JP3678399B2 - Nitride semiconductor laser device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser using a nitride semiconductor material.
[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 characteristics improved to a level at which both reading and writing of a disk are possible, and has already been put into practical use.
[0003]
Blue semiconductor lasers have been actively developed with the aim of further increasing recording density. Although semiconductor lasers based on II-VI groups have already been confirmed to oscillate, there are many barriers to practical use, such as the reliability being limited to about 100 hours, and it is difficult to make wavelengths below 480 nm. For example, there are many material limits for application to next-generation optical disc systems.
[0004]
On the other hand, GaN-based semiconductor lasers are promising and have been actively researched and developed, such as being capable of short wavelengths down to 350 nm or less, and having reliability of over 10,000 hours in LEDs. In some cases, laser oscillation by current injection at room temperature was also confirmed. As described above, the GaN-based material is an excellent material that satisfies the requirements of the next-generation optical disk system light source.
[0005]
Further, in order to make the semiconductor laser applicable to an optical disk system or the like, the oscillation beam characteristic of the laser is important. In order to improve the oscillation beam characteristics, it is essential to form a transverse mode control structure in the light emitting portion in a direction parallel to the bonding plane. The transverse mode control structure can be usually formed by a method such as a mesa structure embedded with semiconductor layers having different refractive indexes.
[0006]
In order to obtain stable fundamental transverse mode characteristics of the mesa structure, it is necessary to make the clad layer sufficiently thick and the ridge width narrow, but the AlGaN layer used as the clad layer is thick due to large lattice mismatch with GaN. Then, there was a problem of cracks. Further, the AlGaN layer has a problem that the voltage cannot be lowered because the resistivity cannot be reduced.
[0007]
For this reason, it is proposed that the clad layer has a superlattice structure of thin film (about 2 nm thick) GaN and a thin film of about 15% Al composition (about 2 nm thick) AlGaN to prevent cracks and reduce the voltage. ing. In this structure, since the cladding layer has a superlattice structure, the critical film thickness against strain increases and cracks are difficult to occur.
[0008]
In such a structure, as shown in FIG. 1A, in a superlattice structure in which an AlGaN barrier layer having a large band gap is doped n-type and a GaN well layer is undoped, a large band bend is formed at the heterointerface. Dengue occurs and the two-dimensional electron gas accumulates. The superlattice is designed to be thin, and the two-dimensional electron gas adjacent to each other is coupled or tunneled to smoothly transport carriers in the vertical direction (left and right in the figure) of the epilayer without going through high resistivity AlGaN. Therefore, the resistance of the cladding layer can be reduced. The same applies to the p-type.
[0009]
However, according to the research of the present inventors, it has been found that the situation changes greatly when a superlattice is formed on a sapphire substrate. Theoretically, experimentally, the superlattice structure made of nitride semiconductor material generates a very large internal electric field due to the piezo effect and spontaneous polarization due to strain compared to the superlattice structure of other semiconductor materials. It became clear.
[0010]
As shown in FIG. 1B, in the superlattice structure using a nitride-based semiconductor material, both the barrier layer and the well layer have a triangular potential due to the influence of an electric field and form a two-dimensional electron gas essential for carrier injection. Therefore, it is very difficult to reduce the operating voltage of the laser to 5 V or less. In addition to such a factor that the two-dimensional electron gas is not formed, an effective high hetero barrier is formed by the triangular potential, and various voltage drop factors are induced.
[0011]
In order to further improve the recording density on the optical desk, it is necessary to further shorten the wavelength of the light source. However, even when an active layer having a promising GaN / AlGaN quantum well is used for the laser, Such a triangular potential was also formed in the well portion, which significantly deteriorated carrier recombination pairing.
[0012]
Thus, with the conventional superlattice structure of nitride-based semiconductor material, it is extremely difficult to manufacture a transverse mode control semiconductor laser device with low operating voltage and good mode characteristics, and reliability characteristics (especially at high temperatures) are also high. It was damaged.
[0013]
[Problems to be solved by the invention]
As described above, in a lateral mode control semiconductor laser device having a superlattice structure using a conventional nitride-based semiconductor material, it is extremely difficult to achieve low voltage and good light emission characteristics due to spontaneous polarization and the piezoelectric effect. There was a problem.
[0014]
The present invention has been made in consideration of the above circumstances, has excellent process reproducibility, is easy to process, has a high yield, has a low threshold and a low operating voltage, and can operate in a basic transverse mode and has excellent characteristics. An object of the present invention is to provide a semiconductor laser device.
[0015]
[Means for Solving the Problems]
In order to solve the above object, the present invention provides an active layer made of a nitride-based semiconductor provided on a substrate, and first and second layers provided on the substrate so as to sandwich the active layer. And at least one of the first and second cladding layers is a barrier layer / well layer ( AlxGa1-xN / AlyGa1-yN The barrier layer ( AlxGa1-xN ) has a superlattice structure of 0 < x ≦ 1,0 ≦ y < 1 ). The nitride-based semiconductor laser device is characterized in that the Al composition of a part or all of 0 < x ≦ 1 ) increases in a graded manner from the substrate side to the surface side .
[0016]
The present invention also includes an active layer made of a nitride-based semiconductor provided on a substrate, and first and second cladding layers provided on the substrate so as to sandwich the active layer. , At least one of the first and second cladding layers is a barrier layer / well layer ( AlxGa1-xN / AlyGa1-yN The well layer ( AlxGa1-xN ) has a superlattice structure of 0 < x ≦ 1,0 ≦ y < 1 ). The nitride-based semiconductor laser device is characterized in that the Al composition of a part or all of 0 < x ≦ 1 ) increases in a graded manner from the substrate side to the surface side .
[0017]
The present invention also provides a nitride semiconductor laser device characterized in that the Al composition of the barrier layer or well layer monotonously increases or decreases. With a superlattice structure composed of at least AlxGa1-xN / AlyGa1-yN (0 <x ≦ 1,0 ≦ y <1) configured on the substrate, the Al composition is not uniform and is formed in a graded shape , By designing the band structure of the superlattice structure in detail, it is possible to provide a nitride semiconductor laser device that prevents the influence of the piezo electric field and spontaneous polarization, has a low voltage, and has a high emission efficiency and gain.
[0018]
In addition, the present invention is excellent in process reproducibility, easy to process, and can operate in a basic transverse mode with a low threshold and a low operating voltage with a high yield. In particular, among the characteristics of the nitride semiconductor laser device, not only the operating voltage can be lowered and the transverse mode characteristics can be stabilized, but also the reliability can be improved.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 2 is a cross-sectional view for explaining a schematic configuration of the blue nitride semiconductor laser device according to the first embodiment of the present invention.
[0020]
All the nitride layers were grown by MOCVD (metal organic chemical vapor deposition). Regarding growth conditions, pressure is normal pressure, GaN other than buffer layer, AlGaN layer is basically in the range of 1000 ° C to 1100 ° C in a mixed atmosphere of nitrogen, hydrogen and ammonia, growth including active layer is nitrogen and ammonia atmosphere Thus, the temperature range was 700 ° C to 850 ° C.
[0021]
In FIG. 2 , 11 is a sapphire substrate, and 12 is a GaN buffer layer (0.03 μm) grown at a low temperature (550 ° C.). Reference numeral 14 denotes GaN grown at a high temperature (1100 ° C.). The lower portion is undoped through the SiO 2 stripe 10 for lateral growth, and the upper portion is an n-GaN contact layer. Reference numeral 13 denotes an n-side electrode.
[0022]
Reference numeral 15 denotes an n-type cladding layer made of a superlattice structure of n-AlGaN / un-GaN having a total thickness of 1.0 μm. Reference numeral 16 denotes an active layer portion including a multiple quantum well structure (MQW) and a light guide layer. It has a light guide layer made of GaN having a thickness of 0.1 μm, the well layer is made of In0.13 Ga0.87 N4 layer having a thickness of 4 nm, and the barrier layer is made of In0.03 Ga0.97N having a thickness of 8 nm.
[0023]
17 is a p-type cladding layer made of p-AlGaN / un-GaN superlattice with a total thickness of 1.0 μm, 20 is a 0.5 μm P-GaN contact layer (Mg-doped), and the outermost surface is made of higher Mg. Concentration. 21 is a SiO ₂ constriction layer, 22 is a p-type electrode, and 23 is an electrode pad.
[0024]
FIG. 3 shows a band diagram of the superlattice structure of the nitride semiconductor laser device.
Here, the n conduction band side is shown. Both the barrier layer and the well layer have a thickness of 2 nm, and only the barrier layer is doped to an n-type of 5 × 10 ¹⁸ cm ⁻³ . By changing the Al composition of the barrier layer from 15% to 20% graded from the substrate side to the surface side, the band bending on the GaN side is sharpened to ensure the formation of the two-dimensional electron gas.
[0025]
In this example, when the ridge width was 3 μm, continuous oscillation at room temperature was performed at a threshold of 35 mA. The oscillation wavelength was 400 nm and the operating voltage was 4.0V. The beam characteristics were unimodal, and the astigmatic difference was as small as 5 μm. The maximum light output was obtained up to 40 mW with continuous oscillation, and the reliability was stable for more than 2000 hours at 50C. These characteristics were obtained with a structure bonded to a heat sink with the substrate still below. As for the noise characteristics, the characteristics of 10 ^-13 dB / Hz or less were obtained even in the presence of return light. The device yield was extremely high, and the transverse mode characteristics described above were obtained with 90% of the devices.
[0028]
FIG. 4 is a cross-sectional view for explaining a schematic configuration of a blue nitride semiconductor laser device according to the second embodiment of the present invention. The difference from the first embodiment is that an n-type GaN substrate 24 grown by a hydride vapor phase growth apparatus is used as the substrate. Lateral growth technology is also incorporated during the growth of the GaN substrate, and the dislocation density is suppressed to 10 4 cm -2 or less. Further, after forming the mesa, n-InGaN absorbing layer (optical waveguide layer) 18 is formed by selective growth by regrowth, growing then contact layer 20.
[0029]
FIG. 5 shows a band diagram of the superlattice structure of the nitride semiconductor laser device. Again, the n side conduction band side is shown. Both the barrier layer and the well layer are 2 nm thick, and only the barrier layer is doped to an n-type of 5 × 10 18 cm −3. Here, by changing the Al composition of the well layer from 0% to 5% graded from the substrate side to the surface side, the band bottom on the GaN side is quasi-flatted to ensure the formation of the two-dimensional electron gas. .
[0032]
In the present invention, the p-type was similarly designed in consideration of the band on the valence band side. Voltage drop in the case of the superlattice is expected to affect mainly piezoelectric field-spontaneous polarization, the substrate - field effect direction of the surface side is as shown.
[0034]
Note that the present invention is not limited to this embodiment, and SiC or the like can be applied as the semiconductor layer or substrate, and II-VI group compound semiconductors, Si, Ge, or the like may be used.
As long as the structure does not adversely affect the laser threshold, various applications are possible.
In addition, the present invention can also be applied to the field of optical devices such as waveguide structures, light receiving elements, and transistors.
[0035]
【The invention's effect】
As described above in detail, according to the present invention, the resistance of the superlattice layer can be reduced, the process can be easily reproducible, the process is easy, the yield is high, and the basic threshold voltage is low and the operating voltage is low. Provided is a nitride-based transverse mode control type structure laser capable of mode operation and having good characteristics. Particularly in the characteristics of the semiconductor laser, not only the operation voltage is lowered and the transverse mode characteristic is stabilized, but also there is a great effect of improving the reliability. Its usefulness is tremendous.
[Brief description of the drawings]
FIG. 1 is a diagram showing a conventional background .
FIG. 2 is a sectional view of the nitride semiconductor laser device according to the first embodiment of the present invention .
FIG. 3 is a band diagram of the superlattice structure of the nitride semiconductor laser device according to the first embodiment of the present invention .
FIG. 4 is a sectional view of a nitride semiconductor laser device according to a second embodiment of the present invention .
FIG. 5 is a band diagram of a superlattice structure of a nitride semiconductor laser device according to a second embodiment of the present invention .
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Sapphire substrate 12 ... Buffer layer 13 ... N side electrode 14 ... n-GaN contact layer 15 ... n-AlGaN / GaN superlattice clad layer 16 ... Active layer part containing multiple quantum well structure (MQW) light guide layer 17 ... p-AlGaN / GaN superlattice cladding layer 18... InGaN absorption layer (optical waveguide layer)
20 ... p-GaN contact layer 21 ... SiO2 constriction layer 22 ... p-type electrode 23 ... electrode pad 10 ... SiO2 stripe mask 24 ... n-GaN substrate

Claims (4)

An active layer made of a nitride-based semiconductor provided on a substrate; and first and second cladding layers provided on the substrate so as to sandwich the active layer . At least one of the second cladding layer, a barrier layer / well layer (AlxGa1-xN / AlyGa1-yN 0 <x ≦ 1,0 ≦ y <1) , such a superlattice structure consisting Rutotomoni, the barrier layer (AlxGa1 -xN 0 <x ≦ 1) of the part or the Al composition of the whole of the layer, the nitride semiconductor laser device characterized in that it increases toward the surface side grayed Rededdo shape from the substrate side. An active layer made of a nitride-based semiconductor provided on a substrate; and first and second cladding layers provided on the substrate so as to sandwich the active layer . At least one of the second cladding layer, a barrier layer / well layer (AlxGa1-xN / AlyGa1-yN 0 <x ≦ 1,0 ≦ y <1) , such a superlattice structure consisting Rutotomoni, the well layer (AlxGa1 -xN 0 <x ≦ 1) of the part or the Al composition of the whole of the layer, the nitride semiconductor laser device characterized in that it increases toward the surface side grayed Rededdo shape from the substrate side. An active layer made of a nitride-based semiconductor provided on a substrate, and an n-type cladding layer and a p-type cladding layer provided on the substrate so as to sandwich the active layer, respectively, The clad layer is a barrier layer / well layer ( AlxGa1-xN / AlyGa1-yN The barrier layer ( AlxGa1-xN ) has a superlattice structure of 0 < x ≦ 1,0 ≦ y < 1 ). 0 < x ≦ 1 ) A nitride semiconductor laser device characterized in that the Al composition of a part or all of the layers increases in a graded manner from the substrate side toward the surface side . An active layer made of a nitride semiconductor provided on a substrate, and an n-type cladding layer and a p-type cladding layer provided on the substrate so as to sandwich the active layer, respectively, The cladding layer is a barrier layer / well layer ( AlxGa1-xN / AlyGa1-yN 0 < x ≦ 1,0 ≦ y < 1 ), And the well layer ( AlxGa1-xN 0 < x ≦ 1 The nitride-based semiconductor laser device is characterized in that the Al composition of some or all of the layers increases in a graded manner from the substrate side to the surface side.

Images