JP2833396B2 - Strained multiple quantum well 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 used in optical communication and optical information processing fields.

[0002]

2. Description of the Related Art In recent years, the demand for semiconductor lasers has been increasing in many fields, and research and development have been actively promoted mainly on GaAs and InP systems. Among them, MBE, MOVPE and other semiconductor laser crystal growth technologies have made great progress, and extremely thin layers such as quantum wells can be fabricated with good reproducibility and uniformly over a large area. These quantum wells are used for the active layer of a semiconductor laser, and it has been demonstrated that the power consumption of the laser can be extremely reduced by the quantum size effect. At present, some of them have been commercialized. More recently, it has been theoretically or experimentally shown that the luminous efficiency is increased by intentionally displacing the lattice constant of the quantum well layer from the substrate and distorting it, thereby enabling lower power consumption. ing.

FIGS. 9A to 9C show light intensity distributions corresponding to the refractive index distribution of the active layer of each semiconductor laser. Generally, in a quantum well laser, the well layer used for the active layer is very thin, 10 nm or less. Therefore, as shown in FIG. The ratio of being confined in the layer (hereinafter, the light confinement coefficient) is much smaller than that of the conventional double hetero type semiconductor laser (a) using a thick active layer. Spontaneous emission is necessary to cause the population inversion of the active region for laser oscillation. If the light confinement coefficient is small, even if the luminous efficiency increases due to the quantum well or the strained quantum well,
Unless larger spontaneous emission light is caused, sufficient light cannot be confined in the active region, and the original properties of the quantum well layer cannot be fully utilized. Therefore, as shown in FIG. 9C, a method of increasing the optical confinement coefficient by using an active layer as a multiple quantum well has been adopted, and a multiple quantum well laser with extremely low power consumption has been realized. It is shown in FIG.

FIG. 10 shows a strained multiple quantum well semiconductor laser in which the active layer has a strained multiple quantum well structure. This structure is n-
On the GaAs substrate 101, a p-AlGaInP cladding layer 4a, a strained quantum well layer 3, and a p-AlGaInP cladding layer 4b are grown, and the cladding layer 4b is etched in a stripe shape and buried with an n-GaAs current blocking layer 102. Furthermore, p-GaAs
The contact layer 103 is grown. Finally n-Ga
As substrate 101, cathode electrode 105, contact layer 10
3, an anode electrode 106 is formed. Since the active layer has a strained multiple quantum well structure, it is composed of a barrier layer having the same lattice constant as the substrate and a well layer having a lattice constant larger than that of the substrate.

FIG. 11A shows the state of the lattice of the n-cladding layer 4a, the strained multiple quantum well 3, and the p-cladding layer 4b in the structure of the conventional strained multiple quantum well semiconductor laser. As shown, the well layer 111 in the strained multiple quantum well 3
Has a larger lattice constant than the barrier layer 112 and the cladding layers 4a4b, and thus has a compressive strain as shown in FIG. Then, tensile strain occurs in the barrier layer 112. The energy gap changes as shown in FIG. 11 (c) due to strain in the well layer 111 and the barrier layer 112.

[0006]

Since the optical confinement coefficient of the strained quantum well laser is also small as described above, multiplexing has been attempted and a strained multiple quantum well has been used. If the distortion is large, the expected multiplexing effect may not be obtained.

FIG. 11 shows an enlarged sectional view of the n-clad layer 4a, the strained quantum well layer 3, and the p-clad layer 4b of FIG. As shown in FIG. 11A, the strained quantum well layer 3 includes a well layer 111 and a barrier layer 112. Here, the well layer 111 has a larger lattice constant than the substrate, and the barrier layer 112 has the same lattice constant as the substrate. The cladding layers 4a and 4b have the same lattice constant as the substrate. Therefore, as shown in FIG. 11A, the barrier layer 112 is surrounded by layers (cladding layers and barrier layers) having smaller lattice constants on both sides thereof, so that the barrier layer 112 becomes smaller than its own lattice constant. ,
As a result, they are subject to compressive strain. Conversely, the barrier layer 112 is a well layer 1 having a larger lattice constant than itself.
11, tensile strain occurs. This is shown in FIG.

As shown in FIG. 11B, a compressive strain is generated in the well layer 111 and a tensile strain is generated in the barrier layer 112. This has various adverse effects on the production and operation of the laser.

As described above, the lattice constant of the strained multiple quantum well layer 3 does not completely coincide with the substrate. As the number of layers increases, the lattice in which the well layer 111 and the barrier layer 112 are pulled and balanced with each other. (Generally called a free-standing lattice constant). Accordingly, since the strain is present on the strained multiple quantum well layer 3 on average, dislocation defects occur when the total film thickness exceeds a certain value (critical film thickness). For this reason, the strained multiple quantum well 3 must be formed in the active layer of the laser so as not to exceed the critical film thickness.

[0010] Further, since the lattice is fixed at both ends of the strained multiple quantum well by the thick cladding layer 4 lattice-matched to the substrate, the magnitude of the strain is distributed as shown in FIG.
As a result, the energy gap becomes non-uniform as shown in (c), and the luminous efficiency of the laser decreases.
Moreover, when tensile strain is applied to the barrier layer 112, the well layer 1
Since the confinement of carriers into the luminous efficiency is reduced, the luminous efficiency is further reduced.

In addition, in GaInP / AlGaInP, InGaAsP / InP, and the like, the effective mass of carriers is several times larger in holes than in electrons. Then, as shown in FIG. 12, holes are distributed only up to about 100 meV from the valence band edge of the well layer, but electrons are distributed over a wide energy range up to about 200 meV. In addition, the ratio of the band offset (barrier barrier) between the conduction band and the valence band when there is no distortion is about 4: 6, which is larger in the valence band, so a band gap material that can sufficiently confine electrons should be used. When used as a barrier layer, a valence band barrier becomes unnecessarily large.

If the quantum wells are multiplexed because the confinement of holes becomes too strong, the diffusion of holes having a large mass is suppressed, and the non-uniformity of the hole density in each well layer becomes remarkable, so that the efficiency of the laser is increased. There is also a problem of lowering the image quality.

As described above, in the conventional strained multiple quantum well laser, (1) a critical thickness exists in the strained multiple quantum well, and even if the number of well layers and barrier layers is increased to increase the multiplexing number. Can not. (2) The strain distribution of the strained quantum well is not uniform, and the energy gap is uneven. (3) An uneven distribution of the injected carrier density among the quantum wells can be obtained.

For these reasons, the characteristics of the semiconductor laser have deteriorated. The present invention has been made in view of the above problems, and has as its object to provide a highly efficient strained multiple quantum well laser having a strained multiple quantum well having uniform and strong strain.

[0015]

In order to achieve the above object, a strained multiple quantum well semiconductor laser according to the present invention comprises: (1) a compound semiconductor substrate and a first cladding layer formed on the compound semiconductor substrate. And an active layer having a strained multiple quantum well structure formed on the first cladding layer, in which well layers and barrier layers having opposite strains are alternately stacked, and a second active layer formed on the active layer. Wherein the free-standing lattice constant of the strained multiple quantum well, which is determined by the lattice constant, the layer thickness, and the elastic constant of the well layer and the barrier layer, substantially matches the compound semiconductor substrate. It is a multiple quantum well semiconductor laser. (2) free standing lattice constant of a strained multiple quantum _{well, a 0 = (a 1 G} 1 d 1 + a 2 G 2 d 2) / (G 1 d 1 + G 2 d 2) (a1, d1 each well layer The lattice constant and layer thickness of
a2 and d2 indicate the lattice constant and the layer thickness of the barrier layer, respectively.
G _1, G ₂ each _{G = 2 (C 11 + C} 12) by the elastic constants C ₁₁ and C ₁₂ of the material constituting the well layer and the barrier layer (1-C
₁₂ / C ₁₁ ). (3) A strained multiple quantum well semiconductor laser in which tensile strain is applied to a barrier layer in a strained multiple quantum well of an active layer.

[0016]

According to the first and second means, the strain in each well layer becomes uniform, and the energy gap in each well layer becomes equal. Therefore, the strained multiple quantum well semiconductor laser of the present invention can obtain a high optical gain.

Further, the third means can reduce the valence band barrier while keeping the conduction band barrier sufficiently large, so that diffusion of holes is likely to occur and uniform carrier density can be obtained in each well.

[0018]

(Embodiment 1) FIG. 1 is a sectional view of a cladding layer and a strained multiple quantum well in a semiconductor laser structure according to a first embodiment. Except for this part, the structure is the same as that of FIG. The compositions of the cladding layer, the well layer, and the barrier layer, and the lattice mismatch with the substrate are described in FIGS.

In the first embodiment, the well layer 11 has a thickness of 5n.
m _0.44 In _0.56 P, the barrier layer 12 is 5 nm (Al _0.45 G
a _0.55 ) _{Consists of 0.57} In _0.43 P. In this case, the well layer 11 and the barrier layer 12 have a compressive strain of 0.5% and a tensile strain of 0.5%, respectively, due to lattice mismatch with the GaAs substrate. This is different from the conventional strained multiple quantum well semiconductor laser in that a strain opposite to that of the well layer is applied to the barrier layer. However, the well layers and barrier layers, lattice constant a ₀ in the free-standing of the strained MQW 3 shown by the following formula satisfies the condition to match the lattice constant of the substrate.

A ₀ = (a ₁ G ₁ d ₁ + a ₂ G ₂ d ₂ ) / (G ₁ d ₁ + G ₂ d ₂ ) where a and d are lattice constants, respectively, and the subscript 1 is the well thickness. Layer 1
Reference numerals 1 and 2 indicate values of the barrier layer 12. G is a constant determined from the elastic constants C ₁₁ and C ₁₂ of each material, and can be obtained from the following equation. G = 2 (C ₁₁ + C ₁₂ ) (1-C ₁₂ / C ₁₁ ) Under the above conditions, even if the strained multiple quantum well is grown on the n-cladding layer 4a (the same lattice constant as the substrate), Since the lattice constant of the free standing of the strained multiple quantum well created under these conditions is the same as that of the substrate (cladding layer), the strain applied to the strained multiple quantum well 3 on average is zero,
Even when epitaxial growth is performed over an infinite period, a crystal of good quality can be produced without generating dislocation defects or the like. Even if this is used as an active layer of a semiconductor laser, an infinite number of well layers can be similarly formed. A strained quantum well semiconductor laser having good characteristics has an optimum multiplex number of well layers and barrier layers. According to the present embodiment, the optimum multiplex number can be adjusted, and a semiconductor laser having good characteristics can be obtained. Can be manufactured.

FIG. 2 shows the structure of the strained multiple quantum well, and the distribution of the strain and the energy gap thereof. Fig. 2 (a)
As described above, since the compressive strain of the well layer 11 and the tensile strain of the barrier layer 12 are in balance at the lattice constant of the substrate, the amount of strain applied to the well layer 11 becomes almost uniform as shown in FIG. And the optical gain, which is a conventional problem, can be prevented.

Further, when tensile strain is applied to the barrier layer 12, the band edge of the valence band becomes a sub-band of the light hole and the mass of the hole can be reduced, so that a problem arises when the number of well layers is increased. Non-uniformity of injected carrier density can also be suppressed, and good laser characteristics can be obtained.

Although the present embodiment uses the GaInP / AlGaInP system, it goes without saying that the same applies to semiconductor lasers of any material. The same applies to the case where tensile strain is applied to the well layer and compressive strain is applied to the barrier layer.

Embodiment 2 FIG. 3 is a sectional view of a strained multiple quantum well of a strained multiple quantum well semiconductor laser according to a second embodiment. 10 is the same as FIG. 10 except for the clad layer, well layer, and barrier layer shown in FIG. 7 and 8
Describes the composition of each layer and the lattice mismatch rate.

FIG. 4 shows the strained multiple quantum well 3 and the cladding layers 4a and 4a.
2 shows the structure of b. The well layer 31 is made of Ga _0.44 In having a thickness of 5 nm.
_0.56 P, barrier layer 32 is 5 nm (Al _0.50 Ga _0.50 ) _0.44 In _0.56
Consists of P. (b) From well layer 31, barrier layer 3
It can be seen that each of the samples No. 2 was in a state where a compression strain of 0.5% was applied.

In this case, since the free standing described in the first embodiment does not coincide with the substrate in the strained multiple quantum well 3, there is a critical film thickness at which dislocation defects occur. Therefore, the film thickness of the entire strained multiple quantum well 3 must be less than the critical film thickness. Therefore, in this embodiment, the number of wells is 3
However, this is because the well layer 31 and the barrier layer 32
It varies depending on the thickness of the film and the magnitude of the applied strain.

Since both the well layer 31 and the barrier layer 32 have the same amount of strain, the two layers do not pull each other and have a uniform strain distribution. This 2
Since each layer has a larger lattice constant than the substrate, it has the same amount of compressive strain with respect to the substrate. Therefore, a large strain can be introduced into the strained quantum well. However, it should be noted here that the thickness of the strained quantum well is set to a critical thickness or less. Otherwise, dislocations will enter the strained quantum well, and the characteristics of the semiconductor laser will be degraded.

As described above, in the second embodiment, a high optical gain can be obtained as in the first embodiment.

In this embodiment, the same applies to semiconductor lasers of any material. Further, contrary to the present embodiment, the same applies when tensile strain is applied to both the well layer 31 and the barrier layer 32 and the thickness is set to be equal to or less than the critical thickness.

(Embodiment 3) FIG. 5 is a sectional view showing a strained multiple quantum well of a strained multiple quantum well semiconductor laser according to a third embodiment. The structure other than this layer is the same as that of FIG. The compositions and lattice mismatch rates are described in FIGS.

The well layer 51 is Ga _0.51 In _0.49 P having a thickness of 5 nm, the barrier layer 52 is composed of _{(Al 0.50 Ga 0.60) 0 .44} In 0.56 P of 5 nm. This structure is characterized in that the barrier layer 52 has a compressive strain. When compressive strain e is applied, the energy at the band edge of the conduction band and the valence band of the barrier layer 52 (see FIG. 6A) changes by ΔE _C and ΔE _V respectively according to the hydrostatic pressure component of the strain as shown in the following equation. I do.

ΔE _C = a _c [(C ₁₁ -C ₁₂ ) / C ₁₁ ] e δE _V = a _v [(C ₁₁ -C ₁₂ ) / C ₁₁ ] e where a _c and a _v are conduction bands , The deformation potential of the valence band. A _c for ordinary compound semiconductors
There the big example GaInP a _c is greater 4 times the a _v than a _v.

FIG. 6 shows the energy gap when there is no distortion (a) and when the barrier layer is subjected to compressive strain (b). As is clear from the figure, (b) shows that the barrier in the conduction band 5 is relatively large and the barrier in the valence band 6 is relatively small. As a result, even if the band gap of the barrier layer is reduced, the conduction band 5 is almost enough to confine electrons in 150 me.
A V barrier is obtained, while the valence band 6 can be suppressed to about 150 meV, which does not hinder the diffusion of holes.
As a result, even if the number of wells is increased, carriers are uniformly distributed in each well, so that low loss and high optical gain can be obtained.

When the tensile strain is applied only to the well layer, the same barrier and barrier as in this embodiment can be obtained. When both are combined, tensile strain is applied to the well layer and compressive strain is applied to the barrier layer. Be effective.

[0035]

As described above, according to the present invention,
A strained multiple quantum well semiconductor laser having good optical characteristics such as a low threshold current can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view showing the configuration of a strained multiple quantum well according to a first embodiment.

FIG. 2 is a diagram showing distributions of lattice constants, strains, and energy gaps according to the first embodiment.

FIG. 3 is a configuration sectional view of a strained multiple quantum well according to a second embodiment.

FIG. 4 is a diagram showing distributions of lattice constant, strain, and energy gap according to a second embodiment.

FIG. 5 is a sectional view showing the configuration of a strained multiple quantum well according to the third invention.

FIG. 6 is a diagram showing an energy gap according to a third embodiment.

FIG. 7 is a diagram showing compositions of a clad layer, a barrier layer, and a well layer in each embodiment.

FIG. 8 is a diagram showing a lattice mismatch ratio between a clad layer, a barrier layer, and a well layer of a substrate according to each embodiment.

FIG. 9 is a diagram showing light confinement due to the difference in the structure of the active layer.

FIG. 10 is a sectional view showing the configuration of a strained multiple quantum well semiconductor laser.

FIG. 11 is a diagram showing distributions of lattice constant, strain, and energy gap in a conventional strained multiple quantum well.

FIG. 12 is a diagram showing energy distribution of electrons and holes.

[Explanation of symbols]

Reference Signs List 3 Strained multiple quantum well 4an n-cladding layer 4b p-cladding layer 5 conduction band 6 valence band 7 electron energy distribution 8 hole energy distribution 11 Ga _0.44 In _0.56 P strained quantum well layer 12 (Al _0.45 Ga _0.55 ) _0.57 In _0.43 P strain barrier layer 31 Ga _0.44 In _0.56 P strain quantum well layer 32 (Al _0.50 Ga _0.50 ) _0.44 In _0.56 P strain barrier layer 51 Ga _0.51 In _0.49 P strain quantum well layer 52 (Al _0.50 Ga _0.50 ) _0.44 In _0.56 P strain barrier layer 111 Ga _0.44 In _0.56 P well layer 112 (Al _0.45 Ga _0.55 ) _0.51 In _0.49 P barrier layer

Continuation of the front page (56) References JP-A-5-267784 (JP, A) JP-A-3-21093 (JP, A) JP-A-4-22185 (JP, A) (58) Fields investigated (Int) .Cl. ⁶ , DB name) H01S 3/18 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A compound semiconductor substrate, a first cladding layer formed on the compound semiconductor substrate, and a well layer and a barrier layer formed on the first cladding layer and having strains opposite to each other. An active layer having a strained multiple quantum well structure and a second cladding layer formed on the active layer, and are determined by a lattice constant, a layer thickness, and an elastic constant of the well layer and the barrier layer. Wherein the free standing lattice constant of the strained multiple quantum well is substantially equal to that of the compound semiconductor substrate. 2. A free-standing lattice constant of a strained multiple quantum _{well, a 0 = (a 1 G} 1 d 1 + a 2 G 2 d 2) / (G 1 d 1 + G 2 d 2) (a1, d1 each The lattice constant and layer thickness of the well layer;
a2 and d2 indicate the lattice constant and the layer thickness of the barrier layer, respectively.
G _1, G ₂ each _{G = 2 (C 11 + C} 12) by the elastic constants C ₁₁ and C ₁₂ of the material constituting the well layer and the barrier layer (1-C
_2. The strained multiple quantum well semiconductor laser according to claim 1, wherein the constant is determined by ( ₁₂ / C ₁₁ ). 3. The semiconductor laser according to claim 1, wherein a tensile strain is applied to the barrier layer in the strained multiple quantum well of the active layer.