JPH06302910A - Quantum well semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a quantum well semiconductor laser wherein the injection efficiency of a carrier into an active layer and the temperature characteristics are high, and the series resistance is low. CONSTITUTION:On an N-type GaAs substrate 1, the following are laminated and formed in order; an N-type light confinement layer 15, an InGaAs strained quantum well active layer 7, a P-type light confinement layer 16, a P-InGaP clad layer 11, and a P-GaAs cap layer 12. The right side and the left side of a semiconductor laser are etched and eliminated in the cap layer 12 and a part of the clad layer 11, and a polyimide film 17 is formed in the eliminated part. In the N-type light confinement layer 15 and the P-type light confinement layer 16, InGaAsP layers different in valence band energy are laminated and formed to constitute a stepwise energy state, and the P-type light confinement layer 16 is made thicker than the N-type light confinement layer 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum well semiconductor laser used as a light source for optical communication, for example.

[0002]

2. Description of the Related Art A semiconductor laser used as a light source for optical communication has a layer structure composed of a clad layer, an active layer and the like on the upper side of a substrate. The semiconductor laser is a lateral laser beam emitted from the laser. The device size of the laser is often limited to control the mode. Further, since it is possible to improve the emission efficiency of the semiconductor laser, the light confinement layers formed of a material system different from that of the active layer are provided on the upper and lower sides of the active layer so as to sandwich the active layer.
A semiconductor laser having a double heterojunction diode is often used.

As described above, the series resistance of a semiconductor laser having a device size limited and having a double heterojunction diode is almost determined by the current-voltage characteristics of the double heterojunction diode. Further, it is known that a semiconductor laser formed of a material system having a large offset on the valence band side of the band gap between the cladding layer and the active layer has a large series resistance and a low characteristic temperature. In particular, in a semiconductor laser having a simple optical-carrier separation confinement structure in which each of the optical confinement layers is formed of a uniform material, the tendency is more remarkable.

That is, when a voltage is applied to the semiconductor laser, electrons or holes (holes) that are carriers are injected into the active layer, but if the material forming the cladding layer and the active layer has a large offset on the valence band side. , The spikes formed at the upper and lower interfaces of the active layer become a quantum barrier, making it difficult for energetically heavy holes to cross the barrier, hindering the injection of holes into the active layer, and increasing the series resistance. Will be done.

Further, in the semiconductor laser having a simple optical-carrier separation confinement structure, as shown in FIG. 4, since the spike 21 is particularly high in energy and wide, the hole 20 below the valence band E _V is formed. Is hard to move to the suspicious Fermi level E _FP side of the hole 20, which hinders injection into the active layer, which is a factor that greatly increases the series resistance. Further, as described above, if the injection of the holes 20 into the active layer is hindered by the spikes 21, the characteristic temperature of the semiconductor laser is also lowered.

By the way, in recent years, there has been a demand for higher output and higher speed of a semiconductor laser, and in order to realize such a high output laser and a laser driven at a high speed, the series resistance of the semiconductor laser is reduced. It is indispensable that a semiconductor laser having a graded index type optical-carrier separation / confinement structure is often used in place of the simple optical-carrier separation / confinement structure semiconductor laser having a large series resistance as described above. Became.

The graded index type semiconductor laser divides the spike into narrow spikes having low energy, or in order to remove the spike completely,
The light confinement layer is a laser configured to gradually change in energy, and as a material for forming the light confinement layer, InGaAsP / InP or InGaAsP /
A lattice matching material such as GaAs is used. In a graded index type semiconductor laser with an optical-carrier separated confinement structure, it is technically difficult to continuously change the composition while maintaining the lattice matching of these lattice-matched materials. By stacking light confinement material layers of different lattice matching systems in multiple layers, the layers are formed so as to be stepwise in terms of energy, and the light confinement layers are configured to gradually change in energy.

[0008]

However, in order to couple a semiconductor laser to a micro waveguide such as an optical fiber, it is necessary to make the beam shape of the laser light circular and narrow. For laser light control,
When the thickness of the optical confinement layer of the graded index type semiconductor laser is reduced, the spikes existing at the interfaces of the optical confinement layers forming the multilayer optical confinement layer come close to each other, and the spikes are low in energy and narrow. But if it is close, for a hole that is heavy in energy and slow in movement,
There was a problem that it became a barrier group that was difficult to cross.
As described above, when it becomes difficult for the holes to cross the close barrier group, the series resistance increases and the characteristic temperature decreases.

The present invention has been made to solve the above problems, and an object thereof is to provide a quantum well semiconductor laser having high carrier injection efficiency and high characteristic temperature and low series resistance.

[0010]

In order to solve the above problems, the present invention is configured as follows. That is, the present invention provides a quantum well semiconductor laser having a graded index type optical-carrier separation confinement structure in which a strained quantum well active layer is sandwiched between a multilayer p-type optical confinement layer and a multilayer n-type optical confinement layer. The p-type light confinement layer is thicker than the n-type light confinement layer.

[0011]

In the present invention having the above-described structure, since the p-type light confinement layer is formed thicker than the n-type light confinement layer, when the holes are injected into the active layer, the multi-layer The influence of the valence band spike existing in the p-type optical confinement layer on the injection of holes into the active layer is reduced. Therefore, the injection efficiency with which holes are injected into the active layer and the characteristic temperature increase, and the series resistance also decreases.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the quantum well semiconductor laser according to the present invention. This laser is a ridge waveguide laser having a width of 3 μm and a cavity length of 1 mm. In the figure,
0.5 μm thick n-GaAs on n-type GaAs substrate 1
A buffer layer 2, an n-InGaP cladding layer 3 having a thickness of 2.0 μm, a multilayer n-type optical confinement layer 15 having a thickness of 30 nm,
A strained quantum well active layer 7 of InGaAs having a film thickness of 9 nm and a multi-layered p-type optical confinement layer 16 having a film thickness of 60 nm are sequentially laminated.

A film thickness of 2.0 is formed on the upper side of the p-type optical confinement layer 16.
A p-InGaP clad layer 11 having a thickness of 0.5 μm and a p-GaAs cap layer 12 having a thickness of 0.5 μm are laminated in this order, and the cap layer 12 and a part of the clad layer 11 are removed by etching on the left and right sides of the laminated semiconductor. After that, a polyimide insulating film 17 is laminated on both left and right sides of the clad layer 11. An n-side electrode 13 is formed on the lower surface side of the substrate 1, and a p-side electrode 14 is formed on the upper surface side of the polyimide insulating film 17 and the cap layer 12.
Is formed.

This quantum well semiconductor laser is a laser having a graded index type light-carrier separation structure. As shown in FIG. 2, the n-type optical confinement layer 15 has a film thickness.
10 nm n-InGaAsP optical confinement layers 4, 5, 6
Of the n-type optical confinement layers 4, 5 and 6 respectively.
Are 1.77 eV, 1.65 eV, and 1.55 eV, respectively, and the n-type optical confinement layer 15 is a stepwise layer in terms of energy. p
The type optical confinement layer 16 is formed by sequentially stacking p-InGaAsP optical confinement layers 8, 9 and 10 having a film thickness of 20 nm,
The Eg of each of the p-type optical confinement layers 8, 9 and 10 is 1.55 eV, 1.65 eV and 1.75 eV, respectively, and the p-type optical confinement layer 16 is also an energy stepwise layer.

The layers 4, 5 of the n-type optical confinement layer 15 are also provided.
6 and the p-type optical confinement layer 16 have a valence band energy difference between the layers 8, 9 and 10 which are symmetrical with respect to each other with the active layer 7 in between, and the optical confinement layer at the bottom of the n-type optical confinement layer 15 is provided. The valence band energy (Eg) of No. 4 and the Eg of the uppermost optical confinement layer 10 of the p-type optical confinement layer 16 are the same. Thus, the Eg of the n-type optical confinement layer 4 and the p-type optical confinement layer 10 are configured to be the same, and the active layer 7 and the optical confinement layers 4 and 10 have the same valence band energy difference. It controls the radiation pattern field of the laser light emitted from the.

The radiation pattern field is controlled by the valence band energy difference between each light confinement layer 4, 10 and the active layer 7, and at the same time is controlled by the sum of the thicknesses of the light confinement layers 15, 16. That is, when the radiation pattern field is set to a certain form, the sum of the thicknesses of the p-type and n-type optical confinement layers is uniquely determined corresponding to the radiation pattern field when the semiconductor laser is manufactured. In the conventional example, the p-type optical confinement layer and the n-type optical confinement layer have the same thickness under the condition that the sum of the thicknesses of the p-type and n-type optical confinement layers is constant. Is characterized in that the thickness of the p-type light confinement layer 16 is thicker than that of the n-type light confinement layer 15 under the condition that the sum of the p-type and n-type light confinement layers is constant. .

The quantum well semiconductor laser of this embodiment is constructed as described above, and its operation will be described below. When a voltage is applied to the n-side electrode 13 and the p-side electrode 14, electrons are injected into the strained quantum well active layer 7 from the n-type optical confinement layer 15 side, and holes are injected from the p-type optical confinement layer 16 side. . In the active layer 7, the injected electrons and holes are recombined, and the oscillated light is amplified by the active layer 7 and emitted as laser light.

By the way, p of this quantum well semiconductor laser is
The type light confinement layer 16 is formed to be thicker than the n type light confinement layer 15, and each p type light confinement layer 8,
Since the thicknesses of 9 and 10 are also large, as shown in FIG. 3, the valence band E _V structure of the p-type optical confinement layer 16 composed of the valence bands E _V of the p-type optical confinement layers 8, 9 and 10 is It has a stepwise energy-like shape. Therefore, the spikes 21 formed at the interfaces of the p-type optical confinement layers 8, 9 and 10
Is a stepwise valence band E _V that is gentle in terms of energy
Is formed on the boundary side of the spike 21 and the width d of the spike 21 and the spike 21 becomes wider.

When a voltage is applied to the electrodes 13 and 14 of the semiconductor laser, the hole 20 below the valence band E _V becomes
As shown by the arrow, the holes 20 move to the quasi-Fermi level E _FP side and are injected into the active layer 7. At this time, since the width d between the spikes 21 formed on the boundary side of the valence band E _V is wide, , The spike 21 does not become a close barrier group that prevents the movement of the hole 20, and the hole 20 moves to the pseudo Fermi level E _FP without any trouble and is injected into the active layer 7. Therefore, the injection efficiency of the holes 20 into the active layer 7 increases, the series resistance of the semiconductor laser decreases, and the characteristic temperature also increases.

On the other hand, the n-type optical confinement layer 15 is thinner than the p-type confinement layer 16, and the n-type optical confinement layer 15 is correspondingly thin.
The width d of the spike 21 generated at the interface between the layers 8, 9, and 10 becomes narrower and closer to each other, but the electron is not heavy in energy like the hole 20, so that the spike 21 may hinder the movement of the electron. However, the electrons are safely injected into the active layer 7.

Therefore, in the quantum well semiconductor laser in which the thickness of the p-type optical confinement layer 16 is larger than that of the n-type optical confinement layer 15 as in the present embodiment, injection of electrons into the active layer 7 and the like are performed as in the conventional case. Since the injection efficiency of the holes 20 into the active layer 7 is improved, the result is an excellent quantum well semiconductor laser with high carrier injection efficiency and high characteristic temperature and low series resistance.

In order to confirm the effect of this embodiment, as a comparative example, a conventional example in which the thicknesses of the light confinement layers on the upper and lower sides of the active layer 7, that is, the p-type light confinement layer and the n-type light confinement layer are made equal. Make a quantum well semiconductor laser similar to
A comparative study was performed with the quantum well semiconductor laser of this example. The laser of this comparative example has the same structure as that of the present embodiment except for the optical confinement layer, and each of the p-InGaAsP optical confinement layers 8 forming the p-type optical confinement layer 16,
In the quantum well semiconductor laser, the thickness of each of the n-InGaAsP optical confinement layers 4, 5 and 6 is 15 nm, and the thickness of each of the optical confinement layers 9 and 10 is 15 nm. is there. That is, the p-type light confinement layer 16 and the n-type light confinement layer 15 are both formed to have the same thickness of 45 nm, and the sum of the thicknesses of the p-type light confinement layer 15 and the n-type light confinement layer 16 is 90 nm. .

The p-type optical confinement layer 16 of the quantum well semiconductor laser of this embodiment is formed of the optical confinement layers 8, 9 and 10 of p-InGaAsP having a thickness of 20 nm, and the n-type optical confinement layer 15 is formed.
Is formed by the optical confinement layers 4, 5 and 6 of n-InGaAsP having a thickness of 10 nm, and the thickness of the p-type optical confinement layer 16 is 60.
The thickness of the n-type light confinement layer 15 is 30 nm, and the thickness of the p-type light confinement layer 16 is larger than that of the n-type light confinement layer 15. The sum of the thicknesses of the p-type light confinement layer 16 and the n-type light confinement layer 15 is 90 nm, which is the same value in this example and the comparative example.

Regarding the quantum well semiconductor laser as described above, the full-width at half maximum of the vertical far-field image of the laser is set to 20 degrees, and
The voltage during driving at 0 mA was measured. As a result, the quantum well semiconductor laser of this example had a voltage of 1.9 V, and the laser of the comparative example had a voltage of 2.1 V. The voltage of the laser of this example was 0.2 V lower than that of the laser of the comparative example. Therefore, according to this study, the laser of the present embodiment in which the thickness of the p-type optical confinement layer 16 is made thicker than the thickness of the n-type optical confinement layer 15, the p-type optical confinement layer 16 and the n-type optical confinement layer 15 are It was proved that the series resistance was lower than that of the conventional example laser having the same thickness, and the excellent laser exhibited the same driving force as that of the conventional example even at a low voltage.

The structure of the present invention is not limited to the above embodiment, and various embodiments can be adopted. For example, in the above embodiment, each layer 4 of the n-type optical confinement layer 15 is
Although the band gap energy difference between the layers 5, 9 and 10 of the p-type optical confinement layer 16 is vertically symmetrical with the active layer 7 in between, the band gap energy difference is not necessarily vertically symmetrical. However, the value of each bandgap energy is not always the value shown in the above embodiment. However, by making the bandgap energies of the light confinement layer 4 at the bottom of the n-type light confinement layer 15 and the light confinement layer 10 at the top of the p-type light confinement layer 15 the same, the pattern field of the laser light is It is desirable to regulate

Further, in the above embodiment, the n-type optical confinement layer
The layers 4, 5, 6 forming 15 and the layers 8, 9, 10 forming the p-type optical confinement layer 16 each have three layers.
The number of layers forming each of the light confinement layers 15 and 16 may be other than three, and the number of layers may be different between the n-type light confinement layer 15 and the p-type light confinement layer.

In the above embodiment, the ridge waveguide laser has a width of 3 μm and a cavity length of 1 mm, but the size of the laser, the structure of the laser, the composition of each layer constituting the laser, the number of layers, etc. The present invention can be applied to all other quantum well semiconductor lasers without limitation.

[0028]

According to the present invention, the thickness of the p-type optical confinement layer sandwiching the active layer is made thicker than the thickness of the n-type confinement layer, so that the p-type optical confinement layer side of the multilayer is injected into the active layer. Holes are less affected by valence band spikes formed in the p-type optical confinement layer, and can be injected into the active layer without any trouble. Therefore, it is possible to increase the injection efficiency and characteristic temperature at which holes are injected into the active layer,
The series resistance can be reduced. On the other hand, since electrons injected from the n-type optical confinement layer side into the active layer are not heavy in energy like holes, they are injected into the active layer as usual. Therefore, the present invention has high carrier injection efficiency and high characteristic temperature. An excellent quantum well semiconductor laser with low series resistance can be obtained. From these facts, the present invention can realize high output and high speed of the semiconductor laser.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a quantum well semiconductor laser according to the present invention.

2 is an explanatory diagram showing a multilayer structure of an active layer 7 and optical confinement layers 15 and 16 of FIG. 1 and their energy states. FIG.

FIG. 3 is an explanatory diagram showing movement of holes 20 on the side of the multilayer p-type optical confinement layer 16 of the present embodiment.

FIG. 4 is an explanatory diagram showing movement of holes 20 of a laser having a simple optical-carrier separation confinement structure.

[Explanation of symbols]

 4, 5, 6, 15 n-type optical confinement layer 7 active layer 8, 9, 10, 16 p-type optical confinement layer 20 hole 21 spike

Claims (1)

[Claims] 1. A quantum well semiconductor laser having a graded index type optical-carrier separation confinement structure in which a strained quantum well active layer is sandwiched between a multilayer p-type optical confinement layer and a multilayer n-type optical confinement layer, A quantum well semiconductor laser, wherein the p-type light confinement layer is thicker than the n-type light confinement layer.