JP2009158807A - Semiconductor laser diode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser diode outputting polarized light generated in an active layer having a semipolar surface as a principal surface, while suppressing increase of a threshold. <P>SOLUTION: The semiconductor laser diode includes: the active layer 30 consisting of a nitride semiconductor having the semipolar surface as a principal surface; a first reflecting film 20 arranged parallel to the principal surface of the active layer 30; and a second reflecting film 40 arranged to face the reflecting film 20 with the active layer 30 therebetween. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a semiconductor laser diode made of a nitride semiconductor.

An element made of a nitride semiconductor is used for a semiconductor laser diode. Examples of nitride semiconductors include aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN). A typical nitride semiconductor is represented by Al _x In _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). Gallium nitride (GaN) is a well-known nitride semiconductor among hexagonal compound semiconductors containing nitrogen. In a semiconductor laser diode using GaN, an active layer (light emitting layer) is generally formed on a GaN substrate, and light generated in the active layer is output to the outside.

An active layer made of a nitride semiconductor grown using a hexagonal semipolar plane as a crystal growth surface can output light in a strongly polarized state (polarized light). Further, in the nitride semiconductor whose main surface is a semipolar surface, the In composition ratio x in In _x Ga _1-x N (0 <x ≦ 1) compared to the case where the main surface is a polar surface or a nonpolar surface. It is known that can be increased. For example, an example in which an In _x Ga _1-x N layer is formed on a semipolar surface of a GaN substrate and orange light is output has been reported (see Non-Patent Document 1).
For example, if a semiconductor laser diode that outputs polarized light is used as a liquid crystal backlight or projector light source, the light component cut by a polarizer such as a deflector is reduced, and the efficiency of the liquid crystal backlight or projector light source is expected to improve. Has been. If green light is generated, an RGB light source can be realized together with red and blue light.
M. Funatao, et al., “Japanese Journal of Applied Physics vol. 45”, 2006, No. 26, p. L659-L662

  However, when an active layer of a nitride semiconductor is grown on the semipolar plane of the substrate, the side surface of the substrate is cleaved because there is no crystal plane perpendicular to the semipolar plane and oriented in the c-axis direction. It does not become a surface. In other words, in order to output polarized light in a strong polarization state, it is necessary to form a resonator in the direction along the c-axis and output the polarized light, but a surface for extracting polarized light to the outside (hereinafter referred to as “output surface”) .) Is difficult to form by cleavage. If the output surface is formed by etching or the like without cleaving, there is a problem that the resonance surface does not become a mirror surface and the threshold value of laser oscillation increases.

  In view of the above problems, the present invention provides a semiconductor laser diode capable of outputting polarized light generated in an active layer having a semipolar plane as a main surface while suppressing an increase in the threshold of laser oscillation.

  According to one aspect of the present invention, an active layer made of a nitride semiconductor having a semipolar plane as a main surface, a first reflective film disposed parallel to the main surface, and a first reflection with the active layer interposed therebetween A semiconductor laser diode is provided that includes a second reflective film disposed opposite the film.

  ADVANTAGE OF THE INVENTION According to this invention, the semiconductor laser diode which can output the polarization | polarized-light generated in the active layer which uses a semipolar surface as a main surface, suppressing the raise of the threshold value of a laser oscillation can be provided.

  Next, first to third embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thickness of each layer, and the like are different from the actual ones. Therefore, specific thicknesses and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  Also, the following first to third embodiments exemplify apparatuses and methods for embodying the technical idea of the present invention, and the technical idea of the present invention is the component parts. The material, shape, structure, arrangement, etc. are not specified below. The technical idea of the present invention can be variously modified within the scope of the claims.

(First embodiment)
As shown in FIG. 1A, the semiconductor laser diode 1 according to the first embodiment of the present invention includes an active layer 30 made of a nitride semiconductor having a semipolar plane (semipolar) as a main surface, and an active layer. The first reflective film 20 disposed in parallel with the main surface 30 and the second reflective film 40 disposed opposite to the first reflective film 20 with the active layer 30 interposed therebetween.

  The light extracted from the active layer made of a nitride semiconductor with the c-plane, which is the polar plane of the GaN crystal, as the crystal growth surface is in a randomly polarized (non-polarized) state. On the other hand, an active layer formed using a nitride semiconductor whose crystal growth surface is a nonpolar plane or semipolar plane such as an a plane or m plane other than the c plane generates light in a highly polarized state (polarized light). Therefore, the active layer 30 of the semiconductor laser diode 1 shown in FIG. 1 generates polarized light. Note that the polarized light indicates that the linearly polarized light component is not uniform (random) but biased, but it may not be 100% linearly polarized light. The direction with the largest linearly polarized light component is defined as the polarization direction of the polarized light. Details of the semipolar plane will be described later.

  As shown in FIG. 1A, the semiconductor laser diode 1 includes a first reflective film 20, an active layer 30, and a second reflective film 40 on a main surface 11 of a GaN substrate 10 that is a semipolar surface. It is the structure which laminated | stacked in this order. The first reflective film 20 and the second reflective film 40 reflect the polarized light generated in the active layer 30, and the first reflective film 20, the active layer 30, and the second reflective film 40 constitute a plate vertical resonator. Is done. The first reflective film 20 and the second reflective film 40 each function as a distributed black reflector (DBR) and can be configured as a laminated structure of a plurality of films having different refractive indexes.

  Further, the first electrode 50 is disposed in contact with the back surface of the GaN substrate 10 facing the main surface 11. Furthermore, the second electrode 60 is disposed on the second reflective film 40. As shown in FIG. 1, the second electrode 60 is disposed so as to expose a part of the upper surface of the second reflective film 40, and the output surface 41 is formed as the exposed upper surface of the second reflective film 40. As shown in FIG. 1A, the semiconductor laser diode 1 outputs output light L from the output surface 41 in a direction perpendicular to the main surface 11 of the GaN substrate 10. FIG. 1B shows a top view of the semiconductor laser diode 1 viewed from the output surface 41 direction.

  A first conductivity type carrier is supplied from the first electrode 50 to the active layer 30, and a second conductivity type carrier is supplied from the second electrode 60 to the active layer 30. When the first conductivity type is n-type and the second conductivity type is p-type, electrons supplied from the first electrode 50 via the first reflection film 20 and from the second electrode 60 to the second reflection film The holes supplied through 40 are recombined in the active layer 30, and light is generated from the active layer 30. Therefore, the first reflective film 20 is formed as an n-type nitride semiconductor stacked body, and the second reflective film 40 is formed as a p-type nitride semiconductor stacked body.

  As described above, the semiconductor laser diode 1 is a surface emitting laser diode including the active layer 30 and a pair of reflective films disposed in parallel with the main surface of the active layer 30 with the active layer 30 interposed therebetween. . That is, the semiconductor laser diode 1 operates as a so-called “vertical cavity surface emitting laser (VCSEL)” in which a resonator is disposed in a direction perpendicular to the main surface 11 of the GaN substrate 10. The surface emitting laser diode has excellent characteristics such as low threshold operation, a circular narrow emission beam, and easy two-dimensional array formation, and can be applied to various fields such as a display and high-speed optical communication.

  The semipolar plane will be described with reference to FIG. FIG. 2 is a schematic diagram showing a unit cell having a nitride semiconductor crystal structure. As shown in FIG. 2A, the crystal structure of the nitride semiconductor is a hexagonal system. The hexagonal c-axis is along the axial direction of the hexagonal column, and the plane (the top surface of the hexagonal column) having the c-axis as a normal line is the c-plane {0001}. When the nitride semiconductor crystal is cleaved by two planes parallel to the c-plane, the + c-axis side plane (+ c plane) becomes a crystal plane in which group III atoms are arranged, and the -c-axis side plane (-c plane) is It becomes a crystal plane in which nitrogen atoms are arranged. For this reason, the c-plane is called a polar plane because it exhibits different properties on the + c-axis side and the −c-axis side.

  As shown in FIG. 2B, four nitrogen atoms are bonded to one group III atom. Four nitrogen atoms are located at four vertices of a regular tetrahedron having a group III atom arranged at the center. Of these four nitrogen atoms, one nitrogen atom is positioned in the + c axis direction with respect to the group III atom, and the other three nitrogen atoms are positioned on the −c axis side with respect to the group III atom. Due to such a structure, in the nitride semiconductor, the polarization direction is along the c-axis.

  In the hexagonal system, the side surfaces of the hexagonal columns are each m-plane {1100}, and the plane passing through a pair of ridge lines that are not adjacent to each other is the a-plane {11-20}. Since these are crystal planes perpendicular to the c-plane and orthogonal to the polarization direction, they are nonpolar planes, that is, nonpolar planes. Further, as shown in FIG. 2C, the crystal plane inclined with respect to the c-plane (not parallel to the c-plane nor perpendicular) intersects with the polarization direction at an angle. It is a plane with a polarity, that is, a semipolar plane. Specific examples of the semipolar plane are {10-11} plane, {10-13} plane, {11-22} plane, and the like. That is, the main surface 11 of the GaN substrate 10 and the main surface of the active layer 30 are {10-11} plane, {10-13} plane, {11-22} plane, and the like.

  The active layer 30 is formed by stacking a lower barrier layer 31, a quantum well layer 32, and an upper barrier layer 33 in this order in the normal direction of the crystal growth surface, with the semipolar plane as the crystal growth surface. The active layer 30 is formed by, for example, metal organic chemical vapor deposition (MOCVD method) or the like. The quantum well layer 32 has a quantum well structure in which a well layer (well layer) 322 is sandwiched between barrier layers (layer barrier layers) 321 having a larger band gap than the well layer 322. Light is generated by recombination of electrons and holes in the quantum well layer 32, and the generated light is amplified.

  The quantum well structure of the quantum well layer 32 may be multiplexed instead of one well layer. For example, a multiple quantum well structure containing indium gallium nitride (InGaN) can be employed for the active layer 30 and is configured by repeatedly laminating InGaN layers and GaN layers having a thickness of several nanometers alternately for a plurality of periods. In this case, the InGaN layer becomes a well layer with a relatively small band gap by setting the composition ratio of indium (In) to, for example, 5% or more. On the other hand, the GaN layer functions as a barrier layer (barrier layer) having a relatively large band gap. The wavelength of light generated in the active layer 30 (hereinafter referred to as “emission wavelength”) can be set by adjusting the In composition ratio in the quantum well layer (InGaN layer) or the like.

  Since the main surface of the active layer 30 is a semipolar surface, InGaN having a higher In composition ratio can be grown than when the main surface is a polar surface or a nonpolar surface. Increasing the In composition ratio in the InGaN layer increases the emission wavelength. For this reason, by forming the active layer 30 including the InGaN film on the semipolar plane, it becomes easy to generate light having a wavelength of 500 nm or more, for example, green light. For example, the wavelength of light generated in the active layer 30 can be set to 500 nm or more by setting the In composition ratio in the InGaN film to about 0.2 to 0.4.

  FIG. 3 shows a configuration example of the active layer 30. In the example shown in FIG. 3, the quantum well layer 32 has a structure in which two well layers 322 are sandwiched between barrier layers 321. The lower barrier layer 31 in contact with the first reflective film 20 is, for example, n-type GaN doped with an n-type dopant such as silicon (Si), and the upper barrier layer 33 in contact with the second reflective film 40 is For example, p-type GaN doped with a p-type dopant such as magnesium (Mg).

The optical film thickness between the first reflective film 20 and the second reflective film 40, that is, the optical film thickness of the entire active layer 30 is set to about one wavelength of the output light L. For example, the lower barrier layer 31 is a GaN layer having a film thickness d ₃₁ , the barrier layer 321 is a GaN layer having a film thickness d ₃₂₁ , the well layer 322 is In _0.192 GaN having a film thickness d ₃₂₂ , and the upper barrier layer 33 is formed with a film thickness d. _When the GaN layer is ₃₃ , and the refractive index n1 of InGaN and the refractive index n2 of GaN, the optical film thickness W of the entire active layer 30 is expressed by the following formula (1):

_{W = 2 × d 322 × n1} + 3 × d 321 × n2 + (d 31 + d 33) × n2 ··· (1)

When the wavelength of the output light L is set to about 500 nm, since the refractive index n1 of InGaN is 2.77 and the refractive index n2 of GaN is 2.405, for example, d ₃₁ = d ₃₃ = 86 nm and d ₃₂₁ = 10 nm. , D ₃₂₂ = 3 nm, the optical film thickness W becomes approximately 500 nm. The film thicknesses of the lower barrier layer 31 and the upper barrier layer 33 can be set for adjusting the optical film thickness between the first reflective film 20 and the second reflective film 40, that is, the cavity length.

FIG. 4 shows the relationship between the band gap and emission wavelength of a nitride semiconductor that can be employed in the active layer 30 and the lattice constant. In FIG. 4, the left vertical axis represents the band gap (energy gap, unit eV), the right vertical axis represents the emission wavelength (unit nm), and the horizontal axis represents the lattice constant (unit angstrom) in the a-axis direction. Each vertex of the region shown by hatching in FIG. 4 indicates the characteristics of boron nitride (BN), aluminum nitride (AlN), and indium nitride (InN), respectively. Moreover, the point GaN in the hatching region indicates the characteristics of GaN. In other words, hatched areas are the possible characteristics of the nitride containing boron (B) semiconductor _{Al x B y In z Ga 1} -xyz N. For example, a straight line connecting vertices indicating BN and InN shows characteristics in which the composition ratio x is changed from 0 to _{1 in a} ternary mixed crystal of B _x In _1-x N.

FIG. 4 shows that the emission wavelength can be changed by changing the composition ratio of the nitride semiconductor while keeping the lattice constant constant. That, Al _x B _y In _z Ga _{by 1-xyz} N appropriately selecting the composition ratio of the barrier layer 321 and the well layer 322 of the quantum well layer 32 made of for, the lattice constant difference between the barrier layer 321 and the well layer 322 A quantum well structure can be realized without causing the above.

For example, consider the generation of light at a lattice constant of 3.189 in the a-axis direction of GaN shown in FIG. At this time, the band gap of GaN is 3.39 eV, and the wavelength of the output light L corresponding to this band gap is 365 nm. The emission wavelength at the straight line connecting the vertices indicating BN and InN at the lattice constant of 3.189 is 515 nm. Since the well layer 322 needs to have a band gap smaller than that of the barrier layer 321, when the barrier layer 321 is made of GaN, the composition of the well layer 322 is set in the range of the region D shown in FIG. Therefore, output light L having a wavelength of 365 nm to 515 nm can be generated in the active layer 30. In this case the lattice matching of the well layer 322 and barrier layer 321 have been made, by employing the _{Al x B y In z Ga 1} -xyz N active layer 30, can suppress the occurrence of cracks and defects.

  Next, the first reflective film 20 and the second reflective film 40 will be described. As already described, the first reflecting film 20 and the second reflecting film 40 function as a reflecting mirror of the vertical resonator. Further, the reflectance of the first reflective film 20 is set larger than that of the second reflective film 40. The polarized light generated in the active layer 30 is amplified while reciprocating between the first reflective film 20 and the second reflective film 40. A part of the amplified polarized light passes through the second reflective film 40 and is output as output light L from the output surface 41 formed on the upper surface of the second reflective film 40 to the outside of the semiconductor laser diode 1. Is done.

The semiconductor laser diode 1 shown in FIG. 1 is an example of a structure in which a plurality of nitride semiconductors having different refractive indexes are alternately stacked on the first reflective film 20 and the second reflective film 40. Here, it is assumed that each of the first reflective film 20 and the second reflective film 40 has a structure in which a plurality of Al _0.30 GaN films and GaN films are alternately stacked. FIG. 5 shows the relationship between the emission wavelength and the reflectivity for each number of stacked pairs in the case where the Al _0.30 GaN film and the GaN film are stacked to form the first reflective film 20 and the second reflective film 40. For example, the reflectivity of 5 pairs shown in FIG. 5 is a reflectivity in a reflective film in which five Al _0.30 GaN films and five GaN films are alternately arranged. The reflectance shown in FIG. 5 is a value calculated assuming that the refractive index of Al _0.30 GaN is 1.929 and the refractive index of GaN is 2.405.

The thickness of each layer included in the first reflective film 20 and the second reflective film 40 is set to λ / 4n according to the emission wavelength λ. n is the refractive index of each layer. For example, when the emission wavelength λ is 500 nm, the thickness of the Al _0.30 GaN layer is set to 64.8 nm, and the thickness of the GaN layer is set to 52.0 nm.

As shown in FIG. 5, when the wavelength of light is 500 nm, a reflectance of 99% or more can be obtained with 15 pairs or more. Although FIG. 5 illustrates the case of Al _0.30 GaN and GaN pairs, the number of pairs can be reduced if a pair of nitride semiconductors having a large refractive index difference is employed.

  As already described, the reflectance of the first reflective film 20 is set larger than that of the second reflective film 40. For example, the reflectance of the first reflective film 20 is set as close to 100% as possible, and the reflectance of the second reflective film 40 is set to about 97%. By setting the reflectance of the second reflective film 40 so high, the polarized light generated in the active layer 30 can be oscillated even with a short resonator length. A part of the polarized light generated in the active layer 30 and amplified while reciprocating between the first reflective film 20 and the second reflective film 40 passes through the second reflective film 40 and passes through the semiconductor laser diode 1. Is output outside of.

  The plurality of nitride semiconductors constituting the first reflective film 20 and the second reflective film 40 have different refractive indexes. For example, when the first reflective film 20 and the second reflective film 40 are formed by alternately laminating GaN and AlGaN, the Al composition ratio in AlGaN is increased in order to increase the difference in refractive index from GaN. It is effective. However, when the Al composition ratio is increased, the difference in lattice constant between the AlGaN layer and the GaN substrate 10 increases, and cracks, defects, and the like are generated. In particular, in the first reflective film 20 and the second reflective film 40, since the thickness of each layer is thicker than that of the active layer 30, cracks, defects, and the like due to lattice constant differences are likely to occur.

In general, a difference in refractive index is large between nitride semiconductors having a large band gap difference. Therefore, as the nitride semiconductor forming the first reflective layer 20 and the second reflective film 40, by employing the _{Al x B y In z Ga 1} -xyz N described with reference to FIG. 4, the first The occurrence of cracks and defects in the reflective film 20 and the second reflective film 40 can be suppressed. That is, as shown in FIG. 4, a plurality of _{Al x B y In z Ga 1} -xyz N set the composition ratios so that the difference in band gap remains to match the lattice constant increases by alternately stacking The first reflective film 20 and the second reflective film 40 are formed. As a result, the first reflective film 20 and the second reflective film 40 in which nitride semiconductors having the same lattice constant and a large difference in refractive index are stacked can be realized. By making the lattice constants of the first reflective film 20 and the second reflective film 40 coincide with the lattice constant of the GaN substrate 10, the occurrence of cracks and defects can be prevented.

  As described above, the semiconductor laser diode 1 operates as a VCSEL in which a resonator is disposed perpendicular to the main surface 11 of the GaN substrate 10. In general, a VCSEL has a lower laser oscillation threshold current than a Fabry-Perot resonator. For this reason, according to the semiconductor laser diode 1, it can be suppressed that the blue shift of the oscillation wavelength accompanying the high injection of carriers becomes an obstruction factor for extending the oscillation wavelength. The “blue shift” is a phenomenon that occurs because the band gap of the well layer (eg, InGaN layer) of the quantum well layer 32 is not uniform. In other words, recombination occurs only in a part of the region where the band gap of the well layer is narrow at low currents, but recombination with a large band gap occurs at the whole well layer at high currents, and the wavelength of the generated light Is a phenomenon that changes. That is, when the threshold value is high, there is a problem that the emission wavelength shifts. Since the threshold value of the semiconductor laser diode 1 is low, the occurrence of this emission wavelength shift is suppressed.

  The semiconductor laser diode 1 uses the exposed region of the upper surface of the second reflective film 40 as an output surface 41, and outputs output light L, which is polarized light generated in the active layer 30, from the output surface 41 to the outside of the semiconductor laser diode. . Therefore, the semiconductor laser diode 1 is flip-chip mounted so that the second electrode 60 is directly connected to a printed circuit board or the like. Although not shown, the bonding pad disposed on the second electrode 60 and the printed board are electrically connected by a bonding wire or the like.

  Below, the manufacturing method of the semiconductor laser diode 1 is demonstrated. Note that the manufacturing method of the semiconductor laser diode 1 described below is an example, and it is needless to say that the semiconductor laser diode 1 can be realized by various other manufacturing methods including this modification.

  (A) The first reflective film 20, the active layer 30, and the second reflective film 40 are epitaxially grown on the main surface 11 of the GaN substrate 10 using MOCVD or the like.

  (B) A conductive layer film constituting the second electrode 60 is formed on the second reflective film 40. Then, a part of the conductive layer film is removed using a lift-off method or the like to form the second electrode 60, and at the same time, the output surface 41 is formed as a part of the exposed second reflective film 40. Further, the first electrode 50 is formed on the back surface of the GaN substrate 10. For the first electrode 50, for example, an aluminum (Al) film, a laminate of Al-titanium (Ti) -gold (Au), or the like can be used. The second electrode 60 may be a palladium (Pd) -gold (Au) laminate, for example.

  As described above, in the semiconductor laser diode 1, the first reflective film 20, the active layer 30, and the second reflective film 40 can be continuously formed in the same growth furnace. Therefore, the process time can be shortened.

  As described above, the semiconductor laser diode 1 according to the first embodiment of the present invention includes the active layer 30 having a semipolar surface as the main surface, and the active layer 30 as the first reflective film 20 and the first reflective film 20. It has a structure sandwiched between two reflective films 40. For this reason, in the semiconductor laser diode 1, the output light L in which the polarization state of the polarized light generated in the active layer 30 is maintained is output in a direction perpendicular to the main surface 11 of the GaN substrate 10. That is, since the side surface of the GaN substrate 10 that cannot be cleaved is not used as the output surface, it is possible to provide the semiconductor laser diode 1 that suppresses an increase in threshold value and is highly efficient and has a small shift in emission wavelength.

  Moreover, since the semipolar plane can increase the In composition ratio in InGaN compared to the polar plane and the nonpolar plane, by adopting an active layer 30 including an InGaN layer having a high In composition ratio in the semiconductor laser diode 1, Polarized light can be output in a wavelength region of 500 nm or more. For example, the semiconductor laser diode 1 can output green light, and can realize an RGB light source in combination with red and blue light. Using this RGB light source, a small display or projector using a semiconductor laser diode can be realized.

(Second Embodiment)
As shown in FIG. 6, the semiconductor laser diode 1 </ b> A according to the second embodiment of the present invention has an active layer 30 formed on the main surface 11 of the GaN substrate 10 and is a transparent electrode on the active layer 30. A second electrode 60 is disposed. The semiconductor laser diode 1 shown in FIG. 1 is different from the semiconductor laser diode 1 shown in FIG. 1 in that the first reflective film 20 is disposed on the back surface of the GaN substrate 10 and the second reflective film 40 is disposed on the second electrode 60.

  In the semiconductor laser diode 1A, each of the first reflective film 20 and the second reflective film 40 has a structure in which a plurality of insulating films having different refractive indexes are alternately stacked. Therefore, the first electrode 50 is disposed on the main surface 11 of the GaN substrate 10 so as to be separated from the active layer 30, and the first conductivity type carriers are supplied from the first electrode 50 to the active layer 30 through the GaN substrate 10. Is done. Further, the second conductivity type carriers are directly supplied from the second electrode 60 to the active layer 30.

  As shown in FIG. 6, an insulating film 70 is disposed so as to cover a part of the side surface and the upper surface of the active layer 30. The second electrode 60 is in contact with the active layer 30 in the opening provided in the insulating film 70. That is, current confinement is performed by limiting the contact area between the active layer 30 and the second electrode 60.

As the second electrode 60, for example, a transparent electrode such as zinc oxide (ZnO) or indium-tin oxide (ITO) can be used. For the insulating film 70, for example, a zirconia (ZrO ₂ ) film, a silicon oxide (SiO ₂ ) film or the like can be employed.

The first reflective layer 20 and the second reflecting film 40, alternately example, a ZrO ₂ film having a thickness of lambda ₂ / (4n _1), and a SiO ₂ film having a thickness of lambda ₂ / (4n ₂₎ Are formed by repeatedly laminating a plurality of times. Here, λ is the emission wavelength, n ₁ is the refractive index of ZrO ₂ , and n ₂ is the refractive index of SiO ₂ . Also in the semiconductor laser diode 1A, the reflectance of the first reflective film 20 is set to be larger than that of the second reflective film 40 corresponding to the emission wavelength λ. For example, by adjusting the number of pairs of the ZrO ₂ film and the SiO ₂ film, the reflectance of the second reflective film 40 is about 97%, and the reflectance of the first reflective film 20 is 100% or a reflectance close thereto. To do. The first reflective film 20 and the second reflective film 40 are formed by, for example, sputtering.

  Since the second electrode 60 is a transparent electrode, the polarized light generated in the active layer 30 uses the GaN substrate 10, the active layer 30, and the second electrode 60 as resonance paths, and the first reflective film 20 and the second reflective film 40. Amplified while reciprocating between. A part of the amplified polarized light passes through the second reflective film 40 and is output from the output surface 41 on the upper surface of the second reflective film 40 to the outside of the semiconductor laser diode 1A.

  The semiconductor laser diode 1A according to the second embodiment of the present invention includes an active layer 30 having a semipolar plane as a main surface, a first reflective film 20 having a structure in which the active layer 30 is laminated with an insulating film, It has a structure sandwiched between two reflective films 40. In the semiconductor laser diode 1 </ b> A, polarized light generated in the active layer 30 is output in a direction perpendicular to the main surface 11 of the GaN substrate 10. For this reason, polarized light is output while suppressing an increase in threshold value. In the semiconductor laser diode 1 </ b> A, generation of cracks and defects can be suppressed as compared with the case where a nitride semiconductor is grown as the first reflective film 20 and the second reflective film 40. Others are substantially the same as those in the first embodiment, and redundant description is omitted.

(Third embodiment)
In the semiconductor laser diode 1B according to the third embodiment of the present invention shown in FIG. 7, the first reflective film 20 has a structure in which a plurality of nitride semiconductors having different refractive indexes are alternately stacked. The two reflective films 40 have a structure in which a plurality of insulating films having different refractive indexes are alternately stacked.

  Therefore, similarly to the semiconductor laser diode 1 shown in FIG. 1, the first reflective film 20 is disposed between the active layer 30 and the main surface 11 of the GaN substrate 10, and the first electrode is formed on the back surface of the GaN substrate 10. 50 is arranged. Further, the second reflective film 40 is disposed on the second electrode 60 in contact with the active layer 30 in the opening provided in the insulating film 70, as in the semiconductor laser diode 1A shown in FIG. The second electrode 60 is a transparent electrode. Then, current confinement is performed by limiting the contact area between the active layer 30 and the second electrode 60.

For example, ZnO or ITO can be used for the second electrode 60. For the insulating film 70, for example, a ZrO ₂ film, a SiO ₂ film, or the like can be employed.

As described in the first embodiment, for example, a structure in which an Al _0.30 GaN film and a GaN film are alternately and repeatedly stacked a plurality of times can be employed for the first reflective film 20. As described in the second embodiment, for example, a structure in which ZrO ₂ films and SiO ₂ films are alternately and repeatedly stacked a plurality of times can be employed for the second reflective film 40. Also in the semiconductor laser diode 1B, the reflectance of the first reflective film 20 is set to be larger than that of the second reflective film 40 corresponding to the emission wavelength λ.

  Polarized light generated in the active layer 30 is amplified while reciprocating between the first reflective film 20 and the second reflective film 40 using the active layer 30 and the second electrode 60 as resonance paths. A part of the amplified polarized light passes through the second reflective film 40 and is output from the output surface 41 on the upper surface of the second reflective film 40 to the outside of the semiconductor laser diode 1B.

  The semiconductor laser diode 1B according to the third embodiment of the present invention includes an active layer 30 having a semipolar plane as a main surface, a first reflective film 20 having a structure in which nitride semiconductors are stacked, and an insulating film. The second reflective film 40 having the above structure is sandwiched. In the semiconductor laser diode 1 </ b> B, polarized light generated in the active layer 30 is output in a direction perpendicular to the main surface 11 of the GaN substrate 10. For this reason, polarized light is output while suppressing an increase in threshold value. In the semiconductor laser diode 1B, one of the first reflective film 20 and the second reflective film 40 is formed by laminating an insulating film, so that both have a structure in which a nitride semiconductor is grown. Generation of cracks and defects can be suppressed. Others are substantially the same as those in the first embodiment, and redundant description is omitted.

(Other embodiments)
As described above, the present invention has been described according to the first to third embodiments. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples, and operational techniques will be apparent to those skilled in the art. For example, the active layer 30 may have a double hetero structure instead of a quantum well structure.

  As described above, the present invention naturally includes various embodiments not described herein. Accordingly, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

1A and 1B are schematic views showing a configuration of a semiconductor laser diode according to a first embodiment of the present invention, in which FIG. 1A is a cross-sectional view along a direction perpendicular to a main surface of an active layer, and FIG. FIG. FIG. 2A is a schematic diagram illustrating a semipolar plane, FIG. 2A is a schematic diagram illustrating a crystal structure of a nitride semiconductor, and FIG. 2B is a schematic diagram illustrating a bond between a group III atom and a nitrogen atom. FIG. 2C is a schematic diagram for explaining the semipolar plane. It is a schematic diagram which shows the structural example of the active layer of the semiconductor laser diode which concerns on the 1st Embodiment of this invention. It is a graph which shows the relationship between the band gap of a nitride semiconductor used for the semiconductor laser diode concerning the 1st Embodiment of this invention, a lattice constant, and light emission wavelength. It is a graph which shows the relationship between the reflectance of the reflective film of the semiconductor laser diode concerning the 1st Embodiment of this invention, and light emission wavelength. It is a schematic diagram which shows the structure of the semiconductor laser diode which concerns on the 2nd Embodiment of this invention. It is a schematic diagram which shows the structure of the semiconductor laser diode which concerns on the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1A, 1B ... Semiconductor laser diode 10 ... GaN board | substrate 11 ... Main surface 20 ... 1st reflective film 30 ... Active layer 31 ... Lower side barrier layer 32 ... Quantum well layer 321 ... Barrier layer 322 ... Well layer 33 ... Upper side Barrier layer 40 ... second reflective film 41 ... output surface 50 ... first electrode 60 ... second electrode 70 ... insulating film

Claims (6)

An active layer made of a nitride semiconductor having a semipolar plane as a main surface;
A first reflective film disposed parallel to the main surface;
A semiconductor laser diode, comprising: a second reflective film disposed opposite to the first reflective film with the active layer interposed therebetween.   2. The polarized light generated in the active layer through the second reflective film is output as the reflectivity of the first reflective film is larger than that of the second reflective film. Semiconductor laser diode.   3. The structure according to claim 1, wherein at least one of the first reflective film and the second reflective film has a structure in which a plurality of nitride semiconductors having different refractive indexes are alternately stacked. Semiconductor laser diode.   4. The semiconductor laser diode according to claim 3, wherein each of the plurality of nitride semiconductors is a nitride semiconductor containing boron and has the same lattice constant as that of the substrate.   3. The semiconductor according to claim 1, wherein at least one of the first reflective film and the second reflective film has a structure in which a plurality of insulating films having different refractive indexes are alternately stacked. Laser diode. 6. The semiconductor laser diode according to claim 1, wherein the active layer includes a nitride semiconductor layer made of In _x Ga _1-x N (0 <x ≦ 1).

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