Patent Description:
The present disclosure relates to a nitride semiconductor laser element.

In recent years, a wide variety of light emitting devices used in lighting or the like by utilizing the emitted excitation light from a semiconductor laser provided as an excitation light source have been proposed.

<CIT> proposes for such a light emitting device a nitride-based semiconductor laser element in which the detachment of the protective film formed on the resonator faces is suppressed.

Besides, <CIT> discloses a light absorption film which is formed on the outermost surface of an end surface on the light emitting side of a chip used in a laser device, typically, a laser chip and which absorbs part of the light emitted.

The invention is defined by the independent claim.

On the other hand, for a high output semiconductor laser, in particular, catastrophic optical damage (COD) is one of the limiting factors for increasing the output. In other words, there is a desire for a higher output high-performance semiconductor laser element with less catastrophic optical damage.

According to an embodiment of the present disclosure, a nitride semiconductor laser element with less catastrophic optical damage can be provided.

Certain embodiments of the present disclosure will be explained below with reference to the accompanying drawings. The embodiments described below are illustrations provided for the purpose of giving shape to the technical ideas of the present invention, and the present invention is not limited to the embodiments described below. In the explanation below, the same designations and reference numerals denote identical members or similar member, for which redundant explanation will be omitted as appropriate.

A nitride semiconductor laser element (hereinafter, occasionally referred to as a semiconductor laser element) according to one embodiment is shown in <FIG>. <FIG> is a perspective view of the nitride semiconductor laser element according to the embodiment. <FIG> is a front view of the nitride semiconductor laser element in <FIG>. <FIG> is a cross-sectional view taken along line IC-IC in <FIG>. In <FIG> and <FIG>, the protective film <NUM> and the protective film <NUM> are not shown.

The nitride semiconductor laser element <NUM> is an edge-emitting laser element. The nitride semiconductor laser element <NUM> has a first nitride semiconductor layer <NUM>, a second nitride semiconductor layer <NUM>, an active layer <NUM> disposed between the first nitride semiconductor layer <NUM> and the second nitride semiconductor layer <NUM>, and a protective film <NUM>. The first nitride semiconductor layer <NUM>, the active layer <NUM>, and the second nitride semiconductor layer <NUM> have a light-emission-side end face <NUM> and a light-reflection-side end face <NUM> as the faces that intersect the face of the active layer <NUM> on the second nitride semiconductor layer <NUM> side. The stack body which includes the first nitride semiconductor layer <NUM>, the active layer <NUM>, and the second nitride semiconductor layer <NUM> may occasionally be referred to as a nitride semiconductor stack body.

A protective film <NUM> is disposed on the light-emission-side end face <NUM>. The protective film <NUM> includes, successively from the light-emission-side end face <NUM> side, a first film <NUM> containing oxygen and aluminum and/or gallium, a second film <NUM> formed of nitride, and a third film <NUM> containing aluminum and oxygen. The first film <NUM> and the second film <NUM> are crystalline films.

With such a structure, the occurrence of catastrophic optical damage in the nitride semiconductor laser element <NUM> can be reduced. The possible reasons for this effect are as follows. First, because the first film <NUM> in the protective film <NUM> is a film containing oxygen, resistance during operation is less likely to be changed, i.e., the resistance is less likely to be reduced. Furthermore, the third film <NUM> containing oxygen makes it less likely to be oxidized. Moreover, because the nitride second film <NUM> is interposed between the first film <NUM> and the third film <NUM>, the second film <NUM> is less likely to react with ambient oxygen to oxidize and expand. The first film <NUM> being a crystalline film facilitates the formation of a crystalline second film <NUM>, and makes the oxidation reaction between the first film <NUM> and the end faces of the nitride semiconductor stack body unlikely. Moreover, the crystalline second film <NUM> can function as an oxygen barrier layer, thereby making the light-emission-side end face <NUM> less likely to be oxidized by the oxygen from the third film <NUM> or the outside of the nitride semiconductor laser element <NUM>. As a result of these, the occurrence of catastrophic optical damage (COD) is believed to be effectively reduced. Reducing the occurrence of catastrophic optical damage can extend the service life of the nitride semiconductor laser element <NUM>. Such an effect becomes prominent particularly in a high output nitride semiconductor laser element <NUM>. A high output nitride semiconductor laser element <NUM> is an element having <NUM> MW/cm<NUM> or higher light density, for example. The light density of the nitride semiconductor laser element <NUM> may be <NUM> MW/cm<NUM> or lower. A high output nitride semiconductor laser element <NUM> is an element outputting <NUM> W or higher, for example, in the case of a multi-transverse mode, and may be an element outputting <NUM> W or higher. A high output nitride semiconductor laser element <NUM> is an element outputting <NUM> W or higher, for example, in the case of a single transverse mode. The output of the nitride semiconductor laser element <NUM> may be <NUM> W or lower.

A first nitride semiconductor layer <NUM>, an active layer <NUM>, and a second nitride semiconductor layer <NUM> are stacked in that order. A nitride semiconductor stack body which includes these semiconductor layers can be formed on a substrate <NUM>.

The first nitride semiconductor layer <NUM> is of a first conductivity type, and the second nitride semiconductor layer <NUM> is of a second conductivity type. The first conductivity type may be n-type or p-type. The second conductivity type means a different conductivity type form the first conductivity type. The first nitride semiconductor layer <NUM>, the active layer <NUM>, and the second nitride semiconductor layer <NUM> can be formed with semiconductor layers made of InxAlyGa<NUM>-x-yN (<NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, <NUM>≤x+y≤<NUM>). The first nitride semiconductor layer <NUM> and the second nitride semiconductor layer <NUM> may contain one or more n-type impurities, such as Si, Ge, and the like. The first nitride semiconductor layer <NUM> and the second nitride semiconductor layer <NUM> may contain one or more p-type impurities, such as Mg, Zn, and the like. The impurity content can be set, for example, in a range of <NUM>×<NUM><NUM>/cm<NUM> to <NUM>×<NUM><NUM>/cm<NUM>. The first nitride semiconductor layer <NUM> and the second nitride semiconductor layer <NUM> may include an undoped layer. An undoped layer refers to a layer to which no n-type or p-type impurity is intentionally added. An undoped layer may have an impurity concentration below the detectable limit in a secondary ion mass spectroscopy (SIMS) analysis or the like, or an impurity concentration lower than <NUM>×<NUM><NUM>/cm<NUM>. The first nitride semiconductor layer <NUM> and the second nitride semiconductor layer <NUM> may each have one semiconductor layer only, but preferably have two or more layers.

The peak oscillation wavelength of the semiconductor laser formed by the first nitride semiconductor layer <NUM>, the active layer <NUM>, and the second nitride semiconductor layer <NUM> is <NUM> to <NUM>, for example.

A first nitride semiconductor layer <NUM> can have a multilayer structure composed of nitride semiconductors, such as GaN, InGaN, AlGaN, or the like. The first nitride semiconductor layer <NUM>, if formed on a substrate <NUM>, can have an underlayer, a clad layer, an intermediate layer, a light guide layer, or the like. It may include a semiconductor layer other than these.

An active layer <NUM> has a quantum well structure. The active layer <NUM> may have a multiple quantum well structure or a single quantum well structure.

The active layer <NUM> can have a multilayer structure composed of nitride semiconductors, such as GaN, InGaN, or the like. In the case where the active layer <NUM> has a multiple quantum well structure, it can have, successively from the first nitride semiconductor layer <NUM> side, a well layer, an intermediate barrier layer, and a well layer. It may have a plurality of well layers and a plurality of intermediate barrier layers. A well layer is, for example, an InGaN layer. An intermediate barrier layer is, for example, an InGaN layer or a GaN Layer. The active layer <NUM> is, for example, an undoped layer.

The composition of the active layer <NUM> can be suitably adjusted according to the oscillation wavelength of the nitride semiconductor laser element to be obtained. For example, the well layers can be InxGa<NUM>-xN layers or the like. The composition ratio x can be selected in a range of the <NUM> to <NUM>. This can set the peak oscillation wavelength of the semiconductor laser in a range of the <NUM> to <NUM>. The well layers may be GaN or AlGaN layers.

A second nitride semiconductor layer <NUM> can have a multilayer structure composed of nitride semiconductors, such as GaN, InGaN, AlGaN, or the like. Examples of the second nitride semiconductor layer <NUM> include those having a clad later, a light guide layer, or the like. It may include a semiconductor layer other than these.

As long as an edge-emitting laser element can be constructed, the second nitride semiconductor layer <NUM> may have a ridge 12a on the surface, i.e., the upper face, or a current constriction layer known in the art formed in the second nitride semiconductor layer <NUM>. For example, the second nitride semiconductor layer <NUM> is a p-type semiconductor layer, and has a ridge 12a formed on the upper face. The second nitride semiconductor layer <NUM> may be an n-type semiconductor layer.

The ridge 12a functions as an optical waveguide region, and has a width of <NUM> to <NUM>, for example. The width of the ridge 12a can be set to <NUM> to <NUM> in the case of driving a nitride semiconductor laser element <NUM> as a high output element. The height of the ridge is <NUM> to <NUM>, for example. By adjusting the thicknesses of and the materials for the layers constituting the second nitride semiconductor layer <NUM>, the extent of the light confinement effect can be suitably adjusted. The ridge 12a can be <NUM> to <NUM> µm in length in the direction of the oscillator. The ridge 12a does not have to have a constant width across the length in the direction of the oscillator, and may have perpendicular or tapered lateral faces. The taper angle in this case, for example, is <NUM> degrees or greater but smaller than <NUM> degrees.

The crystal planes of the layer forming surfaces of the first nitride semiconductor layer <NUM>, the active layer <NUM>, and the second nitride semiconductor layer <NUM> are not particularly limited, and may be C-plane {<NUM>}, M-plane {<NUM>-<NUM>}, A-plane {<NUM>-<NUM>}, R-plane {<NUM>-<NUM>}, or any other plane. For example, the upper face of the second nitride semiconductor layer <NUM> is C-plane. According to the crystallography notation in expressing crystal planes and directions, an overbar is applied to <NUM>, for example, to represent the inverse direction of <NUM>, but is expressed as "-<NUM>" as a matter of convenience. Coordinates in square brackets [ ] denote an individual orientation, coordinates in angle brackets < > denote a family of orientations, indices in parentheses ( ) denote an individual plane, and indices in curly brackets { } denote a family of planes.

A light-emission-side end face <NUM> and a light-reflection-side end face <NUM> are the semiconductor layer end faces which respectively include at least the light emission face 14a, which is the optical waveguide region of the active layer <NUM> or the region corresponding to or the NFP (near field pattern), and the light reflecting face at the other end. The light-emission-side end face <NUM> and the light-reflection-side end face <NUM> are faces defined by the layer forming faces (the X-Y planes in <FIG>) of the first nitride semiconductor layer <NUM>, the active layer <NUM>, and the second nitride semiconductor layer <NUM>. The light-emission-side end face <NUM> and the light-reflection-side end face <NUM> may be oblique to the stacking direction of the semiconductor layers (the arrow Z direction in <FIG>), but are preferably in parallel therewith. They are preferably perpendicular to the semiconductor layer forming faces (the X-Y planes in <FIG>). The light-emission-side end face <NUM> and the light-reflection-side end face <NUM> are at a position opposite to one another, and are preferably in parallel with one another. A resonator is formed between the light-emission-side end face <NUM> and the light-reflection-side end face <NUM>. Being "parallel" here includes a variance of up to ±<NUM>°. Being "perpendicular" here includes a variance of up to ±<NUM>°.

The light-emission-side end face <NUM> and the light-reflection-side end face <NUM> may be M-plane {<NUM>-<NUM>}, A-plane {<NUM>-<NUM>}, C-plane {<NUM>}, R-plane {<NUM>-<NUM>}, or other planes. For example, the light-emission-side end face <NUM> is M-plane. In the case where the light-emission-side end face <NUM> and the light-reflection-side end face <NUM> are M-planes, these faces can be obtained by cleaving. Cleaving can be accomplished, for example, by forming a recess by laser processing followed by pressing. The laser scanning direction may be the same as or different from the cleaving direction. The light-emission-side end face <NUM> does not have to be M-plane in the strict sense.

A protective film (a first protective film) on the light-emission-side end face <NUM> includes, successively from the light-emission-side end face <NUM> side, a first film <NUM> containing oxygen and aluminum and/or gallium, a nitride second film <NUM>, and a third film <NUM> containing aluminum and oxygen. The first film <NUM> and the second film <NUM> are crystalline films.

The protective film <NUM> disposed on the light-emission-side end face <NUM> covers the face of the resonator formed by the semiconductor layers, but does not necessarily have to cover the entire light-emission-side end face <NUM>. The protective film <NUM> covers at least the optical waveguide region of the resonator face or the region corresponding to the NFP, i.e., the light emitting face 14a that is the end face region including the active layer <NUM> and some of the upper and lower layers thereof. The protective film <NUM> may cover the light-emission-side end face <NUM> in its entirety. A portion of the protective film <NUM> may be disposed on other faces beside the resonator face, for example, the upper face and lateral faces of the semiconductor layers. The first film <NUM>, the second film <NUM>, and the third film <NUM> that configure the protective film <NUM> are each formed with a light transmissive material with respect to the oscillation wavelength of the nitride semiconductor laser element <NUM>.

A first film <NUM> is a film disposed in contact with the light-emission-side end face <NUM>. "Being in contact" may include not only the case in which the first film <NUM> is directly in contact with the resonator face, but also the case in which the first film <NUM> is formed on a thin film formed on the resonator face to the extent of having the effect of the present disclosure. For example, there may be a thin film formed by the ambient gas during the pretreatment of the resonator face or when the film formation is initiated. The thickness of such a thin film is, for example, smaller than the thickness of the first film <NUM>. The thickness of such a thin film is, for example, <NUM> or smaller, or <NUM> or smaller. Such a thin film contains, for example, gallium (Ga) and oxygen (O).

The first film <NUM> may be an oxide film containing aluminum (Al), oxide film containing Ga, or oxide film containing Al and Ga. An oxide film containing aluminum includes an aluminum oxide film such as Al<NUM>O<NUM>. An oxide film containing Ga includes a gallium oxide film such as Ga<NUM>O<NUM>. An oxide film containing Al and Ga includes an aluminum gallium oxide film such as AlGaO. Such a film can reduce resistance fluctuations during the operation of the semiconductor laser element. Furthermore, the first film <NUM> being an oxide film containing Al can facilitate the formation of a crystalline film. The first film <NUM> is, for example, an Al<NUM>O<NUM> film. The first film <NUM> may be an insulating film.

The first film <NUM> is a crystalline (monocrystalline or polycrystalline) film. A single crystal is a material having minimal lattice constant variations or lattice-plane tilting. In other words, atoms are regularly arranged in the material, and long-range order is maintained. A polycrystal is composed of a large number of minute monocrystals, i.e., microcrystals. Such a crystal of a film can be determined by observing an electron diffraction image. An electron diffraction image appears in correspondence with the magnitude of lattice constant and lattice plane orientation when an electron beam is incident on the film. For example, in the case of a single crystal, regularly arranged diffraction points will be observed. In the case of a polycrystal, which is composed of microcrystals, the orientations of lattice planes are not aligned, and thus the electron diffraction image will show complexly merged diffraction points or Debye rings. On the other hand, an amorphous material lacks a long-range periodic structure in the atomic arrangement, not allowing electron diffraction to appear. Accordingly, no diffraction points are observed in the diffraction image. An electron diffraction image can be observed by cutting the end face on which the layer is formed so as to expose a cross section, and irradiating an electron beam thereto. Crystalline differences can also be confirmed by observing a cross section, for example, by using a transmission electron microscope, scanning transmission electron microscope, or scanning electron microscope, or based on the etching rate difference by using an appropriate etchant, such as an acidic or alkaline solution. Alternatively, the atomic arrangement can be confirmed by high-resolution transmission electron microscopy imaging.

<FIG> is an example of a high-resolution transmission electron microscope image of a protective film <NUM> and its vicinity. The scale bar shown at bottom right in the image denotes <NUM>. The nitride semiconductor stack body shown in <FIG> includes an active layer <NUM>. <FIG> is an electron diffraction image of the area in the circle A in <FIG>, which is the electron diffraction image of the third film <NUM>. <FIG> is an electron diffraction image of the area in the circle B in <FIG>, which is the electron diffraction image of the second film <NUM>. <FIG> is an electron diffraction image of the area in the circle C in <FIG>, which is the electron diffraction image of the first film <NUM>. <FIG> is an electron diffraction image of the area in the circle D in <FIG>, which is the electron diffraction image of the nitride semiconductor stack body. <FIG> confirms that the first film <NUM> is crystalline, i.e., the first film <NUM> is a film having crystalline quality.

Examples of crystal structures for the first film <NUM> include the cubic crystal system, tetragonal crystal system, hexagonal crystal system, or the like. The materials for, the crystallinity and the orientation of the first film <NUM> can be selected according to the materials for, the crystallinity and the orientation of the light-emission-side end face <NUM> on which the first film <NUM> is to be formed. For example, a crystalline film is a film which includes a single crystal and/or polycrystal in part, or a film composed only of a single crystal or polycrystal. In other words, the first film <NUM> does not necessarily have to be strictly monocrystalline or polycrystalline, and may be one having a crystal structure similar to these, or one having a crystal structure showing the characteristics of these structures. The crystalline quality of the first film <NUM> may differ between the optical waveguide region or the region corresponding to the NFP and the areas of the first nitride semiconductor layer <NUM> and the second nitride semiconductor layer <NUM> distant from the active layer <NUM> in the thickness direction. The first film <NUM> is preferably crystalline practically across the entire thickness in the optical waveguide region or the region corresponding to the NFP. Practically across the entire thickness refers to the portion excluding the portions where the boundaries with adjacent layers are indistinguishable. For example, the first film <NUM> is a film that is polycrystalline at least in part. The first film <NUM> may be such that at least one half of the portion adjacent to the active layer <NUM> is polycrystalline in the cross sections taken in the directions intersecting the principal faces of the active layer <NUM> and intersecting the light-emission-side end face <NUM> (e.g., in the directions perpendicular to both).

The thickness of the first film <NUM> is, for example, <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>. The smaller the thickness of the first film <NUM>, the higher the tendency for achieving good crystalline quality becomes. The thickness of the first film <NUM> can be set to <NUM> or larger, or <NUM> or larger. It may be set to <NUM> or larger. For example, the thickness of the first film <NUM> can be set to <NUM> or larger but smaller than <NUM>. Setting the thickness of the first film <NUM> to <NUM> or larger can further extend the service life of the nitride semiconductor laser element <NUM>. It is likely because this can improve the crystalline quality of the second film <NUM>. The thickness of the first film <NUM> can be <NUM> or smaller, <NUM> or smaller, <NUM> or smaller, or smaller than <NUM>. For example, the thickness of the first film <NUM> can be <NUM> or larger and <NUM> or smaller. The thickness of the first film <NUM> can be <NUM> or smaller, <NUM> or larger and <NUM> or smaller. The thickness of the first film <NUM> refers to the length in the direction parallel to the principal faces of the active layer <NUM>. Likewise, the thicknesses of the second film <NUM> and the third film <NUM> refer to the lengths in the direction parallel to the principal faces of the active layer <NUM>.

A first film <NUM> can be formed by a method known in the art. For example, pulsed sputtering, electron cyclotron resonance (ECR) sputtering, magnetron sputtering, ion beam assisted deposition, laser ablation, chemical vapor deposition (CVD), or a combination of two or more of these methods can be used. Alternatively, any of these methods can be combined with full or partial pretreatment. For pretreatment, any one or more of the following can be used: inert gas (Ar, He, Xe, or the like) or plasma irradiation, oxygen or ozone gas irradiation, oxidation (heat treatment), and exposure treatment.

A first film <NUM> is preferably formed by pulsed sputtering or ECR sputtering among them. For example, a first film can be formed by using an ECR sputtering system. This can form a first film <NUM> of good crystalline quality. In the case of forming a first film <NUM> by using an ECR sputtering system, the oxygen flow rate during film formation is preferably set to <NUM> × <NUM>-<NUM>/cm<NUM> or higher. Pretreatment may be performed before forming a first film <NUM>. For the pretreatment, the light-emission-side end face <NUM> can be treated with oxygen plasma. Pulsed sputtering includes one using an oxide target, and one using a non-oxide targe intermittently sputtering while irradiating oxygen or plasma, or in oxygen environment. ECR sputtering tends to allow for a lower temperature during film formation than pulsed sputtering. This can reduce the degradation of the properties of the electrodes described later.

Forming a first film <NUM> as an oxygen-containing film in contact with the light-emission-side end face <NUM> as described above can reduce resistance fluctuations during the operation of the nitride semiconductor laser element <NUM>. Furthermore, forming a first film <NUM> as a crystalline film can facilitate the formation of a crystalline second film <NUM>. A first film <NUM> having a relatively small thickness, for example, thinner than a third film <NUM>, can reduce the stress in the light-emission-side end face <NUM> attributable to the heat generated during the operation of a nitride semiconductor laser element <NUM>. Moreover, the adhesion between the light-emission-side end face <NUM> and the protective film <NUM> can be improved.

A second film <NUM> is formed in contact with the first film <NUM>. The second film <NUM> is a nitride film. A nitride film can specifically be AlN, GaN, AlGaN, or the like. Among them, an AlN film is preferable. Because an AlN film can be formed by ECR sputtering, for example, both the first film <NUM> and the second film <NUM> can be formed together by using an ECR sputtering system. The first film <NUM> and the second film <NUM> can be grown in a continuous manner.

The second film <NUM> is a crystalline film. Thus, it can effectively function as an oxygen barrier layer. The second film <NUM> may be oriented along M axis <<NUM>-<NUM>>, A axis <<NUM>-<NUM>>, C axis <<NUM>>, or R axis <<NUM>-<NUM>>, or have any other orientation in the thickness direction. For example, the second film <NUM> is C-axis-oriented relative to the M axis of the nitride semiconductor stack body. In this case, the M axis of the nitride semiconductor stack body parallels the C axis of the second film <NUM>. The axial orientation of the crystal in the portion of the second film <NUM> adjacent to the active layer <NUM> may be the same as the axial orientation of the crystal in the portion adjacent to the first nitride semiconductor layer <NUM> and/or the portion adjacent to the second nitride semiconductor layer <NUM>. This makes it easier to form the second film <NUM> with good crystalline quality. For example, in the second film <NUM>, both the portion adjacent to the active layer <NUM> and the portion(s) adjacent to the first nitride semiconductor layer <NUM> and/or the second nitride semiconductor layer <NUM> are C-axis oriented. For example, the C axis in the portion of the second film <NUM> adjacent to the active layer <NUM> and the C axis in the portion(s) of the second film <NUM> adjacent to the first nitride semiconductor layer <NUM> and/or the second nitride semiconductor layer <NUM> are in parallel. A portion of the second film <NUM> adjacent to a certain layer (e.g., the active layer <NUM>) refers to the portion interposed by the imaginary plane which is an extension of one of the principal faces of the layer and the imaginary plane which is an extension of the other principal face. The axial orientation of the crystal may be the same across the entire second film <NUM>. The crystalline quality of each portion of the second film <NUM> can be evaluated by using a cross section taken in the direction intersecting the principal faces of the active layer <NUM> (e.g., perpendicular direction).

The thickness of the second film <NUM> is, for example, <NUM> to <NUM>, and may be <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. Setting the thickness of the second film <NUM> to fall within these ranges can achieve good crystalline quality, and therefor the second film can effectively function as an oxygen barrier layer. The thickness of the second film <NUM> being <NUM> or smaller is believed to readily reduce crack formation. The thickness of the second film <NUM> may be larger than <NUM> in order to adjust the reflectivity of the protective film <NUM>.

The second film <NUM> can be formed by a known method, such as sputtering, ECR sputtering, or the like. Sputtering includes one using a nitride target, and one using a non-nitride targe sputtering while irradiating nitrogen gas or plasma, or in nitrogen environment.

With regard to the example of the second film <NUM> shown in <FIG>, <FIG> confirms that the second film <NUM> is crystalline, i.e., the second film <NUM> is a film having crystalline quality.

A third film <NUM> is a film formed in contact with the second film <NUM>. The third film <NUM> is a film containing Al and O. The third film <NUM> may be an oxide film containing Al or oxynitride film containing Al. The third film <NUM> may be an aluminum oxide film. Because the third film <NUM> is less likely to be oxidized with such a composition, the possibility of oxygen reaching the nitride semiconductor stack body can be reduced. The progression of the third film <NUM> oxidation made of an oxide film containing Al is considered more difficult than an oxynitride film containing Al during the operation of the nitride semiconductor laser element <NUM>. This, as a result, can further extend the service life of the nitride semiconductor laser element <NUM>. The third film <NUM> is, for example, an Al<NUM>O<NUM> film.

The third film <NUM> may be a crystalline film or a film which includes an amorphous structure. An amorphous structure means a structure lacking a periodic structure such as the atomic arrangement of a crystal, i.e., the atomic arrangement is irregular or lacks the long-range order. The third film <NUM> is preferably a film having an amorphous structure, or one that includes an amorphous structure and a crystalline structure. Among them, a film composed only of an amorphous structure is preferable. This makes it easier to make the third film <NUM> thicker than the thickness(es) of the first film <NUM> and/or the second film <NUM>. Furthermore, a portion of the third film <NUM>, for example, a portion on the active layer <NUM> side, can become crystalline during the operation of the nitride semiconductor laser element <NUM>. This can potentially further extend the service life of the nitride semiconductor laser element <NUM>. For example, such crystallization can occur in the case where the third film <NUM> is made of Al<NUM>O<NUM>.

The thickness of the third film <NUM> is preferably larger than the thickness of the first film <NUM>. The thickness of the third film <NUM> may be smaller than the thickness of the second film <NUM>, but preferably larger. The thickness of the third film <NUM> is, for example, three times the thickness of the first film <NUM> or larger, and may be <NUM> times or larger. The thickness of the third film <NUM> is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>. Setting the thickness of the third film <NUM> to fall within such ranges can increase the total thickness of the protective film <NUM>, thereby reducing the possibility of the oxygen from the outside of the nitride semiconductor laser element <NUM> transmitting through the protective film <NUM> to reach the nitride semiconductor stack body. This can potentially be beneficial to service life extension. The effect can be more prominent particularly when the first film <NUM> and the second film <NUM> are formed thinner than the third film <NUM> in order to fulfil their respective functions. The thickness of the third film <NUM> can be in a range of ±<NUM>% of (λ/2n<NUM>) × <NUM>. The λ refers to the oscillation wavelength of nitride semiconductor laser element <NUM>, and the n<NUM> refers to the refraction index of the third film <NUM> with respect to the oscillation wavelength. This can extend the service life of the nitride semiconductor laser element <NUM> as compared to a third film having a thickness smaller than that. It is likely because the protective film <NUM> having a larger thickness can extend the time for the damage progressing from the inside of the nitride semiconductor laser element <NUM> to reach the outer surface of the protective film <NUM>. As shown in <FIG>, a portion of the protective film can be provided on a side of the upper face side and/or the lower face side of the nitride semiconductor stack body. <FIG> shows a cross-sectional view of another example of the nitride semiconductor laser element <NUM>. In this manner, providing the protective film <NUM> can extend, by the thickness of the protective film <NUM>, the distance for the damage to reach the outer surface of the nitride semiconductor laser element <NUM> at the upper face side and/or the lower face side of the nitride semiconductor stack body. This can extend the service life of the nitride semiconductor laser element <NUM>. The protective film <NUM> may further include another film on the outer side of the third film <NUM>. Including such another film results in an increase in the thickness of the protective film <NUM>, and therefore can further extend the service life of the nitride semiconductor laser element <NUM>. As shown in <FIG>, the protective film <NUM> can include the fourth film <NUM> provided on the outer side of the third film <NUM>. <FIG> is an enlarged view showing a portion of another example of the protective film <NUM> at the light-emission-side end face <NUM> side. The fourth film <NUM> is disposed on a surface of the third film <NUM> located opposite to the second film <NUM> of the protective film <NUM>. The thickness of the fourth film can be larger than the thickness of the third film <NUM>. Providing the fourth film <NUM> can extend the distance for the damage progressing from the inside of the nitride semiconductor laser element <NUM> to reach the outer surface of the protective film <NUM> as compared to a case of providing no fourth film <NUM>. The refractive index of the fourth film <NUM> with respect to the wavelength oscillation λ can be smaller than the refractive index of the third film <NUM> with respect to the wavelength oscillation λ. For example, the thickness of the third film <NUM> can be set in a range of ±<NUM>% of λ/2n<NUM>, and the thickness of the fourth film <NUM> can be set in a range of ±<NUM>% of λ/2n<NUM>. The symbol "n<NUM>" refers to the refractive index of the fourth film <NUM> with respect to the wavelength oscillation λ. Example of the fourth film <NUM> includes a film formed of silicone oxide (e.g., SiO<NUM>). This can provide an advantage of suppressing initial characterization degradation or suppressing heat generation due to the light absorption. The third film <NUM> can be disposed as the outermost layer of the protective film <NUM>. In the protective film <NUM>, another film may be provided between the second film <NUM> and the third film <NUM>.

The third film <NUM> can be formed by a known method, such as sputtering, ECR sputtering, or the like. It can be formed by using an ECR sputtering system, for example. In the case of forming the first film <NUM> and the third film <NUM> by using an ECR sputtering system, the oxygen flow rate during the formation of the third film <NUM> is lower than the oxygen flow rate during the formation of the first film <NUM>. In this manner, the first film <NUM> can be formed as a crystalline film, and the third film <NUM> as a film having an amorphous structure.

With respect to the example of the third film <NUM> shown in <FIG>, <FIG> confirms that the third film <NUM> is amorphous, i.e., the third film <NUM> is a film having an amorphous structure.

The construction described above can reduce the occurrence of catastrophic optical damage in the nitride semiconductor laser element <NUM>, making it possible to extend the service life of the nitride semiconductor laser element <NUM>.

A protective film <NUM> (a second protective film or an additional protective film) is disposed on the light-reflection-side end face <NUM>. The protective film <NUM> has a film structure different from or the same as that of the protective film <NUM> provided on the light-emission-side end face <NUM>. The reflectivity of the protective film <NUM> with respect to the oscillation wavelength of the nitride semiconductor laser element <NUM> is higher than the reflectivity of the protective film <NUM> with respect to the oscillation wavelength of the nitride semiconductor laser element <NUM>.

The protective film <NUM> formed on the light-reflection-side end face <NUM> may have the same multilayer structure made up of the first film <NUM>, the second film <NUM>, and the third film <NUM> described above. The protective film <NUM> may be a Si, Mg, Al, Hf, Nb, Zr, Sc, Ta, Ga, Zn, Y, B, or Ti oxide (particularly, Al<NUM>O<NUM>, SiO<NUM>, Nb<NUM>O<NUM>, TiO<NUM>, ZrO<NUM> or the like), a nitride (particularly, AlN, AlGaN, BN, or the like), a fluoride, or a combination of two or more of these. In the case of forming the same multilayer film composed of the first film <NUM>, the second film <NUM>, and the third film <NUM>, the thicknesses of the films may be different from those of the protective film <NUM> formed on the light-emission-side end face <NUM>. The protective film <NUM> is preferably formed with a material having transmissivity and/or a material having reflectivity with respect to the oscillation wavelength of the nitride semiconductor laser element <NUM>. The protective film <NUM> may be a single layer or multiple layers.

Examples of the protective film <NUM> formed on the light-reflection-side end face <NUM> include a multilayer film composed of a Si oxide and a Zr oxide, a multilayer film composed of an Al oxide and a Zr oxide, a multilayer film composed of a Si oxide and a Ti oxide, a multilayer film composed of an Al oxide, a Si oxide, and a Zr oxide, a multilayer film composed of a Si oxide, a Ta oxide, and an Al oxide, and the like. The stacking periodicity or the like can be suitably adjusted according to a desired reflectivity.

The thickness of such a protective film <NUM> is not particularly limited, for example, <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. For example, the thickness of the protective film <NUM> is larger than the thickness of the protective film <NUM>.

A protective film <NUM> is, for example, a multilayer film. A multilayer film can have a structure in which relatively low refractive index films and relatively high refractive index films are alternately stacked. The film in the multilayer film that is in contact with the light-reflection-side end face <NUM> may have a relatively low or relatively high refractive index. As shown in <FIG>, the protective film <NUM> can have a first portion 25a disposed in contact with the light-reflection-side end face <NUM> and a second portion 25b disposed in contact with the first portion 25a. <FIG> is an enlarged view of a portion of an example of the protective film <NUM> formed on the light-reflection-side end face <NUM>. The thickness of the protective film <NUM> is preferably λ/4n multiplied by an odd number ±<NUM>%, more preferably λ/4n multiplied by an odd number. This can reduce the probability of the occurrence of optical damage at the light-reflection-side end face <NUM> during the operation of the nitride semiconductor laser element <NUM>. Here, λ is the oscillation wavelength of a semiconductor laser element, and n is the refractive index of each film with respect to the oscillation wavelength λ. The thickness of the protective film <NUM> refers to the length in the direction parallel to the principal faces of the active layer <NUM>. The second portion 25b is composed of high refractive index films <NUM> and low refractive index films <NUM> which are alternately disposed. The first portion 25a is composed of one or more films. The film in the first portion 25a in contact with the second portion 25b has a refractive index which is different from the refractive indices of the high refractive index films <NUM> and the low refractive index films <NUM>. Combining the first portion 25a and the second portion 25b allows the interface between the first portion 25a and the second portion 25b to reflect light, thereby increasing the reflectivity of the protective film <NUM>. This can increase the light output of the nitride semiconductor laser element <NUM>. The distance from the light-reflection-side end face <NUM> to the interface between the first portion 25a and the second portion 25b is preferably <NUM> to <NUM>. This can further increase the reflectivity of the protective film <NUM> thereby further increasing the light output of the nitride semiconductor laser element <NUM>. The distance here refers to the shortest distance. The protective film <NUM> may be composed only of the first portion 25a and the second portion 25b, or further include another film.

The first portion 25a preferably has a multilayer structure having a relatively low refractive index film and a relatively high refractive index film. This can further increase the reflectivity of the protective film <NUM>. For example, a relatively low refractive index film in the first portion 25a is disposed in contact with a high refractive index film <NUM> of the second portion 25b. Alternatively, a relatively high refractive index film in the first portion 25a may be in contact with a low refractive index film <NUM> of the second portion 25b. The film in the second portion 25b that is in contact with the first portion 25a is preferably a film having a higher refractive index than the refractive index of the film in the first portion 25a that is in contact with the second portion 25b as well as a higher refractive index than the refractive index of any of the films in the first portion 25a. This can further increase the reflectivity of the protective film <NUM>. The low refractive index films <NUM> of the second portion 25b can be, for example, silicon oxide films (e.g., SiO<NUM> films). The high refractive index films <NUM> of the second portion 25b can be, for example, tantalum oxide films (e.g., Ta<NUM>O<NUM>).

The first portion 25a may have a fifth film <NUM>, a sixth film <NUM>, and a seventh film <NUM>. The first portion 25a may have, successively from the light-reflection-side end face <NUM> side, a crystalline fifth film <NUM> containing oxygen and aluminum and/or gallium, a nitride crystalline sixth film <NUM>, and a seventh film <NUM> containing aluminum and oxygen. For the fifth film <NUM>, the materials, the thickness, and the forming methods described with reference to the first film <NUM> can be employed. For the sixth film <NUM>, the materials, the thickness, and the forming methods described with reference to the second film <NUM> can be employed. For the seventh film <NUM>, the materials, the thickness, and the forming methods described with reference to the third film <NUM> can be employed. Because the first portion 25a is the portion in contact with the light-reflection-side end face <NUM>, a similar effect to that achieved by the protective film <NUM> can be expected by having such a fifth film <NUM>, sixth film <NUM>, and seventh film <NUM>. The fifth film <NUM> may be disposed in contact with the light-reflection-side end face <NUM>. The thicknesses of the fifth film <NUM>, the sixth film <NUM>, and the seventh film <NUM> may each be λ/4n or smaller. This can position the interface between the first portion 25a and the second portion 25b relatively close to the light-reflection-side end face <NUM>, thereby further increasing the reflectivity of the protective film <NUM> and further increasing the light output of the nitride semiconductor laser element <NUM>. For example, the first portion 25a is composed only of the fifth film <NUM>, the sixth film <NUM>, and the seventh film <NUM>.

A substrate <NUM> may be an insulating substrate or conductive substrate. For the substrate <NUM>, for example, a nitride semiconductor substrate formed of GaN or the like can be used. The first principal face of the substrate <NUM> which is the semiconductor forming face can be C-plane, R-plane, or M-plane, and is, for example, C-plane. The substrate may have an off-angle of <NUM>° to <NUM>° relative to the first principal face and/or the second principal face located opposite to the first principal face. The thickness of the substrate <NUM> is, for example, <NUM> to <NUM>.

A nitride semiconductor laser element <NUM> can have an embedded layer <NUM> on the upper face of the second nitride semiconductor layer <NUM>, e.g., on the lateral faces of the ridge 12a and the upper face of the second nitride semiconductor layer <NUM> contiguous with the lateral faces of the ridge 12a.

The embedded layer <NUM> is preferably formed with a material having a lower refractive index than that of the second nitride semiconductor layer <NUM>. The embedded layer <NUM> can be a single layer or multilayered insulating film formed of an oxide, nitride, or oxynitride of Zr, Si, V, Hf, Ta, Al, Ce, In, Sb, or Zn. The embedded layer <NUM> can be formed by using any of the methods known in the art described earlier with reference to the first film <NUM>.

A first electrode <NUM> can be disposed on the lower face of the first nitride semiconductor layer <NUM>, or if the substrate <NUM> is provided, on the lower face of the substrate <NUM>. In the case where the substrate <NUM> is a semiconductor substrate, the substrate <NUM> is of the same conductivity type as the first nitride semiconductor layer <NUM>. The first electrode <NUM> is disposed, for example, practically across the entire lower face of the substrate <NUM>.

A second electrode <NUM> can be disposed on the upper face of the second nitride semiconductor layer <NUM>, for example, on the upper face of the ridge 12a, and a pad electrode <NUM> can be further disposed thereon.

The first electrode <NUM> and the second electrode <NUM> can be formed as a single layer or multilayered film of a metal, such as Ni, Rh, Cr, Au, W, Pt, Ti, Al, Pd, or the like, an alloy thereof, or a conductive oxide containing at least one selected from Zn, In, and Sn. Conductive oxide examples include ITO (indium tin oxide), IZO (indium zinc oxide), GZO (gallium-doped zinc oxide), and the like. The thickness of each electrode can be any as long as it can normally function as an electrode for a semiconductor laser element, for example, <NUM> to <NUM>.

The first electrode <NUM> and the second electrode <NUM> may be respectively disposed on the first principal face side and the second principal face side of the nitride semiconductor stack body, or they may both be disposed on either the first principal face side or the second principal side.

The embedded layer <NUM>, the first electrode <NUM>, the second electrode <NUM>, and the pad electrode <NUM> may be disposed at a distance from or in contact with the protective film <NUM> described earlier. The embedded layer <NUM>, the first electrode <NUM>, the second electrode <NUM>, and the pad electrode <NUM> may cover or be covered by the protective film <NUM>. The embedded layer <NUM> and the second electrode <NUM> are preferably covered by the protective film <NUM>. This can reduce the detachment of the embedded layer <NUM> and the second electrode <NUM>.

As a semiconductor laser element shown in <FIG>, a gallium nitride-based semiconductor laser element having an oscillation wavelength peaking at about <NUM> was produced.

A MOCVD system was used to produce an epitaxial wafer for the semiconductor laser element. For the raw materials, trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH<NUM>), silane gas, and bis(cyclopentadienyl)magnesium (Cp<NUM>Mg) were suitably used.

On a C-plane n-type GaN substrate (substrate <NUM>), an n-side semiconductor layer as a first nitride semiconductor layer <NUM>, an active layer <NUM>, and a p-side semiconductor layer as a second nitride semiconductor layer <NUM> were grown, and on the surface of the p-side semiconductor layer, striped ridges 12a of <NUM> in length in the direction parallel to the length of the resonator were formed.

Subsequently, a p-electrode formed of ITO (<NUM>) was formed as a second electrode <NUM> on the surface of the p-side semiconductor layer, and an embedded layer <NUM> formed of SiO<NUM> was formed on the lateral faces of the ridges 12a and the upper face of the p-side semiconductor layer outward from the ridges 12a. The embedded layer <NUM> was formed such that a portion thereof covers the p-electrode.

On the p-electrode, a Ni (<NUM>)/Pd (<NUM>)/Au (<NUM>)/Pt (<NUM>)/Au (<NUM>) pad electrode <NUM> was continuously formed.

Subsequently, the substrate <NUM> was polished from the face opposite the face on which the first nitride semiconductor layer <NUM> was formed such that the thickness became <NUM>, and on the polished face, a Ti (<NUM>)/Pt (<NUM>)/Au (<NUM>) n-electrode as a first electrode <NUM> was formed. Then the body is cleaved from the n-electrode side of the substrate <NUM> into a bar, making the cleaved faces the resonator faces, i.e., the light-emission-side end face <NUM> and the light-reflection-side end face <NUM>.

Surface treatment was performed on the light-emission-side end face <NUM> and the light-reflection-side end face <NUM> of the resultant bar by oxygen plasma exposure using an ECR sputtering system. At this time, the O<NUM> flow rate was set to <NUM> × <NUM>-<NUM> m<NUM>/s, treating the bar at <NUM> W microwave for ten minutes.

Subsequently, an Al<NUM>O<NUM> first film <NUM> was formed on the light-emission-side end face <NUM> by using an Al target at a <NUM>×<NUM>-<NUM> m<NUM>/s Ar flow rate, <NUM>×<NUM>-<NUM> m<NUM>/s oxygen flow rate, and <NUM> W microwave. In a similar manner, a second film <NUM> was formed by changing oxygen gas to nitrogen gas, and a third film <NUM> was formed by changing nitrogen gas to oxygen gas, whereby a protective film <NUM> was formed. For the semiconductor laser elements in Examples <NUM> and <NUM> and Comparative Example, the first film <NUM>, the second film <NUM>, and the third film <NUM> were formed to have the thicknesses shown in Table <NUM>. For each of the semiconductor laser elements in Examples <NUM> and <NUM> and Comparative Example, the reflectivity of the protective film <NUM> was about <NUM>%. The oxygen flow rate during the formation of the first film <NUM> was <NUM>×<NUM>-<NUM> m<NUM>/s, and the oxygen flow rate during the formation of the third film <NUM> was <NUM>×<NUM>-<NUM> m<NUM>/s. On the light-reflection-side end face <NUM>, a protective film <NUM> having a multilayer structure in which multiple SiO<NUM> layers and Ta<NUM>O<NUM> layers were alternately stacked was formed.

Subsequently, nitride semiconductor laser elements <NUM> were obtained by cleaving each bar in the direction perpendicular to the cleaved faces.

For Example <NUM>, Example <NUM>, and Comparative Example, seven pieces of semiconductor laser elements were produced each. Life tests were conducted in which the laser elements were allowed to continuously oscillate for about four months at a light density of about <NUM> MW/cm<NUM>, and catastrophic failure pieces were counted.

The results were three for Example <NUM>, zero for Example <NUM>, and five for Comparative Example.

Claim 1:
A nitride semiconductor laser element (<NUM>) comprising:
a nitride semiconductor stack body including
a first nitride semiconductor layer (<NUM>) of a first conductivity type,
a second nitride semiconductor layer (<NUM>) of a second conductivity type different from the first conductivity type, and
an active layer (<NUM>) disposed between the first nitride semiconductor layer (<NUM>) and the second nitride semiconductor layer (<NUM>),
the nitride semiconductor stack body defining
a light-emission-side end face (<NUM>) intersecting a face of the active layer (<NUM>) on a second nitride semiconductor layer side, and
a light-reflection-side end face (<NUM>) intersecting the face of the active layer (<NUM>) on the second nitride semiconductor layer side; and
a protective film (<NUM>) disposed on the light-emission-side end face (<NUM>) of the nitride semiconductor stack body, wherein
the protective film (<NUM>) includes, in the order from the light-emission-side end face (<NUM>), a first film (<NUM>) that is a crystalline film containing oxygen and aluminum and/or gallium, a second film (<NUM>) that is a nitride crystalline film, and a third film (<NUM>) containing aluminum and oxygen, wherein
each of the first film (<NUM>) and the second film (<NUM>) is thinner than the third film (<NUM>).