JP2001313440A - Nitride semiconductor light-emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a longer lifetime by improving a crystallinity of a GaN layer downward of an optical waveguide and serving as a reflection film in a mask layer, to reduce the threshold of light emission of a nitride semiconductor light-emitting element. SOLUTION: The nitride semiconductor light-emitting element comprises nitride semiconductor layers 3, 7 which are crystal grown on a substrate 2, the optical waveguide formed by laminating a plurality of nitride semiconductor films 8 to 14 on the semiconductor layers 3, 7, and the mask layer 4 embedded in the layers 3, 7 under a light-emitting element region, for confining a light of the waveguide and laminating a plurality of dielectric films at crystal growing. The light-emitting element also comprises a nitride film (e.g. a silicon nitride film or a titanium nitride film) 6 of a semiconductor material or a metal on the uppermost layer of the mask layer 4. In this case, the layer 4 contains a reflection film 5 which increases the reflectivity of the mask layer in a layer which is set lower than the nitride film 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitride semiconductor light emitting device having a nitride semiconductor layer formed by crystal growth on a substrate such as sapphire and forming an optical waveguide of the light emitting device thereon.

[0002]

2. Description of the Related Art In recent years, development of nitride light emitting devices such as blue-violet LEDs and blue-violet LDs using nitride semiconductors such as GaN has been actively pursued.

For example, in a GaN-based LD, (0001)
A GaN layer and an n-type GaN layer are laminated on a surface sapphire substrate, and, for example, a ridge-shaped optical waveguide is formed thereon. The optical waveguide includes an n-type buffer layer, an n-type cladding layer, an n-type light guide layer, an active layer, a p-type buffer layer, a p-type light guide layer, a p-type clad layer, and a p-type contact layer on the n-type GaN layer. It is formed by stacking various nitride semiconductor films. On the uppermost p-type contact layer of this ridge type optical waveguide,
A p-type electrode is formed. On the other hand, the n-type electrode is formed on the n-type GaN surface exposed beside the optical waveguide.

In order to extend the life of a GaN-based light emitting device (LD or LED), Ga caused by factors such as lattice mismatch between the sapphire substrate and the GaN layer and a difference in thermal expansion coefficient.
It is important to suppress the growth of crystal defects in the N layer. This crystal defect tends to be continuously dislocated upward during the crystal growth of the GaN layer, and the crystal defect density increases in the optical waveguide and the GaN layer portion under the large heat generation amount which lowers the life of the GaN-based light emitting device. Because it greatly affects

In order to reduce the crystal defect density, a crystal growth method called an ELOG (Epitaxy Lateral Over Growth) method is effective. In the ELOG method, a mask layer (crystal growth blocking layer) is provided during the growth of the GaN layer to promote the lateral crystal growth in the subsequent GaN crystal growth. Since the lateral growth of GaN reduces crystal defects, the defect density of the GaN crystal, which has grown laterally from the periphery on the mask layer, is at least one order of magnitude lower than that of the GaN crystal below the mask layer. I do. ELOG
Details of the method are disclosed in, for example, Japanese Patent Application Laid-Open No. H11-251631.

[0006]

However, the conventional GaN
GaN based light emitting devices, even when formed using the ELOG method
The defect density of the crystal was high in the past, and this was a problem in further extending the life and improving the light output. Therefore,
Further, a technique for increasing the crystallinity of the GaN layer below the optical waveguide and reducing the defect density has been strongly desired.

On the other hand, a GaN-based light emitting device emits a large amount of spontaneous emission light on the substrate side, which increases the threshold value of laser oscillation and increases the amount of heat generated inside the LD or LED. Therefore, reducing the amount of light leaking to the substrate side is effective for extending the life of the GaN-based light emitting device. Also,
Since the sapphire substrate has light transmittance, spontaneous emission light is emitted to the back surface of the substrate, which causes noise of the LD or LED. In order to suppress this noise, it was necessary to provide a reflective film on the back surface of the sapphire substrate.

For the purpose of reducing the amount of spontaneous emission to the substrate side and lowering the threshold, for example, an n-type cladding layer
GaN with superlattice structure by l _x Ga _1-x N / GaN
System LDs are known. However, in this superlattice structure,
When the mixed crystal ratio of Al is increased, crystal defects increase, so that the light confinement ability by the superlattice structure is also limited.

An object of the present invention is to provide a nitride semiconductor light emitting device having a mask layer having a structure capable of further improving the crystallinity of a GaN layer below an optical waveguide and achieving a long life. Another object of the present invention is to provide a mask layer having the function of a reflective film to lower the threshold value, thereby achieving a longer life and lower noise.

[0010]

A nitride semiconductor light emitting device according to the present invention comprises a nitride semiconductor layer crystal-grown on a substrate and a plurality of nitride semiconductor films stacked on the nitride semiconductor layer. Having an optical waveguide and a mask layer at the time of crystal growth embedded in the nitride semiconductor layer below the light emitting element region where light of the optical waveguide is confined and formed by stacking a plurality of dielectric films. A semiconductor light emitting device having a nitride film of a semiconductor material or metal (for example, a silicon nitride film or a titanium nitride film) as an uppermost layer of the mask layer.

The nitride semiconductor layer is preferably formed by crystal growth on the substrate, and is exposed around the first nitride semiconductor layer in which the mask layer is formed at a predetermined position on the upper surface, and around the mask layer. A second nitride semiconductor layer grown from the surface of the first nitride semiconductor layer.

Alternatively, preferably, the nitride semiconductor layer is formed by crystal growth on the substrate, an upper mask layer is formed at a predetermined position on an upper surface, and a first nitride layer having a step on a surface around the upper mask layer. And a second nitride semiconductor layer crystal-grown from a surface portion of the first nitride semiconductor layer including a step side surface exposed around the upper mask layer. A lower mask layer is preferably formed on the bottom of the step so as to have a thickness that partially exposes the side surface of the step. The uppermost layer of the upper mask layer (and the lower mask layer)
For example, it is formed of either a silicon nitride film or a titanium nitride film.

The mask layer (or the upper mask layer,
The lower mask layer) preferably includes a silicon oxide film below the uppermost layer. The mask layer preferably includes a reflective film that increases the reflectance of the mask layer below the uppermost layer. This reflection film includes a silicon oxide film and a titanium oxide film, or includes a silicon oxide film and an aluminum oxide film. Further, the reflection film is preferably composed of a plurality of dielectric films repeatedly formed in order with a film thickness corresponding to the emission wavelength of the light emitting element.

In the nitride semiconductor light emitting device having such a structure, since the nitride film is provided on the uppermost layer of the mask layer (or the upper mask layer and the lower mask layer), the nitride semiconductor light-emitting element is formed on the mask layer during crystal growth of the nitride semiconductor layer. The crystallinity of the nitride semiconductor crystal that has grown laterally from the periphery is high. This is because the nitride semiconductor layer and the nitride semiconductor layer have good crystal matching. Including a silicon oxide film below the uppermost layer of the mask layer is effective in preventing crystal growth. When a reflective film is provided below the uppermost layer of the mask layer, spontaneous emission light from the optical waveguide above the mask layer is reflected by the mask layer and returns to the waveguide. Therefore, for example, in the case of a semiconductor laser, the threshold value of laser oscillation is reduced, and the noise characteristics are improved.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIG. 1 is a schematic cross-sectional view of a GaN laser diode (LD) according to the first embodiment. This LD1 is made of, for example, sapphire (single crystal Al ₂
O ₃ ) and spinel (MgAl ₂ O ₄ ). Specifically, a first nitride semiconductor layer 3 made of, for example, undoped GaN or GaN doped with a low concentration of n-type impurity is provided on the insulating substrate 2 via a buffer layer (not shown) as necessary. ,
It is formed by a crystal growth method such as MOCVD. The thickness of the first nitride semiconductor layer 3 is about 2 to 3 μm.

The mask layer 4 has, for example, a line width of 2.0-5.
It is formed on the first nitride semiconductor layer 3 in the form of a parallel stripe having a thickness of about 0 μm and a line interval of about 5 to 20 μm. The mask layer 4 includes a reflective film 5 and a nitride film 6 thereon. The reflection film 5 is made of a silicon oxide (SiO ₂ ) thin film, titanium oxide (TiO ₂ ) or aluminum oxide (Al ₂
It has a multilayer structure in which a thin film of O ₃ ) is repeatedly formed a plurality of times, for example, three times. The total thickness of the reflection film 5 is a quarter of the oscillation wavelength λ (400 to four hundred and several tens nm) or a multiple thereof. Thereby, a reflectance of 90% or more is achieved. The nitride film 6 is made of, for example, a silicon nitride (SiN) thin film or a titanium nitride (TiN) thin film, and its thickness is such that it hardly affects the reflectivity of the underlying reflective film 5, for example, several tens nm to 100 nm. Selected from within range.

A second nitride semiconductor layer 7 made of GaN or the like doped with an n-type impurity is formed so as to bury the mask layer 4 with a GaN layer by a so-called ELOG method. The second nitride semiconductor layer 7 includes the first nitride semiconductor layer 3
The thickness above is, for example, about 5 μm.

On the second nitride semiconductor layer 7, for example, a plurality of GaN layer films are laminated to form an optical waveguide structure, which is patterned to form a ridge-type laser stripe.

The optical waveguides are arranged in order from the lower layer, for example,
1.2 μm thick n-type Al _0.08 GaN n clad layer 8, 0.1 μm thick n-type GaN n guide layer 9, 7 nm In _0.08 GaN and 3.5 nm In _0.01 G
An active layer 1 having a multiple quantum well (MQW) structure in which aN is repeatedly formed four times or more, for example.
0, 0.02 μm thick p-type Al _0.2 GaN p-type cap layer 11, 0.05 μm thick p-type Al _0.08 Ga
P guide layer 12 made of N, p-type Al having a thickness of 0.5 μm
_It comprises a p-cladding layer 13 made of _0.06 GaN and a p-contact layer 14 made of p-type GaN having a thickness of 0.1 μm. The active layer 10 has a multiple quantum well structure in order to reduce noise due to the QW (Quantum Well) effect.

In the optical waveguide, the p guide layer 12, the p clad layer 13, and the p contact layer 14 are patterned in a stripe shape to form a ridge. Further, the cap layer 11, the active layer 10, the n guide layer 9, and the n clad layer 8 are etched away for n contact at a position slightly away from the ridge.

For example, a silicon oxide film 15 is coated on the entire surface including the laminated film forming the optical waveguide. The silicon oxide film 15 is formed on the uppermost p-contact layer 1 of the ridge.
4 and on the second nitride semiconductor layer 7 exposed by etching for n-contact. On silicon oxide film 15, p metal electrode 16 and n metal electrode 17 made of, for example, aluminum are formed. The p-metal electrode 16 is connected to the p-contact layer 14 via the opening 15a of the silicon oxide film 15. N-metal electrode 17 is connected to second nitride semiconductor layer 7 via opening 15 b of silicon oxide film 15.

2 to 6 are cross-sectional views of the GaN-LD 1 in the process of being manufactured. In order to form the LD1, first, as shown in FIG. 2A, a first nitride semiconductor layer 3 is formed on an insulating substrate 2 such as a sapphire substrate by MOCVD or the like to a thickness of about 2 to 3 μm. The crystal grows. FIG.
As shown in (B), a reflective film 5 made of silicon oxide and titanium oxide is formed by vapor deposition or ECR sputtering. A nitride film 6 made of silicon nitride or titanium nitride is deposited on the reflective film 5 by vapor deposition, CVD or ECR.
It is formed by sputtering. Thereafter, as shown in FIG. 2C, a parallel stripe-shaped resist R is
Form on top. The resist R is formed at a location where a laser stripe is to be formed later. When the nitride film 6 and the reflection film 5 are etched by RIE or the like using the resist R as a mask,
The mask layer 4 is formed.

After removing the resist R, a second nitride semiconductor layer 7 is formed by a so-called ELOG method. Mask layer 4
When a GaN layer is crystal-grown from the surface of the surrounding first nitride semiconductor layer 3 by MOCVD, crystal growth proceeds in the vertical direction at first, but from the time when the crystal growth surface exceeds the upper surface of the mask layer 4, particularly the mask layer 4 Crystals grow laterally on top.
When the GaN layer grown laterally from the periphery is connected to the mask layer 4 and the upper surface of the mask layer 4 is covered by the GaN layer, crystal growth proceeds vertically again over the entire surface. When the thickness of the GaN layer from the surface of the first nitride semiconductor layer 3 reaches a predetermined value, the crystal growth is stopped. Thereby, as shown in FIG. 3, second nitride semiconductor layer 7 having a thickness of, for example, about 5 μm is obtained.

As shown in FIG. 4, the second nitride semiconductor layer 7
The n-cladding layer 8, the n-guide layer 9, the active layer 10, and the cap layer 1
1, a p guide layer 12, a p clad layer 13, and a p contact layer 14 are sequentially formed.

Next, the p guide layer 12, the p clad layer 1
The 3 and p contact layers 14 are patterned in stripes. Specifically, after a striped resist is formed on the p-contact layer 14, the surrounding p-contact layer 14, p-cladding layer 13 and p-guide layer 12 are removed by etching using the resist as a mask. When the resist is removed, a ridge shown in FIG. 5 is formed.

In a similar manner, the cap layer 11, the active layer 10, the n guide layer 9 and the n clad layer 8 are sequentially etched at a position slightly apart from the ridge to form an n contact portion. Thereafter, a silicon oxide film 15 is formed by CVD on the ridge surface, on the surrounding p-contact layer 14, and on the entire surface including the n-contact portion. A resist opening on the ridge and on the n-contact portion is formed, and etching is performed using the resist as a mask. As a result, as shown in FIG.
5a and 15b are formed.

Thereafter, as shown in FIG. 1, aluminum is vapor-deposited on the entire surface including the silicon oxide film 15 and its opening, and is patterned to form a p-metal electrode 16 and an n-metal electrode 17. The chip is divided by etching, and the GaN-L is divided through processes such as end face coating.
The chip of D1 is completed.

According to the first embodiment, since the uppermost layer of the mask layer 4 has the nitride film 6, the second nitride semiconductor layer 7
The crystallinity of the nitride semiconductor layer, which has grown laterally from the periphery on the mask layer 4 during the crystal growth, is high. This is because the nitride semiconductor layer and the nitride semiconductor layer have good crystal matching. Including a silicon oxide film below the uppermost layer of the mask layer 4 is effective in preventing crystal growth. Since the reflective film 5 is provided below the uppermost layer of the mask layer 4, spontaneous emission light from the optical waveguide above the mask layer 4 is effectively reflected by the mask layer 4, and the reflected light returns to the optical waveguide side. For this reason, the laser oscillation threshold of the GaN-LD 1 is reduced, that is, the laser oscillates with a small input power, power consumption and temperature rise are suppressed, and reliability is improved. Further, noise characteristics are improved.

Second Embodiment FIG. 7 is a schematic sectional view of a GaN-LD according to a second embodiment.

In the first nitride semiconductor layer 20 in the GaN-LD, a portion where a laser stripe is to be formed later is removed by etching, thereby forming a step 20a.
Are formed. An upper mask layer 21 is formed on the first nitride semiconductor layer 20 around the step 20a.
A lower mask layer 24 is formed on the inner insulating substrate 2. The upper and lower mask layers 21 and 24 are composed of lower reflective films 22 and 25 and upper nitride films 23 and 26, respectively. As the material of the reflection films 22 and 25 and the nitride films 23 and 26, the same material as in the first embodiment can be selected. In FIG. 7, the reflection films 2 of the upper and lower mask layers 21 and 24 are shown.
2 and 25 have a multilayer structure as in the first embodiment. Step 20 not covered by mask layers 21 and 24
A second nitride semiconductor layer 27 is crystal-grown from the side wall a by the so-called PENDEO method. The laser structure on the second nitride semiconductor layer 27 is the same as that of the first embodiment, and is denoted by the same reference numerals, and description thereof is omitted.

FIGS. 8 to 13 are cross-sectional views of the GaN-LD in the course of manufacturing. In this GaN-LD, the insulating substrate 2 is substantially similar to the case of the first embodiment shown in FIG.
After forming the first nitride semiconductor layer 20 thereon and forming the mask layer 21 thereon, the mask layer 21 is patterned. However, in the etching of the mask layer 21 in the second embodiment, at the place where the laser stripe is formed,
The mask layer 21 is removed. FIG. 8 is a sectional view immediately after the etching.

In the second embodiment, as shown in FIG. 9, the reflection film 25 and the nitride film 26 are formed by, for example, vapor deposition or ECR sputtering with the resist R attached. At this time, it is desirable to form the film under the condition that the reflection film 25 and the nitride film 26 do not adhere to the side wall of the step 20a of the first nitride semiconductor layer 20. When the reflection film 25 or the nitride film 26 adheres to the side wall of the step 20a, it is removed by light etching, and the first nitride semiconductor layer 20 is removed at that portion.
To expose. After that, when the resist R is removed, the reflection film 25 and the nitride film 26 on the resist R are lifted off, and as shown in FIG.
1, 24 are formed.

In FIG. 11, the second nitride semiconductor layer 27 is grown by MOCVD. At this time, since the exposed surface of the first nitride semiconductor layer 20 is a surface which is substantially vertically raised, the GaN layer grows in the lateral direction so as to fill the step 20a from the beginning. Further, from the time when the crystal growth surface exceeds the upper surface height of the upper mask layer 21, the horizontal crystal growth proceeds on the upper mask layer 21 and the vertical crystal growth proceeds above the step portion. When the thickness of the second nitride semiconductor layer 27 from the upper surface of the lower mask layer 24 reaches a predetermined value, the crystal growth is stopped. Thereafter, similarly to the first embodiment, the n-cladding layer 8, the n-guide layer 9, the active layer 10, the cap layer 11, and the p-guide layer are formed on the second nitride semiconductor layer 27 while appropriately changing the MOCVD conditions. 12, a p-cladding layer 13 and a p-contact layer 14 are sequentially formed.

As shown in FIG. 12, the p guide layers 12, p
The cladding layer 13 and the p-contact layer 14 are patterned in a stripe shape to form a ridge. Further, the cap layer 11 and the active layer 1 are slightly separated from the ridge.
The 0, n guide layer 9 and the n clad layer 8 are sequentially etched to form an n contact portion.

Thereafter, as shown in FIG. 13, a silicon oxide film 15 is formed on the entire surface by CVD, and openings 15a and 15b are formed in the silicon oxide film 15. In addition, as shown in FIG. 7, aluminum is vapor-deposited on the entire surface and patterned to form a p-metal electrode 16 and an n-metal electrode 17. The chip is divided by etching, and the GaN-LD1 chip is completed through steps such as end face coating.

According to the second embodiment, since the upper and lower mask layers 21 and 24 have the nitride films 23 and 26 as the uppermost layers, the mask layers 21 and 24 are formed during the crystal growth of the second nitride semiconductor layer 27. The crystallinity of the nitride semiconductor layer which has grown laterally from the periphery is high. In particular, PEND
In the EO method, the GaN crystal grows in the lateral direction from the beginning due to the presence of the lower mask layer 24 and the step 20a located below the laser stripe. For this reason, the effect that the crystal defects generated due to the lattice mismatch between the first nitride semiconductor layer 20 and the insulating substrate 2 and the like are sealed in the first nitride semiconductor layer 20 is larger than that of the first embodiment. It is difficult for defects to be dislocated in the second nitride semiconductor layer 27. Therefore, a higher quality GaN layer can be obtained, and the life of the laser can be easily prolonged. Further, since the lower mask layer 24 has a reflective film as well as the upper mask layer 21, the threshold value of the laser oscillation is reduced as in the first embodiment, that is, the laser oscillates with a small input power. Power consumption and temperature rise are suppressed, and reliability is improved. Further, noise characteristics are improved.

In the first and second embodiments,
There is no limitation on the laminated film structure and the stripe shape for forming the optical waveguide of the laser. Also, for example, the insulating substrate 2
Can be used instead of a conductive substrate, in which case n
It is also possible to provide a metal electrode on the back surface of the substrate.

[0038]

According to the nitride semiconductor light emitting device according to the present invention, the second nitride semiconductor layer is formed above the mask layer formed on the nitride semiconductor layer (first nitride semiconductor layer). As a result, the reliability and the life of the light-emitting element are improved. In addition, since the mask layer includes the reflective film, the threshold value of laser oscillation or light emission is reduced. As a result, low power consumption, high reliability, long life, and low noise of the light-emitting element are achieved. .

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a GaN-LD according to a first embodiment.

FIG. 2 is a cross-sectional view after etching at the time of forming a mask layer in the manufacture of the GaN-LD according to the first embodiment.

FIG. 3 is a cross-sectional view after a second nitride semiconductor layer is formed in the manufacture of the GaN-LD according to the first embodiment.

FIG. 4 is a cross-sectional view after forming a multilayer film for forming an optical waveguide of a laser in manufacturing the GaN-LD according to the first embodiment.

FIG. 5 is a cross-sectional view after a ridge is formed in the manufacture of the GaN-LD according to the first embodiment.

FIG. 6 is a cross-sectional view after the opening of the silicon oxide film is formed in the manufacture of the GaN-LD according to the first embodiment.

FIG. 7 is a schematic sectional view of a GaN-LD according to a second embodiment.

FIG. 8 is a cross-sectional view after etching a first nitride semiconductor layer in the manufacture of a GaN-LD according to the second embodiment.

FIG. 9 is a cross-sectional view after a reflective film and a nitride film are formed in the manufacture of the GaN-LD according to the second embodiment.

FIG. 10 is a cross-sectional view after lift-off of a reflection film and a nitride film in manufacturing a GaN-LD according to the second embodiment.

FIG. 11 is a cross-sectional view after forming a multilayer film for forming an optical waveguide of a laser in manufacturing the GaN-LD according to the second embodiment.

FIG. 12 is a cross-sectional view after a ridge is formed in the manufacture of the GaN-LD according to the second embodiment.

FIG. 13 is a cross-sectional view after the opening of the silicon oxide film is formed in the manufacture of the GaN-LD according to the second embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... GaN-LD (nitride semiconductor light emitting element), 2 ... insulating substrate (substrate), 3, 20 ... 1st nitride semiconductor layer, 4 ...
Mask layer, 5, 22, 25 ... reflective film, 6, 23, 26 ...
Nitride film, 7, 27 ... second nitride semiconductor layer, 8 ... n cladding layer, 9 ... n guide layer, 10 ... active layer, 11 ... cap layer, 12 ... p guide layer, 13 ... p cladding layer, 14 ... p
Contact layer, 15: silicon oxide film, 15a, 15b
... opening, 16 ... p metal electrode, 17 ... n metal electrode,
20a: step, R: resist.

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Claims (14)

[Claims] 1. A nitride semiconductor layer crystal-grown on a substrate, an optical waveguide formed by stacking a plurality of nitride semiconductor films on the nitride semiconductor layer, and a light emitting element in which light of the optical waveguide is confined. A nitride semiconductor light-emitting device having a mask layer at the time of crystal growth, which is embedded in the nitride semiconductor layer below a region and is formed by stacking a plurality of dielectric films, wherein the uppermost layer of the mask layer Semiconductor light-emitting device having a nitride film of a semiconductor material or metal. 2. The nitride semiconductor layer according to claim 1, wherein said nitride semiconductor layer is crystal-grown on said substrate, and said first nitride semiconductor layer has said mask layer formed at a predetermined position on an upper surface thereof; 2. The nitride semiconductor light emitting device according to claim 1, further comprising: a second nitride semiconductor layer crystal-grown from a surface portion of the first nitride semiconductor layer. 3. 3. The first nitride semiconductor layer, wherein the nitride semiconductor layer is crystal-grown on the substrate, an upper mask layer is formed at a predetermined position on an upper surface, and a first nitride semiconductor layer having a step on a surface around the upper mask layer. And a first portion including a step side surface exposed around the upper mask layer.
The nitride semiconductor light emitting device according to claim 1, further comprising a second nitride semiconductor layer crystal-grown from a surface portion of the nitride semiconductor layer. 4. The nitride semiconductor light emitting device according to claim 3, wherein a lower mask layer is formed at a bottom portion of said step so as to partially expose a side surface of said step. 5. The nitride semiconductor light emitting device according to claim 1, wherein an uppermost layer of said mask layer is formed of one of a silicon nitride film and a titanium nitride film. 6. The nitride semiconductor semiconductor device according to claim 3, wherein an uppermost layer of said upper mask layer is formed of one of a silicon nitride film and a titanium nitride film. 7. The nitride semiconductor semiconductor device according to claim 4, wherein the uppermost layers of said upper mask layer and said lower mask layer are each formed of one of a silicon nitride film and a titanium nitride film. 8. The nitride semiconductor light emitting device according to claim 1, wherein said mask layer includes a silicon oxide film below said uppermost layer. 9. The nitride semiconductor light emitting device according to claim 3, wherein said upper mask layer includes a silicon oxide film below said uppermost layer. 10. The nitride semiconductor light emitting device according to claim 4, wherein said upper mask layer and said lower mask layer include a silicon oxide film below said uppermost layer. 11. The nitride semiconductor light emitting device according to claim 1, wherein said mask layer includes a reflection film for increasing the reflectance of said mask layer below said uppermost layer. 12. The nitride semiconductor light emitting device according to claim 11, wherein said reflection film includes a silicon oxide film and a titanium oxide film. 13. The nitride semiconductor light emitting device according to claim 11, wherein said reflection film includes a silicon oxide film and an aluminum oxide film. 14. The nitride semiconductor light-emitting device according to claim 11, wherein said reflection film is composed of a plurality of dielectric films repeatedly formed in order with a film thickness corresponding to the emission wavelength of said light-emitting device.