JPS6066484A - Manufacture of semiconductor laser device - Google Patents

Abstract

PURPOSE:To reduce optical output fluctuation due to reflecting light and improve mass productivity by a method wherein a current blocking layer is provided on an optical guide layer to perform current strangulation and lateral mode control simultaneously in the horizontal direction on the junction surface while periodical irregularities are provided on the optical guide layer to form a diffraction grid. CONSTITUTION:A clad layer 2, an undope active layer 3, an optical guide layer 4 and a current blocking layer 5 are successively grown to be formed on an N- GaAs substrate 1. Firstly the current blocking layer 5 is coated with photoresist which is exposed to form a stripe pattern. Secondly the current blocking layer 5 is etched utilizing the photoresist as a mask to form periodical irregularities 12 on the current blocking layer 5. Successively, a striped groove 11 is formed on the current blocking layer 5 utilizing the photoresit as a mask in the direction wherein said periodical irregularities intersect at right angles in the direction of stripe. Finally, the photoresist is removed and a clad layer 6 and a contact layer 7 are successively grown to be formed on overall surface. Later the contact layer 7 is coated with an electrode 8 while the lower surface of the substrate 1 is also coated with another electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to an improvement in a distributed feedback semiconductor laser device having both a gain waveguide structure and a refractive index waveguide structure, and a method for manufacturing the same.

[Prior Art and its Problems] Semiconductor lasers have come to be widely used as light sources for optical optical devices and optical communications such as digital audio discs (1) At)), video disks, document files, etc. Among them, a diffraction grating is formed by providing periodic irregularities in the semiconductor laser and in the optical waveguide, and the laser beam can be
A so-called distributed di-reduction semiconductor laser that forms a dragonfly (1) distributedFeedback: D
F"13 laser) (Well, since it oscillates in a stable longitudinal mode, the longitudinal mode hopping phenomenon that occurs due to temperature changes does not occur, so it is the same as that seen in normal) R Briperot type semiconductor lasers. No hopping noise,
This light source is excellent as a light source for piteo disks, which require a low high-frequency noise level in l'f. Also, I)Ii
'13 Rayleigh' has excellent characteristics such as small temperature change in the oscillation wavelength and the ability to select the emission wavelength by changing the period of the diffraction grating. It is also suitable for purposes such as transmission and large-scale communication using wavelength multiplexing.

By the way, as a laser for optical disks, firstly, it is necessary to stably oscillate the laser beam in the lowest order AT mode, which is better than focusing the laser beam on a minute spot with a diameter of about 1 μm. Stabilizes to basic mode,
A waveguide structure is effective for building a laser beam with small astigmatism. In addition, the second emitted laser beam is reflected by the optical surface or the surface of the optical component, and a portion of the laser beam returns to the laser end face, making laser oscillation unstable, and the reflected light noise is minimized. There needs to be.

Conventionally, 1) FB lasers in which the transverse mode is controlled by a built-in waveguide structure have been mainly used with a so-called buried structure. It has been confirmed that the buried structure DF13 laser has very small astigmatism, which indicates the virtual light source position in the horizontal and vertical directions on the cemented surface, and this laser was used as a light source for optical disks. However,
It was found that the laser was difficult to use because the coherence was too strong, and the return light noise caused by the coherence effect was large. On the other hand, information devices such as optical disk devices and optical communication system L are expected to become more widespread in the future, and technology for mass production of semiconductor lasers is required. Conventionally, a liquid phase epitaxial growth method (LPE method) using a sliding boat method has been used as a technology for manufacturing a thin film multilayer ZO-junction crystal for semiconductor lasers, but the LPE method has a limit in increasing the wafer area. For this reason, the metal organic vapor deposition method (MOCVD method), which is characterized by uniformity and controllability, and the molecule #Jj!
Crystal growth techniques such as epitaxy method (FvlBB method) are attracting attention.

Conventionally, semiconductor lasers based on the LPE method have
Various A structures have been developed, including the MOCVJJ method and MD.
Since the crystal growth mechanism is different between I2 method etc. and LPE method, 1. It is difficult to manufacture lasers manufactured by the PE method in seven minutes, and it is necessary to develop a new laser structure that takes advantage of the characteristics of these growth technologies. In fact, the buried structure laser described above is
It was difficult to manufacture using the MOCVD method or the MBE method.

Therefore, we developed a DFB laser with controlled transverse mode in which the coherence of the laser beam is weak and the optical output fluctuation due to the return light is small.
It has been desired to realize a structure suitable for mass production that can be manufactured by OCVD or MBE.

[Purpose of the invention]

The purpose of the present invention is to improve the characteristics of the laser beam with weak iJ coherence and small fluctuations in optical output due to return light, and to improve the characteristics of the MOCVD method and MBE
An object of the present invention is to provide a DFB laser with a controlled transverse mode, which has a highly mass-producible structure that can be manufactured by a method.

[Summary of the Invention] The gist of the invention is to provide a current blocking layer on the optical guide layer formed on the flat IT layer, and to prevent current confinement in the horizontal direction and transverse mode at the bonding surface. By performing control at the same time, a structure having both an advantageous Q waveguide groove structure and a refractive index waveguide structure can be realized, and at the same time, in the part where the current blocking layer is not provided, periodic irregularities are formed in the original guide layer. was provided to form a diffraction grating and constitute a DFB resonator.

[Effects of the present invention]

According to the present invention, the characteristics of Laserb C having weak coherence and small fluctuations in optical output due to returned light, and MOCVD and MBE
It is possible to realize a DFB laser with a controlled transverse mode, which has a highly mass-producible structure that can be manufactured by the method. As a result, semiconductor lasers having characteristics suitable for light sources for optical disks can be produced in large quantities and at low cost.

[Embodiments of the invention]

FIG. 1 shows an embodiment of a semiconductor laser device of the present invention. The p-GaAtAs optical guide layer 4 in the striped groove portion provided in the n-GaAsflj flow blocking layer 5 has a period △ that efficiently reflects and returns light in the emission wavelength band of the active layer.
(Δ-1-λ(, /2neff, t=1.

2.3. ...However, λ. is the emission wavelength in free space,
;'+e f f is the average effective refractive index of the waveguide), and the unevenness 12 is provided to form a DF13 structure. p-
(Since the JaAtAs light guide layer has a larger refractive index than the crat layer, the waveguide mode distribution is sufficiently spread in the light guide layer, which is not shown in Figure 1), allowing efficient reflection feedback. Realized.

In the laser having the structure shown in FIG. 1, current is injected into the active layer through the striped portion from which the current blocking layer has been removed. This is PNPN on both sides of the stripe part.
This is because the current does not flow on both sides of the stripe unless a sufficiently high forward voltage is applied because the junction is -C.Also, the light guided into the active layer is Because it is thin, it leaks into the P-type optical guide layer, and a part of it is affected by the current blocking layer whose complex refractive index is different from that of the cladding layer, and the current confinement effect and built-in structure are formed in a self-aligned manner with the waveguide structure. It has %n that is said to be done.

In such a structure, when fabricating a Brie-Bello type semiconductor laser without a diffraction grating, if the width W of the stripe portion is as wide as 5 μm or more, a transverse mode in the horizontal direction will not be created at the junction surface. The laser beam is kited by the waveguide structure and exhibits stable laser oscillation. Therefore, from a point where the current is slightly higher than the oscillation threshold current, the mode becomes single-axis mode.
The astigmatism difference is also very small, 0 to 5 [μm].

In other words, it shows a laser oscillation state with very strong coherence. On the other hand, when the width W of the stripe portion is gradually narrowed, the distribution of injected carriers in the region where the oscilloscope mode guided by the waveguide structure is distributed,
In other words, the influence of the gain distribution that causes the gain waveguide effect appears strongly. This situation is shown in Figure 2 (a) to (C1).In the figure, the solid line A shows the refractive index distribution, the broken line B shows the damping distribution, and the dotted chain I%1C shows the waveguide mode distribution1. , the so-called built-in oscillation in single mode?H'
j: In contrast to waveguide lasers, which have strong ijJ+ interpolation and have large return light noise, the advantage is that they oscillate in longitudinal multimode;)
It is well known that the waveguide laser has a small noise level, but as the width W of the stripe section is decreased in the laser described in 2. Benefit (4) Being affected by the waveguide effect means that it is being affected by an intermediate effect between the two. Monday 1st Tuesday, the stripe width W is 3 [μm] and T is /
In this laser, although the astigmatism difference is relatively small at 5 [μm] or less, when the optical output is increased to 3 to 5 [mW], it changes from longitudinal multimode oscillation to single-longitudinal mode oscillation. Nevertheless, it was confirmed that the return light noise decreased to 01 to o, ol [11] over the range of 5 to 10 [mW].

On the other hand, the intracavity loss of a gain waveguide laser is 200 [cm
], and the built-in waveguide laser has a small intracavity loss of 20 to 30 [z].

According to experiments conducted by the present inventors, when the intracavity loss was measured by changing the stripe %i W, the stripe width W = 5
The results were as follows: 30 Ccm ] at W = 3 [lLm], 50 [z] at W = 3 [lLm], and 150 [on ] at W = 1.5 (zzm).The narrower the 8-stripe section W, the greater the intra-cavity loss. A small fire grows larger,
As the stripe width W becomes narrower, the gain waveguide effect becomes larger in addition to the waveguide effect.

The phenomenon described in Fabry Be's ``1'' occurs not only in the case of the 1-type laser but also in the JJFB laser.In the DFB laser, the gain waveguide 4t'j M-index waveguide groove Although the difference is that single-longitudinal mode oscillation is performed in the case of , the same results as in the case of the Fabry-Perot laser were obtained regarding astigmatism and return light noise.

This means that in the case of a gain waveguide type DFB laser, the longitudinal mode is a single mode, and since the spectral linewidth is wider than that of a refractive index waveguide type pBV laser, it is coherent. As a result, the return light noise is small 11 yen
It is interpreted that sex has been obtained.

In the laser of the present invention, current confinement and transverse mode control are n-
Since the GaAs' laminar flow blocking layer can be realized in a self-aligned manner, the manufacturing process is simple because only one mask alignment step is required, and the structure is suitable for mass production.

Further, periodic irregularities 1 are formed on the p-GaAtAs optical guide layer 4.
2, once n-GaAs'11L
After forming unevenness on the entire surface of the flow blocking layer, stripe grooves 11 are formed by etching.By using a method that preserves the shape of the unevenness during the etching process, n-GaAs current blocking is achieved. The unevenness on the layer is made of p-GaA
It becomes possible to transfer onto the 4As light guide layer 4, and the process can be simplified. This process can be realized, for example, by using techniques such as active ion etching or ion milling, as described below.

Since these methods can perform so-called vertical etching, it is possible to create unevenness with a rectangular cross section.
This allows the diffraction efficiency of the diffraction grating to be increased compared to other shapes. In this way, by using the above method, it is possible to reduce the number of manufacturing steps by simply adding the step of fabricating the diffraction grating on the n-GaAs current blocking layer, compared to the case of a normal fiber optic/Perot laser. In addition, the structure of the present invention can minimize the increase in
It is possible to create by the method or MBE method. These methods aim at growing a homogeneous thin film crystal over a large 10-layer area with zero efficiency, so they are methods for manufacturing semiconductor lasers with excellent repeatability. The laser of the present invention can be produced in large quantities at low cost.

The laser with the above structure has a first layer extending from the substrate l to the current blocking layer 5.
2nd crystal) Jy, length, 'East b; 2nd crystal to form the light guide 6 and the conotact layer 7 after forming a diffraction grating by etching a part of the stopper layer 5 in a stripe shape. It is created by a two-step crystal growth process called crystal growth. Here, the growth on the cladding layer 7 at the start of the second crystal growth is the growth on the JaA4As surface once the surface is exposed to the air. Difficult to grow, MOCV, L) Law MB
It was only by using the E method that it became possible to manufacture it with good controllability.

addition, ti, MOCVJ) method, MB-E method-r t-
Since there is no problem of disappearance of unevenness due to the melt bank, which is a problem with the PE method, a 74 diffraction grating with uneven shapes with good diffraction efficiency can be built into the element as is. Another advantage is that it can greatly improve the properties of

Like Jin, Akira's laser uses the MOCvIJ method.

It can be considered a laser suitable for fabrication using the MBE method.

Next, the manufacturing method of the present invention will be explained in detail using FIG. 3.

'First, as shown in Fig. 3 FaJ, an n-GaAs substrate 1 (Sl doped, n=1xlo c
m) 2μm thick n Oa o, 6A, L on soil
o, ,i A S cladding layer 2 (Sc dawg, n
=2X10 an )a 0.08 μm thick Andog U aO,9A Z O,] A S 'active layer 3.
0.4 μm thick p G a o, 75 A Z O,2
s A S '7L guide Ij44 (Zn doped, n
= 5xlOan) and u − of thickness 0.6/i m
() a A s Current blocking layer 5 (Se top, n
, -5xtd8Crn-1) were grown sequentially. The MOCVD method was used for this first crystal growth, and the growth conditions were: substrate temperature 750°C, V/1II = 20, total flow rate of carrier gas (H2) and raw material waste 10 t/mi.
n, the raw material is trimethylgalliu A ('l'MO: (
Ckl) 3 (J a ), trimebul aluminum (T MA : (CH3) 3A t ), f 7
(As14), p dopant: diethyl zinc (1)
l; Z: (C211g)2Zn), n dopant: hydrogen selenide (J12Se), growth rate 0.2
5 (it m /min). In addition, the first
Although it is not necessarily necessary to use the MO-CVD method for the second crystal growth, MO-CVD allows crystal growth with good uniformity over a large area.
Using the O-CVD method is LP when considering product production.
It is more advantageous than the E method.

Next, as shown in FIG. 3 (tb), a photoresist is coated on the current blocking layer 5 and exposed by a 2-beam C flux interferometry method using a laser to form a periodic stripe pattern of 022 μm. . This value corresponds to a second-order Mader grating (t=2) for wavelengths from λ0 to 8000A. Next, using this as a mask, C(-t4-C4
7. Etched to a depth of approximately 0.2 μm by reactive ion etching using a combination gas.
Periodic unevenness is formed on the top. At this time, it is advantageous to set the etching conditions so that so-called vertical etching is prevented, since a diffraction grating with a rectangular cross section and high diffraction efficiency can be obtained.

Then, using the photoresist as a mask, the above-mentioned periodic uneven stripes are applied to the flow-up stopper layer 5.1. Striped grooves each having a width of 2.5 μm are formed in a direction perpendicular to the groove. At this time, if the above-mentioned active ion etching method is used to obtain vertical etching, etching is not performed from the 111j surface of the unevenness, so that the period in which the n-GaAst flow blocking laminar flow top layer 5 was raised is not etched. The surface roughness was preserved during etching and was transferred onto the exposed p"0.75 AtAs cladding layer. When a wet etching solution was used with Esotin 0.25, etching was performed even from the sides of the roughness. As the etching progresses, the uneven shape changes, so it is preferable to use the above-mentioned reactive etching method or the ion 7-ring method.

Next, after removing the photoresist and performing surface cleaning treatment, a second crystal growth was performed by MOCVD. That is, as shown in FIGS. 1(a) and 1(b), pGaO6A1 with a thickness of 2μnl is deposited on the entire surface.
o, 4AS cladding layer 6 (Zn doped, p=8X1
0 cnr) and a p-GaAs contact layer 7 (Zn doped +p=5×10 cm) with a thickness of 2 μm were successively grown.

After this, the normal electrode material is formed by forming a Cr/Au electrode layer 8 on the contact layer 7 and an Au electrode layer 8 on the bottom surface of the substrate 1.
A large amount of Ge/AuI was deposited.

Thus obtained! The 1D element oscillated in the fundamental transverse mode, the excavation wavelength was approximately 800oX, and stable single-longitudinal mode oscillation was observed at 14+, confirming DFB operation.

Furthermore, the return C noise is also the return light Ho, oox-10 (
%), the detection limit is 0.0 at a light output of 5 mW.
Six of them were extremely small, less than 5 inches in size.

Note that the present invention is not limited to the embodiments described above. For example, the constituent materials are not limited to Ga AtAs, but also I noaA s P , ALl nGa
P Other III-V group compound semiconductors can be used.

Furthermore, it is also possible to use the Ml(E method) instead of the MOCV L) method as a crystal growth method. It can be implemented.

[Brief explanation of the drawing]

FIG. 431 shows the semiconductor laser device 6. of the present invention. Figure 2 is a diagram for explaining one embodiment, Figure 2 is a diagram showing the shadow of the gain distribution that causes the waveguide effect, Figure 3 is a diagram showing the manufacturing process of the present invention. be. ]...n-GaAs substrate 2 '''n Gao, 6AZO, 4A S cladding layer 3...Undoped Gao, 9Ato,
IAs active layer 4--p-Ga AtAs light guide layer 0.75 0.25 5...n-GaAs current blocking layer 6--p-Ga AtAs crat layer 0.6 0.4 7... p-(JaAs contact layer 8...Cr/Au electrode 9-Au-Ge/A, u electrode 11...stripe groove 12...periodic unevenness (diffraction grating) Yu (ih, 75-1
Figure 1 Figure 2 Figure 3

Claims (2)

[Claims] (1) In a semiconductor laser device made of a compound semiconductor material and having a double Peter junction structure, a first conductivity type cladding layer grown on a fourteenth conduction type substrate, and a first conductivity type cladding layer grown on this cladding layer. A striped 11" portion is grown on the optical waveguide layer of the active layer and the second conductive optical waveguide layer grown on the active layer, and extends to the optical waveguide layer. and a second conductive type cladding layer that includes the groove and is grown on the current blocking layer, at least in the groove, 1. A semiconductor laser device, wherein periodic irregularities are formed on the optical waveguide layer, and the refractive index of the optical waveguide layer is thicker than that of the cladding layer. (2) In the method for manufacturing a semiconductor laser device made of a compound semiconductor material and having a double heterojunction structure, the first
A first conductive layer is formed on the conductive substrate. a step of sequentially growing an active layer, a second conductive type optical waveguide layer, and a first conductive type current blocking layer; a step of forming striped periodic irregularities on the current blocking layer; forming striped grooves in the blocking layer in a direction perpendicular to the direction of the stripes of the striped periodic irregularities until reaching the optical waveguide layer; and forming a second conductive groove on the current blocking layer including the groove portion. The method includes a step of growing a mold cladding layer, and when forming the striped grooves, periodic irregularities formed on the current blocking layer in the groove portions are transferred onto the optical waveguide layer. A method for manufacturing a semiconductor laser device.