JPH0568873B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to improvements in semiconductor laser devices with built-in waveguide structures.

[Technical background of the invention and its problems]

Digital Audio Disk (DAD),
As the application of semiconductor lasers has expanded to optical disk devices such as video disks and document files, and as light sources for optical communications, technology for mass production of semiconductor lasers has become necessary. Conventionally, the liquid phase epitaxial growth method (LPE method) using a sliding boat method has been used as a thin film multilayer heterojunction crystal manipulation technology for semiconductor lasers.
With the LPE method, there is a limit to how large the wafer area can be.
For this reason, crystal growth techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), which have excellent uniformity and controllability over large areas, have attracted particular attention in recent years.

An example of a built-in waveguide laser that takes advantage of the characteristics of the MOCVD method is the semiconductor laser shown in FIG. 4, which was published in Applied Physics Letters, No. 37, No. 3, p. 262, 1980.
In the figure, 1 is an N-GaAs substrate, and 2 is an N-GaAs substrate.
GaAlAs clad layer, 3 is GaAlAs active layer, 4 is P-GaAlAs clad layer, 5 is N-GaAs current blocking layer, 6 is P-GaAlAs covering layer, 7 is P-GaAs
Contact layers 8 and 9 indicate metal electrodes.
In this structure, a current blocking layer 5 of different conductivity type is used.
At the same time, the current injection into the active layer is limited to a stripe pattern, and at the same time, the light guided in the active layer leaks to the current blocking layer 5 and the coating layer, and as a result, there is a difference between the area directly under the stripe and the other part. A difference in birefringence is generated, and a guided mode is formed directly below the stripe. That is,
The current blocking layer 5 forms a gain waveguide structure by current confinement and a built-in refractive index waveguide structure in a self-aligned manner. According to a report by the authors, a laser with this structure has a power consumption of 50 [mA] in room temperature pulse operation.
It has been shown that oscillation occurs at relatively low thresholds, and that single-mode oscillation is achieved and transverse modes are well controlled.

Note that the laser with the above structure requires the first crystal growth from the substrate 1 to the current blocking layer 5, and the first crystal growth from the substrate 1 to the current blocking layer 5.
It is formed by a two-step crystal growth process consisting of etching a part of the crystal in a stripe shape and then a second crystal growth to form the covering layer 6 and the contact layer 7. Here, the growth to the cladding layer 7 at the start of the second crystal growth is growth on the GaAlAs surface whose surface is once exposed to the air. For this reason, it is difficult to grow using the conventional LPE method, and it was only possible to manufacture it with good control using the MOCVD method, which allows easy growth on GaAlAs surfaces.

By the way, the oscillation threshold of a semiconductor laser needs to be low from the viewpoint of reducing operating current and improving life characteristics, and the low threshold is a measure of the structure and performance of the laser. It has also become familiar. Laser structures that exhibit low thresholds include built-in waveguide structures, such as the buried type (BH) and the lateral junction type (TJS), which have a power output of 10 to 20 [mA].
The following threshold values are shown. Compared to these, the threshold value of the laser with the structure shown in Figure 1 is 50
[mB] is more than twice as high as BH and TJS types.
Experiments conducted by the present inventors have confirmed that it is extremely difficult to measure a threshold value even lower than this with the current structure. This difference in threshold value is thought to be due to the difference in waveguide effect between the structure shown in FIG. 4 and the BH, TJS, etc. structure. That is, the fourth
In the structure shown in the figure, light guided through the active layer 3 leaks through the cladding layer 4 to the current blocking layer 5 and is absorbed, resulting in a difference in the imaginary part of the equivalent complex refractive index in the horizontal direction to the junction surface. It is an absorption loss guide formed to guide light. On the other hand, in the case of a BH structure, etc., it is a refractive index guide in which light is guided by the difference in the real part of the complex refractive index. In other words, in the structure shown in FIG. 4, it is thought that the threshold value increases by the amount of absorption loss.

In view of the above-mentioned drawbacks of the loss guide structure, in order to achieve a lower threshold value, it may be possible to improve the laser to a refractive index guided laser that does not need to pay the penalty of such loss. A semiconductor laser devised based on this idea is shown in FIG. That is, the current blocking layer 5 is left to obtain a current blocking effect, and by making the cladding layer 4, which has a lower refractive efficiency than this layer, sufficiently thick, a current blocking layer that has a high refractive index and absorbs laser light is formed. In order to prevent light from seeping out to the laser beam 5, a coating layer 6 having a slightly higher refractive index than the cladding layer 4 and not absorbing laser light is provided in the striped groove portion. In this structure, the light guided to the active layer 3 senses the coating layer 6 with a high refractive index directly below the stripe, while
On both sides of the stripe, a layer with a low refractive index is felt,
As a result, a difference occurs in the real part of the effective refractive index between the inside and outside of the stripe, and light is guided by the refractive index waveguide effect.

However, it has been found that the laser shown in FIG. 5 often fails to achieve an unexpectedly low threshold value. It has also become clear that the cause of this is inherent in the structure itself. That is, in the case of the structure shown in FIG. 5, it has been found that the laser light abnormally leaks into the coating layer 6 on both sides of the striped groove, and this causes a new loss peculiar to this type of laser. This situation is illustrated in FIG. First, p-type/n-type cladding layers 2 and 4
It is assumed that the effective refractive index of the mode perpendicular to the bonded surface at a location where is sufficiently thick is n eff°, and that the refractive index n of the coating layer 6 has a relationship of n>n eff°. By the way, P-
In places where the thickness of the cladding layer is sufficiently thin, the active layer 3
The light guided by the coating layer 6 senses the high refractive index of the coating layer 6,
At this time, it becomes a certain value larger than n effn eff° of the waveguide mode. If the thickness of the P-cladding layer is virtually increased in this state, the light guided to the active layer 3 will no longer feel the coating layer 6, so that n
eff becomes smaller and gradually approaches n eff°. Then, as the coating layer 6 moves further away, n
eff° is smaller than n of the covering layer 6. When such a layer with a refractive index larger than n eff degree of the waveguide mode exists relatively close to each other, light that has been guided to the active layer 3 leaks toward the coating layer 6, and the light that has been guided to the active layer 3 leaks out from the active layer 3. A situation arises in which it is dissipated. The problem is that this phenomenon does not occur when the coating layer 6 is close to the active layer 3, but occurs at a certain distance away.In the case of the laser shown in Fig. 2, the light is guided to the active layer in the stripe groove. Even if the striped grooves are covered, a phenomenon occurs in which light escapes into the coating layer 6 on both sides of the striped groove portion. The phenomenon in which the waveguide light is dissipated on both sides of the stripe as described above can be seen when the end face of this type of laser is observed with an infrared microscope.
This is clear from the fact that radial bright spots are found on both sides of the stripe above the active layer. In order to always satisfy the relationship n eff > n even on both sides of the stripe, the refractive index n of the covering layer 6 should be selected to be sufficiently small so that the relationship n eff > n is satisfied. However, this has the disadvantage that it becomes difficult to increase the effective refractive index in the region immediately below the stripe.

As described above, in the laser with the structure shown in Fig. 5, the first
Compared to the laser with the structure shown in the figure, the threshold current is lower by changing from the loss waveguide structure to the refractive index waveguide structure.
On the other hand, since a new loss occurred, the threshold value could not be lowered sufficiently compared to FIG. 4.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser device that can reliably produce a built-in waveguide effect due to an effective refractive index difference and can achieve a low threshold value.

[Summary of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a laser in which the effective refractive index immediately below the stripe is kept sufficiently larger than that on both sides of the stripe, while minimizing loss of light due to abnormal seepage into the coating layer.

That is, in the present invention, a layer having a conductivity type different from that of the outer cladding layer is formed on the cladding layer on the side opposite to the substrate with respect to the active layer, excluding the striped portion, and a layer having the same conductivity type as the cladding layer is formed on this layer. In a heterojunction semiconductor laser device in which a coating layer is formed and has a current confinement effect and a built-in waveguide effect,
The covering layer is formed in at least two layers, and the first covering layer closer to the active layer has a higher refractive index than the cladding layer, and is formed excluding the side surfaces of the striped groove. The first thing
The second coating layer, which is farther from the active layer than the coating layer, has a refractive index smaller than that of the first coating layer.

〔Effect of the invention〕

According to the present invention, by using at least two layers, a high refractive index layer and a low refractive index layer, to be buried in the striped grooves, the light seeping into the striped grooves senses the light refractive index layer, and the bonding surface This results in an effective refractive index distribution in the horizontal direction. Here, if the refractive index n of the low refractive layer of the coating layer is set to a value sufficiently close to the effective refractive index n eff° of the mode waveguided in the active layer, or if it is set to a value smaller than n eff , it is necessary to form a stripe shape. Even in the area immediately below the side surface of the groove, based on the above discussion, the light guided by the active layer will not dissipate toward the coating layer. Furthermore, by making the refractive index of the high refractive layer of the coating layer sufficiently large or sufficiently thick, there is no effective refractive index layer required to confine the mode directly under the striped groove. Even if the refractive index of the laser is made sufficiently large, the coated light cannot be dissipated as in conventional lasers. Therefore, in the laser of the present invention, excessive increase in threshold current can be suppressed as much as possible, and it has become possible to sufficiently achieve a low threshold value using a refractive index guided laser.

[Embodiments of the invention]

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to an embodiment of the present invention. 21 in the figure is N-
GaAs substrate, 22 is N-Ga _0.65 Al _0.35 As cladding layer, 23 is Ga ₀₉₂ Al ₀₀₈ As active layer, 24 is -Ga _0.65
Al _0.35 As cladding layer, 25 N-GaAs current blocking layer, 26 P-Ga _0.7 Al _0.3 As first covering layer, 27 P-Ga _0.65 Al _0.35 As second covering layer, 28 P-
The GaAS contact layer, 29 and 30 indicate metal electrode layers, respectively.

The laser having the above structure is realized by the steps shown in FIGS. 2a to 2c. First, as shown in FIG. 5a ^, a 1.5 [ ^μm ] thick N-
Ga _0.65 Al _0.35 As cladding layer 22, (Se doped 1×
10 ¹⁷ cm ^-3 ), undoped Ga _0.92 with a thickness of 0.08 [μm]
Al _0.08 As active layer 23, 1.5 [μm] thick P-
Ga _0.65 Al _0.35 As clad layer 24 (Zn doped 7×
10 ¹⁸ cm ^-3 ) and 1 μm thick N-GaAs current blocking layer (heterogeneous layer) 25 (Se doped 5×10 ¹⁸ cm ^-3 )
grew sequentially. In this first crystal growth,
Using MOCVD method, growth conditions are substrate temperature 750
[°C], V/=20, carrier gas (H ₂ ) flow rate ~10 [/min], raw materials are trimethyl gallium (TMG: (CH) ₃ Ga), trimethyl aluminum (TMA: (CH ₃ ) ₃ Al), Arsine (AsH ₃ ), p-
Dopant: diethyl zinc (DEZ: (C ₂ H ₅ ) ₂ Zn),
The n-dopant was hydrogen selenide (H ₂ Se), and the growth rate was 0.25 [μm/min]. Although it is not necessarily necessary to use the MO-CVD method for the first crystal growth, it is possible to use the MO-CMD method, which allows crystal growth with good uniformity over a large area.
It is advantageous compared to the LPE method when considering mass production.

Next, as shown in FIG. 2b, a photoresist 31 is applied on the current blocking layer 25, a striped window with a width of 3 [μm] is formed in the resist 31, and the current blocking layer 25 is selected using this as a mask. Then, the cladding layer 24 was further etched halfway to form striped grooves 32. Then,
After removing the resist 31 and performing surface cleaning treatment, a second crystal growth was performed using the MOCVD method.
That is, as shown in Figure 6c, the entire surface is coated with a thickness of 0.3 [μ
m] P-Ga _0.07 Al _0.03 As first coating layer 26, P-
A Ga _0.65 Al _0.35 As second covering layer 27 and a P-GaAs covering layer (Zn doped 5×10 ¹⁸ cm ^-3 ) 28 were grown.

By the way, the striped groove 32 has a plane orientation of 100.
When a square crystal GaAs substrate is used, it is necessary to form it in the so-called reverse mesa direction. In the reverse mesa direction, when a mesa is formed using an etchant with strong surface anisotropy such as a sulfuric acid etchant containing an oxidizing agent and a mask such as photoresist, a mesa whose width becomes narrower toward the top is formed. 011},
In other words, it is a surface orientation equivalent to {011} that is orthogonal to the forward mesa direction. When striped grooves are selected in the so-called reverse mesa direction and grown using the MOCVD method, growth progresses on the 100 planes at the bottom of the groove, but almost no growth occurs on the sides of the groove.
This phenomenon in MOCVD growth is a relatively well-known fact. This is in sharp contrast to when striped grooves are selected in the so-called forward mesa direction and MOCVD growth is performed, in which growth occurs not only at the bottom of the groove but also on the side surfaces of the groove. The structure of the present invention can be realized by selecting the reverse mesa direction when a 100-plane orientation substrate is used. After this, Cr-
The structure shown in FIG. 1 was obtained by depositing an Au electrode layer 29 and an Au--Ge electrode 30 on the lower surface of the substrate 21.

The resonator length is determined by cleaving the sample thus obtained.
The characteristics of the element cut into a 250 [μm] Fabry-Perot laser are as low as a threshold current of 35 [mA].
The differential and quantum efficiency were also good at 50%. Also,
Good current-light output characteristics with no kinks and good line formation were obtained up to an output of 12 [mW] or more. Furthermore, the beam waist of the laser beam emitted from the laser end face in the horizontal and vertical directions coincided with the end face, confirming that the refractive index guide was sufficiently performed.

FIG. 3 is a sectional view showing a schematic structure of a semiconductor laser according to another embodiment. Note that the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted. This embodiment differs from the previously described embodiments in that there are two different layers. That is, the p-GaAlAs cladding layer 24 and the n-GaAs
A second heterogeneous layer 25 a of n-Ga _0.04 Al _0.6 As is inserted between the first heterogeneous layer 25 . This contributes to reducing the effective refractive index of the mode guided by the active layer 23 in the regions on both sides of the stripe. Therefore, together with the effect of the first coating layer 26,
A large effective refractive index distribution can be generated directly below the stripe. In reality, in order to stably obtain single fundamental transverse mode oscillation, an appropriate amount of effective refractive index difference is sufficient. The advantage of being able to make a large difference is to increase the thickness of the p-cladding layer directly under the stripe, which can be used to ensure process yield and reliability. Another advantage of introducing the second heterogeneous layer 25a is that the reactive current component that spreads through the p-clad layer 24 in the embodiment of FIG. 1 can be reduced by the thickness of the p-clad layer 24. It is a point.

Note that the present invention is not limited to the embodiments described above. For example, the constituent material is GaAlAs
The material is not limited to , and other compound semiconductor materials such as InGaAsP and AlGaInP may be used. Furthermore, it is also possible to use the MBE method instead of the MOCVD method as a crystal growth method. It is also possible to use a P-type substrate as the substrate and reverse the conductivity type of each layer. In addition, various modifications can be made without departing from the gist of the present invention.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a schematic structure of a semiconductor laser according to an embodiment of the present invention, FIGS. 2 a to c are sectional views showing the manufacturing process of the laser of the above embodiment, and FIG. 3 is another embodiment. FIG. 4 is a sectional view showing the schematic structure of a conventional semiconductor laser.
The figure is a cross-sectional view showing a schematic structure of a semiconductor laser with a large striped effective refractive index difference, and FIG. 6 is a schematic diagram showing waveguide characteristics of the laser shown in FIG. 5. 21...n-GaAs substrate, 22...n-Ga _0.65
Al _0.35 As clad layer, 23...Ga _0.92 Al _0.08 As active layer, 24...p-Ga _0.65 Al _0.35 As clad layer,
25...n-GaAs current blocking layer (different layer), 26
...p-Ga _0.3 Al _0.2 As first coating layer, 27...p-
Ga _0.65 Al _0.35 As second coating layer, 28... p-GaAs contact layer, 29, 30... electrode, 31... resist, 32... striped groove.

Claims (1)

[Scope of Claims] 1. A different type of layer having a conductivity type different from that of the cladding layer is formed on the opposite side of the active layer from the substrate, excluding the striped portion, and a layer having the same conductivity as the above cladding layer is formed on the cladding layer on the side opposite to the substrate. In a heterojunction semiconductor laser device in which a conductive coating layer is formed to have a current confinement effect and a built-in waveguide effect, the coating layer is formed in at least two layers, the first one being closer to the active layer. The covering layer is a layer having a higher refractive index than the cladding layer, and is formed excluding the side surfaces of the striped groove, and the second covering layer is farther from the active layer than the first covering layer. A semiconductor laser device characterized in that the layer has a refractive index smaller than that of the first coating layer. 2. The semiconductor laser device according to claim 1, wherein the cladding layer has grooves formed halfway therein to correspond to the striped portions.