JP4953559B2 - Nitride semiconductor laser device - Google Patents

Description

  The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device formed with a cleavage plane as a resonator surface.

  Conventionally, a wide range of light-emitting elements such as green and blue have been studied using nitride semiconductors, particularly GaN-based semiconductors, as semiconductor light-emitting elements. These nitride semiconductors are usually grown on a sapphire substrate as a growth substrate, but sapphire has a lattice mismatch of 13% or more with the nitride semiconductor. For this reason, the nitride semiconductor grown on the sapphire substrate has very many lattice defects. A substrate with many crystal defects is unsuitable for a laser element and is difficult to put into practical use. In addition, sapphire does not have a cleavage property due to the nature of the hexagonal crystal.

Therefore, an attempt has been made to fabricate a nitride semiconductor substrate lattice-matched with a nitride semiconductor (for example, Patent Document 1).
This nitride semiconductor substrate grows a GaN crystal without embedding the facet structure while maintaining a three-dimensional facet structure when the growth surface of the vapor phase growth is not flat when growing the GaN single crystal. As a result, dislocations are reduced.
JP 2001-102307 A

However, in the substrate obtained by such a method, defects are concentrated on the boundary portion of the facet plane, and in the in-plane direction of the substrate, for example, stripe-like regions and crystal defects are concentrated. The area coexists.
Therefore, when a nitride semiconductor layer is laminated on this substrate, an optical waveguide is formed along the stripe direction of the crystal defect, and the substrate is naturally cleaved perpendicular to the stripe to produce a resonator surface. In addition, since the crystal is discontinuous at the interface between the region with many crystal defects and the region with few crystal defects, there is a problem that it cannot be cleaved linearly.

  In addition, in order to avoid the above-mentioned problems due to natural cleavage, when scribing is performed on the entire surface of the substrate and the stacked semiconductor layers are cleaved along this scribe, it is caused by the scribing to the cleavage plane that becomes the resonator surface. There was a problem that fine vertical streaks entered, and the reliability of the laser was impaired.

  The present invention has been made in view of the above problems, and when a nitride semiconductor substrate having a low-density defect region with few crystal defects and a high-density defect region with more crystal defects than the low-density defect region is used. However, an object of the present invention is to provide a highly reliable nitride semiconductor laser device capable of obtaining a resonator surface with excellent flatness and maintaining stable operation.

A first nitride semiconductor laser element of the present invention includes a nitride semiconductor substrate, and a nitride semiconductor layer composed of an n-type semiconductor layer, an active layer, and a p-type semiconductor layer stacked on the nitride semiconductor substrate. A nitride semiconductor laser element having a stripe-shaped optical waveguide region in the nitride semiconductor layer,
The nitride semiconductor substrate includes a lower substrate having two or more different plane orientation on the surface, and the upper layer substrate, is being laminated,
Between the upper substrate and the lower substrate, a cavity is provided,
The upper substrate has a high-density defect region and a low-density defect region above the cavity,
The optical waveguide region is located above the low density defect region .

In this nitride semiconductor laser device, the lower layer substrate has two or more different plane orientations periodically, and at least one plane orientation is a high dislocation density region.

The second nitride semiconductor laser element of the present invention includes a nitride semiconductor substrate, and a nitride semiconductor layer comprising an n-type semiconductor layer, an active layer, and a p-type semiconductor layer stacked on the nitride semiconductor substrate. A nitride semiconductor laser element having a stripe-shaped optical waveguide region in the nitride semiconductor layer,
The nitride semiconductor substrate includes a lower substrate, and the upper layer substrate, is being laminated with a high density defective regions and low density defective regions,
Between the upper substrate and the lower substrate, a cavity is provided,
The upper substrate has a high-density defect region and a low-density defect region above the cavity,
The optical waveguide region is located above the low density defect region .

Further, in the above-described nitride semiconductor laser element, the layer growth suppression region is composed of a cavity formed on the lower layer substrate, the layer growth suppression region is composed of a recess provided on the lower layer substrate, or a layer The growth inhibition region is characterized by comprising a stripe-like growth inhibition film.
In particular, the second nitride semiconductor laser element is characterized in that the base substrate is periodically provided with a high dislocation density region and a low dislocation density region.
In addition, the lower layer substrate has a luminescence region.
Furthermore, the nitride semiconductor substrate includes two or more types of n-type impurities.
In addition, the nitride semiconductor substrate and the n-type semiconductor layer contain different n-type impurities.
A third nitride semiconductor laser device of the present invention includes a nitride semiconductor substrate, and a nitride semiconductor layer including an n-type semiconductor layer, an active layer, and a p-type semiconductor layer stacked on the nitride semiconductor substrate. A nitride semiconductor laser element having a stripe-shaped optical waveguide region in the nitride semiconductor layer,
The nitride semiconductor substrate is a substrate having a high dislocation density region and a low dislocation density region having fewer dislocations than the high dislocation density region.
A base layer is laminated on the substrate,
The substrate has a layer growth inhibition region formed of a recess at an interface with the base layer.

  According to the nitride semiconductor laser device of the present invention, since the upper layer substrate having a plane orientation different from the plane orientation of the lower layer substrate is laminated on the lower layer substrate having two or more different plane orientations on the surface, Alternatively, a lower layer substrate having a high dislocation density region with more crystal defects than other regions and a low dislocation density region with less crystal defects than the high dislocation density region, and a high density defect region and a different period from this lower substrate Since the upper layer substrate having the low density defect region is laminated, it is possible to obtain a resonator surface with a very flat cleaved surface at the end surface near the waveguide region, improving the stability and reliability of the laser device. Can be made. That is, when the cavity surface of a semiconductor laser element is produced by natural cleavage of a nitride semiconductor, it is usually determined by the arrangement of different plane orientations on the surface of the nitride semiconductor substrate, or the arrangement of different dislocation density regions or defect density regions. At the interface, non-linear cracks, undulations, and the like that appear to be caused by the reversal of the crystallinity occur. However, the upper substrate that cancels the crystallinity inversion due to the difference in the plane orientation, that is, the nitride semiconductor layer is laminated on the lower substrate, so that such non-linear cracks and undulations are caused by the upper substrate. As a result of the absorption, the end surface of the optical waveguide region can be a flat cleaved surface without vertical stripes above the lower substrate.

In particular, when two or more different plane orientations are periodically arranged in the lower layer substrate, the distance between the low density defect region and the high density defect region on the upper layer substrate surface can be controlled. Can be arranged in place, and a long-life nitride semiconductor laser device can be obtained.
Further, the high density defect region and the low density defect region on the surface of the upper layer substrate are arranged, and the period of the high density defect region and the low density defect region and the period of two or more different plane orientations in the lower layer substrate are When arranged differently, the high-density defect region and the low-density defect region can be arranged at an arbitrary period on the upper layer substrate surface without being governed by the plane orientation of the lower layer substrate surface, The lifetime and characteristics of the nitride semiconductor laser device due to these crystal defects can be prevented from deteriorating.

  When the high dislocation density region and the low dislocation density region are periodically arranged, regions having different plane orientations are provided corresponding to the cycle. That is, among different plane orientations, for example, a region formed as a (0001) plane is a low dislocation density region, and a region formed as a (000-1 plane) is a high dislocation density region. it can. As described above, when regions having different dislocation densities are periodically arranged, and the crystal plane orientation can be ensured by having a certain area (region), between the plane orientation and the dislocation density, In particular, a high dislocation density region and other dislocation density regions (medium dislocation density region, low dislocation density region) can be disposed with correlation. In addition, when the dislocation density is periodically or randomly arranged in an area that is less than or equal to the crystal plane orientation, only the (0001) plane appears as the plane orientation. In this case, although it is likely to appear as a more stable plane orientation, it can be arbitrarily selected depending on the crystal growth method, growth conditions or composition. When a gallium nitride compound semiconductor layer is grown as the upper substrate, the (0001) plane can be grown stably.

  Furthermore, when the layer growth suppression region disposed in the nitride semiconductor substrate is formed of a cavity, uneven, or striped growth suppression film, the resonator surface is naturally cleaved perpendicular to the optical waveguide. When manufacturing, since the non-linear cracks and undulations from the substrate side and the vertical stripes from the substrate side are surely absorbed by these cavities and the like, the above-described effects can be exhibited more remarkably. Furthermore, the crystal growth inversion on the lower layer substrate surface can be eliminated by the uneven or striped growth inhibiting film, and the above-described effects can be more reliably exhibited.

In addition, when the lower layer substrate has a luminescence region, a region having a very low defect density in the lower layer substrate can be secured, and a nitride semiconductor layer, an n-type semiconductor layer, an active layer, It is possible to suppress the defect density in the p-type semiconductor layer and the like, and to obtain a nitride semiconductor laser element having a longer lifetime.
Furthermore, when the nitride semiconductor substrate contains two or more types of n-type impurities, when the nitride semiconductor substrate and the n-type semiconductor layer contain different n-type impurities, the impurity concentration of each layer can be easily controlled. A nitride semiconductor laser device having a long life, high quality, and high reliability can be obtained.

The nitride semiconductor laser device of the present invention includes a nitride semiconductor substrate and a nitride semiconductor layer made of an n-type semiconductor layer, an active layer, and a p-type semiconductor stacked on the nitride semiconductor substrate.
In the present invention, the nitride semiconductor substrate means a substrate for forming a nitride semiconductor laminated structure that functions as a nitride semiconductor laser element. For example, a single or multiple layer buffer is arbitrarily formed on a lower substrate. And a substrate on which an upper substrate composed of a layer, optionally a growth inhibiting film, and optionally a cavity, a buffer layer or an underlayer is formed. The film thickness of this nitride semiconductor substrate, that is, the total film thickness of the lower substrate, optionally the buffer layer, optionally the growth restraining film, and optionally the upper layer substrate such as the cavity is laminated can be freely set. For example, about 50-1000 micrometers is mentioned.

Here, examples of the nitride semiconductor substrate include a substrate made of a nitride semiconductor such as GaN, AlGaN, AlN, GaInN, AlGaInN, and AlBGaInN. In other words, both the lower layer substrate and the upper layer substrate are preferably made of the nitride semiconductor described above, but both the lower layer substrate and the upper layer substrate are not necessarily the same nitride semiconductor. Especially, it is preferable that both consist of GaN. The nitride semiconductor substrate may be doped with n-type or p-type impurities. Examples of the impurities include Cl, O, S, Se, Te, C, Si, Ge, Zn, Cd, Mg, and Be. Especially, it is preferable that 2 or more types of impurities are contained. In this case, impurities different in the thickness direction may be contained, or may be contained so as to be mixed. Specifically, those containing different impurities between the lower layer substrate and the upper layer substrate can be mentioned. Also, the concentration is not particularly limited, for example, 5 × 10 ¹⁵ / cm ³ approximately ~5 × 10 ²¹ / cm ³ are suitable.

In order to obtain the nitride semiconductor substrate of the present invention, as the lower layer substrate, a commercially available nitride semiconductor substrate may be used, or a heterogeneous substrate (for example, sapphire, SiC, Si, spinel (MgAl ₂ O _{4), ZnO, MgO, SiO} 2, GaAs, GaP, GaN, AlN, NdGaO 3 , etc.), those obtained by laminating nitride semiconductor GaN layer or the like is formed on (e.g., in JP-a-11-238945 Patent Publication The nitride semiconductor substrate described above may be used, or a nitride semiconductor such as a GaN layer is stacked on a heterogeneous substrate such as sapphire and then the sapphire is removed (for example, Japanese Patent Laid-Open No. 2001-102307) JP, 11-4048, Jpn. J. Appl. Phys. Vol.39 (2000) p647-650 etc.) may be used.

The lower substrate is suitable to have a crystal defect of, for example, as small as about 1 × 10 ⁷ cm ⁻² or less, preferably about 1 × 10 ⁶ cm ⁻² , or such a small region. For example, in the method of growing a GaN crystal without embedding the facet structure while maintaining the above-described conventional three-dimensional facet structure, a low dislocation density region with few crystal defects on the surface and a low dislocation density region. And a high dislocation density region with many crystal defects, and a substrate having a medium dislocation density region between the low dislocation density region and the high dislocation density region and having an amount of crystal defects between these regions. can get. In these regions, the boundary between the high dislocation density region and the other regions (for example, the middle dislocation density region and the low dislocation density region) is formed to be relatively easy to understand visually. In other words, a substrate having at least two different plane orientations corresponding to a low dislocation density region and a high dislocation density region can be obtained.

  Here, having two different plane orientations means having different plane orientations. Preferably, the crystal is inverted, that is, the C plane (0001) and the -C plane (000-1). And so on. In addition to the two types, the region corresponding to the intermediate dislocation density region is not necessarily a region having the same plane orientation, a region having a different plane orientation, or a specific plane orientation. There may be a region where the plane orientation is dominant (for example, a region having an off angle of about 0.01 to 2 ° from a specific plane orientation). Such a surface having a low off-angle can be said to be substantially a C-plane, for example, a surface having an off-angle of about 0.01 to 2 ° from the C-plane. These regions or regions having a specific plane orientation may be randomly arranged or may be partially arranged with a period, but are preferably arranged periodically over substantially the whole. Although not limited in the period, for example, the high dislocation density region or the -C plane is arranged at a pitch of about 10 to 50 μm, particularly about 30 to 40 μm, and a pitch of about 300 to 600 μm, for example about 400 μm. Is appropriate. Further, it is appropriate that the low dislocation density region or the C plane (including the substantial C plane) is arranged with a width of about 1 to 600 μm and a pitch of about 300 to 600 μm, for example, about 400 μm. Even if the low, medium, and high dislocation density regions are periodically arranged, these regions do not necessarily have a uniform width, and are partially thick and wide. It may be narrowed or divided.

  The difference in the plane orientation of the surface of the lower layer substrate also varies in the distribution of impurities depending on the type of impurities when doping impurities. For example, impurities are less likely to enter in a part of the C plane, and impurities are more likely to enter the other C plane and -C plane. For this reason, the distribution of impurity concentration differs between the two, and as a result, the resistance value also differs. Furthermore, when a nitride semiconductor laser device is fabricated using the nitride substrate of the present invention, a region (luminescence region) corresponding to a low dislocation density region of the lower substrate emits light due to light emission in the active layer. . Therefore, for example, in the case of the C plane and the −C plane, light emission is observed in the luminescence region of the C plane.

  The nitride semiconductor substrate is configured by laminating an upper layer substrate such as a buffer layer and an under layer on the lower layer substrate, and the surface of the upper layer substrate has a plane orientation having a period different from the plane orientation of the lower layer substrate, or The surface of the upper layer substrate has a low density defect region with few crystal defects and a high density defect region with more crystal defects than the low density defect region. For example, when an upper substrate is grown on a lower substrate, a growth inhibiting film is formed on the lower substrate directly or via a buffer layer or the like, and an upper substrate such as a buffer layer or an underlying layer is formed thereon. The Thereby, for example, the crystal inversion in the lower layer substrate is resolved without being controlled by two or more different plane orientations on the lower layer substrate surface. A region containing only defects will be formed. In addition, it is preferable that the different plane orientations of the upper layer substrate surface are periodic. However, the regions having these plane orientations do not necessarily have a certain uniform width, and may be partially thick and narrow, or may be divided.

As the growth inhibiting film used here, a known mask material used in so-called ELOG growth, for example, an amorphous or polycrystalline material in which a nitride semiconductor layer is difficult to grow on it, _{Specifically, SiO 2, SiN, SiON,} TiO 2, ZrO 2 and the like. Of these, SiO ₂ is preferable. The growth inhibiting film may have any of various pattern shapes, that is, a dot shape (circular, square, polygonal, etc.), a stripe shape, a lattice shape, etc., but a stripe shape is preferable. The pattern shape is preferably a regular, for example, periodic pattern. The growth inhibiting film can be formed by a known method such as vacuum deposition, sputtering, or CVD, and can be patterned by a known method such as photolithography and an etching process. When the buffer layer is formed on the lower substrate, the buffer layer is the same as the nitride semiconductor layer described later, and preferably has the same composition as the nitride semiconductor layer formed thereon. It is done. As for the film thickness of a buffer layer, about 0.01-5 micrometers is mentioned, for example.

  Further, instead of using the growth inhibiting film, irregularities may be formed on the surface of the lower substrate, and an upper substrate such as a buffer layer or an underlayer may be formed thereon. Formation of irregularities on the surface of the lower layer substrate can be realized by a known method, photolithography, and an etching process.

A cavity may be formed inside the nitride semiconductor substrate, that is, at the interface between the lower layer substrate and the upper layer substrate. Usually, by controlling the material, width (size) and thickness of the above-described growth inhibiting film, the period and depth of the unevenness, etc., the lateral growth of the semiconductor constituting the upper substrate to be grown thereon is controlled. As a result, a cavity can be formed on the growth inhibiting film, on the concave portion or on the convex portion. The width of the growth inhibiting film, the width of the concave portion or the convex portion is, for example, about 2 to 400 μm, preferably at least twice the width of the stripe-shaped optical waveguide region described later, and smaller than the high dislocation density region of the lower layer substrate. The thickness of the growth inhibiting film or the height of the irregularities is, for example, about 0.1 to 2 μm. The size of the cavity is not particularly limited and is preferably at least twice the width of the stripe-shaped optical waveguide region described later. Specifically, the width is 3 μm or more, preferably 10 μm or more. Moreover, the height is about 3 micrometers or less, for example, Preferably about 0.1-2 micrometers is mentioned. In other words, the growth inhibiting film (growth inhibiting region) is preferably formed corresponding to the position of the optical waveguide region of the nitride semiconductor laser element formed thereon, and the surface of the nitride semiconductor substrate on the surface of the lower substrate Regardless of orientation, dislocation density, etc., it can be formed in any width, size, shape, etc. That is, the growth inhibiting film (growth inhibiting region) is formed so as to almost completely cover the low dislocation density region of the lower substrate, almost completely cover the high dislocation density region, or almost completely cover the medium dislocation density region. For example, it is appropriate to form with a width, shape, etc. smaller than those regions.

  It is preferable that nothing is clogged in the cavities, but when a buffer layer, a base layer, or the like is formed, some material constituting these layers may be buried. This is because the cavity does not become a strong layer with good crystallinity but becomes a sparse polycrystalline or amorphous layer, and the cleavage from the substrate side can be interrupted. When the nitride semiconductor layer that is laminated and constitutes the laser element is cleaved from the nitride semiconductor substrate side, the vertical stripe from the scribe line formed on the back surface of the lower layer substrate reaches the optical waveguide region. Can be suppressed. That is, the vertical streak extends because the layer growth inhibition region (cavity, recess, growth inhibition film, etc.) provided between the upper layer substrate and the lower layer substrate inhibits the linear continuity of the crystal in the vertical direction. This can also be suppressed. Furthermore, the difference in height of the dislocation density is made smaller than that of the lower layer substrate, and the inhomogeneity of the crystal plane orientation and dislocation density distribution in the plane is alleviated, so that the cleavage property can be improved.

The upper layer substrate such as the buffer layer and / or the underlayer formed on the lower layer substrate on which the growth suppressing film or the unevenness is formed does not need to be a layer in which their functions, functions, etc. are clearly distinguished. This is a layer (substrate) that is grown to obtain a nitride semiconductor substrate surface that does not inherit the surface dislocation density state, plane orientation, etc., and has only different levels of crystal defects. In addition, the film thickness of the upper layer board | substrate formed on a lower layer board | substrate is about 3 micrometers or more, for example, Preferably about 5-20 micrometers is mentioned.
In the nitride semiconductor substrate of the present invention, when the upper layer substrate is formed on the growth inhibiting film or the lower substrate including the unevenness, a cavity is arbitrarily formed on the growth inhibiting film or on the recess, and the lateral growth is performed. As a result, a crystal defect is distributed at a high density on the surface of the finally obtained nitride semiconductor substrate. That is, the high-density defect region is arranged near the center of the growth inhibiting film or the recess. Then, in regions other than this high-density defect region, for example, in the vicinity of the end portion of the growth suppression film or recess, in the region where the growth suppression film is not formed, or in the region where the projection is formed, the crystal defect density is higher than this region. A region having a low is arranged.

As described above, a nitride semiconductor substrate in which an upper layer substrate such as a buffer layer, an arbitrary growth inhibiting film, and an arbitrary cavity is formed on a lower layer substrate has a defect density of 1 × as a low density defect region. A region having a defect density of about 10 ⁴ to 1 × 10 ⁷ cm ⁻² or lower is a higher density defect region. For example, the defect density is about 1 × 10 ⁷ to 1 × 10 ⁹ cm ⁻² or more. In other words, a region having a low defect density is formed at least on the surface thereof.
A nitride semiconductor laser is configured by stacking nitride semiconductor layers having various compositions and film thicknesses on such a nitride semiconductor substrate.
Specifically, an n-type or p-type semiconductor layer, an active layer, a p-type or n-type semiconductor layer is stacked on the above-described nitride semiconductor substrate, and is connected to the n-type and p-type semiconductor layers. An electrode is formed to constitute a nitride semiconductor laser.
For example, an n-type semiconductor layer is formed on the nitride semiconductor substrate. As an n-type semiconductor layer, an n-type contact layer is first grown arbitrarily. The n-type contact layer preferably has a composition larger than the band gap energy of the active layer, and Al _j Ga _1-j N (0 ≦ j <0.3) is an example. The thickness of the n-type contact layer is not particularly limited, but is preferably 1 μm or more.

Next, an n-type cladding layer is grown. The n-type cladding layer preferably contains Al. Further, it may be a single layer or a multilayer layer (superlattice structure). The n-type impurity concentration is not particularly limited, but is preferably 1 × 10 ¹⁷ to 1 × 10 ²⁰ / cm ³ , and the n-type impurity concentration may be inclined in the film thickness direction. Further, it may function as a cladding layer for confining carriers by providing an Al composition gradient.

Next, an n-type light guide layer made of undoped GaN is grown using TMG and ammonia as source gases at the same temperature. The film thickness of the n-type light guide layer is not particularly limited, and an arbitrary film thickness can be selected. For example, about 0.15 μm can be mentioned. This layer is provided as a layer that confines carriers in the active layer and does not confine light, and can be formed of a single layer or a multilayer layer (superlattice structure). Further, an n-type impurity may be doped. However, the n-type light guide layer can be omitted depending on the composition and film thickness of the active layer.
An active layer is formed on the n-type semiconductor layer. The active layer may be either single (SQW) or multiple quantum well structure (MQW). For example, _a well layer composed of Al _a In _b Ga _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, a + b ≦ 1), and Al _c In _d Ga _1-cd N (0 ≦ and a quantum well structure including a barrier layer of c ≦ 1, 0 ≦ d ≦ 1, c + d ≦ 1). The semiconductor layer used for the active layer may be non-doped, n-type impurity doped, or p-type impurity doped, but is preferably non-doped or n-type impurity doped. As a result, the output of the light emitting element can be increased. Furthermore, the well layer may be undoped and the barrier layer may be n-type impurity doped. Thereby, the output and luminous efficiency of the light emitting element can be increased. In addition, by including Al in the well layer, it is possible to obtain a wavelength range that is difficult for a conventional InGaN well layer, specifically, a wavelength near 365 nm, which is the band gap energy of GaN, or a wavelength shorter than that. .

The film thickness of the well layer is preferably about 1 to 30 nm, for example. In particular, when the thickness is 2 nm or more, a layer having a relatively uniform film quality is obtained with no great unevenness in film thickness, and when the thickness is 20 nm or less, the generation of crystal defects is suppressed and crystal growth is possible. Furthermore, the output can be improved by setting the film thickness to 3.5 nm or more. In the single quantum well structure, the effect of improving the output can be obtained by setting the film thickness to 5 nm or more in the same manner as described above. The number of well layers is not particularly limited, but when the number is 4 or more, it is preferable to keep the thickness of the active layer low by setting the thickness of the well layer to 10 nm or less. This is because if the thickness of each layer constituting the active layer is increased, the thickness of the entire active layer is increased, causing an increase in _Vf . In the case of a multiple quantum well structure, it is preferable to have at least one well layer having a film thickness in the range of 10 nm or less among the plurality of wells, and more preferably to set all the well layers to 10 nm or less. .
For example, when the barrier layer is doped with an n-type impurity, the concentration is preferably at least 5 × 10 ¹⁶ / cm ³ or more. For high-power devices, 5 × 10 ¹⁷ / cm ³ or more and 1 × 10 ²⁰ / cm ³ or less are appropriate. When the n-type impurity is doped in the barrier layer, all the barrier layers in the active layer may be doped, or a part may be doped and a part may be non-doped.

A p-type compound semiconductor layer is formed on the active layer.
For example, the temperature is set to 1050 ° C., TMG and ammonia are used as source gases, and a p-type light guide layer made of undoped GaN is grown. The film thickness of the p-type light guide layer is not particularly limited, and an arbitrary film thickness can be selected. For example, about 0.15 μm can be mentioned. The p-type light guide layer is preferably grown undoped, but may be doped with Mg. Further, similarly to the n-type light guide layer, it can be formed of a single layer or a multilayer film layer (superlattice structure). Furthermore, the p-type light guide layer can be omitted depending on the composition and thickness of the active layer.

Next, the p-type cladding layer is not particularly limited as long as it has a composition larger than the band gap energy of the active layer and can confine carriers in the active layer. As an example, Al _k Ga _1-k N (0 ≦ k <1). The thickness of the p-type cladding layer is not particularly limited, but is preferably 0.01 to 0.3 μm. The p-type impurity concentration of the p-type cladding layer is suitably 1 × 10 ¹⁸ to 1 × 10 ²¹ / cm ³ . When the p-type impurity concentration is in the above range, the bulk resistance can be reduced without reducing the crystallinity. The p-type cladding layer may be a single layer or a multilayer film layer (superlattice structure). In the case of a multilayer film layer, it may be a multilayer film layer composed of the above Al _k Ga _1-k N and a nitride semiconductor layer having a smaller band gap energy. For example, as a layer having a small band gap energy, as in the case of the n-type cladding layer, In ₁ Ga _1- N (0 ≦ l <1), Al _m Ga _1-m N (0 ≦ m <1, m> l). In the case of a superlattice structure, the thickness of each layer forming the multilayer film layer is preferably 100 mm or less. In addition, when the p-type cladding layer is a multilayer film composed of a layer having a large band gap energy and a layer having a small band gap energy, at least one of the layer having a large band gap energy and the layer having a small band gap energy is doped with a p-type impurity. You may let them. When doping both the layer with large band gap energy and the layer with small band gap energy, the doping amount may be the same or different.

Next, a p-type contact layer is formed on the p-type cladding layer. p-type contact layer, for example, as an _{example, Al f Ga 1-f N} (0 ≦ f <1) is used. In particular, by using Al _f Ga _1-f N (0 ≦ f <0.3), good ohmic contact with the p-electrode which is an ohmic electrode is possible. The p-type impurity concentration is preferably 1 × 10 ¹⁷ / cm ³ or more. Further, the p-type contact layer preferably has a concentration gradient that increases the p-type impurity concentration and / or a composition gradient that decreases the mixed crystal ratio of Al. In this case, the concentration gradient and the composition gradient may be changed continuously or discontinuously and stepwise. For example, a p-type contact layer is in contact with an ohmic electrode, a first p-type contact layer having a high p-type impurity concentration and a low Al composition ratio, and a second p-type contact having a low p-type impurity concentration and a high Al composition ratio. It can also consist of layers. Good ohmic contact can be obtained by the first p-type contact layer, and self-absorption can be prevented by the second p-type contact layer.

  Note that after the compound semiconductor layer is grown on the substrate, heat treatment is preferably performed at 400 ° C. or higher in an atmosphere containing oxygen and / or nitrogen. As a result, hydrogen bonded to the p-type layer is removed, and a p-type nitride semiconductor layer exhibiting p-type conductivity can be formed.

  In the p-type semiconductor layer, for example, the upper region from the middle of the cladding layer, the interface between the cladding layer and the light guide layer, the middle of the light guide layer, or the like is preferably configured as a striped ridge. That is, the ridge-shaped stripe is formed by etching from the p-type contact layer to the lower side (substrate side) than the p-type contact layer. For example, a stripe formed by etching from the p-type contact layer to the middle of the p-type cladding layer or a stripe formed by etching from the p-type contact layer to the n-type contact layer can be used. With such a configuration, a stripe-shaped optical waveguide region can be formed in the region including the active layer existing therebelow. Here, the optical waveguide region refers to a region including an active layer and a so-called light guide layer disposed arbitrarily above and below the active layer. The width, length, height, and the like of the optical waveguide region can be appropriately set according to the composition and thickness of the semiconductor layer, the characteristics of the nitride semiconductor laser to be obtained, and the like. For example, the width of the ridge-shaped stripe formed by etching is 0.5 to 4 μm, preferably 1 to 3 μm. When the width of the stripe is within this range, it is preferable that the horizontal and transverse mode easily becomes a single mode. Further, if the etching is performed closer to the substrate side than the interface between the p-type cladding layer and the laser waveguide region, it is preferable to bring the aspect ratio closer to 1.

  The stripe-shaped ridge needs to be formed above the low-density defect region on the surface of the nitride semiconductor substrate regardless of the position of the specific plane orientation on the surface of the lower layer substrate of the nitride semiconductor substrate. However, the ridge is not necessarily formed in a region having the lowest crystal defect density, and is preferably formed in a region avoiding a region having the highest crystal defect density, that is, a low-density defect region. This is because, in a region where the crystal defect density is very low, there is no gap in which impurities are mixed in the crystal lattice of the elements constituting the nitride semiconductor, the doping is difficult, and the resistance tends to increase.

  When forming a ridge-shaped stripe, if the substrate for forming the element structure is an ELOG substrate, and if the ELOG growth is performed using a protective film, the ELOG growth is performed with unevenness above the protective film. In this case, it is preferable to form a ridge-shaped stripe in the upper part of the recess in terms of improving the reliability of the device. Further, it is preferable in terms of reliability to avoid the upper portions of the central portion of the protective film and the central portion of the concave portion.

  At least one element selected from the group consisting of, for example, Ti, V, Zr, Nb, Hf, and Ta, on the side surface of the ridge-shaped stripe formed by etching and the plane of the nitride semiconductor layer continuous to the side surface It is preferable that the insulating film having a value smaller than the refractive index of the laser waveguide region is formed as a single layer or a laminated film, at least one of oxides containing BN, SiC, and AlN. Specifically, an oxide containing at least one element selected from the group consisting of Si, V, Zr, Nb, Hf, and Ta having a refractive index of about 1.6 to 2.3. , BN, AlN and the like. Among these, one or more elements of Zr and Hf oxides and BN are more preferable.

Some of these materials have a property of being slightly dissolved in hydrofluoric acid, but if an insulating layer of a laser element is used, the reliability as a buried layer tends to be considerably higher than that of SiO ₂ . In addition, an oxide thin film formed in a gas phase such as PVD or CVD is unlikely to be an oxide in which the element and oxygen are equivalently reacted, and thus the reliability of the insulating property of the oxide thin film is unlikely to be insufficient. Although there is a tendency, the above-described elements of PVD, CVD oxide, BN, SiC, and AlN tend to be more reliable in terms of insulation than Si oxide. In particular, by providing this insulating film in a region where the electrode on the surface of the p-type layer is not formed, the occurrence of electrode migration can be effectively suppressed. In addition, it is very convenient as a buried layer of a laser element if a refractive index of an oxide smaller than that of a nitride semiconductor (for example, other than SiC) is selected.

Thereafter, a p-electrode capable of providing ohmic contact is formed on the surface of the p-type contact layer. The p-electrode is formed of a material, a film thickness, a shape, and the like that can make ohmic contact with the surface of the p-type compound semiconductor layer and efficiently inject current into the element, particularly into the optical waveguide region. As the electrode material, all of those usually used as electrodes can be used. For example, metals such as Co, Ni, Fe, Rh, Ru, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Re, Mn, Al, Zn, Pt, Au, Ru, Pd, Rh Alternatively, a single layer film or a stacked film of a conductive oxide film such as an alloy, ZnO, In ₂ O ₃ , SnO ₂ , ITO (a composite oxide of In and Sn), MgO, or the like can be given. Examples of the method for forming the p electrode include CVD, sputtering, and vapor deposition. The film thickness of the p-electrode is not particularly limited. For example, the sheet resistance can be lowered by setting the thickness to about 500 mm or more.

Usually, in the compound semiconductor element, apart from the ohmic p-electrode, for example, a bonding p-pad electrode connected by wire bonding is formed, and the p-pad electrode is electrically connected to the p-electrode which is an ohmic electrode. Connect to. The p-pad electrode may be provided on the p-type layer, or may be provided by metal wiring and on the outside of the p-type layer, for example, on the n-electrode formation surface via an insulating film. When the p-pad electrode is formed on the p-type layer, the p-pad electrode may be formed so that part of the p-electrode overlaps, or the p-pad electrode may be formed on the p-electrode. . Since the p-pad electrode is an electrode for mounting with a wire or the like, there is no particular limitation as long as it has a film thickness that does not damage the semiconductor element during mounting.
For example, it is preferable to select a p-pad electrode that has high adhesion to the p-electrode. For example, it can be selected from the materials for the p-electrode described above and formed as a single layer film or a laminated film.
The ohmic electrode and the pad electrode constituting the p electrode are preferably formed of, for example, a Ni—Au-based, Ni—Au—Pt-based, Pd—Pt-based, or Ni—Pt-based electrode material.

  Further, the n-type semiconductor layer is formed so that the n-electrode is electrically connected in direct contact or indirectly. For example, the n-electrode is formed on the surface of the n-type contact layer on the same side as the p-electrode or on the back side of the nitride semiconductor substrate (the side opposite to the p-electrode). Examples of the n electrode are the same as those used for the p electrode. Of these, electrode materials such as Ti—Al, Ti—Pt, V—Pt, Ti—Al—Ti—Pt, W—Al—W, and Ti—Mo—Ti—Pt are used. Is appropriate. The thickness of the n electrode is suitably 0.1 to 1.5 μm. As for the n-electrode, the bonding pad electrode and the ohmic electrode in ohmic contact with the n-type layer can be simultaneously formed in substantially the same shape. Further, the ohmic electrode and the n-pad electrode may be stacked to be stacked, or the ohmic n-electrode may be stacked by a shape different from that of the n-pad electrode in a different process.

  Heat treatment is preferably performed after the electrode is formed so as to be connected to the nitride semiconductor layer. The electrode can be alloyed by such heat treatment, and good ohmic contact with the semiconductor layer can be obtained. In addition, the contact resistance between the semiconductor layer and the electrode can be reduced. As heat processing temperature, the range of 200 to 1200 degreeC is preferable. In addition, heat treatment in an atmosphere containing oxygen and / or nitrogen as the atmospheric gas is appropriate.

In the present invention, a layer for improving the function of the semiconductor laser device (for example, a reflection film, an intermediate layer) in or between the nitride semiconductor substrate, the n-type semiconductor layer, the active layer, and the p-type semiconductor substrate. Layer, etc.) may be formed.
Further, an element used as a semiconductor laser element is usually used by being divided and / or cleaved into quadrangular or polygonal chips each having a side of about 150 to 1000 μm. In the present invention, as described above, a substrate having two or more different plane orientations on the surface as a lower substrate, in other words, a substrate having a low dislocation density region and a high dislocation density region, preferably a medium dislocation density region. And a nitride semiconductor substrate having a nitride semiconductor layer having a high-density defect region and a low-density defect region on the surface thereof, and a semiconductor layer constituting an element structure is stacked thereon. Divide / cleave for each chip. However, when a single chip is configured, all of these regions may not necessarily exist on the surface of the lower substrate and the nitride semiconductor layer due to the demand for reduction in size. That is, when the semiconductor layer constituting the element structure is sized so that its function can be sufficiently exerted, part or all of the high dislocation density region in the lower layer substrate of the nitride semiconductor substrate, the middle or low dislocation density region In some cases, a part of the nitride semiconductor laser element is removed and no longer exists in the nitride semiconductor laser element.

Hereinafter, embodiments of the nitride semiconductor laser device of the present invention will be described in detail.
Example 1
(Preparation of lower layer substrate)
A SiO ₂ film having a thickness of 0.1 μm was formed on the entire surface of the a-side (111) of a 2-inch (111) GaAs substrate, and stripe-like windows were formed by photolithography and etching processes. The windows were 4 μm pitch.
The GaAs substrate on which the SiO ₂ film is formed is kept at a low temperature of about 400 ° C., and the GaN film is formed with an NH ₃ partial pressure of 0.2 atm (20 kPa) and an HCl partial pressure of 2 × 10 ⁻³ atm (0.2 kPa) for about 30 minutes. Was deposited at about 800 liters.
Next, the temperature of the GaAs substrate was raised to about 1000 ° C., and the NH ₃ partial pressure was 0.4 atm (40 kPa) and the HCl partial pressure was 3 × 10 ⁻² atm (3 kPa). Thereby, a GaN epitaxial layer of about 500 μm was formed.
The obtained GaAs substrate was sliced by a slicer to produce a wafer made of GaN having a film thickness of about 400 μm, which was used as a lower layer substrate.

  When this lower layer substrate was observed with a microscope, it was confirmed that dislocation bundles 10 having a very high dislocation density appeared at a pitch of about 400 μm, as shown in FIG. Moreover, although the thickness was not uniform between the dislocation bundle 10 and the dislocation bundle 10, the area | region 30 with a very low dislocation density of the width | variety of about 80-100 micrometers was recognized. When these dislocation bundles 10 and the region 30 having a low dislocation density are analyzed by electron diffraction and / or X-ray diffraction, the plane orientation is reversed, that is, it has a -C plane and a C plane. It was confirmed. Further, it was confirmed that a middle dislocation density region 20 having an intermediate dislocation density was disposed between the dislocation bundle 10 and the region 30 having a low dislocation density.

(Production of nitride semiconductor substrate)
Thereafter, a SiO ₂ film having a film thickness of 0.5 μm was formed on the entire surface of the obtained GaN substrate as the lower layer substrate, and the SiO ₂ film was patterned into a stripe shape by photolithography and etching processes. At this time, the pattern width of the stripe shape was about 14 μm and the pitch was about 20 μm.
On the obtained lower layer substrate, 100 μm of a high-temperature growth buffer layer of Al _x Ga _1-x N (0 ≦ x ≦ 1) and 3 μm of GaN were grown as an epitaxial layer. At this time, a cavity was formed on the SiO ₂ film by appropriately setting the growth conditions of the buffer layer and the GaN epitaxial layer. The conditions for forming the cavities are well known in the art, and are omitted here. In this way, a GaN substrate having a buffer layer and a GaN epitaxial layer and having a cavity therein was used as a nitride semiconductor substrate.

In the nitride semiconductor substrate obtained by such a method, a region having a high crystal defect such as a crystal defect of about 3 × 10 ⁷ m ⁻² is formed on the surface above the vicinity of the center of the cavity, Above the periphery, there are regions where crystal defects are lower than this (for example, the crystal defect density is about 3 × 10 ⁵ cm ⁻² ). That is, the cavities are arranged from the high density defect region to a part of the low density defect region.

(Formation of nitride semiconductor laser device)
On the obtained nitride semiconductor substrate, 2 μm of Si-doped GaN was laminated to form an n-type semiconductor layer.
On top of this, an n-type cladding layer made of Si-doped AlGaN was formed to a thickness of 2 μm, and an n-type guide layer made of GaN was formed to a thickness of 1700 mm.
Subsequently, as an active layer of a multi-quantum well serving as a light emitting region, (well layer / barrier layer) = (InGaN, 60Ｓｉ / Si-doped GaN, 250Å), two well layers and three barrier layers alternately. Laminated.

  On the active layer, as a p-type semiconductor layer, 200 Ｍｇ Mg-doped AlGaN, 1500 Å undoped GaN guide layer, 4000 ク ラ ッ ド Mg-doped AlGaN cladding layer, and 200 Ｍｇ Mg-doped GaN contact layer were laminated. An undoped GaN guide layer formed as a p-type semiconductor layer exhibits p-type due to diffusion of Mg from an adjacent layer.

  Next, a striped ridge was formed on the surface of the p-type semiconductor layer. First, a resist is applied to the surface of the p-type semiconductor layer, and etching is performed in the middle of the p-type semiconductor layer, for example, to the middle of the undoped GaN layered at 1000 mm, by a photolithography and etching process, leaving a striped portion. As a result, a ridge was formed. Here, the ridge width is about 2 μm and the height is about 0.5 μm. In addition, the ridge is provided above the region where the cavity exists in the nitride semiconductor substrate, and is further provided so as to be located above the region shifted from the high-density defect region, that is, above the low-density defect region. For example, it is disposed above the low density defect region that is shifted from the high density defect region by about 1 to 3 μm.

Furthermore, a p-electrode made of Ni—Au was formed on the surface of the p-type semiconductor layer, and a p-pad electrode made of Ni—Ti—Au was further formed on the p-electrode. The surface was covered with a protective film, leaving the p-electrode.
Subsequently, scribing was performed from the back side of the nitride semiconductor substrate on which the n-type semiconductor layer, the active layer, and the p-type semiconductor layer were stacked, and the semiconductor layer was naturally cleaved together with the nitride semiconductor substrate. As a result, it is possible to manufacture a bar in which semiconductor laser elements having a cleavage plane as a resonator plane are connected side by side. Furthermore, a chip-shaped semiconductor laser element can be obtained by dividing this bar into individual elements.
The cavity surface of the semiconductor laser device thus obtained has a high density defect region and a low density defect region below the cavity in the nitride semiconductor substrate (on the side opposite to the side where the semiconductor layer is laminated). At the interface, non-linear cracks and waviness, which are considered to be caused by the reversal of the crystallinity, were observed. On the other hand, above the cavity (on the side where the semiconductor layer is laminated), such non-linear cracks and the like were absorbed by the cavity, and a very flat cleavage plane was obtained at the end face near the waveguide. .

  Evaluation of this nitride semiconductor laser device revealed that the cavity surface was flatter and a semiconductor laser device with good performance was obtained compared to a semiconductor laser device manufactured in the same manner except that no cavity was formed. It was confirmed that

Example 2
A growth inhibiting film made of SiO ₂ film is formed, and a high-temperature growth buffer layer and an epitaxial layer of Al _x Ga _1-x N (0 ≦ x ≦ 1) are grown on the lower substrate under conditions that do not cause cavities. A semiconductor laser device was obtained in the same manner as in Example 1 except that a nitride semiconductor substrate was produced.
Evaluation of the obtained nitride semiconductor laser device revealed that non-linear cracks and vertical streaks due to crystal inversion in the lower substrate of the nitride semiconductor substrate were eliminated on the device side of the nitride semiconductor substrate. As in Example 1, it was confirmed that a flat resonator surface was obtained.

  The present invention can be used not only for elements using nitride semiconductors, that is, not only for lasers but also for LEDs and other elements obtained by dividing a substrate or a semiconductor multilayer structure by cleaving. it can.

It is a figure which shows typically the lower layer substrate surface of the nitride semiconductor substrate which forms the nitride semiconductor laser element in this invention.

Explanation of symbols

10 Dislocation bundle 20 Medium dislocation density region 30 Low dislocation density region

Claims (7)

A nitride semiconductor substrate, and a nitride semiconductor layer comprising an n-type semiconductor layer, an active layer, and a p-type semiconductor layer stacked on the nitride semiconductor substrate, and a pair of resonators on side surfaces of the nitride semiconductor layer having a surface, and a nitride semiconductor laser device having a stripe-shaped optical waveguide region in the nitride semiconductor layer,
The nitride semiconductor substrate is formed by laminating a lower layer substrate having two or more different plane orientations on the surface and an upper layer substrate grown through a stripe-shaped growth suppression film ,
A cavity is provided between the lower layer substrate and the upper layer substrate,
The upper layer substrate has a high density defect region and a low density defect region located above the region where the cavity exists,
The nitride semiconductor laser device, wherein the optical waveguide region is located above the low-density defect region in the upper substrate.   The nitride semiconductor laser element according to claim 1, wherein the lower layer substrate has two or more different plane orientations periodically, and at least one plane orientation is a high-density defect region. A nitride semiconductor substrate, and a nitride semiconductor layer comprising an n-type semiconductor layer, an active layer, and a p-type semiconductor layer stacked on the nitride semiconductor substrate, and a pair of resonators on side surfaces of the nitride semiconductor layer A nitride semiconductor laser device having a plane and a stripe-shaped optical waveguide region in the nitride semiconductor layer,
The nitride semiconductor substrate is formed by laminating a lower layer substrate having a high density defect region and a low density defect region and an upper layer substrate grown through a stripe-shaped growth suppression film ,
A cavity is provided between the lower layer substrate and the upper layer substrate,
The upper layer substrate has a high density defect region and a low density defect region located above the region where the cavity exists,
The nitride semiconductor laser device, wherein the optical waveguide region is located above the low-density defect region in the upper substrate.   The nitride semiconductor according to any one of claims 1 to 3, wherein a growth suppression region wider than the cavity is provided between the lower layer substrate and the upper layer substrate and below the cavity. Laser element.   The nitride semiconductor laser element according to any one of claims 1 to 4, wherein the lower substrate is provided with a high-density defect region and a low-density defect region periodically.   The nitride semiconductor laser device according to claim 1, wherein the nitride semiconductor substrate contains two or more types of n-type impurities.   The nitride semiconductor laser element according to claim 1, wherein the nitride semiconductor substrate and the n-type semiconductor layer contain different n-type impurities.

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