JPH07162084A - Quantum well structure of semiconductor laser - Google Patents

Abstract

PURPOSE:To shorten carrier conveyance time between quantum wells, and uniform the density distribution of carriers which are injected into each quantum well. CONSTITUTION:The title quantum well structure of a semiconductor laser is formed by laminating a plurality of quantum well layers 20b1-20b4 and barrier layers 20a1-20a5. The quantum well layer is composed of first semiconductor thinner than or equal to the thickness which generates quantum effect. The barrier layer is composed of second semiconductor whose forbidden bandwidth is larger than or equal to that of the quantum well layer. In order to uniform the carrier density distribution, the forbidden bandwidth of each of barrier layers 20a1-20a5 is changed in order in the thickness direction. The thickness of each of the quantum well layers 20b1-29b4 is so changed that all of the wavelengths lB1-lB2 of the well layers become equal according to the change amount of the forbidden bandwidths of the barrier layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum well structure of a semiconductor laser suitable for reducing the threshold current density of a semiconductor laser having a quantum well layer structure or a semiconductor laser having a multiple quantum well structure.

[0002]

2. Description of the Related Art A semiconductor laser includes a quantum well layer made of a first semiconductor having a thickness equal to or less than a thickness at which a quantum effect is generated in an active layer, and a second semiconductor having a forbidden band width wider than that of the quantum well layer. There is a quantum well structure in which a plurality of barrier layers (also referred to as barrier layers) are laminated.
This quantum well structure was developed as a performance improving measure for a semiconductor laser capable of lowering an oscillation threshold current, expanding a direct modulation band, and reducing an oscillation spectrum line width.

FIG. 4 shows an example of a band (conduction band) diagram near the active layer adopting the quantum well structure.
As shown in FIG. 4, the active layer has a quantum well (multi-quantum well: MQW) structure 10 formed substantially in the center in the thickness direction. The quantum well structure 10 includes three barrier layers 10a.
1 to 10a3 and four quantum well layers 10b1 to 10b4 are alternately laminated. An optical confinement layer (OCL) is formed on both sides of the quantum well structure in the thickness direction.
12 is laminated, and the clad layer 14 is further laminated on the outer side thereof.

[0004]

By the way, in the semiconductor laser using the above quantum well structure, the internal differential efficiency is lowered due to the non-uniformity of the carrier density distribution injected into each quantum well. This decrease in internal differential efficiency occurs because the carrier transport time between quantum wells becomes long and the distribution of carrier density injected into each quantum well becomes non-uniform. According to the band diagram of FIG. 4, it is assumed that holes are injected from the left side of FIG.
The carrier density injected into each of the quantum well layers 10b1 to 10b4 gradually decreases in the thickness direction as indicated by the hatched lines in FIG.

Therefore, as a method of making the injected carrier density distribution uniform, it is conceivable to reduce the barrier layer thickness and strengthen the coupling between the quantum wells by tunneling, but in this case, the quantum confinement is weak. Therefore, the crystallinity becomes worse in the strained quantum well structure.

There is also a method of weakening the quantum confinement by lowering the barrier height of the barrier layer as a whole, but with this method, the first and second quantum wells from the carrier injection side are almost equal. However, the problem of non-uniformity still remains in the subsequent quantum wells.

It is to be noted that FIG. 6 is related to the present invention.
As shown in, a structure has been proposed in which the barrier height (corresponding to the forbidden band width) of the barrier layer is sequentially decreased in the thickness direction in accordance with the carrier density distribution to sequentially weaken the quantum confinement (for example, (See Hei 4-7594). However, in this structure, when the composition of the barrier layers 10a1 to 10a3 is changed, the carrier density becomes uniform in the quantum well layers 10b1 to 10b4, for example, as shown in FIG. The emission wavelength is different.

The emission wavelengths (oscillation wavelengths) of the respective quantum well layers are different, and there is a problem that the threshold current can be reduced only to some extent.

The present invention has been made to solve the above-mentioned conventional problems, in which the carrier transport time between quantum wells is shortened and the distribution of carrier density injected into each quantum well is made uniform. Another object of the present invention is to provide a quantum well structure of a semiconductor laser capable of making the emission wavelength of each quantum well uniform.

[0010]

In the present invention, in order to solve the above-mentioned problems, a quantum well layer made of a first semiconductor having a thickness equal to or less than a thickness at which a quantum effect occurs and a forbidden band width from the quantum well layer. In a quantum well structure of a semiconductor laser in which a plurality of barrier layers made of a wide second semiconductor are stacked, each barrier layer has a forbidden band width sequentially in the thickness direction so that the carrier density distribution becomes uniform. And a semiconductor layer characterized in that the thickness of each quantum well layer is changed so that the wavelengths of the respective well layers are all equal according to the amount of change in the band gap of the barrier layer. It has a structure of a quantum well structure of a laser.

[0011]

In the present invention, since each forbidden band width (barrier height) in each barrier layer is sequentially changed in the thickness direction so that the carrier density distribution becomes uniform, carrier transport between quantum wells is performed. The internal differential efficiency can be improved by shortening the time and making the density distribution of carriers injected into each quantum well uniform. However, since each barrier layer changes the forbidden band width by changing the composition thereof, the wavelength of light in each well layer is different. Therefore, when the forbidden band width of the barrier layer changes, the wavelength λ also changes accordingly, as shown in FIG.

Therefore, in the present invention, the thickness of each quantum well layer is changed so that all the wavelengths of each well layer become equal depending on the amount of change in the forbidden band width of the barrier layer. Thus, the difference in bandgap energy (composition wavelength) of the quantum well due to the change in barrier height is corrected. In this method, the composition of the barrier layer is set to some extent in order to make the carrier density distribution uniform, and the variation of the wavelength due to the setting of the composition is set and corrected by the layer thickness of each quantum well layer. In the vapor phase epitaxy method (for example, MOV method) and the molecular beam epitaxy method (MBE method), which are the current semiconductor film forming techniques, the layer thickness can be set extremely accurately. It is extremely effective in accurately setting the wavelength to the target wavelength. Therefore, it is possible to reliably prevent the increase of the oscillation threshold and the broadening of the oscillation spectrum line width due to the difference in the wavelength of each well in the multiple quantum well (oscillation wavelength when compared with a single quantum well in a laser). is there.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a diagram showing the structure of the semiconductor laser of this embodiment. 1A is an overall perspective configuration diagram including a cross section of a main part, FIG. 1B is an enlarged view of a main part of FIG. 1A, and FIG. 1C is an enlarged explanatory view of the periphery of an active layer of FIG. 1B. .

As shown in FIG. 1, the semiconductor laser is
The active layer 20 having a multiple quantum well (MQW) structure is made of InGa.
A plurality (five layers in the embodiment) of barrier layers 20a (20a1 to 20a5) made of a mixed crystal of AsP and a plurality (four layers in the embodiment) of quantum well layers 20b made of a mixed crystal of InGaAsP.
(20b1 to 20b4) are alternately stacked. Each layer 20a1~20a5 of the barrier layer 20a is, changing the composition ratio by changing the variables X, Y set in composition represented by the following formula _{(In 1-x Ga x As} y P 1-y) ... (1) As a result, the barrier height (corresponding to the forbidden band width) changes sequentially in the thickness direction. In addition, each layer 20b of the quantum well layer 20b
1 to 20b4 have a constant composition ratio (for example, wavelength λ = 1.
7 μm) The layer thickness is changed according to the composition of the barrier layer 20a.

The structure other than the active layer 20 has a structure of a semiconductor laser having a buried hetero structure, and the arrow direction in FIG. 1A is the waveguide direction of laser light. In the semiconductor laser of the embodiment, as shown in FIG. 1B, i-InGaAs is formed above and below the active layer 20 in the thickness direction.
Waveguide layer 2 made of P (for example, wavelength λ = 1.15 μm)
2a and 22b are stacked, and the upper waveguide layer 22
An n-InP clad layer 24 is laminated on a. On both sides in the width direction of the active layer 20, the waveguide layers 22a and 22b, and the cladding layer 24, a block layer 26a made of n-InP and a block made of p-InP sandwiching the block layer 26a from both sides in the thickness direction. The layer 26b is laminated, and the active layer 20 and the like and the block layers 26a and 26b are laminated on the buffer layer 28 made of p-InP. Further, the buffer layer 28 is the substrate 3 made of p-InP.
0 stacked.

A layer 32 of n-InP is formed on the cladding layer 24 and the upper block layer 26b.
And n-InGaAsP or n-In
A layer 34 made of GaAs is laminated. A p-type electrode 36a is formed on the lower surface of the substrate 30, and an n-type electrode 36b is formed on the upper surface of the n-InGaAsP layer 34. The semiconductor laser of the embodiment oscillates when a voltage of more than the threshold current flows through the active layer 20 when a voltage is applied between the electrodes to output laser light in the direction of arrow A.

Here, in the embodiment, the active layer 20 is formed as shown in FIG.
Has an energy band diagram as outlined in FIG. Then, in FIG. 3, in order to make the wavelength of the output laser approximately 1.48 μm, each set barrier layer 20a is set.
Quantum well layers 20b1 to 20b1 that change depending on the composition of 1 to 20a5
An example of the wavelength λg (μm: 1.24 / Eg) of 20b4 is shown. In FIG. 3, the respective wavelengths of the quantum well layers 20b1 to 20b4 are indicated by λ _B1 to λ _B4 .

The composition wavelength of the barrier layer is obtained by 1.24 / barrier height (barrier band width of the barrier layer). Also, the first
The barrier heights of the fifth to fifth barrier layers 20a1 to 20a5 are sequentially reduced, and the barrier heights of the fifth barrier layers 20a1 to 20a5 are sequentially reduced in the thickness direction. _{B1 to} λ _B4 change accordingly. The barrier layers 20a1 to 20a5 are sequentially changed (in the example, they are sequentially lowered), and thus the wavelength is
The relationship is λ _B1 > λ _B2 > λ _B3 > λ _B4 . The wavelength of the first quantum well layer 20b1 is in accordance with the barrier height on the carrier injection side.

[0019] Thus, based on the relationship of FIG. 3, the Symbol first to fourth quantum well layer 20B1～20b4, the single wavelength (embodiments that wavelength lambda _B1 to [lambda] _B4 is intended 1.
A film having a thickness of 48 μm) is formed. As a result, the wavelength of the laser emitted from the semiconductor laser becomes the closest to a single wavelength.

Although the barrier layer and the quantum well layer are both made of InGaAsP in the above-mentioned embodiment, the present invention is not limited to such a configuration and can be applied to semiconductor lasers having other configurations. For example, it is obvious that the present invention can be applied to those having the number of layers different from those of the embodiment, and those having a quantum well structure in which the barrier layer is AlGaAs and the quantum well layer is GaAs. In addition to the MOPVD method, each layer is formed by MB
It can be performed by using an appropriate method depending on the semiconductor such as the E method.

[0021]

As described above, according to the present invention, the carrier transport time between quantum wells can be shortened and the distribution of carrier density injected into each quantum well can be made uniform. Therefore, it is possible to reliably prevent the increase of the oscillation threshold and the broadening of the oscillation spectrum line width due to the difference in the wavelength of each well in the multiple quantum well (oscillation wavelength when compared with a single quantum well in a laser). is there.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of a semiconductor laser of an embodiment of the present invention. (A) is an overall perspective view including a cross section of a main part,
(B) is an enlarged view of a main part of (a), and (c) is an enlarged explanatory view of the periphery of the active layer of (b).

FIG. 2 shows a schematic energy band diagram of the active layer 20 of FIG.

FIG. 3 is a graph showing an example of the relationship between wavelength and quantum well layer thickness according to the present invention.

FIG. 4 is an example of a band structure of a semiconductor laser having a multi-quantum well structure with a separate confinement heterojunction.

FIG. 5 is an example of a band diagram of a conventional multiple quantum well structure.

FIG. 6 is another example of a band diagram of a conventional multiple quantum well structure.

[Explanation of symbols]

20 active layers 20a1 to 20a5 first to fifth barrier layers 20b1 to 20b4 first to fourth quantum well layers λ _{B1 to} λ _B4 wavelengths

Claims (1)

[Claims] 1. A first device having a thickness equal to or less than a thickness at which a quantum effect occurs
In a quantum well structure of a semiconductor laser in which a plurality of quantum well layers made of the above semiconductor and a barrier layer made of a second semiconductor having a forbidden band width wider than that of the quantum well layers are stacked, each barrier layer has a carrier density distribution. , Each forbidden band width changes in order in the thickness direction, and each quantum well layer has the same wavelength in each well layer according to the amount of change in the forbidden band width of the barrier layer. A quantum well structure for a semiconductor laser, characterized in that its thickness is changed so that