JP3891108B2 - Nitride semiconductor light emitting device - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a laser element made of a nitride semiconductor (InaAlbGa1-a-bN, 0 ≦ a, 0 ≦ b, a + b ≦ 1).
[0002]
[Prior art]
We have produced a nitride semiconductor laser device including an active layer on a nitride semiconductor substrate, and have achieved the world's first continuous oscillation at room temperature of 10,000 hours or more (ICNS'97 Proceedings, October 27-31, 1997, P444-446, and Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L1568-1571, Part 2, No. 12A, 1 December 1997). FIG. 8 is a schematic cross-sectional view showing the structure of the laser element. As a basic structure, a plurality of nitride semiconductor layers serving as laser element structures are formed on a nitride semiconductor substrate made of n-GaN selectively grown on a sapphire substrate through a SiO2 film formed in a stripe shape. It is laminated. (For details, refer to Jpn.J.Appl.Phys.Vol.36)
[0003]
The basic laser device structure is a super-layer composed of n-Al0.14Ga0.86N / GaN, in which an active layer in which a barrier layer made of In0.02Ga0.98N and a well layer made of In0.15Ga0.85N are stacked. It has a double heterostructure sandwiched between an n-side cladding layer having a lattice structure and a p-side cladding layer having a superlattice structure made of p-Al0.14Ga0.86N / GaN. Each of the single nitride semiconductor layers constituting the superlattice has a thickness of 25 angstroms. Further, a ridge having the same stripe width is formed on a part of the p-type cladding layer. The light emitted from the active layer is concentrated in the waveguide region under the ridge stripe, resonates between the resonance surfaces, and is emitted as a laser.
[0004]
[Problems to be solved by the invention]
However, in the conventional laser element, the laser light tends to be multimode. The horizontal transverse mode is made single mode by narrowing the ridge stripe, but if the stripe width is narrowed to obtain the single mode, it tends to be difficult to form an ohmic electrode formed on the ridge stripe. . In addition, since the electrode area is reduced, the current concentrates in a minute portion, thereby increasing the threshold value of the laser element or reducing the reliability of the element itself due to heat generation.
[0005]
On the other hand, in the vertical transverse mode, light confinement by both cladding layers tends to be insufficient. For example, light leaked from the n-side cladding layer is reflected by a sapphire substrate having a low refractive index and guided in a GaN layer having a high refractive index between the substrate and the n-side cladding layer. The light guided in the GaN layer overlaps with the laser light emitted from the end face of the active layer and disturbs the shape of the far field pattern (FFP). Observed as a mode. A multimode laser element is very difficult to use as a light source for pickup.
[0006]
Therefore, an object of the present invention is to use a nitride semiconductor laser element as various light sources, so that it has single mode NFP and FFP, and oscillates laser light for a long time with a low threshold.
[0007]
[Means for Solving the Problems]
The nitride semiconductor laser device of the present invention isThis can be achieved by the following configurations (1) to (9).
(1) The substrate has an n-side cladding layer, an active layer, a p-side cladding layer, and a p-side contact layer in this order, and has a ridge above the active layer, with respect to the opposing resonant surface In the nitride semiconductor laser device in which the laser beam resonates substantially vertically, the p-side cladding layer is made of a superlattice having a nitride semiconductor layer containing at least Al, and is one of the p-side contact layer and the p-side cladding layer. The ridge is formed in a part or all of the ridge, the p-side contact layer has a striped p-electrode on the ridge outermost surface, and an insulating film is formed on the exposed surfaces of the p-side contact layer and the p-side cladding layer. A p-pad electrode electrically connected to the p-electrode through the insulating film, and the stripe width of the ridge becomes narrower as it approaches both resonance planes, and on the resonance plane of the laser element Lean Characterized in that it is formed to have a corner.
Furthermore, as a specific nitride semiconductor laser element of the present invention,
(2) When the total thickness of the p-side cladding layer is 50 mm or more and 2.0 μm or less and the Al average composition contained in the p-side cladding layer is expressed in percentage (%), the Al average composition (% ) Is 50% or less, and the product of the total thickness (μm) of the p-side cladding layer and the Al average composition (%) is preferably 4.4 or more.
(3) Preferably, the nitride semiconductor layer including the active layer between the n-side and p-side cladding layers has a thickness in the range of 200 angstroms to 1.0 μm.
(4) The thickness of the p-side cladding layer is preferably smaller than the thickness of the n-side cladding layer.
(5) The nitride semiconductor layer containing Al is made of Al. _X Ga _1-X N (0 <X <1) is preferable.
(6) The inclination angle is preferably 10 ° or less.
(7) The substrate is preferably a nitride semiconductor substrate.
(8) It is preferable that an electrode is provided on the nitride semiconductor substrate.
(9) The nitride semiconductor laser element is preferably formed by cleaving the substrate.
[0008]
Furthermore, the ridge stripe is characterized in that the stripe width on the output side is narrower than the stripe width on the total reflection side. The laser output side and the total reflection side can be determined as appropriate by adjusting the reflectance of the reflective film formed on the resonance surface, for example.
[0009]
In the laser device of the present invention, the n-side cladding layer is made of a superlattice having a nitride semiconductor layer containing at least Al, and the entire thickness of the n-side cladding layer is 0.5 μm or more. When the average Al composition contained in the side cladding layer is expressed as a percentage (%), the product of the total thickness (μm) of the n-side cladding layer and the Al average composition (%) is 4.4 or more. It is configured.
[0010]
Further, when the thickness of the entire p-side cladding layer is 2.0 μm or less and the Al average composition contained in the p-side cladding layer is expressed as a percentage (%), the thickness of the entire p-side cladding layer The product of (μm) and Al average composition (%) is configured to be 4.4 or more.
[0011]
Furthermore, in the laser element of the present invention, the nitride semiconductor layer including the active layer between the n-side and p-side cladding layers has a thickness in the range of 200 angstroms to 1.0 μm. Features.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the laser device of the present invention, the cladding layer is an optical confinement layer including a nitride semiconductor having a refractive index smaller than that of the well layer of the active layer. A superlattice refers to a multilayer structure in which nitride semiconductor layers having a single layer thickness of 100 angstroms or less and different compositions are laminated, preferably 70 angstroms or less, more preferably 40 angstroms or less. A physical semiconductor layer is stacked. As a specific configuration, for example, a superlattice in which an AlXGa1-XN (0 <X <1) layer and another nitride semiconductor layer having a composition different from that of the AlXGa1-XN layer is formed, for example, AlXGa1-XN / GaN. AlxGa1-XN / AlYGa1-YN (0 <Y <1, Y <X), AlXGa1-XN / InZGa1-ZN (0 <Z <1), etc. A superlattice can be formed by a combination of a crystal and a binary mixed crystal. Among them, the superlattice composed of AlxGa1-XN and GaN is most preferable. The same applies to the case where the n-side cladding layer is a superlattice. Needless to say, the cladding layer and the active layer need not be formed in contact with each other.
[0013]
FIG. 1 is a schematic perspective view showing the shape of the laser device of the present invention, and also shows a laminated structure. FIG. 2 is a plan view of the laser element of FIG. 1 viewed from the ridge side. This laser element is formed such that a ridge stripe is formed in a part of the p-side cladding layer 18 made of a superlattice, and one of the stripe widths of the ridge becomes narrower as it approaches the resonance surface. Is formed so as to become wider as it approaches the resonance surface.
[0014]
In this way, preferably, the stripe width on the output side of the ridge is narrowed and the other total reflection side is widened, so that the laser beam under the ridge becomes a single mode in the horizontal transverse mode where the stripe width is narrow. On the other hand, where the stripe width is wide, there is a possibility that it becomes multimode below the ridge, but since it is already single in the narrow part, it is wide while the guided light is resonating between the resonance surfaces. Since the multimode is attenuated in a narrow portion, the laser beam emitted from the resonance surface corresponding to the portion having a wide stripe width is also changed to a single mode. In addition, since there are a portion where the stripe width of the ridge is gradually narrowed and a portion where the stripe width is gradually widened, the surface area is almost the same as that provided with stripe widths at equal intervals or substantially wide. Therefore, the current does not concentrate in a narrow region and flows evenly in the ridge, so that the heat generation of the element itself is reduced and the reliability is improved.
[0015]
FIG. 4 is a plan view showing the shape of the ridge. Unlike FIG. 2, the n-electrode is omitted. Referring to this figure, when the stripe ridge is formed, the stripe width a on the output side is desirably adjusted to a range of 0.5 μm to 5 μm, more preferably 1 μm to 2 μm. If it is narrower than 0.5 μm, the ridge width of the contact layer laminated on the clad layer is also narrowed, and it tends to be difficult to form an ohmic p-electrode. On the other hand, the stripe width b on the total reflection side is desirably adjusted to a range of 2 μm to 20 μm, more preferably 4 μm to 6 μm. If it is narrower than 2 μm, the other stripe width a must be narrower than 2 μm. For this reason, the entire ridge width becomes too narrow, and the current tends to be concentrated on the ridge, so that the device tends to generate heat. On the other hand, if the width is larger than 20 μm, the light loss at the lower portion of the ridge tends to be large and the output tends to decrease.
[0016]
Further, as shown in FIG. 4, it is most preferable that the stripe shape of the ridge does not have “necking” in the middle, and narrows to either one at a certain angle. In the present invention, it is desirable that the angle be as small as possible. For example, the angle θ with respect to the direction horizontal to the laser resonance direction is adjusted to within 10 °, more preferably within 8 °, and most preferably within 5 °. When the angle becomes steep, light leaks from the steep part to the outside, making it difficult for the light to concentrate at the bottom of the ridge, resulting in poor laser efficiency, low output, and high threshold. It is in. Therefore, in the present invention, it is desirable to form a ridge stripe that becomes gradually narrower as it approaches the end surface at an angle as gentle as possible, with the narrower side as the output side and the wider side as the total reflection side. As a preferable ridge stripe, it is most preferable to form a ridge stripe having an output side a of 1 to 3 μm, a total reflection side of 3 to 8 μm, and an inclination angle of 3 ° or less.
[0017]
5 to 7 are plan views showing other ridge stripe shapes according to the present invention. In the present invention, as shown in FIGS. 5 to 6, both ridge stripe widths may be formed so as to become narrower as they approach the resonance surface. Further, as shown in FIG. 7, the stripe may have a shape in which one side becomes narrower and the other side becomes wider as the resonance surface is approached. However, as shown in FIG. 5 to FIG. 7, when the stripe width is abruptly narrowed, the angle [theta] increases, and light tends to leak from the constricted portion, and the laser output tends to decrease. Therefore, when forming the ridge stripe as shown in FIGS. 5 to 7, it is desirable to adjust the inclination angle within 10 °.
[0018]
Furthermore, in the present invention, the n-side cladding layer is made of a superlattice having a nitride semiconductor layer containing at least Al, the entire thickness of the n-side cladding layer is 0.5 μm or more, and the n-side cladding layer includes By representing the Al average composition contained in percentage (%), the product of the total thickness (μm) of the n-side cladding layer and Al average composition (%) is 4.4 or more. The laser beam in the vertical and transverse modes can be controlled, and the single mode is easily obtained. When the thickness of the n-side cladding layer is less than 0.5 μm and the product of the total thickness (μm) of the n-side cladding layer and the Al average composition (%) is less than 4.4, the n-side cladding The light confinement as a layer becomes insufficient, the light is guided by the contact layer on the n side, FFP is disturbed, and the threshold value tends to increase. A preferable product value is 5.0 or more, more preferably 5.4 or more. Adjust to 7 or more as the best mode.
[0019]
For example, the total thickness of the n-side cladding layer is 0.8 μm or more, and the Al average composition contained in the n-side cladding layer is 5.5% or more. The product in this case is 4.4 or more. Preferably, the total thickness of the n-side cladding layer is 1.0 μm or more, and the Al average composition contained in the n-side cladding layer is 5.0% or more. The product in this case is 5.0 or more. More preferably, the total thickness of the n-side cladding layer is 1.2 μm or more, and the Al average composition contained in the n-side cladding layer is 4.5% or more. In this case, the product is 5.4 or more. This specifically shows the relationship between the thickness of the n-side cladding layer and the Al average composition of the n-side cladding layer made of a superlattice.
[0020]
In addition, regarding the p-side cladding layer, when the thickness of the entire p-side cladding layer is 2.0 μm or less and the Al average composition contained in the p-side cladding layer is expressed as a percentage (%), the p-side cladding layer The product of the total thickness (μm) and the Al average composition (%) may be 4.4 or more. With this configuration, the laser beam in the vertical and transverse modes can be controlled, and the single mode is easily obtained. In addition, it is more preferable that the n-side cladding layer has the above configuration than the p-side cladding layer. This is because the p-side cladding layer is formed as a ridge stripe having the shape as in the present application, and light is absorbed by the electrode, so even if there is leakage of light in the vertical transverse mode, it is almost ignored. Because it can.
[0021]
However, when the p-side cladding layer is configured as described above, it is desirable that the thickness of the p-side cladding layer be smaller than that of the n-side cladding layer. This is because when the Al average composition of the p-side cladding layer is increased or the film thickness is increased, the resistance value of the AlGaN layer tends to increase, and when the resistance value of AlGaN increases, the driving voltage tends to increase. Because. The preferred film thickness is 1.5 μm or less, more preferably 1 μm or less. The lower limit is not particularly limited, but in order to act as a cladding layer, it is desirable to have a film thickness of 50 angstroms or more, and the average composition of Al is desirably 50% or less.
[0022]
In general, it is known that AlxGa1-XN has a larger band gap energy and a smaller refractive index as the Al mixed crystal ratio is increased. Ideally, it would be industrially convenient if an AlXGa1-XN layer having a large Al mixed crystal ratio X, for example 0.5 or more, can be grown as a single layer with a film thickness of, for example, several μm. AlXGa1-XN is difficult to grow with a thick film. If AlxGa1-XN having an Al mixed crystal ratio of 0.5 or more is to be grown in a single layer, cracks will occur in the crystal at, for example, 0.1 μm or more. However, if Al x Ga 1-XN is a thin film constituting a superlattice as in the present invention, the single film thickness is less than the critical limit film thickness of Al x Ga 1-XN, so that cracks are unlikely to occur. Therefore, if the cladding layer is a superlattice, even a layer having a high Al mixed crystal ratio can be grown with a thick film, and by combining them, light can be prevented from leaking from the n-side cladding layer to the substrate side. .
[0023]
In the present invention, the Al average composition in the superlattice is obtained by the following calculation method. For example, in the case of a superlattice obtained by laminating 200 pairs (1.0 μm) of 25 Å Al0.5Ga0.5N and 25 Å GaN, one pair is 50 Å, and the Al mixed crystal ratio of the Al-containing layer is 0. Therefore, 0.5 (25/50) = 0.25, and the Al average composition in the superlattice is 25%. On the other hand, when the film thickness is different, when Al0.5Ga0.5N is laminated at 40 angstroms and GaN is laminated at 20 angstroms, the weighted average of the film thickness is obtained, and 0.5 (40/60) = 0.33. The Al average composition is 33.3%. That is, the Al average composition of the superlattice in the present invention is obtained by multiplying the Al mixed crystal ratio of the nitride semiconductor layer containing Al by the ratio of the nitride semiconductor layer to the film thickness of one pair of superlattices. The same applies to the case where both Al are contained. For example, 0.1 (20/50) +0.2 (30/50) = 0.16 in the case of Al0.1Ga0.9N20 angstrom and Al0.2Ga0.8N30 angstrom. In other words, 16% is Al average composition. In the above example, AlGaN / GaN and AlGaN / AlGaN have been described. However, the same calculation method is applied to AlGaN / InGaN. Therefore, when growing the n-side cladding layer, the growth method can be designed based on the above calculation method. Further, the Al average composition of the n-side cladding layer can be detected using an analyzer such as SIMS (secondary ion mass spectrometer) or Auger.
[0024]
【Example】
[Example 1]
A first embodiment will be described with reference to FIGS. FIG. 3 is a schematic cross-sectional view showing the structure after the p-electrode is formed on the laser element of FIG. 1, and the portions denoted by the same reference numerals in FIGS. 1 to 3 indicate the same portions.
[0025]
(Underlayer 2)
A different substrate 1 made of sapphire made of sapphire with a 2-inch φ and C-plane as the main surface is set in a MOVPE reaction vessel, the temperature is set to 500 ° C., and a buffer made of GaN using trimethyl gallium (TMG) and ammonia (NH 3). A layer (not shown) is grown to a thickness of 200 angstroms. After growing the buffer layer, the temperature is set to 1050 ° C., and the underlayer 2 made of GaN is grown to a thickness of 4 μm. This underlayer 2 functions as an underlayer for forming a protective film partially on the surface and then performing selective growth of the nitride semiconductor substrate. The underlayer 2 is preferably grown AlXGa1-XN (0≤X≤0.5) having an Al mixed crystal ratio X value of 0.5 or less. If it exceeds 0.5, the crystal itself tends to crack rather than a crystal defect, so that the crystal growth itself tends to be difficult. Further, it is desirable to grow the film thickness to be thicker than the buffer layer and adjust the film thickness to 10 μm or less. In addition to sapphire, a substrate made of a material different from a nitride semiconductor, such as SiC, ZnO, spinel, GaAs, or the like, which is known for growing a nitride semiconductor, can be used.
[0026]
(Protective film 3)
After the underlayer 2 is grown, the wafer is taken out of the reaction vessel, a striped photomask is formed on the surface of the underlayer 2, and a protective film made of SiO2 having a stripe width of 10 μm and a stripe interval (window portion) of 2 μm by a CVD apparatus. 3 is formed with a film thickness of 1 μm. The shape of the protective film may be any shape such as a stripe shape, a dot shape, or a grid shape, but the second nitride semiconductor layer 3 having fewer crystal defects is formed by increasing the area of the protective film than the window portion. Is easy to grow. Examples of the material of the protective film include oxides and nitrides such as silicon oxide (SiOx), silicon nitride (SiXNY), titanium oxide (TiOx), and zirconium oxide (ZrOx), and multilayer films other than 1200 ° C. A metal having a melting point of These protective film materials withstand the nitride semiconductor growth temperature of 600 ° C. to 1100 ° C., and have a property that the nitride semiconductor does not grow or hardly grow on the surface thereof.
[0027]
(Nitride semiconductor substrate 4)
After the protective film 3 is formed, the wafer is set in the MOVPE reaction vessel again, the temperature is set to 1050 ° C., and a nitride semiconductor substrate 4 made of GaN made of undoped GaN is grown to a thickness of 20 μm using TMG and ammonia. Let The surface of the nitride semiconductor substrate 4 after growth had crystal defects appearing parallel to the stripe-shaped protective film at the stripe central portion of the protective film and the stripe central portion of the window portion. By preventing the ridge stripe from being related to the crystal defect at the time of formation, the crystal defect is not dislocated in the active layer, and the reliability of the device is improved. The nitride semiconductor substrate 4 can be grown using halide vapor phase epitaxy (HVPE), but can also be grown by MOVPE. The nitride semiconductor substrate is most preferably grown with GaN containing no In or Al. As the gas during growth, an organic gallium compound such as triethylgallium (TEG) is used in addition to TMG, and the nitrogen source is ammonia, or Most preferably, hydrazine is used. Further, the carrier concentration may be adjusted to an appropriate range by doping this GaN substrate with n-type impurities such as Si and Ge. In particular, when the heterogeneous substrate 1, the underlayer 2, and the protective film 3 are removed, the nitride semiconductor substrate becomes a contact layer, and therefore it is desirable to dope the nitride semiconductor substrate 4 with n-type impurities.
[0028]
(N-side buffer layer 11 = cum-side contact layer)
Next, an n-side buffer layer 11 made of GaN doped with 3 × 10 18 / cm 3 of Si is grown on the second nitride semiconductor layer 4 to a thickness of 5 μm using ammonia, TMG, and silane gas as an impurity gas. . This buffer layer acts as a contact layer for forming an n-electrode when a light-emitting element having a structure as shown in FIG. 1 is manufactured. Further, when the heterogeneous substrate 1 to the protective film 3 are removed and an electrode is provided on the nitride semiconductor substrate 4, it can be omitted. The n-side buffer layer 11 is a buffer layer grown at a high temperature. For example, GaN, AlN, etc. at a low temperature of 900 ° C. or lower on a substrate made of a material different from a nitride semiconductor body such as sapphire, SiC, spinel. Is distinguished from a buffer layer that is directly grown at a film thickness of 0.5 μm or less.
[0029]
(Crack prevention layer 12)
Next, the crack prevention layer 12 made of In0.06Ga0.94N is grown to a thickness of 0.15 μm using TMG, TMI (trimethylindium), and ammonia at a temperature of 800 ° C.
[0030]
(N-side cladding layer 13 = superlattice layer)
Subsequently, using TMA, TMG, ammonia, and silane gas at 1050 ° C., a first layer made of n-type Al0.16Ga0.84N doped with 1 × 10 19 / cm 3 of Si is grown to a thickness of 25 Å, The silane gas and TMA are stopped, and a second layer made of undoped GaN is grown to a thickness of 25 Å. Then, a superlattice layer is formed as first layer + second layer + first layer + second layer +..., And an n-side cladding layer 13 made of a superlattice having a total film thickness of 1.2 μm is grown. Since the n-side cladding layer made of this superlattice has an Al average composition of 8.0%, the product with the film thickness is 9.6. When a superlattice in which nitride semiconductors with different band gap energies are stacked on the n-side cladding layer, the threshold tends to decrease when so-called modulation doping is performed by doping a large amount of impurities into one of the layers. It is in.
[0031]
(N-side light guide layer 14)
Subsequently, the silane gas is stopped, and an n-side light guide layer 14 made of undoped GaN is grown at 1050 ° C. to a thickness of 0.1 μm. This n-side light guide layer acts as a light guide layer of the active layer, and it is desirable to grow GaN and InGaN, and it is usually desirable to grow with a film thickness of 100 Å to 5 μm, more preferably 200 Å to 1 μm. .
[0032]
(Active layer 15)
Next, the active layer 14 is grown using TMG, TMI, and ammonia. The active layer is maintained at a temperature of 800 ° C., and a well layer made of undoped In 0.2 Ga 0.8 N is grown to a thickness of 40 Å. Next, a barrier layer made of undoped In0.01Ga0.95N is grown to a thickness of 100 angstroms at the same temperature only by changing the molar ratio of TMI. A well layer and a barrier layer are stacked in order, and finally an active layer having a multi-quantum well structure (MQW) having a total thickness of 440 Å is grown by ending with the barrier layer. The active layer may be undoped as in this embodiment, or may be doped with n-type impurities and / or p-type impurities. Impurities may be doped into both the well layer and the barrier layer, or one of them may be doped.
[0033]
(P-side cap layer 16)
Next, the temperature was raised to 1050 ° C., TMG, TMA, ammonia, Cp 2 Mg (cyclopentadienyl magnesium) was used, and the band gap energy was larger than that of the p-side light guide layer 17, and Mg was doped 1 × 10 20 / cm 3. A p-side cap layer 16 made of p-type Al0.3Ga0.7N is grown to a thickness of 300 angstroms. When the p-type cap layer 16 is formed with a film thickness of 0.1 μm or less, the output of the element tends to be improved. The lower limit of the film thickness is not particularly limited, but it is desirable to form the film with a film thickness of 10 angstroms or more.
[0034]
(P-side light guide layer 17)
Subsequently, Cp2Mg and TMA are stopped, and a p-side light guide layer 17 made of undoped GaN having a band gap energy smaller than that of the p-side cap layer 16 is grown at 1050 ° C. to a thickness of 0.1 μm. This layer acts as a light guide layer of the active layer, and is preferably grown by GaN and InGaN, similarly to the n-type light guide layer 14.
[0035]
(P-side cladding layer 18)
Subsequently, a third layer made of p-type Al0.16Ga0.84N doped with 1 × 1020 / cm3 Mg at 1050 ° C. is grown to a thickness of 25 Å, and then only TMA is stopped, and a third layer made of undoped GaN is grown. 4 is grown to a thickness of 25 Å, and a p-side cladding layer 18 made of a superlattice layer having a total thickness of 0.6 μm is grown. Since this p-side cladding layer also has an average Al composition of 8%, the product with the film thickness is 4.8. In addition, when the p-side cladding layer is also made of a superlattice in which at least one of the nitride semiconductor layers containing Al is included and nitride semiconductor layers having different band gap energies are stacked, impurities are highly doped in any one of the layers. Thus, when so-called modulation doping is performed, the threshold tends to be lowered.
[0036]
Here, the film thickness of the core portion (waveguide portion) sandwiched between the clad layers will be described. The core portion is a region where the n-side light guide layer 14, the active layer 15, the p-side cap layer 16, and the p-side light guide layer 17 are combined, that is, between the n-side cladding layer and the p-side cladding layer. A nitride semiconductor layer including an active layer, which is a region that guides light emission of the active layer. In the case of a nitride semiconductor laser element, the FFP does not become a single beam because, as described above, the light emitted from the cladding layer is guided in the n-side contact layer and becomes multimode. is there. In addition, there is a case where a multimode is caused by resonance in the core. In the present invention, first, the thickness of the n-side cladding layer is increased to increase the Al average composition, thereby providing a refractive index difference and confining light in the core with the cladding layer. However, if multi-mode is possible in the core, FFP will be disturbed. Therefore, in relation to the n-side cladding layer of the present invention, it is desirable to adjust the thickness of the core portion so as not to be multimode in the core. A preferable thickness for preventing multimode from occurring in the core portion is 200 angstrom or more and 1.0 μm or less, more desirably 500 angstrom to 0.8 μm, most desirably 0.1 μm to 0.5 μm. It is desirable to adjust to. If it is thinner than 200 angstroms, light leaks from the core part and the threshold tends to increase. If it is thicker than 1.0 μm, it tends to be multimode.
[0037]
(P-side contact layer 19)
Finally, a p-side contact layer 18 made of p-type GaN doped with 2 × 10 20 / cm 3 of Mg is grown on the p-side cladding layer 18 at 1050 ° C. to a thickness of 150 Å. The p-side contact layer 19 can be composed of p-type InXAlYGa1-X-YN (0.ltoreq.X, 0.ltoreq.Y, X + Y.ltoreq.1). A preferred ohmic contact is obtained.
[0038]
The wafer on which the nitride semiconductor is grown as described above is annealed in a reaction vessel at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the layer doped with the p-type impurity.
[0039]
After annealing, the wafer is removed from the reaction vessel, and the uppermost p-side contact layer 18 and a part of the p-side cladding layer 17 are etched by an RIE apparatus to form a ridge having a shape as shown in FIGS. A stripe is formed. In this ridge stripe, the stripe width on one resonance surface side is set to 2.0 μm, the stripe width on the other resonance surface side is set to 4.0 μm, and the resonator length is set to 500 μm, The angle θ with respect to the laser resonance direction is set to 3 ° or less. In this embodiment, a part of the p-side cladding layer is formed as a ridge stripe, but the etching depth can be increased so that the entire p-side cladding layer can be formed as a ridge stripe.
[0040]
Further, when forming the ridge stripe, the ridge stripe is formed at a position where no crystal defect appears on the surface of the nitride semiconductor substrate. Crystal defects tend to appear at the center of the stripe-shaped protective film 3 and at the center of the stripe-shaped window. Therefore, by forming the ridge stripe while avoiding the central portion, the crystal defects tend not to extend to the active layer, so that the element can have a long life and the reliability is improved.
[0041]
Next, a mask is formed on the ridge surface and etching is performed by RIE to expose the surface of the n-side buffer layer 11. The exposed n-side buffer layer 11 also functions as a contact layer for forming the n-electrode 23.
[0042]
Next, a p-electrode 20 made of Ni and Au is formed in a stripe shape on the ridge outermost surface of the p-side contact layer 19, while an n-electrode 22 made of Ti and Al is exposed on the surface of the n-side buffer layer 11 previously exposed. After forming the stripe shape, an insulating film 23 made of SiO 2 is formed on the surface of the nitride semiconductor layer exposed between the p-electrode 20 and the n-electrode 22 as shown in FIG. Thus, the p pad electrode 21 electrically connected to the p electrode 20 is formed. The p pad electrode absorbs light leaking to the p layer side.
[0043]
As described above, after polishing the sapphire substrate of the wafer on which the n-electrode and the p-electrode are formed to 70 μm, the substrate is cleaved in a bar shape from the substrate side in a direction perpendicular to the stripe-shaped electrode, A resonator is manufactured. A dielectric multilayer film made of SiO2 and TiO2 is formed on the resonator surface, and finally a bar is cut in a direction parallel to the p-electrode to form a laser element.
[0044]
When this laser element was placed on a heat sink and each electrode was wire-bonded and laser oscillation was attempted at room temperature, it showed continuous oscillation at room temperature. The single laser beam had a single FFP and its shape was elliptical. A good shape was obtained. As for the characteristics of the laser element, the threshold value was reduced by 15% or more and the lifetime was improved by 60% or more compared to what we published in Jpn.J.Appl.Phys.Vol.36 (1997).
[0045]
[Example 2]
In Example 1, when the n-side cladding layer 13 is grown, an n-side cladding made of a superlattice having a total thickness of 1.0 μm is formed by stacking Si-doped n-type Al0.20Ga0.80N25 angstroms and undoped GaN25 angstroms. A laser device was fabricated in the same manner except that the layer 13 was grown. Since the n-side cladding layer has an Al average composition of 10.0%, the product with the film thickness is 10.0. This laser element also had almost the same characteristics as in Example 1.
[0046]
[Example 3]
In Example 1, when the n-side cladding layer 13 is grown, an n-side cladding made of a superlattice having a total thickness of 0.7 μm is formed by laminating Si-doped n-type Al0.20Ga0.80N25 angstroms and undoped GaN25 angstroms. A laser device was fabricated in the same manner except that the layer 13 was grown. Since the n-side cladding layer has an Al average composition of 1.0%, its product with the film thickness is 7.0. This laser element also had almost the same characteristics as in Example 1.
[0047]
[Example 4]
In Example 1, when the n-side cladding layer 13 is grown, an n-side cladding made of a superlattice having a total thickness of 0.8 μm is formed by stacking Si-doped n-type Al0.12Ga0.88N25 angstroms and undoped GaN25 angstroms. A laser device was fabricated in the same manner except that the layer 13 was grown. Since the n-side cladding layer has an Al average composition of 6.0%, the product with the film thickness is 4.8. This laser device has the same FFP as that of Example 1, and the threshold value is reduced by 8% or more and the lifetime is 30% or more compared with that published in Jpn.J.Appl.Phys.Vol.36 (1997). Improved.
[0048]
[Example 5]
In Example 1, when the n-side cladding layer 18 is grown, the Si-doped n-type Al 0.07 Ga 0.93 N layer 25 Å and the undoped GaN layer 25 Å are grown with a total thickness of 1.4 μm. Thus, a laser device was manufactured. Since the n-side cladding layer has an Al average composition of 3.5%, the product with the film thickness is 4.9. This laser device exhibited almost the same characteristics as those of Example 4.
[0049]
[Example 6]
In Example 1, when forming a striped ridge, the ridge shape is as shown in FIG. However, in FIG. 5, the stripe width on both output sides is 2 μm, the stripe width at the center is 4 μm, and the inclination angle is adjusted within 5 °. This laser device also exhibited almost the same characteristics as Example 1.
[0050]
【The invention's effect】
As described above, according to the present invention, a single mode can be obtained in both the horizontal and vertical modes. Moreover, the pot shape for laser beam extension is also a single ellipse, and a constant FFP is obtained. Nitride semiconductors use a material called sapphire, which has a lower refractive index than nitride semiconductors, so it seems that conventional problems cannot be avoided. Even if a laser element is manufactured on any substrate having a low rate, a laser beam having a clean shape can be obtained in a single mode.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a structure of a laser device according to an embodiment of the present invention.
FIG. 2 is a plan view of the laser device of FIG. 1 as viewed from above the ridge.
3 is a schematic cross-sectional view showing a structure after a p-electrode is formed on the laser element of FIG.
FIG. 4 is a plan view showing the shape of a ridge stripe.
FIG. 5 is a plan view showing one shape of a ridge stripe of the laser element according to the present invention.
FIG. 6 is a plan view showing one shape of a ridge stripe of the laser element according to the present invention.
FIG. 7 is a plan view showing one shape of a ridge stripe of the laser device according to the present invention.
FIG. 8 is a schematic cross-sectional view showing the structure of a conventional laser element.
[Explanation of symbols]
1 ... Different substrates
2 ... GaN underlayer
3 ... Protective film
4 ... Nitride semiconductor substrate
11 ... n-side buffer layer
12 ... Crack prevention layer
13 ... n-side cladding layer
14 ... n-side light guide layer
15 ... Active layer
16 ... p-side cap layer
17 ... p-side light guide layer
18 ... p-side cladding layer
19 ... p-side contact layer
20 ... p electrode
21 ... P pad electrode
22 ... n electrode
23 ... Insulating film

Claims (9)

An n-side cladding layer, an active layer, a p-side cladding layer, and a p-side contact layer in that order on the substrate;
In a nitride semiconductor laser device having a ridge in the layer above the active layer and in which the laser beam resonates substantially perpendicularly to the opposing resonance surface,
The p-side cladding layer is made of a superlattice having a nitride semiconductor layer containing at least Al,
The ridge is formed in part or all of the p-side contact layer and the p-side cladding layer,
The p-side contact layer has a striped p-electrode on the ridge outermost surface, an exposed insulating layer on the exposed p-side contact layer and the p-side cladding layer, and the p-electrode through the insulating film An electrically connected p-pad electrode;
Further nitride semiconductor laser device characterized by being formed so that the have a tilt angle in a narrow of Ri, and the resonance surface of the laser element according to the stripe width of the ridge is close to both of the resonance surface. When the total thickness of the p-side cladding layer is 50 mm or more and 2.0 μm or less and the Al average composition contained in the p-side cladding layer is expressed in percentage (%), the Al average composition (%) is 50 % or less, according to claim 1, characterized in that the product of the thickness of the entire p-side cladding layer ([mu] m) and Al average composition (%) is configured to be 4.4 or more Nitride semiconductor laser device. The thickness of the nitride semiconductor layer including an active layer between the n-side and p-side cladding layer 200 angstroms, to claim 1 or 2, characterized in that in the range of 1.0μm The nitride semiconductor laser device described. 4. The nitride semiconductor laser device according to claim 1, wherein a thickness of the p-side cladding layer is smaller than a thickness of the n-side cladding layer. 5. The nitride semiconductor layer containing Al is made of Al. _X Ga _1-X 5. The nitride semiconductor laser element according to claim 1, wherein the nitride semiconductor laser element is made of N (0 <X <1). The nitride semiconductor laser element according to claim 1, wherein the tilt angle is 10 ° or less. The nitride semiconductor laser device according to claim 1, wherein the substrate is a nitride semiconductor substrate. 8. The nitride semiconductor laser device according to claim 7, wherein an electrode is provided on the nitride semiconductor substrate. 9. The nitride semiconductor laser element according to claim 1, wherein the nitride semiconductor laser element is formed by cleaving the substrate. 10.

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