Patent Application
20120184057
1. Field of the Invention
The present invention relates to a group-III nitride semiconductor laser device, and a method of fabricating the group-III nitride semiconductor laser device.
2. Related Background Art
Non Patent Literature 1 discloses a laser diode made on an m-plane GaN substrate. The laser diode has two cleaved end faces for an optical cavity. One of the cleaved end faces is a +c plane and the other cleaved end faces is a −c plane. In this laser diode, the reflectance of a dielectric multilayer film on the front end face (emitting face) is 70% and the reflectance of a dielectric multilayer film on the rear end face is 99%.
Non Patent Literature 2 discloses a laser diode made on a GaN substrate inclined at the angle of 1 degree with respect to the m-plane to the −c axis direction. The laser diode has two cleaved end faces for an optical cavity. One cleaved end face is a +c plane and the other cleaved end face is a −c plane. In this laser diode, the reflectance of the dielectric multilayer film on the front end face (exit face) is 90% and the reflectance of the dielectric multilayer film on the rear end face is 95%.
Non Patent Literature 1: APPLIED PHYSICS LETTERS 94, (2009), 071105.
Non Patent Literature 2: Applied Physics Express 2, (2009), 082102.
A light emitting device is made on a semipolar surface of a GaN substrate. In the GaN surface having semipolar nature, the c-axis of GaN is inclined with respect to a normal to the semipolar surface of the GaN substrate. In fabrication of a semiconductor laser using the GaN semipolar surface, when the c-axis of GaN is inclined toward an extending direction of a waveguide of the semiconductor laser, it becomes feasible to form the end faces available for an optical cavity. Dielectric multilayer films with desired reflectances are formed on these respective end faces to form the optical cavity. The thicknesses of the dielectric multilayer films on the two end faces are different from each other in order to obtain the dielectric multilayer films with the mutually different reflectances. The reflectance of the dielectric multilayer film on the front end face is set smaller than that of the dielectric multilayer film on the rear end face, and a laser beam is emitted from the front end face.
In nitride semiconductor lasers, catastrophic optical damage (COD) is caused. Inventors' experiments show that, when a number of semiconductor lasers as above, COD levels of these lasers widely distribute, and the reason for the wide distribution is not known.
The inventors' research finds that the distribution of COD levels is associated with the crystal orientation of the end faces for an optical cavity and the thicknesses of dielectric multilayer films.
It is an object of the present invention to provide a III-nitride semiconductor laser device having an improved resistance to COD. It is another object of the present invention to provide a method of fabricating a III-nitride semiconductor laser device with an improved resistance to COD.
A III-nitride semiconductor laser device according to a first aspect of the present invention comprises: (a) a laser structure comprising a support base and a semiconductor region, the support base having a semipolar primary surface of a III-nitride semiconductor, and the semiconductor region being provided on the semipolar primary surface of the support base; and (b) first and second dielectric multilayer films for an optical cavity of the nitride semiconductor laser device, the first and second dielectric multilayer films being provided on first and second end faces of the semiconductor region, respectively, the semiconductor region including a first cladding layer of a first conductivity type gallium nitride-based semiconductor, a second cladding layer of a second conductivity type gallium nitride-based semiconductor, and an active layer, and the an active layer being provided between the first cladding layer and the second cladding layer, the first cladding layer, the second cladding layer, and the active layer being arranged in a normal axis to the semipolar primary surface, the active layer comprising a gallium nitride-based semiconductor layer, a c+ axis vector being inclined at an angle in a range of not less than 45 degrees and not more than 80 degrees and of not less than 100 degrees and not more than 135 degrees toward a direction of any one crystal axis of m- and a-axes of the III-nitride semiconductor with respect to a normal vector, the c+ axis vector indicating a direction of a <0001> axis of the III-nitride semiconductor of the support base, and the normal vector indicating a direction of the normal axis, the first and second end faces intersecting with a reference plane, the reference plane being defined by the normal axis and the one crystal axis of the hexagonal III-nitride semiconductor, the c+ axis vector making an acute angle with a waveguide vector, and the waveguide vector indicating a direction from the second end face to the first end face, and a thickness of the first dielectric multilayer film being smaller than a thickness of the second dielectric multilayer film.
In this III-nitride semiconductor laser device, the c+ axis vector makes the acute angle with the waveguide vector and this waveguide vector is directed in the direction from the second end face to the first end face. An angle between the c+ axis vector and a vector normal to the second end face is larger than an angle between the c+ axis vector and a vector normal to the first end face. In this laser device, since the thickness of the first dielectric multilayer film on the first end face is smaller than the thickness of the second dielectric multilayer film on the second end face, the first dielectric multilayer film on the first end face works as the front side and a laser beam is emitted from this front side. The second dielectric multilayer film on the second end face works as the rear side and most of the laser beam is reflected by this rear side. In the laser device on the semipolar plane, when the thickness of the first dielectric multilayer film on the front side is smaller than the thickness of the second dielectric multilayer film on the rear side, reduction is achieved in device degradation due to the dielectric multilayer film on the end face, so as to provide an improvement in resistance to optical damage of end faces due to COD.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured as follows: the semiconductor region is located between the first surface and the support base, and wherein each of the first and second end faces is included in a fractured face, and the fractured face extends from an edge of the first surface to an edge of the second surface.
Since in the III-nitride semiconductor laser device the first and second end faces of the laser structure intersect with the reference plane defined by the normal axis to the primary surface and the a-axis or m-axis of the hexagonal III-nitride semiconductor, the first and second end faces can be formed as fractured faces, and each of the fractured faces extends from the edge of the first surface to the edge of the second surface.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the c-axis of the III-nitride semiconductor is inclined toward the direction of the m-axis of the nitride semiconductor. In another embodiment, the III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the c-axis of the III-nitride semiconductor is inclined toward the direction of the a-axis of the nitride semiconductor.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the primary surface of the support base is inclined in the range of not less than −4 degrees and not more than +4 degrees with respect to any one of {10-11}, {20-21}, {20-2-1}, and {10-1-1} planes. Furthermore, the III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the primary surface of the support base is any one of the {10-11} plane, {20-21} plane, {20-2-1} plane, and {10-1-1} plane.
In this III-nitride semiconductor laser device, when the c-axis of the III-nitride semiconductor is inclined toward the direction of the m-axis of the nitride semiconductor, practical plane orientations and angular range for the primary surface can include at least the aforementioned plane orientations and angle range.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the primary surface of the support base is inclined in the range of not less than −4 degrees and not more than +4 degrees from any one of {11-22}, {11-21}, {11-2-1}, and {11-2-2} planes. Furthermore, the III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the primary surface of the support base is any one of the {11-22} plane, {11-21} plane, {11-2-1} plane, and {11-2-2} plane.
In this III-nitride semiconductor laser device, when the c-axis of the III-nitride semiconductor is inclined toward the direction of the a-axis of the nitride semiconductor, practical plane orientations and angular range for the primary surface can encompass at least the aforementioned plane orientations and angle range.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the active layer comprises a well layer comprised of a strained gallium nitride-based semiconductor containing indium as a constituent element. Furthermore, the III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the active layer comprises a well layer comprised of strained InGaN.
With this III-nitride semiconductor laser device, the degradation of interest is observed in the GaN-based semiconductor containing indium as a Group III constituent element. The degree of degradation becomes more prominent with increase in the indium composition.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the active layer is adapted to generate light at a wavelength of 430 to 550 nm.
This III-nitride semiconductor laser device can provide the light in the aforementioned wavelength range by use of the well layer that comprises the strained GaN-based semiconductor containing, for example, indium as a Group III constituent element.
The III-nitride semiconductor laser device according to the first aspect of the present invention can be configured so that the III-nitride semiconductor is GaN. With this III-nitride semiconductor laser device, for example, the emission of light in the aforementioned wavelength range (wavelength range from blue to green) can be provided by creation of the laser structure using the GaN primary surface.
In the III-nitride semiconductor laser device according to the first aspect of the present invention, the first dielectric multilayer film has a dielectric layer, and the dielectric layer in the first dielectric multilayer film is comprised of at least one of silicon oxide, silicon nitride, silicon oxynitride, titanium oxide, titanium nitride, titanium oxynitride, zirconium oxide, zirconium nitride, zirconium oxynitride, zirconium fluoride, tantalum oxide, tantalum nitride, tantalum oxynitride, hafnium oxide, hafnium nitride, hafnium oxynitride, hafnium fluoride, aluminum oxide, aluminum nitride, aluminum oxynitride, magnesium fluoride, magnesium oxide, magnesium nitride, magnesium oxynitride, calcium fluoride, barium fluoride, cerium fluoride, antimony oxide, bismuth oxide, and gadolinium oxide. The second dielectric multilayer film has a dielectric layer, and the dielectric layer in the second dielectric multilayer film is comprised of at least one of silicon oxide, silicon nitride, silicon oxynitride, titanium oxide, titanium nitride, titanium oxynitride, zirconium oxide, zirconium nitride, zirconium oxynitride, zirconium fluoride, tantalum oxide, tantalum nitride, tantalum oxynitride, hafnium oxide, hafnium nitride, hafnium oxynitride, hafnium fluoride, aluminum oxide, aluminum nitride, aluminum oxynitride, magnesium fluoride, magnesium oxide, magnesium nitride, magnesium oxynitride, calcium fluoride, barium fluoride, cerium fluoride, antimony oxide, bismuth oxide, and gadolinium oxide.
In this III-nitride semiconductor laser device, practical materials of the dielectric films can include silicon oxide (e.g., SiO2), silicon nitride (e.g., Si3N4), silicon oxynitride (e.g., SiOxN1-x), titanium oxide (e.g., TiO2), titanium nitride (e.g., TiN), titanium oxynitride (e.g., TiOxN1-x), zirconium oxide (e.g., ZrO2), zirconium nitride (e.g., ZrN), zirconium oxynitride (e.g., ZrOxN1-x), zirconium fluoride (e.g., ZrF4), tantalum oxide (e.g., Ta2O5), tantalum nitride (e.g., Ta3N5), tantalum oxynitride (e.g., TaOxN1-x), hafnium oxide (e.g., HfO2), hafnium nitride (e.g., HfN), hafnium oxynitride (e.g., HfOxN1-x), hafnium fluoride (e.g., HEN, aluminum oxide (e.g., Al2O3), aluminum nitride (e.g., AlN), aluminum oxynitride (e.g., AlOxN1-x), magnesium fluoride (e.g., MgF2), magnesium oxide (e.g., MgO), magnesium nitride (e.g., Mg3N2), magnesium oxynitride (e.g., MgOxN1-x), calcium fluoride (e.g., CaF2), barium fluoride (e.g., BaF2), cerium fluoride (e.g., CeF3), antimony oxide (e.g., Sb2O3), bismuth oxide (e.g., Bi2O3), and gadolinium oxide (e.g., Gd2O3).
A second aspect of the present invention relates to a method of fabricating a III-nitride semiconductor laser device. This method comprises the steps of: (a) preparing a substrate with a semipolar primary surface, the semipolar primary surface comprising a hexagonal III-nitride semiconductor; (b) forming a substrate product having a laser structure, an anode electrode, and a cathode electrode, the laser structure comprising a substrate and a semiconductor region, and the semiconductor region being formed on the semipolar primary surface; (c) after forming the substrate product, forming first and second end faces; and (d) forming first and second dielectric multilayer films for an optical cavity of the nitride semiconductor laser device on the first and second end faces, respectively, the first and second end faces intersecting with a reference plane, the reference plane being defined by a normal axis to the semipolar primary surface and any one crystal axis of a- and m-axes of the hexagonal III-nitride semiconductor, the semiconductor region comprising a first cladding layer of a first conductivity type gallium nitride-based semiconductor, a second cladding layer of a second conductivity type gallium nitride-based semiconductor, and an active layer, and the active layer being provided between the first cladding layer and the second cladding layer, the first cladding layer, the second cladding layer, and the active layer being arranged in a direction of the normal axis, the active layer comprising a gallium nitride-based semiconductor layer, the semipolar primary surface of the substrate being inclined at an angle in a range of not less than 45 degrees and not more than 80 degrees and of not less than 100 degrees and not more than 135 degrees with respect to a plane perpendicular to a c+ axis vector, and the c+ axis vector indicating a direction of the <0001> axis of the nitride semiconductor, the c+ axis vector making an acute angle with a waveguide vector, and the waveguide vector indicating a direction from the second end face to the first end face, and a thickness of the first dielectric multilayer film being smaller than a thickness of the second dielectric multilayer film.
According to this method, the waveguide vector making the acute angle with the c+ axis vector corresponds to the direction from the second end face to the first end face, and the first dielectric multilayer film (C+ side) on the first end face is formed so as to be thinner than the second dielectric multilayer film (C− side) on the second end face in thickness; therefore, it is feasible to reduce optical absorption due to the interface between the semiconductor and the dielectric multilayer film on the end face thereby obtaining improvement in the COD level. In this III-nitride semiconductor laser device, the angle between the c+ axis vector and the normal vector to the second end face is larger than the angle between the c+ axis vector and the normal vector to the first end face. When the thickness of the first dielectric multilayer film (C− side) on the front side is smaller than the thickness of the second dielectric multilayer film (C+ side) on the rear side, the first dielectric multilayer film on the first end face works as the front side, and a laser beam is emitted from this front side. The second dielectric multilayer film on the second end face works as the rear side and most of the laser beam is reflected by this rear side.
The method according to the second aspect of the present invention further comprises the step of, prior to forming the first and second dielectric multilayer films, determining plane orientations of the first and second end faces. This method allows the selection of the appropriate dielectric multilayer films for the respective end faces in accordance with the result of determination and allows the growth of the dielectric multilayer films on the respective end faces.
The method according to the second aspect of the present invention can be configured as follows: the step of forming the first and second end faces comprises: the step of forming the first and second end faces comprises the steps of: scribing a first surface of the substrate product; and breaking the substrate product by press against a second surface of the substrate product to form a laser bar having the first and second end faces, the first and second end faces of the laser bar being formed by the breaking, the first surface being opposite to the second surface, the semiconductor region being provided between the first surface and the substrate, and each of the first and second end faces of the laser bar being included in a fractured face, and the fractured face extending from the first surface to the second surface and being formed by the breaking.
In this method, since the first and second end faces of the laser bar intersect with the reference plane defined by the normal axis to the primary surface and the a-axis or m-axis of the hexagonal III-nitride semiconductor, the first and second end faces can be formed as fractured faces by the scribe formation and press, and the fractured faces each extend from an edge of the first surface to an edge of the second surface.
The method according to the second aspect of the present invention can be configured so that the c-axis of the III-nitride semiconductor is inclined toward the direction of the m-axis of the nitride semiconductor. In another embodiment, the method according to the second aspect of the present invention can be configured so that the c-axis of the III-nitride semiconductor is inclined toward the direction of the a-axis of the nitride semiconductor.
The method according to the second aspect of the present invention can be configured so that the primary surface of the substrate is inclined in a range of not less than −4 degrees and not more than +4 degrees with respect to any one of {10-11}, {20-21}, {20-2-1}, and {10-1-1} planes. Furthermore, the method according to the second aspect of the present invention can be configured so that the primary surface of the substrate is any one of the {10-11} plane, {20-21} plane, {20-2-1} plane, and {10-1-1} plane.
In this method, when the c-axis of the III-nitride semiconductor is inclined toward the direction of the m-axis of the nitride semiconductor, practical plane orientations and angular range for the primary surface include at least the aforementioned plane orientations and angle range.
The method according to the second aspect of the present invention can be configured so that the primary surface of the substrate is inclined in the range of not less than −4 degrees and not more than +4 degrees from any one of {11-22}, {11-21}, {11-2-1}, and {11-2-2} planes. Furthermore, the method according to the second aspect of the present invention can be configured so that the primary surface of the substrate is any one of the {11-22} plane, {11-21} plane, {11-2-1} plane, and {11-2-2} plane.
In this substrate, when the c-axis of the III-nitride semiconductor is inclined toward the direction of the a-axis of the nitride semiconductor, practical plane orientations and angular range for the primary surface include at least the aforementioned plane orientations and angle range.
In the method according to the second aspect of the present invention, preferably, formation of the active layer comprises a step of growing a well layer of a strained gallium nitride-based semiconductor, and the strained gallium nitride-based semiconductor contains indium as a constituent element. In this method according to the second aspect of the present invention, the well layer is grown in the formation of the active layer and comprises strained InGaN, and this strain results from stress from and through a semiconductor layer adjacent to the well layer. In this method, the degradation of interest is observed in a GaN-based semiconductor containing indium as a Group III constituent element. The degree of degradation becomes more prominent with increase in the indium composition.
In the method according to the second aspect of the present invention, the active layer can be adapted to generate light at a wavelength of 430 to 550 nm. This method can provide the light in the aforementioned wavelength range by use of the well layer comprised of the strained GaN-based semiconductor containing indium as a constituent element.
In the method according to the second aspect of the present invention, preferably, the III-nitride semiconductor is GaN. In this method, for example, the emission of light in the aforementioned wavelength range (wavelength range from blue to green) can be provided by creation of the laser structure using the GaN primary surface.
In the method according to the second aspect of the present invention, a dielectric layer in the first dielectric multilayer film can be formed using at least one selected from silicon oxide, silicon nitride, silicon oxynitride, titanium oxide, titanium nitride, titanium oxynitride, zirconium oxide, zirconium nitride, zirconium oxynitride, zirconium fluoride, tantalum oxide, tantalum nitride, tantalum oxynitride, hafnium oxide, hafnium nitride, hafnium oxynitride, hafnium fluoride, aluminum oxide, aluminum nitride, aluminum oxynitride, magnesium fluoride, magnesium oxide, magnesium nitride, magnesium oxynitride, calcium fluoride, barium fluoride, cerium fluoride, antimony oxide, bismuth oxide, and gadolinium oxide. A dielectric layer in the second dielectric multilayer film can be formed using at least one selected from silicon oxide, silicon nitride, silicon oxynitride, titanium oxide, titanium nitride, titanium oxynitride, zirconium oxide, zirconium nitride, zirconium oxynitride, zirconium fluoride, tantalum oxide, tantalum nitride, tantalum oxynitride, hafnium oxide, hafnium nitride, hafnium oxynitride, hafnium fluoride, aluminum oxide, aluminum nitride, aluminum oxynitride, magnesium fluoride, magnesium oxide, magnesium nitride, magnesium oxynitride, calcium fluoride, barium fluoride, cerium fluoride, antimony oxide, bismuth oxide, and gadolinium oxide.
In this method, practical dielectric films can include silicon oxide (e.g., SiO2), silicon nitride (e.g., Si3N4), silicon oxynitride (e.g., SiOxN1-x), titanium oxide (e.g., TiO2), titanium nitride (e.g., TiN), titanium oxynitride (e.g., TiOxN1-x), zirconium oxide (e.g., ZrO2), zirconium nitride (e.g., ZrN), zirconium oxynitride (e.g., ZrOxN1-x), zirconium fluoride (e.g., ZrF4), tantalum oxide (e.g., Ta2O5), tantalum nitride (e.g., Ta3N5), tantalum oxynitride (e.g., TaOxN1-x), hafnium oxide (e.g., HfO2), hafnium nitride (e.g., HfN), hafnium oxynitride (e.g., HfOxN1-x), hafnium fluoride (e.g., HfF4), aluminum oxide (e.g., Al2O3), aluminum nitride (e.g., AlN), aluminum oxynitride (e.g., AlOxN1-x), magnesium fluoride (e.g., MgF2), magnesium oxide (e.g., MgO), magnesium nitride (e.g., Mg3N2), magnesium oxynitride (e.g., MgOxN1-x), calcium fluoride (e.g., CaF2), barium fluoride (e.g., BaF2), cerium fluoride (e.g., CeF3), antimony oxide (e.g., Sb2O3), bismuth oxide (e.g., Bi2O3), and gadolinium oxide (e.g., Gd2O3).
The foregoing object and other objects, features, and advantages of the present invention can more readily become apparent in view of the following detailed description of the preferred embodiments of the present invention proceeding with reference to the accompanying drawings.
11: III-nitride semiconductor laser device;
The expertise of the present invention can be readily understood in view of the following detailed description with reference to the accompanying drawings presented by way of illustration. Embodiments of the III-nitride semiconductor laser device and the method for fabricating the III-nitride semiconductor laser device is described with reference to the accompanying drawings. The same portions will be denoted by the same reference signs if possible.
The laser structure 13 includes a first end face 26 and a second end face 28 for an optical cavity. A waveguide for the optical cavity extends from the second end face 28 to the first end face 26, and a waveguide vector WV indicates a direction from the second end face 28 to the first end face 26. The first and second end faces 26 and 28 of the laser structure 13 intersect with a reference plane defined by the normal axis NX and a crystal axis of the hexagonal III-nitride semiconductor (m-axis or a-axis). In
With reference to
The thickness DREF1 of a first dielectric multilayer film (C+ side) 43a is smaller than the thickness DREF2 of a second dielectric multilayer film (C− side) 43b. In the III-nitride semiconductor laser device 11, the c+ axis vector makes an acute angle with the waveguide vector WV, and this waveguide vector WV indicates a direction from the second end face 28 to the first end face 26. In this example, since the thickness DREF1 of the first dielectric multilayer film 43a on the first end face 26 (C+ side) is smaller than the thickness DREF2 of the second dielectric multilayer film 43b on the second end face (C− side) 28, the first dielectric multilayer film 43a forms a front side, and thus a laser beam is emitted from this front side. The second dielectric multilayer film 43b forms the rear side, and most of the laser beam is reflected by this rear side. When the thickness DREF1 of the first dielectric multilayer film 43a on the front side is smaller than the thickness DREF2 of the second dielectric multilayer film 43b on the rear side, reduction is achieved in optical absorption around the interface between the semiconductor end face and the dielectric multilayer film thereon to increase resistance to end face deterioration due to COD.
The III-nitride semiconductor laser device 11 further has an insulating film 31. The insulating film 31 is provided on a top surface 19a of the semiconductor region 19 of the laser structure 13, and covers the top surface 19a. The semiconductor region 19 is located between the insulating film 31 and the support base 17. The support base 17 comprises the hexagonal III-nitride semiconductor. The insulating film 31 has an aperture 31a. The aperture 31a has, for example, a stripe shape. When the c-axis is inclined toward the direction of the m-axis as in the present embodiment, the aperture 31a extends along a direction of an intersecting line LIX between the top surface 19a of the semiconductor region 19 and the m-n plane mentioned above. The intersecting line LIX extends in the direction of the waveguide vector WV. If the c-axis is inclined toward the direction of the a-axis, the aperture 31a extends in a direction of another intersecting line LIX between the a-n plane and the top surface 19a.
The electrode 15 is in contact with the top surface 19a (e.g., second conductivity type contact layer 33) of the semiconductor region 19 through the aperture 31a, and extends along the direction of the aforementioned intersecting line LIX. In the III-nitride semiconductor laser device 11, the laser waveguide includes the first cladding layer 21, second cladding layer 23 and active layer 25, and extends along the direction of the aforementioned intersecting line LIX.
In the III-nitride semiconductor laser device 11, each of the first end face 26 and the second end face 28 can be a fractured face. In the subsequent description, the first end face 26, and the second end face 28 will be referred to as first fractured face 27 and second fractured face 29. The first fractured face 27 and the second fractured face 29 intersect with the m-n plane that is defined by the normal axis NX and the m-axis of the hexagonal III-nitride semiconductor. The optical cavity of the III-nitride semiconductor laser device 11 includes the first and second fractured faces 27 and 29, and the laser waveguide extends from one of the first fractured face 27 and the second fractured face 29 to the other. The laser structure 13 includes a first surface 13a and a second surface 13b, and the first surface 13a is opposite to the second surface 13b. Each of the first and second fractured faces 27 and 29 extends from an edge 13c of the first surface 13a to an edge 13d of the second surface 13b. The first and second fractured faces 27 and 29 are different from the conventional cleaved facets, such as c-plane, m-plane, or a-plane.
In this III-nitride semiconductor laser device 11, the first and second fractured faces 27 and 29 constituting the optical cavity intersect with the m-n plane. For this reason, it is feasible to provide the laser waveguide extending in the direction of the intersecting line between the m-n plane and the semipolar plane 17a. Therefore, the III-nitride semiconductor laser device 11 has the optical cavity enabling a low threshold current.
The III-nitride semiconductor laser device 11 includes an n-side optical guiding layer 35 and a p-side optical guiding layer 37. The n-side optical guiding layer 35 includes a first part 35a and a second part 35b, and the n-side optical guiding layer 35 comprises, for example, GaN, InGaN, or the like. The p-side optical guiding layer 37 includes a first part 37a and a second part 37b, and the p-side optical guiding layer 37 comprises, for example, GaN, InGaN, or the like. A carrier block layer 39 is provided, for example, between the first part 37a and the second part 37b. Another electrode 41 is provided on the back surface 17b of the support base 17, and the electrode 41 covers, for example, the back surface 17b of the support base 17.
As shown in part (b) of
In the III-nitride semiconductor laser device 11, an end face 17c of the support base 17 and an end face 19c of the semiconductor region 19 are exposed in each of the first and second fractured faces 27 and 29, and the end face 17c and the end face 19c are covered with a dielectric multilayer film 43a. An angle BETA between an m-axis vector MA of the active layer 25 and a vector NA normal to an end face 25c of the active layer 25 and the end face 17c of the support base 17 is defined by a component (BETA)1, which is defined on a first plane S1 defined by the c-axis and m-axis of the III-nitride semiconductor, and a component (BETA)2, which is defined on a second plane S2 perpendicular to the first plane S1 and the normal axis NX. The component (BETA)1 is preferably in the range of not less than (ALPHA−4) degrees and not more than (ALPHA+4) degrees on the first plane S1 that is defined by the c-axis and m-axis of the III-nitride semiconductor. This angular range is shown as an angle between a typical m-plane SM and a reference plane FA in
Referring again to
In the III-nitride semiconductor laser device 11, 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, and the angle ALPHA is preferably not less than 100 degrees and not more than 135 degrees. At angles below 45 degrees and above 135 degrees, end faces made by press are highly likely to be composed of m-planes. At angles above 80 degrees and below 100 degrees, its desired flatness and verticality could not be achieved.
In the III-nitride semiconductor laser device 11, in terms of formation of the fractured faces, the angle ALPHA between the normal axis NX and the c-axis of the hexagonal III-nitride semiconductor is more preferably not less than 63 degrees and not more than 80 degrees. Furthermore, the angle ALPHA is preferably not less than 100 degrees and not more than 117 degrees. At angles below 63 degrees and above 117 degrees, an m-plane can appear in part of an end face formed by press. The angle ALPHA in an angle above 80 degrees and below 100 degrees can provide the end faces with desired flatness and verticality.
In the III-nitride semiconductor laser device 11, when the c-axis of the III-nitride semiconductor is inclined toward the direction of the m-axis of the nitride semiconductor, practical plane orientations and angular range include at least the following plane orientations and angular range for the primary surface. For example, the primary surface 17a of the support base 17 can be inclined in the range of not less than −4 degrees and not more than 4 degrees from any one of a {10-11} plane, a {20-21} plane, a {20-2-1} plane, and a {10-1-1} plane. Furthermore, the primary surface 17a of the support base 17 can be any one of the {10-11} plane, {20-21} plane, {20-2-1} plane, and {10-1-1} plane.
In the III-nitride semiconductor laser device 11, when the c-axis of the III-nitride semiconductor is inclined toward the direction of the a-axis of the nitride semiconductor, practical plane orientations and angular range for the primary surface include at least the following plane orientations and angular range. The primary surface 17a of the support base 17 can be inclined in the range of not less than −4 degrees and not more than 4 degrees from any one of a {11-22} plane, a {11-21} plane, a {11-2-1} plane, and a {11-2-2} plane. Furthermore, the primary surface 17a of the support base 17 can be any one of the {11-22} plane, {11-21} plane, {11-2-1} plane, and {11-2-2} plane.
With these typical semipolar planes 17a, it is feasible to provide the first and second end faces 26 and 28 with flatness and verticality enough to constitute the optical cavity of the III-nitride semiconductor laser device 11. In the range of angles encompassing the above typical plane orientations, the end faces with sufficient flatness and verticality are obtained. The first dielectric multilayer film (C− side) 43a having a thickness smaller than that of the second dielectric multilayer film (C+ side) 43b can avoid the degradation due to the interface between the semiconductor light emitting layer and the dielectric multilayer film. The first dielectric multilayer film (C− side) 43a having a reflectance smaller than that of the second dielectric multilayer film (C+ side) 43b can reduce the optical absorption at the interface between the semiconductor end face and the dielectric multilayer film thereon to improve the COD level.
The support base 17 can be constituted by any one of GaN, AlGaN, InGaN, and InAlGaN. When the substrate is composed of any one of these GaN-based semiconductors, it is feasible to obtain the fractured faces 27 and 29 applicable to the optical cavity.
The support base 17 can be made of GaN. In this III-nitride semiconductor laser device, provision of the laser structure using the GaN primary surface leads to provision of emission, for example, in the aforementioned wavelength range (wavelength range from blue to green). When an AlN or AlGaN substrate is used as the substrate, the degree of polarization can be increased and optical confinement can be enhanced by its low refractive index. When an InGaN substrate is used as the substrate, the lattice mismatch rate between the substrate and the light emitting layer can be decreased to improve its crystal quality. In the III-nitride semiconductor laser device 11, the stacking fault density of the support base 17 can be not more than 1×104 cm−1. Since the stacking fault density is not more than 1×104 cm−1, the flatness and/or verticality of the fractured faces is less likely to be disordered for an accidental reason.
In Step S102, a substrate product SP is formed. Although a member of nearly a disk shape is depicted as the substrate product SP in part (a) of
In step S104, an anode electrode 58a and a cathode electrode 58b are formed on the laser structure 55. Before formation of the electrode on the back surface of the substrate 51, the back surface of the substrate used in the crystal growth is polished to form the substrate product SP having a desired thickness DSUB. In the formation of electrodes, for example, the anode electrode 58a is formed on the semiconductor region 53, and the cathode electrode 58b is formed on the back surface (polished surface) 51b of the substrate 51. The anode electrode 58a extends in the X-axis direction, and the cathode electrode 58b covers the entire area of the back surface 51b. Through these steps, the substrate product SP is formed. The substrate product SP includes a first surface 63a and a second surface 63b which is opposite thereto. The semiconductor region 53 is located between the first surface 63a and the substrate 51.
Next, in step S105, the end faces for the optical cavity for laser is formed. In the present embodiment, a laser bar is produced from the substrate product SP. The laser bar has a pair of end faces on which a dielectric multilayer film can be formed. An example of production of the laser bar and end faces will be described below.
In step S106, the first surface 63a of the substrate product SP is scribed as shown in part (b) of
In step S107, as shown in part (c) of
The laser bar LB1 thus formed has the first and second end faces 67a and 67b formed by the above-described breaking work, and each of the end faces 67a and 67b extends from the first surface 63a to the second surface 63b. For this reason, the end faces 67a and 67b constitute the laser optical 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. A waveguide vector WV is shown in each of laser bars LB0 and LB1. The waveguide vector WV is directed in the direction from the end face 67a to the end face 67b. In part (c) of
In this method, the first surface 63a of the substrate product SP is first scribed in the direction of the a-axis of the hexagonal III-nitride semiconductor, and thereafter the substrate product SP is broken by press against the second surface 63b of the substrate product SP to form a new substrate product SP1 and a new laser bar LB1. Accordingly, the first and second end faces 67a and 67b are formed in the laser bar LB1 so as to intersect with the m-n plane. This end-face forming method provides the first and second end faces 67a and 67b with flatness and verticality enough to constitute the laser cavity for the III-nitride semiconductor laser device. The laser waveguide thus formed extends in the direction toward which the c-axis of the hexagonal III-nitride is inclined. This method forms mirror end faces for the optical cavity capable of providing this laser waveguide.
By this method, the new substrate product SP1 and laser bar LB1 are formed by fracture of the substrate product. In Step S108, the breaking by press is repeated to produce many laser bars. This fracture is induced with the scribed grooves 65a shorter than a fracture line BREAK of the laser bar LB1.
In step S109, a dielectric multilayer film is formed on the end faces 67a and 67b of the laser bar LB1 to form a laser bar product. This step is carried out, for example, as follows. Step S110 is carried out to determine plane orientations of the end faces 67a and 67b of the laser bar LB1. This determination can be made, for example, by measuring the orientation of the c+ axis vector. Alternatively, the determination can also be made, for example, by carrying out the following process and/or operation to associate the end faces 67a and 67b with the direction of the c+ axis vector in production of the end faces 67a and 67b: a mark indicative of the direction of the c+ axis vector is formed on the laser bar; and/or the produced laser bar is arranged so as to indicate the direction of the c+ axis vector. After the determination, in the laser bar LB1 an angle between a normal vector normal to the second end face 67b, and the c+ axis vector is larger than an angle between a normal vector to the first end face 67a and the c+ axis vector.
After the determination, step S111 is carried out to form a dielectric multilayer film on each of the end faces 67a and 67b of the laser bar LB1. According to this method, the direction of the waveguide vector WV making the acute angle with the c+ axis vector corresponds to the direction from the second end face 67a to the first end face 67b in the laser bar LB1. In this laser bar product, since the thickness DREF1 of the first dielectric multilayer film (C+) on the first end face 67b is made smaller than the thickness DREF2 of the second dielectric multilayer film (C−) on the second end face 67a, it is feasible to enhance the resistance to end face damage due to COD. When the thickness DREF1 of the first dielectric multilayer film (C+) is smaller than the thickness DREF2 of the second dielectric multilayer film (C−), the first dielectric multilayer film on the first end face is provided for the front side, and a laser beam is emitted from this front side. The second dielectric multilayer film on the second end face is provided for the rear side, and most of the laser beam is reflected by this rear side.
In Step S112, this laser bar product is broken into individual semiconductor laser dies.
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. At angles below 45 degrees and above 135 degrees, an end face formed by press is highly likely to be comprised of an m-plane. At angles above 80 degrees and below 100 degrees, the desired flatness and verticality could not be achieved. More preferably, the angle ALPHA can be in the range of not less than 63 degrees and not more than 80 degrees and in the range of not less than 100 degrees and not more than 117 degrees. At angles below 63 degrees and above 117 degrees, an m-plane could be formed in part of an end face formed by press. At angles above 80 degrees and below 100 degrees, the desired flatness and verticality could not be achieved. The semipolar primary surface 51a can be any one of a {20-21} plane, a {10-11} plane, a {20-2-1} plane, and a {10-1-1} plane; or, when the c-axis is inclined toward the direction of the a-axis, the semipolar primary surface 51a can be any one of a {11-22} plane, a {11-21} plane, a {11-2-1} plane, and a {11-2-2} plane. Furthermore, a plane slightly inclined with respect to the above planes in the range of not less than −4 degrees and not more than +4 degrees is also suitably applicable to the foregoing primary surface. With these typical semipolar planes, it is feasible to provide the end faces for the laser cavity with optical flatness and verticality enough to constitute the laser cavity of the III-nitride semiconductor laser device.
The substrate 51 can be composed of any one of GaN, AlGaN, InGaN, and InAlGaN. When the substrate used is one comprised of any one of these GaN-based semiconductors, it is feasible to obtain the end faces applicable to the laser cavity. The substrate 51 is preferably made of GaN.
In the step S106 in which the substrate product SP is formed, the semiconductor substrate used in the crystal growth is subjected to processing such as slicing or grinding so that the substrate thickness becomes not more than 400 μm, and the second surface 63b can include a processed surface formed by polishing. In this substrate thickness, the use of fracture permits the flatness and verticality, with a good yield, enough to constitute the laser optical cavity of the III-nitride semiconductor laser device. The use of fracture allows formation of the end faces 67a and 67b that are not subjected to any ion damages. More preferably, the second surface 63b is a polished surface made by polishing, and the thickness of the polished substrate is not more than 100 μm. For easier handling of the substrate product SP, the substrate thickness is preferably not less than 50 μm. If the fracture is not used, then the end faces can be, for example, etched faces made by etching and this light emitting layer has end faces exposed at the etched faces.
In the production method of the laser end faces according to the present embodiment, the angle BETA, which was described with reference to
The end faces 67a and 67b are formed by the breaking process by press onto the plurality of GaN-based semiconductor layers that are epitaxially grown on the semipolar plane 51a. Since the semiconductor layers are made of the epitaxial films on the semipolar plane 51a, the end faces 67a and 67b are not cleaved facets with a low plane index such as c-plane, m-plane, or a-plane having been used heretofore as optical cavity mirrors. In breaking the laminate of the epitaxial films on the semipolar plane 51a, however, the end faces 67a and 67b have the flatness and verticality applicable to the optical cavity mirrors.
A laser diode is grown by organometallic vapor phase epitaxy as described below. Raw materials used are as follows: trimethyl gallium (TMGa); trimethyl aluminum (TMAl); trimethyl indium (TMIn); ammonia (NH3); silane (SiH4); and bis(cyclopentadienyl)magnesium (Cp2Mg). A substrate 71 is prepared, which is a {20-21} GaN substrate. This GaN substrate is fabricated by cutting a (0001) GaN ingot, grown thick by HVPE, with a wafer slicing apparatus at an angle of 75 degrees with respect to the m-axis direction.
This substrate is loaded into a susceptor in a growth reactor, and thereafter epitaxial layers for the laser structure shown in
Using this epitaxial substrate, an index guiding type laser is fabricated by photolithography and etching. First, a stripe mask is formed by photolithography, and the mask extends in a direction of the projected c-axis onto the primary surface. Using this mask, a striped ridge structure in the width of 2 μm is formed by dry etching. The dry etching is carried out, for example, using chlorine gas (Cl2). An insulating film 79 with a striped aperture is formed on the surface of the ridge structure. The insulating film 79 used is, for example, SiO2 formed by vacuum evaporation. After the formation of the insulating film 79, a p-side electrode 80a and an n-side electrode 80b are made to obtain a substrate product. The p-side electrode 80a is produced by vacuum evaporation. The p-side electrode 80a is, for example, Ni/Au. A backside of this epitaxial substrate is polished down to 100 μm. The polishing of the backside is carried out using diamond slurry. The n-side electrode 80b is formed on the polished surface by evaporation. The n-side electrode 80b is constituted of Ti/Al/Ti/Au.
A laser bar is produced by scribing along the surface of this substrate product, using a laser scriber capable of applying a YAG laser beam at the wavelength of 355 nm. The conditions for formation of scribed grooves were as follows:
In the laser diode on the {20-21}-plane GaN substrate according to the present example, since the end faces for the optical cavity are inclined with respect to the direction of polarity (e.g., the direction of the c+ axis vector), chemical properties of crystal planes of these end faces are not equivalent to each other. In the subsequent description, the end face 81b close to the +c plane will be referred to as {-1017} end face, and the end face 81a close to the −c plane as {10-1-7} end face. For descriptive purposes, the <-1014> and <10-1-4> directions, which are approximate normal vectors, are used as normal vectors to these end faces.
The end faces of the laser bar are coated with respective dielectric multilayer films 82a and 82b by vacuum evaporation. The dielectric multilayer films are made by alternately depositing two types of layers with mutually different refractive indices, e.g., SiO2 and TiO2. Each of the thicknesses of the films is adjusted in the range of 50 to 100 nm so that the center wavelength of reflectance is designed to fall within the range of 500 to 530 nm. The single wafer is divided into three in advance to produce three types of samples below.
A reflecting film (ten cycles, reflectance 95%) is formed on the {-1017} end face. The {-1017} end face is defined as a reflecting face (rear).
A reflecting film (four cycles, reflectance 60%) is formed on the {4017} end face. The {4017} end face is defined as a light exit face (front).
These laser devices are mounted on a TO header and these mounted devices are energized to evaluate the device lifetime. A DC power supply is used as the power supply. Among the laser diodes thus produced, those with the lasing wavelength of 520 to 530 nm are evaluated as to current versus optical output characteristics. On the occasion of measurement of optical output, emission from the end face of each laser device is detected with a photodiode. These laser devices are fed up to 400 mA, and the current at which COD is caused is measured for each laser device. Criteria for determining whether or not COD is caused in laser devices are as follows: optical output in high current injection decrease in measured current versus optical output characteristics; and physical damage on the end face is observed after the high current injection.
Values in columns for the devices A to C indicate maximum optical outputs in the above feeding up to 400 mA.
The representation using parentheses shows that the relevant device is not damaged in the current injection up to 400 mA, and the values in parentheses indicate optical output levels at the maximum current of 400 mA.
By use of the above measurements, optical output levels (COD levels) at which COD is caused are estimated. The estimated results are as follows:
The above results show that the excellent device lifetimes are achieved by taking account of the relation between the crystal planes and the total numbers of reflecting films in the laser diode chips fabricated from the same epitaxial substrate. Laser chips fabricated from the single epitaxial substrate exhibit different COD levels, i.e., some of them are lower than others, if the relation between the number of reflecting film layers and the crystal orientation is not taken into account. When the emitting end is formed on the {10-1-7} plane side with weaker chemical properties like Device A, the COD level is made lower. COD levels are reduced in high operating current and in high operating voltage. From the above tendency, the more heat is generated in the device and the less heat is dissipated from the device, the lower the COD level is. When the total thickness of the emitting well layers is not more than 6 nanometers, optical density pre one well layer becomes large, thereby further lowering COD levels.
The polarity (plane orientation indicating the direction of the c-axis) in the end faces of the laser bar can be determined, for example, as follows: the laser bar is processed by the focused ion beam (FIB) method to form a plane parallel to the waveguide, and this plane is observed by the transmission electron microscopic (TEM) method through the estimation using convergent beam electron diffraction (CBED) method. The total number of films can be checked by observing the portions of the dielectric multilayer films using a transmission electron microscope. It is presumed that the cause of the device degradation is deterioration of crystal quality of the well layers, having a high In composition, in contact with the reflecting film. In order to suppress this deterioration to obtain a long-lifetime device, it is preferable to increase the thickness of the reflecting film on the end face close to the −c plane and to decrease the thickness of the reflecting film on the end face close to the +c plane.
For evaluating the fundamental characteristics of the fabricated lasers, evaluation by energization is carried out at room temperature. A pulsed power supply is used as the power supply to generate the pulse width of 500 ns and the duty ratio of 0.1%. In measurement of optical output, emission from the laser end face is detected with a photodiode, and the current-optical output characteristic (I-L characteristic) is measured. In measurement of emission wavelength, the emission from the laser end face is made to pass through an optical fiber and a spectrum thereof is measured using a spectrum analyzer as a detector. In estimating the polarization, the emission from the laser is observed through a polarizer, and the polarizer is rotated to estimate the polarization state of the laser beam. In observing LED-mode light, an optical fiber is provided to receive light emitted from the top surface of the laser to measure light emitted from the top surface of the laser device.
The polarization state in the lasing state is estimated for all the lasers, which shows that the emission is polarized in the a-axis direction. The lasing wavelength is in the range of 500 to 530 nm.
The polarization state of the LED-mode light (spontaneous emission) is measured with all the lasers. The degree of polarization ρ is defined as (I1−I2)/(I1+I2), where I1 indicates a polarization component in the direction of the a-axis, and I2 indicates a polarization component in the direction of the projected m-axis onto the primary surface. A relationship of determined polarization degree ρ versus minimum threshold current density is estimated in this way, and the result obtained is shown in
A relationship of inclination angle of the c-axis toward the m-axis direction of the GaN substrate versus lasing yield is estimated, and the result obtained is shown in
The below provides plane indices of primary surfaces of GaN substrates and plane indices perpendicular to the primary surfaces of substrates and nearly perpendicular to the direction of the projected c-axis onto the primary surface. The unit of angle is “degree.” Plane index of primary surface: Angle to (0001), Plane index of first end face perpendicular to primary surface, Angle to primary surface.
It is understood with reference to
The cause of COD is as follows: Interface states, such as defect in the bandgap, absorb light to generate heat to reduce an effective value of the bandgap, which further increases quantity of optical absorption. That is, negative feedback turns on in optical absorption. According to Inventors' knowledge, it is presumed that the above reaction at the surface with the end face coating films is promoted with increase in the occurrence of arrangement of nitrogen atoms bound each through three bonds to the crystal at two or more continuous locations. At this time, the c+ axis vector indicating the direction of the <0001> axis of the GaN substrate is inclined at an angle in the range of approximately 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 toward the direction of any one crystal axis of the m-axis and the a-axis of the GaN substrate with respect to the normal vector indicating the direction of the normal axis to the primary surface of the GaN substrate. For example, part (a) of
In this laser diode, when the waveguide vector WV making the acute angle with the c+ axis vector is directed in the direction from the second end face (e.g., the end face 28 in
According to various experiments including the above examples, 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 lasing chip yield and device lifetime, 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. In the case of the inclination of the <0001> axis toward the m-axis direction, the primary surface can be any one of typical semipolar planes, e.g., the {20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1} plane. Furthermore, the primary surface can be a slightly inclined plane from these semipolar planes. The semipolar principal plane can be a slightly inclined plane off in the range of not less than −4 degrees and not more than +4 degrees toward the m-plane direction, for example, from any one of the {20-21} plane, {10-11} plane, {20-2-1} plane, and {10-1-1} plane. In the case of the inclination of the <0001> axis toward the a-axis direction, the primary surface can be any one of typical semipolar planes, e.g., the {11-22} plane, {11-21} plane, {11-2-1} plane, and {11-2-2} plane. Furthermore, the primary surface can be a slightly inclined surface from these semipolar planes. The semipolar principal plane can be a slightly inclined plane in the range of not less than −4 degrees and not more than +4 degrees toward the a-plane direction, for example, from any one of the {11-22} plane, {11-21} plane, {11-2-1} plane, and {11-2-2} plane.
As described above, the above embodiments provide the III-nitride semiconductor laser device having an improved resistance to COD. Furthermore, the above embodiment provides the method of fabricating the III-nitride semiconductor laser device with an improved resistance to COD.
Having been described and illustrated the principle of the present invention in the preferred embodiments, but it is recognized by those skilled in the art that the present invention can be modified in arrangement and in detail without departing from the principle. The present invention is by no means intended to be limited to the specific configurations disclosed in the embodiments. Therefore, the applicant claims all modifications and changes falling within the scope of claims and resulting from the scope of spirit thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2009-228894 | Sep 2009 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12831557 | Jul 2010 | US |
| Child | 13366748 | US |
