JP2661563B2 - Semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to a quantum well semiconductor laser.

[0002]

2. Description of the Related Art Conventionally, in a quantum well type semiconductor laser, an active layer in which a barrier layer and a quantum well layer are sequentially stacked, and an active layer having a forbidden band width larger than the forbidden band width of the active layer, is used. An n-type clad layer and a p-type clad layer disposed so as to sandwich the layers are formed on a semiconductor substrate.

One method of improving the characteristics of this type of semiconductor laser is to introduce distortion into the active layer.
An example of a semiconductor laser using this method is “Struct.
ures for improved 1,5μm w
average length growers by
LP-OMVPE; InGaAs-InP stra
ined-layer quantum wells
a good candidate "(PJAT
hijs, E .; A. Montie and T.M. van
Dongen, Journal of Crystal
l Growth 107 (1991); 731
740).

In this semiconductor laser, a compression strain layer is used as a quantum well layer. In this case, if the strain of the quantum well layer is increased to further improve the characteristics of the semiconductor laser, the limit of the critical film thickness is exceeded, and defects are introduced into the crystal.

As one method for solving the problem, strains are introduced into the quantum well layer and the barrier layer in opposite directions, respectively.
There is a strain compensation structure that makes the average of the strain amount zero in the entire active layer including the quantum well layer and the barrier layer.

An example of a semiconductor laser having this distortion compensation structure is as follows.
"Strain-compensated strain
ned-layer superlattics f
or1.5μm wavelength laser
s "(BI Miller, U. Koren, M.S.
G. FIG. Young, and M.S. D. Chien, App
l. Phys. Lett. 58 (18), 6 May
1991, p. 1952-1954). In this semiconductor laser, since the limit of the critical film thickness is not exceeded, no defect is introduced into the crystal.

[0007]

In the conventional quantum well type semiconductor laser described above, in order to further improve the characteristics of a semiconductor laser using a compression strained layer as a quantum well layer of an active layer, a quantum well layer and a barrier layer are required. The strain in the quantum well layer is increased by using a strain compensation structure in which strains in opposite directions are respectively introduced to make the average of the strain amount zero in the entire active layer.

However, in this strain compensation structure, the difference in the amount of strain between the quantum well layer and the barrier layer is large. As the process proceeds, the barrier layer loses flatness, and a good strain compensation structure cannot be realized.

Accordingly, an object of the present invention is to provide a semiconductor laser which can solve the above-mentioned problems and realize a good strain compensation structure without losing flatness of a barrier layer during epitaxial growth of an active layer. It is in.

[0010]

A semiconductor laser according to the present invention has an active layer in which a barrier layer and a quantum well layer are sequentially stacked, a forbidden band width larger than the forbidden band width of the active layer, and A semiconductor laser in which an n-type cladding layer and a p-type cladding layer disposed so as to sandwich the active layer are formed on a semiconductor substrate, wherein the quantum well layer has a strain in a stacking direction.
The distribution is such that the volume changes continuously and the average is undistorted.
It is configured to obtain .

[0011]

[0012]

In the semiconductor laser of the present invention, of the barrier layer and the quantum well layer constituting the active layer, the quantum well layer is internally strain-compensated, and the quantum well layer itself is strain-free. For this reason, it is not necessary to introduce distortion for distortion compensation into the barrier layer, and a layer without distortion can be used. Therefore,
It is possible to avoid the problem that it is difficult to stack flat at the atomic level with a multi-element mixed crystal containing distortion,
A good strain quantum well structure can be realized.

In general, in a quantum well structure, it is necessary to set the forbidden band width of the barrier layer to be intermediate between the forbidden band width of the quantum well and the forbidden band width of the cladding layer.
This is because electrons and holes are confined in the quantum well, electrons and holes do not flow into the cladding layer, and the refractive index of the entire active layer is made larger than that of the cladding layer to form an optical waveguide.

Therefore, a multi-element mixed crystal having a higher degree of freedom in adjustment than the quantum well layer is used for the barrier layer. For example, a multi-crystal of a III-V compound semiconductor composed of a plurality of V groups is used for the barrier layer.

In order to epitaxially grow a strained ultrathin film for compensating strain in the quantum well layer, MOVPE (Metal-organics) having a high degree of non-equilibrium is required.
Vapor Phase Epitaxy method or gas source MBE (Molecular Beam Ep)
Although an itaxy) method is used, there is a problem that it is difficult to stack flat at the atomic level in a multi-element mixed crystal containing strain, so that the interface is not flat and a good strained quantum well structure is required. It is hard to realize.

On the other hand, in the present invention, since the strain is compensated inside the quantum well layer, an improvement in the light emission characteristics due to the strain can be expected, and there is no problem in epitaxial growth because the barrier layer has no strain. A good strain quantum well structure can be realized.

[0017]

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention.
In the figure, a semiconductor laser crystal includes an n-type InP substrate 1,
an n-type InP cladding layer 2, a 15-nm-thick InGaAsP barrier layer 3 having a bandgap wavelength of 1.15 μm composition, a quantum well layer 4 interposed between the barrier layers 3, and a p-type I
nP cladding layer 7 and p-type InGaAs contact layer 8
Are sequentially laminated. In this semiconductor laser crystal, a multilayer film having one cycle of the barrier layer 3 and the quantum well layer 4 is formed in four cycles, and these form an active layer.

Here, the quantum well layer 4 has a thickness of 2 nm and is 1.
A main quantum well layer 5 of In _0.8 Ga _0.2 As having a compressive strain of 84%, a thickness of 1.5 nm sandwiching the main quantum well layer 5 and having a thickness of 1.5 nm;
And a sub quantum well layer 6 of In _0.35 Ga _0.65 As having a tensile strain of 1.22%.

Therefore, in the quantum well layer 4, since the main quantum well layer 5 having the compressive strain and the sub quantum well layer 6 having the tensile strain arranged above and below the compressive strain cancel each other out, the strain is apparently no strain. It has become. Therefore, a strain-free InGaAsP mixed crystal can be used for the barrier layer 3, and a flat, high-quality quantum well structure at the atomic layer level can be formed.

Actually, when a conventional strain compensation structure is formed by a gas source MBE method using a strained InGaAsP layer, a pattern corresponding to three-dimensional growth (roughness) of a growth layer is observed by high-speed electron beam diffraction. You. On the other hand, when the strain compensating structure according to one embodiment of the present invention was manufactured, it was observed by high-speed electron diffraction that a flat surface was formed at the atomic layer level. Sex can be confirmed.

FIG. 2 is a schematic diagram of an energy band around a quantum well of a semiconductor laser crystal according to one embodiment of the present invention. In the figure, the band structure of the semiconductor laser crystal having the above configuration is shown for one quantum well layer 4 and two barrier layers 3 sandwiching the quantum well layer 4.

In the quantum well layer 4 and the two barrier layers 3, a conduction band 11 and a valence band (heavy hole band) 12
And a valence band (light hole band) 13 are formed in a convex shape so as to face each other.

In the main quantum well layer 5 of the quantum well layer 4, the valence band edge becomes a heavy hole edge due to compressive strain, and in the sub quantum well layer 6, the valence band edge becomes a light hole edge due to tensile strain. .

According to the energy distribution near the quantum well layer 4 (conduction band 11 and valence bands 12, 13), electron distribution 14, heavy hole distribution 15, and light hole distribution 1
6 are formed.

In FIG. 2, the electron distribution 14, the heavy hole distribution 15, and the light hole distribution 16 are conveniently shown so as to show sub-band energy levels at positions where the distribution is zero in the barrier layer 3 distant from the quantum well layer 4. It is written.

The energy level difference between the heavy hole and the light hole is 85 meV, which is sufficient for separation, and the transition between the level between the electron and the heavy hole is the minimum energy and the wavelength is 1560.
It has energy corresponding to nm and oscillates at a wavelength corresponding to this. Therefore, the oscillation mode is the TE mode.

Here, in the sub quantum well layer 6, the conduction band energy edge is located at a lower energy than the conduction band energy edge of the barrier layer 3, and the valence band edge (heavy hole) is the valence electron energy of the barrier layer 3. The composition of InGaAs which is located between the band edge and the valence band edge of the main quantum well layer 5 and further satisfies the condition of having a tensile strain is, for example, In _0.35 Ga _0.65
The composition of As is used, and another composition may be used.

More specifically, the condition is that the band structure in which the charge carriers involved in the minimum energy transition contributing to the laser oscillation have no maximum value or minimum value in the sub quantum well layer 6. This condition facilitates the injection of charge carriers into the main quantum well layer 5 and is necessary for the oscillation characteristics of the semiconductor laser.

Although the structure of the semiconductor laser is not particularly shown in the embodiment of the present invention, a planar type, SAS (Se
If Aligned Strip) type, BH (Bu
ride Hetero) type, DCPBH (Doubl)
e Channel Planar Burried H
The present invention can be applied to a commonly used structure such as an (etero) type.

FIG. 3 is a sectional view of another embodiment of the present invention. In the figure, the semiconductor laser crystal is an n-type InP substrate 2
1, an n-type InP cladding layer 22, and InGaAsP having a layer thickness of 15 nm and a bandgap wavelength of 1.15 μm.
Barrier layer 23, a quantum well layer 24 interposed between the barrier layers 23, a p-type InP clad layer 25, and a p-type InGaAs
The s-contact layer 26 is sequentially laminated. In the semiconductor laser crystal, a multilayer film having one cycle of the barrier layer 23 and the quantum well layer 24 is formed four times, and these form an active layer.

Here, the quantum well layer 24 has a thickness of 2.5 at the center.
% Of In _0.9 Ga _0.1 As having a compressive strain of 1.7% and In _0.28 Ga _0.72 As having a tensile strain of 1.7% at the peripheral portion. The distortion and the distortion cancel each other, and the distortion is apparently no distortion.

FIG. 4 is a schematic diagram of an energy band around a quantum well of a semiconductor laser crystal according to another embodiment of the present invention. In the figure, the band structure of the semiconductor laser crystal having the above configuration is shown for one quantum well layer 24 and two barrier layers 23 sandwiching the quantum well layer 24.

In the quantum well layer 24 and the two barrier layers 23, the conduction band 31 and the valence band (heavy hole band) are formed.
The cross sections are each formed in a triangular convex shape so that the 32 and the valence band (light hole band) 33 face each other.

At the center of the quantum well layer 24, the valence band edge becomes a heavy hole edge due to compressive strain, and at the peripheral portion, the valence band edge becomes a light hole edge due to tensile strain.

According to the energy distribution near the quantum well layer 24 (conduction band 31 and valence bands 32 and 33),
An electron distribution 34, a heavy hole distribution 35, and a light hole distribution 36 are formed.

In FIG. 4, the electron distribution 34, the heavy hole distribution 35, and the light hole distribution 36 are conveniently shown so as to show the subband energy levels at positions where the distribution becomes zero in the barrier layer 23 remote from the quantum well layer 24. It is written.

The energy level difference between the heavy hole and the light hole is sufficiently large, and the transition between the level between the electron and the heavy hole is the minimum energy transition, so that oscillation occurs in the TE mode.

Here, the composition of the quantum well layer 24 is changed from In _0.9 Ga _0.1 As having a compressive strain of 2.5% at the center to In _0.28 Ga _0.72 As having a tensile strain of 1.7% at the periphery. Uses InGaAs that changes continuously.

In other words, the conduction band energy edge of the peripheral portion is located at a lower energy than the conduction band energy edge of the barrier layer 23,
The In _0.9 As an example the composition of satisfying InGaAs called valence band edge (heavy holes) is positioned intermediate the valence band edge of the valence band edge and the central portion of the barrier layer 23, having a further tensile strain Ga _0.1 As to In _0.28
The composition of Ga _0.72 As is used, and another composition may be used.

More specifically, the band structure in which the charge carriers involved in the minimum energy transition contributing to the laser oscillation have a maximum value in the valence band and a minimum value in the conduction band in the center of the quantum well layer 24. Is a condition. This condition facilitates the injection of charge carriers into the quantum well layer 24 and is necessary for the oscillation characteristics of the semiconductor laser.

Although the structure of the semiconductor laser is not particularly shown in the embodiment of the present invention, a planar type, a SAS type,
The present invention can be applied to commonly used structures such as H type and DCPBH type.

For the semiconductor laser according to the present invention, any method may be used as long as it is an epitaxial growth method capable of steep interfaces and precise composition control. Here the gas source MB
The case of making the epitaxial growth by the E method will be described below.

First, the n-type InP substrates 1 and 21 having the (100) plane are heated to 500 ° C. while irradiating phosphine gas thermally dissociated at 950 ° C.
The n-type InP cladding layers 2 and 22 are grown by irradiating with Si as a type dopant.

Next, In and Ga are irradiated while simultaneously irradiating arsine and phosphine to grow the barrier layers 3 and 23. Thereafter, similarly, group III elements such as In and Ga
And a composition for growing phosphine and arsine, which are group V materials, that is, a composition formed in the order of the sub-quantum well layer 6, the main quantum well layer 5, and the sub-quantum well layer 6, or In at the center.
Irradiation in accordance with the composition from _0.9 Ga _0.1 As to peripheral In _0.28 Ga _0.72 As allows quantum well layers 4 and 24 to be irradiated.
Is formed.

For lateral confinement of current, a confinement structure by buried growth can be used. When the composition is continuously changed, the desired composition change is obtained by controlling the temperatures of the In and Ga K cells to change the amount of molecular beam.

In the above-described embodiment, the quantum well layers 4 and 24 have compositions that oscillate around a wavelength of 1.5 μm.
In addition to nGaAs, a combination of InAsP and InGaAs, InAlAs, or the like may be used as long as the structure can compensate for the strain in the quantum well layers 4 and 24.

In the above-described embodiment, a group III-V compound semiconductor having a composition lattice-matched to InP is used.
The present invention is also applicable to a short-wave or visible semiconductor laser made of a III-V compound semiconductor having a composition lattice-matched to s. For example, a quantum well layer having a similar composition change for InGaP may be used.

Further, in the above-described embodiment, all layers are lattice-matched to InP. However, in order to obtain a required band structure, a composition of III-
Any group V compound semiconductor may be used if it is within the critical film thickness. For example, the barrier layers 3 and 23 need not be completely strain-free, but may have strain within a critical film thickness if there is no problem such as non-planarization during crystal growth.

As described above, the quantum well layer 4 is provided so as to sandwich the main quantum well layer 5 having the strain within the critical thickness and the main quantum well layer 5 so as to compensate for the strain of the main quantum well layer 5. And the auxiliary quantum well layer 6 having a strain within a critical film thickness for making the quantum well layer 4 unstrained, which eliminates the necessity of stacking a strained barrier layer during epitaxial growth. Without lowering
The apparently strain-free quantum well layer 4 can be formed.

In other words, a good strain compensation structure can be realized without losing the flatness of the barrier layer 3 at the time of epitaxial growth of the active layer. Therefore, excellent high-temperature operation characteristics and high-speed modulation characteristics of the strained quantum well structure can be obtained. A semiconductor laser having the same can be obtained.

Further, the quantum well layer 24 is formed such that the direction of the strain in the central portion and the direction of the strain in the peripheral portion of the distribution in which the amount of strain changes continuously in the stacking direction are different from each other, so that the average of the distribution is unstrained. With such a configuration, it is not necessary to stack a barrier layer having strain during epitaxial growth, so that the apparently strain-free quantum well layer 4 can be formed without deteriorating the crystal quality.

In other words, a good strain compensation structure can be realized without losing the flatness of the barrier layer 3 during the epitaxial growth of the active layer, so that the strained quantum well structure provides excellent high-temperature operation characteristics and high-speed modulation characteristics. A semiconductor laser having the same can be obtained.

[0054]

As described above, according to the semiconductor laser of the present invention, in an active layer in which a barrier layer and a quantum well layer are sequentially laminated, the quantum well layer having a strain within a critical thickness is used. A quantum well layer and first and second sub-quantum well layers disposed so as to sandwich the main quantum well layer and having a strain within a critical thickness for compensating for the strain in the main quantum well layer are formed. By doing so, there is an effect that a good strain compensation structure can be realized without losing the flatness of the barrier layer during the epitaxial growth of the active layer.

According to another semiconductor laser of the present invention, the direction of strain in the central portion and the direction of strain in the peripheral portion of the strain distribution in which the amount of strain continuously changes in the stacking direction is changed. By adopting a configuration in which the average is strain-free so as to be different from each other, there is an effect that a good strain compensation structure can be realized without losing the flatness of the barrier layer during the epitaxial growth of the active layer.

[Brief description of the drawings]

FIG. 1 is a sectional view of one embodiment of the present invention.

FIG. 2 is a schematic diagram of an energy band around a quantum well of a semiconductor laser crystal according to one embodiment of the present invention.

FIG. 3 is a sectional view of another embodiment of the present invention.

FIG. 4 is a schematic diagram of an energy band around a quantum well of a semiconductor laser crystal according to another embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1,11 n-type InP substrate 2,22 n-type InP cladding layer 3,23 barrier layer 4,24 quantum well layer 5 main quantum well layer 6 sub-quantum well layer 7,25 p-type InP cladding layer 8,26 p-type InGaAs Contact layer

Claims (3)

(57) [Claims] 1. An active layer having a barrier layer and a quantum well layer sequentially stacked, and an active layer having a forbidden band width larger than a forbidden band width of the active layer and disposed so as to sandwich the active layer. A semiconductor laser in which an n-type cladding layer and a p-type cladding layer are formed on a semiconductor substrate, wherein the quantum well layer has a distribution in which the amount of strain continuously changes in the stacking direction and the average has no strain. A semiconductor laser characterized by having. Wherein said distribution is a semiconductor laser according to claim 1, wherein the direction of the distortion in the direction of the periphery of the distortion of the central portion, characterized in that respectively different. 3. The barrier layer according to claim 1, wherein the barrier layer comprises a plurality of V-groups.
Claim 1 or claim 2 The semiconductor laser according characterized in that it is a multiple mixed crystal of I -V compound semiconductor.