JP2011096870A - Nitride semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor laser element with high reliability that can secure sufficient heat dissipation without generating a crack on a nitride semiconductor layer. <P>SOLUTION: The nitride semiconductor laser element includes a substrate 10, a nitride semiconductor layer 13 laminated over the substrate 10 and having a ridge on its top surface, and an electrode 16 electrically connected to the nitride semiconductor layer 13, wherein first films 15a, formed of Al-containing nitride films differing in lattice constant within the films, are included as protective films 15 on surfaces of the nitride semiconductor layers 13 on both sides of the ridge 14 from side faces of the ridge 14. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor laser device, and more particularly to a nitride semiconductor laser device having a ridge waveguide structure.

Nitride _{semiconductor, In x Al y Ga 1-} xy N (0 ≦ x, 0 ≦ y, 0 ≦ x + y ≦ 1) being formed by a nitride semiconductor containing, as structure being particularly promising, the ridge Some have a waveguide structure. In the semiconductor laser device having such a structure, a protective film is usually formed for light confinement from the side surface of the ridge to the surface of the nitride semiconductor layer on both sides of the ridge.
Such a protective film is formed of, for example, an oxide dielectric such as SiO ₂ or Al ₂ O ₃ or a nitride dielectric such as AlN in consideration of a difference in refractive index and adhesion with the nitride semiconductor layer. (See, for example, Patent Document 1). AlN is useful as a protective film covering both sides of the ridge because of its high thermal conductivity.

JP 2009-4645

  However, if an AlN film on the semiconductor layer is deposited with a relatively thick film that contributes to heat dissipation without impairing the characteristics of the laser element, and with good adhesion to the semiconductor layer, the difference in lattice constant from the semiconductor layer Therefore, there is a problem that cracks occur in the AlN film, and there is a problem that when the film is formed in a thin film shape, insulation cannot be ensured and leakage may occur.

  The present invention has been made in view of the above problems, and in particular, a nitride semiconductor that can ensure sufficient heat dissipation without generating cracks on the nitride semiconductor layer and has high reliability. An object is to provide a laser element.

The nitride semiconductor laser device of the present invention is
A nitride semiconductor laser device comprising a substrate, a nitride semiconductor layer stacked on the substrate and having a ridge on the surface, and an electrode electrically connected to the nitride semiconductor layer,
A first film made of an Al-containing nitride film having two or more portions having different crystallinity in the film thickness direction is provided as a protective film on the surface of the nitride semiconductor layer on both sides of the ridge from the side surface of the ridge. And

Such a nitride semiconductor laser device preferably includes one or more of the following.
The protective film includes a second film made of a nitride film having a composition different from that of the first film, which is polycrystalline or amorphous on the first film.
The first film has a portion having different crystallinity in the film thickness direction, the nitride semiconductor layer side being coaxially aligned with the nitride semiconductor layer.
The first film has a C-axis orientation on the nitride semiconductor layer side, and there are portions having different crystallinity in the film thickness direction.
The first film includes a hexagonal crystal.
The second film is _{In x Al y Ga 1-xy} N (0 ≦ x, 0 ≦ y, 0 ≦ x + y ≦ 1).
The first film contains a single crystal on the nitride semiconductor layer side.

The nitride semiconductor laser element of the present invention is
A nitride semiconductor laser device comprising a substrate, a nitride semiconductor layer stacked on the substrate and having a ridge on the surface, and an electrode electrically connected to the nitride semiconductor layer, wherein the side surface of the ridge To the surface of the nitride semiconductor layer on both sides of the ridge,
(1) The rocking curve measured by the ω scan by the X-ray diffraction method has a plurality of peaks, or
(2) The protective film includes a first film made of an Al-containing nitride film having a plurality of peaks other than a peak having a periodicity whose rocking curve measured by an ω / 2θ scan by an X-ray diffraction method indicates a film thickness. It is characterized by.

  In any of the above-described nitride semiconductor laser elements of the present invention, the first film preferably has a rocking curve half-value width of 0.5 ° or less measured by an ω scan by an X-ray diffraction method.

  According to the present invention, it is possible to obtain a nitride semiconductor laser device that can ensure sufficient heat dissipation without generating cracks on the nitride semiconductor layer and has improved reliability.

It is a schematic sectional drawing of the principal part for demonstrating the structure of the nitride semiconductor laser element of this invention. It is a schematic sectional drawing explaining the lamination | stacking state of the protective film of this invention. It is an electron beam image for demonstrating crystallinity of material. It is a graph which shows the XRD analysis result (omega scan) from the upper surface of the 1st film | membrane of the nitride semiconductor laser element of this invention. It is a graph which shows the XRD analysis result ((omega) / 2 (theta) scan) from the upper surface of the 1st film | membrane of the nitride semiconductor laser element of this invention. It is a graph which shows the XRD analysis result ((omega) / 2 (theta) scan) from the upper surface of the 1st film | membrane of the nitride semiconductor laser element for a comparison.

As shown in FIG. 1, the nitride semiconductor laser device of the present invention mainly includes a substrate 10, a nitride semiconductor layer, that is, an n-side nitride semiconductor layer 11, an active layer 12, a p-side nitride semiconductor layer 13, and an electrode. 16 and the protective film 15.
The protective film is also referred to as a buried film, and is usually formed from the surface of the nitride semiconductor layer to the side surface of the ridge 14. That is, the protective film is formed on the nitride semiconductor layer in a region other than the region in which the nitride semiconductor layer and an electrode described later are in direct contact and electrically connected.
Here, the position, size, shape and the like of the connection region between the nitride semiconductor layer and the electrode are not particularly limited, and are formed on a part of the surface of the nitride semiconductor layer, for example, the surface of the nitride semiconductor layer. Illustrated are almost the entire surface of the stripe-shaped ridge upper surface or the portion extending from almost the entire surface of the ridge upper surface to both sides thereof.

The protective film is composed of at least a first film.
The first film is formed in direct contact with the nitride semiconductor surface on both sides of the ridge from the side surface of the ridge.
The first film is made of an Al-containing nitride film. Examples of the Al-containing nitride film include materials having good thermal conductivity. For example, the thermal conductivity is 10 W / m · K or more at 0 ° C., preferably 50 W / m · K or more, and further 100 W / m · K. The above materials are preferable. Specifically, AlN is mentioned. This ensures better heat dissipation compared to insulating films such as oxides, nitrides, oxynitrides, etc., such as Si, Ce, In, Sb, Zn, etc. that have been conventionally formed as buried films. be able to.

  The first film includes a portion where the crystallinity is different in the film, that is, the crystallinity is changed in the film. That is, since the first film is formed on the nitride semiconductor layer having a considerably different crystal lattice and contains stress, the first film is easily cracked. Therefore, when the first film includes a portion having different crystallinity in the film, the stress applied to the first film due to the difference in crystallinity with respect to the nitride semiconductor layer, particularly the p-side semiconductor layer, is increased. It can be mitigated and can effectively prevent cracks from occurring in the first film. As a result, insulation can be ensured in the vicinity of the ridge base and the like, and the occurrence of leakage current due to cracks and the like can be avoided.

Here, the crystallinity of the first film can be easily determined from a diffraction image by an electron beam.
Generally, it is classified into single crystal, polycrystal, and amorphous depending on the degree of crystallinity of the material of the film.
In a normal state, the single crystal has almost no change in lattice constant in the material and almost no lattice plane inclination. In other words, the atomic arrangement is regularly arranged in the material and the long-range order is maintained, there is almost no change in lattice constant, and there is almost no lattice plane inclination.
The polycrystal is composed of a large number of minute single crystals, that is, microcrystals.
Amorphous means one that does not have a periodic structure as in crystals, that is, one that has an irregular atomic arrangement and no long-range order.

  It is known that when an electron beam is incident on these materials, an electron beam diffraction image appears regularly corresponding to the size of the lattice constant and the surface direction. For example, in the case of a single crystal, since the crystal planes are substantially aligned, diffraction points are regularly arranged as shown in FIG. 3A. In the case of a polycrystal, since it is composed of microcrystals, the directions of the respective lattice planes are not aligned, and it can be seen in a state where diffraction points are intricately combined as shown in FIG. To do. On the other hand, in the case of amorphous, since the atomic arrangement does not have a periodic structure over a long distance, electron beam diffraction does not occur. Therefore, there is no diffraction point in the diffraction image, as shown in FIG. 3C.

  In the present invention, the difference in crystallinity means that the axial direction (orientation) or / and the lattice constant are different. In this case, the portion adjacent to the nitride semiconductor layer of the first film has a crystallinity close to that of the nitride semiconductor layer, and may be different as the distance increases, or in a portion far from the nitride semiconductor layer of the first film, Crystallinity close to that of the nitride semiconductor layer, which may be different as it approaches, and crystallinity close to the nitride semiconductor layer in the plane or in the film thickness direction regardless of whether it is far / close to the nitride semiconductor layer and What is necessary is just to have different crystallinity.

As the crystallinity of the first film, it is preferable that the nitride semiconductor layer is grown in the same orientation as the nitride semiconductor layer, and the crystallinity changes as the distance from the nitride semiconductor layer increases. Thereby, generation | occurrence | production of a crack can be suppressed, ensuring the adhesiveness with the nitride semiconductor layer used as the foundation | substrate of a 1st film | membrane.
The first film preferably contains a single crystal, particularly on the side in contact with the nitride semiconductor layer. In the present application, a film containing a single crystal does not necessarily have to be a single crystal over the entire film, and some polycrystalline portions may exist. In addition, a film containing a single crystal is partially disordered in crystal orientation, in other words, the direction of the orientation axis may be different.
Further, it may be a film containing polycrystal, and may partially contain a single crystal in the film. In this case, even if it is a polycrystal, since the predominance or tendency of the orientation of axial light distribution is recognized, it is only necessary to recognize a portion where the crystallinity is changed (different) in the polycrystal portion.
The first film is preferably oriented coaxially with the nitride semiconductor layer on the nitride semiconductor layer side. The bottom surface of the ridge and the side surface of the ridge may have different orientations. For example, the bottom surface of the ridge may be C-axis oriented, and the side surface of the ridge may be M-axis oriented.
As a result of an experiment by the present inventor, C-axis orientation is obtained with AlN that is C-axis oriented on a C-axis oriented nitride semiconductor layer and AlN that is M-axis oriented on an M-axis oriented nitride semiconductor layer. It was found that AlN tends to have a smaller half width. Note that the smaller the half width, the better the crystallinity of the film. For this reason, the nitride semiconductor layer side is more preferably C-axis oriented.
Furthermore, AlN is M-axis oriented as a first film on the M-axis oriented nitride semiconductor layer, and the first film has a region in which M-axis orientation and C-axis orientation are mixed on the first film. The uppermost surface of each layer may be C-axis oriented.
Further, the first film is preferably a film containing a hexagonal crystal. In particular, the nitride semiconductor layer side preferably contains C-axis-oriented hexagonal crystal.

  The lattice constant of the first film is preferably different within the range from the material eigenvalue of the first film, which is an Al-containing nitride film, to the material eigenvalue of the nitride semiconductor layer. The material eigenvalue here is a lattice constant as a material eigenvalue of the Al-containing nitride or the nitride semiconductor itself, and generally means a value described in the literature. Examples of this document include “advanced electronics I-1 III-V compound semiconductor” (published by Baifukan Co., Ltd.). Specifically, in AlN, the lattice constant in the C-axis direction is 4.980, the lattice constant in the A-axis direction is 3.111, and in GaN, the lattice constant in the C-axis direction is 5.1666, and the lattice constant in the A-axis direction. Is 3.180. In InGaN, depending on the In mixed crystal ratio, if the In mixed crystal ratio is 5 to 50%, the lattice constant in the C-axis direction is 5.210 to 5.442, and the lattice constant in the A-axis direction is 3. 207 to 3.363.

For example, when the nitride semiconductor layer is GaN and the first film is AlN, the lattice constant in the C-axis direction of AlN of the first film is a lattice within the range of 5.1666 to 4.980 of GaN. The lattice constant in the A-axis direction preferably has a lattice constant in the range of 3.180 for GaN to 3.111 for AlN.
In this case, it is preferable that the lattice constant of the first film decreases from the nitride semiconductor layer side toward the surface, that is, changes from the lattice constant of GaN to the lattice constant of AlN. Thereby, the adhesion between the nitride semiconductor layer and the first film can be improved.

  On the other hand, the lattice constant of the first film may increase from the nitride semiconductor layer side toward the surface, that is, may change so as to approach the lattice constant of GaN from the lattice constant of AlN. Thereby, even if it is a case where the 1st film | membrane is made to grow into a thick film, generation | occurrence | production of the crack of a 1st film | membrane can be suppressed.

Thus, the change in the axial direction or lattice constant in the first film is suitably different in the film thickness direction. However, it does not have to change in all of the film thickness direction, and it is only necessary that a part having a different axial direction or lattice constant from the nitride semiconductor layer side exists in a part of the film thickness direction.
Thereby, the lattice constant with the nitride semiconductor layer can be relaxed, and the occurrence of cracks in the first film can be reliably prevented. That is, on the side in contact with the nitride semiconductor layer in the first film, the orientation is controlled by the lattice constant or orientation of the nitride semiconductor layer and is relatively close to the lattice constant inherent to the material of the nitride semiconductor layer. And having a region in which the orientation of the first film differs as the distance from the nitride semiconductor layer increases, lattice relaxation can be caused and cracks can be suppressed. As a result, the stress on the nitride semiconductor layer can be minimized corresponding to the crystal of the nitride semiconductor layer.

The change in the axial direction or lattice constant in the first film can be confirmed, for example, by the X-ray diffraction method of the first film.
In general, when a film having substantially uniform crystallinity is measured in the film, the graph of ω scan shows a single peak (see, for example, the dotted line and peak D in FIG. 4A). When a plurality of peaks appear, it can be determined that the axial direction or the lattice constant in the first film has changed (see, for example, the solid line and peaks A to C in FIG. 4A).
In the graph of ω / 2θ scan, generally, when a film having almost uniform crystallinity is measured in the film, for example, as shown in FIG. 4C, a single peak (peak P _AlN ) is shown, and the film is present on both sides thereof. A peak having a periodicity indicating the thickness (peak Pb) appears. When a peak other than the peak having periodicity indicating the film thickness is observed (for example, see the peak Pa in FIG. 4B), it is determined that the axial direction or the lattice constant in the first film is changed. Can do.
The first film suitably has a half-value width of the rocking curve measured by the ω scan by such an X-ray diffraction method, for example, 0.5 ° or less. In fact, in FIG. 4A, it has been confirmed that the half width of the rocking carp is 0.5 ° or less. Note that the smaller the half width, the better the crystallinity of the film. By setting the full width at half maximum in such a range, a film with good crystallinity can be obtained and heat dissipation can be improved.

In the X-ray diffraction method, for example, the scan described above can be performed under the following conditions.
X-ray used: CuKα ray (λ = 0.154 nm), monochromator: Ge (220), measurement method: ω scan or ω / 2θ scan, step width: 0.01 °, scan speed: 0.2 sec / step .

In the nitride semiconductor laser device of the present invention, in addition to the first film described above, the first film simply contains Al whose rocking curve measured by the ω scan by the X-ray diffraction method has a plurality of peaks. It is made of a nitride film or a film made of an Al-containing nitride film having a plurality of peaks other than a peak having a periodicity whose rocking curve measured by an ω / 2θ scan by an X-ray diffraction method indicates the film thickness. There may be. Also in such a film, it is suitable that the half width of the rocking curve measured by the ω scan by the X-ray diffraction method is, for example, 0.5 ° or less.
By having such a peak, as described above, the occurrence of cracks in the first film can be reliably prevented. As a result, the stress on the nitride semiconductor layer can be minimized corresponding to the crystal of the nitride semiconductor layer.

The protective film of the present invention preferably has a laminated structure in which the second film is laminated on the first film.
Specifically, as shown in FIGS. 2A to 2E, the first film 15a partially exposes the second film 15b regardless of whether or not the side surface of the ridge 14 is completely covered. The stacked state in which the first film 15a is completely covered with the second film 15b (FIGS. 2C and 2D). ) And the like.

The second film has a composition different from that of the first film and is made of a polycrystalline or amorphous nitride film.
The second film may be formed of any of an insulator, a dielectric, a semiconductor, and a conductor (for example, metal), but is preferably made of a material having good heat dissipation. For example, a material having a thermal conductivity of 10 W / m · K or higher at 0 ° C., preferably 50 W / m · K or higher, and more preferably 100 W / m · K or higher is preferable. _{Specifically, In x Al y Ga 1-} xy N (0 ≦ x, 0 ≦ y, 0 ≦ x + y ≦ 1) can be mentioned, GaN, InGaN, AlGaN and the like are suitable.

In other words, when a protective film with good heat dissipation is formed on the nitride semiconductor layer with good adhesion, it may be difficult to form a thick film as desired due to problems such as the characteristics inherent in the material. Many. On the other hand, the protective film has a good heat dissipation property, in order to keep the insulation between the electrodes and the active layer to a minimum so as not to cause leaks or to absorb light by the electrodes formed thereon. The film thickness for earning the distance is required. In addition, it is intended to have various functions of having resistance to chemical substances such as acid or alkali exposed in the manufacturing process.
For this reason, in the present invention, by selecting a material with good heat dissipation as the first film in direct contact with the nitride semiconductor layer, and forming the second film on the first film, The effective function of the first film can be maximized, and the first film can be supplemented with the various functions described above, and the characteristics of the resulting nitride semiconductor laser can be improved and stabilized.

  The second film may be a film made of polycrystalline or amorphous. Since the second film is a film provided to exhibit or supplement the function of the first film, the second film is formed of a material having a composition different from that of the first film. Therefore, when the second film is formed of a single crystal film, a significant lattice constant difference is caused between the second film and the first film. Therefore, by making the second film a film made of polycrystal or amorphous, the generation of further stress due to the difference in the lattice constant from the first film is avoided, and the generation of cracks and leaks is prevented. can do. Furthermore, dislocations propagated from the nitride semiconductor layer to the first film and dislocations generated in the first film are not exposed to the surface in contact with the electrode by making the second film a film made of polycrystalline or amorphous. In addition, it can be blocked at the interface between the first film and the second film.

The second film is preferably thinner than the first film. Thereby, the stress of the protective film can be effectively suppressed.
For example, the protective film suitably has a total film thickness of about 60 nm to 700 nm, and preferably has a total film thickness of about 200 to 500 nm. By setting the total film thickness within this range, it is possible to ensure insulation, have an optical confinement effect, and effectively prevent light absorption by the electrodes. In addition, peeling and cracking of the protective film due to stress can be suppressed.
The second film is suitably about 3 to 20% of the thickness of the protective film, and more preferably about 4 to 10%.
Specifically, the first film has a thickness of 50 nm or more, preferably 80 nm or more.
The second film has a thickness of about 10 to 100 nm, preferably about 20 to 50 nm.
The second film may have a laminated structure. In this case, any of the material or composition, the crystal system, the film formation method, or the like, or a stacked structure of films having two or more different layers can be used.

For example, in the multilayer film forming the protective film, if the heat dissipation of the first film in contact with the nitride semiconductor layer is poor, the heat generated in the active layer cannot be released, resulting in characteristics and stability of characteristics and It is disadvantageous for life. When the first film is an amorphous film, the adhesion with the nitride semiconductor layer is insufficient, and the protective film is peeled off or the heat transfer is deteriorated. This cannot ensure a long life of the nitride semiconductor laser element. If the protective film has a single crystal single layer structure, the stress between the protective film and the nitride semiconductor increases. Therefore, if an attempt is made to secure a desired film thickness, a crack occurs in the protective film and the nitride semiconductor layer, a leak occurs due to the crack, and the deterioration rate of the nitride semiconductor laser element increases.
On the other hand, these problems can be solved by using the first film as the protective film as in the present invention.

  The protective film can be formed by a method known in the art. For example, evaporation method, sputtering method, reactive sputtering method, ECR plasma sputtering method, magnetron sputtering method, ion beam assisted evaporation method, ion plating method, laser ablation method, CVD method, spray method, spin coating method, dip method or Various methods such as a method of combining two or more of these methods or a method of combining these methods and oxidation treatment (heat treatment) can be used.

In particular, the ECR plasma sputtering method is suitable for obtaining a film having crystallinity. As the film forming conditions, for example, when the microwave power is 300 to 800 W, the RF power is 300 to 800 W, and the argon flow rate is 10 to 40 sccm, the nitrogen flow rate is suitably 5 to 20 sccm.
In addition, it is suitable to perform the heat treatment at any stage such as during and / or after the formation of the protective film, that is, the first film, and / or after the formation of the second film. As for heat processing, the temperature of 150 degreeC or more is mentioned, for example.

  The protective film is generally formed of an insulating material having a refractive index smaller than that of the nitride semiconductor layer. Specifically, those having a refractive index of 2.4 or less are suitable. The refractive index is a spectroscopic ellipsometer using ellipsometry. A. It can be measured using HS-190 manufactured by WOOLLAM.

In the nitride semiconductor laser element of the present invention, for example, as shown in FIG. 1, the p-side electrode 16 and the n-side electrode 19 are formed on different principal surface sides with respect to the substrate 10. On the first main surface of the substrate 10, an n-side semiconductor layer 11, an active layer 12, and a p-side semiconductor layer 13 are formed in this order as nitride semiconductor layers. A ridge 14 is formed on the surface of the nitride semiconductor, and the protective film 15 described above is formed in a two-layer structure of the first film 15a and the second film 15b from the side surface of the ridge 14 to the surface of the nitride semiconductor layer. ing.
A p-side electrode 16 is formed on the upper surface of the ridge 14. An n-side electrode 19 is formed on the second main surface side of the substrate 10.
Further, a protective film 15 is formed continuously from the side surface of the ridge 14 to protect the nitride semiconductor layer from the side surface to the upper surface of the nitride semiconductor layer. A p-pad electrode 18 is formed on the upper surfaces of the protective film 15 and the p-side electrode 16.

In the present invention, the substrate 10 is suitably a conductive substrate. The substrate is preferably a nitride semiconductor substrate having an off angle of greater than 0 ° and less than or equal to 10 ° on the first main surface and / or the second main surface, for example. As for the film thickness, 50 micrometers or more and 10 mm or less are mentioned, for example. For example, known substrates such as various substrates exemplified in JP 2006-24703 A, commercially available substrates, and the like may be used.
The nitride semiconductor substrate can be formed by vapor phase growth methods such as MOCVD method, HVPE method, MBE method, hydrothermal synthesis method for crystal growth in a supercritical fluid, high pressure method, flux method, melting method and the like.

As the nitride semiconductor layer, the general formula _{In x Al y Ga 1-xy} N (0 ≦ x ≦ 1,0 ≦ y ≦ 1,0 ≦ x + y ≦ 1) can be used including. In addition to this, a group III element partially substituted with B may be used, or a group V element partially substituted with P and As may be used. The n-side semiconductor layer may contain any one or more of IV group elements or VI group elements such as Si, Ge, Sn, S, O, Ti, Zr, and Cd as n-type impurities. Further, the p-side semiconductor layer may contain Mg, Zn, Be, Mn, Ca, Sr, etc. as p-type impurities. The impurities are preferably contained in a concentration range of, for example, about 5 × 10 ¹⁶ / cm ³ to 1 × 10 ²¹ / cm ³ .

The active layer may have either a multiple quantum well structure or a single quantum well structure.
The nitride semiconductor layer has a light guide layer that constitutes an optical waveguide in the n-side semiconductor layer and the p-side semiconductor layer, so that an SCH (Separate Confinement Heterostructure) structure that is a separated light confinement structure with an active layer sandwiched therebetween. It is preferable that However, the present invention is not limited to these structures.

  The growth method of the nitride semiconductor layer is not particularly limited, but MOVPE (metal organic chemical vapor deposition), MOCVD (metal organic chemical vapor deposition), HVPE (hydride vapor deposition), MBE (molecular beam epitaxy). All methods known as nitride semiconductor growth methods can be suitably used. In particular, MOCVD or MBE is preferable because it can be grown with good crystallinity.

  A ridge is formed on the surface of the nitride semiconductor layer, that is, the p-side semiconductor layer. The ridge functions as a waveguide region, and the width is preferably about 1.0 μm to 30.0 μm, and more preferably about 1.0 μm to 3.0 μm. The height (etching depth) can be adjusted as appropriate, such as the film thickness, material, and the like of the p-side semiconductor layer, and the degree of light confinement, for example, 0.1 to 2 μm. . The ridge is preferably set so that the length in the resonator direction is about 100 μm to 2000 μm. Further, they may not all have the same width in the resonator direction, and the side surfaces thereof may be vertical or tapered. The taper angle in this case is suitably about 45 ° to 90 °.

  The ridge can be formed by a method usually used in this field. For example, photolithography and an etching process are mentioned. The etching at this time may be either dry etching (for example, RIE method) or wet etching, or both may be performed in this order or in the reverse order. In particular, it is preferable to dry-etch the surface of the nitride semiconductor and then perform wet etching. By forming such a ridge, a protective film composed of the first film and the second film described above can be formed.

  The electrodes in the present invention refer to a pair of electrodes electrically connected to the p and n side nitride semiconductor layers. The p-side electrode is preferably formed on the nitride semiconductor layer and the protective film. Since the electrode is continuously formed on the uppermost nitride semiconductor layer and the first film of the protective film, peeling of the first film can be prevented. In particular, if the p-side electrode is formed up to the surface of the protective film formed on the side surface of the ridge, it is possible to effectively prevent the protective film from peeling off.

The electrode is formed of a single layer film or a laminated film of a metal or an alloy such as palladium, platinum, nickel, gold, titanium, tungsten, copper, silver, zinc, tin, indium, aluminum, iridium, rhodium, and ITO. Can do. Specifically, Ni / Pd / Pt / Au, Ti / Pd / Pt / Au, etc. are illustrated. The film thickness of an electrode can be suitably adjusted with the material etc. to be used, for example, about 50-500 nm is suitable.
The electrodes only need to be formed on at least the p-side and n-side semiconductor layers or the substrate, respectively, and one or more conductive layers such as pad electrodes may be formed on the electrodes.

  Further, it is preferable that a second protective film is formed on the protective film. Such a second protective film may be disposed on the protective film at least on the surface of the nitride semiconductor layer, and may be disposed on the side surface of the nitride semiconductor layer and / or the side surface of the substrate with or without the protective film interposed therebetween. Or it is preferable that the surface etc. are further coat | covered. The second protective film can be formed of the same material as that exemplified for the protective film. Thereby, not only the insulating property but also the exposed side surface or surface of the nitride semiconductor layer can be reliably protected.

In addition, it is suitable that a reflection mirror is formed on the end face of the resonator. The reflection mirror can be formed of an oxide film, a nitride film, an oxynitride film, a combination thereof, or the like. Specifically, it is a dielectric multilayer film made of an oxide film such as SiO ₂ , ZrO ₂ , TiO ₂ , Al ₂ O ₃ , Nb ₂ O ₅ , SiN, AlN, SiON, AlON, or a nitride film. The reflection mirror is preferably formed on the light reflection side and / or the light emission surface of the resonance surface, and more preferably on the light reflection side and the light emission surface of the resonance surface. If the resonance surface is formed by cleavage, the reflection mirror can be formed with good reproducibility.

Hereinafter, embodiments of the nitride semiconductor laser device of the present invention will be described in detail with reference to the drawings.
Example 1
In the nitride semiconductor laser device of this embodiment, as shown in FIG. 1A, an n-side semiconductor layer 11, an active layer 12, and a p-side semiconductor layer 13 are formed in this order on a substrate 10 as a nitride semiconductor layer. A ridge 14 is formed on the surface of the p-side semiconductor layer 13. A protective film 15 made of a first film 15a (an AlN film having a portion with different crystallinity in the film) and a second film 15b (polycrystalline or amorphous GaN) extends from the side surface of the ridge 14 to the surface of the nitride semiconductor layer. Is formed.

  A p-side electrode 16 is formed on the upper surface of the ridge 14. An n-side electrode 19 is formed on the second main surface side of the substrate 10. A p-pad electrode 18 is formed on the upper surfaces of the protective film 15, the p-side electrode 16 and the protective film 17.

This laser element can be manufactured by the following method.
(substrate)
First, a GaN substrate containing n-type impurities is prepared. The first main surface of the GaN substrate is set as a growth surface in the MOCVD reaction vessel.

(N-side semiconductor layer 11)
The growth temperature is raised to 1050 ° C., trimethyl gallium (TMG), TMA (trimethyl aluminum), ammonia (NH ₃ ) are used as source gases, silane gas is used as impurity gas, and Si is doped at 1 × 10 ¹⁹ / cm ³ Al _0.03 Ga _0.97 N is grown to a thickness of 2.5 μm. This layer is an n-side cladding layer.

  Subsequently, TMG and ammonia are used as source gases, and an n-side light guide layer made of undoped GaN is grown at a thickness of 170 nm at a temperature of 1050 ° C.

(Active layer 12)
A barrier layer made of In _0.02 Ga _0.98 N doped with 5 × 10 ¹⁸ / cm ^{3 of} Si using trimethylindium (TMI), TMG, ammonia, and silane gas as a source gas at a temperature of 900 ° C. with a film thickness of 14 nm Grow. Subsequently, the temperature is lowered to 820 ° C., the silane gas is stopped, and a well layer made of undoped In _0.07 Ga _0.93 N is grown to a thickness of 8 nm.
The three-layer structure of the barrier layer, the well layer, and the intermediate layer is laminated twice more, and finally the barrier layer is formed to grow the active layer 12 composed of multiple quantum wells (MQW) having a total film thickness of 58 nm. .

(P-side semiconductor layer 13)
Stop the TMI, flow TMA and Cp ₂ Mg, and grow a p-side cap layer made of Al _0.25 Ga _0.75 N doped with 1 × 10 ²⁰ / cm ^{3 of} Mg to a thickness of 10 nm. Subsequently, Cp ₂ Mg and TMA are stopped, and a p-side light guide layer made of undoped GaN is grown to a thickness of 0.15 μm at 1000 ° C. This p-side light guide layer is grown as undoped, but due to the diffusion of Mg from the p-side cap layer, the Mg concentration becomes 5 × 10 ¹⁶ / cm ³ , indicating the p-side.

Cp ₂ Mg is stopped, TMA is flown, and a layer made of undoped Al _0.1 Ga _0.9 N is grown at a thickness of 2.5 nm at 1000 ° C. The TMA is stopped, Cp ₂ Mg is allowed to flow, and a layer made of undoped GaN having an Mg concentration of 1 × 10 ¹⁹ / cm ³ is grown to a thickness of 2.5 nm. By repeating this, a p-side cladding layer made of a superlattice layer having a total film thickness of 0.45 μm is grown.
Finally, a p-side contact layer made of p-GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown on the p-side cladding layer to a thickness of 15 nm.

(Formation of ridge 14)
The obtained nitride semiconductor-grown wafer is taken out of the reaction vessel, and a protective layer made of stripe-shaped SiO ₂ having a width of 2.0 μm is formed on the surface of the uppermost p-side contact layer through a mask having a predetermined shape. A film is formed. Then, using RIE (reactive ion etching), etching was performed to the vicinity of the interface between the p-side cladding layer and the p-side light guide layer, and an acidic solution (for example, a mixed solution of phosphoric acid and sulfuric acid) was arbitrarily used. Wet etching (surface treatment) is performed to form a stripe-shaped ridge 14 having a width of 2.0 μm.

(Formation of protective film 15)
Subsequently, with the SiO ₂ mask on, the surface of the nitride semiconductor layer is subjected to nitrogen power in an argon atmosphere using an ECR plasma sputtering apparatus using an Al target with a microwave power of 500 W, an RF power of 250 W, and an argon atmosphere. At room temperature, an AlN film of 150 nm is formed as the first film 15a. Subsequently, the temperature is raised to 400 ° C., and an AlN film of 50 nm is formed under the same conditions.

In order to verify the protective film of the obtained nitride semiconductor laser device, on the n-GaN substrate (c-axis orientation: C plane), using the same material and substantially the same film formation method as above, First, an AlN film is formed on a pretreated GaN substrate using an ECR sputtering apparatus while flowing nitrogen in an argon atmosphere under conditions of microwave power of 500 W and RF power of 250 W at room temperature, and then The AlN film was formed to a thickness of 50 nm (total film thickness 200 nm) while performing the heat treatment under the same conditions.
For comparison, an AlN film having a thickness of 100 nm was formed under the same conditions except that the film was formed at room temperature.
About the obtained protective film, an XRD apparatus (used X-ray: CuKα ray (λ = 0.154 nm), monochromator: Ge (220), measurement method: ω scan or ω / 2θ scan, step) (Width: 0.01 °, scan speed: 0.2 sec / step). In this apparatus, the vicinity of 18 ° corresponds to a peak derived from AlN having C-axis orientation.
The results are shown in FIGS. 4A to 4C. 4A shows the results of the ω scan, and FIGS. 4B and 4C show the results of the ω / 2θ scan.

When an AlN film having a thickness of 200 nm is formed, as shown in FIG. 4A, which is the ω scan result, one peak appears in the vicinity of the peak derived from AlN having C-axis orientation (solid line, peak A in FIG. 4A). In addition, a plurality of peaks (solid lines, peaks B and C in FIG. 4A) indicating that the axial direction or the lattice constant in the first film was changed were recognized.
Further, as shown in FIG. 4B which is the ω / 2θ scan result, a plurality of peaks other than the peak having periodicity indicating the film thickness are formed on one side in the vicinity of the peak derived from AlN having C-axis orientation (P _AlN ). A peak (Pa) was observed.
This confirmed that AlN was formed in the film having two or more sites having different crystallinity. In other words, by forming the AlN film while performing the heat treatment, the crystallinity of the region adjacent to the nitride semiconductor layer and the region having different crystallinity can coexist.

On the other hand, as shown in FIG. 4A, when an AlN film was formed to a thickness of 100 nm, one peak (dotted line in FIG. 4A, peak D) was observed near the peak derived from AlN having C-axis orientation. As shown in FIG. 4C, which is the ω / 2θ scan result, peaks (Pb) having periodicity indicating the film thickness are recognized on both sides of the vicinity of the peak derived from AlN having C-axis orientation (P _AlN ). It was only.
Note that, as described above, in the protective film for verifying the protective film of the nitride semiconductor laser element, the orientation was confirmed from the ω scan or ω / 2θ scan results. It was confirmed to be a crystal.

Subsequently, as the second film 15b, a GaN film having a thickness of 20 nm is formed at room temperature by using a GaN target and flowing nitrogen in an argon atmosphere using a GaN target in a magnetron sputtering apparatus.
It was found that the obtained GaN film was formed in a polycrystalline state.

Thereafter, it is immersed in buffered hydrofluoric acid to dissolve and remove the SiO ₂ formed on the p-side contact layer, and together with the GaN film as the second film 15b by the lift-off method, the first film 15a on the p-side contact layer. The AlN film is removed.

(Formation of electrodes and pad electrodes)
A p-side electrode 16 formed by sequentially forming Ni / Au / Pt on the ridge outermost surface of the p-side contact layer is formed in a stripe shape. A p-side pad electrode 18 electrically connected to the p-side electrode 16 through the protective film 15 is formed. On the other hand, an n-side electrode 19 formed by sequentially forming Ti / Pt / Au is formed on the second main surface side of the substrate 10.

(Split)
Thereafter, the substrate 10 is cleaved into a bar shape, and a resonance surface is formed on the cleavage surface of the bar. After producing the resonance surface, the bar-shaped wafer is further cut in a direction perpendicular to the resonance surface to obtain a laser chip.

(Evaluation)
When the obtained semiconductor laser is mounted face-down, the protective film made of the first film and the second film is made of SiO ₂ film (thickness: 200 nm), compared with a semiconductor laser manufactured by the same configuration, Thermal resistance can be reduced by about 0.0 ° C./W.
That is, by forming the first film as described above, lattice relaxation can be generated and cracks can be suppressed in the protective film. As a result, while having good adhesion to the nitride semiconductor layer, it is possible to prevent the protective film from peeling off and to form a thick film, thereby improving heat dissipation.

Example 2
The nitride semiconductor laser device of this example has the same configuration as that of Example 1 except that the stacked state of the n-side semiconductor layer is different.
This nitride semiconductor laser device can be manufactured as follows.
(substrate)
First, a GaN substrate containing n-type impurities is prepared. The first main surface of the GaN substrate is set as a growth surface in the MOCVD reaction vessel.
(N-side semiconductor layer 11)
The growth temperature is raised to 1050 ° C., trimethyl gallium (TMG), TMA (trimethyl aluminum), ammonia (NH ₃ ) are used as source gases, silane gas is used as impurity gas, and Si is doped at 3 × 10 ¹⁸ / cm ³ Al _0.02 Ga _0.98 N is grown to a thickness of 1.0 μm. This layer is an n-side contact layer.
Thereafter, an In _y Ga _1-y N (0 <y ≦ 1) layer doped with 3 × 10 ¹⁸ / cm ^{3 of} Si using trimethylindium (TMI), TMG, ammonia, and silane gas as an impurity gas as a source gas. Growth is performed with a film thickness of 150 nm. This layer is an n-side crack prevention layer.
A film having a growth temperature of 1000 ° C., TMG, TMA, NH ₃ as source gas, silane gas as impurity gas, and Al _0.053 Ga _0.947 N doped with Si 3 × 10 ¹⁸ / cm ³ is 2.0 μm Grow with thickness. This layer is an n-side cladding layer.
Subsequently, TMG and ammonia are used as source gases, and an n-side light guide layer made of undoped GaN is grown to a thickness of 300 nm.

(Active layer 12)
A temperature of 920 ° C. is used, and a first barrier layer made of In _0.02 Ga _0.98 N doped with Si at 5 × 10 ¹⁸ / cm ³ is grown to a film thickness of 140 nm using TMI, TMG, ammonia, and silane gas as source gases. . Subsequently, the temperature is lowered to 850 ° C., the silane gas is stopped, and a well layer made of undoped In _0.13 Ga _0.87 N is grown to a thickness of 3 nm. Further, the temperature is raised to 920 ° C., TMI is stopped, and a second barrier layer made of undoped GaN is grown to a thickness of 14 nm. The temperature is lowered again to 850 ° C., TMI is flown to deposit a well layer, and finally the temperature is raised to 920 ° C. to grow a final barrier layer made of undoped In _0.13 Ga _0.87 N with a thickness of 70 nm.

(P-side semiconductor layer 13)
TMI is stopped, TMA and Cp ₂ Mg are allowed to flow, and a p-side cap layer made of Al _0.20 Ga _0.80 N doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown to a thickness of 10 nm. Subsequently, Cp ₂ Mg is stopped, and a p-side light guide layer made of undoped Al _0.02 Ga _0.98 N is grown at a film thickness of 0.3 μm at 1000 ° C. This p-side light guide layer is grown as undoped, but due to diffusion of Mg from the p-side cap layer, the Mg concentration becomes 5 × 10 ¹⁶ / cm ³ and exhibits p-type.
Flowing TMA, growing a layer made of undoped Al _0.1 Ga _0.9 N to a thickness of 2.5 nm at 1000 ° C., stopping TMA, flowing Cp ₂ Mg, undoped with Mg concentration of 1 × 10 ¹⁹ / cm ³ A layer made of GaN is grown to a thickness of 2.5 nm, and a p-side cladding layer made of a superlattice layer having a total thickness of 0.45 μm is grown.
Finally, a p-side contact layer made of p-GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown on the p-side cladding layer to a thickness of 15 nm.
Thereafter, a nitride semiconductor laser element is manufactured in the same manner as in the first embodiment.
By mounting the obtained nitride semiconductor laser face down in the same manner as in the first embodiment, the thermal resistance can be reduced in the same manner as in the first embodiment.

Example 3
In the nitride semiconductor laser device of this embodiment, as shown in FIG. 1, an AlN film having a portion with different crystallinity in the film is formed as the first film 15a, and a protective film made of amorphous GaN is formed as the second film 15b. However, the structure is substantially the same as that of the semiconductor laser device of Example 1 except that the first film 15a is formed under the following film forming conditions.
In an ECR plasma sputtering apparatus, using an Al target, an AlN film of 50 nm is formed as the first film 15a at room temperature while flowing nitrogen in a microwave power of 500 W, an RF power of 250 W, and an argon atmosphere. Subsequently, an AlN film having a thickness of 150 nm is formed under the same conditions at a temperature of 200.degree. Subsequently, an AlN film is formed to a thickness of 50 nm under the same conditions at a temperature of 400 ° C.
The first film 15a formed in this manner has two or more portions having different orientations in the film, as in the first embodiment, and the same effect as in the first embodiment can be obtained.

Example 4
In the nitride semiconductor laser device of this embodiment, as shown in FIG. 1, an AlN film having a portion with different orientation in the film as the first film 15a and a protective film made of amorphous GaN are formed as the second film 15b. However, the structure is substantially the same as that of the semiconductor laser device of Example 1 except that the first film 15a is formed under the following film forming conditions.
In an ECR plasma sputtering apparatus, using an Al target, an AlN film having a thickness of 150 nm is formed as the first film 15a at room temperature while flowing nitrogen in an atmosphere of microwave power 350W, RF power 350W, and argon flow rate. Subsequently, an AlN film is formed to a thickness of 50 nm while flowing nitrogen at a temperature of 400 ° C. in a microwave power of 500 W, an RF power of 250 W, and an argon atmosphere.
The first film 15a formed in this manner has two or more portions having different orientations in the film, as in the first embodiment, and the same effect as in the first embodiment can be obtained.

Example 5
In the nitride semiconductor laser device of this embodiment, as shown in FIG. 1, an AlN film having a different orientation in the film is formed as the first film 15a, and the second film 15b is made of amorphous AlGaN. The structure is substantially the same as that of the semiconductor laser device of Example 1 except that a protective film is formed.
The first film 15a formed in this manner has two or more portions having different orientations in the film, as in the first embodiment, and the same effect as in the first embodiment can be obtained.

  The nitride semiconductor laser of the present invention can be widely used, for example, for illumination light sources, optical disc applications, optical communication systems, displays, printers, exposure applications, measurement, bio-related excitation light sources, and the like.

10 substrate 11 n-side semiconductor layer 12 active layer 13 p-side semiconductor layer 14 ridge 15 protective film 15a first film 15b second film 16 p-side electrode 17 protective electrode 18 p-pad electrode 19 n-side electrode

Claims (10)

A nitride semiconductor laser device comprising a substrate, a nitride semiconductor layer stacked on the substrate and having a ridge on the surface, and an electrode electrically connected to the nitride semiconductor layer,
A first film made of an Al-containing nitride film having two or more portions having different crystallinity in the film thickness direction is provided as a protective film on the surface of the nitride semiconductor layer on both sides of the ridge from the side surface of the ridge. Nitride semiconductor laser device.   2. The nitride semiconductor laser device according to claim 1, wherein the protective film includes a second film made of a nitride film having a composition different from that of the first film on the first film.   3. The nitride semiconductor laser element according to claim 1, wherein the first film is coaxially oriented with the nitride semiconductor layer on the nitride semiconductor layer side and has a portion having different crystallinity in the film thickness direction.   4. The nitride semiconductor laser element according to claim 1, wherein the first film has a C-axis orientation on the nitride semiconductor layer side and a portion having different crystallinity exists in the film thickness direction. 5.   The nitride semiconductor laser element according to claim 1, wherein the first film includes a hexagonal crystal. A second _{layer, In x Al y Ga 1-} xy N (0 ≦ x, 0 ≦ y, 0 ≦ x + y ≦ 1) a nitride semiconductor laser device according to claims 2 to any one of 5 is.   The nitride semiconductor laser element according to claim 1, wherein the first film contains a single crystal on the nitride semiconductor layer side. A nitride semiconductor laser device comprising a substrate, a nitride semiconductor layer stacked on the substrate and having a ridge on the surface, and an electrode electrically connected to the nitride semiconductor layer,
A protective film is provided on the surface of the nitride semiconductor layer on both sides of the ridge from the side surface of the ridge as a protective film with an Al-containing nitride film having a plurality of rocking curves measured by ω-scanning by an X-ray diffraction method. A nitride semiconductor laser device characterized by comprising: A nitride semiconductor laser device comprising a substrate, a nitride semiconductor layer stacked on the substrate and having a ridge on the surface, and an electrode electrically connected to the nitride semiconductor layer,
Al content having a plurality of peaks other than a peak having a periodicity in which a rocking curve measured by a ω / 2θ scan by an X-ray diffraction method is present on the surface of the nitride semiconductor layer on both sides of the ridge from the side surface of the ridge A nitride semiconductor laser device comprising a first film made of a nitride film as a protective film.   10. The nitride semiconductor laser device according to claim 1, wherein the first film has a half-value width of a rocking curve measured by an ω scan by an X-ray diffraction method of 0.5 ° or less. 11.

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