Method of fabricating group-III nitride semiconductor laser device

A method for fabricating a III-nitride semiconductor laser device includes: forming a substrate product having a laser structure; scribing a first surface of the substrate product to form a scribed mark, which extends along a reference line indicative of a direction of the a-axis of the hexagonal III-nitride semiconductor, on the first surface, a scribed mark; mounting the substrate product on a breaking device to support first and second regions of the substrate product by first and second support portions, respectively, of the breaking device; and carrying out breakup of the substrate product by press in alignment with the scribed mark in a third region, without supporting the third region of the substrate product located between the first and second regions, to form another substrate product and a laser bar. First and second end faces of the laser bar form a laser cavity of the III-nitride semiconductor laser device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating a Group III-nitride semiconductor laser device.

2. Related Background Art

Non-patent Literature 1 discloses a semiconductor laser made on a c-plane sapphire substrate. The mirror faces of the semiconductor laser are made by dry etching. It shows micrographs of the cavity mirror faces of the laser and describes that the roughness of the end faces is about 50 nm.

Non-patent Literature 2 discloses a semiconductor laser made on a (11-22)-plane GaN substrate. The mirror faces of the semiconductor laser are formed by dry etching.

Non-patent Literature 3 discloses a gallium nitride (GaN)-based semiconductor laser. It proposes generation of laser light polarized in an off direction of the c-axis of the substrate, in order to use m-planes of cleaved facets for the laser cavity. Specifically, this Literature discloses increase of the well thickness on a nonpolar surface and decrease of the well thickness on a semipolar surface.

SUMMARY OF THE INVENTION

In the band structure of a GaN-based semiconductor, there are a number of transitions capable of lasing. According to Inventors' knowledge, it is considered that in the III-nitride semiconductor laser device using the semipolar-plane support base the c-axis of which is inclined toward the m-axis, the threshold current can be lowered when the laser waveguide extends along a plane defined by the c-axis and the m-axis. When the laser waveguide extends in this orientation, a lasing mode with the smallest transition energy (difference between conduction band energy and valence band energy) among the possible transitions becomes capable of lasing, and this mode becoming capable of lasing can reduce the threshold current.

However, this orientation of the laser waveguide does not allow use of the conventional cleaved facets such as c-planes, a-planes, or m-planes for the cavity mirrors. For this reason, the cavity mirrors have been made heretofore by forming dry-etched facets of semiconductor layers by reactive ion etching (RIE). There are now desires for improvement in the cavity mirrors formed by RIE, in terms of perpendicularity to the laser waveguide, flatness of the dry-etched facets, or ion damage. It becomes a heavy burden to derive process conditions for obtaining excellent dry-etched facets in the current technical level.

As far as the inventors know, no one has succeeded heretofore in achieving the III-nitride semiconductor laser device, which is formed on the semipolar plane, that has both of the laser waveguide extending in the inclination direction (off-axis direction) of the c-axis and the end faces for cavity mirrors formed without use of dry etching.

The inventors filed a Japanese patent application (Japanese Patent Application No. 2009-144442) associated with the present invention.

The present invention has been accomplished in view of the above-described circumstances. It is an object of the present invention to provide a method for fabricating a III-nitride semiconductor laser device, and the method can provide improvement in flatness of cavity mirrors in the III-nitride semiconductor laser device with a laser cavity enabling reduction in threshold current on a semipolar plane of a substrate inclined in a direction from the c-axis of a hexagonal III-nitride toward the m-axis thereof and can reduce the threshold current.

One aspect of the present invention relates to a method for fabricating a III-nitride semiconductor laser device. This method comprises the steps of: (a) forming a substrate product having a laser structure, the laser structure including a substrate and a semiconductor region, the substrate comprising a hexagonal III-nitride semiconductor and having a semipolar primary surface, and the semiconductor region being formed on the semipolar primary surface; (b) scribing a first surface of the substrate product to form the scribed mark on the first surface, the scribed mark extending along a reference line indicative of a direction of an a-axis of the hexagonal III-nitride semiconductor; (c) mounting the substrate product on a breaking device such that first and second support portions of the breaking device support first and second regions of the substrate product, respectively, after forming the scribed mark; and (d) carrying out breakup of the substrate product by press in alignment with the scribed mark in a third region of the substrate product, without supporting the third region, to form another substrate product and a laser bar, the third region being located between the first and second regions. The third region of the substrate product is adjacent to the first and second regions; the press is implemented against a second surface of the substrate product; a first edge of the first support portion is separated from a second edge of the second support portion; the substrate product is mounted on the support device so that the reference line is separated from a center line defining a center between the first edge and the second edge; the first surface is a surface opposite to the second surface; the semiconductor region is located between the first surface and the substrate; the laser bar has first and second end faces extending from the first surface to the second surface and being made by the breakup; the first and second end faces form a laser cavity of the III-nitride semiconductor laser device; the substrate product comprises an anode electrode and a cathode electrode provided on the laser structure; the semiconductor region includes a first cladding layer comprising a first conductivity type GaN-based semiconductor, a second cladding layer comprising a second conductivity type GaN-based semiconductor, and an active layer provided between the first cladding layer and the second cladding layer; the first cladding layer, the second cladding layer, and the active layer are arranged along a normal axis normal to the semipolar primary surface; a c-axis of the hexagonal III-nitride semiconductor of the substrate is inclined at an angle ALPHA with respect to an axis normal to the semipolar primary surface toward the m-axis of the hexagonal III-nitride semiconductor; and the first and second end faces intersect with an m-n plane defined by an m-axis of the hexagonal III-nitride semiconductor and the normal axis normal to the semipolar primary surface.

In this method, the substrate product is divided into the first and second regions of the substrate product to be supported during the breakup, and the third region of the substrate product to be pressed without being supported during the breakup. The substrate product is mounted on the breaking device so that the reference line is separated from the center line that defines the center between the first edge and the second edge. When the press is carried out in a manner such that shear force by press is applied at a position apart from the center line, different forces act in two portions, located on both sides of the reference line extending along the press line, of the third region. During a period of the press, bending moment and shear force act on the first portion that is between the supported first region and the pressed portion on the third region of the substrate product, and bending moment and shear force act on the second portion between the supported second region and the pressed portion on the third region of the substrate product. When the breakup is implemented by application of shear force at the boundary between the first portion and the second portion, a distribution of bending moment in the first portion is different from that in the second portion. This method improves the flatness of the cavity mirrors in the III-nitride semiconductor laser device.

In the fabricating method according to the foregoing aspect of the present invention, the angle ALPHA can be in the range of not less than 45 degrees and not more than 80 degrees or in the range of not less than 100 degrees and not more than 135 degrees. In this fabricating method, when the angle is in the range of less than 45 degrees or in the range of more than 135 degrees, the end faces formed by press are highly likely to be composed of m-planes. When the angle falls within the range of more than 80 degrees and less than 100 degrees, desired flatness and perpendicularity are not achieved.

In the fabricating method according to the foregoing aspect of the present invention, when the substrate product is mounted on the breaking device, a distance between the reference line and the center line is preferably not less than 100 μm. This method reduces difference in yield among substrate products.

In the fabricating method according to the foregoing aspect of the present invention, the distance, LENGTH, between the reference line and the center line can be not less than 200 μm on the first surface. In this method, when the distance LENGTH is not less than 200 μm, the yield improves among substrate products. In the fabricating method according to the foregoing aspect of the present invention, the distance LENGTH between the reference line and the center line can be in the range of not more than 400 μm.

In the fabricating method according to the foregoing aspect of the present invention, a distance between the first edge and the second edge can be not less than 1000 μm. In this method, if the distance between the first edge and the second edge is small, the bending moment on the substrate product is small during the press. For this reason, when the breakup is implemented by applying the shear force at the boundary between the first portion and the second portion of the third region of the substrate product, the difference is also small between the bending moment distribution in the first portion and the bending moment distribution in the second portion.

In the fabricating method according to the foregoing aspect of the present invention, the distance between the first edge and the second edge can be not more than 1500 μm. In this method, when the distance between the first edge and the second edge is more than 1500 μm, the distance between the first edge and the second edge, i.e., the width of the third region of the substrate product is larger than the cavity length and is several times the cavity length and, therefore, fracture will fail to occur at a position of a desired scribed mark but can occur at a position of another scribed mark.

In the fabricating method according to the foregoing aspect of the present invention, preferably, the scribed mark extends in a direction from the first surface to the second surface, along an a-n plane defined by the normal axis to the semipolar primary surface and the direction of the a-axis. In this fabricating method, the scribed mark extending in the direction from the first surface to the second surface along the a-n plane guides generation of the end face.

In the fabricating method according to the foregoing aspect of the present invention, preferably, the step of forming the substrate product comprises subjecting the substrate to such processing that a thickness of the substrate becomes not less than 50 μm; the processing is slicing or grinding; the second surface is a processed surface made by the processing, or a surface including an electrode formed on the processed surface. In this fabricating method, when the substrate thickness is not less than 50 μm, handling becomes easier and production yield becomes higher. With the substrate having such thickness, the first and second end faces can be formed in good yield with flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device, or without ion damage.

In the fabricating method according to the foregoing aspect of the present invention, preferably, the step of forming the substrate product comprises subjecting the substrate to such processing that the thickness of the substrate becomes not more than 160 μm. In this fabricating method, when the substrate thickness is not more than 160 μm, the substrate is more suitable for obtaining excellent-quality fractured faces for the laser cavity.

In the fabricating method according to the foregoing aspect of the present invention, preferably, the scribing is carried out using a laser scriber; the scribed mark comprises a scribed groove; the scribed groove extends in the direction from the first surface toward the second surface, along the a-n plane defined by the direction of the a-axis and the normal axis to the semipolar primary surface. In this fabricating method, the scribed groove extends in the direction from the first surface toward the substrate. The other substrate product and the laser bar are formed by fracture of the substrate product, and propagation of the fracture is guided by the scribed groove extending in the foregoing depth direction.

In the fabricating method according to the foregoing aspect of the present invention, the semipolar primary surface can be any one of a {20-21} plane, a {10-11} plane, a {20-2-1} plane, and a {10-1-1} plane. By this fabricating method, the first and second end faces can be provided with flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device, on any one of these typical semipolar planes.

In the fabricating method according to the foregoing aspect of the present invention, the semipolar primary surface also suitably applicable is a surface with a slant in the range of not less than −4 degrees and not more than +4 degrees from any one semipolar plane of a {20-21} plane, a {10-11} plane, a {20-2-1} plane, and a {10-1-1} plane, toward an m-plane. By this fabricating method, the first and second end faces can be provided with flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device, on the slight slant surface from any one of these typical semipolar planes.

In the fabricating method according to the foregoing aspect of the present invention, the substrate can comprise any one of GaN, AlGaN, AlN, InGaN, and InAlGaN. This fabricating method allows the first and second end faces applicable to the cavity to be obtained through the use of the substrate that comprises one of these GaN-based semiconductors.

In the fabricating method according to the foregoing aspect of the present invention, an end face of the active layer in each of the first and second end faces can make an angle in the range of not less than (ALPHA−5) degrees and not more than (ALPHA+5) degrees, defined on a first plane defined by the c-axis and the m-axis of the hexagonal III-nitride semiconductor, with respect to a reference plane perpendicular to the m-axis of the substrate. This fabricating method allows the formation of the end faces with the foregoing perpendicularity, defined in the angle taken from one to the other of the c-axis and the m-axis, in excellent yield.

In the fabricating method according to the foregoing aspect of the present invention, the angle can be in the range of not less than −5 degrees and not more than +5 degrees on a second plane which is perpendicular to the first plane and the normal axis to the semipolar primary surface. This fabricating method allows the formation of the end faces with the foregoing perpendicularity, defined in the angle formed on the plane perpendicular to the axis normal to the semipolar plane, in excellent yield.

In the fabricating method according to the foregoing aspect of the present invention, preferably, the angle ALPHA is in the range of not less than 63 degrees and not more than 80 degrees or in the range of not less than 100 degrees and not more than 117 degrees. In this fabricating method, when the angle is in the range of less than 63 degrees or in the range of more than 117 degrees, an m-plane can appear in part of an end face made by the press. When the angle is in the range of more than 80 degrees and less than 100 degrees, the desired flatness and perpendicularity are not achieved.

The fabricating method according to the foregoing aspect of the present invention can be configured as follows: the step of forming the scribed mark comprises scribing the first surface of the substrate product to form a plurality of scribed grooves extending in the direction of the a-axis of the hexagonal III-nitride semiconductor; the plurality of scribed grooves are arrayed at a pitch of n (n=1, 2, 3, 4, . . . ) times a device width in a direction intersecting with a direction of the laser cavity. Using the semipolar plane can improve quantum efficiency due to reduction of the piezoelectric field and improvement in crystal quality of the light emitting layer region.

In the fabricating method according to the foregoing aspect of the present invention, preferably, the thickness of the substrate is not less than 50 μm. In this fabricating method, when the thickness is not less than 50 μm, handling becomes easier and it can contribute to improvement in production yield.

In the fabricating method according to the foregoing aspect of the present invention, the thickness of the substrate can be not more than 400 μm. By this fabricating method, the fractured faces for the laser cavity can be obtained.

In the fabricating method according to the foregoing aspect of the present invention, the substrate can comprise any one of GaN, AlGaN, AlN, InGaN, and InAlGaN. In this fabricating method, when the substrate used is one which comprises any one of these GaN-based semiconductors, it becomes feasible to obtain the first and second end faces applicable to the cavity. Use of an AlN substrate or AlGaN substrate allows for increase in degree of polarization and enhancement of optical confinement by virtue of low refractive index. Use of an InGaN substrate allows for decrease in lattice mismatch rate between the substrate and the light emitting layer and improvement in crystal quality.

The fabricating method according to the foregoing aspect of the present invention can further comprise a step of forming a dielectric multilayer film on at least one of the first and second fractured faces. In this III-nitride semiconductor laser device, an end face coat is also applicable to the fractured faces and the end face coat allows for adjustment of reflectance.

In the fabricating method according to the foregoing aspect of the present invention, the active layer can include a light emitting region provided so as to generate light at a wavelength of not less than 360 nm and not more than 600 nm. Since this fabricating method makes use of the semipolar plane, the resultant device is the III-nitride semiconductor laser device making efficient use of polarization in the LED mode and achieves a low threshold current. The light emitting region can be, for example, any one of a single quantum well structure, a multiple quantum well structure, and a bulk structure.

In the fabricating method according to the foregoing aspect of the present invention, the active layer can include a quantum well structure provided so as to generate light at a wavelength of not less than 430 nm and not more than 550 nm. Since this fabricating method makes use of the semipolar plane, it allows for increase in quantum efficiency through both decrease of the piezoelectric field and improvement in crystal quality of the light emitting layer region, and it is thus suitable for generation of light at the wavelength of not less than 430 nm and not more than 550 nm.

In the fabricating method according to the foregoing aspect of the present invention, laser light from the active layer can be polarized in the direction of the a-axis of the hexagonal III-nitride semiconductor. In this fabricating method, a band transition allowing for achievement of a low threshold current has polarized nature. This transition can be used in the semiconductor laser device in which the c-axis is inclined toward the m-axis.

In the fabricating method according to the foregoing aspect of the present invention, light in the LED mode in the III-nitride semiconductor laser device includes a polarization component I1in the direction of the a-axis of the hexagonal III-nitride semiconductor, and a polarization component I2in a projected direction of the c-axis of the hexagonal III-nitride semiconductor on the primary surface, and the polarization component I1is greater than the polarization component I2. Since this fabricating method provides the laser structure on the primary surface of the substrate the c-axis of which is inclined toward the m-axis, it allows light in a mode with large emission intensity in the LED mode to lase with the laser cavity.

In the fabricating method according to the foregoing aspect of the present invention, preferably, a stacking fault density of the support base is not more than 1×104cm−1. When the stacking fault density falls within this range, the flatness and/or perpendicularity of the fractured facets can be made less likely to be disturbed for a certain accidental reason.

In the fabricating method according to the foregoing aspect of the present invention, preferably, a ratio of the distance between the reference line and the center line to a length of the laser cavity is in the range of not less than ⅓ and not more than ⅔, on the first surface. In the fabricating method according to the foregoing aspect of the present invention, preferably, a ratio of a distance between the first edge and the second edge to the length of the laser cavity is in the range of not less than 5/3 and not more than 5/2.

A III-nitride semiconductor laser device according to another aspect of the present invention comprises: (a) a laser structure including a support base which comprises a hexagonal III-nitride semiconductor and has a semipolar primary surface, and a semiconductor region provided on the semipolar primary surface of the support base; and (b) an electrode provided on the semiconductor region of the laser structure. The semiconductor region comprises a first cladding layer comprising a first conductivity type GaN-based semiconductor, a second cladding layer comprising a second conductivity type GaN-based semiconductor, and an active layer provided between the first cladding layer and the second cladding layer; the first cladding layer, the second cladding layer, and the active layer are arranged along a normal axis to the semipolar primary surface; the c-axis of the hexagonal III-nitride semiconductor of the support base is inclined at an angle ALPHA with respect to the normal axis to the semipolar primary surface toward the m-axis of the hexagonal III-nitride semiconductor; the laser structure comprises first and second fractured faces intersecting with an m-n plane defined by the m-axis of the hexagonal III-nitride semiconductor and the normal axis to the semipolar primary surface; a laser cavity of the III-nitride semiconductor laser device comprises the first and second fractured faces; the laser structure comprises first and second surfaces; the first surface is a surface opposite to the second surface; each of the first and second fractured faces extends from an edge of the first surface to an edge of the second surface.

In the foregoing III-nitride semiconductor laser device, since the first and second fractured faces that form the laser cavity intersect with the m-n plane defined by the m-axis of the hexagonal III-nitride semiconductor and the normal axis, it is feasible to provide a laser waveguide extending in a direction of an intersecting line between the m-n plane and the semipolar plane. Therefore, the present invention succeeds in providing the III-nitride semiconductor laser device with the laser cavity enabling a low threshold current. According to Inventors' experiment, when the first and second fractured faces to constitute the laser cavity are formed by the fabricating method according to the present invention, the fractured faces have excellent flatness and the excellent flatness contributes to reduction in threshold current.

The above object and the other objects, features, and advantages of the present invention can more readily become clear in view of the following detailed description of the preferred embodiments of the present invention proceeding with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the method for fabricating the III-nitride semiconductor laser device according to the present invention will be described with reference to the accompanying drawings. The same portions will be denoted by the same reference signs as much as possible.

The expertise of the present invention can be readily understood in view of the following detailed description with reference to the accompanying drawings provided by way of illustration only. The following will describe an embodiment of the method for fabricating the III-nitride semiconductor laser device according to the present invention, with reference to the accompanying drawings. The same portions will be denoted by the same reference signs as much as possible.

FIG. 1is a drawing schematically showing a structure of a III-nitride semiconductor laser device according to an embodiment of the present invention. The III-nitride semiconductor laser device11has a ridge structure, but embodiments of the present invention are not limited to the ridge structure. The III-nitride semiconductor laser device11has a laser structure13and an electrode15. The laser structure13includes a support base17and a semiconductor region19. The support base17comprises a hexagonal III-nitride semiconductor and has a semipolar primary surface17aand a back surface17b. The semiconductor region19is provided on the semipolar primary surface17aof the support base17. The electrode15is provided on the semiconductor region19of the laser structure13. The semiconductor region19includes a first cladding layer21, a second cladding layer23, and an active layer25. The first cladding layer21comprises a first conductivity type GaN-based semiconductor, e.g., n-type AlGaN, n-type InAlGaN, or the like. The second cladding layer23comprises a second conductivity type GaN-based semiconductor, e.g., p-type AlGaN, p-type InAlGaN, or the like. The active layer25is provided between the first cladding layer21and the second cladding layer23. The active layer25includes GaN-based semiconductor layers and the GaN-based semiconductor layers can be, for example, well layers25a. The active layer25includes barrier layers25bwhich comprise a GaN-based semiconductor, and the well layers25aand the barrier layers25bare alternately arranged. The well layers25acomprise, for example, of InGaN or the like, and the barrier layers25bcomprise, for example, GaN, InGaN, or the like. The active layer25can include a quantum well structure provided so as to generate light at the wavelength of not less than 360 nm and not more than 600 nm. Use of a semipolar plane is suitable for generation of light at the wavelength of not less than 430 nm and not more than 550 nm. The first cladding layer21, the second cladding layer23, and the active layer25are arranged along a normal axis NX normal to the semipolar primary surface17a. In the III-nitride semiconductor laser device11, the laser structure13includes a first fractured face27and a second fractured face29which intersect with an m-n plane defined by the normal axis NX and the m-axis of the hexagonal III-nitride semiconductor.

Referring toFIG. 1, an orthogonal coordinate system S and a crystal coordinate system CR are depicted. The normal axis NX is directed along a direction of the Z-axis of the orthogonal coordinate system S. The semipolar primary surface17aextends in parallel with a predetermined plane defined by the X-axis and the Y-axis of the orthogonal coordinate system S. InFIG. 1, a typical c-plane Sc is also depicted. The c-axis of the hexagonal III-nitride semiconductor of the support base17is inclined at a nonzero angle ALPHA with respect to the normal axis NX toward the m-axis of the hexagonal III-nitride semiconductor.

The III-nitride semiconductor laser device11further comprises an insulating film31. The insulating film31covers a surface19aof the semiconductor region19of the laser structure13, and the semiconductor region19is located between the insulating film31and the support base17. The support base17comprises a hexagonal III-nitride semiconductor. The insulating film31has an aperture31a, and the aperture31aextends in a direction of an intersecting line LIX between the surface19aof the semiconductor region19and the foregoing m-n plane and has, for example, a stripe shape. The electrode15is in contact with the surface19aof the semiconductor region19(e.g., a contact layer33of the second conductivity type) through the aperture31a, and extends in the direction of the foregoing intersecting line LIX. In the III-nitride semiconductor laser device11, a laser waveguide includes the first cladding layer21, the second cladding layer23and the active layer25, and extends in the direction of the foregoing intersecting line LIX.

In the III-nitride semiconductor laser device11, the first fractured face27and the second fractured face29intersect with the m-n plane defined by the normal axis NX and the m-axis of the hexagonal III-nitride semiconductor. A laser cavity of the III-nitride semiconductor laser device11includes the first and second fractured faces27and29, and the laser waveguide extends from one to the other of the first fractured face27and the second fractured face29. The laser structure13includes a first surface13aand a second surface13b, and the first surface13ais a surface opposite to the second surface13b. The first and second fractured faces27and29each extend from an edge13cof the first surface13ato an edge13dof the second surface13b. The first and second fractured faces27and29are different from the conventional cleaved facets such as c-planes, m-planes, or a-planes.

In this III-nitride semiconductor laser device11, the first and second fractured faces27and29that form the laser cavity intersect with the m-n plane. This allows for provision of the laser waveguide extending in the direction of the intersecting line between the m-n plane and the semipolar plane17a. Accordingly, the III-nitride semiconductor laser device11has the laser cavity that enables a low threshold current.

In the III-nitride semiconductor laser device11, the fractured face27has a first recess40a. The first recess40aextends in the direction from the first surface13ato the second surface13b, along the a-n plane defined by the normal axis NX and the direction of the a-axis and along a side face20a(which is different from the fractured faces27and29) of the III-nitride semiconductor laser device11. This first recess40ais formed of a scribed mark by fracture. The scribed mark extending in the direction from the first surface (epitaxial surface)13atoward the support base17along the a-n plane guides directions of propagation of fracture. The first recess40ais provided from the epitaxial surface into the support base17. Furthermore, the first fractured face27has a second recess40b, and the second recess40bextends in the direction from the first surface13ato the second surface13b. The second recess40bextends along the a-n plane and along a side face20b(which is different from the fractured faces27and29) of the III-nitride semiconductor laser device11. An end face of the optical waveguide (e.g., the end face of the active layer25) appears in the first fractured face27, and the end face of the active layer25in the first fractured face27is located between the recesses40aand40b. Each of these recesses40aand40bis formed of a scribed mark by fracture. These scribed marks extending in the direction from the first surface (epitaxial surface)13atoward the support base17along the a-n plane can guide the directions of propagation of fracture. The fractured face29can also have recesses42aand42bsimilar to those of the fractured face27.

As shown inFIG. 1, the III-nitride semiconductor laser device11has a ridge structure. In the ridge structure, the second conductivity type contact layer33has a stripe shape, and the second cladding layer23includes a flat portion covering the underlying semiconductor layer, and a ridge portion forming a stripe shape similar to the shape of the second conductivity type contact layer33. The III-nitride semiconductor laser device11includes an n-side optical guide layer35and a p-side optical guide layer37. The n-side optical guide layer35includes a first portion35aand a second portion35b, and the n-side optical guide layer35comprises, for example, GaN, InGaN, or the like. The p-side optical guide layer37includes a first portion37aand a second portion37b, and the p-side optical guide layer37comprises, for example, GaN, InGaN, or the like. A carrier block layer39is provided, for example, between the first portion37aand the second portion37b. Another electrode41is provided on the back surface17bof the support base17, and this electrode41covers, for example, the back surface17bof the support base17.

FIG. 2is a drawing showing a band structure in the active layer in the III-nitride semiconductor laser device.FIG. 3is a drawing showing polarization of emission in the active layer25of the III-nitride semiconductor laser device11.FIG. 4is a drawing schematically showing a cross section defined by the c-axis and the m-axis. With reference to part (a) ofFIG. 2, there are three possible transitions between the conduction band and valence bands in the vicinity of Γ point of the band structure BAND. There is a relatively small energy difference between band A and band B. An emission by transition Ea between the conduction band and band A is polarized in the a-axis direction, and an emission by transition Eb between the conduction band and band B is polarized in the direction of the projected c-axis onto the primary surface. Concerning lasing, a threshold of transition Ea is smaller than a threshold of transition Eb.

With reference to part (b) ofFIG. 2, there are shown spectra of light in the LED mode in the III-nitride semiconductor laser device11. The light in the LED mode includes a polarization component I1in the direction of the a-axis of the hexagonal III-nitride semiconductor, and a polarization component I2in the direction of the c-axis, projected on the primary surface, of the hexagonal III-nitride semiconductor, and the polarization component I1is larger than the polarization component I2. Degree of polarization ρ is defined by (I1−I2)/(I1+I2). By use of the laser cavity of the III-nitride semiconductor laser device11, the device can be lased to emit light in a mode with large emission intensity in the LED mode.

As shown inFIG. 3, the device may be further provided with dielectric multilayer film43a,43bon at least one of the first and second fractured faces27,29or with both on the respective faces. An end face coat is also applicable to the fractured faces27,29. The end face coat allows adjustment of reflectance.

As shown in part (b) ofFIG. 3, the laser light L from the active layer25is polarized in the direction of the a-axis of the hexagonal III-nitride semiconductor. In this III-nitride semiconductor laser device11, a band transition allowing for implementation of a low threshold current has polarized nature. The first and second fractured faces27,29for the laser cavity are different from the conventional cleaved facets such as c-planes, m-planes, or a-planes. However, the first and second fractured faces27,29have flatness and perpendicularity enough as mirrors for the cavity. For this reason, by using the first and second fractured faces27,29and the laser waveguide extending between these fractured faces27,29, as shown in part (b) ofFIG. 3, it becomes feasible to achieve low-threshold lasing through the use of the emission (I1) by transition Ea stronger than the emission (I2) by transition Eb polarized in the projected direction of the c-axis on the primary surface.

In the III-nitride semiconductor laser device11, as shown in part (a) ofFIG. 3, an end face17cof the support base17and an end face19cof the semiconductor region19are exposed in each of the first and second fractured faces27,29, and the end face17cand the end face19care covered by the dielectric multilayer film43a. An angle BETA between a normal vector NA to the end face17cof the support base17and an end face25cin the active layer25, and an m-axis vector MA of the active layer25is defined by component (BETA)1defined on a first plane S1defined by the c-axis and m-axis of the III-nitride semiconductor, and component (BETA)2defined on a second plane S2perpendicular to the first plane S1and the normal axis NX. The component (BETA)1is preferably in the range of not less than (ALPHA−5) degrees and not more than (ALPHA+5) degrees on the first plane S1defined by the c-axis and m-axis of the III-nitride semiconductor. This angle range is shown as an angle between a typical m-plane (standard plane orthogonal to the m-axis) SMand a reference plane FAinFIG. 4. The typical m-plane SMis depicted from the inside to the outside of the laser structure inFIG. 4, for easier understanding. The reference plane FAextends along the end face25cof the active layer25. This III-nitride semiconductor laser device11has the end faces satisfying the aforementioned perpendicularity, as to the angle BETA taken from one to the other of the c-axis and the m-axis. The component (BETA)2is preferably in the range of not less than −5 degrees and not more than +5 degrees on the second plane S2. Here, BETA2=(BETA)12+(BETA)22. In this case, the end faces27,29of the III-nitride semiconductor laser device11satisfy the aforementioned perpendicularity as to the angle defined on the plane perpendicular to the normal axis NX to the semipolar plane17a.

Referring again toFIG. 1, in the III-nitride semiconductor laser device11the thickness DSUB of the support base17is preferably not more than 400 μm. This III-nitride semiconductor laser device is suitable for obtaining good-quality fractured faces for the laser cavity. In the III-nitride semiconductor laser device11, the thickness DSUB of the support base17is more preferably not less than 50 μm and not more than 160 μm. This III-nitride semiconductor laser device11is more suitable for obtaining good-quality fractured faces for the laser cavity. Furthermore, handling becomes easier and the production yield becomes higher.

In the III-nitride semiconductor laser device11, the angle ALPHA between the normal axis NX and the c-axis of the hexagonal III-nitride semiconductor is preferably not less than 45 degrees and preferably not more than 80 degrees. Furthermore, the angle ALPHA is preferably not less than 100 degrees and preferably not more than 135 degrees. When the angle is in the range of less than 45 degrees or in the range of more than 135 degrees, the end faces made by press are highly likely to be comprised of m-planes. When the angle is in the range of more than 80 degrees and less than 100 degrees, it could result in failing to achieve the desired flatness and perpendicularity.

In the III-nitride semiconductor laser device11, more preferably, the angle ALPHA between the normal axis NX and the c-axis of the hexagonal III-nitride semiconductor is not less than 63 degrees and not more than 80 degrees. Furthermore, the angle ALPHA is more preferably not less than 100 degrees and not more than 117 degrees. When the angle is in the range of less than 63 degrees or in the range of more than 117 degrees, an m-plane can appear in part of an end face made by press. When the angle is in the range of more than 80 degrees and less than 100 degrees, it could result in failing to achieve the desired flatness and perpendicularity.

The semipolar primary surface17acan be any one of a {20-21} plane, a {10-11} plane, a {20-2-1} plane, and a {10-1-1} plane. Furthermore, a surface with a slight slant in the range of not less than −4 degrees and not more than +4 degrees from these planes is also suitable for the primary surface. On the semipolar surface17aof one of these typical planes, it is feasible to provide the first and second end faces27,29with flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device11. Furthermore, the end faces with sufficient flatness and perpendicularity are obtained in an angular range across these typical plane orientations.

In the III-nitride semiconductor laser device11, the stacking fault density of the support base17can be not more than 1×104cm−1. Since the stacking fault density is not more than 1×104cm−1, the flatness and/or perpendicularity of the fractured faces is less likely to be disturbed for a certain accidental reason. The stacking fault density of the support base17can be estimated by observing the substrate before the growth by the cathodoluminescence method. The cathodoluminescence is to observe an emission process of carriers excited by an electron beam and if there is a stacking fault, non-radiative recombination of carriers occurs in the vicinity thereof to be observed as a dark line. For example, the stacking fault density can be defined as a density (line density) per unit length of dark lines. The cathodoluminescence method of nondestructive measurement is applied herein in order to check the stacking fault density, but it is also possible to use a transmission electron microscope of destructive measurement. When a cross section of a sample is observed from the a-axis direction with the transmission electron microscope, a defect extending in the m-axis direction from the substrate toward the sample surface is a stacking fault included in the support base and the line density of stacking faults can be determined in the same manner as in the case of the cathodoluminescence method.

The support base17can be comprised of any one of GaN, AlN, AlGaN, InGaN, and InAlGaN. When the substrate comprised of any one of these GaN-based semiconductors is used, the end faces27,29applicable to the cavity can be obtained. When an AlN or AlGaN substrate is used, it is feasible to increase the degree of polarization and to enhance optical confinement by virtue of low refractive index. When an InGaN substrate is used, it is feasible to decrease the lattice mismatch rate between the substrate and the light emitting layer and to improve crystal quality.

FIG. 5is a drawing showing major steps in a method for fabricating a III-nitride semiconductor laser device according to an embodiment of the present invention. With reference to part (a) ofFIG. 6, a substrate51is shown. Step S101is to prepare the substrate51for fabrication of the III-nitride semiconductor laser device. The c-axis (vector VC) of the hexagonal III-nitride semiconductor of the substrate51is inclined at the angle ALPHA with respect to the normal axis NX toward the m-axis (vector VM) of the hexagonal III-nitride semiconductor. For this reason, the substrate51has a semipolar primary surface51acomprised of the hexagonal III-nitride semiconductor.

Step S102is to form a substrate product SP. In part (a) ofFIG. 6, the substrate product SP is depicted as a member of a nearly disklike shape, but the shape of the substrate product SP is not limited to this. For obtaining the substrate product SP, step S103is first carried out to form a laser structure55. The laser structure55includes a semiconductor region53and the substrate51and step S103is to form the semiconductor region53on the semipolar primary surface51a. For forming the semiconductor region53, a first conductivity type GaN-based semiconductor region57, a light emitting layer59, and a second conductivity type GaN-based semiconductor region61are grown in order on the semipolar primary surface51a. The GaN-based semiconductor region57can include, for example, an n-type cladding layer and the GaN-based semiconductor region61can include, for example, a p-type cladding layer. The light emitting layer59is provided between the GaN-based semiconductor region57and the GaN-based semiconductor region61and can include an active layer, optical guide layers, an electron block layer, and so on. The first conductivity type GaN-based semiconductor region57, the light emitting layer59, and the second conductivity type GaN-based semiconductor region61are arranged along the normal axis NX to the semipolar primary surface51a. These semiconductor layers are epitaxially grown. The surface of the semiconductor region53is covered by an insulating film54. The insulating film54is comprised, for example, of silicon oxide. The insulating film54has an aperture54a. The aperture54ais, for example, a stripe shape.

Step S104is to form an anode electrode58aand a cathode electrode58bon the laser structure55. Before forming the electrode on the back surface of the substrate51, the back surface of the substrate used in crystal growth is polished to form a substrate product SP in a desired thickness DSUB. In formation of the electrodes, for example, the anode electrode58ais formed on the semiconductor region53and the cathode electrode58bis formed on the back surface (polished surface)51bof the substrate51. The anode electrode58aextends in the X-axis direction and the cathode electrode58bcovers the entire area of the back surface51b. These steps result in forming the substrate product SP. The substrate product SP includes a first surface63a, and a second surface63blocated opposite thereto. The semiconductor region53is located between the first surface63aand the substrate51.

Step S105is, as shown in part (b) ofFIG. 6, to scribe the first surface63aof the substrate product SP. This scribing step is carried out with a laser scriber10a. This scribing step results in forming scribed grooves65a. In part (b) ofFIG. 6, five scribed grooves are already formed and formation of a scribed groove65bis in progress with a laser beam LB. The length of the scribed grooves65ais shorter than the length of an intersecting line AIS between the a-n plane defined by the a-axis of the hexagonal III-nitride semiconductor and the normal axis NX, and the first surface63a, and the laser beam LB is applied to a part of the intersecting line AIS. By the application with the laser beam LB, grooves extending in the specific direction and reaching the semiconductor region are formed in the first surface63a. The scribed grooves65acan be formed, for example, at an edge of the substrate product SP. The scribed grooves65aextend in the direction from the first surface63ato the second surface63balong the normal axis and a-n plane. The scribed grooves65aextend in the direction from the first surface63atoward the substrate51. Another substrate product and a laser bar are formed from the substrate product SP by press on the substrate product SP and propagation of the fracture is guided by the scribed grooves65aextending in the aforementioned depth direction. In part (b) ofFIG. 6, the substrate product is depicted in the disk-like shape, but the shape of the substrate product is not limited to this.

After forming the scribed grooves65a, step S106is carried out, as shown in part (a) ofFIG. 7, to mount the substrate product on a breaking device71to support first and second regions70a,70bof the substrate product SP by first and second support portions71a,71c, respectively, of the breaking device71. A first edge (e.g., an edge of a support surface)71bof the first support portion71ais separated from a second edge (e.g., an edge of a support surface)71dof the second support portion71c. A third region70cis provided between the first and second regions70a,70bof the substrate product SP and the breaking device71does not support the third region70c. Before mounted, the substrate product SP is placed between two films with flexibility (e.g., flexible films) FLM1, FLM2and the flexible films applicable herein are, for example, polyvinyl chloride films or the like. In the arrangement shown in part (a) ofFIG. 7, a blade69is aligned with an inside scribed groove65ain the substrate product SP of the larger size, in order to form a substrate product of a smaller size by breaking the substrate product SP of the larger size. In the present embodiment, the third region70cincludes two scribed grooves65a.

In part (c) ofFIG. 6, a laser bar LB0is already formed. Step S107is to carry out breakup of the substrate product by press in alignment with a mark of a desired scribed groove65aand thereby form another substrate product SP1and a laser bar LB1. The press is implemented using a breaking tool, e.g., the blade69. The breaking tool includes an edge69aextending in one direction, and at least two blade faces69b,69cdefining the edge69a. The press on the substrate product SP1is carried out on the support device71. The support device71includes support surfaces and edges71b,71dof support members71a,71cand the support surfaces end at the edges71b,71d. In part (c) ofFIG. 6, the substrate product is supported so that the laser bars successively formed are located on the support surface71a.

In step S107, as shown in part (c) ofFIG. 6, breakup of the substrate product is brought about by press against the second surface63bof this substrate product, to form a substrate product SP1and a laser bar LB1. The orientation of the scribed groove65aof the substrate product SP1is aligned with the extending direction of the edges71b,71dof the support device71and the substrate product SP is positioned on the basis of the edges71b,71dof the support device71. The scribed groove65ato cause next fracture is not positioned on the support surfaces71a,71c.

The foregoing breakup will be described below with reference to part (a) ofFIG. 7. In this form, the third region70cincludes two lines of scribed grooves65a. The substrate product SP is divided into the first region70a, second region70b, and third region70cwith respect to reference lines (X-coordinates A1, B1) extending along the edges71b,71d. In the substrate product SP, the first region70aand the second region70bare adjacent to the third region70c, and the third region70cis located between the first region70aand the second region70b.

The press is implemented on the third region70cwhile supporting the first and second regions70a,70bby the support device71but not supporting the third region70c. The direction of the edge69aof the breaking tool is aligned with the extending direction of the edge71band the edge69aof the breaking tool is pressed on the substrate product SP from a direction intersecting with the second surface63b. The intersecting direction is preferably a direction substantially perpendicular to the second surface63b. The press results in breaking the substrate product to form the other substrate product SP1and the laser bar LB1. The laser bar LB1produced as described above by the press has first and second end faces67a,67band the end faces of the laser waveguide in these end faces67a,67bhave perpendicularity and flatness enough to be applicable to the cavity mirrors of the semiconductor laser.

A laser bar producing method will be described with reference to part (a) ofFIG. 7. In the laser bar producing method shown in part (a) ofFIG. 7(which will be referred to as “method A”), the substrate product SP consists of the three regions70a,70b,70cseparated by the first and second reference lines (X-coordinates A1, B1). After formation of the scribed marks in this substrate product SP, the substrate product SP is positioned on the support device of breaking device71so as not to support the face of coordinates (near X-coordinate C1) on the surface63awhere the scribed marks are formed, by the support surfaces71a,71c. The substrate product SP is mounted on the support device71so that the reference lines (X-coordinates A1, B1) are separated from a center line CNT which defines a center between the first edge71band the second edge71d. After the positioning, the breakup of the substrate product SP is brought about by press along the press line (X-coordinate C1) in the central region70cof the substrate product SP while supporting the two-side regions70a,70bof the substrate product SP, whereby the other substrate product SP1and the laser bar LB1are formed as shown in part (c) ofFIG. 6. In the supporting process prior to the foregoing press, the regions70a,70bof the substrate product SP are supported, while the third region70cof the substrate product SP is not supported. For this reason, the third region70cbecomes bent in downwardly convex shape during the press.

When the press process is carried out to implement the press with the blade edge in alignment with the press line in the region70c, the third region70cis divided into two portions with respect to the press line and there are different force distributions in these two portions, as understood from the description below. For explaining this, the third region70cis divided into first and second portions70d,70ewith respect to the press line (X-coordinate C1) and the first portion70din the third region70cis adjacent to the second portion70eand the second region70band is located between them. The foregoing press applies shear force to the boundary (press line) between the first portion70eand the second portion70d. During the period of the press, the bending moment and shear force both act on the second portion70ebetween the supported first region70aand the press line on the third region70cof the substrate product SP, whereas the bending moment and shear force both act on the first portion70dbetween the supported second region70band the press line on the third region70cof the substrate product SP. The bending moment distribution in the first portion70dis different from that in the second portion70e, and the bending moment is asymmetric with respect to the press line.

In this method, the substrate product SP is divided into the first and second regions70a,70bof the substrate product SP to be supported during the breakup, and the third region70cof the substrate product SP to be pressed without being supported during the breakup. The substrate product SP is mounted on the breaking device71so that the reference lines (X-coordinates A1, B1) are separated from the center line CNT defining the center between the first edge71band the second edge71d. The shear force by the press is exerted at the press line (X-coordinate C1) at the position apart from the foregoing center line CNT. When this press is implemented, different forces are applied to the two portions70d,70eof the third region70cseparated with respect to the reference line (X-coordinate C1) along the press line. The bending moment distribution in the first portion70dis different from that in the second portion70e. The shear force is exerted at the press line, while the bending moment distributions are made asymmetric with respect to the press line. This method improves the flatness of the cavity mirrors in the III-nitride semiconductor laser device.

By this fracture method, improvement is made in flatness of the cavity mirrors and the forces for fracture include the shear force and bending moment. It is assumed in part (b) ofFIG. 7that the bending moment linearly varies in the substrate product, for easier understanding, but there is no need for limiting the variation in bending moment to it.

On the other hand, in a laser bar producing method shown in part (a) ofFIG. 8(which will be referred to as “method B”), the substrate product SP is supported on both sides, and the substrate product SP is supported by two support bodies spaced by a distance D between two edges71b(X-coordinates A2, B2). The press is carried out at a position (X-coordinate C2) equal to half (D/2) of the distance D. As shown in part (b) ofFIG. 8, the bending moment is applied on both sides of the press line (X-coordinate C2) CNT2, and it takes a maximum at the press line (X-coordinate C2) and decreases from the maximum toward the two edges71b(X-coordinates A2, B2). This bending moment is directed in a direction to contribute to the fracture. Deformation of the substrate product by the press is sufficiently large at the position of the applying line and the substrate product is curved in downwardly convex shape. Therefore, the fracture is caused by the maximum of bending moment and the shear force. It is seen by comparison betweenFIGS. 7 and 8that the distributions of bending moment and shear force in the substrate product SP are different from each other between the two laser bar producing methods.

The laser bar LB1thus formed has the first and second end faces67a,67bformed by the aforementioned breakup and each of the end faces67a,67bextends from the first surface63ato the second surface63b. For this reason, the end faces67a,67bform the laser cavity of the III-nitride semiconductor laser device and intersect with the XZ plane. This XZ plane corresponds to the m-n plane defined by the m-axis of the hexagonal III-nitride semiconductor and the normal axis NX.

According to this method, the first surface63aof the substrate product SP is scribed in the direction of the a-axis of the hexagonal III-nitride semiconductor and thereafter the breakup of the substrate product SP is carried out by the press against the second surface63bof the substrate product SP, thereby forming the new substrate product SP1and the laser bar LB1. For this reason, the first and second end faces67a,67bare formed in the laser bar LB1so as to intersect with the m-n plane. This end face forming method provides the first and second end faces67a,67bwith flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device.

In this method, the laser waveguide formed extends in the direction of inclination of the c-axis of the hexagonal III-nitride. The cavity mirror end faces allowing for provision of this laser waveguide are formed without use of dry-etched facets.

This method involves the fracture of the substrate product SP, thereby forming the new substrate product SP1and the laser bar LB1.

Step S108is to repeatedly carry out the breakup by the press to produce many laser bars. This fracture is brought about using the scribed groove65ashorter than a fracture line BREAK of the laser bar LB1.

Step S109is to form dielectric multilayer films on the end faces67a,67bof the laser bar LB1to form a laser bar product. Step S110is to break this laser bar product into chips of individual semiconductor lasers.

In the fabrication method according to the present embodiment, the angle ALPHA can be in the range of not less than 45 degrees and not more than 80 degrees or in the range of not less than 100 degrees and not more than 135 degrees. When the angle is in the range of less than 45 degrees or in the range of more than 135 degrees, the end face made by press becomes highly likely to be comprised of an m-plane. When the angle is in the range of more than 80 degrees and less than 100 degrees, it could result in failing to achieve the desired flatness and perpendicularity. More preferably, the angle ALPHA can be in the range of not less than 63 degrees and not more than 80 degrees or in the range of not less than 100 degrees and not more than 117 degrees. When the angle is in the range of less than 63 degrees or in the range of more than 117 degrees, an m-plane can appear in part of an end face formed by press. When the angle is in the range of more than 80 degrees and less than 100 degrees, it could result in failing to achieve the desired flatness and perpendicularity. The semipolar primary surface51acan be any one of a {20-21} plane, a {10-11} plane, a {20-2-1} plane, and a {10-1-1} plane. Furthermore, a surface with a slight slant in the range of not less than −4 degrees and not more than +4 degrees from these planes is also suitable for the primary surface. On these typical semipolar planes, it is feasible to provide the end faces for the laser cavity with flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device.

The substrate51can be comprised of any one of GaN, AlGaN, InGaN, and InAlGaN. When the substrate used is comprised of any one of these GaN-based semiconductors, it is feasible to obtain the end faces applicable to the laser cavity. The substrate51is preferably comprised of GaN.

In the step S104of forming the substrate product SP, the semiconductor substrate used in crystal growth can be one subjected to processing such as slicing or grinding so that the substrate thickness becomes not more than 400 μm, and having the second surface63bof a processed surface formed by polishing. In this substrate thickness, the end faces67a,67bcan be formed in good yield, with flatness and perpendicularity enough to construct the laser cavity of the III-nitride semiconductor laser device or without ion damage. More preferably, the second surface63bis a polished surface formed by polishing and the substrate thickness after polishing is not more than 160 μm. For relatively easily handling the substrate product SP, the substrate thickness is preferably not less than 50 μm.

In the production method of the laser end faces according to the present embodiment, the angle BETA explained with reference toFIG. 3is also defined in the laser bar LB1. In the laser bar LB1, the component (BETA)1of the angle BETA is preferably in the range of not less than (ALPHA−5) degrees and not more than (ALPHA+5) degrees on a first plane (plane corresponding to the first plane S1in the description with reference toFIG. 3) defined by the c-axis and m-axis of the III-nitride semiconductor. The end faces67a,67bof the laser bar LB1satisfy the aforementioned perpendicularity as to the angle component of the angle BETA taken from one to the other of the c-axis and the m-axis. The component (BETA)2of the angle BETA is preferably in the range of not less than −5 degrees and not more than +5 degrees on a second plane (plane corresponding to the second plane S2shown inFIG. 3). In this case, the end faces67a,67bof the laser bar LB1satisfy the aforementioned perpendicularity as to the angle component of the angle BETA defined on the plane perpendicular to the normal axis NX to the semipolar plane51a.

The end faces67a,67bare formed by break by press against the plurality of GaN-based semiconductor layers epitaxially grown on the semipolar plane51a. Since they are epitaxial films on the semipolar plane51a, the end faces67a,67bare not cleaved facets with a low plane index like c-planes, m-planes, or a-planes which have been used heretofore for the conventional cavity mirrors. However, through the break of the stack of epitaxial films on the semipolar plane51a, the end faces67a,67bhave the flatness and perpendicularity applicable as cavity mirrors.

A semipolar-plane GaN substrate is prepared and perpendicularity of a fractured face is observed as described below. The substrate used is a {20-21}-plane GaN substrate cut at the angle of 75 degrees toward the m-axis out of a (0001) GaN ingot thickly grown by HVPE. The primary surface of the GaN substrate is mirror-finished and the back surface is in a ground pear-skin state. The thickness of the substrate is 370 μm.

On the back side in the pear-skin state, a marking line is drawn perpendicularly to the projected direction of the c-axis on the primary surface of the substrate, with a diamond pen and thereafter a substrate product is broken by the press according to the method shown inFIG. 7.

Part (a) ofFIG. 9is a scanning electron microscope image obtained by observing a fractured face from the a-plane direction, in which the right end face is the fractured face. It is seen that the fractured face has flatness and perpendicularity to the semipolar primary surface.

A region around the end face of the active layer in the fractured face formed by the break is observed with a scanning electron microscope, and no prominent unevenness is observed. From this result, the flatness (magnitude of unevenness) of the region around the end face of the active layer is believed to be not more than 20 nm. Furthermore, the perpendicularity of the fractured face to the sample surface is in the range of −5 degrees to +5 degrees.

It is found in Example 1 that in the GaN substrate having the semipolar {20-21} plane, the fractured face obtained by drawing the marking line perpendicular to the projected direction of the c-axis on the primary surface of the substrate and pressing the substrate has the flatness and perpendicularity to the primary surface of the substrate. For checking applicability of this fractured face to the laser cavity, a laser diode shown inFIG. 10is grown by metal-organic vapor phase epitaxy as described below. The raw materials used are trimethyl gallium (TMGa), trimethyl aluminum (TMAl), trimethyl indium (TMIn), ammonia (NH3), and silane (SiH4). A substrate71is prepared. The substrate71prepared is a GaN substrate cut at an angle in the range of 0 degrees to 90 degrees toward the m-axis out of a (0001) GaN ingot thickly grown by HVPE, with a wafer slicing device, in such a manner that the angle ALPHA of inclination of the c-axis toward the m-axis has a desired off angle in the range of 0 degrees to 90 degrees. For example, when the substrate is cut at the angle of 75 degrees, the resultant substrate is a {20-21}-plane GaN substrate and it is represented by reference sign71ain the hexagonal crystal lattice shown in part (b) ofFIG. 9.

This substrate71is placed on a susceptor in a reaction furnace and the epitaxial layers are grown according to the following growth procedure. First, n-type GaN72is grown in the thickness of 1000 nm. Next, an n-type InAlGaN cladding layer73is grown in the thickness of 1200 nm. Thereafter, an n-type GaN guide layer74aand an undoped InGaN guide layer74bare grown in the thickness of 200 nm and in the thickness of 65 nm, respectively, and then a three-cycle MQW 75 consisting of GaN 15 nm thick/InGaN 3 nm thick is grown. Subsequently grown are an undoped InGaN guide layer76ain the thickness of 65 nm, a p-type AlGaN block layer77ain the thickness of 20 nm, and a p-type GaN guide layer76bin the thickness of 200 nm. Then a p-type InAlGaN cladding layer77bis grown in the thickness of 400 nm. Finally, a p-type GaN contact layer78is grown in the thickness of 50 nm.

An insulating film79of SiO2is deposited on the contact layer78and then photolithography is used to form a stripe window WIN in the width of 10 μm by wet etching. In this step, two types of contact windows are formed along two stripe directions. They are the laser stripe along (1) M-direction (direction of the contact window extending along the predetermined plane defined by the c-axis and the m-axis), and the laser stripe along (2) A-direction: <11-20> direction.

After the formation of the stripe window, a p-side electrode80aof Ni/Au and a pad electrode of Ti/Al are made by vapor deposition. Next, the back surface of the GaN substrate (GaN wafer) is polished using a diamond slurry to produce a substrate product with the back surface in a mirror state. At this time, the thickness of the substrate product is measured with a contact film thickness meter. The measurement of thickness may also be carried out from a sample cross section with a microscope. The microscope applicable herein is an optical microscope or a scanning electron microscope. An n-side electrode80bof Ti/Al/Ti/Au is formed by vapor deposition on the back surface (polished surface) of the GaN substrate (GaN wafer).

The cavity mirrors for these two types of laser stripes are produced with a laser scriber using the YAG laser at the wavelength of 355 nm. When the break is implemented with the laser scriber, the oscillating chip yield can be improved as compared with the case using the diamond scribing method. The conditions for formation of the scribed grooves are as follows: laser beam output of 100 mW; scanning speed of 5 mm/s. The scribed grooves thus formed are, for example, grooves having the length of 30 μm, the width of 10 μm, and the depth of 40 μm. The scribed grooves are formed by applying the laser beam directly to the epitaxially grown surface at the pitch of 800 μm and through the aperture of the insulating film of the substrate. The cavity length is 600 μm.

After formation of the scribed marks, a laser bar is produced from the substrate product, using the breaking device. The position of the scribed groove is aligned with the center position of the breaking blade and the blade is pressed from the back surface of the substrate of the substrate product to produce the laser bar. On that occasion, the laser bar is prepared by the following two methods.

“Method A”: method of pressing the blade against the back surface of the substrate of the substrate product away from the center line between the edges of the support tables, while supporting the both-side epitaxial surfaces of the substrate product by the support tables of the breaking device (e.g., bed knives).

“Method B”: method of pressing the blade against the back surface of the substrate of the substrate product in alignment with the center line between the edges of the support tables, while supporting the both-side epitaxial surfaces of the substrate product by the support tables of the breaking device (e.g., bed knives).

In the both methods, the distance between the edges of the support tables (bed knives) is 1000 μm.

The cavity mirrors are made by fracture using the blade. A laser bar is produced by breaking the substrate product by the press against the back side of the substrate. More specifically, part (b) ofFIG. 9and part (c) ofFIG. 9show relations between crystal orientations and laser end faces, for the {20-21}-plane GaN substrate. Part (b) ofFIG. 9shows the case where the laser stripe is provided in the inclination direction of the m-axis direction in accordance with the present embodiment and shows end faces81a,81bfor the laser cavity along with the semipolar plane71a. The end faces81a,81bare approximately perpendicular to the semipolar plane71a, but are different from the conventional cleaved facets such as the hitherto-used c-planes, m-planes, or a-planes. The direction of the laser stripe is different from that shown in part (c) ofFIG. 9. Part (c) ofFIG. 9shows the case where the laser stripe is provided in the a-axis direction and shows end faces81c,81dfor the laser cavity along with the semipolar plane71a. The end faces81c,81dare approximately perpendicular to the semipolar plane71aand are composed of a-planes.

For the laser bars produced by method (A) and method (B) above, angular deviation of the cavity mirrors is measured with an optical microscope. The semiconductor epitaxial region including a plurality of semiconductor epitaxial films is grown on the substrate in the substrate product. In lasing, light traveling back and forth in the cavity propagates mainly in the semiconductor epitaxial region. For this reason, the measurement of the angular deviation of the cavity mirrors is carried out by estimating angular deviation of a cross section of the semiconductor epitaxial region appearing in the fractured face, as shown in part (a) ofFIG. 11. In part (a) ofFIG. 11, a window WIN shown inFIG. 10is indicated by a dashed line, in order to show the position of the stripe waveguide. Part (b) ofFIG. 11is a histogram showing a relation of distributions of angular deviation with the laser bar producing methods. The horizontal axis represents deviation angle Δθ of cavity mirrors on the basis of the direction perpendicular to the direction of the stripe waveguide in the semiconductor epitaxial region. It is shown by the measurement results that the center of the distribution of angular deviation of the cavity mirrors in the laser bar produced by method (A) is closer to the angle of zero than that in the laser bar by method (B). The averages and standard deviations of angular deviations of the laser mirrors in the laser bars produced by methods (A) and (B) are as described below.

It is seen from this result that when the laser bar is produced by method (A), the cavity mirrors with small angular deviation (with the center of deviation angle distribution being closer to zero) can be stably formed. The vertical axis represents density, which is defined by (count in each angular range)/(total count).

When the laser bar is produced by method (A), the distribution of angular deviation of the cavity mirrors shifts toward zero. It is seen from this result that when the laser bar is produced by method (A), the cavity mirrors with small angular deviation can be formed on a stable basis.

FIG. 12is a drawing showing a relation of the inclination angle ALPHA (off-axis angle) of the c-axis toward the m-axis of the GaN substrate versus cavity yield. In the present example, the cavity yield is defined as (the number of cavities with the deviation angle in the epitaxial surface of not more than 1 degree)/(the total number of cavities subjected to the measurement of deviation angle). It is seen fromFIG. 12that there is no significant variation in cavity yield when the semipolar plane to be used is one having the off-axis angle in the range of 45 degrees to 80 degrees.

FIG. 13is a drawing showing a relation of substrate thickness versus cavity yield. With reference toFIG. 13, when the thickness of the substrate product is smaller than 50 μm, the laser bar can be more likely to be broken because handling becomes not easy. According to Inventors' knowledge, when the thickness of the substrate produce is not less than 50 μm, handling is possible without significant difficulty, and it is possible to produce the cavity by fracture. The laser bar can be produced when the thickness of the substrate product is up to 160 μm. As the thickness increases over 150 μm, the cavity yield starts decreasing. From this experiment and other results, the thickness of the substrate product is preferably in the range of not less than 50 μm and not more than 150 μm.

The end faces of the laser bar are coated with a dielectric multilayer film by vacuum vapor deposition. The dielectric multilayer film is composed of an alternate stack of SiO2and TiO2. The thickness of each layer is adjusted in the range of 50 to 100 nm and is designed so that the center wavelength of reflectance fell within the range of 500 to 530 nm. The reflecting surface on one side has ten cycles and the designed value of reflectance of about 95%, and the reflecting surface on the other side has six cycles and the designed value of reflectance of about 80%.

Evaluation by energization is carried out at room temperature. A power supply used is a pulsed power source with the pulse width of 500 ns and the duty ratio of 0.1% and the energization is implemented with needles on the surface electrodes. On the occasion of light output measurement, the emission from the laser bar end face is detected with a photodiode to check the current-optical output characteristic (I-L characteristic). In measurement of emission wavelength, the emission from the laser bar end face is made to pass through an optical fiber and a spectrum thereof is measured with a spectrum analyzer as a detector. In checking a polarization state, the emission from the laser bar is made to pass through a polarizing plate to rotate, thereby checking the polarization state. In observation of LED-mode emission, an optical fiber is arranged on the front surface side of the laser bar to measure light emitted from the front surface.

The polarization state after oscillation is checked for every laser, and it is found that the light is polarized in the a-axis direction. The oscillation wavelength of the light emitting region of the active layer is 500-530 nm.

The polarization state in the LED mode (spontaneous emission) is measured for every laser. When the polarization component in the a-axis direction is I1and the polarization component in the projected direction of the m-axis on the primary surface is I2, the polarization degree ρ is defined as (I1−I2)/(I1+I2). In this way, the relation between determined polarization degree ρ and minimum of threshold current density is investigated and the result obtained is as shown in FIG.14. It is seen fromFIG. 14that the threshold current density demonstrates a significant decrease in the case of the laser (1) with the laser stripe along the M-direction when the polarization degree is positive. Namely, it is seen that when the polarization degree is positive (I1>I2) and when the waveguide is provided along an off direction, the threshold current density is significantly decreased.

FIG. 15is a drawing showing a relation of cavity yield versus distance between the press line for press with the blade and the center line CNT of the spacing between the support portions (bed knives). In part (a) ofFIG. 15, the spacing between the support portions of the support device is referred to as “LENGTH” and the distance between the press line for press with the blade and the center line CNT of the spacing between the support portions is represented by “SHFT” as a relative shift amount of the blade to the center line. In this experiment, the spacing between the support portions is fixed at 1000 μm. As shown in part (b) ofFIG. 15, the shift amount SHFT of the blade defined as the distance between the center line CNT and the press line is varied in the range of 0 μm to 400 μm. Fractured faces are successfully produced by application of shear force with the shift in this range. When the distance between the center line and the press line is smaller than 200 μm, the cavity yield tends to decrease. A conceivable reason for it is that the laser bar producing method becomes closer to method (A) as the shift amount decreases toward zero. When the shift amount is zero, the yield is 29.3%. With the shift amount of 100 μm, improvement in yield is achieved. In this experiment, the cavity yield is high in the range of 200 μm to 400 μm. It is seen therefrom that the cavity yield is maintained at the high level. In order to prevent interference between the blade and the bed knife during the press on the substrate product with the blade, the distance between the center line and the press line is set in the range of not more than 400 μm. From the above result, the distance between the center line and the press line is preferably in the range of not less than 200 μm and not more than 400 μm.

FIG. 16is a drawing showing a relation of cavity yield versus spacing between the support portions. In this case, the shift amount SHFT is fixed at 200 μm and the thickness of the substrate is 80 μm. A laser bar is produced with the cavity length of 600 μm. It is considered that as the spacing between the support portions (LENGTH) becomes smaller than 1000 μm, the bending moment acting on the substrate product decreases, so as to lower the yield in production of the laser bar. In a setup where the spacing between the support portions (LENGTH) is larger than 1500 μm, there are three or more scribed grooves in the spacing between the support portions (LENGTH) in the case of the cavity length of 600 μm in the present example, and fracture may fail to propagate along a scribed line at a desired position, which will make production of the laser bar unstable. This is considered to be the cause to lower the yield in production of the laser bar. In the present example, the spacing between the bed knives is preferably not less than 1000 μm and not more than 1500 μm.

From the above and other experiment results, a ratio (SHFT/LCV) of the distance between the center line CNT and the press line as the reference line on the first surface63a(which is “SHFT” shown inFIG. 7) to the cavity length LCV is preferably not less than ⅓ (200 μm/600 μm). The ratio (SHFT/LCV) is preferably in the range of not more than ⅔ (400 μm/600 μm). From the above description, the ratio (SHFT/LCV) is preferably in the range of not less than ⅓ and not more than ⅔. Furthermore, a ratio (LENGTH/LCV) of the distance between the edges in the support device71(which is the spacing LENGTH between the first edge71band the second edge71d) to the cavity length LCV is preferably not less than 5/3 (1000 μm/600 μm). The ratio (LENGTH/LCV) is preferably not more than 5/2 (1500 μm/600 μm). From the above description, the ratio (LENGTH/LCV) is preferably in the range of not less than 5/3 and not more than 5/2.

In Example 2, the plural epitaxial films for the semiconductor laser are grown on the GaN substrate having the {20-21} plane. The end faces for the optical cavity are formed by the formation of scribed grooves and the press as described above. In order to find candidates for these end faces, plane orientations making an angle near 90 degrees to the (20-21) plane and being different from the a-plane are determined by calculation. The following angles and plane orientations have angles near 90 degrees to the (20-21) plane.

FIG. 17is a drawing showing atomic arrangements in the (20-21) plane, (−101-6) plane, and (−1016) plane.FIG. 18is a drawing showing atomic arrangements in the (20-21) plane, (−101-7) plane, and (−1017) plane.FIG. 19is a drawing showing atomic arrangements in the (20-21) plane, (−101-8) plane, and (−1018) plane. As shown inFIGS. 17 to 19, local atom arrangements indicated by arrows show configurations of neutral atoms in terms of charge and electrically neutral atom arrangements appear periodically. The reason why the relatively normal faces are obtained to the grown surface can be that generation of fractured faces is considered to be relatively stable because of the periodic appearance of the neutral atomic configurations in terms of charge.

According to various experiments including the above-described Examples 1 to 3, the angle ALPHA can be in the range of not less than 45 degrees and not more than 80 degrees or in the range of not less than 100 degrees and not more than 135 degrees. In order to improve the oscillating chip yield, the angle ALPHA can be in the range of not less than 63 degrees and not more than 80 degrees or in the range of not less than 100 degrees and not more than 117 degrees. The typical semipolar primary surface can be any one of the {20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1} plane. Furthermore, the primary surface can be a slight slant surface from these semipolar planes. The semipolar primary surface can be, for example, a slight slant surface off in the range of not less than −4 degrees and not more than +4 degrees toward the m-plane from any one of the {20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1} plane.

The embodiment provides the III-nitride semiconductor laser device with the laser cavity enabling the reduction of threshold current, on the semipolar plane of the support base inclined from the c-axis toward the m-axis of the hexagonal III-nitride. The embodiment also provides the method for fabricating the III-nitride semiconductor laser, which can improve the flatness of the cavity mirrors in the III-nitride semiconductor laser device and which can reduce the threshold current.

The principle of the present invention has been illustrated and described in the preferred embodiments, but it should be noted that it is recognized by those skilled in the art that the present invention can be modified in arrangement and detail without departing from the principle. The present invention is by no means limited to the specific configurations disclosed in the embodiments. Therefore, the inventor claims all modifications and changes falling within the scope of claims and coming from the scope of spirit of the invention.