JPH01183184A - Semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To simplify steps, to eliminate the limit of the composition of a layer for forming a laser and to increase the life of the laser by forming a protrusion on a substrate, forming periodic diffraction gratings on the side faces of the protrusion, and so forming a layer on the substrate as to substantially hold the shape Of the substrate. CONSTITUTION:Protrusions which extend toward a resonator are formed on an active layer 34 on a substrate 31 and layers 32, 33 extending under the layer 34, periodic diffraction gratings 44, 45 are formed on the side faces of the protrusions, and films are grown thereon. Accordingly, these films are formed according to the shape of the substrate. Thus, a semiconductor laser which is scarcely affected by an external influence and has a small oscillation wavelength can be simply obtained. Further, the damage to a grown layer due to the formation of the grating after the growth of the film is temporarily interrupted is reduced to eliminate the limit of the composition of the layer, thereby increasing the life of a semiconductor laser.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser that emits light suitable for optical communication and a method for manufacturing the same.

[Conventional technology]

Currently, optical communication technology is being developed with the goal of high-speed, large-capacity communication. When performing high-speed, large-capacity communication, the light source that serves as the signal source must meet the following conditions: 1) The oscillation wavelength width is narrow, and 2) It is not susceptible to external influences (temperature, humidity, etc.). is necessary.

As shown in FIG. 7(a), the optical fiber through which light propagates has a core 2 made of quartz glass containing P or Ge, and a cladding layer 1 made of 5j02 or plastic containing B or F around the core. Usually covered by 3.

Taking a single mode fiber as an example, light is mainly confined within the core and transmitted. At this time, if the wavelength width of the light oscillated from the light source is wide, the refractive index of the core has wavelength dependence, so when the light is transmitted over a certain distance, the transmission time will differ depending on the difference in wavelength. Therefore, the output pulse width has a temporal spread compared to the manual pulse width. This is usually called refractive index dispersion or wavelength dispersion, and is one of the factors that determines the transmission speed of optical communications. That is, the seventh
As shown in Figures (a) and (b), when a pulse 6 is manually applied into an optical fiber, the light propagating the signal of this pulse 6 has a wavelength width, and is divided into light 5 with wavelength λ1 and light 4 with wavelength λ2. Can be divided. At the rising edge of the pulse 6, the light 5 with the wavelength λ1 and the light 4 with the wavelength λ2 spread to positions 8 and 9, respectively, on the output side due to refractive index dispersion. Normally, if there is no refractive index dispersion, pulse 6 becomes pulse 10, which has the same waveform as pulse 6, but if there is refractive index dispersion, pulse 6 becomes pulse 7, which spreads in time, as described above. For this reason, the first specification strongly required of a light source is that the oscillation wavelength width be narrow.

A light source must not only have a narrow oscillation wavelength width.

That is, the light source is required to have characteristics that do not change due to external influences (temperature, humidity, etc.) for the following reasons. Generally, a semiconductor laser is used as a light source for optical communication because of its narrow oscillation wavelength width. However, the numerical characteristics of semiconductor lasers tend to depend particularly on temperature, and as the temperature rises, the oscillation wavelength becomes longer by about 2 to 3 persons/deg. in this way,
If the oscillation wavelength of the laser is unstable, the signal transmission speed may change or other unstable factors may be introduced. A DFB laser shown in FIG. 8(a) has been developed as a semiconductor laser that has a relatively stable oscillation wavelength and provides a light source suitable for optical communications.

For this DFB laser, “S, Noda, K
.

Kojima, K., Mitsunaga, K.
uma, K., Ilamanaka.

and T, Nakayama, Appl, phys
, Lett, 48. I'767 pages (1986)
It is described in etc.

An example of a general method for manufacturing a DFB laser will be described with reference to FIGS. 8(a) and 8(b).

n-G'aAs buffer layer 1 on n-GaAs substrate 11
2 with a thickness of 0.2 μ'm, n-AIo4Gao, ,
, the thickness of the As cladding layer 13 is 1.5 μm, and the active layer 14
are sequentially stacked. Note that the active layer 14 is nondoped.
-Alo2Gao, 5 layers of 6As barrier layers (35 layers thick) and 6 layers of nc+ndoped-GaAs well layers (120 layers thick) are alternately laminated.

Furthermore, P-A on top of this]42. Ga, , , 3As barrier layer 15 with a thickness of 0.02 μm, 'P-AI. +3G
An ao, a7As guide layer 16 is formed to a thickness of 0.2 μm. Thereafter, a diffraction grating with a period of 2535 is formed on the P-Alo, rsGao, and 67AS guide layer 16 by interference exposure. The etching solution used to form the diffraction grating is H2SO4:1 (202:H2O).The wafer produced through the above steps becomes as shown in Figure 8(a).After this, a second etching process is performed. Film growth is performed. In the second film growth process, the l”Al layer on which the diffraction grating has been formed is
o, +3caO, 8TAS P-A1 on the guide layer 16
．． 3Ga0. , the As subcladding layer 18 has a thickness of 0.
．． 29 μm, P-A1.4Gao6As cladding layer 19 has a thickness of 1.2 μm, P”-GaAs, contact layer 20 has a thickness of 0.j4zm.

Form sequentially. After this, a ridge is formed to improve the efficiency of current injection. The method is to form a strip of resist perpendicular to the diffraction grating using normal photolithography technology, and
P-AI at room temperature using 0□=5:l etching solution.
'0.13Ga0.87AS Etching is performed up to the top of the guide layer 16. Furthermore, in order to prevent current from being injected into parts other than the ridge, a 5in2 layer 17 is formed on the part other than the upper part of the ridge, and finally, the bottom surface of the n-GaAs substrate 11, 5j
Au/Cr and 8uGe/N on the top surface of the 02 layer 17, respectively.
i/Au is deposited to provide a lower electrode 22 and an upper electrode 21.

In the semiconductor laser actually manufactured according to the above process,
The oscillation light of the active layer 14 spreads to the upper and lower layers of the active layer 14,
The wavelength of the oscillated light becomes stable under the influence of the diffraction grating of the guide layer 16. The threshold current of this semiconductor laser is 28mA
The external differential quantum efficiency was 48%, the longitudinal mode was single, and the temperature dependence of wavelength was 0.76 persons/deg. In this way, the conditions for light sources for optical communication are 1)
The oscillation wavelength width is narrow, and 2) the temperature dependence is small.

[Problem to be solved by the invention]

However, in the production of the conventional semiconductor laser mentioned above,
A two-part film growth process is required; 1) the process is complicated; and 2) the composition of the layers constituting the laser for film regrowth is limited.

There is a drawback. The process of 1) is complicated because: - Stopping film growth, processing, and then growing the film.
The continuous film growth used in the production of semiconductor lasers other than DFB lasers complicates the process and means that the grown film is likely to be damaged. 2) The fact that the composition of the layer constituting the laser is limited in order to re-grow the film means the following. In the case of regrowth, P-AI on a film containing a large amount of AI (X≧0.4)
Growth of GaAs is not possible. Therefore, Fig. 8(a)
In the example shown in , it is necessary to use P-AIGaAs with a small X, for example, x = 0.13, for the layer forming the diffraction grating, and the composition of the layer is limited.

[Means to solve the problem]

A semiconductor laser in which one or more semiconductor layers including at least an active layer and a PN junction are laminated on a substrate, and an electrode for passing a forward current to the semiconductor layer is provided, the active layer and the lower part of the active layer A semiconductor laser characterized in that a layer existing in the substrate has a convex portion extending in the direction of the resonator, and a periodic diffraction grating is formed on the side surface of the convex portion, and the convex portion is formed on a substrate. a step of forming a periodic diffraction grating on the side surface of the convex portion; and a step of forming a layer on the substrate so as to substantially maintain the shape of the substrate. It's a method.

[Effect]

In the present invention, a convex portion is formed on a substrate in advance, a diffraction grating is formed on the side surface of the convex portion, and a film is grown on the substrate. As a result, these films are formed to reflect the shape of the substrate, and a diffraction grating is formed on the side surface of the active layer at the ridge. As a result, as shown in FIG. 6, when the pitch of the diffraction grating is d and the incident angle of light is θ, the wavelength λ of the oscillated light satisfies the following equation.

mλ=2dcosθ (m=1.2, 3., , ) Here, since θ is almost 0°, the above equation becomes 0λ=2d. Therefore, the laser oscillates at a wavelength corresponding to the pitch d of the diffraction grating, which can improve wavelength instability caused by external influences (temperature, humidity, injection current, etc.) in conventional semiconductor lasers, and also improve the pitch d. Because it is so small, the mode spacing becomes wide and there are almost no mode spacing transitions.

Furthermore, in the present invention, the semiconductor layer stacked on the pre-processed semiconductor substrate is formed so as to maintain its processed shape, so a semiconductor laser can be manufactured through simple steps without interrupting growth. Therefore, it is possible to have a longer life than a DFB laser. Furthermore, in terms of characteristics, the threshold current and differential quantum efficiency can be improved due to the reduction in defects.

〔Example〕

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

FIG. 1 is a schematic perspective view showing an embodiment of the semiconductor laser of the present invention, FIG. 2 is a perspective view of the substrate 31 in FIG. 1, and FIG.
a) to FIG. 3(e) are schematic diagrams showing a method of manufacturing the substrate 31 of FIG. 2.

In this example, a convex portion extending in the direction of the resonator is provided at the center, and diffraction gratings 44 and 45 extending in the longitudinal direction of the convex portion are formed on the side surfaces of the convex portion and on the substrate 31 on both sides of the convex portion. n
- On the GaAs substrate 31 (thickness 350 μm),
-GaAs buffer layer 32 (0.5 μm), n-A
lo5Gao, sAs cladding layer 33 (1.5 μm
), an active layer 34 is laminated. The active layer 34 includes four layers of nondopen-Alo, , Gao, and As barrier layers (thickness: 100 layers), and nondopen-Alo, , Gao, and As barrier layers (thickness: 100 layers).
d-A1. , +Gao98Swe8genwelllayer0people)
5 layers are alternately laminated. Furthermore, on top of this, P-A1. , 5 Gao, sAs cladding layer 35 (1.5 μm), P-GaAsr contact layer 36 (0.5 μm), 5 layers for current confinement 02
Layer 37 (3,000 people) is stacked. however,
The S i '02 layer 37 is provided so as not to cover the upper part of the ridge.

Further, n=lower surface of GaAs substrate 31, 5in2 layer 37
AuGe/Ni/8u and Cr/heu on the top surface of the
is deposited, and a lower electrode 39 and an upper electrode 38 are provided. When a current 40 is applied to this semiconductor laser, the active layer 3
The upper surface of the convex portion 4 emits light. Since this semiconductor laser has a diffraction grating on the side and bottom surfaces of each layer in the ridge, the wavelength width can be stabilized against external influences. moreover,
Since the semiconductor laser of the present invention has a ridge structure, current can be efficiently injected into the light emitting section, making it the most suitable configuration for a wavelength-stable laser. In this embodiment, Sn and Be are used as n-type and p-type dopants.

In this example, the MBE method was used as the film growth method. The growth conditions were a substrate temperature of 450° C., a Ga:As flux ratio of Ca:As=1:2, and film growth was performed by rotating the substrate. However, the method of the present invention is not limited to only MBE method, but also MOCVD method, LPE method.
It is also possible to form a film by a method. Furthermore, although the wafer of this example is made of a GaAs-based compound,
m-v group compounds such as Ga1nS and InGaAsP,
It may also be composed of other II-Vl group compounds.

Next, a method for manufacturing the substrate shown in FIG. 2 will be explained with reference to FIGS. 3(a) to 3(e).

First, an Si02 mask 41 having a width of 5 μm and a thickness of 2000 is formed at the center of the n-GaAs substrate 31 (FIG. 3(a)).

Next, a resist 42 for producing a diffraction grating was applied thereon (FIG. 3(b)).

Next, using xylene as the impregnating liquid, the substrate incidence angle θ = 37°
, He-Cod laser beam interference exposure with a wavelength of 4,416 beams was performed, and interference fringes 43 with a period of 2,400 beams were formed (FIG. 3(C)).

Next, H2SO4: H2O2: 820 = 7
Diffraction gratings 45 are formed on the sides of the ridge and on the n-GaAs substrate 31 at the bottom of the ridge by wet etching for about 90 seconds using a 1:1 etching solution at a solution temperature of 10°C.
．． 44 were formed. When the etching depth is 1 μm, the width of the top of the ridge is approximately 2 μm narrower than that of the 5in2 mask 41, and the width of the top of the ridge is 3 μm (see Fig. 3 (d).
)). Next, the 5j02 mask 41 was removed (FIG. 3(e)). Through the above steps, the top surface 46 of the ridge becomes flat, and the diffraction grating is formed on the sides of the ridge and at the bottom of the ridge.
It was formed on a GaAs substrate 31.

Note that the resist 42 is a resist manufactured by Sibley (
Product name: Microposit MP1300-1'7) was used.

The substrate manufactured as described above has a (100) crystal surface and the convex portions extend in the [011] direction.

In addition, by extending the convex portion in the [011] direction, the [0
11] The same effect as in the case of the direction can be obtained. However, in this case, when wet etching the substrate, Br2-C
It is necessary to use an H30H-based etching solution. Furthermore, when extending the convex part along the [011] direction, for example, the accuracy needs to be within ±15° in the vertical and horizontal directions from the [011] axis, and the same is true for the [011,] direction. It is.

FIGS. 4(a) to 4(e) are schematic diagrams showing a second example of the method for manufacturing the substrate.

The difference from the cases shown in FIGS. 3(a) to 3(e) is that a diffraction grating is also provided on the upper surface of the convex portion.

First, we want to form a resist 42 with a thickness of 2000 on an n-GaAs substrate (nv7c) with a thickness of 350 μm as shown in FIG.
)).

Next, using xylene as the impregnating liquid, the substrate incidence angle θ = 46°
, three-beam interference exposure was performed using an Ar laser with a wavelength of 4880 rays, and interference fringes 43 with a period of 2260 rays were formed (FIG. 4(b)).

Next, H2SO,: H2O2: H2O= 1:
1:100), :c using a solution temperature of 10
Wet-etch the diffraction grating 44 at ℃ for about 20 seconds.
was formed (Fig. 4(C)).

Further, a 5in2 mask 41 having a width of 5 μm and a thickness of 2000 was formed in a portion corresponding to the upper ridge (FIG. 4(d)).

Next, wet-etch this (for example) + 2sOJ
system) to form diffraction gratings 45 and 47. As a result, the height of the diffraction grating 44 becomes low and the shape becomes gentle (FIG. 4(e)).

Since the diffraction grating 47 is also formed on the upper surface of the convex portion of the substrate processed as described above, the diffraction grating is also formed on the ridge of the semiconductor laser manufactured using this substrate. Therefore, the oscillation wavelength can be further stabilized.

It is clear that the structure shown in FIG. 3 can also be formed using the dry etching method. In addition, in the example,
Although an n-type substrate is used, a p-type substrate may also be used and the same effect can be easily obtained. In this case, p of the layer to be laminated in FIG.

It is necessary to reverse the n polarity. Note that when dry etching a substrate, the angle between the side surface of the ridge and the bottom surface of the ridge can be set arbitrarily by adjusting the direction of the incident beam on the substrate.

Although a single laser structure has been described above, according to the method of the present invention, a plurality of laser structures can be arranged in a single photolithography process, so an array laser can also be produced.

FIGS. 5(a) to 5(e) are schematic diagrams showing a third specific example of the method for manufacturing the substrate.

In this example, a diffraction grating is formed on a substrate by dry etching rather than wet etching.

First, a pattern with a stripe width of 3 μm was formed along the [011] axis at the center of the GaAs substrate 31 having the (100) plane. This pattern has a thickness of 2,000 people and resists 51
, a Ti layer 52 with a thickness of 500 layers, and a resist 53 with a thickness of 2000 layers (FIG. 5(a)).

A resist pattern for forming a diffraction grating was formed on this wafer. This resist pattern consists of a resist 54 with a thickness of 2000, a Ti layer 55 with a thickness of 500, and a thickness of 2000.
This is the laminated structure of the human resist 56 (FIG. 5(b)).

Next, the resist 56 was subjected to interference exposure to form interference fringes 57 on the resist 56 at a cycle of 2450 (FIG. 5(C)).

Further, the Ti layer 55 was dry etched with CF4, and the resist 54 was further etched by 02 to form a mask. Using this mask, diffraction gratings 44 and 45 and ridges were formed by dry etching with CI2 or Ar. These diffraction gratings 44 and 45 are located on the substrate 31 at the bottom of the ridge, respectively.
It was formed on the side surface of the ridge (Fig. 5(d)). Finally, the resists 51 and 53 and the Ti layer 52 were removed (Fig. 5(e)
)).

〔Effect of the invention〕

As explained above, by forming convex portions on the substrate in advance,
By forming a diffraction grating on the side surface of the convex portion and growing a film thereon, these films are formed to reflect the shape of the substrate. As a result, a diffraction grating is formed on the side surface of the active layer in the ridge, which simplifies the manufacturing process of a semiconductor laser that maintains the advantages of being less susceptible to external influences and having a narrow oscillation wavelength width. After temporarily suspending film growth as in the method, damage to the grown layer due to the formation of a diffraction grating is reduced and the composition of the layer is no longer limited. Therefore, it is possible to extend the life of the semiconductor laser having the above-mentioned advantages, and also to reduce damage, reduce threshold current, and improve differential quantum efficiency.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view showing an embodiment of the semiconductor laser of the present invention, FIG. 2 is a perspective view of the substrate of the semiconductor laser shown in FIG. 1, and FIGS. Figures illustrating the processing steps for the substrate in Figure 1, Figures 4(a) to 4(e), and Figure 5(
2.a) to 5(e) respectively show the processing steps of the substrate. Figure 6 shows the third example, Figure 6 shows wavelength stabilization using a diffraction grating, Figures 7(a) and 7(b) show refractive index dispersion in the optical fiber, and Figure 8. (a) is a perspective view of a conventional semiconductor laser, and FIG. 8(b) is FIG. 8(a).
FIG. 3 is a perspective view showing one step in the manufacturing process of the semiconductor laser of FIG. 1.3...Clad layer, 2...Core, 4.5...Light, 6, 7.10...・Pulse, 8.9...position, 11.31...---n-GaAs substrate, 12,
32...n-GaAs buffer layer, 13.
・・・・・・・・・・・・n-Alo, 4Gao68S
Cladding layer, 14, 34... Active layer, 151+++++++++"6+p-AIo27Gao,
73AS barrier layer, 16..."...P-AI
. , 13G80.87AS guide layer, 17, 37...
・・・・・・5i02 layer, 18”・・・・・・P-
AI (+3Gao 7AS subcladding layer, 19・・・・・・・・・・・・l”AI., 4Gao
, 6AS cladding layer, 20...
P''-GaAs contact layer, 21, 38...
...Top electrode, 22, 39...Bottom electrode,
33, 35...=n-Alo, 5Gao,..., As clad layer, 36...=...P-GaAs contact layer, 40...... Current , 41...5i02 mask, 42,
51.53.54.56...Resist, 43, 57
......Interference fringes, 44, 45.47...Diffraction grating, 46......Ridge upper part, 52, 55
......Ti layer. Patent applicant Canon Co., Ltd.

Claims (1)

[Scope of Claims] 1) A semiconductor laser in which one or more semiconductor layers including at least an active layer and a PN junction are laminated on a substrate, and an electrode for passing a forward current to the semiconductor layer is provided, comprising: A semiconductor laser characterized in that an active layer and a layer below the active layer have a convex portion extending in the direction of a resonator, and a periodic diffraction grating is formed on a side surface of the convex portion. 2) The semiconductor laser according to claim 1, wherein the layer on the substrate is formed so as to substantially maintain the shape of the substrate. 3) The semiconductor laser according to claim 1, wherein the convex portion is formed on the substrate having a (100) surface approximately along the [011] axis. 4) The semiconductor laser according to claim 1, wherein the convex portion is formed on the substrate having a (100) surface approximately along the [01@1@] axis. 5) The method includes forming a convex portion on a substrate, forming a periodic diffraction grating on the side surface of the convex portion, and forming a layer on the substrate so as to substantially maintain the shape of the substrate. A method for manufacturing a semiconductor laser, characterized by: