JPH02113586A - Semiconductor laser element - Google Patents

Abstract

PURPOSE:To abate the noise made by returning laser beams by a method wherein the refractive index of a p-type clad layer is differentiated from that of an n-type clad layer while Si is specified to be a dopant of the n-type clad layer. CONSTITUTION:After depositing an n-type GaAs current stopping layer 6 on a p-type GaAs substrate 1, a V-type groove 9 is formed so that the end thereof starting from the surface thereof may reach the p-type GaAs substrate 1 and then a p-type clad layer 2 is deposited again to fill up the V-groove 9. Furthermore, an active layer 3, an Si doped n-type clad layer 4, a gap layer 5 are deposited and an (n) side electrode 7 is formed on the surface furthermore, after forming a (p) side electrode 8, a resonance surface is formed by cleavage. In such a laser element, the refractive index of the p-type clad layer 2 is less than that of the n-type clad layer 4 while the effective difference in refractive index is also small. Through these procedures, the noise properties of returning laser beams can be abated to lower the noise without thickening the clad layers and the active layer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device having a novel configuration that is effective in reducing noise induced by return light of laser light. It is.

(Prior Art) Semiconductor laser elements are widely used as light sources for reading information from optical discs such as compact discs and video discs. When used as a reading light source in this way, it is important that the semiconductor laser device has low light intensity noise. In particular, the biggest problem is the return light noise generated by the return light reflected from the optical disk and re-entering the semiconductor laser element.

This return light noise occurs when a laser beam oscillating at a single wavelength travels to another wavelength due to the return light (mode competition noise), and this return light noise is caused by the interaction between the semiconductor laser element and the optical disk. The closer the distance, the larger it becomes. Several structures have been proposed to reduce return light noise, but the simplest and most reliable structure is to reduce the coherence of the laser beam and shorten the coherence length. The goal is to make the laser element insensitive to returned light.

One of the factors that influences the coherence of laser light is the oscillation spectrum. In other words, a laser beam that oscillates in a single longitudinal mode has high coherence, and a laser beam that oscillates in multiple longitudinal modes has low coherence.Therefore, in order to make the semiconductor laser element strong against return light, it is necessary to oscillate in multiple longitudinal modes. It is conceivable to make it possible to do so. Generally, multi-longitudinal mode oscillation laser light is obtained by a gain waveguide laser, but such laser devices have the problem of high noise level and high oscillation threshold current. Therefore, it is not practical.

Further, single longitudinal mode oscillation laser light is generally obtained by a refractive index waveguide type laser.

One of these refractive index waveguide lasers is VSIS (
V-channeled 5ubstrate Inn
er 5tripe) laser (for example, Appl, Phys, Lett, 40.
p, 372.1982>, this VSIS laser is
A double heterostructure with a flat active layer is grown on a GaAs substrate with letter-shaped striped grooves, and is characterized by light absorption regions provided on both sides of the refractive index waveguide. are doing.

This VSIS laser emits laser light with a wavelength of 780 nm and is often used as a light source for compact disc and video disc reading devices.

FIG. 5 (a) shows a cross-sectional structure of an example of a VSIS laser. This VSIS laser has an n-GaAs current blocking layer 56 formed on a p-GaAs substrate 51, and A V-shaped groove 59 reaching 51 is formed.Above it, Mg-doped p-Gas,s
A 1.5A s cladding layer 52, undoped Ga
Il, eyA 1 s, 13A s active layer 53, Te
Doped n-Ga, sAlg, 6As cladding layer 5
4, and Te-doped n-G a A, s cap N
55 was epitaxially grown, and further n(l
! An It pole 57 and a p-side electrode 58 are provided. In such a VSIS laser, the refractive index distribution in the direction perpendicular to the junction and the refractive index distribution in the direction parallel to the junction are shown in FIGS. 5(b) and 5(C), respectively. The laser beam is guided with a portion of it leaking out to both cladding layers 52 and 54, as indicated by the symbol A in FIG. 5(a).

In this vsrs laser, in order to reduce noise, the thickness d of the p-cladding layer 52 on the substrate 51 side outside the figure 7 groove 59 is
By setting thick l and the thickness d2 of the active layer 53 (for example, d = 0.4 μm, d2 = 0.15 μm
), the refractive index difference Δn (Fig. 5(C)) of the refractive index waveguide is l
By reducing the refractive index difference Δn in this way, a phenomenon called automatic oscillation is utilized to achieve vertical multi-mode laser oscillation.

(Problem to be Solved by the Invention) However, as mentioned above, in the VSIS laser, since the thicknesses d and d2 are large, the current slope becomes large, and the oscillation threshold current value increases to 50 mA or more. There were drawbacks. Furthermore, when the thicknesses d and d2 are increased, the non-uniformity of these 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 produced during manufacturing, resulting in a problem that the manufacturing yield is greatly reduced.

By the way, the n-cladding layer 5 in the double heterojunction structure
Te, which is used as a dopant in No. 4, is known to have a large effect on the longitudinal mode behavior and noise characteristics in the laser device. That is, GaAlA
Te in s forms a deep impurity level, which acts as a saturable absorber for light leaking from the active layer into the n-cladding layer (Copeland et al., IE
EE J, Quant, Elect.

QE-16, p, 721.1980), is known to bring about mode hop suppression effects (N,
one et al, 1EBB J, Quant,
Elect,, Q) i-21, p, 1264.
1985).

The mode hop suppression effect can prevent mode competition noise when the current changes or the temperature changes, so it is an effective means for reducing the noise of the laser element itself. However, when the returned light enters the laser element, due to the existence of the mode hop suppression effect, large mode competition noise will occur.Therefore, in practical terms, the structure has a structure in which the mode hop suppression effect does not occur. A semiconductor laser device is more desirable.

Therefore, from the perspective of mode competition noise suppression, it is desirable to reduce the amount of Te doped into the n-cladding layer as much as possible to reduce the mode hop suppression effect.
If the Te doping amount is reduced too much, the specific resistance will increase,
Since laser oscillation is inhibited, the amount of doping cannot be reduced, and in the end, the mode hop suppression effect cannot be eliminated.

The present invention was made in view of the current situation, and
The purpose is to reduce noise,
Moreover, it is an object of the present invention to provide a semiconductor laser element having a novel structure in which the oscillation threshold current value does not increase and the manufacturing yield does not decrease.

(Means for Solving the Problems) The semiconductor laser device of the present invention includes a substrate in which stripe grooves are formed, and a double layer formed above the substrate in which an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer. A semiconductor laser element having a heterojunction structure, in which an effective refractive index waveguide is formed by the stripe grooves, the P
The refractive index of the type cladding layer and the refractive index of the n-type cladding layer are different from each other, and the dopant of the n-type cladding layer is 81, thereby achieving the above object.

Further, in the semiconductor laser device of the present invention, the refractive index of one of the p-type cladding layer and the n-type cladding layer formed on the substrate side is smaller than the refractive index of the other cladding layer. You can also do that.

Furthermore, in the semiconductor laser device of the present invention, the n-type cladding layer has a laminated structure of two or more layers, and the dopant in the layer of the n-type cladding layer in contact with the active layer is Si, and the dopant in the other layer is Si. can also be set to Te.

(Function) In the present invention, Si is used as a dopant for the n-cladding layer. Si is an amphoteric impurity;
A semiconductor layer doped with , for example, a GaAlAs layer, becomes either p-type or n-type depending on the conditions of its liquid phase epitaxial growth. In a semiconductor laser device with an oscillation wavelength of 780 nm or less manufactured by liquid phase epitaxial method, Si
There has been no report of an example in which SL was used as an n-type dopant.Since Si in a semiconductor layer (e.g., a GaAlAs layer) does not form a saturable absorber, a semiconductor using a semiconductor layer doped with SL as a cladding layer has not been reported. A mode hop suppression effect does not occur in a laser element.

Therefore, in such a laser element, small mode competitive noise is generated due to current changes and temperature changes, but it is insensitive to return light, so no thick mode competitive noise is generated. In the present invention, the refractive index of one cladding layer (first cladding layer) constituting the double heterojunction structure is different from the refractive index of the other cladding layer (second cladding layer) on the opposite side. For example, by making both cladding layers G
In the case of aAlAs, by making the A1 composition ratio of the first cladding layer larger than that of the second cladding layer, the light intensity distribution in the direction perpendicular to the active layer is made asymmetric, and the first cladding layer (e.g. current blocking layer)
The proportion of light absorption by Therefore, the effective refractive index difference Δn of the optical waveguide becomes small, and the above-mentioned automatic oscillation condition is satisfied.

In this case, a large amount of light leaks toward the second cladding layer, but by doping the second cladding layer with Si, there is no possibility that the -de-hop suppressing effect will occur.

In this way, it is no longer necessary to increase the thickness d of the cladding layer and the thickness d2 of the active layer as in the conventional case in order to prevent the mode hop suppression effect, so that the reactive current is reduced. As a result, an increase in the oscillation threshold current and a decrease in yield can be prevented.

(Example) The invention will now be described with reference to examples.

A sectional view of an embodiment of the present invention is shown in FIG. 1(a).The structure of the embodiment will be explained by explaining the manufacturing process of the embodiment.

First, an n-type GaAs current blocking J is placed on a p-type GaAs substrate 1.
'16 (thickness approximately 0.8 μm) was grown by liquid phase epitaxial growth. After that, from the surface of the current blocking layer 6, p-type GaA
A ■-shaped groove 9 was formed so that its tip reached the s-substrate 1. Again, by liquid phase epitaxial method, Mg-doped p
G a e, sA l @5As cladding layer 2 is grown to fill the ■-shaped trench 9, and the p-type cladding layer 2 outside the trench 9 is grown.
The thickness was set to 0.1 μm. Furthermore, an undoped Ga@, 5vA16.13As active layer 3 with a thickness of 0.0
8μm), Si-doped (3x10"cm-") n-type G
a@4sAl essAs cladding layer 4 (thickness 1 μm
), and Te-doped (I x 10"cm-3) n
A type GaAs cap layer 5 (40 μm thick) was grown by liquid phase epitaxial growth. Au-Ge on the surface of the cap layer 5
15 by forming an n-side electrode 8i7 and polishing the back surface of the substrate 1.
After the thickness was reduced to 0 μm, a resonant surface was formed by 0-order cracking on which an Au-Zn p-side electrode 8 was formed. The resonator length is 2
It was set to 50 μm.

The refractive index distribution in the direction perpendicular to the junction and the refractive index distribution in the parallel direction to the junction in this example are shown in FIGS. 1(b) and 1(c), respectively. ,
The refractive index (3, 3) of the p-type cladding layer 2 is smaller than the refractive index (3, 36) of the n-type cladding layer 4. Further, the effective refractive index difference Δn is 0.001.

The laser element of this example oscillated with a threshold current of 35 mA (oscillation wavelength 780 nm), and the oscillation spectrum of this example at an optical output of 3 mW is shown in FIG.

In this way, we were able to obtain an automatic oscillation spectrum with a lower threshold current than conventional laser devices. In addition, as shown in FIG. 3, the return light noise characteristics of this example had a relative noise intensity of -135 dB/Hz or less, indicating low noise.

As mentioned above, when growing an n-type cladding layer by doping with Si, it was found that the higher the growth temperature and the faster the growth rate, the more likely it is to become n-type and the higher the carrier (donor) concentration. . However, depending on the growth conditions,
There were cases in which the cladding layer was inverted to p-type during growth, or a high resistance layer was formed in which donors and acceptors were compensated.

This inconvenience was solved by adopting a configuration like 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 N3 is made of Si.
Doped (3X 1017 cm-') and cap layer 5
The layer 4b in contact with the active layer 3 is doped with Te (1×10"Cm-3>), and Si
The thickness of the doped cladding layer 4a is 0. The thickness was 3 to 0.5 μm. This example also provided longitudinal mode characteristics and return optical noise characteristics similar to those of the previous example.

Although the present invention has been described above in detail using a GaAlAs-based VSIS laser as an example, it goes without saying that the present invention can also be applied to laser elements using other semiconductor materials.

(Effects of the Invention) In the semiconductor laser device of the present invention, as described above, noise reduction is achieved without increasing the thickness of the cladding layer and the active layer. is prevented. Therefore, the laser element of the present invention has a lower oscillation threshold current and an improved manufacturing yield, making it optimal as a light source for optical disks.

, ・ t;H FIG. 1(a) is a cross-sectional view of one embodiment of the present invention, and FIG. 1(b) is a sectional view of an embodiment of the present invention.
is a graph showing the refractive index distribution in the central longitudinal section passing through the ■-shaped groove, Figure (c) is a graph showing the effective refractive index distribution in the direction parallel to the junction, and Figures 2 and 3 are the oscillations of the example. Graphs showing the spectrum and return optical noise characteristics, FIG. 4 is a sectional view of another embodiment, FIG. 5(a) is a sectional view of the conventional example, and FIG. (c) is a graph showing the effective refractive index distribution in the direction parallel to the junction.

2...p-type cladding layer, 3...active layer, 4...S
i-doped n-type cladding layer, 4a...Si-doped n-type cladding layer, 4b...Te-doped n-type cladding layer, 6.
...Current blocking layer, 9...V-shaped groove.

that's all

Claims (1)

[Claims] 1. A substrate in which stripe grooves are formed, and a double heterojunction structure formed above the substrate in which an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer, A semiconductor laser device in which an effective refractive index waveguide is formed by a groove, wherein the refractive index of the p-type cladding layer and the refractive index of the n-type cladding layer are different from each other, and the refractive index of the n-type cladding layer is different from each other.
A semiconductor laser device in which the dopant of the shaped cladding layer is Si. 2. The semiconductor laser device according to claim 1, wherein one of the p-type cladding layer and the n-type cladding layer formed on the substrate side has a smaller refractive index than the other cladding layer. . 3. The n-type cladding layer has a laminated structure of two or more layers, and the dopant in the layer in contact with the active layer of the n-type cladding layer is Si, and the dopant in the other layers is Te. The semiconductor laser device described above.