JPH0260184A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To improve an element in characteristic and reliability by a method wherein a window region of a rib waveguide type is formed of the same crystal structure as a clad layer, and the length of the window region is set to a certain value which makes an effective end face reflectivity larger than a specified percentage. CONSTITUTION:A refractive index optical waveguide mechanism of a rib structure 10 is formed on a central region of the surface of a guide layer 5, and a window region 11, formed of the same crystal structure as a clad layer 12, is formed as being in contact with both the ends of an active layer 4 and guide layers 3 and 5. And, the length l is set to a certain value which makes an effective end face reflectivity equal to 1% or more. As mentioned above, the window region is formed of the same crystal structure as the clad layer 13, so that a semiconductor laser of this design can be manufactured through the crystal growth in twice, and the length of the window region 11 is restricted to a small value which makes the end face reflectivity equal to 1% or more, whereby the end face reflectivity is made not to decrease much and consequently a single mode oscillation can be realized.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to semiconductor lasers.

[Summary of the invention]

The present invention is a rib waveguide type semiconductor laser, in which a window region having the same crystal structure as the cladding layer is formed in contact with both ends of an active layer and a guide layer, and the length of this window region is determined by the effective end face reflection. By selecting a length with an efficiency of 1% or more, it is possible to provide a semiconductor laser that can achieve high optical output, improve device characteristics and reliability, and is easy to manufacture.

[Conventional technology]

2. Description of the Related Art Semiconductor lasers are known in which a region having an effective band gap larger than the light emitting region, that is, a window region transparent to laser oscillation light, is formed at both ends of a light emitting region of an active layer.

In a semiconductor laser having such a window structure, optical damage (catastrophic optical damage) at the end facets occurs.
calDama, ge (COD)) level, that is, the critical optical output at which optical damage occurs, it is possible to increase the optical output, obtain an effective low reflectance, and improve the far field pattern (far field pattern). ) at θN/
Effects such as bringing the θ□ ratio closer to 1.0 and obtaining high reliability can be expected. Until now, window structures have been particularly used in distributed feedback semiconductor lasers (hereinafter referred to as DFB lasers).
However, many examples have been implemented using embedded heterostructures. The reasons for this are: (i) DFB lasers essentially do not require end face reflections, so even if the end face reflectance decreases, the threshold current increases little; (ii) DFB lasers eliminate end face reflections and use diffraction gratings. (iii) It is easier to obtain single mode oscillation using phase shift technology, etc. (iii) Edge anti-reflection coating technology requires strict control of conditions and is not easy. things were important.

The present applicant previously filed Japanese Patent Application No. 177372/1983 (
In JP-A No. 63-33891), in a DFB laser that has an active layer sandwiched between a pair of cladding layers and a diffraction grating, the diffraction grating is formed in a stripe shape, and flat regions are provided on both sides of the stripe-shaped diffraction grating. A DFB laser is proposed in which a dummy region having the same diffraction grating structure as the striped diffraction grating is formed through the stripe-shaped diffraction grating. With this DFB laser, a narrow striped diffraction grating can be formed with good reproducibility. In addition, since the surfaces of the guide layers on both sides of the striped diffraction grating are flat, when each crystal layer is grown on top of the diffraction grating after forming the grating, the crystallinity of the crystal layer is good and the life of the semiconductor laser is improved. and the stability of operation is improved.

[Problem to be solved by the invention]

By the way, in the DFB laser, the use of a window structure with the expectation of the above-mentioned effects has some problems in terms of device characteristics. These are (i) selectivity between TE mode and TM mode is lost and bimodal oscillation is likely to occur, and (ii) external differential quantum efficiency is likely to decrease, resulting in an increase in drive current value.

Furthermore, the structures used so far, especially the embedded heterostructures, have had many manufacturing problems. That is, in the case of a DFB laser, crystal growth is required after forming the diffraction grating, crystal growth after forming the window region, and crystal growth after etching for the buried heterostructure, so it is usually performed at least twice. In this case, three or more crystal growths were required. In addition, when crystal growth is performed in two steps, the window region has the same structure as the buried region (which has a pn junction for current confinement) formed after etching, so the output light is present in the window region. There was a high possibility that refraction and distortion would occur at the pn junction.

Furthermore, when applying a window structure not only to DFB lasers but also to so-called Fabry-Perot semiconductor lasers that utilize end face reflectance, the threshold current will significantly increase unless the window region length is shortened. As a result, the growth surface becomes a flattened Blenner structure, which has disadvantages such as lack of marks and difficulty in microfabrication.

In view of the above-mentioned points, the present invention provides a semiconductor laser with a window structure that has improved device characteristics and reliability and is easy to manufacture.

[Means to solve the problem]

The present invention provides a semiconductor laser having a cladding layer, an active layer, and a guide layer, which has an optical waveguide mechanism having a rib structure in which the center region of the guide layer is thickly formed in a stripe shape. A window region that is in contact with both ends and has the same crystal structure as one of the cladding layers and is transparent to Raded oscillation light is formed, and the length l of this window region is such that the effective end face reflectance at the end face of the window region is 1% or more. Select and configure the length.

The semiconductor laser of the present invention includes a DFB laser and a Fabry laser.
Applicable to Perot type semiconductor lasers.

[Effect]

The semiconductor radar of the present invention achieves effects such as high output and high reliability due to the window structure, and at the same time, it is configured in a rib waveguide type, and the window region is formed with the same crystal structure as the cladding layer. It can be manufactured with multiple crystal growth steps, improving reliability and device characteristics. In particular, compared to the conventional double-growth method, since no pn junction exists in the window region, no refraction or scattering of output light occurs in the window region.

The window region length l is limited to a short length such that the end face reflectance is 1% or more. Therefore, when applied to a DFB radar, the end face reflectance does not decrease significantly, the threshold current decreases, and the TE mode and 7M mode can be selected, resulting in single mode oscillation. In addition, if the end face reflectance is controlled and reduced only by the window region, the external differential quantum efficiency will decrease, but in this configuration, the end face reflectance is not reduced only by the window region, so the end face reflection is The end face reflectance can be easily controlled, and a decrease in external differential quantum efficiency can be prevented.

On the other hand, when applied to a Fabry-Perot type semiconductor laser, the end face reflectance does not decrease significantly, so the laser oscillates.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a semiconductor laser according to the present invention will be described with reference to the drawings, together with a manufacturing method thereof.

FIG. 1 shows a case where the present invention is applied to an Aji'GaAs-based DFB laser. In this example, FIG. 1 AI, A2 and A
3, a cladding layer (2) of n (or p) type Aj2GaAs is formed on an n (or p) type GaAs substrate (1).
), n (or p) form Aj! Guide layer made of GaAs (3
), GaAs active layer (4) and p (or n) type Aj!
A guide layer (5) made of GaAs is sequentially epitaxially grown by, for example, MOCVD (metallic chemical vapor deposition), MBE (molecular beam epitaxy), or LPE (liquid phase epitaxy), so that the surface of the guide layer (5) is A striped diffraction grating (
6) and a diffraction grating (
Dummy region (9) with the same diffraction grating (8) as in 6)
form.

The effective thickness of the guide layer (5) is thick at the central stripe portion where the diffraction grating (6) is formed, and is smaller at the flat regions (7) on both sides. Note that the dummy region (9) in which the diffraction grating (8) is formed is formed thickly like the central stripe portion.

The central stripe portion in which the diffraction grating (6) is formed, the flat region (7), and the dummy region (9) are formed as follows.

After applying a positive photoresist layer on the guide layer (5), parts corresponding to the flat areas are selectively exposed (first exposure), and then a holographic exposure method, e.g.
Laser 351. The latent image of the diffraction grating is exposed by two-beam interferometry using lnm laser beams (second exposure). The order of the first exposure step and the second exposure step may be reversed. Next, after developing to form a resist mask, the guide layer (5) is selectively etched. The development conditions and etching conditions may be exactly the same as those for forming a diffraction grating on the entire surface. By this selective etching, a narrow stripe-like rib structure (10) is formed, and at the same time, periodic unevenness, that is, a diffraction grating (6), is formed on the surface of the rib along the direction of propagation of light. A flat region (7) is formed on both sides of the diffraction grating ω), which is etched to a thickness that does not reach the active layer (4), and a diffraction grating is formed in a relatively wide area outside the flat region (7). (8)
A dummy region (9) is formed. In this way, a refractive index optical waveguide mechanism using the rib structure (lO) is formed. The width Wl of the rib waveguide is, for example, about 2 to 5 μm, the width W2 of the flat region (7) is about 10 μm, and the dummy region (
The width W3 of 9) can be about 270 μm.

Next, after forming a rib waveguide as shown in Figure 1,
The guide layer (5), the active layer (4), and the guide layer (3) are selectively etched away in areas that are to become window areas with a width of 2j2. The etching direction is approximately perpendicular to the longitudinal direction of the rib waveguide and parallel to the crystal opening direction as much as possible. At this time, the guide layer (5), the active layer (4)
Since the total thickness d of the guide layer (3) and the guide layer (3) is about 0.3 μm, the etching groove (11) can be easily formed.

Next, as shown in FIG.
Methods such as CVD or MBE are used for vapor phase growth.

That is, the p (or n) type AlGaA has a larger bite gap than the guide layer (5) (3) and the active layer (4).
The s-layer is grown in a vapor phase, and a window region (
At the same time as forming the guide layer (5), the guide layer (5) is formed continuously.
) on which a cladding layer (12) is grown in a vapor phase, and further on the cladding layer (12) is a P (or n) type Ga having the same conductivity type as the cladding layer (12).
A cap layer (14) containing As is grown in a vapor phase. Here, a step portion (
15) clearly appears.

Next, as shown in the first diagram and D2, the window area (1
3) selectively etching away the nearby cap layer (14); That is, the cap layer (14) is formed in a stripe shape so as to correspond to the rib waveguide, that is, the striped diffraction grating (6). Note that although not shown in the drawings, the cap layer (14) is
, protons, boron, etc. are ion-implanted into the cladding layer (12), the negative part of the window region (11), excluding the central part corresponding to the striped diffraction grating (6), to form an ion-implanted layer for current confinement. It can also be made to form.

Then, ohmic electrodes (16) and (17) are formed on the entire upper surface of the substrate (1) including the back surface and the cap layer (1). Next, using the stepped portion (15) appearing on the surface corresponding to the window region (11) as a mark, the inter-fold formation is divided by selective etching to form a laser beam output surface (17).

In this way, a rib waveguide type DFB laser (19) having the desired window structure as shown in FIGS. 1E and E2 is obtained.
get. The window area (1) indulged in this DFB laser (19)
1) The length β is limited to a length at which the effective end face reflectance at the laser beam output surface (18) is 1% or more. Actually, the window region length l is the refractive index of the active layer (4),
It is determined depending on the thickness, the refractive index of the guide layer (3) (5), the thickness, the height h of the rib, etc. Typically, l can be several tens of micrometers or less.

According to the DFB Ray'j (18) having such a window structure, the crystal growth is performed twice, so that the device characteristics and reliability are improved. In particular, the optical waveguide mechanism is a rib waveguide type, and the window region (11) is a cladding layer (12).
Since it is formed at the same time and has the same crystal structure as the cladding layer (12), there is no p-n junction in the window region (11) compared to a buried hetero DFB laser with a window structure formed by the conventional two-step crystal growth method. Therefore, no refraction or scattering of the output light occurs in the window region (11).

In addition, since the second crystal growth uses vapor phase growth, a step portion (15) indicating the position of the window region (11) appears on the surface after epitaxial growth, and serves as a mark in the subsequent creases and etching. Long β can be easily controlled.

In addition, the length l of the window area is selected to be such that the end face reflectance is 1% or more, and in order to prevent the end face reflectance from being reduced only by the window area (11), the end face reflectance is controlled by methods such as end face coating. In addition, it is possible to prevent a decrease in external differential quantum efficiency. Incidentally, when the end face reflectance is reduced by controlling only the end face reflectance of the window region, the external differential quantum efficiency decreases.

In addition, since the end face reflectance of the window area does not decrease significantly, the DF
The TE mode and 1M mode can be selected in the B radar, and single mode oscillation can be obtained. Furthermore, the overall manufacturing ease is achieved.

In this DFB laser (19), effects such as high output, effectively low reflectance, θN/θ□ ratio approaching 1.0 in the far field pattern, and high reliability can be obtained by the window structure. .

In the upper case, it is applied to a DFB laser, but since the end face reflectance of the window region does not decrease significantly, a rib waveguide-type Fabry-Perot semiconductor laser that uses end face reflection (for example, a diffraction grating is formed in E1 and E2 in Fig. 1) is used. It can also be applied to structures that are not

Furthermore, the material is not limited to ^i' GaAs, but other types of materials can also be used.

〔Effect of the invention〕

According to the present invention, since it has a window structure, it is possible to increase the optical output and obtain a reliable semiconductor laser. In particular, as a rib waveguide type, the window region is formed with the same crystal structure as the cladding layer, and the length of the window region is shortened, so that element characteristics and reliability are improved. That is, when applied to a DFB laser,
Single mode oscillation can be obtained by selecting between TE mode and 1M mode. The external differential quantum efficiency increases and the drive current value can be lowered. There is no refraction or scattering of the output light in the window area. Since it can be formed by two crystal growths, reliability is increased and this semiconductor laser can be manufactured easily.

Further, it can be applied to a Fapley-Perot semiconductor laser.

[Brief explanation of the drawing]

FIG. 1 is a manufacturing process diagram showing an example of a semiconductor laser according to the present invention. (1) is the substrate, (2) (12> is the cladding layer, (3)
(5) is a guide layer, (4) is an active layer, (6) is a diffraction grating, (II) is a window region, and (14) is a cap layer. Agent Sadado Ito Hidemori Matsukuma

Claims (1)

[Claims] A semiconductor laser having a cladding layer, an active layer, and a guide layer, wherein the central region of the guide layer has an optical waveguide mechanism with a rib structure formed thickly in a stripe shape, and the active layer and the guide layer Window regions having the same crystal structure as the cladding layer are formed in contact with both ends of the cladding layer, and the length of the window region is set to a length such that an effective end face reflectance is 1% or more.