JP2001102690A - Nitride semiconductor laser device - Google Patents

Abstract

(57) [Summary] (Problem corrected) [Problem] A nitride semiconductor laser having excellent characteristics such as excellent process reproducibility, low threshold voltage, low operating voltage, and high reliability. A system transverse mode control structure is provided. SOLUTION: An InAlGaN layer serving as a barrier layer and an InAlGaN layer serving as a well-side layer each substantially match a dominant lattice constant of a constituent layer forming a semiconductor laser or a lattice constant of a substrate, or compensate for distortion with each other. the composition so as to reduce the occurrence of generation or piezoelectric field of cracks, to set the thickness and the multiple quantum well structure 16, or a superlattice structure _{Al x Ga 1-x N /} in z Al y Ga 1-y _-Z
A light emitting layer can be formed by using a pair layer composed of N (0 <x, z ≦ 1, 0 ≦ y <1), or by using the multiple quantum well structure 16 or the superlattice structure for at least the n-type cladding layer 15. This also provides a nitride-based laser having high luminous efficiency and high gain.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device using a compound semiconductor material.
The present invention relates to a semiconductor laser made of a compound semiconductor containing nitrogen such as N or InGaN.

[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. Blue semiconductor lasers have been actively developed with the aim of further improving the recording density. The oscillation operation of the semiconductor laser based on II-VI group has already been confirmed. However, there are many barriers to practical use, such as the reliability is limited to about 100 hours, and it is difficult to produce a wavelength of 480 nm or less. There are many.

On the other hand, a GaN-based semiconductor laser has a
Short wavelength down to nm or less, and LED for reliability
The laser oscillation by current injection at room temperature, which is promising and has been actively studied and developed, has been confirmed. As described above, the nitride-based material is an excellent material that satisfies the necessary conditions for the light source of the next-generation optical disk system.

In order to be applicable to an optical disk system or the like, the oscillation beam characteristics of a laser are important, and it is essential to form a transverse mode control structure in a light emitting portion in a direction parallel to a bonding plane. The transverse mode control structure can be usually formed by a method such as embedding with a semiconductor layer having a different refractive index. In order to obtain stable fundamental transverse mode characteristics, a sufficiently thick cladding layer and a narrow ridge width are required, but the AlGaN layer used as the cladding layer is GaN.
There is a problem that cracks occur when the thickness is increased due to lattice irregularity, and that the voltage cannot be reduced because the resistivity cannot be reduced. Therefore, a thin film (2 nm
GaN and a thin film of Al composition of about 15% (2 nm)
It has been proposed that cracks can be prevented and the voltage can be reduced by forming a clad using a superlattice with AlGaN (IEEE JST Quant).
um Electron. 4 , 483, 199
8). Here, the reduction of cracks is an effect of increasing the critical film thickness with respect to strain caused by changing from a bulk to a superlattice. The concept of the voltage reduction effect is shown in FIG. Here, for simplicity, the n-side conduction band side is shown. If the AlGaN side having a large band gap is doped with n-type and GaN is undoped, large band bending occurs at the hetero interface, and a two-dimensional electron gas is accumulated. The superlattice is designed to be thin, and the two-dimensional electron gas next to each other couples or tunnels with each other, resulting in high resistivity AlGaN.
The carrier can be transported smoothly in the vertical direction (left and right in the figure) of the epi layer without passing through. The same applies to the case of the p-type. Although a flat band is shown here, for example, when a superlattice is formed on a sapphire substrate as shown in FIG. The inventors have theoretically and experimentally revealed that an extremely large internal electric field is generated as compared with the lattice and the multiple quantum well structure. In other words, both the barrier layer and the well-side layer have a triangular potential due to the influence of the electric field, so that band bending sufficient to form a two-dimensional electron gas essential for carrier injection cannot be obtained.
It was extremely difficult to lower it below. In addition to the two-dimensional electron gas, an effective high heterobarrier is formed by the triangular potential, which causes various factors of voltage drop.

In order to further increase the recording density on the 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, and significantly deteriorates carrier recombination pairing. Thus, in the prior art, at first glance, it was thought that the characteristics were improved by introducing a superlattice structure, but there was a limit to the reduction in operating voltage,
It is extremely difficult to fabricate a nitride-based transverse mode control semiconductor laser having good mode characteristics. Particularly, with respect to reliability characteristics (especially at high temperatures), dislocations during operation, growth of defects, Propagation was apt to occur, and the yield was not high enough for practical use.

[0006]

As described above, with the conventional nitride-based transverse mode control laser, it is very difficult to produce a laser having a low voltage and good emission characteristics due to spontaneous polarization and piezo effect, and the reliability is also low. There was a problem that it could not be sufficient. The present invention has been made in consideration of the above circumstances, and has as its object the advantage that the basic lateral mode operation can be performed with a low threshold value and a low operating voltage, which is excellent in process reproducibility, easy in the process, and high in yield. It is an object of the present invention to provide a nitride-based transverse mode control type structure laser having good characteristics.

[0007]

The gist of the present invention is InA.
InAlG serving as a barrier layer in a nitride semiconductor laser device having a multiple quantum well structure composed of lGaN (barrier layer) / InAlGaN (well layer) or a superlattice structure
The aN layer and the InAlGaN layer serving as the well-side layer substantially match the dominant lattice constant of the constituent layers forming the semiconductor laser or the lattice constant of the substrate, or compensate for each other's strain, thereby generating cracks and piezo electric fields. the composition so as to reduce the occurrence, and setting the thickness and the multiple quantum well structure or super lattice structure _{Al x Ga 1-x N /} in z Al y Ga
_It can be formed from a pair of layers of 1-yzN (0 <x, z ≦ 1, 0 ≦ y <1), or by using this multiple quantum well structure or superlattice structure in at least the n-type cladding layer. An object of the present invention is to provide a nitride-based laser which suppresses the residual strain, prevents the influence of a piezo electric field and spontaneous polarization, has a low voltage, is highly reliable, and has a high luminous efficiency and a high gain as a light emitting layer.

According to the present invention, the influence of strain can be essentially reduced, the resistance of the superlattice layer can be reduced, the process reproducibility is excellent, the process is easy, and the yield is high. Provided is a nitride-based lateral mode control type laser having good characteristics and capable of performing a basic lateral mode operation at a low threshold and a low 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.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. FIG. 1 is for explaining a schematic configuration of a blue semiconductor laser device according to a first embodiment of the present invention. All the nitride layers were grown by MOCVD (metal organic chemical vapor deposition). Regarding growth conditions,
The pressure is normal pressure, the GaN and AlGaN layers other than the buffer layer are basically in the range of 1000 ° C. to 1100 ° C. in an atmosphere in which nitrogen, hydrogen and ammonia are mixed, and the growth including the active layer is 700 ° C. in the nitrogen and ammonia atmosphere. To 850 ° C. In the figure, 11 is a sapphire substrate, and 12 is a buffer layer (0.03 μm) grown at low temperature (550 ° C.).
It is. 14 is grown at a high temperature (1100 ° C.), undoped at the bottom through an SiO ₂ stripe 10 for lateral growth, n-GaN contact layer at the top, 13 is an n-side electrode,
15 is an n-InAlGaN / 1.5 μm total thickness
An n-type cladding layer 16 made of a superlattice of un-GaN, 16 is an active layer portion including a multiple quantum well structure (MQW) and a light guide layer, and has an optical guide layer made of GaN having a thickness of 0.1 μm. The well layer is 4 nm thick In0.13 Ga0.8.
7N4 layer, the barrier layer is 8 nm thick In0.0
3 Ga0.97N. 17 is a p-type cladding layer made of a p-AlGaN / un-GaN superlattice having a total thickness of 0.8 μm, and 20 is a 0.5 μm P-type cladding layer.
GaN contact layer (Mg-doped)
g. p-GaN contact layer, 21
Is an SiO ₂ confinement layer, 22 is a p-type electrode, and 23 is an electrode pad. FIG. 2 shows the relationship between the composition of the layers constituting the superlattice structure related to the gist of the present invention, the lattice constant, and the band gap. When GaN is grown thick on the underlayer as in the present embodiment, the dominant lattice constant substantially matches that of GaN. Al
As shown in the figure, the lattice constant of the GaN mixed crystal becomes smaller and the lattice constant of the InGaN mixed crystal becomes larger.
When an appropriate composition is set in the aN mixed crystal, lattice matching with GaN is achieved, and a layer having a large band gap can be formed. In this embodiment, as the n-side clad superlattice layer, specifically, the thickness of both the barrier layer and the well layer is 2 nm, and only the barrier layer is 5 × 10 ^18.
cm ⁻³ , and the composition of the barrier layer is set to In.
_0.03 Al _0.15 Ga _0.82 N. In the case of the n-type, growth and doping of the quaternary layer are not problematic. In the growth of the present superlattice structure, high quality growth was achieved by setting the growth temperature in the range of 900 ° C. to 1000 ° C. and using a mixed gas of nitrogen and ammonia as the atmosphere gas. The composition of the InAlGaN mixed crystal does not necessarily have to match that of GaN.
However, the dislocations were reduced by entering only slightly, leading to higher reliability. In addition, an ordered structure at the atomic level was formed in the InAlGaN mixed crystal, which was very convenient for voltage reduction and optical confinement. Also, the p-side GaN guide layer is removed, the superlattice structure is directly formed on the MQW, and modulation such as increasing the Al composition of the superlattice close to the MQW and reducing the layer thickness is performed.
The same or higher maximum oscillation temperature was obtained by manufacturing with a design aiming for the electron reflection effect by the quantum effect.

In this embodiment, when the ridge width is 2 μm, continuous oscillation at room temperature occurs at a threshold value of 18 mA. The oscillation wavelength is 400
nm, and the operating voltage was 3.8 V. AlGaN on n side
In the case of the superlattice structure combining / GaN, the voltage is 4.
It was impossible to reduce the voltage to 5 V or less. 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 in continuous oscillation. FIG. 3 shows the reliability characteristics. 30m at 50C for reliability
W: Stably operated for over 2000 hours. In the case of the AlGaN / GaN combined superlattice structure having a comparatively large residual strain, such reliability was not obtained, and an unexpectedly improved result was obtained. These characteristics were obtained with a structure in which the substrate was bonded to a heat sink while keeping the substrate below. Regarding noise characteristics, even in the presence of return light, 10 ^-13 dB / Hz
The following characteristics were obtained. The element yield is extremely high,
The transverse mode characteristics described above were obtained with 90% of the devices.

FIG. 4 is a view for explaining a schematic configuration of a blue semiconductor laser device according to a second embodiment of the present invention. In this embodiment, all the nitride layers located above the 14 n-GaN layers are MOCVD as in the first embodiment.
(Organic metal vapor phase epitaxy). The same applies to growth conditions and the like. The difference from the first embodiment is that an n-type GaN substrate 24 grown by a hydride vapor phase epitaxy apparatus is used as the substrate. Lateral growth technology is incorporated during GaN substrate growth, and the dislocation density is 10 ⁴ c
m- ² or less. 15 is a total thickness of 1.5
The n-type cladding layer was made of a superlattice of μm n-AlGaN / un-InGaN, and the other growth layers were the same as in the first embodiment. In this embodiment, as the n-side clad superlattice layer, specifically, the thickness of both the barrier layer and the well layer is 3 nm.
Only the barrier layer was doped into an n-type of 5 × 10 ¹⁸ cm ⁻³ , and the composition of the barrier layer was set to Al _0.15 Ga _0.85 N.
The well layer was set to In _0.03 Ga _0.97 N. The distortions compensated each other, and no problems such as cracks and wafer warpage occurred. It is also possible to use an InGaN ternary substrate or an AlGaN ternary substrate containing a small amount of In as the substrate.

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 the beam characteristics were the same as those of the first embodiment, but the maximum continuous oscillation temperature could be increased to 100 ° C. 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 with the first embodiment, stable transverse characteristics were obtained with a high yield.

FIG. 5 is a view for explaining a schematic configuration of a blue semiconductor laser device according to a third embodiment of the present invention. As in the second embodiment, the GaN substrate is used. The difference in structure here is that after forming a mesa, an n-InGaN absorption layer (optical waveguide layer) 18 is formed by selective growth by regrowth.
Thereafter, the contact layer 20 is grown. Other layer structures are the same as in the second embodiment. In this embodiment, the ridge width is 3
In the case of μm, continuous oscillation at room temperature was performed at a threshold value of 35 mA. The oscillation wavelength was 390 nm and the operating voltage was 3.8V. 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 50 mW by continuous oscillation, and the device stably operated at 60 C for 2000 hours or more in terms of reliability. The optical waveguide layer formed by regrowth is AlGa
N or InAlGaN may be used, and good beam characteristics were obtained by setting the composition. InA described in this case in this part
An lGaN / GaN superlattice may be used. Further, the active layer may be etched and embedded with a high-resistance layer. In this case, by reducing the stripe width to about 1 μm, the
The threshold value can be reduced to mA.

A similar design is possible in the case of the p-type,
There was some difficulty in doping control. Mg for p-type
The operation voltage can be further reduced by making the superlattice co-doped with oxygen, Si, etc. together with the impurities, and the p-
Good device characteristics were obtained even if GaN was removed and an electrode contact was made directly to the superlattice. We consider that the voltage drop in the superlattice is mainly due to the piezo electric field and spontaneous polarization.
The field effect direction on the front side is also as illustrated. Regarding the Al composition, the composition may be graded or stepwise changed stepwise. When a superlattice is used as the light emitting layer, the same application is possible and the threshold value can be reduced. In this case as well, setting the directional superlattice for compensating the distortion is the same.
Note that the present invention is not limited to the present embodiment, and SiC or the like may be applied as a semiconductor layer or a substrate, and a III-V group compound semiconductor, a II-VI group 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. In addition, the present invention can be applied to optical device fields such as a waveguide structure, a light receiving element, and a transistor.

[0015]

As described above in detail, according to the present invention, the effect of the essential residual strain can be suppressed, the resistance of the superlattice layer can be reduced, and the process reproducibility is excellent. Abstract: Provided is a nitride-based transverse mode control type laser having good characteristics, capable of performing basic transverse mode operation at a low threshold voltage and a low operating voltage with high yield. 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 first embodiment of the present invention.

FIG. 2 is a diagram related to a first embodiment of the present invention.

FIG. 3 is a diagram related to the reliability of the first embodiment of the present invention.

FIG. 4 is a diagram related to details of a second embodiment of the present invention.

FIG. 5 is a diagram related to a third embodiment of the present invention.

FIG. 6 is a diagram showing a conventional structure.

[Explanation of symbols]

10 SiO ₂ stripe mask, 11 sapphire substrate, 12 buffer, 14 n-GaN contact layer, 13 n-side electrode, 15 n-AlGaN cladding layer, 16 multiple quantum well structure (MQW), active layer section including optical guide layer, 17 p-AlGaN cladding layer, 18 InGaN absorption layer (optical waveguide layer), 20 p-GaN contact layer, 21 SiO ₂ confinement layer, 22 p-type electrode, 23 electrode pad, 24 n-GaN substrate.

Claims (3)

[Claims] At least InAl formed on a substrate
InAlGa serving as a barrier layer in a nitride semiconductor laser device having a multiple quantum well structure composed of GaN (barrier layer) / InAlGaN (well layer) or a superlattice structure
The N layer or the InAlGaN layer serving as the well-side layer substantially matches the dominant lattice constant of the constituent layer forming the semiconductor laser or the lattice constant of the substrate, or compensates for each other, thereby generating cracks and piezo electric fields. A nitride-based semiconductor laser device characterized in that the composition and the thickness are formed so as to reduce the occurrence of occurrence. Wherein said multiple quantum well structure or super lattice structure _{Al x Ga 1- x N / In} z Al y Ga
_2. The nitride-based semiconductor laser device according to claim 1, wherein the nitride-based semiconductor laser device is formed of a pair layer made of 1 _-yzN (0≤x, y, z <1). 3. The nitride semiconductor laser device according to claim 1, wherein said multiple quantum well structure or superlattice structure is used for at least an n-type cladding layer.