JP2699848B2 - Manufacturing method of semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a semiconductor laser using an InP-based material.

[0002]

2. Description of the Related Art An element structure of a conventional semiconductor laser is shown in FIG. 7 (ELECTRONIC LETTERS; 2).
8th October 1982 VoL18, No2
2). In FIG. 7, reference numeral 10 denotes an N-InP substrate. The N-InP cladding layer 9 is disposed on the N-InP substrate 10, and the non-doped InGaAsP active layer 8 is
It is arranged on the nP cladding layer 9. The P-InP clad layer 6 is disposed on the undoped InGaAsP active layer 8. The P-InP current blocking layer 5 is formed such that the P-InP current blocking layer 5 buries the channel 12 formed by etching.
The N-InP current blocking layer 4 is disposed on the P-InP current blocking layer 5. The P-InP layer 3 is disposed so as to cover the P-InP cladding layer and the N-InP current block layer.
Further, the P ⁺ -InGaAsP cap layer 2 is made of P-In
It is arranged on the P buried layer 3. N-side electrode 11 is NI
On the back surface of the nP substrate 10, and the p-side electrode 1 is P ⁺ -InG
It is provided on the aAsP cap layer 2. In order to obtain a desired conductivity type in an InP-based semiconductor, Zn is used as a P-type, and Se or Si is used as an introduced impurity as an N-type.

FIG. 8A shows an example of a concentration profile of an impurity introduced near a non-doped InGaAsP active layer 8 in a conventional semiconductor laser. As shown in FIG. 8A, the impurity concentration of the P-InP cladding layer 6 is constant. This is an example of a case where crystal growth is performed by using the MOVPE method. When a carrier gas containing a constant concentration of (CH ₃ ) ₂ Zn is mixed with another material gas, the crystal growth is made constant. It is for doing. However, Zn used as a P-type impurity has a high diffusion rate in a crystal. As a result, even if the crystal is grown with the impurity concentration as shown in FIG. 8A, the impurity in InP diffuses during the subsequent thermal process and is measured by the SIMS method or the like.
The density profile is as shown in FIG.
In many cases, the nGaAsP active layer is no longer non-doped.

As a method for solving this problem, there is provided a method of providing a spacer layer for suppressing diffusion of impurities into the active layer between the non-doped active layer and at least one of the upper and lower doping layers (for example, Japanese Patent Laid-Open No. 4-49691). And a method of setting the average thickness of the spacer layer to 300 angstroms or less (for example, JP-A-4-74487).

[0005]

A conventional semiconductor laser using an InP-based material is configured as described above.
In many cases, excessive impurities in the P-type InP diffuse during the thermal process, so that the InGaAsP active layer is eventually not non-doped. If one or both of the P-type impurity and the N-type impurity diffuse in the InGaAsP active layer, there is a problem that a crystal defect which is a center of non-radiative recombination is formed and the characteristics of the device are deteriorated.

In order to solve the above-mentioned problem, P-
In the method of lowering the impurity concentration in the InP layer or providing a spacer layer between the active layer and the P-InP layer (for example, Japanese Patent Application Laid-Open No. 4-49691), not only the conductivity is lowered but also the pn junction surface is reduced. Move to form a so-called remote junction, which degrades element characteristics. In the method of preventing the decrease in conductivity by setting the thickness of the spacer layer to 300 angstroms or less (for example, JP-A-4-74487), when the impurity concentration in the P-InP layer is high, the impurity in the non-doped InGaAsP active layer is There was a problem that diffusion could not be controlled.

[0007]

According to a method of manufacturing a semiconductor laser using an InP-based material according to the present invention, the concentration of an impurity introduced into a P-type cladding layer is reduced on the side close to an active layer, and the active layer is formed at a low concentration. It is characterized in that the crystal grows so that the concentration increases as the distance from the layer increases. This suppresses the diffusion of the doping impurity into the active layer, and also suppresses a decrease in conductivity and the formation of a remote junction.

[0008]

FIG. 5 shows the operation of the present invention. Since Zn used as a P-type impurity has a high diffusion rate, in the present invention, the doping concentration of the P-type InP cladding layer is set to be low on the side close to the active layer, and is grown to a high concentration as the distance from the active layer increases. I have. It is also known that the diffusion distance of the doped impurity is longer as the concentration is higher and shorter as the concentration is lower. Therefore, when the crystal is grown with the concentration profile as shown by the curve (a) in FIG. 5, when the device is completed, the concentration profile becomes as shown by the curve (b) in FIG. 5, and the diffusion of impurities into the active layer is suppressed. it can.

Further, if the impurity concentration on the side close to the active layer is sufficiently reduced for the above-mentioned reason regarding the diffusion of impurities, as shown by the curve (a) in FIG. Even when the impurity concentration is high, when the device is completed, the concentration profile is as shown by the curve (b) in FIG.
Diffusion of impurities into the active layer can be suppressed.

Further, according to the present invention, the impurity concentration in the P-type InP cladding layer becomes substantially constant at a relatively high concentration when the device is completed because of the above-mentioned reason for impurity diffusion. And the formation of a remote junction can be suppressed.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing the structure of a semiconductor laser according to a first embodiment of the present invention. N- on the N-InP substrate 10
The InP cladding layer 9, the non-doped InGaAsP active layer 8, the i-InP spacer layer 7, and the P-InP cladding layer 6 are provided by crystal growth. Further, the P-InP current blocking layer 5 has a P-type
The N-InP current blocking layer 4 is disposed on the InP cladding layer 6 and the N-InP current blocking layer 4 is disposed on the P-InP current blocking layer 5. Further, the P-InP buried layer 3 is disposed so as to cover the P-InP cladding layer 6 and the N-InP current blocking layer 4. Further, a P ⁺ -InGaAsP cap layer 2
Is disposed on the P-InP layer 3. The N-side electrode 11 is N-
On the back surface of the InP substrate 10, the P-side electrode 1 is made of P-InG
It is provided on the aAsP cap layer 2.

FIG. 2A shows an impurity concentration distribution near an active layer during crystal growth in a semiconductor laser having the above structure.
FIG. 2B shows a final impurity concentration distribution in the vicinity of the active layer in the above structure.

The semiconductor laser of this embodiment is manufactured as follows. First, N-InP is placed on the N-InP substrate 10.
P substrate cladding layer 9, non-doped InGaAsP active layer 8, i-InP spacer layer 7, P-InP cladding layer 6
Are sequentially grown epitaxially by the MOVPE method. A typical thickness of each layer is 0.5 μm for the N-InP cladding layer 9.
m, InGaAsP active layer 8 is 0.12 μm, i-In
The P spacer layer 7 is 0.02 μm, and the P-InP cladding layer 6 is 0.7 μm. In this embodiment, the impurity concentration of the P-InP cladding layer 6 is set to i-In
The side in contact with the P spacer layer 7 is 3 × 10 ¹⁷ cm ⁻³ ,
The side in contact with the InP current blocking layer 5 is 7 × 10 ¹⁷ cm
^The crystal growth is performed by controlling the flow rate of the (CH ₃ ) ₂ Zn gas so as to be about ⁻³ .

Next, a resist is applied on the P-InP cladding layer 6 and a mask is exposed to form a stripe pattern. Thereafter, the N-InP cladding layer 9, the non-doped InGaAsP active layer 8, the i-InP
The channel 12 is provided by removing the spacer layer 7 and the P-InP clad layer 6 while leaving them in a mesa shape. The width of the active layer at this time is about 1.5 μm. Thereafter, the resist is peeled off, and L
P-InP current blocking layer 5, N-InP by PE method
Current block layer 4, P-InP layer 3, and P-InGa
The AsP cap layer 2 is sequentially grown so as to bury the channel portion 12 to obtain a so-called DC-PBH structure. After that, thermal diffusion of impurities is performed on the P-InGaAsP cap layer 2 in order to reduce the contact resistance. Finally, N-In
An N-side electrode 11 is formed on the back surface of the P substrate 10 by using P ⁺ -InGaAs.
The P-side electrode 1 is formed on the surface of the sP cap layer 2 to complete the device.

The semiconductor laser using the InP-based material according to the present invention is configured as described above. During the crystal growth, impurities are introduced as shown in FIG. Is introduced so that the concentration is low on the side close to the active layer and increases as the distance from the active layer increases. Then, during the heating step, the impurities in the high-concentration portion of the P-InP cladding layer 6 diffuse into the low-concentration portion of the cladding layer because of a long diffusion distance. On the other hand, the impurity in the low concentration portion in the clad layer has a short diffusion distance and therefore does not reach the non-doped InGaAsP active layer, and the non-doped state is maintained. As a result, when the device is completed, the impurity concentration is as shown in FIG.

FIG. 9 shows the difference in light output-current characteristics between the semiconductor lasers of the first embodiment and the conventional example. The curve (a) in FIG. 9 is the light output-current characteristic of the first embodiment, and the curve (b) is the light output-current characteristic of the conventional example. As can be seen from this figure, the laser oscillation threshold value of the present invention is 33 mA.
20% to 27mA from the above, and the slope efficiency is reduced to 0.3mA
It improved about 10% from 5 W / A to 0.39 W / A.

Next, a second embodiment of the present invention will be described with reference to FIG. Referring to FIG. 3, the second embodiment of the present invention differs from the first embodiment shown in FIG. 1 in that the i-InP spacer layer 7 is not provided and the non-doped InGaAsP active layer 8 is provided in the lower SC.
H is applied to the layer 8C, the multiple quantum well active layer 8b, and the upper SCH layer 8a.
It is different in that it is configured. At this time, the upper and lower S
InGaAs with a CH layer thickness of 60 nm and a composition of 1.2 μm
sP, multiple quantum well active layer: barrier layer thickness 8 nm, composition 2
μm InGaAsP, well layer thickness 4 nm, composition 1.5
0.9% compressive strain is applied to 5 μm InGaAsP. FIG. 4A shows a profile of impurity introduction during crystal growth, and FIG. 4B shows an impurity profile when the device is completed.

The method for fabricating the semiconductor laser of the second embodiment will be described with reference to the first embodiment shown in FIG.
The difference is that there is no epitaxial growth of the spacer layer 7 and the lower SCH layer 8c, the multiple quantum well active layer 8b and the upper SCH layer 8a are epitaxially grown instead of the non-doped InGaAsP active layer 8. In the second embodiment, the upper SCH
The layer 8a has the same effect as the i-InP spacer layer of the first embodiment. That is, the impurity concentration introduction profile as shown in FIG. 4A during the crystal growth and the impurity concentration profile as shown in FIG. 4B when the device is completed. As described above, in the second embodiment, the i-InP spacer layer can be omitted, and the Fermi level difference between the upper SCH layer and the P-InP cladding layer can be increased, which is advantageous in terms of temperature characteristics. become. In the second embodiment, the oscillation threshold is 3
Reduced from 0 mA to 25 mA with slope efficiency of 0.36
It improved from W / A to 0.41 W / A.

In each of the above embodiments, the impurity concentration in the P-InP cladding layer is continuously increased linearly from the active layer side to the opposite side, but this is a curve. Needless to say, the same effect can be obtained even if the shape is stepwise.

[0020]

As described above, according to the present invention, in a semiconductor laser using an InP-based material, the impurity concentration of the P-InP cladding layer is set to a non-doped InGaAsP or InGaAsP-based or an active layer composed of multiple quantum wells. It grows so that it is low on the side and high on the opposite side. As a result, diffusion of impurities during the thermal process remains in the P-InP cladding layer and does not reach the active layer. Therefore, since the active layer is kept in a non-doped state, carrier non-radiative coupling in the active layer is suppressed. The value of the for the internal absorption loss was reduced from the conventional methods 7.0 cm ^-1 to 4.5 cm ^-1.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a semiconductor laser manufactured according to a first embodiment of the present invention.

FIGS. 2A and 2B are impurity concentration profiles of a semiconductor laser manufactured according to the first embodiment of the present invention.

FIG. 3 is a sectional view showing a semiconductor laser manufactured according to a second embodiment of the present invention.

FIGS. 4A and 4B are impurity concentration profiles of a semiconductor laser manufactured according to a second embodiment of the present invention.

FIG. 5 is a first example of an impurity concentration profile showing the operation of the present invention.

FIG. 6 is a second example of the impurity concentration profile showing the operation of the present invention.

FIG. 7 is a cross-sectional view showing an element structure of a conventional semiconductor laser.

FIGS. 8A and 8B are impurity concentration profiles of a conventional semiconductor laser.

FIG. 9 is a diagram comparing light output-current characteristics of the device of the present invention and a conventional device.

[Explanation of symbols]

Reference Signs List 1 P-side electrode 2 P ⁺ -InP cap layer 3 P-InP layer 4 N-InP current blocking layer 5 P-InP current blocking layer 6 P-InP cladding layer 7 i-InP spacer layer 8 InGaAsP active layer 9 N-InP Cladding layer 10 N-InP substrate 11 N-side electrode 12 channel

Claims (1)

(57) [Claims] 1. An N-type InP substrate comprising a P-type and an N-type cladding layer comprising an InP compound semiconductor and a non-doped InG
In a method of manufacturing a semiconductor laser having an active layer of aAs or InGaAsP system or an active layer composed of multiple quantum wells, when the crystal is grown, the P-type cladding layer is not
In the vicinity of the active layer, the concentration of the pure substance is about 3 × 10 ¹⁷ cm ^−3.
And the higher the distance from the active layer, the higher the concentration.
In the vicinity of the final growth interface of the P-type cladding layer,
About 7 × 10 ¹⁷ cm ⁻³ , the layer of the P-type cladding layer
A method of manufacturing a semiconductor laser , wherein the thickness is about 0.7 μm to control diffusion of impurities into the active layer.