JP2008004662A - Nitride semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor laser element of which a diffraction grating is precisely formed, the upper surface of a semiconductor layer that buries it is flat, the intrinsic performance of a laser element is assured or improved, refractive index difference of the diffraction grating is effectively increased, and a threshold current is lowered. <P>SOLUTION: The nitride semiconductor laser element comprises a substrate; and a nitride semiconductor layer on which a first semiconductor layer, an active layer, and a second semiconductor layer are sequentially stacked. In the first semiconductor layer, a layer on which convexity and concavity sections are formed and a layer for flattening the convexity and concavity sections which is formed on the layer where the convexity and concavity sections are formed, are formed. The distance from the upper surface of the convexity sections constituting the convexity and concavity sections to the active layer is equal to three times the height of the convexity sections or less, otherwise it is three times the pitch width of the convexity and concavity sections or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  Today, semiconductor lasers using nitride semiconductors are considered capable of oscillating in a wide wavelength range from the ultraviolet to the red, and their application range is large-capacity and high-density information recording / reproduction such as DVD. It is expected not only for use in an optical disc system capable of recording, but also as a light source for laser printers, optical networks, and the like.

  In general, a semiconductor laser element has a width centered on a wavelength at which the optical amplification factor of an active layer is determined by an energy gap, and therefore several longitudinal modes exist in its output. This mode distribution changes according to the output and the driving temperature of the laser element, and tends to move to the longer wavelength side as they increase. For example, when optical communication is performed over a long distance, it is transmitted for each mode. Differences in speed will result in dispersion.

  For this reason, a longitudinal single mode single frequency laser is required for use in applications such as optical communications, and a DFB (distributed feedback type) laser has a distinct longitudinal single mode. It has been proposed to obtain (for example, Patent Documents 1 to 5).

In such a DFB laser, a diffraction grating layer that periodically reflects light is provided in parallel to the active layer in the double heterostructure, and the light generated in the active layer is caused by the grating spacing of the diffraction grating. It is considered that the light is periodically reflected, the peaks of the original light and the reflected light, the peaks and valleys match, and are strengthened to obtain a single-frequency laser light output.
JP-A-8-195530 JP-A-9-191153 JP 2000-223784 A JP 2001-203422 A JP 2002-131567 A

  However, the conventionally proposed DFB laser has not yet brought out a sufficient effect of the diffraction grating. In particular, no single-spectrum peak lasing has been reported for gallium nitride-based DFB lasers.

For example, in the DFB laser, in order to sufficiently draw out the effect of the diffraction grating, it is necessary to accurately perform uneven processing on the semiconductor layer forming the diffraction grating.
Even if such uneven processing can be performed, when the semiconductor is regrown on the diffraction grating, unevenness is generated on the growth surface, making it difficult to ensure the performance of the laser element itself. It is.

  The present invention has been made in view of the above problems, and a diffraction grating is formed with high accuracy, and the upper surface of a semiconductor layer buried in the diffraction grating is flat. An object of the present invention is to provide a nitride semiconductor laser device capable of effectively increasing a refractive index difference in a grating and reducing a threshold current.

  The nitride semiconductor laser device of the present invention is a nitride semiconductor laser device comprising a substrate and a nitride semiconductor layer in which a first semiconductor layer, an active layer, and a second semiconductor layer are sequentially stacked on the substrate. A layer in which irregularities are formed in the first semiconductor layer, and a layer for flattening the irregularities are formed on the layer in which the irregularities are formed, from the upper surface of the convex portion constituting the irregularities to the active layer The distance is 3 times or less the height of the unevenness or 3 times or less the pitch width of the unevenness.

  In the nitride semiconductor laser device of the present invention, irregularities are not formed on the upper side of the active layer, but irregularities are formed on the lower side of the active layer. Therefore, there is no damage given to the active layer, which is a concern when unevenness is formed on the upper side of the active layer. However, if there is an irregular step on the surface of the first semiconductor layer, the active layer formed thereon also has an irregular layer structure, and there is a problem that a desired gain cannot be sufficiently obtained in the wafer or chip. Arise. On the other hand, in order to flatten the step, the thickness of the layer for flattening the unevenness may be increased. However, the distance from the active layer to the unevenness becomes too far, and the effect of the unevenness (diffraction grating) is reduced. It will be.

In the present invention, the effect of unevenness (diffraction grating) can be exhibited without reducing the performance of the laser element itself.
In such a nitride semiconductor laser element, it is preferable that the distance from the upper surface of the convex portion constituting the irregularities to the active layer is 300 nm or less.
Moreover, it is preferable that the width | variety of the convex part in an unevenness | corrugation and the width | variety of a recessed part are the ranges of 1 / 15-8 of an uneven | corrugated height, respectively.
The irregularities are preferably formed in stripes, and the width of the irregularities has a pitch in the range of 40 to 140 nm.
The distance from the upper surface of the convex portion constituting the irregularities to the active layer is preferably 0.5 times or more.

A member having a lower refractive index than that of the nitride semiconductor may be disposed on the upper surface of the convex portion, and the member having a lower refractive index may be made of SiO ₂ , TiO ₂ , ZrO ₂ , Al ₂ O ₃ , SiN, or AlN. And at least one selected from the county.
The active layer is preferably a nitride semiconductor containing In.
The layer on which the irregularities are formed includes a first layer made of Al _a Ga _1-a N (0 ≦ a ≦ 0.10) and Al _b Ga _1-b N (0.01 ≦ b ≦ 0.14). It is preferable to include a superlattice layer composed of the second layer.

Further, the layer for flattening the unevenness is a layer made of Al _c Ga _1-c N (0 ≦ c ≦ 0.075), or from Al _a Ga _1-a N (0 ≦ a ≦ 0.10). a first layer consisting preferably includes a super lattice layer composed of the second layer of _{Al b Ga 1-b N (} 0.01 ≦ b ≦ 0.14).

  Furthermore, it is preferable that the nitride semiconductor laser element has a resonance surface, and the unevenness is provided in a stripe shape substantially parallel to the resonance surface.

  According to the nitride semiconductor laser device of the present invention, the diffraction grating is accurately formed without complicated processing, and the top surface thereof is flat, thereby ensuring or improving the original performance of the laser device. Thus, the effect of the diffraction grating can be sufficiently exhibited, and the threshold current can be reduced.

  The nitride semiconductor laser device of the present invention mainly includes a substrate 11 and a nitride semiconductor layer in which a first semiconductor layer 12, an active layer 13, and a second semiconductor layer 14 are laminated in this order as shown in FIG. Composed. Concavities and convexities (diffraction grating) 22 are formed in the first semiconductor layer 12. FIG. 1A is a schematic cross-sectional view of a resonance surface of a nitride semiconductor laser element. FIG. 1B is a schematic cross-sectional view of the side surface of the nitride semiconductor laser element. Furthermore, the first semiconductor layer and the second semiconductor layer respectively include a first electrode and a second electrode that are electrically connected (not shown). A protective film 16 is formed at the end of the resonance surface. Here, the first semiconductor layer includes at least one layer including an n-type or p-type impurity. The second semiconductor layer includes at least one layer containing a conductivity type opposite to that of the first semiconductor layer, that is, a p-type or n-type impurity. Each of the first and second semiconductor layers may be either a single layer or a multilayer structure. In the case of a multilayer structure, all of the layers constituting them may not exhibit n-type or p-type. Good. In the following description, a layer disposed on the n-type layer side from the active layer is referred to as an n-type layer or n-side layer, and a layer disposed on the p-type layer side from the active layer is referred to as a p-type layer or p-side layer. There is a case.

(substrate)
The substrate used for the nitride semiconductor laser device of the present invention may be a different type substrate different from the nitride semiconductor, or a nitride semiconductor substrate. Examples of the heterogeneous substrate include, for example, an insulating substrate such as sapphire, spinel (MgA1 ₂ O ₄ ) having any one of the C-plane, R-plane, and A-plane, SiC (including 6H, 4H, and 3C), A nitride semiconductor such as an oxide substrate lattice-matched with ZnS, ZnO, GaAs, Si, and a nitride semiconductor can be grown, and a conventionally known substrate material can be used. Among them, sapphire and spinel are mentioned. The substrate may have at least a surface having an off-angle angle of about 0.01 to 0.3 ° and a step-like off-angle angle. Thereby, generation | occurrence | production of a fine crack can be prevented inside the nitride semiconductor layer and active layer which comprise an element.

  When using a heterogeneous substrate, an underlayer (including a protective layer for lateral growth), a contact layer, or the like may be formed, but a nitride semiconductor substrate (for example, a GaN substrate) is used. However, they are not necessarily formed and can be omitted.

  In the case of using a heterogeneous substrate, after growing a base layer made of a nitride semiconductor layer on the lower layer of the element structure on the heterogeneous substrate, the heterogeneous substrate is removed by a method such as polishing, and the nitride semiconductor alone An element structure may be formed as a substrate. Further, the heterogeneous substrate may be removed during or after the element structure is formed.

(Underlayer)
An underlayer (not shown) made of a nitride semiconductor including a buffer layer and the like may be formed on the substrate.
As the buffer layer, for example, InαAlβGa ₁ -α _- βN (0 ≦ α, 0 ≦ β, α + β ≦ 1) or the like (for example, AlN, GaN, AlGaN, InGaN, etc., preferably GaN) is preferably 300 ° C. or higher and 900 ° C. or lower. The film is grown at a temperature of several tens of angstroms to several hundreds of angstroms. This buffer layer is preferable for alleviating the lattice constant irregularity between the heterogeneous substrate and the high-temperature-grown nitride semiconductor layer and preventing the occurrence of dislocations.

The first nitride semiconductor layer is partially grown on the buffer layer, and further, the second nitride semiconductor layer is grown using the first nitride semiconductor layer as a growth nucleus, thereby lateral growth is achieved. It is possible to form an underlayer with reduced dislocations utilized. The underlayer is preferably a nitride semiconductor represented by Al _x Ga _1-x N (0 ≦ x ≦ 1), and preferably has a thickness of 2 to 30 μm. Further, the laterally grown region (low dislocation region) has a dislocation number as small as 1 × 10 ⁷ pieces / cm ² or less, preferably 1 × 10 ⁶ pieces / cm ² or less, or a region with a small amount. What you have is appropriate. Further, the underlayer contains n-type impurities (for example, Si, Sn, Ge, Se, C, Ti, O, etc.) in the range of about 1 × 10 ^{16 to} 5 × 10 ²¹ cm ^−3, for example. May be.

(Semiconductor layer)
In the nitride semiconductor laser element of the present invention, the first and second semiconductor layers are group III-V nitride semiconductors (InαAlβGa ₁ -α _- βN, 0 ≦ α, 0 ≦ β, α + β ≦ 1), Further, it may contain a mixed crystal in which B is used as a group III element or a part of N is substituted with P or As as a group V element. The semiconductor layer can be formed by any method known in the art such as MOVPE (metal organic vapor phase epitaxy), HVPE (halide vapor phase epitaxy), MBE (molecular beam vapor phase epitaxy).

  As the n-type impurity used in the nitride semiconductor layer, a group IV or group VI element such as Si, Ge, Sn, S, O, Ti, or Zr can be used, and Si, Ge, or Sn is preferable. Further, the p-type impurity is not particularly limited, and examples thereof include Be, Zn, Mn, Cr, Mg, and Ca, and Mg is preferable. Thereby, each conductivity type nitride semiconductor layer can be formed, and each conductivity type layer mentioned below can be constituted.

For example, the first semiconductor layer is formed in a multilayer structure on the buffer layer and / or the base layer.
The first n-side semiconductor layer can be formed of Al _d Ga _1-d N (0 ≦ d ≦ 0.5). The first n-side semiconductor layer is doped with n-type impurities. The n-type impurity content of this layer is preferably 5 × 10 ¹⁷ / cm ^{3 to} 5 × 10 ¹⁸ / cm ³ . The film thickness of the first n-side semiconductor layer is 0.5 to 10 μm, preferably 2 to 5 μm. The first n-side semiconductor layer can function as an n-type contact layer or an n-type cladding layer. In the case where the first n-side semiconductor layer functions as an n-type contact layer, an n-electrode is formed on the first n-side semiconductor layer in a later step. Note that the first n-side semiconductor layer can be omitted.

A second n-side semiconductor layer is formed on the first n-side semiconductor layer. This second n-side semiconductor layer is made of Si-doped In _g Ga _1-g N (0.02 ≦ g ≦ 0.20), preferably In _g Ga _{1-g with} g of 0.08 to 0.12. N. This second n-side semiconductor layer can effectively prevent the occurrence of cracks in the nitride semiconductor element. The doping amount of Si is 5 × 10 ¹⁷ / cm ^{3 to} 5 × 10 ¹⁹ / cm ³ . The film thickness of the second n-side semiconductor layer is a thickness that does not impair the crystallinity, and is, for example, 0.10 to 0.20 μm. Note that the second n-side semiconductor layer can be omitted.

A third n-side semiconductor layer is formed on the second n-side semiconductor layer. The third n-side semiconductor layer may be formed of a single layer or a superlattice layer. To form the third n-side semiconductor layer as a superlattice layer, Al _a Ga _1-a N (0 ≦ a ≦ 0.10) is used as the first layer, and Al _b Ga _1-b N ( It is preferable to alternately form layers of 0.01 ≦ b ≦ 0.14). The first layer and the second layer can be formed by stacking layers having a single layer thickness of 100 angstroms or less. By using such a superlattice layer, the occurrence of cracks can be prevented and the crystallinity can be improved despite containing Al. The total film thickness of the third n-side semiconductor layer is preferably 0.45 to 3.0 μm. The average composition is preferably formed as Al _e Ga _1-e N (0.01 ≦ e ≦ 0.14). When the average composition of Al is within this range, cracks can be suppressed and a sufficient difference in refractive index from the laser waveguide can be obtained. The content of the n-type impurity is preferably 1 × 10 ¹⁷ / cm ^{3 to} 5 × 10 ¹⁸ / cm ³ . When the n-type impurity is contained in this range, the resistivity can be lowered and the crystallinity is not impaired. The third n-side semiconductor layer can function as an n-side cladding layer.

A fourth n-side semiconductor layer is formed on the third n-side semiconductor layer. Examples of the fourth n-side semiconductor layer include Al _c Ga _1-c N (0 ≦ c ≦ 0.075). The film thickness of the fourth n-side semiconductor layer is 0.05 to 0.25 μm, preferably 0.14 to 0.16 μm. When the film is grown at this film thickness, the threshold value (I _th ) can be lowered without generating cracks. The fourth n-side semiconductor layer can function as a light guide layer. Note that the fourth n-side semiconductor layer can be omitted.

An active layer is formed on the fourth n-side semiconductor layer. The active layer includes a well layer and a barrier layer, and the well layer includes Al _x In _y Ga _{1 -xy} N (0 ≦ x ≦ 0.05, 0.005 ≦ y ≦ 0.25, x + y <1 ). The barrier layer _{Al u In v Ga 1-u} - is preferably made of a _{v N (0 ≦ u ≦ 0.2,0} ≦ v ≦ 0.15). For example, the barrier layers are stacked so as to sandwich the well layer, and the barrier layer and the well layer are not limited to one layer and may be two or more multilayers. Specifically, an active layer having a multiple quantum well structure in which a barrier layer and a well layer are repeatedly stacked may be used. The film thicknesses of the well layer and the barrier layer are each preferably 50 to 150 Å. The film thickness of the first barrier layer is preferably 30 to 250 angstroms, and the film thickness of the second barrier layer is preferably 30 to 250 angstroms. If the active layer is formed with a film thickness in this range, the occurrence of cracks in the vicinity of the active layer can be suppressed.

One or both of the well layer and the barrier layer may contain impurities.
A second semiconductor layer is formed in a multilayer structure on the active layer.
The first p-side semiconductor layer can be formed as a carrier confinement layer, an electron confinement layer, an optical confinement layer, or the like.

The first p-side semiconductor layer is preferably a nitride semiconductor layer containing Al having a larger band gap energy than the active layer. The carrier confinement layer may be formed of a single film or a multilayer film having a different composition. Examples of the p-type carrier confinement layer include those obtained by growing at least one layer of Mg-doped Al _d Ga _1-d N (0 <d ≦ 1). Preferably d is in the range of 0.1 to 0.5, and more preferably d is in the range of 0.15 to 0.35. The thickness of the p-type carrier confinement layer is preferably 10 to 200 angstroms. When the film thickness is in the above range, electrons in the active layer can be confined well, and the occurrence of cracks can also be suppressed. Also, the bulk resistance can be kept low.

  The p-type electron confinement layer is a layer in which carriers are confined in an active layer or a well layer. In a laser element, a high-power light-emitting element, etc., the carrier is blocked by the active layer due to a temperature rise and current density increase due to element driving. It is possible to prevent overflow, and a structure in which carriers are efficiently injected into the active layer can be obtained.

The p-type electron confinement layer usually contains Mg. Content is ^{1 × 10 19 / cm 3 ~1} × 10 21 / cm 3. If the content is within this range, in addition to lowering the bulk resistance, Mg is well diffused into a p-type guide layer grown by undoped described later, and Mg is 1 × in the p-type guide layer which is a thin film layer. 10 ¹⁶ / cm ³ can be contained in a range of ~1 × 10 ¹⁸ / cm ^3. The p-type electron confinement layer is preferably grown at a low temperature, for example, a temperature similar to the temperature at which the active layer of about 900 to 1000 ° C. is grown, because it can prevent decomposition of the active layer.

  The p-type electron confinement layer may be composed of a single film, or a low-temperature growth layer and a high-temperature layer, for example, a layer grown at a temperature about 100 ° C. higher than the growth temperature of the active layer. It may be composed of layers. Thus, when it is composed of two layers, the low temperature growth layer prevents the active layer from being decomposed, and the high temperature growth layer reduces the bulk resistance. The thickness of each layer when the p-type electron confinement layer is composed of two layers is not particularly limited, but the low temperature growth layer is preferably 10 to 50 Å, and the high temperature growth layer is preferably 50 to 150 Å. Note that the first p-side semiconductor layer can be omitted.

A second p-side semiconductor layer is formed on the first p-side semiconductor layer. An example of the second p-side semiconductor layer is a nitride semiconductor layer made of Al _c Ga _1-c N (0 ≦ c ≦ 0.2). The film thickness of the p-type guide layer is preferably 0.05 to 0.25 μm. Within this range, the threshold can be lowered. In addition, the second p-side semiconductor layer is grown as an undoped layer, but Mg contained in the p-type electron confinement layer diffuses and is 1 × 10 ¹⁶ / cm ³ to 1 × 10 ¹⁸ / cm ³ . Mg may be contained in the range. The second p-side semiconductor layer can function as a light guide layer. Note that the second p-side semiconductor layer can be omitted.

A third p-side semiconductor layer is formed on the second p-side semiconductor layer. The third p-side semiconductor layer may have a single film structure, but preferably has a multilayer structure in which nitride semiconductor layers having different compositions are stacked. For example, in the case of a single layer structure, a layer made of Al _a Ga _1-a N (0 ≦ a ≦ 0.5) can be mentioned. In addition, the first layer is made of Al _a Ga _1-a N (0 ≦ a ≦ 0.10), and the second layer is made of Al _b Ga _1-b N (0.05 ≦ b ≦ 0.14). A lattice layer is mentioned. The content of the p-type impurity is preferably 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ . When the p-type impurity is contained within this range, the bulk resistance can be reduced with a content that does not impair the crystallinity. Such a multilayer film preferably has a single layer thickness of 100 angstroms or less. By forming the third p-side semiconductor layer with a superlattice structure, generation of cracks can be suppressed. The total thickness of the third p-side semiconductor layer is 0.4 to 0.55 μm, and the forward voltage (Vf) can be reduced within this range. Moreover, the average composition of Al of the whole third p-side semiconductor layer is 0.01 to 0.14. When the value is within this range, the occurrence of cracks can be suppressed and a sufficient difference in refractive index from the laser waveguide can be obtained. This third p-side semiconductor layer has a band gap larger than that of the active layer and normally functions as a p-side cladding layer, but may also function as a p-side contact layer at the same time. Thereby, a fourth p-side semiconductor layer to be formed later can be omitted.

A fourth p-side semiconductor layer is formed on the third p-side semiconductor layer. The fourth p-side semiconductor layer is preferably formed by growing a nitride semiconductor layer made of GaN containing Mg. A film thickness of about 10 to 200 angstroms is appropriate. The Mg content is 1 × 10 ¹⁹ / cm ³ to 1 × 10 ²² / cm ³ . By adjusting the film thickness and the Mg content in this way, the carrier concentration of the p-type contact layer is increased and the ohmic contact with the p-electrode described later is improved. As described above, the fourth p-side semiconductor layer can be omitted.

(Unevenness)
In the first semiconductor layer 12 of the present invention, irregularities 22 are provided periodically, and can function as a diffraction grating. In this specification, the unevenness in the semiconductor layer may be referred to as a diffraction grating.

  The size of the diffraction grating is not particularly limited, and can be appropriately adjusted depending on the wavelength of the laser beam to be obtained, the composition of the nitride semiconductor layer to be used, and the like. The period (pitch) P of the unevenness is determined by the wavelength to be oscillated and the effective refractive index, and can be obtained by λ (wavelength) / 2n (refractive index of the semiconductor). For example, when a nitride semiconductor laser element having an oscillation wavelength of 400 nm is formed, the period of unevenness is 400 nm (wavelength) / [2 × 2.5 (semiconductor refractive index)] = 80 nm.

  For example, the unevenness is formed in the third n-side semiconductor layer (n-side cladding layer), and can be flattened by embedding the unevenness in the fourth n-side semiconductor layer. Alternatively, unevenness may be formed in the lower layer portion of the third n-side semiconductor layer (n-side cladding layer), and further, the unevenness may be buried in the upper layer of the third n-side semiconductor layer to be planarized. However, in this case, the composition or the laminated structure is different between the semiconductor layer for forming the unevenness and the semiconductor layer for flattening the unevenness, and it is necessary to adjust the refractive index difference.

  Moreover, in this invention, it is preferable that the distance (refer D in FIG. 2) from the upper surface of the convex part which comprises an unevenness | corrugation is 3 times or less of uneven | corrugated height. From another viewpoint, the distance D from the upper surface of the convex portion to the active layer is preferably not more than three times the pitch width of the concave and convex portions. By setting the distance in such a range, single mode oscillation can be realized. Moreover, it is preferable that this distance D is 0.5 times or more of uneven | corrugated height. Thereby, the surface of the semiconductor layer formed on the unevenness can be planarized. Furthermore, from another viewpoint, the unevenness is preferably formed close to the active layer. For example, the distance of the unevenness closest to the active layer (see D in FIG. 2) is about 300 nm or less and about 100 nm. In the following, it is preferably formed so as to be arranged at about 50 nm or less, or at the height of unevenness or less.

  The shape of the unevenness is not particularly limited, and can be, for example, a sawtooth shape, a sine wave shape, a rectangular shape, a trapezoidal shape, an inverted trapezoidal shape, etc., but may be a rectangular shape, a trapezoidal shape, an inverted trapezoidal shape, or the like. preferable. That is, the side surface of the convex portion constituting the diffraction grating has a shape that matches the normal direction of the substrate, α = 30 ° with respect to the normal direction of the substrate (trapezoid, see FIG. 4A), or close to this. The shape having an inclination in the range of the shape in which the side surface is expanded) to β = −30 ° (inverted trapezoidal shape, see FIG. 4B or a shape in which the side surface is recessed). If the side surface of the convex portion is formed within this range, the surface of the semiconductor layer that fills the concave and convex portions can be flattened without increasing the thickness. Further, α = 20 ° to β = −30 °, α = 10 ° to β = −30 °, α = 5 ° to β = −30 °, α = 30 ° to β = −20 °, α = 30 °. It is more preferable that β = −10 °, α = 30 ° to β = −5, α = 20 ° to β = −20 °, and α = 10 ° to β = −10 °.

  The pitch of the concavo-convex width (one cycle of the concavo-convex) is preferably in the range of about 40 to 140 nm, for example. In addition, although it is preferable that the width B of a convex part and the width A (refer FIG. 2) of a recessed part are respectively the same, you may differ. In this case, it is preferable that one of the widths is in a range of 1/2 to 2 of the other width. From another viewpoint, it is preferable that the width of the convex portion and the width of the concave portion are within a range of about 1/15 to 8 of the height of the convex portion described later.

The depth of the diffraction grating (uneven height H, see FIG. 2) is about 300 nm or less, preferably about 50 to 150 nm.
By making such a size and depth, it is possible to flatten the surface of the first semiconductor layer in which the unevenness is embedded while forming the unevenness with high accuracy, and as a result, the characteristics of the active layer formed thereon Can be secured or improved.

  Further, as shown in FIGS. 5A, 5B, and 6, the concave bottom surface that constitutes the diffraction grating has a length in the range of 1/2 to 1/10 of the uneven height H that constitutes the diffraction grating. The length (length from the bottom surface) G preferably has an inclination (inclination angle = γ) continuous from the side surface of the convex portion to the bottom surface of the concave portion. The inclined surface may be formed on both side surfaces of the convex portion, but may be absent on both side surfaces of the partial convex portion or on one side surface of the partial convex portion (FIG. 5B). reference). Further, when the side surface of the recess is inclined as shown in FIG. 4 (a) or (b), it may be inclined at an inclination angle γ that can be distinguished from the inclination as shown in FIG. preferable. Note that all the inclined surfaces are not formed with the same inclination and height, and the inclination angle and / or height may be different on some or all side surfaces. Specifically, the inclined length G is, for example, about 5 to 150 nm. Moreover, it is preferable that this inclination | tilt angle (gamma) is the range of 20 degrees-70 degrees with respect to the normal line direction of a board | substrate, or a recessed part side surface, for example. When the side surface near the bottom of the concave portion of the diffraction grating is inclined as described above, the concave portion is completely embedded without forming a cavity in the concave portion when the semiconductor layer is regrown on the diffraction grating. Can do. As a result, the surface of the semiconductor layer for flattening the irregularities formed on the diffraction grating can be easily flattened, and a rise in voltage in the laser element, disturbance of the diffraction grating, and the like can be prevented.

  Further, as shown in FIG. 5C, the unevenness constituting the diffraction grating has an upper surface of the convex portion (for example, a surface substantially horizontal to the substrate surface), and is further inclined between the upper surface and the side surface. It may have a surface (inclination angle δ). The inclination angle is not particularly limited, but for example, about 20 ° to 70 ° is appropriate. Moreover, although the depth (Gx) is not specifically limited, For example, about 1-30 nm is mentioned.

  The method for forming irregularities in the first semiconductor layer is not particularly limited. For example, as shown in FIGS. 7A to 7D, a layer (for example, a clad layer) on which irregularities are to be formed. 12a), the mask pattern 20 is formed by a photolithography process and an etching process using methods known in the art such as a double resist method, a contact mask exposure method, an electron beam drawing method, and a phase shift method. Then, the diffraction grating 22 is formed by etching using the mask pattern 20 as a mask. After that, the mask pattern 20 is removed, and the unevenness of the diffraction grating 22 is changed into a layer for flattening the unevenness (for example, the cladding layer 12a, the light It can be formed by embedding with a guide layer 12b).

Mask pattern in this case, various _{resist, Al 2 O 3, ZrO 2} , SiO 2, TiO 2, Ta 2 O 5, AlN, oxide film or a nitride film such as SiN, nickel, single metal film such as chromium It can be formed using a single film or a multilayer film. These film thicknesses are preferably formed to be, for example, about 10 to 500 nm. Thereby, the height of the convex part of the diffraction grating can be formed to a desired height.

In particular, when patterning using a low refractive index member such as SiO ₂ , TiO ₂ , ZrO ₂ , Al ₂ O ₃ , SiN, AlN or the like as a mask pattern, as shown in FIG. A layer (for example, the cladding layer 12a and the light guide layer 12b) for flattening the unevenness may be grown without removing the mask pattern 20. As a result, a member having a lower refractive index than the nitride semiconductor is disposed on the upper surface of the convex portion, but the effect of the diffraction grating can be further improved by the member having a lower refractive index.

  Etching in the case of forming irregularities by etching the semiconductor layer using a mask pattern may be either wet etching or dry etching using an appropriate etchant. For example, when dry etching is used, it is preferable to perform etching at a pressure within a range of 0.05 to 10 Pa (a constant pressure or a pressure changed as appropriate). Thereby, the etching of desired depth can be performed efficiently.

  Furthermore, after forming the diffraction grating, two or more types of semiconductor layers having different Al compositions may be stacked as a superlattice structure while appropriately adjusting conditions such as pressure, temperature, source gas type, and flow rate. Furthermore, when forming each layer of the superlattice structure, the film may be formed with a waiting time of about 5 seconds, or the ratio of the source gas flow rate containing nitrogen atoms to the source gas flow rate containing group III elements is 2000. The semiconductor layer may be formed by setting it to about or more, or when forming each layer of the superlattice structure, the semiconductor layer may be formed at a normal pressure, or a combination thereof may be used.

  In particular, as a method of inclining the side surface near the bottom side surface of the concave portion, a mask of a single film or a multilayer film as described above is formed on the layer on which the diffraction grating is to be formed, and this mask is used, for example, In the range of 0.05 to 10 Pa, a method of dry etching by changing the etching pressure, a method of wet etching or dry etching by changing the kind or composition of the etchant, and the like are advantageous. Further, as a mask material for forming a diffraction grating, a mask material having a structure of two or more layers using a material having low selectivity with a resist as a lower layer and a material having high selectivity with a resist as an upper layer is used. A method of transferring a diffraction grating corresponding pattern from a resist to such a mask material and etching the diffraction grating using a mask having a structure of two or more layers is effective.

(ridge)
After forming the nitride semiconductor layer, a ridge is formed on the surface of the second semiconductor layer.
For example, a protective film made of Si oxide (mainly SiO ₂ ) is formed with a film thickness of 0.5 μm on almost the entire surface of the uppermost fourth p-side semiconductor layer (p-side contact layer) by a CVD apparatus, and thereafter Then, a mask having a predetermined shape is formed on the protective film, and a stripe-shaped protective film is formed by a photolithography technique using CHF ₃ gas using an RIE apparatus or the like. Using this protective film as a mask, the ridge can be formed by etching the semiconductor layer with, for example, SiCl ₄ gas. The ridge is usually etched from the fourth p-side semiconductor layer and is preferably formed above the active layer, and the shape is suitably a stripe. For example, a stripe formed by etching from the p-side contact layer to the middle of the p-side guide layer can be used. In the present invention, when unevenness is provided in the p-side semiconductor layer, a ridge may be formed above the unevenness, etching may be stopped in the middle of the unevenness, or the ridge may be formed below the unevenness. The ridge may be formed by etching. In the case where the ridge is formed by etching below the unevenness, the structure has an unevenness under the ridge. The width of the ridge is in the range of 0.5 to 15 μm, preferably 1.0 to 5.0 μm.

Buried films 15 are formed on both sides of the ridge. This buried film is preferably insulative and has a low refractive index.
In the nitride semiconductor laser element of the present invention, it is not always necessary to form a ridge. For example, a semiconductor laser element in which a current confinement layer is formed in a nitride semiconductor layer may be used.

(electrode)
A p-electrode is formed on the ridge on the p-type contact layer by sputtering. The p electrode 120 preferably has, for example, a multilayer structure of metal layers, and specifically includes Ni / Au.
Further, an n-electrode that takes ohmic contact is also formed on the upper surface of the n-type contact layer. The n-electrode is made of Ti / Al, and is formed in stripes parallel to the ridge and having the same length. The n-electrode is not necessarily formed on the upper surface of the n-type contact layer. For example, if the substrate used has conductivity and can ensure ohmic contact, the lower surface side of the substrate (the side opposite to the element structure) ).
The nitride semiconductor laser element of the present invention usually has a pair of resonator end faces. An insulating film may be formed on the ridge, the resonator surface, or the like, or a pad electrode or the like may be formed on each electrode.

Hereinafter, embodiments of the nitride semiconductor laser device of the present invention will be described in detail with reference to the drawings.
Example 1
As shown in FIG. 1A, for example, the nitride semiconductor laser device of the present invention includes a substrate 11, a first semiconductor layer 12 (for example, n-type), an active layer 13, and a second semiconductor layer 14 ( For example, p-type). The laser device further includes a first electrode (not shown) and a second electrode (not shown) electrically connected to the first semiconductor layer 12 and the second semiconductor layer 14, respectively, and the second semiconductor. A first protective film (buried film) 15 is formed on the layer 14, and a second protective film 16 (see FIG. 1B) is formed on the resonator end face. The second protective film 16 functions as a mirror.

As the substrate 11, a heterogeneous substrate made of sapphire is used, and an underlying layer using lateral growth is formed thereon via a buffer layer (not shown).
The compositions and film thicknesses of the first semiconductor layer 12, the active layer 13, and the second semiconductor layer 14 are as shown in Table 1.

This nitride semiconductor laser has a structure shown in Table 2.

  In this laser element, as shown in FIG. 1B, a diffraction grating 22 is formed in the cladding layer in the first semiconductor layer 12. A light guide layer is formed on the upper surface of the diffraction grating 22. As shown in FIGS. 1B and 2, the diffraction grating 22 has a pitch P of 80 nm, a width A of the convex portions 24 a of 40 nm, a width B of the concave portions 24 b of 40 nm, an uneven height H of 100 nm, and an active layer. The distance D from 13 is 127 nm.

This laser element was manufactured as follows.
First, a sapphire substrate is prepared as a heterogeneous substrate.
This sapphire substrate is set in a reaction vessel, the temperature is set to 510 ° C., hydrogen is used as a carrier gas, ammonia and TMG (trimethyl gallium) are used as a source gas, and a low-temperature growth buffer layer made of GaN is formed on the sapphire substrate. Growing with a film thickness of 15 nm. On top of this, a base layer made of GaN is grown at 2500 nm.

An n-type contact layer made of Al _0.02 Ga _0.98 N doped with Si at 1100 ° C. is grown to a thickness of 3.5 μm on the obtained underlayer using TMG, TMA, ammonia, and silane gas as an impurity gas.
Next, a crack preventing layer made of In _0.06 Ga _0.94 N doped with Si at 5 × 10 ¹⁸ / cm ³ at a temperature of 920 ° C. using TMG, TMI (trimethylindium), and ammonia is formed to a thickness of 0.15 μm. Grow.

The temperature is set to 1000 ° C., TMA, TMG, and ammonia are used as source gases, and an A layer made of undoped Al _0.08 Ga _0.92 N is grown to a thickness of 25 mm, and then Si is used as an impurity gas using Si. A B layer made of GaN doped with 5 × 10 ¹⁸ / cm ³ is grown to a thickness of 25 mm. Then, the operation of alternately laminating the A layer and the B layer is repeated 260 times to laminate the A layer and the B layer, and an n-type clad layer made of a multilayer film (superlattice structure) having a total film thickness of 1.3 μm is formed. Grow. At this time, the average composition of Al in the cladding layer is 0.036.

An SiO ₂ film (thickness 200 nm) is formed on this layer, and a resist layer (thickness 200 nm) is formed thereon. A pattern corresponding to the diffraction grating is formed on the resist layer using an electron beam drawing method. Using the resist pattern as a mask, the pattern is transferred to the SiO ₂ film, and further, using the patterned resist layer and the SiO ₂ film as a mask, by the RIE method, for example, within a pressure range of 0.05 to 10 pa. The n-type cladding layer is dug down by 100 nm as appropriate (see FIGS. 7A and 7B).

  After removing the mask pattern (see FIG. 7 (c)), while adjusting the conditions such as the flow rate and pressure of the source gas, and using TMG and ammonia as the source gas at the same temperature on the cladding layer, GaN An n-type light guide layer having a thickness of 0.17 μm is grown (see FIG. 7D).

Thereby, the light guide layer can be grown flat on the diffraction grating.
Next, the temperature is set to 900 ° C., TMI (trimethylindium) and TMG are used as the source gas, a first barrier layer made of Si-doped In _0.02 Ga _0.98 N, and then undoped In _0.11 Ga _0.89 N Two layers of a well layer made of Si and a barrier layer made of Si-doped In _0.02 Ga _0.98 N are laminated, and a second barrier layer made of undoped In _0.02 Ga _0.98 N is laminated thereon to form an active layer To do. The first barrier layer is 140 Å, the second barrier layer is 140 Å, the well layer 70 Å / barrier layer 140 、, and a quantum well structure with a total film thickness of about 560 形成 is formed.

On the active layer, at the same temperature, TMA, TMG and ammonia are used as source gases, Cp ₂ Mg (cyclopentadienylmagnesium) is used as impurity gas, and Mg is doped at 1 × 10 ¹⁹ / cm ^3. _A p-type second cap layer made of Al _0.25 Ga _0.75 N doped with 1 × 10 ¹⁹ / cm ^{3 of} Mg is grown on a p-type first cap layer made of _0.25 Ga _0.75 N with a thickness of 30 mm. Growing with a film thickness of 70 mm.

  The temperature is set to 1000 ° C., TMG and ammonia are used as source gases, and a 0.15 μm-thick p-side light guide layer made of GaN is grown. This p-type light guide layer is a non-doped layer, but may contain Mg due to diffusion of Mg from adjacent layers such as a p-type cap layer and a p-type cladding layer described later. Further, Mg may be intentionally doped.

Subsequently, an A layer made of undoped Al _0.13 Ga _0.87 N is grown to a thickness of 25 mm at 1000 ° C., and a B layer made of Mg doped Al _0.09 Ga _0.91 N is grown to a thickness of 25 mm using Cp ₂ Mg. Then, the operation of alternately laminating the A layer and the B layer is repeated 7 times to form a superlattice layer having a total film thickness of 35 nm. This superlattice layer constitutes a part of the p-type cladding layer.

Next, an A layer made of undoped Al _0.13 Ga _0.87 N is grown to a thickness of 25 mm at 1000 ° C., and a B layer made of Mg doped Al _0.09 Ga _0.91 N is grown to a thickness of 25 mm using Cp ₂ Mg. The operation of alternately laminating the A layer and the B layer is repeated 87 times to grow a p-type cladding layer made of a superlattice layer having a total film thickness of 0.47 μm.

Finally, a p-type contact layer made of p-type GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown on the p-type cladding layer 110 at a temperature of 1000 ° C. to a thickness of 150 μm. The p-type contact layer can be composed of p-type In _X Al _Y Ga _1-XY N (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), preferably GaN or Al composition ratio doped with p-type impurities If AlGaN is 0.3 or less, the most preferable ohmic contact with the p-electrode can be obtained. Since an electrode is formed on the p-type contact layer, it is desirable to have a high carrier concentration of 1 × 10 ¹⁷ / cm ³ or more.

After the reaction is completed, the wafer is annealed at 700 ° C. in a nitrogen atmosphere in the reaction vessel to reduce the resistance of the p-type layer.
After the nitride semiconductor is grown in this way to form a laminated structure, the wafer is taken out of the reaction vessel, and an etching mask made of SiO ₂ is formed on the surface of the uppermost p-type contact layer. The mask is processed into a predetermined shape, and a p-type semiconductor layer, an active layer, and a part of the n-type semiconductor layer are sequentially etched by RIE (reactive ion etching) using Cl ₂ gas to form an n electrode. Expose the surface of the mold contact layer. At this time, the surface of the n-type contact layer is exposed not only in the region where the n-electrode is formed, but also in the region including the position and / or the periphery where the nitride semiconductor element is cleaved or divided in a later step. .

Next, a ridge stripe is formed as the above-described stripe-shaped waveguide region. First, a first protective film made of Si oxide (mainly SiO ₂ ) is formed to a thickness of 0.5 μm on almost the entire surface of the uppermost p-type contact layer (upper contact layer) 111 by a PVD apparatus. . Next, a mask having a predetermined shape is formed on the first protective film, and the first protective film is formed with a stripe width of 1. with a RIE (reactive ion etching) apparatus using CF ₄ gas and photolithography. 6 μm. Subsequently, using the first protective film as a mask, the p-type contact layer, the p-type cladding layer, and a part of the p-type light guide layer are etched, and the film thickness of the p-type light guide layer becomes 0.1 μm. A ridge stripe is formed by etching to a certain depth.

After forming the ridge stripe, an embedded film made of Zr oxide (mainly ZrO ₂ ) is formed on the first protective film and the p-type light guide layer exposed by etching from the first protective film. It is continuously formed with a film thickness of 0.2 μm.

After forming the protective film, the wafer is heat-treated at 600 ° C. Thus, when the material other than SiO ₂ is formed as the second protective film, the buried film is formed by performing heat treatment at a temperature not less than 300 ° C. and not more than the decomposition temperature of the nitride semiconductor (1200 ° C.) after the formation of the protective film. It is more desirable to add this step because it is difficult to dissolve in the dissolving material (hydrofluoric acid) of the first protective film.

  Next, the wafer is immersed in hydrofluoric acid, and the first protective film is removed by a lift-off method. Thereby, the first protective film provided on the p-type contact layer is removed, and the p-type contact layer is exposed. As described above, as shown in FIG. 1, the buried film is formed on the side surface of the ridge stripe and the plane continuous therewith (exposed surface of the p-type light guide layer).

Thus, after the first protective film provided on the p-type contact layer is removed, a p-electrode made of Ni / Au (100 （/ 1500Å) is formed on the exposed surface of the p-type contact layer. . The p-electrode is formed over the protective film with a stripe width of 100 μm.
Before or after the formation of the p-electrode, a striped n-electrode made of Ti / Al (100 Å / 5000 形成) is formed in a direction parallel to the stripe on the surface of the already exposed n-type contact layer.

Next, in order to provide an extraction electrode for the p electrode and the n electrode, a desired region is masked and a protective film made of SiO ₂ is provided. At this time, a protective film made of SiO ₂ is also provided on the paired resonator surface (reflecting surface side) and resonator surface (light emitting surface side).
Thereafter, a pad electrode made of RhO—Pt—Au (3000 to 1500 to 6000) is provided on the p-electrode 120. A pad electrode made of Ni—Ti—Au (1000 Å−1000 Å−8000 Å) is provided on the n electrode 121.

  After forming the n-electrode and the p-electrode as described above, the sapphire substrate of the wafer is polished to 200 μm, and then the surface of the above-described resonator surface (light emitting surface side), that is, the stripe-shaped p Then, the surface in the direction perpendicular to the n-electrode is etched to remove the protective film and to form a resonance surface (1-100 plane, plane corresponding to the side of the hexagonal columnar crystal = M plane).

Finally, the wafer was cut into chips in a direction parallel to the p-electrode to obtain a laser element (resonator length: 675 μm).
In such a laser element, the light guide layer is embedded flat on the diffraction grating, the upper surface thereof is flattened, and the active layer can be formed flat on the upper surface. Thus, a difference in refractive index can be secured in the diffraction grating. Therefore, when this laser element was actually oscillated, a single spectrum peak was obtained at 30 mW, as shown in FIG. As a reference example, it is understood that when a laser element having the same configuration is oscillated except that no diffraction grating is formed, a single spectrum peak is not obtained at 10 mW, as shown in FIG.

Example 2
This semiconductor laser device has substantially the same configuration as the device of Example 1 except that the height H of the unevenness constituting the diffraction grating is 100 nm and the distance D from the active layer 13 is 100 nm.
Also in this laser element, by shortening the distance from the active layer to the diffraction grating, the light generated in the active layer is remarkably strengthened by the lattice spacing of the diffraction grating, as shown in FIG. A single spectral peak similar to is obtained.

Example 3
In the diffraction grating formed in the cladding layer in the first semiconductor layer 12, as shown in FIG. 5A, the laser element of this embodiment has a tilt angle γ of 20 to 70 ° on the side surface of the recess 22b. The configuration is the same as that of the laser device of Example 1 except that a shape having a slope with a length G from the bottom surface of 30 nm is formed.

Such irregularities are formed as follows.
An SiO ₂ film (film thickness 200 nm) is formed on the n-type cladding layer, and a resist layer (film thickness 200 nm) is formed thereon. A pattern corresponding to the diffraction grating is formed on the resist layer using an electron beam drawing method. Using the resist pattern as a mask to transfer the pattern on the SiO ₂ film, further using these patterned resist layer and the SiO ₂ film as a mask, by RIE method to form a diffraction grating. At this time, the etching pressure is initially set to 8 Pa, for example, and then changed to 4 Pa, thereby forming the bottom surface and the inclined surface (the G region in FIG. 5A) corresponding to the inclined portion. The thickness is adjusted as appropriate within a range of 0.05 to 10 pa, and the n-type cladding layer is dug 100 nm to form side surfaces and form a diffraction grating.

After removing the mask pattern, while adjusting the conditions such as the flow rate and pressure of the source gas, the film thickness of GaN using TMG and ammonia as the source gas on the cladding layer at the same temperature as in Example 1. A 0.17 μm n-type light guide layer is grown.
Thereby, an inclination can be formed from the side surface to the bottom surface of the convex portion constituting the diffraction grating, and the n-type light guide layer can be grown flat on the diffraction grating.
In such a laser element, a difference in refractive index can be ensured in the diffraction grating, and when this laser element is actually oscillated, a single spectral peak of 30 mW is obtained as in FIG.

Example 4
In the laser element of this embodiment, after forming a diffraction grating in the n-type cladding layer, the A layer (Al _0.12 Ga _0.88 N) and the B layer (Al _0.04 Ga _0.96 N) are alternately formed as the upper layer in the n-type cladding layer. 15 pairs. Each film thickness is 25 mm and the total film thickness is 450 mm. Subsequently, a semiconductor laser element is formed in substantially the same manner as in Example 1 except that the light guide layer is formed with a film thickness of 0.17 μm.

  Also in this laser element, a difference in refractive index can be ensured in the diffraction grating, and when this laser element is actually oscillated, a single spectral peak of 30 mW is obtained as in FIG.

It is a schematic cross section explaining the laser element structure of the present invention. It is a schematic cross section for demonstrating the unevenness | corrugation in the laser element of this invention. It is a spectrum figure of the laser element of this invention. It is a schematic cross section for demonstrating the diffraction grating of another aspect in one laser element of this invention. It is a schematic cross section for demonstrating the diffraction grating of another aspect in one laser element of this invention. It is a schematic cross section for demonstrating the diffraction grating of another aspect in one laser element of this invention. It is a schematic cross-sectional process drawing which shows the method of forming the unevenness | corrugation in one laser element of this invention.

Explanation of symbols

10 laser element 11 substrate 12 first semiconductor layer 12a clad layer 12b light guide layer 13 active layer 14 second semiconductor layer 15 buried film 16 protective film 20 mask pattern 22 diffraction grating 22a convex part 22b concave part

Claims (13)

A nitride semiconductor laser device comprising: a substrate; and a nitride semiconductor layer in which a first semiconductor layer, an active layer, and a second semiconductor layer are sequentially stacked on the substrate,
A layer in which unevenness is formed in the first semiconductor layer, and a layer for flattening the unevenness is formed on the layer in which the unevenness is formed;
A nitride semiconductor laser device characterized in that the distance from the upper surface of the convex portion constituting the irregularities to the active layer is not more than three times the height of the irregularities. A nitride semiconductor laser device comprising: a substrate; and a nitride semiconductor layer in which a first semiconductor layer, an active layer, and a second semiconductor layer are sequentially stacked on the substrate,
A layer in which unevenness is formed in the first semiconductor layer, and a layer for flattening the unevenness is formed on the layer in which the unevenness is formed;
A nitride semiconductor laser device characterized in that the distance from the upper surface of the convex portion constituting the irregularities to the active layer is not more than three times the pitch width of the irregularities.   The nitride semiconductor laser element according to claim 1 or 2, wherein a distance from the upper surface of the convex portion constituting the concave and convex portions to the active layer is 300 nm or less.   The nitride semiconductor laser element according to any one of claims 1 to 3, wherein the width of the convex portion and the width of the concave portion in the concave and convex portions are each in a range of 1/15 to 8 of the height of the concave and convex portions.   The nitride semiconductor laser device according to claim 1, wherein the unevenness is formed in a stripe shape, and the width of the unevenness has a pitch in the range of 40 to 140 nm.   The nitride semiconductor laser device according to any one of claims 1 to 5, wherein a distance from the upper surface of the convex portion constituting the irregularities to the active layer is 0.5 times or more of the irregularity height.   The nitride semiconductor laser element according to claim 1, wherein a member having a refractive index lower than that of the nitride semiconductor is disposed on the upper surface of the convex portion. Low refractive index _{member, SiO 2, TiO 2, ZrO} 2, Al 2 O 3, SiN, the nitride semiconductor laser device according to claim 7 at least one selected from the county consisting AlN.   The nitride semiconductor laser element according to claim 1, wherein the active layer is a nitride semiconductor containing In. The layer in which the irregularities are formed includes a first layer made of Al _a Ga _1-a N (0 ≦ a ≦ 0.10) and Al _b Ga _1-b N (0.01 ≦ b ≦ 0.14). The nitride semiconductor laser element according to any one of claims 1 to 9, further comprising a superlattice layer formed of the second layer. The nitride semiconductor laser device according to claim ₁ , wherein the layer for flattening the unevenness is a layer made of Al _c Ga _1-c N (0 ≦ c ≦ 0.075). The layer for flattening the irregularities is _{a first} layer made of Al _a Ga _1-a N (0 ≦ a ≦ 0.10) and Al _b Ga _1-b N (0.01 ≦ b ≦ 0.14). The nitride semiconductor laser element according to any one of claims 1 to 10, further comprising a superlattice layer constituted by the second layer.   The nitride semiconductor laser element according to claim 1, wherein the nitride semiconductor laser element has a resonance surface, and the unevenness is provided in a stripe shape substantially parallel to the resonance surface.

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