JP2009170798A - Group iii nitride semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor laser element capable of reducing influence of piezo polarization, having sufficiently low dislocation density, and having a structure suitable for an industrially-producible GaN wafer. <P>SOLUTION: In this group III nitride semiconductor laser 11, a semiconductor lamination 15 is formed on a principal surface 13a of a substrate 13 formed of a hexagonal n-type gallium nitride. A p-side electrode 17 is formed on a p-type gallium nitride-based semiconductor region 23 of the semiconductor lamination 15. An n-side electrode 19 is formed on the back surface 13b of the substrate 13. The principal surface 13a of the substrate 13 is tilted at an off-angle of 20-45° in the a-axis direction of the gallium nitride from a c-plane of the gallium nitride. When referring to an orthogonal coordinate system Cr, the c-coordinate axis is oriented to the c-axis of the hexagonal n-type gallium nitride of the substrate 13. The angle formed by a crystal axis vector C and the normal N of the principal surface 13a of the substrate 13 is the above off-angle. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a group III nitride semiconductor laser.

  Non-Patent Document 1 describes a 2-inch diameter free-standing c-plane GaN wafer. The GaN wafer is made from a GaN crystal grown by using the HVPE method after forming a mask on the GaAs (111) A plane. The mask has 6-fold symmetry.

  Non-Patent Document 2 describes a 2 inch diameter self-standing c-plane GaN wafer. Non-Patent Document 3 describes the production of a GaN crystal by the Advance-DEEP (hereinafter referred to as A-DEEP) method. An ultra-low dislocation density is realized in the GaN crystal by a method using the polarity difference. After a dot-shaped or striped patterned layer is formed on the GaAs (111) A surface, a GaN thick film is grown on the GaAs (111) A surface by HVPE. In most areas, Ga-polar GaN is grown, but N-polar GaN crystals are grown on the patterned layer. N-polar GaN crystals call a core. In the core, the growth rate of the N-polar GaN crystal is low, so the GaN crystal always forms a concave shape. Therefore, a V-shaped valley or an inverted pyramid-shaped pit is formed at the bottom of the concave portion. During the growth, the positions of the V-shaped valleys and pits are fixed by the core. As growth progresses, dislocations present on the facets of V-shaped valleys and pits progress toward the core. This achieves a low dislocation density. Non-Patent Document 4 describes the production of a GaN crystal by the A-DEEP method.

  Non-Patent Document 5 describes an InGaN / GaN light-emitting diode formed on an m-plane GaN single crystal. Non-Patent Document 6 describes a light emitting diode formed on m-plane GaN.

Patent Document 1 describes a method for producing a GaN single crystal having a low dislocation of 10 ⁶ cm ⁻² or less. In this single crystal gallium nitride crystal growth method, the growth surface of vapor phase growth is not in a planar state. It forms a three-dimensional facet structure and grows without embedding the facet structure while maintaining the facet structure to reduce dislocations.

  Patent Document 2 describes a method for producing a low dislocation GaN single crystal. In this method, a seed pattern is regularly provided on a base substrate, and facet pits are formed and maintained on the seed pattern, and GaN is facet grown to form a closed defect gathering region at the bottom of the pits comprising the facet surface. Dislocations are collected there, and the surrounding single crystal low dislocation associated region and single crystal low dislocation residual region are lowered. Since the closed defect collection region is closed, the dislocation is not confined and released again.

In Patent Document 3, a stripe-mask pattern is regularly provided on a base substrate, and a linear V-groove (valley) made of facets is formed thereon, and while maintaining this, GaN is facet grown from the facet surface. A defect gathering region is formed at the bottom of the V groove (valley), and dislocations are collected there, and the surrounding low defect single crystal region and C-plane growth region are lowered. Since the defect gathering region is closed, the dislocations are not confined and released again.
SEI Technical Review No. 161, page 77 Journal of crystal growth 237 (2002) pp.912-921 Journal of crystal growth 305 (2007) pp.377-383 Japan Society for the Promotion of Science, 161st Committee, 54th meeting, pages 5-10 JJAPVol.46, L126 (2007). JJAPVol.45, L1197 (2006). JP 2001-102307 A JP 2003-165799 A JP 2003-183100 A

  The c-plane of the GaN crystal is a polar plane, and therefore, the influence of piezo polarization appears on the light-emitting element. Therefore, a light emitting device that reduces the influence of piezoelectric polarization is desired. For this purpose, nonpolar planes such as a-plane and m-plane of GaN crystal can be used. In order to realize a semiconductor laser on these nonpolar surfaces, it is necessary to supply a GaN substrate having a nonpolar surface. Unless a GaN wafer having a practical size is supplied, it is difficult to industrialize a nonpolar semiconductor laser.

  However, the size of existing nonpolar plane substrates is only a few millimeters to tens of millimeters. In addition, in order to obtain a semiconductor laser of excellent quality, it must be formed using a GaN wafer having a low dislocation defect density. In order to realize a nonpolar plane semiconductor laser, it is necessary to solve these technical problems.

  On the other hand, regarding c-plane GaN wafers, GaN wafers having a practical size (ie, large diameter) such as 2 inches have been supplied. Large-diameter c-plane GaN wafers can be produced by the methods described in Non-Patent Documents 1, 2, 3, and 6. For control of high dislocation regions, A-DEEP in Non-Patent Documents 3 and 6 are used. The method is preferred.

  In crystal production of a GaN wafer, a GaN crystal is grown by heteroepitaxial growth on a heterogeneous substrate such as a GaAs (111) A-plane substrate using the HVPE method. In the growth of this GaN crystal, c-plane crystal growth occurs, and the GaN crystal grows in the c-axis direction. If an m-plane or a-plane wafer is to be cut out from this GaN crystal, a very thick crystal having the same thickness as the wafer diameter is required. Therefore, industrial realization of GaN crystals with such a thickness is difficult, and even if technically feasible, it is not industrial realization in terms of cost. Therefore, what is required is to produce a nitride semiconductor laser device having a structure suitable for industrially producible GaN wafers.

  Therefore, the present invention has been made in view of the above circumstances, and has a structure that can reduce the influence of piezoelectric polarization and has a sufficiently low dislocation density and is suitable for industrially producible GaN wafers. An object of the present invention is to provide a nitride semiconductor laser having the same.

The inventors examined the above crystal growth technique for c-plane GaN wafers and the influence of piezo polarization on laser emission characteristics, and decided to proceed with the technical investigation based on the following policy. That is, in a semiconductor laser,
(A) Even if the influence of piezoelectric polarization is not zero, the influence of piezoelectric polarization is reduced as much as possible.
(B) In order to avoid the complexity in crystal growth, not only the nonpolar plane but also the semipolar plane is included in the examination range.
(C) The dislocation defect density in the device region of the GaN wafer is sufficiently reduced.
Under these policies, we studied the structure and fabrication of semiconductor lasers.

  It became clear that what is important is what kind of GaN wafer to use. The inventors need to solve a number of technical problems up to mass production of GaN wafers, but have not been limited to the a-plane and m-plane, but have already been demonstrated to be possible to manufacture. I decided to use the surface. The GaN crystal plane has a large off angle from the c-plane. Using a wafer having a large off-angle from the c-plane means that a semipolar surface that can be realized is used instead of a nonpolar surface that is difficult to realize from the viewpoint of manufacturing a large-diameter GaN wafer. . The semipolar plane has a large off-angle from the c-plane and is inclined at an angle between the c-plane and a nonpolar plane perpendicular to the hexagonal system. The influence of piezo polarization on the semipolar plane is smaller than the influence of piezo polarization on the c plane.

As a result of these studies, the substrate to be used is as follows.
(1) Use a semipolar plane having an angle with reduced piezo polarization as much as possible (2) Reduced dislocation density of the fabricated GaN wafer (3) Practical size of 2 inch diameter Able to produce a wafer.
In order to satisfy these three conditions, the following approach was implemented.

  First, in order to satisfy the condition (3), the substrate manufacturing process starts with GaN heteroepitaxial growth by the HVPE method already proven in the production of c-plane wafers, removes the underlying substrate after the growth of the GaN crystal, It is preferred to make GaN crystals for GaN wafers.

  In order to reduce the dislocation density under the condition (2), an attempt was made to reduce the dislocation defect density using the DEEP method or the A-DEEP method. In order to easily obtain the semipolar plane of the condition (1), the semipolar plane is not cut out from the c-plane grown GaN crystal, but a large off angle is given to the base substrate for heteroepitaxial growth of GaN. An attempt was made to grow a GaN crystal on the surface with a large off-angle. However, when the off-angle of the base crystal is large, a good quality GaN crystal cannot be obtained. As a result of various trials, it became clear that there is an off-angle range that can provide good quality. Increasing the off-angle of the GaN wafer as much as possible reduces the influence of piezoelectric polarization. Also, in the production of GaN wafers having a semipolar surface, GaN wafers having a large off-angle can be processed by slicing the epitaxial thick film produced by the A-DEEP method at an angle. is there. In order to efficiently produce a GaN wafer, a substrate manufacturing method in which a GaN crystal is grown on a main surface with an appropriate off angle is preferable to an inclined slice.

  As a result of these attempts, the group III nitride semiconductor laser of the present invention can be fabricated using a GaN wafer of 2 inches or more that can be epitaxially grown well and has a large off-angle GaN main surface. Moreover, in the group III nitride semiconductor laser of the present invention, an active layer can be provided on the low dislocation region of the GaN wafer, and the influence of piezoelectric polarization is reduced. Next, the configuration of the group III nitride semiconductor laser will be described.

  According to one aspect of the present invention, a group III nitride semiconductor laser includes (a) a substrate made of hexagonal n-type gallium nitride, (b) an n-type gallium nitride semiconductor region, and a p-type gallium nitride semiconductor. A semiconductor stack provided on a main surface of the substrate, and a gallium nitride based semiconductor active layer provided between the n-type gallium nitride based semiconductor region and the p-type gallium nitride based semiconductor region, (C) a p-side electrode provided on the p-type gallium nitride based semiconductor region of the semiconductor stack, and (d) an n-side electrode provided on the back surface of the substrate. The main surface of the substrate is inclined from the c-plane of gallium nitride at an angle of 20 degrees or more and 45 degrees or less in the a-axis direction of gallium nitride, and the substrate extends along the c-axis direction of the substrate. A first region extending from the back surface to the main surface, and a second region extending from the back surface to the main surface along the first region, the first region of the substrate being The dislocation density is equal to or less than the dislocation density of the second region of the substrate, and the main surface of the substrate has a first area where the first region appears and a first region where the second region appears. The ohmic contact region between the p-side electrode and the p-type gallium nitride based semiconductor region is provided on the first area.

  According to this group III nitride semiconductor laser, the semiconductor stack is provided on the main surface of the substrate inclined at an angle of 20 degrees or more and 45 degrees or less from the c-plane of gallium nitride to the a-axis direction of gallium nitride. The influence of is reduced. Further, since the dislocation density in the first region of the substrate is smaller than the dislocation density in the second region, a semiconductor stack for the semiconductor laser is disposed on the substrate in which the dislocation density is controlled. Furthermore, the large-diameter GaN made of gallium nitride has a first region extending in the c-axis direction from the back surface to the main surface of the substrate and a second region extending from the back surface to the main surface along the first region. An epitaxial wafer for semiconductor lamination can be produced by performing epitaxial growth on the wafer.

  In the group III nitride semiconductor laser according to the present invention, preferably, the first and second regions extend in a direction of a predetermined axis, and the p-side electrode extends in the direction of the predetermined axis. According to this group III nitride semiconductor laser, since the first and second region alignment p-side electrodes extend in the direction of a predetermined axis, it is easy to orient the optical waveguide for the laser resonator.

  In the group III nitride semiconductor laser according to the present invention, the substrate includes a first portion extending from the back surface of the substrate to the main surface along the c-axis direction, and the first portion along the first portion. A second portion extending from the back surface to the main surface and a third portion extending from the back surface to the main surface along the second portion. The specific resistance of the first part is lower than the specific resistance of the second part. The specific resistance of the third part is lower than the specific resistance of the second part. The second portion is between the first portion and the third portion. The first region includes the first, second and third portions. According to this group III nitride semiconductor laser, the first to third portions are inclined along the c-axis direction with respect to the main surface of the substrate. In the substrate region, one of the first and third portions is inclined along the second portion. Therefore, element resistance can be reduced.

In the group III nitride semiconductor laser according to the present invention, the substrate may include a first portion having a low specific resistance, a first portion having a high specific resistance, and a third portion having a low specific resistance. . The first and second portions have a specific resistance in the range of 5 × 10 ⁻³ Ω · cm to 3 × 10 ⁻² Ω · cm, and are provided in parallel with the gallium nitride based semiconductor active layer. The second portion is adjacent to the first and third portions and has a specific resistance in the range of 4 × 10 ⁻² Ω · cm to 5 × 10 ⁶ Ω · cm.

  According to the group III nitride semiconductor laser, the first and third portions having a low specific resistance and the second portion having a high specific resistance in the above range are adjacent to each other, so that the injection current of the semiconductor laser is high. It flows not only in the specific resistance part but also in the low specific resistance part.

  In the group III nitride semiconductor laser according to the present invention, the second region of the substrate includes a polarity reversal region having a first polarity in a c-axis direction, and the substrate includes the polarity reversal region, A non-inversion region having a polarity opposite to the first polarity. The polarity inversion region in the second region of the substrate may have a dislocation density that is greater than a dislocation density of the polarity non-inversion region. According to this group III nitride semiconductor laser, dislocations gather in the polarity inversion region, resulting in a high dislocation density. Therefore, the dislocation density in the polarity non-inversion region is reduced.

  In the group III nitride semiconductor laser according to the present invention, the second region is a polarity reversal region having a first polarity in the c-axis direction, and the substrate includes the polarity reversal region and the first reversal region. A non-inversion region having a polarity opposite to the polarity. The group III nitride semiconductor laser may further include a side surface extending in the m-axis direction. The polarity inversion region in the second region of the substrate may be present on at least a part of the side surface. According to this group III nitride semiconductor laser, the dislocation density is low by arranging the first and second regions so that the polarity inversion region having a high dislocation density exists in at least a part of the side surface of the semiconductor laser. These regions can be arranged between the side surfaces of the semiconductor laser.

  In the group III nitride semiconductor laser according to the present invention, the second region of the substrate includes a polarity reversal region having a first polarity in a c-axis direction, and the substrate includes the polarity reversal region, A polarity non-inversion region having a polarity opposite to the first polarity, and a boundary between the polarity inversion region and the polarity non-inversion region extends along a reference plane intersecting the main surface. it can.

  According to this group III nitride semiconductor laser, the polarity inversion region and the polarity non-inversion region are related to the dislocation density, and the boundary between the polarity inversion region and the polarity non-inversion region extends along the reference plane where the main surfaces intersect. Therefore, the light emitting region can be arranged on the low dislocation region by adjusting the position of the light emitting region in the semiconductor stack with respect to the polarity inversion region and the polarity non-inversion region.

  In the group III nitride semiconductor laser according to the present invention, the polarity inversion region may extend in the c-axis direction of gallium nitride of the substrate. According to this group III nitride semiconductor laser, most of the dislocations extend in the c-axis direction (growth direction), so that the dislocation density on the main surface of the substrate decreases according to the off angle.

  In the group III nitride semiconductor laser according to the present invention, the specific resistance of the polarity inversion region is lower than the specific resistance of the non-polarity inversion region, and the polarity inversion region has a normal line between the main surface and a c-axis. It may extend along a reference plane inclined with respect to the main surface at an angle formed. According to the group III nitride semiconductor laser, the polarity inversion region is inclined according to the reference plane, thereby approaching a region having non-inversion polarity and located between the p-side electrode and the n-side electrode. Come. Since the specific resistance in the polarity inversion region is lower than the specific resistance in the polarity non-inversion region, the element resistance can be lowered.

In the group III nitride semiconductor laser according to the present invention, the dislocation density of the first region is in the range of 1 × 10 ² cm ^{−2 to} 1 × 10 ⁶ cm ⁻² , and the dislocation in the second region is The density can be in the range of 1 × 10 ⁶ cm ^{−2 to} 1 × 10 ⁸ cm ⁻² . According to this group III nitride semiconductor laser, the region having a low dislocation density can have a dislocation density of 1 × 10 ⁶ cm ⁻² or less.

  In the group III nitride semiconductor laser according to the present invention, the average dislocation density in the second region may be a value that is three times or more the average dislocation density in the first region. In the substrate used for this group III nitride semiconductor laser, the low dislocation GaN region existing between the polarity inversion regions has a dislocation density distribution. When the average dislocation density is measured statistically, the center region has the lowest dislocation density, and the dislocation density decreases toward the outer polarity inversion region. When estimated in consideration of the chip size of the semiconductor laser, the average dislocation density in the GaN region having a high dislocation density in the semiconductor laser is about three times or more the average dislocation density in the GaN region having a low dislocation density.

  The group III nitride semiconductor laser according to the present invention may further include first and second cleavage planes intersecting the m-axis. According to this group III nitride semiconductor laser, the yield of cleavage at the m-plane is improved.

  In the group III nitride semiconductor laser according to the present invention, the main surface of the substrate is preferably inclined at an angle of 28 degrees or more and 32 degrees or less with respect to the a-axis direction of gallium nitride from the c-plane of gallium nitride. According to this group III nitride semiconductor laser, the influence of piezoelectric polarization is reduced.

  In the group III nitride semiconductor laser according to the present invention, the semiconductor stack includes a semiconductor ridge, the gallium nitride based semiconductor active layer is in the semiconductor ridge, and the gallium nitride based semiconductor active layer is on the first area. Can be provided. According to this group III nitride semiconductor laser, the semiconductor ridge structure is formed so that the semiconductor stack does not include a semiconductor region that has inherited a high dislocation density. Also, current confinement is improved.

  In the group III nitride semiconductor laser according to the present invention, the angle at which the main surface is inclined in the m-axis direction from the c-plane is in the range of −1 degree or more and +1 degree or less. According to this group III nitride semiconductor laser, when the angle at which the main surface is inclined in the m-axis direction from the c-plane exceeds ± 1 °, the laser characteristics are deteriorated.

  As described above, according to the present invention, there can be provided a nitride semiconductor laser that can reduce the influence of piezo polarization and has a structure suitable for industrially producible GaN wafers having a sufficiently low dislocation density. The

  The knowledge of the present invention can be easily understood by considering the following detailed description with reference to the accompanying drawings shown as examples. Next, embodiments of the group III nitride semiconductor laser of the present invention will be described with reference to the attached drawings. Where possible, the same parts are denoted by the same reference numerals.

  FIG. 1 schematically shows the structure of a group III nitride semiconductor laser according to the present embodiment. FIG. 2 shows a schematic cross section taken along the line II shown in FIG. The group III nitride semiconductor laser 11 includes a substrate 13 made of hexagonal n-type gallium nitride and a semiconductor stack 15 made of n-type gallium nitride semiconductor. The semiconductor stack 15 is provided on the main surface 13 a of the substrate 13. The semiconductor stack 15 includes an n-type gallium nitride semiconductor region 21, a p-type gallium nitride semiconductor region 23, and a gallium nitride semiconductor active layer 25. The gallium nitride semiconductor active layer 25 is an n-type gallium nitride semiconductor. It is provided between region 21 and p-type gallium nitride semiconductor region 23. The p-side electrode 17 is connected to the p-type gallium nitride semiconductor region 23 of the semiconductor stack 15. The n-side electrode 19 is connected to the back surface 13 b of the substrate 13.

  The main surface 13a of the substrate 13 is inclined at an off angle of 20 degrees or more in the a-axis direction of gallium nitride from the c-plane of gallium nitride. The main surface 13a of the substrate 13 is inclined at an off angle of 45 degrees or less. Referring to the orthogonal coordinate system Cr in FIG. 1, the c coordinate axis is directed to the c axis of the hexagonal n-type gallium nitride of the substrate 13, and the m coordinate axis is the hexagonal n-type gallium nitride of the substrate 13. The a coordinate axis is directed to the a axis of the hexagonal n-type gallium nitride of the substrate 13. The crystal axis vectors C and M indicate the directions of the c coordinate axis and the m coordinate axis, respectively, and the angle formed by the vector C and the normal N of the principal surface 13a of the substrate 13 is the above-described off angle. For example, the m coordinate axis can be oriented in the direction of the Y axis of the Cartesian coordinate system S.

  In order to reduce the influence of piezoelectric polarization, an off angle of about 20 degrees is required. In the growth of GaN crystals in the method of manufacturing a gallium nitride wafer, good crystal growth is performed until the off-angle of the base crystal is about 35 degrees. However, when the off-angle of the base crystal exceeds 45 degrees, polycrystallization is possible. High nature.

  In the substrate 13, the main surface 13a is preferably inclined at an angle of 28 degrees or more from the c-plane of gallium nitride in the a-axis direction of gallium nitride, and the main surface 13a is inclined at an angle of 32 degrees or less. It is preferable. According to this group III nitride semiconductor laser, the influence of piezoelectric polarization is reduced. If the inclination angle is 28 degrees or more, there is an advantage that the polarization is further reduced to an extent effective in use. If the inclination angle is 32 degrees or less, there is an advantage that better crystallinity is maintained in this range.

  The substrate 13 includes first regions 13c, 13f, 13g and second regions 13d, 13e. The first region 13c extends from the back surface 13b of the substrate 13 to the main surface 13a along the c-axis direction. The second region 13d extends from the back surface 13b to the main surface 13a along the first region 13c. The dislocation density of the first regions 13c, 13f, and 13g is less than or equal to the dislocation density of the second regions 13d and 13e. For example, the dislocation density of the first regions 13c, 13f, 13g is smaller than the maximum value of the dislocation density of the second regions 13d, 13e, and the minimum value of the dislocation density of the first regions 13c, 13f, 13g is It is smaller than the minimum value of the dislocation density of the second regions 13d and 13e.

  As shown in FIG. 2, the main surface 13a of the substrate 13 has first areas 13h and 13i and second areas 13j and 13k. In the first areas 13h and 13i of the main surface 13a, first regions 13c and 13f appear, respectively. Second areas 13d and 13e appear in the second areas 13j and 13k, respectively. The back surface 13b of the substrate 13 has third areas 13m and 13q and fourth areas 13n and 13p. First regions 13c and 13g appear in the third areas 13m and 13q of the back surface 13b, respectively. Second areas 13d and 13e appear in the second areas 13n and 13p, respectively.

  The first region 13c is inclined in the direction of the vector C indicating the inclination angle of the c-axis and extends from the back surface 13b to the main surface 13a. Corresponding to this inclination, the X-axis direction of the orthogonal coordinate system S in FIG. The area 13h is shifted with respect to the area 13m, and the coordinates X1, X3 at both ends of the area 13m are different from the coordinates X2, X4 at both ends of the area 13h, respectively. The difference between the coordinates is related to the inclination of the c-axis, and the inclination of the c-axis is expressed by (X2-X1) / (Z1-Z2) or (X4-X3) / (Z1-Z2). The Z coordinate represents the upper and lower limit positions of the substrate 13. The ohmic contact of the p-side electrode 17 is provided on the first area 13, and the n-side electrode 19 is provided on the entire back surface 13b.

  According to this group III nitride semiconductor laser 11, the semiconductor stack 15 is provided on the substrate main surface 13 a inclined from the c-plane of gallium nitride at an angle of 20 degrees or more and 45 degrees or less in the a-axis direction of gallium nitride. The effect of piezo polarization is reduced. Further, since the dislocation density in the first region 13c of the substrate 13 is smaller than the dislocation density in the second regions 13d and 13e, the semiconductor stack 15 for the semiconductor laser is disposed on the substrate 13 in which the dislocation density is controlled. The Further, a semiconductor stack is formed by epitaxial growth on a large-diameter GaN wafer made of gallium nitride including a low dislocation density region corresponding to the first region 13c and a high dislocation density region corresponding to the second regions 13d and 13e. 15 can be produced.

  Referring to FIG. 1 again, in the group III nitride semiconductor laser 11, the first and second regions 13c to 13g extend in a predetermined axis direction (for example, the direction of the vector M indicating the m-axis direction). Yes. Similarly, the p-side electrode 17 preferably extends in the direction of a predetermined axis. This facilitates the orientation of the optical waveguide for the laser resonator. The surface of the semiconductor laminate 15 is covered with an insulating film 27, and the p-side electrode 17 is in ohmic contact with the p-type gallium nitride semiconductor through the opening of the insulating film 27.

  The semiconductor stack 15 can include a semiconductor ridge 29. The gallium nitride based semiconductor active layer 25 is in the semiconductor ridge 29, and a ridge structure is suitable for optical confinement. The gallium nitride based semiconductor active layer 25 is provided on the first area 13h. By forming the semiconductor ridge structure, the semiconductor stack 15 does not include a semiconductor region that has inherited a high dislocation density. Therefore, the dislocation density in the fabricated ridge structure is low, and the lifetime of the fabricated device is extended. The gallium nitride based semiconductor active layer 25 can have a quantum well structure such as MQW or SQW.

  The semiconductor ridge 29 can include a p-type gallium nitride based semiconductor region 23. The p-type gallium nitride based semiconductor region 23 can include, for example, an electron block layer 31, a p-type cladding layer 33, and a p-type contact layer 35. The n-type gallium nitride based semiconductor region 21 can include, for example, an n-type cladding layer 37 and an n-type buffer layer 39. The semiconductor ridge 29 can include, for example, a part or all of the n-type cladding layer 37.

  The group III nitride semiconductor laser 11 has first and second cleavage surfaces 41 and 43. The cleavage planes 41 and 43 constitute an optical resonator of the group III nitride semiconductor laser 11. The angle at which the main surface 13a is inclined in the m-axis direction from the c-plane is preferably in the range of −1 degree to +1 degree. The cleavage yield on the m-plane is improved. When the absolute value of the inclination angle formed by the cleavage plane with respect to the laser stripe exceeds 1 degree, the laser characteristics are deteriorated.

The substrate 13 includes a polarity inversion region having a first polarity in the c-axis direction and a polarity non-inversion region having a polarity opposite to the first polarity. In the plane orthogonal to the c-axis, one of the N plane and the Ga plane appears in the polarity inversion region, and the other of the N plane and the Ga plane appears in the polarity non-inversion region. The second region 13d of the substrate 13 includes a polarity inversion region. The first region 13c of the substrate 13 includes a polarity non-inversion region. The polarity inversion region has a dislocation density larger than the dislocation density of the polarity non-inversion region. The average dislocation density of the polarity inversion region is, for example, in the range of 1 × 10 ⁶ cm ^{−2 to} 1 × 10 ⁸ cm ⁻² , and the polarity non-inversion region is, for example, 1 × 10 ² cm ^{−2 to} 1 × 10 ^6. It is in the range of cm ⁻² or less. According to the group III nitride semiconductor laser 11, the region having a low dislocation density can have a dislocation density of 1 × 10 ⁶ cm ⁻² or less. Dislocations are collected in the polarity inversion region to increase the dislocation density, and the dislocation density in the nonpolarity inversion region is reduced.

  The average dislocation density in the second regions 13d and 13e can be a value that is three times or more the average dislocation density in the first region 13c. In the substrate 13, the low dislocation GaN region existing between the polarity inversion regions has a dislocation density distribution. When the average dislocation density is measured statistically, the central region has the lowest dislocation density, and the dislocation density increases as the outer inversion region is approached. When estimated in consideration of the chip size of the semiconductor laser, the average dislocation density in the high dislocation density GaN region in the semiconductor laser is about three times or more the average dislocation density in the low dislocation density GaN region.

  The first region 13 c is located between the semiconductor ridge 29 and the electrode 19 and extends substantially parallel to the semiconductor ridge 29. The second regions 13 d and 13 e are adjacent to the first region 13 c and extend substantially parallel to the semiconductor ridge 29. In addition, the polarity inversion region extends in the c-axis direction of gallium nitride, and many threading dislocations extend in the c-axis direction (growth direction), so that the dislocation density on the main surface of the substrate decreases according to the off angle. .

  The group III nitride semiconductor laser 11 can further include side surfaces 45 and 47 extending in the m-axis direction. The polarity inversion region in the second region 13 e of the substrate 13 appears on at least a part of the side surface 45, and the polarity non-inversion region appears on at least a part of the side surface 45. Further, the polarity inversion region in the second region 13 d of the substrate 13 appears on at least a part of the side surface 47, and the polarity non-inversion region appears on the side surface 47. The polarity inversion region may appear on the side surfaces 45 and 47 as shown in FIG. 1, but may appear only on one of the side surfaces 45 and 47, and the polarity non-inversion region appears on the other side surface. In this group III nitride semiconductor laser 11, the first and second regions 13 c, 13 d, and 13 e are arranged so that a polarity inversion region having a high dislocation density exists at least at a part of the side surfaces 45 and 47 of the semiconductor laser. By doing so, the region 13c having a low dislocation density can be disposed between the side surfaces 45 and 47 of the semiconductor laser. The boundary between the polarity inversion region and the polarity non-inversion region extends along a reference plane where the main surface 13a intersects. For this reason, the light emitting region can be arranged on the low dislocation region by adjusting the position of the light emitting region in the semiconductor stack 15 with respect to the positions of the polarity inversion region and the polarity non-inversion region.

Referring to FIG. 1, in the group III nitride semiconductor laser 11, the substrate 13 has a first portion 13u, a second portion 13v, and a third portion 13w. The first portion 13u extends from the back surface 13b of the substrate 13 to the main surface 13a along the c-axis direction. The second portion 13v extends from the back surface 13b to the main surface 13a along the first portion 13u. The third portion 13w extends from the back surface 13b to the main surface 13a along the second portion 13v. The specific resistance R _13u of the first portion _13u is lower than the specific resistance R _13v of the second portion 13v. Resistivity _{R 13w} of the third portion 13w is lower than the specific resistance _{R 13v} of the second portion 13v. The second portion 13v is between the first portion 13u and the third portion 13w. The first region 13c includes first, second, and third portions 13u, 13v, and 13w. According to this group III nitride semiconductor laser 11, the first to third portions 13 u to 13 w are inclined along the c-axis direction with respect to the main surface 13 a of the substrate 13. One of the first and third portions 13u and 13w (the third region 13w in the form of FIG. 1) is inclined along the second portion 13v in the substrate region between the semiconductor layer 15b and the semiconductor stack 15. Come. Therefore, element resistance can be reduced. The second region 13d can be between the first portion 13u and the third portion 13w, and the second region 13e is between the first portion 13u and the third portion 13w. . The first, second, and third portions 13u, 13v, and 13w extend in a direction from one end face 41 of the semiconductor laser 11 toward the other end face 43.

  The first area 13h of the main surface 13a of the substrate 13 includes three area portions where the first, second, and third portions 13u, 13v, and 13w respectively appear. When the first area 13h has a stripe shape extending in the Y-axis direction, these area portions also have a stripe shape extending in the Y-axis direction. The third area 13m of the back surface 13b includes three area portions where the first, second, and third portions 13u, 13v, and 13w respectively appear. When the third area 13m has a stripe shape extending in the Y-axis direction, these area portions also have a stripe shape extending in the Y-axis direction.

  The first, second, and third portions 13u, 13v, and 13w of the first region 13c are inclined in the direction of the vector C indicating the inclination angle of the c-axis and extend from the back surface 13b to the main surface, corresponding to this inclination. Thus, the area 13h is shifted with respect to the area 13m with respect to the X-axis direction of the orthogonal coordinate system S of FIG. As shown in FIG. 2, the coordinates X11 and X31 at both ends of the second portion 13v in the area 13m are different from the coordinates X21 and X41 at both ends of the second portion 13v in the area 13h, respectively. The difference between the coordinates is related to the inclination of the c-axis, and the inclination of the c-axis is expressed by (X21-X11) / (Z1-Z2) or (X41-X31) / (Z1-Z2). In the embodiment shown in FIG. 2, most of the current from the p-side electrode 17 reaches the n-side electrode 19 via the semiconductor stack 15, the second portion 13v, and the third portion 13w. A part of the current from the p-side electrode 17 reaches the n-side electrode 19 through the semiconductor stack 15 and the first portion 13v. For this reason, according to the present embodiment, a low-resistance current path for reducing the element resistance is provided.

As already described, the substrate 13 includes a high resistivity second portion and a low resistivity first and third portions. The first portion has a specific resistance in the range of 5 × 10 ⁻³ Ω · cm to 3 × 10 ⁻² Ω · cm and is provided in parallel with the active layer. The third portion has a specific resistance in the range of 5 × 10 ⁻³ Ω · cm to 3 × 10 ⁻² Ω · cm, and is provided in parallel with the active layer. The second portion is provided between the first and third portions and has a specific resistance in the range of 4 × 10 ⁻² Ω · cm to 5 × 10 ⁶ Ω · cm. In the method of manufacturing the substrate 13 used for the semiconductor laser 11, the specific resistance of the region grown on the c-plane tends to be high, but the specific resistance of the region grown on the facet plane such as the {11-22} plane is low. Are distinguished by clear boundaries. The specific resistance of the polarity inversion region is as low as the specific resistance of the first and third portions of the polarity non-inversion region. In one embodiment, the first and third portions are non-polar inversion regions, and the second portion is also a non-polarity inversion region.

  According to the group III nitride semiconductor laser, the first and third portions having a low specific resistance and the second portion having a high specific resistance are adjacent to each other. The element resistance can be reduced by flowing in at least one of the three portions and the second portion.

  For example, the polarity inversion region extends along a reference plane that is inclined with respect to the main surface 13a at an angle formed by the normal line N of the main surface 13a and the c-axis. Since the polarity inversion region is inclined along the reference plane, one of the polarity inversion region (for example, the region 13e) and the first and third portions 13u and 13w is between the p-side electrode 17 and the n-side electrode 19. It approaches the position of the region (region of non-inverted polarity). The current from the p-side electrode 17 to the n-side electrode 19 flows not only in the polarity non-inversion region but also in one of the first and third portions 13u and 13w and the polarity inversion region. Since the specific resistance of one of the first and third portions 13u and 13w and the polarity inversion region is lower than that of the non-polarity inversion region, the element resistance can be lowered.

(Example)
The GaN semiconductor laser according to the present embodiment is fabricated on a GaN wafer formed from a GaN crystal grown by the A-DEEP method on a mask layer having a stripe pattern formed on a GaAs wafer. . The GaN wafer includes polar stripe-shaped regions oriented in the c-axis direction. This region is crystal-grown as a V-shaped valley due to the facet surface of GaN, and thereby the dislocation density can be controlled by collecting dislocations at the bottom of the V-shaped valley.

  A GaN wafer is fabricated from GaN crystals grown on different kinds of underlying start substrates having large off angles. In the GaN growth, a GaN crystal having the same off angle as the off angle of the base is grown. In this crystal growth, the polarity-inverted GaN region grows in the c-axis direction. The GaN crystal thus grown is sliced along a plane parallel to an axis orthogonal to the main surface of the underlying start substrate, and surface polishing is performed to form a GaN wafer having a flat main surface. The polarity inversion region in the GaN wafer is inclined at the above-mentioned off angle with respect to the main surface of the GaN wafer.

  A semiconductor laser device was manufactured using this GaN wafer. First, a plurality of nitride-based semiconductor layers were epitaxially grown on a GaN wafer by a crystal growth method such as metal organic chemical vapor deposition. For example, after growing an n-type GaN-based semiconductor layer, an n-type gallium nitride-based cladding layer is grown, and an active layer including a well layer and a barrier layer is grown to form a multiple quantum structure. An epitaxial wafer having an epitaxial multilayer film was formed by growing an electron block layer, a p-type gallium nitride based cladding layer, and a p-type gallium nitride based contact layer on the multiple quantum structure. Further, after forming a photoresist having a stripe pattern, an epitaxial multilayer film was formed by etching to form a laser ridge structure. After forming an insulating film having an opening on the laser ridge structure, an anode electrode was formed on the laser ridge structure and a cathode electrode was formed on the back surface of the wafer. This laser ridge structure was formed in parallel to the striped polarity inversion region and avoiding the polarity inversion region. This semiconductor laser chip has a cleavage plane for a mirror-like resonance surface. In order to produce this resonator, a semiconductor laser piece was produced by cleaving on the {1-100} plane which is the m plane. The m-plane is perpendicular to the m-axis direction in which the laser ridge structure and the polarity inversion region extend.

  In addition, when this GaN wafer is used, in order to separate the semiconductor laser piece as the final group III nitride semiconductor laser, the polarity-reversed region is diced to form the side surface except for the cleaved surface of the m-plane. Thus, individual semiconductor lasers were manufactured.

  FIG. 3 is a drawing showing the characteristics of a GaN-based semiconductor laser fabricated on a c-plane, m-plane, and semipolar plane GaN wafer. It has been found that the group III nitride semiconductor laser fabricated on the semipolar plane does not show a large difference in characteristics compared to the group III nitride semiconductor laser fabricated on the nonpolar plane. The group III nitride semiconductor laser fabricated on the semipolar plane has sufficient characteristics as a laser element, and has the characteristics that the influence of piezoelectric polarization is reduced compared to the laser element on the polar plane. Have. Although the III-nitride semiconductor laser fabricated in this way has a semipolar plane instead of a nonpolar plane, as shown in FIG. 3, the semiconductor laser on the semipolar plane (off angle of 28 degrees) is shown in FIG. The blue shift is small enough.

  A specific embodiment will be described. A GaN wafer used for manufacturing a semiconductor laser device was manufactured by a stripe A-DEEP method. This GaN wafer has n-type conductivity, and the stripe direction is the <1-100> direction (m-axis direction). The low dislocation density region, the high dislocation density region, and the stripe shape extend in this direction. The polarity inversion regions are alternately arranged in parallel. The main surface of the GaN wafer has an off angle of 20 degrees to 40 degrees in the <11-20> direction (that is, the A-axis direction) with respect to the c-plane. In the dislocation density distribution between the polarity inversion regions, the center of the polarity inversion region has the lowest dislocation density. An epitaxial wafer is manufactured by growing a laser structure (epitaxial multilayer structure) on the main surface of the wafer where the polarity inversion region and the low dislocation density region between them appear. Next, a p-side electrode and an n-side electrode are formed on the upper and lower surfaces of the epitaxial wafer including the epitaxial multilayer structure made of a gallium nitride semiconductor and the n-type conductive GaN wafer, respectively, to produce a substrate product. The resonant surface of the semiconductor laser element is the c-plane m-plane and is substantially perpendicular to the main surface of the GaN wafer. After producing the laser bar from the substrate product, the laser bar is divided to form individual laser elements. Basically, it is preferable to divide at a position in the polarity inversion region. When the polarity inversion region is divided, inversion regions may exist on both side surfaces of the semiconductor laser. This polarity inversion region is inclined at an angle α equal to the off-angle from a cross section perpendicular to the main surface of the GaN substrate of the semiconductor laser, and extends from the main surface of the GaN substrate to the back surface. When the polarity reversal region is cut with a dicing saw during division, the polarity reversal region may disappear from the semiconductor laser due to a loss (cutting off) due to cutting. It is also possible to divide at the center of the polarity non-inversion region and take two laser elements between the polarity inversion regions. As a result of the characteristic evaluation, it was shown that the semiconductor laser device manufactured as described above has a good characteristic with greatly reduced influence of piezoelectric polarization. The amount of wavelength change (blue shift) that accompanies an increase in current injection amount is about 10% to 30% compared to the amount of wavelength change of the laser element structure on the (0001) plane (c-plane) that is the polar plane. Reduced and had good properties.

  Thus, the semiconductor laser device is fabricated on the main surface of the gallium nitride substrate, and this main surface has a crystal plane inclined from 20 degrees to 45 degrees in the a-axis direction from the c plane. The semiconductor laser device uses an m-plane cleavage plane as a resonance surface, a stripe-shaped laser active layer is formed in a low dislocation density region, and the high dislocation density region is separated from the laser active layer in parallel and perpendicular to the resonance surface. In the direction. An n-side electrode is provided on the back surface of the GaN substrate having n-type conductivity, and a p-side electrode is provided on the p-type gallium nitride based semiconductor layer on the stripe-shaped laser active layer. The group III nitride semiconductor laser device has mass productivity capable of industrial production and excellent electrical characteristics.

A group III nitride semiconductor laser device according to the present embodiment was fabricated as follows. First, a GaN substrate was prepared. The base substrate will be described. A GaAs (111) wafer was used as the base start substrate. However, with respect to the normal vector of the surface of the base substrate, the crystal orientation <111> is at angles of 0 degrees, 15 degrees, 20 degrees, 25 degrees, 30 degrees, 35 degrees, and 40 degrees in the <1-10> direction. An inclined GaAs wafer was prepared. At each inclination angle, two types were prepared: a GaAs wafer having an inclination of −1 to +1 degrees and a GaAs wafer having an inclination of about 2 degrees in the <11-2> axis direction. That is, a device was fabricated using 14 types of GaAs wafers. A mask layer having a pattern for the A-DEEP method was formed on the surface of these base start substrates. The pattern of the mask layer has a stripe shape, for example. This mask layer is made of, for example, silicon oxide (SiO ₂ ). The width of the stripe pattern of the mask layer was 50 μm, and the repetition pitch was 400 μm.

Next, growth conditions will be described. A GaN crystal was grown on the surface of the base substrate by the HVPE method on the surface of the above-mentioned 14 types of base substrate. The growth conditions used to form the GaN crystal are shown below. At the beginning of crystal growth, a thin buffer layer was grown at a relatively low temperature. As the buffer layer deposition conditions, a deposition temperature of 500 degrees Celsius, a partial pressure of HCl of 100 Pa (1 × 10 ⁻³ atm), and a partial pressure of NH ₃ of 10,000 Pa (0.1 atm) are used for 60 minutes. The film was formed. The thickness of the buffer layer was 60 nm. Thereafter, a thick GaN epitaxial film was grown on the buffer layer at a high temperature to produce an epitaxial crystal product. As a film forming condition of the GaN epitaxial thick film, a film forming temperature of 1010 degrees Celsius, an HCl partial pressure of 3000 Pa (3 × 10 ⁻² atm), and an NH ₃ partial pressure of 20000 Pa (0.2 atm) are used for 100 hours. The film was formed. The thickness of the epitaxial crystal layer was 10 mm.

  However, in the growth using a wafer with an off angle of 40 degrees, the surface morphology of the wafer with the off angle changes every time the film is formed, and stable growth cannot be performed. However, GaN crystals with relatively good surface morphology were selected.

  The creation of the GaN crystal will be described. Thereafter, the GaAs substrate was removed from the GaN epitaxial thick film by etching the epitaxial crystal product. In this way, a GaN crystal 55 having a thickness of 10 mm as shown in FIG. 4 was obtained. The GaN crystal 55 includes polarity inversion regions 55a and polarity non-inversion regions 55b that are alternately arranged. The GaN crystal 55 was sliced to a thickness of 400 μm using a wire saw along the reference plane R1 indicated by the one-dot chain line in FIG. 4, and the surface was polished to prepare a GaN wafer. The reference plane R1 is defined by a vector r1 and a vector m (indicating the direction of the m-axis). An angle α1 formed between the normal line n1 of the main surface of the GaN wafer and the c-axis C is an off-angle. Substrate evaluation will be described. It was confirmed by X-ray diffraction that the main surfaces of these GaN wafers had the same off angle as the off angle of the underlying GaAs substrate.

  In order to obtain a larger off-angle GaN wafer, after forming a large off-angle GaN crystal grown by epitaxial growth by the A-DEEP method, a reference plane along the reference plane R2 indicated by the one-dot chain line in FIG. It is also possible to slice and process the GaN crystal at an angle with respect to R1. The reference plane R2 is defined by a vector r2 and a vector m. The angle α2 (> α1) formed between the normal line n2 of the main surface of the GaN wafer and the c-axis can be increased.

  When dislocation density is observed by cathodoluminescence (CL), stripe-like polarity inversion regions, which are GaN regions with high dislocation density, are arranged at intervals of 400 μm corresponding to the stripe pattern of the mask layer on the underlying GaAs substrate. I was observed. A region of low dislocation density was observed between the polarity inversion regions. The off-angles of the main surfaces of these 12 types of GaN wafers are about −1 to +1 degrees or −1 to −2 degrees and +1 to +2 degrees with respect to the <1-100> direction, Further, in the <11-20> direction, there were off-angles of almost 0 degrees, 15 degrees, 20 degrees, 25 degrees, 30 degrees, 35 degrees, and 40 degrees. Therefore, it was confirmed that the off angles of all the produced GaN wafers reflected the off angle of the surface of the initial base GaAs substrate. Further, in the measurement result of the cathode luminescence method for these GaN wafers, a stripe-like clear polarity inversion region was observed. Further, in the measurement result of the cathodoluminescence, a band-like dark region of contrast was clearly observed in the substantially central portion between adjacent polarity inversion regions. The width of this region varies and can be narrow or wide. The width of the narrow portion was almost zero, and the width of the wide portion was about 250 μm. The average width was about 100 μm as a result of 10-point average width measurement at an equal interval in a length of 5 mm.

Further, the bulk specific resistance of the wafer was measured by micro rho measurement. As a result, the dark contrast region between the polarity inversion regions showed a high specific resistance, for example, 0.2 Ω · cm. The specific resistance value in this region was in the range of 4 × 10 ⁻² Ω · cm to 5 × 10 ⁶ Ω · cm. The other region showed a low specific resistance, for example, 8 × 10 ⁻³ Ω · cm. The value of the specific resistance in this region was from 5 × 10 ⁻³ Ω · cm to 3 × 10 ⁻² Ω · cm.

The production of the laser element will be described. Further, epitaxial growth was performed on these 14 types of GaN wafers by metal organic vapor phase epitaxy (MOVPE) to produce a gallium nitride based semiconductor film stack. The stack included an n-type buffer layer, an n-type cladding layer, an MQW active layer, an electron block layer, a p-type cladding layer, and a p-type contact layer. In particular,
Substrate: GaN wafer n-type buffer layer: n-AlGaN (20 nm)
n-type cladding layer: n-GaN (2μm)
MQW active layer: 6MQW of InGaN well layer (5nm) / InGaN barrier layer (15nm)
Electron blocking layer: p-AlGaN layer (20 nm)
p-type cladding layer: p-GaN layer (25 nm)
p-type contact layer: p ⁺ -GaN layer (25 nm)
It is.

  Then, after forming a mask by photolithography, the above-described lamination was processed by a dry etching method to form a stripe type laser ridge structure. This laser ridge structure was formed in parallel with the polarity non-inversion region, for example, so as to extend in the direction of the polarity inversion region on the polarity non-inversion region between adjacent stripe-like polarity inversion regions. After forming an insulating film for protecting the laser ridge structure, an opening was formed in accordance with the uppermost p-type conductive layer of the laser ridge structure. A p-side electrode electrically connected to the p-type conductive layer was formed on the p-type conductive layer and the insulating film, and an n-side electrode was formed on the back surface of the GaN wafer. As a result, a plurality of gallium nitride based semiconductor laser elements were obtained.

The evaluation results of these gallium nitride based semiconductor laser elements will be described. After peeling off the electrodes of several semiconductor laser elements, the surface of the epitaxial layer of the semiconductor laser elements was observed by cathodoluminescence. The threading dislocations in the epitaxial layer were observed by cathodoluminescence. The threading dislocation density observed as black spots in the cathodoluminescence image was measured to estimate the threading dislocation density. According to the measurement result of the threading dislocation density, the polarity non-inversion region located between the stripe-type polarity inversion regions is a low dislocation, and the threading dislocation density is 1 × 10 ⁶ cm ⁻² or less at most. . Some semiconductor laser elements have almost no dislocations, and the threading dislocation density in this region is considered to be about 1 × 10 ² cm ⁻² . The laser ridge structure was formed in this low dislocation region. On the other hand, in the vicinity of the polarity inversion region, a region having a threading dislocation density of 1 × 10 ⁶ cm ⁻² to 1 × 10 ⁸ cm ⁻² was often observed. However, depending on the semiconductor laser element, there was a low dislocation region where no threading dislocation was observed even near the polarity inversion region. These semiconductor laser elements are laser elements having a large off angle and a low dislocation density. FIG. 5 shows an example of a dislocation density distribution between high dislocation density regions (cores). There is a high dislocation density region H having a width of 30 μm at the coordinates of 0 μm and 400 μm on the horizontal axis, and a low dislocation density region L is located between the high dislocation density regions H. According to the dislocation density distribution line D, the dislocation density monotonously decreases from the high dislocation density regions H on both sides, and shows the minimum value of the dislocation density at approximately the center. The dislocation density in the high dislocation density region H is larger than the dislocation density in the low dislocation density region L. The specific resistance of the high dislocation density region H is low, and the specific resistance of the low dislocation density region L is high.

  Next, a current was passed through the remaining semiconductor laser element from which the electrode was not peeled, and the light emission characteristics by electroluminescence were investigated. All 14 types of semiconductor laser devices were investigated.

First, we compared the emission brightness and the current injected into the semiconductor laser device LD ₂ of the semiconductor laser device LD about ₁ and the off-angle twice within the <1-100> small off-angle once attached to the directions. As a result, a semiconductor laser device LD _2, the semiconductor laser device LD ₁ has a higher brightness was found to be preferable that the angle with the <1-100> direction small.

Next, in the semiconductor laser element with a small off angle of 1 degree or less attached in the <1-100> direction, the off angle in the <11-20> direction is 0 degree, 15 degrees, 20 degrees, 25 degrees, 30 degrees, Semiconductor laser elements LD ₁ (0), LD ₁ (15), LD ₁ (20), LD ₁ (25), LD ₁ (30), LD ₁ (35), LD ₁ ( 40) was prepared. The light emission characteristics of these semiconductor laser elements were compared by electroluminescence. In addition, the amount of change in emission wavelength due to blue shift when the current injection amount was increased was also measured. As a result, in the semiconductor laser device _{LD 1 (0) ~LD 1 (} 40), as a wavelength change rate of the current injection 200 mA, the following values were obtained. Wavelength change rate, the semiconductor laser device _LD 1 of the ₍₀₎ ~LD 1 (40) the variation of the emission wavelength at _{△ λ BS (0) ~ △} λ BS (40) semiconductor laser element _X 1 (0) a This is a value normalized by the change amount Δλ _BS (0). Correspondence between off angle (deg) and wavelength change (%) is (angle 0 °, change 100%), (angle 15 °, change 82%), (angle 20 °, change 50%), (Angle 25 °, change 32%), (angle 30 °, change 17%), (angle 35 °, change 6%), (angle 40 °, change 5%). From these results, it was confirmed that as the off-angle was increased, the amount of wavelength change Δλ _BS was reduced and the influence of blue shift was reduced. That is, the influence of polarization was reduced. Even if this wavelength change amount is not a nonpolar surface, it has been shown that the reduction amount is large when the off angle is 20 degrees or more. The inventors' experiments have shown that the effect of reducing piezo polarization is large on a semipolar surface with an off angle of 20 degrees to 45 degrees.

  Three effects of the semiconductor laser element will be described. First, the element resistance of the semiconductor laser element will be described. The GaN crystal by the A-DEEP method includes a portion formed by growth on the c-plane during crystal growth and a portion formed by growth on the facet plane consisting of {11-22} plane, and the ratio of these portions There is a difference in resistance. In general, the specific resistance of the crystal part grown on the c-plane tends to be higher than that of the crystal part due to the growth of the facet plane. On the other hand, crystal growth grows in the c-axis direction. At this time, when a c-plane just substrate is used, the crystal grows while forming a crystal region for c-plane growth at the exact center of adjacent polarity inversion regions. Therefore, in the c-plane just GaN wafer produced by this growth, a crystal portion (c-plane growth region) having a high specific resistance extends perpendicularly to the main surface of the substrate in the center of adjacent polarity inversion regions. When a semiconductor laser device is fabricated on this GaN wafer, a GaN-based semiconductor is epitaxially grown on the surface of the GaN wafer. A laser ridge structure is formed at the center of the adjacent polarity inversion regions. A high resistivity crystal portion (c-plane growth region) is located immediately below the laser ridge structure. For this reason, the resistance value of the current path of the semiconductor laser element is large, and the forward voltage Vf during laser operation increases.

  However, when a semiconductor laser device is formed on a GaN wafer having a semipolar surface as described above, the main surface of the GaN wafer itself has a large off angle, so that a high resistivity GaN crystal region (c-plane) The growth region does not extend perpendicularly from the main surface of the substrate directly below the laser ridge structure, but extends obliquely from directly below the laser ridge structure at the same angle as the large off-angle. Similarly, the polarity inversion region also extends from the main surface of the GaN wafer with an inclination that is the same as the large off angle, and approaches the GaN crystal region located between the laser ridge structure and the back surface of the wafer. Therefore, the current flowing out of the laser ridge structure spreads in the GaN wafer and reaches the n-side electrode on the back surface of the wafer after passing through not only the high resistivity c-plane growth region but also the low resistance polarity reversal region. In a preferred example, the GaN crystal region located between the laser ridge structure and the wafer back surface includes not only a high resistivity c-plane growth region but also a low resistance polarity reversal region. As a result, the resistance value of the current path decreases, and the forward voltage Vf during laser operation can be reduced.

  Next, suppression of cracking during back grinding will be described. In manufacturing a semiconductor laser device, there is a step of thinning the wafer by back-grinding the back surface of the GaN wafer after epitaxial growth. This is because when the wafer thickness is reduced, the cleavage performed in the next step is facilitated. By this cleavage, a laser bar is formed. In this step, the use of a GaN wafer having a large off angle significantly reduced the rate of wafer cracking compared to a c-plane just GaN wafer. By using a GaN wafer having an off-angle as in the present embodiment to produce a semiconductor laser element, the frequency of cracks can be reduced and the cost can be reduced.

  Third, the improvement of cleavage is explained. In the manufacture of a semiconductor laser device, a substrate product on which electrodes are formed is cleaved at a cleavage plane to produce a bar-shaped semiconductor piece. This is an important process for forming a mirror-like cleavage surface by cleavage. In general, when a GaN crystal wafer by A-DEEP method is used, this cleavage plane is m-plane {1-100}. Compared with a c-plane just GaN wafer of GaN crystal by A-DEEP method, when using a GaN wafer having a large off angle as in this embodiment, the yield of this cleavage process increased. This is presumably because the state of stress around the cleavage plane propagating at the time of cleavage changes due to the polarity inversion region entering obliquely with respect to the traveling direction of the propagation of cleavage.

  While the principles of the invention have been illustrated and described in the preferred embodiments, it will be appreciated by those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. The present invention is not limited to the specific configuration disclosed in the present embodiment. We therefore claim all modifications and changes that come within the scope and spirit of the following claims.

  As a laser made of a nitride compound semiconductor, a blue-violet laser for reading and writing information on a high-density optical disk has already been put on the market. These semiconductor lasers are manufactured using a conductive GaN wafer. A GaN crystal for a GaN wafer is grown on a heterogeneous substrate by heteroepitaxial growth by a vapor phase growth method called HVPE. Thereafter, a GaN crystal is obtained by separating the heterogeneous substrate. The main surface of all commercially available GaN wafers so far is the c-plane. However, since these GaN crystals use heteroepitaxial growth, they contain a large amount of dislocation defects due to a large crystal lattice mismatch between the underlying heterogeneous substrate and the GaN crystal. Further, in order to improve laser characteristics such as long life and high output, it is necessary to reduce dislocation defects in the substrate crystal. In a semiconductor laser using a crystal having a high dislocation defect density, the laser life is shortened, and a reduction in the dislocation result density is necessary to extend the laser life.

For example, when an epitaxial thick film having a mirror-like c-plane is grown on a heterogeneous substrate such as sapphire by HVPE, and then the sapphire substrate is removed to produce a GaN free-standing wafer, this wafer is excellent. Quality semiconductor lasers are not possible. This is because dislocation defects having a density of about 10 ⁷ cm ⁻² or more based on crystal lattice mismatch were formed.

  In order to reduce the dislocation density, the DEEP method and the A-DEEP method have already been developed as a method for crystal growth of GaN. These crystals grow while forming and maintaining a facet structure composed of facet surfaces in the growth surface shape during crystal growth. Thereby, dislocation defects are reduced.

  In the DEEP method, for example, when a GaN crystal is grown by the HVEP method, a pit having a facet surface is formed on the entire surface and grown, so that a dislocation defect existing in the pit region hits the center of the pit (the bottom in the depth direction). ) To collect. On the contrary, the density of dislocation defects around it is reduced. In the GaN crystal thus obtained, the dislocation defect density is high in the central region of the pits where dislocation defects are collected, and the dislocation defect density is low in other regions. Further, the position of the region corresponding to the bottom of the pit having a high dislocation defect density is random.

  The A-DEEP method is a more advanced method. In the GaN crystal growth, the crystal growth is grown in the c-axis direction, but the c-axis direction of the region corresponding to the bottom (the deepest position in the depth direction) of the structure composed of the crystal facets is reversed 180 degrees in the reverse direction. ing. Using this polarity reversal region as a bottom, pits and V-shaped grooves formed of facets are formed, and crystal growth is performed while maintaining this, and dislocation defects are collected in the polarity reversal region during crystal growth. By gathering dislocation defects in the crystal in the polarity inversion region, dislocation defects in the area around the polarity inversion region can be reduced. This method is characterized in that the density of dislocation defects in the inversion region is high and the density of dislocation defects in the polarity non-inversion region other than the polarity inversion region is low. The polarity inversion region and the polarity non-inversion region have a clear boundary, and grow while extending parallel to the c-axis, which is the normal crystal growth direction. The difference between DEEP and A-DEEP is that the former has a random pit position (that is, a dislocation defect gathering location) and cannot be fixed at a desired location, whereas the latter has a predetermined inversion region location. The point is that it can be fixed in place. Further, by forming the polarity inversion region and growing it, it is possible to grow while maintaining the facet surface stably, and it becomes possible to further reduce the dislocation density. The presence of this polarity reversal region is very important.

For forming the inversion region, for example, a method of forming a mask layer made of a different material by photolithography is used. As this different material, materials such as SiO ₂ , Al ₂ O ₃ , ZrO ₂ , ZnO, MgO, SiN, and AlN can be used. By forming a mask layer that covers a predetermined place, a polarity inversion region is formed immediately above the mask layer during GaN crystal growth. That is, in the regions other than the polarity inversion region, the c-axis of GaN faces the crystal growth direction and the crystal growth proceeds. However, only in the polarity inversion region, the c-axis is opposite to the growth direction (180 ° inversion). The crystal growth proceeds. As a result, a GaN crystal in which polarity inversion regions are regularly arranged at predetermined locations can be obtained. Of course, as described above, these methods form a three-dimensional facet structure and grow crystals, so in order to form a practical wafer, it is flattened by processing and polishing, and has a flat shape. Must be processed into. Further, in order to produce a laser device using the GaN wafer thus obtained, it is necessary to align the laser active layer with low dislocation defect regions which are designed in advance and regularly arranged. Thereby, it is possible to produce a laser device with a long life and excellent quality.

  On the other hand, there is a need for a longer wavelength laser using a nitride-based semiconductor. For example, an attempt has been made to realize a projection type projection television using RGB visible lasers. The blue laser has already been realized, and further research and development is being pursued in search of the realization of the green laser. However, it is predicted that a conventional c-plane GaN wafer cannot be realized for realizing a longer wavelength laser. One of the reasons is said as follows. The GaN crystal is polarized in the c-axis direction. In order to increase the wavelength, it is necessary to increase the In ratio of the InGaN layer, and the distortion caused by the difference in lattice constant due to the increase of the indium ratio increases, and thus the influence of piezo polarization becomes greater. In a laser having a longer wavelength, the polarization effect causes a bias in the presence of holes and electrons in the active layer, and the recombination emission probability is significantly reduced. For this reason, since internal quantum efficiency falls, it does not reach laser oscillation and it is difficult to realize a green laser. This is the reason.

  In order to solve these problems, it is said that a nonpolar plane substrate having a nonpolar plane as a main surface is promising. This is because there is theoretically no influence of piezo polarization. Laser devices using an M-plane {1-100} plane or an A-plane {11-20} plane that are already nonpolar planes have been proposed. However, these non-polar plane substrates grow c-plane GaN crystals on a heterogeneous substrate with a thickness of only a few millimeters, and cut the GaN crystals in a direction perpendicular to the c-plane to obtain M-plane {1-100} planes. And a microcrystal piece having a principal plane of the A-plane {11-20} plane. Accordingly, the size called a substrate here is smaller than the commercially available wafer size, and the c-axis direction is a crystal piece of about several millimeters at most. Thus, industrial production of the laser element is impossible in the future.

  On the other hand, according to the semiconductor laser device of the present embodiment, a group III nitride semiconductor laser using a semipolar surface, not a nonpolar surface, is provided. According to the technical background understood from the above description, the technical significance is fully understood.

FIG. 1 is a drawing schematically showing the structure of a group III nitride semiconductor laser according to the present embodiment. FIG. 2 is a cross-sectional view taken along the line I-I shown in FIG. FIG. 3 is a drawing showing the blue shift of a semiconductor laser fabricated on the c-plane (polar plane), m-plane (nonpolar plane), and semipolar plane. FIG. 4 is a drawing schematically showing this GaN crystal. FIG. 5 is a drawing showing an example of a specific resistance distribution between high dislocation density regions.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Group III nitride semiconductor laser, 13 ... Substrate, 13b ... Substrate back surface, 13a ... Substrate main surface, 13c, 13f, 13g ... First region, 13d, 13e ... Second region, 13h, 13i ... First 13j, 13k ... second area, 13m, 13q ... third area, 13n, 13p ... fourth area, 15 ... semiconductor stack, 17 ... p-side electrode, 19 ... n-side electrode, 21 ... n Type gallium nitride based semiconductor region, 23... P type gallium nitride based semiconductor region, 25... Gallium nitride based semiconductor active layer, C, M... Crystal axis vector, N. 33 ... p-type cladding layer, 35 ... p-type contact layer, 37 ... n-type cladding layer, 39 ... n-type buffer layer, 41, 43 ... cleavage surface, 45, 47 ... side surface

Claims (15)

A substrate made of hexagonal n-type gallium nitride;
an n-type gallium nitride based semiconductor region, a p-type gallium nitride based semiconductor region, and a gallium nitride based semiconductor active layer provided between the n-type gallium nitride based semiconductor region and the p-type gallium nitride based semiconductor region, A semiconductor stack provided on the main surface of the substrate;
A p-side electrode provided on the p-type gallium nitride based semiconductor region of the semiconductor stack;
An n-side electrode provided on the back surface of the substrate,
The main surface of the substrate is inclined at an angle of 20 degrees or more and 45 degrees or less in the a-axis direction of gallium nitride from the c-plane of gallium nitride,
The substrate includes a first region extending from the back surface to the main surface along the c-axis direction, and a second region extending from the back surface to the main surface along the first region. Have
The dislocation density of the first region of the substrate is smaller than the dislocation density of the second region of the substrate,
The main surface of the substrate has a first area where the first region appears, and a second area where the second region appears,
The group III nitride semiconductor laser, wherein an ohmic contact region between the p-side electrode and the p-type gallium nitride based semiconductor region is provided on the first area. The first and second regions extend in the direction of a predetermined axis;
2. The group III nitride semiconductor laser according to claim 1, wherein the p-side electrode extends in the direction of the predetermined axis. The substrate includes a first portion extending from the back surface to the main surface along the c-axis direction, and a second portion extending from the back surface to the main surface along the first portion. A third portion extending from the back surface to the main surface along the second portion,
The specific resistance of the first part is lower than the specific resistance of the second part,
The specific resistance of the third part is lower than the specific resistance of the second part,
The second part is between the first part and the third part;
3. The group III nitride semiconductor laser according to claim 1, wherein the first region includes the first, second, and third portions. 4. The first portion has a specific resistance in the range of 5 × 10 ⁻³ Ω · cm to 3 × 10 ⁻² Ω · cm, and is provided in parallel with the gallium nitride based semiconductor active layer. 3 has a specific resistance in the range of 5 × 10 ⁻³ Ω · cm to 3 × 10 ⁻² Ω · cm, and is provided in parallel with the gallium nitride based semiconductor active layer. The portion is adjacent to the first and third portions and has a specific resistance in the range of 4 × 10 ⁻² Ω · cm to 5 × 10 ⁶ Ω · cm. The group III nitride semiconductor laser described. The second region of the substrate comprises a polarity reversal region having a first polarity in the c-axis direction;
The first region of the substrate comprises a polarity non-inversion region having a polarity opposite to the first polarity of the polarity inversion region;
The polarity reversal region in the second region of the substrate has a dislocation density larger than that of the non-polarity reversal region, according to any one of claims 1 to 4. The group III nitride semiconductor laser described. The second region of the substrate includes a polarity reversal region having a first polarity in a c-axis direction;
The first region of the substrate comprises a polarity non-inversion region having a polarity opposite to the first polarity of the polarity inversion region;
The group III nitride semiconductor laser further includes a side surface extending in the m-axis direction,
5. The group III nitride according to claim 1, wherein the polarity inversion region in the second region of the substrate is present in at least a part of the side surface. Semiconductor laser. The second region of the substrate comprises a polarity reversal region having a first polarity in the c-axis direction;
The first region of the substrate comprises a polarity non-inversion region having a polarity opposite to the first polarity of the polarity inversion region;
5. The boundary between the polarity inversion region and the polarity non-inversion region extends along a reference plane where the main surfaces intersect with each other. Group III nitride semiconductor laser.   8. The group III nitride semiconductor laser according to claim 5, wherein the polarity reversal region extends in a c-axis direction of gallium nitride of the substrate. The specific resistance of the polarity inversion region is lower than the specific resistance of the second portion,
The said polarity inversion area | region is extended along the reference plane inclined with respect to the said main surface by the angle which the normal line of the said main surface and c axis | shaft make. The group III nitride semiconductor laser described in any one of the above. The dislocation density of the first region is in the range of 1 × 10 ² cm ⁻² or more and 1 × 10 ⁶ cm ⁻² or less,
10. The dislocation density in the second region is in a range from 1 × 10 ⁶ cm ^{−2 to} 1 × 10 ⁸ cm ^−2. 10. Group III nitride semiconductor laser.   The average dislocation density in the second region is a value that is three times or more the average dislocation density in the first region, and is described in any one of claims 1 to 10. Group III nitride semiconductor laser.   The group III nitride semiconductor laser according to any one of claims 1 to 11, further comprising first and second cleavage planes intersecting the m-axis.   The main surface of the substrate is inclined at an angle of not less than 28 degrees and not more than 32 degrees in the a-axis direction of gallium nitride from the c-plane of gallium nitride. A group III nitride semiconductor laser as described in one item. The semiconductor stack includes a semiconductor ridge;
The gallium nitride based semiconductor active layer is in the semiconductor ridge;
The group III nitride semiconductor laser according to claim 1, wherein the gallium nitride based semiconductor active layer is provided on the first area.   The angle at which the main surface is inclined in the m-axis direction from the c-plane is in the range of -1 degree or more and +1 degree or less, and is described in any one of claims 1 to 14. Group III nitride semiconductor laser.

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