JPS62123790A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To realize a stable self-exciting oscillation while maintaining a stable basic lateral mode oscillation by a method wherein a double-hetero junction structure is provided along the protruded surface of a substrate and a current blocking layer which has a light absorbing function is provided between the guide layer and the cladding layer in the structure. CONSTITUTION:After a protruded normal mesa-stripe 12 is formed on an N-type GaAs substrate 10 by an etching method, the 2nd and the 1st cladding layers 13 and 14 made of N-type AlGaAs, an active layer 15, the 3rd cladding layer 16 made of P-type AlGaAs, a guide layer 17 and a current blocking layer 18 made of N-type GaAs are successively formed by a vapor phase deposition method with organic metal compounds continuously. An SiO2 film 19 is formed over the whole surface and an aperture is drilled in the film 19 at the position corresponding to the protruded part and etching is performed to expose the guide layer 17. After the SiO2 film 19 is removed, the 4th cladding layer 20 made of P-type AlGaAs and a cap layer 21 made of P-type GaAs are formed and P-type side and N-type side electrodes 22 and 23 are applied.

Description

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

[Conventional technology]

When a semiconductor laser is used for optical information processing, especially as a light source for reading video disks or optical disks, noise characteristics, particularly noise induced by return light, become a problem. Various methods have been attempted to reduce the feedback-induced noise of semiconductor lasers, and among them, reducing output coherence is particularly effective.

As one of these methods, Oishi, Shimone, and Nakamura proposed low-noise semiconductor lasers through high-frequency superposition.

Paper written by Ojima on page 102 of the Proceedings of the Applied Physics Association Lectures published in the fall of 1983, 26a-
It was proposed in P-6 "Low noise and longitudinal mode characteristics of half- and four-body lasers by high-frequency superposition" and has been shown to be effective.

In addition, a semiconductor laser that generates self-excited vibration and multiplexes longitudinal modes to reduce noise is manufactured by Matsumoto.

IEICE technical report by Tamura, Watanabe, and Kurihara.

This method was proposed and attempted in the article "Noise characteristics and self-pulse modulation mechanism of 15SS laser" published in Photon Quantum Electronics 0QE 84-57.39.

FIG. 5 is a cross-sectional view of a conventional l5S8 laser chip.

On the P-type GaAs substrate 50, an N-type GaAs layer 6 is formed.
1°63.65 and Al, ), 45 Ga 0.55
The As layers 62 and 64 are laminated to form a composite current blocking layer, with a single-shaped groove in the center 95. The current injection region to the active layer 55 is determined by the width Wl of the lower part of the single-shaped groove, etc. The measurement light guide area is determined by the width W2 of the top of the 1-shaped groove. This laser facilitates self-pulse modulation by providing a portion of the light guide region with a low injection current density and realizing a saturable absorber built into the light guide.

[Problem that the invention seeks to solve]

The method using high frequency superimposition described above not only requires the addition of a high frequency drive circuit, but also has disadvantages such as leakage of high frequencies to external mechanisms.

Secondly, in conventional semiconductor lasers that generate self-excited vibration to reduce noise, the self-excited vibration characteristics depend extremely sensitively on the structure such as the thickness of one layer and the groove structure, so stable self-excited vibration characteristics cannot be achieved. It had the disadvantage that the yield, which is the proportion of products obtained, was low.

An object of the present invention is to provide a semiconductor laser that produces stable self-excited vibration, has low noise characteristics, maintains fundamental transverse mode oscillation, and has excellent controllability 2 and reproducibility.

[Means for solving problems]

In the semiconductor laser of the present invention, an active layer is sandwiched between a cladding layer having a wider forbidden band width and a guide layer and a cladding layer on both sides of the active layer, and a guide layer is disposed on at least one side, and the guide layer and the cladding layer are sandwiched between the active layer and the cladding layer. between the layers.

A double heterojunction structure having a current blocking layer having a bandgap smaller than the active layer having a current injection region is provided on a substrate having a band-shaped concave or convex surface along the concave or convex surface. and the current injection region has a predetermined width smaller than the width of the central flat portion of the active layer, and the carrier diffusion length in the active layer is equal to or longer than the tube wavelength and is equal to the width of the central flat portion of the active layer. The absolute value of the difference with the width of the current injection region is less than or equal to the absolute value of the width of the current injection region.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings.

1 and 2 are a perspective view and a sectional view, respectively, of a first embodiment of the present invention.

N m AA with thickness o, os μm! 0. Is Ga
0085 As layer (N type impurity concentration: 1.5×1018
A thickness of 0.0 on the underside of the active layer 15 consisting of
A first cladding layer 14 consisting of a 5 μm N-type klO, 5Ga 6.5 As layer and a 1.0 μm thick N-type KlO layer.
A second cladding layer 13 consisting of ,4GaO,gAs is arranged, and on the upper side there is a P-type 10,5Ga with a thickness of 0.05μm.
Third cladding layer 16 consisting of an OlS As layer. Thickness 0.
A guide layer 17 consisting of a 5 μm thick P type kl O, 3Ga O, 7As layer and a 2.0 μm thick P type kl O 14G
a (A fourth cladding layer 20 made of a 1,6As layer is arranged, and a current injection region is provided between the guide layer 17 and the fourth cladding layer 20 with a thickness of 1.0 pm (D N fJ, G
aAs (N-type impurity concentration'?-5 x 1018 pieces/d)
A double heterojunction structure having a current blocking layer 18 consisting of the following is provided along the surface of an NmGaAs substrate 10 whose plane is the (100) plane and which has a band-shaped convex surface. The thickness of each layer is approximately uniform over the entire surface. The band-shaped convex surface of the N-type GaAs substrate 10, that is, the mesa stripe has a 1@ of 1.6 μm [011]
running in the direction Father, stop the flow of 11L/i! 1B has a striped current injection region with a width of 1.6 μm. Further, the central flat part of the active layer 15 (N-type GaAs substrate 1
The width of the portion on the mesa stripe (0) is 4 μm.

The impurity concentration of the active layer 15 is 1.5×10 particles/c! t
If S is set to S, the carrier diffusion length can be made approximately 1 μm. The internal wavelength of this rape is 0.224 μm (=
0.78 μm/3.4s6).

In this embodiment, the first cladding layer and the third cladding layer are provided to make the shape of the output beam nearly symmetrical, and are not necessarily necessary. father.

Although not shown, it goes without saying that in an actual semiconductor laser, a protective film is attached to the end facet.

Next, the manufacturing method of this example will be explained.

FIGS. 3(a) and 3(b) are cross-sectional views of a laser chip shown in order of steps for explaining the manufacturing method of the first embodiment.

First, as shown in FIG. 3 (al), 5io
After providing the Z film 11, selective etching is performed (oT).
An 8i02 film 11 is left in a stripe shape of @2 μm in the
The substrate 10 is etched to a depth of 1.0 μm. At this time, in the (011) direction, a convex forward mesa stripe 12 is formed in the area where the 5i02 film 11 is left. At this time, due to under-etching.

The width of the top of the mesa stripe is 1.6 μm.

Next, as shown in FIG. 3 (bl), this 5iOz film 11
After removing, N-type Al o, a Ga o, 6A
The 5 layer 13 is 1.0 μm thick, N-type Al (,5GaO,S
The As layer 14 has a thickness of 0.05 μm and is made of N-type Al (, 15 Ga
(B5 As layer 15 (N-type impurity concentration -1, 5X10
18 pieces/, ff1) at 0.05 μm.

P-type A1g, 5Ga, As layer 16 with a thickness of 0.05 μm
, P mAlo, x Ga O, 7As layer 17 is Q,
5μm, N-type GaAs layer (N-type impurity concentration ≧5X10”
pieces/cr1% 1.0 μm.

It is grown continuously by a chemical vapor deposition (MOCVD) method using an organometallic compound.

As mentioned above, the N-type impurity concentration of the active layer 15 is set to 1.5×1.
It was revealed that when the carrier diffusion length is set to 0''/d, the carrier diffusion length can be made to be ~1 μm, and the luminous efficiency is also increased.

Regarding the above-mentioned growth, the conventional liquid phase growth method is a method in which a melt with controlled composition for each growth layer is prepared and the substrate is moved to grow each layer. Not only is it extremely difficult to form a multilayer structure as in this embodiment, but it is also impossible to control the composition and thickness of each layer. On the contrary.

Since the MOCVD method is a vapor phase growth method using an organometallic compound, it is possible to easily grow layers of any composition into any multilayer structure by changing the composition of the mixed gas. (This can be easily done.Moreover, since the MOCVD method allows thin film growth and has precise film thickness controllability, the thin second cladding layer 14. Active layer 15.Third Crabstone layer 16t can be grown with good control of thickness.MOCV
In the D method, fine particles of each composition grow while bonding, so there is no dependence on the plane orientation of the growth, and the growth is uniform in thickness in all directions. Therefore, even if multiple layers are grown on a substrate having a convex surface as in the structure of this embodiment, the layer thickness will be uniform along the shape of the convex portion.

Next, a 5i02 film 19 is placed on the growth surface of the N-type GaAs layer 18.
After forming a 2 μm wide window using a photoresist method so as to coincide with the convex region of the substrate having a convex surface, an N-type Ga
The As layer 18 is etched to form P-type AlO, 3GaO.
, r fi, expose the surface of the 5 layer 17.

Next, after removing the 5iCh film 19, the P-type AA! 0.4G
a, ), 6A5 layer 2o 2. Ottm, PffGa
The AS key layer 21 is continuously grown to a thickness of 1.0 μm.

In the commonly used liquid phase growth method, P
It is almost impossible to form an epitaxial layer on the type kl 6.3 Ga 6.7 As layer 17 unless measures are taken to remove the oxide film formed on the surface, and the controllability of the film growth becomes very poor. - MOCVD method is relatively simple. In particular, in this MOCVD method, the fourth cladding layer 2
If the surface of the growing surface is lightly etched with a gas such as HCI immediately before growing 0, the reproducibility and reliability of the film formation and the semiconductor laser can be further improved.

Next, as shown in FIG. 1 or 2, a P-side electrode 22 is attached to the surface of the P-type GaAs cap layer 21, and an N-side electrode 23 is attached to the back surface of the N-type GaAs substrate 10.

FIG. 4 is a sectional view of a second embodiment of the invention.

Thickness 0.05 μm (7) N-type Al o, 1s Ga
O, 115As layer (Nm impurity concentration = 1.5
A first guide layer 24 made of an N-type Al0.3Gao4As layer with a thickness of 1.0 μm and a thickness],, Otxm (7J N
The first layer consists of type AJ o, s Gao, s A s layers.
A cladding layer 14 is arranged, and a P-type layer with a thickness of 0.5 μm is placed on the upper side.
i2 guide layer 2 consisting of lO,3Ga0.7As layer
A P-type AlO, 4G ao, a S-layer caranal fourth cladding layer 20 with a thickness of 2.0 μm is arranged, and a current injection region is provided between the second guide layer 25 and the fourth cladding layer 20. N-type GaAs with a thickness of 1.0 μm (N-type impurity degree n≧5
A double heterojunction structure having a current blocking layer 18 consisting of (X1018 pieces/item) has a band-shaped concave surface (1018 pieces/item).
00) is provided along the surface of the NmGaAs substrate 10 having a flat surface. The thickness of each layer is approximately uniform over the entire surface.

The band-shaped concave surface of the N-type GaAs substrate 10, ie.

The stripe groove has a width of 8 μm and runs in the oTB direction. The current blocking layer 18 will be discussed as having a stripe-shaped current injection region of @1.2 μm. Further, the width of the central flat portion of the active layer 15 (the portion above the striped grooves of the N-type GaAs substrate 10) is 4 μm.

The carrier diffusion length in the active layer is 1 μm, and the tube wavelength is 0.2
It becomes 24 μm.

The manufacturing method of this example is similar to that of the $1 example.

Next, the operation of the semiconductor laser of the present invention will be explained.

The current flowing into the semiconductor laser 2 of the present invention from the entire surface type flows through the cap layer 21. The current flows over the entire surface of the fourth cladding layer 20, but since there is an N-type GaAs layer 18 with a different electrical polarity adjacent to the fourth cladding layer 20, the current is blocked by this current blocking layer. From the current injection region, which is a striped window opened in 18, to the guide layer,
N type //Alo, ts Ga O, 85 is implanted into the AS layer 15. The gear injected into the active layer 15 diffuses toward the horizontal lateral force in the active layer, forms a gain distribution, and starts laser oscillation. At this point, as mentioned above, since the carrier diffusion length in the active layer is short, the gain distribution is mainly formed in the part of the active layer under the striped window formed in the current blocking layer 18, and its shape is steep. (9) As a result, the gain is high only in the lower part of the striped window, and the outside becomes a loss region.

In the semiconductor laser of the present invention, the central flat portion of the active layer is sandwiched between guide layers in the horizontal and lateral directions 2, as seen in FIG. 2 or 4.

Therefore, the active 1- light is focused on the active layer having a high refractive index in the horizontal and lateral directions, and a positive refractive index guiding mechanism based on a positive refractive index distribution is built.

Generally, when both ends of the active layer are sandwiched between cladding layers with a low refractive index, the positive refractive index distribution becomes too sharp, and as a result, −th order transverse mode oscillation occurs at low excitation levels. Therefore, in order to suppress this, it is necessary to narrow the width of the active layer. In contrast, in the structure of the present invention, the guide layers are adjacent to both ends of the active layer either directly or via a thin cladding layer, so that light is influenced by the guide layers.

The refractive index of the guide layer is closer to that of the active layer than the cladding layer, so the difference in refractive index with the active layer is relatively small.
The height of the positive refractive index distribution created in the horizontal direction of the active layer can be made relatively small, and a stable fundamental transverse mode can spread throughout the active layer along the convex or concave regions, and this excavation can be further enhanced. It can be maintained over a wide range of injection current ranges. Therefore, in the structure of the present invention, the width of the spread of light is wider than the width of the gain distribution, and the light spreads from the gain region to the loss region outside of the gain region.

Isometrically speaking, this means that it has a saturable absorber, and self-excited vibration is likely to occur.

Furthermore, in one structure of the present invention, light seeps out of the active layer and is drawn into the guide layer where it spreads out vertically.

(In the first embodiment, it seeps from the active layer into the third cladding layer 16, but since the third cladding layer 16 is thin, it is drawn into the adjacent P-type AJo, 5Gao4AS guide layer 17 with a high refractive index. Furthermore, there is an N-type GaAs layer 18 adjacent to the guide layer, but this layer has a higher refractive index than the guide layer and not only draws in light, but also prohibits laser oscillation light. Band width is narrow < 10000
c1rL-" or more. Therefore,
Light is drawn into the current blocking layer where it undergoes large absorption losses. This means that light outside the striped current injection region formed in the current blocking layer 18 will undergo significant absorption loss, which will aggravate the loss outside the gain region, making the saturable absorber more effective. make a target

Furthermore, in the structure of the present invention, the carrier diffusion length is less than half the absolute value of the difference between the width of the central flat part of the active layer, which determines the width of the refractive index distribution, and the width of the current injection region, which determines the width of the gain distribution. Because the refractive index during laser oscillation is relatively small,
It has the effect of promoting self-excited vibration.

That is, first, the carrier diffusion length is short.

The density distribution of the injected carriers fluctuates sharply, and the width of the fundamental transverse mode fluctuates accordingly.

Its contraction and expansion occur, and as a result, the magnitude of self-excited vibration is promoted. According to the analysis results of the present inventor.

As a result of calculation using the carrier diffusion lengths of 1 μm and 2 μm in the structure of each of the above-mentioned examples, the carrier diffusion length is 1 μm.
It has been revealed that the self-excited vibration in the case of 2 μm is 5.5 to 6 times that in the case of 2 μm.

Furthermore, the fact that the refractive index of the central flat portion of the active layer during laser oscillation is relatively small also promotes fluctuations in the width of the fundamental transverse mode. According to the analysis results of the present inventor, the first peak intensity and the intensity at the first valley of self-excited vibration are calculated using the structure of each of the above-mentioned examples and a carrier diffusion length of 1 μm. The ratio of the refractive index for guiding light is ηB = 1,0xl
O″″2 (this is the value for a normal semiconductor laser) is 16
0, ηB=5×1O−3 (direct in the present invention) is 19
I found out it's going to be 5.

3. If the carrier diffusion length in the active layer becomes less than the tube wavelength, the threshold current for laser oscillation will rise rapidly, so it is not a good idea to make it too small.

As a result of all of the above synergistic effects, self-excited vibration easily occurs in the structure of the present invention, and as a result, the axial mode becomes multi-mode, and since the coherence scar of the axial mode is reduced, the noise for one reflected light is also extremely low.

Low noise characteristics can be obtained. Therefore, the semiconductor laser of the present invention becomes a low-noise laser necessary for optical reading.

As described above, the semiconductor laser of the present invention is essentially different from the conventional semiconductor laser shown in FIG. In the semiconductor laser shown in FIG. 5, a T-shaped groove is formed after a multilayer structure is grown on a substrate.

Next, fill this groove flatly and place a flat active layer on top of it.

A cladding layer is formed and an FC layer structure is formed. The differences between this structure and the structure of the semiconductor laser of the present invention are as follows.

First of all, in the structure shown in Figure 5, the current injection region is farther away from the active layer than the refractive index distribution region formed by the light absorption effect, so the current is equal to or greater than the refractive index distribution region. Since the carriers are spread and injected into the active layer, the carrier distribution formed in the active layer has a width comparable to the refractive index distribution, and the light absorption effect that occurs in the horizontal and lateral directions of the active layer as in the present invention is significantly reduced. self-excited vibration tends to be less likely to occur.

Second, the semiconductor laser of the present invention, as explained above,
It is essentially important that the carrier diffusion length be shortened to less than half the absolute value of the difference between the width of the effective refractive index distribution, that is, the width of the central flat part of the active layer and the width of the current injection region. This has the effect of causing and promoting an excited vibration,
This results in low noise laser characteristics. However, the structure shown in FIG. 5 does not take such effects into consideration, and as a result, the allowable range in which self-excited vibration occurs becomes extremely narrow.

The case of the double heterojunction of A4GaAs/GaAs has been described above, but the present invention
P, InGaAsP/InGaP, InGaAs/In
P.

Needless to say, the present invention can be applied to many semiconductor materials such as GaAsSb/GaAs8b.

〔Effect of the invention〕

As explained above, the present invention provides a double heterojunction structure along the concave or convex surface of a substrate, and has a light absorbing effect between the guide layer and the cladding layer of the double heterojunction structure. By providing a current blocking layer, it is possible to produce a semiconductor laser that maintains stable fundamental transverse mode oscillation, performs stable self-oscillation, has low noise, and can be manufactured at high yield.

[Brief explanation of drawings]

1 and 2 are respectively a perspective view and a sectional view of the first embodiment of the present invention, and FIGS. 3(a+) and 3(b) show the order of steps for explaining the manufacturing method of the first embodiment. 4 is a sectional view of a second embodiment of the present invention, and FIG. 5 is a sectional view of a conventional 15SS laser chip. 10 N-type GaAs substrate , 11--8iO
Z film, 12...Mesa stripe, 13°---
--N saint lo, first cladding layer consisting of 5Ga0.5As layer, 14...N type kl O,s Ga
A first crat layer consisting of an O05As layer. 15------- N-type kl (,,,Ga O, as
Active layer consisting of A s layer, l 6-----・P type k
l O,5Ga (Second cloved layer consisting of 1,5As layer, 17...P-type AJo, 3Gao,, guide layer consisting of A5 layer, 18......N gGa A
Current blocking layer consisting of 41 layers, 19......8i0
z film, 20...P-type AlO, 4Ga6. gAs
The fourth cryo layer consists of layers. 21...P-type GaAs cap layer, 22...
...P side electrode, 23...--N side as pole, 24...
...N type AA! gi guide layer consisting of o, 5Gao, and 7As layers, 25-・-P type P l O63G a
6.7 Second guide layer consisting of As layer. 50------ P type G3 people, board, 51 ・---
-・-NMGaLS? rya, player, 52...N
Side electrode, 53...P side type j, 54--- ・-
Cladding layer consisting of P-type Al o, 35 Ga,), 65 As layer, 55-.-P-type Al o, 13 Ga
0.87 Active layer consisting of A'i layer, cladding layer consisting of 56-...-N type Alo, 3sGao, 5sAs layer,
61...NfiGaAs layer. 62-・-・AlO, 45Ga O,! S As layer,
63-...-N type GaAs layer, 64-...-person 1r
O,45Ga 0055 people-i layer, 65...
・N-type GaAs layer. Agent: Susumu Uchihara, Patent Attorney, 30 years old

Claims (1)

[Claims] The active layer is sandwiched between a cladding layer and/or a guide layer with a larger forbidden band width on both sides of the active layer and a cladding layer with a guide layer disposed on at least one side, and a current injection region is provided between the active layer and the guide layer. A double heterojunction structure comprising a current blocking layer having a smaller band gap than the current blocking layer is provided on a substrate having a band-shaped concave or convex surface along the concave or convex surface, and the current injection region is , has a predetermined width smaller than the width of the central flat part of the active layer, the carrier diffusion length in the active layer is equal to or longer than the tube wavelength, and the width of the central flat part of the active layer is equal to the width of the current injection region. A semiconductor laser characterized in that the absolute value of the difference is less than or equal to the absolute value.