JPH07202328A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To improve characteristics such as a threshold value and slope efficiency of a semiconductor laser by thinly forming barrier layers and having a quantum effect together with effectively injecting carriers into a quantum well. CONSTITUTION:InGaAsP having a wider band gap than InGaAsP of the quantum well layers 4 is used for barrier layers 3. Then, these barrier layers 3 have InGaAsP having two or more different band gaps or are the InGaAsP layers having continuously changing compositions. Then, these barrier layers 3 are InGaAsP layers either having two or more different band gaps or having continuously changing compositions and the barrier layers 3 have a thickness such that wave functions of carriers between the respective quantum wells are not overlapped. In this way, while thinly forming the barrier wall layers 3 having a quantum effect, carriers are injected effectively into the quantum well quantity. Thereby, the characteristics of a threshold value and slope efficiency of a semiconductor laser are improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a long wavelength semiconductor laser which is a light source of an optical communication system and a light source of an optical measuring instrument.

[0002]

2. Description of the Related Art In recent years, there have been many reports that laser characteristics have been remarkably improved by using multiple quantum wells in the active layer of a semiconductor laser.

FIG. 3 schematically shows the band structure in the active layer of a conventional 1.3 μm multiple quantum well semiconductor laser. In the figure, 1 is an n-InP buffer layer formed on an n-type InP substrate, and 2 is an I having a composition of 1.13 μm.
nGaAsP optical waveguide layer, 3 is 1.13 μm composition InG
aAsP barrier layer, 4 is a 1.4 μm composition InGaAsP quantum well layer, the total number is 7, and 5 is a p-InP clad layer.
Reference numeral 1 denotes the first quantum level of electrons, and 12 denotes the first quantum level of holes. In this case, the thickness of the quantum well layer is such that the energy difference between the first quantum levels of electrons and holes is 1.
It is about 42 Å so as to be 3 μm,
The thickness of the barrier layer is about 100 Å so that carrier wave functions do not overlap between adjacent quantum wells. As can be seen from this example, in the conventional multiple quantum well laser, the composition of each barrier layer is the same, and each barrier layer is composed of one layer of uniform composition having a larger band gap than the quantum well layer. It is common to have In a conventional multi-quantum well laser in which such a barrier layer has a single composition, the barrier layer is thickened in order to reduce the overlap of carrier wave functions between the quantum wells, and the carriers distributed on the barrier layer are reduced. The thickness of the barrier layer has been determined as a compromise between the two contradictory factors of making the barrier layer thin in order to reduce the number of carriers and increase the number of carriers injected into the quantum well layer.

On the other hand, in a GaAs-based short wavelength band laser in which the band gap difference between the quantum well layer and the barrier layer can be secured, the barrier layer is composed of two or more kinds of semiconductors having different band gaps as described in "JP-A-62-35591". It has been proposed that the injection efficiency of the carrier and the quantum effect are made compatible by forming the layer by stacking the layers, and dividing the carrier into a part for tunneling the carrier and a part for confining the carrier in the quantum level.

[0005]

In the conventional multiple quantum well laser in which the barrier layer has a single composition, it is necessary to prevent carrier wave functions from crossing between adjacent quantum wells in order to sufficiently exert the quantum effect. It is necessary to increase the barrier layer thickness. As a result, carriers are distributed on the barrier layer, and there is a drawback that carrier injection efficiency into the quantum well is poor. In the case of a long-wavelength laser used for optical communication, a method of tunneling carriers using the second quantum level of carriers as described in "Japanese Patent Laid-Open No. 62-35591" is a band of a quantum well layer and a barrier layer. Since the gap difference cannot be large, the second quantum level cannot exist and cannot be applied.

The present invention has been made in order to solve the above-mentioned problems, and a barrier layer is formed thick to maintain a quantum effect and carriers are effectively injected into a quantum well, so that the threshold value of a semiconductor laser, slope efficiency, etc. The purpose is to improve the characteristics of.

[0007]

The long wavelength multiple quantum well semiconductor laser of the present invention uses InGaAs or InGaAsP for the quantum well layer and InGa of the quantum well layer for the barrier layer.
InGaAs with wider bandgap than AsP
P is used, and the barrier layer has InGaAsP having at least two kinds of different band gaps,
Alternatively, the composition is an InGaAsP layer whose composition changes continuously, and the barrier layer has a thickness such that carrier wave functions do not overlap between the quantum well layers.

[0008]

The carriers injected from the electrode into the active part composed of multiple quantum wells are injected into the quantum well layer at a certain ratio, but the remaining carriers are distributed in the barrier layer and the optical waveguide layer. If the thickness of the barrier layer is increased in order to sufficiently exert the quantum effect, the proportion of carriers distributed in the barrier layer increases and the proportion injected into the quantum well layer decreases. Carriers on the barrier layer have a lower energy by changing from a conventional barrier layer of uniform composition to a barrier layer composed of multiple thin layers of multiple compositions, or a barrier layer in which the composition changes continuously. Since it can be easily relaxed, it is not distributed on the barrier layer and the carrier injection efficiency into the quantum well layer is improved. Further, since the thickness of the entire barrier layer is such that the wave function of carriers trapped in the quantum well layer does not overlap with the wave function of carriers of the adjacent quantum wells, the quantum effect can be sufficiently exhibited.

[0009]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows the first embodiment of 1.55 of the present invention.
1 schematically shows a band structure in an active layer of a μm band multiple quantum well type semiconductor laser. In the figure, 1 is n-
InP buffer layer, 2. InGaA having a composition of 1.13 μm
sP optical waveguide layer, 3 is a 1.2 μm composition InGaAsP barrier layer, 6 is a 1.13 μm composition InGaAsP barrier layer, 4
Is a 1.65 μm compositionally strained InGaAsP quantum well layer, and 5 is a p-InP cladding layer. The thickness of each layer is n
-InP buffer layer is 0.4 μm, InGaAsP optical waveguide layer is 60 nm on p-side and n-side, 1.2 μm composition InGaAsP barrier layer is 3 nm and 1.13 μm InGa on both sides of 1.13 μm InGaAsP barrier layer, respectively.
The AsP barrier layer is 4 nm, the quantum well layer is 4 nm, and p-In
The P clad layer has a thickness of 0.7 μm. The number of quantum wells is 5. Reference numeral 11 denotes the first quantum level of electrons, and 12 denotes the first quantum level of holes. As a method of forming such an extremely thin semiconductor layer, a metal organic chemical vapor deposition method or a molecular beam growth method is used. After this, for example, a photolithography method and an LPE growth method are used to form a buried semiconductor laser in a buried structure such as a DC-PBH structure. In this embodiment, the total thickness of the barrier layer is 10 nm, and it is possible to prevent the carrier wave functions from overlapping between adjacent quantum wells and to fully exert the quantum effect. Further, since the layer thickness of each composition of the barrier layer is 4 nm or less, the carriers on the barrier layer are easily relaxed to a state of low energy, so that the injection efficiency into the quantum well layer is also improved.

In this embodiment, 1.0% and 90% coatings were applied to the front and rear end faces, respectively, with a cavity length of 300 μm, and the optical output characteristics were measured.
Values of mA and slope efficiency of 0.5 W / A were obtained. This value is about 5m in threshold value compared with the conventional multiple quantum well laser.
A was low, and the slope efficiency was about 10% greater.

FIG. 2 shows 1.3 μm of the second embodiment of the present invention.
1 schematically shows a band structure in an active layer of a band multi-quantum well semiconductor laser. In the figure, 1 is n-In
P buffer layer, 2 is InGaAsP having a composition of 1.13 μm
Optical waveguide layer, 3 is InGaAsP barrier layer, 4 is 1.44
μm composition strain InGaAsP quantum well layer, 5 is p-InP
It is a clad layer. In this second embodiment, InGaAs
The P barrier layer has a composition of 1.2 μm at the interface with the quantum well layer, the center of the barrier layer has a composition of 1.13 μm, and the interface with the next quantum well layer has a composition of 1.2 μm.
The feature is that the composition can be continuously changed while maintaining the lattice matching with. In order to form such a barrier layer, for example, a metalorganic vapor phase epitaxy method may be used and the flow rate of the raw material may be changed under the condition of lattice matching with InP. The total thickness of this barrier layer is 10 nm, which is sufficient to prevent overlapping of carrier wave functions between quantum wells. Further, since the potential is inclined on the barrier layer, the carriers cannot stably exist on the barrier layer, so that the carrier injection efficiency into the quantum well layer is extremely high. This second embodiment is useful in that the number of hetero interfaces can be reduced and scattering loss at the hetero interfaces can be reduced when the total number of quantum wells is as large as 10 layers or more in order to realize an extremely low threshold value. The production of a semiconductor laser is exactly the same as in the first embodiment.

In the second embodiment, the threshold is about 5 mA when the cavity length is 300 μm and the low-reflection film and the high-reflection film of 30% and 90% are respectively attached to the end faces, which is about 5 mA compared to the conventional multiple quantum well semiconductor laser. Was as low as 5 mA.

Next, a third embodiment will be described. FIG. 4 shows a sectional view of a lateral injection type multiple quantum well DFB laser of the third embodiment. A diffraction grating is formed on a semi-insulating InP substrate 21, and an active layer 22 composed of multiple quantum wells sandwiched between InGaAsP optical waveguide layers 2 and a high-resistance InP layer 23 are stacked on the diffraction grating. Next, a channel for n-electrode is formed to form n-InP.
26, and a channel is formed for the p-side electrode, and then p-InP 24 and P ⁺ -InGaAs contact layer 25 are sequentially formed. Finally, n28 and p27 type electrode metals separated by the SiO ₂ insulating film 29 are formed.
The multiple quantum well active part is schematically shown in FIG. The quantum well layer 4 is strained InGaAsP having a composition of 1.44 μm. The barrier layer 6 is 1.13 μm in composition at the interface with the quantum well layer, 1.05 μm in composition at the center of the barrier layer, and 1.13 μm in composition at the interface with the next quantum well layer. In whose composition was continuously changed while maintaining
GaAsP has a thickness of 100 Å. The optical waveguide 2 is a semi-insulating InP substrate 2 on which a diffraction grating is formed.
1 and the InGaAsP having a high resistance InP layer 23 having a composition of 1.0 μm and an interface with the quantum well layer of 1.13 μm, the composition of which is continuously changed while maintaining lattice matching with InP. is there. In this example,
Electrons are laterally injected into the quantum well from n-InP26 and holes from p-InP22. The number of quantum well layers is 20. When the photoluminescence vector was measured while changing the thickness of the barrier layer, an increase in the PL half-width, which is considered to be due to the formation of a miniband due to the overlapping of the wave functions, was observed at 80 Å or less.
Therefore, it is considered that if the thickness of the barrier layer is 100 Å, the overlapping of the wave functions can be sufficiently prevented.
In the case of a lateral injection laser, the number of quantum well layers can be increased because carriers are uniformly injected into the quantum wells regardless of the positions of the quantum well layers. The large number of quantum well layers means that the number of barrier layers is also large. As a result, in a conventional barrier layer having a constant composition of the barrier layer, the proportion of carriers distributed on the barrier layer increases. In the third embodiment, as shown in FIG. 5, the potential is inclined on the barrier layer and the carriers cannot stably exist on the barrier layer, so that the efficiency of carrier injection into the quantum well layer is further improved. The third embodiment is useful in that the number of quantum well layers can be extremely increased in order to increase the relaxation oscillation frequency.

In the third embodiment, the relaxation oscillation frequency at a drive current about twice the threshold value is 12 GHz, which is about 1.5 times faster than that of the conventional lateral injection laser.

In the above embodiments, the buried type semiconductor laser is taken as an example, but the present invention can be applied to lasers of any structure having a multiple quantum well layer as an active layer.

[0016]

As described above, in the multiple quantum well semiconductor laser according to the present invention, in order to sufficiently bring out the quantum effect, the barrier layer having a sufficient thickness to suppress the overlapping of carrier wave functions between the quantum wells. However, since the ratio of carriers distributed on the barrier layer can be reduced, carrier injection efficiency into the quantum well layer is improved, and laser characteristics such as threshold value and slope efficiency are improved.

In the first embodiment, when the resonator length is 300 μm, the front and rear end faces are coated with 1.0% and 90%, respectively, and the optical output characteristics are measured.
Values of mA and slope efficiency of 0.5 W / A were obtained. This value is about 5m in threshold value compared with the conventional multiple quantum well laser.
A was low, and the slope efficiency was about 10% greater.

In the second embodiment, the resonator length is 300 μm.
The thresholds when the low reflection film and the high reflection film of 30% and 90% were respectively attached to the end faces of m, were about 5 mA, which was about 5 mA lower than that of the conventional multiple quantum well semiconductor laser.

Further, in the third embodiment, the relaxation oscillation frequency at a drive current about twice the threshold value is 12 GHz, which is about 1.5 times faster than that of the conventional lateral injection laser.

[Brief description of drawings]

FIG. 1 is a diagram showing a band structure of a first embodiment of the present invention.

FIG. 2 is a diagram showing a band structure of a second embodiment of the present invention.

FIG. 3 is a diagram showing a band structure of a conventional multiple quantum well semiconductor laser.

FIG. 4 is a sectional view of a third embodiment of the present invention.

FIG. 5 is a diagram schematically illustrating an active part according to a third embodiment of the present invention.

[Explanation of symbols]

1 n-InP buffer layer 2 InGaAsP optical waveguide layer 3 InGaAsP barrier layer 4 (strain) InGaAsP quantum well layer 5 p-InP clad layer 6 InGaAsP barrier layer 11 First quantum order of electrons 12 First quantum order of holes 21 Semi-insulation (SI) -InP substrate 22 Multiple quantum well active layer 23 High resistance (HR) -InP 24 p-InP 25 p ⁺ -InGaAs contact layer 26 n-InP 27, 28 Electrode metal 29 SiO ₂ insulating film

Claims (1)

[Claims] 1. InGaAs or In for the quantum well layer
InGaAs with a quantum well layer as a barrier layer using GaAsP
In a long wavelength multiple quantum well type semiconductor laser using InGaAsP having a band gap wider than P, the barrier layer has an InGaAsP laminated structure having at least two kinds of different band gaps, or the composition is continuous. A changing InGaAsP layer, and
A long-wavelength multiple quantum well type semiconductor laser, wherein the barrier layer has a thickness such that wave functions of carriers do not overlap between the quantum well layers.