JPH04180685A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To make a threshold current low, to realize a high-efficiency oscillation and to enhance reproducibility and reliability by forming a double heterojunction structure in which an active layer has been sandwiched between clad layers composed of a material whose band gap is wider than that of the active layer. CONSTITUTION:A double heterojunction structure in which an active layer 12 has been sandwiched between a first n-type AlGaAs clad layer 11 and a second p-type AlGaAs clad layer 13 which are composed of a material whose band gap is wider than that of the layer 12 is formed. The layer 13 has a protrusion-shaped region 15; and the region 15 is composed of a protrusion-part region 16 whose shape is close to a rectangle and of a slope-shaped bottom region 17 succeeding to it. In the region 16, its width is narrower than a carrier diffusion length and the region 17 becomes slope-shaped at least over the width of the carrier diffusion length on both sides from the center of the region 16. A multilayer structure in which a light-absorbing layer 18 has been buried on both sides excluding the region 16 is formed. Thereby, a threshold current becomes low, an oscillation can be performed with high efficiency, a functional transverse-mode oscillation is maintained, a noise is low, and this laser can be manufactured with high reproducibility and at a high yield.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser, and particularly to a semiconductor laser used for optical information processing and the like.

[Conventional technology]

Semiconductor lasers using crystalline materials such as A42 GaAs/GaAs are compact, have low power consumption, and can perform continuous oscillation at room temperature with high efficiency. It is suitable as a light source for DAD) and is being put into practical use. The demand for this semiconductor laser as a light source for optical writing of optical discs and the like is increasing, and in order to meet this demand, research and development is underway on semiconductor lasers that can withstand high optical output oscillation. Recently, in response to the rapidly increasing importance of these semiconductor lasers, mass production has begun. This A
I! In addition to the conventional liquid phase growth method, GaAs/GaAs semiconductor laser manufacturing methods include
Vapor phase growth method using organic metals (Melaorgani)
c Vapor Phase Epitaxy, hereinafter M
OVPE method) is widely used. Among these, examples of AAGaAs/GaAs semiconductor lasers with transverse mode control include Coleman (J, J, Coleman)
and Dapkus, R,D, published in the magazine Applied Physics Letters.
cs Letters) 1980, Vol. 37, pp. 262-263, single-axis mode oscillation G by rMOcVD method.
aAuAs-GaAs SAS laser” “Singl
e-1ongitudinal-mode metal
-organic chemical-vapour-
-deposition self-alignedG
a A (l A s -G a A 5double
e-heterostructure 1asers”
SAS (self-align), which was announced under the title
ed 5 structures) are commonly used. In this structure, absorption layers are provided on both sides of a striped region adjacent to the active layer, and the absorption layer absorbs the light seeping out from the active layer to form a lost region. This structure controls the transverse mode by providing an effective refractive index distribution between the two.

[Problem to be solved by the invention]

The so-called SAS laser described above has a narrow tolerance range for maintaining stable fundamental transverse mode oscillation. Furthermore, since the astigmatism difference, which is important in practice, is as long as ~10 μm, the optical output is condensed into DRAW (direct read af) for optical disks.
In order to use it as a laser, it not only requires a complicated lens, but also requires a large optical output oscillation because the proportion of light that enters the lens system decreases.
The reliability of the laser is seriously impaired. Also, if the effective refractive index distribution is reduced, the transverse mode becomes unstable, so
It is difficult to reduce noise due to the low refractive index distribution, and it cannot be used as a DAD or VD laser that requires low noise characteristics. Therefore, its use as a laser for optical information processing is limited, and in addition, the narrow tolerance range causes manufacturing problems such as extremely low yields.

The purpose of the present invention is to eliminate the above-mentioned drawbacks, and to achieve not only high-efficiency oscillation with a low threshold current, but also low astigmatic difference, stable fundamental transverse mode, and large optical output oscillation, as well as low noise. The object of the present invention is to provide a semiconductor laser that can be produced relatively easily and in large quantities and has excellent reproducibility and reliability.

[Means to solve the problem]

A first means provided by the present invention to solve the above-mentioned problem includes a double heterojunction structure in which an active layer is sandwiched between cladding layers made of a material with a wider band gap than the active layer, and the cladding layer one side has a convex area,
The convex region consists of a nearly rectangular convex region and a sloped bottom region following it, the width of the convex region is narrower than the carrier diffusion length, and the bottom region has a width extending from the center of the convex region to both sides. has a sloped shape over at least the width of the carrier diffusion length, and on both sides excluding the convex region are light absorption layers made of a substance that electrically blocks current and absorbs light from the active layer. The light-absorbing layer absorbs light seeping from the active layer over the sloped bottom region from the region excluding the convex region, and the absorption coefficient is in the convex region. This is a semiconductor laser characterized by a decrease in the value as it approaches the target.

A second means of the present invention is that in the first means, the convex region having an inclined bottom region and the rectangular convex region having the same width as the convex region across the bottom form a resonator. This semiconductor laser is characterized by being formed in series in the longitudinal direction.

A third means of the present invention is that in the first means, the convex region formed in the cladding layer and having an inclined bottom region seeps from the active layer in a part of the longitudinal direction of the resonator. This semiconductor laser is characterized in that the thickness of the cladding layer is increased so that the emitted light is not absorbed even outside the convex region.

A fourth means of the present invention is that in the first means, a current blocking layer having a different electrical polarity from that of the substrate is formed on the substrate, and the cladding layer adjacent to the current blocking layer is tilted with respect to the substrate in the thickness direction. The semiconductor laser is characterized in that a groove is formed in the form of a convex region having a bottom region in the shape of a shape, and the tip of the groove is adjacent to a substrate.

A fifth means of the present invention is that in the fourth means, the groove is provided near both end faces, and a rectangular groove having the same width is formed in the current blocking layer in the central region of the resonator between them. The semiconductor laser is characterized in that the tip of the groove is adjacent to the substrate, and the layer thickness of an active layer adjacent to the groove via a cladding layer is thinner near both end faces.

〔Example〕

The present invention will be described in detail below using the drawings.

Figure 1 is a perspective view of the first embodiment, and Figure 2/(a) is a perspective view of the first embodiment.
Figure 2(b) is a gain-loss distribution diagram corresponding to Figure 2(a), and Figure 2(c) is a refractive index distribution diagram corresponding to Figure 2(a). , FIG. 3 is a diagram showing the analysis results of the relationship between the astigmatism difference and the slope of the bottom region, FIG. 4 is a perspective view of the second embodiment, and FIG. 5 is a sectional view taken along line BB' in FIG. 4. , FIG. 6 is a perspective view of the third embodiment, FIG. 7 is a sectional view taken along line c-c' in FIG. 6, FIG. 8 is a perspective view of the fourth embodiment, and FIG. 9 is a perspective view of the fourth embodiment. DD' sectional view, FIG. 10 is a top view of the fifth embodiment,
Figure 11 is a sectional view taken along line E-E' in Figure 10, and Figure 12 is a cross-sectional view of the first
It is a FF' cross-sectional view of FIG.

As shown in FIG. 2(a), an n-type GaAs substrate 1
n-type A on 0 42 a, a s G & o, s s
A S first cladding layer 11 with a thickness of 2.5 pm, undoped A A' oy+s G a o,s 5A S
The active layer 12 is 0.03. um, p type A j' 0.45
G a a, s s A s second cladding layer 13 2
．． 0 μm, p-type GaAs chip layer 14 is 1.0 μmM
Continuous growth using OVPE method. Next, after the entire growth surface is coated with a SiO2 film, a window is opened in the length direction of the resonator leaving a striped region of 4 μm in width by photoresist method, and etching is performed. At this time, the etching is p-type G
aAs +7 layer 14 to p-type AI! a4s G &
This is done over the second cladding layer 13 of 0.55As.

In the area where the window is opened, the second cladding layer thickness is 0.2 μm.
Perform etching to a depth of . If a phosphoric acid-based etching solution is used at this time, the upper part of the striped region becomes a rectangular shape with a width of 3 μm, and the second cladding layer 13 is etched to a depth of about 1.3 μm in the lateral direction of the striped region. Pull the hem into the shape you want. As a result, especially in the second cladding layer 13 which is the second layer from the surface, the convex regions 1
5 is formed, and its convex region 16 has a width of 3 μm and a nearly rectangular shape, while the bottom region 17 has a thickness that gradually decreases from the second cladding layer thickness of 0.7 μm to a width of 4 μm. It coincides with a flat plane when the thickness of the two-clad layer becomes 0.2 μm. As described above, the convex region 15 having the convex region 16 and the inclined bottom region 17 is formed in the second cladding layer. At this time, the carrier diffusion length is ~3 μm, which satisfies the conditions of the present invention.

Next, with a 5in2 film attached to the surface of the p-type GaAs cap layer 14, n-type GaAs light absorption layers 18 are placed on both ends of the 5in2 film.
is grown using the MOVPE method. At this time, the striped convex ends that did not grow on the 5iCh film and remained after etching were n
A GaAs light absorbing layer 14 is formed. After removing 5iOz, a p-type ohmic contact 19. is formed on the p-type GaAs cap layer 14. A first embodiment of the present invention can be obtained by attaching an n-type ohmic contact 20 to the substrate 10 side (
Figures 1 and 2 (a)).

In the case of the second embodiment of the present invention, during etching,
Windows are opened only in the lateral direction at both ends of the striped region forming the convex region. For this region, a window is opened, for example, by 100 .mu.m in the longitudinal direction of the resonator, with a distance of 50 .mu.m, and etched as described above. Next, the etched area is coated with a photoresist, and windows are opened on both sides of the striped area in the remaining area and dry etching is performed so that the second cladding layer has a thickness of 0.2 .mu.m.

At this time, the dry etched region does not need to be adjacent to the convex region in the cavity length direction (there may be unetched regions on both sides of the striped region in the cavity length direction). A second embodiment is obtained by adding a buried electrode with a light absorption layer as in the first embodiment (FIGS. 4 and 5).

In the case of the third embodiment of the present invention, before etching in the first embodiment, first the central portion of the resonator is covered perpendicularly to the length direction of the resonator, and the entire lateral direction is covered with a length of about 80 μm.
Etching is performed to a depth of 0.8 μm at 110 pm near both end faces. After that, the coating is removed, and the thickness of the second cladding layer outside the convex region near both end faces is 0.2 μm as in the first embodiment.
When etched until it becomes 1.0 μm outside the convex region at the center of the resonator, the thickness of the second cladding layer becomes 1.0 μm, and the light in the active layer is not absorbed at the center of the resonator, and this region becomes gain guiding. By filling this with a light absorption layer and attaching electrodes as in the first embodiment, the third embodiment is obtained (FIGS. 6 and 7).

In the fourth embodiment of the present invention shown in FIGS. 8 and 9, p
An n-type GaAs layer 22 is formed on a GaAs-type substrate 21.
．． Grows 0 μm. After that, the entire surface was coated with 5in2 film, and then the width was 8μm in the length direction of the resonator using the photoresist method.
A striped window is opened and etched. If sulfuric acid-based etching is used at this time, the n-type GaAs layer is etched into a gentle mesa shape. I'm scared of etching 0
．． After 5 μm, a window with a width of 3 μm is opened in the center of the mesa using the photoresist method again, and dry etching is performed to a depth of 0.8 μm using the photoresist film as a protective film.

As a result, a rectangular groove whose tip reaches as far as the p-type GaAs substrate 21 and a concave region 23 whose original part has a gently sloped shape and reaches a groove depth of 0.5 μm are formed.

This concave region 23 corresponds to the convex region 15 in the first embodiment of the present invention, and the n-type GaAs layer 22 is made of n-type GaAs which has a current blocking effect and a light absorption effect from the active layer.
It corresponds to the light absorption layer 14. After this, the coating was removed and p
Type A 470.4S Ga a, ss A s The first cladding layer 24 is grown to fill the concave region 23 so that the growth thickness of the n-type GaAs layer 22 outside the concave region is 0.2 μm, and then 7 Ndobu A II c, s Ga o
, ss A s active layer 25 of 0.04 μm, n-type AI
I 111.45 G a rhss A S second cladding layer 26 of 2.0. um, n-type GaAs cap layer 2
Continuously grow 7. At this time, if liquid phase growth is used, the first cladding layer 24 having a flat growth surface can be easily grown to meet the above conditions. n-type ohmic contact on this growth surface)28. A fourth embodiment of the present invention is obtained by forming a p-type ohmic contact 29 on a p-type GaAs substrate 21 (FIGS. 8 and 9).

In the fifth embodiment of the present invention, when forming the concave region shown in the fourth embodiment, mesa-shaped etching is performed only in the vicinity of both end faces by 100 μm. On the other hand, in the central part of the resonator, a window with a width of 3 μm is opened and a rectangular shape is etched to a depth of 0.6 μm. Thereafter, a groove of 3 .mu.m in depth is formed by dry etching to a depth of 0.8 .mu.m over the entire resonator so as to pass through the center of each of the grooves. Next, the film is removed and the same layers as in the fourth example are grown in a liquid phase, but at this time p-type Al1o,
<sG a o, s s A sThe first cladding layer 24 fills the concave regions 23 near both end faces, and grows so that the growth thickness of the n-type GaAs layers 22 at both ends is 0.2 μm. , the grown layer surface is flat near both end faces, whereas the groove at the center of the resonator is depressed by about 0.1 μm. The undoped active layer 25 to be grown next has a flat surface and a thickness of 0 near both end faces.
．． If the thickness is 0.03 μm, an active layer with a thickness of 0.13 μm will grow at the center of the resonator. Otherwise, the fifth embodiment of the present invention can be obtained in the same manner as the fourth embodiment (
Figure 1O, Figure 11, Figure 12).

[Effect]

In the structure of the present invention, the injected current flows through the cap layer 1
4. It is implanted into the active layer 12 through the convex region 15 of the second cladding layer 13 . The carriers injected into the active layer diffuse in the horizontal and lateral directions of the active layer, form a gain distribution, and start laser oscillation. At this time, the convex region 1 of the convex region 15
Since the width of 6 is narrower than the carrier diffusion length, carriers flow concentrated in the convex region 16 and are injected into the active layer.Reflecting this, the gain distribution has a shape with the apex mainly at the convex region. become.

On the other hand, light seeps out of the active layer and spreads widely in the vertical direction. Therefore, light is transmitted to the light absorbing layer 1 on both sides of the convex region 15.
The light leaks out to within 8, where it suffers a large light absorption loss. Furthermore, in the bottom region 17 which is inclined in the convex region 15, the light seeping out from the active layer is absorbed by the light absorption layer 1.
It seeps into the 8 and suffers losses there. This state becomes smaller as the convex region 16 is approached, and the amount of loss decreases.

A large gain loss distribution is formed by the synergistic effect of the optical absorption loss caused by this light and the gain distribution formed by carrier injection (FIG. 2(b)). Furthermore, in response to this change in light absorption loss, a positive refractive index distribution is formed with the convex region 16 as the apex (FIG. 2(C)).

As a result of the above, in the structure of the present invention, light is controlled by the positive refractive index guiding mechanism and spreads not only to the convex region 16 but also to the portions of the bottom regions on both sides of the convex region 16 where light absorption loss is relatively small. Since light undergoes a large optical absorption loss at both ends of this spread, it is also controlled by the gain-loss distribution. Due to the synergistic effect of this and the positive refractive index guiding mechanism, stable fundamental transverse mode oscillation can be maintained even during large optical output oscillation. Furthermore, as described above, since the light spreads laterally from the convex region, the spot size becomes large. As a result, the emitted light power increases and the external differential quantum efficiency increases. Therefore, high output oscillation can be achieved with a relatively low current value.

Furthermore, the structure of the present invention also reduces the astigmatism difference.

In a normal SAS laser, the astigmatism difference is defined by the groove width, but in the structure of the present invention, the astigmatism difference is defined by the central convex region of the convex region.

Therefore, even if the spot size increases, a small astigmatism difference can be maintained. FIG. 3 shows the calculation result of this astigmatism difference.

Figure 3 shows the relationship between the astigmatism difference Δ2 and the slope ratio of the bottom of the convex region. The value was used. In the figure, the width of the convex region is W, the total width of the bottom region is 6 μm, the active layer thickness is 0.03 μm, the thickness of the cladding layer outside the convex region is 0.2 μm, and the other layer thicknesses and compositions are as follows. These are calculation results using Example 1. As is clear from the figure, H=0.5. um, and W≦3. um, it can be seen that Δ2≦3.5 μm can be obtained. Furthermore, the narrower W is, the better; if the slope becomes steep to some extent (H
There is an inflection point where ΔZ increases rapidly (as ΔZ becomes large),
It can be seen that the value becomes smaller as W becomes larger. Therefore,
The narrower W and the gentler the slope, the smaller Δ2 becomes, and the wider its allowable range becomes. In FIG. 3, the shaded area is the allowable range.

In this way, the narrower the astigmatism difference, the better the light collection efficiency.
Even with relatively low optical output, high output can be maintained on the optical disk surface.

In addition, in the structure of the present invention, as mentioned above, the light spreads to the vicinity of the large light absorption loss region, and isometrically, there is a saturable absorber in the horizontal and lateral directions of the active layer, so that the light When affected, self-excited vibration is likely to occur. In the structure of the present invention, the fact that the width of the gain distribution is narrower than the width of the refractive index distribution also promotes self-oscillation. That is, when the injected carrier density distribution fluctuates, the light tends to fluctuate under its influence. When self-excited vibration occurs in this way, the axial mode becomes multi-mode, and the coherence of the axial mode is reduced, so that noise in reflected light can be suppressed to a low level.

In addition to the above structure, the second structure of the present invention has a saturable absorber in the longitudinal direction of the resonator.

That is, in a rectangular groove region, light undergoes absorption loss on both sides of the rectangle. On the other hand, in the convex region, light spreads laterally, and when this light passes through the rectangular groove, it undergoes absorption loss. Therefore, the region in which self-excited vibration occurs more easily than in the first structure and low noise is maintained becomes wider.

The third structure of the present invention includes a region in the longitudinal direction of the resonator that is free from light absorption loss due to the light absorption layer.

This area becomes a gain guiding mechanism. In gain guiding, coherence deteriorates due to multi-axis mode, so it is easier to achieve noise reduction than in the first structure.

Although the fourth structure of the present invention is different in structure from the first structure, it has exactly the same effect. That is, in the fourth structure, the current blocking layer 22 grown on the substrate has a light absorption effect.

The fifth structure of the present invention is effective in increasing output in addition to the effects of the first structure. Since the light spreads widely in the horizontal, lateral, and vertical directions at both end faces, the COD level increases. Furthermore, since the active layer is thick near the center of the resonator, it is possible to reduce the amount of active layer.

〔Effect of the invention〕

The first semiconductor laser of the present invention is:

(1) Fundamental transverse mode oscillation can be maintained (2) Astigmatism difference is small and light can be easily focused (3) Spot size is large and high efficiency (4) Self-excited vibration easily occurs and low (5) Since the structure is relatively simple, it has the advantage of being able to be manufactured with good reproducibility and high yield.

In addition to the above-mentioned effects, the second semiconductor laser of the present invention has the advantage that it has a saturable absorber in the longitudinal direction of the resonator, so that self-excited vibration easily occurs and its permissible range is wide.

In addition to the effects of the first semiconductor laser, the third semiconductor laser of the present invention is equipped with a gain guiding mechanism in the longitudinal direction of the resonator, and has a multi-axis mode, thereby achieving low noise characteristics.

The fourth semiconductor laser of the present invention has a different structure from the first semiconductor laser, but has the same effect.

In addition to the effects of the first semiconductor laser, the fifth semiconductor laser of the present invention (1) is capable of high-output oscillation; (2)
It has the advantage of low threshold oscillation.

Note that in the above embodiment, p and n may be reversed. Furthermore, although the embodiments have been described with respect to the Aj7GaAs/GaAs double heterojunction crystal material, other crystal materials such as InGaP/AAInP may be used.

InGaAsP/InGaP, InGaAsP/InP
.

AI! GaAsSb/GaAsSbII can be applied to many crystal materials.

[Brief explanation of drawings]

Fig. 1 is a perspective view of the first embodiment of the present invention, Fig. 2(a)
is a cross-sectional view taken along line AA' in Figure 1, and Figure 2 (b) is a cross-sectional view taken along line AA' in Figure 1;
), Figure 2(c) is a gain-loss distribution diagram corresponding to Figure 2(c).
The refractive index distribution map corresponding to a), Figure 3, shows the astigmatism difference (Δ2
) and the distance (H) from the bottom of the light absorption layer showing the slope of the bottom region of the convex region. FIG. 4 is a perspective view of the second embodiment of the present invention. Figure 5 shows the fourth B-B'
A sectional view, FIG. 6 is a perspective view of the third embodiment of the present invention, and FIG.
The figure is a sectional view taken along the line CC' in FIG. 6, FIG. 8 is a perspective view of the fourth embodiment, FIG. 9 is a sectional view taken along the line DD' in FIG. 8, and FIG. 10 is a sectional view of the fifth embodiment. The top view of FIG. 11 is EE' in FIG. 10.
The sectional view, FIG. 12, is a sectional view taken along line FF' in FIG. In the figure, 10... n-type GaAs substrate, 11...
n-type AI2゜450&o, ssA S first cladding layer, 12--A 1 o, + s Ga Q, l S
As active layer, 13... p-type A II 0.45
0 a o, s s A S second cladding layer, 14...
...p-type GaAs cap layer, 15...
...Convex region, 16...Convex region, 17...
... Slanted bottom region, 18 ... n-type GaAs
Light absorption layer, 19...p-type ohmic contact, 2o...n-type ohmic contact, 21.
...p-type GaAs substrate, 22...n-type G
a As layer, 23... Concave region, 24...
...p-type A 12 Li2 G & ,,ss A
S first cladding layer, 25--A II O, 18G
&a, ssA sactive layer, 26"""n type A (l
tL4s G a ass A s second cladding layer, 2
7...n-type GaAs cap layer, 28...
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Claims (1)

[Claims] 1. A double heterojunction structure in which an active layer is sandwiched between cladding layers made of a material with a wider band gap than the active layer, one of the cladding layers having a convex region; The convex region consists of a nearly rectangular convex region and an inclined bottom region following it, the width of the convex region is narrower than the carrier diffusion length, and the width of the convex region is narrower than the carrier diffusion length. has a sloped shape over at least the width of the carrier diffusion length, and is embedded with a light absorption layer made of a material that electrically blocks current and absorbs light from the active layer on both sides except for the convex region. has a multilayer structure, and the light absorbing layer absorbs light seeping out from the active layer from the region excluding the convex region to the bottom region forming an inclined shape,
A semiconductor laser characterized in that its absorption coefficient decreases as it approaches a convex region. 2. The semiconductor laser according to claim 1, wherein the convex region having an inclined bottom region and a rectangular convex region having the same width as the convex region across the bottom extend in the longitudinal direction of the resonator. A semiconductor laser characterized by being formed in a series. 3. In the semiconductor laser according to claim 1, the convex region having an inclined bottom region formed in the cladding layer directs light seeping from the active layer in a part of the longitudinal direction of the resonator to the convex region. A semiconductor laser characterized by having a thick cladding layer so as not to absorb even outside the region. 4. In the semiconductor laser according to claim 1, a current blocking layer having an electrical polarity different from that of the substrate is formed on the substrate, and a bottom region where the cladding layer adjacent to the current blocking layer is inclined with respect to the substrate in the layer thickness direction. 1. A semiconductor laser characterized in that a groove is formed to form a convex region having a convex shape, and a tip of the groove is adjacent to the substrate. 5. In the semiconductor laser according to claim 4, the groove is provided near both end faces, and a rectangular groove having the same width is formed in the current blocking layer in the central region of the resonator, and the tip of the groove is adjacent to the substrate. 2. A semiconductor laser characterized in that the thickness of an active layer adjacent to each groove via a cladding layer is thinner near both end faces.