JPS5824456Y2 - semiconductor laser - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to the structure of a semiconductor laser.

With advances in compound semiconductor crystal purification technology and crystal growth technology, devices using compound semiconductors have developed rapidly in recent years.

Semiconductor lasers using GaAs-(GaAl)As-based materials are one such type, and until now they have been used as light sources for optical communications.
Development is proceeding mainly in the near-infrared region of approximately 0.8 μm.

These semiconductor lasers are made of GaAs on a GaAS substrate.
, (GaAl)As-based multilayer structures are generally formed continuously by a liquid phase epitaxial method.

In this case, GaAs or (GaAl)As
The laser active layer has n-type and p-type (GaAl).
It becomes a double heterostructure sandwiched with As layers.

In III-V group compound semiconductors such as (GaAl)As, Zn is most commonly used as a p-type impurity.

However, in the above-mentioned double heterostructure, since the diffusion rate of Zn is high, during the thermal cycle of crystal growth, Zn penetrates through the active layer and forms n(GaAl)As.
The particles diffused into the layers, reducing luminous efficiency and causing an increase in the threshold current value of laser oscillation and deterioration.

To avoid this, Ge, which has a slow diffusion rate, is used as a p-type impurity, especially in (GaAl)As lasers with an oscillation wavelength of 0.8 to 0.9πm, but as the amount of AI increases, holes concentration decreases.

For this reason, Ge is unsuitable as a p-type impurity, especially when the mixed crystal ratio of Al is 0.5 or more, such as in a visible laser.

The present invention aims to solve these conventional problems.

In the present invention, an impurity with a slow intracrystal diffusion rate is added between the active layer and the Zn-doped l)-(GaAl)As layer, which conventionally constitutes a double heterostructure.
aA l ) Soak the As layer.

The thickness of these five layers is determined by Z due to thermal cycles during crystal growth.
The growth temperature, growth time, etc. are selected in consideration so that even if Zn in the n-doped layer diffuses, the diffusion front does not reach into the active layer.

Figure 1 shows a GaAS crystal with Zn added (GaAs
l) When growing an As layer, the depth to which Zn diffuses into the GaAs crystal was plotted against time.

The parameter in the figure is the sample temperature.

Diffusion depth ld: Boltzmann constant T: temperature t: time (sec) is given by

For example, at a growth temperature of 800°C, approximately 0.6
5 μm Zn diffuses into the GaAS crystal.

Therefore, a low concentration layer doped with a p-type impurity (e.g. Ge) having a slow diffusion rate is added between the active layer and the Zn-doped (GaAl)As layer, so that Zn diffusion stops within this layer. By setting the layer thickness, it is possible to prevent Zn from reaching the active layer.

Even in this case, if the distance from the Zn diffusion front to the laser active layer is about 0.2 μm or less, the problem of heat generation can be ignored.

Furthermore, since most of the holes are injected into the active layer, the presence of these five layers does not affect the laser oscillation characteristics.

Therefore, by adopting the structure according to the present invention, it becomes possible to use an added impurity having a high intracrystal diffusion rate without affecting the laser device characteristics in any way.

Therefore, the degree of freedom in selecting materials constituting the semiconductor laser and expanding the range of laser oscillation wavelengths is significantly increased.

In GaAs or GaAIAs-based materials, examples of impurity elements with a high intracrystal diffusion rate as described above include Zn, and examples of impurity elements with a low diffusion rate include Ge, Si, etc.

Hereinafter, the present invention will be explained in detail with reference to examples.

Example 1 FIG. 2 is a sectional view showing the first embodiment.

1 is an n-GaAs substrate (100 plane, Si doped electron concentration n is 1×1018 Sn doped, electron concentration n2 5×1017 cm−3, thickness ~2 μm), 3 is a Ga1-yAlyAS active layer (y,=
0.15, Si doped, hole concentration p25 x 1017 cm
-3, thickness ~0.2 μm), 4 is l) -Ga. -xA
t, As layer (X20.6, Ge doped, hole concentration p21 x 1017 cm-3, thickness 0.3 μm), 5 is p-G
a1-xA l xAs layer (X: 0.6, Zn doped,
Hole concentration p21 x 1018 cm-3, thickness ~2 μm),
6 is an n-GaAs layer (Sn doped, electron concentration n: 5
X 10110l7”).

In this example, the present invention is applied to a (GaAl)As-based visible semiconductor laser (oscillation wavelength ~7600 nm).

In order to realize a visible semiconductor laser, (GaAl
) In the As mixed crystal, it is necessary to increase the A I As mixed crystal ratio and widen the band gap. In this example, the AlAs mixed crystal ratio y of the laser active layer 3 is 0.15, the carrier trapping layer 2, The mixed crystal ratio X of 4.5 is set to 0.6.

However, in (GaAl)As, AIAS
When the mixed crystal ratio of layer 5 becomes large, sufficient carrier concentration cannot be obtained with the commonly used p-type impurity Ge.
Ga1-XAIXAS used Zn as a p-type impurity.

This Zn has a fast diffusion rate and diffuses to other layers during the thermal cycle of crystal growth.

Therefore, as shown in FIG. 2, the laser active layer 3 and Z
A structure was adopted in which a 0.3 pm (7) Ge-doped Ge1-xAIXAS layer 4 was placed between the n-doped layers 5.

To produce such a multilayer crystal, substrates are successively brought into contact with a solution using a conventional slide-board liquid phase growth method.

The contact temperature of the first layer was 760°C, and the cooling rate was 1°C/min.

The growth time was 10 minutes for the first layer, 2 seconds for the second layer, 20 seconds for the third layer, 10 minutes for the fourth layer, and 2 minutes for the fifth layer.

After the growth was completed, the sample was rapidly cooled. During this thermal cycle, Zn diffused from the Zn-doped layer 5 becomes n-(Ga
A1) It did not reach the As layer 2, but stopped within the p-(GaAl)As layer 4 at a distance of 0.05 to 0.1 μm from the active layer.

Width 10 to reach layer 5 through surface layer 6 of this crystal.
Zn is diffused in μm stripes, and Cr-Au is deposited on the surface.
was vacuum-deposited to form an ohmic electrode.

Moreover, Au-Ge-Ni was vapor-deposited on the surface of the substrate 1 to form an ohmic electrode.

This was inserted into a laser chip with a length of 300 μm.

This laser oscillated at a wavelength of 7600 A, and the threshold current was 100 mA (threshold current density ~3 kA/cm2).

Compared to the case of a very common double heterostructure without layer 4, the threshold current was approximately ÷.

Furthermore, in the laser having the structure of the present invention, the average life in continuous oscillation at 70° C. was about 3,000 hours, whereas in the case of the conventional structure, it was about 1,000 hours.

Example 2 FIG. 3 is a sectional view showing the second embodiment.

This is a 7-layer GaAS substrate (100 sides, Si-doped, electron concentration n 1 x 1018 cm), on which a strip-shaped groove 8 is provided.

9 is an n-Ga1-XA1xAS layer (x:=Q.6, Si
doped, electron concentration n: 5 x 10"cm'),
It is formed so as to fill flatly the band-shaped groove 8 provided on the substrate, and to have a thickness of 0.3 to 0.4 μm on the outside of the groove.

10 is a Ga1-yAlyAS active layer (y~0.15, S
i doping, hole concentration p25 x 1017 crn-3)
The thickness is approximately 0.1 μm.

4, 5.6 are similar to FIG. 2, and layer 4 is to prevent the Zn doped in layer 5 from diffusing into the active layer during thermal cycling during growth.

In the semiconductor laser having this structure, light is distributed in the layers 4, 5.9 with the active layer 10 as the center, but in the region outside the band-shaped groove 8, it partially reaches the substrate.

Therefore, absorption of light in the substrate creates an artificial waveguide mechanism in the lateral direction with respect to the active layer, and the transverse mode of oscillation is stabilized.

In order to form a crystal with such a structure, first 100
A band-shaped concave groove is formed in the plane GaAs substrate in the 01-direction using photolithography or chemical etching.

The etching solution used was 1:1:3 of phosphoric acid: hydrogen peroxide: ethylene glycol, and etching was carried out at 20° C. for about 2 minutes to a depth of about 1.3 μm.

The width of the band-shaped groove 8 was approximately 5 μm.

Next, by the usual slide board liquid phase growth method,
Layers 9, 10, 4, 5.6 are grown successively.

In order to grow the groove 9 flatly, it is important to grow it at a relatively slow growth rate.

A semiconductor laser manufactured by subjecting this crystal to processes such as Zn diffusion and electrode formation has a wavelength of 7600 A and a threshold current of 110 mA (L threshold current density -3, 5 kA/cm2
), and oscillated stably in horizontal wheel mode up to twice the threshold current.

Furthermore, operation for more than 3,000 hours under continuous oscillation at 70°C was confirmed.

On the other hand, in this structure, when Zn is used as the doping material for layer 4, it is observed that Zn penetrates through active layer 10 and diffuses into layer 9 during crystal growth, and the threshold current density in this case was approximately 7 kA/cm2.

Further, in this case, the average lifetime of continuous oscillation at 70° C. was 1000 hours.

As explained above, according to the present invention, materials that could not be used in the past can be used as crystal impurities, which greatly contributes to improving the performance of semiconductor lasers, including expanding their oscillation wavelength range and extending their lifetime. It became clear.

In addition, in the above embodiments, in order to specifically illustrate the essential effect of the present invention, the description was limited to a GaAIAS semiconductor laser, but the essence of the present invention is that the light-emitting layer and the P-type (or N-type)
The point is that a crystal layer having a length equal to or less than the diffusion length of the carriers is interposed between the crystal layer doped with impurities that provides conduction carriers at the concentration required for laser oscillation (type).

Therefore, the material constituting the laser is not limited to the above materials, for example, Ga1JnxAst-yPy(
0<x<1.0<Y≦1) system, Gat-x
A l xSbyAsl-y(0<x<1.0<
Needless to say, the present invention can be applied not only to semiconductor lasers using III-V group materials such as y≦1), but also to semiconductor lasers using II-VI group materials.

[Brief explanation of drawings]

FIG. 1 is a plot of Zn diffusion depth versus time, FIG. 2 is a cross-sectional view of the structure showing the first embodiment, and FIG.
The figure is a sectional view of the structure showing the embodiment of FIG.

Claims (1)

[Scope of utility model registration request] Layering multiple compound semiconductor layers on a compound semiconductor substrate,
This stacked compound semiconductor layer has a double heterostructure in which an active region is sandwiched between compound semiconductor layers whose forbidden band width is wider than that of the active region, and each side of the active region is sandwiched between two compound semiconductor layers. In a semiconductor laser in which the semiconductor layers have conductivity types opposite to each other, the structure of the compound semiconductor layer in contact with the active region and having a p-type conductivity type is changed to a first semiconductor layer in contact with the active region and a compound semiconductor layer having a p-type conductivity type. at least two of the second semiconductor layers in contact with each other, and the first
and the main added impurity to the second semiconductor layer is not a different type of impurity, and the intracrystalline diffusion rate of the main added impurity of the first semiconductor layer is the intracrystalline diffusion rate of the main added impurity of the second semiconductor layer. A semiconductor laser characterized in that it is configured to have a structure that is smaller than that of the semiconductor laser.