JPH07183614A - Distorted multiple quantum well optical device - Google Patents

Abstract

PURPOSE:To provide a distorted MQW optical device which enables the property improvement by the introduction of distortion without deteriorating light shut-in effect. CONSTITUTION:MQW laser is made by stacking an n-type InP buffer layer 2, an n-type InGaAsP lower shut-in layer 3, an i-type MQW layer 4, a p-type InGaAsP upper light shut-in layer 5, and a p-type InP clad layer 6 in order on an n-type InP substrate 1. The MQW layer 4 is composed basically of an InxGa1-xAs well layer 12, where pull distortion of --0.5-2.00/o is introduced, and In0.82Ga0.18As0.4P0.6 barrier layers 11 of nondistortion, and an InyGa1-yAs shut-in layer 13, which is of approximately the same size, in the range of 0.5-2.0%, as a well layer 12 and in which compressed distortion of reverse polarity is introduced, is interposed between each well layer 12 and the barrier layer 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device such as a semiconductor laser using a strained multiple quantum well structure.

[0002]

2. Description of the Related Art Conventionally, starting with a semiconductor laser,
Various multi-quantum well optical devices have been studied. The multi-quantum-well (MQW) structure is a structure in which a well layer and a barrier layer are alternately laminated in thin layers with an electron wavelength (10 nm) or less. In the case of an optical modulator, about 30 layers are repeatedly laminated. In this MQW structure, the injected carriers are confined in the well layer and the absorption loss in the optical waveguide region is reduced. As a result, for example, in the case of a semiconductor laser, advantages such as an increase in optical output, a decrease in threshold current, and stabilization of oscillation wavelength can be obtained.

In such an MQW structure, a strained MQW structure has recently been proposed and attracted attention. This is MQ
Strain is introduced by intentionally making the W layer of the well layer lattice-matched with the barrier layer, which has the effect of increasing the differential gain and suppressing the efficiency decrease due to non-radiative recombination.

The specific principle will be described below with reference to FIG. For example, in the InGaAs / InP-based MQW structure, the band structure of the well layer is in the state of FIG. The heavy hole (HH) band and the light hole (LH) band degenerate at the wave vector k = 0. When compressive strain is introduced in the plane direction of such a well layer,
The energy levels of the heavy hole band and the light hole band shift to the low energy side, but the amount of shift of the heavy hole band is large.
As a result, as shown in FIG. 4A, the heavy hole band and the light hole band are separated, that is, the degeneracy is partially released.

When compressive strain is introduced into the well layer as described above, the effective mass of holes in the heavy hole band in the in-plane direction becomes small, so that the oscillation threshold of the laser becomes low and the oscillation efficiency is improved. To do. The decrease in emission efficiency due to the transition to the conduction band (Auger non-radiative recombination) is also suppressed. Further, since the bandgap energy Eg becomes small, the wavelength shifts to the long wavelength side. Further, selective oscillation in TE mode in which heavier holes are dominant can be obtained.

On the contrary, if tensile strain is introduced into the well layer,
Although the heavy hole band and the light hole band shift to the high energy side, the shift amount of the heavy hole band is large at this time as well, and the band structure becomes as shown in FIG. 4C. In this state, the oscillation wavelength shifts to the short wavelength side. In addition, T where light hole band is dominant
Since the gain in the M mode increases, polarization independence tends to occur. Recently, a semiconductor laser that has introduced this tensile strain,
Although optical switches, laser amplifiers, etc. have been proposed, the effective mass of holes in the light hole band becomes small in the case of tensile strain, so that in the case of lasers, the threshold value also decreases and the oscillation efficiency improves. Such an advantage can be obtained. Further, it is said that the saturation of the differential gain is less likely to occur as compared with the case of the compression distortion, and it is said to be advantageous for a high output laser, a high amplification factor laser amplifier and the like.

FIG. 3 shows the structure of an InGaAs / InP MQW laser which is a long wavelength laser in the 1.3 to 1.55 μm band. An n-type InP buffer layer is formed on the n-type InP substrate 1, on which an n-type InGaAsP layer 3 to be a light confining layer and undoped (i-type) to be an active layer are formed.
An MQW layer 4 and a p-type InGaAsP layer 5 to be a light confining layer are sequentially formed, and a p-type InP clad layer 6 is further formed thereon. As shown in the enlarged view, the MQW layer 4 has the same i-type In as the i-type InGaAs well layer 41.
The GaAsP barrier layer 41 and the GaAsP barrier layer 41 are alternately laminated in 3 to 5 pairs.

In the structure of FIG. 3, in order to introduce a strain into the MQW layer 4 and obtain good characteristics, in the case of compressive strain, the strain is preferably in the range of 0 to 1.5%, particularly around 1%. It is said. In the case of tensile strain, it is said that the effect appears at −0.8 to −2%, and the characteristics rather deteriorate at about −0.5%. In this specification, the strain is a deviation of Δ from the lattice constant a of a composition crystal, eg, InP, for obtaining a desired composition wavelength, and the lattice constant a of the InGaAs well layer from a.
It is defined as Δa / a. Δa / a is negative for tensile strain and positive for compressive strain.

InGaAs is lattice-matched to InP.
In the system, it is In _0.53 Ga _0.47 As. Therefore InG
In order to introduce tensile strain to the well layer with an aAs / InP laser, grow a well layer having a composition having a lattice constant smaller than the lattice constant a of InP, for example, a Ga-rich composition such as In _0.45 Ga _0.55 As. Good. Atomic radius is
This is because In> Ga. Theoretically, In _0.53 Ga _0.47
The composition wavelength of the As bulk crystal is 1.65 μm. On the other hand, when a strained MQW laser is constructed using an In _0.45 Ga _0.55 As well layer of 5 to 20 nm, the oscillation wavelength shifts to the short wavelength side due to the quantum effect. To do. Conversely, InGaAs
In order to introduce compressive strain into the well layer with a / InP-based laser,
For example, a well layer having an In-rich composition such as In _0.58 Ga _0.42 As may be grown.

[0010]

DISCLOSURE OF THE INVENTION Distorted MQW as described above
It has been experimentally revealed that in a laser, in order to sufficiently confine strain of the well layer in the well layer, the barrier layer needs to have a thickness twice or more that of the well layer. However, when the barrier layer has a large thickness, the MQW layer as a whole has a large thickness, and the optical confinement coefficient Γ decreases. This is because the optical confinement coefficient Γ depends on the layer thickness of the optical confinement region, that is, the layer thickness of the MQW layer, and becomes smaller as the MQW layer becomes thicker. If the optical confinement coefficient Γ becomes smaller, the effect of reducing the threshold current density and improving the oscillation efficiency due to the introduction of strain cannot be sufficiently obtained.

In an actual InGaAs / InP MQW laser, if an attempt is made to achieve a threshold current density of 2000 A / cm ² or less, the optical confinement coefficient Γ is 0.1.
Although it is desired to set the above, it is difficult to realize this unless the layer thickness of the MQW layer is 0.2 μm or less, preferably 0.1 μm or less.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide a strained MQW optical device capable of improving the characteristics by introducing strain without deteriorating the optical confinement effect.

[0013]

According to the present invention, first, a multiple quantum well layer in which well layers and barrier layers are alternately laminated and strain is introduced into the well layer, and optical confinement sandwiching the multiple quantum well layer are provided. And a strain confinement layer for confining the strain of the well layer is interposed between each well layer and the barrier layer of the multi-quantum well layer.

Secondly, the present invention relates to a multi-quantum well formed by stacking a lower optical confinement layer, a multiple quantum well layer in which well layers and barrier layers are alternately laminated, and an upper optical confinement layer on a semiconductor substrate. In the laser, the multiple quantum well layer is
A strain confinement layer in which a tensile strain of −0.5 to −2.0% is introduced into the well layer and a compressive strain of 0.5 to 2.0% is introduced between each well layer and the barrier layer is interposed. It is characterized by making it.

Thirdly, the present invention relates to a multi-quantum well formed by stacking a lower optical confinement layer, a multiple quantum well layer in which well layers and barrier layers are alternately laminated and an upper optical confinement layer on a semiconductor substrate. In the laser, the multiple quantum well layer is
Strain confinement layer with 0.1-1.0% compressive strain introduced into the well layer and -0.1-1.0% tensile strain introduced between each well layer and barrier layer It is characterized by making it.

[0016]

According to the present invention, the strain confinement layer is interposed between the well layer and the barrier layer so as to confine the strain introduced into the well layer of the strained MQW layer. Specifically, in the case of a multiple quantum well laser, the well layer is a tensile strain layer of -0.5 to -2.0%, and the strain confinement layer has a compressive strain of the same magnitude as that of the well layer. Layer. On the contrary, the well layer is 0.1
When the compressive strain layer is 1.0%, the strain confinement layer is a layer to which tensile strain having the same magnitude as that of the well layer is introduced. As a result, the strain in the well layer is confined in a form of being offset by the reverse strain of the strain confinement layer, so that the barrier layer can be made thinner than before. And as a result, MQ
A large optical confinement effect can be obtained by making the layer thickness of the entire W layer sufficiently small.

Further, since the strain opposite to that of the well layer and the strain confinement layer is introduced, it is equivalent to the fact that a larger strain is effectively introduced into the well layer as compared with the case where the strain is introduced into only the well layer. become. In other words, this means that strain can be effectively introduced without making the layer thickness of the well layer so small. Therefore, the controllability and uniformity of the layer thickness of the well layer can be improved, the steepness of the interface can be improved, and excellent quantum effect characteristics can be obtained. Further, if the layer thickness of the well layer does not have to be so small, the crystallinity will be good, and the reliability of the device will be improved.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows InGa according to the first embodiment of the present invention.
This is an As / InP strained MQW laser. On the n-type InP substrate 1, the n-type InP buffer layer 2 and the n-type InGaAs
P lower optical confinement layer 3, i-type MQW layer 4, p-type InGa
AsP upper optical confinement layer 5 and p-type InP clad layer 6
Are sequentially laminated and obtained. This basic structure is similar to the conventional one.

The MQW layer 4 comprises a barrier layer 11 and a well layer 12.
The basic structure is an alternate laminated structure of
A tensile strain of 0.5 to -2.0% is introduced. For example, the barrier layer 11 is made of In _0.82 Ga _0.18 As _0.4 P _0.6 with a composition wavelength of 1.15 μm, and the well layer 12 is In _x Ga _1-x As (0.4 ≦ x ≦ 0.5).
And In addition, in order to confine the strain in the well layer 12 between each well layer 12 and the barrier layer 11 due to the introduction of tensile strain in the well layer 12, the range of 0.5 to 2.0% is used. A strain confinement layer 13 having the same size as that of the well layer 12 and having a compressive strain of opposite polarity is introduced. Specifically, the strain confinement layer 13 is formed of In _y Ga _1-y As (0.57).
≦ y ≦ 0.83).

By interposing the strain confinement layer 13 in the MQW layer 4 as described above, in this embodiment, it is not necessary to increase the layer thickness of the barrier layer 11 as in the conventional case.
Specifically, in this embodiment, the well layer 12 and the barrier layer 11 have the same layer thickness.

With the structure of this embodiment, a sufficient quantum effect can be obtained with an MQW layer thinner than before. More specifically, in the conventional technique having no strain confinement layer, assuming that the well layer has a thickness of 6 nm, the barrier layer needs to have a thickness of 12 nm. At this time, the MQW layer for 5 periods has a thickness of 5 well layers and 6 well layers. Due to the barrier layer, 6 × 5 + 12 × 6 = 102 [nm].

On the other hand, in the case of this embodiment, the well layer,
If the barrier layer has a thickness of 6 nm and the strain confinement layer has a thickness of 2 nm with 1 nm on each side of each well layer, the layer thickness of the MQW layer for 5 cycles is (6 + 2) × 5 + 6 × 6 = 76 [nm].

As described above, the thickness of the strained MQW layer can be reduced by 26 nm as compared with the conventional one. As a result, an excellent optical confinement effect with an optical confinement coefficient Γ of about 0.1 or less can be obtained, and in combination with the effect of introducing strain, the threshold current of the MQW laser can be reduced and the oscillation efficiency can be improved.

Further, in this embodiment, for example, when it is desired to introduce a tensile strain of -1% into the well layer 12, a tensile strain of -0.5% is actually introduced into the well layer 12 and the strain confinement layer 13 is introduced. By introducing a compression strain of + 0.5% to, a tensile strain of -1% can be effectively realized. This means that desired strain can be introduced into the well layer without making the thickness of the well layer so small.
The uniformity and the like are improved, and excellent laser characteristics and high reliability are obtained.

FIG. 2 shows In according to the second embodiment of the present invention.
It is a GaAs / InP strained MQW laser. Since the basic structure is the same as that of the embodiment shown in FIG. 1, detailed description will be omitted.
In the case of this embodiment, the MQW layer 4 has a well layer 22 into which a compressive strain of 0.1 to 1.0% is introduced, and each well layer 22.
A strain confinement layer 23 in which a tensile strain of −0.1 to −1.0% is introduced is interposed between the barrier layer 21 and the barrier layer 21.

Specifically, for example, the barrier layer 21 is In _0.82 Ga _0.18 As _0.4 P _0.6 as in the above-described embodiment, and the well layer 22 is In _x Ga _1-x As (0.55 ≦ x ≦ 0.6).
8), the strain confinement layer 23 has a size similar to that of the well layer 22 and is In _y Ga _1-y in which tensile strain of opposite polarity is introduced.
As (0.39 ≦ y ≦ 0.52).

Also in this embodiment, as in the previous embodiment, without reducing the effect of introducing strain into the well layer,
A sufficient optical confinement effect can be obtained, and thus an excellent MQW laser can be obtained.

Although lasers have been described in the above embodiments, the present invention is not limited to lasers and can be applied to various optical devices such as optical switches and optical modulators using the same MQW structure. Particularly in an optical device in which the number of repetitions of the MQW layer is larger than that of a laser, the effect of reducing the layer thickness of the entire MQW layer becomes large by reducing the layer thickness of the barrier layer. A specific numerical example will be given.

For example, the well layer is 8 nm and the barrier layer is 16 nm.
When obtaining an MQW structure with 30 cycles, the total layer thickness of the MQW layer is 8 × 30 + 16 × 31 = 736 [nm]. On the other hand, the well layer and the barrier layer both have a thickness of 8 nm,
Assuming that a strain confinement layer of 1 nm is provided on both sides of each well layer, the total layer thickness of the MQW layer is (8 + 2) × 30 + 8 × 31 = 548 [nm]. From the above, the MQW layer as a whole is approximately 20
A layer thickness reduction of about 0 nm and 30% can be achieved. This allows
A large light confinement effect can be obtained.

Further, in the embodiment, the case where the substrate side is n-type and the upper side of the MQW layer is p-type is shown, but this conductivity type may be reversed. Furthermore, the present invention is effective not only in the InGaAs / InP system but also in the case of using other compound semiconductors.

[0031]

As described above, according to the present invention, MQ
Provided is a strained MQW optical device capable of improving characteristics by introducing strain without deteriorating the optical confinement effect by interposing a strain confinement layer for confining the strain of the well layer between each well layer of the W layer and the barrier layer. can do.

[Brief description of drawings]

FIG. 1 shows a strained MQW laser according to a first embodiment of the present invention.

FIG. 2 shows a strained MQW laser according to a second embodiment of the present invention.

FIG. 3 shows a conventional strained MQW laser.

FIG. 4 is a diagram for explaining an effect of distortion MQW.

[Explanation of symbols]

1 ... n-type InP substrate, 2 ... n-type InP buffer layer, 3 ...
n-type InGaAsP lower optical confinement layer, 4 ... Strained MQW
Layer, 5 ... p-type InGaAsP upper optical confinement layer, 6 ... p
Type InP clad layer, 11 ... i type In _0.82 Ga _0.18 As
_0.4 P _0.6 barrier layer, 12 ... i-type In _x Ga _1-x As well layer, 13 ... i-type In _y Ga _1-y As strain confinement layer, 2
1 ... i-type In _0.82 Ga _0.18 As _0.4 P _0.6 barrier layer, 2
2 ... i-type In _x Ga _1-x As well layer, 23 ... i-type In _y
Ga _1-y As strain confinement layer.

Claims (3)

[Claims] 1. A multi-quantum well optical device comprising: a multi-quantum well layer in which well layers and barrier layers are alternately laminated and strain is introduced into the well layer; and a light confinement layer sandwiching the multi-quantum well layer. A strained multiple quantum well optical device, characterized in that a strain confinement layer for confining strain of the well layer is interposed between each well layer and the barrier layer of the multiple quantum well layer. 2. A multi-quantum well laser configured by stacking a lower optical confinement layer, a multiple quantum well layer in which well layers and barrier layers are alternately laminated, and an upper optical confinement layer on a semiconductor substrate. The well layer is -0.5 to -2.0% in the well layer.
Strained multi-quantum well laser characterized in that a strain confinement layer having a tensile strain of 0.5 to 2.0% is interposed between each well layer and the barrier layer. . 3. A multi-quantum well laser configured by stacking a lower optical confinement layer, a multiple quantum well layer in which well layers and barrier layers are alternately laminated, and an upper optical confinement layer on a semiconductor substrate. The well layer has a compressive strain of 0.1 to 1.0% introduced into the well layer, and −0.
A strained multiple quantum well laser having a strain confinement layer having a tensile strain of 1 to −1.0% introduced therein.