JP2000286501A - Distributed feedback semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distributed feedback semiconductor laser element of high single mode yield, low oscillating threshold current and high quantum efficiency. SOLUTION: In a distributed feedback semiconductor laser element that contains a layer structure, wherein a light absorption layer 9 of diffraction grating structure is formed above or below an active layer 4, an equivalent absorption coefficient of the light absorption layer 9 is large in a center part R1 of a resonator, and is small in the vicinity of an end surface R2 thereof. More specifically, the light absorption layer 9 is composed of the same semiconductor material, and its thickness is large at the center part R1 of the resonator, becomes thinner the nearer it gets to the vicinity of end surface R2 thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser device, and more particularly, to a distributed feedback semiconductor laser device which exhibits high quantum efficiency even at a low lasing threshold current and realizes a stable single mode with a high yield. The present invention relates to a laser device.

[0002]

2. Description of the Related Art A wavelength division multiplexing communication system has been realized, but it is necessary for a laser light source used for the same to realize stable single mode oscillation with a high yield, and high operation reliability. It is necessary to reduce the driving power in order to secure the power.

With respect to a laser light source that satisfies such demands, a diffraction grating structure that periodically changes the real and imaginary parts of the refractive index is formed inside a resonator made of a semiconductor, and is fed back only to a laser beam of a specific wavelength. A distributed feedback semiconductor laser device having wavelength selectivity as described above has been studied. FIG. 5 shows an example A of a distributed feedback semiconductor laser device having a light absorbing diffraction grating structure among the distributed feedback semiconductor laser devices.

In this laser device A, a buffer layer 2 made of, for example, n-InP is formed on a substrate 1 made of, for example, n-InP, and further, for example, i-GaIn
Lower optical confinement layer 3 made of AsP, for example, i-GaI
Active layer 4 having a quantum well structure using nAsP, for example, i-
An upper light confinement layer 5 made of GaInAsP is sequentially stacked. Furthermore, an upper cladding layer 6 made of, for example, p-InP and a p-I
A contact layer 7 made of nGaAs is laminated, and the entire side portion is made of a layer 8a made of p-InP and an n-
The layer 8b made of InP is buried in the buried layer 8 which is laminated in this order.

Although the lower cladding layer is not provided in the above-described layer structure, in the case of this material system, both the buffer layer and the cladding layer are of the n-InP type, so they may be regarded as the same. Note that a portion corresponding to the lower cladding layer may be provided separately from the buffer layer. In the upper cladding layer 6, a light absorption layer 9 having a diffraction grating structure composed of, for example, an n-InGaAs diffraction grating 9a is formed at a position separated upward from the upper light confinement layer 5.

The light absorbing layer 9 is formed so as to extend in the longitudinal direction of the element (resonator) within the width sandwiched by the buried layers 8, and each diffraction grating 9a constituting the light absorbing layer 9 is formed. Are all the same in composition and thickness, and the period of the diffraction grating 9a is about 240 nm. In this embedded device, a lower electrode and an upper electrode (both not shown) are provided on the back surface of the substrate 1 and the upper surface of the contact layer 7, respectively.
It is needless to say that a reflection film (not shown) having a predetermined reflectance may be formed on both end surfaces.

[0007] It has been theoretically calculated that the laser device having this structure oscillates on the short wavelength side of the stop band, so that the single-mode yield is higher than that of an ordinary compound with a complex refractive index. Therefore, the distributed feedback semiconductor laser device having the light absorbing diffraction grating structure is positioned as a promising single mode laser device.

[0008]

However, in the case of the laser device having the above structure, the light absorbing layer has a diffraction grating structure in which diffraction gratings having the same composition and thickness are uniformly arranged at equal intervals in the longitudinal direction of the resonator. Therefore, there is a problem that the loss of the oscillation laser light as a whole of the resonator becomes large.

As a result, the oscillation threshold current for driving the laser element increases, and the quantum efficiency of the laser decreases. The present invention solves the above-described problems in a distributed feedback semiconductor laser device having a light absorbing diffraction grating structure,
The object of the present invention is to provide a distributed feedback semiconductor laser device in which the single mode yield is high and the loss is small, so that the oscillation threshold current is low, and the quantum efficiency is high even at a low oscillation threshold current. .

[0010]

According to the present invention, there is provided a distributed feedback semiconductor having a layer structure in which a light absorption layer having a diffraction grating structure is formed above or below an active layer. In the laser device, there is provided a distributed feedback semiconductor laser device, wherein an equivalent absorption coefficient of the light absorption layer is large at a central portion of the resonator and small near an end face of the resonator. .

In this case, the absorption coefficient of the light absorption layer in the cavity length direction is adjusted by combining the thickness of the absorption layer and the semiconductor material constituting the absorption layer. Specifically, a distributed feedback semiconductor laser in which the diffraction grating of the light absorption layer is made of the same semiconductor material, and the thickness of which is thicker at the center of the resonator and becomes thinner toward the end face of the resonator. An element is provided.

[0012]

1 and 2 show an example B of a laser device according to the present invention. FIG. 1 is a partially cutaway perspective view showing the entire layer structure of the laser element B, and FIG. 2 is an enlarged view showing a dashed area C in FIG. In the case of the laser element B shown in FIGS. 1 and 2, a light absorbing layer described later is formed of the same semiconductor material.

The laser element B has the same layer structure as the laser element A shown in FIG. 5 except that the light absorbing layer 9 has a configuration described later. The same is true for the individual diffraction gratings 9a constituting the light absorbing layer 9 being arranged at a predetermined period in the longitudinal direction of the resonator. However, the thickness of the diffraction grating 9a is arranged in a grating structure, the laser device A shown in FIG. 5 in that it is gradually thinner from the central portion R ₁ of the cavity toward the end face neighborhood R ₂ resonators Is different.

By configuring the light absorbing layer 9 as described above, a high single mode yield is secured as in the case of the laser element A, and the equivalent of the light absorbing layer at the central portion R ₁ of the resonator is obtained. absorption coefficient is large, also equivalent absorption coefficient near the end surface R ₂ side as the light-absorbing layer of the resonator is small. Accordingly, since the loss is smaller at the end face neighborhood R ₂ of the oscillation laser beam, is also lowered oscillation threshold current required to drive the entire device, also the quantum efficiency is higher.

That is, the laser element B has a low oscillation threshold current because of a small loss while securing a high single-mode yield, and a laser element driven with high quantum efficiency even at the low oscillation threshold current. It has become. As described above, the laser device of the present invention increases the equivalent absorption coefficient in the portion of the light absorption layer located at the central portion R ₁ of the resonator, and increases the equivalent absorption coefficient in the portion of the light absorption layer located near the end face R _2. the absorption coefficient to reduce the loss of the emitted laser light at the end face neighborhood R ₂ by reducing, with it have been developed in the technical idea that to lower the oscillation threshold current.

The laser element B shown in FIGS. 1 and 2 is an embodiment in which the semiconductor material constituting the light absorbing layer 9 is all the same for each diffraction grating 9a and the above-described technical idea is realized by changing the thickness. . In order to realize the above technical idea, the present invention is not limited to a mode such as the laser element B,
For example, the diffraction grating 9a all thickness same city in the light absorbing layer 9, however, the resonance center portion of the vessel R ₁ and the end face neighborhood R ₂
May be made of semiconductor materials having different compositions and therefore different absorption coefficients, and the light absorbing layer may be formed by changing both the thickness and the composition.

Next, a method for manufacturing a laser device of the present invention will be described.
A description will be given by taking the laser element B shown in FIG. 1 as an example. First,
As shown in FIG. 3, the MOCVD method or the G
A buffer layer 2, a lower optical confinement layer 3, an active layer 4, and an upper optical confinement layer 5 are sequentially formed by a predetermined semiconductor material by the SMBE method or the like, and a semiconductor material for an upper clad is formed on the upper optical confinement layer 5. producing a layer structure B ₀ to form a layer 6a made of. The thickness of this layer 6a differs depending on the semiconductor material in order to exert a function of overlapping a part of the electric field with a light absorption layer described later formed thereon, but is usually set to about 30 to 500 nm. Is done.

Then, after covering the entire surface of the layer 6a of the layer structure B ₀ , for example, a SiNx film is formed, and then, for example, RI
E to form a mask M having a planar pattern in which the openings D shown in FIG.
Manufacture ₁ . The opening M is a central portion D ₁ is a narrow opening, and both ends have become wide opening D _2, the entire longitudinal direction, the same as the longitudinal direction of the resonator in the device to be formed Each is formed at a location corresponding to a plane location where the light absorbing layer is to be formed.

Next, a semiconductor material constituting a light absorbing layer to be formed on the layer structure B ₁ is selectively grown,
The semiconductor material is deposited on the layer 6a (part of the upper clad layer) exposed from the opening D of the mask M. During this selective growth, narrow semiconductor material is deposited thickly in the width opening D _1, the thickness of the semiconductor material deposited as the opening becomes wider becomes thinner, the thinnest at both ends of the opening is widest. Therefore, when the selective growth is completed, a layer is formed in the opening D of the mask, the layer having the largest thickness at the central portion (narrow opening portion D ₁ ) and decreasing in thickness toward both ends. This is the desired light absorbing layer precursor.

Next, the mask M is removed by, for example, RIE. The precursor of the light absorbing layer is arranged on the layer 6a. Then, for example, by performing a two-beam interference exposure method and an etching process on the arranged precursors of the light absorption layer,
The precursor is chopped at a predetermined pitch and processed into a diffraction grating structure having a predetermined period. Accordingly, the diffraction grating in the processed diffraction grating structure, the thickness of those located in the central portion (corresponding to the position of the narrow opening D ₁₎ has become thicker, thinner toward both ends.

Next, a predetermined semiconductor material is deposited on the diffraction grating structure to form an upper cladding layer, a light absorbing layer having the diffraction grating structure is buried therein, and a contact is formed on the upper cladding layer. A layer is formed to obtain the layer structure B shown in FIG. The buried layer is formed in the above-described series of processes. After forming a lower electrode and an upper electrode on the back surface of the substrate and on the contact layer, respectively, the substrate is cleaved to the length of the resonator to produce a bar, and the bar is cut and processed to obtain a desired semiconductor laser device. To

[0022] Thus, the light absorption layer 9 in the laser device obtained in the resonator of the central portion R ₁ in increasing its thickness, cavity near the end R ₂ in its thickness is thin diffraction grating 9a is resonators It has a diffraction grating structure arranged in the longitudinal direction. When viewed in a plan view, the diffraction grating structure is located at the center in the width direction of the element. Although the above description is directed to the case where the light absorbing layer 9 is located above the active layer 4, the laser device of the present invention has the same effect even when the light absorbing layer is located below the active layer. Demonstrate.

[0023]

EXAMPLE The laser device B of the present invention shown in FIGS. 1 and 2 was manufactured as follows. First, the n-I
A buffer layer 2 made of n-InP and having a thickness of 0.5 μm was formed on an nP substrate 1. Next, on this buffer layer 2, a 50 nm-thick lower optical confinement layer 3 made of i-GaInAsP (λg = 1.2 μm)
Six layers of quantum wells composed of nAsP and GaInAsP (λg
= 1.2 μm) and an active layer 4, i-GaInAsP (λg = 1.2 μm) having a quantum well structure composed of a barrier layer composed of
m), an upper optical confinement layer 5 having a thickness of 50 nm was sequentially formed.

Then, on the upper optical confinement layer 5, p-
InP is deposited at a thickness of 50 nm to form a layer structure B ₀ shown in FIG.
Was manufactured, a SiNx film was formed on the layer 6a by a plasma CVD method, and then the plane pattern M shown in FIG. 4 was formed by RIE. The opening D in the plane pattern M is 300 μm corresponding to the length of the resonator formed by the entire length, and the width of the narrow opening D ₁ is 10 μm.
a [mu] m, the width of the wide opening D2 at both ends has become a 100 [mu] m, it has a shape gradually broadening depict curved from the narrow opening D ₁ to wide opening D ₂ at both ends.

Next, the layer structure B₁For n-In
Selective growth of GaAs (λg = 1.65 μm) was performed.
A narrow opening is formed on the layer 6a exposed from the mask M.
Mouth D ₁Has a thickness of 20 nm and a wide opening D_TwoThen the thickness is 5
nm and the narrow opening D₁To wide opening D_TwoThick over
An n-InGaAs layer having a gradually decreasing amount was formed. R
After removing the mask M by IE, the n-InGa
Two-beam interference exposure method and etching treatment were performed on the As layer.
A diffraction grating structure having a pitch of about 240 nm was formed.

Next, a 2.5 μm thick upper cladding layer 6 is formed by depositing p-InP thereon, and a 0.5 μm thick contact layer 7 made of p-InGaAs is further formed thereon. The layer structure B shown in FIG. In this process, the light absorption layer 9 was embedded by forming the embedded layer 8 with the layer 8a made of p-InP and the layer 8b made of n-InP.

Finally, a lower electrode made of AuGeNi is formed on the back surface of the substrate 1, and an upper electrode made of AuZn is formed on the contact layer 7, and the resonator length is 300 μm.
The bar was cleaved and cut into bars to obtain a laser device B of the present invention. For comparison, a laser device A having the same layer structure as that of the example was manufactured except that the thickness of each diffraction grating in the light absorption layer was constant at 20 nm in the longitudinal direction of the resonator.

The characteristics of these two types of laser devices were evaluated. First, both showed a high single mode yield of 80%. In the case of the laser device A of the comparative example, the oscillation threshold current was 15 mA and the quantum efficiency was 0.16 W / A, but in the laser device B of the present invention, the oscillation threshold current was 8 mA and the quantum efficiency was It was 0.28 W / A, and the device characteristics were extremely improved.

[0029]

As is apparent from the above description, the distributed feedback semiconductor laser device of the present invention exhibits a high single mode yield due to the formation of the light-absorbing diffraction grating and, at the same time, has a high efficiency of the cavity. Since the absorption coefficient of the light absorption layer at the center is large and the absorption coefficient of the light absorption layer near the end face of the resonator is small, the loss of the oscillating laser light is reduced. Therefore, the oscillation threshold current becomes low,
Quantum efficiency is high and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view showing a layer structure of a laser device of the present invention.

FIG. 2 is a partially enlarged view of a dashed area C in FIG. 1;

FIG. 3 is a cross-sectional view showing an initial layer structure B ₀ at the time of manufacturing the laser device of the present invention.

Is a perspective view showing a layer structure B ₁ obtained by forming a mask [FIG. 4] layer structure B _0.

FIG. 5 is a partially cutaway perspective view showing a layer structure of a conventional laser element.

[Explanation of symbols]

1 substrate 2 of the buffer layer 3 lower optical confinement layer 4 active layer 5 upper optical confinement layer 6 upper clad 7 contact layer 8 buried layer 9 grating structure of the optical absorption layer 9a diffraction grating R ₁ resonator central portion of the R ₂ resonator wide opening of the narrow opening D ₂ opening D of the opening D ₁ opening D near the end M mask D mask M

Claims (3)

[Claims] 1. A distributed feedback semiconductor laser device having a layer structure in which a light absorption layer having a diffraction grating structure is formed above or below an active layer, wherein an equivalent absorption coefficient of the light absorption layer is equal to that of a resonator. A distributed feedback semiconductor laser device characterized in that it is larger at the center of the laser diode and smaller near the end face of the resonator. 2. The distributed feedback semiconductor laser device according to claim 1, wherein an absorption coefficient of the light absorption layer in a cavity length direction is adjusted by a combination of a thickness of the absorption layer and a semiconductor material forming the absorption layer. 3. The distribution according to claim 1, wherein the diffraction grating of the light absorbing layer is made of the same semiconductor material, and the thickness of the diffraction grating is thicker at the center of the resonator and becomes thinner toward the end face of the resonator. Feedback semiconductor laser device.