JPH08213696A - Semiconductor laser element - Google Patents

Abstract

PURPOSE: To reduce noises, to prevent the increase of an oscillation threshold current value and to obviate the deterioration of yield on manufacture by using Si as a main impurity contained in an n-type clad layer. CONSTITUTION: An n-type GaAs current block layer 6 is liquid-phase epitaxial- grown on a p-type GaAs substrate 1, and a V-shaped groove 9 is formed to the p-type GaAs substrate 1 from the surface of the current block layer 6 so that the front end of the groove 9 reaches the substrate 1. An Mg-doped p-type GaAlAs clad layer 2 is grown through a liquid-phase epitaxial method again to fill the V-shaped groove 9. An undoped GaAlAs active layer 3, an Si-doped n-type GaAlAs clad layer 4 and a Te-doped n-type GaAs cap layer 5 are liquid- phase epitaxial-grown. Accordingly, return-light noises can be reduced and noises are lowered without thickening the clad layers and the active layer by using Si as the dopant of the n-type clad layer 4, thus preventing the increase of a reactive current.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device having a novel structure effective for reducing noise induced by returning light of laser light.

[0002]

2. Description of the Related Art Semiconductor laser devices are widely used as light sources for reading information from optical discs such as compact discs and video discs. When used as such a light source for reading, it is important that the light intensity noise of the semiconductor laser element is small. Particularly, the return light noise generated by the return light reflected from the optical disk and re-incident on the semiconductor laser element is the most serious problem.

This return light noise occurs when laser light oscillating at a certain single wavelength travels to another wavelength due to the return light (mode competing noise). Further, this return optical noise increases as the distance between the semiconductor laser element and the optical disk decreases. Several structures have been proposed in order to reduce the return optical noise, but the simplest and most reliable configuration is to reduce the coherence (coherence) of laser light and shorten the coherence length. This is to make the laser element insensitive to the returning light.

An oscillation spectrum is one of the factors that influence the coherence of laser light. That is, laser light oscillating in a single longitudinal mode has a high coherence, and laser light oscillating in a multi-longitudinal mode has a low coherence. Therefore, in order to make the semiconductor laser element strong against the returning light, it is possible to oscillate in the multi-longitudinal mode. Generally, laser light of multi-longitudinal mode oscillation is obtained by a gain waveguide type laser, but such a laser element has a problem that the noise level of itself is high,
Further, there is a problem that the oscillation threshold current is high, which is not practical.

Further, laser light of single longitudinal mode oscillation is generally obtained by a refractive index waveguide type laser. One of such refractive index waveguide type lasers is VSIS (V-ch
anneled Substrate Inner Stripe) There is something called a laser. (For example, Appl.Phys.Lett. 40, p.372,198
2). This VSIS laser is one in which a double hetero structure having a flat active layer is grown on a GaAs substrate having a V-shaped stripe groove, and light absorption regions are provided on both sides of a refractive index waveguide. It has the characteristics of This VSIS laser oscillates a laser beam having a wavelength of 780 nm and is widely used as a light source of a compact disc or video disc reader.

FIG. 5A shows a sectional structure of an example of the VSIS laser. In this VSIS laser, an n-GaAs current blocking layer 56 is formed on a p-GaAs substrate 51, and a V-shaped groove 59 reaching the substrate 51 from the surface of the current blocking layer 56 is formed. Above it, Mg-doped p-Ga
_0.5 Al _0.5 As clad layer 52, undoped Ga _0.87
Al _0.13 As active layer 53, Te-doped n-Ga _0.5 A
1 _0.5 As clad layer 54 and Te-doped n-Ga
An As cap layer 55 is epitaxially grown, and an n-side electrode 57 and a p-side electrode 58 are further provided. The refractive index distribution in the direction perpendicular to the junction and the refractive index distribution in the direction parallel to the junction in such a VSIS laser are as shown in FIGS. 5B and 5C, respectively, and the laser light is As shown by a symbol A in FIG. 5A, a part of the light is guided to the both clad layers 52 and 54 in a state of being exuded.

In this VSIS laser, in order to reduce noise, the thickness d ₁ of the p-cladding layer 52 on the substrate 51 side outside the V-shaped groove 59 and the thickness d ₂ of the active layer 53 are set thick. By (for example, d ₁ = 0.4 μm, d ₂ =
0.15 μm), the refractive index difference Δn of the refractive index waveguide (see FIG.
(C)) is set to about 1 × 10 ⁻³ . By making the refractive index difference Δn small in this way, a phenomenon called self-excited oscillation is utilized to achieve longitudinal multimode laser oscillation.

[0008]

However, as described above, in the VSIS laser, since the thicknesses d ₁ and d ₂ are large, the current spread becomes large and the oscillation threshold current value becomes 50 mA or more. There was a flaw. When the thicknesses d ₁ and d ₂ are increased, the nonuniformity of the thicknesses is sensitively reflected in the magnitude of the oscillation threshold current value.
That is, a large number of laser elements having an abnormally large oscillation threshold current value are generated during manufacturing, which causes a problem of a large decrease in manufacturing yield.

By the way, n- in the double heterojunction structure
Te used as a dopant for the cladding layer 54
Is known to have a great influence on the behavior of longitudinal modes and noise characteristics in a laser device. That is, Te in GaAlAs forms a deep impurity level,
This acts as a saturable absorber for the light exuding from the active layer in the n-clad layer (Copeland et al, IEEE J, Qu.
ant.Elect., QE-16, p.721,1980), is known to bring about a mode-hop suppression effect (N.Chinone et al, IEE
E J. Quant. Elect., QE-21, p. 1264, 1985).

The mode-hop suppressing effect is an effective means for reducing the noise of the laser device itself, since it can prevent the mode competing noise when the current changes or the temperature changes. However, when the returning light is incident on the laser element, a large mode competition noise is generated due to the existence of the mode hopping suppression effect. Therefore, in practical use, a semiconductor laser device having a structure in which the mode hop suppression effect does not occur is desired.

Therefore, from the viewpoint of suppressing mode competition noise, it is desirable to reduce the amount of Te doped in the n-clad layer as much as possible to reduce the mode hop suppression effect. However, if the doping amount of Te is excessively reduced, the specific resistance becomes high and the laser oscillation is hindered. Therefore, the doping amount cannot be reduced, and in the end, the mode hop suppression effect cannot be removed.

Therefore, the conventional semiconductor laser device having the self-sustained pulsation phenomenon has a mode-hop suppressing effect based on saturable absorption of the n-type dopant, and the suppression of the return optical noise is not sufficient.

The present invention has been made in view of the above circumstances, and an object of the present invention is to achieve a low noise, an oscillation threshold current value does not increase, and a manufacturing yield decreases. An object of the present invention is to provide a semiconductor laser device having a novel structure that does not exist.

[0014]

According to a first aspect of the present invention, there is provided a semiconductor laser device comprising an active layer sandwiched between a p-type cladding layer and an n-type cladding layer, and inside or at an end of the p-type cladding layer. And an n-type current blocking layer having a stripe-shaped cutout extending in the cavity direction, the effective refractive index difference Δn of the optical waveguide of the active layer is selected to be small so as to cause a self-oscillation phenomenon. In the semiconductor laser device consisting of
The main impurity contained in the shaped clad layer is Si.

The operation of the present invention will be described below.

In claim 1 of the present invention, Si is used as a dopant for the n-cladding layer. Si is an amphoteric impurity, and a semiconductor layer to which Si is added, such as a GaAlAs layer, can be either p-type or n-type depending on the conditions of its liquid phase epitaxial growth. No example has been reported in which Si is used as an n-type dopant in a semiconductor laser device having an oscillation wavelength of 780 nm or less produced by a liquid phase epitaxial method. Semiconductor layer (eg, GaA
Since Si in the (1As layer) does not form a saturable absorber,
A mode hop suppression effect does not occur in a semiconductor laser device using a semiconductor layer to which Si is added as a cladding layer.
Therefore, in such a laser device, a small mode competing noise is generated with respect to a current change and a temperature change, but it is insensitive to return light, and thus a large mode competing noise is not generated.

Further, in the present invention, one clad layer (first clad layer) forming the double heterojunction structure.
By making the refractive index of the other cladding layer (second cladding layer) on the opposite side different from that of the other cladding layer, for example, when both cladding layers are made of GaAlAs, the Al composition ratio of the first cladding layer is Is made larger than that of the second clad layer, thereby making the light intensity distribution in the direction perpendicular to the active layer asymmetrical and a layer adjacent to the first clad layer (for example,
The rate of light absorption by the current blocking layer) is reduced.
Therefore, the effective refractive index difference Δn of the optical waveguide becomes small,
The above-mentioned self-oscillation condition is satisfied. In this case, a large amount of light exudes toward the second cladding layer side, but there is no fear that a mode hop suppression effect will be produced by doping the second cladding layer with Si.

As described above, in order to prevent the mode-hop suppressing effect from occurring, the thickness d of the cladding layer is different from the conventional one.
Since it is not necessary to increase the thickness ₁ and the thickness d ₂ of the active layer,
Less reactive current. As a result, it is possible to prevent an increase in oscillation threshold current and a decrease in yield.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

A sectional view of one embodiment of the present invention is shown in FIG. The structure will be described by explaining the manufacturing process of the present embodiment.

First, n-type GaA is formed on the p-type GaAs substrate 1.
The s current blocking layer 6 (thickness: about 0.8 μm) was grown by liquid phase epitaxial growth. Then, from the surface of the current blocking layer 6, p
V-shaped groove 9 so that its tip reaches the GaAs substrate 1
Was formed. Again, the Mg-doped p-type Ga _0.5 Al _0.5 As cladding layer 2 is grown by the liquid phase epitaxial method to fill the V-shaped groove 9 so that the thickness of the p-type cladding layer 2 outside the groove 9 becomes 0.1 μm. I chose Furthermore, undoped Ga
_0.87 Al _0.13 As Active layer 3 (thickness 0.08 μm), Si
Doped (3 × 10 ¹⁷ cm ⁻³ ) n-type Ga _0.55 Al _0.45 As
Cladding layer 4 (thickness 1 μm) and Te-doped (1 × 1
0 ¹⁸ cm ⁻³ ) n-type GaAs cap layer 5 (thickness 40 μm
m) was subjected to liquid phase epitaxial growth. After forming an Au-Ge n-side electrode 7 on the surface of the cap layer 5 and polishing the back surface of the substrate 1 to a thickness of 150 μm, Au-Zn p
The side electrode 8 was formed. Next, a resonance surface was formed by cleavage. The resonator length was 250 μm.

FIG. 1 shows the refractive index distribution in the direction perpendicular to the junction and the refractive index distribution in the direction parallel to the junction in this embodiment.
It is shown in (b) and FIG. In the laser device of this example, the refractive index (3.3) of the p-type cladding layer 2 is
It is made smaller than the refractive index (3.6) of the n-type cladding layer 4. The effective refractive index difference Δn is 0.001.

The laser device of this example oscillated with a threshold current of 35 mA (oscillation wavelength 780 nm). FIG. 2 shows the oscillation spectrum of this example when the optical output was 3 mW. Thus, the self-sustained pulsation spectrum could be obtained with a threshold current lower than that of the conventional laser device. Further, the return optical noise characteristic of this embodiment has a relative noise intensity of −13 as shown in FIG.
It was 5 dB / Hz or less and had low noise.

As described above, in the case of growing an n-type cladding layer by Si doping, the higher the growth temperature and the higher the growth rate, the easier it becomes to become n-type and the higher the carrier (donor) concentration thereof. There was found.

However, depending on the growth conditions, the cladding layer may be inverted to p-type during the growth, or a high resistance layer in which the donor and the acceptor are compensated may occur. This inconvenience has been solved by adopting the configuration of the embodiment shown in FIG. In this embodiment, the n-type cladding layer 4 has a two-layer structure, and the layer 4a on the side in contact with the active layer 3 is Si-doped (3 ×
10 ¹⁷ cm ⁻³ ), and the layer 4 b in contact with the cap layer 5 is T
It was e-doped (1 × 10 ¹⁸ cm ⁻³ ). The Si-doped clad layer 4a is formed so that the laser light that oozes out from the active layer 3 does not leak to the Te-doped clad layer 4b.
Thickness was 0.3 to 0.5 μm. Also in this example, the same longitudinal mode characteristics and return optical noise characteristics as those of the above-mentioned examples were obtained.

Although the embodiment of the present invention has been described by taking the GaAlAs-based VSIS laser as an example, it goes without saying that the present invention can also be applied to a laser element using another semiconductor material.

[0027]

As described above, in the semiconductor laser device of the present invention, by using Si as the dopant for the n-type cladding layer, the return optical noise can be reduced and the noise can be reduced without increasing the thickness of the cladding layer and the active layer. Therefore, the increase of the reactive current due to the large layer thickness of each of these layers is prevented. Therefore, the laser device of the present invention has a reduced oscillation threshold current and improved manufacturing yield, and is optimal as a light source for optical disks.

[Brief description of drawings]

FIG. 1A is a sectional view of an embodiment of the present invention, and FIG.
Is a graph showing the refractive index distribution of the central longitudinal section passing through the V-shaped groove, and (c) is a graph showing the effective refractive index distribution in the direction parallel to the junction.

FIG. 2 is a graph showing an oscillation spectrum of the embodiment shown in FIG. 1 of the present invention.

FIG. 3 is a graph showing return optical noise characteristics of the embodiment shown in FIG. 1 of the present invention.

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

5A is a cross-sectional view of a conventional example, FIG. 5B is a graph showing a refractive index distribution in a central longitudinal section passing through a V-shaped groove, and FIG. 5C is an effective refractive index distribution in a direction parallel to the junction. It is a graph.

[Explanation of symbols]

 2 p-type clad layer 3 active layer 4 Si-doped n-type clad layer 4a Si-doped n-type clad layer 4b Te-doped n-type clad layer 6 current blocking layer 9 V-shaped groove

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaki Kondo 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture

Claims (1)

[Claims] 1. An active layer sandwiched between a p-type cladding layer and an n-type cladding layer, and an n-type having a stripe-shaped cutout portion formed in or at the end of the p-type cladding layer and extending in the cavity direction. In a semiconductor laser device comprising a current blocking layer, and the effective refractive index difference Δn of the optical waveguide of the active layer is selected to be small so as to cause a self-sustained pulsation phenomenon, the main impurities contained in the n-type cladding layer are A semiconductor laser device characterized by being Si.