JPS6180881A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To realize a semiconductor laser device capable of a lower threshold value with help of built-in waveguide effect attributable to the difference in effective refractive indexes by a method wherein two coating layers are formed, with the first coating layer provided with a refractive index higher than that of an clad layer and occupying the entire region except along the sides of a stripe-shaped groove and the second coating layer provided with a refractive index lower than that of the first coating layer. CONSTITUTION:On an n-GaAs substrate 21 of the facial orientation (100), a clad layer 22, undoped activation layer 23, p-clad layer 24, and n-GaAs current-impeding layer 25 are grown in that order. A photoresist 31 is placed on the current-impeding layer 25 by application, and a stripe-shaped window is provided. The current-impeding layer 25 is subjected to selective etching with the stripe-shaped window serving as a mask. Further, the clad layer 24 is exposed to etching until a stripe-shaped groove 32 is formed. On the entire surface, a first coating layer 26, second coating layer 27, and contact layer 28 are grown. On the contact layer 28, a Cr-Au electrode layer 29 is formed and, on the lower surface of the substrate 21, an Au-Ge electrode layer 30 is formed. An element obtained in this way operates on a threshold current that is as low as 35mA, and has an excellent differential.quantative efficiency of 50%.

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] As the application of semiconductor lasers opens up as a light source for optical communication and optical disk devices such as digital audio disks (DAD), videos, disks, document files, etc., the debt of semiconductor lasers has increased. Industrialization technology has become necessary. Conventionally, a liquid phase epitaxial growth method (LPE method) using a sliding boat method has been used as a technology for manufacturing thin film multilayer heterojunction crystals for semiconductor lasers, but the LPE method has a limit in increasing the wafer area. For this reason, the metal organic chemical vapor deposition method (MOCVD method) and the molecular beam epitaxy method (MB
Crystal growth techniques such as method B) have been attracting particular attention in recent years.

What can be called a built-in waveguide laser that takes advantage of the characteristics of the MOCVD method (Applied Physics Letters,
No. 37, No. 3, p. 262, 1980).
There is a semiconductor laser as shown in the figure. In addition, 1 in the figure is N
-GaAs substrate, 2 is N GaAA! As cladding layer, 3 GaAJAs active layer, 4 P-GaA7As cladding layer, 5 N-GaAs current blocking layer, 6 P-Ga
An AJAs coating layer, 7 a P-GaAs27 tact layer, and 8°9 a metal electrode. In this structure, current injection into the active layer is limited to a stripe pattern by the current blocking layer 5 of different conductivity types, and at the same time, light guided in the active layer leaks to the current blocking layer 5 and the coating layer. As a result, a different birefringence difference occurs between the portion directly below the stripe and the other portion, resulting in the formation of a guided mode in the portion 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 oscillates at a fairly low threshold of about 50 (mA) in room temperature pulse operation, and single mode oscillation is achieved and the transverse mode is sufficiently well controlled. It is shown.

Note that the laser having the above structure requires the first crystal growth from the substrate 1 to the current blocking layer 5, and the second step of etching a part of the current blocking layer 5 in a stripe shape to form the covering layer 6 and the contact layer 7. It is created by a two-step crystal growth process consisting of a second crystal growth. Here, the growth on the cladding layer 7 at the start of the second crystal growth is -0 11 A I A when the foot surface is exposed to the air.
This is growth onto the s-plane. For this reason, in the conventional LPE method, growth is limited to r4<, M which is easy to grow on the GaAlAs surface.
This is the first time that it has become possible to manufacture it with good controllability using the OCVD method.

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's also becoming. Laser structures that exhibit a low threshold include a built-in waveguide structure, such as a buried type (BH) and a lateral junction type (TJ8), which exhibit a threshold of 10 to 20 (fflA) or less. Compared to these, the threshold value of the laser having the structure shown in FIG. 1 is 50 [mB], which is more than twice as high as that of the BH and TJS types, as described above. Experiments conducted by the present inventors have confirmed that it is extremely difficult to lower the threshold voltage even further with the current structure. This difference in threshold values is
This is thought to be due to the difference in waveguide effect between the negative 34-diagram structure and the BH, TJ S type, etc. In other words, in the structure shown in FIG. 4, the light guided by the active layer 3 leaks through the cladding layer 4 to the current blocking layer 5 and is absorbed, so that the imaginary part of the isometric complex refractive index is This is an absorption loss guide in which light is guided by a difference formed between the two. 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 realize a low threshold (tlJ), it is possible to improve the laser to a constant refractive index wave type laser that does not need to pay the penalty for such loss. A semiconductor laser devised based on this idea is as shown in Figure 5.That is, although the current blocking layer 5 is left in order to obtain a current blocking effect, a cladding layer with a smaller refractive index than this layer is added. By making the cladding layer 4 sufficiently thick, it is possible to prevent light from seeping into the current blocking layer 5, which has a high refractive index and absorbs laser light. , and is provided with a coating layer 6 that does not absorb laser light.With this structure, the light guided into the active layer 3 senses the coating layer 6 with a high refractive index directly below the stripe, while A layer with a low refractive index is felt on both sides, and 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 a low threshold value unexpectedly. 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, the effective refractive index of the mode perpendicular to the junction surface where the p-type and n-type cladding layers 2 and 4 are sufficiently thick is ne.
It is assumed that the refractive index n of the coating layer 6 has a relationship of n>neff'. By the way, in a place where the thickness of the P-cladding layer is sufficiently thin, the light guided by the active layer 1fj3 senses the high refractive index of the covering layer 6, and the nef of the guided mode at this time is
f takes on a certain value greater than n 6ff'. Then, when the thickness of the P-cladding layer is virtually increased in this state, the light guided into the active layer 3 no longer senses the coating layer 6, so that n eff becomes smaller and gradually approaches n eff'. become. And when 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 higher than n eff' of the waveguide mode is present relatively close to each other, light that has been transmitted into the active layer 3 in four waves flows toward the coating layer 6, and the active layer A situation arises in which the signal is dissipated from 3. This phenomenon is caused by the coating layer 6
The problem is that this does not occur close to the active layer 3, but occurs at a certain distance, and in the case of the laser shown in Figure 2, even though the light is guided to the active layer in the stripe groove,
A phenomenon appears in which light escapes into the coating layer 6 on both sides of the striped groove. The above phenomenon in which the waveguide light is dissipated at both stripes 1u11 is caused by the fact that when this type of laser is observed with an infrared microscope, radial bright spots are found on both sides of the stripe above the active layer. It is clear from this. nef on both sides of the stripe
f) In order to ensure that the relationship n is always satisfied, the refractive index n of the coating layer 6 may 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 threshold current is lowered by changing from the loss waveguide structure to the refractive index waveguide structure compared to the laser with the structure shown in Fig. 1, but on the other hand, there is a new loss. Because of this, the threshold value could not be lowered sufficiently compared to FIG. 4.

[Purpose 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.

[Summary of the Invention] The present invention provides a laser in which the effective refractive index immediately below the stripe is kept sufficiently larger than that at both parts of the stripe, while minimizing loss of light due to abnormal seepage into the coating layer. The purpose is to

That is, in the present invention, a layer having a conductivity type different from that of the cladding layer is formed on the opposite side of the substrate from the active layer, excluding the striped portion, and a layer having the same conductivity type as the cladding layer is formed on the cladding layer on the side opposite to the substrate. 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 coating layer is formed in at least two layers,
The first coating layer closer to the active 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 has a higher refractive index than the first fσ layer. The second coating layer that is farther from the active layer has a refractive index smaller than that of the first coating layer.

〔Effect of the invention〕

According to the present invention, by setting at least two layers, a high refractive index layer and a low refractive index layer, to be embedded in the striped groove,
The light that spills out into the striped grooves senses the high refractive index layer,
This results in an effective refractive index distribution in the horizontal direction on the cemented surface. Here, the refractive index n of the low refractive index layer of the coating layer is defined as the active layer 1 = effective refractive index of the guided mode n eff
If the value is sufficiently close to neff- or is smaller than neff-, then the light guided into the active layer will be affected by 3Pfl? based on the above discussion, even in the area directly under the side surface of the striped groove. Dissipation to ll cannot occur.

In addition, there is no effective refractive index layer required to confine the mode directly under the striped groove by increasing the refractive index of the high refractive index layer of the QD sufficiently or making it sufficiently thick. 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 a low threshold value can be sufficiently achieved 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-Ga A
s substrate, 22 is N-Ga6. as Alo, ss A3
Cladding layer, 23 is G" o ot Al 06@A
s active layer, 24 is P G a (1,65 AI!o
, 5sAs cladding layer, 5 is N-GaAa current blocking layer, 26 is P-Ga, , klo, , As first coating layer, 27 is Poao, 61 "", B5 As second coating scrap, 4 is P- 29 and 30 indicate a QaAs contact layer and a metal electrode layer, respectively.

The laser having the above structure is realized by the steps shown in FIGS. 2(a) to 2(C). First, as shown in Figure 5(a),
N-GaAs substrate 21 with plane orientation (100) (SL doped I x 10m8m') Upper 1-1li-sa 1.5
Cμm) (7) N-Gao, as AA! 6.
B5 As cladding layer 22, (8e doped 1×101?
cIIv”), thickness!: o, Os (μm, l (
7) 7: / F - ふ G” o, ss AJ 6.6@
As active layer 23, thickness 1.5 Cμm] of P-Ga,
, kl, , As cladding layer 24 (Zn doped 7
×IQ"cm-") and N- of thickness 1 (, um)
GaAs current blocking layer (heterogeneous layer) 25 (Be doped 5
Xl□"c'm-") were grown sequentially. This first
The MOCVD method was used for the second crystal growth, and the growth conditions were a substrate temperature of 750 C (O), V/m = 20, a carrier gas (Hl) flow rate of 10 (A!/min), and a raw material of trimethyl gallium. A (TMG: (CH), Ga
), )limethylaluminum (TMA: (CH
), AJ), arsine (A”s), p-dopant: diethylzinc (DEZ: (C,H@)t
Zn), n-dopant: hydrogen selenide (H, 8e
), and the growth rate was 0.25 (μm/min). Although it is not necessarily necessary to use the grain MO-CVD method for the first crystal growth, using the MO-CVD method, which allows crystal growth with good uniformity over a large area, is advantageous when considering mass production. It is more advantageous than the law.

Next, as shown in FIG. 2(b), a photoresist 31 is coated on the current blocking layer 25, and the resist 31 has a width of 3 μm.
], and using this as a mask, the current blocking layer 25 is selectively etched, and then the cladding layer 24 is etched.
was etched halfway to form striped grooves 32. Next, after removing the resist 31 and performing a surface cleaning treatment, a second crystal growth was performed using the MOCVD method. That is, as shown in FIG. 6(C), the entire surface is coated with a thickness of 0.3
[μm] P-Ga0. AJ, As first coating layer 26
, P Ga 6.65 A16.16 As 2nd +
'l i'U % 27 and P-GaAs smoked layer (Zn
A doped 5×IQ” cm−3) was grown.

By the way, the striped grooves 32 have a plane orientation (100) Ga
When a square crystal Aa substrate is used, it is necessary to form it in the so-called reverse mesa direction. In the reverse mesa direction, an etchant with strong plane anisotropy such as a sulfuric acid etchant containing an oxidizing agent is used.
When a mesa is formed using photoresist as a mask,
A mesa is formed that becomes narrower as it goes upwards (01
Orthogonal to the direction equivalent to the moon, i.e. the Mesa direction (
011). When striped grooves are selected in the so-called reverse mesa direction and grown by MOCVD, growth progresses on the (100) plane at the bottom of the groove, but almost no growth occurs on the side surfaces of the groove. This phenomenon in MOCVD growth is a relatively well-known fact. This causes the striped grooves to move in the so-called forward mesa direction! BiMO
In contrast, when CVD growth is performed, growth occurs not only on the bottom of the groove but also on the sides of the groove. The structure of the present invention can be realized by selecting the reverse mesa direction when using a (100) plane orientation substrate. From this point on, a Cr--Au electrode layer 29 was deposited on the contact layer 28 and an Au--Ge electrode 30 was deposited on the lower surface of the substrate 21 in the usual steps to obtain the structure shown in FIG. 1.

The sample thus obtained was cleaved to create a resonator with a cavity length of 250 [
The characteristics of the device cut out into a Fabry Bellows laser with a wavelength of 35 [μm] were as low as a threshold current of 35 [m], and the differential/quantum efficiency was also good at 50 [%]. Also, output 12 [m
W] and above, a kink-free current-light output characteristic with good linearity was obtained. Furthermore, the beam waist of the laser light beam emitted from the laser end facet in the horizontal and vertical directions coincided with the end facet, 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 the number of different layers is two. That is, between the p-GaA/As cladding layer 24 and the n-GaAa first heterogeneous layer 25 1: n −
A second heterogeneous layer 2sa of Ga6.4 Alo, 11 As is inserted. 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, in combination with the effect of the first coating layer 26, a large effective refractive index distribution can be generated directly under 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 increase the thickness of the p-cladding layer directly under the stripe is to increase the thickness of the p-cladding layer, which can be used to ensure process yield and reliability. 2nd food heterogeneous layer 25
Another advantage of reducing a is that the reactive current component that spreads through the p-cladding layer 24 in the embodiment of FIG. 1 can be reduced as the p-cladding layer 24 becomes thinner. be.

Note that the present invention is not limited to the embodiments described above. For example, the constituent material is not limited to G1AlAs (other compound semiconductor materials such as InGaAmP and AJGaInP may also be used.Furthermore, it is also possible to use the MBE method instead of the MOCVD method as the 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 the drawing]

FIG. 1 is a schematic diagram showing a schematic structure of a semiconductor laser according to an embodiment C2 of the present invention, FIGS. 2(1) to (c) are sectional views showing the manufacturing process of the laser of the above embodiment, and FIG. 4 is a cross-sectional view showing the schematic structure of another embodiment, FIG. 4 is a cross-sectional view showing the schematic structure of a conventional semiconductor laser, and FIG. 5 is a schematic structure of a semiconductor laser with a large striped effective refractive index difference. The cross-sectional view shown in FIG. 6 is a schematic diagram showing waveguide characteristics of the laser shown in FIG. 5. 21,,, n-GaAs substrate, 22-' Gao, as klo, ms'' cladding layer, 23- oaoj! Alo, os'' active layer, 24,, p-Gao, 61I) J,,,A
s cladding layer, 25...n = GaAs s current blocking layer (heterogeneous layer), 26- pGaoo, AA! o,t”
First coating layer, 27-poa06ff person lo3. As second covering layer, 28...p-GaAs contact layer, 2
9, 30... Electrode 31... Resist, 32... Striped groove. Agent: Patent Attorney Noriyuki Chika (and 1 other person) No. 1
Figure 2 (C) Figure 3 Figure 4 Figure 6

Claims (2)

[Claims] (1) On the cladding layer on the side opposite to the substrate with respect to the active layer, a layer of a different type having a conductivity type different from that of the cladding layer is formed except for the striped portion, and on top of this, a coating of the same conductivity type as the cladding layer is formed. In a heterojunction semiconductor laser device in which layers are formed to have a current confinement effect and a built-in waveguide effect, the coating layer is formed in at least two layers, and the first coating layer closer to the active layer is The second coating layer, which is a layer having a higher refractive index than the cladding layer and is formed excluding the side surfaces of the striped grooves, is further away from the active layer than the first coating layer. A semiconductor laser device having a refractive index smaller than that of a coating layer. (2) The semiconductor laser device according to claim 1, wherein the cladding layer has grooves corresponding to the striped portions formed halfway therein.