition formula of a nitride semiconductor,
Even though different layers are represented by the same formula, they do not have the same composition.

A substrate 101 made of washed spinel (MgAl ₂ O ₄ ) is set on a tray 34, transferred into a reaction vessel, and the inside of the reaction vessel is sufficiently replaced with hydrogen. Is raised to 1050 ° C. to clean the substrate. On the substrate, besides spinel, A side,
Sapphire having plane orientations such as C plane and R plane can be used,
In addition, a known substrate made of a single crystal such as SiC, MgO, Si, ZnO, or GaN is used.

Next, the temperature is lowered to 500 ° C., and hydrogen is supplied as a carrier gas, and ammonia and TMG (trimethylgallium) are supplied from a nozzle 35 as raw material gases.
A buffer layer 102 of GaN is grown on the substrate 101 at 300 ° C. by 300 Å. During the growth, an inert gas such as hydrogen, nitrogen, or argon is supplied from the conical tube 36 at a rate of 20 liters / minute. The buffer layer 102 is provided to alleviate lattice mismatch between the substrate and the nitride semiconductor, and is usually formed of GaN, AlN, AlGaN
Are grown at a film thickness of 1000 Å or less. However, when a substrate having a lattice constant close to that of the nitride semiconductor or a lattice-matched substrate is used, the growth may not be performed depending on factors such as a growth method and growth conditions. It can be omitted.

After the growth of the buffer layer, the temperature is increased to 1030 ° C., and TMG and ammonia gas
Using SiH ₄ (silane) gas as an impurity gas, an n-type contact layer 103 made of Si-doped GaN is grown to a thickness of 4 μm. The n-type contact layer 103 may have any composition as long as it is an n-type nitride semiconductor, but is preferably Al _Y Ga ₁ -YN (0 ≦ Y ≦ 1). In particular, by using AlGaN for the n-type contact layer, the difference in refractive index from the active layer can be increased, and the n-type contact layer can function as a cladding layer as a light confinement layer and a contact layer for injecting current. Further, this contact layer is
By setting N, the light emission of the active layer can be hardly spread in the n-type contact layer, so that the threshold value decreases. Further, when the n-type contact layer is made of Al _Y Ga ₁ -YN, it is desirable to have a structure having a small Al composition ratio on the substrate side and a large Al composition ratio on the active layer side, that is, a compositionally graded structure. With the above structure, an n-type contact layer with good crystallinity can be obtained, so that the crystallinity of the nitride semiconductor stacked on the n-type contact layer with good crystallinity can be improved. As a result, the threshold value is reduced, and the reliability of the device is significantly improved. Further, since the Al mixed crystal ratio on the active layer side is large, the refractive index difference from the active layer is also large, and effectively acts as a light confinement layer. On the other hand, if GaN is used, n
The ohmic characteristics with the electrodes are very good.
When the contact layer is GaN, it is necessary to provide a light confinement layer made of AlGaN between the GaN contact layer and the active layer. It is desirable that the thickness of the n-type contact layer 103 be adjusted to 0.1 μm or more and 5 μm or less.

Next, the temperature is lowered to 750 ° C., and TMG, TMI (trimethylindium) and ammonia are used as source gases, silane gas is used as impurity gas, and Si-doped In0.
A crack preventing layer (not shown) made of 1Ga0.9N
It is grown to a thickness of 00 Å. The anti-crack layer is an n-type nitride semiconductor containing In, preferably In
By growing with GaN, it becomes possible to grow the n-type optical confinement layer 104 made of a nitride semiconductor containing Al to be grown next with a thick film. In the case of an LD, it is necessary to grow a layer serving as a light confinement layer to a thickness of, for example, 0.1 μm or more. Conventionally, when thick AlGaN was directly grown on the GaN and AlGaN layers, cracks were formed in the AlGaN grown later, making it difficult to fabricate the device. Cracks in the layer 104 can be prevented. Moreover, even if the optical confinement layer 103 to be grown next is grown as a thick film, the film can be grown with good film quality. Preferably, the crack preventing layer is grown to a thickness of 100 Å or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to function as a crack prevention as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black. The crack prevention layer is not shown because it can be omitted depending on the growth method and the growth apparatus, but it is preferable to grow the LD in manufacturing an LD.

Next, the temperature is raised to 1050 ° C.
TEG, TMA (trimethylaluminum),
Near, Si doped n using silane gas as impurity gas
N-type optical confinement layer 104 of Al0.3Ga0.7N
It is grown to a thickness of 0.6 μm. The optical confinement layer 104
Al-containing nitride semiconductor, particularly preferably Al_YGa_{1 -Y}
By constituting N (0 <Y <1), good crystallinity can be obtained.
And a large refractive index difference from the active layer.
This is effective for vertically confining laser light. This layer is usually
It is desirable to grow with a film thickness of 0.1 μm to 1 μm.
No. If it is thinner than 0.1 μm, it acts as a light confinement layer
If it is thicker than 1 μm, even if the crack prevention layer
Even in AlGaN grown on the top, cracks were found in the crystal.
It tends to be difficult to make devices.

Subsequently, TMG, ammonia,
Si-doped n-type GaN using silane gas as impurity gas
An n-type light guide layer 105 is grown to a thickness of 500 angstroms. The n-type light guide layer 105 is made of In
N-type nitride semiconductor or n-type GaN, preferably ternary mixed crystal or binary mixed crystal In _x Ga ₁ -xN (0 ≦
X ≦ 1). This layer is usually preferably grown to a thickness of 100 Å to 1 μm.
By using GaN or GaN, the next active layer 106 can easily have a quantum structure.

Next, the active layer 106 is grown using TMG, TMI, and ammonia as source gases. The temperature of the active layer is maintained at 750 ° C., and first, non-doped In0.2Ga0.8N
A well layer having a thickness of 25 Å is grown. Next, a barrier layer made of non-doped In0.01Ga0.95N was deposited at the same temperature by changing only the molar ratio of TMI.
It is grown to a thickness of 0 Å. This operation 4
This is repeated twice, and finally, a well layer is grown to grow an active layer 106 having a multiple quantum well structure with a total thickness of 325 Å. FIG. 4 shows a state of the surface after the active layer is grown.

After the active layer 106 is grown, the temperature is raised to 1050 ° C., and TMG, TMA, ammonia, Cp 2 Mg (cyclopentadienyl magnesium) as an acceptor impurity source, and a p-type cap made of Mg-doped p-type Al 0.2 Ga 0.8 N are used. A layer (not shown) is grown to a thickness of 100 Å. This p-type cap layer has a thickness of 1 μm or less, preferably 10 Å or more, and 0.1 μm or less.
By growing it with a thickness of 1 μm or less, InGaN
Has a function as a cap layer for preventing the active layer formed from being decomposed, and a p-type nitride semiconductor containing Al, preferably Al _Y Ga _1-Y N (0 <Y <1), on the active layer. By growing the p-type cap layer made of such a material, the light emission output is significantly improved. The thickness of this p-type cap layer is 1 μm.
If the thickness is larger than m, cracks tend to occur in the layer itself, and it tends to be difficult to fabricate the element. Note that this p-type cap layer can also be omitted depending on the growth method, the growth apparatus, and the like, and is not particularly illustrated.

Next, while maintaining the temperature at 1050 ° C., T
Mg-doped p-type G using MG, ammonia, Cp2Mg
A p-type light guide layer 107 made of aN is grown to a thickness of 500 Å. This p-type light guide layer 107
As described above, by using InGaN or GaN, the following p-type optical confinement layer 108 containing Al can be grown with good crystallinity.

Subsequently, TMG, TMA, ammonia, C
Using p2Mg, a p-type optical confinement layer 108 of Mg-doped Al0.3Ga0.7N is grown to a thickness of 0.5 .mu.m. The p-type optical confinement layer is the same as the n-type optical confinement layer,
It is desirable to grow with a film thickness of 0.1 μm to 1 μm. By using a p-type nitride semiconductor containing Al such as AlGaN, the difference in the refractive index from the active layer is increased, and the vertical direction of the laser light is increased. Effectively acts as a light confinement layer.

Finally, TMG, ammonia, Cp2Mg
Is used to grow a p-type contact layer 109 made of Mg-doped p-type GaN with a thickness of 0.2 μm. In particular, the p-type contact layer 109 is made of Al _Y Ga _{1 -Y} N (0 ≦ Y ≦ 1),
Among them, when p-type GaN doped with Mg is used, a p-type layer having the highest carrier concentration can be obtained, good ohmic contact with the positive electrode can be obtained, and the threshold current can be reduced.

The wafer on which the nitride semiconductor is laminated as described above is taken out of the reaction vessel, and as shown in FIG.
Selective etching is performed from the uppermost p-type contact layer 109 to expose the surface of the n-type contact layer 103 where a negative electrode is to be formed. Furthermore, etching is performed from the p-type contact layer side to produce a stripe-shaped ridge-shaped laser device, and then the positive electrode 20 and the negative electrode 21 are formed in stripes at positions parallel to the ridge to form a laser device. .

When this laser device was pulse-oscillated at room temperature, it showed a spectrum as shown in FIG. 2 at each current value, and the light emission output was the same as that of a laser beam of the same structure grown by an MBE (molecular beam vapor phase epitaxy) apparatus. The threshold current was reduced by 20% or more compared to the device, and the output was twice or more. In addition,
According to the MBE apparatus, the nitride semiconductor layer can easily obtain a flat surface at the atomic layer level.

Example 2 In Example 1, when growing the n-type contact layer 103, the flow rate of TMA was changed stepwise, and the Al mixed crystal ratio (Y
Value) was 0, and a laser element was obtained in the same manner as described above except that a layer having a composition gradient of 0.2 on the n-type optical confinement layer 104 side having an Al composition ratio of 0.2 was obtained. 4%, and the output was improved by 10%.

[0032]

As described above, the light emitting device of the present invention is a nitride semiconductor light emitting device having an active layer made of a nitride semiconductor having a quantum structure as a light emitting layer. By having a peak, the output of the light emitting element is improved. Particularly, in the case of a laser element, it is very convenient to use the laser element as a light source of a magneto-optical disk which dislikes single-mode oscillation by using its unique emission spectrum. The present invention is the first to discover that such a phenomenon appears in a laser element, and its industrial utility value is very large.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of a light emitting device according to one embodiment of the present invention.

FIGS. 2A and 2B are diagrams illustrating emission spectra of light-emitting elements according to one embodiment of the present invention by comparing current values.

FIG. 3 is a schematic cross-sectional view showing an enlarged part of an interface of an active layer of the light emitting device of FIG. 1;

FIG. 4 is a perspective view partially showing a state of the surface of the active layer immediately after the active layer is grown.

FIG. 5 is an energy band diagram of a well layer of the light emitting element of the present invention.

FIG. 6 shows an MO used to obtain a light emitting device of the present invention.
FIG. 2 is a schematic cross-sectional view showing a structure inside a reaction container of the VPE device.

[Explanation of symbols]

 101 substrate 102 buffer layer 103 n-type contact layer 104 n-type light confinement layer 105 n-type light guide layer 106 active layer 107 ························································································································ Negative-electrode

────────────────────────────────────────────────── ─── Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 33/00 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. An Al _x Ga ₁ -xN (0 <x <
And n-type contact layer made of _{1), Al y Ga 1-} y N
An n-type optical confinement layer containing (0 <y <1) and In _z Ga
_An n-type optical guide layer made of _1-z N (0 ≦ z ≦ 1) and a film thickness of at least 10 made of a nitride semiconductor containing indium;
An active layer having a quantum well structure having a well layer of 0 ° or less, a nitride semiconductor laser having a ridge shape in a p-type layer, wherein the active layer has irregularities of 5 angstroms or more on its surface. In the emission spectrum of the semiconductor laser,
A nitride semiconductor laser having a plurality of emission peaks that are not equally spaced. 2. An interval between the emission peaks is 1 meV to 10 meV.
2. The nitride semiconductor laser according to claim 1, wherein the distance is 0 meV. 3. The nitride semiconductor laser according to claim 1, wherein a cladding layer having a larger band gap than the active layer is grown on the active layer. 4. The nitride semiconductor laser according to claim 3, wherein said active layer and said clad layer are in contact with each other in a state of lattice mismatch.