JP4718852B2 - Nitride semiconductor device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a semiconductor device using a group III nitride used for a blue light emitting diode, a blue-violet laser diode, and the like, and a method for manufacturing the same.

Group III nitride semiconductors have a very large band gap, such as gallium nitride (GaN) and a band gap of 3.4 keV (room temperature). Therefore, a material capable of realizing visible light emission over a wide range from blue to ultraviolet. As a promising material. Semiconductor lasers and LEDs that actually emit blue light to ultraviolet light have been realized and are commercially available. As an important technique for forming an optical device using such a nitride semiconductor, there are a technique for narrowing a current flowing through the device and a technique for forming an insulating film for insulating between electrodes. In general, a dielectric such as a silicon oxide (SiO ₂ ) film or a silicon nitride (SiN) film formed by a high-frequency sputtering method, a CVD (Chemical Vapor Deposition) method, or a vapor deposition method as an insulating film that blocks a current flowing through the device A membrane is used. Research and development of techniques for improving device characteristics including the technique for forming such an insulating film and the technique for constricting the current flowing through the device are being actively conducted (for example, Non-Patent Document 1).

  FIG. 11 is a cross-sectional view showing an example of a conventional semiconductor light emitting device that functions as a surface emitting laser. The conventional surface emitting laser shown in FIG. 1 includes a sapphire substrate 1101, a low-temperature buffer layer, a nitride semiconductor mirror 1102, an n-type cladding layer 1103, an active layer 1104, and a p-type cladding layer 1105. The low temperature buffer layer is made of AlN or GaN, and the nitride semiconductor mirror 1102 is made of a multilayer film of AlN / GaN. The n-type cladding layer 1103 is made of AlGaN having a thickness of 0.5 μm, and the active layer 1104 is formed of a multilayer film of InGaN / InGaN. The p-type cladding layer 1105 is made of GaN having a thickness of 0.5 μm.

In addition, the conventional surface emitting laser includes an insulating film 1106 made of SiN or SiO ₂ provided on the side surfaces of the active layer 1104 and the p-type cladding layer 1105 on the n-type cladding layer, and the p-type cladding layer 1105. Transparent electrode 1107 provided on top, p-type electrode 1108 provided on insulating film 1106 so as to be in contact with transparent electrode 1107, electrode pad 1109 provided on p-type electrode 1108, and n-type cladding layer 1103 An n-type electrode 1110 provided on the upper surface and an electrode pad 1111 provided on the n-type electrode 1110 are further provided.

  The n-type cladding layer 1103, the active layer 1104, and the p-type cladding layer 1105 are formed in a mesa shape by dry etching. A reflective film 1112 made of a dielectric multilayer film is formed immediately above the transparent electrode 1107.

  The above-described insulating film 1106 provided on the n-type cladding layer 1103, the side wall of the mesa formed by the active layer 1104 and the p-type cladding layer 1105 is provided so that the current flowing through the laser device is concentrated on the active layer 1104. It is formed using a sputtering method or a p-CVD method. The diameters of the upper portion of the n-type clad layer 1103 formed by etching, the p-type clad layer 1105 and the active layer 1104 are about 5 μm, and are formed to have substantially the same size.

  Next, a method for manufacturing a semiconductor laser device in a conventional embodiment will be briefly described. 12A to 12E are cross-sectional views showing a conventional method for manufacturing a surface emitting laser.

  First, as shown in FIG. 12A, a nitride semiconductor mirror 1102 made of an AlN / GaN multilayer film and an n-type clad layer made of AlGaN are formed on a sapphire substrate 1101 using metal organic chemical vapor deposition (MOCVD). 1103, an active layer 1104 having a multiple quantum well formed by stacking InGaN / InGaN, and a p-type cladding layer 1105 made of GaN are epitaxially grown in this order.

Next, as shown in FIG. 12B, a 300 nm thick deposited film made of SiO ₂ is formed on the entire surface of the p-type cladding layer 1105 by using plasma CVD. The deposited film is etched into a disk shape having a diameter of 5 μm by a photolithography process and an RIE dry etching process using CHF ₃ gas, thereby forming a SiO ₂ mask 1113. Next, after removing the resist, chlorine-based inductively coupled plasma (ICP) etching using plasma of a chlorine-based material is performed using the SiO ₂ mask 1113 as a mask. Etching is performed until the n-type cladding layer 1103 is exposed to form a cylindrical structure having a diameter of 5 μm.

Subsequently, as shown in FIG. 12C, after removing the SiO ₂ mask 1113, an SiO ₂ film 1106 having a thickness of 300 nm is deposited again on the entire surface of the device.

Next, as shown in FIG. 12D, a portion of the SiO ₂ film 1106 provided on the cylindrical p-type cladding layer 1105 and a part of the n-type cladding layer 1103 by a photolithography process. The part provided on the top is removed.

Next, as shown in FIG. 12E, a transparent electrode 1107 made of ITO (Indium-Tin Oxide) provided on the p-type cladding layer 1105 and an SiO ₂ film 1106 is connected to the transparent electrode 1107. The formed p-type electrode 1108 is formed. Next, after forming an n-type electrode 1110 on the exposed portion of the n-type cladding layer 1103, electrode pads 1109 and 1111 are formed, respectively. Thereafter, a reflective film 1112 made of a multilayer film composed of TaO ₂ and SiO ₂ is formed on the transparent electrode 1107, whereby the semiconductor laser device shown in FIG. 11 is manufactured.

  On the other hand, a selective oxidation technique for nitride semiconductors is attracting attention in the sense of improving the performance of nitride semiconductor devices. For example, the surface of the GaN layer is selectively oxidized by heat treatment in an oxygen atmosphere using a Si thin film as a mask material. Thereafter, the mask material is removed, and various devices are manufactured. When a field effect transistor is fabricated on the main surface of the GaN layer, device isolation and device breakdown voltage can be achieved by the oxide layer (see Patent Document 1). Such a nitride semiconductor oxide film is also used as a current blocking layer in a semiconductor laser element (see Patent Document 2).

Such an oxidation technique can be applied to current confinement of a semiconductor laser, and a wide range of applications are expected.
JP 2001-267555 A Japanese Patent Application No. 2003-162417 IEEE Journal of Quntum Electronics Vol.39 No.1 p135-p140

  In general, a nitride semiconductor laser has a structure for confining light in an active layer, and the size of an active region that emits light when energized greatly affects laser characteristics such as a threshold value and slope efficiency. Therefore, for example, in the surface emitting laser, the diameter of the active region is set to about 5 μm, and in the ridge type laser, the width of the active region needs to be set to about 2 μm with high accuracy. In the conventional manufacturing method, as shown in FIG. 12B, the nitride semiconductor is etched vertically from the wafer surface side by dry etching to form a mesa shape. For this reason, it is difficult to control the size of the active region, and forming a mesa shape by dry etching has caused a reduction in the yield of the semiconductor laser.

  In the conventional manufacturing method, since the p-type nitride semiconductor layer and the active region are formed by dry etching using the same mask, the size of the p-type nitride semiconductor layer on the surface side from the active region is the size of the active region. It becomes the same level as. For this reason, the resistance in the vertical direction (vertical direction in FIG. 11) of the p-type nitride semiconductor (p-type cladding layer 1105) made of a material with high resistivity is increased, and the operating voltage of the laser element is increased.

  An object of the present invention is to provide a semiconductor light emitting device in which the electrical resistance in a nitride semiconductor layer is reduced and the yield is improved while realizing high luminous efficiency.

In order to achieve the above object, a first nitride semiconductor device of the present invention comprises a substrate and at least two layers of nitride semiconductor layers containing In having different compositions from each other formed on the substrate. A nitride semiconductor layer having a large In composition among the at least two nitride semiconductor layers is an oxidation promoting layer having a higher oxidation rate than a nitride semiconductor layer having a small In composition, and the two-layer nitride semiconductor Part of each of the layers is oxidized.

  With this configuration, the thickness of the insulating film can be changed by appropriately selecting the composition of the nitride semiconductor layer. Therefore, for example, when a plurality of nitride semiconductor layers are stacked in the vertical direction, the area of the layer that becomes a contact with the electrode is increased to reduce the contact resistance and the resistance in the layer while flowing to the active layer. The current can be confined.

The two-layer nitride semiconductor layer of different band gaps from each other, of the nitride semiconductor layer of the two layers, the thickness of the portion of the insulation Enmaku formed by oxidizing a small band gap nitride semiconductor layer The thickness of the insulating film formed by oxidizing the nitride semiconductor layer having a large band gap is preferably larger.

Of the at least two nitride semiconductor layers, the thickness of the insulating film formed by oxidizing the nitride semiconductor layer having a high In composition is obtained by oxidizing the nitride semiconductor layer having a low In composition. By being thicker than the thickness of the formed insulating film, the thickness of the insulating film can be adjusted depending on the level of In composition. For this reason, the optical waveguide of the edge-emitting semiconductor laser and the current confinement layer of the surface-emitting laser can be manufactured in a self-aligned manner using this.

The at least two nitride semiconductor layers are stacked in a direction perpendicular to the main surface,
The insulating film is preferably formed on a side surface of the at least two nitride semiconductor layers.

The nitride semiconductor device further includes a first cladding layer of a first conductivity type provided on or above the substrate, and the at least two nitride semiconductor layers are on the first cladding layer. An active layer that generates light, and a second cladding layer of a second conductivity type that is provided on the active layer and has an In composition lower than that of the active layer. The area is preferably smaller than the planar area of the second cladding layer. Since the area of the active layer into which current flows can be narrowed by reducing the planar area of the active layer, the light emission efficiency can be improved. Further, since the planar area of the second cladding layer is larger than that of the active layer, the contact resistance with the electrode and the series resistance in the second cladding layer can be reduced, and the device characteristics can be improved. . Furthermore, since the nitride semiconductor device of the present invention can be formed without etching the active layer, the occurrence of a leak path caused by etching can be prevented.

  The nitride semiconductor device of the present invention can also function as a surface emitting laser that emits light generated in the active layer upward.

  The nitride semiconductor device of the present invention can also function as a ridge type semiconductor laser that emits light generated in the active layer from the end face of the active layer.

Wherein the at least two layers nitride semiconductor layer is provided on the front Stories second cladding layer includes In, and a second oxidation promoting layer thicker than said active layer, said second oxide promoting layer A third clad layer of the second conductivity type provided thereon, and the planar area of the second oxidation promoting layer is the plane of the second clad layer and the third clad layer. It may be smaller than the area. By increasing the thickness of the oxidation promoting layer, it is possible to form a thick insulating film without increasing the In composition, so that light generated in the active layer can be efficiently extracted .

The nitride semiconductor device further includes a fourth clad layer of a first conductivity type provided on or above the substrate, and an active layer provided on the fourth clad layer and generating light. The at least two nitride semiconductor layers include a fifth conductivity type fifth cladding layer provided on the active layer, and a third oxidation promotion layer having an In composition higher than that of the fifth cladding layer. If the provided on the oxidation-promoting layer, the third has a sixth cladding layer of the lower second conductivity type in composition than the oxidation promoting layer, the plane of said third oxidation-promoting layer The area may be smaller than the planar areas of the fifth cladding layer and the sixth cladding layer.

The thickness of the third oxidation promotion layer is thicker than that of the fifth cladding layer and the sixth cladding layer, and the portion of the insulating film provided on the side surface of the third oxidation promotion layer Is preferably thicker than the thicknesses of the portions provided on the side surfaces of the fifth cladding layer and the sixth cladding layer.

  The nitride semiconductor device of the present invention can also function as a surface emitting laser that emits light generated in the active layer upward.

In addition, the third oxidation promoting layer can function as a waveguide for the light generated in the active layer, and can function as a ridge type semiconductor laser that emits the light from the end face of the active layer.

The method for manufacturing a nitride semiconductor device of the present invention includes a step (a) of forming a plurality of nitride semiconductor layers having two or more types of In compositions on or above a substrate, and a step of forming the plurality of nitride semiconductor layers. An insulating film having a thickness corresponding to the composition of the plurality of nitride semiconductor layers by oxidizing the side wall portion has a higher oxidation rate for a nitride semiconductor layer having a large In composition than a nitride semiconductor layer having a small In composition. And a step (b) of forming as described above .

  By this method, it is possible to form insulating films having different thicknesses depending on the composition of the nitride semiconductor device. For example, the planar area of a selected layer of the nitride semiconductor layers stacked in the vertical direction It can be made smaller than the layer. Therefore, since the formation of the optical waveguide and the formation of the current confinement layer can be performed in a self-aligned manner, a nitride semiconductor device can be manufactured with a high yield.

In the case of further comprising a step (c) of forming a mesa shape by removing a part of the plurality of nitride semiconductor layers after the step (a) and before the step (b). Since the insulating film is formed by the oxidation step (step (b)) after the formation of the film, it is not necessary to make the mesa shape formed in the step (c) as fine as the conventional method, so that the tolerance of processing accuracy increases. It becomes possible to improve the yield .

Prior Symbol step (a) said plurality of nitride semiconductor layer formed of, include In, and an active layer for generating light, provided on the active layer, the In composition is lower than the active layer and the first Since the area of the active layer can be reduced in the step (b), a nitride semiconductor device with improved light emission efficiency can be manufactured. In addition, since the area of the active layer can be reduced without using etching, the occurrence of a leak path in the active layer can be prevented.

  The plurality of nitride semiconductor layers formed in the step (a) are provided on the second cladding layer and the second cladding layer, include In, and are thicker than the second cladding layer. An acceleration layer and a third cladding layer provided on the oxidation promotion layer and thinner than the oxidation promotion layer may be included. By making the oxidation promotion layer thicker than the second cladding layer, the oxidation rate of the oxidation promotion layer can be increased without increasing the In composition in the step (b).

  In the step (b), the insulating film is preferably formed by performing a heat treatment in a gas atmosphere containing an oxygen compound.

  It is particularly preferable that the gas containing the oxygen compound is water vapor.

  According to the method for manufacturing a nitride semiconductor device of the present invention, the active layer having a large In composition or the oxidation promoting layer can be accelerated more quickly than the other layers with respect to the group III nitride semiconductor. Therefore, it is possible to provide a nitride semiconductor device capable of confining a current to be generated and having a reduced internal resistance. In addition, according to the nitride semiconductor device of the present invention, an insulating film obtained by oxidation is used for the nitride semiconductor layer, and the active layer and the oxidation promoting layer can be made smaller than the cladding layer. Controllability and design flexibility are improved, and high-performance device characteristics can be realized.

  In order to solve the above-described conventional problems, the inventor of the present application has adopted a current confinement technique for narrowing a region of an active layer into which a current flows in order to improve luminous efficiency. As a result of examining a number of structures, if the cylindrical portion of the semiconductor light emitting device can be oxidized so as to selectively oxidize the active layer, the active layer in which the current flows without increasing the resistance of the p-type cladding layer can be obtained. We thought that the plane area could be reduced.

  On the other hand, the inventor of the present application has found that the oxidation rate varies greatly depending on the In composition as a result of oxidation experiments on GaInN having various compositions. Therefore, the inventor of the present application actively utilizes the discovery, and selectively oxidizes an active region made of a nitride semiconductor containing In, thereby accurately reducing the active region without increasing the resistance in the cladding layer. We have come up with a configuration that efficiently injects current into the active layer. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a cross-sectional view showing a configuration of a surface emitting laser according to a first embodiment of the present invention.

  As shown in the figure, the surface emitting laser of this embodiment includes a sapphire substrate 101, a nitride semiconductor mirror 102 made of a GaN mixed crystal multilayer film provided on the sapphire substrate 101, and a nitride semiconductor mirror 102. An n-type cladding layer 103 made of AlGaN mixed crystal having a thickness of 0.5 μm, an active layer 104 made of an InGaN / InGaN multilayer film provided on the n-type cladding layer 103, and an active layer 104 A p-type cladding layer 105 made of GaN having a thickness of 0.2 μm, and a thermal oxide film 106 provided on the n-type cladding layer 103 so as to surround the active layer 104 and the p-type cladding layer 105; It has. In addition, the surface emitting laser according to the present embodiment includes a transparent electrode 107 provided on the entire main surface of the p-type cladding layer 105, and a reflective film 112 formed of a dielectric multilayer film provided immediately above the transparent electrode 107. A p-type electrode 108 provided on the thermal oxide film 106 and connected to the transparent electrode 107; a p-type electrode pad 109 provided on the p-type electrode 108; and a part of the n-type cladding layer 103. The n-type electrode 110 provided on the n-type electrode 110 and the n-type electrode pad 111 provided on the n-type electrode 110 are provided.

  The active layer 104 includes a multiple quantum well layer in which an InGaN barrier layer having an In composition of 2% and a thickness of 7.5 nm and an InGaN well layer having an In composition of 10% and a thickness of 3.5 nm are stacked in three cycles, The InGaN guide layer has an In composition of 2% and a thickness of 40 nm, and the emission wavelength from the InGaN well layer is set to 410 nm. In the surface emitting laser of this embodiment, light is emitted from the upper surface.

  The upper part of the n-type cladding layer 103, the active layer 104, the p-type cladding layer 105, and a part of the thermal oxide film 106 are formed in a mesa shape, and the planar shape is, for example, a circle. The diameter of the mesa portion is 20 μm before the formation of the thermal oxide film.

  The thermal oxide film 106 is an insulating film and is formed by thermally oxidizing the side wall portion or the surface portion of the n-type cladding layer 103, the active layer 104, and the p-type cladding layer 105 with water vapor or the like. The thickness of the portion of thermal oxide film 106 located on the side wall of active layer 104 is thicker than the thickness of the portion located on the side walls of n-type cladding layer 103 and p-type cladding layer 105. This is because the n-type cladding layer 103 and the p-type cladding layer 105 do not contain In, whereas the active layer 104 contains In. For this reason, the diameter of the active layer 104 is larger than the diameter of the upper part of the n-type cladding layer 103 and the p-type cladding layer 105. In the example shown in FIG. 1, the diameter of the upper part of the n-type cladding layer 103 and the p-type cladding layer 105 is 15 μm, and the diameter of the active layer 104 is 5 μm. The thickness of the portion of the thermal oxide film 106 provided on the upper portion of the n-type cladding layer 103 and the sidewall of the p-type cladding layer 105 is, for example, 2.5 μm. The thickness is 7.5 μm, for example.

  In the surface emitting laser according to the present embodiment having such a configuration, since the planar area of the active layer 104 is narrowed by the formation of the thermal oxide film 106, the current flowing in the element is passed through the active layer 104 in a constricted state. Can do. For this reason, the luminous efficiency in the active layer 104 can be improved.

  Furthermore, in the surface emitting laser according to the present embodiment, the diameters of the upper portion of the n-type cladding layer 103 and the p-type cladding layer 105 are set larger than the diameter of the active layer 104. Therefore, the vertical resistance in the n-type cladding layer 103 and the p-type cladding layer 105 can be reduced, and the contact resistance between the p-type cladding layer 105 and the transparent electrode 107 can be effectively reduced. As a result, the device characteristics of the surface emitting laser of this embodiment are greatly improved.

  Next, a method for manufacturing the surface emitting laser according to the present embodiment will be described. 2A to 2E are cross-sectional views illustrating a method for manufacturing the surface-emitting laser according to the present embodiment.

  First, as shown in FIG. 2 (a), the nitride semiconductor mirror 102, the n-type cladding layer 103 made of AlGaN, and the activity of forming a multiple quantum well on the sapphire substrate 101 using the MOCVD method, the n-type cladding layer 103 made of AlGaN, and InGaN / InGaN. The layer 104 and the p-type cladding layer 105 made of GaN are epitaxially grown in this order.

  Next, as shown in FIG. 2B, the metal mask patterned by the photolithography process is used to mask the region other than the region for forming the n-type electrode 110, and the n-type cladding layer 103 is exposed. Etch until

Subsequently, as shown in FIG. 2C, a 300 nm-thick deposited film made of a multilayer film of SiN and SiO ₂ is deposited on the entire surface of the device (on the nitride semiconductor) using a plasma CVD method. Thereafter, the oxidation mask 113 is formed in a disk shape having a diameter of 20 μm by a photolithography process and an RIE dry etching process using CHF ₃ gas. At this time, the oxidation prevention mask 113 is left also in the region for forming the n-type electrode. After removing the resist, the upper portion of the n-type clad layer 103, the active layer 104, and a part of the p-type clad layer 105 are removed by chlorine-based ICP dry etching using the antioxidant mask 113 as a mask, and a mesa made of these layers is removed. Form a structure. The mesa shape size at this time is a cylindrical structure having a diameter of 20 μm.

Next, as shown in FIG. 2D, the substrate is introduced into an oxidation furnace, heated to 900 ° C. in a mixed atmosphere of water vapor and nitrogen, and the side walls of the mesa are oxidized for 30 minutes at atmospheric pressure. A thermal oxide film 106 is formed. Water vapor is supplied at a flow rate of 2.3 slm by bubbling 1 slm (slm is L / min at 0 ° C., 101.3 kPa) into H ₂ O at 94 ° C., and the water vapor partial pressure is 69%. In this step, since the active layer 104 is oxidized faster than the n-type cladding layer 103 and the p-type cladding layer 105, the size of the active layer 104 is smaller than that of the n-type cladding layer 103 and the p-type cladding layer 105. For example, the diameter of the active layer 104 viewed from above is 5 μm, and the diameters of the n-type cladding layer 103 and the p-type cladding layer 105 are 15 μm.

Next, as shown in FIG. 2E, the antioxidant mask 113 is removed by etching. Thereafter, the transparent electrode 107 made of ITO is formed on the p-type cladding layer, and then the p-type electrode 1108, the n-type electrode 110, the p-type electrode pad 109, and the n-type electrode pad 111 are formed by a known method. . Thereafter, the reflective film 112 made of a multilayer film of Ta ₂ O ₅ and SiO ₂ is formed on the transparent electrode 107, whereby the surface emitting laser of this embodiment can be manufactured. According to this method, since the diameter of the mesa formed by etching in the step shown in FIG. 2C can be made larger than that of the conventional method, a high degree of etching accuracy is not required, and the heat is self-aligned. Since the oxide film 106 can be formed, a surface emitting laser with uniform performance can be easily manufactured, and the yield of the surface emitting laser can be improved.

  Here, the oxidation process in the formation process of the thermal oxide film 106 shown in FIG. FIGS. 3A and 3B are schematic cross-sectional views of the nitride semiconductor layer for explaining the oxidation process of the mesa sidewall.

In the element shown in FIGS. 3A and 3B, a GaN first semiconductor layer (n-type clad layer) 202 and In _x Ga _1-x N (0 <x ≦ 1) are formed on a sapphire substrate 201. A second semiconductor layer 203 (active layer) and a third semiconductor layer (p-type cladding layer) 204 made of GaN are sequentially stacked. In particular, FIG. 3A shows a surface emitting laser in which a mesa is vertically formed by dry etching after depositing a mask made of SiN by CVD to prevent oxidation on the upper surface of the third semiconductor layer. Show. FIG. 3B shows the surface emitting laser after heat treatment at 900 ° C. for 30 minutes in a steam atmosphere. By this heat treatment, the upper portion of the first semiconductor layer 202 and the side wall portions of the second semiconductor layer 203 and the third semiconductor layer 204 are oxidized to form oxide films 205, 206, and 207, respectively. At this time, since the second semiconductor layer 203 having a large In composition is oxidized faster than the first semiconductor layer 202 and the third semiconductor layer 204, the thickness of the oxide film 206 formed on the sidewall of the second semiconductor layer 203 is increased. t ₂ is thicker than the thickness t _{1 of} the oxide film 205 formed on the sidewall of the first semiconductor layer 202 and the oxide film 207 formed on the sidewall of the third semiconductor layer 204.

FIG. 4A is a view showing the same surface emitting laser as FIG. 3B, and FIG. 4B is a second semiconductor layer (“InGaN layer” in the drawing) in the surface emitting laser shown in FIG. It is a figure which shows the relationship between the composition of this, and the film thickness of the thermal oxide film formed by thermal oxidation. Here, d indicates the film thickness of the active layer. The thermal oxidation was performed at 900 ° C. for 30 minutes in the presence of water vapor. Water vapor was supplied at a flow rate of 2.3 slm by bubbling nitrogen at 1 slm (slm is L / min at 0 ° C., 101.3 kPa) into H ₂ O at 94 ° C., and the water vapor partial pressure was 69%.

  As shown in FIG. 4B, the results of this experiment revealed that the rate of oxidation increases as the In composition increases in the nitride semiconductor layer. This finding has been found for the first time by the present inventors. In other words, since the oxidation rate increases as the In composition increases, in the surface emitting laser shown in FIG. 4A, the second semiconductor layer is more than the oxide films on the sidewalls of the first semiconductor layer 202 and the third semiconductor layer 204. The oxide film on the side wall 203 is formed thicker. As a result of examining the case where the second semiconductor layer is made of another nitride semiconductor, the inventor of the present application has estimated that when there are a plurality of nitride semiconductor layers, a layer having a smaller band gap tends to be oxidized. .

  The inventors of the present application have also found that when the In composition of the second semiconductor layer 203 is the same, the thicker the second semiconductor layer, the thicker the sidewall oxide film. This is presumably because the thicker the second semiconductor layer 203 is, the easier oxygen can enter.

  In the surface emitting laser shown in FIG. 1, the In composition of the active layer 104 is larger than the In compositions of the n-type cladding layer 103 and the p-type cladding layer 105. Therefore, by performing thermal oxidation, in the active layer 104, the oxidation proceeds quickly from the side wall and the thermal oxide film 106 becomes thicker, and the oxidation gradually proceeds on the side walls of the n-type cladding layer 103 and the p-type cladding layer 105. As a result, the thermal oxide film 106 becomes thinner. Therefore, the diameter of the active layer 104 can be made smaller than the diameters of the n-type cladding layer 103 and the p-type cladding layer 105.

  Thus, since it is possible to increase the planar area of the p-type cladding layer 105 without increasing the planar area of the active layer 104, it is possible to realize a surface emitting laser with reduced electrical resistance inside the element. it can. In the surface emitting laser according to the present embodiment, since the active layer 104 is covered with the thermal oxide film 106 that is an insulator, no current flows, and current is efficiently injected from the p-type cladding layer 105 into the active layer 104. It can be configured.

  FIG. 5 is a transmission electron microscope (TEM) image showing a longitudinal section of the semiconductor light emitting device after the thermal oxidation treatment. From the figure, since the oxidation rate of the active layer made of InGaN / InGaN is faster than the n-type clad layer and the p-type clad layer made of GaN or AlGaN not containing In, oxide films formed at both ends of the active layer It can be seen that the width of the active layer is narrowed.

  Note that the knowledge that the oxidation rate differs depending on the In composition contained in the nitride semiconductor can also be used when a plurality of nitride semiconductor layers having different In compositions are arranged in the lateral direction. For example, when an InGaN layer and a GaN layer are disposed adjacent to each other and their upper surfaces are exposed, a deep insulating oxide film can be formed on the InGaN layer by performing thermal oxidation. It can be used to form an insulating film for use.

In the surface emitting laser according to the present embodiment described so far, a sapphire substrate is used as an underlayer. However, a GaN substrate or an AlN layer is formed on a sapphire substrate in order to reduce defects in the nitride semiconductor layer. GaN that has been deposited or that has been subjected to ELO growth (Epitaxial Lateral Overgrowth) on the substrate using patterned SiO ₂ as a mask may be used. By forming a nitride semiconductor layer on a low-defect underlayer, device characteristics can be improved.

  In the surface emitting laser according to the first embodiment of the present invention, a GaN layer is used for the p-type cladding layer 105. The AlGaN layer has a thickness of 10% and a thickness of 1.5 nm. An SLS structure in which 1.5 nm GaN layers are alternately stacked may be used as the p-type cladding layer. With such a configuration, a p-type cladding layer having a low resistance can be obtained, so that element characteristics can be improved.

  In the surface emitting laser according to the first embodiment of the present invention, an InGaN / InGaN layer is used for the active layer 104, but a layer made of AlInGaN quaternary mixed crystal containing Al may be used. In the mixed crystal having the same band gap wavelength, the oxidation rate of the mixed crystal having a larger In composition changes more remarkably, so that selective oxidation can be performed on the same principle as the method described in the present embodiment.

Thickness on the side surface of the nitride semiconductor layer represented by Al _x Ga _1-xy In _y N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 <x + y ≦ 1) described in this embodiment The structure in which an insulating film made of different oxides is formed can be applied not only to a surface-emitting laser but also to an edge-emitting laser or a light-emitting diode (LED) described later.

In the manufacturing method of the present embodiment, the thermal oxide film 106 is formed by heating the substrate in the presence of water vapor, but thermal oxidation by other means may be performed. For example, thermal oxidation may be performed in a gas containing oxygen other than water vapor in its molecule, such as NO, NO ₂ , and methanol.

(Second Embodiment)
FIG. 6 is a cross-sectional view showing a configuration of a surface emitting laser according to the second embodiment of the present invention. The surface emitting laser of this embodiment includes an oxidation promoting layer that utilizes the above-described knowledge that “the thicker the nitride semiconductor layer, the faster the oxidation from the side wall portion of the nitride semiconductor layer proceeds”. It is characterized by that.

  In the surface emitting laser according to the second embodiment of the present invention, the sapphire substrate 301, the nitride semiconductor mirror 302, the n-type cladding layer 303 having a thickness of 0.5 μm made of AlGaInN mixed crystal, and the active made of a multilayer film of InGaN / InGaN. The configuration up to the layer 304 is the same as in the first embodiment.

  In the surface emitting laser of this embodiment, an oxidation promotion layer 313 made of an InGaN mixed crystal having an In composition of 6% and a thickness of 30 nm is provided on the first p-type cladding layer 305 provided on the active layer 304. ing. A second p-type cladding layer 314 is formed on the oxidation promotion layer 313. That is, the mesa is formed by dry-etching up to the second p-type cladding layer 314 without making the active layer 304 a part of the mesa, and the side wall of the oxidation promoting layer 313 is thermally oxidized, whereby the thermal oxide layer. 306 is formed.

  A transparent electrode 307 is formed on the entire surface of the second p-type cladding layer 314, and a p-type electrode 308 connected to the transparent electrode 307 is formed on the thermal oxide film 306. A p-type electrode pad 309 is formed on the p-type electrode 308. A reflective film 312 made of a dielectric multilayer film is formed immediately above the transparent electrode 307.

  The oxidation promoting layer 313 has an In composition higher than that of the second p-type cladding layer 314, and oxidation proceeds faster than the second p-type cladding layer 314. Therefore, the width of the oxidation promoting layer 313 can be made smaller than the width of the second p-type cladding layer 314 in the step of forming the thermal oxide film 306 as described above.

  As described above, in the surface emitting laser according to the present embodiment, the size of the oxidation promoting layer 313 can be reduced, so that the current injected into the active layer 304 can be effectively confined and the light emission efficiency is improved. be able to.

  Further, since the area of the second p-type cladding layer 314 can be made larger than before, the resistance value in the layer can be reduced, and the contact resistance generated between the transparent electrode 307 and the transparent electrode 307 can also be reduced. Further, since the active layer 304 is not exposed by etching, the reliability of the element is improved.

  In the surface emitting laser of the present embodiment, if the oxidation promoting layer 313 is thickened, the planar area of the oxidation promoting layer 313 can be reduced, and if the oxidation promoting layer 313 is thinned, the planar area of the oxidation promoting layer 313 can be increased. Accordingly, it is possible to adjust the current confinement to a desired degree. Further, by thickening the oxidation promoting layer 313, a thick sidewall oxide film (thermal oxide film 306) can be formed with a composition having a lower In composition than the active layer 304. In this case, light absorption by the oxidation promoting layer 313 can be suppressed, so that laser characteristics can be improved.

(Third embodiment)
FIG. 7 is a cross-sectional view showing a configuration of a ridge type semiconductor laser according to the third embodiment of the present invention.

  As shown in the figure, the semiconductor laser according to the present embodiment includes a GaN substrate 401 and a first n-type AlGaN having an Al composition of 5% and a film thickness of 1.2 μm provided on the main surface of the GaN substrate 401. Provided on the n-type clad layer 402 and the first n-type clad layer 402, and on the second n-type clad layer 403 made of n-type GaN having a thickness of 90 nm and on the second n-type clad layer 403 An active layer 404 made of a multilayer film of InGaN / InGaN, and a p-type cladding layer 405 having a thickness of 0.5 μm provided on the active layer 404 and alternately laminated with AlGaN and GaN having an Al composition of 10%. And. The semiconductor laser of this embodiment is provided so as to surround the upper portion of the second n-type cladding layer 403, the active layer 404 and the p-type cladding layer 405, and is formed by oxidizing a nitride semiconductor. On the film 406, the p-type electrode 408 provided on the p-type cladding layer 405 and the thermal oxide film 406, the n-type electrode pad 409 provided on the p-type electrode 408, and the back surface of the GaN substrate 401 An n-type electrode 410 provided and an n-type electrode pad 411 provided on the back surface of the n-type electrode 410 are provided. The “main surface” refers to a surface on which crystal growth is performed.

  The active layer 304 made of InGaN / InGaN has a multiple quantum well structure in which a barrier layer having an In composition of 2% and a thickness of 7.5 nm and a well layer having an In composition of 10% and a thickness of 3.5 nm are alternately stacked in three periods. And a guide layer having an In composition of 2% and a thickness of 40 nm, and an emission wavelength from the well layer is set to 410 nm. In the semiconductor laser of the present embodiment, light generated in the active layer 404 is emitted from the end face of the active layer 404 in the front direction of FIG.

  The upper portion of the second n-type cladding layer 403, the active layer 404, the p-type cladding layer 405, and the thermal oxide film 406 are formed in a stripe shape. Here, the thermal oxide film 406 is formed by thermal oxidation of the upper portion of the second n-type cladding layer 403, the active layer 404, and the p-type cladding layer 405 in water vapor as in the semiconductor light emitting devices according to the first and second embodiments. It is formed by performing. The upper portion of the second n-type cladding layer 403 and the width of the p-type cladding layer 405 are about 5 μm, and the width of the active layer 404 is 1.5 μm.

  The semiconductor laser of this embodiment is different from the semiconductor light emitting device of the first embodiment in that a thermal oxide film 406 formed by oxidizing the second n-type cladding layer 403, the active layer 404, and the p-type cladding layer 405. Is used as the current confinement layer of the ridge stripe semiconductor laser.

  In the semiconductor laser of the present embodiment, the refractive index of the first n-type cladding layer 402, the second n-type cladding layer 403, the p-type cladding layer 405, and the active layer 404 is 2.5 to 2.5 with respect to the oscillation wavelength of 410 nm. Whereas it is about 2.8, the refractive index of the thermal oxide film 406 is about 1.8, which is lower than that of nitride. For this reason, the light is confined in the active layer portion. Thus, since the refractive index of the thermal oxide film 406 is smaller than that of the base nitride semiconductor, it can function as a waveguide by confining light in the active layer.

  In the semiconductor laser of this embodiment, the current flowing through the active layer 404 is narrowed, so that the light emission efficiency is improved. Further, since the area of the p-type cladding layer 405 can be made larger than that of the active layer 404, the resistance in the element can be reduced, and the element characteristics of the semiconductor laser can be improved.

  Thus, the configuration of the present invention can be applied not only to the surface emitting laser but also to the edge emitting laser.

(Fourth embodiment)
FIG. 8 is a cross-sectional view showing a configuration of a ridge type semiconductor laser according to the fourth embodiment of the present invention.

  As shown in the figure, in the semiconductor laser of the present embodiment, the GaN substrate 501, the first n-type cladding layer 502, the second n-type cladding layer 503, the n-type electrode 510 and the n-type electrode pad 511 are the first ones. This is the same configuration as that of the semiconductor laser according to the third embodiment.

  In addition, the semiconductor laser of this embodiment includes an active layer 504 made of an InGaN / InGaN multilayer film provided on the second n-type cladding layer 503, and an AlGaN having an Al composition of 10% provided on the active layer 504. First p-type clad layer 505 having a thickness of 500 nm formed by alternately laminating GaN and GaN, oxidation promoting layer 506 made of InGaN having an In composition of 6% and a thickness of 80 nm, and GaN having a thickness of 200 nm And a second p-type cladding layer 507. Furthermore, the semiconductor laser according to the present embodiment includes a thermal oxide film 513 surrounding the upper side of the first p-type cladding layer 505, the side surfaces of the oxidation promoting layer 506 and the second p-type cladding layer 507, and the second p-type cladding. A p-type electrode 508 provided on the layer 507 and the thermal oxide film 513 and a p-type electrode pad 509 provided on the p-type electrode 508 are provided.

  The semiconductor laser of this embodiment is a ridge stripe type edge emitting laser, and is different from the third embodiment in that the current flowing through the active layer is confined using an oxidation promoting layer. Since the refractive index of the thermal oxide film 513 is smaller than that of the nitride semiconductor, light can be confined in the oxidation promoting layer 506. Also in the semiconductor laser of this embodiment, it is possible to improve the light emission efficiency without increasing the resistance inside the device.

  Further, by using the oxidation promotion layer 506, the current confinement layer and the optical confinement can be designed separately, so that the degree of freedom in design is improved and a high-performance ridge stripe laser is realized. be able to.

  Note that the oxidation promoting layer 506 is preferably thick enough to be easily oxidized. By forming the oxidation promotion layer 506 thick, the diameter of the oxidation promotion layer 506 can be reduced even if the In composition of the oxidation promotion layer 506 is lowered. When the In composition of the oxidation promoting layer 506 is lowered in this way, light generated in the active layer 504 can be made harder to absorb, so that the light emission efficiency can be improved. In addition, the oxidation promotion layer 506 preferably contains In.

  Note that a structure including the oxidation promoting layer 506 for current confinement can also be applied to a self-excited oscillation laser. In this case, the In composition of the oxidation promotion layer 506 may be set high so that the oxidation promotion layer 506 absorbs laser light.

(Fifth embodiment)
FIG. 9 is a sectional view showing the structure of a surface emitting laser according to the fifth embodiment of the present invention.

  As shown in the figure, the surface emitting laser of this embodiment has a configuration similar to that of the surface emitting laser according to the second embodiment shown in FIG. 6, and includes a sapphire substrate 601, a nitride semiconductor mirror 602, and n. The mold cladding layer is the same as that of the surface emitting laser according to the second embodiment. Further, the active layer 604, the first p-type cladding layer 605, the oxidation promoting layer 613, the second p-type cladding layer 614, the transparent electrode 607, the p-type electrode 608, the n-type electrode pad 611, and the p-type electrode pad. The composition and film thickness of 609 are the same as those of the surface emitting laser of the second embodiment.

  The surface emitting laser of this embodiment is different from the surface emitting laser according to the second embodiment in that the thermal oxide film 606 is provided so as to surround the side surface of the active layer 604, and the planar area of the active layer 604 is reduced. It is a point. That is, the thermal oxide film 606 has thick portions provided on the side surfaces of the oxidation promoting layer 613 and the active layer 604, and on the side surfaces of the first p-type cladding layer 605 and the second p-type cladding layer 614. The thickness of the portion provided in is reduced.

  The In composition of the active layer 604 is, for example, 10% in the well layer, and the In composition of the oxidation promoting layer 613 is 5%, which is smaller than that of the well layer of the active layer 604. For this reason, the band gap of the oxidation promoting layer 613 is larger than the emission wavelength of the active layer 604, and the light generated in the active layer 604 is emitted without being absorbed by the oxidation promoting layer 613. The thermal oxide film 606 is formed by performing the etching while exposing the side surface of the active layer 604 by etching. At this time, by making the film thickness of the oxidation promoting layer 613 thicker than that of the active layer 604, the rate at which the oxidation promoting layer 613 is oxidized can be increased even when the In composition of the oxidation promoting layer 613 is smaller than the active layer 604. .

  Thus, in the surface emitting laser according to the present embodiment, the thickness of the oxide film formed on the side surface of the nitride semiconductor layer can be adjusted by changing the film thickness of the oxidation promoting layer 613. For example, by setting the total film thickness of the active layer 604 to 30 nm and the film thickness of the oxidation promoting layer 613 as 80 nm, the diameter of the oxidation promoting layer 613 can be set narrower than the diameter of the active layer 604. . Thus, since the current confinement and the optical confinement structure can be designed by the width and thickness of the oxidation promoting layer and the active layer, the degree of freedom in design can be increased and the device characteristics can be improved.

  In the surface emitting laser of this embodiment, it is preferable to make the diameter of the oxidation promoting layer 613 smaller than the diameter of the active layer 604 in order to efficiently inject current into the active layer 604.

(Sixth embodiment)
FIG. 10 is a sectional view showing a configuration of a ridge type semiconductor laser according to the sixth embodiment of the present invention.

  As shown in the figure, the surface emitting laser of this embodiment has a configuration similar to that of the surface emitting laser according to the fourth embodiment shown in FIG. 8, and includes a GaN substrate 701, a first n-type cladding layer. 702 and the second n-type cladding layer 703 are the same as those of the semiconductor laser according to the fourth embodiment. In addition, the active layer 704, the first p-type cladding layer 705, the oxidation promotion layer 706, the second p-type cladding layer 707, the p-type electrode 708, the n-type electrode 710, the n-type electrode pad 711, and the p-type electrode The composition and film thickness of the pad 709 are the same as those of the semiconductor laser of the fourth embodiment.

  The first p-type cladding layer 705 has a structure in which AlGaN and GaN having an Al composition of 10% are alternately stacked and has a thickness of 0.5 μm. The oxidation promotion layer 706 is made of InGaN having an In composition of 6% and a film thickness of 80 nm.

  The semiconductor laser of this embodiment is the fourth embodiment in that the structure in which the oxidation promotion layer and the active layer described in the fifth embodiment are both oxidized to form a thermal oxide film is applied to a ridge type semiconductor laser. This is different from the semiconductor laser of the form. By setting the oxidation promotion layer 706 to be thicker than the active layer 704, a portion of the thermal oxide film 712 on the side surface of the oxidation promotion layer 706 can be formed thicker than a portion on the side surface of the active layer 704. The oxidation promotion layer 706 and the active layer 704 surrounded by the thermal oxide film having a small refractive index function as an optical waveguide of the ridge semiconductor laser.

  Thus, light confinement can be freely set by the diameter of the oxidation promoting layer 706, and the device characteristics can be improved.

  The nitride semiconductor device of the present invention can be used for various applications such as a light source for DVDs and hard disks.

It is sectional drawing which shows the structure of the surface emitting laser which concerns on the 1st Embodiment of this invention. (A)-(e) is sectional drawing which shows the manufacturing method of the surface emitting laser which concerns on 1st Embodiment. (A), (b) is typical sectional drawing of the nitride semiconductor layer for demonstrating the oxidation process of a mesa side wall. (A) is a figure which shows the same surface emitting laser as FIG.3 (b), (b) is the thermal oxidation formed by the composition and thermal oxidation of the 2nd semiconductor layer in the surface emitting laser shown to (a). It is a figure which shows the relationship with the film thickness of a film | membrane. It is a transmission electron microscope (TEM) image which shows the longitudinal cross-section of the semiconductor light-emitting device after a thermal oxidation process. It is sectional drawing which shows the structure of the surface emitting laser which concerns on 2nd Embodiment of this invention. It is sectional drawing which shows the structure of the ridge type semiconductor laser which concerns on 3rd Embodiment of this invention. It is sectional drawing which shows the structure of the ridge type semiconductor laser which concerns on the 4th Embodiment of this invention. It is sectional drawing which shows the structure of the surface emitting laser which concerns on the 5th Embodiment of this invention. It is sectional drawing which shows the structure of the ridge type semiconductor laser which concerns on the 6th Embodiment of this invention. It is sectional drawing which shows an example of the conventional semiconductor light-emitting device which functions as a surface emitting laser. (A)-(e) is sectional drawing which shows the manufacturing method of the conventional surface emitting laser.

101, 201, 301, 601 Sapphire substrate
102, 302, 602 Nitride semiconductor mirror
103, 303 n-type cladding layer
104, 304, 404, 504, 604, 704 Active layer
105, 405 p-type cladding layer
106, 306, 406, 513, 606, 712 Thermal oxide film
107, 307, 607 Transparent electrode
108, 308, 408, 508, 608, 708 p-type electrode
109,309,409,509,609,709 p-type electrode pad
110, 310, 410, 510, 710 n-type electrode
111, 311, 411, 511, 611, 711 N-type electrode pad
112, 312, 612 Reflective film
113 Antioxidation mask
202 1st semiconductor layer
203 Second semiconductor layer
204 Third semiconductor layer
205, 206, 207 Oxide films 305, 505, 605, 705 First p-type cladding layer
313, 506, 613, 706 Oxidation promoting layer
314, 507, 614, 707 Second p-type cladding layer
401, 501, 701 GaN substrate
402, 502, 702 First n-type cladding layer
403, 503, 703 Second n-type cladding layer

Claims (18)

A substrate,
A nitride semiconductor layer containing In having different compositions from each other, formed on at least two layers on the substrate;
Of the at least two nitride semiconductor layers, a nitride semiconductor layer having a large In composition is an oxidation promoting layer having a higher oxidation rate than a nitride semiconductor layer having a small In composition.
The nitride semiconductor device is characterized in that a part of each of the two nitride semiconductor layers is oxidized . The at least two nitride semiconductor layers have different band gaps from each other ,
Among the nitride semiconductor layer of said at least two layers, the thickness of the portion of the insulation Enmaku formed by oxidizing a small band gap nitride semiconductor layer by oxidizing a band gap larger nitride semiconductor layer The nitride semiconductor device according to claim 1, wherein the nitride semiconductor device is thicker than a thickness of the formed insulating film portion. Of the at least two nitride semiconductor layers, the thickness of the insulating film formed by oxidizing the nitride semiconductor layer having a high In composition is obtained by oxidizing the nitride semiconductor layer having a low In composition. The nitride semiconductor device according to claim 2, wherein the nitride semiconductor device is thicker than a thickness of a portion of the formed insulating film . The at least two nitride semiconductor layers are stacked in a direction perpendicular to the main surface,
The nitride semiconductor device according to claim 3, wherein the insulating film is formed on a side surface of the at least two nitride semiconductor layers. The nitride semiconductor device further includes a first cladding layer of a first conductivity type provided on or above the substrate,
The at least two nitride semiconductor layers are provided on the first cladding layer, and an active layer that generates light, and a second conductive material provided on the active layer and having a lower In composition than the active layer. A second cladding layer of the mold,
The nitride semiconductor device according to claim 4, wherein a planar area of the active layer is smaller than a planar area of the second cladding layer.   6. The nitride semiconductor device according to claim 5, wherein the nitride semiconductor device functions as a surface emitting laser that emits light generated in the active layer upward.   6. The nitride semiconductor device according to claim 5, wherein the nitride semiconductor device functions as a ridge-type semiconductor laser that emits light generated in the active layer from an end face of the active layer. Wherein the at least two layers nitride semiconductor layer is provided on the front Stories second cladding layer includes In, and a second oxidation promoting layer thicker than said active layer, said second oxide promoting layer And a third clad layer of the second conductivity type provided thereon,
The plane area of the second oxidation promoting layer, a nitride semiconductor device according to claim 5, characterized in that less than a plane area of the second cladding layer and the third cladding layer. The nitride semiconductor device further includes a fourth clad layer of a first conductivity type provided on or above the substrate, and an active layer provided on the fourth clad layer and generating light. ,
The at least two nitride semiconductor layers include a fifth conductivity type fifth cladding layer provided on the active layer, and a third oxidation promotion layer having an In composition higher than that of the fifth cladding layer. A second conductivity type sixth cladding layer provided on the oxidation promotion layer and having a lower In composition than the third oxidation promotion layer;
5. The nitride semiconductor device according to claim 4, wherein a planar area of the third oxidation promotion layer is smaller than planar areas of the fifth cladding layer and the sixth cladding layer. 6. The thickness of the third oxidation promotion layer is thicker than the fifth cladding layer and the sixth cladding layer,
Of the insulating film, the thickness of the portion provided on the side surface of the oxidation promoting layer is thicker than the thickness of the portion provided on the side surface of the fifth cladding layer and the sixth cladding layer. The nitride semiconductor device according to claim 9.   The nitride semiconductor device according to claim 9, wherein the nitride semiconductor device functions as a surface emitting laser that emits light generated in the active layer upward. The third oxidation promotion layer serves as a waveguide for light generated in the active layer,
The nitride semiconductor device according to claim 9, wherein the nitride semiconductor device functions as a ridge type semiconductor laser that emits the light from an end face of the active layer. A step (a) of forming a plurality of nitride semiconductor layers having two or more types of In compositions on or above the substrate;
An insulating film having a thickness corresponding to the composition of the plurality of nitride semiconductor layers is formed by oxidizing the side wall portions of the plurality of nitride semiconductor layers, and a nitride semiconductor layer having a large In composition and a nitride semiconductor layer having a small In composition And a step (b) of forming the nitride semiconductor device so as to increase the oxidation rate as compared with the method of manufacturing a nitride semiconductor device.   After the step (a) and before the step (b), the method further includes a step (c) of removing a part of the plurality of nitride semiconductor layers to form a mesa shape. The method for manufacturing a nitride semiconductor device according to claim 13. The plurality of nitride semiconductor layers formed in the step (a) includes an active layer that contains In, generates light, and is provided on the active layer, and has a lower In composition than the active layer. 14. The method for manufacturing a nitride semiconductor device according to claim 13 , further comprising a cladding layer. The plurality of nitride semiconductor layers formed in the step (a) are provided on the second cladding layer and the second cladding layer, include In, and are thicker than the second cladding layer. The method for manufacturing a nitride semiconductor device according to claim 13 , further comprising: an accelerating layer and a third cladding layer provided on the oxidation accelerating layer and thinner than the oxidation accelerating layer.   14. The method of manufacturing a nitride semiconductor device according to claim 13, wherein in the step (b), the insulating film is formed by performing a heat treatment in a gas atmosphere containing an oxygen compound. The method for manufacturing a nitride semiconductor device according to claim 17 , wherein the gas containing the oxygen compound is water vapor.