JPS603177A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To obtain a laser device, an astigmatism and noises therefrom are small and which is suitable for a light source for an optical disk, by simultaneously forming refractive-index waveguide structure and gain waveguide structure in the direction of a resonator and independently selecting structural parameters determining the effective refractive-index difference of a refractive-index waveguide section and the gain distribution of a gain waveguide section. CONSTITUTION:An n type Ga0.55Al0.45As clad layer 2, an un-doped Ga0.85Al0.15 As active layer 3 and a p type Ga0.65Al0.35As optical guide layer 4 are laminated on an n type GaAs substrate 1 and grown in a liquid phase in an epitaxial manner, and a latticed optical guide with striped projecting sections 5 and flat sections 6 is formed on the surface of the layer 4 through etching by a phosphoric acid group etchant while being covered with a resist mask. A p type Ga0.55Al0.45As clad layer 7 and a p type GaAs contact layer 8 are grown on the latticed optical guide through a molecular-beam epitaxial method, the layer 8 is changed into a plurality of striped shapes through etching, and sections among the striped layers 8 are buried with oxide films 9. Metallic electrodes 11 and 12 are each applied on the films 9 and the back of the substrate 1.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a semiconductor laser device having both a gain waveguide (1, f structure and a refractive index waveguide structure).

[Technical background of the invention and its problems]

In recent years, semiconductor lasers using ■-v group compound semiconductor materials such as GaALAs have been increasingly applied to information processing equipment such as DADs (digital audio disks) and optical disk files. In semiconductor lasers for optical disks, it is necessary to narrow the laser beam to a small size, and in order to simplify the optical system, fundamental transverse mode oscillation and small astigmatism are required. Furthermore, the following problems have been found when applied to optical discs. That is, in optical disk files and the like, due to the mechanism of reading out information by detecting the intensity of the reflected light of the light irradiated on the disk, it is inevitable that some of the reflected light returns to the semiconductor laser. Therefore, in addition to the resonator formed by both end faces of the laser, the semiconductor laser also has a resonator formed by the laser end face and the disk surface, resulting in a laser having a double resonator. When the disk surface vibrates while rotating,
The latter change in resonator length causes fluctuations in the spectrum, optical output, etc., and optical noise occurs at some points.

Here, we will review the waveguide structure of a semiconductor laser from the viewpoint of suppressing return optical noise.

Waveguide structures of semiconductor lasers are generally classified into two types: gain waveguide structures and refractive index waveguide structures. In these structures, there is a trade-off between reducing astigmatism and reducing reflected light noise. That is, in the refractive index waveguide structure, the astigmatism is 5 [μ
Since the transverse mode is small and stable at less than rn5], the longitudinal mode also oscillates in a single mode, but because the spectral line width is narrow, the amount of output fluctuation due to feedback noise is large at more than 10[6]. On the other hand, in the gain waveguide structure, the longitudinal mode becomes multi-mode and the spectral linewidth is wide, so the amount of output fluctuation due to return optical noise is less than 1 [μm], but the astigmatism is more than 20 [μm]. growing.

As described above, conventionally, it has been impossible to suppress both astigmatism and return noise within the range that is practically acceptable for semiconductor lasers for optical disks using only the refractive index waveguide structure or the gain waveguide structure. could not.

[Purpose of the invention]

An object of the present invention is to provide a semiconductor laser device which is extremely useful as a light source for optical disks, which can sufficiently reduce the adverse effects of reflected light noise without losing its characteristics of fundamental transverse mode oscillation and small astigmatism. I am determined to provide it.

[Summary of the invention]

The gist of the present invention is to realize a structure that combines the features of both a refractive index waveguide structure and a gain waveguide structure.

As a refractive index waveguide structure, a so-called lipstripe structure in which a light guide layer on an active layer is provided with striped convex portions has recently been proposed. However, in a single refractive index waveguide structure, the transverse mode is very well controlled, the spectrum is a single mode, the linewidth is narrow, the coherency is high, and the amount of output fluctuation due to returned light is large, so it is difficult to use a DAD laser. Not suitable as.

Therefore, the inventors of the present invention have conducted intensive research with the aim of forming both a refractive index waveguide and a gain waveguide in the stripe direction, and as a result, they have found that the width of the striped convex portions is changed.
It has been found that the convex portion may be formed not over the entire area in the direction of the resonator, but only in a portion thereof. That is, it has been found that when the width of the striped convex portion is partially widened, the mosquito net has a gain waveguide structure in the wide part, and a refractive index waveguide structure in other parts. In addition, when a striped convex portion is formed in a part of the resonator direction, the convex portion does not have a refractive index waveguide structure.
It was found that the other parts have a gain waveguide structure.

The present invention focuses on such points, and includes a light guide layer that is grown on a flat active layer and has striped convex portions formed on its surface, and has a smaller refractive index than the active layer. , a cladding layer having a smaller refractive index than that of the optical guide layer grown on the optical guide layer, and a current confinement formed on the cladding layer to inject a current in a stripe pattern into the portion including the stripe-shaped convex portion. In the semiconductor laser device having a structure, the optical guide layer is composed of a portion where the striped convex portion is formed in the direction of the resonator and a flat portion where the convex portion is formed, or the stripe The convex portions are configured to have at least two different widths in one stripe.

Further, the present invention provides a semiconductor laser device in which the striped convex portions are formed extending to a layer closer to the substrate than the active layer, in which the striped convex portions are formed in the semiconductor layer including the active layer in the direction of the resonator. The striped protrusions may have at least two different widths in one stripe.

The effective refractive index difference in the refractive index waveguide structure depends on the composition (band gap, refractive index) and thickness of each semiconductor layer, especially the thickness d of the active layer (d = 0.1 μm), and the striped convexity. It depends on the thickness of the part and its step. Furthermore, for fundamental transverse mode oscillation, there is an upper limit to the width of the convex portion, and there is also a relationship with the effective refractive index difference. FIG. 18 shows an example of the results of mode calculation of these relationships using GaAtAs as the material. Here, the solid line A indicates the relationship between the width W of the lipstripe (stripe-like convex portion) shown in FIG. 2 and the step difference Δt of the optical guide layer,
The broken line B shows the relationship between the effective refractive index difference Δ”eff and the above-mentioned step difference Δt.The area on the diagonal side with the solid line A as the boundary is the region where fundamental transverse mode oscillation can be obtained. The gain distribution of depends on the width of the electrode stripes, the thickness of each layer, especially the distance from the electrode to the active layer, and the resistivity of each layer.

〔Effect of the invention〕

According to the present invention, both the refractive index waveguide structure and the gain waveguide structure can be formed simultaneously in the cavity direction. All of the structural parameters that determine the effective refractive index difference in the refractive index waveguide portion, the gain distribution in the gain waveguide portion, etc. can be independently selected. Furthermore, the ratio of the equal refractive index waveguide portion and the gain waveguide portion in the resonator length can be arbitrarily changed. From the above, it was possible to see the dependence of the two waveguide structures on the return light noise characteristics. Therefore, by appropriately setting the ratio of the two waveguide sections and various structural parameters, it is possible to obtain characteristics suitable for an optical disk laser, that is, fundamental transverse mode oscillation with sufficiently small astigmatism, and the ability to suppress return υ optical noise. Characteristics in which the amount of output fluctuation is also sufficiently small can be obtained.

According to the experiments of the present inventors, using GaAAAs as a material,
Effective refractive index difference! 1=2X10, width of convex part W=2.5 [
If the width of the striped electrode is 3.5 [μm] and the refractive index waveguide is provided only on one side of the resonator, the transverse mode oscillates in the fundamental mode, and there is a convex portion on one side of the resonator of 45% or more. If there is, it indicates the properties of a refractive index waveguide, and the astigmatism is 3 [μm
] Hereinafter, the longitudinal mode was a single mode, and the amount of output fluctuation due to return light noise was 10%E or more. In addition, if a convex portion exists on one side of the resonator (5 to 45 C%), the refractive index
Shows gain waveguide type properties, astigmatism 5-10 [μm]
, the longitudinal mode was multimode up to 3 [mW':], and above that, a single mode tendency appeared, but it was still multimode, and the amount of output variation due to return light noise was less than 1 [mW']. Furthermore, if there is a convex part below 5 [chi] on one side of the resonator,
Shows gain waveguide type properties, astigmatism 30 [μ77]
It was found that the longitudinal mode is multi-mode, and the amount of output fluctuation due to return light noise is less than 1 [chi]. Here, the astigmatism is 5 to 10 [μm] when the convex portion is 5 to 45 [%],
The characteristic that the amount of output variation due to return light noise is 1 [chi] or less is extremely suitable for use as a laser for optical disks.

[Embodiments of the invention]

FIGS. 3(a) to 3(f) are perspective views showing the manufacturing process of a semiconductor laser device according to an embodiment of the present invention.

First, as shown in FIG. 3(a) (N-GaAs substrate 1 (n
-1~2XiOcm) on top of N-Gao, 55A
to, 45 As cladding layer 2 (n=lX10 (M
% thickness 1.511qn), undoped G Ill
o, as AZo, 15 AS active layer 3 (thickness 0
．． 1μm) and P-GaO,65AAo,35A
s light guide M4 (p=1x10 cm, thickness 0.
5 μm) were sequentially grown and formed.

8-3 MO-CVD (metal-organic chemical vapor deposition) was used for this first crystal growth, and the growth conditions were a temperature of 750 [°C], f
=20, carrier gas (H2) flow rate ~10 (A'/
1nin:), the raw material is trimethyl gallium (TMG
: (CH3Ga'), trimethyla, IL/minium (TMA: (CH3)5AA), arsine (As
Hρ, p dopant: diethyl zinc (DEZ: (C
2H5)2Zn), n dopant: hydrogen selenide (H2
S e ), and the growth rate is 0.25 DJ/min
)Met. Note that the first crystal growth does not necessarily require M.
Although it is not necessary to use the O-CVD method, it is possible to use the MO-CVp method, which allows crystal growth with good uniformity in a uniform manner.
When mass production is considered, this method is more advantageous than the LPE method.

Next, a photoresist mask (not shown) is made on the light guide layer 4, and etched with a phosphoric acid etchant (temperature 20°C) for about 10 seconds, so that the photoresist mask (not shown) is exposed as shown in FIG. 3(b). Width 2.5 [7 m above] and length 30 on the surface of guide layer 4
Striped convex portions 5 with a height of 0 [μm] and a step difference of 0.2 Cttm and unetched flat portions 6 with a length of 300 [μm] were formed periodically at a pitch of 300 [μm]. . Note that these values may be determined from FIG. 1 so as to satisfy the condition that the effective refractive index difference is 2×10 −3 or more and fundamental transverse mode oscillation is obtained. In addition, when looking at one chip, the optical waveguide layer 4 has a striped convex portion 5 with a width of 2.5 μm and 300 μm in the cavity direction, and the convex portion as shown in FIG. 3(C). 5 and a flat portion 6 of the same height.

Next, after removing the photoresist mask,
The second crystal growth was performed by MO-CVD method. That is, as shown in FIG. 3(d), on the light guide layer 4)' -
Gao, 5sAto, xs'' cladding layer 7 (p=I
X 10' 8cm-3, thickness 0.3 ttm) and P
-GaAs contact layer 8 (P""lXl019c
m-3, thickness 0.5 μm) were successively grown. At this time, since the At concentration of the light guide layer 4 is 0.35, L
It cannot be grown using the PE method, and requires the MO-CVD method or the MBE method (molecular beam epitaxial method). Then,
Zn was diffused into the contact layer 8 to increase the carrier concentration in the contact layer 8. Next, as shown in FIG.
An oxide film 9 was formed on the exposed surface of the cladding layer 7 for 2000 years by selective etching, leaving stripes as shown in FIG. .

Next, a Gr-Au layer 11 as a P-type electrode and an Au-Ge layer 12 as an N-type electrode were deposited to establish ohmic contact. This sample was cleaved into a chip with a cavity length of 250 [μm] and a width of 300 [μm] as shown in FIG. 3(f), and a semiconductor laser was completed.

When we investigated the characteristics of the semiconductor laser thus created, we found that it oscillated in the fundamental transverse mode, had sufficiently small astigmatism, and
The amount of output fluctuation due to optical noise was also sufficiently small. Furthermore, when the ratio of the refractive index waveguide portion and the gain waveguide portion in the resonator was varied, the results described in the section [Effects of the Invention] regarding the mode guide were obtained.

From the above, in order to obtain a laser with excellent transverse mode control and strong characteristics against returned light, the structure of this example has a cavity length of 250t, a trnC refractive index waveguide portion of 50μ, and a gain waveguide portion of 200μ. ), it can be seen that a chip with a width of 300 [μm] is sufficient. In fact, when we investigated the characteristics of this structure, we found that the fundamental-like mode oscillates below the oscillation threshold of 70 [mA], the differential quantum efficiency is 15 [μm], and the astigmatism is below 10 [μm], and the longitudinal mode is 5 [μm] or less. (argument) in multiple modes,
The amount of output fluctuation due to return source noise was less than 1 [chi], making it extremely suitable as a radar for optical disks.

FIG. 4 is a perspective view schematically showing the structure of another embodiment. Note that the same parts as in FIG. 3(f) are given the same reference numerals, and detailed explanation thereof will be omitted. In the previously described embodiment, the striped convex portions necessary to produce the refractive index waveguide effect are formed only on the optical guide layer 4, whereas in this embodiment, the striped convex portions are formed on the active layer 3. It differs in that it is formed to include In the case of this example, the active layer 3 is surrounded on all sides by a layer (cladding layer 7) with a lower refractive index than the active layer 3, and the effect of providing the refractive index waveguide structure can be obtained more reliably. It will be done. That is, in the case of the previous embodiment, the light guided in the gain waveguide section is coupled to the refractive index waveguide section, and is emitted from the end face of the refractive index waveguide section as a laser beam with small astigmatism. , the laser beam components not coupled to the refractive index waveguide portion are also guided to the active layer on both sides of the striped convex portion, and are similarly emitted from the end face. Therefore, when the laser beam is narrowed down to a minute spot, the pattern on the imaging plane is such that the minute spot due to the laser beam coupled to the refractive index waveguide is superimposed with a wide range of notars due to components that are not coupled to the refractive index waveguide. In particular, when applied to an optical disk device in which it is necessary to image a semiconductor laser beam into a spot as small as possible, problems may arise depending on the signal detection method.

On the other hand, in this example, there is no active layer on both sides of the striped convex portion, and there is no structure for guiding leaked light on both sides of the striped convex portion. For this reason, even the light that has not been coupled to the refractive index waveguide is sufficiently spread out before reaching the end facet, and therefore, in the case of the semiconductor laser of this example, even when the laser beam is narrowed down to a minute spot, it still remains as a minute spot. No hemming components appear around the pattern. As described above, the semiconductor laser of this example has the advantage of better imaging characteristics of the laser light beam than the semiconductor laser of the previous example, but the striped convexity is insufficient to stably realize the fundamental transverse mode. There is also the drawback that the condition for the width of the section is more severe than in the first embodiment.

The manufacturing method of this example is almost the same as that of the previous example.
The only difference is that the etching for forming the striped convex portions is performed deeper than the active layer 3. Regarding the characteristics of the mode guide with respect to the ratio of the refractive index waveguide part and the gain waveguide part in the resonator, results similar to those of the previous example were obtained. Therefore, in the case of this structure as well, the resonator length is 25
0 [μm], the refractive index waveguide part is 50 [μm] and the gain waveguide part is 200 [μm]. The difference is that in the mode guide part there is no guide structure other than the active region, so propagation loss is reduced. Therefore, the oscillation threshold value is 60 [ηLA
), differential quantum efficiency 20 [%], air field
In the cross-turn, the spread caused by the NO8 component, which was present in the previous example, was eliminated, and an isotropic cross-turn was obtained. The astigmatism is less than 10 [μm], fundamental transverse mode oscillation is obtained, the longitudinal mode is multi-mode up to 5 [mW], and the output fluctuation due to return light noise is less than 1 [fissure], making it excellent for optical disks. The characteristics were shown.

Note that the present invention is not limited to the embodiments described above. In the embodiment, the refractive index waveguide structure is provided only on one side of the resonator, but it may be provided on both sides so as to sandwich the gain waveguide portion. Furthermore, the height of the gain waveguide portion does not necessarily have to be the same as the height of the refractive index waveguide portion, and may be smaller than that. In addition, the forming material is GaAtA
Not limited to s, QaInAsP and GaA4As
A compound semiconductor such as Sb may also be used. Furthermore, it is also possible to use a P-type substrate in place of the N-type substrate and reverse the conductivity type of each layer. MO-CVD as a crystal growth method
In addition to the MBE method, it is also possible to use the MBE method. Further, the current confinement structure is not limited to one consisting of a striped contact layer and an oxide film, but may be any structure that includes striped convex portions of the optical guide layer and can inject a current into the active layer in a striped manner. Can be changed. In addition, various modifications can be made without departing from the gist of the present invention.

[Brief explanation of drawings]

Fig. 1 is a characteristic diagram showing the relationship between the guide layer step Δt, the effective refractive index difference Δ”eff, and the rib stripe width W that provides fundamental transverse mode oscillation in a semiconductor laser with a beam stripe structure. , @W and the active layer thickness d are schematic diagrams defining and showing, FIGS. 3(a) to 3(f) are perspective views showing the manufacturing process of a semiconductor laser according to an embodiment of the present invention, and FIG. 4 is a schematic diagram showing another example. It is a perspective view showing a schematic structure of an example. 1---N-GaAs substrate, 2.''N-Gao,
55Ato, 45AS cladding layer, 3... undoped GaO, 85AtO, 15A8 active layer, 4 -P-G
ao, 5AAo, 35As optical guide layer, 5... striped convex portion, 6... flat portion, 5-P-Ga, 5, A/
=, 45As 2 Rad layer, 8... P-GaAs contact layer, 9... Oxide film, 11.12... Metal electrode. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Ichigoichi Figure 3 <a) Figure 3 (c) (d) Figure 3

Claims (2)

[Claims] (1) In a semiconductor laser device having a cedar-pull heterojunction structure made of a compound semiconductor material, there is a flat active layer and a stripe-like structure grown on the side of the active layer opposite to the substrate. a light guide layer having a lower refractive index than the active layer in which a convex portion is formed; a cladding layer having a lower refractive index than the light guide layer grown on the light guide layer; and a current confinement structure for injecting a current in a striped manner into a portion including the striped convex portion, and the light guide layer is arranged between the portion where the striped convex portion is formed and the convex portion in the direction of the resonator. 1. A semiconductor laser device characterized in that the semiconductor laser device has a flat portion with no portion formed therein, or the striped convex portion has at least two different widths in one stripe. (2) In a semiconductor laser device having a cedar pull heterojunction structure made of a compound semiconductor material, a striped convex portion is formed on the surface of the active layer and extends to a layer on the substrate side. a semiconductor layer including at least an active layer, a cladding layer formed on the semiconductor layer and having a lower refractive index than the active layer, and a striped layer formed on the cladding layer in a portion including the striped convex portion. a current confinement structure for injecting current; A semiconductor laser device characterized in that a convex portion has a width of at least 2'#i in one stripe.