JP2000223790A - Nitride-based semiconductor laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride-based semiconductor laser device which is excellent in the reproducibility of a process, which can be operated at a low threshold value, at a low operating voltage and with high reliability, and whose characteristic is good. SOLUTION: In this nitride-based semiconductor laser device, a multiple quantum well structure 16 which is composed of AlxGa1-xN/AlyGa1-yN (0<x<=1 and 0<=y<1) constituted on a substrate 11 or a superlattice structure is provided. The nitride-based semiconductor laser device is formed in such a way that its A composition is not uniform but step-shaped or graded in a part or all of AlxGa1-xN layers as barrier layers, or in the AlyGa1-yN layer to be used as a well-side layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a semiconductor laser using a nitride semiconductor material.

[0002]

2. Description of the Related Art In recent years, short-wavelength semiconductor lasers have been developed for application to high-density optical disk systems and the like. In this type of laser, it is required to shorten the oscillation wavelength in order to increase the recording density. 600n of InGaAlP material as short wavelength semiconductor laser
The m-band light source has already been put to practical use with its characteristics improved to a level at which both reading and writing of a disk are possible.

[0003] Blue semiconductor lasers have been actively developed to further improve the recording density. Already II-VI
Oscillation has been confirmed for group-based semiconductor lasers, but there are many barriers to practical use, such as the reliability is limited to about 100 hours, and it is difficult to produce wavelengths of 480 nm or less. There are many material limitations in the application to optical disc systems and the like.

On the other hand, a GaN-based semiconductor laser has a
Short wavelength down to nm or less, and LED for reliability
Has been confirmed for more than 10,000 hours, and is promising and is being actively researched and developed. In some cases, laser oscillation due to current injection at room temperature was also confirmed.
As described above, the GaN-based material is an excellent material that satisfies the necessary conditions for the light source of the next-generation optical disk system.

[0005] In order to apply a semiconductor laser to an optical disk system or the like, the oscillation beam characteristics of the laser are important. In order to improve the oscillation beam characteristics, it is necessary 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 in which semiconductor layers having different refractive indexes are embedded.

In order to obtain a stable fundamental transverse mode characteristic of the mesa structure, it is necessary to have a sufficiently thick cladding layer and a narrow ridge width.
Since the aN layer has a large lattice mismatch with GaN, there is a problem that cracks occur when the aN layer is made thick. Further, the AlGaN layer has a problem that the voltage cannot be reduced because the resistivity cannot be reduced.

For this reason, as a cladding layer, a thin film (about 2 nm thick) and a thin film of about 15% Al composition (about 2 nm thick)
It has been proposed that a crack can be prevented and the voltage can be reduced by using a superlattice structure with AlGaN. In this structure, since the cladding layer has a superlattice structure, the critical film thickness against strain increases, and cracks hardly occur.

Further, 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 hetero-interface is formed. Large band bending occurs, and two-dimensional electron gas accumulates. The superlattice is designed to be thin, and the two-dimensional electron gas next to each other couples or tunnels with each other, and transports the carrier up and down the epilayer (left and right in the figure) without passing through AlGaN with high resistivity.
Direction, and the resistance of the cladding layer can be reduced. The same applies to the case of the p-type.

However, according to the study of the present inventor,
It has been found that the situation changes significantly when a superlattice is formed on a sapphire substrate. Theoretically and experimentally, the superlattice structure made of nitride semiconductor material generates a very large internal electric field due to the piezo effect and spontaneous polarization caused by strain, compared to the superlattice structure made of other semiconductor materials. Became clear.

As shown in FIG. 1B, in a 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 two-dimensional electrons essential for carrier injection. It is not possible to obtain sufficient band-bending to form gas, and therefore the operating voltage of the laser is 5 V
It is extremely difficult to lower it below. Such 2
In addition to the factors that do not form the three-dimensional electron gas, an effective high heterobarrier is formed due to the triangular potential, causing various factors of voltage drop.

In order to further increase the recording density on an optical disk, it is necessary to further shorten the wavelength of the light source.
Even when a promising active layer using GaN / AlGaN as a quantum well is used for a laser, such a triangular potential is also formed in a well portion, which significantly deteriorates carrier recombination pairing.

As described above, with the conventional superlattice structure of a nitride-based semiconductor material, it is extremely difficult to manufacture a lateral mode control semiconductor laser device having a low operating voltage and good mode characteristics, and its reliability characteristics (particularly at high temperatures). ) Was also impaired.

[0013]

As described above, in a conventional transverse mode control semiconductor laser device having a superlattice structure using a nitride-based semiconductor material, a voltage is low due to spontaneous polarization and a piezo effect, and emission characteristics are excellent. There is a problem that it is very difficult to do so.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has excellent characteristics such as excellent process reproducibility, easy steps, and a high yield, capable of basic lateral mode operation at a low threshold value and a low operating voltage. It is an object to provide a nitride semiconductor laser device with good performance.

[0015]

Means for Solving the Problems To achieve the above object, the present invention comprises an active layer made of a nitride-based semiconductor, and a clad layer formed so as to sandwich the active layer,
The active layer is a barrier layer / well layer (AlxGa1-xN / AlyGa1-yN
It has a superlattice structure of 0 <x ≦ 1,0 ≦ y <1), and is a layer or a part of the barrier layer (AlxGa1-xN0 <x ≦ 1) or the well layer (AlyGa1-yN0). Provided is a nitride-based semiconductor laser device, characterized in that the Al composition of some or all of the layers of ≦ y <1) changes stepwise or graded.

Further, the present invention includes an active layer made of a nitride-based semiconductor and a clad layer formed so as to sandwich the active layer, wherein the clad layer includes a barrier layer / well layer (Al
xGa1-xN / AlyGa1-yN 0 <x ≦ 1,0 ≦ y <1) and a superlattice structure, wherein the barrier layer (AlxGa1-xN0 <x ≦ 1)
Part or all of the layers, or the well layer (AlyGa1
Provided is a nitride-based semiconductor laser device characterized in that the Al composition of part or all of -yN 0 ≤ y <1) changes stepwise or graded.

The present invention also provides a nitride semiconductor laser device characterized in that the Al composition of the barrier layer or the well layer monotonically increases or decreases. At least AlxGa1-xN / AlyGa1-yN (0 <x
≦ 1,0 ≦ y <1) The Al composition is not uniform and is formed in a stepped or graded shape, so that the band structure of the superlattice structure is designed in detail and the piezo electric field and It is possible to provide a nitride semiconductor laser device in which the influence of spontaneous polarization is prevented, the voltage is low, and the light emitting layer has high luminous efficiency and high gain.

Further, according to the present invention, the basic lateral mode operation can be performed with a low threshold voltage and a low operating voltage, which is excellent in process reproducibility, easy in the process, and high in the yield. In particular, among the characteristics of the nitride semiconductor laser device, not only the operating voltage can be reduced and the transverse mode characteristics can be stabilized, but also the reliability can be improved.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 2 is a sectional view for explaining a schematic configuration of the blue nitride semiconductor laser device according to the first embodiment of the present invention.

Each nitride layer was grown by MOCVD (metal organic chemical vapor deposition). Regarding growth conditions,
The pressure is normal pressure, and the GaN and AlGaN layers other than the buffer layer are basically 10 atmospheres in a mixed atmosphere of nitrogen, hydrogen and ammonia.
The growth including the active layer was performed in a range of from 700 ° C. to 850 ° C. in a nitrogen and ammonia atmosphere.

In FIG. 1, reference numeral 11 denotes a sapphire substrate;
Is a GaN buffer layer (0.03
μm). 14 is GaN grown at high temperature (1100 ° C)
The lower portion was undoped, and the upper portion was an n-GaN contact layer via a lateral growth SiO ₂ stripe 10. 13 is an n-side electrode.

Reference numeral 15 denotes n-Al having a total thickness of 1.0 μm.
An n-type cladding layer having a GaN / un-GaN superlattice structure, an active layer portion 16 including a multiple quantum well structure (MQW) and an optical guide layer, and an optical guide layer made of GaN having a thickness of 0.1 μm. And the well layer is made of In0.13 Ga having a thickness of 4 nm.
0.87 N4 layer, the barrier layer is 8 nm thick InO.
03 Ga0.97N.

Reference numeral 17 denotes p-Al having a total thickness of 1.0 μm.
A p-type cladding layer made of a GaN / un-GaN superlattice, 20 is a 0.5 μm P-GaN contact layer (M
(g-doped), and the outermost surface has a higher Mg concentration. 21 is a SiO ₂ confinement layer, 22 is a p-type electrode, and 23 is an electrode pad.

FIG. 3 shows a band diagram of the superlattice structure of the nitride semiconductor laser device. Here, the n-side conduction band side is shown. The thickness of both the barrier layer and the well layer is 2 nm, and only the barrier layer is 5
It is doped into an n-type of × 10 ¹⁸ cm ⁻³ . Al of barrier layer
By changing the composition graded from 15% to 20% from the substrate side to the surface side, the band bending on the GaN side is sharpened and the formation of a two-dimensional electron gas is ensured.

In this embodiment, when the ridge width is 3 μm, continuous oscillation at room temperature occurs at a threshold value of 35 mA. The oscillation wavelength is 400
nm, and the operating voltage was 4.0 V. The beam characteristics were unimodal, and the astigmatic difference was a sufficiently small value of 5 μm.
The maximum light output was obtained up to 40 mW by continuous oscillation, and the device operated stably at 50 C for 2000 hours or more in terms of reliability. These characteristics were obtained with a structure in which the substrate was bonded to a heat sink while keeping the substrate below. Regarding the noise characteristics, a characteristic of 10 ^-13 dB / Hz or less was obtained even in the presence of returning light. The yield of the devices was extremely high, and the above-mentioned transverse mode characteristics were obtained with 90% of the devices.

FIG. 4 is a sectional view for explaining a schematic structure of a blue nitride semiconductor laser device according to a second embodiment of the present invention. In this embodiment, all the nitride layers located above the fourteen n-GaN layers are MO layers as in the first embodiment.
Growth was performed by CVD (metal organic chemical vapor deposition). The same applies to growth conditions and the like. The difference from the first embodiment is that n-type Ga grown as a substrate by a hydride vapor phase epitaxy apparatus is used.
That is, the N substrate 24 is used. Lateral growth technology is incorporated during GaN substrate growth, and dislocation density is reduced to 1
0 ⁴ cm ^-2 is suppressed below.

FIG. 5 shows a band diagram of the superlattice structure of the nitride semiconductor laser device. Here, the n-side conduction band side is also shown. Both the barrier layer and the well layer have a thickness of 2 nm, and both the well layer and the barrier layer are doped with 3 × 10 ¹⁸ cm ⁻³ n-type.
Here, the Al composition of the barrier layer was graded from 12% to 15% from the surface side to the substrate side so as to minimize spikes and bending in the band of the barrier layer. In this example, the effect of the two-dimensional electron gas is not necessarily utilized positively, but a case in which ordinary carrier conduction is used. However, the voltage could be reduced as in the first embodiment. When this laser was mounted on a heat sink with a bonding surface, the threshold current value and beam characteristics were the same as those of the first embodiment, but the maximum continuous oscillation temperature was 100 ° C.
Could be higher. The reliability test can be performed at a high temperature, and it has been confirmed that the device operates stably at 50 ° C. for 10,000 hours or more. As in the case of the first embodiment, stable transverse characteristics were obtained with good yield.

FIG. 6 is a sectional view for explaining a schematic structure of a blue nitride semiconductor laser device according to a third embodiment of the present invention. In this embodiment, the use of a GaN substrate is the same as that of the second embodiment. However, the difference in structure is that after forming the mesa, the n-InGaN absorption layer (optical waveguide layer) 18 is selectively grown by regrowth. Is formed, and then the contact layer 20 is grown.

FIG. 7 shows a band diagram of the superlattice structure of the nitride semiconductor laser device. Here, the n-side conduction band side is also shown. The thickness of both the barrier layer and the well layer is 2 nm, and only the barrier layer is 5
It is doped into an n-type of × 10 ¹⁸ cm ⁻³ . Here, the Al composition of the well layer is 0% to 5% from the substrate side to the surface side.
The band bottom on the GaN side was quasi-flattened by changing to a graded manner to ensure the formation of a two-dimensional electron gas.

FIG. 8 shows a band diagram of an active layer (MQW) of a nitride semiconductor laser device as another embodiment.
Both the barrier layer and the well layer have a thickness of 2 nm, and both are undoped. This laser is an ultraviolet laser oscillating at 350 nm, and the well layer is based on GaN, but as shown in FIG. 8, graded by adding 0 to 5% of the Al composition of the well layer from the substrate side to the surface side. With this structure, the band bottom on the GaN side is flattened, and the light emission characteristics (threshold) are improved.

In this embodiment, when the ridge width was 3 μm, continuous oscillation at room temperature was performed at a threshold value of 65 mA. The oscillation wavelength is 350
nm and the operating voltage was 4.5V. The beam characteristics were unimodal, and the astigmatic difference was a sufficiently small value of 5 μm.
The maximum light output was obtained up to 10 mW by continuous oscillation, and the device stably operated at 40 C for 1,000 hours or more in terms of reliability.

In the present invention, the p-type was also designed and implemented in consideration of the band on the valence band side. We see that the voltage drop in the superlattice is mainly due to the piezo electric field and spontaneous polarization.
The direction of the electric field effect on the substrate-surface side is also as illustrated,
Depending on the Al composition, reversing the composition graded direction may have an effect.

In the above embodiment, the composition is mainly graded, but may be changed stepwise in a stepwise manner. Also,
Also when InGaN is used as the light emitting layer, the well layer or the barrier layer can be graded to reduce the threshold and the like.

It should be noted that the present invention is not limited to this embodiment, and SiC or the like can be applied as a semiconductor layer and a substrate.
A II-VI compound semiconductor, Si, Ge, or the like may be used. Various applications are possible as long as the structure does not adversely affect the threshold value of the laser. Other, waveguide structure,
The present invention is also applicable to the field of optical devices such as light receiving elements and transistors.

[0035]

As described above in detail, according to the present invention, the superlattice layer can be made to have a low resistance, excellent in process reproducibility, easy to process, high in yield and low in threshold and low. Provided is a nitride-based transverse mode control type laser having good characteristics and capable of performing basic transverse mode operation at an operating voltage. In particular, in the characteristics of the semiconductor laser, there is a great effect of improving not only the operating voltage and stabilization of the transverse mode characteristics but also the reliability. Its usefulness is enormous.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional background,

FIG. 2 is a cross-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.

FIG. 6 is a sectional view of a nitride semiconductor laser device according to a third embodiment of the present invention.

FIG. 7 is a band diagram of the superlattice structure of the nitride semiconductor laser device according to the first embodiment of the present invention.

FIG. 8 is a band diagram of a superlattice structure of a nitride semiconductor laser device according to another embodiment of the present invention.

[Explanation of symbols]

REFERENCE SIGNS LIST 11 sapphire substrate 12 buffer layer 13 n-side electrode 14 n-GaN contact layer 15 n-AlGaN / GaN superlattice cladding layer 16 active layer portion including multiple quantum well structure (MQW) light guide layer 17 p-AlGaN / GaN superlattice cladding layer 18 ... InGaN absorbing layer (optical waveguide layer) 20 ... p-GaN contact layer 21 ... SiO ₂ blocking layer 22 ... p-type electrode 23 ... electrode pad 10 ... SiO ₂ stripe masks 24 ... n -GaN substrate

Claims (3)

[Claims] 1. An active layer comprising a nitride-based semiconductor, and a clad layer formed so as to sandwich the active layer, wherein the active layer comprises a barrier layer / well layer (AlxGa1-xN / AlyGa1-yN).
It has a superlattice structure of 0 <x ≦ 1,0 ≦ y <1), and is a layer or a part of the barrier layer (AlxGa1-xN0 <x ≦ 1) or the well layer (AlyGa1-yN0). ≦ y <1) A nitride semiconductor laser device characterized in that the Al composition of some or all of the layers changes stepwise or graded. 2. An active layer comprising a nitride-based semiconductor, and a clad layer formed so as to sandwich the active layer, wherein the clad layer is a barrier layer / well layer (AlxGa1-xN / AlyGa1).
-yN 0 <x ≤1,0 ≤y <1) and has a superlattice structure, and a part or all of the barrier layer (AlxGa1-xN 0 <x ≤1) or the well layer (AlyGa1- yN 0 ≦ y <
1) A nitride semiconductor laser device characterized in that the Al composition of some or all of the layers changes stepwise or graded. 3. The nitride semiconductor laser device according to claim 1, wherein the Al composition of the barrier layer or the well layer monotonously increases or decreases.