JP3147596B2 - Strained multiple quantum well structure and semiconductor laser using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active layer structure of a semiconductor laser device used as a light source for optical communication or an optical disk, or an external modulator.

[0002]

2. Description of the Related Art In recent years, in a quantum well structure, a critical film thickness (transition occurs in a crystal in order to reduce lattice mismatch) by making the lattice constant of a well layer larger or smaller than the lattice constant of a substrate. A technique for applying a compressive or tensile strain to the inside of a well layer set to a thickness of less than or equal to (thickness) has been actively studied. This is because the energy band structure of the quantum well can be freely designed by applying a strain inside the well layer. In particular, when a quantum well structure in which the above-described strain is introduced is used for an active layer of a semiconductor laser, a laser in a wavelength band that cannot be realized by a lattice matching system can be realized.
In addition, improvement of characteristics can be expected in a laser in a wavelength band realized also in a lattice matching system.

[0003] 1.3 used as a light source for optical communication
FIG. 7 shows a conventional example of a strained quantum well structure that emits light in the μm band. An InGaAsP strain well layer 702 in which a compressive strain of about 1% is introduced on an InP substrate 701 and an unstrained InGaAs
It is composed of a P barrier layer (wavelength composition: 1.05 μm) 703, and the number of well layers is 10. The thickness of the well layer is 3 to 6 nm, and the thickness of the barrier layer is 10 nm. A semiconductor laser having a strained multiple quantum well structure as an active layer configured as described above has a lower threshold current characteristic, a higher optical output characteristic, a higher temperature characteristic, and a lower threshold voltage than a semiconductor laser having a lattice-matched multiple quantum well structure as an active layer.
High differential gain characteristics are realized.

[0004]

In order to transmit a large amount of information in optical communication, a semiconductor laser which can operate at a very high speed is required. However, the upper limit of the response speed of a semiconductor laser is limited by the relaxation oscillation frequency ( _fr ) inherent to the laser. The equation of the relaxation oscillation frequency is shown.

[0005] fr = 1 / 2π × SQR { AΓ / V a / q (I-I th)} (1) where A is the differential gain, gamma light confinement coefficient to all the well layer, V _a is the well total volume of the layer, q is a charge amount, I is the amount of injection current, I _th represents the threshold current. In the conventional example, it is possible to increase the differential gain by about 1.5 times as compared with no distortion by introducing strain into the well layer. However, in order to further increase the relaxation oscillation frequency, it is necessary to introduce strain. In addition, it is necessary to further increase the differential gain by reducing the thickness of the well layer to enhance the quantum size effect. However, the thinning of the well layer causes a decrease in the light confinement coefficient. As a result, the relaxation oscillation frequency does not increase so much. Therefore, it is necessary to increase the number of well layers and sufficiently increase the light confinement coefficient.

In a lattice matching system, even if the number of well layers is increased to about 20, the crystallinity is hardly impaired. The white circle in FIG. 8 indicates the full width at half maximum (FWHM) of the photoluminescence spectrum at 77 K from the multiple quantum well structure of the lattice matching system.
Shows the dependence on the number of well layers. It can be seen that even when the number of well layers is changed from 5 to 20, the change in the full width at half maximum of the photoluminescence spectrum is small. On the other hand, the black circle in FIG. 8 shows the dependency of the full width at half maximum (FWHM) of the photoluminescence spectrum at 77 K from the strained multiple quantum well structure having a well layer with a thickness of 3 nm to which 1% compressive strain is applied, on the number of well layers. . It can be seen that the full width at half maximum changes little from the first layer to the tenth layer, but increases sharply in the 14th layer. The sharp increase in the full width at half maximum of the photoluminescence spectrum indicates that the amount of strain confined inside each quantum well is not uniform.When such a quantum well structure is used for the active layer of a semiconductor laser, the threshold current Increases, and the luminous efficiency decreases. In the structure of the conventional example, the number of well layers is 14
A sharp increase in the full width at half maximum of the photoluminescence spectrum was observed, but when the well layer thickness, the barrier layer thickness, and the strain amount were different, the number of well layers in which the full width at half maximum of the photoluminescence spectrum rapidly increased was different. Conceivable.

In view of the above, an object of the present invention is to provide a strained multiple quantum well structure capable of uniformly applying a strain to each quantum well even when the number of well layers is increased.

[0008]

The abrupt increase in the full width at half maximum of the photoluminescence spectrum which occurs when the number of well layers is increased in the strained multiple quantum well structure causes the amount of strain confined in the well layers to increase in each well layer. It is thought to be caused by non-uniformity. In order to make the strain amount of each well layer uniform, the lattice constant of each barrier layer must always be the same as the lattice constant of the substrate.

However, for example, as shown in FIG. 4A, a well layer having a lattice constant different from the lattice constant a0 of the substrate 401 (here, a layer having a larger lattice constant in the bulk state than the substrate and having a lattice constant in the bulk state as a _epi ; a _epi > a0)
When the barrier layer 403 having the same lattice constant as the substrate 402 is alternately laminated, the stress distribution becomes as shown in FIG. 4B, and the partial stress (tensile stress) acting on the barrier layer as the number of well layers increases. Will increase (Preprints of the Japan Society of Applied Physics 1930-ZB-4, 1992).

FIG. 4A schematically shows how the stress on the barrier layer increases as the number of layers increases, with the size of the arrow. When the barrier layer is a quaternary mixed crystal such as InGaAsP, it is conceivable that an extremely large stress acts during crystal growth and an atomic arrangement different from the atomic arrangement when no stress acts. When a very large tensile stress acts on the InGaAsP barrier layer, which has the same lattice constant as the substrate when no stress is applied, it is considered that such a change in the arrangement of atoms causes the lattice constant to be larger than the lattice constant of the substrate. Can be If the lattice constant of the barrier layer is larger than the lattice constant of the substrate, the amount of strain confined in the well layer to be stacked next decreases.

As shown in FIG. 5, when the number of well layers is small, the lattice constant of the n-th well layer 502 in the in-plane direction of the substrate 501 is a ₀ , and the strain amount is (a ₀ -1). / is a a _epi, following the lattice constant of the barrier layer 504 of the m-th well layer 503 due to the stress effects of the number of well layers becomes very large changes to _{a 1 (a 1> a 0} ). Accordingly, the strain amount of the m-th well layer 503 becomes {(a ₀ + a ₁ ) / 2-1} / a
Change to _epi . Furthermore lattice constant of the next barrier layer 506 (m + 1) th well layer 505 is changed to a ₂ (a _2>
a ₁ ). As a result, the strain amount of the (m + 1) th well layer 505 changes to {(a ₁ + a ₂ ) / 2-1} / a _epi .

It is considered that as the number of well layers is increased according to the above principle, the amount of strain confined in the well layers becomes non-uniform. In order to make the strain amount of each well layer uniform, it is conceivable to make the barrier layer a binary crystal such as InP in which the atomic arrangement does not change. However, when the barrier layer is made of InP, the energy band gap difference between the well layer and the well layer becomes very large, so that electrons and holes are not uniformly injected into each well, thereby deteriorating laser characteristics.

Therefore, as shown in FIG. 1, the barrier layer is made of a crystal having a smaller energy band gap than the substrate (the substrate is made of In
P is InGaAsP, and a very thin binary crystal layer that does not act as a barrier for electrons and holes between the barrier layer and the strained well layer located on the substrate side (if the substrate is InP, By inserting (InP), it is possible to make the amount of strain in each well layer uniform and to make the injection of electrons and holes into each well layer uniform.

If the number of well layers is further increased, the thickness of the entire strained multiple quantum well layer exceeds the critical thickness determined by the average amount of strain, and lattice relaxation occurs in the entire strained multiple quantum well layer, Crystallinity is significantly impaired. To prevent this,
As shown in FIG. 6, a strain compensation region 603 in which strain in the direction opposite to that of the well layer 601 is provided in the whole (a) or a part (b) of the barrier layer 602 to reduce the amount of strain in the entire strained multiple quantum well layer. Although the method is performed, even in this case, if the barrier layer is a quaternary mixed crystal such as InGaAsP, the stress acting on the barrier layer causes the above-described problem. When strain in the direction opposite to that of the well layer is introduced into the barrier layer in this way, as shown in FIG. 2, the binary system is extremely thin enough that this reverse strain layer does not act as a barrier for electrons and holes. Crystal layer (In the case where the substrate is InP, In
P) makes it possible to make the amount of strain in each well layer uniform and to make the injection of electrons and holes into each well layer uniform.

[0015]

According to the above-described structure, a plurality of quantum well layers formed on a binary compound semiconductor substrate, into which a plurality of compressive strains or tensile strains are introduced, and a non-strainless, energy band gap smaller than that of the compound semiconductor substrate. In a strained multiple quantum well structure including a barrier layer, a layer of the same material as that of the compound semiconductor substrate is inserted between the barrier layer and the well layer located on the substrate side of the barrier layer, whereby each well layer It is possible to increase the number of well layers while keeping the amount of strain uniform and the injection of electrons and holes into each well layer uniform.

In the strained multiple quantum well structure in which the barrier layer has a strain introduced in a direction opposite to that of the well layer, a layer of the same material as the compound semiconductor substrate is formed between the barrier layer and the well layer. The insertion makes it possible to increase the number of well layers while keeping the amount of strain in each well layer uniform and injecting electrons and holes into each well layer uniformly.

[0017]

(Embodiment 1) FIG. 1 is a diagram for explaining an energy band diagram on a conduction band side of a strained multiple quantum well structure in Embodiment 1 of the present invention. An n-type InGaAsP waveguide layer 102 having a wavelength composition of 1.05 μm and a layer composition of 150 nm (layer thickness: 150 nm) on an n-type InP substrate 101, and 14 layers of 1% InGaAsP having a wavelength composition of 1.5 μm in which 1% compressive strain is introduced. A strained multiple quantum well active layer 105 comprising a well layer (thickness: 3 nm) 103 and an InGaAsP barrier layer (thickness: 10 nm) 104 having no distortion and a wavelength composition of 1.05 μm;
μm p-type InGaAsP waveguide layer 102 (layer thickness 30
nm) 106 and a p-type InP cladding layer 107 are laminated.

The feature of this embodiment is that a 1 nm-thick InP layer 108 is provided between a barrier layer and a strain well layer located on the substrate side of the barrier layer.
Is inserted.

In the strained multiple quantum well structure of this embodiment, since the InP layer 108 does not change its lattice constant against stress from the strained well layer 103, a uniform strain can be accumulated in each strained well layer. . As a result, as shown in FIG.
In the conventional structure in which the P layer 108 is not inserted (black circles), the full width at half maximum of the photoluminescence spectrum sharply increases when the number of well layers is 14, but in the first embodiment (open circles), the photoluminescence spectrum is increased even when the number of well layers is 14. Did not show a sharp increase in the full width at half maximum.

(Embodiment 2) FIG. 2 is a diagram for explaining an energy band diagram on the conduction band side of a strained multiple quantum well structure in Embodiment 2 of the present invention. An n-type InGaAsP waveguide layer 202 having a wavelength composition of 1.05 μm and a layer thickness of 150 nm (layer thickness: 150 nm) on an n-type InP substrate 201, and 20 layers of 1 μm InGaAsP having a wavelength composition of 1.5 μm in which 1% compressive strain is introduced. A strained multiple quantum well active layer 2 composed of a well layer (thickness: 3 nm) 203 and an InGaAsP barrier layer (thickness: 6 nm) 204 having a wavelength composition of 1.05 μm into which a tensile strain of 0.5% is introduced.
05, p-type InGaAs with no distortion and a wavelength composition of 1.05 μm
An sP waveguide layer 206 (thickness: 30 nm) and a p-type InP cladding layer 207 are stacked.

This embodiment is characterized by the barrier layer 204 and the well layer 2
3 is that an InP layer 208 having a layer thickness of 1 nm is inserted between the layers.

In the strained multiple quantum well structure of this embodiment, the InP layer 208 is
Because the lattice constant does not change even with the stress from 4,
A uniform amount of strain can be accumulated in each strain well layer 103. Also,
Since the average strain of the strained multiple quantum well layer is almost equal to 0,
Even if the number of well layers is as large as 20, no sudden increase in the full width at half maximum of the photoluminescence spectrum was observed in the structure of Example 2 (white angle) as shown in FIG.

In this embodiment, an InGaAsP / InP-based material is used as a material constituting the strained multiple quantum well structure. However, the effect remains unchanged in other material systems, for example, an InGaAs / GaAs-based laser.

[0024]

As described above, according to the present invention,
Even if the number of well layers in the strained multiple quantum well is very large, it is possible to confine strain uniformly in each well layer, and a semiconductor laser using such a strained multiple quantum well structure as an active layer has a very high relaxation. Vibration frequency can be obtained, and high-speed operation is possible, and its practical effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an energy band diagram on a conduction band side of a strained multiple quantum well structure according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an energy band diagram on a conduction band side of a strained multiple quantum well structure according to a second embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the effect of the present invention.

4A is a layer structure diagram in which well layers having different lattice constants and lattice constants of a substrate and barrier layers having the same lattice constant as the substrate are alternately layered, and FIG. Stress distribution diagram when barrier layers with the same lattice constant are alternately stacked in multiple layers with different well layers and substrates

FIG. 5 is a diagram showing a change in lattice constant when the number of well layers is increased.

FIG. 6A is a diagram showing a strain distribution of a conventional example in which a strain in a direction opposite to that of a well layer is introduced into the entire barrier layer. FIG. 6B is a diagram showing a strain in a direction opposite to the well layer in a part of the barrier layer. Diagram showing the distribution of strain in the conventional example introducing

FIG. 7 is a diagram illustrating an energy band diagram on a conduction band side of a conventional strained quantum well structure.

FIG. 8 is a view for explaining the dependence of the full width at half maximum of the photoluminescence spectrum at 77K of the structure of the conventional example on the number of well layers.

[Explanation of symbols]

101 n-type InP substrate 102 n-type InGa with no distortion and wavelength composition of 1.05 μm
AsP waveguide layer 103 Wavelength composition 1.5 μm with 1% compressive strain introduced
InGaAsP well layer 104 of InGaAsP with no distortion and wavelength composition of 1.05 μm
Barrier layer 105 Strained multiple quantum well active layer 106 P-type InGa with no distortion and wavelength composition of 1.05 μm
AsP waveguide layer 107 p-type InP cladding layer 108 1-nm thick InP layer 201 n-type InP substrate 202 n-type InGa with no distortion and wavelength composition of 1.05 μm
AsP waveguide layer 203 Wavelength composition 1.5 μm with 1% compressive strain introduced
InGaAsP well layer 204 InGaAsP barrier layer having a wavelength composition of 1.05 μm into which 0.5% tensile strain has been introduced 205 Strained multiple quantum well active layer 206 p-type InGa having no distortion and a wavelength composition of 1.05 μm
AsP waveguide layer 207 p-type InP cladding layer 208 1-nm thick InP layer 401 substrate having lattice constant a0 402 well layer having a different lattice constant from substrate 403 barrier layer having the same lattice constant as the substrate 501 substrate 502 n-th Well layer 503 m-th well layer 504 barrier layer next to m-th well layer 505 m + 1-th well layer 506 barrier layer next to m + 1-th well layer 601 well layer 602 barrier layer 603 well layer Is a strain compensation region in which directional strain is introduced 701 InP substrate 702 InGaAsP strain well layer in which about 1% compressive strain is introduced 703 Unstrained InGaAsP barrier layer (wavelength composition 1.0
5 μm) 704 InP cladding layer

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masato Ishino 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-5-55697 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 21/20

Claims (5)

(57) [Claims] 1. A multi-quantum well structure formed on a substrate, wherein barrier layers and well layers to which strain is introduced are alternately and repeatedly stacked, wherein the well layer and the barrier layer are provided between the well layer and the barrier layer. A strained multiple quantum well structure including a layer having the same composition as a substrate. 2. The strained multiple quantum well according to claim 1, wherein the layer having the same composition as the substrate is provided between the barrier layer and the well layer located on the substrate side of the barrier layer. Structure. 3. The strained multiple quantum well structure according to claim 1, wherein a strain in a direction opposite to that of the well layer is introduced into the barrier layer. 4. The strained multiple quantum well structure according to claim 1, wherein layers having the same composition as the substrate are provided on both sides of the barrier layer. 5. A semiconductor laser having the strained multiple quantum well structure according to claim 1 as an active layer.