JP3385710B2 - Semiconductor laser - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used in the fields of optical communication and optical information processing.

[0002]

2. Description of the Related Art In recent years, the demand for semiconductor lasers has increased in many fields, and research and development have been actively promoted centering on GaAs and InP. Among them, MBE, MO
The crystal growth technology of semiconductor lasers such as VPE has made great progress, and it has become possible to produce very thin layers such as quantum wells with good reproducibility and uniformly over a large area. These quantum wells are used in the active layer of a semiconductor laser, and it has been proved that the power consumption of the laser can be made extremely small by the quantum effect, and some of them are now commercialized.

Among these, there is a strong demand for a short wavelength semiconductor laser for the purpose of improving the recording density in the field of optical information processing.
Currently, GaInP / AlGaI with a wavelength of about 630 nm
nP semiconductor lasers have been commercialized. A quantum well is used in this laser as well, and here again, it greatly contributes to the improvement of characteristics as compared with other lasers.

When attempting to shorten the wavelength with a conventional bulk material, AlGaIn containing Al (aluminum) in the active layer is used.
P mixed crystal must be used. However, since Al is a highly oxidative material, increasing the amount of Al induces defects and the like, resulting in extremely low luminous efficiency. On the other hand, if the quantum well structure is used, even if GaInP having stable quality and easy to manufacture is used, light emission of a short wavelength can be obtained, and thus there is no such problem. Therefore, most of the 630 nm band semiconductor lasers currently commercialized use quantum wells.

FIG. 6A is an energy band diagram of a typical 630 nm band multiple quantum well (MQW) laser.
(B) is a separate confinement type multiple quantum well (SCH-MQ
W) shows the energy band diagram of the laser. Figure 6
The semiconductor laser of (a) has a quantum well layer 41 and a barrier layer 4.
The multi-quantum well 34 composed of 2 and 3 is sandwiched between the first cladding 32 and the second cladding 35. In the figure, the conduction band is “reference number 21” and the valence band is “reference number 2”.
2 ".

The structure of FIG. 6B is different from the structure of FIG. 6A in that an optical guide layer 33 is further formed between the multiple quantum well 34 and the cladding layer. Both of the semiconductor lasers are configured such that the ground levels 36 and 37 of electrons and holes are located on the higher energy side than the band edges of the conduction band 21 and the valence band 22 in order to obtain the emission of 630 nm. . The height difference between the ground levels 36 and 37 and the band edges of the cladding layers 32 and 35 (ΔEC, ΔE
V) is particularly small in the conduction band 21. This means that the carriers are weakly confined in the active region, and a large amount of electrons diffuse into the P-clad layer, causing a leak current to flow. That is, the electrons supplied from the n-clad layer are not sufficiently confined in the quantum well layer 41 and overflow from the quantum well layer 41 and spread three-dimensionally.

Therefore, the number of electrons that do not contribute to laser oscillation increases, the threshold current required for laser oscillation increases, and this threshold current greatly depends on temperature. The structure of FIGS. 6A and 6B is similar to that described above, but the semiconductor laser of FIG. 6B includes the light guide layer 33, and light may be confined in this portion. it can. Therefore, the light emitted from the quantum well can efficiently contribute to stimulated emission, thereby reducing the threshold current of the laser. Further, as a result, the carrier density at the time of laser oscillation is reduced, so that the above-mentioned leak current can be reduced at the same time.

[0008]

However, as shown in FIG.
Separate confinement type multiple quantum well structure (SC) shown in (b)
Even if the (H structure) is adopted, the threshold current is still higher than that of other semiconductor lasers and the stability against temperature change is inferior. Therefore, the reliability of the device is low, and it can be used only in limited applications. This is because a wide-gap material is used, so that the leak current is large and the potential barrier of the quantum well (the difference between the ground level and the conduction band 21 of the barrier layer) is small, as described above. Therefore, the quantum effect cannot be sufficiently obtained.

FIG. 7 shows a well width of 3 nm, a barrier width of 5 nm,
2 shows a two-dimensional density of states of electrons in a separate confinement multiple quantum well structure in which a multiple quantum well having five wells is sandwiched between optical guide layers having a film thickness of 65 nm. According to this figure, in the case of a multiple quantum well, the electrons are not completely confined in the well and the electron waves are coupled to each other, so that the fine stepwise density of states is added. From the figure, the energy is 0.
It can be seen that when the voltage exceeds 114 eV, the density of states increases rapidly. This energy is equivalent to the height of the potential barrier, and the energy above the barrier is
It means that no more electrons can be confined in the well.

Therefore, in this region, electrons overflow from the quantum well layer 41 as shown in FIG. 6 (b), so that the quantum effect hardly occurs, which causes a decrease in luminous efficiency. In this structure, since the pseudo-Fermi level during laser oscillation exceeds 0.1 eV, a large number of electrons can exist in this region, and it is the electrons near the ground level that can contribute to laser oscillation. The electron of becomes invalid.

The high-energy electrons are recombined with holes to become a reactive current, which causes an increase in the threshold current of the laser.

The conventional short wavelength demultiplexing confinement type multi-quantum well laser has obtained only insufficient characteristics for the above reasons. In addition to the short wavelength laser, for example, InGaA
Even in a long-wavelength semiconductor laser such as an sP / InP-based laser, when the quantum well width becomes extremely small, a large amount of electrons similarly exists in the optical guide layer, which may reduce the laser efficiency. The single quantum well semiconductor laser having one well layer has the same problem as described above.

The present invention has been made in view of the above circumstances, and an object of the present invention is to control the energy or spatial distribution of electrons so that the region apart from the oscillation wavelength of a laser can be controlled. Another object of the present invention is to provide a semiconductor laser having a low threshold current, which suppresses the recombination current of

[0014]

[0015]

[0016]

[0017]

Semiconductors laser of the present invention In order to achieve the above object, according to a semiconductor laser with a quantum well layer including a well layer and a barrier layer, and the light guide layer, wherein the light guide layer is, the It is characterized by including a low barrier portion having a bandgap smaller than that of the light guide layer and having a thickness larger than the de Broglie wavelength of electrons.

In one embodiment, the thickness of the light guide layer is larger than the thickness of the quantum well layer.

In one embodiment, the low barrier thickness
Is thicker than the thickness of the quantum well layer .

[0020]

[Function] By forming an out-barrier layer on the outside of the multiple quantum well and further forming an optical guide layer having a bandgap smaller than that of the out-barrier layer on both sides of the out-barrier layer, electrons can be confined in the quantum well layer. Since the effect of confinement of holes and holes in the multiple quantum well is increased, the leak current and the ineffective recombination current in the guide layer are reduced, and a short wavelength semiconductor laser with a low threshold current can be obtained.

Further, according to the present invention, since the band gap of the barrier layer of the multiple quantum well can be made small and the refractive index of this portion can be made large, the confinement of light can be strengthened, and the density of states of electrons can be made large and the band density near the band edge can be increased. Since only the electrons contribute to the radiative recombination, the leak current and the ineffective recombination current in the guide layer decrease, and at the same time, the confinement of light increases, so that a short wavelength semiconductor laser with a low threshold current can be obtained.

[0022]

EXAMPLES The present invention will be described below with reference to examples.

Example 1 FIG. 1A shows a cross section of a semiconductor laser according to the present invention, and FIG. 1B shows the structure of a multistrain quantum well of the semiconductor laser of FIG. 1A. A cross-sectional view is shown.

In the present semiconductor laser, as shown in FIG. 1A, n- (Al0.7
Ga0.3) 0.51In0.49P clad layer 2, (Al0.5Ga
0.5) 0.51In0.49P optical guide layer 3, multiple quantum well layer 4, (Al0.5Ga0.5) 0.51In0.49P optical guide layer 3,
On top of that, stripe-shaped P- (Al0.7Ga0.
3) 0.51In0.49P clad layer 5 is formed. Further, both sides of the striped clad layer 5 are filled with the current block layer 7. The multiple quantum well layer 4 is shown in FIG.
As shown in (b), the barrier layers 1 are alternately laminated.
2 and the well layer 11 and the out barrier layer 12a formed outside them.

In the present specification, for example, (A
l _0.7 Ga _0.3 ) _0.51 In _0.49 P (Al _0.7 Ga _0.3 )
Notated as 0.51In0.49P.

The method of manufacturing the semiconductor laser of this embodiment will be described below.

First, by the first crystal growth, an n- (Al0.7Ga0.3) 0.51In0.49P clad layer 2, an (Al0.5Ga0.5) 0.51In0.49P optical guide layer 3 are formed on the GaAs substrate 1. Multiple quantum well layer 4, (Al0.5Ga0.5) 0.51In0.
49P optical guide layer 3, P- (Al0.7Ga0.3) 0.51In0.
The 49P clad layer 5 and the P-GaInP protective layer 6 are continuously formed.

Next, the cladding layer 5 is etched into a stripe shape, and then the n-GaAs current blocking layer 7 is formed by the second growth. Finally, by the third growth, P-
The GaAs contact layer 8 is formed.

In the multiple quantum well layer 4, the well layer 11
Is formed of Ga0.44In0.56P having a thickness of 3 nm, and the barrier layer 12 has a thickness of (Al0.7Ga0.3) 0.51In0.
It is formed from 49P. The well layer 11 has 0.5%
Is given a compressive strain.

In this embodiment, the outermost out-barrier layer 12a adjacent to the light guide layer 3 has a thickness different from that of the other barrier layers 1.
Thicker than 2. One of the main features of the present semiconductor laser is that the relatively thick out barrier layer 12a is provided in this way to control the electron distribution. A semiconductor laser is known in which a layer having a large band gap is provided in order to prevent overflow of carriers (electrons) injected into an active layer of the semiconductor laser and the carriers are reflected (for example, Japanese Patent Laid-Open No. 4-218994). Unlike this laser, the laser of this embodiment controls the electron distribution.

FIG. 2 shows an energy band diagram of the multiple quantum well structure of the present semiconductor laser. The barrier layer 12 and the out barrier layer 12a are formed of the same material as the clad layers 2 and 5. Further, the band gaps of the barrier layer 12 and the out barrier layer 12a are larger than the band gap of the light guide layer 3.

In such a multiple quantum well structure, the two-dimensional confinement effect of carriers in the well layer 11 is strengthened by the potential barrier formed by the barrier layer 12 and the out barrier layer 12a.

Further, the thickness (Wb) of the light guide layer is made larger than the thickness (Wa) of the multiple quantum well layer 4, and
The out barrier layer 12a is preferably thicker than the de Broglie wavelength of electrons. By adopting such a configuration, most of the electrons having energy higher than the conduction band 21 of the light guide layer 3 will be distributed in the light guide layer 3 and will not exist in the multiple quantum well region. In other words, in the multiple quantum well region, from the ground level to the optical guide layer 3
Thus, there are electrons having energy in the region up to the conduction band 21 of. As a result, electrons having energy in the region from the ground level to the conduction band 21 of the light guide layer 3 are two-dimensionalized in the multiple quantum well region, and electrons having energy above the conduction band 21 of the light guide layer 3 are formed. Are two-dimensionalized (localized) in the light guide layer 3. According to this embodiment, by providing the out barrier layer 12a,
The distribution of electrons is controlled so as to reduce the density of states of electrons that do not contribute to laser emission. Therefore, the recombination current at wavelengths other than the oscillation wavelength is reduced, and a semiconductor laser having a low threshold current is provided.

FIG. 4A shows the two-dimensional state density of electrons. In the multiple quantum well of the present invention, the density of states after the energy corresponding to the potential barrier of the barrier layer is exceeded is smaller than that in the conventional multiple quantum well. Therefore, even if the electron density is the same, the ratio of electrons that can contribute to laser oscillation, that is, the number of electrons existing near the band edge increases, and a larger gain can be generated.

FIG. 4 (b) shows the two-dimensional density of states of the holes. Regarding holes, since the original potential barrier is large, there is no significant difference between the present invention and the conventional example as much as the difference shown in FIG. .

As a result, in the multiple quantum well structure according to the present invention, it is possible to obtain the gain required for laser oscillation with less carriers, so that it is possible to suppress the recombination current which does not contribute to the oscillation in the well and at the same time the quantum well. It is possible to reduce the leakage current to the outside.

FIG. 5 shows the relationship between the injected current density and the maximum gain. As described above, it can be seen that the current density required to obtain the same gain is greatly reduced in the present invention. When this is used for the active layer of the laser,
If the threshold gain of laser oscillation is 200 cm ^-1 , the threshold current can be reduced by about 50% as compared with the conventional case. Naturally, the temperature dependence of the threshold current can be reduced, and a highly reliable short wavelength semiconductor laser can be realized. As described above, the present invention is extremely effective in improving the performance of a short wavelength multiple quantum well semiconductor laser, which has been difficult in the past.

Also in a semiconductor laser having a single quantum well structure, if an out barrier layer 12a having a bandgap larger than the bandgap of the light guide layer is inserted between the quantum well layer and the light guide layer, a multiple quantum well structure is obtained. The characteristics can be improved as in the case of the structured semiconductor laser.

Example 2 Another semiconductor laser according to the present invention will be described below. The structure of this semiconductor laser is basically the same as the structure of the semiconductor laser shown in FIG. 1 and FIG.
And the multi-quantum well layer 4 to the optical guide layer 3 provided between the n-clad layer 2 and the n-clad layer 2 have different structures.

FIGS. 3A to 3D show energy band diagrams of this portion.

FIG. 3A is a band diagram of the structure described above and is shown for reference. 3 (b) to (d)
The energy band structure of another embodiment is shown in FIG.

The structure of FIG. 3B is different from the structure of FIG. 3A in that the band gap of the barrier layer 12 is relatively small. Even if the bandgap of the barrier layer 12 is smaller than that of the out barrier layer 12a, if the bandgap of the barrier layer 12 is larger than that of the light guide layer 3, electrons that contribute to laser emission are sufficiently confined in the well layer 11, and thus laser emission is performed. The density of states of electrons that do not contribute to the can be reduced. In addition, according to the structure of FIG.
The refractive index of the barrier layer 12 can be made larger than that of the out barrier layer 12a, and light can be effectively confined in the multiple quantum wells.

In the structure of FIG. 3C, the band gap of the out barrier layer 12a is relatively smaller than that of the structure of FIG. 3B, and is the same size as the barrier layer 12. According to this structure, the refractive index can be further increased as compared with the structure of FIG.

The structure of FIG. 3D is different from the structures of FIGS. 3A to 3C in that the light guide 3 and the barrier layer 12b are different.
The point is that the graded layer is used. Figure 3 (d)
The light guide layer of (Al0.7Ga0.
3) From the composition of 0.51In0.49P to (Al0.4Ga0.6) 0.51
It is gradually changing to In0.49P. The out-barrier layer is (Al0.7Ga0.3) 0.51In0.
The composition of 49P gradually changed to (Al0.6Ga0.4) 0.51In0.49P.

FIGS. 3 (b) to 3 (d) show a variation of the laser structure based on FIG. 3 (a), but the present invention is not limited to this, and FIG.
A combination of the structures (a) to (d) may be used. For example, the structure of FIG. 3 (a) and the structure of FIG. 3 (d) may be combined. In this case, for example, a structure can be obtained in which only the light guide layer has a graded structure and the out barrier layer is formed of (Al0.7Ga0.3) 0.51In0.49P. Further, when the structure of FIG. 3B and the structure of FIG. 3C are combined, the band gap of the barrier layer 12 is smaller than the band gap of the out barrier layer 12a and the band gap of the out barrier layer 12a is smaller. A structure in which the gap is smaller than the band gap of the cladding layer 2 is obtained. With the semiconductor laser obtained from these combinations, the same effects as those of the above-described embodiment can be obtained.

Reference Example 1 FIG. 8 is a sectional view showing the element structure of a multiple quantum well semiconductor laser as a reference example . The basic structure is the same as the structure (Example 1) of the semiconductor laser of FIG. The difference from the first embodiment is that the high barrier portion 88, which is a region having a large band gap, is formed in the optical guide layer 82, and the band gap of the barrier layer 91 in the multiple quantum well layer 83 is small. It is that. The manufacture of this semiconductor laser
The same method as the manufacturing method of Example 1 is performed.

As shown in FIG. 8, the light guide layer 82
A high barrier portion 88 is formed therein, and this layer prevents electrons that do not contribute to laser emission from overflowing onto the multiple quantum well layer 83 as in the case of Example 1 and multiplexes only electrons that contribute to laser emission. Since it can be confined in the quantum well layer 83, a low threshold current semiconductor laser can be realized.

In the multiple quantum well layer 83, the well layer 90 is made of Ga0.44In0.56P having a thickness of 3 nm, and the barrier layer 91 is (Al0.5Ga0.5) 0.51In0.49P having a thickness of 5 nm.
Are formed from. The light guide layer 82 has a part thereof.
The high barrier portion 88 having a band gap larger than that of the light guide layer 82 is included. The film thickness of the high barrier portion 88 is 20 nm. The quantum well layer 90 is given 0.5% compressive strain.

The structure of this reference example will be described in more detail with reference to FIG. FIG. 9 shows the band structure of the semiconductor laser according to the present reference example . In FIG. 9, the conduction band is indicated by “reference numeral 95” and the valence band is indicated by “reference numeral 96” in the figure.

As can be seen from FIG. 9, the high barrier portion 88 provided in the light guide layer 82 is formed by the p and n cladding layers 8.
It is made of the same material as 1, 84, and has a high barrier portion 88.
Is larger than the band gap of the light guide layer 82.

According to the multiple quantum well having such a structure,
The subband spacing becomes wider after the energy of the barrier layer 91 is exceeded, and as a result, the density of states in both the conduction band 95 1 and the valence band 96 2 becomes small.

In this reference example , the thickness of the portion excluding the high barrier portion 88 from the optical guide layer 82 is preferably thicker than the thickness of the multiple quantum well layer 83. By doing so, it is possible to localize the electrons having the energy equal to or higher than the conduction band 95 of the light guide layer 82 to the light guide layer 82, not to the multiple quantum well region. The same effect as can be obtained.

FIG. 10 shows the refractive index distribution and the light intensity distribution of this structure. In FIG. 10, the refractive index of the semiconductor laser of this reference example is shown by the solid line, and
The refractive index of the semiconductor laser is shown by a broken line. In this way, the refractive index becomes the largest around the quantum well, so
The confinement of light becomes stronger and the coupling between the optical gain generated in the quantum well and the optical mode becomes stronger. As a result, Example 1
It is possible to reach the oscillation threshold with a carrier density of the same level as.

In the multiple quantum well of the present reference example , since the state density is large, the pseudo Fermi level is low at the same carrier density, so that the current due to the radiative recombination of carriers near the band edge is reduced.

In the semiconductor laser of this reference example , the barrier layer 9
1 or the density of states of carriers (electrons and holes) at an energy higher than that of the light guide layer 82 rapidly increases as the energy increases. However, as is clear from the carrier distribution function shown in FIG. 11, the carriers in such a high energy region almost never exist near the quantum well due to the effect of the high barrier portion, as in the first embodiment. Instead, it is confined in the light guide layer. In this way, the electrons that do not contribute to laser emission are confined in the light guide layer,
Since it does not overflow onto the multiple quantum well layer, recombination current that does not contribute to oscillation in the multiple quantum well layer can be suppressed.

Both the conduction band and the valence band show the same distribution, but the height of the barrier barrier is higher in the valence band, and the density of states in the valence band is also larger. Although a large number of holes are present in the light guide layer, they are confined in the multiple quantum well layer. Therefore, a large number of electrons existing in the light guide layer cannot be recombined with holes existing in the light guide layer, and a reactive current unnecessary for laser oscillation does not flow.

As described above, in the semiconductor laser according to the present reference example , the spatial distribution of carriers and the energy distribution of the entire active region including the optical guide layer are changed, whereby the recombination current during laser oscillation is greatly reduced. It is possible to

Further, since the pseudo Fermi level of the conduction band at the time of laser oscillation can be lowered, the p-clad layer 8 is formed.
It is possible to suppress the leak current flowing by diffusing carriers into No. 4 and lowering the threshold current of the laser, operating at high temperature, and improving reliability.

In this reference example , GaInP / AlG is used.
The aInP-based short wavelength semiconductor laser is mentioned,
The same effect can be obtained by using other materials. Also books
In the reference example , the case where the light guide layers are symmetrically provided on both sides of the quantum well is described.
The light guide layer may be present only on the side where there is.

Also in a semiconductor laser having a single quantum well structure, if an out-barrier layer 12a having a band gap larger than the band gap of the optical guide layer is inserted between the quantum well layer and the optical guide layer, the multiple quantum well structure is obtained. The characteristics can be improved as in the case of the structured semiconductor laser.

(Embodiment 3 ) FIG. 12 is a sectional view showing the device structure of still another semiconductor laser according to the present invention. The basic structure is the same as the structure (Example 1) of the semiconductor laser of FIG. The difference from Example 1 is that the low barrier portion 128, which is a region having a small band gap, is formed in the optical guide layer 122, and the band gap of the barrier layer 131 in the multiple quantum well layer 123 is small. It is that. This semiconductor laser is also manufactured by the same method as the manufacturing method of the first embodiment.

By providing the low barrier portion 128 having a small bandgap in the light guide layer 122 as described above, electrons which do not contribute to laser emission can be confined in this region, which contributes to laser oscillation in the multiple quantum well. Not recombination current can be suppressed.

The well layer 130 of the multiple quantum well layer 123 is formed of Ga0.44In0.56P having a thickness of 3 nm, and the barrier layer 131 is (Al0.5Ga0.5) 0.51I having a thickness of 5 nm.
It is formed from n0.49P. The low barrier portion 128 in the light guide layer 122 has a film thickness of 30 nm. The quantum well layer 130 is given a compressive strain of 0.5%.

FIG. 13 shows the band structure of this multiple quantum well semiconductor laser. The band gap of the low barrier portion 128 is smaller than that of the optical guide layer 122 and the barrier layer 131, and is, for example, only 30 meV higher than the ground level of electrons in the quantum well in the conduction band. Therefore, the density of states in the conduction band increases at a lower energy as compared with the conventional separate confinement type multiple quantum well structure (SCH structure) without the low barrier portion 128 as shown in FIG. Similarly, in the valence band, the density of states rapidly increases when the energy exceeds a certain energy, but since this region is sufficiently separated from the band edge, it has almost no effect on the energy distribution of holes. For this reason, even if the carrier density increases, the pseudo-Fermi level in the conduction band slightly increases, and a high carrier density is required to obtain an arbitrary maximum gain.

However, since the refractive index around the quantum well becomes large as shown in FIG. 15, the confinement of light becomes stronger and the coupling between the optical gain generated in the quantum well and the optical mode becomes stronger. As a result, it is possible to reach the oscillation threshold with the same carrier density as that of the conventional SCH structure laser.

Further, in the multiple quantum well of the present invention, since the state density is large, the pseudo Fermi level is low at the same carrier density, so that the current due to radiative recombination of carriers near the band edge is reduced. Also, electrons with high energy in the conduction band are confined by the potential valleys of the low barrier portion as shown in FIG. 16, but since holes hardly exist in this region, they do not flow as an invalid current. Therefore, even if the carrier density increases, the oscillation threshold can be reached with a small current, and the pseudo-Fermi level of the conduction band can be lowered, so that the leakage current can be suppressed and the low threshold current of the laser can be suppressed. And
High temperature operation and high reliability are possible.

In the case of this embodiment, it is preferable that the thickness of the low barrier portion is larger than the thickness of the multiple quantum well layer.

In the first to third embodiments, GaI
Although the nP / AlGaInP-based short wavelength semiconductor laser has been described, the same effect can be obtained with a semiconductor laser formed of another material. Further, although the semiconductor laser in which the light guide layers are symmetrically provided on both sides of the quantum well has been described, the light guide layers may be provided asymmetrically or only on the P clad layer side.

Even in a semiconductor laser having a single quantum well structure, if a high barrier portion or a small barrier portion having a bandgap larger than that of the optical guide layer is inserted in the optical guide layer, the same as in the case of a multiple quantum well semiconductor laser. Similarly, the characteristics can be improved.

Next, the conventional SCH structure laser and the first embodiment will be compared. FIG. 17A shows a typical 630 nm.
FIG. 17B shows a band-separated confinement type multiple quantum well (SCH-MQW) structure, and FIG. 17B shows an energy band diagram of the semiconductor laser of Example 1. Each structure has the light guide layer 33, and has a mechanism for confining light therein. Therefore, the light emitted from the quantum well can efficiently contribute to stimulated emission, and the threshold current of the laser can be reduced. Since the carrier density during laser oscillation is reduced, the leak current can be reduced at the same time.

However, the difference in height between the pseudo Fermi level of the conduction band 21 and the band edge of the cladding layer is still small. This means that the carriers are weakly confined in the active region, and a large amount of electrons diffuse into the P-clad layer, causing a leak current to flow. Therefore,
Particularly, in the structure of FIG. 17A, the current required for laser oscillation becomes large, or this current greatly depends on the temperature change.

In the structure of FIG. 17B, the barrier layer 12 and the out-barrier layer 12a are made of the same material as the cladding layers 2 and 5, and the band gap is larger than that of the optical guide layer 3. When the multiple quantum well having such a configuration is used, the potential barrier by the barrier layer 12 and the out barrier layer 12a is increased, and the effect of confining carriers in the well layer 11 is enhanced. Therefore, the density of states in both the conduction band and the valence band is reduced, and a large gain can be generated with a small carrier density.

18 (a) and 18 (b) are similar to FIG.
It corresponds to (a) and (b), and shows the spatial distribution of electrons in a region higher than the energy of the light guide layer 3. FIG.
As shown in (a), in the conventional SCH structure, electrons also exist near the central multiple quantum well. On the other hand, as shown in FIG. 18B, in the high barrier barrier type structure of Example 1, most are distributed in the light guide layer 3.

Holes also show a similar spatial distribution, but in either structure, holes cannot energetically exist in the light guide layer 3. Therefore, even with the same electron density, the high barrier barrier type structure of Example 1 has a larger proportion of electrons existing near the band edge that can contribute to laser oscillation, and a larger gain can be generated. However, the outer barrier layer 1
When 2a is thicker than the de Broglie wavelength of electrons, stronger carrier confinement is sufficiently realized.

19 (a) and 19 (b) show the refractive index and the light intensity distribution of the conventional SCH semiconductor laser and the high barrier barrier type multiple quantum well of the first embodiment.

[0076] (Embodiment 4) FIGS. 20 and 21 show the energy band of the lifting one semiconductive <br/> body lasers combined each feature described above.

[0077] Figure 20 (a) and (b), reference example of the improvement of the first semiconductor laser, reference bandgap of the optical guide layer 3 showed a greater example than the band gap of the barrier layer 12 Example 2 Is . As shown in FIG. 20B, the band gap of the high barrier portion 88 does not have to be equal to the band gap of the cladding layer 2.

[0078] Figure 20 (c) and (d) of the refinement of the semiconductor laser of Example 3, in the embodiment bandgap of the optical guide layer 3 showed a greater example than the band gap of the barrier layer 12 There is . As shown in FIG. 20 (d),
The bandgap of the light guide layer 3 may not be equal on both sides of the low barrier portion 28. Although not shown, the light guide layer 3 may have a graded structure.

FIG. 21A shows a high barrier portion 88 having a band gap larger than that of the light guide layer 3 in the light guide layer 3.
And a low barrier portion 28 having a band gap smaller than that of the optical guide layer 3 is shown.

FIG. 21B shows a semiconductor laser in which a low barrier portion 28 having a band gap smaller than that of the optical guide layer 3 is provided in the optical guide layer 3 and an out barrier layer 12a is provided as the outermost layer of the quantum well. Is shown.

These semiconductor lasers can also exhibit the above-mentioned effects.

According to the present invention, an out-barrier layer is formed on the outermost side of a multiple quantum well, and an optical guide layer having a bandgap smaller than that of the out-barrier layer is provided on the outer side of the multi-quantum well. The electrons hardly overflow, and the electrons contributing to the laser emission can be confined in the well layer, and the density of the electrons not contributing to the laser emission overflowing on the well layer can be reduced. Therefore, Ru can realize a semiconductor laser with a low threshold current.

[0082] Further, the light guide layer provided small low barrier unit band gap, and to reduce the band gap of the barrier layer of the multiple quantum well layer by increasing the refractive index of this portion, multiple light quantum Confine electrons that do not contribute to laser oscillation in the well layer and in the optical guide layer,
Only electrons that contribute to light emission can be confined in the multiple quantum well layer. Therefore, it is possible to realize a low threshold semiconductor laser that suppresses the reactive current that does not contribute to light emission.

[Brief description of drawings]

FIG. 1 is a view showing a structural cross section of a semiconductor laser according to the present invention and a structural section of its multiple quantum well.

FIG. 2 is a diagram showing energy bands of multiple quantum wells of a semiconductor laser according to the present invention.

FIG. 3 is a diagram showing energy bands of multiple quantum wells of a semiconductor laser according to the present invention.

FIG. 4 is a diagram showing a two-dimensional density of states of electrons in a multiple quantum well according to the present invention.

FIG. 5 is a diagram showing the relationship between the injection current density and the maximum gain of the present invention and the conventional multiple quantum well.

FIG. 6 is a diagram showing an energy band of a conventional multiple quantum well.

FIG. 7 is a diagram showing a two-dimensional state density of electrons in a conventional multiple quantum well.

FIG. 8 is a diagram showing a structural cross section of a semiconductor laser using a multiple quantum well as a reference example and a structural section of the multiple quantum well.

FIG. 9 is a diagram showing energy bands of a multiple quantum well which is a reference example .

FIG. 10 is a diagram showing a refractive index and a light intensity distribution of a multiple quantum well which is a reference example .

FIG. 11 is a diagram showing an electron distribution in a conduction band of a multiple quantum well which is a reference example .

FIG. 12 is a diagram showing a structure cross section of a semiconductor laser using another multiple quantum well according to the present invention and a structure cross section of the multiple quantum well.

FIG. 13 is a diagram showing energy bands of a multiple quantum well according to the present invention.

FIG. 14 is a diagram showing the density of states of electrons in the present invention and the conventional multiple quantum well.

FIG. 15 is a diagram showing a refractive index and a light intensity distribution of a multiple quantum well according to the present invention.

FIG. 16 is a diagram showing an electron distribution in a conduction band of a multiple quantum well according to the present invention.

FIG. 17 is a diagram showing an energy band of a semiconductor laser having a conventional SCH structure and an energy band of a multiple quantum well of the present invention.

FIG. 18 is a diagram showing an electron distribution of a conventional SCH semiconductor laser and an electron distribution of a multiple quantum well of the present invention.

FIG. 19 is a diagram showing the refractive index and light intensity distribution of a conventional SCH structure semiconductor laser, and the refractive index and light intensity distribution of a multiple quantum well of the present invention.

FIG. 20 is a diagram showing energy bands of multiple quantum wells of a semiconductor laser according to the present invention.

FIG. 21 is a diagram showing energy bands of a multiple quantum well of a semiconductor laser according to the present invention.

[Explanation of symbols]

1 n-GaAs substrate 2 n- (Al0.7Ga0.3) 0.51In0.49P cladding layer 3 (Al0.5Ga0.5) 0.51In0.49P optical guide layer 4 multiple quantum well layer 5 P- (Al0.7Ga0.3) ) 0.51In0.49P clad layer 6 P-GaInP protective layer 7 n-GaAs current blocking layer 8 P-GaAs contact layer 11 Ga0.44In0.56P well layer 12 (Al0.7Ga0.3) 0.51In0.49P barrier layer 12a ( Al0.7Ga0.3) 0.51In0.49P Out barrier layer 21 Conduction band 22 Valence band 32 First cladding layer 33 Optical guide layer 34 Multiple quantum well layer 35 Second cladding layer 36 Ground level of electron 37 Ground level of hole 41 quantum well layer 42 barrier layer 80 n-GaAs substrate 81 n- (Al0.7Ga0.3) 0.51In0.49P cladding layer 82 (Al0.5Ga0.5) 0.51In0.49P optical guide layer 83 multiple quantum well layer 84 P- (Al0.7Ga0.3 0.51In0.49P clad layer 85 P-GaInP protective layer 86 n-GaAs current blocking layer 87 P-GaAs contact layer 88 (Al0.7Ga0.3) 0.51In0.49P high barrier portion 90 Ga0.44In0.56P well layer 91 ( Al0.7Ga0.3) 0.51In0.49P barrier layer 120 n-GaAs substrate 121 n- (Al0.7Ga0.3) 0.51In0.49P cladding layer 122 (Al0.5Ga0.5) 0.51In0.49P optical guide layer 122a ( Al0.5Ga0.5) 0.51In0.49P optical guide layer 123 multiple quantum well layer 124 P- (Al0.7Ga0.3) 0.51In0.49P clad layer 125 P-GaInP protective layer 126 n-GaAs current blocking layer 127 P- GaAs contact layer 128 (Al0.3Ga0.7) 0.51In0.49P low barrier portion 130 Ga0.44In0.56P well layer 131 (Al0.7Ga0.3) 0.51In0.49P barrier layer

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yuu Uenoyama 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-6-132604 (JP, A) JP-A-6- 196801 (JP, A) JP-A-6-69593 (JP, A) JP-A-6-237041 (JP, A) JP-A-6-268320 (JP, A) JP-A-4-218994 (JP, A) JP 64-7587 (JP, A) JP 62-29189 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (3)

(57) [Claims] 1. A semiconductor laser comprising a quantum well layer including a well layer and a barrier layer, and an optical guide layer, wherein the optical guide layer has a bandgap smaller than a bandgap of the optical guide layer. A semiconductor laser having a low barrier portion having a thickness greater than a de Broglie wavelength of electrons. 2. The semiconductor laser according to claim 1, wherein the optical guide layer is thicker than the quantum well layer. 3. The semiconductor laser according to claim 1, wherein the thickness of the low barrier portion is thicker than the thickness of the quantum well layer.