JPH1084170A - Quantum well semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To emit light of a wavelength band required for optical communication by a method wherein at least either of quantum well layers and barrier layers are respectively constituted of a GaAlInAs layer and the composition of the quantum well layers is set into a composition of a grating constant larger than that of an InP substrate. SOLUTION: An N-type buffer layer 12, an optical confinement layer 18, an active layer 17, an optical confinement layer 19, a P-type InP clad layer 13, a P-type GaInAsP cap layer 14, an N-type electrode 15 and a P-type electrode 16 are formed on an N-type InP substrate 11 to constitute the main part of a quantum well semiconductor laser element. Quantum well layers 20 and barrier layers 21 are alternately laminated as the layer 17 to constitute the layer 17. At least either of the layers 20 and the layers 21 are respectively constituted of an AlGaInAs layer, the layers 20 are respectively constituted of a GaAlInAs layer and. the layers 21 are respectively constituted of a GaInAsP layer. Moreover, it is also possible that the layers 20 and the layers 21 are respectively constituted of a GaInAsP layer and a GaAlInAs layer. The composition of the layers 20 are set so as to become a composition of a grating constant larger than that of the InP substarte.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor laser using a quantum well structure used as a light source for optical communication and optical information processing.

[0002]

2. Description of the Related Art Desirable characteristics of a semiconductor laser device include a low threshold current density, a small temperature dependence of the threshold current density, a high modulation frequency, and a small wavelength chirping. These characteristics are improved by a quantum well semiconductor laser device having an active layer usually made of a layer thinner than 30 nm.

An active layer of a quantum well semiconductor laser device is composed of a layer having a small energy band gap called a quantum well and a layer having a large energy band gap called a barrier layer. In such a quantum well active layer, electrons and holes are confined in the quantum well and behave according to quantum mechanics.

The characteristics of a quantum well semiconductor laser device are improved by making the lattice constant of the quantum well larger than the lattice constant of the barrier layer and introducing strain into the quantum well. The reason for this is that the heavy holes in the valence band have their effective mass lightened in the direction perpendicular to the thin film layer and form a ground quantum level in the valence band. As a result, an optical transition between electrons and heavy holes is promoted in the quantum well layer. This is because electrons and heavy holes have substantially the same effective mass, so that it is easy to form a population inversion necessary for laser oscillation. It should be noted that the magnitude of the strain and the thickness of the quantum well layer must be within a certain critical film thickness value so that no dislocation is induced by the strain.

As shown in FIG. 4A, for example, a conventional strained quantum well semiconductor laser device has an n-type GaAs buffer layer 2 and an n-type Ga _0.6 Al
_0.4 As clad layer 3 is sequentially laminated, then 0.2 μm
It is composed of a Ga _0.63 In _0.37 As quantum well layer 4 having a thickness of 4 nm and having strains 5 and 6 on both sides, which have inclined regions 5 and 6 in which the Al component continuously changes from 40% to 0% with a thickness of m. An active layer is stacked, and a p-type G
a _0.6 Al _0.4 As clad layer 7 and p-type GaAsP
Cap layers are sequentially stacked, and finally, an n-type electrode 9 and a p-type electrode 10 are deposited.

This quantum well semiconductor laser device has an oscillation wavelength of 0.99 μm and a threshold current density of 195 Acm ^−2.
Met. FIG. 4 (b) shows the conduction band side of the band gap corresponding to FIG. 4 (a).

[0007]

However, the conventional strained quantum well semiconductor laser device cannot obtain an oscillation of 1.3 to 1.55 μm which is an important wavelength in optical fiber communication. In order to obtain an oscillation having a wavelength of 1.3 μm or longer from the active layer of Ga _1-x In _x As, from the magnitude of the energy band gap, X ≧
It must have an In composition of 0.5. However, in such a high X Ga _1-x In _x As, the lattice constant becomes large, and if it is not applied to the quantum well (4) of the conventional strained quantum well laser shown in FIG. There is a problem that the above-mentioned large strain is generated and the laser characteristics are degraded due to the generation of dislocations.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and it is an object of the present invention to provide a wavelength band of 1.3 μm to 1.55 which is an important wavelength band in optical communication.
An object of the present invention is to provide a high-performance strained quantum well semiconductor laser device that emits light in a long wavelength band of μm. A quantum well semiconductor laser device, wherein at least one of the quantum well layer and the barrier layer is GaAl.
InAs, and the composition of the quantum well layer is In
A quantum well semiconductor laser device having a composition having a lattice constant larger than that of P.

[0009]

The bandgap and lattice constant of a group III-V compound semiconductor vary depending on its composition. In the present invention, the band gap satisfies the condition of laser oscillation, and the lattice constant is appropriately selected to improve the performance of the quantum well semiconductor laser device.

That is, the composition of the quantum well layer is selected so that the lattice constant of the quantum well layer is larger than the lattice constant of the InP substrate, and the quantum well layer is designed so that a compressive strain is applied to the quantum well layer. The effective mass in the direction perpendicular to the film surface of the heavy holes in the valence band of the layer becomes small, the population inversion occurs at low injection carriers, and laser oscillation becomes possible at low injection current.

In the present invention, it is more preferable that the barrier layer has a lattice constant smaller than that of the InP substrate. By configuring the quantum well layer and the barrier layer so that strains are applied in opposite directions, the strain of the quantum well active layer as a whole can be reduced, and dislocations that adversely affect laser characteristics can be prevented. is there.

Further, in the present invention, the average lattice constant of the entire active layer can be made equal to the lattice constant of the InP substrate by adjusting the thicknesses of the quantum well layer and the barrier layer.

In the present invention, at least one of the quantum well layer and the barrier layer is made of AlGaInAs. Thereby, under the condition that a compressive strain is applied to the quantum well layer, it is important in optical communication to be 1.3 to 1.3.
This is because a high-performance laser that oscillates at a wavelength of 1.55 μm can be realized. In the present invention, when both the quantum well layer and the barrier layer are made of AlGaInAs, the quantum well layer is made of IGaInAs.
When nGaAsP is used and the barrier layer is made of AlGaInAs, three modes in which the quantum well layer is made of AlGaInAs and the barrier layer is made of InGaAsP are included.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1A is a cross-sectional view of a main part of a quantum well semiconductor laser according to the present invention. The structure is such that an n-type InP buffer layer 12 is
It is epitaxially grown to a thickness of about 0.2 μm.
The n-type InP substrate 11 and the n-type InP buffer layer 12
i or Se is doped at 2 × 10 ^{17 to} 5 × 10 ¹⁸ cm ⁻³ . Next, the light confinement layer 18, the active layer 17, and the light confinement layer 19 are sequentially stacked. In addition, thickness 0.1
0.2 μm, 2 × 10 ^{17 to} 5 × 10 ¹⁸ cm ⁻³ doped p-type InP cladding 13 and thickness 0.1 to 2.0
μm, p-type GaI highly doped to ５5 × 10 ¹⁸ cm ^-3
An nAsP cap layer 14 is sequentially stacked, and finally an n-type electrode 15 and a p-type electrode 16 are formed.

The optical confinement layer 18 is made of InGaAsP having the same lattice constant as InP, and has a composition of n-type InP.
The band gap is selected so as to gradually decrease as the buffer layer approaches the active layer 17. That is, the composition of the optical confinement layer 18 is always on the 0.585 nm isolattice constant line (solid line) L in the InGaAsP diagram of FIG. 2, and the final composition, that is, the composition of the portion in contact with the active layer 17 is: It has an energy band gap larger than the oscillation wavelength of 1.3 μm.
The composition is on the L line on the left side of the intersection P between the equiband gap line (dotted line) C line corresponding to a band gap of 3 μm and the solid line L. Note that L is InP and Ga _0.53 In.
_0.47 As is tied. In addition, the light confinement layer 18 may be non-doped, or the buffer layer 12 may
It may be doped so that the doping amount gradually decreases toward 7.

The light confinement layer 19 has a relationship that can be said to be a mirror image with the light confinement layer 18 except that the light confinement layer 19 is doped to the p-type. Changes gradually. The active layer 17 is composed of n-thick quantum well layers 20 each having a thickness of 2.5 to 30 nm and alternately separated by (n-1) barrier layers 21 each having a thickness of 2.5 to 30 nm. Have been. In this case, both side surfaces of the active layer 17 become the quantum well layers 20, but (n + 1) barrier layers 21 may be provided, and both side surfaces of the active layer 17 may be the barrier layers 21.

The quantum well layer 20 uses GaAlInAs having an oscillation wavelength of 1.3 μm. FIG. 3 shows GaAlInAs
The solid line L ′ in the figure is an isolattice constant line having the same lattice constant as InP, and Ga _0.5 In
_0.5 As and Al _0.5 In _0.5 As are connected.

The composition of GaAlInAs in the quantum well layer 20 is selected to be a composition having a lattice constant larger than that of InP (composition in the hatched portion in FIG. 3) and an oscillation wavelength of 1.3 μm. Further, the barrier layer 21 includes the light confinement layer 18,
GaAlInAs having the same band gap as the band gap of the portion in contact with the active layer 17 of 19.

The composition of the barrier layer 21 is selected on the solid line L 'in FIG. 3 so as to have the same lattice constant as that of InP.

The number n of quantum wells is a small integer, for example, n
= 3. There is an upper limit for the thickness of each quantum well, and the value is determined by the occurrence of dislocation induced by strain. In this embodiment, the oscillation wavelength corresponding to n = 3 is 1.3 μm.
The value slightly deviates from m. The reason for this is that the band gap is narrowed due to the strain, and the wavelength is increased, and the wavelength is shortened by the quantum confinement of electrons.

In order to make the oscillation wavelength exactly coincide with 1.3 μm, the composition may be adjusted in consideration of the effect of band gap reduction due to the above-described strain and the effect of band gap expansion due to the quantum confinement effect.

The composition of the blank portion (the portion other than the hatched portion) in FIG. 3 may be selected so that the barrier layer has a lattice constant smaller than that of InP. This makes it possible to make the average lattice constant of the active layer 17 close to the lattice constant of InP. By adjusting the film thickness and the composition, the lattice constant of the active layer 17 and the lattice constant of InP are made equal. You can also.

In this case, it is possible to grow up to the number of quantum well layers and barrier layers where n is several hundreds without generating strain-induced dislocations. Such a quantum well semiconductor laser device is suitable for realizing a surface emitting laser having a vertical cavity.

In the above embodiment, both the quantum well layer and the barrier layer are made of GaAlInAs. However, the quantum well layer may be made of GaAlInAs, the barrier layer may be made of GaInAsP, the quantum well layer may be made of GaInAsP, and the barrier layer may be made of GaAlInAs. Is also good.

Here, when the quantum well layer is made of GaInAsP, the composition thereof is 1.3 μm in the oscillation wavelength shown in FIG.
It is preferable that the composition be on the equiband gap line C corresponding to m and have a composition close to T between PTs whose lattice constant is larger than that of the InP substrate, and when the barrier layer is GaInAsP,
The composition is a composition on the L line on the left side of the point P in FIG.

In the above embodiment, the light confinement layer 1
Although the quantum well semiconductor laser having 8 and 19 has been described as an example, the optical confinement layer is not an essential component of the present invention, and a quantum well semiconductor laser without an optical confinement layer is also included in the scope of the present invention.

In the above embodiment, the buffer layer 12, the cladding layers 12 and 13 are made of InP, and the optical confinement layer is made of InGa.
Although AsP was used, they may be made of GaAlInAs.

[0028]

As described above, according to the present invention,
There is an excellent effect that a high performance quantum well semiconductor laser having an oscillation wavelength in the 1.3 to 1.55 μm band can be obtained.

[Brief description of the drawings]

FIG. 1A is a sectional view of a main part of an embodiment according to the present invention, and FIG. 1B is a view showing a conduction band side of a band gap thereof.

FIG. 2 is a diagram of GaInAsP.

FIG. 3 is a diagram of AlGaInAs.

FIG. 4 is a sectional view of a main part of a conventional example (a) and a view showing a conduction band side of a band gap thereof (b).

[Explanation of symbols]

 1 is an n-type GaAs substrate 2 is an n-type GaAs buffer layer 3 is an n-type GaAlAs cladding layer 4 is a GaInAs quantum well layer 5, 6 is an inclined region 7 is a p-type GaAlAs cladding layer 8 is a p-type GaAs cap layer 9, 15 is n-type electrodes 10 and 16 are p-type electrodes 11 are n-type InP substrates 12 are buffer layers 13 are p-type cladding layers 14 are p-type cap layers 17 are active layers 18 and 19 are optical confinement layers 20 are quantum well layers 21 Barrier layer

Claims (1)

[Claims] 1. A quantum well semiconductor laser device having a quantum well structure comprising at least one quantum well layer and at least one barrier layer on an InP substrate, wherein at least one of the quantum well layer and the barrier layer is made of GaAlInAs. And a composition of the quantum well layer having a lattice constant larger than that of InP.