JP3617910B2 - Nitride compound semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device using a compound semiconductor material, and more particularly to a nitride-based compound semiconductor laser made of a compound semiconductor containing nitrogen such as GaN, AlGaN, or InGaN.
[0002]
[Prior art]
In recent years, development of short-wavelength semiconductor lasers has been promoted for the purpose of application to high-density optical disk systems and the like. This type of laser is required to shorten the oscillation wavelength in order to increase the recording density. A 600 nm band light source made of InGaAlP material as a short-wavelength semiconductor laser has already been put into practical use because its characteristics have been improved to a level at which both reading and writing can be performed.
[0003]
Further, blue semiconductor lasers have been actively developed with the aim of further improving the recording density, and the operation of semiconductor lasers based on II-VI group has already been confirmed. However, there are many barriers to practical use such as the reliability being limited to about 100 hours, and it is difficult to make the wavelength below 480 nm. There are many.
[0004]
On the other hand, GaN-based semiconductor lasers have a short wavelength down to 350 nm or less, and are promising and have been actively researched and developed with regard to reliability. Laser oscillation due to current injection at room temperature was also confirmed. Thus, the nitride system is an excellent material that satisfies the requirements of the next-generation optical disk system light source.
[0005]
In order to make it applicable to an optical disk system or the like, the oscillation beam characteristic of the laser is important, and it is essential to form a transverse mode control structure in the light emitting part in a direction parallel to the bonding plane. The transverse mode control structure can be usually formed by a method of leaving a part of the active layer and embedding with a semiconductor layer having a different refractive index.
[0006]
However, in the real refractive index waveguide type such as the BH (Buried Hetero) structure, the mesa width needs to be reduced to about the wavelength so that the higher-order mode is cut off, but since the oscillation wavelength is short in the first place, Control of a very narrow mesa width of about 0.5 μm is required. Manufacturing such a narrow-width semiconductor laser itself is difficult in terms of process, the yield is extremely poor, the device resistance exceeds 50Ω, and laser oscillation itself becomes difficult. And even if it oscillates, the reliability of the element was significantly impaired.
[0007]
Note that if the buried layer is formed of a material having a loss with respect to the oscillation light, the higher-order mode is cut off, so that the mesa width can be designed to be as wide as several μm. However, in this case, there is a problem that the threshold value increases because the fundamental mode also receives optical loss.
[0008]
[Problems to be solved by the invention]
Thus, conventionally, in a transverse mode control laser using a GaN-based material, it is necessary to make the mesa width very narrow, and there is a problem that the process is very difficult. Further, if a material having a loss with respect to oscillation light is used as the buried layer in order to widen the mesa width, there is a problem that the threshold value increases.
[0009]
The present invention has been made in consideration of the above circumstances, and the object of the present invention is to realize lateral mode control with excellent characteristics without extremely narrowing the mesa width, and a threshold value. It is an object of the present invention to provide a nitride-based compound semiconductor laser capable of reducing the above.
[0010]
[Means for Solving the Problems]
(Constitution)
The gist of the present invention relates to the structure of a nitride-based semiconductor laser device formed on a substrate such as sapphire, and a guide for cutting off higher-order modes separately from the waveguide structure for obtaining the fundamental transverse mode. By creating a separate wave structure and selectively controlling the fundamental mode and higher-order mode, the laser dimensions that ensure process tolerance and reproducibility are realized, and good characteristics are obtained. is there.
[0011]
That is, the present invention relates to a nitride compound semiconductor laser in which a nitride compound semiconductor layer is laminated on a substrate , the outer side of the mesa including the active layer is buried with the buried layer, and the refractive index is parallel to the substrate surface. The anti-waveguide-type first waveguide structure built using the difference has a refractive index different from that of the material having a loss with respect to the oscillation light or the buried layer forming the first waveguide structure. A second waveguide structure made of a material and formed on the outer side of the first waveguide structure and absorbing higher-order modes other than the fundamental transverse mode is provided.
[0012]
Here, preferred embodiments of the present invention include the following.
(1) The distance between the first waveguide structure and the second waveguide structure, that is, the thickness in the direction parallel to the bonding surface of the buried layer in the first waveguide structure is set in the range of 1 to 10 μm. about.
(2) The distance between the first waveguide structure and the second waveguide structure, that is, the thickness in the direction parallel to the bonding surface of the buried layer in the first waveguide structure is set in the range of 1 to 4 μm. about.
[0014]
(3) The first waveguide structure is formed from a semiconductor layer, and the second waveguide structure is formed from a material other than a semiconductor.
(4) Both the first waveguide structure and the second waveguide structure are formed from a semiconductor layer.
[0015]
(5) nitride compound semiconductor _{layer, In x Al y Ga z N} (x + y + z = 1,0 ≦ x, y, z ≦ 1) it is to be.
(6) The substrate is sapphire or SiC.
[0016]
(Function)
According to the present invention, the second waveguide structure is provided outside the first waveguide structure, and the second waveguide structure gives a large loss to higher-order modes other than the fundamental transverse mode, or the mode Therefore, it is possible to cut off the higher order mode without losing the fundamental mode. In this case, the high-order mode is cut off even if the mesa width is not extremely narrowed. Therefore, the mesa width can be sufficiently widened, and the yield can be improved and the element resistance can be reduced. Furthermore, since the fundamental mode is not subject to loss, there is no inconvenience that the threshold value increases. Therefore, the process is excellent in reproducibility, the manufacture is gentle, the low threshold operation is possible, and its usefulness is great.
[0017]
Moreover, the process tolerance can be further ensured by making the first waveguide structure an anti-waveguide structure. In addition, a self-aligned process can be realized by forming a waveguide structure with a semiconductor layer, and regrowth and the like can be omitted by appropriately adopting a material other than a semiconductor.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
The details of the present invention will be described below with reference to the illustrated embodiments.
(First embodiment)
FIG. 1 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the first embodiment of the present invention.
[0019]
All the nitride layers were grown by MOCVD (metal organic chemical vapor deposition). Regarding the growth conditions, the pressure is normal pressure, the growth of the GaN and AlGaN layers other than the buffer layer is basically in the range of 1000 ° C. to 1100 ° C. in an atmosphere in which nitrogen, hydrogen, and ammonia are mixed. The range was 700 ° C. to 850 ° C. in a nitrogen and ammonia atmosphere.
[0020]
In the figure, 11 is a sapphire substrate, and 12 is a GaN buffer layer (thickness 0.03 μm) grown at a low temperature (550 ° C.). 13 is an n-GaN contact layer (Si-doped; 5 × 10 ¹⁸ cm ⁻³ , thickness 3 μm) grown at high temperature (1100 ° C.), 14 is an n-side electrode made of Al / Ti / Au, and 15 is n-Al _{It is a 0.15} Ga _0.85 N clad layer (Si-doped; 1 × 10 ¹⁸ cm ⁻³ , thickness 0.32 μm). Reference numeral 16 denotes an active layer portion including a multiple quantum well structure (MQW) and a light guide layer, which has a light guide layer made of GaN having a thickness of 0.05 μm, and the well layer has a thickness of In _0.20 Ga _{0. The} barrier layer is composed of In _0.03 Ga _0.97 N having a thickness of 6 nm.
[0021]
17 is a p-Al _0.15 Ga _0.85 N clad layer (Mg doped; 5 × 10 ¹⁹ cm ⁻³ , thickness 0.35 μm), 18 is a p-GaN regrowth cap layer (Mg doped; 5 × 10 ¹⁹ cm ⁻³ , thickness 0.3 μm). Reference numeral 19 denotes a p-GaN buried contact layer (Mg doped; 8 × 10 ¹⁹ cm ⁻³ , thickness 0.8 μm) formed by regrowth, and the uppermost portion is made highly concentrated (Mg doped; 2 × 10 ²⁰ cm ^{− 3} and a thickness of 0.1 μm). 20 is a p-side electrode made of Pt / Ti / Pt / Au, 21 is a SiO ₂ dielectric film, and 22 is an Au electrode pad. The sapphire substrate 11 has a (0001) c plane, and the laser mirror is formed by cleavage.
[0022]
In order to realize this structure, after crystal growth from 12 to 18 in the figure, a first mesa as shown by 16 to 18 in the figure is formed by using an optical lithography technique and a dry etching technique, and then 19, 20 Regrown the layer. Thereafter, the second mesa is formed outside the first mesa by repeatedly using the photolithography technique and the dry etching technique, the electrode pad 22 is further formed on the dielectric film 21 having the opening, and the contact layer 13 is formed. An n-side electrode 14 was formed thereon.
[0023]
Here, in the direction parallel to the bonding surface, the first and second mesas, that is, the first mesa and the buried contact layer 19 form the first waveguide structure, and the electrode pad 22 forms the second waveguide structure. Will be formed. The width of the first mesa in the first waveguide structure, that is, the width of the remaining active layer was 3 μm, and the width of the outer mesa (second mesa) was 6 μm. Au as the electrode pad 22 is in contact with the outside of the second mesa.
[0024]
This structure is an anti-waveguide structure in which the effective refractive index including the cladding of the active layer portion is smaller than that of GaN which is the buried layer of the first mesa. The mode light that stands as a steady solution in the active layer part oozes out to the GaN part in both the fundamental mode and the higher-order mode, but the higher-order mode spreads outside the fundamental mode, so it is absorbed by the outer Au and cut off. Become. Since the anti-waveguide structure has a larger spread of higher-order modes than the waveguide structure, the mesa width can be widened and the distance to the outer mesa can be widened. On the other hand, the threshold current value itself can be reduced to a certain range where the current injection area is reduced. Therefore, if the accuracy of the process is increased, the inner mesa width can be reduced to about 1 μm.
[0025]
The process tolerance increases as the distance between the first waveguide structure and the second waveguide structure, that is, the distance between the inner mesa and the outer mesa (which is the thickness of the GaN buried contact layer 19 in this embodiment) increases. If the electric field intensity of the high-order mode light that oozes out becomes too small, the cutoff becomes impossible, so that a suitable region exists. In the case of forming by a process as in the present embodiment, 1 μm or more is necessary from the viewpoint of reproducibility, and the upper limit is preferably 10 μm or less, which is required to be cut off, and more preferably about 10 times the oscillation wavelength or less, and about 4 μm or less. In the case of a self-alignment type as will be described later, the lower limit condition also depends on the cross-sectional structure, and can be set to 1 μm or less.
[0026]
In this embodiment, when the inner mesa width is 3 μm, room temperature continuous oscillation was performed at a threshold of 65 mA. The oscillation wavelength was 415 nm and the operating voltage was 5.5V. When the mesa width was 1 μm, the threshold voltage decreased to 35 mA, but the operating voltage increased to 7V. The beam characteristics were unimodal, and the astigmatic difference was as small as 5 μm. The maximum optical output was obtained up to 10 mW with continuous oscillation, and the reliability was stable for over 1000 hours at room temperature.
[0027]
These characteristics were obtained with a structure in which the substrate was bonded to the heat sink with the substrate below. In the actual process, the inner mesa is not necessarily located at the center of the outer mesa, and it may be decentered due to misalignment during the photolithography process. There is no big problem.
[0028]
(Second Embodiment)
FIG. 2 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the second embodiment of the present invention. Here, the structure near the mesa structure is shown enlarged. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0029]
All the nitride layers were grown by MOCVD as in the first embodiment. The growth conditions are the same. And after forming each nitride layer of 13-20 in a figure, the 2nd mesa was formed by digging into a channel type. The dielectric film 21 and the electrode pad 22 were also formed in the same manner as in the first embodiment.
[0030]
In this embodiment, the p-side portion of the laser structure can be made relatively flat. When this laser was mounted on the heat sink at the junction surface (p side), the threshold current value and beam characteristics were the same as in the first embodiment, but the maximum continuous oscillation temperature was high from 50 ° C to 80 ° C. We were able to. The reliability test can be tested at a high temperature, and it was confirmed that it operates stably at 50 ° C. for 1000 hours or more.
[0031]
(Third embodiment)
FIG. 3 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the third embodiment of the present invention. Here, as in the second embodiment, the structure near the mesa structure is shown enlarged. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0032]
Each nitride layer was grown by the MOCVD method as in the first and second embodiments. The growth conditions are the same. In the present embodiment, the second mesa is formed by digging into a channel type as in the second embodiment. In addition, a ZnO film 23 is formed in the channel, and this is formed into the second waveguide. Used as structure.
[0033]
That is, the ZnO film 23 was formed on the entire surface by spin coating before the Au pad 22 was formed, and the ZnO film 23 was left only in the channel by performing back etching by RIE. Here, the band gap of ZnO is 3.35 eV, which is similar to GaN. However, when the film is formed by spin coating, the band gap is distorted, and the oscillation wavelength light can be moderately absorbed.
[0034]
In this embodiment, the controllability of the transverse mode is very good, and mode instabilities such as kinks are hardly observed. This is thought to be due to the fact that the absorption is increased by using ZnO rather than the absorption of higher order modes using Au. This absorption layer may be organic such as polyimide formed by spin coating. Further, poly-Si or the like may be attached by a CVD method, or a metal such as W or Mo may be used. In this structure, various absorption layers can be applied.
[0035]
(Fourth embodiment)
FIG. 4 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the fourth embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part as FIG. 1, and the detailed description is abbreviate | omitted.
[0036]
This embodiment is different from the first embodiment in that the first mesa is not buried in the GaN contact layer 19 but in the p-Al _0.07 Ga _0.93 N layer 24 (Mg doped; 5 × 10 ¹⁹ cm ^-3 , thickness 0.45 μm). This p-AlGaN buried layer 24 was formed by leaving a SiO ₂ stripe on the first mesa and performing selective growth before growing the p-GaN contact layer 19.
[0037]
Further, since the p-AlGaN buried layer 24 is formed, the thickness of the p-GaN contact layer 19 is 0.5 μm, which is thinner than that of the first embodiment. Further, the width of the first mesa in the first waveguide structure, that is, the width of the remaining active layer 16 was 1.5 μm, and the width of the outer mesa (second mesa) was 3 μm.
[0038]
In this embodiment, the effective refractive index including the cladding of the active layer portion has a waveguide structure that is larger than that of AlGaN that is the buried layer of the first mesa. In this case, room temperature continuous oscillation was performed at a threshold of 25 mA. The oscillation wavelength was 415 nm and the operating voltage was 5.5V. The beam characteristics were unimodal, and the astigmatic difference was as small as 3 μm. The maximum optical output was obtained up to 10 mW with continuous oscillation, and the reliability was stable for over 1000 hours at room temperature.
[0039]
These characteristics were obtained with a structure in which the substrate was bonded to the heat sink with the substrate below. In this embodiment, since the buried layer is made of p-AlGaN, the built-in potential at the junction with n-AlGaN increases, and the leakage current outside the active layer can be reduced. The buried layer AlGaN may be a high resistance layer doped with Zn or the like.
[0040]
(Fifth embodiment)
FIG. 5 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the fifth embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part as FIG. 1, and the detailed description is abbreviate | omitted.
[0041]
11 to 22 in the figure are the same as those described in the first embodiment, and 25 is a Zn-doped high-resistance GaN buried layer. However, the Au electrode pad 22 also serves as a p-side electrode. The manufacturing method is similar to the previous embodiments. The width of the mesa, that is, the width of the remaining active layer 16 was 2 μm. This structure is an anti-waveguide structure in which the effective refractive index including the cladding of the active layer portion is smaller than that of GaN which is the buried layer 25 of the mesa.
[0042]
In this embodiment, the mesa for exposing the n-plane and the active layer are brought close to 1.5 μm without bothering the outer mesa. Higher order modes that ooze out from the active layer are absorbed by Au outside the n-plane mesa and cut off. Although such an asymmetrical absorption layer setting is used, there is no problem from the viewpoint of making it difficult for higher-order modes to stand.
[0043]
In this embodiment, room temperature continuous oscillation was performed at a threshold of 70 mA. The oscillation wavelength was 415 nm and the operating voltage was 5.5V. In the present embodiment, the n-plane mesa can be used as the second mesa in the process, so that the process can be greatly shortened.
[0044]
(Sixth embodiment)
FIG. 6 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the sixth embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part as FIG. 1, and the detailed description is abbreviate | omitted.
[0045]
In the present embodiment, the first mesa is embedded at 26, 27, and 28. That is, after the first mesa is formed, SiO ₂ is left on the mesa, and the p-Al _0.07 Ga _0.93 N buried layer 26 and n-In _0.2 Ga are sequentially formed in a self-aligned manner by selective growth. It is obtained by forming a _0.8 N current confinement / light absorption layer 27 and a GaN current confinement layer 28. Here, 26 is provided as a low refractive index layer for fundamental mode, 27 is provided as a light absorption layer for higher order mode, and 28 is provided for stability of current confinement.
[0046]
In this embodiment, after forming the first mesa, the p-AlGaN buried layer 26, the n-InGaN current confinement / light absorption layer 27, and the GaN current confinement layer 28 are formed, whereby both sides of the first mesa are formed. In addition, a low refractive index layer for the fundamental mode and a light absorption layer for the higher order mode could be built in. The width of the mesa, that is, the width of the remaining active layer 16 was 2 μm. This structure is a waveguide structure in which the effective refractive index including the cladding of the active layer portion is larger than that of AlGaN which is a mesa buried layer.
[0047]
In this embodiment, room temperature continuous oscillation was performed at a threshold of 30 mA. The oscillation wavelength was 415 nm and the operating voltage was 5.5V. In the present embodiment, reproducibility in the process is remarkably reproducible and the number of steps is reduced.
[0048]
(Seventh embodiment)
FIG. 7 is a sectional view showing a schematic configuration of a blue semiconductor laser according to the seventh embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part as FIG. 1, and the detailed description is abbreviate | omitted.
[0049]
This embodiment is different from the sixth embodiment in that a p-GaN buried layer 29 is used instead of the p-AlGaN buried layer 26. The width of the mesa, that is, the width of the remaining active layer 16 was 3 μm. This structure is an anti-waveguide structure in which the effective refractive index including the cladding of the active layer portion is smaller than that of GaN which is a mesa buried layer.
[0050]
Also in this embodiment, after the first mesa is formed, SiO ₂ is left on the mesa, and 29, 27, 28, the buried layer for controlling the fundamental mode, and the high-order mode are sequentially formed in a self-aligned manner by selective growth. The light absorption layer was able to be accurately built on the side of the mesa.
[0051]
In this embodiment, room temperature continuous oscillation was performed at a threshold of 38 mA. The oscillation wavelength was 415 nm and the operating voltage was 4.5V. Similar to the sixth embodiment, this embodiment also brings about remarkable reproducibility and reduction in the number of steps in process reproducibility.
[0052]
The present invention is not limited to the above-described embodiments. In the embodiment, sapphire is used as the substrate. However, the present invention is not limited to this, and SiC or the like can be applied as long as it is a single crystal. In the embodiment, the target layer is described as a gallium nitride-based material such as AlGaN. However, an element such as In, Ti, Si, C, or Ni may be included in an amount of impurities that do not become mixed crystals. Further, it is not always necessary to make the substrate side n-type, and all p and n conductivity types may be reversed.
[0053]
In the embodiment, the mesa is formed by etching up to the active layer, and the so-called BH structure in which the mesa side is embedded is described in detail. However, the active layer is not etched and a structure using a waveguide structure above the active layer surface is used. Also good. Furthermore, the present invention can also be applied to a structure in which the clad on the light emitting portion is etched in the concave portion and embedded with a high refractive index layer. In addition, various layers and structures that absorb higher-order modes can be applied as long as they do not adversely affect the laser threshold. Furthermore, it can be applied to the field of optical devices such as waveguide structures, light receiving elements, and transistors. In addition, various modifications can be made without departing from the scope of the present invention.
[0054]
【The invention's effect】
As described above in detail, according to the present invention, a waveguide structure for cutting off a higher-order mode is created separately from a waveguide structure for obtaining a fundamental transverse mode, and a fundamental mode and a higher-order mode are selected. Thus, an excellent gallium nitride-based lateral mode semiconductor laser with a sufficiently wide mesa width, a sufficiently low element resistance, and a simple manufacturing method can be realized. Since this semiconductor laser can reduce the threshold, has good beam characteristics, and greatly improves reliability, its usefulness is tremendous.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a schematic configuration of a blue semiconductor laser according to a first embodiment.
FIG. 2 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a second embodiment.
FIG. 3 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a third embodiment.
FIG. 4 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a fourth embodiment.
FIG. 5 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a fifth embodiment.
FIG. 6 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a sixth embodiment.
FIG. 7 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a seventh embodiment.
[Explanation of symbols]
11 ... sapphire substrate 12 ... GaN buffer layer 13 ... n-GaN contact layer 14 ... n-side electrode _{15 ... n-Al 0.15 Ga 0.85} N clad layer 16 ... active layer 17 including the MQW and the light guide layer ... p-Al _0.15 Ga _0.85 N cladding layer 18 ... p-GaN regrowth cap layer 19 ... p-GaN contact layer 20 ... p-side electrode 21 ... SiO ₂ dielectric film 22 ... Au electrode pad 23 ... ZnO Buried layer 24... P-Al _0.07 Ga _0.93 N buried layer 25... High resistance GaN buried layer

Claims (3)

A nitride compound semiconductor laser comprising a nitride compound semiconductor layer laminated on a substrate,
The first waveguide structure of the anti-waveguide type, which is formed by using the refractive index difference in the direction parallel to the substrate surface by filling the outer side of the mesa including the active layer with the buried layer, and has a loss with respect to the oscillation light A material or a material having a refractive index different from that of the buried layer forming the first waveguide structure is formed outside the first waveguide structure and absorbs higher-order modes other than the fundamental transverse mode . A nitride-based compound semiconductor laser comprising two waveguide structures. 2. The nitride-based compound semiconductor laser according to claim 1, wherein a thickness of the buried layer in the first waveguide structure in a direction parallel to the substrate surface is set in a range of 1 to 4 [mu] m. The nitride-based compound semiconductor laser according to claim 1, wherein the first waveguide structure is formed of a semiconductor layer, and the second waveguide structure is formed of a semiconductor layer or a material other than a semiconductor. .

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