JPH0528515B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a semiconductor laser, and particularly to the structure of a cladding layer.

[Prior Art] FIG. 2 is a sectional view showing a conventional semiconductor laser. First, the configuration of this semiconductor laser will be explained. In the figure, an n-type substrate is placed on a p-type GaAs substrate 1.
A GaAs current blocking layer 2 is formed and a V-shaped groove 9 is formed in this substrate and in this current blocking layer. On a part of the p-type GaAs substrate 1 and on the n-type GaAs current blocking layer 2, p-type Al _x Ga _1-x
As lower cladding layer 3, Al _y Ga _1-y As active layer 4, n
A type Al _x Ga _1-x As upper cladding layer 5 and an n-type GaAs contact layer 6 are formed in this order. Here, x and y are composition ratios. n-type Al _x Ga _1-x As upper cladding layer 5 and Al _y Ga _1-y active layer 4 and p-type Al _x
The Ga _1-x As lower cladding layer 3 forms a so-called double heterostructure. The n-type GaAs contact layer 6 is a layer for forming an electrode, and an upper electrode 7 is formed on this contact layer. 8 is a lower electrode. In addition, 10 is an Al _g Ga _1-y As active layer 4
It shows the light distribution of light seeping out from the

Next, the operation of this semiconductor laser will be explained. When a voltage is applied between the upper electrode 7 and the lower electrode 8 to apply a forward bias to the double heterojunction, the n-type Al _x Ga _1-x As upper cladding layer 5 and the p-type Al _x
Electron/hole pairs are effectively confined within the Al _y Ga _1-y As active layer 4, which has an energy gap narrower than that of the Ga 1-x As lower cladding layer 3 _, and the n-type Al _x Ga _1-x As upper Cladding layer 5, p
Light is effectively confined in the Al _y Ga _1-y As active layer 4 which has a refractive index higher than that of the Al _x Ga _1-x As lower cladding layer 3 . The above is an explanation of the vertical confinement of electron/hole pairs and light within the cross section of FIG. 2, but regarding the confinement of horizontal electron/hole pairs and light within this cross section, The n-type GaAs current blocking layer 2 has p-type Al _x Ga _1-x
Since it has a large number of carriers different from the As lower cladding layer 3 and the p-type GaAs substrate 1, it becomes a potential wall that impedes carrier conduction, and the carriers flow in the V-shaped groove 9 of the n-type GaAs current blocking layer 2, causing electrons and The hole pair will be confined within the V-shaped groove 9. Furthermore, an n-type GaAs current blocking layer 2
has an energy gap narrower than that of the Al _y Ga _1-y As active layer 4, so the light generated from the Al _y Ga _1-y As active layer 4 forms a carrier in this current blocking layer, which is conducted and The effect of reducing the potential wall occurs, but this effect is
This can be eliminated by creating many non-radiative recombination centers in the n-type GaAs current blocking layer 2. In addition, confinement of light in the horizontal direction is caused by a rapid increase in the absorption coefficient of light in the n-type GaAs current blocking layer 2 outside the V-shaped groove 9, which lowers its effective refractive index.
Eventually, the refractive index inside the V-shaped groove 9 increases, so the light
The laser oscillation is effectively performed by being effectively confined within the shaped groove 9.

[Problems to be Solved by the Invention] Incidentally, in a conventional semiconductor laser having such a structure, the oscillation mode is the fundamental mode in the lateral direction,
Single mode is normal in the vertical direction. However, because of its single mode, it has the drawbacks of noise generation due to changes in temperature, injection current, and mode hopping that occurs when light returns. For example, the dependence of the S/N characteristic on the light return rate is as shown in FIG. 3, and there is a problem that the S/N characteristic is poor when the light return rate is relatively small. As a method to eliminate this drawback, for example, when Al _x Ga _1-x As (x = 0.45) doped with Te is used as the n-type Al _x Ga _1-x As upper cladding layer 5, Te/Al doping is used. There is a way to lower the ratio. By lowering the Te/Al doping ratio, the longitudinal mode changes from single mode to multimode, as shown in FIG. 4, and the S/N characteristics are improved. In other words, mode hopping noise is less likely to occur. This occurs because the light seeping into the upper cladding layer is less likely to be absorbed in this cladding layer, as shown in the light distribution 10 in FIG. However, when the Te concentration of the n-type cladding layer is lowered, the resistance of the n-type cladding layer increases, which increases heat generation, reduces the life of the semiconductor laser, and poses problems in terms of reliability. Also, n
When the Te concentration in the type cladding layer is lowered, solid phase diffusion increases when the n-type cladding layer is grown from the p-type active layer, making it easier for heliodesis on the n-type layer side. Therefore, the potential of the p-n interface increases, and the potential drop of electrons injected into the active layer increases. This also leads to deterioration of the life span of the semiconductor laser and poses a problem in terms of reliability. Furthermore, in general, too high a Te concentration also causes deterioration of crystallinity, which shortens the life of the semiconductor laser. Therefore, the lifetime of the semiconductor laser exhibits a chevron-shaped characteristic with respect to the Te concentration, as shown in FIG. As described above, the suppression of mode hopping noise and the lifetime of the semiconductor laser were in a contradictory relationship with respect to the direction of lowering the Te concentration.

This invention was made to solve the above-mentioned problems, and the object is to obtain a semiconductor laser that can simultaneously suppress mode hopping noise, that is, improve S/N characteristics and extend the lifespan. .

[Means for Solving the Problems] A semiconductor laser according to the present invention includes a first conductivity type cladding layer, an active layer formed on the first conductivity type cladding layer, and a semiconductor laser formed on the first conductivity type cladding layer. A semiconductor layer having a double heterostructure formed with a second conductivity type clad layer, each of the first conductivity type clad layer, the active layer, and the second conductivity type clad layer containing at least Al, Ca, and As. This semiconductor laser has the following features. 1st
At least one of the conductive type clad layer and the second conductive type clad layer includes a first clad layer and a second clad layer. The first cladding layer is located on the active layer side. The first cladding layer and the second cladding layer contain the same type of impurity. The concentration of the same type of impurity contained in the first cladding layer is higher than the concentration of the same type of impurity contained in the second cladding layer.

[Function] In the present invention, at least one of the first conductivity type cladding layer and the second conductivity type cladding layer includes a plurality of layers. The first cladding layer is located on the active layer side and contains the same type of impurity at a higher concentration than the second cladding layer. Therefore, the first
The cladding layer can improve the lifetime of the semiconductor laser.

Furthermore, the second cladding layer having a low carrier concentration changes the longitudinal mode from single mode to multimode, thereby improving the S/N characteristics.

Furthermore, since the first cladding layer and the second cladding layer contain the same type of impurity, the carrier concentration can be easily changed by simply controlling the amount of impurity added in the growth process of these two layers. Therefore, it is possible to easily realize a semiconductor laser that can simultaneously achieve improved S/N characteristics and longer life simply by controlling the amount of impurities added.

[Embodiments of the Invention] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description of this embodiment, the description of parts that overlap with the description of the conventional technology will be omitted as appropriate.

FIG. 1 is a sectional view showing a semiconductor laser according to an embodiment of the present invention. The configuration of this example is the second
The difference from the structure shown in the figure is that the upper clad layer is composed of an n-type Al _x Ga _1-x As upper first clad layer 5a and an n-type Al _x Ga _1-x As upper second clad layer 5b. It is. The n-type Al _x Ga _1-x As upper first cladding layer 5a is made of, for example, 5×10 ¹⁸ cc of Te-doped Al _x Ga _1-x As (x=0.45). The thickness is set to be sufficiently smaller than the distance of the light distribution 10 seeping into the upper cladding layer. In addition, the n-type Al _x Ga _1-x As upper second cladding layer 5b is doped with, for example, 1×10 ¹⁷ cc of Te.
It consists of Al _x Ga _1-x As (x = 0.45). n-type
Al _x Ga _1-x As upper first cladding layer 5a and n-type
The energy gap of the Al _x Ga _1-x As upper second cladding layer 5b is wider than that of the Al _y Ga _1-y As active layer 4, and the energy gap of the Al _x Ga _1-x As upper first cladding layer 5a and the n-type Al The refractive index of _{the x} Ga _1-x As upper second cladding layer 5 b is lower than the refractive index of the Al _y Ga _1-y As active layer 4 . Also, Al _y at this time
The Ga _1-y As active layer 4 is, for example, Al _y Ga _1-y As (y=
0.15), and its thickness is set to be smaller than 0.1 μm.

Next, the operation of this semiconductor laser will be explained. n-type Al _x Ga _1-x As upper first cladding layer 5a;
The energy gap of each of the n-type Al _x Ga _1-x As upper second cladding layer 5b and the p-type Al _x Ga _1-x As lower cladding layer 3 is wider than the energy gap of the Al _y Ga _1-y As active layer 4. , also n-type Al _x Ga _1-x
The refractive index of each of the As upper first cladding layer 5a, the n-type Al _x Ga _1-x As upper second cladding layer 5b, and the p-type Al _x Ga _1-x As lower cladding layer 3 is Al _y Ga _1-y.
Since the refractive index is lower than that of the As active layer 4, electron-hole pairs and light are transferred to the Al _y Ga _1-y As upper first cladding layer 5.
Since the carrier concentration of a is as high as 5×10 ¹⁸ cc, as can be seen from FIG. 5, the life of the semiconductor laser is improved. Furthermore, from the Al _y Ga _1-y As active layer 4 to the n-type Al _x
Ga _1-x AS upper first cladding layer 5a, n-type Al _x
The light seepage into the upper second cladding layer 5b of Ga _1-x As occurs in the same way as the light seepage from the active layer to the upper cladding layer in a conventional semiconductor laser, and the light distribution becomes as shown in 10. Since the thickness of the first cladding layer 5a on the upper side of the n-type Al _x Ga 1- _x AS is set to be sufficiently smaller than the seepage distance of the light distribution 10, most of the seeped light is transmitted to the upper side of the n-type Al _x Ga _1-x As. This will seep into the second cladding layer 5b. At this time, n
Since the carrier concentration of the upper second cladding layer 5b of the Al _x Ga _1-x As type is as low as 1×10 ¹⁷ cc, the leaked light is hardly absorbed by this upper second cladding layer, and the longitudinal mode changes from the single mode to the single mode. Becomes polymodal. For this reason, mode hopping noise caused by changes in temperature, injection current, and light return is suppressed, and S/N characteristics are improved. By constructing the upper second cladding layer with a low
The effect of improving characteristics can be obtained independently.

Further, in the above embodiment, the upper cladding layer is composed of the upper first cladding layer and the upper second cladding layer, but the lower cladding layer is composed of the lower first cladding layer and the lower second cladding layer. It may also be constructed from the following, and in this case as well, the same effects as in the above embodiment can be achieved.

Further, in the above embodiment, the upper cladding layer is composed of the upper first cladding layer and the upper second cladding layer, but the upper cladding layer or the lower cladding layer, or the upper cladding layer and the lower cladding layer It is also possible to have multiple layers, each of which has the same conductivity type, is made of the same base material, and has a different carrier concentration, and in these cases, the same effect as in the above embodiment can be obtained. play.

Further, the present invention can also be applied to a semiconductor laser in which the conductivity type of each layer shown in FIG. 1 is reversed, and the same effects as in the above embodiment can be obtained in this case as well.

In addition, in the above embodiment, a GaAs substrate is used.
Although an AlGaAs-based semiconductor laser has been described, it goes without saying that the present invention can be applied to other types of semiconductor lasers.

In addition, this invention also improves the life span and S/N of semiconductor lasers.
Although applied to improve properties, the invention can also be applied when changing the diffusion length of the implanted carrier or changing the impurity diffusion profile.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain a semiconductor laser that can suppress mode hopping noise, that is, improve S/N characteristics and extend the life span at the same time.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a semiconductor laser according to an embodiment of the present invention. FIG. 2 is a sectional view showing a conventional semiconductor laser. FIG. 3 is a diagram showing S/N characteristics with respect to the light return rate in a semiconductor laser. FIG. 4 is a diagram showing longitudinal mode characteristics with respect to upper cladding layer carrier concentration in a semiconductor laser. FIG. 5 is a diagram showing the lifetime characteristics of a semiconductor laser with respect to carrier concentration in the upper cladding layer. In the figure, 1 is a p-type GaAs substrate, 2 is an n-type
GaAs current blocking layer, 3 p-type Al _x Ga _1-x As lower cladding layer, 4 Al _y Ga _1-y As active layer, 5a n
Type Al _x Ga _1-x As upper first cladding layer, 5b is n type
Al _x Ga _1-x As upper second cladding layer, 6 is n-type GaAs
Contact layer, 7 is upper electrode, 8 is lower electrode, 9
is a V-shaped groove, and 10 is a light distribution. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1 Consisting of a first conductivity type cladding layer, an active layer formed on the first conductivity type cladding layer, and a second conductivity type cladding layer formed on the active layer. The first conductivity type cladding layer, the active layer and the second conductivity type cladding layer each contain at least Al, Ca and As.
In the semiconductor laser which is a semiconductor layer including the first conductivity type clad layer and the second conductivity type clad layer, at least one includes a first clad layer and a second clad layer, and the first clad layer includes the first clad layer. located on the active layer side, the first cladding layer and the second cladding layer contain the same type of impurity, and the concentration of the same type of impurity contained in the first cladding layer is equal to the concentration of the same type of impurity contained in the second cladding layer. A semiconductor laser characterized by a higher concentration than similar impurities.