JPS6136720B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of a semiconductor laser that emits body mode oscillation.

Continuous oscillation of a semiconductor laser at high temperatures requires confining the optical energy and injected current to a specific region that provides the best thermal path to remove heat from the junction and at the same time minimizes losses and wasteful recombination. A so-called electrode stripe type semiconductor laser has appeared. Since then, various striped structures have been developed and are still available today, but all of them have their own drawbacks and are unsatisfactory in terms of characteristics.

For example, electrode stripe type semiconductor laser, etc.
When only the current distribution is specified, the laser light is mainly guided in the stripe direction by the gain distribution, but the waveguide effect due to this gain is also unstable and easily causes higher-order transverse mode oscillation or multimode oscillation. Furthermore, this often distorts the current-light output characteristics.

Therefore, attempts have been made to correct these drawbacks by structurally creating a waveguide effect inside the semiconductor laser.

The so-called rib guide stripe type semiconductor laser can also be understood as one such attempt. In this structure, as described below, the thickness inside the striped region of the active layer is made thicker than the outside thereof, thereby providing a waveguide effect and attempting to obtain body mode oscillation.

This rib guide stripe type semiconductor laser should be cited as the prior art prior to the present invention, and below we will briefly explain the structure, functions, etc. of this type using drawings, and what problems should be solved by the present invention. do. FIG. 1 is a cross-sectional view showing the outline thereof.

For example, {100} plane n-type InP substrate 1 has [0|T]
A striped structure 7 parallel to the direction is formed. On this InP substrate _, a P-type In _{0.77 Ga 0.23} As _0.51 P _0.49 active layer 2 and two layers of P-type InP are sequentially formed using a _liquid phase epitaxial _method . A SiO ₂ film is formed on the P-type InP layer 3 , and a window of SiO ₂ is provided in a portion corresponding to the striped current region so that the current flows uniformly to the active layer region directly above the striped groove 7 . , 6 can be attached to create a rib guide stripe type semiconductor laser.

The active layer corresponding to the oscillation region of this semiconductor laser is thick inside the groove and slightly thinner at the outer periphery. In addition, the crystal layers sandwiching this active layer from above and below are made of InP with a refractive index smaller than that of the active layer.
It is a layer. Since the thickness of the active layer differs in the width direction, light within the active layer is more susceptible to the influence of the InP crystal substrate in areas where the layer is thinner than in thicker areas. This is equivalent to the fact that the refractive index of the thinner part of the active layer is effectively smaller than the refractive index of the thicker part.

That is, even if this is just an optical waveguide, a striped waveguide structure having a certain direction and an appropriate width is provided within the plane of the active layer. For example, in the case of a rib-guide stripe type semiconductor laser, the transverse mode control function does not depend on the gain distribution as in the electrode stripe type, and as a result, stable fundamental transverse mode oscillation can be easily achieved over a wide range of injected power levels. The shortcomings of the conventional electrode stripe type were greatly improved. In this respect, it can be said that this rib-structured semiconductor laser was an innovative proposal.

However, this rib guide stripe type semiconductor laser has a drawback in that it is difficult to manufacture due to its structural dimensions. For example, if the active layer is 2000 Å, the actual size for forming an equivalent refractive index difference in the active layer region to realize the optical waveguide function necessary for fundamental transverse mode oscillation is the depth of the groove formed in the semiconductor substrate. Optimum dimensions are 400 Å and a width of about 8 μm, and when the depth of the groove is more than twice the above value, the difference in equivalent refractive index easily becomes large and higher-order mode oscillation is likely to occur.
The magnitude of this equivalent refractive index difference can be controlled to an optimum value for fundamental mode oscillation by changing the thickness of the active layer in addition to setting the depth of the groove to an appropriate depth.
If the groove depth is constant, increasing the layer thickness will decrease the equivalent refractive index difference, and conversely, decreasing the layer thickness will increase it. It is convenient for the active layer to be thick to some extent from the viewpoint of ease of manufacture, controllability of layer thickness, etc. However, this has the disadvantage of increasing the oscillation threshold current. On the other hand, when the active layer is made thinner, the oscillation threshold current becomes lower, but the equivalent refractive index difference tends to increase, and even slight non-uniformity in groove depth causes the equivalent refractive index difference to fluctuate greatly.

Furthermore, the depth of the grooves actually needs to be formed uniformly and with good reproducibility to a depth of 400 Å or less. Such structural dimensional conditions necessary for characteristics naturally cause a significant deterioration in manufacturing controllability and reproducibility.

Furthermore, structural designs such as widening the groove width and increasing the thickness of the active layer in order to increase the output have disadvantages such as easily causing high-order transverse mode oscillation and increasing the oscillation threshold current. Another drawback is that the injected current spreads in the light emitting region, that is, the current confinement effect is weak. As a result, the conversion efficiency of the current that contributes to oscillation becomes smaller, lowering the differential quantum efficiency,
This increases the oscillation threshold current.

The purpose of this invention is to eliminate the drawbacks of conventional semiconductor lasers, to facilitate fundamental transverse mode oscillation, to have a low oscillation threshold current, to have high differential quantum efficiency, to enable high output, to be easy to manufacture, and to have a high yield. An object of the present invention is to provide a semiconductor laser structure suitable for mass production.

The present invention is solved by the structure of a semiconductor laser as described below.

The main structure of the semiconductor laser of the present invention is as follows.

First, a long and narrow groove is formed on the surface of an n-type semiconductor substrate. Subsequently, a light guide and carrier confinement layer (hereinafter abbreviated as the light guide layer), an active layer, and finally a light and carrier confinement layer (hereinafter abbreviated as the carrier confinement layer) are grown in the trenches of the semiconductor substrate by conventional epitaxial growth. grown continuously on a surface. Each layer is then doped with impurities to become an n-type conductor. A light guide layer is formed so that the groove is completely filled with the light guide layer, and an active layer is grown on the flattened surface of the light guide layer. Regions of each layer opposite to the positions of the striped grooves are converted into p-type semiconductors in a stripe pattern, and electrodes are provided on the p-type layer and the n-type layer, respectively.

The basic principle of the present invention will be explained with reference to FIGS. 2 and 3. FIG. 2 is a typical example of a semiconductor laser in which the present invention is implemented, and shows a main cross-sectional view of the element perpendicular to the laser beam.

8 is a semiconductor substrate in which rectangular grooves 15 are formed in a stripe pattern, and the following layers are sequentially grown on this substrate. They are a light guide layer 9 of the first semiconductor layer, an active layer 10 of the second semiconductor layer, and a carrier confinement layer 11 of the third semiconductor layer. Layers 9, 10 and 11 are formed to have the same conductivity type as the semiconductor substrate. 16, 17, 18 are
The electrodes 13 and 14 are formed in a region converted into a conductivity type different from that of the semiconductor substrate by impurity diffusion from the surface of the electrode 11.
are provided in contact with the semiconductor substrate 8 and the carrier confinement layer 11, respectively, and form a forward bias rectifying junction surface 19 within the active layer 10.

When the rectifying junction 19 is forward biased, carriers are injected into the region 17 of the active layer 10 and recombine to generate light. Above and below the active layer 10 are optical guide layers 9 having a larger forbidden band width and a lower refractive index.
The photons are sandwiched between the carrier confinement layer 11 and the carrier confinement layer 11 to confine the carrier, while the generated light leaks out to the light guide layer 9 having a slightly lower refractive index, and the semiconductor substrate 8 and the carrier confinement layer 11 completely confine the photon. In other words, it has a double heterostructure in which photon confinement and carrier confinement are separated. A case in which InP-InGaAsP is used as a semiconductor layer will be specifically described.

After attaching a photoresist film to the surface of the n-type InP substrate 8 and exposing it to light, a long and narrow rectangular groove 15 with a width of 5 μm is formed.
form. The groove dimensions are 0.1 μm deep and 5 μm wide. The remaining photoresist film is removed from the surface of the InP substrate and the following layers are successively grown by liquid phase epitaxial growth. n-type light guide layer
In _{0.88 Ga 0.12 As 0.26 P 0.74 layer} 9 is grown _. This growth continues until the groove 15 portion is completely filled and the entire top surface is substantially flat. Next, the n-type _{In0.77Ga0.23As0.51P0.49} layer 10, _which is the active layer _of the second semiconductor _{layer ,} and the n-type _InP layer _{11 ,} which is a carrier confinement layer, are grown.

Typically, the layer thicknesses in the region of the grooves are respectively 0.3 .mu.m for the light guide layer 9, 0.1 .mu.m for the active layer 10 and 1.5 .mu.m for the carrier confinement layer 11.

Subsequently, SiO ₂ is applied to the surface of the carrier confinement layer 11.
A film 12 is deposited and removed in stripes using photoresist. This window position is placed directly above the groove 15 provided in the InP substrate 8. Next, zinc is diffused from the surface, and n-type InP11 and n-type
In _0.77 Ga _0.23 As _{0.51 P 0.49 10} , n _{- type}
Part of In ₀ . ₈₈ Ga _{0 . 12 As 0} . ₂₆ P ₀ . ₇₄ 9 is converted to P _type , and P type _{InP18 and} P type _{In 0 .}
17, P- _{type In 0.88} Ga _{0.12 As 0.26 P 0.74 16} . P
The type conversion region may be formed up to the region in contact with the reflective surface, or may be formed only in the region away from the reflective surface. Further, electrodes 13 and 14 are formed on the front and back sides respectively to complete the process.

In operation, the rectifying junction 19 is forward biased by applying a positive voltage to the electrode 14 and a negative voltage to the electrode 13. Rectifying junction 19 is an InP homojunction,
_{In 0.77} Ga _0.23 As _{0.51 P 0.49 homozygous ,}
Since it consists of a parallel connection with an In _{0 . 88} Ga ₀ . ₁₂ As 0 . ₂₆ P ₀ . ₇₄ homojunction, most of the injected current is concentrated in the junction where the diffusion potential is low. Since the forbidden band width _{of the active layer 10 is the narrowest, current flows through the In 0.77 Ga 0.23 As 0.51 P 0.49 homojunction .} Therefore, laser oscillation occurs in P region 17 of active layer 10.

An active layer 10 is sandwiched above and below by a light guide layer 9 having a large forbidden band width and a carrier confinement layer 11. That is _, while the forbidden band width of the active layer ₁₀ In _0.77 Ga _0.23 As _{0.51 P 0.48} is 0.98 _eV , the optical guide layer 9
The _P region 1 of the active layer 10 is formed by a double heterojunction in which _the forbidden band width of In _{0 . 88 Ga 0 .} 12 As 0 . 26 P 0 .
The carriers injected into region 7 are confined within this region without diffusing. On the other hand, light is generated by recombination within the P region 17 of the active layer 10, and when the gain overcomes the loss due to sufficient injection current, laser light is generated from the P region 17. This light seeps into the light guide layer 9 of the first semiconductor layer. Since the light guide layer 9 is sufficiently transparent to the laser light (oscillation wavelength is approximately 1.26 μm), the laser light is not lost within the light guide layer 9. There, the laser light spreads and propagates between the light guide layer 9 and the active layer 10. In this case, the refractive index of the active layer 10 is n ₂ =3.5, whereas the refractive index of the light guide layer 9 is n ₁ =3.46. Since the difference in refractive index between the two is small, the active layer 10 and the light guide layer 9
The waveguiding effect at the interface is very weak. But layer 9,
The refractive index of the semiconductor substrate 8 and the carrier confinement layer 11 sandwiching the layers 9 and 10 is (the refractive index of InP n=3.2).
Since it is small compared to its value of 0, it constitutes a strong waveguide. In particular, the laser beam is guided by the semiconductor substrate 8 and the carrier confinement layer 11, which have a small refractive index, and is confined in the region of the layers 9 and 10. The thickness of the light guide layer 9 is different between the groove 15 region and the outside of the groove 15. Therefore, the lateral refractive index distribution of the light guide layer 9 is larger in the groove area than outside the groove. In other words, it is equivalent to creating an optical waveguide mechanism similar to a rib structure in the light emitting region.

According to this embodiment, an optical waveguide that enables fundamental transverse mode oscillation can be provided by appropriately controlling the thickness of the optical guide layer 9 and the depth of the groove 15 without changing the active layer thickness. It will be done. Further, the carrier confinement region and the light confinement region are completely separated from each other. These characteristics mean that the fundamental transverse mode oscillation is maintained over a wide operating current range;
The oscillation threshold current also offers the advantage of being small.

Also, the large thickness of the light emitting region in the layer direction provides another advantage in that high output can be easily obtained.

Furthermore, by forming a PN junction in the lateral direction of the active layer, for example, the carrier concentration in the outer n region can be reduced to n>1×
10 ¹⁸ cm ^-3 , the carrier concentration in the P region is P<5×10 ¹⁸
When controlled within the cm ^-3 range, a refractive index difference is realized due to the concentration difference.

This refractive index difference, combined with the refractive index distribution created in the groove region, provides a feature that constitutes a more reliable optical waveguide mechanism.

Another feature is that a semiconductor laser in which relaxation oscillations in dynamic characteristics are suppressed can be obtained. That is, by forming a light emitting region as narrow as the carrier diffusion length in the active layer, the so-called hole burning phenomenon in which the carrier concentration rapidly decreases with light emission becomes less likely to occur.

A feature of the manufacturing method is that, compared to conventional rib structures, the depth of the groove formed in the semiconductor substrate does not have a sharp effect on the difference in the effective scrap refractive index that constitutes the optical waveguide mechanism, making it easy to manufacture. It is highly reproducible and mass-producible. Furthermore, in a structure in which the p-type conversion region is separated from the reflective surface, by controlling the p-type impurity concentration, the n-type region near the reflective surface becomes transparent to laser light, resulting in high output and long life. have Since current injection in this embodiment is based on a current concentration mechanism based on differences in diffusion potential, that is, a homojunction with the lowest diffusion potential is provided in the active layer 10,
Current is concentrated at this homojunction. This increases the efficiency of current injection into the light emitting region,
Improve the differential quantum efficiency of light output. It also has the advantage of reducing operating current.

The embodiment shown in FIG. 3 is similar to the embodiment shown in FIG. 2, but is different from the embodiment shown in FIG.
layer _{11 and n-type In 0.77 Ga 0.23 As 0.51 P 0.49 layer} 10. (No p-type conversion region is formed in _{the n-type In 0.88 Ga 0.12 As 0.26 P 0.74 layer 9. )} In this case,
Carriers are injected into the p-type region 17 _, which is a light-emitting _region provided in the active layer 10, from the n-type regions on both sides of the p-type region _{17 and from the n-type In 0.88 Ga 0.12 As 0.26 P 0.74 layer 9 .} It is done from three sides. On the other hand, the diffusion length of the carrier is several μ
In the structure shown in FIG. 2, when the width of the striped light emitting region 17 is increased, the light emission often separates into two points, but in the structure shown in FIG. 3, such a phenomenon does not occur. Therefore, it is possible to increase the stripe width of the light emitting region 17 and increase the output. At this time, the same effects and effects can be obtained even with a structure in which only a portion of the active layer 10 is converted to p-type and a homo-PN junction is formed within the active layer.

In the above embodiment, the groove formed in the semiconductor substrate is rectangular, but the same effect as the present invention can be obtained with grooves of other shapes, such as a V-shape, a U-shape, or a semicircle shape.

Furthermore, in addition to the above effects, in semiconductor lasers in which grooves such as V-shaped grooves are formed with different depths depending on the location, the effective refractive index of the grooves in the optical guide layer changes depending on the depth of the grooves. It has the advantage of producing a light-concentrating effect and producing a light-emitting spot with a small diameter. Further, such characteristics are not only advantageous for efficient coupling with optical fibers, but also advantageous for application to optical equipment such as optical video disc players.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view of a conventional semiconductor laser.
FIG. 2 is a schematic cross-sectional view of a semiconductor laser according to one embodiment of the present invention, and FIG. 3 is a schematic cross-sectional view of another embodiment of the present invention. In the drawings, 1, 8...semiconductor substrate, 2, 1
0...Active layer, 3,11...Photon and carrier confinement layer, 9...Light guide and carrier confinement layer, 4,12...SiO ₂ film, 6,14...P-type electrode, 5,13... N-type electrode, 16, 17, 18...
... P-type conversion region, 19 ... homozygous, respectively.

Claims (1)

[Claims] 1 a semiconductor substrate of a first conductivity type having a groove, and a first semiconductor substrate formed on the semiconductor substrate, the semiconductor substrate having a narrower forbidden band width and a larger refractive index than the semiconductor substrate;
a conductive type light guide and carrier confinement layer; and a first conductive type, which has a narrower band gap and a larger refractive index than the light guide and carrier confinement layer, and is formed on the light guide and carrier confinement layer. an active layer having a band gap wider than that of either of the light guide and carrier confinement layer and the active layer, and a refractive index smaller than the refractive index of each of the two layers. a heterostructure including at least a photon and carrier confinement layer of a first conductivity type formed on the active layer, and a second conductivity type formed in stripes to a depth penetrating at least the photon and carrier confinement layer. What is claimed is: 1. A semiconductor laser comprising a semiconductor region formed therein.